3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar and list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one list argument.) For instance, splice() has three scalar
19 arguments followed by a list.
21 In the syntax descriptions that follow, list operators that expect a
22 list (and provide list context for the elements of the list) are shown
23 with LIST as an argument. Such a list may consist of any combination
24 of scalar arguments or list values; the list values will be included
25 in the list as if each individual element were interpolated at that
26 point in the list, forming a longer single-dimensional list value.
27 Elements of the LIST should be separated by commas.
29 Any function in the list below may be used either with or without
30 parentheses around its arguments. (The syntax descriptions omit the
31 parentheses.) If you use the parentheses, the simple (but occasionally
32 surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33 function, and precedence doesn't matter. Otherwise it's a list
34 operator or unary operator, and precedence does matter. And whitespace
35 between the function and left parenthesis doesn't count--so you need to
38 print 1+2+4; # Prints 7.
39 print(1+2) + 4; # Prints 3.
40 print (1+2)+4; # Also prints 3!
41 print +(1+2)+4; # Prints 7.
42 print ((1+2)+4); # Prints 7.
44 If you run Perl with the B<-w> switch it can warn you about this. For
45 example, the third line above produces:
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
50 For functions that can be used in either a scalar or list context,
51 nonabortive failure is generally indicated in a scalar context by
52 returning the undefined value, and in a list context by returning the
55 Remember the following important rule: There is B<no rule> that relates
56 the behavior of an expression in list context to its behavior in scalar
57 context, or vice versa. It might do two totally different things.
58 Each operator and function decides which sort of value it would be most
59 appropriate to return in a scalar context. Some operators return the
60 length of the list that would have been returned in list context. Some
61 operators return the first value in the list. Some operators return the
62 last value in the list. Some operators return a count of successful
63 operations. In general, they do what you want, unless you want
66 An named array in scalar context is quite different from what would at
67 first glance appear to be a list in scalar context. You can't get a list
68 like C<(1,2,3)> into being in scalar context, because the compiler knows
69 the context at compile time. It would generate the scalar comma operator
70 there, not the list construction version of the comma. That means it
71 was never a list to start with.
73 In general, functions in Perl that serve as wrappers for system calls
74 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
75 true when they succeed and C<undef> otherwise, as is usually mentioned
76 in the descriptions below. This is different from the C interfaces,
77 which return C<-1> on failure. Exceptions to this rule are C<wait()>,
78 C<waitpid()>, and C<syscall()>. System calls also set the special C<$!>
79 variable on failure. Other functions do not, except accidentally.
81 =head2 Perl Functions by Category
83 Here are Perl's functions (including things that look like
84 functions, like some keywords and named operators)
85 arranged by category. Some functions appear in more
90 =item Functions for SCALARs or strings
92 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
93 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
94 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
96 =item Regular expressions and pattern matching
98 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
100 =item Numeric functions
102 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
103 C<sin>, C<sqrt>, C<srand>
105 =item Functions for real @ARRAYs
107 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
109 =item Functions for list data
111 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
113 =item Functions for real %HASHes
115 C<delete>, C<each>, C<exists>, C<keys>, C<values>
117 =item Input and output functions
119 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
120 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
121 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
122 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
125 =item Functions for fixed length data or records
127 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
129 =item Functions for filehandles, files, or directories
131 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
132 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
133 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
136 =item Keywords related to the control flow of your perl program
138 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<else>, C<elsif>,
139 C<eval>, C<exit>, C<for>, C<foreach>, C<goto>, C<if>, C<last>,
140 C<next>, C<redo>, C<return>, C<sub>, C<unless>, C<wantarray>,
143 =item Keywords related to scoping
145 C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
147 =item Miscellaneous functions
149 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
150 C<scalar>, C<undef>, C<wantarray>
152 =item Functions for processes and process groups
154 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
155 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
156 C<times>, C<wait>, C<waitpid>
158 =item Keywords related to perl modules
160 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
162 =item Keywords related to classes and object-orientedness
164 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
167 =item Low-level socket functions
169 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
170 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
171 C<socket>, C<socketpair>
173 =item System V interprocess communication functions
175 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
176 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
178 =item Fetching user and group info
180 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
181 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
182 C<getpwuid>, C<setgrent>, C<setpwent>
184 =item Fetching network info
186 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
187 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
188 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
189 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
190 C<setnetent>, C<setprotoent>, C<setservent>
192 =item Time-related functions
194 C<gmtime>, C<localtime>, C<time>, C<times>
196 =item Functions new in perl5
198 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
199 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
200 C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
201 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
203 * - C<sub> was a keyword in perl4, but in perl5 it is an
204 operator, which can be used in expressions.
206 =item Functions obsoleted in perl5
208 C<dbmclose>, C<dbmopen>
214 Perl was born in UNIX and therefore it can access all the common UNIX
215 system calls. In non-UNIX environments the functionality of many
216 UNIX system calls may not be available or the details of the available
217 functionality may be slightly different. The Perl functions affected
220 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
221 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
222 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
223 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
224 C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
225 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
226 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
227 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
228 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
229 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<select>, C<semctl>,
230 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
231 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
232 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
233 C<shmwrite>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
234 C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<utime>,
237 For more information about the portability of these functions, see
238 L<perlport> and other available platform-specific documentation.
240 =head2 Alphabetical Listing of Perl Functions
244 =item I<-X> FILEHANDLE
250 A file test, where X is one of the letters listed below. This unary
251 operator takes one argument, either a filename or a filehandle, and
252 tests the associated file to see if something is true about it. If the
253 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
254 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
255 the undefined value if the file doesn't exist. Despite the funny
256 names, precedence is the same as any other named unary operator, and
257 the argument may be parenthesized like any other unary operator. The
258 operator may be any of:
259 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
260 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
262 -r File is readable by effective uid/gid.
263 -w File is writable by effective uid/gid.
264 -x File is executable by effective uid/gid.
265 -o File is owned by effective uid.
267 -R File is readable by real uid/gid.
268 -W File is writable by real uid/gid.
269 -X File is executable by real uid/gid.
270 -O File is owned by real uid.
273 -z File has zero size.
274 -s File has nonzero size (returns size).
276 -f File is a plain file.
277 -d File is a directory.
278 -l File is a symbolic link.
279 -p File is a named pipe (FIFO), or Filehandle is a pipe.
281 -b File is a block special file.
282 -c File is a character special file.
283 -t Filehandle is opened to a tty.
285 -u File has setuid bit set.
286 -g File has setgid bit set.
287 -k File has sticky bit set.
289 -T File is a text file.
290 -B File is a binary file (opposite of -T).
292 -M Age of file in days when script started.
293 -A Same for access time.
294 -C Same for inode change time.
300 next unless -f $_; # ignore specials
304 The interpretation of the file permission operators C<-r>, C<-R>,
305 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
306 of the file and the uids and gids of the user. There may be other
307 reasons you can't actually read, write, or execute the file. Such
308 reasons may be for example network filesystem access controls, ACLs
309 (access control lists), read-only filesystems, and unrecognized
312 Also note that, for the superuser on the local filesystems, C<-r>,
313 C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return 1
314 if any execute bit is set in the mode. Scripts run by the superuser
315 may thus need to do a stat() to determine the actual mode of the file,
316 or temporarily set the uid to something else.
318 If you are using ACLs, there is a pragma called C<filetest> that may
319 produce more accurate results than the bare stat() mode bits.
321 When under the C<use filetest 'access'> the above-mentioned filetests
322 will test whether the permission can (not) be granted using the
323 access() family of system calls. Also note that the C<-x> and C<-X> may
324 under this pragma return true even if there are no execute permission
325 bits set (nor any extra execute permission ACLs). This strangeness is
326 due to the underlying system calls' definitions. Read the
327 documentation for the C<filetest> pragma for more information.
329 Note that C<-s/a/b/> does not do a negated substitution. Saying
330 C<-exp($foo)> still works as expected, however--only single letters
331 following a minus are interpreted as file tests.
333 The C<-T> and C<-B> switches work as follows. The first block or so of the
334 file is examined for odd characters such as strange control codes or
335 characters with the high bit set. If too many strange characters (E<gt>30%)
336 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
337 containing null in the first block is considered a binary file. If C<-T>
338 or C<-B> is used on a filehandle, the current stdio buffer is examined
339 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
340 file, or a file at EOF when testing a filehandle. Because you have to
341 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
342 against the file first, as in C<next unless -f $file && -T $file>.
344 If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
345 the special filehandle consisting of a solitary underline, then the stat
346 structure of the previous file test (or stat operator) is used, saving
347 a system call. (This doesn't work with C<-t>, and you need to remember
348 that lstat() and C<-l> will leave values in the stat structure for the
349 symbolic link, not the real file.) Example:
351 print "Can do.\n" if -r $a || -w _ || -x _;
354 print "Readable\n" if -r _;
355 print "Writable\n" if -w _;
356 print "Executable\n" if -x _;
357 print "Setuid\n" if -u _;
358 print "Setgid\n" if -g _;
359 print "Sticky\n" if -k _;
360 print "Text\n" if -T _;
361 print "Binary\n" if -B _;
367 Returns the absolute value of its argument.
368 If VALUE is omitted, uses C<$_>.
370 =item accept NEWSOCKET,GENERICSOCKET
372 Accepts an incoming socket connect, just as the accept(2) system call
373 does. Returns the packed address if it succeeded, FALSE otherwise.
374 See example in L<perlipc/"Sockets: Client/Server Communication">.
380 Arranges to have a SIGALRM delivered to this process after the
381 specified number of seconds have elapsed. If SECONDS is not specified,
382 the value stored in C<$_> is used. (On some machines,
383 unfortunately, the elapsed time may be up to one second less than you
384 specified because of how seconds are counted.) Only one timer may be
385 counting at once. Each call disables the previous timer, and an
386 argument of C<0> may be supplied to cancel the previous timer without
387 starting a new one. The returned value is the amount of time remaining
388 on the previous timer.
390 For delays of finer granularity than one second, you may use Perl's
391 C<syscall()> interface to access setitimer(2) if your system supports it,
392 or else see L</select()>. It is usually a mistake to intermix C<alarm()>
393 and C<sleep()> calls.
395 If you want to use C<alarm()> to time out a system call you need to use an
396 C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
397 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
398 restart system calls on some systems. Using C<eval()>/C<die()> always works,
399 modulo the caveats given in L<perlipc/"Signals">.
402 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
404 $nread = sysread SOCKET, $buffer, $size;
408 die unless $@ eq "alarm\n"; # propagate unexpected errors
417 Returns the arctangent of Y/X in the range -PI to PI.
419 For the tangent operation, you may use the C<POSIX::tan()>
420 function, or use the familiar relation:
422 sub tan { sin($_[0]) / cos($_[0]) }
424 =item bind SOCKET,NAME
426 Binds a network address to a socket, just as the bind system call
427 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
428 packed address of the appropriate type for the socket. See the examples in
429 L<perlipc/"Sockets: Client/Server Communication">.
431 =item binmode FILEHANDLE
433 Arranges for the file to be read or written in "binary" mode in operating
434 systems that distinguish between binary and text files. Files that are
435 not in binary mode have CR LF sequences translated to LF on input and LF
436 translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
437 and similarly archaic systems, it may be imperative--otherwise your
438 MS-DOS-damaged C library may mangle your file. The key distinction between
439 systems that need C<binmode()> and those that don't is their text file
440 formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
441 character, and that encode that character in C as C<"\n">, do not need
442 C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value
443 is taken as the name of the filehandle.
445 =item bless REF,CLASSNAME
449 This function tells the thingy referenced by REF that it is now
450 an object in the CLASSNAME package--or the current package if no CLASSNAME
451 is specified, which is often the case. It returns the reference for
452 convenience, because a C<bless()> is often the last thing in a constructor.
453 Always use the two-argument version if the function doing the blessing
454 might be inherited by a derived class. See L<perltoot> and L<perlobj>
455 for more about the blessing (and blessings) of objects.
457 Consider always blessing objects in CLASSNAMEs that are mixed case.
458 Namespaces with all lowercase names are considered reserved for Perl
459 pragmata. Builtin types have all uppercase names, so to prevent confusion,
460 it is best to avoid such package names as well.
462 See L<perlmod/"Perl Modules">.
468 Returns the context of the current subroutine call. In scalar context,
469 returns the caller's package name if there is a caller, that is, if
470 we're in a subroutine or C<eval()> or C<require()>, and the undefined value
471 otherwise. In list context, returns
473 ($package, $filename, $line) = caller;
475 With EXPR, it returns some extra information that the debugger uses to
476 print a stack trace. The value of EXPR indicates how many call frames
477 to go back before the current one.
479 ($package, $filename, $line, $subroutine,
480 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
482 Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
483 call, but an C<eval()>. In such a case additional elements C<$evaltext> and
484 C<$is_require> are set: C<$is_require> is true if the frame is created by a
485 C<require> or C<use> statement, C<$evaltext> contains the text of the
486 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
487 C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
488 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
491 Furthermore, when called from within the DB package, caller returns more
492 detailed information: it sets the list variable C<@DB::args> to be the
493 arguments with which the subroutine was invoked.
495 Be aware that the optimizer might have optimized call frames away before
496 C<caller()> had a chance to get the information. That means that C<caller(N)>
497 might not return information about the call frame you expect it do, for
498 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
499 previous time C<caller()> was called.
503 Changes the working directory to EXPR, if possible. If EXPR is
504 omitted, changes to home directory. Returns TRUE upon success, FALSE
505 otherwise. See example under C<die()>.
509 Changes the permissions of a list of files. The first element of the
510 list must be the numerical mode, which should probably be an octal
511 number, and which definitely should I<not> a string of octal digits:
512 C<0644> is okay, C<'0644'> is not. Returns the number of files
513 successfully changed. See also L</oct>, if all you have is a string.
515 $cnt = chmod 0755, 'foo', 'bar';
516 chmod 0755, @executables;
517 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
519 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
520 $mode = 0644; chmod $mode, 'foo'; # this is best
528 This is a slightly safer version of L</chop>. It removes any
529 line ending that corresponds to the current value of C<$/> (also known as
530 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
531 number of characters removed from all its arguments. It's often used to
532 remove the newline from the end of an input record when you're worried
533 that the final record may be missing its newline. When in paragraph mode
534 (C<$/ = "">), it removes all trailing newlines from the string. If
535 VARIABLE is omitted, it chomps C<$_>. Example:
538 chomp; # avoid \n on last field
543 You can actually chomp anything that's an lvalue, including an assignment:
546 chomp($answer = <STDIN>);
548 If you chomp a list, each element is chomped, and the total number of
549 characters removed is returned.
557 Chops off the last character of a string and returns the character
558 chopped. It's used primarily to remove the newline from the end of an
559 input record, but is much more efficient than C<s/\n//> because it neither
560 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
564 chop; # avoid \n on last field
569 You can actually chop anything that's an lvalue, including an assignment:
572 chop($answer = <STDIN>);
574 If you chop a list, each element is chopped. Only the value of the
575 last C<chop()> is returned.
577 Note that C<chop()> returns the last character. To return all but the last
578 character, use C<substr($string, 0, -1)>.
582 Changes the owner (and group) of a list of files. The first two
583 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
584 Returns the number of files successfully changed.
586 $cnt = chown $uid, $gid, 'foo', 'bar';
587 chown $uid, $gid, @filenames;
589 Here's an example that looks up nonnumeric uids in the passwd file:
592 chop($user = <STDIN>);
594 chop($pattern = <STDIN>);
596 ($login,$pass,$uid,$gid) = getpwnam($user)
597 or die "$user not in passwd file";
599 @ary = glob($pattern); # expand filenames
600 chown $uid, $gid, @ary;
602 On most systems, you are not allowed to change the ownership of the
603 file unless you're the superuser, although you should be able to change
604 the group to any of your secondary groups. On insecure systems, these
605 restrictions may be relaxed, but this is not a portable assumption.
611 Returns the character represented by that NUMBER in the character set.
612 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
613 chr(0x263a) is a Unicode smiley face (but only within the scope of a
614 C<use utf8>). For the reverse, use L</ord>.
616 If NUMBER is omitted, uses C<$_>.
618 =item chroot FILENAME
622 This function works like the system call by the same name: it makes the
623 named directory the new root directory for all further pathnames that
624 begin with a C<"/"> by your process and all its children. (It doesn't
625 change your current working directory, which is unaffected.) For security
626 reasons, this call is restricted to the superuser. If FILENAME is
627 omitted, does a C<chroot()> to C<$_>.
629 =item close FILEHANDLE
633 Closes the file or pipe associated with the file handle, returning TRUE
634 only if stdio successfully flushes buffers and closes the system file
635 descriptor. Closes the currently selected filehandle if the argument
638 You don't have to close FILEHANDLE if you are immediately going to do
639 another C<open()> on it, because C<open()> will close it for you. (See
640 C<open()>.) However, an explicit C<close()> on an input file resets the line
641 counter (C<$.>), while the implicit close done by C<open()> does not.
643 If the file handle came from a piped open C<close()> will additionally
644 return FALSE if one of the other system calls involved fails or if the
645 program exits with non-zero status. (If the only problem was that the
646 program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
647 waits for the process executing on the pipe to complete, in case you
648 want to look at the output of the pipe afterwards. Closing a pipe
649 explicitly also puts the exit status value of the command into C<$?>.
