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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,
51non-abortive failure is generally indicated in a scalar context by
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
129=item Keywords related to scoping
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.
228 -s File has non-zero size (returns size).
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
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278file, or a file at EOF when testing a filehandle. Because you have to
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
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303=item abs
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
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316=item alarm
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
a0d0e21e 329syscall() interface to access setitimer(2) if your system supports it,
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330or else see L</select()> below. It is not advised to intermix alarm()
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
373translated to CR LF on output. Binmode has no effect under Unix; in DOS
374and similarly archaic systems, it may be imperative--otherwise your
375DOS-damaged C library may mangle your file. The key distinction between
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, $line) = caller;
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404
405With EXPR, it returns some extra information that the debugger uses to
406print a stack trace. The value of EXPR indicates how many call frames
407to go back before the current one.
408
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409 ($package, $filename, $line, $subroutine,
410 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
411
412Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
2f9daede 413call, but C<L<eval>>. In such a case additional elements $evaltext and
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414$is_require are set: $is_require is true if the frame is created by
415C<L<require>> or C<L<use>> statement, $evaltext contains the text of
2f9daede 416C<L<eval EXPR>> statement. In particular, for C<L<eval BLOCK>>
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417statement $filename is C<"(eval)">, but $evaltext is undefined. (Note
418also that C<L<use>> statement creates a C<L<require>> frame inside
419an C<L<eval EXPR>>) frame.
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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
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423arguments with which that subroutine was invoked.
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
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, $gid, @filenames;
511
512Here's an example that looks up non-numeric uids in the passwd file:
513
514 print "User: ";
515 chop($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
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525On most systems, you are not allowed to change the ownership of the
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
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532=item chr
533
a0d0e21e 534Returns the character represented by that NUMBER in the character set.
2f9daede 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
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541=item chroot
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
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591=item cos EXPR
592
593Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
594takes cosine of $_.
595
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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 $pwd) {
627 die "Sorry...\n";
628 } else {
629 print "ok\n";
630 }
631
5f05dabc 632Of course, typing in your own password to whomever asks you
748a9306 633for it is unwise.
a0d0e21e 634
aa689395 635=item dbmclose HASH
a0d0e21e
LW
636
637[This function has been superseded by the untie() function.]
638
aa689395 639Breaks the binding between a DBM file and a hash.
a0d0e21e 640
aa689395 641=item dbmopen HASH,DBNAME,MODE
a0d0e21e
LW
642
643[This function has been superseded by the tie() function.]
644
aa689395
PP
645This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to a
646hash. HASH is the name of the hash. (Unlike normal open, the first
647argument is I<NOT> a filehandle, even though it looks like one). DBNAME
648is the name of the database (without the F<.dir> or F<.pag> extension if
649any). If the database does not exist, it is created with protection
650specified by MODE (as modified by the umask()). If your system supports
651only the older DBM functions, you may perform only one dbmopen() in your
652program. In older versions of Perl, if your system had neither DBM nor
653ndbm, calling dbmopen() produced a fatal error; it now falls back to
654sdbm(3).
655
656If you don't have write access to the DBM file, you can only read hash
657variables, not set them. If you want to test whether you can write,
658either use file tests or try setting a dummy hash entry inside an eval(),
659which will trap the error.
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LW
660
661Note that functions such as keys() and values() may return huge array
662values when used on large DBM files. You may prefer to use the each()
663function to iterate over large DBM files. Example:
664
665 # print out history file offsets
666 dbmopen(%HIST,'/usr/lib/news/history',0666);
667 while (($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
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LW
676=item defined EXPR
677
bbce6d69
PP
678=item defined
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,
700not whether the key exists in the hash. Use L<exists> for the latter
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 $1 is defined, despite the fact that it
719matched "nothing". But it didn't really match nothing--rather, it
720matched something that happened to be 0 characters long. This is all
721very above-board and honest. When a function returns an undefined value,
2f9daede
TPG
722it's an admission that it couldn't give you an honest answer. So you
723should use defined() only when you're questioning the integrity of what
724you're trying to do. At other times, a simple comparison to 0 or "" is
725what you want.
726
727Currently, using defined() on an entire array or hash reports whether
728memory for that aggregate has ever been allocated. So an array you set
729to the empty list appears undefined initially, and one that once was full
730and that you then set to the empty list still appears defined. You
731should instead use a simple test for size:
28757baa
PP
732
733 if (@an_array) { print "has array elements\n" }
734 if (%a_hash) { print "has hash members\n" }
735
736Using undef() on these, however, does clear their memory and then report
737them as not defined anymore, but you shoudln't do that unless you don't
738plan to use them again, because it saves time when you load them up
739again to have memory already ready to be filled.
740
741This counter-intuitive behaviour of defined() on aggregates may be
742changed, fixed, or broken in a future release of Perl.
743
2f9daede
TPG
744See also L<undef>, L<exists>, L<ref>.
745
a0d0e21e
LW
746=item delete EXPR
747
aa689395
PP
748Deletes the specified key(s) and their associated values from a hash.
749For each key, returns the deleted value associated with that key, or
750the undefined value if there was no such key. Deleting from C<$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<$!> is 0, exits with the value of
28757baa
PP
776C<($? E<gt>E<gt> 8)> (back-tick `command` status). If C<($? E<gt>E<gt> 8)>
777is 0, exits with 255. Inside an eval(), the error message is stuffed into
778C<$@>, 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';
784 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
785
786If the value of EXPR does not end in a newline, the current script line
787number and input line number (if any) are also printed, and a newline
788is supplied. Hint: sometimes appending ", stopped" to your message
789will cause it to make better sense when the string "at foo line 123" is
790appended. Suppose you are running script "canasta".
791
792 die "/etc/games is no good";
793 die "/etc/games is no good, stopped";
794
795produce, respectively
796
797 /etc/games is no good at canasta line 123.
798 /etc/games is no good, stopped at canasta line 123.
799
800See also exit() and warn().
801
774d564b
PP
802You can arrange for a callback to be called just before the die() does
803its deed, by setting the C<$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
5f05dabc 835re-parse 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
37798a01 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
974 # make divide-by-zero non-fatal
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
a0d0e21e
LW
1004With an eval(), you should be especially careful to remember what's
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
1023normally you I<WOULD> like to use double quotes, except that in that
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
1050LIST as a multi-valued list, even if there is only a single scalar in
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
PP
1087See also die(). If EXPR is omitted, exits with 0 status. The only
1088univerally portable values for EXPR are 0 for success and 1 for error;
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
bbce6d69
PP
1098=item exp
1099
a0d0e21e
LW
1100Returns I<e> (the natural logarithm base) to the power of EXPR.
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
LW
1192
1193There's also the double-fork trick (error checking on
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
1221 format Something =
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 {
cb1a09d0 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
f86702cc 1279Determination of whether $BSD_STYLE should be set
cb1a09d0
AD
1280is left as an exercise to the reader.
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;
28757baa 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
4633a7c4 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
5f05dabc 1449with the time localized for the standard Greenwich time zone.
4633a7c4 1450Typically used as follows:
a0d0e21e
LW
1451
1452
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
2f9daede
TPG
1458the range 0..6. Also, $year is the number of years since 1900, I<not>
1459simply the last two digits of the year.
1460
1461If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1462
0a753a76
PP
1463In a scalar context, prints out the ctime(3) value:
1464
1465 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1466
1467Also see the F<timegm.pl> library, and the strftime(3) function available
1468via the POSIX module.
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
2f9daede
TPG
1504This is similar in spirit to, but not the same as, L<grep(1)>
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
bbce6d69
PP
1529=item hex
1530
2f9daede
TPG
1531Interprets EXPR as a hex string and returns the corresponding
1532value. (To convert strings that might start with either 0 or 0x
1533see L<oct>.) If EXPR is omitted, uses $_.
1534
1535 print hex '0xAf'; # prints '175'
1536 print hex 'aF'; # same
a0d0e21e
LW
1537
1538=item import
1539
1540There is no built-in 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
4633a7c4 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
bbce6d69
PP
1557=item int
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
1571may help you in this, but it's non-trivial.) SCALAR will be read and/or
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
1608Joins the separate strings of LIST or ARRAY into a single string with
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
aa689395
PP
1638To sort an array by value, you'll need to use a C<sort{}> function.
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
4633a7c4
LW
1661Sends a signal to a list of processes. The first element of
1662the list must be the signal to send. Returns the number of
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
bbce6d69
PP
1690=item lc
1691
a0d0e21e 1692Returns an lowercased version of EXPR. This is the internal function
4633a7c4 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
bbce6d69
PP
1700=item lcfirst
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
bbce6d69
PP
1710=item length
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
1742 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1743 localtime(time);
1744
1745All array elements are numeric, and come straight out of a struct tm.
1746In particular this means that $mon has the range 0..11 and $wday has
0a753a76
PP
1747the range 0..6 and $year is year-1900, that is, $year is 123 in year
17482023. If EXPR is omitted, uses the current time ("localtime(time)").
a0d0e21e 1749
0a753a76 1750In a scalar context, returns the ctime(3) value:
a0d0e21e 1751
5f05dabc 1752 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 1753
0a753a76 1754Also see the Time::Local module, and the strftime(3) function available
da0045b7 1755via the POSIX module.
a0d0e21e
LW
1756
1757=item log EXPR
1758
bbce6d69
PP
1759=item log
1760
a0d0e21e
LW
1761Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1762of $_.
1763
1764=item lstat FILEHANDLE
1765
1766=item lstat EXPR
1767
bbce6d69
PP
1768=item lstat
1769
a0d0e21e
LW
1770Does the same thing as the stat() function, but stats a symbolic link
1771instead of the file the symbolic link points to. If symbolic links are
1772unimplemented on your system, a normal stat() is done.
1773
bbce6d69
PP
1774If EXPR is omitted, stats $_.
1775
a0d0e21e
LW
1776=item m//
1777
1778The match operator. See L<perlop>.
1779
1780=item map BLOCK LIST
1781
1782=item map EXPR,LIST
1783
1784Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1785element) and returns the list value composed of the results of each such
1786evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1787may produce zero, one, or more elements in the returned value.
1788
1789 @chars = map(chr, @nums);
1790
1791translates a list of numbers to the corresponding characters. And
1792
4633a7c4 1793 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
1794
1795is just a funny way to write
1796
1797 %hash = ();
1798 foreach $_ (@array) {
4633a7c4 1799 $hash{getkey($_)} = $_;
a0d0e21e
LW
1800 }
1801
1802=item mkdir FILENAME,MODE
1803
1804Creates the directory specified by FILENAME, with permissions specified
1805by MODE (as modified by umask). If it succeeds it returns 1, otherwise
184e9718 1806it returns 0 and sets C<$!> (errno).
a0d0e21e
LW
1807
1808=item msgctl ID,CMD,ARG
1809
4633a7c4 1810Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
a0d0e21e
LW
1811must be a variable which will hold the returned msqid_ds structure.
