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1 =head1 NAME 2 X<function> 3 4 perlfunc - Perl builtin functions 5 6 =head1 DESCRIPTION 7 8 The functions in this section can serve as terms in an expression. 9 They fall into two major categories: list operators and named unary 10 operators. These differ in their precedence relationship with a 11 following comma. (See the precedence table in L<perlop>.) List 12 operators take more than one argument, while unary operators can never 13 take more than one argument. Thus, a comma terminates the argument of 14 a unary operator, but merely separates the arguments of a list 15 operator. A unary operator generally provides a scalar context to its 16 argument, while a list operator may provide either scalar or list 17 contexts for its arguments. If it does both, the scalar arguments will 18 be first, and the list argument will follow. (Note that there can ever 19 be only one such list argument.) For instance, splice() has three scalar 20 arguments followed by a list, whereas gethostbyname() has four scalar 21 arguments. 22 23 In the syntax descriptions that follow, list operators that expect a 24 list (and provide list context for the elements of the list) are shown 25 with LIST as an argument. Such a list may consist of any combination 26 of scalar arguments or list values; the list values will be included 27 in the list as if each individual element were interpolated at that 28 point in the list, forming a longer single-dimensional list value. 29 Commas should separate elements of the LIST. 30 31 Any function in the list below may be used either with or without 32 parentheses around its arguments. (The syntax descriptions omit the 33 parentheses.) If you use the parentheses, the simple (but occasionally 34 surprising) rule is this: It I<looks> like a function, therefore it I<is> a 35 function, and precedence doesn't matter. Otherwise it's a list 36 operator or unary operator, and precedence does matter. And whitespace 37 between the function and left parenthesis doesn't count--so you need to 38 be careful sometimes: 39 40 print 1+2+4; # Prints 7. 41 print(1+2) + 4; # Prints 3. 42 print (1+2)+4; # Also prints 3! 43 print +(1+2)+4; # Prints 7. 44 print ((1+2)+4); # Prints 7. 45 46 If you run Perl with the B<-w> switch it can warn you about this. For 47 example, the third line above produces: 48 49 print (...) interpreted as function at - line 1. 50 Useless use of integer addition in void context at - line 1. 51 52 A few functions take no arguments at all, and therefore work as neither 53 unary nor list operators. These include such functions as C<time> 54 and C<endpwent>. For example, C<time+86_400> always means 55 C<time() + 86_400>. 56 57 For functions that can be used in either a scalar or list context, 58 nonabortive failure is generally indicated in a scalar context by 59 returning the undefined value, and in a list context by returning the 60 null list. 61 62 Remember the following important rule: There is B<no rule> that relates 63 the behavior of an expression in list context to its behavior in scalar 64 context, or vice versa. It might do two totally different things. 65 Each operator and function decides which sort of value it would be most 66 appropriate to return in scalar context. Some operators return the 67 length of the list that would have been returned in list context. Some 68 operators return the first value in the list. Some operators return the 69 last value in the list. Some operators return a count of successful 70 operations. In general, they do what you want, unless you want 71 consistency. 72 X<context> 73 74 A named array in scalar context is quite different from what would at 75 first glance appear to be a list in scalar context. You can't get a list 76 like C<(1,2,3)> into being in scalar context, because the compiler knows 77 the context at compile time. It would generate the scalar comma operator 78 there, not the list construction version of the comma. That means it 79 was never a list to start with. 80 81 In general, functions in Perl that serve as wrappers for system calls 82 of the same name (like chown(2), fork(2), closedir(2), etc.) all return 83 true when they succeed and C<undef> otherwise, as is usually mentioned 84 in the descriptions below. This is different from the C interfaces, 85 which return C<-1> on failure. Exceptions to this rule are C<wait>, 86 C<waitpid>, and C<syscall>. System calls also set the special C<$!> 87 variable on failure. Other functions do not, except accidentally. 88 89 =head2 Perl Functions by Category 90 X<function> 91 92 Here are Perl's functions (including things that look like 93 functions, like some keywords and named operators) 94 arranged by category. Some functions appear in more 95 than one place. 96 97 =over 4 98 99 =item Functions for SCALARs or strings 100 X<scalar> X<string> X<character> 101 102 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>, 103 C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>, 104 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///> 105 106 =item Regular expressions and pattern matching 107 X<regular expression> X<regex> X<regexp> 108 109 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//> 110 111 =item Numeric functions 112 X<numeric> X<number> X<trigonometric> X<trigonometry> 113 114 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>, 115 C<sin>, C<sqrt>, C<srand> 116 117 =item Functions for real @ARRAYs 118 X<array> 119 120 C<pop>, C<push>, C<shift>, C<splice>, C<unshift> 121 122 =item Functions for list data 123 X<list> 124 125 C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack> 126 127 =item Functions for real %HASHes 128 X<hash> 129 130 C<delete>, C<each>, C<exists>, C<keys>, C<values> 131 132 =item Input and output functions 133 X<I/O> X<input> X<output> X<dbm> 134 135 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>, 136 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>, 137 C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>, 138 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>, 139 C<warn>, C<write> 140 141 =item Functions for fixed length data or records 142 143 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec> 144 145 =item Functions for filehandles, files, or directories 146 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink> 147 148 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>, 149 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, 150 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>, 151 C<umask>, C<unlink>, C<utime> 152 153 =item Keywords related to the control flow of your Perl program 154 X<control flow> 155 156 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>, 157 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray> 158 159 =item Keywords related to switch 160 161 C<break>, C<continue>, C<given>, C<when>, C<default> 162 163 (These are only available if you enable the "switch" feature. 164 See L<feature> and L<perlsyn/"Switch statements">.) 165 166 =item Keywords related to scoping 167 168 C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>, 169 C<use> 170 171 (C<state> is only available if the "state" feature is enabled. See 172 L<feature>.) 173 174 =item Miscellaneous functions 175 176 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, 177 C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray> 178 179 =item Functions for processes and process groups 180 X<process> X<pid> X<process id> 181 182 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>, 183 C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>, 184 C<times>, C<wait>, C<waitpid> 185 186 =item Keywords related to perl modules 187 X<module> 188 189 C<do>, C<import>, C<no>, C<package>, C<require>, C<use> 190 191 =item Keywords related to classes and object-orientation 192 X<object> X<class> X<package> 193 194 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>, 195 C<untie>, C<use> 196 197 =item Low-level socket functions 198 X<socket> X<sock> 199 200 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>, 201 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>, 202 C<socket>, C<socketpair> 203 204 =item System V interprocess communication functions 205 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message> 206 207 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>, 208 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite> 209 210 =item Fetching user and group info 211 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd> 212 213 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>, 214 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>, 215 C<getpwuid>, C<setgrent>, C<setpwent> 216 217 =item Fetching network info 218 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service> 219 220 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>, 221 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, 222 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>, 223 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>, 224 C<setnetent>, C<setprotoent>, C<setservent> 225 226 =item Time-related functions 227 X<time> X<date> 228 229 C<gmtime>, C<localtime>, C<time>, C<times> 230 231 =item Functions new in perl5 232 X<perl5> 233 234 C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>, 235 C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>, 236 C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>, 237 C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>, 238 C<ucfirst>, C<untie>, C<use>, C<when> 239 240 * - C<sub> was a keyword in perl4, but in perl5 it is an 241 operator, which can be used in expressions. 242 243 =item Functions obsoleted in perl5 244 245 C<dbmclose>, C<dbmopen> 246 247 =back 248 249 =head2 Portability 250 X<portability> X<Unix> X<portable> 251 252 Perl was born in Unix and can therefore access all common Unix 253 system calls. In non-Unix environments, the functionality of some 254 Unix system calls may not be available, or details of the available 255 functionality may differ slightly. The Perl functions affected 256 by this are: 257 258 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>, 259 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>, 260 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>, 261 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>, 262 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, 263 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>, 264 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>, 265 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>, 266 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>, 267 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>, 268 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>, 269 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>, 270 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>, 271 C<shmwrite>, C<socket>, C<socketpair>, 272 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>, 273 C<times>, C<truncate>, C<umask>, C<unlink>, 274 C<utime>, C<wait>, C<waitpid> 275 276 For more information about the portability of these functions, see 277 L<perlport> and other available platform-specific documentation. 278 279 =head2 Alphabetical Listing of Perl Functions 280 281 =over 8 282 283 =item -X FILEHANDLE 284 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p> 285 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C> 286 287 =item -X EXPR 288 289 =item -X DIRHANDLE 290 291 =item -X 292 293 A file test, where X is one of the letters listed below. This unary 294 operator takes one argument, either a filename, a filehandle, or a dirhandle, 295 and tests the associated file to see if something is true about it. If the 296 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN. 297 Unless otherwise documented, it returns C<1> for true and C<''> for false, or 298 the undefined value if the file doesn't exist. Despite the funny 299 names, precedence is the same as any other named unary operator. The 300 operator may be any of: 301 302 -r File is readable by effective uid/gid. 303 -w File is writable by effective uid/gid. 304 -x File is executable by effective uid/gid. 305 -o File is owned by effective uid. 306 307 -R File is readable by real uid/gid. 308 -W File is writable by real uid/gid. 309 -X File is executable by real uid/gid. 310 -O File is owned by real uid. 311 312 -e File exists. 313 -z File has zero size (is empty). 314 -s File has nonzero size (returns size in bytes). 315 316 -f File is a plain file. 317 -d File is a directory. 318 -l File is a symbolic link. 319 -p File is a named pipe (FIFO), or Filehandle is a pipe. 320 -S File is a socket. 321 -b File is a block special file. 322 -c File is a character special file. 323 -t Filehandle is opened to a tty. 324 325 -u File has setuid bit set. 326 -g File has setgid bit set. 327 -k File has sticky bit set. 328 329 -T File is an ASCII text file (heuristic guess). 330 -B File is a "binary" file (opposite of -T). 331 332 -M Script start time minus file modification time, in days. 333 -A Same for access time. 334 -C Same for inode change time (Unix, may differ for other platforms) 335 336 Example: 337 338 while (<>) { 339 chomp; 340 next unless -f $_; # ignore specials 341 #... 342 } 343 344 The interpretation of the file permission operators C<-r>, C<-R>, 345 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode 346 of the file and the uids and gids of the user. There may be other 347 reasons you can't actually read, write, or execute the file: for 348 example network filesystem access controls, ACLs (access control lists), 349 read-only filesystems, and unrecognized executable formats. Note 350 that the use of these six specific operators to verify if some operation 351 is possible is usually a mistake, because it may be open to race 352 conditions. 353 354 Also note that, for the superuser on the local filesystems, the C<-r>, 355 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1 356 if any execute bit is set in the mode. Scripts run by the superuser 357 may thus need to do a stat() to determine the actual mode of the file, 358 or temporarily set their effective uid to something else. 359 360 If you are using ACLs, there is a pragma called C<filetest> that may 361 produce more accurate results than the bare stat() mode bits. 362 When under the C<use filetest 'access'> the above-mentioned filetests 363 will test whether the permission can (not) be granted using the 364 access() family of system calls. Also note that the C<-x> and C<-X> may 365 under this pragma return true even if there are no execute permission 366 bits set (nor any extra execute permission ACLs). This strangeness is 367 due to the underlying system calls' definitions. Note also that, due to 368 the implementation of C<use filetest 'access'>, the C<_> special 369 filehandle won't cache the results of the file tests when this pragma is 370 in effect. Read the documentation for the C<filetest> pragma for more 371 information. 372 373 Note that C<-s/a/b/> does not do a negated substitution. Saying 374 C<-exp($foo)> still works as expected, however--only single letters 375 following a minus are interpreted as file tests. 376 377 The C<-T> and C<-B> switches work as follows. The first block or so of the 378 file is examined for odd characters such as strange control codes or 379 characters with the high bit set. If too many strange characters (>30%) 380 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file 381 containing null in the first block is considered a binary file. If C<-T> 382 or C<-B> is used on a filehandle, the current IO buffer is examined 383 rather than the first block. Both C<-T> and C<-B> return true on a null 384 file, or a file at EOF when testing a filehandle. Because you have to 385 read a file to do the C<-T> test, on most occasions you want to use a C<-f> 386 against the file first, as in C<next unless -f $file && -T $file>. 387 388 If any of the file tests (or either the C<stat> or C<lstat> operators) are given 389 the special filehandle consisting of a solitary underline, then the stat 390 structure of the previous file test (or stat operator) is used, saving 391 a system call. (This doesn't work with C<-t>, and you need to remember 392 that lstat() and C<-l> will leave values in the stat structure for the 393 symbolic link, not the real file.) (Also, if the stat buffer was filled by 394 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>). 395 Example: 396 397 print "Can do.\n" if -r $a || -w _ || -x _; 398 399 stat($filename); 400 print "Readable\n" if -r _; 401 print "Writable\n" if -w _; 402 print "Executable\n" if -x _; 403 print "Setuid\n" if -u _; 404 print "Setgid\n" if -g _; 405 print "Sticky\n" if -k _; 406 print "Text\n" if -T _; 407 print "Binary\n" if -B _; 408 409 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file 410 test operators, in a way that C<-f -w -x $file> is equivalent to 411 C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use 412 the return value of C<-f $file> as an argument to another filetest 413 operator, no special magic will happen.) 414 415 =item abs VALUE 416 X<abs> X<absolute> 417 418 =item abs 419 420 Returns the absolute value of its argument. 421 If VALUE is omitted, uses C<$_>. 422 423 =item accept NEWSOCKET,GENERICSOCKET 424 X<accept> 425 426 Accepts an incoming socket connect, just as the accept(2) system call 427 does. Returns the packed address if it succeeded, false otherwise. 428 See the example in L<perlipc/"Sockets: Client/Server Communication">. 429 430 On systems that support a close-on-exec flag on files, the flag will 431 be set for the newly opened file descriptor, as determined by the 432 value of $^F. See L<perlvar/$^F>. 433 434 =item alarm SECONDS 435 X<alarm> 436 X<SIGALRM> 437 X<timer> 438 439 =item alarm 440 441 Arranges to have a SIGALRM delivered to this process after the 442 specified number of wallclock seconds has elapsed. If SECONDS is not 443 specified, the value stored in C<$_> is used. (On some machines, 444 unfortunately, the elapsed time may be up to one second less or more 445 than you specified because of how seconds are counted, and process 446 scheduling may delay the delivery of the signal even further.) 447 448 Only one timer may be counting at once. Each call disables the 449 previous timer, and an argument of C<0> may be supplied to cancel the 450 previous timer without starting a new one. The returned value is the 451 amount of time remaining on the previous timer. 452 453 For delays of finer granularity than one second, the Time::HiRes module 454 (from CPAN, and starting from Perl 5.8 part of the standard 455 distribution) provides ualarm(). You may also use Perl's four-argument 456 version of select() leaving the first three arguments undefined, or you 457 might be able to use the C<syscall> interface to access setitimer(2) if 458 your system supports it. See L<perlfaq8> for details. 459 460 It is usually a mistake to intermix C<alarm> and C<sleep> calls. 461 (C<sleep> may be internally implemented in your system with C<alarm>) 462 463 If you want to use C<alarm> to time out a system call you need to use an 464 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to 465 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to 466 restart system calls on some systems. Using C<eval>/C<die> always works, 467 modulo the caveats given in L<perlipc/"Signals">. 468 469 eval { 470 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required 471 alarm $timeout; 472 $nread = sysread SOCKET, $buffer, $size; 473 alarm 0; 474 }; 475 if ($@) { 476 die unless $@ eq "alarm\n"; # propagate unexpected errors 477 # timed out 478 } 479 else { 480 # didn't 481 } 482 483 For more information see L<perlipc>. 484 485 =item atan2 Y,X 486 X<atan2> X<arctangent> X<tan> X<tangent> 487 488 Returns the arctangent of Y/X in the range -PI to PI. 489 490 For the tangent operation, you may use the C<Math::Trig::tan> 491 function, or use the familiar relation: 492 493 sub tan { sin($_[0]) / cos($_[0]) } 494 495 Note that atan2(0, 0) is not well-defined. 496 497 =item bind SOCKET,NAME 498 X<bind> 499 500 Binds a network address to a socket, just as the bind system call 501 does. Returns true if it succeeded, false otherwise. NAME should be a 502 packed address of the appropriate type for the socket. See the examples in 503 L<perlipc/"Sockets: Client/Server Communication">. 504 505 =item binmode FILEHANDLE, LAYER 506 X<binmode> X<binary> X<text> X<DOS> X<Windows> 507 508 =item binmode FILEHANDLE 509 510 Arranges for FILEHANDLE to be read or written in "binary" or "text" 511 mode on systems where the run-time libraries distinguish between 512 binary and text files. If FILEHANDLE is an expression, the value is 513 taken as the name of the filehandle. Returns true on success, 514 otherwise it returns C<undef> and sets C<$!> (errno). 515 516 On some systems (in general, DOS and Windows-based systems) binmode() 517 is necessary when you're not working with a text file. For the sake 518 of portability it is a good idea to always use it when appropriate, 519 and to never use it when it isn't appropriate. Also, people can 520 set their I/O to be by default UTF-8 encoded Unicode, not bytes. 521 522 In other words: regardless of platform, use binmode() on binary data, 523 like for example images. 524 525 If LAYER is present it is a single string, but may contain multiple 526 directives. The directives alter the behaviour of the file handle. 527 When LAYER is present using binmode on text file makes sense. 528 529 If LAYER is omitted or specified as C<:raw> the filehandle is made 530 suitable for passing binary data. This includes turning off possible CRLF 531 translation and marking it as bytes (as opposed to Unicode characters). 532 Note that, despite what may be implied in I<"Programming Perl"> (the 533 Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf> 534 -- other layers which would affect the binary nature of the stream are 535 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the 536 PERLIO environment variable. 537 538 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the 539 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to 540 establish default I/O layers. See L<open>. 541 542 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE" 543 in "Programming Perl, 3rd Edition". However, since the publishing of this 544 book, by many known as "Camel III", the consensus of the naming of this 545 functionality has moved from "discipline" to "layer". All documentation 546 of this version of Perl therefore refers to "layers" rather than to 547 "disciplines". Now back to the regularly scheduled documentation...> 548 549 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>. 550 C<:utf8> just marks the data as UTF-8 without further checking, 551 while C<:encoding(utf8)> checks the data for actually being valid 552 UTF-8. More details can be found in L<PerlIO::encoding>. 553 554 In general, binmode() should be called after open() but before any I/O 555 is done on the filehandle. Calling binmode() will normally flush any 556 pending buffered output data (and perhaps pending input data) on the 557 handle. An exception to this is the C<:encoding> layer that 558 changes the default character encoding of the handle, see L<open>. 559 The C<:encoding> layer sometimes needs to be called in 560 mid-stream, and it doesn't flush the stream. The C<:encoding> 561 also implicitly pushes on top of itself the C<:utf8> layer because 562 internally Perl will operate on UTF-8 encoded Unicode characters. 563 564 The operating system, device drivers, C libraries, and Perl run-time 565 system all work together to let the programmer treat a single 566 character (C<\n>) as the line terminator, irrespective of the external 567 representation. On many operating systems, the native text file 568 representation matches the internal representation, but on some 569 platforms the external representation of C<\n> is made up of more than 570 one character. 571 572 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single 573 character to end each line in the external representation of text (even 574 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED 575 on Unix and most VMS files). In other systems like OS/2, DOS and the 576 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, 577 but what's stored in text files are the two characters C<\cM\cJ>. That 578 means that, if you don't use binmode() on these systems, C<\cM\cJ> 579 sequences on disk will be converted to C<\n> on input, and any C<\n> in 580 your program will be converted back to C<\cM\cJ> on output. This is what 581 you want for text files, but it can be disastrous for binary files. 582 583 Another consequence of using binmode() (on some systems) is that 584 special end-of-file markers will be seen as part of the data stream. 585 For systems from the Microsoft family this means that if your binary 586 data contains C<\cZ>, the I/O subsystem will regard it as the end of 587 the file, unless you use binmode(). 588 589 binmode() is not only important for readline() and print() operations, 590 but also when using read(), seek(), sysread(), syswrite() and tell() 591 (see L<perlport> for more details). See the C<$/> and C<$\> variables 592 in L<perlvar> for how to manually set your input and output 593 line-termination sequences. 594 595 =item bless REF,CLASSNAME 596 X<bless> 597 598 =item bless REF 599 600 This function tells the thingy referenced by REF that it is now an object 601 in the CLASSNAME package. If CLASSNAME is omitted, the current package 602 is used. Because a C<bless> is often the last thing in a constructor, 603 it returns the reference for convenience. Always use the two-argument 604 version if a derived class might inherit the function doing the blessing. 605 See L<perltoot> and L<perlobj> for more about the blessing (and blessings) 606 of objects. 607 608 Consider always blessing objects in CLASSNAMEs that are mixed case. 609 Namespaces with all lowercase names are considered reserved for 610 Perl pragmata. Builtin types have all uppercase names. To prevent 611 confusion, you may wish to avoid such package names as well. Make sure 612 that CLASSNAME is a true value. 613 614 See L<perlmod/"Perl Modules">. 615 616 =item break 617 618 Break out of a C<given()> block. 619 620 This keyword is enabled by the "switch" feature: see L<feature> 621 for more information. 622 623 =item caller EXPR 624 X<caller> X<call stack> X<stack> X<stack trace> 625 626 =item caller 627 628 Returns the context of the current subroutine call. In scalar context, 629 returns the caller's package name if there is a caller, that is, if 630 we're in a subroutine or C<eval> or C<require>, and the undefined value 631 otherwise. In list context, returns 632 633 # 0 1 2 634 ($package, $filename, $line) = caller; 635 636 With EXPR, it returns some extra information that the debugger uses to 637 print a stack trace. The value of EXPR indicates how many call frames 638 to go back before the current one. 639 640 # 0 1 2 3 4 641 ($package, $filename, $line, $subroutine, $hasargs, 642 643 # 5 6 7 8 9 10 644 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash) 645 = caller($i); 646 647 Here $subroutine may be C<(eval)> if the frame is not a subroutine 648 call, but an C<eval>. In such a case additional elements $evaltext and 649 C<$is_require> are set: C<$is_require> is true if the frame is created by a 650 C<require> or C<use> statement, $evaltext contains the text of the 651 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement, 652 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that 653 each C<use> statement creates a C<require> frame inside an C<eval EXPR> 654 frame.) $subroutine may also be C<(unknown)> if this particular 655 subroutine happens to have been deleted from the symbol table. 656 C<$hasargs> is true if a new instance of C<@_> was set up for the frame. 657 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was 658 compiled with. The C<$hints> and C<$bitmask> values are subject to change 659 between versions of Perl, and are not meant for external use. 660 661 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the 662 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values 663 of this hash, as they are the actual values stored in the optree. 664 665 Furthermore, when called from within the DB package, caller returns more 666 detailed information: it sets the list variable C<@DB::args> to be the 667 arguments with which the subroutine was invoked. 668 669 Be aware that the optimizer might have optimized call frames away before 670 C<caller> had a chance to get the information. That means that C<caller(N)> 671 might not return information about the call frame you expect it do, for 672 C<< N > 1 >>. In particular, C<@DB::args> might have information from the 673 previous time C<caller> was called. 674 675 =item chdir EXPR 676 X<chdir> 677 X<cd> 678 X<directory, change> 679 680 =item chdir FILEHANDLE 681 682 =item chdir DIRHANDLE 683 684 =item chdir 685 686 Changes the working directory to EXPR, if possible. If EXPR is omitted, 687 changes to the directory specified by C<$ENV{HOME}>, if set; if not, 688 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the 689 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If 690 neither is set, C<chdir> does nothing. It returns true upon success, 691 false otherwise. See the example under C<die>. 692 693 On systems that support fchdir, you might pass a file handle or 694 directory handle as argument. On systems that don't support fchdir, 695 passing handles produces a fatal error at run time. 696 697 =item chmod LIST 698 X<chmod> X<permission> X<mode> 699 700 Changes the permissions of a list of files. The first element of the 701 list must be the numerical mode, which should probably be an octal 702 number, and which definitely should I<not> be a string of octal digits: 703 C<0644> is okay, C<'0644'> is not. Returns the number of files 704 successfully changed. See also L</oct>, if all you have is a string. 705 706 $cnt = chmod 0755, 'foo', 'bar'; 707 chmod 0755, @executables; 708 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to 709 # --w----r-T 710 $mode = '0644'; chmod oct($mode), 'foo'; # this is better 711 $mode = 0644; chmod $mode, 'foo'; # this is best 712 713 On systems that support fchmod, you might pass file handles among the 714 files. On systems that don't support fchmod, passing file handles 715 produces a fatal error at run time. The file handles must be passed 716 as globs or references to be recognized. Barewords are considered 717 file names. 718 719 open(my $fh, "<", "foo"); 720 my $perm = (stat $fh)[2] & 07777; 721 chmod($perm | 0600, $fh); 722 723 You can also import the symbolic C<S_I*> constants from the Fcntl 724 module: 725 726 use Fcntl ':mode'; 727 728 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; 729 # This is identical to the chmod 0755 of the above example. 730 731 =item chomp VARIABLE 732 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol> 733 734 =item chomp( LIST ) 735 736 =item chomp 737 738 This safer version of L</chop> removes any trailing string 739 that corresponds to the current value of C<$/> (also known as 740 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total 741 number of characters removed from all its arguments. It's often used to 742 remove the newline from the end of an input record when you're worried 743 that the final record may be missing its newline. When in paragraph 744 mode (C<$/ = "">), it removes all trailing newlines from the string. 745 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is 746 a reference to an integer or the like, see L<perlvar>) chomp() won't 747 remove anything. 748 If VARIABLE is omitted, it chomps C<$_>. Example: 749 750 while (<>) { 751 chomp; # avoid \n on last field 752 @array = split(/:/); 753 # ... 754 } 755 756 If VARIABLE is a hash, it chomps the hash's values, but not its keys. 757 758 You can actually chomp anything that's an lvalue, including an assignment: 759 760 chomp($cwd = `pwd`); 761 chomp($answer = <STDIN>); 762 763 If you chomp a list, each element is chomped, and the total number of 764 characters removed is returned. 765 766 Note that parentheses are necessary when you're chomping anything 767 that is not a simple variable. This is because C<chomp $cwd = `pwd`;> 768 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as 769 C<chomp( $cwd = `pwd` )> which you might expect. Similarly, 770 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than 771 as C<chomp($a, $b)>. 772 773 =item chop VARIABLE 774 X<chop> 775 776 =item chop( LIST ) 777 778 =item chop 779 780 Chops off the last character of a string and returns the character 781 chopped. It is much more efficient than C<s/.$//s> because it neither 782 scans nor copies the string. If VARIABLE is omitted, chops C<$_>. 783 If VARIABLE is a hash, it chops the hash's values, but not its keys. 784 785 You can actually chop anything that's an lvalue, including an assignment. 786 787 If you chop a list, each element is chopped. Only the value of the 788 last C<chop> is returned. 789 790 Note that C<chop> returns the last character. To return all but the last 791 character, use C<substr($string, 0, -1)>. 792 793 See also L</chomp>. 794 795 =item chown LIST 796 X<chown> X<owner> X<user> X<group> 797 798 Changes the owner (and group) of a list of files. The first two 799 elements of the list must be the I<numeric> uid and gid, in that 800 order. A value of -1 in either position is interpreted by most 801 systems to leave that value unchanged. Returns the number of files 802 successfully changed. 803 804 $cnt = chown $uid, $gid, 'foo', 'bar'; 805 chown $uid, $gid, @filenames; 806 807 On systems that support fchown, you might pass file handles among the 808 files. On systems that don't support fchown, passing file handles 809 produces a fatal error at run time. The file handles must be passed 810 as globs or references to be recognized. Barewords are considered 811 file names. 812 813 Here's an example that looks up nonnumeric uids in the passwd file: 814 815 print "User: "; 816 chomp($user = <STDIN>); 817 print "Files: "; 818 chomp($pattern = <STDIN>); 819 820 ($login,$pass,$uid,$gid) = getpwnam($user) 821 or die "$user not in passwd file"; 822 823 @ary = glob($pattern); # expand filenames 824 chown $uid, $gid, @ary; 825 826 On most systems, you are not allowed to change the ownership of the 827 file unless you're the superuser, although you should be able to change 828 the group to any of your secondary groups. On insecure systems, these 829 restrictions may be relaxed, but this is not a portable assumption. 830 On POSIX systems, you can detect this condition this way: 831 832 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED); 833 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED); 834 835 =item chr NUMBER 836 X<chr> X<character> X<ASCII> X<Unicode> 837 838 =item chr 839 840 Returns the character represented by that NUMBER in the character set. 841 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and 842 chr(0x263a) is a Unicode smiley face. 843 844 Negative values give the Unicode replacement character (chr(0xfffd)), 845 except under the L<bytes> pragma, where low eight bits of the value 846 (truncated to an integer) are used. 847 848 If NUMBER is omitted, uses C<$_>. 849 850 For the reverse, use L</ord>. 851 852 Note that characters from 128 to 255 (inclusive) are by default 853 internally not encoded as UTF-8 for backward compatibility reasons. 854 855 See L<perlunicode> for more about Unicode. 856 857 =item chroot FILENAME 858 X<chroot> X<root> 859 860 =item chroot 861 862 This function works like the system call by the same name: it makes the 863 named directory the new root directory for all further pathnames that 864 begin with a C</> by your process and all its children. (It doesn't 865 change your current working directory, which is unaffected.) For security 866 reasons, this call is restricted to the superuser. If FILENAME is 867 omitted, does a C<chroot> to C<$_>. 868 869 =item close FILEHANDLE 870 X<close> 871 872 =item close 873 874 Closes the file or pipe associated with the file handle, flushes the IO 875 buffers, and closes the system file descriptor. Returns true if those 876 operations have succeeded and if no error was reported by any PerlIO 877 layer. Closes the currently selected filehandle if the argument is 878 omitted. 879 880 You don't have to close FILEHANDLE if you are immediately going to do 881 another C<open> on it, because C<open> will close it for you. (See 882 C<open>.) However, an explicit C<close> on an input file resets the line 883 counter (C<$.>), while the implicit close done by C<open> does not. 884 885 If the file handle came from a piped open, C<close> will additionally 886 return false if one of the other system calls involved fails, or if the 887 program exits with non-zero status. (If the only problem was that the 888 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe 889 also waits for the process executing on the pipe to complete, in case you 890 want to look at the output of the pipe afterwards, and 891 implicitly puts the exit status value of that command into C<$?> and 892 C<${^CHILD_ERROR_NATIVE}>. 893 894 Prematurely closing the read end of a pipe (i.e. before the process 895 writing to it at the other end has closed it) will result in a 896 SIGPIPE being delivered to the writer. If the other end can't 897 handle that, be sure to read all the data before closing the pipe. 898 899 Example: 900 901 open(OUTPUT, '|sort >foo') # pipe to sort 902 or die "Can't start sort: $!"; 903 #... # print stuff to output 904 close OUTPUT # wait for sort to finish 905 or warn $! ? "Error closing sort pipe: $!" 906 : "Exit status $? from sort"; 907 open(INPUT, 'foo') # get sort's results 908 or die "Can't open 'foo' for input: $!"; 909 910 FILEHANDLE may be an expression whose value can be used as an indirect 911 filehandle, usually the real filehandle name. 912 913 =item closedir DIRHANDLE 914 X<closedir> 915 916 Closes a directory opened by C<opendir> and returns the success of that 917 system call. 918 919 =item connect SOCKET,NAME 920 X<connect> 921 922 Attempts to connect to a remote socket, just as the connect system call 923 does. Returns true if it succeeded, false otherwise. NAME should be a 924 packed address of the appropriate type for the socket. See the examples in 925 L<perlipc/"Sockets: Client/Server Communication">. 