This is a live mirror of the Perl 5 development currently hosted at https://github.com/perl/perl5
perlfunc.pod: s/definetely/definitely/
[perl5.git] / pod / perlfunc.pod
CommitLineData
a0d0e21e
LW
1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
2b5ab1e7 15argument, while a list operator may provide either scalar or list
a0d0e21e 16contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 17be first, and the list argument will follow. (Note that there can ever
0f31cffe 18be only one such list argument.) For instance, splice() has three scalar
2b5ab1e7
TC
19arguments followed by a list, whereas gethostbyname() has four scalar
20arguments.
a0d0e21e
LW
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
19799a22 33surprising) rule is this: It I<looks> like a function, therefore it I<is> a
a0d0e21e
LW
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
PP
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.
a0d0e21e
LW
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
2b5ab1e7
TC
51A few functions take no arguments at all, and therefore work as neither
52unary nor list operators. These include such functions as C<time>
53and C<endpwent>. For example, C<time+86_400> always means
54C<time() + 86_400>.
55
a0d0e21e 56For functions that can be used in either a scalar or list context,
54310121 57nonabortive failure is generally indicated in a scalar context by
a0d0e21e
LW
58returning the undefined value, and in a list context by returning the
59null list.
60
5a964f20
TC
61Remember the following important rule: There is B<no rule> that relates
62the behavior of an expression in list context to its behavior in scalar
63context, or vice versa. It might do two totally different things.
a0d0e21e 64Each operator and function decides which sort of value it would be most
2b5ab1e7 65appropriate to return in scalar context. Some operators return the
5a964f20 66length of the list that would have been returned in list context. Some
a0d0e21e
LW
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
d1be9408 72A named array in scalar context is quite different from what would at
5a964f20
TC
73first glance appear to be a list in scalar context. You can't get a list
74like C<(1,2,3)> into being in scalar context, because the compiler knows
75the context at compile time. It would generate the scalar comma operator
76there, not the list construction version of the comma. That means it
77was never a list to start with.
78
79In general, functions in Perl that serve as wrappers for system calls
f86cebdf 80of the same name (like chown(2), fork(2), closedir(2), etc.) all return
5a964f20
TC
81true when they succeed and C<undef> otherwise, as is usually mentioned
82in the descriptions below. This is different from the C interfaces,
19799a22
GS
83which return C<-1> on failure. Exceptions to this rule are C<wait>,
84C<waitpid>, and C<syscall>. System calls also set the special C<$!>
5a964f20
TC
85variable on failure. Other functions do not, except accidentally.
86
cb1a09d0
AD
87=head2 Perl Functions by Category
88
89Here are Perl's functions (including things that look like
5a964f20 90functions, like some keywords and named operators)
cb1a09d0
AD
91arranged by category. Some functions appear in more
92than one place.
93
13a2d996 94=over 4
cb1a09d0
AD
95
96=item Functions for SCALARs or strings
97
22fae026 98C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
945c54fd
JH
99C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
cb1a09d0
AD
101
102=item Regular expressions and pattern matching
103
ab4f32c2 104C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
cb1a09d0
AD
105
106=item Numeric functions
107
22fae026
TM
108C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109C<sin>, C<sqrt>, C<srand>
cb1a09d0
AD
110
111=item Functions for real @ARRAYs
112
22fae026 113C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
cb1a09d0
AD
114
115=item Functions for list data
116
ab4f32c2 117C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
cb1a09d0
AD
118
119=item Functions for real %HASHes
120
22fae026 121C<delete>, C<each>, C<exists>, C<keys>, C<values>
cb1a09d0
AD
122
123=item Input and output functions
124
22fae026
TM
125C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
129C<warn>, C<write>
cb1a09d0
AD
130
131=item Functions for fixed length data or records
132
22fae026 133C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
cb1a09d0
AD
134
135=item Functions for filehandles, files, or directories
136
22fae026 137C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 138C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
1e278fd9
JH
139C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
140C<umask>, C<unlink>, C<utime>
cb1a09d0
AD
141
142=item Keywords related to the control flow of your perl program
143
98293880
JH
144C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 146
54310121 147=item Keywords related to scoping
cb1a09d0 148
4375e838 149C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
cb1a09d0
AD
150
151=item Miscellaneous functions
152
4375e838 153C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
22fae026 154C<scalar>, C<undef>, C<wantarray>
cb1a09d0
AD
155
156=item Functions for processes and process groups
157
22fae026 158C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 159C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 160C<times>, C<wait>, C<waitpid>
cb1a09d0
AD
161
162=item Keywords related to perl modules
163
22fae026 164C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0
AD
165
166=item Keywords related to classes and object-orientedness
167
22fae026
TM
168C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
169C<untie>, C<use>
cb1a09d0
AD
170
171=item Low-level socket functions
172
22fae026
TM
173C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 175C<socket>, C<socketpair>
cb1a09d0
AD
176
177=item System V interprocess communication functions
178
22fae026
TM
179C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
cb1a09d0
AD
181
182=item Fetching user and group info
183
22fae026
TM
184C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186C<getpwuid>, C<setgrent>, C<setpwent>
cb1a09d0
AD
187
188=item Fetching network info
189
22fae026
TM
190C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194C<setnetent>, C<setprotoent>, C<setservent>
cb1a09d0
AD
195
196=item Time-related functions
197
22fae026 198C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 199
37798a01
PP
200=item Functions new in perl5
201
22fae026 202C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
b76cc8ba 203C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
4375e838 204C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
22fae026 205C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
37798a01
PP
206
207* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 208operator, which can be used in expressions.
37798a01
PP
209
210=item Functions obsoleted in perl5
211
22fae026 212C<dbmclose>, C<dbmopen>
37798a01 213
cb1a09d0
AD
214=back
215
60f9f73c
JH
216=head2 Portability
217
2b5ab1e7
TC
218Perl was born in Unix and can therefore access all common Unix
219system calls. In non-Unix environments, the functionality of some
220Unix system calls may not be available, or details of the available
221functionality may differ slightly. The Perl functions affected
60f9f73c
JH
222by this are:
223
224C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
ef5a6dd7
JH
227C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
228C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
60f9f73c
JH
229C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 233C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
60f9f73c
JH
234C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 237C<shmwrite>, C<socket>, C<socketpair>,
80cbd5ad
JH
238C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 240C<utime>, C<wait>, C<waitpid>
60f9f73c
JH
241
242For more information about the portability of these functions, see
243L<perlport> and other available platform-specific documentation.
244
cb1a09d0
AD
245=head2 Alphabetical Listing of Perl Functions
246
a0d0e21e
LW
247=over 8
248
5b3c99c0 249=item -X FILEHANDLE
a0d0e21e 250
5b3c99c0 251=item -X EXPR
a0d0e21e 252
5b3c99c0 253=item -X
a0d0e21e
LW
254
255A file test, where X is one of the letters listed below. This unary
256operator takes one argument, either a filename or a filehandle, and
257tests the associated file to see if something is true about it. If the
7660c0ab 258argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 259Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e
LW
260the undefined value if the file doesn't exist. Despite the funny
261names, precedence is the same as any other named unary operator, and
262the argument may be parenthesized like any other unary operator. The
263operator may be any of:
7e778d91
IZ
264X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e
LW
266
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
271
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
276
277 -e File exists.
8e7e0aa8
MJD
278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
a0d0e21e
LW
280
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
9c4d0f16 284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
a0d0e21e
LW
285 -S File is a socket.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
289
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
293
121910a4 294 -T File is an ASCII text file (heuristic guess).
2cdbc966 295 -B File is a "binary" file (opposite of -T).
a0d0e21e 296
95a3fe12 297 -M Script start time minus file modification time, in days.
a0d0e21e 298 -A Same for access time.
95a3fe12 299 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 300
a0d0e21e
LW
301Example:
302
303 while (<>) {
5b3eff12 304 chomp;
a0d0e21e 305 next unless -f $_; # ignore specials
5a964f20 306 #...
a0d0e21e
LW
307 }
308
5ff3f7a4
GS
309The interpretation of the file permission operators C<-r>, C<-R>,
310C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311of the file and the uids and gids of the user. There may be other
312reasons you can't actually read, write, or execute the file. Such
313reasons may be for example network filesystem access controls, ACLs
314(access control lists), read-only filesystems, and unrecognized
315executable formats.
316
2b5ab1e7
TC
317Also note that, for the superuser on the local filesystems, the C<-r>,
318C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
5ff3f7a4
GS
319if any execute bit is set in the mode. Scripts run by the superuser
320may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 321or temporarily set their effective uid to something else.
5ff3f7a4
GS
322
323If you are using ACLs, there is a pragma called C<filetest> that may
324produce more accurate results than the bare stat() mode bits.
5ff3f7a4
GS
325When under the C<use filetest 'access'> the above-mentioned filetests
326will test whether the permission can (not) be granted using the
468541a8 327access() family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
328under this pragma return true even if there are no execute permission
329bits set (nor any extra execute permission ACLs). This strangeness is
330due to the underlying system calls' definitions. Read the
331documentation for the C<filetest> pragma for more information.
332
a0d0e21e
LW
333Note that C<-s/a/b/> does not do a negated substitution. Saying
334C<-exp($foo)> still works as expected, however--only single letters
335following a minus are interpreted as file tests.
336
337The C<-T> and C<-B> switches work as follows. The first block or so of the
338file is examined for odd characters such as strange control codes or
61eff3bc 339characters with the high bit set. If too many strange characters (>30%)
a0d0e21e
LW
340are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341containing null in the first block is considered a binary file. If C<-T>
9124316e 342or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 343rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 344file, or a file at EOF when testing a filehandle. Because you have to
4633a7c4
LW
345read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 347
19799a22 348If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 349the special filehandle consisting of a solitary underline, then the stat
a0d0e21e
LW
350structure of the previous file test (or stat operator) is used, saving
351a system call. (This doesn't work with C<-t>, and you need to remember
352that lstat() and C<-l> will leave values in the stat structure for the
5c9aa243
RGS
353symbolic link, not the real file.) (Also, if the stat buffer was filled by
354a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
355Example:
a0d0e21e
LW
356
357 print "Can do.\n" if -r $a || -w _ || -x _;
358
359 stat($filename);
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
368
fbb0b3b3
RGS
369As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
370test operators, in a way that C<-f -w -x $file> is equivalent to
371C<-x $file && -w _ && -f _>. (This is only syntax fancy : if you use
372the return value of C<-f $file> as an argument to another filetest
373operator, no special magic will happen.)
374
a0d0e21e
LW
375=item abs VALUE
376
54310121 377=item abs
bbce6d69 378
a0d0e21e 379Returns the absolute value of its argument.
7660c0ab 380If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
381
382=item accept NEWSOCKET,GENERICSOCKET
383
f86cebdf 384Accepts an incoming socket connect, just as the accept(2) system call
19799a22 385does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 386See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 387
8d2a6795
GS
388On systems that support a close-on-exec flag on files, the flag will
389be set for the newly opened file descriptor, as determined by the
390value of $^F. See L<perlvar/$^F>.
391
a0d0e21e
LW
392=item alarm SECONDS
393
54310121 394=item alarm
bbce6d69 395
a0d0e21e 396Arranges to have a SIGALRM delivered to this process after the
d400eac8
JH
397specified number of wallclock seconds have elapsed. If SECONDS is not
398specified, the value stored in C<$_> is used. (On some machines,
399unfortunately, the elapsed time may be up to one second less or more
400than you specified because of how seconds are counted, and process
401scheduling may delay the delivery of the signal even further.)
402
403Only one timer may be counting at once. Each call disables the
404previous timer, and an argument of C<0> may be supplied to cancel the
405previous timer without starting a new one. The returned value is the
406amount of time remaining on the previous timer.
a0d0e21e 407
4633a7c4 408For delays of finer granularity than one second, you may use Perl's
19799a22
GS
409four-argument version of select() leaving the first three arguments
410undefined, or you might be able to use the C<syscall> interface to
83df6a1d
JH
411access setitimer(2) if your system supports it. The Time::HiRes
412module (from CPAN, and starting from Perl 5.8 part of the standard
413distribution) may also prove useful.
2b5ab1e7 414
68f8bed4
JH
415It is usually a mistake to intermix C<alarm> and C<sleep> calls.
416(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 417
19799a22
GS
418If you want to use C<alarm> to time out a system call you need to use an
419C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 420fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 421restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 422modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
423
424 eval {
f86cebdf 425 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 426 alarm $timeout;
ff68c719 427 $nread = sysread SOCKET, $buffer, $size;
36477c24 428 alarm 0;
ff68c719 429 };
ff68c719 430 if ($@) {
f86cebdf 431 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719
PP
432 # timed out
433 }
434 else {
435 # didn't
436 }
437
91d81acc
JH
438For more information see L<perlipc>.
439
a0d0e21e
LW
440=item atan2 Y,X
441
442Returns the arctangent of Y/X in the range -PI to PI.
443
ca6e1c26 444For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
445function, or use the familiar relation:
446
447 sub tan { sin($_[0]) / cos($_[0]) }
448
a0d0e21e
LW
449=item bind SOCKET,NAME
450
451Binds a network address to a socket, just as the bind system call
19799a22 452does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
453packed address of the appropriate type for the socket. See the examples in
454L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 455
fae2c0fb 456=item binmode FILEHANDLE, LAYER
1c1fc3ea 457
a0d0e21e
LW
458=item binmode FILEHANDLE
459
1cbfc93d
NIS
460Arranges for FILEHANDLE to be read or written in "binary" or "text"
461mode on systems where the run-time libraries distinguish between
462binary and text files. If FILEHANDLE is an expression, the value is
463taken as the name of the filehandle. Returns true on success,
b5fe5ca2 464otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 465
d807c6f4
JH
466On some systems (in general, DOS and Windows-based systems) binmode()
467is necessary when you're not working with a text file. For the sake
468of portability it is a good idea to always use it when appropriate,
469and to never use it when it isn't appropriate. Also, people can
470set their I/O to be by default UTF-8 encoded Unicode, not bytes.
471
472In other words: regardless of platform, use binmode() on binary data,
473like for example images.
474
475If LAYER is present it is a single string, but may contain multiple
476directives. The directives alter the behaviour of the file handle.
477When LAYER is present using binmode on text file makes sense.
