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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
17be first, and the list argument will follow. (Note that there can only
18ever be one list argument.) For instance, splice() has three scalar
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
31parens.) If you use the parens, the simple (but occasionally
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
38 print 1+2+3; # Prints 6.
39 print(1+2) + 3; # Prints 3.
40 print (1+2)+3; # Also prints 3!
41 print +(1+2)+3; # Prints 6.
42 print ((1+2)+3); # Prints 6.
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
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cb1a09d0 59=item
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60
61I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
62
63=back
64
65Each operator and function decides which sort of value it would be most
66appropriate to return in a scalar context. Some operators return the
67length of the list that would have been returned in a list context. Some
68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
72
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73=head2 Perl Functions by Category
74
75Here are Perl's functions (including things that look like
76functions, like some of the keywords and named operators)
77arranged by category. Some functions appear in more
78than one place.
79
80=over
81
82=item Functions for SCALARs or strings
83
84chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
85oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
86sprintf, substr, tr///, uc, ucfirst, y///
87
88=item Regular expressions and pattern matching
89
90m//, pos, quotemeta, s///, split, study
91
92=item Numeric functions
93
94abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
95srand
96
97=item Functions for real @ARRAYs
98
99pop, push, shift, splice, unshift
100
101=item Functions for list data
102
103grep, join, map, qw/STRING/, reverse, sort, unpack
104
105=item Functions for real %HASHes
106
107delete, each, exists, keys, values
108
109=item Input and output functions
110
111binmode, close, closedir, dbmclose, dbmopen, die, eof,
112fileno, flock, format, getc, print, printf, read, readdir,
113rewinddir, seek, seekdir, select, syscall, sysread,
114syswrite, tell, telldir, truncate, warn, write
115
116=item Functions for fixed length data or records
117
118pack, read, syscall, sysread, syswrite, unpack, vec
119
120=item Functions for filehandles, files, or directories
121
da0045b7 122I<-X>, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
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123lstat, mkdir, open, opendir, readlink, rename, rmdir,
124stat, symlink, umask, unlink, utime
125
126=item Keywords related to the control flow of your perl program
127
128caller, continue, die, do, dump, eval, exit, goto, last,
129next, redo, return, sub, wantarray
130
131=item Keywords related to scoping
132
133caller, import, local, my, package, use
134
135=item Miscellaneous functions
136
137defined, dump, eval, formline, local, my, reset, scalar,
138undef, wantarray
139
140=item Functions for processes and process groups
141
142alarm, exec, fork, getpgrp, getppid, getpriority, kill,
143pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
144times, wait, waitpid
145
146=item Keywords related to perl modules
147
148do, import, no, package, require, use
149
150=item Keywords related to classes and object-orientedness
151
f3cbc334 152bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use
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153
154=item Low-level socket functions
155
156accept, bind, connect, getpeername, getsockname,
157getsockopt, listen, recv, send, setsockopt, shutdown,
158socket, socketpair
159
160=item System V interprocess communication functions
161
162msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
163shmctl, shmget, shmread, shmwrite
164
165=item Fetching user and group info
166
167endgrent, endhostent, endnetent, endpwent, getgrent,
168getgrgid, getgrnam, getlogin, getpwent, getpwnam,
169getpwuid, setgrent, setpwent
170
171=item Fetching network info
172
173endprotoent, endservent, gethostbyaddr, gethostbyname,
174gethostent, getnetbyaddr, getnetbyname, getnetent,
175getprotobyname, getprotobynumber, getprotoent,
176getservbyname, getservbyport, getservent, sethostent,
177setnetent, setprotoent, setservent
178
179=item Time-related functions
180
181gmtime, localtime, time, times
182
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183=item Functions new in perl5
184
185abs, bless, chomp, chr, exists, formline, glob, import, lc,
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186lcfirst, map, my, no, prototype, qx, qw, readline, readpipe,
187ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use
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188
189* - C<sub> was a keyword in perl4, but in perl5 it is an
190operator which can be used in expressions.
191
192=item Functions obsoleted in perl5
193
194dbmclose, dbmopen
195
196
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197=back
198
199=head2 Alphabetical Listing of Perl Functions
200
201
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202=over 8
203
204=item -X FILEHANDLE
205
206=item -X EXPR
207
208=item -X
209
210A file test, where X is one of the letters listed below. This unary
211operator takes one argument, either a filename or a filehandle, and
212tests the associated file to see if something is true about it. If the
213argument is omitted, tests $_, except for C<-t>, which tests STDIN.
214Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
215the undefined value if the file doesn't exist. Despite the funny
216names, precedence is the same as any other named unary operator, and
217the argument may be parenthesized like any other unary operator. The
218operator may be any of:
219
220 -r File is readable by effective uid/gid.
221 -w File is writable by effective uid/gid.
222 -x File is executable by effective uid/gid.
223 -o File is owned by effective uid.
224
225 -R File is readable by real uid/gid.
226 -W File is writable by real uid/gid.
227 -X File is executable by real uid/gid.
228 -O File is owned by real uid.
229
230 -e File exists.
231 -z File has zero size.
232 -s File has non-zero size (returns size).
233
234 -f File is a plain file.
235 -d File is a directory.
236 -l File is a symbolic link.
237 -p File is a named pipe (FIFO).
238 -S File is a socket.
239 -b File is a block special file.
240 -c File is a character special file.
241 -t Filehandle is opened to a tty.
242
243 -u File has setuid bit set.
244 -g File has setgid bit set.
245 -k File has sticky bit set.
246
247 -T File is a text file.
248 -B File is a binary file (opposite of -T).
249
250 -M Age of file in days when script started.
251 -A Same for access time.
252 -C Same for inode change time.
253
254The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
255C<-W>, C<-x> and C<-X> is based solely on the mode of the file and the
256uids and gids of the user. There may be other reasons you can't actually
257read, write or execute the file. Also note that, for the superuser,
258C<-r>, C<-R>, C<-w> and C<-W> always return 1, and C<-x> and C<-X> return
2591 if any execute bit is set in the mode. Scripts run by the superuser may
260thus need to do a stat() in order to determine the actual mode of the
261file, or temporarily set the uid to something else.
262
263Example:
264
265 while (<>) {
266 chop;
267 next unless -f $_; # ignore specials
268 ...
269 }
270
271Note that C<-s/a/b/> does not do a negated substitution. Saying
272C<-exp($foo)> still works as expected, however--only single letters
273following a minus are interpreted as file tests.
274
275The C<-T> and C<-B> switches work as follows. The first block or so of the
276file is examined for odd characters such as strange control codes or
277characters with the high bit set. If too many odd characters (>30%)
278are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
279containing null in the first block is considered a binary file. If C<-T>
280or C<-B> is used on a filehandle, the current stdio buffer is examined
281rather than the first block. Both C<-T> and C<-B> return TRUE on a null
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282file, or a file at EOF when testing a filehandle. Because you have to
283read a file to do the C<-T> test, on most occasions you want to use a C<-f>
284against the file first, as in C<next unless -f $file && -T $file>.
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285
286If any of the file tests (or either the stat() or lstat() operators) are given the
287special filehandle consisting of a solitary underline, then the stat
288structure of the previous file test (or stat operator) is used, saving
289a system call. (This doesn't work with C<-t>, and you need to remember
290that lstat() and C<-l> will leave values in the stat structure for the
291symbolic link, not the real file.) Example:
292
293 print "Can do.\n" if -r $a || -w _ || -x _;
294
295 stat($filename);
296 print "Readable\n" if -r _;
297 print "Writable\n" if -w _;
298 print "Executable\n" if -x _;
299 print "Setuid\n" if -u _;
300 print "Setgid\n" if -g _;
301 print "Sticky\n" if -k _;
302 print "Text\n" if -T _;
303 print "Binary\n" if -B _;
304
305=item abs VALUE
306
307Returns the absolute value of its argument.
308
309=item accept NEWSOCKET,GENERICSOCKET
310
311Accepts an incoming socket connect, just as the accept(2) system call
312does. Returns the packed address if it succeeded, FALSE otherwise.
4633a7c4 313See example in L<perlipc/"Sockets: Client/Server Communication">.
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314
315=item alarm SECONDS
316
317Arranges to have a SIGALRM delivered to this process after the
318specified number of seconds have elapsed. (On some machines,
319unfortunately, the elapsed time may be up to one second less than you
320specified because of how seconds are counted.) Only one timer may be
321counting at once. Each call disables the previous timer, and an
322argument of 0 may be supplied to cancel the previous timer without
323starting a new one. The returned value is the amount of time remaining
324on the previous timer.
325
4633a7c4 326For delays of finer granularity than one second, you may use Perl's
a0d0e21e 327syscall() interface to access setitimer(2) if your system supports it,
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328or else see L</select()> below. It is not advised to intermix alarm()
329and sleep() calls.
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330
331=item atan2 Y,X
332
333Returns the arctangent of Y/X in the range -PI to PI.
334
335=item bind SOCKET,NAME
336
337Binds a network address to a socket, just as the bind system call
338does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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339packed address of the appropriate type for the socket. See the examples in
340L<perlipc/"Sockets: Client/Server Communication">.
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341
342=item binmode FILEHANDLE
343
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344Arranges for the file to be read or written in "binary" mode in operating
345systems that distinguish between binary and text files. Files that are
346not in binary mode have CR LF sequences translated to LF on input and LF
347translated to CR LF on output. Binmode has no effect under Unix; in DOS
348and similarly archaic systems, it may be imperative--otherwise your
349DOS-damaged C library may mangle your file. The key distinction between
350systems that need binmode and those that don't is their text file
351formats. Systems like Unix and Plan9 that delimit lines with a single
352character, and that encode that character in C as '\n', do not need
353C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
354is taken as the name of the filehandle.
a0d0e21e 355
4633a7c4 356=item bless REF,CLASSNAME
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357
358=item bless REF
359
360This function tells the referenced object (passed as REF) that it is now
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361an object in the CLASSNAME package--or the current package if no CLASSNAME
362is specified, which is often the case. It returns the reference for
363convenience, since a bless() is often the last thing in a constructor.
364Always use the two-argument version if the function doing the blessing
365might be inherited by a derived class. See L<perlobj> for more about the
366blessing (and blessings) of objects.
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367
368=item caller EXPR
369
370=item caller
371
372Returns the context of the current subroutine call. In a scalar context,
373returns TRUE if there is a caller, that is, if we're in a subroutine or
374eval() or require(), and FALSE otherwise. In a list context, returns
375
748a9306 376 ($package, $filename, $line) = caller;
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377
378With EXPR, it returns some extra information that the debugger uses to
379print a stack trace. The value of EXPR indicates how many call frames
380to go back before the current one.
381
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382 ($package, $filename, $line,
383 $subroutine, $hasargs, $wantargs) = caller($i);
384
385Furthermore, when called from within the DB package, caller returns more
4633a7c4 386detailed information: it sets the list variable @DB::args to be the
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387arguments with which that subroutine was invoked.
388
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389=item chdir EXPR
390
391Changes the working directory to EXPR, if possible. If EXPR is
392omitted, changes to home directory. Returns TRUE upon success, FALSE
393otherwise. See example under die().
394
395=item chmod LIST
396
397Changes the permissions of a list of files. The first element of the
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398list must be the numerical mode, which should probably be an octal
399number. Returns the number of files successfully changed.
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400
401 $cnt = chmod 0755, 'foo', 'bar';
402 chmod 0755, @executables;
403
404=item chomp VARIABLE
405
406=item chomp LIST
407
408=item chomp
409
410This is a slightly safer version of chop (see below). It removes any
411line ending that corresponds to the current value of C<$/> (also known as
412$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the number
413of characters removed. It's often used to remove the newline from the
414end of an input record when you're worried that the final record may be
415missing its newline. When in paragraph mode (C<$/ = "">), it removes all
416trailing newlines from the string. If VARIABLE is omitted, it chomps
417$_. Example:
418
419 while (<>) {
420 chomp; # avoid \n on last field
421 @array = split(/:/);
422 ...
423 }
424
425You can actually chomp anything that's an lvalue, including an assignment:
426
427 chomp($cwd = `pwd`);
428 chomp($answer = <STDIN>);
429
430If you chomp a list, each element is chomped, and the total number of
431characters removed is returned.
432
433=item chop VARIABLE
434
435=item chop LIST
436
437=item chop
438
439Chops off the last character of a string and returns the character
440chopped. It's used primarily to remove the newline from the end of an
441input record, but is much more efficient than C<s/\n//> because it neither
442scans nor copies the string. If VARIABLE is omitted, chops $_.
443Example:
444
445 while (<>) {
446 chop; # avoid \n on last field
447 @array = split(/:/);
448 ...
449 }
450
451You can actually chop anything that's an lvalue, including an assignment:
452
453 chop($cwd = `pwd`);
454 chop($answer = <STDIN>);
455
456If you chop a list, each element is chopped. Only the value of the
457last chop is returned.
458
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459Note that chop returns the last character. To return all but the last
460character, use C<substr($string, 0, -1)>.
461
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462=item chown LIST
463
464Changes the owner (and group) of a list of files. The first two
465elements of the list must be the I<NUMERICAL> uid and gid, in that order.
466Returns the number of files successfully changed.
467
468 $cnt = chown $uid, $gid, 'foo', 'bar';
469 chown $uid, $gid, @filenames;
470
471Here's an example that looks up non-numeric uids in the passwd file:
472
473 print "User: ";
474 chop($user = <STDIN>);
475 print "Files: "
476 chop($pattern = <STDIN>);
477
478 ($login,$pass,$uid,$gid) = getpwnam($user)
479 or die "$user not in passwd file";
480
481 @ary = <${pattern}>; # expand filenames
482 chown $uid, $gid, @ary;
483
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484On most systems, you are not allowed to change the ownership of the
485file unless you're the superuser, although you should be able to change
486the group to any of your secondary groups. On insecure systems, these
487restrictions may be relaxed, but this is not a portable assumption.
488
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489=item chr NUMBER
490
491Returns the character represented by that NUMBER in the character set.
492For example, C<chr(65)> is "A" in ASCII.
493
494=item chroot FILENAME
495
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496This function works as the system call by the same name: it makes the
497named directory the new root directory for all further pathnames that
498begin with a "/" by your process and all of its children. (It doesn't
499change your current working directory is unaffected.) For security
500reasons, this call is restricted to the superuser. If FILENAME is
501omitted, does chroot to $_.
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502
503=item close FILEHANDLE
504
505Closes the file or pipe associated with the file handle, returning TRUE
506only if stdio successfully flushes buffers and closes the system file
507descriptor. You don't have to close FILEHANDLE if you are immediately
4633a7c4 508going to do another open() on it, since open() will close it for you. (See
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509open().) However, an explicit close on an input file resets the line
510counter ($.), while the implicit close done by open() does not. Also,
511closing a pipe will wait for the process executing on the pipe to
512complete, in case you want to look at the output of the pipe
513afterwards. Closing a pipe explicitly also puts the status value of
514the command into C<$?>. Example:
515
516 open(OUTPUT, '|sort >foo'); # pipe to sort
517 ... # print stuff to output
518 close OUTPUT; # wait for sort to finish
519 open(INPUT, 'foo'); # get sort's results
520
521FILEHANDLE may be an expression whose value gives the real filehandle name.
