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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
57=over 5
58
59=item *
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
73=over 8
74
75=item -X FILEHANDLE
76
77=item -X EXPR
78
79=item -X
80
81A file test, where X is one of the letters listed below. This unary
82operator takes one argument, either a filename or a filehandle, and
83tests the associated file to see if something is true about it. If the
84argument is omitted, tests $_, except for C<-t>, which tests STDIN.
85Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
86the undefined value if the file doesn't exist. Despite the funny
87names, precedence is the same as any other named unary operator, and
88the argument may be parenthesized like any other unary operator. The
89operator may be any of:
90
91 -r File is readable by effective uid/gid.
92 -w File is writable by effective uid/gid.
93 -x File is executable by effective uid/gid.
94 -o File is owned by effective uid.
95
96 -R File is readable by real uid/gid.
97 -W File is writable by real uid/gid.
98 -X File is executable by real uid/gid.
99 -O File is owned by real uid.
100
101 -e File exists.
102 -z File has zero size.
103 -s File has non-zero size (returns size).
104
105 -f File is a plain file.
106 -d File is a directory.
107 -l File is a symbolic link.
108 -p File is a named pipe (FIFO).
109 -S File is a socket.
110 -b File is a block special file.
111 -c File is a character special file.
112 -t Filehandle is opened to a tty.
113
114 -u File has setuid bit set.
115 -g File has setgid bit set.
116 -k File has sticky bit set.
117
118 -T File is a text file.
119 -B File is a binary file (opposite of -T).
120
121 -M Age of file in days when script started.
122 -A Same for access time.
123 -C Same for inode change time.
124
125The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
126C<-W>, C<-x> and C<-X> is based solely on the mode of the file and the
127uids and gids of the user. There may be other reasons you can't actually
128read, write or execute the file. Also note that, for the superuser,
129C<-r>, C<-R>, C<-w> and C<-W> always return 1, and C<-x> and C<-X> return
1301 if any execute bit is set in the mode. Scripts run by the superuser may
131thus need to do a stat() in order to determine the actual mode of the
132file, or temporarily set the uid to something else.
133
134Example:
135
136 while (<>) {
137 chop;
138 next unless -f $_; # ignore specials
139 ...
140 }
141
142Note that C<-s/a/b/> does not do a negated substitution. Saying
143C<-exp($foo)> still works as expected, however--only single letters
144following a minus are interpreted as file tests.
145
146The C<-T> and C<-B> switches work as follows. The first block or so of the
147file is examined for odd characters such as strange control codes or
148characters with the high bit set. If too many odd characters (>30%)
149are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
150containing null in the first block is considered a binary file. If C<-T>
151or C<-B> is used on a filehandle, the current stdio buffer is examined
152rather than the first block. Both C<-T> and C<-B> return TRUE on a null
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153file, or a file at EOF when testing a filehandle. Because you have to
154read a file to do the C<-T> test, on most occasions you want to use a C<-f>
155against the file first, as in C<next unless -f $file && -T $file>.
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156
157If any of the file tests (or either the stat() or lstat() operators) are given the
158special filehandle consisting of a solitary underline, then the stat
159structure of the previous file test (or stat operator) is used, saving
160a system call. (This doesn't work with C<-t>, and you need to remember
161that lstat() and C<-l> will leave values in the stat structure for the
162symbolic link, not the real file.) Example:
163
164 print "Can do.\n" if -r $a || -w _ || -x _;
165
166 stat($filename);
167 print "Readable\n" if -r _;
168 print "Writable\n" if -w _;
169 print "Executable\n" if -x _;
170 print "Setuid\n" if -u _;
171 print "Setgid\n" if -g _;
172 print "Sticky\n" if -k _;
173 print "Text\n" if -T _;
174 print "Binary\n" if -B _;
175
176=item abs VALUE
177
178Returns the absolute value of its argument.
179
180=item accept NEWSOCKET,GENERICSOCKET
181
182Accepts an incoming socket connect, just as the accept(2) system call
183does. Returns the packed address if it succeeded, FALSE otherwise.
4633a7c4 184See example in L<perlipc/"Sockets: Client/Server Communication">.
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185
186=item alarm SECONDS
187
188Arranges to have a SIGALRM delivered to this process after the
189specified number of seconds have elapsed. (On some machines,
190unfortunately, the elapsed time may be up to one second less than you
191specified because of how seconds are counted.) Only one timer may be
192counting at once. Each call disables the previous timer, and an
193argument of 0 may be supplied to cancel the previous timer without
194starting a new one. The returned value is the amount of time remaining
195on the previous timer.
196
4633a7c4 197For delays of finer granularity than one second, you may use Perl's
a0d0e21e 198syscall() interface to access setitimer(2) if your system supports it,
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199or else see L</select()> below. It is not advised to intermix alarm()
200and sleep() calls.
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201
202=item atan2 Y,X
203
204Returns the arctangent of Y/X in the range -PI to PI.
205
206=item bind SOCKET,NAME
207
208Binds a network address to a socket, just as the bind system call
209does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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210packed address of the appropriate type for the socket. See the examples in
211L<perlipc/"Sockets: Client/Server Communication">.
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212
213=item binmode FILEHANDLE
214
215Arranges for the file to be read or written in "binary" mode in
216operating systems that distinguish between binary and text files.
217Files that are not in binary mode have CR LF sequences translated to LF
218on input and LF translated to CR LF on output. Binmode has no effect
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219under Unix; in DOS, it may be imperative--otherwise your DOS C library
220may mangle your file. If FILEHANDLE is an expression,
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221the value is taken as the name of the filehandle.
222
4633a7c4 223=item bless REF,CLASSNAME
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224
225=item bless REF
226
227This function tells the referenced object (passed as REF) that it is now
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228an object in the CLASSNAME package--or the current package if no CLASSNAME
229is specified, which is often the case. It returns the reference for
230convenience, since a bless() is often the last thing in a constructor.
231Always use the two-argument version if the function doing the blessing
232might be inherited by a derived class. See L<perlobj> for more about the
233blessing (and blessings) of objects.
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234
235=item caller EXPR
236
237=item caller
238
239Returns the context of the current subroutine call. In a scalar context,
240returns TRUE if there is a caller, that is, if we're in a subroutine or
241eval() or require(), and FALSE otherwise. In a list context, returns
242
748a9306 243 ($package, $filename, $line) = caller;
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244
245With EXPR, it returns some extra information that the debugger uses to
246print a stack trace. The value of EXPR indicates how many call frames
247to go back before the current one.
248
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249 ($package, $filename, $line,
250 $subroutine, $hasargs, $wantargs) = caller($i);
251
252Furthermore, when called from within the DB package, caller returns more
4633a7c4 253detailed information: it sets the list variable @DB::args to be the
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254arguments with which that subroutine was invoked.
255
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256=item chdir EXPR
257
258Changes the working directory to EXPR, if possible. If EXPR is
259omitted, changes to home directory. Returns TRUE upon success, FALSE
260otherwise. See example under die().
261
262=item chmod LIST
263
264Changes the permissions of a list of files. The first element of the
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265list must be the numerical mode, which should probably be an octal
266number. Returns the number of files successfully changed.
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267
268 $cnt = chmod 0755, 'foo', 'bar';
269 chmod 0755, @executables;
270
271=item chomp VARIABLE
272
273=item chomp LIST
274
275=item chomp
276
277This is a slightly safer version of chop (see below). It removes any
278line ending that corresponds to the current value of C<$/> (also known as
279$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the number
280of characters removed. It's often used to remove the newline from the
281end of an input record when you're worried that the final record may be
282missing its newline. When in paragraph mode (C<$/ = "">), it removes all
283trailing newlines from the string. If VARIABLE is omitted, it chomps
284$_. Example:
285
286 while (<>) {
287 chomp; # avoid \n on last field
288 @array = split(/:/);
289 ...
290 }
291
292You can actually chomp anything that's an lvalue, including an assignment:
293
294 chomp($cwd = `pwd`);
295 chomp($answer = <STDIN>);
296
297If you chomp a list, each element is chomped, and the total number of
298characters removed is returned.
299
300=item chop VARIABLE
301
302=item chop LIST
303
304=item chop
305
306Chops off the last character of a string and returns the character
307chopped. It's used primarily to remove the newline from the end of an
308input record, but is much more efficient than C<s/\n//> because it neither
309scans nor copies the string. If VARIABLE is omitted, chops $_.
310Example:
311
312 while (<>) {
313 chop; # avoid \n on last field
314 @array = split(/:/);
315 ...
316 }
317
318You can actually chop anything that's an lvalue, including an assignment:
319
320 chop($cwd = `pwd`);
321 chop($answer = <STDIN>);
322
323If you chop a list, each element is chopped. Only the value of the
324last chop is returned.
325
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326Note that chop returns the last character. To return all but the last
327character, use C<substr($string, 0, -1)>.
328
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329=item chown LIST
330
331Changes the owner (and group) of a list of files. The first two
332elements of the list must be the I<NUMERICAL> uid and gid, in that order.
333Returns the number of files successfully changed.
334
335 $cnt = chown $uid, $gid, 'foo', 'bar';
336 chown $uid, $gid, @filenames;
337
338Here's an example that looks up non-numeric uids in the passwd file:
339
340 print "User: ";
341 chop($user = <STDIN>);
342 print "Files: "
343 chop($pattern = <STDIN>);
344
345 ($login,$pass,$uid,$gid) = getpwnam($user)
346 or die "$user not in passwd file";
347
348 @ary = <${pattern}>; # expand filenames
349 chown $uid, $gid, @ary;
350
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351On most systems, you are not allowed to change the ownership of the
352file unless you're the superuser, although you should be able to change
353the group to any of your secondary groups. On insecure systems, these
354restrictions may be relaxed, but this is not a portable assumption.
355
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356=item chr NUMBER
357
358Returns the character represented by that NUMBER in the character set.
359For example, C<chr(65)> is "A" in ASCII.
360
361=item chroot FILENAME
362
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363This function works as the system call by the same name: it makes the
364named directory the new root directory for all further pathnames that
365begin with a "/" by your process and all of its children. (It doesn't
366change your current working directory is unaffected.) For security
367reasons, this call is restricted to the superuser. If FILENAME is
368omitted, does chroot to $_.
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369
370=item close FILEHANDLE
371
372Closes the file or pipe associated with the file handle, returning TRUE
373only if stdio successfully flushes buffers and closes the system file
374descriptor. You don't have to close FILEHANDLE if you are immediately
4633a7c4 375going to do another open() on it, since open() will close it for you. (See
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376open().) However, an explicit close on an input file resets the line
377counter ($.), while the implicit close done by open() does not. Also,
378closing a pipe will wait for the process executing on the pipe to
379complete, in case you want to look at the output of the pipe
380afterwards. Closing a pipe explicitly also puts the status value of
381the command into C<$?>. Example:
382
383 open(OUTPUT, '|sort >foo'); # pipe to sort
384 ... # print stuff to output
385 close OUTPUT; # wait for sort to finish
386 open(INPUT, 'foo'); # get sort's results
387
388FILEHANDLE may be an expression whose value gives the real filehandle name.
389
390=item closedir DIRHANDLE
391
392Closes a directory opened by opendir().
393
394=item connect SOCKET,NAME
395
396Attempts to connect to a remote socket, just as the connect system call
397does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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398packed address of the appropriate type for the socket. See the examples in
399L<perlipc/"Sockets: Client/Server Communication">.
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400
401=item cos EXPR
402
403Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
404takes cosine of $_.
405
406=item crypt PLAINTEXT,SALT
407
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408Encrypts a string exactly like the crypt(3) function in the C library
409(assuming that you actually have a version there that has not been
410extirpated as a potential munition). This can prove useful for checking
411the password file for lousy passwords, amongst other things. Only the
412guys wearing white hats should do this.
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413
414Here's an example that makes sure that whoever runs this program knows
415their own password:
416
417 $pwd = (getpwuid($<))[1];
418 $salt = substr($pwd, 0, 2);
419
420 system "stty -echo";
421 print "Password: ";
422 chop($word = <STDIN>);
423 print "\n";
424 system "stty echo";
425
426 if (crypt($word, $salt) ne $pwd) {
427 die "Sorry...\n";
428 } else {
429 print "ok\n";
430 }
431
432Of course, typing in your own password to whoever asks you
748a9306 433for it is unwise.
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434
435=item dbmclose ASSOC_ARRAY
436
437[This function has been superseded by the untie() function.]
438
439Breaks the binding between a DBM file and an associative array.
440
441=item dbmopen ASSOC,DBNAME,MODE
442
443[This function has been superseded by the tie() function.]
444
4633a7c4 445This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an associative array. ASSOC is the
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446name of the associative array. (Unlike normal open, the first argument
447is I<NOT> a filehandle, even though it looks like one). DBNAME is the
4633a7c4 448name of the database (without the F<.dir> or F<.pag> extension if any). If the
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449database does not exist, it is created with protection specified by
450MODE (as modified by the umask()). If your system only supports the
451older DBM functions, you may perform only one dbmopen() in your program.
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452In order versions of Perl,
453if your system had neither DBM nor ndbm, calling dbmopen() produced a
454fatal error; it now falls back to sdbm(3).
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455
456If you don't have write access to the DBM file, you can only read
457associative array variables, not set them. If you want to test whether
458you can write, either use file tests or try setting a dummy array entry
459inside an eval(), which will trap the error.
