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2=head1 NAME
3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
15operator. A unary operator generally provides a scalar context to its
16argument, while a list operator may provide either scalar and list
17contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 18be first, and the list argument will follow. (Note that there can ever
19be only one list argument.) For instance, splice() has three scalar
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20arguments followed by a list.
21
22In the syntax descriptions that follow, list operators that expect a
23list (and provide list context for the elements of the list) are shown
24with LIST as an argument. Such a list may consist of any combination
25of scalar arguments or list values; the list values will be included
26in the list as if each individual element were interpolated at that
27point in the list, forming a longer single-dimensional list value.
28Elements of the LIST should be separated by commas.
29
30Any function in the list below may be used either with or without
31parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 32parentheses.) If you use the parentheses, the simple (but occasionally
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33surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
34function, and precedence doesn't matter. Otherwise it's a list
35operator or unary operator, and precedence does matter. And whitespace
36between the function and left parenthesis doesn't count--so you need to
37be careful sometimes:
38
68dc0745 39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
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44
45If you run Perl with the B<-w> switch it can warn you about this. For
46example, the third line above produces:
47
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
50
51For functions that can be used in either a scalar or list context,
54310121 52nonabortive failure is generally indicated in a scalar context by
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53returning the undefined value, and in a list context by returning the
54null list.
55
56Remember the following rule:
57
cb1a09d0 58=over 8
a0d0e21e 59
8ebc5c01 60=item I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>
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61
62=back
63
64Each operator and function decides which sort of value it would be most
65appropriate to return in a scalar context. Some operators return the
66length of the list that would have been returned in a list context. Some
67operators return the first value in the list. Some operators return the
68last value in the list. Some operators return a count of successful
69operations. In general, they do what you want, unless you want
70consistency.
71
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72=head2 Perl Functions by Category
73
74Here are Perl's functions (including things that look like
75functions, like some of the keywords and named operators)
76arranged by category. Some functions appear in more
77than one place.
78
79=over
80
81=item Functions for SCALARs or strings
82
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83C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
84C<length>, C<oct>, C<ord>, C<pack>, C<q>/STRING/, C<qq>/STRING/, C<reverse>,
85C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y>///
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86
87=item Regular expressions and pattern matching
88
22fae026 89C<m>//, C<pos>, C<quotemeta>, C<s>///, C<split>, C<study>
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90
91=item Numeric functions
92
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93C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
94C<sin>, C<sqrt>, C<srand>
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95
96=item Functions for real @ARRAYs
97
22fae026 98C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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99
100=item Functions for list data
101
22fae026 102C<grep>, C<join>, C<map>, C<qw>/STRING/, C<reverse>, C<sort>, C<unpack>
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103
104=item Functions for real %HASHes
105
22fae026 106C<delete>, C<each>, C<exists>, C<keys>, C<values>
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107
108=item Input and output functions
109
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110C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
111C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
112C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
113C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
114C<warn>, C<write>
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115
116=item Functions for fixed length data or records
117
22fae026 118C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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119
120=item Functions for filehandles, files, or directories
121
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122C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
123C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, C<readlink>,
124C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, C<unlink>, C<utime>
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125
126=item Keywords related to the control flow of your perl program
127
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128C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
129C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 130
54310121 131=item Keywords related to scoping
cb1a09d0 132
22fae026 133C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
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134
135=item Miscellaneous functions
136
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137C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
138C<scalar>, C<undef>, C<wantarray>
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139
140=item Functions for processes and process groups
141
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142C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
143C<pipe>, C<qx>/STRING/, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
144C<times>, C<wait>, C<waitpid>
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145
146=item Keywords related to perl modules
147
22fae026 148C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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149
150=item Keywords related to classes and object-orientedness
151
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152C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
153C<untie>, C<use>
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154
155=item Low-level socket functions
156
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157C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
158C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
159C<socket>, C<socketpair>
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160
161=item System V interprocess communication functions
162
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163C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
164C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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165
166=item Fetching user and group info
167
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168C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
169C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
170C<getpwuid>, C<setgrent>, C<setpwent>
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171
172=item Fetching network info
173
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174C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
175C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
176C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
177C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
178C<setnetent>, C<setprotoent>, C<setservent>
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179
180=item Time-related functions
181
22fae026 182C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 183
37798a01 184=item Functions new in perl5
185
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186C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
187C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
188C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
189C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
37798a01 190
191* - C<sub> was a keyword in perl4, but in perl5 it is an
192operator which can be used in expressions.
193
194=item Functions obsoleted in perl5
195
22fae026 196C<dbmclose>, C<dbmopen>
37798a01 197
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198=back
199
200=head2 Alphabetical Listing of Perl Functions
201
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202=over 8
203
22fae026 204=item I<-X> FILEHANDLE
a0d0e21e 205
22fae026 206=item I<-X> EXPR
a0d0e21e 207
22fae026 208=item I<-X>
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209
210A file test, where X is one of the letters listed below. This unary
211operator takes one argument, either a filename or a filehandle, and
212tests the associated file to see if something is true about it. If the
213argument is omitted, tests $_, except for C<-t>, which tests STDIN.
214Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
215the undefined value if the file doesn't exist. Despite the funny
216names, precedence is the same as any other named unary operator, and
217the argument may be parenthesized like any other unary operator. The
218operator may be any of:
219
220 -r File is readable by effective uid/gid.
221 -w File is writable by effective uid/gid.
222 -x File is executable by effective uid/gid.
223 -o File is owned by effective uid.
224
225 -R File is readable by real uid/gid.
226 -W File is writable by real uid/gid.
227 -X File is executable by real uid/gid.
228 -O File is owned by real uid.
229
230 -e File exists.
231 -z File has zero size.
54310121 232 -s File has nonzero size (returns size).
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233
234 -f File is a plain file.
235 -d File is a directory.
236 -l File is a symbolic link.
237 -p File is a named pipe (FIFO).
238 -S File is a socket.
239 -b File is a block special file.
240 -c File is a character special file.
241 -t Filehandle is opened to a tty.
242
243 -u File has setuid bit set.
244 -g File has setgid bit set.
245 -k File has sticky bit set.
246
247 -T File is a text file.
248 -B File is a binary file (opposite of -T).
249
250 -M Age of file in days when script started.
251 -A Same for access time.
252 -C Same for inode change time.
253
254The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
5f05dabc 255C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the
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256uids and gids of the user. There may be other reasons you can't actually
257read, write or execute the file. Also note that, for the superuser,
5f05dabc 258C<-r>, C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return
a0d0e21e 2591 if any execute bit is set in the mode. Scripts run by the superuser may
5f05dabc 260thus need to do a stat() to determine the actual mode of the
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261file, or temporarily set the uid to something else.
262
263Example:
264
265 while (<>) {
266 chop;
267 next unless -f $_; # ignore specials
268 ...
269 }
270
271Note that C<-s/a/b/> does not do a negated substitution. Saying
272C<-exp($foo)> still works as expected, however--only single letters
273following a minus are interpreted as file tests.
274
275The C<-T> and C<-B> switches work as follows. The first block or so of the
276file is examined for odd characters such as strange control codes or
184e9718 277characters with the high bit set. If too many odd characters (E<gt>30%)
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278are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
279containing null in the first block is considered a binary file. If C<-T>
280or C<-B> is used on a filehandle, the current stdio buffer is examined
281rather than the first block. Both C<-T> and C<-B> return TRUE on a null
54310121 282file, or a file at EOF when testing a filehandle. Because you have to
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283read a file to do the C<-T> test, on most occasions you want to use a C<-f>
284against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 285
28757baa 286If any of the file tests (or either the stat() or lstat() operators) are given
287the special filehandle consisting of a solitary underline, then the stat
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288structure of the previous file test (or stat operator) is used, saving
289a system call. (This doesn't work with C<-t>, and you need to remember
290that lstat() and C<-l> will leave values in the stat structure for the
291symbolic link, not the real file.) Example:
292
293 print "Can do.\n" if -r $a || -w _ || -x _;
294
295 stat($filename);
296 print "Readable\n" if -r _;
297 print "Writable\n" if -w _;
298 print "Executable\n" if -x _;
299 print "Setuid\n" if -u _;
300 print "Setgid\n" if -g _;
301 print "Sticky\n" if -k _;
302 print "Text\n" if -T _;
303 print "Binary\n" if -B _;
304
305=item abs VALUE
306
54310121 307=item abs
bbce6d69 308
a0d0e21e 309Returns the absolute value of its argument.
bbce6d69 310If VALUE is omitted, uses $_.
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311
312=item accept NEWSOCKET,GENERICSOCKET
313
314Accepts an incoming socket connect, just as the accept(2) system call
315does. Returns the packed address if it succeeded, FALSE otherwise.
4633a7c4 316See example in L<perlipc/"Sockets: Client/Server Communication">.
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317
318=item alarm SECONDS
319
54310121 320=item alarm
bbce6d69 321
a0d0e21e 322Arranges to have a SIGALRM delivered to this process after the
bbce6d69 323specified number of seconds have elapsed. If SECONDS is not specified,
324the value stored in $_ is used. (On some machines,
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325unfortunately, the elapsed time may be up to one second less than you
326specified because of how seconds are counted.) Only one timer may be
327counting at once. Each call disables the previous timer, and an
328argument of 0 may be supplied to cancel the previous timer without
329starting a new one. The returned value is the amount of time remaining
330on the previous timer.
331
4633a7c4 332For delays of finer granularity than one second, you may use Perl's
54310121 333syscall() interface to access setitimer(2) if your system supports it,
334or else see L</select()>. It is usually a mistake to intermix alarm()
4633a7c4 335and sleep() calls.
a0d0e21e 336
ff68c719 337If you want to use alarm() to time out a system call you need to use an
2f9daede 338eval/die pair. You can't rely on the alarm causing the system call to
ff68c719 339fail with $! set to EINTR because Perl sets up signal handlers to
340restart system calls on some systems. Using eval/die always works.
341
342 eval {
28757baa 343 local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
36477c24 344 alarm $timeout;
ff68c719 345 $nread = sysread SOCKET, $buffer, $size;
36477c24 346 alarm 0;
ff68c719 347 };
348 die if $@ && $@ ne "alarm\n"; # propagate errors
349 if ($@) {
350 # timed out
351 }
352 else {
353 # didn't
354 }
355
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356=item atan2 Y,X
357
358Returns the arctangent of Y/X in the range -PI to PI.
359
28757baa 360For the tangent operation, you may use the POSIX::tan()
361function, or use the familiar relation:
362
363 sub tan { sin($_[0]) / cos($_[0]) }
364
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365=item bind SOCKET,NAME
366
367Binds a network address to a socket, just as the bind system call
368does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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369packed address of the appropriate type for the socket. See the examples in
370L<perlipc/"Sockets: Client/Server Communication">.
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371
372=item binmode FILEHANDLE
373
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374Arranges for the file to be read or written in "binary" mode in operating
375systems that distinguish between binary and text files. Files that are
376not in binary mode have CR LF sequences translated to LF on input and LF
54310121 377translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
cb1a09d0 378and similarly archaic systems, it may be imperative--otherwise your
54310121 379MS-DOS-damaged C library may mangle your file. The key distinction between
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380systems that need binmode and those that don't is their text file
381formats. Systems like Unix and Plan9 that delimit lines with a single
382character, and that encode that character in C as '\n', do not need
383C<binmode>. The rest need it. If FILEHANDLE is an expression, the value
384is taken as the name of the filehandle.
a0d0e21e 385
4633a7c4 386=item bless REF,CLASSNAME
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387
388=item bless REF
389
28757baa 390This function tells the thingy referenced by REF that it is now
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391an object in the CLASSNAME package--or the current package if no CLASSNAME
392is specified, which is often the case. It returns the reference for
5f05dabc 393convenience, because a bless() is often the last thing in a constructor.
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394Always use the two-argument version if the function doing the blessing
395might be inherited by a derived class. See L<perlobj> for more about the
396blessing (and blessings) of objects.
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397
398=item caller EXPR
399
400=item caller
401
402Returns the context of the current subroutine call. In a scalar context,
28757baa 403returns the caller's package name if there is a caller, that is, if
404we're in a subroutine or eval() or require(), and the undefined value
405otherwise. In a list context, returns
a0d0e21e 406
748a9306 407 ($package, $filename, $line) = caller;
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408
409With EXPR, it returns some extra information that the debugger uses to
410print a stack trace. The value of EXPR indicates how many call frames
411to go back before the current one.
412
54310121 413 ($package, $filename, $line, $subroutine,
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414 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
415
416Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
dc848c6f 417call, but an C<eval>. In such a case additional elements $evaltext and
418$is_require are set: $is_require is true if the frame is created by a
419C<require> or C<use> statement, $evaltext contains the text of the
420C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
421$filename is C<"(eval)">, but $evaltext is undefined. (Note also that
422each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
423frame.
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424
425Furthermore, when called from within the DB package, caller returns more
4633a7c4 426detailed information: it sets the list variable @DB::args to be the
54310121 427arguments with which the subroutine was invoked.
748a9306 428
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429=item chdir EXPR
430
431Changes the working directory to EXPR, if possible. If EXPR is
432omitted, changes to home directory. Returns TRUE upon success, FALSE
433otherwise. See example under die().
434
435=item chmod LIST
436
437Changes the permissions of a list of files. The first element of the
4633a7c4 438list must be the numerical mode, which should probably be an octal
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439number, and which definitely should I<not> a string of octal digits:
440C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 441successfully changed. See also L</oct>, if all you have is a string.
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442
443 $cnt = chmod 0755, 'foo', 'bar';
444 chmod 0755, @executables;
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445 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to --w----r-T
446 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
447 $mode = 0644; chmod $mode, 'foo'; # this is best
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448
449=item chomp VARIABLE
450
451=item chomp LIST
452
453=item chomp
454
3e3baf6d 455This is a slightly safer version of L</chop>. It removes any
a0d0e21e 456line ending that corresponds to the current value of C<$/> (also known as
28757baa 457$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
458number of characters removed from all its arguments. It's often used to
459remove the newline from the end of an input record when you're worried
460that the final record may be missing its newline. When in paragraph mode
461(C<$/ = "">), it removes all trailing newlines from the string. If
462VARIABLE is omitted, it chomps $_. Example:
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463
464 while (<>) {
465 chomp; # avoid \n on last field
466 @array = split(/:/);
467 ...
468 }
469
470You can actually chomp anything that's an lvalue, including an assignment:
471
472 chomp($cwd = `pwd`);
473 chomp($answer = <STDIN>);
474
475If you chomp a list, each element is chomped, and the total number of
476characters removed is returned.
477
478=item chop VARIABLE
479
480=item chop LIST
481
482=item chop
483
484Chops off the last character of a string and returns the character
485chopped. It's used primarily to remove the newline from the end of an
486input record, but is much more efficient than C<s/\n//> because it neither
487scans nor copies the string. If VARIABLE is omitted, chops $_.
488Example:
489
490 while (<>) {
491 chop; # avoid \n on last field
492 @array = split(/:/);
493 ...
494 }
495
496You can actually chop anything that's an lvalue, including an assignment:
497
498 chop($cwd = `pwd`);
499 chop($answer = <STDIN>);
500
501If you chop a list, each element is chopped. Only the value of the
502last chop is returned.
503
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504Note that chop returns the last character. To return all but the last
505character, use C<substr($string, 0, -1)>.
506
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507=item chown LIST
508
509Changes the owner (and group) of a list of files. The first two
510elements of the list must be the I<NUMERICAL> uid and gid, in that order.
511Returns the number of files successfully changed.
512
513 $cnt = chown $uid, $gid, 'foo', 'bar';
514 chown $uid, $gid, @filenames;
515
54310121 516Here's an example that looks up nonnumeric uids in the passwd file:
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517
518 print "User: ";
519 chop($user = <STDIN>);
520 print "Files: "
521 chop($pattern = <STDIN>);
522
523 ($login,$pass,$uid,$gid) = getpwnam($user)
524 or die "$user not in passwd file";
525
526 @ary = <${pattern}>; # expand filenames
527 chown $uid, $gid, @ary;
528
54310121 529On most systems, you are not allowed to change the ownership of the
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530file unless you're the superuser, although you should be able to change
531the group to any of your secondary groups. On insecure systems, these
532restrictions may be relaxed, but this is not a portable assumption.
533
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534=item chr NUMBER
535
54310121 536=item chr
bbce6d69 537
a0d0e21e 538Returns the character represented by that NUMBER in the character set.
dc848c6f 539For example, C<chr(65)> is "A" in ASCII. For the reverse, use L</ord>.
a0d0e21e 540
bbce6d69 541If NUMBER is omitted, uses $_.
542
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543=item chroot FILENAME
544
54310121 545=item chroot
bbce6d69 546
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547This function works as the system call by the same name: it makes the
548named directory the new root directory for all further pathnames that
549begin with a "/" by your process and all of its children. (It doesn't
28757baa 550change your current working directory, which is unaffected.) For security
4633a7c4
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551reasons, this call is restricted to the superuser. If FILENAME is
552omitted, does chroot to $_.
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553
554=item close FILEHANDLE
555
556Closes the file or pipe associated with the file handle, returning TRUE
557only if stdio successfully flushes buffers and closes the system file
fb73857a 558descriptor.