522
523=item closedir DIRHANDLE
524
525Closes a directory opened by opendir().
526
527=item connect SOCKET,NAME
528
529Attempts to connect to a remote socket, just as the connect system call
530does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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531packed address of the appropriate type for the socket. See the examples in
532L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 533
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534=item continue BLOCK
535
536Actually a flow control statement rather than a function. If there is a
537C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
538C<foreach>), it is always executed just before the conditional is about to
539be evaluated again, just like the third part of a C<for> loop in C. Thus
540it can be used to increment a loop variable, even when the loop has been
541continued via the C<next> statement (which is similar to the C C<continue>
542statement).
543
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544=item cos EXPR
545
546Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
547takes cosine of $_.
548
549=item crypt PLAINTEXT,SALT
550
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551Encrypts a string exactly like the crypt(3) function in the C library
552(assuming that you actually have a version there that has not been
553extirpated as a potential munition). This can prove useful for checking
554the password file for lousy passwords, amongst other things. Only the
555guys wearing white hats should do this.
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556
557Here's an example that makes sure that whoever runs this program knows
558their own password:
559
560 $pwd = (getpwuid($<))[1];
561 $salt = substr($pwd, 0, 2);
562
563 system "stty -echo";
564 print "Password: ";
565 chop($word = <STDIN>);
566 print "\n";
567 system "stty echo";
568
569 if (crypt($word, $salt) ne $pwd) {
570 die "Sorry...\n";
571 } else {
572 print "ok\n";
573 }
574
575Of course, typing in your own password to whoever asks you
748a9306 576for it is unwise.
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577
578=item dbmclose ASSOC_ARRAY
579
580[This function has been superseded by the untie() function.]
581
582Breaks the binding between a DBM file and an associative array.
583
584=item dbmopen ASSOC,DBNAME,MODE
585
586[This function has been superseded by the tie() function.]
587
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588This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an
589associative array. ASSOC is the name of the associative array. (Unlike
590normal open, the first argument is I<NOT> a filehandle, even though it
591looks like one). DBNAME is the name of the database (without the F<.dir>
592or F<.pag> extension if any). If the database does not exist, it is
593created with protection specified by MODE (as modified by the umask()).
594If your system only supports the older DBM functions, you may perform only
595one dbmopen() in your program. In older versions of Perl, if your system
596had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now
597falls back to sdbm(3).
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598
599If you don't have write access to the DBM file, you can only read
600associative array variables, not set them. If you want to test whether
601you can write, either use file tests or try setting a dummy array entry
602inside an eval(), which will trap the error.
603
604Note that functions such as keys() and values() may return huge array
605values when used on large DBM files. You may prefer to use the each()
606function to iterate over large DBM files. Example:
607
608 # print out history file offsets
609 dbmopen(%HIST,'/usr/lib/news/history',0666);
610 while (($key,$val) = each %HIST) {
611 print $key, ' = ', unpack('L',$val), "\n";
612 }
613 dbmclose(%HIST);
614
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615See also L<AnyDBM_File> for a more general description of the pros and
616cons of the various dbm apparoches, as well as L<DB_File> for a particularly
617rich implementation.
4633a7c4 618
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619=item defined EXPR
620
cb1a09d0 621Returns a boolean value saying whether EXPR has a real value
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622or not. Many operations return the undefined value under exceptional
623conditions, such as end of file, uninitialized variable, system error
624and such. This function allows you to distinguish between an undefined
625null scalar and a defined null scalar with operations that might return
626a real null string, such as referencing elements of an array. You may
627also check to see if arrays or subroutines exist. Use of defined on
628predefined variables is not guaranteed to produce intuitive results.
629
630When used on a hash array element, it tells you whether the value
631is defined, not whether the key exists in the hash. Use exists() for that.
632
633Examples:
634
635 print if defined $switch{'D'};
636 print "$val\n" while defined($val = pop(@ary));
637 die "Can't readlink $sym: $!"
638 unless defined($value = readlink $sym);
639 eval '@foo = ()' if defined(@foo);
640 die "No XYZ package defined" unless defined %_XYZ;
641 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
642
643See also undef().
644
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645Note: many folks tend to overuse defined(), and then are surprised to
646discover that the number 0 and the null string are, in fact, defined
647concepts. For example, if you say
648
649 "ab" =~ /a(.*)b/;
650
651the pattern match succeeds, and $1 is defined, despite the fact that it
652matched "nothing". But it didn't really match nothing--rather, it
653matched something that happened to be 0 characters long. This is all
654very above-board and honest. When a function returns an undefined value,
655it's an admission that it couldn't give you an honest answer. So
656you should only use defined() when you're questioning the integrity
657of what you're trying to do. At other times, a simple comparison to
6580 or "" is what you want.
659
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660=item delete EXPR
661
662Deletes the specified value from its hash array. Returns the deleted
663value, or the undefined value if nothing was deleted. Deleting from
664C<$ENV{}> modifies the environment. Deleting from an array tied to a DBM
665file deletes the entry from the DBM file. (But deleting from a tie()d
666hash doesn't necessarily return anything.)
667
668The following deletes all the values of an associative array:
669
670 foreach $key (keys %ARRAY) {
671 delete $ARRAY{$key};
672 }
673
674(But it would be faster to use the undef() command.) Note that the
675EXPR can be arbitrarily complicated as long as the final operation is
676a hash key lookup:
677
678 delete $ref->[$x][$y]{$key};
679
680=item die LIST
681
682Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
4633a7c4 683the current value of $! (errno). If $! is 0, exits with the value of
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684C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)> is 0,
685exits with 255. Inside an eval(), the error message is stuffed into C<$@>,
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686and the eval() is terminated with the undefined value; this makes die()
687the way to raise an exception.
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688
689Equivalent examples:
690
691 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
692 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
693
694If the value of EXPR does not end in a newline, the current script line
695number and input line number (if any) are also printed, and a newline
696is supplied. Hint: sometimes appending ", stopped" to your message
697will cause it to make better sense when the string "at foo line 123" is
698appended. Suppose you are running script "canasta".
699
700 die "/etc/games is no good";
701 die "/etc/games is no good, stopped";
702
703produce, respectively
704
705 /etc/games is no good at canasta line 123.
706 /etc/games is no good, stopped at canasta line 123.
707
708See also exit() and warn().
709
710=item do BLOCK
711
712Not really a function. Returns the value of the last command in the
713sequence of commands indicated by BLOCK. When modified by a loop
714modifier, executes the BLOCK once before testing the loop condition.
715(On other statements the loop modifiers test the conditional first.)
716
717=item do SUBROUTINE(LIST)
718
719A deprecated form of subroutine call. See L<perlsub>.
720
721=item do EXPR
722
723Uses the value of EXPR as a filename and executes the contents of the
724file as a Perl script. Its primary use is to include subroutines
725from a Perl subroutine library.
726
727 do 'stat.pl';
728
729is just like
730
731 eval `cat stat.pl`;
732
733except that it's more efficient, more concise, keeps track of the
734current filename for error messages, and searches all the B<-I>
735libraries if the file isn't in the current directory (see also the @INC
736array in L<perlvar/Predefined Names>). It's the same, however, in that it does
737reparse the file every time you call it, so you probably don't want to
738do this inside a loop.
739
740Note that inclusion of library modules is better done with the
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741use() and require() operators, which also do error checking
742and raise an exception if there's a problem.
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743
744=item dump LABEL
745
746This causes an immediate core dump. Primarily this is so that you can
747use the B<undump> program to turn your core dump into an executable binary
748after having initialized all your variables at the beginning of the
749program. When the new binary is executed it will begin by executing a
750C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
751it as a goto with an intervening core dump and reincarnation. If LABEL
752is omitted, restarts the program from the top. WARNING: any files
753opened at the time of the dump will NOT be open any more when the
754program is reincarnated, with possible resulting confusion on the part
755of Perl. See also B<-u> option in L<perlrun>.
756
757Example:
758
759 #!/usr/bin/perl
760 require 'getopt.pl';
761 require 'stat.pl';
762 %days = (
763 'Sun' => 1,
764 'Mon' => 2,
765 'Tue' => 3,
766 'Wed' => 4,
767 'Thu' => 5,
768 'Fri' => 6,
769 'Sat' => 7,
770 );
771
772 dump QUICKSTART if $ARGV[0] eq '-d';
773
774 QUICKSTART:
775 Getopt('f');
776
777=item each ASSOC_ARRAY
778
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779When called in a list context, returns a 2-element array consisting
780of the key and value for the next element of an associative array,
781so that you can iterate over it. When called in a scalar context,
782returns the key only for the next element in the associative array.
a0d0e21e 783Entries are returned in an apparently random order. When the array is
da0045b7
PP
784entirely read, a null array is returned in list context (which when
785assigned produces a FALSE (0) value), and C<undef> is returned in a
786scalar context. The next call to each() after that will start
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787iterating again. The iterator can be reset only by reading all the
788elements from the array. You should not add elements to an array while
789you're iterating over it. There is a single iterator for each
790associative array, shared by all each(), keys() and values() function
791calls in the program. The following prints out your environment like
792the printenv(1) program, only in a different order:
793
794 while (($key,$value) = each %ENV) {
795 print "$key=$value\n";
796 }
797
798See also keys() and values().
799
800=item eof FILEHANDLE
801
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802=item eof ()
803
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804=item eof
805
806Returns 1 if the next read on FILEHANDLE will return end of file, or if
807FILEHANDLE is not open. FILEHANDLE may be an expression whose value
808gives the real filehandle name. (Note that this function actually
809reads a character and then ungetc()s it, so it is not very useful in an
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810interactive context.) Do not read from a terminal file (or call
811C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
812as terminals may lose the end-of-file condition if you do.
813
814An C<eof> without an argument uses the last file read as argument.
815Empty parentheses () may be used to indicate
4633a7c4 816the pseudofile formed of the files listed on the command line, i.e.
37798a01 817C<eof()> is reasonable to use inside a while (E<lt>E<gt>) loop to detect the end
a0d0e21e 818of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
37798a01 819test I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 820
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821 # reset line numbering on each input file
822 while (<>) {
823 print "$.\t$_";
824 close(ARGV) if (eof); # Not eof().
825 }
826
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827 # insert dashes just before last line of last file
828 while (<>) {
829 if (eof()) {
830 print "--------------\n";
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831 close(ARGV); # close or break; is needed if we
832 # are reading from the terminal
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833 }
834 print;
835 }
836
a0d0e21e 837Practical hint: you almost never need to use C<eof> in Perl, because the
37798a01 838input operators return undef when they run out of data.
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839
840=item eval EXPR
841
842=item eval BLOCK
843
844EXPR is parsed and executed as if it were a little Perl program. It
845is executed in the context of the current Perl program, so that any
846variable settings, subroutine or format definitions remain afterwards.
847The value returned is the value of the last expression evaluated, or a
848return statement may be used, just as with subroutines.
849
850If there is a syntax error or runtime error, or a die() statement is
851executed, an undefined value is returned by eval(), and C<$@> is set to the
852error message. If there was no error, C<$@> is guaranteed to be a null
853string. If EXPR is omitted, evaluates $_. The final semicolon, if
854any, may be omitted from the expression.
855
856Note that, since eval() traps otherwise-fatal errors, it is useful for
4633a7c4 857determining whether a particular feature (such as socket() or symlink())
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858is implemented. It is also Perl's exception trapping mechanism, where
859the die operator is used to raise exceptions.
860
861If the code to be executed doesn't vary, you may use the eval-BLOCK
862form to trap run-time errors without incurring the penalty of
863recompiling each time. The error, if any, is still returned in C<$@>.
864Examples:
865
866 # make divide-by-zero non-fatal
867 eval { $answer = $a / $b; }; warn $@ if $@;
868
869 # same thing, but less efficient
870 eval '$answer = $a / $b'; warn $@ if $@;
871
872 # a compile-time error
873 eval { $answer = };
874
875 # a run-time error
876 eval '$answer ='; # sets $@
877
878With an eval(), you should be especially careful to remember what's
879being looked at when:
880
881 eval $x; # CASE 1
882 eval "$x"; # CASE 2
883
884 eval '$x'; # CASE 3
885 eval { $x }; # CASE 4
886
887 eval "\$$x++" # CASE 5
888 $$x++; # CASE 6
889
890Cases 1 and 2 above behave identically: they run the code contained in the
891variable $x. (Although case 2 has misleading double quotes making the
892reader wonder what else might be happening (nothing is).) Cases 3 and 4
893likewise behave in the same way: they run the code <$x>, which does
894nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
895is a place where normally you I<WOULD> like to use double quotes, except
cb1a09d0 896that in that particular situation, you can just use symbolic references
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897instead, as in case 6.
898
899=item exec LIST
900
901The exec() function executes a system command I<AND NEVER RETURNS>. Use
902the system() function if you want it to return.
903
904If there is more than one argument in LIST, or if LIST is an array with
905more than one value, calls execvp(3) with the arguments in LIST. If
906there is only one scalar argument, the argument is checked for shell
907metacharacters. If there are any, the entire argument is passed to
908C</bin/sh -c> for parsing. If there are none, the argument is split
909into words and passed directly to execvp(), which is more efficient.
37798a01 910Note: exec() and system() do not flush your output buffer, so you may
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911need to set C<$|> to avoid lost output. Examples:
912
913 exec '/bin/echo', 'Your arguments are: ', @ARGV;
914 exec "sort $outfile | uniq";
915
916If you don't really want to execute the first argument, but want to lie
917to the program you are executing about its own name, you can specify
918the program you actually want to run as an "indirect object" (without a
919comma) in front of the LIST. (This always forces interpretation of the
920LIST as a multi-valued list, even if there is only a single scalar in
921the list.) Example:
922
923 $shell = '/bin/csh';
924 exec $shell '-sh'; # pretend it's a login shell
925
926or, more directly,
927
928 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
929
930=item exists EXPR
931
932Returns TRUE if the specified hash key exists in its hash array, even
933if the corresponding value is undefined.
934
935 print "Exists\n" if exists $array{$key};
936 print "Defined\n" if defined $array{$key};
937 print "True\n" if $array{$key};
938
939A hash element can only be TRUE if it's defined, and defined if
940it exists, but the reverse doesn't necessarily hold true.
