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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
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4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
15operator. A unary operator generally provides a scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
a0d0e21e 17contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 18be first, and the list argument will follow. (Note that there can ever
0f31cffe 19be only one such list argument.) For instance, splice() has three scalar
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20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
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22
23In the syntax descriptions that follow, list operators that expect a
24list (and provide list context for the elements of the list) are shown
25with LIST as an argument. Such a list may consist of any combination
26of scalar arguments or list values; the list values will be included
27in the list as if each individual element were interpolated at that
28point in the list, forming a longer single-dimensional list value.
cf264981 29Commas should separate elements of the LIST.
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30
31Any function in the list below may be used either with or without
32parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 33parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 34surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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35function, and precedence doesn't matter. Otherwise it's a list
36operator or unary operator, and precedence does matter. And whitespace
37between the function and left parenthesis doesn't count--so you need to
38be careful sometimes:
39
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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.
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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
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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
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59returning the undefined value, and in a list context by returning the
60null list.
61
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62Remember the following important rule: There is B<no rule> that relates
63the behavior of an expression in list context to its behavior in scalar
64context, or vice versa. It might do two totally different things.
a0d0e21e 65Each operator and function decides which sort of value it would be most
2b5ab1e7 66appropriate to return in scalar context. Some operators return the
5a964f20 67length of the list that would have been returned in list context. Some
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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
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75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
81In general, functions in Perl that serve as wrappers for system calls
f86cebdf 82of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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83true when they succeed and C<undef> otherwise, as is usually mentioned
84in the descriptions below. This is different from the C interfaces,
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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<$!>
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87variable on failure. Other functions do not, except accidentally.
88
cb1a09d0 89=head2 Perl Functions by Category
d74e8afc 90X<function>
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91
92Here are Perl's functions (including things that look like
5a964f20 93functions, like some keywords and named operators)
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94arranged by category. Some functions appear in more
95than one place.
96
13a2d996 97=over 4
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98
99=item Functions for SCALARs or strings
d74e8afc 100X<scalar> X<string> X<character>
cb1a09d0 101
22fae026 102C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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103C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
104C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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105
106=item Regular expressions and pattern matching
d74e8afc 107X<regular expression> X<regex> X<regexp>
cb1a09d0 108
ab4f32c2 109C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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110
111=item Numeric functions
d74e8afc 112X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 113
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114C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
115C<sin>, C<sqrt>, C<srand>
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116
117=item Functions for real @ARRAYs
d74e8afc 118X<array>
cb1a09d0 119
22fae026 120C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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121
122=item Functions for list data
d74e8afc 123X<list>
cb1a09d0 124
ab4f32c2 125C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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126
127=item Functions for real %HASHes
d74e8afc 128X<hash>
cb1a09d0 129
22fae026 130C<delete>, C<each>, C<exists>, C<keys>, C<values>
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131
132=item Input and output functions
d74e8afc 133X<I/O> X<input> X<output> X<dbm>
cb1a09d0 134
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135C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
136C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
137C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
138C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
139C<warn>, C<write>
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140
141=item Functions for fixed length data or records
142
22fae026 143C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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144
145=item Functions for filehandles, files, or directories
d74e8afc 146X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 147
22fae026 148C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 149C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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150C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
151C<umask>, C<unlink>, C<utime>
cb1a09d0 152
cf264981 153=item Keywords related to the control flow of your Perl program
d74e8afc 154X<control flow>
cb1a09d0 155
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156C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
157C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 158
54310121 159=item Keywords related to scoping
cb1a09d0 160
4375e838 161C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
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162
163=item Miscellaneous functions
164
4375e838 165C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
22fae026 166C<scalar>, C<undef>, C<wantarray>
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167
168=item Functions for processes and process groups
d74e8afc 169X<process> X<pid> X<process id>
cb1a09d0 170
22fae026 171C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 172C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 173C<times>, C<wait>, C<waitpid>
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174
175=item Keywords related to perl modules
d74e8afc 176X<module>
cb1a09d0 177
22fae026 178C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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179
180=item Keywords related to classes and object-orientedness
d74e8afc 181X<object> X<class> X<package>
cb1a09d0 182
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183C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
184C<untie>, C<use>
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185
186=item Low-level socket functions
d74e8afc 187X<socket> X<sock>
cb1a09d0 188
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189C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
190C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 191C<socket>, C<socketpair>
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192
193=item System V interprocess communication functions
d74e8afc 194X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 195
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196C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
197C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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198
199=item Fetching user and group info
d74e8afc 200X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 201
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202C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
203C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
204C<getpwuid>, C<setgrent>, C<setpwent>
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205
206=item Fetching network info
d74e8afc 207X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 208
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209C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
210C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
211C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
212C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
213C<setnetent>, C<setprotoent>, C<setservent>
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214
215=item Time-related functions
d74e8afc 216X<time> X<date>
cb1a09d0 217
22fae026 218C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 219
37798a01 220=item Functions new in perl5
d74e8afc 221X<perl5>
37798a01 222
22fae026 223C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
b76cc8ba 224C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
4375e838 225C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
22fae026 226C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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227
228* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 229operator, which can be used in expressions.
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230
231=item Functions obsoleted in perl5
232
22fae026 233C<dbmclose>, C<dbmopen>
37798a01 234
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235=back
236
60f9f73c 237=head2 Portability
d74e8afc 238X<portability> X<Unix> X<portable>
60f9f73c 239
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240Perl was born in Unix and can therefore access all common Unix
241system calls. In non-Unix environments, the functionality of some
242Unix system calls may not be available, or details of the available
243functionality may differ slightly. The Perl functions affected
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244by this are:
245
246C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
247C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
248C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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249C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
250C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 251C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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252C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
253C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
254C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 255C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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256C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
257C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
258C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 259C<shmwrite>, C<socket>, C<socketpair>,
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260C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
261C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 262C<utime>, C<wait>, C<waitpid>
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263
264For more information about the portability of these functions, see
265L<perlport> and other available platform-specific documentation.
266
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267=head2 Alphabetical Listing of Perl Functions
268
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269=over 8
270
5b3c99c0 271=item -X FILEHANDLE
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272X<-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>
273X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 274
5b3c99c0 275=item -X EXPR
a0d0e21e 276
5b3c99c0 277=item -X
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278
279A file test, where X is one of the letters listed below. This unary
280operator takes one argument, either a filename or a filehandle, and
281tests the associated file to see if something is true about it. If the
7660c0ab 282argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 283Unless otherwise documented, it returns C<1> for true and C<''> for false, or
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284the undefined value if the file doesn't exist. Despite the funny
285names, precedence is the same as any other named unary operator, and
286the argument may be parenthesized like any other unary operator. The
287operator may be any of:
288
289 -r File is readable by effective uid/gid.
290 -w File is writable by effective uid/gid.
291 -x File is executable by effective uid/gid.
292 -o File is owned by effective uid.
293
294 -R File is readable by real uid/gid.
295 -W File is writable by real uid/gid.
296 -X File is executable by real uid/gid.
297 -O File is owned by real uid.
298
299 -e File exists.
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300 -z File has zero size (is empty).
301 -s File has nonzero size (returns size in bytes).
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302
303 -f File is a plain file.
304 -d File is a directory.
305 -l File is a symbolic link.
9c4d0f16 306 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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307 -S File is a socket.
308 -b File is a block special file.
309 -c File is a character special file.
310 -t Filehandle is opened to a tty.
311
312 -u File has setuid bit set.
313 -g File has setgid bit set.
314 -k File has sticky bit set.
315
121910a4 316 -T File is an ASCII text file (heuristic guess).
2cdbc966 317 -B File is a "binary" file (opposite of -T).
a0d0e21e 318
95a3fe12 319 -M Script start time minus file modification time, in days.
a0d0e21e 320 -A Same for access time.
95a3fe12 321 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 322
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323Example:
324
325 while (<>) {
5b3eff12 326 chomp;
a0d0e21e 327 next unless -f $_; # ignore specials
5a964f20 328 #...
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329 }
330
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331The interpretation of the file permission operators C<-r>, C<-R>,
332C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
333of the file and the uids and gids of the user. There may be other
334reasons you can't actually read, write, or execute the file. Such
335reasons may be for example network filesystem access controls, ACLs
336(access control lists), read-only filesystems, and unrecognized
337executable formats.
338
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339Also note that, for the superuser on the local filesystems, the C<-r>,
340C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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341if any execute bit is set in the mode. Scripts run by the superuser
342may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 343or temporarily set their effective uid to something else.
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344
345If you are using ACLs, there is a pragma called C<filetest> that may
346produce more accurate results than the bare stat() mode bits.
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347When under the C<use filetest 'access'> the above-mentioned filetests
348will test whether the permission can (not) be granted using the
468541a8 349access() family of system calls. Also note that the C<-x> and C<-X> may
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350under this pragma return true even if there are no execute permission
351bits set (nor any extra execute permission ACLs). This strangeness is
352due to the underlying system calls' definitions. Read the
353documentation for the C<filetest> pragma for more information.
354
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355Note that C<-s/a/b/> does not do a negated substitution. Saying
356C<-exp($foo)> still works as expected, however--only single letters
357following a minus are interpreted as file tests.
358
359The C<-T> and C<-B> switches work as follows. The first block or so of the
360file is examined for odd characters such as strange control codes or
61eff3bc 361characters with the high bit set. If too many strange characters (>30%)
cf264981 362are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
a0d0e21e 363containing null in the first block is considered a binary file. If C<-T>
9124316e 364or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 365rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 366file, or a file at EOF when testing a filehandle. Because you have to
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367read a file to do the C<-T> test, on most occasions you want to use a C<-f>
368against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 369
19799a22 370If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 371the special filehandle consisting of a solitary underline, then the stat
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372structure of the previous file test (or stat operator) is used, saving
373a system call. (This doesn't work with C<-t>, and you need to remember
374that lstat() and C<-l> will leave values in the stat structure for the
5c9aa243 375symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 376an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 377Example:
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378
379 print "Can do.\n" if -r $a || -w _ || -x _;
380
381 stat($filename);
382 print "Readable\n" if -r _;
383 print "Writable\n" if -w _;
384 print "Executable\n" if -x _;
385 print "Setuid\n" if -u _;
386 print "Setgid\n" if -g _;
387 print "Sticky\n" if -k _;
388 print "Text\n" if -T _;
389 print "Binary\n" if -B _;
390
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391As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
392test operators, in a way that C<-f -w -x $file> is equivalent to
cf264981 393C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
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394the return value of C<-f $file> as an argument to another filetest
395operator, no special magic will happen.)
396
a0d0e21e 397=item abs VALUE
d74e8afc 398X<abs> X<absolute>
a0d0e21e 399
54310121 400=item abs
bbce6d69 401
a0d0e21e 402Returns the absolute value of its argument.
7660c0ab 403If VALUE is omitted, uses C<$_>.
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404
405=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 406X<accept>
a0d0e21e 407
f86cebdf 408Accepts an incoming socket connect, just as the accept(2) system call
19799a22 409does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 410See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 411
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412On systems that support a close-on-exec flag on files, the flag will
413be set for the newly opened file descriptor, as determined by the
414value of $^F. See L<perlvar/$^F>.
415
a0d0e21e 416=item alarm SECONDS
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417X<alarm>
418X<SIGALRM>
419X<timer>
a0d0e21e 420
54310121 421=item alarm
bbce6d69 422
a0d0e21e 423Arranges to have a SIGALRM delivered to this process after the
cf264981 424specified number of wallclock seconds has elapsed. If SECONDS is not
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425specified, the value stored in C<$_> is used. (On some machines,
426unfortunately, the elapsed time may be up to one second less or more
427than you specified because of how seconds are counted, and process
428scheduling may delay the delivery of the signal even further.)
429
430Only one timer may be counting at once. Each call disables the
431previous timer, and an argument of C<0> may be supplied to cancel the
432previous timer without starting a new one. The returned value is the
433amount of time remaining on the previous timer.
a0d0e21e 434
4633a7c4 435For delays of finer granularity than one second, you may use Perl's
19799a22
GS
436four-argument version of select() leaving the first three arguments
437undefined, or you might be able to use the C<syscall> interface to
83df6a1d
JH
438access setitimer(2) if your system supports it. The Time::HiRes
439module (from CPAN, and starting from Perl 5.8 part of the standard
440distribution) may also prove useful.
2b5ab1e7 441
68f8bed4
JH
442It is usually a mistake to intermix C<alarm> and C<sleep> calls.
443(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 444
19799a22
GS
445If you want to use C<alarm> to time out a system call you need to use an
446C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 447fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 448restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 449modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
450
451 eval {
f86cebdf 452 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 453 alarm $timeout;
ff68c719 454 $nread = sysread SOCKET, $buffer, $size;
36477c24 455 alarm 0;
ff68c719 456 };
ff68c719 457 if ($@) {
f86cebdf 458 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719
PP
459 # timed out
460 }
461 else {
462 # didn't
463 }
464
91d81acc
JH
465For more information see L<perlipc>.
466
a0d0e21e 467=item atan2 Y,X
d74e8afc 468X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
469
470Returns the arctangent of Y/X in the range -PI to PI.
471
ca6e1c26 472For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
473function, or use the familiar relation:
474
475 sub tan { sin($_[0]) / cos($_[0]) }
476
bf5f1b4c
JH
477Note that atan2(0, 0) is not well-defined.
