This is a live mirror of the Perl 5 development currently hosted at https://github.com/perl/perl5
Re: [perl #40262] kill( 0, ... ) does not return alive status of child process
[perl5.git] / pod / perlfunc.pod
CommitLineData
a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
a0d0e21e
LW
3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
15operator. A unary operator generally provides a scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
a0d0e21e 17contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 18be first, and the list argument will follow. (Note that there can ever
0f31cffe 19be only one such list argument.) For instance, splice() has three scalar
2b5ab1e7
TC
20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
a0d0e21e
LW
22
23In the syntax descriptions that follow, list operators that expect a
24list (and provide list context for the elements of the list) are shown
25with LIST as an argument. Such a list may consist of any combination
26of scalar arguments or list values; the list values will be included
27in the list as if each individual element were interpolated at that
28point in the list, forming a longer single-dimensional list value.
cf264981 29Commas should separate elements of the LIST.
a0d0e21e
LW
30
31Any function in the list below may be used either with or without
32parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 33parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 34surprising) rule is this: It I<looks> like a function, therefore it I<is> a
a0d0e21e
LW
35function, and precedence doesn't matter. Otherwise it's a list
36operator or unary operator, and precedence does matter. And whitespace
37between the function and left parenthesis doesn't count--so you need to
38be careful sometimes:
39
68dc0745 40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
a0d0e21e
LW
45
46If you run Perl with the B<-w> switch it can warn you about this. For
47example, the third line above produces:
48
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
51
2b5ab1e7
TC
52A few functions take no arguments at all, and therefore work as neither
53unary nor list operators. These include such functions as C<time>
54and C<endpwent>. For example, C<time+86_400> always means
55C<time() + 86_400>.
56
a0d0e21e 57For functions that can be used in either a scalar or list context,
54310121 58nonabortive failure is generally indicated in a scalar context by
a0d0e21e
LW
59returning the undefined value, and in a list context by returning the
60null list.
61
5a964f20
TC
62Remember the following important rule: There is B<no rule> that relates
63the behavior of an expression in list context to its behavior in scalar
64context, or vice versa. It might do two totally different things.
a0d0e21e 65Each operator and function decides which sort of value it would be most
2b5ab1e7 66appropriate to return in scalar context. Some operators return the
5a964f20 67length of the list that would have been returned in list context. Some
a0d0e21e
LW
68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
d74e8afc 72X<context>
a0d0e21e 73
d1be9408 74A named array in scalar context is quite different from what would at
5a964f20
TC
75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
81In general, functions in Perl that serve as wrappers for system calls
f86cebdf 82of the same name (like chown(2), fork(2), closedir(2), etc.) all return
5a964f20
TC
83true when they succeed and C<undef> otherwise, as is usually mentioned
84in the descriptions below. This is different from the C interfaces,
19799a22
GS
85which return C<-1> on failure. Exceptions to this rule are C<wait>,
86C<waitpid>, and C<syscall>. System calls also set the special C<$!>
5a964f20
TC
87variable on failure. Other functions do not, except accidentally.
88
cb1a09d0 89=head2 Perl Functions by Category
d74e8afc 90X<function>
cb1a09d0
AD
91
92Here are Perl's functions (including things that look like
5a964f20 93functions, like some keywords and named operators)
cb1a09d0
AD
94arranged by category. Some functions appear in more
95than one place.
96
13a2d996 97=over 4
cb1a09d0
AD
98
99=item Functions for SCALARs or strings
d74e8afc 100X<scalar> X<string> X<character>
cb1a09d0 101
22fae026 102C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
945c54fd
JH
103C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
104C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
cb1a09d0
AD
105
106=item Regular expressions and pattern matching
d74e8afc 107X<regular expression> X<regex> X<regexp>
cb1a09d0 108
ab4f32c2 109C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
cb1a09d0
AD
110
111=item Numeric functions
d74e8afc 112X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 113
22fae026
TM
114C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
115C<sin>, C<sqrt>, C<srand>
cb1a09d0
AD
116
117=item Functions for real @ARRAYs
d74e8afc 118X<array>
cb1a09d0 119
22fae026 120C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
cb1a09d0
AD
121
122=item Functions for list data
d74e8afc 123X<list>
cb1a09d0 124
ab4f32c2 125C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
cb1a09d0
AD
126
127=item Functions for real %HASHes
d74e8afc 128X<hash>
cb1a09d0 129
22fae026 130C<delete>, C<each>, C<exists>, C<keys>, C<values>
cb1a09d0
AD
131
132=item Input and output functions
d74e8afc 133X<I/O> X<input> X<output> X<dbm>
cb1a09d0 134
22fae026
TM
135C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
136C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
0d863452 137C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
22fae026
TM
138C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
139C<warn>, C<write>
cb1a09d0
AD
140
141=item Functions for fixed length data or records
142
22fae026 143C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
cb1a09d0
AD
144
145=item Functions for filehandles, files, or directories
d74e8afc 146X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 147
22fae026 148C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 149C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
1e278fd9
JH
150C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
151C<umask>, C<unlink>, C<utime>
cb1a09d0 152
cf264981 153=item Keywords related to the control flow of your Perl program
d74e8afc 154X<control flow>
cb1a09d0 155
98293880
JH
156C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
157C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 158
0d863452
RH
159=item Keywords related to switch
160
36fb85f3 161C<break>, C<continue>, C<given>, C<when>, C<default>
0d863452
RH
162
163(These are only available if you enable the "switch" feature.
164See L<feature> and L<perlsyn/"Switch statements">.)
165
54310121 166=item Keywords related to scoping
cb1a09d0 167
36fb85f3
RGS
168C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
169C<use>
170
171(C<state> is only available if the "state" feature is enabled. See
172L<feature>.)
cb1a09d0
AD
173
174=item Miscellaneous functions
175
36fb85f3 176C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
834df1c5 177C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
cb1a09d0
AD
178
179=item Functions for processes and process groups
d74e8afc 180X<process> X<pid> X<process id>
cb1a09d0 181
22fae026 182C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 183C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 184C<times>, C<wait>, C<waitpid>
cb1a09d0
AD
185
186=item Keywords related to perl modules
d74e8afc 187X<module>
cb1a09d0 188
22fae026 189C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0
AD
190
191=item Keywords related to classes and object-orientedness
d74e8afc 192X<object> X<class> X<package>
cb1a09d0 193
22fae026
TM
194C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
195C<untie>, C<use>
cb1a09d0
AD
196
197=item Low-level socket functions
d74e8afc 198X<socket> X<sock>
cb1a09d0 199
22fae026
TM
200C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
201C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 202C<socket>, C<socketpair>
cb1a09d0
AD
203
204=item System V interprocess communication functions
d74e8afc 205X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 206
22fae026
TM
207C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
208C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
cb1a09d0
AD
209
210=item Fetching user and group info
d74e8afc 211X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 212
22fae026
TM
213C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
214C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
215C<getpwuid>, C<setgrent>, C<setpwent>
cb1a09d0
AD
216
217=item Fetching network info
d74e8afc 218X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 219
22fae026
TM
220C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
221C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
222C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
223C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
224C<setnetent>, C<setprotoent>, C<setservent>
cb1a09d0
AD
225
226=item Time-related functions
d74e8afc 227X<time> X<date>
cb1a09d0 228
22fae026 229C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 230
37798a01 231=item Functions new in perl5
d74e8afc 232X<perl5>
37798a01 233
834df1c5
SP
234C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
235C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
236C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr>, C<qw>, C<qx>,
237C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
238C<ucfirst>, C<untie>, C<use>, C<when>
37798a01 239
240* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 241operator, which can be used in expressions.
37798a01 242
243=item Functions obsoleted in perl5
244
22fae026 245C<dbmclose>, C<dbmopen>
37798a01 246
cb1a09d0
AD
247=back
248
60f9f73c 249=head2 Portability
d74e8afc 250X<portability> X<Unix> X<portable>
60f9f73c 251
2b5ab1e7
TC
252Perl was born in Unix and can therefore access all common Unix
253system calls. In non-Unix environments, the functionality of some
254Unix system calls may not be available, or details of the available
255functionality may differ slightly. The Perl functions affected
60f9f73c
JH
256by this are:
257
258C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
259C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
260C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
ef5a6dd7
JH
261C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
262C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 263C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
60f9f73c
JH
264C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
265C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
266C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 267C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
60f9f73c
JH
268C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
269C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
270C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 271C<shmwrite>, C<socket>, C<socketpair>,
80cbd5ad
JH
272C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
273C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 274C<utime>, C<wait>, C<waitpid>
60f9f73c
JH
275
276For more information about the portability of these functions, see
277L<perlport> and other available platform-specific documentation.
278
cb1a09d0
AD
279=head2 Alphabetical Listing of Perl Functions
280
a0d0e21e
LW
281=over 8
282
5b3c99c0 283=item -X FILEHANDLE
d74e8afc
ITB
284X<-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>
285X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 286
5b3c99c0 287=item -X EXPR
a0d0e21e 288
5228a96c
SP
289=item -X DIRHANDLE
290
5b3c99c0 291=item -X
a0d0e21e
LW
292
293A file test, where X is one of the letters listed below. This unary
5228a96c
SP
294operator takes one argument, either a filename, a filehandle, or a dirhandle,
295and tests the associated file to see if something is true about it. If the
7660c0ab 296argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 297Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e
LW
298the undefined value if the file doesn't exist. Despite the funny
299names, precedence is the same as any other named unary operator, and
300the argument may be parenthesized like any other unary operator. The
301operator may be any of:
302
303 -r File is readable by effective uid/gid.
304 -w File is writable by effective uid/gid.
305 -x File is executable by effective uid/gid.
306 -o File is owned by effective uid.
307
308 -R File is readable by real uid/gid.
309 -W File is writable by real uid/gid.
310 -X File is executable by real uid/gid.
311 -O File is owned by real uid.
312
313 -e File exists.
8e7e0aa8
MJD
314 -z File has zero size (is empty).
315 -s File has nonzero size (returns size in bytes).
a0d0e21e
LW
316
317 -f File is a plain file.
318 -d File is a directory.
319 -l File is a symbolic link.
9c4d0f16 320 -p File is a named pipe (FIFO), or Filehandle is a pipe.
a0d0e21e
LW
321 -S File is a socket.
322 -b File is a block special file.
323 -c File is a character special file.
324 -t Filehandle is opened to a tty.
325
326 -u File has setuid bit set.
327 -g File has setgid bit set.
328 -k File has sticky bit set.
329
121910a4 330 -T File is an ASCII text file (heuristic guess).
2cdbc966 331 -B File is a "binary" file (opposite of -T).
a0d0e21e 332
95a3fe12 333 -M Script start time minus file modification time, in days.
a0d0e21e 334 -A Same for access time.
95a3fe12 335 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 336
a0d0e21e
LW
337Example:
338
339 while (<>) {
5b3eff12 340 chomp;
a0d0e21e 341 next unless -f $_; # ignore specials
5a964f20 342 #...
a0d0e21e
LW
343 }
344
5ff3f7a4
GS
345The interpretation of the file permission operators C<-r>, C<-R>,
346C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
347of the file and the uids and gids of the user. There may be other
348reasons you can't actually read, write, or execute the file. Such
349reasons may be for example network filesystem access controls, ACLs
350(access control lists), read-only filesystems, and unrecognized
351executable formats.
352
2b5ab1e7
TC
353Also note that, for the superuser on the local filesystems, the C<-r>,
354C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
5ff3f7a4
GS
355if any execute bit is set in the mode. Scripts run by the superuser
356may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 357or temporarily set their effective uid to something else.
5ff3f7a4
GS
358
359If you are using ACLs, there is a pragma called C<filetest> that may
360produce more accurate results than the bare stat() mode bits.
5ff3f7a4
GS
361When under the C<use filetest 'access'> the above-mentioned filetests
362will test whether the permission can (not) be granted using the
468541a8 363access() family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
364under this pragma return true even if there are no execute permission
365bits set (nor any extra execute permission ACLs). This strangeness is
366due to the underlying system calls' definitions. Read the
367documentation for the C<filetest> pragma for more information.
368
a0d0e21e
LW
369Note that C<-s/a/b/> does not do a negated substitution. Saying
370C<-exp($foo)> still works as expected, however--only single letters
371following a minus are interpreted as file tests.
372
373The C<-T> and C<-B> switches work as follows. The first block or so of the
374file is examined for odd characters such as strange control codes or
61eff3bc 375characters with the high bit set. If too many strange characters (>30%)
cf264981 376are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
a0d0e21e 377containing null in the first block is considered a binary file. If C<-T>
9124316e 378or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 379rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 380file, or a file at EOF when testing a filehandle. Because you have to
4633a7c4
LW
381read a file to do the C<-T> test, on most occasions you want to use a C<-f>
382against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 383
19799a22 384If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 385the special filehandle consisting of a solitary underline, then the stat
a0d0e21e
LW
386structure of the previous file test (or stat operator) is used, saving
387a system call. (This doesn't work with C<-t>, and you need to remember
388that lstat() and C<-l> will leave values in the stat structure for the
5c9aa243 389symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 390an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 391Example:
a0d0e21e
LW
392
393 print "Can do.\n" if -r $a || -w _ || -x _;
394
395 stat($filename);
396 print "Readable\n" if -r _;
397 print "Writable\n" if -w _;
398 print "Executable\n" if -x _;
399 print "Setuid\n" if -u _;
400 print "Setgid\n" if -g _;
401 print "Sticky\n" if -k _;
402 print "Text\n" if -T _;
403 print "Binary\n" if -B _;
404
fbb0b3b3
RGS
405As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
406test operators, in a way that C<-f -w -x $file> is equivalent to
cf264981 407C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
fbb0b3b3
RGS
408the return value of C<-f $file> as an argument to another filetest
409operator, no special magic will happen.)
410
a0d0e21e 411=item abs VALUE
d74e8afc 412X<abs> X<absolute>
a0d0e21e 413
54310121 414=item abs
bbce6d69 415
a0d0e21e 416Returns the absolute value of its argument.
