This is a live mirror of the Perl 5 development currently hosted at https://github.com/perl/perl5
perldoc improvements for map
[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
3b10bc60 17contexts for its arguments. If it does both, scalar arguments
18come first and list argument follow, and there can only ever
19be 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
3b10bc60 24list (and provide list context for elements of the list) are shown
a0d0e21e
LW
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.
8bdbc703 29Commas should separate literal 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
3b10bc60 33parentheses.) If you use parentheses, the simple but occasionally
34surprising rule is this: It I<looks> like a function, therefore it I<is> a
a0d0e21e 35function, and precedence doesn't matter. Otherwise it's a list
3b10bc60 36operator or unary operator, and precedence does matter. Whitespace
37between the function and left parenthesis doesn't count, so sometimes
38you need to be careful:
a0d0e21e 39
5ed4f2ec 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 59returning the undefined value, and in a list context by returning the
3b10bc60 60empty list.
a0d0e21e 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.
80d38338 65Each operator and function decides which sort of value 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
3b10bc60 81In general, functions in Perl that serve as wrappers for system calls ("syscalls")
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
88e1f1a2
JV
89Extension modules can also hook into the Perl parser to define new
90kinds of keyword-headed expression. These may look like functions, but
91may also look completely different. The syntax following the keyword
92is defined entirely by the extension. If you are an implementor, see
93L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
94a module, see the module's documentation for details of the syntax that
95it defines.
96
cb1a09d0 97=head2 Perl Functions by Category
d74e8afc 98X<function>
cb1a09d0
AD
99
100Here are Perl's functions (including things that look like
5a964f20 101functions, like some keywords and named operators)
cb1a09d0
AD
102arranged by category. Some functions appear in more
103than one place.
104
13a2d996 105=over 4
cb1a09d0
AD
106
107=item Functions for SCALARs or strings
d74e8afc 108X<scalar> X<string> X<character>
cb1a09d0 109
22fae026 110C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
1dc8ecb8 111C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
945c54fd 112C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
cb1a09d0
AD
113
114=item Regular expressions and pattern matching
d74e8afc 115X<regular expression> X<regex> X<regexp>
cb1a09d0 116
ab4f32c2 117C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
cb1a09d0
AD
118
119=item Numeric functions
d74e8afc 120X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 121
22fae026
TM
122C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
123C<sin>, C<sqrt>, C<srand>
cb1a09d0
AD
124
125=item Functions for real @ARRAYs
d74e8afc 126X<array>
cb1a09d0 127
a5ce339c 128C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
cb1a09d0
AD
129
130=item Functions for list data
d74e8afc 131X<list>
cb1a09d0 132
1dc8ecb8 133C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
cb1a09d0
AD
134
135=item Functions for real %HASHes
d74e8afc 136X<hash>
cb1a09d0 137
22fae026 138C<delete>, C<each>, C<exists>, C<keys>, C<values>
cb1a09d0
AD
139
140=item Input and output functions
d74e8afc 141X<I/O> X<input> X<output> X<dbm>
cb1a09d0 142
22fae026
TM
143C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
144C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
0d863452 145C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
22fae026
TM
146C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
147C<warn>, C<write>
cb1a09d0
AD
148
149=item Functions for fixed length data or records
150
22fae026 151C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
cb1a09d0
AD
152
153=item Functions for filehandles, files, or directories
d74e8afc 154X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 155
22fae026 156C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 157C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
1e278fd9
JH
158C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
159C<umask>, C<unlink>, C<utime>
cb1a09d0 160
cf264981 161=item Keywords related to the control flow of your Perl program
d74e8afc 162X<control flow>
cb1a09d0 163
98293880
JH
164C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
165C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 166
0d863452
RH
167=item Keywords related to switch
168
36fb85f3 169C<break>, C<continue>, C<given>, C<when>, C<default>
0d863452 170
3b10bc60 171(These are available only if you enable the C<"switch"> feature.
0d863452
RH
172See L<feature> and L<perlsyn/"Switch statements">.)
173
54310121 174=item Keywords related to scoping
cb1a09d0 175
36fb85f3
RGS
176C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
177C<use>
178
3b10bc60 179(C<state> is available only if the C<"state"> feature is enabled. See
36fb85f3 180L<feature>.)
cb1a09d0
AD
181
182=item Miscellaneous functions
183
36fb85f3 184C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
834df1c5 185C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
cb1a09d0
AD
186
187=item Functions for processes and process groups
d74e8afc 188X<process> X<pid> X<process id>
cb1a09d0 189
22fae026 190C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
1dc8ecb8 191C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 192C<times>, C<wait>, C<waitpid>
cb1a09d0 193
3b10bc60 194=item Keywords related to Perl modules
d74e8afc 195X<module>
cb1a09d0 196
22fae026 197C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 198
353c6505 199=item Keywords related to classes and object-orientation
d74e8afc 200X<object> X<class> X<package>
cb1a09d0 201
22fae026
TM
202C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
203C<untie>, C<use>
cb1a09d0
AD
204
205=item Low-level socket functions
d74e8afc 206X<socket> X<sock>
cb1a09d0 207
22fae026
TM
208C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
209C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 210C<socket>, C<socketpair>
cb1a09d0
AD
211
212=item System V interprocess communication functions
d74e8afc 213X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 214
22fae026
TM
215C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
216C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
cb1a09d0
AD
217
218=item Fetching user and group info
d74e8afc 219X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 220
22fae026
TM
221C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
222C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
223C<getpwuid>, C<setgrent>, C<setpwent>
cb1a09d0
AD
224
225=item Fetching network info
d74e8afc 226X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 227
22fae026
TM
228C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
229C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
230C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
231C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
232C<setnetent>, C<setprotoent>, C<setservent>
cb1a09d0
AD
233
234=item Time-related functions
d74e8afc 235X<time> X<date>
cb1a09d0 236
22fae026 237C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 238
37798a01 239=item Functions new in perl5
d74e8afc 240X<perl5>
37798a01 241
834df1c5
SP
242C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
243C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
1dc8ecb8 244C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
834df1c5
SP
245C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
246C<ucfirst>, C<untie>, C<use>, C<when>
37798a01 247
3b10bc60 248* C<sub> was a keyword in Perl 4, but in Perl 5 it is an
5a964f20 249operator, which can be used in expressions.
37798a01 250
251=item Functions obsoleted in perl5
252
22fae026 253C<dbmclose>, C<dbmopen>
37798a01 254
cb1a09d0
AD
255=back
256
60f9f73c 257=head2 Portability
d74e8afc 258X<portability> X<Unix> X<portable>
60f9f73c 259
2b5ab1e7
TC
260Perl was born in Unix and can therefore access all common Unix
261system calls. In non-Unix environments, the functionality of some
262Unix system calls may not be available, or details of the available
263functionality may differ slightly. The Perl functions affected
60f9f73c
JH
264by this are:
265
266C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
267C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
268C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
ef5a6dd7
JH
269C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
270C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 271C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
60f9f73c
JH
272C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
273C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
274C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 275C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
60f9f73c
JH
276C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
277C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
278C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 279C<shmwrite>, C<socket>, C<socketpair>,
80cbd5ad
JH
280C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
281C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 282C<utime>, C<wait>, C<waitpid>
60f9f73c
JH
283
284For more information about the portability of these functions, see
285L<perlport> and other available platform-specific documentation.
286
cb1a09d0
AD
287=head2 Alphabetical Listing of Perl Functions
288
3b10bc60 289=over
a0d0e21e 290
5b3c99c0 291=item -X FILEHANDLE
d74e8afc
ITB
292X<-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>
293X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 294
5b3c99c0 295=item -X EXPR
a0d0e21e 296
5228a96c
SP
297=item -X DIRHANDLE
298
5b3c99c0 299=item -X
a0d0e21e
LW
300
301A file test, where X is one of the letters listed below. This unary
5228a96c
SP
302operator takes one argument, either a filename, a filehandle, or a dirhandle,
303and tests the associated file to see if something is true about it. If the
7660c0ab 304argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 305Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 306the undefined value if the file doesn't exist. Despite the funny
d0821a6a 307names, precedence is the same as any other named unary operator. The
a0d0e21e
LW
308operator may be any of:
309
5ed4f2ec 310 -r File is readable by effective uid/gid.
311 -w File is writable by effective uid/gid.
312 -x File is executable by effective uid/gid.
313 -o File is owned by effective uid.
a0d0e21e 314
5ed4f2ec 315 -R File is readable by real uid/gid.
316 -W File is writable by real uid/gid.
317 -X File is executable by real uid/gid.
318 -O File is owned by real uid.
a0d0e21e 319
5ed4f2ec 320 -e File exists.
321 -z File has zero size (is empty).
322 -s File has nonzero size (returns size in bytes).
a0d0e21e 323
5ed4f2ec 324 -f File is a plain file.
325 -d File is a directory.
326 -l File is a symbolic link.
327 -p File is a named pipe (FIFO), or Filehandle is a pipe.
328 -S File is a socket.
329 -b File is a block special file.
330 -c File is a character special file.
331 -t Filehandle is opened to a tty.
a0d0e21e 332
5ed4f2ec 333 -u File has setuid bit set.
334 -g File has setgid bit set.
335 -k File has sticky bit set.
a0d0e21e 336
5ed4f2ec 337 -T File is an ASCII text file (heuristic guess).
338 -B File is a "binary" file (opposite of -T).
a0d0e21e 339
5ed4f2ec 340 -M Script start time minus file modification time, in days.
341 -A Same for access time.
342 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 343
a0d0e21e
LW
344Example:
345
346 while (<>) {
a9a5a0dc
VP
347 chomp;
348 next unless -f $_; # ignore specials
349 #...
a0d0e21e
LW
350 }
351
5ff3f7a4
GS
352The interpretation of the file permission operators C<-r>, C<-R>,
353C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
354of the file and the uids and gids of the user. There may be other
ecae030f
MO
355reasons you can't actually read, write, or execute the file: for
356example network filesystem access controls, ACLs (access control lists),
357read-only filesystems, and unrecognized executable formats. Note
358that the use of these six specific operators to verify if some operation
359is possible is usually a mistake, because it may be open to race
360conditions.
5ff3f7a4 361
2b5ab1e7
TC
362Also note that, for the superuser on the local filesystems, the C<-r>,
363C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
5ff3f7a4
GS
364if any execute bit is set in the mode. Scripts run by the superuser
365may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 366or temporarily set their effective uid to something else.
5ff3f7a4
GS
367
368If you are using ACLs, there is a pragma called C<filetest> that may
369produce more accurate results than the bare stat() mode bits.
5ff3f7a4 370When under the C<use filetest 'access'> the above-mentioned filetests
3b10bc60 371test whether the permission can (not) be granted using the
372access(2) family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
373under this pragma return true even if there are no execute permission
374bits set (nor any extra execute permission ACLs). This strangeness is
ecae030f
MO
375due to the underlying system calls' definitions. Note also that, due to
376the implementation of C<use filetest 'access'>, the C<_> special
377filehandle won't cache the results of the file tests when this pragma is
378in effect. Read the documentation for the C<filetest> pragma for more
379information.
5ff3f7a4 380
a0d0e21e 381Note that C<-s/a/b/> does not do a negated substitution. Saying
3b10bc60 382C<-exp($foo)> still works as expected, however: only single letters
a0d0e21e
LW
383following a minus are interpreted as file tests.
384
385The C<-T> and C<-B> switches work as follows. The first block or so of the
386file is examined for odd characters such as strange control codes or
61eff3bc 387characters with the high bit set. If too many strange characters (>30%)
cf264981 388are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 389containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 390or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 391rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 392file, or a file at EOF when testing a filehandle. Because you have to
4633a7c4
LW
393read a file to do the C<-T> test, on most occasions you want to use a C<-f>
394against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 395
19799a22 396If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 397the special filehandle consisting of a solitary underline, then the stat
a0d0e21e
LW
398structure of the previous file test (or stat operator) is used, saving
399a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 400that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 401symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 402an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 403Example:
a0d0e21e
LW
404
405 print "Can do.\n" if -r $a || -w _ || -x _;
406
407 stat($filename);
408 print "Readable\n" if -r _;
409 print "Writable\n" if -w _;
410 print "Executable\n" if -x _;
411 print "Setuid\n" if -u _;
412 print "Setgid\n" if -g _;
413 print "Sticky\n" if -k _;
414 print "Text\n" if -T _;
415 print "Binary\n" if -B _;
416
fbb0b3b3
RGS
417As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
418test operators, in a way that C<-f -w -x $file> is equivalent to
3b10bc60 419C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
fbb0b3b3
RGS
420the return value of C<-f $file> as an argument to another filetest
421operator, no special magic will happen.)
422
a0d0e21e 423=item abs VALUE
d74e8afc 424X<abs> X<absolute>
a0d0e21e 425
54310121 426=item abs
bbce6d69 427
a0d0e21e 428Returns the absolute value of its argument.
7660c0ab 429If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
430
431=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 432X<accept>
a0d0e21e 433
3b10bc60 434Accepts an incoming socket connect, just as accept(2)
19799a22 435does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 436See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 437
8d2a6795
GS
438On systems that support a close-on-exec flag on files, the flag will
439be set for the newly opened file descriptor, as determined by the
440value of $^F. See L<perlvar/$^F>.
441
a0d0e21e 442=item alarm SECONDS
d74e8afc
ITB
443X<alarm>
444X<SIGALRM>
445X<timer>
a0d0e21e 446
54310121 447=item alarm
bbce6d69 448
a0d0e21e 449Arranges to have a SIGALRM delivered to this process after the
cf264981 450specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
451specified, the value stored in C<$_> is used. (On some machines,
452unfortunately, the elapsed time may be up to one second less or more
453than you specified because of how seconds are counted, and process
454scheduling may delay the delivery of the signal even further.)