941
942Note that the EXPR can be arbitrarily complicated as long as the final
943operation is a hash key lookup:
944
945 if (exists $ref->[$x][$y]{$key}) { ... }
946
947=item exit EXPR
948
949Evaluates EXPR and exits immediately with that value. (Actually, it
950calls any defined C<END> routines first, but the C<END> routines may not
951abort the exit. Likewise any object destructors that need to be called
952are called before exit.) Example:
953
954 $ans = <STDIN>;
955 exit 0 if $ans =~ /^[Xx]/;
956
957See also die(). If EXPR is omitted, exits with 0 status.
958
959=item exp EXPR
960
961Returns I<e> (the natural logarithm base) to the power of EXPR.
962If EXPR is omitted, gives C<exp($_)>.
963
964=item fcntl FILEHANDLE,FUNCTION,SCALAR
965
966Implements the fcntl(2) function. You'll probably have to say
967
968 use Fcntl;
969
970first to get the correct function definitions. Argument processing and
971value return works just like ioctl() below. Note that fcntl() will produce
972a fatal error if used on a machine that doesn't implement fcntl(2).
973For example:
974
975 use Fcntl;
976 fcntl($filehandle, F_GETLK, $packed_return_buffer);
977
978=item fileno FILEHANDLE
979
980Returns the file descriptor for a filehandle. This is useful for
981constructing bitmaps for select(). If FILEHANDLE is an expression, the
982value is taken as the name of the filehandle.
983
984=item flock FILEHANDLE,OPERATION
985
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986Calls flock(2) on FILEHANDLE. See L<flock(2)> for definition of
987OPERATION. Returns TRUE for success, FALSE on failure. Will produce a
988fatal error if used on a machine that doesn't implement either flock(2) or
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989fcntl(2). The fcntl(2) system call will be automatically used if flock(2)
990is missing from your system. This makes flock() the portable file locking
991strategy, although it will only lock entire files, not records. Note also
992that some versions of flock() cannot lock things over the network; you
993would need to use the more system-specific fcntl() for that.
4633a7c4
LW
994
995Here's a mailbox appender for BSD systems.
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LW
996
997 $LOCK_SH = 1;
998 $LOCK_EX = 2;
999 $LOCK_NB = 4;
1000 $LOCK_UN = 8;
1001
1002 sub lock {
1003 flock(MBOX,$LOCK_EX);
1004 # and, in case someone appended
1005 # while we were waiting...
1006 seek(MBOX, 0, 2);
1007 }
1008
1009 sub unlock {
1010 flock(MBOX,$LOCK_UN);
1011 }
1012
1013 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1014 or die "Can't open mailbox: $!";
1015
1016 lock();
1017 print MBOX $msg,"\n\n";
1018 unlock();
1019
cb1a09d0 1020See also L<DB_File> for other flock() examples.
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1021
1022=item fork
1023
1024Does a fork(2) system call. Returns the child pid to the parent process
4633a7c4 1025and 0 to the child process, or C<undef> if the fork is unsuccessful.
a0d0e21e
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1026Note: unflushed buffers remain unflushed in both processes, which means
1027you may need to set C<$|> ($AUTOFLUSH in English) or call the
1028autoflush() FileHandle method to avoid duplicate output.
1029
1030If you fork() without ever waiting on your children, you will accumulate
1031zombies:
1032
4633a7c4 1033 $SIG{CHLD} = sub { wait };
a0d0e21e
LW
1034
1035There's also the double-fork trick (error checking on
1036fork() returns omitted);
1037
1038 unless ($pid = fork) {
1039 unless (fork) {
1040 exec "what you really wanna do";
1041 die "no exec";
1042 # ... or ...
4633a7c4 1043 ## (some_perl_code_here)
a0d0e21e
LW
1044 exit 0;
1045 }
1046 exit 0;
1047 }
1048 waitpid($pid,0);
1049
cb1a09d0
AD
1050See also L<perlipc> for more examples of forking and reaping
1051moribund children.
1052
1053=item format
1054
1055Declare a picture format with use by the write() function. For
1056example:
1057
1058 format Something =
1059 Test: @<<<<<<<< @||||| @>>>>>
1060 $str, $%, '$' . int($num)
1061 .
1062
1063 $str = "widget";
1064 $num = $cost/$quantiy;
1065 $~ = 'Something';
1066 write;
1067
1068See L<perlform> for many details and examples.
1069
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1070
1071=item formline PICTURE, LIST
1072
4633a7c4 1073This is an internal function used by C<format>s, though you may call it
a0d0e21e
LW
1074too. It formats (see L<perlform>) a list of values according to the
1075contents of PICTURE, placing the output into the format output
4633a7c4
LW
1076accumulator, C<$^A> (or $ACCUMULATOR in English).
1077Eventually, when a write() is done, the contents of
a0d0e21e
LW
1078C<$^A> are written to some filehandle, but you could also read C<$^A>
1079yourself and then set C<$^A> back to "". Note that a format typically
1080does one formline() per line of form, but the formline() function itself
748a9306 1081doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1082that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
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1083You may therefore need to use multiple formlines to implement a single
1084record format, just like the format compiler.
1085
1086Be careful if you put double quotes around the picture, since an "C<@>"
1087character may be taken to mean the beginning of an array name.
4633a7c4 1088formline() always returns TRUE. See L<perlform> for other examples.
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1089
1090=item getc FILEHANDLE
1091
1092=item getc
1093
1094Returns the next character from the input file attached to FILEHANDLE,
1095or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
4633a7c4 1096This is not particularly efficient. It cannot be used to get unbuffered
cb1a09d0 1097single-characters, however. For that, try something more like:
4633a7c4
LW
1098
1099 if ($BSD_STYLE) {
1100 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1101 }
1102 else {
cb1a09d0 1103 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1104 }
1105
1106 $key = getc(STDIN);
1107
1108 if ($BSD_STYLE) {
1109 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1110 }
1111 else {
cb1a09d0 1112 system "stty", 'icanon', 'eol', '^@'; # ascii null
4633a7c4
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1113 }
1114 print "\n";
1115
1116Determination of whether to whether $BSD_STYLE should be set
cb1a09d0
AD
1117is left as an exercise to the reader.
1118
1119See also the C<Term::ReadKey> module from your nearest CPAN site;
1120details on CPAN can be found on L<perlmod/CPAN>
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1121
1122=item getlogin
1123
1124Returns the current login from F</etc/utmp>, if any. If null, use
4633a7c4 1125getpwuid().
a0d0e21e
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1126
1127 $login = getlogin || (getpwuid($<))[0] || "Kilroy";
1128
da0045b7 1129Do not consider getlogin() for authentication: it is not as
4633a7c4
LW
1130secure as getpwuid().
1131
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1132=item getpeername SOCKET
1133
1134Returns the packed sockaddr address of other end of the SOCKET connection.
1135
4633a7c4
LW
1136 use Socket;
1137 $hersockaddr = getpeername(SOCK);
1138 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1139 $herhostname = gethostbyaddr($iaddr, AF_INET);
1140 $herstraddr = inet_ntoa($iaddr);
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1141
1142=item getpgrp PID
1143
47e29363
PP
1144Returns the current process group for the specified PID. Use
1145a PID of 0 to get the current process group for the
4633a7c4 1146current process. Will raise an exception if used on a machine that
a0d0e21e 1147doesn't implement getpgrp(2). If PID is omitted, returns process
47e29363
PP
1148group of current process. Note that the POSIX version of getpgrp()
1149does not accept a PID argument, so only PID==0 is truly portable.
a0d0e21e
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1150
1151=item getppid
1152
1153Returns the process id of the parent process.
1154
1155=item getpriority WHICH,WHO
1156
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LW
1157Returns the current priority for a process, a process group, or a user.
1158(See L<getpriority(2)>.) Will raise a fatal exception if used on a
a0d0e21e
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1159machine that doesn't implement getpriority(2).
1160
1161=item getpwnam NAME
1162
1163=item getgrnam NAME
1164
1165=item gethostbyname NAME
1166
1167=item getnetbyname NAME
1168
1169=item getprotobyname NAME
1170
1171=item getpwuid UID
1172
1173=item getgrgid GID
1174
1175=item getservbyname NAME,PROTO
1176
1177=item gethostbyaddr ADDR,ADDRTYPE
1178
1179=item getnetbyaddr ADDR,ADDRTYPE
1180
1181=item getprotobynumber NUMBER
1182
1183=item getservbyport PORT,PROTO
1184
1185=item getpwent
1186
1187=item getgrent
1188
1189=item gethostent
1190
1191=item getnetent
1192
1193=item getprotoent
1194
1195=item getservent
1196
1197=item setpwent
1198
1199=item setgrent
1200
1201=item sethostent STAYOPEN
1202
1203=item setnetent STAYOPEN
1204
1205=item setprotoent STAYOPEN
1206
1207=item setservent STAYOPEN
1208
1209=item endpwent
1210
1211=item endgrent
1212
1213=item endhostent
1214
1215=item endnetent
1216
1217=item endprotoent
1218
1219=item endservent
1220
1221These routines perform the same functions as their counterparts in the
1222system library. Within a list context, the return values from the
1223various get routines are as follows:
1224
1225 ($name,$passwd,$uid,$gid,
1226 $quota,$comment,$gcos,$dir,$shell) = getpw*
1227 ($name,$passwd,$gid,$members) = getgr*
1228 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1229 ($name,$aliases,$addrtype,$net) = getnet*
1230 ($name,$aliases,$proto) = getproto*
1231 ($name,$aliases,$port,$proto) = getserv*
1232
1233(If the entry doesn't exist you get a null list.)
1234
1235Within a scalar context, you get the name, unless the function was a
1236lookup by name, in which case you get the other thing, whatever it is.
1237(If the entry doesn't exist you get the undefined value.) For example:
1238
1239 $uid = getpwnam
1240 $name = getpwuid
1241 $name = getpwent
1242 $gid = getgrnam
1243 $name = getgrgid
1244 $name = getgrent
1245 etc.
1246
1247The $members value returned by I<getgr*()> is a space separated list of
1248the login names of the members of the group.
1249
1250For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1251C, it will be returned to you via C<$?> if the function call fails. The
1252@addrs value returned by a successful call is a list of the raw
1253addresses returned by the corresponding system library call. In the
1254Internet domain, each address is four bytes long and you can unpack it
1255by saying something like:
1256
1257 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1258
1259=item getsockname SOCKET
1260
1261Returns the packed sockaddr address of this end of the SOCKET connection.
1262
4633a7c4
LW
1263 use Socket;
1264 $mysockaddr = getsockname(SOCK);
1265 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1266
1267=item getsockopt SOCKET,LEVEL,OPTNAME
1268
1269Returns the socket option requested, or undefined if there is an error.
1270
1271=item glob EXPR
1272
1273Returns the value of EXPR with filename expansions such as a shell
1274would do. This is the internal function implementing the <*.*>
4633a7c4 1275operator, except it's easier to use.
a0d0e21e
LW
1276
1277=item gmtime EXPR
1278
1279Converts a time as returned by the time function to a 9-element array
4633a7c4
LW
1280with the time localized for the standard Greenwich timezone.
1281Typically used as follows:
a0d0e21e
LW
1282
1283
1284 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1285 gmtime(time);
1286
1287All array elements are numeric, and come straight out of a struct tm.
1288In particular this means that $mon has the range 0..11 and $wday has
1289the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
1290
1291=item goto LABEL
1292
748a9306
LW
1293=item goto EXPR
1294
a0d0e21e
LW
1295=item goto &NAME
1296
1297The goto-LABEL form finds the statement labeled with LABEL and resumes
1298execution there. It may not be used to go into any construct that
1299requires initialization, such as a subroutine or a foreach loop. It
1300also can't be used to go into a construct that is optimized away. It
1301can be used to go almost anywhere else within the dynamic scope,
1302including out of subroutines, but it's usually better to use some other
1303construct such as last or die. The author of Perl has never felt the
1304need to use this form of goto (in Perl, that is--C is another matter).
1305
748a9306
LW
1306The goto-EXPR form expects a label name, whose scope will be resolved
1307dynamically. This allows for computed gotos per FORTRAN, but isn't
1308necessarily recommended if you're optimizing for maintainability:
1309
1310 goto ("FOO", "BAR", "GLARCH")[$i];
1311
a0d0e21e
LW
1312The goto-&NAME form is highly magical, and substitutes a call to the
1313named subroutine for the currently running subroutine. This is used by
1314AUTOLOAD subroutines that wish to load another subroutine and then
1315pretend that the other subroutine had been called in the first place
1316(except that any modifications to @_ in the current subroutine are
1317propagated to the other subroutine.) After the goto, not even caller()
1318will be able to tell that this routine was called first.
1319
1320=item grep BLOCK LIST
1321
1322=item grep EXPR,LIST
1323
1324Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1325$_ to each element) and returns the list value consisting of those
1326elements for which the expression evaluated to TRUE. In a scalar
1327context, returns the number of times the expression was TRUE.
1328
1329 @foo = grep(!/^#/, @bar); # weed out comments
1330
1331or equivalently,
1332
1333 @foo = grep {!/^#/} @bar; # weed out comments
1334
1335Note that, since $_ is a reference into the list value, it can be used
1336to modify the elements of the array. While this is useful and
1337supported, it can cause bizarre results if the LIST is not a named
1338array.
1339
1340=item hex EXPR
1341
4633a7c4
LW
1342Interprets EXPR as a hex string and returns the corresponding decimal
1343value. (To convert strings that might start with 0 or 0x see
1344oct().) If EXPR is omitted, uses $_.
a0d0e21e
LW
1345
1346=item import
1347
1348There is no built-in import() function. It is merely an ordinary
4633a7c4 1349method (subroutine) defined (or inherited) by modules that wish to export
a0d0e21e 1350names to another module. The use() function calls the import() method
4633a7c4 1351for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1352
1353=item index STR,SUBSTR,POSITION
1354
1355=item index STR,SUBSTR
1356
4633a7c4
LW
1357Returns the position of the first occurrence of SUBSTR in STR at or after
1358POSITION. If POSITION is omitted, starts searching from the beginning of
1359the string. The return value is based at 0 (or whatever you've set the $[
1360variable to--but don't do that). If the substring is not found, returns
a0d0e21e
LW
1361one less than the base, ordinarily -1.
