perlfunc.pod revision 1.6
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 4 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<our>, C<package>, C<use> 150 151=item Miscellaneous functions 152 153C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, 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<our>, C<prototype>, 204C<qx>, C<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 (is empty). 278 -s File has nonzero size (returns size in bytes). 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 chomp; 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 regard 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, $evaltext contains the text of the 542C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement, 543$filename is C<(eval)>, but $evaltext is undefined. (Note also that 544each C<use> statement creates a C<require> frame inside an C<eval EXPR>) 545frame. C<$hasargs> is true if a new instance of C<@_> was set up for the 546frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller 547was compiled with. The C<$hints> and C<$bitmask> values are subject to 548change between versions of Perl, and are not meant for external use. 549 550Furthermore, when called from within the DB package, caller returns more 551detailed information: it sets the list variable C<@DB::args> to be the 552arguments with which the subroutine was invoked. 553 554Be aware that the optimizer might have optimized call frames away before 555C<caller> had a chance to get the information. That means that C<caller(N)> 556might not return information about the call frame you expect it do, for 557C<< N > 1 >>. In particular, C<@DB::args> might have information from the 558previous time C<caller> was called. 559 560=item chdir EXPR 561 562Changes the working directory to EXPR, if possible. If EXPR is omitted, 563changes to the directory specified by C<$ENV{HOME}>, if set; if not, 564changes to the directory specified by C<$ENV{LOGDIR}>. If neither is 565set, C<chdir> does nothing. It returns true upon success, false 566otherwise. See the example under C<die>. 567 568=item chmod LIST 569 570Changes the permissions of a list of files. The first element of the 571list must be the numerical mode, which should probably be an octal 572number, and which definitely should I<not> a string of octal digits: 573C<0644> is okay, C<'0644'> is not. Returns the number of files 574successfully changed. See also L</oct>, if all you have is a string. 575 576 $cnt = chmod 0755, 'foo', 'bar'; 577 chmod 0755, @executables; 578 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to 579 # --w----r-T 580 $mode = '0644'; chmod oct($mode), 'foo'; # this is better 581 $mode = 0644; chmod $mode, 'foo'; # this is best 582 583You can also import the symbolic C<S_I*> constants from the Fcntl 584module: 585 586 use Fcntl ':mode'; 587 588 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; 589 # This is identical to the chmod 0755 of the above example. 590 591=item chomp VARIABLE 592 593=item chomp LIST 594 595=item chomp 596 597This safer version of L</chop> removes any trailing string 598that corresponds to the current value of C<$/> (also known as 599$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total 600number of characters removed from all its arguments. It's often used to 601remove the newline from the end of an input record when you're worried 602that the final record may be missing its newline. When in paragraph 603mode (C<$/ = "">), it removes all trailing newlines from the string. 604When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is 605a reference to an integer or the like, see L<perlvar>) chomp() won't 606remove anything. 607If VARIABLE is omitted, it chomps C<$_>. Example: 608 609 while (<>) { 610 chomp; # avoid \n on last field 611 @array = split(/:/); 612 # ... 613 } 614 615If VARIABLE is a hash, it chomps the hash's values, but not its keys. 616 617You can actually chomp anything that's an lvalue, including an assignment: 618 619 chomp($cwd = `pwd`); 620 chomp($answer = <STDIN>); 621 622If you chomp a list, each element is chomped, and the total number of 623characters removed is returned. 624 625=item chop VARIABLE 626 627=item chop LIST 628 629=item chop 630 631Chops off the last character of a string and returns the character 632chopped. It is much more efficient than C<s/.$//s> because it neither 633scans nor copies the string. If VARIABLE is omitted, chops C<$_>. 634If VARIABLE is a hash, it chops the hash's values, but not its keys. 635 636You can actually chop anything that's an lvalue, including an assignment. 637 638If you chop a list, each element is chopped. Only the value of the 639last C<chop> is returned. 640 641Note that C<chop> returns the last character. To return all but the last 642character, use C<substr($string, 0, -1)>. 643 644=item chown LIST 645 646Changes the owner (and group) of a list of files. The first two 647elements of the list must be the I<numeric> uid and gid, in that 648order. A value of -1 in either position is interpreted by most 649systems to leave that value unchanged. Returns the number of files 650successfully changed. 651 652 $cnt = chown $uid, $gid, 'foo', 'bar'; 653 chown $uid, $gid, @filenames; 654 655Here's an example that looks up nonnumeric uids in the passwd file: 656 657 print "User: "; 658 chomp($user = <STDIN>); 659 print "Files: "; 660 chomp($pattern = <STDIN>); 661 662 ($login,$pass,$uid,$gid) = getpwnam($user) 663 or die "$user not in passwd file"; 664 665 @ary = glob($pattern); # expand filenames 666 chown $uid, $gid, @ary; 667 668On most systems, you are not allowed to change the ownership of the 669file unless you're the superuser, although you should be able to change 670the group to any of your secondary groups. On insecure systems, these 671restrictions may be relaxed, but this is not a portable assumption. 672On POSIX systems, you can detect this condition this way: 673 674 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED); 675 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED); 676 677=item chr NUMBER 678 679=item chr 680 681Returns the character represented by that NUMBER in the character set. 682For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and 683chr(0x263a) is a Unicode smiley face (but only within the scope of 684a C<use utf8>). For the reverse, use L</ord>. 685See L<utf8> for more about Unicode. 686 687If NUMBER is omitted, uses C<$_>. 688 689=item chroot FILENAME 690 691=item chroot 692 693This function works like the system call by the same name: it makes the 694named directory the new root directory for all further pathnames that 695begin with a C</> by your process and all its children. (It doesn't 696change your current working directory, which is unaffected.) For security 697reasons, this call is restricted to the superuser. If FILENAME is 698omitted, does a C<chroot> to C<$_>. 699 700=item close FILEHANDLE 701 702=item close 703 704Closes the file or pipe associated with the file handle, returning true 705only if stdio successfully flushes buffers and closes the system file 706descriptor. Closes the currently selected filehandle if the argument 707is omitted. 708 709You don't have to close FILEHANDLE if you are immediately going to do 710another C<open> on it, because C<open> will close it for you. (See 711C<open>.) However, an explicit C<close> on an input file resets the line 712counter (C<$.>), while the implicit close done by C<open> does not. 713 714If the file handle came from a piped open C<close> will additionally 715return false if one of the other system calls involved fails or if the 716program exits with non-zero status. (If the only problem was that the 717program exited non-zero C<$!> will be set to C<0>.) Closing a pipe 718also waits for the process executing on the pipe to complete, in case you 719want to look at the output of the pipe afterwards, and 720implicitly puts the exit status value of that command into C<$?>. 721 722Prematurely closing the read end of a pipe (i.e. before the process 723writing to it at the other end has closed it) will result in a 724SIGPIPE being delivered to the writer. If the other end can't 725handle that, be sure to read all the data before closing the pipe. 726 727Example: 728 729 open(OUTPUT, '|sort >foo') # pipe to sort 730 or die "Can't start sort: $!"; 731 #... # print stuff to output 732 close OUTPUT # wait for sort to finish 733 or warn $! ? "Error closing sort pipe: $!" 734 : "Exit status $? from sort"; 735 open(INPUT, 'foo') # get sort's results 736 or die "Can't open 'foo' for input: $!"; 737 738FILEHANDLE may be an expression whose value can be used as an indirect 739filehandle, usually the real filehandle name. 740 741=item closedir DIRHANDLE 742 743Closes a directory opened by C<opendir> and returns the success of that 744system call. 745 746DIRHANDLE may be an expression whose value can be used as an indirect 747dirhandle, usually the real dirhandle name. 748 749=item connect SOCKET,NAME 750 751Attempts to connect to a remote socket, just as the connect system call 752does. Returns true if it succeeded, false otherwise. NAME should be a 753packed address of the appropriate type for the socket. See the examples in 754L<perlipc/"Sockets: Client/Server Communication">. 755 756=item continue BLOCK 757 758Actually a flow control statement rather than a function. If there is a 759C<continue> BLOCK attached to a BLOCK (typically in a C<while> or 760C<foreach>), it is always executed just before the conditional is about to 761be evaluated again, just like the third part of a C<for> loop in C. Thus 762it can be used to increment a loop variable, even when the loop has been 763continued via the C<next> statement (which is similar to the C C<continue> 764statement). 765 766C<last>, C<next>, or C<redo> may appear within a C<continue> 767block. C<last> and C<redo> will behave as if they had been executed within 768the main block. So will C<next>, but since it will execute a C<continue> 769block, it may be more entertaining. 770 771 while (EXPR) { 772 ### redo always comes here 773 do_something; 774 } continue { 775 ### next always comes here 776 do_something_else; 777 # then back the top to re-check EXPR 778 } 779 ### last always comes here 780 781Omitting the C<continue> section is semantically equivalent to using an 782empty one, logically enough. In that case, C<next> goes directly back 783to check the condition at the top of the loop. 784 785=item cos EXPR 786 787=item cos 788 789Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, 790takes cosine of C<$_>. 791 792For the inverse cosine operation, you may use the C<Math::Trig::acos()> 793function, or use this relation: 794 795 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } 796 797=item crypt PLAINTEXT,SALT 798 799Encrypts a string exactly like the crypt(3) function in the C library 800(assuming that you actually have a version there that has not been 801extirpated as a potential munition). This can prove useful for checking 802the password file for lousy passwords, amongst other things. Only the 803guys wearing white hats should do this. 804 805Note that C<crypt> is intended to be a one-way function, much like breaking 806eggs to make an omelette. There is no (known) corresponding decrypt 807function. As a result, this function isn't all that useful for 808cryptography. (For that, see your nearby CPAN mirror.) 809 810When verifying an existing encrypted string you should use the encrypted 811text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This 812allows your code to work with the standard C<crypt> and with more 813exotic implementations. When choosing a new salt create a random two 814character string whose characters come from the set C<[./0-9A-Za-z]> 815(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). 816 817Here's an example that makes sure that whoever runs this program knows 818their own password: 819 820 $pwd = (getpwuid($<))[1]; 821 822 system "stty -echo"; 823 print "Password: "; 824 chomp($word = <STDIN>); 825 print "\n"; 826 system "stty echo"; 827 828 if (crypt($word, $pwd) ne $pwd) { 829 die "Sorry...\n"; 830 } else { 831 print "ok\n"; 832 } 833 834Of course, typing in your own password to whoever asks you 835for it is unwise. 836 837The C<crypt> function is unsuitable for encrypting large quantities 838of data, not least of all because you can't get the information 839back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories 840on your favorite CPAN mirror for a slew of potentially useful 841modules. 842 843=item dbmclose HASH 844 845[This function has been largely superseded by the C<untie> function.] 846 847Breaks the binding between a DBM file and a hash. 848 849=item dbmopen HASH,DBNAME,MASK 850 851[This function has been largely superseded by the C<tie> function.] 852 853This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a 854hash. HASH is the name of the hash. (Unlike normal C<open>, the first 855argument is I<not> a filehandle, even though it looks like one). DBNAME 856is the name of the database (without the F<.dir> or F<.pag> extension if 857any). If the database does not exist, it is created with protection 858specified by MASK (as modified by the C<umask>). If your system supports 859only the older DBM functions, you may perform only one C<dbmopen> in your 860program. In older versions of Perl, if your system had neither DBM nor 861ndbm, calling C<dbmopen> produced a fatal error; it now falls back to 862sdbm(3). 863 864If you don't have write access to the DBM file, you can only read hash 865variables, not set them. If you want to test whether you can write, 866either use file tests or try setting a dummy hash entry inside an C<eval>, 867which will trap the error. 868 869Note that functions such as C<keys> and C<values> may return huge lists 870when used on large DBM files. You may prefer to use the C<each> 871function to iterate over large DBM files. Example: 872 873 # print out history file offsets 874 dbmopen(%HIST,'/usr/lib/news/history',0666); 875 while (($key,$val) = each %HIST) { 876 print $key, ' = ', unpack('L',$val), "\n"; 877 } 878 dbmclose(%HIST); 879 880See also L<AnyDBM_File> for a more general description of the pros and 881cons of the various dbm approaches, as well as L<DB_File> for a particularly 882rich implementation. 883 884You can control which DBM library you use by loading that library 885before you call dbmopen(): 886 887 use DB_File; 888 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db") 889 or die "Can't open netscape history file: $!"; 890 891=item defined EXPR 892 893=item defined 894 895Returns a Boolean value telling whether EXPR has a value other than 896the undefined value C<undef>. If EXPR is not present, C<$_> will be 897checked. 898 899Many operations return C<undef> to indicate failure, end of file, 900system error, uninitialized variable, and other exceptional 901conditions. This function allows you to distinguish C<undef> from 902other values. (A simple Boolean test will not distinguish among 903C<undef>, zero, the empty string, and C<"0">, which are all equally 904false.) Note that since C<undef> is a valid scalar, its presence 905doesn't I<necessarily> indicate an exceptional condition: C<pop> 906returns C<undef> when its argument is an empty array, I<or> when the 907element to return happens to be C<undef>. 908 909You may also use C<defined(&func)> to check whether subroutine C<&func> 910has ever been defined. The return value is unaffected by any forward 911declarations of C<&foo>. Note that a subroutine which is not defined 912may still be callable: its package may have an C<AUTOLOAD> method that 913makes it spring into existence the first time that it is called -- see 914L<perlsub>. 915 916Use of C<defined> on aggregates (hashes and arrays) is deprecated. It 917used to report whether memory for that aggregate has ever been 918allocated. This behavior may disappear in future versions of Perl. 919You should instead use a simple test for size: 920 921 if (@an_array) { print "has array elements\n" } 922 if (%a_hash) { print "has hash members\n" } 923 924When used on a hash element, it tells you whether the value is defined, 925not whether the key exists in the hash. Use L</exists> for the latter 926purpose. 927 928Examples: 929 930 print if defined $switch{'D'}; 931 print "$val\n" while defined($val = pop(@ary)); 932 die "Can't readlink $sym: $!" 933 unless defined($value = readlink $sym); 934 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } 935 $debugging = 0 unless defined $debugging; 936 937Note: Many folks tend to overuse C<defined>, and then are surprised to 938discover that the number C<0> and C<""> (the zero-length string) are, in fact, 939defined values. For example, if you say 940 941 "ab" =~ /a(.*)b/; 942 943The pattern match succeeds, and C<$1> is defined, despite the fact that it 944matched "nothing". But it didn't really match nothing--rather, it 945matched something that happened to be zero characters long. This is all 946very above-board and honest. When a function returns an undefined value, 947it's an admission that it couldn't give you an honest answer. So you 948should use C<defined> only when you're questioning the integrity of what 949you're trying to do. At other times, a simple comparison to C<0> or C<""> is 950what you want. 951 952See also L</undef>, L</exists>, L</ref>. 953 954=item delete EXPR 955 956Given an expression that specifies a hash element, array element, hash slice, 957or array slice, deletes the specified element(s) from the hash or array. 958In the case of an array, if the array elements happen to be at the end, 959the size of the array will shrink to the highest element that tests 960true for exists() (or 0 if no such element exists). 961 962Returns each element so deleted or the undefined value if there was no such 963element. Deleting from C<$ENV{}> modifies the environment. Deleting from 964a hash tied to a DBM file deletes the entry from the DBM file. Deleting 965from a C<tie>d hash or array may not necessarily return anything. 966 967Deleting an array element effectively returns that position of the array 968to its initial, uninitialized state. Subsequently testing for the same 969element with exists() will return false. Note that deleting array 970elements in the middle of an array will not shift the index of the ones 971after them down--use splice() for that. See L</exists>. 972 973The following (inefficiently) deletes all the values of %HASH and @ARRAY: 974 975 foreach $key (keys %HASH) { 976 delete $HASH{$key}; 977 } 978 979 foreach $index (0 .. $#ARRAY) { 980 delete $ARRAY[$index]; 981 } 982 983And so do these: 984 985 delete @HASH{keys %HASH}; 986 987 delete @ARRAY[0 .. $#ARRAY]; 988 989But both of these are slower than just assigning the empty list 990or undefining %HASH or @ARRAY: 991 992 %HASH = (); # completely empty %HASH 993 undef %HASH; # forget %HASH ever existed 994 995 @ARRAY = (); # completely empty @ARRAY 996 undef @ARRAY; # forget @ARRAY ever existed 997 998Note that the EXPR can be arbitrarily complicated as long as the final 999operation is a hash element, array element, hash slice, or array slice 1000lookup: 1001 1002 delete $ref->[$x][$y]{$key}; 1003 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; 1004 1005 delete $ref->[$x][$y][$index]; 1006 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices]; 1007 1008=item die LIST 1009 1010Outside an C<eval>, prints the value of LIST to C<STDERR> and 1011exits with the current value of C<$!> (errno). If C<$!> is C<0>, 1012exits with the value of C<<< ($? >> 8) >>> (backtick `command` 1013status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside 1014an C<eval(),> the error message is stuffed into C<$@> and the 1015C<eval> is terminated with the undefined value. This makes 1016C<die> the way to raise an exception. 1017 1018Equivalent examples: 1019 1020 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; 1021 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" 1022 1023If the value of EXPR does not end in a newline, the current script line 1024number and input line number (if any) are also printed, and a newline 1025is supplied. Note that the "input line number" (also known as "chunk") 1026is subject to whatever notion of "line" happens to be currently in 1027effect, and is also available as the special variable C<$.>. 1028See L<perlvar/"$/"> and L<perlvar/"$.">. 1029 1030Hint: sometimes appending C<", stopped"> to your message 1031will cause it to make better sense when the string C<"at foo line 123"> is 1032appended. Suppose you are running script "canasta". 1033 1034 die "/etc/games is no good"; 1035 die "/etc/games is no good, stopped"; 1036 1037produce, respectively 1038 1039 /etc/games is no good at canasta line 123. 1040 /etc/games is no good, stopped at canasta line 123. 1041 1042See also exit(), warn(), and the Carp module. 1043 1044If LIST is empty and C<$@> already contains a value (typically from a 1045previous eval) that value is reused after appending C<"\t...propagated">. 1046This is useful for propagating exceptions: 1047 1048 eval { ... }; 1049 die unless $@ =~ /Expected exception/; 1050 1051If C<$@> is empty then the string C<"Died"> is used. 1052 1053die() can also be called with a reference argument. If this happens to be 1054trapped within an eval(), $@ contains the reference. This behavior permits 1055a more elaborate exception handling implementation using objects that 1056maintain arbitrary state about the nature of the exception. Such a scheme 1057is sometimes preferable to matching particular string values of $@ using 1058regular expressions. Here's an example: 1059 1060 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; 1061 if ($@) { 1062 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) { 1063 # handle Some::Module::Exception 1064 } 1065 else { 1066 # handle all other possible exceptions 1067 } 1068 } 1069 1070Because perl will stringify uncaught exception messages before displaying 1071them, you may want to overload stringification operations on such custom 1072exception objects. See L<overload> for details about that. 1073 1074You can arrange for a callback to be run just before the C<die> 1075does its deed, by setting the C<$SIG{__DIE__}> hook. The associated 1076handler will be called with the error text and can change the error 1077message, if it sees fit, by calling C<die> again. See 1078L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and 1079L<"eval BLOCK"> for some examples. Although this feature was meant 1080to be run only right before your program was to exit, this is not 1081currently the case--the C<$SIG{__DIE__}> hook is currently called 1082even inside eval()ed blocks/strings! If one wants the hook to do 1083nothing in such situations, put 1084 1085 die @_ if $^S; 1086 1087as the first line of the handler (see L<perlvar/$^S>). Because 1088this promotes strange action at a distance, this counterintuitive 1089behavior may be fixed in a future release. 1090 1091=item do BLOCK 1092 1093Not really a function. Returns the value of the last command in the 1094sequence of commands indicated by BLOCK. When modified by a loop 1095modifier, executes the BLOCK once before testing the loop condition. 1096(On other statements the loop modifiers test the conditional first.) 1097 1098C<do BLOCK> does I<not> count as a loop, so the loop control statements 1099C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. 1100See L<perlsyn> for alternative strategies. 1101 1102=item do SUBROUTINE(LIST) 1103 1104A deprecated form of subroutine call. See L<perlsub>. 1105 1106=item do EXPR 1107 1108Uses the value of EXPR as a filename and executes the contents of the 1109file as a Perl script. Its primary use is to include subroutines 1110from a Perl subroutine library. 1111 1112 do 'stat.pl'; 1113 1114is just like 1115 1116 scalar eval `cat stat.pl`; 1117 1118except that it's more efficient and concise, keeps track of the current 1119filename for error messages, searches the @INC libraries, and updates 1120C<%INC> if the file is found. See L<perlvar/Predefined Names> for these 1121variables. It also differs in that code evaluated with C<do FILENAME> 1122cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the 1123same, however, in that it does reparse the file every time you call it, 1124so you probably don't want to do this inside a loop. 1125 1126If C<do> cannot read the file, it returns undef and sets C<$!> to the 1127error. If C<do> can read the file but cannot compile it, it 1128returns undef and sets an error message in C<$@>. If the file is 1129successfully compiled, C<do> returns the value of the last expression 1130evaluated. 1131 1132Note that inclusion of library modules is better done with the 1133C<use> and C<require> operators, which also do automatic error checking 1134and raise an exception if there's a problem. 1135 1136You might like to use C<do> to read in a program configuration 1137file. Manual error checking can be done this way: 1138 1139 # read in config files: system first, then user 1140 for $file ("/share/prog/defaults.rc", 1141 "$ENV{HOME}/.someprogrc") 1142 { 1143 unless ($return = do $file) { 1144 warn "couldn't parse $file: $@" if $@; 1145 warn "couldn't do $file: $!" unless defined $return; 1146 warn "couldn't run $file" unless $return; 1147 } 1148 } 1149 1150=item dump LABEL 1151 1152=item dump 1153 1154This function causes an immediate core dump. See also the B<-u> 1155command-line switch in L<perlrun>, which does the same thing. 1156Primarily this is so that you can use the B<undump> program (not 1157supplied) to turn your core dump into an executable binary after 1158having initialized all your variables at the beginning of the 1159program. When the new binary is executed it will begin by executing 1160a C<goto LABEL> (with all the restrictions that C<goto> suffers). 1161Think of it as a goto with an intervening core dump and reincarnation. 1162If C<LABEL> is omitted, restarts the program from the top. 1163 1164B<WARNING>: Any files opened at the time of the dump will I<not> 1165be open any more when the program is reincarnated, with possible 1166resulting confusion on the part of Perl. 1167 1168This function is now largely obsolete, partly because it's very 1169hard to convert a core file into an executable, and because the 1170real compiler backends for generating portable bytecode and compilable 1171C code have superseded it. 1172 1173If you're looking to use L<dump> to speed up your program, consider 1174generating bytecode or native C code as described in L<perlcc>. If 1175you're just trying to accelerate a CGI script, consider using the 1176C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI. 1177You might also consider autoloading or selfloading, which at least 1178make your program I<appear> to run faster. 