7660c0ab 417If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
418
419=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 420X<accept>
a0d0e21e 421
f86cebdf 422Accepts an incoming socket connect, just as the accept(2) system call
19799a22 423does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 424See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 425
8d2a6795
GS
426On systems that support a close-on-exec flag on files, the flag will
427be set for the newly opened file descriptor, as determined by the
428value of $^F. See L<perlvar/$^F>.
429
a0d0e21e 430=item alarm SECONDS
d74e8afc
ITB
431X<alarm>
432X<SIGALRM>
433X<timer>
a0d0e21e 434
54310121 435=item alarm
bbce6d69 436
a0d0e21e 437Arranges to have a SIGALRM delivered to this process after the
cf264981 438specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
439specified, the value stored in C<$_> is used. (On some machines,
440unfortunately, the elapsed time may be up to one second less or more
441than you specified because of how seconds are counted, and process
442scheduling may delay the delivery of the signal even further.)
443
444Only one timer may be counting at once. Each call disables the
445previous timer, and an argument of C<0> may be supplied to cancel the
446previous timer without starting a new one. The returned value is the
447amount of time remaining on the previous timer.
a0d0e21e 448
4633a7c4 449For delays of finer granularity than one second, you may use Perl's
19799a22
GS
450four-argument version of select() leaving the first three arguments
451undefined, or you might be able to use the C<syscall> interface to
83df6a1d
JH
452access setitimer(2) if your system supports it. The Time::HiRes
453module (from CPAN, and starting from Perl 5.8 part of the standard
454distribution) may also prove useful.
2b5ab1e7 455
68f8bed4
JH
456It is usually a mistake to intermix C<alarm> and C<sleep> calls.
457(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 458
19799a22
GS
459If you want to use C<alarm> to time out a system call you need to use an
460C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 461fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 462restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 463modulo the caveats given in L<perlipc/"Signals">.
ff68c719 464
465 eval {
f86cebdf 466 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 467 alarm $timeout;
ff68c719 468 $nread = sysread SOCKET, $buffer, $size;
36477c24 469 alarm 0;
ff68c719 470 };
ff68c719 471 if ($@) {
f86cebdf 472 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719 473 # timed out
474 }
475 else {
476 # didn't
477 }
478
91d81acc
JH
479For more information see L<perlipc>.
480
a0d0e21e 481=item atan2 Y,X
d74e8afc 482X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
483
484Returns the arctangent of Y/X in the range -PI to PI.
485
ca6e1c26 486For the tangent operation, you may use the C<Math::Trig::tan>
28757baa 487function, or use the familiar relation:
488
489 sub tan { sin($_[0]) / cos($_[0]) }
490
bf5f1b4c
JH
491Note that atan2(0, 0) is not well-defined.
492
a0d0e21e 493=item bind SOCKET,NAME
d74e8afc 494X<bind>
a0d0e21e
LW
495
496Binds a network address to a socket, just as the bind system call
19799a22 497does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
498packed address of the appropriate type for the socket. See the examples in
499L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 500
fae2c0fb 501=item binmode FILEHANDLE, LAYER
d74e8afc 502X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 503
a0d0e21e
LW
504=item binmode FILEHANDLE
505
1cbfc93d
NIS
506Arranges for FILEHANDLE to be read or written in "binary" or "text"
507mode on systems where the run-time libraries distinguish between
508binary and text files. If FILEHANDLE is an expression, the value is
509taken as the name of the filehandle. Returns true on success,
b5fe5ca2 510otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 511
d807c6f4
JH
512On some systems (in general, DOS and Windows-based systems) binmode()
513is necessary when you're not working with a text file. For the sake
514of portability it is a good idea to always use it when appropriate,
515and to never use it when it isn't appropriate. Also, people can
516set their I/O to be by default UTF-8 encoded Unicode, not bytes.
517
518In other words: regardless of platform, use binmode() on binary data,
519like for example images.
520
521If LAYER is present it is a single string, but may contain multiple
522directives. The directives alter the behaviour of the file handle.
523When LAYER is present using binmode on text file makes sense.
524
fae2c0fb 525If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
526suitable for passing binary data. This includes turning off possible CRLF
527translation and marking it as bytes (as opposed to Unicode characters).
749683d2
YST
528Note that, despite what may be implied in I<"Programming Perl"> (the
529Camel) or elsewhere, C<:raw> is I<not> the simply inverse of C<:crlf>
fae2c0fb 530-- other layers which would affect binary nature of the stream are
0226bbdb
NIS
531I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
532PERLIO environment variable.
01e6739c 533
d807c6f4
JH
534The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
535form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
536establish default I/O layers. See L<open>.
537
fae2c0fb
RGS
538I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
539in "Programming Perl, 3rd Edition". However, since the publishing of this
540book, by many known as "Camel III", the consensus of the naming of this
541functionality has moved from "discipline" to "layer". All documentation
542of this version of Perl therefore refers to "layers" rather than to
543"disciplines". Now back to the regularly scheduled documentation...>
544
01e6739c 545To mark FILEHANDLE as UTF-8, use C<:utf8>.
1cbfc93d 546
ed53a2bb 547In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
548is done on the filehandle. Calling binmode() will normally flush any
549pending buffered output data (and perhaps pending input data) on the
fae2c0fb 550handle. An exception to this is the C<:encoding> layer that
01e6739c 551changes the default character encoding of the handle, see L<open>.
fae2c0fb 552The C<:encoding> layer sometimes needs to be called in
3874323d
JH
553mid-stream, and it doesn't flush the stream. The C<:encoding>
554also implicitly pushes on top of itself the C<:utf8> layer because
555internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 556
19799a22 557The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
558system all work together to let the programmer treat a single
559character (C<\n>) as the line terminator, irrespective of the external
560representation. On many operating systems, the native text file
561representation matches the internal representation, but on some
562platforms the external representation of C<\n> is made up of more than
563one character.
564
68bd7414
NIS
565Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
566character to end each line in the external representation of text (even
5e12dbfa 567though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
568on Unix and most VMS files). In other systems like OS/2, DOS and the
569various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
570but what's stored in text files are the two characters C<\cM\cJ>. That
571means that, if you don't use binmode() on these systems, C<\cM\cJ>
572sequences on disk will be converted to C<\n> on input, and any C<\n> in
573your program will be converted back to C<\cM\cJ> on output. This is what
574you want for text files, but it can be disastrous for binary files.
30168b04
GS
575
576Another consequence of using binmode() (on some systems) is that
577special end-of-file markers will be seen as part of the data stream.
578For systems from the Microsoft family this means that if your binary
4375e838 579data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
580the file, unless you use binmode().
581
582binmode() is not only important for readline() and print() operations,
583but also when using read(), seek(), sysread(), syswrite() and tell()
584(see L<perlport> for more details). See the C<$/> and C<$\> variables
585in L<perlvar> for how to manually set your input and output
586line-termination sequences.
a0d0e21e 587
4633a7c4 588=item bless REF,CLASSNAME
d74e8afc 589X<bless>
a0d0e21e
LW
590
591=item bless REF
592
2b5ab1e7
TC
593This function tells the thingy referenced by REF that it is now an object
594in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 595is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 596it returns the reference for convenience. Always use the two-argument
cf264981
SP
597version if a derived class might inherit the function doing the blessing.
598See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
599of objects.
a0d0e21e 600
57668c4d 601Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 602Namespaces with all lowercase names are considered reserved for
cf264981 603Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
604confusion, you may wish to avoid such package names as well. Make sure
605that CLASSNAME is a true value.
60ad88b8
GS
606
607See L<perlmod/"Perl Modules">.
608
0d863452
RH
609=item break
610
611Break out of a C<given()> block.
612
613This keyword is enabled by the "switch" feature: see L<feature>
614for more information.
615
a0d0e21e 616=item caller EXPR
d74e8afc 617X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
618
619=item caller
620
5a964f20 621Returns the context of the current subroutine call. In scalar context,
28757baa 622returns the caller's package name if there is a caller, that is, if
19799a22 623we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 624otherwise. In list context, returns
a0d0e21e 625
ee6b43cc 626 # 0 1 2
748a9306 627 ($package, $filename, $line) = caller;
a0d0e21e
LW
628
629With EXPR, it returns some extra information that the debugger uses to
630print a stack trace. The value of EXPR indicates how many call frames
631to go back before the current one.
632
ee6b43cc 633 # 0 1 2 3 4
f3aa04c2 634 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 635
636 # 5 6 7 8 9 10
b3ca2e83 637 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 638 = caller($i);
e7ea3e70 639
951ba7fe 640Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 641call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 642C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 643C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 644C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 645$filename is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
646each C<use> statement creates a C<require> frame inside an C<eval EXPR>
647frame.) $subroutine may also be C<(unknown)> if this particular
648subroutine happens to have been deleted from the symbol table.
649C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
650C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
651compiled with. The C<$hints> and C<$bitmask> values are subject to change
652between versions of Perl, and are not meant for external use.
748a9306 653
b3ca2e83
NC
654C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
655caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
656of this hash, as they are the actual values stored in the optree.
657
748a9306 658Furthermore, when called from within the DB package, caller returns more
7660c0ab 659detailed information: it sets the list variable C<@DB::args> to be the
54310121 660arguments with which the subroutine was invoked.
748a9306 661
7660c0ab 662Be aware that the optimizer might have optimized call frames away before
19799a22 663C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 664might not return information about the call frame you expect it do, for
b76cc8ba 665C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 666previous time C<caller> was called.
7660c0ab 667
a0d0e21e 668=item chdir EXPR
d74e8afc
ITB
669X<chdir>
670X<cd>
f723aae1 671X<directory, change>
a0d0e21e 672
c4aca7d0
GA
673=item chdir FILEHANDLE
674
675=item chdir DIRHANDLE
676
ce2984c3
PF
677=item chdir
678
ffce7b87 679Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 680changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 681changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
682variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
683neither is set, C<chdir> does nothing. It returns true upon success,
684false otherwise. See the example under C<die>.
a0d0e21e 685
c4aca7d0
GA
686On systems that support fchdir, you might pass a file handle or
687directory handle as argument. On systems that don't support fchdir,
688passing handles produces a fatal error at run time.
689
a0d0e21e 690=item chmod LIST
d74e8afc 691X<chmod> X<permission> X<mode>
a0d0e21e
LW
692
693Changes the permissions of a list of files. The first element of the
4633a7c4 694list must be the numerical mode, which should probably be an octal
4ad40acf 695number, and which definitely should I<not> be a string of octal digits:
2f9daede 696C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 697successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
698
699 $cnt = chmod 0755, 'foo', 'bar';
700 chmod 0755, @executables;
f86cebdf
GS
701 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
702 # --w----r-T
2f9daede
TP
703 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
704 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 705
c4aca7d0
GA
706On systems that support fchmod, you might pass file handles among the
707files. On systems that don't support fchmod, passing file handles
345da378
GA
708produces a fatal error at run time. The file handles must be passed
709as globs or references to be recognized. Barewords are considered
710file names.
c4aca7d0
GA
711
712 open(my $fh, "<", "foo");
713 my $perm = (stat $fh)[2] & 07777;
714 chmod($perm | 0600, $fh);
715
ca6e1c26
JH
716You can also import the symbolic C<S_I*> constants from the Fcntl
717module:
718
719 use Fcntl ':mode';
720
721 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
722 # This is identical to the chmod 0755 of the above example.
723
a0d0e21e 724=item chomp VARIABLE
d74e8afc 725X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 726
313c9f5c 727=item chomp( LIST )
a0d0e21e
LW
728
729=item chomp
730
2b5ab1e7
TC
731This safer version of L</chop> removes any trailing string
732that corresponds to the current value of C<$/> (also known as
28757baa 733$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
734number of characters removed from all its arguments. It's often used to
735remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
736that the final record may be missing its newline. When in paragraph
737mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
738When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
739a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 740remove anything.
19799a22 741If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
742
743 while (<>) {
744 chomp; # avoid \n on last field
745 @array = split(/:/);
5a964f20 746 # ...
a0d0e21e
LW
747 }
748
4bf21a6d
RD
749If VARIABLE is a hash, it chomps the hash's values, but not its keys.
750
a0d0e21e
LW
751You can actually chomp anything that's an lvalue, including an assignment:
752
753 chomp($cwd = `pwd`);
754 chomp($answer = <STDIN>);
755
756If you chomp a list, each element is chomped, and the total number of
757characters removed is returned.
758
442a8c12
NC
759If the C<encoding> pragma is in scope then the lengths returned are
760calculated from the length of C<$/> in Unicode characters, which is not
761always the same as the length of C<$/> in the native encoding.
762
15e44fd8
RGS
763Note that parentheses are necessary when you're chomping anything
764that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
765is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
766C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
767C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
768as C<chomp($a, $b)>.
769
a0d0e21e 770=item chop VARIABLE
d74e8afc 771X<chop>
a0d0e21e 772
313c9f5c 773=item chop( LIST )
a0d0e21e
LW
774
775=item chop
776
777Chops off the last character of a string and returns the character
5b3eff12 778chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 779scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
780If VARIABLE is a hash, it chops the hash's values, but not its keys.
781
5b3eff12 782You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
783
784If you chop a list, each element is chopped. Only the value of the
19799a22 785last C<chop> is returned.
a0d0e21e 786
19799a22 787Note that C<chop> returns the last character. To return all but the last
748a9306
LW
788character, use C<substr($string, 0, -1)>.
789
15e44fd8
RGS
790See also L</chomp>.
