1=head1 NAME 2 3perlfunc - Perl builtin functions 4 5=head1 DESCRIPTION 6 7The functions in this section can serve as terms in an expression. 8They fall into two major categories: list operators and named unary 9operators. These differ in their precedence relationship with a 10following comma. (See the precedence table in L<perlop>.) List 11operators take more than one argument, while unary operators can never 12take more than one argument. Thus, a comma terminates the argument of 13a unary operator, but merely separates the arguments of a list 14operator. A unary operator generally provides a scalar context to its 15argument, while a list operator may provide either scalar or list 16contexts for its arguments. If it does both, the scalar arguments will 17be first, and the list argument will follow. (Note that there can ever 18be only one such list argument.) For instance, splice() has three scalar 19arguments followed by a list, whereas gethostbyname() has four scalar 20arguments. 21 22In the syntax descriptions that follow, list operators that expect a 23list (and provide list context for the elements of the list) are shown 24with LIST as an argument. Such a list may consist of any combination 25of scalar arguments or list values; the list values will be included 26in the list as if each individual element were interpolated at that 27point in the list, forming a longer single-dimensional list value. 28Elements of the LIST should be separated by commas. 29 30Any function in the list below may be used either with or without 31parentheses around its arguments. (The syntax descriptions omit the 32parentheses.) If you use the parentheses, the simple (but occasionally 33surprising) rule is this: It I<looks> like a function, therefore it I<is> a 34function, and precedence doesn't matter. Otherwise it's a list 35operator or unary operator, and precedence does matter. And whitespace 36between the function and left parenthesis doesn't count--so you need to 37be careful sometimes: 38 39 print 1+2+4; # Prints 7. 40 print(1+2) + 4; # Prints 3. 41 print (1+2)+4; # Also prints 3! 42 print +(1+2)+4; # Prints 7. 43 print ((1+2)+4); # Prints 7. 44 45If you run Perl with the B<-w> switch it can warn you about this. For 46example, the third line above produces: 47 48 print (...) interpreted as function at - line 1. 49 Useless use of integer addition in void context at - line 1. 50 51A few functions take no arguments at all, and therefore work as neither 52unary nor list operators. These include such functions as C<time> 53and C<endpwent>. For example, C<time+86_400> always means 54C<time() + 86_400>. 55 56For functions that can be used in either a scalar or list context, 57nonabortive failure is generally indicated in a scalar context by 58returning the undefined value, and in a list context by returning the 59null list. 60 61Remember the following important rule: There is B<no rule> that relates 62the behavior of an expression in list context to its behavior in scalar 63context, or vice versa. It might do two totally different things. 64Each operator and function decides which sort of value it would be most 65appropriate to return in scalar context. Some operators return the 66length of the list that would have been returned in list context. Some 67operators return the first value in the list. Some operators return the 68last value in the list. Some operators return a count of successful 69operations. In general, they do what you want, unless you want 70consistency. 71 72An named array in scalar context is quite different from what would at 73first glance appear to be a list in scalar context. You can't get a list 74like C<(1,2,3)> into being in scalar context, because the compiler knows 75the context at compile time. It would generate the scalar comma operator 76there, not the list construction version of the comma. That means it 77was never a list to start with. 78 79In general, functions in Perl that serve as wrappers for system calls 80of the same name (like chown(2), fork(2), closedir(2), etc.) all return 81true when they succeed and C<undef> otherwise, as is usually mentioned 82in the descriptions below. This is different from the C interfaces, 83which return C<-1> on failure. Exceptions to this rule are C<wait>, 84C<waitpid>, and C<syscall>. System calls also set the special C<$!> 85variable on failure. Other functions do not, except accidentally. 86 87=head2 Perl Functions by Category 88 89Here are Perl's functions (including things that look like 90functions, like some keywords and named operators) 91arranged by category. Some functions appear in more 92than one place. 93 94=over 95 96=item Functions for SCALARs or strings 97 98C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>, 99C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>, 100C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///> 101 102=item Regular expressions and pattern matching 103 104C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//> 105 106=item Numeric functions 107 108C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>, 109C<sin>, C<sqrt>, C<srand> 110 111=item Functions for real @ARRAYs 112 113C<pop>, C<push>, C<shift>, C<splice>, C<unshift> 114 115=item Functions for list data 116 117C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack> 118 119=item Functions for real %HASHes 120 121C<delete>, C<each>, C<exists>, C<keys>, C<values> 122 123=item Input and output functions 124 125C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>, 126C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>, 127C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>, 128C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>, 129C<warn>, C<write> 130 131=item Functions for fixed length data or records 132 133C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec> 134 135=item Functions for filehandles, files, or directories 136 137C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>, 138C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, 139C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, 140C<unlink>, C<utime> 141 142=item Keywords related to the control flow of your perl program 143 144C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>, 145C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray> 146 147=item Keywords related to scoping 148 149C<caller>, C<import>, C<local>, C<my>, C<package>, C<use> 150 151=item Miscellaneous functions 152 153C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>, 154C<scalar>, C<undef>, C<wantarray> 155 156=item Functions for processes and process groups 157 158C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>, 159C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>, 160C<times>, C<wait>, C<waitpid> 161 162=item Keywords related to perl modules 163 164C<do>, C<import>, C<no>, C<package>, C<require>, C<use> 165 166=item Keywords related to classes and object-orientedness 167 168C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>, 169C<untie>, C<use> 170 171=item Low-level socket functions 172 173C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>, 174C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>, 175C<socket>, C<socketpair> 176 177=item System V interprocess communication functions 178 179C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>, 180C<shmctl>, C<shmget>, C<shmread>, C<shmwrite> 181 182=item Fetching user and group info 183 184C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>, 185C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>, 186C<getpwuid>, C<setgrent>, C<setpwent> 187 188=item Fetching network info 189 190C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>, 191C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, 192C<getprotobyname>, C<getprotobynumber>, C<getprotoent>, 193C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>, 194C<setnetent>, C<setprotoent>, C<setservent> 195 196=item Time-related functions 197 198C<gmtime>, C<localtime>, C<time>, C<times> 199 200=item Functions new in perl5 201 202C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>, 203C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>, 204C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>, 205C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use> 206 207* - C<sub> was a keyword in perl4, but in perl5 it is an 208operator, which can be used in expressions. 209 210=item Functions obsoleted in perl5 211 212C<dbmclose>, C<dbmopen> 213 214=back 215 216=head2 Portability 217 218Perl was born in Unix and can therefore access all common Unix 219system calls. In non-Unix environments, the functionality of some 220Unix system calls may not be available, or details of the available 221functionality may differ slightly. The Perl functions affected 222by this are: 223 224C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>, 225C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>, 226C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>, 227C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>, 228C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, 229C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>, 230C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>, 231C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>, 232C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>, 233C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>, 234C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>, 235C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>, 236C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>, 237C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>, 238C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>, 239C<utime>, C<wait>, C<waitpid> 240 241For more information about the portability of these functions, see 242L<perlport> and other available platform-specific documentation. 243 244=head2 Alphabetical Listing of Perl Functions 245 246=over 8 247 248=item I<-X> FILEHANDLE 249 250=item I<-X> EXPR 251 252=item I<-X> 253 254A file test, where X is one of the letters listed below. This unary 255operator takes one argument, either a filename or a filehandle, and 256tests the associated file to see if something is true about it. If the 257argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN. 258Unless otherwise documented, it returns C<1> for true and C<''> for false, or 259the undefined value if the file doesn't exist. Despite the funny 260names, precedence is the same as any other named unary operator, and 261the argument may be parenthesized like any other unary operator. The 262operator may be any of: 263X<-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> 264X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C> 265 266 -r File is readable by effective uid/gid. 267 -w File is writable by effective uid/gid. 268 -x File is executable by effective uid/gid. 269 -o File is owned by effective uid. 270 271 -R File is readable by real uid/gid. 272 -W File is writable by real uid/gid. 273 -X File is executable by real uid/gid. 274 -O File is owned by real uid. 275 276 -e File exists. 277 -z File has zero size. 278 -s File has nonzero size (returns size). 279 280 -f File is a plain file. 281 -d File is a directory. 282 -l File is a symbolic link. 283 -p File is a named pipe (FIFO), or Filehandle is a pipe. 284 -S File is a socket. 285 -b File is a block special file. 286 -c File is a character special file. 287 -t Filehandle is opened to a tty. 288 289 -u File has setuid bit set. 290 -g File has setgid bit set. 291 -k File has sticky bit set. 292 293 -T File is an ASCII text file. 294 -B File is a "binary" file (opposite of -T). 295 296 -M Age of file in days when script started. 297 -A Same for access time. 298 -C Same for inode change time. 299 300Example: 301 302 while (<>) { 303 chop; 304 next unless -f $_; # ignore specials 305 #... 306 } 307 308The interpretation of the file permission operators C<-r>, C<-R>, 309C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode 310of the file and the uids and gids of the user. There may be other 311reasons you can't actually read, write, or execute the file. Such 312reasons may be for example network filesystem access controls, ACLs 313(access control lists), read-only filesystems, and unrecognized 314executable formats. 315 316Also note that, for the superuser on the local filesystems, the C<-r>, 317C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1 318if any execute bit is set in the mode. Scripts run by the superuser 319may thus need to do a stat() to determine the actual mode of the file, 320or temporarily set their effective uid to something else. 321 322If you are using ACLs, there is a pragma called C<filetest> that may 323produce more accurate results than the bare stat() mode bits. 324When under the C<use filetest 'access'> the above-mentioned filetests 325will test whether the permission can (not) be granted using the 326access() family of system calls. Also note that the C<-x> and C<-X> may 327under this pragma return true even if there are no execute permission 328bits set (nor any extra execute permission ACLs). This strangeness is 329due to the underlying system calls' definitions. Read the 330documentation for the C<filetest> pragma for more information. 331 332Note that C<-s/a/b/> does not do a negated substitution. Saying 333C<-exp($foo)> still works as expected, however--only single letters 334following a minus are interpreted as file tests. 335 336The C<-T> and C<-B> switches work as follows. The first block or so of the 337file is examined for odd characters such as strange control codes or 338characters with the high bit set. If too many strange characters (>30%) 339are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file 340containing null in the first block is considered a binary file. If C<-T> 341or C<-B> is used on a filehandle, the current stdio buffer is examined 342rather than the first block. Both C<-T> and C<-B> return true on a null 343file, or a file at EOF when testing a filehandle. Because you have to 344read a file to do the C<-T> test, on most occasions you want to use a C<-f> 345against the file first, as in C<next unless -f $file && -T $file>. 346 347If any of the file tests (or either the C<stat> or C<lstat> operators) are given 348the special filehandle consisting of a solitary underline, then the stat 349structure of the previous file test (or stat operator) is used, saving 350a system call. (This doesn't work with C<-t>, and you need to remember 351that lstat() and C<-l> will leave values in the stat structure for the 352symbolic link, not the real file.) Example: 353 354 print "Can do.\n" if -r $a || -w _ || -x _; 355 356 stat($filename); 357 print "Readable\n" if -r _; 358 print "Writable\n" if -w _; 359 print "Executable\n" if -x _; 360 print "Setuid\n" if -u _; 361 print "Setgid\n" if -g _; 362 print "Sticky\n" if -k _; 363 print "Text\n" if -T _; 364 print "Binary\n" if -B _; 365 366=item abs VALUE 367 368=item abs 369 370Returns the absolute value of its argument. 371If VALUE is omitted, uses C<$_>. 372 373=item accept NEWSOCKET,GENERICSOCKET 374 375Accepts an incoming socket connect, just as the accept(2) system call 376does. Returns the packed address if it succeeded, false otherwise. 377See the example in L<perlipc/"Sockets: Client/Server Communication">. 378 379On systems that support a close-on-exec flag on files, the flag will 380be set for the newly opened file descriptor, as determined by the 381value of $^F. See L<perlvar/$^F>. 382 383=item alarm SECONDS 384 385=item alarm 386 387Arranges to have a SIGALRM delivered to this process after the 388specified number of seconds have elapsed. If SECONDS is not specified, 389the value stored in C<$_> is used. (On some machines, 390unfortunately, the elapsed time may be up to one second less than you 391specified because of how seconds are counted.) Only one timer may be 392counting at once. Each call disables the previous timer, and an 393argument of C<0> may be supplied to cancel the previous timer without 394starting a new one. The returned value is the amount of time remaining 395on the previous timer. 396 397For delays of finer granularity than one second, you may use Perl's 398four-argument version of select() leaving the first three arguments 399undefined, or you might be able to use the C<syscall> interface to 400access setitimer(2) if your system supports it. The Time::HiRes module 401from CPAN may also prove useful. 402 403It is usually a mistake to intermix C<alarm> and C<sleep> calls. 404(C<sleep> may be internally implemented in your system with C<alarm>) 405 406If you want to use C<alarm> to time out a system call you need to use an 407C<eval>/C<die> pair. You can't rely on the alarm causing the system call to 408fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to 409restart system calls on some systems. Using C<eval>/C<die> always works, 410modulo the caveats given in L<perlipc/"Signals">. 411 412 eval { 413 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required 414 alarm $timeout; 415 $nread = sysread SOCKET, $buffer, $size; 416 alarm 0; 417 }; 418 if ($@) { 419 die unless $@ eq "alarm\n"; # propagate unexpected errors 420 # timed out 421 } 422 else { 423 # didn't 424 } 425 426=item atan2 Y,X 427 428Returns the arctangent of Y/X in the range -PI to PI. 429 430For the tangent operation, you may use the C<Math::Trig::tan> 431function, or use the familiar relation: 432 433 sub tan { sin($_[0]) / cos($_[0]) } 434 435=item bind SOCKET,NAME 436 437Binds a network address to a socket, just as the bind system call 438does. Returns true if it succeeded, false otherwise. NAME should be a 439packed address of the appropriate type for the socket. See the examples in 440L<perlipc/"Sockets: Client/Server Communication">. 441 442=item binmode FILEHANDLE, DISCIPLINE 443 444=item binmode FILEHANDLE 445 446Arranges for FILEHANDLE to be read or written in "binary" or "text" mode 447on systems where the run-time libraries distinguish between binary and 448text files. If FILEHANDLE is an expression, the value is taken as the 449name of the filehandle. DISCIPLINE can be either of C<":raw"> for 450binary mode or C<":crlf"> for "text" mode. If the DISCIPLINE is 451omitted, it defaults to C<":raw">. 452 453binmode() should be called after open() but before any I/O is done on 454the filehandle. 455 456On many systems binmode() currently has no effect, but in future, it 457will be extended to support user-defined input and output disciplines. 458On some systems binmode() is necessary when you're not working with a 459text file. For the sake of portability it is a good idea to always use 460it when appropriate, and to never use it when it isn't appropriate. 461 462In other words: Regardless of platform, use binmode() on binary 463files, and do not use binmode() on text files. 464 465The C<open> pragma can be used to establish default disciplines. 466See L<open>. 467 468The operating system, device drivers, C libraries, and Perl run-time 469system all work together to let the programmer treat a single 470character (C<\n>) as the line terminator, irrespective of the external 471representation. On many operating systems, the native text file 472representation matches the internal representation, but on some 473platforms the external representation of C<\n> is made up of more than 474one character. 475 476Mac OS and all variants of Unix use a single character to end each line 477in the external representation of text (even though that single 478character is not necessarily the same across these platforms). 479Consequently binmode() has no effect on these operating systems. In 480other systems like VMS, MS-DOS and the various flavors of MS-Windows 481your program sees a C<\n> as a simple C<\cJ>, but what's stored in text 482files are the two characters C<\cM\cJ>. That means that, if you don't 483use binmode() on these systems, C<\cM\cJ> sequences on disk will be 484converted to C<\n> on input, and any C<\n> in your program will be 485converted back to C<\cM\cJ> on output. This is what you want for text 486files, but it can be disastrous for binary files. 487 488Another consequence of using binmode() (on some systems) is that 489special end-of-file markers will be seen as part of the data stream. 490For systems from the Microsoft family this means that if your binary 491data contains C<\cZ>, the I/O subsystem will ragard it as the end of 492the file, unless you use binmode(). 493 494binmode() is not only important for readline() and print() operations, 495but also when using read(), seek(), sysread(), syswrite() and tell() 496(see L<perlport> for more details). See the C<$/> and C<$\> variables 497in L<perlvar> for how to manually set your input and output 498line-termination sequences. 499 500=item bless REF,CLASSNAME 501 502=item bless REF 503 504This function tells the thingy referenced by REF that it is now an object 505in the CLASSNAME package. If CLASSNAME is omitted, the current package 506is used. Because a C<bless> is often the last thing in a constructor, 507it returns the reference for convenience. Always use the two-argument 508version if the function doing the blessing might be inherited by a 509derived class. See L<perltoot> and L<perlobj> for more about the blessing 510(and blessings) of objects. 511 512Consider always blessing objects in CLASSNAMEs that are mixed case. 513Namespaces with all lowercase names are considered reserved for 514Perl pragmata. Builtin types have all uppercase names, so to prevent 515confusion, you may wish to avoid such package names as well. Make sure 516that CLASSNAME is a true value. 517 518See L<perlmod/"Perl Modules">. 519 520=item caller EXPR 521 522=item caller 523 524Returns the context of the current subroutine call. In scalar context, 525returns the caller's package name if there is a caller, that is, if 526we're in a subroutine or C<eval> or C<require>, and the undefined value 527otherwise. In list context, returns 528 529 ($package, $filename, $line) = caller; 530 531With EXPR, it returns some extra information that the debugger uses to 532print a stack trace. The value of EXPR indicates how many call frames 533to go back before the current one. 534 535 ($package, $filename, $line, $subroutine, $hasargs, 536 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i); 537 538Here $subroutine may be C<(eval)> if the frame is not a subroutine 539call, but an C<eval>. In such a case additional elements $evaltext and 540C<$is_require> are set: C<$is_require> is true if the frame is created by a 541C<require> or C<use> statement, $evaltext contains the text of the 542C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement, 543$filename is C<(eval)>, but $evaltext is undefined. (Note also that 544each C<use> statement creates a C<require> frame inside an C<eval EXPR>) 545frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller 546was compiled with. The C<$hints> and C<$bitmask> values are subject to 547change between versions of Perl, and are not meant for external use. 548 549Furthermore, when called from within the DB package, caller returns more 550detailed information: it sets the list variable C<@DB::args> to be the 551arguments with which the subroutine was invoked. 552 553Be aware that the optimizer might have optimized call frames away before 554C<caller> had a chance to get the information. That means that C<caller(N)> 555might not return information about the call frame you expect it do, for 556C<< N > 1 >>. In particular, C<@DB::args> might have information from the 557previous time C<caller> was called. 558 559=item chdir EXPR 560 561Changes the working directory to EXPR, if possible. If EXPR is omitted, 562changes to the directory specified by C<$ENV{HOME}>, if set; if not, 563changes to the directory specified by C<$ENV{LOGDIR}>. If neither is 564set, C<chdir> does nothing. It returns true upon success, false 565otherwise. See the example under C<die>. 566 567=item chmod LIST 568 569Changes the permissions of a list of files. The first element of the 570list must be the numerical mode, which should probably be an octal 571number, and which definitely should I<not> a string of octal digits: 572C<0644> is okay, C<'0644'> is not. Returns the number of files 573successfully changed. See also L</oct>, if all you have is a string. 574 575 $cnt = chmod 0755, 'foo', 'bar'; 576 chmod 0755, @executables; 577 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to 578 # --w----r-T 579 $mode = '0644'; chmod oct($mode), 'foo'; # this is better 580 $mode = 0644; chmod $mode, 'foo'; # this is best 581 582You can also import the symbolic C<S_I*> constants from the Fcntl 583module: 584 585 use Fcntl ':mode'; 586 587 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; 588 # This is identical to the chmod 0755 of the above example. 589 590=item chomp VARIABLE 591 592=item chomp LIST 593 594=item chomp 595 596This safer version of L</chop> removes any trailing string 597that corresponds to the current value of C<$/> (also known as 598$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total 599number of characters removed from all its arguments. It's often used to 600remove the newline from the end of an input record when you're worried 601that the final record may be missing its newline. When in paragraph 602mode (C<$/ = "">), it removes all trailing newlines from the string. 603When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is 604a reference to an integer or the like, see L<perlvar>) chomp() won't 605remove anything. 606If VARIABLE is omitted, it chomps C<$_>. Example: 607 608 while (<>) { 609 chomp; # avoid \n on last field 610 @array = split(/:/); 611 # ... 612 } 613 614You can actually chomp anything that's an lvalue, including an assignment: 615 616 chomp($cwd = `pwd`); 617 chomp($answer = <STDIN>); 618 619If you chomp a list, each element is chomped, and the total number of 620characters removed is returned. 621 622=item chop VARIABLE 623 624=item chop LIST 625 626=item chop 627 628Chops off the last character of a string and returns the character 629chopped. It's used primarily to remove the newline from the end of an 630input record, but is much more efficient than C<s/\n//> because it neither 631scans nor copies the string. If VARIABLE is omitted, chops C<$_>. 632Example: 633 634 while (<>) { 635 chop; # avoid \n on last field 636 @array = split(/:/); 637 #... 638 } 639 640You can actually chop anything that's an lvalue, including an assignment: 641 642 chop($cwd = `pwd`); 643 chop($answer = <STDIN>); 644 645If you chop a list, each element is chopped. Only the value of the 646last C<chop> is returned. 647 648Note that C<chop> returns the last character. To return all but the last 649character, use C<substr($string, 0, -1)>. 650 651=item chown LIST 652 653Changes the owner (and group) of a list of files. The first two 654elements of the list must be the I<numeric> uid and gid, in that 655order. A value of -1 in either position is interpreted by most 656systems to leave that value unchanged. Returns the number of files 657successfully changed. 658 659 $cnt = chown $uid, $gid, 'foo', 'bar'; 660 chown $uid, $gid, @filenames; 661 662Here's an example that looks up nonnumeric uids in the passwd file: 663 664 print "User: "; 665 chomp($user = <STDIN>); 666 print "Files: "; 667 chomp($pattern = <STDIN>); 668 669 ($login,$pass,$uid,$gid) = getpwnam($user) 670 or die "$user not in passwd file"; 671 672 @ary = glob($pattern); # expand filenames 673 chown $uid, $gid, @ary; 674 675On most systems, you are not allowed to change the ownership of the 676file unless you're the superuser, although you should be able to change 677the group to any of your secondary groups. On insecure systems, these 678restrictions may be relaxed, but this is not a portable assumption. 679On POSIX systems, you can detect this condition this way: 680 681 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED); 682 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED); 683 684=item chr NUMBER 685 686=item chr 687 688Returns the character represented by that NUMBER in the character set. 689For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and 690chr(0x263a) is a Unicode smiley face (but only within the scope of 691a C<use utf8>). For the reverse, use L</ord>. 692See L<utf8> for more about Unicode. 693 694If NUMBER is omitted, uses C<$_>. 695 696=item chroot FILENAME 697 698=item chroot 699 700This function works like the system call by the same name: it makes the 701named directory the new root directory for all further pathnames that 702begin with a C</> by your process and all its children. (It doesn't 703change your current working directory, which is unaffected.) For security 704reasons, this call is restricted to the superuser. If FILENAME is 705omitted, does a C<chroot> to C<$_>. 706 707=item close FILEHANDLE 708 709=item close 710 711Closes the file or pipe associated with the file handle, returning true 712only if stdio successfully flushes buffers and closes the system file 713descriptor. Closes the currently selected filehandle if the argument 714is omitted. 715 716You don't have to close FILEHANDLE if you are immediately going to do 717another C<open> on it, because C<open> will close it for you. (See 718C<open>.) However, an explicit C<close> on an input file resets the line 719counter (C<$.>), while the implicit close done by C<open> does not. 720 721If the file handle came from a piped open C<close> will additionally 722return false if one of the other system calls involved fails or if the 723program exits with non-zero status. (If the only problem was that the 724program exited non-zero C<$!