926 927 =item continue BLOCK 928 X<continue> 929 930 =item continue 931 932 C<continue> is actually a flow control statement rather than a function. If 933 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or 934 C<foreach>), it is always executed just before the conditional is about to 935 be evaluated again, just like the third part of a C<for> loop in C. Thus 936 it can be used to increment a loop variable, even when the loop has been 937 continued via the C<next> statement (which is similar to the C C<continue> 938 statement). 939 940 C<last>, C<next>, or C<redo> may appear within a C<continue> 941 block. C<last> and C<redo> will behave as if they had been executed within 942 the main block. So will C<next>, but since it will execute a C<continue> 943 block, it may be more entertaining. 944 945 while (EXPR) { 946 ### redo always comes here 947 do_something; 948 } continue { 949 ### next always comes here 950 do_something_else; 951 # then back the top to re-check EXPR 952 } 953 ### last always comes here 954 955 Omitting the C<continue> section is semantically equivalent to using an 956 empty one, logically enough. In that case, C<next> goes directly back 957 to check the condition at the top of the loop. 958 959 If the "switch" feature is enabled, C<continue> is also a 960 function that will break out of the current C<when> or C<default> 961 block, and fall through to the next case. See L<feature> and 962 L<perlsyn/"Switch statements"> for more information. 963 964 965 =item cos EXPR 966 X<cos> X<cosine> X<acos> X<arccosine> 967 968 =item cos 969 970 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, 971 takes cosine of C<$_>. 972 973 For the inverse cosine operation, you may use the C<Math::Trig::acos()> 974 function, or use this relation: 975 976 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } 977 978 =item crypt PLAINTEXT,SALT 979 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password> 980 X<decrypt> X<cryptography> X<passwd> X<encrypt> 981 982 Creates a digest string exactly like the crypt(3) function in the C 983 library (assuming that you actually have a version there that has not 984 been extirpated as a potential munitions). 985 986 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned 987 into a short string, called a digest, which is returned. The same 988 PLAINTEXT and SALT will always return the same string, but there is no 989 (known) way to get the original PLAINTEXT from the hash. Small 990 changes in the PLAINTEXT or SALT will result in large changes in the 991 digest. 992 993 There is no decrypt function. This function isn't all that useful for 994 cryptography (for that, look for F<Crypt> modules on your nearby CPAN 995 mirror) and the name "crypt" is a bit of a misnomer. Instead it is 996 primarily used to check if two pieces of text are the same without 997 having to transmit or store the text itself. An example is checking 998 if a correct password is given. The digest of the password is stored, 999 not the password itself. The user types in a password that is 1000 crypt()'d with the same salt as the stored digest. If the two digests 1001 match the password is correct. 1002 1003 When verifying an existing digest string you should use the digest as 1004 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used 1005 to create the digest is visible as part of the digest. This ensures 1006 crypt() will hash the new string with the same salt as the digest. 1007 This allows your code to work with the standard L<crypt|/crypt> and 1008 with more exotic implementations. In other words, do not assume 1009 anything about the returned string itself, or how many bytes in the 1010 digest matter. 1011 1012 Traditionally the result is a string of 13 bytes: two first bytes of 1013 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only 1014 the first eight bytes of the digest string mattered, but alternative 1015 hashing schemes (like MD5), higher level security schemes (like C2), 1016 and implementations on non-UNIX platforms may produce different 1017 strings. 1018 1019 When choosing a new salt create a random two character string whose 1020 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.', 1021 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of 1022 characters is just a recommendation; the characters allowed in 1023 the salt depend solely on your system's crypt library, and Perl can't 1024 restrict what salts C<crypt()> accepts. 1025 1026 Here's an example that makes sure that whoever runs this program knows 1027 their password: 1028 1029 $pwd = (getpwuid($<))[1]; 1030 1031 system "stty -echo"; 1032 print "Password: "; 1033 chomp($word = <STDIN>); 1034 print "\n"; 1035 system "stty echo"; 1036 1037 if (crypt($word, $pwd) ne $pwd) { 1038 die "Sorry...\n"; 1039 } else { 1040 print "ok\n"; 1041 } 1042 1043 Of course, typing in your own password to whoever asks you 1044 for it is unwise. 1045 1046 The L<crypt|/crypt> function is unsuitable for hashing large quantities 1047 of data, not least of all because you can't get the information 1048 back. Look at the L<Digest> module for more robust algorithms. 1049 1050 If using crypt() on a Unicode string (which I<potentially> has 1051 characters with codepoints above 255), Perl tries to make sense 1052 of the situation by trying to downgrade (a copy of the string) 1053 the string back to an eight-bit byte string before calling crypt() 1054 (on that copy). If that works, good. If not, crypt() dies with 1055 C<Wide character in crypt>. 1056 1057 =item dbmclose HASH 1058 X<dbmclose> 1059 1060 [This function has been largely superseded by the C<untie> function.] 1061 1062 Breaks the binding between a DBM file and a hash. 1063 1064 =item dbmopen HASH,DBNAME,MASK 1065 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm> 1066 1067 [This function has been largely superseded by the C<tie> function.] 1068 1069 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a 1070 hash. HASH is the name of the hash. (Unlike normal C<open>, the first 1071 argument is I<not> a filehandle, even though it looks like one). DBNAME 1072 is the name of the database (without the F<.dir> or F<.pag> extension if 1073 any). If the database does not exist, it is created with protection 1074 specified by MASK (as modified by the C<umask>). If your system supports 1075 only the older DBM functions, you may perform only one C<dbmopen> in your 1076 program. In older versions of Perl, if your system had neither DBM nor 1077 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to 1078 sdbm(3). 1079 1080 If you don't have write access to the DBM file, you can only read hash 1081 variables, not set them. If you want to test whether you can write, 1082 either use file tests or try setting a dummy hash entry inside an C<eval>, 1083 which will trap the error. 1084 1085 Note that functions such as C<keys> and C<values> may return huge lists 1086 when used on large DBM files. You may prefer to use the C<each> 1087 function to iterate over large DBM files. Example: 1088 1089 # print out history file offsets 1090 dbmopen(%HIST,'/usr/lib/news/history',0666); 1091 while (($key,$val) = each %HIST) { 1092 print $key, ' = ', unpack('L',$val), "\n"; 1093 } 1094 dbmclose(%HIST); 1095 1096 See also L<AnyDBM_File> for a more general description of the pros and 1097 cons of the various dbm approaches, as well as L<DB_File> for a particularly 1098 rich implementation. 1099 1100 You can control which DBM library you use by loading that library 1101 before you call dbmopen(): 1102 1103 use DB_File; 1104 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db") 1105 or die "Can't open netscape history file: $!"; 1106 1107 =item defined EXPR 1108 X<defined> X<undef> X<undefined> 1109 1110 =item defined 1111 1112 Returns a Boolean value telling whether EXPR has a value other than 1113 the undefined value C<undef>. If EXPR is not present, C<$_> will be 1114 checked. 1115 1116 Many operations return C<undef> to indicate failure, end of file, 1117 system error, uninitialized variable, and other exceptional 1118 conditions. This function allows you to distinguish C<undef> from 1119 other values. (A simple Boolean test will not distinguish among 1120 C<undef>, zero, the empty string, and C<"0">, which are all equally 1121 false.) Note that since C<undef> is a valid scalar, its presence 1122 doesn't I<necessarily> indicate an exceptional condition: C<pop> 1123 returns C<undef> when its argument is an empty array, I<or> when the 1124 element to return happens to be C<undef>. 1125 1126 You may also use C<defined(&func)> to check whether subroutine C<&func> 1127 has ever been defined. The return value is unaffected by any forward 1128 declarations of C<&func>. Note that a subroutine which is not defined 1129 may still be callable: its package may have an C<AUTOLOAD> method that 1130 makes it spring into existence the first time that it is called -- see 1131 L<perlsub>. 1132 1133 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It 1134 used to report whether memory for that aggregate has ever been 1135 allocated. This behavior may disappear in future versions of Perl. 1136 You should instead use a simple test for size: 1137 1138 if (@an_array) { print "has array elements\n" } 1139 if (%a_hash) { print "has hash members\n" } 1140 1141 When used on a hash element, it tells you whether the value is defined, 1142 not whether the key exists in the hash. Use L</exists> for the latter 1143 purpose. 1144 1145 Examples: 1146 1147 print if defined $switch{'D'}; 1148 print "$val\n" while defined($val = pop(@ary)); 1149 die "Can't readlink $sym: $!" 1150 unless defined($value = readlink $sym); 1151 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } 1152 $debugging = 0 unless defined $debugging; 1153 1154 Note: Many folks tend to overuse C<defined>, and then are surprised to 1155 discover that the number C<0> and C<""> (the zero-length string) are, in fact, 1156 defined values. For example, if you say 1157 1158 "ab" =~ /a(.*)b/; 1159 1160 The pattern match succeeds, and C<$1> is defined, despite the fact that it 1161 matched "nothing". It didn't really fail to match anything. Rather, it 1162 matched something that happened to be zero characters long. This is all 1163 very above-board and honest. When a function returns an undefined value, 1164 it's an admission that it couldn't give you an honest answer. So you 1165 should use C<defined> only when you're questioning the integrity of what 1166 you're trying to do. At other times, a simple comparison to C<0> or C<""> is 1167 what you want. 1168 1169 See also L</undef>, L</exists>, L</ref>. 1170 1171 =item delete EXPR 1172 X<delete> 1173 1174 Given an expression that specifies a hash element, array element, hash slice, 1175 or array slice, deletes the specified element(s) from the hash or array. 1176 In the case of an array, if the array elements happen to be at the end, 1177 the size of the array will shrink to the highest element that tests 1178 true for exists() (or 0 if no such element exists). 1179 1180 Returns a list with the same number of elements as the number of elements 1181 for which deletion was attempted. Each element of that list consists of 1182 either the value of the element deleted, or the undefined value. In scalar 1183 context, this means that you get the value of the last element deleted (or 1184 the undefined value if that element did not exist). 1185 1186 %hash = (foo => 11, bar => 22, baz => 33); 1187 $scalar = delete $hash{foo}; # $scalar is 11 1188 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22 1189 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33) 1190 1191 Deleting from C<%ENV> modifies the environment. Deleting from 1192 a hash tied to a DBM file deletes the entry from the DBM file. Deleting 1193 from a C<tie>d hash or array may not necessarily return anything. 1194 1195 Deleting an array element effectively returns that position of the array 1196 to its initial, uninitialized state. Subsequently testing for the same 1197 element with exists() will return false. Also, deleting array elements 1198 in the middle of an array will not shift the index of the elements 1199 after them down. Use splice() for that. See L</exists>. 1200 1201 The following (inefficiently) deletes all the values of %HASH and @ARRAY: 1202 1203 foreach $key (keys %HASH) { 1204 delete $HASH{$key}; 1205 } 1206 1207 foreach $index (0 .. $#ARRAY) { 1208 delete $ARRAY[$index]; 1209 } 1210 1211 And so do these: 1212 1213 delete @HASH{keys %HASH}; 1214 1215 delete @ARRAY[0 .. $#ARRAY]; 1216 1217 But both of these are slower than just assigning the empty list 1218 or undefining %HASH or @ARRAY: 1219 1220 %HASH = (); # completely empty %HASH 1221 undef %HASH; # forget %HASH ever existed 1222 1223 @ARRAY = (); # completely empty @ARRAY 1224 undef @ARRAY; # forget @ARRAY ever existed 1225 1226 Note that the EXPR can be arbitrarily complicated as long as the final 1227 operation is a hash element, array element, hash slice, or array slice 1228 lookup: 1229 1230 delete $ref->[$x][$y]{$key}; 1231 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; 1232 1233 delete $ref->[$x][$y][$index]; 1234 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices]; 1235 1236 =item die LIST 1237 X<die> X<throw> X<exception> X<raise> X<$@> X<abort> 1238 1239 Outside an C<eval>, prints the value of LIST to C<STDERR> and 1240 exits with the current value of C<$!> (errno). If C<$!> is C<0>, 1241 exits with the value of C<<< ($? >> 8) >>> (backtick `command` 1242 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside 1243 an C<eval(),> the error message is stuffed into C<$@> and the 1244 C<eval> is terminated with the undefined value. This makes 1245 C<die> the way to raise an exception. 1246 1247 Equivalent examples: 1248 1249 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; 1250 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" 1251 1252 If the last element of LIST does not end in a newline, the current 1253 script line number and input line number (if any) are also printed, 1254 and a newline is supplied. Note that the "input line number" (also 1255 known as "chunk") is subject to whatever notion of "line" happens to 1256 be currently in effect, and is also available as the special variable 1257 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">. 1258 1259 Hint: sometimes appending C<", stopped"> to your message will cause it 1260 to make better sense when the string C<"at foo line 123"> is appended. 1261 Suppose you are running script "canasta". 1262 1263 die "/etc/games is no good"; 1264 die "/etc/games is no good, stopped"; 1265 1266 produce, respectively 1267 1268 /etc/games is no good at canasta line 123. 1269 /etc/games is no good, stopped at canasta line 123. 1270 1271 See also exit(), warn(), and the Carp module. 1272 1273 If LIST is empty and C<$@> already contains a value (typically from a 1274 previous eval) that value is reused after appending C<"\t...propagated">. 1275 This is useful for propagating exceptions: 1276 1277 eval { ... }; 1278 die unless $@ =~ /Expected exception/; 1279 1280 If LIST is empty and C<$@> contains an object reference that has a 1281 C<PROPAGATE> method, that method will be called with additional file 1282 and line number parameters. The return value replaces the value in 1283 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >> 1284 were called. 1285 1286 If C<$@> is empty then the string C<"Died"> is used. 1287 1288 die() can also be called with a reference argument. If this happens to be 1289 trapped within an eval(), $@ contains the reference. This behavior permits 1290 a more elaborate exception handling implementation using objects that 1291 maintain arbitrary state about the nature of the exception. Such a scheme 1292 is sometimes preferable to matching particular string values of $@ using 1293 regular expressions. Because $@ is a global variable, and eval() may be 1294 used within object implementations, care must be taken that analyzing the 1295 error object doesn't replace the reference in the global variable. The 1296 easiest solution is to make a local copy of the reference before doing 1297 other manipulations. Here's an example: 1298 1299 use Scalar::Util 'blessed'; 1300 1301 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; 1302 if (my $ev_err = $@) { 1303 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) { 1304 # handle Some::Module::Exception 1305 } 1306 else { 1307 # handle all other possible exceptions 1308 } 1309 } 1310 1311 Because perl will stringify uncaught exception messages before displaying 1312 them, you may want to overload stringification operations on such custom 1313 exception objects. See L<overload> for details about that. 1314 1315 You can arrange for a callback to be run just before the C<die> 1316 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated 1317 handler will be called with the error text and can change the error 1318 message, if it sees fit, by calling C<die> again. See 1319 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and 1320 L<"eval BLOCK"> for some examples. Although this feature was 1321 to be run only right before your program was to exit, this is not 1322 currently the case--the C<$SIG{__DIE__}> hook is currently called 1323 even inside eval()ed blocks/strings! If one wants the hook to do 1324 nothing in such situations, put 1325 1326 die @_ if $^S; 1327 1328 as the first line of the handler (see L<perlvar/$^S>). Because 1329 this promotes strange action at a distance, this counterintuitive 1330 behavior may be fixed in a future release. 1331 1332 =item do BLOCK 1333 X<do> X<block> 1334 1335 Not really a function. Returns the value of the last command in the 1336 sequence of commands indicated by BLOCK. When modified by the C<while> or 1337 C<until> loop modifier, executes the BLOCK once before testing the loop 1338 condition. (On other statements the loop modifiers test the conditional 1339 first.) 1340 1341 C<do BLOCK> does I<not> count as a loop, so the loop control statements 1342 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. 1343 See L<perlsyn> for alternative strategies. 1344 1345 =item do SUBROUTINE(LIST) 1346 X<do> 1347 1348 This form of subroutine call is deprecated. See L<perlsub>. 1349 1350 =item do EXPR 1351 X<do> 1352 1353 Uses the value of EXPR as a filename and executes the contents of the 1354 file as a Perl script. 1355 1356 do 'stat.pl'; 1357 1358 is just like 1359 1360 eval `cat stat.pl`; 1361 1362 except that it's more efficient and concise, keeps track of the current 1363 filename for error messages, searches the @INC directories, and updates 1364 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these 1365 variables. It also differs in that code evaluated with C<do FILENAME> 1366 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the 1367 same, however, in that it does reparse the file every time you call it, 1368 so you probably don't want to do this inside a loop. 1369 1370 If C<do> cannot read the file, it returns undef and sets C<$!> to the 1371 error. If C<do> can read the file but cannot compile it, it 1372 returns undef and sets an error message in C<$@>. If the file is 1373 successfully compiled, C<do> returns the value of the last expression 1374 evaluated. 1375 1376 Note that inclusion of library modules is better done with the 1377 C<use> and C<require> operators, which also do automatic error checking 1378 and raise an exception if there's a problem. 1379 1380 You might like to use C<do> to read in a program configuration 1381 file. Manual error checking can be done this way: 1382 1383 # read in config files: system first, then user 1384 for $file ("/share/prog/defaults.rc", 1385 "$ENV{HOME}/.someprogrc") 1386 { 1387 unless ($return = do $file) { 1388 warn "couldn't parse $file: $@" if $@; 1389 warn "couldn't do $file: $!" unless defined $return; 1390 warn "couldn't run $file" unless $return; 1391 } 1392 } 1393 1394 =item dump LABEL 1395 X<dump> X<core> X<undump> 1396 1397 =item dump 1398 1399 This function causes an immediate core dump. See also the B<-u> 1400 command-line switch in L<perlrun>, which does the same thing. 1401 Primarily this is so that you can use the B<undump> program (not 1402 supplied) to turn your core dump into an executable binary after 1403 having initialized all your variables at the beginning of the 1404 program. When the new binary is executed it will begin by executing 1405 a C<goto LABEL> (with all the restrictions that C<goto> suffers). 1406 Think of it as a goto with an intervening core dump and reincarnation. 1407 If C<LABEL> is omitted, restarts the program from the top. 1408 1409 B<WARNING>: Any files opened at the time of the dump will I<not> 1410 be open any more when the program is reincarnated, with possible 1411 resulting confusion on the part of Perl. 1412 1413 This function is now largely obsolete, mostly because it's very hard to 1414 convert a core file into an executable. That's why you should now invoke 1415 it as C<CORE::dump()>, if you don't want to be warned against a possible 1416 typo. 1417 1418 =item each HASH 1419 X<each> X<hash, iterator> 1420 1421 When called in list context, returns a 2-element list consisting of the 1422 key and value for the next element of a hash, so that you can iterate over 1423 it. When called in scalar context, returns only the key for the next 1424 element in the hash. 1425 1426 Entries are returned in an apparently random order. The actual random 1427 order is subject to change in future versions of perl, but it is 1428 guaranteed to be in the same order as either the C<keys> or C<values> 1429 function would produce on the same (unmodified) hash. Since Perl 1430 5.8.1 the ordering is different even between different runs of Perl 1431 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">). 1432 1433 When the hash is entirely read, a null array is returned in list context 1434 (which when assigned produces a false (C<0>) value), and C<undef> in 1435 scalar context. The next call to C<each> after that will start iterating 1436 again. There is a single iterator for each hash, shared by all C<each>, 1437 C<keys>, and C<values> function calls in the program; it can be reset by 1438 reading all the elements from the hash, or by evaluating C<keys HASH> or 1439 C<values HASH>. If you add or delete elements of a hash while you're 1440 iterating over it, you may get entries skipped or duplicated, so 1441 don't. Exception: It is always safe to delete the item most recently 1442 returned by C<each()>, which means that the following code will work: 1443 1444 while (($key, $value) = each %hash) { 1445 print $key, "\n"; 1446 delete $hash{$key}; # This is safe 1447 } 1448 1449 The following prints out your environment like the printenv(1) program, 1450 only in a different order: 1451 1452 while (($key,$value) = each %ENV) { 1453 print "$key=$value\n"; 1454 } 1455 1456 See also C<keys>, C<values> and C<sort>. 1457 1458 =item eof FILEHANDLE 1459 X<eof> 1460 X<end of file> 1461 X<end-of-file> 1462 1463 =item eof () 1464 1465 =item eof 1466 1467 Returns 1 if the next read on FILEHANDLE will return end of file, or if 1468 FILEHANDLE is not open. FILEHANDLE may be an expression whose value 1469 gives the real filehandle. (Note that this function actually 1470 reads a character and then C<ungetc>s it, so isn't very useful in an 1471 interactive context.) Do not read from a terminal file (or call 1472 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such 1473 as terminals may lose the end-of-file condition if you do. 1474 1475 An C<eof> without an argument uses the last file read. Using C<eof()> 1476 with empty parentheses is very different. It refers to the pseudo file 1477 formed from the files listed on the command line and accessed via the 1478 C<< <> >> operator. Since C<< <> >> isn't explicitly opened, 1479 as a normal filehandle is, an C<eof()> before C<< <> >> has been 1480 used will cause C<@ARGV> to be examined to determine if input is 1481 available. Similarly, an C<eof()> after C<< <> >> has returned 1482 end-of-file will assume you are processing another C<@ARGV> list, 1483 and if you haven't set C<@ARGV>, will read input from C<STDIN>; 1484 see L<perlop/"I/O Operators">. 1485 1486 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to 1487 detect the end of each file, C<eof()> will only detect the end of the 1488 last file. Examples: 1489 1490 # reset line numbering on each input file 1491 while (<>) { 1492 next if /^\s*#/; # skip comments 1493 print "$.\t$_"; 1494 } continue { 1495 close ARGV if eof; # Not eof()! 1496 } 1497 1498 # insert dashes just before last line of last file 1499 while (<>) { 1500 if (eof()) { # check for end of last file 1501 print "--------------\n"; 1502 } 1503 print; 1504 last if eof(); # needed if we're reading from a terminal 1505 } 1506 1507 Practical hint: you almost never need to use C<eof> in Perl, because the 1508 input operators typically return C<undef> when they run out of data, or if 1509 there was an error. 1510 1511 =item eval EXPR 1512 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute> 1513 X<error, handling> X<exception, handling> 1514 1515 =item eval BLOCK 1516 1517 =item eval 1518 1519 In the first form, the return value of EXPR is parsed and executed as if it 1520 were a little Perl program. The value of the expression (which is itself 1521 determined within scalar context) is first parsed, and if there weren't any 1522 errors, executed in the lexical context of the current Perl program, so 1523 that any variable settings or subroutine and format definitions remain 1524 afterwards. Note that the value is parsed every time the C<eval> executes. 1525 If EXPR is omitted, evaluates C<$_>. This form is typically used to 1526 delay parsing and subsequent execution of the text of EXPR until run time. 1527 1528 In the second form, the code within the BLOCK is parsed only once--at the 1529 same time the code surrounding the C<eval> itself was parsed--and executed 1530 within the context of the current Perl program. This form is typically 1531 used to trap exceptions more efficiently than the first (see below), while 1532 also providing the benefit of checking the code within BLOCK at compile 1533 time. 1534 1535 The final semicolon, if any, may be omitted from the value of EXPR or within 1536 the BLOCK. 1537 1538 In both forms, the value returned is the value of the last expression 1539 evaluated inside the mini-program; a return statement may be also used, just 1540 as with subroutines. The expression providing the return value is evaluated 1541 in void, scalar, or list context, depending on the context of the C<eval> 1542 itself. See L</wantarray> for more on how the evaluation context can be 1543 determined. 1544 1545 If there is a syntax error or runtime error, or a C<die> statement is 1546 executed, an undefined value is returned by C<eval>, and C<$@> is set to the 1547 error message. If there was no error, C<$@> is guaranteed to be a null 1548 string. Beware that using C<eval> neither silences perl from printing 1549 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>. 1550 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or 1551 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>. 1552 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>. 1553 1554 Note that, because C<eval> traps otherwise-fatal errors, it is useful for 1555 determining whether a particular feature (such as C<socket> or C<symlink>) 1556 is implemented. It is also Perl's exception trapping mechanism, where 1557 the die operator is used to raise exceptions. 1558 1559 If the code to be executed doesn't vary, you may use the eval-BLOCK 1560 form to trap run-time errors without incurring the penalty of 1561 recompiling each time. The error, if any, is still returned in C<$@>. 1562 Examples: 1563 1564 # make divide-by-zero nonfatal 1565 eval { $answer = $a / $b; }; warn $@ if $@; 1566 1567 # same thing, but less efficient 1568 eval '$answer = $a / $b'; warn $@ if $@; 1569 1570 # a compile-time error 1571 eval { $answer = }; # WRONG 1572 1573 # a run-time error 1574 eval '$answer ='; # sets $@ 1575 1576 Using the C<eval{}> form as an exception trap in libraries does have some 1577 issues. Due to the current arguably broken state of C<__DIE__> hooks, you 1578 may wish not to trigger any C<__DIE__> hooks that user code may have installed. 1579 You can use the C<local $SIG{__DIE__}> construct for this purpose, 1580 as shown in this example: 1581 1582 # a very private exception trap for divide-by-zero 1583 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; 1584 warn $@ if $@; 1585 1586 This is especially significant, given that C<__DIE__> hooks can call 1587 C<die> again, which has the effect of changing their error messages: 1588 1589 # __DIE__ hooks may modify error messages 1590 { 1591 local $SIG{'__DIE__'} = 1592 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; 1593 eval { die "foo lives here" }; 1594 print $@ if $@; # prints "bar lives here" 1595 } 1596 1597 Because this promotes action at a distance, this counterintuitive behavior 1598 may be fixed in a future release. 1599 1600 With an C<eval>, you should be especially careful to remember what's 1601 being looked at when: 1602 1603 eval $x; # CASE 1 1604 eval "$x"; # CASE 2 1605 1606 eval '$x'; # CASE 3 1607 eval { $x }; # CASE 4 1608 1609 eval "\$$x++"; # CASE 5 1610 $$x++; # CASE 6 1611 1612 Cases 1 and 2 above behave identically: they run the code contained in 1613 the variable $x. (Although case 2 has misleading double quotes making 1614 the reader wonder what else might be happening (nothing is).) Cases 3 1615 and 4 likewise behave in the same way: they run the code C<'$x'>, which 1616 does nothing but return the value of $x. (Case 4 is preferred for 1617 purely visual reasons, but it also has the advantage of compiling at 1618 compile-time instead of at run-time.) Case 5 is a place where 1619 normally you I<would> like to use double quotes, except that in this 1620 particular situation, you can just use symbolic references instead, as 1621 in case 6. 1622 1623 C<eval BLOCK> does I<not> count as a loop, so the loop control statements 1624 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. 1625 1626 Note that as a very special case, an C<eval ''> executed within the C<DB> 1627 package doesn't see the usual surrounding lexical scope, but rather the 1628 scope of the first non-DB piece of code that called it. You don't normally 1629 need to worry about this unless you are writing a Perl debugger. 1630 1631 =item exec LIST 1632 X<exec> X<execute> 1633 1634 =item exec PROGRAM LIST 1635 1636 The C<exec> function executes a system command I<and never returns>-- 1637 use C<system> instead of C<exec> if you want it to return. It fails and 1638 returns false only if the command does not exist I<and> it is executed 1639 directly instead of via your system's command shell (see below). 1640 1641 Since it's a common mistake to use C<exec> instead of C<system>, Perl 1642 warns you if there is a following statement which isn't C<die>, C<warn>, 1643 or C<exit> (if C<-w> is set - but you always do that). If you 1644 I<really> want to follow an C<exec> with some other statement, you 1645 can use one of these styles to avoid the warning: 1646 1647 exec ('foo') or print STDERR "couldn't exec foo: $!"; 1648 { exec ('foo') }; print STDERR "couldn't exec foo: $!"; 1649 1650 If there is more than one argument in LIST, or if LIST is an array 1651 with more than one value, calls execvp(3) with the arguments in LIST. 1652 If there is only one scalar argument or an array with one element in it, 1653 the argument is checked for shell metacharacters, and if there are any, 1654 the entire argument is passed to the system's command shell for parsing 1655 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms). 1656 If there are no shell metacharacters in the argument, it is split into 1657 words and passed directly to C<execvp>, which is more efficient. 1658 Examples: 1659 1660 exec '/bin/echo', 'Your arguments are: ', @ARGV; 1661 exec "sort $outfile | uniq"; 1662 1663 If you don't really want to execute the first argument, but want to lie 1664 to the program you are executing about its own name, you can specify 1665 the program you actually want to run as an "indirect object" (without a 1666 comma) in front of the LIST. (This always forces interpretation of the 1667 LIST as a multivalued list, even if there is only a single scalar in 1668 the list.) Example: 1669 1670 $shell = '/bin/csh'; 1671 exec $shell '-sh'; # pretend it's a login shell 1672 1673 or, more directly, 1674 1675 exec {'/bin/csh'} '-sh'; # pretend it's a login shell 1676 1677 When the arguments get executed via the system shell, results will 1678 be subject to its quirks and capabilities. See L<perlop/"`STRING`"> 1679 for details. 1680 1681 Using an indirect object with C<exec> or C<system> is also more 1682 secure. This usage (which also works fine with system()) forces 1683 interpretation of the arguments as a multivalued list, even if the 1684 list had just one argument. That way you're safe from the shell 1685 expanding wildcards or splitting up words with whitespace in them. 1686 1687 @args = ( "echo surprise" ); 1688 1689 exec @args; # subject to shell escapes 1690 # if @args == 1 1691 exec { $args[0] } @args; # safe even with one-arg list 1692 1693 The first version, the one without the indirect object, ran the I<echo> 1694 program, passing it C<"surprise"> an argument. The second version 1695 didn't--it tried to run a program literally called I<"echo surprise">, 1696 didn't find it, and set C<$?> to a non-zero value indicating failure. 1697 1698 Beginning with v5.6.0, Perl will attempt to flush all files opened for 1699 output before the exec, but this may not be supported on some platforms 1700 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH 1701 in English) or call the C<autoflush()> method of C<IO::Handle> on any 1702 open handles in order to avoid lost output. 1703 1704 Note that C<exec> will not call your C<END> blocks, nor will it call 1705 any C<DESTROY> methods in your objects. 1706 1707 =item exists EXPR 1708 X<exists> X<autovivification> 1709 1710 Given an expression that specifies a hash element or array element, 1711 returns true if the specified element in the hash or array has ever 1712 been initialized, even if the corresponding value is undefined. The 1713 element is not autovivified if it doesn't exist. 1714 1715 print "Exists\n" if exists $hash{$key}; 1716 print "Defined\n" if defined $hash{$key}; 1717 print "True\n" if $hash{$key}; 1718 1719 print "Exists\n" if exists $array[$index]; 1720 print "Defined\n" if defined $array[$index]; 1721 print "True\n" if $array[$index]; 1722 1723 A hash or array element can be true only if it's defined, and defined if 1724 it exists, but the reverse doesn't necessarily hold true. 1725 1726 Given an expression that specifies the name of a subroutine, 1727 returns true if the specified subroutine has ever been declared, even 1728 if it is undefined. Mentioning a subroutine name for exists or defined 1729 does not count as declaring it. Note that a subroutine which does not 1730 exist may still be callable: its package may have an C<AUTOLOAD> 1731 method that makes it spring into existence the first time that it is 1732 called -- see L<perlsub>. 1733 1734 print "Exists\n" if exists &subroutine; 1735 print "Defined\n" if defined &subroutine; 1736 1737 Note that the EXPR can be arbitrarily complicated as long as the final 1738 operation is a hash or array key lookup or subroutine name: 1739 1740 if (exists $ref->{A}->{B}->{$key}) { } 1741 if (exists $hash{A}{B}{$key}) { } 1742 1743 if (exists $ref->{A}->{B}->[$ix]) { } 1744 if (exists $hash{A}{B}[$ix]) { } 1745 1746 if (exists &{$ref->{A}{B}{$key}}) { } 1747 1748 Although the deepest nested array or hash will not spring into existence 1749 just because its existence was tested, any intervening ones will. 1750 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring 1751 into existence due to the existence test for the $key element above. 1752 This happens anywhere the arrow operator is used, including even: 1753 1754 undef $ref; 1755 if (exists $ref->{"Some key"}) { } 1756 print $ref; # prints HASH(0x80d3d5c) 1757 1758 This surprising autovivification in what does not at first--or even 1759 second--glance appear to be an lvalue context may be fixed in a future 1760 release. 1761 1762 Use of a subroutine call, rather than a subroutine name, as an argument 1763 to exists() is an error. 1764 1765 exists ⊂ # OK 1766 exists &sub(); # Error 1767 1768 =item exit EXPR 1769 X<exit> X<terminate> X<abort> 1770 1771 =item exit 1772 1773 Evaluates EXPR and exits immediately with that value. Example: 1774 1775 $ans = <STDIN>; 1776 exit 0 if $ans =~ /^[Xx]/; 1777 1778 See also C<die>. If EXPR is omitted, exits with C<0> status. The only 1779 universally recognized values for EXPR are C<0> for success and C<1> 1780 for error; other values are subject to interpretation depending on the 1781 environment in which the Perl program is running. For example, exiting 1782 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause 1783 the mailer to return the item undelivered, but that's not true everywhere. 1784 1785 Don't use C<exit> to abort a subroutine if there's any chance that 1786 someone might want to trap whatever error happened. Use C<die> instead, 1787 which can be trapped by an C<eval>. 1788 1789 The exit() function does not always exit immediately. It calls any 1790 defined C<END> routines first, but these C<END> routines may not 1791 themselves abort the exit. Likewise any object destructors that need to 1792 be called are called before the real exit. If this is a problem, you 1793 can call C<POSIX:_exit($status)> to avoid END and destructor processing. 1794 See L<perlmod> for details. 1795 1796 =item exp EXPR 1797 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e> 1798 1799 =item exp 1800 1801 Returns I<e> (the natural logarithm base) to the power of EXPR. 1802 If EXPR is omitted, gives C<exp($_)>. 1803 1804 =item fcntl FILEHANDLE,FUNCTION,SCALAR 1805 X<fcntl> 1806 1807 Implements the fcntl(2) function. You'll probably have to say 1808 1809 use Fcntl; 1810 1811 first to get the correct constant definitions. Argument processing and 1812 value return works just like C<ioctl> below. 1813 For example: 1814 1815 use Fcntl; 1816 fcntl($filehandle, F_GETFL, $packed_return_buffer) 1817 or die "can't fcntl F_GETFL: $!"; 1818 1819 You don't have to check for C<defined> on the return from C<fcntl>. 