1812Returns like ioctl: the undefined value for error, "0 but true" for
1813zero, or the actual return value otherwise.
1814
1815=item msgget KEY,FLAGS
1816
4633a7c4 1817Calls the System V IPC function msgget(2). Returns the message queue id,
a0d0e21e
LW
1818or the undefined value if there is an error.
1819
1820=item msgsnd ID,MSG,FLAGS
1821
1822Calls the System V IPC function msgsnd to send the message MSG to the
1823message queue ID. MSG must begin with the long integer message type,
c07a80fd 1824which may be created with C<pack("l", $type)>. Returns TRUE if
a0d0e21e
LW
1825successful, or FALSE if there is an error.
1826
1827=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1828
1829Calls the System V IPC function msgrcv to receive a message from
1830message queue ID into variable VAR with a maximum message size of
1831SIZE. Note that if a message is received, the message type will be the
1832first thing in VAR, and the maximum length of VAR is SIZE plus the size
1833of the message type. Returns TRUE if successful, or FALSE if there is
1834an error.
1835
1836=item my EXPR
1837
1838A "my" declares the listed variables to be local (lexically) to the
cb1a09d0 1839enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
5f05dabc 1840more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 1841L<perlsub/"Private Variables via my()"> for details.
4633a7c4 1842
a0d0e21e
LW
1843=item next LABEL
1844
1845=item next
1846
1847The C<next> command is like the C<continue> statement in C; it starts
1848the next iteration of the loop:
1849
4633a7c4
LW
1850 LINE: while (<STDIN>) {
1851 next LINE if /^#/; # discard comments
a0d0e21e
LW
1852 ...
1853 }
1854
1855Note that if there were a C<continue> block on the above, it would get
1856executed even on discarded lines. If the LABEL is omitted, the command
1857refers to the innermost enclosing loop.
1858
1859=item no Module LIST
1860
1861See the "use" function, which "no" is the opposite of.
1862
1863=item oct EXPR
1864
bbce6d69
PP
1865=item oct
1866
4633a7c4 1867Interprets EXPR as an octal string and returns the corresponding
2f9daede 1868value. (If EXPR happens to start off with 0x, interprets it as
4633a7c4
LW
1869a hex string instead.) The following will handle decimal, octal, and
1870hex in the standard Perl or C notation:
a0d0e21e
LW
1871
1872 $val = oct($val) if $val =~ /^0/;
1873
2f9daede
TPG
1874If EXPR is omitted, uses $_. This function is commonly used when
1875a string such as "644" needs to be converted into a file mode, for
1876example. (Although perl will automatically convert strings into
1877numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
1878
1879=item open FILEHANDLE,EXPR
1880
1881=item open FILEHANDLE
1882
1883Opens the file whose filename is given by EXPR, and associates it with
5f05dabc
PP
1884FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1885name of the real filehandle wanted. If EXPR is omitted, the scalar
1886variable of the same name as the FILEHANDLE contains the filename.
1887(Note that lexical variables--those declared with C<my>--will not work
1888for this purpose; so if you're using C<my>, specify EXPR in your call
1889to open.)
1890
1891If the filename begins with '<' or nothing, the file is opened for input.
1892If the filename begins with '>', the file is truncated and opened for
1893output. If the filename begins with '>>', the file is opened for
1894appending. You can put a '+' in front of the '>' or '<' to indicate that
1895you want both read and write access to the file; thus '+<' is almost
1896always preferred for read/write updates--the '+>' mode would clobber the
1897file first. The prefix and the filename may be separated with spaces.
1898These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
1899'w+', 'a', and 'a+'.
1900
1901If the filename begins with "|", the filename is interpreted as a command
1902to which output is to be piped, and if the filename ends with a "|", the
1903filename is interpreted See L<perlipc/"Using open() for IPC"> for more
1904examples of this. as command which pipes input to us. (You may not have
7e1af8bc
PP
1905a raw open() to a command that pipes both in I<and> out, but see
1906L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
1907for alternatives.)
cb1a09d0 1908
184e9718 1909Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
4633a7c4
LW
1910non-zero upon success, the undefined value otherwise. If the open
1911involved a pipe, the return value happens to be the pid of the
cb1a09d0
AD
1912subprocess.
1913
1914If you're unfortunate enough to be running Perl on a system that
1915distinguishes between text files and binary files (modern operating
1916systems don't care), then you should check out L</binmode> for tips for
1917dealing with this. The key distinction between systems that need binmode
1918and those that don't is their text file formats. Systems like Unix and
1919Plan9 that delimit lines with a single character, and that encode that
1920character in C as '\n', do not need C<binmode>. The rest need it.
1921
cb1a09d0 1922Examples:
a0d0e21e
LW
1923
1924 $ARTICLE = 100;
1925 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1926 while (<ARTICLE>) {...
1927
1928 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1929
cb1a09d0
AD
1930 open(DBASE, '+<dbase.mine'); # open for update
1931
4633a7c4 1932 open(ARTICLE, "caesar <$article |"); # decrypt article
a0d0e21e 1933
4633a7c4 1934 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
a0d0e21e
LW
1935
1936 # process argument list of files along with any includes
1937
1938 foreach $file (@ARGV) {
1939 process($file, 'fh00');
1940 }
1941
1942 sub process {
1943 local($filename, $input) = @_;
1944 $input++; # this is a string increment
1945 unless (open($input, $filename)) {
1946 print STDERR "Can't open $filename: $!\n";
1947 return;
1948 }
1949
1950 while (<$input>) { # note use of indirection
1951 if (/^#include "(.*)"/) {
1952 process($1, $input);
1953 next;
1954 }
1955 ... # whatever
1956 }
1957 }
1958
1959You may also, in the Bourne shell tradition, specify an EXPR beginning
184e9718 1960with "E<gt>&", in which case the rest of the string is interpreted as the
a0d0e21e 1961name of a filehandle (or file descriptor, if numeric) which is to be
184e9718 1962duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
5f05dabc 1963+E<gt>E<gt>, and +E<lt>. The
a0d0e21e 1964mode you specify should match the mode of the original filehandle.
184e9718 1965(Duping a filehandle does not take into account any existing contents of
cb1a09d0 1966stdio buffers.)
a0d0e21e
LW
1967Here is a script that saves, redirects, and restores STDOUT and
1968STDERR:
1969
1970 #!/usr/bin/perl
1971 open(SAVEOUT, ">&STDOUT");
1972 open(SAVEERR, ">&STDERR");
1973
1974 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1975 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1976
1977 select(STDERR); $| = 1; # make unbuffered
1978 select(STDOUT); $| = 1; # make unbuffered
1979
1980 print STDOUT "stdout 1\n"; # this works for
1981 print STDERR "stderr 1\n"; # subprocesses too
1982
1983 close(STDOUT);
1984 close(STDERR);
1985
1986 open(STDOUT, ">&SAVEOUT");
1987 open(STDERR, ">&SAVEERR");
1988
1989 print STDOUT "stdout 2\n";
1990 print STDERR "stderr 2\n";
1991
1992
184e9718 1993If you specify "E<lt>&=N", where N is a number, then Perl will do an
4633a7c4
LW
1994equivalent of C's fdopen() of that file descriptor; this is more
1995parsimonious of file descriptors. For example:
a0d0e21e
LW
1996
1997 open(FILEHANDLE, "<&=$fd")
1998
5f05dabc 1999If you open a pipe on the command "-", i.e., either "|-" or "-|", then
a0d0e21e
LW
2000there is an implicit fork done, and the return value of open is the pid
2001of the child within the parent process, and 0 within the child
184e9718 2002process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2003The filehandle behaves normally for the parent, but i/o to that
2004filehandle is piped from/to the STDOUT/STDIN of the child process.
2005In the child process the filehandle isn't opened--i/o happens from/to
2006the new STDOUT or STDIN. Typically this is used like the normal
2007piped open when you want to exercise more control over just how the
2008pipe command gets executed, such as when you are running setuid, and
4633a7c4
LW
2009don't want to have to scan shell commands for metacharacters.
2010The following pairs are more or less equivalent:
a0d0e21e
LW
2011
2012 open(FOO, "|tr '[a-z]' '[A-Z]'");
2013 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2014
2015 open(FOO, "cat -n '$file'|");
2016 open(FOO, "-|") || exec 'cat', '-n', $file;
2017
4633a7c4
LW
2018See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2019
a0d0e21e 2020Explicitly closing any piped filehandle causes the parent process to
184e9718 2021wait for the child to finish, and returns the status value in C<$?>.
a0d0e21e 2022Note: on any operation which may do a fork, unflushed buffers remain
184e9718 2023unflushed in both processes, which means you may need to set C<$|> to
a0d0e21e
LW
2024avoid duplicate output.
2025
5f05dabc
PP
2026Using the constructor from the IO::Handle package (or one of its
2027subclasses, such as IO::File or IO::Socket),
c07a80fd
PP
2028you can generate anonymous filehandles which have the scope of whatever
2029variables hold references to them, and automatically close whenever
2030and however you leave that scope:
2031
5f05dabc 2032 use IO::File;
c07a80fd
PP
2033 ...
2034 sub read_myfile_munged {
2035 my $ALL = shift;
5f05dabc 2036 my $handle = new IO::File;
c07a80fd
PP
2037 open($handle, "myfile") or die "myfile: $!";
2038 $first = <$handle>
2039 or return (); # Automatically closed here.
2040 mung $first or die "mung failed"; # Or here.
2041 return $first, <$handle> if $ALL; # Or here.
2042 $first; # Or here.
2043 }
2044
a0d0e21e 2045The filename that is passed to open will have leading and trailing
5f05dabc 2046whitespace deleted. To open a file with arbitrary weird
a0d0e21e
LW
2047characters in it, it's necessary to protect any leading and trailing
2048whitespace thusly:
2049
cb1a09d0
AD
2050 $file =~ s#^(\s)#./$1#;
2051 open(FOO, "< $file\0");
2052
c07a80fd
PP
2053If you want a "real" C open() (see L<open(2)> on your system), then
2054you should use the sysopen() function. This is another way to
2055protect your filenames from interpretation. For example:
cb1a09d0 2056
28757baa 2057 use IO::Handle;
c07a80fd
PP
2058 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
2059 or die "sysopen $path: $!";
2060 HANDLE->autoflush(1);
2061 HANDLE->print("stuff $$\n");
2062 seek(HANDLE, 0, 0);
2063 print "File contains: ", <HANDLE>;
cb1a09d0
AD
2064
2065See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
2066
2067=item opendir DIRHANDLE,EXPR
2068
2069Opens a directory named EXPR for processing by readdir(), telldir(),
5f05dabc 2070seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
a0d0e21e
LW
2071DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2072
2073=item ord EXPR
2074
bbce6d69
PP
2075=item ord
2076
a0d0e21e 2077Returns the numeric ascii value of the first character of EXPR. If
2f9daede 2078EXPR is omitted, uses $_. For the reverse, see L<chr>.
a0d0e21e
LW
2079
2080=item pack TEMPLATE,LIST
2081
2082Takes an array or list of values and packs it into a binary structure,
2083returning the string containing the structure. The TEMPLATE is a
2084sequence of characters that give the order and type of values, as
2085follows:
2086
2087 A An ascii string, will be space padded.
2088 a An ascii string, will be null padded.