478
fae2c0fb 479If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
480suitable for passing binary data. This includes turning off possible CRLF
481translation and marking it as bytes (as opposed to Unicode characters).
749683d2
YST
482Note that, despite what may be implied in I<"Programming Perl"> (the
483Camel) or elsewhere, C<:raw> is I<not> the simply inverse of C<:crlf>
fae2c0fb 484-- other layers which would affect binary nature of the stream are
0226bbdb
NIS
485I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
486PERLIO environment variable.
01e6739c 487
d807c6f4
JH
488The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
489form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
490establish default I/O layers. See L<open>.
491
fae2c0fb
RGS
492I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
493in "Programming Perl, 3rd Edition". However, since the publishing of this
494book, by many known as "Camel III", the consensus of the naming of this
495functionality has moved from "discipline" to "layer". All documentation
496of this version of Perl therefore refers to "layers" rather than to
497"disciplines". Now back to the regularly scheduled documentation...>
498
01e6739c 499To mark FILEHANDLE as UTF-8, use C<:utf8>.
1cbfc93d 500
ed53a2bb 501In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
502is done on the filehandle. Calling binmode() will normally flush any
503pending buffered output data (and perhaps pending input data) on the
fae2c0fb 504handle. An exception to this is the C<:encoding> layer that
01e6739c 505changes the default character encoding of the handle, see L<open>.
fae2c0fb 506The C<:encoding> layer sometimes needs to be called in
3874323d
JH
507mid-stream, and it doesn't flush the stream. The C<:encoding>
508also implicitly pushes on top of itself the C<:utf8> layer because
509internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 510
19799a22 511The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
512system all work together to let the programmer treat a single
513character (C<\n>) as the line terminator, irrespective of the external
514representation. On many operating systems, the native text file
515representation matches the internal representation, but on some
516platforms the external representation of C<\n> is made up of more than
517one character.
518
68bd7414
NIS
519Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
520character to end each line in the external representation of text (even
5e12dbfa 521though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
522on Unix and most VMS files). In other systems like OS/2, DOS and the
523various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
524but what's stored in text files are the two characters C<\cM\cJ>. That
525means that, if you don't use binmode() on these systems, C<\cM\cJ>
526sequences on disk will be converted to C<\n> on input, and any C<\n> in
527your program will be converted back to C<\cM\cJ> on output. This is what
528you want for text files, but it can be disastrous for binary files.
30168b04
GS
529
530Another consequence of using binmode() (on some systems) is that
531special end-of-file markers will be seen as part of the data stream.
532For systems from the Microsoft family this means that if your binary
4375e838 533data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
534the file, unless you use binmode().
535
536binmode() is not only important for readline() and print() operations,
537but also when using read(), seek(), sysread(), syswrite() and tell()
538(see L<perlport> for more details). See the C<$/> and C<$\> variables
539in L<perlvar> for how to manually set your input and output
540line-termination sequences.
a0d0e21e 541
4633a7c4 542=item bless REF,CLASSNAME
a0d0e21e
LW
543
544=item bless REF
545
2b5ab1e7
TC
546This function tells the thingy referenced by REF that it is now an object
547in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 548is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
549it returns the reference for convenience. Always use the two-argument
550version if the function doing the blessing might be inherited by a
551derived class. See L<perltoot> and L<perlobj> for more about the blessing
552(and blessings) of objects.
a0d0e21e 553
57668c4d 554Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
555Namespaces with all lowercase names are considered reserved for
556Perl pragmata. Builtin types have all uppercase names, so to prevent
557confusion, you may wish to avoid such package names as well. Make sure
558that CLASSNAME is a true value.
60ad88b8
GS
559
560See L<perlmod/"Perl Modules">.
561
a0d0e21e
LW
562=item caller EXPR
563
564=item caller
565
5a964f20 566Returns the context of the current subroutine call. In scalar context,
28757baa 567returns the caller's package name if there is a caller, that is, if
19799a22 568we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 569otherwise. In list context, returns
a0d0e21e 570
748a9306 571 ($package, $filename, $line) = caller;
a0d0e21e
LW
572
573With EXPR, it returns some extra information that the debugger uses to
574print a stack trace. The value of EXPR indicates how many call frames
575to go back before the current one.
576
f3aa04c2 577 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 578 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 579
951ba7fe 580Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 581call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 582C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 583C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 584C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 585$filename is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
586each C<use> statement creates a C<require> frame inside an C<eval EXPR>
587frame.) $subroutine may also be C<(unknown)> if this particular
588subroutine happens to have been deleted from the symbol table.
589C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
590C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
591compiled with. The C<$hints> and C<$bitmask> values are subject to change
592between versions of Perl, and are not meant for external use.
748a9306
LW
593
594Furthermore, when called from within the DB package, caller returns more
7660c0ab 595detailed information: it sets the list variable C<@DB::args> to be the
54310121 596arguments with which the subroutine was invoked.
748a9306 597
7660c0ab 598Be aware that the optimizer might have optimized call frames away before
19799a22 599C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 600might not return information about the call frame you expect it do, for
b76cc8ba 601C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 602previous time C<caller> was called.
7660c0ab 603
a0d0e21e
LW
604=item chdir EXPR
605
c4aca7d0
GA
606=item chdir FILEHANDLE
607
608=item chdir DIRHANDLE
609
ce2984c3
PF
610=item chdir
611
ffce7b87 612Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 613changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 614changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
615variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
616neither is set, C<chdir> does nothing. It returns true upon success,
617false otherwise. See the example under C<die>.
a0d0e21e 618
c4aca7d0
GA
619On systems that support fchdir, you might pass a file handle or
620directory handle as argument. On systems that don't support fchdir,
621passing handles produces a fatal error at run time.
622
a0d0e21e
LW
623=item chmod LIST
624
625Changes the permissions of a list of files. The first element of the
4633a7c4 626list must be the numerical mode, which should probably be an octal
4ad40acf 627number, and which definitely should I<not> be a string of octal digits:
2f9daede 628C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 629successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
630
631 $cnt = chmod 0755, 'foo', 'bar';
632 chmod 0755, @executables;
f86cebdf
GS
633 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
634 # --w----r-T
2f9daede
TPG
635 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
636 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 637
c4aca7d0
GA
638On systems that support fchmod, you might pass file handles among the
639files. On systems that don't support fchmod, passing file handles
640produces a fatal error at run time.
641
642 open(my $fh, "<", "foo");
643 my $perm = (stat $fh)[2] & 07777;
644 chmod($perm | 0600, $fh);
645
ca6e1c26
JH
646You can also import the symbolic C<S_I*> constants from the Fcntl
647module:
648
649 use Fcntl ':mode';
650
651 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
652 # This is identical to the chmod 0755 of the above example.
653
a0d0e21e
LW
654=item chomp VARIABLE
655
313c9f5c 656=item chomp( LIST )
a0d0e21e
LW
657
658=item chomp
659
2b5ab1e7
TC
660This safer version of L</chop> removes any trailing string
661that corresponds to the current value of C<$/> (also known as
28757baa
PP
662$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
663number of characters removed from all its arguments. It's often used to
664remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
665that the final record may be missing its newline. When in paragraph
666mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
667When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
668a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 669remove anything.
19799a22 670If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
671
672 while (<>) {
673 chomp; # avoid \n on last field
674 @array = split(/:/);
5a964f20 675 # ...
a0d0e21e
LW
676 }
677
4bf21a6d
RD
678If VARIABLE is a hash, it chomps the hash's values, but not its keys.
679
a0d0e21e
LW
680You can actually chomp anything that's an lvalue, including an assignment:
681
682 chomp($cwd = `pwd`);
683 chomp($answer = <STDIN>);
684
685If you chomp a list, each element is chomped, and the total number of
686characters removed is returned.
687
442a8c12
NC
688If the C<encoding> pragma is in scope then the lengths returned are
689calculated from the length of C<$/> in Unicode characters, which is not
690always the same as the length of C<$/> in the native encoding.
691
15e44fd8
RGS
692Note that parentheses are necessary when you're chomping anything
693that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
694is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
695C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
696C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
697as C<chomp($a, $b)>.
698
a0d0e21e
LW
699=item chop VARIABLE
700
313c9f5c 701=item chop( LIST )
a0d0e21e
LW
702
703=item chop
704
705Chops off the last character of a string and returns the character
5b3eff12 706chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 707scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
708If VARIABLE is a hash, it chops the hash's values, but not its keys.
709
5b3eff12 710You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
711
712If you chop a list, each element is chopped. Only the value of the
19799a22 713last C<chop> is returned.
a0d0e21e 714
19799a22 715Note that C<chop> returns the last character. To return all but the last
748a9306
LW
716character, use C<substr($string, 0, -1)>.
717
15e44fd8
RGS
718See also L</chomp>.
719
a0d0e21e
LW
720=item chown LIST
721
722Changes the owner (and group) of a list of files. The first two
19799a22
GS
723elements of the list must be the I<numeric> uid and gid, in that
724order. A value of -1 in either position is interpreted by most
725systems to leave that value unchanged. Returns the number of files
726successfully changed.
a0d0e21e
LW
727
728 $cnt = chown $uid, $gid, 'foo', 'bar';
729 chown $uid, $gid, @filenames;
730
c4aca7d0
GA
731On systems that support fchown, you might pass file handles among the
732files. On systems that don't support fchown, passing file handles
733produces a fatal error at run time.
734
54310121 735Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
736
737 print "User: ";
19799a22 738 chomp($user = <STDIN>);
5a964f20 739 print "Files: ";
19799a22 740 chomp($pattern = <STDIN>);
a0d0e21e
LW
741
742 ($login,$pass,$uid,$gid) = getpwnam($user)
743 or die "$user not in passwd file";
744
5a964f20 745 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
746 chown $uid, $gid, @ary;
747
54310121 748On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
749file unless you're the superuser, although you should be able to change
750the group to any of your secondary groups. On insecure systems, these
751restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
752On POSIX systems, you can detect this condition this way:
753
754 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
755 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 756
a0d0e21e
LW
757=item chr NUMBER
758
54310121 759=item chr
bbce6d69 760
a0d0e21e 761Returns the character represented by that NUMBER in the character set.
a0ed51b3 762For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1e54db1a
JH
763chr(0x263a) is a Unicode smiley face. Note that characters from 128
764to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
765backward compatibility reasons (but see L<encoding>).
aaa68c4a 766
8a064bd6
JH
767Negative values give the Unicode replacement character (chr(0xfffd)),
768except under the L</bytes> pragma, where low eight bits of the value
769(truncated to an integer) are used.
770
974da8e5
JH
771If NUMBER is omitted, uses C<$_>.
772
b76cc8ba 773For the reverse, use L</ord>.
a0d0e21e 774
974da8e5
JH
775Note that under the C<bytes> pragma the NUMBER is masked to
776the low eight bits.
777
778See L<perlunicode> and L<encoding> for more about Unicode.
bbce6d69 779
a0d0e21e
LW
780=item chroot FILENAME
781
54310121 782=item chroot
bbce6d69 783
5a964f20 784This function works like the system call by the same name: it makes the
4633a7c4 785named directory the new root directory for all further pathnames that
951ba7fe 786begin with a C</> by your process and all its children. (It doesn't
28757baa 787change your current working directory, which is unaffected.) For security
4633a7c4 788reasons, this call is restricted to the superuser. If FILENAME is
19799a22 789omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
790
791=item close FILEHANDLE
792
6a518fbc
TP
793=item close
794
9124316e
JH
795Closes the file or pipe associated with the file handle, returning
796true only if IO buffers are successfully flushed and closes the system
797file descriptor. Closes the currently selected filehandle if the
798argument is omitted.
fb73857a
PP
799
800You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
801another C<open> on it, because C<open> will close it for you. (See
802C<open>.) However, an explicit C<close> on an input file resets the line
803counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 804
dede8123
RGS
805If the file handle came from a piped open, C<close> will additionally
806return false if one of the other system calls involved fails, or if the
fb73857a 807program exits with non-zero status. (If the only problem was that the
dede8123 808program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 809also waits for the process executing on the pipe to complete, in case you
b76cc8ba 810want to look at the output of the pipe afterwards, and
e5218da5
GA
811implicitly puts the exit status value of that command into C<$?> and
812C<${^CHILD_ERROR_NATIVE}>.
5a964f20 813
73689b13
GS
814Prematurely closing the read end of a pipe (i.e. before the process
815writing to it at the other end has closed it) will result in a
816SIGPIPE being delivered to the writer. If the other end can't
817handle that, be sure to read all the data before closing the pipe.
818
fb73857a 819Example:
a0d0e21e 820
fb73857a
PP
821 open(OUTPUT, '|sort >foo') # pipe to sort
822 or die "Can't start sort: $!";
5a964f20 823 #... # print stuff to output
fb73857a
PP
824 close OUTPUT # wait for sort to finish
825 or warn $! ? "Error closing sort pipe: $!"
826 : "Exit status $? from sort";
827 open(INPUT, 'foo') # get sort's results
828 or die "Can't open 'foo' for input: $!";
a0d0e21e 829
5a964f20
TC
830FILEHANDLE may be an expression whose value can be used as an indirect
831filehandle, usually the real filehandle name.
a0d0e21e
LW
832
833=item closedir DIRHANDLE
834
19799a22 835Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
836system call.
837
a0d0e21e
LW
838=item connect SOCKET,NAME
839
840Attempts to connect to a remote socket, just as the connect system call
19799a22 841does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
842packed address of the appropriate type for the socket. See the examples in
843L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 844
cb1a09d0
AD
845=item continue BLOCK
846
847Actually a flow control statement rather than a function. If there is a
98293880
JH
848C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
849C<foreach>), it is always executed just before the conditional is about to
850be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
851it can be used to increment a loop variable, even when the loop has been
852continued via the C<next> statement (which is similar to the C C<continue>
853statement).
854
98293880 855C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
856block. C<last> and C<redo> will behave as if they had been executed within
857the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
858block, it may be more entertaining.
859
860 while (EXPR) {
861 ### redo always comes here
862 do_something;
863 } continue {
864 ### next always comes here
865 do_something_else;
866 # then back the top to re-check EXPR
867 }
868 ### last always comes here
869
870Omitting the C<continue> section is semantically equivalent to using an
19799a22 871empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
872to check the condition at the top of the loop.
