3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar and list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one list argument.) For instance, splice() has three scalar
19 arguments followed by a list.
21 In the syntax descriptions that follow, list operators that expect a
22 list (and provide list context for the elements of the list) are shown
23 with LIST as an argument. Such a list may consist of any combination
24 of scalar arguments or list values; the list values will be included
25 in the list as if each individual element were interpolated at that
26 point in the list, forming a longer single-dimensional list value.
27 Elements of the LIST should be separated by commas.
29 Any function in the list below may be used either with or without
30 parentheses around its arguments. (The syntax descriptions omit the
31 parentheses.) If you use the parentheses, the simple (but occasionally
32 surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33 function, and precedence doesn't matter. Otherwise it's a list
34 operator or unary operator, and precedence does matter. And whitespace
35 between the function and left parenthesis doesn't count--so you need to
38 print 1+2+4; # Prints 7.
39 print(1+2) + 4; # Prints 3.
40 print (1+2)+4; # Also prints 3!
41 print +(1+2)+4; # Prints 7.
42 print ((1+2)+4); # Prints 7.
44 If you run Perl with the B<-w> switch it can warn you about this. For
45 example, the third line above produces:
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
50 For functions that can be used in either a scalar or list context,
51 nonabortive failure is generally indicated in a scalar context by
52 returning the undefined value, and in a list context by returning the
55 Remember the following important rule: There is B<no rule> that relates
56 the behavior of an expression in list context to its behavior in scalar
57 context, or vice versa. It might do two totally different things.
58 Each operator and function decides which sort of value it would be most
59 appropriate to return in a scalar context. Some operators return the
60 length of the list that would have been returned in list context. Some
61 operators return the first value in the list. Some operators return the
62 last value in the list. Some operators return a count of successful
63 operations. In general, they do what you want, unless you want
66 An named array in scalar context is quite different from what would at
67 first glance appear to be a list in scalar context. You can't get a list
68 like C<(1,2,3)> into being in scalar context, because the compiler knows
69 the context at compile time. It would generate the scalar comma operator
70 there, not the list construction version of the comma. That means it
71 was never a list to start with.
73 In general, functions in Perl that serve as wrappers for system calls
74 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
75 true when they succeed and C<undef> otherwise, as is usually mentioned
76 in the descriptions below. This is different from the C interfaces,
77 which return C<-1> on failure. Exceptions to this rule are C<wait()>,
78 C<waitpid()>, and C<syscall()>. System calls also set the special C<$!>
79 variable on failure. Other functions do not, except accidentally.
81 =head2 Perl Functions by Category
83 Here are Perl's functions (including things that look like
84 functions, like some keywords and named operators)
85 arranged by category. Some functions appear in more
90 =item Functions for SCALARs or strings
92 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
93 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
94 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
96 =item Regular expressions and pattern matching
98 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
100 =item Numeric functions
102 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
103 C<sin>, C<sqrt>, C<srand>
105 =item Functions for real @ARRAYs
107 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
109 =item Functions for list data
111 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
113 =item Functions for real %HASHes
115 C<delete>, C<each>, C<exists>, C<keys>, C<values>
117 =item Input and output functions
119 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
120 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
121 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
122 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
125 =item Functions for fixed length data or records
127 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
129 =item Functions for filehandles, files, or directories
131 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
132 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, C<readlink>,
133 C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, C<unlink>, C<utime>
135 =item Keywords related to the control flow of your perl program
137 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
138 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
140 =item Keywords related to scoping
142 C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
144 =item Miscellaneous functions
146 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
147 C<scalar>, C<undef>, C<wantarray>
149 =item Functions for processes and process groups
151 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
152 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
153 C<times>, C<wait>, C<waitpid>
155 =item Keywords related to perl modules
157 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
159 =item Keywords related to classes and object-orientedness
161 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
164 =item Low-level socket functions
166 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
167 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
168 C<socket>, C<socketpair>
170 =item System V interprocess communication functions
172 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
173 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
175 =item Fetching user and group info
177 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
178 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
179 C<getpwuid>, C<setgrent>, C<setpwent>
181 =item Fetching network info
183 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
184 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
185 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
186 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
187 C<setnetent>, C<setprotoent>, C<setservent>
189 =item Time-related functions
191 C<gmtime>, C<localtime>, C<time>, C<times>
193 =item Functions new in perl5
195 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
196 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
197 C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
198 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
200 * - C<sub> was a keyword in perl4, but in perl5 it is an
201 operator, which can be used in expressions.
203 =item Functions obsoleted in perl5
205 C<dbmclose>, C<dbmopen>
209 =head2 Alphabetical Listing of Perl Functions
213 =item I<-X> FILEHANDLE
219 A file test, where X is one of the letters listed below. This unary
220 operator takes one argument, either a filename or a filehandle, and
221 tests the associated file to see if something is true about it. If the
222 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
223 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
224 the undefined value if the file doesn't exist. Despite the funny
225 names, precedence is the same as any other named unary operator, and
226 the argument may be parenthesized like any other unary operator. The
227 operator may be any of:
228 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
229 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
231 -r File is readable by effective uid/gid.
232 -w File is writable by effective uid/gid.
233 -x File is executable by effective uid/gid.
234 -o File is owned by effective uid.
236 -R File is readable by real uid/gid.
237 -W File is writable by real uid/gid.
238 -X File is executable by real uid/gid.
239 -O File is owned by real uid.
242 -z File has zero size.
243 -s File has nonzero size (returns size).
245 -f File is a plain file.
246 -d File is a directory.
247 -l File is a symbolic link.
248 -p File is a named pipe (FIFO), or Filehandle is a pipe.
250 -b File is a block special file.
251 -c File is a character special file.
252 -t Filehandle is opened to a tty.
254 -u File has setuid bit set.
255 -g File has setgid bit set.
256 -k File has sticky bit set.
258 -T File is a text file.
259 -B File is a binary file (opposite of -T).
261 -M Age of file in days when script started.
262 -A Same for access time.
263 -C Same for inode change time.
265 The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
266 C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
267 uids and gids of the user. There may be other reasons you can't actually
268 read, write, or execute the file, such as AFS access control lists. Also note that, for the superuser,
269 C<-r>, C<-R>, C<-w>, and C<-W> always return C<1>, and C<-x> and C<-X> return
270 C<1> if any execute bit is set in the mode. Scripts run by the superuser may
271 thus need to do a C<stat()> to determine the actual mode of the
272 file, or temporarily set the uid to something else.
278 next unless -f $_; # ignore specials
282 Note that C<-s/a/b/> does not do a negated substitution. Saying
283 C<-exp($foo)> still works as expected, however--only single letters
284 following a minus are interpreted as file tests.
286 The C<-T> and C<-B> switches work as follows. The first block or so of the
287 file is examined for odd characters such as strange control codes or
288 characters with the high bit set. If too many strange characters (E<gt>30%)
289 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
290 containing null in the first block is considered a binary file. If C<-T>
291 or C<-B> is used on a filehandle, the current stdio buffer is examined
292 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
293 file, or a file at EOF when testing a filehandle. Because you have to
294 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
295 against the file first, as in C<next unless -f $file && -T $file>.
297 If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
298 the special filehandle consisting of a solitary underline, then the stat
299 structure of the previous file test (or stat operator) is used, saving
300 a system call. (This doesn't work with C<-t>, and you need to remember
301 that lstat() and C<-l> will leave values in the stat structure for the
302 symbolic link, not the real file.) Example:
304 print "Can do.\n" if -r $a || -w _ || -x _;
307 print "Readable\n" if -r _;
308 print "Writable\n" if -w _;
309 print "Executable\n" if -x _;
310 print "Setuid\n" if -u _;
311 print "Setgid\n" if -g _;
312 print "Sticky\n" if -k _;
313 print "Text\n" if -T _;
314 print "Binary\n" if -B _;
320 Returns the absolute value of its argument.
321 If VALUE is omitted, uses C<$_>.
323 =item accept NEWSOCKET,GENERICSOCKET
325 Accepts an incoming socket connect, just as the accept(2) system call
326 does. Returns the packed address if it succeeded, FALSE otherwise.
327 See example in L<perlipc/"Sockets: Client/Server Communication">.
333 Arranges to have a SIGALRM delivered to this process after the
334 specified number of seconds have elapsed. If SECONDS is not specified,
335 the value stored in C<$_> is used. (On some machines,
336 unfortunately, the elapsed time may be up to one second less than you
337 specified because of how seconds are counted.) Only one timer may be
338 counting at once. Each call disables the previous timer, and an
339 argument of C<0> may be supplied to cancel the previous timer without
340 starting a new one. The returned value is the amount of time remaining
341 on the previous timer.
343 For delays of finer granularity than one second, you may use Perl's
344 C<syscall()> interface to access setitimer(2) if your system supports it,
345 or else see L</select()>. It is usually a mistake to intermix C<alarm()>
346 and C<sleep()> calls.
348 If you want to use C<alarm()> to time out a system call you need to use an
349 C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
350 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
351 restart system calls on some systems. Using C<eval()>/C<die()> always works,
352 modulo the caveats given in L<perlipc/"Signals">.
355 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
357 $nread = sysread SOCKET, $buffer, $size;
361 die unless $@ eq "alarm\n"; # propagate unexpected errors
370 Returns the arctangent of Y/X in the range -PI to PI.
372 For the tangent operation, you may use the C<POSIX::tan()>
373 function, or use the familiar relation:
375 sub tan { sin($_[0]) / cos($_[0]) }
377 =item bind SOCKET,NAME
379 Binds a network address to a socket, just as the bind system call
380 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
381 packed address of the appropriate type for the socket. See the examples in
382 L<perlipc/"Sockets: Client/Server Communication">.
384 =item binmode FILEHANDLE
386 Arranges for the file to be read or written in "binary" mode in operating
387 systems that distinguish between binary and text files. Files that are
388 not in binary mode have CR LF sequences translated to LF on input and LF
389 translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
390 and similarly archaic systems, it may be imperative--otherwise your
391 MS-DOS-damaged C library may mangle your file. The key distinction between
392 systems that need C<binmode()> and those that don't is their text file
393 formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
394 character, and that encode that character in C as C<"\n">, do not need
395 C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value
396 is taken as the name of the filehandle.
398 =item bless REF,CLASSNAME
402 This function tells the thingy referenced by REF that it is now
403 an object in the CLASSNAME package--or the current package if no CLASSNAME
404 is specified, which is often the case. It returns the reference for
405 convenience, because a C<bless()> is often the last thing in a constructor.
406 Always use the two-argument version if the function doing the blessing
407 might be inherited by a derived class. See L<perltoot> and L<perlobj>
408 for more about the blessing (and blessings) of objects.
414 Returns the context of the current subroutine call. In scalar context,
415 returns the caller's package name if there is a caller, that is, if
416 we're in a subroutine or C<eval()> or C<require()>, and the undefined value
417 otherwise. In list context, returns
419 ($package, $filename, $line) = caller;
421 With EXPR, it returns some extra information that the debugger uses to
422 print a stack trace. The value of EXPR indicates how many call frames
423 to go back before the current one.
425 ($package, $filename, $line, $subroutine,
426 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
428 Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
429 call, but an C<eval()>. In such a case additional elements C<$evaltext> and
430 C<$is_require> are set: C<$is_require> is true if the frame is created by a
431 C<require> or C<use> statement, C<$evaltext> contains the text of the
432 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
433 C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
434 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
437 Furthermore, when called from within the DB package, caller returns more
438 detailed information: it sets the list variable C<@DB::args> to be the
439 arguments with which the subroutine was invoked.
441 Be aware that the optimizer might have optimized call frames away before
442 C<caller()> had a chance to get the information. That means that C<caller(N)>
443 might not return information about the call frame you expect it do, for
444 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
445 previous time C<caller()> was called.
449 Changes the working directory to EXPR, if possible. If EXPR is
450 omitted, changes to home directory. Returns TRUE upon success, FALSE
451 otherwise. See example under C<die()>.
455 Changes the permissions of a list of files. The first element of the
456 list must be the numerical mode, which should probably be an octal
457 number, and which definitely should I<not> a string of octal digits:
458 C<0644> is okay, C<'0644'> is not. Returns the number of files
459 successfully changed. See also L</oct>, if all you have is a string.
461 $cnt = chmod 0755, 'foo', 'bar';
462 chmod 0755, @executables;
463 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
465 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
466 $mode = 0644; chmod $mode, 'foo'; # this is best
474 This is a slightly safer version of L</chop>. It removes any
475 line ending that corresponds to the current value of C<$/> (also known as
476 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
477 number of characters removed from all its arguments. It's often used to
478 remove the newline from the end of an input record when you're worried
479 that the final record may be missing its newline. When in paragraph mode
480 (C<$/ = "">), it removes all trailing newlines from the string. If
481 VARIABLE is omitted, it chomps C<$_>. Example:
484 chomp; # avoid \n on last field
489 You can actually chomp anything that's an lvalue, including an assignment:
492 chomp($answer = <STDIN>);
494 If you chomp a list, each element is chomped, and the total number of
495 characters removed is returned.
503 Chops off the last character of a string and returns the character
504 chopped. It's used primarily to remove the newline from the end of an
505 input record, but is much more efficient than C<s/\n//> because it neither
506 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
510 chop; # avoid \n on last field
515 You can actually chop anything that's an lvalue, including an assignment:
518 chop($answer = <STDIN>);
520 If you chop a list, each element is chopped. Only the value of the
521 last C<chop()> is returned.
523 Note that C<chop()> returns the last character. To return all but the last
524 character, use C<substr($string, 0, -1)>.
528 Changes the owner (and group) of a list of files. The first two
529 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
530 Returns the number of files successfully changed.
532 $cnt = chown $uid, $gid, 'foo', 'bar';
533 chown $uid, $gid, @filenames;
535 Here's an example that looks up nonnumeric uids in the passwd file:
538 chop($user = <STDIN>);
540 chop($pattern = <STDIN>);
542 ($login,$pass,$uid,$gid) = getpwnam($user)
543 or die "$user not in passwd file";
545 @ary = glob($pattern); # expand filenames
546 chown $uid, $gid, @ary;
548 On most systems, you are not allowed to change the ownership of the
549 file unless you're the superuser, although you should be able to change
550 the group to any of your secondary groups. On insecure systems, these
551 restrictions may be relaxed, but this is not a portable assumption.
557 Returns the character represented by that NUMBER in the character set.
558 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
559 chr(0x263a) is a Unicode smiley face (but only within the scope of a
560 C<use utf8>). For the reverse, use L</ord>.
562 If NUMBER is omitted, uses C<$_>.
564 =item chroot FILENAME
568 This function works like the system call by the same name: it makes the
569 named directory the new root directory for all further pathnames that
570 begin with a C<"/"> by your process and all its children. (It doesn't
571 change your current working directory, which is unaffected.) For security
572 reasons, this call is restricted to the superuser. If FILENAME is
573 omitted, does a C<chroot()> to C<$_>.
575 =item close FILEHANDLE
579 Closes the file or pipe associated with the file handle, returning TRUE
580 only if stdio successfully flushes buffers and closes the system file
581 descriptor. Closes the currently selected filehandle if the argument
584 You don't have to close FILEHANDLE if you are immediately going to do
585 another C<open()> on it, because C<open()> will close it for you. (See
586 C<open()>.) However, an explicit C<close()> on an input file resets the line
587 counter (C<$.>), while the implicit close done by C<open()> does not.
589 If the file handle came from a piped open C<close()> will additionally
590 return FALSE if one of the other system calls involved fails or if the
591 program exits with non-zero status. (If the only problem was that the
592 program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
593 waits for the process executing on the pipe to complete, in case you
594 want to look at the output of the pipe afterwards. Closing a pipe
595 explicitly also puts the exit status value of the command into C<$?>.
599 open(OUTPUT, '|sort >foo') # pipe to sort
600 or die "Can't start sort: $!";
601 #... # print stuff to output
602 close OUTPUT # wait for sort to finish
603 or warn $! ? "Error closing sort pipe: $!"
604 : "Exit status $? from sort";
605 open(INPUT, 'foo') # get sort's results
606 or die "Can't open 'foo' for input: $!";
608 FILEHANDLE may be an expression whose value can be used as an indirect
609 filehandle, usually the real filehandle name.
611 =item closedir DIRHANDLE
613 Closes a directory opened by C<opendir()> and returns the success of that
616 DIRHANDLE may be an expression whose value can be used as an indirect
617 dirhandle, usually the real dirhandle name.