1820 Like C<ioctl>, it maps a C<0> return from the system call into 1821 C<"0 but true"> in Perl. This string is true in boolean context and C<0> 1822 in numeric context. It is also exempt from the normal B<-w> warnings 1823 on improper numeric conversions. 1824 1825 Note that C<fcntl> will produce a fatal error if used on a machine that 1826 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) 1827 manpage to learn what functions are available on your system. 1828 1829 Here's an example of setting a filehandle named C<REMOTE> to be 1830 non-blocking at the system level. You'll have to negotiate C<$|> 1831 on your own, though. 1832 1833 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK); 1834 1835 $flags = fcntl(REMOTE, F_GETFL, 0) 1836 or die "Can't get flags for the socket: $!\n"; 1837 1838 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK) 1839 or die "Can't set flags for the socket: $!\n"; 1840 1841 =item fileno FILEHANDLE 1842 X<fileno> 1843 1844 Returns the file descriptor for a filehandle, or undefined if the 1845 filehandle is not open. This is mainly useful for constructing 1846 bitmaps for C<select> and low-level POSIX tty-handling operations. 1847 If FILEHANDLE is an expression, the value is taken as an indirect 1848 filehandle, generally its name. 1849 1850 You can use this to find out whether two handles refer to the 1851 same underlying descriptor: 1852 1853 if (fileno(THIS) == fileno(THAT)) { 1854 print "THIS and THAT are dups\n"; 1855 } 1856 1857 (Filehandles connected to memory objects via new features of C<open> may 1858 return undefined even though they are open.) 1859 1860 1861 =item flock FILEHANDLE,OPERATION 1862 X<flock> X<lock> X<locking> 1863 1864 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true 1865 for success, false on failure. Produces a fatal error if used on a 1866 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). 1867 C<flock> is Perl's portable file locking interface, although it locks 1868 only entire files, not records. 1869 1870 Two potentially non-obvious but traditional C<flock> semantics are 1871 that it waits indefinitely until the lock is granted, and that its locks 1872 B<merely advisory>. Such discretionary locks are more flexible, but offer 1873 fewer guarantees. This means that programs that do not also use C<flock> 1874 may modify files locked with C<flock>. See L<perlport>, 1875 your port's specific documentation, or your system-specific local manpages 1876 for details. It's best to assume traditional behavior if you're writing 1877 portable programs. (But if you're not, you should as always feel perfectly 1878 free to write for your own system's idiosyncrasies (sometimes called 1879 "features"). Slavish adherence to portability concerns shouldn't get 1880 in the way of your getting your job done.) 1881 1882 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with 1883 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but 1884 you can use the symbolic names if you import them from the Fcntl module, 1885 either individually, or as a group using the ':flock' tag. LOCK_SH 1886 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN 1887 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with 1888 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking 1889 waiting for the lock (check the return status to see if you got it). 1890 1891 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE 1892 before locking or unlocking it. 1893 1894 Note that the emulation built with lockf(3) doesn't provide shared 1895 locks, and it requires that FILEHANDLE be open with write intent. These 1896 are the semantics that lockf(3) implements. Most if not all systems 1897 implement lockf(3) in terms of fcntl(2) locking, though, so the 1898 differing semantics shouldn't bite too many people. 1899 1900 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE 1901 be open with read intent to use LOCK_SH and requires that it be open 1902 with write intent to use LOCK_EX. 1903 1904 Note also that some versions of C<flock> cannot lock things over the 1905 network; you would need to use the more system-specific C<fcntl> for 1906 that. If you like you can force Perl to ignore your system's flock(2) 1907 function, and so provide its own fcntl(2)-based emulation, by passing 1908 the switch C<-Ud_flock> to the F<Configure> program when you configure 1909 perl. 1910 1911 Here's a mailbox appender for BSD systems. 1912 1913 use Fcntl ':flock'; # import LOCK_* constants 1914 1915 sub lock { 1916 flock(MBOX,LOCK_EX); 1917 # and, in case someone appended 1918 # while we were waiting... 1919 seek(MBOX, 0, 2); 1920 } 1921 1922 sub unlock { 1923 flock(MBOX,LOCK_UN); 1924 } 1925 1926 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}") 1927 or die "Can't open mailbox: $!"; 1928 1929 lock(); 1930 print MBOX $msg,"\n\n"; 1931 unlock(); 1932 1933 On systems that support a real flock(), locks are inherited across fork() 1934 calls, whereas those that must resort to the more capricious fcntl() 1935 function lose the locks, making it harder to write servers. 1936 1937 See also L<DB_File> for other flock() examples. 1938 1939 =item fork 1940 X<fork> X<child> X<parent> 1941 1942 Does a fork(2) system call to create a new process running the 1943 same program at the same point. It returns the child pid to the 1944 parent process, C<0> to the child process, or C<undef> if the fork is 1945 unsuccessful. File descriptors (and sometimes locks on those descriptors) 1946 are shared, while everything else is copied. On most systems supporting 1947 fork(), great care has gone into making it extremely efficient (for 1948 example, using copy-on-write technology on data pages), making it the 1949 dominant paradigm for multitasking over the last few decades. 1950 1951 Beginning with v5.6.0, Perl will attempt to flush all files opened for 1952 output before forking the child process, but this may not be supported 1953 on some platforms (see L<perlport>). To be safe, you may need to set 1954 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of 1955 C<IO::Handle> on any open handles in order to avoid duplicate output. 1956 1957 If you C<fork> without ever waiting on your children, you will 1958 accumulate zombies. On some systems, you can avoid this by setting 1959 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of 1960 forking and reaping moribund children. 1961 1962 Note that if your forked child inherits system file descriptors like 1963 STDIN and STDOUT that are actually connected by a pipe or socket, even 1964 if you exit, then the remote server (such as, say, a CGI script or a 1965 backgrounded job launched from a remote shell) won't think you're done. 1966 You should reopen those to F</dev/null> if it's any issue. 1967 1968 =item format 1969 X<format> 1970 1971 Declare a picture format for use by the C<write> function. For 1972 example: 1973 1974 format Something = 1975 Test: @<<<<<<<< @||||| @>>>>> 1976 $str, $%, '$' . int($num) 1977 . 1978 1979 $str = "widget"; 1980 $num = $cost/$quantity; 1981 $~ = 'Something'; 1982 write; 1983 1984 See L<perlform> for many details and examples. 1985 1986 =item formline PICTURE,LIST 1987 X<formline> 1988 1989 This is an internal function used by C<format>s, though you may call it, 1990 too. It formats (see L<perlform>) a list of values according to the 1991 contents of PICTURE, placing the output into the format output 1992 accumulator, C<$^A> (or C<$ACCUMULATOR> in English). 1993 Eventually, when a C<write> is done, the contents of 1994 C<$^A> are written to some filehandle. You could also read C<$^A> 1995 and then set C<$^A> back to C<"">. Note that a format typically 1996 does one C<formline> per line of form, but the C<formline> function itself 1997 doesn't care how many newlines are embedded in the PICTURE. This means 1998 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line. 1999 You may therefore need to use multiple formlines to implement a single 2000 record format, just like the format compiler. 2001 2002 Be careful if you put double quotes around the picture, because an C<@> 2003 character may be taken to mean the beginning of an array name. 2004 C<formline> always returns true. See L<perlform> for other examples. 2005 2006 =item getc FILEHANDLE 2007 X<getc> X<getchar> X<character> X<file, read> 2008 2009 =item getc 2010 2011 Returns the next character from the input file attached to FILEHANDLE, 2012 or the undefined value at end of file, or if there was an error (in 2013 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from 2014 STDIN. This is not particularly efficient. However, it cannot be 2015 used by itself to fetch single characters without waiting for the user 2016 to hit enter. For that, try something more like: 2017 2018 if ($BSD_STYLE) { 2019 system "stty cbreak </dev/tty >/dev/tty 2>&1"; 2020 } 2021 else { 2022 system "stty", '-icanon', 'eol', "\001"; 2023 } 2024 2025 $key = getc(STDIN); 2026 2027 if ($BSD_STYLE) { 2028 system "stty -cbreak </dev/tty >/dev/tty 2>&1"; 2029 } 2030 else { 2031 system "stty", 'icanon', 'eol', '^@'; # ASCII null 2032 } 2033 print "\n"; 2034 2035 Determination of whether $BSD_STYLE should be set 2036 is left as an exercise to the reader. 2037 2038 The C<POSIX::getattr> function can do this more portably on 2039 systems purporting POSIX compliance. See also the C<Term::ReadKey> 2040 module from your nearest CPAN site; details on CPAN can be found on 2041 L<perlmodlib/CPAN>. 2042 2043 =item getlogin 2044 X<getlogin> X<login> 2045 2046 This implements the C library function of the same name, which on most 2047 systems returns the current login from F</etc/utmp>, if any. If null, 2048 use C<getpwuid>. 2049 2050 $login = getlogin || getpwuid($<) || "Kilroy"; 2051 2052 Do not consider C<getlogin> for authentication: it is not as 2053 secure as C<getpwuid>. 2054 2055 =item getpeername SOCKET 2056 X<getpeername> X<peer> 2057 2058 Returns the packed sockaddr address of other end of the SOCKET connection. 2059 2060 use Socket; 2061 $hersockaddr = getpeername(SOCK); 2062 ($port, $iaddr) = sockaddr_in($hersockaddr); 2063 $herhostname = gethostbyaddr($iaddr, AF_INET); 2064 $herstraddr = inet_ntoa($iaddr); 2065 2066 =item getpgrp PID 2067 X<getpgrp> X<group> 2068 2069 Returns the current process group for the specified PID. Use 2070 a PID of C<0> to get the current process group for the 2071 current process. Will raise an exception if used on a machine that 2072 doesn't implement getpgrp(2). If PID is omitted, returns process 2073 group of current process. Note that the POSIX version of C<getpgrp> 2074 does not accept a PID argument, so only C<PID==0> is truly portable. 2075 2076 =item getppid 2077 X<getppid> X<parent> X<pid> 2078 2079 Returns the process id of the parent process. 2080 2081 Note for Linux users: on Linux, the C functions C<getpid()> and 2082 C<getppid()> return different values from different threads. In order to 2083 be portable, this behavior is not reflected by the perl-level function 2084 C<getppid()>, that returns a consistent value across threads. If you want 2085 to call the underlying C<getppid()>, you may use the CPAN module 2086 C<Linux::Pid>. 2087 2088 =item getpriority WHICH,WHO 2089 X<getpriority> X<priority> X<nice> 2090 2091 Returns the current priority for a process, a process group, or a user. 2092 (See L<getpriority(2)>.) Will raise a fatal exception if used on a 2093 machine that doesn't implement getpriority(2). 2094 2095 =item getpwnam NAME 2096 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname> 2097 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr> 2098 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent> 2099 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent> 2100 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent> 2101 X<endnetent> X<endprotoent> X<endservent> 2102 2103 =item getgrnam NAME 2104 2105 =item gethostbyname NAME 2106 2107 =item getnetbyname NAME 2108 2109 =item getprotobyname NAME 2110 2111 =item getpwuid UID 2112 2113 =item getgrgid GID 2114 2115 =item getservbyname NAME,PROTO 2116 2117 =item gethostbyaddr ADDR,ADDRTYPE 2118 2119 =item getnetbyaddr ADDR,ADDRTYPE 2120 2121 =item getprotobynumber NUMBER 2122 2123 =item getservbyport PORT,PROTO 2124 2125 =item getpwent 2126 2127 =item getgrent 2128 2129 =item gethostent 2130 2131 =item getnetent 2132 2133 =item getprotoent 2134 2135 =item getservent 2136 2137 =item setpwent 2138 2139 =item setgrent 2140 2141 =item sethostent STAYOPEN 2142 2143 =item setnetent STAYOPEN 2144 2145 =item setprotoent STAYOPEN 2146 2147 =item setservent STAYOPEN 2148 2149 =item endpwent 2150 2151 =item endgrent 2152 2153 =item endhostent 2154 2155 =item endnetent 2156 2157 =item endprotoent 2158 2159 =item endservent 2160 2161 These routines perform the same functions as their counterparts in the 2162 system library. In list context, the return values from the 2163 various get routines are as follows: 2164 2165 ($name,$passwd,$uid,$gid, 2166 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw* 2167 ($name,$passwd,$gid,$members) = getgr* 2168 ($name,$aliases,$addrtype,$length,@addrs) = gethost* 2169 ($name,$aliases,$addrtype,$net) = getnet* 2170 ($name,$aliases,$proto) = getproto* 2171 ($name,$aliases,$port,$proto) = getserv* 2172 2173 (If the entry doesn't exist you get a null list.) 2174 2175 The exact meaning of the $gcos field varies but it usually contains 2176 the real name of the user (as opposed to the login name) and other 2177 information pertaining to the user. Beware, however, that in many 2178 system users are able to change this information and therefore it 2179 cannot be trusted and therefore the $gcos is tainted (see 2180 L<perlsec>). The $passwd and $shell, user's encrypted password and 2181 login shell, are also tainted, because of the same reason. 2182 2183 In scalar context, you get the name, unless the function was a 2184 lookup by name, in which case you get the other thing, whatever it is. 2185 (If the entry doesn't exist you get the undefined value.) For example: 2186 2187 $uid = getpwnam($name); 2188 $name = getpwuid($num); 2189 $name = getpwent(); 2190 $gid = getgrnam($name); 2191 $name = getgrgid($num); 2192 $name = getgrent(); 2193 #etc. 2194 2195 In I<getpw*()> the fields $quota, $comment, and $expire are special 2196 cases in the sense that in many systems they are unsupported. If the 2197 $quota is unsupported, it is an empty scalar. If it is supported, it 2198 usually encodes the disk quota. If the $comment field is unsupported, 2199 it is an empty scalar. If it is supported it usually encodes some 2200 administrative comment about the user. In some systems the $quota 2201 field may be $change or $age, fields that have to do with password 2202 aging. In some systems the $comment field may be $class. The $expire 2203 field, if present, encodes the expiration period of the account or the 2204 password. For the availability and the exact meaning of these fields 2205 in your system, please consult your getpwnam(3) documentation and your 2206 F<pwd.h> file. You can also find out from within Perl what your 2207 $quota and $comment fields mean and whether you have the $expire field 2208 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>, 2209 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password 2210 files are only supported if your vendor has implemented them in the 2211 intuitive fashion that calling the regular C library routines gets the 2212 shadow versions if you're running under privilege or if there exists 2213 the shadow(3) functions as found in System V (this includes Solaris 2214 and Linux.) Those systems that implement a proprietary shadow password 2215 facility are unlikely to be supported. 2216 2217 The $members value returned by I<getgr*()> is a space separated list of 2218 the login names of the members of the group. 2219 2220 For the I<gethost*()> functions, if the C<h_errno> variable is supported in 2221 C, it will be returned to you via C<$?> if the function call fails. The 2222 C<@addrs> value returned by a successful call is a list of the raw 2223 addresses returned by the corresponding system library call. In the 2224 Internet domain, each address is four bytes long and you can unpack it 2225 by saying something like: 2226 2227 ($a,$b,$c,$d) = unpack('W4',$addr[0]); 2228 2229 The Socket library makes this slightly easier: 2230 2231 use Socket; 2232 $iaddr = inet_aton("127.1"); # or whatever address 2233 $name = gethostbyaddr($iaddr, AF_INET); 2234 2235 # or going the other way 2236 $straddr = inet_ntoa($iaddr); 2237 2238 In the opposite way, to resolve a hostname to the IP address 2239 you can write this: 2240 2241 use Socket; 2242 $packed_ip = gethostbyname("www.perl.org"); 2243 if (defined $packed_ip) { 2244 $ip_address = inet_ntoa($packed_ip); 2245 } 2246 2247 Make sure <gethostbyname()> is called in SCALAR context and that 2248 its return value is checked for definedness. 2249 2250 If you get tired of remembering which element of the return list 2251 contains which return value, by-name interfaces are provided 2252 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>, 2253 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>, 2254 and C<User::grent>. These override the normal built-ins, supplying 2255 versions that return objects with the appropriate names 2256 for each field. For example: 2257 2258 use File::stat; 2259 use User::pwent; 2260 $is_his = (stat($filename)->uid == pwent($whoever)->uid); 2261 2262 Even though it looks like they're the same method calls (uid), 2263 they aren't, because a C<File::stat> object is different from 2264 a C<User::pwent> object. 2265 2266 =item getsockname SOCKET 2267 X<getsockname> 2268 2269 Returns the packed sockaddr address of this end of the SOCKET connection, 2270 in case you don't know the address because you have several different 2271 IPs that the connection might have come in on. 2272 2273 use Socket; 2274 $mysockaddr = getsockname(SOCK); 2275 ($port, $myaddr) = sockaddr_in($mysockaddr); 2276 printf "Connect to %s [%s]\n", 2277 scalar gethostbyaddr($myaddr, AF_INET), 2278 inet_ntoa($myaddr); 2279 2280 =item getsockopt SOCKET,LEVEL,OPTNAME 2281 X<getsockopt> 2282 2283 Queries the option named OPTNAME associated with SOCKET at a given LEVEL. 2284 Options may exist at multiple protocol levels depending on the socket 2285 type, but at least the uppermost socket level SOL_SOCKET (defined in the 2286 C<Socket> module) will exist. To query options at another level the 2287 protocol number of the appropriate protocol controlling the option 2288 should be supplied. For example, to indicate that an option is to be 2289 interpreted by the TCP protocol, LEVEL should be set to the protocol 2290 number of TCP, which you can get using getprotobyname. 2291 2292 The call returns a packed string representing the requested socket option, 2293 or C<undef> if there is an error (the error reason will be in $!). What 2294 exactly is in the packed string depends in the LEVEL and OPTNAME, consult 2295 your system documentation for details. A very common case however is that 2296 the option is an integer, in which case the result will be a packed 2297 integer which you can decode using unpack with the C<i> (or C<I>) format. 2298 2299 An example testing if Nagle's algorithm is turned on on a socket: 2300 2301 use Socket qw(:all); 2302 2303 defined(my $tcp = getprotobyname("tcp")) 2304 or die "Could not determine the protocol number for tcp"; 2305 # my $tcp = IPPROTO_TCP; # Alternative 2306 my $packed = getsockopt($socket, $tcp, TCP_NODELAY) 2307 or die "Could not query TCP_NODELAY socket option: $!"; 2308 my $nodelay = unpack("I", $packed); 2309 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n"; 2310 2311 2312 =item glob EXPR 2313 X<glob> X<wildcard> X<filename, expansion> X<expand> 2314 2315 =item glob 2316 2317 In list context, returns a (possibly empty) list of filename expansions on 2318 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In 2319 scalar context, glob iterates through such filename expansions, returning 2320 undef when the list is exhausted. This is the internal function 2321 implementing the C<< <*.c> >> operator, but you can use it directly. If 2322 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in 2323 more detail in L<perlop/"I/O Operators">. 2324 2325 Beginning with v5.6.0, this operator is implemented using the standard 2326 C<File::Glob> extension. See L<File::Glob> for details. 2327 2328 =item gmtime EXPR 2329 X<gmtime> X<UTC> X<Greenwich> 2330 2331 =item gmtime 2332 2333 Works just like L<localtime> but the returned values are 2334 localized for the standard Greenwich time zone. 2335 2336 Note: when called in list context, $isdst, the last value 2337 returned by gmtime is always C<0>. There is no 2338 Daylight Saving Time in GMT. 2339 2340 See L<perlport/gmtime> for portability concerns. 2341 2342 =item goto LABEL 2343 X<goto> X<jump> X<jmp> 2344 2345 =item goto EXPR 2346 2347 =item goto &NAME 2348 2349 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes 2350 execution there. It may not be used to go into any construct that 2351 requires initialization, such as a subroutine or a C<foreach> loop. It 2352 also can't be used to go into a construct that is optimized away, 2353 or to get out of a block or subroutine given to C<sort>. 2354 It can be used to go almost anywhere else within the dynamic scope, 2355 including out of subroutines, but it's usually better to use some other 2356 construct such as C<last> or C<die>. The author of Perl has never felt the 2357 need to use this form of C<goto> (in Perl, that is--C is another matter). 2358 (The difference being that C does not offer named loops combined with 2359 loop control. Perl does, and this replaces most structured uses of C<goto> 2360 in other languages.) 2361 2362 The C<goto-EXPR> form expects a label name, whose scope will be resolved 2363 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't 2364 necessarily recommended if you're optimizing for maintainability: 2365 2366 goto ("FOO", "BAR", "GLARCH")[$i]; 2367 2368 The C<goto-&NAME> form is quite different from the other forms of 2369 C<goto>. In fact, it isn't a goto in the normal sense at all, and 2370 doesn't have the stigma associated with other gotos. Instead, it 2371 exits the current subroutine (losing any changes set by local()) and 2372 immediately calls in its place the named subroutine using the current 2373 value of @_. This is used by C<AUTOLOAD> subroutines that wish to 2374 load another subroutine and then pretend that the other subroutine had 2375 been called in the first place (except that any modifications to C<@_> 2376 in the current subroutine are propagated to the other subroutine.) 2377 After the C<goto>, not even C<caller> will be able to tell that this 2378 routine was called first. 2379 2380 NAME needn't be the name of a subroutine; it can be a scalar variable 2381 containing a code reference, or a block that evaluates to a code 2382 reference. 2383 2384 =item grep BLOCK LIST 2385 X<grep> 2386 2387 =item grep EXPR,LIST 2388 2389 This is similar in spirit to, but not the same as, grep(1) and its 2390 relatives. In particular, it is not limited to using regular expressions. 2391 2392 Evaluates the BLOCK or EXPR for each element of LIST (locally setting 2393 C<$_> to each element) and returns the list value consisting of those 2394 elements for which the expression evaluated to true. In scalar 2395 context, returns the number of times the expression was true. 2396 2397 @foo = grep(!/^#/, @bar); # weed out comments 2398 2399 or equivalently, 2400 2401 @foo = grep {!/^#/} @bar; # weed out comments 2402 2403 Note that C<$_> is an alias to the list value, so it can be used to 2404 modify the elements of the LIST. While this is useful and supported, 2405 it can cause bizarre results if the elements of LIST are not variables. 2406 Similarly, grep returns aliases into the original list, much as a for 2407 loop's index variable aliases the list elements. That is, modifying an 2408 element of a list returned by grep (for example, in a C<foreach>, C<map> 2409 or another C<grep>) actually modifies the element in the original list. 2410 This is usually something to be avoided when writing clear code. 2411 2412 If C<$_> is lexical in the scope where the C<grep> appears (because it has 2413 been declared with C<my $_>) then, in addition to being locally aliased to 2414 the list elements, C<$_> keeps being lexical inside the block; i.e. it 2415 can't be seen from the outside, avoiding any potential side-effects. 2416 2417 See also L</map> for a list composed of the results of the BLOCK or EXPR. 2418 2419 =item hex EXPR 2420 X<hex> X<hexadecimal> 2421 2422 =item hex 2423 2424 Interprets EXPR as a hex string and returns the corresponding value. 2425 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see 2426 L</oct>.) If EXPR is omitted, uses C<$_>. 2427 2428 print hex '0xAf'; # prints '175' 2429 print hex 'aF'; # same 2430 2431 Hex strings may only represent integers. Strings that would cause 2432 integer overflow trigger a warning. Leading whitespace is not stripped, 2433 unlike oct(). To present something as hex, look into L</printf>, 2434 L</sprintf>, or L</unpack>. 2435 2436 =item import LIST 2437 X<import> 2438 2439 There is no builtin C<import> function. It is just an ordinary 2440 method (subroutine) defined (or inherited) by modules that wish to export 2441 names to another module. The C<use> function calls the C<import> method 2442 for the package used. See also L</use>, L<perlmod>, and L<Exporter>. 2443 2444 =item index STR,SUBSTR,POSITION 2445 X<index> X<indexOf> X<InStr> 2446 2447 =item index STR,SUBSTR 2448 2449 The index function searches for one string within another, but without 2450 the wildcard-like behavior of a full regular-expression pattern match. 2451 It returns the position of the first occurrence of SUBSTR in STR at 2452 or after POSITION. If POSITION is omitted, starts searching from the 2453 beginning of the string. POSITION before the beginning of the string 2454 or after its end is treated as if it were the beginning or the end, 2455 respectively. POSITION and the return value are based at C<0> (or whatever 2456 you've set the C<$[> variable to--but don't do that). If the substring 2457 is not found, C<index> returns one less than the base, ordinarily C<-1>. 2458 2459 =item int EXPR 2460 X<int> X<integer> X<truncate> X<trunc> X<floor> 2461 2462 =item int 2463 2464 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>. 2465 You should not use this function for rounding: one because it truncates 2466 towards C<0>, and two because machine representations of floating point 2467 numbers can sometimes produce counterintuitive results. For example, 2468 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's 2469 because it's really more like -268.99999999999994315658 instead. Usually, 2470 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil> 2471 functions will serve you better than will int(). 2472 2473 =item ioctl FILEHANDLE,FUNCTION,SCALAR 2474 X<ioctl> 2475 2476 Implements the ioctl(2) function. You'll probably first have to say 2477 2478 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph 2479 2480 to get the correct function definitions. If F<sys/ioctl.ph> doesn't 2481 exist or doesn't have the correct definitions you'll have to roll your 2482 own, based on your C header files such as F<< <sys/ioctl.h> >>. 2483 (There is a Perl script called B<h2ph> that comes with the Perl kit that 2484 may help you in this, but it's nontrivial.) SCALAR will be read and/or 2485 written depending on the FUNCTION--a pointer to the string value of SCALAR 2486 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR 2487 has no string value but does have a numeric value, that value will be 2488 passed rather than a pointer to the string value. To guarantee this to be 2489 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack> 2490 functions may be needed to manipulate the values of structures used by 2491 C<ioctl>. 2492 2493 The return value of C<ioctl> (and C<fcntl>) is as follows: 2494 2495 if OS returns: then Perl returns: 2496 -1 undefined value 2497 0 string "0 but true" 2498 anything else that number 2499 2500 Thus Perl returns true on success and false on failure, yet you can 2501 still easily determine the actual value returned by the operating 2502 system: 2503 2504 $retval = ioctl(...) || -1; 2505 printf "System returned %d\n", $retval; 2506 2507 The special string C<"0 but true"> is exempt from B<-w> complaints 2508 about improper numeric conversions. 2509 2510 =item join EXPR,LIST 2511 X<join> 2512 2513 Joins the separate strings of LIST into a single string with fields 2514 separated by the value of EXPR, and returns that new string. Example: 2515 2516 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); 2517 2518 Beware that unlike C<split>, C<join> doesn't take a pattern as its 2519 first argument. Compare L</split>. 2520 2521 =item keys HASH 2522 X<keys> X<key> 2523 2524 Returns a list consisting of all the keys of the named hash. 2525 (In scalar context, returns the number of keys.) 2526 2527 The keys are returned in an apparently random order. The actual 2528 random order is subject to change in future versions of perl, but it 2529 is guaranteed to be the same order as either the C<values> or C<each> 2530 function produces (given that the hash has not been modified). Since 2531 Perl 5.8.1 the ordering is different even between different runs of 2532 Perl for security reasons (see L<perlsec/"Algorithmic Complexity 2533 Attacks">). 2534 2535 As a side effect, calling keys() resets the HASH's internal iterator 2536 (see L</each>). In particular, calling keys() in void context resets 2537 the iterator with no other overhead. 2538 2539 Here is yet another way to print your environment: 2540 2541 @keys = keys %ENV; 2542 @values = values %ENV; 2543 while (@keys) { 2544 print pop(@keys), '=', pop(@values), "\n"; 2545 } 2546 2547 or how about sorted by key: 2548 2549 foreach $key (sort(keys %ENV)) { 2550 print $key, '=', $ENV{$key}, "\n"; 2551 } 2552 2553 The returned values are copies of the original keys in the hash, so 2554 modifying them will not affect the original hash. Compare L</values>. 2555 2556 To sort a hash by value, you'll need to use a C<sort> function. 2557 Here's a descending numeric sort of a hash by its values: 2558 2559 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { 2560 printf "%4d %s\n", $hash{$key}, $key; 2561 } 2562 2563 As an lvalue C<keys> allows you to increase the number of hash buckets 2564 allocated for the given hash. This can gain you a measure of efficiency if 2565 you know the hash is going to get big. (This is similar to pre-extending 2566 an array by assigning a larger number to $#array.) If you say 2567 2568 keys %hash = 200; 2569 2570 then C<%hash> will have at least 200 buckets allocated for it--256 of them, 2571 in fact, since it rounds up to the next power of two. These 2572 buckets will be retained even if you do C<%hash = ()>, use C<undef 2573 %hash> if you want to free the storage while C<%hash> is still in scope. 2574 You can't shrink the number of buckets allocated for the hash using 2575 C<keys> in this way (but you needn't worry about doing this by accident, 2576 as trying has no effect). 2577 2578 See also C<each>, C<values> and C<sort>. 2579 2580 =item kill SIGNAL, LIST 2581 X<kill> X<signal> 2582 2583 Sends a signal to a list of processes. Returns the number of 2584 processes successfully signaled (which is not necessarily the 2585 same as the number actually killed). 2586 2587 $cnt = kill 1, $child1, $child2; 2588 kill 9, @goners; 2589 2590 If SIGNAL is zero, no signal is sent to the process, but the kill(2) 2591 system call will check whether it's possible to send a signal to it (that 2592 means, to be brief, that the process is owned by the same user, or we are 2593 the super-user). This is a useful way to check that a child process is 2594 alive (even if only as a zombie) and hasn't changed its UID. See 2595 L<perlport> for notes on the portability of this construct. 2596 2597 Unlike in the shell, if SIGNAL is negative, it kills 2598 process groups instead of processes. (On System V, a negative I<PROCESS> 2599 number will also kill process groups, but that's not portable.) That 2600 means you usually want to use positive not negative signals. You may also 2601 use a signal name in quotes. 2602 2603 See L<perlipc/"Signals"> for more details. 2604 2605 =item last LABEL 2606 X<last> X<break> 2607 2608 =item last 2609 2610 The C<last> command is like the C<break> statement in C (as used in 2611 loops); it immediately exits the loop in question. If the LABEL is 2612 omitted, the command refers to the innermost enclosing loop. The 2613 C<continue> block, if any, is not executed: 2614 2615 LINE: while (<STDIN>) { 2616 last LINE if /^$/; # exit when done with header 2617 #... 2618 } 2619 2620 C<last> cannot be used to exit a block which returns a value such as 2621 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit 2622 a grep() or map() operation. 2623 2624 Note that a block by itself is semantically identical to a loop 2625 that executes once. Thus C<last> can be used to effect an early 2626 exit out of such a block. 2627 2628 See also L</continue> for an illustration of how C<last>, C<next>, and 2629 C<redo> work. 2630 2631 =item lc EXPR 2632 X<lc> X<lowercase> 2633 2634 =item lc 2635 2636 Returns a lowercased version of EXPR. This is the internal function 2637 implementing the C<\L> escape in double-quoted strings. Respects 2638 current LC_CTYPE locale if C<use locale> in force. See L<perllocale> 2639 and L<perlunicode> for more details about locale and Unicode support. 2640 2641 If EXPR is omitted, uses C<$_>. 2642 2643 =item lcfirst EXPR 2644 X<lcfirst> X<lowercase> 2645 2646 =item lcfirst 2647 2648 Returns the value of EXPR with the first character lowercased. This 2649 is the internal function implementing the C<\l> escape in 2650 double-quoted strings. Respects current LC_CTYPE locale if C<use 2651 locale> in force. See L<perllocale> and L<perlunicode> for more 2652 details about locale and Unicode support. 2653 2654 If EXPR is omitted, uses C<$_>. 2655 2656 =item length EXPR 2657 X<length> X<size> 2658 2659 =item length 2660 2661 Returns the length in I<characters> of the value of EXPR. If EXPR is 2662 omitted, returns length of C<$_>. Note that this cannot be used on 2663 an entire array or hash to find out how many elements these have. 2664 For that, use C<scalar @array> and C<scalar keys %hash> respectively. 2665 2666 Note the I<characters>: if the EXPR is in Unicode, you will get the 2667 number of characters, not the number of bytes. To get the length 2668 of the internal string in bytes, use C<bytes::length(EXPR)>, see 2669 L<bytes>. Note that the internal encoding is variable, and the number 2670 of bytes usually meaningless. To get the number of bytes that the 2671 string would have when encoded as UTF-8, use 2672 C<length(Encoding::encode_utf8(EXPR))>. 2673 2674 =item link OLDFILE,NEWFILE 2675 X<link> 2676 2677 Creates a new filename linked to the old filename. Returns true for 2678 success, false otherwise. 2679 2680 =item listen SOCKET,QUEUESIZE 2681 X<listen> 2682 2683 Does the same thing that the listen system call does. Returns true if 2684 it succeeded, false otherwise. See the example in 2685 L<perlipc/"Sockets: Client/Server Communication">. 2686 2687 =item local EXPR 2688 X<local> 2689 2690 You really probably want to be using C<my> instead, because C<local> isn't 2691 what most people think of as "local". See 2692 L<perlsub/"Private Variables via my()"> for details. 2693 2694 A local modifies the listed variables to be local to the enclosing 2695 block, file, or eval. If more than one value is listed, the list must 2696 be placed in parentheses. See L<perlsub/"Temporary Values via local()"> 2697 for details, including issues with tied arrays and hashes. 2698 2699 =item localtime EXPR 2700 X<localtime> X<ctime> 2701 2702 =item localtime 2703 2704 Converts a time as returned by the time function to a 9-element list 2705 with the time analyzed for the local time zone. Typically used as 2706 follows: 2707 2708 # 0 1 2 3 4 5 6 7 8 2709 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = 2710 localtime(time); 2711 2712 All list elements are numeric, and come straight out of the C `struct 2713 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours 2714 of the specified time. 2715 2716 C<$mday> is the day of the month, and C<$mon> is the month itself, in 2717 the range C<0..11> with 0 indicating January and 11 indicating December. 2718 This makes it easy to get a month name from a list: 2719 2720 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec ); 2721 print "$abbr[$mon] $mday"; 2722 # $mon=9, $mday=18 gives "Oct 18" 2723 2724 C<$year> is the number of years since 1900, not just the last two digits 2725 of the year. That is, C<$year> is C<123> in year 2023. The proper way 2726 to get a complete 4-digit year is simply: 2727 2728 $year += 1900; 2729 2730 Otherwise you create non-Y2K-compliant programs--and you wouldn't want 2731 to do that, would you? 2732 2733 To get the last two digits of the year (e.g., '01' in 2001) do: 2734 2735 $year = sprintf("%02d", $year % 100); 2736 2737 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating 2738 Wednesday. C<$yday> is the day of the year, in the range C<0..364> 2739 (or C<0..365> in leap years.) 2740 2741 C<$isdst> is true if the specified time occurs during Daylight Saving 2742 Time, false otherwise. 2743 2744 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>). 2745 2746 In scalar context, C<localtime()> returns the ctime(3) value: 2747 2748 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" 2749 2750 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT 2751 instead of local time use the L</gmtime> builtin. See also the 2752 C<Time::Local> module (to convert the second, minutes, hours, ... back to 2753 the integer value returned by time()), and the L<POSIX> module's strftime(3) 2754 and mktime(3) functions. 2755 2756 To get somewhat similar but locale dependent date strings, set up your 2757 locale environment variables appropriately (please see L<perllocale>) and 2758 try for example: 2759 2760 use POSIX qw(strftime); 2761 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; 2762 # or for GMT formatted appropriately for your locale: 2763 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; 2764 2765 Note that the C<%a> and C<%b>, the short forms of the day of the week 2766 and the month of the year, may not necessarily be three characters wide. 2767 2768 See L<perlport/localtime> for portability concerns. 