873
a0d0e21e
LW
874=item cos EXPR
875
d6217f1e
GS
876=item cos
877
5a964f20 878Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 879takes cosine of C<$_>.
a0d0e21e 880
ca6e1c26 881For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
882function, or use this relation:
883
884 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
885
a0d0e21e
LW
886=item crypt PLAINTEXT,SALT
887
f86cebdf 888Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
889(assuming that you actually have a version there that has not been
890extirpated as a potential munition). This can prove useful for checking
891the password file for lousy passwords, amongst other things. Only the
892guys wearing white hats should do this.
a0d0e21e 893
a6d05634 894Note that L<crypt|/crypt> is intended to be a one-way function, much like
85c16d83
JH
895breaking eggs to make an omelette. There is no (known) corresponding
896decrypt function (in other words, the crypt() is a one-way hash
897function). As a result, this function isn't all that useful for
11155c91 898cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 899
85c16d83
JH
900When verifying an existing encrypted string you should use the
901encrypted text as the salt (like C<crypt($plain, $crypted) eq
8e2ffcbe 902$crypted>). This allows your code to work with the standard L<crypt|/crypt>
85c16d83
JH
903and with more exotic implementations. In other words, do not assume
904anything about the returned string itself, or how many bytes in
905the encrypted string matter.
906
907Traditionally the result is a string of 13 bytes: two first bytes of
908the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
909the first eight bytes of the encrypted string mattered, but
910alternative hashing schemes (like MD5), higher level security schemes
911(like C2), and implementations on non-UNIX platforms may produce
912different strings.
913
914When choosing a new salt create a random two character string whose
915characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
916'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
917characters is just a recommendation; the characters allowed in
918the salt depend solely on your system's crypt library, and Perl can't
919restrict what salts C<crypt()> accepts.
e71965be 920
a0d0e21e
LW
921Here's an example that makes sure that whoever runs this program knows
922their own password:
923
924 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
925
926 system "stty -echo";
927 print "Password: ";
e71965be 928 chomp($word = <STDIN>);
a0d0e21e
LW
929 print "\n";
930 system "stty echo";
931
e71965be 932 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
933 die "Sorry...\n";
934 } else {
935 print "ok\n";
54310121 936 }
a0d0e21e 937
9f8f0c9d 938Of course, typing in your own password to whoever asks you
748a9306 939for it is unwise.
a0d0e21e 940
8e2ffcbe 941The L<crypt|/crypt> function is unsuitable for encrypting large quantities
19799a22
GS
942of data, not least of all because you can't get the information
943back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
944on your favorite CPAN mirror for a slew of potentially useful
945modules.
946
f2791508
JH
947If using crypt() on a Unicode string (which I<potentially> has
948characters with codepoints above 255), Perl tries to make sense
949of the situation by trying to downgrade (a copy of the string)
950the string back to an eight-bit byte string before calling crypt()
951(on that copy). If that works, good. If not, crypt() dies with
952C<Wide character in crypt>.
85c16d83 953
aa689395 954=item dbmclose HASH
a0d0e21e 955
19799a22 956[This function has been largely superseded by the C<untie> function.]
a0d0e21e 957
aa689395 958Breaks the binding between a DBM file and a hash.
a0d0e21e 959
19799a22 960=item dbmopen HASH,DBNAME,MASK
a0d0e21e 961
19799a22 962[This function has been largely superseded by the C<tie> function.]
a0d0e21e 963
7b8d334a 964This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
965hash. HASH is the name of the hash. (Unlike normal C<open>, the first
966argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
967is the name of the database (without the F<.dir> or F<.pag> extension if
968any). If the database does not exist, it is created with protection
19799a22
GS
969specified by MASK (as modified by the C<umask>). If your system supports
970only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 971program. In older versions of Perl, if your system had neither DBM nor
19799a22 972ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
973sdbm(3).
974
975If you don't have write access to the DBM file, you can only read hash
976variables, not set them. If you want to test whether you can write,
19799a22 977either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 978which will trap the error.
a0d0e21e 979
19799a22
GS
980Note that functions such as C<keys> and C<values> may return huge lists
981when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
982function to iterate over large DBM files. Example:
983
984 # print out history file offsets
985 dbmopen(%HIST,'/usr/lib/news/history',0666);
986 while (($key,$val) = each %HIST) {
987 print $key, ' = ', unpack('L',$val), "\n";
988 }
989 dbmclose(%HIST);
990
cb1a09d0 991See also L<AnyDBM_File> for a more general description of the pros and
184e9718 992cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 993rich implementation.
4633a7c4 994
2b5ab1e7
TC
995You can control which DBM library you use by loading that library
996before you call dbmopen():
997
998 use DB_File;
999 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1000 or die "Can't open netscape history file: $!";
1001
a0d0e21e
LW
1002=item defined EXPR
1003
54310121 1004=item defined
bbce6d69 1005
2f9daede
TPG
1006Returns a Boolean value telling whether EXPR has a value other than
1007the undefined value C<undef>. If EXPR is not present, C<$_> will be
1008checked.
1009
1010Many operations return C<undef> to indicate failure, end of file,
1011system error, uninitialized variable, and other exceptional
1012conditions. This function allows you to distinguish C<undef> from
1013other values. (A simple Boolean test will not distinguish among
7660c0ab 1014C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1015false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1016doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1017returns C<undef> when its argument is an empty array, I<or> when the
1018element to return happens to be C<undef>.
1019
f10b0346
GS
1020You may also use C<defined(&func)> to check whether subroutine C<&func>
1021has ever been defined. The return value is unaffected by any forward
04891299 1022declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1023may still be callable: its package may have an C<AUTOLOAD> method that
1024makes it spring into existence the first time that it is called -- see
1025L<perlsub>.
f10b0346
GS
1026
1027Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1028used to report whether memory for that aggregate has ever been
1029allocated. This behavior may disappear in future versions of Perl.
1030You should instead use a simple test for size:
1031
1032 if (@an_array) { print "has array elements\n" }
1033 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1034
1035When used on a hash element, it tells you whether the value is defined,
dc848c6f 1036not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1037purpose.
a0d0e21e
LW
1038
1039Examples:
1040
1041 print if defined $switch{'D'};
1042 print "$val\n" while defined($val = pop(@ary));
1043 die "Can't readlink $sym: $!"
1044 unless defined($value = readlink $sym);
a0d0e21e 1045 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1046 $debugging = 0 unless defined $debugging;
a0d0e21e 1047
19799a22 1048Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1049discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1050defined values. For example, if you say
a5f75d66
AD
1051
1052 "ab" =~ /a(.*)b/;
1053
7660c0ab 1054The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 1055matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 1056matched something that happened to be zero characters long. This is all
a5f75d66 1057very above-board and honest. When a function returns an undefined value,
2f9daede 1058it's an admission that it couldn't give you an honest answer. So you
19799a22 1059should use C<defined> only when you're questioning the integrity of what
7660c0ab 1060you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1061what you want.
1062
dc848c6f 1063See also L</undef>, L</exists>, L</ref>.
2f9daede 1064
a0d0e21e
LW
1065=item delete EXPR
1066
01020589
GS
1067Given an expression that specifies a hash element, array element, hash slice,
1068or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1069In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1070the size of the array will shrink to the highest element that tests
8216c1fd 1071true for exists() (or 0 if no such element exists).
a0d0e21e 1072
eba0920a
EM
1073Returns a list with the same number of elements as the number of elements
1074for which deletion was attempted. Each element of that list consists of
1075either the value of the element deleted, or the undefined value. In scalar
1076context, this means that you get the value of the last element deleted (or
1077the undefined value if that element did not exist).
1078
1079 %hash = (foo => 11, bar => 22, baz => 33);
1080 $scalar = delete $hash{foo}; # $scalar is 11
1081 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1082 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1083
1084Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1085a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1086from a C<tie>d hash or array may not necessarily return anything.
1087
8ea97a1e
GS
1088Deleting an array element effectively returns that position of the array
1089to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
1090element with exists() will return false. Note that deleting array
1091elements in the middle of an array will not shift the index of the ones
1092after them down--use splice() for that. See L</exists>.
8ea97a1e 1093
01020589 1094The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1095
5f05dabc
PP
1096 foreach $key (keys %HASH) {
1097 delete $HASH{$key};
a0d0e21e
LW
1098 }
1099
01020589
GS
1100 foreach $index (0 .. $#ARRAY) {
1101 delete $ARRAY[$index];
1102 }
1103
1104And so do these:
5f05dabc 1105
01020589
GS
1106 delete @HASH{keys %HASH};
1107
9740c838 1108 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1109
2b5ab1e7 1110But both of these are slower than just assigning the empty list
01020589
GS
1111or undefining %HASH or @ARRAY:
1112
1113 %HASH = (); # completely empty %HASH
1114 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1115
01020589
GS
1116 @ARRAY = (); # completely empty @ARRAY
1117 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1118
1119Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1120operation is a hash element, array element, hash slice, or array slice
1121lookup:
a0d0e21e
LW
1122
1123 delete $ref->[$x][$y]{$key};
5f05dabc 1124 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1125
01020589
GS
1126 delete $ref->[$x][$y][$index];
1127 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1128
a0d0e21e
LW
1129=item die LIST
1130
19799a22
GS
1131Outside an C<eval>, prints the value of LIST to C<STDERR> and
1132exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1133exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1134status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1135an C<eval(),> the error message is stuffed into C<$@> and the
1136C<eval> is terminated with the undefined value. This makes
1137C<die> the way to raise an exception.
a0d0e21e
LW
1138
1139Equivalent examples:
1140
1141 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1142 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1143
ccac6780 1144If the last element of LIST does not end in a newline, the current
df37ec69
WW
1145script line number and input line number (if any) are also printed,
1146and a newline is supplied. Note that the "input line number" (also
1147known as "chunk") is subject to whatever notion of "line" happens to
1148be currently in effect, and is also available as the special variable
1149C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1150
1151Hint: sometimes appending C<", stopped"> to your message will cause it
1152to make better sense when the string C<"at foo line 123"> is appended.
1153Suppose you are running script "canasta".
a0d0e21e
LW
1154
1155 die "/etc/games is no good";
1156 die "/etc/games is no good, stopped";
1157
1158produce, respectively
1159
1160 /etc/games is no good at canasta line 123.
1161 /etc/games is no good, stopped at canasta line 123.
1162
2b5ab1e7 1163See also exit(), warn(), and the Carp module.
a0d0e21e 1164
7660c0ab
A
1165If LIST is empty and C<$@> already contains a value (typically from a
1166previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1167This is useful for propagating exceptions:
1168
1169 eval { ... };
1170 die unless $@ =~ /Expected exception/;
1171
ad216e65
JH
1172If LIST is empty and C<$@> contains an object reference that has a
1173C<PROPAGATE> method, that method will be called with additional file
1174and line number parameters. The return value replaces the value in
16869676 1175C<$@>. ie. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1176were called.
1177
7660c0ab 1178If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1179
52531d10
GS
1180die() can also be called with a reference argument. If this happens to be
1181trapped within an eval(), $@ contains the reference. This behavior permits
1182a more elaborate exception handling implementation using objects that
4375e838 1183maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1184is sometimes preferable to matching particular string values of $@ using
1185regular expressions. Here's an example:
1186
da279afe 1187 use Scalar::Util 'blessed';
1188
52531d10
GS
1189 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1190 if ($@) {
da279afe 1191 if (blessed($@) && $@->isa("Some::Module::Exception")) {
52531d10
GS
1192 # handle Some::Module::Exception
1193 }
1194 else {
1195 # handle all other possible exceptions
1196 }
1197 }
1198
19799a22 1199Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1200them, you may want to overload stringification operations on such custom
1201exception objects. See L<overload> for details about that.
1202
19799a22
GS
1203You can arrange for a callback to be run just before the C<die>
1204does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1205handler will be called with the error text and can change the error
1206message, if it sees fit, by calling C<die> again. See
1207L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1208L<"eval BLOCK"> for some examples. Although this feature was meant
1209to be run only right before your program was to exit, this is not
1210currently the case--the C<$SIG{__DIE__}> hook is currently called
1211even inside eval()ed blocks/strings! If one wants the hook to do
1212nothing in such situations, put
fb73857a
PP
1213
1214 die @_ if $^S;
1215
19799a22
GS
1216as the first line of the handler (see L<perlvar/$^S>). Because
1217this promotes strange action at a distance, this counterintuitive
b76cc8ba 1218behavior may be fixed in a future release.
774d564b 1219
a0d0e21e
LW
1220=item do BLOCK
1221
1222Not really a function. Returns the value of the last command in the
1223sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1224modifier, executes the BLOCK once before testing the loop condition.
1225(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1226
4968c1e4 1227C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1228C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1229See L<perlsyn> for alternative strategies.
4968c1e4 1230
a0d0e21e
LW
1231=item do SUBROUTINE(LIST)
1232
1233A deprecated form of subroutine call. See L<perlsub>.
1234
1235=item do EXPR
1236
1237Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1238file as a Perl script.
a0d0e21e
LW
1239
1240 do 'stat.pl';
1241
1242is just like
1243
986b19de 1244 eval `cat stat.pl`;
a0d0e21e 1245
2b5ab1e7 1246except that it's more efficient and concise, keeps track of the current
ea63ef19 1247filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1248C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1249variables. It also differs in that code evaluated with C<do FILENAME>
1250cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1251same, however, in that it does reparse the file every time you call it,
1252so you probably don't want to do this inside a loop.
a0d0e21e 1253
8e30cc93 1254If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1255error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1256returns undef and sets an error message in C<$@>. If the file is
1257successfully compiled, C<do> returns the value of the last expression
1258evaluated.
1259
a0d0e21e 1260Note that inclusion of library modules is better done with the
19799a22 1261C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1262and raise an exception if there's a problem.
a0d0e21e 1263
5a964f20
TC
1264You might like to use C<do> to read in a program configuration
1265file. Manual error checking can be done this way:
1266
b76cc8ba 1267 # read in config files: system first, then user
f86cebdf 1268 for $file ("/share/prog/defaults.rc",
b76cc8ba 1269 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1270 {
5a964f20 1271 unless ($return = do $file) {
f86cebdf
GS
1272 warn "couldn't parse $file: $@" if $@;
1273 warn "couldn't do $file: $!" unless defined $return;
1274 warn "couldn't run $file" unless $return;
5a964f20
TC
1275 }
1276 }
1277
a0d0e21e
LW
1278=item dump LABEL
1279
1614b0e3
JD
1280=item dump
1281
19799a22
GS
1282This function causes an immediate core dump. See also the B<-u>
1283command-line switch in L<perlrun>, which does the same thing.
