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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
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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
184e9718 277characters with the high bit set. If too many odd characters (E<gt>30%)
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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
cb1a09d0 615See also L<AnyDBM_File> for a more general description of the pros and
184e9718 616cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 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
184e9718 683the current value of C<$!> (errno). If C<$!> 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
184e9718 893likewise behave in the same way: they run the code E<lt>$xE<gt>, which does
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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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AD
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)
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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";
184e9718 1064 $num = $cost/$quantity;
cb1a09d0
AD
1065 $~ = 'Something';
1066 write;
1067
1068See L<perlform> for many details and examples.
1069
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LW
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
LW
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
4633a7c4
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
184e9718 1274would do. This is the internal function implementing the E<lt>*.*E<gt>
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
184e9718 1359the string. The return value is based at 0 (or whatever you've set the C<$[>
4633a7c4 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
184e9718 1580it returns 0 and sets C<$!> (errno).
a0d0e21e
LW
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
184e9718
PP
1656begins with "E<lt>" or nothing, the file is opened for input. If the filename
1657begins with "E<gt>", the file is opened for output. If the filename begins
1658with "E<gt>E<gt>", the file is opened for appending. You can put a '+' in
1659front of the 'E<gt>' or 'E<lt>' to indicate that you want both read and write
1660access to the file; thus '+E<lt>' is usually preferred for read/write
1661updates--the '+E<gt>' mode would clobber the file first. These correspond to
1662the fopen(3) modes of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.
cb1a09d0
AD
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
184e9718 1671Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
4633a7c4
LW
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
184e9718 1722with "E<gt>&", in which case the rest of the string is interpreted as the
a0d0e21e 1723name of a filehandle (or file descriptor, if numeric) which is to be
184e9718
PP
1724duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
1725+E<gt>E<gt> and +E<lt>. The
a0d0e21e 1726mode you specify should match the mode of the original filehandle.
184e9718 1727(Duping a filehandle does not take into account any existing contents of
cb1a09d0 1728stdio buffers.)
a0d0e21e
LW
1729Here is a script that saves, redirects, and restores STDOUT and
1730STDERR:
1731
1732 #!/usr/bin/perl
1733 open(SAVEOUT, ">&STDOUT");
1734 open(SAVEERR, ">&STDERR");
1735
1736 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1737 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1738
1739 select(STDERR); $| = 1; # make unbuffered
1740 select(STDOUT); $| = 1; # make unbuffered
1741
1742 print STDOUT "stdout 1\n"; # this works for
1743 print STDERR "stderr 1\n"; # subprocesses too
1744
1745 close(STDOUT);
1746 close(STDERR);
1747
1748 open(STDOUT, ">&SAVEOUT");
1749 open(STDERR, ">&SAVEERR");
1750
1751 print STDOUT "stdout 2\n";
1752 print STDERR "stderr 2\n";
1753
1754
184e9718 1755If you specify "E<lt>&=N", where N is a number, then Perl will do an
4633a7c4
LW
1756equivalent of C's fdopen() of that file descriptor; this is more
1757parsimonious of file descriptors. For example:
a0d0e21e
LW
1758
1759 open(FILEHANDLE, "<&=$fd")
1760
1761If you open a pipe on the command "-", i.e. either "|-" or "-|", then
1762there is an implicit fork done, and the return value of open is the pid
1763of the child within the parent process, and 0 within the child
184e9718 1764process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
1765The filehandle behaves normally for the parent, but i/o to that
1766filehandle is piped from/to the STDOUT/STDIN of the child process.
1767In the child process the filehandle isn't opened--i/o happens from/to
1768the new STDOUT or STDIN. Typically this is used like the normal
1769piped open when you want to exercise more control over just how the
1770pipe command gets executed, such as when you are running setuid, and
4633a7c4
LW
1771don't want to have to scan shell commands for metacharacters.
1772The following pairs are more or less equivalent:
a0d0e21e
LW
1773
1774 open(FOO, "|tr '[a-z]' '[A-Z]'");
1775 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1776
1777 open(FOO, "cat -n '$file'|");
1778 open(FOO, "-|") || exec 'cat', '-n', $file;
1779
4633a7c4
LW
1780See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1781
a0d0e21e 1782Explicitly closing any piped filehandle causes the parent process to
184e9718 1783wait for the child to finish, and returns the status value in C<$?>.
a0d0e21e 1784Note: on any operation which may do a fork, unflushed buffers remain
184e9718 1785unflushed in both processes, which means you may need to set C<$|> to
a0d0e21e
LW
1786avoid duplicate output.
1787
c07a80fd
PP
1788Using the FileHandle constructor from the FileHandle package,
1789you can generate anonymous filehandles which have the scope of whatever
1790variables hold references to them, and automatically close whenever
1791and however you leave that scope:
1792
1793 use FileHandle;
1794 ...
1795 sub read_myfile_munged {
1796 my $ALL = shift;
1797 my $handle = new FileHandle;
1798 open($handle, "myfile") or die "myfile: $!";
1799 $first = <$handle>
1800 or return (); # Automatically closed here.
1801 mung $first or die "mung failed"; # Or here.
1802 return $first, <$handle> if $ALL; # Or here.
1803 $first; # Or here.
1804 }
1805
a0d0e21e
LW
1806The filename that is passed to open will have leading and trailing
1807whitespace deleted. In order to open a file with arbitrary weird
1808characters in it, it's necessary to protect any leading and trailing
1809whitespace thusly:
1810
cb1a09d0
AD
1811 $file =~ s#^(\s)#./$1#;
1812 open(FOO, "< $file\0");
1813
c07a80fd
PP
1814If you want a "real" C open() (see L<open(2)> on your system), then
1815you should use the sysopen() function. This is another way to
1816protect your filenames from interpretation. For example:
cb1a09d0
AD
1817
1818 use FileHandle;
c07a80fd
PP
1819 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
1820 or die "sysopen $path: $!";
1821 HANDLE->autoflush(1);
1822 HANDLE->print("stuff $$\n");
1823 seek(HANDLE, 0, 0);
1824 print "File contains: ", <HANDLE>;
cb1a09d0
AD
1825
1826See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
1827
1828=item opendir DIRHANDLE,EXPR
1829
1830Opens a directory named EXPR for processing by readdir(), telldir(),
1831seekdir(), rewinddir() and closedir(). Returns TRUE if successful.
1832DIRHANDLEs have their own namespace separate from FILEHANDLEs.
1833
1834=item ord EXPR
1835
1836Returns the numeric ascii value of the first character of EXPR. If
1837EXPR is omitted, uses $_.
1838
1839=item pack TEMPLATE,LIST
1840
1841Takes an array or list of values and packs it into a binary structure,
1842returning the string containing the structure. The TEMPLATE is a
1843sequence of characters that give the order and type of values, as
1844follows:
1845
1846 A An ascii string, will be space padded.
1847 a An ascii string, will be null padded.
1848 b A bit string (ascending bit order, like vec()).
1849 B A bit string (descending bit order).
1850 h A hex string (low nybble first).
1851 H A hex string (high nybble first).
1852
1853 c A signed char value.
1854 C An unsigned char value.
1855 s A signed short value.
1856 S An unsigned short value.
1857 i A signed integer value.
1858 I An unsigned integer value.
1859 l A signed long value.
1860 L An unsigned long value.
1861
1862 n A short in "network" order.
1863 N A long in "network" order.
1864 v A short in "VAX" (little-endian) order.
1865 V A long in "VAX" (little-endian) order.
1866
1867 f A single-precision float in the native format.
1868 d A double-precision float in the native format.
1869
1870 p A pointer to a null-terminated string.
1871 P A pointer to a structure (fixed-length string).
1872
1873 u A uuencoded string.
1874
def98dd4
UP
1875 w A BER compressed integer. Bytes give an unsigned integer base
1876 128, most significant digit first, with as few digits as
1877 possible, and with the bit 8 of each byte except the last set
1878 to "1."
1879
a0d0e21e
LW
1880 x A null byte.
1881 X Back up a byte.
1882 @ Null fill to absolute position.
