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1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
15argument, while a list operator may provide either scalar and list
16contexts for its arguments. If it does both, the scalar arguments will
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17be first, and the list argument will follow. (Note that there can ever
18be only one list argument.) For instance, splice() has three scalar
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19arguments followed by a list.
20
21In the syntax descriptions that follow, list operators that expect a
22list (and provide list context for the elements of the list) are shown
23with LIST as an argument. Such a list may consist of any combination
24of scalar arguments or list values; the list values will be included
25in the list as if each individual element were interpolated at that
26point in the list, forming a longer single-dimensional list value.
27Elements of the LIST should be separated by commas.
28
29Any function in the list below may be used either with or without
30parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 31parentheses.) If you use the parentheses, the simple (but occasionally
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32surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33function, and precedence doesn't matter. Otherwise it's a list
34operator or unary operator, and precedence does matter. And whitespace
35between the function and left parenthesis doesn't count--so you need to
36be careful sometimes:
37
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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.
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43
44If you run Perl with the B<-w> switch it can warn you about this. For
45example, the third line above produces:
46
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
49
50For functions that can be used in either a scalar or list context,
54310121 51nonabortive failure is generally indicated in a scalar context by
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52returning the undefined value, and in a list context by returning the
53null list.
54
55Remember the following rule:
56
cb1a09d0 57=over 8
a0d0e21e 58
8ebc5c01 59=item I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
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60
61=back
62
63Each operator and function decides which sort of value it would be most
64appropriate to return in a scalar context. Some operators return the
65length of the list that would have been returned in a list context. Some
66operators return the first value in the list. Some operators return the
67last value in the list. Some operators return a count of successful
68operations. In general, they do what you want, unless you want
69consistency.
70
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71=head2 Perl Functions by Category
72
73Here are Perl's functions (including things that look like
74functions, like some of the keywords and named operators)
75arranged by category. Some functions appear in more
76than one place.
77
78=over
79
80=item Functions for SCALARs or strings
81
82chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
83oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
84sprintf, substr, tr///, uc, ucfirst, y///
85
86=item Regular expressions and pattern matching
87
88m//, pos, quotemeta, s///, split, study
89
90=item Numeric functions
91
92abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
93srand
94
95=item Functions for real @ARRAYs
96
97pop, push, shift, splice, unshift
98
99=item Functions for list data
100
101grep, join, map, qw/STRING/, reverse, sort, unpack
102
103=item Functions for real %HASHes
104
105delete, each, exists, keys, values
106
107=item Input and output functions
108
109binmode, close, closedir, dbmclose, dbmopen, die, eof,
110fileno, flock, format, getc, print, printf, read, readdir,
111rewinddir, seek, seekdir, select, syscall, sysread,
112syswrite, tell, telldir, truncate, warn, write
113
114=item Functions for fixed length data or records
115
116pack, read, syscall, sysread, syswrite, unpack, vec
117
118=item Functions for filehandles, files, or directories
119
da0045b7 120I<-X>, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
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121lstat, mkdir, open, opendir, readlink, rename, rmdir,
122stat, symlink, umask, unlink, utime
123
124=item Keywords related to the control flow of your perl program
125
126caller, continue, die, do, dump, eval, exit, goto, last,
127next, redo, return, sub, wantarray
128
54310121 129=item Keywords related to scoping
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130
131caller, import, local, my, package, use
132
133=item Miscellaneous functions
134
135defined, dump, eval, formline, local, my, reset, scalar,
136undef, wantarray
137
138=item Functions for processes and process groups
139
140alarm, exec, fork, getpgrp, getppid, getpriority, kill,
141pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
142times, wait, waitpid
143
144=item Keywords related to perl modules
145
146do, import, no, package, require, use
147
148=item Keywords related to classes and object-orientedness
149
f3cbc334 150bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
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151
152=item Low-level socket functions
153
154accept, bind, connect, getpeername, getsockname,
155getsockopt, listen, recv, send, setsockopt, shutdown,
156socket, socketpair
157
158=item System V interprocess communication functions
159
160msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
161shmctl, shmget, shmread, shmwrite
162
163=item Fetching user and group info
164
165endgrent, endhostent, endnetent, endpwent, getgrent,
166getgrgid, getgrnam, getlogin, getpwent, getpwnam,
167getpwuid, setgrent, setpwent
168
169=item Fetching network info
170
171endprotoent, endservent, gethostbyaddr, gethostbyname,
172gethostent, getnetbyaddr, getnetbyname, getnetent,
173getprotobyname, getprotobynumber, getprotoent,
174getservbyname, getservbyport, getservent, sethostent,
175setnetent, setprotoent, setservent
176
177=item Time-related functions
178
179gmtime, localtime, time, times
180
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181=item Functions new in perl5
182
183abs, bless, chomp, chr, exists, formline, glob, import, lc,
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184lcfirst, map, my, no, prototype, qx, qw, readline, readpipe,
185ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use
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186
187* - C<sub> was a keyword in perl4, but in perl5 it is an
188operator which can be used in expressions.
189
190=item Functions obsoleted in perl5
191
192dbmclose, dbmopen
193
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194=back
195
196=head2 Alphabetical Listing of Perl Functions
197
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198=over 8
199
200=item -X FILEHANDLE
201
202=item -X EXPR
203
204=item -X
205
206A file test, where X is one of the letters listed below. This unary
207operator takes one argument, either a filename or a filehandle, and
208tests the associated file to see if something is true about it. If the
209argument is omitted, tests $_, except for C<-t>, which tests STDIN.
210Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
211the undefined value if the file doesn't exist. Despite the funny
212names, precedence is the same as any other named unary operator, and
213the argument may be parenthesized like any other unary operator. The
214operator may be any of:
215
216 -r File is readable by effective uid/gid.
217 -w File is writable by effective uid/gid.
218 -x File is executable by effective uid/gid.
219 -o File is owned by effective uid.
220
221 -R File is readable by real uid/gid.
222 -W File is writable by real uid/gid.
223 -X File is executable by real uid/gid.
224 -O File is owned by real uid.
225
226 -e File exists.
227 -z File has zero size.
54310121 228 -s File has nonzero size (returns size).
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229
230 -f File is a plain file.
231 -d File is a directory.
232 -l File is a symbolic link.
233 -p File is a named pipe (FIFO).
234 -S File is a socket.
235 -b File is a block special file.
236 -c File is a character special file.
237 -t Filehandle is opened to a tty.
238
239 -u File has setuid bit set.
240 -g File has setgid bit set.
241 -k File has sticky bit set.
242
243 -T File is a text file.
244 -B File is a binary file (opposite of -T).
245
246 -M Age of file in days when script started.
247 -A Same for access time.
248 -C Same for inode change time.
249
250The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
5f05dabc 251C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
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252uids and gids of the user. There may be other reasons you can't actually
253read, write or execute the file. Also note that, for the superuser,
5f05dabc 254C<-r>, C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return
a0d0e21e 2551 if any execute bit is set in the mode. Scripts run by the superuser may
5f05dabc 256thus need to do a stat() to determine the actual mode of the
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257file, or temporarily set the uid to something else.
258
259Example:
260
261 while (<>) {
262 chop;
263 next unless -f $_; # ignore specials
264 ...
265 }
266
267Note that C<-s/a/b/> does not do a negated substitution. Saying
268C<-exp($foo)> still works as expected, however--only single letters
269following a minus are interpreted as file tests.
270
271The C<-T> and C<-B> switches work as follows. The first block or so of the
272file is examined for odd characters such as strange control codes or
184e9718 273characters with the high bit set. If too many odd characters (E<gt>30%)
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274are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
275containing null in the first block is considered a binary file. If C<-T>
276or C<-B> is used on a filehandle, the current stdio buffer is examined
277rather than the first block. Both C<-T> and C<-B> return TRUE on a null
54310121 278file, or a file at EOF when testing a filehandle. Because you have to
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279read a file to do the C<-T> test, on most occasions you want to use a C<-f>
280against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 281
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282If any of the file tests (or either the stat() or lstat() operators) are given
283the special filehandle consisting of a solitary underline, then the stat
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284structure of the previous file test (or stat operator) is used, saving
285a system call. (This doesn't work with C<-t>, and you need to remember
286that lstat() and C<-l> will leave values in the stat structure for the
287symbolic link, not the real file.) Example:
288
289 print "Can do.\n" if -r $a || -w _ || -x _;
290
291 stat($filename);
292 print "Readable\n" if -r _;
293 print "Writable\n" if -w _;
294 print "Executable\n" if -x _;
295 print "Setuid\n" if -u _;
296 print "Setgid\n" if -g _;
297 print "Sticky\n" if -k _;
298 print "Text\n" if -T _;
299 print "Binary\n" if -B _;
300
301=item abs VALUE
302
54310121 303=item abs
bbce6d69 304
a0d0e21e 305Returns the absolute value of its argument.
bbce6d69 306If VALUE is omitted, uses $_.
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307
308=item accept NEWSOCKET,GENERICSOCKET
309
310Accepts an incoming socket connect, just as the accept(2) system call
311does. Returns the packed address if it succeeded, FALSE otherwise.
4633a7c4 312See example in L<perlipc/"Sockets: Client/Server Communication">.
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313
314=item alarm SECONDS
315
54310121 316=item alarm
bbce6d69 317
a0d0e21e 318Arranges to have a SIGALRM delivered to this process after the
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319specified number of seconds have elapsed. If SECONDS is not specified,
320the value stored in $_ is used. (On some machines,
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321unfortunately, the elapsed time may be up to one second less than you
322specified because of how seconds are counted.) Only one timer may be
323counting at once. Each call disables the previous timer, and an
324argument of 0 may be supplied to cancel the previous timer without
325starting a new one. The returned value is the amount of time remaining
326on the previous timer.
327
4633a7c4 328For delays of finer granularity than one second, you may use Perl's
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329syscall() interface to access setitimer(2) if your system supports it,
330or else see L</select()>. It is usually a mistake to intermix alarm()
4633a7c4 331and sleep() calls.
a0d0e21e 332
ff68c719 333If you want to use alarm() to time out a system call you need to use an
2f9daede 334eval/die pair. You can't rely on the alarm causing the system call to
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335fail with $! set to EINTR because Perl sets up signal handlers to
336restart system calls on some systems. Using eval/die always works.
337
338 eval {
28757baa 339 local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
36477c24 340 alarm $timeout;
ff68c719 341 $nread = sysread SOCKET, $buffer, $size;
36477c24 342 alarm 0;
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343 };
344 die if $@ && $@ ne "alarm\n"; # propagate errors
345 if ($@) {
346 # timed out
347 }
348 else {
349 # didn't
350 }
351
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352=item atan2 Y,X
353
354Returns the arctangent of Y/X in the range -PI to PI.
355
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356For the tangent operation, you may use the POSIX::tan()
357function, or use the familiar relation:
358
359 sub tan { sin($_[0]) / cos($_[0]) }
360
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361=item bind SOCKET,NAME
362
363Binds a network address to a socket, just as the bind system call
364does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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365packed address of the appropriate type for the socket. See the examples in
366L<perlipc/"Sockets: Client/Server Communication">.
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367
368=item binmode FILEHANDLE
369
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370Arranges for the file to be read or written in "binary" mode in operating
371systems that distinguish between binary and text files. Files that are
372not in binary mode have CR LF sequences translated to LF on input and LF
54310121 373translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
cb1a09d0 374and similarly archaic systems, it may be imperative--otherwise your
54310121 375MS-DOS-damaged C library may mangle your file. The key distinction between
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376systems that need binmode and those that don't is their text file
377formats. Systems like Unix and Plan9 that delimit lines with a single
378character, and that encode that character in C as '\n', do not need
379C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
380is taken as the name of the filehandle.
a0d0e21e 381
4633a7c4 382=item bless REF,CLASSNAME
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383
384=item bless REF
385
28757baa 386This function tells the thingy referenced by REF that it is now
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387an object in the CLASSNAME package--or the current package if no CLASSNAME
388is specified, which is often the case. It returns the reference for
5f05dabc 389convenience, because a bless() is often the last thing in a constructor.
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390Always use the two-argument version if the function doing the blessing
391might be inherited by a derived class. See L<perlobj> for more about the
392blessing (and blessings) of objects.
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393
394=item caller EXPR
395
396=item caller
397
398Returns the context of the current subroutine call. In a scalar context,
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399returns the caller's package name if there is a caller, that is, if
400we're in a subroutine or eval() or require(), and the undefined value
401otherwise. In a list context, returns
a0d0e21e 402
748a9306 403 ($package, $filename, $line) = caller;
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404
405With EXPR, it returns some extra information that the debugger uses to
406print a stack trace. The value of EXPR indicates how many call frames
407to go back before the current one.
408
54310121 409 ($package, $filename, $line, $subroutine,
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410 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
411
412Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
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413call, but an C<eval>. In such a case additional elements $evaltext and
414$is_require are set: $is_require is true if the frame is created by a
415C<require> or C<use> statement, $evaltext contains the text of the
416C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
417$filename is C<"(eval)">, but $evaltext is undefined. (Note also that
418each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
419frame.
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420
421Furthermore, when called from within the DB package, caller returns more
4633a7c4 422detailed information: it sets the list variable @DB::args to be the
54310121 423arguments with which the subroutine was invoked.
748a9306 424
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425=item chdir EXPR
426
427Changes the working directory to EXPR, if possible. If EXPR is
428omitted, changes to home directory. Returns TRUE upon success, FALSE
429otherwise. See example under die().
430
431=item chmod LIST
432
433Changes the permissions of a list of files. The first element of the
4633a7c4 434list must be the numerical mode, which should probably be an octal
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435number, and which definitely should I<not> a string of octal digits:
436C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 437successfully changed. See also L</oct>, if all you have is a string.
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438
439 $cnt = chmod 0755, 'foo', 'bar';
440 chmod 0755, @executables;
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441 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to --w----r-T
442 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
443 $mode = 0644; chmod $mode, 'foo'; # this is best
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444
445=item chomp VARIABLE
446
447=item chomp LIST
448
449=item chomp
450
451This is a slightly safer version of chop (see below). It removes any
452line ending that corresponds to the current value of C<$/> (also known as
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453$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
454number of characters removed from all its arguments. It's often used to
455remove the newline from the end of an input record when you're worried
456that the final record may be missing its newline. When in paragraph mode
457(C<$/ = "">), it removes all trailing newlines from the string. If
458VARIABLE is omitted, it chomps $_. Example:
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459
460 while (<>) {
461 chomp; # avoid \n on last field
462 @array = split(/:/);
463 ...
464 }
465
466You can actually chomp anything that's an lvalue, including an assignment:
467
468 chomp($cwd = `pwd`);
469 chomp($answer = <STDIN>);
470
471If you chomp a list, each element is chomped, and the total number of
472characters removed is returned.
473
474=item chop VARIABLE
475
476=item chop LIST
477
478=item chop
479
480Chops off the last character of a string and returns the character
481chopped. It's used primarily to remove the newline from the end of an
482input record, but is much more efficient than C<s/\n//> because it neither
483scans nor copies the string. If VARIABLE is omitted, chops $_.
484Example:
485
486 while (<>) {
487 chop; # avoid \n on last field
488 @array = split(/:/);
489 ...
490 }
491
492You can actually chop anything that's an lvalue, including an assignment:
493
494 chop($cwd = `pwd`);
495 chop($answer = <STDIN>);
496
497If you chop a list, each element is chopped. Only the value of the
498last chop is returned.
499
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500Note that chop returns the last character. To return all but the last
501character, use C<substr($string, 0, -1)>.
502
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503=item chown LIST
504
505Changes the owner (and group) of a list of files. The first two
506elements of the list must be the I<NUMERICAL> uid and gid, in that order.
507Returns the number of files successfully changed.
508
509 $cnt = chown $uid, $gid, 'foo', 'bar';
510 chown $uid, $gid, @filenames;
511
54310121 512Here's an example that looks up nonnumeric uids in the passwd file:
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513
514 print "User: ";
515 chop($user = <STDIN>);
516 print "Files: "
517 chop($pattern = <STDIN>);
518
519 ($login,$pass,$uid,$gid) = getpwnam($user)
520 or die "$user not in passwd file";
521
522 @ary = <${pattern}>; # expand filenames
523 chown $uid, $gid, @ary;
524
54310121 525On most systems, you are not allowed to change the ownership of the
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526file unless you're the superuser, although you should be able to change
527the group to any of your secondary groups. On insecure systems, these
528restrictions may be relaxed, but this is not a portable assumption.
529
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530=item chr NUMBER
531
54310121 532=item chr
bbce6d69 533
a0d0e21e 534Returns the character represented by that NUMBER in the character set.
dc848c6f 535For example, C<chr(65)> is "A" in ASCII. For the reverse, use L</ord>.
a0d0e21e 536
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537If NUMBER is omitted, uses $_.