791
a0d0e21e 792=item chown LIST
d74e8afc 793X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
794
795Changes the owner (and group) of a list of files. The first two
19799a22
GS
796elements of the list must be the I<numeric> uid and gid, in that
797order. A value of -1 in either position is interpreted by most
798systems to leave that value unchanged. Returns the number of files
799successfully changed.
a0d0e21e
LW
800
801 $cnt = chown $uid, $gid, 'foo', 'bar';
802 chown $uid, $gid, @filenames;
803
c4aca7d0
GA
804On systems that support fchown, you might pass file handles among the
805files. On systems that don't support fchown, passing file handles
345da378
GA
806produces a fatal error at run time. The file handles must be passed
807as globs or references to be recognized. Barewords are considered
808file names.
c4aca7d0 809
54310121 810Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
811
812 print "User: ";
19799a22 813 chomp($user = <STDIN>);
5a964f20 814 print "Files: ";
19799a22 815 chomp($pattern = <STDIN>);
a0d0e21e
LW
816
817 ($login,$pass,$uid,$gid) = getpwnam($user)
818 or die "$user not in passwd file";
819
5a964f20 820 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
821 chown $uid, $gid, @ary;
822
54310121 823On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
824file unless you're the superuser, although you should be able to change
825the group to any of your secondary groups. On insecure systems, these
826restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
827On POSIX systems, you can detect this condition this way:
828
829 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
830 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 831
a0d0e21e 832=item chr NUMBER
d74e8afc 833X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 834
54310121 835=item chr
bbce6d69 836
a0d0e21e 837Returns the character represented by that NUMBER in the character set.
a0ed51b3 838For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1e54db1a
JH
839chr(0x263a) is a Unicode smiley face. Note that characters from 128
840to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
841backward compatibility reasons (but see L<encoding>).
aaa68c4a 842
8a064bd6 843Negative values give the Unicode replacement character (chr(0xfffd)),
5f0135eb 844except under the L<bytes> pragma, where low eight bits of the value
8a064bd6
JH
845(truncated to an integer) are used.
846
974da8e5
JH
847If NUMBER is omitted, uses C<$_>.
848
b76cc8ba 849For the reverse, use L</ord>.
a0d0e21e 850
974da8e5
JH
851Note that under the C<bytes> pragma the NUMBER is masked to
852the low eight bits.
853
854See L<perlunicode> and L<encoding> for more about Unicode.
bbce6d69 855
a0d0e21e 856=item chroot FILENAME
d74e8afc 857X<chroot> X<root>
a0d0e21e 858
54310121 859=item chroot
bbce6d69 860
5a964f20 861This function works like the system call by the same name: it makes the
4633a7c4 862named directory the new root directory for all further pathnames that
951ba7fe 863begin with a C</> by your process and all its children. (It doesn't
28757baa 864change your current working directory, which is unaffected.) For security
4633a7c4 865reasons, this call is restricted to the superuser. If FILENAME is
19799a22 866omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
867
868=item close FILEHANDLE
d74e8afc 869X<close>
a0d0e21e 870
6a518fbc
TP
871=item close
872
9124316e
JH
873Closes the file or pipe associated with the file handle, returning
874true only if IO buffers are successfully flushed and closes the system
875file descriptor. Closes the currently selected filehandle if the
876argument is omitted.
fb73857a 877
878You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
879another C<open> on it, because C<open> will close it for you. (See
880C<open>.) However, an explicit C<close> on an input file resets the line
881counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 882
dede8123
RGS
883If the file handle came from a piped open, C<close> will additionally
884return false if one of the other system calls involved fails, or if the
fb73857a 885program exits with non-zero status. (If the only problem was that the
dede8123 886program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 887also waits for the process executing on the pipe to complete, in case you
b76cc8ba 888want to look at the output of the pipe afterwards, and
e5218da5
GA
889implicitly puts the exit status value of that command into C<$?> and
890C<${^CHILD_ERROR_NATIVE}>.
5a964f20 891
73689b13
GS
892Prematurely closing the read end of a pipe (i.e. before the process
893writing to it at the other end has closed it) will result in a
894SIGPIPE being delivered to the writer. If the other end can't
895handle that, be sure to read all the data before closing the pipe.
896
fb73857a 897Example:
a0d0e21e 898
fb73857a 899 open(OUTPUT, '|sort >foo') # pipe to sort
900 or die "Can't start sort: $!";
5a964f20 901 #... # print stuff to output
fb73857a 902 close OUTPUT # wait for sort to finish
903 or warn $! ? "Error closing sort pipe: $!"
904 : "Exit status $? from sort";
905 open(INPUT, 'foo') # get sort's results
906 or die "Can't open 'foo' for input: $!";
a0d0e21e 907
5a964f20
TC
908FILEHANDLE may be an expression whose value can be used as an indirect
909filehandle, usually the real filehandle name.
a0d0e21e
LW
910
911=item closedir DIRHANDLE
d74e8afc 912X<closedir>
a0d0e21e 913
19799a22 914Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
915system call.
916
a0d0e21e 917=item connect SOCKET,NAME
d74e8afc 918X<connect>
a0d0e21e
LW
919
920Attempts to connect to a remote socket, just as the connect system call
19799a22 921does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
922packed address of the appropriate type for the socket. See the examples in
923L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 924
cb1a09d0 925=item continue BLOCK
d74e8afc 926X<continue>
cb1a09d0 927
0d863452
RH
928=item continue
929
cf264981
SP
930C<continue> is actually a flow control statement rather than a function. If
931there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
932C<foreach>), it is always executed just before the conditional is about to
933be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
934it can be used to increment a loop variable, even when the loop has been
935continued via the C<next> statement (which is similar to the C C<continue>
936statement).
937
98293880 938C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
939block. C<last> and C<redo> will behave as if they had been executed within
940the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
941block, it may be more entertaining.
942
943 while (EXPR) {
944 ### redo always comes here
945 do_something;
946 } continue {
947 ### next always comes here
948 do_something_else;
949 # then back the top to re-check EXPR
950 }
951 ### last always comes here
952
953Omitting the C<continue> section is semantically equivalent to using an
19799a22 954empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
955to check the condition at the top of the loop.
956
0d863452
RH
957If the "switch" feature is enabled, C<continue> is also a
958function that will break out of the current C<when> or C<default>
959block, and fall through to the next case. See L<feature> and
960L<perlsyn/"Switch statements"> for more information.
961
962
a0d0e21e 963=item cos EXPR
d74e8afc 964X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 965
d6217f1e
GS
966=item cos
967
5a964f20 968Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 969takes cosine of C<$_>.
a0d0e21e 970
ca6e1c26 971For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa 972function, or use this relation:
973
974 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
975
a0d0e21e 976=item crypt PLAINTEXT,SALT
d74e8afc 977X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 978X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 979
ef2e6798
MS
980Creates a digest string exactly like the crypt(3) function in the C
981library (assuming that you actually have a version there that has not
cf264981 982been extirpated as a potential munitions).
ef2e6798
MS
983
984crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
985into a short string, called a digest, which is returned. The same
986PLAINTEXT and SALT will always return the same string, but there is no
987(known) way to get the original PLAINTEXT from the hash. Small
988changes in the PLAINTEXT or SALT will result in large changes in the
989digest.
990
991There is no decrypt function. This function isn't all that useful for
992cryptography (for that, look for F<Crypt> modules on your nearby CPAN
993mirror) and the name "crypt" is a bit of a misnomer. Instead it is
994primarily used to check if two pieces of text are the same without
995having to transmit or store the text itself. An example is checking
996if a correct password is given. The digest of the password is stored,
cf264981 997not the password itself. The user types in a password that is
ef2e6798
MS
998crypt()'d with the same salt as the stored digest. If the two digests
999match the password is correct.
1000
1001When verifying an existing digest string you should use the digest as
1002the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1003to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1004crypt() will hash the new string with the same salt as the digest.
1005This allows your code to work with the standard L<crypt|/crypt> and
1006with more exotic implementations. In other words, do not assume
1007anything about the returned string itself, or how many bytes in the
1008digest matter.
85c16d83
JH
1009
1010Traditionally the result is a string of 13 bytes: two first bytes of
1011the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
ef2e6798
MS
1012the first eight bytes of the digest string mattered, but alternative
1013hashing schemes (like MD5), higher level security schemes (like C2),
1014and implementations on non-UNIX platforms may produce different
1015strings.
85c16d83
JH
1016
1017When choosing a new salt create a random two character string whose
1018characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1019'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1020characters is just a recommendation; the characters allowed in
1021the salt depend solely on your system's crypt library, and Perl can't
1022restrict what salts C<crypt()> accepts.
e71965be 1023
a0d0e21e 1024Here's an example that makes sure that whoever runs this program knows
cf264981 1025their password:
a0d0e21e
LW
1026
1027 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1028
1029 system "stty -echo";
1030 print "Password: ";
e71965be 1031 chomp($word = <STDIN>);
a0d0e21e
LW
1032 print "\n";
1033 system "stty echo";
1034
e71965be 1035 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
1036 die "Sorry...\n";
1037 } else {
1038 print "ok\n";
54310121 1039 }
a0d0e21e 1040
9f8f0c9d 1041Of course, typing in your own password to whoever asks you
748a9306 1042for it is unwise.
a0d0e21e 1043
ef2e6798 1044The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1045of data, not least of all because you can't get the information
ef2e6798 1046back. Look at the L<Digest> module for more robust algorithms.
19799a22 1047
f2791508
JH
1048If using crypt() on a Unicode string (which I<potentially> has
1049characters with codepoints above 255), Perl tries to make sense
1050of the situation by trying to downgrade (a copy of the string)
1051the string back to an eight-bit byte string before calling crypt()
1052(on that copy). If that works, good. If not, crypt() dies with
1053C<Wide character in crypt>.
85c16d83 1054
aa689395 1055=item dbmclose HASH
d74e8afc 1056X<dbmclose>
a0d0e21e 1057
19799a22 1058[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1059
aa689395 1060Breaks the binding between a DBM file and a hash.
a0d0e21e 1061
19799a22 1062=item dbmopen HASH,DBNAME,MASK
d74e8afc 1063X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1064
19799a22 1065[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1066
7b8d334a 1067This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1068hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1069argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395 1070is the name of the database (without the F<.dir> or F<.pag> extension if
1071any). If the database does not exist, it is created with protection
19799a22
GS
1072specified by MASK (as modified by the C<umask>). If your system supports
1073only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 1074program. In older versions of Perl, if your system had neither DBM nor
19799a22 1075ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395 1076sdbm(3).
1077
1078If you don't have write access to the DBM file, you can only read hash
1079variables, not set them. If you want to test whether you can write,
19799a22 1080either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 1081which will trap the error.
a0d0e21e 1082
19799a22
GS
1083Note that functions such as C<keys> and C<values> may return huge lists
1084when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1085function to iterate over large DBM files. Example:
1086
1087 # print out history file offsets
1088 dbmopen(%HIST,'/usr/lib/news/history',0666);
1089 while (($key,$val) = each %HIST) {
1090 print $key, ' = ', unpack('L',$val), "\n";
1091 }
1092 dbmclose(%HIST);
1093
cb1a09d0 1094See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1095cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1096rich implementation.
4633a7c4 1097
2b5ab1e7
TC
1098You can control which DBM library you use by loading that library
1099before you call dbmopen():
1100
1101 use DB_File;
1102 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1103 or die "Can't open netscape history file: $!";
1104
a0d0e21e 1105=item defined EXPR
d74e8afc 1106X<defined> X<undef> X<undefined>
a0d0e21e 1107
54310121 1108=item defined
bbce6d69 1109
2f9daede
TP
1110Returns a Boolean value telling whether EXPR has a value other than
1111the undefined value C<undef>. If EXPR is not present, C<$_> will be
1112checked.
1113
1114Many operations return C<undef> to indicate failure, end of file,
1115system error, uninitialized variable, and other exceptional
1116conditions. This function allows you to distinguish C<undef> from
1117other values. (A simple Boolean test will not distinguish among
7660c0ab 1118C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1119false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1120doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TP
1121returns C<undef> when its argument is an empty array, I<or> when the
1122element to return happens to be C<undef>.
1123
f10b0346
GS
1124You may also use C<defined(&func)> to check whether subroutine C<&func>
1125has ever been defined. The return value is unaffected by any forward
04891299 1126declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1127may still be callable: its package may have an C<AUTOLOAD> method that
1128makes it spring into existence the first time that it is called -- see
1129L<perlsub>.
f10b0346
GS
1130
1131Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1132used to report whether memory for that aggregate has ever been
1133allocated. This behavior may disappear in future versions of Perl.
1134You should instead use a simple test for size:
1135
1136 if (@an_array) { print "has array elements\n" }
1137 if (%a_hash) { print "has hash members\n" }
2f9daede
TP
1138
1139When used on a hash element, it tells you whether the value is defined,
dc848c6f 1140not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1141purpose.
a0d0e21e
LW
1142
1143Examples:
1144
1145 print if defined $switch{'D'};
1146 print "$val\n" while defined($val = pop(@ary));
1147 die "Can't readlink $sym: $!"
1148 unless defined($value = readlink $sym);
a0d0e21e 1149 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1150 $debugging = 0 unless defined $debugging;
a0d0e21e 1151
19799a22 1152Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1153discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1154defined values. For example, if you say
a5f75d66
AD
1155
1156 "ab" =~ /a(.*)b/;
1157
7660c0ab 1158The pattern match succeeds, and C<$1> is defined, despite the fact that it
cf264981 1159matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1160matched something that happened to be zero characters long. This is all
a5f75d66 1161very above-board and honest. When a function returns an undefined value,
2f9daede 1162it's an admission that it couldn't give you an honest answer. So you
19799a22 1163should use C<defined> only when you're questioning the integrity of what
7660c0ab 1164you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TP
1165what you want.
1166
dc848c6f 1167See also L</undef>, L</exists>, L</ref>.
2f9daede 1168
a0d0e21e 1169=item delete EXPR
d74e8afc 1170X<delete>
a0d0e21e 1171
01020589
GS
1172Given an expression that specifies a hash element, array element, hash slice,
1173or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1174In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1175the size of the array will shrink to the highest element that tests
8216c1fd 1176true for exists() (or 0 if no such element exists).
a0d0e21e 1177
eba0920a
EM
1178Returns a list with the same number of elements as the number of elements
1179for which deletion was attempted. Each element of that list consists of
1180either the value of the element deleted, or the undefined value. In scalar
1181context, this means that you get the value of the last element deleted (or
1182the undefined value if that element did not exist).
1183
1184 %hash = (foo => 11, bar => 22, baz => 33);
1185 $scalar = delete $hash{foo}; # $scalar is 11
1186 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1187 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1188
1189Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1190a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1191from a C<tie>d hash or array may not necessarily return anything.