1883
1884Each letter may optionally be followed by a number which gives a repeat
1885count. With all types except "a", "A", "b", "B", "h" and "H", and "P" the
1886pack function will gobble up that many values from the LIST. A * for the
1887repeat count means to use however many items are left. The "a" and "A"
1888types gobble just one value, but pack it as a string of length count,
1889padding with nulls or spaces as necessary. (When unpacking, "A" strips
1890trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
1891fields pack a string that many bits long. The "h" and "H" fields pack a
1892string that many nybbles long. The "P" packs a pointer to a structure of
1893the size indicated by the length. Real numbers (floats and doubles) are
1894in the native machine format only; due to the multiplicity of floating
1895formats around, and the lack of a standard "network" representation, no
1896facility for interchange has been made. This means that packed floating
1897point data written on one machine may not be readable on another - even if
1898both use IEEE floating point arithmetic (as the endian-ness of the memory
1899representation is not part of the IEEE spec). Note that Perl uses doubles
1900internally for all numeric calculation, and converting from double into
1901float and thence back to double again will lose precision (i.e.
1902C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
1903
1904Examples:
1905
1906 $foo = pack("cccc",65,66,67,68);
1907 # foo eq "ABCD"
1908 $foo = pack("c4",65,66,67,68);
1909 # same thing
1910
1911 $foo = pack("ccxxcc",65,66,67,68);
1912 # foo eq "AB\0\0CD"
1913
1914 $foo = pack("s2",1,2);
1915 # "\1\0\2\0" on little-endian
1916 # "\0\1\0\2" on big-endian
1917
1918 $foo = pack("a4","abcd","x","y","z");
1919 # "abcd"
1920
1921 $foo = pack("aaaa","abcd","x","y","z");
1922 # "axyz"
1923
1924 $foo = pack("a14","abcdefg");
1925 # "abcdefg\0\0\0\0\0\0\0"
1926
1927 $foo = pack("i9pl", gmtime);
1928 # a real struct tm (on my system anyway)
1929
1930 sub bintodec {
1931 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
1932 }
1933
1934The same template may generally also be used in the unpack function.
1935
cb1a09d0
AD
1936=item package NAMESPACE
1937
1938Declares the compilation unit as being in the given namespace. The scope
1939of the package declaration is from the declaration itself through the end of
1940the enclosing block (the same scope as the local() operator). All further
1941unqualified dynamic identifiers will be in this namespace. A package
1942statement only affects dynamic variables--including those you've used
1943local() on--but I<not> lexical variables created with my(). Typically it
1944would be the first declaration in a file to be included by the C<require>
1945or C<use> operator. You can switch into a package in more than one place;
1946it merely influences which symbol table is used by the compiler for the
1947rest of that block. You can refer to variables and filehandles in other
1948packages by prefixing the identifier with the package name and a double
1949colon: C<$Package::Variable>. If the package name is null, the C<main>
1950package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
1951
1952See L<perlmod/"Packages"> for more information about packages, modules,
1953and classes. See L<perlsub> for other scoping issues.
1954
a0d0e21e
LW
1955=item pipe READHANDLE,WRITEHANDLE
1956
1957Opens a pair of connected pipes like the corresponding system call.
1958Note that if you set up a loop of piped processes, deadlock can occur
1959unless you are very careful. In addition, note that Perl's pipes use
184e9718 1960stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
1961after each command, depending on the application.
1962
4633a7c4
LW
1963See L<open2>, L<open3>, and L<perlipc/"Bidirectional Communication">
1964for examples of such things.
1965
a0d0e21e
LW
1966=item pop ARRAY
1967
1968Pops and returns the last value of the array, shortening the array by
19691. Has a similar effect to
1970
1971 $tmp = $ARRAY[$#ARRAY--];
1972
1973If there are no elements in the array, returns the undefined value.
cb1a09d0
AD
1974If ARRAY is omitted, pops the
1975@ARGV array in the main program, and the @_ array in subroutines, just
1976like shift().
a0d0e21e
LW
1977
1978=item pos SCALAR
1979
4633a7c4 1980Returns the offset of where the last C<m//g> search left off for the variable
a0d0e21e
LW
1981in question. May be modified to change that offset.
1982
1983=item print FILEHANDLE LIST
1984
1985=item print LIST
1986
1987=item print
1988
cb1a09d0 1989Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 1990if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 1991the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
1992level of indirection. (NOTE: If FILEHANDLE is a variable and the next
1993token is a term, it may be misinterpreted as an operator unless you
1994interpose a + or put parens around the arguments.) If FILEHANDLE is
1995omitted, prints by default to standard output (or to the last selected
da0045b7 1996output channel--see L</select>). If LIST is also omitted, prints $_ to
a0d0e21e
LW
1997STDOUT. To set the default output channel to something other than
1998STDOUT use the select operation. Note that, because print takes a
1999LIST, anything in the LIST is evaluated in a list context, and any
2000subroutine that you call will have one or more of its expressions
2001evaluated in a list context. Also be careful not to follow the print
2002keyword with a left parenthesis unless you want the corresponding right
2003parenthesis to terminate the arguments to the print--interpose a + or
2004put parens around all the arguments.
2005
4633a7c4 2006Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2007you will have to use a block returning its value instead:
4633a7c4
LW
2008
2009 print { $files[$i] } "stuff\n";
2010 print { $OK ? STDOUT : STDERR } "stuff\n";
2011
a0d0e21e
LW
2012=item printf FILEHANDLE LIST
2013
2014=item printf LIST
2015
2016Equivalent to a "print FILEHANDLE sprintf(LIST)". The first argument
2017of the list will be interpreted as the printf format.
2018
da0045b7
PP
2019=item prototype FUNCTION
2020
2021Returns the prototype of a function as a string (or C<undef> if the
2022function has no prototype). FUNCTION is a reference to the the
2023function whose prototype you want to retrieve.
2024
a0d0e21e
LW
2025=item push ARRAY,LIST
2026
2027Treats ARRAY as a stack, and pushes the values of LIST
2028onto the end of ARRAY. The length of ARRAY increases by the length of
2029LIST. Has the same effect as
2030
2031 for $value (LIST) {
2032 $ARRAY[++$#ARRAY] = $value;
2033 }
2034
2035but is more efficient. Returns the new number of elements in the array.
2036
2037=item q/STRING/
2038
2039=item qq/STRING/
2040
2041=item qx/STRING/
2042
2043=item qw/STRING/
2044
2045Generalized quotes. See L<perlop>.
2046
2047=item quotemeta EXPR
2048
2049Returns the value of EXPR with with all regular expression
2050metacharacters backslashed. This is the internal function implementing
2051the \Q escape in double-quoted strings.
2052
2053=item rand EXPR
2054
2055=item rand
2056
2057Returns a random fractional number between 0 and the value of EXPR.
2058(EXPR should be positive.) If EXPR is omitted, returns a value between
20590 and 1. This function produces repeatable sequences unless srand()
2060is invoked. See also srand().
2061
2062(Note: if your rand function consistently returns numbers that are too
2063large or too small, then your version of Perl was probably compiled
2064with the wrong number of RANDBITS. As a workaround, you can usually
2065multiply EXPR by the correct power of 2 to get the range you want.
2066This will make your script unportable, however. It's better to recompile
2067if you can.)
2068
2069=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2070
2071=item read FILEHANDLE,SCALAR,LENGTH
2072
2073Attempts to read LENGTH bytes of data into variable SCALAR from the
2074specified FILEHANDLE. Returns the number of bytes actually read, or
2075undef if there was an error. SCALAR will be grown or shrunk to the
2076length actually read. An OFFSET may be specified to place the read
2077data at some other place than the beginning of the string. This call
2078is actually implemented in terms of stdio's fread call. To get a true
2079read system call, see sysread().
2080
2081=item readdir DIRHANDLE
2082
2083Returns the next directory entry for a directory opened by opendir().
2084If used in a list context, returns all the rest of the entries in the
2085directory. If there are no more entries, returns an undefined value in
2086a scalar context or a null list in a list context.
2087
cb1a09d0
AD
2088If you're planning to filetest the return values out of a readdir(), you'd
2089better prepend the directory in question. Otherwise, since we didn't
2090chdir() there, it would have been testing the wrong file.