455
456Only one timer may be counting at once. Each call disables the
457previous timer, and an argument of C<0> may be supplied to cancel the
458previous timer without starting a new one. The returned value is the
459amount of time remaining on the previous timer.
a0d0e21e 460
2bc69794
BS
461For delays of finer granularity than one second, the Time::HiRes module
462(from CPAN, and starting from Perl 5.8 part of the standard
463distribution) provides ualarm(). You may also use Perl's four-argument
464version of select() leaving the first three arguments undefined, or you
465might be able to use the C<syscall> interface to access setitimer(2) if
466your system supports it. See L<perlfaq8> for details.
2b5ab1e7 467
80d38338
TC
468It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
469C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 470
19799a22
GS
471If you want to use C<alarm> to time out a system call you need to use an
472C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 473fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 474restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 475modulo the caveats given in L<perlipc/"Signals">.
ff68c719 476
477 eval {
a9a5a0dc
VP
478 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
479 alarm $timeout;
480 $nread = sysread SOCKET, $buffer, $size;
481 alarm 0;
ff68c719 482 };
ff68c719 483 if ($@) {
a9a5a0dc 484 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 485 # timed out
ff68c719 486 }
487 else {
5ed4f2ec 488 # didn't
ff68c719 489 }
490
91d81acc
JH
491For more information see L<perlipc>.
492
a0d0e21e 493=item atan2 Y,X
d74e8afc 494X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
495
496Returns the arctangent of Y/X in the range -PI to PI.
497
ca6e1c26 498For the tangent operation, you may use the C<Math::Trig::tan>
28757baa 499function, or use the familiar relation:
500
501 sub tan { sin($_[0]) / cos($_[0]) }
502
a1021d57
RGS
503The return value for C<atan2(0,0)> is implementation-defined; consult
504your atan2(3) manpage for more information.
bf5f1b4c 505
a0d0e21e 506=item bind SOCKET,NAME
d74e8afc 507X<bind>
a0d0e21e 508
3b10bc60 509Binds a network address to a socket, just as bind(2)
19799a22 510does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
511packed address of the appropriate type for the socket. See the examples in
512L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 513
fae2c0fb 514=item binmode FILEHANDLE, LAYER
d74e8afc 515X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 516
a0d0e21e
LW
517=item binmode FILEHANDLE
518
1cbfc93d
NIS
519Arranges for FILEHANDLE to be read or written in "binary" or "text"
520mode on systems where the run-time libraries distinguish between
521binary and text files. If FILEHANDLE is an expression, the value is
522taken as the name of the filehandle. Returns true on success,
b5fe5ca2 523otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 524
d807c6f4
JH
525On some systems (in general, DOS and Windows-based systems) binmode()
526is necessary when you're not working with a text file. For the sake
527of portability it is a good idea to always use it when appropriate,
528and to never use it when it isn't appropriate. Also, people can
529set their I/O to be by default UTF-8 encoded Unicode, not bytes.
530
531In other words: regardless of platform, use binmode() on binary data,
532like for example images.
533
534If LAYER is present it is a single string, but may contain multiple
3b10bc60 535directives. The directives alter the behaviour of the filehandle.
920f5fe1 536When LAYER is present using binmode on a text file makes sense.
d807c6f4 537
fae2c0fb 538If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
539suitable for passing binary data. This includes turning off possible CRLF
540translation and marking it as bytes (as opposed to Unicode characters).
749683d2 541Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 542Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
543Other layers that would affect the binary nature of the stream are
544I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 545PERLIO environment variable.
01e6739c 546
3b10bc60 547The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
548form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
549establish default I/O layers. See L<open>.
550
fae2c0fb
RGS
551I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
552in "Programming Perl, 3rd Edition". However, since the publishing of this
553book, by many known as "Camel III", the consensus of the naming of this
554functionality has moved from "discipline" to "layer". All documentation
555of this version of Perl therefore refers to "layers" rather than to
556"disciplines". Now back to the regularly scheduled documentation...>
557
6902c96a
T
558To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
559C<:utf8> just marks the data as UTF-8 without further checking,
560while C<:encoding(utf8)> checks the data for actually being valid
561UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 562
ed53a2bb 563In general, binmode() should be called after open() but before any I/O
3b10bc60 564is done on the filehandle. Calling binmode() normally flushes any
01e6739c 565pending buffered output data (and perhaps pending input data) on the
fae2c0fb 566handle. An exception to this is the C<:encoding> layer that
01e6739c 567changes the default character encoding of the handle, see L<open>.
fae2c0fb 568The C<:encoding> layer sometimes needs to be called in
3874323d
JH
569mid-stream, and it doesn't flush the stream. The C<:encoding>
570also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 571internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 572
19799a22 573The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
574system all work together to let the programmer treat a single
575character (C<\n>) as the line terminator, irrespective of the external
576representation. On many operating systems, the native text file
577representation matches the internal representation, but on some
578platforms the external representation of C<\n> is made up of more than
579one character.
580
68bd7414
NIS
581Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
582character to end each line in the external representation of text (even
5e12dbfa 583though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
584on Unix and most VMS files). In other systems like OS/2, DOS and the
585various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
586but what's stored in text files are the two characters C<\cM\cJ>. That
587means that, if you don't use binmode() on these systems, C<\cM\cJ>
588sequences on disk will be converted to C<\n> on input, and any C<\n> in
589your program will be converted back to C<\cM\cJ> on output. This is what
590you want for text files, but it can be disastrous for binary files.
30168b04
GS
591
592Another consequence of using binmode() (on some systems) is that
593special end-of-file markers will be seen as part of the data stream.
594For systems from the Microsoft family this means that if your binary
4375e838 595data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
596the file, unless you use binmode().
597
3b10bc60 598binmode() is important not only for readline() and print() operations,
30168b04
GS
599but also when using read(), seek(), sysread(), syswrite() and tell()
600(see L<perlport> for more details). See the C<$/> and C<$\> variables
601in L<perlvar> for how to manually set your input and output
602line-termination sequences.
a0d0e21e 603
4633a7c4 604=item bless REF,CLASSNAME
d74e8afc 605X<bless>
a0d0e21e
LW
606
607=item bless REF
608
2b5ab1e7
TC
609This function tells the thingy referenced by REF that it is now an object
610in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 611is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 612it returns the reference for convenience. Always use the two-argument
cf264981
SP
613version if a derived class might inherit the function doing the blessing.
614See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
615of objects.
a0d0e21e 616
57668c4d 617Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 618Namespaces with all lowercase names are considered reserved for
cf264981 619Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
620confusion, you may wish to avoid such package names as well. Make sure
621that CLASSNAME is a true value.
60ad88b8
GS
622
623See L<perlmod/"Perl Modules">.
624
0d863452
RH
625=item break
626
627Break out of a C<given()> block.
628
3b10bc60 629This keyword is enabled by the C<"switch"> feature: see L<feature>
0d863452
RH
630for more information.
631
a0d0e21e 632=item caller EXPR
d74e8afc 633X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
634
635=item caller
636
5a964f20 637Returns the context of the current subroutine call. In scalar context,
80d38338
TC
638returns the caller's package name if there I<is> a caller (that is, if
639we're in a subroutine or C<eval> or C<require>) and the undefined value
5a964f20 640otherwise. In list context, returns
a0d0e21e 641
ee6b43cc 642 # 0 1 2
748a9306 643 ($package, $filename, $line) = caller;
a0d0e21e
LW
644
645With EXPR, it returns some extra information that the debugger uses to
646print a stack trace. The value of EXPR indicates how many call frames
647to go back before the current one.
648
ee6b43cc 649 # 0 1 2 3 4
f3aa04c2 650 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 651
652 # 5 6 7 8 9 10
b3ca2e83 653 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 654 = caller($i);
e7ea3e70 655
951ba7fe 656Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 657call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 658C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 659C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 660C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 661$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
662each C<use> statement creates a C<require> frame inside an C<eval EXPR>
663frame.) $subroutine may also be C<(unknown)> if this particular
664subroutine happens to have been deleted from the symbol table.
665C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
666C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
667compiled with. The C<$hints> and C<$bitmask> values are subject to change
668between versions of Perl, and are not meant for external use.
748a9306 669
b3ca2e83
NC
670C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
671caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
672of this hash, as they are the actual values stored in the optree.
673
748a9306 674Furthermore, when called from within the DB package, caller returns more
7660c0ab 675detailed information: it sets the list variable C<@DB::args> to be the
54310121 676arguments with which the subroutine was invoked.
748a9306 677
7660c0ab 678Be aware that the optimizer might have optimized call frames away before
19799a22 679C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 680might not return information about the call frame you expect it to, for
b76cc8ba 681C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 682previous time C<caller> was called.
7660c0ab 683
ca9f0cb5
NC
684Also be aware that setting C<@DB::args> is I<best effort>, intended for
685debugging or generating backtraces, and should not be relied upon. In
686particular, as C<@_> contains aliases to the caller's arguments, Perl does
687not take a copy of C<@_>, so C<@DB::args> will contain modifications the
688subroutine makes to C<@_> or its contents, not the original values at call
689time. C<@DB::args>, like C<@_>, does not hold explicit references to its
690elements, so under certain cases its elements may have become freed and
691reallocated for other variables or temporary values. Finally, a side effect
692of the current implementation means that the effects of C<shift @_> can
693I<normally> be undone (but not C<pop @_> or other splicing, and not if a
694reference to C<@_> has been taken, and subject to the caveat about reallocated
695elements), so C<@DB::args> is actually a hybrid of the current state and
696initial state of C<@_>. Buyer beware.
697
a0d0e21e 698=item chdir EXPR
d74e8afc
ITB
699X<chdir>
700X<cd>
f723aae1 701X<directory, change>
a0d0e21e 702
c4aca7d0
GA
703=item chdir FILEHANDLE
704
705=item chdir DIRHANDLE
706
ce2984c3
PF
707=item chdir
708
ffce7b87 709Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 710changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 711changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0 712variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
80d38338 713neither is set, C<chdir> does nothing. It returns true on success,
b4ad75f0 714false otherwise. See the example under C<die>.
a0d0e21e 715
3b10bc60 716On systems that support fchdir(2), you may pass a filehandle or
717directory handle as argument. On systems that don't support fchdir(2),
718passing handles raises an exception.
c4aca7d0 719
a0d0e21e 720=item chmod LIST
d74e8afc 721X<chmod> X<permission> X<mode>
a0d0e21e
LW
722
723Changes the permissions of a list of files. The first element of the
4633a7c4 724list must be the numerical mode, which should probably be an octal
4ad40acf 725number, and which definitely should I<not> be a string of octal digits:
3b10bc60 726C<0644> is okay, but C<"0644"> is not. Returns the number of files
dc848c6f 727successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e 728
3b10bc60 729 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 730 chmod 0755, @executables;
3b10bc60 731 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 732 # --w----r-T
3b10bc60 733 $mode = "0644"; chmod oct($mode), "foo"; # this is better
734 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 735
3b10bc60 736On systems that support fchmod(2), you may pass filehandles among the
737files. On systems that don't support fchmod(2), passing filehandles raises
738an exception. Filehandles must be passed as globs or glob references to be
739recognized; barewords are considered filenames.
c4aca7d0
GA
740
741 open(my $fh, "<", "foo");
742 my $perm = (stat $fh)[2] & 07777;
743 chmod($perm | 0600, $fh);
744
3b10bc60 745You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
746module:
747
3b10bc60 748 use Fcntl qw( :mode );
ca6e1c26 749 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 750 # Identical to the chmod 0755 of the example above.
ca6e1c26 751
a0d0e21e 752=item chomp VARIABLE
d74e8afc 753X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 754
313c9f5c 755=item chomp( LIST )
a0d0e21e
LW
756
757=item chomp
758
2b5ab1e7
TC
759This safer version of L</chop> removes any trailing string
760that corresponds to the current value of C<$/> (also known as
28757baa 761$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
762number of characters removed from all its arguments. It's often used to
763remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
764that the final record may be missing its newline. When in paragraph
765mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
766When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
767a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 768remove anything.
19799a22 769If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
770
771 while (<>) {
a9a5a0dc
VP
772 chomp; # avoid \n on last field
773 @array = split(/:/);
774 # ...
a0d0e21e
LW
775 }
776
4bf21a6d
RD
777If VARIABLE is a hash, it chomps the hash's values, but not its keys.
778
a0d0e21e
LW
779You can actually chomp anything that's an lvalue, including an assignment:
780
781 chomp($cwd = `pwd`);
782 chomp($answer = <STDIN>);
783
784If you chomp a list, each element is chomped, and the total number of
785characters removed is returned.
786
15e44fd8
RGS
787Note that parentheses are necessary when you're chomping anything
788that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
789is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
790C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
791C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
792as C<chomp($a, $b)>.
793
a0d0e21e 794=item chop VARIABLE
d74e8afc 795X<chop>
a0d0e21e 796
313c9f5c 797=item chop( LIST )
a0d0e21e
LW
798
799=item chop
800
801Chops off the last character of a string and returns the character
5b3eff12 802chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 803scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
804If VARIABLE is a hash, it chops the hash's values, but not its keys.
805
5b3eff12 806You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
807
808If you chop a list, each element is chopped. Only the value of the
19799a22 809last C<chop> is returned.
a0d0e21e 810
19799a22 811Note that C<chop> returns the last character. To return all but the last
748a9306
LW
812character, use C<substr($string, 0, -1)>.
813
15e44fd8
RGS
814See also L</chomp>.
