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 $_; # ignore specials
304 The interpretation of the file permission operators C<-r>, C<-R>,
305 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
306 of the file and the uids and gids of the user. There may be other
307 reasons you can't actually read, write, or execute the file. Such
308 reasons may be for example network filesystem access controls, ACLs
309 (access control lists), read-only filesystems, and unrecognized
312 Also note that, for the superuser on the local filesystems, C<-r>,
313 C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return 1
314 if any execute bit is set in the mode. Scripts run by the superuser
315 may thus need to do a stat() to determine the actual mode of the file,
316 or temporarily set the uid to something else.
318 If you are using ACLs, there is a pragma called C<filetest> that may
319 produce more accurate results than the bare stat() mode bits.
321 When under the C<use filetest 'access'> the above-mentioned filetests
322 will test whether the permission can (not) be granted using the
323 access() family of system calls. Also note that the -x and -X may
324 under this pragma return true even if there are no execute permission
325 bits set (nor any extra execute permission ACLs). This strangeness is
326 due to the underlying system calls' definitions. Read the
327 documentation for the C<filetest> pragma for more information.
329 Note that C<-s/a/b/> does not do a negated substitution. Saying
330 C<-exp($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($_[0]) }
424 =item bind SOCKET,NAME
426 Binds a network address to a socket, just as the bind system call
427 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
428 packed address of the appropriate type for the socket. See the examples in
429 L<perlipc/"Sockets: Client/Server Communication">.
431 =item binmode FILEHANDLE
433 Arranges for the file to be read or written in "binary" mode in operating
434 systems that distinguish between binary and text files. Files that are
435 not in binary mode have CR LF sequences translated to LF on input and LF
436 translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
437 and similarly archaic systems, it may be imperative--otherwise your
438 MS-DOS-damaged C library may mangle your file. The key distinction between
439 systems that need C<binmode()> and those that don't is their text file
440 formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
441 character, and that encode that character in C as C<"\n">, do not need
442 C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value
443 is taken as the name of the filehandle.
445 =item bless REF,CLASSNAME
449 This function tells the thingy referenced by REF that it is now
450 an object in the CLASSNAME package--or the current package if no CLASSNAME
451 is specified, which is often the case. It returns the reference for
452 convenience, because a C<bless()> is often the last thing in a constructor.
453 Always use the two-argument version if the function doing the blessing
454 might be inherited by a derived class. See L<perltoot> and L<perlobj>
455 for more about the blessing (and blessings) of objects.
461 Returns the context of the current subroutine call. In scalar context,
462 returns the caller's package name if there is a caller, that is, if
463 we're in a subroutine or C<eval()> or C<require()>, and the undefined value
464 otherwise. In list context, returns
466 ($package, $filename, $line) = caller;
468 With EXPR, it returns some extra information that the debugger uses to
469 print a stack trace. The value of EXPR indicates how many call frames
470 to go back before the current one.
472 ($package, $filename, $line, $subroutine,
473 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
475 Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
476 call, but an C<eval()>. In such a case additional elements C<$evaltext> and
477 C<$is_require> are set: C<$is_require> is true if the frame is created by a
478 C<require> or C<use> statement, C<$evaltext> contains the text of the
479 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
480 C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
481 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
484 Furthermore, when called from within the DB package, caller returns more
485 detailed information: it sets the list variable C<@DB::args> to be the
486 arguments with which the subroutine was invoked.
488 Be aware that the optimizer might have optimized call frames away before
489 C<caller()> had a chance to get the information. That means that C<caller(N)>
490 might not return information about the call frame you expect it do, for
491 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
492 previous time C<caller()> was called.
496 Changes the working directory to EXPR, if possible. If EXPR is
497 omitted, changes to home directory. Returns TRUE upon success, FALSE
498 otherwise. See example under C<die()>.
502 Changes the permissions of a list of files. The first element of the
503 list must be the numerical mode, which should probably be an octal
504 number, and which definitely should I<not> a string of octal digits:
505 C<0644> is okay, C<'0644'> is not. Returns the number of files
506 successfully changed. See also L</oct>, if all you have is a string.
508 $cnt = chmod 0755, 'foo', 'bar';
509 chmod 0755, @executables;
510 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
512 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
513 $mode = 0644; chmod $mode, 'foo'; # this is best
521 This is a slightly safer version of L</chop>. It removes any
522 line ending that corresponds to the current value of C<$/> (also known as
523 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
524 number of characters removed from all its arguments. It's often used to
525 remove the newline from the end of an input record when you're worried
526 that the final record may be missing its newline. When in paragraph mode
527 (C<$/ = "">), it removes all trailing newlines from the string. If
528 VARIABLE is omitted, it chomps C<$_>. Example:
531 chomp; # avoid \n on last field
536 You can actually chomp anything that's an lvalue, including an assignment:
539 chomp($answer = <STDIN>);
541 If you chomp a list, each element is chomped, and the total number of
542 characters removed is returned.
550 Chops off the last character of a string and returns the character
551 chopped. It's used primarily to remove the newline from the end of an
552 input record, but is much more efficient than C<s/\n//> because it neither
553 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
557 chop; # avoid \n on last field
562 You can actually chop anything that's an lvalue, including an assignment:
565 chop($answer = <STDIN>);
567 If you chop a list, each element is chopped. Only the value of the
568 last C<chop()> is returned.
570 Note that C<chop()> returns the last character. To return all but the last
571 character, use C<substr($string, 0, -1)>.
575 Changes the owner (and group) of a list of files. The first two
576 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
577 Returns the number of files successfully changed.
579 $cnt = chown $uid, $gid, 'foo', 'bar';
580 chown $uid, $gid, @filenames;
582 Here's an example that looks up nonnumeric uids in the passwd file:
585 chop($user = <STDIN>);
587 chop($pattern = <STDIN>);
589 ($login,$pass,$uid,$gid) = getpwnam($user)
590 or die "$user not in passwd file";
592 @ary = glob($pattern); # expand filenames
593 chown $uid, $gid, @ary;
595 On most systems, you are not allowed to change the ownership of the
596 file unless you're the superuser, although you should be able to change
597 the group to any of your secondary groups. On insecure systems, these
598 restrictions may be relaxed, but this is not a portable assumption.
604 Returns the character represented by that NUMBER in the character set.
605 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
606 chr(0x263a) is a Unicode smiley face (but only within the scope of a
607 C<use utf8>). For the reverse, use L</ord>.
609 If NUMBER is omitted, uses C<$_>.
611 =item chroot FILENAME
615 This function works like the system call by the same name: it makes the
616 named directory the new root directory for all further pathnames that
617 begin with a C<"/"> by your process and all its children. (It doesn't
618 change your current working directory, which is unaffected.) For security
619 reasons, this call is restricted to the superuser. If FILENAME is
620 omitted, does a C<chroot()> to C<$_>.
622 =item close FILEHANDLE
626 Closes the file or pipe associated with the file handle, returning TRUE
627 only if stdio successfully flushes buffers and closes the system file
628 descriptor. Closes the currently selected filehandle if the argument
631 You don't have to close FILEHANDLE if you are immediately going to do
632 another C<open()> on it, because C<open()> will close it for you. (See
633 C<open()>.) However, an explicit C<close()> on an input file resets the line
634 counter (C<$.>), while the implicit close done by C<open()> does not.
636 If the file handle came from a piped open C<close()> will additionally
637 return FALSE if one of the other system calls involved fails or if the
638 program exits with non-zero status. (If the only problem was that the
639 program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
640 waits for the process executing on the pipe to complete, in case you
641 want to look at the output of the pipe afterwards. Closing a pipe
642 explicitly also puts the exit status value of the command into C<$?>.
646 open(OUTPUT, '|sort >foo') # pipe to sort
647 or die "Can't start sort: $!";
648 #... # print stuff to output
649 close OUTPUT # wait for sort to finish
650 or warn $! ? "Error closing sort pipe: $!"
651 : "Exit status $? from sort";
652 open(INPUT, 'foo') # get sort's results
653 or die "Can't open 'foo' for input: $!";
655 FILEHANDLE may be an expression whose value can be used as an indirect
656 filehandle, usually the real filehandle name.
658 =item closedir DIRHANDLE
660 Closes a directory opened by C<opendir()> and returns the success of that
663 DIRHANDLE may be an expression whose value can be used as an indirect
664 dirhandle, usually the real dirhandle name.
666 =item connect SOCKET,NAME
668 Attempts to connect to a remote socket, just as the connect system call
669 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
670 packed address of the appropriate type for the socket. See the examples in
671 L<perlipc/"Sockets: Client/Server Communication">.
675 Actually a flow control statement rather than a function. If there is a
676 C<continue> BLOCK attached to a BLOCK (typically in a L<(while> or
677 L</foreach>), it is always executed just before the conditional is about to
678 be evaluated again, just like the third part of a L</for> loop in C. Thus
679 it can be used to increment a loop variable, even when the loop has been
680 continued via the C<next> statement (which is similar to the C C<continue>
683 L</last>, L</next>, or L</redo> may appear within a C<continue>
684 block. C<last> and C<redo> will behave as if they had been executed within
685 the main block. So will C<next>, but since it will execute a C<continue>
686 block, it may be more entertaining.
689 ### redo always comes here
692 ### next always comes here
694 # then back the top to re-check EXPR
696 ### last always comes here
698 Omitting the C<continue> section is semantically equivalent to using an
699 empty one, logically enough. In that case, C<next> goes directly back
700 to check the condition at the top of the loop.
706 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
707 takes cosine of C<$_>.
709 For the inverse cosine operation, you may use the C<POSIX::acos()>
710 function, or use this relation:
712 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
714 =item crypt PLAINTEXT,SALT
716 Encrypts a string exactly like the crypt(3) function in the C library
717 (assuming that you actually have a version there that has not been
718 extirpated as a potential munition). This can prove useful for checking
719 the password file for lousy passwords, amongst other things. Only the
720 guys wearing white hats should do this.
722 Note that C<crypt()> is intended to be a one-way function, much like breaking
723 eggs to make an omelette. There is no (known) corresponding decrypt
724 function. As a result, this function isn't all that useful for
725 cryptography. (For that, see your nearby CPAN mirror.)
727 When verifying an existing encrypted string you should use the encrypted
728 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
729 allows your code to work with the standard C<crypt()> and with more
730 exotic implementations. When choosing a new salt create a random two
731 character string whose characters come from the set C<[./0-9A-Za-z]>
732 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
734 Here's an example that makes sure that whoever runs this program knows
737 $pwd = (getpwuid($<))[1];
741 chomp($word = <STDIN>);
745 if (crypt($word, $pwd) ne $pwd) {
751 Of course, typing in your own password to whoever asks you
756 [This function has been superseded by the C<untie()> function.]
758 Breaks the binding between a DBM file and a hash.
760 =item dbmopen HASH,DBNAME,MODE
762 [This function has been superseded by the C<tie()> function.]
764 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
765 hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
766 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
767 is the name of the database (without the F<.dir> or F<.pag> extension if
768 any). If the database does not exist, it is created with protection
769 specified by MODE (as modified by the C<umask()>). If your system supports
770 only the older DBM functions, you may perform only one C<dbmopen()> in your
771 program. In older versions of Perl, if your system had neither DBM nor
772 ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
775 If you don't have write access to the DBM file, you can only read hash
776 variables, not set them. If you want to test whether you can write,
777 either use file tests or try setting a dummy hash entry inside an C<eval()>,
778 which will trap the error.
780 Note that functions such as C<keys()> and C<values()> may return huge lists
781 when used on large DBM files. You may prefer to use the C<each()>
782 function to iterate over large DBM files. Example:
784 # print out history file offsets
785 dbmopen(%HIST,'/usr/lib/news/history',0666);
786 while (($key,$val) = each %HIST) {
787 print $key, ' = ', unpack('L',$val), "\n";
791 See also L<AnyDBM_File> for a more general description of the pros and
792 cons of the various dbm approaches, as well as L<DB_File> for a particularly
799 Returns a Boolean value telling whether EXPR has a value other than
800 the undefined value C<undef>. If EXPR is not present, C<$_> will be
803 Many operations return C<undef> to indicate failure, end of file,
804 system error, uninitialized variable, and other exceptional
805 conditions. This function allows you to distinguish C<undef> from
806 other values. (A simple Boolean test will not distinguish among
807 C<undef>, zero, the empty string, and C<"0">, which are all equally
808 false.) Note that since C<undef> is a valid scalar, its presence
809 doesn't I<necessarily> indicate an exceptional condition: C<pop()>
810 returns C<undef> when its argument is an empty array, I<or> when the
811 element to return happens to be C<undef>.
813 You may also use C<defined()> to check whether a subroutine exists, by
814 saying C<defined &func> without parentheses. On the other hand, use
815 of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
816 produce intuitive results, and should probably be avoided.
818 When used on a hash element, it tells you whether the value is defined,
819 not whether the key exists in the hash. Use L</exists> for the latter
824 print if defined $switch{'D'};
825 print "$val\n" while defined($val = pop(@ary));
826 die "Can't readlink $sym: $!"
827 unless defined($value = readlink $sym);
828 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
829 $debugging = 0 unless defined $debugging;
831 Note: Many folks tend to overuse C<defined()>, and then are surprised to
832 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
833 defined values. For example, if you say
837 The pattern match succeeds, and C<$1> is defined, despite the fact that it
838 matched "nothing". But it didn't really match nothing--rather, it
839 matched something that happened to be C<0> characters long. This is all
840 very above-board and honest. When a function returns an undefined value,
841 it's an admission that it couldn't give you an honest answer. So you
842 should use C<defined()> only when you're questioning the integrity of what
843 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
846 Currently, using C<defined()> on an entire array or hash reports whether
847 memory for that aggregate has ever been allocated. So an array you set
848 to the empty list appears undefined initially, and one that once was full
849 and that you then set to the empty list still appears defined. You
850 should instead use a simple test for size:
852 if (@an_array) { print "has array elements\n" }
853 if (%a_hash) { print "has hash members\n" }
855 Using C<undef()> on these, however, does clear their memory and then report
856 them as not defined anymore, but you shouldn't do that unless you don't
857 plan to use them again, because it saves time when you load them up
858 again to have memory already ready to be filled. The normal way to
859 free up space used by an aggregate is to assign the empty list.
861 This counterintuitive behavior of C<defined()> on aggregates may be
862 changed, fixed, or broken in a future release of Perl.
864 See also L</undef>, L</exists>, L</ref>.
868 Deletes the specified key(s) and their associated values from a hash.
869 For each key, returns the deleted value associated with that key, or
870 the undefined value if there was no such key. Deleting from C<$ENV{}>
871 modifies the environment. Deleting from a hash tied to a DBM file
872 deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
873 doesn't necessarily return anything.)
875 The following deletes all the values of a hash:
877 foreach $key (keys %HASH) {
883 delete @HASH{keys %HASH}
885 (But both of these are slower than just assigning the empty list, or
886 using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
887 long as the final operation is a hash element lookup or hash slice:
889 delete $ref->[$x][$y]{$key};
890 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
894 Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
895 the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
896 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
897 is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
898 C<$@> and the C<eval()> is terminated with the undefined value. This makes
899 C<die()> the way to raise an exception.
903 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
904 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
906 If the value of EXPR does not end in a newline, the current script line
907 number and input line number (if any) are also printed, and a newline
908 is supplied. Hint: sometimes appending C<", stopped"> to your message
909 will cause it to make better sense when the string C<"at foo line 123"> is
910 appended. Suppose you are running script "canasta".
912 die "/etc/games is no good";
913 die "/etc/games is no good, stopped";
915 produce, respectively
917 /etc/games is no good at canasta line 123.
918 /etc/games is no good, stopped at canasta line 123.
920 See also C<exit()> and C<warn()>.
922 If LIST is empty and C<$@> already contains a value (typically from a
923 previous eval) that value is reused after appending C<"\t...propagated">.