2769 2770 The L<Time::gmtime> and L<Time::localtime> modules provides a convenient, 2771 by-name access mechanism to the gmtime() and localtime() functions, 2772 respectively. 2773 2774 For a comprehensive date and time representation look at the 2775 L<DateTime> module on CPAN. 2776 2777 =item lock THING 2778 X<lock> 2779 2780 This function places an advisory lock on a shared variable, or referenced 2781 object contained in I<THING> until the lock goes out of scope. 2782 2783 lock() is a "weak keyword" : this means that if you've defined a function 2784 by this name (before any calls to it), that function will be called 2785 instead. (However, if you've said C<use threads>, lock() is always a 2786 keyword.) See L<threads>. 2787 2788 =item log EXPR 2789 X<log> X<logarithm> X<e> X<ln> X<base> 2790 2791 =item log 2792 2793 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, 2794 returns log of C<$_>. To get the log of another base, use basic algebra: 2795 The base-N log of a number is equal to the natural log of that number 2796 divided by the natural log of N. For example: 2797 2798 sub log10 { 2799 my $n = shift; 2800 return log($n)/log(10); 2801 } 2802 2803 See also L</exp> for the inverse operation. 2804 2805 =item lstat EXPR 2806 X<lstat> 2807 2808 =item lstat 2809 2810 Does the same thing as the C<stat> function (including setting the 2811 special C<_> filehandle) but stats a symbolic link instead of the file 2812 the symbolic link points to. If symbolic links are unimplemented on 2813 your system, a normal C<stat> is done. For much more detailed 2814 information, please see the documentation for C<stat>. 2815 2816 If EXPR is omitted, stats C<$_>. 2817 2818 =item m// 2819 2820 The match operator. See L<perlop>. 2821 2822 =item map BLOCK LIST 2823 X<map> 2824 2825 =item map EXPR,LIST 2826 2827 Evaluates the BLOCK or EXPR for each element of LIST (locally setting 2828 C<$_> to each element) and returns the list value composed of the 2829 results of each such evaluation. In scalar context, returns the 2830 total number of elements so generated. Evaluates BLOCK or EXPR in 2831 list context, so each element of LIST may produce zero, one, or 2832 more elements in the returned value. 2833 2834 @chars = map(chr, @nums); 2835 2836 translates a list of numbers to the corresponding characters. And 2837 2838 %hash = map { get_a_key_for($_) => $_ } @array; 2839 2840 is just a funny way to write 2841 2842 %hash = (); 2843 foreach (@array) { 2844 $hash{get_a_key_for($_)} = $_; 2845 } 2846 2847 Note that C<$_> is an alias to the list value, so it can be used to 2848 modify the elements of the LIST. While this is useful and supported, 2849 it can cause bizarre results if the elements of LIST are not variables. 2850 Using a regular C<foreach> loop for this purpose would be clearer in 2851 most cases. See also L</grep> for an array composed of those items of 2852 the original list for which the BLOCK or EXPR evaluates to true. 2853 2854 If C<$_> is lexical in the scope where the C<map> appears (because it has 2855 been declared with C<my $_>), then, in addition to being locally aliased to 2856 the list elements, C<$_> keeps being lexical inside the block; that is, it 2857 can't be seen from the outside, avoiding any potential side-effects. 2858 2859 C<{> starts both hash references and blocks, so C<map { ...> could be either 2860 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look 2861 ahead for the closing C<}> it has to take a guess at which its dealing with 2862 based what it finds just after the C<{>. Usually it gets it right, but if it 2863 doesn't it won't realize something is wrong until it gets to the C<}> and 2864 encounters the missing (or unexpected) comma. The syntax error will be 2865 reported close to the C<}> but you'll need to change something near the C<{> 2866 such as using a unary C<+> to give perl some help: 2867 2868 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong 2869 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right 2870 %hash = map { ("\L$_", 1) } @array # this also works 2871 %hash = map { lc($_), 1 } @array # as does this. 2872 %hash = map +( lc($_), 1 ), @array # this is EXPR and works! 2873 2874 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array) 2875 2876 or to force an anon hash constructor use C<+{>: 2877 2878 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end 2879 2880 and you get list of anonymous hashes each with only 1 entry. 2881 2882 =item mkdir FILENAME,MASK 2883 X<mkdir> X<md> X<directory, create> 2884 2885 =item mkdir FILENAME 2886 2887 =item mkdir 2888 2889 Creates the directory specified by FILENAME, with permissions 2890 specified by MASK (as modified by C<umask>). If it succeeds it 2891 returns true, otherwise it returns false and sets C<$!> (errno). 2892 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults 2893 to C<$_>. 2894 2895 In general, it is better to create directories with permissive MASK, 2896 and let the user modify that with their C<umask>, than it is to supply 2897 a restrictive MASK and give the user no way to be more permissive. 2898 The exceptions to this rule are when the file or directory should be 2899 kept private (mail files, for instance). The perlfunc(1) entry on 2900 C<umask> discusses the choice of MASK in more detail. 2901 2902 Note that according to the POSIX 1003.1-1996 the FILENAME may have any 2903 number of trailing slashes. Some operating and filesystems do not get 2904 this right, so Perl automatically removes all trailing slashes to keep 2905 everyone happy. 2906 2907 In order to recursively create a directory structure look at 2908 the C<mkpath> function of the L<File::Path> module. 2909 2910 =item msgctl ID,CMD,ARG 2911 X<msgctl> 2912 2913 Calls the System V IPC function msgctl(2). You'll probably have to say 2914 2915 use IPC::SysV; 2916 2917 first to get the correct constant definitions. If CMD is C<IPC_STAT>, 2918 then ARG must be a variable that will hold the returned C<msqid_ds> 2919 structure. Returns like C<ioctl>: the undefined value for error, 2920 C<"0 but true"> for zero, or the actual return value otherwise. See also 2921 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation. 2922 2923 =item msgget KEY,FLAGS 2924 X<msgget> 2925 2926 Calls the System V IPC function msgget(2). Returns the message queue 2927 id, or the undefined value if there is an error. See also 2928 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation. 2929 2930 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS 2931 X<msgrcv> 2932 2933 Calls the System V IPC function msgrcv to receive a message from 2934 message queue ID into variable VAR with a maximum message size of 2935 SIZE. Note that when a message is received, the message type as a 2936 native long integer will be the first thing in VAR, followed by the 2937 actual message. This packing may be opened with C<unpack("l! a*")>. 2938 Taints the variable. Returns true if successful, or false if there is 2939 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and 2940 C<IPC::SysV::Msg> documentation. 2941 2942 =item msgsnd ID,MSG,FLAGS 2943 X<msgsnd> 2944 2945 Calls the System V IPC function msgsnd to send the message MSG to the 2946 message queue ID. MSG must begin with the native long integer message 2947 type, and be followed by the length of the actual message, and finally 2948 the message itself. This kind of packing can be achieved with 2949 C<pack("l! a*", $type, $message)>. Returns true if successful, 2950 or false if there is an error. See also C<IPC::SysV> 2951 and C<IPC::SysV::Msg> documentation. 2952 2953 =item my EXPR 2954 X<my> 2955 2956 =item my TYPE EXPR 2957 2958 =item my EXPR : ATTRS 2959 2960 =item my TYPE EXPR : ATTRS 2961 2962 A C<my> declares the listed variables to be local (lexically) to the 2963 enclosing block, file, or C<eval>. If more than one value is listed, 2964 the list must be placed in parentheses. 2965 2966 The exact semantics and interface of TYPE and ATTRS are still 2967 evolving. TYPE is currently bound to the use of C<fields> pragma, 2968 and attributes are handled using the C<attributes> pragma, or starting 2969 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See 2970 L<perlsub/"Private Variables via my()"> for details, and L<fields>, 2971 L<attributes>, and L<Attribute::Handlers>. 2972 2973 =item next LABEL 2974 X<next> X<continue> 2975 2976 =item next 2977 2978 The C<next> command is like the C<continue> statement in C; it starts 2979 the next iteration of the loop: 2980 2981 LINE: while (<STDIN>) { 2982 next LINE if /^#/; # discard comments 2983 #... 2984 } 2985 2986 Note that if there were a C<continue> block on the above, it would get 2987 executed even on discarded lines. If the LABEL is omitted, the command 2988 refers to the innermost enclosing loop. 2989 2990 C<next> cannot be used to exit a block which returns a value such as 2991 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit 2992 a grep() or map() operation. 2993 2994 Note that a block by itself is semantically identical to a loop 2995 that executes once. Thus C<next> will exit such a block early. 2996 2997 See also L</continue> for an illustration of how C<last>, C<next>, and 2998 C<redo> work. 2999 3000 =item no Module VERSION LIST 3001 X<no> 3002 3003 =item no Module VERSION 3004 3005 =item no Module LIST 3006 3007 =item no Module 3008 3009 =item no VERSION 3010 3011 See the C<use> function, of which C<no> is the opposite. 3012 3013 =item oct EXPR 3014 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin> 3015 3016 =item oct 3017 3018 Interprets EXPR as an octal string and returns the corresponding 3019 value. (If EXPR happens to start off with C<0x>, interprets it as a 3020 hex string. If EXPR starts off with C<0b>, it is interpreted as a 3021 binary string. Leading whitespace is ignored in all three cases.) 3022 The following will handle decimal, binary, octal, and hex in the standard 3023 Perl or C notation: 3024 3025 $val = oct($val) if $val =~ /^0/; 3026 3027 If EXPR is omitted, uses C<$_>. To go the other way (produce a number 3028 in octal), use sprintf() or printf(): 3029 3030 $perms = (stat("filename"))[2] & 07777; 3031 $oct_perms = sprintf "%lo", $perms; 3032 3033 The oct() function is commonly used when a string such as C<644> needs 3034 to be converted into a file mode, for example. (Although perl will 3035 automatically convert strings into numbers as needed, this automatic 3036 conversion assumes base 10.) 3037 3038 =item open FILEHANDLE,EXPR 3039 X<open> X<pipe> X<file, open> X<fopen> 3040 3041 =item open FILEHANDLE,MODE,EXPR 3042 3043 =item open FILEHANDLE,MODE,EXPR,LIST 3044 3045 =item open FILEHANDLE,MODE,REFERENCE 3046 3047 =item open FILEHANDLE 3048 3049 Opens the file whose filename is given by EXPR, and associates it with 3050 FILEHANDLE. 3051 3052 (The following is a comprehensive reference to open(): for a gentler 3053 introduction you may consider L<perlopentut>.) 3054 3055 If FILEHANDLE is an undefined scalar variable (or array or hash element) 3056 the variable is assigned a reference to a new anonymous filehandle, 3057 otherwise if FILEHANDLE is an expression, its value is used as the name of 3058 the real filehandle wanted. (This is considered a symbolic reference, so 3059 C<use strict 'refs'> should I<not> be in effect.) 3060 3061 If EXPR is omitted, the scalar variable of the same name as the 3062 FILEHANDLE contains the filename. (Note that lexical variables--those 3063 declared with C<my>--will not work for this purpose; so if you're 3064 using C<my>, specify EXPR in your call to open.) 3065 3066 If three or more arguments are specified then the mode of opening and 3067 the file name are separate. If MODE is C<< '<' >> or nothing, the file 3068 is opened for input. If MODE is C<< '>' >>, the file is truncated and 3069 opened for output, being created if necessary. If MODE is C<<< '>>' >>>, 3070 the file is opened for appending, again being created if necessary. 3071 3072 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to 3073 indicate that you want both read and write access to the file; thus 3074 C<< '+<' >> is almost always preferred for read/write updates--the C<< 3075 '+>' >> mode would clobber the file first. You can't usually use 3076 either read-write mode for updating textfiles, since they have 3077 variable length records. See the B<-i> switch in L<perlrun> for a 3078 better approach. The file is created with permissions of C<0666> 3079 modified by the process' C<umask> value. 3080 3081 These various prefixes correspond to the fopen(3) modes of C<'r'>, 3082 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>. 3083 3084 In the 2-arguments (and 1-argument) form of the call the mode and 3085 filename should be concatenated (in this order), possibly separated by 3086 spaces. It is possible to omit the mode in these forms if the mode is 3087 C<< '<' >>. 3088 3089 If the filename begins with C<'|'>, the filename is interpreted as a 3090 command to which output is to be piped, and if the filename ends with a 3091 C<'|'>, the filename is interpreted as a command which pipes output to 3092 us. See L<perlipc/"Using open() for IPC"> 3093 for more examples of this. (You are not allowed to C<open> to a command 3094 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, 3095 and L<perlipc/"Bidirectional Communication with Another Process"> 3096 for alternatives.) 3097 3098 For three or more arguments if MODE is C<'|-'>, the filename is 3099 interpreted as a command to which output is to be piped, and if MODE 3100 is C<'-|'>, the filename is interpreted as a command which pipes 3101 output to us. In the 2-arguments (and 1-argument) form one should 3102 replace dash (C<'-'>) with the command. 3103 See L<perlipc/"Using open() for IPC"> for more examples of this. 3104 (You are not allowed to C<open> to a command that pipes both in I<and> 3105 out, but see L<IPC::Open2>, L<IPC::Open3>, and 3106 L<perlipc/"Bidirectional Communication"> for alternatives.) 3107 3108 In the three-or-more argument form of pipe opens, if LIST is specified 3109 (extra arguments after the command name) then LIST becomes arguments 3110 to the command invoked if the platform supports it. The meaning of 3111 C<open> with more than three arguments for non-pipe modes is not yet 3112 specified. Experimental "layers" may give extra LIST arguments 3113 meaning. 3114 3115 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN 3116 and opening C<< '>-' >> opens STDOUT. 3117 3118 You may use the three-argument form of open to specify IO "layers" 3119 (sometimes also referred to as "disciplines") to be applied to the handle 3120 that affect how the input and output are processed (see L<open> and 3121 L<PerlIO> for more details). For example 3122 3123 open(FH, "<:encoding(UTF-8)", "file") 3124 3125 will open the UTF-8 encoded file containing Unicode characters, 3126 see L<perluniintro>. Note that if layers are specified in the 3127 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>; 3128 usually set by the B<open> pragma or the switch B<-CioD>) are ignored. 3129 3130 Open returns nonzero upon success, the undefined value otherwise. If 3131 the C<open> involved a pipe, the return value happens to be the pid of 3132 the subprocess. 3133 3134 If you're running Perl on a system that distinguishes between text 3135 files and binary files, then you should check out L</binmode> for tips 3136 for dealing with this. The key distinction between systems that need 3137 C<binmode> and those that don't is their text file formats. Systems 3138 like Unix, Mac OS, and Plan 9, which delimit lines with a single 3139 character, and which encode that character in C as C<"\n">, do not 3140 need C<binmode>. The rest need it. 3141 3142 When opening a file, it's usually a bad idea to continue normal execution 3143 if the request failed, so C<open> is frequently used in connection with 3144 C<die>. Even if C<die> won't do what you want (say, in a CGI script, 3145 where you want to make a nicely formatted error message (but there are 3146 modules that can help with that problem)) you should always check 3147 the return value from opening a file. The infrequent exception is when 3148 working with an unopened filehandle is actually what you want to do. 3149 3150 As a special case the 3-arg form with a read/write mode and the third 3151 argument being C<undef>: 3152 3153 open(TMP, "+>", undef) or die ... 3154 3155 opens a filehandle to an anonymous temporary file. Also using "+<" 3156 works for symmetry, but you really should consider writing something 3157 to the temporary file first. You will need to seek() to do the 3158 reading. 3159 3160 Since v5.8.0, perl has built using PerlIO by default. Unless you've 3161 changed this (i.e. Configure -Uuseperlio), you can open file handles to 3162 "in memory" files held in Perl scalars via: 3163 3164 open($fh, '>', \$variable) || .. 3165 3166 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory" 3167 file, you have to close it first: 3168 3169 close STDOUT; 3170 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!"; 3171 3172 Examples: 3173 3174 $ARTICLE = 100; 3175 open ARTICLE or die "Can't find article $ARTICLE: $!\n"; 3176 while (<ARTICLE>) {... 3177 3178 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) 3179 # if the open fails, output is discarded 3180 3181 open(DBASE, '+<', 'dbase.mine') # open for update 3182 or die "Can't open 'dbase.mine' for update: $!"; 3183 3184 open(DBASE, '+<dbase.mine') # ditto 3185 or die "Can't open 'dbase.mine' for update: $!"; 3186 3187 open(ARTICLE, '-|', "caesar <$article") # decrypt article 3188 or die "Can't start caesar: $!"; 3189 3190 open(ARTICLE, "caesar <$article |") # ditto 3191 or die "Can't start caesar: $!"; 3192 3193 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id 3194 or die "Can't start sort: $!"; 3195 3196 # in memory files 3197 open(MEMORY,'>', \$var) 3198 or die "Can't open memory file: $!"; 3199 print MEMORY "foo!\n"; # output will end up in $var 3200 3201 # process argument list of files along with any includes 3202 3203 foreach $file (@ARGV) { 3204 process($file, 'fh00'); 3205 } 3206 3207 sub process { 3208 my($filename, $input) = @_; 3209 $input++; # this is a string increment 3210 unless (open($input, $filename)) { 3211 print STDERR "Can't open $filename: $!\n"; 3212 return; 3213 } 3214 3215 local $_; 3216 while (<$input>) { # note use of indirection 3217 if (/^#include "(.*)"/) { 3218 process($1, $input); 3219 next; 3220 } 3221 #... # whatever 3222 } 3223 } 3224 3225 See L<perliol> for detailed info on PerlIO. 3226 3227 You may also, in the Bourne shell tradition, specify an EXPR beginning 3228 with C<< '>&' >>, in which case the rest of the string is interpreted 3229 as the name of a filehandle (or file descriptor, if numeric) to be 3230 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>, 3231 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. 3232 The mode you specify should match the mode of the original filehandle. 3233 (Duping a filehandle does not take into account any existing contents 3234 of IO buffers.) If you use the 3-arg form then you can pass either a 3235 number, the name of a filehandle or the normal "reference to a glob". 3236 3237 Here is a script that saves, redirects, and restores C<STDOUT> and 3238 C<STDERR> using various methods: 3239 3240 #!/usr/bin/perl 3241 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!"; 3242 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!"; 3243 3244 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!"; 3245 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!"; 3246 3247 select STDERR; $| = 1; # make unbuffered 3248 select STDOUT; $| = 1; # make unbuffered 3249 3250 print STDOUT "stdout 1\n"; # this works for 3251 print STDERR "stderr 1\n"; # subprocesses too 3252 3253 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!"; 3254 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!"; 3255 3256 print STDOUT "stdout 2\n"; 3257 print STDERR "stderr 2\n"; 3258 3259 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number 3260 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of 3261 that file descriptor (and not call L<dup(2)>); this is more 3262 parsimonious of file descriptors. For example: 3263 3264 # open for input, reusing the fileno of $fd 3265 open(FILEHANDLE, "<&=$fd") 3266 3267 or 3268 3269 open(FILEHANDLE, "<&=", $fd) 3270 3271 or 3272 3273 # open for append, using the fileno of OLDFH 3274 open(FH, ">>&=", OLDFH) 3275 3276 or 3277 3278 open(FH, ">>&=OLDFH") 3279 3280 Being parsimonious on filehandles is also useful (besides being 3281 parsimonious) for example when something is dependent on file 3282 descriptors, like for example locking using flock(). If you do just 3283 C<< open(A, '>>&B') >>, the filehandle A will not have the same file 3284 descriptor as B, and therefore flock(A) will not flock(B), and vice 3285 versa. But with C<< open(A, '>>&=B') >> the filehandles will share 3286 the same file descriptor. 3287 3288 Note that if you are using Perls older than 5.8.0, Perl will be using 3289 the standard C libraries' fdopen() to implement the "=" functionality. 3290 On many UNIX systems fdopen() fails when file descriptors exceed a 3291 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is 3292 most often the default. 3293 3294 You can see whether Perl has been compiled with PerlIO or not by 3295 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio> 3296 is C<define>, you have PerlIO, otherwise you don't. 3297 3298 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'> 3299 with 2-arguments (or 1-argument) form of open(), then 3300 there is an implicit fork done, and the return value of open is the pid 3301 of the child within the parent process, and C<0> within the child 3302 process. (Use C<defined($pid)> to determine whether the open was successful.) 3303 The filehandle behaves normally for the parent, but i/o to that 3304 filehandle is piped from/to the STDOUT/STDIN of the child process. 3305 In the child process the filehandle isn't opened--i/o happens from/to 3306 the new STDOUT or STDIN. Typically this is used like the normal 3307 piped open when you want to exercise more control over just how the 3308 pipe command gets executed, such as when you are running setuid, and 3309 don't want to have to scan shell commands for metacharacters. 3310 The following triples are more or less equivalent: 3311 3312 open(FOO, "|tr '[a-z]' '[A-Z]'"); 3313 open(FOO, '|-', "tr '[a-z]' '[A-Z]'"); 3314 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]'; 3315 open(FOO, '|-', "tr", '[a-z]', '[A-Z]'); 3316 3317 open(FOO, "cat -n '$file'|"); 3318 open(FOO, '-|', "cat -n '$file'"); 3319 open(FOO, '-|') || exec 'cat', '-n', $file; 3320 open(FOO, '-|', "cat", '-n', $file); 3321 3322 The last example in each block shows the pipe as "list form", which is 3323 not yet supported on all platforms. A good rule of thumb is that if 3324 your platform has true C<fork()> (in other words, if your platform is 3325 UNIX) you can use the list form. 3326 3327 See L<perlipc/"Safe Pipe Opens"> for more examples of this. 3328 3329 Beginning with v5.6.0, Perl will attempt to flush all files opened for 3330 output before any operation that may do a fork, but this may not be 3331 supported on some platforms (see L<perlport>). To be safe, you may need 3332 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method 3333 of C<IO::Handle> on any open handles. 3334 3335 On systems that support a close-on-exec flag on files, the flag will 3336 be set for the newly opened file descriptor as determined by the value 3337 of $^F. See L<perlvar/$^F>. 3338 3339 Closing any piped filehandle causes the parent process to wait for the 3340 child to finish, and returns the status value in C<$?> and 3341 C<${^CHILD_ERROR_NATIVE}>. 3342 3343 The filename passed to 2-argument (or 1-argument) form of open() will 3344 have leading and trailing whitespace deleted, and the normal 3345 redirection characters honored. This property, known as "magic open", 3346 can often be used to good effect. A user could specify a filename of 3347 F<"rsh cat file |">, or you could change certain filenames as needed: 3348 3349 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; 3350 open(FH, $filename) or die "Can't open $filename: $!"; 3351 3352 Use 3-argument form to open a file with arbitrary weird characters in it, 3353 3354 open(FOO, '<', $file); 3355 3356 otherwise it's necessary to protect any leading and trailing whitespace: 3357 3358 $file =~ s#^(\s)#./$1#; 3359 open(FOO, "< $file\0"); 3360 3361 (this may not work on some bizarre filesystems). One should 3362 conscientiously choose between the I<magic> and 3-arguments form 3363 of open(): 3364 3365 open IN, $ARGV[0]; 3366 3367 will allow the user to specify an argument of the form C<"rsh cat file |">, 3368 but will not work on a filename which happens to have a trailing space, while 3369 3370 open IN, '<', $ARGV[0]; 3371 3372 will have exactly the opposite restrictions. 3373 3374 If you want a "real" C C<open> (see L<open(2)> on your system), then you 3375 should use the C<sysopen> function, which involves no such magic (but 3376 may use subtly different filemodes than Perl open(), which is mapped 3377 to C fopen()). This is 3378 another way to protect your filenames from interpretation. For example: 3379 3380 use IO::Handle; 3381 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) 3382 or die "sysopen $path: $!"; 3383 $oldfh = select(HANDLE); $| = 1; select($oldfh); 3384 print HANDLE "stuff $$\n"; 3385 seek(HANDLE, 0, 0); 3386 print "File contains: ", <HANDLE>; 3387 3388 Using the constructor from the C<IO::Handle> package (or one of its 3389 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous 3390 filehandles that have the scope of whatever variables hold references to 3391 them, and automatically close whenever and however you leave that scope: 3392 3393 use IO::File; 3394 #... 3395 sub read_myfile_munged { 3396 my $ALL = shift; 3397 my $handle = new IO::File; 3398 open($handle, "myfile") or die "myfile: $!"; 3399 $first = <$handle> 3400 or return (); # Automatically closed here. 3401 mung $first or die "mung failed"; # Or here. 3402 return $first, <$handle> if $ALL; # Or here. 3403 $first; # Or here. 3404 } 3405 3406 See L</seek> for some details about mixing reading and writing. 3407 3408 =item opendir DIRHANDLE,EXPR 3409 X<opendir> 3410 3411 Opens a directory named EXPR for processing by C<readdir>, C<telldir>, 3412 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful. 3413 DIRHANDLE may be an expression whose value can be used as an indirect 3414 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined 3415 scalar variable (or array or hash element), the variable is assigned a 3416 reference to a new anonymous dirhandle. 3417 DIRHANDLEs have their own namespace separate from FILEHANDLEs. 3418 3419 =item ord EXPR 3420 X<ord> X<encoding> 3421 3422 =item ord 3423 3424 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC, 3425 or Unicode) value of the first character of EXPR. If EXPR is omitted, 3426 uses C<$_>. 3427 3428 For the reverse, see L</chr>. 3429 See L<perlunicode> for more about Unicode. 3430 3431 =item our EXPR 3432 X<our> X<global> 3433 3434 =item our TYPE EXPR 3435 3436 =item our EXPR : ATTRS 3437 3438 =item our TYPE EXPR : ATTRS 3439 3440 C<our> associates a simple name with a package variable in the current 3441 package for use within the current scope. When C<use strict 'vars'> is in 3442 effect, C<our> lets you use declared global variables without qualifying 3443 them with package names, within the lexical scope of the C<our> declaration. 3444 In this way C<our> differs from C<use vars>, which is package scoped. 3445 3446 Unlike C<my>, which both allocates storage for a variable and associates 3447 a simple name with that storage for use within the current scope, C<our> 3448 associates a simple name with a package variable in the current package, 3449 for use within the current scope. In other words, C<our> has the same 3450 scoping rules as C<my>, but does not necessarily create a 3451 variable. 3452 3453 If more than one value is listed, the list must be placed 3454 in parentheses. 3455 3456 our $foo; 3457 our($bar, $baz); 3458 3459 An C<our> declaration declares a global variable that will be visible 3460 across its entire lexical scope, even across package boundaries. The 3461 package in which the variable is entered is determined at the point 3462 of the declaration, not at the point of use. This means the following 3463 behavior holds: 3464 3465 package Foo; 3466 our $bar; # declares $Foo::bar for rest of lexical scope 3467 $bar = 20; 3468 3469 package Bar; 3470 print $bar; # prints 20, as it refers to $Foo::bar 3471 3472 Multiple C<our> declarations with the same name in the same lexical 3473 scope are allowed if they are in different packages. If they happen 3474 to be in the same package, Perl will emit warnings if you have asked 3475 for them, just like multiple C<my> declarations. Unlike a second 3476 C<my> declaration, which will bind the name to a fresh variable, a 3477 second C<our> declaration in the same package, in the same scope, is 3478 merely redundant. 3479 3480 use warnings; 3481 package Foo; 3482 our $bar; # declares $Foo::bar for rest of lexical scope 3483 $bar = 20; 3484 3485 package Bar; 3486 our $bar = 30; # declares $Bar::bar for rest of lexical scope 3487 print $bar; # prints 30 3488 3489 our $bar; # emits warning but has no other effect 3490 print $bar; # still prints 30 3491 3492 An C<our> declaration may also have a list of attributes associated 3493 with it. 3494 3495 The exact semantics and interface of TYPE and ATTRS are still 3496 evolving. TYPE is currently bound to the use of C<fields> pragma, 3497 and attributes are handled using the C<attributes> pragma, or starting 3498 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See 3499 L<perlsub/"Private Variables via my()"> for details, and L<fields>, 3500 L<attributes>, and L<Attribute::Handlers>. 3501 3502 =item pack TEMPLATE,LIST 3503 X<pack> 3504 3505 Takes a LIST of values and converts it into a string using the rules 3506 given by the TEMPLATE. The resulting string is the concatenation of 3507 the converted values. Typically, each converted value looks 3508 like its machine-level representation. For example, on 32-bit machines 3509 an integer may be represented by a sequence of 4 bytes that will be 3510 converted to a sequence of 4 characters. 3511 3512 The TEMPLATE is a sequence of characters that give the order and type 3513 of values, as follows: 3514 3515 a A string with arbitrary binary data, will be null padded. 3516 A A text (ASCII) string, will be space padded. 3517 Z A null terminated (ASCIZ) string, will be null padded. 3518 3519 b A bit string (ascending bit order inside each byte, like vec()). 3520 B A bit string (descending bit order inside each byte). 3521 h A hex string (low nybble first). 3522 H A hex string (high nybble first). 3523 3524 c A signed char (8-bit) value. 3525 C An unsigned char (octet) value. 3526 W An unsigned char value (can be greater than 255). 3527 3528 s A signed short (16-bit) value. 3529 S An unsigned short value. 3530 3531 l A signed long (32-bit) value. 3532 L An unsigned long value. 3533 3534 q A signed quad (64-bit) value. 3535 Q An unsigned quad value. 3536 (Quads are available only if your system supports 64-bit 3537 integer values _and_ if Perl has been compiled to support those. 3538 Causes a fatal error otherwise.) 3539 3540 i A signed integer value. 3541 I A unsigned integer value. 3542 (This 'integer' is _at_least_ 32 bits wide. Its exact 3543 size depends on what a local C compiler calls 'int'.) 3544 3545 n An unsigned short (16-bit) in "network" (big-endian) order. 3546 N An unsigned long (32-bit) in "network" (big-endian) order. 3547 v An unsigned short (16-bit) in "VAX" (little-endian) order. 3548 V An unsigned long (32-bit) in "VAX" (little-endian) order. 3549 3550 j A Perl internal signed integer value (IV). 3551 J A Perl internal unsigned integer value (UV). 3552 3553 f A single-precision float in the native format. 3554 d A double-precision float in the native format. 3555 3556 F A Perl internal floating point value (NV) in the native format 3557 D A long double-precision float in the native format. 3558 (Long doubles are available only if your system supports long 3559 double values _and_ if Perl has been compiled to support those. 3560 Causes a fatal error otherwise.) 3561 3562 p A pointer to a null-terminated string. 3563 P A pointer to a structure (fixed-length string). 3564 3565 u A uuencoded string. 3566 U A Unicode character number. Encodes to a character in character mode 3567 and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode. 3568 3569 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for 3570 details). Its bytes represent an unsigned integer in base 128, 3571 most significant digit first, with as few digits as possible. Bit 3572 eight (the high bit) is set on each byte except the last. 3573 3574 x A null byte. 3575 X Back up a byte. 3576 @ Null fill or truncate to absolute position, counted from the 3577 start of the innermost ()-group. 3578 . Null fill or truncate to absolute position specified by value. 3579 ( Start of a ()-group. 3580 3581 One or more of the modifiers below may optionally follow some letters in the 3582 TEMPLATE (the second column lists the letters for which the modifier is 3583 valid): 3584 3585 ! sSlLiI Forces native (short, long, int) sizes instead 3586 of fixed (16-/32-bit) sizes. 3587 3588 xX Make x and X act as alignment commands. 3589 3590 nNvV Treat integers as signed instead of unsigned. 3591 3592 @. Specify position as byte offset in the internal 3593 representation of the packed string. Efficient but 3594 dangerous. 3595 3596 > sSiIlLqQ Force big-endian byte-order on the type. 3597 jJfFdDpP (The "big end" touches the construct.) 3598 3599 < sSiIlLqQ Force little-endian byte-order on the type. 3600 jJfFdDpP (The "little end" touches the construct.) 3601 3602 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups, 3603 in which case they force a certain byte-order on all components of 3604 that group, including subgroups. 3605 3606 The following rules apply: 3607 3608 =over 8 3609 3610 =item * 3611 3612 Each letter may optionally be followed by a number giving a repeat 3613 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>, 3614 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up 3615 that many values from the LIST. A C<*> for the repeat count means to 3616 use however many items are left, except for C<@>, C<x>, C<X>, where it 3617 is equivalent to C<0>, for <.> where it means relative to string start 3618 and C<u>, where it is equivalent to 1 (or 45, which is the same). 3619 A numeric repeat count may optionally be enclosed in brackets, as in 3620 C<pack 'C[80]', @arr>. 3621 3622 One can replace the numeric repeat count by a template enclosed in brackets; 3623 then the packed length of this template in bytes is used as a count. 3624 For example, C<x[L]> skips a long (it skips the number of bytes in a long); 3625 the template C<$t X[$t] $t> unpack()s twice what $t unpacks. 3626 If the template in brackets contains alignment commands (such as C<x![d]>), 3627 its packed length is calculated as if the start of the template has the maximal 3628 possible alignment. 3629 3630 When used with C<Z>, C<*> results in the addition of a trailing null 3631 byte (so the packed result will be one longer than the byte C<length> 3632 of the item). 3633 3634 When used with C<@>, the repeat count represents an offset from the start 3635 of the innermost () group. 3636 3637 When used with C<.>, the repeat count is used to determine the starting 3638 position from where the value offset is calculated. If the repeat count 3639 is 0, it's relative to the current position. If the repeat count is C<*>, 3640 the offset is relative to the start of the packed string. And if its an 3641 integer C<n> the offset is relative to the start of the n-th innermost 3642 () group (or the start of the string if C<n> is bigger then the group 3643 level). 3644 3645 The repeat count for C<u> is interpreted as the maximal number of bytes 3646 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat 3647 count should not be more than 65. 3648 3649 =item * 3650 3651 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a 3652 string of length count, padding with nulls or spaces as necessary. When 3653 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything 3654 after the first null, and C<a> returns data verbatim. 3655 3656 If the value-to-pack is too long, it is truncated. If too long and an 3657 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed 3658 by a null byte. Thus C<Z> always packs a trailing null (except when the 3659 count is 0). 3660 3661 =item * 3662 3663 Likewise, the C<b> and C<B> fields pack a string that many bits long. 3664 Each character of the input field of pack() generates 1 bit of the result. 3665 Each result bit is based on the least-significant bit of the corresponding 3666 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0"> 3667 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">. 3668 3669 Starting from the beginning of the input string of pack(), each 8-tuple 3670 of characters is converted to 1 character of output. With format C<b> 3671 the first character of the 8-tuple determines the least-significant bit of a 3672 character, and with format C<B> it determines the most-significant bit of 3673 a character. 3674 3675 If the length of the input string is not exactly divisible by 8, the 3676 remainder is packed as if the input string were padded by null characters 3677 at the end. Similarly, during unpack()ing the "extra" bits are ignored. 3678 3679 If the input string of pack() is longer than needed, extra characters are 3680 ignored. A C<*> for the repeat count of pack() means to use all the 3681 characters of the input field. On unpack()ing the bits are converted to a 3682 string of C<"0">s and C<"1">s. 3683 3684 =item * 3685 3686 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups, 3687 representable as hexadecimal digits, 0-9a-f) long. 3688 3689 Each character of the input field of pack() generates 4 bits of the result. 