1179 1180=item each HASH 1181 1182When called in list context, returns a 2-element list consisting of the 1183key and value for the next element of a hash, so that you can iterate over 1184it. When called in scalar context, returns only the key for the next 1185element in the hash. 1186 1187Entries are returned in an apparently random order. The actual random 1188order is subject to change in future versions of perl, but it is guaranteed 1189to be in the same order as either the C<keys> or C<values> function 1190would produce on the same (unmodified) hash. 1191 1192When the hash is entirely read, a null array is returned in list context 1193(which when assigned produces a false (C<0>) value), and C<undef> in 1194scalar context. The next call to C<each> after that will start iterating 1195again. There is a single iterator for each hash, shared by all C<each>, 1196C<keys>, and C<values> function calls in the program; it can be reset by 1197reading all the elements from the hash, or by evaluating C<keys HASH> or 1198C<values HASH>. If you add or delete elements of a hash while you're 1199iterating over it, you may get entries skipped or duplicated, so 1200don't. Exception: It is always safe to delete the item most recently 1201returned by C<each()>, which means that the following code will work: 1202 1203 while (($key, $value) = each %hash) { 1204 print $key, "\n"; 1205 delete $hash{$key}; # This is safe 1206 } 1207 1208The following prints out your environment like the printenv(1) program, 1209only in a different order: 1210 1211 while (($key,$value) = each %ENV) { 1212 print "$key=$value\n"; 1213 } 1214 1215See also C<keys>, C<values> and C<sort>. 1216 1217=item eof FILEHANDLE 1218 1219=item eof () 1220 1221=item eof 1222 1223Returns 1 if the next read on FILEHANDLE will return end of file, or if 1224FILEHANDLE is not open. FILEHANDLE may be an expression whose value 1225gives the real filehandle. (Note that this function actually 1226reads a character and then C<ungetc>s it, so isn't very useful in an 1227interactive context.) Do not read from a terminal file (or call 1228C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such 1229as terminals may lose the end-of-file condition if you do. 1230 1231An C<eof> without an argument uses the last file read. Using C<eof()> 1232with empty parentheses is very different. It refers to the pseudo file 1233formed from the files listed on the command line and accessed via the 1234C<< <> >> operator. Since C<< <> >> isn't explicitly opened, 1235as a normal filehandle is, an C<eof()> before C<< <> >> has been 1236used will cause C<@ARGV> to be examined to determine if input is 1237available. 1238 1239In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to 1240detect the end of each file, C<eof()> will only detect the end of the 1241last file. Examples: 1242 1243 # reset line numbering on each input file 1244 while (<>) { 1245 next if /^\s*#/; # skip comments 1246 print "$.\t$_"; 1247 } continue { 1248 close ARGV if eof; # Not eof()! 1249 } 1250 1251 # insert dashes just before last line of last file 1252 while (<>) { 1253 if (eof()) { # check for end of current file 1254 print "--------------\n"; 1255 close(ARGV); # close or last; is needed if we 1256 # are reading from the terminal 1257 } 1258 print; 1259 } 1260 1261Practical hint: you almost never need to use C<eof> in Perl, because the 1262input operators typically return C<undef> when they run out of data, or if 1263there was an error. 1264 1265=item eval EXPR 1266 1267=item eval BLOCK 1268 1269In the first form, the return value of EXPR is parsed and executed as if it 1270were a little Perl program. The value of the expression (which is itself 1271determined within scalar context) is first parsed, and if there weren't any 1272errors, executed in the lexical context of the current Perl program, so 1273that any variable settings or subroutine and format definitions remain 1274afterwards. Note that the value is parsed every time the eval executes. 1275If EXPR is omitted, evaluates C<$_>. This form is typically used to 1276delay parsing and subsequent execution of the text of EXPR until run time. 1277 1278In the second form, the code within the BLOCK is parsed only once--at the 1279same time the code surrounding the eval itself was parsed--and executed 1280within the context of the current Perl program. This form is typically 1281used to trap exceptions more efficiently than the first (see below), while 1282also providing the benefit of checking the code within BLOCK at compile 1283time. 1284 1285The final semicolon, if any, may be omitted from the value of EXPR or within 1286the BLOCK. 1287 1288In both forms, the value returned is the value of the last expression 1289evaluated inside the mini-program; a return statement may be also used, just 1290as with subroutines. The expression providing the return value is evaluated 1291in void, scalar, or list context, depending on the context of the eval itself. 1292See L</wantarray> for more on how the evaluation context can be determined. 1293 1294If there is a syntax error or runtime error, or a C<die> statement is 1295executed, an undefined value is returned by C<eval>, and C<$@> is set to the 1296error message. If there was no error, C<$@> is guaranteed to be a null 1297string. Beware that using C<eval> neither silences perl from printing 1298warnings to STDERR, nor does it stuff the text of warning messages into C<$@>. 1299To do either of those, you have to use the C<$SIG{__WARN__}> facility. See 1300L</warn> and L<perlvar>. 1301 1302Note that, because C<eval> traps otherwise-fatal errors, it is useful for 1303determining whether a particular feature (such as C<socket> or C<symlink>) 1304is implemented. It is also Perl's exception trapping mechanism, where 1305the die operator is used to raise exceptions. 1306 1307If the code to be executed doesn't vary, you may use the eval-BLOCK 1308form to trap run-time errors without incurring the penalty of 1309recompiling each time. The error, if any, is still returned in C<$@>. 1310Examples: 1311 1312 # make divide-by-zero nonfatal 1313 eval { $answer = $a / $b; }; warn $@ if $@; 1314 1315 # same thing, but less efficient 1316 eval '$answer = $a / $b'; warn $@ if $@; 1317 1318 # a compile-time error 1319 eval { $answer = }; # WRONG 1320 1321 # a run-time error 1322 eval '$answer ='; # sets $@ 1323 1324Due to the current arguably broken state of C<__DIE__> hooks, when using 1325the C<eval{}> form as an exception trap in libraries, you may wish not 1326to trigger any C<__DIE__> hooks that user code may have installed. 1327You can use the C<local $SIG{__DIE__}> construct for this purpose, 1328as shown in this example: 1329 1330 # a very private exception trap for divide-by-zero 1331 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; 1332 warn $@ if $@; 1333 1334This is especially significant, given that C<__DIE__> hooks can call 1335C<die> again, which has the effect of changing their error messages: 1336 1337 # __DIE__ hooks may modify error messages 1338 { 1339 local $SIG{'__DIE__'} = 1340 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; 1341 eval { die "foo lives here" }; 1342 print $@ if $@; # prints "bar lives here" 1343 } 1344 1345Because this promotes action at a distance, this counterintuitive behavior 1346may be fixed in a future release. 1347 1348With an C<eval>, you should be especially careful to remember what's 1349being looked at when: 1350 1351 eval $x; # CASE 1 1352 eval "$x"; # CASE 2 1353 1354 eval '$x'; # CASE 3 1355 eval { $x }; # CASE 4 1356 1357 eval "\$$x++"; # CASE 5 1358 $$x++; # CASE 6 1359 1360Cases 1 and 2 above behave identically: they run the code contained in 1361the variable $x. (Although case 2 has misleading double quotes making 1362the reader wonder what else might be happening (nothing is).) Cases 3 1363and 4 likewise behave in the same way: they run the code C<'$x'>, which 1364does nothing but return the value of $x. (Case 4 is preferred for 1365purely visual reasons, but it also has the advantage of compiling at 1366compile-time instead of at run-time.) Case 5 is a place where 1367normally you I<would> like to use double quotes, except that in this 1368particular situation, you can just use symbolic references instead, as 1369in case 6. 1370 1371C<eval BLOCK> does I<not> count as a loop, so the loop control statements 1372C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. 1373 1374=item exec LIST 1375 1376=item exec PROGRAM LIST 1377 1378The C<exec> function executes a system command I<and never returns>-- 1379use C<system> instead of C<exec> if you want it to return. It fails and 1380returns false only if the command does not exist I<and> it is executed 1381directly instead of via your system's command shell (see below). 1382 1383Since it's a common mistake to use C<exec> instead of C<system>, Perl 1384warns you if there is a following statement which isn't C<die>, C<warn>, 1385or C<exit> (if C<-w> is set - but you always do that). If you 1386I<really> want to follow an C<exec> with some other statement, you 1387can use one of these styles to avoid the warning: 1388 1389 exec ('foo') or print STDERR "couldn't exec foo: $!"; 1390 { exec ('foo') }; print STDERR "couldn't exec foo: $!"; 1391 1392If there is more than one argument in LIST, or if LIST is an array 1393with more than one value, calls execvp(3) with the arguments in LIST. 1394If there is only one scalar argument or an array with one element in it, 1395the argument is checked for shell metacharacters, and if there are any, 1396the entire argument is passed to the system's command shell for parsing 1397(this is C</bin/sh -c> on Unix platforms, but varies on other platforms). 1398If there are no shell metacharacters in the argument, it is split into 1399words and passed directly to C<execvp>, which is more efficient. 1400Examples: 1401 1402 exec '/bin/echo', 'Your arguments are: ', @ARGV; 1403 exec "sort $outfile | uniq"; 1404 1405If you don't really want to execute the first argument, but want to lie 1406to the program you are executing about its own name, you can specify 1407the program you actually want to run as an "indirect object" (without a 1408comma) in front of the LIST. (This always forces interpretation of the 1409LIST as a multivalued list, even if there is only a single scalar in 1410the list.) Example: 1411 1412 $shell = '/bin/csh'; 1413 exec $shell '-sh'; # pretend it's a login shell 1414 1415or, more directly, 1416 1417 exec {'/bin/csh'} '-sh'; # pretend it's a login shell 1418 1419When the arguments get executed via the system shell, results will 1420be subject to its quirks and capabilities. See L<perlop/"`STRING`"> 1421for details. 1422 1423Using an indirect object with C<exec> or C<system> is also more 1424secure. This usage (which also works fine with system()) forces 1425interpretation of the arguments as a multivalued list, even if the 1426list had just one argument. That way you're safe from the shell 1427expanding wildcards or splitting up words with whitespace in them. 1428 1429 @args = ( "echo surprise" ); 1430 1431 exec @args; # subject to shell escapes 1432 # if @args == 1 1433 exec { $args[0] } @args; # safe even with one-arg list 1434 1435The first version, the one without the indirect object, ran the I<echo> 1436program, passing it C<"surprise"> an argument. The second version 1437didn't--it tried to run a program literally called I<"echo surprise">, 1438didn't find it, and set C<$?> to a non-zero value indicating failure. 1439 1440Beginning with v5.6.0, Perl will attempt to flush all files opened for 1441output before the exec, but this may not be supported on some platforms 1442(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH 1443in English) or call the C<autoflush()> method of C<IO::Handle> on any 1444open handles in order to avoid lost output. 1445 1446Note that C<exec> will not call your C<END> blocks, nor will it call 1447any C<DESTROY> methods in your objects. 1448 1449=item exists EXPR 1450 1451Given an expression that specifies a hash element or array element, 1452returns true if the specified element in the hash or array has ever 1453been initialized, even if the corresponding value is undefined. The 1454element is not autovivified if it doesn't exist. 1455 1456 print "Exists\n" if exists $hash{$key}; 1457 print "Defined\n" if defined $hash{$key}; 1458 print "True\n" if $hash{$key}; 1459 1460 print "Exists\n" if exists $array[$index]; 1461 print "Defined\n" if defined $array[$index]; 1462 print "True\n" if $array[$index]; 1463 1464A hash or array element can be true only if it's defined, and defined if 1465it exists, but the reverse doesn't necessarily hold true. 1466 1467Given an expression that specifies the name of a subroutine, 1468returns true if the specified subroutine has ever been declared, even 1469if it is undefined. Mentioning a subroutine name for exists or defined 1470does not count as declaring it. Note that a subroutine which does not 1471exist may still be callable: its package may have an C<AUTOLOAD> 1472method that makes it spring into existence the first time that it is 1473called -- see L<perlsub>. 1474 1475 print "Exists\n" if exists &subroutine; 1476 print "Defined\n" if defined &subroutine; 1477 1478Note that the EXPR can be arbitrarily complicated as long as the final 1479operation is a hash or array key lookup or subroutine name: 1480 1481 if (exists $ref->{A}->{B}->{$key}) { } 1482 if (exists $hash{A}{B}{$key}) { } 1483 1484 if (exists $ref->{A}->{B}->[$ix]) { } 1485 if (exists $hash{A}{B}[$ix]) { } 1486 1487 if (exists &{$ref->{A}{B}{$key}}) { } 1488 1489Although the deepest nested array or hash will not spring into existence 1490just because its existence was tested, any intervening ones will. 1491Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring 1492into existence due to the existence test for the $key element above. 1493This happens anywhere the arrow operator is used, including even: 1494 1495 undef $ref; 1496 if (exists $ref->{"Some key"}) { } 1497 print $ref; # prints HASH(0x80d3d5c) 1498 1499This surprising autovivification in what does not at first--or even 1500second--glance appear to be an lvalue context may be fixed in a future 1501release. 1502 1503See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics 1504on how exists() acts when used on a pseudo-hash. 1505 1506Use of a subroutine call, rather than a subroutine name, as an argument 1507to exists() is an error. 1508 1509 exists ⊂ # OK 1510 exists &sub(); # Error 1511 1512=item exit EXPR 1513 1514Evaluates EXPR and exits immediately with that value. Example: 1515 1516 $ans = <STDIN>; 1517 exit 0 if $ans =~ /^[Xx]/; 1518 1519See also C<die>. If EXPR is omitted, exits with C<0> status. The only 1520universally recognized values for EXPR are C<0> for success and C<1> 1521for error; other values are subject to interpretation depending on the 1522environment in which the Perl program is running. For example, exiting 152369 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause 1524the mailer to return the item undelivered, but that's not true everywhere. 1525 1526Don't use C<exit> to abort a subroutine if there's any chance that 1527someone might want to trap whatever error happened. Use C<die> instead, 1528which can be trapped by an C<eval>. 1529 1530The exit() function does not always exit immediately. It calls any 1531defined C<END> routines first, but these C<END> routines may not 1532themselves abort the exit. Likewise any object destructors that need to 1533be called are called before the real exit. If this is a problem, you 1534can call C<POSIX:_exit($status)> to avoid END and destructor processing. 1535See L<perlmod> for details. 1536 1537=item exp EXPR 1538 1539=item exp 1540 1541Returns I<e> (the natural logarithm base) to the power of EXPR. 1542If EXPR is omitted, gives C<exp($_)>. 1543 1544=item fcntl FILEHANDLE,FUNCTION,SCALAR 1545 1546Implements the fcntl(2) function. You'll probably have to say 1547 1548 use Fcntl; 1549 1550first to get the correct constant definitions. Argument processing and 1551value return works just like C<ioctl> below. 1552For example: 1553 1554 use Fcntl; 1555 fcntl($filehandle, F_GETFL, $packed_return_buffer) 1556 or die "can't fcntl F_GETFL: $!"; 1557 1558You don't have to check for C<defined> on the return from C<fnctl>. 1559Like C<ioctl>, it maps a C<0> return from the system call into 1560C<"0 but true"> in Perl. This string is true in boolean context and C<0> 1561in numeric context. It is also exempt from the normal B<-w> warnings 1562on improper numeric conversions. 1563 1564Note that C<fcntl> will produce a fatal error if used on a machine that 1565doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) 1566manpage to learn what functions are available on your system. 1567 1568=item fileno FILEHANDLE 1569 1570Returns the file descriptor for a filehandle, or undefined if the 1571filehandle is not open. This is mainly useful for constructing 1572bitmaps for C<select> and low-level POSIX tty-handling operations. 1573If FILEHANDLE is an expression, the value is taken as an indirect 1574filehandle, generally its name. 1575 1576You can use this to find out whether two handles refer to the 1577same underlying descriptor: 1578 1579 if (fileno(THIS) == fileno(THAT)) { 1580 print "THIS and THAT are dups\n"; 1581 } 1582 1583=item flock FILEHANDLE,OPERATION 1584 1585Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true 1586for success, false on failure. Produces a fatal error if used on a 1587machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). 1588C<flock> is Perl's portable file locking interface, although it locks 1589only entire files, not records. 1590 1591Two potentially non-obvious but traditional C<flock> semantics are 1592that it waits indefinitely until the lock is granted, and that its locks 1593B<merely advisory>. Such discretionary locks are more flexible, but offer 1594fewer guarantees. This means that files locked with C<flock> may be 1595modified by programs that do not also use C<flock>. See L<perlport>, 1596your port's specific documentation, or your system-specific local manpages 1597for details. It's best to assume traditional behavior if you're writing 1598portable programs. (But if you're not, you should as always feel perfectly 1599free to write for your own system's idiosyncrasies (sometimes called 1600"features"). Slavish adherence to portability concerns shouldn't get 1601in the way of your getting your job done.) 1602 1603OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with 1604LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but 1605you can use the symbolic names if you import them from the Fcntl module, 1606either individually, or as a group using the ':flock' tag. LOCK_SH 1607requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN 1608releases a previously requested lock. If LOCK_NB is bitwise-or'ed with 1609LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking 1610waiting for the lock (check the return status to see if you got it). 1611 1612To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE 1613before locking or unlocking it. 1614 1615Note that the emulation built with lockf(3) doesn't provide shared 1616locks, and it requires that FILEHANDLE be open with write intent. These 1617are the semantics that lockf(3) implements. Most if not all systems 1618implement lockf(3) in terms of fcntl(2) locking, though, so the 1619differing semantics shouldn't bite too many people. 1620 1621Note also that some versions of C<flock> cannot lock things over the 1622network; you would need to use the more system-specific C<fcntl> for 1623that. If you like you can force Perl to ignore your system's flock(2) 1624function, and so provide its own fcntl(2)-based emulation, by passing 1625the switch C<-Ud_flock> to the F<Configure> program when you configure 1626perl. 1627 1628Here's a mailbox appender for BSD systems. 1629 1630 use Fcntl ':flock'; # import LOCK_* constants 1631 1632 sub lock { 1633 flock(MBOX,LOCK_EX); 1634 # and, in case someone appended 1635 # while we were waiting... 1636 seek(MBOX, 0, 2); 1637 } 1638 1639 sub unlock { 1640 flock(MBOX,LOCK_UN); 1641 } 1642 1643 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}") 1644 or die "Can't open mailbox: $!"; 1645 1646 lock(); 1647 print MBOX $msg,"\n\n"; 1648 unlock(); 1649 1650On systems that support a real flock(), locks are inherited across fork() 1651calls, whereas those that must resort to the more capricious fcntl() 1652function lose the locks, making it harder to write servers. 1653 1654See also L<DB_File> for other flock() examples. 1655 1656=item fork 1657 1658Does a fork(2) system call to create a new process running the 1659same program at the same point. It returns the child pid to the 1660parent process, C<0> to the child process, or C<undef> if the fork is 1661unsuccessful. File descriptors (and sometimes locks on those descriptors) 1662are shared, while everything else is copied. On most systems supporting 1663fork(), great care has gone into making it extremely efficient (for 1664example, using copy-on-write technology on data pages), making it the 1665dominant paradigm for multitasking over the last few decades. 1666 1667Beginning with v5.6.0, Perl will attempt to flush all files opened for 1668output before forking the child process, but this may not be supported 1669on some platforms (see L<perlport>). To be safe, you may need to set 1670C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of 1671C<IO::Handle> on any open handles in order to avoid duplicate output. 1672 1673If you C<fork> without ever waiting on your children, you will 1674accumulate zombies. On some systems, you can avoid this by setting 1675C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of 1676forking and reaping moribund children. 1677 1678Note that if your forked child inherits system file descriptors like 1679STDIN and STDOUT that are actually connected by a pipe or socket, even 1680if you exit, then the remote server (such as, say, a CGI script or a 1681backgrounded job launched from a remote shell) won't think you're done. 1682You should reopen those to F</dev/null> if it's any issue. 1683 1684=item format 1685 1686Declare a picture format for use by the C<write> function. For 1687example: 1688 1689 format Something = 1690 Test: @<<<<<<<< @||||| @>>>>> 1691 $str, $%, '$' . int($num) 1692 . 1693 1694 $str = "widget"; 1695 $num = $cost/$quantity; 1696 $~ = 'Something'; 1697 write; 1698 1699See L<perlform> for many details and examples. 1700 1701=item formline PICTURE,LIST 1702 1703This is an internal function used by C<format>s, though you may call it, 1704too. It formats (see L<perlform>) a list of values according to the 1705contents of PICTURE, placing the output into the format output 1706accumulator, C<$^A> (or C<$ACCUMULATOR> in English). 1707Eventually, when a C<write> is done, the contents of 1708C<$^A> are written to some filehandle, but you could also read C<$^A> 1709yourself and then set C<$^A> back to C<"">. Note that a format typically 1710does one C<formline> per line of form, but the C<formline> function itself 1711doesn't care how many newlines are embedded in the PICTURE. This means 1712that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line. 1713You may therefore need to use multiple formlines to implement a single 1714record format, just like the format compiler. 1715 1716Be careful if you put double quotes around the picture, because an C<@> 1717character may be taken to mean the beginning of an array name. 1718C<formline> always returns true. See L<perlform> for other examples. 1719 1720=item getc FILEHANDLE 1721 1722=item getc 1723 1724Returns the next character from the input file attached to FILEHANDLE, 1725or the undefined value at end of file, or if there was an error. 1726If FILEHANDLE is omitted, reads from STDIN. This is not particularly 1727efficient. However, it cannot be used by itself to fetch single 1728characters without waiting for the user to hit enter. For that, try 1729something more like: 1730 1731 if ($BSD_STYLE) { 1732 system "stty cbreak </dev/tty >/dev/tty 2>&1"; 1733 } 1734 else { 1735 system "stty", '-icanon', 'eol', "\001"; 1736 } 1737 1738 $key = getc(STDIN); 1739 1740 if ($BSD_STYLE) { 1741 system "stty -cbreak </dev/tty >/dev/tty 2>&1"; 1742 } 1743 else { 1744 system "stty", 'icanon', 'eol', '^@'; # ASCII null 1745 } 1746 print "\n"; 1747 1748Determination of whether $BSD_STYLE should be set 1749is left as an exercise to the reader. 1750 1751The C<POSIX::getattr> function can do this more portably on 1752systems purporting POSIX compliance. See also the C<Term::ReadKey> 1753module from your nearest CPAN site; details on CPAN can be found on 1754L<perlmodlib/CPAN>. 1755 1756=item getlogin 1757 1758Implements the C library function of the same name, which on most 1759systems returns the current login from F</etc/utmp>, if any. If null, 1760use C<getpwuid>. 1761 1762 $login = getlogin || getpwuid($<) || "Kilroy"; 1763 1764Do not consider C<getlogin> for authentication: it is not as 1765secure as C<getpwuid>. 1766 1767=item getpeername SOCKET 1768 1769Returns the packed sockaddr address of other end of the SOCKET connection. 1770 1771 use Socket; 1772 $hersockaddr = getpeername(SOCK); 1773 ($port, $iaddr) = sockaddr_in($hersockaddr); 1774 $herhostname = gethostbyaddr($iaddr, AF_INET); 1775 $herstraddr = inet_ntoa($iaddr); 1776 1777=item getpgrp PID 1778 1779Returns the current process group for the specified PID. Use 1780a PID of C<0> to get the current process group for the 1781current process. Will raise an exception if used on a machine that 1782doesn't implement getpgrp(2). If PID is omitted, returns process 1783group of current process. Note that the POSIX version of C<getpgrp> 1784does not accept a PID argument, so only C<PID==0> is truly portable. 1785 1786=item getppid 1787 1788Returns the process id of the parent process. 1789 1790=item getpriority WHICH,WHO 1791 1792Returns the current priority for a process, a process group, or a user. 1793(See L<getpriority(2)>.) Will raise a fatal exception if used on a 1794machine that doesn't implement getpriority(2). 1795 1796=item getpwnam NAME 1797 1798=item getgrnam NAME 1799 1800=item gethostbyname NAME 1801 1802=item getnetbyname NAME 1803 1804=item getprotobyname NAME 1805 1806=item getpwuid UID 1807 1808=item getgrgid GID 1809 1810=item getservbyname NAME,PROTO 1811 1812=item gethostbyaddr ADDR,ADDRTYPE 1813 1814=item getnetbyaddr ADDR,ADDRTYPE 1815 1816=item getprotobynumber NUMBER 1817 1818=item getservbyport PORT,PROTO 1819 1820=item getpwent 1821 1822=item getgrent 1823 1824=item gethostent 1825 1826=item getnetent 1827 1828=item getprotoent 1829 1830=item getservent 1831 1832=item setpwent 1833 1834=item setgrent 1835 1836=item sethostent STAYOPEN 1837 1838=item setnetent STAYOPEN 1839 1840=item setprotoent STAYOPEN 1841 1842=item setservent STAYOPEN 1843 1844=item endpwent 1845 1846=item endgrent 1847 1848=item endhostent 1849 1850=item endnetent 1851 1852=item endprotoent 1853 1854=item endservent 1855 1856These routines perform the same functions as their counterparts in the 1857system library. In list context, the return values from the 1858various get routines are as follows: 1859 1860 ($name,$passwd,$uid,$gid, 1861 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw* 1862 ($name,$passwd,$gid,$members) = getgr* 1863 ($name,$aliases,$addrtype,$length,@addrs) = gethost* 1864 ($name,$aliases,$addrtype,$net) = getnet* 1865 ($name,$aliases,$proto) = getproto* 1866 ($name,$aliases,$port,$proto) = getserv* 1867 1868(If the entry doesn't exist you get a null list.) 1869 1870The exact meaning of the $gcos field varies but it usually contains 1871the real name of the user (as opposed to the login name) and other 1872information pertaining to the user. Beware, however, that in many 1873system users are able to change this information and therefore it 1874cannot be trusted and therefore the $gcos is tainted (see 1875L<perlsec>). The $passwd and $shell, user's encrypted password and 1876login shell, are also tainted, because of the same reason. 