460
461Note that functions such as keys() and values() may return huge array
462values when used on large DBM files. You may prefer to use the each()
463function to iterate over large DBM files. Example:
464
465 # print out history file offsets
466 dbmopen(%HIST,'/usr/lib/news/history',0666);
467 while (($key,$val) = each %HIST) {
468 print $key, ' = ', unpack('L',$val), "\n";
469 }
470 dbmclose(%HIST);
471
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472See also L<DB_File> for many other interesting possibilities.
473
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474=item defined EXPR
475
476Returns a boolean value saying whether the lvalue EXPR has a real value
477or not. Many operations return the undefined value under exceptional
478conditions, such as end of file, uninitialized variable, system error
479and such. This function allows you to distinguish between an undefined
480null scalar and a defined null scalar with operations that might return
481a real null string, such as referencing elements of an array. You may
482also check to see if arrays or subroutines exist. Use of defined on
483predefined variables is not guaranteed to produce intuitive results.
484
485When used on a hash array element, it tells you whether the value
486is defined, not whether the key exists in the hash. Use exists() for that.
487
488Examples:
489
490 print if defined $switch{'D'};
491 print "$val\n" while defined($val = pop(@ary));
492 die "Can't readlink $sym: $!"
493 unless defined($value = readlink $sym);
494 eval '@foo = ()' if defined(@foo);
495 die "No XYZ package defined" unless defined %_XYZ;
496 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
497
498See also undef().
499
500=item delete EXPR
501
502Deletes the specified value from its hash array. Returns the deleted
503value, or the undefined value if nothing was deleted. Deleting from
504C<$ENV{}> modifies the environment. Deleting from an array tied to a DBM
505file deletes the entry from the DBM file. (But deleting from a tie()d
506hash doesn't necessarily return anything.)
507
508The following deletes all the values of an associative array:
509
510 foreach $key (keys %ARRAY) {
511 delete $ARRAY{$key};
512 }
513
514(But it would be faster to use the undef() command.) Note that the
515EXPR can be arbitrarily complicated as long as the final operation is
516a hash key lookup:
517
518 delete $ref->[$x][$y]{$key};
519
520=item die LIST
521
522Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
4633a7c4 523the current value of $! (errno). If $! is 0, exits with the value of
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524C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)> is 0,
525exits with 255. Inside an eval(), the error message is stuffed into C<$@>,
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526and the eval() is terminated with the undefined value; this makes die()
527the way to raise an exception.
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528
529Equivalent examples:
530
531 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
532 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
533
534If the value of EXPR does not end in a newline, the current script line
535number and input line number (if any) are also printed, and a newline
536is supplied. Hint: sometimes appending ", stopped" to your message
537will cause it to make better sense when the string "at foo line 123" is
538appended. Suppose you are running script "canasta".
539
540 die "/etc/games is no good";
541 die "/etc/games is no good, stopped";
542
543produce, respectively
544
545 /etc/games is no good at canasta line 123.
546 /etc/games is no good, stopped at canasta line 123.
547
548See also exit() and warn().
549
550=item do BLOCK
551
552Not really a function. Returns the value of the last command in the
553sequence of commands indicated by BLOCK. When modified by a loop
554modifier, executes the BLOCK once before testing the loop condition.
555(On other statements the loop modifiers test the conditional first.)
556
557=item do SUBROUTINE(LIST)
558
559A deprecated form of subroutine call. See L<perlsub>.
560
561=item do EXPR
562
563Uses the value of EXPR as a filename and executes the contents of the
564file as a Perl script. Its primary use is to include subroutines
565from a Perl subroutine library.
566
567 do 'stat.pl';
568
569is just like
570
571 eval `cat stat.pl`;
572
573except that it's more efficient, more concise, keeps track of the
574current filename for error messages, and searches all the B<-I>
575libraries if the file isn't in the current directory (see also the @INC
576array in L<perlvar/Predefined Names>). It's the same, however, in that it does
577reparse the file every time you call it, so you probably don't want to
578do this inside a loop.
579
580Note that inclusion of library modules is better done with the
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581use() and require() operators, which also do error checking
582and raise an exception if there's a problem.
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583
584=item dump LABEL
585
586This causes an immediate core dump. Primarily this is so that you can
587use the B<undump> program to turn your core dump into an executable binary
588after having initialized all your variables at the beginning of the
589program. When the new binary is executed it will begin by executing a
590C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
591it as a goto with an intervening core dump and reincarnation. If LABEL
592is omitted, restarts the program from the top. WARNING: any files
593opened at the time of the dump will NOT be open any more when the
594program is reincarnated, with possible resulting confusion on the part
595of Perl. See also B<-u> option in L<perlrun>.
596
597Example:
598
599 #!/usr/bin/perl
600 require 'getopt.pl';
601 require 'stat.pl';
602 %days = (
603 'Sun' => 1,
604 'Mon' => 2,
605 'Tue' => 3,
606 'Wed' => 4,
607 'Thu' => 5,
608 'Fri' => 6,
609 'Sat' => 7,
610 );
611
612 dump QUICKSTART if $ARGV[0] eq '-d';
613
614 QUICKSTART:
615 Getopt('f');
616
617=item each ASSOC_ARRAY
618
4633a7c4 619Returns a 2-element array consisting of the key and value for the next
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620value of an associative array, so that you can iterate over it.
621Entries are returned in an apparently random order. When the array is
622entirely read, a null array is returned (which when assigned produces a
623FALSE (0) value). The next call to each() after that will start
624iterating again. The iterator can be reset only by reading all the
625elements from the array. You should not add elements to an array while
626you're iterating over it. There is a single iterator for each
627associative array, shared by all each(), keys() and values() function
628calls in the program. The following prints out your environment like
629the printenv(1) program, only in a different order:
630
631 while (($key,$value) = each %ENV) {
632 print "$key=$value\n";
633 }
634
635See also keys() and values().
636
637=item eof FILEHANDLE
638
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639=item eof ()
640
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641=item eof
642
643Returns 1 if the next read on FILEHANDLE will return end of file, or if
644FILEHANDLE is not open. FILEHANDLE may be an expression whose value
645gives the real filehandle name. (Note that this function actually
646reads a character and then ungetc()s it, so it is not very useful in an
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647interactive context.) Do not read from a terminal file (or call
648C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
649as terminals may lose the end-of-file condition if you do.
650
651An C<eof> without an argument uses the last file read as argument.
652Empty parentheses () may be used to indicate
4633a7c4 653the pseudofile formed of the files listed on the command line, i.e.
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654C<eof()> is reasonable to use inside a while (<>) loop to detect the end
655of only the last file. Use C<eof(ARGV)> or eof without the parentheses to
656test I<EACH> file in a while (<>) loop. Examples:
657
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658 # reset line numbering on each input file
659 while (<>) {
660 print "$.\t$_";
661 close(ARGV) if (eof); # Not eof().
662 }
663
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664 # insert dashes just before last line of last file
665 while (<>) {
666 if (eof()) {
667 print "--------------\n";
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668 close(ARGV); # close or break; is needed if we
669 # are reading from the terminal
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670 }
671 print;
672 }
673
a0d0e21e 674Practical hint: you almost never need to use C<eof> in Perl, because the
4633a7c4 675input operators return undef when they run out of data. Testing C<eof>
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676
677=item eval EXPR
678
679=item eval BLOCK
680
681EXPR is parsed and executed as if it were a little Perl program. It
682is executed in the context of the current Perl program, so that any
683variable settings, subroutine or format definitions remain afterwards.
684The value returned is the value of the last expression evaluated, or a
685return statement may be used, just as with subroutines.
686
687If there is a syntax error or runtime error, or a die() statement is
688executed, an undefined value is returned by eval(), and C<$@> is set to the
689error message. If there was no error, C<$@> is guaranteed to be a null
690string. If EXPR is omitted, evaluates $_. The final semicolon, if
691any, may be omitted from the expression.
692
693Note that, since eval() traps otherwise-fatal errors, it is useful for
4633a7c4 694determining whether a particular feature (such as socket() or symlink())
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695is implemented. It is also Perl's exception trapping mechanism, where
696the die operator is used to raise exceptions.
697
698If the code to be executed doesn't vary, you may use the eval-BLOCK
699form to trap run-time errors without incurring the penalty of
700recompiling each time. The error, if any, is still returned in C<$@>.
701Examples:
702
703 # make divide-by-zero non-fatal
704 eval { $answer = $a / $b; }; warn $@ if $@;
705
706 # same thing, but less efficient
707 eval '$answer = $a / $b'; warn $@ if $@;
708
709 # a compile-time error
710 eval { $answer = };
711
712 # a run-time error
713 eval '$answer ='; # sets $@
714
715With an eval(), you should be especially careful to remember what's
716being looked at when:
717
718 eval $x; # CASE 1
719 eval "$x"; # CASE 2
720
721 eval '$x'; # CASE 3
722 eval { $x }; # CASE 4
723
724 eval "\$$x++" # CASE 5
725 $$x++; # CASE 6
726
727Cases 1 and 2 above behave identically: they run the code contained in the
728variable $x. (Although case 2 has misleading double quotes making the
729reader wonder what else might be happening (nothing is).) Cases 3 and 4
730likewise behave in the same way: they run the code <$x>, which does
731nothing at all. (Case 4 is preferred for purely visual reasons.) Case 5
732is a place where normally you I<WOULD> like to use double quotes, except
748a9306 733in that particular situation, you can just use symbolic references
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734instead, as in case 6.
735
736=item exec LIST
737
738The exec() function executes a system command I<AND NEVER RETURNS>. Use
739the system() function if you want it to return.
740
741If there is more than one argument in LIST, or if LIST is an array with
742more than one value, calls execvp(3) with the arguments in LIST. If
743there is only one scalar argument, the argument is checked for shell
744metacharacters. If there are any, the entire argument is passed to
745C</bin/sh -c> for parsing. If there are none, the argument is split
746into words and passed directly to execvp(), which is more efficient.
747Note: exec() (and system(0) do not flush your output buffer, so you may
748need to set C<$|> to avoid lost output. Examples:
749
750 exec '/bin/echo', 'Your arguments are: ', @ARGV;
751 exec "sort $outfile | uniq";
752
753If you don't really want to execute the first argument, but want to lie
754to the program you are executing about its own name, you can specify
755the program you actually want to run as an "indirect object" (without a
756comma) in front of the LIST. (This always forces interpretation of the
757LIST as a multi-valued list, even if there is only a single scalar in
758the list.) Example:
759
760 $shell = '/bin/csh';
761 exec $shell '-sh'; # pretend it's a login shell
762
763or, more directly,
764
765 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
766
767=item exists EXPR
768
769Returns TRUE if the specified hash key exists in its hash array, even
770if the corresponding value is undefined.
771
772 print "Exists\n" if exists $array{$key};
773 print "Defined\n" if defined $array{$key};
774 print "True\n" if $array{$key};
775
776A hash element can only be TRUE if it's defined, and defined if
777it exists, but the reverse doesn't necessarily hold true.
778
779Note that the EXPR can be arbitrarily complicated as long as the final
780operation is a hash key lookup:
781
782 if (exists $ref->[$x][$y]{$key}) { ... }
783
784=item exit EXPR
785
786Evaluates EXPR and exits immediately with that value. (Actually, it
787calls any defined C<END> routines first, but the C<END> routines may not
788abort the exit. Likewise any object destructors that need to be called
789are called before exit.) Example:
790
791 $ans = <STDIN>;
792 exit 0 if $ans =~ /^[Xx]/;
793
794See also die(). If EXPR is omitted, exits with 0 status.
795
796=item exp EXPR
797
798Returns I<e> (the natural logarithm base) to the power of EXPR.
799If EXPR is omitted, gives C<exp($_)>.
800
801=item fcntl FILEHANDLE,FUNCTION,SCALAR
802
803Implements the fcntl(2) function. You'll probably have to say
804
805 use Fcntl;
806
807first to get the correct function definitions. Argument processing and
808value return works just like ioctl() below. Note that fcntl() will produce
809a fatal error if used on a machine that doesn't implement fcntl(2).
810For example:
811
812 use Fcntl;
813 fcntl($filehandle, F_GETLK, $packed_return_buffer);
814
815=item fileno FILEHANDLE
816
817Returns the file descriptor for a filehandle. This is useful for
818constructing bitmaps for select(). If FILEHANDLE is an expression, the
819value is taken as the name of the filehandle.
820
821=item flock FILEHANDLE,OPERATION
822
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823Calls flock(2) on FILEHANDLE. See L<flock(2)> for definition of
824OPERATION. Returns TRUE for success, FALSE on failure. Will produce a
825fatal error if used on a machine that doesn't implement either flock(2) or
826fcntl(2). (fcntl(2) will be automatically used if flock(2) is missing.)
827
828Here's a mailbox appender for BSD systems.
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829
830 $LOCK_SH = 1;
831 $LOCK_EX = 2;
832 $LOCK_NB = 4;
833 $LOCK_UN = 8;
834
835 sub lock {
836 flock(MBOX,$LOCK_EX);
837 # and, in case someone appended
838 # while we were waiting...