2091
2092 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2093 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2094 closedir DIR;
2095
a0d0e21e
LW
2096=item readlink EXPR
2097
2098Returns the value of a symbolic link, if symbolic links are
2099implemented. If not, gives a fatal error. If there is some system
184e9718 2100error, returns the undefined value and sets C<$!> (errno). If EXPR is
a0d0e21e
LW
2101omitted, uses $_.
2102
2103=item recv SOCKET,SCALAR,LEN,FLAGS
2104
2105Receives a message on a socket. Attempts to receive LENGTH bytes of
2106data into variable SCALAR from the specified SOCKET filehandle.
2107Actually does a C recvfrom(), so that it can returns the address of the
2108sender. Returns the undefined value if there's an error. SCALAR will
2109be grown or shrunk to the length actually read. Takes the same flags
4633a7c4
LW
2110as the system call of the same name.
2111See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2112
2113=item redo LABEL
2114
2115=item redo
2116
2117The C<redo> command restarts the loop block without evaluating the
2118conditional again. The C<continue> block, if any, is not executed. If
2119the LABEL is omitted, the command refers to the innermost enclosing
2120loop. This command is normally used by programs that want to lie to
2121themselves about what was just input:
2122
2123 # a simpleminded Pascal comment stripper
2124 # (warning: assumes no { or } in strings)
4633a7c4 2125 LINE: while (<STDIN>) {
a0d0e21e
LW
2126 while (s|({.*}.*){.*}|$1 |) {}
2127 s|{.*}| |;
2128 if (s|{.*| |) {
2129 $front = $_;
2130 while (<STDIN>) {
2131 if (/}/) { # end of comment?
2132 s|^|$front{|;
4633a7c4 2133 redo LINE;
a0d0e21e
LW
2134 }
2135 }
2136 }
2137 print;
2138 }
2139
2140=item ref EXPR
2141
2142Returns a TRUE value if EXPR is a reference, FALSE otherwise. The value
2143returned depends on the type of thing the reference is a reference to.
2144Builtin types include:
2145
2146 REF
2147 SCALAR
2148 ARRAY
2149 HASH
2150 CODE
2151 GLOB
2152
2153If the referenced object has been blessed into a package, then that package
2154name is returned instead. You can think of ref() as a typeof() operator.
2155
2156 if (ref($r) eq "HASH") {
2157 print "r is a reference to an associative array.\n";
2158 }
2159 if (!ref ($r) {
2160 print "r is not a reference at all.\n";
2161 }
2162
2163See also L<perlref>.
2164
2165=item rename OLDNAME,NEWNAME
2166
2167Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2168not work across filesystem boundaries.
2169
2170=item require EXPR
2171
2172=item require
2173
2174Demands some semantics specified by EXPR, or by $_ if EXPR is not
2175supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2176(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2177
2178Otherwise, demands that a library file be included if it hasn't already
2179been included. The file is included via the do-FILE mechanism, which is
2180essentially just a variety of eval(). Has semantics similar to the following
2181subroutine:
2182
2183 sub require {
2184 local($filename) = @_;
2185 return 1 if $INC{$filename};
2186 local($realfilename,$result);
2187 ITER: {
2188 foreach $prefix (@INC) {
2189 $realfilename = "$prefix/$filename";
2190 if (-f $realfilename) {
2191 $result = do $realfilename;
2192 last ITER;
2193 }
2194 }
2195 die "Can't find $filename in \@INC";
2196 }
2197 die $@ if $@;
2198 die "$filename did not return true value" unless $result;
2199 $INC{$filename} = $realfilename;
2200 $result;
2201 }
2202
2203Note that the file will not be included twice under the same specified
2204name. The file must return TRUE as the last statement to indicate
2205successful execution of any initialization code, so it's customary to
2206end such a file with "1;" unless you're sure it'll return TRUE
2207otherwise. But it's better just to put the "C<1;>", in case you add more
2208statements.
2209
da0045b7
PP
2210If EXPR is a bare word, the require assumes a "F<.pm>" extension and
2211replaces "F<::>" with "F</>" in the filename for you,
a0d0e21e
LW
2212to make it easy to load standard modules. This form of loading of
2213modules does not risk altering your namespace.
2214
da0045b7 2215For a yet-more-powerful import facility, see L</use> and
748a9306 2216L<perlmod>.
a0d0e21e
LW
2217
2218=item reset EXPR
2219
2220=item reset
2221
2222Generally used in a C<continue> block at the end of a loop to clear
2223variables and reset ?? searches so that they work again. The
2224expression is interpreted as a list of single characters (hyphens
2225allowed for ranges). All variables and arrays beginning with one of
2226those letters are reset to their pristine state. If the expression is
2227omitted, one-match searches (?pattern?) are reset to match again. Only
2228resets variables or searches in the current package. Always returns
22291. Examples:
2230
2231 reset 'X'; # reset all X variables
2232 reset 'a-z'; # reset lower case variables
2233 reset; # just reset ?? searches
2234
2235Resetting "A-Z" is not recommended since you'll wipe out your
2236ARGV and ENV arrays. Only resets package variables--lexical variables
2237are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2238so you'll probably want to use them instead. See L</my>.
a0d0e21e
LW
2239
2240=item return LIST
2241
2242Returns from a subroutine or eval with the value specified. (Note that
4633a7c4 2243in the absence of a return a subroutine or eval() will automatically
a0d0e21e
LW
2244return the value of the last expression evaluated.)
2245
2246=item reverse LIST
2247
2248In a list context, returns a list value consisting of the elements
2249of LIST in the opposite order. In a scalar context, returns a string
2250value consisting of the bytes of the first element of LIST in the
4633a7c4
LW
2251opposite order.
2252
2253 print reverse <>; # line tac
2254
2255 undef $/;
2256 print scalar reverse scalar <>; # byte tac
a0d0e21e
LW
2257
2258=item rewinddir DIRHANDLE
2259
2260Sets the current position to the beginning of the directory for the
2261readdir() routine on DIRHANDLE.
2262
2263=item rindex STR,SUBSTR,POSITION
2264
2265=item rindex STR,SUBSTR
2266
2267Works just like index except that it returns the position of the LAST
2268occurrence of SUBSTR in STR. If POSITION is specified, returns the
2269last occurrence at or before that position.
2270
2271=item rmdir FILENAME
2272
2273Deletes the directory specified by FILENAME if it is empty. If it
184e9718 2274succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
a0d0e21e
LW
2275FILENAME is omitted, uses $_.
2276
2277=item s///
2278
2279The substitution operator. See L<perlop>.
2280
2281=item scalar EXPR
2282
2283Forces EXPR to be interpreted in a scalar context and returns the value
cb1a09d0
AD
2284of EXPR.
2285
2286 @counts = ( scalar @a, scalar @b, scalar @c );
2287
2288There is no equivalent operator to force an expression to
2289be interpolated in a list context because it's in practice never
2290needed. If you really wanted to do so, however, you could use
2291the construction C<@{[ (some expression) ]}>, but usually a simple
2292C<(some expression)> suffices.
a0d0e21e
LW
2293
2294=item seek FILEHANDLE,POSITION,WHENCE
2295
2296Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2297call of stdio. FILEHANDLE may be an expression whose value gives the name
2298of the filehandle. The values for WHENCE are 0 to set the file pointer to
2299POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2300plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
4633a7c4 2301this from POSIX module. Returns 1 upon success, 0 otherwise.
a0d0e21e 2302
cb1a09d0
AD
2303On some systems you have to do a seek whenever you switch between reading
2304and writing. Amongst other things, this may have the effect of calling
2305stdio's clearerr(3). A "whence" of 1 (SEEK_CUR) is useful for not moving
2306the file pointer:
2307
2308 seek(TEST,0,1);
2309
2310This is also useful for applications emulating C<tail -f>. Once you hit
2311EOF on your read, and then sleep for a while, you might have to stick in a
2312seek() to reset things. First the simple trick listed above to clear the
2313filepointer. The seek() doesn't change the current position, but it
2314I<does> clear the end-of-file condition on the handle, so that the next
37798a01 2315C<E<lt>FILEE<gt>> makes Perl try again to read something. Hopefully.
cb1a09d0
AD
2316
2317If that doesn't work (some stdios are particularly cantankerous), then
2318you may need something more like this:
2319
2320 for (;;) {
2321 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2322 # search for some stuff and put it into files
2323 }
2324 sleep($for_a_while);
2325 seek(FILE, $curpos, 0);
2326 }
2327
a0d0e21e
LW
2328=item seekdir DIRHANDLE,POS
2329
2330Sets the current position for the readdir() routine on DIRHANDLE. POS
2331must be a value returned by telldir(). Has the same caveats about
2332possible directory compaction as the corresponding system library
2333routine.