815
a0d0e21e 816=item chown LIST
d74e8afc 817X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
818
819Changes the owner (and group) of a list of files. The first two
19799a22
GS
820elements of the list must be the I<numeric> uid and gid, in that
821order. A value of -1 in either position is interpreted by most
822systems to leave that value unchanged. Returns the number of files
823successfully changed.
a0d0e21e
LW
824
825 $cnt = chown $uid, $gid, 'foo', 'bar';
826 chown $uid, $gid, @filenames;
827
3b10bc60 828On systems that support fchown(2), you may pass filehandles among the
829files. On systems that don't support fchown(2), passing filehandles raises
830an exception. Filehandles must be passed as globs or glob references to be
831recognized; barewords are considered filenames.
c4aca7d0 832
54310121 833Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
834
835 print "User: ";
19799a22 836 chomp($user = <STDIN>);
5a964f20 837 print "Files: ";
19799a22 838 chomp($pattern = <STDIN>);
a0d0e21e
LW
839
840 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 841 or die "$user not in passwd file";
a0d0e21e 842
5ed4f2ec 843 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
844 chown $uid, $gid, @ary;
845
54310121 846On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
847file unless you're the superuser, although you should be able to change
848the group to any of your secondary groups. On insecure systems, these
849restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
850On POSIX systems, you can detect this condition this way:
851
852 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
853 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 854
a0d0e21e 855=item chr NUMBER
d74e8afc 856X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 857
54310121 858=item chr
bbce6d69 859
a0d0e21e 860Returns the character represented by that NUMBER in the character set.
a0ed51b3 861For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 862chr(0x263a) is a Unicode smiley face.
aaa68c4a 863
8a064bd6 864Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 865except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
866(truncated to an integer) are used.
867
974da8e5
JH
868If NUMBER is omitted, uses C<$_>.
869
b76cc8ba 870For the reverse, use L</ord>.
a0d0e21e 871
2575c402
JW
872Note that characters from 128 to 255 (inclusive) are by default
873internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 874
2575c402 875See L<perlunicode> for more about Unicode.
bbce6d69 876
a0d0e21e 877=item chroot FILENAME
d74e8afc 878X<chroot> X<root>
a0d0e21e 879
54310121 880=item chroot
bbce6d69 881
5a964f20 882This function works like the system call by the same name: it makes the
4633a7c4 883named directory the new root directory for all further pathnames that
951ba7fe 884begin with a C</> by your process and all its children. (It doesn't
28757baa 885change your current working directory, which is unaffected.) For security
4633a7c4 886reasons, this call is restricted to the superuser. If FILENAME is
19799a22 887omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
888
889=item close FILEHANDLE
d74e8afc 890X<close>
a0d0e21e 891
6a518fbc
TP
892=item close
893
3b10bc60 894Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26
RGS
895buffers, and closes the system file descriptor. Returns true if those
896operations have succeeded and if no error was reported by any PerlIO
897layer. Closes the currently selected filehandle if the argument is
898omitted.
fb73857a 899
900You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 901another C<open> on it, because C<open> closes it for you. (See
19799a22
GS
902C<open>.) However, an explicit C<close> on an input file resets the line
903counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 904
3b10bc60 905If the filehandle came from a piped open, C<close> returns false if one of
906the other syscalls involved fails or if its program exits with non-zero
907status. If the only problem was that the program exited non-zero, C<$!>
908will be set to C<0>. Closing a pipe also waits for the process executing
909on the pipe to exit--in case you wish to look at the output of the pipe
910afterwards--and implicitly puts the exit status value of that command into
911C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 912
80d38338
TC
913Closing the read end of a pipe before the process writing to it at the
914other end is done writing results in the writer receiving a SIGPIPE. If
915the other end can't handle that, be sure to read all the data before
916closing the pipe.
73689b13 917
fb73857a 918Example:
a0d0e21e 919
fb73857a 920 open(OUTPUT, '|sort >foo') # pipe to sort
921 or die "Can't start sort: $!";
5ed4f2ec 922 #... # print stuff to output
923 close OUTPUT # wait for sort to finish
fb73857a 924 or warn $! ? "Error closing sort pipe: $!"
925 : "Exit status $? from sort";
5ed4f2ec 926 open(INPUT, 'foo') # get sort's results
fb73857a 927 or die "Can't open 'foo' for input: $!";
a0d0e21e 928
5a964f20
TC
929FILEHANDLE may be an expression whose value can be used as an indirect
930filehandle, usually the real filehandle name.
a0d0e21e
LW
931
932=item closedir DIRHANDLE
d74e8afc 933X<closedir>
a0d0e21e 934
19799a22 935Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
936system call.
937
a0d0e21e 938=item connect SOCKET,NAME
d74e8afc 939X<connect>
a0d0e21e 940
80d38338
TC
941Attempts to connect to a remote socket, just like connect(2).
942Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
943packed address of the appropriate type for the socket. See the examples in
944L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 945
cb1a09d0 946=item continue BLOCK
d74e8afc 947X<continue>
cb1a09d0 948
0d863452
RH
949=item continue
950
cf264981
SP
951C<continue> is actually a flow control statement rather than a function. If
952there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
953C<foreach>), it is always executed just before the conditional is about to
954be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
955it can be used to increment a loop variable, even when the loop has been
956continued via the C<next> statement (which is similar to the C C<continue>
957statement).
958
98293880 959C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 960block; C<last> and C<redo> behave as if they had been executed within
19799a22 961the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
962block, it may be more entertaining.
963
964 while (EXPR) {
a9a5a0dc
VP
965 ### redo always comes here
966 do_something;
1d2dff63 967 } continue {
a9a5a0dc
VP
968 ### next always comes here
969 do_something_else;
970 # then back the top to re-check EXPR
1d2dff63
GS
971 }
972 ### last always comes here
973
3b10bc60 974Omitting the C<continue> section is equivalent to using an
975empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
976to check the condition at the top of the loop.
977
3b10bc60 978If the C<"switch"> feature is enabled, C<continue> is also a
979function that exits the current C<when> (or C<default>) block and
980falls through to the next one. See L<feature> and
0d863452
RH
981L<perlsyn/"Switch statements"> for more information.
982
983
a0d0e21e 984=item cos EXPR
d74e8afc 985X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 986
d6217f1e
GS
987=item cos
988
5a964f20 989Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 990takes cosine of C<$_>.
a0d0e21e 991
ca6e1c26 992For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa 993function, or use this relation:
994
995 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
996
a0d0e21e 997=item crypt PLAINTEXT,SALT
d74e8afc 998X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 999X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1000
ef2e6798
MS
1001Creates a digest string exactly like the crypt(3) function in the C
1002library (assuming that you actually have a version there that has not
bb23f8d1 1003been extirpated as a potential munition).
ef2e6798
MS
1004
1005crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
1006into a short string, called a digest, which is returned. The same
1007PLAINTEXT and SALT will always return the same string, but there is no
1008(known) way to get the original PLAINTEXT from the hash. Small
1009changes in the PLAINTEXT or SALT will result in large changes in the
1010digest.
1011
1012There is no decrypt function. This function isn't all that useful for
1013cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1014mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1015primarily used to check if two pieces of text are the same without
1016having to transmit or store the text itself. An example is checking
1017if a correct password is given. The digest of the password is stored,
cf264981 1018not the password itself. The user types in a password that is
ef2e6798
MS
1019crypt()'d with the same salt as the stored digest. If the two digests
1020match the password is correct.
1021
1022When verifying an existing digest string you should use the digest as
1023the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1024to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1025crypt() will hash the new string with the same salt as the digest.
1026This allows your code to work with the standard L<crypt|/crypt> and
1027with more exotic implementations. In other words, do not assume
1028anything about the returned string itself, or how many bytes in the
1029digest matter.
85c16d83
JH
1030
1031Traditionally the result is a string of 13 bytes: two first bytes of
1032the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
bb23f8d1 1033the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1034hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1035and implementations on non-Unix platforms may produce different
ef2e6798 1036strings.
85c16d83
JH
1037
1038When choosing a new salt create a random two character string whose
1039characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1040'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1041characters is just a recommendation; the characters allowed in
1042the salt depend solely on your system's crypt library, and Perl can't
1043restrict what salts C<crypt()> accepts.
e71965be 1044
a0d0e21e 1045Here's an example that makes sure that whoever runs this program knows
cf264981 1046their password:
a0d0e21e
LW
1047
1048 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1049
1050 system "stty -echo";
1051 print "Password: ";
e71965be 1052 chomp($word = <STDIN>);
a0d0e21e
LW
1053 print "\n";
1054 system "stty echo";
1055
e71965be 1056 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1057 die "Sorry...\n";
a0d0e21e 1058 } else {
a9a5a0dc 1059 print "ok\n";
54310121 1060 }
a0d0e21e 1061
9f8f0c9d 1062Of course, typing in your own password to whoever asks you
748a9306 1063for it is unwise.
a0d0e21e 1064
ef2e6798 1065The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1066of data, not least of all because you can't get the information
ef2e6798 1067back. Look at the L<Digest> module for more robust algorithms.
19799a22 1068
f2791508
JH
1069If using crypt() on a Unicode string (which I<potentially> has
1070characters with codepoints above 255), Perl tries to make sense
1071of the situation by trying to downgrade (a copy of the string)
1072the string back to an eight-bit byte string before calling crypt()
1073(on that copy). If that works, good. If not, crypt() dies with
1074C<Wide character in crypt>.
85c16d83 1075
aa689395 1076=item dbmclose HASH
d74e8afc 1077X<dbmclose>
a0d0e21e 1078
19799a22 1079[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1080
aa689395 1081Breaks the binding between a DBM file and a hash.
a0d0e21e 1082
19799a22 1083=item dbmopen HASH,DBNAME,MASK
d74e8afc 1084X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1085
19799a22 1086[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1087
7b8d334a 1088This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1089hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1090argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395 1091is the name of the database (without the F<.dir> or F<.pag> extension if
1092any). If the database does not exist, it is created with protection
19799a22 1093specified by MASK (as modified by the C<umask>). If your system supports
80d38338 1094only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1095program. In older versions of Perl, if your system had neither DBM nor
19799a22 1096ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395 1097sdbm(3).
1098
1099If you don't have write access to the DBM file, you can only read hash
1100variables, not set them. If you want to test whether you can write,
3b10bc60 1101either use file tests or try setting a dummy hash entry inside an C<eval>
1102to trap the error.
a0d0e21e 1103
19799a22
GS
1104Note that functions such as C<keys> and C<values> may return huge lists
1105when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1106function to iterate over large DBM files. Example:
1107
1108 # print out history file offsets
1109 dbmopen(%HIST,'/usr/lib/news/history',0666);
1110 while (($key,$val) = each %HIST) {
a9a5a0dc 1111 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1112 }
1113 dbmclose(%HIST);
1114
cb1a09d0 1115See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1116cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1117rich implementation.
4633a7c4 1118
2b5ab1e7
TC
1119You can control which DBM library you use by loading that library
1120before you call dbmopen():
1121
1122 use DB_File;
1123 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1124 or die "Can't open netscape history file: $!";
2b5ab1e7 1125
a0d0e21e 1126=item defined EXPR
d74e8afc 1127X<defined> X<undef> X<undefined>
a0d0e21e 1128
54310121 1129=item defined
bbce6d69 1130
2f9daede 1131Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1132the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TP
1133checked.
1134
1135Many operations return C<undef> to indicate failure, end of file,
1136system error, uninitialized variable, and other exceptional
1137conditions. This function allows you to distinguish C<undef> from
1138other values. (A simple Boolean test will not distinguish among
7660c0ab 1139C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1140false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1141doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TP
1142returns C<undef> when its argument is an empty array, I<or> when the
1143element to return happens to be C<undef>.
1144
f10b0346
GS
1145You may also use C<defined(&func)> to check whether subroutine C<&func>
1146has ever been defined. The return value is unaffected by any forward
80d38338 1147declarations of C<&func>. A subroutine that is not defined
847c7ebe 1148may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1149makes it spring into existence the first time that it is called; see
847c7ebe 1150L<perlsub>.
f10b0346
GS
1151
1152Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1153used to report whether memory for that aggregate has ever been
1154allocated. This behavior may disappear in future versions of Perl.
1155You should instead use a simple test for size:
1156
1157 if (@an_array) { print "has array elements\n" }
1158 if (%a_hash) { print "has hash members\n" }
2f9daede
TP
1159
1160When used on a hash element, it tells you whether the value is defined,
dc848c6f 1161not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1162purpose.
a0d0e21e
LW
1163
1164Examples:
1165
1166 print if defined $switch{'D'};
1167 print "$val\n" while defined($val = pop(@ary));
1168 die "Can't readlink $sym: $!"
a9a5a0dc 1169 unless defined($value = readlink $sym);
a0d0e21e 1170 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1171 $debugging = 0 unless defined $debugging;
a0d0e21e 1172
19799a22 1173Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1174discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1175defined values. For example, if you say
a5f75d66
AD
1176
1177 "ab" =~ /a(.*)b/;
1178
80d38338 1179The pattern match succeeds and C<$1> is defined, although it
cf264981 1180matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1181matched something that happened to be zero characters long. This is all
a5f75d66 1182very above-board and honest. When a function returns an undefined value,
2f9daede 1183it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1184should use C<defined> only when questioning the integrity of what
7660c0ab 1185you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TP
1186what you want.
1187
dc848c6f 1188See also L</undef>, L</exists>, L</ref>.
2f9daede 1189
a0d0e21e 1190=item delete EXPR
d74e8afc 1191X<delete>
a0d0e21e 1192
d0a76353
RS
1193Given an expression that specifies an element or slice of a hash, C<delete>
1194deletes the specified elements from that hash so that exists() on that element
1195no longer returns true. Setting a hash element to the undefined value does
1196not remove its key, but deleting it does; see L</exists>.
80d38338 1197
d0a76353 1198It returns the value or values deleted in list context, or the last such
80d38338 1199element in scalar context. The return list's length always matches that of
d0a76353
RS
1200the argument list: deleting non-existent elements returns the undefined value
1201in their corresponding positions.
80d38338 1202
d0a76353
RS
1203delete() may also be used on arrays and array slices, but its behavior is less
1204straightforward. Although exists() will return false for deleted entries,
1205deleting array elements never changes indices of existing values; use shift()
1206or splice() for that. However, if all deleted elements fall at the end of an
1207array, the array's size shrinks to the position of the highest element that
1208still tests true for exists(), or to 0 if none do.