924 This is useful for propagating exceptions:
927 die unless $@ =~ /Expected exception/;
929 If C<$@> is empty then the string C<"Died"> is used.
931 You can arrange for a callback to be run just before the C<die()> does
932 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
933 will be called with the error text and can change the error message, if
934 it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
935 setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
937 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
938 blocks/strings. If one wants the hook to do nothing in such
943 as the first line of the handler (see L<perlvar/$^S>).
947 Not really a function. Returns the value of the last command in the
948 sequence of commands indicated by BLOCK. When modified by a loop
949 modifier such as L</while> or L</until>, executes the BLOCK once
950 before testing the loop condition. (On other statements the loop
951 modifiers test the conditional first.)
953 C<do BLOCK> does I<not> count as a loop, so the loop control statements
954 L</next>, L</last> or L</redo> cannot be used to leave or restart the block.
956 =item do SUBROUTINE(LIST)
958 A deprecated form of subroutine call. See L<perlsub>.
962 Uses the value of EXPR as a filename and executes the contents of the
963 file as a Perl script. Its primary use is to include subroutines
964 from a Perl subroutine library.
970 scalar eval `cat stat.pl`;
972 except that it's more efficient and concise, keeps track of the
973 current filename for error messages, and searches all the B<-I>
974 libraries if the file isn't in the current directory (see also the @INC
975 array in L<perlvar/Predefined Names>). It is also different in how
976 code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
977 scope like C<eval STRING> does. It's the same, however, in that it does
978 reparse the file every time you call it, so you probably don't want to
979 do this inside a loop.
981 If C<do> cannot read the file, it returns undef and sets C<$!> to the
982 error. If C<do> can read the file but cannot compile it, it
983 returns undef and sets an error message in C<$@>. If the file is
984 successfully compiled, C<do> returns the value of the last expression
987 Note that inclusion of library modules is better done with the
988 C<use()> and C<require()> operators, which also do automatic error checking
989 and raise an exception if there's a problem.
991 You might like to use C<do> to read in a program configuration
992 file. Manual error checking can be done this way:
994 # read in config files: system first, then user
995 for $file ("/share/prog/defaults.rc",
996 "$ENV{HOME}/.someprogrc") {
997 unless ($return = do $file) {
998 warn "couldn't parse $file: $@" if $@;
999 warn "couldn't do $file: $!" unless defined $return;
1000 warn "couldn't run $file" unless $return;
1006 This causes an immediate core dump. Primarily this is so that you can
1007 use the B<undump> program to turn your core dump into an executable binary
1008 after having initialized all your variables at the beginning of the
1009 program. When the new binary is executed it will begin by executing a
1010 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
1011 it as a goto with an intervening core dump and reincarnation. If C<LABEL>
1012 is omitted, restarts the program from the top. WARNING: Any files
1013 opened at the time of the dump will NOT be open any more when the
1014 program is reincarnated, with possible resulting confusion on the part
1015 of Perl. See also B<-u> option in L<perlrun>.
1020 require 'getopt.pl';
1032 dump QUICKSTART if $ARGV[0] eq '-d';
1037 This operator is largely obsolete, partly because it's very hard to
1038 convert a core file into an executable, and because the real perl-to-C
1039 compiler has superseded it.
1043 When called in list context, returns a 2-element list consisting of the
1044 key and value for the next element of a hash, so that you can iterate over
1045 it. When called in scalar context, returns the key for only the "next"
1046 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
1047 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1050 Entries are returned in an apparently random order. When the hash is
1051 entirely read, a null array is returned in list context (which when
1052 assigned produces a FALSE (C<0>) value), and C<undef> in
1053 scalar context. The next call to C<each()> after that will start iterating
1054 again. There is a single iterator for each hash, shared by all C<each()>,
1055 C<keys()>, and C<values()> function calls in the program; it can be reset by
1056 reading all the elements from the hash, or by evaluating C<keys HASH> or
1057 C<values HASH>. If you add or delete elements of a hash while you're
1058 iterating over it, you may get entries skipped or duplicated, so don't.
1060 The following prints out your environment like the printenv(1) program,
1061 only in a different order:
1063 while (($key,$value) = each %ENV) {
1064 print "$key=$value\n";
1067 See also C<keys()> and C<values()>.
1071 =item elsif (EXPR) BLOCK
1075 =item eof FILEHANDLE
1081 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1082 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1083 gives the real filehandle. (Note that this function actually
1084 reads a character and then C<ungetc()>s it, so isn't very useful in an
1085 interactive context.) Do not read from a terminal file (or call
1086 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1087 as terminals may lose the end-of-file condition if you do.
1089 An C<eof> without an argument uses the last file read as argument.
1090 Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
1091 the files listed on the command line, i.e., C<eof()> is reasonable to
1092 use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1093 last file. Use C<eof(ARGV)> or eof without the parentheses to test
1094 I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
1096 # reset line numbering on each input file
1098 next if /^\s*#/; # skip comments
1101 close ARGV if eof; # Not eof()!
1104 # insert dashes just before last line of last file
1106 if (eof()) { # check for end of current file
1107 print "--------------\n";
1108 close(ARGV); # close or break; is needed if we
1109 # are reading from the terminal
1114 Practical hint: you almost never need to use C<eof> in Perl, because the
1115 input operators return false values when they run out of data, or if there
1122 In the first form, the return value of EXPR is parsed and executed as if it
1123 were a little Perl program. The value of the expression (which is itself
1124 determined within scalar context) is first parsed, and if there weren't any
1125 errors, executed in the context of the current Perl program, so that any
1126 variable settings or subroutine and format definitions remain afterwards.
1127 Note that the value is parsed every time the eval executes. If EXPR is
1128 omitted, evaluates C<$_>. This form is typically used to delay parsing
1129 and subsequent execution of the text of EXPR until run time.
1131 In the second form, the code within the BLOCK is parsed only once--at the
1132 same time the code surrounding the eval itself was parsed--and executed
1133 within the context of the current Perl program. This form is typically
1134 used to trap exceptions more efficiently than the first (see below), while
1135 also providing the benefit of checking the code within BLOCK at compile
1138 The final semicolon, if any, may be omitted from the value of EXPR or within
1141 In both forms, the value returned is the value of the last expression
1142 evaluated inside the mini-program; a return statement may be also used, just
1143 as with subroutines. The expression providing the return value is evaluated
1144 in void, scalar, or list context, depending on the context of the eval itself.
1145 See L</wantarray> for more on how the evaluation context can be determined.
1147 If there is a syntax error or runtime error, or a C<die()> statement is
1148 executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
1149 error message. If there was no error, C<$@> is guaranteed to be a null
1150 string. Beware that using C<eval()> neither silences perl from printing
1151 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1152 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1153 L</warn> and L<perlvar>.
1155 Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1156 determining whether a particular feature (such as C<socket()> or C<symlink()>)
1157 is implemented. It is also Perl's exception trapping mechanism, where
1158 the die operator is used to raise exceptions.
1160 If the code to be executed doesn't vary, you may use the eval-BLOCK
1161 form to trap run-time errors without incurring the penalty of
1162 recompiling each time. The error, if any, is still returned in C<$@>.
1165 # make divide-by-zero nonfatal
1166 eval { $answer = $a / $b; }; warn $@ if $@;
1168 # same thing, but less efficient
1169 eval '$answer = $a / $b'; warn $@ if $@;
1171 # a compile-time error
1172 eval { $answer = }; # WRONG
1175 eval '$answer ='; # sets $@
1177 When using the C<eval{}> form as an exception trap in libraries, you may
1178 wish not to trigger any C<__DIE__> hooks that user code may have
1179 installed. You can use the C<local $SIG{__DIE__}> construct for this
1180 purpose, as shown in this example:
1182 # a very private exception trap for divide-by-zero
1183 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1186 This is especially significant, given that C<__DIE__> hooks can call
1187 C<die()> again, which has the effect of changing their error messages:
1189 # __DIE__ hooks may modify error messages
1191 local $SIG{'__DIE__'} =
1192 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1193 eval { die "foo lives here" };
1194 print $@ if $@; # prints "bar lives here"
1197 With an C<eval()>, you should be especially careful to remember what's
1198 being looked at when:
1204 eval { $x }; # CASE 4
1206 eval "\$$x++"; # CASE 5
1209 Cases 1 and 2 above behave identically: they run the code contained in
1210 the variable C<$x>. (Although case 2 has misleading double quotes making
1211 the reader wonder what else might be happening (nothing is).) Cases 3
1212 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1213 does nothing but return the value of C<$x>. (Case 4 is preferred for
1214 purely visual reasons, but it also has the advantage of compiling at
1215 compile-time instead of at run-time.) Case 5 is a place where
1216 normally you I<WOULD> like to use double quotes, except that in this
1217 particular situation, you can just use symbolic references instead, as
1220 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1221 C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
1226 =item exec PROGRAM LIST
1228 The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1229 use C<system()> instead of C<exec()> if you want it to return. It fails and
1230 returns FALSE only if the command does not exist I<and> it is executed
1231 directly instead of via your system's command shell (see below).
1233 Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1234 warns you if there is a following statement which isn't C<die()>, C<warn()>,
1235 or C<exit()> (if C<-w> is set - but you always do that). If you
1236 I<really> want to follow an C<exec()> with some other statement, you
1237 can use one of these styles to avoid the warning:
1239 exec ('foo') or print STDERR "couldn't exec foo: $!";
1240 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1242 If there is more than one argument in LIST, or if LIST is an array
1243 with more than one value, calls execvp(3) with the arguments in LIST.
1244 If there is only one scalar argument or an array with one element in it,
1245 the argument is checked for shell metacharacters, and if there are any,
1246 the entire argument is passed to the system's command shell for parsing
1247 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1248 If there are no shell metacharacters in the argument, it is split into
1249 words and passed directly to C<execvp()>, which is more efficient. Note:
1250 C<exec()> and C<system()> do not flush your output buffer, so you may need to
1251 set C<$|> to avoid lost output. Examples:
1253 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1254 exec "sort $outfile | uniq";
1256 If you don't really want to execute the first argument, but want to lie
1257 to the program you are executing about its own name, you can specify
1258 the program you actually want to run as an "indirect object" (without a
1259 comma) in front of the LIST. (This always forces interpretation of the
1260 LIST as a multivalued list, even if there is only a single scalar in
1263 $shell = '/bin/csh';
1264 exec $shell '-sh'; # pretend it's a login shell
1268 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1270 When the arguments get executed via the system shell, results will
1271 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1274 Using an indirect object with C<exec()> or C<system()> is also more secure.
1275 This usage forces interpretation of the arguments as a multivalued list,
1276 even if the list had just one argument. That way you're safe from the
1277 shell expanding wildcards or splitting up words with whitespace in them.
1279 @args = ( "echo surprise" );
1281 system @args; # subject to shell escapes
1283 system { $args[0] } @args; # safe even with one-arg list
1285 The first version, the one without the indirect object, ran the I<echo>
1286 program, passing it C<"surprise"> an argument. The second version
1287 didn't--it tried to run a program literally called I<"echo surprise">,
1288 didn't find it, and set C<$?> to a non-zero value indicating failure.
1290 Note that C<exec()> will not call your C<END> blocks, nor will it call
1291 any C<DESTROY> methods in your objects.
1295 Returns TRUE if the specified hash key exists in its hash array, even
1296 if the corresponding value is undefined.
1298 print "Exists\n" if exists $array{$key};
1299 print "Defined\n" if defined $array{$key};
1300 print "True\n" if $array{$key};
1302 A hash element can be TRUE only if it's defined, and defined if
1303 it exists, but the reverse doesn't necessarily hold true.
1305 Note that the EXPR can be arbitrarily complicated as long as the final
1306 operation is a hash key lookup:
1308 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1310 Although the last element will not spring into existence just because its
1311 existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1312 C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1313 test for a $key element. This autovivification may be fixed in a later
1318 Evaluates EXPR and exits immediately with that value. (Actually, it
1319 calls any defined C<END> routines first, but the C<END> routines may not
1320 abort the exit. Likewise any object destructors that need to be called
1321 are called before exit.) Example:
1324 exit 0 if $ans =~ /^[Xx]/;
1326 See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1327 universally portable values for EXPR are C<0> for success and C<1> for error;
1328 all other values are subject to unpredictable interpretation depending
1329 on the environment in which the Perl program is running.
1331 You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1332 someone might want to trap whatever error happened. Use C<die()> instead,
1333 which can be trapped by an C<eval()>.
1335 All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1341 Returns I<e> (the natural logarithm base) to the power of EXPR.
1342 If EXPR is omitted, gives C<exp($_)>.
1344 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1346 Implements the fcntl(2) function. You'll probably have to say
1350 first to get the correct constant definitions. Argument processing and
1351 value return works just like C<ioctl()> below.
1355 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1356 or die "can't fcntl F_GETFL: $!";
1358 You don't have to check for C<defined()> on the return from
1359 C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
1360 call into "C<0> but true" in Perl. This string is true in
1361 boolean context and C<0> in numeric context. It is also
1362 exempt from the normal B<-w> warnings on improper numeric
1365 Note that C<fcntl()> will produce a fatal error if used on a machine that
1366 doesn't implement fcntl(2).
1368 =item fileno FILEHANDLE
1370 Returns the file descriptor for a filehandle. This is useful for
1371 constructing bitmaps for C<select()> and low-level POSIX tty-handling
1372 operations. If FILEHANDLE is an expression, the value is taken as
1373 an indirect filehandle, generally its name.
1375 You can use this to find out whether two handles refer to the
1376 same underlying descriptor:
1378 if (fileno(THIS) == fileno(THAT)) {
1379 print "THIS and THAT are dups\n";
1382 =item flock FILEHANDLE,OPERATION
1384 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1385 success, FALSE on failure. Produces a fatal error if used on a machine
1386 that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
1387 is Perl's portable file locking interface, although it locks only entire
1390 On many platforms (including most versions or clones of Unix), locks
1391 established by C<flock()> are B<merely advisory>. Such discretionary locks
1392 are more flexible, but offer fewer guarantees. This means that files
1393 locked with C<flock()> may be modified by programs that do not also use
1394 C<flock()>. Windows NT and OS/2 are among the platforms which
1395 enforce mandatory locking. See your local documentation for details.
1397 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1398 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1399 you can use the symbolic names if import them from the Fcntl module,
1400 either individually, or as a group using the ':flock' tag. LOCK_SH
1401 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1402 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1403 LOCK_EX then C<flock()> will return immediately rather than blocking
1404 waiting for the lock (check the return status to see if you got it).
1406 To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1407 before (un)locking it.
1409 Note that the emulation built with lockf(3) doesn't provide shared
1410 locks, and it requires that FILEHANDLE be open with write intent. These
1411 are the semantics that lockf(3) implements. Most (all?) systems
1412 implement lockf(3) in terms of fcntl(2) locking, though, so the
1413 differing semantics shouldn't bite too many people.
1415 Note also that some versions of C<flock()> cannot lock things over the
1416 network; you would need to use the more system-specific C<fcntl()> for
1417 that. If you like you can force Perl to ignore your system's flock(2)
1418 function, and so provide its own fcntl(2)-based emulation, by passing
1419 the switch C<-Ud_flock> to the F<Configure> program when you configure
1422 Here's a mailbox appender for BSD systems.
1424 use Fcntl ':flock'; # import LOCK_* constants
1427 flock(MBOX,LOCK_EX);
1428 # and, in case someone appended
1429 # while we were waiting...
1434 flock(MBOX,LOCK_UN);
1437 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1438 or die "Can't open mailbox: $!";
1441 print MBOX $msg,"\n\n";
1444 See also L<DB_File> for other flock() examples.
1446 =item for (INITIAL; WHILE; EACH) BLOCK
1448 Do INITIAL, enter BLOCK while EXPR is true, at the end of each round
1449 do EACH. For example:
1451 for ($i = 0, $j = 0; $i < 10; $i++) {
1452 if ($i % 3 == 0) { $j++ }
1453 print "i = $i, j = $j\n";
1456 See L<perlsyn> for more details. See also L</foreach>, a twin of
1457 C<for>, L</while> and L</until>, close cousins of L<for>, and
1458 L</last>, L</next>, and L</redo> for additional control flow.
1460 =item foreach LOOPVAR (LIST) BLOCK
1462 Enter BLOCK as LOOPVAR set in turn to each element of LIST.
1465 foreach $rolling (@stones) { print "rolling $stone\n" }
1467 foreach my $file (@files) { print "file $file\n" }
1469 The LOOPVAR is optional and defaults to C<$_>. If the elements are
1470 modifiable (as opposed to constants or tied variables) you can modify them.