538
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539=item chroot FILENAME
540
54310121 541=item chroot
bbce6d69 542
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543This function works as the system call by the same name: it makes the
544named directory the new root directory for all further pathnames that
545begin with a "/" by your process and all of its children. (It doesn't
28757baa 546change your current working directory, which is unaffected.) For security
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547reasons, this call is restricted to the superuser. If FILENAME is
548omitted, does chroot to $_.
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549
550=item close FILEHANDLE
551
552Closes the file or pipe associated with the file handle, returning TRUE
553only if stdio successfully flushes buffers and closes the system file
554descriptor. You don't have to close FILEHANDLE if you are immediately
5f05dabc 555going to do another open() on it, because open() will close it for you. (See
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556open().) However, an explicit close on an input file resets the line
557counter ($.), while the implicit close done by open() does not. Also,
558closing a pipe will wait for the process executing on the pipe to
559complete, in case you want to look at the output of the pipe
560afterwards. Closing a pipe explicitly also puts the status value of
561the command into C<$?>. Example:
562
563 open(OUTPUT, '|sort >foo'); # pipe to sort
564 ... # print stuff to output
565 close OUTPUT; # wait for sort to finish
566 open(INPUT, 'foo'); # get sort's results
567
568FILEHANDLE may be an expression whose value gives the real filehandle name.
569
570=item closedir DIRHANDLE
571
572Closes a directory opened by opendir().
573
574=item connect SOCKET,NAME
575
576Attempts to connect to a remote socket, just as the connect system call
577does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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578packed address of the appropriate type for the socket. See the examples in
579L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 580
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581=item continue BLOCK
582
583Actually a flow control statement rather than a function. If there is a
584C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
585C<foreach>), it is always executed just before the conditional is about to
586be evaluated again, just like the third part of a C<for> loop in C. Thus
587it can be used to increment a loop variable, even when the loop has been
588continued via the C<next> statement (which is similar to the C C<continue>
589statement).
590
a0d0e21e
LW
591=item cos EXPR
592
593Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
594takes cosine of $_.
595
28757baa
PP
596For the inverse cosine operation, you may use the POSIX::acos()
597function, or use this relation:
598
599 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
600
a0d0e21e
LW
601=item crypt PLAINTEXT,SALT
602
4633a7c4
LW
603Encrypts a string exactly like the crypt(3) function in the C library
604(assuming that you actually have a version there that has not been
605extirpated as a potential munition). This can prove useful for checking
606the password file for lousy passwords, amongst other things. Only the
607guys wearing white hats should do this.
a0d0e21e 608
11155c91
CS
609Note that crypt is intended to be a one-way function, much like breaking
610eggs to make an omelette. There is no (known) corresponding decrypt
611function. As a result, this function isn't all that useful for
612cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 613
a0d0e21e
LW
614Here's an example that makes sure that whoever runs this program knows
615their own password:
616
617 $pwd = (getpwuid($<))[1];
618 $salt = substr($pwd, 0, 2);
619
620 system "stty -echo";
621 print "Password: ";
622 chop($word = <STDIN>);
623 print "\n";
624 system "stty echo";
625
626 if (crypt($word, $salt) ne $pwd) {
627 die "Sorry...\n";
628 } else {
629 print "ok\n";
54310121 630 }
a0d0e21e 631
54310121 632Of course, typing in your own password to whomever asks you
748a9306 633for it is unwise.
a0d0e21e 634
aa689395 635=item dbmclose HASH
a0d0e21e
LW
636
637[This function has been superseded by the untie() function.]
638
aa689395 639Breaks the binding between a DBM file and a hash.
a0d0e21e 640
aa689395 641=item dbmopen HASH,DBNAME,MODE
a0d0e21e
LW
642
643[This function has been superseded by the tie() function.]
644
aa689395
PP
645This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to a
646hash. HASH is the name of the hash. (Unlike normal open, the first
647argument is I<NOT> a filehandle, even though it looks like one). DBNAME
648is the name of the database (without the F<.dir> or F<.pag> extension if
649any). If the database does not exist, it is created with protection
650specified by MODE (as modified by the umask()). If your system supports
651only the older DBM functions, you may perform only one dbmopen() in your
652program. In older versions of Perl, if your system had neither DBM nor
653ndbm, calling dbmopen() produced a fatal error; it now falls back to
654sdbm(3).
655
656If you don't have write access to the DBM file, you can only read hash
657variables, not set them. If you want to test whether you can write,
658either use file tests or try setting a dummy hash entry inside an eval(),
659which will trap the error.
a0d0e21e
LW
660
661Note that functions such as keys() and values() may return huge array
662values when used on large DBM files. You may prefer to use the each()
663function to iterate over large DBM files. Example:
664
665 # print out history file offsets
666 dbmopen(%HIST,'/usr/lib/news/history',0666);
667 while (($key,$val) = each %HIST) {
668 print $key, ' = ', unpack('L',$val), "\n";
669 }
670 dbmclose(%HIST);
671
cb1a09d0 672See also L<AnyDBM_File> for a more general description of the pros and
184e9718 673cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 674rich implementation.
4633a7c4 675
a0d0e21e
LW
676=item defined EXPR
677
54310121 678=item defined
bbce6d69 679
2f9daede
TPG
680Returns a Boolean value telling whether EXPR has a value other than
681the undefined value C<undef>. If EXPR is not present, C<$_> will be
682checked.
683
684Many operations return C<undef> to indicate failure, end of file,
685system error, uninitialized variable, and other exceptional
686conditions. This function allows you to distinguish C<undef> from
687other values. (A simple Boolean test will not distinguish among
688C<undef>, zero, the empty string, and "0", which are all equally
689false.) Note that since C<undef> is a valid scalar, its presence
690doesn't I<necessarily> indicate an exceptional condition: pop()
691returns C<undef> when its argument is an empty array, I<or> when the
692element to return happens to be C<undef>.
693
694You may also use defined() to check whether a subroutine exists. On
695the other hand, use of defined() upon aggregates (hashes and arrays)
696is not guaranteed to produce intuitive results, and should probably be
697avoided.
698
699When used on a hash element, it tells you whether the value is defined,
dc848c6f 700not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 701purpose.
a0d0e21e
LW
702
703Examples:
704
705 print if defined $switch{'D'};
706 print "$val\n" while defined($val = pop(@ary));
707 die "Can't readlink $sym: $!"
708 unless defined($value = readlink $sym);
a0d0e21e 709 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 710 $debugging = 0 unless defined $debugging;
a0d0e21e 711
2f9daede
TPG
712Note: Many folks tend to overuse defined(), and then are surprised to
713discover that the number 0 and "" (the zero-length string) are, in fact,
714defined values. For example, if you say
a5f75d66
AD
715
716 "ab" =~ /a(.*)b/;
717
718the pattern match succeeds, and $1 is defined, despite the fact that it
719matched "nothing". But it didn't really match nothing--rather, it
720matched something that happened to be 0 characters long. This is all
721very above-board and honest. When a function returns an undefined value,
2f9daede
TPG
722it's an admission that it couldn't give you an honest answer. So you
723should use defined() only when you're questioning the integrity of what
724you're trying to do. At other times, a simple comparison to 0 or "" is
725what you want.
726
727Currently, using defined() on an entire array or hash reports whether
728memory for that aggregate has ever been allocated. So an array you set
729to the empty list appears undefined initially, and one that once was full
730and that you then set to the empty list still appears defined. You
731should instead use a simple test for size:
28757baa
PP
732
733 if (@an_array) { print "has array elements\n" }
734 if (%a_hash) { print "has hash members\n" }
735
736Using undef() on these, however, does clear their memory and then report
737them as not defined anymore, but you shoudln't do that unless you don't
738plan to use them again, because it saves time when you load them up
739again to have memory already ready to be filled.
740
54310121 741This counterintuitive behaviour of defined() on aggregates may be
28757baa
PP
742changed, fixed, or broken in a future release of Perl.
743
dc848c6f 744See also L</undef>, L</exists>, L</ref>.
2f9daede 745
a0d0e21e
LW
746=item delete EXPR
747
aa689395
PP
748Deletes the specified key(s) and their associated values from a hash.
749For each key, returns the deleted value associated with that key, or
750the undefined value if there was no such key. Deleting from C<$ENV{}>
751modifies the environment. Deleting from a hash tied to a DBM file
5f05dabc
PP
752deletes the entry from the DBM file. (But deleting from a tie()d hash
753doesn't necessarily return anything.)
a0d0e21e 754
aa689395 755The following deletes all the values of a hash:
a0d0e21e 756
5f05dabc
PP
757 foreach $key (keys %HASH) {
758 delete $HASH{$key};
a0d0e21e
LW
759 }
760
5f05dabc
PP
761And so does this:
762
763 delete @HASH{keys %HASH}
764
765(But both of these are slower than the undef() command.) Note that the
766EXPR can be arbitrarily complicated as long as the final operation is a
767hash element lookup or hash slice:
a0d0e21e
LW
768
769 delete $ref->[$x][$y]{$key};
5f05dabc 770 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
771
772=item die LIST
773
774Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
184e9718 775the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
54310121 776C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
28757baa
PP
777is 0, exits with 255. Inside an eval(), the error message is stuffed into
778C<$@>, and the eval() is terminated with the undefined value; this makes
779die() the way to raise an exception.
a0d0e21e
LW
780
781Equivalent examples:
782
783 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 784 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
785
786If the value of EXPR does not end in a newline, the current script line
787number and input line number (if any) are also printed, and a newline
788is supplied. Hint: sometimes appending ", stopped" to your message
789will cause it to make better sense when the string "at foo line 123" is
790appended. Suppose you are running script "canasta".
791
792 die "/etc/games is no good";
793 die "/etc/games is no good, stopped";
794
795produce, respectively
796
797 /etc/games is no good at canasta line 123.
798 /etc/games is no good, stopped at canasta line 123.
799
800See also exit() and warn().
801
774d564b
PP
802You can arrange for a callback to be called just before the die() does
803its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
804will be called with the error text and can change the error message, if
805it sees fit, by calling die() again. See L<perlvar> for details on
806setting C<%SIG> entries, and eval() for some examples.
807
a0d0e21e
LW
808=item do BLOCK
809
810Not really a function. Returns the value of the last command in the
811sequence of commands indicated by BLOCK. When modified by a loop
812modifier, executes the BLOCK once before testing the loop condition.
813(On other statements the loop modifiers test the conditional first.)
814
815=item do SUBROUTINE(LIST)
816
817A deprecated form of subroutine call. See L<perlsub>.
818
819=item do EXPR
820
821Uses the value of EXPR as a filename and executes the contents of the
822file as a Perl script. Its primary use is to include subroutines
823from a Perl subroutine library.
824
825 do 'stat.pl';
826
827is just like
828
829 eval `cat stat.pl`;
830
831except that it's more efficient, more concise, keeps track of the
832current filename for error messages, and searches all the B<-I>
833libraries if the file isn't in the current directory (see also the @INC
834array in L<perlvar/Predefined Names>). It's the same, however, in that it does
54310121 835reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
836do this inside a loop.
837
838Note that inclusion of library modules is better done with the
4633a7c4
LW
839use() and require() operators, which also do error checking
840and raise an exception if there's a problem.
a0d0e21e
LW
841
842=item dump LABEL
843
844This causes an immediate core dump. Primarily this is so that you can
845use the B<undump> program to turn your core dump into an executable binary
846after having initialized all your variables at the beginning of the
847program. When the new binary is executed it will begin by executing a
848C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
849it as a goto with an intervening core dump and reincarnation. If LABEL
850is omitted, restarts the program from the top. WARNING: any files
851opened at the time of the dump will NOT be open any more when the
852program is reincarnated, with possible resulting confusion on the part
853of Perl. See also B<-u> option in L<perlrun>.
854
855Example:
856
857 #!/usr/bin/perl
858 require 'getopt.pl';
859 require 'stat.pl';
860 %days = (
861 'Sun' => 1,
862 'Mon' => 2,
863 'Tue' => 3,
864 'Wed' => 4,
865 'Thu' => 5,
866 'Fri' => 6,
867 'Sat' => 7,
868 );
869
870 dump QUICKSTART if $ARGV[0] eq '-d';
871
872 QUICKSTART:
873 Getopt('f');
874
aa689395
PP
875=item each HASH
876
877When called in a list context, returns a 2-element array consisting of the
878key and value for the next element of a hash, so that you can iterate over
879it. When called in a scalar context, returns the key for only the next
2f9daede
TPG
880element in the hash. (Note: Keys may be "0" or "", which are logically
881false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
882for this reason.)
883
884Entries are returned in an apparently random order. When the hash is
885entirely read, a null array is returned in list context (which when
886assigned produces a FALSE (0) value), and C<undef> is returned in a
887scalar context. The next call to each() after that will start iterating
888again. There is a single iterator for each hash, shared by all each(),
889keys(), and values() function calls in the program; it can be reset by
890reading all the elements from the hash, or by evaluating C<keys HASH> or
891C<values HASH>. If you add or delete elements of a hash while you're
892iterating over it, you may get entries skipped or duplicated, so don't.
aa689395
PP
893
894The following prints out your environment like the printenv(1) program,
895only in a different order:
a0d0e21e
LW
896
897 while (($key,$value) = each %ENV) {
898 print "$key=$value\n";
899 }
900
901See also keys() and values().
902
903=item eof FILEHANDLE
904
4633a7c4
LW
905=item eof ()
906
a0d0e21e
LW
907=item eof
908
909Returns 1 if the next read on FILEHANDLE will return end of file, or if
910FILEHANDLE is not open. FILEHANDLE may be an expression whose value
911gives the real filehandle name. (Note that this function actually
912reads a character and then ungetc()s it, so it is not very useful in an
748a9306
LW
913interactive context.) Do not read from a terminal file (or call
914C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
915as terminals may lose the end-of-file condition if you do.
916
917An C<eof> without an argument uses the last file read as argument.
2f9daede
TPG
918Empty parentheses () may be used to indicate the pseudo file formed of
919the files listed on the command line, i.e., C<eof()> is reasonable to
920use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
921last file. Use C<eof(ARGV)> or eof without the parentheses to test
922I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 923
748a9306
LW
924 # reset line numbering on each input file
925 while (<>) {
926 print "$.\t$_";
927 close(ARGV) if (eof); # Not eof().
928 }
929
a0d0e21e
LW
930 # insert dashes just before last line of last file
931 while (<>) {
932 if (eof()) {
933 print "--------------\n";
748a9306
LW
934 close(ARGV); # close or break; is needed if we
935 # are reading from the terminal
a0d0e21e
LW
936 }
937 print;
938 }
939
a0d0e21e 940Practical hint: you almost never need to use C<eof> in Perl, because the
54310121 941input operators return undef when they run out of data.
a0d0e21e
LW
942
943=item eval EXPR
944
945=item eval BLOCK
946
947EXPR is parsed and executed as if it were a little Perl program. It
948is executed in the context of the current Perl program, so that any
5f05dabc 949variable settings or subroutine and format definitions remain afterwards.
a0d0e21e 950The value returned is the value of the last expression evaluated, or a
55497cff
PP
951return statement may be used, just as with subroutines. The last
952expression is evaluated in scalar or array context, depending on the
953context of the eval.
a0d0e21e
LW
954
955If there is a syntax error or runtime error, or a die() statement is
956executed, an undefined value is returned by eval(), and C<$@> is set to the
957error message. If there was no error, C<$@> is guaranteed to be a null
774d564b
PP
958string. If EXPR is omitted, evaluates C<$_>. The final semicolon, if
959any, may be omitted from the expression. Beware that using eval()
960neither silences perl from printing warnings to STDERR, nor does it
961stuff the text of warning messages into C<$@>. To do either of those,
962you have to use the C<$SIG{__WARN__}> facility. See warn() and L<perlvar>.
a0d0e21e 963
5f05dabc 964Note that, because eval() traps otherwise-fatal errors, it is useful for
4633a7c4 965determining whether a particular feature (such as socket() or symlink())
a0d0e21e
LW
966is implemented. It is also Perl's exception trapping mechanism, where
967the die operator is used to raise exceptions.
968
969If the code to be executed doesn't vary, you may use the eval-BLOCK
970form to trap run-time errors without incurring the penalty of
971recompiling each time. The error, if any, is still returned in C<$@>.