559
560You don't have to close FILEHANDLE if you are immediately going to do
561another open() on it, because open() will close it for you. (See
a0d0e21e 562open().) However, an explicit close on an input file resets the line
fb73857a 563counter ($.), while the implicit close done by open() does not.
564
565If the file handle came from a piped open C<close> will additionally
566return FALSE if one of the other system calls involved fails or if the
567program exits with non-zero status. (If the only problem was that the
568program exited non-zero $! will be set to 0.) Also, closing a pipe will
569wait for the process executing on the pipe to complete, in case you
570want to look at the output of the pipe afterwards. Closing a pipe
571explicitly also puts the exit status value of the command into C<$?>.
572Example:
a0d0e21e 573
fb73857a 574 open(OUTPUT, '|sort >foo') # pipe to sort
575 or die "Can't start sort: $!";
a0d0e21e 576 ... # print stuff to output
fb73857a 577 close OUTPUT # wait for sort to finish
578 or warn $! ? "Error closing sort pipe: $!"
579 : "Exit status $? from sort";
580 open(INPUT, 'foo') # get sort's results
581 or die "Can't open 'foo' for input: $!";
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582
583FILEHANDLE may be an expression whose value gives the real filehandle name.
584
585=item closedir DIRHANDLE
586
587Closes a directory opened by opendir().
588
589=item connect SOCKET,NAME
590
591Attempts to connect to a remote socket, just as the connect system call
592does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
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593packed address of the appropriate type for the socket. See the examples in
594L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 595
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596=item continue BLOCK
597
598Actually a flow control statement rather than a function. If there is a
599C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
600C<foreach>), it is always executed just before the conditional is about to
601be evaluated again, just like the third part of a C<for> loop in C. Thus
602it can be used to increment a loop variable, even when the loop has been
603continued via the C<next> statement (which is similar to the C C<continue>
604statement).
605
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606=item cos EXPR
607
608Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
609takes cosine of $_.
610
28757baa 611For the inverse cosine operation, you may use the POSIX::acos()
612function, or use this relation:
613
614 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
615
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616=item crypt PLAINTEXT,SALT
617
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618Encrypts a string exactly like the crypt(3) function in the C library
619(assuming that you actually have a version there that has not been
620extirpated as a potential munition). This can prove useful for checking
621the password file for lousy passwords, amongst other things. Only the
622guys wearing white hats should do this.
a0d0e21e 623
11155c91
CS
624Note that crypt is intended to be a one-way function, much like breaking
625eggs to make an omelette. There is no (known) corresponding decrypt
626function. As a result, this function isn't all that useful for
627cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 628
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629Here's an example that makes sure that whoever runs this program knows
630their own password:
631
632 $pwd = (getpwuid($<))[1];
633 $salt = substr($pwd, 0, 2);
634
635 system "stty -echo";
636 print "Password: ";
637 chop($word = <STDIN>);
638 print "\n";
639 system "stty echo";
640
641 if (crypt($word, $salt) ne $pwd) {
642 die "Sorry...\n";
643 } else {
644 print "ok\n";
54310121 645 }
a0d0e21e 646
9f8f0c9d 647Of course, typing in your own password to whoever asks you
748a9306 648for it is unwise.
a0d0e21e 649
aa689395 650=item dbmclose HASH
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651
652[This function has been superseded by the untie() function.]
653
aa689395 654Breaks the binding between a DBM file and a hash.
a0d0e21e 655
aa689395 656=item dbmopen HASH,DBNAME,MODE
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LW
657
658[This function has been superseded by the tie() function.]
659
7b8d334a 660This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
aa689395 661hash. HASH is the name of the hash. (Unlike normal open, the first
662argument is I<NOT> a filehandle, even though it looks like one). DBNAME
663is the name of the database (without the F<.dir> or F<.pag> extension if
664any). If the database does not exist, it is created with protection
665specified by MODE (as modified by the umask()). If your system supports
666only the older DBM functions, you may perform only one dbmopen() in your
667program. In older versions of Perl, if your system had neither DBM nor
668ndbm, calling dbmopen() produced a fatal error; it now falls back to
669sdbm(3).
670
671If you don't have write access to the DBM file, you can only read hash
672variables, not set them. If you want to test whether you can write,
673either use file tests or try setting a dummy hash entry inside an eval(),
674which will trap the error.
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675
676Note that functions such as keys() and values() may return huge array
677values when used on large DBM files. You may prefer to use the each()
678function to iterate over large DBM files. Example:
679
680 # print out history file offsets
681 dbmopen(%HIST,'/usr/lib/news/history',0666);
682 while (($key,$val) = each %HIST) {
683 print $key, ' = ', unpack('L',$val), "\n";
684 }
685 dbmclose(%HIST);
686
cb1a09d0 687See also L<AnyDBM_File> for a more general description of the pros and
184e9718 688cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 689rich implementation.
4633a7c4 690
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691=item defined EXPR
692
54310121 693=item defined
bbce6d69 694
2f9daede
TP
695Returns a Boolean value telling whether EXPR has a value other than
696the undefined value C<undef>. If EXPR is not present, C<$_> will be
697checked.
698
699Many operations return C<undef> to indicate failure, end of file,
700system error, uninitialized variable, and other exceptional
701conditions. This function allows you to distinguish C<undef> from
702other values. (A simple Boolean test will not distinguish among
703C<undef>, zero, the empty string, and "0", which are all equally
704false.) Note that since C<undef> is a valid scalar, its presence
705doesn't I<necessarily> indicate an exceptional condition: pop()
706returns C<undef> when its argument is an empty array, I<or> when the
707element to return happens to be C<undef>.
708
709You may also use defined() to check whether a subroutine exists. On
710the other hand, use of defined() upon aggregates (hashes and arrays)
711is not guaranteed to produce intuitive results, and should probably be
712avoided.
713
714When used on a hash element, it tells you whether the value is defined,
dc848c6f 715not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 716purpose.
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LW
717
718Examples:
719
720 print if defined $switch{'D'};
721 print "$val\n" while defined($val = pop(@ary));
722 die "Can't readlink $sym: $!"
723 unless defined($value = readlink $sym);
a0d0e21e 724 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 725 $debugging = 0 unless defined $debugging;
a0d0e21e 726
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TP
727Note: Many folks tend to overuse defined(), and then are surprised to
728discover that the number 0 and "" (the zero-length string) are, in fact,
729defined values. For example, if you say
a5f75d66
AD
730
731 "ab" =~ /a(.*)b/;
732
733the pattern match succeeds, and $1 is defined, despite the fact that it
734matched "nothing". But it didn't really match nothing--rather, it
735matched something that happened to be 0 characters long. This is all
736very above-board and honest. When a function returns an undefined value,
2f9daede
TP
737it's an admission that it couldn't give you an honest answer. So you
738should use defined() only when you're questioning the integrity of what
739you're trying to do. At other times, a simple comparison to 0 or "" is
740what you want.
741
742Currently, using defined() on an entire array or hash reports whether
743memory for that aggregate has ever been allocated. So an array you set
744to the empty list appears undefined initially, and one that once was full
745and that you then set to the empty list still appears defined. You
746should instead use a simple test for size:
28757baa 747
748 if (@an_array) { print "has array elements\n" }
749 if (%a_hash) { print "has hash members\n" }
750
751Using undef() on these, however, does clear their memory and then report
752them as not defined anymore, but you shoudln't do that unless you don't
753plan to use them again, because it saves time when you load them up
754again to have memory already ready to be filled.
755
54310121 756This counterintuitive behaviour of defined() on aggregates may be
28757baa 757changed, fixed, or broken in a future release of Perl.
758
dc848c6f 759See also L</undef>, L</exists>, L</ref>.
2f9daede 760
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LW
761=item delete EXPR
762
aa689395 763Deletes the specified key(s) and their associated values from a hash.
764For each key, returns the deleted value associated with that key, or
765the undefined value if there was no such key. Deleting from C<$ENV{}>
766modifies the environment. Deleting from a hash tied to a DBM file
5f05dabc 767deletes the entry from the DBM file. (But deleting from a tie()d hash
768doesn't necessarily return anything.)
a0d0e21e 769
aa689395 770The following deletes all the values of a hash:
a0d0e21e 771
5f05dabc 772 foreach $key (keys %HASH) {
773 delete $HASH{$key};
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LW
774 }
775
5f05dabc 776And so does this:
777
778 delete @HASH{keys %HASH}
779
780(But both of these are slower than the undef() command.) Note that the
781EXPR can be arbitrarily complicated as long as the final operation is a
782hash element lookup or hash slice:
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LW
783
784 delete $ref->[$x][$y]{$key};
5f05dabc 785 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e
LW
786
787=item die LIST
788
789Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
184e9718 790the current value of C<$!> (errno). If C<$!> is 0, exits with the value of
54310121 791C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
28757baa 792is 0, exits with 255. Inside an eval(), the error message is stuffed into
793C<$@>, and the eval() is terminated with the undefined value; this makes
794die() the way to raise an exception.
a0d0e21e
LW
795
796Equivalent examples:
797
798 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 799 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
800
801If the value of EXPR does not end in a newline, the current script line
802number and input line number (if any) are also printed, and a newline
803is supplied. Hint: sometimes appending ", stopped" to your message
804will cause it to make better sense when the string "at foo line 123" is
805appended. Suppose you are running script "canasta".
806
807 die "/etc/games is no good";
808 die "/etc/games is no good, stopped";
809
810produce, respectively
811
812 /etc/games is no good at canasta line 123.
813 /etc/games is no good, stopped at canasta line 123.
814
815See also exit() and warn().
816
fb73857a 817If LIST is empty and $@ already contains a value (typically from a
818previous eval) that value is reused after appending "\t...propagated".
819This is useful for propagating exceptions:
820
821 eval { ... };
822 die unless $@ =~ /Expected exception/;
823
824If $@ is empty then the string "Died" is used.
825
774d564b 826You can arrange for a callback to be called just before the die() does
827its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
828will be called with the error text and can change the error message, if
fb73857a 829it sees fit, by calling die() again. See L<perlvar/$SIG{expr}> for details on
830setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
831
832Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
833blocks/strings. If one wants the hook to do nothing in such
834situations, put
835
836 die @_ if $^S;
837
838as the first line of the handler (see L<perlvar/$^S>).
774d564b 839
a0d0e21e
LW
840=item do BLOCK
841
842Not really a function. Returns the value of the last command in the
843sequence of commands indicated by BLOCK. When modified by a loop
844modifier, executes the BLOCK once before testing the loop condition.
845(On other statements the loop modifiers test the conditional first.)
846
847=item do SUBROUTINE(LIST)
848
849A deprecated form of subroutine call. See L<perlsub>.
850
851=item do EXPR
852
853Uses the value of EXPR as a filename and executes the contents of the
854file as a Perl script. Its primary use is to include subroutines
855from a Perl subroutine library.
856
857 do 'stat.pl';
858
859is just like
860
fb73857a 861 scalar eval `cat stat.pl`;
a0d0e21e
LW
862
863except that it's more efficient, more concise, keeps track of the
864current filename for error messages, and searches all the B<-I>
865libraries if the file isn't in the current directory (see also the @INC
dc1be6b5
GS
866array in L<perlvar/Predefined Names>). It is also different in how
867code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
868scope like C<eval STRING> does. It's the same, however, in that it does
54310121 869reparse the file every time you call it, so you probably don't want to
a0d0e21e
LW
870do this inside a loop.
871
872Note that inclusion of library modules is better done with the
4633a7c4
LW
873use() and require() operators, which also do error checking
874and raise an exception if there's a problem.
a0d0e21e
LW
875
876=item dump LABEL
877
878This causes an immediate core dump. Primarily this is so that you can
879use the B<undump> program to turn your core dump into an executable binary
880after having initialized all your variables at the beginning of the
881program. When the new binary is executed it will begin by executing a
882C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
883it as a goto with an intervening core dump and reincarnation. If LABEL
884is omitted, restarts the program from the top. WARNING: any files
885opened at the time of the dump will NOT be open any more when the
886program is reincarnated, with possible resulting confusion on the part
887of Perl. See also B<-u> option in L<perlrun>.
888
889Example:
890
891 #!/usr/bin/perl
892 require 'getopt.pl';
893 require 'stat.pl';
894 %days = (
895 'Sun' => 1,
896 'Mon' => 2,
897 'Tue' => 3,
898 'Wed' => 4,
899 'Thu' => 5,
900 'Fri' => 6,
901 'Sat' => 7,
902 );
903
904 dump QUICKSTART if $ARGV[0] eq '-d';
905
906 QUICKSTART:
907 Getopt('f');
908
aa689395 909=item each HASH
910
911When called in a list context, returns a 2-element array consisting of the
912key and value for the next element of a hash, so that you can iterate over
913it. When called in a scalar context, returns the key for only the next
2f9daede
TP
914element in the hash. (Note: Keys may be "0" or "", which are logically
915false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
916for this reason.)
917
918Entries are returned in an apparently random order. When the hash is
919entirely read, a null array is returned in list context (which when
920assigned produces a FALSE (0) value), and C<undef> is returned in a
921scalar context. The next call to each() after that will start iterating
922again. There is a single iterator for each hash, shared by all each(),
923keys(), and values() function calls in the program; it can be reset by
924reading all the elements from the hash, or by evaluating C<keys HASH> or
925C<values HASH>. If you add or delete elements of a hash while you're
926iterating over it, you may get entries skipped or duplicated, so don't.
aa689395 927
928The following prints out your environment like the printenv(1) program,
929only in a different order:
a0d0e21e
LW
930
931 while (($key,$value) = each %ENV) {
932 print "$key=$value\n";
933 }
934
935See also keys() and values().
936
937=item eof FILEHANDLE
938
4633a7c4
LW
939=item eof ()
940
a0d0e21e
LW
941=item eof
942
943Returns 1 if the next read on FILEHANDLE will return end of file, or if
944FILEHANDLE is not open. FILEHANDLE may be an expression whose value
945gives the real filehandle name. (Note that this function actually
946reads a character and then ungetc()s it, so it is not very useful in an
748a9306
LW
947interactive context.) Do not read from a terminal file (or call
948C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
949as terminals may lose the end-of-file condition if you do.
950
951An C<eof> without an argument uses the last file read as argument.
2f9daede
TP
952Empty parentheses () may be used to indicate the pseudo file formed of
953the files listed on the command line, i.e., C<eof()> is reasonable to
954use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
955last file. Use C<eof(ARGV)> or eof without the parentheses to test
956I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
a0d0e21e 957
748a9306
LW
958 # reset line numbering on each input file
959 while (<>) {
960 print "$.\t$_";
961 close(ARGV) if (eof); # Not eof().
962 }
963
a0d0e21e
LW
964 # insert dashes just before last line of last file
965 while (<>) {
966 if (eof()) {
967 print "--------------\n";
748a9306
LW
968 close(ARGV); # close or break; is needed if we
969 # are reading from the terminal
a0d0e21e
LW
970 }
971 print;
972 }
973
a0d0e21e 974Practical hint: you almost never need to use C<eof> in Perl, because the
54310121 975input operators return undef when they run out of data.
a0d0e21e
LW
976
977=item eval EXPR
978
979=item eval BLOCK
980
c7cc6f1c
GS
981In the first form, the return value of EXPR is parsed and executed as if it
982were a little Perl program. The value of the expression (which is itself
983determined within a scalar context) is first parsed, and if there are no
984errors, executed in the context of the current Perl program, so that any
5f05dabc 985variable settings or subroutine and format definitions remain afterwards.
c7cc6f1c
GS
986Note that the value is parsed every time the eval executes. If EXPR is
987omitted, evaluates C<$_>. This form is typically used to delay parsing
988and subsequent execution of the text of EXPR until run time.
989
990In the second form, the code within the BLOCK is parsed only once--at the
991same time the code surrounding the eval itself was parsed--and executed
992within the context of the current Perl program. This form is typically
993used to trap exceptions more efficiently than the first (see below), while
994also providing the benefit of checking the code within BLOCK at compile
995time.
996
997The final semicolon, if any, may be omitted from the value of EXPR or within
998the BLOCK.
999
1000In both forms, the value returned is the value of the last expression
1001evaluated inside the mini-program, or a return statement may be used, just
1002as with subroutines. The expression providing the return value is evaluated
1003in void, scalar or array context, depending on the context of the eval itself.
1004See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e
LW
1005
1006If there is a syntax error or runtime error, or a die() statement is
1007executed, an undefined value is returned by eval(), and C<$@> is set to the
1008error message. If there was no error, C<$@> is guaranteed to be a null
c7cc6f1c
GS
1009string. Beware that using eval() neither silences perl from printing
1010warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1011To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1012L</warn> and L<perlvar>.
a0d0e21e 1013
5f05dabc 1014Note that, because eval() traps otherwise-fatal errors, it is useful for
4633a7c4 1015determining whether a particular feature (such as socket() or symlink())
a0d0e21e
LW
1016is implemented. It is also Perl's exception trapping mechanism, where
1017the die operator is used to raise exceptions.
1018
1019If the code to be executed doesn't vary, you may use the eval-BLOCK
1020form to trap run-time errors without incurring the penalty of
1021recompiling each time. The error, if any, is still returned in C<$@>.