1192
8ea97a1e
GS
1193Deleting an array element effectively returns that position of the array
1194to its initial, uninitialized state. Subsequently testing for the same
cf264981
SP
1195element with exists() will return false. Also, deleting array elements
1196in the middle of an array will not shift the index of the elements
1197after them down. Use splice() for that. See L</exists>.
8ea97a1e 1198
01020589 1199The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1200
5f05dabc 1201 foreach $key (keys %HASH) {
1202 delete $HASH{$key};
a0d0e21e
LW
1203 }
1204
01020589
GS
1205 foreach $index (0 .. $#ARRAY) {
1206 delete $ARRAY[$index];
1207 }
1208
1209And so do these:
5f05dabc 1210
01020589
GS
1211 delete @HASH{keys %HASH};
1212
9740c838 1213 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1214
2b5ab1e7 1215But both of these are slower than just assigning the empty list
01020589
GS
1216or undefining %HASH or @ARRAY:
1217
1218 %HASH = (); # completely empty %HASH
1219 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1220
01020589
GS
1221 @ARRAY = (); # completely empty @ARRAY
1222 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1223
1224Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1225operation is a hash element, array element, hash slice, or array slice
1226lookup:
a0d0e21e
LW
1227
1228 delete $ref->[$x][$y]{$key};
5f05dabc 1229 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1230
01020589
GS
1231 delete $ref->[$x][$y][$index];
1232 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1233
a0d0e21e 1234=item die LIST
d74e8afc 1235X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1236
19799a22
GS
1237Outside an C<eval>, prints the value of LIST to C<STDERR> and
1238exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1239exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1240status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1241an C<eval(),> the error message is stuffed into C<$@> and the
1242C<eval> is terminated with the undefined value. This makes
1243C<die> the way to raise an exception.
a0d0e21e
LW
1244
1245Equivalent examples:
1246
1247 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1248 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1249
ccac6780 1250If the last element of LIST does not end in a newline, the current
df37ec69
WW
1251script line number and input line number (if any) are also printed,
1252and a newline is supplied. Note that the "input line number" (also
1253known as "chunk") is subject to whatever notion of "line" happens to
1254be currently in effect, and is also available as the special variable
1255C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1256
1257Hint: sometimes appending C<", stopped"> to your message will cause it
1258to make better sense when the string C<"at foo line 123"> is appended.
1259Suppose you are running script "canasta".
a0d0e21e
LW
1260
1261 die "/etc/games is no good";
1262 die "/etc/games is no good, stopped";
1263
1264produce, respectively
1265
1266 /etc/games is no good at canasta line 123.
1267 /etc/games is no good, stopped at canasta line 123.
1268
2b5ab1e7 1269See also exit(), warn(), and the Carp module.
a0d0e21e 1270
7660c0ab
A
1271If LIST is empty and C<$@> already contains a value (typically from a
1272previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a 1273This is useful for propagating exceptions:
1274
1275 eval { ... };
1276 die unless $@ =~ /Expected exception/;
1277
ad216e65
JH
1278If LIST is empty and C<$@> contains an object reference that has a
1279C<PROPAGATE> method, that method will be called with additional file
1280and line number parameters. The return value replaces the value in
28a5cf3b 1281C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1282were called.
1283
7660c0ab 1284If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1285
52531d10
GS
1286die() can also be called with a reference argument. If this happens to be
1287trapped within an eval(), $@ contains the reference. This behavior permits
1288a more elaborate exception handling implementation using objects that
4375e838 1289maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1290is sometimes preferable to matching particular string values of $@ using
1291regular expressions. Here's an example:
1292
da279afe 1293 use Scalar::Util 'blessed';
1294
52531d10
GS
1295 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1296 if ($@) {
da279afe 1297 if (blessed($@) && $@->isa("Some::Module::Exception")) {
52531d10
GS
1298 # handle Some::Module::Exception
1299 }
1300 else {
1301 # handle all other possible exceptions
1302 }
1303 }
1304
19799a22 1305Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1306them, you may want to overload stringification operations on such custom
1307exception objects. See L<overload> for details about that.
1308
19799a22
GS
1309You can arrange for a callback to be run just before the C<die>
1310does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1311handler will be called with the error text and can change the error
1312message, if it sees fit, by calling C<die> again. See
1313L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1314L<"eval BLOCK"> for some examples. Although this feature was
19799a22
GS
1315to be run only right before your program was to exit, this is not
1316currently the case--the C<$SIG{__DIE__}> hook is currently called
1317even inside eval()ed blocks/strings! If one wants the hook to do
1318nothing in such situations, put
fb73857a 1319
1320 die @_ if $^S;
1321
19799a22
GS
1322as the first line of the handler (see L<perlvar/$^S>). Because
1323this promotes strange action at a distance, this counterintuitive
b76cc8ba 1324behavior may be fixed in a future release.
774d564b 1325
a0d0e21e 1326=item do BLOCK
d74e8afc 1327X<do> X<block>
a0d0e21e
LW
1328
1329Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1330sequence of commands indicated by BLOCK. When modified by the C<while> or
1331C<until> loop modifier, executes the BLOCK once before testing the loop
1332condition. (On other statements the loop modifiers test the conditional
1333first.)
a0d0e21e 1334
4968c1e4 1335C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1336C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1337See L<perlsyn> for alternative strategies.
4968c1e4 1338
a0d0e21e 1339=item do SUBROUTINE(LIST)
d74e8afc 1340X<do>
a0d0e21e 1341
cf264981 1342This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1343
1344=item do EXPR
d74e8afc 1345X<do>
a0d0e21e
LW
1346
1347Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1348file as a Perl script.
a0d0e21e
LW
1349
1350 do 'stat.pl';
1351
1352is just like
1353
986b19de 1354 eval `cat stat.pl`;
a0d0e21e 1355
2b5ab1e7 1356except that it's more efficient and concise, keeps track of the current
ea63ef19 1357filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1358C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1359variables. It also differs in that code evaluated with C<do FILENAME>
1360cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1361same, however, in that it does reparse the file every time you call it,
1362so you probably don't want to do this inside a loop.
a0d0e21e 1363
8e30cc93 1364If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1365error. If C<do> can read the file but cannot compile it, it
8e30cc93
MG
1366returns undef and sets an error message in C<$@>. If the file is
1367successfully compiled, C<do> returns the value of the last expression
1368evaluated.
1369
a0d0e21e 1370Note that inclusion of library modules is better done with the
19799a22 1371C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1372and raise an exception if there's a problem.
a0d0e21e 1373
5a964f20
TC
1374You might like to use C<do> to read in a program configuration
1375file. Manual error checking can be done this way:
1376
b76cc8ba 1377 # read in config files: system first, then user
f86cebdf 1378 for $file ("/share/prog/defaults.rc",
b76cc8ba 1379 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1380 {
5a964f20 1381 unless ($return = do $file) {
f86cebdf
GS
1382 warn "couldn't parse $file: $@" if $@;
1383 warn "couldn't do $file: $!" unless defined $return;
1384 warn "couldn't run $file" unless $return;
5a964f20
TC
1385 }
1386 }
1387
a0d0e21e 1388=item dump LABEL
d74e8afc 1389X<dump> X<core> X<undump>
a0d0e21e 1390
1614b0e3
JD
1391=item dump
1392
19799a22
GS
1393This function causes an immediate core dump. See also the B<-u>
1394command-line switch in L<perlrun>, which does the same thing.
1395Primarily this is so that you can use the B<undump> program (not
1396supplied) to turn your core dump into an executable binary after
1397having initialized all your variables at the beginning of the
1398program. When the new binary is executed it will begin by executing
1399a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1400Think of it as a goto with an intervening core dump and reincarnation.
1401If C<LABEL> is omitted, restarts the program from the top.
1402
1403B<WARNING>: Any files opened at the time of the dump will I<not>
1404be open any more when the program is reincarnated, with possible
b76cc8ba 1405resulting confusion on the part of Perl.
19799a22
GS
1406
1407This function is now largely obsolete, partly because it's very
1408hard to convert a core file into an executable, and because the
1409real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1410C code have superseded it. That's why you should now invoke it as
1411C<CORE::dump()>, if you don't want to be warned against a possible
1412typo.
19799a22
GS
1413
1414If you're looking to use L<dump> to speed up your program, consider
1415generating bytecode or native C code as described in L<perlcc>. If
1416you're just trying to accelerate a CGI script, consider using the
210b36aa 1417C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1418You might also consider autoloading or selfloading, which at least
b76cc8ba 1419make your program I<appear> to run faster.
5a964f20 1420
aa689395 1421=item each HASH
d74e8afc 1422X<each> X<hash, iterator>
aa689395 1423
5a964f20 1424When called in list context, returns a 2-element list consisting of the
aa689395 1425key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1426it. When called in scalar context, returns only the key for the next
e902a979 1427element in the hash.
2f9daede 1428
ab192400 1429Entries are returned in an apparently random order. The actual random
504f80c1
JH
1430order is subject to change in future versions of perl, but it is
1431guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1432function would produce on the same (unmodified) hash. Since Perl
14335.8.1 the ordering is different even between different runs of Perl
1434for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1435
1436When the hash is entirely read, a null array is returned in list context
19799a22
GS
1437(which when assigned produces a false (C<0>) value), and C<undef> in
1438scalar context. The next call to C<each> after that will start iterating
1439again. There is a single iterator for each hash, shared by all C<each>,
1440C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TP
1441reading all the elements from the hash, or by evaluating C<keys HASH> or
1442C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1443iterating over it, you may get entries skipped or duplicated, so
1444don't. Exception: It is always safe to delete the item most recently
1445returned by C<each()>, which means that the following code will work:
1446
1447 while (($key, $value) = each %hash) {
1448 print $key, "\n";
1449 delete $hash{$key}; # This is safe
1450 }
aa689395 1451
f86cebdf 1452The following prints out your environment like the printenv(1) program,
aa689395 1453only in a different order:
a0d0e21e
LW
1454
1455 while (($key,$value) = each %ENV) {
1456 print "$key=$value\n";
1457 }
1458
19799a22 1459See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1460
1461=item eof FILEHANDLE
d74e8afc
ITB
1462X<eof>
1463X<end of file>
1464X<end-of-file>
a0d0e21e 1465
4633a7c4
LW
1466=item eof ()
1467
a0d0e21e
LW
1468=item eof
1469
1470Returns 1 if the next read on FILEHANDLE will return end of file, or if
1471FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1472gives the real filehandle. (Note that this function actually
19799a22 1473reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1474interactive context.) Do not read from a terminal file (or call
19799a22 1475C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1476as terminals may lose the end-of-file condition if you do.
1477
820475bd
GS
1478An C<eof> without an argument uses the last file read. Using C<eof()>
1479with empty parentheses is very different. It refers to the pseudo file
1480formed from the files listed on the command line and accessed via the
61eff3bc
JH
1481C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1482as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1483used will cause C<@ARGV> to be examined to determine if input is
67408cae 1484available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1485end-of-file will assume you are processing another C<@ARGV> list,
1486and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1487see L<perlop/"I/O Operators">.
820475bd 1488
61eff3bc 1489In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1490detect the end of each file, C<eof()> will only detect the end of the
1491last file. Examples:
a0d0e21e 1492
748a9306
LW
1493 # reset line numbering on each input file
1494 while (<>) {
b76cc8ba 1495 next if /^\s*#/; # skip comments
748a9306 1496 print "$.\t$_";
5a964f20
TC
1497 } continue {
1498 close ARGV if eof; # Not eof()!
748a9306
LW
1499 }
1500
a0d0e21e
LW
1501 # insert dashes just before last line of last file
1502 while (<>) {
6ac88b13 1503 if (eof()) { # check for end of last file
a0d0e21e
LW
1504 print "--------------\n";
1505 }
1506 print;
6ac88b13 1507 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1508 }
1509
a0d0e21e 1510Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1511input operators typically return C<undef> when they run out of data, or if
1512there was an error.
a0d0e21e
LW
1513
1514=item eval EXPR
d74e8afc 1515X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1516X<error, handling> X<exception, handling>
a0d0e21e
LW
1517
1518=item eval BLOCK
1519
ce2984c3
PF
1520=item eval
1521
c7cc6f1c
GS
1522In the first form, the return value of EXPR is parsed and executed as if it
1523were a little Perl program. The value of the expression (which is itself
5a964f20 1524determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1525errors, executed in the lexical context of the current Perl program, so
1526that any variable settings or subroutine and format definitions remain
cf264981 1527afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1528If EXPR is omitted, evaluates C<$_>. This form is typically used to
1529delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1530
1531In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1532same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1533within the context of the current Perl program. This form is typically
1534used to trap exceptions more efficiently than the first (see below), while
1535also providing the benefit of checking the code within BLOCK at compile
1536time.
1537
1538The final semicolon, if any, may be omitted from the value of EXPR or within
1539the BLOCK.
1540
1541In both forms, the value returned is the value of the last expression
5a964f20 1542evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1543as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1544in void, scalar, or list context, depending on the context of the C<eval>
1545itself. See L</wantarray> for more on how the evaluation context can be
1546determined.
a0d0e21e 1547
19799a22
GS
1548If there is a syntax error or runtime error, or a C<die> statement is
1549executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1550error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1551string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1552warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1553To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1554turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1555See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1556
19799a22
GS
1557Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1558determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1559is implemented. It is also Perl's exception trapping mechanism, where
1560the die operator is used to raise exceptions.
1561
1562If the code to be executed doesn't vary, you may use the eval-BLOCK
1563form to trap run-time errors without incurring the penalty of
1564recompiling each time. The error, if any, is still returned in C<$@>.
1565Examples:
1566
54310121 1567 # make divide-by-zero nonfatal
a0d0e21e
LW
1568 eval { $answer = $a / $b; }; warn $@ if $@;
1569
1570 # same thing, but less efficient
1571 eval '$answer = $a / $b'; warn $@ if $@;
1572
1573 # a compile-time error
5a964f20 1574 eval { $answer = }; # WRONG
a0d0e21e
LW
1575
1576 # a run-time error
1577 eval '$answer ='; # sets $@
1578
cf264981
SP
1579Using the C<eval{}> form as an exception trap in libraries does have some
1580issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1581may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7
TC
1582You can use the C<local $SIG{__DIE__}> construct for this purpose,
1583as shown in this example:
774d564b 1584
1585 # a very private exception trap for divide-by-zero
f86cebdf
GS
1586 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1587 warn $@ if $@;
774d564b 1588
1589This is especially significant, given that C<__DIE__> hooks can call
19799a22 1590C<die> again, which has the effect of changing their error messages:
774d564b 1591
1592 # __DIE__ hooks may modify error messages
1593 {
f86cebdf
GS
1594 local $SIG{'__DIE__'} =
1595 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1596 eval { die "foo lives here" };
1597 print $@ if $@; # prints "bar lives here"
774d564b 1598 }
1599
19799a22 1600Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1601may be fixed in a future release.