478
a0d0e21e 479=item bind SOCKET,NAME
d74e8afc 480X<bind>
a0d0e21e
LW
481
482Binds a network address to a socket, just as the bind system call
19799a22 483does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
484packed address of the appropriate type for the socket. See the examples in
485L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 486
fae2c0fb 487=item binmode FILEHANDLE, LAYER
d74e8afc 488X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 489
a0d0e21e
LW
490=item binmode FILEHANDLE
491
1cbfc93d
NIS
492Arranges for FILEHANDLE to be read or written in "binary" or "text"
493mode on systems where the run-time libraries distinguish between
494binary and text files. If FILEHANDLE is an expression, the value is
495taken as the name of the filehandle. Returns true on success,
b5fe5ca2 496otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 497
d807c6f4
JH
498On some systems (in general, DOS and Windows-based systems) binmode()
499is necessary when you're not working with a text file. For the sake
500of portability it is a good idea to always use it when appropriate,
501and to never use it when it isn't appropriate. Also, people can
502set their I/O to be by default UTF-8 encoded Unicode, not bytes.
503
504In other words: regardless of platform, use binmode() on binary data,
505like for example images.
506
507If LAYER is present it is a single string, but may contain multiple
508directives. The directives alter the behaviour of the file handle.
509When LAYER is present using binmode on text file makes sense.
510
fae2c0fb 511If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
512suitable for passing binary data. This includes turning off possible CRLF
513translation and marking it as bytes (as opposed to Unicode characters).
749683d2
YST
514Note that, despite what may be implied in I<"Programming Perl"> (the
515Camel) or elsewhere, C<:raw> is I<not> the simply inverse of C<:crlf>
fae2c0fb 516-- other layers which would affect binary nature of the stream are
0226bbdb
NIS
517I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
518PERLIO environment variable.
01e6739c 519
d807c6f4
JH
520The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
521form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
522establish default I/O layers. See L<open>.
523
fae2c0fb
RGS
524I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
525in "Programming Perl, 3rd Edition". However, since the publishing of this
526book, by many known as "Camel III", the consensus of the naming of this
527functionality has moved from "discipline" to "layer". All documentation
528of this version of Perl therefore refers to "layers" rather than to
529"disciplines". Now back to the regularly scheduled documentation...>
530
01e6739c 531To mark FILEHANDLE as UTF-8, use C<:utf8>.
1cbfc93d 532
ed53a2bb 533In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
534is done on the filehandle. Calling binmode() will normally flush any
535pending buffered output data (and perhaps pending input data) on the
fae2c0fb 536handle. An exception to this is the C<:encoding> layer that
01e6739c 537changes the default character encoding of the handle, see L<open>.
fae2c0fb 538The C<:encoding> layer sometimes needs to be called in
3874323d
JH
539mid-stream, and it doesn't flush the stream. The C<:encoding>
540also implicitly pushes on top of itself the C<:utf8> layer because
541internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 542
19799a22 543The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
544system all work together to let the programmer treat a single
545character (C<\n>) as the line terminator, irrespective of the external
546representation. On many operating systems, the native text file
547representation matches the internal representation, but on some
548platforms the external representation of C<\n> is made up of more than
549one character.
550
68bd7414
NIS
551Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
552character to end each line in the external representation of text (even
5e12dbfa 553though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
554on Unix and most VMS files). In other systems like OS/2, DOS and the
555various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
556but what's stored in text files are the two characters C<\cM\cJ>. That
557means that, if you don't use binmode() on these systems, C<\cM\cJ>
558sequences on disk will be converted to C<\n> on input, and any C<\n> in
559your program will be converted back to C<\cM\cJ> on output. This is what
560you want for text files, but it can be disastrous for binary files.
30168b04
GS
561
562Another consequence of using binmode() (on some systems) is that
563special end-of-file markers will be seen as part of the data stream.
564For systems from the Microsoft family this means that if your binary
4375e838 565data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
566the file, unless you use binmode().
567
568binmode() is not only important for readline() and print() operations,
569but also when using read(), seek(), sysread(), syswrite() and tell()
570(see L<perlport> for more details). See the C<$/> and C<$\> variables
571in L<perlvar> for how to manually set your input and output
572line-termination sequences.
a0d0e21e 573
4633a7c4 574=item bless REF,CLASSNAME
d74e8afc 575X<bless>
a0d0e21e
LW
576
577=item bless REF
578
2b5ab1e7
TC
579This function tells the thingy referenced by REF that it is now an object
580in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 581is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 582it returns the reference for convenience. Always use the two-argument
cf264981
SP
583version if a derived class might inherit the function doing the blessing.
584See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
585of objects.
a0d0e21e 586
57668c4d 587Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 588Namespaces with all lowercase names are considered reserved for
cf264981 589Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
590confusion, you may wish to avoid such package names as well. Make sure
591that CLASSNAME is a true value.
60ad88b8
GS
592
593See L<perlmod/"Perl Modules">.
594
a0d0e21e 595=item caller EXPR
d74e8afc 596X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
597
598=item caller
599
5a964f20 600Returns the context of the current subroutine call. In scalar context,
28757baa 601returns the caller's package name if there is a caller, that is, if
19799a22 602we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 603otherwise. In list context, returns
a0d0e21e 604
748a9306 605 ($package, $filename, $line) = caller;
a0d0e21e
LW
606
607With EXPR, it returns some extra information that the debugger uses to
608print a stack trace. The value of EXPR indicates how many call frames
609to go back before the current one.
610
f3aa04c2 611 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 612 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 613
951ba7fe 614Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 615call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 616C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 617C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 618C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 619$filename is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
620each C<use> statement creates a C<require> frame inside an C<eval EXPR>
621frame.) $subroutine may also be C<(unknown)> if this particular
622subroutine happens to have been deleted from the symbol table.
623C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
624C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
625compiled with. The C<$hints> and C<$bitmask> values are subject to change
626between versions of Perl, and are not meant for external use.
748a9306
LW
627
628Furthermore, when called from within the DB package, caller returns more
7660c0ab 629detailed information: it sets the list variable C<@DB::args> to be the
54310121 630arguments with which the subroutine was invoked.
748a9306 631
7660c0ab 632Be aware that the optimizer might have optimized call frames away before
19799a22 633C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 634might not return information about the call frame you expect it do, for
b76cc8ba 635C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 636previous time C<caller> was called.
7660c0ab 637
a0d0e21e 638=item chdir EXPR
d74e8afc
ITB
639X<chdir>
640X<cd>
a0d0e21e 641
c4aca7d0
GA
642=item chdir FILEHANDLE
643
644=item chdir DIRHANDLE
645
ce2984c3
PF
646=item chdir
647
ffce7b87 648Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 649changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 650changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
651variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
652neither is set, C<chdir> does nothing. It returns true upon success,
653false otherwise. See the example under C<die>.
a0d0e21e 654
c4aca7d0
GA
655On systems that support fchdir, you might pass a file handle or
656directory handle as argument. On systems that don't support fchdir,
657passing handles produces a fatal error at run time.
658
a0d0e21e 659=item chmod LIST
d74e8afc 660X<chmod> X<permission> X<mode>
a0d0e21e
LW
661
662Changes the permissions of a list of files. The first element of the
4633a7c4 663list must be the numerical mode, which should probably be an octal
4ad40acf 664number, and which definitely should I<not> be a string of octal digits:
2f9daede 665C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 666successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
667
668 $cnt = chmod 0755, 'foo', 'bar';
669 chmod 0755, @executables;
f86cebdf
GS
670 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
671 # --w----r-T
2f9daede
TPG
672 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
673 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 674
c4aca7d0
GA
675On systems that support fchmod, you might pass file handles among the
676files. On systems that don't support fchmod, passing file handles
677produces a fatal error at run time.
678
679 open(my $fh, "<", "foo");
680 my $perm = (stat $fh)[2] & 07777;
681 chmod($perm | 0600, $fh);
682
ca6e1c26
JH
683You can also import the symbolic C<S_I*> constants from the Fcntl
684module:
685
686 use Fcntl ':mode';
687
688 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
689 # This is identical to the chmod 0755 of the above example.
690
a0d0e21e 691=item chomp VARIABLE
d74e8afc 692X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 693
313c9f5c 694=item chomp( LIST )
a0d0e21e
LW
695
696=item chomp
697
2b5ab1e7
TC
698This safer version of L</chop> removes any trailing string
699that corresponds to the current value of C<$/> (also known as
28757baa
PP
700$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
701number of characters removed from all its arguments. It's often used to
702remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
703that the final record may be missing its newline. When in paragraph
704mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
705When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
706a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 707remove anything.
19799a22 708If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
709
710 while (<>) {
711 chomp; # avoid \n on last field
712 @array = split(/:/);
5a964f20 713 # ...
a0d0e21e
LW
714 }
715
4bf21a6d
RD
716If VARIABLE is a hash, it chomps the hash's values, but not its keys.
717
a0d0e21e
LW
718You can actually chomp anything that's an lvalue, including an assignment:
719
720 chomp($cwd = `pwd`);
721 chomp($answer = <STDIN>);
722
723If you chomp a list, each element is chomped, and the total number of
724characters removed is returned.
725
442a8c12
NC
726If the C<encoding> pragma is in scope then the lengths returned are
727calculated from the length of C<$/> in Unicode characters, which is not
728always the same as the length of C<$/> in the native encoding.
729
15e44fd8
RGS
730Note that parentheses are necessary when you're chomping anything
731that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
732is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
733C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
734C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
735as C<chomp($a, $b)>.
736
a0d0e21e 737=item chop VARIABLE
d74e8afc 738X<chop>
a0d0e21e 739
313c9f5c 740=item chop( LIST )
a0d0e21e
LW
741
742=item chop
743
744Chops off the last character of a string and returns the character
5b3eff12 745chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 746scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
747If VARIABLE is a hash, it chops the hash's values, but not its keys.
748
5b3eff12 749You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
750
751If you chop a list, each element is chopped. Only the value of the
19799a22 752last C<chop> is returned.
a0d0e21e 753
19799a22 754Note that C<chop> returns the last character. To return all but the last
748a9306
LW
755character, use C<substr($string, 0, -1)>.
756
15e44fd8
RGS
757See also L</chomp>.
758
a0d0e21e 759=item chown LIST
d74e8afc 760X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
761
762Changes the owner (and group) of a list of files. The first two
19799a22
GS
763elements of the list must be the I<numeric> uid and gid, in that
764order. A value of -1 in either position is interpreted by most
765systems to leave that value unchanged. Returns the number of files
766successfully changed.
a0d0e21e
LW
767
768 $cnt = chown $uid, $gid, 'foo', 'bar';
769 chown $uid, $gid, @filenames;
770
c4aca7d0
GA
771On systems that support fchown, you might pass file handles among the
772files. On systems that don't support fchown, passing file handles
773produces a fatal error at run time.
774
54310121 775Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
776
777 print "User: ";
19799a22 778 chomp($user = <STDIN>);
5a964f20 779 print "Files: ";
19799a22 780 chomp($pattern = <STDIN>);
a0d0e21e
LW
781
782 ($login,$pass,$uid,$gid) = getpwnam($user)
783 or die "$user not in passwd file";
784
5a964f20 785 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
786 chown $uid, $gid, @ary;
787
54310121 788On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
789file unless you're the superuser, although you should be able to change
790the group to any of your secondary groups. On insecure systems, these
791restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
792On POSIX systems, you can detect this condition this way:
793
794 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
795 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 796
a0d0e21e 797=item chr NUMBER
d74e8afc 798X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 799
54310121 800=item chr
bbce6d69 801
a0d0e21e 802Returns the character represented by that NUMBER in the character set.
a0ed51b3 803For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1e54db1a
JH
804chr(0x263a) is a Unicode smiley face. Note that characters from 128
805to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
806backward compatibility reasons (but see L<encoding>).
aaa68c4a 807
8a064bd6 808Negative values give the Unicode replacement character (chr(0xfffd)),
5f0135eb 809except under the L<bytes> pragma, where low eight bits of the value
8a064bd6
JH
810(truncated to an integer) are used.
811
974da8e5
JH
812If NUMBER is omitted, uses C<$_>.
813
b76cc8ba 814For the reverse, use L</ord>.
a0d0e21e 815
974da8e5
JH
816Note that under the C<bytes> pragma the NUMBER is masked to
817the low eight bits.
818
819See L<perlunicode> and L<encoding> for more about Unicode.
bbce6d69 820
a0d0e21e 821=item chroot FILENAME
d74e8afc 822X<chroot> X<root>
a0d0e21e 823
54310121 824=item chroot
bbce6d69 825
5a964f20 826This function works like the system call by the same name: it makes the
4633a7c4 827named directory the new root directory for all further pathnames that
951ba7fe 828begin with a C</> by your process and all its children. (It doesn't
28757baa 829change your current working directory, which is unaffected.) For security
4633a7c4 830reasons, this call is restricted to the superuser. If FILENAME is
19799a22 831omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
832
833=item close FILEHANDLE
d74e8afc 834X<close>
a0d0e21e 835
6a518fbc
TP
836=item close
837
9124316e
JH
838Closes the file or pipe associated with the file handle, returning
839true only if IO buffers are successfully flushed and closes the system
840file descriptor. Closes the currently selected filehandle if the
841argument is omitted.
fb73857a
PP
842
843You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
844another C<open> on it, because C<open> will close it for you. (See
845C<open>.) However, an explicit C<close> on an input file resets the line
846counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 847
dede8123
RGS
848If the file handle came from a piped open, C<close> will additionally
849return false if one of the other system calls involved fails, or if the
fb73857a 850program exits with non-zero status. (If the only problem was that the
dede8123 851program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 852also waits for the process executing on the pipe to complete, in case you
b76cc8ba 853want to look at the output of the pipe afterwards, and
e5218da5
GA
854implicitly puts the exit status value of that command into C<$?> and
855C<${^CHILD_ERROR_NATIVE}>.