1284Primarily this is so that you can use the B<undump> program (not
1285supplied) to turn your core dump into an executable binary after
1286having initialized all your variables at the beginning of the
1287program. When the new binary is executed it will begin by executing
1288a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1289Think of it as a goto with an intervening core dump and reincarnation.
1290If C<LABEL> is omitted, restarts the program from the top.
1291
1292B<WARNING>: Any files opened at the time of the dump will I<not>
1293be open any more when the program is reincarnated, with possible
b76cc8ba 1294resulting confusion on the part of Perl.
19799a22
GS
1295
1296This function is now largely obsolete, partly because it's very
1297hard to convert a core file into an executable, and because the
1298real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1299C code have superseded it. That's why you should now invoke it as
1300C<CORE::dump()>, if you don't want to be warned against a possible
1301typo.
19799a22
GS
1302
1303If you're looking to use L<dump> to speed up your program, consider
1304generating bytecode or native C code as described in L<perlcc>. If
1305you're just trying to accelerate a CGI script, consider using the
210b36aa 1306C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1307You might also consider autoloading or selfloading, which at least
b76cc8ba 1308make your program I<appear> to run faster.
5a964f20 1309
aa689395
PP
1310=item each HASH
1311
5a964f20 1312When called in list context, returns a 2-element list consisting of the
aa689395 1313key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1314it. When called in scalar context, returns only the key for the next
e902a979 1315element in the hash.
2f9daede 1316
ab192400 1317Entries are returned in an apparently random order. The actual random
504f80c1
JH
1318order is subject to change in future versions of perl, but it is
1319guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1320function would produce on the same (unmodified) hash. Since Perl
13215.8.1 the ordering is different even between different runs of Perl
1322for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1323
1324When the hash is entirely read, a null array is returned in list context
19799a22
GS
1325(which when assigned produces a false (C<0>) value), and C<undef> in
1326scalar context. The next call to C<each> after that will start iterating
1327again. There is a single iterator for each hash, shared by all C<each>,
1328C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1329reading all the elements from the hash, or by evaluating C<keys HASH> or
1330C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1331iterating over it, you may get entries skipped or duplicated, so
1332don't. Exception: It is always safe to delete the item most recently
1333returned by C<each()>, which means that the following code will work:
1334
1335 while (($key, $value) = each %hash) {
1336 print $key, "\n";
1337 delete $hash{$key}; # This is safe
1338 }
aa689395 1339
f86cebdf 1340The following prints out your environment like the printenv(1) program,
aa689395 1341only in a different order:
a0d0e21e
LW
1342
1343 while (($key,$value) = each %ENV) {
1344 print "$key=$value\n";
1345 }
1346
19799a22 1347See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1348
1349=item eof FILEHANDLE
1350
4633a7c4
LW
1351=item eof ()
1352
a0d0e21e
LW
1353=item eof
1354
1355Returns 1 if the next read on FILEHANDLE will return end of file, or if
1356FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1357gives the real filehandle. (Note that this function actually
19799a22 1358reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1359interactive context.) Do not read from a terminal file (or call
19799a22 1360C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1361as terminals may lose the end-of-file condition if you do.
1362
820475bd
GS
1363An C<eof> without an argument uses the last file read. Using C<eof()>
1364with empty parentheses is very different. It refers to the pseudo file
1365formed from the files listed on the command line and accessed via the
61eff3bc
JH
1366C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1367as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1368used will cause C<@ARGV> to be examined to determine if input is
67408cae 1369available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1370end-of-file will assume you are processing another C<@ARGV> list,
1371and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1372see L<perlop/"I/O Operators">.
820475bd 1373
61eff3bc 1374In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1375detect the end of each file, C<eof()> will only detect the end of the
1376last file. Examples:
a0d0e21e 1377
748a9306
LW
1378 # reset line numbering on each input file
1379 while (<>) {
b76cc8ba 1380 next if /^\s*#/; # skip comments
748a9306 1381 print "$.\t$_";
5a964f20
TC
1382 } continue {
1383 close ARGV if eof; # Not eof()!
748a9306
LW
1384 }
1385
a0d0e21e
LW
1386 # insert dashes just before last line of last file
1387 while (<>) {
6ac88b13 1388 if (eof()) { # check for end of last file
a0d0e21e
LW
1389 print "--------------\n";
1390 }
1391 print;
6ac88b13 1392 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1393 }
1394
a0d0e21e 1395Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1396input operators typically return C<undef> when they run out of data, or if
1397there was an error.
a0d0e21e
LW
1398
1399=item eval EXPR
1400
1401=item eval BLOCK
1402
ce2984c3
PF
1403=item eval
1404
c7cc6f1c
GS
1405In the first form, the return value of EXPR is parsed and executed as if it
1406were a little Perl program. The value of the expression (which is itself
5a964f20 1407determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1408errors, executed in the lexical context of the current Perl program, so
1409that any variable settings or subroutine and format definitions remain
1410afterwards. Note that the value is parsed every time the eval executes.
1411If EXPR is omitted, evaluates C<$_>. This form is typically used to
1412delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1413
1414In the second form, the code within the BLOCK is parsed only once--at the
1415same time the code surrounding the eval itself was parsed--and executed
1416within the context of the current Perl program. This form is typically
1417used to trap exceptions more efficiently than the first (see below), while
1418also providing the benefit of checking the code within BLOCK at compile
1419time.
1420
1421The final semicolon, if any, may be omitted from the value of EXPR or within
1422the BLOCK.
1423
1424In both forms, the value returned is the value of the last expression
5a964f20 1425evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1426as with subroutines. The expression providing the return value is evaluated
5a964f20 1427in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1428See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1429
19799a22
GS
1430If there is a syntax error or runtime error, or a C<die> statement is
1431executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1432error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1433string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1434warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1435To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1436turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1437See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1438
19799a22
GS
1439Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1440determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1441is implemented. It is also Perl's exception trapping mechanism, where
1442the die operator is used to raise exceptions.
1443
1444If the code to be executed doesn't vary, you may use the eval-BLOCK
1445form to trap run-time errors without incurring the penalty of
1446recompiling each time. The error, if any, is still returned in C<$@>.
1447Examples:
1448
54310121 1449 # make divide-by-zero nonfatal
a0d0e21e
LW
1450 eval { $answer = $a / $b; }; warn $@ if $@;
1451
1452 # same thing, but less efficient
1453 eval '$answer = $a / $b'; warn $@ if $@;
1454
1455 # a compile-time error
5a964f20 1456 eval { $answer = }; # WRONG
a0d0e21e
LW
1457
1458 # a run-time error
1459 eval '$answer ='; # sets $@
1460
2b5ab1e7
TC
1461Due to the current arguably broken state of C<__DIE__> hooks, when using
1462the C<eval{}> form as an exception trap in libraries, you may wish not
1463to trigger any C<__DIE__> hooks that user code may have installed.
1464You can use the C<local $SIG{__DIE__}> construct for this purpose,
1465as shown in this example:
774d564b
PP
1466
1467 # a very private exception trap for divide-by-zero
f86cebdf
GS
1468 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1469 warn $@ if $@;
774d564b
PP
1470
1471This is especially significant, given that C<__DIE__> hooks can call
19799a22 1472C<die> again, which has the effect of changing their error messages:
774d564b
PP
1473
1474 # __DIE__ hooks may modify error messages
1475 {
f86cebdf
GS
1476 local $SIG{'__DIE__'} =
1477 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1478 eval { die "foo lives here" };
1479 print $@ if $@; # prints "bar lives here"
774d564b
PP
1480 }
1481
19799a22 1482Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1483may be fixed in a future release.
1484
19799a22 1485With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1486being looked at when:
1487
1488 eval $x; # CASE 1
1489 eval "$x"; # CASE 2
1490
1491 eval '$x'; # CASE 3
1492 eval { $x }; # CASE 4
1493
5a964f20 1494 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1495 $$x++; # CASE 6
1496
2f9daede 1497Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1498the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1499the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1500and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1501does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1502purely visual reasons, but it also has the advantage of compiling at
1503compile-time instead of at run-time.) Case 5 is a place where
19799a22 1504normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1505particular situation, you can just use symbolic references instead, as
1506in case 6.
a0d0e21e 1507
4968c1e4 1508C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1509C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1510
d819b83a
DM
1511Note that as a very special case, an C<eval ''> executed within the C<DB>
1512package doesn't see the usual surrounding lexical scope, but rather the
1513scope of the first non-DB piece of code that called it. You don't normally
1514need to worry about this unless you are writing a Perl debugger.
1515
a0d0e21e
LW
1516=item exec LIST
1517
8bf3b016
GS
1518=item exec PROGRAM LIST
1519
19799a22
GS
1520The C<exec> function executes a system command I<and never returns>--
1521use C<system> instead of C<exec> if you want it to return. It fails and
1522returns false only if the command does not exist I<and> it is executed
fb73857a 1523directly instead of via your system's command shell (see below).
a0d0e21e 1524
19799a22
GS
1525Since it's a common mistake to use C<exec> instead of C<system>, Perl
1526warns you if there is a following statement which isn't C<die>, C<warn>,
1527or C<exit> (if C<-w> is set - but you always do that). If you
1528I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1529can use one of these styles to avoid the warning:
1530
5a964f20
TC
1531 exec ('foo') or print STDERR "couldn't exec foo: $!";
1532 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1533
5a964f20 1534If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1535with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1536If there is only one scalar argument or an array with one element in it,
1537the argument is checked for shell metacharacters, and if there are any,
1538the entire argument is passed to the system's command shell for parsing
1539(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1540If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1541words and passed directly to C<execvp>, which is more efficient.
19799a22 1542Examples:
a0d0e21e 1543
19799a22
GS
1544 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1545 exec "sort $outfile | uniq";
a0d0e21e
LW
1546
1547If you don't really want to execute the first argument, but want to lie
1548to the program you are executing about its own name, you can specify
1549the program you actually want to run as an "indirect object" (without a
1550comma) in front of the LIST. (This always forces interpretation of the
54310121 1551LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1552the list.) Example:
1553
1554 $shell = '/bin/csh';
1555 exec $shell '-sh'; # pretend it's a login shell
1556
1557or, more directly,
1558
1559 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1560
bb32b41a
GS
1561When the arguments get executed via the system shell, results will
1562be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1563for details.
1564
19799a22
GS
1565Using an indirect object with C<exec> or C<system> is also more
1566secure. This usage (which also works fine with system()) forces
1567interpretation of the arguments as a multivalued list, even if the
1568list had just one argument. That way you're safe from the shell
1569expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1570
1571 @args = ( "echo surprise" );
1572
2b5ab1e7 1573 exec @args; # subject to shell escapes
f86cebdf 1574 # if @args == 1
2b5ab1e7 1575 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1576
1577The first version, the one without the indirect object, ran the I<echo>
1578program, passing it C<"surprise"> an argument. The second version
1579didn't--it tried to run a program literally called I<"echo surprise">,
1580didn't find it, and set C<$?> to a non-zero value indicating failure.
1581
0f897271
GS
1582Beginning with v5.6.0, Perl will attempt to flush all files opened for
1583output before the exec, but this may not be supported on some platforms
1584(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1585in English) or call the C<autoflush()> method of C<IO::Handle> on any
1586open handles in order to avoid lost output.
1587
19799a22 1588Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1589any C<DESTROY> methods in your objects.
1590
a0d0e21e
LW
1591=item exists EXPR
1592
01020589 1593Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1594returns true if the specified element in the hash or array has ever
1595been initialized, even if the corresponding value is undefined. The
1596element is not autovivified if it doesn't exist.
a0d0e21e 1597
01020589
GS
1598 print "Exists\n" if exists $hash{$key};
1599 print "Defined\n" if defined $hash{$key};
1600 print "True\n" if $hash{$key};
1601
1602 print "Exists\n" if exists $array[$index];
1603 print "Defined\n" if defined $array[$index];
1604 print "True\n" if $array[$index];
a0d0e21e 1605
8ea97a1e 1606A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1607it exists, but the reverse doesn't necessarily hold true.
1608
afebc493
GS
1609Given an expression that specifies the name of a subroutine,
1610returns true if the specified subroutine has ever been declared, even
1611if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1612does not count as declaring it. Note that a subroutine which does not
1613exist may still be callable: its package may have an C<AUTOLOAD>
1614method that makes it spring into existence the first time that it is
1615called -- see L<perlsub>.
afebc493
GS
1616
1617 print "Exists\n" if exists &subroutine;
1618 print "Defined\n" if defined &subroutine;
1619
a0d0e21e 1620Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1621operation is a hash or array key lookup or subroutine name:
a0d0e21e 1622
2b5ab1e7
TC
1623 if (exists $ref->{A}->{B}->{$key}) { }
1624 if (exists $hash{A}{B}{$key}) { }
1625
01020589
GS
1626 if (exists $ref->{A}->{B}->[$ix]) { }
1627 if (exists $hash{A}{B}[$ix]) { }
1628
afebc493
GS
1629 if (exists &{$ref->{A}{B}{$key}}) { }
1630
01020589
GS
1631Although the deepest nested array or hash will not spring into existence
1632just because its existence was tested, any intervening ones will.
61eff3bc 1633Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1634into existence due to the existence test for the $key element above.
1635This happens anywhere the arrow operator is used, including even:
5a964f20 1636
2b5ab1e7
TC
1637 undef $ref;
1638 if (exists $ref->{"Some key"}) { }
1639 print $ref; # prints HASH(0x80d3d5c)
1640
1641This surprising autovivification in what does not at first--or even
1642second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1643release.
a0d0e21e 1644
afebc493
GS
1645Use of a subroutine call, rather than a subroutine name, as an argument
1646to exists() is an error.