839 seek(MBOX, 0, 2);
840 }
841
842 sub unlock {
843 flock(MBOX,$LOCK_UN);
844 }
845
846 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
847 or die "Can't open mailbox: $!";
848
849 lock();
850 print MBOX $msg,"\n\n";
851 unlock();
852
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LW
853Note that many versions of flock() cannot lock things over the network.
854You need to do locking with fcntl() for that.
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855
856=item fork
857
858Does a fork(2) system call. Returns the child pid to the parent process
4633a7c4 859and 0 to the child process, or C<undef> if the fork is unsuccessful.
a0d0e21e
LW
860Note: unflushed buffers remain unflushed in both processes, which means
861you may need to set C<$|> ($AUTOFLUSH in English) or call the
862autoflush() FileHandle method to avoid duplicate output.
863
864If you fork() without ever waiting on your children, you will accumulate
865zombies:
866
4633a7c4 867 $SIG{CHLD} = sub { wait };
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868
869There's also the double-fork trick (error checking on
870fork() returns omitted);
871
872 unless ($pid = fork) {
873 unless (fork) {
874 exec "what you really wanna do";
875 die "no exec";
876 # ... or ...
4633a7c4 877 ## (some_perl_code_here)
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878 exit 0;
879 }
880 exit 0;
881 }
882 waitpid($pid,0);
883
884
885=item formline PICTURE, LIST
886
4633a7c4 887This is an internal function used by C<format>s, though you may call it
a0d0e21e
LW
888too. It formats (see L<perlform>) a list of values according to the
889contents of PICTURE, placing the output into the format output
4633a7c4
LW
890accumulator, C<$^A> (or $ACCUMULATOR in English).
891Eventually, when a write() is done, the contents of
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892C<$^A> are written to some filehandle, but you could also read C<$^A>
893yourself and then set C<$^A> back to "". Note that a format typically
894does one formline() per line of form, but the formline() function itself
748a9306 895doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 896that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
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897You may therefore need to use multiple formlines to implement a single
898record format, just like the format compiler.
899
900Be careful if you put double quotes around the picture, since an "C<@>"
901character may be taken to mean the beginning of an array name.
4633a7c4 902formline() always returns TRUE. See L<perlform> for other examples.
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903
904=item getc FILEHANDLE
905
906=item getc
907
908Returns the next character from the input file attached to FILEHANDLE,
909or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
4633a7c4
LW
910This is not particularly efficient. It cannot be used to get unbuffered
911single-character
912
913 if ($BSD_STYLE) {
914 system "stty cbreak </dev/tty >/dev/tty 2>&1";
915 }
916 else {
917 system "stty", '-icanon',
918 system "stty", 'eol', "\001";
919 }
920
921 $key = getc(STDIN);
922
923 if ($BSD_STYLE) {
924 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
925 }
926 else {
927 system "stty", 'icanon';
928 system "stty", 'eol', '^@'; # ascii null
929 }
930 print "\n";
931
932Determination of whether to whether $BSD_STYLE should be set
933is left as an exercise to the reader.
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934
935=item getlogin
936
937Returns the current login from F</etc/utmp>, if any. If null, use
4633a7c4 938getpwuid().
a0d0e21e
LW
939
940 $login = getlogin || (getpwuid($<))[0] || "Kilroy";
941
4633a7c4
LW
942Do not consider getlogin() for authorentication: it is not as
943secure as getpwuid().
944
a0d0e21e
LW
945=item getpeername SOCKET
946
947Returns the packed sockaddr address of other end of the SOCKET connection.
948
4633a7c4
LW
949 use Socket;
950 $hersockaddr = getpeername(SOCK);
951 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
952 $herhostname = gethostbyaddr($iaddr, AF_INET);
953 $herstraddr = inet_ntoa($iaddr);
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954
955=item getpgrp PID
956
957Returns the current process group for the specified PID, 0 for the
4633a7c4 958current process. Will raise an exception if used on a machine that
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959doesn't implement getpgrp(2). If PID is omitted, returns process
960group of current process.
961
962=item getppid
963
964Returns the process id of the parent process.
965
966=item getpriority WHICH,WHO
967
4633a7c4
LW
968Returns the current priority for a process, a process group, or a user.
969(See L<getpriority(2)>.) Will raise a fatal exception if used on a
a0d0e21e
LW
970machine that doesn't implement getpriority(2).
971
972=item getpwnam NAME
973
974=item getgrnam NAME
975
976=item gethostbyname NAME
977
978=item getnetbyname NAME
979
980=item getprotobyname NAME
981
982=item getpwuid UID
983
984=item getgrgid GID
985
986=item getservbyname NAME,PROTO
987
988=item gethostbyaddr ADDR,ADDRTYPE
989
990=item getnetbyaddr ADDR,ADDRTYPE
991
992=item getprotobynumber NUMBER
993
994=item getservbyport PORT,PROTO
995
996=item getpwent
997
998=item getgrent
999
1000=item gethostent
1001
1002=item getnetent
1003
1004=item getprotoent
1005
1006=item getservent
1007
1008=item setpwent
1009
1010=item setgrent
1011
1012=item sethostent STAYOPEN
1013
1014=item setnetent STAYOPEN
1015
1016=item setprotoent STAYOPEN
1017
1018=item setservent STAYOPEN
1019
1020=item endpwent
1021
1022=item endgrent
1023
1024=item endhostent
1025
1026=item endnetent
1027
1028=item endprotoent
1029
1030=item endservent
1031
1032These routines perform the same functions as their counterparts in the
1033system library. Within a list context, the return values from the
1034various get routines are as follows:
1035
1036 ($name,$passwd,$uid,$gid,
1037 $quota,$comment,$gcos,$dir,$shell) = getpw*
1038 ($name,$passwd,$gid,$members) = getgr*
1039 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1040 ($name,$aliases,$addrtype,$net) = getnet*
1041 ($name,$aliases,$proto) = getproto*
1042 ($name,$aliases,$port,$proto) = getserv*
1043
1044(If the entry doesn't exist you get a null list.)
1045
1046Within a scalar context, you get the name, unless the function was a
1047lookup by name, in which case you get the other thing, whatever it is.
1048(If the entry doesn't exist you get the undefined value.) For example:
1049
1050 $uid = getpwnam
1051 $name = getpwuid
1052 $name = getpwent
1053 $gid = getgrnam
1054 $name = getgrgid
1055 $name = getgrent
1056 etc.
1057
1058The $members value returned by I<getgr*()> is a space separated list of
1059the login names of the members of the group.
1060
1061For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1062C, it will be returned to you via C<$?> if the function call fails. The
1063@addrs value returned by a successful call is a list of the raw
1064addresses returned by the corresponding system library call. In the
1065Internet domain, each address is four bytes long and you can unpack it
1066by saying something like:
1067
1068 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1069
1070=item getsockname SOCKET
1071
1072Returns the packed sockaddr address of this end of the SOCKET connection.
1073
4633a7c4
LW
1074 use Socket;
1075 $mysockaddr = getsockname(SOCK);
1076 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1077
1078=item getsockopt SOCKET,LEVEL,OPTNAME
1079
1080Returns the socket option requested, or undefined if there is an error.
1081
1082=item glob EXPR
1083
1084Returns the value of EXPR with filename expansions such as a shell
1085would do. This is the internal function implementing the <*.*>
4633a7c4 1086operator, except it's easier to use.
a0d0e21e
LW
1087
1088=item gmtime EXPR
1089
1090Converts a time as returned by the time function to a 9-element array
4633a7c4
LW
1091with the time localized for the standard Greenwich timezone.
1092Typically used as follows:
a0d0e21e
LW
1093
1094
1095 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1096 gmtime(time);
1097
1098All array elements are numeric, and come straight out of a struct tm.
1099In particular this means that $mon has the range 0..11 and $wday has
1100the range 0..6. If EXPR is omitted, does C<gmtime(time())>.
1101
1102=item goto LABEL
1103
748a9306
LW
1104=item goto EXPR
1105
a0d0e21e
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1106=item goto &NAME
1107
1108The goto-LABEL form finds the statement labeled with LABEL and resumes
1109execution there. It may not be used to go into any construct that
1110requires initialization, such as a subroutine or a foreach loop. It
1111also can't be used to go into a construct that is optimized away. It
1112can be used to go almost anywhere else within the dynamic scope,
1113including out of subroutines, but it's usually better to use some other
1114construct such as last or die. The author of Perl has never felt the
1115need to use this form of goto (in Perl, that is--C is another matter).
1116
748a9306
LW
1117The goto-EXPR form expects a label name, whose scope will be resolved
1118dynamically. This allows for computed gotos per FORTRAN, but isn't
1119necessarily recommended if you're optimizing for maintainability:
1120
1121 goto ("FOO", "BAR", "GLARCH")[$i];
1122
a0d0e21e
LW
1123The goto-&NAME form is highly magical, and substitutes a call to the
1124named subroutine for the currently running subroutine. This is used by
1125AUTOLOAD subroutines that wish to load another subroutine and then
1126pretend that the other subroutine had been called in the first place
1127(except that any modifications to @_ in the current subroutine are
1128propagated to the other subroutine.) After the goto, not even caller()
1129will be able to tell that this routine was called first.
1130
1131=item grep BLOCK LIST
1132
1133=item grep EXPR,LIST
1134
1135Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1136$_ to each element) and returns the list value consisting of those
1137elements for which the expression evaluated to TRUE. In a scalar
1138context, returns the number of times the expression was TRUE.
1139
1140 @foo = grep(!/^#/, @bar); # weed out comments
1141
1142or equivalently,
1143
1144 @foo = grep {!/^#/} @bar; # weed out comments
1145
1146Note that, since $_ is a reference into the list value, it can be used
1147to modify the elements of the array. While this is useful and
1148supported, it can cause bizarre results if the LIST is not a named
1149array.
1150
1151=item hex EXPR
1152
4633a7c4
LW
1153Interprets EXPR as a hex string and returns the corresponding decimal
1154value. (To convert strings that might start with 0 or 0x see
1155oct().) If EXPR is omitted, uses $_.
a0d0e21e
LW
1156
1157=item import
1158
1159There is no built-in import() function. It is merely an ordinary
4633a7c4 1160method (subroutine) defined (or inherited) by modules that wish to export
a0d0e21e 1161names to another module. The use() function calls the import() method
4633a7c4 1162for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1163
1164=item index STR,SUBSTR,POSITION
1165
1166=item index STR,SUBSTR
1167
4633a7c4
LW
1168Returns the position of the first occurrence of SUBSTR in STR at or after
1169POSITION. If POSITION is omitted, starts searching from the beginning of
1170the string. The return value is based at 0 (or whatever you've set the $[
1171variable to--but don't do that). If the substring is not found, returns
a0d0e21e
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1172one less than the base, ordinarily -1.
1173
1174=item int EXPR
1175
1176Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1177
1178=item ioctl FILEHANDLE,FUNCTION,SCALAR
1179
1180Implements the ioctl(2) function. You'll probably have to say
1181
4633a7c4 1182 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1183
4633a7c4 1184first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1185exist or doesn't have the correct definitions you'll have to roll your
4633a7c4
LW
1186own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1187(There is a Perl script called B<h2ph> that comes with the Perl kit which
1188may help you in this, but it's non-trivial.) SCALAR will be read and/or
1189written depending on the FUNCTION--a pointer to the string value of SCALAR
1190will be passed as the third argument of the actual ioctl call. (If SCALAR
1191has no string value but does have a numeric value, that value will be
1192passed rather than a pointer to the string value. To guarantee this to be
1193TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1194functions are useful for manipulating the values of structures used by
1195ioctl(). The following example sets the erase character to DEL.
a0d0e21e
LW
1196
1197 require 'ioctl.ph';
4633a7c4
LW
1198 $getp = &TIOCGETP;
1199 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1200 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1201 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1202 @ary = unpack($sgttyb_t,$sgttyb);
1203 $ary[2] = 127;
1204 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1205 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1206 || die "Can't ioctl: $!";
1207 }
1208
1209The return value of ioctl (and fcntl) is as follows:
1210
1211 if OS returns: then Perl returns:
1212 -1 undefined value
1213 0 string "0 but true"
1214 anything else that number
1215
1216Thus Perl returns TRUE on success and FALSE on failure, yet you can
1217still easily determine the actual value returned by the operating
1218system:
1219
1220 ($retval = ioctl(...)) || ($retval = -1);
1221 printf "System returned %d\n", $retval;
1222
1223=item join EXPR,LIST
1224
1225Joins the separate strings of LIST or ARRAY into a single string with
1226fields separated by the value of EXPR, and returns the string.
1227Example:
1228
1229 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1230
1231See L<perlfunc/split>.
1232
1233=item keys ASSOC_ARRAY
1234
1235Returns a normal array consisting of all the keys of the named
1236associative array. (In a scalar context, returns the number of keys.)