> will be set to C<0>.) Closing a pipe 725also waits for the process executing on the pipe to complete, in case you 726want to look at the output of the pipe afterwards, and 727implicitly puts the exit status value of that command into C<$?>. 728 729Prematurely closing the read end of a pipe (i.e. before the process 730writing to it at the other end has closed it) will result in a 731SIGPIPE being delivered to the writer. If the other end can't 732handle that, be sure to read all the data before closing the pipe. 733 734Example: 735 736 open(OUTPUT, '|sort >foo') # pipe to sort 737 or die "Can't start sort: $!"; 738 #... # print stuff to output 739 close OUTPUT # wait for sort to finish 740 or warn $! ? "Error closing sort pipe: $!" 741 : "Exit status $? from sort"; 742 open(INPUT, 'foo') # get sort's results 743 or die "Can't open 'foo' for input: $!"; 744 745FILEHANDLE may be an expression whose value can be used as an indirect 746filehandle, usually the real filehandle name. 747 748=item closedir DIRHANDLE 749 750Closes a directory opened by C<opendir> and returns the success of that 751system call. 752 753DIRHANDLE may be an expression whose value can be used as an indirect 754dirhandle, usually the real dirhandle name. 755 756=item connect SOCKET,NAME 757 758Attempts to connect to a remote socket, just as the connect system call 759does. Returns true if it succeeded, false otherwise. NAME should be a 760packed address of the appropriate type for the socket. See the examples in 761L<perlipc/"Sockets: Client/Server Communication">. 762 763=item continue BLOCK 764 765Actually a flow control statement rather than a function. If there is a 766C<continue> BLOCK attached to a BLOCK (typically in a C<while> or 767C<foreach>), it is always executed just before the conditional is about to 768be evaluated again, just like the third part of a C<for> loop in C. Thus 769it can be used to increment a loop variable, even when the loop has been 770continued via the C<next> statement (which is similar to the C C<continue> 771statement). 772 773C<last>, C<next>, or C<redo> may appear within a C<continue> 774block. C<last> and C<redo> will behave as if they had been executed within 775the main block. So will C<next>, but since it will execute a C<continue> 776block, it may be more entertaining. 777 778 while (EXPR) { 779 ### redo always comes here 780 do_something; 781 } continue { 782 ### next always comes here 783 do_something_else; 784 # then back the top to re-check EXPR 785 } 786 ### last always comes here 787 788Omitting the C<continue> section is semantically equivalent to using an 789empty one, logically enough. In that case, C<next> goes directly back 790to check the condition at the top of the loop. 791 792=item cos EXPR 793 794Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, 795takes cosine of C<$_>. 796 797For the inverse cosine operation, you may use the C<Math::Trig::acos()> 798function, or use this relation: 799 800 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } 801 802=item crypt PLAINTEXT,SALT 803 804Encrypts a string exactly like the crypt(3) function in the C library 805(assuming that you actually have a version there that has not been 806extirpated as a potential munition). This can prove useful for checking 807the password file for lousy passwords, amongst other things. Only the 808guys wearing white hats should do this. 809 810Note that C<crypt> is intended to be a one-way function, much like breaking 811eggs to make an omelette. There is no (known) corresponding decrypt 812function. As a result, this function isn't all that useful for 813cryptography. (For that, see your nearby CPAN mirror.) 814 815When verifying an existing encrypted string you should use the encrypted 816text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This 817allows your code to work with the standard C<crypt> and with more 818exotic implementations. When choosing a new salt create a random two 819character string whose characters come from the set C<[./0-9A-Za-z]> 820(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). 821 822Here's an example that makes sure that whoever runs this program knows 823their own password: 824 825 $pwd = (getpwuid($<))[1]; 826 827 system "stty -echo"; 828 print "Password: "; 829 chomp($word = <STDIN>); 830 print "\n"; 831 system "stty echo"; 832 833 if (crypt($word, $pwd) ne $pwd) { 834 die "Sorry...\n"; 835 } else { 836 print "ok\n"; 837 } 838 839Of course, typing in your own password to whoever asks you 840for it is unwise. 841 842The L<crypt> function is unsuitable for encrypting large quantities 843of data, not least of all because you can't get the information 844back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories 845on your favorite CPAN mirror for a slew of potentially useful 846modules. 847 848=item dbmclose HASH 849 850[This function has been largely superseded by the C<untie> function.] 851 852Breaks the binding between a DBM file and a hash. 853 854=item dbmopen HASH,DBNAME,MASK 855 856[This function has been largely superseded by the C<tie> function.] 857 858This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a 859hash. HASH is the name of the hash. (Unlike normal C<open>, the first 860argument is I<not> a filehandle, even though it looks like one). DBNAME 861is the name of the database (without the F<.dir> or F<.pag> extension if 862any). If the database does not exist, it is created with protection 863specified by MASK (as modified by the C<umask>). If your system supports 864only the older DBM functions, you may perform only one C<dbmopen> in your 865program. In older versions of Perl, if your system had neither DBM nor 866ndbm, calling C<dbmopen> produced a fatal error; it now falls back to 867sdbm(3). 868 869If you don't have write access to the DBM file, you can only read hash 870variables, not set them. If you want to test whether you can write, 871either use file tests or try setting a dummy hash entry inside an C<eval>, 872which will trap the error. 873 874Note that functions such as C<keys> and C<values> may return huge lists 875when used on large DBM files. You may prefer to use the C<each> 876function to iterate over large DBM files. Example: 877 878 # print out history file offsets 879 dbmopen(%HIST,'/usr/lib/news/history',0666); 880 while (($key,$val) = each %HIST) { 881 print $key, ' = ', unpack('L',$val), "\n"; 882 } 883 dbmclose(%HIST); 884 885See also L<AnyDBM_File> for a more general description of the pros and 886cons of the various dbm approaches, as well as L<DB_File> for a particularly 887rich implementation. 888 889You can control which DBM library you use by loading that library 890before you call dbmopen(): 891 892 use DB_File; 893 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db") 894 or die "Can't open netscape history file: $!"; 895 896=item defined EXPR 897 898=item defined 899 900Returns a Boolean value telling whether EXPR has a value other than 901the undefined value C<undef>. If EXPR is not present, C<$_> will be 902checked. 903 904Many operations return C<undef> to indicate failure, end of file, 905system error, uninitialized variable, and other exceptional 906conditions. This function allows you to distinguish C<undef> from 907other values. (A simple Boolean test will not distinguish among 908C<undef>, zero, the empty string, and C<"0">, which are all equally 909false.) Note that since C<undef> is a valid scalar, its presence 910doesn't I<necessarily> indicate an exceptional condition: C<pop> 911returns C<undef> when its argument is an empty array, I<or> when the 912element to return happens to be C<undef>. 913 914You may also use C<defined(&func)> to check whether subroutine C<&func> 915has ever been defined. The return value is unaffected by any forward 916declarations of C<&foo>. 917 918Use of C<defined> on aggregates (hashes and arrays) is deprecated. It 919used to report whether memory for that aggregate has ever been 920allocated. This behavior may disappear in future versions of Perl. 921You should instead use a simple test for size: 922 923 if (@an_array) { print "has array elements\n" } 924 if (%a_hash) { print "has hash members\n" } 925 926When used on a hash element, it tells you whether the value is defined, 927not whether the key exists in the hash. Use L</exists> for the latter 928purpose. 929 930Examples: 931 932 print if defined $switch{'D'}; 933 print "$val\n" while defined($val = pop(@ary)); 934 die "Can't readlink $sym: $!" 935 unless defined($value = readlink $sym); 936 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } 937 $debugging = 0 unless defined $debugging; 938 939Note: Many folks tend to overuse C<defined>, and then are surprised to 940discover that the number C<0> and C<""> (the zero-length string) are, in fact, 941defined values. For example, if you say 942 943 "ab" =~ /a(.*)b/; 944 945The pattern match succeeds, and C<$1> is defined, despite the fact that it 946matched "nothing". But it didn't really match nothing--rather, it 947matched something that happened to be zero characters long. This is all 948very above-board and honest. When a function returns an undefined value, 949it's an admission that it couldn't give you an honest answer. So you 950should use C<defined> only when you're questioning the integrity of what 951you're trying to do. At other times, a simple comparison to C<0> or C<""> is 952what you want. 953 954See also L</undef>, L</exists>, L</ref>. 955 956=item delete EXPR 957 958Given an expression that specifies a hash element, array element, hash slice, 959or array slice, deletes the specified element(s) from the hash or array. 960In the case of an array, if the array elements happen to be at the end, 961the size of the array will shrink to the highest element that tests 962true for exists() (or 0 if no such element exists). 963 964Returns each element so deleted or the undefined value if there was no such 965element. Deleting from C<$ENV{}> modifies the environment. Deleting from 966a hash tied to a DBM file deletes the entry from the DBM file. Deleting 967from a C<tie>d hash or array may not necessarily return anything. 968 969Deleting an array element effectively returns that position of the array 970to its initial, uninitialized state. Subsequently testing for the same 971element with exists() will return false. Note that deleting array 972elements in the middle of an array will not shift the index of the ones 973after them down--use splice() for that. See L</exists>. 974 975The following (inefficiently) deletes all the values of %HASH and @ARRAY: 976 977 foreach $key (keys %HASH) { 978 delete $HASH{$key}; 979 } 980 981 foreach $index (0 .. $#ARRAY) { 982 delete $ARRAY[$index]; 983 } 984 985And so do these: 986 987 delete @HASH{keys %HASH}; 988 989 delete @ARRAY[0 .. $#ARRAY]; 990 991But both of these are slower than just assigning the empty list 992or undefining %HASH or @ARRAY: 993 994 %HASH = (); # completely empty %HASH 995 undef %HASH; # forget %HASH ever existed 996 997 @ARRAY = (); # completely empty @ARRAY 998 undef @ARRAY; # forget @ARRAY ever existed 999 1000Note that the EXPR can be arbitrarily complicated as long as the final 1001operation is a hash element, array element, hash slice, or array slice 1002lookup: 1003 1004 delete $ref->[$x][$y]{$key}; 1005 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; 1006 1007 delete $ref->[$x][$y][$index]; 1008 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices]; 1009 1010=item die LIST 1011 1012Outside an C<eval>, prints the value of LIST to C<STDERR> and 1013exits with the current value of C<$!> (errno). If C<$!> is C<0>, 1014exits with the value of C<<< ($? >> 8) >>> (backtick `command` 1015status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside 1016an C<eval(),> the error message is stuffed into C<$@> and the 1017C<eval> is terminated with the undefined value. This makes 1018C<die> the way to raise an exception. 1019 1020Equivalent examples: 1021 1022 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; 1023 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" 1024 1025If the value of EXPR does not end in a newline, the current script line 1026number and input line number (if any) are also printed, and a newline 1027is supplied. Note that the "input line number" (also known as "chunk") 1028is subject to whatever notion of "line" happens to be currently in 1029effect, and is also available as the special variable C<$.>. 1030See L<perlvar/"$/"> and L<perlvar/"$.">. 1031 1032Hint: sometimes appending C<", stopped"> to your message 1033will cause it to make better sense when the string C<"at foo line 123"> is 1034appended. Suppose you are running script "canasta". 1035 1036 die "/etc/games is no good"; 1037 die "/etc/games is no good, stopped"; 1038 1039produce, respectively 1040 1041 /etc/games is no good at canasta line 123. 1042 /etc/games is no good, stopped at canasta line 123. 1043 1044See also exit(), warn(), and the Carp module. 1045 1046If LIST is empty and C<$@> already contains a value (typically from a 1047previous eval) that value is reused after appending C<"\t...propagated">. 1048This is useful for propagating exceptions: 1049 1050 eval { ... }; 1051 die unless $@ =~ /Expected exception/; 1052 1053If C<$@> is empty then the string C<"Died"> is used. 1054 1055die() can also be called with a reference argument. If this happens to be 1056trapped within an eval(), $@ contains the reference. This behavior permits 1057a more elaborate exception handling implementation using objects that 1058maintain arbitary state about the nature of the exception. Such a scheme 1059is sometimes preferable to matching particular string values of $@ using 1060regular expressions. Here's an example: 1061 1062 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; 1063 if ($@) { 1064 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) { 1065 # handle Some::Module::Exception 1066 } 1067 else { 1068 # handle all other possible exceptions 1069 } 1070 } 1071 1072Because perl will stringify uncaught exception messages before displaying 1073them, you may want to overload stringification operations on such custom 1074exception objects. See L<overload> for details about that. 1075 1076You can arrange for a callback to be run just before the C<die> 1077does its deed, by setting the C<$SIG{__DIE__}> hook. The associated 1078handler will be called with the error text and can change the error 1079message, if it sees fit, by calling C<die> again. See 1080L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and 1081L<"eval BLOCK"> for some examples. Although this feature was meant 1082to be run only right before your program was to exit, this is not 1083currently the case--the C<$SIG{__DIE__}> hook is currently called 1084even inside eval()ed blocks/strings! If one wants the hook to do 1085nothing in such situations, put 1086 1087 die @_ if $^S; 1088 1089as the first line of the handler (see L<perlvar/$^S>). Because 1090this promotes strange action at a distance, this counterintuitive 1091behavior may be fixed in a future release. 1092 1093=item do BLOCK 1094 1095Not really a function. Returns the value of the last command in the 1096sequence of commands indicated by BLOCK. When modified by a loop 1097modifier, executes the BLOCK once before testing the loop condition. 1098(On other statements the loop modifiers test the conditional first.) 1099 1100C<do BLOCK> does I<not> count as a loop, so the loop control statements 1101C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. 1102See L<perlsyn> for alternative strategies. 1103 1104=item do SUBROUTINE(LIST) 1105 1106A deprecated form of subroutine call. See L<perlsub>. 1107 1108=item do EXPR 1109 1110Uses the value of EXPR as a filename and executes the contents of the 1111file as a Perl script. Its primary use is to include subroutines 1112from a Perl subroutine library. 1113 1114 do 'stat.pl'; 1115 1116is just like 1117 1118 scalar eval `cat stat.pl`; 1119 1120except that it's more efficient and concise, keeps track of the current 1121filename for error messages, searches the @INC libraries, and updates 1122C<%INC> if the file is found. See L<perlvar/Predefined Names> for these 1123variables. It also differs in that code evaluated with C<do FILENAME> 1124cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the 1125same, however, in that it does reparse the file every time you call it, 1126so you probably don't want to do this inside a loop. 1127 1128If C<do> cannot read the file, it returns undef and sets C<$!> to the 1129error. If C<do> can read the file but cannot compile it, it 1130returns undef and sets an error message in C<$@>. If the file is 1131successfully compiled, C<do> returns the value of the last expression 1132evaluated. 1133 1134Note that inclusion of library modules is better done with the 1135C<use> and C<require> operators, which also do automatic error checking 1136and raise an exception if there's a problem. 1137 1138You might like to use C<do> to read in a program configuration 1139file. Manual error checking can be done this way: 1140 1141 # read in config files: system first, then user 1142 for $file ("/share/prog/defaults.rc", 1143 "$ENV{HOME}/.someprogrc") 1144 { 1145 unless ($return = do $file) { 1146 warn "couldn't parse $file: $@" if $@; 1147 warn "couldn't do $file: $!" unless defined $return; 1148 warn "couldn't run $file" unless $return; 1149 } 1150 } 1151 1152=item dump LABEL 1153 1154=item dump 1155 1156This function causes an immediate core dump. See also the B<-u> 1157command-line switch in L<perlrun>, which does the same thing. 1158Primarily this is so that you can use the B<undump> program (not 1159supplied) to turn your core dump into an executable binary after 1160having initialized all your variables at the beginning of the 1161program. When the new binary is executed it will begin by executing 1162a C<goto LABEL> (with all the restrictions that C<goto> suffers). 1163Think of it as a goto with an intervening core dump and reincarnation. 1164If C<LABEL> is omitted, restarts the program from the top. 1165 1166B<WARNING>: Any files opened at the time of the dump will I<not> 1167be open any more when the program is reincarnated, with possible 1168resulting confusion on the part of Perl. 1169 1170This function is now largely obsolete, partly because it's very 1171hard to convert a core file into an executable, and because the 1172real compiler backends for generating portable bytecode and compilable 1173C code have superseded it. 1174 1175If you're looking to use L<dump> to speed up your program, consider 1176generating bytecode or native C code as described in L<perlcc>. If 1177you're just trying to accelerate a CGI script, consider using the 1178C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI. 1179You might also consider autoloading or selfloading, which at least 1180make your program I<appear> to run faster. 1181 1182=item each HASH 1183 1184When called in list context, returns a 2-element list consisting of the 1185key and value for the next element of a hash, so that you can iterate over 1186it. When called in scalar context, returns the key for only the "next" 1187element in the hash. 1188 1189Entries are returned in an apparently random order. The actual random 1190order is subject to change in future versions of perl, but it is guaranteed 1191to be in the same order as either the C<keys> or C<values> function 1192would produce on the same (unmodified) hash. 1193 1194When the hash is entirely read, a null array is returned in list context 1195(which when assigned produces a false (C<0>) value), and C<undef> in 1196scalar context. The next call to C<each> after that will start iterating 1197again. There is a single iterator for each hash, shared by all C<each>, 1198C<keys>, and C<values> function calls in the program; it can be reset by 1199reading all the elements from the hash, or by evaluating C<keys HASH> or 1200C<values HASH>. If you add or delete elements of a hash while you're 1201iterating over it, you may get entries skipped or duplicated, so don't. 1202 1203The following prints out your environment like the printenv(1) program, 1204only in a different order: 1205 1206 while (($key,$value) = each %ENV) { 1207 print "$key=$value\n"; 1208 } 1209 1210See also C<keys>, C<values> and C<sort>. 1211 1212=item eof FILEHANDLE 1213 1214=item eof () 1215 1216=item eof 1217 1218Returns 1 if the next read on FILEHANDLE will return end of file, or if 1219FILEHANDLE is not open. FILEHANDLE may be an expression whose value 1220gives the real filehandle. (Note that this function actually 1221reads a character and then C<ungetc>s it, so isn't very useful in an 1222interactive context.) Do not read from a terminal file (or call 1223C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such 1224as terminals may lose the end-of-file condition if you do. 1225 1226An C<eof> without an argument uses the last file read. Using C<eof()> 1227with empty parentheses is very different. It refers to the pseudo file 1228formed from the files listed on the command line and accessed via the 1229C<< <> >> operator. Since C<< <> >> isn't explicitly opened, 1230as a normal filehandle is, an C<eof()> before C<< <> >> has been 1231used will cause C<@ARGV> to be examined to determine if input is 1232available. 1233 1234In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to 1235detect the end of each file, C<eof()> will only detect the end of the 1236last file. Examples: 1237 1238 # reset line numbering on each input file 1239 while (<>) { 1240 next if /^\s*#/; # skip comments 1241 print "$.\t$_"; 1242 } continue { 1243 close ARGV if eof; # Not eof()! 1244 } 1245 1246 # insert dashes just before last line of last file 1247 while (<>) { 1248 if (eof()) { # check for end of current file 1249 print "--------------\n"; 1250 close(ARGV); # close or last; is needed if we 1251 # are reading from the terminal 1252 } 1253 print; 1254 } 1255 1256Practical hint: you almost never need to use C<eof> in Perl, because the 1257input operators typically return C<undef> when they run out of data, or if 1258there was an error. 1259 1260=item eval EXPR 1261 1262=item eval BLOCK 1263 1264In the first form, the return value of EXPR is parsed and executed as if it 1265were a little Perl program. The value of the expression (which is itself 1266determined within scalar context) is first parsed, and if there weren't any 1267errors, executed in the context of the current Perl program, so that any 1268variable settings or subroutine and format definitions remain afterwards. 1269Note that the value is parsed every time the eval executes. If EXPR is 1270omitted, evaluates C<$_>. This form is typically used to delay parsing 1271and subsequent execution of the text of EXPR until run time. 1272 1273In the second form, the code within the BLOCK is parsed only once--at the 1274same time the code surrounding the eval itself was parsed--and executed 1275within the context of the current Perl program. This form is typically 1276used to trap exceptions more efficiently than the first (see below), while 1277also providing the benefit of checking the code within BLOCK at compile 1278time. 1279 1280The final semicolon, if any, may be omitted from the value of EXPR or within 1281the BLOCK. 1282 1283In both forms, the value returned is the value of the last expression 1284evaluated inside the mini-program; a return statement may be also used, just 1285as with subroutines. The expression providing the return value is evaluated 1286in void, scalar, or list context, depending on the context of the eval itself. 1287See L</wantarray> for more on how the evaluation context can be determined. 1288 1289If there is a syntax error or runtime error, or a C<die> statement is 1290executed, an undefined value is returned by C<eval>, and C<$@> is set to the 1291error message. If there was no error, C<$@> is guaranteed to be a null 1292string. Beware that using C<eval> neither silences perl from printing 1293warnings to STDERR, nor does it stuff the text of warning messages into C<$@>. 1294To do either of those, you have to use the C<$SIG{__WARN__}> facility. See 1295L</warn> and L<perlvar>. 1296 1297Note that, because C<eval> traps otherwise-fatal errors, it is useful for 1298determining whether a particular feature (such as C<socket> or C<symlink>) 1299is implemented. It is also Perl's exception trapping mechanism, where 1300the die operator is used to raise exceptions. 1301 1302If the code to be executed doesn't vary, you may use the eval-BLOCK 1303form to trap run-time errors without incurring the penalty of 1304recompiling each time. The error, if any, is still returned in C<$@>. 1305Examples: 1306 1307 # make divide-by-zero nonfatal 1308 eval { $answer = $a / $b; }; warn $@ if $@; 1309 1310 # same thing, but less efficient 1311 eval '$answer = $a / $b'; warn $@ if $@; 1312 1313 # a compile-time error 1314 eval { $answer = }; # WRONG 1315 1316 # a run-time error 1317 eval '$answer ='; # sets $@ 1318 1319Due to the current arguably broken state of C<__DIE__> hooks, when using 1320the C<eval{}> form as an exception trap in libraries, you may wish not 1321to trigger any C<__DIE__> hooks that user code may have installed. 1322You can use the C<local $SIG{__DIE__}> construct for this purpose, 1323as shown in this example: 1324 1325 # a very private exception trap for divide-by-zero 1326 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; 1327 warn $@ if $@; 1328 1329This is especially significant, given that C<__DIE__> hooks can call 1330C<die> again, which has the effect of changing their error messages: 1331 1332 # __DIE__ hooks may modify error messages 1333 { 1334 local $SIG{'__DIE__'} = 1335 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; 1336 eval { die "foo lives here" }; 1337 print $@ if $@; # prints "bar lives here" 1338 } 1339 1340Because this promotes action at a distance, this counterintuitive behavior 1341may be fixed in a future release. 1342 1343With an C<eval>, you should be especially careful to remember what's 1344being looked at when: 1345 1346 eval $x; # CASE 1 1347 eval "$x"; # CASE 2 1348 1349 eval '$x'; # CASE 3 1350 eval { $x }; # CASE 4 1351 1352 eval "\$$x++"; # CASE 5 1353 $$x++; # CASE 6 1354 1355Cases 1 and 2 above behave identically: they run the code contained in 1356the variable $x. (Although case 2 has misleading double quotes making 1357the reader wonder what else might be happening (nothing is).) Cases 3 1358and 4 likewise behave in the same way: they run the code C<'$x'>, which 1359does nothing but return the value of $x. (Case 4 is preferred for 1360purely visual reasons, but it also has the advantage of compiling at 1361compile-time instead of at run-time.) Case 5 is a place where 1362normally you I<would> like to use double quotes, except that in this 1363particular situation, you can just use symbolic references instead, as 1364in case 6. 1365 1366C<eval BLOCK> does I<not> count as a loop, so the loop control statements 1367C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. 1368 1369=item exec LIST 1370 1371=item exec PROGRAM LIST 1372 1373The C<exec> function executes a system command I<and never returns>-- 1374use C<system> instead of C<exec> if you want it to return. It fails and 1375returns false only if the command does not exist I<and> it is executed 1376directly instead of via your system's command shell (see below). 1377 1378Since it's a common mistake to use C<exec> instead of C<system>, Perl 1379warns you if there is a following statement which isn't C<die>, C<warn>, 1380or C<exit> (if C<-w> is set - but you always do that). If you 1381I<really> want to follow an C<exec> with some other statement, you 1382can use one of these styles to avoid the warning: 1383 1384 exec ('foo') or print STDERR "couldn't exec foo: $!"; 1385 { exec ('foo') }; print STDERR "couldn't exec foo: $!"; 1386 1387If there is more than one argument in LIST, or if LIST is an array 1388with more than one value, calls execvp(3) with the arguments in LIST. 1389If there is only one scalar argument or an array with one element in it, 1390the argument is checked for shell metacharacters, and if there are any, 1391the entire argument is passed to the system's command shell for parsing 1392(this is C</bin/sh -c> on Unix platforms, but varies on other platforms). 1393If there are no shell metacharacters in the argument, it is split into 1394words and passed directly to C<execvp>, which is more efficient. 1395Examples: 1396 1397 exec '/bin/echo', 'Your arguments are: ', @ARGV; 1398 exec "sort $outfile | uniq"; 1399 1400If you don't really want to execute the first argument, but want to lie 1401to the program you are executing about its own name, you can specify 1402the program you actually want to run as an "indirect object" (without a 1403comma) in front of the LIST. (This always forces interpretation of the 1404LIST as a multivalued list, even if there is only a single scalar in 1405the list.) Example: 1406 1407 $shell = '/bin/csh'; 1408 exec $shell '-sh'; # pretend it's a login shell 1409 1410or, more directly, 1411 1412 exec {'/bin/csh'} '-sh'; # pretend it's a login shell 1413 1414When the arguments get executed via the system shell, results will 1415be subject to its quirks and capabilities. See L<perlop/"`STRING`"> 1416for details. 