1472 foreach (@words) { tr/abc/xyz/ }
1474 See L<perlsyn> for more details. See also L</for>, a twin of
1475 C<foreach>, L</while> and L</until>, close cousins of L<for>, and
1476 L</last>, L</next>, and L</redo> for additional control flow.
1480 Does a fork(2) system call. Returns the child pid to the parent process,
1481 C<0> to the child process, or C<undef> if the fork is unsuccessful.
1483 Note: unflushed buffers remain unflushed in both processes, which means
1484 you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1485 method of C<IO::Handle> to avoid duplicate output.
1487 If you C<fork()> without ever waiting on your children, you will accumulate
1490 $SIG{CHLD} = sub { wait };
1492 There's also the double-fork trick (error checking on
1493 C<fork()> returns omitted);
1495 unless ($pid = fork) {
1497 exec "what you really wanna do";
1500 ## (some_perl_code_here)
1507 See also L<perlipc> for more examples of forking and reaping
1510 Note that if your forked child inherits system file descriptors like
1511 STDIN and STDOUT that are actually connected by a pipe or socket, even
1512 if you exit, then the remote server (such as, say, httpd or rsh) won't think
1513 you're done. You should reopen those to F</dev/null> if it's any issue.
1517 Declare a picture format for use by the C<write()> function. For
1521 Test: @<<<<<<<< @||||| @>>>>>
1522 $str, $%, '$' . int($num)
1526 $num = $cost/$quantity;
1530 See L<perlform> for many details and examples.
1532 =item formline PICTURE,LIST
1534 This is an internal function used by C<format>s, though you may call it,
1535 too. It formats (see L<perlform>) a list of values according to the
1536 contents of PICTURE, placing the output into the format output
1537 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1538 Eventually, when a C<write()> is done, the contents of
1539 C<$^A> are written to some filehandle, but you could also read C<$^A>
1540 yourself and then set C<$^A> back to C<"">. Note that a format typically
1541 does one C<formline()> per line of form, but the C<formline()> function itself
1542 doesn't care how many newlines are embedded in the PICTURE. This means
1543 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1544 You may therefore need to use multiple formlines to implement a single
1545 record format, just like the format compiler.
1547 Be careful if you put double quotes around the picture, because an "C<@>"
1548 character may be taken to mean the beginning of an array name.
1549 C<formline()> always returns TRUE. See L<perlform> for other examples.
1551 =item getc FILEHANDLE
1555 Returns the next character from the input file attached to FILEHANDLE,
1556 or the undefined value at end of file, or if there was an error. If
1557 FILEHANDLE is omitted, reads from STDIN. This is not particularly
1558 efficient. It cannot be used to get unbuffered single-characters,
1559 however. For that, try something more like:
1562 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1565 system "stty", '-icanon', 'eol', "\001";
1571 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1574 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1578 Determination of whether $BSD_STYLE should be set
1579 is left as an exercise to the reader.
1581 The C<POSIX::getattr()> function can do this more portably on systems
1582 purporting POSIX compliance.
1583 See also the C<Term::ReadKey> module from your nearest CPAN site;
1584 details on CPAN can be found on L<perlmod/CPAN>.
1588 Implements the C library function of the same name, which on most
1589 systems returns the current login from F</etc/utmp>, if any. If null,
1592 $login = getlogin || getpwuid($<) || "Kilroy";
1594 Do not consider C<getlogin()> for authentication: it is not as
1595 secure as C<getpwuid()>.
1597 =item getpeername SOCKET
1599 Returns the packed sockaddr address of other end of the SOCKET connection.
1602 $hersockaddr = getpeername(SOCK);
1603 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1604 $herhostname = gethostbyaddr($iaddr, AF_INET);
1605 $herstraddr = inet_ntoa($iaddr);
1609 Returns the current process group for the specified PID. Use
1610 a PID of C<0> to get the current process group for the
1611 current process. Will raise an exception if used on a machine that
1612 doesn't implement getpgrp(2). If PID is omitted, returns process
1613 group of current process. Note that the POSIX version of C<getpgrp()>
1614 does not accept a PID argument, so only C<PID==0> is truly portable.
1618 Returns the process id of the parent process.
1620 =item getpriority WHICH,WHO
1622 Returns the current priority for a process, a process group, or a user.
1623 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1624 machine that doesn't implement getpriority(2).
1630 =item gethostbyname NAME
1632 =item getnetbyname NAME
1634 =item getprotobyname NAME
1640 =item getservbyname NAME,PROTO
1642 =item gethostbyaddr ADDR,ADDRTYPE
1644 =item getnetbyaddr ADDR,ADDRTYPE
1646 =item getprotobynumber NUMBER
1648 =item getservbyport PORT,PROTO
1666 =item sethostent STAYOPEN
1668 =item setnetent STAYOPEN
1670 =item setprotoent STAYOPEN
1672 =item setservent STAYOPEN
1686 These routines perform the same functions as their counterparts in the
1687 system library. In list context, the return values from the
1688 various get routines are as follows:
1690 ($name,$passwd,$uid,$gid,
1691 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1692 ($name,$passwd,$gid,$members) = getgr*
1693 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1694 ($name,$aliases,$addrtype,$net) = getnet*
1695 ($name,$aliases,$proto) = getproto*
1696 ($name,$aliases,$port,$proto) = getserv*
1698 (If the entry doesn't exist you get a null list.)
1700 In scalar context, you get the name, unless the function was a
1701 lookup by name, in which case you get the other thing, whatever it is.
1702 (If the entry doesn't exist you get the undefined value.) For example:
1704 $uid = getpwnam($name);
1705 $name = getpwuid($num);
1707 $gid = getgrnam($name);
1708 $name = getgrgid($num;
1712 In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
1713 cases in the sense that in many systems they are unsupported. If the
1714 C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1715 usually encodes the disk quota. If the C<$comment> field is unsupported,
1716 it is an empty scalar. If it is supported it usually encodes some
1717 administrative comment about the user. In some systems the $quota
1718 field may be C<$change> or C<$age>, fields that have to do with password
1719 aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
1720 field, if present, encodes the expiration period of the account or the
1721 password. For the availability and the exact meaning of these fields
1722 in your system, please consult your getpwnam(3) documentation and your
1723 F<pwd.h> file. You can also find out from within Perl which meaning
1724 your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1725 field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1726 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
1728 The C<$members> value returned by I<getgr*()> is a space separated list of
1729 the login names of the members of the group.
1731 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1732 C, it will be returned to you via C<$?> if the function call fails. The
1733 C<@addrs> value returned by a successful call is a list of the raw
1734 addresses returned by the corresponding system library call. In the
1735 Internet domain, each address is four bytes long and you can unpack it
1736 by saying something like:
1738 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1740 If you get tired of remembering which element of the return list contains
1741 which return value, by-name interfaces are also provided in modules:
1742 C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1743 C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
1744 normal built-in, replacing them with versions that return objects with
1745 the appropriate names for each field. For example:
1749 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1751 Even though it looks like they're the same method calls (uid),
1752 they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
1754 =item getsockname SOCKET
1756 Returns the packed sockaddr address of this end of the SOCKET connection.
1759 $mysockaddr = getsockname(SOCK);
1760 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1762 =item getsockopt SOCKET,LEVEL,OPTNAME
1764 Returns the socket option requested, or undef if there is an error.
1770 Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
1771 do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1772 operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
1773 The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1774 L<perlop/"I/O Operators">.
1778 Converts a time as returned by the time function to a 9-element array
1779 with the time localized for the standard Greenwich time zone.
1780 Typically used as follows:
1783 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1786 All array elements are numeric, and come straight out of a struct tm.
1787 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1788 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1789 years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
1791 If EXPR is omitted, does C<gmtime(time())>.
1793 In scalar context, returns the ctime(3) value:
1795 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1797 Also see the C<timegm()> function provided by the C<Time::Local> module,
1798 and the strftime(3) function available via the POSIX module.
1800 This scalar value is B<not> locale dependent, see L<perllocale>, but
1801 instead a Perl builtin. Also see the C<Time::Local> module, and the
1802 strftime(3) and mktime(3) function available via the POSIX module. To
1803 get somewhat similar but locale dependent date strings, set up your
1804 locale environment variables appropriately (please see L<perllocale>)
1805 and try for example:
1807 use POSIX qw(strftime);
1808 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1810 Note that the C<%a> and C<%b>, the short forms of the day of the week
1811 and the month of the year, may not necessarily be three characters wide.
1819 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1820 execution there. It may not be used to go into any construct that
1821 requires initialization, such as a subroutine or a C<foreach> loop. It
1822 also can't be used to go into a construct that is optimized away,
1823 or to get out of a block or subroutine given to C<sort()>.
1824 It can be used to go almost anywhere else within the dynamic scope,
1825 including out of subroutines, but it's usually better to use some other
1826 construct such as C<last> or C<die()>. The author of Perl has never felt the
1827 need to use this form of C<goto> (in Perl, that is--C is another matter).
1829 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1830 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1831 necessarily recommended if you're optimizing for maintainability:
1833 goto ("FOO", "BAR", "GLARCH")[$i];
1835 The C<goto-&NAME> form is highly magical, and substitutes a call to the
1836 named subroutine for the currently running subroutine. This is used by
1837 C<AUTOLOAD> subroutines that wish to load another subroutine and then
1838 pretend that the other subroutine had been called in the first place
1839 (except that any modifications to C<@_> in the current subroutine are
1840 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
1841 will be able to tell that this routine was called first.
1843 =item grep BLOCK LIST
1845 =item grep EXPR,LIST
1847 This is similar in spirit to, but not the same as, grep(1)
1848 and its relatives. In particular, it is not limited to using
1849 regular expressions.
1851 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1852 C<$_> to each element) and returns the list value consisting of those
1853 elements for which the expression evaluated to TRUE. In a scalar
1854 context, returns the number of times the expression was TRUE.
1856 @foo = grep(!/^#/, @bar); # weed out comments
1860 @foo = grep {!/^#/} @bar; # weed out comments
1862 Note that, because C<$_> is a reference into the list value, it can be used
1863 to modify the elements of the array. While this is useful and
1864 supported, it can cause bizarre results if the LIST is not a named
1865 array. Similarly, grep returns aliases into the original list,
1866 much like the way that a for loop's index variable aliases the list
1867 elements. That is, modifying an element of a list returned by grep
1868 (for example, in a C<foreach>, C<map()> or another C<grep()>)
1869 actually modifies the element in the original list.
1871 See also L</map> for an array composed of the results of the BLOCK or EXPR.
1877 Interprets EXPR as a hex string and returns the corresponding
1878 value. (To convert strings that might start with either 0 or 0x
1879 see L</oct>.) If EXPR is omitted, uses C<$_>.
1881 print hex '0xAf'; # prints '175'
1882 print hex 'aF'; # same
1884 =item if (EXPR) BLOCK
1886 =item if (EXPR) BLOCK else BLOCK2
1888 =item if (EXPR) BLOCK elsif (EXPR2) BLOCK2
1890 Enter BLOCKs conditionally. The first EXPR to return true
1891 causes the corresponding BLOCK to be entered, or, in the case
1892 of C<else>, the fall-through default BLOCK.
1894 Take notice: Perl wants BLOCKS, expressions (like e.g. in C, C++, or
1897 See L<perlsyn> for more details. See also C<unless>.
1901 There is no builtin C<import()> function. It is just an ordinary
1902 method (subroutine) defined (or inherited) by modules that wish to export
1903 names to another module. The C<use()> function calls the C<import()> method
1904 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1906 =item index STR,SUBSTR,POSITION
1908 =item index STR,SUBSTR
1910 Returns the position of the first occurrence of SUBSTR in STR at or after
1911 POSITION. If POSITION is omitted, starts searching from the beginning of
1912 the string. The return value is based at C<0> (or whatever you've set the C<$[>
1913 variable to--but don't do that). If the substring is not found, returns
1914 one less than the base, ordinarily C<-1>.
1920 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
1921 You should not use this for rounding, because it truncates
1922 towards C<0>, and because machine representations of floating point
1923 numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1924 or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
1926 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1928 Implements the ioctl(2) function. You'll probably have to say
1930 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1932 first to get the correct function definitions. If F<ioctl.ph> doesn't
1933 exist or doesn't have the correct definitions you'll have to roll your
1934 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1935 (There is a Perl script called B<h2ph> that comes with the Perl kit that
1936 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1937 written depending on the FUNCTION--a pointer to the string value of SCALAR
1938 will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
1939 has no string value but does have a numeric value, that value will be
1940 passed rather than a pointer to the string value. To guarantee this to be
1941 TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
1942 functions are useful for manipulating the values of structures used by
1943 C<ioctl()>. The following example sets the erase character to DEL.
1947 die "NO TIOCGETP" if $@ || !$getp;
1948 $sgttyb_t = "ccccs"; # 4 chars and a short
1949 if (ioctl(STDIN,$getp,$sgttyb)) {
1950 @ary = unpack($sgttyb_t,$sgttyb);
1952 $sgttyb = pack($sgttyb_t,@ary);
1953 ioctl(STDIN,&TIOCSETP,$sgttyb)
1954 || die "Can't ioctl: $!";
1957 The return value of C<ioctl()> (and C<fcntl()>) is as follows:
1959 if OS returns: then Perl returns:
1961 0 string "0 but true"
1962 anything else that number
1964 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1965 still easily determine the actual value returned by the operating
1968 ($retval = ioctl(...)) || ($retval = -1);
1969 printf "System returned %d\n", $retval;
1971 The special string "C<0> but true" is excempt from B<-w> complaints
1972 about improper numeric conversions.
1974 =item join EXPR,LIST
1976 Joins the separate strings of LIST into a single string with
1977 fields separated by the value of EXPR, and returns the string.
1980 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1986 Returns a list consisting of all the keys of the named hash. (In a
1987 scalar context, returns the number of keys.) The keys are returned in
1988 an apparently random order, but it is the same order as either the
1989 C<values()> or C<each()> function produces (given that the hash has not been
1990 modified). As a side effect, it resets HASH's iterator.
1992 Here is yet another way to print your environment:
1995 @values = values %ENV;
1996 while ($#keys >= 0) {
1997 print pop(@keys), '=', pop(@values), "\n";
2000 or how about sorted by key:
2002 foreach $key (sort(keys %ENV)) {
2003 print $key, '=', $ENV{$key}, "\n";
2006 To sort a hash by value, you'll need to use a C<sort()> function.
2007 Here's a descending numeric sort of a hash by its values:
2009 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2010 printf "%4d %s\n", $hash{$key}, $key;
2013 As an lvalue C<keys()> allows you to increase the number of hash buckets
2014 allocated for the given hash. This can gain you a measure of efficiency if
2015 you know the hash is going to get big. (This is similar to pre-extending
2016 an array by assigning a larger number to $#array.) If you say
2020 then C<%hash> will have at least 200 buckets allocated for it--256 of them, in fact, since
2021 it rounds up to the next power of two. These
2022 buckets will be retained even if you do C<%hash = ()>, use C<undef
2023 %hash> if you want to free the storage while C<%hash> is still in scope.
2024 You can't shrink the number of buckets allocated for the hash using
2025 C<keys()> in this way (but you needn't worry about doing this by accident,
2026 as trying has no effect).
2030 Sends a signal to a list of processes. The first element of
2031 the list must be the signal to send. Returns the number of
2032 processes successfully signaled.