1209
1210B<Be aware> that calling delete on array values is deprecated and likely to
1211be removed in a future version of Perl.
80d38338
TC
1212
1213Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1214a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1215or array may not necessarily return anything; it depends on the implementation
1216of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1217
80d38338
TC
1218The C<delete local EXPR> construct localizes the deletion to the current
1219block at run time. Until the block exits, elements locally deleted
1220temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1221of composite types">.
eba0920a
EM
1222
1223 %hash = (foo => 11, bar => 22, baz => 33);
1224 $scalar = delete $hash{foo}; # $scalar is 11
1225 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1226 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1227
01020589 1228The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1229
5f05dabc 1230 foreach $key (keys %HASH) {
a9a5a0dc 1231 delete $HASH{$key};
a0d0e21e
LW
1232 }
1233
01020589 1234 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1235 delete $ARRAY[$index];
01020589
GS
1236 }
1237
1238And so do these:
5f05dabc 1239
01020589
GS
1240 delete @HASH{keys %HASH};
1241
9740c838 1242 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1243
80d38338
TC
1244But both are slower than assigning the empty list
1245or undefining %HASH or @ARRAY, which is the customary
1246way to empty out an aggregate:
01020589 1247
5ed4f2ec 1248 %HASH = (); # completely empty %HASH
1249 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1250
5ed4f2ec 1251 @ARRAY = (); # completely empty @ARRAY
1252 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1253
80d38338
TC
1254The EXPR can be arbitrarily complicated provided its
1255final operation is an element or slice of an aggregate:
a0d0e21e
LW
1256
1257 delete $ref->[$x][$y]{$key};
5f05dabc 1258 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1259
01020589
GS
1260 delete $ref->[$x][$y][$index];
1261 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1262
a0d0e21e 1263=item die LIST
d74e8afc 1264X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1265
4c050ad5
NC
1266C<die> raises an exception. Inside an C<eval> the error message is stuffed
1267into C<$@> and the C<eval> is terminated with the undefined value.
1268If the exception is outside of all enclosing C<eval>s, then the uncaught
1269exception prints LIST to C<STDERR> and exits with a non-zero value. If you
1270need to exit the process with a specific exit code, see L<exit>.
a0d0e21e
LW
1271
1272Equivalent examples:
1273
1274 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1275 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1276
ccac6780 1277If the last element of LIST does not end in a newline, the current
df37ec69
WW
1278script line number and input line number (if any) are also printed,
1279and a newline is supplied. Note that the "input line number" (also
1280known as "chunk") is subject to whatever notion of "line" happens to
1281be currently in effect, and is also available as the special variable
1282C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1283
1284Hint: sometimes appending C<", stopped"> to your message will cause it
1285to make better sense when the string C<"at foo line 123"> is appended.
1286Suppose you are running script "canasta".
a0d0e21e
LW
1287
1288 die "/etc/games is no good";
1289 die "/etc/games is no good, stopped";
1290
1291produce, respectively
1292
1293 /etc/games is no good at canasta line 123.
1294 /etc/games is no good, stopped at canasta line 123.
1295
a96d0188 1296If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1297previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a 1298This is useful for propagating exceptions:
1299
1300 eval { ... };
1301 die unless $@ =~ /Expected exception/;
1302
a96d0188 1303If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1304C<PROPAGATE> method, that method will be called with additional file
1305and line number parameters. The return value replaces the value in
80d38338 1306C<$@>. i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1307were called.
1308
7660c0ab 1309If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1310
4c050ad5
NC
1311If an uncaught exception results in interpreter exit, the exit code is
1312determined from the values of C<$!> and C<$?> with this pseudocode:
1313
1314 exit $! if $!; # errno
1315 exit $? >> 8 if $? >> 8; # child exit status
1316 exit 255; # last resort
1317
1318The intent is to squeeze as much possible information about the likely cause
1319into the limited space of the system exit code. However, as C<$!> is the value
1320of C's C<errno>, which can be set by any system call, this means that the value
1321of the exit code used by C<die> can be non-predictable, so should not be relied
1322upon, other than to be non-zero.
1323
80d38338
TC
1324You can also call C<die> with a reference argument, and if this is trapped
1325within an C<eval>, C<$@> contains that reference. This permits more
1326elaborate exception handling using objects that maintain arbitrary state
1327about the exception. Such a scheme is sometimes preferable to matching
1328particular string values of C<$@> with regular expressions. Because C<$@>
1329is a global variable and C<eval> may be used within object implementations,
1330be careful that analyzing the error object doesn't replace the reference in
1331the global variable. It's easiest to make a local copy of the reference
1332before any manipulations. Here's an example:
52531d10 1333
80d38338 1334 use Scalar::Util "blessed";
da279afe 1335
52531d10 1336 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1337 if (my $ev_err = $@) {
1338 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1339 # handle Some::Module::Exception
1340 }
1341 else {
1342 # handle all other possible exceptions
1343 }
1344 }
1345
3b10bc60 1346Because Perl stringifies uncaught exception messages before display,
80d38338 1347you'll probably want to overload stringification operations on
52531d10
GS
1348exception objects. See L<overload> for details about that.
1349
19799a22
GS
1350You can arrange for a callback to be run just before the C<die>
1351does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1352handler is called with the error text and can change the error
19799a22
GS
1353message, if it sees fit, by calling C<die> again. See
1354L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1355L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1356to be run only right before your program was to exit, this is not
3b10bc60 1357currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1358even inside eval()ed blocks/strings! If one wants the hook to do
1359nothing in such situations, put
fb73857a 1360
5ed4f2ec 1361 die @_ if $^S;
fb73857a 1362
19799a22
GS
1363as the first line of the handler (see L<perlvar/$^S>). Because
1364this promotes strange action at a distance, this counterintuitive
b76cc8ba 1365behavior may be fixed in a future release.
774d564b 1366
4c050ad5
NC
1367See also exit(), warn(), and the Carp module.
1368
a0d0e21e 1369=item do BLOCK
d74e8afc 1370X<do> X<block>
a0d0e21e
LW
1371
1372Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1373sequence of commands indicated by BLOCK. When modified by the C<while> or
1374C<until> loop modifier, executes the BLOCK once before testing the loop
1375condition. (On other statements the loop modifiers test the conditional
1376first.)
a0d0e21e 1377
4968c1e4 1378C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1379C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1380See L<perlsyn> for alternative strategies.
4968c1e4 1381
a0d0e21e 1382=item do SUBROUTINE(LIST)
d74e8afc 1383X<do>
a0d0e21e 1384
cf264981 1385This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1386
1387=item do EXPR
d74e8afc 1388X<do>
a0d0e21e
LW
1389
1390Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1391file as a Perl script.
a0d0e21e
LW
1392
1393 do 'stat.pl';
1394
1395is just like
1396
986b19de 1397 eval `cat stat.pl`;
a0d0e21e 1398
2b5ab1e7 1399except that it's more efficient and concise, keeps track of the current
ea63ef19 1400filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1401C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1402variables. It also differs in that code evaluated with C<do FILENAME>
1403cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1404same, however, in that it does reparse the file every time you call it,
1405so you probably don't want to do this inside a loop.
a0d0e21e 1406
8e30cc93 1407If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1408error. If C<do> can read the file but cannot compile it, it
8e30cc93
MG
1409returns undef and sets an error message in C<$@>. If the file is
1410successfully compiled, C<do> returns the value of the last expression
1411evaluated.
1412
80d38338 1413Inclusion of library modules is better done with the
19799a22 1414C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1415and raise an exception if there's a problem.
a0d0e21e 1416
5a964f20
TC
1417You might like to use C<do> to read in a program configuration
1418file. Manual error checking can be done this way:
1419
b76cc8ba 1420 # read in config files: system first, then user
f86cebdf 1421 for $file ("/share/prog/defaults.rc",
b76cc8ba 1422 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1423 {
1424 unless ($return = do $file) {
1425 warn "couldn't parse $file: $@" if $@;
1426 warn "couldn't do $file: $!" unless defined $return;
1427 warn "couldn't run $file" unless $return;
1428 }
5a964f20
TC
1429 }
1430
a0d0e21e 1431=item dump LABEL
d74e8afc 1432X<dump> X<core> X<undump>
a0d0e21e 1433
1614b0e3
JD
1434=item dump
1435
19799a22
GS
1436This function causes an immediate core dump. See also the B<-u>
1437command-line switch in L<perlrun>, which does the same thing.
1438Primarily this is so that you can use the B<undump> program (not
1439supplied) to turn your core dump into an executable binary after
1440having initialized all your variables at the beginning of the
1441program. When the new binary is executed it will begin by executing
1442a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1443Think of it as a goto with an intervening core dump and reincarnation.
1444If C<LABEL> is omitted, restarts the program from the top.
1445
1446B<WARNING>: Any files opened at the time of the dump will I<not>
1447be open any more when the program is reincarnated, with possible
80d38338 1448resulting confusion by Perl.
19799a22 1449
59f521f4
RGS
1450This function is now largely obsolete, mostly because it's very hard to
1451convert a core file into an executable. That's why you should now invoke
1452it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1453typo.
19799a22 1454
cba5a3b0 1455=item each HASH (or HASHREF)
d74e8afc 1456X<each> X<hash, iterator>
aa689395 1457
cba5a3b0 1458=item each ARRAY (or ARRAYREF)
aeedbbed
NC
1459X<array, iterator>
1460
80d38338
TC
1461When called in list context, returns a 2-element list consisting of the key
1462and value for the next element of a hash, or the index and value for the
1463next element of an array, so that you can iterate over it. When called in
1464scalar context, returns only the key (not the value) in a hash, or the index
1465in an array.
2f9daede 1466
aeedbbed 1467Hash entries are returned in an apparently random order. The actual random
3b10bc60 1468order is subject to change in future versions of Perl, but it is
504f80c1 1469guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1470function would produce on the same (unmodified) hash. Since Perl
22883ac5 14715.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1472for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1473
80d38338
TC
1474After C<each> has returned all entries from the hash or array, the next
1475call to C<each> returns the empty list in list context and C<undef> in
1476scalar context. The next call following that one restarts iteration. Each
1477hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1478and C<values>. The iterator is implicitly reset when C<each> has reached
1479the end as just described; it can be explicitly reset by calling C<keys> or
1480C<values> on the hash or array. If you add or delete a hash's elements
1481while iterating over it, entries may be skipped or duplicated--so don't do
1482that. Exception: It is always safe to delete the item most recently
3b10bc60 1483returned by C<each()>, so the following code works properly:
74fc8b5f
MJD
1484
1485 while (($key, $value) = each %hash) {
1486 print $key, "\n";
1487 delete $hash{$key}; # This is safe
1488 }
aa689395 1489
80d38338 1490This prints out your environment like the printenv(1) program,
3b10bc60 1491but in a different order:
a0d0e21e
LW
1492
1493 while (($key,$value) = each %ENV) {
a9a5a0dc 1494 print "$key=$value\n";
a0d0e21e
LW
1495 }
1496
cba5a3b0
DG
1497When given a reference to a hash or array, the argument will be
1498dereferenced automatically.
1499
1500 while (($key,$value) = each $hashref) { ... }
1501
1502If the reference is a blessed object that overrides either C<%{}> or
1503C<@{}>, the override will be used instead of dereferencing the underlying
1504variable type. If both overrides are provided, C<%{}> will be the default.
1505If this is not desired, you must dereference the argument yourself.
1506
19799a22 1507See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1508
1509=item eof FILEHANDLE
d74e8afc
ITB
1510X<eof>
1511X<end of file>
1512X<end-of-file>
a0d0e21e 1513
4633a7c4
LW
1514=item eof ()
1515
a0d0e21e
LW
1516=item eof
1517
1518Returns 1 if the next read on FILEHANDLE will return end of file, or if
1519FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1520gives the real filehandle. (Note that this function actually
80d38338 1521reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1522interactive context.) Do not read from a terminal file (or call
19799a22 1523C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1524as terminals may lose the end-of-file condition if you do.
1525
820475bd 1526An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1527with empty parentheses is different. It refers to the pseudo file
820475bd 1528formed from the files listed on the command line and accessed via the
61eff3bc
JH
1529C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1530as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1531used will cause C<@ARGV> to be examined to determine if input is
67408cae 1532available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1533end-of-file will assume you are processing another C<@ARGV> list,
1534and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1535see L<perlop/"I/O Operators">.
820475bd 1536
61eff3bc 1537In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
3b10bc60 1538detect the end of each file, C<eof()> will detect the end of only the
820475bd 1539last file. Examples:
a0d0e21e 1540
748a9306
LW
1541 # reset line numbering on each input file
1542 while (<>) {
a9a5a0dc
VP
1543 next if /^\s*#/; # skip comments
1544 print "$.\t$_";
5a964f20 1545 } continue {
a9a5a0dc 1546 close ARGV if eof; # Not eof()!
748a9306
LW
1547 }
1548
a0d0e21e
LW
1549 # insert dashes just before last line of last file
1550 while (<>) {
a9a5a0dc
VP
1551 if (eof()) { # check for end of last file
1552 print "--------------\n";
1553 }
1554 print;
1555 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1556 }
1557
a0d0e21e 1558Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1559input operators typically return C<undef> when they run out of data, or if
1560there was an error.
a0d0e21e
LW
1561
1562=item eval EXPR
d74e8afc 1563X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1564X<error, handling> X<exception, handling>
a0d0e21e
LW
1565
1566=item eval BLOCK
1567
ce2984c3
PF
1568=item eval
1569
c7cc6f1c
GS
1570In the first form, the return value of EXPR is parsed and executed as if it
1571were a little Perl program. The value of the expression (which is itself
5a964f20 1572determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1573errors, executed in the lexical context of the current Perl program, so
1574that any variable settings or subroutine and format definitions remain
cf264981 1575afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1576If EXPR is omitted, evaluates C<$_>. This form is typically used to
1577delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1578
1579In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1580same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1581within the context of the current Perl program. This form is typically
1582used to trap exceptions more efficiently than the first (see below), while
1583also providing the benefit of checking the code within BLOCK at compile
1584time.