1237The keys are returned in an apparently random order, but it is the same
1238order as either the values() or each() function produces (given that
1239the associative array has not been modified). Here is yet another way
1240to print your environment:
1241
1242 @keys = keys %ENV;
1243 @values = values %ENV;
1244 while ($#keys >= 0) {
1245 print pop(@keys), '=', pop(@values), "\n";
1246 }
1247
1248or how about sorted by key:
1249
1250 foreach $key (sort(keys %ENV)) {
1251 print $key, '=', $ENV{$key}, "\n";
1252 }
1253
4633a7c4
LW
1254To sort an array by value, you'll need to use a C<sort{}>
1255function. Here's a descending numeric sort by value:
1256
1257 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1258 printf "%4d %s\n", $hash{$key}, $key;
1259 }
1260
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1261=item kill LIST
1262
4633a7c4
LW
1263Sends a signal to a list of processes. The first element of
1264the list must be the signal to send. Returns the number of
1265processes successfully signaled.
a0d0e21e
LW
1266
1267 $cnt = kill 1, $child1, $child2;
1268 kill 9, @goners;
1269
4633a7c4
LW
1270Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1271process groups instead of processes. (On System V, a negative I<PROCESS>
1272number will also kill process groups, but that's not portable.) That
1273means you usually want to use positive not negative signals. You may also
1274use a signal name in quotes. See the L<perlipc/"Signals"> man page for details.
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LW
1275
1276=item last LABEL
1277
1278=item last
1279
1280The C<last> command is like the C<break> statement in C (as used in
1281loops); it immediately exits the loop in question. If the LABEL is
1282omitted, the command refers to the innermost enclosing loop. The
1283C<continue> block, if any, is not executed:
1284
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LW
1285 LINE: while (<STDIN>) {
1286 last LINE if /^$/; # exit when done with header
a0d0e21e
LW
1287 ...
1288 }
1289
1290=item lc EXPR
1291
1292Returns an lowercased version of EXPR. This is the internal function
4633a7c4
LW
1293implementing the \L escape in double-quoted strings.
1294Should respect any POSIX setlocale() settings.
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1295
1296=item lcfirst EXPR
1297
1298Returns the value of EXPR with the first character lowercased. This is
1299the internal function implementing the \l escape in double-quoted strings.
4633a7c4 1300Should respect any POSIX setlocale() settings.
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1301
1302=item length EXPR
1303
1304Returns the length in characters of the value of EXPR. If EXPR is
1305omitted, returns length of $_.
1306
1307=item link OLDFILE,NEWFILE
1308
1309Creates a new filename linked to the old filename. Returns 1 for
1310success, 0 otherwise.
1311
1312=item listen SOCKET,QUEUESIZE
1313
1314Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 1315it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
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LW
1316
1317=item local EXPR
1318
1319In general, you should be using "my" instead of "local", because it's
748a9306
LW
1320faster and safer. Format variables often use "local" though, as
1321do other variables whose current value must be visible to called
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1322subroutines. This is known as dynamic scoping. Lexical scoping is
1323done with "my", which works more like C's auto declarations.
1324
1325A local modifies the listed variables to be local to the enclosing block,
1326subroutine, eval or "do". If more than one value is listed, the list
1327must be placed in parens. All the listed elements must be legal
1328lvalues. This operator works by saving the current values of those
1329variables in LIST on a hidden stack and restoring them upon exiting the
1330block, subroutine or eval. This means that called subroutines can also
1331reference the local variable, but not the global one. The LIST may be
1332assigned to if desired, which allows you to initialize your local
1333variables. (If no initializer is given for a particular variable, it
1334is created with an undefined value.) Commonly this is used to name the
1335parameters to a subroutine. Examples:
1336
1337 sub RANGEVAL {
1338 local($min, $max, $thunk) = @_;
1339 local $result = '';
1340 local $i;
1341
1342 # Presumably $thunk makes reference to $i
1343
1344 for ($i = $min; $i < $max; $i++) {
1345 $result .= eval $thunk;
1346 }
1347
1348 $result;
1349 }
1350
1351
1352 if ($sw eq '-v') {
1353 # init local array with global array
1354 local @ARGV = @ARGV;
1355 unshift(@ARGV,'echo');
1356 system @ARGV;
1357 }
1358 # @ARGV restored
1359
1360
1361 # temporarily add to digits associative array
1362 if ($base12) {
1363 # (NOTE: not claiming this is efficient!)
1364 local(%digits) = (%digits,'t',10,'e',11);
1365 parse_num();
1366 }
1367
1368Note that local() is a run-time command, and so gets executed every
748a9306 1369time through a loop. In Perl 4 it used more stack storage each
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LW
1370time until the loop was exited. Perl 5 reclaims the space each time
1371through, but it's still more efficient to declare your variables
1372outside the loop.
1373
748a9306 1374A local is simply a modifier on an lvalue expression.
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LW
1375When you assign to a localized EXPR, the local doesn't change whether
1376EXPR is viewed as a scalar or an array. So
1377
1378 local($foo) = <STDIN>;
1379 local @FOO = <STDIN>;
1380
1381both supply a list context to the righthand side, while
1382
1383 local $foo = <STDIN>;
1384
1385supplies a scalar context.
1386
1387=item localtime EXPR
1388
1389Converts a time as returned by the time function to a 9-element array
1390with the time analyzed for the local timezone. Typically used as
1391follows:
1392
1393 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1394 localtime(time);
1395
1396All array elements are numeric, and come straight out of a struct tm.
1397In particular this means that $mon has the range 0..11 and $wday has
1398the range 0..6. If EXPR is omitted, does localtime(time).
1399
1400In a scalar context, prints out the ctime(3) value:
1401
1402 $now_string = localtime; # e.g. "Thu Oct 13 04:54:34 1994"
1403
1404See also L<perlmod/timelocal> and the strftime(3) function available
1405via the POSIX modulie.
1406
1407=item log EXPR
1408
1409Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1410of $_.
1411
1412=item lstat FILEHANDLE
1413
1414=item lstat EXPR
1415
1416Does the same thing as the stat() function, but stats a symbolic link
1417instead of the file the symbolic link points to. If symbolic links are
1418unimplemented on your system, a normal stat() is done.
1419
1420=item m//
1421
1422The match operator. See L<perlop>.
1423
1424=item map BLOCK LIST
1425
1426=item map EXPR,LIST
1427
1428Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1429element) and returns the list value composed of the results of each such
1430evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1431may produce zero, one, or more elements in the returned value.
1432
1433 @chars = map(chr, @nums);
1434
1435translates a list of numbers to the corresponding characters. And
1436
4633a7c4 1437 %hash = map { getkey($_) => $_ } @array;
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1438
1439is just a funny way to write
1440
1441 %hash = ();
1442 foreach $_ (@array) {
4633a7c4 1443 $hash{getkey($_)} = $_;
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LW
1444 }
1445
1446=item mkdir FILENAME,MODE
1447
1448Creates the directory specified by FILENAME, with permissions specified
1449by MODE (as modified by umask). If it succeeds it returns 1, otherwise
1450it returns 0 and sets $! (errno).
1451
1452=item msgctl ID,CMD,ARG
1453
4633a7c4 1454Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
a0d0e21e
LW
1455must be a variable which will hold the returned msqid_ds structure.
1456Returns like ioctl: the undefined value for error, "0 but true" for
1457zero, or the actual return value otherwise.
1458
1459=item msgget KEY,FLAGS
1460
4633a7c4 1461Calls the System V IPC function msgget(2). Returns the message queue id,
a0d0e21e
LW
1462or the undefined value if there is an error.
1463
1464=item msgsnd ID,MSG,FLAGS
1465
1466Calls the System V IPC function msgsnd to send the message MSG to the
1467message queue ID. MSG must begin with the long integer message type,
1468which may be created with C<pack("L", $type)>. Returns TRUE if
1469successful, or FALSE if there is an error.
1470
1471=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1472
1473Calls the System V IPC function msgrcv to receive a message from
1474message queue ID into variable VAR with a maximum message size of
1475SIZE. Note that if a message is received, the message type will be the
1476first thing in VAR, and the maximum length of VAR is SIZE plus the size
1477of the message type. Returns TRUE if successful, or FALSE if there is
1478an error.
1479
1480=item my EXPR
1481
1482A "my" declares the listed variables to be local (lexically) to the
4633a7c4 1483enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If more than one value is
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LW
1484listed, the list must be placed in parens. All the listed elements
1485must be legal lvalues. Only alphanumeric identifiers may be lexically
1486scoped--magical builtins like $/ must be localized with "local"
4633a7c4
LW
1487instead. You also cannot use my() on a package variable.
1488In particular, you're not allowed to say
a0d0e21e 1489
4633a7c4
LW
1490 my $_; # Illegal!
1491 my $pack::$var; # Illegal!
a0d0e21e
LW
1492
1493Unlike the "local" declaration, variables declared with "my"
1494are totally hidden from the outside world, including any called
1495subroutines (even if it's the same subroutine--every call gets its own
4633a7c4 1496copy).
a0d0e21e
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1497
1498(An eval(), however, can see the lexical variables of the scope it is
1499being evaluated in so long as the names aren't hidden by declarations within
1500the eval() itself. See L<perlref>.)
1501
1502The EXPR may be assigned to if desired, which allows you to initialize
1503your variables. (If no initializer is given for a particular
1504variable, it is created with an undefined value.) Commonly this is
1505used to name the parameters to a subroutine. Examples:
1506
1507 sub RANGEVAL {
1508 my($min, $max, $thunk) = @_;
1509 my $result = '';
1510 my $i;
1511
1512 # Presumably $thunk makes reference to $i
1513
1514 for ($i = $min; $i < $max; $i++) {
1515 $result .= eval $thunk;
1516 }
1517
1518 $result;
1519 }
1520
1521
1522 if ($sw eq '-v') {
1523 # init my array with global array
1524 my @ARGV = @ARGV;
1525 unshift(@ARGV,'echo');
1526 system @ARGV;
1527 }
1528 # Outer @ARGV again visible
1529
748a9306
LW
1530The "my" is simply a modifier on something you might assign to.
1531So when you do assign to the EXPR, the "my" doesn't change whether
a0d0e21e
LW
1532EXPR is viewed as a scalar or an array. So
1533
748a9306 1534 my ($foo) = <STDIN>;
a0d0e21e
LW
1535 my @FOO = <STDIN>;
1536
1537both supply a list context to the righthand side, while
1538
1539 my $foo = <STDIN>;
1540
748a9306
LW
1541supplies a scalar context. But the following only declares one variable:
1542
1543 my $foo, $bar = 1;
1544
1545That has the same effect as
1546
1547 my $foo;
1548 $bar = 1;
1549
1550The declared variable is not introduced (is not visible) until after
1551the current statement. Thus,
1552
1553 my $x = $x;
1554
1555can be used to initialize the new $x with the value of the old $x, and
1556the expression
1557
1558 my $x = 123 and $x == 123
1559
1560is false unless the old $x happened to have the value 123.
a0d0e21e
LW
1561
1562Some users may wish to encourage the use of lexically scoped variables.
1563As an aid to catching implicit references to package variables,
1564if you say
1565
1566 use strict 'vars';
1567
1568then any variable reference from there to the end of the enclosing
1569block must either refer to a lexical variable, or must be fully
1570qualified with the package name. A compilation error results
1571otherwise. An inner block may countermand this with S<"no strict 'vars'">.
1572
4633a7c4
LW
1573Variables declared with "my" are not part of any package and
1574are therefore never fully qualified with the package name.
1575However, you may declare a "my" variable at the outer most
1576scope of a file to totally hide any such identifiers from the
1577outside world. This is similar to a C's static variables
1578at the file level. To do this with a subroutine requires the
1579use of a closure (anonymous function):
1580
1581 my $secret_version = '1.001-beta';
1582 my $secret_sub = { print $secret_version };
1583 &$secret_sub();
1584
1585This does not work with object methods, however;
1586all object methods have to be in the symbol table of some
1587package to be found.
1588
1589Just because the "my" variable is lexically scoped doesn't mean that
1590within a function it works like a C static. Here's a mechanism for giving
1591a function private variables with both lexical scoping and a static
1592lifetime.
1593
1594 #!/usr/bin/perl -l
1595 $var = "global";
1596 { my $count = 0;
1597 my $var = "static";
1598 sub foo {
1599 $count++;
1600 print "$var (call # $count)";
1601 }
1602 }
1603 print $var; foo();
1604 print $var; foo();
1605 print $var; foo();
1606
a0d0e21e
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1607=item next LABEL
1608
1609=item next
1610
1611The C<next> command is like the C<continue> statement in C; it starts
1612the next iteration of the loop:
1613
4633a7c4
LW
1614 LINE: while (<STDIN>) {
1615 next LINE if /^#/; # discard comments
a0d0e21e
LW
1616 ...
1617 }
1618
1619Note that if there were a C<continue> block on the above, it would get
1620executed even on discarded lines. If the LABEL is omitted, the command
1621refers to the innermost enclosing loop.
1622
1623=item no Module LIST
1624
1625See the "use" function, which "no" is the opposite of.
1626
1627=item oct EXPR
1628
4633a7c4
LW
1629Interprets EXPR as an octal string and returns the corresponding
1630decimal value. (If EXPR happens to start off with 0x, interprets it as
1631a hex string instead.) The following will handle decimal, octal, and
1632hex in the standard Perl or C notation:
a0d0e21e
LW
1633
1634 $val = oct($val) if $val =~ /^0/;
1635
1636If EXPR is omitted, uses $_.