2034 $cnt = kill 1, $child1, $child2;
2037 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
2038 process groups instead of processes. (On System V, a negative I<PROCESS>
2039 number will also kill process groups, but that's not portable.) That
2040 means you usually want to use positive not negative signals. You may also
2041 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2047 The C<last> command is like the C<break> statement in C (as used in
2048 loops); it immediately exits the loop in question. If the LABEL is
2049 omitted, the command refers to the innermost enclosing loop. The
2050 C<continue> block, if any, is not executed:
2052 LINE: while (<STDIN>) {
2053 last LINE if /^$/; # exit when done with header
2057 C<last> cannot be used to exit a block which returns a value such as
2058 C<eval {}>, C<sub {}> or C<do {}>.
2060 See also L</continue> for an illustration of how C<last>, L</next>, and
2063 See also L<perlsyn>.
2069 Returns an lowercased version of EXPR. This is the internal function
2070 implementing the C<\L> escape in double-quoted strings.
2071 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2073 If EXPR is omitted, uses C<$_>.
2079 Returns the value of EXPR with the first character lowercased. This is
2080 the internal function implementing the C<\l> escape in double-quoted strings.
2081 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2083 If EXPR is omitted, uses C<$_>.
2089 Returns the length in characters of the value of EXPR. If EXPR is
2090 omitted, returns length of C<$_>.
2092 =item link OLDFILE,NEWFILE
2094 Creates a new filename linked to the old filename. Returns TRUE for
2095 success, FALSE otherwise.
2097 =item listen SOCKET,QUEUESIZE
2099 Does the same thing that the listen system call does. Returns TRUE if
2100 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
2104 A local modifies the listed variables to be local to the enclosing
2105 block, file, or eval. If more than one value is listed, the list must
2106 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2107 for details, including issues with tied arrays and hashes.
2109 You really probably want to be using C<my()> instead, because C<local()> isn't
2110 what most people think of as "local". See L<perlsub/"Private Variables
2111 via my()"> for details.
2113 =item localtime EXPR
2115 Converts a time as returned by the time function to a 9-element array
2116 with the time analyzed for the local time zone. Typically used as
2120 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2123 All array elements are numeric, and come straight out of a struct tm.
2124 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
2125 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
2126 years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year.
2128 If EXPR is omitted, uses the current time (C<localtime(time)>).
2130 In scalar context, returns the ctime(3) value:
2132 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2134 This scalar value is B<not> locale dependent, see L<perllocale>, but
2135 instead a Perl builtin. Also see the C<Time::Local> module, and the
2136 strftime(3) and mktime(3) function available via the POSIX module. To
2137 get somewhat similar but locale dependent date strings, set up your
2138 locale environment variables appropriately (please see L<perllocale>)
2139 and try for example:
2141 use POSIX qw(strftime);
2142 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2144 Note that the C<%a> and C<%b>, the short forms of the day of the week
2145 and the month of the year, may not necessarily be three characters wide.
2151 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
2154 =item lstat FILEHANDLE
2160 Does the same thing as the C<stat()> function (including setting the
2161 special C<_> filehandle) but stats a symbolic link instead of the file
2162 the symbolic link points to. If symbolic links are unimplemented on
2163 your system, a normal C<stat()> is done.
2165 If EXPR is omitted, stats C<$_>.
2169 The match operator. See L<perlop>.
2171 =item map BLOCK LIST
2175 Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
2176 element) and returns the list value composed of the results of each such
2177 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2178 may produce zero, one, or more elements in the returned value.
2180 @chars = map(chr, @nums);
2182 translates a list of numbers to the corresponding characters. And
2184 %hash = map { getkey($_) => $_ } @array;
2186 is just a funny way to write
2189 foreach $_ (@array) {
2190 $hash{getkey($_)} = $_;
2193 Note that, because C<$_> is a reference into the list value, it can be used
2194 to modify the elements of the array. While this is useful and
2195 supported, it can cause bizarre results if the LIST is not a named
2196 array. See also L</grep> for an array composed of those items of the
2197 original list for which the BLOCK or EXPR evaluates to true.
2199 =item mkdir FILENAME,MODE
2201 Creates the directory specified by FILENAME, with permissions
2202 specified by MODE (as modified by C<umask>). If it succeeds it
2203 returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
2205 In general, it is better to create directories with permissive MODEs,
2206 and let the user modify that with their C<umask>, than it is to supply
2207 a restrictive MODE and give the user no way to be more permissive.
2208 The exceptions to this rule are when the file or directory should be
2209 kept private (mail files, for instance). The perlfunc(1) entry on
2210 C<umask> discusses the choice of MODE in more detail.
2212 =item msgctl ID,CMD,ARG
2214 Calls the System V IPC function msgctl(2). You'll probably have to say
2218 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2219 then ARG must be a variable which will hold the returned C<msqid_ds>
2220 structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
2221 true" for zero, or the actual return value otherwise. See also
2222 C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
2224 =item msgget KEY,FLAGS
2226 Calls the System V IPC function msgget(2). Returns the message queue
2227 id, or the undefined value if there is an error. See also C<IPC::SysV>
2228 and C<IPC::SysV::Msg> documentation.
2230 =item msgsnd ID,MSG,FLAGS
2232 Calls the System V IPC function msgsnd to send the message MSG to the
2233 message queue ID. MSG must begin with the long integer message type,
2234 which may be created with C<pack("l", $type)>. Returns TRUE if
2235 successful, or FALSE if there is an error. See also C<IPC::SysV>
2236 and C<IPC::SysV::Msg> documentation.
2238 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2240 Calls the System V IPC function msgrcv to receive a message from
2241 message queue ID into variable VAR with a maximum message size of
2242 SIZE. Note that if a message is received, the message type will be
2243 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2244 size of the message type. Returns TRUE if successful, or FALSE if
2245 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2249 A C<my()> declares the listed variables to be local (lexically) to the
2250 enclosing block, file, or C<eval()>. If
2251 more than one value is listed, the list must be placed in parentheses. See
2252 L<perlsub/"Private Variables via my()"> for details.
2258 The C<next> command is like the C<continue> statement in C; it starts
2259 the next iteration of the loop:
2261 LINE: while (<STDIN>) {
2262 next LINE if /^#/; # discard comments
2266 Note that if there were a C<continue> block on the above, it would get
2267 executed even on discarded lines. If the LABEL is omitted, the command
2268 refers to the innermost enclosing loop.
2270 C<next> cannot be used to exit a block which returns a value such as
2271 C<eval {}>, C<sub {}> or C<do {}>.
2273 See also L</continue> for an illustration of how L</last>, C<next>, and
2276 See also L<perlsyn>.
2278 =item no Module LIST
2280 See the L</use> function, which C<no> is the opposite of.
2286 Interprets EXPR as an octal string and returns the corresponding
2287 value. (If EXPR happens to start off with C<0x>, interprets it as
2288 a hex string instead.) The following will handle decimal, octal, and
2289 hex in the standard Perl or C notation:
2291 $val = oct($val) if $val =~ /^0/;
2293 If EXPR is omitted, uses C<$_>. This function is commonly used when
2294 a string such as C<644> needs to be converted into a file mode, for
2295 example. (Although perl will automatically convert strings into
2296 numbers as needed, this automatic conversion assumes base 10.)
2298 =item open FILEHANDLE,EXPR
2300 =item open FILEHANDLE
2302 Opens the file whose filename is given by EXPR, and associates it with
2303 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2304 name of the real filehandle wanted. If EXPR is omitted, the scalar
2305 variable of the same name as the FILEHANDLE contains the filename.
2306 (Note that lexical variables--those declared with C<my()>--will not work
2307 for this purpose; so if you're using C<my()>, specify EXPR in your call
2310 If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2311 If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2312 output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
2313 the file is opened for appending, again being created if necessary.
2314 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2315 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2316 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2317 file first. You can't usually use either read-write mode for updating
2318 textfiles, since they have variable length records. See the B<-i>
2319 switch in L<perlrun> for a better approach. The file is created with
2320 permissions of C<0666> modified by the process' C<umask> value.
2322 The prefix and the filename may be separated with spaces.
2323 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2324 C<'w+'>, C<'a'>, and C<'a+'>.
2326 If the filename begins with C<'|'>, the filename is interpreted as a
2327 command to which output is to be piped, and if the filename ends with a
2328 C<'|'>, the filename is interpreted See L<perlipc/"Using open() for IPC">
2329 for more examples of this. (You are not allowed to C<open()> to a command
2330 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2331 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2333 Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
2334 nonzero upon success, the undefined value otherwise. If the C<open()>
2335 involved a pipe, the return value happens to be the pid of the
2338 If you're unfortunate enough to be running Perl on a system that
2339 distinguishes between text files and binary files (modern operating
2340 systems don't care), then you should check out L</binmode> for tips for
2341 dealing with this. The key distinction between systems that need C<binmode()>
2342 and those that don't is their text file formats. Systems like Unix, MacOS, and
2343 Plan9, which delimit lines with a single character, and which encode that
2344 character in C as C<"\n">, do not need C<binmode()>. The rest need it.
2346 When opening a file, it's usually a bad idea to continue normal execution
2347 if the request failed, so C<open()> is frequently used in connection with
2348 C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
2349 where you want to make a nicely formatted error message (but there are
2350 modules that can help with that problem)) you should always check
2351 the return value from opening a file. The infrequent exception is when
2352 working with an unopened filehandle is actually what you want to do.
2357 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2358 while (<ARTICLE>) {...
2360 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2361 # if the open fails, output is discarded
2363 open(DBASE, '+<dbase.mine') # open for update
2364 or die "Can't open 'dbase.mine' for update: $!";
2366 open(ARTICLE, "caesar <$article |") # decrypt article
2367 or die "Can't start caesar: $!";
2369 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2370 or die "Can't start sort: $!";
2372 # process argument list of files along with any includes
2374 foreach $file (@ARGV) {
2375 process($file, 'fh00');
2379 my($filename, $input) = @_;
2380 $input++; # this is a string increment
2381 unless (open($input, $filename)) {
2382 print STDERR "Can't open $filename: $!\n";
2387 while (<$input>) { # note use of indirection
2388 if (/^#include "(.*)"/) {
2389 process($1, $input);
2396 You may also, in the Bourne shell tradition, specify an EXPR beginning
2397 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2398 name of a filehandle (or file descriptor, if numeric) to be
2399 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2400 C<+E<gt>E<gt>>, and C<+E<lt>>. The
2401 mode you specify should match the mode of the original filehandle.
2402 (Duping a filehandle does not take into account any existing contents of
2404 Here is a script that saves, redirects, and restores STDOUT and
2408 open(OLDOUT, ">&STDOUT");
2409 open(OLDERR, ">&STDERR");
2411 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2412 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2414 select(STDERR); $| = 1; # make unbuffered
2415 select(STDOUT); $| = 1; # make unbuffered
2417 print STDOUT "stdout 1\n"; # this works for
2418 print STDERR "stderr 1\n"; # subprocesses too
2423 open(STDOUT, ">&OLDOUT");
2424 open(STDERR, ">&OLDERR");
2426 print STDOUT "stdout 2\n";
2427 print STDERR "stderr 2\n";
2430 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2431 equivalent of C's C<fdopen()> of that file descriptor; this is more
2432 parsimonious of file descriptors. For example:
2434 open(FILEHANDLE, "<&=$fd")
2436 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
2437 there is an implicit fork done, and the return value of open is the pid
2438 of the child within the parent process, and C<0> within the child
2439 process. (Use C<defined($pid)> to determine whether the open was successful.)
2440 The filehandle behaves normally for the parent, but i/o to that
2441 filehandle is piped from/to the STDOUT/STDIN of the child process.
2442 In the child process the filehandle isn't opened--i/o happens from/to
2443 the new STDOUT or STDIN. Typically this is used like the normal
2444 piped open when you want to exercise more control over just how the
2445 pipe command gets executed, such as when you are running setuid, and
2446 don't want to have to scan shell commands for metacharacters.
2447 The following pairs are more or less equivalent:
2449 open(FOO, "|tr '[a-z]' '[A-Z]'");
2450 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2452 open(FOO, "cat -n '$file'|");
2453 open(FOO, "-|") || exec 'cat', '-n', $file;
2455 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2457 NOTE: On any operation that may do a fork, any unflushed buffers remain
2458 unflushed in both processes, which means you may need to set C<$|> to
2459 avoid duplicate output.
2461 Closing any piped filehandle causes the parent process to wait for the
2462 child to finish, and returns the status value in C<$?>.
2464 The filename passed to open will have leading and trailing
2465 whitespace deleted, and the normal redirection characters
2466 honored. This property, known as "magic open",
2467 can often be used to good effect. A user could specify a filename of
2468 F<"rsh cat file |">, or you could change certain filenames as needed:
2470 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2471 open(FH, $filename) or die "Can't open $filename: $!";
2473 However, to open a file with arbitrary weird characters in it, it's
2474 necessary to protect any leading and trailing whitespace:
2476 $file =~ s#^(\s)#./$1#;
2477 open(FOO, "< $file\0");
2479 If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2480 should use the C<sysopen()> function, which involves no such magic. This is
2481 another way to protect your filenames from interpretation. For example:
2484 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2485 or die "sysopen $path: $!";
2486 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2487 print HANDLE "stuff $$\n");
2489 print "File contains: ", <HANDLE>;
2491 Using the constructor from the C<IO::Handle> package (or one of its
2492 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2493 filehandles that have the scope of whatever variables hold references to
2494 them, and automatically close whenever and however you leave that scope:
2498 sub read_myfile_munged {
2500 my $handle = new IO::File;
2501 open($handle, "myfile") or die "myfile: $!";
2503 or return (); # Automatically closed here.
2504 mung $first or die "mung failed"; # Or here.
2505 return $first, <$handle> if $ALL; # Or here.
2509 See L</seek()> for some details about mixing reading and writing.
2511 =item opendir DIRHANDLE,EXPR
2513 Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2514 C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
2515 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2521 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2522 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2524 =item pack TEMPLATE,LIST
2526 Takes an array or list of values and packs it into a binary structure,
2527 returning the string containing the structure. The TEMPLATE is a
2528 sequence of characters that give the order and type of values, as
2531 A An ascii string, will be space padded.
2532 a An ascii string, will be null padded.
2533 b A bit string (ascending bit order, like vec()).
2534 B A bit string (descending bit order).
2535 h A hex string (low nybble first).
2536 H A hex string (high nybble first).
2538 c A signed char value.
2539 C An unsigned char value. Only does bytes. See U for Unicode.
2541 s A signed short value.
2542 S An unsigned short value.
2543 (This 'short' is _exactly_ 16 bits, which may differ from
2544 what a local C compiler calls 'short'.)
2546 i A signed integer value.
2547 I An unsigned integer value.
2548 (This 'integer' is _at_least_ 32 bits wide. Its exact
2549 size depends on what a local C compiler calls 'int',
2550 and may even be larger than the 'long' described in
2553 l A signed long value.
2554 L An unsigned long value.
2555 (This 'long' is _exactly_ 32 bits, which may differ from
2556 what a local C compiler calls 'long'.)
2558 n A short in "network" (big-endian) order.
2559 N A long in "network" (big-endian) order.
2560 v A short in "VAX" (little-endian) order.
2561 V A long in "VAX" (little-endian) order.
2562 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2563 _exactly_ 32 bits, respectively.)
2565 q A signed quad (64-bit) value.
2566 Q An unsigned quad value.
2567 (Available only if your system supports 64-bit integer values
2568 _and_ if Perl has been compiled to support those.
2569 Causes a fatal error otherwise.)
2571 f A single-precision float in the native format.
2572 d A double-precision float in the native format.
2574 p A pointer to a null-terminated string.
2575 P A pointer to a structure (fixed-length string).
2577 u A uuencoded string.
2578 U A Unicode character number. Encodes to UTF-8 internally.
2579 Works even if C<use utf8> is not in effect.
2581 w A BER compressed integer. Its bytes represent an unsigned
2582 integer in base 128, most significant digit first, with as
2583 few digits as possible. Bit eight (the high bit) is set
2584 on each byte except the last.
2588 @ Null fill to absolute position.