653 open(OUTPUT, '|sort >foo') # pipe to sort
654 or die "Can't start sort: $!";
655 #... # print stuff to output
656 close OUTPUT # wait for sort to finish
657 or warn $! ? "Error closing sort pipe: $!"
658 : "Exit status $? from sort";
659 open(INPUT, 'foo') # get sort's results
660 or die "Can't open 'foo' for input: $!";
662 FILEHANDLE may be an expression whose value can be used as an indirect
663 filehandle, usually the real filehandle name.
665 =item closedir DIRHANDLE
667 Closes a directory opened by C<opendir()> and returns the success of that
670 DIRHANDLE may be an expression whose value can be used as an indirect
671 dirhandle, usually the real dirhandle name.
673 =item connect SOCKET,NAME
675 Attempts to connect to a remote socket, just as the connect system call
676 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
677 packed address of the appropriate type for the socket. See the examples in
678 L<perlipc/"Sockets: Client/Server Communication">.
682 Actually a flow control statement rather than a function. If there is a
683 C<continue> BLOCK attached to a BLOCK (typically in a L</while> or
684 L</foreach>), it is always executed just before the conditional is about to
685 be evaluated again, just like the third part of a L</for> loop in C. Thus
686 it can be used to increment a loop variable, even when the loop has been
687 continued via the C<next> statement (which is similar to the C C<continue>
690 L</last>, L</next>, or L</redo> may appear within a C<continue>
691 block. C<last> and C<redo> will behave as if they had been executed within
692 the main block. So will C<next>, but since it will execute a C<continue>
693 block, it may be more entertaining.
696 ### redo always comes here
699 ### next always comes here
701 # then back the top to re-check EXPR
703 ### last always comes here
705 Omitting the C<continue> section is semantically equivalent to using an
706 empty one, logically enough. In that case, C<next> goes directly back
707 to check the condition at the top of the loop.
713 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
714 takes cosine of C<$_>.
716 For the inverse cosine operation, you may use the C<POSIX::acos()>
717 function, or use this relation:
719 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
721 =item crypt PLAINTEXT,SALT
723 Encrypts a string exactly like the crypt(3) function in the C library
724 (assuming that you actually have a version there that has not been
725 extirpated as a potential munition). This can prove useful for checking
726 the password file for lousy passwords, amongst other things. Only the
727 guys wearing white hats should do this.
729 Note that C<crypt()> is intended to be a one-way function, much like breaking
730 eggs to make an omelette. There is no (known) corresponding decrypt
731 function. As a result, this function isn't all that useful for
732 cryptography. (For that, see your nearby CPAN mirror.)
734 When verifying an existing encrypted string you should use the encrypted
735 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
736 allows your code to work with the standard C<crypt()> and with more
737 exotic implementations. When choosing a new salt create a random two
738 character string whose characters come from the set C<[./0-9A-Za-z]>
739 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
741 Here's an example that makes sure that whoever runs this program knows
744 $pwd = (getpwuid($<))[1];
748 chomp($word = <STDIN>);
752 if (crypt($word, $pwd) ne $pwd) {
758 Of course, typing in your own password to whoever asks you
763 [This function has been superseded by the C<untie()> function.]
765 Breaks the binding between a DBM file and a hash.
767 =item dbmopen HASH,DBNAME,MODE
769 [This function has been superseded by the C<tie()> function.]
771 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
772 hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
773 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
774 is the name of the database (without the F<.dir> or F<.pag> extension if
775 any). If the database does not exist, it is created with protection
776 specified by MODE (as modified by the C<umask()>). If your system supports
777 only the older DBM functions, you may perform only one C<dbmopen()> in your
778 program. In older versions of Perl, if your system had neither DBM nor
779 ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
782 If you don't have write access to the DBM file, you can only read hash
783 variables, not set them. If you want to test whether you can write,
784 either use file tests or try setting a dummy hash entry inside an C<eval()>,
785 which will trap the error.
787 Note that functions such as C<keys()> and C<values()> may return huge lists
788 when used on large DBM files. You may prefer to use the C<each()>
789 function to iterate over large DBM files. Example:
791 # print out history file offsets
792 dbmopen(%HIST,'/usr/lib/news/history',0666);
793 while (($key,$val) = each %HIST) {
794 print $key, ' = ', unpack('L',$val), "\n";
798 See also L<AnyDBM_File> for a more general description of the pros and
799 cons of the various dbm approaches, as well as L<DB_File> for a particularly
806 Returns a Boolean value telling whether EXPR has a value other than
807 the undefined value C<undef>. If EXPR is not present, C<$_> will be
810 Many operations return C<undef> to indicate failure, end of file,
811 system error, uninitialized variable, and other exceptional
812 conditions. This function allows you to distinguish C<undef> from
813 other values. (A simple Boolean test will not distinguish among
814 C<undef>, zero, the empty string, and C<"0">, which are all equally
815 false.) Note that since C<undef> is a valid scalar, its presence
816 doesn't I<necessarily> indicate an exceptional condition: C<pop()>
817 returns C<undef> when its argument is an empty array, I<or> when the
818 element to return happens to be C<undef>.
820 You may also use C<defined()> to check whether a subroutine exists, by
821 saying C<defined &func> without parentheses. On the other hand, use
822 of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
823 produce intuitive results, and should probably be avoided.
825 When used on a hash element, it tells you whether the value is defined,
826 not whether the key exists in the hash. Use L</exists> for the latter
831 print if defined $switch{'D'};
832 print "$val\n" while defined($val = pop(@ary));
833 die "Can't readlink $sym: $!"
834 unless defined($value = readlink $sym);
835 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
836 $debugging = 0 unless defined $debugging;
838 Note: Many folks tend to overuse C<defined()>, and then are surprised to
839 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
840 defined values. For example, if you say
844 The pattern match succeeds, and C<$1> is defined, despite the fact that it
845 matched "nothing". But it didn't really match nothing--rather, it
846 matched something that happened to be C<0> characters long. This is all
847 very above-board and honest. When a function returns an undefined value,
848 it's an admission that it couldn't give you an honest answer. So you
849 should use C<defined()> only when you're questioning the integrity of what
850 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
853 Currently, using C<defined()> on an entire array or hash reports whether
854 memory for that aggregate has ever been allocated. So an array you set
855 to the empty list appears undefined initially, and one that once was full
856 and that you then set to the empty list still appears defined. You
857 should instead use a simple test for size:
859 if (@an_array) { print "has array elements\n" }
860 if (%a_hash) { print "has hash members\n" }
862 Using C<undef()> on these, however, does clear their memory and then report
863 them as not defined anymore, but you shouldn't do that unless you don't
864 plan to use them again, because it saves time when you load them up
865 again to have memory already ready to be filled. The normal way to
866 free up space used by an aggregate is to assign the empty list.
868 This counterintuitive behavior of C<defined()> on aggregates may be
869 changed, fixed, or broken in a future release of Perl.
871 See also L</undef>, L</exists>, L</ref>.
875 Deletes the specified key(s) and their associated values from a hash.
876 For each key, returns the deleted value associated with that key, or
877 the undefined value if there was no such key. Deleting from C<$ENV{}>
878 modifies the environment. Deleting from a hash tied to a DBM file
879 deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
880 doesn't necessarily return anything.)
882 The following deletes all the values of a hash:
884 foreach $key (keys %HASH) {
890 delete @HASH{keys %HASH}
892 (But both of these are slower than just assigning the empty list, or
893 using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
894 long as the final operation is a hash element lookup or hash slice:
896 delete $ref->[$x][$y]{$key};
897 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
901 Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
902 the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
903 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
904 is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
905 C<$@> and the C<eval()> is terminated with the undefined value. This makes
906 C<die()> the way to raise an exception.
910 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
911 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
913 If the value of EXPR does not end in a newline, the current script line
914 number and input line number (if any) are also printed, and a newline
915 is supplied. Hint: sometimes appending C<", stopped"> to your message
916 will cause it to make better sense when the string C<"at foo line 123"> is
917 appended. Suppose you are running script "canasta".
919 die "/etc/games is no good";
920 die "/etc/games is no good, stopped";
922 produce, respectively
924 /etc/games is no good at canasta line 123.
925 /etc/games is no good, stopped at canasta line 123.
927 See also C<exit()> and C<warn()>.
929 If LIST is empty and C<$@> already contains a value (typically from a
930 previous eval) that value is reused after appending C<"\t...propagated">.
931 This is useful for propagating exceptions:
934 die unless $@ =~ /Expected exception/;
936 If C<$@> is empty then the string C<"Died"> is used.
938 You can arrange for a callback to be run just before the C<die()> does
939 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
940 will be called with the error text and can change the error message, if
941 it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
942 setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
944 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
945 blocks/strings. If one wants the hook to do nothing in such
950 as the first line of the handler (see L<perlvar/$^S>).
954 Not really a function. Returns the value of the last command in the
955 sequence of commands indicated by BLOCK. When modified by a loop
956 modifier such as L</while> or L</until>, executes the BLOCK once
957 before testing the loop condition. (On other statements the loop
958 modifiers test the conditional first.)
960 C<do BLOCK> does I<not> count as a loop, so the loop control statements
961 L</next>, L</last> or L</redo> cannot be used to leave or restart the block.
963 =item do SUBROUTINE(LIST)
965 A deprecated form of subroutine call. See L<perlsub>.
969 Uses the value of EXPR as a filename and executes the contents of the
970 file as a Perl script. Its primary use is to include subroutines
971 from a Perl subroutine library.
977 scalar eval `cat stat.pl`;
979 except that it's more efficient and concise, keeps track of the
980 current filename for error messages, and searches all the B<-I>
981 libraries if the file isn't in the current directory (see also the @INC
982 array in L<perlvar/Predefined Names>). It is also different in how
983 code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
984 scope like C<eval STRING> does. It's the same, however, in that it does
985 reparse the file every time you call it, so you probably don't want to
986 do this inside a loop.
988 If C<do> cannot read the file, it returns undef and sets C<$!> to the
989 error. If C<do> can read the file but cannot compile it, it
990 returns undef and sets an error message in C<$@>. If the file is
991 successfully compiled, C<do> returns the value of the last expression
994 Note that inclusion of library modules is better done with the
995 C<use()> and C<require()> operators, which also do automatic error checking
996 and raise an exception if there's a problem.
998 You might like to use C<do> to read in a program configuration
999 file. Manual error checking can be done this way:
1001 # read in config files: system first, then user
1002 for $file ("/share/prog/defaults.rc",
1003 "$ENV{HOME}/.someprogrc") {
1004 unless ($return = do $file) {
1005 warn "couldn't parse $file: $@" if $@;
1006 warn "couldn't do $file: $!" unless defined $return;
1007 warn "couldn't run $file" unless $return;
1013 This causes an immediate core dump. Primarily this is so that you can
1014 use the B<undump> program to turn your core dump into an executable binary
1015 after having initialized all your variables at the beginning of the
1016 program. When the new binary is executed it will begin by executing a
1017 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
1018 it as a goto with an intervening core dump and reincarnation. If C<LABEL>
1019 is omitted, restarts the program from the top. WARNING: Any files
1020 opened at the time of the dump will NOT be open any more when the
1021 program is reincarnated, with possible resulting confusion on the part
1022 of Perl. See also B<-u> option in L<perlrun>.
1027 require 'getopt.pl';
1039 dump QUICKSTART if $ARGV[0] eq '-d';
1044 This operator is largely obsolete, partly because it's very hard to
1045 convert a core file into an executable, and because the real perl-to-C
1046 compiler has superseded it.
1050 When called in list context, returns a 2-element list consisting of the
1051 key and value for the next element of a hash, so that you can iterate over
1052 it. When called in scalar context, returns the key for only the "next"
1053 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
1054 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1057 Entries are returned in an apparently random order. The actual random
1058 order is subject to change in future versions of perl, but it is guaranteed
1059 to be in the same order as either the C<keys()> or C<values()> function
1060 would produce on the same (unmodified) hash.
1062 When the hash is entirely read, a null array is returned in list context
1063 (which when assigned produces a FALSE (C<0>) value), and C<undef> in
1064 scalar context. The next call to C<each()> after that will start iterating
1065 again. There is a single iterator for each hash, shared by all C<each()>,
1066 C<keys()>, and C<values()> function calls in the program; it can be reset by
1067 reading all the elements from the hash, or by evaluating C<keys HASH> or
1068 C<values HASH>. If you add or delete elements of a hash while you're
1069 iterating over it, you may get entries skipped or duplicated, so don't.
1071 The following prints out your environment like the printenv(1) program,
1072 only in a different order:
1074 while (($key,$value) = each %ENV) {
1075 print "$key=$value\n";
1078 See also C<keys()>, C<values()> and C<sort()>.
1082 =item elsif (EXPR) BLOCK
1086 =item eof FILEHANDLE
1092 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1093 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1094 gives the real filehandle. (Note that this function actually
1095 reads a character and then C<ungetc()>s it, so isn't very useful in an
1096 interactive context.) Do not read from a terminal file (or call
1097 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1098 as terminals may lose the end-of-file condition if you do.
1100 An C<eof> without an argument uses the last file read as argument.
1101 Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
1102 the files listed on the command line, i.e., C<eof()> is reasonable to
1103 use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1104 last file. Use C<eof(ARGV)> or eof without the parentheses to test
1105 I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
1107 # reset line numbering on each input file
1109 next if /^\s*#/; # skip comments
1112 close ARGV if eof; # Not eof()!
1115 # insert dashes just before last line of last file
1117 if (eof()) { # check for end of current file
1118 print "--------------\n";
1119 close(ARGV); # close or break; is needed if we
1120 # are reading from the terminal
1125 Practical hint: you almost never need to use C<eof> in Perl, because the
1126 input operators return false values when they run out of data, or if there
1133 In the first form, the return value of EXPR is parsed and executed as if it
1134 were a little Perl program. The value of the expression (which is itself
1135 determined within scalar context) is first parsed, and if there weren't any
1136 errors, executed in the context of the current Perl program, so that any
1137 variable settings or subroutine and format definitions remain afterwards.
1138 Note that the value is parsed every time the eval executes. If EXPR is
1139 omitted, evaluates C<$_>. This form is typically used to delay parsing
1140 and subsequent execution of the text of EXPR until run time.
1142 In the second form, the code within the BLOCK is parsed only once--at the
1143 same time the code surrounding the eval itself was parsed--and executed
1144 within the context of the current Perl program. This form is typically
1145 used to trap exceptions more efficiently than the first (see below), while
1146 also providing the benefit of checking the code within BLOCK at compile
1149 The final semicolon, if any, may be omitted from the value of EXPR or within
1152 In both forms, the value returned is the value of the last expression
1153 evaluated inside the mini-program; a return statement may be also used, just
1154 as with subroutines. The expression providing the return value is evaluated
1155 in void, scalar, or list context, depending on the context of the eval itself.
1156 See L</wantarray> for more on how the evaluation context can be determined.
1158 If there is a syntax error or runtime error, or a C<die()> statement is
1159 executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
1160 error message. If there was no error, C<$@> is guaranteed to be a null
1161 string. Beware that using C<eval()> neither silences perl from printing
1162 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1163 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1164 L</warn> and L<perlvar>.
1166 Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1167 determining whether a particular feature (such as C<socket()> or C<symlink()>)
1168 is implemented. It is also Perl's exception trapping mechanism, where
1169 the die operator is used to raise exceptions.
1171 If the code to be executed doesn't vary, you may use the eval-BLOCK
1172 form to trap run-time errors without incurring the penalty of
1173 recompiling each time. The error, if any, is still returned in C<$@>.
1176 # make divide-by-zero nonfatal
1177 eval { $answer = $a / $b; }; warn $@ if $@;
1179 # same thing, but less efficient
1180 eval '$answer = $a / $b'; warn $@ if $@;
1182 # a compile-time error
1183 eval { $answer = }; # WRONG
1186 eval '$answer ='; # sets $@
1188 When using the C<eval{}> form as an exception trap in libraries, you may
1189 wish not to trigger any C<__DIE__> hooks that user code may have
1190 installed. You can use the C<local $SIG{__DIE__}> construct for this
1191 purpose, as shown in this example:
1193 # a very private exception trap for divide-by-zero
1194 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1197 This is especially significant, given that C<__DIE__> hooks can call
1198 C<die()> again, which has the effect of changing their error messages:
1200 # __DIE__ hooks may modify error messages
1202 local $SIG{'__DIE__'} =
1203 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1204 eval { die "foo lives here" };
1205 print $@ if $@; # prints "bar lives here"
1208 With an C<eval()>, you should be especially careful to remember what's
1209 being looked at when:
1215 eval { $x }; # CASE 4
1217 eval "\$$x++"; # CASE 5
1220 Cases 1 and 2 above behave identically: they run the code contained in
1221 the variable C<$x>. (Although case 2 has misleading double quotes making
1222 the reader wonder what else might be happening (nothing is).) Cases 3
1223 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1224 does nothing but return the value of C<$x>. (Case 4 is preferred for
1225 purely visual reasons, but it also has the advantage of compiling at
1226 compile-time instead of at run-time.) Case 5 is a place where
1227 normally you I<WOULD> like to use double quotes, except that in this
1228 particular situation, you can just use symbolic references instead, as
1231 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1232 C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
1237 =item exec PROGRAM LIST
1239 The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1240 use C<system()> instead of C<exec()> if you want it to return. It fails and
1241 returns FALSE only if the command does not exist I<and> it is executed
1242 directly instead of via your system's command shell (see below).