2334
2335=item select FILEHANDLE
2336
2337=item select
2338
2339Returns the currently selected filehandle. Sets the current default
2340filehandle for output, if FILEHANDLE is supplied. This has two
2341effects: first, a C<write> or a C<print> without a filehandle will
2342default to this FILEHANDLE. Second, references to variables related to
2343output will refer to this output channel. For example, if you have to
2344set the top of form format for more than one output channel, you might
2345do the following:
2346
2347 select(REPORT1);
2348 $^ = 'report1_top';
2349 select(REPORT2);
2350 $^ = 'report2_top';
2351
2352FILEHANDLE may be an expression whose value gives the name of the
2353actual filehandle. Thus:
2354
2355 $oldfh = select(STDERR); $| = 1; select($oldfh);
2356
4633a7c4
LW
2357Some programmers may prefer to think of filehandles as objects with
2358methods, preferring to write the last example as:
a0d0e21e
LW
2359
2360 use FileHandle;
2361 STDERR->autoflush(1);
2362
2363=item select RBITS,WBITS,EBITS,TIMEOUT
2364
4633a7c4 2365This calls the select(2) system call with the bitmasks specified, which
a0d0e21e
LW
2366can be constructed using fileno() and vec(), along these lines:
2367
2368 $rin = $win = $ein = '';
2369 vec($rin,fileno(STDIN),1) = 1;
2370 vec($win,fileno(STDOUT),1) = 1;
2371 $ein = $rin | $win;
2372
2373If you want to select on many filehandles you might wish to write a
2374subroutine:
2375
2376 sub fhbits {
2377 local(@fhlist) = split(' ',$_[0]);
2378 local($bits);
2379 for (@fhlist) {
2380 vec($bits,fileno($_),1) = 1;
2381 }
2382 $bits;
2383 }
4633a7c4 2384 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
2385
2386The usual idiom is:
2387
2388 ($nfound,$timeleft) =
2389 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2390
c07a80fd 2391or to block until something becomes ready just do this
a0d0e21e
LW
2392
2393 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2394
c07a80fd
PP
2395Most systems do not both to return anything useful in $timeleft, so
2396calling select() in a scalar context just returns $nfound.
2397
a0d0e21e
LW
2398Any of the bitmasks can also be undef. The timeout, if specified, is
2399in seconds, which may be fractional. Note: not all implementations are
2400capable of returning the $timeleft. If not, they always return
2401$timeleft equal to the supplied $timeout.
2402
da0045b7 2403You can effect a 250-millisecond sleep this way:
a0d0e21e
LW
2404
2405 select(undef, undef, undef, 0.25);
2406
184e9718 2407B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
cb1a09d0 2408with select(). You have to use sysread() instead.
a0d0e21e
LW
2409
2410=item semctl ID,SEMNUM,CMD,ARG
2411
2412Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2413&GETALL, then ARG must be a variable which will hold the returned
2414semid_ds structure or semaphore value array. Returns like ioctl: the
2415undefined value for error, "0 but true" for zero, or the actual return
2416value otherwise.
2417
2418=item semget KEY,NSEMS,FLAGS
2419
2420Calls the System V IPC function semget. Returns the semaphore id, or
2421the undefined value if there is an error.
2422
2423=item semop KEY,OPSTRING
2424
2425Calls the System V IPC function semop to perform semaphore operations
2426such as signaling and waiting. OPSTRING must be a packed array of
2427semop structures. Each semop structure can be generated with
2428C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2429operations is implied by the length of OPSTRING. Returns TRUE if
2430successful, or FALSE if there is an error. As an example, the
2431following code waits on semaphore $semnum of semaphore id $semid:
2432
2433 $semop = pack("sss", $semnum, -1, 0);
2434 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2435
2436To signal the semaphore, replace "-1" with "1".
2437
2438=item send SOCKET,MSG,FLAGS,TO
2439
2440=item send SOCKET,MSG,FLAGS
2441
2442Sends a message on a socket. Takes the same flags as the system call
2443of the same name. On unconnected sockets you must specify a
2444destination to send TO, in which case it does a C sendto(). Returns
2445the number of characters sent, or the undefined value if there is an
2446error.
4633a7c4 2447See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2448
2449=item setpgrp PID,PGRP
2450
2451Sets the current process group for the specified PID, 0 for the current
2452process. Will produce a fatal error if used on a machine that doesn't
47e29363
PP
2453implement setpgrp(2). If the arguments are ommitted, it defaults to
24540,0. Note that the POSIX version of setpgrp() does not accept any
2455arguments, so only setpgrp 0,0 is portable.
a0d0e21e
LW
2456
2457=item setpriority WHICH,WHO,PRIORITY
2458
2459Sets the current priority for a process, a process group, or a user.
748a9306 2460(See setpriority(2).) Will produce a fatal error if used on a machine
a0d0e21e
LW
2461that doesn't implement setpriority(2).
2462
2463=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2464
2465Sets the socket option requested. Returns undefined if there is an
2466error. OPTVAL may be specified as undef if you don't want to pass an
2467argument.
2468
2469=item shift ARRAY
2470
2471=item shift
2472
2473Shifts the first value of the array off and returns it, shortening the
2474array by 1 and moving everything down. If there are no elements in the
2475array, returns the undefined value. If ARRAY is omitted, shifts the
2476@ARGV array in the main program, and the @_ array in subroutines.
2477(This is determined lexically.) See also unshift(), push(), and pop().
2478Shift() and unshift() do the same thing to the left end of an array
2479that push() and pop() do to the right end.
2480
2481=item shmctl ID,CMD,ARG
2482
2483Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2484must be a variable which will hold the returned shmid_ds structure.
2485Returns like ioctl: the undefined value for error, "0 but true" for
2486zero, or the actual return value otherwise.
2487
2488=item shmget KEY,SIZE,FLAGS
2489
2490Calls the System V IPC function shmget. Returns the shared memory
2491segment id, or the undefined value if there is an error.
2492
2493=item shmread ID,VAR,POS,SIZE
2494
2495=item shmwrite ID,STRING,POS,SIZE
2496
2497Reads or writes the System V shared memory segment ID starting at
2498position POS for size SIZE by attaching to it, copying in/out, and
2499detaching from it. When reading, VAR must be a variable which will
2500hold the data read. When writing, if STRING is too long, only SIZE
2501bytes are used; if STRING is too short, nulls are written to fill out
2502SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2503
2504=item shutdown SOCKET,HOW
2505
2506Shuts down a socket connection in the manner indicated by HOW, which
2507has the same interpretation as in the system call of the same name.
2508
2509=item sin EXPR
2510
2511Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2512returns sine of $_.
2513
2514=item sleep EXPR
2515
2516=item sleep
2517
2518Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2519May be interrupted by sending the process a SIGALRM. Returns the
2520number of seconds actually slept. You probably cannot mix alarm() and
2521sleep() calls, since sleep() is often implemented using alarm().
2522
2523On some older systems, it may sleep up to a full second less than what
2524you requested, depending on how it counts seconds. Most modern systems
2525always sleep the full amount.
2526
cb1a09d0
AD
2527For delays of finer granularity than one second, you may use Perl's
2528syscall() interface to access setitimer(2) if your system supports it,
2529or else see L</select()> below.
2530
a0d0e21e
LW
2531=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2532
2533Opens a socket of the specified kind and attaches it to filehandle
2534SOCKET. DOMAIN, TYPE and PROTOCOL are specified the same as for the
2535system call of the same name. You should "use Socket;" first to get
4633a7c4 2536the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2537
2538=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2539
2540Creates an unnamed pair of sockets in the specified domain, of the
2541specified type. DOMAIN, TYPE and PROTOCOL are specified the same as
2542for the system call of the same name. If unimplemented, yields a fatal
2543error. Returns TRUE if successful.