2590 Each letter may optionally be followed by a number giving a repeat
2591 count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the
2592 pack function will gobble up that many values from the LIST. A C<*> for the
2593 repeat count means to use however many items are left. The C<"a"> and C<"A">
2594 types gobble just one value, but pack it as a string of length count,
2595 padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips
2596 trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B">
2597 fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a
2598 string that many nybbles long. The C<"p"> type packs a pointer to a null-
2599 terminated string. You are responsible for ensuring the string is not a
2600 temporary value (which can potentially get deallocated before you get
2601 around to using the packed result). The C<"P"> packs a pointer to a structure
2602 of the size indicated by the length. A NULL pointer is created if the
2603 corresponding value for C<"p"> or C<"P"> is C<undef>.
2604 Real numbers (floats and doubles) are
2605 in the native machine format only; due to the multiplicity of floating
2606 formats around, and the lack of a standard "network" representation, no
2607 facility for interchange has been made. This means that packed floating
2608 point data written on one machine may not be readable on another - even if
2609 both use IEEE floating point arithmetic (as the endian-ness of the memory
2610 representation is not part of the IEEE spec). Note that Perl uses doubles
2611 internally for all numeric calculation, and converting from double into
2612 float and thence back to double again will lose precision (i.e.,
2613 C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>).
2617 $foo = pack("CCCC",65,66,67,68);
2619 $foo = pack("C4",65,66,67,68);
2621 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2622 # same thing with Unicode circled letters
2624 $foo = pack("ccxxcc",65,66,67,68);
2627 $foo = pack("s2",1,2);
2628 # "\1\0\2\0" on little-endian
2629 # "\0\1\0\2" on big-endian
2631 $foo = pack("a4","abcd","x","y","z");
2634 $foo = pack("aaaa","abcd","x","y","z");
2637 $foo = pack("a14","abcdefg");
2638 # "abcdefg\0\0\0\0\0\0\0"
2640 $foo = pack("i9pl", gmtime);
2641 # a real struct tm (on my system anyway)
2644 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2647 The same template may generally also be used in the unpack function.
2651 =item package NAMESPACE
2653 Declares the compilation unit as being in the given namespace. The scope
2654 of the package declaration is from the declaration itself through the end of
2655 the enclosing block (the same scope as the C<local()> operator). All further
2656 unqualified dynamic identifiers will be in this namespace. A package
2657 statement affects only dynamic variables--including those you've used
2658 C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
2659 would be the first declaration in a file to be included by the C<require>
2660 or C<use> operator. You can switch into a package in more than one place;
2661 it merely influences which symbol table is used by the compiler for the
2662 rest of that block. You can refer to variables and filehandles in other
2663 packages by prefixing the identifier with the package name and a double
2664 colon: C<$Package::Variable>. If the package name is null, the C<main>
2665 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2667 If NAMESPACE is omitted, then there is no current package, and all
2668 identifiers must be fully qualified or lexicals. This is stricter
2669 than C<use strict>, since it also extends to function names.
2671 See L<perlmod/"Packages"> for more information about packages, modules,
2672 and classes. See L<perlsub> for other scoping issues.
2674 =item pipe READHANDLE,WRITEHANDLE
2676 Opens a pair of connected pipes like the corresponding system call.
2677 Note that if you set up a loop of piped processes, deadlock can occur
2678 unless you are very careful. In addition, note that Perl's pipes use
2679 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2680 after each command, depending on the application.
2682 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2683 for examples of such things.
2689 Pops and returns the last value of the array, shortening the array by
2690 1. Has a similar effect to
2692 $tmp = $ARRAY[$#ARRAY--];
2694 If there are no elements in the array, returns the undefined value.
2695 If ARRAY is omitted, pops the
2696 C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2703 Returns the offset of where the last C<m//g> search left off for the variable
2704 is in question (C<$_> is used when the variable is not specified). May be
2705 modified to change that offset. Such modification will also influence
2706 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2709 =item print FILEHANDLE LIST
2715 Prints a string or a comma-separated list of strings. Returns TRUE
2716 if successful. FILEHANDLE may be a scalar variable name, in which case
2717 the variable contains the name of or a reference to the filehandle, thus introducing one
2718 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2719 token is a term, it may be misinterpreted as an operator unless you
2720 interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
2721 omitted, prints by default to standard output (or to the last selected
2722 output channel--see L</select>). If LIST is also omitted, prints C<$_> to
2723 the currently selected output channel. To set the default output channel to something other than
2724 STDOUT use the select operation. Note that, because print takes a
2725 LIST, anything in the LIST is evaluated in list context, and any
2726 subroutine that you call will have one or more of its expressions
2727 evaluated in list context. Also be careful not to follow the print
2728 keyword with a left parenthesis unless you want the corresponding right
2729 parenthesis to terminate the arguments to the print--interpose a C<+> or
2730 put parentheses around all the arguments.
2732 Note that if you're storing FILEHANDLES in an array or other expression,
2733 you will have to use a block returning its value instead:
2735 print { $files[$i] } "stuff\n";
2736 print { $OK ? STDOUT : STDERR } "stuff\n";
2738 =item printf FILEHANDLE FORMAT, LIST
2740 =item printf FORMAT, LIST
2742 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
2743 (the output record separator) is not appended. The first argument
2744 of the list will be interpreted as the C<printf()> format. If C<use locale> is
2745 in effect, the character used for the decimal point in formatted real numbers
2746 is affected by the LC_NUMERIC locale. See L<perllocale>.
2748 Don't fall into the trap of using a C<printf()> when a simple
2749 C<print()> would do. The C<print()> is more efficient and less
2752 =item prototype FUNCTION
2754 Returns the prototype of a function as a string (or C<undef> if the
2755 function has no prototype). FUNCTION is a reference to, or the name of,
2756 the function whose prototype you want to retrieve.
2758 If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2759 a name for Perl builtin. If builtin is not I<overridable> (such as
2760 C<qw//>) or its arguments cannot be expressed by a prototype (such as
2761 C<system()>) - in other words, the builtin does not behave like a Perl
2762 function - returns C<undef>. Otherwise, the string describing the
2763 equivalent prototype is returned.
2765 =item push ARRAY,LIST
2767 Treats ARRAY as a stack, and pushes the values of LIST
2768 onto the end of ARRAY. The length of ARRAY increases by the length of
2769 LIST. Has the same effect as
2772 $ARRAY[++$#ARRAY] = $value;
2775 but is more efficient. Returns the new number of elements in the array.
2787 Generalized quotes. See L<perlop>.
2789 =item quotemeta EXPR
2793 Returns the value of EXPR with all non-alphanumeric
2794 characters backslashed. (That is, all characters not matching
2795 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2796 returned string, regardless of any locale settings.)
2797 This is the internal function implementing
2798 the C<\Q> escape in double-quoted strings.
2800 If EXPR is omitted, uses C<$_>.
2806 Returns a random fractional number greater than or equal to C<0> and less
2807 than the value of EXPR. (EXPR should be positive.) If EXPR is
2808 omitted, the value C<1> is used. Automatically calls C<srand()> unless
2809 C<srand()> has already been called. See also C<srand()>.
2811 (Note: If your rand function consistently returns numbers that are too
2812 large or too small, then your version of Perl was probably compiled
2813 with the wrong number of RANDBITS.)
2815 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2817 =item read FILEHANDLE,SCALAR,LENGTH
2819 Attempts to read LENGTH bytes of data into variable SCALAR from the
2820 specified FILEHANDLE. Returns the number of bytes actually read,
2821 C<0> at end of file, or undef if there was an error. SCALAR will be grown
2822 or shrunk to the length actually read. An OFFSET may be specified to
2823 place the read data at some other place than the beginning of the
2824 string. This call is actually implemented in terms of stdio's fread(3)
2825 call. To get a true read(2) system call, see C<sysread()>.
2827 =item readdir DIRHANDLE
2829 Returns the next directory entry for a directory opened by C<opendir()>.
2830 If used in list context, returns all the rest of the entries in the
2831 directory. If there are no more entries, returns an undefined value in
2832 scalar context or a null list in list context.
2834 If you're planning to filetest the return values out of a C<readdir()>, you'd
2835 better prepend the directory in question. Otherwise, because we didn't
2836 C<chdir()> there, it would have been testing the wrong file.
2838 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2839 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2844 Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
2845 is read and returned. In list context, reads until end-of-file is
2846 reached and returns a list of lines (however you've defined lines
2847 with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2848 This is the internal function implementing the C<E<lt>EXPRE<gt>>
2849 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2850 operator is discussed in more detail in L<perlop/"I/O Operators">.
2853 $line = readline(*STDIN); # same thing
2859 Returns the value of a symbolic link, if symbolic links are
2860 implemented. If not, gives a fatal error. If there is some system
2861 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2862 omitted, uses C<$_>.
2866 EXPR is executed as a system command.
2867 The collected standard output of the command is returned.
2868 In scalar context, it comes back as a single (potentially
2869 multi-line) string. In list context, returns a list of lines
2870 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2871 This is the internal function implementing the C<qx/EXPR/>
2872 operator, but you can use it directly. The C<qx/EXPR/>
2873 operator is discussed in more detail in L<perlop/"I/O Operators">.
2875 =item recv SOCKET,SCALAR,LEN,FLAGS
2877 Receives a message on a socket. Attempts to receive LENGTH bytes of
2878 data into variable SCALAR from the specified SOCKET filehandle.
2879 Actually does a C C<recvfrom()>, so that it can return the address of the
2880 sender. Returns the undefined value if there's an error. SCALAR will
2881 be grown or shrunk to the length actually read. Takes the same flags
2882 as the system call of the same name.
2883 See L<perlipc/"UDP: Message Passing"> for examples.
2889 The C<redo> command restarts the loop block without evaluating the
2890 conditional again. The L</continue> block, if any, is not executed. If
2891 the LABEL is omitted, the command refers to the innermost enclosing
2892 loop. This command is normally used by programs that want to lie to
2893 themselves about what was just input:
2895 # a simpleminded Pascal comment stripper
2896 # (warning: assumes no { or } in strings)
2897 LINE: while (<STDIN>) {
2898 while (s|({.*}.*){.*}|$1 |) {}
2903 if (/}/) { # end of comment?
2912 C<redo> cannot be used to retry a block which returns a value such as
2913 C<eval {}>, C<sub {}> or C<do {}>.
2915 See also L</continue> for an illustration of how L</last>, L</next>, and
2918 See also L<perlsyn>.
2924 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2925 is not specified, C<$_> will be used. The value returned depends on the
2926 type of thing the reference is a reference to.
2927 Builtin types include:
2936 If the referenced object has been blessed into a package, then that package
2937 name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
2939 if (ref($r) eq "HASH") {
2940 print "r is a reference to a hash.\n";
2943 print "r is not a reference at all.\n";
2946 See also L<perlref>.
2948 =item rename OLDNAME,NEWNAME
2950 Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
2951 not work across file system boundaries.
2957 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
2958 supplied. If EXPR is numeric, demands that the current version of Perl
2959 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2961 Otherwise, demands that a library file be included if it hasn't already
2962 been included. The file is included via the do-FILE mechanism, which is
2963 essentially just a variety of C<eval()>. Has semantics similar to the following
2968 return 1 if $INC{$filename};
2969 my($realfilename,$result);
2971 foreach $prefix (@INC) {
2972 $realfilename = "$prefix/$filename";
2973 if (-f $realfilename) {
2974 $result = do $realfilename;
2978 die "Can't find $filename in \@INC";
2981 die "$filename did not return true value" unless $result;
2982 $INC{$filename} = $realfilename;
2986 Note that the file will not be included twice under the same specified
2987 name. The file must return TRUE as the last statement to indicate
2988 successful execution of any initialization code, so it's customary to
2989 end such a file with "C<1;>" unless you're sure it'll return TRUE
2990 otherwise. But it's better just to put the "C<1;>", in case you add more
2993 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
2994 replaces "F<::>" with "F</>" in the filename for you,
2995 to make it easy to load standard modules. This form of loading of
2996 modules does not risk altering your namespace.
2998 In other words, if you try this:
3000 require Foo::Bar; # a splendid bareword
3002 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3003 directories specified in the C<@INC> array.
3005 But if you try this:
3007 $class = 'Foo::Bar';
3008 require $class; # $class is not a bareword
3010 require "Foo::Bar"; # not a bareword because of the ""
3012 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3013 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3015 eval "require $class";
3017 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3023 Generally used in a C<continue> block at the end of a loop to clear
3024 variables and reset C<??> searches so that they work again. The
3025 expression is interpreted as a list of single characters (hyphens
3026 allowed for ranges). All variables and arrays beginning with one of
3027 those letters are reset to their pristine state. If the expression is
3028 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3029 only variables or searches in the current package. Always returns
3032 reset 'X'; # reset all X variables
3033 reset 'a-z'; # reset lower case variables
3034 reset; # just reset ?? searches
3036 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3037 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
3038 are unaffected, but they clean themselves up on scope exit anyway,
3039 so you'll probably want to use them instead. See L</my>.
3045 Returns from a subroutine, C<eval()>, or C<do FILE> with the value
3046 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3047 context, depending on how the return value will be used, and the context
3048 may vary from one execution to the next (see C<wantarray()>). If no EXPR
3049 is given, returns an empty list in list context, an undefined value in
3050 scalar context, or nothing in a void context.
3052 (Note that in the absence of a return, a subroutine, eval, or do FILE
3053 will automatically return the value of the last expression evaluated.)
3057 In list context, returns a list value consisting of the elements
3058 of LIST in the opposite order. In scalar context, concatenates the
3059 elements of LIST, and returns a string value with all the characters
3060 in the opposite order.
3062 print reverse <>; # line tac, last line first
3064 undef $/; # for efficiency of <>
3065 print scalar reverse <>; # character tac, last line tsrif
3067 This operator is also handy for inverting a hash, although there are some
3068 caveats. If a value is duplicated in the original hash, only one of those
3069 can be represented as a key in the inverted hash. Also, this has to
3070 unwind one hash and build a whole new one, which may take some time
3073 %by_name = reverse %by_address; # Invert the hash
3075 =item rewinddir DIRHANDLE
3077 Sets the current position to the beginning of the directory for the
3078 C<readdir()> routine on DIRHANDLE.
3080 =item rindex STR,SUBSTR,POSITION
3082 =item rindex STR,SUBSTR
3084 Works just like index except that it returns the position of the LAST
3085 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3086 last occurrence at or before that position.
3088 =item rmdir FILENAME
3092 Deletes the directory specified by FILENAME if that directory is empty. If it
3093 succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
3094 FILENAME is omitted, uses C<$_>.
3098 The substitution operator. See L<perlop>.
3102 Forces EXPR to be interpreted in scalar context and returns the value
3105 @counts = ( scalar @a, scalar @b, scalar @c );
3107 There is no equivalent operator to force an expression to
3108 be interpolated in list context because it's in practice never
3109 needed. If you really wanted to do so, however, you could use
3110 the construction C<@{[ (some expression) ]}>, but usually a simple
3111 C<(some expression)> suffices.
3113 =item seek FILEHANDLE,POSITION,WHENCE
3115 Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
3116 FILEHANDLE may be an expression whose value gives the name of the
3117 filehandle. The values for WHENCE are C<0> to set the new position to
3118 POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
3119 set it to EOF plus POSITION (typically negative). For WHENCE you may
3120 use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3121 C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
3123 If you want to position file for C<sysread()> or C<syswrite()>, don't use
3124 C<seek()> -- buffering makes its effect on the file's system position
3125 unpredictable and non-portable. Use C<sysseek()> instead.