972Examples:
973
54310121 974 # make divide-by-zero nonfatal
a0d0e21e
LW
975 eval { $answer = $a / $b; }; warn $@ if $@;
976
977 # same thing, but less efficient
978 eval '$answer = $a / $b'; warn $@ if $@;
979
980 # a compile-time error
981 eval { $answer = };
982
983 # a run-time error
984 eval '$answer ='; # sets $@
985
774d564b
PP
986When using the eval{} form as an exception trap in libraries, you may
987wish not to trigger any C<__DIE__> hooks that user code may have
988installed. You can use the C<local $SIG{__DIE__}> construct for this
989purpose, as shown in this example:
990
991 # a very private exception trap for divide-by-zero
992 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
993
994This is especially significant, given that C<__DIE__> hooks can call
995die() again, which has the effect of changing their error messages:
996
997 # __DIE__ hooks may modify error messages
998 {
999 local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1000 eval { die "foo foofs here" };
1001 print $@ if $@; # prints "bar barfs here"
1002 }
1003
54310121 1004With an eval(), you should be especially careful to remember what's
a0d0e21e
LW
1005being looked at when:
1006
1007 eval $x; # CASE 1
1008 eval "$x"; # CASE 2
1009
1010 eval '$x'; # CASE 3
1011 eval { $x }; # CASE 4
1012
1013 eval "\$$x++" # CASE 5
1014 $$x++; # CASE 6
1015
2f9daede
TPG
1016Cases 1 and 2 above behave identically: they run the code contained in
1017the variable $x. (Although case 2 has misleading double quotes making
1018the reader wonder what else might be happening (nothing is).) Cases 3
1019and 4 likewise behave in the same way: they run the code '$x', which
1020does nothing but return the value of C<$x>. (Case 4 is preferred for
1021purely visual reasons, but it also has the advantage of compiling at
1022compile-time instead of at run-time.) Case 5 is a place where
54310121 1023normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TPG
1024particular situation, you can just use symbolic references instead, as
1025in case 6.
a0d0e21e
LW
1026
1027=item exec LIST
1028
55497cff
PP
1029The exec() function executes a system command I<AND NEVER RETURNS>,
1030unless the command does not exist and is executed directly instead of
1031via C</bin/sh -c> (see below). Use system() instead of exec() if you
1032want it to return.
a0d0e21e
LW
1033
1034If there is more than one argument in LIST, or if LIST is an array with
1035more than one value, calls execvp(3) with the arguments in LIST. If
1036there is only one scalar argument, the argument is checked for shell
1037metacharacters. If there are any, the entire argument is passed to
1038C</bin/sh -c> for parsing. If there are none, the argument is split
1039into words and passed directly to execvp(), which is more efficient.
37798a01 1040Note: exec() and system() do not flush your output buffer, so you may
a0d0e21e
LW
1041need to set C<$|> to avoid lost output. Examples:
1042
1043 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1044 exec "sort $outfile | uniq";
1045
1046If you don't really want to execute the first argument, but want to lie
1047to the program you are executing about its own name, you can specify
1048the program you actually want to run as an "indirect object" (without a
1049comma) in front of the LIST. (This always forces interpretation of the
54310121 1050LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1051the list.) Example:
1052
1053 $shell = '/bin/csh';
1054 exec $shell '-sh'; # pretend it's a login shell
1055
1056or, more directly,
1057
1058 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1059
1060=item exists EXPR
1061
1062Returns TRUE if the specified hash key exists in its hash array, even
1063if the corresponding value is undefined.
1064
1065 print "Exists\n" if exists $array{$key};
1066 print "Defined\n" if defined $array{$key};
1067 print "True\n" if $array{$key};
1068
5f05dabc 1069A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1070it exists, but the reverse doesn't necessarily hold true.
1071
1072Note that the EXPR can be arbitrarily complicated as long as the final
1073operation is a hash key lookup:
1074
1075 if (exists $ref->[$x][$y]{$key}) { ... }
1076
1077=item exit EXPR
1078
1079Evaluates EXPR and exits immediately with that value. (Actually, it
1080calls any defined C<END> routines first, but the C<END> routines may not
1081abort the exit. Likewise any object destructors that need to be called
1082are called before exit.) Example:
1083
1084 $ans = <STDIN>;
1085 exit 0 if $ans =~ /^[Xx]/;
1086
f86702cc 1087See also die(). If EXPR is omitted, exits with 0 status. The only
54310121 1088universally portable values for EXPR are 0 for success and 1 for error;
f86702cc
PP
1089all other values are subject to unpredictable interpretation depending
1090on the environment in which the Perl program is running.
a0d0e21e 1091
28757baa
PP
1092You shouldn't use exit() to abort a subroutine if there's any chance that
1093someone might want to trap whatever error happened. Use die() instead,
1094which can be trapped by an eval().
1095
a0d0e21e
LW
1096=item exp EXPR
1097
54310121 1098=item exp
bbce6d69 1099
54310121 1100Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1101If EXPR is omitted, gives C<exp($_)>.
1102
1103=item fcntl FILEHANDLE,FUNCTION,SCALAR
1104
1105Implements the fcntl(2) function. You'll probably have to say
1106
1107 use Fcntl;
1108
1109first to get the correct function definitions. Argument processing and
1110value return works just like ioctl() below. Note that fcntl() will produce
1111a fatal error if used on a machine that doesn't implement fcntl(2).
1112For example:
1113
1114 use Fcntl;
1115 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1116
1117=item fileno FILEHANDLE
1118
1119Returns the file descriptor for a filehandle. This is useful for
1120constructing bitmaps for select(). If FILEHANDLE is an expression, the
1121value is taken as the name of the filehandle.
1122
1123=item flock FILEHANDLE,OPERATION
1124
8ebc5c01 1125Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745
PP
1126success, FALSE on failure. Produces a fatal error if used on a machine
1127that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock()
1128is Perl's portable file locking interface, although it locks only entire
1129files, not records.
8ebc5c01
PP
1130
1131OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1132LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745
PP
1133you can use the symbolic names if import them from the Fcntl module,
1134either individually, or as a group using the ':flock' tag. LOCK_SH
1135requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1136releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1137LOCK_EX then flock() will return immediately rather than blocking
1138waiting for the lock (check the return status to see if you got it).
1139
1140To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1141before (un)locking it.
8ebc5c01
PP
1142
1143Note that the emulation built with lockf(3) doesn't provide shared
1144locks, and it requires that FILEHANDLE be open with write intent. These
1145are the semantics that lockf(3) implements. Most (all?) systems
1146implement lockf(3) in terms of fcntl(2) locking, though, so the
1147differing semantics shouldn't bite too many people.
1148
1149Note also that some versions of flock() cannot lock things over the
1150network; you would need to use the more system-specific fcntl() for
1151that. If you like you can force Perl to ignore your system's flock(2)
1152function, and so provide its own fcntl(2)-based emulation, by passing
1153the switch C<-Ud_flock> to the F<Configure> program when you configure
1154perl.
4633a7c4
LW
1155
1156Here's a mailbox appender for BSD systems.
a0d0e21e 1157
7e1af8bc 1158 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1159
1160 sub lock {
7e1af8bc 1161 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1162 # and, in case someone appended
1163 # while we were waiting...
1164 seek(MBOX, 0, 2);
1165 }
1166
1167 sub unlock {
7e1af8bc 1168 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1169 }
1170
1171 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1172 or die "Can't open mailbox: $!";
1173
1174 lock();
1175 print MBOX $msg,"\n\n";
1176 unlock();
1177
cb1a09d0 1178See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1179
1180=item fork
1181
1182Does a fork(2) system call. Returns the child pid to the parent process
4633a7c4 1183and 0 to the child process, or C<undef> if the fork is unsuccessful.
a0d0e21e 1184Note: unflushed buffers remain unflushed in both processes, which means
28757baa
PP
1185you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1186method of IO::Handle to avoid duplicate output.
a0d0e21e
LW
1187
1188If you fork() without ever waiting on your children, you will accumulate
1189zombies:
1190
4633a7c4 1191 $SIG{CHLD} = sub { wait };
a0d0e21e 1192
54310121 1193There's also the double-fork trick (error checking on
a0d0e21e
LW
1194fork() returns omitted);
1195
1196 unless ($pid = fork) {
1197 unless (fork) {
1198 exec "what you really wanna do";
1199 die "no exec";
1200 # ... or ...
4633a7c4 1201 ## (some_perl_code_here)
a0d0e21e
LW
1202 exit 0;
1203 }
1204 exit 0;
1205 }
1206 waitpid($pid,0);
1207
cb1a09d0
AD
1208See also L<perlipc> for more examples of forking and reaping
1209moribund children.
1210
28757baa
PP
1211Note that if your forked child inherits system file descriptors like
1212STDIN and STDOUT that are actually connected by a pipe or socket, even
1213if you exit, the remote server (such as, say, httpd or rsh) won't think
1214you're done. You should reopen those to /dev/null if it's any issue.
1215
cb1a09d0
AD
1216=item format
1217
1218Declare a picture format with use by the write() function. For
1219example:
1220
54310121 1221 format Something =
cb1a09d0
AD
1222 Test: @<<<<<<<< @||||| @>>>>>
1223 $str, $%, '$' . int($num)
1224 .
1225
1226 $str = "widget";
184e9718 1227 $num = $cost/$quantity;
cb1a09d0
AD
1228 $~ = 'Something';
1229 write;
1230
1231See L<perlform> for many details and examples.
1232
a0d0e21e
LW
1233
1234=item formline PICTURE, LIST
1235
4633a7c4 1236This is an internal function used by C<format>s, though you may call it
a0d0e21e
LW
1237too. It formats (see L<perlform>) a list of values according to the
1238contents of PICTURE, placing the output into the format output
4633a7c4
LW
1239accumulator, C<$^A> (or $ACCUMULATOR in English).
1240Eventually, when a write() is done, the contents of
a0d0e21e
LW
1241C<$^A> are written to some filehandle, but you could also read C<$^A>
1242yourself and then set C<$^A> back to "". Note that a format typically
1243does one formline() per line of form, but the formline() function itself
748a9306 1244doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1245that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1246You may therefore need to use multiple formlines to implement a single
1247record format, just like the format compiler.
1248
5f05dabc 1249Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1250character may be taken to mean the beginning of an array name.
4633a7c4 1251formline() always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1252
1253=item getc FILEHANDLE
1254
1255=item getc
1256
1257Returns the next character from the input file attached to FILEHANDLE,
1258or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
4633a7c4 1259This is not particularly efficient. It cannot be used to get unbuffered
cb1a09d0 1260single-characters, however. For that, try something more like:
4633a7c4
LW
1261
1262 if ($BSD_STYLE) {
1263 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1264 }
1265 else {
54310121 1266 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1267 }
1268
1269 $key = getc(STDIN);
1270
1271 if ($BSD_STYLE) {
1272 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1273 }
1274 else {
5f05dabc 1275 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1276 }
1277 print "\n";
1278
54310121
PP
1279Determination of whether $BSD_STYLE should be set
1280is left as an exercise to the reader.
cb1a09d0 1281
28757baa
PP
1282The POSIX::getattr() function can do this more portably on systems
1283alleging POSIX compliance.
cb1a09d0 1284See also the C<Term::ReadKey> module from your nearest CPAN site;
54310121 1285details on CPAN can be found on L<perlmod/CPAN>.
a0d0e21e
LW
1286
1287=item getlogin
1288
1289Returns the current login from F</etc/utmp>, if any. If null, use
54310121 1290getpwuid().
a0d0e21e 1291
f86702cc 1292 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1293
da0045b7 1294Do not consider getlogin() for authentication: it is not as
4633a7c4
LW
1295secure as getpwuid().
1296
a0d0e21e
LW
1297=item getpeername SOCKET
1298
1299Returns the packed sockaddr address of other end of the SOCKET connection.
1300
4633a7c4
LW
1301 use Socket;
1302 $hersockaddr = getpeername(SOCK);
1303 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1304 $herhostname = gethostbyaddr($iaddr, AF_INET);
1305 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1306
1307=item getpgrp PID
1308
47e29363
PP
1309Returns the current process group for the specified PID. Use
1310a PID of 0 to get the current process group for the
4633a7c4 1311current process. Will raise an exception if used on a machine that
a0d0e21e 1312doesn't implement getpgrp(2). If PID is omitted, returns process
47e29363
PP
1313group of current process. Note that the POSIX version of getpgrp()
1314does not accept a PID argument, so only PID==0 is truly portable.
a0d0e21e
LW
1315
1316=item getppid
1317
1318Returns the process id of the parent process.
1319
1320=item getpriority WHICH,WHO
1321
4633a7c4
LW
1322Returns the current priority for a process, a process group, or a user.
1323(See L<getpriority(2)>.) Will raise a fatal exception if used on a
a0d0e21e
LW
1324machine that doesn't implement getpriority(2).
1325
1326=item getpwnam NAME
1327
1328=item getgrnam NAME
1329
1330=item gethostbyname NAME
1331
1332=item getnetbyname NAME
1333
1334=item getprotobyname NAME
1335
1336=item getpwuid UID
1337
1338=item getgrgid GID
1339
1340=item getservbyname NAME,PROTO
1341
1342=item gethostbyaddr ADDR,ADDRTYPE
1343
1344=item getnetbyaddr ADDR,ADDRTYPE
1345
1346=item getprotobynumber NUMBER
1347
1348=item getservbyport PORT,PROTO
1349
1350=item getpwent
1351
1352=item getgrent
1353
1354=item gethostent
1355
1356=item getnetent
1357
1358=item getprotoent
1359
1360=item getservent
1361
1362=item setpwent
1363
1364=item setgrent
1365
1366=item sethostent STAYOPEN
1367
1368=item setnetent STAYOPEN
1369
1370=item setprotoent STAYOPEN
1371
1372=item setservent STAYOPEN
1373
1374=item endpwent
1375
1376=item endgrent
1377
1378=item endhostent
1379
1380=item endnetent
1381
1382=item endprotoent
1383
1384=item endservent
1385
1386These routines perform the same functions as their counterparts in the
1387system library. Within a list context, the return values from the
1388various get routines are as follows:
1389
1390 ($name,$passwd,$uid,$gid,
1391 $quota,$comment,$gcos,$dir,$shell) = getpw*
1392 ($name,$passwd,$gid,$members) = getgr*
1393 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1394 ($name,$aliases,$addrtype,$net) = getnet*
1395 ($name,$aliases,$proto) = getproto*
1396 ($name,$aliases,$port,$proto) = getserv*
1397
1398(If the entry doesn't exist you get a null list.)
1399
1400Within a scalar context, you get the name, unless the function was a
1401lookup by name, in which case you get the other thing, whatever it is.
1402(If the entry doesn't exist you get the undefined value.) For example:
1403
1404 $uid = getpwnam
1405 $name = getpwuid
1406 $name = getpwent
1407 $gid = getgrnam
1408 $name = getgrgid
1409 $name = getgrent
1410 etc.
1411
1412The $members value returned by I<getgr*()> is a space separated list of
1413the login names of the members of the group.
1414
1415For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1416C, it will be returned to you via C<$?> if the function call fails. The
1417@addrs value returned by a successful call is a list of the raw
1418addresses returned by the corresponding system library call. In the
1419Internet domain, each address is four bytes long and you can unpack it
1420by saying something like:
1421
1422 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1423
1424=item getsockname SOCKET
1425
1426Returns the packed sockaddr address of this end of the SOCKET connection.
1427
4633a7c4
LW
1428 use Socket;
1429 $mysockaddr = getsockname(SOCK);
1430 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1431
1432=item getsockopt SOCKET,LEVEL,OPTNAME
1433
1434Returns the socket option requested, or undefined if there is an error.
1435
1436=item glob EXPR
1437
0a753a76
PP
1438=item glob
1439
68dc0745
PP
1440Returns the value of EXPR with filename expansions such as a shell would
1441do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1442operator, but you can use it directly. If EXPR is omitted, $_ is used.
1443The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1444L<perlop/"I/O Operators">.
a0d0e21e
LW
1445
1446=item gmtime EXPR
1447
1448Converts a time as returned by the time function to a 9-element array
54310121 1449with the time localized for the standard Greenwich time zone.
4633a7c4 1450Typically used as follows:
a0d0e21e 1451
54310121 1452 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1453 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1454 gmtime(time);
1455
1456All array elements are numeric, and come straight out of a struct tm.
1457In particular this means that $mon has the range 0..11 and $wday has
54310121
PP
1458the range 0..6 with sunday as day 0. Also, $year is the number of
1459years since 1900, I<not> simply the last two digits of the year.
2f9daede
TPG
1460
1461If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1462
54310121 1463In a scalar context, returns the ctime(3) value:
0a753a76
PP
1464
1465 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1466
54310121
PP
1467Also see the timegm() function provided by the Time::Local module,
1468and the strftime(3) function available via the POSIX module.
0a753a76 1469
a0d0e21e
LW
1470=item goto LABEL
1471
748a9306
LW
1472=item goto EXPR
1473
a0d0e21e
LW
1474=item goto &NAME
1475
1476The goto-LABEL form finds the statement labeled with LABEL and resumes
1477execution there. It may not be used to go into any construct that
1478requires initialization, such as a subroutine or a foreach loop. It
0a753a76
PP
1479also can't be used to go into a construct that is optimized away,
1480or to get out of a block or subroutine given to sort().