1022Examples:
1023
54310121 1024 # make divide-by-zero nonfatal
a0d0e21e
LW
1025 eval { $answer = $a / $b; }; warn $@ if $@;
1026
1027 # same thing, but less efficient
1028 eval '$answer = $a / $b'; warn $@ if $@;
1029
1030 # a compile-time error
1031 eval { $answer = };
1032
1033 # a run-time error
1034 eval '$answer ='; # sets $@
1035
774d564b 1036When using the eval{} form as an exception trap in libraries, you may
1037wish not to trigger any C<__DIE__> hooks that user code may have
1038installed. You can use the C<local $SIG{__DIE__}> construct for this
1039purpose, as shown in this example:
1040
1041 # a very private exception trap for divide-by-zero
1042 eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
1043
1044This is especially significant, given that C<__DIE__> hooks can call
1045die() again, which has the effect of changing their error messages:
1046
1047 # __DIE__ hooks may modify error messages
1048 {
1049 local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1050 eval { die "foo lives here" };
1051 print $@ if $@; # prints "bar lives here"
774d564b 1052 }
1053
54310121 1054With an eval(), you should be especially careful to remember what's
a0d0e21e
LW
1055being looked at when:
1056
1057 eval $x; # CASE 1
1058 eval "$x"; # CASE 2
1059
1060 eval '$x'; # CASE 3
1061 eval { $x }; # CASE 4
1062
1063 eval "\$$x++" # CASE 5
1064 $$x++; # CASE 6
1065
2f9daede
TP
1066Cases 1 and 2 above behave identically: they run the code contained in
1067the variable $x. (Although case 2 has misleading double quotes making
1068the reader wonder what else might be happening (nothing is).) Cases 3
1069and 4 likewise behave in the same way: they run the code '$x', which
1070does nothing but return the value of C<$x>. (Case 4 is preferred for
1071purely visual reasons, but it also has the advantage of compiling at
1072compile-time instead of at run-time.) Case 5 is a place where
54310121 1073normally you I<WOULD> like to use double quotes, except that in this
2f9daede
TP
1074particular situation, you can just use symbolic references instead, as
1075in case 6.
a0d0e21e
LW
1076
1077=item exec LIST
1078
8bf3b016
GS
1079=item exec PROGRAM LIST
1080
fb73857a 1081The exec() function executes a system command I<AND NEVER RETURNS> -
1082use system() instead of exec() if you want it to return. It fails and
1083returns FALSE only if the command does not exist I<and> it is executed
1084directly instead of via your system's command shell (see below).
a0d0e21e
LW
1085
1086If there is more than one argument in LIST, or if LIST is an array with
1087more than one value, calls execvp(3) with the arguments in LIST. If
1088there is only one scalar argument, the argument is checked for shell
bb32b41a
GS
1089metacharacters, and if there are any, the entire argument is passed to
1090the system's command shell for parsing (this is C</bin/sh -c> on Unix
1091platforms, but varies on other platforms). If there are no shell
1092metacharacters in the argument, it is split into words and passed
1093directly to execvp(), which is more efficient. Note: exec() and
1094system() do not flush your output buffer, so you may need to set C<$|>
1095to avoid lost output. Examples:
a0d0e21e
LW
1096
1097 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1098 exec "sort $outfile | uniq";
1099
1100If you don't really want to execute the first argument, but want to lie
1101to the program you are executing about its own name, you can specify
1102the program you actually want to run as an "indirect object" (without a
1103comma) in front of the LIST. (This always forces interpretation of the
54310121 1104LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1105the list.) Example:
1106
1107 $shell = '/bin/csh';
1108 exec $shell '-sh'; # pretend it's a login shell
1109
1110or, more directly,
1111
1112 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1113
bb32b41a
GS
1114When the arguments get executed via the system shell, results will
1115be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1116for details.
1117
a0d0e21e
LW
1118=item exists EXPR
1119
1120Returns TRUE if the specified hash key exists in its hash array, even
1121if the corresponding value is undefined.
1122
1123 print "Exists\n" if exists $array{$key};
1124 print "Defined\n" if defined $array{$key};
1125 print "True\n" if $array{$key};
1126
5f05dabc 1127A hash element can be TRUE only if it's defined, and defined if
a0d0e21e
LW
1128it exists, but the reverse doesn't necessarily hold true.
1129
1130Note that the EXPR can be arbitrarily complicated as long as the final
1131operation is a hash key lookup:
1132
1133 if (exists $ref->[$x][$y]{$key}) { ... }
1134
1135=item exit EXPR
1136
1137Evaluates EXPR and exits immediately with that value. (Actually, it
1138calls any defined C<END> routines first, but the C<END> routines may not
1139abort the exit. Likewise any object destructors that need to be called
1140are called before exit.) Example:
1141
1142 $ans = <STDIN>;
1143 exit 0 if $ans =~ /^[Xx]/;
1144
f86702cc 1145See also die(). If EXPR is omitted, exits with 0 status. The only
54310121 1146universally portable values for EXPR are 0 for success and 1 for error;
f86702cc 1147all other values are subject to unpredictable interpretation depending
1148on the environment in which the Perl program is running.
a0d0e21e 1149
28757baa 1150You shouldn't use exit() to abort a subroutine if there's any chance that
1151someone might want to trap whatever error happened. Use die() instead,
1152which can be trapped by an eval().
1153
a0d0e21e
LW
1154=item exp EXPR
1155
54310121 1156=item exp
bbce6d69 1157
54310121 1158Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1159If EXPR is omitted, gives C<exp($_)>.
1160
1161=item fcntl FILEHANDLE,FUNCTION,SCALAR
1162
1163Implements the fcntl(2) function. You'll probably have to say
1164
1165 use Fcntl;
1166
1167first to get the correct function definitions. Argument processing and
1168value return works just like ioctl() below. Note that fcntl() will produce
1169a fatal error if used on a machine that doesn't implement fcntl(2).
1170For example:
1171
1172 use Fcntl;
1173 fcntl($filehandle, F_GETLK, $packed_return_buffer);
1174
1175=item fileno FILEHANDLE
1176
1177Returns the file descriptor for a filehandle. This is useful for
1178constructing bitmaps for select(). If FILEHANDLE is an expression, the
1179value is taken as the name of the filehandle.
1180
1181=item flock FILEHANDLE,OPERATION
1182
8ebc5c01 1183Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
68dc0745 1184success, FALSE on failure. Produces a fatal error if used on a machine
1185that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock()
1186is Perl's portable file locking interface, although it locks only entire
1187files, not records.
8ebc5c01 1188
a3cb178b
GS
1189On many platforms (including most versions or clones of Unix), locks
1190established by flock() are B<merely advisory>. This means that files
1191locked with flock() may be modified by programs which do not also use
1192flock(). Windows NT and OS/2, however, are among the platforms which
1193supply mandatory locking. See your local documentation for details.
1194
8ebc5c01 1195OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1196LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
68dc0745 1197you can use the symbolic names if import them from the Fcntl module,
1198either individually, or as a group using the ':flock' tag. LOCK_SH
1199requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1200releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1201LOCK_EX then flock() will return immediately rather than blocking
1202waiting for the lock (check the return status to see if you got it).
1203
1204To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1205before (un)locking it.
8ebc5c01 1206
1207Note that the emulation built with lockf(3) doesn't provide shared
1208locks, and it requires that FILEHANDLE be open with write intent. These
1209are the semantics that lockf(3) implements. Most (all?) systems
1210implement lockf(3) in terms of fcntl(2) locking, though, so the
1211differing semantics shouldn't bite too many people.
1212
1213Note also that some versions of flock() cannot lock things over the
1214network; you would need to use the more system-specific fcntl() for
1215that. If you like you can force Perl to ignore your system's flock(2)
1216function, and so provide its own fcntl(2)-based emulation, by passing
1217the switch C<-Ud_flock> to the F<Configure> program when you configure
1218perl.
4633a7c4
LW
1219
1220Here's a mailbox appender for BSD systems.
a0d0e21e 1221
7e1af8bc 1222 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1223
1224 sub lock {
7e1af8bc 1225 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1226 # and, in case someone appended
1227 # while we were waiting...
1228 seek(MBOX, 0, 2);
1229 }
1230
1231 sub unlock {
7e1af8bc 1232 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1233 }
1234
1235 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1236 or die "Can't open mailbox: $!";
1237
1238 lock();
1239 print MBOX $msg,"\n\n";
1240 unlock();
1241
cb1a09d0 1242See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1243
1244=item fork
1245
1246Does a fork(2) system call. Returns the child pid to the parent process
4633a7c4 1247and 0 to the child process, or C<undef> if the fork is unsuccessful.
a0d0e21e 1248Note: unflushed buffers remain unflushed in both processes, which means
28757baa 1249you may need to set C<$|> ($AUTOFLUSH in English) or call the autoflush()
1250method of IO::Handle to avoid duplicate output.
a0d0e21e
LW
1251
1252If you fork() without ever waiting on your children, you will accumulate
1253zombies:
1254
4633a7c4 1255 $SIG{CHLD} = sub { wait };
a0d0e21e 1256
54310121 1257There's also the double-fork trick (error checking on
a0d0e21e
LW
1258fork() returns omitted);
1259
1260 unless ($pid = fork) {
1261 unless (fork) {
1262 exec "what you really wanna do";
1263 die "no exec";
1264 # ... or ...
4633a7c4 1265 ## (some_perl_code_here)
a0d0e21e
LW
1266 exit 0;
1267 }
1268 exit 0;
1269 }
1270 waitpid($pid,0);
1271
cb1a09d0
AD
1272See also L<perlipc> for more examples of forking and reaping
1273moribund children.
1274
28757baa 1275Note that if your forked child inherits system file descriptors like
1276STDIN and STDOUT that are actually connected by a pipe or socket, even
1277if you exit, the remote server (such as, say, httpd or rsh) won't think
1278you're done. You should reopen those to /dev/null if it's any issue.
1279
cb1a09d0
AD
1280=item format
1281
7b8d334a 1282Declare a picture format for use by the write() function. For
cb1a09d0
AD
1283example:
1284
54310121 1285 format Something =
cb1a09d0
AD
1286 Test: @<<<<<<<< @||||| @>>>>>
1287 $str, $%, '$' . int($num)
1288 .
1289
1290 $str = "widget";
184e9718 1291 $num = $cost/$quantity;
cb1a09d0
AD
1292 $~ = 'Something';
1293 write;
1294
1295See L<perlform> for many details and examples.
1296
a0d0e21e 1297
8903cb82 1298=item formline PICTURE,LIST
a0d0e21e 1299
4633a7c4 1300This is an internal function used by C<format>s, though you may call it
a0d0e21e
LW
1301too. It formats (see L<perlform>) a list of values according to the
1302contents of PICTURE, placing the output into the format output
4633a7c4
LW
1303accumulator, C<$^A> (or $ACCUMULATOR in English).
1304Eventually, when a write() is done, the contents of
a0d0e21e
LW
1305C<$^A> are written to some filehandle, but you could also read C<$^A>
1306yourself and then set C<$^A> back to "". Note that a format typically
1307does one formline() per line of form, but the formline() function itself
748a9306 1308doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1309that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1310You may therefore need to use multiple formlines to implement a single
1311record format, just like the format compiler.
1312
5f05dabc 1313Be careful if you put double quotes around the picture, because an "C<@>"
748a9306 1314character may be taken to mean the beginning of an array name.
4633a7c4 1315formline() always returns TRUE. See L<perlform> for other examples.
a0d0e21e
LW
1316
1317=item getc FILEHANDLE
1318
1319=item getc
1320
1321Returns the next character from the input file attached to FILEHANDLE,
1322or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
4633a7c4 1323This is not particularly efficient. It cannot be used to get unbuffered
cb1a09d0 1324single-characters, however. For that, try something more like:
4633a7c4
LW
1325
1326 if ($BSD_STYLE) {
1327 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1328 }
1329 else {
54310121 1330 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1331 }
1332
1333 $key = getc(STDIN);
1334
1335 if ($BSD_STYLE) {
1336 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1337 }
1338 else {
5f05dabc 1339 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1340 }
1341 print "\n";
1342
54310121 1343Determination of whether $BSD_STYLE should be set
1344is left as an exercise to the reader.
cb1a09d0 1345
28757baa 1346The POSIX::getattr() function can do this more portably on systems
1347alleging POSIX compliance.
cb1a09d0 1348See also the C<Term::ReadKey> module from your nearest CPAN site;
54310121 1349details on CPAN can be found on L<perlmod/CPAN>.
a0d0e21e
LW
1350
1351=item getlogin
1352
1353Returns the current login from F</etc/utmp>, if any. If null, use
54310121 1354getpwuid().
a0d0e21e 1355
f86702cc 1356 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1357
da0045b7 1358Do not consider getlogin() for authentication: it is not as
4633a7c4
LW
1359secure as getpwuid().
1360
a0d0e21e
LW
1361=item getpeername SOCKET
1362
1363Returns the packed sockaddr address of other end of the SOCKET connection.
1364
4633a7c4
LW
1365 use Socket;
1366 $hersockaddr = getpeername(SOCK);
1367 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1368 $herhostname = gethostbyaddr($iaddr, AF_INET);
1369 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1370
1371=item getpgrp PID
1372
47e29363 1373Returns the current process group for the specified PID. Use
1374a PID of 0 to get the current process group for the
4633a7c4 1375current process. Will raise an exception if used on a machine that
a0d0e21e 1376doesn't implement getpgrp(2). If PID is omitted, returns process
47e29363 1377group of current process. Note that the POSIX version of getpgrp()
1378does not accept a PID argument, so only PID==0 is truly portable.
a0d0e21e
LW
1379
1380=item getppid
1381
1382Returns the process id of the parent process.
1383
1384=item getpriority WHICH,WHO
1385
4633a7c4
LW
1386Returns the current priority for a process, a process group, or a user.
1387(See L<getpriority(2)>.) Will raise a fatal exception if used on a
a0d0e21e
LW
1388machine that doesn't implement getpriority(2).
1389
1390=item getpwnam NAME
1391
1392=item getgrnam NAME
1393
1394=item gethostbyname NAME
1395
1396=item getnetbyname NAME
1397
1398=item getprotobyname NAME
1399
1400=item getpwuid UID
1401
1402=item getgrgid GID
1403
1404=item getservbyname NAME,PROTO
1405
1406=item gethostbyaddr ADDR,ADDRTYPE
1407
1408=item getnetbyaddr ADDR,ADDRTYPE
1409
1410=item getprotobynumber NUMBER
1411
1412=item getservbyport PORT,PROTO
1413
1414=item getpwent
1415
1416=item getgrent
1417
1418=item gethostent
1419
1420=item getnetent
1421
1422=item getprotoent
1423
1424=item getservent
1425
1426=item setpwent
1427
1428=item setgrent
1429
1430=item sethostent STAYOPEN
1431
1432=item setnetent STAYOPEN
1433
1434=item setprotoent STAYOPEN
1435
1436=item setservent STAYOPEN
1437
1438=item endpwent
1439
1440=item endgrent
1441
1442=item endhostent
1443
1444=item endnetent
1445
1446=item endprotoent
1447
1448=item endservent
1449
1450These routines perform the same functions as their counterparts in the
1451system library. Within a list context, the return values from the
1452various get routines are as follows:
1453
1454 ($name,$passwd,$uid,$gid,
6ee623d5 1455 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1456 ($name,$passwd,$gid,$members) = getgr*
1457 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1458 ($name,$aliases,$addrtype,$net) = getnet*
1459 ($name,$aliases,$proto) = getproto*
1460 ($name,$aliases,$port,$proto) = getserv*
1461
1462(If the entry doesn't exist you get a null list.)
1463
1464Within a scalar context, you get the name, unless the function was a
1465lookup by name, in which case you get the other thing, whatever it is.
1466(If the entry doesn't exist you get the undefined value.) For example:
1467
1468 $uid = getpwnam
1469 $name = getpwuid
1470 $name = getpwent
1471 $gid = getgrnam
1472 $name = getgrgid
1473 $name = getgrent
1474 etc.
1475
6ee623d5
GS
1476In I<getpw*()> the fields $quota, $comment, and $expire are special
1477cases in the sense that in many systems they are unsupported. If the
1478$quota is unsupported, it is an empty scalar. If it is supported, it
1479usually encodes the disk quota. If the $comment field is unsupported,
1480it is an empty scalar. If it is supported it usually encodes some
1481administrative comment about the user. In some systems the $quota
1482field may be $change or $age, fields that have to do with password
1483aging. In some systems the $comment field may be $class. The $expire
1484field, if present, encodes the expiration period of the account or the
1485password. For the availability and the exact meaning of these fields
1486in your system, please consult your getpwnam(3) documentation and your
1487<pwd.h> file. You can also find out from within Perl which meaning
1488your $quota and $comment fields have and whether you have the $expire
1489field by using the Config module and the values d_pwquota, d_pwage,
1490d_pwchange, d_pwcomment, and d_pwexpire.
1491
a0d0e21e
LW
1492The $members value returned by I<getgr*()> is a space separated list of
1493the login names of the members of the group.