1362
1363=item int EXPR
1364
1365Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1366
1367=item ioctl FILEHANDLE,FUNCTION,SCALAR
1368
1369Implements the ioctl(2) function. You'll probably have to say
1370
4633a7c4 1371 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1372
4633a7c4 1373first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1374exist or doesn't have the correct definitions you'll have to roll your
4633a7c4
LW
1375own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1376(There is a Perl script called B<h2ph> that comes with the Perl kit which
1377may help you in this, but it's non-trivial.) SCALAR will be read and/or
1378written depending on the FUNCTION--a pointer to the string value of SCALAR
1379will be passed as the third argument of the actual ioctl call. (If SCALAR
1380has no string value but does have a numeric value, that value will be
1381passed rather than a pointer to the string value. To guarantee this to be
1382TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1383functions are useful for manipulating the values of structures used by
1384ioctl(). The following example sets the erase character to DEL.
a0d0e21e
LW
1385
1386 require 'ioctl.ph';
4633a7c4
LW
1387 $getp = &TIOCGETP;
1388 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1389 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1390 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1391 @ary = unpack($sgttyb_t,$sgttyb);
1392 $ary[2] = 127;
1393 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1394 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1395 || die "Can't ioctl: $!";
1396 }
1397
1398The return value of ioctl (and fcntl) is as follows:
1399
1400 if OS returns: then Perl returns:
1401 -1 undefined value
1402 0 string "0 but true"
1403 anything else that number
1404
1405Thus Perl returns TRUE on success and FALSE on failure, yet you can
1406still easily determine the actual value returned by the operating
1407system:
1408
1409 ($retval = ioctl(...)) || ($retval = -1);
1410 printf "System returned %d\n", $retval;
1411
1412=item join EXPR,LIST
1413
1414Joins the separate strings of LIST or ARRAY into a single string with
1415fields separated by the value of EXPR, and returns the string.
1416Example:
1417
1418 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1419
1420See L<perlfunc/split>.
1421
1422=item keys ASSOC_ARRAY
1423
1424Returns a normal array consisting of all the keys of the named
1425associative array. (In a scalar context, returns the number of keys.)
1426The keys are returned in an apparently random order, but it is the same
1427order as either the values() or each() function produces (given that
1428the associative array has not been modified). Here is yet another way
1429to print your environment:
1430
1431 @keys = keys %ENV;
1432 @values = values %ENV;
1433 while ($#keys >= 0) {
1434 print pop(@keys), '=', pop(@values), "\n";
1435 }
1436
1437or how about sorted by key:
1438
1439 foreach $key (sort(keys %ENV)) {
1440 print $key, '=', $ENV{$key}, "\n";
1441 }
1442
4633a7c4 1443To sort an array by value, you'll need to use a C<sort{}>
cb1a09d0 1444function. Here's a descending numeric sort of a hash by its values:
4633a7c4
LW
1445
1446 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1447 printf "%4d %s\n", $hash{$key}, $key;
1448 }
1449
a0d0e21e
LW
1450=item kill LIST
1451
4633a7c4
LW
1452Sends a signal to a list of processes. The first element of
1453the list must be the signal to send. Returns the number of
1454processes successfully signaled.
a0d0e21e
LW
1455
1456 $cnt = kill 1, $child1, $child2;
1457 kill 9, @goners;
1458
4633a7c4
LW
1459Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1460process groups instead of processes. (On System V, a negative I<PROCESS>
1461number will also kill process groups, but that's not portable.) That
1462means you usually want to use positive not negative signals. You may also
da0045b7 1463use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
1464
1465=item last LABEL
1466
1467=item last
1468
1469The C<last> command is like the C<break> statement in C (as used in
1470loops); it immediately exits the loop in question. If the LABEL is
1471omitted, the command refers to the innermost enclosing loop. The
1472C<continue> block, if any, is not executed:
1473
4633a7c4
LW
1474 LINE: while (<STDIN>) {
1475 last LINE if /^$/; # exit when done with header
a0d0e21e
LW
1476 ...
1477 }
1478
1479=item lc EXPR
1480
1481Returns an lowercased version of EXPR. This is the internal function
4633a7c4
LW
1482implementing the \L escape in double-quoted strings.
1483Should respect any POSIX setlocale() settings.
a0d0e21e
LW
1484
1485=item lcfirst EXPR
1486
1487Returns the value of EXPR with the first character lowercased. This is
1488the internal function implementing the \l escape in double-quoted strings.
4633a7c4 1489Should respect any POSIX setlocale() settings.
a0d0e21e
LW
1490
1491=item length EXPR
1492
1493Returns the length in characters of the value of EXPR. If EXPR is
1494omitted, returns length of $_.
1495
1496=item link OLDFILE,NEWFILE
1497
1498Creates a new filename linked to the old filename. Returns 1 for
1499success, 0 otherwise.
1500
1501=item listen SOCKET,QUEUESIZE
1502
1503Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 1504it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
1505
1506=item local EXPR
1507
a0d0e21e 1508A local modifies the listed variables to be local to the enclosing block,
cb1a09d0
AD
1509subroutine, C<eval{}> or C<do>. If more than one value is listed, the
1510list must be placed in parens. See L<perlsub/"Temporary Values via
1511local()"> for details.
a0d0e21e 1512
cb1a09d0
AD
1513But you really probably want to be using my() instead, because local() isn't
1514what most people think of as "local"). See L<perlsub/"Private Variables
1515via my()"> for details.
a0d0e21e
LW
1516
1517=item localtime EXPR
1518
1519Converts a time as returned by the time function to a 9-element array
1520with the time analyzed for the local timezone. Typically used as
1521follows:
1522
1523 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1524 localtime(time);
1525
1526All array elements are numeric, and come straight out of a struct tm.
1527In particular this means that $mon has the range 0..11 and $wday has
1528the range 0..6. If EXPR is omitted, does localtime(time).
1529
1530In a scalar context, prints out the ctime(3) value:
1531
1532 $now_string = localtime; # e.g. "Thu Oct 13 04:54:34 1994"
1533
37798a01 1534Also see the F<timelocal.pl> library, and the strftime(3) function available
da0045b7 1535via the POSIX module.
a0d0e21e
LW
1536
1537=item log EXPR
1538
1539Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1540of $_.
1541
1542=item lstat FILEHANDLE
1543
1544=item lstat EXPR
1545
1546Does the same thing as the stat() function, but stats a symbolic link
1547instead of the file the symbolic link points to. If symbolic links are
1548unimplemented on your system, a normal stat() is done.
1549
1550=item m//
1551
1552The match operator. See L<perlop>.
1553
1554=item map BLOCK LIST
1555
1556=item map EXPR,LIST
1557
1558Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1559element) and returns the list value composed of the results of each such
1560evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1561may produce zero, one, or more elements in the returned value.
1562
1563 @chars = map(chr, @nums);
1564
1565translates a list of numbers to the corresponding characters. And
1566
4633a7c4 1567 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
1568
1569is just a funny way to write
1570
1571 %hash = ();
1572 foreach $_ (@array) {
4633a7c4 1573 $hash{getkey($_)} = $_;
a0d0e21e
LW
1574 }
1575
1576=item mkdir FILENAME,MODE
1577
1578Creates the directory specified by FILENAME, with permissions specified
1579by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1580it returns 0 and sets $! (errno).
1581
1582=item msgctl ID,CMD,ARG
1583
4633a7c4 1584Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
a0d0e21e
LW
1585must be a variable which will hold the returned msqid_ds structure.
1586Returns like ioctl: the undefined value for error, "0 but true" for
1587zero, or the actual return value otherwise.
1588
1589=item msgget KEY,FLAGS
1590
4633a7c4 1591Calls the System V IPC function msgget(2). Returns the message queue id,
a0d0e21e
LW
1592or the undefined value if there is an error.
1593
1594=item msgsnd ID,MSG,FLAGS
1595
1596Calls the System V IPC function msgsnd to send the message MSG to the
1597message queue ID. MSG must begin with the long integer message type,
c07a80fd 1598which may be created with C<pack("l", $type)>. Returns TRUE if
a0d0e21e
LW
1599successful, or FALSE if there is an error.
1600
1601=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1602
1603Calls the System V IPC function msgrcv to receive a message from
1604message queue ID into variable VAR with a maximum message size of
1605SIZE. Note that if a message is received, the message type will be the
1606first thing in VAR, and the maximum length of VAR is SIZE plus the size
1607of the message type. Returns TRUE if successful, or FALSE if there is
1608an error.
1609
1610=item my EXPR
1611
1612A "my" declares the listed variables to be local (lexically) to the
cb1a09d0
AD
1613enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
1614more than one value is listed, the list must be placed in parens. See
1615L<perlsub/"Private Variables via my()"> for details.
4633a7c4 1616
a0d0e21e
LW
1617=item next LABEL
1618
1619=item next
1620
1621The C<next> command is like the C<continue> statement in C; it starts
1622the next iteration of the loop:
1623
4633a7c4
LW
1624 LINE: while (<STDIN>) {
1625 next LINE if /^#/; # discard comments
a0d0e21e
LW
1626 ...
1627 }
1628
1629Note that if there were a C<continue> block on the above, it would get
1630executed even on discarded lines. If the LABEL is omitted, the command
1631refers to the innermost enclosing loop.
1632
1633=item no Module LIST
1634
1635See the "use" function, which "no" is the opposite of.
1636
1637=item oct EXPR
1638
4633a7c4
LW
1639Interprets EXPR as an octal string and returns the corresponding
1640decimal value. (If EXPR happens to start off with 0x, interprets it as
1641a hex string instead.) The following will handle decimal, octal, and
1642hex in the standard Perl or C notation:
a0d0e21e
LW
1643
1644 $val = oct($val) if $val =~ /^0/;
1645
1646If EXPR is omitted, uses $_.
1647
1648=item open FILEHANDLE,EXPR
1649
1650=item open FILEHANDLE
1651
1652Opens the file whose filename is given by EXPR, and associates it with
4633a7c4
LW
1653FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name
1654of the real filehandle wanted. If EXPR is omitted, the scalar variable of
1655the same name as the FILEHANDLE contains the filename. If the filename
1656begins with "<" or nothing, the file is opened for input. If the filename
1657begins with ">", the file is opened for output. If the filename begins
cb1a09d0 1658with ">>", the file is opened for appending. You can put a '+' in front
4633a7c4 1659of the '>' or '<' to indicate that you want both read and write access to
cb1a09d0
AD
1660the file; thus '+<' is usually preferred for read/write updates--the '+>'
1661mode would clobber the file first. These correspond to the fopen(3) modes
1662of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.
1663
1664If the filename begins with "|", the filename is interpreted
4633a7c4
LW
1665as a command to which output is to be piped, and if the filename ends with
1666a "|", the filename is interpreted See L<perlipc/"Using open() for IPC">
1667for more examples of this. as command which pipes input to us. (You may
da0045b7 1668not have a raw open() to a command that pipes both in I<and> out, but see L<open2>,
4633a7c4 1669L<open3>, and L<perlipc/"Bidirectional Communication"> for alternatives.)
cb1a09d0 1670
4633a7c4
LW
1671Opening '-' opens STDIN and opening '>-' opens STDOUT. Open returns
1672non-zero upon success, the undefined value otherwise. If the open
1673involved a pipe, the return value happens to be the pid of the
cb1a09d0
AD
1674subprocess.
1675
1676If you're unfortunate enough to be running Perl on a system that
1677distinguishes between text files and binary files (modern operating
1678systems don't care), then you should check out L</binmode> for tips for
1679dealing with this. The key distinction between systems that need binmode
1680and those that don't is their text file formats. Systems like Unix and
1681Plan9 that delimit lines with a single character, and that encode that
1682character in C as '\n', do not need C<binmode>. The rest need it.
1683
cb1a09d0 1684Examples:
a0d0e21e
LW
1685
1686 $ARTICLE = 100;
1687 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1688 while (<ARTICLE>) {...
1689
1690 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1691
cb1a09d0
AD
1692 open(DBASE, '+<dbase.mine'); # open for update
1693
4633a7c4 1694 open(ARTICLE, "caesar <$article |"); # decrypt article
a0d0e21e 1695
4633a7c4 1696 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
a0d0e21e
LW
1697
1698 # process argument list of files along with any includes
1699
1700 foreach $file (@ARGV) {
1701 process($file, 'fh00');
1702 }
1703
1704 sub process {
1705 local($filename, $input) = @_;
1706 $input++; # this is a string increment
1707 unless (open($input, $filename)) {
1708 print STDERR "Can't open $filename: $!\n";
1709 return;
1710 }
1711
1712 while (<$input>) { # note use of indirection
1713 if (/^#include "(.*)"/) {
1714 process($1, $input);
1715 next;
1716 }
1717 ... # whatever
1718 }
1719 }
1720
1721You may also, in the Bourne shell tradition, specify an EXPR beginning
1722with ">&", in which case the rest of the string is interpreted as the
1723name of a filehandle (or file descriptor, if numeric) which is to be
1724duped and opened. You may use & after >, >>, <, +>, +>> and +<. The
1725mode you specify should match the mode of the original filehandle.
cb1a09d0
AD
1726(Duping a filehandle does not take into acount any existing contents of
1727stdio buffers.)
a0d0e21e
LW
1728Here is a script that saves, redirects, and restores STDOUT and
1729STDERR:
1730
1731 #!/usr/bin/perl
1732 open(SAVEOUT, ">&STDOUT");
1733 open(SAVEERR, ">&STDERR");
1734
1735 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1736 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1737
1738 select(STDERR); $| = 1; # make unbuffered
1739 select(STDOUT); $| = 1; # make unbuffered
1740
1741 print STDOUT "stdout 1\n"; # this works for
1742 print STDERR "stderr 1\n"; # subprocesses too
1743
1744 close(STDOUT);
1745 close(STDERR);
1746
1747 open(STDOUT, ">&SAVEOUT");
1748 open(STDERR, ">&SAVEERR");
1749
1750 print STDOUT "stdout 2\n";
1751 print STDERR "stderr 2\n";
1752
1753
1754If you specify "<&=N", where N is a number, then Perl will do an
4633a7c4
LW
1755equivalent of C's fdopen() of that file descriptor; this is more
1756parsimonious of file descriptors. For example:
a0d0e21e
LW
1757
1758 open(FILEHANDLE, "<&=$fd")
1759
1760If you open a pipe on the command "-", i.e. either "|-" or "-|", then
1761there is an implicit fork done, and the return value of open is the pid
1762of the child within the parent process, and 0 within the child
1763process. (Use defined($pid) to determine whether the open was successful.)
1764The filehandle behaves normally for the parent, but i/o to that
1765filehandle is piped from/to the STDOUT/STDIN of the child process.
1766In the child process the filehandle isn't opened--i/o happens from/to
1767the new STDOUT or STDIN. Typically this is used like the normal
1768piped open when you want to exercise more control over just how the
1769pipe command gets executed, such as when you are running setuid, and
4633a7c4
LW
1770don't want to have to scan shell commands for metacharacters.
1771The following pairs are more or less equivalent:
a0d0e21e
LW
1772
1773 open(FOO, "|tr '[a-z]' '[A-Z]'");
1774 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1775
1776 open(FOO, "cat -n '$file'|");
1777 open(FOO, "-|") || exec 'cat', '-n', $file;
1778
4633a7c4
LW
1779See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1780
a0d0e21e
LW
1781Explicitly closing any piped filehandle causes the parent process to
1782wait for the child to finish, and returns the status value in $?.
1783Note: on any operation which may do a fork, unflushed buffers remain
1784unflushed in both processes, which means you may need to set $| to
1785avoid duplicate output.