619 =item connect SOCKET,NAME
621 Attempts to connect to a remote socket, just as the connect system call
622 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
623 packed address of the appropriate type for the socket. See the examples in
624 L<perlipc/"Sockets: Client/Server Communication">.
628 Actually a flow control statement rather than a function. If there is a
629 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
630 C<foreach>), it is always executed just before the conditional is about to
631 be evaluated again, just like the third part of a C<for> loop in C. Thus
632 it can be used to increment a loop variable, even when the loop has been
633 continued via the C<next> statement (which is similar to the C C<continue>
636 C<last>, C<next>, or C<redo> may appear within a C<continue>
637 block. C<last> and C<redo> will behave as if they had been executed within
638 the main block. So will C<next>, but since it will execute a C<continue>
639 block, it may be more entertaining.
642 ### redo always comes here
645 ### next always comes here
647 # then back the top to re-check EXPR
649 ### last always comes here
651 Omitting the C<continue> section is semantically equivalent to using an
652 empty one, logically enough. In that case, C<next> goes directly back
653 to check the condition at the top of the loop.
657 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
658 takes cosine of C<$_>.
660 For the inverse cosine operation, you may use the C<POSIX::acos()>
661 function, or use this relation:
663 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
665 =item crypt PLAINTEXT,SALT
667 Encrypts a string exactly like the crypt(3) function in the C library
668 (assuming that you actually have a version there that has not been
669 extirpated as a potential munition). This can prove useful for checking
670 the password file for lousy passwords, amongst other things. Only the
671 guys wearing white hats should do this.
673 Note that C<crypt()> is intended to be a one-way function, much like breaking
674 eggs to make an omelette. There is no (known) corresponding decrypt
675 function. As a result, this function isn't all that useful for
676 cryptography. (For that, see your nearby CPAN mirror.)
678 When verifying an existing encrypted string you should use the encrypted
679 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
680 allows your code to work with the standard C<crypt()> and with more
681 exotic implementations. When choosing a new salt create a random two
682 character string whose characters come from the set C<[./0-9A-Za-z]>
683 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
685 Here's an example that makes sure that whoever runs this program knows
688 $pwd = (getpwuid($<))[1];
692 chomp($word = <STDIN>);
696 if (crypt($word, $pwd) ne $pwd) {
702 Of course, typing in your own password to whoever asks you
707 [This function has been superseded by the C<untie()> function.]
709 Breaks the binding between a DBM file and a hash.
711 =item dbmopen HASH,DBNAME,MODE
713 [This function has been superseded by the C<tie()> function.]
715 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
716 hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
717 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
718 is the name of the database (without the F<.dir> or F<.pag> extension if
719 any). If the database does not exist, it is created with protection
720 specified by MODE (as modified by the C<umask()>). If your system supports
721 only the older DBM functions, you may perform only one C<dbmopen()> in your
722 program. In older versions of Perl, if your system had neither DBM nor
723 ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
726 If you don't have write access to the DBM file, you can only read hash
727 variables, not set them. If you want to test whether you can write,
728 either use file tests or try setting a dummy hash entry inside an C<eval()>,
729 which will trap the error.
731 Note that functions such as C<keys()> and C<values()> may return huge lists
732 when used on large DBM files. You may prefer to use the C<each()>
733 function to iterate over large DBM files. Example:
735 # print out history file offsets
736 dbmopen(%HIST,'/usr/lib/news/history',0666);
737 while (($key,$val) = each %HIST) {
738 print $key, ' = ', unpack('L',$val), "\n";
742 See also L<AnyDBM_File> for a more general description of the pros and
743 cons of the various dbm approaches, as well as L<DB_File> for a particularly
750 Returns a Boolean value telling whether EXPR has a value other than
751 the undefined value C<undef>. If EXPR is not present, C<$_> will be
754 Many operations return C<undef> to indicate failure, end of file,
755 system error, uninitialized variable, and other exceptional
756 conditions. This function allows you to distinguish C<undef> from
757 other values. (A simple Boolean test will not distinguish among
758 C<undef>, zero, the empty string, and C<"0">, which are all equally
759 false.) Note that since C<undef> is a valid scalar, its presence
760 doesn't I<necessarily> indicate an exceptional condition: C<pop()>
761 returns C<undef> when its argument is an empty array, I<or> when the
762 element to return happens to be C<undef>.
764 You may also use C<defined()> to check whether a subroutine exists, by
765 saying C<defined &func> without parentheses. On the other hand, use
766 of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
767 produce intuitive results, and should probably be avoided.
769 When used on a hash element, it tells you whether the value is defined,
770 not whether the key exists in the hash. Use L</exists> for the latter
775 print if defined $switch{'D'};
776 print "$val\n" while defined($val = pop(@ary));
777 die "Can't readlink $sym: $!"
778 unless defined($value = readlink $sym);
779 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
780 $debugging = 0 unless defined $debugging;
782 Note: Many folks tend to overuse C<defined()>, and then are surprised to
783 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
784 defined values. For example, if you say
788 The pattern match succeeds, and C<$1> is defined, despite the fact that it
789 matched "nothing". But it didn't really match nothing--rather, it
790 matched something that happened to be C<0> characters long. This is all
791 very above-board and honest. When a function returns an undefined value,
792 it's an admission that it couldn't give you an honest answer. So you
793 should use C<defined()> only when you're questioning the integrity of what
794 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
797 Currently, using C<defined()> on an entire array or hash reports whether
798 memory for that aggregate has ever been allocated. So an array you set
799 to the empty list appears undefined initially, and one that once was full
800 and that you then set to the empty list still appears defined. You
801 should instead use a simple test for size:
803 if (@an_array) { print "has array elements\n" }
804 if (%a_hash) { print "has hash members\n" }
806 Using C<undef()> on these, however, does clear their memory and then report
807 them as not defined anymore, but you shouldn't do that unless you don't
808 plan to use them again, because it saves time when you load them up
809 again to have memory already ready to be filled. The normal way to
810 free up space used by an aggregate is to assign the empty list.
812 This counterintuitive behavior of C<defined()> on aggregates may be
813 changed, fixed, or broken in a future release of Perl.
815 See also L</undef>, L</exists>, L</ref>.
819 Deletes the specified key(s) and their associated values from a hash.
820 For each key, returns the deleted value associated with that key, or
821 the undefined value if there was no such key. Deleting from C<$ENV{}>
822 modifies the environment. Deleting from a hash tied to a DBM file
823 deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
824 doesn't necessarily return anything.)
826 The following deletes all the values of a hash:
828 foreach $key (keys %HASH) {
834 delete @HASH{keys %HASH}
836 (But both of these are slower than just assigning the empty list, or
837 using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
838 long as the final operation is a hash element lookup or hash slice:
840 delete $ref->[$x][$y]{$key};
841 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
845 Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
846 the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
847 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
848 is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
849 C<$@> and the C<eval()> is terminated with the undefined value. This makes
850 C<die()> the way to raise an exception.
854 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
855 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
857 If the value of EXPR does not end in a newline, the current script line
858 number and input line number (if any) are also printed, and a newline
859 is supplied. Hint: sometimes appending C<", stopped"> to your message
860 will cause it to make better sense when the string C<"at foo line 123"> is
861 appended. Suppose you are running script "canasta".
863 die "/etc/games is no good";
864 die "/etc/games is no good, stopped";
866 produce, respectively
868 /etc/games is no good at canasta line 123.
869 /etc/games is no good, stopped at canasta line 123.
871 See also C<exit()> and C<warn()>.
873 If LIST is empty and C<$@> already contains a value (typically from a
874 previous eval) that value is reused after appending C<"\t...propagated">.
875 This is useful for propagating exceptions:
878 die unless $@ =~ /Expected exception/;
880 If C<$@> is empty then the string C<"Died"> is used.
882 You can arrange for a callback to be run just before the C<die()> does
883 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
884 will be called with the error text and can change the error message, if
885 it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
886 setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
888 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
889 blocks/strings. If one wants the hook to do nothing in such
894 as the first line of the handler (see L<perlvar/$^S>).
898 Not really a function. Returns the value of the last command in the
899 sequence of commands indicated by BLOCK. When modified by a loop
900 modifier, executes the BLOCK once before testing the loop condition.
901 (On other statements the loop modifiers test the conditional first.)
903 C<do BLOCK> does I<not> count as a loop, so the loop control statements
904 C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
906 =item do SUBROUTINE(LIST)
908 A deprecated form of subroutine call. See L<perlsub>.
912 Uses the value of EXPR as a filename and executes the contents of the
913 file as a Perl script. Its primary use is to include subroutines
914 from a Perl subroutine library.
920 scalar eval `cat stat.pl`;
922 except that it's more efficient and concise, keeps track of the
923 current filename for error messages, and searches all the B<-I>
924 libraries if the file isn't in the current directory (see also the @INC
925 array in L<perlvar/Predefined Names>). It is also different in how
926 code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
927 scope like C<eval STRING> does. It's the same, however, in that it does
928 reparse the file every time you call it, so you probably don't want to
929 do this inside a loop.
931 If C<do> cannot read the file, it returns undef and sets C<$!> to the
932 error. If C<do> can read the file but cannot compile it, it
933 returns undef and sets an error message in C<$@>. If the file is
934 successfully compiled, C<do> returns the value of the last expression
937 Note that inclusion of library modules is better done with the
938 C<use()> and C<require()> operators, which also do automatic error checking
939 and raise an exception if there's a problem.
941 You might like to use C<do> to read in a program configuration
942 file. Manual error checking can be done this way:
944 # read in config files: system first, then user
945 for $file ("/share/prog/defaults.rc",
946 "$ENV{HOME}/.someprogrc") {
947 unless ($return = do $file) {
948 warn "couldn't parse $file: $@" if $@;
949 warn "couldn't do $file: $!" unless defined $return;
950 warn "couldn't run $file" unless $return;
956 This causes an immediate core dump. Primarily this is so that you can
957 use the B<undump> program to turn your core dump into an executable binary
958 after having initialized all your variables at the beginning of the
959 program. When the new binary is executed it will begin by executing a
960 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
961 it as a goto with an intervening core dump and reincarnation. If C<LABEL>
962 is omitted, restarts the program from the top. WARNING: Any files
963 opened at the time of the dump will NOT be open any more when the
964 program is reincarnated, with possible resulting confusion on the part
965 of Perl. See also B<-u> option in L<perlrun>.
982 dump QUICKSTART if $ARGV[0] eq '-d';
987 This operator is largely obsolete, partly because it's very hard to
988 convert a core file into an executable, and because the real perl-to-C
989 compiler has superseded it.
993 When called in list context, returns a 2-element list consisting of the
994 key and value for the next element of a hash, so that you can iterate over
995 it. When called in scalar context, returns the key for only the "next"
996 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
997 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1000 Entries are returned in an apparently random order. When the hash is
1001 entirely read, a null array is returned in list context (which when
1002 assigned produces a FALSE (C<0>) value), and C<undef> in
1003 scalar context. The next call to C<each()> after that will start iterating
1004 again. There is a single iterator for each hash, shared by all C<each()>,
1005 C<keys()>, and C<values()> function calls in the program; it can be reset by
1006 reading all the elements from the hash, or by evaluating C<keys HASH> or
1007 C<values HASH>. If you add or delete elements of a hash while you're
1008 iterating over it, you may get entries skipped or duplicated, so don't.
1010 The following prints out your environment like the printenv(1) program,
1011 only in a different order:
1013 while (($key,$value) = each %ENV) {
1014 print "$key=$value\n";
1017 See also C<keys()> and C<values()>.
1019 =item eof FILEHANDLE
1025 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1026 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1027 gives the real filehandle. (Note that this function actually
1028 reads a character and then C<ungetc()>s it, so isn't very useful in an
1029 interactive context.) Do not read from a terminal file (or call
1030 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1031 as terminals may lose the end-of-file condition if you do.
1033 An C<eof> without an argument uses the last file read as argument.
1034 Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
1035 the files listed on the command line, i.e., C<eof()> is reasonable to
1036 use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1037 last file. Use C<eof(ARGV)> or eof without the parentheses to test
1038 I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
1040 # reset line numbering on each input file
1042 next if /^\s*#/; # skip comments
1045 close ARGV if eof; # Not eof()!
1048 # insert dashes just before last line of last file
1050 if (eof()) { # check for end of current file
1051 print "--------------\n";
1052 close(ARGV); # close or break; is needed if we
1053 # are reading from the terminal
1058 Practical hint: you almost never need to use C<eof> in Perl, because the
1059 input operators return false values when they run out of data, or if there
1066 In the first form, the return value of EXPR is parsed and executed as if it
1067 were a little Perl program. The value of the expression (which is itself
1068 determined within scalar context) is first parsed, and if there weren't any
1069 errors, executed in the context of the current Perl program, so that any
1070 variable settings or subroutine and format definitions remain afterwards.
1071 Note that the value is parsed every time the eval executes. If EXPR is
1072 omitted, evaluates C<$_>. This form is typically used to delay parsing
1073 and subsequent execution of the text of EXPR until run time.
1075 In the second form, the code within the BLOCK is parsed only once--at the
1076 same time the code surrounding the eval itself was parsed--and executed
1077 within the context of the current Perl program. This form is typically
1078 used to trap exceptions more efficiently than the first (see below), while
1079 also providing the benefit of checking the code within BLOCK at compile
1082 The final semicolon, if any, may be omitted from the value of EXPR or within
1085 In both forms, the value returned is the value of the last expression
1086 evaluated inside the mini-program; a return statement may be also used, just
1087 as with subroutines. The expression providing the return value is evaluated
1088 in void, scalar, or list context, depending on the context of the eval itself.
1089 See L</wantarray> for more on how the evaluation context can be determined.
1091 If there is a syntax error or runtime error, or a C<die()> statement is
1092 executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
1093 error message. If there was no error, C<$@> is guaranteed to be a null
1094 string. Beware that using C<eval()> neither silences perl from printing
1095 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1096 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1097 L</warn> and L<perlvar>.
1099 Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1100 determining whether a particular feature (such as C<socket()> or C<symlink()>)
1101 is implemented. It is also Perl's exception trapping mechanism, where
1102 the die operator is used to raise exceptions.
1104 If the code to be executed doesn't vary, you may use the eval-BLOCK
1105 form to trap run-time errors without incurring the penalty of
1106 recompiling each time. The error, if any, is still returned in C<$@>.
1109 # make divide-by-zero nonfatal
1110 eval { $answer = $a / $b; }; warn $@ if $@;
1112 # same thing, but less efficient
1113 eval '$answer = $a / $b'; warn $@ if $@;
1115 # a compile-time error
1116 eval { $answer = }; # WRONG
1119 eval '$answer ='; # sets $@
1121 When using the C<eval{}> form as an exception trap in libraries, you may
1122 wish not to trigger any C<__DIE__> hooks that user code may have
1123 installed. You can use the C<local $SIG{__DIE__}> construct for this
1124 purpose, as shown in this example:
1126 # a very private exception trap for divide-by-zero
1127 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1130 This is especially significant, given that C<__DIE__> hooks can call
1131 C<die()> again, which has the effect of changing their error messages:
1133 # __DIE__ hooks may modify error messages
1135 local $SIG{'__DIE__'} =
1136 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1137 eval { die "foo lives here" };
1138 print $@ if $@; # prints "bar lives here"
1141 With an C<eval()>, you should be especially careful to remember what's
1142 being looked at when:
1148 eval { $x }; # CASE 4
1150 eval "\$$x++"; # CASE 5
1153 Cases 1 and 2 above behave identically: they run the code contained in
1154 the variable C<$x>. (Although case 2 has misleading double quotes making
1155 the reader wonder what else might be happening (nothing is).) Cases 3
1156 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1157 does nothing but return the value of C<$x>. (Case 4 is preferred for
1158 purely visual reasons, but it also has the advantage of compiling at
1159 compile-time instead of at run-time.) Case 5 is a place where
1160 normally you I<WOULD> like to use double quotes, except that in this
1161 particular situation, you can just use symbolic references instead, as
1164 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1165 C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
1170 =item exec PROGRAM LIST
1172 The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1173 use C<system()> instead of C<exec()> if you want it to return. It fails and
1174 returns FALSE only if the command does not exist I<and> it is executed
1175 directly instead of via your system's command shell (see below).
1177 Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1178 warns you if there is a following statement which isn't C<die()>, C<warn()>,
1179 or C<exit()> (if C<-w> is set - but you always do that). If you
1180 I<really> want to follow an C<exec()> with some other statement, you
1181 can use one of these styles to avoid the warning:
1183 exec ('foo') or print STDERR "couldn't exec foo: $!";
1184 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1186 If there is more than one argument in LIST, or if LIST is an array
1187 with more than one value, calls execvp(3) with the arguments in LIST.