1481It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e
LW
1482including out of subroutines, but it's usually better to use some other
1483construct such as last or die. The author of Perl has never felt the
1484need to use this form of goto (in Perl, that is--C is another matter).
1485
748a9306
LW
1486The goto-EXPR form expects a label name, whose scope will be resolved
1487dynamically. This allows for computed gotos per FORTRAN, but isn't
1488necessarily recommended if you're optimizing for maintainability:
1489
1490 goto ("FOO", "BAR", "GLARCH")[$i];
1491
a0d0e21e
LW
1492The goto-&NAME form is highly magical, and substitutes a call to the
1493named subroutine for the currently running subroutine. This is used by
1494AUTOLOAD subroutines that wish to load another subroutine and then
1495pretend that the other subroutine had been called in the first place
1496(except that any modifications to @_ in the current subroutine are
1497propagated to the other subroutine.) After the goto, not even caller()
1498will be able to tell that this routine was called first.
1499
1500=item grep BLOCK LIST
1501
1502=item grep EXPR,LIST
1503
54310121 1504This is similar in spirit to, but not the same as, grep(1)
2f9daede
TPG
1505and its relatives. In particular, it is not limited to using
1506regular expressions.
1507
a0d0e21e
LW
1508Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1509$_ to each element) and returns the list value consisting of those
1510elements for which the expression evaluated to TRUE. In a scalar
1511context, returns the number of times the expression was TRUE.
1512
1513 @foo = grep(!/^#/, @bar); # weed out comments
1514
1515or equivalently,
1516
1517 @foo = grep {!/^#/} @bar; # weed out comments
1518
5f05dabc 1519Note that, because $_ is a reference into the list value, it can be used
a0d0e21e
LW
1520to modify the elements of the array. While this is useful and
1521supported, it can cause bizarre results if the LIST is not a named
2f9daede 1522array. Similarly, grep returns aliases into the original list,
2ae324a7 1523much like the way that L<Foreach Loops>'s index variable aliases the list
2f9daede
TPG
1524elements. That is, modifying an element of a list returned by grep
1525actually modifies the element in the original list.
a0d0e21e
LW
1526
1527=item hex EXPR
1528
54310121 1529=item hex
bbce6d69 1530
54310121 1531Interprets EXPR as a hex string and returns the corresponding
2f9daede 1532value. (To convert strings that might start with either 0 or 0x
dc848c6f 1533see L</oct>.) If EXPR is omitted, uses $_.
2f9daede
TPG
1534
1535 print hex '0xAf'; # prints '175'
1536 print hex 'aF'; # same
a0d0e21e
LW
1537
1538=item import
1539
54310121 1540There is no builtin import() function. It is merely an ordinary
4633a7c4 1541method (subroutine) defined (or inherited) by modules that wish to export
a0d0e21e 1542names to another module. The use() function calls the import() method
54310121 1543for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1544
1545=item index STR,SUBSTR,POSITION
1546
1547=item index STR,SUBSTR
1548
4633a7c4
LW
1549Returns the position of the first occurrence of SUBSTR in STR at or after
1550POSITION. If POSITION is omitted, starts searching from the beginning of
184e9718 1551the string. The return value is based at 0 (or whatever you've set the C<$[>
4633a7c4 1552variable to--but don't do that). If the substring is not found, returns
a0d0e21e
LW
1553one less than the base, ordinarily -1.
1554
1555=item int EXPR
1556
54310121 1557=item int
bbce6d69 1558
a0d0e21e
LW
1559Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1560
1561=item ioctl FILEHANDLE,FUNCTION,SCALAR
1562
1563Implements the ioctl(2) function. You'll probably have to say
1564
4633a7c4 1565 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1566
4633a7c4 1567first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1568exist or doesn't have the correct definitions you'll have to roll your
4633a7c4
LW
1569own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1570(There is a Perl script called B<h2ph> that comes with the Perl kit which
54310121 1571may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4
LW
1572written depending on the FUNCTION--a pointer to the string value of SCALAR
1573will be passed as the third argument of the actual ioctl call. (If SCALAR
1574has no string value but does have a numeric value, that value will be
1575passed rather than a pointer to the string value. To guarantee this to be
1576TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1577functions are useful for manipulating the values of structures used by
1578ioctl(). The following example sets the erase character to DEL.
a0d0e21e
LW
1579
1580 require 'ioctl.ph';
4633a7c4
LW
1581 $getp = &TIOCGETP;
1582 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1583 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1584 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1585 @ary = unpack($sgttyb_t,$sgttyb);
1586 $ary[2] = 127;
1587 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1588 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1589 || die "Can't ioctl: $!";
1590 }
1591
1592The return value of ioctl (and fcntl) is as follows:
1593
1594 if OS returns: then Perl returns:
1595 -1 undefined value
1596 0 string "0 but true"
1597 anything else that number
1598
1599Thus Perl returns TRUE on success and FALSE on failure, yet you can
1600still easily determine the actual value returned by the operating
1601system:
1602
1603 ($retval = ioctl(...)) || ($retval = -1);
1604 printf "System returned %d\n", $retval;
1605
1606=item join EXPR,LIST
1607
54310121 1608Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1609fields separated by the value of EXPR, and returns the string.
1610Example:
1611
1612 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1613
1614See L<perlfunc/split>.
1615
aa689395
PP
1616=item keys HASH
1617
1618Returns a normal array consisting of all the keys of the named hash. (In
1619a scalar context, returns the number of keys.) The keys are returned in
1620an apparently random order, but it is the same order as either the
1621values() or each() function produces (given that the hash has not been
1622modified). As a side effect, it resets HASH's iterator.
a0d0e21e 1623
aa689395 1624Here is yet another way to print your environment:
a0d0e21e
LW
1625
1626 @keys = keys %ENV;
1627 @values = values %ENV;
1628 while ($#keys >= 0) {
1629 print pop(@keys), '=', pop(@values), "\n";
1630 }
1631
1632or how about sorted by key:
1633
1634 foreach $key (sort(keys %ENV)) {
1635 print $key, '=', $ENV{$key}, "\n";
1636 }
1637
54310121 1638To sort an array by value, you'll need to use a C<sort> function.
aa689395 1639Here's a descending numeric sort of a hash by its values:
4633a7c4
LW
1640
1641 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1642 printf "%4d %s\n", $hash{$key}, $key;
1643 }
1644
55497cff 1645As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
1646allocated for the given hash. This can gain you a measure of efficiency if
1647you know the hash is going to get big. (This is similar to pre-extending
1648an array by assigning a larger number to $#array.) If you say
55497cff
PP
1649
1650 keys %hash = 200;
1651
1652then C<%hash> will have at least 200 buckets allocated for it. These
1653buckets will be retained even if you do C<%hash = ()>, use C<undef
1654%hash> if you want to free the storage while C<%hash> is still in scope.
1655You can't shrink the number of buckets allocated for the hash using
1656C<keys> in this way (but you needn't worry about doing this by accident,
1657as trying has no effect).
1658
a0d0e21e
LW
1659=item kill LIST
1660
54310121
PP
1661Sends a signal to a list of processes. The first element of
1662the list must be the signal to send. Returns the number of
4633a7c4 1663processes successfully signaled.
a0d0e21e
LW
1664
1665 $cnt = kill 1, $child1, $child2;
1666 kill 9, @goners;
1667
4633a7c4
LW
1668Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1669process groups instead of processes. (On System V, a negative I<PROCESS>
1670number will also kill process groups, but that's not portable.) That
1671means you usually want to use positive not negative signals. You may also
da0045b7 1672use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
1673
1674=item last LABEL
1675
1676=item last
1677
1678The C<last> command is like the C<break> statement in C (as used in
1679loops); it immediately exits the loop in question. If the LABEL is
1680omitted, the command refers to the innermost enclosing loop. The
1681C<continue> block, if any, is not executed:
1682
4633a7c4
LW
1683 LINE: while (<STDIN>) {
1684 last LINE if /^$/; # exit when done with header
a0d0e21e
LW
1685 ...
1686 }
1687
1688=item lc EXPR
1689
54310121 1690=item lc
bbce6d69 1691
a0d0e21e 1692Returns an lowercased version of EXPR. This is the internal function
54310121 1693implementing the \L escape in double-quoted strings.
a034a98d 1694Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1695
bbce6d69
PP
1696If EXPR is omitted, uses $_.
1697
a0d0e21e
LW
1698=item lcfirst EXPR
1699
54310121 1700=item lcfirst
bbce6d69 1701
a0d0e21e
LW
1702Returns the value of EXPR with the first character lowercased. This is
1703the internal function implementing the \l escape in double-quoted strings.
a034a98d 1704Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1705
bbce6d69
PP
1706If EXPR is omitted, uses $_.
1707
a0d0e21e
LW
1708=item length EXPR
1709
54310121 1710=item length
bbce6d69 1711
a0d0e21e
LW
1712Returns the length in characters of the value of EXPR. If EXPR is
1713omitted, returns length of $_.
1714
1715=item link OLDFILE,NEWFILE
1716
1717Creates a new filename linked to the old filename. Returns 1 for
1718success, 0 otherwise.
1719
1720=item listen SOCKET,QUEUESIZE
1721
1722Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 1723it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
1724
1725=item local EXPR
1726
a0d0e21e 1727A local modifies the listed variables to be local to the enclosing block,
5f05dabc
PP
1728subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1729list must be placed in parentheses. See L<perlsub/"Temporary Values via
cb1a09d0 1730local()"> for details.
a0d0e21e 1731
cb1a09d0
AD
1732But you really probably want to be using my() instead, because local() isn't
1733what most people think of as "local"). See L<perlsub/"Private Variables
1734via my()"> for details.
a0d0e21e
LW
1735
1736=item localtime EXPR
1737
1738Converts a time as returned by the time function to a 9-element array
5f05dabc 1739with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
1740follows:
1741
54310121 1742 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1743 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1744 localtime(time);
1745
1746All array elements are numeric, and come straight out of a struct tm.
1747In particular this means that $mon has the range 0..11 and $wday has
54310121
PP
1748the range 0..6 with sunday as day 0. Also, $year is the number of
1749years since 1900, that is, $year is 123 in year 2023.
1750
1751If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 1752
0a753a76 1753In a scalar context, returns the ctime(3) value:
a0d0e21e 1754
5f05dabc 1755 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 1756
54310121
PP
1757Also see the Time::Local module, and the strftime(3) and mktime(3)
1758function available via the POSIX module.
a0d0e21e
LW
1759
1760=item log EXPR
1761
54310121 1762=item log
bbce6d69 1763
a0d0e21e
LW
1764Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1765of $_.
1766
1767=item lstat FILEHANDLE
1768
1769=item lstat EXPR
1770
54310121 1771=item lstat
bbce6d69 1772
a0d0e21e
LW
1773Does the same thing as the stat() function, but stats a symbolic link
1774instead of the file the symbolic link points to. If symbolic links are
1775unimplemented on your system, a normal stat() is done.
1776
bbce6d69
PP
1777If EXPR is omitted, stats $_.
1778
a0d0e21e
LW
1779=item m//
1780
1781The match operator. See L<perlop>.
1782
1783=item map BLOCK LIST
1784
1785=item map EXPR,LIST
1786
1787Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1788element) and returns the list value composed of the results of each such
1789evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1790may produce zero, one, or more elements in the returned value.
1791
1792 @chars = map(chr, @nums);
1793
1794translates a list of numbers to the corresponding characters. And
1795
4633a7c4 1796 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
1797
1798is just a funny way to write
1799
1800 %hash = ();
1801 foreach $_ (@array) {
4633a7c4 1802 $hash{getkey($_)} = $_;
a0d0e21e
LW
1803 }
1804
1805=item mkdir FILENAME,MODE
1806
1807Creates the directory specified by FILENAME, with permissions specified
1808by MODE (as modified by umask). If it succeeds it returns 1, otherwise
184e9718 1809it returns 0 and sets C<$!> (errno).
a0d0e21e
LW
1810
1811=item msgctl ID,CMD,ARG
1812
4633a7c4 1813Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
a0d0e21e
LW
1814must be a variable which will hold the returned msqid_ds structure.
1815Returns like ioctl: the undefined value for error, "0 but true" for
1816zero, or the actual return value otherwise.
1817
1818=item msgget KEY,FLAGS
1819
4633a7c4 1820Calls the System V IPC function msgget(2). Returns the message queue id,
a0d0e21e
LW
1821or the undefined value if there is an error.
1822
1823=item msgsnd ID,MSG,FLAGS
1824
1825Calls the System V IPC function msgsnd to send the message MSG to the
1826message queue ID. MSG must begin with the long integer message type,
c07a80fd 1827which may be created with C<pack("l", $type)>. Returns TRUE if
a0d0e21e
LW
1828successful, or FALSE if there is an error.
1829
1830=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1831
1832Calls the System V IPC function msgrcv to receive a message from
1833message queue ID into variable VAR with a maximum message size of
1834SIZE. Note that if a message is received, the message type will be the
1835first thing in VAR, and the maximum length of VAR is SIZE plus the size
1836of the message type. Returns TRUE if successful, or FALSE if there is
1837an error.
1838
1839=item my EXPR
1840
1841A "my" declares the listed variables to be local (lexically) to the
cb1a09d0 1842enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
5f05dabc 1843more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 1844L<perlsub/"Private Variables via my()"> for details.
4633a7c4 1845
a0d0e21e
LW
1846=item next LABEL
1847
1848=item next
1849
1850The C<next> command is like the C<continue> statement in C; it starts
1851the next iteration of the loop:
1852
4633a7c4
LW
1853 LINE: while (<STDIN>) {
1854 next LINE if /^#/; # discard comments
a0d0e21e
LW
1855 ...
1856 }
1857
1858Note that if there were a C<continue> block on the above, it would get
1859executed even on discarded lines. If the LABEL is omitted, the command
1860refers to the innermost enclosing loop.
1861
1862=item no Module LIST
1863
1864See the "use" function, which "no" is the opposite of.
1865
1866=item oct EXPR
1867
54310121 1868=item oct
bbce6d69 1869
4633a7c4 1870Interprets EXPR as an octal string and returns the corresponding
2f9daede 1871value. (If EXPR happens to start off with 0x, interprets it as
4633a7c4
LW
1872a hex string instead.) The following will handle decimal, octal, and
1873hex in the standard Perl or C notation:
a0d0e21e
LW
1874
1875 $val = oct($val) if $val =~ /^0/;
1876
2f9daede
TPG
1877If EXPR is omitted, uses $_. This function is commonly used when
1878a string such as "644" needs to be converted into a file mode, for
1879example. (Although perl will automatically convert strings into
1880numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
1881
1882=item open FILEHANDLE,EXPR
1883
1884=item open FILEHANDLE
1885
1886Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
1887FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1888name of the real filehandle wanted. If EXPR is omitted, the scalar
1889variable of the same name as the FILEHANDLE contains the filename.
1890(Note that lexical variables--those declared with C<my>--will not work
1891for this purpose; so if you're using C<my>, specify EXPR in your call
1892to open.)
1893
1894If the filename begins with '<' or nothing, the file is opened for input.
1895If the filename begins with '>', the file is truncated and opened for
1896output. If the filename begins with '>>', the file is opened for
1897appending. You can put a '+' in front of the '>' or '<' to indicate that
1898you want both read and write access to the file; thus '+<' is almost
1899always preferred for read/write updates--the '+>' mode would clobber the
1900file first. The prefix and the filename may be separated with spaces.
1901These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
1902'w+', 'a', and 'a+'.
1903
1904If the filename begins with "|", the filename is interpreted as a command
1905to which output is to be piped, and if the filename ends with a "|", the
1906filename is interpreted See L<perlipc/"Using open() for IPC"> for more
1907examples of this. as command which pipes input to us. (You may not have
7e1af8bc
PP
1908a raw open() to a command that pipes both in I<and> out, but see
1909L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
1910for alternatives.)
cb1a09d0 1911
184e9718 1912Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
54310121 1913nonzero upon success, the undefined value otherwise. If the open
4633a7c4 1914involved a pipe, the return value happens to be the pid of the
54310121 1915subprocess.
cb1a09d0
AD
1916
1917If you're unfortunate enough to be running Perl on a system that
1918distinguishes between text files and binary files (modern operating
1919systems don't care), then you should check out L</binmode> for tips for
1920dealing with this. The key distinction between systems that need binmode
1921and those that don't is their text file formats. Systems like Unix and
1922Plan9 that delimit lines with a single character, and that encode that
1923character in C as '\n', do not need C<binmode>. The rest need it.