1786
c07a80fd
PP
1787Using the FileHandle constructor from the FileHandle package,
1788you can generate anonymous filehandles which have the scope of whatever
1789variables hold references to them, and automatically close whenever
1790and however you leave that scope:
1791
1792 use FileHandle;
1793 ...
1794 sub read_myfile_munged {
1795 my $ALL = shift;
1796 my $handle = new FileHandle;
1797 open($handle, "myfile") or die "myfile: $!";
1798 $first = <$handle>
1799 or return (); # Automatically closed here.
1800 mung $first or die "mung failed"; # Or here.
1801 return $first, <$handle> if $ALL; # Or here.
1802 $first; # Or here.
1803 }
1804
a0d0e21e
LW
1805The filename that is passed to open will have leading and trailing
1806whitespace deleted. In order to open a file with arbitrary weird
1807characters in it, it's necessary to protect any leading and trailing
1808whitespace thusly:
1809
cb1a09d0
AD
1810 $file =~ s#^(\s)#./$1#;
1811 open(FOO, "< $file\0");
1812
c07a80fd
PP
1813If you want a "real" C open() (see L<open(2)> on your system), then
1814you should use the sysopen() function. This is another way to
1815protect your filenames from interpretation. For example:
cb1a09d0
AD
1816
1817 use FileHandle;
c07a80fd
PP
1818 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
1819 or die "sysopen $path: $!";
1820 HANDLE->autoflush(1);
1821 HANDLE->print("stuff $$\n");
1822 seek(HANDLE, 0, 0);
1823 print "File contains: ", <HANDLE>;
cb1a09d0
AD
1824
1825See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
1826
1827=item opendir DIRHANDLE,EXPR
1828
1829Opens a directory named EXPR for processing by readdir(), telldir(),
1830seekdir(), rewinddir() and closedir(). Returns TRUE if successful.
1831DIRHANDLEs have their own namespace separate from FILEHANDLEs.
1832
1833=item ord EXPR
1834
1835Returns the numeric ascii value of the first character of EXPR. If
1836EXPR is omitted, uses $_.
1837
1838=item pack TEMPLATE,LIST
1839
1840Takes an array or list of values and packs it into a binary structure,
1841returning the string containing the structure. The TEMPLATE is a
1842sequence of characters that give the order and type of values, as
1843follows:
1844
1845 A An ascii string, will be space padded.
1846 a An ascii string, will be null padded.
1847 b A bit string (ascending bit order, like vec()).
1848 B A bit string (descending bit order).
1849 h A hex string (low nybble first).
1850 H A hex string (high nybble first).
1851
1852 c A signed char value.
1853 C An unsigned char value.
1854 s A signed short value.
1855 S An unsigned short value.
1856 i A signed integer value.
1857 I An unsigned integer value.
1858 l A signed long value.
1859 L An unsigned long value.
1860
1861 n A short in "network" order.
1862 N A long in "network" order.
1863 v A short in "VAX" (little-endian) order.
1864 V A long in "VAX" (little-endian) order.
1865
1866 f A single-precision float in the native format.
1867 d A double-precision float in the native format.
1868
1869 p A pointer to a null-terminated string.
1870 P A pointer to a structure (fixed-length string).
1871
1872 u A uuencoded string.
1873
1874 x A null byte.
1875 X Back up a byte.
1876 @ Null fill to absolute position.
1877
1878Each letter may optionally be followed by a number which gives a repeat
1879count. With all types except "a", "A", "b", "B", "h" and "H", and "P" the
1880pack function will gobble up that many values from the LIST. A * for the
1881repeat count means to use however many items are left. The "a" and "A"
1882types gobble just one value, but pack it as a string of length count,
1883padding with nulls or spaces as necessary. (When unpacking, "A" strips
1884trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
1885fields pack a string that many bits long. The "h" and "H" fields pack a
1886string that many nybbles long. The "P" packs a pointer to a structure of
1887the size indicated by the length. Real numbers (floats and doubles) are
1888in the native machine format only; due to the multiplicity of floating
1889formats around, and the lack of a standard "network" representation, no
1890facility for interchange has been made. This means that packed floating
1891point data written on one machine may not be readable on another - even if
1892both use IEEE floating point arithmetic (as the endian-ness of the memory
1893representation is not part of the IEEE spec). Note that Perl uses doubles
1894internally for all numeric calculation, and converting from double into
1895float and thence back to double again will lose precision (i.e.
1896C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
1897
1898Examples:
1899
1900 $foo = pack("cccc",65,66,67,68);
1901 # foo eq "ABCD"
1902 $foo = pack("c4",65,66,67,68);
1903 # same thing
1904
1905 $foo = pack("ccxxcc",65,66,67,68);
1906 # foo eq "AB\0\0CD"
1907
1908 $foo = pack("s2",1,2);
1909 # "\1\0\2\0" on little-endian
1910 # "\0\1\0\2" on big-endian
1911
1912 $foo = pack("a4","abcd","x","y","z");
1913 # "abcd"
1914
1915 $foo = pack("aaaa","abcd","x","y","z");
1916 # "axyz"
1917
1918 $foo = pack("a14","abcdefg");
1919 # "abcdefg\0\0\0\0\0\0\0"
1920
1921 $foo = pack("i9pl", gmtime);
1922 # a real struct tm (on my system anyway)
1923
1924 sub bintodec {
1925 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
1926 }
1927
1928The same template may generally also be used in the unpack function.
1929
cb1a09d0
AD
1930=item package NAMESPACE
1931
1932Declares the compilation unit as being in the given namespace. The scope
1933of the package declaration is from the declaration itself through the end of
1934the enclosing block (the same scope as the local() operator). All further
1935unqualified dynamic identifiers will be in this namespace. A package
1936statement only affects dynamic variables--including those you've used
1937local() on--but I<not> lexical variables created with my(). Typically it
1938would be the first declaration in a file to be included by the C<require>
1939or C<use> operator. You can switch into a package in more than one place;
1940it merely influences which symbol table is used by the compiler for the
1941rest of that block. You can refer to variables and filehandles in other
1942packages by prefixing the identifier with the package name and a double
1943colon: C<$Package::Variable>. If the package name is null, the C<main>
1944package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
1945
1946See L<perlmod/"Packages"> for more information about packages, modules,
1947and classes. See L<perlsub> for other scoping issues.
1948
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LW
1949=item pipe READHANDLE,WRITEHANDLE
1950
1951Opens a pair of connected pipes like the corresponding system call.
1952Note that if you set up a loop of piped processes, deadlock can occur
1953unless you are very careful. In addition, note that Perl's pipes use
1954stdio buffering, so you may need to set $| to flush your WRITEHANDLE
1955after each command, depending on the application.
1956
4633a7c4
LW
1957See L<open2>, L<open3>, and L<perlipc/"Bidirectional Communication">
1958for examples of such things.
1959
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1960=item pop ARRAY
1961
1962Pops and returns the last value of the array, shortening the array by
19631. Has a similar effect to
1964
1965 $tmp = $ARRAY[$#ARRAY--];
1966
1967If there are no elements in the array, returns the undefined value.
cb1a09d0
AD
1968If ARRAY is omitted, pops the
1969@ARGV array in the main program, and the @_ array in subroutines, just
1970like shift().
a0d0e21e
LW
1971
1972=item pos SCALAR
1973
4633a7c4 1974Returns the offset of where the last C<m//g> search left off for the variable
a0d0e21e
LW
1975in question. May be modified to change that offset.
1976
1977=item print FILEHANDLE LIST
1978
1979=item print LIST
1980
1981=item print
1982
cb1a09d0 1983Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 1984if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 1985the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
1986level of indirection. (NOTE: If FILEHANDLE is a variable and the next
1987token is a term, it may be misinterpreted as an operator unless you
1988interpose a + or put parens around the arguments.) If FILEHANDLE is
1989omitted, prints by default to standard output (or to the last selected
da0045b7 1990output channel--see L</select>). If LIST is also omitted, prints $_ to
a0d0e21e
LW
1991STDOUT. To set the default output channel to something other than
1992STDOUT use the select operation. Note that, because print takes a
1993LIST, anything in the LIST is evaluated in a list context, and any
1994subroutine that you call will have one or more of its expressions
1995evaluated in a list context. Also be careful not to follow the print
1996keyword with a left parenthesis unless you want the corresponding right
1997parenthesis to terminate the arguments to the print--interpose a + or
1998put parens around all the arguments.
1999
4633a7c4 2000Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2001you will have to use a block returning its value instead:
4633a7c4
LW
2002
2003 print { $files[$i] } "stuff\n";
2004 print { $OK ? STDOUT : STDERR } "stuff\n";
2005
a0d0e21e
LW
2006=item printf FILEHANDLE LIST
2007
2008=item printf LIST
2009
2010Equivalent to a "print FILEHANDLE sprintf(LIST)". The first argument
2011of the list will be interpreted as the printf format.
2012
da0045b7
PP
2013=item prototype FUNCTION
2014
2015Returns the prototype of a function as a string (or C<undef> if the
2016function has no prototype). FUNCTION is a reference to the the
2017function whose prototype you want to retrieve.
2018
a0d0e21e
LW
2019=item push ARRAY,LIST
2020
2021Treats ARRAY as a stack, and pushes the values of LIST
2022onto the end of ARRAY. The length of ARRAY increases by the length of
2023LIST. Has the same effect as
2024
2025 for $value (LIST) {
2026 $ARRAY[++$#ARRAY] = $value;
2027 }
2028
2029but is more efficient. Returns the new number of elements in the array.
2030
2031=item q/STRING/
2032
2033=item qq/STRING/
2034
2035=item qx/STRING/
2036
2037=item qw/STRING/
2038
2039Generalized quotes. See L<perlop>.
2040
2041=item quotemeta EXPR
2042
2043Returns the value of EXPR with with all regular expression
2044metacharacters backslashed. This is the internal function implementing
2045the \Q escape in double-quoted strings.
2046
2047=item rand EXPR
2048
2049=item rand
2050
2051Returns a random fractional number between 0 and the value of EXPR.
2052(EXPR should be positive.) If EXPR is omitted, returns a value between
20530 and 1. This function produces repeatable sequences unless srand()
2054is invoked. See also srand().
2055
2056(Note: if your rand function consistently returns numbers that are too
2057large or too small, then your version of Perl was probably compiled
2058with the wrong number of RANDBITS. As a workaround, you can usually
2059multiply EXPR by the correct power of 2 to get the range you want.
2060This will make your script unportable, however. It's better to recompile
2061if you can.)
2062
2063=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2064
2065=item read FILEHANDLE,SCALAR,LENGTH
2066
2067Attempts to read LENGTH bytes of data into variable SCALAR from the
2068specified FILEHANDLE. Returns the number of bytes actually read, or
2069undef if there was an error. SCALAR will be grown or shrunk to the
2070length actually read. An OFFSET may be specified to place the read
2071data at some other place than the beginning of the string. This call
2072is actually implemented in terms of stdio's fread call. To get a true
2073read system call, see sysread().
2074
2075=item readdir DIRHANDLE
2076
2077Returns the next directory entry for a directory opened by opendir().
2078If used in a list context, returns all the rest of the entries in the
2079directory. If there are no more entries, returns an undefined value in
2080a scalar context or a null list in a list context.
2081
cb1a09d0
AD
2082If you're planning to filetest the return values out of a readdir(), you'd
2083better prepend the directory in question. Otherwise, since we didn't
2084chdir() there, it would have been testing the wrong file.
2085
2086 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2087 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2088 closedir DIR;
2089
a0d0e21e
LW
2090=item readlink EXPR
2091
2092Returns the value of a symbolic link, if symbolic links are
2093implemented. If not, gives a fatal error. If there is some system
2094error, returns the undefined value and sets $! (errno). If EXPR is
2095omitted, uses $_.
2096
2097=item recv SOCKET,SCALAR,LEN,FLAGS
2098
2099Receives a message on a socket. Attempts to receive LENGTH bytes of
2100data into variable SCALAR from the specified SOCKET filehandle.
2101Actually does a C recvfrom(), so that it can returns the address of the
2102sender. Returns the undefined value if there's an error. SCALAR will
2103be grown or shrunk to the length actually read. Takes the same flags
4633a7c4
LW
2104as the system call of the same name.
2105See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2106
2107=item redo LABEL
2108
2109=item redo
2110
2111The C<redo> command restarts the loop block without evaluating the
2112conditional again. The C<continue> block, if any, is not executed. If
2113the LABEL is omitted, the command refers to the innermost enclosing
2114loop. This command is normally used by programs that want to lie to
2115themselves about what was just input:
2116
2117 # a simpleminded Pascal comment stripper
2118 # (warning: assumes no { or } in strings)
4633a7c4 2119 LINE: while (<STDIN>) {
a0d0e21e
LW
2120 while (s|({.*}.*){.*}|$1 |) {}
2121 s|{.*}| |;
2122 if (s|{.*| |) {
2123 $front = $_;
2124 while (<STDIN>) {
2125 if (/}/) { # end of comment?
2126 s|^|$front{|;
4633a7c4 2127 redo LINE;
a0d0e21e
LW
2128 }
2129 }
2130 }
2131 print;
2132 }
2133
2134=item ref EXPR
2135
2136Returns a TRUE value if EXPR is a reference, FALSE otherwise. The value
2137returned depends on the type of thing the reference is a reference to.
2138Builtin types include:
2139
2140 REF
2141 SCALAR
2142 ARRAY
2143 HASH
2144 CODE
2145 GLOB
2146
2147If the referenced object has been blessed into a package, then that package
2148name is returned instead. You can think of ref() as a typeof() operator.
2149
2150 if (ref($r) eq "HASH") {
2151 print "r is a reference to an associative array.\n";
2152 }
2153 if (!ref ($r) {
2154 print "r is not a reference at all.\n";
2155 }
2156
2157See also L<perlref>.
2158
2159=item rename OLDNAME,NEWNAME
2160
2161Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2162not work across filesystem boundaries.
2163
2164=item require EXPR
2165
2166=item require
2167
2168Demands some semantics specified by EXPR, or by $_ if EXPR is not
2169supplied. If EXPR is numeric, demands that the current version of Perl
2170($] or $PERL_VERSION) be equal or greater than EXPR.
2171
2172Otherwise, demands that a library file be included if it hasn't already
2173been included. The file is included via the do-FILE mechanism, which is
2174essentially just a variety of eval(). Has semantics similar to the following
2175subroutine:
2176
2177 sub require {
2178 local($filename) = @_;
2179 return 1 if $INC{$filename};
2180 local($realfilename,$result);
2181 ITER: {
2182 foreach $prefix (@INC) {
2183 $realfilename = "$prefix/$filename";
2184 if (-f $realfilename) {
2185 $result = do $realfilename;
2186 last ITER;
2187 }
2188 }
2189 die "Can't find $filename in \@INC";
2190 }
2191 die $@ if $@;
2192 die "$filename did not return true value" unless $result;
2193 $INC{$filename} = $realfilename;
2194 $result;
2195 }
2196
2197Note that the file will not be included twice under the same specified
2198name. The file must return TRUE as the last statement to indicate
2199successful execution of any initialization code, so it's customary to
2200end such a file with "1;" unless you're sure it'll return TRUE
2201otherwise. But it's better just to put the "C<1;>", in case you add more
2202statements.