3690 For non-alphabetical characters the result is based on the 4 least-significant 3691 bits of the input character, i.e., on C<ord($char)%16>. In particular, 3692 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes 3693 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result 3694 is compatible with the usual hexadecimal digits, so that C<"a"> and 3695 C<"A"> both generate the nybble C<0xa==10>. The result for characters 3696 C<"g".."z"> and C<"G".."Z"> is not well-defined. 3697 3698 Starting from the beginning of the input string of pack(), each pair 3699 of characters is converted to 1 character of output. With format C<h> the 3700 first character of the pair determines the least-significant nybble of the 3701 output character, and with format C<H> it determines the most-significant 3702 nybble. 3703 3704 If the length of the input string is not even, it behaves as if padded 3705 by a null character at the end. Similarly, during unpack()ing the "extra" 3706 nybbles are ignored. 3707 3708 If the input string of pack() is longer than needed, extra characters are 3709 ignored. 3710 A C<*> for the repeat count of pack() means to use all the characters of 3711 the input field. On unpack()ing the nybbles are converted to a string 3712 of hexadecimal digits. 3713 3714 =item * 3715 3716 The C<p> type packs a pointer to a null-terminated string. You are 3717 responsible for ensuring the string is not a temporary value (which can 3718 potentially get deallocated before you get around to using the packed result). 3719 The C<P> type packs a pointer to a structure of the size indicated by the 3720 length. A NULL pointer is created if the corresponding value for C<p> or 3721 C<P> is C<undef>, similarly for unpack(). 3722 3723 If your system has a strange pointer size (i.e. a pointer is neither as 3724 big as an int nor as big as a long), it may not be possible to pack or 3725 unpack pointers in big- or little-endian byte order. Attempting to do 3726 so will result in a fatal error. 3727 3728 =item * 3729 3730 The C</> template character allows packing and unpacking of a sequence of 3731 items where the packed structure contains a packed item count followed by 3732 the packed items themselves. 3733 3734 For C<pack> you write I<length-item>C</>I<sequence-item> and the 3735 I<length-item> describes how the length value is packed. The ones likely 3736 to be of most use are integer-packing ones like C<n> (for Java strings), 3737 C<w> (for ASN.1 or SNMP) and C<N> (for Sun XDR). 3738 3739 For C<pack>, the I<sequence-item> may have a repeat count, in which case 3740 the minimum of that and the number of available items is used as argument 3741 for the I<length-item>. If it has no repeat count or uses a '*', the number 3742 of available items is used. 3743 3744 For C<unpack> an internal stack of integer arguments unpacked so far is 3745 used. You write C</>I<sequence-item> and the repeat count is obtained by 3746 popping off the last element from the stack. The I<sequence-item> must not 3747 have a repeat count. 3748 3749 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">), 3750 the I<length-item> is a string length, not a number of strings. If there is 3751 an explicit repeat count for pack, the packed string will be adjusted to that 3752 given length. 3753 3754 unpack 'W/a', "\04Gurusamy"; gives ('Guru') 3755 unpack 'a3/A A*', '007 Bond J '; gives (' Bond', 'J') 3756 unpack 'a3 x2 /A A*', '007: Bond, J.'; gives ('Bond, J', '.') 3757 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world" 3758 pack 'a/W2', ord('a') .. ord('z'); gives '2ab' 3759 3760 The I<length-item> is not returned explicitly from C<unpack>. 3761 3762 Adding a count to the I<length-item> letter is unlikely to do anything 3763 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a 3764 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters, 3765 which Perl does not regard as legal in numeric strings. 3766 3767 =item * 3768 3769 The integer types C<s>, C<S>, C<l>, and C<L> may be 3770 followed by a C<!> modifier to signify native shorts or 3771 longs--as you can see from above for example a bare C<l> does mean 3772 exactly 32 bits, the native C<long> (as seen by the local C compiler) 3773 may be larger. This is an issue mainly in 64-bit platforms. You can 3774 see whether using C<!> makes any difference by 3775 3776 print length(pack("s")), " ", length(pack("s!")), "\n"; 3777 print length(pack("l")), " ", length(pack("l!")), "\n"; 3778 3779 C<i!> and C<I!> also work but only because of completeness; 3780 they are identical to C<i> and C<I>. 3781 3782 The actual sizes (in bytes) of native shorts, ints, longs, and long 3783 longs on the platform where Perl was built are also available via 3784 L<Config>: 3785 3786 use Config; 3787 print $Config{shortsize}, "\n"; 3788 print $Config{intsize}, "\n"; 3789 print $Config{longsize}, "\n"; 3790 print $Config{longlongsize}, "\n"; 3791 3792 (The C<$Config{longlongsize}> will be undefined if your system does 3793 not support long longs.) 3794 3795 =item * 3796 3797 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> 3798 are inherently non-portable between processors and operating systems 3799 because they obey the native byteorder and endianness. For example a 3800 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively 3801 (arranged in and handled by the CPU registers) into bytes as 3802 3803 0x12 0x34 0x56 0x78 # big-endian 3804 0x78 0x56 0x34 0x12 # little-endian 3805 3806 Basically, the Intel and VAX CPUs are little-endian, while everybody 3807 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and 3808 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq 3809 used/uses them in little-endian mode; SGI/Cray uses them in big-endian 3810 mode. 3811 3812 The names `big-endian' and `little-endian' are comic references to 3813 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a 3814 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and 3815 the egg-eating habits of the Lilliputians. 3816 3817 Some systems may have even weirder byte orders such as 3818 3819 0x56 0x78 0x12 0x34 3820 0x34 0x12 0x78 0x56 3821 3822 You can see your system's preference with 3823 3824 print join(" ", map { sprintf "%#02x", $_ } 3825 unpack("W*",pack("L",0x12345678))), "\n"; 3826 3827 The byteorder on the platform where Perl was built is also available 3828 via L<Config>: 3829 3830 use Config; 3831 print $Config{byteorder}, "\n"; 3832 3833 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'> 3834 and C<'87654321'> are big-endian. 3835 3836 If you want portable packed integers you can either use the formats 3837 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>> 3838 modifiers. These modifiers are only available as of perl 5.9.2. 3839 See also L<perlport>. 3840 3841 =item * 3842 3843 All integer and floating point formats as well as C<p> and C<P> and 3844 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers 3845 to force big- or little- endian byte-order, respectively. 3846 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover 3847 signed integers, 64-bit integers and floating point values. However, 3848 there are some things to keep in mind. 3849 3850 Exchanging signed integers between different platforms only works 3851 if all platforms store them in the same format. Most platforms store 3852 signed integers in two's complement, so usually this is not an issue. 3853 3854 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point 3855 formats on big- or little-endian machines. Otherwise, attempting to 3856 do so will result in a fatal error. 3857 3858 Forcing big- or little-endian byte-order on floating point values for 3859 data exchange can only work if all platforms are using the same 3860 binary representation (e.g. IEEE floating point format). Even if all 3861 platforms are using IEEE, there may be subtle differences. Being able 3862 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful, 3863 but also very dangerous if you don't know exactly what you're doing. 3864 It is definitely not a general way to portably store floating point 3865 values. 3866 3867 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect 3868 all types inside the group that accept the byte-order modifiers, 3869 including all subgroups. It will silently be ignored for all other 3870 types. You are not allowed to override the byte-order within a group 3871 that already has a byte-order modifier suffix. 3872 3873 =item * 3874 3875 Real numbers (floats and doubles) are in the native machine format only; 3876 due to the multiplicity of floating formats around, and the lack of a 3877 standard "network" representation, no facility for interchange has been 3878 made. This means that packed floating point data written on one machine 3879 may not be readable on another - even if both use IEEE floating point 3880 arithmetic (as the endian-ness of the memory representation is not part 3881 of the IEEE spec). See also L<perlport>. 3882 3883 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>> 3884 modifiers to force big- or little-endian byte-order on floating point values. 3885 3886 Note that Perl uses doubles (or long doubles, if configured) internally for 3887 all numeric calculation, and converting from double into float and thence back 3888 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>) 3889 will not in general equal $foo). 3890 3891 =item * 3892 3893 Pack and unpack can operate in two modes, character mode (C<C0> mode) where 3894 the packed string is processed per character and UTF-8 mode (C<U0> mode) 3895 where the packed string is processed in its UTF-8-encoded Unicode form on 3896 a byte by byte basis. Character mode is the default unless the format string 3897 starts with an C<U>. You can switch mode at any moment with an explicit 3898 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch 3899 or until the end of the ()-group in which it was entered. 3900 3901 =item * 3902 3903 You must yourself do any alignment or padding by inserting for example 3904 enough C<'x'>es while packing. There is no way to pack() and unpack() 3905 could know where the characters are going to or coming from. Therefore 3906 C<pack> (and C<unpack>) handle their output and input as flat 3907 sequences of characters. 3908 3909 =item * 3910 3911 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may 3912 take a repeat count, both as postfix, and for unpack() also via the C</> 3913 template character. Within each repetition of a group, positioning with 3914 C<@> starts again at 0. Therefore, the result of 3915 3916 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' ) 3917 3918 is the string "\0a\0\0bc". 3919 3920 =item * 3921 3922 C<x> and C<X> accept C<!> modifier. In this case they act as 3923 alignment commands: they jump forward/back to the closest position 3924 aligned at a multiple of C<count> characters. For example, to pack() or 3925 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to 3926 use the template C<W x![d] d W[2]>; this assumes that doubles must be 3927 aligned on the double's size. 3928 3929 For alignment commands C<count> of 0 is equivalent to C<count> of 1; 3930 both result in no-ops. 3931 3932 =item * 3933 3934 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they 3935 will represent signed 16-/32-bit integers in big-/little-endian order. 3936 This is only portable if all platforms sharing the packed data use the 3937 same binary representation for signed integers (e.g. all platforms are 3938 using two's complement representation). 3939 3940 =item * 3941 3942 A comment in a TEMPLATE starts with C<#> and goes to the end of line. 3943 White space may be used to separate pack codes from each other, but 3944 modifiers and a repeat count must follow immediately. 3945 3946 =item * 3947 3948 If TEMPLATE requires more arguments to pack() than actually given, pack() 3949 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments 3950 to pack() than actually given, extra arguments are ignored. 3951 3952 =back 3953 3954 Examples: 3955 3956 $foo = pack("WWWW",65,66,67,68); 3957 # foo eq "ABCD" 3958 $foo = pack("W4",65,66,67,68); 3959 # same thing 3960 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9); 3961 # same thing with Unicode circled letters. 3962 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); 3963 # same thing with Unicode circled letters. You don't get the UTF-8 3964 # bytes because the U at the start of the format caused a switch to 3965 # U0-mode, so the UTF-8 bytes get joined into characters 3966 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9); 3967 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9" 3968 # This is the UTF-8 encoding of the string in the previous example 3969 3970 $foo = pack("ccxxcc",65,66,67,68); 3971 # foo eq "AB\0\0CD" 3972 3973 # note: the above examples featuring "W" and "c" are true 3974 # only on ASCII and ASCII-derived systems such as ISO Latin 1 3975 # and UTF-8. In EBCDIC the first example would be 3976 # $foo = pack("WWWW",193,194,195,196); 3977 3978 $foo = pack("s2",1,2); 3979 # "\1\0\2\0" on little-endian 3980 # "\0\1\0\2" on big-endian 3981 3982 $foo = pack("a4","abcd","x","y","z"); 3983 # "abcd" 3984 3985 $foo = pack("aaaa","abcd","x","y","z"); 3986 # "axyz" 3987 3988 $foo = pack("a14","abcdefg"); 3989 # "abcdefg\0\0\0\0\0\0\0" 3990 3991 $foo = pack("i9pl", gmtime); 3992 # a real struct tm (on my system anyway) 3993 3994 $utmp_template = "Z8 Z8 Z16 L"; 3995 $utmp = pack($utmp_template, @utmp1); 3996 # a struct utmp (BSDish) 3997 3998 @utmp2 = unpack($utmp_template, $utmp); 3999 # "@utmp1" eq "@utmp2" 4000 4001 sub bintodec { 4002 unpack("N", pack("B32", substr("0" x 32 . shift, -32))); 4003 } 4004 4005 $foo = pack('sx2l', 12, 34); 4006 # short 12, two zero bytes padding, long 34 4007 $bar = pack('s@4l', 12, 34); 4008 # short 12, zero fill to position 4, long 34 4009 # $foo eq $bar 4010 $baz = pack('s.l', 12, 4, 34); 4011 # short 12, zero fill to position 4, long 34 4012 4013 $foo = pack('nN', 42, 4711); 4014 # pack big-endian 16- and 32-bit unsigned integers 4015 $foo = pack('S>L>', 42, 4711); 4016 # exactly the same 4017 $foo = pack('s<l<', -42, 4711); 4018 # pack little-endian 16- and 32-bit signed integers 4019 $foo = pack('(sl)<', -42, 4711); 4020 # exactly the same 4021 4022 The same template may generally also be used in unpack(). 4023 4024 =item package NAMESPACE 4025 X<package> X<module> X<namespace> 4026 4027 =item package 4028 4029 Declares the compilation unit as being in the given namespace. The scope 4030 of the package declaration is from the declaration itself through the end 4031 of the enclosing block, file, or eval (the same as the C<my> operator). 4032 All further unqualified dynamic identifiers will be in this namespace. 4033 A package statement affects only dynamic variables--including those 4034 you've used C<local> on--but I<not> lexical variables, which are created 4035 with C<my>. Typically it would be the first declaration in a file to 4036 be included by the C<require> or C<use> operator. You can switch into a 4037 package in more than one place; it merely influences which symbol table 4038 is used by the compiler for the rest of that block. You can refer to 4039 variables and filehandles in other packages by prefixing the identifier 4040 with the package name and a double colon: C<$Package::Variable>. 4041 If the package name is null, the C<main> package as assumed. That is, 4042 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>, 4043 still seen in older code). 4044 4045 If NAMESPACE is omitted, then there is no current package, and all 4046 identifiers must be fully qualified or lexicals. However, you are 4047 strongly advised not to make use of this feature. Its use can cause 4048 unexpected behaviour, even crashing some versions of Perl. It is 4049 deprecated, and will be removed from a future release. 4050 4051 See L<perlmod/"Packages"> for more information about packages, modules, 4052 and classes. See L<perlsub> for other scoping issues. 4053 4054 =item pipe READHANDLE,WRITEHANDLE 4055 X<pipe> 4056 4057 Opens a pair of connected pipes like the corresponding system call. 4058 Note that if you set up a loop of piped processes, deadlock can occur 4059 unless you are very careful. In addition, note that Perl's pipes use 4060 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE 4061 after each command, depending on the application. 4062 4063 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication"> 4064 for examples of such things. 4065 4066 On systems that support a close-on-exec flag on files, the flag will be set 4067 for the newly opened file descriptors as determined by the value of $^F. 4068 See L<perlvar/$^F>. 4069 4070 =item pop ARRAY 4071 X<pop> X<stack> 4072 4073 =item pop 4074 4075 Pops and returns the last value of the array, shortening the array by 4076 one element. 4077 4078 If there are no elements in the array, returns the undefined value 4079 (although this may happen at other times as well). If ARRAY is 4080 omitted, pops the C<@ARGV> array in the main program, and the C<@_> 4081 array in subroutines, just like C<shift>. 4082 4083 =item pos SCALAR 4084 X<pos> X<match, position> 4085 4086 =item pos 4087 4088 Returns the offset of where the last C<m//g> search left off for the variable 4089 in question (C<$_> is used when the variable is not specified). Note that 4090 0 is a valid match offset. C<undef> indicates that the search position 4091 is reset (usually due to match failure, but can also be because no match has 4092 yet been performed on the scalar). C<pos> directly accesses the location used 4093 by the regexp engine to store the offset, so assigning to C<pos> will change 4094 that offset, and so will also influence the C<\G> zero-width assertion in 4095 regular expressions. Because a failed C<m//gc> match doesn't reset the offset, 4096 the return from C<pos> won't change either in this case. See L<perlre> and 4097 L<perlop>. 4098 4099 =item print FILEHANDLE LIST 4100 X<print> 4101 4102 =item print LIST 4103 4104 =item print 4105 4106 Prints a string or a list of strings. Returns true if successful. 4107 FILEHANDLE may be a scalar variable name, in which case the variable 4108 contains the name of or a reference to the filehandle, thus introducing 4109 one level of indirection. (NOTE: If FILEHANDLE is a variable and 4110 the next token is a term, it may be misinterpreted as an operator 4111 unless you interpose a C<+> or put parentheses around the arguments.) 4112 If FILEHANDLE is omitted, prints by default to standard output (or 4113 to the last selected output channel--see L</select>). If LIST is 4114 also omitted, prints C<$_> to the currently selected output channel. 4115 To set the default output channel to something other than STDOUT 4116 use the select operation. The current value of C<$,> (if any) is 4117 printed between each LIST item. The current value of C<$\> (if 4118 any) is printed after the entire LIST has been printed. Because 4119 print takes a LIST, anything in the LIST is evaluated in list 4120 context, and any subroutine that you call will have one or more of 4121 its expressions evaluated in list context. Also be careful not to 4122 follow the print keyword with a left parenthesis unless you want 4123 the corresponding right parenthesis to terminate the arguments to 4124 the print--interpose a C<+> or put parentheses around all the 4125 arguments. 4126 4127 Note that if you're storing FILEHANDLEs in an array, or if you're using 4128 any other expression more complex than a scalar variable to retrieve it, 4129 you will have to use a block returning the filehandle value instead: 4130 4131 print { $files[$i] } "stuff\n"; 4132 print { $OK ? STDOUT : STDERR } "stuff\n"; 4133 4134 =item printf FILEHANDLE FORMAT, LIST 4135 X<printf> 4136 4137 =item printf FORMAT, LIST 4138 4139 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\> 4140 (the output record separator) is not appended. The first argument 4141 of the list will be interpreted as the C<printf> format. See C<sprintf> 4142 for an explanation of the format argument. If C<use locale> is in effect, 4143 and POSIX::setlocale() has been called, the character used for the decimal 4144 separator in formatted floating point numbers is affected by the LC_NUMERIC 4145 locale. See L<perllocale> and L<POSIX>. 4146 4147 Don't fall into the trap of using a C<printf> when a simple 4148 C<print> would do. The C<print> is more efficient and less 4149 error prone. 4150 4151 =item prototype FUNCTION 4152 X<prototype> 4153 4154 Returns the prototype of a function as a string (or C<undef> if the 4155 function has no prototype). FUNCTION is a reference to, or the name of, 4156 the function whose prototype you want to retrieve. 4157 4158 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a 4159 name for Perl builtin. If the builtin is not I<overridable> (such as 4160 C<qw//>) or if its arguments cannot be adequately expressed by a prototype 4161 (such as C<system>), prototype() returns C<undef>, because the builtin 4162 does not really behave like a Perl function. Otherwise, the string 4163 describing the equivalent prototype is returned. 4164 4165 =item push ARRAY,LIST 4166 X<push> X<stack> 4167 4168 Treats ARRAY as a stack, and pushes the values of LIST 4169 onto the end of ARRAY. The length of ARRAY increases by the length of 4170 LIST. Has the same effect as 4171 4172 for $value (LIST) { 4173 $ARRAY[++$#ARRAY] = $value; 4174 } 4175 4176 but is more efficient. Returns the number of elements in the array following 4177 the completed C<push>. 4178 4179 =item q/STRING/ 4180 4181 =item qq/STRING/ 4182 4183 =item qr/STRING/ 4184 4185 =item qx/STRING/ 4186 4187 =item qw/STRING/ 4188 4189 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">. 4190 4191 =item quotemeta EXPR 4192 X<quotemeta> X<metacharacter> 4193 4194 =item quotemeta 4195 4196 Returns the value of EXPR with all non-"word" 4197 characters backslashed. (That is, all characters not matching 4198 C</[A-Za-z_0-9]/> will be preceded by a backslash in the 4199 returned string, regardless of any locale settings.) 4200 This is the internal function implementing 4201 the C<\Q> escape in double-quoted strings. 4202 4203 If EXPR is omitted, uses C<$_>. 4204 4205 =item rand EXPR 4206 X<rand> X<random> 4207 4208 =item rand 4209 4210 Returns a random fractional number greater than or equal to C<0> and less 4211 than the value of EXPR. (EXPR should be positive.) If EXPR is 4212 omitted, the value C<1> is used. Currently EXPR with the value C<0> is 4213 also special-cased as C<1> - this has not been documented before perl 5.8.0 4214 and is subject to change in future versions of perl. Automatically calls 4215 C<srand> unless C<srand> has already been called. See also C<srand>. 4216 4217 Apply C<int()> to the value returned by C<rand()> if you want random 4218 integers instead of random fractional numbers. For example, 4219 4220 int(rand(10)) 4221 4222 returns a random integer between C<0> and C<9>, inclusive. 4223 4224 (Note: If your rand function consistently returns numbers that are too 4225 large or too small, then your version of Perl was probably compiled 4226 with the wrong number of RANDBITS.) 4227 4228 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET 4229 X<read> X<file, read> 4230 4231 =item read FILEHANDLE,SCALAR,LENGTH 4232 4233 Attempts to read LENGTH I<characters> of data into variable SCALAR 4234 from the specified FILEHANDLE. Returns the number of characters 4235 actually read, C<0> at end of file, or undef if there was an error (in 4236 the latter case C<$!> is also set). SCALAR will be grown or shrunk 4237 so that the last character actually read is the last character of the 4238 scalar after the read. 4239 4240 An OFFSET may be specified to place the read data at some place in the 4241 string other than the beginning. A negative OFFSET specifies 4242 placement at that many characters counting backwards from the end of 4243 the string. A positive OFFSET greater than the length of SCALAR 4244 results in the string being padded to the required size with C<"\0"> 4245 bytes before the result of the read is appended. 4246 4247 The call is actually implemented in terms of either Perl's or system's 4248 fread() call. To get a true read(2) system call, see C<sysread>. 4249 4250 Note the I<characters>: depending on the status of the filehandle, 4251 either (8-bit) bytes or characters are read. By default all 4252 filehandles operate on bytes, but for example if the filehandle has 4253 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open> 4254 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode 4255 characters, not bytes. Similarly for the C<:encoding> pragma: 4256 in that case pretty much any characters can be read. 4257 4258 =item readdir DIRHANDLE 4259 X<readdir> 4260 4261 Returns the next directory entry for a directory opened by C<opendir>. 4262 If used in list context, returns all the rest of the entries in the 4263 directory. If there are no more entries, returns an undefined value in 4264 scalar context or a null list in list context. 4265 4266 If you're planning to filetest the return values out of a C<readdir>, you'd 4267 better prepend the directory in question. Otherwise, because we didn't 4268 C<chdir> there, it would have been testing the wrong file. 4269 4270 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!"; 4271 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR); 4272 closedir DIR; 4273 4274 =item readline EXPR 4275 4276 =item readline 4277 X<readline> X<gets> X<fgets> 4278 4279 Reads from the filehandle whose typeglob is contained in EXPR (or from 4280 *ARGV if EXPR is not provided). In scalar context, each call reads and 4281 returns the next line, until end-of-file is reached, whereupon the 4282 subsequent call returns undef. In list context, reads until end-of-file 4283 is reached and returns a list of lines. Note that the notion of "line" 4284 used here is however you may have defined it with C<$/> or 4285 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">. 4286 4287 When C<$/> is set to C<undef>, when readline() is in scalar 4288 context (i.e. file slurp mode), and when an empty file is read, it 4289 returns C<''> the first time, followed by C<undef> subsequently. 4290 4291 This is the internal function implementing the C<< <EXPR> >> 4292 operator, but you can use it directly. The C<< <EXPR> >> 4293 operator is discussed in more detail in L<perlop/"I/O Operators">. 4294 4295 $line = <STDIN>; 4296 $line = readline(*STDIN); # same thing 4297 4298 If readline encounters an operating system error, C<$!> will be set with the 4299 corresponding error message. It can be helpful to check C<$!> when you are 4300 reading from filehandles you don't trust, such as a tty or a socket. The 4301 following example uses the operator form of C<readline>, and takes the necessary 4302 steps to ensure that C<readline> was successful. 4303 4304 for (;;) { 4305 undef $!; 4306 unless (defined( $line = <> )) { 4307 die $! if $!; 4308 last; # reached EOF 4309 } 4310 # ... 4311 } 4312 4313 =item readlink EXPR 4314 X<readlink> 4315 4316 =item readlink 4317 4318 Returns the value of a symbolic link, if symbolic links are 4319 implemented. If not, gives a fatal error. If there is some system 4320 error, returns the undefined value and sets C<$!> (errno). If EXPR is 4321 omitted, uses C<$_>. 4322 4323 =item readpipe EXPR 4324 4325 =item readpipe 4326 X<readpipe> 4327 4328 EXPR is executed as a system command. 4329 The collected standard output of the command is returned. 4330 In scalar context, it comes back as a single (potentially 4331 multi-line) string. In list context, returns a list of lines 4332 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>). 4333 This is the internal function implementing the C<qx/EXPR/> 4334 operator, but you can use it directly. The C<qx/EXPR/> 4335 operator is discussed in more detail in L<perlop/"I/O Operators">. 4336 If EXPR is omitted, uses C<$_>. 4337 4338 =item recv SOCKET,SCALAR,LENGTH,FLAGS 4339 X<recv> 4340 4341 Receives a message on a socket. Attempts to receive LENGTH characters 4342 of data into variable SCALAR from the specified SOCKET filehandle. 4343 SCALAR will be grown or shrunk to the length actually read. Takes the 4344 same flags as the system call of the same name. Returns the address 4345 of the sender if SOCKET's protocol supports this; returns an empty 4346 string otherwise. If there's an error, returns the undefined value. 4347 This call is actually implemented in terms of recvfrom(2) system call. 4348 See L<perlipc/"UDP: Message Passing"> for examples. 4349 4350 Note the I<characters>: depending on the status of the socket, either 4351 (8-bit) bytes or characters are received. By default all sockets 4352 operate on bytes, but for example if the socket has been changed using 4353 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the 4354 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode 4355 characters, not bytes. Similarly for the C<:encoding> pragma: in that 4356 case pretty much any characters can be read. 4357 4358 =item redo LABEL 4359 X<redo> 4360 4361 =item redo 4362 4363 The C<redo> command restarts the loop block without evaluating the 4364 conditional again. The C<continue> block, if any, is not executed. If 4365 the LABEL is omitted, the command refers to the innermost enclosing 4366 loop. Programs that want to lie to themselves about what was just input 4367 normally use this command: 4368 4369 # a simpleminded Pascal comment stripper 4370 # (warning: assumes no { or } in strings) 4371 LINE: while (<STDIN>) { 4372 while (s|({.*}.*){.*}|$1 |) {} 4373 s|{.*}| |; 4374 if (s|{.*| |) { 4375 $front = $_; 4376 while (<STDIN>) { 4377 if (/}/) { # end of comment? 4378 s|^|$front\{|; 4379 redo LINE; 4380 } 4381 } 4382 } 4383 print; 4384 } 4385 4386 C<redo> cannot be used to retry a block which returns a value such as 4387 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit 4388 a grep() or map() operation. 4389 4390 Note that a block by itself is semantically identical to a loop 4391 that executes once. Thus C<redo> inside such a block will effectively 4392 turn it into a looping construct. 4393 4394 See also L</continue> for an illustration of how C<last>, C<next>, and 4395 C<redo> work. 4396 4397 =item ref EXPR 4398 X<ref> X<reference> 4399 4400 =item ref 4401 4402 Returns a non-empty string if EXPR is a reference, the empty 4403 string otherwise. If EXPR 4404 is not specified, C<$_> will be used. The value returned depends on the 4405 type of thing the reference is a reference to. 4406 Builtin types include: 4407 4408 SCALAR 4409 ARRAY 4410 HASH 4411 CODE 4412 REF 4413 GLOB 4414 LVALUE 4415 FORMAT 4416 IO 4417 VSTRING 4418 Regexp 4419 4420 If the referenced object has been blessed into a package, then that package 4421 name is returned instead. You can think of C<ref> as a C<typeof> operator. 4422 4423 if (ref($r) eq "HASH") { 4424 print "r is a reference to a hash.\n"; 4425 } 4426 unless (ref($r)) { 4427 print "r is not a reference at all.\n"; 4428 } 4429 4430 The return value C<LVALUE> indicates a reference to an lvalue that is not 4431 a variable. You get this from taking the reference of function calls like 4432 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points 4433 to a L<version string|perldata/"Version Strings">. 4434 4435 The result C<Regexp> indicates that the argument is a regular expression 4436 resulting from C<qr//>. 4437 4438 See also L<perlref>. 4439 4440 =item rename OLDNAME,NEWNAME 4441 X<rename> X<move> X<mv> X<ren> 4442 4443 Changes the name of a file; an existing file NEWNAME will be 4444 clobbered. Returns true for success, false otherwise. 4445 4446 Behavior of this function varies wildly depending on your system 4447 implementation. For example, it will usually not work across file system 4448 boundaries, even though the system I<mv> command sometimes compensates 4449 for this. Other restrictions include whether it works on directories, 4450 open files, or pre-existing files. Check L<perlport> and either the 4451 rename(2) manpage or equivalent system documentation for details. 4452 4453 For a platform independent C<move> function look at the L<File::Copy> 4454 module. 4455 4456 =item require VERSION 4457 X<require> 4458 4459 =item require EXPR 4460 4461 =item require 4462 4463 Demands a version of Perl specified by VERSION, or demands some semantics 4464 specified by EXPR or by C<$_> if EXPR is not supplied. 4465 4466 VERSION may be either a numeric argument such as 5.006, which will be 4467 compared to C<$]>, or a literal of the form v5.6.1, which will be compared 4468 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if 4469 VERSION is greater than the version of the current Perl interpreter. 4470 Compare with L</use>, which can do a similar check at compile time. 4471 4472 Specifying VERSION as a literal of the form v5.6.1 should generally be 4473 avoided, because it leads to misleading error messages under earlier 4474 versions of Perl that do not support this syntax. The equivalent numeric 4475 version should be used instead. 4476 4477 require v5.6.1; # run time version check 4478 require 5.6.1; # ditto 4479 require 5.006_001; # ditto; preferred for backwards compatibility 4480 4481 Otherwise, C<require> demands that a library file be included if it 4482 hasn't already been included. The file is included via the do-FILE 4483 mechanism, which is essentially just a variety of C<eval> with the 4484 caveat that lexical variables in the invoking script will be invisible 4485 to the included code. Has semantics similar to the following subroutine: 4486 4487 sub require { 4488 my ($filename) = @_; 4489 if (exists $INC{$filename}) { 4490 return 1 if $INC{$filename}; 4491 die "Compilation failed in require"; 4492 } 4493 my ($realfilename,$result); 4494 ITER: { 4495 foreach $prefix (@INC) { 4496 $realfilename = "$prefix/$filename"; 4497 if (-f $realfilename) { 4498 $INC{$filename} = $realfilename; 4499 $result = do $realfilename; 4500 last ITER; 4501 } 4502 } 4503 die "Can't find $filename in \@INC"; 4504 } 4505 if ($@) { 4506 $INC{$filename} = undef; 4507 die $@; 4508 } elsif (!$result) { 4509 delete $INC{$filename}; 4510 die "$filename did not return true value"; 4511 } else { 4512 return $result; 4513 } 4514 } 4515 4516 Note that the file will not be included twice under the same specified 4517 name. 4518 4519 The file must return true as the last statement to indicate 4520 successful execution of any initialization code, so it's customary to 4521 end such a file with C<1;> unless you're sure it'll return true 4522 otherwise. But it's better just to put the C<1;>, in case you add more 4523 statements. 4524 4525 If EXPR is a bareword, the require assumes a "F<.pm>" extension and 4526 replaces "F<::>" with "F</>" in the filename for you, 4527 to make it easy to load standard modules. This form of loading of 4528 modules does not risk altering your namespace. 4529 4530 In other words, if you try this: 4531 4532 require Foo::Bar; # a splendid bareword 4533 4534 The require function will actually look for the "F<Foo/Bar.pm>" file in the 4535 directories specified in the C<@INC> array. 4536 4537 But if you try this: 4538 4539 $class = 'Foo::Bar'; 4540 require $class; # $class is not a bareword 4541 #or 4542 require "Foo::Bar"; # not a bareword because of the "" 4543 4544 The require function will look for the "F<Foo::Bar>" file in the @INC array and 4545 will complain about not finding "F<Foo::Bar>" there. In this case you can do: 4546 4547 eval "require $class"; 4548 4549 Now that you understand how C<require> looks for files in the case of a 4550 bareword argument, there is a little extra functionality going on behind 4551 the scenes. Before C<require> looks for a "F<.pm>" extension, it will 4552 first look for a similar filename with a "F<.pmc>" extension. If this file 4553 is found, it will be loaded in place of any file ending in a "F<.pm>" 4554 extension. 4555 4556 You can also insert hooks into the import facility, by putting directly 4557 Perl code into the @INC array. There are three forms of hooks: subroutine 4558 references, array references and blessed objects. 4559 4560 Subroutine references are the simplest case. When the inclusion system 4561 walks through @INC and encounters a subroutine, this subroutine gets 4562 called with two parameters, the first being a reference to itself, and the 4563 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The 4564 subroutine should return nothing, or a list of up to three values in the 4565 following order: 4566 4567 =over 4568 4569 =item 1 4570 4571 A filehandle, from which the file will be read. 4572 4573 =item 2 4574 4575 A reference to a subroutine. If there is no filehandle (previous item), 4576 then this subroutine is expected to generate one line of source code per 4577 call, writing the line into C<$_> and returning 1, then returning 0 at 4578 "end of file". If there is a filehandle, then the subroutine will be 4579 called to act a simple source filter, with the line as read in C<$_>. 4580 Again, return 1 for each valid line, and 0 after all lines have been 4581 returned. 4582 4583 =item 3 4584 4585 Optional state for the subroutine. The state is passed in as C<$_[1]>. A 4586 reference to the subroutine itself is passed in as C<$_[0]>. 4587 4588 =back 4589 4590 If an empty list, C<undef>, or nothing that matches the first 3 values above 4591 is returned then C<require> will look at the remaining elements of @INC. 4592 Note that this file handle must be a real file handle (strictly a typeglob, 4593 or reference to a typeglob, blessed or unblessed) - tied file handles will be 4594 ignored and return value processing will stop there. 4595 4596 If the hook is an array reference, its first element must be a subroutine 4597 reference. This subroutine is called as above, but the first parameter is 4598 the array reference. This enables to pass indirectly some arguments to 4599 the subroutine. 4600 4601 In other words, you can write: 4602 4603 push @INC, \&my_sub; 4604 sub my_sub { 4605 my ($coderef, $filename) = @_; # $coderef is \&my_sub 4606 ... 4607 } 4608 4609 or: 4610 4611 push @INC, [ \&my_sub, $x, $y, ... ]; 4612 sub my_sub { 4613 my ($arrayref, $filename) = @_; 4614 # Retrieve $x, $y, ... 4615 my @parameters = @$arrayref[1..$#$arrayref]; 4616 ... 