5a964f20 856
73689b13
GS
857Prematurely closing the read end of a pipe (i.e. before the process
858writing to it at the other end has closed it) will result in a
859SIGPIPE being delivered to the writer. If the other end can't
860handle that, be sure to read all the data before closing the pipe.
861
fb73857a 862Example:
a0d0e21e 863
fb73857a
PP
864 open(OUTPUT, '|sort >foo') # pipe to sort
865 or die "Can't start sort: $!";
5a964f20 866 #... # print stuff to output
fb73857a
PP
867 close OUTPUT # wait for sort to finish
868 or warn $! ? "Error closing sort pipe: $!"
869 : "Exit status $? from sort";
870 open(INPUT, 'foo') # get sort's results
871 or die "Can't open 'foo' for input: $!";
a0d0e21e 872
5a964f20
TC
873FILEHANDLE may be an expression whose value can be used as an indirect
874filehandle, usually the real filehandle name.
a0d0e21e
LW
875
876=item closedir DIRHANDLE
d74e8afc 877X<closedir>
a0d0e21e 878
19799a22 879Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
880system call.
881
a0d0e21e 882=item connect SOCKET,NAME
d74e8afc 883X<connect>
a0d0e21e
LW
884
885Attempts to connect to a remote socket, just as the connect system call
19799a22 886does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
887packed address of the appropriate type for the socket. See the examples in
888L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 889
cb1a09d0 890=item continue BLOCK
d74e8afc 891X<continue>
cb1a09d0 892
cf264981
SP
893C<continue> is actually a flow control statement rather than a function. If
894there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
895C<foreach>), it is always executed just before the conditional is about to
896be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
897it can be used to increment a loop variable, even when the loop has been
898continued via the C<next> statement (which is similar to the C C<continue>
899statement).
900
98293880 901C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
902block. C<last> and C<redo> will behave as if they had been executed within
903the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
904block, it may be more entertaining.
905
906 while (EXPR) {
907 ### redo always comes here
908 do_something;
909 } continue {
910 ### next always comes here
911 do_something_else;
912 # then back the top to re-check EXPR
913 }
914 ### last always comes here
915
916Omitting the C<continue> section is semantically equivalent to using an
19799a22 917empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
918to check the condition at the top of the loop.
919
a0d0e21e 920=item cos EXPR
d74e8afc 921X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 922
d6217f1e
GS
923=item cos
924
5a964f20 925Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 926takes cosine of C<$_>.
a0d0e21e 927
ca6e1c26 928For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
929function, or use this relation:
930
931 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
932
a0d0e21e 933=item crypt PLAINTEXT,SALT
d74e8afc
ITB
934X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
935X<decrypt> X<cryptography> X<passwd>
a0d0e21e 936
ef2e6798
MS
937Creates a digest string exactly like the crypt(3) function in the C
938library (assuming that you actually have a version there that has not
cf264981 939been extirpated as a potential munitions).
ef2e6798
MS
940
941crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
942into a short string, called a digest, which is returned. The same
943PLAINTEXT and SALT will always return the same string, but there is no
944(known) way to get the original PLAINTEXT from the hash. Small
945changes in the PLAINTEXT or SALT will result in large changes in the
946digest.
947
948There is no decrypt function. This function isn't all that useful for
949cryptography (for that, look for F<Crypt> modules on your nearby CPAN
950mirror) and the name "crypt" is a bit of a misnomer. Instead it is
951primarily used to check if two pieces of text are the same without
952having to transmit or store the text itself. An example is checking
953if a correct password is given. The digest of the password is stored,
cf264981 954not the password itself. The user types in a password that is
ef2e6798
MS
955crypt()'d with the same salt as the stored digest. If the two digests
956match the password is correct.
957
958When verifying an existing digest string you should use the digest as
959the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 960to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
961crypt() will hash the new string with the same salt as the digest.
962This allows your code to work with the standard L<crypt|/crypt> and
963with more exotic implementations. In other words, do not assume
964anything about the returned string itself, or how many bytes in the
965digest matter.
85c16d83
JH
966
967Traditionally the result is a string of 13 bytes: two first bytes of
968the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
ef2e6798
MS
969the first eight bytes of the digest string mattered, but alternative
970hashing schemes (like MD5), higher level security schemes (like C2),
971and implementations on non-UNIX platforms may produce different
972strings.
85c16d83
JH
973
974When choosing a new salt create a random two character string whose
975characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
976'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
977characters is just a recommendation; the characters allowed in
978the salt depend solely on your system's crypt library, and Perl can't
979restrict what salts C<crypt()> accepts.
e71965be 980
a0d0e21e 981Here's an example that makes sure that whoever runs this program knows
cf264981 982their password:
a0d0e21e
LW
983
984 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
985
986 system "stty -echo";
987 print "Password: ";
e71965be 988 chomp($word = <STDIN>);
a0d0e21e
LW
989 print "\n";
990 system "stty echo";
991
e71965be 992 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
993 die "Sorry...\n";
994 } else {
995 print "ok\n";
54310121 996 }
a0d0e21e 997
9f8f0c9d 998Of course, typing in your own password to whoever asks you
748a9306 999for it is unwise.
a0d0e21e 1000
ef2e6798 1001The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1002of data, not least of all because you can't get the information
ef2e6798 1003back. Look at the L<Digest> module for more robust algorithms.
19799a22 1004
f2791508
JH
1005If using crypt() on a Unicode string (which I<potentially> has
1006characters with codepoints above 255), Perl tries to make sense
1007of the situation by trying to downgrade (a copy of the string)
1008the string back to an eight-bit byte string before calling crypt()
1009(on that copy). If that works, good. If not, crypt() dies with
1010C<Wide character in crypt>.
85c16d83 1011
aa689395 1012=item dbmclose HASH
d74e8afc 1013X<dbmclose>
a0d0e21e 1014
19799a22 1015[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1016
aa689395 1017Breaks the binding between a DBM file and a hash.
a0d0e21e 1018
19799a22 1019=item dbmopen HASH,DBNAME,MASK
d74e8afc 1020X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1021
19799a22 1022[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1023
7b8d334a 1024This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1025hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1026argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1027is the name of the database (without the F<.dir> or F<.pag> extension if
1028any). If the database does not exist, it is created with protection
19799a22
GS
1029specified by MASK (as modified by the C<umask>). If your system supports
1030only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 1031program. In older versions of Perl, if your system had neither DBM nor
19799a22 1032ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1033sdbm(3).
1034
1035If you don't have write access to the DBM file, you can only read hash
1036variables, not set them. If you want to test whether you can write,
19799a22 1037either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 1038which will trap the error.
a0d0e21e 1039
19799a22
GS
1040Note that functions such as C<keys> and C<values> may return huge lists
1041when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1042function to iterate over large DBM files. Example:
1043
1044 # print out history file offsets
1045 dbmopen(%HIST,'/usr/lib/news/history',0666);
1046 while (($key,$val) = each %HIST) {
1047 print $key, ' = ', unpack('L',$val), "\n";
1048 }
1049 dbmclose(%HIST);
1050
cb1a09d0 1051See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1052cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1053rich implementation.
4633a7c4 1054
2b5ab1e7
TC
1055You can control which DBM library you use by loading that library
1056before you call dbmopen():
1057
1058 use DB_File;
1059 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1060 or die "Can't open netscape history file: $!";
1061
a0d0e21e 1062=item defined EXPR
d74e8afc 1063X<defined> X<undef> X<undefined>
a0d0e21e 1064
54310121 1065=item defined
bbce6d69 1066
2f9daede
TPG
1067Returns a Boolean value telling whether EXPR has a value other than
1068the undefined value C<undef>. If EXPR is not present, C<$_> will be
1069checked.
1070
1071Many operations return C<undef> to indicate failure, end of file,
1072system error, uninitialized variable, and other exceptional
1073conditions. This function allows you to distinguish C<undef> from
1074other values. (A simple Boolean test will not distinguish among
7660c0ab 1075C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1076false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1077doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1078returns C<undef> when its argument is an empty array, I<or> when the
1079element to return happens to be C<undef>.
1080
f10b0346
GS
1081You may also use C<defined(&func)> to check whether subroutine C<&func>
1082has ever been defined. The return value is unaffected by any forward
04891299 1083declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1084may still be callable: its package may have an C<AUTOLOAD> method that
1085makes it spring into existence the first time that it is called -- see
1086L<perlsub>.
f10b0346
GS
1087
1088Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1089used to report whether memory for that aggregate has ever been
1090allocated. This behavior may disappear in future versions of Perl.
1091You should instead use a simple test for size:
1092
1093 if (@an_array) { print "has array elements\n" }
1094 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1095
1096When used on a hash element, it tells you whether the value is defined,
dc848c6f 1097not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1098purpose.
a0d0e21e
LW
1099
1100Examples:
1101
1102 print if defined $switch{'D'};
1103 print "$val\n" while defined($val = pop(@ary));
1104 die "Can't readlink $sym: $!"
1105 unless defined($value = readlink $sym);
a0d0e21e 1106 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1107 $debugging = 0 unless defined $debugging;
a0d0e21e 1108
19799a22 1109Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1110discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1111defined values. For example, if you say
a5f75d66
AD
1112
1113 "ab" =~ /a(.*)b/;
1114
7660c0ab 1115The pattern match succeeds, and C<$1> is defined, despite the fact that it
cf264981 1116matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1117matched something that happened to be zero characters long. This is all
a5f75d66 1118very above-board and honest. When a function returns an undefined value,
2f9daede 1119it's an admission that it couldn't give you an honest answer. So you
19799a22 1120should use C<defined> only when you're questioning the integrity of what
7660c0ab 1121you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1122what you want.
1123
dc848c6f 1124See also L</undef>, L</exists>, L</ref>.
2f9daede 1125
a0d0e21e 1126=item delete EXPR
d74e8afc 1127X<delete>
a0d0e21e 1128
01020589
GS
1129Given an expression that specifies a hash element, array element, hash slice,
1130or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1131In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1132the size of the array will shrink to the highest element that tests
8216c1fd 1133true for exists() (or 0 if no such element exists).
a0d0e21e 1134
eba0920a
EM
1135Returns a list with the same number of elements as the number of elements
1136for which deletion was attempted. Each element of that list consists of
1137either the value of the element deleted, or the undefined value. In scalar
1138context, this means that you get the value of the last element deleted (or
1139the undefined value if that element did not exist).
1140
1141 %hash = (foo => 11, bar => 22, baz => 33);
1142 $scalar = delete $hash{foo}; # $scalar is 11
1143 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1144 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1145
1146Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1147a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1148from a C<tie>d hash or array may not necessarily return anything.
1149
8ea97a1e
GS
1150Deleting an array element effectively returns that position of the array
1151to its initial, uninitialized state. Subsequently testing for the same
cf264981
SP
1152element with exists() will return false. Also, deleting array elements
1153in the middle of an array will not shift the index of the elements
1154after them down. Use splice() for that. See L</exists>.
8ea97a1e 1155
01020589 1156The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1157
5f05dabc
PP
1158 foreach $key (keys %HASH) {
1159 delete $HASH{$key};
a0d0e21e
LW
1160 }
1161
01020589
GS
1162 foreach $index (0 .. $#ARRAY) {
1163 delete $ARRAY[$index];
1164 }
1165
1166And so do these:
5f05dabc 1167
01020589
GS
1168 delete @HASH{keys %HASH};
1169
9740c838 1170 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1171
2b5ab1e7 1172But both of these are slower than just assigning the empty list
01020589
GS
1173or undefining %HASH or @ARRAY:
1174
1175 %HASH = (); # completely empty %HASH
1176 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1177
01020589
GS
1178 @ARRAY = (); # completely empty @ARRAY
1179 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1180
1181Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1182operation is a hash element, array element, hash slice, or array slice
1183lookup:
a0d0e21e
LW
1184
1185 delete $ref->[$x][$y]{$key};
5f05dabc 1186 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1187
01020589
GS
1188 delete $ref->[$x][$y][$index];
1189 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1190
a0d0e21e 1191=item die LIST
d74e8afc 1192X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1193
19799a22
GS
1194Outside an C<eval>, prints the value of LIST to C<STDERR> and
1195exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1196exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1197status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1198an C<eval(),> the error message is stuffed into C<$@> and the
1199C<eval> is terminated with the undefined value. This makes
1200C<die> the way to raise an exception.
a0d0e21e
LW
1201
1202Equivalent examples:
1203
1204 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1205 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1206
ccac6780 1207If the last element of LIST does not end in a newline, the current
df37ec69
WW
1208script line number and input line number (if any) are also printed,
1209and a newline is supplied. Note that the "input line number" (also
1210known as "chunk") is subject to whatever notion of "line" happens to
1211be currently in effect, and is also available as the special variable
1212C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1213
1214Hint: sometimes appending C<", stopped"> to your message will cause it
1215to make better sense when the string C<"at foo line 123"> is appended.