2203
da0045b7
PP
2204If EXPR is a bare word, the require assumes a "F<.pm>" extension and
2205replaces "F<::>" with "F</>" in the filename for you,
a0d0e21e
LW
2206to make it easy to load standard modules. This form of loading of
2207modules does not risk altering your namespace.
2208
da0045b7 2209For a yet-more-powerful import facility, see L</use> and
748a9306 2210L<perlmod>.
a0d0e21e
LW
2211
2212=item reset EXPR
2213
2214=item reset
2215
2216Generally used in a C<continue> block at the end of a loop to clear
2217variables and reset ?? searches so that they work again. The
2218expression is interpreted as a list of single characters (hyphens
2219allowed for ranges). All variables and arrays beginning with one of
2220those letters are reset to their pristine state. If the expression is
2221omitted, one-match searches (?pattern?) are reset to match again. Only
2222resets variables or searches in the current package. Always returns
22231. Examples:
2224
2225 reset 'X'; # reset all X variables
2226 reset 'a-z'; # reset lower case variables
2227 reset; # just reset ?? searches
2228
2229Resetting "A-Z" is not recommended since you'll wipe out your
2230ARGV and ENV arrays. Only resets package variables--lexical variables
2231are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2232so you'll probably want to use them instead. See L</my>.
a0d0e21e
LW
2233
2234=item return LIST
2235
2236Returns from a subroutine or eval with the value specified. (Note that
4633a7c4 2237in the absence of a return a subroutine or eval() will automatically
a0d0e21e
LW
2238return the value of the last expression evaluated.)
2239
2240=item reverse LIST
2241
2242In a list context, returns a list value consisting of the elements
2243of LIST in the opposite order. In a scalar context, returns a string
2244value consisting of the bytes of the first element of LIST in the
4633a7c4
LW
2245opposite order.
2246
2247 print reverse <>; # line tac
2248
2249 undef $/;
2250 print scalar reverse scalar <>; # byte tac
a0d0e21e
LW
2251
2252=item rewinddir DIRHANDLE
2253
2254Sets the current position to the beginning of the directory for the
2255readdir() routine on DIRHANDLE.
2256
2257=item rindex STR,SUBSTR,POSITION
2258
2259=item rindex STR,SUBSTR
2260
2261Works just like index except that it returns the position of the LAST
2262occurrence of SUBSTR in STR. If POSITION is specified, returns the
2263last occurrence at or before that position.
2264
2265=item rmdir FILENAME
2266
2267Deletes the directory specified by FILENAME if it is empty. If it
2268succeeds it returns 1, otherwise it returns 0 and sets $! (errno). If
2269FILENAME is omitted, uses $_.
2270
2271=item s///
2272
2273The substitution operator. See L<perlop>.
2274
2275=item scalar EXPR
2276
2277Forces EXPR to be interpreted in a scalar context and returns the value
cb1a09d0
AD
2278of EXPR.
2279
2280 @counts = ( scalar @a, scalar @b, scalar @c );
2281
2282There is no equivalent operator to force an expression to
2283be interpolated in a list context because it's in practice never
2284needed. If you really wanted to do so, however, you could use
2285the construction C<@{[ (some expression) ]}>, but usually a simple
2286C<(some expression)> suffices.
a0d0e21e
LW
2287
2288=item seek FILEHANDLE,POSITION,WHENCE
2289
2290Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2291call of stdio. FILEHANDLE may be an expression whose value gives the name
2292of the filehandle. The values for WHENCE are 0 to set the file pointer to
2293POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2294plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
4633a7c4 2295this from POSIX module. Returns 1 upon success, 0 otherwise.
a0d0e21e 2296
cb1a09d0
AD
2297On some systems you have to do a seek whenever you switch between reading
2298and writing. Amongst other things, this may have the effect of calling
2299stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2300the file pointer:
2301
2302 seek(TEST,0,1);
2303
2304This is also useful for applications emulating C<tail -f>. Once you hit
2305EOF on your read, and then sleep for a while, you might have to stick in a
2306seek() to reset things. First the simple trick listed above to clear the
2307filepointer. The seek() doesn't change the current position, but it
2308I<does> clear the end-of-file condition on the handle, so that the next
37798a01 2309C<E<lt>FILEE<gt>> makes Perl try again to read something. Hopefully.
cb1a09d0
AD
2310
2311If that doesn't work (some stdios are particularly cantankerous), then
2312you may need something more like this:
2313
2314 for (;;) {
2315 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2316 # search for some stuff and put it into files
2317 }
2318 sleep($for_a_while);
2319 seek(FILE, $curpos, 0);
2320 }
2321
a0d0e21e
LW
2322=item seekdir DIRHANDLE,POS
2323
2324Sets the current position for the readdir() routine on DIRHANDLE. POS
2325must be a value returned by telldir(). Has the same caveats about
2326possible directory compaction as the corresponding system library
2327routine.
2328
2329=item select FILEHANDLE
2330
2331=item select
2332
2333Returns the currently selected filehandle. Sets the current default
2334filehandle for output, if FILEHANDLE is supplied. This has two
2335effects: first, a C<write> or a C<print> without a filehandle will
2336default to this FILEHANDLE. Second, references to variables related to
2337output will refer to this output channel. For example, if you have to
2338set the top of form format for more than one output channel, you might
2339do the following:
2340
2341 select(REPORT1);
2342 $^ = 'report1_top';
2343 select(REPORT2);
2344 $^ = 'report2_top';
2345
2346FILEHANDLE may be an expression whose value gives the name of the
2347actual filehandle. Thus:
2348
2349 $oldfh = select(STDERR); $| = 1; select($oldfh);
2350
4633a7c4
LW
2351Some programmers may prefer to think of filehandles as objects with
2352methods, preferring to write the last example as:
a0d0e21e
LW
2353
2354 use FileHandle;
2355 STDERR->autoflush(1);
2356
2357=item select RBITS,WBITS,EBITS,TIMEOUT
2358
4633a7c4 2359This calls the select(2) system call with the bitmasks specified, which
a0d0e21e
LW
2360can be constructed using fileno() and vec(), along these lines:
2361
2362 $rin = $win = $ein = '';
2363 vec($rin,fileno(STDIN),1) = 1;
2364 vec($win,fileno(STDOUT),1) = 1;
2365 $ein = $rin | $win;
2366
2367If you want to select on many filehandles you might wish to write a
2368subroutine:
2369
2370 sub fhbits {
2371 local(@fhlist) = split(' ',$_[0]);
2372 local($bits);
2373 for (@fhlist) {
2374 vec($bits,fileno($_),1) = 1;
2375 }
2376 $bits;
2377 }
4633a7c4 2378 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
2379
2380The usual idiom is:
2381
2382 ($nfound,$timeleft) =
2383 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2384
c07a80fd 2385or to block until something becomes ready just do this
a0d0e21e
LW
2386
2387 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2388
c07a80fd
PP
2389Most systems do not both to return anything useful in $timeleft, so
2390calling select() in a scalar context just returns $nfound.
2391
a0d0e21e
LW
2392Any of the bitmasks can also be undef. The timeout, if specified, is
2393in seconds, which may be fractional. Note: not all implementations are
2394capable of returning the $timeleft. If not, they always return
2395$timeleft equal to the supplied $timeout.
2396
da0045b7 2397You can effect a 250-millisecond sleep this way:
a0d0e21e
LW
2398
2399 select(undef, undef, undef, 0.25);
2400
cb1a09d0
AD
2401B<WARNING>: Do not attempt to mix buffered I/O (like read() or <FH>)
2402with select(). You have to use sysread() instead.
a0d0e21e
LW
2403
2404=item semctl ID,SEMNUM,CMD,ARG
2405
2406Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2407&GETALL, then ARG must be a variable which will hold the returned
2408semid_ds structure or semaphore value array. Returns like ioctl: the
2409undefined value for error, "0 but true" for zero, or the actual return
2410value otherwise.
2411
2412=item semget KEY,NSEMS,FLAGS
2413
2414Calls the System V IPC function semget. Returns the semaphore id, or
2415the undefined value if there is an error.
2416
2417=item semop KEY,OPSTRING
2418
2419Calls the System V IPC function semop to perform semaphore operations
2420such as signaling and waiting. OPSTRING must be a packed array of
2421semop structures. Each semop structure can be generated with
2422C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2423operations is implied by the length of OPSTRING. Returns TRUE if
2424successful, or FALSE if there is an error. As an example, the
2425following code waits on semaphore $semnum of semaphore id $semid:
2426
2427 $semop = pack("sss", $semnum, -1, 0);
2428 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2429
2430To signal the semaphore, replace "-1" with "1".
2431
2432=item send SOCKET,MSG,FLAGS,TO
2433
2434=item send SOCKET,MSG,FLAGS
2435
2436Sends a message on a socket. Takes the same flags as the system call
2437of the same name. On unconnected sockets you must specify a
2438destination to send TO, in which case it does a C sendto(). Returns
2439the number of characters sent, or the undefined value if there is an
2440error.
4633a7c4 2441See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2442
2443=item setpgrp PID,PGRP
2444
2445Sets the current process group for the specified PID, 0 for the current
2446process. Will produce a fatal error if used on a machine that doesn't
47e29363
PP
2447implement setpgrp(2). If the arguments are ommitted, it defaults to
24480,0. Note that the POSIX version of setpgrp() does not accept any
2449arguments, so only setpgrp 0,0 is portable.
a0d0e21e
LW
2450
2451=item setpriority WHICH,WHO,PRIORITY
2452
2453Sets the current priority for a process, a process group, or a user.
748a9306 2454(See setpriority(2).) Will produce a fatal error if used on a machine
a0d0e21e
LW
2455that doesn't implement setpriority(2).
2456
2457=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2458
2459Sets the socket option requested. Returns undefined if there is an
2460error. OPTVAL may be specified as undef if you don't want to pass an
2461argument.
2462
2463=item shift ARRAY
2464
2465=item shift
2466
2467Shifts the first value of the array off and returns it, shortening the
2468array by 1 and moving everything down. If there are no elements in the
2469array, returns the undefined value. If ARRAY is omitted, shifts the
2470@ARGV array in the main program, and the @_ array in subroutines.
2471(This is determined lexically.) See also unshift(), push(), and pop().
2472Shift() and unshift() do the same thing to the left end of an array
2473that push() and pop() do to the right end.
2474
2475=item shmctl ID,CMD,ARG
2476
2477Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2478must be a variable which will hold the returned shmid_ds structure.
2479Returns like ioctl: the undefined value for error, "0 but true" for
2480zero, or the actual return value otherwise.
2481
2482=item shmget KEY,SIZE,FLAGS
2483
2484Calls the System V IPC function shmget. Returns the shared memory
2485segment id, or the undefined value if there is an error.
2486
2487=item shmread ID,VAR,POS,SIZE
2488
2489=item shmwrite ID,STRING,POS,SIZE
2490
2491Reads or writes the System V shared memory segment ID starting at
2492position POS for size SIZE by attaching to it, copying in/out, and
2493detaching from it. When reading, VAR must be a variable which will
2494hold the data read. When writing, if STRING is too long, only SIZE
2495bytes are used; if STRING is too short, nulls are written to fill out
2496SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2497
2498=item shutdown SOCKET,HOW
2499
2500Shuts down a socket connection in the manner indicated by HOW, which
2501has the same interpretation as in the system call of the same name.
2502
2503=item sin EXPR
2504
2505Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2506returns sine of $_.
2507
2508=item sleep EXPR
2509
2510=item sleep
2511
2512Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2513May be interrupted by sending the process a SIGALRM. Returns the
2514number of seconds actually slept. You probably cannot mix alarm() and
2515sleep() calls, since sleep() is often implemented using alarm().
2516
2517On some older systems, it may sleep up to a full second less than what
2518you requested, depending on how it counts seconds. Most modern systems
2519always sleep the full amount.
2520
cb1a09d0
AD
2521For delays of finer granularity than one second, you may use Perl's
2522syscall() interface to access setitimer(2) if your system supports it,
2523or else see L</select()> below.
2524
a0d0e21e
LW
2525=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2526
2527Opens a socket of the specified kind and attaches it to filehandle
2528SOCKET. DOMAIN, TYPE and PROTOCOL are specified the same as for the
2529system call of the same name. You should "use Socket;" first to get
4633a7c4 2530the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2531
2532=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2533
2534Creates an unnamed pair of sockets in the specified domain, of the
2535specified type. DOMAIN, TYPE and PROTOCOL are specified the same as
2536for the system call of the same name. If unimplemented, yields a fatal
2537error. Returns TRUE if successful.
2538
2539=item sort SUBNAME LIST
2540
2541=item sort BLOCK LIST
2542
2543=item sort LIST
2544
2545Sorts the LIST and returns the sorted list value. Nonexistent values
2546of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2547in standard string comparison order. If SUBNAME is specified, it
2548gives the name of a subroutine that returns an integer less than, equal
2549to, or greater than 0, depending on how the elements of the array are
2550to be ordered. (The <=> and cmp operators are extremely useful in such
2551routines.) SUBNAME may be a scalar variable name, in which case the
2552value provides the name of the subroutine to use. In place of a
2553SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2554subroutine.