2544
2545=item sort SUBNAME LIST
2546
2547=item sort BLOCK LIST
2548
2549=item sort LIST
2550
2551Sorts the LIST and returns the sorted list value. Nonexistent values
2552of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2553in standard string comparison order. If SUBNAME is specified, it
2554gives the name of a subroutine that returns an integer less than, equal
2555to, or greater than 0, depending on how the elements of the array are
184e9718 2556to be ordered. (The E<lt>=E<gt> and cmp operators are extremely useful in such
a0d0e21e
LW
2557routines.) SUBNAME may be a scalar variable name, in which case the
2558value provides the name of the subroutine to use. In place of a
2559SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2560subroutine.
2561
cb1a09d0
AD
2562In the interests of efficiency the normal calling code for subroutines is
2563bypassed, with the following effects: the subroutine may not be a
2564recursive subroutine, and the two elements to be compared are passed into
2565the subroutine not via @_ but as the package global variables $a and
2566$b (see example below). They are passed by reference, so don't
2567modify $a and $b. And don't try to declare them as lexicals either.
a0d0e21e
LW
2568
2569Examples:
2570
2571 # sort lexically
2572 @articles = sort @files;
2573
2574 # same thing, but with explicit sort routine
2575 @articles = sort {$a cmp $b} @files;
2576
cb1a09d0
AD
2577 # now case-insensitively
2578 @articles = sort { uc($a) cmp uc($b)} @files;
2579
a0d0e21e
LW
2580 # same thing in reversed order
2581 @articles = sort {$b cmp $a} @files;
2582
2583 # sort numerically ascending
2584 @articles = sort {$a <=> $b} @files;
2585
2586 # sort numerically descending
2587 @articles = sort {$b <=> $a} @files;
2588
2589 # sort using explicit subroutine name
2590 sub byage {
2591 $age{$a} <=> $age{$b}; # presuming integers
2592 }
2593 @sortedclass = sort byage @class;
2594
c07a80fd
PP
2595 # this sorts the %age associative arrays by value
2596 # instead of key using an inline function
2597 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
2598
a0d0e21e
LW
2599 sub backwards { $b cmp $a; }
2600 @harry = ('dog','cat','x','Cain','Abel');
2601 @george = ('gone','chased','yz','Punished','Axed');
2602 print sort @harry;
2603 # prints AbelCaincatdogx
2604 print sort backwards @harry;
2605 # prints xdogcatCainAbel
2606 print sort @george, 'to', @harry;
2607 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2608
cb1a09d0
AD
2609 # inefficiently sort by descending numeric compare using
2610 # the first integer after the first = sign, or the
2611 # whole record case-insensitively otherwise
2612
2613 @new = sort {
2614 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
2615 ||
2616 uc($a) cmp uc($b)
2617 } @old;
2618
2619 # same thing, but much more efficiently;
2620 # we'll build auxiliary indices instead
2621 # for speed
2622 @nums = @caps = ();
2623 for (@old) {
2624 push @nums, /=(\d+)/;
2625 push @caps, uc($_);
2626 }
2627
2628 @new = @old[ sort {
2629 $nums[$b] <=> $nums[$a]
2630 ||
2631 $caps[$a] cmp $caps[$b]
2632 } 0..$#old
2633 ];
2634
2635 # same thing using a Schwartzian Transform (no temps)
2636 @new = map { $_->[0] }
2637 sort { $b->[1] <=> $a->[1]
2638 ||
2639 $a->[2] cmp $b->[2]
2640 } map { [$_, /=(\d+)/, uc($_)] } @old;
2641
184e9718 2642If you're using strict, you I<MUST NOT> declare $a
cb1a09d0
AD
2643and $b as lexicals. They are package globals. That means
2644if you're in the C<main> package, it's
2645
2646 @articles = sort {$main::b <=> $main::a} @files;
2647
2648or just
2649
2650 @articles = sort {$::b <=> $::a} @files;
2651
2652but if you're in the C<FooPack> package, it's
2653
2654 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
2655
a0d0e21e
LW
2656=item splice ARRAY,OFFSET,LENGTH,LIST
2657
2658=item splice ARRAY,OFFSET,LENGTH
2659
2660=item splice ARRAY,OFFSET
2661
2662Removes the elements designated by OFFSET and LENGTH from an array, and
2663replaces them with the elements of LIST, if any. Returns the elements
2664removed from the array. The array grows or shrinks as necessary. If
2665LENGTH is omitted, removes everything from OFFSET onward. The
184e9718 2666following equivalencies hold (assuming C<$[ == 0>):
a0d0e21e
LW
2667
2668 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2669 pop(@a) splice(@a,-1)
2670 shift(@a) splice(@a,0,1)
2671 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2672 $a[$x] = $y splice(@a,$x,1,$y);
2673
2674Example, assuming array lengths are passed before arrays:
2675
2676 sub aeq { # compare two list values
2677 local(@a) = splice(@_,0,shift);
2678 local(@b) = splice(@_,0,shift);
2679 return 0 unless @a == @b; # same len?
2680 while (@a) {
2681 return 0 if pop(@a) ne pop(@b);
2682 }
2683 return 1;
2684 }
2685 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2686
2687=item split /PATTERN/,EXPR,LIMIT
2688
2689=item split /PATTERN/,EXPR
2690
2691=item split /PATTERN/
2692
2693=item split
2694
2695Splits a string into an array of strings, and returns it.
2696
2697If not in a list context, returns the number of fields found and splits into
2698the @_ array. (In a list context, you can force the split into @_ by
2699using C<??> as the pattern delimiters, but it still returns the array
2700value.) The use of implicit split to @_ is deprecated, however.
2701
2702If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
4633a7c4
LW
2703splits on whitespace (after skipping any leading whitespace). Anything
2704matching PATTERN is taken to be a delimiter separating the fields. (Note
2705that the delimiter may be longer than one character.) If LIMIT is
2706specified and is not negative, splits into no more than that many fields
2707(though it may split into fewer). If LIMIT is unspecified, trailing null
2708fields are stripped (which potential users of pop() would do well to
2709remember). If LIMIT is negative, it is treated as if an arbitrarily large
2710LIMIT had been specified.
a0d0e21e
LW
2711
2712A pattern matching the null string (not to be confused with
748a9306 2713a null pattern C<//>, which is just one member of the set of patterns
a0d0e21e
LW
2714matching a null string) will split the value of EXPR into separate
2715characters at each point it matches that way. For example:
2716
2717 print join(':', split(/ */, 'hi there'));
2718
2719produces the output 'h:i:t:h:e:r:e'.
2720
2721The LIMIT parameter can be used to partially split a line
2722
2723 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2724
2725When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2726one larger than the number of variables in the list, to avoid
2727unnecessary work. For the list above LIMIT would have been 4 by
2728default. In time critical applications it behooves you not to split
2729into more fields than you really need.
2730
2731If the PATTERN contains parentheses, additional array elements are
2732created from each matching substring in the delimiter.
2733
da0045b7 2734 split(/([,-])/, "1-10,20", 3);
a0d0e21e
LW
2735
2736produces the list value
2737
2738 (1, '-', 10, ',', 20)
2739
4633a7c4
LW
2740If you had the entire header of a normal Unix email message in $header,
2741you could split it up into fields and their values this way:
2742
2743 $header =~ s/\n\s+/ /g; # fix continuation lines
2744 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2745
a0d0e21e
LW
2746The pattern C</PATTERN/> may be replaced with an expression to specify
2747patterns that vary at runtime. (To do runtime compilation only once,
748a9306
LW
2748use C</$variable/o>.)
2749
2750As a special case, specifying a PATTERN of space (C<' '>) will split on
2751white space just as split with no arguments does. Thus, split(' ') can
2752be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
2753will give you as many null initial fields as there are leading spaces.