3127 On some systems you have to do a seek whenever you switch between reading
3128 and writing. Amongst other things, this may have the effect of calling
3129 stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
3134 This is also useful for applications emulating C<tail -f>. Once you hit
3135 EOF on your read, and then sleep for a while, you might have to stick in a
3136 seek() to reset things. The C<seek()> doesn't change the current position,
3137 but it I<does> clear the end-of-file condition on the handle, so that the
3138 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3140 If that doesn't work (some stdios are particularly cantankerous), then
3141 you may need something more like this:
3144 for ($curpos = tell(FILE); $_ = <FILE>;
3145 $curpos = tell(FILE)) {
3146 # search for some stuff and put it into files
3148 sleep($for_a_while);
3149 seek(FILE, $curpos, 0);
3152 =item seekdir DIRHANDLE,POS
3154 Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3155 must be a value returned by C<telldir()>. Has the same caveats about
3156 possible directory compaction as the corresponding system library
3159 =item select FILEHANDLE
3163 Returns the currently selected filehandle. Sets the current default
3164 filehandle for output, if FILEHANDLE is supplied. This has two
3165 effects: first, a C<write()> or a C<print()> without a filehandle will
3166 default to this FILEHANDLE. Second, references to variables related to
3167 output will refer to this output channel. For example, if you have to
3168 set the top of form format for more than one output channel, you might
3176 FILEHANDLE may be an expression whose value gives the name of the
3177 actual filehandle. Thus:
3179 $oldfh = select(STDERR); $| = 1; select($oldfh);
3181 Some programmers may prefer to think of filehandles as objects with
3182 methods, preferring to write the last example as:
3185 STDERR->autoflush(1);
3187 =item select RBITS,WBITS,EBITS,TIMEOUT
3189 This calls the select(2) system call with the bit masks specified, which
3190 can be constructed using C<fileno()> and C<vec()>, along these lines:
3192 $rin = $win = $ein = '';
3193 vec($rin,fileno(STDIN),1) = 1;
3194 vec($win,fileno(STDOUT),1) = 1;
3197 If you want to select on many filehandles you might wish to write a
3201 my(@fhlist) = split(' ',$_[0]);
3204 vec($bits,fileno($_),1) = 1;
3208 $rin = fhbits('STDIN TTY SOCK');
3212 ($nfound,$timeleft) =
3213 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3215 or to block until something becomes ready just do this
3217 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3219 Most systems do not bother to return anything useful in C<$timeleft>, so
3220 calling select() in scalar context just returns C<$nfound>.
3222 Any of the bit masks can also be undef. The timeout, if specified, is
3223 in seconds, which may be fractional. Note: not all implementations are
3224 capable of returning theC<$timeleft>. If not, they always return
3225 C<$timeleft> equal to the supplied C<$timeout>.
3227 You can effect a sleep of 250 milliseconds this way:
3229 select(undef, undef, undef, 0.25);
3231 B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3232 or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3233 then only on POSIX systems. You have to use C<sysread()> instead.
3235 =item semctl ID,SEMNUM,CMD,ARG
3237 Calls the System V IPC function C<semctl()>. You'll probably have to say
3241 first to get the correct constant definitions. If CMD is IPC_STAT or
3242 GETALL, then ARG must be a variable which will hold the returned
3243 semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
3244 undefined value for error, "C<0> but true" for zero, or the actual return
3245 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3247 =item semget KEY,NSEMS,FLAGS
3249 Calls the System V IPC function semget. Returns the semaphore id, or
3250 the undefined value if there is an error. See also C<IPC::SysV> and
3251 C<IPC::SysV::Semaphore> documentation.
3253 =item semop KEY,OPSTRING
3255 Calls the System V IPC function semop to perform semaphore operations
3256 such as signaling and waiting. OPSTRING must be a packed array of
3257 semop structures. Each semop structure can be generated with
3258 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3259 operations is implied by the length of OPSTRING. Returns TRUE if
3260 successful, or FALSE if there is an error. As an example, the
3261 following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
3263 $semop = pack("sss", $semnum, -1, 0);
3264 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3266 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3267 and C<IPC::SysV::Semaphore> documentation.
3269 =item send SOCKET,MSG,FLAGS,TO
3271 =item send SOCKET,MSG,FLAGS
3273 Sends a message on a socket. Takes the same flags as the system call
3274 of the same name. On unconnected sockets you must specify a
3275 destination to send TO, in which case it does a C C<sendto()>. Returns
3276 the number of characters sent, or the undefined value if there is an
3278 See L<perlipc/"UDP: Message Passing"> for examples.
3280 =item setpgrp PID,PGRP
3282 Sets the current process group for the specified PID, C<0> for the current
3283 process. Will produce a fatal error if used on a machine that doesn't
3284 implement setpgrp(2). If the arguments are omitted, it defaults to
3285 C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3286 arguments, so only setpgrp C<0,0> is portable.
3288 =item setpriority WHICH,WHO,PRIORITY
3290 Sets the current priority for a process, a process group, or a user.
3291 (See setpriority(2).) Will produce a fatal error if used on a machine
3292 that doesn't implement setpriority(2).
3294 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3296 Sets the socket option requested. Returns undefined if there is an
3297 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3304 Shifts the first value of the array off and returns it, shortening the
3305 array by 1 and moving everything down. If there are no elements in the
3306 array, returns the undefined value. If ARRAY is omitted, shifts the
3307 C<@_> array within the lexical scope of subroutines and formats, and the
3308 C<@ARGV> array at file scopes or within the lexical scopes established by
3309 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3310 See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3311 same thing to the left end of an array that C<pop()> and C<push()> do to the
3314 =item shmctl ID,CMD,ARG
3316 Calls the System V IPC function shmctl. You'll probably have to say
3320 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3321 then ARG must be a variable which will hold the returned C<shmid_ds>
3322 structure. Returns like ioctl: the undefined value for error, "C<0> but
3323 true" for zero, or the actual return value otherwise.
3324 See also C<IPC::SysV> documentation.
3326 =item shmget KEY,SIZE,FLAGS
3328 Calls the System V IPC function shmget. Returns the shared memory
3329 segment id, or the undefined value if there is an error.
3330 See also C<IPC::SysV> documentation.
3332 =item shmread ID,VAR,POS,SIZE
3334 =item shmwrite ID,STRING,POS,SIZE
3336 Reads or writes the System V shared memory segment ID starting at
3337 position POS for size SIZE by attaching to it, copying in/out, and
3338 detaching from it. When reading, VAR must be a variable that will
3339 hold the data read. When writing, if STRING is too long, only SIZE
3340 bytes are used; if STRING is too short, nulls are written to fill out
3341 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
3342 See also C<IPC::SysV> documentation.
3344 =item shutdown SOCKET,HOW
3346 Shuts down a socket connection in the manner indicated by HOW, which
3347 has the same interpretation as in the system call of the same name.
3349 shutdown(SOCKET, 0); # I/we have stopped reading data
3350 shutdown(SOCKET, 1); # I/we have stopped writing data
3351 shutdown(SOCKET, 2); # I/we have stopped using this socket
3353 This is useful with sockets when you want to tell the other
3354 side you're done writing but not done reading, or vice versa.
3355 It's also a more insistent form of close because it also
3356 disables the filedescriptor in any forked copies in other
3363 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3364 returns sine of C<$_>.
3366 For the inverse sine operation, you may use the C<POSIX::asin()>
3367 function, or use this relation:
3369 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3375 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3376 May be interrupted if the process receives a signal such as C<SIGALRM>.
3377 Returns the number of seconds actually slept. You probably cannot
3378 mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
3381 On some older systems, it may sleep up to a full second less than what
3382 you requested, depending on how it counts seconds. Most modern systems
3383 always sleep the full amount. They may appear to sleep longer than that,
3384 however, because your process might not be scheduled right away in a
3385 busy multitasking system.
3387 For delays of finer granularity than one second, you may use Perl's
3388 C<syscall()> interface to access setitimer(2) if your system supports it,
3389 or else see L</select()> above.
3391 See also the POSIX module's C<sigpause()> function.
3393 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3395 Opens a socket of the specified kind and attaches it to filehandle
3396 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
3397 system call of the same name. You should "C<use Socket;>" first to get
3398 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
3400 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3402 Creates an unnamed pair of sockets in the specified domain, of the
3403 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3404 for the system call of the same name. If unimplemented, yields a fatal
3405 error. Returns TRUE if successful.
3407 Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call
3408 to C<pipe(Rdr, Wtr)> is essentially:
3411 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3412 shutdown(Rdr, 1); # no more writing for reader
3413 shutdown(Wtr, 0); # no more reading for writer
3415 See L<perlipc> for an example of socketpair use.
3417 =item sort SUBNAME LIST
3419 =item sort BLOCK LIST
3423 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3424 is omitted, C<sort()>s in standard string comparison order. If SUBNAME is
3425 specified, it gives the name of a subroutine that returns an integer
3426 less than, equal to, or greater than C<0>, depending on how the elements
3427 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3428 operators are extremely useful in such routines.) SUBNAME may be a
3429 scalar variable name (unsubscripted), in which case the value provides
3430 the name of (or a reference to) the actual subroutine to use. In place
3431 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3434 In the interests of efficiency the normal calling code for subroutines is
3435 bypassed, with the following effects: the subroutine may not be a
3436 recursive subroutine, and the two elements to be compared are passed into
3437 the subroutine not via C<@_> but as the package global variables C<$a> and
3438 C<$b> (see example below). They are passed by reference, so don't
3439 modify C<$a> and C<$b>. And don't try to declare them as lexicals either.
3441 You also cannot exit out of the sort block or subroutine using any of the
3442 loop control operators described in L<perlsyn> or with C<goto()>.
3444 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3445 current collation locale. See L<perllocale>.
3450 @articles = sort @files;
3452 # same thing, but with explicit sort routine
3453 @articles = sort {$a cmp $b} @files;
3455 # now case-insensitively
3456 @articles = sort {uc($a) cmp uc($b)} @files;
3458 # same thing in reversed order
3459 @articles = sort {$b cmp $a} @files;
3461 # sort numerically ascending
3462 @articles = sort {$a <=> $b} @files;
3464 # sort numerically descending
3465 @articles = sort {$b <=> $a} @files;
3467 # sort using explicit subroutine name
3469 $age{$a} <=> $age{$b}; # presuming numeric
3471 @sortedclass = sort byage @class;
3473 # this sorts the %age hash by value instead of key
3474 # using an in-line function
3475 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3477 sub backwards { $b cmp $a; }
3478 @harry = ('dog','cat','x','Cain','Abel');
3479 @george = ('gone','chased','yz','Punished','Axed');
3481 # prints AbelCaincatdogx
3482 print sort backwards @harry;
3483 # prints xdogcatCainAbel
3484 print sort @george, 'to', @harry;
3485 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3487 # inefficiently sort by descending numeric compare using
3488 # the first integer after the first = sign, or the
3489 # whole record case-insensitively otherwise
3492 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3497 # same thing, but much more efficiently;
3498 # we'll build auxiliary indices instead
3502 push @nums, /=(\d+)/;
3507 $nums[$b] <=> $nums[$a]
3509 $caps[$a] cmp $caps[$b]
3513 # same thing using a Schwartzian Transform (no temps)
3514 @new = map { $_->[0] }
3515 sort { $b->[1] <=> $a->[1]
3518 } map { [$_, /=(\d+)/, uc($_)] } @old;
3520 If you're using strict, you I<MUST NOT> declare C<$a>
3521 and C<$b> as lexicals. They are package globals. That means
3522 if you're in the C<main> package, it's
3524 @articles = sort {$main::b <=> $main::a} @files;
3528 @articles = sort {$::b <=> $::a} @files;
3530 but if you're in the C<FooPack> package, it's
3532 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3534 The comparison function is required to behave. If it returns
3535 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
3536 sometimes saying the opposite, for example) the results are not
3539 =item splice ARRAY,OFFSET,LENGTH,LIST
3541 =item splice ARRAY,OFFSET,LENGTH
3543 =item splice ARRAY,OFFSET
3545 Removes the elements designated by OFFSET and LENGTH from an array, and
3546 replaces them with the elements of LIST, if any. In list context,
3547 returns the elements removed from the array. In scalar context,
3548 returns the last element removed, or C<undef> if no elements are
3549 removed. The array grows or shrinks as necessary.
3550 If OFFSET is negative then it start that far from the end of the array.
3551 If LENGTH is omitted, removes everything from OFFSET onward.
3552 If LENGTH is negative, leave that many elements off the end of the array.
3553 The following equivalences hold (assuming C<$[ == 0>):
3555 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3556 pop(@a) splice(@a,-1)
3557 shift(@a) splice(@a,0,1)
3558 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3559 $a[$x] = $y splice(@a,$x,1,$y)
3561 Example, assuming array lengths are passed before arrays:
3563 sub aeq { # compare two list values
3564 my(@a) = splice(@_,0,shift);
3565 my(@b) = splice(@_,0,shift);
3566 return 0 unless @a == @b; # same len?
3568 return 0 if pop(@a) ne pop(@b);
3572 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3574 =item split /PATTERN/,EXPR,LIMIT
3576 =item split /PATTERN/,EXPR
3578 =item split /PATTERN/
3582 Splits a string into an array of strings, and returns it. By default,
3583 empty leading fields are preserved, and empty trailing ones are deleted.
3585 If not in list context, returns the number of fields found and splits into
3586 the C<@_> array. (In list context, you can force the split into C<@_> by
3587 using C<??> as the pattern delimiters, but it still returns the list
3588 value.) The use of implicit split to C<@_> is deprecated, however, because
3589 it clobbers your subroutine arguments.
3591 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
3592 splits on whitespace (after skipping any leading whitespace). Anything
3593 matching PATTERN is taken to be a delimiter separating the fields. (Note
3594 that the delimiter may be longer than one character.)
3596 If LIMIT is specified and positive, splits into no more than that
3597 many fields (though it may split into fewer). If LIMIT is unspecified
3598 or zero, trailing null fields are stripped (which potential users
3599 of C<pop()> would do well to remember). If LIMIT is negative, it is
3600 treated as if an arbitrarily large LIMIT had been specified.
3602 A pattern matching the null string (not to be confused with
3603 a null pattern C<//>, which is just one member of the set of patterns
3604 matching a null string) will split the value of EXPR into separate
3605 characters at each point it matches that way. For example:
3607 print join(':', split(/ */, 'hi there'));
3609 produces the output 'h:i:t:h:e:r:e'.
3611 The LIMIT parameter can be used to split a line partially
3613 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3615 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3616 one larger than the number of variables in the list, to avoid
3617 unnecessary work. For the list above LIMIT would have been 4 by
3618 default. In time critical applications it behooves you not to split
3619 into more fields than you really need.
3621 If the PATTERN contains parentheses, additional array elements are
3622 created from each matching substring in the delimiter.
3624 split(/([,-])/, "1-10,20", 3);
3626 produces the list value
3628 (1, '-', 10, ',', 20)
3630 If you had the entire header of a normal Unix email message in C<$header>,
3631 you could split it up into fields and their values this way:
3633 $header =~ s/\n\s+/ /g; # fix continuation lines
3634 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3636 The pattern C</PATTERN/> may be replaced with an expression to specify
3637 patterns that vary at runtime. (To do runtime compilation only once,
3638 use C</$variable/o>.)
3640 As a special case, specifying a PATTERN of space (C<' '>) will split on
3641 white space just as C<split()> with no arguments does. Thus, C<split(' ')> can
3642 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3643 will give you as many null initial fields as there are leading spaces.
3644 A C<split()> on C</\s+/> is like a C<split(' ')> except that any leading
3645 whitespace produces a null first field. A C<split()> with no arguments
3646 really does a C<split(' ', $_)> internally.
3650 open(PASSWD, '/etc/passwd');
3652 ($login, $passwd, $uid, $gid,
3653 $gcos, $home, $shell) = split(/:/);
3657 (Note that C<$shell> above will still have a newline on it. See L</chop>,
3658 L</chomp>, and L</join>.)
3660 =item sprintf FORMAT, LIST
3662 Returns a string formatted by the usual C<printf()> conventions of the
3663 C library function C<sprintf()>. See L<sprintf(3)> or L<printf(3)>
3664 on your system for an explanation of the general principles.
3666 Perl does its own C<sprintf()> formatting -- it emulates the C
3667 function C<sprintf()>, but it doesn't use it (except for floating-point
3668 numbers, and even then only the standard modifiers are allowed). As a
3669 result, any non-standard extensions in your local C<sprintf()> are not
3670 available from Perl.