1647
1648 exists &sub; # OK
1649 exists &sub(); # Error
1650
a0d0e21e
LW
1651=item exit EXPR
1652
ce2984c3
PF
1653=item exit
1654
2b5ab1e7 1655Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1656
1657 $ans = <STDIN>;
1658 exit 0 if $ans =~ /^[Xx]/;
1659
19799a22 1660See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1661universally recognized values for EXPR are C<0> for success and C<1>
1662for error; other values are subject to interpretation depending on the
1663environment in which the Perl program is running. For example, exiting
166469 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1665the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1666
19799a22
GS
1667Don't use C<exit> to abort a subroutine if there's any chance that
1668someone might want to trap whatever error happened. Use C<die> instead,
1669which can be trapped by an C<eval>.
28757baa 1670
19799a22 1671The exit() function does not always exit immediately. It calls any
2b5ab1e7 1672defined C<END> routines first, but these C<END> routines may not
19799a22 1673themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1674be called are called before the real exit. If this is a problem, you
1675can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1676See L<perlmod> for details.
5a964f20 1677
a0d0e21e
LW
1678=item exp EXPR
1679
54310121 1680=item exp
bbce6d69 1681
b76cc8ba 1682Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1683If EXPR is omitted, gives C<exp($_)>.
1684
1685=item fcntl FILEHANDLE,FUNCTION,SCALAR
1686
f86cebdf 1687Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1688
1689 use Fcntl;
1690
0ade1984 1691first to get the correct constant definitions. Argument processing and
b76cc8ba 1692value return works just like C<ioctl> below.
a0d0e21e
LW
1693For example:
1694
1695 use Fcntl;
5a964f20
TC
1696 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1697 or die "can't fcntl F_GETFL: $!";
1698
554ad1fc 1699You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1700Like C<ioctl>, it maps a C<0> return from the system call into
1701C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1702in numeric context. It is also exempt from the normal B<-w> warnings
1703on improper numeric conversions.
5a964f20 1704
19799a22 1705Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1706doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1707manpage to learn what functions are available on your system.
a0d0e21e 1708
be2f7487 1709Here's an example of setting a filehandle named C<REMOTE> to be
1710non-blocking at the system level. You'll have to negotiate C<$|>
1711on your own, though.
1712
1713 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1714
1715 $flags = fcntl(REMOTE, F_GETFL, 0)
1716 or die "Can't get flags for the socket: $!\n";
1717
1718 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1719 or die "Can't set flags for the socket: $!\n";
1720
a0d0e21e
LW
1721=item fileno FILEHANDLE
1722
2b5ab1e7
TC
1723Returns the file descriptor for a filehandle, or undefined if the
1724filehandle is not open. This is mainly useful for constructing
19799a22 1725bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1726If FILEHANDLE is an expression, the value is taken as an indirect
1727filehandle, generally its name.
5a964f20 1728
b76cc8ba 1729You can use this to find out whether two handles refer to the
5a964f20
TC
1730same underlying descriptor:
1731
1732 if (fileno(THIS) == fileno(THAT)) {
1733 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1734 }
1735
1736(Filehandles connected to memory objects via new features of C<open> may
1737return undefined even though they are open.)
1738
a0d0e21e
LW
1739
1740=item flock FILEHANDLE,OPERATION
1741
19799a22
GS
1742Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1743for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1744machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1745C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1746only entire files, not records.
1747
1748Two potentially non-obvious but traditional C<flock> semantics are
1749that it waits indefinitely until the lock is granted, and that its locks
1750B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1751fewer guarantees. This means that files locked with C<flock> may be
1752modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1753your port's specific documentation, or your system-specific local manpages
1754for details. It's best to assume traditional behavior if you're writing
1755portable programs. (But if you're not, you should as always feel perfectly
1756free to write for your own system's idiosyncrasies (sometimes called
1757"features"). Slavish adherence to portability concerns shouldn't get
1758in the way of your getting your job done.)
a3cb178b 1759
8ebc5c01
PP
1760OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1761LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1762you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1763either individually, or as a group using the ':flock' tag. LOCK_SH
1764requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1765releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1766LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1767waiting for the lock (check the return status to see if you got it).
1768
2b5ab1e7
TC
1769To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1770before locking or unlocking it.
8ebc5c01 1771
f86cebdf 1772Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1773locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1774are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1775implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1776differing semantics shouldn't bite too many people.
1777
becacb53
TM
1778Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1779be open with read intent to use LOCK_SH and requires that it be open
1780with write intent to use LOCK_EX.
1781
19799a22
GS
1782Note also that some versions of C<flock> cannot lock things over the
1783network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1784that. If you like you can force Perl to ignore your system's flock(2)
1785function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1786the switch C<-Ud_flock> to the F<Configure> program when you configure
1787perl.
4633a7c4
LW
1788
1789Here's a mailbox appender for BSD systems.
a0d0e21e 1790
7e1af8bc 1791 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1792
1793 sub lock {
7e1af8bc 1794 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1795 # and, in case someone appended
1796 # while we were waiting...
1797 seek(MBOX, 0, 2);
1798 }
1799
1800 sub unlock {
7e1af8bc 1801 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1802 }
1803
1804 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1805 or die "Can't open mailbox: $!";
1806
1807 lock();
1808 print MBOX $msg,"\n\n";
1809 unlock();
1810
2b5ab1e7
TC
1811On systems that support a real flock(), locks are inherited across fork()
1812calls, whereas those that must resort to the more capricious fcntl()
1813function lose the locks, making it harder to write servers.
1814
cb1a09d0 1815See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1816
1817=item fork
1818
2b5ab1e7
TC
1819Does a fork(2) system call to create a new process running the
1820same program at the same point. It returns the child pid to the
1821parent process, C<0> to the child process, or C<undef> if the fork is
1822unsuccessful. File descriptors (and sometimes locks on those descriptors)
1823are shared, while everything else is copied. On most systems supporting
1824fork(), great care has gone into making it extremely efficient (for
1825example, using copy-on-write technology on data pages), making it the
1826dominant paradigm for multitasking over the last few decades.
5a964f20 1827
0f897271
GS
1828Beginning with v5.6.0, Perl will attempt to flush all files opened for
1829output before forking the child process, but this may not be supported
1830on some platforms (see L<perlport>). To be safe, you may need to set
1831C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1832C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1833
19799a22 1834If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1835accumulate zombies. On some systems, you can avoid this by setting
1836C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1837forking and reaping moribund children.
cb1a09d0 1838
28757baa
PP
1839Note that if your forked child inherits system file descriptors like
1840STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1841if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1842backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1843You should reopen those to F</dev/null> if it's any issue.
28757baa 1844
cb1a09d0
AD
1845=item format
1846
19799a22 1847Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1848example:
1849
54310121 1850 format Something =
cb1a09d0
AD
1851 Test: @<<<<<<<< @||||| @>>>>>
1852 $str, $%, '$' . int($num)
1853 .
1854
1855 $str = "widget";
184e9718 1856 $num = $cost/$quantity;
cb1a09d0
AD
1857 $~ = 'Something';
1858 write;
1859
1860See L<perlform> for many details and examples.
1861
8903cb82 1862=item formline PICTURE,LIST
a0d0e21e 1863
5a964f20 1864This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1865too. It formats (see L<perlform>) a list of values according to the
1866contents of PICTURE, placing the output into the format output
7660c0ab 1867accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1868Eventually, when a C<write> is done, the contents of
a0d0e21e 1869C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1870yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1871does one C<formline> per line of form, but the C<formline> function itself
748a9306 1872doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1873that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1874You may therefore need to use multiple formlines to implement a single
1875record format, just like the format compiler.
1876
19799a22 1877Be careful if you put double quotes around the picture, because an C<@>
748a9306 1878character may be taken to mean the beginning of an array name.
19799a22 1879C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1880
1881=item getc FILEHANDLE
1882
1883=item getc
1884
1885Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
1886or the undefined value at end of file, or if there was an error (in
1887the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1888STDIN. This is not particularly efficient. However, it cannot be
1889used by itself to fetch single characters without waiting for the user
1890to hit enter. For that, try something more like:
4633a7c4
LW
1891
1892 if ($BSD_STYLE) {
1893 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1894 }
1895 else {
54310121 1896 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1897 }
1898
1899 $key = getc(STDIN);
1900
1901 if ($BSD_STYLE) {
1902 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1903 }
1904 else {
5f05dabc 1905 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1906 }
1907 print "\n";
1908
54310121
PP
1909Determination of whether $BSD_STYLE should be set
1910is left as an exercise to the reader.
cb1a09d0 1911
19799a22 1912The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1913systems purporting POSIX compliance. See also the C<Term::ReadKey>
1914module from your nearest CPAN site; details on CPAN can be found on
1915L<perlmodlib/CPAN>.
a0d0e21e
LW
1916
1917=item getlogin
1918
5a964f20
TC
1919Implements the C library function of the same name, which on most
1920systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1921use C<getpwuid>.
a0d0e21e 1922
f86702cc 1923 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1924
19799a22
GS
1925Do not consider C<getlogin> for authentication: it is not as
1926secure as C<getpwuid>.
4633a7c4 1927
a0d0e21e
LW
1928=item getpeername SOCKET
1929
1930Returns the packed sockaddr address of other end of the SOCKET connection.
1931
4633a7c4
LW
1932 use Socket;
1933 $hersockaddr = getpeername(SOCK);
19799a22 1934 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1935 $herhostname = gethostbyaddr($iaddr, AF_INET);
1936 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1937
1938=item getpgrp PID
1939
47e29363 1940Returns the current process group for the specified PID. Use
7660c0ab 1941a PID of C<0> to get the current process group for the
4633a7c4 1942current process. Will raise an exception if used on a machine that
f86cebdf 1943doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1944group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1945does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1946
1947=item getppid
1948
1949Returns the process id of the parent process.
1950
4d76a344
RGS
1951Note for Linux users: on Linux, the C functions C<getpid()> and
1952C<getppid()> return different values from different threads. In order to
1953be portable, this behavior is not reflected by the perl-level function
1954C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
1955to call the underlying C<getppid()>, you may use the CPAN module
1956C<Linux::Pid>.
4d76a344 1957
a0d0e21e
LW
1958=item getpriority WHICH,WHO
1959
4633a7c4
LW
1960Returns the current priority for a process, a process group, or a user.
1961(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1962machine that doesn't implement getpriority(2).
a0d0e21e
LW
1963
1964=item getpwnam NAME
1965
1966=item getgrnam NAME
1967
1968=item gethostbyname NAME
1969
1970=item getnetbyname NAME
1971
1972=item getprotobyname NAME
1973
1974=item getpwuid UID
1975
1976=item getgrgid GID
1977
1978=item getservbyname NAME,PROTO
1979
1980=item gethostbyaddr ADDR,ADDRTYPE
1981
1982=item getnetbyaddr ADDR,ADDRTYPE
1983
1984=item getprotobynumber NUMBER
1985
1986=item getservbyport PORT,PROTO
1987
1988=item getpwent
1989
1990=item getgrent
1991
1992=item gethostent
1993
1994=item getnetent
1995
1996=item getprotoent
1997
1998=item getservent
1999
2000=item setpwent
2001
2002=item setgrent
2003
2004=item sethostent STAYOPEN
2005
2006=item setnetent STAYOPEN
2007
2008=item setprotoent STAYOPEN
2009
2010=item setservent STAYOPEN
2011
2012=item endpwent
2013
2014=item endgrent
2015
2016=item endhostent
2017
2018=item endnetent
2019
2020=item endprotoent
2021
2022=item endservent
2023
2024These routines perform the same functions as their counterparts in the
5a964f20 2025system library. In list context, the return values from the
a0d0e21e
LW
2026various get routines are as follows:
2027
2028 ($name,$passwd,$uid,$gid,
6ee623d5 2029 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2030 ($name,$passwd,$gid,$members) = getgr*
2031 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2032 ($name,$aliases,$addrtype,$net) = getnet*
2033 ($name,$aliases,$proto) = getproto*
2034 ($name,$aliases,$port,$proto) = getserv*
2035
2036(If the entry doesn't exist you get a null list.)
2037
4602f195
JH
2038The exact meaning of the $gcos field varies but it usually contains
2039the real name of the user (as opposed to the login name) and other
2040information pertaining to the user. Beware, however, that in many
2041system users are able to change this information and therefore it
106325ad 2042cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2043L<perlsec>). The $passwd and $shell, user's encrypted password and
2044login shell, are also tainted, because of the same reason.
4602f195 2045
5a964f20 2046In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2047lookup by name, in which case you get the other thing, whatever it is.
2048(If the entry doesn't exist you get the undefined value.) For example:
2049
5a964f20
TC
2050 $uid = getpwnam($name);
2051 $name = getpwuid($num);
2052 $name = getpwent();
2053 $gid = getgrnam($name);
08a33e13 2054 $name = getgrgid($num);
5a964f20
TC
2055 $name = getgrent();
2056 #etc.
a0d0e21e 2057
4602f195
JH
2058In I<getpw*()> the fields $quota, $comment, and $expire are special
2059cases in the sense that in many systems they are unsupported. If the
2060$quota is unsupported, it is an empty scalar. If it is supported, it
2061usually encodes the disk quota. If the $comment field is unsupported,
2062it is an empty scalar. If it is supported it usually encodes some
2063administrative comment about the user. In some systems the $quota
2064field may be $change or $age, fields that have to do with password
2065aging. In some systems the $comment field may be $class. The $expire
2066field, if present, encodes the expiration period of the account or the
2067password. For the availability and the exact meaning of these fields
2068in your system, please consult your getpwnam(3) documentation and your
2069F<pwd.h> file. You can also find out from within Perl what your
2070$quota and $comment fields mean and whether you have the $expire field
2071by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2072C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2073files are only supported if your vendor has implemented them in the
2074intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
2075shadow versions if you're running under privilege or if there exists
2076the shadow(3) functions as found in System V ( this includes Solaris
2077and Linux.) Those systems which implement a proprietary shadow password
2078facility are unlikely to be supported.
6ee623d5 2079
19799a22 2080The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2081the login names of the members of the group.