1417 1418Using an indirect object with C<exec> or C<system> is also more 1419secure. This usage (which also works fine with system()) forces 1420interpretation of the arguments as a multivalued list, even if the 1421list had just one argument. That way you're safe from the shell 1422expanding wildcards or splitting up words with whitespace in them. 1423 1424 @args = ( "echo surprise" ); 1425 1426 exec @args; # subject to shell escapes 1427 # if @args == 1 1428 exec { $args[0] } @args; # safe even with one-arg list 1429 1430The first version, the one without the indirect object, ran the I<echo> 1431program, passing it C<"surprise"> an argument. The second version 1432didn't--it tried to run a program literally called I<"echo surprise">, 1433didn't find it, and set C<$?> to a non-zero value indicating failure. 1434 1435Beginning with v5.6.0, Perl will attempt to flush all files opened for 1436output before the exec, but this may not be supported on some platforms 1437(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH 1438in English) or call the C<autoflush()> method of C<IO::Handle> on any 1439open handles in order to avoid lost output. 1440 1441Note that C<exec> will not call your C<END> blocks, nor will it call 1442any C<DESTROY> methods in your objects. 1443 1444=item exists EXPR 1445 1446Given an expression that specifies a hash element or array element, 1447returns true if the specified element in the hash or array has ever 1448been initialized, even if the corresponding value is undefined. The 1449element is not autovivified if it doesn't exist. 1450 1451 print "Exists\n" if exists $hash{$key}; 1452 print "Defined\n" if defined $hash{$key}; 1453 print "True\n" if $hash{$key}; 1454 1455 print "Exists\n" if exists $array[$index]; 1456 print "Defined\n" if defined $array[$index]; 1457 print "True\n" if $array[$index]; 1458 1459A hash or array element can be true only if it's defined, and defined if 1460it exists, but the reverse doesn't necessarily hold true. 1461 1462Given an expression that specifies the name of a subroutine, 1463returns true if the specified subroutine has ever been declared, even 1464if it is undefined. Mentioning a subroutine name for exists or defined 1465does not count as declaring it. 1466 1467 print "Exists\n" if exists &subroutine; 1468 print "Defined\n" if defined &subroutine; 1469 1470Note that the EXPR can be arbitrarily complicated as long as the final 1471operation is a hash or array key lookup or subroutine name: 1472 1473 if (exists $ref->{A}->{B}->{$key}) { } 1474 if (exists $hash{A}{B}{$key}) { } 1475 1476 if (exists $ref->{A}->{B}->[$ix]) { } 1477 if (exists $hash{A}{B}[$ix]) { } 1478 1479 if (exists &{$ref->{A}{B}{$key}}) { } 1480 1481Although the deepest nested array or hash will not spring into existence 1482just because its existence was tested, any intervening ones will. 1483Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring 1484into existence due to the existence test for the $key element above. 1485This happens anywhere the arrow operator is used, including even: 1486 1487 undef $ref; 1488 if (exists $ref->{"Some key"}) { } 1489 print $ref; # prints HASH(0x80d3d5c) 1490 1491This surprising autovivification in what does not at first--or even 1492second--glance appear to be an lvalue context may be fixed in a future 1493release. 1494 1495See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics 1496on how exists() acts when used on a pseudo-hash. 1497 1498Use of a subroutine call, rather than a subroutine name, as an argument 1499to exists() is an error. 1500 1501 exists ⊂ # OK 1502 exists &sub(); # Error 1503 1504=item exit EXPR 1505 1506Evaluates EXPR and exits immediately with that value. Example: 1507 1508 $ans = <STDIN>; 1509 exit 0 if $ans =~ /^[Xx]/; 1510 1511See also C<die>. If EXPR is omitted, exits with C<0> status. The only 1512universally recognized values for EXPR are C<0> for success and C<1> 1513for error; other values are subject to interpretation depending on the 1514environment in which the Perl program is running. For example, exiting 151569 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause 1516the mailer to return the item undelivered, but that's not true everywhere. 1517 1518Don't use C<exit> to abort a subroutine if there's any chance that 1519someone might want to trap whatever error happened. Use C<die> instead, 1520which can be trapped by an C<eval>. 1521 1522The exit() function does not always exit immediately. It calls any 1523defined C<END> routines first, but these C<END> routines may not 1524themselves abort the exit. Likewise any object destructors that need to 1525be called are called before the real exit. If this is a problem, you 1526can call C<POSIX:_exit($status)> to avoid END and destructor processing. 1527See L<perlmod> for details. 1528 1529=item exp EXPR 1530 1531=item exp 1532 1533Returns I<e> (the natural logarithm base) to the power of EXPR. 1534If EXPR is omitted, gives C<exp($_)>. 1535 1536=item fcntl FILEHANDLE,FUNCTION,SCALAR 1537 1538Implements the fcntl(2) function. You'll probably have to say 1539 1540 use Fcntl; 1541 1542first to get the correct constant definitions. Argument processing and 1543value return works just like C<ioctl> below. 1544For example: 1545 1546 use Fcntl; 1547 fcntl($filehandle, F_GETFL, $packed_return_buffer) 1548 or die "can't fcntl F_GETFL: $!"; 1549 1550You don't have to check for C<defined> on the return from C<fnctl>. 1551Like C<ioctl>, it maps a C<0> return from the system call into 1552C<"0 but true"> in Perl. This string is true in boolean context and C<0> 1553in numeric context. It is also exempt from the normal B<-w> warnings 1554on improper numeric conversions. 1555 1556Note that C<fcntl> will produce a fatal error if used on a machine that 1557doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) 1558manpage to learn what functions are available on your system. 1559 1560=item fileno FILEHANDLE 1561 1562Returns the file descriptor for a filehandle, or undefined if the 1563filehandle is not open. This is mainly useful for constructing 1564bitmaps for C<select> and low-level POSIX tty-handling operations. 1565If FILEHANDLE is an expression, the value is taken as an indirect 1566filehandle, generally its name. 1567 1568You can use this to find out whether two handles refer to the 1569same underlying descriptor: 1570 1571 if (fileno(THIS) == fileno(THAT)) { 1572 print "THIS and THAT are dups\n"; 1573 } 1574 1575=item flock FILEHANDLE,OPERATION 1576 1577Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true 1578for success, false on failure. Produces a fatal error if used on a 1579machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). 1580C<flock> is Perl's portable file locking interface, although it locks 1581only entire files, not records. 1582 1583Two potentially non-obvious but traditional C<flock> semantics are 1584that it waits indefinitely until the lock is granted, and that its locks 1585B<merely advisory>. Such discretionary locks are more flexible, but offer 1586fewer guarantees. This means that files locked with C<flock> may be 1587modified by programs that do not also use C<flock>. See L<perlport>, 1588your port's specific documentation, or your system-specific local manpages 1589for details. It's best to assume traditional behavior if you're writing 1590portable programs. (But if you're not, you should as always feel perfectly 1591free to write for your own system's idiosyncrasies (sometimes called 1592"features"). Slavish adherence to portability concerns shouldn't get 1593in the way of your getting your job done.) 1594 1595OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with 1596LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but 1597you can use the symbolic names if you import them from the Fcntl module, 1598either individually, or as a group using the ':flock' tag. LOCK_SH 1599requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN 1600releases a previously requested lock. If LOCK_NB is bitwise-or'ed with 1601LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking 1602waiting for the lock (check the return status to see if you got it). 1603 1604To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE 1605before locking or unlocking it. 1606 1607Note that the emulation built with lockf(3) doesn't provide shared 1608locks, and it requires that FILEHANDLE be open with write intent. These 1609are the semantics that lockf(3) implements. Most if not all systems 1610implement lockf(3) in terms of fcntl(2) locking, though, so the 1611differing semantics shouldn't bite too many people. 1612 1613Note also that some versions of C<flock> cannot lock things over the 1614network; you would need to use the more system-specific C<fcntl> for 1615that. If you like you can force Perl to ignore your system's flock(2) 1616function, and so provide its own fcntl(2)-based emulation, by passing 1617the switch C<-Ud_flock> to the F<Configure> program when you configure 1618perl. 1619 1620Here's a mailbox appender for BSD systems. 1621 1622 use Fcntl ':flock'; # import LOCK_* constants 1623 1624 sub lock { 1625 flock(MBOX,LOCK_EX); 1626 # and, in case someone appended 1627 # while we were waiting... 1628 seek(MBOX, 0, 2); 1629 } 1630 1631 sub unlock { 1632 flock(MBOX,LOCK_UN); 1633 } 1634 1635 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}") 1636 or die "Can't open mailbox: $!"; 1637 1638 lock(); 1639 print MBOX $msg,"\n\n"; 1640 unlock(); 1641 1642On systems that support a real flock(), locks are inherited across fork() 1643calls, whereas those that must resort to the more capricious fcntl() 1644function lose the locks, making it harder to write servers. 1645 1646See also L<DB_File> for other flock() examples. 1647 1648=item fork 1649 1650Does a fork(2) system call to create a new process running the 1651same program at the same point. It returns the child pid to the 1652parent process, C<0> to the child process, or C<undef> if the fork is 1653unsuccessful. File descriptors (and sometimes locks on those descriptors) 1654are shared, while everything else is copied. On most systems supporting 1655fork(), great care has gone into making it extremely efficient (for 1656example, using copy-on-write technology on data pages), making it the 1657dominant paradigm for multitasking over the last few decades. 1658 1659Beginning with v5.6.0, Perl will attempt to flush all files opened for 1660output before forking the child process, but this may not be supported 1661on some platforms (see L<perlport>). To be safe, you may need to set 1662C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of 1663C<IO::Handle> on any open handles in order to avoid duplicate output. 1664 1665If you C<fork> without ever waiting on your children, you will 1666accumulate zombies. On some systems, you can avoid this by setting 1667C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of 1668forking and reaping moribund children. 1669 1670Note that if your forked child inherits system file descriptors like 1671STDIN and STDOUT that are actually connected by a pipe or socket, even 1672if you exit, then the remote server (such as, say, a CGI script or a 1673backgrounded job launched from a remote shell) won't think you're done. 1674You should reopen those to F</dev/null> if it's any issue. 1675 1676=item format 1677 1678Declare a picture format for use by the C<write> function. For 1679example: 1680 1681 format Something = 1682 Test: @<<<<<<<< @||||| @>>>>> 1683 $str, $%, '$' . int($num) 1684 . 1685 1686 $str = "widget"; 1687 $num = $cost/$quantity; 1688 $~ = 'Something'; 1689 write; 1690 1691See L<perlform> for many details and examples. 1692 1693=item formline PICTURE,LIST 1694 1695This is an internal function used by C<format>s, though you may call it, 1696too. It formats (see L<perlform>) a list of values according to the 1697contents of PICTURE, placing the output into the format output 1698accumulator, C<$^A> (or C<$ACCUMULATOR> in English). 1699Eventually, when a C<write> is done, the contents of 1700C<$^A> are written to some filehandle, but you could also read C<$^A> 1701yourself and then set C<$^A> back to C<"">. Note that a format typically 1702does one C<formline> per line of form, but the C<formline> function itself 1703doesn't care how many newlines are embedded in the PICTURE. This means 1704that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line. 1705You may therefore need to use multiple formlines to implement a single 1706record format, just like the format compiler. 1707 1708Be careful if you put double quotes around the picture, because an C<@> 1709character may be taken to mean the beginning of an array name. 1710C<formline> always returns true. See L<perlform> for other examples. 1711 1712=item getc FILEHANDLE 1713 1714=item getc 1715 1716Returns the next character from the input file attached to FILEHANDLE, 1717or the undefined value at end of file, or if there was an error. 1718If FILEHANDLE is omitted, reads from STDIN. This is not particularly 1719efficient. However, it cannot be used by itself to fetch single 1720characters without waiting for the user to hit enter. For that, try 1721something more like: 1722 1723 if ($BSD_STYLE) { 1724 system "stty cbreak </dev/tty >/dev/tty 2>&1"; 1725 } 1726 else { 1727 system "stty", '-icanon', 'eol', "\001"; 1728 } 1729 1730 $key = getc(STDIN); 1731 1732 if ($BSD_STYLE) { 1733 system "stty -cbreak </dev/tty >/dev/tty 2>&1"; 1734 } 1735 else { 1736 system "stty", 'icanon', 'eol', '^@'; # ASCII null 1737 } 1738 print "\n"; 1739 1740Determination of whether $BSD_STYLE should be set 1741is left as an exercise to the reader. 1742 1743The C<POSIX::getattr> function can do this more portably on 1744systems purporting POSIX compliance. See also the C<Term::ReadKey> 1745module from your nearest CPAN site; details on CPAN can be found on 1746L<perlmodlib/CPAN>. 1747 1748=item getlogin 1749 1750Implements the C library function of the same name, which on most 1751systems returns the current login from F</etc/utmp>, if any. If null, 1752use C<getpwuid>. 1753 1754 $login = getlogin || getpwuid($<) || "Kilroy"; 1755 1756Do not consider C<getlogin> for authentication: it is not as 1757secure as C<getpwuid>. 1758 1759=item getpeername SOCKET 1760 1761Returns the packed sockaddr address of other end of the SOCKET connection. 1762 1763 use Socket; 1764 $hersockaddr = getpeername(SOCK); 1765 ($port, $iaddr) = sockaddr_in($hersockaddr); 1766 $herhostname = gethostbyaddr($iaddr, AF_INET); 1767 $herstraddr = inet_ntoa($iaddr); 1768 1769=item getpgrp PID 1770 1771Returns the current process group for the specified PID. Use 1772a PID of C<0> to get the current process group for the 1773current process. Will raise an exception if used on a machine that 1774doesn't implement getpgrp(2). If PID is omitted, returns process 1775group of current process. Note that the POSIX version of C<getpgrp> 1776does not accept a PID argument, so only C<PID==0> is truly portable. 1777 1778=item getppid 1779 1780Returns the process id of the parent process. 1781 1782=item getpriority WHICH,WHO 1783 1784Returns the current priority for a process, a process group, or a user. 1785(See L<getpriority(2)>.) Will raise a fatal exception if used on a 1786machine that doesn't implement getpriority(2). 1787 1788=item getpwnam NAME 1789 1790=item getgrnam NAME 1791 1792=item gethostbyname NAME 1793 1794=item getnetbyname NAME 1795 1796=item getprotobyname NAME 1797 1798=item getpwuid UID 1799 1800=item getgrgid GID 1801 1802=item getservbyname NAME,PROTO 1803 1804=item gethostbyaddr ADDR,ADDRTYPE 1805 1806=item getnetbyaddr ADDR,ADDRTYPE 1807 1808=item getprotobynumber NUMBER 1809 1810=item getservbyport PORT,PROTO 1811 1812=item getpwent 1813 1814=item getgrent 1815 1816=item gethostent 1817 1818=item getnetent 1819 1820=item getprotoent 1821 1822=item getservent 1823 1824=item setpwent 1825 1826=item setgrent 1827 1828=item sethostent STAYOPEN 1829 1830=item setnetent STAYOPEN 1831 1832=item setprotoent STAYOPEN 1833 1834=item setservent STAYOPEN 1835 1836=item endpwent 1837 1838=item endgrent 1839 1840=item endhostent 1841 1842=item endnetent 1843 1844=item endprotoent 1845 1846=item endservent 1847 1848These routines perform the same functions as their counterparts in the 1849system library. In list context, the return values from the 1850various get routines are as follows: 1851 1852 ($name,$passwd,$uid,$gid, 1853 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw* 1854 ($name,$passwd,$gid,$members) = getgr* 1855 ($name,$aliases,$addrtype,$length,@addrs) = gethost* 1856 ($name,$aliases,$addrtype,$net) = getnet* 1857 ($name,$aliases,$proto) = getproto* 1858 ($name,$aliases,$port,$proto) = getserv* 1859 1860(If the entry doesn't exist you get a null list.) 1861 1862The exact meaning of the $gcos field varies but it usually contains 1863the real name of the user (as opposed to the login name) and other 1864information pertaining to the user. Beware, however, that in many 1865system users are able to change this information and therefore it 1866cannot be trusted and therefore the $gcos is is tainted (see 1867L<perlsec>). The $passwd and $shell, user's encrypted password and 1868login shell, are also tainted, because of the same reason. 1869 1870In scalar context, you get the name, unless the function was a 1871lookup by name, in which case you get the other thing, whatever it is. 1872(If the entry doesn't exist you get the undefined value.) For example: 1873 1874 $uid = getpwnam($name); 1875 $name = getpwuid($num); 1876 $name = getpwent(); 1877 $gid = getgrnam($name); 1878 $name = getgrgid($num; 1879 $name = getgrent(); 1880 #etc. 1881 1882In I<getpw*()> the fields $quota, $comment, and $expire are special 1883cases in the sense that in many systems they are unsupported. If the 1884$quota is unsupported, it is an empty scalar. If it is supported, it 1885usually encodes the disk quota. If the $comment field is unsupported, 1886it is an empty scalar. If it is supported it usually encodes some 1887administrative comment about the user. In some systems the $quota 1888field may be $change or $age, fields that have to do with password 1889aging. In some systems the $comment field may be $class. The $expire 1890field, if present, encodes the expiration period of the account or the 1891password. For the availability and the exact meaning of these fields 1892in your system, please consult your getpwnam(3) documentation and your 1893F<pwd.h> file. You can also find out from within Perl what your 1894$quota and $comment fields mean and whether you have the $expire field 1895by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>, 1896C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password 1897files are only supported if your vendor has implemented them in the 1898intuitive fashion that calling the regular C library routines gets the 1899shadow versions if you're running under privilege. Those that 1900incorrectly implement a separate library call are not supported. 1901 1902The $members value returned by I<getgr*()> is a space separated list of 1903the login names of the members of the group. 1904 1905For the I<gethost*()> functions, if the C<h_errno> variable is supported in 1906C, it will be returned to you via C<$?> if the function call fails. The 1907C<@addrs> value returned by a successful call is a list of the raw 1908addresses returned by the corresponding system library call. In the 1909Internet domain, each address is four bytes long and you can unpack it 1910by saying something like: 1911 1912 ($a,$b,$c,$d) = unpack('C4',$addr[0]); 1913 1914The Socket library makes this slightly easier: 1915 1916 use Socket; 1917 $iaddr = inet_aton("127.1"); # or whatever address 1918 $name = gethostbyaddr($iaddr, AF_INET); 1919 1920 # or going the other way 1921 $straddr = inet_ntoa($iaddr); 1922 1923If you get tired of remembering which element of the return list 1924contains which return value, by-name interfaces are provided 1925in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>, 1926C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>, 1927and C<User::grent>. These override the normal built-ins, supplying 1928versions that return objects with the appropriate names 1929for each field. For example: 1930 1931 use File::stat; 1932 use User::pwent; 1933 $is_his = (stat($filename)->uid == pwent($whoever)->uid); 1934 1935Even though it looks like they're the same method calls (uid), 1936they aren't, because a C<File::stat> object is different from 1937a C<User::pwent> object. 1938 1939=item getsockname SOCKET 1940 1941Returns the packed sockaddr address of this end of the SOCKET connection, 1942in case you don't know the address because you have several different 1943IPs that the connection might have come in on. 1944 1945 use Socket; 1946 $mysockaddr = getsockname(SOCK); 1947 ($port, $myaddr) = sockaddr_in($mysockaddr); 1948 printf "Connect to %s [%s]\n", 1949 scalar gethostbyaddr($myaddr, AF_INET), 1950 inet_ntoa($myaddr); 1951 1952=item getsockopt SOCKET,LEVEL,OPTNAME 1953 1954Returns the socket option requested, or undef if there is an error. 1955 1956=item glob EXPR 1957 1958=item glob 1959 1960Returns the value of EXPR with filename expansions such as the 1961standard Unix shell F</bin/csh> would do. This is the internal function 1962implementing the C<< <*.c> >> operator, but you can use it directly. 1963If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is 1964discussed in more detail in L<perlop/"I/O Operators">. 1965 1966Beginning with v5.6.0, this operator is implemented using the standard 1967C<File::Glob> extension. See L<File::Glob> for details. 1968 1969=item gmtime EXPR 1970 1971Converts a time as returned by the time function to a 8-element list 1972with the time localized for the standard Greenwich time zone. 1973Typically used as follows: 1974 1975 # 0 1 2 3 4 5 6 7 1976 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) = 1977 gmtime(time); 1978 1979All list elements are numeric, and come straight out of the C `struct 1980tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the 1981specified time. $mday is the day of the month, and $mon is the month 1982itself, in the range C<0..11> with 0 indicating January and 11 1983indicating December. $year is the number of years since 1900. That 1984is, $year is C<123> in year 2023. $wday is the day of the week, with 19850 indicating Sunday and 3 indicating Wednesday. $yday is the day of 1986the year, in the range C<1..365> (or C<1..366> in leap years.) 1987 1988Note that the $year element is I<not> simply the last two digits of 1989the year. If you assume it is, then you create non-Y2K-compliant 1990programs--and you wouldn't want to do that, would you? 1991 1992The proper way to get a complete 4-digit year is simply: 1993 1994 $year += 1900; 1995 1996And to get the last two digits of the year (e.g., '01' in 2001) do: 1997 1998 $year = sprintf("%02d", $year % 100); 1999 2000If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>). 2001 2002In scalar context, C<gmtime()> returns the ctime(3) value: 2003 2004 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994" 2005 2006Also see the C<timegm> function provided by the C<Time::Local> module, 2007and the strftime(3) function available via the POSIX module. 2008 2009This scalar value is B<not> locale dependent (see L<perllocale>), but 2010is instead a Perl builtin. Also see the C<Time::Local> module, and the 2011strftime(3) and mktime(3) functions available via the POSIX module. To 2012get somewhat similar but locale dependent date strings, set up your 2013locale environment variables appropriately (please see L<perllocale>) 2014and try for example: 2015 2016 use POSIX qw(strftime); 2017 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; 2018 2019Note that the C<%a> and C<%b> escapes, which represent the short forms 2020of the day of the week and the month of the year, may not necessarily 2021be three characters wide in all locales. 2022 2023=item goto LABEL 2024 2025=item goto EXPR 2026 2027=item goto &NAME 2028 2029The C<goto-LABEL> form finds the statement labeled with LABEL and resumes 2030execution there. It may not be used to go into any construct that 2031requires initialization, such as a subroutine or a C<foreach> loop. It 2032also can't be used to go into a construct that is optimized away, 2033or to get out of a block or subroutine given to C<sort>. 2034It can be used to go almost anywhere else within the dynamic scope, 2035including out of subroutines, but it's usually better to use some other 2036construct such as C<last> or C<die>. The author of Perl has never felt the 2037need to use this form of C<goto> (in Perl, that is--C is another matter). 2038 2039The C<goto-EXPR> form expects a label name, whose scope will be resolved 2040dynamically. This allows for computed C<goto>s per FORTRAN, but isn't 2041necessarily recommended if you're optimizing for maintainability: 2042 2043 goto ("FOO", "BAR", "GLARCH")[$i]; 2044 2045The C<goto-&NAME> form is quite different from the other forms of C<goto>. 2046In fact, it isn't a goto in the normal sense at all, and doesn't have 2047the stigma associated with other gotos. Instead, it 2048substitutes a call to the named subroutine for the currently running 2049subroutine. This is used by C<AUTOLOAD> subroutines that wish to load 2050another subroutine and then pretend that the other subroutine had been 2051called in the first place (except that any modifications to C<@_> 2052in the current subroutine are propagated to the other subroutine.) 2053After the C<goto>, not even C<caller> will be able to tell that this 2054routine was called first. 2055 2056NAME needn't be the name of a subroutine; it can be a scalar variable 2057containing a code reference, or a block which evaluates to a code 2058reference. 2059 2060=item grep BLOCK LIST 2061 2062=item grep EXPR,LIST 2063 2064This is similar in spirit to, but not the same as, grep(1) and its 2065relatives. In particular, it is not limited to using regular expressions. 2066 2067Evaluates the BLOCK or EXPR for each element of LIST (locally setting 2068C<$_> to each element) and returns the list value consisting of those 2069elements for which the expression evaluated to true. In scalar 2070context, returns the number of times the expression was true. 2071 2072 @foo = grep(!/^#/, @bar); # weed out comments 2073 2074or equivalently, 2075 2076 @foo = grep {!/^#/} @bar; # weed out comments 2077 2078Note that, because C<$_> is a reference into the list value, it can 2079be used to modify the elements of the array. While this is useful and 2080supported, it can cause bizarre results if the LIST is not a named array. 2081Similarly, grep returns aliases into the original list, much as a for 2082loop's index variable aliases the list elements. That is, modifying an 2083element of a list returned by grep (for example, in a C<foreach>, C<map> 2084or another C<grep>) actually modifies the element in the original list. 2085This is usually something to be avoided when writing clear code. 2086 2087See also L</map> for a list composed of the results of the BLOCK or EXPR. 2088 2089=item hex EXPR 2090 2091=item hex 2092 2093Interprets EXPR as a hex string and returns the corresponding value. 2094(To convert strings that might start with either 0, 0x, or 0b, see 2095L</oct>.) If EXPR is omitted, uses C<$_>. 2096 2097 print hex '0xAf'; # prints '175' 2098 print hex 'aF'; # same 2099 2100Hex strings may only represent integers. Strings that would cause 2101integer overflow trigger a warning. 2102 2103=item import 2104 2105There is no builtin C<import> function. It is just an ordinary 2106method (subroutine) defined (or inherited) by modules that wish to export 2107names to another module. The C<use> function calls the C<import> method 2108for the package used. See also L</use()>, L<perlmod>, and L<Exporter>. 2109 2110=item index STR,SUBSTR,POSITION 2111 2112=item index STR,SUBSTR 2113 2114The index function searches for one string within another, but without 2115the wildcard-like behavior of a full regular-expression pattern match. 2116It returns the position of the first occurrence of SUBSTR in STR at 2117or after POSITION. If POSITION is omitted, starts searching from the 2118beginning of the string. The return value is based at C<0> (or whatever 2119you've set the C<$[> variable to--but don't do that). If the substring 2120is not found, returns one less than the base, ordinarily C<-1>. 2121 2122=item int EXPR 2123 2124=item int 2125 2126Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>. 2127You should not use this function for rounding: one because it truncates 2128towards C<0>, and two because machine representations of floating point 2129numbers can sometimes produce counterintuitive results. For example, 2130C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's 2131because it's really more like -268.99999999999994315658 instead. Usually, 2132the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil> 2133functions will serve you better than will int(). 2134 2135=item ioctl FILEHANDLE,FUNCTION,SCALAR 2136 2137Implements the ioctl(2) function. You'll probably first have to say 2138 2139 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph 2140 2141to get the correct function definitions. If F<ioctl.ph> doesn't 2142exist or doesn't have the correct definitions you'll have to roll your 2143own, based on your C header files such as F<< <sys/ioctl.h> >>. 2144(There is a Perl script called B<h2ph> that comes with the Perl kit that 2145may help you in this, but it's nontrivial.) SCALAR will be read and/or 2146written depending on the FUNCTION--a pointer to the string value of SCALAR 2147will be passed as the third argument of the actual C<ioctl> call. (If SCALAR 2148has no string value but does have a numeric value, that value will be 2149passed rather than a pointer to the string value. To guarantee this to be 2150true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack> 2151functions may be needed to manipulate the values of structures used by 2152C<ioctl>. 