3672 Perl's C<sprintf()> permits the following universally-known conversions:
3675 %c a character with the given number
3677 %d a signed integer, in decimal
3678 %u an unsigned integer, in decimal
3679 %o an unsigned integer, in octal
3680 %x an unsigned integer, in hexadecimal
3681 %e a floating-point number, in scientific notation
3682 %f a floating-point number, in fixed decimal notation
3683 %g a floating-point number, in %e or %f notation
3685 In addition, Perl permits the following widely-supported conversions:
3687 %X like %x, but using upper-case letters
3688 %E like %e, but using an upper-case "E"
3689 %G like %g, but with an upper-case "E" (if applicable)
3690 %p a pointer (outputs the Perl value's address in hexadecimal)
3691 %n special: *stores* the number of characters output so far
3692 into the next variable in the parameter list
3694 Finally, for backward (and we do mean "backward") compatibility, Perl
3695 permits these unnecessary but widely-supported conversions:
3698 %D a synonym for %ld
3699 %U a synonym for %lu
3700 %O a synonym for %lo
3703 Perl permits the following universally-known flags between the C<%>
3704 and the conversion letter:
3706 space prefix positive number with a space
3707 + prefix positive number with a plus sign
3708 - left-justify within the field
3709 0 use zeros, not spaces, to right-justify
3710 # prefix non-zero octal with "0", non-zero hex with "0x"
3711 number minimum field width
3712 .number "precision": digits after decimal point for
3713 floating-point, max length for string, minimum length
3715 l interpret integer as C type "long" or "unsigned long"
3716 h interpret integer as C type "short" or "unsigned short"
3718 There is also one Perl-specific flag:
3720 V interpret integer as Perl's standard integer type
3722 Where a number would appear in the flags, an asterisk ("C<*>") may be
3723 used instead, in which case Perl uses the next item in the parameter
3724 list as the given number (that is, as the field width or precision).
3725 If a field width obtained through "C<*>" is negative, it has the same
3726 effect as the "C<->" flag: left-justification.
3728 If C<use locale> is in effect, the character used for the decimal
3729 point in formatted real numbers is affected by the LC_NUMERIC locale.
3736 Return the square root of EXPR. If EXPR is omitted, returns square
3743 Sets the random number seed for the C<rand()> operator. If EXPR is
3744 omitted, uses a semi-random value supplied by the kernel (if it supports
3745 the F</dev/urandom> device) or based on the current time and process
3746 ID, among other things. In versions of Perl prior to 5.004 the default
3747 seed was just the current C<time()>. This isn't a particularly good seed,
3748 so many old programs supply their own seed value (often C<time ^ $$> or
3749 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
3751 In fact, it's usually not necessary to call C<srand()> at all, because if
3752 it is not called explicitly, it is called implicitly at the first use of
3753 the C<rand()> operator. However, this was not the case in version of Perl
3754 before 5.004, so if your script will run under older Perl versions, it
3755 should call C<srand()>.
3757 Note that you need something much more random than the default seed for
3758 cryptographic purposes. Checksumming the compressed output of one or more
3759 rapidly changing operating system status programs is the usual method. For
3762 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3764 If you're particularly concerned with this, see the C<Math::TrulyRandom>
3767 Do I<not> call C<srand()> multiple times in your program unless you know
3768 exactly what you're doing and why you're doing it. The point of the
3769 function is to "seed" the C<rand()> function so that C<rand()> can produce
3770 a different sequence each time you run your program. Just do it once at the
3771 top of your program, or you I<won't> get random numbers out of C<rand()>!
3773 Frequently called programs (like CGI scripts) that simply use
3777 for a seed can fall prey to the mathematical property that
3781 one-third of the time. So don't do that.
3783 =item stat FILEHANDLE
3789 Returns a 13-element list giving the status info for a file, either
3790 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
3791 it stats C<$_>. Returns a null list if the stat fails. Typically used
3794 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3795 $atime,$mtime,$ctime,$blksize,$blocks)
3798 Not all fields are supported on all filesystem types. Here are the
3799 meaning of the fields:
3801 0 dev device number of filesystem
3803 2 mode file mode (type and permissions)
3804 3 nlink number of (hard) links to the file
3805 4 uid numeric user ID of file's owner
3806 5 gid numeric group ID of file's owner
3807 6 rdev the device identifier (special files only)
3808 7 size total size of file, in bytes
3809 8 atime last access time since the epoch
3810 9 mtime last modify time since the epoch
3811 10 ctime inode change time (NOT creation time!) since the epoch
3812 11 blksize preferred block size for file system I/O
3813 12 blocks actual number of blocks allocated
3815 (The epoch was at 00:00 January 1, 1970 GMT.)
3817 If stat is passed the special filehandle consisting of an underline, no
3818 stat is done, but the current contents of the stat structure from the
3819 last stat or filetest are returned. Example:
3821 if (-x $file && (($d) = stat(_)) && $d < 0) {
3822 print "$file is executable NFS file\n";
3825 (This works on machines only for which the device number is negative under NFS.)
3827 In scalar context, C<stat()> returns a boolean value indicating success
3828 or failure, and, if successful, sets the information associated with
3829 the special filehandle C<_>.
3835 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3836 doing many pattern matches on the string before it is next modified.
3837 This may or may not save time, depending on the nature and number of
3838 patterns you are searching on, and on the distribution of character
3839 frequencies in the string to be searched -- you probably want to compare
3840 run times with and without it to see which runs faster. Those loops
3841 which scan for many short constant strings (including the constant
3842 parts of more complex patterns) will benefit most. You may have only
3843 one C<study()> active at a time -- if you study a different scalar the first
3844 is "unstudied". (The way C<study()> works is this: a linked list of every
3845 character in the string to be searched is made, so we know, for
3846 example, where all the C<'k'> characters are. From each search string,
3847 the rarest character is selected, based on some static frequency tables
3848 constructed from some C programs and English text. Only those places
3849 that contain this "rarest" character are examined.)
3851 For example, here is a loop that inserts index producing entries
3852 before any line containing a certain pattern:
3856 print ".IX foo\n" if /\bfoo\b/;
3857 print ".IX bar\n" if /\bbar\b/;
3858 print ".IX blurfl\n" if /\bblurfl\b/;
3863 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
3864 will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
3865 a big win except in pathological cases. The only question is whether
3866 it saves you more time than it took to build the linked list in the
3869 Note that if you have to look for strings that you don't know till
3870 runtime, you can build an entire loop as a string and C<eval()> that to
3871 avoid recompiling all your patterns all the time. Together with
3872 undefining C<$/> to input entire files as one record, this can be very
3873 fast, often faster than specialized programs like fgrep(1). The following
3874 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3875 out the names of those files that contain a match:
3877 $search = 'while (<>) { study;';
3878 foreach $word (@words) {
3879 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3884 eval $search; # this screams
3885 $/ = "\n"; # put back to normal input delimiter
3886 foreach $file (sort keys(%seen)) {
3894 =item sub NAME BLOCK
3896 This is subroutine definition, not a real function I<per se>. With just a
3897 NAME (and possibly prototypes), it's just a forward declaration. Without
3898 a NAME, it's an anonymous function declaration, and does actually return a
3899 value: the CODE ref of the closure you just created. See L<perlsub> and
3900 L<perlref> for details.
3902 =item substr EXPR,OFFSET,LEN,REPLACEMENT
3904 =item substr EXPR,OFFSET,LEN
3906 =item substr EXPR,OFFSET
3908 Extracts a substring out of EXPR and returns it. First character is at
3909 offset C<0>, or whatever you've set C<$[> to (but don't do that).
3910 If OFFSET is negative (or more precisely, less than C<$[>), starts
3911 that far from the end of the string. If LEN is omitted, returns
3912 everything to the end of the string. If LEN is negative, leaves that
3913 many characters off the end of the string.
3915 If you specify a substring that is partly outside the string, the part
3916 within the string is returned. If the substring is totally outside
3917 the string a warning is produced.
3919 You can use the C<substr()> function
3920 as an lvalue, in which case EXPR must be an lvalue. If you assign
3921 something shorter than LEN, the string will shrink, and if you assign
3922 something longer than LEN, the string will grow to accommodate it. To
3923 keep the string the same length you may need to pad or chop your value
3926 An alternative to using C<substr()> as an lvalue is to specify the
3927 replacement string as the 4th argument. This allows you to replace
3928 parts of the EXPR and return what was there before in one operation.
3930 =item symlink OLDFILE,NEWFILE
3932 Creates a new filename symbolically linked to the old filename.
3933 Returns C<1> for success, C<0> otherwise. On systems that don't support
3934 symbolic links, produces a fatal error at run time. To check for that,
3937 $symlink_exists = eval { symlink("",""); 1 };
3941 Calls the system call specified as the first element of the list,
3942 passing the remaining elements as arguments to the system call. If
3943 unimplemented, produces a fatal error. The arguments are interpreted
3944 as follows: if a given argument is numeric, the argument is passed as
3945 an int. If not, the pointer to the string value is passed. You are
3946 responsible to make sure a string is pre-extended long enough to
3947 receive any result that might be written into a string. You can't use a
3948 string literal (or other read-only string) as an argument to C<syscall()>
3949 because Perl has to assume that any string pointer might be written
3951 integer arguments are not literals and have never been interpreted in a
3952 numeric context, you may need to add C<0> to them to force them to look
3953 like numbers. This emulates the C<syswrite()> function (or vice versa):
3955 require 'syscall.ph'; # may need to run h2ph
3957 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
3959 Note that Perl supports passing of up to only 14 arguments to your system call,
3960 which in practice should usually suffice.
3962 Syscall returns whatever value returned by the system call it calls.
3963 If the system call fails, C<syscall()> returns C<-1> and sets C<$!> (errno).
3964 Note that some system calls can legitimately return C<-1>. The proper
3965 way to handle such calls is to assign C<$!=0;> before the call and
3966 check the value of C<$!> if syscall returns C<-1>.
3968 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
3969 number of the read end of the pipe it creates. There is no way
3970 to retrieve the file number of the other end. You can avoid this
3971 problem by using C<pipe()> instead.
3973 =item sysopen FILEHANDLE,FILENAME,MODE
3975 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3977 Opens the file whose filename is given by FILENAME, and associates it
3978 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3979 the name of the real filehandle wanted. This function calls the
3980 underlying operating system's C<open()> function with the parameters
3981 FILENAME, MODE, PERMS.
3983 The possible values and flag bits of the MODE parameter are
3984 system-dependent; they are available via the standard module C<Fcntl>.
3985 For historical reasons, some values work on almost every system
3986 supported by perl: zero means read-only, one means write-only, and two
3987 means read/write. We know that these values do I<not> work under
3988 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
3989 use them in new code.
3991 If the file named by FILENAME does not exist and the C<open()> call creates
3992 it (typically because MODE includes the C<O_CREAT> flag), then the value of
3993 PERMS specifies the permissions of the newly created file. If you omit
3994 the PERMS argument to C<sysopen()>, Perl uses the octal value C<0666>.
3995 These permission values need to be in octal, and are modified by your
3996 process's current C<umask>.
3998 Seldom if ever use C<0644> as argument to C<sysopen()> because that
3999 takes away the user's option to have a more permissive umask. Better
4000 to omit it. See the perlfunc(1) entry on C<umask> for more on this.
4002 The C<IO::File> module provides a more object-oriented approach, if you're
4003 into that kind of thing.
4005 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4007 =item sysread FILEHANDLE,SCALAR,LENGTH
4009 Attempts to read LENGTH bytes of data into variable SCALAR from the
4010 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4011 so mixing this with other kinds of reads, C<print()>, C<write()>,
4012 C<seek()>, C<tell()>, or C<eof()> can cause confusion because stdio
4013 usually buffers data. Returns the number of bytes actually read, C<0>
4014 at end of file, or undef if there was an error. SCALAR will be grown or
4015 shrunk so that the last byte actually read is the last byte of the
4016 scalar after the read.
4018 An OFFSET may be specified to place the read data at some place in the
4019 string other than the beginning. A negative OFFSET specifies
4020 placement at that many bytes counting backwards from the end of the
4021 string. A positive OFFSET greater than the length of SCALAR results
4022 in the string being padded to the required size with C<"\0"> bytes before
4023 the result of the read is appended.
4025 =item sysseek FILEHANDLE,POSITION,WHENCE
4027 Sets FILEHANDLE's system position using the system call lseek(2). It
4028 bypasses stdio, so mixing this with reads (other than C<sysread()>),
4029 C<print()>, C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause
4030 confusion. FILEHANDLE may be an expression whose value gives the name
4031 of the filehandle. The values for WHENCE are C<0> to set the new
4032 position to POSITION, C<1> to set the it to the current position plus
4033 POSITION, and C<2> to set it to EOF plus POSITION (typically negative).
4034 For WHENCE, you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4035 C<SEEK_END> from either the C<IO::Seekable> or the POSIX module.
4037 Returns the new position, or the undefined value on failure. A position
4038 of zero is returned as the string "C<0> but true"; thus C<sysseek()> returns
4039 TRUE on success and FALSE on failure, yet you can still easily determine
4044 =item system PROGRAM LIST
4046 Does exactly the same thing as "C<exec LIST>" except that a fork is done
4047 first, and the parent process waits for the child process to complete.
4048 Note that argument processing varies depending on the number of
4049 arguments. If there is more than one argument in LIST, or if LIST is
4050 an array with more than one value, starts the program given by the
4051 first element of the list with arguments given by the rest of the list.
4052 If there is only one scalar argument, the argument is
4053 checked for shell metacharacters, and if there are any, the entire
4054 argument is passed to the system's command shell for parsing (this is
4055 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
4056 there are no shell metacharacters in the argument, it is split into
4057 words and passed directly to C<execvp()>, which is more efficient.
4059 The return value is the exit status of the program as
4060 returned by the C<wait()> call. To get the actual exit value divide by
4061 256. See also L</exec>. This is I<NOT> what you want to use to capture
4062 the output from a command, for that you should use merely backticks or
4063 C<qx//>, as described in L<perlop/"`STRING`">.
4065 Like C<exec()>, C<system()> allows you to lie to a program about its name if
4066 you use the "C<system PROGRAM LIST>" syntax. Again, see L</exec>.
4068 Because C<system()> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4069 program they're running doesn't actually interrupt your program.
4071 @args = ("command", "arg1", "arg2");
4073 or die "system @args failed: $?"
4075 You can check all the failure possibilities by inspecting
4078 $exit_value = $? >> 8;
4079 $signal_num = $? & 127;
4080 $dumped_core = $? & 128;
4082 When the arguments get executed via the system shell, results
4083 and return codes will be subject to its quirks and capabilities.
4084 See L<perlop/"`STRING`"> and L</exec> for details.
4086 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4088 =item syswrite FILEHANDLE,SCALAR,LENGTH
4090 =item syswrite FILEHANDLE,SCALAR
4092 Attempts to write LENGTH bytes of data from variable SCALAR to the
4093 specified FILEHANDLE, using the system call write(2). If LENGTH is
4094 not specified, writes whole SCALAR. It bypasses
4095 stdio, so mixing this with reads (other than C<sysread())>, C<print()>,
4096 C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause confusion
4097 because stdio usually buffers data. Returns the number of bytes
4098 actually written, or C<undef> if there was an error. If the LENGTH is
4099 greater than the available data in the SCALAR after the OFFSET, only as
4100 much data as is available will be written.
4102 An OFFSET may be specified to write the data from some part of the
4103 string other than the beginning. A negative OFFSET specifies writing
4104 that many bytes counting backwards from the end of the string. In the
4105 case the SCALAR is empty you can use OFFSET but only zero offset.
4107 =item tell FILEHANDLE
4111 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4112 expression whose value gives the name of the actual filehandle. If
4113 FILEHANDLE is omitted, assumes the file last read.
4115 =item telldir DIRHANDLE
4117 Returns the current position of the C<readdir()> routines on DIRHANDLE.
4118 Value may be given to C<seekdir()> to access a particular location in a
4119 directory. Has the same caveats about possible directory compaction as
4120 the corresponding system library routine.