2754A split on /\s+/ is like a split(' ') except that any leading
2755whitespace produces a null first field. A split with no arguments
2756really does a C<split(' ', $_)> internally.
a0d0e21e
LW
2757
2758Example:
2759
2760 open(passwd, '/etc/passwd');
2761 while (<passwd>) {
748a9306
LW
2762 ($login, $passwd, $uid, $gid, $gcos,
2763 $home, $shell) = split(/:/);
a0d0e21e
LW
2764 ...
2765 }
2766
2767(Note that $shell above will still have a newline on it. See L</chop>,
2768L</chomp>, and L</join>.)
2769
2770=item sprintf FORMAT,LIST
2771
2772Returns a string formatted by the usual printf conventions of the C
cb1a09d0
AD
2773language. See L<sprintf(3)> or L<printf(3)> on your system for details.
2774(The * character for an indirectly specified length is not
a0d0e21e 2775supported, but you can get the same effect by interpolating a variable
cb1a09d0
AD
2776into the pattern.) Some C libraries' implementations of sprintf() can
2777dump core when fed ludicrous arguments.
a0d0e21e
LW
2778
2779=item sqrt EXPR
2780
2781Return the square root of EXPR. If EXPR is omitted, returns square
2782root of $_.
2783
2784=item srand EXPR
2785
cb1a09d0 2786Sets the random number seed for the C<rand> operator. If EXPR is omitted,
da0045b7
PP
2787uses a semirandom value based on the current time and process ID, among
2788other things. Of course, you'd need something much more random than that for
cb1a09d0
AD
2789cryptographic purposes, since it's easy to guess the current time.
2790Checksumming the compressed output of rapidly changing operating system
2791status programs is the usual method. Examples are posted regularly to
2792the comp.security.unix newsgroup.
a0d0e21e
LW
2793
2794=item stat FILEHANDLE
2795
2796=item stat EXPR
2797
2798Returns a 13-element array giving the status info for a file, either the
2799file opened via FILEHANDLE, or named by EXPR. Returns a null list if
2800the stat fails. Typically used as follows:
2801
2802 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
2803 $atime,$mtime,$ctime,$blksize,$blocks)
2804 = stat($filename);
2805
c07a80fd
PP
2806Not all fields are supported on all filesystem types. Here are the
2807meaning of the fields:
2808
2809 dev device number of filesystem
2810 ino inode number
2811 mode file mode (type and permissions)
2812 nlink number of (hard) links to the file
2813 uid numeric user ID of file's owner
2814 gid numer group ID of file's owner
2815 rdev the device identifier (special files only)
2816 size total size of file, in bytes
2817 atime last access time since the epoch
2818 mtime last modify time since the epoch
2819 ctime inode change time (NOT creation type!) since the epoch
2820 blksize preferred blocksize for file system I/O
2821 blocks actual number of blocks allocated
2822
2823(The epoch was at 00:00 January 1, 1970 GMT.)
2824
a0d0e21e
LW
2825If stat is passed the special filehandle consisting of an underline, no
2826stat is done, but the current contents of the stat structure from the
2827last stat or filetest are returned. Example:
2828
2829 if (-x $file && (($d) = stat(_)) && $d < 0) {
2830 print "$file is executable NFS file\n";
2831 }
2832
2833(This only works on machines for which the device number is negative under NFS.)
2834
2835=item study SCALAR
2836
2837=item study
2838
184e9718 2839Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
a0d0e21e
LW
2840doing many pattern matches on the string before it is next modified.
2841This may or may not save time, depending on the nature and number of
2842patterns you are searching on, and on the distribution of character
2843frequencies in the string to be searched--you probably want to compare
2844runtimes with and without it to see which runs faster. Those loops
2845which scan for many short constant strings (including the constant
2846parts of more complex patterns) will benefit most. You may have only
2847one study active at a time--if you study a different scalar the first
2848is "unstudied". (The way study works is this: a linked list of every
2849character in the string to be searched is made, so we know, for
2850example, where all the 'k' characters are. From each search string,
2851the rarest character is selected, based on some static frequency tables
2852constructed from some C programs and English text. Only those places
2853that contain this "rarest" character are examined.)
2854
2855For example, here is a loop which inserts index producing entries
2856before any line containing a certain pattern:
2857
2858 while (<>) {
2859 study;
2860 print ".IX foo\n" if /\bfoo\b/;
2861 print ".IX bar\n" if /\bbar\b/;
2862 print ".IX blurfl\n" if /\bblurfl\b/;
2863 ...
2864 print;
2865 }
2866
2867In searching for /\bfoo\b/, only those locations in $_ that contain "f"
2868will be looked at, because "f" is rarer than "o". In general, this is
2869a big win except in pathological cases. The only question is whether
2870it saves you more time than it took to build the linked list in the
2871first place.
2872
2873Note that if you have to look for strings that you don't know till
2874runtime, you can build an entire loop as a string and eval that to
2875avoid recompiling all your patterns all the time. Together with
2876undefining $/ to input entire files as one record, this can be very
2877fast, often faster than specialized programs like fgrep(1). The following
184e9718 2878scans a list of files (C<@files>) for a list of words (C<@words>), and prints
a0d0e21e
LW
2879out the names of those files that contain a match:
2880
2881 $search = 'while (<>) { study;';
2882 foreach $word (@words) {
2883 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
2884 }
2885 $search .= "}";
2886 @ARGV = @files;
2887 undef $/;
2888 eval $search; # this screams
2889 $/ = "\n"; # put back to normal input delim
2890 foreach $file (sort keys(%seen)) {
2891 print $file, "\n";
2892 }
2893
cb1a09d0
AD
2894=item sub BLOCK
2895
2896=item sub NAME
2897
2898=item sub NAME BLOCK
2899
2900This is subroutine definition, not a real function I<per se>. With just a
2901NAME (and possibly prototypes), it's just a forward declaration. Without
2902a NAME, it's an anonymous function declaration, and does actually return a
2903value: the CODE ref of the closure you just created. See L<perlsub> and
2904L<perlref> for details.
2905
a0d0e21e
LW
2906=item substr EXPR,OFFSET,LEN
2907
2908=item substr EXPR,OFFSET
2909
2910Extracts a substring out of EXPR and returns it. First character is at
2911offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
2912that far from the end of the string. If LEN is omitted, returns
748a9306
LW
2913everything to the end of the string. If LEN is negative, leaves that
2914many characters off the end of the string.
2915
2916You can use the substr() function
a0d0e21e
LW
2917as an lvalue, in which case EXPR must be an lvalue. If you assign
2918something shorter than LEN, the string will shrink, and if you assign
2919something longer than LEN, the string will grow to accommodate it. To
2920keep the string the same length you may need to pad or chop your value
2921using sprintf().
2922
2923=item symlink OLDFILE,NEWFILE
2924
2925Creates a new filename symbolically linked to the old filename.
2926Returns 1 for success, 0 otherwise. On systems that don't support
2927symbolic links, produces a fatal error at run time. To check for that,
2928use eval:
2929
2930 $symlink_exists = (eval 'symlink("","");', $@ eq '');
2931
2932=item syscall LIST
2933
2934Calls the system call specified as the first element of the list,
2935passing the remaining elements as arguments to the system call. If
2936unimplemented, produces a fatal error. The arguments are interpreted
2937as follows: if a given argument is numeric, the argument is passed as
2938an int. If not, the pointer to the string value is passed. You are
2939responsible to make sure a string is pre-extended long enough to
2940receive any result that might be written into a string. If your
2941integer arguments are not literals and have never been interpreted in a
2942numeric context, you may need to add 0 to them to force them to look
2943like numbers.
2944
2945 require 'syscall.ph'; # may need to run h2ph
2946 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
2947
2948Note that Perl only supports passing of up to 14 arguments to your system call,
2949which in practice should usually suffice.
2950
c07a80fd
PP
2951=item sysopen FILEHANDLE,FILENAME,MODE
2952
2953=item sysopen FILEHANDLE,FILENAME,MODE,PERMS
2954
2955Opens the file whose filename is given by FILENAME, and associates it
2956with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
2957the name of the real filehandle wanted. This function calls the
2958underlying operating system's C<open> function with the parameters
2959FILENAME, MODE, PERMS.
2960
2961The possible values and flag bits of the MODE parameter are
2962system-dependent; they are available via the standard module C<Fcntl>.