1585
1586The final semicolon, if any, may be omitted from the value of EXPR or within
1587the BLOCK.
1588
1589In both forms, the value returned is the value of the last expression
5a964f20 1590evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1591as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1592in void, scalar, or list context, depending on the context of the C<eval>
1593itself. See L</wantarray> for more on how the evaluation context can be
1594determined.
a0d0e21e 1595
19799a22 1596If there is a syntax error or runtime error, or a C<die> statement is
bbead3ca
BL
1597executed, C<eval> returns an undefined value in scalar context
1598or an empty list in list context, and C<$@> is set to the
3b10bc60 1599error message. If there was no error, C<$@> is guaranteed to be the empty
1600string. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1601warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1602To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1603turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1604See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1605
19799a22
GS
1606Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1607determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1608is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1609the die operator is used to raise exceptions.
1610
5f1da31c
NT
1611If you want to trap errors when loading an XS module, some problems with
1612the binary interface (such as Perl version skew) may be fatal even with
1613C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1614
a0d0e21e
LW
1615If the code to be executed doesn't vary, you may use the eval-BLOCK
1616form to trap run-time errors without incurring the penalty of
1617recompiling each time. The error, if any, is still returned in C<$@>.
1618Examples:
1619
54310121 1620 # make divide-by-zero nonfatal
a0d0e21e
LW
1621 eval { $answer = $a / $b; }; warn $@ if $@;
1622
1623 # same thing, but less efficient
1624 eval '$answer = $a / $b'; warn $@ if $@;
1625
1626 # a compile-time error
5ed4f2ec 1627 eval { $answer = }; # WRONG
a0d0e21e
LW
1628
1629 # a run-time error
5ed4f2ec 1630 eval '$answer ='; # sets $@
a0d0e21e 1631
cf264981
SP
1632Using the C<eval{}> form as an exception trap in libraries does have some
1633issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1634may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1635You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1636as this example shows:
774d564b 1637
80d38338 1638 # a private exception trap for divide-by-zero
f86cebdf
GS
1639 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1640 warn $@ if $@;
774d564b 1641
1642This is especially significant, given that C<__DIE__> hooks can call
19799a22 1643C<die> again, which has the effect of changing their error messages:
774d564b 1644
1645 # __DIE__ hooks may modify error messages
1646 {
f86cebdf
GS
1647 local $SIG{'__DIE__'} =
1648 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1649 eval { die "foo lives here" };
1650 print $@ if $@; # prints "bar lives here"
774d564b 1651 }
1652
19799a22 1653Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1654may be fixed in a future release.
1655
19799a22 1656With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1657being looked at when:
1658
5ed4f2ec 1659 eval $x; # CASE 1
1660 eval "$x"; # CASE 2
a0d0e21e 1661
5ed4f2ec 1662 eval '$x'; # CASE 3
1663 eval { $x }; # CASE 4
a0d0e21e 1664
5ed4f2ec 1665 eval "\$$x++"; # CASE 5
1666 $$x++; # CASE 6
a0d0e21e 1667
2f9daede 1668Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1669the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1670the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1671and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1672does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TP
1673purely visual reasons, but it also has the advantage of compiling at
1674compile-time instead of at run-time.) Case 5 is a place where
19799a22 1675normally you I<would> like to use double quotes, except that in this
2f9daede
TP
1676particular situation, you can just use symbolic references instead, as
1677in case 6.
a0d0e21e 1678
b208c909
FC
1679Before Perl 5.14, the assignment to C<$@> occured before restoration of localised variables, which means that, if your code is to run on older
1680versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1681errors:
1682
1683 # alter $@ on nefarious repugnancy only
1684 {
1685 my $e;
1686 {
1687 local $@; # protect existing $@
1688 eval { test_repugnancy() };
b208c909 1689 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
8a5a710d
DN
1690 $@ =~ /nefarious/ and $e = $@;
1691 }
1692 die $e if defined $e
1693 }
1694
4968c1e4 1695C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1696C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1697
3b10bc60 1698An C<eval ''> executed within the C<DB> package doesn't see the usual
1699surrounding lexical scope, but rather the scope of the first non-DB piece
1700of code that called it. You don't normally need to worry about this unless
1701you are writing a Perl debugger.
d819b83a 1702
a0d0e21e 1703=item exec LIST
d74e8afc 1704X<exec> X<execute>
a0d0e21e 1705
8bf3b016
GS
1706=item exec PROGRAM LIST
1707
3b10bc60 1708The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1709use C<system> instead of C<exec> if you want it to return. It fails and
1710returns false only if the command does not exist I<and> it is executed
fb73857a 1711directly instead of via your system's command shell (see below).
a0d0e21e 1712
19799a22 1713Since it's a common mistake to use C<exec> instead of C<system>, Perl
80d38338 1714warns you if there is a following statement that isn't C<die>, C<warn>,
3b10bc60 1715or C<exit> (if C<-w> is set--but you always do that, right?). If you
19799a22 1716I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1717can use one of these styles to avoid the warning:
1718
5a964f20
TC
1719 exec ('foo') or print STDERR "couldn't exec foo: $!";
1720 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1721
5a964f20 1722If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1723with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1724If there is only one scalar argument or an array with one element in it,
1725the argument is checked for shell metacharacters, and if there are any,
1726the entire argument is passed to the system's command shell for parsing
1727(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1728If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1729words and passed directly to C<execvp>, which is more efficient.
19799a22 1730Examples:
a0d0e21e 1731
19799a22
GS
1732 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1733 exec "sort $outfile | uniq";
a0d0e21e
LW
1734
1735If you don't really want to execute the first argument, but want to lie
1736to the program you are executing about its own name, you can specify
1737the program you actually want to run as an "indirect object" (without a
1738comma) in front of the LIST. (This always forces interpretation of the
54310121 1739LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1740the list.) Example:
1741
1742 $shell = '/bin/csh';
5ed4f2ec 1743 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1744
1745or, more directly,
1746
5ed4f2ec 1747 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1748
3b10bc60 1749When the arguments get executed via the system shell, results are
1750subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1751for details.
1752
19799a22
GS
1753Using an indirect object with C<exec> or C<system> is also more
1754secure. This usage (which also works fine with system()) forces
1755interpretation of the arguments as a multivalued list, even if the
1756list had just one argument. That way you're safe from the shell
1757expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1758
1759 @args = ( "echo surprise" );
1760
2b5ab1e7 1761 exec @args; # subject to shell escapes
f86cebdf 1762 # if @args == 1
2b5ab1e7 1763 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1764
1765The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1766program, passing it C<"surprise"> an argument. The second version didn't;
1767it tried to run a program named I<"echo surprise">, didn't find it, and set
1768C<$?> to a non-zero value indicating failure.
5a964f20 1769
80d38338 1770Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1771output before the exec, but this may not be supported on some platforms
1772(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1773in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 1774open handles to avoid lost output.
0f897271 1775
80d38338
TC
1776Note that C<exec> will not call your C<END> blocks, nor will it invoke
1777C<DESTROY> methods on your objects.
7660c0ab 1778
a0d0e21e 1779=item exists EXPR
d74e8afc 1780X<exists> X<autovivification>
a0d0e21e 1781
d0a76353
RS
1782Given an expression that specifies an element of a hash, returns true if the
1783specified element in the hash has ever been initialized, even if the
1784corresponding value is undefined.
a0d0e21e 1785
5ed4f2ec 1786 print "Exists\n" if exists $hash{$key};
1787 print "Defined\n" if defined $hash{$key};
01020589
GS
1788 print "True\n" if $hash{$key};
1789
d0a76353
RS
1790exists may also be called on array elements, but its behavior is much less
1791obvious, and is strongly tied to the use of L</delete> on arrays. B<Be aware>
1792that calling exists on array values is deprecated and likely to be removed in
1793a future version of Perl.
1794
5ed4f2ec 1795 print "Exists\n" if exists $array[$index];
1796 print "Defined\n" if defined $array[$index];
01020589 1797 print "True\n" if $array[$index];
a0d0e21e 1798
8ea97a1e 1799A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1800it exists, but the reverse doesn't necessarily hold true.
1801
afebc493
GS
1802Given an expression that specifies the name of a subroutine,
1803returns true if the specified subroutine has ever been declared, even
1804if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 1805does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
1806exist may still be callable: its package may have an C<AUTOLOAD>
1807method that makes it spring into existence the first time that it is
3b10bc60 1808called; see L<perlsub>.
afebc493 1809
5ed4f2ec 1810 print "Exists\n" if exists &subroutine;
1811 print "Defined\n" if defined &subroutine;
afebc493 1812
a0d0e21e 1813Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1814operation is a hash or array key lookup or subroutine name:
a0d0e21e 1815
5ed4f2ec 1816 if (exists $ref->{A}->{B}->{$key}) { }
1817 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 1818
5ed4f2ec 1819 if (exists $ref->{A}->{B}->[$ix]) { }
1820 if (exists $hash{A}{B}[$ix]) { }
01020589 1821
afebc493
GS
1822 if (exists &{$ref->{A}{B}{$key}}) { }
1823
3b10bc60 1824Although the mostly deeply nested array or hash will not spring into
1825existence just because its existence was tested, any intervening ones will.
61eff3bc 1826Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 1827into existence due to the existence test for the $key element above.
3b10bc60 1828This happens anywhere the arrow operator is used, including even here:
5a964f20 1829
2b5ab1e7 1830 undef $ref;
5ed4f2ec 1831 if (exists $ref->{"Some key"}) { }
1832 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
1833
1834This surprising autovivification in what does not at first--or even
1835second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1836release.
a0d0e21e 1837
afebc493
GS
1838Use of a subroutine call, rather than a subroutine name, as an argument
1839to exists() is an error.
1840
5ed4f2ec 1841 exists &sub; # OK
1842 exists &sub(); # Error
afebc493 1843
a0d0e21e 1844=item exit EXPR
d74e8afc 1845X<exit> X<terminate> X<abort>
a0d0e21e 1846
ce2984c3
PF
1847=item exit
1848
2b5ab1e7 1849Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1850
1851 $ans = <STDIN>;
1852 exit 0 if $ans =~ /^[Xx]/;
1853
19799a22 1854See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1855universally recognized values for EXPR are C<0> for success and C<1>
1856for error; other values are subject to interpretation depending on the
1857environment in which the Perl program is running. For example, exiting
185869 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1859the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1860
19799a22
GS
1861Don't use C<exit> to abort a subroutine if there's any chance that
1862someone might want to trap whatever error happened. Use C<die> instead,
1863which can be trapped by an C<eval>.
28757baa 1864
19799a22 1865The exit() function does not always exit immediately. It calls any
2b5ab1e7 1866defined C<END> routines first, but these C<END> routines may not
19799a22 1867themselves abort the exit. Likewise any object destructors that need to
60275626
FC
1868be called are called before the real exit. C<END> routines and destructors
1869can change the exit status by modifying C<$?>. If this is a problem, you
2b5ab1e7 1870can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1871See L<perlmod> for details.
5a964f20 1872
a0d0e21e 1873=item exp EXPR
d74e8afc 1874X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1875
54310121 1876=item exp
bbce6d69 1877
b76cc8ba 1878Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1879If EXPR is omitted, gives C<exp($_)>.
1880
1881=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1882X<fcntl>
a0d0e21e 1883
f86cebdf 1884Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1885
1886 use Fcntl;
1887
0ade1984 1888first to get the correct constant definitions. Argument processing and
3b10bc60 1889value returned work just like C<ioctl> below.
a0d0e21e
LW
1890For example:
1891
1892 use Fcntl;
5a964f20 1893 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 1894 or die "can't fcntl F_GETFL: $!";
5a964f20 1895
554ad1fc 1896You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1897Like C<ioctl>, it maps a C<0> return from the system call into
1898C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1899in numeric context. It is also exempt from the normal B<-w> warnings
1900on improper numeric conversions.
5a964f20 1901
3b10bc60 1902Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
1903doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1904manpage to learn what functions are available on your system.
a0d0e21e 1905
be2f7487
TH
1906Here's an example of setting a filehandle named C<REMOTE> to be
1907non-blocking at the system level. You'll have to negotiate C<$|>
1908on your own, though.
1909
1910 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1911
1912 $flags = fcntl(REMOTE, F_GETFL, 0)
1913 or die "Can't get flags for the socket: $!\n";
1914
1915 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1916 or die "Can't set flags for the socket: $!\n";
1917
a0d0e21e 1918=item fileno FILEHANDLE
d74e8afc 1919X<fileno>
a0d0e21e 1920
2b5ab1e7
TC
1921Returns the file descriptor for a filehandle, or undefined if the
1922filehandle is not open. This is mainly useful for constructing
19799a22 1923bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1924If FILEHANDLE is an expression, the value is taken as an indirect
1925filehandle, generally its name.
5a964f20 1926
b76cc8ba 1927You can use this to find out whether two handles refer to the
5a964f20
TC
1928same underlying descriptor:
1929
1930 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 1931 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1932 }
1933
1934(Filehandles connected to memory objects via new features of C<open> may
1935return undefined even though they are open.)
1936
a0d0e21e
LW
1937
1938=item flock FILEHANDLE,OPERATION
d74e8afc 1939X<flock> X<lock> X<locking>
a0d0e21e 1940
19799a22
GS
1941Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1942for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1943machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1944C<flock> is Perl's portable file locking interface, although it locks
3b10bc60 1945entire files only, not records.