1244 Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1245 warns you if there is a following statement which isn't C<die()>, C<warn()>,
1246 or C<exit()> (if C<-w> is set - but you always do that). If you
1247 I<really> want to follow an C<exec()> with some other statement, you
1248 can use one of these styles to avoid the warning:
1250 exec ('foo') or print STDERR "couldn't exec foo: $!";
1251 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1253 If there is more than one argument in LIST, or if LIST is an array
1254 with more than one value, calls execvp(3) with the arguments in LIST.
1255 If there is only one scalar argument or an array with one element in it,
1256 the argument is checked for shell metacharacters, and if there are any,
1257 the entire argument is passed to the system's command shell for parsing
1258 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1259 If there are no shell metacharacters in the argument, it is split into
1260 words and passed directly to C<execvp()>, which is more efficient. Note:
1261 C<exec()> and C<system()> do not flush your output buffer, so you may need to
1262 set C<$|> to avoid lost output. Examples:
1264 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1265 exec "sort $outfile | uniq";
1267 If you don't really want to execute the first argument, but want to lie
1268 to the program you are executing about its own name, you can specify
1269 the program you actually want to run as an "indirect object" (without a
1270 comma) in front of the LIST. (This always forces interpretation of the
1271 LIST as a multivalued list, even if there is only a single scalar in
1274 $shell = '/bin/csh';
1275 exec $shell '-sh'; # pretend it's a login shell
1279 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1281 When the arguments get executed via the system shell, results will
1282 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1285 Using an indirect object with C<exec()> or C<system()> is also more secure.
1286 This usage forces interpretation of the arguments as a multivalued list,
1287 even if the list had just one argument. That way you're safe from the
1288 shell expanding wildcards or splitting up words with whitespace in them.
1290 @args = ( "echo surprise" );
1292 system @args; # subject to shell escapes
1294 system { $args[0] } @args; # safe even with one-arg list
1296 The first version, the one without the indirect object, ran the I<echo>
1297 program, passing it C<"surprise"> an argument. The second version
1298 didn't--it tried to run a program literally called I<"echo surprise">,
1299 didn't find it, and set C<$?> to a non-zero value indicating failure.
1301 Note that C<exec()> will not call your C<END> blocks, nor will it call
1302 any C<DESTROY> methods in your objects.
1306 Returns TRUE if the specified hash key exists in its hash array, even
1307 if the corresponding value is undefined.
1309 print "Exists\n" if exists $array{$key};
1310 print "Defined\n" if defined $array{$key};
1311 print "True\n" if $array{$key};
1313 A hash element can be TRUE only if it's defined, and defined if
1314 it exists, but the reverse doesn't necessarily hold true.
1316 Note that the EXPR can be arbitrarily complicated as long as the final
1317 operation is a hash key lookup:
1319 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1321 Although the last element will not spring into existence just because its
1322 existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1323 C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1324 test for a $key element. This autovivification may be fixed in a later
1329 Evaluates EXPR and exits immediately with that value. (Actually, it
1330 calls any defined C<END> routines first, but the C<END> routines may not
1331 abort the exit. Likewise any object destructors that need to be called
1332 are called before exit.) Example:
1335 exit 0 if $ans =~ /^[Xx]/;
1337 See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1338 universally portable values for EXPR are C<0> for success and C<1> for error;
1339 all other values are subject to unpredictable interpretation depending
1340 on the environment in which the Perl program is running.
1342 You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1343 someone might want to trap whatever error happened. Use C<die()> instead,
1344 which can be trapped by an C<eval()>.
1346 All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1352 Returns I<e> (the natural logarithm base) to the power of EXPR.
1353 If EXPR is omitted, gives C<exp($_)>.
1355 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1357 Implements the fcntl(2) function. You'll probably have to say
1361 first to get the correct constant definitions. Argument processing and
1362 value return works just like C<ioctl()> below.
1366 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1367 or die "can't fcntl F_GETFL: $!";
1369 You don't have to check for C<defined()> on the return from
1370 C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
1371 call into "C<0> but true" in Perl. This string is true in
1372 boolean context and C<0> in numeric context. It is also
1373 exempt from the normal B<-w> warnings on improper numeric
1376 Note that C<fcntl()> will produce a fatal error if used on a machine that
1377 doesn't implement fcntl(2).
1379 =item fileno FILEHANDLE
1381 Returns the file descriptor for a filehandle. This is useful for
1382 constructing bitmaps for C<select()> and low-level POSIX tty-handling
1383 operations. If FILEHANDLE is an expression, the value is taken as
1384 an indirect filehandle, generally its name.
1386 You can use this to find out whether two handles refer to the
1387 same underlying descriptor:
1389 if (fileno(THIS) == fileno(THAT)) {
1390 print "THIS and THAT are dups\n";
1393 =item flock FILEHANDLE,OPERATION
1395 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1396 success, FALSE on failure. Produces a fatal error if used on a machine
1397 that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
1398 is Perl's portable file locking interface, although it locks only entire
1401 On many platforms (including most versions or clones of Unix), locks
1402 established by C<flock()> are B<merely advisory>. Such discretionary locks
1403 are more flexible, but offer fewer guarantees. This means that files
1404 locked with C<flock()> may be modified by programs that do not also use
1405 C<flock()>. Windows NT and OS/2 are among the platforms which
1406 enforce mandatory locking. See your local documentation for details.
1408 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1409 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1410 you can use the symbolic names if import them from the Fcntl module,
1411 either individually, or as a group using the ':flock' tag. LOCK_SH
1412 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1413 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1414 LOCK_EX then C<flock()> will return immediately rather than blocking
1415 waiting for the lock (check the return status to see if you got it).
1417 To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1418 before (un)locking it.
1420 Note that the emulation built with lockf(3) doesn't provide shared
1421 locks, and it requires that FILEHANDLE be open with write intent. These
1422 are the semantics that lockf(3) implements. Most (all?) systems
1423 implement lockf(3) in terms of fcntl(2) locking, though, so the
1424 differing semantics shouldn't bite too many people.
1426 Note also that some versions of C<flock()> cannot lock things over the
1427 network; you would need to use the more system-specific C<fcntl()> for
1428 that. If you like you can force Perl to ignore your system's flock(2)
1429 function, and so provide its own fcntl(2)-based emulation, by passing
1430 the switch C<-Ud_flock> to the F<Configure> program when you configure
1433 Here's a mailbox appender for BSD systems.
1435 use Fcntl ':flock'; # import LOCK_* constants
1438 flock(MBOX,LOCK_EX);
1439 # and, in case someone appended
1440 # while we were waiting...
1445 flock(MBOX,LOCK_UN);
1448 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1449 or die "Can't open mailbox: $!";
1452 print MBOX $msg,"\n\n";
1455 See also L<DB_File> for other flock() examples.
1457 =item for (INITIAL; WHILE; EACH) BLOCK
1459 Do INITIAL, enter BLOCK while EXPR is true, at the end of each round
1460 do EACH. For example:
1462 for ($i = 0, $j = 0; $i < 10; $i++) {
1463 if ($i % 3 == 0) { $j++ }
1464 print "i = $i, j = $j\n";
1467 See L<perlsyn> for more details. See also L</foreach>, a twin of
1468 C<for>, L</while> and L</until>, close cousins of L<for>, and
1469 L</last>, L</next>, and L</redo> for additional control flow.
1471 =item foreach LOOPVAR (LIST) BLOCK
1473 Enter BLOCK as LOOPVAR set in turn to each element of LIST.
1476 foreach $rolling (@stones) { print "rolling $stone\n" }
1478 foreach my $file (@files) { print "file $file\n" }
1480 The LOOPVAR is optional and defaults to C<$_>. If the elements are
1481 modifiable (as opposed to constants or tied variables) you can modify them.
1483 foreach (@words) { tr/abc/xyz/ }
1485 See L<perlsyn> for more details. See also L</for>, a twin of
1486 C<foreach>, L</while> and L</until>, close cousins of L<for>, and
1487 L</last>, L</next>, and L</redo> for additional control flow.
1491 Does a fork(2) system call. Returns the child pid to the parent process,
1492 C<0> to the child process, or C<undef> if the fork is unsuccessful.
1494 Note: unflushed buffers remain unflushed in both processes, which means
1495 you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1496 method of C<IO::Handle> to avoid duplicate output.
1498 If you C<fork()> without ever waiting on your children, you will accumulate
1501 $SIG{CHLD} = sub { wait };
1503 There's also the double-fork trick (error checking on
1504 C<fork()> returns omitted);
1506 unless ($pid = fork) {
1508 exec "what you really wanna do";
1511 ## (some_perl_code_here)
1518 See also L<perlipc> for more examples of forking and reaping
1521 Note that if your forked child inherits system file descriptors like
1522 STDIN and STDOUT that are actually connected by a pipe or socket, even
1523 if you exit, then the remote server (such as, say, httpd or rsh) won't think
1524 you're done. You should reopen those to F</dev/null> if it's any issue.
1528 Declare a picture format for use by the C<write()> function. For
1532 Test: @<<<<<<<< @||||| @>>>>>
1533 $str, $%, '$' . int($num)
1537 $num = $cost/$quantity;
1541 See L<perlform> for many details and examples.
1543 =item formline PICTURE,LIST
1545 This is an internal function used by C<format>s, though you may call it,
1546 too. It formats (see L<perlform>) a list of values according to the
1547 contents of PICTURE, placing the output into the format output
1548 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1549 Eventually, when a C<write()> is done, the contents of
1550 C<$^A> are written to some filehandle, but you could also read C<$^A>
1551 yourself and then set C<$^A> back to C<"">. Note that a format typically
1552 does one C<formline()> per line of form, but the C<formline()> function itself
1553 doesn't care how many newlines are embedded in the PICTURE. This means
1554 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1555 You may therefore need to use multiple formlines to implement a single
1556 record format, just like the format compiler.
1558 Be careful if you put double quotes around the picture, because an "C<@>"
1559 character may be taken to mean the beginning of an array name.
1560 C<formline()> always returns TRUE. See L<perlform> for other examples.
1562 =item getc FILEHANDLE
1566 Returns the next character from the input file attached to FILEHANDLE,
1567 or the undefined value at end of file, or if there was an error. If
1568 FILEHANDLE is omitted, reads from STDIN. This is not particularly
1569 efficient. It cannot be used to get unbuffered single-characters,
1570 however. For that, try something more like:
1573 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1576 system "stty", '-icanon', 'eol', "\001";
1582 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1585 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1589 Determination of whether $BSD_STYLE should be set
1590 is left as an exercise to the reader.
1592 The C<POSIX::getattr()> function can do this more portably on systems
1593 purporting POSIX compliance.
1594 See also the C<Term::ReadKey> module from your nearest CPAN site;
1595 details on CPAN can be found on L<perlmod/CPAN>.
1599 Implements the C library function of the same name, which on most
1600 systems returns the current login from F</etc/utmp>, if any. If null,
1603 $login = getlogin || getpwuid($<) || "Kilroy";
1605 Do not consider C<getlogin()> for authentication: it is not as
1606 secure as C<getpwuid()>.
1608 =item getpeername SOCKET
1610 Returns the packed sockaddr address of other end of the SOCKET connection.
1613 $hersockaddr = getpeername(SOCK);
1614 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1615 $herhostname = gethostbyaddr($iaddr, AF_INET);
1616 $herstraddr = inet_ntoa($iaddr);
1620 Returns the current process group for the specified PID. Use
1621 a PID of C<0> to get the current process group for the
1622 current process. Will raise an exception if used on a machine that
1623 doesn't implement getpgrp(2). If PID is omitted, returns process
1624 group of current process. Note that the POSIX version of C<getpgrp()>
1625 does not accept a PID argument, so only C<PID==0> is truly portable.
1629 Returns the process id of the parent process.
1631 =item getpriority WHICH,WHO
1633 Returns the current priority for a process, a process group, or a user.
1634 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1635 machine that doesn't implement getpriority(2).
1641 =item gethostbyname NAME
1643 =item getnetbyname NAME
1645 =item getprotobyname NAME
1651 =item getservbyname NAME,PROTO
1653 =item gethostbyaddr ADDR,ADDRTYPE
1655 =item getnetbyaddr ADDR,ADDRTYPE
1657 =item getprotobynumber NUMBER
1659 =item getservbyport PORT,PROTO
1677 =item sethostent STAYOPEN
1679 =item setnetent STAYOPEN
1681 =item setprotoent STAYOPEN
1683 =item setservent STAYOPEN
1697 These routines perform the same functions as their counterparts in the
1698 system library. In list context, the return values from the
1699 various get routines are as follows:
1701 ($name,$passwd,$uid,$gid,
1702 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1703 ($name,$passwd,$gid,$members) = getgr*
1704 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1705 ($name,$aliases,$addrtype,$net) = getnet*
1706 ($name,$aliases,$proto) = getproto*
1707 ($name,$aliases,$port,$proto) = getserv*
1709 (If the entry doesn't exist you get a null list.)
1711 In scalar context, you get the name, unless the function was a
1712 lookup by name, in which case you get the other thing, whatever it is.
1713 (If the entry doesn't exist you get the undefined value.) For example:
1715 $uid = getpwnam($name);
1716 $name = getpwuid($num);
1718 $gid = getgrnam($name);
1719 $name = getgrgid($num;
1723 In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
1724 cases in the sense that in many systems they are unsupported. If the
1725 C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1726 usually encodes the disk quota. If the C<$comment> field is unsupported,
1727 it is an empty scalar. If it is supported it usually encodes some
1728 administrative comment about the user. In some systems the $quota
1729 field may be C<$change> or C<$age>, fields that have to do with password
1730 aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
1731 field, if present, encodes the expiration period of the account or the
1732 password. For the availability and the exact meaning of these fields
1733 in your system, please consult your getpwnam(3) documentation and your
1734 F<pwd.h> file. You can also find out from within Perl which meaning
1735 your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1736 field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1737 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
1739 The C<$members> value returned by I<getgr*()> is a space separated list of
1740 the login names of the members of the group.
1742 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1743 C, it will be returned to you via C<$?> if the function call fails. The
1744 C<@addrs> value returned by a successful call is a list of the raw
1745 addresses returned by the corresponding system library call. In the
1746 Internet domain, each address is four bytes long and you can unpack it
1747 by saying something like:
1749 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1751 If you get tired of remembering which element of the return list contains
1752 which return value, by-name interfaces are also provided in modules:
1753 C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1754 C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
1755 normal built-in, replacing them with versions that return objects with
1756 the appropriate names for each field. For example:
1760 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1762 Even though it looks like they're the same method calls (uid),
1763 they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
1765 =item getsockname SOCKET
1767 Returns the packed sockaddr address of this end of the SOCKET connection.
1770 $mysockaddr = getsockname(SOCK);
1771 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1773 =item getsockopt SOCKET,LEVEL,OPTNAME
1775 Returns the socket option requested, or undef if there is an error.
1781 Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
1782 do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1783 operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
1784 The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1785 L<perlop/"I/O Operators">.
1789 Converts a time as returned by the time function to a 9-element array
1790 with the time localized for the standard Greenwich time zone.
1791 Typically used as follows:
1794 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1797 All array elements are numeric, and come straight out of a struct tm.
1798 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1799 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1800 years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
1802 If EXPR is omitted, does C<gmtime(time())>.
1804 In scalar context, returns the ctime(3) value:
1806 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1808 Also see the C<timegm()> function provided by the C<Time::Local> module,
1809 and the strftime(3) function available via the POSIX module.
1811 This scalar value is B<not> locale dependent, see L<perllocale>, but
1812 instead a Perl builtin. Also see the C<Time::Local> module, and the
1813 strftime(3) and mktime(3) function available via the POSIX module. To
1814 get somewhat similar but locale dependent date strings, set up your
1815 locale environment variables appropriately (please see L<perllocale>)
1816 and try for example:
1818 use POSIX qw(strftime);
1819 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1821 Note that the C<%a> and C<%b>, the short forms of the day of the week
1822 and the month of the year, may not necessarily be three characters wide.
1830 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1831 execution there. It may not be used to go into any construct that
1832 requires initialization, such as a subroutine or a C<foreach> loop. It
1833 also can't be used to go into a construct that is optimized away,
1834 or to get out of a block or subroutine given to C<sort()>.
1835 It can be used to go almost anywhere else within the dynamic scope,
1836 including out of subroutines, but it's usually better to use some other
1837 construct such as C<last> or C<die()>. The author of Perl has never felt the
1838 need to use this form of C<goto> (in Perl, that is--C is another matter).
1840 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1841 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1842 necessarily recommended if you're optimizing for maintainability:
1844 goto ("FOO", "BAR", "GLARCH")[$i];
1846 The C<goto-&NAME> form is highly magical, and substitutes a call to the
1847 named subroutine for the currently running subroutine. This is used by
1848 C<AUTOLOAD> subroutines that wish to load another subroutine and then
1849 pretend that the other subroutine had been called in the first place
1850 (except that any modifications to C<@_> in the current subroutine are
1851 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
1852 will be able to tell that this routine was called first.
1854 =item grep BLOCK LIST
1856 =item grep EXPR,LIST
1858 This is similar in spirit to, but not the same as, grep(1)
1859 and its relatives. In particular, it is not limited to using
1860 regular expressions.