2082
2083For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2084C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2085C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2086addresses returned by the corresponding system library call. In the
2087Internet domain, each address is four bytes long and you can unpack it
2088by saying something like:
2089
f337b084 2090 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2091
2b5ab1e7
TC
2092The Socket library makes this slightly easier:
2093
2094 use Socket;
2095 $iaddr = inet_aton("127.1"); # or whatever address
2096 $name = gethostbyaddr($iaddr, AF_INET);
2097
2098 # or going the other way
19799a22 2099 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2100
19799a22
GS
2101If you get tired of remembering which element of the return list
2102contains which return value, by-name interfaces are provided
2103in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2104C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2105and C<User::grent>. These override the normal built-ins, supplying
2106versions that return objects with the appropriate names
2107for each field. For example:
5a964f20
TC
2108
2109 use File::stat;
2110 use User::pwent;
2111 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2112
b76cc8ba
NIS
2113Even though it looks like they're the same method calls (uid),
2114they aren't, because a C<File::stat> object is different from
19799a22 2115a C<User::pwent> object.
5a964f20 2116
a0d0e21e
LW
2117=item getsockname SOCKET
2118
19799a22
GS
2119Returns the packed sockaddr address of this end of the SOCKET connection,
2120in case you don't know the address because you have several different
2121IPs that the connection might have come in on.
a0d0e21e 2122
4633a7c4
LW
2123 use Socket;
2124 $mysockaddr = getsockname(SOCK);
19799a22 2125 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2126 printf "Connect to %s [%s]\n",
19799a22
GS
2127 scalar gethostbyaddr($myaddr, AF_INET),
2128 inet_ntoa($myaddr);
a0d0e21e
LW
2129
2130=item getsockopt SOCKET,LEVEL,OPTNAME
2131
636e6b1f
TH
2132Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2133Options may exist at multiple protocol levels depending on the socket
2134type, but at least the uppermost socket level SOL_SOCKET (defined in the
2135C<Socket> module) will exist. To query options at another level the
2136protocol number of the appropriate protocol controlling the option
2137should be supplied. For example, to indicate that an option is to be
2138interpreted by the TCP protocol, LEVEL should be set to the protocol
2139number of TCP, which you can get using getprotobyname.
2140
2141The call returns a packed string representing the requested socket option,
2142or C<undef> if there is an error (the error reason will be in $!). What
2143exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2144your system documentation for details. A very common case however is that
2145the option is an integer, in which case the result will be an packed
2146integer which you can decode using unpack with the C<i> (or C<I>) format.
2147
2148An example testing if Nagle's algorithm is turned on on a socket:
2149
4852725b 2150 use Socket qw(:all);
636e6b1f
TH
2151
2152 defined(my $tcp = getprotobyname("tcp"))
2153 or die "Could not determine the protocol number for tcp";
4852725b
DD
2154 # my $tcp = IPPROTO_TCP; # Alternative
2155 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2156 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2157 my $nodelay = unpack("I", $packed);
2158 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2159
a0d0e21e
LW
2160
2161=item glob EXPR
2162
0a753a76
PP
2163=item glob
2164
d9a9d457
JL
2165In list context, returns a (possibly empty) list of filename expansions on
2166the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2167scalar context, glob iterates through such filename expansions, returning
2168undef when the list is exhausted. This is the internal function
2169implementing the C<< <*.c> >> operator, but you can use it directly. If
2170EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2171more detail in L<perlop/"I/O Operators">.
a0d0e21e 2172
3a4b19e4
GS
2173Beginning with v5.6.0, this operator is implemented using the standard
2174C<File::Glob> extension. See L<File::Glob> for details.
2175
a0d0e21e
LW
2176=item gmtime EXPR
2177
ce2984c3
PF
2178=item gmtime
2179
d1be9408 2180Converts a time as returned by the time function to an 8-element list
54310121 2181with the time localized for the standard Greenwich time zone.
4633a7c4 2182Typically used as follows:
a0d0e21e 2183
b76cc8ba 2184 # 0 1 2 3 4 5 6 7
48a26b3a 2185 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
2186 gmtime(time);
2187
48a26b3a
GS
2188All list elements are numeric, and come straight out of the C `struct
2189tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2190specified time. $mday is the day of the month, and $mon is the month
2191itself, in the range C<0..11> with 0 indicating January and 11
2192indicating December. $year is the number of years since 1900. That
2193is, $year is C<123> in year 2023. $wday is the day of the week, with
21940 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2195the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2196
2197Note that the $year element is I<not> simply the last two digits of
2198the year. If you assume it is, then you create non-Y2K-compliant
2199programs--and you wouldn't want to do that, would you?
2f9daede 2200
abd75f24
GS
2201The proper way to get a complete 4-digit year is simply:
2202
2203 $year += 1900;
2204
2205And to get the last two digits of the year (e.g., '01' in 2001) do:
2206
2207 $year = sprintf("%02d", $year % 100);
2208
48a26b3a 2209If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2210
48a26b3a 2211In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2212
2213 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2214
fe86afc2
NC
2215If you need local time instead of GMT use the L</localtime> builtin.
2216See also the C<timegm> function provided by the C<Time::Local> module,
2217and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
7660c0ab 2218
fe86afc2
NC
2219This scalar value is B<not> locale dependent (see L<perllocale>), but is
2220instead a Perl builtin. To get somewhat similar but locale dependent date
2221strings, see the example in L</localtime>.
0a753a76 2222
62aa5637
MS
2223See L<perlport/gmtime> for portability concerns.
2224
a0d0e21e
LW
2225=item goto LABEL
2226
748a9306
LW
2227=item goto EXPR
2228
a0d0e21e
LW
2229=item goto &NAME
2230
7660c0ab 2231The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2232execution there. It may not be used to go into any construct that
7660c0ab 2233requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2234also can't be used to go into a construct that is optimized away,
19799a22 2235or to get out of a block or subroutine given to C<sort>.
0a753a76 2236It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2237including out of subroutines, but it's usually better to use some other
19799a22 2238construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2239need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2240(The difference being that C does not offer named loops combined with
2241loop control. Perl does, and this replaces most structured uses of C<goto>
2242in other languages.)
a0d0e21e 2243
7660c0ab
A
2244The C<goto-EXPR> form expects a label name, whose scope will be resolved
2245dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2246necessarily recommended if you're optimizing for maintainability:
2247
2248 goto ("FOO", "BAR", "GLARCH")[$i];
2249
1b6921cb
BT
2250The C<goto-&NAME> form is quite different from the other forms of
2251C<goto>. In fact, it isn't a goto in the normal sense at all, and
2252doesn't have the stigma associated with other gotos. Instead, it
2253exits the current subroutine (losing any changes set by local()) and
2254immediately calls in its place the named subroutine using the current
2255value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2256load another subroutine and then pretend that the other subroutine had
2257been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2258in the current subroutine are propagated to the other subroutine.)
2259After the C<goto>, not even C<caller> will be able to tell that this
2260routine was called first.
2261
2262NAME needn't be the name of a subroutine; it can be a scalar variable
2263containing a code reference, or a block which evaluates to a code
2264reference.
a0d0e21e
LW
2265
2266=item grep BLOCK LIST
2267
2268=item grep EXPR,LIST
2269
2b5ab1e7
TC
2270This is similar in spirit to, but not the same as, grep(1) and its
2271relatives. In particular, it is not limited to using regular expressions.
2f9daede 2272
a0d0e21e 2273Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2274C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2275elements for which the expression evaluated to true. In scalar
2276context, returns the number of times the expression was true.
a0d0e21e
LW
2277
2278 @foo = grep(!/^#/, @bar); # weed out comments
2279
2280or equivalently,
2281
2282 @foo = grep {!/^#/} @bar; # weed out comments
2283
be3174d2
GS
2284Note that C<$_> is an alias to the list value, so it can be used to
2285modify the elements of the LIST. While this is useful and supported,
2286it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2287Similarly, grep returns aliases into the original list, much as a for
2288loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2289element of a list returned by grep (for example, in a C<foreach>, C<map>
2290or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2291This is usually something to be avoided when writing clear code.
a0d0e21e 2292
a4fb8298
RGS
2293If C<$_> is lexical in the scope where the C<grep> appears (because it has
2294been declared with C<my $_>) then, in addition the be locally aliased to
2295the list elements, C<$_> keeps being lexical inside the block; i.e. it
2296can't be seen from the outside, avoiding any potential side-effects.
2297
19799a22 2298See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2299
a0d0e21e
LW
2300=item hex EXPR
2301
54310121 2302=item hex
bbce6d69 2303
2b5ab1e7 2304Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2305(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2306L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2307
2308 print hex '0xAf'; # prints '175'
2309 print hex 'aF'; # same
a0d0e21e 2310
19799a22 2311Hex strings may only represent integers. Strings that would cause
53305cf1 2312integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2313unlike oct(). To present something as hex, look into L</printf>,
2314L</sprintf>, or L</unpack>.
19799a22 2315
ce2984c3 2316=item import LIST
a0d0e21e 2317
19799a22 2318There is no builtin C<import> function. It is just an ordinary
4633a7c4 2319method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2320names to another module. The C<use> function calls the C<import> method
cea6626f 2321for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2322
2323=item index STR,SUBSTR,POSITION
2324
2325=item index STR,SUBSTR
2326
2b5ab1e7
TC
2327The index function searches for one string within another, but without
2328the wildcard-like behavior of a full regular-expression pattern match.
2329It returns the position of the first occurrence of SUBSTR in STR at
2330or after POSITION. If POSITION is omitted, starts searching from the
2331beginning of the string. The return value is based at C<0> (or whatever
2332you've set the C<$[> variable to--but don't do that). If the substring
2333is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2334
2335=item int EXPR
2336
54310121 2337=item int
bbce6d69 2338
7660c0ab 2339Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2340You should not use this function for rounding: one because it truncates
2341towards C<0>, and two because machine representations of floating point
2342numbers can sometimes produce counterintuitive results. For example,
2343C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2344because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2345the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2346functions will serve you better than will int().
a0d0e21e
LW
2347
2348=item ioctl FILEHANDLE,FUNCTION,SCALAR
2349
2b5ab1e7 2350Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2351
a11c483f 2352 require "sys/ioctl.ph"; # probably in $Config{archlib}/ioctl.ph
a0d0e21e 2353
a11c483f 2354to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2355exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2356own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2357(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2358may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2359written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2360will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2361has no string value but does have a numeric value, that value will be
2362passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2363true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2364functions may be needed to manipulate the values of structures used by
b76cc8ba 2365C<ioctl>.
a0d0e21e 2366
19799a22 2367The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2368
2369 if OS returns: then Perl returns:
2370 -1 undefined value
2371 0 string "0 but true"
2372 anything else that number
2373
19799a22 2374Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2375still easily determine the actual value returned by the operating
2376system:
2377
2b5ab1e7 2378 $retval = ioctl(...) || -1;
a0d0e21e
LW
2379 printf "System returned %d\n", $retval;
2380
be2f7487 2381The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2382about improper numeric conversions.
2383
a0d0e21e
LW
2384=item join EXPR,LIST
2385
2b5ab1e7
TC
2386Joins the separate strings of LIST into a single string with fields
2387separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2388
2b5ab1e7 2389 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2390
eb6e2d6f
GS
2391Beware that unlike C<split>, C<join> doesn't take a pattern as its
2392first argument. Compare L</split>.
a0d0e21e 2393
aa689395
PP
2394=item keys HASH
2395
504f80c1
JH
2396Returns a list consisting of all the keys of the named hash.
2397(In scalar context, returns the number of keys.)
2398
2399The keys are returned in an apparently random order. The actual
2400random order is subject to change in future versions of perl, but it
2401is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2402function produces (given that the hash has not been modified). Since
2403Perl 5.8.1 the ordering is different even between different runs of
2404Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2405Attacks">).
504f80c1
JH
2406
2407As a side effect, calling keys() resets the HASH's internal iterator,
2f65b2f0
RGS
2408see L</each>. (In particular, calling keys() in void context resets
2409the iterator with no other overhead.)
a0d0e21e 2410
aa689395 2411Here is yet another way to print your environment:
a0d0e21e
LW
2412
2413 @keys = keys %ENV;
2414 @values = values %ENV;
b76cc8ba 2415 while (@keys) {
a0d0e21e
LW
2416 print pop(@keys), '=', pop(@values), "\n";
2417 }
2418
2419or how about sorted by key:
2420
2421 foreach $key (sort(keys %ENV)) {
2422 print $key, '=', $ENV{$key}, "\n";
2423 }
2424
8ea1e5d4
GS
2425The returned values are copies of the original keys in the hash, so
2426modifying them will not affect the original hash. Compare L</values>.
2427
19799a22 2428To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2429Here's a descending numeric sort of a hash by its values:
4633a7c4 2430
5a964f20 2431 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2432 printf "%4d %s\n", $hash{$key}, $key;
2433 }
2434
19799a22 2435As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2436allocated for the given hash. This can gain you a measure of efficiency if
2437you know the hash is going to get big. (This is similar to pre-extending
2438an array by assigning a larger number to $#array.) If you say
55497cff
PP
2439
2440 keys %hash = 200;
2441
ab192400
GS
2442then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2443in fact, since it rounds up to the next power of two. These
55497cff
PP
2444buckets will be retained even if you do C<%hash = ()>, use C<undef
2445%hash> if you want to free the storage while C<%hash> is still in scope.
2446You can't shrink the number of buckets allocated for the hash using
19799a22 2447C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2448as trying has no effect).
2449
19799a22 2450See also C<each>, C<values> and C<sort>.
ab192400 2451
b350dd2f 2452=item kill SIGNAL, LIST
a0d0e21e 2453
b350dd2f 2454Sends a signal to a list of processes. Returns the number of
517db077
GS
2455processes successfully signaled (which is not necessarily the
2456same as the number actually killed).
a0d0e21e
LW
2457
2458 $cnt = kill 1, $child1, $child2;
2459 kill 9, @goners;
2460
b350dd2f 2461If SIGNAL is zero, no signal is sent to the process. This is a
1e9c1022 2462useful way to check that a child process is alive and hasn't changed
b350dd2f
GS
2463its UID. See L<perlport> for notes on the portability of this
2464construct.
2465
2466Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2467process groups instead of processes. (On System V, a negative I<PROCESS>
2468number will also kill process groups, but that's not portable.) That
2469means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2470use a signal name in quotes.