2b5ab1e7
TC
1946
1947Two potentially non-obvious but traditional C<flock> semantics are
1948that it waits indefinitely until the lock is granted, and that its locks
1949B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1950fewer guarantees. This means that programs that do not also use C<flock>
1951may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1952your port's specific documentation, or your system-specific local manpages
1953for details. It's best to assume traditional behavior if you're writing
1954portable programs. (But if you're not, you should as always feel perfectly
1955free to write for your own system's idiosyncrasies (sometimes called
1956"features"). Slavish adherence to portability concerns shouldn't get
1957in the way of your getting your job done.)
a3cb178b 1958
8ebc5c01 1959OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1960LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1961you can use the symbolic names if you import them from the Fcntl module,
68dc0745 1962either individually, or as a group using the ':flock' tag. LOCK_SH
1963requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 1964releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
3b10bc60 1965LOCK_SH or LOCK_EX then C<flock> returns immediately rather than blocking
1966waiting for the lock; check the return status to see if you got it.
68dc0745 1967
2b5ab1e7
TC
1968To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1969before locking or unlocking it.
8ebc5c01 1970
f86cebdf 1971Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1972locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1973are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1974implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01 1975differing semantics shouldn't bite too many people.
1976
becacb53
TM
1977Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1978be open with read intent to use LOCK_SH and requires that it be open
1979with write intent to use LOCK_EX.
1980
19799a22
GS
1981Note also that some versions of C<flock> cannot lock things over the
1982network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1983that. If you like you can force Perl to ignore your system's flock(2)
1984function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 1985the switch C<-Ud_flock> to the F<Configure> program when you configure
3b10bc60 1986Perl.
4633a7c4
LW
1987
1988Here's a mailbox appender for BSD systems.
a0d0e21e 1989
7ed5353d 1990 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
1991
1992 sub lock {
a9a5a0dc
VP
1993 my ($fh) = @_;
1994 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 1995
a9a5a0dc
VP
1996 # and, in case someone appended while we were waiting...
1997 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
1998 }
1999
2000 sub unlock {
a9a5a0dc
VP
2001 my ($fh) = @_;
2002 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2003 }
2004
b0169937 2005 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2006 or die "Can't open mailbox: $!";
a0d0e21e 2007
7ed5353d 2008 lock($mbox);
b0169937 2009 print $mbox $msg,"\n\n";
7ed5353d 2010 unlock($mbox);
a0d0e21e 2011
3b10bc60 2012On systems that support a real flock(2), locks are inherited across fork()
2013calls, whereas those that must resort to the more capricious fcntl(2)
2014function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2015
cb1a09d0 2016See also L<DB_File> for other flock() examples.
a0d0e21e
LW
2017
2018=item fork
d74e8afc 2019X<fork> X<child> X<parent>
a0d0e21e 2020
2b5ab1e7
TC
2021Does a fork(2) system call to create a new process running the
2022same program at the same point. It returns the child pid to the
2023parent process, C<0> to the child process, or C<undef> if the fork is
2024unsuccessful. File descriptors (and sometimes locks on those descriptors)
2025are shared, while everything else is copied. On most systems supporting
2026fork(), great care has gone into making it extremely efficient (for
2027example, using copy-on-write technology on data pages), making it the
2028dominant paradigm for multitasking over the last few decades.
5a964f20 2029
80d38338 2030Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
2031output before forking the child process, but this may not be supported
2032on some platforms (see L<perlport>). To be safe, you may need to set
2033C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2034C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2035
19799a22 2036If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2037accumulate zombies. On some systems, you can avoid this by setting
2038C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2039forking and reaping moribund children.
cb1a09d0 2040
28757baa 2041Note that if your forked child inherits system file descriptors like
2042STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2043if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2044backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2045You should reopen those to F</dev/null> if it's any issue.
28757baa 2046
cb1a09d0 2047=item format
d74e8afc 2048X<format>
cb1a09d0 2049
19799a22 2050Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2051example:
2052
54310121 2053 format Something =
a9a5a0dc
VP
2054 Test: @<<<<<<<< @||||| @>>>>>
2055 $str, $%, '$' . int($num)
cb1a09d0
AD
2056 .
2057
2058 $str = "widget";
184e9718 2059 $num = $cost/$quantity;
cb1a09d0
AD
2060 $~ = 'Something';
2061 write;
2062
2063See L<perlform> for many details and examples.
2064
8903cb82 2065=item formline PICTURE,LIST
d74e8afc 2066X<formline>
a0d0e21e 2067
5a964f20 2068This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2069too. It formats (see L<perlform>) a list of values according to the
2070contents of PICTURE, placing the output into the format output
7660c0ab 2071accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2072Eventually, when a C<write> is done, the contents of
cf264981
SP
2073C<$^A> are written to some filehandle. You could also read C<$^A>
2074and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2075does one C<formline> per line of form, but the C<formline> function itself
748a9306 2076doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2077that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2078You may therefore need to use multiple formlines to implement a single
3b10bc60 2079record format, just like the C<format> compiler.
748a9306 2080
19799a22 2081Be careful if you put double quotes around the picture, because an C<@>
748a9306 2082character may be taken to mean the beginning of an array name.
19799a22 2083C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
2084
2085=item getc FILEHANDLE
f723aae1 2086X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2087
2088=item getc
2089
2090Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2091or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2092the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2093STDIN. This is not particularly efficient. However, it cannot be
2094used by itself to fetch single characters without waiting for the user
2095to hit enter. For that, try something more like:
4633a7c4
LW
2096
2097 if ($BSD_STYLE) {
a9a5a0dc 2098 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2099 }
2100 else {
a9a5a0dc 2101 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2102 }
2103
2104 $key = getc(STDIN);
2105
2106 if ($BSD_STYLE) {
a9a5a0dc 2107 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2108 }
2109 else {
3b10bc60 2110 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2111 }
2112 print "\n";
2113
54310121 2114Determination of whether $BSD_STYLE should be set
2115is left as an exercise to the reader.
cb1a09d0 2116
19799a22 2117The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2118systems purporting POSIX compliance. See also the C<Term::ReadKey>
2119module from your nearest CPAN site; details on CPAN can be found on
2120L<perlmodlib/CPAN>.
a0d0e21e
LW
2121
2122=item getlogin
d74e8afc 2123X<getlogin> X<login>
a0d0e21e 2124
cf264981 2125This implements the C library function of the same name, which on most
3b10bc60 2126systems returns the current login from F</etc/utmp>, if any. If it
2127returns the empty string, use C<getpwuid>.
a0d0e21e 2128
f86702cc 2129 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2130
19799a22
GS
2131Do not consider C<getlogin> for authentication: it is not as
2132secure as C<getpwuid>.
4633a7c4 2133
a0d0e21e 2134=item getpeername SOCKET
d74e8afc 2135X<getpeername> X<peer>
a0d0e21e
LW
2136
2137Returns the packed sockaddr address of other end of the SOCKET connection.
2138
4633a7c4
LW
2139 use Socket;
2140 $hersockaddr = getpeername(SOCK);
19799a22 2141 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2142 $herhostname = gethostbyaddr($iaddr, AF_INET);
2143 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2144
2145=item getpgrp PID
d74e8afc 2146X<getpgrp> X<group>
a0d0e21e 2147
47e29363 2148Returns the current process group for the specified PID. Use
7660c0ab 2149a PID of C<0> to get the current process group for the
4633a7c4 2150current process. Will raise an exception if used on a machine that
f86cebdf 2151doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2152group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2153does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2154
2155=item getppid
d74e8afc 2156X<getppid> X<parent> X<pid>
a0d0e21e
LW
2157
2158Returns the process id of the parent process.
2159
4d76a344
RGS
2160Note for Linux users: on Linux, the C functions C<getpid()> and
2161C<getppid()> return different values from different threads. In order to
3b10bc60 2162be portable, this behavior is not reflected by the Perl-level function
4d76a344 2163C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2164to call the underlying C<getppid()>, you may use the CPAN module
2165C<Linux::Pid>.
4d76a344 2166
a0d0e21e 2167=item getpriority WHICH,WHO
d74e8afc 2168X<getpriority> X<priority> X<nice>
a0d0e21e 2169
4633a7c4 2170Returns the current priority for a process, a process group, or a user.
f4084e39 2171(See C<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2172machine that doesn't implement getpriority(2).
a0d0e21e
LW
2173
2174=item getpwnam NAME
d74e8afc
ITB
2175X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2176X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2177X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2178X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2179X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2180X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2181
2182=item getgrnam NAME
2183
2184=item gethostbyname NAME
2185
2186=item getnetbyname NAME
2187
2188=item getprotobyname NAME
2189
2190=item getpwuid UID
2191
2192=item getgrgid GID
2193
2194=item getservbyname NAME,PROTO
2195
2196=item gethostbyaddr ADDR,ADDRTYPE
2197
2198=item getnetbyaddr ADDR,ADDRTYPE
2199
2200=item getprotobynumber NUMBER
2201
2202=item getservbyport PORT,PROTO
2203
2204=item getpwent
2205
2206=item getgrent
2207
2208=item gethostent
2209
2210=item getnetent
2211
2212=item getprotoent
2213
2214=item getservent
2215
2216=item setpwent
2217
2218=item setgrent
2219
2220=item sethostent STAYOPEN
2221
2222=item setnetent STAYOPEN
2223
2224=item setprotoent STAYOPEN
2225
2226=item setservent STAYOPEN
2227
2228=item endpwent
2229
2230=item endgrent
2231
2232=item endhostent
2233
2234=item endnetent
2235
2236=item endprotoent
2237
2238=item endservent
2239
80d38338
TC
2240These routines are the same as their counterparts in the
2241system C library. In list context, the return values from the
a0d0e21e
LW
2242various get routines are as follows:
2243
2244 ($name,$passwd,$uid,$gid,
6ee623d5 2245 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2246 ($name,$passwd,$gid,$members) = getgr*
2247 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2248 ($name,$aliases,$addrtype,$net) = getnet*
2249 ($name,$aliases,$proto) = getproto*
2250 ($name,$aliases,$port,$proto) = getserv*
2251
3b10bc60 2252(If the entry doesn't exist you get an empty list.)
a0d0e21e 2253
4602f195
JH
2254The exact meaning of the $gcos field varies but it usually contains
2255the real name of the user (as opposed to the login name) and other
2256information pertaining to the user. Beware, however, that in many
2257system users are able to change this information and therefore it
106325ad 2258cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2259L<perlsec>). The $passwd and $shell, user's encrypted password and
2260login shell, are also tainted, because of the same reason.
4602f195 2261
5a964f20 2262In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2263lookup by name, in which case you get the other thing, whatever it is.
2264(If the entry doesn't exist you get the undefined value.) For example:
2265
5a964f20
TC
2266 $uid = getpwnam($name);
2267 $name = getpwuid($num);
2268 $name = getpwent();
2269 $gid = getgrnam($name);
08a33e13 2270 $name = getgrgid($num);
5a964f20
TC
2271 $name = getgrent();
2272 #etc.
a0d0e21e 2273
4602f195 2274In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2275in that they are unsupported on many systems. If the
4602f195
JH
2276$quota is unsupported, it is an empty scalar. If it is supported, it
2277usually encodes the disk quota. If the $comment field is unsupported,
2278it is an empty scalar. If it is supported it usually encodes some
2279administrative comment about the user. In some systems the $quota
2280field may be $change or $age, fields that have to do with password
2281aging. In some systems the $comment field may be $class. The $expire
2282field, if present, encodes the expiration period of the account or the
2283password. For the availability and the exact meaning of these fields
2284in your system, please consult your getpwnam(3) documentation and your
2285F<pwd.h> file. You can also find out from within Perl what your
2286$quota and $comment fields mean and whether you have the $expire field
2287by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2288C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2289files are supported only if your vendor has implemented them in the
4602f195 2290intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2291shadow versions if you're running under privilege or if there exists
cf264981
SP
2292the shadow(3) functions as found in System V (this includes Solaris
2293and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2294facility are unlikely to be supported.
6ee623d5 2295
19799a22 2296The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2297the login names of the members of the group.
2298
2299For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2300C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2301C<@addrs> value returned by a successful call is a list of raw
2302addresses returned by the corresponding library call. In the
2303Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2304by saying something like:
2305
f337b084 2306 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2307
2b5ab1e7
TC
2308The Socket library makes this slightly easier:
2309
2310 use Socket;
2311 $iaddr = inet_aton("127.1"); # or whatever address
2312 $name = gethostbyaddr($iaddr, AF_INET);
2313
2314 # or going the other way
19799a22 2315 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2316
d760c846
GS
2317In the opposite way, to resolve a hostname to the IP address
2318you can write this:
2319
2320 use Socket;
2321 $packed_ip = gethostbyname("www.perl.org");
2322 if (defined $packed_ip) {
2323 $ip_address = inet_ntoa($packed_ip);
2324 }
2325
2326Make sure <gethostbyname()> is called in SCALAR context and that
2327its return value is checked for definedness.
2328
19799a22
GS
2329If you get tired of remembering which element of the return list
2330contains which return value, by-name interfaces are provided
2331in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2332C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2333and C<User::grent>. These override the normal built-ins, supplying
2334versions that return objects with the appropriate names
2335for each field. For example:
5a964f20
TC
2336
2337 use File::stat;
2338 use User::pwent;
2339 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2340
b76cc8ba
NIS
2341Even though it looks like they're the same method calls (uid),
2342they aren't, because a C<File::stat> object is different from
19799a22 2343a C<User::pwent> object.