1862 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1863 C<$_> to each element) and returns the list value consisting of those
1864 elements for which the expression evaluated to TRUE. In a scalar
1865 context, returns the number of times the expression was TRUE.
1867 @foo = grep(!/^#/, @bar); # weed out comments
1871 @foo = grep {!/^#/} @bar; # weed out comments
1873 Note that, because C<$_> is a reference into the list value, it can be used
1874 to modify the elements of the array. While this is useful and
1875 supported, it can cause bizarre results if the LIST is not a named
1876 array. Similarly, grep returns aliases into the original list,
1877 much like the way that a for loop's index variable aliases the list
1878 elements. That is, modifying an element of a list returned by grep
1879 (for example, in a C<foreach>, C<map()> or another C<grep()>)
1880 actually modifies the element in the original list.
1882 See also L</map> for an array composed of the results of the BLOCK or EXPR.
1888 Interprets EXPR as a hex string and returns the corresponding
1889 value. (To convert strings that might start with either 0 or 0x
1890 see L</oct>.) If EXPR is omitted, uses C<$_>.
1892 print hex '0xAf'; # prints '175'
1893 print hex 'aF'; # same
1895 =item if (EXPR) BLOCK
1897 =item if (EXPR) BLOCK else BLOCK2
1899 =item if (EXPR) BLOCK elsif (EXPR2) BLOCK2
1901 Enter BLOCKs conditionally. The first EXPR to return true
1902 causes the corresponding BLOCK to be entered, or, in the case
1903 of C<else>, the fall-through default BLOCK.
1905 Take notice: Perl wants BLOCKS, expressions (like e.g. in C, C++, or
1908 See L<perlsyn> for more details. See also C<unless>.
1912 There is no builtin C<import()> function. It is just an ordinary
1913 method (subroutine) defined (or inherited) by modules that wish to export
1914 names to another module. The C<use()> function calls the C<import()> method
1915 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1917 =item index STR,SUBSTR,POSITION
1919 =item index STR,SUBSTR
1921 Returns the position of the first occurrence of SUBSTR in STR at or after
1922 POSITION. If POSITION is omitted, starts searching from the beginning of
1923 the string. The return value is based at C<0> (or whatever you've set the C<$[>
1924 variable to--but don't do that). If the substring is not found, returns
1925 one less than the base, ordinarily C<-1>.
1931 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
1932 You should not use this for rounding, because it truncates
1933 towards C<0>, and because machine representations of floating point
1934 numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1935 or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
1937 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1939 Implements the ioctl(2) function. You'll probably have to say
1941 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1943 first to get the correct function definitions. If F<ioctl.ph> doesn't
1944 exist or doesn't have the correct definitions you'll have to roll your
1945 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1946 (There is a Perl script called B<h2ph> that comes with the Perl kit that
1947 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1948 written depending on the FUNCTION--a pointer to the string value of SCALAR
1949 will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
1950 has no string value but does have a numeric value, that value will be
1951 passed rather than a pointer to the string value. To guarantee this to be
1952 TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
1953 functions are useful for manipulating the values of structures used by
1954 C<ioctl()>. The following example sets the erase character to DEL.
1958 die "NO TIOCGETP" if $@ || !$getp;
1959 $sgttyb_t = "ccccs"; # 4 chars and a short
1960 if (ioctl(STDIN,$getp,$sgttyb)) {
1961 @ary = unpack($sgttyb_t,$sgttyb);
1963 $sgttyb = pack($sgttyb_t,@ary);
1964 ioctl(STDIN,&TIOCSETP,$sgttyb)
1965 || die "Can't ioctl: $!";
1968 The return value of C<ioctl()> (and C<fcntl()>) is as follows:
1970 if OS returns: then Perl returns:
1972 0 string "0 but true"
1973 anything else that number
1975 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1976 still easily determine the actual value returned by the operating
1979 ($retval = ioctl(...)) || ($retval = -1);
1980 printf "System returned %d\n", $retval;
1982 The special string "C<0> but true" is excempt from B<-w> complaints
1983 about improper numeric conversions.
1985 =item join EXPR,LIST
1987 Joins the separate strings of LIST into a single string with
1988 fields separated by the value of EXPR, and returns the string.
1991 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1997 Returns a list consisting of all the keys of the named hash. (In a
1998 scalar context, returns the number of keys.) The keys are returned in
1999 an apparently random order. The actual random order is subject to
2000 change in future versions of perl, but it is guaranteed to be the same
2001 order as either the C<values()> or C<each()> function produces (given
2002 that the hash has not been modified). As a side effect, it resets
2005 Here is yet another way to print your environment:
2008 @values = values %ENV;
2009 while ($#keys >= 0) {
2010 print pop(@keys), '=', pop(@values), "\n";
2013 or how about sorted by key:
2015 foreach $key (sort(keys %ENV)) {
2016 print $key, '=', $ENV{$key}, "\n";
2019 To sort a hash by value, you'll need to use a C<sort()> function.
2020 Here's a descending numeric sort of a hash by its values:
2022 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2023 printf "%4d %s\n", $hash{$key}, $key;
2026 As an lvalue C<keys()> allows you to increase the number of hash buckets
2027 allocated for the given hash. This can gain you a measure of efficiency if
2028 you know the hash is going to get big. (This is similar to pre-extending
2029 an array by assigning a larger number to $#array.) If you say
2033 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2034 in fact, since it rounds up to the next power of two. These
2035 buckets will be retained even if you do C<%hash = ()>, use C<undef
2036 %hash> if you want to free the storage while C<%hash> is still in scope.
2037 You can't shrink the number of buckets allocated for the hash using
2038 C<keys()> in this way (but you needn't worry about doing this by accident,
2039 as trying has no effect).
2041 See also C<each()>, C<values()> and C<sort()>.
2045 Sends a signal to a list of processes. The first element of
2046 the list must be the signal to send. Returns the number of
2047 processes successfully signaled.
2049 $cnt = kill 1, $child1, $child2;
2052 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
2053 process groups instead of processes. (On System V, a negative I<PROCESS>
2054 number will also kill process groups, but that's not portable.) That
2055 means you usually want to use positive not negative signals. You may also
2056 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2062 The C<last> command is like the C<break> statement in C (as used in
2063 loops); it immediately exits the loop in question. If the LABEL is
2064 omitted, the command refers to the innermost enclosing loop. The
2065 C<continue> block, if any, is not executed:
2067 LINE: while (<STDIN>) {
2068 last LINE if /^$/; # exit when done with header
2072 C<last> cannot be used to exit a block which returns a value such as
2073 C<eval {}>, C<sub {}> or C<do {}>.
2075 See also L</continue> for an illustration of how C<last>, L</next>, and
2078 See also L<perlsyn>.
2084 Returns an lowercased version of EXPR. This is the internal function
2085 implementing the C<\L> escape in double-quoted strings.
2086 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2088 If EXPR is omitted, uses C<$_>.
2094 Returns the value of EXPR with the first character lowercased. This is
2095 the internal function implementing the C<\l> escape in double-quoted strings.
2096 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2098 If EXPR is omitted, uses C<$_>.
2104 Returns the length in characters of the value of EXPR. If EXPR is
2105 omitted, returns length of C<$_>.
2107 =item link OLDFILE,NEWFILE
2109 Creates a new filename linked to the old filename. Returns TRUE for
2110 success, FALSE otherwise.
2112 =item listen SOCKET,QUEUESIZE
2114 Does the same thing that the listen system call does. Returns TRUE if
2115 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
2119 A local modifies the listed variables to be local to the enclosing
2120 block, file, or eval. If more than one value is listed, the list must
2121 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2122 for details, including issues with tied arrays and hashes.
2124 You really probably want to be using C<my()> instead, because C<local()> isn't
2125 what most people think of as "local". See L<perlsub/"Private Variables
2126 via my()"> for details.
2128 =item localtime EXPR
2130 Converts a time as returned by the time function to a 9-element array
2131 with the time analyzed for the local time zone. Typically used as
2135 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2138 All array elements are numeric, and come straight out of a struct tm.
2139 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
2140 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
2141 years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year.
2143 If EXPR is omitted, uses the current time (C<localtime(time)>).
2145 In scalar context, returns the ctime(3) value:
2147 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2149 This scalar value is B<not> locale dependent, see L<perllocale>, but
2150 instead a Perl builtin. Also see the C<Time::Local> module, and the
2151 strftime(3) and mktime(3) function available via the POSIX module. To
2152 get somewhat similar but locale dependent date strings, set up your
2153 locale environment variables appropriately (please see L<perllocale>)
2154 and try for example:
2156 use POSIX qw(strftime);
2157 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2159 Note that the C<%a> and C<%b>, the short forms of the day of the week
2160 and the month of the year, may not necessarily be three characters wide.
2166 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
2169 =item lstat FILEHANDLE
2175 Does the same thing as the C<stat()> function (including setting the
2176 special C<_> filehandle) but stats a symbolic link instead of the file
2177 the symbolic link points to. If symbolic links are unimplemented on
2178 your system, a normal C<stat()> is done.
2180 If EXPR is omitted, stats C<$_>.
2184 The match operator. See L<perlop>.
2186 =item map BLOCK LIST
2190 Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
2191 element) and returns the list value composed of the results of each such
2192 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2193 may produce zero, one, or more elements in the returned value.
2195 @chars = map(chr, @nums);
2197 translates a list of numbers to the corresponding characters. And
2199 %hash = map { getkey($_) => $_ } @array;
2201 is just a funny way to write
2204 foreach $_ (@array) {
2205 $hash{getkey($_)} = $_;
2208 Note that, because C<$_> is a reference into the list value, it can be used
2209 to modify the elements of the array. While this is useful and
2210 supported, it can cause bizarre results if the LIST is not a named
2211 array. See also L</grep> for an array composed of those items of the
2212 original list for which the BLOCK or EXPR evaluates to true.
2214 =item mkdir FILENAME,MODE
2216 Creates the directory specified by FILENAME, with permissions
2217 specified by MODE (as modified by C<umask>). If it succeeds it
2218 returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
2220 In general, it is better to create directories with permissive MODEs,
2221 and let the user modify that with their C<umask>, than it is to supply
2222 a restrictive MODE and give the user no way to be more permissive.
2223 The exceptions to this rule are when the file or directory should be
2224 kept private (mail files, for instance). The perlfunc(1) entry on
2225 C<umask> discusses the choice of MODE in more detail.
2227 =item msgctl ID,CMD,ARG
2229 Calls the System V IPC function msgctl(2). You'll probably have to say
2233 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2234 then ARG must be a variable which will hold the returned C<msqid_ds>
2235 structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
2236 true" for zero, or the actual return value otherwise. See also
2237 C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
2239 =item msgget KEY,FLAGS
2241 Calls the System V IPC function msgget(2). Returns the message queue
2242 id, or the undefined value if there is an error. See also C<IPC::SysV>
2243 and C<IPC::SysV::Msg> documentation.
2245 =item msgsnd ID,MSG,FLAGS
2247 Calls the System V IPC function msgsnd to send the message MSG to the
2248 message queue ID. MSG must begin with the long integer message type,
2249 which may be created with C<pack("l", $type)>. Returns TRUE if
2250 successful, or FALSE if there is an error. See also C<IPC::SysV>
2251 and C<IPC::SysV::Msg> documentation.
2253 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2255 Calls the System V IPC function msgrcv to receive a message from
2256 message queue ID into variable VAR with a maximum message size of
2257 SIZE. Note that if a message is received, the message type will be
2258 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2259 size of the message type. Returns TRUE if successful, or FALSE if
2260 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2264 A C<my()> declares the listed variables to be local (lexically) to the
2265 enclosing block, file, or C<eval()>. If
2266 more than one value is listed, the list must be placed in parentheses. See
2267 L<perlsub/"Private Variables via my()"> for details.
2273 The C<next> command is like the C<continue> statement in C; it starts
2274 the next iteration of the loop:
2276 LINE: while (<STDIN>) {
2277 next LINE if /^#/; # discard comments
2281 Note that if there were a C<continue> block on the above, it would get
2282 executed even on discarded lines. If the LABEL is omitted, the command
2283 refers to the innermost enclosing loop.
2285 C<next> cannot be used to exit a block which returns a value such as
2286 C<eval {}>, C<sub {}> or C<do {}>.
2288 See also L</continue> for an illustration of how L</last>, C<next>, and
2291 See also L<perlsyn>.
2293 =item no Module LIST
2295 See the L</use> function, which C<no> is the opposite of.
2301 Interprets EXPR as an octal string and returns the corresponding
2302 value. (If EXPR happens to start off with C<0x>, interprets it as
2303 a hex string instead.) The following will handle decimal, octal, and
2304 hex in the standard Perl or C notation:
2306 $val = oct($val) if $val =~ /^0/;
2308 If EXPR is omitted, uses C<$_>. This function is commonly used when
2309 a string such as C<644> needs to be converted into a file mode, for
2310 example. (Although perl will automatically convert strings into
2311 numbers as needed, this automatic conversion assumes base 10.)
2313 =item open FILEHANDLE,EXPR
2315 =item open FILEHANDLE
2317 Opens the file whose filename is given by EXPR, and associates it with
2318 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2319 name of the real filehandle wanted. If EXPR is omitted, the scalar
2320 variable of the same name as the FILEHANDLE contains the filename.
2321 (Note that lexical variables--those declared with C<my()>--will not work
2322 for this purpose; so if you're using C<my()>, specify EXPR in your call
2325 If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2326 If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2327 output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
2328 the file is opened for appending, again being created if necessary.
2329 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2330 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2331 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2332 file first. You can't usually use either read-write mode for updating
2333 textfiles, since they have variable length records. See the B<-i>
2334 switch in L<perlrun> for a better approach. The file is created with
2335 permissions of C<0666> modified by the process' C<umask> value.
2337 The prefix and the filename may be separated with spaces.
2338 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2339 C<'w+'>, C<'a'>, and C<'a+'>.
2341 If the filename begins with C<'|'>, the filename is interpreted as a
2342 command to which output is to be piped, and if the filename ends with a
2343 C<'|'>, the filename is interpreted See L<perlipc/"Using open() for IPC">
2344 for more examples of this. (You are not allowed to C<open()> to a command
2345 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2346 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2348 Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
2349 nonzero upon success, the undefined value otherwise. If the C<open()>
2350 involved a pipe, the return value happens to be the pid of the
2353 If you're unfortunate enough to be running Perl on a system that
2354 distinguishes between text files and binary files (modern operating
2355 systems don't care), then you should check out L</binmode> for tips for
2356 dealing with this. The key distinction between systems that need C<binmode()>
2357 and those that don't is their text file formats. Systems like Unix, MacOS, and
2358 Plan9, which delimit lines with a single character, and which encode that
2359 character in C as C<"\n">, do not need C<binmode()>. The rest need it.
2361 When opening a file, it's usually a bad idea to continue normal execution
2362 if the request failed, so C<open()> is frequently used in connection with
2363 C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
2364 where you want to make a nicely formatted error message (but there are
2365 modules that can help with that problem)) you should always check
2366 the return value from opening a file. The infrequent exception is when
2367 working with an unopened filehandle is actually what you want to do.
2372 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2373 while (<ARTICLE>) {...
2375 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2376 # if the open fails, output is discarded
2378 open(DBASE, '+<dbase.mine') # open for update
2379 or die "Can't open 'dbase.mine' for update: $!";
2381 open(ARTICLE, "caesar <$article |") # decrypt article
2382 or die "Can't start caesar: $!";
2384 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2385 or die "Can't start sort: $!";
2387 # process argument list of files along with any includes
2389 foreach $file (@ARGV) {
2390 process($file, 'fh00');
2394 my($filename, $input) = @_;
2395 $input++; # this is a string increment
2396 unless (open($input, $filename)) {
2397 print STDERR "Can't open $filename: $!\n";
2402 while (<$input>) { # note use of indirection
2403 if (/^#include "(.*)"/) {
2404 process($1, $input);
2411 You may also, in the Bourne shell tradition, specify an EXPR beginning
2412 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2413 name of a filehandle (or file descriptor, if numeric) to be
2414 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2415 C<+E<gt>E<gt>>, and C<+E<lt>>. The
2416 mode you specify should match the mode of the original filehandle.
2417 (Duping a filehandle does not take into account any existing contents of
2419 Here is a script that saves, redirects, and restores STDOUT and
2423 open(OLDOUT, ">&STDOUT");
2424 open(OLDERR, ">&STDERR");
2426 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2427 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2429 select(STDERR); $| = 1; # make unbuffered
2430 select(STDOUT); $| = 1; # make unbuffered
2432 print STDOUT "stdout 1\n"; # this works for
2433 print STDERR "stderr 1\n"; # subprocesses too
2438 open(STDOUT, ">&OLDOUT");
2439 open(STDERR, ">&OLDERR");
2441 print STDOUT "stdout 2\n";
2442 print STDERR "stderr 2\n";
2445 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2446 equivalent of C's C<fdopen()> of that file descriptor; this is more
2447 parsimonious of file descriptors. For example:
2449 open(FILEHANDLE, "<&=$fd")
2451 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
2452 there is an implicit fork done, and the return value of open is the pid
2453 of the child within the parent process, and C<0> within the child
2454 process. (Use C<defined($pid)> to determine whether the open was successful.)