2089 b A bit string (ascending bit order, like vec()).
2090 B A bit string (descending bit order).
2091 h A hex string (low nybble first).
2092 H A hex string (high nybble first).
2093
2094 c A signed char value.
2095 C An unsigned char value.
2096 s A signed short value.
2097 S An unsigned short value.
2098 i A signed integer value.
2099 I An unsigned integer value.
2100 l A signed long value.
2101 L An unsigned long value.
2102
2103 n A short in "network" order.
2104 N A long in "network" order.
2105 v A short in "VAX" (little-endian) order.
2106 V A long in "VAX" (little-endian) order.
2107
2108 f A single-precision float in the native format.
2109 d A double-precision float in the native format.
2110
2111 p A pointer to a null-terminated string.
2112 P A pointer to a structure (fixed-length string).
2113
2114 u A uuencoded string.
2115
2f9daede 2116 w A BER compressed integer. Bytes give an unsigned integer base
def98dd4
UP
2117 128, most significant digit first, with as few digits as
2118 possible, and with the bit 8 of each byte except the last set
2119 to "1."
2120
a0d0e21e
LW
2121 x A null byte.
2122 X Back up a byte.
2123 @ Null fill to absolute position.
2124
2125Each letter may optionally be followed by a number which gives a repeat
5f05dabc 2126count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
a0d0e21e
LW
2127pack function will gobble up that many values from the LIST. A * for the
2128repeat count means to use however many items are left. The "a" and "A"
2129types gobble just one value, but pack it as a string of length count,
2130padding with nulls or spaces as necessary. (When unpacking, "A" strips
2131trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2132fields pack a string that many bits long. The "h" and "H" fields pack a
2133string that many nybbles long. The "P" packs a pointer to a structure of
2134the size indicated by the length. Real numbers (floats and doubles) are
2135in the native machine format only; due to the multiplicity of floating
2136formats around, and the lack of a standard "network" representation, no
2137facility for interchange has been made. This means that packed floating
2138point data written on one machine may not be readable on another - even if
2139both use IEEE floating point arithmetic (as the endian-ness of the memory
2140representation is not part of the IEEE spec). Note that Perl uses doubles
2141internally for all numeric calculation, and converting from double into
5f05dabc 2142float and thence back to double again will lose precision (i.e.,
a0d0e21e
LW
2143C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2144
2145Examples:
2146
2147 $foo = pack("cccc",65,66,67,68);
2148 # foo eq "ABCD"
2149 $foo = pack("c4",65,66,67,68);
2150 # same thing
2151
2152 $foo = pack("ccxxcc",65,66,67,68);
2153 # foo eq "AB\0\0CD"
2154
2155 $foo = pack("s2",1,2);
2156 # "\1\0\2\0" on little-endian
2157 # "\0\1\0\2" on big-endian
2158
2159 $foo = pack("a4","abcd","x","y","z");
2160 # "abcd"
2161
2162 $foo = pack("aaaa","abcd","x","y","z");
2163 # "axyz"
2164
2165 $foo = pack("a14","abcdefg");
2166 # "abcdefg\0\0\0\0\0\0\0"
2167
2168 $foo = pack("i9pl", gmtime);
2169 # a real struct tm (on my system anyway)
2170
2171 sub bintodec {
2172 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2173 }
2174
2175The same template may generally also be used in the unpack function.
2176
cb1a09d0
AD
2177=item package NAMESPACE
2178
2179Declares the compilation unit as being in the given namespace. The scope
2180of the package declaration is from the declaration itself through the end of
2181the enclosing block (the same scope as the local() operator). All further
2182unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2183statement affects only dynamic variables--including those you've used
cb1a09d0
AD
2184local() on--but I<not> lexical variables created with my(). Typically it
2185would be the first declaration in a file to be included by the C<require>
2186or C<use> operator. You can switch into a package in more than one place;
5f05dabc 2187it influences merely which symbol table is used by the compiler for the
cb1a09d0
AD
2188rest of that block. You can refer to variables and filehandles in other
2189packages by prefixing the identifier with the package name and a double
2190colon: C<$Package::Variable>. If the package name is null, the C<main>
2191package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2192
2193See L<perlmod/"Packages"> for more information about packages, modules,
2194and classes. See L<perlsub> for other scoping issues.
2195
a0d0e21e
LW
2196=item pipe READHANDLE,WRITEHANDLE
2197
2198Opens a pair of connected pipes like the corresponding system call.
2199Note that if you set up a loop of piped processes, deadlock can occur
2200unless you are very careful. In addition, note that Perl's pipes use
184e9718 2201stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2202after each command, depending on the application.
2203
7e1af8bc 2204See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2205for examples of such things.
2206
a0d0e21e
LW
2207=item pop ARRAY
2208
28757baa
PP
2209=item pop
2210
a0d0e21e
LW
2211Pops and returns the last value of the array, shortening the array by
22121. Has a similar effect to
2213
2214 $tmp = $ARRAY[$#ARRAY--];
2215
2216If there are no elements in the array, returns the undefined value.
cb1a09d0
AD
2217If ARRAY is omitted, pops the
2218@ARGV array in the main program, and the @_ array in subroutines, just
2219like shift().
a0d0e21e
LW
2220
2221=item pos SCALAR
2222
bbce6d69
PP
2223=item pos
2224
4633a7c4 2225Returns the offset of where the last C<m//g> search left off for the variable
2f9daede 2226is in question ($_ is used when the variable is not specified). May be
44a8e56a
PP
2227modified to change that offset. Such modification will also influence
2228the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2229L<perlop>.
a0d0e21e
LW
2230
2231=item print FILEHANDLE LIST
2232
2233=item print LIST
2234
2235=item print
2236
cb1a09d0 2237Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2238if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2239the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2240level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2241token is a term, it may be misinterpreted as an operator unless you
5f05dabc 2242interpose a + or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2243omitted, prints by default to standard output (or to the last selected
da0045b7 2244output channel--see L</select>). If LIST is also omitted, prints $_ to
a0d0e21e
LW
2245STDOUT. To set the default output channel to something other than
2246STDOUT use the select operation. Note that, because print takes a
2247LIST, anything in the LIST is evaluated in a list context, and any
2248subroutine that you call will have one or more of its expressions
2249evaluated in a list context. Also be careful not to follow the print
2250keyword with a left parenthesis unless you want the corresponding right
2251parenthesis to terminate the arguments to the print--interpose a + or
5f05dabc 2252put parentheses around all the arguments.
a0d0e21e 2253
4633a7c4 2254Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2255you will have to use a block returning its value instead:
4633a7c4
LW
2256
2257 print { $files[$i] } "stuff\n";
2258 print { $OK ? STDOUT : STDERR } "stuff\n";
2259
5f05dabc 2260=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2261
5f05dabc 2262=item printf FORMAT, LIST
a0d0e21e 2263
a034a98d
DD
2264Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>. The first argument
2265of the list will be interpreted as the printf format. If C<use locale> is
2266in effect, the character used for the decimal point in formatted real numbers
2267is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2268
28757baa
PP
2269Don't fall into the trap of using a printf() when a simple
2270print() would do. The print() is more efficient, and less
2271error prone.
2272
da0045b7
PP
2273=item prototype FUNCTION
2274
2275Returns the prototype of a function as a string (or C<undef> if the
5f05dabc
PP
2276function has no prototype). FUNCTION is a reference to, or the name of,
2277the function whose prototype you want to retrieve.
da0045b7 2278
a0d0e21e
LW
2279=item push ARRAY,LIST
2280
2281Treats ARRAY as a stack, and pushes the values of LIST
2282onto the end of ARRAY. The length of ARRAY increases by the length of
2283LIST. Has the same effect as
2284
2285 for $value (LIST) {
2286 $ARRAY[++$#ARRAY] = $value;
2287 }
2288
2289but is more efficient. Returns the new number of elements in the array.
2290
2291=item q/STRING/
2292
2293=item qq/STRING/
2294
2295=item qx/STRING/
2296
2297=item qw/STRING/
2298
2299Generalized quotes. See L<perlop>.
2300
2301=item quotemeta EXPR
2302
bbce6d69
PP
2303=item quotemeta
2304
68dc0745 2305Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2306characters backslashed. (That is, all characters not matching
2307C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2308returned string, regardless of any locale settings.)
2309This is the internal function implementing
a0d0e21e
LW
2310the \Q escape in double-quoted strings.
2311
bbce6d69
PP
2312If EXPR is omitted, uses $_.
2313
a0d0e21e
LW
2314=item rand EXPR
2315
2316=item rand
2317
2318Returns a random fractional number between 0 and the value of EXPR.
2319(EXPR should be positive.) If EXPR is omitted, returns a value between
93dc8474
CS
23200 and 1. Automatically calls srand() unless srand() has already been
2321called. See also srand().
a0d0e21e 2322
2f9daede 2323(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2324large or too small, then your version of Perl was probably compiled
2f9daede 2325with the wrong number of RANDBITS.)
a0d0e21e
LW
2326
2327=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2328
2329=item read FILEHANDLE,SCALAR,LENGTH
2330
2331Attempts to read LENGTH bytes of data into variable SCALAR from the
2332specified FILEHANDLE. Returns the number of bytes actually read, or
2333undef if there was an error. SCALAR will be grown or shrunk to the
2334length actually read. An OFFSET may be specified to place the read
2335data at some other place than the beginning of the string. This call
2336is actually implemented in terms of stdio's fread call. To get a true
2337read system call, see sysread().
2338
2339=item readdir DIRHANDLE
2340
2341Returns the next directory entry for a directory opened by opendir().
2342If used in a list context, returns all the rest of the entries in the
2343directory. If there are no more entries, returns an undefined value in
2344a scalar context or a null list in a list context.
2345
cb1a09d0 2346If you're planning to filetest the return values out of a readdir(), you'd
5f05dabc 2347better prepend the directory in question. Otherwise, because we didn't
cb1a09d0
AD
2348chdir() there, it would have been testing the wrong file.