2555
cb1a09d0
AD
2556In the interests of efficiency the normal calling code for subroutines is
2557bypassed, with the following effects: the subroutine may not be a
2558recursive subroutine, and the two elements to be compared are passed into
2559the subroutine not via @_ but as the package global variables $a and
2560$b (see example below). They are passed by reference, so don't
2561modify $a and $b. And don't try to declare them as lexicals either.
a0d0e21e
LW
2562
2563Examples:
2564
2565 # sort lexically
2566 @articles = sort @files;
2567
2568 # same thing, but with explicit sort routine
2569 @articles = sort {$a cmp $b} @files;
2570
cb1a09d0
AD
2571 # now case-insensitively
2572 @articles = sort { uc($a) cmp uc($b)} @files;
2573
a0d0e21e
LW
2574 # same thing in reversed order
2575 @articles = sort {$b cmp $a} @files;
2576
2577 # sort numerically ascending
2578 @articles = sort {$a <=> $b} @files;
2579
2580 # sort numerically descending
2581 @articles = sort {$b <=> $a} @files;
2582
2583 # sort using explicit subroutine name
2584 sub byage {
2585 $age{$a} <=> $age{$b}; # presuming integers
2586 }
2587 @sortedclass = sort byage @class;
2588
c07a80fd
PP
2589 # this sorts the %age associative arrays by value
2590 # instead of key using an inline function
2591 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2592
a0d0e21e
LW
2593 sub backwards { $b cmp $a; }
2594 @harry = ('dog','cat','x','Cain','Abel');
2595 @george = ('gone','chased','yz','Punished','Axed');
2596 print sort @harry;
2597 # prints AbelCaincatdogx
2598 print sort backwards @harry;
2599 # prints xdogcatCainAbel
2600 print sort @george, 'to', @harry;
2601 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2602
cb1a09d0
AD
2603 # inefficiently sort by descending numeric compare using
2604 # the first integer after the first = sign, or the
2605 # whole record case-insensitively otherwise
2606
2607 @new = sort {
2608 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2609 ||
2610 uc($a) cmp uc($b)
2611 } @old;
2612
2613 # same thing, but much more efficiently;
2614 # we'll build auxiliary indices instead
2615 # for speed
2616 @nums = @caps = ();
2617 for (@old) {
2618 push @nums, /=(\d+)/;
2619 push @caps, uc($_);
2620 }
2621
2622 @new = @old[ sort {
2623 $nums[$b] <=> $nums[$a]
2624 ||
2625 $caps[$a] cmp $caps[$b]
2626 } 0..$#old
2627 ];
2628
2629 # same thing using a Schwartzian Transform (no temps)
2630 @new = map { $_->[0] }
2631 sort { $b->[1] <=> $a->[1]
2632 ||
2633 $a->[2] cmp $b->[2]
2634 } map { [$_, /=(\d+)/, uc($_)] } @old;
2635
2636If you're and using strict, you I<MUST NOT> declare $a
2637and $b as lexicals. They are package globals. That means
2638if you're in the C<main> package, it's
2639
2640 @articles = sort {$main::b <=> $main::a} @files;
2641
2642or just
2643
2644 @articles = sort {$::b <=> $::a} @files;
2645
2646but if you're in the C<FooPack> package, it's
2647
2648 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2649
a0d0e21e
LW
2650=item splice ARRAY,OFFSET,LENGTH,LIST
2651
2652=item splice ARRAY,OFFSET,LENGTH
2653
2654=item splice ARRAY,OFFSET
2655
2656Removes the elements designated by OFFSET and LENGTH from an array, and
2657replaces them with the elements of LIST, if any. Returns the elements
2658removed from the array. The array grows or shrinks as necessary. If
2659LENGTH is omitted, removes everything from OFFSET onward. The
2660following equivalencies hold (assuming $[ == 0):
2661
2662 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2663 pop(@a) splice(@a,-1)
2664 shift(@a) splice(@a,0,1)
2665 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2666 $a[$x] = $y splice(@a,$x,1,$y);
2667
2668Example, assuming array lengths are passed before arrays:
2669
2670 sub aeq { # compare two list values
2671 local(@a) = splice(@_,0,shift);
2672 local(@b) = splice(@_,0,shift);
2673 return 0 unless @a == @b; # same len?
2674 while (@a) {
2675 return 0 if pop(@a) ne pop(@b);
2676 }
2677 return 1;
2678 }
2679 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2680
2681=item split /PATTERN/,EXPR,LIMIT
2682
2683=item split /PATTERN/,EXPR
2684
2685=item split /PATTERN/
2686
2687=item split
2688
2689Splits a string into an array of strings, and returns it.
2690
2691If not in a list context, returns the number of fields found and splits into
2692the @_ array. (In a list context, you can force the split into @_ by
2693using C<??> as the pattern delimiters, but it still returns the array
2694value.) The use of implicit split to @_ is deprecated, however.
2695
2696If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
4633a7c4
LW
2697splits on whitespace (after skipping any leading whitespace). Anything
2698matching PATTERN is taken to be a delimiter separating the fields. (Note
2699that the delimiter may be longer than one character.) If LIMIT is
2700specified and is not negative, splits into no more than that many fields
2701(though it may split into fewer). If LIMIT is unspecified, trailing null
2702fields are stripped (which potential users of pop() would do well to
2703remember). If LIMIT is negative, it is treated as if an arbitrarily large
2704LIMIT had been specified.
a0d0e21e
LW
2705
2706A pattern matching the null string (not to be confused with
748a9306 2707a null pattern C<//>, which is just one member of the set of patterns
a0d0e21e
LW
2708matching a null string) will split the value of EXPR into separate
2709characters at each point it matches that way. For example:
2710
2711 print join(':', split(/ */, 'hi there'));
2712
2713produces the output 'h:i:t:h:e:r:e'.
2714
2715The LIMIT parameter can be used to partially split a line
2716
2717 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2718
2719When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2720one larger than the number of variables in the list, to avoid
2721unnecessary work. For the list above LIMIT would have been 4 by
2722default. In time critical applications it behooves you not to split
2723into more fields than you really need.
2724
2725If the PATTERN contains parentheses, additional array elements are
2726created from each matching substring in the delimiter.
2727
da0045b7 2728 split(/([,-])/, "1-10,20", 3);
a0d0e21e
LW
2729
2730produces the list value
2731
2732 (1, '-', 10, ',', 20)
2733
4633a7c4
LW
2734If you had the entire header of a normal Unix email message in $header,
2735you could split it up into fields and their values this way:
2736
2737 $header =~ s/\n\s+/ /g; # fix continuation lines
2738 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2739
a0d0e21e
LW
2740The pattern C</PATTERN/> may be replaced with an expression to specify
2741patterns that vary at runtime. (To do runtime compilation only once,
748a9306
LW
2742use C</$variable/o>.)
2743
2744As a special case, specifying a PATTERN of space (C<' '>) will split on
2745white space just as split with no arguments does. Thus, split(' ') can
2746be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
2747will give you as many null initial fields as there are leading spaces.
2748A split on /\s+/ is like a split(' ') except that any leading
2749whitespace produces a null first field. A split with no arguments
2750really does a C<split(' ', $_)> internally.
a0d0e21e
LW
2751
2752Example:
2753
2754 open(passwd, '/etc/passwd');
2755 while (<passwd>) {
748a9306
LW
2756 ($login, $passwd, $uid, $gid, $gcos,
2757 $home, $shell) = split(/:/);
a0d0e21e
LW
2758 ...
2759 }
2760
2761(Note that $shell above will still have a newline on it. See L</chop>,
2762L</chomp>, and L</join>.)
2763
2764=item sprintf FORMAT,LIST
2765
2766Returns a string formatted by the usual printf conventions of the C
cb1a09d0
AD
2767language. See L<sprintf(3)> or L<printf(3)> on your system for details.
2768(The * character for an indirectly specified length is not
a0d0e21e 2769supported, but you can get the same effect by interpolating a variable
cb1a09d0
AD
2770into the pattern.) Some C libraries' implementations of sprintf() can
2771dump core when fed ludicrous arguments.
a0d0e21e
LW
2772
2773=item sqrt EXPR
2774
2775Return the square root of EXPR. If EXPR is omitted, returns square
2776root of $_.
2777
2778=item srand EXPR
2779
cb1a09d0 2780Sets the random number seed for the C<rand> operator. If EXPR is omitted,
da0045b7
PP
2781uses a semirandom value based on the current time and process ID, among
2782other things. Of course, you'd need something much more random than that for
cb1a09d0
AD
2783cryptographic purposes, since it's easy to guess the current time.
2784Checksumming the compressed output of rapidly changing operating system
2785status programs is the usual method. Examples are posted regularly to
2786the comp.security.unix newsgroup.
a0d0e21e
LW
2787
2788=item stat FILEHANDLE
2789
2790=item stat EXPR
2791
2792Returns a 13-element array giving the status info for a file, either the
2793file opened via FILEHANDLE, or named by EXPR. Returns a null list if
2794the stat fails. Typically used as follows:
2795
2796 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
2797 $atime,$mtime,$ctime,$blksize,$blocks)
2798 = stat($filename);
2799
c07a80fd
PP
2800Not all fields are supported on all filesystem types. Here are the
2801meaning of the fields:
2802
2803 dev device number of filesystem
2804 ino inode number
2805 mode file mode (type and permissions)
2806 nlink number of (hard) links to the file
2807 uid numeric user ID of file's owner
2808 gid numer group ID of file's owner
2809 rdev the device identifier (special files only)
2810 size total size of file, in bytes
2811 atime last access time since the epoch
2812 mtime last modify time since the epoch
2813 ctime inode change time (NOT creation type!) since the epoch
2814 blksize preferred blocksize for file system I/O
2815 blocks actual number of blocks allocated
2816
2817(The epoch was at 00:00 January 1, 1970 GMT.)
2818
a0d0e21e
LW
2819If stat is passed the special filehandle consisting of an underline, no
2820stat is done, but the current contents of the stat structure from the
2821last stat or filetest are returned. Example:
2822
2823 if (-x $file && (($d) = stat(_)) && $d < 0) {
2824 print "$file is executable NFS file\n";
2825 }
2826
2827(This only works on machines for which the device number is negative under NFS.)
2828
2829=item study SCALAR
2830
2831=item study
2832
2833Takes extra time to study SCALAR ($_ if unspecified) in anticipation of
2834doing many pattern matches on the string before it is next modified.
2835This may or may not save time, depending on the nature and number of
2836patterns you are searching on, and on the distribution of character
2837frequencies in the string to be searched--you probably want to compare
2838runtimes with and without it to see which runs faster. Those loops
2839which scan for many short constant strings (including the constant
2840parts of more complex patterns) will benefit most. You may have only
2841one study active at a time--if you study a different scalar the first
2842is "unstudied". (The way study works is this: a linked list of every
2843character in the string to be searched is made, so we know, for
2844example, where all the 'k' characters are. From each search string,
2845the rarest character is selected, based on some static frequency tables
2846constructed from some C programs and English text. Only those places
2847that contain this "rarest" character are examined.)
2848
2849For example, here is a loop which inserts index producing entries
2850before any line containing a certain pattern:
2851
2852 while (<>) {
2853 study;
2854 print ".IX foo\n" if /\bfoo\b/;
2855 print ".IX bar\n" if /\bbar\b/;
2856 print ".IX blurfl\n" if /\bblurfl\b/;
2857 ...
2858 print;
2859 }
2860
2861In searching for /\bfoo\b/, only those locations in $_ that contain "f"
2862will be looked at, because "f" is rarer than "o". In general, this is
2863a big win except in pathological cases. The only question is whether
2864it saves you more time than it took to build the linked list in the
2865first place.
2866
2867Note that if you have to look for strings that you don't know till
2868runtime, you can build an entire loop as a string and eval that to
2869avoid recompiling all your patterns all the time. Together with
2870undefining $/ to input entire files as one record, this can be very
2871fast, often faster than specialized programs like fgrep(1). The following
2872scans a list of files (@files) for a list of words (@words), and prints
2873out the names of those files that contain a match:
2874
2875 $search = 'while (<>) { study;';
2876 foreach $word (@words) {
2877 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
2878 }
2879 $search .= "}";
2880 @ARGV = @files;
2881 undef $/;
2882 eval $search; # this screams
2883 $/ = "\n"; # put back to normal input delim
2884 foreach $file (sort keys(%seen)) {
2885 print $file, "\n";
2886 }
2887
cb1a09d0
AD
2888=item sub BLOCK
2889
2890=item sub NAME
2891
2892=item sub NAME BLOCK
2893
2894This is subroutine definition, not a real function I<per se>. With just a
2895NAME (and possibly prototypes), it's just a forward declaration. Without
2896a NAME, it's an anonymous function declaration, and does actually return a
2897value: the CODE ref of the closure you just created. See L<perlsub> and
2898L<perlref> for details.
2899
a0d0e21e
LW
2900=item substr EXPR,OFFSET,LEN
2901
2902=item substr EXPR,OFFSET
2903
2904Extracts a substring out of EXPR and returns it. First character is at
2905offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
2906that far from the end of the string. If LEN is omitted, returns
748a9306
LW
2907everything to the end of the string. If LEN is negative, leaves that
2908many characters off the end of the string.
2909
2910You can use the substr() function
a0d0e21e
LW
2911as an lvalue, in which case EXPR must be an lvalue. If you assign
2912something shorter than LEN, the string will shrink, and if you assign
2913something longer than LEN, the string will grow to accommodate it. To
2914keep the string the same length you may need to pad or chop your value
2915using sprintf().
2916
2917=item symlink OLDFILE,NEWFILE
2918
2919Creates a new filename symbolically linked to the old filename.
2920Returns 1 for success, 0 otherwise. On systems that don't support
2921symbolic links, produces a fatal error at run time. To check for that,
2922use eval:
2923
2924 $symlink_exists = (eval 'symlink("","");', $@ eq '');
2925
2926=item syscall LIST
2927
2928Calls the system call specified as the first element of the list,
2929passing the remaining elements as arguments to the system call. If
2930unimplemented, produces a fatal error. The arguments are interpreted
2931as follows: if a given argument is numeric, the argument is passed as
2932an int. If not, the pointer to the string value is passed. You are
2933responsible to make sure a string is pre-extended long enough to
2934receive any result that might be written into a string. If your
2935integer arguments are not literals and have never been interpreted in a
2936numeric context, you may need to add 0 to them to force them to look
2937like numbers.
2938
2939 require 'syscall.ph'; # may need to run h2ph
2940 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
2941
2942Note that Perl only supports passing of up to 14 arguments to your system call,
2943which in practice should usually suffice.
2944
c07a80fd
PP
2945=item sysopen FILEHANDLE,FILENAME,MODE
2946
2947=item sysopen FILEHANDLE,FILENAME,MODE,PERMS
2948
2949Opens the file whose filename is given by FILENAME, and associates it
2950with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
2951the name of the real filehandle wanted. This function calls the
2952underlying operating system's C<open> function with the parameters
2953FILENAME, MODE, PERMS.
2954
2955The possible values and flag bits of the MODE parameter are
2956system-dependent; they are available via the standard module C<Fcntl>.
2957However, for historical reasons, some values are universal: zero means
2958read-only, one means write-only, and two means read/write.
2959
2960If the file named by FILENAME does not exist and the C<open> call
2961creates it (typically because MODE includes the O_CREAT flag), then
2962the value of PERMS specifies the permissions of the newly created
2963file. If PERMS is omitted, the default value is 0666, which allows
2964read and write for all. This default is reasonable: see C<umask>.
2965
a0d0e21e
LW
2966=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
2967
2968=item sysread FILEHANDLE,SCALAR,LENGTH
2969
2970Attempts to read LENGTH bytes of data into variable SCALAR from the
2971specified FILEHANDLE, using the system call read(2). It bypasses
2972stdio, so mixing this with other kinds of reads may cause confusion.
2973Returns the number of bytes actually read, or undef if there was an
2974error. SCALAR will be grown or shrunk to the length actually read. An
2975OFFSET may be specified to place the read data at some other place than
2976the beginning of the string.
2977
2978=item system LIST
2979
2980Does exactly the same thing as "exec LIST" except that a fork is done
2981first, and the parent process waits for the child process to complete.