2455 The filehandle behaves normally for the parent, but i/o to that
2456 filehandle is piped from/to the STDOUT/STDIN of the child process.
2457 In the child process the filehandle isn't opened--i/o happens from/to
2458 the new STDOUT or STDIN. Typically this is used like the normal
2459 piped open when you want to exercise more control over just how the
2460 pipe command gets executed, such as when you are running setuid, and
2461 don't want to have to scan shell commands for metacharacters.
2462 The following pairs are more or less equivalent:
2464 open(FOO, "|tr '[a-z]' '[A-Z]'");
2465 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2467 open(FOO, "cat -n '$file'|");
2468 open(FOO, "-|") || exec 'cat', '-n', $file;
2470 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2472 NOTE: On any operation that may do a fork, any unflushed buffers remain
2473 unflushed in both processes, which means you may need to set C<$|> to
2474 avoid duplicate output. On systems that support a close-on-exec flag on
2475 files, the flag will be set for the newly opened file descriptor as
2476 determined by the value of $^F. See L<perlvar/$^F>.
2478 Closing any piped filehandle causes the parent process to wait for the
2479 child to finish, and returns the status value in C<$?>.
2481 The filename passed to open will have leading and trailing
2482 whitespace deleted, and the normal redirection characters
2483 honored. This property, known as "magic open",
2484 can often be used to good effect. A user could specify a filename of
2485 F<"rsh cat file |">, or you could change certain filenames as needed:
2487 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2488 open(FH, $filename) or die "Can't open $filename: $!";
2490 However, to open a file with arbitrary weird characters in it, it's
2491 necessary to protect any leading and trailing whitespace:
2493 $file =~ s#^(\s)#./$1#;
2494 open(FOO, "< $file\0");
2496 If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2497 should use the C<sysopen()> function, which involves no such magic. This is
2498 another way to protect your filenames from interpretation. For example:
2501 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2502 or die "sysopen $path: $!";
2503 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2504 print HANDLE "stuff $$\n");
2506 print "File contains: ", <HANDLE>;
2508 Using the constructor from the C<IO::Handle> package (or one of its
2509 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2510 filehandles that have the scope of whatever variables hold references to
2511 them, and automatically close whenever and however you leave that scope:
2515 sub read_myfile_munged {
2517 my $handle = new IO::File;
2518 open($handle, "myfile") or die "myfile: $!";
2520 or return (); # Automatically closed here.
2521 mung $first or die "mung failed"; # Or here.
2522 return $first, <$handle> if $ALL; # Or here.
2526 See L</seek()> for some details about mixing reading and writing.
2528 =item opendir DIRHANDLE,EXPR
2530 Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2531 C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
2532 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2538 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2539 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2541 =item pack TEMPLATE,LIST
2543 Takes an array or list of values and packs it into a binary structure,
2544 returning the string containing the structure. The TEMPLATE is a
2545 sequence of characters that give the order and type of values, as
2548 A An ascii string, will be space padded.
2549 a An ascii string, will be null padded.
2550 b A bit string (ascending bit order, like vec()).
2551 B A bit string (descending bit order).
2552 h A hex string (low nybble first).
2553 H A hex string (high nybble first).
2555 c A signed char value.
2556 C An unsigned char value. Only does bytes. See U for Unicode.
2558 s A signed short value.
2559 S An unsigned short value.
2560 (This 'short' is _exactly_ 16 bits, which may differ from
2561 what a local C compiler calls 'short'.)
2563 i A signed integer value.
2564 I An unsigned integer value.
2565 (This 'integer' is _at_least_ 32 bits wide. Its exact
2566 size depends on what a local C compiler calls 'int',
2567 and may even be larger than the 'long' described in
2570 l A signed long value.
2571 L An unsigned long value.
2572 (This 'long' is _exactly_ 32 bits, which may differ from
2573 what a local C compiler calls 'long'.)
2575 n A short in "network" (big-endian) order.
2576 N A long in "network" (big-endian) order.
2577 v A short in "VAX" (little-endian) order.
2578 V A long in "VAX" (little-endian) order.
2579 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2580 _exactly_ 32 bits, respectively.)
2582 q A signed quad (64-bit) value.
2583 Q An unsigned quad value.
2584 (Available only if your system supports 64-bit integer values
2585 _and_ if Perl has been compiled to support those.
2586 Causes a fatal error otherwise.)
2588 f A single-precision float in the native format.
2589 d A double-precision float in the native format.
2591 p A pointer to a null-terminated string.
2592 P A pointer to a structure (fixed-length string).
2594 u A uuencoded string.
2595 U A Unicode character number. Encodes to UTF-8 internally.
2596 Works even if C<use utf8> is not in effect.
2598 w A BER compressed integer. Its bytes represent an unsigned
2599 integer in base 128, most significant digit first, with as
2600 few digits as possible. Bit eight (the high bit) is set
2601 on each byte except the last.
2605 @ Null fill to absolute position.
2607 Each letter may optionally be followed by a number giving a repeat
2608 count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the
2609 pack function will gobble up that many values from the LIST. A C<*> for the
2610 repeat count means to use however many items are left. The C<"a"> and C<"A">
2611 types gobble just one value, but pack it as a string of length count,
2612 padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips
2613 trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B">
2614 fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a
2615 string that many nybbles long. The C<"p"> type packs a pointer to a null-
2616 terminated string. You are responsible for ensuring the string is not a
2617 temporary value (which can potentially get deallocated before you get
2618 around to using the packed result). The C<"P"> packs a pointer to a structure
2619 of the size indicated by the length. A NULL pointer is created if the
2620 corresponding value for C<"p"> or C<"P"> is C<undef>.
2621 Real numbers (floats and doubles) are
2622 in the native machine format only; due to the multiplicity of floating
2623 formats around, and the lack of a standard "network" representation, no
2624 facility for interchange has been made. This means that packed floating
2625 point data written on one machine may not be readable on another - even if
2626 both use IEEE floating point arithmetic (as the endian-ness of the memory
2627 representation is not part of the IEEE spec). Note that Perl uses doubles
2628 internally for all numeric calculation, and converting from double into
2629 float and thence back to double again will lose precision (i.e.,
2630 C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>).
2634 $foo = pack("CCCC",65,66,67,68);
2636 $foo = pack("C4",65,66,67,68);
2638 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2639 # same thing with Unicode circled letters
2641 $foo = pack("ccxxcc",65,66,67,68);
2644 $foo = pack("s2",1,2);
2645 # "\1\0\2\0" on little-endian
2646 # "\0\1\0\2" on big-endian
2648 $foo = pack("a4","abcd","x","y","z");
2651 $foo = pack("aaaa","abcd","x","y","z");
2654 $foo = pack("a14","abcdefg");
2655 # "abcdefg\0\0\0\0\0\0\0"
2657 $foo = pack("i9pl", gmtime);
2658 # a real struct tm (on my system anyway)
2661 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2664 The same template may generally also be used in the unpack function.
2668 =item package NAMESPACE
2670 Declares the compilation unit as being in the given namespace. The scope
2671 of the package declaration is from the declaration itself through the end of
2672 the enclosing block (the same scope as the C<local()> operator). All further
2673 unqualified dynamic identifiers will be in this namespace. A package
2674 statement affects only dynamic variables--including those you've used
2675 C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
2676 would be the first declaration in a file to be included by the C<require>
2677 or C<use> operator. You can switch into a package in more than one place;
2678 it merely influences which symbol table is used by the compiler for the
2679 rest of that block. You can refer to variables and filehandles in other
2680 packages by prefixing the identifier with the package name and a double
2681 colon: C<$Package::Variable>. If the package name is null, the C<main>
2682 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2684 If NAMESPACE is omitted, then there is no current package, and all
2685 identifiers must be fully qualified or lexicals. This is stricter
2686 than C<use strict>, since it also extends to function names.
2688 See L<perlmod/"Packages"> for more information about packages, modules,
2689 and classes. See L<perlsub> for other scoping issues.
2691 =item pipe READHANDLE,WRITEHANDLE
2693 Opens a pair of connected pipes like the corresponding system call.
2694 Note that if you set up a loop of piped processes, deadlock can occur
2695 unless you are very careful. In addition, note that Perl's pipes use
2696 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2697 after each command, depending on the application.
2699 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2700 for examples of such things.
2702 On systems that support a close-on-exec flag on files, the flag will be set
2703 for the newly opened file descriptors as determined by the value of $^F.
2710 Pops and returns the last value of the array, shortening the array by
2711 1. Has a similar effect to
2713 $tmp = $ARRAY[$#ARRAY--];
2715 If there are no elements in the array, returns the undefined value.
2716 If ARRAY is omitted, pops the
2717 C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2724 Returns the offset of where the last C<m//g> search left off for the variable
2725 is in question (C<$_> is used when the variable is not specified). May be
2726 modified to change that offset. Such modification will also influence
2727 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2730 =item print FILEHANDLE LIST
2736 Prints a string or a comma-separated list of strings. Returns TRUE
2737 if successful. FILEHANDLE may be a scalar variable name, in which case
2738 the variable contains the name of or a reference to the filehandle, thus introducing one
2739 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2740 token is a term, it may be misinterpreted as an operator unless you
2741 interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
2742 omitted, prints by default to standard output (or to the last selected
2743 output channel--see L</select>). If LIST is also omitted, prints C<$_> to
2744 the currently selected output channel. To set the default output channel to something other than
2745 STDOUT use the select operation. Note that, because print takes a
2746 LIST, anything in the LIST is evaluated in list context, and any
2747 subroutine that you call will have one or more of its expressions
2748 evaluated in list context. Also be careful not to follow the print
2749 keyword with a left parenthesis unless you want the corresponding right
2750 parenthesis to terminate the arguments to the print--interpose a C<+> or
2751 put parentheses around all the arguments.
2753 Note that if you're storing FILEHANDLES in an array or other expression,
2754 you will have to use a block returning its value instead:
2756 print { $files[$i] } "stuff\n";
2757 print { $OK ? STDOUT : STDERR } "stuff\n";
2759 =item printf FILEHANDLE FORMAT, LIST
2761 =item printf FORMAT, LIST
2763 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
2764 (the output record separator) is not appended. The first argument
2765 of the list will be interpreted as the C<printf()> format. If C<use locale> is
2766 in effect, the character used for the decimal point in formatted real numbers
2767 is affected by the LC_NUMERIC locale. See L<perllocale>.
2769 Don't fall into the trap of using a C<printf()> when a simple
2770 C<print()> would do. The C<print()> is more efficient and less
2773 =item prototype FUNCTION
2775 Returns the prototype of a function as a string (or C<undef> if the
2776 function has no prototype). FUNCTION is a reference to, or the name of,
2777 the function whose prototype you want to retrieve.
2779 If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2780 a name for Perl builtin. If builtin is not I<overridable> (such as
2781 C<qw//>) or its arguments cannot be expressed by a prototype (such as
2782 C<system()>) - in other words, the builtin does not behave like a Perl
2783 function - returns C<undef>. Otherwise, the string describing the
2784 equivalent prototype is returned.
2786 =item push ARRAY,LIST
2788 Treats ARRAY as a stack, and pushes the values of LIST
2789 onto the end of ARRAY. The length of ARRAY increases by the length of
2790 LIST. Has the same effect as
2793 $ARRAY[++$#ARRAY] = $value;
2796 but is more efficient. Returns the new number of elements in the array.
2808 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
2810 =item quotemeta EXPR
2814 Returns the value of EXPR with all non-alphanumeric
2815 characters backslashed. (That is, all characters not matching
2816 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2817 returned string, regardless of any locale settings.)
2818 This is the internal function implementing
2819 the C<\Q> escape in double-quoted strings.
2821 If EXPR is omitted, uses C<$_>.
2827 Returns a random fractional number greater than or equal to C<0> and less
2828 than the value of EXPR. (EXPR should be positive.) If EXPR is
2829 omitted, the value C<1> is used. Automatically calls C<srand()> unless
2830 C<srand()> has already been called. See also C<srand()>.
2832 (Note: If your rand function consistently returns numbers that are too
2833 large or too small, then your version of Perl was probably compiled
2834 with the wrong number of RANDBITS.)
2836 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2838 =item read FILEHANDLE,SCALAR,LENGTH
2840 Attempts to read LENGTH bytes of data into variable SCALAR from the
2841 specified FILEHANDLE. Returns the number of bytes actually read,
2842 C<0> at end of file, or undef if there was an error. SCALAR will be grown
2843 or shrunk to the length actually read. An OFFSET may be specified to
2844 place the read data at some other place than the beginning of the
2845 string. This call is actually implemented in terms of stdio's fread(3)
2846 call. To get a true read(2) system call, see C<sysread()>.
2848 =item readdir DIRHANDLE
2850 Returns the next directory entry for a directory opened by C<opendir()>.
2851 If used in list context, returns all the rest of the entries in the
2852 directory. If there are no more entries, returns an undefined value in
2853 scalar context or a null list in list context.
2855 If you're planning to filetest the return values out of a C<readdir()>, you'd
2856 better prepend the directory in question. Otherwise, because we didn't
2857 C<chdir()> there, it would have been testing the wrong file.
2859 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2860 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2865 Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
2866 is read and returned. In list context, reads until end-of-file is
2867 reached and returns a list of lines (however you've defined lines
2868 with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2869 This is the internal function implementing the C<E<lt>EXPRE<gt>>
2870 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2871 operator is discussed in more detail in L<perlop/"I/O Operators">.
2874 $line = readline(*STDIN); # same thing
2880 Returns the value of a symbolic link, if symbolic links are
2881 implemented. If not, gives a fatal error. If there is some system
2882 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2883 omitted, uses C<$_>.
2887 EXPR is executed as a system command.
2888 The collected standard output of the command is returned.
2889 In scalar context, it comes back as a single (potentially
2890 multi-line) string. In list context, returns a list of lines
2891 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2892 This is the internal function implementing the C<qx/EXPR/>
2893 operator, but you can use it directly. The C<qx/EXPR/>
2894 operator is discussed in more detail in L<perlop/"I/O Operators">.
2896 =item recv SOCKET,SCALAR,LEN,FLAGS
2898 Receives a message on a socket. Attempts to receive LENGTH bytes of
2899 data into variable SCALAR from the specified SOCKET filehandle.
2900 Actually does a C C<recvfrom()>, so that it can return the address of the
2901 sender. Returns the undefined value if there's an error. SCALAR will
2902 be grown or shrunk to the length actually read. Takes the same flags
2903 as the system call of the same name.
2904 See L<perlipc/"UDP: Message Passing"> for examples.
2910 The C<redo> command restarts the loop block without evaluating the
2911 conditional again. The L</continue> block, if any, is not executed. If
2912 the LABEL is omitted, the command refers to the innermost enclosing
2913 loop. This command is normally used by programs that want to lie to
2914 themselves about what was just input:
2916 # a simpleminded Pascal comment stripper
2917 # (warning: assumes no { or } in strings)
2918 LINE: while (<STDIN>) {
2919 while (s|({.*}.*){.*}|$1 |) {}
2924 if (/}/) { # end of comment?
2933 C<redo> cannot be used to retry a block which returns a value such as
2934 C<eval {}>, C<sub {}> or C<do {}>.
2936 See also L</continue> for an illustration of how L</last>, L</next>, and
2939 See also L<perlsyn>.
2945 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2946 is not specified, C<$_> will be used. The value returned depends on the
2947 type of thing the reference is a reference to.
2948 Builtin types include:
2957 If the referenced object has been blessed into a package, then that package
2958 name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
2960 if (ref($r) eq "HASH") {
2961 print "r is a reference to a hash.\n";
2964 print "r is not a reference at all.\n";
2967 See also L<perlref>.
2969 =item rename OLDNAME,NEWNAME
2971 Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
2972 not work across file system boundaries.
2978 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
2979 supplied. If EXPR is numeric, demands that the current version of Perl
2980 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2982 Otherwise, demands that a library file be included if it hasn't already
2983 been included. The file is included via the do-FILE mechanism, which is
2984 essentially just a variety of C<eval()>. Has semantics similar to the following
2989 return 1 if $INC{$filename};
2990 my($realfilename,$result);
2992 foreach $prefix (@INC) {
2993 $realfilename = "$prefix/$filename";
2994 if (-f $realfilename) {
2995 $result = do $realfilename;
2999 die "Can't find $filename in \@INC";
3002 die "$filename did not return true value" unless $result;
3003 $INC{$filename} = $realfilename;
3007 Note that the file will not be included twice under the same specified
3008 name. The file must return TRUE as the last statement to indicate
3009 successful execution of any initialization code, so it's customary to
3010 end such a file with "C<1;>" unless you're sure it'll return TRUE
3011 otherwise. But it's better just to put the "C<1;>", in case you add more
3014 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3015 replaces "F<::>" with "F</>" in the filename for you,
3016 to make it easy to load standard modules. This form of loading of
3017 modules does not risk altering your namespace.
3019 In other words, if you try this:
3021 require Foo::Bar; # a splendid bareword
3023 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3024 directories specified in the C<@INC> array.