2349
2350 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2351 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2352 closedir DIR;
2353
a0d0e21e
LW
2354=item readlink EXPR
2355
bbce6d69
PP
2356=item readlink
2357
a0d0e21e
LW
2358Returns the value of a symbolic link, if symbolic links are
2359implemented. If not, gives a fatal error. If there is some system
184e9718 2360error, returns the undefined value and sets C<$!> (errno). If EXPR is
a0d0e21e
LW
2361omitted, uses $_.
2362
2363=item recv SOCKET,SCALAR,LEN,FLAGS
2364
2365Receives a message on a socket. Attempts to receive LENGTH bytes of
2366data into variable SCALAR from the specified SOCKET filehandle.
2367Actually does a C recvfrom(), so that it can returns the address of the
2368sender. Returns the undefined value if there's an error. SCALAR will
2369be grown or shrunk to the length actually read. Takes the same flags
4633a7c4
LW
2370as the system call of the same name.
2371See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2372
2373=item redo LABEL
2374
2375=item redo
2376
2377The C<redo> command restarts the loop block without evaluating the
2378conditional again. The C<continue> block, if any, is not executed. If
2379the LABEL is omitted, the command refers to the innermost enclosing
2380loop. This command is normally used by programs that want to lie to
2381themselves about what was just input:
2382
2383 # a simpleminded Pascal comment stripper
2384 # (warning: assumes no { or } in strings)
4633a7c4 2385 LINE: while (<STDIN>) {
a0d0e21e
LW
2386 while (s|({.*}.*){.*}|$1 |) {}
2387 s|{.*}| |;
2388 if (s|{.*| |) {
2389 $front = $_;
2390 while (<STDIN>) {
2391 if (/}/) { # end of comment?
2392 s|^|$front{|;
4633a7c4 2393 redo LINE;
a0d0e21e
LW
2394 }
2395 }
2396 }
2397 print;
2398 }
2399
2400=item ref EXPR
2401
bbce6d69
PP
2402=item ref
2403
2f9daede
TPG
2404Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2405is not specified, $_ will be used. The value returned depends on the
bbce6d69 2406type of thing the reference is a reference to.
a0d0e21e
LW
2407Builtin types include:
2408
2409 REF
2410 SCALAR
2411 ARRAY
2412 HASH
2413 CODE
2414 GLOB
2415
2416If the referenced object has been blessed into a package, then that package
2417name is returned instead. You can think of ref() as a typeof() operator.
2418
2419 if (ref($r) eq "HASH") {
aa689395 2420 print "r is a reference to a hash.\n";
a0d0e21e
LW
2421 }
2422 if (!ref ($r) {
2423 print "r is not a reference at all.\n";
2424 }
2425
2426See also L<perlref>.
2427
2428=item rename OLDNAME,NEWNAME
2429
2430Changes the name of a file. Returns 1 for success, 0 otherwise. Will
5f05dabc 2431not work across file system boundaries.
a0d0e21e
LW
2432
2433=item require EXPR
2434
2435=item require
2436
2437Demands some semantics specified by EXPR, or by $_ if EXPR is not
2438supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2439(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2440
2441Otherwise, demands that a library file be included if it hasn't already
2442been included. The file is included via the do-FILE mechanism, which is
2443essentially just a variety of eval(). Has semantics similar to the following
2444subroutine:
2445
2446 sub require {
2447 local($filename) = @_;
2448 return 1 if $INC{$filename};
2449 local($realfilename,$result);
2450 ITER: {
2451 foreach $prefix (@INC) {
2452 $realfilename = "$prefix/$filename";
2453 if (-f $realfilename) {
2454 $result = do $realfilename;
2455 last ITER;
2456 }
2457 }
2458 die "Can't find $filename in \@INC";
2459 }
2460 die $@ if $@;
2461 die "$filename did not return true value" unless $result;
2462 $INC{$filename} = $realfilename;
2463 $result;
2464 }
2465
2466Note that the file will not be included twice under the same specified
2467name. The file must return TRUE as the last statement to indicate
2468successful execution of any initialization code, so it's customary to
2469end such a file with "1;" unless you're sure it'll return TRUE
2470otherwise. But it's better just to put the "C<1;>", in case you add more
2471statements.
2472
da0045b7
PP
2473If EXPR is a bare word, the require assumes a "F<.pm>" extension and
2474replaces "F<::>" with "F</>" in the filename for you,
a0d0e21e
LW
2475to make it easy to load standard modules. This form of loading of
2476modules does not risk altering your namespace.
2477
da0045b7 2478For a yet-more-powerful import facility, see L</use> and
748a9306 2479L<perlmod>.
a0d0e21e
LW
2480
2481=item reset EXPR
2482
2483=item reset
2484
2485Generally used in a C<continue> block at the end of a loop to clear
2486variables and reset ?? searches so that they work again. The
2487expression is interpreted as a list of single characters (hyphens
2488allowed for ranges). All variables and arrays beginning with one of
2489those letters are reset to their pristine state. If the expression is
5f05dabc
PP
2490omitted, one-match searches (?pattern?) are reset to match again. Resets
2491only variables or searches in the current package. Always returns
a0d0e21e
LW
24921. Examples:
2493
2494 reset 'X'; # reset all X variables
2495 reset 'a-z'; # reset lower case variables
2496 reset; # just reset ?? searches
2497
5f05dabc
PP
2498Resetting "A-Z" is not recommended because you'll wipe out your
2499ARGV and ENV arrays. Resets only package variables--lexical variables
a0d0e21e 2500are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2501so you'll probably want to use them instead. See L</my>.
a0d0e21e
LW
2502
2503=item return LIST
2504
68dc0745
PP
2505Returns from a subroutine, eval(), or do FILE with the value specified.
2506(Note that in the absence of a return, a subroutine, eval, or do FILE
2507will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
2508
2509=item reverse LIST
2510
2511In a list context, returns a list value consisting of the elements
2f9daede
TPG
2512of LIST in the opposite order. In a scalar context, concatenates the
2513elements of LIST, and returns a string value consisting of those bytes,
2514but in the opposite order.
4633a7c4 2515
2f9daede 2516 print reverse <>; # line tac, last line first
4633a7c4 2517
2f9daede
TPG
2518 undef $/; # for efficiency of <>
2519 print scalar reverse <>; # byte tac, last line tsrif
2520
2521This operator is also handy for inverting a hash, although there are some
2522caveats. If a value is duplicated in the original hash, only one of those
2523can be represented as a key in the inverted hash. Also, this has to
2524unwind one hash and build a whole new one, which may take some time
2525on a large hash.
2526
2527 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
2528
2529=item rewinddir DIRHANDLE
2530
2531Sets the current position to the beginning of the directory for the
2532readdir() routine on DIRHANDLE.
2533
2534=item rindex STR,SUBSTR,POSITION
2535
2536=item rindex STR,SUBSTR
2537
2538Works just like index except that it returns the position of the LAST
2539occurrence of SUBSTR in STR. If POSITION is specified, returns the
2540last occurrence at or before that position.
2541
2542=item rmdir FILENAME
2543
bbce6d69
PP
2544=item rmdir
2545
a0d0e21e 2546Deletes the directory specified by FILENAME if it is empty. If it
184e9718 2547succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
a0d0e21e
LW
2548FILENAME is omitted, uses $_.
2549
2550=item s///
2551
2552The substitution operator. See L<perlop>.
2553
2554=item scalar EXPR
2555
2556Forces EXPR to be interpreted in a scalar context and returns the value
cb1a09d0
AD
2557of EXPR.
2558
2559 @counts = ( scalar @a, scalar @b, scalar @c );
2560
2561There is no equivalent operator to force an expression to
2562be interpolated in a list context because it's in practice never
2563needed. If you really wanted to do so, however, you could use
2564the construction C<@{[ (some expression) ]}>, but usually a simple
2565C<(some expression)> suffices.
a0d0e21e
LW
2566
2567=item seek FILEHANDLE,POSITION,WHENCE
2568
2569Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2570call of stdio. FILEHANDLE may be an expression whose value gives the name
2571of the filehandle. The values for WHENCE are 0 to set the file pointer to
2572POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2573plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
4633a7c4 2574this from POSIX module. Returns 1 upon success, 0 otherwise.
a0d0e21e 2575
cb1a09d0
AD
2576On some systems you have to do a seek whenever you switch between reading
2577and writing. Amongst other things, this may have the effect of calling
2578stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2579the file pointer:
2580
2581 seek(TEST,0,1);
2582
2583This is also useful for applications emulating C<tail -f>. Once you hit
2584EOF on your read, and then sleep for a while, you might have to stick in a
2585seek() to reset things. First the simple trick listed above to clear the
2586filepointer. The seek() doesn't change the current position, but it
2587I<does> clear the end-of-file condition on the handle, so that the next
5f05dabc 2588C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
2589
2590If that doesn't work (some stdios are particularly cantankerous), then
2591you may need something more like this:
2592
2593 for (;;) {
2594 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2595 # search for some stuff and put it into files
2596 }
2597 sleep($for_a_while);
2598 seek(FILE, $curpos, 0);
2599 }
2600
a0d0e21e
LW
2601=item seekdir DIRHANDLE,POS
2602
2603Sets the current position for the readdir() routine on DIRHANDLE. POS
2604must be a value returned by telldir(). Has the same caveats about
2605possible directory compaction as the corresponding system library
2606routine.
2607
2608=item select FILEHANDLE
2609
2610=item select
2611
2612Returns the currently selected filehandle. Sets the current default
2613filehandle for output, if FILEHANDLE is supplied. This has two
2614effects: first, a C<write> or a C<print> without a filehandle will
2615default to this FILEHANDLE. Second, references to variables related to
2616output will refer to this output channel. For example, if you have to
2617set the top of form format for more than one output channel, you might
2618do the following:
2619
2620 select(REPORT1);
2621 $^ = 'report1_top';
2622 select(REPORT2);
2623 $^ = 'report2_top';
2624
2625FILEHANDLE may be an expression whose value gives the name of the
2626actual filehandle. Thus:
2627
2628 $oldfh = select(STDERR); $| = 1; select($oldfh);
2629
4633a7c4
LW
2630Some programmers may prefer to think of filehandles as objects with
2631methods, preferring to write the last example as:
a0d0e21e 2632
28757baa 2633 use IO::Handle;
a0d0e21e
LW
2634 STDERR->autoflush(1);
2635
2636=item select RBITS,WBITS,EBITS,TIMEOUT
2637
5f05dabc 2638This calls the select(2) system call with the bit masks specified, which
a0d0e21e
LW
2639can be constructed using fileno() and vec(), along these lines:
2640
2641 $rin = $win = $ein = '';
2642 vec($rin,fileno(STDIN),1) = 1;
2643 vec($win,fileno(STDOUT),1) = 1;
2644 $ein = $rin | $win;
2645
2646If you want to select on many filehandles you might wish to write a
2647subroutine:
2648
2649 sub fhbits {
2650 local(@fhlist) = split(' ',$_[0]);
2651 local($bits);
2652 for (@fhlist) {
2653 vec($bits,fileno($_),1) = 1;
2654 }
2655 $bits;
2656 }
4633a7c4 2657 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
2658
2659The usual idiom is:
2660
2661 ($nfound,$timeleft) =
2662 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2663
c07a80fd 2664or to block until something becomes ready just do this
a0d0e21e
LW
2665
2666 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2667
5f05dabc 2668Most systems do not bother to return anything useful in $timeleft, so
c07a80fd
PP
2669calling select() in a scalar context just returns $nfound.