1924
cb1a09d0 1925Examples:
a0d0e21e
LW
1926
1927 $ARTICLE = 100;
1928 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1929 while (<ARTICLE>) {...
1930
1931 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1932
cb1a09d0
AD
1933 open(DBASE, '+<dbase.mine'); # open for update
1934
4633a7c4 1935 open(ARTICLE, "caesar <$article |"); # decrypt article
a0d0e21e 1936
4633a7c4 1937 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
a0d0e21e
LW
1938
1939 # process argument list of files along with any includes
1940
1941 foreach $file (@ARGV) {
1942 process($file, 'fh00');
1943 }
1944
1945 sub process {
1946 local($filename, $input) = @_;
1947 $input++; # this is a string increment
1948 unless (open($input, $filename)) {
1949 print STDERR "Can't open $filename: $!\n";
1950 return;
1951 }
1952
1953 while (<$input>) { # note use of indirection
1954 if (/^#include "(.*)"/) {
1955 process($1, $input);
1956 next;
1957 }
1958 ... # whatever
1959 }
1960 }
1961
1962You may also, in the Bourne shell tradition, specify an EXPR beginning
184e9718 1963with "E<gt>&", in which case the rest of the string is interpreted as the
a0d0e21e 1964name of a filehandle (or file descriptor, if numeric) which is to be
184e9718 1965duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
5f05dabc 1966+E<gt>E<gt>, and +E<lt>. The
a0d0e21e 1967mode you specify should match the mode of the original filehandle.
184e9718 1968(Duping a filehandle does not take into account any existing contents of
cb1a09d0 1969stdio buffers.)
a0d0e21e
LW
1970Here is a script that saves, redirects, and restores STDOUT and
1971STDERR:
1972
1973 #!/usr/bin/perl
1974 open(SAVEOUT, ">&STDOUT");
1975 open(SAVEERR, ">&STDERR");
1976
1977 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1978 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1979
1980 select(STDERR); $| = 1; # make unbuffered
1981 select(STDOUT); $| = 1; # make unbuffered
1982
1983 print STDOUT "stdout 1\n"; # this works for
1984 print STDERR "stderr 1\n"; # subprocesses too
1985
1986 close(STDOUT);
1987 close(STDERR);
1988
1989 open(STDOUT, ">&SAVEOUT");
1990 open(STDERR, ">&SAVEERR");
1991
1992 print STDOUT "stdout 2\n";
1993 print STDERR "stderr 2\n";
1994
1995
184e9718 1996If you specify "E<lt>&=N", where N is a number, then Perl will do an
4633a7c4
LW
1997equivalent of C's fdopen() of that file descriptor; this is more
1998parsimonious of file descriptors. For example:
a0d0e21e
LW
1999
2000 open(FILEHANDLE, "<&=$fd")
2001
5f05dabc 2002If you open a pipe on the command "-", i.e., either "|-" or "-|", then
a0d0e21e
LW
2003there is an implicit fork done, and the return value of open is the pid
2004of the child within the parent process, and 0 within the child
184e9718 2005process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2006The filehandle behaves normally for the parent, but i/o to that
2007filehandle is piped from/to the STDOUT/STDIN of the child process.
2008In the child process the filehandle isn't opened--i/o happens from/to
2009the new STDOUT or STDIN. Typically this is used like the normal
2010piped open when you want to exercise more control over just how the
2011pipe command gets executed, such as when you are running setuid, and
54310121 2012don't want to have to scan shell commands for metacharacters.
4633a7c4 2013The following pairs are more or less equivalent:
a0d0e21e
LW
2014
2015 open(FOO, "|tr '[a-z]' '[A-Z]'");
2016 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2017
2018 open(FOO, "cat -n '$file'|");
2019 open(FOO, "-|") || exec 'cat', '-n', $file;
2020
4633a7c4
LW
2021See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2022
a0d0e21e 2023Explicitly closing any piped filehandle causes the parent process to
184e9718 2024wait for the child to finish, and returns the status value in C<$?>.
a0d0e21e 2025Note: on any operation which may do a fork, unflushed buffers remain
184e9718 2026unflushed in both processes, which means you may need to set C<$|> to
a0d0e21e
LW
2027avoid duplicate output.
2028
5f05dabc
PP
2029Using the constructor from the IO::Handle package (or one of its
2030subclasses, such as IO::File or IO::Socket),
c07a80fd
PP
2031you can generate anonymous filehandles which have the scope of whatever
2032variables hold references to them, and automatically close whenever
2033and however you leave that scope:
2034
5f05dabc 2035 use IO::File;
c07a80fd
PP
2036 ...
2037 sub read_myfile_munged {
2038 my $ALL = shift;
5f05dabc 2039 my $handle = new IO::File;
c07a80fd
PP
2040 open($handle, "myfile") or die "myfile: $!";
2041 $first = <$handle>
2042 or return (); # Automatically closed here.
2043 mung $first or die "mung failed"; # Or here.
2044 return $first, <$handle> if $ALL; # Or here.
2045 $first; # Or here.
2046 }
2047
a0d0e21e 2048The filename that is passed to open will have leading and trailing
5f05dabc 2049whitespace deleted. To open a file with arbitrary weird
a0d0e21e
LW
2050characters in it, it's necessary to protect any leading and trailing
2051whitespace thusly:
2052
cb1a09d0
AD
2053 $file =~ s#^(\s)#./$1#;
2054 open(FOO, "< $file\0");
2055
c07a80fd
PP
2056If you want a "real" C open() (see L<open(2)> on your system), then
2057you should use the sysopen() function. This is another way to
2058protect your filenames from interpretation. For example:
cb1a09d0 2059
28757baa 2060 use IO::Handle;
c07a80fd
PP
2061 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
2062 or die "sysopen $path: $!";
2063 HANDLE->autoflush(1);
2064 HANDLE->print("stuff $$\n");
2065 seek(HANDLE, 0, 0);
2066 print "File contains: ", <HANDLE>;
cb1a09d0
AD
2067
2068See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
2069
2070=item opendir DIRHANDLE,EXPR
2071
2072Opens a directory named EXPR for processing by readdir(), telldir(),
5f05dabc 2073seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
a0d0e21e
LW
2074DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2075
2076=item ord EXPR
2077
54310121 2078=item ord
bbce6d69 2079
a0d0e21e 2080Returns the numeric ascii value of the first character of EXPR. If
dc848c6f 2081EXPR is omitted, uses $_. For the reverse, see L</chr>.
a0d0e21e
LW
2082
2083=item pack TEMPLATE,LIST
2084
2085Takes an array or list of values and packs it into a binary structure,
2086returning the string containing the structure. The TEMPLATE is a
2087sequence of characters that give the order and type of values, as
2088follows:
2089
2090 A An ascii string, will be space padded.
2091 a An ascii string, will be null padded.
2092 b A bit string (ascending bit order, like vec()).
2093 B A bit string (descending bit order).
2094 h A hex string (low nybble first).
2095 H A hex string (high nybble first).
2096
2097 c A signed char value.
2098 C An unsigned char value.
2099 s A signed short value.
2100 S An unsigned short value.
2101 i A signed integer value.
2102 I An unsigned integer value.
2103 l A signed long value.
2104 L An unsigned long value.
2105
2106 n A short in "network" order.
2107 N A long in "network" order.
2108 v A short in "VAX" (little-endian) order.
2109 V A long in "VAX" (little-endian) order.
2110
2111 f A single-precision float in the native format.
2112 d A double-precision float in the native format.
2113
2114 p A pointer to a null-terminated string.
2115 P A pointer to a structure (fixed-length string).
2116
2117 u A uuencoded string.
2118
2f9daede 2119 w A BER compressed integer. Bytes give an unsigned integer base
def98dd4
UP
2120 128, most significant digit first, with as few digits as
2121 possible, and with the bit 8 of each byte except the last set
2122 to "1."
2123
a0d0e21e
LW
2124 x A null byte.
2125 X Back up a byte.
2126 @ Null fill to absolute position.
2127
2128Each letter may optionally be followed by a number which gives a repeat
5f05dabc 2129count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
a0d0e21e
LW
2130pack function will gobble up that many values from the LIST. A * for the
2131repeat count means to use however many items are left. The "a" and "A"
2132types gobble just one value, but pack it as a string of length count,
2133padding with nulls or spaces as necessary. (When unpacking, "A" strips
2134trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2135fields pack a string that many bits long. The "h" and "H" fields pack a
2136string that many nybbles long. The "P" packs a pointer to a structure of
2137the size indicated by the length. Real numbers (floats and doubles) are
2138in the native machine format only; due to the multiplicity of floating
2139formats around, and the lack of a standard "network" representation, no
2140facility for interchange has been made. This means that packed floating
2141point data written on one machine may not be readable on another - even if
2142both use IEEE floating point arithmetic (as the endian-ness of the memory
2143representation is not part of the IEEE spec). Note that Perl uses doubles
2144internally for all numeric calculation, and converting from double into
5f05dabc 2145float and thence back to double again will lose precision (i.e.,
a0d0e21e
LW
2146C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2147
2148Examples:
2149
2150 $foo = pack("cccc",65,66,67,68);
2151 # foo eq "ABCD"
2152 $foo = pack("c4",65,66,67,68);
2153 # same thing
2154
2155 $foo = pack("ccxxcc",65,66,67,68);
2156 # foo eq "AB\0\0CD"
2157
2158 $foo = pack("s2",1,2);
2159 # "\1\0\2\0" on little-endian
2160 # "\0\1\0\2" on big-endian
2161
2162 $foo = pack("a4","abcd","x","y","z");
2163 # "abcd"
2164
2165 $foo = pack("aaaa","abcd","x","y","z");
2166 # "axyz"
2167
2168 $foo = pack("a14","abcdefg");
2169 # "abcdefg\0\0\0\0\0\0\0"
2170
2171 $foo = pack("i9pl", gmtime);
2172 # a real struct tm (on my system anyway)
2173
2174 sub bintodec {
2175 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2176 }
2177
2178The same template may generally also be used in the unpack function.
2179
cb1a09d0
AD
2180=item package NAMESPACE
2181
2182Declares the compilation unit as being in the given namespace. The scope
2183of the package declaration is from the declaration itself through the end of
2184the enclosing block (the same scope as the local() operator). All further
2185unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2186statement affects only dynamic variables--including those you've used
cb1a09d0
AD
2187local() on--but I<not> lexical variables created with my(). Typically it
2188would be the first declaration in a file to be included by the C<require>
2189or C<use> operator. You can switch into a package in more than one place;
5f05dabc 2190it influences merely which symbol table is used by the compiler for the
cb1a09d0
AD
2191rest of that block. You can refer to variables and filehandles in other
2192packages by prefixing the identifier with the package name and a double
2193colon: C<$Package::Variable>. If the package name is null, the C<main>
2194package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2195
2196See L<perlmod/"Packages"> for more information about packages, modules,
2197and classes. See L<perlsub> for other scoping issues.
2198
a0d0e21e
LW
2199=item pipe READHANDLE,WRITEHANDLE
2200
2201Opens a pair of connected pipes like the corresponding system call.
2202Note that if you set up a loop of piped processes, deadlock can occur
2203unless you are very careful. In addition, note that Perl's pipes use
184e9718 2204stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2205after each command, depending on the application.
2206
7e1af8bc 2207See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2208for examples of such things.
2209
a0d0e21e
LW
2210=item pop ARRAY
2211
54310121 2212=item pop
28757baa 2213
a0d0e21e
LW
2214Pops and returns the last value of the array, shortening the array by
22151. Has a similar effect to
2216
2217 $tmp = $ARRAY[$#ARRAY--];
2218
2219If there are no elements in the array, returns the undefined value.
cb1a09d0
AD
2220If ARRAY is omitted, pops the
2221@ARGV array in the main program, and the @_ array in subroutines, just
2222like shift().
a0d0e21e
LW
2223
2224=item pos SCALAR
2225
54310121 2226=item pos
bbce6d69 2227
4633a7c4 2228Returns the offset of where the last C<m//g> search left off for the variable
2f9daede 2229is in question ($_ is used when the variable is not specified). May be
44a8e56a
PP
2230modified to change that offset. Such modification will also influence
2231the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2232L<perlop>.
a0d0e21e
LW
2233
2234=item print FILEHANDLE LIST
2235
2236=item print LIST
2237
2238=item print
2239
cb1a09d0 2240Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2241if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2242the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2243level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2244token is a term, it may be misinterpreted as an operator unless you
5f05dabc 2245interpose a + or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2246omitted, prints by default to standard output (or to the last selected
da0045b7 2247output channel--see L</select>). If LIST is also omitted, prints $_ to
a0d0e21e
LW
2248STDOUT. To set the default output channel to something other than
2249STDOUT use the select operation. Note that, because print takes a
2250LIST, anything in the LIST is evaluated in a list context, and any
2251subroutine that you call will have one or more of its expressions
2252evaluated in a list context. Also be careful not to follow the print
2253keyword with a left parenthesis unless you want the corresponding right
2254parenthesis to terminate the arguments to the print--interpose a + or
5f05dabc 2255put parentheses around all the arguments.
a0d0e21e 2256
4633a7c4 2257Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2258you will have to use a block returning its value instead:
4633a7c4
LW
2259
2260 print { $files[$i] } "stuff\n";
2261 print { $OK ? STDOUT : STDERR } "stuff\n";
2262
5f05dabc 2263=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2264
5f05dabc 2265=item printf FORMAT, LIST
a0d0e21e 2266
a034a98d
DD
2267Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
2268of the list will be interpreted as the printf format. If C<use locale> is
2269in effect, the character used for the decimal point in formatted real numbers
2270is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2271
28757baa
PP
2272Don't fall into the trap of using a printf() when a simple
2273print() would do. The print() is more efficient, and less
2274error prone.
2275
da0045b7
PP
2276=item prototype FUNCTION
2277
2278Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
2279function has no prototype). FUNCTION is a reference to, or the name of,
2280the function whose prototype you want to retrieve.
da0045b7 2281
a0d0e21e
LW
2282=item push ARRAY,LIST
2283
2284Treats ARRAY as a stack, and pushes the values of LIST
2285onto the end of ARRAY. The length of ARRAY increases by the length of
2286LIST. Has the same effect as
2287
2288 for $value (LIST) {
2289 $ARRAY[++$#ARRAY] = $value;
2290 }
2291
2292but is more efficient. Returns the new number of elements in the array.
2293
2294=item q/STRING/
2295
2296=item qq/STRING/
2297
2298=item qx/STRING/
2299
2300=item qw/STRING/
2301
2302Generalized quotes. See L<perlop>.
2303
2304=item quotemeta EXPR
2305
54310121 2306=item quotemeta
bbce6d69 2307
68dc0745 2308Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2309characters backslashed. (That is, all characters not matching
2310C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2311returned string, regardless of any locale settings.)
2312This is the internal function implementing
a0d0e21e
LW
2313the \Q escape in double-quoted strings.
2314
bbce6d69
PP
2315If EXPR is omitted, uses $_.
2316
a0d0e21e
LW
2317=item rand EXPR
2318
2319=item rand
2320
2321Returns a random fractional number between 0 and the value of EXPR.
54310121 2322(EXPR should be positive.) If EXPR is omitted, returns a value between
93dc8474
CS
23230 and 1. Automatically calls srand() unless srand() has already been
2324called. See also srand().
a0d0e21e 2325
2f9daede 2326(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2327large or too small, then your version of Perl was probably compiled
2f9daede 2328with the wrong number of RANDBITS.)
a0d0e21e
LW
2329
2330=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2331
2332=item read FILEHANDLE,SCALAR,LENGTH
2333
2334Attempts to read LENGTH bytes of data into variable SCALAR from the
2335specified FILEHANDLE. Returns the number of bytes actually read, or
2336undef if there was an error. SCALAR will be grown or shrunk to the
2337length actually read. An OFFSET may be specified to place the read
2338data at some other place than the beginning of the string. This call
2339is actually implemented in terms of stdio's fread call. To get a true
2340read system call, see sysread().
2341
2342=item readdir DIRHANDLE
2343
2344Returns the next directory entry for a directory opened by opendir().
2345If used in a list context, returns all the rest of the entries in the
2346directory. If there are no more entries, returns an undefined value in
2347a scalar context or a null list in a list context.
2348
cb1a09d0 2349If you're planning to filetest the return values out of a readdir(), you'd
5f05dabc 2350better prepend the directory in question. Otherwise, because we didn't
cb1a09d0
AD
2351chdir() there, it would have been testing the wrong file.
2352
2353 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2354 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2355 closedir DIR;
2356
a0d0e21e
LW
2357=item readlink EXPR
2358
54310121 2359=item readlink
bbce6d69 2360
a0d0e21e
LW
2361Returns the value of a symbolic link, if symbolic links are
2362implemented. If not, gives a fatal error. If there is some system
184e9718 2363error, returns the undefined value and sets C<$!> (errno). If EXPR is
a0d0e21e
LW
2364omitted, uses $_.