3026 But if you try this:
3028 $class = 'Foo::Bar';
3029 require $class; # $class is not a bareword
3031 require "Foo::Bar"; # not a bareword because of the ""
3033 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3034 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3036 eval "require $class";
3038 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3044 Generally used in a C<continue> block at the end of a loop to clear
3045 variables and reset C<??> searches so that they work again. The
3046 expression is interpreted as a list of single characters (hyphens
3047 allowed for ranges). All variables and arrays beginning with one of
3048 those letters are reset to their pristine state. If the expression is
3049 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3050 only variables or searches in the current package. Always returns
3053 reset 'X'; # reset all X variables
3054 reset 'a-z'; # reset lower case variables
3055 reset; # just reset ?? searches
3057 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3058 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
3059 are unaffected, but they clean themselves up on scope exit anyway,
3060 so you'll probably want to use them instead. See L</my>.
3066 Returns from a subroutine, C<eval()>, or C<do FILE> with the value
3067 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3068 context, depending on how the return value will be used, and the context
3069 may vary from one execution to the next (see C<wantarray()>). If no EXPR
3070 is given, returns an empty list in list context, an undefined value in
3071 scalar context, or nothing in a void context.
3073 (Note that in the absence of a return, a subroutine, eval, or do FILE
3074 will automatically return the value of the last expression evaluated.)
3078 In list context, returns a list value consisting of the elements
3079 of LIST in the opposite order. In scalar context, concatenates the
3080 elements of LIST, and returns a string value with all the characters
3081 in the opposite order.
3083 print reverse <>; # line tac, last line first
3085 undef $/; # for efficiency of <>
3086 print scalar reverse <>; # character tac, last line tsrif
3088 This operator is also handy for inverting a hash, although there are some
3089 caveats. If a value is duplicated in the original hash, only one of those
3090 can be represented as a key in the inverted hash. Also, this has to
3091 unwind one hash and build a whole new one, which may take some time
3094 %by_name = reverse %by_address; # Invert the hash
3096 =item rewinddir DIRHANDLE
3098 Sets the current position to the beginning of the directory for the
3099 C<readdir()> routine on DIRHANDLE.
3101 =item rindex STR,SUBSTR,POSITION
3103 =item rindex STR,SUBSTR
3105 Works just like index except that it returns the position of the LAST
3106 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3107 last occurrence at or before that position.
3109 =item rmdir FILENAME
3113 Deletes the directory specified by FILENAME if that directory is empty. If it
3114 succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
3115 FILENAME is omitted, uses C<$_>.
3119 The substitution operator. See L<perlop>.
3123 Forces EXPR to be interpreted in scalar context and returns the value
3126 @counts = ( scalar @a, scalar @b, scalar @c );
3128 There is no equivalent operator to force an expression to
3129 be interpolated in list context because it's in practice never
3130 needed. If you really wanted to do so, however, you could use
3131 the construction C<@{[ (some expression) ]}>, but usually a simple
3132 C<(some expression)> suffices.
3134 =item seek FILEHANDLE,POSITION,WHENCE
3136 Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
3137 FILEHANDLE may be an expression whose value gives the name of the
3138 filehandle. The values for WHENCE are C<0> to set the new position to
3139 POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
3140 set it to EOF plus POSITION (typically negative). For WHENCE you may
3141 use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3142 C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
3144 If you want to position file for C<sysread()> or C<syswrite()>, don't use
3145 C<seek()> -- buffering makes its effect on the file's system position
3146 unpredictable and non-portable. Use C<sysseek()> instead.
3148 On some systems you have to do a seek whenever you switch between reading
3149 and writing. Amongst other things, this may have the effect of calling
3150 stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
3155 This is also useful for applications emulating C<tail -f>. Once you hit
3156 EOF on your read, and then sleep for a while, you might have to stick in a
3157 seek() to reset things. The C<seek()> doesn't change the current position,
3158 but it I<does> clear the end-of-file condition on the handle, so that the
3159 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3161 If that doesn't work (some stdios are particularly cantankerous), then
3162 you may need something more like this:
3165 for ($curpos = tell(FILE); $_ = <FILE>;
3166 $curpos = tell(FILE)) {
3167 # search for some stuff and put it into files
3169 sleep($for_a_while);
3170 seek(FILE, $curpos, 0);
3173 =item seekdir DIRHANDLE,POS
3175 Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3176 must be a value returned by C<telldir()>. Has the same caveats about
3177 possible directory compaction as the corresponding system library
3180 =item select FILEHANDLE
3184 Returns the currently selected filehandle. Sets the current default
3185 filehandle for output, if FILEHANDLE is supplied. This has two
3186 effects: first, a C<write()> or a C<print()> without a filehandle will
3187 default to this FILEHANDLE. Second, references to variables related to
3188 output will refer to this output channel. For example, if you have to
3189 set the top of form format for more than one output channel, you might
3197 FILEHANDLE may be an expression whose value gives the name of the
3198 actual filehandle. Thus:
3200 $oldfh = select(STDERR); $| = 1; select($oldfh);
3202 Some programmers may prefer to think of filehandles as objects with
3203 methods, preferring to write the last example as:
3206 STDERR->autoflush(1);
3208 =item select RBITS,WBITS,EBITS,TIMEOUT
3210 This calls the select(2) system call with the bit masks specified, which
3211 can be constructed using C<fileno()> and C<vec()>, along these lines:
3213 $rin = $win = $ein = '';
3214 vec($rin,fileno(STDIN),1) = 1;
3215 vec($win,fileno(STDOUT),1) = 1;
3218 If you want to select on many filehandles you might wish to write a
3222 my(@fhlist) = split(' ',$_[0]);
3225 vec($bits,fileno($_),1) = 1;
3229 $rin = fhbits('STDIN TTY SOCK');
3233 ($nfound,$timeleft) =
3234 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3236 or to block until something becomes ready just do this
3238 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3240 Most systems do not bother to return anything useful in C<$timeleft>, so
3241 calling select() in scalar context just returns C<$nfound>.
3243 Any of the bit masks can also be undef. The timeout, if specified, is
3244 in seconds, which may be fractional. Note: not all implementations are
3245 capable of returning theC<$timeleft>. If not, they always return
3246 C<$timeleft> equal to the supplied C<$timeout>.
3248 You can effect a sleep of 250 milliseconds this way:
3250 select(undef, undef, undef, 0.25);
3252 B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3253 or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3254 then only on POSIX systems. You have to use C<sysread()> instead.
3256 =item semctl ID,SEMNUM,CMD,ARG
3258 Calls the System V IPC function C<semctl()>. You'll probably have to say
3262 first to get the correct constant definitions. If CMD is IPC_STAT or
3263 GETALL, then ARG must be a variable which will hold the returned
3264 semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
3265 undefined value for error, "C<0> but true" for zero, or the actual return
3266 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3268 =item semget KEY,NSEMS,FLAGS
3270 Calls the System V IPC function semget. Returns the semaphore id, or
3271 the undefined value if there is an error. See also C<IPC::SysV> and
3272 C<IPC::SysV::Semaphore> documentation.
3274 =item semop KEY,OPSTRING
3276 Calls the System V IPC function semop to perform semaphore operations
3277 such as signaling and waiting. OPSTRING must be a packed array of
3278 semop structures. Each semop structure can be generated with
3279 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3280 operations is implied by the length of OPSTRING. Returns TRUE if
3281 successful, or FALSE if there is an error. As an example, the
3282 following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
3284 $semop = pack("sss", $semnum, -1, 0);
3285 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3287 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3288 and C<IPC::SysV::Semaphore> documentation.
3290 =item send SOCKET,MSG,FLAGS,TO
3292 =item send SOCKET,MSG,FLAGS
3294 Sends a message on a socket. Takes the same flags as the system call
3295 of the same name. On unconnected sockets you must specify a
3296 destination to send TO, in which case it does a C C<sendto()>. Returns
3297 the number of characters sent, or the undefined value if there is an
3299 See L<perlipc/"UDP: Message Passing"> for examples.
3301 =item setpgrp PID,PGRP
3303 Sets the current process group for the specified PID, C<0> for the current
3304 process. Will produce a fatal error if used on a machine that doesn't
3305 implement setpgrp(2). If the arguments are omitted, it defaults to
3306 C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3307 arguments, so only setpgrp C<0,0> is portable.
3309 =item setpriority WHICH,WHO,PRIORITY
3311 Sets the current priority for a process, a process group, or a user.
3312 (See setpriority(2).) Will produce a fatal error if used on a machine
3313 that doesn't implement setpriority(2).
3315 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3317 Sets the socket option requested. Returns undefined if there is an
3318 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3325 Shifts the first value of the array off and returns it, shortening the
3326 array by 1 and moving everything down. If there are no elements in the
3327 array, returns the undefined value. If ARRAY is omitted, shifts the
3328 C<@_> array within the lexical scope of subroutines and formats, and the
3329 C<@ARGV> array at file scopes or within the lexical scopes established by
3330 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3331 See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3332 same thing to the left end of an array that C<pop()> and C<push()> do to the
3335 =item shmctl ID,CMD,ARG
3337 Calls the System V IPC function shmctl. You'll probably have to say
3341 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3342 then ARG must be a variable which will hold the returned C<shmid_ds>
3343 structure. Returns like ioctl: the undefined value for error, "C<0> but
3344 true" for zero, or the actual return value otherwise.
3345 See also C<IPC::SysV> documentation.
3347 =item shmget KEY,SIZE,FLAGS
3349 Calls the System V IPC function shmget. Returns the shared memory
3350 segment id, or the undefined value if there is an error.
3351 See also C<IPC::SysV> documentation.
3353 =item shmread ID,VAR,POS,SIZE
3355 =item shmwrite ID,STRING,POS,SIZE
3357 Reads or writes the System V shared memory segment ID starting at
3358 position POS for size SIZE by attaching to it, copying in/out, and
3359 detaching from it. When reading, VAR must be a variable that will
3360 hold the data read. When writing, if STRING is too long, only SIZE
3361 bytes are used; if STRING is too short, nulls are written to fill out
3362 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
3363 See also C<IPC::SysV> documentation.
3365 =item shutdown SOCKET,HOW
3367 Shuts down a socket connection in the manner indicated by HOW, which
3368 has the same interpretation as in the system call of the same name.
3370 shutdown(SOCKET, 0); # I/we have stopped reading data
3371 shutdown(SOCKET, 1); # I/we have stopped writing data
3372 shutdown(SOCKET, 2); # I/we have stopped using this socket
3374 This is useful with sockets when you want to tell the other
3375 side you're done writing but not done reading, or vice versa.
3376 It's also a more insistent form of close because it also
3377 disables the filedescriptor in any forked copies in other
3384 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3385 returns sine of C<$_>.
3387 For the inverse sine operation, you may use the C<POSIX::asin()>
3388 function, or use this relation:
3390 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3396 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3397 May be interrupted if the process receives a signal such as C<SIGALRM>.
3398 Returns the number of seconds actually slept. You probably cannot
3399 mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
3402 On some older systems, it may sleep up to a full second less than what
3403 you requested, depending on how it counts seconds. Most modern systems
3404 always sleep the full amount. They may appear to sleep longer than that,
3405 however, because your process might not be scheduled right away in a
3406 busy multitasking system.
3408 For delays of finer granularity than one second, you may use Perl's
3409 C<syscall()> interface to access setitimer(2) if your system supports it,
3410 or else see L</select()> above.
3412 See also the POSIX module's C<sigpause()> function.
3414 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3416 Opens a socket of the specified kind and attaches it to filehandle
3417 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
3418 system call of the same name. You should "C<use Socket;>" first to get
3419 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
3421 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3423 Creates an unnamed pair of sockets in the specified domain, of the
3424 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3425 for the system call of the same name. If unimplemented, yields a fatal
3426 error. Returns TRUE if successful.
3428 Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call
3429 to C<pipe(Rdr, Wtr)> is essentially:
3432 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3433 shutdown(Rdr, 1); # no more writing for reader
3434 shutdown(Wtr, 0); # no more reading for writer
3436 See L<perlipc> for an example of socketpair use.
3438 =item sort SUBNAME LIST
3440 =item sort BLOCK LIST
3444 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3445 is omitted, C<sort()>s in standard string comparison order. If SUBNAME is
3446 specified, it gives the name of a subroutine that returns an integer
3447 less than, equal to, or greater than C<0>, depending on how the elements
3448 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3449 operators are extremely useful in such routines.) SUBNAME may be a
3450 scalar variable name (unsubscripted), in which case the value provides
3451 the name of (or a reference to) the actual subroutine to use. In place
3452 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3455 In the interests of efficiency the normal calling code for subroutines is
3456 bypassed, with the following effects: the subroutine may not be a
3457 recursive subroutine, and the two elements to be compared are passed into
3458 the subroutine not via C<@_> but as the package global variables C<$a> and
3459 C<$b> (see example below). They are passed by reference, so don't
3460 modify C<$a> and C<$b>. And don't try to declare them as lexicals either.
3462 You also cannot exit out of the sort block or subroutine using any of the
3463 loop control operators described in L<perlsyn> or with C<goto()>.
3465 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3466 current collation locale. See L<perllocale>.
3471 @articles = sort @files;
3473 # same thing, but with explicit sort routine
3474 @articles = sort {$a cmp $b} @files;
3476 # now case-insensitively
3477 @articles = sort {uc($a) cmp uc($b)} @files;
3479 # same thing in reversed order
3480 @articles = sort {$b cmp $a} @files;
3482 # sort numerically ascending
3483 @articles = sort {$a <=> $b} @files;
3485 # sort numerically descending
3486 @articles = sort {$b <=> $a} @files;
3488 # sort using explicit subroutine name
3490 $age{$a} <=> $age{$b}; # presuming numeric
3492 @sortedclass = sort byage @class;
3494 # this sorts the %age hash by value instead of key
3495 # using an in-line function
3496 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3498 sub backwards { $b cmp $a; }
3499 @harry = ('dog','cat','x','Cain','Abel');
3500 @george = ('gone','chased','yz','Punished','Axed');
3502 # prints AbelCaincatdogx
3503 print sort backwards @harry;
3504 # prints xdogcatCainAbel
3505 print sort @george, 'to', @harry;
3506 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3508 # inefficiently sort by descending numeric compare using
3509 # the first integer after the first = sign, or the
3510 # whole record case-insensitively otherwise
3513 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3518 # same thing, but much more efficiently;
3519 # we'll build auxiliary indices instead
3523 push @nums, /=(\d+)/;
3528 $nums[$b] <=> $nums[$a]
3530 $caps[$a] cmp $caps[$b]
3534 # same thing using a Schwartzian Transform (no temps)
3535 @new = map { $_->[0] }
3536 sort { $b->[1] <=> $a->[1]
3539 } map { [$_, /=(\d+)/, uc($_)] } @old;
3541 If you're using strict, you I<MUST NOT> declare C<$a>
3542 and C<$b> as lexicals. They are package globals. That means
3543 if you're in the C<main> package, it's
3545 @articles = sort {$main::b <=> $main::a} @files;
3549 @articles = sort {$::b <=> $::a} @files;
3551 but if you're in the C<FooPack> package, it's
3553 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3555 The comparison function is required to behave. If it returns
3556 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
3557 sometimes saying the opposite, for example) the results are not
3560 =item splice ARRAY,OFFSET,LENGTH,LIST
3562 =item splice ARRAY,OFFSET,LENGTH
3564 =item splice ARRAY,OFFSET
3566 Removes the elements designated by OFFSET and LENGTH from an array, and
3567 replaces them with the elements of LIST, if any. In list context,
3568 returns the elements removed from the array. In scalar context,
3569 returns the last element removed, or C<undef> if no elements are
3570 removed. The array grows or shrinks as necessary.
3571 If OFFSET is negative then it start that far from the end of the array.
3572 If LENGTH is omitted, removes everything from OFFSET onward.
3573 If LENGTH is negative, leave that many elements off the end of the array.
3574 The following equivalences hold (assuming C<$[ == 0>):
3576 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3577 pop(@a) splice(@a,-1)
3578 shift(@a) splice(@a,0,1)
3579 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3580 $a[$x] = $y splice(@a,$x,1,$y)
3582 Example, assuming array lengths are passed before arrays:
3584 sub aeq { # compare two list values
3585 my(@a) = splice(@_,0,shift);
3586 my(@b) = splice(@_,0,shift);
3587 return 0 unless @a == @b; # same len?
3589 return 0 if pop(@a) ne pop(@b);
3593 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3595 =item split /PATTERN/,EXPR,LIMIT
3597 =item split /PATTERN/,EXPR
3599 =item split /PATTERN/
3603 Splits a string into an array of strings, and returns it. By default,
3604 empty leading fields are preserved, and empty trailing ones are deleted.
3606 If not in list context, returns the number of fields found and splits into
3607 the C<@_> array. (In list context, you can force the split into C<@_> by
3608 using C<??> as the pattern delimiters, but it still returns the list
3609 value.) The use of implicit split to C<@_> is deprecated, however, because
3610 it clobbers your subroutine arguments.
3612 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
3613 splits on whitespace (after skipping any leading whitespace). Anything
3614 matching PATTERN is taken to be a delimiter separating the fields. (Note
3615 that the delimiter may be longer than one character.)
3617 If LIMIT is specified and positive, splits into no more than that
3618 many fields (though it may split into fewer). If LIMIT is unspecified
3619 or zero, trailing null fields are stripped (which potential users
3620 of C<pop()> would do well to remember). If LIMIT is negative, it is
3621 treated as if an arbitrarily large LIMIT had been specified.