1877 1878In scalar context, you get the name, unless the function was a 1879lookup by name, in which case you get the other thing, whatever it is. 1880(If the entry doesn't exist you get the undefined value.) For example: 1881 1882 $uid = getpwnam($name); 1883 $name = getpwuid($num); 1884 $name = getpwent(); 1885 $gid = getgrnam($name); 1886 $name = getgrgid($num; 1887 $name = getgrent(); 1888 #etc. 1889 1890In I<getpw*()> the fields $quota, $comment, and $expire are special 1891cases in the sense that in many systems they are unsupported. If the 1892$quota is unsupported, it is an empty scalar. If it is supported, it 1893usually encodes the disk quota. If the $comment field is unsupported, 1894it is an empty scalar. If it is supported it usually encodes some 1895administrative comment about the user. In some systems the $quota 1896field may be $change or $age, fields that have to do with password 1897aging. In some systems the $comment field may be $class. The $expire 1898field, if present, encodes the expiration period of the account or the 1899password. For the availability and the exact meaning of these fields 1900in your system, please consult your getpwnam(3) documentation and your 1901F<pwd.h> file. You can also find out from within Perl what your 1902$quota and $comment fields mean and whether you have the $expire field 1903by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>, 1904C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password 1905files are only supported if your vendor has implemented them in the 1906intuitive fashion that calling the regular C library routines gets the 1907shadow versions if you're running under privilege or if there exists 1908the shadow(3) functions as found in System V ( this includes Solaris 1909and Linux.) Those systems which implement a proprietary shadow password 1910facility are unlikely to be supported. 1911 1912The $members value returned by I<getgr*()> is a space separated list of 1913the login names of the members of the group. 1914 1915For the I<gethost*()> functions, if the C<h_errno> variable is supported in 1916C, it will be returned to you via C<$?> if the function call fails. The 1917C<@addrs> value returned by a successful call is a list of the raw 1918addresses returned by the corresponding system library call. In the 1919Internet domain, each address is four bytes long and you can unpack it 1920by saying something like: 1921 1922 ($a,$b,$c,$d) = unpack('C4',$addr[0]); 1923 1924The Socket library makes this slightly easier: 1925 1926 use Socket; 1927 $iaddr = inet_aton("127.1"); # or whatever address 1928 $name = gethostbyaddr($iaddr, AF_INET); 1929 1930 # or going the other way 1931 $straddr = inet_ntoa($iaddr); 1932 1933If you get tired of remembering which element of the return list 1934contains which return value, by-name interfaces are provided 1935in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>, 1936C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>, 1937and C<User::grent>. These override the normal built-ins, supplying 1938versions that return objects with the appropriate names 1939for each field. For example: 1940 1941 use File::stat; 1942 use User::pwent; 1943 $is_his = (stat($filename)->uid == pwent($whoever)->uid); 1944 1945Even though it looks like they're the same method calls (uid), 1946they aren't, because a C<File::stat> object is different from 1947a C<User::pwent> object. 1948 1949=item getsockname SOCKET 1950 1951Returns the packed sockaddr address of this end of the SOCKET connection, 1952in case you don't know the address because you have several different 1953IPs that the connection might have come in on. 1954 1955 use Socket; 1956 $mysockaddr = getsockname(SOCK); 1957 ($port, $myaddr) = sockaddr_in($mysockaddr); 1958 printf "Connect to %s [%s]\n", 1959 scalar gethostbyaddr($myaddr, AF_INET), 1960 inet_ntoa($myaddr); 1961 1962=item getsockopt SOCKET,LEVEL,OPTNAME 1963 1964Returns the socket option requested, or undef if there is an error. 1965 1966=item glob EXPR 1967 1968=item glob 1969 1970Returns the value of EXPR with filename expansions such as the 1971standard Unix shell F</bin/csh> would do. This is the internal function 1972implementing the C<< <*.c> >> operator, but you can use it directly. 1973If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is 1974discussed in more detail in L<perlop/"I/O Operators">. 1975 1976Beginning with v5.6.0, this operator is implemented using the standard 1977C<File::Glob> extension. See L<File::Glob> for details. 1978 1979=item gmtime EXPR 1980 1981Converts a time as returned by the time function to a 8-element list 1982with the time localized for the standard Greenwich time zone. 1983Typically used as follows: 1984 1985 # 0 1 2 3 4 5 6 7 1986 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) = 1987 gmtime(time); 1988 1989All list elements are numeric, and come straight out of the C `struct 1990tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the 1991specified time. $mday is the day of the month, and $mon is the month 1992itself, in the range C<0..11> with 0 indicating January and 11 1993indicating December. $year is the number of years since 1900. That 1994is, $year is C<123> in year 2023. $wday is the day of the week, with 19950 indicating Sunday and 3 indicating Wednesday. $yday is the day of 1996the year, in the range C<0..364> (or C<0..365> in leap years.) 1997 1998Note that the $year element is I<not> simply the last two digits of 1999the year. If you assume it is, then you create non-Y2K-compliant 2000programs--and you wouldn't want to do that, would you? 2001 2002The proper way to get a complete 4-digit year is simply: 2003 2004 $year += 1900; 2005 2006And to get the last two digits of the year (e.g., '01' in 2001) do: 2007 2008 $year = sprintf("%02d", $year % 100); 2009 2010If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>). 2011 2012In scalar context, C<gmtime()> returns the ctime(3) value: 2013 2014 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994" 2015 2016Also see the C<timegm> function provided by the C<Time::Local> module, 2017and the strftime(3) function available via the POSIX module. 2018 2019This scalar value is B<not> locale dependent (see L<perllocale>), but 2020is instead a Perl builtin. Also see the C<Time::Local> module, and the 2021strftime(3) and mktime(3) functions available via the POSIX module. To 2022get somewhat similar but locale dependent date strings, set up your 2023locale environment variables appropriately (please see L<perllocale>) 2024and try for example: 2025 2026 use POSIX qw(strftime); 2027 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; 2028 2029Note that the C<%a> and C<%b> escapes, which represent the short forms 2030of the day of the week and the month of the year, may not necessarily 2031be three characters wide in all locales. 2032 2033=item goto LABEL 2034 2035=item goto EXPR 2036 2037=item goto &NAME 2038 2039The C<goto-LABEL> form finds the statement labeled with LABEL and resumes 2040execution there. It may not be used to go into any construct that 2041requires initialization, such as a subroutine or a C<foreach> loop. It 2042also can't be used to go into a construct that is optimized away, 2043or to get out of a block or subroutine given to C<sort>. 2044It can be used to go almost anywhere else within the dynamic scope, 2045including out of subroutines, but it's usually better to use some other 2046construct such as C<last> or C<die>. The author of Perl has never felt the 2047need to use this form of C<goto> (in Perl, that is--C is another matter). 2048 2049The C<goto-EXPR> form expects a label name, whose scope will be resolved 2050dynamically. This allows for computed C<goto>s per FORTRAN, but isn't 2051necessarily recommended if you're optimizing for maintainability: 2052 2053 goto ("FOO", "BAR", "GLARCH")[$i]; 2054 2055The C<goto-&NAME> form is quite different from the other forms of C<goto>. 2056In fact, it isn't a goto in the normal sense at all, and doesn't have 2057the stigma associated with other gotos. Instead, it 2058substitutes a call to the named subroutine for the currently running 2059subroutine. This is used by C<AUTOLOAD> subroutines that wish to load 2060another subroutine and then pretend that the other subroutine had been 2061called in the first place (except that any modifications to C<@_> 2062in the current subroutine are propagated to the other subroutine.) 2063After the C<goto>, not even C<caller> will be able to tell that this 2064routine was called first. 2065 2066NAME needn't be the name of a subroutine; it can be a scalar variable 2067containing a code reference, or a block which evaluates to a code 2068reference. 2069 2070=item grep BLOCK LIST 2071 2072=item grep EXPR,LIST 2073 2074This is similar in spirit to, but not the same as, grep(1) and its 2075relatives. In particular, it is not limited to using regular expressions. 2076 2077Evaluates the BLOCK or EXPR for each element of LIST (locally setting 2078C<$_> to each element) and returns the list value consisting of those 2079elements for which the expression evaluated to true. In scalar 2080context, returns the number of times the expression was true. 2081 2082 @foo = grep(!/^#/, @bar); # weed out comments 2083 2084or equivalently, 2085 2086 @foo = grep {!/^#/} @bar; # weed out comments 2087 2088Note that C<$_> is an alias to the list value, so it can be used to 2089modify the elements of the LIST. While this is useful and supported, 2090it can cause bizarre results if the elements of LIST are not variables. 2091Similarly, grep returns aliases into the original list, much as a for 2092loop's index variable aliases the list elements. That is, modifying an 2093element of a list returned by grep (for example, in a C<foreach>, C<map> 2094or another C<grep>) actually modifies the element in the original list. 2095This is usually something to be avoided when writing clear code. 2096 2097See also L</map> for a list composed of the results of the BLOCK or EXPR. 2098 2099=item hex EXPR 2100 2101=item hex 2102 2103Interprets EXPR as a hex string and returns the corresponding value. 2104(To convert strings that might start with either 0, 0x, or 0b, see 2105L</oct>.) If EXPR is omitted, uses C<$_>. 2106 2107 print hex '0xAf'; # prints '175' 2108 print hex 'aF'; # same 2109 2110Hex strings may only represent integers. Strings that would cause 2111integer overflow trigger a warning. 2112 2113=item import 2114 2115There is no builtin C<import> function. It is just an ordinary 2116method (subroutine) defined (or inherited) by modules that wish to export 2117names to another module. The C<use> function calls the C<import> method 2118for the package used. See also L</use>, L<perlmod>, and L<Exporter>. 2119 2120=item index STR,SUBSTR,POSITION 2121 2122=item index STR,SUBSTR 2123 2124The index function searches for one string within another, but without 2125the wildcard-like behavior of a full regular-expression pattern match. 2126It returns the position of the first occurrence of SUBSTR in STR at 2127or after POSITION. If POSITION is omitted, starts searching from the 2128beginning of the string. The return value is based at C<0> (or whatever 2129you've set the C<$[> variable to--but don't do that). If the substring 2130is not found, returns one less than the base, ordinarily C<-1>. 2131 2132=item int EXPR 2133 2134=item int 2135 2136Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>. 2137You should not use this function for rounding: one because it truncates 2138towards C<0>, and two because machine representations of floating point 2139numbers can sometimes produce counterintuitive results. For example, 2140C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's 2141because it's really more like -268.99999999999994315658 instead. Usually, 2142the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil> 2143functions will serve you better than will int(). 2144 2145=item ioctl FILEHANDLE,FUNCTION,SCALAR 2146 2147Implements the ioctl(2) function. You'll probably first have to say 2148 2149 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph 2150 2151to get the correct function definitions. If F<ioctl.ph> doesn't 2152exist or doesn't have the correct definitions you'll have to roll your 2153own, based on your C header files such as F<< <sys/ioctl.h> >>. 2154(There is a Perl script called B<h2ph> that comes with the Perl kit that 2155may help you in this, but it's nontrivial.) SCALAR will be read and/or 2156written depending on the FUNCTION--a pointer to the string value of SCALAR 2157will be passed as the third argument of the actual C<ioctl> call. (If SCALAR 2158has no string value but does have a numeric value, that value will be 2159passed rather than a pointer to the string value. To guarantee this to be 2160true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack> 2161functions may be needed to manipulate the values of structures used by 2162C<ioctl>. 2163 2164The return value of C<ioctl> (and C<fcntl>) is as follows: 2165 2166 if OS returns: then Perl returns: 2167 -1 undefined value 2168 0 string "0 but true" 2169 anything else that number 2170 2171Thus Perl returns true on success and false on failure, yet you can 2172still easily determine the actual value returned by the operating 2173system: 2174 2175 $retval = ioctl(...) || -1; 2176 printf "System returned %d\n", $retval; 2177 2178The special string "C<0> but true" is exempt from B<-w> complaints 2179about improper numeric conversions. 2180 2181Here's an example of setting a filehandle named C<REMOTE> to be 2182non-blocking at the system level. You'll have to negotiate C<$|> 2183on your own, though. 2184 2185 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK); 2186 2187 $flags = fcntl(REMOTE, F_GETFL, 0) 2188 or die "Can't get flags for the socket: $!\n"; 2189 2190 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK) 2191 or die "Can't set flags for the socket: $!\n"; 2192 2193=item join EXPR,LIST 2194 2195Joins the separate strings of LIST into a single string with fields 2196separated by the value of EXPR, and returns that new string. Example: 2197 2198 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); 2199 2200Beware that unlike C<split>, C<join> doesn't take a pattern as its 2201first argument. Compare L</split>. 2202 2203=item keys HASH 2204 2205Returns a list consisting of all the keys of the named hash. (In 2206scalar context, returns the number of keys.) The keys are returned in 2207an apparently random order. The actual random order is subject to 2208change in future versions of perl, but it is guaranteed to be the same 2209order as either the C<values> or C<each> function produces (given 2210that the hash has not been modified). As a side effect, it resets 2211HASH's iterator. 2212 2213Here is yet another way to print your environment: 2214 2215 @keys = keys %ENV; 2216 @values = values %ENV; 2217 while (@keys) { 2218 print pop(@keys), '=', pop(@values), "\n"; 2219 } 2220 2221or how about sorted by key: 2222 2223 foreach $key (sort(keys %ENV)) { 2224 print $key, '=', $ENV{$key}, "\n"; 2225 } 2226 2227The returned values are copies of the original keys in the hash, so 2228modifying them will not affect the original hash. Compare L</values>. 2229 2230To sort a hash by value, you'll need to use a C<sort> function. 2231Here's a descending numeric sort of a hash by its values: 2232 2233 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { 2234 printf "%4d %s\n", $hash{$key}, $key; 2235 } 2236 2237As an lvalue C<keys> allows you to increase the number of hash buckets 2238allocated for the given hash. This can gain you a measure of efficiency if 2239you know the hash is going to get big. (This is similar to pre-extending 2240an array by assigning a larger number to $#array.) If you say 2241 2242 keys %hash = 200; 2243 2244then C<%hash> will have at least 200 buckets allocated for it--256 of them, 2245in fact, since it rounds up to the next power of two. These 2246buckets will be retained even if you do C<%hash = ()>, use C<undef 2247%hash> if you want to free the storage while C<%hash> is still in scope. 2248You can't shrink the number of buckets allocated for the hash using 2249C<keys> in this way (but you needn't worry about doing this by accident, 2250as trying has no effect). 2251 2252See also C<each>, C<values> and C<sort>. 2253 2254=item kill SIGNAL, LIST 2255 2256Sends a signal to a list of processes. Returns the number of 2257processes successfully signaled (which is not necessarily the 2258same as the number actually killed). 2259 2260 $cnt = kill 1, $child1, $child2; 2261 kill 9, @goners; 2262 2263If SIGNAL is zero, no signal is sent to the process. This is a 2264useful way to check that the process is alive and hasn't changed 2265its UID. See L<perlport> for notes on the portability of this 2266construct. 2267 2268Unlike in the shell, if SIGNAL is negative, it kills 2269process groups instead of processes. (On System V, a negative I<PROCESS> 2270number will also kill process groups, but that's not portable.) That 2271means you usually want to use positive not negative signals. You may also 2272use a signal name in quotes. See L<perlipc/"Signals"> for details. 2273 2274=item last LABEL 2275 2276=item last 2277 2278The C<last> command is like the C<break> statement in C (as used in 2279loops); it immediately exits the loop in question. If the LABEL is 2280omitted, the command refers to the innermost enclosing loop. The 2281C<continue> block, if any, is not executed: 2282 2283 LINE: while (<STDIN>) { 2284 last LINE if /^$/; # exit when done with header 2285 #... 2286 } 2287 2288C<last> cannot be used to exit a block which returns a value such as 2289C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit 2290a grep() or map() operation. 2291 2292Note that a block by itself is semantically identical to a loop 2293that executes once. Thus C<last> can be used to effect an early 2294exit out of such a block. 2295 2296See also L</continue> for an illustration of how C<last>, C<next>, and 2297C<redo> work. 2298 2299=item lc EXPR 2300 2301=item lc 2302 2303Returns an lowercased version of EXPR. This is the internal function 2304implementing the C<\L> escape in double-quoted strings. 2305Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale> 2306and L<utf8>. 2307 2308If EXPR is omitted, uses C<$_>. 2309 2310=item lcfirst EXPR 2311 2312=item lcfirst 2313 2314Returns the value of EXPR with the first character lowercased. This is 2315the internal function implementing the C<\l> escape in double-quoted strings. 2316Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. 2317 2318If EXPR is omitted, uses C<$_>. 2319 2320=item length EXPR 2321 2322=item length 2323 2324Returns the length in characters of the value of EXPR. If EXPR is 2325omitted, returns length of C<$_>. Note that this cannot be used on 2326an entire array or hash to find out how many elements these have. 2327For that, use C<scalar @array> and C<scalar keys %hash> respectively. 2328 2329=item link OLDFILE,NEWFILE 2330 2331Creates a new filename linked to the old filename. Returns true for 2332success, false otherwise. 2333 2334=item listen SOCKET,QUEUESIZE 2335 2336Does the same thing that the listen system call does. Returns true if 2337it succeeded, false otherwise. See the example in 2338L<perlipc/"Sockets: Client/Server Communication">. 2339 2340=item local EXPR 2341 2342You really probably want to be using C<my> instead, because C<local> isn't 2343what most people think of as "local". See 2344L<perlsub/"Private Variables via my()"> for details. 2345 2346A local modifies the listed variables to be local to the enclosing 2347block, file, or eval. If more than one value is listed, the list must 2348be placed in parentheses. See L<perlsub/"Temporary Values via local()"> 2349for details, including issues with tied arrays and hashes. 2350 2351=item localtime EXPR 2352 2353Converts a time as returned by the time function to a 9-element list 2354with the time analyzed for the local time zone. Typically used as 2355follows: 2356 2357 # 0 1 2 3 4 5 6 7 8 2358 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = 2359 localtime(time); 2360 2361All list elements are numeric, and come straight out of the C `struct 2362tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the 2363specified time. $mday is the day of the month, and $mon is the month 2364itself, in the range C<0..11> with 0 indicating January and 11 2365indicating December. $year is the number of years since 1900. That 2366is, $year is C<123> in year 2023. $wday is the day of the week, with 23670 indicating Sunday and 3 indicating Wednesday. $yday is the day of 2368the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst 2369is true if the specified time occurs during daylight savings time, 2370false otherwise. 2371 2372Note that the $year element is I<not> simply the last two digits of 2373the year. If you assume it is, then you create non-Y2K-compliant 2374programs--and you wouldn't want to do that, would you? 2375 2376The proper way to get a complete 4-digit year is simply: 2377 2378 $year += 1900; 2379 2380And to get the last two digits of the year (e.g., '01' in 2001) do: 2381 2382 $year = sprintf("%02d", $year % 100); 2383 2384If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>). 2385 2386In scalar context, C<localtime()> returns the ctime(3) value: 2387 2388 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" 2389 2390This scalar value is B<not> locale dependent, see L<perllocale>, but 2391instead a Perl builtin. Also see the C<Time::Local> module 2392(to convert the second, minutes, hours, ... back to seconds since the 2393stroke of midnight the 1st of January 1970, the value returned by 2394time()), and the strftime(3) and mktime(3) functions available via the 2395POSIX module. To get somewhat similar but locale dependent date 2396strings, set up your locale environment variables appropriately 2397(please see L<perllocale>) and try for example: 2398 2399 use POSIX qw(strftime); 2400 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; 2401 2402Note that the C<%a> and C<%b>, the short forms of the day of the week 2403and the month of the year, may not necessarily be three characters wide. 2404 2405=item lock 2406 2407 lock I<THING> 2408 2409This function places an advisory lock on a variable, subroutine, 2410or referenced object contained in I<THING> until the lock goes out 2411of scope. This is a built-in function only if your version of Perl 2412was built with threading enabled, and if you've said C<use Threads>. 2413Otherwise a user-defined function by this name will be called. See 2414L<Thread>. 2415 2416=item log EXPR 2417 2418=item log 2419 2420Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, 2421returns log of C<$_>. To get the log of another base, use basic algebra: 2422The base-N log of a number is equal to the natural log of that number 2423divided by the natural log of N. For example: 2424 2425 sub log10 { 2426 my $n = shift; 2427 return log($n)/log(10); 2428 } 2429 2430See also L</exp> for the inverse operation. 2431 2432=item lstat FILEHANDLE 2433 2434=item lstat EXPR 2435 2436=item lstat 2437 2438Does the same thing as the C<stat> function (including setting the 2439special C<_> filehandle) but stats a symbolic link instead of the file 2440the symbolic link points to. If symbolic links are unimplemented on 2441your system, a normal C<stat> is done. 2442 2443If EXPR is omitted, stats C<$_>. 2444 2445=item m// 2446 2447The match operator. See L<perlop>. 2448 2449=item map BLOCK LIST 2450 2451=item map EXPR,LIST 2452 2453Evaluates the BLOCK or EXPR for each element of LIST (locally setting 2454C<$_> to each element) and returns the list value composed of the 2455results of each such evaluation. In scalar context, returns the 2456total number of elements so generated. Evaluates BLOCK or EXPR in 2457list context, so each element of LIST may produce zero, one, or 2458more elements in the returned value. 2459 2460 @chars = map(chr, @nums); 2461 2462translates a list of numbers to the corresponding characters. And 2463 2464 %hash = map { getkey($_) => $_ } @array; 2465 2466is just a funny way to write 2467 2468 %hash = (); 2469 foreach $_ (@array) { 2470 $hash{getkey($_)} = $_; 2471 } 2472 2473Note that C<$_> is an alias to the list value, so it can be used to 2474modify the elements of the LIST. While this is useful and supported, 2475it can cause bizarre results if the elements of LIST are not variables. 2476Using a regular C<foreach> loop for this purpose would be clearer in 2477most cases. See also L</grep> for an array composed of those items of 2478the original list for which the BLOCK or EXPR evaluates to true. 2479 2480C<{> starts both hash references and blocks, so C<map { ...> could be either 2481the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look 2482ahead for the closing C<}> it has to take a guess at which its dealing with 2483based what it finds just after the C<{>. Usually it gets it right, but if it 2484doesn't it won't realize something is wrong until it gets to the C<}> and 2485encounters the missing (or unexpected) comma. The syntax error will be 2486reported close to the C<}> but you'll need to change something near the C<{> 2487such as using a unary C<+> to give perl some help: 2488 2489 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong 2490 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right 2491 %hash = map { ("\L$_", 1) } @array # this also works 2492 %hash = map { lc($_), 1 } @array # as does this. 2493 %hash = map +( lc($_), 1 ), @array # this is EXPR and works! 2494 2495 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array) 2496 2497or to force an anon hash constructor use C<+{> 2498 2499 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end 2500 2501and you get list of anonymous hashes each with only 1 entry. 2502 2503=item mkdir FILENAME,MASK 2504 2505=item mkdir FILENAME 2506 2507Creates the directory specified by FILENAME, with permissions 2508specified by MASK (as modified by C<umask>). If it succeeds it 2509returns true, otherwise it returns false and sets C<$!> (errno). 2510If omitted, MASK defaults to 0777. 2511 2512In general, it is better to create directories with permissive MASK, 2513and let the user modify that with their C<umask>, than it is to supply 2514a restrictive MASK and give the user no way to be more permissive. 2515The exceptions to this rule are when the file or directory should be 2516kept private (mail files, for instance). The perlfunc(1) entry on 2517C<umask> discusses the choice of MASK in more detail. 2518 2519=item msgctl ID,CMD,ARG 2520 2521Calls the System V IPC function msgctl(2). You'll probably have to say 2522 2523 use IPC::SysV; 2524 2525first to get the correct constant definitions. If CMD is C<IPC_STAT>, 2526then ARG must be a variable which will hold the returned C<msqid_ds> 2527structure. Returns like C<ioctl>: the undefined value for error, 2528C<"0 but true"> for zero, or the actual return value otherwise. See also 2529L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation. 2530 2531=item msgget KEY,FLAGS 2532 2533Calls the System V IPC function msgget(2). Returns the message queue 2534id, or the undefined value if there is an error. See also 2535L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation. 