1637
1638=item open FILEHANDLE,EXPR
1639
1640=item open FILEHANDLE
1641
1642Opens the file whose filename is given by EXPR, and associates it with
4633a7c4
LW
1643FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name
1644of the real filehandle wanted. If EXPR is omitted, the scalar variable of
1645the same name as the FILEHANDLE contains the filename. If the filename
1646begins with "<" or nothing, the file is opened for input. If the filename
1647begins with ">", the file is opened for output. If the filename begins
1648with ">>", the file is opened for appending. (You can put a '+' in front
1649of the '>' or '<' to indicate that you want both read and write access to
1650the file.) If the filename begins with "|", the filename is interpreted
1651as a command to which output is to be piped, and if the filename ends with
1652a "|", the filename is interpreted See L<perlipc/"Using open() for IPC">
1653for more examples of this. as command which pipes input to us. (You may
1654not have a command that pipes both in and out, but see See L<open2>,
1655L<open3>, and L<perlipc/"Bidirectional Communication"> for alternatives.)
1656Opening '-' opens STDIN and opening '>-' opens STDOUT. Open returns
1657non-zero upon success, the undefined value otherwise. If the open
1658involved a pipe, the return value happens to be the pid of the
1659subprocess. Examples:
a0d0e21e
LW
1660
1661 $ARTICLE = 100;
1662 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
1663 while (<ARTICLE>) {...
1664
1665 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
1666
4633a7c4 1667 open(ARTICLE, "caesar <$article |"); # decrypt article
a0d0e21e 1668
4633a7c4 1669 open(EXTRACT, "|sort >/tmp/Tmp$$"); # $$ is our process id
a0d0e21e
LW
1670
1671 # process argument list of files along with any includes
1672
1673 foreach $file (@ARGV) {
1674 process($file, 'fh00');
1675 }
1676
1677 sub process {
1678 local($filename, $input) = @_;
1679 $input++; # this is a string increment
1680 unless (open($input, $filename)) {
1681 print STDERR "Can't open $filename: $!\n";
1682 return;
1683 }
1684
1685 while (<$input>) { # note use of indirection
1686 if (/^#include "(.*)"/) {
1687 process($1, $input);
1688 next;
1689 }
1690 ... # whatever
1691 }
1692 }
1693
1694You may also, in the Bourne shell tradition, specify an EXPR beginning
1695with ">&", in which case the rest of the string is interpreted as the
1696name of a filehandle (or file descriptor, if numeric) which is to be
1697duped and opened. You may use & after >, >>, <, +>, +>> and +<. The
1698mode you specify should match the mode of the original filehandle.
1699Here is a script that saves, redirects, and restores STDOUT and
1700STDERR:
1701
1702 #!/usr/bin/perl
1703 open(SAVEOUT, ">&STDOUT");
1704 open(SAVEERR, ">&STDERR");
1705
1706 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
1707 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
1708
1709 select(STDERR); $| = 1; # make unbuffered
1710 select(STDOUT); $| = 1; # make unbuffered
1711
1712 print STDOUT "stdout 1\n"; # this works for
1713 print STDERR "stderr 1\n"; # subprocesses too
1714
1715 close(STDOUT);
1716 close(STDERR);
1717
1718 open(STDOUT, ">&SAVEOUT");
1719 open(STDERR, ">&SAVEERR");
1720
1721 print STDOUT "stdout 2\n";
1722 print STDERR "stderr 2\n";
1723
1724
1725If you specify "<&=N", where N is a number, then Perl will do an
4633a7c4
LW
1726equivalent of C's fdopen() of that file descriptor; this is more
1727parsimonious of file descriptors. For example:
a0d0e21e
LW
1728
1729 open(FILEHANDLE, "<&=$fd")
1730
1731If you open a pipe on the command "-", i.e. either "|-" or "-|", then
1732there is an implicit fork done, and the return value of open is the pid
1733of the child within the parent process, and 0 within the child
1734process. (Use defined($pid) to determine whether the open was successful.)
1735The filehandle behaves normally for the parent, but i/o to that
1736filehandle is piped from/to the STDOUT/STDIN of the child process.
1737In the child process the filehandle isn't opened--i/o happens from/to
1738the new STDOUT or STDIN. Typically this is used like the normal
1739piped open when you want to exercise more control over just how the
1740pipe command gets executed, such as when you are running setuid, and
4633a7c4
LW
1741don't want to have to scan shell commands for metacharacters.
1742The following pairs are more or less equivalent:
a0d0e21e
LW
1743
1744 open(FOO, "|tr '[a-z]' '[A-Z]'");
1745 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
1746
1747 open(FOO, "cat -n '$file'|");
1748 open(FOO, "-|") || exec 'cat', '-n', $file;
1749
4633a7c4
LW
1750See L<perlipc/"Safe Pipe Opens"> for more examples of this.
1751
a0d0e21e
LW
1752Explicitly closing any piped filehandle causes the parent process to
1753wait for the child to finish, and returns the status value in $?.
1754Note: on any operation which may do a fork, unflushed buffers remain
1755unflushed in both processes, which means you may need to set $| to
1756avoid duplicate output.
1757
1758The filename that is passed to open will have leading and trailing
1759whitespace deleted. In order to open a file with arbitrary weird
1760characters in it, it's necessary to protect any leading and trailing
1761whitespace thusly:
1762
1763 $file =~ s#^(\s)#./$1#;
1764 open(FOO, "< $file\0");
1765
1766=item opendir DIRHANDLE,EXPR
1767
1768Opens a directory named EXPR for processing by readdir(), telldir(),
1769seekdir(), rewinddir() and closedir(). Returns TRUE if successful.
1770DIRHANDLEs have their own namespace separate from FILEHANDLEs.
1771
1772=item ord EXPR
1773
1774Returns the numeric ascii value of the first character of EXPR. If
1775EXPR is omitted, uses $_.
1776
1777=item pack TEMPLATE,LIST
1778
1779Takes an array or list of values and packs it into a binary structure,
1780returning the string containing the structure. The TEMPLATE is a
1781sequence of characters that give the order and type of values, as
1782follows:
1783
1784 A An ascii string, will be space padded.
1785 a An ascii string, will be null padded.
1786 b A bit string (ascending bit order, like vec()).
1787 B A bit string (descending bit order).
1788 h A hex string (low nybble first).
1789 H A hex string (high nybble first).
1790
1791 c A signed char value.
1792 C An unsigned char value.
1793 s A signed short value.
1794 S An unsigned short value.
1795 i A signed integer value.
1796 I An unsigned integer value.
1797 l A signed long value.
1798 L An unsigned long value.
1799
1800 n A short in "network" order.
1801 N A long in "network" order.
1802 v A short in "VAX" (little-endian) order.
1803 V A long in "VAX" (little-endian) order.
1804
1805 f A single-precision float in the native format.
1806 d A double-precision float in the native format.
1807
1808 p A pointer to a null-terminated string.
1809 P A pointer to a structure (fixed-length string).
1810
1811 u A uuencoded string.
1812
1813 x A null byte.
1814 X Back up a byte.
1815 @ Null fill to absolute position.
1816
1817Each letter may optionally be followed by a number which gives a repeat
1818count. With all types except "a", "A", "b", "B", "h" and "H", and "P" the
1819pack function will gobble up that many values from the LIST. A * for the
1820repeat count means to use however many items are left. The "a" and "A"
1821types gobble just one value, but pack it as a string of length count,
1822padding with nulls or spaces as necessary. (When unpacking, "A" strips
1823trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
1824fields pack a string that many bits long. The "h" and "H" fields pack a
1825string that many nybbles long. The "P" packs a pointer to a structure of
1826the size indicated by the length. Real numbers (floats and doubles) are
1827in the native machine format only; due to the multiplicity of floating
1828formats around, and the lack of a standard "network" representation, no
1829facility for interchange has been made. This means that packed floating
1830point data written on one machine may not be readable on another - even if
1831both use IEEE floating point arithmetic (as the endian-ness of the memory
1832representation is not part of the IEEE spec). Note that Perl uses doubles
1833internally for all numeric calculation, and converting from double into
1834float and thence back to double again will lose precision (i.e.
1835C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
1836
1837Examples:
1838
1839 $foo = pack("cccc",65,66,67,68);
1840 # foo eq "ABCD"
1841 $foo = pack("c4",65,66,67,68);
1842 # same thing
1843
1844 $foo = pack("ccxxcc",65,66,67,68);
1845 # foo eq "AB\0\0CD"
1846
1847 $foo = pack("s2",1,2);
1848 # "\1\0\2\0" on little-endian
1849 # "\0\1\0\2" on big-endian
1850
1851 $foo = pack("a4","abcd","x","y","z");
1852 # "abcd"
1853
1854 $foo = pack("aaaa","abcd","x","y","z");
1855 # "axyz"
1856
1857 $foo = pack("a14","abcdefg");
1858 # "abcdefg\0\0\0\0\0\0\0"
1859
1860 $foo = pack("i9pl", gmtime);
1861 # a real struct tm (on my system anyway)
1862
1863 sub bintodec {
1864 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
1865 }
1866
1867The same template may generally also be used in the unpack function.
1868
1869=item pipe READHANDLE,WRITEHANDLE
1870
1871Opens a pair of connected pipes like the corresponding system call.
1872Note that if you set up a loop of piped processes, deadlock can occur
1873unless you are very careful. In addition, note that Perl's pipes use
1874stdio buffering, so you may need to set $| to flush your WRITEHANDLE
1875after each command, depending on the application.
1876
4633a7c4
LW
1877See L<open2>, L<open3>, and L<perlipc/"Bidirectional Communication">
1878for examples of such things.
1879
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LW
1880=item pop ARRAY
1881
1882Pops and returns the last value of the array, shortening the array by
18831. Has a similar effect to
1884
1885 $tmp = $ARRAY[$#ARRAY--];
1886
1887If there are no elements in the array, returns the undefined value.
1888
1889=item pos SCALAR
1890
4633a7c4 1891Returns the offset of where the last C<m//g> search left off for the variable
a0d0e21e
LW
1892in question. May be modified to change that offset.
1893
1894=item print FILEHANDLE LIST
1895
1896=item print LIST
1897
1898=item print
1899
1900Prints a string or a comma-separated list of strings. Returns non-zero
1901if successful. FILEHANDLE may be a scalar variable name, in which case
1902the variable contains the name of the filehandle, thus introducing one
1903level of indirection. (NOTE: If FILEHANDLE is a variable and the next
1904token is a term, it may be misinterpreted as an operator unless you
1905interpose a + or put parens around the arguments.) If FILEHANDLE is
1906omitted, prints by default to standard output (or to the last selected
1907output channel--see select()). If LIST is also omitted, prints $_ to
1908STDOUT. To set the default output channel to something other than
1909STDOUT use the select operation. Note that, because print takes a
1910LIST, anything in the LIST is evaluated in a list context, and any
1911subroutine that you call will have one or more of its expressions
1912evaluated in a list context. Also be careful not to follow the print
1913keyword with a left parenthesis unless you want the corresponding right
1914parenthesis to terminate the arguments to the print--interpose a + or
1915put parens around all the arguments.
1916
4633a7c4
LW
1917Note that if you're storing FILEHANDLES in an array or other expression,
1918you will have to use a block returning its value instead
1919
1920 print { $files[$i] } "stuff\n";
1921 print { $OK ? STDOUT : STDERR } "stuff\n";
1922
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1923=item printf FILEHANDLE LIST
1924
1925=item printf LIST
1926
1927Equivalent to a "print FILEHANDLE sprintf(LIST)". The first argument
1928of the list will be interpreted as the printf format.
1929
1930=item push ARRAY,LIST
1931
1932Treats ARRAY as a stack, and pushes the values of LIST
1933onto the end of ARRAY. The length of ARRAY increases by the length of
1934LIST. Has the same effect as
1935
1936 for $value (LIST) {
1937 $ARRAY[++$#ARRAY] = $value;
1938 }
1939
1940but is more efficient. Returns the new number of elements in the array.
1941
1942=item q/STRING/
1943
1944=item qq/STRING/
1945
1946=item qx/STRING/
1947
1948=item qw/STRING/
1949
1950Generalized quotes. See L<perlop>.
1951
1952=item quotemeta EXPR
1953
1954Returns the value of EXPR with with all regular expression
1955metacharacters backslashed. This is the internal function implementing
1956the \Q escape in double-quoted strings.
1957
1958=item rand EXPR
1959
1960=item rand
1961
1962Returns a random fractional number between 0 and the value of EXPR.
1963(EXPR should be positive.) If EXPR is omitted, returns a value between
19640 and 1. This function produces repeatable sequences unless srand()
1965is invoked. See also srand().
1966
1967(Note: if your rand function consistently returns numbers that are too
1968large or too small, then your version of Perl was probably compiled
1969with the wrong number of RANDBITS. As a workaround, you can usually
1970multiply EXPR by the correct power of 2 to get the range you want.
1971This will make your script unportable, however. It's better to recompile
1972if you can.)
1973
1974=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
1975
1976=item read FILEHANDLE,SCALAR,LENGTH
1977
1978Attempts to read LENGTH bytes of data into variable SCALAR from the
1979specified FILEHANDLE. Returns the number of bytes actually read, or
1980undef if there was an error. SCALAR will be grown or shrunk to the
1981length actually read. An OFFSET may be specified to place the read
1982data at some other place than the beginning of the string. This call
1983is actually implemented in terms of stdio's fread call. To get a true
1984read system call, see sysread().
1985
1986=item readdir DIRHANDLE
1987
1988Returns the next directory entry for a directory opened by opendir().
1989If used in a list context, returns all the rest of the entries in the
1990directory. If there are no more entries, returns an undefined value in
1991a scalar context or a null list in a list context.