2365
2366=item recv SOCKET,SCALAR,LEN,FLAGS
2367
2368Receives a message on a socket. Attempts to receive LENGTH bytes of
2369data into variable SCALAR from the specified SOCKET filehandle.
2370Actually does a C recvfrom(), so that it can returns the address of the
2371sender. Returns the undefined value if there's an error. SCALAR will
2372be grown or shrunk to the length actually read. Takes the same flags
54310121 2373as the system call of the same name.
4633a7c4 2374See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2375
2376=item redo LABEL
2377
2378=item redo
2379
2380The C<redo> command restarts the loop block without evaluating the
2381conditional again. The C<continue> block, if any, is not executed. If
2382the LABEL is omitted, the command refers to the innermost enclosing
2383loop. This command is normally used by programs that want to lie to
2384themselves about what was just input:
2385
2386 # a simpleminded Pascal comment stripper
2387 # (warning: assumes no { or } in strings)
4633a7c4 2388 LINE: while (<STDIN>) {
a0d0e21e
LW
2389 while (s|({.*}.*){.*}|$1 |) {}
2390 s|{.*}| |;
2391 if (s|{.*| |) {
2392 $front = $_;
2393 while (<STDIN>) {
2394 if (/}/) { # end of comment?
2395 s|^|$front{|;
4633a7c4 2396 redo LINE;
a0d0e21e
LW
2397 }
2398 }
2399 }
2400 print;
2401 }
2402
2403=item ref EXPR
2404
54310121 2405=item ref
bbce6d69 2406
2f9daede
TPG
2407Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2408is not specified, $_ will be used. The value returned depends on the
bbce6d69 2409type of thing the reference is a reference to.
a0d0e21e
LW
2410Builtin types include:
2411
2412 REF
2413 SCALAR
2414 ARRAY
2415 HASH
2416 CODE
2417 GLOB
2418
54310121 2419If the referenced object has been blessed into a package, then that package
a0d0e21e
LW
2420name is returned instead. You can think of ref() as a typeof() operator.
2421
2422 if (ref($r) eq "HASH") {
aa689395 2423 print "r is a reference to a hash.\n";
54310121 2424 }
a0d0e21e
LW
2425 if (!ref ($r) {
2426 print "r is not a reference at all.\n";
54310121 2427 }
a0d0e21e
LW
2428
2429See also L<perlref>.
2430
2431=item rename OLDNAME,NEWNAME
2432
2433Changes the name of a file. Returns 1 for success, 0 otherwise. Will
5f05dabc 2434not work across file system boundaries.
a0d0e21e
LW
2435
2436=item require EXPR
2437
2438=item require
2439
2440Demands some semantics specified by EXPR, or by $_ if EXPR is not
2441supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2442(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2443
2444Otherwise, demands that a library file be included if it hasn't already
2445been included. The file is included via the do-FILE mechanism, which is
2446essentially just a variety of eval(). Has semantics similar to the following
2447subroutine:
2448
2449 sub require {
2450 local($filename) = @_;
2451 return 1 if $INC{$filename};
2452 local($realfilename,$result);
2453 ITER: {
2454 foreach $prefix (@INC) {
2455 $realfilename = "$prefix/$filename";
2456 if (-f $realfilename) {
2457 $result = do $realfilename;
2458 last ITER;
2459 }
2460 }
2461 die "Can't find $filename in \@INC";
2462 }
2463 die $@ if $@;
2464 die "$filename did not return true value" unless $result;
2465 $INC{$filename} = $realfilename;
2466 $result;
2467 }
2468
2469Note that the file will not be included twice under the same specified
2470name. The file must return TRUE as the last statement to indicate
2471successful execution of any initialization code, so it's customary to
2472end such a file with "1;" unless you're sure it'll return TRUE
2473otherwise. But it's better just to put the "C<1;>", in case you add more
2474statements.
2475
54310121 2476If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 2477replaces "F<::>" with "F</>" in the filename for you,
54310121 2478to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
2479modules does not risk altering your namespace.
2480
54310121 2481For a yet-more-powerful import facility, see L</use> and
748a9306 2482L<perlmod>.
a0d0e21e
LW
2483
2484=item reset EXPR
2485
2486=item reset
2487
2488Generally used in a C<continue> block at the end of a loop to clear
2489variables and reset ?? searches so that they work again. The
2490expression is interpreted as a list of single characters (hyphens
2491allowed for ranges). All variables and arrays beginning with one of
2492those letters are reset to their pristine state. If the expression is
5f05dabc
PP
2493omitted, one-match searches (?pattern?) are reset to match again. Resets
2494only variables or searches in the current package. Always returns
a0d0e21e
LW
24951. Examples:
2496
2497 reset 'X'; # reset all X variables
2498 reset 'a-z'; # reset lower case variables
2499 reset; # just reset ?? searches
2500
5f05dabc
PP
2501Resetting "A-Z" is not recommended because you'll wipe out your
2502ARGV and ENV arrays. Resets only package variables--lexical variables
a0d0e21e 2503are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2504so you'll probably want to use them instead. See L</my>.
a0d0e21e 2505
54310121
PP
2506=item return EXPR
2507
2508=item return
2509
2510Returns from a subroutine, eval(), or do FILE with the value of the
2511given EXPR. Evaluation of EXPR may be in a list, scalar, or void
2512context, depending on how the return value will be used, and the context
2513may vary from one execution to the next (see wantarray()). If no EXPR
2514is given, returns an empty list in a list context, an undefined value in
2515a scalar context, or nothing in a void context.
a0d0e21e 2516
68dc0745
PP
2517(Note that in the absence of a return, a subroutine, eval, or do FILE
2518will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
2519
2520=item reverse LIST
2521
2522In a list context, returns a list value consisting of the elements
2f9daede
TPG
2523of LIST in the opposite order. In a scalar context, concatenates the
2524elements of LIST, and returns a string value consisting of those bytes,
2525but in the opposite order.
4633a7c4 2526
2f9daede 2527 print reverse <>; # line tac, last line first
4633a7c4 2528
2f9daede
TPG
2529 undef $/; # for efficiency of <>
2530 print scalar reverse <>; # byte tac, last line tsrif
2531
2532This operator is also handy for inverting a hash, although there are some
2533caveats. If a value is duplicated in the original hash, only one of those
2534can be represented as a key in the inverted hash. Also, this has to
2535unwind one hash and build a whole new one, which may take some time
2536on a large hash.
2537
2538 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
2539
2540=item rewinddir DIRHANDLE
2541
2542Sets the current position to the beginning of the directory for the
2543readdir() routine on DIRHANDLE.
2544
2545=item rindex STR,SUBSTR,POSITION
2546
2547=item rindex STR,SUBSTR
2548
2549Works just like index except that it returns the position of the LAST
2550occurrence of SUBSTR in STR. If POSITION is specified, returns the
2551last occurrence at or before that position.
2552
2553=item rmdir FILENAME
2554
54310121 2555=item rmdir
bbce6d69 2556
a0d0e21e 2557Deletes the directory specified by FILENAME if it is empty. If it
184e9718 2558succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
a0d0e21e
LW
2559FILENAME is omitted, uses $_.
2560
2561=item s///
2562
2563The substitution operator. See L<perlop>.
2564
2565=item scalar EXPR
2566
2567Forces EXPR to be interpreted in a scalar context and returns the value
54310121 2568of EXPR.
cb1a09d0
AD
2569
2570 @counts = ( scalar @a, scalar @b, scalar @c );
2571
54310121 2572There is no equivalent operator to force an expression to
cb1a09d0
AD
2573be interpolated in a list context because it's in practice never
2574needed. If you really wanted to do so, however, you could use
2575the construction C<@{[ (some expression) ]}>, but usually a simple
2576C<(some expression)> suffices.
a0d0e21e
LW
2577
2578=item seek FILEHANDLE,POSITION,WHENCE
2579
2580Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2581call of stdio. FILEHANDLE may be an expression whose value gives the name
2582of the filehandle. The values for WHENCE are 0 to set the file pointer to
2583POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2584plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
137443ea
PP
2585this from either the IO::Seekable or the POSIX module. Returns 1 upon
2586success, 0 otherwise.
2587
2588If you want to position a file pointer for sysread() or syswrite(), don't
2589use seek() -- buffering makes its effect on the system file pointer
2590unpredictable and non-portable. Use sysseek() instead.
a0d0e21e 2591
cb1a09d0
AD
2592On some systems you have to do a seek whenever you switch between reading
2593and writing. Amongst other things, this may have the effect of calling
2594stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2595the file pointer:
2596
2597 seek(TEST,0,1);
2598
2599This is also useful for applications emulating C<tail -f>. Once you hit
2600EOF on your read, and then sleep for a while, you might have to stick in a
2601seek() to reset things. First the simple trick listed above to clear the
2602filepointer. The seek() doesn't change the current position, but it
2603I<does> clear the end-of-file condition on the handle, so that the next
5f05dabc 2604C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
2605
2606If that doesn't work (some stdios are particularly cantankerous), then
2607you may need something more like this:
2608
2609 for (;;) {
2610 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2611 # search for some stuff and put it into files
2612 }
2613 sleep($for_a_while);
2614 seek(FILE, $curpos, 0);
2615 }
2616
a0d0e21e
LW
2617=item seekdir DIRHANDLE,POS
2618
2619Sets the current position for the readdir() routine on DIRHANDLE. POS
2620must be a value returned by telldir(). Has the same caveats about
2621possible directory compaction as the corresponding system library
2622routine.
2623
2624=item select FILEHANDLE
2625
2626=item select
2627
2628Returns the currently selected filehandle. Sets the current default
2629filehandle for output, if FILEHANDLE is supplied. This has two
2630effects: first, a C<write> or a C<print> without a filehandle will
2631default to this FILEHANDLE. Second, references to variables related to
2632output will refer to this output channel. For example, if you have to
2633set the top of form format for more than one output channel, you might
2634do the following:
2635
2636 select(REPORT1);
2637 $^ = 'report1_top';
2638 select(REPORT2);
2639 $^ = 'report2_top';
2640
2641FILEHANDLE may be an expression whose value gives the name of the
2642actual filehandle. Thus:
2643
2644 $oldfh = select(STDERR); $| = 1; select($oldfh);
2645
4633a7c4
LW
2646Some programmers may prefer to think of filehandles as objects with
2647methods, preferring to write the last example as:
a0d0e21e 2648
28757baa 2649 use IO::Handle;
a0d0e21e
LW
2650 STDERR->autoflush(1);
2651
2652=item select RBITS,WBITS,EBITS,TIMEOUT
2653
5f05dabc 2654This calls the select(2) system call with the bit masks specified, which
a0d0e21e
LW
2655can be constructed using fileno() and vec(), along these lines:
2656
2657 $rin = $win = $ein = '';
2658 vec($rin,fileno(STDIN),1) = 1;
2659 vec($win,fileno(STDOUT),1) = 1;
2660 $ein = $rin | $win;
2661
2662If you want to select on many filehandles you might wish to write a
2663subroutine:
2664
2665 sub fhbits {
2666 local(@fhlist) = split(' ',$_[0]);
2667 local($bits);
2668 for (@fhlist) {
2669 vec($bits,fileno($_),1) = 1;
2670 }
2671 $bits;
2672 }
4633a7c4 2673 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
2674
2675The usual idiom is:
2676
2677 ($nfound,$timeleft) =
2678 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2679
54310121 2680or to block until something becomes ready just do this
a0d0e21e
LW
2681
2682 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2683
5f05dabc 2684Most systems do not bother to return anything useful in $timeleft, so
c07a80fd
PP
2685calling select() in a scalar context just returns $nfound.
2686
5f05dabc 2687Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e
LW
2688in seconds, which may be fractional. Note: not all implementations are
2689capable of returning the $timeleft. If not, they always return
2690$timeleft equal to the supplied $timeout.
2691
ff68c719 2692You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
2693
2694 select(undef, undef, undef, 0.25);
2695
184e9718 2696B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
cb1a09d0 2697with select(). You have to use sysread() instead.
a0d0e21e
LW
2698
2699=item semctl ID,SEMNUM,CMD,ARG
2700
2701Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2702&GETALL, then ARG must be a variable which will hold the returned
2703semid_ds structure or semaphore value array. Returns like ioctl: the
2704undefined value for error, "0 but true" for zero, or the actual return
2705value otherwise.
2706
2707=item semget KEY,NSEMS,FLAGS
2708
2709Calls the System V IPC function semget. Returns the semaphore id, or
2710the undefined value if there is an error.
2711
2712=item semop KEY,OPSTRING
2713
2714Calls the System V IPC function semop to perform semaphore operations
2715such as signaling and waiting. OPSTRING must be a packed array of
2716semop structures. Each semop structure can be generated with
2717C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2718operations is implied by the length of OPSTRING. Returns TRUE if
2719successful, or FALSE if there is an error. As an example, the
2720following code waits on semaphore $semnum of semaphore id $semid:
2721
2722 $semop = pack("sss", $semnum, -1, 0);
2723 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2724
2725To signal the semaphore, replace "-1" with "1".
2726
2727=item send SOCKET,MSG,FLAGS,TO
2728
2729=item send SOCKET,MSG,FLAGS
2730
2731Sends a message on a socket. Takes the same flags as the system call
2732of the same name. On unconnected sockets you must specify a
2733destination to send TO, in which case it does a C sendto(). Returns
2734the number of characters sent, or the undefined value if there is an
2735error.
4633a7c4 2736See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2737
2738=item setpgrp PID,PGRP
2739
2740Sets the current process group for the specified PID, 0 for the current
2741process. Will produce a fatal error if used on a machine that doesn't
5f05dabc 2742implement setpgrp(2). If the arguments are omitted, it defaults to
47e29363
PP
27430,0. Note that the POSIX version of setpgrp() does not accept any
2744arguments, so only setpgrp 0,0 is portable.
a0d0e21e
LW
2745
2746=item setpriority WHICH,WHO,PRIORITY
2747
2748Sets the current priority for a process, a process group, or a user.
748a9306 2749(See setpriority(2).) Will produce a fatal error if used on a machine
a0d0e21e
LW
2750that doesn't implement setpriority(2).
2751
2752=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2753
2754Sets the socket option requested. Returns undefined if there is an
2755error. OPTVAL may be specified as undef if you don't want to pass an
2756argument.
2757
2758=item shift ARRAY
2759
2760=item shift
2761
2762Shifts the first value of the array off and returns it, shortening the
2763array by 1 and moving everything down. If there are no elements in the
2764array, returns the undefined value. If ARRAY is omitted, shifts the
2765@ARGV array in the main program, and the @_ array in subroutines.
2766(This is determined lexically.) See also unshift(), push(), and pop().
2767Shift() and unshift() do the same thing to the left end of an array
2f9daede 2768that pop() and push() do to the right end.
a0d0e21e
LW
2769
2770=item shmctl ID,CMD,ARG
2771
2772Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2773must be a variable which will hold the returned shmid_ds structure.
2774Returns like ioctl: the undefined value for error, "0 but true" for
2775zero, or the actual return value otherwise.
2776
2777=item shmget KEY,SIZE,FLAGS
2778
2779Calls the System V IPC function shmget. Returns the shared memory
2780segment id, or the undefined value if there is an error.
2781
2782=item shmread ID,VAR,POS,SIZE
2783
2784=item shmwrite ID,STRING,POS,SIZE
2785
2786Reads or writes the System V shared memory segment ID starting at
2787position POS for size SIZE by attaching to it, copying in/out, and
2788detaching from it. When reading, VAR must be a variable which will
2789hold the data read. When writing, if STRING is too long, only SIZE
2790bytes are used; if STRING is too short, nulls are written to fill out
2791SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2792
2793=item shutdown SOCKET,HOW
2794
2795Shuts down a socket connection in the manner indicated by HOW, which
2796has the same interpretation as in the system call of the same name.
2797
2798=item sin EXPR
2799
54310121 2800=item sin
bbce6d69 2801
a0d0e21e
LW
2802Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2803returns sine of $_.
2804
54310121 2805For the inverse sine operation, you may use the POSIX::asin()
28757baa
PP
2806function, or use this relation:
2807
2808 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
2809
a0d0e21e
LW
2810=item sleep EXPR
2811
2812=item sleep
2813
2814Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2815May be interrupted by sending the process a SIGALRM. Returns the
2816number of seconds actually slept. You probably cannot mix alarm() and
5f05dabc 2817sleep() calls, because sleep() is often implemented using alarm().
a0d0e21e
LW
2818
2819On some older systems, it may sleep up to a full second less than what
2820you requested, depending on how it counts seconds. Most modern systems
2821always sleep the full amount.