2536 2537=item msgrcv ID,VAR,SIZE,TYPE,FLAGS 2538 2539Calls the System V IPC function msgrcv to receive a message from 2540message queue ID into variable VAR with a maximum message size of 2541SIZE. Note that when a message is received, the message type as a 2542native long integer will be the first thing in VAR, followed by the 2543actual message. This packing may be opened with C<unpack("l! a*")>. 2544Taints the variable. Returns true if successful, or false if there is 2545an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and 2546C<IPC::SysV::Msg> documentation. 2547 2548=item msgsnd ID,MSG,FLAGS 2549 2550Calls the System V IPC function msgsnd to send the message MSG to the 2551message queue ID. MSG must begin with the native long integer message 2552type, and be followed by the length of the actual message, and finally 2553the message itself. This kind of packing can be achieved with 2554C<pack("l! a*", $type, $message)>. Returns true if successful, 2555or false if there is an error. See also C<IPC::SysV> 2556and C<IPC::SysV::Msg> documentation. 2557 2558=item my EXPR 2559 2560=item my EXPR : ATTRIBUTES 2561 2562A C<my> declares the listed variables to be local (lexically) to the 2563enclosing block, file, or C<eval>. If 2564more than one value is listed, the list must be placed in parentheses. See 2565L<perlsub/"Private Variables via my()"> for details. 2566 2567=item next LABEL 2568 2569=item next 2570 2571The C<next> command is like the C<continue> statement in C; it starts 2572the next iteration of the loop: 2573 2574 LINE: while (<STDIN>) { 2575 next LINE if /^#/; # discard comments 2576 #... 2577 } 2578 2579Note that if there were a C<continue> block on the above, it would get 2580executed even on discarded lines. If the LABEL is omitted, the command 2581refers to the innermost enclosing loop. 2582 2583C<next> cannot be used to exit a block which returns a value such as 2584C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit 2585a grep() or map() operation. 2586 2587Note that a block by itself is semantically identical to a loop 2588that executes once. Thus C<next> will exit such a block early. 2589 2590See also L</continue> for an illustration of how C<last>, C<next>, and 2591C<redo> work. 2592 2593=item no Module LIST 2594 2595See the C<use> function, which C<no> is the opposite of. 2596 2597=item oct EXPR 2598 2599=item oct 2600 2601Interprets EXPR as an octal string and returns the corresponding 2602value. (If EXPR happens to start off with C<0x>, interprets it as a 2603hex string. If EXPR starts off with C<0b>, it is interpreted as a 2604binary string.) The following will handle decimal, binary, octal, and 2605hex in the standard Perl or C notation: 2606 2607 $val = oct($val) if $val =~ /^0/; 2608 2609If EXPR is omitted, uses C<$_>. To go the other way (produce a number 2610in octal), use sprintf() or printf(): 2611 2612 $perms = (stat("filename"))[2] & 07777; 2613 $oct_perms = sprintf "%lo", $perms; 2614 2615The oct() function is commonly used when a string such as C<644> needs 2616to be converted into a file mode, for example. (Although perl will 2617automatically convert strings into numbers as needed, this automatic 2618conversion assumes base 10.) 2619 2620=item open FILEHANDLE,MODE,LIST 2621 2622=item open FILEHANDLE,EXPR 2623 2624=item open FILEHANDLE 2625 2626Opens the file whose filename is given by EXPR, and associates it with 2627FILEHANDLE. If FILEHANDLE is an expression, its value is used as the 2628name of the real filehandle wanted. (This is considered a symbolic 2629reference, so C<use strict 'refs'> should I<not> be in effect.) 2630 2631If EXPR is omitted, the scalar 2632variable of the same name as the FILEHANDLE contains the filename. 2633(Note that lexical variables--those declared with C<my>--will not work 2634for this purpose; so if you're using C<my>, specify EXPR in your call 2635to open.) See L<perlopentut> for a kinder, gentler explanation of opening 2636files. 2637 2638If MODE is C<< '<' >> or nothing, the file is opened for input. 2639If MODE is C<< '>' >>, the file is truncated and opened for 2640output, being created if necessary. If MODE is C<<< '>>' >>>, 2641the file is opened for appending, again being created if necessary. 2642You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that 2643you want both read and write access to the file; thus C<< '+<' >> is almost 2644always preferred for read/write updates--the C<< '+>' >> mode would clobber the 2645file first. You can't usually use either read-write mode for updating 2646textfiles, since they have variable length records. See the B<-i> 2647switch in L<perlrun> for a better approach. The file is created with 2648permissions of C<0666> modified by the process' C<umask> value. 2649 2650These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, 2651C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>. 2652 2653In the 2-arguments (and 1-argument) form of the call the mode and 2654filename should be concatenated (in this order), possibly separated by 2655spaces. It is possible to omit the mode if the mode is C<< '<' >>. 2656 2657If the filename begins with C<'|'>, the filename is interpreted as a 2658command to which output is to be piped, and if the filename ends with a 2659C<'|'>, the filename is interpreted as a command which pipes output to 2660us. See L<perlipc/"Using open() for IPC"> 2661for more examples of this. (You are not allowed to C<open> to a command 2662that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, 2663and L<perlipc/"Bidirectional Communication with Another Process"> 2664for alternatives.) 2665 2666If MODE is C<'|-'>, the filename is interpreted as a 2667command to which output is to be piped, and if MODE is 2668C<'-|'>, the filename is interpreted as a command which pipes output to 2669us. In the 2-arguments (and 1-argument) form one should replace dash 2670(C<'-'>) with the command. See L<perlipc/"Using open() for IPC"> 2671for more examples of this. (You are not allowed to C<open> to a command 2672that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, 2673and L<perlipc/"Bidirectional Communication"> for alternatives.) 2674 2675In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN 2676and opening C<< '>-' >> opens STDOUT. 2677 2678Open returns 2679nonzero upon success, the undefined value otherwise. If the C<open> 2680involved a pipe, the return value happens to be the pid of the 2681subprocess. 2682 2683If you're unfortunate enough to be running Perl on a system that 2684distinguishes between text files and binary files (modern operating 2685systems don't care), then you should check out L</binmode> for tips for 2686dealing with this. The key distinction between systems that need C<binmode> 2687and those that don't is their text file formats. Systems like Unix, MacOS, and 2688Plan9, which delimit lines with a single character, and which encode that 2689character in C as C<"\n">, do not need C<binmode>. The rest need it. 2690 2691When opening a file, it's usually a bad idea to continue normal execution 2692if the request failed, so C<open> is frequently used in connection with 2693C<die>. Even if C<die> won't do what you want (say, in a CGI script, 2694where you want to make a nicely formatted error message (but there are 2695modules that can help with that problem)) you should always check 2696the return value from opening a file. The infrequent exception is when 2697working with an unopened filehandle is actually what you want to do. 2698 2699Examples: 2700 2701 $ARTICLE = 100; 2702 open ARTICLE or die "Can't find article $ARTICLE: $!\n"; 2703 while (<ARTICLE>) {... 2704 2705 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) 2706 # if the open fails, output is discarded 2707 2708 open(DBASE, '+<', 'dbase.mine') # open for update 2709 or die "Can't open 'dbase.mine' for update: $!"; 2710 2711 open(DBASE, '+<dbase.mine') # ditto 2712 or die "Can't open 'dbase.mine' for update: $!"; 2713 2714 open(ARTICLE, '-|', "caesar <$article") # decrypt article 2715 or die "Can't start caesar: $!"; 2716 2717 open(ARTICLE, "caesar <$article |") # ditto 2718 or die "Can't start caesar: $!"; 2719 2720 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id 2721 or die "Can't start sort: $!"; 2722 2723 # process argument list of files along with any includes 2724 2725 foreach $file (@ARGV) { 2726 process($file, 'fh00'); 2727 } 2728 2729 sub process { 2730 my($filename, $input) = @_; 2731 $input++; # this is a string increment 2732 unless (open($input, $filename)) { 2733 print STDERR "Can't open $filename: $!\n"; 2734 return; 2735 } 2736 2737 local $_; 2738 while (<$input>) { # note use of indirection 2739 if (/^#include "(.*)"/) { 2740 process($1, $input); 2741 next; 2742 } 2743 #... # whatever 2744 } 2745 } 2746 2747You may also, in the Bourne shell tradition, specify an EXPR beginning 2748with C<< '>&' >>, in which case the rest of the string is interpreted as the 2749name of a filehandle (or file descriptor, if numeric) to be 2750duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>, 2751C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The 2752mode you specify should match the mode of the original filehandle. 2753(Duping a filehandle does not take into account any existing contents of 2754stdio buffers.) Duping file handles is not yet supported for 3-argument 2755open(). 2756 2757Here is a script that saves, redirects, and restores STDOUT and 2758STDERR: 2759 2760 #!/usr/bin/perl 2761 open(OLDOUT, ">&STDOUT"); 2762 open(OLDERR, ">&STDERR"); 2763 2764 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout"; 2765 open(STDERR, ">&STDOUT") || die "Can't dup stdout"; 2766 2767 select(STDERR); $| = 1; # make unbuffered 2768 select(STDOUT); $| = 1; # make unbuffered 2769 2770 print STDOUT "stdout 1\n"; # this works for 2771 print STDERR "stderr 1\n"; # subprocesses too 2772 2773 close(STDOUT); 2774 close(STDERR); 2775 2776 open(STDOUT, ">&OLDOUT"); 2777 open(STDERR, ">&OLDERR"); 2778 2779 print STDOUT "stdout 2\n"; 2780 print STDERR "stderr 2\n"; 2781 2782If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an 2783equivalent of C's C<fdopen> of that file descriptor; this is more 2784parsimonious of file descriptors. For example: 2785 2786 open(FILEHANDLE, "<&=$fd") 2787 2788Note that this feature depends on the fdopen() C library function. 2789On many UNIX systems, fdopen() is known to fail when file descriptors 2790exceed a certain value, typically 255. If you need more file 2791descriptors than that, consider rebuilding Perl to use the C<sfio> 2792library. 2793 2794If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'> 2795with 2-arguments (or 1-argument) form of open(), then 2796there is an implicit fork done, and the return value of open is the pid 2797of the child within the parent process, and C<0> within the child 2798process. (Use C<defined($pid)> to determine whether the open was successful.) 2799The filehandle behaves normally for the parent, but i/o to that 2800filehandle is piped from/to the STDOUT/STDIN of the child process. 2801In the child process the filehandle isn't opened--i/o happens from/to 2802the new STDOUT or STDIN. Typically this is used like the normal 2803piped open when you want to exercise more control over just how the 2804pipe command gets executed, such as when you are running setuid, and 2805don't want to have to scan shell commands for metacharacters. 2806The following triples are more or less equivalent: 2807 2808 open(FOO, "|tr '[a-z]' '[A-Z]'"); 2809 open(FOO, '|-', "tr '[a-z]' '[A-Z]'"); 2810 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]'; 2811 2812 open(FOO, "cat -n '$file'|"); 2813 open(FOO, '-|', "cat -n '$file'"); 2814 open(FOO, '-|') || exec 'cat', '-n', $file; 2815 2816See L<perlipc/"Safe Pipe Opens"> for more examples of this. 2817 2818Beginning with v5.6.0, Perl will attempt to flush all files opened for 2819output before any operation that may do a fork, but this may not be 2820supported on some platforms (see L<perlport>). To be safe, you may need 2821to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method 2822of C<IO::Handle> on any open handles. 2823 2824On systems that support a 2825close-on-exec flag on files, the flag will be set for the newly opened 2826file descriptor as determined by the value of $^F. See L<perlvar/$^F>. 2827 2828Closing any piped filehandle causes the parent process to wait for the 2829child to finish, and returns the status value in C<$?>. 2830 2831The filename passed to 2-argument (or 1-argument) form of open() 2832will have leading and trailing 2833whitespace deleted, and the normal redirection characters 2834honored. This property, known as "magic open", 2835can often be used to good effect. A user could specify a filename of 2836F<"rsh cat file |">, or you could change certain filenames as needed: 2837 2838 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; 2839 open(FH, $filename) or die "Can't open $filename: $!"; 2840 2841Use 3-argument form to open a file with arbitrary weird characters in it, 2842 2843 open(FOO, '<', $file); 2844 2845otherwise it's necessary to protect any leading and trailing whitespace: 2846 2847 $file =~ s#^(\s)#./$1#; 2848 open(FOO, "< $file\0"); 2849 2850(this may not work on some bizarre filesystems). One should 2851conscientiously choose between the I<magic> and 3-arguments form 2852of open(): 2853 2854 open IN, $ARGV[0]; 2855 2856will allow the user to specify an argument of the form C<"rsh cat file |">, 2857but will not work on a filename which happens to have a trailing space, while 2858 2859 open IN, '<', $ARGV[0]; 2860 2861will have exactly the opposite restrictions. 2862 2863If you want a "real" C C<open> (see L<open(2)> on your system), then you 2864should use the C<sysopen> function, which involves no such magic (but 2865may use subtly different filemodes than Perl open(), which is mapped 2866to C fopen()). This is 2867another way to protect your filenames from interpretation. For example: 2868 2869 use IO::Handle; 2870 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) 2871 or die "sysopen $path: $!"; 2872 $oldfh = select(HANDLE); $| = 1; select($oldfh); 2873 print HANDLE "stuff $$\n"; 2874 seek(HANDLE, 0, 0); 2875 print "File contains: ", <HANDLE>; 2876 2877Using the constructor from the C<IO::Handle> package (or one of its 2878subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous 2879filehandles that have the scope of whatever variables hold references to 2880them, and automatically close whenever and however you leave that scope: 2881 2882 use IO::File; 2883 #... 2884 sub read_myfile_munged { 2885 my $ALL = shift; 2886 my $handle = new IO::File; 2887 open($handle, "myfile") or die "myfile: $!"; 2888 $first = <$handle> 2889 or return (); # Automatically closed here. 2890 mung $first or die "mung failed"; # Or here. 2891 return $first, <$handle> if $ALL; # Or here. 2892 $first; # Or here. 2893 } 2894 2895See L</seek> for some details about mixing reading and writing. 2896 2897=item opendir DIRHANDLE,EXPR 2898 2899Opens a directory named EXPR for processing by C<readdir>, C<telldir>, 2900C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful. 2901DIRHANDLEs have their own namespace separate from FILEHANDLEs. 2902 2903=item ord EXPR 2904 2905=item ord 2906 2907Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If 2908EXPR is omitted, uses C<$_>. For the reverse, see L</chr>. 2909See L<utf8> for more about Unicode. 2910 2911=item our EXPR 2912 2913An C<our> declares the listed variables to be valid globals within 2914the enclosing block, file, or C<eval>. That is, it has the same 2915scoping rules as a "my" declaration, but does not create a local 2916variable. If more than one value is listed, the list must be placed 2917in parentheses. The C<our> declaration has no semantic effect unless 2918"use strict vars" is in effect, in which case it lets you use the 2919declared global variable without qualifying it with a package name. 2920(But only within the lexical scope of the C<our> declaration. In this 2921it differs from "use vars", which is package scoped.) 2922 2923An C<our> declaration declares a global variable that will be visible 2924across its entire lexical scope, even across package boundaries. The 2925package in which the variable is entered is determined at the point 2926of the declaration, not at the point of use. This means the following 2927behavior holds: 2928 2929 package Foo; 2930 our $bar; # declares $Foo::bar for rest of lexical scope 2931 $bar = 20; 2932 2933 package Bar; 2934 print $bar; # prints 20 2935 2936Multiple C<our> declarations in the same lexical scope are allowed 2937if they are in different packages. If they happened to be in the same 2938package, Perl will emit warnings if you have asked for them. 2939 2940 use warnings; 2941 package Foo; 2942 our $bar; # declares $Foo::bar for rest of lexical scope 2943 $bar = 20; 2944 2945 package Bar; 2946 our $bar = 30; # declares $Bar::bar for rest of lexical scope 2947 print $bar; # prints 30 2948 2949 our $bar; # emits warning 2950 2951=item pack TEMPLATE,LIST 2952 2953Takes a LIST of values and converts it into a string using the rules 2954given by the TEMPLATE. The resulting string is the concatenation of 2955the converted values. Typically, each converted value looks 2956like its machine-level representation. For example, on 32-bit machines 2957a converted integer may be represented by a sequence of 4 bytes. 2958 2959The TEMPLATE is a 2960sequence of characters that give the order and type of values, as 2961follows: 2962 2963 a A string with arbitrary binary data, will be null padded. 2964 A An ASCII string, will be space padded. 2965 Z A null terminated (asciz) string, will be null padded. 2966 2967 b A bit string (ascending bit order inside each byte, like vec()). 2968 B A bit string (descending bit order inside each byte). 2969 h A hex string (low nybble first). 2970 H A hex string (high nybble first). 2971 2972 c A signed char value. 2973 C An unsigned char value. Only does bytes. See U for Unicode. 2974 2975 s A signed short value. 2976 S An unsigned short value. 2977 (This 'short' is _exactly_ 16 bits, which may differ from 2978 what a local C compiler calls 'short'. If you want 2979 native-length shorts, use the '!' suffix.) 2980 2981 i A signed integer value. 2982 I An unsigned integer value. 2983 (This 'integer' is _at_least_ 32 bits wide. Its exact 2984 size depends on what a local C compiler calls 'int', 2985 and may even be larger than the 'long' described in 2986 the next item.) 2987 2988 l A signed long value. 2989 L An unsigned long value. 2990 (This 'long' is _exactly_ 32 bits, which may differ from 2991 what a local C compiler calls 'long'. If you want 2992 native-length longs, use the '!' suffix.) 2993 2994 n An unsigned short in "network" (big-endian) order. 2995 N An unsigned long in "network" (big-endian) order. 2996 v An unsigned short in "VAX" (little-endian) order. 2997 V An unsigned long in "VAX" (little-endian) order. 2998 (These 'shorts' and 'longs' are _exactly_ 16 bits and 2999 _exactly_ 32 bits, respectively.) 3000 3001 q A signed quad (64-bit) value. 3002 Q An unsigned quad value. 3003 (Quads are available only if your system supports 64-bit 3004 integer values _and_ if Perl has been compiled to support those. 3005 Causes a fatal error otherwise.) 3006 3007 f A single-precision float in the native format. 3008 d A double-precision float in the native format. 3009 3010 p A pointer to a null-terminated string. 3011 P A pointer to a structure (fixed-length string). 3012 3013 u A uuencoded string. 3014 U A Unicode character number. Encodes to UTF-8 internally. 3015 Works even if C<use utf8> is not in effect. 3016 3017 w A BER compressed integer. Its bytes represent an unsigned 3018 integer in base 128, most significant digit first, with as 3019 few digits as possible. Bit eight (the high bit) is set 3020 on each byte except the last. 3021 3022 x A null byte. 3023 X Back up a byte. 3024 @ Null fill to absolute position. 3025 3026The following rules apply: 3027 3028=over 8 3029 3030=item * 3031 3032Each letter may optionally be followed by a number giving a repeat 3033count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>, 3034C<H>, and C<P> the pack function will gobble up that many values from 3035the LIST. A C<*> for the repeat count means to use however many items are 3036left, except for C<@>, C<x>, C<X>, where it is equivalent 3037to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the 3038same). 3039 3040When used with C<Z>, C<*> results in the addition of a trailing null 3041byte (so the packed result will be one longer than the byte C<length> 3042of the item). 3043 3044The repeat count for C<u> is interpreted as the maximal number of bytes 3045to encode per line of output, with 0 and 1 replaced by 45. 3046 3047=item * 3048 3049The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a 3050string of length count, padding with nulls or spaces as necessary. When 3051unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything 3052after the first null, and C<a> returns data verbatim. When packing, 3053C<a>, and C<Z> are equivalent. 3054 3055If the value-to-pack is too long, it is truncated. If too long and an 3056explicit count is provided, C<Z> packs only C<$count-1> bytes, followed 3057by a null byte. Thus C<Z> always packs a trailing null byte under 3058all circumstances. 3059 3060=item * 3061 3062Likewise, the C<b> and C<B> fields pack a string that many bits long. 3063Each byte of the input field of pack() generates 1 bit of the result. 3064Each result bit is based on the least-significant bit of the corresponding 3065input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and 3066C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">. 3067 3068Starting from the beginning of the input string of pack(), each 8-tuple 3069of bytes is converted to 1 byte of output. With format C<b> 3070the first byte of the 8-tuple determines the least-significant bit of a 3071byte, and with format C<B> it determines the most-significant bit of 3072a byte. 3073 3074If the length of the input string is not exactly divisible by 8, the 3075remainder is packed as if the input string were padded by null bytes 3076at the end. Similarly, during unpack()ing the "extra" bits are ignored. 3077 3078If the input string of pack() is longer than needed, extra bytes are ignored. 3079A C<*> for the repeat count of pack() means to use all the bytes of 3080the input field. On unpack()ing the bits are converted to a string 3081of C<"0">s and C<"1">s. 3082 3083=item * 3084 3085The C<h> and C<H> fields pack a string that many nybbles (4-bit groups, 3086representable as hexadecimal digits, 0-9a-f) long. 3087 3088Each byte of the input field of pack() generates 4 bits of the result. 3089For non-alphabetical bytes the result is based on the 4 least-significant 3090bits of the input byte, i.e., on C<ord($byte)%16>. In particular, 3091bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes 3092C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result 3093is compatible with the usual hexadecimal digits, so that C<"a"> and 3094C<"A"> both generate the nybble C<0xa==10>. The result for bytes 3095C<"g".."z"> and C<"G".."Z"> is not well-defined. 3096 3097Starting from the beginning of the input string of pack(), each pair 3098of bytes is converted to 1 byte of output. With format C<h> the 3099first byte of the pair determines the least-significant nybble of the 3100output byte, and with format C<H> it determines the most-significant 3101nybble. 3102 3103If the length of the input string is not even, it behaves as if padded 3104by a null byte at the end. Similarly, during unpack()ing the "extra" 3105nybbles are ignored. 3106 3107If the input string of pack() is longer than needed, extra bytes are ignored. 3108A C<*> for the repeat count of pack() means to use all the bytes of 3109the input field. On unpack()ing the bits are converted to a string 3110of hexadecimal digits. 3111 3112=item * 3113 3114The C<p> type packs a pointer to a null-terminated string. You are 3115responsible for ensuring the string is not a temporary value (which can 3116potentially get deallocated before you get around to using the packed result). 3117The C<P> type packs a pointer to a structure of the size indicated by the 3118length. A NULL pointer is created if the corresponding value for C<p> or 3119C<P> is C<undef>, similarly for unpack(). 3120 3121=item * 3122 3123The C</> template character allows packing and unpacking of strings where 3124the packed structure contains a byte count followed by the string itself. 3125You write I<length-item>C</>I<string-item>. 3126 3127The I<length-item> can be any C<pack> template letter, 3128and describes how the length value is packed. 3129The ones likely to be of most use are integer-packing ones like 3130C<n> (for Java strings), C<w> (for ASN.1 or SNMP) 3131and C<N> (for Sun XDR). 3132 3133The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">. 3134For C<unpack> the length of the string is obtained from the I<length-item>, 3135but if you put in the '*' it will be ignored. 3136 3137 unpack 'C/a', "\04Gurusamy"; gives 'Guru' 3138 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J') 3139 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world" 3140 3141The I<length-item> is not returned explicitly from C<unpack>. 3142 3143Adding a count to the I<length-item> letter is unlikely to do anything 3144useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a 3145I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters, 3146which Perl does not regard as legal in numeric strings. 3147 3148=item * 3149 3150The integer types C<s>, C<S>, C<l>, and C<L> may be 3151immediately followed by a C<!> suffix to signify native shorts or 3152longs--as you can see from above for example a bare C<l> does mean 3153exactly 32 bits, the native C<long> (as seen by the local C compiler) 3154may be larger. This is an issue mainly in 64-bit platforms. You can 3155see whether using C<!> makes any difference by 3156 3157 print length(pack("s")), " ", length(pack("s!")), "\n"; 3158 print length(pack("l")), " ", length(pack("l!")), "\n"; 3159 3160C<i!> and C<I!> also work but only because of completeness; 3161they are identical to C<i> and C<I>. 3162 3163The actual sizes (in bytes) of native shorts, ints, longs, and long 3164longs on the platform where Perl was built are also available via 3165L<Config>: 3166 3167 use Config; 3168 print $Config{shortsize}, "\n"; 3169 print $Config{intsize}, "\n"; 3170 print $Config{longsize}, "\n"; 3171 print $Config{longlongsize}, "\n"; 3172 3173(The C<$Config{longlongsize}> will be undefine if your system does 3174not support long longs.) 3175 3176=item * 3177 3178The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L> 3179are inherently non-portable between processors and operating systems 3180because they obey the native byteorder and endianness. For example a 31814-byte integer 0x12345678 (305419896 decimal) be ordered natively 3182(arranged in and handled by the CPU registers) into bytes as 3183 3184 0x12 0x34 0x56 0x78 # big-endian 3185 0x78 0x56 0x34 0x12 # little-endian 3186 3187Basically, the Intel and VAX CPUs are little-endian, while everybody 3188else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and 3189Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq 3190used/uses them in little-endian mode; SGI/Cray uses them in big-endian mode. 3191 3192The names `big-endian' and `little-endian' are comic references to 3193the classic "Gulliver's Travels" (via the paper "On Holy Wars and a 3194Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and 3195the egg-eating habits of the Lilliputians. 