1602
19799a22 1603With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1604being looked at when:
1605
1606 eval $x; # CASE 1
1607 eval "$x"; # CASE 2
1608
1609 eval '$x'; # CASE 3
1610 eval { $x }; # CASE 4
1611
5a964f20 1612 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1613 $$x++; # CASE 6
1614
2f9daede 1615Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1616the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1617the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1618and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1619does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TP
1620purely visual reasons, but it also has the advantage of compiling at
1621compile-time instead of at run-time.) Case 5 is a place where
19799a22 1622normally you I<would> like to use double quotes, except that in this
2f9daede
TP
1623particular situation, you can just use symbolic references instead, as
1624in case 6.
a0d0e21e 1625
4968c1e4 1626C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1627C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1628
d819b83a
DM
1629Note that as a very special case, an C<eval ''> executed within the C<DB>
1630package doesn't see the usual surrounding lexical scope, but rather the
1631scope of the first non-DB piece of code that called it. You don't normally
1632need to worry about this unless you are writing a Perl debugger.
1633
a0d0e21e 1634=item exec LIST
d74e8afc 1635X<exec> X<execute>
a0d0e21e 1636
8bf3b016
GS
1637=item exec PROGRAM LIST
1638
19799a22
GS
1639The C<exec> function executes a system command I<and never returns>--
1640use C<system> instead of C<exec> if you want it to return. It fails and
1641returns false only if the command does not exist I<and> it is executed
fb73857a 1642directly instead of via your system's command shell (see below).
a0d0e21e 1643
19799a22
GS
1644Since it's a common mistake to use C<exec> instead of C<system>, Perl
1645warns you if there is a following statement which isn't C<die>, C<warn>,
1646or C<exit> (if C<-w> is set - but you always do that). If you
1647I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1648can use one of these styles to avoid the warning:
1649
5a964f20
TC
1650 exec ('foo') or print STDERR "couldn't exec foo: $!";
1651 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1652
5a964f20 1653If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1654with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1655If there is only one scalar argument or an array with one element in it,
1656the argument is checked for shell metacharacters, and if there are any,
1657the entire argument is passed to the system's command shell for parsing
1658(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1659If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1660words and passed directly to C<execvp>, which is more efficient.
19799a22 1661Examples:
a0d0e21e 1662
19799a22
GS
1663 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1664 exec "sort $outfile | uniq";
a0d0e21e
LW
1665
1666If you don't really want to execute the first argument, but want to lie
1667to the program you are executing about its own name, you can specify
1668the program you actually want to run as an "indirect object" (without a
1669comma) in front of the LIST. (This always forces interpretation of the
54310121 1670LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1671the list.) Example:
1672
1673 $shell = '/bin/csh';
1674 exec $shell '-sh'; # pretend it's a login shell
1675
1676or, more directly,
1677
1678 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1679
bb32b41a
GS
1680When the arguments get executed via the system shell, results will
1681be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1682for details.
1683
19799a22
GS
1684Using an indirect object with C<exec> or C<system> is also more
1685secure. This usage (which also works fine with system()) forces
1686interpretation of the arguments as a multivalued list, even if the
1687list had just one argument. That way you're safe from the shell
1688expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1689
1690 @args = ( "echo surprise" );
1691
2b5ab1e7 1692 exec @args; # subject to shell escapes
f86cebdf 1693 # if @args == 1
2b5ab1e7 1694 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1695
1696The first version, the one without the indirect object, ran the I<echo>
1697program, passing it C<"surprise"> an argument. The second version
1698didn't--it tried to run a program literally called I<"echo surprise">,
1699didn't find it, and set C<$?> to a non-zero value indicating failure.
1700
0f897271
GS
1701Beginning with v5.6.0, Perl will attempt to flush all files opened for
1702output before the exec, but this may not be supported on some platforms
1703(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1704in English) or call the C<autoflush()> method of C<IO::Handle> on any
1705open handles in order to avoid lost output.
1706
19799a22 1707Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1708any C<DESTROY> methods in your objects.
1709
a0d0e21e 1710=item exists EXPR
d74e8afc 1711X<exists> X<autovivification>
a0d0e21e 1712
01020589 1713Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1714returns true if the specified element in the hash or array has ever
1715been initialized, even if the corresponding value is undefined. The
1716element is not autovivified if it doesn't exist.
a0d0e21e 1717
01020589
GS
1718 print "Exists\n" if exists $hash{$key};
1719 print "Defined\n" if defined $hash{$key};
1720 print "True\n" if $hash{$key};
1721
1722 print "Exists\n" if exists $array[$index];
1723 print "Defined\n" if defined $array[$index];
1724 print "True\n" if $array[$index];
a0d0e21e 1725
8ea97a1e 1726A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1727it exists, but the reverse doesn't necessarily hold true.
1728
afebc493
GS
1729Given an expression that specifies the name of a subroutine,
1730returns true if the specified subroutine has ever been declared, even
1731if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1732does not count as declaring it. Note that a subroutine which does not
1733exist may still be callable: its package may have an C<AUTOLOAD>
1734method that makes it spring into existence the first time that it is
1735called -- see L<perlsub>.
afebc493
GS
1736
1737 print "Exists\n" if exists &subroutine;
1738 print "Defined\n" if defined &subroutine;
1739
a0d0e21e 1740Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1741operation is a hash or array key lookup or subroutine name:
a0d0e21e 1742
2b5ab1e7
TC
1743 if (exists $ref->{A}->{B}->{$key}) { }
1744 if (exists $hash{A}{B}{$key}) { }
1745
01020589
GS
1746 if (exists $ref->{A}->{B}->[$ix]) { }
1747 if (exists $hash{A}{B}[$ix]) { }
1748
afebc493
GS
1749 if (exists &{$ref->{A}{B}{$key}}) { }
1750
01020589
GS
1751Although the deepest nested array or hash will not spring into existence
1752just because its existence was tested, any intervening ones will.
61eff3bc 1753Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1754into existence due to the existence test for the $key element above.
1755This happens anywhere the arrow operator is used, including even:
5a964f20 1756
2b5ab1e7
TC
1757 undef $ref;
1758 if (exists $ref->{"Some key"}) { }
1759 print $ref; # prints HASH(0x80d3d5c)
1760
1761This surprising autovivification in what does not at first--or even
1762second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1763release.
a0d0e21e 1764
afebc493
GS
1765Use of a subroutine call, rather than a subroutine name, as an argument
1766to exists() is an error.
1767
1768 exists &sub; # OK
1769 exists &sub(); # Error
1770
a0d0e21e 1771=item exit EXPR
d74e8afc 1772X<exit> X<terminate> X<abort>
a0d0e21e 1773
ce2984c3
PF
1774=item exit
1775
2b5ab1e7 1776Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1777
1778 $ans = <STDIN>;
1779 exit 0 if $ans =~ /^[Xx]/;
1780
19799a22 1781See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1782universally recognized values for EXPR are C<0> for success and C<1>
1783for error; other values are subject to interpretation depending on the
1784environment in which the Perl program is running. For example, exiting
178569 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1786the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1787
19799a22
GS
1788Don't use C<exit> to abort a subroutine if there's any chance that
1789someone might want to trap whatever error happened. Use C<die> instead,
1790which can be trapped by an C<eval>.
28757baa 1791
19799a22 1792The exit() function does not always exit immediately. It calls any
2b5ab1e7 1793defined C<END> routines first, but these C<END> routines may not
19799a22 1794themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1795be called are called before the real exit. If this is a problem, you
1796can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1797See L<perlmod> for details.
5a964f20 1798
a0d0e21e 1799=item exp EXPR
d74e8afc 1800X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1801
54310121 1802=item exp
bbce6d69 1803
b76cc8ba 1804Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1805If EXPR is omitted, gives C<exp($_)>.
1806
1807=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1808X<fcntl>
a0d0e21e 1809
f86cebdf 1810Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1811
1812 use Fcntl;
1813
0ade1984 1814first to get the correct constant definitions. Argument processing and
b76cc8ba 1815value return works just like C<ioctl> below.
a0d0e21e
LW
1816For example:
1817
1818 use Fcntl;
5a964f20
TC
1819 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1820 or die "can't fcntl F_GETFL: $!";
1821
554ad1fc 1822You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1823Like C<ioctl>, it maps a C<0> return from the system call into
1824C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1825in numeric context. It is also exempt from the normal B<-w> warnings
1826on improper numeric conversions.
5a964f20 1827
19799a22 1828Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1829doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1830manpage to learn what functions are available on your system.
a0d0e21e 1831
be2f7487
TH
1832Here's an example of setting a filehandle named C<REMOTE> to be
1833non-blocking at the system level. You'll have to negotiate C<$|>
1834on your own, though.
1835
1836 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1837
1838 $flags = fcntl(REMOTE, F_GETFL, 0)
1839 or die "Can't get flags for the socket: $!\n";
1840
1841 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1842 or die "Can't set flags for the socket: $!\n";
1843
a0d0e21e 1844=item fileno FILEHANDLE
d74e8afc 1845X<fileno>
a0d0e21e 1846
2b5ab1e7
TC
1847Returns the file descriptor for a filehandle, or undefined if the
1848filehandle is not open. This is mainly useful for constructing
19799a22 1849bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1850If FILEHANDLE is an expression, the value is taken as an indirect
1851filehandle, generally its name.
5a964f20 1852
b76cc8ba 1853You can use this to find out whether two handles refer to the
5a964f20
TC
1854same underlying descriptor:
1855
1856 if (fileno(THIS) == fileno(THAT)) {
1857 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1858 }
1859
1860(Filehandles connected to memory objects via new features of C<open> may
1861return undefined even though they are open.)
1862
a0d0e21e
LW
1863
1864=item flock FILEHANDLE,OPERATION
d74e8afc 1865X<flock> X<lock> X<locking>
a0d0e21e 1866
19799a22
GS
1867Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1868for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1869machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1870C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1871only entire files, not records.
1872
1873Two potentially non-obvious but traditional C<flock> semantics are
1874that it waits indefinitely until the lock is granted, and that its locks
1875B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1876fewer guarantees. This means that programs that do not also use C<flock>
1877may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1878your port's specific documentation, or your system-specific local manpages
1879for details. It's best to assume traditional behavior if you're writing
1880portable programs. (But if you're not, you should as always feel perfectly
1881free to write for your own system's idiosyncrasies (sometimes called
1882"features"). Slavish adherence to portability concerns shouldn't get
1883in the way of your getting your job done.)
a3cb178b 1884
8ebc5c01 1885OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1886LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1887you can use the symbolic names if you import them from the Fcntl module,
68dc0745 1888either individually, or as a group using the ':flock' tag. LOCK_SH
1889requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1890releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1891LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745 1892waiting for the lock (check the return status to see if you got it).
1893
2b5ab1e7
TC
1894To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1895before locking or unlocking it.
8ebc5c01 1896
f86cebdf 1897Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1898locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1899are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1900implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01 1901differing semantics shouldn't bite too many people.
1902
becacb53
TM
1903Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1904be open with read intent to use LOCK_SH and requires that it be open
1905with write intent to use LOCK_EX.
1906
19799a22
GS
1907Note also that some versions of C<flock> cannot lock things over the
1908network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1909that. If you like you can force Perl to ignore your system's flock(2)
1910function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 1911the switch C<-Ud_flock> to the F<Configure> program when you configure
1912perl.
4633a7c4
LW
1913
1914Here's a mailbox appender for BSD systems.
a0d0e21e 1915
7e1af8bc 1916 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1917
1918 sub lock {
7e1af8bc 1919 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1920 # and, in case someone appended
1921 # while we were waiting...
1922 seek(MBOX, 0, 2);
1923 }
1924
1925 sub unlock {
7e1af8bc 1926 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1927 }
1928
1929 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1930 or die "Can't open mailbox: $!";
1931
1932 lock();
1933 print MBOX $msg,"\n\n";
1934 unlock();
1935
2b5ab1e7
TC
1936On systems that support a real flock(), locks are inherited across fork()
1937calls, whereas those that must resort to the more capricious fcntl()
1938function lose the locks, making it harder to write servers.
1939
cb1a09d0 1940See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1941
1942=item fork
d74e8afc 1943X<fork> X<child> X<parent>
a0d0e21e 1944
2b5ab1e7
TC
1945Does a fork(2) system call to create a new process running the
1946same program at the same point. It returns the child pid to the
1947parent process, C<0> to the child process, or C<undef> if the fork is
1948unsuccessful. File descriptors (and sometimes locks on those descriptors)
1949are shared, while everything else is copied. On most systems supporting
1950fork(), great care has gone into making it extremely efficient (for
1951example, using copy-on-write technology on data pages), making it the
1952dominant paradigm for multitasking over the last few decades.
5a964f20 1953
0f897271
GS
1954Beginning with v5.6.0, Perl will attempt to flush all files opened for
1955output before forking the child process, but this may not be supported
1956on some platforms (see L<perlport>). To be safe, you may need to set
1957C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1958C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1959
19799a22 1960If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1961accumulate zombies. On some systems, you can avoid this by setting
1962C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1963forking and reaping moribund children.
cb1a09d0 1964
28757baa 1965Note that if your forked child inherits system file descriptors like
1966STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1967if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1968backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1969You should reopen those to F</dev/null> if it's any issue.
28757baa 1970
cb1a09d0 1971=item format
d74e8afc 1972X<format>
cb1a09d0 1973
19799a22 1974Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1975example:
1976
54310121 1977 format Something =
cb1a09d0
AD
1978 Test: @<<<<<<<< @||||| @>>>>>
1979 $str, $%, '$' . int($num)
1980 .