5a964f20 2344
a0d0e21e 2345=item getsockname SOCKET
d74e8afc 2346X<getsockname>
a0d0e21e 2347
19799a22
GS
2348Returns the packed sockaddr address of this end of the SOCKET connection,
2349in case you don't know the address because you have several different
2350IPs that the connection might have come in on.
a0d0e21e 2351
4633a7c4
LW
2352 use Socket;
2353 $mysockaddr = getsockname(SOCK);
19799a22 2354 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2355 printf "Connect to %s [%s]\n",
19799a22
GS
2356 scalar gethostbyaddr($myaddr, AF_INET),
2357 inet_ntoa($myaddr);
a0d0e21e
LW
2358
2359=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2360X<getsockopt>
a0d0e21e 2361
636e6b1f
TH
2362Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2363Options may exist at multiple protocol levels depending on the socket
2364type, but at least the uppermost socket level SOL_SOCKET (defined in the
2365C<Socket> module) will exist. To query options at another level the
2366protocol number of the appropriate protocol controlling the option
2367should be supplied. For example, to indicate that an option is to be
2368interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2369number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2370
80d38338 2371The function returns a packed string representing the requested socket
3b10bc60 2372option, or C<undef> on error, with the reason for the error placed in
2373C<$!>). Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2374consult getsockopt(2) for details. A common case is that the option is an
2375integer, in which case the result is a packed integer, which you can decode
2376using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2377
80d38338 2378An example to test whether Nagle's algorithm is turned on on a socket:
636e6b1f 2379
4852725b 2380 use Socket qw(:all);
636e6b1f
TH
2381
2382 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2383 or die "Could not determine the protocol number for tcp";
4852725b
DD
2384 # my $tcp = IPPROTO_TCP; # Alternative
2385 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2386 or die "getsockopt TCP_NODELAY: $!";
636e6b1f
TH
2387 my $nodelay = unpack("I", $packed);
2388 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2389
a0d0e21e
LW
2390
2391=item glob EXPR
d74e8afc 2392X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2393
0a753a76 2394=item glob
2395
d9a9d457
JL
2396In list context, returns a (possibly empty) list of filename expansions on
2397the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2398scalar context, glob iterates through such filename expansions, returning
2399undef when the list is exhausted. This is the internal function
2400implementing the C<< <*.c> >> operator, but you can use it directly. If
2401EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2402more detail in L<perlop/"I/O Operators">.
a0d0e21e 2403
80d38338
TC
2404Note that C<glob> splits its arguments on whitespace and treats
2405each segment as separate pattern. As such, C<glob("*.c *.h")>
2406matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2407C<glob(".* *")> matches all files in the current working directory.
80d38338
TC
2408
2409If non-empty braces are the only wildcard characters used in the
2410C<glob>, no filenames are matched, but potentially many strings
2411are returned. For example, this produces nine strings, one for
2412each pairing of fruits and colors:
2413
2414 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2415
3a4b19e4 2416Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2417C<File::Glob> extension. See L<File::Glob> for details, including
2418C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2419
a0d0e21e 2420=item gmtime EXPR
d74e8afc 2421X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2422
ce2984c3
PF
2423=item gmtime
2424
435fbc73
GS
2425Works just like L<localtime> but the returned values are
2426localized for the standard Greenwich time zone.
a0d0e21e 2427
435fbc73
GS
2428Note: when called in list context, $isdst, the last value
2429returned by gmtime is always C<0>. There is no
2430Daylight Saving Time in GMT.
0a753a76 2431
62aa5637
MS
2432See L<perlport/gmtime> for portability concerns.
2433
a0d0e21e 2434=item goto LABEL
d74e8afc 2435X<goto> X<jump> X<jmp>
a0d0e21e 2436
748a9306
LW
2437=item goto EXPR
2438
a0d0e21e
LW
2439=item goto &NAME
2440
b500e03b
GG
2441The C<goto-LABEL> form finds the statement labeled with LABEL and
2442resumes execution there. It can't be used to get out of a block or
2443subroutine given to C<sort>. It can be used to go almost anywhere
2444else within the dynamic scope, including out of subroutines, but it's
2445usually better to use some other construct such as C<last> or C<die>.
2446The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2447(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2448does not offer named loops combined with loop control. Perl does, and
2449this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2450
7660c0ab
A
2451The C<goto-EXPR> form expects a label name, whose scope will be resolved
2452dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2453necessarily recommended if you're optimizing for maintainability:
2454
2455 goto ("FOO", "BAR", "GLARCH")[$i];
2456
b500e03b 2457Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2458deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2459go into any construct that requires initialization, such as a
2460subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2461construct that is optimized away.
b500e03b 2462
1b6921cb
BT
2463The C<goto-&NAME> form is quite different from the other forms of
2464C<goto>. In fact, it isn't a goto in the normal sense at all, and
2465doesn't have the stigma associated with other gotos. Instead, it
2466exits the current subroutine (losing any changes set by local()) and
2467immediately calls in its place the named subroutine using the current
2468value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2469load another subroutine and then pretend that the other subroutine had
2470been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2471in the current subroutine are propagated to the other subroutine.)
2472After the C<goto>, not even C<caller> will be able to tell that this
2473routine was called first.
2474
2475NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2476containing a code reference, or a block that evaluates to a code
6cb9131c 2477reference.
a0d0e21e
LW
2478
2479=item grep BLOCK LIST
d74e8afc 2480X<grep>
a0d0e21e
LW
2481
2482=item grep EXPR,LIST
2483
2b5ab1e7
TC
2484This is similar in spirit to, but not the same as, grep(1) and its
2485relatives. In particular, it is not limited to using regular expressions.
2f9daede 2486
a0d0e21e 2487Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2488C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2489elements for which the expression evaluated to true. In scalar
2490context, returns the number of times the expression was true.
a0d0e21e
LW
2491
2492 @foo = grep(!/^#/, @bar); # weed out comments
2493
2494or equivalently,
2495
2496 @foo = grep {!/^#/} @bar; # weed out comments
2497
be3174d2
GS
2498Note that C<$_> is an alias to the list value, so it can be used to
2499modify the elements of the LIST. While this is useful and supported,
2500it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2501Similarly, grep returns aliases into the original list, much as a for
2502loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2503element of a list returned by grep (for example, in a C<foreach>, C<map>
2504or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2505This is usually something to be avoided when writing clear code.
a0d0e21e 2506
a4fb8298 2507If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2508been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2509the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2510can't be seen from the outside, avoiding any potential side-effects.
2511
19799a22 2512See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2513
a0d0e21e 2514=item hex EXPR
d74e8afc 2515X<hex> X<hexadecimal>
a0d0e21e 2516
54310121 2517=item hex
bbce6d69 2518
2b5ab1e7 2519Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2520(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2521L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TP
2522
2523 print hex '0xAf'; # prints '175'
2524 print hex 'aF'; # same
a0d0e21e 2525
19799a22 2526Hex strings may only represent integers. Strings that would cause
53305cf1 2527integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2528unlike oct(). To present something as hex, look into L</printf>,
2529L</sprintf>, or L</unpack>.
19799a22 2530
ce2984c3 2531=item import LIST
d74e8afc 2532X<import>
a0d0e21e 2533
19799a22 2534There is no builtin C<import> function. It is just an ordinary
4633a7c4 2535method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2536names to another module. The C<use> function calls the C<import> method
cea6626f 2537for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2538
2539=item index STR,SUBSTR,POSITION
d74e8afc 2540X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2541
2542=item index STR,SUBSTR
2543
2b5ab1e7
TC
2544The index function searches for one string within another, but without
2545the wildcard-like behavior of a full regular-expression pattern match.
2546It returns the position of the first occurrence of SUBSTR in STR at
2547or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2548beginning of the string. POSITION before the beginning of the string
2549or after its end is treated as if it were the beginning or the end,
2550respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2551you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2552is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2553
2554=item int EXPR
f723aae1 2555X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2556
54310121 2557=item int
bbce6d69 2558
7660c0ab 2559Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 2560You should not use this function for rounding: one because it truncates
3b10bc60 2561towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
2562numbers can sometimes produce counterintuitive results. For example,
2563C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2564because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2565the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2566functions will serve you better than will int().
a0d0e21e
LW
2567
2568=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2569X<ioctl>
a0d0e21e 2570
2b5ab1e7 2571Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2572
5ed4f2ec 2573 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2574
a11c483f 2575to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2576exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2577own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2578(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2579may help you in this, but it's nontrivial.) SCALAR will be read and/or
3b10bc60 2580written depending on the FUNCTION; a C pointer to the string value of SCALAR
19799a22 2581will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2582has no string value but does have a numeric value, that value will be
2583passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2584true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2585functions may be needed to manipulate the values of structures used by
b76cc8ba 2586C<ioctl>.
a0d0e21e 2587
19799a22 2588The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e 2589
5ed4f2ec 2590 if OS returns: then Perl returns:
2591 -1 undefined value
2592 0 string "0 but true"
2593 anything else that number
a0d0e21e 2594
19799a22 2595Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2596still easily determine the actual value returned by the operating
2597system:
2598
2b5ab1e7 2599 $retval = ioctl(...) || -1;
a0d0e21e
LW
2600 printf "System returned %d\n", $retval;
2601
be2f7487 2602The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2603about improper numeric conversions.
2604
a0d0e21e 2605=item join EXPR,LIST
d74e8afc 2606X<join>
a0d0e21e 2607
2b5ab1e7
TC
2608Joins the separate strings of LIST into a single string with fields
2609separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2610
2b5ab1e7 2611 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2612
eb6e2d6f
GS
2613Beware that unlike C<split>, C<join> doesn't take a pattern as its
2614first argument. Compare L</split>.
a0d0e21e 2615
cba5a3b0 2616=item keys HASH (or HASHREF)
d74e8afc 2617X<keys> X<key>
aa689395 2618
cba5a3b0 2619=item keys ARRAY (or ARRAYREF)
aeedbbed
NC
2620
2621Returns a list consisting of all the keys of the named hash, or the indices
2622of an array. (In scalar context, returns the number of keys or indices.)
504f80c1 2623
aeedbbed 2624The keys of a hash are returned in an apparently random order. The actual
3b10bc60 2625random order is subject to change in future versions of Perl, but it
504f80c1 2626is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6 2627function produces (given that the hash has not been modified). Since
c5f61d2f 2628Perl 5.8.1 the ordering can be different even between different runs of
4546b9e6 2629Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2630Attacks">).
504f80c1 2631
aeedbbed 2632As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
cf264981
SP
2633(see L</each>). In particular, calling keys() in void context resets
2634the iterator with no other overhead.
a0d0e21e 2635
aa689395 2636Here is yet another way to print your environment:
a0d0e21e
LW
2637
2638 @keys = keys %ENV;
2639 @values = values %ENV;
b76cc8ba 2640 while (@keys) {
a9a5a0dc 2641 print pop(@keys), '=', pop(@values), "\n";
a0d0e21e
LW
2642 }
2643
2644or how about sorted by key:
2645
2646 foreach $key (sort(keys %ENV)) {
a9a5a0dc 2647 print $key, '=', $ENV{$key}, "\n";
a0d0e21e
LW
2648 }
2649
8ea1e5d4
GS
2650The returned values are copies of the original keys in the hash, so
2651modifying them will not affect the original hash. Compare L</values>.
2652
19799a22 2653To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2654Here's a descending numeric sort of a hash by its values:
4633a7c4 2655
5a964f20 2656 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
a9a5a0dc 2657 printf "%4d %s\n", $hash{$key}, $key;
4633a7c4
LW
2658 }
2659
3b10bc60 2660Used as an lvalue, C<keys> allows you to increase the number of hash buckets
aa689395 2661allocated for the given hash. This can gain you a measure of efficiency if
2662you know the hash is going to get big. (This is similar to pre-extending
2663an array by assigning a larger number to $#array.) If you say
55497cff 2664
2665 keys %hash = 200;
2666
ab192400
GS
2667then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2668in fact, since it rounds up to the next power of two. These
55497cff 2669buckets will be retained even if you do C<%hash = ()>, use C<undef
2670%hash> if you want to free the storage while C<%hash> is still in scope.
2671You can't shrink the number of buckets allocated for the hash using
19799a22 2672C<keys> in this way (but you needn't worry about doing this by accident,
aeedbbed
NC
2673as trying has no effect). C<keys @array> in an lvalue context is a syntax
2674error.
55497cff 2675
cba5a3b0
DG
2676When given a reference to a hash or array, the argument will be
2677dereferenced automatically.
2678
2679 for (keys $hashref) { ... }
2680 for (keys $obj->get_arrayref) { ... }
2681
2682If the reference is a blessed object that overrides either C<%{}> or
2683C<@{}>, the override will be used instead of dereferencing the underlying
2684variable type. If both overrides are provided, C<%{}> will be the default.
2685If this is not desired, you must dereference the argument yourself.
2686
19799a22 2687See also C<each>, C<values> and C<sort>.
ab192400 2688
b350dd2f 2689=item kill SIGNAL, LIST
d74e8afc 2690X<kill> X<signal>
a0d0e21e 2691
b350dd2f 2692Sends a signal to a list of processes. Returns the number of
517db077
GS
2693processes successfully signaled (which is not necessarily the
2694same as the number actually killed).
a0d0e21e
LW
2695
2696 $cnt = kill 1, $child1, $child2;
2697 kill 9, @goners;
2698
3b10bc60 2699If SIGNAL is zero, no signal is sent to the process, but C<kill>
2700checks whether it's I<possible> to send a signal to it (that
70fb64f6 2701means, to be brief, that the process is owned by the same user, or we are
3b10bc60 2702the super-user). This is useful to check that a child process is still
81fd35db
DN
2703alive (even if only as a zombie) and hasn't changed its UID. See
2704L<perlport> for notes on the portability of this construct.
b350dd2f 2705
e2c0f81f
DG
2706Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2707of processes. That means you usually want to use positive not negative signals.
2708You may also use a signal name in quotes.
2709
2710The behavior of kill when a I<PROCESS> number is zero or negative depends on
2711the operating system. For example, on POSIX-conforming systems, zero will
2712signal the current process group and -1 will signal all processes.