2670
5f05dabc 2671Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e
LW
2672in seconds, which may be fractional. Note: not all implementations are
2673capable of returning the $timeleft. If not, they always return
2674$timeleft equal to the supplied $timeout.
2675
ff68c719 2676You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
2677
2678 select(undef, undef, undef, 0.25);
2679
184e9718 2680B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
cb1a09d0 2681with select(). You have to use sysread() instead.
a0d0e21e
LW
2682
2683=item semctl ID,SEMNUM,CMD,ARG
2684
2685Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2686&GETALL, then ARG must be a variable which will hold the returned
2687semid_ds structure or semaphore value array. Returns like ioctl: the
2688undefined value for error, "0 but true" for zero, or the actual return
2689value otherwise.
2690
2691=item semget KEY,NSEMS,FLAGS
2692
2693Calls the System V IPC function semget. Returns the semaphore id, or
2694the undefined value if there is an error.
2695
2696=item semop KEY,OPSTRING
2697
2698Calls the System V IPC function semop to perform semaphore operations
2699such as signaling and waiting. OPSTRING must be a packed array of
2700semop structures. Each semop structure can be generated with
2701C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2702operations is implied by the length of OPSTRING. Returns TRUE if
2703successful, or FALSE if there is an error. As an example, the
2704following code waits on semaphore $semnum of semaphore id $semid:
2705
2706 $semop = pack("sss", $semnum, -1, 0);
2707 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2708
2709To signal the semaphore, replace "-1" with "1".
2710
2711=item send SOCKET,MSG,FLAGS,TO
2712
2713=item send SOCKET,MSG,FLAGS
2714
2715Sends a message on a socket. Takes the same flags as the system call
2716of the same name. On unconnected sockets you must specify a
2717destination to send TO, in which case it does a C sendto(). Returns
2718the number of characters sent, or the undefined value if there is an
2719error.
4633a7c4 2720See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2721
2722=item setpgrp PID,PGRP
2723
2724Sets the current process group for the specified PID, 0 for the current
2725process. Will produce a fatal error if used on a machine that doesn't
5f05dabc 2726implement setpgrp(2). If the arguments are omitted, it defaults to
47e29363
PP
27270,0. Note that the POSIX version of setpgrp() does not accept any
2728arguments, so only setpgrp 0,0 is portable.
a0d0e21e
LW
2729
2730=item setpriority WHICH,WHO,PRIORITY
2731
2732Sets the current priority for a process, a process group, or a user.
748a9306 2733(See setpriority(2).) Will produce a fatal error if used on a machine
a0d0e21e
LW
2734that doesn't implement setpriority(2).
2735
2736=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2737
2738Sets the socket option requested. Returns undefined if there is an
2739error. OPTVAL may be specified as undef if you don't want to pass an
2740argument.
2741
2742=item shift ARRAY
2743
2744=item shift
2745
2746Shifts the first value of the array off and returns it, shortening the
2747array by 1 and moving everything down. If there are no elements in the
2748array, returns the undefined value. If ARRAY is omitted, shifts the
2749@ARGV array in the main program, and the @_ array in subroutines.
2750(This is determined lexically.) See also unshift(), push(), and pop().
2751Shift() and unshift() do the same thing to the left end of an array
2f9daede 2752that pop() and push() do to the right end.
a0d0e21e
LW
2753
2754=item shmctl ID,CMD,ARG
2755
2756Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2757must be a variable which will hold the returned shmid_ds structure.
2758Returns like ioctl: the undefined value for error, "0 but true" for
2759zero, or the actual return value otherwise.
2760
2761=item shmget KEY,SIZE,FLAGS
2762
2763Calls the System V IPC function shmget. Returns the shared memory
2764segment id, or the undefined value if there is an error.
2765
2766=item shmread ID,VAR,POS,SIZE
2767
2768=item shmwrite ID,STRING,POS,SIZE
2769
2770Reads or writes the System V shared memory segment ID starting at
2771position POS for size SIZE by attaching to it, copying in/out, and
2772detaching from it. When reading, VAR must be a variable which will
2773hold the data read. When writing, if STRING is too long, only SIZE
2774bytes are used; if STRING is too short, nulls are written to fill out
2775SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2776
2777=item shutdown SOCKET,HOW
2778
2779Shuts down a socket connection in the manner indicated by HOW, which
2780has the same interpretation as in the system call of the same name.
2781
2782=item sin EXPR
2783
bbce6d69
PP
2784=item sin
2785
a0d0e21e
LW
2786Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2787returns sine of $_.
2788
28757baa
PP
2789For the inverse sine operation, you may use the POSIX::sin()
2790function, or use this relation:
2791
2792 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
2793
a0d0e21e
LW
2794=item sleep EXPR
2795
2796=item sleep
2797
2798Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2799May be interrupted by sending the process a SIGALRM. Returns the
2800number of seconds actually slept. You probably cannot mix alarm() and
5f05dabc 2801sleep() calls, because sleep() is often implemented using alarm().
a0d0e21e
LW
2802
2803On some older systems, it may sleep up to a full second less than what
2804you requested, depending on how it counts seconds. Most modern systems
2805always sleep the full amount.
2806
cb1a09d0
AD
2807For delays of finer granularity than one second, you may use Perl's
2808syscall() interface to access setitimer(2) if your system supports it,
2809or else see L</select()> below.
2810
5f05dabc
PP
2811See also the POSIX module's sigpause() function.
2812
a0d0e21e
LW
2813=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2814
2815Opens a socket of the specified kind and attaches it to filehandle
5f05dabc 2816SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
a0d0e21e 2817system call of the same name. You should "use Socket;" first to get
4633a7c4 2818the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2819
2820=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2821
2822Creates an unnamed pair of sockets in the specified domain, of the
5f05dabc 2823specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
a0d0e21e
LW
2824for the system call of the same name. If unimplemented, yields a fatal
2825error. Returns TRUE if successful.
2826
2827=item sort SUBNAME LIST
2828
2829=item sort BLOCK LIST
2830
2831=item sort LIST
2832
2f9daede
TPG
2833Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
2834is omitted, sorts in standard string comparison order. If SUBNAME is
2835specified, it gives the name of a subroutine that returns an integer
2836less than, equal to, or greater than 0, depending on how the elements
2837of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
2838operators are extremely useful in such routines.) SUBNAME may be a
2839scalar variable name, in which case the value provides the name of the
2840subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
2841an anonymous, in-line sort subroutine.
a0d0e21e 2842
cb1a09d0
AD
2843In the interests of efficiency the normal calling code for subroutines is
2844bypassed, with the following effects: the subroutine may not be a
2845recursive subroutine, and the two elements to be compared are passed into
2846the subroutine not via @_ but as the package global variables $a and
2847$b (see example below). They are passed by reference, so don't
2848modify $a and $b. And don't try to declare them as lexicals either.
a0d0e21e 2849
0a753a76
PP
2850You also cannot exit out of the sort block or subroutine using any of the
2851loop control operators described in L<perlsyn> or with goto().
2852
a034a98d
DD
2853When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
2854current collation locale. See L<perllocale>.
2855
a0d0e21e
LW
2856Examples:
2857
2858 # sort lexically
2859 @articles = sort @files;
2860
2861 # same thing, but with explicit sort routine
2862 @articles = sort {$a cmp $b} @files;
2863
cb1a09d0
AD
2864 # now case-insensitively
2865 @articles = sort { uc($a) cmp uc($b)} @files;
2866
a0d0e21e
LW
2867 # same thing in reversed order
2868 @articles = sort {$b cmp $a} @files;
2869
2870 # sort numerically ascending
2871 @articles = sort {$a <=> $b} @files;
2872
2873 # sort numerically descending
2874 @articles = sort {$b <=> $a} @files;
2875
2876 # sort using explicit subroutine name
2877 sub byage {
2f9daede 2878 $age{$a} <=> $age{$b}; # presuming numeric
a0d0e21e
LW
2879 }
2880 @sortedclass = sort byage @class;
2881
aa689395
PP
2882 # this sorts the %age hash by value instead of key
2883 # using an in-line function
c07a80fd
PP
2884 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2885
a0d0e21e
LW
2886 sub backwards { $b cmp $a; }
2887 @harry = ('dog','cat','x','Cain','Abel');
2888 @george = ('gone','chased','yz','Punished','Axed');
2889 print sort @harry;
2890 # prints AbelCaincatdogx
2891 print sort backwards @harry;
2892 # prints xdogcatCainAbel
2893 print sort @george, 'to', @harry;
2894 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2895
cb1a09d0
AD
2896 # inefficiently sort by descending numeric compare using
2897 # the first integer after the first = sign, or the
2898 # whole record case-insensitively otherwise
2899
2900 @new = sort {
2901 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2902 ||
2903 uc($a) cmp uc($b)
2904 } @old;
2905
2906 # same thing, but much more efficiently;
2907 # we'll build auxiliary indices instead
2908 # for speed
2909 @nums = @caps = ();
2910 for (@old) {
2911 push @nums, /=(\d+)/;
2912 push @caps, uc($_);
2913 }
2914
2915 @new = @old[ sort {
2916 $nums[$b] <=> $nums[$a]
2917 ||
2918 $caps[$a] cmp $caps[$b]
2919 } 0..$#old
2920 ];
2921
2922 # same thing using a Schwartzian Transform (no temps)
2923 @new = map { $_->[0] }
2924 sort { $b->[1] <=> $a->[1]
2925 ||
2926 $a->[2] cmp $b->[2]
2927 } map { [$_, /=(\d+)/, uc($_)] } @old;
2928
184e9718 2929If you're using strict, you I<MUST NOT> declare $a
cb1a09d0
AD
2930and $b as lexicals. They are package globals. That means
2931if you're in the C<main> package, it's
2932
2933 @articles = sort {$main::b <=> $main::a} @files;
2934
2935or just
2936
2937 @articles = sort {$::b <=> $::a} @files;
2938
2939but if you're in the C<FooPack> package, it's
2940
2941 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2942
55497cff
PP
2943The comparison function is required to behave. If it returns
2944inconsistent results (sometimes saying $x[1] is less than $x[2] and
2945sometimes saying the opposite, for example) the Perl interpreter will
2946probably crash and dump core. This is entirely due to and dependent
2947upon your system's qsort(3) library routine; this routine often avoids
2948sanity checks in the interest of speed.