2822
cb1a09d0 2823For delays of finer granularity than one second, you may use Perl's
54310121
PP
2824syscall() interface to access setitimer(2) if your system supports it,
2825or else see L</select()> below.
cb1a09d0 2826
5f05dabc
PP
2827See also the POSIX module's sigpause() function.
2828
a0d0e21e
LW
2829=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2830
2831Opens a socket of the specified kind and attaches it to filehandle
5f05dabc 2832SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
a0d0e21e 2833system call of the same name. You should "use Socket;" first to get
4633a7c4 2834the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2835
2836=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2837
2838Creates an unnamed pair of sockets in the specified domain, of the
5f05dabc 2839specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
a0d0e21e
LW
2840for the system call of the same name. If unimplemented, yields a fatal
2841error. Returns TRUE if successful.
2842
2843=item sort SUBNAME LIST
2844
2845=item sort BLOCK LIST
2846
2847=item sort LIST
2848
2f9daede
TPG
2849Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
2850is omitted, sorts in standard string comparison order. If SUBNAME is
2851specified, it gives the name of a subroutine that returns an integer
2852less than, equal to, or greater than 0, depending on how the elements
2853of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
2854operators are extremely useful in such routines.) SUBNAME may be a
2855scalar variable name, in which case the value provides the name of the
2856subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
2857an anonymous, in-line sort subroutine.
a0d0e21e 2858
cb1a09d0
AD
2859In the interests of efficiency the normal calling code for subroutines is
2860bypassed, with the following effects: the subroutine may not be a
2861recursive subroutine, and the two elements to be compared are passed into
2862the subroutine not via @_ but as the package global variables $a and
2863$b (see example below). They are passed by reference, so don't
2864modify $a and $b. And don't try to declare them as lexicals either.
a0d0e21e 2865
0a753a76
PP
2866You also cannot exit out of the sort block or subroutine using any of the
2867loop control operators described in L<perlsyn> or with goto().
2868
a034a98d
DD
2869When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
2870current collation locale. See L<perllocale>.
2871
a0d0e21e
LW
2872Examples:
2873
2874 # sort lexically
2875 @articles = sort @files;
2876
2877 # same thing, but with explicit sort routine
2878 @articles = sort {$a cmp $b} @files;
2879
cb1a09d0 2880 # now case-insensitively
54310121 2881 @articles = sort {uc($a) cmp uc($b)} @files;
cb1a09d0 2882
a0d0e21e
LW
2883 # same thing in reversed order
2884 @articles = sort {$b cmp $a} @files;
2885
2886 # sort numerically ascending
2887 @articles = sort {$a <=> $b} @files;
2888
2889 # sort numerically descending
2890 @articles = sort {$b <=> $a} @files;
2891
2892 # sort using explicit subroutine name
2893 sub byage {
2f9daede 2894 $age{$a} <=> $age{$b}; # presuming numeric
a0d0e21e
LW
2895 }
2896 @sortedclass = sort byage @class;
2897
aa689395
PP
2898 # this sorts the %age hash by value instead of key
2899 # using an in-line function
c07a80fd
PP
2900 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2901
a0d0e21e
LW
2902 sub backwards { $b cmp $a; }
2903 @harry = ('dog','cat','x','Cain','Abel');
2904 @george = ('gone','chased','yz','Punished','Axed');
2905 print sort @harry;
2906 # prints AbelCaincatdogx
2907 print sort backwards @harry;
2908 # prints xdogcatCainAbel
2909 print sort @george, 'to', @harry;
2910 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2911
54310121
PP
2912 # inefficiently sort by descending numeric compare using
2913 # the first integer after the first = sign, or the
cb1a09d0
AD
2914 # whole record case-insensitively otherwise
2915
2916 @new = sort {
2917 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2918 ||
2919 uc($a) cmp uc($b)
2920 } @old;
2921
2922 # same thing, but much more efficiently;
2923 # we'll build auxiliary indices instead
2924 # for speed
2925 @nums = @caps = ();
54310121 2926 for (@old) {
cb1a09d0
AD
2927 push @nums, /=(\d+)/;
2928 push @caps, uc($_);
54310121 2929 }
cb1a09d0
AD
2930
2931 @new = @old[ sort {
2932 $nums[$b] <=> $nums[$a]
2933 ||
2934 $caps[$a] cmp $caps[$b]
2935 } 0..$#old
2936 ];
2937
2938 # same thing using a Schwartzian Transform (no temps)
2939 @new = map { $_->[0] }
2940 sort { $b->[1] <=> $a->[1]
2941 ||
2942 $a->[2] cmp $b->[2]
2943 } map { [$_, /=(\d+)/, uc($_)] } @old;
2944
184e9718 2945If you're using strict, you I<MUST NOT> declare $a
cb1a09d0
AD
2946and $b as lexicals. They are package globals. That means
2947if you're in the C<main> package, it's
2948
2949 @articles = sort {$main::b <=> $main::a} @files;
2950
2951or just
2952
2953 @articles = sort {$::b <=> $::a} @files;
2954
2955but if you're in the C<FooPack> package, it's
2956
2957 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2958
55497cff
PP
2959The comparison function is required to behave. If it returns
2960inconsistent results (sometimes saying $x[1] is less than $x[2] and
2961sometimes saying the opposite, for example) the Perl interpreter will
2962probably crash and dump core. This is entirely due to and dependent
2963upon your system's qsort(3) library routine; this routine often avoids
2964sanity checks in the interest of speed.
2965
a0d0e21e
LW
2966=item splice ARRAY,OFFSET,LENGTH,LIST
2967
2968=item splice ARRAY,OFFSET,LENGTH
2969
2970=item splice ARRAY,OFFSET
2971
2972Removes the elements designated by OFFSET and LENGTH from an array, and
2973replaces them with the elements of LIST, if any. Returns the elements
2974removed from the array. The array grows or shrinks as necessary. If
2975LENGTH is omitted, removes everything from OFFSET onward. The
5f05dabc 2976following equivalences hold (assuming C<$[ == 0>):
a0d0e21e
LW
2977
2978 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2979 pop(@a) splice(@a,-1)
2980 shift(@a) splice(@a,0,1)
2981 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2982 $a[$x] = $y splice(@a,$x,1,$y);
2983
2984Example, assuming array lengths are passed before arrays:
2985
2986 sub aeq { # compare two list values
2987 local(@a) = splice(@_,0,shift);
2988 local(@b) = splice(@_,0,shift);
2989 return 0 unless @a == @b; # same len?
2990 while (@a) {
2991 return 0 if pop(@a) ne pop(@b);
2992 }
2993 return 1;
2994 }
2995 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2996
2997=item split /PATTERN/,EXPR,LIMIT
2998
2999=item split /PATTERN/,EXPR
3000
3001=item split /PATTERN/
3002
3003=item split
3004
3005Splits a string into an array of strings, and returns it.
3006
3007If not in a list context, returns the number of fields found and splits into
3008the @_ array. (In a list context, you can force the split into @_ by
3009using C<??> as the pattern delimiters, but it still returns the array
3010value.) The use of implicit split to @_ is deprecated, however.
3011
3012If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
4633a7c4
LW
3013splits on whitespace (after skipping any leading whitespace). Anything
3014matching PATTERN is taken to be a delimiter separating the fields. (Note
3015that the delimiter may be longer than one character.) If LIMIT is
3016specified and is not negative, splits into no more than that many fields
3017(though it may split into fewer). If LIMIT is unspecified, trailing null
3018fields are stripped (which potential users of pop() would do well to
3019remember). If LIMIT is negative, it is treated as if an arbitrarily large
3020LIMIT had been specified.
a0d0e21e
LW
3021
3022A pattern matching the null string (not to be confused with
748a9306 3023a null pattern C<//>, which is just one member of the set of patterns
a0d0e21e
LW
3024matching a null string) will split the value of EXPR into separate
3025characters at each point it matches that way. For example:
3026
3027 print join(':', split(/ */, 'hi there'));
3028
3029produces the output 'h:i:t:h:e:r:e'.
3030
5f05dabc 3031The LIMIT parameter can be used to split a line partially
a0d0e21e
LW
3032
3033 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3034
3035When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3036one larger than the number of variables in the list, to avoid
3037unnecessary work. For the list above LIMIT would have been 4 by
3038default. In time critical applications it behooves you not to split
3039into more fields than you really need.
3040
3041If the PATTERN contains parentheses, additional array elements are
3042created from each matching substring in the delimiter.
3043
da0045b7 3044 split(/([,-])/, "1-10,20", 3);
a0d0e21e
LW
3045
3046produces the list value
3047
3048 (1, '-', 10, ',', 20)
3049
54310121 3050If you had the entire header of a normal Unix email message in $header,
4633a7c4
LW
3051you could split it up into fields and their values this way:
3052
3053 $header =~ s/\n\s+/ /g; # fix continuation lines
3054 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
3055
a0d0e21e
LW
3056The pattern C</PATTERN/> may be replaced with an expression to specify
3057patterns that vary at runtime. (To do runtime compilation only once,
748a9306
LW
3058use C</$variable/o>.)
3059
3060As a special case, specifying a PATTERN of space (C<' '>) will split on
3061white space just as split with no arguments does. Thus, split(' ') can
3062be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3063will give you as many null initial fields as there are leading spaces.
3064A split on /\s+/ is like a split(' ') except that any leading
3065whitespace produces a null first field. A split with no arguments
3066really does a C<split(' ', $_)> internally.
a0d0e21e
LW
3067
3068Example:
3069
3070 open(passwd, '/etc/passwd');
3071 while (<passwd>) {
54310121 3072 ($login, $passwd, $uid, $gid, $gcos,
748a9306 3073 $home, $shell) = split(/:/);
a0d0e21e
LW
3074 ...
3075 }
3076
54310121 3077(Note that $shell above will still have a newline on it. See L</chop>,
a0d0e21e
LW
3078L</chomp>, and L</join>.)
3079
5f05dabc 3080=item sprintf FORMAT, LIST
a0d0e21e
LW
3081
3082Returns a string formatted by the usual printf conventions of the C
cb1a09d0
AD
3083language. See L<sprintf(3)> or L<printf(3)> on your system for details.
3084(The * character for an indirectly specified length is not
a0d0e21e 3085supported, but you can get the same effect by interpolating a variable
a034a98d
DD
3086into the pattern.) If C<use locale> is
3087in effect, the character used for the decimal point in formatted real numbers
3088is affected by the LC_NUMERIC locale. See L<perllocale>.
3089Some C libraries' implementations of sprintf() can
cb1a09d0 3090dump core when fed ludicrous arguments.
a0d0e21e
LW
3091
3092=item sqrt EXPR
3093
54310121 3094=item sqrt
bbce6d69 3095
a0d0e21e
LW
3096Return the square root of EXPR. If EXPR is omitted, returns square
3097root of $_.
3098
3099=item srand EXPR
3100
93dc8474
CS
3101=item srand
3102
3103Sets the random number seed for the C<rand> operator. If EXPR is
3104omitted, uses a semi-random value based on the current time and process
3105ID, among other things. In versions of Perl prior to 5.004 the default
3106seed was just the current time(). This isn't a particularly good seed,
3107so many old programs supply their own seed value (often C<time ^ $$> or
3108C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
3109
3110In fact, it's usually not necessary to call srand() at all, because if
3111it is not called explicitly, it is called implicitly at the first use of
2f9daede
TPG
3112the C<rand> operator. However, this was not the case in version of Perl
3113before 5.004, so if your script will run under older Perl versions, it
3114should call srand().
93dc8474 3115
2f9daede
TPG
3116Note that you need something much more random than the default seed for
3117cryptographic purposes. Checksumming the compressed output of one or more
3118rapidly changing operating system status programs is the usual method. For
3119example:
28757baa
PP
3120
3121 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3122
0078ec44
RS
3123If you're particularly concerned with this, see the Math::TrulyRandom
3124module in CPAN.
3125
3126Do I<not> call srand() multiple times in your program unless you know
28757baa
PP
3127exactly what you're doing and why you're doing it. The point of the
3128function is to "seed" the rand() function so that rand() can produce
3129a different sequence each time you run your program. Just do it once at the
3130top of your program, or you I<won't> get random numbers out of rand()!
3131
54310121 3132Frequently called programs (like CGI scripts) that simply use
28757baa
PP
3133
3134 time ^ $$
3135
54310121 3136for a seed can fall prey to the mathematical property that
28757baa
PP
3137
3138 a^b == (a+1)^(b+1)
3139
0078ec44 3140one-third of the time. So don't do that.
f86702cc 3141
a0d0e21e
LW
3142=item stat FILEHANDLE
3143
3144=item stat EXPR
3145
54310121 3146=item stat
bbce6d69 3147
a0d0e21e 3148Returns a 13-element array giving the status info for a file, either the
2f9daede 3149file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
bbce6d69
PP
3150stats $_. Returns a null list if the stat fails. Typically used as
3151follows:
3152
a0d0e21e
LW
3153
3154 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3155 $atime,$mtime,$ctime,$blksize,$blocks)
3156 = stat($filename);
3157
54310121 3158Not all fields are supported on all filesystem types. Here are the
c07a80fd
PP
3159meaning of the fields:
3160
54310121
PP
3161 0 dev device number of filesystem
3162 1 ino inode number
3163 2 mode file mode (type and permissions)
3164 3 nlink number of (hard) links to the file
3165 4 uid numeric user ID of file's owner
3166 5 gid numeric group ID of file's owner
3167 6 rdev the device identifier (special files only)
3168 7 size total size of file, in bytes
3169 8 atime last access time since the epoch
3170 9 mtime last modify time since the epoch
3171 10 ctime inode change time (NOT creation time!) since the epoch
3172 11 blksize preferred block size for file system I/O
3173 12 blocks actual number of blocks allocated
c07a80fd
PP
3174
3175(The epoch was at 00:00 January 1, 1970 GMT.)
3176
a0d0e21e
LW
3177If stat is passed the special filehandle consisting of an underline, no
3178stat is done, but the current contents of the stat structure from the
3179last stat or filetest are returned. Example:
3180
3181 if (-x $file && (($d) = stat(_)) && $d < 0) {
3182 print "$file is executable NFS file\n";
3183 }
3184
5f05dabc 3185(This works on machines only for which the device number is negative under NFS.)
a0d0e21e
LW
3186
3187=item study SCALAR
3188
3189=item study
3190
184e9718 3191Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
a0d0e21e
LW
3192doing many pattern matches on the string before it is next modified.
3193This may or may not save time, depending on the nature and number of
3194patterns you are searching on, and on the distribution of character
54310121 3195frequencies in the string to be searched -- you probably want to compare
5f05dabc 3196run times with and without it to see which runs faster. Those loops
a0d0e21e
LW
3197which scan for many short constant strings (including the constant
3198parts of more complex patterns) will benefit most. You may have only
54310121 3199one study active at a time -- if you study a different scalar the first
a0d0e21e
LW
3200is "unstudied". (The way study works is this: a linked list of every
3201character in the string to be searched is made, so we know, for
3202example, where all the 'k' characters are. From each search string,
3203the rarest character is selected, based on some static frequency tables
3204constructed from some C programs and English text. Only those places
3205that contain this "rarest" character are examined.)
3206
3207For example, here is a loop which inserts index producing entries
3208before any line containing a certain pattern:
3209
3210 while (<>) {
3211 study;
3212 print ".IX foo\n" if /\bfoo\b/;
3213 print ".IX bar\n" if /\bbar\b/;
3214 print ".IX blurfl\n" if /\bblurfl\b/;
3215 ...
3216 print;
3217 }
3218
3219In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3220will be looked at, because "f" is rarer than "o". In general, this is
3221a big win except in pathological cases. The only question is whether
3222it saves you more time than it took to build the linked list in the
3223first place.
3224
3225Note that if you have to look for strings that you don't know till
3226runtime, you can build an entire loop as a string and eval that to
3227avoid recompiling all your patterns all the time. Together with
3228undefining $/ to input entire files as one record, this can be very
3229fast, often faster than specialized programs like fgrep(1). The following
184e9718 3230scans a list of files (C<@files>) for a list of words (C<@words>), and prints
a0d0e21e
LW
3231out the names of those files that contain a match:
3232
3233 $search = 'while (<>) { study;';
3234 foreach $word (@words) {
3235 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3236 }
3237 $search .= "}";
3238 @ARGV = @files;
3239 undef $/;
3240 eval $search; # this screams
5f05dabc 3241 $/ = "\n"; # put back to normal input delimiter
a0d0e21e
LW
3242 foreach $file (sort keys(%seen)) {
3243 print $file, "\n";
3244 }
3245
cb1a09d0
AD
3246=item sub BLOCK
3247
3248=item sub NAME
3249
3250=item sub NAME BLOCK
3251
3252This is subroutine definition, not a real function I<per se>. With just a
3253NAME (and possibly prototypes), it's just a forward declaration. Without
3254a NAME, it's an anonymous function declaration, and does actually return a
2f9daede 3255value: the CODE ref of the closure you just created. See L<perlsub> and
cb1a09d0
AD
3256L<perlref> for details.