1e9c1022
JL
2713
2714See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2715
2716=item last LABEL
d74e8afc 2717X<last> X<break>
a0d0e21e
LW
2718
2719=item last
2720
2721The C<last> command is like the C<break> statement in C (as used in
2722loops); it immediately exits the loop in question. If the LABEL is
2723omitted, the command refers to the innermost enclosing loop. The
2724C<continue> block, if any, is not executed:
2725
4633a7c4 2726 LINE: while (<STDIN>) {
a9a5a0dc
VP
2727 last LINE if /^$/; # exit when done with header
2728 #...
a0d0e21e
LW
2729 }
2730
80d38338 2731C<last> cannot be used to exit a block that returns a value such as
2b5ab1e7
TC
2732C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2733a grep() or map() operation.
4968c1e4 2734
6c1372ed
GS
2735Note that a block by itself is semantically identical to a loop
2736that executes once. Thus C<last> can be used to effect an early
2737exit out of such a block.
2738
98293880
JH
2739See also L</continue> for an illustration of how C<last>, C<next>, and
2740C<redo> work.
1d2dff63 2741
a0d0e21e 2742=item lc EXPR
d74e8afc 2743X<lc> X<lowercase>
a0d0e21e 2744
54310121 2745=item lc
bbce6d69 2746
d1be9408 2747Returns a lowercased version of EXPR. This is the internal function
3980dc9c 2748implementing the C<\L> escape in double-quoted strings.
a0d0e21e 2749
7660c0ab 2750If EXPR is omitted, uses C<$_>.
bbce6d69 2751
3980dc9c
KW
2752What gets returned depends on several factors:
2753
2754=over
2755
2756=item If C<use bytes> is in effect:
2757
2758=over
2759
2760=item On EBCDIC platforms
2761
2762The results are what the C language system call C<tolower()> returns.
2763
2764=item On ASCII platforms
2765
2766The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
2767respectively.
2768
2769=back
2770
2771=item Otherwise, If EXPR has the UTF8 flag set
2772
2773If the current package has a subroutine named C<ToLower>, it will be used to
2774change the case (See L<perlunicode/User-Defined Case Mappings>.)
2775Otherwise Unicode semantics are used for the case change.
2776
2777=item Otherwise, if C<use locale> is in effect
2778
2779Respects current LC_CTYPE locale. See L<perllocale>.
2780
2781=item Otherwise, if C<use feature 'unicode_strings'> is in effect:
2782
2783Unicode semantics are used for the case change. Any subroutine named
2784C<ToLower> will not be used.
2785
2786=item Otherwise:
2787
2788=over
2789
2790=item On EBCDIC platforms
2791
2792The results are what the C language system call C<tolower()> returns.
2793
2794=item On ASCII platforms
2795
2796ASCII semantics are used for the case change. The lowercase of any character
2797outside the ASCII range is the character itself.
2798
2799=back
2800
2801=back
2802
a0d0e21e 2803=item lcfirst EXPR
d74e8afc 2804X<lcfirst> X<lowercase>
a0d0e21e 2805
54310121 2806=item lcfirst
bbce6d69 2807
ad0029c4
JH
2808Returns the value of EXPR with the first character lowercased. This
2809is the internal function implementing the C<\l> escape in
3980dc9c 2810double-quoted strings.
a0d0e21e 2811
7660c0ab 2812If EXPR is omitted, uses C<$_>.
bbce6d69 2813
3980dc9c
KW
2814This function behaves the same way under various pragma, such as in a locale,
2815as L</lc> does.
2816
a0d0e21e 2817=item length EXPR
d74e8afc 2818X<length> X<size>
a0d0e21e 2819
54310121 2820=item length
bbce6d69 2821
974da8e5 2822Returns the length in I<characters> of the value of EXPR. If EXPR is
9f621bb0 2823omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
3b10bc60 2824
2825This function cannot be used on an entire array or hash to find out how
2826many elements these have. For that, use C<scalar @array> and C<scalar keys
2827%hash>, respectively.
2828
2829Like all Perl character operations, length() normally deals in logical
2830characters, not physical bytes. For how many bytes a string encoded as
2831UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
2832to C<use Encode> first). See L<Encode> and L<perlunicode>.
974da8e5 2833
a0d0e21e 2834=item link OLDFILE,NEWFILE
d74e8afc 2835X<link>
a0d0e21e 2836
19799a22 2837Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2838success, false otherwise.
a0d0e21e
LW
2839
2840=item listen SOCKET,QUEUESIZE
d74e8afc 2841X<listen>
a0d0e21e 2842
3b10bc60 2843Does the same thing that the listen(2) system call does. Returns true if
b76cc8ba 2844it succeeded, false otherwise. See the example in
cea6626f 2845L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2846
2847=item local EXPR
d74e8afc 2848X<local>
a0d0e21e 2849
19799a22 2850You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2851what most people think of as "local". See
13a2d996 2852L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2853
5a964f20
TC
2854A local modifies the listed variables to be local to the enclosing
2855block, file, or eval. If more than one value is listed, the list must
2856be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2857for details, including issues with tied arrays and hashes.
a0d0e21e 2858
d361fafa
VP
2859The C<delete local EXPR> construct can also be used to localize the deletion
2860of array/hash elements to the current block.
2861See L<perlsub/"Localized deletion of elements of composite types">.
2862
a0d0e21e 2863=item localtime EXPR
435fbc73 2864X<localtime> X<ctime>
a0d0e21e 2865
ba053783
AL
2866=item localtime
2867
19799a22 2868Converts a time as returned by the time function to a 9-element list
5f05dabc 2869with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2870follows:
2871
54310121 2872 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2873 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2874 localtime(time);
a0d0e21e 2875
48a26b3a 2876All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2877tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2878of the specified time.
48a26b3a 2879
ba053783
AL
2880C<$mday> is the day of the month, and C<$mon> is the month itself, in
2881the range C<0..11> with 0 indicating January and 11 indicating December.
2882This makes it easy to get a month name from a list:
54310121 2883
ba053783
AL
2884 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2885 print "$abbr[$mon] $mday";
2886 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2887
ba053783
AL
2888C<$year> is the number of years since 1900, not just the last two digits
2889of the year. That is, C<$year> is C<123> in year 2023. The proper way
80d38338 2890to get a 4-digit year is simply:
abd75f24 2891
ba053783 2892 $year += 1900;
abd75f24 2893
435fbc73
GS
2894Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2895to do that, would you?
2896
ba053783
AL
2897To get the last two digits of the year (e.g., '01' in 2001) do:
2898
2899 $year = sprintf("%02d", $year % 100);
2900
2901C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2902Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2903(or C<0..365> in leap years.)
2904
2905C<$isdst> is true if the specified time occurs during Daylight Saving
2906Time, false otherwise.
abd75f24 2907
e1998452 2908If EXPR is omitted, C<localtime()> uses the current time (as returned
e3176d09 2909by time(3)).
a0d0e21e 2910
48a26b3a 2911In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2912
5f05dabc 2913 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2914
fe86afc2
NC
2915This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2916instead of local time use the L</gmtime> builtin. See also the
2917C<Time::Local> module (to convert the second, minutes, hours, ... back to
2918the integer value returned by time()), and the L<POSIX> module's strftime(3)
2919and mktime(3) functions.
2920
2921To get somewhat similar but locale dependent date strings, set up your
2922locale environment variables appropriately (please see L<perllocale>) and
2923try for example:
a3cb178b 2924
5a964f20 2925 use POSIX qw(strftime);
2b5ab1e7 2926 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2927 # or for GMT formatted appropriately for your locale:
2928 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2929
2930Note that the C<%a> and C<%b>, the short forms of the day of the week
2931and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2932
62aa5637
MS
2933See L<perlport/localtime> for portability concerns.
2934
435fbc73
GS
2935The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2936by-name access mechanism to the gmtime() and localtime() functions,
2937respectively.
2938
2939For a comprehensive date and time representation look at the
2940L<DateTime> module on CPAN.
2941
07698885 2942=item lock THING
d74e8afc 2943X<lock>
19799a22 2944
01e6739c 2945This function places an advisory lock on a shared variable, or referenced
03730085 2946object contained in I<THING> until the lock goes out of scope.
a6d5524e 2947
f3a23afb 2948lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2949by this name (before any calls to it), that function will be called
7b043ca5
RGS
2950instead. If you are not under C<use threads::shared> this does nothing.
2951See L<threads::shared>.
19799a22 2952
a0d0e21e 2953=item log EXPR
d74e8afc 2954X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2955
54310121 2956=item log
bbce6d69 2957
2b5ab1e7
TC
2958Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2959returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2960The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2961divided by the natural log of N. For example:
2962
2963 sub log10 {
a9a5a0dc
VP
2964 my $n = shift;
2965 return log($n)/log(10);
b76cc8ba 2966 }
2b5ab1e7
TC
2967
2968See also L</exp> for the inverse operation.
a0d0e21e 2969
a0d0e21e 2970=item lstat EXPR
d74e8afc 2971X<lstat>
a0d0e21e 2972
54310121 2973=item lstat
bbce6d69 2974
19799a22 2975Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2976special C<_> filehandle) but stats a symbolic link instead of the file
2977the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2978your system, a normal C<stat> is done. For much more detailed
2979information, please see the documentation for C<stat>.
a0d0e21e 2980
7660c0ab 2981If EXPR is omitted, stats C<$_>.
bbce6d69 2982
a0d0e21e
LW
2983=item m//
2984
9f4b9cd0 2985The match operator. See L<perlop/"Regexp Quote-Like Operators">.
a0d0e21e
LW
2986
2987=item map BLOCK LIST
d74e8afc 2988X<map>
a0d0e21e
LW
2989
2990=item map EXPR,LIST
2991
19799a22
GS
2992Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2993C<$_> to each element) and returns the list value composed of the
2994results of each such evaluation. In scalar context, returns the
2995total number of elements so generated. Evaluates BLOCK or EXPR in
2996list context, so each element of LIST may produce zero, one, or
2997more elements in the returned value.
dd99ebda 2998
f9476272 2999 @chars = map(chr, @numbers);
a0d0e21e 3000
f9476272
AH
3001translates a list of numbers to the corresponding characters.
3002
3003 my @squares = map { $_ * $_ } @numbers;
3004
3005translates a list of numbers to their squared values.
3006
3007 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3008
3009shows that number of returned elements can differ from the number of
3010input elements. To omit an element, return an empty list ().
3011This could also be achieved by writing
3012
3013 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3014
3015which makes the intention more clear.
3016
3017Map always returns a list which can be assigned to a hash where the elements
3018become key/value pairs. See L<perldata> for more details.
a0d0e21e 3019
d8216f19 3020 %hash = map { get_a_key_for($_) => $_ } @array;
a0d0e21e
LW
3021
3022is just a funny way to write
3023
3024 %hash = ();
d8216f19 3025 foreach (@array) {
a9a5a0dc 3026 $hash{get_a_key_for($_)} = $_;
a0d0e21e
LW
3027 }
3028
be3174d2
GS
3029Note that C<$_> is an alias to the list value, so it can be used to
3030modify the elements of the LIST. While this is useful and supported,
3031it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
3032Using a regular C<foreach> loop for this purpose would be clearer in
3033most cases. See also L</grep> for an array composed of those items of
3034the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 3035
a4fb8298 3036If C<$_> is lexical in the scope where the C<map> appears (because it has
d8216f19
RGS
3037been declared with C<my $_>), then, in addition to being locally aliased to
3038the list elements, C<$_> keeps being lexical inside the block; that is, it
a4fb8298
RGS
3039can't be seen from the outside, avoiding any potential side-effects.
3040
205fdb4d 3041C<{> starts both hash references and blocks, so C<map { ...> could be either
3b10bc60 3042the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
80d38338
TC
3043ahead for the closing C<}> it has to take a guess at which it's dealing with
3044based on what it finds just after the C<{>. Usually it gets it right, but if it
205fdb4d
NC
3045doesn't it won't realize something is wrong until it gets to the C<}> and
3046encounters the missing (or unexpected) comma. The syntax error will be
80d38338 3047reported close to the C<}>, but you'll need to change something near the C<{>
3b10bc60 3048such as using a unary C<+> to give Perl some help:
205fdb4d 3049
3b10bc60 3050 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3051 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3052 %hash = map { ("\L$_" => 1) } @array # this also works
3053 %hash = map { lc($_) => 1 } @array # as does this.
3054 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
cea6626f 3055
3b10bc60 3056 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
205fdb4d 3057
d8216f19 3058or to force an anon hash constructor use C<+{>:
205fdb4d 3059
3b10bc60 3060 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end
205fdb4d 3061
3b10bc60 3062to get a list of anonymous hashes each with only one entry apiece.
205fdb4d 3063
19799a22 3064=item mkdir FILENAME,MASK
d74e8afc 3065X<mkdir> X<md> X<directory, create>
a0d0e21e 3066
5a211162
GS
3067=item mkdir FILENAME
3068
491873e5
RGS
3069=item mkdir
3070
0591cd52 3071Creates the directory specified by FILENAME, with permissions
19799a22
GS
3072specified by MASK (as modified by C<umask>). If it succeeds it
3073returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
3074If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
3075to C<$_>.
0591cd52 3076
19799a22 3077In general, it is better to create directories with permissive MASK,
0591cd52 3078and let the user modify that with their C<umask>, than it is to supply
19799a22 3079a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
3080The exceptions to this rule are when the file or directory should be
3081kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 3082C<umask> discusses the choice of MASK in more detail.
a0d0e21e 3083
cc1852e8
JH
3084Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3085number of trailing slashes. Some operating and filesystems do not get
3086this right, so Perl automatically removes all trailing slashes to keep
3087everyone happy.
3088
80d38338 3089To recursively create a directory structure, look at
dd184578
RGS
3090the C<mkpath> function of the L<File::Path> module.
3091
a0d0e21e 3092=item msgctl ID,CMD,ARG
d74e8afc 3093X<msgctl>
a0d0e21e 3094
f86cebdf 3095Calls the System V IPC function msgctl(2). You'll probably have to say