1188 If there is only one scalar argument or an array with one element in it,
1189 the argument is checked for shell metacharacters, and if there are any,
1190 the entire argument is passed to the system's command shell for parsing
1191 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1192 If there are no shell metacharacters in the argument, it is split into
1193 words and passed directly to C<execvp()>, which is more efficient. Note:
1194 C<exec()> and C<system()> do not flush your output buffer, so you may need to
1195 set C<$|> to avoid lost output. Examples:
1197 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1198 exec "sort $outfile | uniq";
1200 If you don't really want to execute the first argument, but want to lie
1201 to the program you are executing about its own name, you can specify
1202 the program you actually want to run as an "indirect object" (without a
1203 comma) in front of the LIST. (This always forces interpretation of the
1204 LIST as a multivalued list, even if there is only a single scalar in
1207 $shell = '/bin/csh';
1208 exec $shell '-sh'; # pretend it's a login shell
1212 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1214 When the arguments get executed via the system shell, results will
1215 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1218 Using an indirect object with C<exec()> or C<system()> is also more secure.
1219 This usage forces interpretation of the arguments as a multivalued list,
1220 even if the list had just one argument. That way you're safe from the
1221 shell expanding wildcards or splitting up words with whitespace in them.
1223 @args = ( "echo surprise" );
1225 system @args; # subject to shell escapes
1227 system { $args[0] } @args; # safe even with one-arg list
1229 The first version, the one without the indirect object, ran the I<echo>
1230 program, passing it C<"surprise"> an argument. The second version
1231 didn't--it tried to run a program literally called I<"echo surprise">,
1232 didn't find it, and set C<$?> to a non-zero value indicating failure.
1234 Note that C<exec()> will not call your C<END> blocks, nor will it call
1235 any C<DESTROY> methods in your objects.
1239 Returns TRUE if the specified hash key exists in its hash array, even
1240 if the corresponding value is undefined.
1242 print "Exists\n" if exists $array{$key};
1243 print "Defined\n" if defined $array{$key};
1244 print "True\n" if $array{$key};
1246 A hash element can be TRUE only if it's defined, and defined if
1247 it exists, but the reverse doesn't necessarily hold true.
1249 Note that the EXPR can be arbitrarily complicated as long as the final
1250 operation is a hash key lookup:
1252 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1254 Although the last element will not spring into existence just because its
1255 existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1256 C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1257 test for a $key element. This autovivification may be fixed in a later
1262 Evaluates EXPR and exits immediately with that value. (Actually, it
1263 calls any defined C<END> routines first, but the C<END> routines may not
1264 abort the exit. Likewise any object destructors that need to be called
1265 are called before exit.) Example:
1268 exit 0 if $ans =~ /^[Xx]/;
1270 See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1271 universally portable values for EXPR are C<0> for success and C<1> for error;
1272 all other values are subject to unpredictable interpretation depending
1273 on the environment in which the Perl program is running.
1275 You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1276 someone might want to trap whatever error happened. Use C<die()> instead,
1277 which can be trapped by an C<eval()>.
1279 All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1285 Returns I<e> (the natural logarithm base) to the power of EXPR.
1286 If EXPR is omitted, gives C<exp($_)>.
1288 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1290 Implements the fcntl(2) function. You'll probably have to say
1294 first to get the correct constant definitions. Argument processing and
1295 value return works just like C<ioctl()> below.
1299 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1300 or die "can't fcntl F_GETFL: $!";
1302 You don't have to check for C<defined()> on the return from
1303 C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
1304 call into "C<0> but true" in Perl. This string is true in
1305 boolean context and C<0> in numeric context. It is also
1306 exempt from the normal B<-w> warnings on improper numeric
1309 Note that C<fcntl()> will produce a fatal error if used on a machine that
1310 doesn't implement fcntl(2).
1312 =item fileno FILEHANDLE
1314 Returns the file descriptor for a filehandle. This is useful for
1315 constructing bitmaps for C<select()> and low-level POSIX tty-handling
1316 operations. If FILEHANDLE is an expression, the value is taken as
1317 an indirect filehandle, generally its name.
1319 You can use this to find out whether two handles refer to the
1320 same underlying descriptor:
1322 if (fileno(THIS) == fileno(THAT)) {
1323 print "THIS and THAT are dups\n";
1326 =item flock FILEHANDLE,OPERATION
1328 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1329 success, FALSE on failure. Produces a fatal error if used on a machine
1330 that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
1331 is Perl's portable file locking interface, although it locks only entire
1334 On many platforms (including most versions or clones of Unix), locks
1335 established by C<flock()> are B<merely advisory>. Such discretionary locks
1336 are more flexible, but offer fewer guarantees. This means that files
1337 locked with C<flock()> may be modified by programs that do not also use
1338 C<flock()>. Windows NT and OS/2 are among the platforms which
1339 enforce mandatory locking. See your local documentation for details.
1341 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1342 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1343 you can use the symbolic names if import them from the Fcntl module,
1344 either individually, or as a group using the ':flock' tag. LOCK_SH
1345 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1346 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1347 LOCK_EX then C<flock()> will return immediately rather than blocking
1348 waiting for the lock (check the return status to see if you got it).
1350 To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1351 before (un)locking it.
1353 Note that the emulation built with lockf(3) doesn't provide shared
1354 locks, and it requires that FILEHANDLE be open with write intent. These
1355 are the semantics that lockf(3) implements. Most (all?) systems
1356 implement lockf(3) in terms of fcntl(2) locking, though, so the
1357 differing semantics shouldn't bite too many people.
1359 Note also that some versions of C<flock()> cannot lock things over the
1360 network; you would need to use the more system-specific C<fcntl()> for
1361 that. If you like you can force Perl to ignore your system's flock(2)
1362 function, and so provide its own fcntl(2)-based emulation, by passing
1363 the switch C<-Ud_flock> to the F<Configure> program when you configure
1366 Here's a mailbox appender for BSD systems.
1368 use Fcntl ':flock'; # import LOCK_* constants
1371 flock(MBOX,LOCK_EX);
1372 # and, in case someone appended
1373 # while we were waiting...
1378 flock(MBOX,LOCK_UN);
1381 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1382 or die "Can't open mailbox: $!";
1385 print MBOX $msg,"\n\n";
1388 See also L<DB_File> for other flock() examples.
1392 Does a fork(2) system call. Returns the child pid to the parent process,
1393 C<0> to the child process, or C<undef> if the fork is unsuccessful.
1395 Note: unflushed buffers remain unflushed in both processes, which means
1396 you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1397 method of C<IO::Handle> to avoid duplicate output.
1399 If you C<fork()> without ever waiting on your children, you will accumulate
1402 $SIG{CHLD} = sub { wait };
1404 There's also the double-fork trick (error checking on
1405 C<fork()> returns omitted);
1407 unless ($pid = fork) {
1409 exec "what you really wanna do";
1412 ## (some_perl_code_here)
1419 See also L<perlipc> for more examples of forking and reaping
1422 Note that if your forked child inherits system file descriptors like
1423 STDIN and STDOUT that are actually connected by a pipe or socket, even
1424 if you exit, then the remote server (such as, say, httpd or rsh) won't think
1425 you're done. You should reopen those to F</dev/null> if it's any issue.
1429 Declare a picture format for use by the C<write()> function. For
1433 Test: @<<<<<<<< @||||| @>>>>>
1434 $str, $%, '$' . int($num)
1438 $num = $cost/$quantity;
1442 See L<perlform> for many details and examples.
1444 =item formline PICTURE,LIST
1446 This is an internal function used by C<format>s, though you may call it,
1447 too. It formats (see L<perlform>) a list of values according to the
1448 contents of PICTURE, placing the output into the format output
1449 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1450 Eventually, when a C<write()> is done, the contents of
1451 C<$^A> are written to some filehandle, but you could also read C<$^A>
1452 yourself and then set C<$^A> back to C<"">. Note that a format typically
1453 does one C<formline()> per line of form, but the C<formline()> function itself
1454 doesn't care how many newlines are embedded in the PICTURE. This means
1455 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1456 You may therefore need to use multiple formlines to implement a single
1457 record format, just like the format compiler.
1459 Be careful if you put double quotes around the picture, because an "C<@>"
1460 character may be taken to mean the beginning of an array name.
1461 C<formline()> always returns TRUE. See L<perlform> for other examples.
1463 =item getc FILEHANDLE
1467 Returns the next character from the input file attached to FILEHANDLE,
1468 or the undefined value at end of file, or if there was an error. If
1469 FILEHANDLE is omitted, reads from STDIN. This is not particularly
1470 efficient. It cannot be used to get unbuffered single-characters,
1471 however. For that, try something more like:
1474 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1477 system "stty", '-icanon', 'eol', "\001";
1483 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1486 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1490 Determination of whether $BSD_STYLE should be set
1491 is left as an exercise to the reader.
1493 The C<POSIX::getattr()> function can do this more portably on systems
1494 purporting POSIX compliance.
1495 See also the C<Term::ReadKey> module from your nearest CPAN site;
1496 details on CPAN can be found on L<perlmod/CPAN>.
1500 Implements the C library function of the same name, which on most
1501 systems returns the current login from F</etc/utmp>, if any. If null,
1504 $login = getlogin || getpwuid($<) || "Kilroy";
1506 Do not consider C<getlogin()> for authentication: it is not as
1507 secure as C<getpwuid()>.
1509 =item getpeername SOCKET
1511 Returns the packed sockaddr address of other end of the SOCKET connection.
1514 $hersockaddr = getpeername(SOCK);
1515 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1516 $herhostname = gethostbyaddr($iaddr, AF_INET);
1517 $herstraddr = inet_ntoa($iaddr);
1521 Returns the current process group for the specified PID. Use
1522 a PID of C<0> to get the current process group for the
1523 current process. Will raise an exception if used on a machine that
1524 doesn't implement getpgrp(2). If PID is omitted, returns process
1525 group of current process. Note that the POSIX version of C<getpgrp()>
1526 does not accept a PID argument, so only C<PID==0> is truly portable.
1530 Returns the process id of the parent process.
1532 =item getpriority WHICH,WHO
1534 Returns the current priority for a process, a process group, or a user.
1535 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1536 machine that doesn't implement getpriority(2).
1542 =item gethostbyname NAME
1544 =item getnetbyname NAME
1546 =item getprotobyname NAME
1552 =item getservbyname NAME,PROTO
1554 =item gethostbyaddr ADDR,ADDRTYPE
1556 =item getnetbyaddr ADDR,ADDRTYPE
1558 =item getprotobynumber NUMBER
1560 =item getservbyport PORT,PROTO
1578 =item sethostent STAYOPEN
1580 =item setnetent STAYOPEN
1582 =item setprotoent STAYOPEN
1584 =item setservent STAYOPEN
1598 These routines perform the same functions as their counterparts in the
1599 system library. In list context, the return values from the
1600 various get routines are as follows:
1602 ($name,$passwd,$uid,$gid,
1603 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1604 ($name,$passwd,$gid,$members) = getgr*
1605 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1606 ($name,$aliases,$addrtype,$net) = getnet*
1607 ($name,$aliases,$proto) = getproto*
1608 ($name,$aliases,$port,$proto) = getserv*
1610 (If the entry doesn't exist you get a null list.)
1612 In scalar context, you get the name, unless the function was a
1613 lookup by name, in which case you get the other thing, whatever it is.
1614 (If the entry doesn't exist you get the undefined value.) For example:
1616 $uid = getpwnam($name);
1617 $name = getpwuid($num);
1619 $gid = getgrnam($name);
1620 $name = getgrgid($num;
1624 In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
1625 cases in the sense that in many systems they are unsupported. If the
1626 C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1627 usually encodes the disk quota. If the C<$comment> field is unsupported,
1628 it is an empty scalar. If it is supported it usually encodes some
1629 administrative comment about the user. In some systems the $quota
1630 field may be C<$change> or C<$age>, fields that have to do with password
1631 aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
1632 field, if present, encodes the expiration period of the account or the
1633 password. For the availability and the exact meaning of these fields
1634 in your system, please consult your getpwnam(3) documentation and your
1635 F<pwd.h> file. You can also find out from within Perl which meaning
1636 your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1637 field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1638 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
1640 The C<$members> value returned by I<getgr*()> is a space separated list of
1641 the login names of the members of the group.
1643 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1644 C, it will be returned to you via C<$?> if the function call fails. The
1645 C<@addrs> value returned by a successful call is a list of the raw
1646 addresses returned by the corresponding system library call. In the
1647 Internet domain, each address is four bytes long and you can unpack it
1648 by saying something like:
1650 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1652 If you get tired of remembering which element of the return list contains
1653 which return value, by-name interfaces are also provided in modules:
1654 C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1655 C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
1656 normal built-in, replacing them with versions that return objects with
1657 the appropriate names for each field. For example:
1661 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1663 Even though it looks like they're the same method calls (uid),
1664 they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
1666 =item getsockname SOCKET
1668 Returns the packed sockaddr address of this end of the SOCKET connection.
1671 $mysockaddr = getsockname(SOCK);
1672 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1674 =item getsockopt SOCKET,LEVEL,OPTNAME
1676 Returns the socket option requested, or undef if there is an error.
1682 Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
1683 do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1684 operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
1685 The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1686 L<perlop/"I/O Operators">.
1690 Converts a time as returned by the time function to a 9-element array
1691 with the time localized for the standard Greenwich time zone.
1692 Typically used as follows:
1695 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1698 All array elements are numeric, and come straight out of a struct tm.
1699 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1700 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1701 years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
1703 If EXPR is omitted, does C<gmtime(time())>.
1705 In scalar context, returns the ctime(3) value:
1707 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1709 Also see the C<timegm()> function provided by the C<Time::Local> module,
1710 and the strftime(3) function available via the POSIX module.
1712 This scalar value is B<not> locale dependent, see L<perllocale>, but
1713 instead a Perl builtin. Also see the C<Time::Local> module, and the
1714 strftime(3) and mktime(3) function available via the POSIX module. To
1715 get somewhat similar but locale dependent date strings, set up your
1716 locale environment variables appropriately (please see L<perllocale>)
1717 and try for example:
1719 use POSIX qw(strftime);
1720 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1722 Note that the C<%a> and C<%b>, the short forms of the day of the week
1723 and the month of the year, may not necessarily be three characters wide.
1731 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1732 execution there. It may not be used to go into any construct that
1733 requires initialization, such as a subroutine or a C<foreach> loop. It
1734 also can't be used to go into a construct that is optimized away,
1735 or to get out of a block or subroutine given to C<sort()>.
1736 It can be used to go almost anywhere else within the dynamic scope,
1737 including out of subroutines, but it's usually better to use some other
1738 construct such as C<last> or C<die()>. The author of Perl has never felt the
1739 need to use this form of C<goto> (in Perl, that is--C is another matter).
1741 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1742 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1743 necessarily recommended if you're optimizing for maintainability:
1745 goto ("FOO", "BAR", "GLARCH")[$i];
1747 The C<goto-&NAME> form is highly magical, and substitutes a call to the
1748 named subroutine for the currently running subroutine. This is used by
1749 C<AUTOLOAD> subroutines that wish to load another subroutine and then
1750 pretend that the other subroutine had been called in the first place
1751 (except that any modifications to C<@_> in the current subroutine are
1752 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
1753 will be able to tell that this routine was called first.
1755 =item grep BLOCK LIST
1757 =item grep EXPR,LIST
1759 This is similar in spirit to, but not the same as, grep(1)
1760 and its relatives. In particular, it is not limited to using
1761 regular expressions.
1763 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1764 C<$_> to each element) and returns the list value consisting of those
1765 elements for which the expression evaluated to TRUE. In a scalar
1766 context, returns the number of times the expression was TRUE.
1768 @foo = grep(!/^#/, @bar); # weed out comments
1772 @foo = grep {!/^#/} @bar; # weed out comments
1774 Note that, because C<$_> is a reference into the list value, it can be used
1775 to modify the elements of the array. While this is useful and
1776 supported, it can cause bizarre results if the LIST is not a named
1777 array. Similarly, grep returns aliases into the original list,
1778 much like the way that a for loop's index variable aliases the list
1779 elements. That is, modifying an element of a list returned by grep
1780 (for example, in a C<foreach>, C<map()> or another C<grep()>)
1781 actually modifies the element in the original list.
1783 See also L</map> for an array composed of the results of the BLOCK or EXPR.
1789 Interprets EXPR as a hex string and returns the corresponding
1790 value. (To convert strings that might start with either 0 or 0x
1791 see L</oct>.) If EXPR is omitted, uses C<$_>.