2949
a0d0e21e
LW
2950=item splice ARRAY,OFFSET,LENGTH,LIST
2951
2952=item splice ARRAY,OFFSET,LENGTH
2953
2954=item splice ARRAY,OFFSET
2955
2956Removes the elements designated by OFFSET and LENGTH from an array, and
2957replaces them with the elements of LIST, if any. Returns the elements
2958removed from the array. The array grows or shrinks as necessary. If
2959LENGTH is omitted, removes everything from OFFSET onward. The
5f05dabc 2960following equivalences hold (assuming C<$[ == 0>):
a0d0e21e
LW
2961
2962 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2963 pop(@a) splice(@a,-1)
2964 shift(@a) splice(@a,0,1)
2965 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2966 $a[$x] = $y splice(@a,$x,1,$y);
2967
2968Example, assuming array lengths are passed before arrays:
2969
2970 sub aeq { # compare two list values
2971 local(@a) = splice(@_,0,shift);
2972 local(@b) = splice(@_,0,shift);
2973 return 0 unless @a == @b; # same len?
2974 while (@a) {
2975 return 0 if pop(@a) ne pop(@b);
2976 }
2977 return 1;
2978 }
2979 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2980
2981=item split /PATTERN/,EXPR,LIMIT
2982
2983=item split /PATTERN/,EXPR
2984
2985=item split /PATTERN/
2986
2987=item split
2988
2989Splits a string into an array of strings, and returns it.
2990
2991If not in a list context, returns the number of fields found and splits into
2992the @_ array. (In a list context, you can force the split into @_ by
2993using C<??> as the pattern delimiters, but it still returns the array
2994value.) The use of implicit split to @_ is deprecated, however.
2995
2996If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
4633a7c4
LW
2997splits on whitespace (after skipping any leading whitespace). Anything
2998matching PATTERN is taken to be a delimiter separating the fields. (Note
2999that the delimiter may be longer than one character.) If LIMIT is
3000specified and is not negative, splits into no more than that many fields
3001(though it may split into fewer). If LIMIT is unspecified, trailing null
3002fields are stripped (which potential users of pop() would do well to
3003remember). If LIMIT is negative, it is treated as if an arbitrarily large
3004LIMIT had been specified.
a0d0e21e
LW
3005
3006A pattern matching the null string (not to be confused with
748a9306 3007a null pattern C<//>, which is just one member of the set of patterns
a0d0e21e
LW
3008matching a null string) will split the value of EXPR into separate
3009characters at each point it matches that way. For example:
3010
3011 print join(':', split(/ */, 'hi there'));
3012
3013produces the output 'h:i:t:h:e:r:e'.
3014
5f05dabc 3015The LIMIT parameter can be used to split a line partially
a0d0e21e
LW
3016
3017 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3018
3019When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3020one larger than the number of variables in the list, to avoid
3021unnecessary work. For the list above LIMIT would have been 4 by
3022default. In time critical applications it behooves you not to split
3023into more fields than you really need.
3024
3025If the PATTERN contains parentheses, additional array elements are
3026created from each matching substring in the delimiter.
3027
da0045b7 3028 split(/([,-])/, "1-10,20", 3);
a0d0e21e
LW
3029
3030produces the list value
3031
3032 (1, '-', 10, ',', 20)
3033
4633a7c4
LW
3034If you had the entire header of a normal Unix email message in $header,
3035you could split it up into fields and their values this way:
3036
3037 $header =~ s/\n\s+/ /g; # fix continuation lines
3038 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
3039
a0d0e21e
LW
3040The pattern C</PATTERN/> may be replaced with an expression to specify
3041patterns that vary at runtime. (To do runtime compilation only once,
748a9306
LW
3042use C</$variable/o>.)
3043
3044As a special case, specifying a PATTERN of space (C<' '>) will split on
3045white space just as split with no arguments does. Thus, split(' ') can
3046be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3047will give you as many null initial fields as there are leading spaces.
3048A split on /\s+/ is like a split(' ') except that any leading
3049whitespace produces a null first field. A split with no arguments
3050really does a C<split(' ', $_)> internally.
a0d0e21e
LW
3051
3052Example:
3053
3054 open(passwd, '/etc/passwd');
3055 while (<passwd>) {
748a9306
LW
3056 ($login, $passwd, $uid, $gid, $gcos,
3057 $home, $shell) = split(/:/);
a0d0e21e
LW
3058 ...
3059 }
3060
3061(Note that $shell above will still have a newline on it. See L</chop>,
3062L</chomp>, and L</join>.)
3063
5f05dabc 3064=item sprintf FORMAT, LIST
a0d0e21e
LW
3065
3066Returns a string formatted by the usual printf conventions of the C
cb1a09d0
AD
3067language. See L<sprintf(3)> or L<printf(3)> on your system for details.
3068(The * character for an indirectly specified length is not
a0d0e21e 3069supported, but you can get the same effect by interpolating a variable
a034a98d
DD
3070into the pattern.) If C<use locale> is
3071in effect, the character used for the decimal point in formatted real numbers
3072is affected by the LC_NUMERIC locale. See L<perllocale>.
3073Some C libraries' implementations of sprintf() can
cb1a09d0 3074dump core when fed ludicrous arguments.
a0d0e21e
LW
3075
3076=item sqrt EXPR
3077
bbce6d69
PP
3078=item sqrt
3079
a0d0e21e
LW
3080Return the square root of EXPR. If EXPR is omitted, returns square
3081root of $_.
3082
3083=item srand EXPR
3084
93dc8474
CS
3085=item srand
3086
3087Sets the random number seed for the C<rand> operator. If EXPR is
3088omitted, uses a semi-random value based on the current time and process
3089ID, among other things. In versions of Perl prior to 5.004 the default
3090seed was just the current time(). This isn't a particularly good seed,
3091so many old programs supply their own seed value (often C<time ^ $$> or
3092C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
3093
3094In fact, it's usually not necessary to call srand() at all, because if
3095it is not called explicitly, it is called implicitly at the first use of
2f9daede
TPG
3096the C<rand> operator. However, this was not the case in version of Perl
3097before 5.004, so if your script will run under older Perl versions, it
3098should call srand().
93dc8474 3099
2f9daede
TPG
3100Note that you need something much more random than the default seed for
3101cryptographic purposes. Checksumming the compressed output of one or more
3102rapidly changing operating system status programs is the usual method. For
3103example:
28757baa
PP
3104
3105 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3106
0078ec44
RS
3107If you're particularly concerned with this, see the Math::TrulyRandom
3108module in CPAN.
3109
3110Do I<not> call srand() multiple times in your program unless you know
28757baa
PP
3111exactly what you're doing and why you're doing it. The point of the
3112function is to "seed" the rand() function so that rand() can produce
3113a different sequence each time you run your program. Just do it once at the
3114top of your program, or you I<won't> get random numbers out of rand()!
3115
3116Frequently called programs (like CGI scripts) that simply use
3117
3118 time ^ $$
3119
3120for a seed can fall prey to the mathematical property that
3121
3122 a^b == (a+1)^(b+1)
3123
0078ec44 3124one-third of the time. So don't do that.
f86702cc 3125
a0d0e21e
LW
3126=item stat FILEHANDLE
3127
3128=item stat EXPR
3129
bbce6d69
PP
3130=item stat
3131
a0d0e21e 3132Returns a 13-element array giving the status info for a file, either the
2f9daede 3133file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
bbce6d69
PP
3134stats $_. Returns a null list if the stat fails. Typically used as
3135follows:
3136
a0d0e21e
LW
3137
3138 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3139 $atime,$mtime,$ctime,$blksize,$blocks)
3140 = stat($filename);
3141
c07a80fd
PP
3142Not all fields are supported on all filesystem types. Here are the
3143meaning of the fields:
3144
3145 dev device number of filesystem
3146 ino inode number
3147 mode file mode (type and permissions)
3148 nlink number of (hard) links to the file
3149 uid numeric user ID of file's owner
5f05dabc 3150 gid numeric group ID of file's owner
c07a80fd
PP
3151 rdev the device identifier (special files only)
3152 size total size of file, in bytes
3153 atime last access time since the epoch
3154 mtime last modify time since the epoch
774d564b 3155 ctime inode change time (NOT creation time!) since the epoch
5f05dabc 3156 blksize preferred block size for file system I/O
c07a80fd
PP
3157 blocks actual number of blocks allocated
3158
3159(The epoch was at 00:00 January 1, 1970 GMT.)
3160
a0d0e21e
LW
3161If stat is passed the special filehandle consisting of an underline, no
3162stat is done, but the current contents of the stat structure from the
3163last stat or filetest are returned. Example:
3164
3165 if (-x $file && (($d) = stat(_)) && $d < 0) {
3166 print "$file is executable NFS file\n";
3167 }
3168
5f05dabc 3169(This works on machines only for which the device number is negative under NFS.)
a0d0e21e
LW
3170
3171=item study SCALAR
3172
3173=item study
3174
184e9718 3175Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
a0d0e21e
LW
3176doing many pattern matches on the string before it is next modified.
3177This may or may not save time, depending on the nature and number of
3178patterns you are searching on, and on the distribution of character
3179frequencies in the string to be searched--you probably want to compare
5f05dabc 3180run times with and without it to see which runs faster. Those loops
a0d0e21e
LW
3181which scan for many short constant strings (including the constant
3182parts of more complex patterns) will benefit most. You may have only
3183one study active at a time--if you study a different scalar the first
3184is "unstudied". (The way study works is this: a linked list of every
3185character in the string to be searched is made, so we know, for
3186example, where all the 'k' characters are. From each search string,
3187the rarest character is selected, based on some static frequency tables
3188constructed from some C programs and English text. Only those places
3189that contain this "rarest" character are examined.)
3190
3191For example, here is a loop which inserts index producing entries
3192before any line containing a certain pattern:
3193
3194 while (<>) {
3195 study;
3196 print ".IX foo\n" if /\bfoo\b/;
3197 print ".IX bar\n" if /\bbar\b/;
3198 print ".IX blurfl\n" if /\bblurfl\b/;
3199 ...
3200 print;
3201 }
3202
3203In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3204will be looked at, because "f" is rarer than "o". In general, this is
3205a big win except in pathological cases. The only question is whether
3206it saves you more time than it took to build the linked list in the
3207first place.