1216Suppose you are running script "canasta".
a0d0e21e
LW
1217
1218 die "/etc/games is no good";
1219 die "/etc/games is no good, stopped";
1220
1221produce, respectively
1222
1223 /etc/games is no good at canasta line 123.
1224 /etc/games is no good, stopped at canasta line 123.
1225
2b5ab1e7 1226See also exit(), warn(), and the Carp module.
a0d0e21e 1227
7660c0ab
A
1228If LIST is empty and C<$@> already contains a value (typically from a
1229previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1230This is useful for propagating exceptions:
1231
1232 eval { ... };
1233 die unless $@ =~ /Expected exception/;
1234
ad216e65
JH
1235If LIST is empty and C<$@> contains an object reference that has a
1236C<PROPAGATE> method, that method will be called with additional file
1237and line number parameters. The return value replaces the value in
28a5cf3b 1238C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1239were called.
1240
7660c0ab 1241If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1242
52531d10
GS
1243die() can also be called with a reference argument. If this happens to be
1244trapped within an eval(), $@ contains the reference. This behavior permits
1245a more elaborate exception handling implementation using objects that
4375e838 1246maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1247is sometimes preferable to matching particular string values of $@ using
1248regular expressions. Here's an example:
1249
da279afe 1250 use Scalar::Util 'blessed';
1251
52531d10
GS
1252 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1253 if ($@) {
da279afe 1254 if (blessed($@) && $@->isa("Some::Module::Exception")) {
52531d10
GS
1255 # handle Some::Module::Exception
1256 }
1257 else {
1258 # handle all other possible exceptions
1259 }
1260 }
1261
19799a22 1262Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1263them, you may want to overload stringification operations on such custom
1264exception objects. See L<overload> for details about that.
1265
19799a22
GS
1266You can arrange for a callback to be run just before the C<die>
1267does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1268handler will be called with the error text and can change the error
1269message, if it sees fit, by calling C<die> again. See
1270L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1271L<"eval BLOCK"> for some examples. Although this feature was
19799a22
GS
1272to be run only right before your program was to exit, this is not
1273currently the case--the C<$SIG{__DIE__}> hook is currently called
1274even inside eval()ed blocks/strings! If one wants the hook to do
1275nothing in such situations, put
fb73857a
PP
1276
1277 die @_ if $^S;
1278
19799a22
GS
1279as the first line of the handler (see L<perlvar/$^S>). Because
1280this promotes strange action at a distance, this counterintuitive
b76cc8ba 1281behavior may be fixed in a future release.
774d564b 1282
a0d0e21e 1283=item do BLOCK
d74e8afc 1284X<do> X<block>
a0d0e21e
LW
1285
1286Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1287sequence of commands indicated by BLOCK. When modified by the C<while> or
1288C<until> loop modifier, executes the BLOCK once before testing the loop
1289condition. (On other statements the loop modifiers test the conditional
1290first.)
a0d0e21e 1291
4968c1e4 1292C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1293C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1294See L<perlsyn> for alternative strategies.
4968c1e4 1295
a0d0e21e 1296=item do SUBROUTINE(LIST)
d74e8afc 1297X<do>
a0d0e21e 1298
cf264981 1299This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1300
1301=item do EXPR
d74e8afc 1302X<do>
a0d0e21e
LW
1303
1304Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1305file as a Perl script.
a0d0e21e
LW
1306
1307 do 'stat.pl';
1308
1309is just like
1310
986b19de 1311 eval `cat stat.pl`;
a0d0e21e 1312
2b5ab1e7 1313except that it's more efficient and concise, keeps track of the current
ea63ef19 1314filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1315C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1316variables. It also differs in that code evaluated with C<do FILENAME>
1317cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1318same, however, in that it does reparse the file every time you call it,
1319so you probably don't want to do this inside a loop.
a0d0e21e 1320
8e30cc93 1321If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1322error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1323returns undef and sets an error message in C<$@>. If the file is
1324successfully compiled, C<do> returns the value of the last expression
1325evaluated.
1326
a0d0e21e 1327Note that inclusion of library modules is better done with the
19799a22 1328C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1329and raise an exception if there's a problem.
a0d0e21e 1330
5a964f20
TC
1331You might like to use C<do> to read in a program configuration
1332file. Manual error checking can be done this way:
1333
b76cc8ba 1334 # read in config files: system first, then user
f86cebdf 1335 for $file ("/share/prog/defaults.rc",
b76cc8ba 1336 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1337 {
5a964f20 1338 unless ($return = do $file) {
f86cebdf
GS
1339 warn "couldn't parse $file: $@" if $@;
1340 warn "couldn't do $file: $!" unless defined $return;
1341 warn "couldn't run $file" unless $return;
5a964f20
TC
1342 }
1343 }
1344
a0d0e21e 1345=item dump LABEL
d74e8afc 1346X<dump> X<core> X<undump>
a0d0e21e 1347
1614b0e3
JD
1348=item dump
1349
19799a22
GS
1350This function causes an immediate core dump. See also the B<-u>
1351command-line switch in L<perlrun>, which does the same thing.
1352Primarily this is so that you can use the B<undump> program (not
1353supplied) to turn your core dump into an executable binary after
1354having initialized all your variables at the beginning of the
1355program. When the new binary is executed it will begin by executing
1356a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1357Think of it as a goto with an intervening core dump and reincarnation.
1358If C<LABEL> is omitted, restarts the program from the top.
1359
1360B<WARNING>: Any files opened at the time of the dump will I<not>
1361be open any more when the program is reincarnated, with possible
b76cc8ba 1362resulting confusion on the part of Perl.
19799a22
GS
1363
1364This function is now largely obsolete, partly because it's very
1365hard to convert a core file into an executable, and because the
1366real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1367C code have superseded it. That's why you should now invoke it as
1368C<CORE::dump()>, if you don't want to be warned against a possible
1369typo.
19799a22
GS
1370
1371If you're looking to use L<dump> to speed up your program, consider
1372generating bytecode or native C code as described in L<perlcc>. If
1373you're just trying to accelerate a CGI script, consider using the
210b36aa 1374C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1375You might also consider autoloading or selfloading, which at least
b76cc8ba 1376make your program I<appear> to run faster.
5a964f20 1377
aa689395 1378=item each HASH
d74e8afc 1379X<each> X<hash, iterator>
aa689395 1380
5a964f20 1381When called in list context, returns a 2-element list consisting of the
aa689395 1382key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1383it. When called in scalar context, returns only the key for the next
e902a979 1384element in the hash.
2f9daede 1385
ab192400 1386Entries are returned in an apparently random order. The actual random
504f80c1
JH
1387order is subject to change in future versions of perl, but it is
1388guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1389function would produce on the same (unmodified) hash. Since Perl
13905.8.1 the ordering is different even between different runs of Perl
1391for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1392
1393When the hash is entirely read, a null array is returned in list context
19799a22
GS
1394(which when assigned produces a false (C<0>) value), and C<undef> in
1395scalar context. The next call to C<each> after that will start iterating
1396again. There is a single iterator for each hash, shared by all C<each>,
1397C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1398reading all the elements from the hash, or by evaluating C<keys HASH> or
1399C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1400iterating over it, you may get entries skipped or duplicated, so
1401don't. Exception: It is always safe to delete the item most recently
1402returned by C<each()>, which means that the following code will work:
1403
1404 while (($key, $value) = each %hash) {
1405 print $key, "\n";
1406 delete $hash{$key}; # This is safe
1407 }
aa689395 1408
f86cebdf 1409The following prints out your environment like the printenv(1) program,
aa689395 1410only in a different order:
a0d0e21e
LW
1411
1412 while (($key,$value) = each %ENV) {
1413 print "$key=$value\n";
1414 }
1415
19799a22 1416See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1417
1418=item eof FILEHANDLE
d74e8afc
ITB
1419X<eof>
1420X<end of file>
1421X<end-of-file>
a0d0e21e 1422
4633a7c4
LW
1423=item eof ()
1424
a0d0e21e
LW
1425=item eof
1426
1427Returns 1 if the next read on FILEHANDLE will return end of file, or if
1428FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1429gives the real filehandle. (Note that this function actually
19799a22 1430reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1431interactive context.) Do not read from a terminal file (or call
19799a22 1432C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1433as terminals may lose the end-of-file condition if you do.
1434
820475bd
GS
1435An C<eof> without an argument uses the last file read. Using C<eof()>
1436with empty parentheses is very different. It refers to the pseudo file
1437formed from the files listed on the command line and accessed via the
61eff3bc
JH
1438C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1439as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1440used will cause C<@ARGV> to be examined to determine if input is
67408cae 1441available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1442end-of-file will assume you are processing another C<@ARGV> list,
1443and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1444see L<perlop/"I/O Operators">.
820475bd 1445
61eff3bc 1446In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1447detect the end of each file, C<eof()> will only detect the end of the
1448last file. Examples:
a0d0e21e 1449
748a9306
LW
1450 # reset line numbering on each input file
1451 while (<>) {
b76cc8ba 1452 next if /^\s*#/; # skip comments
748a9306 1453 print "$.\t$_";
5a964f20
TC
1454 } continue {
1455 close ARGV if eof; # Not eof()!
748a9306
LW
1456 }
1457
a0d0e21e
LW
1458 # insert dashes just before last line of last file
1459 while (<>) {
6ac88b13 1460 if (eof()) { # check for end of last file
a0d0e21e
LW
1461 print "--------------\n";
1462 }
1463 print;
6ac88b13 1464 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1465 }
1466
a0d0e21e 1467Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1468input operators typically return C<undef> when they run out of data, or if
1469there was an error.
a0d0e21e
LW
1470
1471=item eval EXPR
d74e8afc 1472X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
a0d0e21e
LW
1473
1474=item eval BLOCK
1475
ce2984c3
PF
1476=item eval
1477
c7cc6f1c
GS
1478In the first form, the return value of EXPR is parsed and executed as if it
1479were a little Perl program. The value of the expression (which is itself
5a964f20 1480determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1481errors, executed in the lexical context of the current Perl program, so
1482that any variable settings or subroutine and format definitions remain
cf264981 1483afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1484If EXPR is omitted, evaluates C<$_>. This form is typically used to
1485delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1486
1487In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1488same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1489within the context of the current Perl program. This form is typically
1490used to trap exceptions more efficiently than the first (see below), while
1491also providing the benefit of checking the code within BLOCK at compile
1492time.
1493
1494The final semicolon, if any, may be omitted from the value of EXPR or within
1495the BLOCK.
1496
1497In both forms, the value returned is the value of the last expression
5a964f20 1498evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1499as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1500in void, scalar, or list context, depending on the context of the C<eval>
1501itself. See L</wantarray> for more on how the evaluation context can be
1502determined.
a0d0e21e 1503
19799a22
GS
1504If there is a syntax error or runtime error, or a C<die> statement is
1505executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1506error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1507string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1508warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1509To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1510turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1511See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1512
19799a22
GS
1513Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1514determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1515is implemented. It is also Perl's exception trapping mechanism, where
1516the die operator is used to raise exceptions.
1517
1518If the code to be executed doesn't vary, you may use the eval-BLOCK
1519form to trap run-time errors without incurring the penalty of
1520recompiling each time. The error, if any, is still returned in C<$@>.
1521Examples:
1522
54310121 1523 # make divide-by-zero nonfatal
a0d0e21e
LW
1524 eval { $answer = $a / $b; }; warn $@ if $@;
1525
1526 # same thing, but less efficient
1527 eval '$answer = $a / $b'; warn $@ if $@;
1528
1529 # a compile-time error
5a964f20 1530 eval { $answer = }; # WRONG
a0d0e21e
LW
1531
1532 # a run-time error
1533 eval '$answer ='; # sets $@
1534
cf264981
SP
1535Using the C<eval{}> form as an exception trap in libraries does have some
1536issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1537may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7
TC
1538You can use the C<local $SIG{__DIE__}> construct for this purpose,
1539as shown in this example:
774d564b
PP
1540
1541 # a very private exception trap for divide-by-zero
f86cebdf
GS
1542 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1543 warn $@ if $@;
774d564b
PP
1544
1545This is especially significant, given that C<__DIE__> hooks can call
19799a22 1546C<die> again, which has the effect of changing their error messages:
774d564b
PP
1547
1548 # __DIE__ hooks may modify error messages
1549 {
f86cebdf
GS
1550 local $SIG{'__DIE__'} =
1551 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1552 eval { die "foo lives here" };
1553 print $@ if $@; # prints "bar lives here"
774d564b
PP
1554 }
1555
19799a22 1556Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1557may be fixed in a future release.
1558
19799a22 1559With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1560being looked at when:
1561
1562 eval $x; # CASE 1
1563 eval "$x"; # CASE 2
1564
1565 eval '$x'; # CASE 3
1566 eval { $x }; # CASE 4
1567
5a964f20 1568 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1569 $$x++; # CASE 6
1570
2f9daede 1571Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1572the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1573the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1574and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1575does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1576purely visual reasons, but it also has the advantage of compiling at
1577compile-time instead of at run-time.) Case 5 is a place where
19799a22 1578normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1579particular situation, you can just use symbolic references instead, as
1580in case 6.
a0d0e21e 1581
4968c1e4 1582C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1583C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1584
d819b83a
DM
1585Note that as a very special case, an C<eval ''> executed within the C<DB>
1586package doesn't see the usual surrounding lexical scope, but rather the
1587scope of the first non-DB piece of code that called it. You don't normally
1588need to worry about this unless you are writing a Perl debugger.