1793 print hex '0xAf'; # prints '175'
1794 print hex 'aF'; # same
1798 There is no builtin C<import()> function. It is just an ordinary
1799 method (subroutine) defined (or inherited) by modules that wish to export
1800 names to another module. The C<use()> function calls the C<import()> method
1801 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1803 =item index STR,SUBSTR,POSITION
1805 =item index STR,SUBSTR
1807 Returns the position of the first occurrence of SUBSTR in STR at or after
1808 POSITION. If POSITION is omitted, starts searching from the beginning of
1809 the string. The return value is based at C<0> (or whatever you've set the C<$[>
1810 variable to--but don't do that). If the substring is not found, returns
1811 one less than the base, ordinarily C<-1>.
1817 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
1818 You should not use this for rounding, because it truncates
1819 towards C<0>, and because machine representations of floating point
1820 numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1821 or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
1823 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1825 Implements the ioctl(2) function. You'll probably have to say
1827 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1829 first to get the correct function definitions. If F<ioctl.ph> doesn't
1830 exist or doesn't have the correct definitions you'll have to roll your
1831 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1832 (There is a Perl script called B<h2ph> that comes with the Perl kit that
1833 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1834 written depending on the FUNCTION--a pointer to the string value of SCALAR
1835 will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
1836 has no string value but does have a numeric value, that value will be
1837 passed rather than a pointer to the string value. To guarantee this to be
1838 TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
1839 functions are useful for manipulating the values of structures used by
1840 C<ioctl()>. The following example sets the erase character to DEL.
1844 die "NO TIOCGETP" if $@ || !$getp;
1845 $sgttyb_t = "ccccs"; # 4 chars and a short
1846 if (ioctl(STDIN,$getp,$sgttyb)) {
1847 @ary = unpack($sgttyb_t,$sgttyb);
1849 $sgttyb = pack($sgttyb_t,@ary);
1850 ioctl(STDIN,&TIOCSETP,$sgttyb)
1851 || die "Can't ioctl: $!";
1854 The return value of C<ioctl()> (and C<fcntl()>) is as follows:
1856 if OS returns: then Perl returns:
1858 0 string "0 but true"
1859 anything else that number
1861 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1862 still easily determine the actual value returned by the operating
1865 ($retval = ioctl(...)) || ($retval = -1);
1866 printf "System returned %d\n", $retval;
1868 The special string "C<0> but true" is excempt from B<-w> complaints
1869 about improper numeric conversions.
1871 =item join EXPR,LIST
1873 Joins the separate strings of LIST into a single string with
1874 fields separated by the value of EXPR, and returns the string.
1877 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1883 Returns a list consisting of all the keys of the named hash. (In a
1884 scalar context, returns the number of keys.) The keys are returned in
1885 an apparently random order, but it is the same order as either the
1886 C<values()> or C<each()> function produces (given that the hash has not been
1887 modified). As a side effect, it resets HASH's iterator.
1889 Here is yet another way to print your environment:
1892 @values = values %ENV;
1893 while ($#keys >= 0) {
1894 print pop(@keys), '=', pop(@values), "\n";
1897 or how about sorted by key:
1899 foreach $key (sort(keys %ENV)) {
1900 print $key, '=', $ENV{$key}, "\n";
1903 To sort a hash by value, you'll need to use a C<sort()> function.
1904 Here's a descending numeric sort of a hash by its values:
1906 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
1907 printf "%4d %s\n", $hash{$key}, $key;
1910 As an lvalue C<keys()> allows you to increase the number of hash buckets
1911 allocated for the given hash. This can gain you a measure of efficiency if
1912 you know the hash is going to get big. (This is similar to pre-extending
1913 an array by assigning a larger number to $#array.) If you say
1917 then C<%hash> will have at least 200 buckets allocated for it--256 of them, in fact, since
1918 it rounds up to the next power of two. These
1919 buckets will be retained even if you do C<%hash = ()>, use C<undef
1920 %hash> if you want to free the storage while C<%hash> is still in scope.
1921 You can't shrink the number of buckets allocated for the hash using
1922 C<keys()> in this way (but you needn't worry about doing this by accident,
1923 as trying has no effect).
1927 Sends a signal to a list of processes. The first element of
1928 the list must be the signal to send. Returns the number of
1929 processes successfully signaled.
1931 $cnt = kill 1, $child1, $child2;
1934 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1935 process groups instead of processes. (On System V, a negative I<PROCESS>
1936 number will also kill process groups, but that's not portable.) That
1937 means you usually want to use positive not negative signals. You may also
1938 use a signal name in quotes. See L<perlipc/"Signals"> for details.
1944 The C<last> command is like the C<break> statement in C (as used in
1945 loops); it immediately exits the loop in question. If the LABEL is
1946 omitted, the command refers to the innermost enclosing loop. The
1947 C<continue> block, if any, is not executed:
1949 LINE: while (<STDIN>) {
1950 last LINE if /^$/; # exit when done with header
1954 C<last> cannot be used to exit a block which returns a value such as
1955 C<eval {}>, C<sub {}> or C<do {}>.
1957 See also L</continue> for an illustration of how C<last>, C<next>, and
1964 Returns an lowercased version of EXPR. This is the internal function
1965 implementing the C<\L> escape in double-quoted strings.
1966 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1968 If EXPR is omitted, uses C<$_>.
1974 Returns the value of EXPR with the first character lowercased. This is
1975 the internal function implementing the C<\l> escape in double-quoted strings.
1976 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
1978 If EXPR is omitted, uses C<$_>.
1984 Returns the length in characters of the value of EXPR. If EXPR is
1985 omitted, returns length of C<$_>.
1987 =item link OLDFILE,NEWFILE
1989 Creates a new filename linked to the old filename. Returns TRUE for
1990 success, FALSE otherwise.
1992 =item listen SOCKET,QUEUESIZE
1994 Does the same thing that the listen system call does. Returns TRUE if
1995 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
1999 A local modifies the listed variables to be local to the enclosing
2000 block, file, or eval. If more than one value is listed, the list must
2001 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2002 for details, including issues with tied arrays and hashes.
2004 You really probably want to be using C<my()> instead, because C<local()> isn't
2005 what most people think of as "local". See L<perlsub/"Private Variables
2006 via my()"> for details.
2008 =item localtime EXPR
2010 Converts a time as returned by the time function to a 9-element array
2011 with the time analyzed for the local time zone. Typically used as
2015 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2018 All array elements are numeric, and come straight out of a struct tm.
2019 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
2020 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
2021 years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year.
2023 If EXPR is omitted, uses the current time (C<localtime(time)>).
2025 In scalar context, returns the ctime(3) value:
2027 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2029 This scalar value is B<not> locale dependent, see L<perllocale>, but
2030 instead a Perl builtin. Also see the C<Time::Local> module, and the
2031 strftime(3) and mktime(3) function available via the POSIX module. To
2032 get somewhat similar but locale dependent date strings, set up your
2033 locale environment variables appropriately (please see L<perllocale>)
2034 and try for example:
2036 use POSIX qw(strftime);
2037 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2039 Note that the C<%a> and C<%b>, the short forms of the day of the week
2040 and the month of the year, may not necessarily be three characters wide.
2046 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
2049 =item lstat FILEHANDLE
2055 Does the same thing as the C<stat()> function (including setting the
2056 special C<_> filehandle) but stats a symbolic link instead of the file
2057 the symbolic link points to. If symbolic links are unimplemented on
2058 your system, a normal C<stat()> is done.
2060 If EXPR is omitted, stats C<$_>.
2064 The match operator. See L<perlop>.
2066 =item map BLOCK LIST
2070 Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
2071 element) and returns the list value composed of the results of each such
2072 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2073 may produce zero, one, or more elements in the returned value.
2075 @chars = map(chr, @nums);
2077 translates a list of numbers to the corresponding characters. And
2079 %hash = map { getkey($_) => $_ } @array;
2081 is just a funny way to write
2084 foreach $_ (@array) {
2085 $hash{getkey($_)} = $_;
2088 Note that, because C<$_> is a reference into the list value, it can be used
2089 to modify the elements of the array. While this is useful and
2090 supported, it can cause bizarre results if the LIST is not a named
2091 array. See also L</grep> for an array composed of those items of the
2092 original list for which the BLOCK or EXPR evaluates to true.
2094 =item mkdir FILENAME,MODE
2096 Creates the directory specified by FILENAME, with permissions
2097 specified by MODE (as modified by C<umask>). If it succeeds it
2098 returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
2100 In general, it is better to create directories with permissive MODEs,
2101 and let the user modify that with their C<umask>, than it is to supply
2102 a restrictive MODE and give the user no way to be more permissive.
2103 The exceptions to this rule are when the file or directory should be
2104 kept private (mail files, for instance). The perlfunc(1) entry on
2105 C<umask> discusses the choice of MODE in more detail.
2107 =item msgctl ID,CMD,ARG
2109 Calls the System V IPC function msgctl(2). You'll probably have to say
2113 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2114 then ARG must be a variable which will hold the returned C<msqid_ds>
2115 structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
2116 true" for zero, or the actual return value otherwise. See also
2117 C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
2119 =item msgget KEY,FLAGS
2121 Calls the System V IPC function msgget(2). Returns the message queue
2122 id, or the undefined value if there is an error. See also C<IPC::SysV>
2123 and C<IPC::SysV::Msg> documentation.
2125 =item msgsnd ID,MSG,FLAGS
2127 Calls the System V IPC function msgsnd to send the message MSG to the
2128 message queue ID. MSG must begin with the long integer message type,
2129 which may be created with C<pack("l", $type)>. Returns TRUE if
2130 successful, or FALSE if there is an error. See also C<IPC::SysV>
2131 and C<IPC::SysV::Msg> documentation.
2133 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2135 Calls the System V IPC function msgrcv to receive a message from
2136 message queue ID into variable VAR with a maximum message size of
2137 SIZE. Note that if a message is received, the message type will be
2138 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2139 size of the message type. Returns TRUE if successful, or FALSE if
2140 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2144 A C<my()> declares the listed variables to be local (lexically) to the
2145 enclosing block, file, or C<eval()>. If
2146 more than one value is listed, the list must be placed in parentheses. See
2147 L<perlsub/"Private Variables via my()"> for details.
2153 The C<next> command is like the C<continue> statement in C; it starts
2154 the next iteration of the loop:
2156 LINE: while (<STDIN>) {
2157 next LINE if /^#/; # discard comments
2161 Note that if there were a C<continue> block on the above, it would get
2162 executed even on discarded lines. If the LABEL is omitted, the command
2163 refers to the innermost enclosing loop.
2165 C<next> cannot be used to exit a block which returns a value such as
2166 C<eval {}>, C<sub {}> or C<do {}>.
2168 See also L</continue> for an illustration of how C<last>, C<next>, and
2171 =item no Module LIST
2173 See the L</use> function, which C<no> is the opposite of.
2179 Interprets EXPR as an octal string and returns the corresponding
2180 value. (If EXPR happens to start off with C<0x>, interprets it as
2181 a hex string instead.) The following will handle decimal, octal, and
2182 hex in the standard Perl or C notation:
2184 $val = oct($val) if $val =~ /^0/;
2186 If EXPR is omitted, uses C<$_>. This function is commonly used when
2187 a string such as C<644> needs to be converted into a file mode, for
2188 example. (Although perl will automatically convert strings into
2189 numbers as needed, this automatic conversion assumes base 10.)
2191 =item open FILEHANDLE,EXPR
2193 =item open FILEHANDLE
2195 Opens the file whose filename is given by EXPR, and associates it with
2196 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2197 name of the real filehandle wanted. If EXPR is omitted, the scalar
2198 variable of the same name as the FILEHANDLE contains the filename.
2199 (Note that lexical variables--those declared with C<my()>--will not work
2200 for this purpose; so if you're using C<my()>, specify EXPR in your call
2203 If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2204 If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2205 output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
2206 the file is opened for appending, again being created if necessary.
2207 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2208 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2209 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2210 file first. You can't usually use either read-write mode for updating
2211 textfiles, since they have variable length records. See the B<-i>
2212 switch in L<perlrun> for a better approach. The file is created with
2213 permissions of C<0666> modified by the process' C<umask> value.
2215 The prefix and the filename may be separated with spaces.
2216 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2217 C<'w+'>, C<'a'>, and C<'a+'>.
2219 If the filename begins with C<'|'>, the filename is interpreted as a
2220 command to which output is to be piped, and if the filename ends with a
2221 C<'|'>, the filename is interpreted See L<perlipc/"Using open() for IPC">
2222 for more examples of this. (You are not allowed to C<open()> to a command
2223 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2224 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2226 Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
2227 nonzero upon success, the undefined value otherwise. If the C<open()>
2228 involved a pipe, the return value happens to be the pid of the
2231 If you're unfortunate enough to be running Perl on a system that
2232 distinguishes between text files and binary files (modern operating
2233 systems don't care), then you should check out L</binmode> for tips for
2234 dealing with this. The key distinction between systems that need C<binmode()>
2235 and those that don't is their text file formats. Systems like Unix, MacOS, and
2236 Plan9, which delimit lines with a single character, and which encode that
2237 character in C as C<"\n">, do not need C<binmode()>. The rest need it.
2239 When opening a file, it's usually a bad idea to continue normal execution
2240 if the request failed, so C<open()> is frequently used in connection with
2241 C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
2242 where you want to make a nicely formatted error message (but there are
2243 modules that can help with that problem)) you should always check
2244 the return value from opening a file. The infrequent exception is when
2245 working with an unopened filehandle is actually what you want to do.
2250 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2251 while (<ARTICLE>) {...
2253 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2254 # if the open fails, output is discarded
2256 open(DBASE, '+<dbase.mine') # open for update
2257 or die "Can't open 'dbase.mine' for update: $!";
2259 open(ARTICLE, "caesar <$article |") # decrypt article
2260 or die "Can't start caesar: $!";
2262 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2263 or die "Can't start sort: $!";
2265 # process argument list of files along with any includes
2267 foreach $file (@ARGV) {
2268 process($file, 'fh00');
2272 my($filename, $input) = @_;
2273 $input++; # this is a string increment
2274 unless (open($input, $filename)) {
2275 print STDERR "Can't open $filename: $!\n";
2280 while (<$input>) { # note use of indirection
2281 if (/^#include "(.*)"/) {
2282 process($1, $input);
2289 You may also, in the Bourne shell tradition, specify an EXPR beginning
2290 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2291 name of a filehandle (or file descriptor, if numeric) to be
2292 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2293 C<+E<gt>E<gt>>, and C<+E<lt>>. The
2294 mode you specify should match the mode of the original filehandle.
2295 (Duping a filehandle does not take into account any existing contents of
2297 Here is a script that saves, redirects, and restores STDOUT and
2301 open(OLDOUT, ">&STDOUT");
2302 open(OLDERR, ">&STDERR");
2304 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2305 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2307 select(STDERR); $| = 1; # make unbuffered
2308 select(STDOUT); $| = 1; # make unbuffered
2310 print STDOUT "stdout 1\n"; # this works for
2311 print STDERR "stderr 1\n"; # subprocesses too
2316 open(STDOUT, ">&OLDOUT");
2317 open(STDERR, ">&OLDERR");
2319 print STDOUT "stdout 2\n";
2320 print STDERR "stderr 2\n";
2323 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2324 equivalent of C's C<fdopen()> of that file descriptor; this is more
2325 parsimonious of file descriptors. For example:
2327 open(FILEHANDLE, "<&=$fd")
2329 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
2330 there is an implicit fork done, and the return value of open is the pid
2331 of the child within the parent process, and C<0> within the child
2332 process. (Use C<defined($pid)> to determine whether the open was successful.)
2333 The filehandle behaves normally for the parent, but i/o to that
2334 filehandle is piped from/to the STDOUT/STDIN of the child process.
2335 In the child process the filehandle isn't opened--i/o happens from/to
2336 the new STDOUT or STDIN. Typically this is used like the normal
2337 piped open when you want to exercise more control over just how the
2338 pipe command gets executed, such as when you are running setuid, and
2339 don't want to have to scan shell commands for metacharacters.
2340 The following pairs are more or less equivalent:
2342 open(FOO, "|tr '[a-z]' '[A-Z]'");
2343 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2345 open(FOO, "cat -n '$file'|");
2346 open(FOO, "-|") || exec 'cat', '-n', $file;
2348 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2350 NOTE: On any operation that may do a fork, any unflushed buffers remain
2351 unflushed in both processes, which means you may need to set C<$|> to
2352 avoid duplicate output.
2354 Closing any piped filehandle causes the parent process to wait for the
2355 child to finish, and returns the status value in C<$?>.