3196 3197Some systems may have even weirder byte orders such as 3198 3199 0x56 0x78 0x12 0x34 3200 0x34 0x12 0x78 0x56 3201 3202You can see your system's preference with 3203 3204 print join(" ", map { sprintf "%#02x", $_ } 3205 unpack("C*",pack("L",0x12345678))), "\n"; 3206 3207The byteorder on the platform where Perl was built is also available 3208via L<Config>: 3209 3210 use Config; 3211 print $Config{byteorder}, "\n"; 3212 3213Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'> 3214and C<'87654321'> are big-endian. 3215 3216If you want portable packed integers use the formats C<n>, C<N>, 3217C<v>, and C<V>, their byte endianness and size is known. 3218See also L<perlport>. 3219 3220=item * 3221 3222Real numbers (floats and doubles) are in the native machine format only; 3223due to the multiplicity of floating formats around, and the lack of a 3224standard "network" representation, no facility for interchange has been 3225made. This means that packed floating point data written on one machine 3226may not be readable on another - even if both use IEEE floating point 3227arithmetic (as the endian-ness of the memory representation is not part 3228of the IEEE spec). See also L<perlport>. 3229 3230Note that Perl uses doubles internally for all numeric calculation, and 3231converting from double into float and thence back to double again will 3232lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general 3233equal $foo). 3234 3235=item * 3236 3237If the pattern begins with a C<U>, the resulting string will be treated 3238as Unicode-encoded. You can force UTF8 encoding on in a string with an 3239initial C<U0>, and the bytes that follow will be interpreted as Unicode 3240characters. If you don't want this to happen, you can begin your pattern 3241with C<C0> (or anything else) to force Perl not to UTF8 encode your 3242string, and then follow this with a C<U*> somewhere in your pattern. 3243 3244=item * 3245 3246You must yourself do any alignment or padding by inserting for example 3247enough C<'x'>es while packing. There is no way to pack() and unpack() 3248could know where the bytes are going to or coming from. Therefore 3249C<pack> (and C<unpack>) handle their output and input as flat 3250sequences of bytes. 3251 3252=item * 3253 3254A comment in a TEMPLATE starts with C<#> and goes to the end of line. 3255 3256=item * 3257 3258If TEMPLATE requires more arguments to pack() than actually given, pack() 3259assumes additional C<""> arguments. If TEMPLATE requires less arguments 3260to pack() than actually given, extra arguments are ignored. 3261 3262=back 3263 3264Examples: 3265 3266 $foo = pack("CCCC",65,66,67,68); 3267 # foo eq "ABCD" 3268 $foo = pack("C4",65,66,67,68); 3269 # same thing 3270 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); 3271 # same thing with Unicode circled letters 3272 3273 $foo = pack("ccxxcc",65,66,67,68); 3274 # foo eq "AB\0\0CD" 3275 3276 # note: the above examples featuring "C" and "c" are true 3277 # only on ASCII and ASCII-derived systems such as ISO Latin 1 3278 # and UTF-8. In EBCDIC the first example would be 3279 # $foo = pack("CCCC",193,194,195,196); 3280 3281 $foo = pack("s2",1,2); 3282 # "\1\0\2\0" on little-endian 3283 # "\0\1\0\2" on big-endian 3284 3285 $foo = pack("a4","abcd","x","y","z"); 3286 # "abcd" 3287 3288 $foo = pack("aaaa","abcd","x","y","z"); 3289 # "axyz" 3290 3291 $foo = pack("a14","abcdefg"); 3292 # "abcdefg\0\0\0\0\0\0\0" 3293 3294 $foo = pack("i9pl", gmtime); 3295 # a real struct tm (on my system anyway) 3296 3297 $utmp_template = "Z8 Z8 Z16 L"; 3298 $utmp = pack($utmp_template, @utmp1); 3299 # a struct utmp (BSDish) 3300 3301 @utmp2 = unpack($utmp_template, $utmp); 3302 # "@utmp1" eq "@utmp2" 3303 3304 sub bintodec { 3305 unpack("N", pack("B32", substr("0" x 32 . shift, -32))); 3306 } 3307 3308 $foo = pack('sx2l', 12, 34); 3309 # short 12, two zero bytes padding, long 34 3310 $bar = pack('s@4l', 12, 34); 3311 # short 12, zero fill to position 4, long 34 3312 # $foo eq $bar 3313 3314The same template may generally also be used in unpack(). 3315 3316=item package NAMESPACE 3317 3318=item package 3319 3320Declares the compilation unit as being in the given namespace. The scope 3321of the package declaration is from the declaration itself through the end 3322of the enclosing block, file, or eval (the same as the C<my> operator). 3323All further unqualified dynamic identifiers will be in this namespace. 3324A package statement affects only dynamic variables--including those 3325you've used C<local> on--but I<not> lexical variables, which are created 3326with C<my>. Typically it would be the first declaration in a file to 3327be included by the C<require> or C<use> operator. You can switch into a 3328package in more than one place; it merely influences which symbol table 3329is used by the compiler for the rest of that block. You can refer to 3330variables and filehandles in other packages by prefixing the identifier 3331with the package name and a double colon: C<$Package::Variable>. 3332If the package name is null, the C<main> package as assumed. That is, 3333C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>, 3334still seen in older code). 3335 3336If NAMESPACE is omitted, then there is no current package, and all 3337identifiers must be fully qualified or lexicals. This is stricter 3338than C<use strict>, since it also extends to function names. 3339 3340See L<perlmod/"Packages"> for more information about packages, modules, 3341and classes. See L<perlsub> for other scoping issues. 3342 3343=item pipe READHANDLE,WRITEHANDLE 3344 3345Opens a pair of connected pipes like the corresponding system call. 3346Note that if you set up a loop of piped processes, deadlock can occur 3347unless you are very careful. In addition, note that Perl's pipes use 3348stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE 3349after each command, depending on the application. 3350 3351See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication"> 3352for examples of such things. 3353 3354On systems that support a close-on-exec flag on files, the flag will be set 3355for the newly opened file descriptors as determined by the value of $^F. 3356See L<perlvar/$^F>. 3357 3358=item pop ARRAY 3359 3360=item pop 3361 3362Pops and returns the last value of the array, shortening the array by 3363one element. Has an effect similar to 3364 3365 $ARRAY[$#ARRAY--] 3366 3367If there are no elements in the array, returns the undefined value 3368(although this may happen at other times as well). If ARRAY is 3369omitted, pops the C<@ARGV> array in the main program, and the C<@_> 3370array in subroutines, just like C<shift>. 3371 3372=item pos SCALAR 3373 3374=item pos 3375 3376Returns the offset of where the last C<m//g> search left off for the variable 3377in question (C<$_> is used when the variable is not specified). May be 3378modified to change that offset. Such modification will also influence 3379the C<\G> zero-width assertion in regular expressions. See L<perlre> and 3380L<perlop>. 3381 3382=item print FILEHANDLE LIST 3383 3384=item print LIST 3385 3386=item print 3387 3388Prints a string or a list of strings. Returns true if successful. 3389FILEHANDLE may be a scalar variable name, in which case the variable 3390contains the name of or a reference to the filehandle, thus introducing 3391one level of indirection. (NOTE: If FILEHANDLE is a variable and 3392the next token is a term, it may be misinterpreted as an operator 3393unless you interpose a C<+> or put parentheses around the arguments.) 3394If FILEHANDLE is omitted, prints by default to standard output (or 3395to the last selected output channel--see L</select>). If LIST is 3396also omitted, prints C<$_> to the currently selected output channel. 3397To set the default output channel to something other than STDOUT 3398use the select operation. The current value of C<$,> (if any) is 3399printed between each LIST item. The current value of C<$\> (if 3400any) is printed after the entire LIST has been printed. Because 3401print takes a LIST, anything in the LIST is evaluated in list 3402context, and any subroutine that you call will have one or more of 3403its expressions evaluated in list context. Also be careful not to 3404follow the print keyword with a left parenthesis unless you want 3405the corresponding right parenthesis to terminate the arguments to 3406the print--interpose a C<+> or put parentheses around all the 3407arguments. 3408 3409Note that if you're storing FILEHANDLES in an array or other expression, 3410you will have to use a block returning its value instead: 3411 3412 print { $files[$i] } "stuff\n"; 3413 print { $OK ? STDOUT : STDERR } "stuff\n"; 3414 3415=item printf FILEHANDLE FORMAT, LIST 3416 3417=item printf FORMAT, LIST 3418 3419Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\> 3420(the output record separator) is not appended. The first argument 3421of the list will be interpreted as the C<printf> format. If C<use locale> is 3422in effect, the character used for the decimal point in formatted real numbers 3423is affected by the LC_NUMERIC locale. See L<perllocale>. 3424 3425Don't fall into the trap of using a C<printf> when a simple 3426C<print> would do. The C<print> is more efficient and less 3427error prone. 3428 3429=item prototype FUNCTION 3430 3431Returns the prototype of a function as a string (or C<undef> if the 3432function has no prototype). FUNCTION is a reference to, or the name of, 3433the function whose prototype you want to retrieve. 3434 3435If FUNCTION is a string starting with C<CORE::>, the rest is taken as a 3436name for Perl builtin. If the builtin is not I<overridable> (such as 3437C<qw//>) or its arguments cannot be expressed by a prototype (such as 3438C<system>) returns C<undef> because the builtin does not really behave 3439like a Perl function. Otherwise, the string describing the equivalent 3440prototype is returned. 3441 3442=item push ARRAY,LIST 3443 3444Treats ARRAY as a stack, and pushes the values of LIST 3445onto the end of ARRAY. The length of ARRAY increases by the length of 3446LIST. Has the same effect as 3447 3448 for $value (LIST) { 3449 $ARRAY[++$#ARRAY] = $value; 3450 } 3451 3452but is more efficient. Returns the new number of elements in the array. 3453 3454=item q/STRING/ 3455 3456=item qq/STRING/ 3457 3458=item qr/STRING/ 3459 3460=item qx/STRING/ 3461 3462=item qw/STRING/ 3463 3464Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">. 3465 3466=item quotemeta EXPR 3467 3468=item quotemeta 3469 3470Returns the value of EXPR with all non-"word" 3471characters backslashed. (That is, all characters not matching 3472C</[A-Za-z_0-9]/> will be preceded by a backslash in the 3473returned string, regardless of any locale settings.) 3474This is the internal function implementing 3475the C<\Q> escape in double-quoted strings. 3476 3477If EXPR is omitted, uses C<$_>. 3478 3479=item rand EXPR 3480 3481=item rand 3482 3483Returns a random fractional number greater than or equal to C<0> and less 3484than the value of EXPR. (EXPR should be positive.) If EXPR is 3485omitted, the value C<1> is used. Automatically calls C<srand> unless 3486C<srand> has already been called. See also C<srand>. 3487 3488(Note: If your rand function consistently returns numbers that are too 3489large or too small, then your version of Perl was probably compiled 3490with the wrong number of RANDBITS.) 3491 3492=item read FILEHANDLE,SCALAR,LENGTH,OFFSET 3493 3494=item read FILEHANDLE,SCALAR,LENGTH 3495 3496Attempts to read LENGTH bytes of data into variable SCALAR from the 3497specified FILEHANDLE. Returns the number of bytes actually read, C<0> 3498at end of file, or undef if there was an error. SCALAR will be grown 3499or shrunk to the length actually read. If SCALAR needs growing, the 3500new bytes will be zero bytes. An OFFSET may be specified to place 3501the read data into some other place in SCALAR than the beginning. 3502The call is actually implemented in terms of stdio's fread(3) call. 3503To get a true read(2) system call, see C<sysread>. 3504 3505=item readdir DIRHANDLE 3506 3507Returns the next directory entry for a directory opened by C<opendir>. 3508If used in list context, returns all the rest of the entries in the 3509directory. If there are no more entries, returns an undefined value in 3510scalar context or a null list in list context. 3511 3512If you're planning to filetest the return values out of a C<readdir>, you'd 3513better prepend the directory in question. Otherwise, because we didn't 3514C<chdir> there, it would have been testing the wrong file. 3515 3516 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!"; 3517 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR); 3518 closedir DIR; 3519 3520=item readline EXPR 3521 3522Reads from the filehandle whose typeglob is contained in EXPR. In scalar 3523context, each call reads and returns the next line, until end-of-file is 3524reached, whereupon the subsequent call returns undef. In list context, 3525reads until end-of-file is reached and returns a list of lines. Note that 3526the notion of "line" used here is however you may have defined it 3527with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">. 3528 3529When C<$/> is set to C<undef>, when readline() is in scalar 3530context (i.e. file slurp mode), and when an empty file is read, it 3531returns C<''> the first time, followed by C<undef> subsequently. 3532 3533This is the internal function implementing the C<< <EXPR> >> 3534operator, but you can use it directly. The C<< <EXPR> >> 3535operator is discussed in more detail in L<perlop/"I/O Operators">. 3536 3537 $line = <STDIN>; 3538 $line = readline(*STDIN); # same thing 3539 3540=item readlink EXPR 3541 3542=item readlink 3543 3544Returns the value of a symbolic link, if symbolic links are 3545implemented. If not, gives a fatal error. If there is some system 3546error, returns the undefined value and sets C<$!> (errno). If EXPR is 3547omitted, uses C<$_>. 3548 3549=item readpipe EXPR 3550 3551EXPR is executed as a system command. 3552The collected standard output of the command is returned. 3553In scalar context, it comes back as a single (potentially 3554multi-line) string. In list context, returns a list of lines 3555(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>). 3556This is the internal function implementing the C<qx/EXPR/> 3557operator, but you can use it directly. The C<qx/EXPR/> 3558operator is discussed in more detail in L<perlop/"I/O Operators">. 3559 3560=item recv SOCKET,SCALAR,LENGTH,FLAGS 3561 3562Receives a message on a socket. Attempts to receive LENGTH bytes of 3563data into variable SCALAR from the specified SOCKET filehandle. SCALAR 3564will be grown or shrunk to the length actually read. Takes the same 3565flags as the system call of the same name. Returns the address of the 3566sender if SOCKET's protocol supports this; returns an empty string 3567otherwise. If there's an error, returns the undefined value. This call 3568is actually implemented in terms of recvfrom(2) system call. See 3569L<perlipc/"UDP: Message Passing"> for examples. 3570 3571=item redo LABEL 3572 3573=item redo 3574 3575The C<redo> command restarts the loop block without evaluating the 3576conditional again. The C<continue> block, if any, is not executed. If 3577the LABEL is omitted, the command refers to the innermost enclosing 3578loop. This command is normally used by programs that want to lie to 3579themselves about what was just input: 3580 3581 # a simpleminded Pascal comment stripper 3582 # (warning: assumes no { or } in strings) 3583 LINE: while (<STDIN>) { 3584 while (s|({.*}.*){.*}|$1 |) {} 3585 s|{.*}| |; 3586 if (s|{.*| |) { 3587 $front = $_; 3588 while (<STDIN>) { 3589 if (/}/) { # end of comment? 3590 s|^|$front\{|; 3591 redo LINE; 3592 } 3593 } 3594 } 3595 print; 3596 } 3597 3598C<redo> cannot be used to retry a block which returns a value such as 3599C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit 3600a grep() or map() operation. 3601 3602Note that a block by itself is semantically identical to a loop 3603that executes once. Thus C<redo> inside such a block will effectively 3604turn it into a looping construct. 3605 3606See also L</continue> for an illustration of how C<last>, C<next>, and 3607C<redo> work. 3608 3609=item ref EXPR 3610 3611=item ref 3612 3613Returns a true value if EXPR is a reference, false otherwise. If EXPR 3614is not specified, C<$_> will be used. The value returned depends on the 3615type of thing the reference is a reference to. 3616Builtin types include: 3617 3618 SCALAR 3619 ARRAY 3620 HASH 3621 CODE 3622 REF 3623 GLOB 3624 LVALUE 3625 3626If the referenced object has been blessed into a package, then that package 3627name is returned instead. You can think of C<ref> as a C<typeof> operator. 3628 3629 if (ref($r) eq "HASH") { 3630 print "r is a reference to a hash.\n"; 3631 } 3632 unless (ref($r)) { 3633 print "r is not a reference at all.\n"; 3634 } 3635 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing 3636 print "r is a reference to something that isa hash.\n"; 3637 } 3638 3639See also L<perlref>. 3640 3641=item rename OLDNAME,NEWNAME 3642 3643Changes the name of a file; an existing file NEWNAME will be 3644clobbered. Returns true for success, false otherwise. 3645 3646Behavior of this function varies wildly depending on your system 3647implementation. For example, it will usually not work across file system 3648boundaries, even though the system I<mv> command sometimes compensates 3649for this. Other restrictions include whether it works on directories, 3650open files, or pre-existing files. Check L<perlport> and either the 3651rename(2) manpage or equivalent system documentation for details. 3652 3653=item require VERSION 3654 3655=item require EXPR 3656 3657=item require 3658 3659Demands some semantics specified by EXPR, or by C<$_> if EXPR is not 3660supplied. 3661 3662If a VERSION is specified as a literal of the form v5.6.1, 3663demands that the current version of Perl (C<$^V> or $PERL_VERSION) be 3664at least as recent as that version, at run time. (For compatibility 3665with older versions of Perl, a numeric argument will also be interpreted 3666as VERSION.) Compare with L</use>, which can do a similar check at 3667compile time. 3668 3669 require v5.6.1; # run time version check 3670 require 5.6.1; # ditto 3671 require 5.005_03; # float version allowed for compatibility 3672 3673Otherwise, demands that a library file be included if it hasn't already 3674been included. The file is included via the do-FILE mechanism, which is 3675essentially just a variety of C<eval>. Has semantics similar to the following 3676subroutine: 3677 3678 sub require { 3679 my($filename) = @_; 3680 return 1 if $INC{$filename}; 3681 my($realfilename,$result); 3682 ITER: { 3683 foreach $prefix (@INC) { 3684 $realfilename = "$prefix/$filename"; 3685 if (-f $realfilename) { 3686 $INC{$filename} = $realfilename; 3687 $result = do $realfilename; 3688 last ITER; 3689 } 3690 } 3691 die "Can't find $filename in \@INC"; 3692 } 3693 delete $INC{$filename} if $@ || !$result; 3694 die $@ if $@; 3695 die "$filename did not return true value" unless $result; 3696 return $result; 3697 } 3698 3699Note that the file will not be included twice under the same specified 3700name. The file must return true as the last statement to indicate 3701successful execution of any initialization code, so it's customary to 3702end such a file with C<1;> unless you're sure it'll return true 3703otherwise. But it's better just to put the C<1;>, in case you add more 3704statements. 3705 3706If EXPR is a bareword, the require assumes a "F<.pm>" extension and 3707replaces "F<::>" with "F</>" in the filename for you, 3708to make it easy to load standard modules. This form of loading of 3709modules does not risk altering your namespace. 3710 3711In other words, if you try this: 3712 3713 require Foo::Bar; # a splendid bareword 3714 3715The require function will actually look for the "F<Foo/Bar.pm>" file in the 3716directories specified in the C<@INC> array. 3717 3718But if you try this: 3719 3720 $class = 'Foo::Bar'; 3721 require $class; # $class is not a bareword 3722 #or 3723 require "Foo::Bar"; # not a bareword because of the "" 3724 3725The require function will look for the "F<Foo::Bar>" file in the @INC array and 3726will complain about not finding "F<Foo::Bar>" there. In this case you can do: 3727 3728 eval "require $class"; 3729 3730For a yet-more-powerful import facility, see L</use> and L<perlmod>. 3731 3732=item reset EXPR 3733 3734=item reset 3735 3736Generally used in a C<continue> block at the end of a loop to clear 3737variables and reset C<??> searches so that they work again. The 3738expression is interpreted as a list of single characters (hyphens 3739allowed for ranges). All variables and arrays beginning with one of 3740those letters are reset to their pristine state. If the expression is 3741omitted, one-match searches (C<?pattern?>) are reset to match again. Resets 3742only variables or searches in the current package. Always returns 37431. Examples: 3744 3745 reset 'X'; # reset all X variables 3746 reset 'a-z'; # reset lower case variables 3747 reset; # just reset ?one-time? searches 3748 3749Resetting C<"A-Z"> is not recommended because you'll wipe out your 3750C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package 3751variables--lexical variables are unaffected, but they clean themselves 3752up on scope exit anyway, so you'll probably want to use them instead. 3753See L</my>. 3754 3755=item return EXPR 3756 3757=item return 3758 3759Returns from a subroutine, C<eval>, or C<do FILE> with the value 3760given in EXPR. Evaluation of EXPR may be in list, scalar, or void 3761context, depending on how the return value will be used, and the context 3762may vary from one execution to the next (see C<wantarray>). If no EXPR 3763is given, returns an empty list in list context, the undefined value in 3764scalar context, and (of course) nothing at all in a void context. 3765 3766(Note that in the absence of a explicit C<return>, a subroutine, eval, 3767or do FILE will automatically return the value of the last expression 3768evaluated.) 3769 3770=item reverse LIST 3771 3772In list context, returns a list value consisting of the elements 3773of LIST in the opposite order. In scalar context, concatenates the 3774elements of LIST and returns a string value with all characters 3775in the opposite order. 3776 3777 print reverse <>; # line tac, last line first 3778 3779 undef $/; # for efficiency of <> 3780 print scalar reverse <>; # character tac, last line tsrif 3781 3782This operator is also handy for inverting a hash, although there are some 3783caveats. If a value is duplicated in the original hash, only one of those 3784can be represented as a key in the inverted hash. Also, this has to 3785unwind one hash and build a whole new one, which may take some time 3786on a large hash, such as from a DBM file. 3787 3788 %by_name = reverse %by_address; # Invert the hash 3789 3790=item rewinddir DIRHANDLE 3791 3792Sets the current position to the beginning of the directory for the 3793C<readdir> routine on DIRHANDLE. 3794 3795=item rindex STR,SUBSTR,POSITION 3796 3797=item rindex STR,SUBSTR 3798 3799Works just like index() except that it returns the position of the LAST 3800occurrence of SUBSTR in STR. If POSITION is specified, returns the 3801last occurrence at or before that position. 3802 3803=item rmdir FILENAME 3804 3805=item rmdir 3806 3807Deletes the directory specified by FILENAME if that directory is empty. If it 3808succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If 3809FILENAME is omitted, uses C<$_>. 3810 3811=item s/// 3812 3813The substitution operator. See L<perlop>. 3814 3815=item scalar EXPR 3816 3817Forces EXPR to be interpreted in scalar context and returns the value 3818of EXPR. 3819 3820 @counts = ( scalar @a, scalar @b, scalar @c ); 3821 3822There is no equivalent operator to force an expression to 3823be interpolated in list context because in practice, this is never 3824needed. If you really wanted to do so, however, you could use 3825the construction C<@{[ (some expression) ]}>, but usually a simple 3826C<(some expression)> suffices. 3827 3828Because C<scalar> is unary operator, if you accidentally use for EXPR a 3829parenthesized list, this behaves as a scalar comma expression, evaluating 3830all but the last element in void context and returning the final element 3831evaluated in scalar context. This is seldom what you want. 3832 3833The following single statement: 3834 3835 print uc(scalar(&foo,$bar)),$baz; 3836 3837is the moral equivalent of these two: 3838 3839 &foo; 3840 print(uc($bar),$baz); 3841 3842See L<perlop> for more details on unary operators and the comma operator. 3843 3844=item seek FILEHANDLE,POSITION,WHENCE 3845 3846Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>. 3847FILEHANDLE may be an expression whose value gives the name of the 3848filehandle. The values for WHENCE are C<0> to set the new position to 3849POSITION, C<1> to set it to the current position plus POSITION, and 3850C<2> to set it to EOF plus POSITION (typically negative). For WHENCE 3851you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> 3852(start of the file, current position, end of the file) from the Fcntl 3853module. Returns C<1> upon success, C<0> otherwise. 3854 3855If you want to position file for C<sysread> or C<syswrite>, don't use 3856C<seek>--buffering makes its effect on the file's system position 3857unpredictable and non-portable. Use C<sysseek> instead. 3858 3859Due to the rules and rigors of ANSI C, on some systems you have to do a 3860seek whenever you switch between reading and writing. Amongst other 3861things, this may have the effect of calling stdio's clearerr(3). 3862A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position: 3863 3864 seek(TEST,0,1); 3865 3866This is also useful for applications emulating C<tail -f>. Once you hit 3867EOF on your read, and then sleep for a while, you might have to stick in a 3868seek() to reset things. The C<seek> doesn't change the current position, 3869but it I<does> clear the end-of-file condition on the handle, so that the 3870next C<< <FILE> >> makes Perl try again to read something. We hope. 3871 3872If that doesn't work (some stdios are particularly cantankerous), then 3873you may need something more like this: 3874 3875 for (;;) { 3876 for ($curpos = tell(FILE); $_ = <FILE>; 3877 $curpos = tell(FILE)) { 3878 # search for some stuff and put it into files 3879 } 3880 sleep($for_a_while); 3881 seek(FILE, $curpos, 0); 3882 } 3883 3884=item seekdir DIRHANDLE,POS 3885 3886Sets the current position for the C<readdir> routine on DIRHANDLE. POS 3887must be a value returned by C<telldir>. Has the same caveats about 3888possible directory compaction as the corresponding system library 3889routine. 