1981
1982 $str = "widget";
184e9718 1983 $num = $cost/$quantity;
cb1a09d0
AD
1984 $~ = 'Something';
1985 write;
1986
1987See L<perlform> for many details and examples.
1988
8903cb82 1989=item formline PICTURE,LIST
d74e8afc 1990X<formline>
a0d0e21e 1991
5a964f20 1992This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1993too. It formats (see L<perlform>) a list of values according to the
1994contents of PICTURE, placing the output into the format output
7660c0ab 1995accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1996Eventually, when a C<write> is done, the contents of
cf264981
SP
1997C<$^A> are written to some filehandle. You could also read C<$^A>
1998and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1999does one C<formline> per line of form, but the C<formline> function itself
748a9306 2000doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 2001that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
2002You may therefore need to use multiple formlines to implement a single
2003record format, just like the format compiler.
2004
19799a22 2005Be careful if you put double quotes around the picture, because an C<@>
748a9306 2006character may be taken to mean the beginning of an array name.
19799a22 2007C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
2008
2009=item getc FILEHANDLE
f723aae1 2010X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2011
2012=item getc
2013
2014Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
2015or the undefined value at end of file, or if there was an error (in
2016the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2017STDIN. This is not particularly efficient. However, it cannot be
2018used by itself to fetch single characters without waiting for the user
2019to hit enter. For that, try something more like:
4633a7c4
LW
2020
2021 if ($BSD_STYLE) {
2022 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2023 }
2024 else {
54310121 2025 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2026 }
2027
2028 $key = getc(STDIN);
2029
2030 if ($BSD_STYLE) {
2031 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2032 }
2033 else {
5f05dabc 2034 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
2035 }
2036 print "\n";
2037
54310121 2038Determination of whether $BSD_STYLE should be set
2039is left as an exercise to the reader.
cb1a09d0 2040
19799a22 2041The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2042systems purporting POSIX compliance. See also the C<Term::ReadKey>
2043module from your nearest CPAN site; details on CPAN can be found on
2044L<perlmodlib/CPAN>.
a0d0e21e
LW
2045
2046=item getlogin
d74e8afc 2047X<getlogin> X<login>
a0d0e21e 2048
cf264981 2049This implements the C library function of the same name, which on most
5a964f20 2050systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 2051use C<getpwuid>.
a0d0e21e 2052
f86702cc 2053 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2054
19799a22
GS
2055Do not consider C<getlogin> for authentication: it is not as
2056secure as C<getpwuid>.
4633a7c4 2057
a0d0e21e 2058=item getpeername SOCKET
d74e8afc 2059X<getpeername> X<peer>
a0d0e21e
LW
2060
2061Returns the packed sockaddr address of other end of the SOCKET connection.
2062
4633a7c4
LW
2063 use Socket;
2064 $hersockaddr = getpeername(SOCK);
19799a22 2065 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2066 $herhostname = gethostbyaddr($iaddr, AF_INET);
2067 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2068
2069=item getpgrp PID
d74e8afc 2070X<getpgrp> X<group>
a0d0e21e 2071
47e29363 2072Returns the current process group for the specified PID. Use
7660c0ab 2073a PID of C<0> to get the current process group for the
4633a7c4 2074current process. Will raise an exception if used on a machine that
f86cebdf 2075doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2076group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2077does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2078
2079=item getppid
d74e8afc 2080X<getppid> X<parent> X<pid>
a0d0e21e
LW
2081
2082Returns the process id of the parent process.
2083
4d76a344
RGS
2084Note for Linux users: on Linux, the C functions C<getpid()> and
2085C<getppid()> return different values from different threads. In order to
2086be portable, this behavior is not reflected by the perl-level function
2087C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2088to call the underlying C<getppid()>, you may use the CPAN module
2089C<Linux::Pid>.
4d76a344 2090
a0d0e21e 2091=item getpriority WHICH,WHO
d74e8afc 2092X<getpriority> X<priority> X<nice>
a0d0e21e 2093
4633a7c4
LW
2094Returns the current priority for a process, a process group, or a user.
2095(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2096machine that doesn't implement getpriority(2).
a0d0e21e
LW
2097
2098=item getpwnam NAME
d74e8afc
ITB
2099X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2100X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2101X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2102X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2103X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2104X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2105
2106=item getgrnam NAME
2107
2108=item gethostbyname NAME
2109
2110=item getnetbyname NAME
2111
2112=item getprotobyname NAME
2113
2114=item getpwuid UID
2115
2116=item getgrgid GID
2117
2118=item getservbyname NAME,PROTO
2119
2120=item gethostbyaddr ADDR,ADDRTYPE
2121
2122=item getnetbyaddr ADDR,ADDRTYPE
2123
2124=item getprotobynumber NUMBER
2125
2126=item getservbyport PORT,PROTO
2127
2128=item getpwent
2129
2130=item getgrent
2131
2132=item gethostent
2133
2134=item getnetent
2135
2136=item getprotoent
2137
2138=item getservent
2139
2140=item setpwent
2141
2142=item setgrent
2143
2144=item sethostent STAYOPEN
2145
2146=item setnetent STAYOPEN
2147
2148=item setprotoent STAYOPEN
2149
2150=item setservent STAYOPEN
2151
2152=item endpwent
2153
2154=item endgrent
2155
2156=item endhostent
2157
2158=item endnetent
2159
2160=item endprotoent
2161
2162=item endservent
2163
2164These routines perform the same functions as their counterparts in the
5a964f20 2165system library. In list context, the return values from the
a0d0e21e
LW
2166various get routines are as follows:
2167
2168 ($name,$passwd,$uid,$gid,
6ee623d5 2169 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2170 ($name,$passwd,$gid,$members) = getgr*
2171 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2172 ($name,$aliases,$addrtype,$net) = getnet*
2173 ($name,$aliases,$proto) = getproto*
2174 ($name,$aliases,$port,$proto) = getserv*
2175
2176(If the entry doesn't exist you get a null list.)
2177
4602f195
JH
2178The exact meaning of the $gcos field varies but it usually contains
2179the real name of the user (as opposed to the login name) and other
2180information pertaining to the user. Beware, however, that in many
2181system users are able to change this information and therefore it
106325ad 2182cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2183L<perlsec>). The $passwd and $shell, user's encrypted password and
2184login shell, are also tainted, because of the same reason.
4602f195 2185
5a964f20 2186In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2187lookup by name, in which case you get the other thing, whatever it is.
2188(If the entry doesn't exist you get the undefined value.) For example:
2189
5a964f20
TC
2190 $uid = getpwnam($name);
2191 $name = getpwuid($num);
2192 $name = getpwent();
2193 $gid = getgrnam($name);
08a33e13 2194 $name = getgrgid($num);
5a964f20
TC
2195 $name = getgrent();
2196 #etc.
a0d0e21e 2197
4602f195
JH
2198In I<getpw*()> the fields $quota, $comment, and $expire are special
2199cases in the sense that in many systems they are unsupported. If the
2200$quota is unsupported, it is an empty scalar. If it is supported, it
2201usually encodes the disk quota. If the $comment field is unsupported,
2202it is an empty scalar. If it is supported it usually encodes some
2203administrative comment about the user. In some systems the $quota
2204field may be $change or $age, fields that have to do with password
2205aging. In some systems the $comment field may be $class. The $expire
2206field, if present, encodes the expiration period of the account or the
2207password. For the availability and the exact meaning of these fields
2208in your system, please consult your getpwnam(3) documentation and your
2209F<pwd.h> file. You can also find out from within Perl what your
2210$quota and $comment fields mean and whether you have the $expire field
2211by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2212C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2213files are only supported if your vendor has implemented them in the
2214intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2215shadow versions if you're running under privilege or if there exists
cf264981
SP
2216the shadow(3) functions as found in System V (this includes Solaris
2217and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2218facility are unlikely to be supported.
6ee623d5 2219
19799a22 2220The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2221the login names of the members of the group.
2222
2223For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2224C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2225C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2226addresses returned by the corresponding system library call. In the
2227Internet domain, each address is four bytes long and you can unpack it
2228by saying something like:
2229
f337b084 2230 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2231
2b5ab1e7
TC
2232The Socket library makes this slightly easier:
2233
2234 use Socket;
2235 $iaddr = inet_aton("127.1"); # or whatever address
2236 $name = gethostbyaddr($iaddr, AF_INET);
2237
2238 # or going the other way
19799a22 2239 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2240
19799a22
GS
2241If you get tired of remembering which element of the return list
2242contains which return value, by-name interfaces are provided
2243in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2244C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2245and C<User::grent>. These override the normal built-ins, supplying
2246versions that return objects with the appropriate names
2247for each field. For example:
5a964f20
TC
2248
2249 use File::stat;
2250 use User::pwent;
2251 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2252
b76cc8ba
NIS
2253Even though it looks like they're the same method calls (uid),
2254they aren't, because a C<File::stat> object is different from
19799a22 2255a C<User::pwent> object.
5a964f20 2256
a0d0e21e 2257=item getsockname SOCKET
d74e8afc 2258X<getsockname>
a0d0e21e 2259
19799a22
GS
2260Returns the packed sockaddr address of this end of the SOCKET connection,
2261in case you don't know the address because you have several different
2262IPs that the connection might have come in on.
a0d0e21e 2263
4633a7c4
LW
2264 use Socket;
2265 $mysockaddr = getsockname(SOCK);
19799a22 2266 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2267 printf "Connect to %s [%s]\n",
19799a22
GS
2268 scalar gethostbyaddr($myaddr, AF_INET),
2269 inet_ntoa($myaddr);
a0d0e21e
LW
2270
2271=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2272X<getsockopt>
a0d0e21e 2273
636e6b1f
TH
2274Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2275Options may exist at multiple protocol levels depending on the socket
2276type, but at least the uppermost socket level SOL_SOCKET (defined in the
2277C<Socket> module) will exist. To query options at another level the
2278protocol number of the appropriate protocol controlling the option
2279should be supplied. For example, to indicate that an option is to be
2280interpreted by the TCP protocol, LEVEL should be set to the protocol
2281number of TCP, which you can get using getprotobyname.
2282
2283The call returns a packed string representing the requested socket option,
2284or C<undef> if there is an error (the error reason will be in $!). What
2285exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2286your system documentation for details. A very common case however is that
cf264981 2287the option is an integer, in which case the result will be a packed
636e6b1f
TH
2288integer which you can decode using unpack with the C<i> (or C<I>) format.
2289
2290An example testing if Nagle's algorithm is turned on on a socket:
2291
4852725b 2292 use Socket qw(:all);
636e6b1f
TH
2293
2294 defined(my $tcp = getprotobyname("tcp"))
2295 or die "Could not determine the protocol number for tcp";
4852725b
DD
2296 # my $tcp = IPPROTO_TCP; # Alternative
2297 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2298 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2299 my $nodelay = unpack("I", $packed);
2300 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2301
a0d0e21e
LW
2302
2303=item glob EXPR
d74e8afc 2304X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2305
0a753a76 2306=item glob
2307
d9a9d457
JL
2308In list context, returns a (possibly empty) list of filename expansions on
2309the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2310scalar context, glob iterates through such filename expansions, returning
2311undef when the list is exhausted. This is the internal function
2312implementing the C<< <*.c> >> operator, but you can use it directly. If
2313EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2314more detail in L<perlop/"I/O Operators">.
a0d0e21e 2315
3a4b19e4
GS
2316Beginning with v5.6.0, this operator is implemented using the standard
2317C<File::Glob> extension. See L<File::Glob> for details.
2318
a0d0e21e 2319=item gmtime EXPR
d74e8afc 2320X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2321
ce2984c3
PF
2322=item gmtime
2323
435fbc73
GS
2324Works just like L<localtime> but the returned values are
2325localized for the standard Greenwich time zone.
a0d0e21e 2326
435fbc73
GS
2327Note: when called in list context, $isdst, the last value
2328returned by gmtime is always C<0>. There is no
2329Daylight Saving Time in GMT.
0a753a76 2330
62aa5637
MS
2331See L<perlport/gmtime> for portability concerns.
2332
a0d0e21e 2333=item goto LABEL
d74e8afc 2334X<goto> X<jump> X<jmp>
a0d0e21e 2335
748a9306
LW
2336=item goto EXPR
2337
a0d0e21e
LW
2338=item goto &NAME
2339
7660c0ab 2340The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2341execution there. It may not be used to go into any construct that
7660c0ab 2342requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2343also can't be used to go into a construct that is optimized away,
19799a22 2344or to get out of a block or subroutine given to C<sort>.
0a753a76 2345It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2346including out of subroutines, but it's usually better to use some other
19799a22 2347construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2348need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2349(The difference being that C does not offer named loops combined with
2350loop control. Perl does, and this replaces most structured uses of C<goto>
2351in other languages.)
a0d0e21e 2352
7660c0ab
A
2353The C<goto-EXPR> form expects a label name, whose scope will be resolved
2354dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2355necessarily recommended if you're optimizing for maintainability:
2356
2357 goto ("FOO", "BAR", "GLARCH")[$i];
2358
1b6921cb
BT
2359The C<goto-&NAME> form is quite different from the other forms of
2360C<goto>. In fact, it isn't a goto in the normal sense at all, and
2361doesn't have the stigma associated with other gotos. Instead, it
2362exits the current subroutine (losing any changes set by local()) and
2363immediately calls in its place the named subroutine using the current
2364value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2365load another subroutine and then pretend that the other subroutine had
2366been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2367in the current subroutine are propagated to the other subroutine.)
2368After the C<goto>, not even C<caller> will be able to tell that this
2369routine was called first.
2370
2371NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2372containing a code reference, or a block that evaluates to a code
6cb9131c 2373reference.
a0d0e21e
LW
2374
2375=item grep BLOCK LIST
d74e8afc 2376X<grep>
a0d0e21e
LW
2377
2378=item grep EXPR,LIST
2379
2b5ab1e7
TC
2380This is similar in spirit to, but not the same as, grep(1) and its
2381relatives. In particular, it is not limited to using regular expressions.