2357 The filename passed to open will have leading and trailing
2358 whitespace deleted, and the normal redirection characters
2359 honored. This property, known as "magic open",
2360 can often be used to good effect. A user could specify a filename of
2361 F<"rsh cat file |">, or you could change certain filenames as needed:
2363 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2364 open(FH, $filename) or die "Can't open $filename: $!";
2366 However, to open a file with arbitrary weird characters in it, it's
2367 necessary to protect any leading and trailing whitespace:
2369 $file =~ s#^(\s)#./$1#;
2370 open(FOO, "< $file\0");
2372 If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2373 should use the C<sysopen()> function, which involves no such magic. This is
2374 another way to protect your filenames from interpretation. For example:
2377 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2378 or die "sysopen $path: $!";
2379 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2380 print HANDLE "stuff $$\n");
2382 print "File contains: ", <HANDLE>;
2384 Using the constructor from the C<IO::Handle> package (or one of its
2385 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2386 filehandles that have the scope of whatever variables hold references to
2387 them, and automatically close whenever and however you leave that scope:
2391 sub read_myfile_munged {
2393 my $handle = new IO::File;
2394 open($handle, "myfile") or die "myfile: $!";
2396 or return (); # Automatically closed here.
2397 mung $first or die "mung failed"; # Or here.
2398 return $first, <$handle> if $ALL; # Or here.
2402 See L</seek()> for some details about mixing reading and writing.
2404 =item opendir DIRHANDLE,EXPR
2406 Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2407 C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
2408 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2414 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2415 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2417 =item pack TEMPLATE,LIST
2419 Takes an array or list of values and packs it into a binary structure,
2420 returning the string containing the structure. The TEMPLATE is a
2421 sequence of characters that give the order and type of values, as
2424 A An ascii string, will be space padded.
2425 a An ascii string, will be null padded.
2426 b A bit string (ascending bit order, like vec()).
2427 B A bit string (descending bit order).
2428 h A hex string (low nybble first).
2429 H A hex string (high nybble first).
2431 c A signed char value.
2432 C An unsigned char value. Only does bytes. See U for Unicode.
2434 s A signed short value.
2435 S An unsigned short value.
2436 (This 'short' is _exactly_ 16 bits, which may differ from
2437 what a local C compiler calls 'short'.)
2439 i A signed integer value.
2440 I An unsigned integer value.
2441 (This 'integer' is _at_least_ 32 bits wide. Its exact
2442 size depends on what a local C compiler calls 'int',
2443 and may even be larger than the 'long' described in
2446 l A signed long value.
2447 L An unsigned long value.
2448 (This 'long' is _exactly_ 32 bits, which may differ from
2449 what a local C compiler calls 'long'.)
2451 n A short in "network" (big-endian) order.
2452 N A long in "network" (big-endian) order.
2453 v A short in "VAX" (little-endian) order.
2454 V A long in "VAX" (little-endian) order.
2455 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2456 _exactly_ 32 bits, respectively.)
2458 f A single-precision float in the native format.
2459 d A double-precision float in the native format.
2461 p A pointer to a null-terminated string.
2462 P A pointer to a structure (fixed-length string).
2464 u A uuencoded string.
2465 U A Unicode character number. Encodes to UTF-8 internally.
2466 Works even if C<use utf8> is not in effect.
2468 w A BER compressed integer. Its bytes represent an unsigned
2469 integer in base 128, most significant digit first, with as
2470 few digits as possible. Bit eight (the high bit) is set
2471 on each byte except the last.
2475 @ Null fill to absolute position.
2477 Each letter may optionally be followed by a number giving a repeat
2478 count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the
2479 pack function will gobble up that many values from the LIST. A C<*> for the
2480 repeat count means to use however many items are left. The C<"a"> and C<"A">
2481 types gobble just one value, but pack it as a string of length count,
2482 padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips
2483 trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B">
2484 fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a
2485 string that many nybbles long. The C<"p"> type packs a pointer to a null-
2486 terminated string. You are responsible for ensuring the string is not a
2487 temporary value (which can potentially get deallocated before you get
2488 around to using the packed result). The C<"P"> packs a pointer to a structure
2489 of the size indicated by the length. A NULL pointer is created if the
2490 corresponding value for C<"p"> or C<"P"> is C<undef>.
2491 Real numbers (floats and doubles) are
2492 in the native machine format only; due to the multiplicity of floating
2493 formats around, and the lack of a standard "network" representation, no
2494 facility for interchange has been made. This means that packed floating
2495 point data written on one machine may not be readable on another - even if
2496 both use IEEE floating point arithmetic (as the endian-ness of the memory
2497 representation is not part of the IEEE spec). Note that Perl uses doubles
2498 internally for all numeric calculation, and converting from double into
2499 float and thence back to double again will lose precision (i.e.,
2500 C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>).
2504 $foo = pack("CCCC",65,66,67,68);
2506 $foo = pack("C4",65,66,67,68);
2508 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2509 # same thing with Unicode circled letters
2511 $foo = pack("ccxxcc",65,66,67,68);
2514 $foo = pack("s2",1,2);
2515 # "\1\0\2\0" on little-endian
2516 # "\0\1\0\2" on big-endian
2518 $foo = pack("a4","abcd","x","y","z");
2521 $foo = pack("aaaa","abcd","x","y","z");
2524 $foo = pack("a14","abcdefg");
2525 # "abcdefg\0\0\0\0\0\0\0"
2527 $foo = pack("i9pl", gmtime);
2528 # a real struct tm (on my system anyway)
2531 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2534 The same template may generally also be used in the unpack function.
2538 =item package NAMESPACE
2540 Declares the compilation unit as being in the given namespace. The scope
2541 of the package declaration is from the declaration itself through the end of
2542 the enclosing block (the same scope as the C<local()> operator). All further
2543 unqualified dynamic identifiers will be in this namespace. A package
2544 statement affects only dynamic variables--including those you've used
2545 C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
2546 would be the first declaration in a file to be included by the C<require>
2547 or C<use> operator. You can switch into a package in more than one place;
2548 it merely influences which symbol table is used by the compiler for the
2549 rest of that block. You can refer to variables and filehandles in other
2550 packages by prefixing the identifier with the package name and a double
2551 colon: C<$Package::Variable>. If the package name is null, the C<main>
2552 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2554 If NAMESPACE is omitted, then there is no current package, and all
2555 identifiers must be fully qualified or lexicals. This is stricter
2556 than C<use strict>, since it also extends to function names.
2558 See L<perlmod/"Packages"> for more information about packages, modules,
2559 and classes. See L<perlsub> for other scoping issues.
2561 =item pipe READHANDLE,WRITEHANDLE
2563 Opens a pair of connected pipes like the corresponding system call.
2564 Note that if you set up a loop of piped processes, deadlock can occur
2565 unless you are very careful. In addition, note that Perl's pipes use
2566 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2567 after each command, depending on the application.
2569 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2570 for examples of such things.
2576 Pops and returns the last value of the array, shortening the array by
2577 1. Has a similar effect to
2579 $tmp = $ARRAY[$#ARRAY--];
2581 If there are no elements in the array, returns the undefined value.
2582 If ARRAY is omitted, pops the
2583 C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2590 Returns the offset of where the last C<m//g> search left off for the variable
2591 is in question (C<$_> is used when the variable is not specified). May be
2592 modified to change that offset. Such modification will also influence
2593 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2596 =item print FILEHANDLE LIST
2602 Prints a string or a comma-separated list of strings. Returns TRUE
2603 if successful. FILEHANDLE may be a scalar variable name, in which case
2604 the variable contains the name of or a reference to the filehandle, thus introducing one
2605 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2606 token is a term, it may be misinterpreted as an operator unless you
2607 interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
2608 omitted, prints by default to standard output (or to the last selected
2609 output channel--see L</select>). If LIST is also omitted, prints C<$_> to
2610 the currently selected output channel. To set the default output channel to something other than
2611 STDOUT use the select operation. Note that, because print takes a
2612 LIST, anything in the LIST is evaluated in list context, and any
2613 subroutine that you call will have one or more of its expressions
2614 evaluated in list context. Also be careful not to follow the print
2615 keyword with a left parenthesis unless you want the corresponding right
2616 parenthesis to terminate the arguments to the print--interpose a C<+> or
2617 put parentheses around all the arguments.
2619 Note that if you're storing FILEHANDLES in an array or other expression,
2620 you will have to use a block returning its value instead:
2622 print { $files[$i] } "stuff\n";
2623 print { $OK ? STDOUT : STDERR } "stuff\n";
2625 =item printf FILEHANDLE FORMAT, LIST
2627 =item printf FORMAT, LIST
2629 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
2630 (the output record separator) is not appended. The first argument
2631 of the list will be interpreted as the C<printf()> format. If C<use locale> is
2632 in effect, the character used for the decimal point in formatted real numbers
2633 is affected by the LC_NUMERIC locale. See L<perllocale>.
2635 Don't fall into the trap of using a C<printf()> when a simple
2636 C<print()> would do. The C<print()> is more efficient and less
2639 =item prototype FUNCTION
2641 Returns the prototype of a function as a string (or C<undef> if the
2642 function has no prototype). FUNCTION is a reference to, or the name of,
2643 the function whose prototype you want to retrieve.
2645 If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2646 a name for Perl builtin. If builtin is not I<overridable> (such as
2647 C<qw//>) or its arguments cannot be expressed by a prototype (such as
2648 C<system()>) - in other words, the builtin does not behave like a Perl
2649 function - returns C<undef>. Otherwise, the string describing the
2650 equivalent prototype is returned.
2652 =item push ARRAY,LIST
2654 Treats ARRAY as a stack, and pushes the values of LIST
2655 onto the end of ARRAY. The length of ARRAY increases by the length of
2656 LIST. Has the same effect as
2659 $ARRAY[++$#ARRAY] = $value;
2662 but is more efficient. Returns the new number of elements in the array.
2674 Generalized quotes. See L<perlop>.
2676 =item quotemeta EXPR
2680 Returns the value of EXPR with all non-alphanumeric
2681 characters backslashed. (That is, all characters not matching
2682 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2683 returned string, regardless of any locale settings.)
2684 This is the internal function implementing
2685 the C<\Q> escape in double-quoted strings.
2687 If EXPR is omitted, uses C<$_>.
2693 Returns a random fractional number greater than or equal to C<0> and less
2694 than the value of EXPR. (EXPR should be positive.) If EXPR is
2695 omitted, the value C<1> is used. Automatically calls C<srand()> unless
2696 C<srand()> has already been called. See also C<srand()>.
2698 (Note: If your rand function consistently returns numbers that are too
2699 large or too small, then your version of Perl was probably compiled
2700 with the wrong number of RANDBITS.)
2702 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2704 =item read FILEHANDLE,SCALAR,LENGTH
2706 Attempts to read LENGTH bytes of data into variable SCALAR from the
2707 specified FILEHANDLE. Returns the number of bytes actually read,
2708 C<0> at end of file, or undef if there was an error. SCALAR will be grown
2709 or shrunk to the length actually read. An OFFSET may be specified to
2710 place the read data at some other place than the beginning of the
2711 string. This call is actually implemented in terms of stdio's fread(3)
2712 call. To get a true read(2) system call, see C<sysread()>.
2714 =item readdir DIRHANDLE
2716 Returns the next directory entry for a directory opened by C<opendir()>.
2717 If used in list context, returns all the rest of the entries in the
2718 directory. If there are no more entries, returns an undefined value in
2719 scalar context or a null list in list context.
2721 If you're planning to filetest the return values out of a C<readdir()>, you'd
2722 better prepend the directory in question. Otherwise, because we didn't
2723 C<chdir()> there, it would have been testing the wrong file.
2725 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2726 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2731 Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
2732 is read and returned. In list context, reads until end-of-file is
2733 reached and returns a list of lines (however you've defined lines
2734 with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2735 This is the internal function implementing the C<E<lt>EXPRE<gt>>
2736 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2737 operator is discussed in more detail in L<perlop/"I/O Operators">.
2740 $line = readline(*STDIN); # same thing
2746 Returns the value of a symbolic link, if symbolic links are
2747 implemented. If not, gives a fatal error. If there is some system
2748 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2749 omitted, uses C<$_>.
2753 EXPR is executed as a system command.
2754 The collected standard output of the command is returned.
2755 In scalar context, it comes back as a single (potentially
2756 multi-line) string. In list context, returns a list of lines
2757 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2758 This is the internal function implementing the C<qx/EXPR/>
2759 operator, but you can use it directly. The C<qx/EXPR/>
2760 operator is discussed in more detail in L<perlop/"I/O Operators">.
2762 =item recv SOCKET,SCALAR,LEN,FLAGS
2764 Receives a message on a socket. Attempts to receive LENGTH bytes of
2765 data into variable SCALAR from the specified SOCKET filehandle.
2766 Actually does a C C<recvfrom()>, so that it can return the address of the
2767 sender. Returns the undefined value if there's an error. SCALAR will
2768 be grown or shrunk to the length actually read. Takes the same flags
2769 as the system call of the same name.
2770 See L<perlipc/"UDP: Message Passing"> for examples.
2776 The C<redo> command restarts the loop block without evaluating the
2777 conditional again. The C<continue> block, if any, is not executed. If
2778 the LABEL is omitted, the command refers to the innermost enclosing
2779 loop. This command is normally used by programs that want to lie to
2780 themselves about what was just input:
2782 # a simpleminded Pascal comment stripper
2783 # (warning: assumes no { or } in strings)
2784 LINE: while (<STDIN>) {
2785 while (s|({.*}.*){.*}|$1 |) {}
2790 if (/}/) { # end of comment?
2799 C<redo> cannot be used to retry a block which returns a value such as
2800 C<eval {}>, C<sub {}> or C<do {}>.
2802 See also L</continue> for an illustration of how C<last>, C<next>, and
2809 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2810 is not specified, C<$_> will be used. The value returned depends on the
2811 type of thing the reference is a reference to.
2812 Builtin types include:
2821 If the referenced object has been blessed into a package, then that package
2822 name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
2824 if (ref($r) eq "HASH") {
2825 print "r is a reference to a hash.\n";
2828 print "r is not a reference at all.\n";
2831 See also L<perlref>.
2833 =item rename OLDNAME,NEWNAME
2835 Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
2836 not work across file system boundaries.
2842 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
2843 supplied. If EXPR is numeric, demands that the current version of Perl
2844 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2846 Otherwise, demands that a library file be included if it hasn't already
2847 been included. The file is included via the do-FILE mechanism, which is
2848 essentially just a variety of C<eval()>. Has semantics similar to the following
2853 return 1 if $INC{$filename};
2854 my($realfilename,$result);
2856 foreach $prefix (@INC) {
2857 $realfilename = "$prefix/$filename";
2858 if (-f $realfilename) {
2859 $result = do $realfilename;
2863 die "Can't find $filename in \@INC";
2866 die "$filename did not return true value" unless $result;
2867 $INC{$filename} = $realfilename;
2871 Note that the file will not be included twice under the same specified
2872 name. The file must return TRUE as the last statement to indicate
2873 successful execution of any initialization code, so it's customary to
2874 end such a file with "C<1;>" unless you're sure it'll return TRUE
2875 otherwise. But it's better just to put the "C<1;>", in case you add more
2878 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
2879 replaces "F<::>" with "F</>" in the filename for you,
2880 to make it easy to load standard modules. This form of loading of
2881 modules does not risk altering your namespace.
2883 In other words, if you try this:
2885 require Foo::Bar; # a splendid bareword
2887 The require function will actually look for the "F<Foo/Bar.pm>" file in the
2888 directories specified in the C<@INC> array.
2890 But if you try this:
2892 $class = 'Foo::Bar';
2893 require $class; # $class is not a bareword
2895 require "Foo::Bar"; # not a bareword because of the ""
2897 The require function will look for the "F<Foo::Bar>" file in the @INC array and
2898 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
2900 eval "require $class";
2902 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
2908 Generally used in a C<continue> block at the end of a loop to clear
2909 variables and reset C<??> searches so that they work again. The
2910 expression is interpreted as a list of single characters (hyphens
2911 allowed for ranges). All variables and arrays beginning with one of
2912 those letters are reset to their pristine state. If the expression is
2913 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
2914 only variables or searches in the current package. Always returns
2917 reset 'X'; # reset all X variables
2918 reset 'a-z'; # reset lower case variables
2919 reset; # just reset ?? searches
2921 Resetting C<"A-Z"> is not recommended because you'll wipe out your
2922 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
2923 are unaffected, but they clean themselves up on scope exit anyway,
2924 so you'll probably want to use them instead. See L</my>.
2930 Returns from a subroutine, C<eval()>, or C<do FILE> with the value
2931 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
2932 context, depending on how the return value will be used, and the context
2933 may vary from one execution to the next (see C<wantarray()>). If no EXPR
2934 is given, returns an empty list in list context, an undefined value in
2935 scalar context, or nothing in a void context.