2471
2472See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2473
2474=item last LABEL
2475
2476=item last
2477
2478The C<last> command is like the C<break> statement in C (as used in
2479loops); it immediately exits the loop in question. If the LABEL is
2480omitted, the command refers to the innermost enclosing loop. The
2481C<continue> block, if any, is not executed:
2482
4633a7c4
LW
2483 LINE: while (<STDIN>) {
2484 last LINE if /^$/; # exit when done with header
5a964f20 2485 #...
a0d0e21e
LW
2486 }
2487
4968c1e4 2488C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2489C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2490a grep() or map() operation.
4968c1e4 2491
6c1372ed
GS
2492Note that a block by itself is semantically identical to a loop
2493that executes once. Thus C<last> can be used to effect an early
2494exit out of such a block.
2495
98293880
JH
2496See also L</continue> for an illustration of how C<last>, C<next>, and
2497C<redo> work.
1d2dff63 2498
a0d0e21e
LW
2499=item lc EXPR
2500
54310121 2501=item lc
bbce6d69 2502
d1be9408 2503Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2504implementing the C<\L> escape in double-quoted strings. Respects
2505current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2506and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2507
7660c0ab 2508If EXPR is omitted, uses C<$_>.
bbce6d69 2509
a0d0e21e
LW
2510=item lcfirst EXPR
2511
54310121 2512=item lcfirst
bbce6d69 2513
ad0029c4
JH
2514Returns the value of EXPR with the first character lowercased. This
2515is the internal function implementing the C<\l> escape in
2516double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2517locale> in force. See L<perllocale> and L<perlunicode> for more
2518details about locale and Unicode support.
a0d0e21e 2519
7660c0ab 2520If EXPR is omitted, uses C<$_>.
bbce6d69 2521
a0d0e21e
LW
2522=item length EXPR
2523
54310121 2524=item length
bbce6d69 2525
974da8e5 2526Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2527omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2528an entire array or hash to find out how many elements these have.
2529For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2530
974da8e5
JH
2531Note the I<characters>: if the EXPR is in Unicode, you will get the
2532number of characters, not the number of bytes. To get the length
2533in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2534
a0d0e21e
LW
2535=item link OLDFILE,NEWFILE
2536
19799a22 2537Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2538success, false otherwise.
a0d0e21e
LW
2539
2540=item listen SOCKET,QUEUESIZE
2541
19799a22 2542Does the same thing that the listen system call does. Returns true if
b76cc8ba 2543it succeeded, false otherwise. See the example in
cea6626f 2544L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2545
2546=item local EXPR
2547
19799a22 2548You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2549what most people think of as "local". See
13a2d996 2550L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2551
5a964f20
TC
2552A local modifies the listed variables to be local to the enclosing
2553block, file, or eval. If more than one value is listed, the list must
2554be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2555for details, including issues with tied arrays and hashes.
a0d0e21e 2556
a0d0e21e
LW
2557=item localtime EXPR
2558
ba053783
AL
2559=item localtime
2560
19799a22 2561Converts a time as returned by the time function to a 9-element list
5f05dabc 2562with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2563follows:
2564
54310121 2565 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2566 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2567 localtime(time);
a0d0e21e 2568
48a26b3a 2569All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2570tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2571of the specified time.
48a26b3a 2572
ba053783
AL
2573C<$mday> is the day of the month, and C<$mon> is the month itself, in
2574the range C<0..11> with 0 indicating January and 11 indicating December.
2575This makes it easy to get a month name from a list:
54310121 2576
ba053783
AL
2577 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2578 print "$abbr[$mon] $mday";
2579 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2580
ba053783
AL
2581C<$year> is the number of years since 1900, not just the last two digits
2582of the year. That is, C<$year> is C<123> in year 2023. The proper way
2583to get a complete 4-digit year is simply:
abd75f24 2584
ba053783 2585 $year += 1900;
abd75f24 2586
ba053783
AL
2587To get the last two digits of the year (e.g., '01' in 2001) do:
2588
2589 $year = sprintf("%02d", $year % 100);
2590
2591C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2592Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2593(or C<0..365> in leap years.)
2594
2595C<$isdst> is true if the specified time occurs during Daylight Saving
2596Time, false otherwise.
abd75f24 2597
48a26b3a 2598If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2599
48a26b3a 2600In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2601
5f05dabc 2602 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2603
fe86afc2
NC
2604This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2605instead of local time use the L</gmtime> builtin. See also the
2606C<Time::Local> module (to convert the second, minutes, hours, ... back to
2607the integer value returned by time()), and the L<POSIX> module's strftime(3)
2608and mktime(3) functions.
2609
2610To get somewhat similar but locale dependent date strings, set up your
2611locale environment variables appropriately (please see L<perllocale>) and
2612try for example:
a3cb178b 2613
5a964f20 2614 use POSIX qw(strftime);
2b5ab1e7 2615 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2616 # or for GMT formatted appropriately for your locale:
2617 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2618
2619Note that the C<%a> and C<%b>, the short forms of the day of the week
2620and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2621
62aa5637
MS
2622See L<perlport/localtime> for portability concerns.
2623
07698885 2624=item lock THING
19799a22 2625
01e6739c 2626This function places an advisory lock on a shared variable, or referenced
03730085 2627object contained in I<THING> until the lock goes out of scope.
a6d5524e 2628
f3a23afb 2629lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2630by this name (before any calls to it), that function will be called
03730085
AB
2631instead. (However, if you've said C<use threads>, lock() is always a
2632keyword.) See L<threads>.
19799a22 2633
a0d0e21e
LW
2634=item log EXPR
2635
54310121 2636=item log
bbce6d69 2637
2b5ab1e7
TC
2638Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2639returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2640The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2641divided by the natural log of N. For example:
2642
2643 sub log10 {
2644 my $n = shift;
2645 return log($n)/log(10);
b76cc8ba 2646 }
2b5ab1e7
TC
2647
2648See also L</exp> for the inverse operation.
a0d0e21e 2649
a0d0e21e
LW
2650=item lstat EXPR
2651
54310121 2652=item lstat
bbce6d69 2653
19799a22 2654Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2655special C<_> filehandle) but stats a symbolic link instead of the file
2656the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2657your system, a normal C<stat> is done. For much more detailed
2658information, please see the documentation for C<stat>.
a0d0e21e 2659
7660c0ab 2660If EXPR is omitted, stats C<$_>.
bbce6d69 2661
a0d0e21e
LW
2662=item m//
2663
2664The match operator. See L<perlop>.
2665
2666=item map BLOCK LIST
2667
2668=item map EXPR,LIST
2669
19799a22
GS
2670Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2671C<$_> to each element) and returns the list value composed of the
2672results of each such evaluation. In scalar context, returns the
2673total number of elements so generated. Evaluates BLOCK or EXPR in
2674list context, so each element of LIST may produce zero, one, or
2675more elements in the returned value.
dd99ebda 2676
a0d0e21e
LW
2677 @chars = map(chr, @nums);
2678
2679translates a list of numbers to the corresponding characters. And
2680
4633a7c4 2681 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2682
2683is just a funny way to write
2684
2685 %hash = ();
2686 foreach $_ (@array) {
4633a7c4 2687 $hash{getkey($_)} = $_;
a0d0e21e
LW
2688 }
2689
be3174d2
GS
2690Note that C<$_> is an alias to the list value, so it can be used to
2691modify the elements of the LIST. While this is useful and supported,
2692it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2693Using a regular C<foreach> loop for this purpose would be clearer in
2694most cases. See also L</grep> for an array composed of those items of
2695the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2696
a4fb8298
RGS
2697If C<$_> is lexical in the scope where the C<map> appears (because it has
2698been declared with C<my $_>) then, in addition the be locally aliased to
2699the list elements, C<$_> keeps being lexical inside the block; i.e. it
2700can't be seen from the outside, avoiding any potential side-effects.
2701
205fdb4d
NC
2702C<{> starts both hash references and blocks, so C<map { ...> could be either
2703the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2704ahead for the closing C<}> it has to take a guess at which its dealing with
2705based what it finds just after the C<{>. Usually it gets it right, but if it
2706doesn't it won't realize something is wrong until it gets to the C<}> and
2707encounters the missing (or unexpected) comma. The syntax error will be
2708reported close to the C<}> but you'll need to change something near the C<{>
2709such as using a unary C<+> to give perl some help:
2710
2711 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2712 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2713 %hash = map { ("\L$_", 1) } @array # this also works
2714 %hash = map { lc($_), 1 } @array # as does this.
2715 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2716
205fdb4d
NC
2717 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2718
2719or to force an anon hash constructor use C<+{>
2720
2721 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2722
2723and you get list of anonymous hashes each with only 1 entry.
2724
19799a22 2725=item mkdir FILENAME,MASK
a0d0e21e 2726
5a211162
GS
2727=item mkdir FILENAME
2728
491873e5
RGS
2729=item mkdir
2730
0591cd52 2731Creates the directory specified by FILENAME, with permissions
19799a22
GS
2732specified by MASK (as modified by C<umask>). If it succeeds it
2733returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2734If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2735to C<$_>.
0591cd52 2736
19799a22 2737In general, it is better to create directories with permissive MASK,
0591cd52 2738and let the user modify that with their C<umask>, than it is to supply
19799a22 2739a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2740The exceptions to this rule are when the file or directory should be
2741kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2742C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2743
cc1852e8
JH
2744Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2745number of trailing slashes. Some operating and filesystems do not get
2746this right, so Perl automatically removes all trailing slashes to keep
2747everyone happy.
2748
a0d0e21e
LW
2749=item msgctl ID,CMD,ARG
2750
f86cebdf 2751Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2752
2753 use IPC::SysV;
2754
7660c0ab
A
2755first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2756then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2757structure. Returns like C<ioctl>: the undefined value for error,
2758C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2759L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2760
2761=item msgget KEY,FLAGS
2762
f86cebdf 2763Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2764id, or the undefined value if there is an error. See also
2765L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2766
a0d0e21e
LW
2767=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2768
2769Calls the System V IPC function msgrcv to receive a message from
2770message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2771SIZE. Note that when a message is received, the message type as a
2772native long integer will be the first thing in VAR, followed by the
2773actual message. This packing may be opened with C<unpack("l! a*")>.
2774Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2775an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2776C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2777
2778=item msgsnd ID,MSG,FLAGS
2779
2780Calls the System V IPC function msgsnd to send the message MSG to the
2781message queue ID. MSG must begin with the native long integer message
2782type, and be followed by the length of the actual message, and finally
2783the message itself. This kind of packing can be achieved with
2784C<pack("l! a*", $type, $message)>. Returns true if successful,
2785or false if there is an error. See also C<IPC::SysV>
2786and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2787
2788=item my EXPR
2789
307ea6df
JH
2790=item my TYPE EXPR
2791
1d2de774 2792=item my EXPR : ATTRS
09bef843 2793
1d2de774 2794=item my TYPE EXPR : ATTRS
307ea6df 2795
19799a22 2796A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2797enclosing block, file, or C<eval>. If more than one value is listed,
2798the list must be placed in parentheses.
307ea6df 2799
1d2de774
JH
2800The exact semantics and interface of TYPE and ATTRS are still
2801evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2802and attributes are handled using the C<attributes> pragma, or starting
2803from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2804L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2805L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2806
a0d0e21e
LW
2807=item next LABEL
2808
2809=item next
2810
2811The C<next> command is like the C<continue> statement in C; it starts
2812the next iteration of the loop:
2813
4633a7c4
LW
2814 LINE: while (<STDIN>) {
2815 next LINE if /^#/; # discard comments
5a964f20 2816 #...
a0d0e21e
LW
2817 }
2818
2819Note that if there were a C<continue> block on the above, it would get
2820executed even on discarded lines. If the LABEL is omitted, the command
2821refers to the innermost enclosing loop.
2822
4968c1e4 2823C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2824C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2825a grep() or map() operation.
4968c1e4 2826
6c1372ed
GS
2827Note that a block by itself is semantically identical to a loop
2828that executes once. Thus C<next> will exit such a block early.
2829
98293880
JH
2830See also L</continue> for an illustration of how C<last>, C<next>, and
2831C<redo> work.
1d2dff63 2832
4a66ea5a
RGS
2833=item no Module VERSION LIST
2834
2835=item no Module VERSION
2836
a0d0e21e
LW
2837=item no Module LIST
2838
4a66ea5a
RGS
2839=item no Module
2840
593b9c14 2841See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
2842
2843=item oct EXPR
2844
54310121 2845=item oct
bbce6d69 2846
4633a7c4 2847Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2848value. (If EXPR happens to start off with C<0x>, interprets it as a
2849hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2850binary string. Leading whitespace is ignored in all three cases.)
2851The following will handle decimal, binary, octal, and hex in the standard
2852Perl or C notation:
a0d0e21e
LW
2853
2854 $val = oct($val) if $val =~ /^0/;
2855
19799a22
GS
2856If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2857in octal), use sprintf() or printf():
2858
2859 $perms = (stat("filename"))[2] & 07777;
2860 $oct_perms = sprintf "%lo", $perms;
2861
2862The oct() function is commonly used when a string such as C<644> needs
2863to be converted into a file mode, for example. (Although perl will
2864automatically convert strings into numbers as needed, this automatic
2865conversion assumes base 10.)
a0d0e21e
LW
2866
2867=item open FILEHANDLE,EXPR
2868
68bd7414
NIS
2869=item open FILEHANDLE,MODE,EXPR
2870
2871=item open FILEHANDLE,MODE,EXPR,LIST
2872
ba964c95
T
2873=item open FILEHANDLE,MODE,REFERENCE
2874
a0d0e21e
LW
2875=item open FILEHANDLE
2876
2877Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
2878FILEHANDLE.
2879
2880(The following is a comprehensive reference to open(): for a gentler
2881introduction you may consider L<perlopentut>.)
2882
a28cd5c9
NT
2883If FILEHANDLE is an undefined scalar variable (or array or hash element)
2884the variable is assigned a reference to a new anonymous filehandle,
2885otherwise if FILEHANDLE is an expression, its value is used as the name of
2886the real filehandle wanted. (This is considered a symbolic reference, so
2887C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
2888
2889If EXPR is omitted, the scalar variable of the same name as the
2890FILEHANDLE contains the filename. (Note that lexical variables--those
2891declared with C<my>--will not work for this purpose; so if you're
67408cae 2892using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
2893
2894If three or more arguments are specified then the mode of opening and
2895the file name are separate. If MODE is C<< '<' >> or nothing, the file
2896is opened for input. If MODE is C<< '>' >>, the file is truncated and
2897opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2898the file is opened for appending, again being created if necessary.