4617 } 4618 4619 If the hook is an object, it must provide an INC method that will be 4620 called as above, the first parameter being the object itself. (Note that 4621 you must fully qualify the sub's name, as unqualified C<INC> is always forced 4622 into package C<main>.) Here is a typical code layout: 4623 4624 # In Foo.pm 4625 package Foo; 4626 sub new { ... } 4627 sub Foo::INC { 4628 my ($self, $filename) = @_; 4629 ... 4630 } 4631 4632 # In the main program 4633 push @INC, new Foo(...); 4634 4635 Note that these hooks are also permitted to set the %INC entry 4636 corresponding to the files they have loaded. See L<perlvar/%INC>. 4637 4638 For a yet-more-powerful import facility, see L</use> and L<perlmod>. 4639 4640 =item reset EXPR 4641 X<reset> 4642 4643 =item reset 4644 4645 Generally used in a C<continue> block at the end of a loop to clear 4646 variables and reset C<??> searches so that they work again. The 4647 expression is interpreted as a list of single characters (hyphens 4648 allowed for ranges). All variables and arrays beginning with one of 4649 those letters are reset to their pristine state. If the expression is 4650 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets 4651 only variables or searches in the current package. Always returns 4652 1. Examples: 4653 4654 reset 'X'; # reset all X variables 4655 reset 'a-z'; # reset lower case variables 4656 reset; # just reset ?one-time? searches 4657 4658 Resetting C<"A-Z"> is not recommended because you'll wipe out your 4659 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package 4660 variables--lexical variables are unaffected, but they clean themselves 4661 up on scope exit anyway, so you'll probably want to use them instead. 4662 See L</my>. 4663 4664 =item return EXPR 4665 X<return> 4666 4667 =item return 4668 4669 Returns from a subroutine, C<eval>, or C<do FILE> with the value 4670 given in EXPR. Evaluation of EXPR may be in list, scalar, or void 4671 context, depending on how the return value will be used, and the context 4672 may vary from one execution to the next (see C<wantarray>). If no EXPR 4673 is given, returns an empty list in list context, the undefined value in 4674 scalar context, and (of course) nothing at all in a void context. 4675 4676 (Note that in the absence of an explicit C<return>, a subroutine, eval, 4677 or do FILE will automatically return the value of the last expression 4678 evaluated.) 4679 4680 =item reverse LIST 4681 X<reverse> X<rev> X<invert> 4682 4683 In list context, returns a list value consisting of the elements 4684 of LIST in the opposite order. In scalar context, concatenates the 4685 elements of LIST and returns a string value with all characters 4686 in the opposite order. 4687 4688 print reverse <>; # line tac, last line first 4689 4690 undef $/; # for efficiency of <> 4691 print scalar reverse <>; # character tac, last line tsrif 4692 4693 Used without arguments in scalar context, reverse() reverses C<$_>. 4694 4695 This operator is also handy for inverting a hash, although there are some 4696 caveats. If a value is duplicated in the original hash, only one of those 4697 can be represented as a key in the inverted hash. Also, this has to 4698 unwind one hash and build a whole new one, which may take some time 4699 on a large hash, such as from a DBM file. 4700 4701 %by_name = reverse %by_address; # Invert the hash 4702 4703 =item rewinddir DIRHANDLE 4704 X<rewinddir> 4705 4706 Sets the current position to the beginning of the directory for the 4707 C<readdir> routine on DIRHANDLE. 4708 4709 =item rindex STR,SUBSTR,POSITION 4710 X<rindex> 4711 4712 =item rindex STR,SUBSTR 4713 4714 Works just like index() except that it returns the position of the I<last> 4715 occurrence of SUBSTR in STR. If POSITION is specified, returns the 4716 last occurrence beginning at or before that position. 4717 4718 =item rmdir FILENAME 4719 X<rmdir> X<rd> X<directory, remove> 4720 4721 =item rmdir 4722 4723 Deletes the directory specified by FILENAME if that directory is 4724 empty. If it succeeds it returns true, otherwise it returns false and 4725 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>. 4726 4727 To remove a directory tree recursively (C<rm -rf> on unix) look at 4728 the C<rmtree> function of the L<File::Path> module. 4729 4730 =item s/// 4731 4732 The substitution operator. See L<perlop>. 4733 4734 =item say FILEHANDLE LIST 4735 X<say> 4736 4737 =item say LIST 4738 4739 =item say 4740 4741 Just like C<print>, but implicitly appends a newline. 4742 C<say LIST> is simply an abbreviation for C<{ local $\ = "\n"; print 4743 LIST }>. 4744 4745 This keyword is only available when the "say" feature is 4746 enabled: see L<feature>. 4747 4748 =item scalar EXPR 4749 X<scalar> X<context> 4750 4751 Forces EXPR to be interpreted in scalar context and returns the value 4752 of EXPR. 4753 4754 @counts = ( scalar @a, scalar @b, scalar @c ); 4755 4756 There is no equivalent operator to force an expression to 4757 be interpolated in list context because in practice, this is never 4758 needed. If you really wanted to do so, however, you could use 4759 the construction C<@{[ (some expression) ]}>, but usually a simple 4760 C<(some expression)> suffices. 4761 4762 Because C<scalar> is unary operator, if you accidentally use for EXPR a 4763 parenthesized list, this behaves as a scalar comma expression, evaluating 4764 all but the last element in void context and returning the final element 4765 evaluated in scalar context. This is seldom what you want. 4766 4767 The following single statement: 4768 4769 print uc(scalar(&foo,$bar)),$baz; 4770 4771 is the moral equivalent of these two: 4772 4773 &foo; 4774 print(uc($bar),$baz); 4775 4776 See L<perlop> for more details on unary operators and the comma operator. 4777 4778 =item seek FILEHANDLE,POSITION,WHENCE 4779 X<seek> X<fseek> X<filehandle, position> 4780 4781 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>. 4782 FILEHANDLE may be an expression whose value gives the name of the 4783 filehandle. The values for WHENCE are C<0> to set the new position 4784 I<in bytes> to POSITION, C<1> to set it to the current position plus 4785 POSITION, and C<2> to set it to EOF plus POSITION (typically 4786 negative). For WHENCE you may use the constants C<SEEK_SET>, 4787 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end 4788 of the file) from the Fcntl module. Returns C<1> upon success, C<0> 4789 otherwise. 4790 4791 Note the I<in bytes>: even if the filehandle has been set to 4792 operate on characters (for example by using the C<:encoding(utf8)> open 4793 layer), tell() will return byte offsets, not character offsets 4794 (because implementing that would render seek() and tell() rather slow). 4795 4796 If you want to position file for C<sysread> or C<syswrite>, don't use 4797 C<seek>--buffering makes its effect on the file's system position 4798 unpredictable and non-portable. Use C<sysseek> instead. 4799 4800 Due to the rules and rigors of ANSI C, on some systems you have to do a 4801 seek whenever you switch between reading and writing. Amongst other 4802 things, this may have the effect of calling stdio's clearerr(3). 4803 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position: 4804 4805 seek(TEST,0,1); 4806 4807 This is also useful for applications emulating C<tail -f>. Once you hit 4808 EOF on your read, and then sleep for a while, you might have to stick in a 4809 seek() to reset things. The C<seek> doesn't change the current position, 4810 but it I<does> clear the end-of-file condition on the handle, so that the 4811 next C<< <FILE> >> makes Perl try again to read something. We hope. 4812 4813 If that doesn't work (some IO implementations are particularly 4814 cantankerous), then you may need something more like this: 4815 4816 for (;;) { 4817 for ($curpos = tell(FILE); $_ = <FILE>; 4818 $curpos = tell(FILE)) { 4819 # search for some stuff and put it into files 4820 } 4821 sleep($for_a_while); 4822 seek(FILE, $curpos, 0); 4823 } 4824 4825 =item seekdir DIRHANDLE,POS 4826 X<seekdir> 4827 4828 Sets the current position for the C<readdir> routine on DIRHANDLE. POS 4829 must be a value returned by C<telldir>. C<seekdir> also has the same caveats 4830 about possible directory compaction as the corresponding system library 4831 routine. 4832 4833 =item select FILEHANDLE 4834 X<select> X<filehandle, default> 4835 4836 =item select 4837 4838 Returns the currently selected filehandle. If FILEHANDLE is supplied, 4839 sets the new current default filehandle for output. This has two 4840 effects: first, a C<write> or a C<print> without a filehandle will 4841 default to this FILEHANDLE. Second, references to variables related to 4842 output will refer to this output channel. For example, if you have to 4843 set the top of form format for more than one output channel, you might 4844 do the following: 4845 4846 select(REPORT1); 4847 $^ = 'report1_top'; 4848 select(REPORT2); 4849 $^ = 'report2_top'; 4850 4851 FILEHANDLE may be an expression whose value gives the name of the 4852 actual filehandle. Thus: 4853 4854 $oldfh = select(STDERR); $| = 1; select($oldfh); 4855 4856 Some programmers may prefer to think of filehandles as objects with 4857 methods, preferring to write the last example as: 4858 4859 use IO::Handle; 4860 STDERR->autoflush(1); 4861 4862 =item select RBITS,WBITS,EBITS,TIMEOUT 4863 X<select> 4864 4865 This calls the select(2) system call with the bit masks specified, which 4866 can be constructed using C<fileno> and C<vec>, along these lines: 4867 4868 $rin = $win = $ein = ''; 4869 vec($rin,fileno(STDIN),1) = 1; 4870 vec($win,fileno(STDOUT),1) = 1; 4871 $ein = $rin | $win; 4872 4873 If you want to select on many filehandles you might wish to write a 4874 subroutine: 4875 4876 sub fhbits { 4877 my(@fhlist) = split(' ',$_[0]); 4878 my($bits); 4879 for (@fhlist) { 4880 vec($bits,fileno($_),1) = 1; 4881 } 4882 $bits; 4883 } 4884 $rin = fhbits('STDIN TTY SOCK'); 4885 4886 The usual idiom is: 4887 4888 ($nfound,$timeleft) = 4889 select($rout=$rin, $wout=$win, $eout=$ein, $timeout); 4890 4891 or to block until something becomes ready just do this 4892 4893 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); 4894 4895 Most systems do not bother to return anything useful in $timeleft, so 4896 calling select() in scalar context just returns $nfound. 4897 4898 Any of the bit masks can also be undef. The timeout, if specified, is 4899 in seconds, which may be fractional. Note: not all implementations are 4900 capable of returning the $timeleft. If not, they always return 4901 $timeleft equal to the supplied $timeout. 4902 4903 You can effect a sleep of 250 milliseconds this way: 4904 4905 select(undef, undef, undef, 0.25); 4906 4907 Note that whether C<select> gets restarted after signals (say, SIGALRM) 4908 is implementation-dependent. See also L<perlport> for notes on the 4909 portability of C<select>. 4910 4911 On error, C<select> behaves like the select(2) system call : it returns 4912 -1 and sets C<$!>. 4913 4914 Note: on some Unixes, the select(2) system call may report a socket file 4915 descriptor as "ready for reading", when actually no data is available, 4916 thus a subsequent read blocks. It can be avoided using always the 4917 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further 4918 details. 4919 4920 B<WARNING>: One should not attempt to mix buffered I/O (like C<read> 4921 or <FH>) with C<select>, except as permitted by POSIX, and even 4922 then only on POSIX systems. You have to use C<sysread> instead. 4923 4924 =item semctl ID,SEMNUM,CMD,ARG 4925 X<semctl> 4926 4927 Calls the System V IPC function C<semctl>. You'll probably have to say 4928 4929 use IPC::SysV; 4930 4931 first to get the correct constant definitions. If CMD is IPC_STAT or 4932 GETALL, then ARG must be a variable that will hold the returned 4933 semid_ds structure or semaphore value array. Returns like C<ioctl>: 4934 the undefined value for error, "C<0 but true>" for zero, or the actual 4935 return value otherwise. The ARG must consist of a vector of native 4936 short integers, which may be created with C<pack("s!",(0)x$nsem)>. 4937 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore> 4938 documentation. 4939 4940 =item semget KEY,NSEMS,FLAGS 4941 X<semget> 4942 4943 Calls the System V IPC function semget. Returns the semaphore id, or 4944 the undefined value if there is an error. See also 4945 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore> 4946 documentation. 4947 4948 =item semop KEY,OPSTRING 4949 X<semop> 4950 4951 Calls the System V IPC function semop to perform semaphore operations 4952 such as signalling and waiting. OPSTRING must be a packed array of 4953 semop structures. Each semop structure can be generated with 4954 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING 4955 implies the number of semaphore operations. Returns true if 4956 successful, or false if there is an error. As an example, the 4957 following code waits on semaphore $semnum of semaphore id $semid: 4958 4959 $semop = pack("s!3", $semnum, -1, 0); 4960 die "Semaphore trouble: $!\n" unless semop($semid, $semop); 4961 4962 To signal the semaphore, replace C<-1> with C<1>. See also 4963 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore> 4964 documentation. 4965 4966 =item send SOCKET,MSG,FLAGS,TO 4967 X<send> 4968 4969 =item send SOCKET,MSG,FLAGS 4970 4971 Sends a message on a socket. Attempts to send the scalar MSG to the 4972 SOCKET filehandle. Takes the same flags as the system call of the 4973 same name. On unconnected sockets you must specify a destination to 4974 send TO, in which case it does a C C<sendto>. Returns the number of 4975 characters sent, or the undefined value if there is an error. The C 4976 system call sendmsg(2) is currently unimplemented. See 4977 L<perlipc/"UDP: Message Passing"> for examples. 4978 4979 Note the I<characters>: depending on the status of the socket, either 4980 (8-bit) bytes or characters are sent. By default all sockets operate 4981 on bytes, but for example if the socket has been changed using 4982 binmode() to operate with the C<:encoding(utf8)> I/O layer (see 4983 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8 4984 encoded Unicode characters, not bytes. Similarly for the C<:encoding> 4985 pragma: in that case pretty much any characters can be sent. 4986 4987 =item setpgrp PID,PGRP 4988 X<setpgrp> X<group> 4989 4990 Sets the current process group for the specified PID, C<0> for the current 4991 process. Will produce a fatal error if used on a machine that doesn't 4992 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted, 4993 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not 4994 accept any arguments, so only C<setpgrp(0,0)> is portable. See also 4995 C<POSIX::setsid()>. 4996 4997 =item setpriority WHICH,WHO,PRIORITY 4998 X<setpriority> X<priority> X<nice> X<renice> 4999 5000 Sets the current priority for a process, a process group, or a user. 5001 (See setpriority(2).) Will produce a fatal error if used on a machine 5002 that doesn't implement setpriority(2). 5003 5004 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL 5005 X<setsockopt> 5006 5007 Sets the socket option requested. Returns undefined if there is an 5008 error. Use integer constants provided by the C<Socket> module for 5009 LEVEL and OPNAME. Values for LEVEL can also be obtained from 5010 getprotobyname. OPTVAL might either be a packed string or an integer. 5011 An integer OPTVAL is shorthand for pack("i", OPTVAL). 5012 5013 An example disabling the Nagle's algorithm for a socket: 5014 5015 use Socket qw(IPPROTO_TCP TCP_NODELAY); 5016 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1); 5017 5018 =item shift ARRAY 5019 X<shift> 5020 5021 =item shift 5022 5023 Shifts the first value of the array off and returns it, shortening the 5024 array by 1 and moving everything down. If there are no elements in the 5025 array, returns the undefined value. If ARRAY is omitted, shifts the 5026 C<@_> array within the lexical scope of subroutines and formats, and the 5027 C<@ARGV> array outside of a subroutine and also within the lexical scopes 5028 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, 5029 C<UNITCHECK {}> and C<END {}> constructs. 5030 5031 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the 5032 same thing to the left end of an array that C<pop> and C<push> do to the 5033 right end. 5034 5035 =item shmctl ID,CMD,ARG 5036 X<shmctl> 5037 5038 Calls the System V IPC function shmctl. You'll probably have to say 5039 5040 use IPC::SysV; 5041 5042 first to get the correct constant definitions. If CMD is C<IPC_STAT>, 5043 then ARG must be a variable that will hold the returned C<shmid_ds> 5044 structure. Returns like ioctl: the undefined value for error, "C<0> but 5045 true" for zero, or the actual return value otherwise. 5046 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. 5047 5048 =item shmget KEY,SIZE,FLAGS 5049 X<shmget> 5050 5051 Calls the System V IPC function shmget. Returns the shared memory 5052 segment id, or the undefined value if there is an error. 5053 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. 5054 5055 =item shmread ID,VAR,POS,SIZE 5056 X<shmread> 5057 X<shmwrite> 5058 5059 =item shmwrite ID,STRING,POS,SIZE 5060 5061 Reads or writes the System V shared memory segment ID starting at 5062 position POS for size SIZE by attaching to it, copying in/out, and 5063 detaching from it. When reading, VAR must be a variable that will 5064 hold the data read. When writing, if STRING is too long, only SIZE 5065 bytes are used; if STRING is too short, nulls are written to fill out 5066 SIZE bytes. Return true if successful, or false if there is an error. 5067 shmread() taints the variable. See also L<perlipc/"SysV IPC">, 5068 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN. 5069 5070 =item shutdown SOCKET,HOW 5071 X<shutdown> 5072 5073 Shuts down a socket connection in the manner indicated by HOW, which 5074 has the same interpretation as in the system call of the same name. 5075 5076 shutdown(SOCKET, 0); # I/we have stopped reading data 5077 shutdown(SOCKET, 1); # I/we have stopped writing data 5078 shutdown(SOCKET, 2); # I/we have stopped using this socket 5079 5080 This is useful with sockets when you want to tell the other 5081 side you're done writing but not done reading, or vice versa. 5082 It's also a more insistent form of close because it also 5083 disables the file descriptor in any forked copies in other 5084 processes. 5085 5086 =item sin EXPR 5087 X<sin> X<sine> X<asin> X<arcsine> 5088 5089 =item sin 5090 5091 Returns the sine of EXPR (expressed in radians). If EXPR is omitted, 5092 returns sine of C<$_>. 5093 5094 For the inverse sine operation, you may use the C<Math::Trig::asin> 5095 function, or use this relation: 5096 5097 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } 5098 5099 =item sleep EXPR 5100 X<sleep> X<pause> 5101 5102 =item sleep 5103 5104 Causes the script to sleep for EXPR seconds, or forever if no EXPR. 5105 May be interrupted if the process receives a signal such as C<SIGALRM>. 5106 Returns the number of seconds actually slept. You probably cannot 5107 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented 5108 using C<alarm>. 5109 5110 On some older systems, it may sleep up to a full second less than what 5111 you requested, depending on how it counts seconds. Most modern systems 5112 always sleep the full amount. They may appear to sleep longer than that, 5113 however, because your process might not be scheduled right away in a 5114 busy multitasking system. 5115 5116 For delays of finer granularity than one second, the Time::HiRes module 5117 (from CPAN, and starting from Perl 5.8 part of the standard 5118 distribution) provides usleep(). You may also use Perl's four-argument 5119 version of select() leaving the first three arguments undefined, or you 5120 might be able to use the C<syscall> interface to access setitimer(2) if 5121 your system supports it. See L<perlfaq8> for details. 5122 5123 See also the POSIX module's C<pause> function. 5124 5125 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL 5126 X<socket> 5127 5128 Opens a socket of the specified kind and attaches it to filehandle 5129 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for 5130 the system call of the same name. You should C<use Socket> first 5131 to get the proper definitions imported. See the examples in 5132 L<perlipc/"Sockets: Client/Server Communication">. 5133 5134 On systems that support a close-on-exec flag on files, the flag will 5135 be set for the newly opened file descriptor, as determined by the 5136 value of $^F. See L<perlvar/$^F>. 5137 5138 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL 5139 X<socketpair> 5140 5141 Creates an unnamed pair of sockets in the specified domain, of the 5142 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as 5143 for the system call of the same name. If unimplemented, yields a fatal 5144 error. Returns true if successful. 5145 5146 On systems that support a close-on-exec flag on files, the flag will 5147 be set for the newly opened file descriptors, as determined by the value 5148 of $^F. See L<perlvar/$^F>. 5149 5150 Some systems defined C<pipe> in terms of C<socketpair>, in which a call 5151 to C<pipe(Rdr, Wtr)> is essentially: 5152 5153 use Socket; 5154 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); 5155 shutdown(Rdr, 1); # no more writing for reader 5156 shutdown(Wtr, 0); # no more reading for writer 5157 5158 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will 5159 emulate socketpair using IP sockets to localhost if your system implements 5160 sockets but not socketpair. 5161 5162 =item sort SUBNAME LIST 5163 X<sort> X<qsort> X<quicksort> X<mergesort> 5164 5165 =item sort BLOCK LIST 5166 5167 =item sort LIST 5168 5169 In list context, this sorts the LIST and returns the sorted list value. 5170 In scalar context, the behaviour of C<sort()> is undefined. 5171 5172 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison 5173 order. If SUBNAME is specified, it gives the name of a subroutine 5174 that returns an integer less than, equal to, or greater than C<0>, 5175 depending on how the elements of the list are to be ordered. (The C<< 5176 <=> >> and C<cmp> operators are extremely useful in such routines.) 5177 SUBNAME may be a scalar variable name (unsubscripted), in which case 5178 the value provides the name of (or a reference to) the actual 5179 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as 5180 an anonymous, in-line sort subroutine. 5181 5182 If the subroutine's prototype is C<($$)>, the elements to be compared 5183 are passed by reference in C<@_>, as for a normal subroutine. This is 5184 slower than unprototyped subroutines, where the elements to be 5185 compared are passed into the subroutine 5186 as the package global variables $a and $b (see example below). Note that 5187 in the latter case, it is usually counter-productive to declare $a and 5188 $b as lexicals. 5189 5190 The values to be compared are always passed by reference and should not 5191 be modified. 5192 5193 You also cannot exit out of the sort block or subroutine using any of the 5194 loop control operators described in L<perlsyn> or with C<goto>. 5195 5196 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the 5197 current collation locale. See L<perllocale>. 5198 5199 sort() returns aliases into the original list, much as a for loop's index 5200 variable aliases the list elements. That is, modifying an element of a 5201 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>) 5202 actually modifies the element in the original list. This is usually 5203 something to be avoided when writing clear code. 5204 5205 Perl 5.6 and earlier used a quicksort algorithm to implement sort. 5206 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort 5207 preserves the input order of elements that compare equal. Although 5208 quicksort's run time is O(NlogN) when averaged over all arrays of 5209 length N, the time can be O(N**2), I<quadratic> behavior, for some 5210 inputs.) In 5.7, the quicksort implementation was replaced with 5211 a stable mergesort algorithm whose worst-case behavior is O(NlogN). 5212 But benchmarks indicated that for some inputs, on some platforms, 5213 the original quicksort was faster. 5.8 has a sort pragma for 5214 limited control of the sort. Its rather blunt control of the 5215 underlying algorithm may not persist into future Perls, but the 5216 ability to characterize the input or output in implementation 5217 independent ways quite probably will. See L<sort>. 5218 5219 Examples: 5220 5221 # sort lexically 5222 @articles = sort @files; 5223 5224 # same thing, but with explicit sort routine 5225 @articles = sort {$a cmp $b} @files; 5226 5227 # now case-insensitively 5228 @articles = sort {uc($a) cmp uc($b)} @files; 5229 5230 # same thing in reversed order 5231 @articles = sort {$b cmp $a} @files; 5232 5233 # sort numerically ascending 5234 @articles = sort {$a <=> $b} @files; 5235 5236 # sort numerically descending 5237 @articles = sort {$b <=> $a} @files; 5238 5239 # this sorts the %age hash by value instead of key 5240 # using an in-line function 5241 @eldest = sort { $age{$b} <=> $age{$a} } keys %age; 5242 5243 # sort using explicit subroutine name 5244 sub byage { 5245 $age{$a} <=> $age{$b}; # presuming numeric 5246 } 5247 @sortedclass = sort byage @class; 5248 5249 sub backwards { $b cmp $a } 5250 @harry = qw(dog cat x Cain Abel); 5251 @george = qw(gone chased yz Punished Axed); 5252 print sort @harry; 5253 # prints AbelCaincatdogx 5254 print sort backwards @harry; 5255 # prints xdogcatCainAbel 5256 print sort @george, 'to', @harry; 5257 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz 5258 5259 # inefficiently sort by descending numeric compare using 5260 # the first integer after the first = sign, or the 5261 # whole record case-insensitively otherwise 5262 5263 @new = sort { 5264 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] 5265 || 5266 uc($a) cmp uc($b) 5267 } @old; 5268 5269 # same thing, but much more efficiently; 5270 # we'll build auxiliary indices instead 5271 # for speed 5272 @nums = @caps = (); 5273 for (@old) { 5274 push @nums, /=(\d+)/; 5275 push @caps, uc($_); 5276 } 5277 5278 @new = @old[ sort { 5279 $nums[$b] <=> $nums[$a] 5280 || 5281 $caps[$a] cmp $caps[$b] 5282 } 0..$#old 5283 ]; 5284 5285 # same thing, but without any temps 5286 @new = map { $_->[0] } 5287 sort { $b->[1] <=> $a->[1] 5288 || 5289 $a->[2] cmp $b->[2] 5290 } map { [$_, /=(\d+)/, uc($_)] } @old; 5291 5292 # using a prototype allows you to use any comparison subroutine 5293 # as a sort subroutine (including other package's subroutines) 5294 package other; 5295 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here 5296 5297 package main; 5298 @new = sort other::backwards @old; 5299 5300 # guarantee stability, regardless of algorithm 5301 use sort 'stable'; 5302 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old; 5303 5304 # force use of mergesort (not portable outside Perl 5.8) 5305 use sort '_mergesort'; # note discouraging _ 5306 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old; 5307 5308 If you're using strict, you I<must not> declare $a 5309 and $b as lexicals. They are package globals. That means 5310 if you're in the C<main> package and type 5311 5312 @articles = sort {$b <=> $a} @files; 5313 5314 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>), 5315 but if you're in the C<FooPack> package, it's the same as typing 5316 5317 @articles = sort {$FooPack::b <=> $FooPack::a} @files; 5318 5319 The comparison function is required to behave. If it returns 5320 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and 5321 sometimes saying the opposite, for example) the results are not 5322 well-defined. 5323 5324 Because C<< <=> >> returns C<undef> when either operand is C<NaN> 5325 (not-a-number), and because C<sort> will trigger a fatal error unless the 5326 result of a comparison is defined, when sorting with a comparison function 5327 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>. 5328 The following example takes advantage of the fact that C<NaN != NaN> to 5329 eliminate any C<NaN>s from the input. 5330 5331 @result = sort { $a <=> $b } grep { $_ == $_ } @input; 5332 5333 =item splice ARRAY,OFFSET,LENGTH,LIST 5334 X<splice> 5335 5336 =item splice ARRAY,OFFSET,LENGTH 5337 5338 =item splice ARRAY,OFFSET 5339 5340 =item splice ARRAY 5341 5342 Removes the elements designated by OFFSET and LENGTH from an array, and 5343 replaces them with the elements of LIST, if any. In list context, 5344 returns the elements removed from the array. In scalar context, 5345 returns the last element removed, or C<undef> if no elements are 5346 removed. The array grows or shrinks as necessary. 5347 If OFFSET is negative then it starts that far from the end of the array. 5348 If LENGTH is omitted, removes everything from OFFSET onward. 5349 If LENGTH is negative, removes the elements from OFFSET onward 5350 except for -LENGTH elements at the end of the array. 5351 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is 5352 past the end of the array, perl issues a warning, and splices at the 5353 end of the array. 5354 5355 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> ) 5356 5357 push(@a,$x,$y) splice(@a,@a,0,$x,$y) 5358 pop(@a) splice(@a,-1) 5359 shift(@a) splice(@a,0,1) 5360 unshift(@a,$x,$y) splice(@a,0,0,$x,$y) 5361 $a[$i] = $y splice(@a,$i,1,$y) 5362 5363 Example, assuming array lengths are passed before arrays: 5364 5365 sub aeq { # compare two list values 5366 my(@a) = splice(@_,0,shift); 5367 my(@b) = splice(@_,0,shift); 5368 return 0 unless @a == @b; # same len? 5369 while (@a) { 5370 return 0 if pop(@a) ne pop(@b); 5371 } 5372 return 1; 5373 } 5374 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } 5375 5376 =item split /PATTERN/,EXPR,LIMIT 5377 X<split> 5378 5379 =item split /PATTERN/,EXPR 5380 5381 =item split /PATTERN/ 5382 5383 =item split 5384 5385 Splits the string EXPR into a list of strings and returns that list. By 5386 default, empty leading fields are preserved, and empty trailing ones are 5387 deleted. (If all fields are empty, they are considered to be trailing.) 5388 5389 In scalar context, returns the number of fields found and splits into 5390 the C<@_> array. Use of split in scalar context is deprecated, however, 5391 because it clobbers your subroutine arguments. 5392 5393 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted, 5394 splits on whitespace (after skipping any leading whitespace). Anything 5395 matching PATTERN is taken to be a delimiter separating the fields. (Note 5396 that the delimiter may be longer than one character.) 5397 5398 If LIMIT is specified and positive, it represents the maximum number 5399 of fields the EXPR will be split into, though the actual number of 5400 fields returned depends on the number of times PATTERN matches within 5401 EXPR. If LIMIT is unspecified or zero, trailing null fields are 5402 stripped (which potential users of C<pop> would do well to remember). 5403 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT 5404 had been specified. Note that splitting an EXPR that evaluates to the 5405 empty string always returns the empty list, regardless of the LIMIT 5406 specified. 5407 5408 A pattern matching the null string (not to be confused with 5409 a null pattern C<//>, which is just one member of the set of patterns 5410 matching a null string) will split the value of EXPR into separate 5411 characters at each point it matches that way. For example: 5412 5413 print join(':', split(/ */, 'hi there')); 5414 5415 produces the output 'h:i:t:h:e:r:e'. 5416 5417 As a special case for C<split>, using the empty pattern C<//> specifically 5418 matches only the null string, and is not be confused with the regular use 5419 of C<//> to mean "the last successful pattern match". So, for C<split>, 5420 the following: 5421 5422 print join(':', split(//, 'hi there')); 5423 5424 produces the output 'h:i: :t:h:e:r:e'. 5425 5426 Empty leading fields are produced when there are positive-width matches at 5427 the beginning of the string; a zero-width match at the beginning of 5428 the string does not produce an empty field. For example: 5429 5430 print join(':', split(/(?=\w)/, 'hi there!')); 5431 5432 produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other 5433 hand, are produced when there is a match at the end of the string (and 5434 when LIMIT is given and is not 0), regardless of the length of the match. 5435 For example: 5436 5437 print join(':', split(//, 'hi there!', -1)); 5438 print join(':', split(/\W/, 'hi there!', -1)); 5439 5440 produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively, 5441 both with an empty trailing field. 5442 5443 The LIMIT parameter can be used to split a line partially 5444 5445 ($login, $passwd, $remainder) = split(/:/, $_, 3); 5446 5447 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies 5448 a LIMIT one larger than the number of variables in the list, to avoid 5449 unnecessary work. For the list above LIMIT would have been 4 by 5450 default. In time critical applications it behooves you not to split 5451 into more fields than you really need. 5452 5453 If the PATTERN contains parentheses, additional list elements are 5454 created from each matching substring in the delimiter. 5455 5456 split(/([,-])/, "1-10,20", 3); 5457 5458 produces the list value 5459 5460 (1, '-', 10, ',', 20) 5461 5462 If you had the entire header of a normal Unix email message in $header, 5463 you could split it up into fields and their values this way: 5464 5465 $header =~ s/\n\s+/ /g; # fix continuation lines 5466 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header); 5467 5468 The pattern C</PATTERN/> may be replaced with an expression to specify 5469 patterns that vary at runtime. (To do runtime compilation only once, 5470 use C</$variable/o>.) 5471 5472 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on 5473 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can 5474 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>> 5475 will give you as many null initial fields as there are leading spaces. 5476 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading 5477 whitespace produces a null first field. A C<split> with no arguments 5478 really does a S<C<split(' ', $_)>> internally. 5479 5480 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't 5481 much use otherwise. 5482 5483 Example: 5484 5485 open(PASSWD, '/etc/passwd'); 5486 while (<PASSWD>) { 5487 chomp; 5488 ($login, $passwd, $uid, $gid, 5489 $gcos, $home, $shell) = split(/:/); 5490 #... 5491 } 5492 5493 As with regular pattern matching, any capturing parentheses that are not 5494 matched in a C<split()> will be set to C<undef> when returned: 5495 5496 @fields = split /(A)|B/, "1A2B3"; 5497 # @fields is (1, 'A', 2, undef, 3) 5498 5499 =item sprintf FORMAT, LIST 5500 X<sprintf> 5501 5502 Returns a string formatted by the usual C<printf> conventions of the C 5503 library function C<sprintf>. See below for more details 5504 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of 5505 the general principles. 5506 5507 For example: 5508 5509 # Format number with up to 8 leading zeroes 5510 $result = sprintf("%08d", $number); 5511 5512 # Round number to 3 digits after decimal point 5513 $rounded = sprintf("%.3f", $number); 5514 5515 Perl does its own C<sprintf> formatting--it emulates the C 5516 function C<sprintf>, but it doesn't use it (except for floating-point 5517 numbers, and even then only the standard modifiers are allowed). As a 5518 result, any non-standard extensions in your local C<sprintf> are not 5519 available from Perl. 5520 5521 Unlike C<printf>, C<sprintf> does not do what you probably mean when you 5522 pass it an array as your first argument. The array is given scalar context, 5523 and instead of using the 0th element of the array as the format, Perl will 5524 use the count of elements in the array as the format, which is almost never 5525 useful. 5526 5527 Perl's C<sprintf> permits the following universally-known conversions: 5528 5529 %% a percent sign 5530 %c a character with the given number 5531 %s a string 5532 %d a signed integer, in decimal 5533 %u an unsigned integer, in decimal 5534 %o an unsigned integer, in octal 5535 %x an unsigned integer, in hexadecimal 5536 %e a floating-point number, in scientific notation 5537 %f a floating-point number, in fixed decimal notation 5538 %g a floating-point number, in %e or %f notation 5539 5540 In addition, Perl permits the following widely-supported conversions: 5541 5542 %X like %x, but using upper-case letters 5543 %E like %e, but using an upper-case "E" 5544 %G like %g, but with an upper-case "E" (if applicable) 5545 %b an unsigned integer, in binary 5546 %B like %b, but using an upper-case "B" with the # flag 5547 %p a pointer (outputs the Perl value's address in hexadecimal) 5548 %n special: *stores* the number of characters output so far 5549 into the next variable in the parameter list 5550 5551 Finally, for backward (and we do mean "backward") compatibility, Perl 5552 permits these unnecessary but widely-supported conversions: 5553 5554 %i a synonym for %d 5555 %D a synonym for %ld 5556 %U a synonym for %lu 5557 %O a synonym for %lo 5558 %F a synonym for %f 5559 5560 Note that the number of exponent digits in the scientific notation produced 5561 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the 5562 exponent less than 100 is system-dependent: it may be three or less 5563 (zero-padded as necessary). In other words, 1.23 times ten to the 5564 99th may be either "1.23e99" or "1.23e099". 5565 5566 Between the C<%> and the format letter, you may specify a number of 5567 additional attributes controlling the interpretation of the format. 5568 In order, these are: 5569 5570 =over 4 5571 5572 =item format parameter index 5573 5574 An explicit format parameter index, such as C<2$>. By default sprintf 5575 will format the next unused argument in the list, but this allows you 5576 to take the arguments out of order, e.g.: 5577 5578 printf '%2$d %1$d', 12, 34; # prints "34 12" 5579 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1" 5580 5581 =item flags 5582 5583 one or more of: 5584 5585 space prefix positive number with a space 5586 + prefix positive number with a plus sign 5587 - left-justify within the field 5588 0 use zeros, not spaces, to right-justify 5589 # ensure the leading "0" for any octal, 5590 prefix non-zero hexadecimal with "0x" or "0X", 5591 prefix non-zero binary with "0b" or "0B" 5592 5593 For example: 5594 5595 printf '<% d>', 12; # prints "< 12>" 5596 printf '<%+d>', 12; # prints "<+12>" 5597 printf '<%6s>', 12; # prints "< 12>" 5598 printf '<%-6s>', 12; # prints "<12 >" 5599 printf '<%06s>', 12; # prints "<000012>" 5600 printf '<%#o>', 12; # prints "<014>" 5601 printf '<%#x>', 12; # prints "<0xc>" 5602 printf '<%#X>', 12; # prints "<0XC>" 5603 printf '<%#b>', 12; # prints "<0b1100>" 5604 printf '<%#B>', 12; # prints "<0B1100>" 5605 5606 When a space and a plus sign are given as the flags at once, 5607 a plus sign is used to prefix a positive number. 