1992
1993=item readlink EXPR
1994
1995Returns the value of a symbolic link, if symbolic links are
1996implemented. If not, gives a fatal error. If there is some system
1997error, returns the undefined value and sets $! (errno). If EXPR is
1998omitted, uses $_.
1999
2000=item recv SOCKET,SCALAR,LEN,FLAGS
2001
2002Receives a message on a socket. Attempts to receive LENGTH bytes of
2003data into variable SCALAR from the specified SOCKET filehandle.
2004Actually does a C recvfrom(), so that it can returns the address of the
2005sender. Returns the undefined value if there's an error. SCALAR will
2006be grown or shrunk to the length actually read. Takes the same flags
4633a7c4
LW
2007as the system call of the same name.
2008See L<perlipc/"UDP: Message Passing"> for examples.
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2009
2010=item redo LABEL
2011
2012=item redo
2013
2014The C<redo> command restarts the loop block without evaluating the
2015conditional again. The C<continue> block, if any, is not executed. If
2016the LABEL is omitted, the command refers to the innermost enclosing
2017loop. This command is normally used by programs that want to lie to
2018themselves about what was just input:
2019
2020 # a simpleminded Pascal comment stripper
2021 # (warning: assumes no { or } in strings)
4633a7c4 2022 LINE: while (<STDIN>) {
a0d0e21e
LW
2023 while (s|({.*}.*){.*}|$1 |) {}
2024 s|{.*}| |;
2025 if (s|{.*| |) {
2026 $front = $_;
2027 while (<STDIN>) {
2028 if (/}/) { # end of comment?
2029 s|^|$front{|;
4633a7c4 2030 redo LINE;
a0d0e21e
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2031 }
2032 }
2033 }
2034 print;
2035 }
2036
2037=item ref EXPR
2038
2039Returns a TRUE value if EXPR is a reference, FALSE otherwise. The value
2040returned depends on the type of thing the reference is a reference to.
2041Builtin types include:
2042
2043 REF
2044 SCALAR
2045 ARRAY
2046 HASH
2047 CODE
2048 GLOB
2049
2050If the referenced object has been blessed into a package, then that package
2051name is returned instead. You can think of ref() as a typeof() operator.
2052
2053 if (ref($r) eq "HASH") {
2054 print "r is a reference to an associative array.\n";
2055 }
2056 if (!ref ($r) {
2057 print "r is not a reference at all.\n";
2058 }
2059
2060See also L<perlref>.
2061
2062=item rename OLDNAME,NEWNAME
2063
2064Changes the name of a file. Returns 1 for success, 0 otherwise. Will
2065not work across filesystem boundaries.
2066
2067=item require EXPR
2068
2069=item require
2070
2071Demands some semantics specified by EXPR, or by $_ if EXPR is not
2072supplied. If EXPR is numeric, demands that the current version of Perl
2073($] or $PERL_VERSION) be equal or greater than EXPR.
2074
2075Otherwise, demands that a library file be included if it hasn't already
2076been included. The file is included via the do-FILE mechanism, which is
2077essentially just a variety of eval(). Has semantics similar to the following
2078subroutine:
2079
2080 sub require {
2081 local($filename) = @_;
2082 return 1 if $INC{$filename};
2083 local($realfilename,$result);
2084 ITER: {
2085 foreach $prefix (@INC) {
2086 $realfilename = "$prefix/$filename";
2087 if (-f $realfilename) {
2088 $result = do $realfilename;
2089 last ITER;
2090 }
2091 }
2092 die "Can't find $filename in \@INC";
2093 }
2094 die $@ if $@;
2095 die "$filename did not return true value" unless $result;
2096 $INC{$filename} = $realfilename;
2097 $result;
2098 }
2099
2100Note that the file will not be included twice under the same specified
2101name. The file must return TRUE as the last statement to indicate
2102successful execution of any initialization code, so it's customary to
2103end such a file with "1;" unless you're sure it'll return TRUE
2104otherwise. But it's better just to put the "C<1;>", in case you add more
2105statements.
2106
2107If EXPR is a bare word, the require assumes a "F<.pm>" extension for you,
2108to make it easy to load standard modules. This form of loading of
2109modules does not risk altering your namespace.
2110
748a9306
LW
2111For a yet-more-powerful import facility, see the L</use()> and
2112L<perlmod>.
a0d0e21e
LW
2113
2114=item reset EXPR
2115
2116=item reset
2117
2118Generally used in a C<continue> block at the end of a loop to clear
2119variables and reset ?? searches so that they work again. The
2120expression is interpreted as a list of single characters (hyphens
2121allowed for ranges). All variables and arrays beginning with one of
2122those letters are reset to their pristine state. If the expression is
2123omitted, one-match searches (?pattern?) are reset to match again. Only
2124resets variables or searches in the current package. Always returns
21251. Examples:
2126
2127 reset 'X'; # reset all X variables
2128 reset 'a-z'; # reset lower case variables
2129 reset; # just reset ?? searches
2130
2131Resetting "A-Z" is not recommended since you'll wipe out your
2132ARGV and ENV arrays. Only resets package variables--lexical variables
2133are unaffected, but they clean themselves up on scope exit anyway,
2134so anymore you probably want to use them instead. See L</my>.
2135
2136=item return LIST
2137
2138Returns from a subroutine or eval with the value specified. (Note that
4633a7c4 2139in the absence of a return a subroutine or eval() will automatically
a0d0e21e
LW
2140return the value of the last expression evaluated.)
2141
2142=item reverse LIST
2143
2144In a list context, returns a list value consisting of the elements
2145of LIST in the opposite order. In a scalar context, returns a string
2146value consisting of the bytes of the first element of LIST in the
4633a7c4
LW
2147opposite order.
2148
2149 print reverse <>; # line tac
2150
2151 undef $/;
2152 print scalar reverse scalar <>; # byte tac
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LW
2153
2154=item rewinddir DIRHANDLE
2155
2156Sets the current position to the beginning of the directory for the
2157readdir() routine on DIRHANDLE.
2158
2159=item rindex STR,SUBSTR,POSITION
2160
2161=item rindex STR,SUBSTR
2162
2163Works just like index except that it returns the position of the LAST
2164occurrence of SUBSTR in STR. If POSITION is specified, returns the
2165last occurrence at or before that position.
2166
2167=item rmdir FILENAME
2168
2169Deletes the directory specified by FILENAME if it is empty. If it
2170succeeds it returns 1, otherwise it returns 0 and sets $! (errno). If
2171FILENAME is omitted, uses $_.
2172
2173=item s///
2174
2175The substitution operator. See L<perlop>.
2176
2177=item scalar EXPR
2178
2179Forces EXPR to be interpreted in a scalar context and returns the value
2180of EXPR.
2181
2182=item seek FILEHANDLE,POSITION,WHENCE
2183
2184Randomly positions the file pointer for FILEHANDLE, just like the fseek()
2185call of stdio. FILEHANDLE may be an expression whose value gives the name
2186of the filehandle. The values for WHENCE are 0 to set the file pointer to
2187POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
2188plus offset. You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
4633a7c4 2189this from POSIX module. Returns 1 upon success, 0 otherwise.
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2190
2191=item seekdir DIRHANDLE,POS
2192
2193Sets the current position for the readdir() routine on DIRHANDLE. POS
2194must be a value returned by telldir(). Has the same caveats about
2195possible directory compaction as the corresponding system library
2196routine.
2197
2198=item select FILEHANDLE
2199
2200=item select
2201
2202Returns the currently selected filehandle. Sets the current default
2203filehandle for output, if FILEHANDLE is supplied. This has two
2204effects: first, a C<write> or a C<print> without a filehandle will
2205default to this FILEHANDLE. Second, references to variables related to
2206output will refer to this output channel. For example, if you have to
2207set the top of form format for more than one output channel, you might
2208do the following:
2209
2210 select(REPORT1);
2211 $^ = 'report1_top';
2212 select(REPORT2);
2213 $^ = 'report2_top';
2214
2215FILEHANDLE may be an expression whose value gives the name of the
2216actual filehandle. Thus:
2217
2218 $oldfh = select(STDERR); $| = 1; select($oldfh);
2219
4633a7c4
LW
2220Some programmers may prefer to think of filehandles as objects with
2221methods, preferring to write the last example as:
a0d0e21e
LW
2222
2223 use FileHandle;
2224 STDERR->autoflush(1);
2225
2226=item select RBITS,WBITS,EBITS,TIMEOUT
2227
4633a7c4 2228This calls the select(2) system call with the bitmasks specified, which
a0d0e21e
LW
2229can be constructed using fileno() and vec(), along these lines:
2230
2231 $rin = $win = $ein = '';
2232 vec($rin,fileno(STDIN),1) = 1;
2233 vec($win,fileno(STDOUT),1) = 1;
2234 $ein = $rin | $win;
2235
2236If you want to select on many filehandles you might wish to write a
2237subroutine:
2238
2239 sub fhbits {
2240 local(@fhlist) = split(' ',$_[0]);
2241 local($bits);
2242 for (@fhlist) {
2243 vec($bits,fileno($_),1) = 1;
2244 }
2245 $bits;
2246 }
4633a7c4 2247 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
2248
2249The usual idiom is:
2250
2251 ($nfound,$timeleft) =
2252 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2253
2254or to block until something becomes ready:
2255
2256 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2257
2258Any of the bitmasks can also be undef. The timeout, if specified, is
2259in seconds, which may be fractional. Note: not all implementations are
2260capable of returning the $timeleft. If not, they always return
2261$timeleft equal to the supplied $timeout.
2262
4633a7c4 2263You can effect a 250-microsecond sleep this way:
a0d0e21e
LW
2264
2265 select(undef, undef, undef, 0.25);
2266
2267
2268=item semctl ID,SEMNUM,CMD,ARG
2269
2270Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2271&GETALL, then ARG must be a variable which will hold the returned
2272semid_ds structure or semaphore value array. Returns like ioctl: the
2273undefined value for error, "0 but true" for zero, or the actual return
2274value otherwise.
2275
2276=item semget KEY,NSEMS,FLAGS
2277
2278Calls the System V IPC function semget. Returns the semaphore id, or
2279the undefined value if there is an error.
2280
2281=item semop KEY,OPSTRING
2282
2283Calls the System V IPC function semop to perform semaphore operations
2284such as signaling and waiting. OPSTRING must be a packed array of
2285semop structures. Each semop structure can be generated with
2286C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2287operations is implied by the length of OPSTRING. Returns TRUE if
2288successful, or FALSE if there is an error. As an example, the
2289following code waits on semaphore $semnum of semaphore id $semid:
2290
2291 $semop = pack("sss", $semnum, -1, 0);
2292 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2293
2294To signal the semaphore, replace "-1" with "1".
2295
2296=item send SOCKET,MSG,FLAGS,TO
2297
2298=item send SOCKET,MSG,FLAGS
2299
2300Sends a message on a socket. Takes the same flags as the system call
2301of the same name. On unconnected sockets you must specify a
2302destination to send TO, in which case it does a C sendto(). Returns
2303the number of characters sent, or the undefined value if there is an
2304error.
4633a7c4 2305See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2306
2307=item setpgrp PID,PGRP
2308
2309Sets the current process group for the specified PID, 0 for the current
2310process. Will produce a fatal error if used on a machine that doesn't
2311implement setpgrp(2).
2312
2313=item setpriority WHICH,WHO,PRIORITY
2314
2315Sets the current priority for a process, a process group, or a user.
748a9306 2316(See setpriority(2).) Will produce a fatal error if used on a machine
a0d0e21e
LW
2317that doesn't implement setpriority(2).
2318
2319=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2320
2321Sets the socket option requested. Returns undefined if there is an
2322error. OPTVAL may be specified as undef if you don't want to pass an
2323argument.
2324
2325=item shift ARRAY
2326
2327=item shift
2328
2329Shifts the first value of the array off and returns it, shortening the
2330array by 1 and moving everything down. If there are no elements in the
2331array, returns the undefined value. If ARRAY is omitted, shifts the
2332@ARGV array in the main program, and the @_ array in subroutines.
2333(This is determined lexically.) See also unshift(), push(), and pop().
2334Shift() and unshift() do the same thing to the left end of an array
2335that push() and pop() do to the right end.
2336
2337=item shmctl ID,CMD,ARG
2338
2339Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2340must be a variable which will hold the returned shmid_ds structure.
2341Returns like ioctl: the undefined value for error, "0 but true" for
2342zero, or the actual return value otherwise.
2343
2344=item shmget KEY,SIZE,FLAGS
2345
2346Calls the System V IPC function shmget. Returns the shared memory
2347segment id, or the undefined value if there is an error.
2348
2349=item shmread ID,VAR,POS,SIZE
2350
2351=item shmwrite ID,STRING,POS,SIZE
2352
2353Reads or writes the System V shared memory segment ID starting at
2354position POS for size SIZE by attaching to it, copying in/out, and
2355detaching from it. When reading, VAR must be a variable which will
2356hold the data read. When writing, if STRING is too long, only SIZE
2357bytes are used; if STRING is too short, nulls are written to fill out
2358SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2359
2360=item shutdown SOCKET,HOW
2361
2362Shuts down a socket connection in the manner indicated by HOW, which
2363has the same interpretation as in the system call of the same name.
2364
2365=item sin EXPR
2366
2367Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2368returns sine of $_.