4122 =item tie VARIABLE,CLASSNAME,LIST
4124 This function binds a variable to a package class that will provide the
4125 implementation for the variable. VARIABLE is the name of the variable
4126 to be enchanted. CLASSNAME is the name of a class implementing objects
4127 of correct type. Any additional arguments are passed to the "C<new()>"
4128 method of the class (meaning C<TIESCALAR>, C<TIEARRAY>, or C<TIEHASH>).
4129 Typically these are arguments such as might be passed to the C<dbm_open()>
4130 function of C. The object returned by the "C<new()>" method is also
4131 returned by the C<tie()> function, which would be useful if you want to
4132 access other methods in CLASSNAME.
4134 Note that functions such as C<keys()> and C<values()> may return huge lists
4135 when used on large objects, like DBM files. You may prefer to use the
4136 C<each()> function to iterate over such. Example:
4138 # print out history file offsets
4140 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4141 while (($key,$val) = each %HIST) {
4142 print $key, ' = ', unpack('L',$val), "\n";
4146 A class implementing a hash should have the following methods:
4148 TIEHASH classname, LIST
4151 STORE this, key, value
4155 NEXTKEY this, lastkey
4157 A class implementing an ordinary array should have the following methods:
4159 TIEARRAY classname, LIST
4162 STORE this, key, value
4165 A class implementing a scalar should have the following methods:
4167 TIESCALAR classname, LIST
4172 Unlike C<dbmopen()>, the C<tie()> function will not use or require a module
4173 for you--you need to do that explicitly yourself. See L<DB_File>
4174 or the F<Config> module for interesting C<tie()> implementations.
4176 For further details see L<perltie>, L<tied VARIABLE>.
4180 Returns a reference to the object underlying VARIABLE (the same value
4181 that was originally returned by the C<tie()> call that bound the variable
4182 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4187 Returns the number of non-leap seconds since whatever time the system
4188 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4189 and 00:00:00 UTC, January 1, 1970 for most other systems).
4190 Suitable for feeding to C<gmtime()> and C<localtime()>.
4194 Returns a four-element list giving the user and system times, in
4195 seconds, for this process and the children of this process.
4197 ($user,$system,$cuser,$csystem) = times;
4201 The transliteration operator. Same as C<y///>. See L<perlop>.
4203 =item truncate FILEHANDLE,LENGTH
4205 =item truncate EXPR,LENGTH
4207 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4208 specified length. Produces a fatal error if truncate isn't implemented
4209 on your system. Returns TRUE if successful, the undefined value
4216 Returns an uppercased version of EXPR. This is the internal function
4217 implementing the C<\U> escape in double-quoted strings.
4218 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4219 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
4220 does not attempt to do titlecase mapping on initial letters. See C<ucfirst()> for that.)
4222 If EXPR is omitted, uses C<$_>.
4228 Returns the value of EXPR with the first character
4229 in uppercase (titlecase in Unicode). This is
4230 the internal function implementing the C<\u> escape in double-quoted strings.
4231 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4233 If EXPR is omitted, uses C<$_>.
4239 Sets the umask for the process to EXPR and returns the previous value.
4240 If EXPR is omitted, merely returns the current umask.
4242 The Unix permission C<rwxr-x---> is represented as three sets of three
4243 bits, or three octal digits: C<0750> (the leading 0 indicates octal
4244 and isn't one of the the digits). The C<umask> value is such a number
4245 representing disabled permissions bits. The permission (or "mode")
4246 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
4247 even if you tell C<sysopen> to create a file with permissions C<0777>,
4248 if your umask is C<0022> then the file will actually be created with
4249 permissions C<0755>. If your C<umask> were C<0027> (group can't
4250 write; others can't read, write, or execute), then passing
4251 C<sysopen()> C<0666> would create a file with mode C<0640> (C<0666 &~
4254 Here's some advice: supply a creation mode of C<0666> for regular
4255 files (in C<sysopen()>) and one of C<0777> for directories (in
4256 C<mkdir()>) and executable files. This gives users the freedom of
4257 choice: if they want protected files, they might choose process umasks
4258 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
4259 Programs should rarely if ever make policy decisions better left to
4260 the user. The exception to this is when writing files that should be
4261 kept private: mail files, web browser cookies, I<.rhosts> files, and
4264 If umask(2) is not implemented on your system and you are trying to
4265 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
4266 fatal error at run time. If umask(2) is not implemented and you are
4267 not trying to restrict access for yourself, returns C<undef>.
4269 Remember that a umask is a number, usually given in octal; it is I<not> a
4270 string of octal digits. See also L</oct>, if all you have is a string.
4276 Undefines the value of EXPR, which must be an lvalue. Use only on a
4277 scalar value, an array (using "C<@>"), a hash (using "C<%>"), a subroutine
4278 (using "C<&>"), or a typeglob (using "<*>"). (Saying C<undef $hash{$key}>
4279 will probably not do what you expect on most predefined variables or
4280 DBM list values, so don't do that; see L<delete>.) Always returns the
4281 undefined value. You can omit the EXPR, in which case nothing is
4282 undefined, but you still get an undefined value that you could, for
4283 instance, return from a subroutine, assign to a variable or pass as a
4284 parameter. Examples:
4287 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4291 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4292 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4293 select undef, undef, undef, 0.25;
4294 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4296 Note that this is a unary operator, not a list operator.
4298 =item unless (EXPR) BLOCK
4300 The negative counterpart of L</if>. If the EXPR returns false the
4303 See also L<perlsyn>.
4309 Deletes a list of files. Returns the number of files successfully
4312 $cnt = unlink 'a', 'b', 'c';
4316 Note: C<unlink()> will not delete directories unless you are superuser and
4317 the B<-U> flag is supplied to Perl. Even if these conditions are
4318 met, be warned that unlinking a directory can inflict damage on your
4319 filesystem. Use C<rmdir()> instead.
4321 If LIST is omitted, uses C<$_>.
4323 =item unpack TEMPLATE,EXPR
4325 C<Unpack()> does the reverse of C<pack()>: it takes a string representing a
4326 structure and expands it out into a list value, returning the array
4327 value. (In scalar context, it returns merely the first value
4328 produced.) The TEMPLATE has the same format as in the C<pack()> function.
4329 Here's a subroutine that does substring:
4332 my($what,$where,$howmuch) = @_;
4333 unpack("x$where a$howmuch", $what);
4338 sub ordinal { unpack("c",$_[0]); } # same as ord()
4340 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4341 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4342 themselves. Default is a 16-bit checksum. For example, the following
4343 computes the same number as the System V sum program:
4346 $checksum += unpack("%16C*", $_);
4350 The following efficiently counts the number of set bits in a bit vector:
4352 $setbits = unpack("%32b*", $selectmask);
4354 =item unshift ARRAY,LIST
4356 Does the opposite of a C<shift()>. Or the opposite of a C<push()>,
4357 depending on how you look at it. Prepends list to the front of the
4358 array, and returns the new number of elements in the array.
4360 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4362 Note the LIST is prepended whole, not one element at a time, so the
4363 prepended elements stay in the same order. Use C<reverse()> to do the
4366 =item until (EXPR) BLOCK
4368 =item do BLOCK until (EXPR)
4370 Enter BLOCK until EXPR returns false. The first form may avoid entering
4371 the BLOCK, the second form enters the BLOCK at least once.
4373 See L</do>, L</while>, and L</for>.
4375 See also L<perlsyn>.
4377 =item untie VARIABLE
4379 Breaks the binding between a variable and a package. (See C<tie()>.)
4381 =item use Module LIST
4385 =item use Module VERSION LIST
4389 Imports some semantics into the current package from the named module,
4390 generally by aliasing certain subroutine or variable names into your
4391 package. It is exactly equivalent to
4393 BEGIN { require Module; import Module LIST; }
4395 except that Module I<must> be a bareword.
4397 If the first argument to C<use> is a number, it is treated as a version
4398 number instead of a module name. If the version of the Perl interpreter
4399 is less than VERSION, then an error message is printed and Perl exits
4400 immediately. This is often useful if you need to check the current
4401 Perl version before C<use>ing library modules that have changed in
4402 incompatible ways from older versions of Perl. (We try not to do
4403 this more than we have to.)
4405 The C<BEGIN> forces the C<require> and C<import()> to happen at compile time. The
4406 C<require> makes sure the module is loaded into memory if it hasn't been
4407 yet. The C<import()> is not a builtin--it's just an ordinary static method
4408 call into the "C<Module>" package to tell the module to import the list of
4409 features back into the current package. The module can implement its
4410 C<import()> method any way it likes, though most modules just choose to
4411 derive their C<import()> method via inheritance from the C<Exporter> class that
4412 is defined in the C<Exporter> module. See L<Exporter>. If no C<import()>
4413 method can be found then the error is currently silently ignored. This
4414 may change to a fatal error in a future version.
4416 If you don't want your namespace altered, explicitly supply an empty list:
4420 That is exactly equivalent to
4422 BEGIN { require Module }
4424 If the VERSION argument is present between Module and LIST, then the
4425 C<use> will call the VERSION method in class Module with the given
4426 version as an argument. The default VERSION method, inherited from
4427 the Universal class, croaks if the given version is larger than the
4428 value of the variable C<$Module::VERSION>. (Note that there is not a
4429 comma after VERSION!)
4431 Because this is a wide-open interface, pragmas (compiler directives)
4432 are also implemented this way. Currently implemented pragmas are:
4436 use sigtrap qw(SEGV BUS);
4437 use strict qw(subs vars refs);
4438 use subs qw(afunc blurfl);
4440 Some of these these pseudo-modules import semantics into the current
4441 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4442 which import symbols into the current package (which are effective
4443 through the end of the file).
4445 There's a corresponding "C<no>" command that unimports meanings imported
4446 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import()>.
4451 If no C<unimport()> method can be found the call fails with a fatal error.
4453 See L<perlmod> for a list of standard modules and pragmas.
4457 Changes the access and modification times on each file of a list of
4458 files. The first two elements of the list must be the NUMERICAL access
4459 and modification times, in that order. Returns the number of files
4460 successfully changed. The inode modification time of each file is set
4461 to the current time. This code has the same effect as the "C<touch>"
4462 command if the files already exist:
4466 utime $now, $now, @ARGV;
4470 Returns a list consisting of all the values of the named hash. (In a
4471 scalar context, returns the number of values.) The values are
4472 returned in an apparently random order, but it is the same order as
4473 either the C<keys()> or C<each()> function would produce on the same hash.
4474 As a side effect, it resets HASH's iterator. See also C<keys()>, C<each()>,
4477 =item vec EXPR,OFFSET,BITS
4479 Treats the string in EXPR as a vector of unsigned integers, and
4480 returns the value of the bit field specified by OFFSET. BITS specifies
4481 the number of bits that are reserved for each entry in the bit
4482 vector. This must be a power of two from 1 to 32. C<vec()> may also be
4483 assigned to, in which case parentheses are needed to give the expression
4484 the correct precedence as in
4486 vec($image, $max_x * $x + $y, 8) = 3;
4488 Vectors created with C<vec()> can also be manipulated with the logical
4489 operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
4490 desired when both operands are strings.
4492 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
4493 The comments show the string after each step. Note that this code works
4494 in the same way on big-endian or little-endian machines.
4497 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
4498 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
4499 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
4500 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
4501 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
4502 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
4503 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
4505 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
4506 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
4507 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
4510 To transform a bit vector into a string or array of 0's and 1's, use these:
4512 $bits = unpack("b*", $vector);
4513 @bits = split(//, unpack("b*", $vector));
4515 If you know the exact length in bits, it can be used in place of the C<*>.
4519 Waits for a child process to terminate and returns the pid of the
4520 deceased process, or C<-1> if there are no child processes. The status is
4521 returned in C<$?>. Note that a return value of C<-1> could mean that
4522 child processes are being automatically reaped, as described in L<perlipc>.
4524 =item waitpid PID,FLAGS
4526 Waits for a particular child process to terminate and returns the pid
4527 of the deceased process, or C<-1> if there is no such child process. The
4528 status is returned in C<$?>. If you say
4530 use POSIX ":sys_wait_h";
4532 waitpid(-1,&WNOHANG);
4534 then you can do a non-blocking wait for any process. Non-blocking wait
4535 is available on machines supporting either the waitpid(2) or
4536 wait4(2) system calls. However, waiting for a particular pid with
4537 FLAGS of C<0> is implemented everywhere. (Perl emulates the system call
4538 by remembering the status values of processes that have exited but have
4539 not been harvested by the Perl script yet.)
4541 Note that a return value of C<-1> could mean that child processes are being
4542 automatically reaped. See L<perlipc> for details, and for other examples.
4546 Returns TRUE if the context of the currently executing subroutine is
4547 looking for a list value. Returns FALSE if the context is looking
4548 for a scalar. Returns the undefined value if the context is looking
4549 for no value (void context).
4551 return unless defined wantarray; # don't bother doing more
4552 my @a = complex_calculation();
4553 return wantarray ? @a : "@a";
4557 Produces a message on STDERR just like C<die()>, but doesn't exit or throw
4560 If LIST is empty and C<$@> already contains a value (typically from a
4561 previous eval) that value is used after appending C<"\t...caught">
4562 to C<$@>. This is useful for staying almost, but not entirely similar to
4565 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
4567 No message is printed if there is a C<$SIG{__WARN__}> handler
4568 installed. It is the handler's responsibility to deal with the message
4569 as it sees fit (like, for instance, converting it into a C<die()>). Most
4570 handlers must therefore make arrangements to actually display the
4571 warnings that they are not prepared to deal with, by calling C<warn()>
4572 again in the handler. Note that this is quite safe and will not
4573 produce an endless loop, since C<__WARN__> hooks are not called from
4576 You will find this behavior is slightly different from that of
4577 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4578 instead call C<die()> again to change it).
4580 Using a C<__WARN__> handler provides a powerful way to silence all
4581 warnings (even the so-called mandatory ones). An example:
4583 # wipe out *all* compile-time warnings
4584 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4586 my $foo = 20; # no warning about duplicate my $foo,
4587 # but hey, you asked for it!
4588 # no compile-time or run-time warnings before here
4591 # run-time warnings enabled after here
4592 warn "\$foo is alive and $foo!"; # does show up
4594 See L<perlvar> for details on setting C<%SIG> entries, and for more
4597 =item while (EXPR) BLOCK
4599 =item do BLOCK while (EXPR)
4601 Enter BLOCK while EXPR is true. The first form may avoid entering the
4602 BLOCK, the second form enters the BLOCK at least once.
4604 See also L<perlsyn>, L</for>, L</until>, and L</continue>.
4606 =item write FILEHANDLE
4612 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
4613 using the format associated with that file. By default the format for
4614 a file is the one having the same name as the filehandle, but the
4615 format for the current output channel (see the C<select()> function) may be set
4616 explicitly by assigning the name of the format to the C<$~> variable.
4618 Top of form processing is handled automatically: if there is
4619 insufficient room on the current page for the formatted record, the
4620 page is advanced by writing a form feed, a special top-of-page format
4621 is used to format the new page header, and then the record is written.
4622 By default the top-of-page format is the name of the filehandle with
4623 "_TOP" appended, but it may be dynamically set to the format of your
4624 choice by assigning the name to the C<$^> variable while the filehandle is
4625 selected. The number of lines remaining on the current page is in
4626 variable C<$->, which can be set to C<0> to force a new page.
4628 If FILEHANDLE is unspecified, output goes to the current default output
4629 channel, which starts out as STDOUT but may be changed by the
4630 C<select()> operator. If the FILEHANDLE is an EXPR, then the expression
4631 is evaluated and the resulting string is used to look up the name of
4632 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4634 Note that write is I<NOT> the opposite of C<read()>. Unfortunately.
4638 The transliteration operator. Same as C<tr///>. See L<perlop>.