3208
3209Note that if you have to look for strings that you don't know till
3210runtime, you can build an entire loop as a string and eval that to
3211avoid recompiling all your patterns all the time. Together with
3212undefining $/ to input entire files as one record, this can be very
3213fast, often faster than specialized programs like fgrep(1). The following
184e9718 3214scans a list of files (C<@files>) for a list of words (C<@words>), and prints
a0d0e21e
LW
3215out the names of those files that contain a match:
3216
3217 $search = 'while (<>) { study;';
3218 foreach $word (@words) {
3219 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3220 }
3221 $search .= "}";
3222 @ARGV = @files;
3223 undef $/;
3224 eval $search; # this screams
5f05dabc 3225 $/ = "\n"; # put back to normal input delimiter
a0d0e21e
LW
3226 foreach $file (sort keys(%seen)) {
3227 print $file, "\n";
3228 }
3229
cb1a09d0
AD
3230=item sub BLOCK
3231
3232=item sub NAME
3233
3234=item sub NAME BLOCK
3235
3236This is subroutine definition, not a real function I<per se>. With just a
3237NAME (and possibly prototypes), it's just a forward declaration. Without
3238a NAME, it's an anonymous function declaration, and does actually return a
2f9daede 3239value: the CODE ref of the closure you just created. See L<perlsub> and
cb1a09d0
AD
3240L<perlref> for details.
3241
a0d0e21e
LW
3242=item substr EXPR,OFFSET,LEN
3243
3244=item substr EXPR,OFFSET
3245
3246Extracts a substring out of EXPR and returns it. First character is at
2f9daede
TPG
3247offset 0, or whatever you've set C<$[> to (but don't do that).
3248If OFFSET is negative, starts
a0d0e21e 3249that far from the end of the string. If LEN is omitted, returns
748a9306
LW
3250everything to the end of the string. If LEN is negative, leaves that
3251many characters off the end of the string.
3252
3253You can use the substr() function
a0d0e21e
LW
3254as an lvalue, in which case EXPR must be an lvalue. If you assign
3255something shorter than LEN, the string will shrink, and if you assign
3256something longer than LEN, the string will grow to accommodate it. To
3257keep the string the same length you may need to pad or chop your value
3258using sprintf().
3259
3260=item symlink OLDFILE,NEWFILE
3261
3262Creates a new filename symbolically linked to the old filename.
3263Returns 1 for success, 0 otherwise. On systems that don't support
3264symbolic links, produces a fatal error at run time. To check for that,
3265use eval:
3266
3267 $symlink_exists = (eval 'symlink("","");', $@ eq '');
3268
3269=item syscall LIST
3270
3271Calls the system call specified as the first element of the list,
3272passing the remaining elements as arguments to the system call. If
3273unimplemented, produces a fatal error. The arguments are interpreted
3274as follows: if a given argument is numeric, the argument is passed as
3275an int. If not, the pointer to the string value is passed. You are
3276responsible to make sure a string is pre-extended long enough to
3277receive any result that might be written into a string. If your
3278integer arguments are not literals and have never been interpreted in a
3279numeric context, you may need to add 0 to them to force them to look
3280like numbers.
3281
3282 require 'syscall.ph'; # may need to run h2ph
3283 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
3284
5f05dabc 3285Note that Perl supports passing of up to only 14 arguments to your system call,
a0d0e21e
LW
3286which in practice should usually suffice.
3287
c07a80fd
PP
3288=item sysopen FILEHANDLE,FILENAME,MODE
3289
3290=item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3291
3292Opens the file whose filename is given by FILENAME, and associates it
3293with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3294the name of the real filehandle wanted. This function calls the
3295underlying operating system's C<open> function with the parameters
3296FILENAME, MODE, PERMS.
3297
3298The possible values and flag bits of the MODE parameter are
3299system-dependent; they are available via the standard module C<Fcntl>.
3300However, for historical reasons, some values are universal: zero means
3301read-only, one means write-only, and two means read/write.
3302
3303If the file named by FILENAME does not exist and the C<open> call
3304creates it (typically because MODE includes the O_CREAT flag), then
3305the value of PERMS specifies the permissions of the newly created
3306file. If PERMS is omitted, the default value is 0666, which allows
3307read and write for all. This default is reasonable: see C<umask>.
3308
28757baa
PP
3309The IO::File module provides a more object-oriented approach, if you're
3310into that kind of thing.
3311
a0d0e21e
LW
3312=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3313
3314=item sysread FILEHANDLE,SCALAR,LENGTH
3315
3316Attempts to read LENGTH bytes of data into variable SCALAR from the
3317specified FILEHANDLE, using the system call read(2). It bypasses
3318stdio, so mixing this with other kinds of reads may cause confusion.
3319Returns the number of bytes actually read, or undef if there was an
ff68c719
PP
3320error. SCALAR will be grown or shrunk so that the last byte actually
3321read is the last byte of the scalar after the read.
3322
3323An OFFSET may be specified to place the read data at some place in the
3324string other than the beginning. A negative OFFSET specifies
3325placement at that many bytes counting backwards from the end of the
3326string. A positive OFFSET greater than the length of SCALAR results
3327in the string being padded to the required size with "\0" bytes before
3328the result of the read is appended.
a0d0e21e
LW
3329
3330=item system LIST
3331
3332Does exactly the same thing as "exec LIST" except that a fork is done
3333first, and the parent process waits for the child process to complete.
3334Note that argument processing varies depending on the number of
3335arguments. The return value is the exit status of the program as
3336returned by the wait() call. To get the actual exit value divide by
cb1a09d0 3337256. See also L</exec>. This is I<NOT> what you want to use to capture
28757baa
PP
3338the output from a command, for that you should use merely back-ticks or
3339qx//, as described in L<perlop/"`STRING`">.
a0d0e21e 3340
28757baa
PP
3341Because system() and back-ticks block SIGINT and SIGQUIT, killing the
3342program they're running doesn't actually interrupt your program.
3343
3344 @args = ("command", "arg1", "arg2");
3345 system(@args) == 0
3346 or die "system @args failed: $?"
3347
3348Here's a more elaborate example of analysing the return value from
3349system() on a UNIX system to check for all possibilities, including for
3350signals and coredumps.
3351
3352 $rc = 0xffff & system @args;
3353 printf "system(%s) returned %#04x: ", "@args", $rc;
3354 if ($rc == 0) {
3355 print "ran with normal exit\n";
3356 }
3357 elsif ($rc == 0xff00) {
3358 print "command failed: $!\n";
3359 }
3360 elsif ($rc > 0x80) {
3361 $rc >>= 8;
3362 print "ran with non-zero exit status $rc\n";
3363 }
3364 else {
3365 print "ran with ";
3366 if ($rc & 0x80) {
3367 $rc &= ~0x80;
3368 print "coredump from ";
3369 }
3370 print "signal $rc\n"
3371 }
3372 $ok = ($rc != 0);
f86702cc 3373
a0d0e21e
LW
3374=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3375
3376=item syswrite FILEHANDLE,SCALAR,LENGTH
3377
3378Attempts to write LENGTH bytes of data from variable SCALAR to the
3379specified FILEHANDLE, using the system call write(2). It bypasses
3380stdio, so mixing this with prints may cause confusion. Returns the
bbce6d69
PP
3381number of bytes actually written, or undef if there was an error.
3382If the length is greater than the available data, only as much data as
ff68c719
PP
3383is available will be written.
3384
3385An OFFSET may be specified to write the data from some part of the
3386string other than the beginning. A negative OFFSET specifies writing
3387from that many bytes counting backwards from the end of the string.
a0d0e21e
LW
3388
3389=item tell FILEHANDLE
3390
3391=item tell
3392
3393Returns the current file position for FILEHANDLE. FILEHANDLE may be an
3394expression whose value gives the name of the actual filehandle. If
3395FILEHANDLE is omitted, assumes the file last read.
3396
3397=item telldir DIRHANDLE
3398
3399Returns the current position of the readdir() routines on DIRHANDLE.
3400Value may be given to seekdir() to access a particular location in a
3401directory. Has the same caveats about possible directory compaction as
3402the corresponding system library routine.
3403
4633a7c4 3404=item tie VARIABLE,CLASSNAME,LIST
a0d0e21e 3405
4633a7c4
LW
3406This function binds a variable to a package class that will provide the
3407implementation for the variable. VARIABLE is the name of the variable
3408to be enchanted. CLASSNAME is the name of a class implementing objects
3409of correct type. Any additional arguments are passed to the "new"
3410method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3411Typically these are arguments such as might be passed to the dbm_open()
cb1a09d0
AD
3412function of C. The object returned by the "new" method is also
3413returned by the tie() function, which would be useful if you want to
4633a7c4 3414access other methods in CLASSNAME.
a0d0e21e
LW
3415
3416Note that functions such as keys() and values() may return huge array
748a9306
LW
3417values when used on large objects, like DBM files. You may prefer to
3418use the each() function to iterate over such. Example:
a0d0e21e
LW
3419
3420 # print out history file offsets
4633a7c4 3421 use NDBM_File;
da0045b7 3422 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
a0d0e21e
LW
3423 while (($key,$val) = each %HIST) {
3424 print $key, ' = ', unpack('L',$val), "\n";
3425 }
3426 untie(%HIST);
3427
aa689395 3428A class implementing a hash should have the following methods:
a0d0e21e 3429
4633a7c4 3430 TIEHASH classname, LIST
a0d0e21e
LW
3431 DESTROY this
3432 FETCH this, key
3433 STORE this, key, value
3434 DELETE this, key
3435 EXISTS this, key
3436 FIRSTKEY this
3437 NEXTKEY this, lastkey
3438
4633a7c4 3439A class implementing an ordinary array should have the following methods:
a0d0e21e 3440
4633a7c4 3441 TIEARRAY classname, LIST
a0d0e21e
LW
3442 DESTROY this
3443 FETCH this, key
3444 STORE this, key, value
3445 [others TBD]
3446
4633a7c4 3447A class implementing a scalar should have the following methods:
a0d0e21e 3448
4633a7c4 3449 TIESCALAR classname, LIST
a0d0e21e
LW
3450 DESTROY this
3451 FETCH this,
3452 STORE this, value
3453
4633a7c4
LW
3454Unlike dbmopen(), the tie() function will not use or require a module
3455for you--you need to do that explicitly yourself. See L<DB_File>
3456or the F<Config> module for interesting tie() implementations.
3457
f3cbc334
RS
3458=item tied VARIABLE
3459
3460Returns a reference to the object underlying VARIABLE (the same value
3461that was originally returned by the tie() call which bound the variable
3462to a package.) Returns the undefined value if VARIABLE isn't tied to a
3463package.
3464
a0d0e21e
LW
3465=item time
3466
da0045b7
PP
3467Returns the number of non-leap seconds since whatever time the system
3468considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3469and 00:00:00 UTC, January 1, 1970 for most other systems).