2937 (Note that in the absence of a return, a subroutine, eval, or do FILE
2938 will automatically return the value of the last expression evaluated.)
2942 In list context, returns a list value consisting of the elements
2943 of LIST in the opposite order. In scalar context, concatenates the
2944 elements of LIST, and returns a string value with all the characters
2945 in the opposite order.
2947 print reverse <>; # line tac, last line first
2949 undef $/; # for efficiency of <>
2950 print scalar reverse <>; # character tac, last line tsrif
2952 This operator is also handy for inverting a hash, although there are some
2953 caveats. If a value is duplicated in the original hash, only one of those
2954 can be represented as a key in the inverted hash. Also, this has to
2955 unwind one hash and build a whole new one, which may take some time
2958 %by_name = reverse %by_address; # Invert the hash
2960 =item rewinddir DIRHANDLE
2962 Sets the current position to the beginning of the directory for the
2963 C<readdir()> routine on DIRHANDLE.
2965 =item rindex STR,SUBSTR,POSITION
2967 =item rindex STR,SUBSTR
2969 Works just like index except that it returns the position of the LAST
2970 occurrence of SUBSTR in STR. If POSITION is specified, returns the
2971 last occurrence at or before that position.
2973 =item rmdir FILENAME
2977 Deletes the directory specified by FILENAME if that directory is empty. If it
2978 succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
2979 FILENAME is omitted, uses C<$_>.
2983 The substitution operator. See L<perlop>.
2987 Forces EXPR to be interpreted in scalar context and returns the value
2990 @counts = ( scalar @a, scalar @b, scalar @c );
2992 There is no equivalent operator to force an expression to
2993 be interpolated in list context because it's in practice never
2994 needed. If you really wanted to do so, however, you could use
2995 the construction C<@{[ (some expression) ]}>, but usually a simple
2996 C<(some expression)> suffices.
2998 =item seek FILEHANDLE,POSITION,WHENCE
3000 Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
3001 FILEHANDLE may be an expression whose value gives the name of the
3002 filehandle. The values for WHENCE are C<0> to set the new position to
3003 POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
3004 set it to EOF plus POSITION (typically negative). For WHENCE you may
3005 use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3006 C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
3008 If you want to position file for C<sysread()> or C<syswrite()>, don't use
3009 C<seek()> -- buffering makes its effect on the file's system position
3010 unpredictable and non-portable. Use C<sysseek()> instead.
3012 On some systems you have to do a seek whenever you switch between reading
3013 and writing. Amongst other things, this may have the effect of calling
3014 stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
3019 This is also useful for applications emulating C<tail -f>. Once you hit
3020 EOF on your read, and then sleep for a while, you might have to stick in a
3021 seek() to reset things. The C<seek()> doesn't change the current position,
3022 but it I<does> clear the end-of-file condition on the handle, so that the
3023 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3025 If that doesn't work (some stdios are particularly cantankerous), then
3026 you may need something more like this:
3029 for ($curpos = tell(FILE); $_ = <FILE>;
3030 $curpos = tell(FILE)) {
3031 # search for some stuff and put it into files
3033 sleep($for_a_while);
3034 seek(FILE, $curpos, 0);
3037 =item seekdir DIRHANDLE,POS
3039 Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3040 must be a value returned by C<telldir()>. Has the same caveats about
3041 possible directory compaction as the corresponding system library
3044 =item select FILEHANDLE
3048 Returns the currently selected filehandle. Sets the current default
3049 filehandle for output, if FILEHANDLE is supplied. This has two
3050 effects: first, a C<write()> or a C<print()> without a filehandle will
3051 default to this FILEHANDLE. Second, references to variables related to
3052 output will refer to this output channel. For example, if you have to
3053 set the top of form format for more than one output channel, you might
3061 FILEHANDLE may be an expression whose value gives the name of the
3062 actual filehandle. Thus:
3064 $oldfh = select(STDERR); $| = 1; select($oldfh);
3066 Some programmers may prefer to think of filehandles as objects with
3067 methods, preferring to write the last example as:
3070 STDERR->autoflush(1);
3072 =item select RBITS,WBITS,EBITS,TIMEOUT
3074 This calls the select(2) system call with the bit masks specified, which
3075 can be constructed using C<fileno()> and C<vec()>, along these lines:
3077 $rin = $win = $ein = '';
3078 vec($rin,fileno(STDIN),1) = 1;
3079 vec($win,fileno(STDOUT),1) = 1;
3082 If you want to select on many filehandles you might wish to write a
3086 my(@fhlist) = split(' ',$_[0]);
3089 vec($bits,fileno($_),1) = 1;
3093 $rin = fhbits('STDIN TTY SOCK');
3097 ($nfound,$timeleft) =
3098 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3100 or to block until something becomes ready just do this
3102 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3104 Most systems do not bother to return anything useful in C<$timeleft>, so
3105 calling select() in scalar context just returns C<$nfound>.
3107 Any of the bit masks can also be undef. The timeout, if specified, is
3108 in seconds, which may be fractional. Note: not all implementations are
3109 capable of returning theC<$timeleft>. If not, they always return
3110 C<$timeleft> equal to the supplied C<$timeout>.
3112 You can effect a sleep of 250 milliseconds this way:
3114 select(undef, undef, undef, 0.25);
3116 B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3117 or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3118 then only on POSIX systems. You have to use C<sysread()> instead.
3120 =item semctl ID,SEMNUM,CMD,ARG
3122 Calls the System V IPC function C<semctl()>. You'll probably have to say
3126 first to get the correct constant definitions. If CMD is IPC_STAT or
3127 GETALL, then ARG must be a variable which will hold the returned
3128 semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
3129 undefined value for error, "C<0> but true" for zero, or the actual return
3130 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3132 =item semget KEY,NSEMS,FLAGS
3134 Calls the System V IPC function semget. Returns the semaphore id, or
3135 the undefined value if there is an error. See also C<IPC::SysV> and
3136 C<IPC::SysV::Semaphore> documentation.
3138 =item semop KEY,OPSTRING
3140 Calls the System V IPC function semop to perform semaphore operations
3141 such as signaling and waiting. OPSTRING must be a packed array of
3142 semop structures. Each semop structure can be generated with
3143 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3144 operations is implied by the length of OPSTRING. Returns TRUE if
3145 successful, or FALSE if there is an error. As an example, the
3146 following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
3148 $semop = pack("sss", $semnum, -1, 0);
3149 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3151 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3152 and C<IPC::SysV::Semaphore> documentation.
3154 =item send SOCKET,MSG,FLAGS,TO
3156 =item send SOCKET,MSG,FLAGS
3158 Sends a message on a socket. Takes the same flags as the system call
3159 of the same name. On unconnected sockets you must specify a
3160 destination to send TO, in which case it does a C C<sendto()>. Returns
3161 the number of characters sent, or the undefined value if there is an
3163 See L<perlipc/"UDP: Message Passing"> for examples.
3165 =item setpgrp PID,PGRP
3167 Sets the current process group for the specified PID, C<0> for the current
3168 process. Will produce a fatal error if used on a machine that doesn't
3169 implement setpgrp(2). If the arguments are omitted, it defaults to
3170 C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3171 arguments, so only setpgrp C<0,0> is portable.
3173 =item setpriority WHICH,WHO,PRIORITY
3175 Sets the current priority for a process, a process group, or a user.
3176 (See setpriority(2).) Will produce a fatal error if used on a machine
3177 that doesn't implement setpriority(2).
3179 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3181 Sets the socket option requested. Returns undefined if there is an
3182 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3189 Shifts the first value of the array off and returns it, shortening the
3190 array by 1 and moving everything down. If there are no elements in the
3191 array, returns the undefined value. If ARRAY is omitted, shifts the
3192 C<@_> array within the lexical scope of subroutines and formats, and the
3193 C<@ARGV> array at file scopes or within the lexical scopes established by
3194 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3195 See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3196 same thing to the left end of an array that C<pop()> and C<push()> do to the
3199 =item shmctl ID,CMD,ARG
3201 Calls the System V IPC function shmctl. You'll probably have to say
3205 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3206 then ARG must be a variable which will hold the returned C<shmid_ds>
3207 structure. Returns like ioctl: the undefined value for error, "C<0> but
3208 true" for zero, or the actual return value otherwise.
3209 See also C<IPC::SysV> documentation.
3211 =item shmget KEY,SIZE,FLAGS
3213 Calls the System V IPC function shmget. Returns the shared memory
3214 segment id, or the undefined value if there is an error.
3215 See also C<IPC::SysV> documentation.
3217 =item shmread ID,VAR,POS,SIZE
3219 =item shmwrite ID,STRING,POS,SIZE
3221 Reads or writes the System V shared memory segment ID starting at
3222 position POS for size SIZE by attaching to it, copying in/out, and
3223 detaching from it. When reading, VAR must be a variable that will
3224 hold the data read. When writing, if STRING is too long, only SIZE
3225 bytes are used; if STRING is too short, nulls are written to fill out
3226 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
3227 See also C<IPC::SysV> documentation.
3229 =item shutdown SOCKET,HOW
3231 Shuts down a socket connection in the manner indicated by HOW, which
3232 has the same interpretation as in the system call of the same name.
3234 shutdown(SOCKET, 0); # I/we have stopped reading data
3235 shutdown(SOCKET, 1); # I/we have stopped writing data
3236 shutdown(SOCKET, 2); # I/we have stopped using this socket
3238 This is useful with sockets when you want to tell the other
3239 side you're done writing but not done reading, or vice versa.
3240 It's also a more insistent form of close because it also
3241 disables the filedescriptor in any forked copies in other
3248 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3249 returns sine of C<$_>.
3251 For the inverse sine operation, you may use the C<POSIX::asin()>
3252 function, or use this relation:
3254 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3260 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3261 May be interrupted if the process receives a signal such as C<SIGALRM>.
3262 Returns the number of seconds actually slept. You probably cannot
3263 mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
3266 On some older systems, it may sleep up to a full second less than what
3267 you requested, depending on how it counts seconds. Most modern systems
3268 always sleep the full amount. They may appear to sleep longer than that,
3269 however, because your process might not be scheduled right away in a
3270 busy multitasking system.
3272 For delays of finer granularity than one second, you may use Perl's
3273 C<syscall()> interface to access setitimer(2) if your system supports it,
3274 or else see L</select()> above.
3276 See also the POSIX module's C<sigpause()> function.
3278 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3280 Opens a socket of the specified kind and attaches it to filehandle
3281 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
3282 system call of the same name. You should "C<use Socket;>" first to get
3283 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
3285 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3287 Creates an unnamed pair of sockets in the specified domain, of the
3288 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3289 for the system call of the same name. If unimplemented, yields a fatal
3290 error. Returns TRUE if successful.
3292 Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call
3293 to C<pipe(Rdr, Wtr)> is essentially:
3296 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3297 shutdown(Rdr, 1); # no more writing for reader
3298 shutdown(Wtr, 0); # no more reading for writer
3300 See L<perlipc> for an example of socketpair use.
3302 =item sort SUBNAME LIST
3304 =item sort BLOCK LIST
3308 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3309 is omitted, C<sort()>s in standard string comparison order. If SUBNAME is
3310 specified, it gives the name of a subroutine that returns an integer
3311 less than, equal to, or greater than C<0>, depending on how the elements
3312 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3313 operators are extremely useful in such routines.) SUBNAME may be a
3314 scalar variable name (unsubscripted), in which case the value provides
3315 the name of (or a reference to) the actual subroutine to use. In place
3316 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3319 In the interests of efficiency the normal calling code for subroutines is
3320 bypassed, with the following effects: the subroutine may not be a
3321 recursive subroutine, and the two elements to be compared are passed into
3322 the subroutine not via C<@_> but as the package global variables C<$a> and
3323 C<$b> (see example below). They are passed by reference, so don't
3324 modify C<$a> and C<$b>. And don't try to declare them as lexicals either.
3326 You also cannot exit out of the sort block or subroutine using any of the
3327 loop control operators described in L<perlsyn> or with C<goto()>.
3329 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3330 current collation locale. See L<perllocale>.
3335 @articles = sort @files;
3337 # same thing, but with explicit sort routine
3338 @articles = sort {$a cmp $b} @files;
3340 # now case-insensitively
3341 @articles = sort {uc($a) cmp uc($b)} @files;
3343 # same thing in reversed order
3344 @articles = sort {$b cmp $a} @files;
3346 # sort numerically ascending
3347 @articles = sort {$a <=> $b} @files;
3349 # sort numerically descending
3350 @articles = sort {$b <=> $a} @files;
3352 # sort using explicit subroutine name
3354 $age{$a} <=> $age{$b}; # presuming numeric
3356 @sortedclass = sort byage @class;
3358 # this sorts the %age hash by value instead of key
3359 # using an in-line function
3360 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3362 sub backwards { $b cmp $a; }
3363 @harry = ('dog','cat','x','Cain','Abel');
3364 @george = ('gone','chased','yz','Punished','Axed');
3366 # prints AbelCaincatdogx
3367 print sort backwards @harry;
3368 # prints xdogcatCainAbel
3369 print sort @george, 'to', @harry;
3370 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3372 # inefficiently sort by descending numeric compare using
3373 # the first integer after the first = sign, or the
3374 # whole record case-insensitively otherwise
3377 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3382 # same thing, but much more efficiently;
3383 # we'll build auxiliary indices instead
3387 push @nums, /=(\d+)/;
3392 $nums[$b] <=> $nums[$a]
3394 $caps[$a] cmp $caps[$b]
3398 # same thing using a Schwartzian Transform (no temps)
3399 @new = map { $_->[0] }
3400 sort { $b->[1] <=> $a->[1]
3403 } map { [$_, /=(\d+)/, uc($_)] } @old;
3405 If you're using strict, you I<MUST NOT> declare C<$a>
3406 and C<$b> as lexicals. They are package globals. That means
3407 if you're in the C<main> package, it's
3409 @articles = sort {$main::b <=> $main::a} @files;
3413 @articles = sort {$::b <=> $::a} @files;
3415 but if you're in the C<FooPack> package, it's
3417 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3419 The comparison function is required to behave. If it returns
3420 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
3421 sometimes saying the opposite, for example) the results are not
3424 =item splice ARRAY,OFFSET,LENGTH,LIST
3426 =item splice ARRAY,OFFSET,LENGTH
3428 =item splice ARRAY,OFFSET
3430 Removes the elements designated by OFFSET and LENGTH from an array, and
3431 replaces them with the elements of LIST, if any. In list context,
3432 returns the elements removed from the array. In scalar context,
3433 returns the last element removed, or C<undef> if no elements are
3434 removed. The array grows or shrinks as necessary.
3435 If OFFSET is negative then it start that far from the end of the array.
3436 If LENGTH is omitted, removes everything from OFFSET onward.
3437 If LENGTH is negative, leave that many elements off the end of the array.
3438 The following equivalences hold (assuming C<$[ == 0>):
3440 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3441 pop(@a) splice(@a,-1)
3442 shift(@a) splice(@a,0,1)
3443 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3444 $a[$x] = $y splice(@a,$x,1,$y)
3446 Example, assuming array lengths are passed before arrays:
3448 sub aeq { # compare two list values
3449 my(@a) = splice(@_,0,shift);
3450 my(@b) = splice(@_,0,shift);
3451 return 0 unless @a == @b; # same len?
3453 return 0 if pop(@a) ne pop(@b);
3457 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3459 =item split /PATTERN/,EXPR,LIMIT
3461 =item split /PATTERN/,EXPR
3463 =item split /PATTERN/
3467 Splits a string into an array of strings, and returns it. By default,
3468 empty leading fields are preserved, and empty trailing ones are deleted.
3470 If not in list context, returns the number of fields found and splits into
3471 the C<@_> array. (In list context, you can force the split into C<@_> by
3472 using C<??> as the pattern delimiters, but it still returns the list
3473 value.) The use of implicit split to C<@_> is deprecated, however, because
3474 it clobbers your subroutine arguments.
3476 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
3477 splits on whitespace (after skipping any leading whitespace). Anything
3478 matching PATTERN is taken to be a delimiter separating the fields. (Note
3479 that the delimiter may be longer than one character.)
3481 If LIMIT is specified and positive, splits into no more than that
3482 many fields (though it may split into fewer). If LIMIT is unspecified
3483 or zero, trailing null fields are stripped (which potential users
3484 of C<pop()> would do well to remember). If LIMIT is negative, it is
3485 treated as if an arbitrarily large LIMIT had been specified.