3890 3891=item select FILEHANDLE 3892 3893=item select 3894 3895Returns the currently selected filehandle. Sets the current default 3896filehandle for output, if FILEHANDLE is supplied. This has two 3897effects: first, a C<write> or a C<print> without a filehandle will 3898default to this FILEHANDLE. Second, references to variables related to 3899output will refer to this output channel. For example, if you have to 3900set the top of form format for more than one output channel, you might 3901do the following: 3902 3903 select(REPORT1); 3904 $^ = 'report1_top'; 3905 select(REPORT2); 3906 $^ = 'report2_top'; 3907 3908FILEHANDLE may be an expression whose value gives the name of the 3909actual filehandle. Thus: 3910 3911 $oldfh = select(STDERR); $| = 1; select($oldfh); 3912 3913Some programmers may prefer to think of filehandles as objects with 3914methods, preferring to write the last example as: 3915 3916 use IO::Handle; 3917 STDERR->autoflush(1); 3918 3919=item select RBITS,WBITS,EBITS,TIMEOUT 3920 3921This calls the select(2) system call with the bit masks specified, which 3922can be constructed using C<fileno> and C<vec>, along these lines: 3923 3924 $rin = $win = $ein = ''; 3925 vec($rin,fileno(STDIN),1) = 1; 3926 vec($win,fileno(STDOUT),1) = 1; 3927 $ein = $rin | $win; 3928 3929If you want to select on many filehandles you might wish to write a 3930subroutine: 3931 3932 sub fhbits { 3933 my(@fhlist) = split(' ',$_[0]); 3934 my($bits); 3935 for (@fhlist) { 3936 vec($bits,fileno($_),1) = 1; 3937 } 3938 $bits; 3939 } 3940 $rin = fhbits('STDIN TTY SOCK'); 3941 3942The usual idiom is: 3943 3944 ($nfound,$timeleft) = 3945 select($rout=$rin, $wout=$win, $eout=$ein, $timeout); 3946 3947or to block until something becomes ready just do this 3948 3949 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); 3950 3951Most systems do not bother to return anything useful in $timeleft, so 3952calling select() in scalar context just returns $nfound. 3953 3954Any of the bit masks can also be undef. The timeout, if specified, is 3955in seconds, which may be fractional. Note: not all implementations are 3956capable of returning the$timeleft. If not, they always return 3957$timeleft equal to the supplied $timeout. 3958 3959You can effect a sleep of 250 milliseconds this way: 3960 3961 select(undef, undef, undef, 0.25); 3962 3963B<WARNING>: One should not attempt to mix buffered I/O (like C<read> 3964or <FH>) with C<select>, except as permitted by POSIX, and even 3965then only on POSIX systems. You have to use C<sysread> instead. 3966 3967=item semctl ID,SEMNUM,CMD,ARG 3968 3969Calls the System V IPC function C<semctl>. You'll probably have to say 3970 3971 use IPC::SysV; 3972 3973first to get the correct constant definitions. If CMD is IPC_STAT or 3974GETALL, then ARG must be a variable which will hold the returned 3975semid_ds structure or semaphore value array. Returns like C<ioctl>: 3976the undefined value for error, "C<0 but true>" for zero, or the actual 3977return value otherwise. The ARG must consist of a vector of native 3978short integers, which may be created with C<pack("s!",(0)x$nsem)>. 3979See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore> 3980documentation. 3981 3982=item semget KEY,NSEMS,FLAGS 3983 3984Calls the System V IPC function semget. Returns the semaphore id, or 3985the undefined value if there is an error. See also 3986L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore> 3987documentation. 3988 3989=item semop KEY,OPSTRING 3990 3991Calls the System V IPC function semop to perform semaphore operations 3992such as signaling and waiting. OPSTRING must be a packed array of 3993semop structures. Each semop structure can be generated with 3994C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore 3995operations is implied by the length of OPSTRING. Returns true if 3996successful, or false if there is an error. As an example, the 3997following code waits on semaphore $semnum of semaphore id $semid: 3998 3999 $semop = pack("sss", $semnum, -1, 0); 4000 die "Semaphore trouble: $!\n" unless semop($semid, $semop); 4001 4002To signal the semaphore, replace C<-1> with C<1>. See also 4003L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore> 4004documentation. 4005 4006=item send SOCKET,MSG,FLAGS,TO 4007 4008=item send SOCKET,MSG,FLAGS 4009 4010Sends a message on a socket. Takes the same flags as the system call 4011of the same name. On unconnected sockets you must specify a 4012destination to send TO, in which case it does a C C<sendto>. Returns 4013the number of characters sent, or the undefined value if there is an 4014error. The C system call sendmsg(2) is currently unimplemented. 4015See L<perlipc/"UDP: Message Passing"> for examples. 4016 4017=item setpgrp PID,PGRP 4018 4019Sets the current process group for the specified PID, C<0> for the current 4020process. Will produce a fatal error if used on a machine that doesn't 4021implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted, 4022it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not 4023accept any arguments, so only C<setpgrp(0,0)> is portable. See also 4024C<POSIX::setsid()>. 4025 4026=item setpriority WHICH,WHO,PRIORITY 4027 4028Sets the current priority for a process, a process group, or a user. 4029(See setpriority(2).) Will produce a fatal error if used on a machine 4030that doesn't implement setpriority(2). 4031 4032=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL 4033 4034Sets the socket option requested. Returns undefined if there is an 4035error. OPTVAL may be specified as C<undef> if you don't want to pass an 4036argument. 4037 4038=item shift ARRAY 4039 4040=item shift 4041 4042Shifts the first value of the array off and returns it, shortening the 4043array by 1 and moving everything down. If there are no elements in the 4044array, returns the undefined value. If ARRAY is omitted, shifts the 4045C<@_> array within the lexical scope of subroutines and formats, and the 4046C<@ARGV> array at file scopes or within the lexical scopes established by 4047the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}> 4048constructs. 4049 4050See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the 4051same thing to the left end of an array that C<pop> and C<push> do to the 4052right end. 4053 4054=item shmctl ID,CMD,ARG 4055 4056Calls the System V IPC function shmctl. You'll probably have to say 4057 4058 use IPC::SysV; 4059 4060first to get the correct constant definitions. If CMD is C<IPC_STAT>, 4061then ARG must be a variable which will hold the returned C<shmid_ds> 4062structure. Returns like ioctl: the undefined value for error, "C<0> but 4063true" for zero, or the actual return value otherwise. 4064See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. 4065 4066=item shmget KEY,SIZE,FLAGS 4067 4068Calls the System V IPC function shmget. Returns the shared memory 4069segment id, or the undefined value if there is an error. 4070See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. 4071 4072=item shmread ID,VAR,POS,SIZE 4073 4074=item shmwrite ID,STRING,POS,SIZE 4075 4076Reads or writes the System V shared memory segment ID starting at 4077position POS for size SIZE by attaching to it, copying in/out, and 4078detaching from it. When reading, VAR must be a variable that will 4079hold the data read. When writing, if STRING is too long, only SIZE 4080bytes are used; if STRING is too short, nulls are written to fill out 4081SIZE bytes. Return true if successful, or false if there is an error. 4082shmread() taints the variable. See also L<perlipc/"SysV IPC">, 4083C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN. 4084 4085=item shutdown SOCKET,HOW 4086 4087Shuts down a socket connection in the manner indicated by HOW, which 4088has the same interpretation as in the system call of the same name. 4089 4090 shutdown(SOCKET, 0); # I/we have stopped reading data 4091 shutdown(SOCKET, 1); # I/we have stopped writing data 4092 shutdown(SOCKET, 2); # I/we have stopped using this socket 4093 4094This is useful with sockets when you want to tell the other 4095side you're done writing but not done reading, or vice versa. 4096It's also a more insistent form of close because it also 4097disables the file descriptor in any forked copies in other 4098processes. 4099 4100=item sin EXPR 4101 4102=item sin 4103 4104Returns the sine of EXPR (expressed in radians). If EXPR is omitted, 4105returns sine of C<$_>. 4106 4107For the inverse sine operation, you may use the C<Math::Trig::asin> 4108function, or use this relation: 4109 4110 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } 4111 4112=item sleep EXPR 4113 4114=item sleep 4115 4116Causes the script to sleep for EXPR seconds, or forever if no EXPR. 4117May be interrupted if the process receives a signal such as C<SIGALRM>. 4118Returns the number of seconds actually slept. You probably cannot 4119mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented 4120using C<alarm>. 4121 4122On some older systems, it may sleep up to a full second less than what 4123you requested, depending on how it counts seconds. Most modern systems 4124always sleep the full amount. They may appear to sleep longer than that, 4125however, because your process might not be scheduled right away in a 4126busy multitasking system. 4127 4128For delays of finer granularity than one second, you may use Perl's 4129C<syscall> interface to access setitimer(2) if your system supports 4130it, or else see L</select> above. The Time::HiRes module from CPAN 4131may also help. 4132 4133See also the POSIX module's C<pause> function. 4134 4135=item socket SOCKET,DOMAIN,TYPE,PROTOCOL 4136 4137Opens a socket of the specified kind and attaches it to filehandle 4138SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for 4139the system call of the same name. You should C<use Socket> first 4140to get the proper definitions imported. See the examples in 4141L<perlipc/"Sockets: Client/Server Communication">. 4142 4143On systems that support a close-on-exec flag on files, the flag will 4144be set for the newly opened file descriptor, as determined by the 4145value of $^F. See L<perlvar/$^F>. 4146 4147=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL 4148 4149Creates an unnamed pair of sockets in the specified domain, of the 4150specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as 4151for the system call of the same name. If unimplemented, yields a fatal 4152error. Returns true if successful. 4153 4154On systems that support a close-on-exec flag on files, the flag will 4155be set for the newly opened file descriptors, as determined by the value 4156of $^F. See L<perlvar/$^F>. 4157 4158Some systems defined C<pipe> in terms of C<socketpair>, in which a call 4159to C<pipe(Rdr, Wtr)> is essentially: 4160 4161 use Socket; 4162 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); 4163 shutdown(Rdr, 1); # no more writing for reader 4164 shutdown(Wtr, 0); # no more reading for writer 4165 4166See L<perlipc> for an example of socketpair use. 4167 4168=item sort SUBNAME LIST 4169 4170=item sort BLOCK LIST 4171 4172=item sort LIST 4173 4174Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK 4175is omitted, C<sort>s in standard string comparison order. If SUBNAME is 4176specified, it gives the name of a subroutine that returns an integer 4177less than, equal to, or greater than C<0>, depending on how the elements 4178of the list are to be ordered. (The C<< <=> >> and C<cmp> 4179operators are extremely useful in such routines.) SUBNAME may be a 4180scalar variable name (unsubscripted), in which case the value provides 4181the name of (or a reference to) the actual subroutine to use. In place 4182of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort 4183subroutine. 4184 4185If the subroutine's prototype is C<($$)>, the elements to be compared 4186are passed by reference in C<@_>, as for a normal subroutine. This is 4187slower than unprototyped subroutines, where the elements to be 4188compared are passed into the subroutine 4189as the package global variables $a and $b (see example below). Note that 4190in the latter case, it is usually counter-productive to declare $a and 4191$b as lexicals. 4192 4193In either case, the subroutine may not be recursive. The values to be 4194compared are always passed by reference, so don't modify them. 4195 4196You also cannot exit out of the sort block or subroutine using any of the 4197loop control operators described in L<perlsyn> or with C<goto>. 4198 4199When C<use locale> is in effect, C<sort LIST> sorts LIST according to the 4200current collation locale. See L<perllocale>. 4201 4202Examples: 4203 4204 # sort lexically 4205 @articles = sort @files; 4206 4207 # same thing, but with explicit sort routine 4208 @articles = sort {$a cmp $b} @files; 4209 4210 # now case-insensitively 4211 @articles = sort {uc($a) cmp uc($b)} @files; 4212 4213 # same thing in reversed order 4214 @articles = sort {$b cmp $a} @files; 4215 4216 # sort numerically ascending 4217 @articles = sort {$a <=> $b} @files; 4218 4219 # sort numerically descending 4220 @articles = sort {$b <=> $a} @files; 4221 4222 # this sorts the %age hash by value instead of key 4223 # using an in-line function 4224 @eldest = sort { $age{$b} <=> $age{$a} } keys %age; 4225 4226 # sort using explicit subroutine name 4227 sub byage { 4228 $age{$a} <=> $age{$b}; # presuming numeric 4229 } 4230 @sortedclass = sort byage @class; 4231 4232 sub backwards { $b cmp $a } 4233 @harry = qw(dog cat x Cain Abel); 4234 @george = qw(gone chased yz Punished Axed); 4235 print sort @harry; 4236 # prints AbelCaincatdogx 4237 print sort backwards @harry; 4238 # prints xdogcatCainAbel 4239 print sort @george, 'to', @harry; 4240 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz 4241 4242 # inefficiently sort by descending numeric compare using 4243 # the first integer after the first = sign, or the 4244 # whole record case-insensitively otherwise 4245 4246 @new = sort { 4247 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] 4248 || 4249 uc($a) cmp uc($b) 4250 } @old; 4251 4252 # same thing, but much more efficiently; 4253 # we'll build auxiliary indices instead 4254 # for speed 4255 @nums = @caps = (); 4256 for (@old) { 4257 push @nums, /=(\d+)/; 4258 push @caps, uc($_); 4259 } 4260 4261 @new = @old[ sort { 4262 $nums[$b] <=> $nums[$a] 4263 || 4264 $caps[$a] cmp $caps[$b] 4265 } 0..$#old 4266 ]; 4267 4268 # same thing, but without any temps 4269 @new = map { $_->[0] } 4270 sort { $b->[1] <=> $a->[1] 4271 || 4272 $a->[2] cmp $b->[2] 4273 } map { [$_, /=(\d+)/, uc($_)] } @old; 4274 4275 # using a prototype allows you to use any comparison subroutine 4276 # as a sort subroutine (including other package's subroutines) 4277 package other; 4278 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here 4279 4280 package main; 4281 @new = sort other::backwards @old; 4282 4283If you're using strict, you I<must not> declare $a 4284and $b as lexicals. They are package globals. That means 4285if you're in the C<main> package and type 4286 4287 @articles = sort {$b <=> $a} @files; 4288 4289then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>), 4290but if you're in the C<FooPack> package, it's the same as typing 4291 4292 @articles = sort {$FooPack::b <=> $FooPack::a} @files; 4293 4294The comparison function is required to behave. If it returns 4295inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and 4296sometimes saying the opposite, for example) the results are not 4297well-defined. 4298 4299=item splice ARRAY,OFFSET,LENGTH,LIST 4300 4301=item splice ARRAY,OFFSET,LENGTH 4302 4303=item splice ARRAY,OFFSET 4304 4305=item splice ARRAY 4306 4307Removes the elements designated by OFFSET and LENGTH from an array, and 4308replaces them with the elements of LIST, if any. In list context, 4309returns the elements removed from the array. In scalar context, 4310returns the last element removed, or C<undef> if no elements are 4311removed. The array grows or shrinks as necessary. 4312If OFFSET is negative then it starts that far from the end of the array. 4313If LENGTH is omitted, removes everything from OFFSET onward. 4314If LENGTH is negative, leaves that many elements off the end of the array. 4315If both OFFSET and LENGTH are omitted, removes everything. 4316 4317The following equivalences hold (assuming C<$[ == 0>): 4318 4319 push(@a,$x,$y) splice(@a,@a,0,$x,$y) 4320 pop(@a) splice(@a,-1) 4321 shift(@a) splice(@a,0,1) 4322 unshift(@a,$x,$y) splice(@a,0,0,$x,$y) 4323 $a[$x] = $y splice(@a,$x,1,$y) 4324 4325Example, assuming array lengths are passed before arrays: 4326 4327 sub aeq { # compare two list values 4328 my(@a) = splice(@_,0,shift); 4329 my(@b) = splice(@_,0,shift); 4330 return 0 unless @a == @b; # same len? 4331 while (@a) { 4332 return 0 if pop(@a) ne pop(@b); 4333 } 4334 return 1; 4335 } 4336 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } 4337 4338=item split /PATTERN/,EXPR,LIMIT 4339 4340=item split /PATTERN/,EXPR 4341 4342=item split /PATTERN/ 4343 4344=item split 4345 4346Splits a string into a list of strings and returns that list. By default, 4347empty leading fields are preserved, and empty trailing ones are deleted. 4348 4349In scalar context, returns the number of fields found and splits into 4350the C<@_> array. Use of split in scalar context is deprecated, however, 4351because it clobbers your subroutine arguments. 4352 4353If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted, 4354splits on whitespace (after skipping any leading whitespace). Anything 4355matching PATTERN is taken to be a delimiter separating the fields. (Note 4356that the delimiter may be longer than one character.) 4357 4358If LIMIT is specified and positive, splits into no more than that 4359many fields (though it may split into fewer). If LIMIT is unspecified 4360or zero, trailing null fields are stripped (which potential users 4361of C<pop> would do well to remember). If LIMIT is negative, it is 4362treated as if an arbitrarily large LIMIT had been specified. 4363 4364A pattern matching the null string (not to be confused with 4365a null pattern C<//>, which is just one member of the set of patterns 4366matching a null string) will split the value of EXPR into separate 4367characters at each point it matches that way. For example: 4368 4369 print join(':', split(/ */, 'hi there')); 4370 4371produces the output 'h:i:t:h:e:r:e'. 4372 4373Empty leading (or trailing) fields are produced when there positive width 4374matches at the beginning (or end) of the string; a zero-width match at the 4375beginning (or end) of the string does not produce an empty field. For 4376example: 4377 4378 print join(':', split(/(?=\w)/, 'hi there!')); 4379 4380produces the output 'h:i :t:h:e:r:e!'. 4381 4382The LIMIT parameter can be used to split a line partially 4383 4384 ($login, $passwd, $remainder) = split(/:/, $_, 3); 4385 4386When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT 4387one larger than the number of variables in the list, to avoid 4388unnecessary work. For the list above LIMIT would have been 4 by 4389default. In time critical applications it behooves you not to split 4390into more fields than you really need. 4391 4392If the PATTERN contains parentheses, additional list elements are 4393created from each matching substring in the delimiter. 4394 4395 split(/([,-])/, "1-10,20", 3); 4396 4397produces the list value 4398 4399 (1, '-', 10, ',', 20) 4400 4401If you had the entire header of a normal Unix email message in $header, 4402you could split it up into fields and their values this way: 4403 4404 $header =~ s/\n\s+/ /g; # fix continuation lines 4405 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header); 4406 4407The pattern C</PATTERN/> may be replaced with an expression to specify 4408patterns that vary at runtime. (To do runtime compilation only once, 4409use C</$variable/o>.) 4410 4411As a special case, specifying a PATTERN of space (C<' '>) will split on 4412white space just as C<split> with no arguments does. Thus, C<split(' ')> can 4413be used to emulate B<awk>'s default behavior, whereas C<split(/ /)> 4414will give you as many null initial fields as there are leading spaces. 4415A C<split> on C</\s+/> is like a C<split(' ')> except that any leading 4416whitespace produces a null first field. A C<split> with no arguments 4417really does a C<split(' ', $_)> internally. 4418 4419A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't 4420much use otherwise. 4421 4422Example: 4423 4424 open(PASSWD, '/etc/passwd'); 4425 while (<PASSWD>) { 4426 chomp; 4427 ($login, $passwd, $uid, $gid, 4428 $gcos, $home, $shell) = split(/:/); 4429 #... 4430 } 4431 4432 4433=item sprintf FORMAT, LIST 4434 4435Returns a string formatted by the usual C<printf> conventions of the C 4436library function C<sprintf>. See below for more details 4437and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of 4438the general principles. 4439 4440For example: 4441 4442 # Format number with up to 8 leading zeroes 4443 $result = sprintf("%08d", $number); 4444 4445 # Round number to 3 digits after decimal point 4446 $rounded = sprintf("%.3f", $number); 4447 4448Perl does its own C<sprintf> formatting--it emulates the C 4449function C<sprintf>, but it doesn't use it (except for floating-point 4450numbers, and even then only the standard modifiers are allowed). As a 4451result, any non-standard extensions in your local C<sprintf> are not 4452available from Perl. 4453 4454Unlike C<printf>, C<sprintf> does not do what you probably mean when you 4455pass it an array as your first argument. The array is given scalar context, 4456and instead of using the 0th element of the array as the format, Perl will 4457use the count of elements in the array as the format, which is almost never 4458useful. 4459 4460Perl's C<sprintf> permits the following universally-known conversions: 4461 4462 %% a percent sign 4463 %c a character with the given number 4464 %s a string 4465 %d a signed integer, in decimal 4466 %u an unsigned integer, in decimal 4467 %o an unsigned integer, in octal 4468 %x an unsigned integer, in hexadecimal 4469 %e a floating-point number, in scientific notation 4470 %f a floating-point number, in fixed decimal notation 4471 %g a floating-point number, in %e or %f notation 4472 4473In addition, Perl permits the following widely-supported conversions: 4474 4475 %X like %x, but using upper-case letters 4476 %E like %e, but using an upper-case "E" 4477 %G like %g, but with an upper-case "E" (if applicable) 4478 %b an unsigned integer, in binary 4479 %p a pointer (outputs the Perl value's address in hexadecimal) 4480 %n special: *stores* the number of characters output so far 4481 into the next variable in the parameter list 4482 4483Finally, for backward (and we do mean "backward") compatibility, Perl 4484permits these unnecessary but widely-supported conversions: 4485 4486 %i a synonym for %d 4487 %D a synonym for %ld 4488 %U a synonym for %lu 4489 %O a synonym for %lo 4490 %F a synonym for %f 4491 4492Note that the number of exponent digits in the scientific notation by 4493C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the 4494exponent less than 100 is system-dependent: it may be three or less 4495(zero-padded as necessary). In other words, 1.23 times ten to the 449699th may be either "1.23e99" or "1.23e099". 4497 4498Perl permits the following universally-known flags between the C<%> 4499and the conversion letter: 4500 4501 space prefix positive number with a space 4502 + prefix positive number with a plus sign 4503 - left-justify within the field 4504 0 use zeros, not spaces, to right-justify 4505 # prefix non-zero octal with "0", non-zero hex with "0x" 4506 number minimum field width 4507 .number "precision": digits after decimal point for 4508 floating-point, max length for string, minimum length 4509 for integer 4510 l interpret integer as C type "long" or "unsigned long" 4511 h interpret integer as C type "short" or "unsigned short" 4512 If no flags, interpret integer as C type "int" or "unsigned" 4513 4514There are also two Perl-specific flags: 4515 4516 V interpret integer as Perl's standard integer type 4517 v interpret string as a vector of integers, output as 4518 numbers separated either by dots, or by an arbitrary 4519 string received from the argument list when the flag 4520 is preceded by C<*> 4521 4522Where a number would appear in the flags, an asterisk (C<*>) may be 4523used instead, in which case Perl uses the next item in the parameter 4524list as the given number (that is, as the field width or precision). 4525If a field width obtained through C<*> is negative, it has the same 4526effect as the C<-> flag: left-justification. 4527 4528The C<v> flag is useful for displaying ordinal values of characters 4529in arbitrary strings: 4530 4531 printf "version is v%vd\n", $^V; # Perl's version 4532 printf "address is %*vX\n", ":", $addr; # IPv6 address 4533 printf "bits are %*vb\n", " ", $bits; # random bitstring 4534 4535If C<use locale> is in effect, the character used for the decimal 4536point in formatted real numbers is affected by the LC_NUMERIC locale. 4537See L<perllocale>. 4538 4539If Perl understands "quads" (64-bit integers) (this requires 4540either that the platform natively support quads or that Perl 4541be specifically compiled to support quads), the characters 4542 4543 d u o x X b i D U O 4544 4545print quads, and they may optionally be preceded by 4546 4547 ll L q 4548 4549For example 4550 4551 %lld %16LX %qo 4552 4553You can find out whether your Perl supports quads via L<Config>: 4554 4555 use Config; 4556 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) && 4557 print "quads\n"; 4558 4559If Perl understands "long doubles" (this requires that the platform 4560support long doubles), the flags 4561 4562 e f g E F G 4563 4564may optionally be preceded by 4565 4566 ll L 4567 4568For example 4569 4570 %llf %Lg 4571 4572You can find out whether your Perl supports long doubles via L<Config>: 4573 4574 use Config; 4575 $Config{d_longdbl} eq 'define' && print "long doubles\n"; 4576 4577=item sqrt EXPR 4578 4579=item sqrt 4580 4581Return the square root of EXPR. If EXPR is omitted, returns square 4582root of C<$_>. Only works on non-negative operands, unless you've 4583loaded the standard Math::Complex module. 4584 4585 use Math::Complex; 4586 print sqrt(-2); # prints 1.4142135623731i 4587 4588=item srand EXPR 4589 4590=item srand 4591 4592Sets the random number seed for the C<rand> operator. If EXPR is 4593omitted, uses a semi-random value supplied by the kernel (if it supports 4594the F</dev/urandom> device) or based on the current time and process 4595ID, among other things. In versions of Perl prior to 5.004 the default 4596seed was just the current C<time>. This isn't a particularly good seed, 4597so many old programs supply their own seed value (often C<time ^ $$> or 4598C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more. 4599 4600In fact, it's usually not necessary to call C<srand> at all, because if 4601it is not called explicitly, it is called implicitly at the first use of 4602the C<rand> operator. However, this was not the case in version of Perl 4603before 5.004, so if your script will run under older Perl versions, it 4604should call C<srand>. 4605 4606Note that you need something much more random than the default seed for 4607cryptographic purposes. Checksumming the compressed output of one or more 4608rapidly changing operating system status programs is the usual method. For 4609example: 4610 4611 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`); 4612 4613If you're particularly concerned with this, see the C<Math::TrulyRandom> 4614module in CPAN. 4615 4616Do I<not> call C<srand> multiple times in your program unless you know 4617exactly what you're doing and why you're doing it. The point of the 4618function is to "seed" the C<rand> function so that C<rand> can produce 4619a different sequence each time you run your program. Just do it once at the 4620top of your program, or you I<won't> get random numbers out of C<rand>! 