2963However, for historical reasons, some values are universal: zero means
2964read-only, one means write-only, and two means read/write.
2965
2966If the file named by FILENAME does not exist and the C<open> call
2967creates it (typically because MODE includes the O_CREAT flag), then
2968the value of PERMS specifies the permissions of the newly created
2969file. If PERMS is omitted, the default value is 0666, which allows
2970read and write for all. This default is reasonable: see C<umask>.
2971
a0d0e21e
LW
2972=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
2973
2974=item sysread FILEHANDLE,SCALAR,LENGTH
2975
2976Attempts to read LENGTH bytes of data into variable SCALAR from the
2977specified FILEHANDLE, using the system call read(2). It bypasses
2978stdio, so mixing this with other kinds of reads may cause confusion.
2979Returns the number of bytes actually read, or undef if there was an
2980error. SCALAR will be grown or shrunk to the length actually read. An
2981OFFSET may be specified to place the read data at some other place than
2982the beginning of the string.
2983
2984=item system LIST
2985
2986Does exactly the same thing as "exec LIST" except that a fork is done
2987first, and the parent process waits for the child process to complete.
2988Note that argument processing varies depending on the number of
2989arguments. The return value is the exit status of the program as
2990returned by the wait() call. To get the actual exit value divide by
cb1a09d0
AD
2991256. See also L</exec>. This is I<NOT> what you want to use to capture
2992the output from a command, for that you should merely use backticks, as
2993described in L<perlop/"`STRING`">.
a0d0e21e
LW
2994
2995=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
2996
2997=item syswrite FILEHANDLE,SCALAR,LENGTH
2998
2999Attempts to write LENGTH bytes of data from variable SCALAR to the
3000specified FILEHANDLE, using the system call write(2). It bypasses
3001stdio, so mixing this with prints may cause confusion. Returns the
3002number of bytes actually written, or undef if there was an error. An
22dc801b 3003OFFSET may be specified to get the write data from some other place than
a0d0e21e
LW
3004the beginning of the string.
3005
3006=item tell FILEHANDLE
3007
3008=item tell
3009
3010Returns the current file position for FILEHANDLE. FILEHANDLE may be an
3011expression whose value gives the name of the actual filehandle. If
3012FILEHANDLE is omitted, assumes the file last read.
3013
3014=item telldir DIRHANDLE
3015
3016Returns the current position of the readdir() routines on DIRHANDLE.
3017Value may be given to seekdir() to access a particular location in a
3018directory. Has the same caveats about possible directory compaction as
3019the corresponding system library routine.
3020
4633a7c4 3021=item tie VARIABLE,CLASSNAME,LIST
a0d0e21e 3022
4633a7c4
LW
3023This function binds a variable to a package class that will provide the
3024implementation for the variable. VARIABLE is the name of the variable
3025to be enchanted. CLASSNAME is the name of a class implementing objects
3026of correct type. Any additional arguments are passed to the "new"
3027method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
3028Typically these are arguments such as might be passed to the dbm_open()
cb1a09d0
AD
3029function of C. The object returned by the "new" method is also
3030returned by the tie() function, which would be useful if you want to
4633a7c4 3031access other methods in CLASSNAME.
a0d0e21e
LW
3032
3033Note that functions such as keys() and values() may return huge array
748a9306
LW
3034values when used on large objects, like DBM files. You may prefer to
3035use the each() function to iterate over such. Example:
a0d0e21e
LW
3036
3037 # print out history file offsets
4633a7c4 3038 use NDBM_File;
da0045b7 3039 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
a0d0e21e
LW
3040 while (($key,$val) = each %HIST) {
3041 print $key, ' = ', unpack('L',$val), "\n";
3042 }
3043 untie(%HIST);
3044
4633a7c4 3045A class implementing an associative array should have the following
a0d0e21e
LW
3046methods:
3047
4633a7c4 3048 TIEHASH classname, LIST
a0d0e21e
LW
3049 DESTROY this
3050 FETCH this, key
3051 STORE this, key, value
3052 DELETE this, key
3053 EXISTS this, key
3054 FIRSTKEY this
3055 NEXTKEY this, lastkey
3056
4633a7c4 3057A class implementing an ordinary array should have the following methods:
a0d0e21e 3058
4633a7c4 3059 TIEARRAY classname, LIST
a0d0e21e
LW
3060 DESTROY this
3061 FETCH this, key
3062 STORE this, key, value
3063 [others TBD]
3064
4633a7c4 3065A class implementing a scalar should have the following methods:
a0d0e21e 3066
4633a7c4 3067 TIESCALAR classname, LIST
a0d0e21e
LW
3068 DESTROY this
3069 FETCH this,
3070 STORE this, value
3071
4633a7c4
LW
3072Unlike dbmopen(), the tie() function will not use or require a module
3073for you--you need to do that explicitly yourself. See L<DB_File>
3074or the F<Config> module for interesting tie() implementations.
3075
f3cbc334
RS
3076=item tied VARIABLE
3077
3078Returns a reference to the object underlying VARIABLE (the same value
3079that was originally returned by the tie() call which bound the variable
3080to a package.) Returns the undefined value if VARIABLE isn't tied to a
3081package.
3082
a0d0e21e
LW
3083=item time
3084
da0045b7
PP
3085Returns the number of non-leap seconds since whatever time the system
3086considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
3087and 00:00:00 UTC, January 1, 1970 for most other systems).
3088Suitable for feeding to gmtime() and localtime().
a0d0e21e
LW
3089
3090=item times
3091
3092Returns a four-element array giving the user and system times, in
3093seconds, for this process and the children of this process.
3094
3095 ($user,$system,$cuser,$csystem) = times;
3096
3097=item tr///
3098
3099The translation operator. See L<perlop>.
3100
3101=item truncate FILEHANDLE,LENGTH
3102
3103=item truncate EXPR,LENGTH
3104
3105Truncates the file opened on FILEHANDLE, or named by EXPR, to the
3106specified length. Produces a fatal error if truncate isn't implemented
3107on your system.
3108
3109=item uc EXPR
3110
3111Returns an uppercased version of EXPR. This is the internal function
3112implementing the \U escape in double-quoted strings.
4633a7c4 3113Should respect any POSIX setlocale() settings.
a0d0e21e
LW
3114
3115=item ucfirst EXPR
3116
3117Returns the value of EXPR with the first character uppercased. This is
3118the internal function implementing the \u escape in double-quoted strings.
4633a7c4 3119Should respect any POSIX setlocale() settings.
a0d0e21e
LW
3120
3121=item umask EXPR
3122
3123=item umask
3124
3125Sets the umask for the process and returns the old one. If EXPR is
3126omitted, merely returns current umask.
3127
3128=item undef EXPR
3129
3130=item undef
3131
3132Undefines the value of EXPR, which must be an lvalue. Use only on a
3133scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
3134will probably not do what you expect on most predefined variables or
3135DBM list values, so don't do that.) Always returns the undefined value. You can omit
3136the EXPR, in which case nothing is undefined, but you still get an
3137undefined value that you could, for instance, return from a
3138subroutine. Examples:
3139
3140 undef $foo;
3141 undef $bar{'blurfl'};
3142 undef @ary;
3143 undef %assoc;
3144 undef &mysub;
3145 return (wantarray ? () : undef) if $they_blew_it;
3146
3147=item unlink LIST
3148
3149Deletes a list of files. Returns the number of files successfully
3150deleted.
3151
3152 $cnt = unlink 'a', 'b', 'c';
3153 unlink @goners;
3154 unlink <*.bak>;
3155
3156Note: unlink will not delete directories unless you are superuser and
3157the B<-U> flag is supplied to Perl. Even if these conditions are
3158met, be warned that unlinking a directory can inflict damage on your
3159filesystem. Use rmdir instead.
3160
3161=item unpack TEMPLATE,EXPR
3162
3163Unpack does the reverse of pack: it takes a string representing a
3164structure and expands it out into a list value, returning the array
3165value. (In a scalar context, it merely returns the first value
3166produced.) The TEMPLATE has the same format as in the pack function.