3487 A pattern matching the null string (not to be confused with
3488 a null pattern C<//>, which is just one member of the set of patterns
3489 matching a null string) will split the value of EXPR into separate
3490 characters at each point it matches that way. For example:
3492 print join(':', split(/ */, 'hi there'));
3494 produces the output 'h:i:t:h:e:r:e'.
3496 The LIMIT parameter can be used to split a line partially
3498 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3500 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3501 one larger than the number of variables in the list, to avoid
3502 unnecessary work. For the list above LIMIT would have been 4 by
3503 default. In time critical applications it behooves you not to split
3504 into more fields than you really need.
3506 If the PATTERN contains parentheses, additional array elements are
3507 created from each matching substring in the delimiter.
3509 split(/([,-])/, "1-10,20", 3);
3511 produces the list value
3513 (1, '-', 10, ',', 20)
3515 If you had the entire header of a normal Unix email message in C<$header>,
3516 you could split it up into fields and their values this way:
3518 $header =~ s/\n\s+/ /g; # fix continuation lines
3519 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3521 The pattern C</PATTERN/> may be replaced with an expression to specify
3522 patterns that vary at runtime. (To do runtime compilation only once,
3523 use C</$variable/o>.)
3525 As a special case, specifying a PATTERN of space (C<' '>) will split on
3526 white space just as C<split()> with no arguments does. Thus, C<split(' ')> can
3527 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3528 will give you as many null initial fields as there are leading spaces.
3529 A C<split()> on C</\s+/> is like a C<split(' ')> except that any leading
3530 whitespace produces a null first field. A C<split()> with no arguments
3531 really does a C<split(' ', $_)> internally.
3535 open(PASSWD, '/etc/passwd');
3537 ($login, $passwd, $uid, $gid,
3538 $gcos, $home, $shell) = split(/:/);
3542 (Note that C<$shell> above will still have a newline on it. See L</chop>,
3543 L</chomp>, and L</join>.)
3545 =item sprintf FORMAT, LIST
3547 Returns a string formatted by the usual C<printf()> conventions of the
3548 C library function C<sprintf()>. See L<sprintf(3)> or L<printf(3)>
3549 on your system for an explanation of the general principles.
3551 Perl does its own C<sprintf()> formatting -- it emulates the C
3552 function C<sprintf()>, but it doesn't use it (except for floating-point
3553 numbers, and even then only the standard modifiers are allowed). As a
3554 result, any non-standard extensions in your local C<sprintf()> are not
3555 available from Perl.
3557 Perl's C<sprintf()> permits the following universally-known conversions:
3560 %c a character with the given number
3562 %d a signed integer, in decimal
3563 %u an unsigned integer, in decimal
3564 %o an unsigned integer, in octal
3565 %x an unsigned integer, in hexadecimal
3566 %e a floating-point number, in scientific notation
3567 %f a floating-point number, in fixed decimal notation
3568 %g a floating-point number, in %e or %f notation
3570 In addition, Perl permits the following widely-supported conversions:
3572 %X like %x, but using upper-case letters
3573 %E like %e, but using an upper-case "E"
3574 %G like %g, but with an upper-case "E" (if applicable)
3575 %p a pointer (outputs the Perl value's address in hexadecimal)
3576 %n special: *stores* the number of characters output so far
3577 into the next variable in the parameter list
3579 Finally, for backward (and we do mean "backward") compatibility, Perl
3580 permits these unnecessary but widely-supported conversions:
3583 %D a synonym for %ld
3584 %U a synonym for %lu
3585 %O a synonym for %lo
3588 Perl permits the following universally-known flags between the C<%>
3589 and the conversion letter:
3591 space prefix positive number with a space
3592 + prefix positive number with a plus sign
3593 - left-justify within the field
3594 0 use zeros, not spaces, to right-justify
3595 # prefix non-zero octal with "0", non-zero hex with "0x"
3596 number minimum field width
3597 .number "precision": digits after decimal point for
3598 floating-point, max length for string, minimum length
3600 l interpret integer as C type "long" or "unsigned long"
3601 h interpret integer as C type "short" or "unsigned short"
3603 There is also one Perl-specific flag:
3605 V interpret integer as Perl's standard integer type
3607 Where a number would appear in the flags, an asterisk ("C<*>") may be
3608 used instead, in which case Perl uses the next item in the parameter
3609 list as the given number (that is, as the field width or precision).
3610 If a field width obtained through "C<*>" is negative, it has the same
3611 effect as the "C<->" flag: left-justification.
3613 If C<use locale> is in effect, the character used for the decimal
3614 point in formatted real numbers is affected by the LC_NUMERIC locale.
3621 Return the square root of EXPR. If EXPR is omitted, returns square
3628 Sets the random number seed for the C<rand()> operator. If EXPR is
3629 omitted, uses a semi-random value supplied by the kernel (if it supports
3630 the F</dev/urandom> device) or based on the current time and process
3631 ID, among other things. In versions of Perl prior to 5.004 the default
3632 seed was just the current C<time()>. This isn't a particularly good seed,
3633 so many old programs supply their own seed value (often C<time ^ $$> or
3634 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
3636 In fact, it's usually not necessary to call C<srand()> at all, because if
3637 it is not called explicitly, it is called implicitly at the first use of
3638 the C<rand()> operator. However, this was not the case in version of Perl
3639 before 5.004, so if your script will run under older Perl versions, it
3640 should call C<srand()>.
3642 Note that you need something much more random than the default seed for
3643 cryptographic purposes. Checksumming the compressed output of one or more
3644 rapidly changing operating system status programs is the usual method. For
3647 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3649 If you're particularly concerned with this, see the C<Math::TrulyRandom>
3652 Do I<not> call C<srand()> multiple times in your program unless you know
3653 exactly what you're doing and why you're doing it. The point of the
3654 function is to "seed" the C<rand()> function so that C<rand()> can produce
3655 a different sequence each time you run your program. Just do it once at the
3656 top of your program, or you I<won't> get random numbers out of C<rand()>!
3658 Frequently called programs (like CGI scripts) that simply use
3662 for a seed can fall prey to the mathematical property that
3666 one-third of the time. So don't do that.
3668 =item stat FILEHANDLE
3674 Returns a 13-element list giving the status info for a file, either
3675 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
3676 it stats C<$_>. Returns a null list if the stat fails. Typically used
3679 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3680 $atime,$mtime,$ctime,$blksize,$blocks)
3683 Not all fields are supported on all filesystem types. Here are the
3684 meaning of the fields:
3686 0 dev device number of filesystem
3688 2 mode file mode (type and permissions)
3689 3 nlink number of (hard) links to the file
3690 4 uid numeric user ID of file's owner
3691 5 gid numeric group ID of file's owner
3692 6 rdev the device identifier (special files only)
3693 7 size total size of file, in bytes
3694 8 atime last access time since the epoch
3695 9 mtime last modify time since the epoch
3696 10 ctime inode change time (NOT creation time!) since the epoch
3697 11 blksize preferred block size for file system I/O
3698 12 blocks actual number of blocks allocated
3700 (The epoch was at 00:00 January 1, 1970 GMT.)
3702 If stat is passed the special filehandle consisting of an underline, no
3703 stat is done, but the current contents of the stat structure from the
3704 last stat or filetest are returned. Example:
3706 if (-x $file && (($d) = stat(_)) && $d < 0) {
3707 print "$file is executable NFS file\n";
3710 (This works on machines only for which the device number is negative under NFS.)
3712 In scalar context, C<stat()> returns a boolean value indicating success
3713 or failure, and, if successful, sets the information associated with
3714 the special filehandle C<_>.
3720 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3721 doing many pattern matches on the string before it is next modified.
3722 This may or may not save time, depending on the nature and number of
3723 patterns you are searching on, and on the distribution of character
3724 frequencies in the string to be searched -- you probably want to compare
3725 run times with and without it to see which runs faster. Those loops
3726 which scan for many short constant strings (including the constant
3727 parts of more complex patterns) will benefit most. You may have only
3728 one C<study()> active at a time -- if you study a different scalar the first
3729 is "unstudied". (The way C<study()> works is this: a linked list of every
3730 character in the string to be searched is made, so we know, for
3731 example, where all the C<'k'> characters are. From each search string,
3732 the rarest character is selected, based on some static frequency tables
3733 constructed from some C programs and English text. Only those places
3734 that contain this "rarest" character are examined.)
3736 For example, here is a loop that inserts index producing entries
3737 before any line containing a certain pattern:
3741 print ".IX foo\n" if /\bfoo\b/;
3742 print ".IX bar\n" if /\bbar\b/;
3743 print ".IX blurfl\n" if /\bblurfl\b/;
3748 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
3749 will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
3750 a big win except in pathological cases. The only question is whether
3751 it saves you more time than it took to build the linked list in the
3754 Note that if you have to look for strings that you don't know till
3755 runtime, you can build an entire loop as a string and C<eval()> that to
3756 avoid recompiling all your patterns all the time. Together with
3757 undefining C<$/> to input entire files as one record, this can be very
3758 fast, often faster than specialized programs like fgrep(1). The following
3759 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3760 out the names of those files that contain a match:
3762 $search = 'while (<>) { study;';
3763 foreach $word (@words) {
3764 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3769 eval $search; # this screams
3770 $/ = "\n"; # put back to normal input delimiter
3771 foreach $file (sort keys(%seen)) {
3779 =item sub NAME BLOCK
3781 This is subroutine definition, not a real function I<per se>. With just a
3782 NAME (and possibly prototypes), it's just a forward declaration. Without
3783 a NAME, it's an anonymous function declaration, and does actually return a
3784 value: the CODE ref of the closure you just created. See L<perlsub> and
3785 L<perlref> for details.
3787 =item substr EXPR,OFFSET,LEN,REPLACEMENT
3789 =item substr EXPR,OFFSET,LEN
3791 =item substr EXPR,OFFSET
3793 Extracts a substring out of EXPR and returns it. First character is at
3794 offset C<0>, or whatever you've set C<$[> to (but don't do that).
3795 If OFFSET is negative (or more precisely, less than C<$[>), starts
3796 that far from the end of the string. If LEN is omitted, returns
3797 everything to the end of the string. If LEN is negative, leaves that
3798 many characters off the end of the string.
3800 If you specify a substring that is partly outside the string, the part
3801 within the string is returned. If the substring is totally outside
3802 the string a warning is produced.
3804 You can use the C<substr()> function
3805 as an lvalue, in which case EXPR must be an lvalue. If you assign
3806 something shorter than LEN, the string will shrink, and if you assign
3807 something longer than LEN, the string will grow to accommodate it. To
3808 keep the string the same length you may need to pad or chop your value
3811 An alternative to using C<substr()> as an lvalue is to specify the
3812 replacement string as the 4th argument. This allows you to replace
3813 parts of the EXPR and return what was there before in one operation.
3815 =item symlink OLDFILE,NEWFILE
3817 Creates a new filename symbolically linked to the old filename.
3818 Returns C<1> for success, C<0> otherwise. On systems that don't support
3819 symbolic links, produces a fatal error at run time. To check for that,
3822 $symlink_exists = eval { symlink("",""); 1 };
3826 Calls the system call specified as the first element of the list,
3827 passing the remaining elements as arguments to the system call. If
3828 unimplemented, produces a fatal error. The arguments are interpreted
3829 as follows: if a given argument is numeric, the argument is passed as
3830 an int. If not, the pointer to the string value is passed. You are
3831 responsible to make sure a string is pre-extended long enough to
3832 receive any result that might be written into a string. You can't use a
3833 string literal (or other read-only string) as an argument to C<syscall()>
3834 because Perl has to assume that any string pointer might be written
3836 integer arguments are not literals and have never been interpreted in a
3837 numeric context, you may need to add C<0> to them to force them to look
3838 like numbers. This emulates the C<syswrite()> function (or vice versa):
3840 require 'syscall.ph'; # may need to run h2ph
3842 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
3844 Note that Perl supports passing of up to only 14 arguments to your system call,
3845 which in practice should usually suffice.
3847 Syscall returns whatever value returned by the system call it calls.
3848 If the system call fails, C<syscall()> returns C<-1> and sets C<$!> (errno).
3849 Note that some system calls can legitimately return C<-1>. The proper
3850 way to handle such calls is to assign C<$!=0;> before the call and
3851 check the value of C<$!> if syscall returns C<-1>.
3853 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
3854 number of the read end of the pipe it creates. There is no way
3855 to retrieve the file number of the other end. You can avoid this
3856 problem by using C<pipe()> instead.
3858 =item sysopen FILEHANDLE,FILENAME,MODE
3860 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3862 Opens the file whose filename is given by FILENAME, and associates it
3863 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3864 the name of the real filehandle wanted. This function calls the
3865 underlying operating system's C<open()> function with the parameters
3866 FILENAME, MODE, PERMS.
3868 The possible values and flag bits of the MODE parameter are
3869 system-dependent; they are available via the standard module C<Fcntl>.
3870 For historical reasons, some values work on almost every system
3871 supported by perl: zero means read-only, one means write-only, and two
3872 means read/write. We know that these values do I<not> work under
3873 OS/390 Unix and on the Macintosh; you probably don't want to use them
3876 If the file named by FILENAME does not exist and the C<open()> call creates
3877 it (typically because MODE includes the C<O_CREAT> flag), then the value of
3878 PERMS specifies the permissions of the newly created file. If you omit
3879 the PERMS argument to C<sysopen()>, Perl uses the octal value C<0666>.
3880 These permission values need to be in octal, and are modified by your
3881 process's current C<umask>.
3883 Seldom if ever use C<0644> as argument to C<sysopen()> because that
3884 takes away the user's option to have a more permissive umask. Better
3885 to omit it. See the perlfunc(1) entry on C<umask> for more on this.
3887 The C<IO::File> module provides a more object-oriented approach, if you're
3888 into that kind of thing.
3890 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3892 =item sysread FILEHANDLE,SCALAR,LENGTH
3894 Attempts to read LENGTH bytes of data into variable SCALAR from the
3895 specified FILEHANDLE, using the system call read(2). It bypasses
3896 stdio, so mixing this with other kinds of reads, C<print()>, C<write()>,
3897 C<seek()>, or C<tell()> can cause confusion because stdio usually buffers
3898 data. Returns the number of bytes actually read, C<0> at end of file,
3899 or undef if there was an error. SCALAR will be grown or shrunk so that
3900 the last byte actually read is the last byte of the scalar after the read.
3902 An OFFSET may be specified to place the read data at some place in the
3903 string other than the beginning. A negative OFFSET specifies
3904 placement at that many bytes counting backwards from the end of the
3905 string. A positive OFFSET greater than the length of SCALAR results
3906 in the string being padded to the required size with C<"\0"> bytes before
3907 the result of the read is appended.
3909 =item sysseek FILEHANDLE,POSITION,WHENCE
3911 Sets FILEHANDLE's system position using the system call lseek(2). It
3912 bypasses stdio, so mixing this with reads (other than C<sysread()>),
3913 C<print()>, C<write()>, C<seek()>, or C<tell()> may cause confusion. FILEHANDLE may
3914 be an expression whose value gives the name of the filehandle. The
3915 values for WHENCE are C<0> to set the new position to POSITION, C<1> to set
3916 the it to the current position plus POSITION, and C<2> to set it to EOF
3917 plus POSITION (typically negative). For WHENCE, you may use the
3918 constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the C<IO::Seekable>
3919 or the POSIX module.
3921 Returns the new position, or the undefined value on failure. A position
3922 of zero is returned as the string "C<0> but true"; thus C<sysseek()> returns
3923 TRUE on success and FALSE on failure, yet you can still easily determine
3928 =item system PROGRAM LIST
3930 Does exactly the same thing as "C<exec LIST>" except that a fork is done
3931 first, and the parent process waits for the child process to complete.
3932 Note that argument processing varies depending on the number of
3933 arguments. If there is more than one argument in LIST, or if LIST is
3934 an array with more than one value, starts the program given by the
3935 first element of the list with arguments given by the rest of the list.
3936 If there is only one scalar argument, the argument is
3937 checked for shell metacharacters, and if there are any, the entire
3938 argument is passed to the system's command shell for parsing (this is
3939 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
3940 there are no shell metacharacters in the argument, it is split into
3941 words and passed directly to C<execvp()>, which is more efficient.
3943 The return value is the exit status of the program as
3944 returned by the C<wait()> call. To get the actual exit value divide by
3945 256. See also L</exec>. This is I<NOT> what you want to use to capture
3946 the output from a command, for that you should use merely backticks or
3947 C<qx//>, as described in L<perlop/"`STRING`">.
3949 Like C<exec()>, C<system()> allows you to lie to a program about its name if
3950 you use the "C<system PROGRAM LIST>" syntax. Again, see L</exec>.