3623 A pattern matching the null string (not to be confused with
3624 a null pattern C<//>, which is just one member of the set of patterns
3625 matching a null string) will split the value of EXPR into separate
3626 characters at each point it matches that way. For example:
3628 print join(':', split(/ */, 'hi there'));
3630 produces the output 'h:i:t:h:e:r:e'.
3632 The LIMIT parameter can be used to split a line partially
3634 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3636 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3637 one larger than the number of variables in the list, to avoid
3638 unnecessary work. For the list above LIMIT would have been 4 by
3639 default. In time critical applications it behooves you not to split
3640 into more fields than you really need.
3642 If the PATTERN contains parentheses, additional array elements are
3643 created from each matching substring in the delimiter.
3645 split(/([,-])/, "1-10,20", 3);
3647 produces the list value
3649 (1, '-', 10, ',', 20)
3651 If you had the entire header of a normal Unix email message in C<$header>,
3652 you could split it up into fields and their values this way:
3654 $header =~ s/\n\s+/ /g; # fix continuation lines
3655 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3657 The pattern C</PATTERN/> may be replaced with an expression to specify
3658 patterns that vary at runtime. (To do runtime compilation only once,
3659 use C</$variable/o>.)
3661 As a special case, specifying a PATTERN of space (C<' '>) will split on
3662 white space just as C<split()> with no arguments does. Thus, C<split(' ')> can
3663 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3664 will give you as many null initial fields as there are leading spaces.
3665 A C<split()> on C</\s+/> is like a C<split(' ')> except that any leading
3666 whitespace produces a null first field. A C<split()> with no arguments
3667 really does a C<split(' ', $_)> internally.
3671 open(PASSWD, '/etc/passwd');
3673 ($login, $passwd, $uid, $gid,
3674 $gcos, $home, $shell) = split(/:/);
3678 (Note that C<$shell> above will still have a newline on it. See L</chop>,
3679 L</chomp>, and L</join>.)
3681 =item sprintf FORMAT, LIST
3683 Returns a string formatted by the usual C<printf()> conventions of the
3684 C library function C<sprintf()>. See L<sprintf(3)> or L<printf(3)>
3685 on your system for an explanation of the general principles.
3687 Perl does its own C<sprintf()> formatting -- it emulates the C
3688 function C<sprintf()>, but it doesn't use it (except for floating-point
3689 numbers, and even then only the standard modifiers are allowed). As a
3690 result, any non-standard extensions in your local C<sprintf()> are not
3691 available from Perl.
3693 Perl's C<sprintf()> permits the following universally-known conversions:
3696 %c a character with the given number
3698 %d a signed integer, in decimal
3699 %u an unsigned integer, in decimal
3700 %o an unsigned integer, in octal
3701 %x an unsigned integer, in hexadecimal
3702 %e a floating-point number, in scientific notation
3703 %f a floating-point number, in fixed decimal notation
3704 %g a floating-point number, in %e or %f notation
3706 In addition, Perl permits the following widely-supported conversions:
3708 %X like %x, but using upper-case letters
3709 %E like %e, but using an upper-case "E"
3710 %G like %g, but with an upper-case "E" (if applicable)
3711 %p a pointer (outputs the Perl value's address in hexadecimal)
3712 %n special: *stores* the number of characters output so far
3713 into the next variable in the parameter list
3715 Finally, for backward (and we do mean "backward") compatibility, Perl
3716 permits these unnecessary but widely-supported conversions:
3719 %D a synonym for %ld
3720 %U a synonym for %lu
3721 %O a synonym for %lo
3724 Perl permits the following universally-known flags between the C<%>
3725 and the conversion letter:
3727 space prefix positive number with a space
3728 + prefix positive number with a plus sign
3729 - left-justify within the field
3730 0 use zeros, not spaces, to right-justify
3731 # prefix non-zero octal with "0", non-zero hex with "0x"
3732 number minimum field width
3733 .number "precision": digits after decimal point for
3734 floating-point, max length for string, minimum length
3736 l interpret integer as C type "long" or "unsigned long"
3737 h interpret integer as C type "short" or "unsigned short"
3739 There is also one Perl-specific flag:
3741 V interpret integer as Perl's standard integer type
3743 Where a number would appear in the flags, an asterisk ("C<*>") may be
3744 used instead, in which case Perl uses the next item in the parameter
3745 list as the given number (that is, as the field width or precision).
3746 If a field width obtained through "C<*>" is negative, it has the same
3747 effect as the "C<->" flag: left-justification.
3749 If C<use locale> is in effect, the character used for the decimal
3750 point in formatted real numbers is affected by the LC_NUMERIC locale.
3757 Return the square root of EXPR. If EXPR is omitted, returns square
3764 Sets the random number seed for the C<rand()> operator. If EXPR is
3765 omitted, uses a semi-random value supplied by the kernel (if it supports
3766 the F</dev/urandom> device) or based on the current time and process
3767 ID, among other things. In versions of Perl prior to 5.004 the default
3768 seed was just the current C<time()>. This isn't a particularly good seed,
3769 so many old programs supply their own seed value (often C<time ^ $$> or
3770 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
3772 In fact, it's usually not necessary to call C<srand()> at all, because if
3773 it is not called explicitly, it is called implicitly at the first use of
3774 the C<rand()> operator. However, this was not the case in version of Perl
3775 before 5.004, so if your script will run under older Perl versions, it
3776 should call C<srand()>.
3778 Note that you need something much more random than the default seed for
3779 cryptographic purposes. Checksumming the compressed output of one or more
3780 rapidly changing operating system status programs is the usual method. For
3783 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3785 If you're particularly concerned with this, see the C<Math::TrulyRandom>
3788 Do I<not> call C<srand()> multiple times in your program unless you know
3789 exactly what you're doing and why you're doing it. The point of the
3790 function is to "seed" the C<rand()> function so that C<rand()> can produce
3791 a different sequence each time you run your program. Just do it once at the
3792 top of your program, or you I<won't> get random numbers out of C<rand()>!
3794 Frequently called programs (like CGI scripts) that simply use
3798 for a seed can fall prey to the mathematical property that
3802 one-third of the time. So don't do that.
3804 =item stat FILEHANDLE
3810 Returns a 13-element list giving the status info for a file, either
3811 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
3812 it stats C<$_>. Returns a null list if the stat fails. Typically used
3815 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3816 $atime,$mtime,$ctime,$blksize,$blocks)
3819 Not all fields are supported on all filesystem types. Here are the
3820 meaning of the fields:
3822 0 dev device number of filesystem
3824 2 mode file mode (type and permissions)
3825 3 nlink number of (hard) links to the file
3826 4 uid numeric user ID of file's owner
3827 5 gid numeric group ID of file's owner
3828 6 rdev the device identifier (special files only)
3829 7 size total size of file, in bytes
3830 8 atime last access time since the epoch
3831 9 mtime last modify time since the epoch
3832 10 ctime inode change time (NOT creation time!) since the epoch
3833 11 blksize preferred block size for file system I/O
3834 12 blocks actual number of blocks allocated
3836 (The epoch was at 00:00 January 1, 1970 GMT.)
3838 If stat is passed the special filehandle consisting of an underline, no
3839 stat is done, but the current contents of the stat structure from the
3840 last stat or filetest are returned. Example:
3842 if (-x $file && (($d) = stat(_)) && $d < 0) {
3843 print "$file is executable NFS file\n";
3846 (This works on machines only for which the device number is negative under NFS.)
3848 In scalar context, C<stat()> returns a boolean value indicating success
3849 or failure, and, if successful, sets the information associated with
3850 the special filehandle C<_>.
3856 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3857 doing many pattern matches on the string before it is next modified.
3858 This may or may not save time, depending on the nature and number of
3859 patterns you are searching on, and on the distribution of character
3860 frequencies in the string to be searched -- you probably want to compare
3861 run times with and without it to see which runs faster. Those loops
3862 which scan for many short constant strings (including the constant
3863 parts of more complex patterns) will benefit most. You may have only
3864 one C<study()> active at a time -- if you study a different scalar the first
3865 is "unstudied". (The way C<study()> works is this: a linked list of every
3866 character in the string to be searched is made, so we know, for
3867 example, where all the C<'k'> characters are. From each search string,
3868 the rarest character is selected, based on some static frequency tables
3869 constructed from some C programs and English text. Only those places
3870 that contain this "rarest" character are examined.)
3872 For example, here is a loop that inserts index producing entries
3873 before any line containing a certain pattern:
3877 print ".IX foo\n" if /\bfoo\b/;
3878 print ".IX bar\n" if /\bbar\b/;
3879 print ".IX blurfl\n" if /\bblurfl\b/;
3884 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
3885 will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
3886 a big win except in pathological cases. The only question is whether
3887 it saves you more time than it took to build the linked list in the
3890 Note that if you have to look for strings that you don't know till
3891 runtime, you can build an entire loop as a string and C<eval()> that to
3892 avoid recompiling all your patterns all the time. Together with
3893 undefining C<$/> to input entire files as one record, this can be very
3894 fast, often faster than specialized programs like fgrep(1). The following
3895 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3896 out the names of those files that contain a match:
3898 $search = 'while (<>) { study;';
3899 foreach $word (@words) {
3900 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3905 eval $search; # this screams
3906 $/ = "\n"; # put back to normal input delimiter
3907 foreach $file (sort keys(%seen)) {
3915 =item sub NAME BLOCK
3917 This is subroutine definition, not a real function I<per se>. With just a
3918 NAME (and possibly prototypes), it's just a forward declaration. Without
3919 a NAME, it's an anonymous function declaration, and does actually return a
3920 value: the CODE ref of the closure you just created. See L<perlsub> and
3921 L<perlref> for details.
3923 =item substr EXPR,OFFSET,LEN,REPLACEMENT
3925 =item substr EXPR,OFFSET,LEN
3927 =item substr EXPR,OFFSET
3929 Extracts a substring out of EXPR and returns it. First character is at
3930 offset C<0>, or whatever you've set C<$[> to (but don't do that).
3931 If OFFSET is negative (or more precisely, less than C<$[>), starts
3932 that far from the end of the string. If LEN is omitted, returns
3933 everything to the end of the string. If LEN is negative, leaves that
3934 many characters off the end of the string.
3936 If you specify a substring that is partly outside the string, the part
3937 within the string is returned. If the substring is totally outside
3938 the string a warning is produced.
3940 You can use the C<substr()> function
3941 as an lvalue, in which case EXPR must be an lvalue. If you assign
3942 something shorter than LEN, the string will shrink, and if you assign
3943 something longer than LEN, the string will grow to accommodate it. To
3944 keep the string the same length you may need to pad or chop your value
3947 An alternative to using C<substr()> as an lvalue is to specify the
3948 replacement string as the 4th argument. This allows you to replace
3949 parts of the EXPR and return what was there before in one operation.
3951 =item symlink OLDFILE,NEWFILE
3953 Creates a new filename symbolically linked to the old filename.
3954 Returns C<1> for success, C<0> otherwise. On systems that don't support
3955 symbolic links, produces a fatal error at run time. To check for that,
3958 $symlink_exists = eval { symlink("",""); 1 };
3962 Calls the system call specified as the first element of the list,
3963 passing the remaining elements as arguments to the system call. If
3964 unimplemented, produces a fatal error. The arguments are interpreted
3965 as follows: if a given argument is numeric, the argument is passed as
3966 an int. If not, the pointer to the string value is passed. You are
3967 responsible to make sure a string is pre-extended long enough to
3968 receive any result that might be written into a string. You can't use a
3969 string literal (or other read-only string) as an argument to C<syscall()>
3970 because Perl has to assume that any string pointer might be written
3972 integer arguments are not literals and have never been interpreted in a
3973 numeric context, you may need to add C<0> to them to force them to look
3974 like numbers. This emulates the C<syswrite()> function (or vice versa):
3976 require 'syscall.ph'; # may need to run h2ph
3978 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
3980 Note that Perl supports passing of up to only 14 arguments to your system call,
3981 which in practice should usually suffice.
3983 Syscall returns whatever value returned by the system call it calls.
3984 If the system call fails, C<syscall()> returns C<-1> and sets C<$!> (errno).
3985 Note that some system calls can legitimately return C<-1>. The proper
3986 way to handle such calls is to assign C<$!=0;> before the call and
3987 check the value of C<$!> if syscall returns C<-1>.
3989 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
3990 number of the read end of the pipe it creates. There is no way
3991 to retrieve the file number of the other end. You can avoid this
3992 problem by using C<pipe()> instead.
3994 =item sysopen FILEHANDLE,FILENAME,MODE
3996 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3998 Opens the file whose filename is given by FILENAME, and associates it
3999 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4000 the name of the real filehandle wanted. This function calls the
4001 underlying operating system's C<open()> function with the parameters
4002 FILENAME, MODE, PERMS.
4004 The possible values and flag bits of the MODE parameter are
4005 system-dependent; they are available via the standard module C<Fcntl>.
4006 For historical reasons, some values work on almost every system
4007 supported by perl: zero means read-only, one means write-only, and two
4008 means read/write. We know that these values do I<not> work under
4009 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4010 use them in new code.
4012 If the file named by FILENAME does not exist and the C<open()> call creates
4013 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4014 PERMS specifies the permissions of the newly created file. If you omit
4015 the PERMS argument to C<sysopen()>, Perl uses the octal value C<0666>.
4016 These permission values need to be in octal, and are modified by your
4017 process's current C<umask>.
4019 Seldom if ever use C<0644> as argument to C<sysopen()> because that
4020 takes away the user's option to have a more permissive umask. Better
4021 to omit it. See the perlfunc(1) entry on C<umask> for more on this.
4023 The C<IO::File> module provides a more object-oriented approach, if you're
4024 into that kind of thing.
4026 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4028 =item sysread FILEHANDLE,SCALAR,LENGTH
4030 Attempts to read LENGTH bytes of data into variable SCALAR from the
4031 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4032 so mixing this with other kinds of reads, C<print()>, C<write()>,
4033 C<seek()>, C<tell()>, or C<eof()> can cause confusion because stdio
4034 usually buffers data. Returns the number of bytes actually read, C<0>
4035 at end of file, or undef if there was an error. SCALAR will be grown or
4036 shrunk so that the last byte actually read is the last byte of the
4037 scalar after the read.
4039 An OFFSET may be specified to place the read data at some place in the
4040 string other than the beginning. A negative OFFSET specifies
4041 placement at that many bytes counting backwards from the end of the
4042 string. A positive OFFSET greater than the length of SCALAR results
4043 in the string being padded to the required size with C<"\0"> bytes before
4044 the result of the read is appended.
4046 =item sysseek FILEHANDLE,POSITION,WHENCE
4048 Sets FILEHANDLE's system position using the system call lseek(2). It
4049 bypasses stdio, so mixing this with reads (other than C<sysread()>),
4050 C<print()>, C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause
4051 confusion. FILEHANDLE may be an expression whose value gives the name
4052 of the filehandle. The values for WHENCE are C<0> to set the new
4053 position to POSITION, C<1> to set the it to the current position plus
4054 POSITION, and C<2> to set it to EOF plus POSITION (typically negative).
4055 For WHENCE, you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4056 C<SEEK_END> from either the C<IO::Seekable> or the POSIX module.
4058 Returns the new position, or the undefined value on failure. A position
4059 of zero is returned as the string "C<0> but true"; thus C<sysseek()> returns
4060 TRUE on success and FALSE on failure, yet you can still easily determine
4065 =item system PROGRAM LIST
4067 Does exactly the same thing as "C<exec LIST>" except that a fork is done
4068 first, and the parent process waits for the child process to complete.
4069 Note that argument processing varies depending on the number of
4070 arguments. If there is more than one argument in LIST, or if LIST is
4071 an array with more than one value, starts the program given by the
4072 first element of the list with arguments given by the rest of the list.
4073 If there is only one scalar argument, the argument is
4074 checked for shell metacharacters, and if there are any, the entire
4075 argument is passed to the system's command shell for parsing (this is
4076 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
4077 there are no shell metacharacters in the argument, it is split into
4078 words and passed directly to C<execvp()>, which is more efficient.
4080 The return value is the exit status of the program as
4081 returned by the C<wait()> call. To get the actual exit value divide by
4082 256. See also L</exec>. This is I<NOT> what you want to use to capture
4083 the output from a command, for that you should use merely backticks or
4084 C<qx//>, as described in L<perlop/"`STRING`">.
4086 Like C<exec()>, C<system()> allows you to lie to a program about its name if
4087 you use the "C<system PROGRAM LIST>" syntax. Again, see L</exec>.
4089 Because C<system()> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4090 program they're running doesn't actually interrupt your program.
4092 @args = ("command", "arg1", "arg2");
4094 or die "system @args failed: $?"
4096 You can check all the failure possibilities by inspecting
4099 $exit_value = $? >> 8;
4100 $signal_num = $? & 127;
4101 $dumped_core = $? & 128;
4103 When the arguments get executed via the system shell, results
4104 and return codes will be subject to its quirks and capabilities.