1494
1495For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1496C, it will be returned to you via C<$?> if the function call fails. The
1497@addrs value returned by a successful call is a list of the raw
1498addresses returned by the corresponding system library call. In the
1499Internet domain, each address is four bytes long and you can unpack it
1500by saying something like:
1501
1502 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1503
1504=item getsockname SOCKET
1505
1506Returns the packed sockaddr address of this end of the SOCKET connection.
1507
4633a7c4
LW
1508 use Socket;
1509 $mysockaddr = getsockname(SOCK);
1510 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
a0d0e21e
LW
1511
1512=item getsockopt SOCKET,LEVEL,OPTNAME
1513
1514Returns the socket option requested, or undefined if there is an error.
1515
1516=item glob EXPR
1517
0a753a76 1518=item glob
1519
68dc0745 1520Returns the value of EXPR with filename expansions such as a shell would
1521do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1522operator, but you can use it directly. If EXPR is omitted, $_ is used.
1523The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1524L<perlop/"I/O Operators">.
a0d0e21e
LW
1525
1526=item gmtime EXPR
1527
1528Converts a time as returned by the time function to a 9-element array
54310121 1529with the time localized for the standard Greenwich time zone.
4633a7c4 1530Typically used as follows:
a0d0e21e 1531
54310121 1532 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1533 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1534 gmtime(time);
1535
1536All array elements are numeric, and come straight out of a struct tm.
1537In particular this means that $mon has the range 0..11 and $wday has
54310121 1538the range 0..6 with sunday as day 0. Also, $year is the number of
1539years since 1900, I<not> simply the last two digits of the year.
2f9daede
TP
1540
1541If EXPR is omitted, does C<gmtime(time())>.
a0d0e21e 1542
54310121 1543In a scalar context, returns the ctime(3) value:
0a753a76 1544
1545 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1546
54310121 1547Also see the timegm() function provided by the Time::Local module,
1548and the strftime(3) function available via the POSIX module.
0a753a76 1549
a0d0e21e
LW
1550=item goto LABEL
1551
748a9306
LW
1552=item goto EXPR
1553
a0d0e21e
LW
1554=item goto &NAME
1555
1556The goto-LABEL form finds the statement labeled with LABEL and resumes
1557execution there. It may not be used to go into any construct that
1558requires initialization, such as a subroutine or a foreach loop. It
0a753a76 1559also can't be used to go into a construct that is optimized away,
1560or to get out of a block or subroutine given to sort().
1561It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e
LW
1562including out of subroutines, but it's usually better to use some other
1563construct such as last or die. The author of Perl has never felt the
1564need to use this form of goto (in Perl, that is--C is another matter).
1565
748a9306
LW
1566The goto-EXPR form expects a label name, whose scope will be resolved
1567dynamically. This allows for computed gotos per FORTRAN, but isn't
1568necessarily recommended if you're optimizing for maintainability:
1569
1570 goto ("FOO", "BAR", "GLARCH")[$i];
1571
a0d0e21e
LW
1572The goto-&NAME form is highly magical, and substitutes a call to the
1573named subroutine for the currently running subroutine. This is used by
1574AUTOLOAD subroutines that wish to load another subroutine and then
1575pretend that the other subroutine had been called in the first place
1576(except that any modifications to @_ in the current subroutine are
1577propagated to the other subroutine.) After the goto, not even caller()
1578will be able to tell that this routine was called first.
1579
1580=item grep BLOCK LIST
1581
1582=item grep EXPR,LIST
1583
54310121 1584This is similar in spirit to, but not the same as, grep(1)
2f9daede
TP
1585and its relatives. In particular, it is not limited to using
1586regular expressions.
1587
a0d0e21e
LW
1588Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1589$_ to each element) and returns the list value consisting of those
1590elements for which the expression evaluated to TRUE. In a scalar
1591context, returns the number of times the expression was TRUE.
1592
1593 @foo = grep(!/^#/, @bar); # weed out comments
1594
1595or equivalently,
1596
1597 @foo = grep {!/^#/} @bar; # weed out comments
1598
5f05dabc 1599Note that, because $_ is a reference into the list value, it can be used
a0d0e21e
LW
1600to modify the elements of the array. While this is useful and
1601supported, it can cause bizarre results if the LIST is not a named
2f9daede 1602array. Similarly, grep returns aliases into the original list,
7b8d334a 1603much like the way that a for loops's index variable aliases the list
2f9daede 1604elements. That is, modifying an element of a list returned by grep
fb73857a 1605(for example, in a C<foreach>, C<map> or another C<grep>)
2f9daede 1606actually modifies the element in the original list.
a0d0e21e 1607
fb73857a 1608See also L</map> for an array composed of the results of the BLOCK or EXPR.
38325410 1609
a0d0e21e
LW
1610=item hex EXPR
1611
54310121 1612=item hex
bbce6d69 1613
54310121 1614Interprets EXPR as a hex string and returns the corresponding
2f9daede 1615value. (To convert strings that might start with either 0 or 0x
dc848c6f 1616see L</oct>.) If EXPR is omitted, uses $_.
2f9daede
TP
1617
1618 print hex '0xAf'; # prints '175'
1619 print hex 'aF'; # same
a0d0e21e
LW
1620
1621=item import
1622
54310121 1623There is no builtin import() function. It is merely an ordinary
4633a7c4 1624method (subroutine) defined (or inherited) by modules that wish to export
a0d0e21e 1625names to another module. The use() function calls the import() method
54310121 1626for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
1627
1628=item index STR,SUBSTR,POSITION
1629
1630=item index STR,SUBSTR
1631
4633a7c4
LW
1632Returns the position of the first occurrence of SUBSTR in STR at or after
1633POSITION. If POSITION is omitted, starts searching from the beginning of
184e9718 1634the string. The return value is based at 0 (or whatever you've set the C<$[>
4633a7c4 1635variable to--but don't do that). If the substring is not found, returns
a0d0e21e
LW
1636one less than the base, ordinarily -1.
1637
1638=item int EXPR
1639
54310121 1640=item int
bbce6d69 1641
a0d0e21e
LW
1642Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
1643
1644=item ioctl FILEHANDLE,FUNCTION,SCALAR
1645
1646Implements the ioctl(2) function. You'll probably have to say
1647
4633a7c4 1648 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 1649
4633a7c4 1650first to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 1651exist or doesn't have the correct definitions you'll have to roll your
4633a7c4
LW
1652own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1653(There is a Perl script called B<h2ph> that comes with the Perl kit which
54310121 1654may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4
LW
1655written depending on the FUNCTION--a pointer to the string value of SCALAR
1656will be passed as the third argument of the actual ioctl call. (If SCALAR
1657has no string value but does have a numeric value, that value will be
1658passed rather than a pointer to the string value. To guarantee this to be
1659TRUE, add a 0 to the scalar before using it.) The pack() and unpack()
1660functions are useful for manipulating the values of structures used by
1661ioctl(). The following example sets the erase character to DEL.
a0d0e21e
LW
1662
1663 require 'ioctl.ph';
4633a7c4
LW
1664 $getp = &TIOCGETP;
1665 die "NO TIOCGETP" if $@ || !$getp;
a0d0e21e 1666 $sgttyb_t = "ccccs"; # 4 chars and a short
4633a7c4 1667 if (ioctl(STDIN,$getp,$sgttyb)) {
a0d0e21e
LW
1668 @ary = unpack($sgttyb_t,$sgttyb);
1669 $ary[2] = 127;
1670 $sgttyb = pack($sgttyb_t,@ary);
4633a7c4 1671 ioctl(STDIN,&TIOCSETP,$sgttyb)
a0d0e21e
LW
1672 || die "Can't ioctl: $!";
1673 }
1674
1675The return value of ioctl (and fcntl) is as follows:
1676
1677 if OS returns: then Perl returns:
1678 -1 undefined value
1679 0 string "0 but true"
1680 anything else that number
1681
1682Thus Perl returns TRUE on success and FALSE on failure, yet you can
1683still easily determine the actual value returned by the operating
1684system:
1685
1686 ($retval = ioctl(...)) || ($retval = -1);
1687 printf "System returned %d\n", $retval;
1688
1689=item join EXPR,LIST
1690
54310121 1691Joins the separate strings of LIST into a single string with
a0d0e21e
LW
1692fields separated by the value of EXPR, and returns the string.
1693Example:
1694
1695 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1696
1697See L<perlfunc/split>.
1698
aa689395 1699=item keys HASH
1700
1701Returns a normal array consisting of all the keys of the named hash. (In
1702a scalar context, returns the number of keys.) The keys are returned in
1703an apparently random order, but it is the same order as either the
1704values() or each() function produces (given that the hash has not been
1705modified). As a side effect, it resets HASH's iterator.
a0d0e21e 1706
aa689395 1707Here is yet another way to print your environment:
a0d0e21e
LW
1708
1709 @keys = keys %ENV;
1710 @values = values %ENV;
1711 while ($#keys >= 0) {
1712 print pop(@keys), '=', pop(@values), "\n";
1713 }
1714
1715or how about sorted by key:
1716
1717 foreach $key (sort(keys %ENV)) {
1718 print $key, '=', $ENV{$key}, "\n";
1719 }
1720
54310121 1721To sort an array by value, you'll need to use a C<sort> function.
aa689395 1722Here's a descending numeric sort of a hash by its values:
4633a7c4
LW
1723
1724 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
1725 printf "%4d %s\n", $hash{$key}, $key;
1726 }
1727
55497cff 1728As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395 1729allocated for the given hash. This can gain you a measure of efficiency if
1730you know the hash is going to get big. (This is similar to pre-extending
1731an array by assigning a larger number to $#array.) If you say
55497cff 1732
1733 keys %hash = 200;
1734
1735then C<%hash> will have at least 200 buckets allocated for it. These
1736buckets will be retained even if you do C<%hash = ()>, use C<undef
1737%hash> if you want to free the storage while C<%hash> is still in scope.
1738You can't shrink the number of buckets allocated for the hash using
1739C<keys> in this way (but you needn't worry about doing this by accident,
1740as trying has no effect).
1741
a0d0e21e
LW
1742=item kill LIST
1743
54310121 1744Sends a signal to a list of processes. The first element of
1745the list must be the signal to send. Returns the number of
4633a7c4 1746processes successfully signaled.
a0d0e21e
LW
1747
1748 $cnt = kill 1, $child1, $child2;
1749 kill 9, @goners;
1750
4633a7c4
LW
1751Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1752process groups instead of processes. (On System V, a negative I<PROCESS>
1753number will also kill process groups, but that's not portable.) That
1754means you usually want to use positive not negative signals. You may also
da0045b7 1755use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
1756
1757=item last LABEL
1758
1759=item last
1760
1761The C<last> command is like the C<break> statement in C (as used in
1762loops); it immediately exits the loop in question. If the LABEL is
1763omitted, the command refers to the innermost enclosing loop. The
1764C<continue> block, if any, is not executed:
1765
4633a7c4
LW
1766 LINE: while (<STDIN>) {
1767 last LINE if /^$/; # exit when done with header
a0d0e21e
LW
1768 ...
1769 }
1770
1771=item lc EXPR
1772
54310121 1773=item lc
bbce6d69 1774
a0d0e21e 1775Returns an lowercased version of EXPR. This is the internal function
54310121 1776implementing the \L escape in double-quoted strings.
a034a98d 1777Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1778
bbce6d69 1779If EXPR is omitted, uses $_.
1780
a0d0e21e
LW
1781=item lcfirst EXPR
1782
54310121 1783=item lcfirst
bbce6d69 1784
a0d0e21e
LW
1785Returns the value of EXPR with the first character lowercased. This is
1786the internal function implementing the \l escape in double-quoted strings.
a034a98d 1787Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
a0d0e21e 1788
bbce6d69 1789If EXPR is omitted, uses $_.
1790
a0d0e21e
LW
1791=item length EXPR
1792
54310121 1793=item length
bbce6d69 1794
a0d0e21e
LW
1795Returns the length in characters of the value of EXPR. If EXPR is
1796omitted, returns length of $_.
1797
1798=item link OLDFILE,NEWFILE
1799
1800Creates a new filename linked to the old filename. Returns 1 for
1801success, 0 otherwise.
1802
1803=item listen SOCKET,QUEUESIZE
1804
1805Does the same thing that the listen system call does. Returns TRUE if
4633a7c4 1806it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
1807
1808=item local EXPR
1809
a0d0e21e 1810A local modifies the listed variables to be local to the enclosing block,
5f05dabc 1811subroutine, C<eval{}>, or C<do>. If more than one value is listed, the
1812list must be placed in parentheses. See L<perlsub/"Temporary Values via
3e3baf6d 1813local()"> for details, including issues with tied arrays and hashes.
a0d0e21e 1814
7b8d334a
GS
1815You really probably want to be using my() instead, because local() isn't
1816what most people think of as "local". See L<perlsub/"Private Variables
cb1a09d0 1817via my()"> for details.
a0d0e21e
LW
1818
1819=item localtime EXPR
1820
1821Converts a time as returned by the time function to a 9-element array
5f05dabc 1822with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
1823follows:
1824
54310121 1825 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
1826 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1827 localtime(time);
1828
1829All array elements are numeric, and come straight out of a struct tm.
1830In particular this means that $mon has the range 0..11 and $wday has
54310121 1831the range 0..6 with sunday as day 0. Also, $year is the number of
1832years since 1900, that is, $year is 123 in year 2023.
1833
1834If EXPR is omitted, uses the current time (C<localtime(time)>).
a0d0e21e 1835
0a753a76 1836In a scalar context, returns the ctime(3) value:
a0d0e21e 1837
5f05dabc 1838 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 1839
a3cb178b
GS
1840This scalar value is B<not> locale dependent, see L<perllocale>, but
1841instead a Perl builtin. Also see the Time::Local module, and the
1842strftime(3) and mktime(3) function available via the POSIX module. To
1843get somewhat similar but locale dependent date strings, set up your
1844locale environment variables appropriately (please see L<perllocale>)
1845and try for example
1846
1847 use POSIX qw(strftime)
1848 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
1849
1850Note that the C<%a> and C<%b>, the short forms of the day of the week
1851and the month of the year, may not necessarily be three characters wide.
a0d0e21e
LW
1852
1853=item log EXPR
1854
54310121 1855=item log
bbce6d69 1856
a0d0e21e
LW
1857Returns logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
1858of $_.
1859
1860=item lstat FILEHANDLE
1861
1862=item lstat EXPR
1863
54310121 1864=item lstat
bbce6d69 1865
a0d0e21e
LW
1866Does the same thing as the stat() function, but stats a symbolic link
1867instead of the file the symbolic link points to. If symbolic links are
1868unimplemented on your system, a normal stat() is done.
1869
bbce6d69 1870If EXPR is omitted, stats $_.
1871
a0d0e21e
LW
1872=item m//
1873
1874The match operator. See L<perlop>.
1875
1876=item map BLOCK LIST
1877
1878=item map EXPR,LIST
1879
1880Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
1881element) and returns the list value composed of the results of each such
1882evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
1883may produce zero, one, or more elements in the returned value.
1884
1885 @chars = map(chr, @nums);
1886
1887translates a list of numbers to the corresponding characters. And
1888
4633a7c4 1889 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
1890
1891is just a funny way to write
1892
1893 %hash = ();
1894 foreach $_ (@array) {
4633a7c4 1895 $hash{getkey($_)} = $_;
a0d0e21e
LW
1896 }
1897
fb73857a 1898Note that, because $_ is a reference into the list value, it can be used
1899to modify the elements of the array. While this is useful and
1900supported, it can cause bizarre results if the LIST is not a named
1901array. See also L</grep> for an array composed of those items of the
1902original list for which the BLOCK or EXPR evaluates to true.
1903
a0d0e21e
LW
1904=item mkdir FILENAME,MODE
1905
1906Creates the directory specified by FILENAME, with permissions specified
1907by MODE (as modified by umask). If it succeeds it returns 1, otherwise
184e9718 1908it returns 0 and sets C<$!> (errno).
a0d0e21e
LW
1909
1910=item msgctl ID,CMD,ARG
1911
4633a7c4 1912Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG
a0d0e21e
LW
1913must be a variable which will hold the returned msqid_ds structure.
1914Returns like ioctl: the undefined value for error, "0 but true" for
1915zero, or the actual return value otherwise.
1916
1917=item msgget KEY,FLAGS
1918
4633a7c4 1919Calls the System V IPC function msgget(2). Returns the message queue id,
a0d0e21e
LW
1920or the undefined value if there is an error.
1921
1922=item msgsnd ID,MSG,FLAGS
1923
1924Calls the System V IPC function msgsnd to send the message MSG to the
1925message queue ID. MSG must begin with the long integer message type,
c07a80fd 1926which may be created with C<pack("l", $type)>. Returns TRUE if
a0d0e21e
LW
1927successful, or FALSE if there is an error.
1928
1929=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
1930
1931Calls the System V IPC function msgrcv to receive a message from
1932message queue ID into variable VAR with a maximum message size of
1933SIZE. Note that if a message is received, the message type will be the
1934first thing in VAR, and the maximum length of VAR is SIZE plus the size
1935of the message type. Returns TRUE if successful, or FALSE if there is
1936an error.
1937
1938=item my EXPR
1939
1940A "my" declares the listed variables to be local (lexically) to the
cb1a09d0 1941enclosing block, subroutine, C<eval>, or C<do/require/use>'d file. If
5f05dabc 1942more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 1943L<perlsub/"Private Variables via my()"> for details.