3952 Because C<system()> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
3953 program they're running doesn't actually interrupt your program.
3955 @args = ("command", "arg1", "arg2");
3957 or die "system @args failed: $?"
3959 You can check all the failure possibilities by inspecting
3962 $exit_value = $? >> 8;
3963 $signal_num = $? & 127;
3964 $dumped_core = $? & 128;
3966 When the arguments get executed via the system shell, results
3967 and return codes will be subject to its quirks and capabilities.
3968 See L<perlop/"`STRING`"> and L</exec> for details.
3970 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
3972 =item syswrite FILEHANDLE,SCALAR,LENGTH
3974 Attempts to write LENGTH bytes of data from variable SCALAR to the
3975 specified FILEHANDLE, using the system call write(2). It bypasses
3976 stdio, so mixing this with reads (other than C<sysread())>, C<print()>,
3977 C<write()>, C<seek()>, or C<tell()> may cause confusion because stdio usually
3978 buffers data. Returns the number of bytes actually written, or C<undef>
3979 if there was an error. If the LENGTH is greater than the available
3980 data in the SCALAR after the OFFSET, only as much data as is available
3983 An OFFSET may be specified to write the data from some part of the
3984 string other than the beginning. A negative OFFSET specifies writing
3985 that many bytes counting backwards from the end of the string. In the
3986 case the SCALAR is empty you can use OFFSET but only zero offset.
3988 =item tell FILEHANDLE
3992 Returns the current position for FILEHANDLE. FILEHANDLE may be an
3993 expression whose value gives the name of the actual filehandle. If
3994 FILEHANDLE is omitted, assumes the file last read.
3996 =item telldir DIRHANDLE
3998 Returns the current position of the C<readdir()> routines on DIRHANDLE.
3999 Value may be given to C<seekdir()> to access a particular location in a
4000 directory. Has the same caveats about possible directory compaction as
4001 the corresponding system library routine.
4003 =item tie VARIABLE,CLASSNAME,LIST
4005 This function binds a variable to a package class that will provide the
4006 implementation for the variable. VARIABLE is the name of the variable
4007 to be enchanted. CLASSNAME is the name of a class implementing objects
4008 of correct type. Any additional arguments are passed to the "C<new()>"
4009 method of the class (meaning C<TIESCALAR>, C<TIEARRAY>, or C<TIEHASH>).
4010 Typically these are arguments such as might be passed to the C<dbm_open()>
4011 function of C. The object returned by the "C<new()>" method is also
4012 returned by the C<tie()> function, which would be useful if you want to
4013 access other methods in CLASSNAME.
4015 Note that functions such as C<keys()> and C<values()> may return huge lists
4016 when used on large objects, like DBM files. You may prefer to use the
4017 C<each()> function to iterate over such. Example:
4019 # print out history file offsets
4021 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4022 while (($key,$val) = each %HIST) {
4023 print $key, ' = ', unpack('L',$val), "\n";
4027 A class implementing a hash should have the following methods:
4029 TIEHASH classname, LIST
4032 STORE this, key, value
4036 NEXTKEY this, lastkey
4038 A class implementing an ordinary array should have the following methods:
4040 TIEARRAY classname, LIST
4043 STORE this, key, value
4046 A class implementing a scalar should have the following methods:
4048 TIESCALAR classname, LIST
4053 Unlike C<dbmopen()>, the C<tie()> function will not use or require a module
4054 for you--you need to do that explicitly yourself. See L<DB_File>
4055 or the F<Config> module for interesting C<tie()> implementations.
4057 For further details see L<perltie>, L<tied VARIABLE>.
4061 Returns a reference to the object underlying VARIABLE (the same value
4062 that was originally returned by the C<tie()> call that bound the variable
4063 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4068 Returns the number of non-leap seconds since whatever time the system
4069 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4070 and 00:00:00 UTC, January 1, 1970 for most other systems).
4071 Suitable for feeding to C<gmtime()> and C<localtime()>.
4075 Returns a four-element list giving the user and system times, in
4076 seconds, for this process and the children of this process.
4078 ($user,$system,$cuser,$csystem) = times;
4082 The transliteration operator. Same as C<y///>. See L<perlop>.
4084 =item truncate FILEHANDLE,LENGTH
4086 =item truncate EXPR,LENGTH
4088 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4089 specified length. Produces a fatal error if truncate isn't implemented
4090 on your system. Returns TRUE if successful, the undefined value
4097 Returns an uppercased version of EXPR. This is the internal function
4098 implementing the C<\U> escape in double-quoted strings.
4099 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4100 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
4101 does not attempt to do titlecase mapping on initial letters. See C<ucfirst()> for that.)
4103 If EXPR is omitted, uses C<$_>.
4109 Returns the value of EXPR with the first character
4110 in uppercase (titlecase in Unicode). This is
4111 the internal function implementing the C<\u> escape in double-quoted strings.
4112 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4114 If EXPR is omitted, uses C<$_>.
4120 Sets the umask for the process to EXPR and returns the previous value.
4121 If EXPR is omitted, merely returns the current umask.
4123 The Unix permission C<rwxr-x---> is represented as three sets of three
4124 bits, or three octal digits: C<0750> (the leading 0 indicates octal
4125 and isn't one of the the digits). The C<umask> value is such a number
4126 representing disabled permissions bits. The permission (or "mode")
4127 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
4128 even if you tell C<sysopen> to create a file with permissions C<0777>,
4129 if your umask is C<0022> then the file will actually be created with
4130 permissions C<0755>. If your C<umask> were C<0027> (group can't
4131 write; others can't read, write, or execute), then passing
4132 C<sysopen()> C<0666> would create a file with mode C<0640> (C<0666 &~
4135 Here's some advice: supply a creation mode of C<0666> for regular
4136 files (in C<sysopen()>) and one of C<0777> for directories (in
4137 C<mkdir()>) and executable files. This gives users the freedom of
4138 choice: if they want protected files, they might choose process umasks
4139 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
4140 Programs should rarely if ever make policy decisions better left to
4141 the user. The exception to this is when writing files that should be
4142 kept private: mail files, web browser cookies, I<.rhosts> files, and
4145 If umask(2) is not implemented on your system and you are trying to
4146 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
4147 fatal error at run time. If umask(2) is not implemented and you are
4148 not trying to restrict access for yourself, returns C<undef>.
4150 Remember that a umask is a number, usually given in octal; it is I<not> a
4151 string of octal digits. See also L</oct>, if all you have is a string.
4159 Undefines the value of EXPR, which must be an lvalue. Use only on a
4160 scalar value, an array (using "C<@>"), a hash (using "C<%>"), a subroutine
4161 (using "C<&>"), or a typeglob (using "<*>"). (Saying C<undef $hash{$key}>
4162 will probably not do what you expect on most predefined variables or
4163 DBM list values, so don't do that; see L<delete>.) Always returns the
4164 undefined value. You can omit the EXPR, in which case nothing is
4165 undefined, but you still get an undefined value that you could, for
4166 instance, return from a subroutine, assign to a variable or pass as a
4167 parameter. Examples:
4170 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4174 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4175 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4176 select undef, undef, undef, 0.25;
4177 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4179 Note that this is a unary operator, not a list operator.
4185 Deletes a list of files. Returns the number of files successfully
4188 $cnt = unlink 'a', 'b', 'c';
4192 Note: C<unlink()> will not delete directories unless you are superuser and
4193 the B<-U> flag is supplied to Perl. Even if these conditions are
4194 met, be warned that unlinking a directory can inflict damage on your
4195 filesystem. Use C<rmdir()> instead.
4197 If LIST is omitted, uses C<$_>.
4199 =item unpack TEMPLATE,EXPR
4201 C<Unpack()> does the reverse of C<pack()>: it takes a string representing a
4202 structure and expands it out into a list value, returning the array
4203 value. (In scalar context, it returns merely the first value
4204 produced.) The TEMPLATE has the same format as in the C<pack()> function.
4205 Here's a subroutine that does substring:
4208 my($what,$where,$howmuch) = @_;
4209 unpack("x$where a$howmuch", $what);
4214 sub ordinal { unpack("c",$_[0]); } # same as ord()
4216 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4217 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4218 themselves. Default is a 16-bit checksum. For example, the following
4219 computes the same number as the System V sum program:
4222 $checksum += unpack("%16C*", $_);
4226 The following efficiently counts the number of set bits in a bit vector:
4228 $setbits = unpack("%32b*", $selectmask);
4230 =item untie VARIABLE
4232 Breaks the binding between a variable and a package. (See C<tie()>.)
4234 =item unshift ARRAY,LIST
4236 Does the opposite of a C<shift()>. Or the opposite of a C<push()>,
4237 depending on how you look at it. Prepends list to the front of the
4238 array, and returns the new number of elements in the array.
4240 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4242 Note the LIST is prepended whole, not one element at a time, so the
4243 prepended elements stay in the same order. Use C<reverse()> to do the
4246 =item use Module LIST
4250 =item use Module VERSION LIST
4254 Imports some semantics into the current package from the named module,
4255 generally by aliasing certain subroutine or variable names into your
4256 package. It is exactly equivalent to
4258 BEGIN { require Module; import Module LIST; }
4260 except that Module I<must> be a bareword.
4262 If the first argument to C<use> is a number, it is treated as a version
4263 number instead of a module name. If the version of the Perl interpreter
4264 is less than VERSION, then an error message is printed and Perl exits
4265 immediately. This is often useful if you need to check the current
4266 Perl version before C<use>ing library modules that have changed in
4267 incompatible ways from older versions of Perl. (We try not to do
4268 this more than we have to.)
4270 The C<BEGIN> forces the C<require> and C<import()> to happen at compile time. The
4271 C<require> makes sure the module is loaded into memory if it hasn't been
4272 yet. The C<import()> is not a builtin--it's just an ordinary static method
4273 call into the "C<Module>" package to tell the module to import the list of
4274 features back into the current package. The module can implement its
4275 C<import()> method any way it likes, though most modules just choose to
4276 derive their C<import()> method via inheritance from the C<Exporter> class that
4277 is defined in the C<Exporter> module. See L<Exporter>. If no C<import()>
4278 method can be found then the error is currently silently ignored. This
4279 may change to a fatal error in a future version.
4281 If you don't want your namespace altered, explicitly supply an empty list:
4285 That is exactly equivalent to
4287 BEGIN { require Module }
4289 If the VERSION argument is present between Module and LIST, then the
4290 C<use> will call the VERSION method in class Module with the given
4291 version as an argument. The default VERSION method, inherited from
4292 the Universal class, croaks if the given version is larger than the
4293 value of the variable C<$Module::VERSION>. (Note that there is not a
4294 comma after VERSION!)
4296 Because this is a wide-open interface, pragmas (compiler directives)
4297 are also implemented this way. Currently implemented pragmas are:
4301 use sigtrap qw(SEGV BUS);
4302 use strict qw(subs vars refs);
4303 use subs qw(afunc blurfl);
4305 Some of these these pseudo-modules import semantics into the current
4306 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4307 which import symbols into the current package (which are effective
4308 through the end of the file).
4310 There's a corresponding "C<no>" command that unimports meanings imported
4311 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import()>.
4316 If no C<unimport()> method can be found the call fails with a fatal error.
4318 See L<perlmod> for a list of standard modules and pragmas.
4322 Changes the access and modification times on each file of a list of
4323 files. The first two elements of the list must be the NUMERICAL access
4324 and modification times, in that order. Returns the number of files
4325 successfully changed. The inode modification time of each file is set
4326 to the current time. This code has the same effect as the "C<touch>"
4327 command if the files already exist:
4331 utime $now, $now, @ARGV;
4335 Returns a list consisting of all the values of the named hash. (In a
4336 scalar context, returns the number of values.) The values are
4337 returned in an apparently random order, but it is the same order as
4338 either the C<keys()> or C<each()> function would produce on the same hash.
4339 As a side effect, it resets HASH's iterator. See also C<keys()>, C<each()>,
4342 =item vec EXPR,OFFSET,BITS
4344 Treats the string in EXPR as a vector of unsigned integers, and
4345 returns the value of the bit field specified by OFFSET. BITS specifies
4346 the number of bits that are reserved for each entry in the bit
4347 vector. This must be a power of two from 1 to 32. C<vec()> may also be
4348 assigned to, in which case parentheses are needed to give the expression
4349 the correct precedence as in
4351 vec($image, $max_x * $x + $y, 8) = 3;
4353 Vectors created with C<vec()> can also be manipulated with the logical
4354 operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
4355 desired when both operands are strings.
4357 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
4358 The comments show the string after each step. Note that this code works
4359 in the same way on big-endian or little-endian machines.
4362 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
4363 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
4364 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
4365 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
4366 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
4367 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
4368 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
4370 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
4371 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
4372 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
4375 To transform a bit vector into a string or array of 0's and 1's, use these:
4377 $bits = unpack("b*", $vector);
4378 @bits = split(//, unpack("b*", $vector));
4380 If you know the exact length in bits, it can be used in place of the C<*>.
4384 Waits for a child process to terminate and returns the pid of the
4385 deceased process, or C<-1> if there are no child processes. The status is
4388 =item waitpid PID,FLAGS
4390 Waits for a particular child process to terminate and returns the pid
4391 of the deceased process, or C<-1> if there is no such child process. The
4392 status is returned in C<$?>. If you say
4394 use POSIX ":sys_wait_h";
4396 waitpid(-1,&WNOHANG);
4398 then you can do a non-blocking wait for any process. Non-blocking wait
4399 is available on machines supporting either the waitpid(2) or
4400 wait4(2) system calls. However, waiting for a particular pid with
4401 FLAGS of C<0> is implemented everywhere. (Perl emulates the system call
4402 by remembering the status values of processes that have exited but have
4403 not been harvested by the Perl script yet.)
4405 See L<perlipc> for other examples.
4409 Returns TRUE if the context of the currently executing subroutine is
4410 looking for a list value. Returns FALSE if the context is looking
4411 for a scalar. Returns the undefined value if the context is looking
4412 for no value (void context).
4414 return unless defined wantarray; # don't bother doing more
4415 my @a = complex_calculation();
4416 return wantarray ? @a : "@a";
4420 Produces a message on STDERR just like C<die()>, but doesn't exit or throw
4423 If LIST is empty and C<$@> already contains a value (typically from a
4424 previous eval) that value is used after appending C<"\t...caught">
4425 to C<$@>. This is useful for staying almost, but not entirely similar to
4428 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
4430 No message is printed if there is a C<$SIG{__WARN__}> handler
4431 installed. It is the handler's responsibility to deal with the message
4432 as it sees fit (like, for instance, converting it into a C<die()>). Most
4433 handlers must therefore make arrangements to actually display the
4434 warnings that they are not prepared to deal with, by calling C<warn()>
4435 again in the handler. Note that this is quite safe and will not
4436 produce an endless loop, since C<__WARN__> hooks are not called from
4439 You will find this behavior is slightly different from that of
4440 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4441 instead call C<die()> again to change it).
4443 Using a C<__WARN__> handler provides a powerful way to silence all
4444 warnings (even the so-called mandatory ones). An example:
4446 # wipe out *all* compile-time warnings
4447 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4449 my $foo = 20; # no warning about duplicate my $foo,
4450 # but hey, you asked for it!
4451 # no compile-time or run-time warnings before here
4454 # run-time warnings enabled after here
4455 warn "\$foo is alive and $foo!"; # does show up
4457 See L<perlvar> for details on setting C<%SIG> entries, and for more
4460 =item write FILEHANDLE
4466 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
4467 using the format associated with that file. By default the format for
4468 a file is the one having the same name as the filehandle, but the
4469 format for the current output channel (see the C<select()> function) may be set
4470 explicitly by assigning the name of the format to the C<$~> variable.
4472 Top of form processing is handled automatically: if there is
4473 insufficient room on the current page for the formatted record, the
4474 page is advanced by writing a form feed, a special top-of-page format
4475 is used to format the new page header, and then the record is written.
4476 By default the top-of-page format is the name of the filehandle with
4477 "_TOP" appended, but it may be dynamically set to the format of your
4478 choice by assigning the name to the C<$^> variable while the filehandle is
4479 selected. The number of lines remaining on the current page is in
4480 variable C<$->, which can be set to C<0> to force a new page.
4482 If FILEHANDLE is unspecified, output goes to the current default output
4483 channel, which starts out as STDOUT but may be changed by the
4484 C<select()> operator. If the FILEHANDLE is an EXPR, then the expression
4485 is evaluated and the resulting string is used to look up the name of
4486 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4488 Note that write is I<NOT> the opposite of C<read()>. Unfortunately.
4492 The transliteration operator. Same as C<tr///>. See L<perlop>.