2369
2370=item sleep EXPR
2371
2372=item sleep
2373
2374Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2375May be interrupted by sending the process a SIGALRM. Returns the
2376number of seconds actually slept. You probably cannot mix alarm() and
2377sleep() calls, since sleep() is often implemented using alarm().
2378
2379On some older systems, it may sleep up to a full second less than what
2380you requested, depending on how it counts seconds. Most modern systems
2381always sleep the full amount.
2382
2383=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2384
2385Opens a socket of the specified kind and attaches it to filehandle
2386SOCKET. DOMAIN, TYPE and PROTOCOL are specified the same as for the
2387system call of the same name. You should "use Socket;" first to get
4633a7c4 2388the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2389
2390=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2391
2392Creates an unnamed pair of sockets in the specified domain, of the
2393specified type. DOMAIN, TYPE and PROTOCOL are specified the same as
2394for the system call of the same name. If unimplemented, yields a fatal
2395error. Returns TRUE if successful.
2396
2397=item sort SUBNAME LIST
2398
2399=item sort BLOCK LIST
2400
2401=item sort LIST
2402
2403Sorts the LIST and returns the sorted list value. Nonexistent values
2404of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
2405in standard string comparison order. If SUBNAME is specified, it
2406gives the name of a subroutine that returns an integer less than, equal
2407to, or greater than 0, depending on how the elements of the array are
2408to be ordered. (The <=> and cmp operators are extremely useful in such
2409routines.) SUBNAME may be a scalar variable name, in which case the
2410value provides the name of the subroutine to use. In place of a
2411SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
2412subroutine.
2413
2414In the interests of efficiency the normal calling code for subroutines
2415is bypassed, with the following effects: the subroutine may not be a
2416recursive subroutine, and the two elements to be compared are passed
2417into the subroutine not via @_ but as $a and $b (see example below).
2418They are passed by reference, so don't modify $a and $b.
2419
2420Examples:
2421
2422 # sort lexically
2423 @articles = sort @files;
2424
2425 # same thing, but with explicit sort routine
2426 @articles = sort {$a cmp $b} @files;
2427
2428 # same thing in reversed order
2429 @articles = sort {$b cmp $a} @files;
2430
2431 # sort numerically ascending
2432 @articles = sort {$a <=> $b} @files;
2433
2434 # sort numerically descending
2435 @articles = sort {$b <=> $a} @files;
2436
2437 # sort using explicit subroutine name
2438 sub byage {
2439 $age{$a} <=> $age{$b}; # presuming integers
2440 }
2441 @sortedclass = sort byage @class;
2442
2443 sub backwards { $b cmp $a; }
2444 @harry = ('dog','cat','x','Cain','Abel');
2445 @george = ('gone','chased','yz','Punished','Axed');
2446 print sort @harry;
2447 # prints AbelCaincatdogx
2448 print sort backwards @harry;
2449 # prints xdogcatCainAbel
2450 print sort @george, 'to', @harry;
2451 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
2452
2453=item splice ARRAY,OFFSET,LENGTH,LIST
2454
2455=item splice ARRAY,OFFSET,LENGTH
2456
2457=item splice ARRAY,OFFSET
2458
2459Removes the elements designated by OFFSET and LENGTH from an array, and
2460replaces them with the elements of LIST, if any. Returns the elements
2461removed from the array. The array grows or shrinks as necessary. If
2462LENGTH is omitted, removes everything from OFFSET onward. The
2463following equivalencies hold (assuming $[ == 0):
2464
2465 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
2466 pop(@a) splice(@a,-1)
2467 shift(@a) splice(@a,0,1)
2468 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
2469 $a[$x] = $y splice(@a,$x,1,$y);
2470
2471Example, assuming array lengths are passed before arrays:
2472
2473 sub aeq { # compare two list values
2474 local(@a) = splice(@_,0,shift);
2475 local(@b) = splice(@_,0,shift);
2476 return 0 unless @a == @b; # same len?
2477 while (@a) {
2478 return 0 if pop(@a) ne pop(@b);
2479 }
2480 return 1;
2481 }
2482 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
2483
2484=item split /PATTERN/,EXPR,LIMIT
2485
2486=item split /PATTERN/,EXPR
2487
2488=item split /PATTERN/
2489
2490=item split
2491
2492Splits a string into an array of strings, and returns it.
2493
2494If not in a list context, returns the number of fields found and splits into
2495the @_ array. (In a list context, you can force the split into @_ by
2496using C<??> as the pattern delimiters, but it still returns the array
2497value.) The use of implicit split to @_ is deprecated, however.
2498
2499If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
4633a7c4
LW
2500splits on whitespace (after skipping any leading whitespace). Anything
2501matching PATTERN is taken to be a delimiter separating the fields. (Note
2502that the delimiter may be longer than one character.) If LIMIT is
2503specified and is not negative, splits into no more than that many fields
2504(though it may split into fewer). If LIMIT is unspecified, trailing null
2505fields are stripped (which potential users of pop() would do well to
2506remember). If LIMIT is negative, it is treated as if an arbitrarily large
2507LIMIT had been specified.
a0d0e21e
LW
2508
2509A pattern matching the null string (not to be confused with
748a9306 2510a null pattern C<//>, which is just one member of the set of patterns
a0d0e21e
LW
2511matching a null string) will split the value of EXPR into separate
2512characters at each point it matches that way. For example:
2513
2514 print join(':', split(/ */, 'hi there'));
2515
2516produces the output 'h:i:t:h:e:r:e'.
2517
2518The LIMIT parameter can be used to partially split a line
2519
2520 ($login, $passwd, $remainder) = split(/:/, $_, 3);
2521
2522When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
2523one larger than the number of variables in the list, to avoid
2524unnecessary work. For the list above LIMIT would have been 4 by
2525default. In time critical applications it behooves you not to split
2526into more fields than you really need.
2527
2528If the PATTERN contains parentheses, additional array elements are
2529created from each matching substring in the delimiter.
2530
2531 split(/([,-])/, "1-10,20");
2532
2533produces the list value
2534
2535 (1, '-', 10, ',', 20)
2536
4633a7c4
LW
2537If you had the entire header of a normal Unix email message in $header,
2538you could split it up into fields and their values this way:
2539
2540 $header =~ s/\n\s+/ /g; # fix continuation lines
2541 %hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
2542
a0d0e21e
LW
2543The pattern C</PATTERN/> may be replaced with an expression to specify
2544patterns that vary at runtime. (To do runtime compilation only once,
748a9306
LW
2545use C</$variable/o>.)
2546
2547As a special case, specifying a PATTERN of space (C<' '>) will split on
2548white space just as split with no arguments does. Thus, split(' ') can
2549be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
2550will give you as many null initial fields as there are leading spaces.
2551A split on /\s+/ is like a split(' ') except that any leading
2552whitespace produces a null first field. A split with no arguments
2553really does a C<split(' ', $_)> internally.
a0d0e21e
LW
2554
2555Example:
2556
2557 open(passwd, '/etc/passwd');
2558 while (<passwd>) {
748a9306
LW
2559 ($login, $passwd, $uid, $gid, $gcos,
2560 $home, $shell) = split(/:/);
a0d0e21e
LW
2561 ...
2562 }
2563
2564(Note that $shell above will still have a newline on it. See L</chop>,
2565L</chomp>, and L</join>.)
2566
2567=item sprintf FORMAT,LIST
2568
2569Returns a string formatted by the usual printf conventions of the C
2570language. (The * character for an indirectly specified length is not
2571supported, but you can get the same effect by interpolating a variable
4633a7c4
LW
2572into the pattern.) Some C libraries' implementations of sprintf() can dump core
2573when fed ludiocrous arguments.
a0d0e21e
LW
2574
2575=item sqrt EXPR
2576
2577Return the square root of EXPR. If EXPR is omitted, returns square
2578root of $_.
2579
2580=item srand EXPR
2581
2582Sets the random number seed for the C<rand> operator. If EXPR is
2583omitted, does C<srand(time)>. Of course, you'd need something much more
2584random than that for cryptographic purposes, since it's easy to guess
2585the current time. Checksumming the compressed output of rapidly
2586changing operating system status programs is the usual method.
2587Examples are posted regularly to comp.security.unix.
2588
2589=item stat FILEHANDLE
2590
2591=item stat EXPR
2592
2593Returns a 13-element array giving the status info for a file, either the
2594file opened via FILEHANDLE, or named by EXPR. Returns a null list if
2595the stat fails. Typically used as follows:
2596
2597 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
2598 $atime,$mtime,$ctime,$blksize,$blocks)
2599 = stat($filename);
2600
2601If stat is passed the special filehandle consisting of an underline, no
2602stat is done, but the current contents of the stat structure from the
2603last stat or filetest are returned. Example:
2604
2605 if (-x $file && (($d) = stat(_)) && $d < 0) {
2606 print "$file is executable NFS file\n";
2607 }
2608
2609(This only works on machines for which the device number is negative under NFS.)
2610
2611=item study SCALAR
2612
2613=item study
2614
2615Takes extra time to study SCALAR ($_ if unspecified) in anticipation of
2616doing many pattern matches on the string before it is next modified.
2617This may or may not save time, depending on the nature and number of
2618patterns you are searching on, and on the distribution of character
2619frequencies in the string to be searched--you probably want to compare
2620runtimes with and without it to see which runs faster. Those loops
2621which scan for many short constant strings (including the constant
2622parts of more complex patterns) will benefit most. You may have only
2623one study active at a time--if you study a different scalar the first
2624is "unstudied". (The way study works is this: a linked list of every
2625character in the string to be searched is made, so we know, for
2626example, where all the 'k' characters are. From each search string,
2627the rarest character is selected, based on some static frequency tables
2628constructed from some C programs and English text. Only those places
2629that contain this "rarest" character are examined.)
2630
2631For example, here is a loop which inserts index producing entries
2632before any line containing a certain pattern:
2633
2634 while (<>) {
2635 study;
2636 print ".IX foo\n" if /\bfoo\b/;
2637 print ".IX bar\n" if /\bbar\b/;
2638 print ".IX blurfl\n" if /\bblurfl\b/;
2639 ...
2640 print;
2641 }
2642
2643In searching for /\bfoo\b/, only those locations in $_ that contain "f"
2644will be looked at, because "f" is rarer than "o". In general, this is
2645a big win except in pathological cases. The only question is whether
2646it saves you more time than it took to build the linked list in the
2647first place.
2648
2649Note that if you have to look for strings that you don't know till
2650runtime, you can build an entire loop as a string and eval that to
2651avoid recompiling all your patterns all the time. Together with
2652undefining $/ to input entire files as one record, this can be very
2653fast, often faster than specialized programs like fgrep(1). The following
2654scans a list of files (@files) for a list of words (@words), and prints
2655out the names of those files that contain a match:
2656
2657 $search = 'while (<>) { study;';
2658 foreach $word (@words) {
2659 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
2660 }
2661 $search .= "}";
2662 @ARGV = @files;
2663 undef $/;
2664 eval $search; # this screams
2665 $/ = "\n"; # put back to normal input delim
2666 foreach $file (sort keys(%seen)) {
2667 print $file, "\n";
2668 }
2669
2670=item substr EXPR,OFFSET,LEN
2671
2672=item substr EXPR,OFFSET
2673
2674Extracts a substring out of EXPR and returns it. First character is at
2675offset 0, or whatever you've set $[ to. If OFFSET is negative, starts
2676that far from the end of the string. If LEN is omitted, returns
748a9306
LW
2677everything to the end of the string. If LEN is negative, leaves that
2678many characters off the end of the string.
2679
2680You can use the substr() function
a0d0e21e
LW
2681as an lvalue, in which case EXPR must be an lvalue. If you assign
2682something shorter than LEN, the string will shrink, and if you assign
2683something longer than LEN, the string will grow to accommodate it. To
2684keep the string the same length you may need to pad or chop your value
2685using sprintf().
2686
2687=item symlink OLDFILE,NEWFILE
2688
2689Creates a new filename symbolically linked to the old filename.
2690Returns 1 for success, 0 otherwise. On systems that don't support
2691symbolic links, produces a fatal error at run time. To check for that,
2692use eval:
2693
2694 $symlink_exists = (eval 'symlink("","");', $@ eq '');
2695
2696=item syscall LIST
2697
2698Calls the system call specified as the first element of the list,
2699passing the remaining elements as arguments to the system call. If
2700unimplemented, produces a fatal error. The arguments are interpreted
2701as follows: if a given argument is numeric, the argument is passed as
2702an int. If not, the pointer to the string value is passed. You are
2703responsible to make sure a string is pre-extended long enough to
2704receive any result that might be written into a string. If your
2705integer arguments are not literals and have never been interpreted in a
2706numeric context, you may need to add 0 to them to force them to look
2707like numbers.
2708
2709 require 'syscall.ph'; # may need to run h2ph
2710 syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
2711
2712Note that Perl only supports passing of up to 14 arguments to your system call,
2713which in practice should usually suffice.
2714
2715=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
2716
2717=item sysread FILEHANDLE,SCALAR,LENGTH
2718
2719Attempts to read LENGTH bytes of data into variable SCALAR from the
2720specified FILEHANDLE, using the system call read(2). It bypasses
2721stdio, so mixing this with other kinds of reads may cause confusion.