2153 2154The return value of C<ioctl> (and C<fcntl>) is as follows: 2155 2156 if OS returns: then Perl returns: 2157 -1 undefined value 2158 0 string "0 but true" 2159 anything else that number 2160 2161Thus Perl returns true on success and false on failure, yet you can 2162still easily determine the actual value returned by the operating 2163system: 2164 2165 $retval = ioctl(...) || -1; 2166 printf "System returned %d\n", $retval; 2167 2168The special string "C<0> but true" is exempt from B<-w> complaints 2169about improper numeric conversions. 2170 2171Here's an example of setting a filehandle named C<REMOTE> to be 2172non-blocking at the system level. You'll have to negotiate C<$|> 2173on your own, though. 2174 2175 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK); 2176 2177 $flags = fcntl(REMOTE, F_GETFL, 0) 2178 or die "Can't get flags for the socket: $!\n"; 2179 2180 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK) 2181 or die "Can't set flags for the socket: $!\n"; 2182 2183=item join EXPR,LIST 2184 2185Joins the separate strings of LIST into a single string with fields 2186separated by the value of EXPR, and returns that new string. Example: 2187 2188 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); 2189 2190Beware that unlike C<split>, C<join> doesn't take a pattern as its 2191first argument. Compare L</split>. 2192 2193=item keys HASH 2194 2195Returns a list consisting of all the keys of the named hash. (In 2196scalar context, returns the number of keys.) The keys are returned in 2197an apparently random order. The actual random order is subject to 2198change in future versions of perl, but it is guaranteed to be the same 2199order as either the C<values> or C<each> function produces (given 2200that the hash has not been modified). As a side effect, it resets 2201HASH's iterator. 2202 2203Here is yet another way to print your environment: 2204 2205 @keys = keys %ENV; 2206 @values = values %ENV; 2207 while (@keys) { 2208 print pop(@keys), '=', pop(@values), "\n"; 2209 } 2210 2211or how about sorted by key: 2212 2213 foreach $key (sort(keys %ENV)) { 2214 print $key, '=', $ENV{$key}, "\n"; 2215 } 2216 2217To sort a hash by value, you'll need to use a C<sort> function. 2218Here's a descending numeric sort of a hash by its values: 2219 2220 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { 2221 printf "%4d %s\n", $hash{$key}, $key; 2222 } 2223 2224As an lvalue C<keys> allows you to increase the number of hash buckets 2225allocated for the given hash. This can gain you a measure of efficiency if 2226you know the hash is going to get big. (This is similar to pre-extending 2227an array by assigning a larger number to $#array.) If you say 2228 2229 keys %hash = 200; 2230 2231then C<%hash> will have at least 200 buckets allocated for it--256 of them, 2232in fact, since it rounds up to the next power of two. These 2233buckets will be retained even if you do C<%hash = ()>, use C<undef 2234%hash> if you want to free the storage while C<%hash> is still in scope. 2235You can't shrink the number of buckets allocated for the hash using 2236C<keys> in this way (but you needn't worry about doing this by accident, 2237as trying has no effect). 2238 2239See also C<each>, C<values> and C<sort>. 2240 2241=item kill SIGNAL, LIST 2242 2243Sends a signal to a list of processes. Returns the number of 2244processes successfully signaled (which is not necessarily the 2245same as the number actually killed). 2246 2247 $cnt = kill 1, $child1, $child2; 2248 kill 9, @goners; 2249 2250If SIGNAL is zero, no signal is sent to the process. This is a 2251useful way to check that the process is alive and hasn't changed 2252its UID. See L<perlport> for notes on the portability of this 2253construct. 2254 2255Unlike in the shell, if SIGNAL is negative, it kills 2256process groups instead of processes. (On System V, a negative I<PROCESS> 2257number will also kill process groups, but that's not portable.) That 2258means you usually want to use positive not negative signals. You may also 2259use a signal name in quotes. See L<perlipc/"Signals"> for details. 2260 2261=item last LABEL 2262 2263=item last 2264 2265The C<last> command is like the C<break> statement in C (as used in 2266loops); it immediately exits the loop in question. If the LABEL is 2267omitted, the command refers to the innermost enclosing loop. The 2268C<continue> block, if any, is not executed: 2269 2270 LINE: while (<STDIN>) { 2271 last LINE if /^$/; # exit when done with header 2272 #... 2273 } 2274 2275C<last> cannot be used to exit a block which returns a value such as 2276C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit 2277a grep() or map() operation. 2278 2279Note that a block by itself is semantically identical to a loop 2280that executes once. Thus C<last> can be used to effect an early 2281exit out of such a block. 2282 2283See also L</continue> for an illustration of how C<last>, C<next>, and 2284C<redo> work. 2285 2286=item lc EXPR 2287 2288=item lc 2289 2290Returns an lowercased version of EXPR. This is the internal function 2291implementing the C<\L> escape in double-quoted strings. 2292Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale> 2293and L<utf8>. 2294 2295If EXPR is omitted, uses C<$_>. 2296 2297=item lcfirst EXPR 2298 2299=item lcfirst 2300 2301Returns the value of EXPR with the first character lowercased. This is 2302the internal function implementing the C<\l> escape in double-quoted strings. 2303Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. 2304 2305If EXPR is omitted, uses C<$_>. 2306 2307=item length EXPR 2308 2309=item length 2310 2311Returns the length in characters of the value of EXPR. If EXPR is 2312omitted, returns length of C<$_>. Note that this cannot be used on 2313an entire array or hash to find out how many elements these have. 2314For that, use C<scalar @array> and C<scalar keys %hash> respectively. 2315 2316=item link OLDFILE,NEWFILE 2317 2318Creates a new filename linked to the old filename. Returns true for 2319success, false otherwise. 2320 2321=item listen SOCKET,QUEUESIZE 2322 2323Does the same thing that the listen system call does. Returns true if 2324it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">. 2325 2326=item local EXPR 2327 2328You really probably want to be using C<my> instead, because C<local> isn't 2329what most people think of as "local". See L<perlsub/"Private Variables 2330via my()"> for details. 2331 2332A local modifies the listed variables to be local to the enclosing 2333block, file, or eval. If more than one value is listed, the list must 2334be placed in parentheses. See L<perlsub/"Temporary Values via local()"> 2335for details, including issues with tied arrays and hashes. 2336 2337=item localtime EXPR 2338 2339Converts a time as returned by the time function to a 9-element list 2340with the time analyzed for the local time zone. Typically used as 2341follows: 2342 2343 # 0 1 2 3 4 5 6 7 8 2344 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = 2345 localtime(time); 2346 2347All list elements are numeric, and come straight out of the C `struct 2348tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the 2349specified time. $mday is the day of the month, and $mon is the month 2350itself, in the range C<0..11> with 0 indicating January and 11 2351indicating December. $year is the number of years since 1900. That 2352is, $year is C<123> in year 2023. $wday is the day of the week, with 23530 indicating Sunday and 3 indicating Wednesday. $yday is the day of 2354the year, in the range C<1..365> (or C<1..366> in leap years.) $isdst 2355is true if the specified time occurs during daylight savings time, 2356false otherwise. 2357 2358Note that the $year element is I<not> simply the last two digits of 2359the year. If you assume it is, then you create non-Y2K-compliant 2360programs--and you wouldn't want to do that, would you? 2361 2362The proper way to get a complete 4-digit year is simply: 2363 2364 $year += 1900; 2365 2366And to get the last two digits of the year (e.g., '01' in 2001) do: 2367 2368 $year = sprintf("%02d", $year % 100); 2369 2370If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>). 2371 2372In scalar context, C<localtime()> returns the ctime(3) value: 2373 2374 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" 2375 2376This scalar value is B<not> locale dependent, see L<perllocale>, but 2377instead a Perl builtin. Also see the C<Time::Local> module 2378(to convert the second, minutes, hours, ... back to seconds since the 2379stroke of midnight the 1st of January 1970, the value returned by 2380time()), and the strftime(3) and mktime(3) functions available via the 2381POSIX module. To get somewhat similar but locale dependent date 2382strings, set up your locale environment variables appropriately 2383(please see L<perllocale>) and try for example: 2384 2385 use POSIX qw(strftime); 2386 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; 2387 2388Note that the C<%a> and C<%b>, the short forms of the day of the week 2389and the month of the year, may not necessarily be three characters wide. 2390 2391=item lock 2392 2393 lock I<THING> 2394 2395This function places an advisory lock on a variable, subroutine, 2396or referenced object contained in I<THING> until the lock goes out 2397of scope. This is a built-in function only if your version of Perl 2398was built with threading enabled, and if you've said C<use Threads>. 2399Otherwise a user-defined function by this name will be called. See 2400L<Thread>. 2401 2402=item log EXPR 2403 2404=item log 2405 2406Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, 2407returns log of C<$_>. To get the log of another base, use basic algebra: 2408The base-N log of a number is equal to the natural log of that number 2409divided by the natural log of N. For example: 2410 2411 sub log10 { 2412 my $n = shift; 2413 return log($n)/log(10); 2414 } 2415 2416See also L</exp> for the inverse operation. 2417 2418=item lstat FILEHANDLE 2419 2420=item lstat EXPR 2421 2422=item lstat 2423 2424Does the same thing as the C<stat> function (including setting the 2425special C<_> filehandle) but stats a symbolic link instead of the file 2426the symbolic link points to. If symbolic links are unimplemented on 2427your system, a normal C<stat> is done. 2428 2429If EXPR is omitted, stats C<$_>. 2430 2431=item m// 2432 2433The match operator. See L<perlop>. 2434 2435=item map BLOCK LIST 2436 2437=item map EXPR,LIST 2438 2439Evaluates the BLOCK or EXPR for each element of LIST (locally setting 2440C<$_> to each element) and returns the list value composed of the 2441results of each such evaluation. In scalar context, returns the 2442total number of elements so generated. Evaluates BLOCK or EXPR in 2443list context, so each element of LIST may produce zero, one, or 2444more elements in the returned value. 2445 2446 @chars = map(chr, @nums); 2447 2448translates a list of numbers to the corresponding characters. And 2449 2450 %hash = map { getkey($_) => $_ } @array; 2451 2452is just a funny way to write 2453 2454 %hash = (); 2455 foreach $_ (@array) { 2456 $hash{getkey($_)} = $_; 2457 } 2458 2459Note that, because C<$_> is a reference into the list value, it can 2460be used to modify the elements of the array. While this is useful and 2461supported, it can cause bizarre results if the LIST is not a named array. 2462Using a regular C<foreach> loop for this purpose would be clearer in 2463most cases. See also L</grep> for an array composed of those items of 2464the original list for which the BLOCK or EXPR evaluates to true. 2465 2466=item mkdir FILENAME,MASK 2467 2468=item mkdir FILENAME 2469 2470Creates the directory specified by FILENAME, with permissions 2471specified by MASK (as modified by C<umask>). If it succeeds it 2472returns true, otherwise it returns false and sets C<$!> (errno). 2473If omitted, MASK defaults to 0777. 2474 2475In general, it is better to create directories with permissive MASK, 2476and let the user modify that with their C<umask>, than it is to supply 2477a restrictive MASK and give the user no way to be more permissive. 2478The exceptions to this rule are when the file or directory should be 2479kept private (mail files, for instance). The perlfunc(1) entry on 2480C<umask> discusses the choice of MASK in more detail. 2481 2482=item msgctl ID,CMD,ARG 2483 2484Calls the System V IPC function msgctl(2). You'll probably have to say 2485 2486 use IPC::SysV; 2487 2488first to get the correct constant definitions. If CMD is C<IPC_STAT>, 2489then ARG must be a variable which will hold the returned C<msqid_ds> 2490structure. Returns like C<ioctl>: the undefined value for error, 2491C<"0 but true"> for zero, or the actual return value otherwise. See also 2492C<IPC::SysV> and C<IPC::Semaphore> documentation. 2493 2494=item msgget KEY,FLAGS 2495 2496Calls the System V IPC function msgget(2). Returns the message queue 2497id, or the undefined value if there is an error. See also C<IPC::SysV> 2498and C<IPC::Msg> documentation. 2499 2500=item msgrcv ID,VAR,SIZE,TYPE,FLAGS 2501 2502Calls the System V IPC function msgrcv to receive a message from 2503message queue ID into variable VAR with a maximum message size of 2504SIZE. Note that when a message is received, the message type as a 2505native long integer will be the first thing in VAR, followed by the 2506actual message. This packing may be opened with C<unpack("l! a*")>. 2507Taints the variable. Returns true if successful, or false if there is 2508an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation. 2509 2510=item msgsnd ID,MSG,FLAGS 2511 2512Calls the System V IPC function msgsnd to send the message MSG to the 2513message queue ID. MSG must begin with the native long integer message 2514type, and be followed by the length of the actual message, and finally 2515the message itself. This kind of packing can be achieved with 2516C<pack("l! a*", $type, $message)>. Returns true if successful, 2517or false if there is an error. See also C<IPC::SysV> 2518and C<IPC::SysV::Msg> documentation. 2519 2520=item my EXPR 2521 2522=item my EXPR : ATTRIBUTES 2523 2524A C<my> declares the listed variables to be local (lexically) to the 2525enclosing block, file, or C<eval>. If 2526more than one value is listed, the list must be placed in parentheses. See 2527L<perlsub/"Private Variables via my()"> for details. 2528 2529=item next LABEL 2530 2531=item next 2532 2533The C<next> command is like the C<continue> statement in C; it starts 2534the next iteration of the loop: 2535 2536 LINE: while (<STDIN>) { 2537 next LINE if /^#/; # discard comments 2538 #... 2539 } 2540 2541Note that if there were a C<continue> block on the above, it would get 2542executed even on discarded lines. If the LABEL is omitted, the command 2543refers to the innermost enclosing loop. 2544 2545C<next> cannot be used to exit a block which returns a value such as 2546C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit 2547a grep() or map() operation. 2548 2549Note that a block by itself is semantically identical to a loop 2550that executes once. Thus C<next> will exit such a block early. 2551 2552See also L</continue> for an illustration of how C<last>, C<next>, and 2553C<redo> work. 2554 2555=item no Module LIST 2556 2557See the L</use> function, which C<no> is the opposite of. 2558 2559=item oct EXPR 2560 2561=item oct 2562 2563Interprets EXPR as an octal string and returns the corresponding 2564value. (If EXPR happens to start off with C<0x>, interprets it as a 2565hex string. If EXPR starts off with C<0b>, it is interpreted as a 2566binary string.) The following will handle decimal, binary, octal, and 2567hex in the standard Perl or C notation: 2568 2569 $val = oct($val) if $val =~ /^0/; 2570 2571If EXPR is omitted, uses C<$_>. To go the other way (produce a number 2572in octal), use sprintf() or printf(): 2573 2574 $perms = (stat("filename"))[2] & 07777; 2575 $oct_perms = sprintf "%lo", $perms; 2576 2577The oct() function is commonly used when a string such as C<644> needs 2578to be converted into a file mode, for example. (Although perl will 2579automatically convert strings into numbers as needed, this automatic 2580conversion assumes base 10.) 2581 2582=item open FILEHANDLE,MODE,LIST 2583 2584=item open FILEHANDLE,EXPR 2585 2586=item open FILEHANDLE 2587 2588Opens the file whose filename is given by EXPR, and associates it with 2589FILEHANDLE. If FILEHANDLE is an expression, its value is used as the 2590name of the real filehandle wanted. (This is considered a symbolic 2591reference, so C<use strict 'refs'> should I<not> be in effect.) 2592 2593If EXPR is omitted, the scalar 2594variable of the same name as the FILEHANDLE contains the filename. 2595(Note that lexical variables--those declared with C<my>--will not work 2596for this purpose; so if you're using C<my>, specify EXPR in your call 2597to open.) See L<perlopentut> for a kinder, gentler explanation of opening 2598files. 2599 2600If MODE is C<< '<' >> or nothing, the file is opened for input. 2601If MODE is C<< '>' >>, the file is truncated and opened for 2602output, being created if necessary. If MODE is C<<< '>>' >>>, 2603the file is opened for appending, again being created if necessary. 2604You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that 2605you want both read and write access to the file; thus C<< '+<' >> is almost 2606always preferred for read/write updates--the C<< '+>' >> mode would clobber the 2607file first. You can't usually use either read-write mode for updating 2608textfiles, since they have variable length records. See the B<-i> 2609switch in L<perlrun> for a better approach. The file is created with 2610permissions of C<0666> modified by the process' C<umask> value. 2611 2612These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, 2613C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>. 2614 2615In the 2-arguments (and 1-argument) form of the call the mode and 2616filename should be concatenated (in this order), possibly separated by 2617spaces. It is possible to omit the mode if the mode is C<< '<' >>. 2618 2619If the filename begins with C<'|'>, the filename is interpreted as a 2620command to which output is to be piped, and if the filename ends with a 2621C<'|'>, the filename is interpreted as a command which pipes output to 2622us. See L<perlipc/"Using open() for IPC"> 2623for more examples of this. (You are not allowed to C<open> to a command 2624that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, 2625and L<perlipc/"Bidirectional Communication with Another Process"> 2626for alternatives.) 2627 2628If MODE is C<'|-'>, the filename is interpreted as a 2629command to which output is to be piped, and if MODE is 2630C<'-|'>, the filename is interpreted as a command which pipes output to 2631us. In the 2-arguments (and 1-argument) form one should replace dash 2632(C<'-'>) with the command. See L<perlipc/"Using open() for IPC"> 2633for more examples of this. (You are not allowed to C<open> to a command 2634that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, 2635and L<perlipc/"Bidirectional Communication"> for alternatives.) 2636 2637In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN 2638and opening C<< '>-' >> opens STDOUT. 2639 2640Open returns 2641nonzero upon success, the undefined value otherwise. If the C<open> 2642involved a pipe, the return value happens to be the pid of the 2643subprocess. 2644 2645If you're unfortunate enough to be running Perl on a system that 2646distinguishes between text files and binary files (modern operating 2647systems don't care), then you should check out L</binmode> for tips for 2648dealing with this. The key distinction between systems that need C<binmode> 2649and those that don't is their text file formats. Systems like Unix, MacOS, and 2650Plan9, which delimit lines with a single character, and which encode that 2651character in C as C<"\n">, do not need C<binmode>. The rest need it. 2652 2653When opening a file, it's usually a bad idea to continue normal execution 2654if the request failed, so C<open> is frequently used in connection with 2655C<die>. Even if C<die> won't do what you want (say, in a CGI script, 2656where you want to make a nicely formatted error message (but there are 2657modules that can help with that problem)) you should always check 2658the return value from opening a file. The infrequent exception is when 2659working with an unopened filehandle is actually what you want to do. 2660 2661Examples: 2662 2663 $ARTICLE = 100; 2664 open ARTICLE or die "Can't find article $ARTICLE: $!\n"; 2665 while (<ARTICLE>) {... 2666 2667 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) 2668 # if the open fails, output is discarded 2669 2670 open(DBASE, '+<', 'dbase.mine') # open for update 2671 or die "Can't open 'dbase.mine' for update: $!"; 2672 2673 open(DBASE, '+<dbase.mine') # ditto 2674 or die "Can't open 'dbase.mine' for update: $!"; 2675 2676 open(ARTICLE, '-|', "caesar <$article") # decrypt article 2677 or die "Can't start caesar: $!"; 2678 2679 open(ARTICLE, "caesar <$article |") # ditto 2680 or die "Can't start caesar: $!"; 2681 2682 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id 2683 or die "Can't start sort: $!"; 2684 2685 # process argument list of files along with any includes 2686 2687 foreach $file (@ARGV) { 2688 process($file, 'fh00'); 2689 } 2690 2691 sub process { 2692 my($filename, $input) = @_; 2693 $input++; # this is a string increment 2694 unless (open($input, $filename)) { 2695 print STDERR "Can't open $filename: $!\n"; 2696 return; 2697 } 2698 2699 local $_; 2700 while (<$input>) { # note use of indirection 2701 if (/^#include "(.*)"/) { 2702 process($1, $input); 2703 next; 2704 } 2705 #... # whatever 2706 } 2707 } 2708 2709You may also, in the Bourne shell tradition, specify an EXPR beginning 2710with C<< '>&' >>, in which case the rest of the string is interpreted as the 2711name of a filehandle (or file descriptor, if numeric) to be 2712duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>, 2713C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The 2714mode you specify should match the mode of the original filehandle. 2715(Duping a filehandle does not take into account any existing contents of 2716stdio buffers.) Duping file handles is not yet supported for 3-argument 2717open(). 2718 2719Here is a script that saves, redirects, and restores STDOUT and 2720STDERR: 2721 2722 #!/usr/bin/perl 2723 open(OLDOUT, ">&STDOUT"); 2724 open(OLDERR, ">&STDERR"); 2725 2726 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout"; 2727 open(STDERR, ">&STDOUT") || die "Can't dup stdout"; 2728 2729 select(STDERR); $| = 1; # make unbuffered 2730 select(STDOUT); $| = 1; # make unbuffered 2731 2732 print STDOUT "stdout 1\n"; # this works for 2733 print STDERR "stderr 1\n"; # subprocesses too 2734 2735 close(STDOUT); 2736 close(STDERR); 2737 2738 open(STDOUT, ">&OLDOUT"); 2739 open(STDERR, ">&OLDERR"); 2740 2741 print STDOUT "stdout 2\n"; 2742 print STDERR "stderr 2\n"; 2743 2744If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an 2745equivalent of C's C<fdopen> of that file descriptor; this is more 2746parsimonious of file descriptors. For example: 2747 2748 open(FILEHANDLE, "<&=$fd") 2749 2750Note that this feature depends on the fdopen() C library function. 2751On many UNIX systems, fdopen() is known to fail when file descriptors 2752exceed a certain value, typically 255. If you need more file 2753descriptors than that, consider rebuilding Perl to use the C<sfio> 2754library. 2755 2756If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'> 2757with 2-arguments (or 1-argument) form of open(), then 2758there is an implicit fork done, and the return value of open is the pid 2759of the child within the parent process, and C<0> within the child 2760process. (Use C<defined($pid)> to determine whether the open was successful.) 2761The filehandle behaves normally for the parent, but i/o to that 2762filehandle is piped from/to the STDOUT/STDIN of the child process. 2763In the child process the filehandle isn't opened--i/o happens from/to 2764the new STDOUT or STDIN. Typically this is used like the normal 2765piped open when you want to exercise more control over just how the 2766pipe command gets executed, such as when you are running setuid, and 2767don't want to have to scan shell commands for metacharacters. 2768The following triples are more or less equivalent: 2769 2770 open(FOO, "|tr '[a-z]' '[A-Z]'"); 2771 open(FOO, '|-', "tr '[a-z]' '[A-Z]'"); 2772 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]'; 2773 2774 open(FOO, "cat -n '$file'|"); 2775 open(FOO, '-|', "cat -n '$file'"); 2776 open(FOO, '-|') || exec 'cat', '-n', $file; 2777 2778See L<perlipc/"Safe Pipe Opens"> for more examples of this. 2779 2780Beginning with v5.6.0, Perl will attempt to flush all files opened for 2781output before any operation that may do a fork, but this may not be 2782supported on some platforms (see L<perlport>). To be safe, you may need 2783to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method 2784of C<IO::Handle> on any open handles. 2785 2786On systems that support a 2787close-on-exec flag on files, the flag will be set for the newly opened 2788file descriptor as determined by the value of $^F. See L<perlvar/$^F>. 2789 2790Closing any piped filehandle causes the parent process to wait for the 2791child to finish, and returns the status value in C<$?>. 2792 2793The filename passed to 2-argument (or 1-argument) form of open() 2794will have leading and trailing 2795whitespace deleted, and the normal redirection characters 2796honored. This property, known as "magic open", 2797can often be used to good effect. A user could specify a filename of 2798F<"rsh cat file |">, or you could change certain filenames as needed: 2799 2800 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; 2801 open(FH, $filename) or die "Can't open $filename: $!"; 2802 2803Use 3-argument form to open a file with arbitrary weird characters in it, 2804 2805 open(FOO, '<', $file); 2806 2807otherwise it's necessary to protect any leading and trailing whitespace: 2808 2809 $file =~ s#^(\s)#./$1#; 2810 open(FOO, "< $file\0"); 2811 2812(this may not work on some bizzare filesystems). One should 2813conscientiously choose between the the I<magic> and 3-arguments form 2814of open(): 2815 2816 open IN, $ARGV[0]; 2817 2818will allow the user to specify an argument of the form C<"rsh cat file |">, 2819but will not work on a filename which happens to have a trailing space, while 2820 2821 open IN, '<', $ARGV[0]; 2822 2823will have exactly the opposite restrictions. 2824 2825If you want a "real" C C<open> (see L<open(2)> on your system), then you 2826should use the C<sysopen> function, which involves no such magic (but 2827may use subtly different filemodes than Perl open(), which is mapped 2828to C fopen()). This is 2829another way to protect your filenames from interpretation. For example: 2830 2831 use IO::Handle; 2832 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) 2833 or die "sysopen $path: $!"; 2834 $oldfh = select(HANDLE); $| = 1; select($oldfh); 2835 print HANDLE "stuff $$\n"); 2836 seek(HANDLE, 0, 0); 2837 print "File contains: ", <HANDLE>; 2838 2839Using the constructor from the C<IO::Handle> package (or one of its 2840subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous 2841filehandles that have the scope of whatever variables hold references to 2842them, and automatically close whenever and however you leave that scope: 2843 2844 use IO::File; 2845 #... 2846 sub read_myfile_munged { 2847 my $ALL = shift; 2848 my $handle = new IO::File; 2849 open($handle, "myfile") or die "myfile: $!"; 2850 $first = <$handle> 2851 or return (); # Automatically closed here. 2852 mung $first or die "mung failed"; # Or here. 2853 return $first, <$handle> if $ALL; # Or here. 2854 $first; # Or here. 2855 } 2856 2857See L</seek> for some details about mixing reading and writing. 2858 2859=item opendir DIRHANDLE,EXPR 2860 2861Opens a directory named EXPR for processing by C<readdir>, C<telldir>, 2862C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful. 2863DIRHANDLEs have their own namespace separate from FILEHANDLEs. 2864 2865=item ord EXPR 2866 2867=item ord 2868 2869Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If 2870EXPR is omitted, uses C<$_>. For the reverse, see L</chr>. 2871See L<utf8> for more about Unicode. 2872 2873=item our EXPR 2874 2875An C<our> declares the listed variables to be valid globals within 2876the enclosing block, file, or C<eval>. That is, it has the same 2877scoping rules as a "my" declaration, but does not create a local 2878variable. If more than one value is listed, the list must be placed 2879in parentheses. The C<our> declaration has no semantic effect unless 2880"use strict vars" is in effect, in which case it lets you use the 2881declared global variable without qualifying it with a package name. 2882(But only within the lexical scope of the C<our> declaration. In this 2883it differs from "use vars", which is package scoped.) 