2f9daede 2382
a0d0e21e 2383Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2384C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2385elements for which the expression evaluated to true. In scalar
2386context, returns the number of times the expression was true.
a0d0e21e
LW
2387
2388 @foo = grep(!/^#/, @bar); # weed out comments
2389
2390or equivalently,
2391
2392 @foo = grep {!/^#/} @bar; # weed out comments
2393
be3174d2
GS
2394Note that C<$_> is an alias to the list value, so it can be used to
2395modify the elements of the LIST. While this is useful and supported,
2396it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2397Similarly, grep returns aliases into the original list, much as a for
2398loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2399element of a list returned by grep (for example, in a C<foreach>, C<map>
2400or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2401This is usually something to be avoided when writing clear code.
a0d0e21e 2402
a4fb8298 2403If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2404been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2405the list elements, C<$_> keeps being lexical inside the block; i.e. it
2406can't be seen from the outside, avoiding any potential side-effects.
2407
19799a22 2408See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2409
a0d0e21e 2410=item hex EXPR
d74e8afc 2411X<hex> X<hexadecimal>
a0d0e21e 2412
54310121 2413=item hex
bbce6d69 2414
2b5ab1e7 2415Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2416(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2417L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TP
2418
2419 print hex '0xAf'; # prints '175'
2420 print hex 'aF'; # same
a0d0e21e 2421
19799a22 2422Hex strings may only represent integers. Strings that would cause
53305cf1 2423integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2424unlike oct(). To present something as hex, look into L</printf>,
2425L</sprintf>, or L</unpack>.
19799a22 2426
ce2984c3 2427=item import LIST
d74e8afc 2428X<import>
a0d0e21e 2429
19799a22 2430There is no builtin C<import> function. It is just an ordinary
4633a7c4 2431method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2432names to another module. The C<use> function calls the C<import> method
cea6626f 2433for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2434
2435=item index STR,SUBSTR,POSITION
d74e8afc 2436X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2437
2438=item index STR,SUBSTR
2439
2b5ab1e7
TC
2440The index function searches for one string within another, but without
2441the wildcard-like behavior of a full regular-expression pattern match.
2442It returns the position of the first occurrence of SUBSTR in STR at
2443or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2444beginning of the string. POSITION before the beginning of the string
2445or after its end is treated as if it were the beginning or the end,
2446respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2447you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2448is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2449
2450=item int EXPR
f723aae1 2451X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2452
54310121 2453=item int
bbce6d69 2454
7660c0ab 2455Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2456You should not use this function for rounding: one because it truncates
2457towards C<0>, and two because machine representations of floating point
2458numbers can sometimes produce counterintuitive results. For example,
2459C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2460because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2461the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2462functions will serve you better than will int().
a0d0e21e
LW
2463
2464=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2465X<ioctl>
a0d0e21e 2466
2b5ab1e7 2467Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2468
6c567752 2469 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2470
a11c483f 2471to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2472exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2473own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2474(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2475may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2476written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2477will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2478has no string value but does have a numeric value, that value will be
2479passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2480true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2481functions may be needed to manipulate the values of structures used by
b76cc8ba 2482C<ioctl>.
a0d0e21e 2483
19799a22 2484The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2485
2486 if OS returns: then Perl returns:
2487 -1 undefined value
2488 0 string "0 but true"
2489 anything else that number
2490
19799a22 2491Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2492still easily determine the actual value returned by the operating
2493system:
2494
2b5ab1e7 2495 $retval = ioctl(...) || -1;
a0d0e21e
LW
2496 printf "System returned %d\n", $retval;
2497
be2f7487 2498The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2499about improper numeric conversions.
2500
a0d0e21e 2501=item join EXPR,LIST
d74e8afc 2502X<join>
a0d0e21e 2503
2b5ab1e7
TC
2504Joins the separate strings of LIST into a single string with fields
2505separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2506
2b5ab1e7 2507 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2508
eb6e2d6f
GS
2509Beware that unlike C<split>, C<join> doesn't take a pattern as its
2510first argument. Compare L</split>.
a0d0e21e 2511
aa689395 2512=item keys HASH
d74e8afc 2513X<keys> X<key>
aa689395 2514
504f80c1
JH
2515Returns a list consisting of all the keys of the named hash.
2516(In scalar context, returns the number of keys.)
2517
2518The keys are returned in an apparently random order. The actual
2519random order is subject to change in future versions of perl, but it
2520is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2521function produces (given that the hash has not been modified). Since
2522Perl 5.8.1 the ordering is different even between different runs of
2523Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2524Attacks">).
504f80c1 2525
cf264981
SP
2526As a side effect, calling keys() resets the HASH's internal iterator
2527(see L</each>). In particular, calling keys() in void context resets
2528the iterator with no other overhead.
a0d0e21e 2529
aa689395 2530Here is yet another way to print your environment:
a0d0e21e
LW
2531
2532 @keys = keys %ENV;
2533 @values = values %ENV;
b76cc8ba 2534 while (@keys) {
a0d0e21e
LW
2535 print pop(@keys), '=', pop(@values), "\n";
2536 }
2537
2538or how about sorted by key:
2539
2540 foreach $key (sort(keys %ENV)) {
2541 print $key, '=', $ENV{$key}, "\n";
2542 }
2543
8ea1e5d4
GS
2544The returned values are copies of the original keys in the hash, so
2545modifying them will not affect the original hash. Compare L</values>.
2546
19799a22 2547To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2548Here's a descending numeric sort of a hash by its values:
4633a7c4 2549
5a964f20 2550 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2551 printf "%4d %s\n", $hash{$key}, $key;
2552 }
2553
19799a22 2554As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395 2555allocated for the given hash. This can gain you a measure of efficiency if
2556you know the hash is going to get big. (This is similar to pre-extending
2557an array by assigning a larger number to $#array.) If you say
55497cff 2558
2559 keys %hash = 200;
2560
ab192400
GS
2561then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2562in fact, since it rounds up to the next power of two. These
55497cff 2563buckets will be retained even if you do C<%hash = ()>, use C<undef
2564%hash> if you want to free the storage while C<%hash> is still in scope.
2565You can't shrink the number of buckets allocated for the hash using
19799a22 2566C<keys> in this way (but you needn't worry about doing this by accident,
55497cff 2567as trying has no effect).
2568
19799a22 2569See also C<each>, C<values> and C<sort>.
ab192400 2570
b350dd2f 2571=item kill SIGNAL, LIST
d74e8afc 2572X<kill> X<signal>
a0d0e21e 2573
b350dd2f 2574Sends a signal to a list of processes. Returns the number of
517db077
GS
2575processes successfully signaled (which is not necessarily the
2576same as the number actually killed).
a0d0e21e
LW
2577
2578 $cnt = kill 1, $child1, $child2;
2579 kill 9, @goners;
2580
70fb64f6 2581If SIGNAL is zero, no signal is sent to the process, but the kill(2)
6cb9d3e4 2582system call will check whether it's possible to send a signal to it (that
70fb64f6
RGS
2583means, to be brief, that the process is owned by the same user, or we are
2584the super-user). This is a useful way to check that a child process is
81fd35db
DN
2585alive (even if only as a zombie) and hasn't changed its UID. See
2586L<perlport> for notes on the portability of this construct.
b350dd2f
GS
2587
2588Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2589process groups instead of processes. (On System V, a negative I<PROCESS>
2590number will also kill process groups, but that's not portable.) That
2591means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2592use a signal name in quotes.
2593
2594See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2595
2596=item last LABEL
d74e8afc 2597X<last> X<break>
a0d0e21e
LW
2598
2599=item last
2600
2601The C<last> command is like the C<break> statement in C (as used in
2602loops); it immediately exits the loop in question. If the LABEL is
2603omitted, the command refers to the innermost enclosing loop. The
2604C<continue> block, if any, is not executed:
2605
4633a7c4
LW
2606 LINE: while (<STDIN>) {
2607 last LINE if /^$/; # exit when done with header
5a964f20 2608 #...
a0d0e21e
LW
2609 }
2610
4968c1e4 2611C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2612C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2613a grep() or map() operation.
4968c1e4 2614
6c1372ed
GS
2615Note that a block by itself is semantically identical to a loop
2616that executes once. Thus C<last> can be used to effect an early
2617exit out of such a block.
2618
98293880
JH
2619See also L</continue> for an illustration of how C<last>, C<next>, and
2620C<redo> work.
1d2dff63 2621
a0d0e21e 2622=item lc EXPR
d74e8afc 2623X<lc> X<lowercase>
a0d0e21e 2624
54310121 2625=item lc
bbce6d69 2626
d1be9408 2627Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2628implementing the C<\L> escape in double-quoted strings. Respects
2629current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2630and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2631
7660c0ab 2632If EXPR is omitted, uses C<$_>.
bbce6d69 2633
a0d0e21e 2634=item lcfirst EXPR
d74e8afc 2635X<lcfirst> X<lowercase>
a0d0e21e 2636
54310121 2637=item lcfirst
bbce6d69 2638
ad0029c4
JH
2639Returns the value of EXPR with the first character lowercased. This
2640is the internal function implementing the C<\l> escape in
2641double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2642locale> in force. See L<perllocale> and L<perlunicode> for more
2643details about locale and Unicode support.
a0d0e21e 2644
7660c0ab 2645If EXPR is omitted, uses C<$_>.
bbce6d69 2646
a0d0e21e 2647=item length EXPR
d74e8afc 2648X<length> X<size>
a0d0e21e 2649
54310121 2650=item length
bbce6d69 2651
974da8e5 2652Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2653omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2654an entire array or hash to find out how many elements these have.
2655For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2656
974da8e5
JH
2657Note the I<characters>: if the EXPR is in Unicode, you will get the
2658number of characters, not the number of bytes. To get the length
2659in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2660
a0d0e21e 2661=item link OLDFILE,NEWFILE
d74e8afc 2662X<link>
a0d0e21e 2663
19799a22 2664Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2665success, false otherwise.
a0d0e21e
LW
2666
2667=item listen SOCKET,QUEUESIZE
d74e8afc 2668X<listen>
a0d0e21e 2669
19799a22 2670Does the same thing that the listen system call does. Returns true if
b76cc8ba 2671it succeeded, false otherwise. See the example in
cea6626f 2672L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2673
2674=item local EXPR
d74e8afc 2675X<local>
a0d0e21e 2676
19799a22 2677You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2678what most people think of as "local". See
13a2d996 2679L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2680
5a964f20
TC
2681A local modifies the listed variables to be local to the enclosing
2682block, file, or eval. If more than one value is listed, the list must
2683be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2684for details, including issues with tied arrays and hashes.
a0d0e21e 2685
a0d0e21e 2686=item localtime EXPR
435fbc73 2687X<localtime> X<ctime>
a0d0e21e 2688
ba053783
AL
2689=item localtime
2690
19799a22 2691Converts a time as returned by the time function to a 9-element list
5f05dabc 2692with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2693follows:
2694
54310121 2695 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2696 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2697 localtime(time);
a0d0e21e 2698
48a26b3a 2699All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2700tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2701of the specified time.
48a26b3a 2702
ba053783
AL
2703C<$mday> is the day of the month, and C<$mon> is the month itself, in
2704the range C<0..11> with 0 indicating January and 11 indicating December.
2705This makes it easy to get a month name from a list:
54310121 2706
ba053783
AL
2707 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2708 print "$abbr[$mon] $mday";
2709 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2710
ba053783
AL
2711C<$year> is the number of years since 1900, not just the last two digits
2712of the year. That is, C<$year> is C<123> in year 2023. The proper way
2713to get a complete 4-digit year is simply:
abd75f24 2714
ba053783 2715 $year += 1900;
abd75f24 2716
435fbc73
GS
2717Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2718to do that, would you?
2719
ba053783
AL
2720To get the last two digits of the year (e.g., '01' in 2001) do:
2721
2722 $year = sprintf("%02d", $year % 100);
2723
2724C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2725Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2726(or C<0..365> in leap years.)
2727
2728C<$isdst> is true if the specified time occurs during Daylight Saving
2729Time, false otherwise.
abd75f24 2730
48a26b3a 2731If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2732
48a26b3a 2733In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2734
5f05dabc 2735 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2736
fe86afc2
NC
2737This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2738instead of local time use the L</gmtime> builtin. See also the
2739C<Time::Local> module (to convert the second, minutes, hours, ... back to
2740the integer value returned by time()), and the L<POSIX> module's strftime(3)
2741and mktime(3) functions.
2742
2743To get somewhat similar but locale dependent date strings, set up your
2744locale environment variables appropriately (please see L<perllocale>) and
2745try for example:
a3cb178b 2746
5a964f20 2747 use POSIX qw(strftime);
2b5ab1e7 2748 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2749 # or for GMT formatted appropriately for your locale:
2750 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2751
2752Note that the C<%a> and C<%b>, the short forms of the day of the week
2753and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2754
62aa5637
MS
2755See L<perlport/localtime> for portability concerns.
2756
435fbc73
GS
2757The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2758by-name access mechanism to the gmtime() and localtime() functions,
2759respectively.
2760
2761For a comprehensive date and time representation look at the
2762L<DateTime> module on CPAN.
2763
07698885 2764=item lock THING
d74e8afc 2765X<lock>
19799a22 2766
01e6739c 2767This function places an advisory lock on a shared variable, or referenced
03730085 2768object contained in I<THING> until the lock goes out of scope.
a6d5524e 2769
f3a23afb 2770lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2771by this name (before any calls to it), that function will be called
03730085
AB
2772instead. (However, if you've said C<use threads>, lock() is always a
2773keyword.) See L<threads>.
19799a22 2774
a0d0e21e 2775=item log EXPR
d74e8afc 2776X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2777
54310121 2778=item log
bbce6d69 2779
2b5ab1e7
TC
2780Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2781returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2782The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2783divided by the natural log of N. For example:
2784
2785 sub log10 {
2786 my $n = shift;
2787 return log($n)/log(10);
b76cc8ba 2788 }
2b5ab1e7
TC
2789
2790See also L</exp> for the inverse operation.
a0d0e21e 2791
a0d0e21e 2792=item lstat EXPR
d74e8afc 2793X<lstat>
a0d0e21e 2794
54310121 2795=item lstat
bbce6d69 2796
19799a22 2797Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2798special C<_> filehandle) but stats a symbolic link instead of the file
2799the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2800your system, a normal C<stat> is done. For much more detailed
2801information, please see the documentation for C<stat>.
a0d0e21e 2802
7660c0ab 2803If EXPR is omitted, stats C<$_>.
bbce6d69 2804
a0d0e21e
LW
2805=item m//
2806
2807The match operator. See L<perlop>.