1589
a0d0e21e 1590=item exec LIST
d74e8afc 1591X<exec> X<execute>
a0d0e21e 1592
8bf3b016
GS
1593=item exec PROGRAM LIST
1594
19799a22
GS
1595The C<exec> function executes a system command I<and never returns>--
1596use C<system> instead of C<exec> if you want it to return. It fails and
1597returns false only if the command does not exist I<and> it is executed
fb73857a 1598directly instead of via your system's command shell (see below).
a0d0e21e 1599
19799a22
GS
1600Since it's a common mistake to use C<exec> instead of C<system>, Perl
1601warns you if there is a following statement which isn't C<die>, C<warn>,
1602or C<exit> (if C<-w> is set - but you always do that). If you
1603I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1604can use one of these styles to avoid the warning:
1605
5a964f20
TC
1606 exec ('foo') or print STDERR "couldn't exec foo: $!";
1607 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1608
5a964f20 1609If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1610with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1611If there is only one scalar argument or an array with one element in it,
1612the argument is checked for shell metacharacters, and if there are any,
1613the entire argument is passed to the system's command shell for parsing
1614(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1615If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1616words and passed directly to C<execvp>, which is more efficient.
19799a22 1617Examples:
a0d0e21e 1618
19799a22
GS
1619 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1620 exec "sort $outfile | uniq";
a0d0e21e
LW
1621
1622If you don't really want to execute the first argument, but want to lie
1623to the program you are executing about its own name, you can specify
1624the program you actually want to run as an "indirect object" (without a
1625comma) in front of the LIST. (This always forces interpretation of the
54310121 1626LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1627the list.) Example:
1628
1629 $shell = '/bin/csh';
1630 exec $shell '-sh'; # pretend it's a login shell
1631
1632or, more directly,
1633
1634 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1635
bb32b41a
GS
1636When the arguments get executed via the system shell, results will
1637be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1638for details.
1639
19799a22
GS
1640Using an indirect object with C<exec> or C<system> is also more
1641secure. This usage (which also works fine with system()) forces
1642interpretation of the arguments as a multivalued list, even if the
1643list had just one argument. That way you're safe from the shell
1644expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1645
1646 @args = ( "echo surprise" );
1647
2b5ab1e7 1648 exec @args; # subject to shell escapes
f86cebdf 1649 # if @args == 1
2b5ab1e7 1650 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1651
1652The first version, the one without the indirect object, ran the I<echo>
1653program, passing it C<"surprise"> an argument. The second version
1654didn't--it tried to run a program literally called I<"echo surprise">,
1655didn't find it, and set C<$?> to a non-zero value indicating failure.
1656
0f897271
GS
1657Beginning with v5.6.0, Perl will attempt to flush all files opened for
1658output before the exec, but this may not be supported on some platforms
1659(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1660in English) or call the C<autoflush()> method of C<IO::Handle> on any
1661open handles in order to avoid lost output.
1662
19799a22 1663Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1664any C<DESTROY> methods in your objects.
1665
a0d0e21e 1666=item exists EXPR
d74e8afc 1667X<exists> X<autovivification>
a0d0e21e 1668
01020589 1669Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1670returns true if the specified element in the hash or array has ever
1671been initialized, even if the corresponding value is undefined. The
1672element is not autovivified if it doesn't exist.
a0d0e21e 1673
01020589
GS
1674 print "Exists\n" if exists $hash{$key};
1675 print "Defined\n" if defined $hash{$key};
1676 print "True\n" if $hash{$key};
1677
1678 print "Exists\n" if exists $array[$index];
1679 print "Defined\n" if defined $array[$index];
1680 print "True\n" if $array[$index];
a0d0e21e 1681
8ea97a1e 1682A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1683it exists, but the reverse doesn't necessarily hold true.
1684
afebc493
GS
1685Given an expression that specifies the name of a subroutine,
1686returns true if the specified subroutine has ever been declared, even
1687if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1688does not count as declaring it. Note that a subroutine which does not
1689exist may still be callable: its package may have an C<AUTOLOAD>
1690method that makes it spring into existence the first time that it is
1691called -- see L<perlsub>.
afebc493
GS
1692
1693 print "Exists\n" if exists &subroutine;
1694 print "Defined\n" if defined &subroutine;
1695
a0d0e21e 1696Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1697operation is a hash or array key lookup or subroutine name:
a0d0e21e 1698
2b5ab1e7
TC
1699 if (exists $ref->{A}->{B}->{$key}) { }
1700 if (exists $hash{A}{B}{$key}) { }
1701
01020589
GS
1702 if (exists $ref->{A}->{B}->[$ix]) { }
1703 if (exists $hash{A}{B}[$ix]) { }
1704
afebc493
GS
1705 if (exists &{$ref->{A}{B}{$key}}) { }
1706
01020589
GS
1707Although the deepest nested array or hash will not spring into existence
1708just because its existence was tested, any intervening ones will.
61eff3bc 1709Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1710into existence due to the existence test for the $key element above.
1711This happens anywhere the arrow operator is used, including even:
5a964f20 1712
2b5ab1e7
TC
1713 undef $ref;
1714 if (exists $ref->{"Some key"}) { }
1715 print $ref; # prints HASH(0x80d3d5c)
1716
1717This surprising autovivification in what does not at first--or even
1718second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1719release.
a0d0e21e 1720
afebc493
GS
1721Use of a subroutine call, rather than a subroutine name, as an argument
1722to exists() is an error.
1723
1724 exists &sub; # OK
1725 exists &sub(); # Error
1726
a0d0e21e 1727=item exit EXPR
d74e8afc 1728X<exit> X<terminate> X<abort>
a0d0e21e 1729
ce2984c3
PF
1730=item exit
1731
2b5ab1e7 1732Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1733
1734 $ans = <STDIN>;
1735 exit 0 if $ans =~ /^[Xx]/;
1736
19799a22 1737See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1738universally recognized values for EXPR are C<0> for success and C<1>
1739for error; other values are subject to interpretation depending on the
1740environment in which the Perl program is running. For example, exiting
174169 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1742the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1743
19799a22
GS
1744Don't use C<exit> to abort a subroutine if there's any chance that
1745someone might want to trap whatever error happened. Use C<die> instead,
1746which can be trapped by an C<eval>.
28757baa 1747
19799a22 1748The exit() function does not always exit immediately. It calls any
2b5ab1e7 1749defined C<END> routines first, but these C<END> routines may not
19799a22 1750themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1751be called are called before the real exit. If this is a problem, you
1752can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1753See L<perlmod> for details.
5a964f20 1754
a0d0e21e 1755=item exp EXPR
d74e8afc 1756X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1757
54310121 1758=item exp
bbce6d69 1759
b76cc8ba 1760Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1761If EXPR is omitted, gives C<exp($_)>.
1762
1763=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1764X<fcntl>
a0d0e21e 1765
f86cebdf 1766Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1767
1768 use Fcntl;
1769
0ade1984 1770first to get the correct constant definitions. Argument processing and
b76cc8ba 1771value return works just like C<ioctl> below.
a0d0e21e
LW
1772For example:
1773
1774 use Fcntl;
5a964f20
TC
1775 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1776 or die "can't fcntl F_GETFL: $!";
1777
554ad1fc 1778You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1779Like C<ioctl>, it maps a C<0> return from the system call into
1780C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1781in numeric context. It is also exempt from the normal B<-w> warnings
1782on improper numeric conversions.
5a964f20 1783
19799a22 1784Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1785doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1786manpage to learn what functions are available on your system.
a0d0e21e 1787
be2f7487 1788Here's an example of setting a filehandle named C<REMOTE> to be
1789non-blocking at the system level. You'll have to negotiate C<$|>
1790on your own, though.
1791
1792 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1793
1794 $flags = fcntl(REMOTE, F_GETFL, 0)
1795 or die "Can't get flags for the socket: $!\n";
1796
1797 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1798 or die "Can't set flags for the socket: $!\n";
1799
a0d0e21e 1800=item fileno FILEHANDLE
d74e8afc 1801X<fileno>
a0d0e21e 1802
2b5ab1e7
TC
1803Returns the file descriptor for a filehandle, or undefined if the
1804filehandle is not open. This is mainly useful for constructing
19799a22 1805bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1806If FILEHANDLE is an expression, the value is taken as an indirect
1807filehandle, generally its name.
5a964f20 1808
b76cc8ba 1809You can use this to find out whether two handles refer to the
5a964f20
TC
1810same underlying descriptor:
1811
1812 if (fileno(THIS) == fileno(THAT)) {
1813 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1814 }
1815
1816(Filehandles connected to memory objects via new features of C<open> may
1817return undefined even though they are open.)
1818
a0d0e21e
LW
1819
1820=item flock FILEHANDLE,OPERATION
d74e8afc 1821X<flock> X<lock> X<locking>
a0d0e21e 1822
19799a22
GS
1823Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1824for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1825machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1826C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1827only entire files, not records.
1828
1829Two potentially non-obvious but traditional C<flock> semantics are
1830that it waits indefinitely until the lock is granted, and that its locks
1831B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1832fewer guarantees. This means that programs that do not also use C<flock>
1833may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1834your port's specific documentation, or your system-specific local manpages
1835for details. It's best to assume traditional behavior if you're writing
1836portable programs. (But if you're not, you should as always feel perfectly
1837free to write for your own system's idiosyncrasies (sometimes called
1838"features"). Slavish adherence to portability concerns shouldn't get
1839in the way of your getting your job done.)
a3cb178b 1840
8ebc5c01
PP
1841OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1842LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1843you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1844either individually, or as a group using the ':flock' tag. LOCK_SH
1845requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1846releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1847LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1848waiting for the lock (check the return status to see if you got it).
1849
2b5ab1e7
TC
1850To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1851before locking or unlocking it.
8ebc5c01 1852
f86cebdf 1853Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1854locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1855are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1856implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1857differing semantics shouldn't bite too many people.
1858
becacb53
TM
1859Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1860be open with read intent to use LOCK_SH and requires that it be open
1861with write intent to use LOCK_EX.
1862
19799a22
GS
1863Note also that some versions of C<flock> cannot lock things over the
1864network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1865that. If you like you can force Perl to ignore your system's flock(2)
1866function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1867the switch C<-Ud_flock> to the F<Configure> program when you configure
1868perl.
4633a7c4
LW
1869
1870Here's a mailbox appender for BSD systems.
a0d0e21e 1871
7e1af8bc 1872 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1873
1874 sub lock {
7e1af8bc 1875 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1876 # and, in case someone appended
1877 # while we were waiting...
1878 seek(MBOX, 0, 2);
1879 }
1880
1881 sub unlock {
7e1af8bc 1882 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1883 }
1884
1885 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1886 or die "Can't open mailbox: $!";
1887
1888 lock();
1889 print MBOX $msg,"\n\n";
1890 unlock();
1891
2b5ab1e7
TC
1892On systems that support a real flock(), locks are inherited across fork()
1893calls, whereas those that must resort to the more capricious fcntl()
1894function lose the locks, making it harder to write servers.
1895
cb1a09d0 1896See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1897
1898=item fork
d74e8afc 1899X<fork> X<child> X<parent>
a0d0e21e 1900
2b5ab1e7
TC
1901Does a fork(2) system call to create a new process running the
1902same program at the same point. It returns the child pid to the
1903parent process, C<0> to the child process, or C<undef> if the fork is
1904unsuccessful. File descriptors (and sometimes locks on those descriptors)
1905are shared, while everything else is copied. On most systems supporting
1906fork(), great care has gone into making it extremely efficient (for
1907example, using copy-on-write technology on data pages), making it the
1908dominant paradigm for multitasking over the last few decades.
5a964f20 1909
0f897271
GS
1910Beginning with v5.6.0, Perl will attempt to flush all files opened for
1911output before forking the child process, but this may not be supported
1912on some platforms (see L<perlport>). To be safe, you may need to set
1913C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1914C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1915
19799a22 1916If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1917accumulate zombies. On some systems, you can avoid this by setting
1918C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1919forking and reaping moribund children.
cb1a09d0 1920
28757baa
PP
1921Note that if your forked child inherits system file descriptors like
1922STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1923if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1924backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1925You should reopen those to F</dev/null> if it's any issue.
28757baa 1926
cb1a09d0 1927=item format
d74e8afc 1928X<format>
cb1a09d0 1929
19799a22 1930Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1931example:
1932
54310121 1933 format Something =
cb1a09d0
AD
1934 Test: @<<<<<<<< @||||| @>>>>>
1935 $str, $%, '$' . int($num)
1936 .
1937
1938 $str = "widget";
184e9718 1939 $num = $cost/$quantity;
cb1a09d0
AD
1940 $~ = 'Something';
1941 write;
1942
1943See L<perlform> for many details and examples.