3470Suitable for feeding to gmtime() and localtime().
a0d0e21e
LW
3471
3472=item times
3473
3474Returns a four-element array giving the user and system times, in
3475seconds, for this process and the children of this process.
3476
3477 ($user,$system,$cuser,$csystem) = times;
3478
3479=item tr///
3480
3481The translation operator. See L<perlop>.
3482
3483=item truncate FILEHANDLE,LENGTH
3484
3485=item truncate EXPR,LENGTH
3486
3487Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3488specified length. Produces a fatal error if truncate isn't implemented
3489on your system.
3490
3491=item uc EXPR
3492
bbce6d69
PP
3493=item uc
3494
a0d0e21e
LW
3495Returns an uppercased version of EXPR. This is the internal function
3496implementing the \U escape in double-quoted strings.
a034a98d 3497Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 3498
bbce6d69
PP
3499If EXPR is omitted, uses $_.
3500
a0d0e21e
LW
3501=item ucfirst EXPR
3502
bbce6d69
PP
3503=item ucfirst
3504
a0d0e21e
LW
3505Returns the value of EXPR with the first character uppercased. This is
3506the internal function implementing the \u escape in double-quoted strings.
a034a98d 3507Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 3508
bbce6d69
PP
3509If EXPR is omitted, uses $_.
3510
a0d0e21e
LW
3511=item umask EXPR
3512
3513=item umask
3514
2f9daede
TPG
3515Sets the umask for the process to EXPR and returns the previous value.
3516If EXPR is omitted, merely returns the current umask. Remember that a
3517umask is a number, usually given in octal; it is I<not> a string of octal
3518digits. See also L<oct>, if all you have is a string.
a0d0e21e
LW
3519
3520=item undef EXPR
3521
3522=item undef
3523
5f05dabc 3524Undefines the value of EXPR, which must be an lvalue. Use on only a
2f9daede
TPG
3525scalar value, an entire array or hash, or a subroutine name (using
3526"&"). (Using undef() will probably not do what you expect on most
3527predefined variables or DBM list values, so don't do that.) Always
3528returns the undefined value. You can omit the EXPR, in which case
3529nothing is undefined, but you still get an undefined value that you
3530could, for instance, return from a subroutine, assign to a variable or
3531pass as a parameter. Examples:
a0d0e21e
LW
3532
3533 undef $foo;
2f9daede 3534 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
a0d0e21e 3535 undef @ary;
aa689395 3536 undef %hash;
a0d0e21e
LW
3537 undef &mysub;
3538 return (wantarray ? () : undef) if $they_blew_it;
2f9daede
TPG
3539 select undef, undef, undef, 0.25;
3540 ($a, $b, undef, $c) = &foo; # Ignore third value returned
a0d0e21e
LW
3541
3542=item unlink LIST
3543
bbce6d69
PP
3544=item unlink
3545
a0d0e21e
LW
3546Deletes a list of files. Returns the number of files successfully
3547deleted.
3548
3549 $cnt = unlink 'a', 'b', 'c';
3550 unlink @goners;
3551 unlink <*.bak>;
3552
3553Note: unlink will not delete directories unless you are superuser and
3554the B<-U> flag is supplied to Perl. Even if these conditions are
3555met, be warned that unlinking a directory can inflict damage on your
3556filesystem. Use rmdir instead.
3557
bbce6d69
PP
3558If LIST is omitted, uses $_.
3559
a0d0e21e
LW
3560=item unpack TEMPLATE,EXPR
3561
3562Unpack does the reverse of pack: it takes a string representing a
3563structure and expands it out into a list value, returning the array
5f05dabc 3564value. (In a scalar context, it returns merely the first value
a0d0e21e
LW
3565produced.) The TEMPLATE has the same format as in the pack function.
3566Here's a subroutine that does substring:
3567
3568 sub substr {
3569 local($what,$where,$howmuch) = @_;
3570 unpack("x$where a$howmuch", $what);
3571 }
3572
3573and then there's
3574
3575 sub ordinal { unpack("c",$_[0]); } # same as ord()
3576
184e9718
PP
3577In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3578you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
a0d0e21e
LW
3579themselves. Default is a 16-bit checksum. For example, the following
3580computes the same number as the System V sum program:
3581
3582 while (<>) {
3583 $checksum += unpack("%16C*", $_);
3584 }
3585 $checksum %= 65536;
3586
3587The following efficiently counts the number of set bits in a bit vector:
3588
3589 $setbits = unpack("%32b*", $selectmask);
3590
3591=item untie VARIABLE
3592
3593Breaks the binding between a variable and a package. (See tie().)
3594
3595=item unshift ARRAY,LIST
3596
3597Does the opposite of a C<shift>. Or the opposite of a C<push>,
3598depending on how you look at it. Prepends list to the front of the
3599array, and returns the new number of elements in the array.
3600
3601 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3602
3603Note the LIST is prepended whole, not one element at a time, so the
3604prepended elements stay in the same order. Use reverse to do the
3605reverse.
3606
3607=item use Module LIST
3608
3609=item use Module
3610
da0045b7
PP
3611=item use Module VERSION LIST
3612
3613=item use VERSION
3614
a0d0e21e
LW
3615Imports some semantics into the current package from the named module,
3616generally by aliasing certain subroutine or variable names into your
3617package. It is exactly equivalent to
3618
3619 BEGIN { require Module; import Module LIST; }
3620
da0045b7
PP
3621except that Module I<must> be a bare word.
3622
3623If the first argument to C<use> is a number, it is treated as a version
3624number instead of a module name. If the version of the Perl interpreter
3625is less than VERSION, then an error message is printed and Perl exits
3626immediately. This is often useful if you need to check the current
3627Perl version before C<use>ing library modules which have changed in
3628incompatible ways from older versions of Perl. (We try not to do
3629this more than we have to.)
3630
a0d0e21e
LW
3631The BEGIN forces the require and import to happen at compile time. The
3632require makes sure the module is loaded into memory if it hasn't been
3633yet. The import is not a builtin--it's just an ordinary static method
3634call into the "Module" package to tell the module to import the list of
3635features back into the current package. The module can implement its
3636import method any way it likes, though most modules just choose to
3637derive their import method via inheritance from the Exporter class that
2f9daede
TPG
3638is defined in the Exporter module. See L<Exporter>. If no import
3639method can be found then the error is currently silently ignored. This
55497cff 3640may change to a fatal error in a future version.
cb1a09d0
AD
3641
3642If you don't want your namespace altered, explicitly supply an empty list:
3643
3644 use Module ();
3645
3646That is exactly equivalent to
3647
3648 BEGIN { require Module; }
a0d0e21e 3649
da0045b7 3650If the VERSION argument is present between Module and LIST, then the
71be2cbc
PP
3651C<use> will call the VERSION method in class Module with the given
3652version as an argument. The default VERSION method, inherited from
3653the Universal class, croaks if the given version is larger than the
3654value of the variable $Module::VERSION. (Note that there is not a
3655comma after VERSION!)
da0045b7 3656
a0d0e21e
LW
3657Because this is a wide-open interface, pragmas (compiler directives)
3658are also implemented this way. Currently implemented pragmas are:
3659
3660 use integer;
4633a7c4 3661 use diagnostics;
a0d0e21e
LW
3662 use sigtrap qw(SEGV BUS);
3663 use strict qw(subs vars refs);
3664 use subs qw(afunc blurfl);
3665
5f05dabc 3666These pseudo-modules import semantics into the current block scope, unlike
a0d0e21e
LW
3667ordinary modules, which import symbols into the current package (which are
3668effective through the end of the file).
3669
3670There's a corresponding "no" command that unimports meanings imported
5f05dabc 3671by use, i.e., it calls C<unimport Module LIST> instead of C<import>.
a0d0e21e
LW
3672
3673 no integer;
3674 no strict 'refs';
3675
55497cff
PP
3676If no unimport method can be found the call fails with a fatal error.
3677
a0d0e21e
LW
3678See L<perlmod> for a list of standard modules and pragmas.
3679
3680=item utime LIST
3681
3682Changes the access and modification times on each file of a list of
3683files. The first two elements of the list must be the NUMERICAL access
3684and modification times, in that order. Returns the number of files
3685successfully changed. The inode modification time of each file is set
3686to the current time. Example of a "touch" command:
3687
3688 #!/usr/bin/perl
3689 $now = time;
3690 utime $now, $now, @ARGV;
3691
aa689395 3692=item values HASH
a0d0e21e 3693
aa689395
PP
3694Returns a normal array consisting of all the values of the named hash.
3695(In a scalar context, returns the number of values.) The values are
3696returned in an apparently random order, but it is the same order as either
3697the keys() or each() function would produce on the same hash. As a side
3698effect, it resets HASH's iterator. See also keys(), each(), and sort().
a0d0e21e
LW
3699
3700=item vec EXPR,OFFSET,BITS
3701
22dc801b 3702Treats the string in EXPR as a vector of unsigned integers, and
5f05dabc 3703returns the value of the bit field specified by OFFSET. BITS specifies
22dc801b 3704the number of bits that are reserved for each entry in the bit
2f9daede 3705vector. This must be a power of two from 1 to 32. vec() may also be
5f05dabc 3706assigned to, in which case parentheses are needed to give the expression
22dc801b
PP
3707the correct precedence as in
3708
3709 vec($image, $max_x * $x + $y, 8) = 3;
a0d0e21e
LW
3710
3711Vectors created with vec() can also be manipulated with the logical
5f05dabc 3712operators |, &, and ^, which will assume a bit vector operation is
a0d0e21e
LW
3713desired when both operands are strings.
3714
3715To transform a bit vector into a string or array of 0's and 1's, use these:
3716
3717 $bits = unpack("b*", $vector);
3718 @bits = split(//, unpack("b*", $vector));
3719
3720If you know the exact length in bits, it can be used in place of the *.
3721
3722=item wait
3723
3724Waits for a child process to terminate and returns the pid of the
3725deceased process, or -1 if there are no child processes. The status is
184e9718 3726returned in C<$?>.
a0d0e21e
LW
3727
3728=item waitpid PID,FLAGS
3729
3730Waits for a particular child process to terminate and returns the pid
3731of the deceased process, or -1 if there is no such child process. The
184e9718 3732status is returned in C<$?>. If you say
a0d0e21e 3733
5f05dabc 3734 use POSIX ":sys_wait_h";
a0d0e21e
LW
3735 ...
3736 waitpid(-1,&WNOHANG);
3737
3738then you can do a non-blocking wait for any process. Non-blocking wait
5f05dabc 3739is available on machines supporting either the waitpid(2) or
a0d0e21e
LW
3740wait4(2) system calls. However, waiting for a particular pid with
3741FLAGS of 0 is implemented everywhere. (Perl emulates the system call