2884 2885An C<our> declaration declares a global variable that will be visible 2886across its entire lexical scope, even across package boundaries. The 2887package in which the variable is entered is determined at the point 2888of the declaration, not at the point of use. This means the following 2889behavior holds: 2890 2891 package Foo; 2892 our $bar; # declares $Foo::bar for rest of lexical scope 2893 $bar = 20; 2894 2895 package Bar; 2896 print $bar; # prints 20 2897 2898Multiple C<our> declarations in the same lexical scope are allowed 2899if they are in different packages. If they happened to be in the same 2900package, Perl will emit warnings if you have asked for them. 2901 2902 use warnings; 2903 package Foo; 2904 our $bar; # declares $Foo::bar for rest of lexical scope 2905 $bar = 20; 2906 2907 package Bar; 2908 our $bar = 30; # declares $Bar::bar for rest of lexical scope 2909 print $bar; # prints 30 2910 2911 our $bar; # emits warning 2912 2913=item pack TEMPLATE,LIST 2914 2915Takes a LIST of values and converts it into a string using the rules 2916given by the TEMPLATE. The resulting string is the concatenation of 2917the converted values. Typically, each converted value looks 2918like its machine-level representation. For example, on 32-bit machines 2919a converted integer may be represented by a sequence of 4 bytes. 2920 2921The TEMPLATE is a 2922sequence of characters that give the order and type of values, as 2923follows: 2924 2925 a A string with arbitrary binary data, will be null padded. 2926 A An ascii string, will be space padded. 2927 Z A null terminated (asciz) string, will be null padded. 2928 2929 b A bit string (ascending bit order inside each byte, like vec()). 2930 B A bit string (descending bit order inside each byte). 2931 h A hex string (low nybble first). 2932 H A hex string (high nybble first). 2933 2934 c A signed char value. 2935 C An unsigned char value. Only does bytes. See U for Unicode. 2936 2937 s A signed short value. 2938 S An unsigned short value. 2939 (This 'short' is _exactly_ 16 bits, which may differ from 2940 what a local C compiler calls 'short'. If you want 2941 native-length shorts, use the '!' suffix.) 2942 2943 i A signed integer value. 2944 I An unsigned integer value. 2945 (This 'integer' is _at_least_ 32 bits wide. Its exact 2946 size depends on what a local C compiler calls 'int', 2947 and may even be larger than the 'long' described in 2948 the next item.) 2949 2950 l A signed long value. 2951 L An unsigned long value. 2952 (This 'long' is _exactly_ 32 bits, which may differ from 2953 what a local C compiler calls 'long'. If you want 2954 native-length longs, use the '!' suffix.) 2955 2956 n An unsigned short in "network" (big-endian) order. 2957 N An unsigned long in "network" (big-endian) order. 2958 v An unsigned short in "VAX" (little-endian) order. 2959 V An unsigned long in "VAX" (little-endian) order. 2960 (These 'shorts' and 'longs' are _exactly_ 16 bits and 2961 _exactly_ 32 bits, respectively.) 2962 2963 q A signed quad (64-bit) value. 2964 Q An unsigned quad value. 2965 (Quads are available only if your system supports 64-bit 2966 integer values _and_ if Perl has been compiled to support those. 2967 Causes a fatal error otherwise.) 2968 2969 f A single-precision float in the native format. 2970 d A double-precision float in the native format. 2971 2972 p A pointer to a null-terminated string. 2973 P A pointer to a structure (fixed-length string). 2974 2975 u A uuencoded string. 2976 U A Unicode character number. Encodes to UTF-8 internally. 2977 Works even if C<use utf8> is not in effect. 2978 2979 w A BER compressed integer. Its bytes represent an unsigned 2980 integer in base 128, most significant digit first, with as 2981 few digits as possible. Bit eight (the high bit) is set 2982 on each byte except the last. 2983 2984 x A null byte. 2985 X Back up a byte. 2986 @ Null fill to absolute position. 2987 2988The following rules apply: 2989 2990=over 8 2991 2992=item * 2993 2994Each letter may optionally be followed by a number giving a repeat 2995count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>, 2996C<H>, and C<P> the pack function will gobble up that many values from 2997the LIST. A C<*> for the repeat count means to use however many items are 2998left, except for C<@>, C<x>, C<X>, where it is equivalent 2999to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the 3000same). 3001 3002When used with C<Z>, C<*> results in the addition of a trailing null 3003byte (so the packed result will be one longer than the byte C<length> 3004of the item). 3005 3006The repeat count for C<u> is interpreted as the maximal number of bytes 3007to encode per line of output, with 0 and 1 replaced by 45. 3008 3009=item * 3010 3011The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a 3012string of length count, padding with nulls or spaces as necessary. When 3013unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything 3014after the first null, and C<a> returns data verbatim. When packing, 3015C<a>, and C<Z> are equivalent. 3016 3017If the value-to-pack is too long, it is truncated. If too long and an 3018explicit count is provided, C<Z> packs only C<$count-1> bytes, followed 3019by a null byte. Thus C<Z> always packs a trailing null byte under 3020all circumstances. 3021 3022=item * 3023 3024Likewise, the C<b> and C<B> fields pack a string that many bits long. 3025Each byte of the input field of pack() generates 1 bit of the result. 3026Each result bit is based on the least-significant bit of the corresponding 3027input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and 3028C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">. 3029 3030Starting from the beginning of the input string of pack(), each 8-tuple 3031of bytes is converted to 1 byte of output. With format C<b> 3032the first byte of the 8-tuple determines the least-significant bit of a 3033byte, and with format C<B> it determines the most-significant bit of 3034a byte. 3035 3036If the length of the input string is not exactly divisible by 8, the 3037remainder is packed as if the input string were padded by null bytes 3038at the end. Similarly, during unpack()ing the "extra" bits are ignored. 3039 3040If the input string of pack() is longer than needed, extra bytes are ignored. 3041A C<*> for the repeat count of pack() means to use all the bytes of 3042the input field. On unpack()ing the bits are converted to a string 3043of C<"0">s and C<"1">s. 3044 3045=item * 3046 3047The C<h> and C<H> fields pack a string that many nybbles (4-bit groups, 3048representable as hexadecimal digits, 0-9a-f) long. 3049 3050Each byte of the input field of pack() generates 4 bits of the result. 3051For non-alphabetical bytes the result is based on the 4 least-significant 3052bits of the input byte, i.e., on C<ord($byte)%16>. In particular, 3053bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes 3054C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result 3055is compatible with the usual hexadecimal digits, so that C<"a"> and 3056C<"A"> both generate the nybble C<0xa==10>. The result for bytes 3057C<"g".."z"> and C<"G".."Z"> is not well-defined. 3058 3059Starting from the beginning of the input string of pack(), each pair 3060of bytes is converted to 1 byte of output. With format C<h> the 3061first byte of the pair determines the least-significant nybble of the 3062output byte, and with format C<H> it determines the most-significant 3063nybble. 3064 3065If the length of the input string is not even, it behaves as if padded 3066by a null byte at the end. Similarly, during unpack()ing the "extra" 3067nybbles are ignored. 3068 3069If the input string of pack() is longer than needed, extra bytes are ignored. 3070A C<*> for the repeat count of pack() means to use all the bytes of 3071the input field. On unpack()ing the bits are converted to a string 3072of hexadecimal digits. 3073 3074=item * 3075 3076The C<p> type packs a pointer to a null-terminated string. You are 3077responsible for ensuring the string is not a temporary value (which can 3078potentially get deallocated before you get around to using the packed result). 3079The C<P> type packs a pointer to a structure of the size indicated by the 3080length. A NULL pointer is created if the corresponding value for C<p> or 3081C<P> is C<undef>, similarly for unpack(). 3082 3083=item * 3084 3085The C</> template character allows packing and unpacking of strings where 3086the packed structure contains a byte count followed by the string itself. 3087You write I<length-item>C</>I<string-item>. 3088 3089The I<length-item> can be any C<pack> template letter, 3090and describes how the length value is packed. 3091The ones likely to be of most use are integer-packing ones like 3092C<n> (for Java strings), C<w> (for ASN.1 or SNMP) 3093and C<N> (for Sun XDR). 3094 3095The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">. 3096For C<unpack> the length of the string is obtained from the I<length-item>, 3097but if you put in the '*' it will be ignored. 3098 3099 unpack 'C/a', "\04Gurusamy"; gives 'Guru' 3100 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J') 3101 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world" 3102 3103The I<length-item> is not returned explicitly from C<unpack>. 3104 3105Adding a count to the I<length-item> letter is unlikely to do anything 3106useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a 3107I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters, 3108which Perl does not regard as legal in numeric strings. 3109 3110=item * 3111 3112The integer types C<s>, C<S>, C<l>, and C<L> may be 3113immediately followed by a C<!> suffix to signify native shorts or 3114longs--as you can see from above for example a bare C<l> does mean 3115exactly 32 bits, the native C<long> (as seen by the local C compiler) 3116may be larger. This is an issue mainly in 64-bit platforms. You can 3117see whether using C<!> makes any difference by 3118 3119 print length(pack("s")), " ", length(pack("s!")), "\n"; 3120 print length(pack("l")), " ", length(pack("l!")), "\n"; 3121 3122C<i!> and C<I!> also work but only because of completeness; 3123they are identical to C<i> and C<I>. 3124 3125The actual sizes (in bytes) of native shorts, ints, longs, and long 3126longs on the platform where Perl was built are also available via 3127L<Config>: 3128 3129 use Config; 3130 print $Config{shortsize}, "\n"; 3131 print $Config{intsize}, "\n"; 3132 print $Config{longsize}, "\n"; 3133 print $Config{longlongsize}, "\n"; 3134 3135(The C<$Config{longlongsize}> will be undefine if your system does 3136not support long longs.) 3137 3138=item * 3139 3140The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L> 3141are inherently non-portable between processors and operating systems 3142because they obey the native byteorder and endianness. For example a 31434-byte integer 0x12345678 (305419896 decimal) be ordered natively 3144(arranged in and handled by the CPU registers) into bytes as 3145 3146 0x12 0x34 0x56 0x78 # little-endian 3147 0x78 0x56 0x34 0x12 # big-endian 3148 3149Basically, the Intel, Alpha, and VAX CPUs are little-endian, while 3150everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA, 3151Power, and Cray are big-endian. MIPS can be either: Digital used it 3152in little-endian mode; SGI uses it in big-endian mode. 3153 3154The names `big-endian' and `little-endian' are comic references to 3155the classic "Gulliver's Travels" (via the paper "On Holy Wars and a 3156Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and 3157the egg-eating habits of the Lilliputians. 3158 3159Some systems may have even weirder byte orders such as 3160 3161 0x56 0x78 0x12 0x34 3162 0x34 0x12 0x78 0x56 3163 3164You can see your system's preference with 3165 3166 print join(" ", map { sprintf "%#02x", $_ } 3167 unpack("C*",pack("L",0x12345678))), "\n"; 3168 3169The byteorder on the platform where Perl was built is also available 3170via L<Config>: 3171 3172 use Config; 3173 print $Config{byteorder}, "\n"; 3174 3175Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'> 3176and C<'87654321'> are big-endian. 3177 3178If you want portable packed integers use the formats C<n>, C<N>, 3179C<v>, and C<V>, their byte endianness and size is known. 3180See also L<perlport>. 3181 3182=item * 3183 3184Real numbers (floats and doubles) are in the native machine format only; 3185due to the multiplicity of floating formats around, and the lack of a 3186standard "network" representation, no facility for interchange has been 3187made. This means that packed floating point data written on one machine 3188may not be readable on another - even if both use IEEE floating point 3189arithmetic (as the endian-ness of the memory representation is not part 3190of the IEEE spec). See also L<perlport>. 3191 3192Note that Perl uses doubles internally for all numeric calculation, and 3193converting from double into float and thence back to double again will 3194lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general 3195equal $foo). 3196 3197=item * 3198 3199You must yourself do any alignment or padding by inserting for example 3200enough C<'x'>es while packing. There is no way to pack() and unpack() 3201could know where the bytes are going to or coming from. Therefore 3202C<pack> (and C<unpack>) handle their output and input as flat 3203sequences of bytes. 3204 3205=item * 3206 3207A comment in a TEMPLATE starts with C<#> and goes to the end of line. 3208 3209=item * 3210 3211If TEMPLATE requires more arguments to pack() than actually given, pack() 3212assumes additional C<""> arguments. If TEMPLATE requires less arguments 3213to pack() than actually given, extra arguments are ignored. 3214 3215=back 3216 3217Examples: 3218 3219 $foo = pack("CCCC",65,66,67,68); 3220 # foo eq "ABCD" 3221 $foo = pack("C4",65,66,67,68); 3222 # same thing 3223 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); 3224 # same thing with Unicode circled letters 3225 3226 $foo = pack("ccxxcc",65,66,67,68); 3227 # foo eq "AB\0\0CD" 3228 3229 # note: the above examples featuring "C" and "c" are true 3230 # only on ASCII and ASCII-derived systems such as ISO Latin 1 3231 # and UTF-8. In EBCDIC the first example would be 3232 # $foo = pack("CCCC",193,194,195,196); 3233 3234 $foo = pack("s2",1,2); 3235 # "\1\0\2\0" on little-endian 3236 # "\0\1\0\2" on big-endian 3237 3238 $foo = pack("a4","abcd","x","y","z"); 3239 # "abcd" 3240 3241 $foo = pack("aaaa","abcd","x","y","z"); 3242 # "axyz" 3243 3244 $foo = pack("a14","abcdefg"); 3245 # "abcdefg\0\0\0\0\0\0\0" 3246 3247 $foo = pack("i9pl", gmtime); 3248 # a real struct tm (on my system anyway) 3249 3250 $utmp_template = "Z8 Z8 Z16 L"; 3251 $utmp = pack($utmp_template, @utmp1); 3252 # a struct utmp (BSDish) 3253 3254 @utmp2 = unpack($utmp_template, $utmp); 3255 # "@utmp1" eq "@utmp2" 3256 3257 sub bintodec { 3258 unpack("N", pack("B32", substr("0" x 32 . shift, -32))); 3259 } 3260 3261 $foo = pack('sx2l', 12, 34); 3262 # short 12, two zero bytes padding, long 34 3263 $bar = pack('s@4l', 12, 34); 3264 # short 12, zero fill to position 4, long 34 3265 # $foo eq $bar 3266 3267The same template may generally also be used in unpack(). 3268 3269=item package 3270 3271=item package NAMESPACE 3272 3273Declares the compilation unit as being in the given namespace. The scope 3274of the package declaration is from the declaration itself through the end 3275of the enclosing block, file, or eval (the same as the C<my> operator). 3276All further unqualified dynamic identifiers will be in this namespace. 3277A package statement affects only dynamic variables--including those 3278you've used C<local> on--but I<not> lexical variables, which are created 3279with C<my>. Typically it would be the first declaration in a file to 3280be included by the C<require> or C<use> operator. You can switch into a 3281package in more than one place; it merely influences which symbol table 3282is used by the compiler for the rest of that block. You can refer to 3283variables and filehandles in other packages by prefixing the identifier 3284with the package name and a double colon: C<$Package::Variable>. 3285If the package name is null, the C<main> package as assumed. That is, 3286C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>, 3287still seen in older code). 3288 3289If NAMESPACE is omitted, then there is no current package, and all 3290identifiers must be fully qualified or lexicals. This is stricter 3291than C<use strict>, since it also extends to function names. 3292 3293See L<perlmod/"Packages"> for more information about packages, modules, 3294and classes. See L<perlsub> for other scoping issues. 3295 3296=item pipe READHANDLE,WRITEHANDLE 3297 3298Opens a pair of connected pipes like the corresponding system call. 3299Note that if you set up a loop of piped processes, deadlock can occur 3300unless you are very careful. In addition, note that Perl's pipes use 3301stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE 3302after each command, depending on the application. 3303 3304See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication"> 3305for examples of such things. 3306 3307On systems that support a close-on-exec flag on files, the flag will be set 3308for the newly opened file descriptors as determined by the value of $^F. 3309See L<perlvar/$^F>. 3310 3311=item pop ARRAY 3312 3313=item pop 3314 3315Pops and returns the last value of the array, shortening the array by 3316one element. Has an effect similar to 3317 3318 $ARRAY[$#ARRAY--] 3319 3320If there are no elements in the array, returns the undefined value 3321(although this may happen at other times as well). If ARRAY is 3322omitted, pops the C<@ARGV> array in the main program, and the C<@_> 3323array in subroutines, just like C<shift>. 3324 3325=item pos SCALAR 3326 3327=item pos 3328 3329Returns the offset of where the last C<m//g> search left off for the variable 3330is in question (C<$_> is used when the variable is not specified). May be 3331modified to change that offset. Such modification will also influence 3332the C<\G> zero-width assertion in regular expressions. See L<perlre> and 3333L<perlop>. 3334 3335=item print FILEHANDLE LIST 3336 3337=item print LIST 3338 3339=item print 3340 3341Prints a string or a list of strings. Returns true if successful. 3342FILEHANDLE may be a scalar variable name, in which case the variable 3343contains the name of or a reference to the filehandle, thus introducing 3344one level of indirection. (NOTE: If FILEHANDLE is a variable and 3345the next token is a term, it may be misinterpreted as an operator 3346unless you interpose a C<+> or put parentheses around the arguments.) 3347If FILEHANDLE is omitted, prints by default to standard output (or 3348to the last selected output channel--see L</select>). If LIST is 3349also omitted, prints C<$_> to the currently selected output channel. 3350To set the default output channel to something other than STDOUT 3351use the select operation. The current value of C<$,> (if any) is 3352printed between each LIST item. The current value of C<$\> (if 3353any) is printed after the entire LIST has been printed. Because 3354print takes a LIST, anything in the LIST is evaluated in list 3355context, and any subroutine that you call will have one or more of 3356its expressions evaluated in list context. Also be careful not to 3357follow the print keyword with a left parenthesis unless you want 3358the corresponding right parenthesis to terminate the arguments to 3359the print--interpose a C<+> or put parentheses around all the 3360arguments. 3361 3362Note that if you're storing FILEHANDLES in an array or other expression, 3363you will have to use a block returning its value instead: 3364 3365 print { $files[$i] } "stuff\n"; 3366 print { $OK ? STDOUT : STDERR } "stuff\n"; 3367 3368=item printf FILEHANDLE FORMAT, LIST 3369 3370=item printf FORMAT, LIST 3371 3372Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\> 3373(the output record separator) is not appended. The first argument 3374of the list will be interpreted as the C<printf> format. If C<use locale> is 3375in effect, the character used for the decimal point in formatted real numbers 3376is affected by the LC_NUMERIC locale. See L<perllocale>. 3377 3378Don't fall into the trap of using a C<printf> when a simple 3379C<print> would do. The C<print> is more efficient and less 3380error prone. 3381 3382=item prototype FUNCTION 3383 3384Returns the prototype of a function as a string (or C<undef> if the 3385function has no prototype). FUNCTION is a reference to, or the name of, 3386the function whose prototype you want to retrieve. 3387 3388If FUNCTION is a string starting with C<CORE::>, the rest is taken as a 3389name for Perl builtin. If the builtin is not I<overridable> (such as 3390C<qw//>) or its arguments cannot be expressed by a prototype (such as 3391C<system>) returns C<undef> because the builtin does not really behave 3392like a Perl function. Otherwise, the string describing the equivalent 3393prototype is returned. 3394 3395=item push ARRAY,LIST 3396 3397Treats ARRAY as a stack, and pushes the values of LIST 3398onto the end of ARRAY. The length of ARRAY increases by the length of 3399LIST. Has the same effect as 3400 3401 for $value (LIST) { 3402 $ARRAY[++$#ARRAY] = $value; 3403 } 3404 3405but is more efficient. Returns the new number of elements in the array. 3406 3407=item q/STRING/ 3408 3409=item qq/STRING/ 3410 3411=item qr/STRING/ 3412 3413=item qx/STRING/ 3414 3415=item qw/STRING/ 3416 3417Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">. 3418 3419=item quotemeta EXPR 3420 3421=item quotemeta 3422 3423Returns the value of EXPR with all non-alphanumeric 3424characters backslashed. (That is, all characters not matching 3425C</[A-Za-z_0-9]/> will be preceded by a backslash in the 3426returned string, regardless of any locale settings.) 3427This is the internal function implementing 3428the C<\Q> escape in double-quoted strings. 3429 3430If EXPR is omitted, uses C<$_>. 3431 3432=item rand EXPR 3433 3434=item rand 3435 3436Returns a random fractional number greater than or equal to C<0> and less 3437than the value of EXPR. (EXPR should be positive.) If EXPR is 3438omitted, the value C<1> is used. Automatically calls C<srand> unless 3439C<srand> has already been called. See also C<srand>. 3440 3441(Note: If your rand function consistently returns numbers that are too 3442large or too small, then your version of Perl was probably compiled 3443with the wrong number of RANDBITS.) 3444 3445=item read FILEHANDLE,SCALAR,LENGTH,OFFSET 3446 3447=item read FILEHANDLE,SCALAR,LENGTH 3448 3449Attempts to read LENGTH bytes of data into variable SCALAR from the 3450specified FILEHANDLE. Returns the number of bytes actually read, 3451C<0> at end of file, or undef if there was an error. SCALAR will be grown 3452or shrunk to the length actually read. An OFFSET may be specified to 3453place the read data at some other place than the beginning of the 3454string. This call is actually implemented in terms of stdio's fread(3) 3455call. To get a true read(2) system call, see C<sysread>. 3456 3457=item readdir DIRHANDLE 3458 3459Returns the next directory entry for a directory opened by C<opendir>. 3460If used in list context, returns all the rest of the entries in the 3461directory. If there are no more entries, returns an undefined value in 3462scalar context or a null list in list context. 3463 3464If you're planning to filetest the return values out of a C<readdir>, you'd 3465better prepend the directory in question. Otherwise, because we didn't 3466C<chdir> there, it would have been testing the wrong file. 3467 3468 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!"; 3469 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR); 3470 closedir DIR; 3471 3472=item readline EXPR 3473 3474Reads from the filehandle whose typeglob is contained in EXPR. In scalar 3475context, each call reads and returns the next line, until end-of-file is 3476reached, whereupon the subsequent call returns undef. In list context, 3477reads until end-of-file is reached and returns a list of lines. Note that 3478the notion of "line" used here is however you may have defined it 3479with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">. 3480 3481When C<$/> is set to C<undef>, when readline() is in scalar 3482context (i.e. file slurp mode), and when an empty file is read, it 3483returns C<''> the first time, followed by C<undef> subsequently. 3484 3485This is the internal function implementing the C<< <EXPR> >> 3486operator, but you can use it directly. The C<< <EXPR> >> 3487operator is discussed in more detail in L<perlop/"I/O Operators">. 3488 3489 $line = <STDIN>; 3490 $line = readline(*STDIN); # same thing 3491 3492=item readlink EXPR 3493 3494=item readlink 3495 3496Returns the value of a symbolic link, if symbolic links are 3497implemented. If not, gives a fatal error. If there is some system 3498error, returns the undefined value and sets C<$!> (errno). If EXPR is 3499omitted, uses C<$_>. 3500 3501=item readpipe EXPR 3502 3503EXPR is executed as a system command. 3504The collected standard output of the command is returned. 3505In scalar context, it comes back as a single (potentially 3506multi-line) string. In list context, returns a list of lines 3507(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>). 3508This is the internal function implementing the C<qx/EXPR/> 3509operator, but you can use it directly. The C<qx/EXPR/> 3510operator is discussed in more detail in L<perlop/"I/O Operators">. 3511 3512=item recv SOCKET,SCALAR,LENGTH,FLAGS 3513 3514Receives a message on a socket. Attempts to receive LENGTH bytes of 3515data into variable SCALAR from the specified SOCKET filehandle. SCALAR 3516will be grown or shrunk to the length actually read. Takes the same 3517flags as the system call of the same name. Returns the address of the 3518sender if SOCKET's protocol supports this; returns an empty string 3519otherwise. If there's an error, returns the undefined value. This call 3520is actually implemented in terms of recvfrom(2) system call. See 3521L<perlipc/"UDP: Message Passing"> for examples. 3522 3523=item redo LABEL 3524 3525=item redo 3526 3527The C<redo> command restarts the loop block without evaluating the 3528conditional again. The C<continue> block, if any, is not executed. If 3529the LABEL is omitted, the command refers to the innermost enclosing 3530loop. This command is normally used by programs that want to lie to 3531themselves about what was just input: 3532 3533 # a simpleminded Pascal comment stripper 3534 # (warning: assumes no { or } in strings) 3535 LINE: while (<STDIN>) { 3536 while (s|({.*}.*){.*}|$1 |) {} 3537 s|{.*}| |; 3538 if (s|{.*| |) { 3539 $front = $_; 3540 while (<STDIN>) { 3541 if (/}/) { # end of comment? 3542 s|^|$front\{|; 3543 redo LINE; 3544 } 3545 } 3546 } 3547 print; 3548 } 3549 3550C<redo> cannot be used to retry a block which returns a value such as 3551C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit 3552a grep() or map() operation. 3553 3554Note that a block by itself is semantically identical to a loop 3555that executes once. Thus C<redo> inside such a block will effectively 3556turn it into a looping construct. 3557 3558See also L</continue> for an illustration of how C<last>, C<next>, and 3559C<redo> work. 3560 3561=item ref EXPR 3562 3563=item ref 3564 3565Returns a true value if EXPR is a reference, false otherwise. If EXPR 3566is not specified, C<$_> will be used. The value returned depends on the 3567type of thing the reference is a reference to. 3568Builtin types include: 3569 3570 SCALAR 3571 ARRAY 3572 HASH 3573 CODE 3574 REF 3575 GLOB 3576 LVALUE 3577 3578If the referenced object has been blessed into a package, then that package 3579name is returned instead. You can think of C<ref> as a C<typeof> operator. 3580 3581 if (ref($r) eq "HASH") { 3582 print "r is a reference to a hash.\n"; 3583 } 3584 unless (ref($r)) { 3585 print "r is not a reference at all.\n"; 3586 } 3587 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing 3588 print "r is a reference to something that isa hash.\n"; 3589 } 3590 3591See also L<perlref>. 3592 3593=item rename OLDNAME,NEWNAME 3594 3595Changes the name of a file; an existing file NEWNAME will be 3596clobbered. Returns true for success, false otherwise. 3597 3598Behavior of this function varies wildly depending on your system 3599implementation. For example, it will usually not work across file system 3600boundaries, even though the system I<mv> command sometimes compensates 3601for this. Other restrictions include whether it works on directories, 3602open files, or pre-existing files. Check L<perlport> and either the 3603rename(2) manpage or equivalent system documentation for details. 3604 3605=item require VERSION 3606 3607=item require EXPR 3608 3609=item require 3610 3611Demands some semantics specified by EXPR, or by C<$_> if EXPR is not 3612supplied. 3613 3614If a VERSION is specified as a literal of the form v5.6.1, 3615demands that the current version of Perl (C<$^V> or $PERL_VERSION) be 3616at least as recent as that version, at run time. (For compatibility 3617with older versions of Perl, a numeric argument will also be interpreted 3618as VERSION.) Compare with L</use>, which can do a similar check at 3619compile time. 3620 3621 require v5.6.1; # run time version check 3622 require 5.6.1; # ditto 3623 require 5.005_03; # float version allowed for compatibility 3624 3625Otherwise, demands that a library file be included if it hasn't already 3626been included. The file is included via the do-FILE mechanism, which is 3627essentially just a variety of C<eval>. Has semantics similar to the following 3628subroutine: 3629 3630 sub require { 3631 my($filename) = @_; 3632 return 1 if $INC{$filename}; 3633 my($realfilename,$result); 3634 ITER: { 3635 foreach $prefix (@INC) { 3636 $realfilename = "$prefix/$filename"; 3637 if (-f $realfilename) { 3638 $INC{$filename} = $realfilename; 3639 $result = do $realfilename; 3640 last ITER; 3641 } 3642 } 3643 die "Can't find $filename in \@INC"; 3644 } 3645 delete $INC{$filename} if $@ || !