4105 See L<perlop/"`STRING`"> and L</exec> for details.
4107 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4109 =item syswrite FILEHANDLE,SCALAR,LENGTH
4111 =item syswrite FILEHANDLE,SCALAR
4113 Attempts to write LENGTH bytes of data from variable SCALAR to the
4114 specified FILEHANDLE, using the system call write(2). If LENGTH is
4115 not specified, writes whole SCALAR. It bypasses
4116 stdio, so mixing this with reads (other than C<sysread())>, C<print()>,
4117 C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause confusion
4118 because stdio usually buffers data. Returns the number of bytes
4119 actually written, or C<undef> if there was an error. If the LENGTH is
4120 greater than the available data in the SCALAR after the OFFSET, only as
4121 much data as is available will be written.
4123 An OFFSET may be specified to write the data from some part of the
4124 string other than the beginning. A negative OFFSET specifies writing
4125 that many bytes counting backwards from the end of the string. In the
4126 case the SCALAR is empty you can use OFFSET but only zero offset.
4128 =item tell FILEHANDLE
4132 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4133 expression whose value gives the name of the actual filehandle. If
4134 FILEHANDLE is omitted, assumes the file last read.
4136 =item telldir DIRHANDLE
4138 Returns the current position of the C<readdir()> routines on DIRHANDLE.
4139 Value may be given to C<seekdir()> to access a particular location in a
4140 directory. Has the same caveats about possible directory compaction as
4141 the corresponding system library routine.
4143 =item tie VARIABLE,CLASSNAME,LIST
4145 This function binds a variable to a package class that will provide the
4146 implementation for the variable. VARIABLE is the name of the variable
4147 to be enchanted. CLASSNAME is the name of a class implementing objects
4148 of correct type. Any additional arguments are passed to the "C<new()>"
4149 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4150 or C<TIEHASH>). Typically these are arguments such as might be passed
4151 to the C<dbm_open()> function of C. The object returned by the "C<new()>"
4152 method is also returned by the C<tie()> function, which would be useful
4153 if you want to access other methods in CLASSNAME.
4155 Note that functions such as C<keys()> and C<values()> may return huge lists
4156 when used on large objects, like DBM files. You may prefer to use the
4157 C<each()> function to iterate over such. Example:
4159 # print out history file offsets
4161 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4162 while (($key,$val) = each %HIST) {
4163 print $key, ' = ', unpack('L',$val), "\n";
4167 A class implementing a hash should have the following methods:
4169 TIEHASH classname, LIST
4171 STORE this, key, value
4176 NEXTKEY this, lastkey
4179 A class implementing an ordinary array should have the following methods:
4181 TIEARRAY classname, LIST
4183 STORE this, key, value
4185 STORESIZE this, count
4191 SPLICE this, offset, length, LIST
4195 A class implementing a file handle should have the following methods:
4197 TIEHANDLE classname, LIST
4198 READ this, scalar, length, offset
4201 WRITE this, scalar, length, offset
4203 PRINTF this, format, LIST
4207 A class implementing a scalar should have the following methods:
4209 TIESCALAR classname, LIST
4214 Not all methods indicated above need be implemented. See L<perltie>,
4215 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar> and L<Tie::Handle>.
4217 Unlike C<dbmopen()>, the C<tie()> function will not use or require a module
4218 for you--you need to do that explicitly yourself. See L<DB_File>
4219 or the F<Config> module for interesting C<tie()> implementations.
4221 For further details see L<perltie>, L<tied VARIABLE>.
4225 Returns a reference to the object underlying VARIABLE (the same value
4226 that was originally returned by the C<tie()> call that bound the variable
4227 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4232 Returns the number of non-leap seconds since whatever time the system
4233 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4234 and 00:00:00 UTC, January 1, 1970 for most other systems).
4235 Suitable for feeding to C<gmtime()> and C<localtime()>.
4239 Returns a four-element list giving the user and system times, in
4240 seconds, for this process and the children of this process.
4242 ($user,$system,$cuser,$csystem) = times;
4246 The transliteration operator. Same as C<y///>. See L<perlop>.
4248 =item truncate FILEHANDLE,LENGTH
4250 =item truncate EXPR,LENGTH
4252 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4253 specified length. Produces a fatal error if truncate isn't implemented
4254 on your system. Returns TRUE if successful, the undefined value
4261 Returns an uppercased version of EXPR. This is the internal function
4262 implementing the C<\U> escape in double-quoted strings.
4263 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4264 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
4265 does not attempt to do titlecase mapping on initial letters. See C<ucfirst()> for that.)
4267 If EXPR is omitted, uses C<$_>.
4273 Returns the value of EXPR with the first character
4274 in uppercase (titlecase in Unicode). This is
4275 the internal function implementing the C<\u> escape in double-quoted strings.
4276 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4278 If EXPR is omitted, uses C<$_>.
4284 Sets the umask for the process to EXPR and returns the previous value.
4285 If EXPR is omitted, merely returns the current umask.
4287 The Unix permission C<rwxr-x---> is represented as three sets of three
4288 bits, or three octal digits: C<0750> (the leading 0 indicates octal
4289 and isn't one of the the digits). The C<umask> value is such a number
4290 representing disabled permissions bits. The permission (or "mode")
4291 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
4292 even if you tell C<sysopen> to create a file with permissions C<0777>,
4293 if your umask is C<0022> then the file will actually be created with
4294 permissions C<0755>. If your C<umask> were C<0027> (group can't
4295 write; others can't read, write, or execute), then passing
4296 C<sysopen()> C<0666> would create a file with mode C<0640> (C<0666 &~
4299 Here's some advice: supply a creation mode of C<0666> for regular
4300 files (in C<sysopen()>) and one of C<0777> for directories (in
4301 C<mkdir()>) and executable files. This gives users the freedom of
4302 choice: if they want protected files, they might choose process umasks
4303 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
4304 Programs should rarely if ever make policy decisions better left to
4305 the user. The exception to this is when writing files that should be
4306 kept private: mail files, web browser cookies, I<.rhosts> files, and
4309 If umask(2) is not implemented on your system and you are trying to
4310 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
4311 fatal error at run time. If umask(2) is not implemented and you are
4312 not trying to restrict access for yourself, returns C<undef>.
4314 Remember that a umask is a number, usually given in octal; it is I<not> a
4315 string of octal digits. See also L</oct>, if all you have is a string.
4321 Undefines the value of EXPR, which must be an lvalue. Use only on a
4322 scalar value, an array (using "C<@>"), a hash (using "C<%>"), a subroutine
4323 (using "C<&>"), or a typeglob (using "<*>"). (Saying C<undef $hash{$key}>
4324 will probably not do what you expect on most predefined variables or
4325 DBM list values, so don't do that; see L<delete>.) Always returns the
4326 undefined value. You can omit the EXPR, in which case nothing is
4327 undefined, but you still get an undefined value that you could, for
4328 instance, return from a subroutine, assign to a variable or pass as a
4329 parameter. Examples:
4332 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4336 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4337 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4338 select undef, undef, undef, 0.25;
4339 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4341 Note that this is a unary operator, not a list operator.
4343 =item unless (EXPR) BLOCK
4345 The negative counterpart of L</if>. If the EXPR returns false the
4348 See also L<perlsyn>.
4354 Deletes a list of files. Returns the number of files successfully
4357 $cnt = unlink 'a', 'b', 'c';
4361 Note: C<unlink()> will not delete directories unless you are superuser and
4362 the B<-U> flag is supplied to Perl. Even if these conditions are
4363 met, be warned that unlinking a directory can inflict damage on your
4364 filesystem. Use C<rmdir()> instead.
4366 If LIST is omitted, uses C<$_>.
4368 =item unpack TEMPLATE,EXPR
4370 C<Unpack()> does the reverse of C<pack()>: it takes a string representing a
4371 structure and expands it out into a list value, returning the array
4372 value. (In scalar context, it returns merely the first value
4373 produced.) The TEMPLATE has the same format as in the C<pack()> function.
4374 Here's a subroutine that does substring:
4377 my($what,$where,$howmuch) = @_;
4378 unpack("x$where a$howmuch", $what);
4383 sub ordinal { unpack("c",$_[0]); } # same as ord()
4385 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4386 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4387 themselves. Default is a 16-bit checksum. For example, the following
4388 computes the same number as the System V sum program:
4391 $checksum += unpack("%16C*", $_);
4395 The following efficiently counts the number of set bits in a bit vector:
4397 $setbits = unpack("%32b*", $selectmask);
4399 =item unshift ARRAY,LIST
4401 Does the opposite of a C<shift()>. Or the opposite of a C<push()>,
4402 depending on how you look at it. Prepends list to the front of the
4403 array, and returns the new number of elements in the array.
4405 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4407 Note the LIST is prepended whole, not one element at a time, so the
4408 prepended elements stay in the same order. Use C<reverse()> to do the
4411 =item until (EXPR) BLOCK
4413 =item do BLOCK until (EXPR)
4415 Enter BLOCK until EXPR returns false. The first form may avoid entering
4416 the BLOCK, the second form enters the BLOCK at least once.
4418 See L</do>, L</while>, and L</for>.
4420 See also L<perlsyn>.
4422 =item untie VARIABLE
4424 Breaks the binding between a variable and a package. (See C<tie()>.)
4426 =item use Module LIST
4430 =item use Module VERSION LIST
4434 Imports some semantics into the current package from the named module,
4435 generally by aliasing certain subroutine or variable names into your
4436 package. It is exactly equivalent to
4438 BEGIN { require Module; import Module LIST; }
4440 except that Module I<must> be a bareword.
4442 If the first argument to C<use> is a number, it is treated as a version
4443 number instead of a module name. If the version of the Perl interpreter
4444 is less than VERSION, then an error message is printed and Perl exits
4445 immediately. This is often useful if you need to check the current
4446 Perl version before C<use>ing library modules that have changed in
4447 incompatible ways from older versions of Perl. (We try not to do
4448 this more than we have to.)
4450 The C<BEGIN> forces the C<require> and C<import()> to happen at compile time. The
4451 C<require> makes sure the module is loaded into memory if it hasn't been
4452 yet. The C<import()> is not a builtin--it's just an ordinary static method
4453 call into the "C<Module>" package to tell the module to import the list of
4454 features back into the current package. The module can implement its
4455 C<import()> method any way it likes, though most modules just choose to
4456 derive their C<import()> method via inheritance from the C<Exporter> class that
4457 is defined in the C<Exporter> module. See L<Exporter>. If no C<import()>
4458 method can be found then the error is currently silently ignored. This
4459 may change to a fatal error in a future version.
4461 If you don't want your namespace altered, explicitly supply an empty list:
4465 That is exactly equivalent to
4467 BEGIN { require Module }
4469 If the VERSION argument is present between Module and LIST, then the
4470 C<use> will call the VERSION method in class Module with the given
4471 version as an argument. The default VERSION method, inherited from
4472 the Universal class, croaks if the given version is larger than the
4473 value of the variable C<$Module::VERSION>. (Note that there is not a
4474 comma after VERSION!)
4476 Because this is a wide-open interface, pragmas (compiler directives)
4477 are also implemented this way. Currently implemented pragmas are:
4481 use sigtrap qw(SEGV BUS);
4482 use strict qw(subs vars refs);
4483 use subs qw(afunc blurfl);
4485 Some of these these pseudo-modules import semantics into the current
4486 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4487 which import symbols into the current package (which are effective
4488 through the end of the file).
4490 There's a corresponding "C<no>" command that unimports meanings imported
4491 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import()>.
4496 If no C<unimport()> method can be found the call fails with a fatal error.
4498 See L<perlmod> for a list of standard modules and pragmas.
4502 Changes the access and modification times on each file of a list of
4503 files. The first two elements of the list must be the NUMERICAL access
4504 and modification times, in that order. Returns the number of files
4505 successfully changed. The inode modification time of each file is set
4506 to the current time. This code has the same effect as the "C<touch>"
4507 command if the files already exist:
4511 utime $now, $now, @ARGV;
4515 Returns a list consisting of all the values of the named hash. (In a
4516 scalar context, returns the number of values.) The values are
4517 returned in an apparently random order. The actual random order is
4518 subject to change in future versions of perl, but it is guaranteed to
4519 be the same order as either the C<keys()> or C<each()> function would
4520 produce on the same (unmodified) hash.
4522 As a side effect, it resets HASH's iterator. See also C<keys()>, C<each()>,
4525 =item vec EXPR,OFFSET,BITS
4527 Treats the string in EXPR as a vector of unsigned integers, and
4528 returns the value of the bit field specified by OFFSET. BITS specifies
4529 the number of bits that are reserved for each entry in the bit
4530 vector. This must be a power of two from 1 to 32. C<vec()> may also be
4531 assigned to, in which case parentheses are needed to give the expression
4532 the correct precedence as in
4534 vec($image, $max_x * $x + $y, 8) = 3;
4536 Vectors created with C<vec()> can also be manipulated with the logical
4537 operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
4538 desired when both operands are strings.
4540 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
4541 The comments show the string after each step. Note that this code works
4542 in the same way on big-endian or little-endian machines.
4545 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
4546 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
4547 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
4548 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
4549 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
4550 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
4551 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
4553 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
4554 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
4555 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
4558 To transform a bit vector into a string or array of 0's and 1's, use these:
4560 $bits = unpack("b*", $vector);
4561 @bits = split(//, unpack("b*", $vector));
4563 If you know the exact length in bits, it can be used in place of the C<*>.
4567 Waits for a child process to terminate and returns the pid of the
4568 deceased process, or C<-1> if there are no child processes. The status is
4569 returned in C<$?>. Note that a return value of C<-1> could mean that
4570 child processes are being automatically reaped, as described in L<perlipc>.
4572 =item waitpid PID,FLAGS
4574 Waits for a particular child process to terminate and returns the pid
4575 of the deceased process, or C<-1> if there is no such child process. The
4576 status is returned in C<$?>. If you say
4578 use POSIX ":sys_wait_h";
4580 waitpid(-1,&WNOHANG);
4582 then you can do a non-blocking wait for any process. Non-blocking wait
4583 is available on machines supporting either the waitpid(2) or
4584 wait4(2) system calls. However, waiting for a particular pid with
4585 FLAGS of C<0> is implemented everywhere. (Perl emulates the system call
4586 by remembering the status values of processes that have exited but have
4587 not been harvested by the Perl script yet.)
4589 Note that a return value of C<-1> could mean that child processes are being
4590 automatically reaped. See L<perlipc> for details, and for other examples.
4594 Returns TRUE if the context of the currently executing subroutine is
4595 looking for a list value. Returns FALSE if the context is looking
4596 for a scalar. Returns the undefined value if the context is looking
4597 for no value (void context).
4599 return unless defined wantarray; # don't bother doing more
4600 my @a = complex_calculation();
4601 return wantarray ? @a : "@a";
4605 Produces a message on STDERR just like C<die()>, but doesn't exit or throw
4608 If LIST is empty and C<$@> already contains a value (typically from a
4609 previous eval) that value is used after appending C<"\t...caught">
4610 to C<$@>. This is useful for staying almost, but not entirely similar to
4613 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
4615 No message is printed if there is a C<$SIG{__WARN__}> handler
4616 installed. It is the handler's responsibility to deal with the message
4617 as it sees fit (like, for instance, converting it into a C<die()>). Most
4618 handlers must therefore make arrangements to actually display the
4619 warnings that they are not prepared to deal with, by calling C<warn()>
4620 again in the handler. Note that this is quite safe and will not
4621 produce an endless loop, since C<__WARN__> hooks are not called from
4624 You will find this behavior is slightly different from that of
4625 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4626 instead call C<die()> again to change it).
4628 Using a C<__WARN__> handler provides a powerful way to silence all
4629 warnings (even the so-called mandatory ones). An example:
4631 # wipe out *all* compile-time warnings
4632 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4634 my $foo = 20; # no warning about duplicate my $foo,
4635 # but hey, you asked for it!
4636 # no compile-time or run-time warnings before here
4639 # run-time warnings enabled after here
4640 warn "\$foo is alive and $foo!"; # does show up
4642 See L<perlvar> for details on setting C<%SIG> entries, and for more
4645 =item while (EXPR) BLOCK
4647 =item do BLOCK while (EXPR)
4649 Enter BLOCK while EXPR is true. The first form may avoid entering the
4650 BLOCK, the second form enters the BLOCK at least once.
4652 See also L<perlsyn>, L</for>, L</until>, and L</continue>.
4654 =item write FILEHANDLE
4660 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
4661 using the format associated with that file. By default the format for
4662 a file is the one having the same name as the filehandle, but the
4663 format for the current output channel (see the C<select()> function) may be set
4664 explicitly by assigning the name of the format to the C<$~> variable.
4666 Top of form processing is handled automatically: if there is
4667 insufficient room on the current page for the formatted record, the
4668 page is advanced by writing a form feed, a special top-of-page format
4669 is used to format the new page header, and then the record is written.
4670 By default the top-of-page format is the name of the filehandle with
4671 "_TOP" appended, but it may be dynamically set to the format of your
4672 choice by assigning the name to the C<$^> variable while the filehandle is
4673 selected. The number of lines remaining on the current page is in
4674 variable C<$->, which can be set to C<0> to force a new page.
4676 If FILEHANDLE is unspecified, output goes to the current default output
4677 channel, which starts out as STDOUT but may be changed by the
4678 C<select()> operator. If the FILEHANDLE is an EXPR, then the expression
4679 is evaluated and the resulting string is used to look up the name of
4680 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4682 Note that write is I<NOT> the opposite of C<read()>. Unfortunately.
4686 The transliteration operator. Same as C<tr///>. See L<perlop>.