4633a7c4 1944
a0d0e21e
LW
1945=item next LABEL
1946
1947=item next
1948
1949The C<next> command is like the C<continue> statement in C; it starts
1950the next iteration of the loop:
1951
4633a7c4
LW
1952 LINE: while (<STDIN>) {
1953 next LINE if /^#/; # discard comments
a0d0e21e
LW
1954 ...
1955 }
1956
1957Note that if there were a C<continue> block on the above, it would get
1958executed even on discarded lines. If the LABEL is omitted, the command
1959refers to the innermost enclosing loop.
1960
1961=item no Module LIST
1962
1963See the "use" function, which "no" is the opposite of.
1964
1965=item oct EXPR
1966
54310121 1967=item oct
bbce6d69 1968
4633a7c4 1969Interprets EXPR as an octal string and returns the corresponding
2f9daede 1970value. (If EXPR happens to start off with 0x, interprets it as
4633a7c4
LW
1971a hex string instead.) The following will handle decimal, octal, and
1972hex in the standard Perl or C notation:
a0d0e21e
LW
1973
1974 $val = oct($val) if $val =~ /^0/;
1975
2f9daede
TP
1976If EXPR is omitted, uses $_. This function is commonly used when
1977a string such as "644" needs to be converted into a file mode, for
1978example. (Although perl will automatically convert strings into
1979numbers as needed, this automatic conversion assumes base 10.)
a0d0e21e
LW
1980
1981=item open FILEHANDLE,EXPR
1982
1983=item open FILEHANDLE
1984
1985Opens the file whose filename is given by EXPR, and associates it with
5f05dabc 1986FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
1987name of the real filehandle wanted. If EXPR is omitted, the scalar
1988variable of the same name as the FILEHANDLE contains the filename.
1989(Note that lexical variables--those declared with C<my>--will not work
1990for this purpose; so if you're using C<my>, specify EXPR in your call
1991to open.)
1992
1993If the filename begins with '<' or nothing, the file is opened for input.
1994If the filename begins with '>', the file is truncated and opened for
1995output. If the filename begins with '>>', the file is opened for
1996appending. You can put a '+' in front of the '>' or '<' to indicate that
1997you want both read and write access to the file; thus '+<' is almost
1998always preferred for read/write updates--the '+>' mode would clobber the
1999file first. The prefix and the filename may be separated with spaces.
2000These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w',
2001'w+', 'a', and 'a+'.
2002
2003If the filename begins with "|", the filename is interpreted as a command
2004to which output is to be piped, and if the filename ends with a "|", the
2005filename is interpreted See L<perlipc/"Using open() for IPC"> for more
2006examples of this. as command which pipes input to us. (You may not have
7e1af8bc 2007a raw open() to a command that pipes both in I<and> out, but see
2008L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2009for alternatives.)
cb1a09d0 2010
184e9718 2011Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT. Open returns
54310121 2012nonzero upon success, the undefined value otherwise. If the open
4633a7c4 2013involved a pipe, the return value happens to be the pid of the
54310121 2014subprocess.
cb1a09d0
AD
2015
2016If you're unfortunate enough to be running Perl on a system that
2017distinguishes between text files and binary files (modern operating
2018systems don't care), then you should check out L</binmode> for tips for
2019dealing with this. The key distinction between systems that need binmode
2020and those that don't is their text file formats. Systems like Unix and
2021Plan9 that delimit lines with a single character, and that encode that
2022character in C as '\n', do not need C<binmode>. The rest need it.
2023
fb73857a 2024When opening a file, it's usually a bad idea to continue normal execution
2025if the request failed, so C<open> is frequently used in connection with
2026C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2027where you want to make a nicely formatted error message (but there are
2028modules which can help with that problem)) you should always check
2029the return value from opening a file. The infrequent exception is when
2030working with an unopened filehandle is actually what you want to do.
2031
cb1a09d0 2032Examples:
a0d0e21e
LW
2033
2034 $ARTICLE = 100;
2035 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2036 while (<ARTICLE>) {...
2037
2038 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2039 # if the open fails, output is discarded
a0d0e21e 2040
fb73857a 2041 open(DBASE, '+<dbase.mine') # open for update
2042 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2043
fb73857a 2044 open(ARTICLE, "caesar <$article |") # decrypt article
2045 or die "Can't start caesar: $!";
a0d0e21e 2046
fb73857a 2047 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2048 or die "Can't start sort: $!";
a0d0e21e
LW
2049
2050 # process argument list of files along with any includes
2051
2052 foreach $file (@ARGV) {
2053 process($file, 'fh00');
2054 }
2055
2056 sub process {
2057 local($filename, $input) = @_;
2058 $input++; # this is a string increment
2059 unless (open($input, $filename)) {
2060 print STDERR "Can't open $filename: $!\n";
2061 return;
2062 }
2063
2064 while (<$input>) { # note use of indirection
2065 if (/^#include "(.*)"/) {
2066 process($1, $input);
2067 next;
2068 }
2069 ... # whatever
2070 }
2071 }
2072
2073You may also, in the Bourne shell tradition, specify an EXPR beginning
184e9718 2074with "E<gt>&", in which case the rest of the string is interpreted as the
a0d0e21e 2075name of a filehandle (or file descriptor, if numeric) which is to be
184e9718 2076duped and opened. You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
5f05dabc 2077+E<gt>E<gt>, and +E<lt>. The
a0d0e21e 2078mode you specify should match the mode of the original filehandle.
184e9718 2079(Duping a filehandle does not take into account any existing contents of
cb1a09d0 2080stdio buffers.)
a0d0e21e
LW
2081Here is a script that saves, redirects, and restores STDOUT and
2082STDERR:
2083
2084 #!/usr/bin/perl
2085 open(SAVEOUT, ">&STDOUT");
2086 open(SAVEERR, ">&STDERR");
2087
2088 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2089 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2090
2091 select(STDERR); $| = 1; # make unbuffered
2092 select(STDOUT); $| = 1; # make unbuffered
2093
2094 print STDOUT "stdout 1\n"; # this works for
2095 print STDERR "stderr 1\n"; # subprocesses too
2096
2097 close(STDOUT);
2098 close(STDERR);
2099
2100 open(STDOUT, ">&SAVEOUT");
2101 open(STDERR, ">&SAVEERR");
2102
2103 print STDOUT "stdout 2\n";
2104 print STDERR "stderr 2\n";
2105
2106
184e9718 2107If you specify "E<lt>&=N", where N is a number, then Perl will do an
4633a7c4
LW
2108equivalent of C's fdopen() of that file descriptor; this is more
2109parsimonious of file descriptors. For example:
a0d0e21e
LW
2110
2111 open(FILEHANDLE, "<&=$fd")
2112
5f05dabc 2113If you open a pipe on the command "-", i.e., either "|-" or "-|", then
a0d0e21e
LW
2114there is an implicit fork done, and the return value of open is the pid
2115of the child within the parent process, and 0 within the child
184e9718 2116process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2117The filehandle behaves normally for the parent, but i/o to that
2118filehandle is piped from/to the STDOUT/STDIN of the child process.
2119In the child process the filehandle isn't opened--i/o happens from/to
2120the new STDOUT or STDIN. Typically this is used like the normal
2121piped open when you want to exercise more control over just how the
2122pipe command gets executed, such as when you are running setuid, and
54310121 2123don't want to have to scan shell commands for metacharacters.
4633a7c4 2124The following pairs are more or less equivalent:
a0d0e21e
LW
2125
2126 open(FOO, "|tr '[a-z]' '[A-Z]'");
2127 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2128
2129 open(FOO, "cat -n '$file'|");
2130 open(FOO, "-|") || exec 'cat', '-n', $file;
2131
4633a7c4
LW
2132See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2133
0dccf244 2134NOTE: On any operation which may do a fork, unflushed buffers remain
184e9718 2135unflushed in both processes, which means you may need to set C<$|> to
a0d0e21e
LW
2136avoid duplicate output.
2137
0dccf244
CS
2138Closing any piped filehandle causes the parent process to wait for the
2139child to finish, and returns the status value in C<$?>.
2140
5f05dabc 2141Using the constructor from the IO::Handle package (or one of its
2142subclasses, such as IO::File or IO::Socket),
c07a80fd 2143you can generate anonymous filehandles which have the scope of whatever
2144variables hold references to them, and automatically close whenever
2145and however you leave that scope:
2146
5f05dabc 2147 use IO::File;
c07a80fd 2148 ...
2149 sub read_myfile_munged {
2150 my $ALL = shift;
5f05dabc 2151 my $handle = new IO::File;
c07a80fd 2152 open($handle, "myfile") or die "myfile: $!";
2153 $first = <$handle>
2154 or return (); # Automatically closed here.
2155 mung $first or die "mung failed"; # Or here.
2156 return $first, <$handle> if $ALL; # Or here.
2157 $first; # Or here.
2158 }
2159
a0d0e21e 2160The filename that is passed to open will have leading and trailing
5f05dabc 2161whitespace deleted. To open a file with arbitrary weird
a0d0e21e
LW
2162characters in it, it's necessary to protect any leading and trailing
2163whitespace thusly:
2164
cb1a09d0
AD
2165 $file =~ s#^(\s)#./$1#;
2166 open(FOO, "< $file\0");
2167
c07a80fd 2168If you want a "real" C open() (see L<open(2)> on your system), then
2169you should use the sysopen() function. This is another way to
2170protect your filenames from interpretation. For example:
cb1a09d0 2171
28757baa 2172 use IO::Handle;
c07a80fd 2173 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
2174 or die "sysopen $path: $!";
2175 HANDLE->autoflush(1);
2176 HANDLE->print("stuff $$\n");
2177 seek(HANDLE, 0, 0);
2178 print "File contains: ", <HANDLE>;
cb1a09d0
AD
2179
2180See L</seek()> for some details about mixing reading and writing.
a0d0e21e
LW
2181
2182=item opendir DIRHANDLE,EXPR
2183
2184Opens a directory named EXPR for processing by readdir(), telldir(),
5f05dabc 2185seekdir(), rewinddir(), and closedir(). Returns TRUE if successful.
a0d0e21e
LW
2186DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2187
2188=item ord EXPR
2189
54310121 2190=item ord
bbce6d69 2191
a0d0e21e 2192Returns the numeric ascii value of the first character of EXPR. If
dc848c6f 2193EXPR is omitted, uses $_. For the reverse, see L</chr>.
a0d0e21e
LW
2194
2195=item pack TEMPLATE,LIST
2196
2197Takes an array or list of values and packs it into a binary structure,
2198returning the string containing the structure. The TEMPLATE is a
2199sequence of characters that give the order and type of values, as
2200follows:
2201
2202 A An ascii string, will be space padded.
2203 a An ascii string, will be null padded.
2204 b A bit string (ascending bit order, like vec()).
2205 B A bit string (descending bit order).
2206 h A hex string (low nybble first).
2207 H A hex string (high nybble first).
2208
2209 c A signed char value.
2210 C An unsigned char value.
96e4d5b1 2211
a0d0e21e
LW
2212 s A signed short value.
2213 S An unsigned short value.
96e4d5b1 2214 (This 'short' is _exactly_ 16 bits, which may differ from
2215 what a local C compiler calls 'short'.)
2216
a0d0e21e
LW
2217 i A signed integer value.
2218 I An unsigned integer value.
96e4d5b1 2219 (This 'integer' is _at_least_ 32 bits wide. Its exact size
2220 depends on what a local C compiler calls 'int', and may
2221 even be larger than the 'long' described in the next item.)
2222
a0d0e21e
LW
2223 l A signed long value.
2224 L An unsigned long value.
96e4d5b1 2225 (This 'long' is _exactly_ 32 bits, which may differ from
2226 what a local C compiler calls 'long'.)
a0d0e21e 2227
96e4d5b1 2228 n A short in "network" (big-endian) order.
2229 N A long in "network" (big-endian) order.
a0d0e21e
LW
2230 v A short in "VAX" (little-endian) order.
2231 V A long in "VAX" (little-endian) order.
96e4d5b1 2232 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2233 _exactly_ 32 bits, respectively.)
a0d0e21e
LW
2234
2235 f A single-precision float in the native format.
2236 d A double-precision float in the native format.
2237
2238 p A pointer to a null-terminated string.
2239 P A pointer to a structure (fixed-length string).
2240
2241 u A uuencoded string.
2242
96e4d5b1 2243 w A BER compressed integer. Its bytes represent an unsigned
2244 integer in base 128, most significant digit first, with as few
2245 digits as possible. Bit eight (the high bit) is set on each
2246 byte except the last.
def98dd4 2247
a0d0e21e
LW
2248 x A null byte.
2249 X Back up a byte.
2250 @ Null fill to absolute position.
2251
2252Each letter may optionally be followed by a number which gives a repeat
5f05dabc 2253count. With all types except "a", "A", "b", "B", "h", "H", and "P" the
a0d0e21e
LW
2254pack function will gobble up that many values from the LIST. A * for the
2255repeat count means to use however many items are left. The "a" and "A"
2256types gobble just one value, but pack it as a string of length count,
2257padding with nulls or spaces as necessary. (When unpacking, "A" strips
2258trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B"
2259fields pack a string that many bits long. The "h" and "H" fields pack a
84902520
TB
2260string that many nybbles long. The "p" type packs a pointer to a null-
2261terminated string. You are responsible for ensuring the string is not a
2262temporary value (which can potentially get deallocated before you get
2263around to using the packed result). The "P" packs a pointer to a structure
61167c6f
PM
2264of the size indicated by the length. A NULL pointer is created if the
2265corresponding value for "p" or "P" is C<undef>.
2266Real numbers (floats and doubles) are
a0d0e21e
LW
2267in the native machine format only; due to the multiplicity of floating
2268formats around, and the lack of a standard "network" representation, no
2269facility for interchange has been made. This means that packed floating
2270point data written on one machine may not be readable on another - even if
2271both use IEEE floating point arithmetic (as the endian-ness of the memory
2272representation is not part of the IEEE spec). Note that Perl uses doubles
2273internally for all numeric calculation, and converting from double into
5f05dabc 2274float and thence back to double again will lose precision (i.e.,
a0d0e21e
LW
2275C<unpack("f", pack("f", $foo)>) will not in general equal $foo).
2276
2277Examples:
2278
2279 $foo = pack("cccc",65,66,67,68);
2280 # foo eq "ABCD"
2281 $foo = pack("c4",65,66,67,68);
2282 # same thing
2283
2284 $foo = pack("ccxxcc",65,66,67,68);
2285 # foo eq "AB\0\0CD"
2286
2287 $foo = pack("s2",1,2);
2288 # "\1\0\2\0" on little-endian
2289 # "\0\1\0\2" on big-endian
2290
2291 $foo = pack("a4","abcd","x","y","z");
2292 # "abcd"
2293
2294 $foo = pack("aaaa","abcd","x","y","z");
2295 # "axyz"
2296
2297 $foo = pack("a14","abcdefg");
2298 # "abcdefg\0\0\0\0\0\0\0"
2299
2300 $foo = pack("i9pl", gmtime);
2301 # a real struct tm (on my system anyway)
2302
2303 sub bintodec {
2304 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2305 }
2306
2307The same template may generally also be used in the unpack function.
2308
cb1a09d0
AD
2309=item package NAMESPACE
2310
2311Declares the compilation unit as being in the given namespace. The scope
2312of the package declaration is from the declaration itself through the end of
2313the enclosing block (the same scope as the local() operator). All further
2314unqualified dynamic identifiers will be in this namespace. A package
5f05dabc 2315statement affects only dynamic variables--including those you've used
cb1a09d0
AD
2316local() on--but I<not> lexical variables created with my(). Typically it
2317would be the first declaration in a file to be included by the C<require>
2318or C<use> operator. You can switch into a package in more than one place;
5f05dabc 2319it influences merely which symbol table is used by the compiler for the
cb1a09d0
AD
2320rest of that block. You can refer to variables and filehandles in other
2321packages by prefixing the identifier with the package name and a double
2322colon: C<$Package::Variable>. If the package name is null, the C<main>
2323package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2324
2325See L<perlmod/"Packages"> for more information about packages, modules,
2326and classes. See L<perlsub> for other scoping issues.
2327
a0d0e21e
LW
2328=item pipe READHANDLE,WRITEHANDLE
2329
2330Opens a pair of connected pipes like the corresponding system call.
2331Note that if you set up a loop of piped processes, deadlock can occur
2332unless you are very careful. In addition, note that Perl's pipes use
184e9718 2333stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
a0d0e21e
LW
2334after each command, depending on the application.
2335
7e1af8bc 2336See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4633a7c4
LW
2337for examples of such things.