3167Here's a subroutine that does substring:
3168
3169 sub substr {
3170 local($what,$where,$howmuch) = @_;
3171 unpack("x$where a$howmuch", $what);
3172 }
3173
3174and then there's
3175
3176 sub ordinal { unpack("c",$_[0]); } # same as ord()
3177
184e9718
PP
3178In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
3179you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
a0d0e21e
LW
3180themselves. Default is a 16-bit checksum. For example, the following
3181computes the same number as the System V sum program:
3182
3183 while (<>) {
3184 $checksum += unpack("%16C*", $_);
3185 }
3186 $checksum %= 65536;
3187
3188The following efficiently counts the number of set bits in a bit vector:
3189
3190 $setbits = unpack("%32b*", $selectmask);
3191
3192=item untie VARIABLE
3193
3194Breaks the binding between a variable and a package. (See tie().)
3195
3196=item unshift ARRAY,LIST
3197
3198Does the opposite of a C<shift>. Or the opposite of a C<push>,
3199depending on how you look at it. Prepends list to the front of the
3200array, and returns the new number of elements in the array.
3201
3202 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
3203
3204Note the LIST is prepended whole, not one element at a time, so the
3205prepended elements stay in the same order. Use reverse to do the
3206reverse.
3207
3208=item use Module LIST
3209
3210=item use Module
3211
da0045b7
PP
3212=item use Module VERSION LIST
3213
3214=item use VERSION
3215
a0d0e21e
LW
3216Imports some semantics into the current package from the named module,
3217generally by aliasing certain subroutine or variable names into your
3218package. It is exactly equivalent to
3219
3220 BEGIN { require Module; import Module LIST; }
3221
da0045b7
PP
3222except that Module I<must> be a bare word.
3223
3224If the first argument to C<use> is a number, it is treated as a version
3225number instead of a module name. If the version of the Perl interpreter
3226is less than VERSION, then an error message is printed and Perl exits
3227immediately. This is often useful if you need to check the current
3228Perl version before C<use>ing library modules which have changed in
3229incompatible ways from older versions of Perl. (We try not to do
3230this more than we have to.)
3231
a0d0e21e
LW
3232The BEGIN forces the require and import to happen at compile time. The
3233require makes sure the module is loaded into memory if it hasn't been
3234yet. The import is not a builtin--it's just an ordinary static method
3235call into the "Module" package to tell the module to import the list of
3236features back into the current package. The module can implement its
3237import method any way it likes, though most modules just choose to
3238derive their import method via inheritance from the Exporter class that
cb1a09d0
AD
3239is defined in the Exporter module. See L<Exporter>.
3240
3241If you don't want your namespace altered, explicitly supply an empty list:
3242
3243 use Module ();
3244
3245That is exactly equivalent to
3246
3247 BEGIN { require Module; }
a0d0e21e 3248
da0045b7
PP
3249If the VERSION argument is present between Module and LIST, then the
3250C<use> will fail if the C<$VERSION> variable in package Module is
3251less than VERSION.
3252
a0d0e21e
LW
3253Because this is a wide-open interface, pragmas (compiler directives)
3254are also implemented this way. Currently implemented pragmas are:
3255
3256 use integer;
4633a7c4 3257 use diagnostics;
a0d0e21e
LW
3258 use sigtrap qw(SEGV BUS);
3259 use strict qw(subs vars refs);
3260 use subs qw(afunc blurfl);
3261
3262These pseudomodules import semantics into the current block scope, unlike
3263ordinary modules, which import symbols into the current package (which are
3264effective through the end of the file).
3265
3266There's a corresponding "no" command that unimports meanings imported
da0045b7 3267by use, i.e. it calls C<unimport Module LIST> instead of C<import>.
a0d0e21e
LW
3268
3269 no integer;
3270 no strict 'refs';
3271
3272See L<perlmod> for a list of standard modules and pragmas.
3273
3274=item utime LIST
3275
3276Changes the access and modification times on each file of a list of
3277files. The first two elements of the list must be the NUMERICAL access
3278and modification times, in that order. Returns the number of files
3279successfully changed. The inode modification time of each file is set
3280to the current time. Example of a "touch" command:
3281
3282 #!/usr/bin/perl
3283 $now = time;
3284 utime $now, $now, @ARGV;
3285
3286=item values ASSOC_ARRAY
3287
3288Returns a normal array consisting of all the values of the named
3289associative array. (In a scalar context, returns the number of
3290values.) The values are returned in an apparently random order, but it
3291is the same order as either the keys() or each() function would produce
c07a80fd 3292on the same array. See also keys(), each(), and sort().
a0d0e21e
LW
3293
3294=item vec EXPR,OFFSET,BITS
3295
22dc801b
PP
3296Treats the string in EXPR as a vector of unsigned integers, and
3297returns the value of the bitfield specified by OFFSET. BITS specifies
3298the number of bits that are reserved for each entry in the bit
3299vector. This must be a power of two from 1 to 32. vec() may also be
3300assigned to, in which case parens are needed to give the expression
3301the correct precedence as in
3302
3303 vec($image, $max_x * $x + $y, 8) = 3;
a0d0e21e
LW
3304
3305Vectors created with vec() can also be manipulated with the logical
3306operators |, & and ^, which will assume a bit vector operation is
3307desired when both operands are strings.
3308
3309To transform a bit vector into a string or array of 0's and 1's, use these:
3310
3311 $bits = unpack("b*", $vector);
3312 @bits = split(//, unpack("b*", $vector));
3313
3314If you know the exact length in bits, it can be used in place of the *.
3315
3316=item wait
3317
3318Waits for a child process to terminate and returns the pid of the
3319deceased process, or -1 if there are no child processes. The status is
184e9718 3320returned in C<$?>.
a0d0e21e
LW
3321
3322=item waitpid PID,FLAGS
3323
3324Waits for a particular child process to terminate and returns the pid
3325of the deceased process, or -1 if there is no such child process. The
184e9718 3326status is returned in C<$?>. If you say
a0d0e21e 3327
47e29363 3328 use POSIX ":wait_h";
a0d0e21e
LW
3329 ...
3330 waitpid(-1,&WNOHANG);
3331
3332then you can do a non-blocking wait for any process. Non-blocking wait
3333is only available on machines supporting either the waitpid(2) or
3334wait4(2) system calls. However, waiting for a particular pid with
3335FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3336by remembering the status values of processes that have exited but have
3337not been harvested by the Perl script yet.)
3338
3339=item wantarray
3340
3341Returns TRUE if the context of the currently executing subroutine is
3342looking for a list value. Returns FALSE if the context is looking
3343for a scalar.
3344
3345 return wantarray ? () : undef;
3346
3347=item warn LIST
3348
3349Produces a message on STDERR just like die(), but doesn't exit or
4633a7c4 3350on an exception.
a0d0e21e
LW
3351
3352=item write FILEHANDLE
3353
3354=item write EXPR
3355
3356=item write
3357
3358Writes a formatted record (possibly multi-line) to the specified file,
3359using the format associated with that file. By default the format for
3360a file is the one having the same name is the filehandle, but the
3361format for the current output channel (see the select() function) may be set
184e9718 3362explicitly by assigning the name of the format to the C<$~> variable.
a0d0e21e
LW
3363
3364Top of form processing is handled automatically: if there is
3365insufficient room on the current page for the formatted record, the
3366page is advanced by writing a form feed, a special top-of-page format
3367is used to format the new page header, and then the record is written.
3368By default the top-of-page format is the name of the filehandle with
3369"_TOP" appended, but it may be dynamically set to the format of your
184e9718 3370choice by assigning the name to the C<$^> variable while the filehandle is
a0d0e21e 3371selected. The number of lines remaining on the current page is in
184e9718 3372variable C<$->, which can be set to 0 to force a new page.
a0d0e21e
LW
3373
3374If FILEHANDLE is unspecified, output goes to the current default output
3375channel, which starts out as STDOUT but may be changed by the
3376C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3377is evaluated and the resulting string is used to look up the name of
3378the FILEHANDLE at run time. For more on formats, see L<perlform>.
3379
3380Note that write is I<NOT> the opposite of read. Unfortunately.
3381
3382=item y///
3383
37798a01 3384The translation operator. See L<perlop>.
a0d0e21e
LW
3385
3386=back