4621 4622Frequently called programs (like CGI scripts) that simply use 4623 4624 time ^ $$ 4625 4626for a seed can fall prey to the mathematical property that 4627 4628 a^b == (a+1)^(b+1) 4629 4630one-third of the time. So don't do that. 4631 4632=item stat FILEHANDLE 4633 4634=item stat EXPR 4635 4636=item stat 4637 4638Returns a 13-element list giving the status info for a file, either 4639the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, 4640it stats C<$_>. Returns a null list if the stat fails. Typically used 4641as follows: 4642 4643 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, 4644 $atime,$mtime,$ctime,$blksize,$blocks) 4645 = stat($filename); 4646 4647Not all fields are supported on all filesystem types. Here are the 4648meaning of the fields: 4649 4650 0 dev device number of filesystem 4651 1 ino inode number 4652 2 mode file mode (type and permissions) 4653 3 nlink number of (hard) links to the file 4654 4 uid numeric user ID of file's owner 4655 5 gid numeric group ID of file's owner 4656 6 rdev the device identifier (special files only) 4657 7 size total size of file, in bytes 4658 8 atime last access time in seconds since the epoch 4659 9 mtime last modify time in seconds since the epoch 4660 10 ctime inode change time (NOT creation time!) in seconds since the epoch 4661 11 blksize preferred block size for file system I/O 4662 12 blocks actual number of blocks allocated 4663 4664(The epoch was at 00:00 January 1, 1970 GMT.) 4665 4666If stat is passed the special filehandle consisting of an underline, no 4667stat is done, but the current contents of the stat structure from the 4668last stat or filetest are returned. Example: 4669 4670 if (-x $file && (($d) = stat(_)) && $d < 0) { 4671 print "$file is executable NFS file\n"; 4672 } 4673 4674(This works on machines only for which the device number is negative 4675under NFS.) 4676 4677Because the mode contains both the file type and its permissions, you 4678should mask off the file type portion and (s)printf using a C<"%o"> 4679if you want to see the real permissions. 4680 4681 $mode = (stat($filename))[2]; 4682 printf "Permissions are %04o\n", $mode & 07777; 4683 4684In scalar context, C<stat> returns a boolean value indicating success 4685or failure, and, if successful, sets the information associated with 4686the special filehandle C<_>. 4687 4688The File::stat module provides a convenient, by-name access mechanism: 4689 4690 use File::stat; 4691 $sb = stat($filename); 4692 printf "File is %s, size is %s, perm %04o, mtime %s\n", 4693 $filename, $sb->size, $sb->mode & 07777, 4694 scalar localtime $sb->mtime; 4695 4696You can import symbolic mode constants (C<S_IF*>) and functions 4697(C<S_IS*>) from the Fcntl module: 4698 4699 use Fcntl ':mode'; 4700 4701 $mode = (stat($filename))[2]; 4702 4703 $user_rwx = ($mode & S_IRWXU) >> 6; 4704 $group_read = ($mode & S_IRGRP) >> 3; 4705 $other_execute = $mode & S_IXOTH; 4706 4707 printf "Permissions are %04o\n", S_ISMODE($mode), "\n"; 4708 4709 $is_setuid = $mode & S_ISUID; 4710 $is_setgid = S_ISDIR($mode); 4711 4712You could write the last two using the C<-u> and C<-d> operators. 4713The commonly available S_IF* constants are 4714 4715 # Permissions: read, write, execute, for user, group, others. 4716 4717 S_IRWXU S_IRUSR S_IWUSR S_IXUSR 4718 S_IRWXG S_IRGRP S_IWGRP S_IXGRP 4719 S_IRWXO S_IROTH S_IWOTH S_IXOTH 4720 4721 # Setuid/Setgid/Stickiness. 4722 4723 S_ISUID S_ISGID S_ISVTX S_ISTXT 4724 4725 # File types. Not necessarily all are available on your system. 4726 4727 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT 4728 4729 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR. 4730 4731 S_IREAD S_IWRITE S_IEXEC 4732 4733and the S_IF* functions are 4734 4735 S_IFMODE($mode) the part of $mode containing the permission bits 4736 and the setuid/setgid/sticky bits 4737 4738 S_IFMT($mode) the part of $mode containing the file type 4739 which can be bit-anded with e.g. S_IFREG 4740 or with the following functions 4741 4742 # The operators -f, -d, -l, -b, -c, -p, and -s. 4743 4744 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode) 4745 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode) 4746 4747 # No direct -X operator counterpart, but for the first one 4748 # the -g operator is often equivalent. The ENFMT stands for 4749 # record flocking enforcement, a platform-dependent feature. 4750 4751 S_ISENFMT($mode) S_ISWHT($mode) 4752 4753See your native chmod(2) and stat(2) documentation for more details 4754about the S_* constants. 4755 4756=item study SCALAR 4757 4758=item study 4759 4760Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of 4761doing many pattern matches on the string before it is next modified. 4762This may or may not save time, depending on the nature and number of 4763patterns you are searching on, and on the distribution of character 4764frequencies in the string to be searched--you probably want to compare 4765run times with and without it to see which runs faster. Those loops 4766which scan for many short constant strings (including the constant 4767parts of more complex patterns) will benefit most. You may have only 4768one C<study> active at a time--if you study a different scalar the first 4769is "unstudied". (The way C<study> works is this: a linked list of every 4770character in the string to be searched is made, so we know, for 4771example, where all the C<'k'> characters are. From each search string, 4772the rarest character is selected, based on some static frequency tables 4773constructed from some C programs and English text. Only those places 4774that contain this "rarest" character are examined.) 4775 4776For example, here is a loop that inserts index producing entries 4777before any line containing a certain pattern: 4778 4779 while (<>) { 4780 study; 4781 print ".IX foo\n" if /\bfoo\b/; 4782 print ".IX bar\n" if /\bbar\b/; 4783 print ".IX blurfl\n" if /\bblurfl\b/; 4784 # ... 4785 print; 4786 } 4787 4788In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f> 4789will be looked at, because C<f> is rarer than C<o>. In general, this is 4790a big win except in pathological cases. The only question is whether 4791it saves you more time than it took to build the linked list in the 4792first place. 4793 4794Note that if you have to look for strings that you don't know till 4795runtime, you can build an entire loop as a string and C<eval> that to 4796avoid recompiling all your patterns all the time. Together with 4797undefining C<$/> to input entire files as one record, this can be very 4798fast, often faster than specialized programs like fgrep(1). The following 4799scans a list of files (C<@files>) for a list of words (C<@words>), and prints 4800out the names of those files that contain a match: 4801 4802 $search = 'while (<>) { study;'; 4803 foreach $word (@words) { 4804 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; 4805 } 4806 $search .= "}"; 4807 @ARGV = @files; 4808 undef $/; 4809 eval $search; # this screams 4810 $/ = "\n"; # put back to normal input delimiter 4811 foreach $file (sort keys(%seen)) { 4812 print $file, "\n"; 4813 } 4814 4815=item sub BLOCK 4816 4817=item sub NAME 4818 4819=item sub NAME BLOCK 4820 4821This is subroutine definition, not a real function I<per se>. With just a 4822NAME (and possibly prototypes or attributes), it's just a forward declaration. 4823Without a NAME, it's an anonymous function declaration, and does actually 4824return a value: the CODE ref of the closure you just created. See L<perlsub> 4825and L<perlref> for details. 4826 4827=item substr EXPR,OFFSET,LENGTH,REPLACEMENT 4828 4829=item substr EXPR,OFFSET,LENGTH 4830 4831=item substr EXPR,OFFSET 4832 4833Extracts a substring out of EXPR and returns it. First character is at 4834offset C<0>, or whatever you've set C<$[> to (but don't do that). 4835If OFFSET is negative (or more precisely, less than C<$[>), starts 4836that far from the end of the string. If LENGTH is omitted, returns 4837everything to the end of the string. If LENGTH is negative, leaves that 4838many characters off the end of the string. 4839 4840You can use the substr() function as an lvalue, in which case EXPR 4841must itself be an lvalue. If you assign something shorter than LENGTH, 4842the string will shrink, and if you assign something longer than LENGTH, 4843the string will grow to accommodate it. To keep the string the same 4844length you may need to pad or chop your value using C<sprintf>. 4845 4846If OFFSET and LENGTH specify a substring that is partly outside the 4847string, only the part within the string is returned. If the substring 4848is beyond either end of the string, substr() returns the undefined 4849value and produces a warning. When used as an lvalue, specifying a 4850substring that is entirely outside the string is a fatal error. 4851Here's an example showing the behavior for boundary cases: 4852 4853 my $name = 'fred'; 4854 substr($name, 4) = 'dy'; # $name is now 'freddy' 4855 my $null = substr $name, 6, 2; # returns '' (no warning) 4856 my $oops = substr $name, 7; # returns undef, with warning 4857 substr($name, 7) = 'gap'; # fatal error 4858 4859An alternative to using substr() as an lvalue is to specify the 4860replacement string as the 4th argument. This allows you to replace 4861parts of the EXPR and return what was there before in one operation, 4862just as you can with splice(). 4863 4864=item symlink OLDFILE,NEWFILE 4865 4866Creates a new filename symbolically linked to the old filename. 4867Returns C<1> for success, C<0> otherwise. On systems that don't support 4868symbolic links, produces a fatal error at run time. To check for that, 4869use eval: 4870 4871 $symlink_exists = eval { symlink("",""); 1 }; 4872 4873=item syscall LIST 4874 4875Calls the system call specified as the first element of the list, 4876passing the remaining elements as arguments to the system call. If 4877unimplemented, produces a fatal error. The arguments are interpreted 4878as follows: if a given argument is numeric, the argument is passed as 4879an int. If not, the pointer to the string value is passed. You are 4880responsible to make sure a string is pre-extended long enough to 4881receive any result that might be written into a string. You can't use a 4882string literal (or other read-only string) as an argument to C<syscall> 4883because Perl has to assume that any string pointer might be written 4884through. If your 4885integer arguments are not literals and have never been interpreted in a 4886numeric context, you may need to add C<0> to them to force them to look 4887like numbers. This emulates the C<syswrite> function (or vice versa): 4888 4889 require 'syscall.ph'; # may need to run h2ph 4890 $s = "hi there\n"; 4891 syscall(&SYS_write, fileno(STDOUT), $s, length $s); 4892 4893Note that Perl supports passing of up to only 14 arguments to your system call, 4894which in practice should usually suffice. 4895 4896Syscall returns whatever value returned by the system call it calls. 4897If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno). 4898Note that some system calls can legitimately return C<-1>. The proper 4899way to handle such calls is to assign C<$!=0;> before the call and 4900check the value of C<$!> if syscall returns C<-1>. 4901 4902There's a problem with C<syscall(&SYS_pipe)>: it returns the file 4903number of the read end of the pipe it creates. There is no way 4904to retrieve the file number of the other end. You can avoid this 4905problem by using C<pipe> instead. 4906 4907=item sysopen FILEHANDLE,FILENAME,MODE 4908 4909=item sysopen FILEHANDLE,FILENAME,MODE,PERMS 4910 4911Opens the file whose filename is given by FILENAME, and associates it 4912with FILEHANDLE. If FILEHANDLE is an expression, its value is used as 4913the name of the real filehandle wanted. This function calls the 4914underlying operating system's C<open> function with the parameters 4915FILENAME, MODE, PERMS. 4916 4917The possible values and flag bits of the MODE parameter are 4918system-dependent; they are available via the standard module C<Fcntl>. 4919See the documentation of your operating system's C<open> to see which 4920values and flag bits are available. You may combine several flags 4921using the C<|>-operator. 4922 4923Some of the most common values are C<O_RDONLY> for opening the file in 4924read-only mode, C<O_WRONLY> for opening the file in write-only mode, 4925and C<O_RDWR> for opening the file in read-write mode, and. 4926 4927For historical reasons, some values work on almost every system 4928supported by perl: zero means read-only, one means write-only, and two 4929means read/write. We know that these values do I<not> work under 4930OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to 4931use them in new code. 4932 4933If the file named by FILENAME does not exist and the C<open> call creates 4934it (typically because MODE includes the C<O_CREAT> flag), then the value of 4935PERMS specifies the permissions of the newly created file. If you omit 4936the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>. 4937These permission values need to be in octal, and are modified by your 4938process's current C<umask>. 4939 4940In many systems the C<O_EXCL> flag is available for opening files in 4941exclusive mode. This is B<not> locking: exclusiveness means here that 4942if the file already exists, sysopen() fails. The C<O_EXCL> wins 4943C<O_TRUNC>. 4944 4945Sometimes you may want to truncate an already-existing file: C<O_TRUNC>. 4946 4947You should seldom if ever use C<0644> as argument to C<sysopen>, because 4948that takes away the user's option to have a more permissive umask. 4949Better to omit it. See the perlfunc(1) entry on C<umask> for more 4950on this. 4951 4952Note that C<sysopen> depends on the fdopen() C library function. 4953On many UNIX systems, fdopen() is known to fail when file descriptors 4954exceed a certain value, typically 255. If you need more file 4955descriptors than that, consider rebuilding Perl to use the C<sfio> 4956library, or perhaps using the POSIX::open() function. 4957 4958See L<perlopentut> for a kinder, gentler explanation of opening files. 4959 4960=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET 4961 4962=item sysread FILEHANDLE,SCALAR,LENGTH 4963 4964Attempts to read LENGTH bytes of data into variable SCALAR from the 4965specified FILEHANDLE, using the system call read(2). It bypasses stdio, 4966so mixing this with other kinds of reads, C<print>, C<write>, 4967C<seek>, C<tell>, or C<eof> can cause confusion because stdio 4968usually buffers data. Returns the number of bytes actually read, C<0> 4969at end of file, or undef if there was an error. SCALAR will be grown or 4970shrunk so that the last byte actually read is the last byte of the 4971scalar after the read. 4972 4973An OFFSET may be specified to place the read data at some place in the 4974string other than the beginning. A negative OFFSET specifies 4975placement at that many bytes counting backwards from the end of the 4976string. A positive OFFSET greater than the length of SCALAR results 4977in the string being padded to the required size with C<"\0"> bytes before 4978the result of the read is appended. 4979 4980There is no syseof() function, which is ok, since eof() doesn't work 4981very well on device files (like ttys) anyway. Use sysread() and check 4982for a return value for 0 to decide whether you're done. 4983 4984=item sysseek FILEHANDLE,POSITION,WHENCE 4985 4986Sets FILEHANDLE's system position using the system call lseek(2). It 4987bypasses stdio, so mixing this with reads (other than C<sysread>), 4988C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion. 4989FILEHANDLE may be an expression whose value gives the name of the 4990filehandle. The values for WHENCE are C<0> to set the new position to 4991POSITION, C<1> to set the it to the current position plus POSITION, 4992and C<2> to set it to EOF plus POSITION (typically negative). For 4993WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and 4994C<SEEK_END> (start of the file, current position, end of the file) 4995from the Fcntl module. 4996 4997Returns the new position, or the undefined value on failure. A position 4998of zero is returned as the string C<"0 but true">; thus C<sysseek> returns 4999true on success and false on failure, yet you can still easily determine 5000the new position. 5001 5002=item system LIST 5003 5004=item system PROGRAM LIST 5005 5006Does exactly the same thing as C<exec LIST>, except that a fork is 5007done first, and the parent process waits for the child process to 5008complete. Note that argument processing varies depending on the 5009number of arguments. If there is more than one argument in LIST, 5010or if LIST is an array with more than one value, starts the program 5011given by the first element of the list with arguments given by the 5012rest of the list. If there is only one scalar argument, the argument 5013is checked for shell metacharacters, and if there are any, the 5014entire argument is passed to the system's command shell for parsing 5015(this is C</bin/sh -c> on Unix platforms, but varies on other 5016platforms). If there are no shell metacharacters in the argument, 5017it is split into words and passed directly to C<execvp>, which is 5018more efficient. 5019 5020Beginning with v5.6.0, Perl will attempt to flush all files opened for 5021output before any operation that may do a fork, but this may not be 5022supported on some platforms (see L<perlport>). To be safe, you may need 5023to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method 5024of C<IO::Handle> on any open handles. 5025 5026The return value is the exit status of the program as 5027returned by the C<wait> call. To get the actual exit value divide by 5028256. See also L</exec>. This is I<not> what you want to use to capture 5029the output from a command, for that you should use merely backticks or 5030C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1 5031indicates a failure to start the program (inspect $! for the reason). 5032 5033Like C<exec>, C<system> allows you to lie to a program about its name if 5034you use the C<system PROGRAM LIST> syntax. Again, see L</exec>. 5035 5036Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the 5037program they're running doesn't actually interrupt your program. 5038 5039 @args = ("command", "arg1", "arg2"); 5040 system(@args) == 0 5041 or die "system @args failed: $?" 5042 5043You can check all the failure possibilities by inspecting 5044C<$?> like this: 5045 5046 $exit_value = $? >> 8; 5047 $signal_num = $? & 127; 5048 $dumped_core = $? & 128; 5049 5050When the arguments get executed via the system shell, results 5051and return codes will be subject to its quirks and capabilities. 5052See L<perlop/"`STRING`"> and L</exec> for details. 5053 5054=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET 5055 5056=item syswrite FILEHANDLE,SCALAR,LENGTH 5057 5058=item syswrite FILEHANDLE,SCALAR 5059 5060Attempts to write LENGTH bytes of data from variable SCALAR to the 5061specified FILEHANDLE, using the system call write(2). If LENGTH 5062is not specified, writes whole SCALAR. It bypasses stdio, so mixing 5063this with reads (other than C<sysread())>, C<print>, C<write>, 5064C<seek>, C<tell>, or C<eof> may cause confusion because stdio 5065usually buffers data. Returns the number of bytes actually written, 5066or C<undef> if there was an error. If the LENGTH is greater than 5067the available data in the SCALAR after the OFFSET, only as much 5068data as is available will be written. 5069 5070An OFFSET may be specified to write the data from some part of the 5071string other than the beginning. A negative OFFSET specifies writing 5072that many bytes counting backwards from the end of the string. In the 5073case the SCALAR is empty you can use OFFSET but only zero offset. 5074 5075=item tell FILEHANDLE 5076 5077=item tell 5078 5079Returns the current position for FILEHANDLE, or -1 on error. FILEHANDLE 5080may be an expression whose value gives the name of the actual filehandle. 5081If FILEHANDLE is omitted, assumes the file last read. 5082 5083The return value of tell() for the standard streams like the STDIN 5084depends on the operating system: it may return -1 or something else. 5085tell() on pipes, fifos, and sockets usually returns -1. 5086 5087There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that. 5088 5089=item telldir DIRHANDLE 5090 5091Returns the current position of the C<readdir> routines on DIRHANDLE. 5092Value may be given to C<seekdir> to access a particular location in a 5093directory. Has the same caveats about possible directory compaction as 5094the corresponding system library routine. 5095 5096=item tie VARIABLE,CLASSNAME,LIST 5097 5098This function binds a variable to a package class that will provide the 5099implementation for the variable. VARIABLE is the name of the variable 5100to be enchanted. CLASSNAME is the name of a class implementing objects 5101of correct type. Any additional arguments are passed to the C<new> 5102method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>, 5103or C<TIEHASH>). Typically these are arguments such as might be passed 5104to the C<dbm_open()> function of C. The object returned by the C<new> 5105method is also returned by the C<tie> function, which would be useful 5106if you want to access other methods in CLASSNAME. 5107 5108Note that functions such as C<keys> and C<values> may return huge lists 5109when used on large objects, like DBM files. You may prefer to use the 5110C<each> function to iterate over such. Example: 5111 5112 # print out history file offsets 5113 use NDBM_File; 5114 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); 5115 while (($key,$val) = each %HIST) { 5116 print $key, ' = ', unpack('L',$val), "\n"; 5117 } 5118 untie(%HIST); 5119 5120A class implementing a hash should have the following methods: 5121 5122 TIEHASH classname, LIST 5123 FETCH this, key 5124 STORE this, key, value 5125 DELETE this, key 5126 CLEAR this 5127 EXISTS this, key 5128 FIRSTKEY this 5129 NEXTKEY this, lastkey 5130 DESTROY this 5131 UNTIE this 5132 5133A class implementing an ordinary array should have the following methods: 5134 5135 TIEARRAY classname, LIST 5136 FETCH this, key 5137 STORE this, key, value 5138 FETCHSIZE this 5139 STORESIZE this, count 5140 CLEAR this 5141 PUSH this, LIST 5142 POP this 5143 SHIFT this 5144 UNSHIFT this, LIST 5145 SPLICE this, offset, length, LIST 5146 EXTEND this, count 5147 DESTROY this 5148 UNTIE this 5149 5150A class implementing a file handle should have the following methods: 5151 5152 TIEHANDLE classname, LIST 5153 READ this, scalar, length, offset 5154 READLINE this 5155 GETC this 5156 WRITE this, scalar, length, offset 5157 PRINT this, LIST 5158 PRINTF this, format, LIST 5159 BINMODE this 5160 EOF this 5161 FILENO this 5162 SEEK this, position, whence 5163 TELL this 5164 OPEN this, mode, LIST 5165 CLOSE this 5166 DESTROY this 5167 UNTIE this 5168 5169A class implementing a scalar should have the following methods: 5170 5171 TIESCALAR classname, LIST 5172 FETCH this, 5173 STORE this, value 5174 DESTROY this 5175 UNTIE this 5176 5177Not all methods indicated above need be implemented. See L<perltie>, 5178L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>. 5179 5180Unlike C<dbmopen>, the C<tie> function will not use or require a module 5181for you--you need to do that explicitly yourself. See L<DB_File> 5182or the F<Config> module for interesting C<tie> implementations. 5183 5184For further details see L<perltie>, L<"tied VARIABLE">. 5185 5186=item tied VARIABLE 5187 5188Returns a reference to the object underlying VARIABLE (the same value 5189that was originally returned by the C<tie> call that bound the variable 5190to a package.) Returns the undefined value if VARIABLE isn't tied to a 5191package. 5192 5193=item time 5194 5195Returns the number of non-leap seconds since whatever time the system 5196considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS, 5197and 00:00:00 UTC, January 1, 1970 for most other systems). 5198Suitable for feeding to C<gmtime> and C<localtime>. 5199 5200For measuring time in better granularity than one second, 5201you may use either the Time::HiRes module from CPAN, or 5202if you have gettimeofday(2), you may be able to use the 5203C<syscall> interface of Perl, see L<perlfaq8> for details. 5204 5205=item times 5206 5207Returns a four-element list giving the user and system times, in 5208seconds, for this process and the children of this process. 5209 5210 ($user,$system,$cuser,$csystem) = times; 5211 5212=item tr/// 5213 5214The transliteration operator. Same as C<y///>. See L<perlop>. 5215 5216=item truncate FILEHANDLE,LENGTH 5217 5218=item truncate EXPR,LENGTH 5219 5220Truncates the file opened on FILEHANDLE, or named by EXPR, to the 5221specified length. Produces a fatal error if truncate isn't implemented 5222on your system. Returns true if successful, the undefined value 5223otherwise. 5224 5225=item uc EXPR 5226 5227=item uc 5228 5229Returns an uppercased version of EXPR. This is the internal function 5230implementing the C<\U> escape in double-quoted strings. 5231Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. 5232Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It 5233does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.) 5234 5235If EXPR is omitted, uses C<$_>. 5236 5237=item ucfirst EXPR 5238 5239=item ucfirst 5240 5241Returns the value of EXPR with the first character 5242in uppercase (titlecase in Unicode). This is 5243the internal function implementing the C<\u> escape in double-quoted strings. 5244Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale> 5245and L<utf8>. 5246 5247If EXPR is omitted, uses C<$_>. 5248 5249=item umask EXPR 5250 5251=item umask 5252 5253Sets the umask for the process to EXPR and returns the previous value. 5254If EXPR is omitted, merely returns the current umask. 5255 5256The Unix permission C<rwxr-x---> is represented as three sets of three 5257bits, or three octal digits: C<0750> (the leading 0 indicates octal 5258and isn't one of the digits). The C<umask> value is such a number 5259representing disabled permissions bits. The permission (or "mode") 5260values you pass C<mkdir> or C<sysopen> are modified by your umask, so 5261even if you tell C<sysopen> to create a file with permissions C<0777>, 5262if your umask is C<0022> then the file will actually be created with 5263permissions C<0755>. If your C<umask> were C<0027> (group can't 5264write; others can't read, write, or execute), then passing 5265C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~ 5266027> is C<0640>). 5267 5268Here's some advice: supply a creation mode of C<0666> for regular 5269files (in C<sysopen>) and one of C<0777> for directories (in 5270C<mkdir>) and executable files. This gives users the freedom of 5271choice: if they want protected files, they might choose process umasks 5272of C<022>, C<027>, or even the particularly antisocial mask of C<077>. 5273Programs should rarely if ever make policy decisions better left to 5274the user. The exception to this is when writing files that should be 5275kept private: mail files, web browser cookies, I<.rhosts> files, and 5276so on. 5277 5278If umask(2) is not implemented on your system and you are trying to 5279restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a 5280fatal error at run time. If umask(2) is not implemented and you are 5281not trying to restrict access for yourself, returns C<undef>. 