$result; 3646 die $@ if $@; 3647 die "$filename did not return true value" unless $result; 3648 return $result; 3649 } 3650 3651Note that the file will not be included twice under the same specified 3652name. The file must return true as the last statement to indicate 3653successful execution of any initialization code, so it's customary to 3654end such a file with C<1;> unless you're sure it'll return true 3655otherwise. But it's better just to put the C<1;>, in case you add more 3656statements. 3657 3658If EXPR is a bareword, the require assumes a "F<.pm>" extension and 3659replaces "F<::>" with "F</>" in the filename for you, 3660to make it easy to load standard modules. This form of loading of 3661modules does not risk altering your namespace. 3662 3663In other words, if you try this: 3664 3665 require Foo::Bar; # a splendid bareword 3666 3667The require function will actually look for the "F<Foo/Bar.pm>" file in the 3668directories specified in the C<@INC> array. 3669 3670But if you try this: 3671 3672 $class = 'Foo::Bar'; 3673 require $class; # $class is not a bareword 3674 #or 3675 require "Foo::Bar"; # not a bareword because of the "" 3676 3677The require function will look for the "F<Foo::Bar>" file in the @INC array and 3678will complain about not finding "F<Foo::Bar>" there. In this case you can do: 3679 3680 eval "require $class"; 3681 3682For a yet-more-powerful import facility, see L</use> and L<perlmod>. 3683 3684=item reset EXPR 3685 3686=item reset 3687 3688Generally used in a C<continue> block at the end of a loop to clear 3689variables and reset C<??> searches so that they work again. The 3690expression is interpreted as a list of single characters (hyphens 3691allowed for ranges). All variables and arrays beginning with one of 3692those letters are reset to their pristine state. If the expression is 3693omitted, one-match searches (C<?pattern?>) are reset to match again. Resets 3694only variables or searches in the current package. Always returns 36951. Examples: 3696 3697 reset 'X'; # reset all X variables 3698 reset 'a-z'; # reset lower case variables 3699 reset; # just reset ?one-time? searches 3700 3701Resetting C<"A-Z"> is not recommended because you'll wipe out your 3702C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package 3703variables--lexical variables are unaffected, but they clean themselves 3704up on scope exit anyway, so you'll probably want to use them instead. 3705See L</my>. 3706 3707=item return EXPR 3708 3709=item return 3710 3711Returns from a subroutine, C<eval>, or C<do FILE> with the value 3712given in EXPR. Evaluation of EXPR may be in list, scalar, or void 3713context, depending on how the return value will be used, and the context 3714may vary from one execution to the next (see C<wantarray>). If no EXPR 3715is given, returns an empty list in list context, the undefined value in 3716scalar context, and (of course) nothing at all in a void context. 3717 3718(Note that in the absence of a explicit C<return>, a subroutine, eval, 3719or do FILE will automatically return the value of the last expression 3720evaluated.) 3721 3722=item reverse LIST 3723 3724In list context, returns a list value consisting of the elements 3725of LIST in the opposite order. In scalar context, concatenates the 3726elements of LIST and returns a string value with all characters 3727in the opposite order. 3728 3729 print reverse <>; # line tac, last line first 3730 3731 undef $/; # for efficiency of <> 3732 print scalar reverse <>; # character tac, last line tsrif 3733 3734This operator is also handy for inverting a hash, although there are some 3735caveats. If a value is duplicated in the original hash, only one of those 3736can be represented as a key in the inverted hash. Also, this has to 3737unwind one hash and build a whole new one, which may take some time 3738on a large hash, such as from a DBM file. 3739 3740 %by_name = reverse %by_address; # Invert the hash 3741 3742=item rewinddir DIRHANDLE 3743 3744Sets the current position to the beginning of the directory for the 3745C<readdir> routine on DIRHANDLE. 3746 3747=item rindex STR,SUBSTR,POSITION 3748 3749=item rindex STR,SUBSTR 3750 3751Works just like index() except that it returns the position of the LAST 3752occurrence of SUBSTR in STR. If POSITION is specified, returns the 3753last occurrence at or before that position. 3754 3755=item rmdir FILENAME 3756 3757=item rmdir 3758 3759Deletes the directory specified by FILENAME if that directory is empty. If it 3760succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If 3761FILENAME is omitted, uses C<$_>. 3762 3763=item s/// 3764 3765The substitution operator. See L<perlop>. 3766 3767=item scalar EXPR 3768 3769Forces EXPR to be interpreted in scalar context and returns the value 3770of EXPR. 3771 3772 @counts = ( scalar @a, scalar @b, scalar @c ); 3773 3774There is no equivalent operator to force an expression to 3775be interpolated in list context because in practice, this is never 3776needed. If you really wanted to do so, however, you could use 3777the construction C<@{[ (some expression) ]}>, but usually a simple 3778C<(some expression)> suffices. 3779 3780Because C<scalar> is unary operator, if you accidentally use for EXPR a 3781parenthesized list, this behaves as a scalar comma expression, evaluating 3782all but the last element in void context and returning the final element 3783evaluated in scalar context. This is seldom what you want. 3784 3785The following single statement: 3786 3787 print uc(scalar(&foo,$bar)),$baz; 3788 3789is the moral equivalent of these two: 3790 3791 &foo; 3792 print(uc($bar),$baz); 3793 3794See L<perlop> for more details on unary operators and the comma operator. 3795 3796=item seek FILEHANDLE,POSITION,WHENCE 3797 3798Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>. 3799FILEHANDLE may be an expression whose value gives the name of the 3800filehandle. The values for WHENCE are C<0> to set the new position to 3801POSITION, C<1> to set it to the current position plus POSITION, and 3802C<2> to set it to EOF plus POSITION (typically negative). For WHENCE 3803you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> 3804(start of the file, current position, end of the file) from the Fcntl 3805module. Returns C<1> upon success, C<0> otherwise. 3806 3807If you want to position file for C<sysread> or C<syswrite>, don't use 3808C<seek>--buffering makes its effect on the file's system position 3809unpredictable and non-portable. Use C<sysseek> instead. 3810 3811Due to the rules and rigors of ANSI C, on some systems you have to do a 3812seek whenever you switch between reading and writing. Amongst other 3813things, this may have the effect of calling stdio's clearerr(3). 3814A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position: 3815 3816 seek(TEST,0,1); 3817 3818This is also useful for applications emulating C<tail -f>. Once you hit 3819EOF on your read, and then sleep for a while, you might have to stick in a 3820seek() to reset things. The C<seek> doesn't change the current position, 3821but it I<does> clear the end-of-file condition on the handle, so that the 3822next C<< <FILE> >> makes Perl try again to read something. We hope. 3823 3824If that doesn't work (some stdios are particularly cantankerous), then 3825you may need something more like this: 3826 3827 for (;;) { 3828 for ($curpos = tell(FILE); $_ = <FILE>; 3829 $curpos = tell(FILE)) { 3830 # search for some stuff and put it into files 3831 } 3832 sleep($for_a_while); 3833 seek(FILE, $curpos, 0); 3834 } 3835 3836=item seekdir DIRHANDLE,POS 3837 3838Sets the current position for the C<readdir> routine on DIRHANDLE. POS 3839must be a value returned by C<telldir>. Has the same caveats about 3840possible directory compaction as the corresponding system library 3841routine. 3842 3843=item select FILEHANDLE 3844 3845=item select 3846 3847Returns the currently selected filehandle. Sets the current default 3848filehandle for output, if FILEHANDLE is supplied. This has two 3849effects: first, a C<write> or a C<print> without a filehandle will 3850default to this FILEHANDLE. Second, references to variables related to 3851output will refer to this output channel. For example, if you have to 3852set the top of form format for more than one output channel, you might 3853do the following: 3854 3855 select(REPORT1); 3856 $^ = 'report1_top'; 3857 select(REPORT2); 3858 $^ = 'report2_top'; 3859 3860FILEHANDLE may be an expression whose value gives the name of the 3861actual filehandle. Thus: 3862 3863 $oldfh = select(STDERR); $| = 1; select($oldfh); 3864 3865Some programmers may prefer to think of filehandles as objects with 3866methods, preferring to write the last example as: 3867 3868 use IO::Handle; 3869 STDERR->autoflush(1); 3870 3871=item select RBITS,WBITS,EBITS,TIMEOUT 3872 3873This calls the select(2) system call with the bit masks specified, which 3874can be constructed using C<fileno> and C<vec>, along these lines: 3875 3876 $rin = $win = $ein = ''; 3877 vec($rin,fileno(STDIN),1) = 1; 3878 vec($win,fileno(STDOUT),1) = 1; 3879 $ein = $rin | $win; 3880 3881If you want to select on many filehandles you might wish to write a 3882subroutine: 3883 3884 sub fhbits { 3885 my(@fhlist) = split(' ',$_[0]); 3886 my($bits); 3887 for (@fhlist) { 3888 vec($bits,fileno($_),1) = 1; 3889 } 3890 $bits; 3891 } 3892 $rin = fhbits('STDIN TTY SOCK'); 3893 3894The usual idiom is: 3895 3896 ($nfound,$timeleft) = 3897 select($rout=$rin, $wout=$win, $eout=$ein, $timeout); 3898 3899or to block until something becomes ready just do this 3900 3901 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); 3902 3903Most systems do not bother to return anything useful in $timeleft, so 3904calling select() in scalar context just returns $nfound. 3905 3906Any of the bit masks can also be undef. The timeout, if specified, is 3907in seconds, which may be fractional. Note: not all implementations are 3908capable of returning the$timeleft. If not, they always return 3909$timeleft equal to the supplied $timeout. 3910 3911You can effect a sleep of 250 milliseconds this way: 3912 3913 select(undef, undef, undef, 0.25); 3914 3915B<WARNING>: One should not attempt to mix buffered I/O (like C<read> 3916or <FH>) with C<select>, except as permitted by POSIX, and even 3917then only on POSIX systems. You have to use C<sysread> instead. 3918 3919=item semctl ID,SEMNUM,CMD,ARG 3920 3921Calls the System V IPC function C<semctl>. You'll probably have to say 3922 3923 use IPC::SysV; 3924 3925first to get the correct constant definitions. If CMD is IPC_STAT or 3926GETALL, then ARG must be a variable which will hold the returned 3927semid_ds structure or semaphore value array. Returns like C<ioctl>: 3928the undefined value for error, "C<0 but true>" for zero, or the actual 3929return value otherwise. The ARG must consist of a vector of native 3930short integers, which may may be created with C<pack("s!",(0)x$nsem)>. 3931See also C<IPC::SysV> and C<IPC::Semaphore> documentation. 3932 3933=item semget KEY,NSEMS,FLAGS 3934 3935Calls the System V IPC function semget. Returns the semaphore id, or 3936the undefined value if there is an error. See also C<IPC::SysV> and 3937C<IPC::SysV::Semaphore> documentation. 3938 3939=item semop KEY,OPSTRING 3940 3941Calls the System V IPC function semop to perform semaphore operations 3942such as signaling and waiting. OPSTRING must be a packed array of 3943semop structures. Each semop structure can be generated with 3944C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore 3945operations is implied by the length of OPSTRING. Returns true if 3946successful, or false if there is an error. As an example, the 3947following code waits on semaphore $semnum of semaphore id $semid: 3948 3949 $semop = pack("sss", $semnum, -1, 0); 3950 die "Semaphore trouble: $!\n" unless semop($semid, $semop); 3951 3952To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV> 3953and C<IPC::SysV::Semaphore> documentation. 3954 3955=item send SOCKET,MSG,FLAGS,TO 3956 3957=item send SOCKET,MSG,FLAGS 3958 3959Sends a message on a socket. Takes the same flags as the system call 3960of the same name. On unconnected sockets you must specify a 3961destination to send TO, in which case it does a C C<sendto>. Returns 3962the number of characters sent, or the undefined value if there is an 3963error. The C system call sendmsg(2) is currently unimplemented. 3964See L<perlipc/"UDP: Message Passing"> for examples. 3965 3966=item setpgrp PID,PGRP 3967 3968Sets the current process group for the specified PID, C<0> for the current 3969process. Will produce a fatal error if used on a machine that doesn't 3970implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted, 3971it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not 3972accept any arguments, so only C<setpgrp(0,0)> is portable. See also 3973C<POSIX::setsid()>. 3974 3975=item setpriority WHICH,WHO,PRIORITY 3976 3977Sets the current priority for a process, a process group, or a user. 3978(See setpriority(2).) Will produce a fatal error if used on a machine 3979that doesn't implement setpriority(2). 3980 3981=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL 3982 3983Sets the socket option requested. Returns undefined if there is an 3984error. OPTVAL may be specified as C<undef> if you don't want to pass an 3985argument. 3986 3987=item shift ARRAY 3988 3989=item shift 3990 3991Shifts the first value of the array off and returns it, shortening the 3992array by 1 and moving everything down. If there are no elements in the 3993array, returns the undefined value. If ARRAY is omitted, shifts the 3994C<@_> array within the lexical scope of subroutines and formats, and the 3995C<@ARGV> array at file scopes or within the lexical scopes established by 3996the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}> 3997constructs. 3998 3999See also C<unshift>, C<push>, and C<pop>. C<Shift()> and C<unshift> do the 4000same thing to the left end of an array that C<pop> and C<push> do to the 4001right end. 4002 4003=item shmctl ID,CMD,ARG 4004 4005Calls the System V IPC function shmctl. You'll probably have to say 4006 4007 use IPC::SysV; 4008 4009first to get the correct constant definitions. If CMD is C<IPC_STAT>, 4010then ARG must be a variable which will hold the returned C<shmid_ds> 4011structure. Returns like ioctl: the undefined value for error, "C<0> but 4012true" for zero, or the actual return value otherwise. 4013See also C<IPC::SysV> documentation. 4014 4015=item shmget KEY,SIZE,FLAGS 4016 4017Calls the System V IPC function shmget. Returns the shared memory 4018segment id, or the undefined value if there is an error. 4019See also C<IPC::SysV> documentation. 4020 4021=item shmread ID,VAR,POS,SIZE 4022 4023=item shmwrite ID,STRING,POS,SIZE 4024 4025Reads or writes the System V shared memory segment ID starting at 4026position POS for size SIZE by attaching to it, copying in/out, and 4027detaching from it. When reading, VAR must be a variable that will 4028hold the data read. When writing, if STRING is too long, only SIZE 4029bytes are used; if STRING is too short, nulls are written to fill out 4030SIZE bytes. Return true if successful, or false if there is an error. 4031shmread() taints the variable. See also C<IPC::SysV> documentation and 4032the C<IPC::Shareable> module from CPAN. 4033 4034=item shutdown SOCKET,HOW 4035 4036Shuts down a socket connection in the manner indicated by HOW, which 4037has the same interpretation as in the system call of the same name. 4038 4039 shutdown(SOCKET, 0); # I/we have stopped reading data 4040 shutdown(SOCKET, 1); # I/we have stopped writing data 4041 shutdown(SOCKET, 2); # I/we have stopped using this socket 4042 4043This is useful with sockets when you want to tell the other 4044side you're done writing but not done reading, or vice versa. 4045It's also a more insistent form of close because it also 4046disables the file descriptor in any forked copies in other 4047processes. 4048 4049=item sin EXPR 4050 4051=item sin 4052 4053Returns the sine of EXPR (expressed in radians). If EXPR is omitted, 4054returns sine of C<$_>. 4055 4056For the inverse sine operation, you may use the C<Math::Trig::asin> 4057function, or use this relation: 4058 4059 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } 4060 4061=item sleep EXPR 4062 4063=item sleep 4064 4065Causes the script to sleep for EXPR seconds, or forever if no EXPR. 4066May be interrupted if the process receives a signal such as C<SIGALRM>. 4067Returns the number of seconds actually slept. You probably cannot 4068mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented 4069using C<alarm>. 4070 4071On some older systems, it may sleep up to a full second less than what 4072you requested, depending on how it counts seconds. Most modern systems 4073always sleep the full amount. They may appear to sleep longer than that, 4074however, because your process might not be scheduled right away in a 4075busy multitasking system. 4076 4077For delays of finer granularity than one second, you may use Perl's 4078C<syscall> interface to access setitimer(2) if your system supports 4079it, or else see L</select> above. The Time::HiRes module from CPAN 4080may also help. 4081 4082See also the POSIX module's C<sigpause> function. 4083 4084=item socket SOCKET,DOMAIN,TYPE,PROTOCOL 4085 4086Opens a socket of the specified kind and attaches it to filehandle 4087SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for 4088the system call of the same name. You should C<use Socket> first 4089to get the proper definitions imported. See the examples in 4090L<perlipc/"Sockets: Client/Server Communication">. 4091 4092On systems that support a close-on-exec flag on files, the flag will 4093be set for the newly opened file descriptor, as determined by the 4094value of $^F. See L<perlvar/$^F>. 4095 4096=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL 4097 4098Creates an unnamed pair of sockets in the specified domain, of the 4099specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as 4100for the system call of the same name. If unimplemented, yields a fatal 4101error. Returns true if successful. 4102 4103On systems that support a close-on-exec flag on files, the flag will 4104be set for the newly opened file descriptors, as determined by the value 4105of $^F. See L<perlvar/$^F>. 4106 4107Some systems defined C<pipe> in terms of C<socketpair>, in which a call 4108to C<pipe(Rdr, Wtr)> is essentially: 4109 4110 use Socket; 4111 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); 4112 shutdown(Rdr, 1); # no more writing for reader 4113 shutdown(Wtr, 0); # no more reading for writer 4114 4115See L<perlipc> for an example of socketpair use. 4116 4117=item sort SUBNAME LIST 4118 4119=item sort BLOCK LIST 4120 4121=item sort LIST 4122 4123Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK 4124is omitted, C<sort>s in standard string comparison order. If SUBNAME is 4125specified, it gives the name of a subroutine that returns an integer 4126less than, equal to, or greater than C<0>, depending on how the elements 4127of the list are to be ordered. (The C<< <=> >> and C<cmp> 4128operators are extremely useful in such routines.) SUBNAME may be a 4129scalar variable name (unsubscripted), in which case the value provides 4130the name of (or a reference to) the actual subroutine to use. In place 4131of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort 4132subroutine. 4133 4134If the subroutine's prototype is C<($$)>, the elements to be compared 4135are passed by reference in C<@_>, as for a normal subroutine. This is 4136slower than unprototyped subroutines, where the elements to be 4137compared are passed into the subroutine 4138as the package global variables $a and $b (see example below). Note that 4139in the latter case, it is usually counter-productive to declare $a and 4140$b as lexicals. 4141 4142In either case, the subroutine may not be recursive. The values to be 4143compared are always passed by reference, so don't modify them. 4144 4145You also cannot exit out of the sort block or subroutine using any of the 4146loop control operators described in L<perlsyn> or with C<goto>. 4147 4148When C<use locale> is in effect, C<sort LIST> sorts LIST according to the 4149current collation locale. See L<perllocale>. 4150 4151Examples: 4152 4153 # sort lexically 4154 @articles = sort @files; 4155 4156 # same thing, but with explicit sort routine 4157 @articles = sort {$a cmp $b} @files; 4158 4159 # now case-insensitively 4160 @articles = sort {uc($a) cmp uc($b)} @files; 4161 4162 # same thing in reversed order 4163 @articles = sort {$b cmp $a} @files; 4164 4165 # sort numerically ascending 4166 @articles = sort {$a <=> $b} @files; 4167 4168 # sort numerically descending 4169 @articles = sort {$b <=> $a} @files; 4170 4171 # this sorts the %age hash by value instead of key 4172 # using an in-line function 4173 @eldest = sort { $age{$b} <=> $age{$a} } keys %age; 4174 4175 # sort using explicit subroutine name 4176 sub byage { 4177 $age{$a} <=> $age{$b}; # presuming numeric 4178 } 4179 @sortedclass = sort byage @class; 4180 4181 sub backwards { $b cmp $a } 4182 @harry = qw(dog cat x Cain Abel); 4183 @george = qw(gone chased yz Punished Axed); 4184 print sort @harry; 4185 # prints AbelCaincatdogx 4186 print sort backwards @harry; 4187 # prints xdogcatCainAbel 4188 print sort @george, 'to', @harry; 4189 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz 4190 4191 # inefficiently sort by descending numeric compare using 4192 # the first integer after the first = sign, or the 4193 # whole record case-insensitively otherwise 4194 4195 @new = sort { 4196 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] 4197 || 4198 uc($a) cmp uc($b) 4199 } @old; 4200 4201 # same thing, but much more efficiently; 4202 # we'll build auxiliary indices instead 4203 # for speed 4204 @nums = @caps = (); 4205 for (@old) { 4206 push @nums, /=(\d+)/; 4207 push @caps, uc($_); 4208 } 4209 4210 @new = @old[ sort { 4211 $nums[$b] <=> $nums[$a] 4212 || 4213 $caps[$a] cmp $caps[$b] 4214 } 0..$#old 4215 ]; 4216 4217 # same thing, but without any temps 4218 @new = map { $_->[0] } 4219 sort { $b->[1] <=> $a->[1] 4220 || 4221 $a->[2] cmp $b->[2] 4222 } map { [$_, /=(\d+)/, uc($_)] } @old; 4223 4224 # using a prototype allows you to use any comparison subroutine 4225 # as a sort subroutine (including other package's subroutines) 4226 package other; 4227 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here 4228 4229 package main; 4230 @new = sort other::backwards @old; 4231 4232If you're using strict, you I<must not> declare $a 4233and $b as lexicals. They are package globals. That means 4234if you're in the C<main> package, it's 4235 4236 @articles = sort {$main::b <=> $main::a} @files; 4237 4238or just 4239 4240 @articles = sort {$::b <=> $::a} @files; 4241 4242but if you're in the C<FooPack> package, it's 4243 4244 @articles = sort {$FooPack::b <=> $FooPack::a} @files; 4245 4246The comparison function is required to behave. If it returns 4247inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and 4248sometimes saying the opposite, for example) the results are not 4249well-defined. 4250 4251=item splice ARRAY,OFFSET,LENGTH,LIST 4252 4253=item splice ARRAY,OFFSET,LENGTH 4254 4255=item splice ARRAY,OFFSET 4256 4257=item splice ARRAY 4258 4259Removes the elements designated by OFFSET and LENGTH from an array, and 4260replaces them with the elements of LIST, if any. In list context, 4261returns the elements removed from the array. In scalar context, 4262returns the last element removed, or C<undef> if no elements are 4263removed. The array grows or shrinks as necessary. 4264If OFFSET is negative then it starts that far from the end of the array. 4265If LENGTH is omitted, removes everything from OFFSET onward. 4266If LENGTH is negative, leaves that many elements off the end of the array. 4267If both OFFSET and LENGTH are omitted, removes everything. 4268 4269The following equivalences hold (assuming C<$[ == 0>): 4270 4271 push(@a,$x,$y) splice(@a,@a,0,$x,$y) 4272 pop(@a) splice(@a,-1) 4273 shift(@a) splice(@a,0,1) 4274 unshift(@a,$x,$y) splice(@a,0,0,$x,$y) 4275 $a[$x] = $y splice(@a,$x,1,$y) 4276 4277Example, assuming array lengths are passed before arrays: 4278 4279 sub aeq { # compare two list values 4280 my(@a) = splice(@_,0,shift); 4281 my(@b) = splice(@_,0,shift); 4282 return 0 unless @a == @b; # same len? 4283 while (@a) { 4284 return 0 if pop(@a) ne pop(@b); 4285 } 4286 return 1; 4287 } 4288 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } 4289 4290=item split /PATTERN/,EXPR,LIMIT 4291 4292=item split /PATTERN/,EXPR 4293 4294=item split /PATTERN/ 4295 4296=item split 4297 4298Splits a string into a list of strings and returns that list. By default, 4299empty leading fields are preserved, and empty trailing ones are deleted. 4300 4301If not in list context, returns the number of fields found and splits into 4302the C<@_> array. (In list context, you can force the split into C<@_> by 4303using C<??> as the pattern delimiters, but it still returns the list 4304value.) The use of implicit split to C<@_> is deprecated, however, because 4305it clobbers your subroutine arguments. 4306 4307If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted, 4308splits on whitespace (after skipping any leading whitespace). Anything 4309matching PATTERN is taken to be a delimiter separating the fields. (Note 4310that the delimiter may be longer than one character.) 4311 4312If LIMIT is specified and positive, splits into no more than that 4313many fields (though it may split into fewer). If LIMIT is unspecified 4314or zero, trailing null fields are stripped (which potential users 4315of C<pop> would do well to remember). If LIMIT is negative, it is 4316treated as if an arbitrarily large LIMIT had been specified. 4317 4318A pattern matching the null string (not to be confused with 4319a null pattern C<//>, which is just one member of the set of patterns 4320matching a null string) will split the value of EXPR into separate 4321characters at each point it matches that way. For example: 4322 4323 print join(':', split(/ */, 'hi there')); 4324 4325produces the output 'h:i:t:h:e:r:e'. 4326 4327The LIMIT parameter can be used to split a line partially 4328 4329 ($login, $passwd, $remainder) = split(/:/, $_, 3); 4330 4331When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT 4332one larger than the number of variables in the list, to avoid 4333unnecessary work. For the list above LIMIT would have been 4 by 4334default. In time critical applications it behooves you not to split 4335into more fields than you really need. 4336 4337If the PATTERN contains parentheses, additional list elements are 4338created from each matching substring in the delimiter. 4339 4340 split(/([,-])/, "1-10,20", 3); 4341 4342produces the list value 4343 4344 (1, '-', 10, ',', 20) 4345 4346If you had the entire header of a normal Unix email message in $header, 4347you could split it up into fields and their values this way: 4348 4349 $header =~ s/\n\s+/ /g; # fix continuation lines 4350 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header); 4351 4352The pattern C</PATTERN/> may be replaced with an expression to specify 4353patterns that vary at runtime. (To do runtime compilation only once, 4354use C</$variable/o>.) 4355 4356As a special case, specifying a PATTERN of space (C<' '>) will split on 4357white space just as C<split> with no arguments does. Thus, C<split(' ')> can 4358be used to emulate B<awk>'s default behavior, whereas C<split(/ /)> 4359will give you as many null initial fields as there are leading spaces. 4360A C<split> on C</\s+/> is like a C<split(' ')> except that any leading 4361whitespace produces a null first field. A C<split> with no arguments 4362really does a C<split(' ', $_)> internally. 4363 4364Example: 4365 4366 open(PASSWD, '/etc/passwd'); 4367 while (<PASSWD>) { 4368 ($login, $passwd, $uid, $gid, 4369 $gcos, $home, $shell) = split(/:/); 4370 #... 4371 } 4372 4373(Note that $shell above will still have a newline on it. See L</chop>, 4374L</chomp>, and L</join>.) 4375 4376=item sprintf FORMAT, LIST 4377 4378Returns a string formatted by the usual C<printf> conventions of the 4379C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)> 4380on your system for an explanation of the general principles. 