2338
a0d0e21e
LW
2339=item pop ARRAY
2340
54310121 2341=item pop
28757baa 2342
a0d0e21e
LW
2343Pops and returns the last value of the array, shortening the array by
23441. Has a similar effect to
2345
2346 $tmp = $ARRAY[$#ARRAY--];
2347
2348If there are no elements in the array, returns the undefined value.
cb1a09d0
AD
2349If ARRAY is omitted, pops the
2350@ARGV array in the main program, and the @_ array in subroutines, just
2351like shift().
a0d0e21e
LW
2352
2353=item pos SCALAR
2354
54310121 2355=item pos
bbce6d69 2356
4633a7c4 2357Returns the offset of where the last C<m//g> search left off for the variable
2f9daede 2358is in question ($_ is used when the variable is not specified). May be
44a8e56a 2359modified to change that offset. Such modification will also influence
2360the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2361L<perlop>.
a0d0e21e
LW
2362
2363=item print FILEHANDLE LIST
2364
2365=item print LIST
2366
2367=item print
2368
cb1a09d0 2369Prints a string or a comma-separated list of strings. Returns TRUE
a0d0e21e 2370if successful. FILEHANDLE may be a scalar variable name, in which case
cb1a09d0 2371the variable contains the name of or a reference to the filehandle, thus introducing one
a0d0e21e
LW
2372level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2373token is a term, it may be misinterpreted as an operator unless you
5f05dabc 2374interpose a + or put parentheses around the arguments.) If FILEHANDLE is
a0d0e21e 2375omitted, prints by default to standard output (or to the last selected
da0045b7 2376output channel--see L</select>). If LIST is also omitted, prints $_ to
a0d0e21e
LW
2377STDOUT. To set the default output channel to something other than
2378STDOUT use the select operation. Note that, because print takes a
2379LIST, anything in the LIST is evaluated in a list context, and any
2380subroutine that you call will have one or more of its expressions
2381evaluated in a list context. Also be careful not to follow the print
2382keyword with a left parenthesis unless you want the corresponding right
2383parenthesis to terminate the arguments to the print--interpose a + or
5f05dabc 2384put parentheses around all the arguments.
a0d0e21e 2385
4633a7c4 2386Note that if you're storing FILEHANDLES in an array or other expression,
da0045b7 2387you will have to use a block returning its value instead:
4633a7c4
LW
2388
2389 print { $files[$i] } "stuff\n";
2390 print { $OK ? STDOUT : STDERR } "stuff\n";
2391
5f05dabc 2392=item printf FILEHANDLE FORMAT, LIST
a0d0e21e 2393
5f05dabc 2394=item printf FORMAT, LIST
a0d0e21e 2395
a3cb178b
GS
2396Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that $\
2397(the output record separator) is not appended. The first argument
a034a98d
DD
2398of the list will be interpreted as the printf format. If C<use locale> is
2399in effect, the character used for the decimal point in formatted real numbers
2400is affected by the LC_NUMERIC locale. See L<perllocale>.
a0d0e21e 2401
28757baa 2402Don't fall into the trap of using a printf() when a simple
2403print() would do. The print() is more efficient, and less
2404error prone.
2405
da0045b7 2406=item prototype FUNCTION
2407
2408Returns the prototype of a function as a string (or C<undef> if the
5f05dabc 2409function has no prototype). FUNCTION is a reference to, or the name of,
2410the function whose prototype you want to retrieve.
da0045b7 2411
b6c543e3
IZ
2412If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2413a name for Perl builtin. If builtin is not I<overridable> (such as
2414C<qw>) or its arguments cannot be expressed by a prototype (such as
2415C<system>) - in other words, the builtin does not behave like a Perl
2416function - returns C<undef>. Otherwise, the string describing the
2417equivalent prototype is returned.
2418
a0d0e21e
LW
2419=item push ARRAY,LIST
2420
2421Treats ARRAY as a stack, and pushes the values of LIST
2422onto the end of ARRAY. The length of ARRAY increases by the length of
2423LIST. Has the same effect as
2424
2425 for $value (LIST) {
2426 $ARRAY[++$#ARRAY] = $value;
2427 }
2428
2429but is more efficient. Returns the new number of elements in the array.
2430
2431=item q/STRING/
2432
2433=item qq/STRING/
2434
2435=item qx/STRING/
2436
2437=item qw/STRING/
2438
2439Generalized quotes. See L<perlop>.
2440
2441=item quotemeta EXPR
2442
54310121 2443=item quotemeta
bbce6d69 2444
68dc0745 2445Returns the value of EXPR with all non-alphanumeric
a034a98d
DD
2446characters backslashed. (That is, all characters not matching
2447C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2448returned string, regardless of any locale settings.)
2449This is the internal function implementing
a0d0e21e
LW
2450the \Q escape in double-quoted strings.
2451
bbce6d69 2452If EXPR is omitted, uses $_.
2453
a0d0e21e
LW
2454=item rand EXPR
2455
2456=item rand
2457
3e3baf6d
TB
2458Returns a random fractional number greater than or equal to 0 and less
2459than the value of EXPR. (EXPR should be positive.) If EXPR is
2460omitted, the value 1 is used. Automatically calls srand() unless
2461srand() has already been called. See also srand().
a0d0e21e 2462
2f9daede 2463(Note: If your rand function consistently returns numbers that are too
a0d0e21e 2464large or too small, then your version of Perl was probably compiled
2f9daede 2465with the wrong number of RANDBITS.)
a0d0e21e
LW
2466
2467=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2468
2469=item read FILEHANDLE,SCALAR,LENGTH
2470
2471Attempts to read LENGTH bytes of data into variable SCALAR from the
2472specified FILEHANDLE. Returns the number of bytes actually read, or
2473undef if there was an error. SCALAR will be grown or shrunk to the
2474length actually read. An OFFSET may be specified to place the read
2475data at some other place than the beginning of the string. This call
2476is actually implemented in terms of stdio's fread call. To get a true
2477read system call, see sysread().
2478
2479=item readdir DIRHANDLE
2480
2481Returns the next directory entry for a directory opened by opendir().
2482If used in a list context, returns all the rest of the entries in the
2483directory. If there are no more entries, returns an undefined value in
2484a scalar context or a null list in a list context.
2485
cb1a09d0 2486If you're planning to filetest the return values out of a readdir(), you'd
5f05dabc 2487better prepend the directory in question. Otherwise, because we didn't
cb1a09d0
AD
2488chdir() there, it would have been testing the wrong file.
2489
2490 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2491 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2492 closedir DIR;
2493
84902520
TB
2494=item readline EXPR
2495
2496Reads from the file handle EXPR. In scalar context, a single line
2497is read and returned. In list context, reads until end-of-file is
2498reached and returns a list of lines (however you've defined lines
2499with $/ or $INPUT_RECORD_SEPARATOR).
2500This is the internal function implementing the C<E<lt>EXPRE<gt>>
2501operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2502operator is discussed in more detail in L<perlop/"I/O Operators">.
2503
a0d0e21e
LW
2504=item readlink EXPR
2505
54310121 2506=item readlink
bbce6d69 2507
a0d0e21e
LW
2508Returns the value of a symbolic link, if symbolic links are
2509implemented. If not, gives a fatal error. If there is some system
184e9718 2510error, returns the undefined value and sets C<$!> (errno). If EXPR is
a0d0e21e
LW
2511omitted, uses $_.
2512
84902520
TB
2513=item readpipe EXPR
2514
2515EXPR is interpolated and then executed as a system command.
2516The collected standard output of the command is returned.
2517In scalar context, it comes back as a single (potentially
2518multi-line) string. In list context, returns a list of lines
2519(however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR).
2520This is the internal function implementing the C<qx/EXPR/>
2521operator, but you can use it directly. The C<qx/EXPR/>
2522operator is discussed in more detail in L<perlop/"I/O Operators">.
2523
a0d0e21e
LW
2524=item recv SOCKET,SCALAR,LEN,FLAGS
2525
2526Receives a message on a socket. Attempts to receive LENGTH bytes of
2527data into variable SCALAR from the specified SOCKET filehandle.
a3cb178b 2528Actually does a C recvfrom(), so that it can return the address of the
a0d0e21e
LW
2529sender. Returns the undefined value if there's an error. SCALAR will
2530be grown or shrunk to the length actually read. Takes the same flags
54310121 2531as the system call of the same name.
4633a7c4 2532See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2533
2534=item redo LABEL
2535
2536=item redo
2537
2538The C<redo> command restarts the loop block without evaluating the
2539conditional again. The C<continue> block, if any, is not executed. If
2540the LABEL is omitted, the command refers to the innermost enclosing
2541loop. This command is normally used by programs that want to lie to
2542themselves about what was just input:
2543
2544 # a simpleminded Pascal comment stripper
2545 # (warning: assumes no { or } in strings)
4633a7c4 2546 LINE: while (<STDIN>) {
a0d0e21e
LW
2547 while (s|({.*}.*){.*}|$1 |) {}
2548 s|{.*}| |;
2549 if (s|{.*| |) {
2550 $front = $_;
2551 while (<STDIN>) {
2552 if (/}/) { # end of comment?
2553 s|^|$front{|;
4633a7c4 2554 redo LINE;
a0d0e21e
LW
2555 }
2556 }
2557 }
2558 print;
2559 }
2560
2561=item ref EXPR
2562
54310121 2563=item ref
bbce6d69 2564
2f9daede
TP
2565Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2566is not specified, $_ will be used. The value returned depends on the
bbce6d69 2567type of thing the reference is a reference to.
a0d0e21e
LW
2568Builtin types include:
2569
2570 REF
2571 SCALAR
2572 ARRAY
2573 HASH
2574 CODE
2575 GLOB
2576
54310121 2577If the referenced object has been blessed into a package, then that package
a0d0e21e
LW
2578name is returned instead. You can think of ref() as a typeof() operator.
2579
2580 if (ref($r) eq "HASH") {
aa689395 2581 print "r is a reference to a hash.\n";
54310121 2582 }
a0d0e21e
LW
2583 if (!ref ($r) {
2584 print "r is not a reference at all.\n";
54310121 2585 }
a0d0e21e
LW
2586
2587See also L<perlref>.
2588
2589=item rename OLDNAME,NEWNAME
2590
2591Changes the name of a file. Returns 1 for success, 0 otherwise. Will
5f05dabc 2592not work across file system boundaries.
a0d0e21e
LW
2593
2594=item require EXPR
2595
2596=item require
2597
2598Demands some semantics specified by EXPR, or by $_ if EXPR is not
2599supplied. If EXPR is numeric, demands that the current version of Perl
184e9718 2600(C<$]> or $PERL_VERSION) be equal or greater than EXPR.
a0d0e21e
LW
2601
2602Otherwise, demands that a library file be included if it hasn't already
2603been included. The file is included via the do-FILE mechanism, which is
2604essentially just a variety of eval(). Has semantics similar to the following
2605subroutine:
2606
2607 sub require {
2608 local($filename) = @_;
2609 return 1 if $INC{$filename};
2610 local($realfilename,$result);
2611 ITER: {
2612 foreach $prefix (@INC) {
2613 $realfilename = "$prefix/$filename";
2614 if (-f $realfilename) {
2615 $result = do $realfilename;
2616 last ITER;
2617 }
2618 }
2619 die "Can't find $filename in \@INC";
2620 }
2621 die $@ if $@;
2622 die "$filename did not return true value" unless $result;
2623 $INC{$filename} = $realfilename;
2624 $result;
2625 }
2626
2627Note that the file will not be included twice under the same specified
2628name. The file must return TRUE as the last statement to indicate
2629successful execution of any initialization code, so it's customary to
2630end such a file with "1;" unless you're sure it'll return TRUE
2631otherwise. But it's better just to put the "C<1;>", in case you add more
2632statements.
2633
54310121 2634If EXPR is a bareword, the require assumes a "F<.pm>" extension and
da0045b7 2635replaces "F<::>" with "F</>" in the filename for you,
54310121 2636to make it easy to load standard modules. This form of loading of
a0d0e21e
LW
2637modules does not risk altering your namespace.
2638
54310121 2639For a yet-more-powerful import facility, see L</use> and
748a9306 2640L<perlmod>.
a0d0e21e
LW
2641
2642=item reset EXPR
2643
2644=item reset
2645
2646Generally used in a C<continue> block at the end of a loop to clear
2647variables and reset ?? searches so that they work again. The
2648expression is interpreted as a list of single characters (hyphens
2649allowed for ranges). All variables and arrays beginning with one of
2650those letters are reset to their pristine state. If the expression is
5f05dabc 2651omitted, one-match searches (?pattern?) are reset to match again. Resets
2652only variables or searches in the current package. Always returns
a0d0e21e
LW
26531. Examples:
2654
2655 reset 'X'; # reset all X variables
2656 reset 'a-z'; # reset lower case variables
2657 reset; # just reset ?? searches
2658
5f05dabc 2659Resetting "A-Z" is not recommended because you'll wipe out your
2660ARGV and ENV arrays. Resets only package variables--lexical variables
a0d0e21e 2661are unaffected, but they clean themselves up on scope exit anyway,
da0045b7 2662so you'll probably want to use them instead. See L</my>.
a0d0e21e 2663
54310121 2664=item return EXPR
2665
2666=item return
2667
2668Returns from a subroutine, eval(), or do FILE with the value of the
2669given EXPR. Evaluation of EXPR may be in a list, scalar, or void
2670context, depending on how the return value will be used, and the context
2671may vary from one execution to the next (see wantarray()). If no EXPR
2672is given, returns an empty list in a list context, an undefined value in
2673a scalar context, or nothing in a void context.
a0d0e21e 2674
68dc0745 2675(Note that in the absence of a return, a subroutine, eval, or do FILE
2676will automatically return the value of the last expression evaluated.)
a0d0e21e
LW
2677
2678=item reverse LIST
2679
2680In a list context, returns a list value consisting of the elements
2f9daede
TP
2681of LIST in the opposite order. In a scalar context, concatenates the
2682elements of LIST, and returns a string value consisting of those bytes,
2683but in the opposite order.
4633a7c4 2684
2f9daede 2685 print reverse <>; # line tac, last line first
4633a7c4 2686
2f9daede
TP
2687 undef $/; # for efficiency of <>
2688 print scalar reverse <>; # byte tac, last line tsrif
2689
2690This operator is also handy for inverting a hash, although there are some
2691caveats. If a value is duplicated in the original hash, only one of those
2692can be represented as a key in the inverted hash. Also, this has to
2693unwind one hash and build a whole new one, which may take some time
2694on a large hash.
2695
2696 %by_name = reverse %by_address; # Invert the hash
a0d0e21e
LW
2697
2698=item rewinddir DIRHANDLE
2699
2700Sets the current position to the beginning of the directory for the
2701readdir() routine on DIRHANDLE.
2702
2703=item rindex STR,SUBSTR,POSITION
2704
2705=item rindex STR,SUBSTR
2706
2707Works just like index except that it returns the position of the LAST
2708occurrence of SUBSTR in STR. If POSITION is specified, returns the
2709last occurrence at or before that position.
2710
2711=item rmdir FILENAME
2712
54310121 2713=item rmdir
bbce6d69 2714
a0d0e21e 2715Deletes the directory specified by FILENAME if it is empty. If it
184e9718 2716succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno). If
a0d0e21e
LW
2717FILENAME is omitted, uses $_.
2718
2719=item s///
2720
2721The substitution operator. See L<perlop>.
2722
2723=item scalar EXPR
2724
2725Forces EXPR to be interpreted in a scalar context and returns the value
54310121 2726of EXPR.
cb1a09d0
AD
2727
2728 @counts = ( scalar @a, scalar @b, scalar @c );
2729
54310121 2730There is no equivalent operator to force an expression to
cb1a09d0
AD
2731be interpolated in a list context because it's in practice never
2732needed. If you really wanted to do so, however, you could use
2733the construction C<@{[ (some expression) ]}>, but usually a simple
2734C<(some expression)> suffices.
a0d0e21e
LW
2735
2736=item seek FILEHANDLE,POSITION,WHENCE
2737
8903cb82 2738Sets FILEHANDLE's position, just like the fseek() call of stdio.
2739FILEHANDLE may be an expression whose value gives the name of the
2740filehandle. The values for WHENCE are 0 to set the new position to
2741POSITION, 1 to set it to the current position plus POSITION, and 2 to
2742set it to EOF plus POSITION (typically negative). For WHENCE you may
2743use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the
2744IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.
2745
2746If you want to position file for sysread() or syswrite(), don't use
2747seek() -- buffering makes its effect on the file's system position
137443ea 2748unpredictable and non-portable. Use sysseek() instead.
a0d0e21e 2749
cb1a09d0
AD
2750On some systems you have to do a seek whenever you switch between reading
2751and writing. Amongst other things, this may have the effect of calling
8903cb82 2752stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving
2753the file position:
cb1a09d0
AD
2754
2755 seek(TEST,0,1);
2756
2757This is also useful for applications emulating C<tail -f>. Once you hit
2758EOF on your read, and then sleep for a while, you might have to stick in a
8903cb82 2759seek() to reset things. The seek() doesn't change the current position,
2760but it I<does> clear the end-of-file condition on the handle, so that the
2761next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
cb1a09d0
AD
2762
2763If that doesn't work (some stdios are particularly cantankerous), then
2764you may need something more like this:
2765
2766 for (;;) {
2767 for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
2768 # search for some stuff and put it into files
2769 }
2770 sleep($for_a_while);
2771 seek(FILE, $curpos, 0);
2772 }
2773
a0d0e21e
LW
2774=item seekdir DIRHANDLE,POS
2775
2776Sets the current position for the readdir() routine on DIRHANDLE. POS
2777must be a value returned by telldir(). Has the same caveats about
2778possible directory compaction as the corresponding system library
2779routine.