3257
a0d0e21e
LW
3258=item substr EXPR,OFFSET,LEN
3259
3260=item substr EXPR,OFFSET
3261
3262Extracts a substring out of EXPR and returns it. First character is at
2f9daede
TPG
3263offset 0, or whatever you've set C<$[> to (but don't do that).
3264If OFFSET is negative, starts
a0d0e21e 3265that far from the end of the string. If LEN is omitted, returns
748a9306
LW
3266everything to the end of the string. If LEN is negative, leaves that
3267many characters off the end of the string.
3268
3269You can use the substr() function
a0d0e21e
LW
3270as an lvalue, in which case EXPR must be an lvalue. If you assign
3271something shorter than LEN, the string will shrink, and if you assign
3272something longer than LEN, the string will grow to accommodate it. To
3273keep the string the same length you may need to pad or chop your value
3274using sprintf().
3275
3276=item symlink OLDFILE,NEWFILE
3277
3278Creates a new filename symbolically linked to the old filename.
3279Returns 1 for success, 0 otherwise. On systems that don't support
3280symbolic links, produces a fatal error at run time. To check for that,
3281use eval:
3282
54310121 3283 $symlink_exists = (eval {symlink("","")};, $@ eq '');
a0d0e21e
LW
3284
3285=item syscall LIST
3286
3287Calls the system call specified as the first element of the list,
3288passing the remaining elements as arguments to the system call. If
3289unimplemented, produces a fatal error. The arguments are interpreted
3290as follows: if a given argument is numeric, the argument is passed as
3291an int. If not, the pointer to the string value is passed. You are
3292responsible to make sure a string is pre-extended long enough to
3293receive any result that might be written into a string. If your
3294integer arguments are not literals and have never been interpreted in a
3295numeric context, you may need to add 0 to them to force them to look
3296like numbers.
3297
3298 require 'syscall.ph'; # may need to run h2ph
3299 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3300
5f05dabc 3301Note that Perl supports passing of up to only 14 arguments to your system call,
a0d0e21e
LW
3302which in practice should usually suffice.
3303
c07a80fd
PP
3304=item sysopen FILEHANDLE,FILENAME,MODE
3305
3306=item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3307
3308Opens the file whose filename is given by FILENAME, and associates it
3309with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3310the name of the real filehandle wanted. This function calls the
3311underlying operating system's C<open> function with the parameters
3312FILENAME, MODE, PERMS.
3313
3314The possible values and flag bits of the MODE parameter are
3315system-dependent; they are available via the standard module C<Fcntl>.
3316However, for historical reasons, some values are universal: zero means
3317read-only, one means write-only, and two means read/write.
3318
3319If the file named by FILENAME does not exist and the C<open> call
3320creates it (typically because MODE includes the O_CREAT flag), then
3321the value of PERMS specifies the permissions of the newly created
3322file. If PERMS is omitted, the default value is 0666, which allows
3323read and write for all. This default is reasonable: see C<umask>.
3324
28757baa
PP
3325The IO::File module provides a more object-oriented approach, if you're
3326into that kind of thing.
3327
a0d0e21e
LW
3328=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3329
3330=item sysread FILEHANDLE,SCALAR,LENGTH
3331
3332Attempts to read LENGTH bytes of data into variable SCALAR from the
3333specified FILEHANDLE, using the system call read(2). It bypasses
137443ea
PP
3334stdio, so mixing this with other kinds of reads or with seek() may
3335cause confusion. Returns the number of bytes actually read, or undef
3336if there was an error. SCALAR will be grown or shrunk so that the
3337last byte actually read is the last byte of the scalar after the read.
ff68c719
PP
3338
3339An OFFSET may be specified to place the read data at some place in the
3340string other than the beginning. A negative OFFSET specifies
3341placement at that many bytes counting backwards from the end of the
3342string. A positive OFFSET greater than the length of SCALAR results
3343in the string being padded to the required size with "\0" bytes before
3344the result of the read is appended.
a0d0e21e 3345
137443ea
PP
3346=item sysseek FILEHANDLE,POSITION,WHENCE
3347
3348Randomly positions the system file pointer for FILEHANDLE using the
3349system call lseek(2). It bypasses stdio, so mixing this with read(),
3350print(), write(), or seek() may cause confusion. FILEHANDLE may be an
3351expression whose value gives the name of the filehandle. The values for
3352WHENCE are 0 to set the file pointer to POSITION, 1 to set the it to
3353current plus POSITION, and 2 to set it to EOF plus offset. You may use
3354the values SEEK_SET, SEEK_CUR, and SEEK_END for this from either the
3355IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.
3356
a0d0e21e
LW
3357=item system LIST
3358
3359Does exactly the same thing as "exec LIST" except that a fork is done
3360first, and the parent process waits for the child process to complete.
3361Note that argument processing varies depending on the number of
3362arguments. The return value is the exit status of the program as
3363returned by the wait() call. To get the actual exit value divide by
54310121
PP
3364256. See also L</exec>. This is I<NOT> what you want to use to capture
3365the output from a command, for that you should use merely backticks or
28757baa 3366qx//, as described in L<perlop/"`STRING`">.
a0d0e21e 3367
54310121 3368Because system() and backticks block SIGINT and SIGQUIT, killing the
28757baa
PP
3369program they're running doesn't actually interrupt your program.
3370
3371 @args = ("command", "arg1", "arg2");
54310121
PP
3372 system(@args) == 0
3373 or die "system @args failed: $?"
28757baa
PP
3374
3375Here's a more elaborate example of analysing the return value from
54310121
PP
3376system() on a Unix system to check for all possibilities, including for
3377signals and core dumps.
28757baa
PP
3378
3379 $rc = 0xffff & system @args;
3380 printf "system(%s) returned %#04x: ", "@args", $rc;
3381 if ($rc == 0) {
3382 print "ran with normal exit\n";
54310121 3383 }
28757baa
PP
3384 elsif ($rc == 0xff00) {
3385 print "command failed: $!\n";
54310121 3386 }
28757baa
PP
3387 elsif ($rc > 0x80) {
3388 $rc >>= 8;
3389 print "ran with non-zero exit status $rc\n";
54310121 3390 }
28757baa
PP
3391 else {
3392 print "ran with ";
3393 if ($rc & 0x80) {
3394 $rc &= ~0x80;
54310121
PP
3395 print "core dump from ";
3396 }
28757baa 3397 print "signal $rc\n"
54310121 3398 }
28757baa 3399 $ok = ($rc != 0);
f86702cc 3400
a0d0e21e
LW
3401=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3402
3403=item syswrite FILEHANDLE,SCALAR,LENGTH
3404
3405Attempts to write LENGTH bytes of data from variable SCALAR to the
3406specified FILEHANDLE, using the system call write(2). It bypasses
137443ea
PP
3407stdio, so mixing this with prints or with seek() may cause confusion.
3408Returns the number of bytes actually written, or undef if there was an
3409error. If the length is greater than the available data, only as much
3410data as is available will be written.
ff68c719
PP
3411
3412An OFFSET may be specified to write the data from some part of the
3413string other than the beginning. A negative OFFSET specifies writing
54310121 3414that many bytes counting backwards from the end of the string.
a0d0e21e
LW
3415
3416=item tell FILEHANDLE
3417
3418=item tell
3419
3420Returns the current file position for FILEHANDLE. FILEHANDLE may be an
3421expression whose value gives the name of the actual filehandle. If
3422FILEHANDLE is omitted, assumes the file last read.
3423
3424=item telldir DIRHANDLE
3425
3426Returns the current position of the readdir() routines on DIRHANDLE.
3427Value may be given to seekdir() to access a particular location in a
3428directory. Has the same caveats about possible directory compaction as
3429the corresponding system library routine.
3430
4633a7c4 3431=item tie VARIABLE,CLASSNAME,LIST
a0d0e21e 3432
4633a7c4
LW
3433This function binds a variable to a package class that will provide the
3434implementation for the variable. VARIABLE is the name of the variable
3435to be enchanted. CLASSNAME is the name of a class implementing objects
3436of correct type. Any additional arguments are passed to the "new"
3437method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3438Typically these are arguments such as might be passed to the dbm_open()
cb1a09d0
AD
3439function of C. The object returned by the "new" method is also
3440returned by the tie() function, which would be useful if you want to
4633a7c4 3441access other methods in CLASSNAME.
a0d0e21e
LW
3442
3443Note that functions such as keys() and values() may return huge array
748a9306
LW
3444values when used on large objects, like DBM files. You may prefer to
3445use the each() function to iterate over such. Example:
a0d0e21e
LW
3446
3447 # print out history file offsets
4633a7c4 3448 use NDBM_File;
da0045b7 3449 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
a0d0e21e
LW
3450 while (($key,$val) = each %HIST) {
3451 print $key, ' = ', unpack('L',$val), "\n";
3452 }
3453 untie(%HIST);
3454
aa689395 3455A class implementing a hash should have the following methods:
a0d0e21e 3456
4633a7c4 3457 TIEHASH classname, LIST
a0d0e21e
LW
3458 DESTROY this
3459 FETCH this, key
3460 STORE this, key, value
3461 DELETE this, key
3462 EXISTS this, key
3463 FIRSTKEY this
3464 NEXTKEY this, lastkey
3465
4633a7c4 3466A class implementing an ordinary array should have the following methods:
a0d0e21e 3467
4633a7c4 3468 TIEARRAY classname, LIST
a0d0e21e
LW
3469 DESTROY this
3470 FETCH this, key
3471 STORE this, key, value
3472 [others TBD]
3473
4633a7c4 3474A class implementing a scalar should have the following methods:
a0d0e21e 3475
4633a7c4 3476 TIESCALAR classname, LIST
a0d0e21e 3477 DESTROY this
54310121 3478 FETCH this,
a0d0e21e
LW
3479 STORE this, value
3480
4633a7c4
LW
3481Unlike dbmopen(), the tie() function will not use or require a module
3482for you--you need to do that explicitly yourself. See L<DB_File>
3483or the F<Config> module for interesting tie() implementations.
3484
f3cbc334
RS
3485=item tied VARIABLE
3486
3487Returns a reference to the object underlying VARIABLE (the same value
3488that was originally returned by the tie() call which bound the variable
3489to a package.) Returns the undefined value if VARIABLE isn't tied to a
3490package.
3491
a0d0e21e
LW
3492=item time
3493
da0045b7
PP
3494Returns the number of non-leap seconds since whatever time the system
3495considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3496and 00:00:00 UTC, January 1, 1970 for most other systems).
3497Suitable for feeding to gmtime() and localtime().
a0d0e21e
LW
3498
3499=item times
3500
3501Returns a four-element array giving the user and system times, in
3502seconds, for this process and the children of this process.
3503
3504 ($user,$system,$cuser,$csystem) = times;
3505
3506=item tr///
3507
54310121 3508The translation operator. Same as y///. See L<perlop>.
a0d0e21e
LW
3509
3510=item truncate FILEHANDLE,LENGTH
3511
3512=item truncate EXPR,LENGTH
3513
3514Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3515specified length. Produces a fatal error if truncate isn't implemented
3516on your system.
3517
3518=item uc EXPR
3519
54310121 3520=item uc
bbce6d69 3521
a0d0e21e
LW
3522Returns an uppercased version of EXPR. This is the internal function
3523implementing the \U escape in double-quoted strings.
a034a98d 3524Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 3525
bbce6d69
PP
3526If EXPR is omitted, uses $_.
3527
a0d0e21e
LW
3528=item ucfirst EXPR
3529
54310121 3530=item ucfirst
bbce6d69 3531
a0d0e21e
LW
3532Returns the value of EXPR with the first character uppercased. This is
3533the internal function implementing the \u escape in double-quoted strings.
a034a98d 3534Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 3535
bbce6d69
PP
3536If EXPR is omitted, uses $_.
3537
a0d0e21e
LW
3538=item umask EXPR
3539
3540=item umask
3541
2f9daede
TPG
3542Sets the umask for the process to EXPR and returns the previous value.
3543If EXPR is omitted, merely returns the current umask. Remember that a
3544umask is a number, usually given in octal; it is I<not> a string of octal
dc848c6f 3545digits. See also L</oct>, if all you have is a string.
a0d0e21e
LW
3546
3547=item undef EXPR
3548
3549=item undef
3550
54310121
PP
3551Undefines the value of EXPR, which must be an lvalue. Use only on a
3552scalar value, an entire array, an entire hash, or a subroutine name (using
2f9daede
TPG
3553"&"). (Using undef() will probably not do what you expect on most
3554predefined variables or DBM list values, so don't do that.) Always
3555returns the undefined value. You can omit the EXPR, in which case
3556nothing is undefined, but you still get an undefined value that you
3557could, for instance, return from a subroutine, assign to a variable or
3558pass as a parameter. Examples:
a0d0e21e
LW
3559
3560 undef $foo;
2f9daede 3561 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
a0d0e21e 3562 undef @ary;
aa689395 3563 undef %hash;
a0d0e21e 3564 undef &mysub;
54310121 3565 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
2f9daede
TPG
3566 select undef, undef, undef, 0.25;
3567 ($a, $b, undef, $c) = &foo; # Ignore third value returned
a0d0e21e
LW
3568
3569=item unlink LIST
3570
54310121 3571=item unlink
bbce6d69 3572
a0d0e21e
LW
3573Deletes a list of files. Returns the number of files successfully
3574deleted.
3575
3576 $cnt = unlink 'a', 'b', 'c';
3577 unlink @goners;
3578 unlink <*.bak>;
3579
3580Note: unlink will not delete directories unless you are superuser and
3581the B<-U> flag is supplied to Perl. Even if these conditions are
3582met, be warned that unlinking a directory can inflict damage on your
3583filesystem. Use rmdir instead.
3584
bbce6d69
PP
3585If LIST is omitted, uses $_.
3586
a0d0e21e
LW
3587=item unpack TEMPLATE,EXPR
3588
3589Unpack does the reverse of pack: it takes a string representing a
3590structure and expands it out into a list value, returning the array
5f05dabc 3591value. (In a scalar context, it returns merely the first value
a0d0e21e
LW
3592produced.) The TEMPLATE has the same format as in the pack function.
3593Here's a subroutine that does substring:
3594
3595 sub substr {
3596 local($what,$where,$howmuch) = @_;
3597 unpack("x$where a$howmuch", $what);
3598 }
3599
3600and then there's
3601
3602 sub ordinal { unpack("c",$_[0]); } # same as ord()
3603
184e9718
PP
3604In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3605you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
a0d0e21e
LW
3606themselves. Default is a 16-bit checksum. For example, the following
3607computes the same number as the System V sum program:
3608
3609 while (<>) {
3610 $checksum += unpack("%16C*", $_);
3611 }
3612 $checksum %= 65536;
3613
3614The following efficiently counts the number of set bits in a bit vector:
3615
3616 $setbits = unpack("%32b*", $selectmask);
3617
3618=item untie VARIABLE
3619
3620Breaks the binding between a variable and a package. (See tie().)
3621
3622=item unshift ARRAY,LIST
3623
3624Does the opposite of a C<shift>. Or the opposite of a C<push>,
3625depending on how you look at it. Prepends list to the front of the
3626array, and returns the new number of elements in the array.
3627
3628 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3629
3630Note the LIST is prepended whole, not one element at a time, so the
3631prepended elements stay in the same order. Use reverse to do the
3632reverse.
3633
3634=item use Module LIST
3635
3636=item use Module
3637
da0045b7
PP
3638=item use Module VERSION LIST
3639
3640=item use VERSION
3641
a0d0e21e
LW
3642Imports some semantics into the current package from the named module,
3643generally by aliasing certain subroutine or variable names into your
3644package. It is exactly equivalent to
3645
3646 BEGIN { require Module; import Module LIST; }
3647
54310121 3648except that Module I<must> be a bareword.
da0045b7
PP
3649
3650If the first argument to C<use> is a number, it is treated as a version
3651number instead of a module name. If the version of the Perl interpreter
3652is less than VERSION, then an error message is printed and Perl exits
3653immediately. This is often useful if you need to check the current
3654Perl version before C<use>ing library modules which have changed in
3655incompatible ways from older versions of Perl. (We try not to do
3656this more than we have to.)
3657
a0d0e21e
LW
3658The BEGIN forces the require and import to happen at compile time. The
3659require makes sure the module is loaded into memory if it hasn't been
3660yet. The import is not a builtin--it's just an ordinary static method
3661call into the "Module" package to tell the module to import the list of
3662features back into the current package. The module can implement its
3663import method any way it likes, though most modules just choose to