2808
2809=item map BLOCK LIST
d74e8afc 2810X<map>
a0d0e21e
LW
2811
2812=item map EXPR,LIST
2813
19799a22
GS
2814Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2815C<$_> to each element) and returns the list value composed of the
2816results of each such evaluation. In scalar context, returns the
2817total number of elements so generated. Evaluates BLOCK or EXPR in
2818list context, so each element of LIST may produce zero, one, or
2819more elements in the returned value.
dd99ebda 2820
a0d0e21e
LW
2821 @chars = map(chr, @nums);
2822
2823translates a list of numbers to the corresponding characters. And
2824
4633a7c4 2825 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2826
2827is just a funny way to write
2828
2829 %hash = ();
2830 foreach $_ (@array) {
4633a7c4 2831 $hash{getkey($_)} = $_;
a0d0e21e
LW
2832 }
2833
be3174d2
GS
2834Note that C<$_> is an alias to the list value, so it can be used to
2835modify the elements of the LIST. While this is useful and supported,
2836it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2837Using a regular C<foreach> loop for this purpose would be clearer in
2838most cases. See also L</grep> for an array composed of those items of
2839the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2840
a4fb8298 2841If C<$_> is lexical in the scope where the C<map> appears (because it has
cf264981 2842been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2843the list elements, C<$_> keeps being lexical inside the block; i.e. it
2844can't be seen from the outside, avoiding any potential side-effects.
2845
205fdb4d
NC
2846C<{> starts both hash references and blocks, so C<map { ...> could be either
2847the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2848ahead for the closing C<}> it has to take a guess at which its dealing with
2849based what it finds just after the C<{>. Usually it gets it right, but if it
2850doesn't it won't realize something is wrong until it gets to the C<}> and
2851encounters the missing (or unexpected) comma. The syntax error will be
2852reported close to the C<}> but you'll need to change something near the C<{>
2853such as using a unary C<+> to give perl some help:
2854
2855 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2856 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2857 %hash = map { ("\L$_", 1) } @array # this also works
2858 %hash = map { lc($_), 1 } @array # as does this.
2859 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2860
205fdb4d
NC
2861 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2862
2863or to force an anon hash constructor use C<+{>
2864
2865 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2866
2867and you get list of anonymous hashes each with only 1 entry.
2868
19799a22 2869=item mkdir FILENAME,MASK
d74e8afc 2870X<mkdir> X<md> X<directory, create>
a0d0e21e 2871
5a211162
GS
2872=item mkdir FILENAME
2873
491873e5
RGS
2874=item mkdir
2875
0591cd52 2876Creates the directory specified by FILENAME, with permissions
19799a22
GS
2877specified by MASK (as modified by C<umask>). If it succeeds it
2878returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2879If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2880to C<$_>.
0591cd52 2881
19799a22 2882In general, it is better to create directories with permissive MASK,
0591cd52 2883and let the user modify that with their C<umask>, than it is to supply
19799a22 2884a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2885The exceptions to this rule are when the file or directory should be
2886kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2887C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2888
cc1852e8
JH
2889Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2890number of trailing slashes. Some operating and filesystems do not get
2891this right, so Perl automatically removes all trailing slashes to keep
2892everyone happy.
2893
dd184578
RGS
2894In order to recursively create a directory structure look at
2895the C<mkpath> function of the L<File::Path> module.
2896
a0d0e21e 2897=item msgctl ID,CMD,ARG
d74e8afc 2898X<msgctl>
a0d0e21e 2899
f86cebdf 2900Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2901
2902 use IPC::SysV;
2903
7660c0ab 2904first to get the correct constant definitions. If CMD is C<IPC_STAT>,
cf264981 2905then ARG must be a variable that will hold the returned C<msqid_ds>
951ba7fe
GS
2906structure. Returns like C<ioctl>: the undefined value for error,
2907C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2908L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2909
2910=item msgget KEY,FLAGS
d74e8afc 2911X<msgget>
a0d0e21e 2912
f86cebdf 2913Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2914id, or the undefined value if there is an error. See also
2915L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2916
a0d0e21e 2917=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
d74e8afc 2918X<msgrcv>
a0d0e21e
LW
2919
2920Calls the System V IPC function msgrcv to receive a message from
2921message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2922SIZE. Note that when a message is received, the message type as a
2923native long integer will be the first thing in VAR, followed by the
2924actual message. This packing may be opened with C<unpack("l! a*")>.
2925Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2926an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2927C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2928
2929=item msgsnd ID,MSG,FLAGS
d74e8afc 2930X<msgsnd>
41d6edb2
JH
2931
2932Calls the System V IPC function msgsnd to send the message MSG to the
2933message queue ID. MSG must begin with the native long integer message
2934type, and be followed by the length of the actual message, and finally
2935the message itself. This kind of packing can be achieved with
2936C<pack("l! a*", $type, $message)>. Returns true if successful,
2937or false if there is an error. See also C<IPC::SysV>
2938and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2939
2940=item my EXPR
d74e8afc 2941X<my>
a0d0e21e 2942
307ea6df
JH
2943=item my TYPE EXPR
2944
1d2de774 2945=item my EXPR : ATTRS
09bef843 2946
1d2de774 2947=item my TYPE EXPR : ATTRS
307ea6df 2948
19799a22 2949A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2950enclosing block, file, or C<eval>. If more than one value is listed,
2951the list must be placed in parentheses.
307ea6df 2952
1d2de774
JH
2953The exact semantics and interface of TYPE and ATTRS are still
2954evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2955and attributes are handled using the C<attributes> pragma, or starting
2956from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2957L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2958L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2959
a0d0e21e 2960=item next LABEL
d74e8afc 2961X<next> X<continue>
a0d0e21e
LW
2962
2963=item next
2964
2965The C<next> command is like the C<continue> statement in C; it starts
2966the next iteration of the loop:
2967
4633a7c4
LW
2968 LINE: while (<STDIN>) {
2969 next LINE if /^#/; # discard comments
5a964f20 2970 #...
a0d0e21e
LW
2971 }
2972
2973Note that if there were a C<continue> block on the above, it would get
2974executed even on discarded lines. If the LABEL is omitted, the command
2975refers to the innermost enclosing loop.
2976
4968c1e4 2977C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2978C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2979a grep() or map() operation.
4968c1e4 2980
6c1372ed
GS
2981Note that a block by itself is semantically identical to a loop
2982that executes once. Thus C<next> will exit such a block early.
2983
98293880
JH
2984See also L</continue> for an illustration of how C<last>, C<next>, and
2985C<redo> work.
1d2dff63 2986
4a66ea5a 2987=item no Module VERSION LIST
d74e8afc 2988X<no>
4a66ea5a
RGS
2989
2990=item no Module VERSION
2991
a0d0e21e
LW
2992=item no Module LIST
2993
4a66ea5a
RGS
2994=item no Module
2995
593b9c14 2996See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
2997
2998=item oct EXPR
d74e8afc 2999X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
a0d0e21e 3000
54310121 3001=item oct
bbce6d69 3002
4633a7c4 3003Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
3004value. (If EXPR happens to start off with C<0x>, interprets it as a
3005hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
3006binary string. Leading whitespace is ignored in all three cases.)
3007The following will handle decimal, binary, octal, and hex in the standard
3008Perl or C notation:
a0d0e21e
LW
3009
3010 $val = oct($val) if $val =~ /^0/;
3011
19799a22
GS
3012If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3013in octal), use sprintf() or printf():
3014
3015 $perms = (stat("filename"))[2] & 07777;
3016 $oct_perms = sprintf "%lo", $perms;
3017
3018The oct() function is commonly used when a string such as C<644> needs
3019to be converted into a file mode, for example. (Although perl will
3020automatically convert strings into numbers as needed, this automatic
3021conversion assumes base 10.)
a0d0e21e
LW
3022
3023=item open FILEHANDLE,EXPR
d74e8afc 3024X<open> X<pipe> X<file, open> X<fopen>
a0d0e21e 3025
68bd7414
NIS
3026=item open FILEHANDLE,MODE,EXPR
3027
3028=item open FILEHANDLE,MODE,EXPR,LIST
3029
ba964c95
T
3030=item open FILEHANDLE,MODE,REFERENCE
3031
a0d0e21e
LW
3032=item open FILEHANDLE
3033
3034Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
3035FILEHANDLE.
3036
3037(The following is a comprehensive reference to open(): for a gentler
3038introduction you may consider L<perlopentut>.)
3039
a28cd5c9
NT
3040If FILEHANDLE is an undefined scalar variable (or array or hash element)
3041the variable is assigned a reference to a new anonymous filehandle,
3042otherwise if FILEHANDLE is an expression, its value is used as the name of
3043the real filehandle wanted. (This is considered a symbolic reference, so
3044C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
3045
3046If EXPR is omitted, the scalar variable of the same name as the
3047FILEHANDLE contains the filename. (Note that lexical variables--those
3048declared with C<my>--will not work for this purpose; so if you're
67408cae 3049using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
3050
3051If three or more arguments are specified then the mode of opening and
3052the file name are separate. If MODE is C<< '<' >> or nothing, the file
3053is opened for input. If MODE is C<< '>' >>, the file is truncated and
3054opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 3055the file is opened for appending, again being created if necessary.
5a964f20 3056
ed53a2bb
JH
3057You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3058indicate that you want both read and write access to the file; thus
3059C<< '+<' >> is almost always preferred for read/write updates--the C<<
3060'+>' >> mode would clobber the file first. You can't usually use
3061either read-write mode for updating textfiles, since they have
3062variable length records. See the B<-i> switch in L<perlrun> for a
3063better approach. The file is created with permissions of C<0666>
3064modified by the process' C<umask> value.
3065
3066These various prefixes correspond to the fopen(3) modes of C<'r'>,
3067C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 3068
6170680b
IZ
3069In the 2-arguments (and 1-argument) form of the call the mode and
3070filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
3071spaces. It is possible to omit the mode in these forms if the mode is
3072C<< '<' >>.
6170680b 3073
7660c0ab 3074If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 3075command to which output is to be piped, and if the filename ends with a
f244e06d
GS
3076C<'|'>, the filename is interpreted as a command which pipes output to
3077us. See L<perlipc/"Using open() for IPC">
19799a22 3078for more examples of this. (You are not allowed to C<open> to a command
5a964f20 3079that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
3080and L<perlipc/"Bidirectional Communication with Another Process">
3081for alternatives.)
cb1a09d0 3082
ed53a2bb
JH
3083For three or more arguments if MODE is C<'|-'>, the filename is
3084interpreted as a command to which output is to be piped, and if MODE
3085is C<'-|'>, the filename is interpreted as a command which pipes
3086output to us. In the 2-arguments (and 1-argument) form one should
3087replace dash (C<'-'>) with the command.
3088See L<perlipc/"Using open() for IPC"> for more examples of this.
3089(You are not allowed to C<open> to a command that pipes both in I<and>
3090out, but see L<IPC::Open2>, L<IPC::Open3>, and
3091L<perlipc/"Bidirectional Communication"> for alternatives.)
3092
3093In the three-or-more argument form of pipe opens, if LIST is specified
3094(extra arguments after the command name) then LIST becomes arguments
3095to the command invoked if the platform supports it. The meaning of
3096C<open> with more than three arguments for non-pipe modes is not yet
3097specified. Experimental "layers" may give extra LIST arguments
3098meaning.
6170680b
IZ
3099
3100In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 3101and opening C<< '>-' >> opens STDOUT.
6170680b 3102
fae2c0fb
RGS
3103You may use the three-argument form of open to specify IO "layers"
3104(sometimes also referred to as "disciplines") to be applied to the handle
3105that affect how the input and output are processed (see L<open> and
3106L<PerlIO> for more details). For example
7207e29d 3107
9124316e
JH
3108 open(FH, "<:utf8", "file")
3109
3110will open the UTF-8 encoded file containing Unicode characters,
6d5e88a0
TS
3111see L<perluniintro>. Note that if layers are specified in the
3112three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3113usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
ed53a2bb
JH
3114
3115Open returns nonzero upon success, the undefined value otherwise. If
3116the C<open> involved a pipe, the return value happens to be the pid of
3117the subprocess.
cb1a09d0 3118
ed53a2bb
JH
3119If you're running Perl on a system that distinguishes between text
3120files and binary files, then you should check out L</binmode> for tips
3121for dealing with this. The key distinction between systems that need
3122C<binmode> and those that don't is their text file formats. Systems
8939ba94 3123like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
3124character, and which encode that character in C as C<"\n">, do not
3125need C<binmode>. The rest need it.
cb1a09d0 3126
fb73857a 3127When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
3128if the request failed, so C<open> is frequently used in connection with
3129C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 3130where you want to make a nicely formatted error message (but there are
5a964f20 3131modules that can help with that problem)) you should always check
19799a22 3132the return value from opening a file. The infrequent exception is when
fb73857a 3133working with an unopened filehandle is actually what you want to do.
3134
cf264981 3135As a special case the 3-arg form with a read/write mode and the third
ed53a2bb 3136argument being C<undef>:
b76cc8ba
NIS
3137
3138 open(TMP, "+>", undef) or die ...
3139
f253e835
JH
3140opens a filehandle to an anonymous temporary file. Also using "+<"
3141works for symmetry, but you really should consider writing something
3142to the temporary file first. You will need to seek() to do the
3143reading.
b76cc8ba 3144
2ce64696 3145Since v5.8.0, perl has built using PerlIO by default. Unless you've
28a5cf3b 3146changed this (i.e. Configure -Uuseperlio), you can open file handles to
2ce64696 3147"in memory" files held in Perl scalars via:
ba964c95 3148
b996200f
SB
3149 open($fh, '>', \$variable) || ..
3150
3151Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3152file, you have to close it first:
3153
3154 close STDOUT;
3155 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
ba964c95 3156
cb1a09d0