2722Returns the number of bytes actually read, or undef if there was an
2723error. SCALAR will be grown or shrunk to the length actually read. An
2724OFFSET may be specified to place the read data at some other place than
2725the beginning of the string.
2726
2727=item system LIST
2728
2729Does exactly the same thing as "exec LIST" except that a fork is done
2730first, and the parent process waits for the child process to complete.
2731Note that argument processing varies depending on the number of
2732arguments. The return value is the exit status of the program as
2733returned by the wait() call. To get the actual exit value divide by
2734256. See also L</exec>.
2735
2736=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
2737
2738=item syswrite FILEHANDLE,SCALAR,LENGTH
2739
2740Attempts to write LENGTH bytes of data from variable SCALAR to the
2741specified FILEHANDLE, using the system call write(2). It bypasses
2742stdio, so mixing this with prints may cause confusion. Returns the
2743number of bytes actually written, or undef if there was an error. An
2744OFFSET may be specified to place the read data at some other place than
2745the beginning of the string.
2746
2747=item tell FILEHANDLE
2748
2749=item tell
2750
2751Returns the current file position for FILEHANDLE. FILEHANDLE may be an
2752expression whose value gives the name of the actual filehandle. If
2753FILEHANDLE is omitted, assumes the file last read.
2754
2755=item telldir DIRHANDLE
2756
2757Returns the current position of the readdir() routines on DIRHANDLE.
2758Value may be given to seekdir() to access a particular location in a
2759directory. Has the same caveats about possible directory compaction as
2760the corresponding system library routine.
2761
4633a7c4 2762=item tie VARIABLE,CLASSNAME,LIST
a0d0e21e 2763
4633a7c4
LW
2764This function binds a variable to a package class that will provide the
2765implementation for the variable. VARIABLE is the name of the variable
2766to be enchanted. CLASSNAME is the name of a class implementing objects
2767of correct type. Any additional arguments are passed to the "new"
2768method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
2769Typically these are arguments such as might be passed to the dbm_open()
2770function of C. The object returned by the "new" method +is also
2771returned by the tie() function, which would be useful if you +want to
2772access other methods in CLASSNAME.
a0d0e21e
LW
2773
2774Note that functions such as keys() and values() may return huge array
748a9306
LW
2775values when used on large objects, like DBM files. You may prefer to
2776use the each() function to iterate over such. Example:
a0d0e21e
LW
2777
2778 # print out history file offsets
4633a7c4 2779 use NDBM_File;
a0d0e21e
LW
2780 tie(%HIST, NDBM_File, '/usr/lib/news/history', 1, 0);
2781 while (($key,$val) = each %HIST) {
2782 print $key, ' = ', unpack('L',$val), "\n";
2783 }
2784 untie(%HIST);
2785
4633a7c4 2786A class implementing an associative array should have the following
a0d0e21e
LW
2787methods:
2788
4633a7c4 2789 TIEHASH classname, LIST
a0d0e21e
LW
2790 DESTROY this
2791 FETCH this, key
2792 STORE this, key, value
2793 DELETE this, key
2794 EXISTS this, key
2795 FIRSTKEY this
2796 NEXTKEY this, lastkey
2797
4633a7c4 2798A class implementing an ordinary array should have the following methods:
a0d0e21e 2799
4633a7c4 2800 TIEARRAY classname, LIST
a0d0e21e
LW
2801 DESTROY this
2802 FETCH this, key
2803 STORE this, key, value
2804 [others TBD]
2805
4633a7c4 2806A class implementing a scalar should have the following methods:
a0d0e21e 2807
4633a7c4 2808 TIESCALAR classname, LIST
a0d0e21e
LW
2809 DESTROY this
2810 FETCH this,
2811 STORE this, value
2812
4633a7c4
LW
2813Unlike dbmopen(), the tie() function will not use or require a module
2814for you--you need to do that explicitly yourself. See L<DB_File>
2815or the F<Config> module for interesting tie() implementations.
2816
a0d0e21e
LW
2817=item time
2818
2819Returns the number of non-leap seconds since 00:00:00 UTC, January 1,
28201970. Suitable for feeding to gmtime() and localtime().
2821
2822=item times
2823
2824Returns a four-element array giving the user and system times, in
2825seconds, for this process and the children of this process.
2826
2827 ($user,$system,$cuser,$csystem) = times;
2828
2829=item tr///
2830
2831The translation operator. See L<perlop>.
2832
2833=item truncate FILEHANDLE,LENGTH
2834
2835=item truncate EXPR,LENGTH
2836
2837Truncates the file opened on FILEHANDLE, or named by EXPR, to the
2838specified length. Produces a fatal error if truncate isn't implemented
2839on your system.
2840
2841=item uc EXPR
2842
2843Returns an uppercased version of EXPR. This is the internal function
2844implementing the \U escape in double-quoted strings.
4633a7c4 2845Should respect any POSIX setlocale() settings.
a0d0e21e
LW
2846
2847=item ucfirst EXPR
2848
2849Returns the value of EXPR with the first character uppercased. This is
2850the internal function implementing the \u escape in double-quoted strings.
4633a7c4 2851Should respect any POSIX setlocale() settings.
a0d0e21e
LW
2852
2853=item umask EXPR
2854
2855=item umask
2856
2857Sets the umask for the process and returns the old one. If EXPR is
2858omitted, merely returns current umask.
2859
2860=item undef EXPR
2861
2862=item undef
2863
2864Undefines the value of EXPR, which must be an lvalue. Use only on a
2865scalar value, an entire array, or a subroutine name (using "&"). (Using undef()
2866will probably not do what you expect on most predefined variables or
2867DBM list values, so don't do that.) Always returns the undefined value. You can omit
2868the EXPR, in which case nothing is undefined, but you still get an
2869undefined value that you could, for instance, return from a
2870subroutine. Examples:
2871
2872 undef $foo;
2873 undef $bar{'blurfl'};
2874 undef @ary;
2875 undef %assoc;
2876 undef &mysub;
2877 return (wantarray ? () : undef) if $they_blew_it;
2878
2879=item unlink LIST
2880
2881Deletes a list of files. Returns the number of files successfully
2882deleted.
2883
2884 $cnt = unlink 'a', 'b', 'c';
2885 unlink @goners;
2886 unlink <*.bak>;
2887
2888Note: unlink will not delete directories unless you are superuser and
2889the B<-U> flag is supplied to Perl. Even if these conditions are
2890met, be warned that unlinking a directory can inflict damage on your
2891filesystem. Use rmdir instead.
2892
2893=item unpack TEMPLATE,EXPR
2894
2895Unpack does the reverse of pack: it takes a string representing a
2896structure and expands it out into a list value, returning the array
2897value. (In a scalar context, it merely returns the first value
2898produced.) The TEMPLATE has the same format as in the pack function.
2899Here's a subroutine that does substring:
2900
2901 sub substr {
2902 local($what,$where,$howmuch) = @_;
2903 unpack("x$where a$howmuch", $what);
2904 }
2905
2906and then there's
2907
2908 sub ordinal { unpack("c",$_[0]); } # same as ord()
2909
2910In addition, you may prefix a field with a %<number> to indicate that
2911you want a <number>-bit checksum of the items instead of the items
2912themselves. Default is a 16-bit checksum. For example, the following
2913computes the same number as the System V sum program:
2914
2915 while (<>) {
2916 $checksum += unpack("%16C*", $_);
2917 }
2918 $checksum %= 65536;
2919
2920The following efficiently counts the number of set bits in a bit vector:
2921
2922 $setbits = unpack("%32b*", $selectmask);
2923
2924=item untie VARIABLE
2925
2926Breaks the binding between a variable and a package. (See tie().)
2927
2928=item unshift ARRAY,LIST
2929
2930Does the opposite of a C<shift>. Or the opposite of a C<push>,
2931depending on how you look at it. Prepends list to the front of the
2932array, and returns the new number of elements in the array.
2933
2934 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
2935
2936Note the LIST is prepended whole, not one element at a time, so the
2937prepended elements stay in the same order. Use reverse to do the
2938reverse.
2939
2940=item use Module LIST
2941
2942=item use Module
2943
2944Imports some semantics into the current package from the named module,
2945generally by aliasing certain subroutine or variable names into your
2946package. It is exactly equivalent to
2947
2948 BEGIN { require Module; import Module LIST; }
2949
2950If you don't want your namespace altered, use require instead.
2951
2952The BEGIN forces the require and import to happen at compile time. The
2953require makes sure the module is loaded into memory if it hasn't been
2954yet. The import is not a builtin--it's just an ordinary static method
2955call into the "Module" package to tell the module to import the list of
2956features back into the current package. The module can implement its
2957import method any way it likes, though most modules just choose to
2958derive their import method via inheritance from the Exporter class that
2959is defined in the Exporter module.
2960
2961Because this is a wide-open interface, pragmas (compiler directives)
2962are also implemented this way. Currently implemented pragmas are:
2963
2964 use integer;
4633a7c4 2965 use diagnostics;
a0d0e21e
LW
2966 use sigtrap qw(SEGV BUS);
2967 use strict qw(subs vars refs);
2968 use subs qw(afunc blurfl);
2969
2970These pseudomodules import semantics into the current block scope, unlike
2971ordinary modules, which import symbols into the current package (which are
2972effective through the end of the file).
2973
2974There's a corresponding "no" command that unimports meanings imported
2975by use.
2976
2977 no integer;
2978 no strict 'refs';
2979
2980See L<perlmod> for a list of standard modules and pragmas.
2981
2982=item utime LIST
2983
2984Changes the access and modification times on each file of a list of
2985files. The first two elements of the list must be the NUMERICAL access
2986and modification times, in that order. Returns the number of files
2987successfully changed. The inode modification time of each file is set
2988to the current time. Example of a "touch" command:
2989
2990 #!/usr/bin/perl
2991 $now = time;
2992 utime $now, $now, @ARGV;
2993
2994=item values ASSOC_ARRAY
2995
2996Returns a normal array consisting of all the values of the named
2997associative array. (In a scalar context, returns the number of
2998values.) The values are returned in an apparently random order, but it
2999is the same order as either the keys() or each() function would produce
3000on the same array. See also keys() and each().
3001
3002=item vec EXPR,OFFSET,BITS
3003
3004Treats a string as a vector of unsigned integers, and returns the value
3005of the bitfield specified. May also be assigned to. BITS must be a
3006power of two from 1 to 32.
3007
3008Vectors created with vec() can also be manipulated with the logical
3009operators |, & and ^, which will assume a bit vector operation is
3010desired when both operands are strings.
3011
3012To transform a bit vector into a string or array of 0's and 1's, use these:
3013
3014 $bits = unpack("b*", $vector);
3015 @bits = split(//, unpack("b*", $vector));
3016
3017If you know the exact length in bits, it can be used in place of the *.
3018
3019=item wait
3020
3021Waits for a child process to terminate and returns the pid of the
3022deceased process, or -1 if there are no child processes. The status is
3023returned in $?.
3024
3025=item waitpid PID,FLAGS
3026
3027Waits for a particular child process to terminate and returns the pid
3028of the deceased process, or -1 if there is no such child process. The
3029status is returned in $?. If you say
3030
3031 use POSIX "wait_h";
3032 ...
3033 waitpid(-1,&WNOHANG);
3034
3035then you can do a non-blocking wait for any process. Non-blocking wait
3036is only available on machines supporting either the waitpid(2) or
3037wait4(2) system calls. However, waiting for a particular pid with
3038FLAGS of 0 is implemented everywhere. (Perl emulates the system call
3039by remembering the status values of processes that have exited but have
3040not been harvested by the Perl script yet.)
3041
3042=item wantarray
3043
3044Returns TRUE if the context of the currently executing subroutine is
3045looking for a list value. Returns FALSE if the context is looking
3046for a scalar.
3047
3048 return wantarray ? () : undef;
3049
3050=item warn LIST
3051
3052Produces a message on STDERR just like die(), but doesn't exit or
4633a7c4 3053on an exception.
a0d0e21e
LW
3054
3055=item write FILEHANDLE
3056
3057=item write EXPR
3058
3059=item write
3060
3061Writes a formatted record (possibly multi-line) to the specified file,
3062using the format associated with that file. By default the format for
3063a file is the one having the same name is the filehandle, but the
3064format for the current output channel (see the select() function) may be set
3065explicitly by assigning the name of the format to the $~ variable.
3066
3067Top of form processing is handled automatically: if there is
3068insufficient room on the current page for the formatted record, the
3069page is advanced by writing a form feed, a special top-of-page format
3070is used to format the new page header, and then the record is written.
3071By default the top-of-page format is the name of the filehandle with
3072"_TOP" appended, but it may be dynamically set to the format of your
3073choice by assigning the name to the $^ variable while the filehandle is
3074selected. The number of lines remaining on the current page is in
3075variable $-, which can be set to 0 to force a new page.
3076
3077If FILEHANDLE is unspecified, output goes to the current default output
3078channel, which starts out as STDOUT but may be changed by the
3079C<select> operator. If the FILEHANDLE is an EXPR, then the expression
3080is evaluated and the resulting string is used to look up the name of
3081the FILEHANDLE at run time. For more on formats, see L<perlform>.
3082
3083Note that write is I<NOT> the opposite of read. Unfortunately.
3084
3085=item y///
3086
3087The translation operator. See L<perlop/tr///>.
3088
3089=back