5282 5283Remember that a umask is a number, usually given in octal; it is I<not> a 5284string of octal digits. See also L</oct>, if all you have is a string. 5285 5286=item undef EXPR 5287 5288=item undef 5289 5290Undefines the value of EXPR, which must be an lvalue. Use only on a 5291scalar value, an array (using C<@>), a hash (using C<%>), a subroutine 5292(using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}> 5293will probably not do what you expect on most predefined variables or 5294DBM list values, so don't do that; see L<delete>.) Always returns the 5295undefined value. You can omit the EXPR, in which case nothing is 5296undefined, but you still get an undefined value that you could, for 5297instance, return from a subroutine, assign to a variable or pass as a 5298parameter. Examples: 5299 5300 undef $foo; 5301 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; 5302 undef @ary; 5303 undef %hash; 5304 undef &mysub; 5305 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. 5306 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; 5307 select undef, undef, undef, 0.25; 5308 ($a, $b, undef, $c) = &foo; # Ignore third value returned 5309 5310Note that this is a unary operator, not a list operator. 5311 5312=item unlink LIST 5313 5314=item unlink 5315 5316Deletes a list of files. Returns the number of files successfully 5317deleted. 5318 5319 $cnt = unlink 'a', 'b', 'c'; 5320 unlink @goners; 5321 unlink <*.bak>; 5322 5323Note: C<unlink> will not delete directories unless you are superuser and 5324the B<-U> flag is supplied to Perl. Even if these conditions are 5325met, be warned that unlinking a directory can inflict damage on your 5326filesystem. Use C<rmdir> instead. 5327 5328If LIST is omitted, uses C<$_>. 5329 5330=item unpack TEMPLATE,EXPR 5331 5332C<unpack> does the reverse of C<pack>: it takes a string 5333and expands it out into a list of values. 5334(In scalar context, it returns merely the first value produced.) 5335 5336The string is broken into chunks described by the TEMPLATE. Each chunk 5337is converted separately to a value. Typically, either the string is a result 5338of C<pack>, or the bytes of the string represent a C structure of some 5339kind. 5340 5341The TEMPLATE has the same format as in the C<pack> function. 5342Here's a subroutine that does substring: 5343 5344 sub substr { 5345 my($what,$where,$howmuch) = @_; 5346 unpack("x$where a$howmuch", $what); 5347 } 5348 5349and then there's 5350 5351 sub ordinal { unpack("c",$_[0]); } # same as ord() 5352 5353In addition to fields allowed in pack(), you may prefix a field with 5354a %<number> to indicate that 5355you want a <number>-bit checksum of the items instead of the items 5356themselves. Default is a 16-bit checksum. Checksum is calculated by 5357summing numeric values of expanded values (for string fields the sum of 5358C<ord($char)> is taken, for bit fields the sum of zeroes and ones). 5359 5360For example, the following 5361computes the same number as the System V sum program: 5362 5363 $checksum = do { 5364 local $/; # slurp! 5365 unpack("%32C*",<>) % 65535; 5366 }; 5367 5368The following efficiently counts the number of set bits in a bit vector: 5369 5370 $setbits = unpack("%32b*", $selectmask); 5371 5372The C<p> and C<P> formats should be used with care. Since Perl 5373has no way of checking whether the value passed to C<unpack()> 5374corresponds to a valid memory location, passing a pointer value that's 5375not known to be valid is likely to have disastrous consequences. 5376 5377If the repeat count of a field is larger than what the remainder of 5378the input string allows, repeat count is decreased. If the input string 5379is longer than one described by the TEMPLATE, the rest is ignored. 5380 5381See L</pack> for more examples and notes. 5382 5383=item untie VARIABLE 5384 5385Breaks the binding between a variable and a package. (See C<tie>.) 5386 5387=item unshift ARRAY,LIST 5388 5389Does the opposite of a C<shift>. Or the opposite of a C<push>, 5390depending on how you look at it. Prepends list to the front of the 5391array, and returns the new number of elements in the array. 5392 5393 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/; 5394 5395Note the LIST is prepended whole, not one element at a time, so the 5396prepended elements stay in the same order. Use C<reverse> to do the 5397reverse. 5398 5399=item use Module VERSION LIST 5400 5401=item use Module VERSION 5402 5403=item use Module LIST 5404 5405=item use Module 5406 5407=item use VERSION 5408 5409Imports some semantics into the current package from the named module, 5410generally by aliasing certain subroutine or variable names into your 5411package. It is exactly equivalent to 5412 5413 BEGIN { require Module; import Module LIST; } 5414 5415except that Module I<must> be a bareword. 5416 5417VERSION, which can be specified as a literal of the form v5.6.1, demands 5418that the current version of Perl (C<$^V> or $PERL_VERSION) be at least 5419as recent as that version. (For compatibility with older versions of Perl, 5420a numeric literal will also be interpreted as VERSION.) If the version 5421of the running Perl interpreter is less than VERSION, then an error 5422message is printed and Perl exits immediately without attempting to 5423parse the rest of the file. Compare with L</require>, which can do a 5424similar check at run time. 5425 5426 use v5.6.1; # compile time version check 5427 use 5.6.1; # ditto 5428 use 5.005_03; # float version allowed for compatibility 5429 5430This is often useful if you need to check the current Perl version before 5431C<use>ing library modules that have changed in incompatible ways from 5432older versions of Perl. (We try not to do this more than we have to.) 5433 5434The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The 5435C<require> makes sure the module is loaded into memory if it hasn't been 5436yet. The C<import> is not a builtin--it's just an ordinary static method 5437call into the C<Module> package to tell the module to import the list of 5438features back into the current package. The module can implement its 5439C<import> method any way it likes, though most modules just choose to 5440derive their C<import> method via inheritance from the C<Exporter> class that 5441is defined in the C<Exporter> module. See L<Exporter>. If no C<import> 5442method can be found then the call is skipped. 5443 5444If you do not want to call the package's C<import> method (for instance, 5445to stop your namespace from being altered), explicitly supply the empty list: 5446 5447 use Module (); 5448 5449That is exactly equivalent to 5450 5451 BEGIN { require Module } 5452 5453If the VERSION argument is present between Module and LIST, then the 5454C<use> will call the VERSION method in class Module with the given 5455version as an argument. The default VERSION method, inherited from 5456the UNIVERSAL class, croaks if the given version is larger than the 5457value of the variable C<$Module::VERSION>. 5458 5459Again, there is a distinction between omitting LIST (C<import> called 5460with no arguments) and an explicit empty LIST C<()> (C<import> not 5461called). Note that there is no comma after VERSION! 5462 5463Because this is a wide-open interface, pragmas (compiler directives) 5464are also implemented this way. Currently implemented pragmas are: 5465 5466 use constant; 5467 use diagnostics; 5468 use integer; 5469 use sigtrap qw(SEGV BUS); 5470 use strict qw(subs vars refs); 5471 use subs qw(afunc blurfl); 5472 use warnings qw(all); 5473 5474Some of these pseudo-modules import semantics into the current 5475block scope (like C<strict> or C<integer>, unlike ordinary modules, 5476which import symbols into the current package (which are effective 5477through the end of the file). 5478 5479There's a corresponding C<no> command that unimports meanings imported 5480by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>. 5481 5482 no integer; 5483 no strict 'refs'; 5484 no warnings; 5485 5486If no C<unimport> method can be found the call fails with a fatal error. 5487 5488See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun> 5489for the C<-M> and C<-m> command-line options to perl that give C<use> 5490functionality from the command-line. 5491 5492=item utime LIST 5493 5494Changes the access and modification times on each file of a list of 5495files. The first two elements of the list must be the NUMERICAL access 5496and modification times, in that order. Returns the number of files 5497successfully changed. The inode change time of each file is set 5498to the current time. This code has the same effect as the C<touch> 5499command if the files already exist: 5500 5501 #!/usr/bin/perl 5502 $now = time; 5503 utime $now, $now, @ARGV; 5504 5505=item values HASH 5506 5507Returns a list consisting of all the values of the named hash. (In a 5508scalar context, returns the number of values.) The values are 5509returned in an apparently random order. The actual random order is 5510subject to change in future versions of perl, but it is guaranteed to 5511be the same order as either the C<keys> or C<each> function would 5512produce on the same (unmodified) hash. 5513 5514Note that the values are not copied, which means modifying them will 5515modify the contents of the hash: 5516 5517 for (values %hash) { s/foo/bar/g } # modifies %hash values 5518 for (@hash{keys %hash}) { s/foo/bar/g } # same 5519 5520As a side effect, calling values() resets the HASH's internal iterator. 5521See also C<keys>, C<each>, and C<sort>. 5522 5523=item vec EXPR,OFFSET,BITS 5524 5525Treats the string in EXPR as a bit vector made up of elements of 5526width BITS, and returns the value of the element specified by OFFSET 5527as an unsigned integer. BITS therefore specifies the number of bits 5528that are reserved for each element in the bit vector. This must 5529be a power of two from 1 to 32 (or 64, if your platform supports 5530that). 5531 5532If BITS is 8, "elements" coincide with bytes of the input string. 5533 5534If BITS is 16 or more, bytes of the input string are grouped into chunks 5535of size BITS/8, and each group is converted to a number as with 5536pack()/unpack() with big-endian formats C<n>/C<N> (and analogously 5537for BITS==64). See L<"pack"> for details. 5538 5539If bits is 4 or less, the string is broken into bytes, then the bits 5540of each byte are broken into 8/BITS groups. Bits of a byte are 5541numbered in a little-endian-ish way, as in C<0x01>, C<0x02>, 5542C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example, 5543breaking the single input byte C<chr(0x36)> into two groups gives a list 5544C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>. 5545 5546C<vec> may also be assigned to, in which case parentheses are needed 5547to give the expression the correct precedence as in 5548 5549 vec($image, $max_x * $x + $y, 8) = 3; 5550 5551If the selected element is outside the string, the value 0 is returned. 5552If an element off the end of the string is written to, Perl will first 5553extend the string with sufficiently many zero bytes. It is an error 5554to try to write off the beginning of the string (i.e. negative OFFSET). 5555 5556The string should not contain any character with the value > 255 (which 5557can only happen if you're using UTF8 encoding). If it does, it will be 5558treated as something which is not UTF8 encoded. When the C<vec> was 5559assigned to, other parts of your program will also no longer consider the 5560string to be UTF8 encoded. In other words, if you do have such characters 5561in your string, vec() will operate on the actual byte string, and not the 5562conceptual character string. 5563 5564Strings created with C<vec> can also be manipulated with the logical 5565operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit 5566vector operation is desired when both operands are strings. 5567See L<perlop/"Bitwise String Operators">. 5568 5569The following code will build up an ASCII string saying C<'PerlPerlPerl'>. 5570The comments show the string after each step. Note that this code works 5571in the same way on big-endian or little-endian machines. 5572 5573 my $foo = ''; 5574 vec($foo, 0, 32) = 0x5065726C; # 'Perl' 5575 5576 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits 5577 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P') 5578 5579 vec($foo, 2, 16) = 0x5065; # 'PerlPe' 5580 vec($foo, 3, 16) = 0x726C; # 'PerlPerl' 5581 vec($foo, 8, 8) = 0x50; # 'PerlPerlP' 5582 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' 5583 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" 5584 vec($foo, 21, 4) = 7; # 'PerlPerlPer' 5585 # 'r' is "\x72" 5586 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" 5587 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" 5588 vec($foo, 94, 1) = 1; # 'PerlPerlPerl' 5589 # 'l' is "\x6c" 5590 5591To transform a bit vector into a string or list of 0's and 1's, use these: 5592 5593 $bits = unpack("b*", $vector); 5594 @bits = split(//, unpack("b*", $vector)); 5595 5596If you know the exact length in bits, it can be used in place of the C<*>. 5597 5598Here is an example to illustrate how the bits actually fall in place: 5599 5600 #!/usr/bin/perl -wl 5601 5602 print <<'EOT'; 5603 0 1 2 3 5604 unpack("V",$_) 01234567890123456789012345678901 5605 ------------------------------------------------------------------ 5606 EOT 5607 5608 for $w (0..3) { 5609 $width = 2**$w; 5610 for ($shift=0; $shift < $width; ++$shift) { 5611 for ($off=0; $off < 32/$width; ++$off) { 5612 $str = pack("B*", "0"x32); 5613 $bits = (1<<$shift); 5614 vec($str, $off, $width) = $bits; 5615 $res = unpack("b*",$str); 5616 $val = unpack("V", $str); 5617 write; 5618 } 5619 } 5620 } 5621 5622 format STDOUT = 5623 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 5624 $off, $width, $bits, $val, $res 5625 . 5626 __END__ 5627 5628Regardless of the machine architecture on which it is run, the above 5629example should print the following table: 5630 5631 0 1 2 3 5632 unpack("V",$_) 01234567890123456789012345678901 5633 ------------------------------------------------------------------ 5634 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000 5635 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000 5636 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000 5637 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000 5638 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000 5639 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000 5640 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000 5641 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000 5642 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000 5643 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000 5644 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000 5645 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000 5646 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000 5647 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000 5648 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000 5649 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000 5650 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000 5651 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000 5652 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000 5653 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000 5654 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000 5655 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000 5656 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000 5657 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000 5658 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000 5659 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000 5660 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000 5661 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000 5662 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000 5663 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100 5664 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010 5665 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001 5666 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000 5667 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000 5668 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000 5669 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000 5670 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000 5671 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000 5672 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000 5673 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000 5674 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000 5675 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000 5676 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000 5677 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000 5678 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000 5679 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000 5680 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000 5681 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010 5682 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000 5683 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000 5684 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000 5685 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000 5686 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000 5687 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000 5688 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000 5689 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000 5690 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000 5691 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000 5692 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000 5693 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000 5694 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000 5695 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000 5696 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100 5697 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001 5698 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000 5699 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000 5700 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000 5701 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000 5702 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000 5703 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000 5704 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000 5705 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000 5706 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000 5707 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000 5708 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000 5709 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000 5710 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000 5711 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000 5712 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000 5713 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100 5714 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000 5715 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000 5716 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000 5717 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000 5718 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000 5719 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000 5720 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000 5721 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010 5722 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000 5723 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000 5724 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000 5725 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000 5726 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000 5727 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000 5728 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000 5729 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001 5730 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000 5731 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000 5732 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000 5733 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000 5734 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000 5735 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000 5736 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000 5737 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000 5738 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000 5739 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000 5740 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000 5741 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000 5742 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000 5743 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000 5744 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000 5745 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000 5746 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000 5747 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000 5748 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000 5749 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000 5750 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000 5751 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000 5752 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000 5753 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100 5754 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000 5755 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000 5756 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000 5757 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010 5758 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000 5759 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000 5760 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000 5761 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001 5762 5763=item wait 5764 5765Behaves like the wait(2) system call on your system: it waits for a child 5766process to terminate and returns the pid of the deceased process, or 5767C<-1> if there are no child processes. The status is returned in C<$?>. 5768Note that a return value of C<-1> could mean that child processes are 5769being automatically reaped, as described in L<perlipc>. 5770 5771=item waitpid PID,FLAGS 5772 5773Waits for a particular child process to terminate and returns the pid of 5774the deceased process, or C<-1> if there is no such child process. On some 5775systems, a value of 0 indicates that there are processes still running. 5776The status is returned in C<$?>. If you say 5777 5778 use POSIX ":sys_wait_h"; 5779 #... 5780 do { 5781 $kid = waitpid(-1,&WNOHANG); 5782 } until $kid == -1; 5783 5784then you can do a non-blocking wait for all pending zombie processes. 5785Non-blocking wait is available on machines supporting either the 5786waitpid(2) or wait4(2) system calls. However, waiting for a particular 5787pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the 5788system call by remembering the status values of processes that have 5789exited but have not been harvested by the Perl script yet.) 5790 5791Note that on some systems, a return value of C<-1> could mean that child 5792processes are being automatically reaped. See L<perlipc> for details, 5793and for other examples. 5794 5795=item wantarray 5796 5797Returns true if the context of the currently executing subroutine is 5798looking for a list value. Returns false if the context is looking 5799for a scalar. Returns the undefined value if the context is looking 5800for no value (void context). 5801 5802 return unless defined wantarray; # don't bother doing more 5803 my @a = complex_calculation(); 5804 return wantarray ? @a : "@a"; 5805 5806This function should have been named wantlist() instead. 5807 5808=item warn LIST 5809 5810Produces a message on STDERR just like C<die>, but doesn't exit or throw 5811an exception. 5812 5813If LIST is empty and C<$@> already contains a value (typically from a 5814previous eval) that value is used after appending C<"\t...caught"> 5815to C<$@>. This is useful for staying almost, but not entirely similar to 5816C<die>. 5817 5818If C<$@> is empty then the string C<"Warning: Something's wrong"> is used. 5819 5820No message is printed if there is a C<$SIG{__WARN__}> handler 5821installed. It is the handler's responsibility to deal with the message 5822as it sees fit (like, for instance, converting it into a C<die>). Most 5823handlers must therefore make arrangements to actually display the 5824warnings that they are not prepared to deal with, by calling C<warn> 5825again in the handler. Note that this is quite safe and will not 5826produce an endless loop, since C<__WARN__> hooks are not called from 5827inside one. 5828 5829You will find this behavior is slightly different from that of 5830C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can 5831instead call C<die> again to change it). 5832 5833Using a C<__WARN__> handler provides a powerful way to silence all 5834warnings (even the so-called mandatory ones). An example: 5835 5836 # wipe out *all* compile-time warnings 5837 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } 5838 my $foo = 10; 5839 my $foo = 20; # no warning about duplicate my $foo, 5840 # but hey, you asked for it! 5841 # no compile-time or run-time warnings before here 5842 $DOWARN = 1; 5843 5844 # run-time warnings enabled after here 5845 warn "\$foo is alive and $foo!"; # does show up 5846 5847See L<perlvar> for details on setting C<%SIG> entries, and for more 5848examples. See the Carp module for other kinds of warnings using its 5849carp() and cluck() functions. 5850 5851=item write FILEHANDLE 5852 5853=item write EXPR 5854 5855=item write 5856 5857Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, 5858using the format associated with that file. By default the format for 5859a file is the one having the same name as the filehandle, but the 5860format for the current output channel (see the C<select> function) may be set 5861explicitly by assigning the name of the format to the C<$~> variable. 5862 5863Top of form processing is handled automatically: if there is 5864insufficient room on the current page for the formatted record, the 5865page is advanced by writing a form feed, a special top-of-page format 5866is used to format the new page header, and then the record is written. 5867By default the top-of-page format is the name of the filehandle with 5868"_TOP" appended, but it may be dynamically set to the format of your 5869choice by assigning the name to the C<$^> variable while the filehandle is 5870selected. The number of lines remaining on the current page is in 5871variable C<$->, which can be set to C<0> to force a new page. 5872 5873If FILEHANDLE is unspecified, output goes to the current default output 5874channel, which starts out as STDOUT but may be changed by the 5875C<select> operator. If the FILEHANDLE is an EXPR, then the expression 5876is evaluated and the resulting string is used to look up the name of 5877the FILEHANDLE at run time. For more on formats, see L<perlform>. 5878 5879Note that write is I<not> the opposite of C<read>. Unfortunately. 5880 5881=item y/// 5882 5883The transliteration operator. Same as C<tr///>. See L<perlop>. 5884 5885=back 5886