5608 5609 printf '<%+ d>', 12; # prints "<+12>" 5610 printf '<% +d>', 12; # prints "<+12>" 5611 5612 When the # flag and a precision are given in the %o conversion, 5613 the precision is incremented if it's necessary for the leading "0". 5614 5615 printf '<%#.5o>', 012; # prints "<00012>" 5616 printf '<%#.5o>', 012345; # prints "<012345>" 5617 printf '<%#.0o>', 0; # prints "<0>" 5618 5619 =item vector flag 5620 5621 This flag tells perl to interpret the supplied string as a vector of 5622 integers, one for each character in the string. Perl applies the format to 5623 each integer in turn, then joins the resulting strings with a separator (a 5624 dot C<.> by default). This can be useful for displaying ordinal values of 5625 characters in arbitrary strings: 5626 5627 printf "%vd", "AB\x{100}"; # prints "65.66.256" 5628 printf "version is v%vd\n", $^V; # Perl's version 5629 5630 Put an asterisk C<*> before the C<v> to override the string to 5631 use to separate the numbers: 5632 5633 printf "address is %*vX\n", ":", $addr; # IPv6 address 5634 printf "bits are %0*v8b\n", " ", $bits; # random bitstring 5635 5636 You can also explicitly specify the argument number to use for 5637 the join string using e.g. C<*2$v>: 5638 5639 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses 5640 5641 =item (minimum) width 5642 5643 Arguments are usually formatted to be only as wide as required to 5644 display the given value. You can override the width by putting 5645 a number here, or get the width from the next argument (with C<*>) 5646 or from a specified argument (with e.g. C<*2$>): 5647 5648 printf '<%s>', "a"; # prints "<a>" 5649 printf '<%6s>', "a"; # prints "< a>" 5650 printf '<%*s>', 6, "a"; # prints "< a>" 5651 printf '<%*2$s>', "a", 6; # prints "< a>" 5652 printf '<%2s>', "long"; # prints "<long>" (does not truncate) 5653 5654 If a field width obtained through C<*> is negative, it has the same 5655 effect as the C<-> flag: left-justification. 5656 5657 =item precision, or maximum width 5658 X<precision> 5659 5660 You can specify a precision (for numeric conversions) or a maximum 5661 width (for string conversions) by specifying a C<.> followed by a number. 5662 For floating point formats, with the exception of 'g' and 'G', this specifies 5663 the number of decimal places to show (the default being 6), e.g.: 5664 5665 # these examples are subject to system-specific variation 5666 printf '<%f>', 1; # prints "<1.000000>" 5667 printf '<%.1f>', 1; # prints "<1.0>" 5668 printf '<%.0f>', 1; # prints "<1>" 5669 printf '<%e>', 10; # prints "<1.000000e+01>" 5670 printf '<%.1e>', 10; # prints "<1.0e+01>" 5671 5672 For 'g' and 'G', this specifies the maximum number of digits to show, 5673 including prior to the decimal point as well as after it, e.g.: 5674 5675 # these examples are subject to system-specific variation 5676 printf '<%g>', 1; # prints "<1>" 5677 printf '<%.10g>', 1; # prints "<1>" 5678 printf '<%g>', 100; # prints "<100>" 5679 printf '<%.1g>', 100; # prints "<1e+02>" 5680 printf '<%.2g>', 100.01; # prints "<1e+02>" 5681 printf '<%.5g>', 100.01; # prints "<100.01>" 5682 printf '<%.4g>', 100.01; # prints "<100>" 5683 5684 For integer conversions, specifying a precision implies that the 5685 output of the number itself should be zero-padded to this width, 5686 where the 0 flag is ignored: 5687 5688 printf '<%.6d>', 1; # prints "<000001>" 5689 printf '<%+.6d>', 1; # prints "<+000001>" 5690 printf '<%-10.6d>', 1; # prints "<000001 >" 5691 printf '<%10.6d>', 1; # prints "< 000001>" 5692 printf '<%010.6d>', 1; # prints "< 000001>" 5693 printf '<%+10.6d>', 1; # prints "< +000001>" 5694 5695 printf '<%.6x>', 1; # prints "<000001>" 5696 printf '<%#.6x>', 1; # prints "<0x000001>" 5697 printf '<%-10.6x>', 1; # prints "<000001 >" 5698 printf '<%10.6x>', 1; # prints "< 000001>" 5699 printf '<%010.6x>', 1; # prints "< 000001>" 5700 printf '<%#10.6x>', 1; # prints "< 0x000001>" 5701 5702 For string conversions, specifying a precision truncates the string 5703 to fit in the specified width: 5704 5705 printf '<%.5s>', "truncated"; # prints "<trunc>" 5706 printf '<%10.5s>', "truncated"; # prints "< trunc>" 5707 5708 You can also get the precision from the next argument using C<.*>: 5709 5710 printf '<%.6x>', 1; # prints "<000001>" 5711 printf '<%.*x>', 6, 1; # prints "<000001>" 5712 5713 If a precision obtained through C<*> is negative, it has the same 5714 effect as no precision. 5715 5716 printf '<%.*s>', 7, "string"; # prints "<string>" 5717 printf '<%.*s>', 3, "string"; # prints "<str>" 5718 printf '<%.*s>', 0, "string"; # prints "<>" 5719 printf '<%.*s>', -1, "string"; # prints "<string>" 5720 5721 printf '<%.*d>', 1, 0; # prints "<0>" 5722 printf '<%.*d>', 0, 0; # prints "<>" 5723 printf '<%.*d>', -1, 0; # prints "<0>" 5724 5725 You cannot currently get the precision from a specified number, 5726 but it is intended that this will be possible in the future using 5727 e.g. C<.*2$>: 5728 5729 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>" 5730 5731 =item size 5732 5733 For numeric conversions, you can specify the size to interpret the 5734 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer 5735 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be 5736 whatever the default integer size is on your platform (usually 32 or 64 5737 bits), but you can override this to use instead one of the standard C types, 5738 as supported by the compiler used to build Perl: 5739 5740 l interpret integer as C type "long" or "unsigned long" 5741 h interpret integer as C type "short" or "unsigned short" 5742 q, L or ll interpret integer as C type "long long", "unsigned long long". 5743 or "quads" (typically 64-bit integers) 5744 5745 The last will produce errors if Perl does not understand "quads" in your 5746 installation. (This requires that either the platform natively supports quads 5747 or Perl was specifically compiled to support quads.) You can find out 5748 whether your Perl supports quads via L<Config>: 5749 5750 use Config; 5751 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) && 5752 print "quads\n"; 5753 5754 For floating point conversions (C<e f g E F G>), numbers are usually assumed 5755 to be the default floating point size on your platform (double or long double), 5756 but you can force 'long double' with C<q>, C<L>, or C<ll> if your 5757 platform supports them. You can find out whether your Perl supports long 5758 doubles via L<Config>: 5759 5760 use Config; 5761 $Config{d_longdbl} eq 'define' && print "long doubles\n"; 5762 5763 You can find out whether Perl considers 'long double' to be the default 5764 floating point size to use on your platform via L<Config>: 5765 5766 use Config; 5767 ($Config{uselongdouble} eq 'define') && 5768 print "long doubles by default\n"; 5769 5770 It can also be the case that long doubles and doubles are the same thing: 5771 5772 use Config; 5773 ($Config{doublesize} == $Config{longdblsize}) && 5774 print "doubles are long doubles\n"; 5775 5776 The size specifier C<V> has no effect for Perl code, but it is supported 5777 for compatibility with XS code; it means 'use the standard size for 5778 a Perl integer (or floating-point number)', which is already the 5779 default for Perl code. 5780 5781 =item order of arguments 5782 5783 Normally, sprintf takes the next unused argument as the value to 5784 format for each format specification. If the format specification 5785 uses C<*> to require additional arguments, these are consumed from 5786 the argument list in the order in which they appear in the format 5787 specification I<before> the value to format. Where an argument is 5788 specified using an explicit index, this does not affect the normal 5789 order for the arguments (even when the explicitly specified index 5790 would have been the next argument in any case). 5791 5792 So: 5793 5794 printf '<%*.*s>', $a, $b, $c; 5795 5796 would use C<$a> for the width, C<$b> for the precision and C<$c> 5797 as the value to format, while: 5798 5799 printf '<%*1$.*s>', $a, $b; 5800 5801 would use C<$a> for the width and the precision, and C<$b> as the 5802 value to format. 5803 5804 Here are some more examples - beware that when using an explicit 5805 index, the C<$> may need to be escaped: 5806 5807 printf "%2\$d %d\n", 12, 34; # will print "34 12\n" 5808 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n" 5809 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n" 5810 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n" 5811 5812 =back 5813 5814 If C<use locale> is in effect, and POSIX::setlocale() has been called, 5815 the character used for the decimal separator in formatted floating 5816 point numbers is affected by the LC_NUMERIC locale. See L<perllocale> 5817 and L<POSIX>. 5818 5819 =item sqrt EXPR 5820 X<sqrt> X<root> X<square root> 5821 5822 =item sqrt 5823 5824 Return the square root of EXPR. If EXPR is omitted, returns square 5825 root of C<$_>. Only works on non-negative operands, unless you've 5826 loaded the standard Math::Complex module. 5827 5828 use Math::Complex; 5829 print sqrt(-2); # prints 1.4142135623731i 5830 5831 =item srand EXPR 5832 X<srand> X<seed> X<randseed> 5833 5834 =item srand 5835 5836 Sets the random number seed for the C<rand> operator. 5837 5838 The point of the function is to "seed" the C<rand> function so that 5839 C<rand> can produce a different sequence each time you run your 5840 program. 5841 5842 If srand() is not called explicitly, it is called implicitly at the 5843 first use of the C<rand> operator. However, this was not the case in 5844 versions of Perl before 5.004, so if your script will run under older 5845 Perl versions, it should call C<srand>. 5846 5847 Most programs won't even call srand() at all, except those that 5848 need a cryptographically-strong starting point rather than the 5849 generally acceptable default, which is based on time of day, 5850 process ID, and memory allocation, or the F</dev/urandom> device, 5851 if available. 5852 5853 You can call srand($seed) with the same $seed to reproduce the 5854 I<same> sequence from rand(), but this is usually reserved for 5855 generating predictable results for testing or debugging. 5856 Otherwise, don't call srand() more than once in your program. 5857 5858 Do B<not> call srand() (i.e. without an argument) more than once in 5859 a script. The internal state of the random number generator should 5860 contain more entropy than can be provided by any seed, so calling 5861 srand() again actually I<loses> randomness. 5862 5863 Most implementations of C<srand> take an integer and will silently 5864 truncate decimal numbers. This means C<srand(42)> will usually 5865 produce the same results as C<srand(42.1)>. To be safe, always pass 5866 C<srand> an integer. 5867 5868 In versions of Perl prior to 5.004 the default seed was just the 5869 current C<time>. This isn't a particularly good seed, so many old 5870 programs supply their own seed value (often C<time ^ $$> or C<time ^ 5871 ($$ + ($$ << 15))>), but that isn't necessary any more. 5872 5873 For cryptographic purposes, however, you need something much more random 5874 than the default seed. Checksumming the compressed output of one or more 5875 rapidly changing operating system status programs is the usual method. For 5876 example: 5877 5878 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`); 5879 5880 If you're particularly concerned with this, see the C<Math::TrulyRandom> 5881 module in CPAN. 5882 5883 Frequently called programs (like CGI scripts) that simply use 5884 5885 time ^ $$ 5886 5887 for a seed can fall prey to the mathematical property that 5888 5889 a^b == (a+1)^(b+1) 5890 5891 one-third of the time. So don't do that. 5892 5893 =item stat FILEHANDLE 5894 X<stat> X<file, status> X<ctime> 5895 5896 =item stat EXPR 5897 5898 =item stat DIRHANDLE 5899 5900 =item stat 5901 5902 Returns a 13-element list giving the status info for a file, either 5903 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is 5904 omitted, it stats C<$_>. Returns a null list if the stat fails. Typically 5905 used as follows: 5906 5907 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, 5908 $atime,$mtime,$ctime,$blksize,$blocks) 5909 = stat($filename); 5910 5911 Not all fields are supported on all filesystem types. Here are the 5912 meanings of the fields: 5913 5914 0 dev device number of filesystem 5915 1 ino inode number 5916 2 mode file mode (type and permissions) 5917 3 nlink number of (hard) links to the file 5918 4 uid numeric user ID of file's owner 5919 5 gid numeric group ID of file's owner 5920 6 rdev the device identifier (special files only) 5921 7 size total size of file, in bytes 5922 8 atime last access time in seconds since the epoch 5923 9 mtime last modify time in seconds since the epoch 5924 10 ctime inode change time in seconds since the epoch (*) 5925 11 blksize preferred block size for file system I/O 5926 12 blocks actual number of blocks allocated 5927 5928 (The epoch was at 00:00 January 1, 1970 GMT.) 5929 5930 (*) Not all fields are supported on all filesystem types. Notably, the 5931 ctime field is non-portable. In particular, you cannot expect it to be a 5932 "creation time", see L<perlport/"Files and Filesystems"> for details. 5933 5934 If C<stat> is passed the special filehandle consisting of an underline, no 5935 stat is done, but the current contents of the stat structure from the 5936 last C<stat>, C<lstat>, or filetest are returned. Example: 5937 5938 if (-x $file && (($d) = stat(_)) && $d < 0) { 5939 print "$file is executable NFS file\n"; 5940 } 5941 5942 (This works on machines only for which the device number is negative 5943 under NFS.) 5944 5945 Because the mode contains both the file type and its permissions, you 5946 should mask off the file type portion and (s)printf using a C<"%o"> 5947 if you want to see the real permissions. 5948 5949 $mode = (stat($filename))[2]; 5950 printf "Permissions are %04o\n", $mode & 07777; 5951 5952 In scalar context, C<stat> returns a boolean value indicating success 5953 or failure, and, if successful, sets the information associated with 5954 the special filehandle C<_>. 5955 5956 The L<File::stat> module provides a convenient, by-name access mechanism: 5957 5958 use File::stat; 5959 $sb = stat($filename); 5960 printf "File is %s, size is %s, perm %04o, mtime %s\n", 5961 $filename, $sb->size, $sb->mode & 07777, 5962 scalar localtime $sb->mtime; 5963 5964 You can import symbolic mode constants (C<S_IF*>) and functions 5965 (C<S_IS*>) from the Fcntl module: 5966 5967 use Fcntl ':mode'; 5968 5969 $mode = (stat($filename))[2]; 5970 5971 $user_rwx = ($mode & S_IRWXU) >> 6; 5972 $group_read = ($mode & S_IRGRP) >> 3; 5973 $other_execute = $mode & S_IXOTH; 5974 5975 printf "Permissions are %04o\n", S_IMODE($mode), "\n"; 5976 5977 $is_setuid = $mode & S_ISUID; 5978 $is_directory = S_ISDIR($mode); 5979 5980 You could write the last two using the C<-u> and C<-d> operators. 5981 The commonly available C<S_IF*> constants are 5982 5983 # Permissions: read, write, execute, for user, group, others. 5984 5985 S_IRWXU S_IRUSR S_IWUSR S_IXUSR 5986 S_IRWXG S_IRGRP S_IWGRP S_IXGRP 5987 S_IRWXO S_IROTH S_IWOTH S_IXOTH 5988 5989 # Setuid/Setgid/Stickiness/SaveText. 5990 # Note that the exact meaning of these is system dependent. 5991 5992 S_ISUID S_ISGID S_ISVTX S_ISTXT 5993 5994 # File types. Not necessarily all are available on your system. 5995 5996 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT 5997 5998 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR. 5999 6000 S_IREAD S_IWRITE S_IEXEC 6001 6002 and the C<S_IF*> functions are 6003 6004 S_IMODE($mode) the part of $mode containing the permission bits 6005 and the setuid/setgid/sticky bits 6006 6007 S_IFMT($mode) the part of $mode containing the file type 6008 which can be bit-anded with e.g. S_IFREG 6009 or with the following functions 6010 6011 # The operators -f, -d, -l, -b, -c, -p, and -S. 6012 6013 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode) 6014 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode) 6015 6016 # No direct -X operator counterpart, but for the first one 6017 # the -g operator is often equivalent. The ENFMT stands for 6018 # record flocking enforcement, a platform-dependent feature. 6019 6020 S_ISENFMT($mode) S_ISWHT($mode) 6021 6022 See your native chmod(2) and stat(2) documentation for more details 6023 about the C<S_*> constants. To get status info for a symbolic link 6024 instead of the target file behind the link, use the C<lstat> function. 6025 6026 =item state EXPR 6027 X<state> 6028 6029 =item state TYPE EXPR 6030 6031 =item state EXPR : ATTRS 6032 6033 =item state TYPE EXPR : ATTRS 6034 6035 C<state> declares a lexically scoped variable, just like C<my> does. 6036 However, those variables will never be reinitialized, contrary to 6037 lexical variables that are reinitialized each time their enclosing block 6038 is entered. 6039 6040 C<state> variables are only enabled when the C<feature 'state'> pragma is 6041 in effect. See L<feature>. 6042 6043 =item study SCALAR 6044 X<study> 6045 6046 =item study 6047 6048 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of 6049 doing many pattern matches on the string before it is next modified. 6050 This may or may not save time, depending on the nature and number of 6051 patterns you are searching on, and on the distribution of character 6052 frequencies in the string to be searched--you probably want to compare 6053 run times with and without it to see which runs faster. Those loops 6054 that scan for many short constant strings (including the constant 6055 parts of more complex patterns) will benefit most. You may have only 6056 one C<study> active at a time--if you study a different scalar the first 6057 is "unstudied". (The way C<study> works is this: a linked list of every 6058 character in the string to be searched is made, so we know, for 6059 example, where all the C<'k'> characters are. From each search string, 6060 the rarest character is selected, based on some static frequency tables 6061 constructed from some C programs and English text. Only those places 6062 that contain this "rarest" character are examined.) 6063 6064 For example, here is a loop that inserts index producing entries 6065 before any line containing a certain pattern: 6066 6067 while (<>) { 6068 study; 6069 print ".IX foo\n" if /\bfoo\b/; 6070 print ".IX bar\n" if /\bbar\b/; 6071 print ".IX blurfl\n" if /\bblurfl\b/; 6072 # ... 6073 print; 6074 } 6075 6076 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f> 6077 will be looked at, because C<f> is rarer than C<o>. In general, this is 6078 a big win except in pathological cases. The only question is whether 6079 it saves you more time than it took to build the linked list in the 6080 first place. 6081 6082 Note that if you have to look for strings that you don't know till 6083 runtime, you can build an entire loop as a string and C<eval> that to 6084 avoid recompiling all your patterns all the time. Together with 6085 undefining C<$/> to input entire files as one record, this can be very 6086 fast, often faster than specialized programs like fgrep(1). The following 6087 scans a list of files (C<@files>) for a list of words (C<@words>), and prints 6088 out the names of those files that contain a match: 6089 6090 $search = 'while (<>) { study;'; 6091 foreach $word (@words) { 6092 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; 6093 } 6094 $search .= "}"; 6095 @ARGV = @files; 6096 undef $/; 6097 eval $search; # this screams 6098 $/ = "\n"; # put back to normal input delimiter 6099 foreach $file (sort keys(%seen)) { 6100 print $file, "\n"; 6101 } 6102 6103 =item sub NAME BLOCK 6104 X<sub> 6105 6106 =item sub NAME (PROTO) BLOCK 6107 6108 =item sub NAME : ATTRS BLOCK 6109 6110 =item sub NAME (PROTO) : ATTRS BLOCK 6111 6112 This is subroutine definition, not a real function I<per se>. 6113 Without a BLOCK it's just a forward declaration. Without a NAME, 6114 it's an anonymous function declaration, and does actually return 6115 a value: the CODE ref of the closure you just created. 6116 6117 See L<perlsub> and L<perlref> for details about subroutines and 6118 references, and L<attributes> and L<Attribute::Handlers> for more 6119 information about attributes. 6120 6121 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT 6122 X<substr> X<substring> X<mid> X<left> X<right> 6123 6124 =item substr EXPR,OFFSET,LENGTH 6125 6126 =item substr EXPR,OFFSET 6127 6128 Extracts a substring out of EXPR and returns it. First character is at 6129 offset C<0>, or whatever you've set C<$[> to (but don't do that). 6130 If OFFSET is negative (or more precisely, less than C<$[>), starts 6131 that far from the end of the string. If LENGTH is omitted, returns 6132 everything to the end of the string. If LENGTH is negative, leaves that 6133 many characters off the end of the string. 6134 6135 my $s = "The black cat climbed the green tree"; 6136 my $color = substr $s, 4, 5; # black 6137 my $middle = substr $s, 4, -11; # black cat climbed the 6138 my $end = substr $s, 14; # climbed the green tree 6139 my $tail = substr $s, -4; # tree 6140 my $z = substr $s, -4, 2; # tr 6141 6142 You can use the substr() function as an lvalue, in which case EXPR 6143 must itself be an lvalue. If you assign something shorter than LENGTH, 6144 the string will shrink, and if you assign something longer than LENGTH, 6145 the string will grow to accommodate it. To keep the string the same 6146 length you may need to pad or chop your value using C<sprintf>. 6147 6148 If OFFSET and LENGTH specify a substring that is partly outside the 6149 string, only the part within the string is returned. If the substring 6150 is beyond either end of the string, substr() returns the undefined 6151 value and produces a warning. When used as an lvalue, specifying a 6152 substring that is entirely outside the string is a fatal error. 6153 Here's an example showing the behavior for boundary cases: 6154 6155 my $name = 'fred'; 6156 substr($name, 4) = 'dy'; # $name is now 'freddy' 6157 my $null = substr $name, 6, 2; # returns '' (no warning) 6158 my $oops = substr $name, 7; # returns undef, with warning 6159 substr($name, 7) = 'gap'; # fatal error 6160 6161 An alternative to using substr() as an lvalue is to specify the 6162 replacement string as the 4th argument. This allows you to replace 6163 parts of the EXPR and return what was there before in one operation, 6164 just as you can with splice(). 6165 6166 my $s = "The black cat climbed the green tree"; 6167 my $z = substr $s, 14, 7, "jumped from"; # climbed 6168 # $s is now "The black cat jumped from the green tree" 6169 6170 Note that the lvalue returned by the 3-arg version of substr() acts as 6171 a 'magic bullet'; each time it is assigned to, it remembers which part 6172 of the original string is being modified; for example: 6173 6174 $x = '1234'; 6175 for (substr($x,1,2)) { 6176 $_ = 'a'; print $x,"\n"; # prints 1a4 6177 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4 6178 $x = '56789'; 6179 $_ = 'pq'; print $x,"\n"; # prints 5pq9 6180 } 6181 6182 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was 6183 unspecified. 6184 6185 =item symlink OLDFILE,NEWFILE 6186 X<symlink> X<link> X<symbolic link> X<link, symbolic> 6187 6188 Creates a new filename symbolically linked to the old filename. 6189 Returns C<1> for success, C<0> otherwise. On systems that don't support 6190 symbolic links, produces a fatal error at run time. To check for that, 6191 use eval: 6192 6193 $symlink_exists = eval { symlink("",""); 1 }; 6194 6195 =item syscall NUMBER, LIST 6196 X<syscall> X<system call> 6197 6198 Calls the system call specified as the first element of the list, 6199 passing the remaining elements as arguments to the system call. If 6200 unimplemented, produces a fatal error. The arguments are interpreted 6201 as follows: if a given argument is numeric, the argument is passed as 6202 an int. If not, the pointer to the string value is passed. You are 6203 responsible to make sure a string is pre-extended long enough to 6204 receive any result that might be written into a string. You can't use a 6205 string literal (or other read-only string) as an argument to C<syscall> 6206 because Perl has to assume that any string pointer might be written 6207 through. If your 6208 integer arguments are not literals and have never been interpreted in a 6209 numeric context, you may need to add C<0> to them to force them to look 6210 like numbers. This emulates the C<syswrite> function (or vice versa): 6211 6212 require 'syscall.ph'; # may need to run h2ph 6213 $s = "hi there\n"; 6214 syscall(&SYS_write, fileno(STDOUT), $s, length $s); 6215 6216 Note that Perl supports passing of up to only 14 arguments to your system call, 6217 which in practice should usually suffice. 6218 6219 Syscall returns whatever value returned by the system call it calls. 6220 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno). 6221 Note that some system calls can legitimately return C<-1>. The proper 6222 way to handle such calls is to assign C<$!=0;> before the call and 6223 check the value of C<$!> if syscall returns C<-1>. 6224 6225 There's a problem with C<syscall(&SYS_pipe)>: it returns the file 6226 number of the read end of the pipe it creates. There is no way 6227 to retrieve the file number of the other end. You can avoid this 6228 problem by using C<pipe> instead. 6229 6230 =item sysopen FILEHANDLE,FILENAME,MODE 6231 X<sysopen> 6232 6233 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS 6234 6235 Opens the file whose filename is given by FILENAME, and associates it 6236 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as 6237 the name of the real filehandle wanted. This function calls the 6238 underlying operating system's C<open> function with the parameters 6239 FILENAME, MODE, PERMS. 6240 6241 The possible values and flag bits of the MODE parameter are 6242 system-dependent; they are available via the standard module C<Fcntl>. 6243 See the documentation of your operating system's C<open> to see which 6244 values and flag bits are available. You may combine several flags 6245 using the C<|>-operator. 6246 6247 Some of the most common values are C<O_RDONLY> for opening the file in 6248 read-only mode, C<O_WRONLY> for opening the file in write-only mode, 6249 and C<O_RDWR> for opening the file in read-write mode. 6250 X<O_RDONLY> X<O_RDWR> X<O_WRONLY> 6251 6252 For historical reasons, some values work on almost every system 6253 supported by perl: zero means read-only, one means write-only, and two 6254 means read/write. We know that these values do I<not> work under 6255 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to 6256 use them in new code. 6257 6258 If the file named by FILENAME does not exist and the C<open> call creates 6259 it (typically because MODE includes the C<O_CREAT> flag), then the value of 6260 PERMS specifies the permissions of the newly created file. If you omit 6261 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>. 6262 These permission values need to be in octal, and are modified by your 6263 process's current C<umask>. 6264 X<O_CREAT> 6265 6266 In many systems the C<O_EXCL> flag is available for opening files in 6267 exclusive mode. This is B<not> locking: exclusiveness means here that 6268 if the file already exists, sysopen() fails. C<O_EXCL> may not work 6269 on network filesystems, and has no effect unless the C<O_CREAT> flag 6270 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from 6271 being opened if it is a symbolic link. It does not protect against 6272 symbolic links in the file's path. 6273 X<O_EXCL> 6274 6275 Sometimes you may want to truncate an already-existing file. This 6276 can be done using the C<O_TRUNC> flag. The behavior of 6277 C<O_TRUNC> with C<O_RDONLY> is undefined. 6278 X<O_TRUNC> 6279 6280 You should seldom if ever use C<0644> as argument to C<sysopen>, because 6281 that takes away the user's option to have a more permissive umask. 6282 Better to omit it. See the perlfunc(1) entry on C<umask> for more 6283 on this. 6284 6285 Note that C<sysopen> depends on the fdopen() C library function. 6286 On many UNIX systems, fdopen() is known to fail when file descriptors 6287 exceed a certain value, typically 255. If you need more file 6288 descriptors than that, consider rebuilding Perl to use the C<sfio> 6289 library, or perhaps using the POSIX::open() function. 6290 6291 See L<perlopentut> for a kinder, gentler explanation of opening files. 6292 6293 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET 6294 X<sysread> 6295 6296 =item sysread FILEHANDLE,SCALAR,LENGTH 6297 6298 Attempts to read LENGTH bytes of data into variable SCALAR from the 6299 specified FILEHANDLE, using the system call read(2). It bypasses 6300 buffered IO, so mixing this with other kinds of reads, C<print>, 6301 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the 6302 perlio or stdio layers usually buffers data. Returns the number of 6303 bytes actually read, C<0> at end of file, or undef if there was an 6304 error (in the latter case C<$!> is also set). SCALAR will be grown or 6305 shrunk so that the last byte actually read is the last byte of the 6306 scalar after the read. 6307 6308 An OFFSET may be specified to place the read data at some place in the 6309 string other than the beginning. A negative OFFSET specifies 6310 placement at that many characters counting backwards from the end of 6311 the string. A positive OFFSET greater than the length of SCALAR 6312 results in the string being padded to the required size with C<"\0"> 6313 bytes before the result of the read is appended. 6314 6315 There is no syseof() function, which is ok, since eof() doesn't work 6316 very well on device files (like ttys) anyway. Use sysread() and check 6317 for a return value for 0 to decide whether you're done. 6318 6319 Note that if the filehandle has been marked as C<:utf8> Unicode 6320 characters are read instead of bytes (the LENGTH, OFFSET, and the 6321 return value of sysread() are in Unicode characters). 6322 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer. 6323 See L</binmode>, L</open>, and the C<open> pragma, L<open>. 6324 6325 =item sysseek FILEHANDLE,POSITION,WHENCE 6326 X<sysseek> X<lseek> 6327 6328 Sets FILEHANDLE's system position in bytes using the system call 6329 lseek(2). FILEHANDLE may be an expression whose value gives the name 6330 of the filehandle. The values for WHENCE are C<0> to set the new 6331 position to POSITION, C<1> to set the it to the current position plus 6332 POSITION, and C<2> to set it to EOF plus POSITION (typically 6333 negative). 6334 6335 Note the I<in bytes>: even if the filehandle has been set to operate 6336 on characters (for example by using the C<:encoding(utf8)> I/O layer), 6337 tell() will return byte offsets, not character offsets (because 6338 implementing that would render sysseek() very slow). 6339 6340 sysseek() bypasses normal buffered IO, so mixing this with reads (other 6341 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>, 6342 C<seek>, C<tell>, or C<eof> may cause confusion. 6343 6344 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, 6345 and C<SEEK_END> (start of the file, current position, end of the file) 6346 from the Fcntl module. Use of the constants is also more portable 6347 than relying on 0, 1, and 2. For example to define a "systell" function: 6348 6349 use Fcntl 'SEEK_CUR'; 6350 sub systell { sysseek($_[0], 0, SEEK_CUR) } 6351 6352 Returns the new position, or the undefined value on failure. A position 6353 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns 6354 true on success and false on failure, yet you can still easily determine 6355 the new position. 6356 6357 =item system LIST 6358 X<system> X<shell> 6359 6360 =item system PROGRAM LIST 6361 6362 Does exactly the same thing as C<exec LIST>, except that a fork is 6363 done first, and the parent process waits for the child process to 6364 complete. Note that argument processing varies depending on the 6365 number of arguments. If there is more than one argument in LIST, 6366 or if LIST is an array with more than one value, starts the program 6367 given by the first element of the list with arguments given by the 6368 rest of the list. If there is only one scalar argument, the argument 6369 is checked for shell metacharacters, and if there are any, the 6370 entire argument is passed to the system's command shell for parsing 6371 (this is C</bin/sh -c> on Unix platforms, but varies on other 6372 platforms). If there are no shell metacharacters in the argument, 6373 it is split into words and passed directly to C<execvp>, which is 6374 more efficient. 6375 6376 Beginning with v5.6.0, Perl will attempt to flush all files opened for 6377 output before any operation that may do a fork, but this may not be 6378 supported on some platforms (see L<perlport>). To be safe, you may need 6379 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method 6380 of C<IO::Handle> on any open handles. 6381 6382 The return value is the exit status of the program as returned by the 6383 C<wait> call. To get the actual exit value, shift right by eight (see 6384 below). See also L</exec>. This is I<not> what you want to use to capture 6385 the output from a command, for that you should use merely backticks or 6386 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1 6387 indicates a failure to start the program or an error of the wait(2) system 6388 call (inspect $! for the reason). 6389 6390 Like C<exec>, C<system> allows you to lie to a program about its name if 6391 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>. 6392 6393 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of 6394 C<system>, if you expect your program to terminate on receipt of these 6395 signals you will need to arrange to do so yourself based on the return 6396 value. 6397 6398 @args = ("command", "arg1", "arg2"); 6399 system(@args) == 0 6400 or die "system @args failed: $?" 6401 6402 You can check all the failure possibilities by inspecting 6403 C<$?> like this: 6404 6405 if ($? == -1) { 6406 print "failed to execute: $!\n"; 6407 } 6408 elsif ($? & 127) { 6409 printf "child died with signal %d, %s coredump\n", 6410 ($? & 127), ($? & 128) ? 'with' : 'without'; 6411 } 6412 else { 6413 printf "child exited with value %d\n", $? >> 8; 6414 } 6415 6416 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}> 6417 with the W*() calls of the POSIX extension. 6418 6419 When the arguments get executed via the system shell, results 6420 and return codes will be subject to its quirks and capabilities. 6421 See L<perlop/"`STRING`"> and L</exec> for details. 6422 6423 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET 6424 X<syswrite> 6425 6426 =item syswrite FILEHANDLE,SCALAR,LENGTH 6427 6428 =item syswrite FILEHANDLE,SCALAR 6429 6430 Attempts to write LENGTH bytes of data from variable SCALAR to the 6431 specified FILEHANDLE, using the system call write(2). If LENGTH is 6432 not specified, writes whole SCALAR. It bypasses buffered IO, so 6433 mixing this with reads (other than C<sysread())>, C<print>, C<write>, 6434 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and 6435 stdio layers usually buffers data. Returns the number of bytes 6436 actually written, or C<undef> if there was an error (in this case the 6437 errno variable C<$!> is also set). If the LENGTH is greater than the 6438 available data in the SCALAR after the OFFSET, only as much data as is 6439 available will be written. 6440 6441 An OFFSET may be specified to write the data from some part of the 6442 string other than the beginning. A negative OFFSET specifies writing 6443 that many characters counting backwards from the end of the string. 6444 In the case the SCALAR is empty you can use OFFSET but only zero offset. 6445 6446 Note that if the filehandle has been marked as C<:utf8>, Unicode 6447 characters are written instead of bytes (the LENGTH, OFFSET, and the 6448 return value of syswrite() are in UTF-8 encoded Unicode characters). 6449 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer. 6450 See L</binmode>, L</open>, and the C<open> pragma, L<open>. 6451 6452 =item tell FILEHANDLE 6453 X<tell> 6454 6455 =item tell 6456 6457 Returns the current position I<in bytes> for FILEHANDLE, or -1 on 6458 error. FILEHANDLE may be an expression whose value gives the name of 6459 the actual filehandle. If FILEHANDLE is omitted, assumes the file 6460 last read. 6461 6462 Note the I<in bytes>: even if the filehandle has been set to 6463 operate on characters (for example by using the C<:encoding(utf8)> open 6464 layer), tell() will return byte offsets, not character offsets (because 6465 that would render seek() and tell() rather slow). 6466 6467 The return value of tell() for the standard streams like the STDIN 6468 depends on the operating system: it may return -1 or something else. 6469 tell() on pipes, fifos, and sockets usually returns -1. 6470 6471 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that. 6472 6473 Do not use tell() (or other buffered I/O operations) on a file handle 6474 that has been manipulated by sysread(), syswrite() or sysseek(). 6475 Those functions ignore the buffering, while tell() does not. 6476 6477 =item telldir DIRHANDLE 6478 X<telldir> 6479 6480 Returns the current position of the C<readdir> routines on DIRHANDLE. 6481 Value may be given to C<seekdir> to access a particular location in a 6482 directory. C<telldir> has the same caveats about possible directory 6483 compaction as the corresponding system library routine. 6484 6485 =item tie VARIABLE,CLASSNAME,LIST 6486 X<tie> 6487 6488 This function binds a variable to a package class that will provide the 6489 implementation for the variable. VARIABLE is the name of the variable 6490 to be enchanted. CLASSNAME is the name of a class implementing objects 6491 of correct type. Any additional arguments are passed to the C<new> 6492 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>, 6493 or C<TIEHASH>). Typically these are arguments such as might be passed 6494 to the C<dbm_open()> function of C. The object returned by the C<new> 6495 method is also returned by the C<tie> function, which would be useful 6496 if you want to access other methods in CLASSNAME. 