5a964f20 2899
ed53a2bb
JH
2900You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2901indicate that you want both read and write access to the file; thus
2902C<< '+<' >> is almost always preferred for read/write updates--the C<<
2903'+>' >> mode would clobber the file first. You can't usually use
2904either read-write mode for updating textfiles, since they have
2905variable length records. See the B<-i> switch in L<perlrun> for a
2906better approach. The file is created with permissions of C<0666>
2907modified by the process' C<umask> value.
2908
2909These various prefixes correspond to the fopen(3) modes of C<'r'>,
2910C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2911
6170680b
IZ
2912In the 2-arguments (and 1-argument) form of the call the mode and
2913filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2914spaces. It is possible to omit the mode in these forms if the mode is
2915C<< '<' >>.
6170680b 2916
7660c0ab 2917If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2918command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2919C<'|'>, the filename is interpreted as a command which pipes output to
2920us. See L<perlipc/"Using open() for IPC">
19799a22 2921for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2922that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2923and L<perlipc/"Bidirectional Communication with Another Process">
2924for alternatives.)
cb1a09d0 2925
ed53a2bb
JH
2926For three or more arguments if MODE is C<'|-'>, the filename is
2927interpreted as a command to which output is to be piped, and if MODE
2928is C<'-|'>, the filename is interpreted as a command which pipes
2929output to us. In the 2-arguments (and 1-argument) form one should
2930replace dash (C<'-'>) with the command.
2931See L<perlipc/"Using open() for IPC"> for more examples of this.
2932(You are not allowed to C<open> to a command that pipes both in I<and>
2933out, but see L<IPC::Open2>, L<IPC::Open3>, and
2934L<perlipc/"Bidirectional Communication"> for alternatives.)
2935
2936In the three-or-more argument form of pipe opens, if LIST is specified
2937(extra arguments after the command name) then LIST becomes arguments
2938to the command invoked if the platform supports it. The meaning of
2939C<open> with more than three arguments for non-pipe modes is not yet
2940specified. Experimental "layers" may give extra LIST arguments
2941meaning.
6170680b
IZ
2942
2943In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2944and opening C<< '>-' >> opens STDOUT.
6170680b 2945
fae2c0fb
RGS
2946You may use the three-argument form of open to specify IO "layers"
2947(sometimes also referred to as "disciplines") to be applied to the handle
2948that affect how the input and output are processed (see L<open> and
2949L<PerlIO> for more details). For example
7207e29d 2950
9124316e
JH
2951 open(FH, "<:utf8", "file")
2952
2953will open the UTF-8 encoded file containing Unicode characters,
fae2c0fb
RGS
2954see L<perluniintro>. (Note that if layers are specified in the
2955three-arg form then default layers set by the C<open> pragma are
01e6739c 2956ignored.)
ed53a2bb
JH
2957
2958Open returns nonzero upon success, the undefined value otherwise. If
2959the C<open> involved a pipe, the return value happens to be the pid of
2960the subprocess.
cb1a09d0 2961
ed53a2bb
JH
2962If you're running Perl on a system that distinguishes between text
2963files and binary files, then you should check out L</binmode> for tips
2964for dealing with this. The key distinction between systems that need
2965C<binmode> and those that don't is their text file formats. Systems
8939ba94 2966like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
2967character, and which encode that character in C as C<"\n">, do not
2968need C<binmode>. The rest need it.
cb1a09d0 2969
fb73857a 2970When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2971if the request failed, so C<open> is frequently used in connection with
2972C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2973where you want to make a nicely formatted error message (but there are
5a964f20 2974modules that can help with that problem)) you should always check
19799a22 2975the return value from opening a file. The infrequent exception is when
fb73857a
PP
2976working with an unopened filehandle is actually what you want to do.
2977
ed53a2bb
JH
2978As a special case the 3 arg form with a read/write mode and the third
2979argument being C<undef>:
b76cc8ba
NIS
2980
2981 open(TMP, "+>", undef) or die ...
2982
f253e835
JH
2983opens a filehandle to an anonymous temporary file. Also using "+<"
2984works for symmetry, but you really should consider writing something
2985to the temporary file first. You will need to seek() to do the
2986reading.
b76cc8ba 2987
2ce64696
JC
2988Since v5.8.0, perl has built using PerlIO by default. Unless you've
2989changed this (ie Configure -Uuseperlio), you can open file handles to
2990"in memory" files held in Perl scalars via:
ba964c95 2991
b996200f
SB
2992 open($fh, '>', \$variable) || ..
2993
2994Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2995file, you have to close it first:
2996
2997 close STDOUT;
2998 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
ba964c95 2999
cb1a09d0 3000Examples:
a0d0e21e
LW
3001
3002 $ARTICLE = 100;
3003 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3004 while (<ARTICLE>) {...
3005
6170680b 3006 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 3007 # if the open fails, output is discarded
a0d0e21e 3008
6170680b 3009 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 3010 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 3011
6170680b
IZ
3012 open(DBASE, '+<dbase.mine') # ditto
3013 or die "Can't open 'dbase.mine' for update: $!";
3014
3015 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 3016 or die "Can't start caesar: $!";
a0d0e21e 3017
6170680b
IZ
3018 open(ARTICLE, "caesar <$article |") # ditto
3019 or die "Can't start caesar: $!";
3020
2359510d 3021 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
fb73857a 3022 or die "Can't start sort: $!";
a0d0e21e 3023
ba964c95
T
3024 # in memory files
3025 open(MEMORY,'>', \$var)
3026 or die "Can't open memory file: $!";
3027 print MEMORY "foo!\n"; # output will end up in $var
3028
a0d0e21e
LW
3029 # process argument list of files along with any includes
3030
3031 foreach $file (@ARGV) {
3032 process($file, 'fh00');
3033 }
3034
3035 sub process {
5a964f20 3036 my($filename, $input) = @_;
a0d0e21e
LW
3037 $input++; # this is a string increment
3038 unless (open($input, $filename)) {
3039 print STDERR "Can't open $filename: $!\n";
3040 return;
3041 }
3042
5a964f20 3043 local $_;
a0d0e21e
LW
3044 while (<$input>) { # note use of indirection
3045 if (/^#include "(.*)"/) {
3046 process($1, $input);
3047 next;
3048 }
5a964f20 3049 #... # whatever
a0d0e21e
LW
3050 }
3051 }
3052
ae4c5402 3053See L<perliol> for detailed info on PerlIO.
2ce64696 3054
a0d0e21e 3055You may also, in the Bourne shell tradition, specify an EXPR beginning
00cafafa
JH
3056with C<< '>&' >>, in which case the rest of the string is interpreted
3057as the name of a filehandle (or file descriptor, if numeric) to be
3058duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3059C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3060The mode you specify should match the mode of the original filehandle.
3061(Duping a filehandle does not take into account any existing contents
3062of IO buffers.) If you use the 3 arg form then you can pass either a
3063number, the name of a filehandle or the normal "reference to a glob".
6170680b 3064
eae1b76b
SB
3065Here is a script that saves, redirects, and restores C<STDOUT> and
3066C<STDERR> using various methods:
a0d0e21e
LW
3067
3068 #!/usr/bin/perl
eae1b76b
SB
3069 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3070 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
818c4caa 3071
eae1b76b
SB
3072 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3073 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
a0d0e21e 3074
eae1b76b
SB
3075 select STDERR; $| = 1; # make unbuffered
3076 select STDOUT; $| = 1; # make unbuffered
a0d0e21e
LW
3077
3078 print STDOUT "stdout 1\n"; # this works for
3079 print STDERR "stderr 1\n"; # subprocesses too
3080
eae1b76b
SB
3081 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3082 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
a0d0e21e
LW
3083
3084 print STDOUT "stdout 2\n";
3085 print STDERR "stderr 2\n";
3086
ef8b303f
JH
3087If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3088or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3089that file descriptor (and not call L<dup(2)>); this is more
3090parsimonious of file descriptors. For example:
a0d0e21e 3091
00cafafa 3092 # open for input, reusing the fileno of $fd
a0d0e21e 3093 open(FILEHANDLE, "<&=$fd")
df632fdf 3094
b76cc8ba 3095or
df632fdf 3096
b76cc8ba 3097 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 3098
00cafafa
JH
3099or
3100
3101 # open for append, using the fileno of OLDFH
3102 open(FH, ">>&=", OLDFH)
3103
3104or
3105
3106 open(FH, ">>&=OLDFH")
3107
ef8b303f
JH
3108Being parsimonious on filehandles is also useful (besides being
3109parsimonious) for example when something is dependent on file
3110descriptors, like for example locking using flock(). If you do just
3111C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3112descriptor as B, and therefore flock(A) will not flock(B), and vice
3113versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3114the same file descriptor.
3115
3116Note that if you are using Perls older than 5.8.0, Perl will be using
3117the standard C libraries' fdopen() to implement the "=" functionality.
3118On many UNIX systems fdopen() fails when file descriptors exceed a
3119certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3120most often the default.
4af147f6 3121
df632fdf
JH
3122You can see whether Perl has been compiled with PerlIO or not by
3123running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3124is C<define>, you have PerlIO, otherwise you don't.
3125
6170680b
IZ
3126If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3127with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 3128there is an implicit fork done, and the return value of open is the pid
7660c0ab 3129of the child within the parent process, and C<0> within the child
184e9718 3130process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
3131The filehandle behaves normally for the parent, but i/o to that
3132filehandle is piped from/to the STDOUT/STDIN of the child process.
3133In the child process the filehandle isn't opened--i/o happens from/to
3134the new STDOUT or STDIN. Typically this is used like the normal
3135piped open when you want to exercise more control over just how the
3136pipe command gets executed, such as when you are running setuid, and
54310121 3137don't want to have to scan shell commands for metacharacters.
6170680b 3138The following triples are more or less equivalent:
a0d0e21e
LW
3139
3140 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
3141 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3142 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 3143 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
3144
3145 open(FOO, "cat -n '$file'|");
6170680b
IZ
3146 open(FOO, '-|', "cat -n '$file'");
3147 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
3148 open(FOO, '-|', "cat", '-n', $file);
3149
3150The last example in each block shows the pipe as "list form", which is
64da03b2
JH
3151not yet supported on all platforms. A good rule of thumb is that if
3152your platform has true C<fork()> (in other words, if your platform is
3153UNIX) you can use the list form.
a0d0e21e 3154
4633a7c4
LW
3155See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3156
0f897271
GS
3157Beginning with v5.6.0, Perl will attempt to flush all files opened for
3158output before any operation that may do a fork, but this may not be
3159supported on some platforms (see L<perlport>). To be safe, you may need
3160to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3161of C<IO::Handle> on any open handles.
3162
ed53a2bb
JH
3163On systems that support a close-on-exec flag on files, the flag will
3164be set for the newly opened file descriptor as determined by the value
3165of $^F. See L<perlvar/$^F>.
a0d0e21e 3166
0dccf244 3167Closing any piped filehandle causes the parent process to wait for the
e5218da5
GA
3168child to finish, and returns the status value in C<$?> and
3169C<${^CHILD_ERROR_NATIVE}>.
0dccf244 3170
ed53a2bb
JH
3171The filename passed to 2-argument (or 1-argument) form of open() will
3172have leading and trailing whitespace deleted, and the normal
3173redirection characters honored. This property, known as "magic open",
5a964f20 3174can often be used to good effect. A user could specify a filename of
7660c0ab 3175F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
3176
3177 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3178 open(FH, $filename) or die "Can't open $filename: $!";
3179
6170680b
IZ
3180Use 3-argument form to open a file with arbitrary weird characters in it,
3181
3182 open(FOO, '<', $file);
3183
3184otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
3185
3186 $file =~ s#^(\s)#./$1#;
3187 open(FOO, "< $file\0");
3188
a31a806a 3189(this may not work on some bizarre filesystems). One should
106325ad 3190conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
3191of open():
3192
3193 open IN, $ARGV[0];
3194
3195will allow the user to specify an argument of the form C<"rsh cat file |">,
3196but will not work on a filename which happens to have a trailing space, while
3197
3198 open IN, '<', $ARGV[0];
3199
3200will have exactly the opposite restrictions.
3201
19799a22 3202If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
3203should use the C<sysopen> function, which involves no such magic (but
3204may use subtly different filemodes than Perl open(), which is mapped
3205to C fopen()). This is
5a964f20
TC
3206another way to protect your filenames from interpretation. For example:
3207
3208 use IO::Handle;
3209 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3210 or die "sysopen $path: $!";
3211 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 3212 print HANDLE "stuff $$\n";
5a964f20
TC
3213 seek(HANDLE, 0, 0);
3214 print "File contains: ", <HANDLE>;
3215
7660c0ab
A
3216Using the constructor from the C<IO::Handle> package (or one of its
3217subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
3218filehandles that have the scope of whatever variables hold references to
3219them, and automatically close whenever and however you leave that scope:
c07a80fd 3220
5f05dabc 3221 use IO::File;
5a964f20 3222 #...
c07a80fd
PP
3223 sub read_myfile_munged {
3224 my $ALL = shift;
5f05dabc 3225 my $handle = new IO::File;
c07a80fd
PP
3226 open($handle, "myfile") or die "myfile: $!";
3227 $first = <$handle>
3228 or return (); # Automatically closed here.
3229 mung $first or die "mung failed"; # Or here.
3230 return $first, <$handle> if $ALL; # Or here.
3231 $first; # Or here.
3232 }
3233
b687b08b 3234See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
3235
3236=item opendir DIRHANDLE,EXPR
3237
19799a22
GS
3238Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3239C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a28cd5c9
NT
3240DIRHANDLE may be an expression whose value can be used as an indirect
3241dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3242scalar variable (or array or hash element), the variable is assigned a
3243reference to a new anonymous dirhandle.
a0d0e21e
LW
3244DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3245
3246=item ord EXPR
3247
54310121 3248=item ord
bbce6d69 3249
121910a4
JH
3250Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3251or Unicode) value of the first character of EXPR. If EXPR is omitted,
3252uses C<$_>.
3253
3254For the reverse, see L</chr>.
3255See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e