2982Note that argument processing varies depending on the number of
2983arguments. The return value is the exit status of the program as
2984returned by the wait() call. To get the actual exit value divide by
cb1a09d0
AD
2985256. See also L</exec>. This is I<NOT> what you want to use to capture
2986the output from a command, for that you should merely use backticks, as
2987described in L<perlop/"`STRING`">.
a0d0e21e
LW
2988
2989=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
2990
2991=item syswrite FILEHANDLE,SCALAR,LENGTH
2992
2993Attempts to write LENGTH bytes of data from variable SCALAR to the
2994specified FILEHANDLE, using the system call write(2). It bypasses
2995stdio, so mixing this with prints may cause confusion. Returns the
2996number of bytes actually written, or undef if there was an error. An
22dc801b 2997OFFSET may be specified to get the write data from some other place than
a0d0e21e
LW
2998the beginning of the string.
2999
3000=item tell FILEHANDLE
3001
3002=item tell
3003
3004Returns the current file position for FILEHANDLE. FILEHANDLE may be an
3005expression whose value gives the name of the actual filehandle. If
3006FILEHANDLE is omitted, assumes the file last read.
3007
3008=item telldir DIRHANDLE
3009
3010Returns the current position of the readdir() routines on DIRHANDLE.
3011Value may be given to seekdir() to access a particular location in a
3012directory. Has the same caveats about possible directory compaction as
3013the corresponding system library routine.
3014
4633a7c4 3015=item tie VARIABLE,CLASSNAME,LIST
a0d0e21e 3016
4633a7c4
LW
3017This function binds a variable to a package class that will provide the
3018implementation for the variable. VARIABLE is the name of the variable
3019to be enchanted. CLASSNAME is the name of a class implementing objects
3020of correct type. Any additional arguments are passed to the "new"
3021method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3022Typically these are arguments such as might be passed to the dbm_open()
cb1a09d0
AD
3023function of C. The object returned by the "new" method is also
3024returned by the tie() function, which would be useful if you want to
4633a7c4 3025access other methods in CLASSNAME.
a0d0e21e
LW
3026
3027Note that functions such as keys() and values() may return huge array
748a9306
LW
3028values when used on large objects, like DBM files. You may prefer to
3029use the each() function to iterate over such. Example:
a0d0e21e
LW
3030
3031 # print out history file offsets
4633a7c4 3032 use NDBM_File;
da0045b7 3033 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
a0d0e21e
LW
3034 while (($key,$val) = each %HIST) {
3035 print $key, ' = ', unpack('L',$val), "\n";
3036 }
3037 untie(%HIST);
3038
4633a7c4 3039A class implementing an associative array should have the following
a0d0e21e
LW
3040methods:
3041
4633a7c4 3042 TIEHASH classname, LIST
a0d0e21e
LW
3043 DESTROY this
3044 FETCH this, key
3045 STORE this, key, value
3046 DELETE this, key
3047 EXISTS this, key
3048 FIRSTKEY this
3049 NEXTKEY this, lastkey
3050
4633a7c4 3051A class implementing an ordinary array should have the following methods:
a0d0e21e 3052
4633a7c4 3053 TIEARRAY classname, LIST
a0d0e21e
LW
3054 DESTROY this
3055 FETCH this, key
3056 STORE this, key, value
3057 [others TBD]
3058
4633a7c4 3059A class implementing a scalar should have the following methods:
a0d0e21e 3060
4633a7c4 3061 TIESCALAR classname, LIST
a0d0e21e
LW
3062 DESTROY this
3063 FETCH this,
3064 STORE this, value
3065
4633a7c4
LW
3066Unlike dbmopen(), the tie() function will not use or require a module
3067for you--you need to do that explicitly yourself. See L<DB_File>
3068or the F<Config> module for interesting tie() implementations.
3069
f3cbc334
RS
3070=item tied VARIABLE
3071
3072Returns a reference to the object underlying VARIABLE (the same value
3073that was originally returned by the tie() call which bound the variable
3074to a package.) Returns the undefined value if VARIABLE isn't tied to a
3075package.
3076
a0d0e21e
LW
3077=item time
3078
da0045b7
PP
3079Returns the number of non-leap seconds since whatever time the system
3080considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3081and 00:00:00 UTC, January 1, 1970 for most other systems).
3082Suitable for feeding to gmtime() and localtime().
a0d0e21e
LW
3083
3084=item times
3085
3086Returns a four-element array giving the user and system times, in
3087seconds, for this process and the children of this process.
3088
3089 ($user,$system,$cuser,$csystem) = times;
3090
3091=item tr///
3092
3093The translation operator. See L<perlop>.
3094
3095=item truncate FILEHANDLE,LENGTH
3096
3097=item truncate EXPR,LENGTH
3098
3099Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3100specified length. Produces a fatal error if truncate isn't implemented
3101on your system.
3102
3103=item uc EXPR
3104
3105Returns an uppercased version of EXPR. This is the internal function
3106implementing the \U escape in double-quoted strings.
4633a7c4 3107Should respect any POSIX setlocale() settings.
a0d0e21e
LW
3108
3109=item ucfirst EXPR
3110
3111Returns the value of EXPR with the first character uppercased. This is
3112the internal function implementing the \u escape in double-quoted strings.
4633a7c4 3113Should respect any POSIX setlocale() settings.
a0d0e21e
LW
3114
3115=item umask EXPR
3116
3117=item umask
3118
3119Sets the umask for the process and returns the old one. If EXPR is
3120omitted, merely returns current umask.
3121
3122=item undef EXPR
3123
3124=item undef
3125
3126Undefines the value of EXPR, which must be an lvalue. Use only on a
3127scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
3128will probably not do what you expect on most predefined variables or
3129DBM list values, so don't do that.) Always returns the undefined value. You can omit
3130the EXPR, in which case nothing is undefined, but you still get an
3131undefined value that you could, for instance, return from a
3132subroutine. Examples:
3133
3134 undef $foo;
3135 undef $bar{'blurfl'};
3136 undef @ary;
3137 undef %assoc;
3138 undef &mysub;
3139 return (wantarray ? () : undef) if $they_blew_it;
3140
3141=item unlink LIST
3142
3143Deletes a list of files. Returns the number of files successfully
3144deleted.
3145
3146 $cnt = unlink 'a', 'b', 'c';
3147 unlink @goners;
3148 unlink <*.bak>;
3149
3150Note: unlink will not delete directories unless you are superuser and
3151the B<-U> flag is supplied to Perl. Even if these conditions are
3152met, be warned that unlinking a directory can inflict damage on your
3153filesystem. Use rmdir instead.
3154
3155=item unpack TEMPLATE,EXPR
3156
3157Unpack does the reverse of pack: it takes a string representing a
3158structure and expands it out into a list value, returning the array
3159value. (In a scalar context, it merely returns the first value
3160produced.) The TEMPLATE has the same format as in the pack function.
3161Here's a subroutine that does substring:
3162
3163 sub substr {
3164 local($what,$where,$howmuch) = @_;
3165 unpack("x$where a$howmuch", $what);
3166 }
3167
3168and then there's
3169
3170 sub ordinal { unpack("c",$_[0]); } # same as ord()
3171
3172In addition, you may prefix a field with a %<number> to indicate that
3173you want a <number>-bit checksum of the items instead of the items
3174themselves. Default is a 16-bit checksum. For example, the following
3175computes the same number as the System V sum program:
3176
3177 while (<>) {
3178 $checksum += unpack("%16C*", $_);
3179 }
3180 $checksum %= 65536;
3181
3182The following efficiently counts the number of set bits in a bit vector:
3183
3184 $setbits = unpack("%32b*", $selectmask);
3185
3186=item untie VARIABLE
3187
3188Breaks the binding between a variable and a package. (See tie().)
3189
3190=item unshift ARRAY,LIST
3191
3192Does the opposite of a C<shift>. Or the opposite of a C<push>,
3193depending on how you look at it. Prepends list to the front of the
3194array, and returns the new number of elements in the array.
3195
3196 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3197
3198Note the LIST is prepended whole, not one element at a time, so the
3199prepended elements stay in the same order. Use reverse to do the
3200reverse.
3201
3202=item use Module LIST
3203
3204=item use Module
3205
da0045b7
PP
3206=item use Module VERSION LIST
3207
3208=item use VERSION
3209
a0d0e21e
LW
3210Imports some semantics into the current package from the named module,
3211generally by aliasing certain subroutine or variable names into your
3212package. It is exactly equivalent to
3213
3214 BEGIN { require Module; import Module LIST; }
3215
da0045b7
PP
3216except that Module I<must> be a bare word.
3217
3218If the first argument to C<use> is a number, it is treated as a version
3219number instead of a module name. If the version of the Perl interpreter
3220is less than VERSION, then an error message is printed and Perl exits
3221immediately. This is often useful if you need to check the current
3222Perl version before C<use>ing library modules which have changed in
3223incompatible ways from older versions of Perl. (We try not to do
3224this more than we have to.)
3225
a0d0e21e
LW
3226The BEGIN forces the require and import to happen at compile time. The
3227require makes sure the module is loaded into memory if it hasn't been
3228yet. The import is not a builtin--it's just an ordinary static method
3229call into the "Module" package to tell the module to import the list of
3230features back into the current package. The module can implement its
3231import method any way it likes, though most modules just choose to
3232derive their import method via inheritance from the Exporter class that
cb1a09d0
AD
3233is defined in the Exporter module. See L<Exporter>.
3234
3235If you don't want your namespace altered, explicitly supply an empty list:
3236
3237 use Module ();
3238
3239That is exactly equivalent to
3240
3241 BEGIN { require Module; }
a0d0e21e 3242
da0045b7
PP
3243If the VERSION argument is present between Module and LIST, then the
3244C<use> will fail if the C<$VERSION> variable in package Module is
3245less than VERSION.
3246
a0d0e21e
LW
3247Because this is a wide-open interface, pragmas (compiler directives)
3248are also implemented this way. Currently implemented pragmas are:
3249
3250 use integer;
4633a7c4 3251 use diagnostics;
a0d0e21e
LW
3252 use sigtrap qw(SEGV BUS);
3253 use strict qw(subs vars refs);
3254 use subs qw(afunc blurfl);
3255
3256These pseudomodules import semantics into the current block scope, unlike
3257ordinary modules, which import symbols into the current package (which are
3258effective through the end of the file).
3259
3260There's a corresponding "no" command that unimports meanings imported
da0045b7 3261by use, i.e. it calls C<unimport Module LIST> instead of C<import>.
a0d0e21e
LW
3262
3263 no integer;
3264 no strict 'refs';
3265
3266See L<perlmod> for a list of standard modules and pragmas.
3267
3268=item utime LIST
3269
3270Changes the access and modification times on each file of a list of
3271files. The first two elements of the list must be the NUMERICAL access
3272and modification times, in that order. Returns the number of files
3273successfully changed. The inode modification time of each file is set
3274to the current time. Example of a "touch" command:
3275
3276 #!/usr/bin/perl
3277 $now = time;
3278 utime $now, $now, @ARGV;
3279
3280=item values ASSOC_ARRAY
3281
3282Returns a normal array consisting of all the values of the named
3283associative array. (In a scalar context, returns the number of
3284values.) The values are returned in an apparently random order, but it
3285is the same order as either the keys() or each() function would produce
c07a80fd 3286on the same array. See also keys(), each(), and sort().
a0d0e21e
LW
3287
3288=item vec EXPR,OFFSET,BITS
3289
22dc801b
PP
3290Treats the string in EXPR as a vector of unsigned integers, and
3291returns the value of the bitfield specified by OFFSET. BITS specifies
3292the number of bits that are reserved for each entry in the bit
3293vector. This must be a power of two from 1 to 32. vec() may also be
3294assigned to, in which case parens are needed to give the expression
3295the correct precedence as in
3296
3297 vec($image, $max_x * $x + $y, 8) = 3;
a0d0e21e
LW
3298
3299Vectors created with vec() can also be manipulated with the logical
3300operators |, & and ^, which will assume a bit vector operation is
3301desired when both operands are strings.
3302
3303To transform a bit vector into a string or array of 0's and 1's, use these:
3304
3305 $bits = unpack("b*", $vector);
3306 @bits = split(//, unpack("b*", $vector));
3307
3308If you know the exact length in bits, it can be used in place of the *.
3309
3310=item wait
3311
3312Waits for a child process to terminate and returns the pid of the
3313deceased process, or -1 if there are no child processes. The status is
3314returned in $?.
3315
3316=item waitpid PID,FLAGS
3317
3318Waits for a particular child process to terminate and returns the pid
3319of the deceased process, or -1 if there is no such child process. The
3320status is returned in $?. If you say
3321
47e29363 3322 use POSIX ":wait_h";
a0d0e21e
LW
3323 ...
3324 waitpid(-1,&WNOHANG);
3325
3326then you can do a non-blocking wait for any process. Non-blocking wait
3327is only available on machines supporting either the waitpid(2) or
3328wait4(2) system calls. However, waiting for a particular pid with
3329FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3330by remembering the status values of processes that have exited but have
3331not been harvested by the Perl script yet.)
3332
3333=item wantarray
3334
3335Returns TRUE if the context of the currently executing subroutine is
3336looking for a list value. Returns FALSE if the context is looking
3337for a scalar.
3338
3339 return wantarray ? () : undef;
3340
3341=item warn LIST
3342
3343Produces a message on STDERR just like die(), but doesn't exit or
4633a7c4 3344on an exception.
a0d0e21e
LW
3345
3346=item write FILEHANDLE
3347
3348=item write EXPR
3349
3350=item write
3351
3352Writes a formatted record (possibly multi-line) to the specified file,
3353using the format associated with that file. By default the format for
3354a file is the one having the same name is the filehandle, but the
3355format for the current output channel (see the select() function) may be set
3356explicitly by assigning the name of the format to the $~ variable.
3357
3358Top of form processing is handled automatically: if there is
3359insufficient room on the current page for the formatted record, the
3360page is advanced by writing a form feed, a special top-of-page format
3361is used to format the new page header, and then the record is written.
3362By default the top-of-page format is the name of the filehandle with
3363"_TOP" appended, but it may be dynamically set to the format of your
3364choice by assigning the name to the $^ variable while the filehandle is
3365selected. The number of lines remaining on the current page is in
3366variable $-, which can be set to 0 to force a new page.
3367
3368If FILEHANDLE is unspecified, output goes to the current default output
3369channel, which starts out as STDOUT but may be changed by the
3370C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3371is evaluated and the resulting string is used to look up the name of
3372the FILEHANDLE at run time. For more on formats, see L<perlform>.
3373
3374Note that write is I<NOT> the opposite of read. Unfortunately.
3375
3376=item y///
3377
37798a01 3378The translation operator. See L<perlop>.
a0d0e21e
LW
3379
3380=back