1944
8903cb82 1945=item formline PICTURE,LIST
d74e8afc 1946X<formline>
a0d0e21e 1947
5a964f20 1948This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1949too. It formats (see L<perlform>) a list of values according to the
1950contents of PICTURE, placing the output into the format output
7660c0ab 1951accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1952Eventually, when a C<write> is done, the contents of
cf264981
SP
1953C<$^A> are written to some filehandle. You could also read C<$^A>
1954and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1955does one C<formline> per line of form, but the C<formline> function itself
748a9306 1956doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1957that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1958You may therefore need to use multiple formlines to implement a single
1959record format, just like the format compiler.
1960
19799a22 1961Be careful if you put double quotes around the picture, because an C<@>
748a9306 1962character may be taken to mean the beginning of an array name.
19799a22 1963C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1964
1965=item getc FILEHANDLE
d74e8afc 1966X<getc> X<getchar>
a0d0e21e
LW
1967
1968=item getc
1969
1970Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
1971or the undefined value at end of file, or if there was an error (in
1972the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1973STDIN. This is not particularly efficient. However, it cannot be
1974used by itself to fetch single characters without waiting for the user
1975to hit enter. For that, try something more like:
4633a7c4
LW
1976
1977 if ($BSD_STYLE) {
1978 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1979 }
1980 else {
54310121 1981 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1982 }
1983
1984 $key = getc(STDIN);
1985
1986 if ($BSD_STYLE) {
1987 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1988 }
1989 else {
5f05dabc 1990 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1991 }
1992 print "\n";
1993
54310121
PP
1994Determination of whether $BSD_STYLE should be set
1995is left as an exercise to the reader.
cb1a09d0 1996
19799a22 1997The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1998systems purporting POSIX compliance. See also the C<Term::ReadKey>
1999module from your nearest CPAN site; details on CPAN can be found on
2000L<perlmodlib/CPAN>.
a0d0e21e
LW
2001
2002=item getlogin
d74e8afc 2003X<getlogin> X<login>
a0d0e21e 2004
cf264981 2005This implements the C library function of the same name, which on most
5a964f20 2006systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 2007use C<getpwuid>.
a0d0e21e 2008
f86702cc 2009 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2010
19799a22
GS
2011Do not consider C<getlogin> for authentication: it is not as
2012secure as C<getpwuid>.
4633a7c4 2013
a0d0e21e 2014=item getpeername SOCKET
d74e8afc 2015X<getpeername> X<peer>
a0d0e21e
LW
2016
2017Returns the packed sockaddr address of other end of the SOCKET connection.
2018
4633a7c4
LW
2019 use Socket;
2020 $hersockaddr = getpeername(SOCK);
19799a22 2021 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2022 $herhostname = gethostbyaddr($iaddr, AF_INET);
2023 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2024
2025=item getpgrp PID
d74e8afc 2026X<getpgrp> X<group>
a0d0e21e 2027
47e29363 2028Returns the current process group for the specified PID. Use
7660c0ab 2029a PID of C<0> to get the current process group for the
4633a7c4 2030current process. Will raise an exception if used on a machine that
f86cebdf 2031doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2032group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2033does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2034
2035=item getppid
d74e8afc 2036X<getppid> X<parent> X<pid>
a0d0e21e
LW
2037
2038Returns the process id of the parent process.
2039
4d76a344
RGS
2040Note for Linux users: on Linux, the C functions C<getpid()> and
2041C<getppid()> return different values from different threads. In order to
2042be portable, this behavior is not reflected by the perl-level function
2043C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2044to call the underlying C<getppid()>, you may use the CPAN module
2045C<Linux::Pid>.
4d76a344 2046
a0d0e21e 2047=item getpriority WHICH,WHO
d74e8afc 2048X<getpriority> X<priority> X<nice>
a0d0e21e 2049
4633a7c4
LW
2050Returns the current priority for a process, a process group, or a user.
2051(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2052machine that doesn't implement getpriority(2).
a0d0e21e
LW
2053
2054=item getpwnam NAME
d74e8afc
ITB
2055X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2056X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2057X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2058X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2059X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2060X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2061
2062=item getgrnam NAME
2063
2064=item gethostbyname NAME
2065
2066=item getnetbyname NAME
2067
2068=item getprotobyname NAME
2069
2070=item getpwuid UID
2071
2072=item getgrgid GID
2073
2074=item getservbyname NAME,PROTO
2075
2076=item gethostbyaddr ADDR,ADDRTYPE
2077
2078=item getnetbyaddr ADDR,ADDRTYPE
2079
2080=item getprotobynumber NUMBER
2081
2082=item getservbyport PORT,PROTO
2083
2084=item getpwent
2085
2086=item getgrent
2087
2088=item gethostent
2089
2090=item getnetent
2091
2092=item getprotoent
2093
2094=item getservent
2095
2096=item setpwent
2097
2098=item setgrent
2099
2100=item sethostent STAYOPEN
2101
2102=item setnetent STAYOPEN
2103
2104=item setprotoent STAYOPEN
2105
2106=item setservent STAYOPEN
2107
2108=item endpwent
2109
2110=item endgrent
2111
2112=item endhostent
2113
2114=item endnetent
2115
2116=item endprotoent
2117
2118=item endservent
2119
2120These routines perform the same functions as their counterparts in the
5a964f20 2121system library. In list context, the return values from the
a0d0e21e
LW
2122various get routines are as follows:
2123
2124 ($name,$passwd,$uid,$gid,
6ee623d5 2125 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2126 ($name,$passwd,$gid,$members) = getgr*
2127 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2128 ($name,$aliases,$addrtype,$net) = getnet*
2129 ($name,$aliases,$proto) = getproto*
2130 ($name,$aliases,$port,$proto) = getserv*
2131
2132(If the entry doesn't exist you get a null list.)
2133
4602f195
JH
2134The exact meaning of the $gcos field varies but it usually contains
2135the real name of the user (as opposed to the login name) and other
2136information pertaining to the user. Beware, however, that in many
2137system users are able to change this information and therefore it
106325ad 2138cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2139L<perlsec>). The $passwd and $shell, user's encrypted password and
2140login shell, are also tainted, because of the same reason.
4602f195 2141
5a964f20 2142In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2143lookup by name, in which case you get the other thing, whatever it is.
2144(If the entry doesn't exist you get the undefined value.) For example:
2145
5a964f20
TC
2146 $uid = getpwnam($name);
2147 $name = getpwuid($num);
2148 $name = getpwent();
2149 $gid = getgrnam($name);
08a33e13 2150 $name = getgrgid($num);
5a964f20
TC
2151 $name = getgrent();
2152 #etc.
a0d0e21e 2153
4602f195
JH
2154In I<getpw*()> the fields $quota, $comment, and $expire are special
2155cases in the sense that in many systems they are unsupported. If the
2156$quota is unsupported, it is an empty scalar. If it is supported, it
2157usually encodes the disk quota. If the $comment field is unsupported,
2158it is an empty scalar. If it is supported it usually encodes some
2159administrative comment about the user. In some systems the $quota
2160field may be $change or $age, fields that have to do with password
2161aging. In some systems the $comment field may be $class. The $expire
2162field, if present, encodes the expiration period of the account or the
2163password. For the availability and the exact meaning of these fields
2164in your system, please consult your getpwnam(3) documentation and your
2165F<pwd.h> file. You can also find out from within Perl what your
2166$quota and $comment fields mean and whether you have the $expire field
2167by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2168C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2169files are only supported if your vendor has implemented them in the
2170intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2171shadow versions if you're running under privilege or if there exists
cf264981
SP
2172the shadow(3) functions as found in System V (this includes Solaris
2173and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2174facility are unlikely to be supported.
6ee623d5 2175
19799a22 2176The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2177the login names of the members of the group.
2178
2179For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2180C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2181C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2182addresses returned by the corresponding system library call. In the
2183Internet domain, each address is four bytes long and you can unpack it
2184by saying something like:
2185
f337b084 2186 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2187
2b5ab1e7
TC
2188The Socket library makes this slightly easier:
2189
2190 use Socket;
2191 $iaddr = inet_aton("127.1"); # or whatever address
2192 $name = gethostbyaddr($iaddr, AF_INET);
2193
2194 # or going the other way
19799a22 2195 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2196
19799a22
GS
2197If you get tired of remembering which element of the return list
2198contains which return value, by-name interfaces are provided
2199in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2200C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2201and C<User::grent>. These override the normal built-ins, supplying
2202versions that return objects with the appropriate names
2203for each field. For example:
5a964f20
TC
2204
2205 use File::stat;
2206 use User::pwent;
2207 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2208
b76cc8ba
NIS
2209Even though it looks like they're the same method calls (uid),
2210they aren't, because a C<File::stat> object is different from
19799a22 2211a C<User::pwent> object.
5a964f20 2212
a0d0e21e 2213=item getsockname SOCKET
d74e8afc 2214X<getsockname>
a0d0e21e 2215
19799a22
GS
2216Returns the packed sockaddr address of this end of the SOCKET connection,
2217in case you don't know the address because you have several different
2218IPs that the connection might have come in on.
a0d0e21e 2219
4633a7c4
LW
2220 use Socket;
2221 $mysockaddr = getsockname(SOCK);
19799a22 2222 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2223 printf "Connect to %s [%s]\n",
19799a22
GS
2224 scalar gethostbyaddr($myaddr, AF_INET),
2225 inet_ntoa($myaddr);
a0d0e21e
LW
2226
2227=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2228X<getsockopt>
a0d0e21e 2229
636e6b1f
TH
2230Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2231Options may exist at multiple protocol levels depending on the socket
2232type, but at least the uppermost socket level SOL_SOCKET (defined in the
2233C<Socket> module) will exist. To query options at another level the
2234protocol number of the appropriate protocol controlling the option
2235should be supplied. For example, to indicate that an option is to be
2236interpreted by the TCP protocol, LEVEL should be set to the protocol
2237number of TCP, which you can get using getprotobyname.
2238
2239The call returns a packed string representing the requested socket option,
2240or C<undef> if there is an error (the error reason will be in $!). What
2241exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2242your system documentation for details. A very common case however is that
cf264981 2243the option is an integer, in which case the result will be a packed
636e6b1f
TH
2244integer which you can decode using unpack with the C<i> (or C<I>) format.
2245
2246An example testing if Nagle's algorithm is turned on on a socket:
2247
4852725b 2248 use Socket qw(:all);
636e6b1f
TH
2249
2250 defined(my $tcp = getprotobyname("tcp"))
2251 or die "Could not determine the protocol number for tcp";
4852725b
DD
2252 # my $tcp = IPPROTO_TCP; # Alternative
2253 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2254 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2255 my $nodelay = unpack("I", $packed);
2256 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2257
a0d0e21e
LW
2258
2259=item glob EXPR
d74e8afc 2260X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2261
0a753a76
PP
2262=item glob
2263
d9a9d457
JL
2264In list context, returns a (possibly empty) list of filename expansions on
2265the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2266scalar context, glob iterates through such filename expansions, returning
2267undef when the list is exhausted. This is the internal function
2268implementing the C<< <*.c> >> operator, but you can use it directly. If
2269EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2270more detail in L<perlop/"I/O Operators">.
a0d0e21e 2271
3a4b19e4
GS
2272Beginning with v5.6.0, this operator is implemented using the standard
2273C<File::Glob> extension. See L<File::Glob> for details.
2274
a0d0e21e 2275=item gmtime EXPR
d74e8afc 2276X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2277
ce2984c3
PF
2278=item gmtime
2279
82dd32fe 2280Converts a time as returned by the time function to an 9-element list
54310121 2281with the time localized for the standard Greenwich time zone.
4633a7c4 2282Typically used as follows:
a0d0e21e 2283
82dd32fe
SP
2284 # 0 1 2 3 4 5 6 7 8
2285 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
a0d0e21e
LW
2286 gmtime(time);
2287
48a26b3a
GS
2288All list elements are numeric, and come straight out of the C `struct
2289tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2290specified time. $mday is the day of the month, and $mon is the month
2291itself, in the range C<0..11> with 0 indicating January and 11
2292indicating December. $year is the number of years since 1900. That
2293is, $year is C<123> in year 2023. $wday is the day of the week, with
22940 indicating Sunday and 3 indicating Wednesday. $yday is the day of
82dd32fe
SP
2295the year, in the range C<0..364> (or C<0..365> in leap years). $isdst
2296is always C<0>.
48a26b3a
GS
2297
2298Note that the $year element is I<not> simply the last two digits of
cf264981 2299the year. If you assume it is then you create non-Y2K-compliant
48a26b3a 2300programs--and you wouldn't want to do that, would you?
2f9daede 2301
abd75f24
GS
2302The proper way to get a complete 4-digit year is simply:
2303
2304 $year += 1900;
2305
2306And to get the last two digits of the year (e.g., '01' in 2001) do:
2307
2308 $year = sprintf("%02d", $year % 100);
2309
48a26b3a 2310If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2311
48a26b3a 2312In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2313
2314 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2315
fe86afc2
NC
2316If you need local time instead of GMT use the L</localtime> builtin.
2317See also the C<timegm> function provided by the C<Time::Local> module,
2318and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
7660c0ab 2319
fe86afc2
NC
2320This scalar value is B<not> locale dependent (see L<perllocale>), but is
2321instead a Perl builtin. To get somewhat similar but locale dependent date
2322strings, see the example in L</localtime>.
0a753a76 2323
62aa5637
MS
2324See L<perlport/gmtime> for portability concerns.