4381 4382Perl does its own C<sprintf> formatting--it emulates the C 4383function C<sprintf>, but it doesn't use it (except for floating-point 4384numbers, and even then only the standard modifiers are allowed). As a 4385result, any non-standard extensions in your local C<sprintf> are not 4386available from Perl. 4387 4388Perl's C<sprintf> permits the following universally-known conversions: 4389 4390 %% a percent sign 4391 %c a character with the given number 4392 %s a string 4393 %d a signed integer, in decimal 4394 %u an unsigned integer, in decimal 4395 %o an unsigned integer, in octal 4396 %x an unsigned integer, in hexadecimal 4397 %e a floating-point number, in scientific notation 4398 %f a floating-point number, in fixed decimal notation 4399 %g a floating-point number, in %e or %f notation 4400 4401In addition, Perl permits the following widely-supported conversions: 4402 4403 %X like %x, but using upper-case letters 4404 %E like %e, but using an upper-case "E" 4405 %G like %g, but with an upper-case "E" (if applicable) 4406 %b an unsigned integer, in binary 4407 %p a pointer (outputs the Perl value's address in hexadecimal) 4408 %n special: *stores* the number of characters output so far 4409 into the next variable in the parameter list 4410 4411Finally, for backward (and we do mean "backward") compatibility, Perl 4412permits these unnecessary but widely-supported conversions: 4413 4414 %i a synonym for %d 4415 %D a synonym for %ld 4416 %U a synonym for %lu 4417 %O a synonym for %lo 4418 %F a synonym for %f 4419 4420Perl permits the following universally-known flags between the C<%> 4421and the conversion letter: 4422 4423 space prefix positive number with a space 4424 + prefix positive number with a plus sign 4425 - left-justify within the field 4426 0 use zeros, not spaces, to right-justify 4427 # prefix non-zero octal with "0", non-zero hex with "0x" 4428 number minimum field width 4429 .number "precision": digits after decimal point for 4430 floating-point, max length for string, minimum length 4431 for integer 4432 l interpret integer as C type "long" or "unsigned long" 4433 h interpret integer as C type "short" or "unsigned short" 4434 If no flags, interpret integer as C type "int" or "unsigned" 4435 4436There are also two Perl-specific flags: 4437 4438 V interpret integer as Perl's standard integer type 4439 v interpret string as a vector of integers, output as 4440 numbers separated either by dots, or by an arbitrary 4441 string received from the argument list when the flag 4442 is preceded by C<*> 4443 4444Where a number would appear in the flags, an asterisk (C<*>) may be 4445used instead, in which case Perl uses the next item in the parameter 4446list as the given number (that is, as the field width or precision). 4447If a field width obtained through C<*> is negative, it has the same 4448effect as the C<-> flag: left-justification. 4449 4450The C<v> flag is useful for displaying ordinal values of characters 4451in arbitrary strings: 4452 4453 printf "version is v%vd\n", $^V; # Perl's version 4454 printf "address is %*vX\n", ":", $addr; # IPv6 address 4455 printf "bits are %*vb\n", " ", $bits; # random bitstring 4456 4457If C<use locale> is in effect, the character used for the decimal 4458point in formatted real numbers is affected by the LC_NUMERIC locale. 4459See L<perllocale>. 4460 4461If Perl understands "quads" (64-bit integers) (this requires 4462either that the platform natively support quads or that Perl 4463be specifically compiled to support quads), the characters 4464 4465 d u o x X b i D U O 4466 4467print quads, and they may optionally be preceded by 4468 4469 ll L q 4470 4471For example 4472 4473 %lld %16LX %qo 4474 4475You can find out whether your Perl supports quads via L<Config>: 4476 4477 use Config; 4478 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) && 4479 print "quads\n"; 4480 4481If Perl understands "long doubles" (this requires that the platform 4482support long doubles), the flags 4483 4484 e f g E F G 4485 4486may optionally be preceded by 4487 4488 ll L 4489 4490For example 4491 4492 %llf %Lg 4493 4494You can find out whether your Perl supports long doubles via L<Config>: 4495 4496 use Config; 4497 $Config{d_longdbl} eq 'define' && print "long doubles\n"; 4498 4499=item sqrt EXPR 4500 4501=item sqrt 4502 4503Return the square root of EXPR. If EXPR is omitted, returns square 4504root of C<$_>. Only works on non-negative operands, unless you've 4505loaded the standard Math::Complex module. 4506 4507 use Math::Complex; 4508 print sqrt(-2); # prints 1.4142135623731i 4509 4510=item srand EXPR 4511 4512=item srand 4513 4514Sets the random number seed for the C<rand> operator. If EXPR is 4515omitted, uses a semi-random value supplied by the kernel (if it supports 4516the F</dev/urandom> device) or based on the current time and process 4517ID, among other things. In versions of Perl prior to 5.004 the default 4518seed was just the current C<time>. This isn't a particularly good seed, 4519so many old programs supply their own seed value (often C<time ^ $$> or 4520C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more. 4521 4522In fact, it's usually not necessary to call C<srand> at all, because if 4523it is not called explicitly, it is called implicitly at the first use of 4524the C<rand> operator. However, this was not the case in version of Perl 4525before 5.004, so if your script will run under older Perl versions, it 4526should call C<srand>. 4527 4528Note that you need something much more random than the default seed for 4529cryptographic purposes. Checksumming the compressed output of one or more 4530rapidly changing operating system status programs is the usual method. For 4531example: 4532 4533 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`); 4534 4535If you're particularly concerned with this, see the C<Math::TrulyRandom> 4536module in CPAN. 4537 4538Do I<not> call C<srand> multiple times in your program unless you know 4539exactly what you're doing and why you're doing it. The point of the 4540function is to "seed" the C<rand> function so that C<rand> can produce 4541a different sequence each time you run your program. Just do it once at the 4542top of your program, or you I<won't> get random numbers out of C<rand>! 4543 4544Frequently called programs (like CGI scripts) that simply use 4545 4546 time ^ $$ 4547 4548for a seed can fall prey to the mathematical property that 4549 4550 a^b == (a+1)^(b+1) 4551 4552one-third of the time. So don't do that. 4553 4554=item stat FILEHANDLE 4555 4556=item stat EXPR 4557 4558=item stat 4559 4560Returns a 13-element list giving the status info for a file, either 4561the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, 4562it stats C<$_>. Returns a null list if the stat fails. Typically used 4563as follows: 4564 4565 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, 4566 $atime,$mtime,$ctime,$blksize,$blocks) 4567 = stat($filename); 4568 4569Not all fields are supported on all filesystem types. Here are the 4570meaning of the fields: 4571 4572 0 dev device number of filesystem 4573 1 ino inode number 4574 2 mode file mode (type and permissions) 4575 3 nlink number of (hard) links to the file 4576 4 uid numeric user ID of file's owner 4577 5 gid numeric group ID of file's owner 4578 6 rdev the device identifier (special files only) 4579 7 size total size of file, in bytes 4580 8 atime last access time in seconds since the epoch 4581 9 mtime last modify time in seconds since the epoch 4582 10 ctime inode change time (NOT creation time!) in seconds since the epoch 4583 11 blksize preferred block size for file system I/O 4584 12 blocks actual number of blocks allocated 4585 4586(The epoch was at 00:00 January 1, 1970 GMT.) 4587 4588If stat is passed the special filehandle consisting of an underline, no 4589stat is done, but the current contents of the stat structure from the 4590last stat or filetest are returned. Example: 4591 4592 if (-x $file && (($d) = stat(_)) && $d < 0) { 4593 print "$file is executable NFS file\n"; 4594 } 4595 4596(This works on machines only for which the device number is negative 4597under NFS.) 4598 4599Because the mode contains both the file type and its permissions, you 4600should mask off the file type portion and (s)printf using a C<"%o"> 4601if you want to see the real permissions. 4602 4603 $mode = (stat($filename))[2]; 4604 printf "Permissions are %04o\n", $mode & 07777; 4605 4606In scalar context, C<stat> returns a boolean value indicating success 4607or failure, and, if successful, sets the information associated with 4608the special filehandle C<_>. 4609 4610The File::stat module provides a convenient, by-name access mechanism: 4611 4612 use File::stat; 4613 $sb = stat($filename); 4614 printf "File is %s, size is %s, perm %04o, mtime %s\n", 4615 $filename, $sb->size, $sb->mode & 07777, 4616 scalar localtime $sb->mtime; 4617 4618You can import symbolic mode constants (C<S_IF*>) and functions 4619(C<S_IS*>) from the Fcntl module: 4620 4621 use Fcntl ':mode'; 4622 4623 $mode = (stat($filename))[2]; 4624 4625 $user_rwx = ($mode & S_IRWXU) >> 6; 4626 $group_read = ($mode & S_IRGRP) >> 3; 4627 $other_execute = $mode & S_IXOTH; 4628 4629 printf "Permissions are %04o\n", S_ISMODE($mode), "\n"; 4630 4631 $is_setuid = $mode & S_ISUID; 4632 $is_setgid = S_ISDIR($mode); 4633 4634You could write the last two using the C<-u> and C<-d> operators. 4635The commonly available S_IF* constants are 4636 4637 # Permissions: read, write, execute, for user, group, others. 4638 4639 S_IRWXU S_IRUSR S_IWUSR S_IXUSR 4640 S_IRWXG S_IRGRP S_IWGRP S_IXGRP 4641 S_IRWXO S_IROTH S_IWOTH S_IXOTH 4642 4643 # Setuid/Setgid/Stickiness. 4644 4645 S_ISUID S_ISGID S_ISVTX S_ISTXT 4646 4647 # File types. Not necessarily all are available on your system. 4648 4649 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT 4650 4651 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR. 4652 4653 S_IREAD S_IWRITE S_IEXEC 4654 4655and the S_IF* functions are 4656 4657 S_IFMODE($mode) the part of $mode containg the permission bits 4658 and the setuid/setgid/sticky bits 4659 4660 S_IFMT($mode) the part of $mode containing the file type 4661 which can be bit-anded with e.g. S_IFREG 4662 or with the following functions 4663 4664 # The operators -f, -d, -l, -b, -c, -p, and -s. 4665 4666 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode) 4667 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode) 4668 4669 # No direct -X operator counterpart, but for the first one 4670 # the -g operator is often equivalent. The ENFMT stands for 4671 # record flocking enforcement, a platform-dependent feature. 4672 4673 S_ISENFMT($mode) S_ISWHT($mode) 4674 4675See your native chmod(2) and stat(2) documentation for more details 4676about the S_* constants. 4677 4678=item study SCALAR 4679 4680=item study 4681 4682Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of 4683doing many pattern matches on the string before it is next modified. 4684This may or may not save time, depending on the nature and number of 4685patterns you are searching on, and on the distribution of character 4686frequencies in the string to be searched--you probably want to compare 4687run times with and without it to see which runs faster. Those loops 4688which scan for many short constant strings (including the constant 4689parts of more complex patterns) will benefit most. You may have only 4690one C<study> active at a time--if you study a different scalar the first 4691is "unstudied". (The way C<study> works is this: a linked list of every 4692character in the string to be searched is made, so we know, for 4693example, where all the C<'k'> characters are. From each search string, 4694the rarest character is selected, based on some static frequency tables 4695constructed from some C programs and English text. Only those places 4696that contain this "rarest" character are examined.) 4697 4698For example, here is a loop that inserts index producing entries 4699before any line containing a certain pattern: 4700 4701 while (<>) { 4702 study; 4703 print ".IX foo\n" if /\bfoo\b/; 4704 print ".IX bar\n" if /\bbar\b/; 4705 print ".IX blurfl\n" if /\bblurfl\b/; 4706 # ... 4707 print; 4708 } 4709 4710In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f> 4711will be looked at, because C<f> is rarer than C<o>. In general, this is 4712a big win except in pathological cases. The only question is whether 4713it saves you more time than it took to build the linked list in the 4714first place. 4715 4716Note that if you have to look for strings that you don't know till 4717runtime, you can build an entire loop as a string and C<eval> that to 4718avoid recompiling all your patterns all the time. Together with 4719undefining C<$/> to input entire files as one record, this can be very 4720fast, often faster than specialized programs like fgrep(1). The following 4721scans a list of files (C<@files>) for a list of words (C<@words>), and prints 4722out the names of those files that contain a match: 4723 4724 $search = 'while (<>) { study;'; 4725 foreach $word (@words) { 4726 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; 4727 } 4728 $search .= "}"; 4729 @ARGV = @files; 4730 undef $/; 4731 eval $search; # this screams 4732 $/ = "\n"; # put back to normal input delimiter 4733 foreach $file (sort keys(%seen)) { 4734 print $file, "\n"; 4735 } 4736 4737=item sub BLOCK 4738 4739=item sub NAME 4740 4741=item sub NAME BLOCK 4742 4743This is subroutine definition, not a real function I<per se>. With just a 4744NAME (and possibly prototypes or attributes), it's just a forward declaration. 4745Without a NAME, it's an anonymous function declaration, and does actually 4746return a value: the CODE ref of the closure you just created. See L<perlsub> 4747and L<perlref> for details. 4748 4749=item substr EXPR,OFFSET,LENGTH,REPLACEMENT 4750 4751=item substr EXPR,OFFSET,LENGTH 4752 4753=item substr EXPR,OFFSET 4754 4755Extracts a substring out of EXPR and returns it. First character is at 4756offset C<0>, or whatever you've set C<$[> to (but don't do that). 4757If OFFSET is negative (or more precisely, less than C<$[>), starts 4758that far from the end of the string. If LENGTH is omitted, returns 4759everything to the end of the string. If LENGTH is negative, leaves that 4760many characters off the end of the string. 4761 4762You can use the substr() function as an lvalue, in which case EXPR 4763must itself be an lvalue. If you assign something shorter than LENGTH, 4764the string will shrink, and if you assign something longer than LENGTH, 4765the string will grow to accommodate it. To keep the string the same 4766length you may need to pad or chop your value using C<sprintf>. 4767 4768If OFFSET and LENGTH specify a substring that is partly outside the 4769string, only the part within the string is returned. If the substring 4770is beyond either end of the string, substr() returns the undefined 4771value and produces a warning. When used as an lvalue, specifying a 4772substring that is entirely outside the string is a fatal error. 4773Here's an example showing the behavior for boundary cases: 4774 4775 my $name = 'fred'; 4776 substr($name, 4) = 'dy'; # $name is now 'freddy' 4777 my $null = substr $name, 6, 2; # returns '' (no warning) 4778 my $oops = substr $name, 7; # returns undef, with warning 4779 substr($name, 7) = 'gap'; # fatal error 4780 4781An alternative to using substr() as an lvalue is to specify the 4782replacement string as the 4th argument. This allows you to replace 4783parts of the EXPR and return what was there before in one operation, 4784just as you can with splice(). 4785 4786=item symlink OLDFILE,NEWFILE 4787 4788Creates a new filename symbolically linked to the old filename. 4789Returns C<1> for success, C<0> otherwise. On systems that don't support 4790symbolic links, produces a fatal error at run time. To check for that, 4791use eval: 4792 4793 $symlink_exists = eval { symlink("",""); 1 }; 4794 4795=item syscall LIST 4796 4797Calls the system call specified as the first element of the list, 4798passing the remaining elements as arguments to the system call. If 4799unimplemented, produces a fatal error. The arguments are interpreted 4800as follows: if a given argument is numeric, the argument is passed as 4801an int. If not, the pointer to the string value is passed. You are 4802responsible to make sure a string is pre-extended long enough to 4803receive any result that might be written into a string. You can't use a 4804string literal (or other read-only string) as an argument to C<syscall> 4805because Perl has to assume that any string pointer might be written 4806through. If your 4807integer arguments are not literals and have never been interpreted in a 4808numeric context, you may need to add C<0> to them to force them to look 4809like numbers. This emulates the C<syswrite> function (or vice versa): 4810 4811 require 'syscall.ph'; # may need to run h2ph 4812 $s = "hi there\n"; 4813 syscall(&SYS_write, fileno(STDOUT), $s, length $s); 4814 4815Note that Perl supports passing of up to only 14 arguments to your system call, 4816which in practice should usually suffice. 4817 4818Syscall returns whatever value returned by the system call it calls. 4819If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno). 4820Note that some system calls can legitimately return C<-1>. The proper 4821way to handle such calls is to assign C<$!=0;> before the call and 4822check the value of C<$!> if syscall returns C<-1>. 4823 4824There's a problem with C<syscall(&SYS_pipe)>: it returns the file 4825number of the read end of the pipe it creates. There is no way 4826to retrieve the file number of the other end. You can avoid this 4827problem by using C<pipe> instead. 4828 4829=item sysopen FILEHANDLE,FILENAME,MODE 4830 4831=item sysopen FILEHANDLE,FILENAME,MODE,PERMS 4832 4833Opens the file whose filename is given by FILENAME, and associates it 4834with FILEHANDLE. If FILEHANDLE is an expression, its value is used as 4835the name of the real filehandle wanted. This function calls the 4836underlying operating system's C<open> function with the parameters 4837FILENAME, MODE, PERMS. 4838 4839The possible values and flag bits of the MODE parameter are 4840system-dependent; they are available via the standard module C<Fcntl>. 4841See the documentation of your operating system's C<open> to see which 4842values and flag bits are available. You may combine several flags 4843using the C<|>-operator. 4844 4845Some of the most common values are C<O_RDONLY> for opening the file in 4846read-only mode, C<O_WRONLY> for opening the file in write-only mode, 4847and C<O_RDWR> for opening the file in read-write mode, and. 4848 4849For historical reasons, some values work on almost every system 4850supported by perl: zero means read-only, one means write-only, and two 4851means read/write. We know that these values do I<not> work under 4852OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to 4853use them in new code. 4854 4855If the file named by FILENAME does not exist and the C<open> call creates 4856it (typically because MODE includes the C<O_CREAT> flag), then the value of 4857PERMS specifies the permissions of the newly created file. If you omit 4858the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>. 4859These permission values need to be in octal, and are modified by your 4860process's current C<umask>. 4861 4862In many systems the C<O_EXCL> flag is available for opening files in 4863exclusive mode. This is B<not> locking: exclusiveness means here that 4864if the file already exists, sysopen() fails. The C<O_EXCL> wins 4865C<O_TRUNC>. 4866 4867Sometimes you may want to truncate an already-existing file: C<O_TRUNC>. 4868 4869You should seldom if ever use C<0644> as argument to C<sysopen>, because 4870that takes away the user's option to have a more permissive umask. 4871Better to omit it. See the perlfunc(1) entry on C<umask> for more 4872on this. 4873 4874Note that C<sysopen> depends on the fdopen() C library function. 4875On many UNIX systems, fdopen() is known to fail when file descriptors 4876exceed a certain value, typically 255. If you need more file 4877descriptors than that, consider rebuilding Perl to use the C<sfio> 4878library, or perhaps using the POSIX::open() function. 4879 4880See L<perlopentut> for a kinder, gentler explanation of opening files. 4881 4882=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET 4883 4884=item sysread FILEHANDLE,SCALAR,LENGTH 4885 4886Attempts to read LENGTH bytes of data into variable SCALAR from the 4887specified FILEHANDLE, using the system call read(2). It bypasses stdio, 4888so mixing this with other kinds of reads, C<print>, C<write>, 4889C<seek>, C<tell>, or C<eof> can cause confusion because stdio 4890usually buffers data. Returns the number of bytes actually read, C<0> 4891at end of file, or undef if there was an error. SCALAR will be grown or 4892shrunk so that the last byte actually read is the last byte of the 4893scalar after the read. 4894 4895An OFFSET may be specified to place the read data at some place in the 4896string other than the beginning. A negative OFFSET specifies 4897placement at that many bytes counting backwards from the end of the 4898string. A positive OFFSET greater than the length of SCALAR results 4899in the string being padded to the required size with C<"\0"> bytes before 4900the result of the read is appended. 4901 4902There is no syseof() function, which is ok, since eof() doesn't work 4903very well on device files (like ttys) anyway. Use sysread() and check 4904for a return value for 0 to decide whether you're done. 4905 4906=item sysseek FILEHANDLE,POSITION,WHENCE 4907 4908Sets FILEHANDLE's system position using the system call lseek(2). It 4909bypasses stdio, so mixing this with reads (other than C<sysread>), 4910C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion. 4911FILEHANDLE may be an expression whose value gives the name of the 4912filehandle. The values for WHENCE are C<0> to set the new position to 4913POSITION, C<1> to set the it to the current position plus POSITION, 4914and C<2> to set it to EOF plus POSITION (typically negative). For 4915WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and 4916C<SEEK_END> (start of the file, current position, end of the file) 4917from the Fcntl module. 4918 4919Returns the new position, or the undefined value on failure. A position 4920of zero is returned as the string C<"0 but true">; thus C<sysseek> returns 4921true on success and false on failure, yet you can still easily determine 4922the new position. 4923 4924=item system LIST 4925 4926=item system PROGRAM LIST 4927 4928Does exactly the same thing as C<exec LIST>, except that a fork is 4929done first, and the parent process waits for the child process to 4930complete. Note that argument processing varies depending on the 4931number of arguments. If there is more than one argument in LIST, 4932or if LIST is an array with more than one value, starts the program 4933given by the first element of the list with arguments given by the 4934rest of the list. If there is only one scalar argument, the argument 4935is checked for shell metacharacters, and if there are any, the 4936entire argument is passed to the system's command shell for parsing 4937(this is C</bin/sh -c> on Unix platforms, but varies on other 4938platforms). If there are no shell metacharacters in the argument, 4939it is split into words and passed directly to C<execvp>, which is 4940more efficient. 4941 4942Beginning with v5.6.0, Perl will attempt to flush all files opened for 4943output before any operation that may do a fork, but this may not be 4944supported on some platforms (see L<perlport>). To be safe, you may need 4945to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method 4946of C<IO::Handle> on any open handles. 4947 4948The return value is the exit status of the program as 4949returned by the C<wait> call. To get the actual exit value divide by 4950256. See also L</exec>. This is I<not> what you want to use to capture 4951the output from a command, for that you should use merely backticks or 4952C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1 4953indicates a failure to start the program (inspect $! for the reason). 4954 4955Like C<exec>, C<system> allows you to lie to a program about its name if 4956you use the C<system PROGRAM LIST> syntax. Again, see L</exec>. 4957 4958Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the 4959program they're running doesn't actually interrupt your program. 4960 4961 @args = ("command", "arg1", "arg2"); 4962 system(@args) == 0 4963 or die "system @args failed: $?" 4964 4965You can check all the failure possibilities by inspecting 4966C<$?> like this: 4967 4968 $exit_value = $? >> 8; 4969 $signal_num = $? & 127; 4970 $dumped_core = $? & 128; 4971 4972When the arguments get executed via the system shell, results 4973and return codes will be subject to its quirks and capabilities. 4974See L<perlop/"`STRING`"> and L</exec> for details. 4975 4976=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET 4977 4978=item syswrite FILEHANDLE,SCALAR,LENGTH 4979 4980=item syswrite FILEHANDLE,SCALAR 4981 4982Attempts to write LENGTH bytes of data from variable SCALAR to the 4983specified FILEHANDLE, using the system call write(2). If LENGTH 4984is not specified, writes whole SCALAR. It bypasses stdio, so mixing 4985this with reads (other than C<sysread())>, C<print>, C<write>, 4986C<seek>, C<tell>, or C<eof> may cause confusion because stdio 4987usually buffers data. Returns the number of bytes actually written, 4988or C<undef> if there was an error. If the LENGTH is greater than 4989the available data in the SCALAR after the OFFSET, only as much 4990data as is available will be written. 4991 4992An OFFSET may be specified to write the data from some part of the 4993string other than the beginning. A negative OFFSET specifies writing 4994that many bytes counting backwards from the end of the string. In the 4995case the SCALAR is empty you can use OFFSET but only zero offset. 4996 4997=item tell FILEHANDLE 4998 4999=item tell 5000 5001Returns the current position for FILEHANDLE. FILEHANDLE may be an 5002expression whose value gives the name of the actual filehandle. If 5003FILEHANDLE is omitted, assumes the file last read. 5004 5005There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that. 5006 5007=item telldir DIRHANDLE 5008 5009Returns the current position of the C<readdir> routines on DIRHANDLE. 5010Value may be given to C<seekdir> to access a particular location in a 5011directory. Has the same caveats about possible directory compaction as 5012the corresponding system library routine. 5013 5014=item tie VARIABLE,CLASSNAME,LIST 5015 5016This function binds a variable to a package class that will provide the 5017implementation for the variable. VARIABLE is the name of the variable 5018to be enchanted. CLASSNAME is the name of a class implementing objects 5019of correct type. Any additional arguments are passed to the C<new> 5020method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>, 5021or C<TIEHASH>). Typically these are arguments such as might be passed 5022to the C<dbm_open()> function of C. The object returned by the C<new> 5023method is also returned by the C<tie> function, which would be useful 5024if you want to access other methods in CLASSNAME. 5025 5026Note that functions such as C<keys> and C<values> may return huge lists 5027when used on large objects, like DBM files. You may prefer to use the 5028C<each> function to iterate over such. Example: 5029 5030 # print out history file offsets 5031 use NDBM_File; 5032 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); 5033 while (($key,$val) = each %HIST) { 5034 print $key, ' = ', unpack('L',$val), "\n"; 5035 } 5036 untie(%HIST); 5037 5038A class implementing a hash should have the following methods: 5039 5040 TIEHASH classname, LIST 5041 FETCH this, key 5042 STORE this, key, value 5043 DELETE this, key 5044 CLEAR this 5045 EXISTS this, key 5046 FIRSTKEY this 5047 NEXTKEY this, lastkey 5048 DESTROY this 5049 5050A class implementing an ordinary array should have the following methods: 5051 5052 TIEARRAY classname, LIST 5053 FETCH this, key 5054 STORE this, key, value 5055 FETCHSIZE this 5056 STORESIZE this, count 5057 CLEAR this 5058 PUSH this, LIST 5059 POP this 5060 SHIFT this 5061 UNSHIFT this, LIST 5062 SPLICE this, offset, length, LIST 5063 EXTEND this, count 5064 DESTROY this 5065 5066A class implementing a file handle should have the following methods: 5067 5068 TIEHANDLE classname, LIST 5069 READ this, scalar, length, offset 5070 READLINE this 5071 GETC this 5072 WRITE this, scalar, length, offset 5073 PRINT this, LIST 5074 PRINTF this, format, LIST 5075 CLOSE this 5076 DESTROY this 5077 5078A class implementing a scalar should have the following methods: 5079 5080 TIESCALAR classname, LIST 5081 FETCH this, 5082 STORE this, value 5083 DESTROY this 5084 5085Not all methods indicated above need be implemented. See L<perltie>, 5086L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>. 5087 5088Unlike C<dbmopen>, the C<tie> function will not use or require a module 5089for you--you need to do that explicitly yourself. See L<DB_File> 5090or the F<Config> module for interesting C<tie> implementations. 5091 5092For further details see L<perltie>, L<"tied VARIABLE">. 5093 5094=item tied VARIABLE 5095 5096Returns a reference to the object underlying VARIABLE (the same value 5097that was originally returned by the C<tie> call that bound the variable 5098to a package.) Returns the undefined value if VARIABLE isn't tied to a 5099package. 5100 5101=item time 5102 5103Returns the number of non-leap seconds since whatever time the system 5104considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS, 5105and 00:00:00 UTC, January 1, 1970 for most other systems). 5106Suitable for feeding to C<gmtime> and C<localtime>. 5107 5108For measuring time in better granularity than one second, 5109you may use either the Time::HiRes module from CPAN, or 5110if you have gettimeofday(2), you may be able to use the 5111C<syscall> interface of Perl, see L<perlfaq8> for details. 5112 5113=item times 5114 5115Returns a four-element list giving the user and system times, in 5116seconds, for this process and the children of this process. 5117 5118 ($user,$system,$cuser,$csystem) = times; 5119 5120=item tr/// 5121 5122The transliteration operator. Same as C<y///>. See L<perlop>. 