2780
2781=item select FILEHANDLE
2782
2783=item select
2784
2785Returns the currently selected filehandle. Sets the current default
2786filehandle for output, if FILEHANDLE is supplied. This has two
2787effects: first, a C<write> or a C<print> without a filehandle will
2788default to this FILEHANDLE. Second, references to variables related to
2789output will refer to this output channel. For example, if you have to
2790set the top of form format for more than one output channel, you might
2791do the following:
2792
2793 select(REPORT1);
2794 $^ = 'report1_top';
2795 select(REPORT2);
2796 $^ = 'report2_top';
2797
2798FILEHANDLE may be an expression whose value gives the name of the
2799actual filehandle. Thus:
2800
2801 $oldfh = select(STDERR); $| = 1; select($oldfh);
2802
4633a7c4
LW
2803Some programmers may prefer to think of filehandles as objects with
2804methods, preferring to write the last example as:
a0d0e21e 2805
28757baa 2806 use IO::Handle;
a0d0e21e
LW
2807 STDERR->autoflush(1);
2808
2809=item select RBITS,WBITS,EBITS,TIMEOUT
2810
5f05dabc 2811This calls the select(2) system call with the bit masks specified, which
a0d0e21e
LW
2812can be constructed using fileno() and vec(), along these lines:
2813
2814 $rin = $win = $ein = '';
2815 vec($rin,fileno(STDIN),1) = 1;
2816 vec($win,fileno(STDOUT),1) = 1;
2817 $ein = $rin | $win;
2818
2819If you want to select on many filehandles you might wish to write a
2820subroutine:
2821
2822 sub fhbits {
2823 local(@fhlist) = split(' ',$_[0]);
2824 local($bits);
2825 for (@fhlist) {
2826 vec($bits,fileno($_),1) = 1;
2827 }
2828 $bits;
2829 }
4633a7c4 2830 $rin = fhbits('STDIN TTY SOCK');
a0d0e21e
LW
2831
2832The usual idiom is:
2833
2834 ($nfound,$timeleft) =
2835 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
2836
54310121 2837or to block until something becomes ready just do this
a0d0e21e
LW
2838
2839 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
2840
5f05dabc 2841Most systems do not bother to return anything useful in $timeleft, so
c07a80fd 2842calling select() in a scalar context just returns $nfound.
2843
5f05dabc 2844Any of the bit masks can also be undef. The timeout, if specified, is
a0d0e21e
LW
2845in seconds, which may be fractional. Note: not all implementations are
2846capable of returning the $timeleft. If not, they always return
2847$timeleft equal to the supplied $timeout.
2848
ff68c719 2849You can effect a sleep of 250 milliseconds this way:
a0d0e21e
LW
2850
2851 select(undef, undef, undef, 0.25);
2852
184e9718 2853B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
cb1a09d0 2854with select(). You have to use sysread() instead.
a0d0e21e
LW
2855
2856=item semctl ID,SEMNUM,CMD,ARG
2857
2858Calls the System V IPC function semctl. If CMD is &IPC_STAT or
2859&GETALL, then ARG must be a variable which will hold the returned
2860semid_ds structure or semaphore value array. Returns like ioctl: the
2861undefined value for error, "0 but true" for zero, or the actual return
2862value otherwise.
2863
2864=item semget KEY,NSEMS,FLAGS
2865
2866Calls the System V IPC function semget. Returns the semaphore id, or
2867the undefined value if there is an error.
2868
2869=item semop KEY,OPSTRING
2870
2871Calls the System V IPC function semop to perform semaphore operations
2872such as signaling and waiting. OPSTRING must be a packed array of
2873semop structures. Each semop structure can be generated with
2874C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
2875operations is implied by the length of OPSTRING. Returns TRUE if
2876successful, or FALSE if there is an error. As an example, the
2877following code waits on semaphore $semnum of semaphore id $semid:
2878
2879 $semop = pack("sss", $semnum, -1, 0);
2880 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
2881
2882To signal the semaphore, replace "-1" with "1".
2883
2884=item send SOCKET,MSG,FLAGS,TO
2885
2886=item send SOCKET,MSG,FLAGS
2887
2888Sends a message on a socket. Takes the same flags as the system call
2889of the same name. On unconnected sockets you must specify a
2890destination to send TO, in which case it does a C sendto(). Returns
2891the number of characters sent, or the undefined value if there is an
2892error.
4633a7c4 2893See L<perlipc/"UDP: Message Passing"> for examples.
a0d0e21e
LW
2894
2895=item setpgrp PID,PGRP
2896
2897Sets the current process group for the specified PID, 0 for the current
2898process. Will produce a fatal error if used on a machine that doesn't
5f05dabc 2899implement setpgrp(2). If the arguments are omitted, it defaults to
47e29363 29000,0. Note that the POSIX version of setpgrp() does not accept any
2901arguments, so only setpgrp 0,0 is portable.
a0d0e21e
LW
2902
2903=item setpriority WHICH,WHO,PRIORITY
2904
2905Sets the current priority for a process, a process group, or a user.
748a9306 2906(See setpriority(2).) Will produce a fatal error if used on a machine
a0d0e21e
LW
2907that doesn't implement setpriority(2).
2908
2909=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
2910
2911Sets the socket option requested. Returns undefined if there is an
2912error. OPTVAL may be specified as undef if you don't want to pass an
2913argument.
2914
2915=item shift ARRAY
2916
2917=item shift
2918
2919Shifts the first value of the array off and returns it, shortening the
2920array by 1 and moving everything down. If there are no elements in the
2921array, returns the undefined value. If ARRAY is omitted, shifts the
977336f5
GS
2922@_ array within the lexical scope of subroutines and formats, and the
2923@ARGV array at file scopes or within the lexical scopes established by
2924the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
2925See also unshift(), push(), and pop(). Shift() and unshift() do the
2926same thing to the left end of an array that pop() and push() do to the
2927right end.
a0d0e21e
LW
2928
2929=item shmctl ID,CMD,ARG
2930
2931Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
2932must be a variable which will hold the returned shmid_ds structure.
2933Returns like ioctl: the undefined value for error, "0 but true" for
2934zero, or the actual return value otherwise.
2935
2936=item shmget KEY,SIZE,FLAGS
2937
2938Calls the System V IPC function shmget. Returns the shared memory
2939segment id, or the undefined value if there is an error.
2940
2941=item shmread ID,VAR,POS,SIZE
2942
2943=item shmwrite ID,STRING,POS,SIZE
2944
2945Reads or writes the System V shared memory segment ID starting at
2946position POS for size SIZE by attaching to it, copying in/out, and
2947detaching from it. When reading, VAR must be a variable which will
2948hold the data read. When writing, if STRING is too long, only SIZE
2949bytes are used; if STRING is too short, nulls are written to fill out
2950SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
2951
2952=item shutdown SOCKET,HOW
2953
2954Shuts down a socket connection in the manner indicated by HOW, which
2955has the same interpretation as in the system call of the same name.
2956
2957=item sin EXPR
2958
54310121 2959=item sin
bbce6d69 2960
a0d0e21e
LW
2961Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
2962returns sine of $_.
2963
54310121 2964For the inverse sine operation, you may use the POSIX::asin()
28757baa 2965function, or use this relation:
2966
2967 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
2968
a0d0e21e
LW
2969=item sleep EXPR
2970
2971=item sleep
2972
2973Causes the script to sleep for EXPR seconds, or forever if no EXPR.
2974May be interrupted by sending the process a SIGALRM. Returns the
2975number of seconds actually slept. You probably cannot mix alarm() and
5f05dabc 2976sleep() calls, because sleep() is often implemented using alarm().
a0d0e21e
LW
2977
2978On some older systems, it may sleep up to a full second less than what
2979you requested, depending on how it counts seconds. Most modern systems
2980always sleep the full amount.
2981
cb1a09d0 2982For delays of finer granularity than one second, you may use Perl's
54310121 2983syscall() interface to access setitimer(2) if your system supports it,
7b8d334a 2984or else see L</select()> above.
cb1a09d0 2985
5f05dabc 2986See also the POSIX module's sigpause() function.
2987
a0d0e21e
LW
2988=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
2989
2990Opens a socket of the specified kind and attaches it to filehandle
5f05dabc 2991SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
a0d0e21e 2992system call of the same name. You should "use Socket;" first to get
4633a7c4 2993the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2994
2995=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
2996
2997Creates an unnamed pair of sockets in the specified domain, of the
5f05dabc 2998specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
a0d0e21e
LW
2999for the system call of the same name. If unimplemented, yields a fatal
3000error. Returns TRUE if successful.
3001
3002=item sort SUBNAME LIST
3003
3004=item sort BLOCK LIST
3005
3006=item sort LIST
3007
2f9daede
TP
3008Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3009is omitted, sorts in standard string comparison order. If SUBNAME is
3010specified, it gives the name of a subroutine that returns an integer
3011less than, equal to, or greater than 0, depending on how the elements
3012of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3013operators are extremely useful in such routines.) SUBNAME may be a
3014scalar variable name, in which case the value provides the name of the
3015subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
3016an anonymous, in-line sort subroutine.
a0d0e21e 3017
cb1a09d0
AD
3018In the interests of efficiency the normal calling code for subroutines is
3019bypassed, with the following effects: the subroutine may not be a
3020recursive subroutine, and the two elements to be compared are passed into
3021the subroutine not via @_ but as the package global variables $a and
3022$b (see example below). They are passed by reference, so don't
3023modify $a and $b. And don't try to declare them as lexicals either.
a0d0e21e 3024
0a753a76 3025You also cannot exit out of the sort block or subroutine using any of the
3026loop control operators described in L<perlsyn> or with goto().
3027
a034a98d
DD
3028When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3029current collation locale. See L<perllocale>.
3030
a0d0e21e
LW
3031Examples:
3032
3033 # sort lexically
3034 @articles = sort @files;
3035
3036 # same thing, but with explicit sort routine
3037 @articles = sort {$a cmp $b} @files;
3038
cb1a09d0 3039 # now case-insensitively
54310121 3040 @articles = sort {uc($a) cmp uc($b)} @files;
cb1a09d0 3041
a0d0e21e
LW
3042 # same thing in reversed order
3043 @articles = sort {$b cmp $a} @files;
3044
3045 # sort numerically ascending
3046 @articles = sort {$a <=> $b} @files;
3047
3048 # sort numerically descending
3049 @articles = sort {$b <=> $a} @files;
3050
3051 # sort using explicit subroutine name
3052 sub byage {
2f9daede 3053 $age{$a} <=> $age{$b}; # presuming numeric
a0d0e21e
LW
3054 }
3055 @sortedclass = sort byage @class;
3056
aa689395 3057 # this sorts the %age hash by value instead of key
3058 # using an in-line function
c07a80fd 3059 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3060
a0d0e21e
LW
3061 sub backwards { $b cmp $a; }
3062 @harry = ('dog','cat','x','Cain','Abel');
3063 @george = ('gone','chased','yz','Punished','Axed');
3064 print sort @harry;
3065 # prints AbelCaincatdogx
3066 print sort backwards @harry;
3067 # prints xdogcatCainAbel
3068 print sort @george, 'to', @harry;
3069 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3070
54310121 3071 # inefficiently sort by descending numeric compare using
3072 # the first integer after the first = sign, or the
cb1a09d0
AD
3073 # whole record case-insensitively otherwise
3074
3075 @new = sort {
3076 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3077 ||
3078 uc($a) cmp uc($b)
3079 } @old;
3080
3081 # same thing, but much more efficiently;
3082 # we'll build auxiliary indices instead
3083 # for speed
3084 @nums = @caps = ();
54310121 3085 for (@old) {
cb1a09d0
AD
3086 push @nums, /=(\d+)/;
3087 push @caps, uc($_);
54310121 3088 }
cb1a09d0
AD
3089
3090 @new = @old[ sort {
3091 $nums[$b] <=> $nums[$a]
3092 ||
3093 $caps[$a] cmp $caps[$b]
3094 } 0..$#old
3095 ];
3096
3097 # same thing using a Schwartzian Transform (no temps)
3098 @new = map { $_->[0] }
3099 sort { $b->[1] <=> $a->[1]
3100 ||
3101 $a->[2] cmp $b->[2]
3102 } map { [$_, /=(\d+)/, uc($_)] } @old;
3103
184e9718 3104If you're using strict, you I<MUST NOT> declare $a
cb1a09d0
AD
3105and $b as lexicals. They are package globals. That means
3106if you're in the C<main> package, it's
3107
3108 @articles = sort {$main::b <=> $main::a} @files;
3109
3110or just
3111
3112 @articles = sort {$::b <=> $::a} @files;
3113
3114but if you're in the C<FooPack> package, it's
3115
3116 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3117
55497cff 3118The comparison function is required to behave. If it returns
3119inconsistent results (sometimes saying $x[1] is less than $x[2] and
3120sometimes saying the opposite, for example) the Perl interpreter will
3121probably crash and dump core. This is entirely due to and dependent
3122upon your system's qsort(3) library routine; this routine often avoids
3123sanity checks in the interest of speed.
3124
a0d0e21e
LW
3125=item splice ARRAY,OFFSET,LENGTH,LIST
3126
3127=item splice ARRAY,OFFSET,LENGTH
3128
3129=item splice ARRAY,OFFSET
3130
3131Removes the elements designated by OFFSET and LENGTH from an array, and
43051805
GS
3132replaces them with the elements of LIST, if any. In a list context,
3133returns the elements removed from the array. In a scalar context,
3134returns the last element removed, or C<undef> if no elements are
3135removed. The array grows or shrinks as necessary. If LENGTH is
3136omitted, removes everything from OFFSET onward. The following
3137equivalences hold (assuming C<$[ == 0>):
a0d0e21e
LW
3138
3139 push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
3140 pop(@a) splice(@a,-1)
3141 shift(@a) splice(@a,0,1)
3142 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3143 $a[$x] = $y splice(@a,$x,1,$y);
3144
3145Example, assuming array lengths are passed before arrays:
3146
3147 sub aeq { # compare two list values
3148 local(@a) = splice(@_,0,shift);
3149 local(@b) = splice(@_,0,shift);
3150 return 0 unless @a == @b; # same len?
3151 while (@a) {
3152 return 0 if pop(@a) ne pop(@b);
3153 }
3154 return 1;
3155 }
3156 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3157
3158=item split /PATTERN/,EXPR,LIMIT
3159
3160=item split /PATTERN/,EXPR
3161
3162=item split /PATTERN/
3163
3164=item split
3165
3166Splits a string into an array of strings, and returns it.
3167
3168If not in a list context, returns the number of fields found and splits into
3169the @_ array. (In a list context, you can force the split into @_ by
3170using C<??> as the pattern delimiters, but it still returns the array
3171value.) The use of implicit split to @_ is deprecated, however.
3172
3173If EXPR is omitted, splits the $_ string. If PATTERN is also omitted,
4633a7c4
LW
3174splits on whitespace (after skipping any leading whitespace). Anything
3175matching PATTERN is taken to be a delimiter separating the fields. (Note
fb73857a 3176that the delimiter may be longer than one character.)
3177
7b8d334a
GS
3178If LIMIT is specified and is positive, splits into no more than that
3179many fields (though it may split into fewer). If LIMIT is unspecified
3180or zero, trailing null fields are stripped (which potential users
fb73857a 3181of pop() would do well to remember). If LIMIT is negative, it is
3182treated as if an arbitrarily large LIMIT had been specified.
a0d0e21e
LW
3183
3184A pattern matching the null string (not to be confused with
748a9306 3185a null pattern C<//>, which is just one member of the set of patterns
a0d0e21e
LW
3186matching a null string) will split the value of EXPR into separate
3187characters at each point it matches that way. For example:
3188
3189 print join(':', split(/ */, 'hi there'));
3190
3191produces the output 'h:i:t:h:e:r:e'.
3192
5f05dabc 3193The LIMIT parameter can be used to split a line partially
a0d0e21e
LW
3194
3195 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3196
3197When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3198one larger than the number of variables in the list, to avoid
3199unnecessary work. For the list above LIMIT would have been 4 by
3200default. In time critical applications it behooves you not to split
3201into more fields than you really need.
3202
3203If the PATTERN contains parentheses, additional array elements are
3204created from each matching substring in the delimiter.
3205
da0045b7 3206 split(/([,-])/, "1-10,20", 3);
a0d0e21e
LW
3207
3208produces the list value
3209
3210 (1, '-', 10, ',', 20)
3211
54310121 3212If you had the entire header of a normal Unix email message in $header,
4633a7c4
LW
3213you could split it up into fields and their values this way:
3214
3215 $header =~ s/\n\s+/ /g; # fix continuation lines
fb73857a 3216 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4633a7c4 3217
a0d0e21e
LW
3218The pattern C</PATTERN/> may be replaced with an expression to specify
3219patterns that vary at runtime. (To do runtime compilation only once,
748a9306
LW
3220use C</$variable/o>.)
3221
3222As a special case, specifying a PATTERN of space (C<' '>) will split on
3223white space just as split with no arguments does. Thus, split(' ') can
3224be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3225will give you as many null initial fields as there are leading spaces.