2325
a0d0e21e 2326=item goto LABEL
d74e8afc 2327X<goto> X<jump> X<jmp>
a0d0e21e 2328
748a9306
LW
2329=item goto EXPR
2330
a0d0e21e
LW
2331=item goto &NAME
2332
7660c0ab 2333The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2334execution there. It may not be used to go into any construct that
7660c0ab 2335requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2336also can't be used to go into a construct that is optimized away,
19799a22 2337or to get out of a block or subroutine given to C<sort>.
0a753a76 2338It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2339including out of subroutines, but it's usually better to use some other
19799a22 2340construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2341need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2342(The difference being that C does not offer named loops combined with
2343loop control. Perl does, and this replaces most structured uses of C<goto>
2344in other languages.)
a0d0e21e 2345
7660c0ab
A
2346The C<goto-EXPR> form expects a label name, whose scope will be resolved
2347dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2348necessarily recommended if you're optimizing for maintainability:
2349
2350 goto ("FOO", "BAR", "GLARCH")[$i];
2351
1b6921cb
BT
2352The C<goto-&NAME> form is quite different from the other forms of
2353C<goto>. In fact, it isn't a goto in the normal sense at all, and
2354doesn't have the stigma associated with other gotos. Instead, it
2355exits the current subroutine (losing any changes set by local()) and
2356immediately calls in its place the named subroutine using the current
2357value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2358load another subroutine and then pretend that the other subroutine had
2359been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2360in the current subroutine are propagated to the other subroutine.)
2361After the C<goto>, not even C<caller> will be able to tell that this
2362routine was called first.
2363
2364NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2365containing a code reference, or a block that evaluates to a code
6cb9131c 2366reference.
a0d0e21e
LW
2367
2368=item grep BLOCK LIST
d74e8afc 2369X<grep>
a0d0e21e
LW
2370
2371=item grep EXPR,LIST
2372
2b5ab1e7
TC
2373This is similar in spirit to, but not the same as, grep(1) and its
2374relatives. In particular, it is not limited to using regular expressions.
2f9daede 2375
a0d0e21e 2376Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2377C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2378elements for which the expression evaluated to true. In scalar
2379context, returns the number of times the expression was true.
a0d0e21e
LW
2380
2381 @foo = grep(!/^#/, @bar); # weed out comments
2382
2383or equivalently,
2384
2385 @foo = grep {!/^#/} @bar; # weed out comments
2386
be3174d2
GS
2387Note that C<$_> is an alias to the list value, so it can be used to
2388modify the elements of the LIST. While this is useful and supported,
2389it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2390Similarly, grep returns aliases into the original list, much as a for
2391loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2392element of a list returned by grep (for example, in a C<foreach>, C<map>
2393or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2394This is usually something to be avoided when writing clear code.
a0d0e21e 2395
a4fb8298 2396If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2397been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2398the list elements, C<$_> keeps being lexical inside the block; i.e. it
2399can't be seen from the outside, avoiding any potential side-effects.
2400
19799a22 2401See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2402
a0d0e21e 2403=item hex EXPR
d74e8afc 2404X<hex> X<hexadecimal>
a0d0e21e 2405
54310121 2406=item hex
bbce6d69 2407
2b5ab1e7 2408Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2409(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2410L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2411
2412 print hex '0xAf'; # prints '175'
2413 print hex 'aF'; # same
a0d0e21e 2414
19799a22 2415Hex strings may only represent integers. Strings that would cause
53305cf1 2416integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2417unlike oct(). To present something as hex, look into L</printf>,
2418L</sprintf>, or L</unpack>.
19799a22 2419
ce2984c3 2420=item import LIST
d74e8afc 2421X<import>
a0d0e21e 2422
19799a22 2423There is no builtin C<import> function. It is just an ordinary
4633a7c4 2424method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2425names to another module. The C<use> function calls the C<import> method
cea6626f 2426for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2427
2428=item index STR,SUBSTR,POSITION
d74e8afc 2429X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2430
2431=item index STR,SUBSTR
2432
2b5ab1e7
TC
2433The index function searches for one string within another, but without
2434the wildcard-like behavior of a full regular-expression pattern match.
2435It returns the position of the first occurrence of SUBSTR in STR at
2436or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2437beginning of the string. POSITION before the beginning of the string
2438or after its end is treated as if it were the beginning or the end,
2439respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2440you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2441is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2442
2443=item int EXPR
d74e8afc 2444X<int> X<integer> X<truncate> X<trunc>
a0d0e21e 2445
54310121 2446=item int
bbce6d69 2447
7660c0ab 2448Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2449You should not use this function for rounding: one because it truncates
2450towards C<0>, and two because machine representations of floating point
2451numbers can sometimes produce counterintuitive results. For example,
2452C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2453because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2454the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2455functions will serve you better than will int().
a0d0e21e
LW
2456
2457=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2458X<ioctl>
a0d0e21e 2459
2b5ab1e7 2460Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2461
6c567752 2462 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2463
a11c483f 2464to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2465exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2466own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2467(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2468may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2469written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2470will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2471has no string value but does have a numeric value, that value will be
2472passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2473true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2474functions may be needed to manipulate the values of structures used by
b76cc8ba 2475C<ioctl>.
a0d0e21e 2476
19799a22 2477The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2478
2479 if OS returns: then Perl returns:
2480 -1 undefined value
2481 0 string "0 but true"
2482 anything else that number
2483
19799a22 2484Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2485still easily determine the actual value returned by the operating
2486system:
2487
2b5ab1e7 2488 $retval = ioctl(...) || -1;
a0d0e21e
LW
2489 printf "System returned %d\n", $retval;
2490
be2f7487 2491The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2492about improper numeric conversions.
2493
a0d0e21e 2494=item join EXPR,LIST
d74e8afc 2495X<join>
a0d0e21e 2496
2b5ab1e7
TC
2497Joins the separate strings of LIST into a single string with fields
2498separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2499
2b5ab1e7 2500 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2501
eb6e2d6f
GS
2502Beware that unlike C<split>, C<join> doesn't take a pattern as its
2503first argument. Compare L</split>.
a0d0e21e 2504
aa689395 2505=item keys HASH
d74e8afc 2506X<keys> X<key>
aa689395 2507
504f80c1
JH
2508Returns a list consisting of all the keys of the named hash.
2509(In scalar context, returns the number of keys.)
2510
2511The keys are returned in an apparently random order. The actual
2512random order is subject to change in future versions of perl, but it
2513is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2514function produces (given that the hash has not been modified). Since
2515Perl 5.8.1 the ordering is different even between different runs of
2516Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2517Attacks">).
504f80c1 2518
cf264981
SP
2519As a side effect, calling keys() resets the HASH's internal iterator
2520(see L</each>). In particular, calling keys() in void context resets
2521the iterator with no other overhead.
a0d0e21e 2522
aa689395 2523Here is yet another way to print your environment:
a0d0e21e
LW
2524
2525 @keys = keys %ENV;
2526 @values = values %ENV;
b76cc8ba 2527 while (@keys) {
a0d0e21e
LW
2528 print pop(@keys), '=', pop(@values), "\n";
2529 }
2530
2531or how about sorted by key:
2532
2533 foreach $key (sort(keys %ENV)) {
2534 print $key, '=', $ENV{$key}, "\n";
2535 }
2536
8ea1e5d4
GS
2537The returned values are copies of the original keys in the hash, so
2538modifying them will not affect the original hash. Compare L</values>.
2539
19799a22 2540To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2541Here's a descending numeric sort of a hash by its values:
4633a7c4 2542
5a964f20 2543 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2544 printf "%4d %s\n", $hash{$key}, $key;
2545 }
2546
19799a22 2547As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2548allocated for the given hash. This can gain you a measure of efficiency if
2549you know the hash is going to get big. (This is similar to pre-extending
2550an array by assigning a larger number to $#array.) If you say
55497cff
PP
2551
2552 keys %hash = 200;
2553
ab192400
GS
2554then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2555in fact, since it rounds up to the next power of two. These
55497cff
PP
2556buckets will be retained even if you do C<%hash = ()>, use C<undef
2557%hash> if you want to free the storage while C<%hash> is still in scope.
2558You can't shrink the number of buckets allocated for the hash using
19799a22 2559C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2560as trying has no effect).
2561
19799a22 2562See also C<each>, C<values> and C<sort>.
ab192400 2563
b350dd2f 2564=item kill SIGNAL, LIST
d74e8afc 2565X<kill> X<signal>
a0d0e21e 2566
b350dd2f 2567Sends a signal to a list of processes. Returns the number of
517db077
GS
2568processes successfully signaled (which is not necessarily the
2569same as the number actually killed).
a0d0e21e
LW
2570
2571 $cnt = kill 1, $child1, $child2;
2572 kill 9, @goners;
2573
b350dd2f 2574If SIGNAL is zero, no signal is sent to the process. This is a
1e9c1022 2575useful way to check that a child process is alive and hasn't changed
b350dd2f
GS
2576its UID. See L<perlport> for notes on the portability of this
2577construct.
2578
2579Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2580process groups instead of processes. (On System V, a negative I<PROCESS>
2581number will also kill process groups, but that's not portable.) That
2582means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2583use a signal name in quotes.
2584
2585See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2586
2587=item last LABEL
d74e8afc 2588X<last> X<break>
a0d0e21e
LW
2589
2590=item last
2591
2592The C<last> command is like the C<break> statement in C (as used in
2593loops); it immediately exits the loop in question. If the LABEL is
2594omitted, the command refers to the innermost enclosing loop. The
2595C<continue> block, if any, is not executed:
2596
4633a7c4
LW
2597 LINE: while (<STDIN>) {
2598 last LINE if /^$/; # exit when done with header
5a964f20 2599 #...
a0d0e21e
LW
2600 }
2601
4968c1e4 2602C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2603C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2604a grep() or map() operation.
4968c1e4 2605
6c1372ed
GS
2606Note that a block by itself is semantically identical to a loop
2607that executes once. Thus C<last> can be used to effect an early
2608exit out of such a block.
2609
98293880
JH
2610See also L</continue> for an illustration of how C<last>, C<next>, and
2611C<redo> work.
1d2dff63 2612
a0d0e21e 2613=item lc EXPR
d74e8afc 2614X<lc> X<lowercase>
a0d0e21e 2615
54310121 2616=item lc
bbce6d69 2617
d1be9408 2618Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2619implementing the C<\L> escape in double-quoted strings. Respects
2620current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2621and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2622
7660c0ab 2623If EXPR is omitted, uses C<$_>.
bbce6d69 2624
a0d0e21e 2625=item lcfirst EXPR
d74e8afc 2626X<lcfirst> X<lowercase>
a0d0e21e 2627
54310121 2628=item lcfirst
bbce6d69 2629
ad0029c4
JH
2630Returns the value of EXPR with the first character lowercased. This
2631is the internal function implementing the C<\l> escape in
2632double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2633locale> in force. See L<perllocale> and L<perlunicode> for more
2634details about locale and Unicode support.
a0d0e21e 2635
7660c0ab 2636If EXPR is omitted, uses C<$_>.
bbce6d69 2637
a0d0e21e 2638=item length EXPR
d74e8afc 2639X<length> X<size>
a0d0e21e 2640
54310121 2641=item length
bbce6d69 2642
974da8e5 2643Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2644omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2645an entire array or hash to find out how many elements these have.
2646For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2647
974da8e5
JH
2648Note the I<characters>: if the EXPR is in Unicode, you will get the
2649number of characters, not the number of bytes. To get the length
2650in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2651
a0d0e21e 2652=item link OLDFILE,NEWFILE
d74e8afc 2653X<link>
a0d0e21e 2654
19799a22 2655Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2656success, false otherwise.
a0d0e21e
LW
2657
2658=item listen SOCKET,QUEUESIZE
d74e8afc 2659X<listen>
a0d0e21e 2660
19799a22 2661Does the same thing that the listen system call does. Returns true if
b76cc8ba 2662it succeeded, false otherwise. See the example in
cea6626f 2663L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2664
2665=item local EXPR
d74e8afc 2666X<local>
a0d0e21e 2667
19799a22 2668You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2669what most people think of as "local". See
13a2d996 2670L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2671
5a964f20
TC
2672A local modifies the listed variables to be local to the enclosing
2673block, file, or eval. If more than one value is listed, the list must
2674be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2675for details, including issues with tied arrays and hashes.
a0d0e21e 2676
a0d0e21e 2677=item localtime EXPR
d74e8afc 2678X<localtime>
a0d0e21e 2679
ba053783
AL
2680=item localtime
2681
19799a22 2682Converts a time as returned by the time function to a 9-element list
5f05dabc 2683with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2684follows:
2685
54310121 2686 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2687 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2688 localtime(time);
a0d0e21e 2689
48a26b3a 2690All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2691tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2692of the specified time.
48a26b3a 2693
ba053783
AL
2694C<$mday> is the day of the month, and C<$mon> is the month itself, in
2695the range C<0..11> with 0 indicating January and 11 indicating December.
2696This makes it easy to get a month name from a list:
54310121 2697
ba053783
AL
2698 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2699 print "$abbr[$mon] $mday";
2700 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2701
ba053783
AL
2702C<$year> is the number of years since 1900, not just the last two digits
2703of the year. That is, C<$year> is C<123> in year 2023. The proper way
2704to get a complete 4-digit year is simply:
abd75f24 2705
ba053783 2706 $year += 1900;
abd75f24 2707
ba053783
AL
2708To get the last two digits of the year (e.g., '01' in 2001) do:
2709
2710 $year = sprintf("%02d", $year % 100);
2711
2712C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2713Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2714(or C<0..365> in leap years.)