5123 5124=item truncate FILEHANDLE,LENGTH 5125 5126=item truncate EXPR,LENGTH 5127 5128Truncates the file opened on FILEHANDLE, or named by EXPR, to the 5129specified length. Produces a fatal error if truncate isn't implemented 5130on your system. Returns true if successful, the undefined value 5131otherwise. 5132 5133=item uc EXPR 5134 5135=item uc 5136 5137Returns an uppercased version of EXPR. This is the internal function 5138implementing the C<\U> escape in double-quoted strings. 5139Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. 5140Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It 5141does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.) 5142 5143If EXPR is omitted, uses C<$_>. 5144 5145=item ucfirst EXPR 5146 5147=item ucfirst 5148 5149Returns the value of EXPR with the first character 5150in uppercase (titlecase in Unicode). This is 5151the internal function implementing the C<\u> escape in double-quoted strings. 5152Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale> 5153and L<utf8>. 5154 5155If EXPR is omitted, uses C<$_>. 5156 5157=item umask EXPR 5158 5159=item umask 5160 5161Sets the umask for the process to EXPR and returns the previous value. 5162If EXPR is omitted, merely returns the current umask. 5163 5164The Unix permission C<rwxr-x---> is represented as three sets of three 5165bits, or three octal digits: C<0750> (the leading 0 indicates octal 5166and isn't one of the digits). The C<umask> value is such a number 5167representing disabled permissions bits. The permission (or "mode") 5168values you pass C<mkdir> or C<sysopen> are modified by your umask, so 5169even if you tell C<sysopen> to create a file with permissions C<0777>, 5170if your umask is C<0022> then the file will actually be created with 5171permissions C<0755>. If your C<umask> were C<0027> (group can't 5172write; others can't read, write, or execute), then passing 5173C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~ 5174027> is C<0640>). 5175 5176Here's some advice: supply a creation mode of C<0666> for regular 5177files (in C<sysopen>) and one of C<0777> for directories (in 5178C<mkdir>) and executable files. This gives users the freedom of 5179choice: if they want protected files, they might choose process umasks 5180of C<022>, C<027>, or even the particularly antisocial mask of C<077>. 5181Programs should rarely if ever make policy decisions better left to 5182the user. The exception to this is when writing files that should be 5183kept private: mail files, web browser cookies, I<.rhosts> files, and 5184so on. 5185 5186If umask(2) is not implemented on your system and you are trying to 5187restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a 5188fatal error at run time. If umask(2) is not implemented and you are 5189not trying to restrict access for yourself, returns C<undef>. 5190 5191Remember that a umask is a number, usually given in octal; it is I<not> a 5192string of octal digits. See also L</oct>, if all you have is a string. 5193 5194=item undef EXPR 5195 5196=item undef 5197 5198Undefines the value of EXPR, which must be an lvalue. Use only on a 5199scalar value, an array (using C<@>), a hash (using C<%>), a subroutine 5200(using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}> 5201will probably not do what you expect on most predefined variables or 5202DBM list values, so don't do that; see L<delete>.) Always returns the 5203undefined value. You can omit the EXPR, in which case nothing is 5204undefined, but you still get an undefined value that you could, for 5205instance, return from a subroutine, assign to a variable or pass as a 5206parameter. Examples: 5207 5208 undef $foo; 5209 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; 5210 undef @ary; 5211 undef %hash; 5212 undef &mysub; 5213 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. 5214 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; 5215 select undef, undef, undef, 0.25; 5216 ($a, $b, undef, $c) = &foo; # Ignore third value returned 5217 5218Note that this is a unary operator, not a list operator. 5219 5220=item unlink LIST 5221 5222=item unlink 5223 5224Deletes a list of files. Returns the number of files successfully 5225deleted. 5226 5227 $cnt = unlink 'a', 'b', 'c'; 5228 unlink @goners; 5229 unlink <*.bak>; 5230 5231Note: C<unlink> will not delete directories unless you are superuser and 5232the B<-U> flag is supplied to Perl. Even if these conditions are 5233met, be warned that unlinking a directory can inflict damage on your 5234filesystem. Use C<rmdir> instead. 5235 5236If LIST is omitted, uses C<$_>. 5237 5238=item unpack TEMPLATE,EXPR 5239 5240C<unpack> does the reverse of C<pack>: it takes a string 5241and expands it out into a list of values. 5242(In scalar context, it returns merely the first value produced.) 5243 5244The string is broken into chunks described by the TEMPLATE. Each chunk 5245is converted separately to a value. Typically, either the string is a result 5246of C<pack>, or the bytes of the string represent a C structure of some 5247kind. 5248 5249The TEMPLATE has the same format as in the C<pack> function. 5250Here's a subroutine that does substring: 5251 5252 sub substr { 5253 my($what,$where,$howmuch) = @_; 5254 unpack("x$where a$howmuch", $what); 5255 } 5256 5257and then there's 5258 5259 sub ordinal { unpack("c",$_[0]); } # same as ord() 5260 5261In addition to fields allowed in pack(), you may prefix a field with 5262a %<number> to indicate that 5263you want a <number>-bit checksum of the items instead of the items 5264themselves. Default is a 16-bit checksum. Checksum is calculated by 5265summing numeric values of expanded values (for string fields the sum of 5266C<ord($char)> is taken, for bit fields the sum of zeroes and ones). 5267 5268For example, the following 5269computes the same number as the System V sum program: 5270 5271 $checksum = do { 5272 local $/; # slurp! 5273 unpack("%32C*",<>) % 65535; 5274 }; 5275 5276The following efficiently counts the number of set bits in a bit vector: 5277 5278 $setbits = unpack("%32b*", $selectmask); 5279 5280The C<p> and C<P> formats should be used with care. Since Perl 5281has no way of checking whether the value passed to C<unpack()> 5282corresponds to a valid memory location, passing a pointer value that's 5283not known to be valid is likely to have disastrous consequences. 5284 5285If the repeat count of a field is larger than what the remainder of 5286the input string allows, repeat count is decreased. If the input string 5287is longer than one described by the TEMPLATE, the rest is ignored. 5288 5289See L</pack> for more examples and notes. 5290 5291=item untie VARIABLE 5292 5293Breaks the binding between a variable and a package. (See C<tie>.) 5294 5295=item unshift ARRAY,LIST 5296 5297Does the opposite of a C<shift>. Or the opposite of a C<push>, 5298depending on how you look at it. Prepends list to the front of the 5299array, and returns the new number of elements in the array. 5300 5301 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/; 5302 5303Note the LIST is prepended whole, not one element at a time, so the 5304prepended elements stay in the same order. Use C<reverse> to do the 5305reverse. 5306 5307=item use Module VERSION LIST 5308 5309=item use Module VERSION 5310 5311=item use Module LIST 5312 5313=item use Module 5314 5315=item use VERSION 5316 5317Imports some semantics into the current package from the named module, 5318generally by aliasing certain subroutine or variable names into your 5319package. It is exactly equivalent to 5320 5321 BEGIN { require Module; import Module LIST; } 5322 5323except that Module I<must> be a bareword. 5324 5325VERSION, which can be specified as a literal of the form v5.6.1, demands 5326that the current version of Perl (C<$^V> or $PERL_VERSION) be at least 5327as recent as that version. (For compatibility with older versions of Perl, 5328a numeric literal will also be interpreted as VERSION.) If the version 5329of the running Perl interpreter is less than VERSION, then an error 5330message is printed and Perl exits immediately without attempting to 5331parse the rest of the file. Compare with L</require>, which can do a 5332similar check at run time. 5333 5334 use v5.6.1; # compile time version check 5335 use 5.6.1; # ditto 5336 use 5.005_03; # float version allowed for compatibility 5337 5338This is often useful if you need to check the current Perl version before 5339C<use>ing library modules that have changed in incompatible ways from 5340older versions of Perl. (We try not to do this more than we have to.) 5341 5342The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The 5343C<require> makes sure the module is loaded into memory if it hasn't been 5344yet. The C<import> is not a builtin--it's just an ordinary static method 5345call into the C<Module> package to tell the module to import the list of 5346features back into the current package. The module can implement its 5347C<import> method any way it likes, though most modules just choose to 5348derive their C<import> method via inheritance from the C<Exporter> class that 5349is defined in the C<Exporter> module. See L<Exporter>. If no C<import> 5350method can be found then the call is skipped. 5351 5352If you don't want your namespace altered, explicitly supply an empty list: 5353 5354 use Module (); 5355 5356That is exactly equivalent to 5357 5358 BEGIN { require Module } 5359 5360If the VERSION argument is present between Module and LIST, then the 5361C<use> will call the VERSION method in class Module with the given 5362version as an argument. The default VERSION method, inherited from 5363the UNIVERSAL class, croaks if the given version is larger than the 5364value of the variable C<$Module::VERSION>. 5365 5366Again, there is a distinction between omitting LIST (C<import> called 5367with no arguments) and an explicit empty LIST C<()> (C<import> not 5368called). Note that there is no comma after VERSION! 5369 5370Because this is a wide-open interface, pragmas (compiler directives) 5371are also implemented this way. Currently implemented pragmas are: 5372 5373 use integer; 5374 use diagnostics; 5375 use sigtrap qw(SEGV BUS); 5376 use strict qw(subs vars refs); 5377 use subs qw(afunc blurfl); 5378 use warnings qw(all); 5379 5380Some of these pseudo-modules import semantics into the current 5381block scope (like C<strict> or C<integer>, unlike ordinary modules, 5382which import symbols into the current package (which are effective 5383through the end of the file). 5384 5385There's a corresponding C<no> command that unimports meanings imported 5386by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>. 5387 5388 no integer; 5389 no strict 'refs'; 5390 no warnings; 5391 5392If no C<unimport> method can be found the call fails with a fatal error. 5393 5394See L<perlmod> for a list of standard modules and pragmas. 5395 5396=item utime LIST 5397 5398Changes the access and modification times on each file of a list of 5399files. The first two elements of the list must be the NUMERICAL access 5400and modification times, in that order. Returns the number of files 5401successfully changed. The inode change time of each file is set 5402to the current time. This code has the same effect as the C<touch> 5403command if the files already exist: 5404 5405 #!/usr/bin/perl 5406 $now = time; 5407 utime $now, $now, @ARGV; 5408 5409=item values HASH 5410 5411Returns a list consisting of all the values of the named hash. (In a 5412scalar context, returns the number of values.) The values are 5413returned in an apparently random order. The actual random order is 5414subject to change in future versions of perl, but it is guaranteed to 5415be the same order as either the C<keys> or C<each> function would 5416produce on the same (unmodified) hash. 5417 5418Note that you cannot modify the values of a hash this way, because the 5419returned list is just a copy. You need to use a hash slice for that, 5420since it's lvaluable in a way that values() is not. 5421 5422 for (values %hash) { s/foo/bar/g } # FAILS! 5423 for (@hash{keys %hash}) { s/foo/bar/g } # ok 5424 5425As a side effect, calling values() resets the HASH's internal iterator. 5426See also C<keys>, C<each>, and C<sort>. 5427 5428=item vec EXPR,OFFSET,BITS 5429 5430Treats the string in EXPR as a bit vector made up of elements of 5431width BITS, and returns the value of the element specified by OFFSET 5432as an unsigned integer. BITS therefore specifies the number of bits 5433that are reserved for each element in the bit vector. This must 5434be a power of two from 1 to 32 (or 64, if your platform supports 5435that). 5436 5437If BITS is 8, "elements" coincide with bytes of the input string. 5438 5439If BITS is 16 or more, bytes of the input string are grouped into chunks 5440of size BITS/8, and each group is converted to a number as with 5441pack()/unpack() with big-endian formats C<n>/C<N> (and analoguously 5442for BITS==64). See L<"pack"> for details. 5443 5444If bits is 4 or less, the string is broken into bytes, then the bits 5445of each byte are broken into 8/BITS groups. Bits of a byte are 5446numbered in a little-endian-ish way, as in C<0x01>, C<0x02>, 5447C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example, 5448breaking the single input byte C<chr(0x36)> into two groups gives a list 5449C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>. 5450 5451C<vec> may also be assigned to, in which case parentheses are needed 5452to give the expression the correct precedence as in 5453 5454 vec($image, $max_x * $x + $y, 8) = 3; 5455 5456If the selected element is off the end of the string, the value 0 is 5457returned. If an element off the end of the string is written to, 5458Perl will first extend the string with sufficiently many zero bytes. 5459 5460Strings created with C<vec> can also be manipulated with the logical 5461operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit 5462vector operation is desired when both operands are strings. 5463See L<perlop/"Bitwise String Operators">. 5464 5465The following code will build up an ASCII string saying C<'PerlPerlPerl'>. 5466The comments show the string after each step. Note that this code works 5467in the same way on big-endian or little-endian machines. 5468 5469 my $foo = ''; 5470 vec($foo, 0, 32) = 0x5065726C; # 'Perl' 5471 5472 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits 5473 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P') 5474 5475 vec($foo, 2, 16) = 0x5065; # 'PerlPe' 5476 vec($foo, 3, 16) = 0x726C; # 'PerlPerl' 5477 vec($foo, 8, 8) = 0x50; # 'PerlPerlP' 5478 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' 5479 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" 5480 vec($foo, 21, 4) = 7; # 'PerlPerlPer' 5481 # 'r' is "\x72" 5482 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" 5483 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" 5484 vec($foo, 94, 1) = 1; # 'PerlPerlPerl' 5485 # 'l' is "\x6c" 5486 5487To transform a bit vector into a string or list of 0's and 1's, use these: 5488 5489 $bits = unpack("b*", $vector); 5490 @bits = split(//, unpack("b*", $vector)); 5491 5492If you know the exact length in bits, it can be used in place of the C<*>. 5493 5494Here is an example to illustrate how the bits actually fall in place: 5495 5496 #!/usr/bin/perl -wl 5497 5498 print <<'EOT'; 5499 0 1 2 3 5500 unpack("V",$_) 01234567890123456789012345678901 5501 ------------------------------------------------------------------ 5502 EOT 5503 5504 for $w (0..3) { 5505 $width = 2**$w; 5506 for ($shift=0; $shift < $width; ++$shift) { 5507 for ($off=0; $off < 32/$width; ++$off) { 5508 $str = pack("B*", "0"x32); 5509 $bits = (1<<$shift); 5510 vec($str, $off, $width) = $bits; 5511 $res = unpack("b*",$str); 5512 $val = unpack("V", $str); 5513 write; 5514 } 5515 } 5516 } 5517 5518 format STDOUT = 5519 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 5520 $off, $width, $bits, $val, $res 5521 . 5522 __END__ 5523 5524Regardless of the machine architecture on which it is run, the above 5525example should print the following table: 5526 5527 0 1 2 3 5528 unpack("V",$_) 01234567890123456789012345678901 5529 ------------------------------------------------------------------ 5530 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000 5531 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000 5532 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000 5533 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000 5534 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000 5535 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000 5536 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000 5537 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000 5538 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000 5539 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000 5540 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000 5541 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000 5542 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000 5543 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000 5544 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000 5545 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000 5546 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000 5547 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000 5548 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000 5549 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000 5550 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000 5551 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000 5552 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000 5553 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000 5554 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000 5555 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000 5556 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000 5557 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000 5558 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000 5559 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100 5560 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010 5561 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001 5562 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000 5563 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000 5564 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000 5565 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000 5566 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000 5567 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000 5568 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000 5569 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000 5570 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000 5571 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000 5572 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000 5573 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000 5574 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000 5575 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000 5576 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000 5577 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010 5578 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000 5579 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000 5580 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000 5581 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000 5582 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000 5583 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000 5584 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000 5585 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000 5586 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000 5587 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000 5588 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000 5589 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000 5590 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000 5591 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000 5592 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100 5593 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001 5594 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000 5595 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000 5596 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000 5597 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000 5598 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000 5599 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000 5600 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000 5601 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000 5602 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000 5603 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000 5604 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000 5605 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000 5606 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000 5607 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000 5608 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000 5609 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100 5610 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000 5611 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000 5612 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000 5613 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000 5614 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000 5615 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000 5616 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000 5617 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010 5618 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000 5619 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000 5620 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000 5621 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000 5622 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000 5623 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000 5624 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000 5625 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001 5626 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000 5627 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000 5628 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000 5629 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000 5630 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000 5631 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000 5632 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000 5633 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000 5634 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000 5635 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000 5636 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000 5637 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000 5638 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000 5639 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000 5640 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000 5641 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000 5642 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000 5643 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000 5644 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000 5645 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000 5646 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000 5647 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000 5648 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000 5649 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100 5650 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000 5651 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000 5652 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000 5653 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010 5654 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000 5655 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000 5656 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000 5657 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001 5658 5659=item wait 5660 5661Behaves like the wait(2) system call on your system: it waits for a child 5662process to terminate and returns the pid of the deceased process, or 5663C<-1> if there are no child processes. The status is returned in C<$?>. 5664Note that a return value of C<-1> could mean that child processes are 5665being automatically reaped, as described in L<perlipc>. 5666 5667=item waitpid PID,FLAGS 5668 5669Waits for a particular child process to terminate and returns the pid of 5670the deceased process, or C<-1> if there is no such child process. On some 5671systems, a value of 0 indicates that there are processes still running. 5672The status is returned in C<$?>. If you say 5673 5674 use POSIX ":sys_wait_h"; 5675 #... 5676 do { 5677 $kid = waitpid(-1,&WNOHANG); 5678 } until $kid == -1; 5679 5680then you can do a non-blocking wait for all pending zombie processes. 5681Non-blocking wait is available on machines supporting either the 5682waitpid(2) or wait4(2) system calls. However, waiting for a particular 5683pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the 5684system call by remembering the status values of processes that have 5685exited but have not been harvested by the Perl script yet.) 5686 5687Note that on some systems, a return value of C<-1> could mean that child 5688processes are being automatically reaped. See L<perlipc> for details, 5689and for other examples. 5690 5691=item wantarray 5692 5693Returns true if the context of the currently executing subroutine is 5694looking for a list value. Returns false if the context is looking 5695for a scalar. Returns the undefined value if the context is looking 5696for no value (void context). 5697 5698 return unless defined wantarray; # don't bother doing more 5699 my @a = complex_calculation(); 5700 return wantarray ? @a : "@a"; 5701 5702This function should have been named wantlist() instead. 5703 5704=item warn LIST 5705 5706Produces a message on STDERR just like C<die>, but doesn't exit or throw 5707an exception. 5708 5709If LIST is empty and C<$@> already contains a value (typically from a 5710previous eval) that value is used after appending C<"\t...caught"> 5711to C<$@>. This is useful for staying almost, but not entirely similar to 5712C<die>. 5713 5714If C<$@> is empty then the string C<"Warning: Something's wrong"> is used. 5715 5716No message is printed if there is a C<$SIG{__WARN__}> handler 5717installed. It is the handler's responsibility to deal with the message 5718as it sees fit (like, for instance, converting it into a C<die>). Most 5719handlers must therefore make arrangements to actually display the 5720warnings that they are not prepared to deal with, by calling C<warn> 5721again in the handler. Note that this is quite safe and will not 5722produce an endless loop, since C<__WARN__> hooks are not called from 5723inside one. 5724 5725You will find this behavior is slightly different from that of 5726C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can 5727instead call C<die> again to change it). 5728 5729Using a C<__WARN__> handler provides a powerful way to silence all 5730warnings (even the so-called mandatory ones). An example: 5731 5732 # wipe out *all* compile-time warnings 5733 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } 5734 my $foo = 10; 5735 my $foo = 20; # no warning about duplicate my $foo, 5736 # but hey, you asked for it! 5737 # no compile-time or run-time warnings before here 5738 $DOWARN = 1; 5739 5740 # run-time warnings enabled after here 5741 warn "\$foo is alive and $foo!"; # does show up 5742 5743See L<perlvar> for details on setting C<%SIG> entries, and for more 5744examples. See the Carp module for other kinds of warnings using its 5745carp() and cluck() functions. 5746 5747=item write FILEHANDLE 5748 5749=item write EXPR 5750 5751=item write 5752 5753Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, 5754using the format associated with that file. By default the format for 5755a file is the one having the same name as the filehandle, but the 5756format for the current output channel (see the C<select> function) may be set 5757explicitly by assigning the name of the format to the C<$~> variable. 5758 5759Top of form processing is handled automatically: if there is 5760insufficient room on the current page for the formatted record, the 5761page is advanced by writing a form feed, a special top-of-page format 5762is used to format the new page header, and then the record is written. 5763By default the top-of-page format is the name of the filehandle with 5764"_TOP" appended, but it may be dynamically set to the format of your 5765choice by assigning the name to the C<$^> variable while the filehandle is 5766selected. The number of lines remaining on the current page is in 5767variable C<$->, which can be set to C<0> to force a new page. 5768 5769If FILEHANDLE is unspecified, output goes to the current default output 5770channel, which starts out as STDOUT but may be changed by the 5771C<select> operator. If the FILEHANDLE is an EXPR, then the expression 5772is evaluated and the resulting string is used to look up the name of 5773the FILEHANDLE at run time. For more on formats, see L<perlform>. 5774 5775Note that write is I<not> the opposite of C<read>. Unfortunately. 5776 5777=item y/// 5778 5779The transliteration operator. Same as C<tr///>. See L<perlop>. 5780 5781=back 5782