6497 6498 Note that functions such as C<keys> and C<values> may return huge lists 6499 when used on large objects, like DBM files. You may prefer to use the 6500 C<each> function to iterate over such. Example: 6501 6502 # print out history file offsets 6503 use NDBM_File; 6504 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); 6505 while (($key,$val) = each %HIST) { 6506 print $key, ' = ', unpack('L',$val), "\n"; 6507 } 6508 untie(%HIST); 6509 6510 A class implementing a hash should have the following methods: 6511 6512 TIEHASH classname, LIST 6513 FETCH this, key 6514 STORE this, key, value 6515 DELETE this, key 6516 CLEAR this 6517 EXISTS this, key 6518 FIRSTKEY this 6519 NEXTKEY this, lastkey 6520 SCALAR this 6521 DESTROY this 6522 UNTIE this 6523 6524 A class implementing an ordinary array should have the following methods: 6525 6526 TIEARRAY classname, LIST 6527 FETCH this, key 6528 STORE this, key, value 6529 FETCHSIZE this 6530 STORESIZE this, count 6531 CLEAR this 6532 PUSH this, LIST 6533 POP this 6534 SHIFT this 6535 UNSHIFT this, LIST 6536 SPLICE this, offset, length, LIST 6537 EXTEND this, count 6538 DESTROY this 6539 UNTIE this 6540 6541 A class implementing a file handle should have the following methods: 6542 6543 TIEHANDLE classname, LIST 6544 READ this, scalar, length, offset 6545 READLINE this 6546 GETC this 6547 WRITE this, scalar, length, offset 6548 PRINT this, LIST 6549 PRINTF this, format, LIST 6550 BINMODE this 6551 EOF this 6552 FILENO this 6553 SEEK this, position, whence 6554 TELL this 6555 OPEN this, mode, LIST 6556 CLOSE this 6557 DESTROY this 6558 UNTIE this 6559 6560 A class implementing a scalar should have the following methods: 6561 6562 TIESCALAR classname, LIST 6563 FETCH this, 6564 STORE this, value 6565 DESTROY this 6566 UNTIE this 6567 6568 Not all methods indicated above need be implemented. See L<perltie>, 6569 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>. 6570 6571 Unlike C<dbmopen>, the C<tie> function will not use or require a module 6572 for you--you need to do that explicitly yourself. See L<DB_File> 6573 or the F<Config> module for interesting C<tie> implementations. 6574 6575 For further details see L<perltie>, L<"tied VARIABLE">. 6576 6577 =item tied VARIABLE 6578 X<tied> 6579 6580 Returns a reference to the object underlying VARIABLE (the same value 6581 that was originally returned by the C<tie> call that bound the variable 6582 to a package.) Returns the undefined value if VARIABLE isn't tied to a 6583 package. 6584 6585 =item time 6586 X<time> X<epoch> 6587 6588 Returns the number of non-leap seconds since whatever time the system 6589 considers to be the epoch, suitable for feeding to C<gmtime> and 6590 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970; 6591 a prominent exception being Mac OS Classic which uses 00:00:00, January 1, 6592 1904 in the current local time zone for its epoch. 6593 6594 For measuring time in better granularity than one second, 6595 you may use either the L<Time::HiRes> module (from CPAN, and starting from 6596 Perl 5.8 part of the standard distribution), or if you have 6597 gettimeofday(2), you may be able to use the C<syscall> interface of Perl. 6598 See L<perlfaq8> for details. 6599 6600 For date and time processing look at the many related modules on CPAN. 6601 For a comprehensive date and time representation look at the 6602 L<DateTime> module. 6603 6604 =item times 6605 X<times> 6606 6607 Returns a four-element list giving the user and system times, in 6608 seconds, for this process and the children of this process. 6609 6610 ($user,$system,$cuser,$csystem) = times; 6611 6612 In scalar context, C<times> returns C<$user>. 6613 6614 Note that times for children are included only after they terminate. 6615 6616 =item tr/// 6617 6618 The transliteration operator. Same as C<y///>. See L<perlop>. 6619 6620 =item truncate FILEHANDLE,LENGTH 6621 X<truncate> 6622 6623 =item truncate EXPR,LENGTH 6624 6625 Truncates the file opened on FILEHANDLE, or named by EXPR, to the 6626 specified length. Produces a fatal error if truncate isn't implemented 6627 on your system. Returns true if successful, the undefined value 6628 otherwise. 6629 6630 The behavior is undefined if LENGTH is greater than the length of the 6631 file. 6632 6633 The position in the file of FILEHANDLE is left unchanged. You may want to 6634 call L<seek> before writing to the file. 6635 6636 =item uc EXPR 6637 X<uc> X<uppercase> X<toupper> 6638 6639 =item uc 6640 6641 Returns an uppercased version of EXPR. This is the internal function 6642 implementing the C<\U> escape in double-quoted strings. Respects 6643 current LC_CTYPE locale if C<use locale> in force. See L<perllocale> 6644 and L<perlunicode> for more details about locale and Unicode support. 6645 It does not attempt to do titlecase mapping on initial letters. See 6646 C<ucfirst> for that. 6647 6648 If EXPR is omitted, uses C<$_>. 6649 6650 =item ucfirst EXPR 6651 X<ucfirst> X<uppercase> 6652 6653 =item ucfirst 6654 6655 Returns the value of EXPR with the first character in uppercase 6656 (titlecase in Unicode). This is the internal function implementing 6657 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE 6658 locale if C<use locale> in force. See L<perllocale> and L<perlunicode> 6659 for more details about locale and Unicode support. 6660 6661 If EXPR is omitted, uses C<$_>. 6662 6663 =item umask EXPR 6664 X<umask> 6665 6666 =item umask 6667 6668 Sets the umask for the process to EXPR and returns the previous value. 6669 If EXPR is omitted, merely returns the current umask. 6670 6671 The Unix permission C<rwxr-x---> is represented as three sets of three 6672 bits, or three octal digits: C<0750> (the leading 0 indicates octal 6673 and isn't one of the digits). The C<umask> value is such a number 6674 representing disabled permissions bits. The permission (or "mode") 6675 values you pass C<mkdir> or C<sysopen> are modified by your umask, so 6676 even if you tell C<sysopen> to create a file with permissions C<0777>, 6677 if your umask is C<0022> then the file will actually be created with 6678 permissions C<0755>. If your C<umask> were C<0027> (group can't 6679 write; others can't read, write, or execute), then passing 6680 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~ 6681 027> is C<0640>). 6682 6683 Here's some advice: supply a creation mode of C<0666> for regular 6684 files (in C<sysopen>) and one of C<0777> for directories (in 6685 C<mkdir>) and executable files. This gives users the freedom of 6686 choice: if they want protected files, they might choose process umasks 6687 of C<022>, C<027>, or even the particularly antisocial mask of C<077>. 6688 Programs should rarely if ever make policy decisions better left to 6689 the user. The exception to this is when writing files that should be 6690 kept private: mail files, web browser cookies, I<.rhosts> files, and 6691 so on. 6692 6693 If umask(2) is not implemented on your system and you are trying to 6694 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a 6695 fatal error at run time. If umask(2) is not implemented and you are 6696 not trying to restrict access for yourself, returns C<undef>. 6697 6698 Remember that a umask is a number, usually given in octal; it is I<not> a 6699 string of octal digits. See also L</oct>, if all you have is a string. 6700 6701 =item undef EXPR 6702 X<undef> X<undefine> 6703 6704 =item undef 6705 6706 Undefines the value of EXPR, which must be an lvalue. Use only on a 6707 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine 6708 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}> 6709 will probably not do what you expect on most predefined variables or 6710 DBM list values, so don't do that; see L<delete>.) Always returns the 6711 undefined value. You can omit the EXPR, in which case nothing is 6712 undefined, but you still get an undefined value that you could, for 6713 instance, return from a subroutine, assign to a variable or pass as a 6714 parameter. Examples: 6715 6716 undef $foo; 6717 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; 6718 undef @ary; 6719 undef %hash; 6720 undef &mysub; 6721 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. 6722 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; 6723 select undef, undef, undef, 0.25; 6724 ($a, $b, undef, $c) = &foo; # Ignore third value returned 6725 6726 Note that this is a unary operator, not a list operator. 6727 6728 =item unlink LIST 6729 X<unlink> X<delete> X<remove> X<rm> X<del> 6730 6731 =item unlink 6732 6733 Deletes a list of files. Returns the number of files successfully 6734 deleted. 6735 6736 $cnt = unlink 'a', 'b', 'c'; 6737 unlink @goners; 6738 unlink <*.bak>; 6739 6740 Note: C<unlink> will not attempt to delete directories unless you are superuser 6741 and the B<-U> flag is supplied to Perl. Even if these conditions are 6742 met, be warned that unlinking a directory can inflict damage on your 6743 filesystem. Finally, using C<unlink> on directories is not supported on 6744 many operating systems. Use C<rmdir> instead. 6745 6746 If LIST is omitted, uses C<$_>. 6747 6748 =item unpack TEMPLATE,EXPR 6749 X<unpack> 6750 6751 =item unpack TEMPLATE 6752 6753 C<unpack> does the reverse of C<pack>: it takes a string 6754 and expands it out into a list of values. 6755 (In scalar context, it returns merely the first value produced.) 6756 6757 If EXPR is omitted, unpacks the C<$_> string. 6758 6759 The string is broken into chunks described by the TEMPLATE. Each chunk 6760 is converted separately to a value. Typically, either the string is a result 6761 of C<pack>, or the characters of the string represent a C structure of some 6762 kind. 6763 6764 The TEMPLATE has the same format as in the C<pack> function. 6765 Here's a subroutine that does substring: 6766 6767 sub substr { 6768 my($what,$where,$howmuch) = @_; 6769 unpack("x$where a$howmuch", $what); 6770 } 6771 6772 and then there's 6773 6774 sub ordinal { unpack("W",$_[0]); } # same as ord() 6775 6776 In addition to fields allowed in pack(), you may prefix a field with 6777 a %<number> to indicate that 6778 you want a <number>-bit checksum of the items instead of the items 6779 themselves. Default is a 16-bit checksum. Checksum is calculated by 6780 summing numeric values of expanded values (for string fields the sum of 6781 C<ord($char)> is taken, for bit fields the sum of zeroes and ones). 6782 6783 For example, the following 6784 computes the same number as the System V sum program: 6785 6786 $checksum = do { 6787 local $/; # slurp! 6788 unpack("%32W*",<>) % 65535; 6789 }; 6790 6791 The following efficiently counts the number of set bits in a bit vector: 6792 6793 $setbits = unpack("%32b*", $selectmask); 6794 6795 The C<p> and C<P> formats should be used with care. Since Perl 6796 has no way of checking whether the value passed to C<unpack()> 6797 corresponds to a valid memory location, passing a pointer value that's 6798 not known to be valid is likely to have disastrous consequences. 6799 6800 If there are more pack codes or if the repeat count of a field or a group 6801 is larger than what the remainder of the input string allows, the result 6802 is not well defined: in some cases, the repeat count is decreased, or 6803 C<unpack()> will produce null strings or zeroes, or terminate with an 6804 error. If the input string is longer than one described by the TEMPLATE, 6805 the rest is ignored. 6806 6807 See L</pack> for more examples and notes. 6808 6809 =item untie VARIABLE 6810 X<untie> 6811 6812 Breaks the binding between a variable and a package. (See C<tie>.) 6813 Has no effect if the variable is not tied. 6814 6815 =item unshift ARRAY,LIST 6816 X<unshift> 6817 6818 Does the opposite of a C<shift>. Or the opposite of a C<push>, 6819 depending on how you look at it. Prepends list to the front of the 6820 array, and returns the new number of elements in the array. 6821 6822 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/; 6823 6824 Note the LIST is prepended whole, not one element at a time, so the 6825 prepended elements stay in the same order. Use C<reverse> to do the 6826 reverse. 6827 6828 =item use Module VERSION LIST 6829 X<use> X<module> X<import> 6830 6831 =item use Module VERSION 6832 6833 =item use Module LIST 6834 6835 =item use Module 6836 6837 =item use VERSION 6838 6839 Imports some semantics into the current package from the named module, 6840 generally by aliasing certain subroutine or variable names into your 6841 package. It is exactly equivalent to 6842 6843 BEGIN { require Module; Module->import( LIST ); } 6844 6845 except that Module I<must> be a bareword. 6846 6847 In the peculiar C<use VERSION> form, VERSION may be either a numeric 6848 argument such as 5.006, which will be compared to C<$]>, or a literal of 6849 the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). A 6850 fatal error is produced if VERSION is greater than the version of the 6851 current Perl interpreter; Perl will not attempt to parse the rest of the 6852 file. Compare with L</require>, which can do a similar check at run time. 6853 Symmetrically, C<no VERSION> allows you to specify that you want a version 6854 of perl older than the specified one. 6855 6856 Specifying VERSION as a literal of the form v5.6.1 should generally be 6857 avoided, because it leads to misleading error messages under earlier 6858 versions of Perl that do not support this syntax. The equivalent numeric 6859 version should be used instead. 6860 6861 Alternatively, you can use a numeric version C<use 5.006> followed by a 6862 v-string version like C<use v5.10.1>, to avoid the unintuitive C<use 6863 5.010_001>. (older perl versions fail gracefully at the first C<use>, 6864 later perl versions understand the v-string syntax in the second). 6865 6866 use v5.6.1; # compile time version check 6867 use 5.6.1; # ditto 6868 use 5.006_001; # ditto; preferred for backwards compatibility 6869 use 5.006; use 5.6.1; # ditto, for compatibility and readability 6870 6871 This is often useful if you need to check the current Perl version before 6872 C<use>ing library modules that have changed in incompatible ways from 6873 older versions of Perl. (We try not to do this more than we have to.) 6874 6875 Also, if the specified perl version is greater than or equal to 5.9.5, 6876 C<use VERSION> will also load the C<feature> pragma and enable all 6877 features available in the requested version. See L<feature>. 6878 6879 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The 6880 C<require> makes sure the module is loaded into memory if it hasn't been 6881 yet. The C<import> is not a builtin--it's just an ordinary static method 6882 call into the C<Module> package to tell the module to import the list of 6883 features back into the current package. The module can implement its 6884 C<import> method any way it likes, though most modules just choose to 6885 derive their C<import> method via inheritance from the C<Exporter> class that 6886 is defined in the C<Exporter> module. See L<Exporter>. If no C<import> 6887 method can be found then the call is skipped, even if there is an AUTOLOAD 6888 method. 6889 6890 If you do not want to call the package's C<import> method (for instance, 6891 to stop your namespace from being altered), explicitly supply the empty list: 6892 6893 use Module (); 6894 6895 That is exactly equivalent to 6896 6897 BEGIN { require Module } 6898 6899 If the VERSION argument is present between Module and LIST, then the 6900 C<use> will call the VERSION method in class Module with the given 6901 version as an argument. The default VERSION method, inherited from 6902 the UNIVERSAL class, croaks if the given version is larger than the 6903 value of the variable C<$Module::VERSION>. 6904 6905 Again, there is a distinction between omitting LIST (C<import> called 6906 with no arguments) and an explicit empty LIST C<()> (C<import> not 6907 called). Note that there is no comma after VERSION! 6908 6909 Because this is a wide-open interface, pragmas (compiler directives) 6910 are also implemented this way. Currently implemented pragmas are: 6911 6912 use constant; 6913 use diagnostics; 6914 use integer; 6915 use sigtrap qw(SEGV BUS); 6916 use strict qw(subs vars refs); 6917 use subs qw(afunc blurfl); 6918 use warnings qw(all); 6919 use sort qw(stable _quicksort _mergesort); 6920 6921 Some of these pseudo-modules import semantics into the current 6922 block scope (like C<strict> or C<integer>, unlike ordinary modules, 6923 which import symbols into the current package (which are effective 6924 through the end of the file). 6925 6926 There's a corresponding C<no> command that unimports meanings imported 6927 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>. 6928 It behaves exactly as C<import> does with respect to VERSION, an 6929 omitted LIST, empty LIST, or no unimport method being found. 6930 6931 no integer; 6932 no strict 'refs'; 6933 no warnings; 6934 6935 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun> 6936 for the C<-M> and C<-m> command-line options to perl that give C<use> 6937 functionality from the command-line. 6938 6939 =item utime LIST 6940 X<utime> 6941 6942 Changes the access and modification times on each file of a list of 6943 files. The first two elements of the list must be the NUMERICAL access 6944 and modification times, in that order. Returns the number of files 6945 successfully changed. The inode change time of each file is set 6946 to the current time. For example, this code has the same effect as the 6947 Unix touch(1) command when the files I<already exist> and belong to 6948 the user running the program: 6949 6950 #!/usr/bin/perl 6951 $atime = $mtime = time; 6952 utime $atime, $mtime, @ARGV; 6953 6954 Since perl 5.7.2, if the first two elements of the list are C<undef>, then 6955 the utime(2) function in the C library will be called with a null second 6956 argument. On most systems, this will set the file's access and 6957 modification times to the current time (i.e. equivalent to the example 6958 above) and will even work on other users' files where you have write 6959 permission: 6960 6961 utime undef, undef, @ARGV; 6962 6963 Under NFS this will use the time of the NFS server, not the time of 6964 the local machine. If there is a time synchronization problem, the 6965 NFS server and local machine will have different times. The Unix 6966 touch(1) command will in fact normally use this form instead of the 6967 one shown in the first example. 6968 6969 Note that only passing one of the first two elements as C<undef> will 6970 be equivalent of passing it as 0 and will not have the same effect as 6971 described when they are both C<undef>. This case will also trigger an 6972 uninitialized warning. 6973 6974 On systems that support futimes, you might pass file handles among the 6975 files. On systems that don't support futimes, passing file handles 6976 produces a fatal error at run time. The file handles must be passed 6977 as globs or references to be recognized. Barewords are considered 6978 file names. 6979 6980 =item values HASH 6981 X<values> 6982 6983 Returns a list consisting of all the values of the named hash. 6984 (In a scalar context, returns the number of values.) 6985 6986 The values are returned in an apparently random order. The actual 6987 random order is subject to change in future versions of perl, but it 6988 is guaranteed to be the same order as either the C<keys> or C<each> 6989 function would produce on the same (unmodified) hash. Since Perl 6990 5.8.1 the ordering is different even between different runs of Perl 6991 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">). 6992 6993 As a side effect, calling values() resets the HASH's internal iterator, 6994 see L</each>. (In particular, calling values() in void context resets 6995 the iterator with no other overhead.) 6996 6997 Note that the values are not copied, which means modifying them will 6998 modify the contents of the hash: 6999 7000 for (values %hash) { s/foo/bar/g } # modifies %hash values 7001 for (@hash{keys %hash}) { s/foo/bar/g } # same 7002 7003 See also C<keys>, C<each>, and C<sort>. 7004 7005 =item vec EXPR,OFFSET,BITS 7006 X<vec> X<bit> X<bit vector> 7007 7008 Treats the string in EXPR as a bit vector made up of elements of 7009 width BITS, and returns the value of the element specified by OFFSET 7010 as an unsigned integer. BITS therefore specifies the number of bits 7011 that are reserved for each element in the bit vector. This must 7012 be a power of two from 1 to 32 (or 64, if your platform supports 7013 that). 7014 7015 If BITS is 8, "elements" coincide with bytes of the input string. 7016 7017 If BITS is 16 or more, bytes of the input string are grouped into chunks 7018 of size BITS/8, and each group is converted to a number as with 7019 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously 7020 for BITS==64). See L<"pack"> for details. 7021 7022 If bits is 4 or less, the string is broken into bytes, then the bits 7023 of each byte are broken into 8/BITS groups. Bits of a byte are 7024 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>, 7025 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example, 7026 breaking the single input byte C<chr(0x36)> into two groups gives a list 7027 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>. 7028 7029 C<vec> may also be assigned to, in which case parentheses are needed 7030 to give the expression the correct precedence as in 7031 7032 vec($image, $max_x * $x + $y, 8) = 3; 7033 7034 If the selected element is outside the string, the value 0 is returned. 7035 If an element off the end of the string is written to, Perl will first 7036 extend the string with sufficiently many zero bytes. It is an error 7037 to try to write off the beginning of the string (i.e. negative OFFSET). 7038 7039 If the string happens to be encoded as UTF-8 internally (and thus has 7040 the UTF8 flag set), this is ignored by C<vec>, and it operates on the 7041 internal byte string, not the conceptual character string, even if you 7042 only have characters with values less than 256. 7043 7044 Strings created with C<vec> can also be manipulated with the logical 7045 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit 7046 vector operation is desired when both operands are strings. 7047 See L<perlop/"Bitwise String Operators">. 7048 7049 The following code will build up an ASCII string saying C<'PerlPerlPerl'>. 7050 The comments show the string after each step. Note that this code works 7051 in the same way on big-endian or little-endian machines. 7052 7053 my $foo = ''; 7054 vec($foo, 0, 32) = 0x5065726C; # 'Perl' 7055 7056 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits 7057 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P') 7058 7059 vec($foo, 2, 16) = 0x5065; # 'PerlPe' 7060 vec($foo, 3, 16) = 0x726C; # 'PerlPerl' 7061 vec($foo, 8, 8) = 0x50; # 'PerlPerlP' 7062 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' 7063 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" 7064 vec($foo, 21, 4) = 7; # 'PerlPerlPer' 7065 # 'r' is "\x72" 7066 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" 7067 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" 7068 vec($foo, 94, 1) = 1; # 'PerlPerlPerl' 7069 # 'l' is "\x6c" 7070 7071 To transform a bit vector into a string or list of 0's and 1's, use these: 7072 7073 $bits = unpack("b*", $vector); 7074 @bits = split(//, unpack("b*", $vector)); 7075 7076 If you know the exact length in bits, it can be used in place of the C<*>. 7077 7078 Here is an example to illustrate how the bits actually fall in place: 7079 7080 #!/usr/bin/perl -wl 7081 7082 print <<'EOT'; 7083 0 1 2 3 7084 unpack("V",$_) 01234567890123456789012345678901 7085 ------------------------------------------------------------------ 7086 EOT 7087 7088 for $w (0..3) { 7089 $width = 2**$w; 7090 for ($shift=0; $shift < $width; ++$shift) { 7091 for ($off=0; $off < 32/$width; ++$off) { 7092 $str = pack("B*", "0"x32); 7093 $bits = (1<<$shift); 7094 vec($str, $off, $width) = $bits; 7095 $res = unpack("b*",$str); 7096 $val = unpack("V", $str); 7097 write; 7098 } 7099 } 7100 } 7101 7102 format STDOUT = 7103 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 7104 $off, $width, $bits, $val, $res 7105 . 7106 __END__ 7107 7108 Regardless of the machine architecture on which it is run, the above 7109 example should print the following table: 7110 7111 0 1 2 3 7112 unpack("V",$_) 01234567890123456789012345678901 7113 ------------------------------------------------------------------ 7114 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000 7115 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000 7116 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000 7117 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000 7118 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000 7119 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000 7120 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000 7121 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000 7122 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000 7123 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000 7124 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000 7125 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000 7126 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000 7127 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000 7128 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000 7129 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000 7130 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000 7131 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000 7132 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000 7133 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000 7134 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000 7135 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000 7136 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000 7137 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000 7138 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000 7139 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000 7140 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000 7141 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000 7142 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000 7143 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100 7144 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010 7145 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001 7146 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000 7147 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000 7148 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000 7149 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000 7150 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000 7151 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000 7152 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000 7153 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000 7154 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000 7155 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000 7156 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000 7157 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000 7158 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000 7159 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000 7160 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000 7161 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010 7162 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000 7163 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000 7164 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000 7165 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000 7166 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000 7167 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000 7168 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000 7169 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000 7170 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000 7171 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000 7172 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000 7173 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000 7174 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000 7175 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000 7176 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100 7177 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001 7178 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000 7179 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000 7180 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000 7181 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000 7182 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000 7183 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000 7184 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000 7185 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000 7186 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000 7187 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000 7188 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000 7189 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000 7190 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000 7191 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000 7192 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000 7193 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100 7194 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000 7195 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000 7196 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000 7197 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000 7198 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000 7199 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000 7200 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000 7201 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010 7202 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000 7203 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000 7204 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000 7205 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000 7206 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000 7207 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000 7208 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000 7209 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001 7210 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000 7211 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000 7212 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000 7213 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000 7214 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000 7215 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000 7216 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000 7217 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000 7218 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000 7219 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000 7220 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000 7221 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000 7222 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000 7223 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000 7224 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000 7225 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000 7226 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000 7227 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000 7228 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000 7229 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000 7230 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000 7231 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000 7232 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000 7233 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100 7234 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000 7235 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000 7236 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000 7237 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010 7238 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000 7239 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000 7240 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000 7241 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001 7242 7243 =item wait 7244 X<wait> 7245 7246 Behaves like the wait(2) system call on your system: it waits for a child 7247 process to terminate and returns the pid of the deceased process, or 7248 C<-1> if there are no child processes. The status is returned in C<$?> 7249 and C<{^CHILD_ERROR_NATIVE}>. 7250 Note that a return value of C<-1> could mean that child processes are 7251 being automatically reaped, as described in L<perlipc>. 7252 7253 =item waitpid PID,FLAGS 7254 X<waitpid> 7255 7256 Waits for a particular child process to terminate and returns the pid of 7257 the deceased process, or C<-1> if there is no such child process. On some 7258 systems, a value of 0 indicates that there are processes still running. 7259 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say 7260 7261 use POSIX ":sys_wait_h"; 7262 #... 7263 do { 7264 $kid = waitpid(-1, WNOHANG); 7265 } while $kid > 0; 7266 7267 then you can do a non-blocking wait for all pending zombie processes. 7268 Non-blocking wait is available on machines supporting either the 7269 waitpid(2) or wait4(2) system calls. However, waiting for a particular 7270 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the 7271 system call by remembering the status values of processes that have 7272 exited but have not been harvested by the Perl script yet.) 7273 7274 Note that on some systems, a return value of C<-1> could mean that child 7275 processes are being automatically reaped. See L<perlipc> for details, 7276 and for other examples. 7277 7278 =item wantarray 7279 X<wantarray> X<context> 7280 7281 Returns true if the context of the currently executing subroutine or 7282 C<eval> is looking for a list value. Returns false if the context is 7283 looking for a scalar. Returns the undefined value if the context is 7284 looking for no value (void context). 7285 7286 return unless defined wantarray; # don't bother doing more 7287 my @a = complex_calculation(); 7288 return wantarray ? @a : "@a"; 7289 7290 C<wantarray()>'s result is unspecified in the top level of a file, 7291 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or 7292 in a C<DESTROY> method. 7293 7294 This function should have been named wantlist() instead. 7295 7296 =item warn LIST 7297 X<warn> X<warning> X<STDERR> 7298 7299 Prints the value of LIST to STDERR. If the last element of LIST does 7300 not end in a newline, it appends the same file/line number text as C<die> 7301 does. 7302 7303 If LIST is empty and C<$@> already contains a value (typically from a 7304 previous eval) that value is used after appending C<"\t...caught"> 7305 to C<$@>. This is useful for staying almost, but not entirely similar to 7306 C<die>. 7307 7308 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used. 7309 7310 No message is printed if there is a C<$SIG{__WARN__}> handler 7311 installed. It is the handler's responsibility to deal with the message 7312 as it sees fit (like, for instance, converting it into a C<die>). Most 7313 handlers must therefore make arrangements to actually display the 7314 warnings that they are not prepared to deal with, by calling C<warn> 7315 again in the handler. Note that this is quite safe and will not 7316 produce an endless loop, since C<__WARN__> hooks are not called from 7317 inside one. 7318 7319 You will find this behavior is slightly different from that of 7320 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can 7321 instead call C<die> again to change it). 7322 7323 Using a C<__WARN__> handler provides a powerful way to silence all 7324 warnings (even the so-called mandatory ones). An example: 7325 7326 # wipe out *all* compile-time warnings 7327 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } 7328 my $foo = 10; 7329 my $foo = 20; # no warning about duplicate my $foo, 7330 # but hey, you asked for it! 7331 # no compile-time or run-time warnings before here 7332 $DOWARN = 1; 7333 7334 # run-time warnings enabled after here 7335 warn "\$foo is alive and $foo!"; # does show up 7336 7337 See L<perlvar> for details on setting C<%SIG> entries, and for more 7338 examples. See the Carp module for other kinds of warnings using its 7339 carp() and cluck() functions. 7340 7341 =item write FILEHANDLE 7342 X<write> 7343 7344 =item write EXPR 7345 7346 =item write 7347 7348 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, 7349 using the format associated with that file. By default the format for 7350 a file is the one having the same name as the filehandle, but the 7351 format for the current output channel (see the C<select> function) may be set 7352 explicitly by assigning the name of the format to the C<$~> variable. 7353 7354 Top of form processing is handled automatically: if there is 7355 insufficient room on the current page for the formatted record, the 7356 page is advanced by writing a form feed, a special top-of-page format 7357 is used to format the new page header, and then the record is written. 7358 By default the top-of-page format is the name of the filehandle with 7359 "_TOP" appended, but it may be dynamically set to the format of your 7360 choice by assigning the name to the C<$^> variable while the filehandle is 7361 selected. The number of lines remaining on the current page is in 7362 variable C<$->, which can be set to C<0> to force a new page. 7363 7364 If FILEHANDLE is unspecified, output goes to the current default output 7365 channel, which starts out as STDOUT but may be changed by the 7366 C<select> operator. If the FILEHANDLE is an EXPR, then the expression 7367 is evaluated and the resulting string is used to look up the name of 7368 the FILEHANDLE at run time. For more on formats, see L<perlform>. 7369 7370 Note that write is I<not> the opposite of C<read>. Unfortunately. 7371 7372 =item y/// 7373 7374 The transliteration operator. Same as C<tr///>. See L<perlop>. 7375 7376 =back
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