3226A split on /\s+/ is like a split(' ') except that any leading
3227whitespace produces a null first field. A split with no arguments
3228really does a C<split(' ', $_)> internally.
a0d0e21e
LW
3229
3230Example:
3231
3232 open(passwd, '/etc/passwd');
3233 while (<passwd>) {
54310121 3234 ($login, $passwd, $uid, $gid, $gcos,
748a9306 3235 $home, $shell) = split(/:/);
a0d0e21e
LW
3236 ...
3237 }
3238
54310121 3239(Note that $shell above will still have a newline on it. See L</chop>,
a0d0e21e
LW
3240L</chomp>, and L</join>.)
3241
5f05dabc 3242=item sprintf FORMAT, LIST
a0d0e21e 3243
74a77017
CS
3244Returns a string formatted by the usual printf conventions of the
3245C library function sprintf(). See L<sprintf(3)> or L<printf(3)>
3246on your system for an explanation of the general principles.
3247
3248Perl does all of its own sprintf() formatting -- it emulates the C
3249function sprintf(), but it doesn't use it (except for floating-point
3250numbers, and even then only the standard modifiers are allowed). As a
3251result, any non-standard extensions in your local sprintf() are not
3252available from Perl.
3253
3254Perl's sprintf() permits the following universally-known conversions:
3255
3256 %% a percent sign
3257 %c a character with the given number
3258 %s a string
3259 %d a signed integer, in decimal
3260 %u an unsigned integer, in decimal
3261 %o an unsigned integer, in octal
3262 %x an unsigned integer, in hexadecimal
3263 %e a floating-point number, in scientific notation
3264 %f a floating-point number, in fixed decimal notation
3265 %g a floating-point number, in %e or %f notation
3266
1b3f7d21 3267In addition, Perl permits the following widely-supported conversions:
74a77017 3268
74a77017
CS
3269 %X like %x, but using upper-case letters
3270 %E like %e, but using an upper-case "E"
3271 %G like %g, but with an upper-case "E" (if applicable)
3272 %p a pointer (outputs the Perl value's address in hexadecimal)
1b3f7d21
CS
3273 %n special: *stores* the number of characters output so far
3274 into the next variable in the parameter list
74a77017 3275
1b3f7d21
CS
3276Finally, for backward (and we do mean "backward") compatibility, Perl
3277permits these unnecessary but widely-supported conversions:
74a77017 3278
1b3f7d21 3279 %i a synonym for %d
74a77017
CS
3280 %D a synonym for %ld
3281 %U a synonym for %lu
3282 %O a synonym for %lo
3283 %F a synonym for %f
3284
3285Perl permits the following universally-known flags between the C<%>
3286and the conversion letter:
3287
3288 space prefix positive number with a space
3289 + prefix positive number with a plus sign
3290 - left-justify within the field
3291 0 use zeros, not spaces, to right-justify
a3cb178b 3292 # prefix non-zero octal with "0", non-zero hex with "0x"
74a77017
CS
3293 number minimum field width
3294 .number "precision": digits after decimal point for floating-point,
3295 max length for string, minimum length for integer
3296 l interpret integer as C type "long" or "unsigned long"
74a77017
CS
3297 h interpret integer as C type "short" or "unsigned short"
3298
1b3f7d21 3299There is also one Perl-specific flag:
74a77017
CS
3300
3301 V interpret integer as Perl's standard integer type
3302
3303Where a number would appear in the flags, an asterisk ("*") may be
3304used instead, in which case Perl uses the next item in the parameter
3305list as the given number (that is, as the field width or precision).
3306If a field width obtained through "*" is negative, it has the same
3307effect as the '-' flag: left-justification.
3308
3309If C<use locale> is in effect, the character used for the decimal
3310point in formatted real numbers is affected by the LC_NUMERIC locale.
3311See L<perllocale>.
a0d0e21e
LW
3312
3313=item sqrt EXPR
3314
54310121 3315=item sqrt
bbce6d69 3316
a0d0e21e
LW
3317Return the square root of EXPR. If EXPR is omitted, returns square
3318root of $_.
3319
3320=item srand EXPR
3321
93dc8474
CS
3322=item srand
3323
3324Sets the random number seed for the C<rand> operator. If EXPR is
3325omitted, uses a semi-random value based on the current time and process
3326ID, among other things. In versions of Perl prior to 5.004 the default
3327seed was just the current time(). This isn't a particularly good seed,
3328so many old programs supply their own seed value (often C<time ^ $$> or
7b8d334a 3329C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
93dc8474
CS
3330
3331In fact, it's usually not necessary to call srand() at all, because if
3332it is not called explicitly, it is called implicitly at the first use of
2f9daede
TP
3333the C<rand> operator. However, this was not the case in version of Perl
3334before 5.004, so if your script will run under older Perl versions, it
3335should call srand().
93dc8474 3336
2f9daede
TP
3337Note that you need something much more random than the default seed for
3338cryptographic purposes. Checksumming the compressed output of one or more
3339rapidly changing operating system status programs is the usual method. For
3340example:
28757baa 3341
3342 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3343
0078ec44
RS
3344If you're particularly concerned with this, see the Math::TrulyRandom
3345module in CPAN.
3346
3347Do I<not> call srand() multiple times in your program unless you know
28757baa 3348exactly what you're doing and why you're doing it. The point of the
3349function is to "seed" the rand() function so that rand() can produce
3350a different sequence each time you run your program. Just do it once at the
3351top of your program, or you I<won't> get random numbers out of rand()!
3352
54310121 3353Frequently called programs (like CGI scripts) that simply use
28757baa 3354
3355 time ^ $$
3356
54310121 3357for a seed can fall prey to the mathematical property that
28757baa 3358
3359 a^b == (a+1)^(b+1)
3360
0078ec44 3361one-third of the time. So don't do that.
f86702cc 3362
a0d0e21e
LW
3363=item stat FILEHANDLE
3364
3365=item stat EXPR
3366
54310121 3367=item stat
bbce6d69 3368
a0d0e21e 3369Returns a 13-element array giving the status info for a file, either the
2f9daede 3370file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
bbce6d69 3371stats $_. Returns a null list if the stat fails. Typically used as
3372follows:
3373
a0d0e21e
LW
3374
3375 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3376 $atime,$mtime,$ctime,$blksize,$blocks)
3377 = stat($filename);
3378
54310121 3379Not all fields are supported on all filesystem types. Here are the
c07a80fd 3380meaning of the fields:
3381
54310121 3382 0 dev device number of filesystem
3383 1 ino inode number
3384 2 mode file mode (type and permissions)
3385 3 nlink number of (hard) links to the file
3386 4 uid numeric user ID of file's owner
3387 5 gid numeric group ID of file's owner
3388 6 rdev the device identifier (special files only)
3389 7 size total size of file, in bytes
3390 8 atime last access time since the epoch
3391 9 mtime last modify time since the epoch
3392 10 ctime inode change time (NOT creation time!) since the epoch
3393 11 blksize preferred block size for file system I/O
3394 12 blocks actual number of blocks allocated
c07a80fd 3395
3396(The epoch was at 00:00 January 1, 1970 GMT.)
3397
a0d0e21e
LW
3398If stat is passed the special filehandle consisting of an underline, no
3399stat is done, but the current contents of the stat structure from the
3400last stat or filetest are returned. Example:
3401
3402 if (-x $file && (($d) = stat(_)) && $d < 0) {
3403 print "$file is executable NFS file\n";
3404 }
3405
5f05dabc 3406(This works on machines only for which the device number is negative under NFS.)
a0d0e21e
LW
3407
3408=item study SCALAR
3409
3410=item study
3411
184e9718 3412Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
a0d0e21e
LW
3413doing many pattern matches on the string before it is next modified.
3414This may or may not save time, depending on the nature and number of
3415patterns you are searching on, and on the distribution of character
54310121 3416frequencies in the string to be searched -- you probably want to compare
5f05dabc 3417run times with and without it to see which runs faster. Those loops
a0d0e21e
LW
3418which scan for many short constant strings (including the constant
3419parts of more complex patterns) will benefit most. You may have only
54310121 3420one study active at a time -- if you study a different scalar the first
a0d0e21e
LW
3421is "unstudied". (The way study works is this: a linked list of every
3422character in the string to be searched is made, so we know, for
3423example, where all the 'k' characters are. From each search string,
3424the rarest character is selected, based on some static frequency tables
3425constructed from some C programs and English text. Only those places
3426that contain this "rarest" character are examined.)
3427
3428For example, here is a loop which inserts index producing entries
3429before any line containing a certain pattern:
3430
3431 while (<>) {
3432 study;
3433 print ".IX foo\n" if /\bfoo\b/;
3434 print ".IX bar\n" if /\bbar\b/;
3435 print ".IX blurfl\n" if /\bblurfl\b/;
3436 ...
3437 print;
3438 }
3439
3440In searching for /\bfoo\b/, only those locations in $_ that contain "f"
3441will be looked at, because "f" is rarer than "o". In general, this is
3442a big win except in pathological cases. The only question is whether
3443it saves you more time than it took to build the linked list in the
3444first place.
3445
3446Note that if you have to look for strings that you don't know till
3447runtime, you can build an entire loop as a string and eval that to
3448avoid recompiling all your patterns all the time. Together with
3449undefining $/ to input entire files as one record, this can be very
3450fast, often faster than specialized programs like fgrep(1). The following
184e9718 3451scans a list of files (C<@files>) for a list of words (C<@words>), and prints
a0d0e21e
LW
3452out the names of those files that contain a match:
3453
3454 $search = 'while (<>) { study;';
3455 foreach $word (@words) {
3456 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3457 }
3458 $search .= "}";
3459 @ARGV = @files;
3460 undef $/;
3461 eval $search; # this screams
5f05dabc 3462 $/ = "\n"; # put back to normal input delimiter
a0d0e21e
LW
3463 foreach $file (sort keys(%seen)) {
3464 print $file, "\n";
3465 }
3466
cb1a09d0
AD
3467=item sub BLOCK
3468
3469=item sub NAME
3470
3471=item sub NAME BLOCK
3472
3473This is subroutine definition, not a real function I<per se>. With just a
3474NAME (and possibly prototypes), it's just a forward declaration. Without
3475a NAME, it's an anonymous function declaration, and does actually return a
2f9daede 3476value: the CODE ref of the closure you just created. See L<perlsub> and
cb1a09d0
AD
3477L<perlref> for details.
3478
7b8d334a
GS
3479=item substr EXPR,OFFSET,LEN,REPLACEMENT
3480
a0d0e21e
LW
3481=item substr EXPR,OFFSET,LEN
3482
3483=item substr EXPR,OFFSET
3484
3485Extracts a substring out of EXPR and returns it. First character is at
2f9daede 3486offset 0, or whatever you've set C<$[> to (but don't do that).
84902520 3487If OFFSET is negative (or more precisely, less than C<$[>), starts
a0d0e21e 3488that far from the end of the string. If LEN is omitted, returns
748a9306
LW
3489everything to the end of the string. If LEN is negative, leaves that
3490many characters off the end of the string.
3491
84902520
TB
3492If you specify a substring which is partly outside the string, the part
3493within the string is returned. If the substring is totally outside
3494the string a warning is produced.
3495
748a9306 3496You can use the substr() function
a0d0e21e
LW
3497as an lvalue, in which case EXPR must be an lvalue. If you assign
3498something shorter than LEN, the string will shrink, and if you assign
3499something longer than LEN, the string will grow to accommodate it. To
3500keep the string the same length you may need to pad or chop your value
3501using sprintf().
3502
7b8d334a
GS
3503An alternative to using substr() as an lvalue is to specify the
3504replacement string as the 4th argument. This allows you to replace
3505parts of the EXPR and return what was there before in one operation.
3506In this case LEN can be C<undef> if you want to affect everything to
3507the end of the string.
3508
a0d0e21e
LW
3509=item symlink OLDFILE,NEWFILE
3510
3511Creates a new filename symbolically linked to the old filename.
3512Returns 1 for success, 0 otherwise. On systems that don't support
3513symbolic links, produces a fatal error at run time. To check for that,
3514use eval:
3515
54310121 3516 $symlink_exists = (eval {symlink("","")};, $@ eq '');
a0d0e21e
LW
3517
3518=item syscall LIST
3519
3520Calls the system call specified as the first element of the list,
3521passing the remaining elements as arguments to the system call. If
3522unimplemented, produces a fatal error. The arguments are interpreted
3523as follows: if a given argument is numeric, the argument is passed as
3524an int. If not, the pointer to the string value is passed. You are
3525responsible to make sure a string is pre-extended long enough to
a3cb178b
GS
3526receive any result that might be written into a string. You can't use a
3527string literal (or other read-only string) as an argument to syscall()
3528because Perl has to assume that any string pointer might be written
3529through. If your
a0d0e21e
LW
3530integer arguments are not literals and have never been interpreted in a
3531numeric context, you may need to add 0 to them to force them to look
3532like numbers.
3533
3534 require 'syscall.ph'; # may need to run h2ph
a3cb178b
GS
3535 $s = "hi there\n";
3536 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
a0d0e21e 3537
5f05dabc 3538Note that Perl supports passing of up to only 14 arguments to your system call,
a0d0e21e
LW
3539which in practice should usually suffice.
3540
fb73857a 3541Syscall returns whatever value returned by the system call it calls.
3542If the system call fails, syscall returns -1 and sets C<$!> (errno).
3543Note that some system calls can legitimately return -1. The proper
3544way to handle such calls is to assign C<$!=0;> before the call and
7b8d334a 3545check the value of C<$!> if syscall returns -1.
fb73857a 3546
3547There's a problem with C<syscall(&SYS_pipe)>: it returns the file
3548number of the read end of the pipe it creates. There is no way
3549to retrieve the file number of the other end. You can avoid this
3550problem by using C<pipe> instead.
3551
c07a80fd 3552=item sysopen FILEHANDLE,FILENAME,MODE
3553
3554=item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3555
3556Opens the file whose filename is given by FILENAME, and associates it
3557with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3558the name of the real filehandle wanted. This function calls the
3559underlying operating system's C<open> function with the parameters
3560FILENAME, MODE, PERMS.
3561
3562The possible values and flag bits of the MODE parameter are
3563system-dependent; they are available via the standard module C<Fcntl>.
3564However, for historical reasons, some values are universal: zero means
3565read-only, one means write-only, and two means read/write.
3566
3567If the file named by FILENAME does not exist and the C<open> call
3568creates it (typically because MODE includes the O_CREAT flag), then
3569the value of PERMS specifies the permissions of the newly created
3570file. If PERMS is omitted, the default value is 0666, which allows
3571read and write for all. This default is reasonable: see C<umask>.
3572
28757baa 3573The IO::File module provides a more object-oriented approach, if you're
3574into that kind of thing.
3575
a0d0e21e
LW
3576=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3577
3578=item sysread FILEHANDLE,SCALAR,LENGTH
3579
3580Attempts to read LENGTH bytes of data into variable SCALAR from the
fb73857a 3581specified FILEHANDLE, using the system call read(2). It bypasses
3582stdio, so mixing this with other kinds of reads, print(), write(),
3583seek(), or tell() can cause confusion because stdio usually buffers
3584data. Returns the number of bytes actually read, or undef if there
3585was an error. SCALAR will be grown or shrunk so that the last byte
3586actually read is the last byte of the scalar after the read.
ff68c719 3587
3588An OFFSET may be specified to place the read data at some place in the
3589string other than the beginning. A negative OFFSET specifies
3590placement at that many bytes counting backwards from the end of the
3591string. A positive OFFSET greater than the length of SCALAR results
3592in the string being padded to the required size with "\0" bytes before
3593the result of the read is appended.
a0d0e21e 3594
137443ea 3595=item sysseek FILEHANDLE,POSITION,WHENCE
3596
8903cb82 3597Sets FILEHANDLE's system position using the system call lseek(2). It
3598bypasses stdio, so mixing this with reads (other than sysread()),
3599print(), write(), seek(), or tell() may cause confusion. FILEHANDLE may
3600be an expression whose value gives the name of the filehandle. The
3601values for WHENCE are 0 to set the new position to POSITION, 1 to set
3602the it to the current position plus POSITION, and 2 to set it to EOF
3603plus POSITION (typically negative). For WHENCE, you may use the
3604constants SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable
3605or the POSIX module.
3606
3607Returns the new position, or the undefined value on failure. A position
3608of zero is returned as the string "0 but true"; thus sysseek() returns
3609TRUE on success and FALSE on failure, yet you can still easily determine
3610the new position.
137443ea 3611
a0d0e21e
LW
3612=item system LIST
3613
8bf3b016
GS
3614=item system PROGRAM LIST
3615
a0d0e21e
LW
3616Does exactly the same thing as "exec LIST" except that a fork is done
3617first, and the parent process waits for the child process to complete.
3618Note that argument processing varies depending on the number of
3619arguments. The return value is the exit status of the program as
3620returned by the wait() call. To get the actual exit value divide by
54310121 3621256. See also L</exec>. This is I<NOT> what you want to use to capture
3622the output from a command, for that you should use merely backticks or
28757baa 3623qx//, as described in L<perlop/"`STRING`">.
a0d0e21e 3624
8bf3b016
GS
3625Like exec(), system() allows you to lie to a program about its name if
3626you use the "system PROGRAM LIST" syntax. Again, see L</exec>.
3627