2715
2716C<$isdst> is true if the specified time occurs during Daylight Saving
2717Time, false otherwise.
abd75f24 2718
48a26b3a 2719If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2720
48a26b3a 2721In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2722
5f05dabc 2723 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2724
fe86afc2
NC
2725This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2726instead of local time use the L</gmtime> builtin. See also the
2727C<Time::Local> module (to convert the second, minutes, hours, ... back to
2728the integer value returned by time()), and the L<POSIX> module's strftime(3)
2729and mktime(3) functions.
2730
2731To get somewhat similar but locale dependent date strings, set up your
2732locale environment variables appropriately (please see L<perllocale>) and
2733try for example:
a3cb178b 2734
5a964f20 2735 use POSIX qw(strftime);
2b5ab1e7 2736 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2737 # or for GMT formatted appropriately for your locale:
2738 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2739
2740Note that the C<%a> and C<%b>, the short forms of the day of the week
2741and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2742
62aa5637
MS
2743See L<perlport/localtime> for portability concerns.
2744
07698885 2745=item lock THING
d74e8afc 2746X<lock>
19799a22 2747
01e6739c 2748This function places an advisory lock on a shared variable, or referenced
03730085 2749object contained in I<THING> until the lock goes out of scope.
a6d5524e 2750
f3a23afb 2751lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2752by this name (before any calls to it), that function will be called
03730085
AB
2753instead. (However, if you've said C<use threads>, lock() is always a
2754keyword.) See L<threads>.
19799a22 2755
a0d0e21e 2756=item log EXPR
d74e8afc 2757X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2758
54310121 2759=item log
bbce6d69 2760
2b5ab1e7
TC
2761Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2762returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2763The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2764divided by the natural log of N. For example:
2765
2766 sub log10 {
2767 my $n = shift;
2768 return log($n)/log(10);
b76cc8ba 2769 }
2b5ab1e7
TC
2770
2771See also L</exp> for the inverse operation.
a0d0e21e 2772
a0d0e21e 2773=item lstat EXPR
d74e8afc 2774X<lstat>
a0d0e21e 2775
54310121 2776=item lstat
bbce6d69 2777
19799a22 2778Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2779special C<_> filehandle) but stats a symbolic link instead of the file
2780the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2781your system, a normal C<stat> is done. For much more detailed
2782information, please see the documentation for C<stat>.
a0d0e21e 2783
7660c0ab 2784If EXPR is omitted, stats C<$_>.
bbce6d69 2785
a0d0e21e
LW
2786=item m//
2787
2788The match operator. See L<perlop>.
2789
2790=item map BLOCK LIST
d74e8afc 2791X<map>
a0d0e21e
LW
2792
2793=item map EXPR,LIST
2794
19799a22
GS
2795Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2796C<$_> to each element) and returns the list value composed of the
2797results of each such evaluation. In scalar context, returns the
2798total number of elements so generated. Evaluates BLOCK or EXPR in
2799list context, so each element of LIST may produce zero, one, or
2800more elements in the returned value.
dd99ebda 2801
a0d0e21e
LW
2802 @chars = map(chr, @nums);
2803
2804translates a list of numbers to the corresponding characters. And
2805
4633a7c4 2806 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2807
2808is just a funny way to write
2809
2810 %hash = ();
2811 foreach $_ (@array) {
4633a7c4 2812 $hash{getkey($_)} = $_;
a0d0e21e
LW
2813 }
2814
be3174d2
GS
2815Note that C<$_> is an alias to the list value, so it can be used to
2816modify the elements of the LIST. While this is useful and supported,
2817it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2818Using a regular C<foreach> loop for this purpose would be clearer in
2819most cases. See also L</grep> for an array composed of those items of
2820the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2821
a4fb8298 2822If C<$_> is lexical in the scope where the C<map> appears (because it has
cf264981 2823been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2824the list elements, C<$_> keeps being lexical inside the block; i.e. it
2825can't be seen from the outside, avoiding any potential side-effects.
2826
205fdb4d
NC
2827C<{> starts both hash references and blocks, so C<map { ...> could be either
2828the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2829ahead for the closing C<}> it has to take a guess at which its dealing with
2830based what it finds just after the C<{>. Usually it gets it right, but if it
2831doesn't it won't realize something is wrong until it gets to the C<}> and
2832encounters the missing (or unexpected) comma. The syntax error will be
2833reported close to the C<}> but you'll need to change something near the C<{>
2834such as using a unary C<+> to give perl some help:
2835
2836 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2837 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2838 %hash = map { ("\L$_", 1) } @array # this also works
2839 %hash = map { lc($_), 1 } @array # as does this.
2840 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2841
205fdb4d
NC
2842 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2843
2844or to force an anon hash constructor use C<+{>
2845
2846 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2847
2848and you get list of anonymous hashes each with only 1 entry.
2849
19799a22 2850=item mkdir FILENAME,MASK
d74e8afc 2851X<mkdir> X<md> X<directory, create>
a0d0e21e 2852
5a211162
GS
2853=item mkdir FILENAME
2854
491873e5
RGS
2855=item mkdir
2856
0591cd52 2857Creates the directory specified by FILENAME, with permissions
19799a22
GS
2858specified by MASK (as modified by C<umask>). If it succeeds it
2859returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2860If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2861to C<$_>.
0591cd52 2862
19799a22 2863In general, it is better to create directories with permissive MASK,
0591cd52 2864and let the user modify that with their C<umask>, than it is to supply
19799a22 2865a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2866The exceptions to this rule are when the file or directory should be
2867kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2868C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2869
cc1852e8
JH
2870Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2871number of trailing slashes. Some operating and filesystems do not get
2872this right, so Perl automatically removes all trailing slashes to keep
2873everyone happy.
2874
a0d0e21e 2875=item msgctl ID,CMD,ARG
d74e8afc 2876X<msgctl>
a0d0e21e 2877
f86cebdf 2878Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2879
2880 use IPC::SysV;
2881
7660c0ab 2882first to get the correct constant definitions. If CMD is C<IPC_STAT>,
cf264981 2883then ARG must be a variable that will hold the returned C<msqid_ds>
951ba7fe
GS
2884structure. Returns like C<ioctl>: the undefined value for error,
2885C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2886L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2887
2888=item msgget KEY,FLAGS
d74e8afc 2889X<msgget>
a0d0e21e 2890
f86cebdf 2891Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2892id, or the undefined value if there is an error. See also
2893L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2894
a0d0e21e 2895=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
d74e8afc 2896X<msgrcv>
a0d0e21e
LW
2897
2898Calls the System V IPC function msgrcv to receive a message from
2899message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2900SIZE. Note that when a message is received, the message type as a
2901native long integer will be the first thing in VAR, followed by the
2902actual message. This packing may be opened with C<unpack("l! a*")>.
2903Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2904an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2905C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2906
2907=item msgsnd ID,MSG,FLAGS
d74e8afc 2908X<msgsnd>
41d6edb2
JH
2909
2910Calls the System V IPC function msgsnd to send the message MSG to the
2911message queue ID. MSG must begin with the native long integer message
2912type, and be followed by the length of the actual message, and finally
2913the message itself. This kind of packing can be achieved with
2914C<pack("l! a*", $type, $message)>. Returns true if successful,
2915or false if there is an error. See also C<IPC::SysV>
2916and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2917
2918=item my EXPR
d74e8afc 2919X<my>
a0d0e21e 2920
307ea6df
JH
2921=item my TYPE EXPR
2922
1d2de774 2923=item my EXPR : ATTRS
09bef843 2924
1d2de774 2925=item my TYPE EXPR : ATTRS
307ea6df 2926
19799a22 2927A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2928enclosing block, file, or C<eval>. If more than one value is listed,
2929the list must be placed in parentheses.
307ea6df 2930
1d2de774
JH
2931The exact semantics and interface of TYPE and ATTRS are still
2932evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2933and attributes are handled using the C<attributes> pragma, or starting
2934from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2935L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2936L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2937
a0d0e21e 2938=item next LABEL
d74e8afc 2939X<next> X<continue>
a0d0e21e
LW
2940
2941=item next
2942
2943The C<next> command is like the C<continue> statement in C; it starts
2944the next iteration of the loop:
2945
4633a7c4
LW
2946 LINE: while (<STDIN>) {
2947 next LINE if /^#/; # discard comments
5a964f20 2948 #...
a0d0e21e
LW
2949 }
2950
2951Note that if there were a C<continue> block on the above, it would get
2952executed even on discarded lines. If the LABEL is omitted, the command
2953refers to the innermost enclosing loop.
2954
4968c1e4 2955C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2956C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2957a grep() or map() operation.
4968c1e4 2958
6c1372ed
GS
2959Note that a block by itself is semantically identical to a loop
2960that executes once. Thus C<next> will exit such a block early.
2961
98293880
JH
2962See also L</continue> for an illustration of how C<last>, C<next>, and
2963C<redo> work.
1d2dff63 2964
4a66ea5a 2965=item no Module VERSION LIST
d74e8afc 2966X<no>
4a66ea5a
RGS
2967
2968=item no Module VERSION
2969
a0d0e21e
LW
2970=item no Module LIST
2971
4a66ea5a
RGS
2972=item no Module
2973
593b9c14 2974See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
2975
2976=item oct EXPR
d74e8afc 2977X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
a0d0e21e 2978
54310121 2979=item oct
bbce6d69 2980
4633a7c4 2981Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2982value. (If EXPR happens to start off with C<0x>, interprets it as a
2983hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2984binary string. Leading whitespace is ignored in all three cases.)
2985The following will handle decimal, binary, octal, and hex in the standard
2986Perl or C notation:
a0d0e21e
LW
2987
2988 $val = oct($val) if $val =~ /^0/;
2989
19799a22
GS
2990If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2991in octal), use sprintf() or printf():
2992
2993 $perms = (stat("filename"))[2] & 07777;
2994 $oct_perms = sprintf "%lo", $perms;
2995
2996The oct() function is commonly used when a string such as C<644> needs
2997to be converted into a file mode, for example. (Although perl will
2998automatically convert strings into numbers as needed, this automatic
2999conversion assumes base 10.)
a0d0e21e
LW
3000
3001=item open FILEHANDLE,EXPR
d74e8afc 3002X<open> X<pipe> X<file, open> X<fopen>
a0d0e21e 3003
68bd7414
NIS
3004=item open FILEHANDLE,MODE,EXPR
3005
3006=item open FILEHANDLE,MODE,EXPR,LIST
3007
ba964c95
T
3008=item open FILEHANDLE,MODE,REFERENCE
3009
a0d0e21e
LW
3010=item open FILEHANDLE
3011
3012Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
3013FILEHANDLE.
3014
3015(The following is a comprehensive reference to open(): for a gentler
3016introduction you may consider L<perlopentut>.)
3017
a28cd5c9
NT
3018If FILEHANDLE is an undefined scalar variable (or array or hash element)
3019the variable is assigned a reference to a new anonymous filehandle,
3020otherwise if FILEHANDLE is an expression, its value is used as the name of
3021the real filehandle wanted. (This is considered a symbolic reference, so
3022C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
3023
3024If EXPR is omitted, the scalar variable of the same name as the
3025FILEHANDLE contains the filename. (Note that lexical variables--those
3026declared with C<my>--will not work for this purpose; so if you're
67408cae 3027using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
3028
3029If three or more arguments are specified then the mode of opening and
3030the file name are separate. If MODE is C<< '<' >> or nothing, the file
3031is opened for input. If MODE is C<< '>' >>, the file is truncated and
3032opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 3033the file is opened for appending, again being created if necessary.
5a964f20 3034
ed53a2bb
JH
3035You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3036indicate that you want both read and write access to the file; thus
3037C<< '+<' >> is almost always preferred for read/write updates--the C<<
3038'+>' >> mode would clobber the file first. You can't usually use
3039either read-write mode for updating textfiles, since they have
3040variable length records. See the B<-i> switch in L<perlrun> for a
3041better approach. The file is created with permissions of C<0666>
3042modified by the process' C<umask> value.
3043
3044These various prefixes correspond to the fopen(3) modes of C<'r'>,
3045C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 3046
6170680b
IZ
3047In the 2-arguments (and 1-argument) form of the call the mode and
3048filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
3049spaces. It is possible to omit the mode in these forms if the mode is
3050C<< '<' >>.
6170680b 3051
7660c0ab 3052If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 3053command to which output is to be piped, and if the filename ends with a
f244e06d
GS
3054C<'|'>, the filename is interpreted as a command which pipes output to
3055us. See L<perlipc/"Using open() for IPC">
19799a22 3056for more examples of this. (You are not allowed to C<open> to a command
5a964f20 3057that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
3058and L<perlipc/"Bidirectional Communication with Another Process">
3059for alternatives.)
cb1a09d0 3060
ed53a2bb
JH
3061For three or more arguments if MODE is C<'|-'>, the filename is
3062interpreted as a command to which output is to be piped, and if MODE
3063is C<'-|'>, the filename is interpreted as a command which pipes
3064output to us. In the 2-arguments (and 1-argument) form one should
3065replace dash (C<'-'>) with the command.
3066See L<perlipc/"Using open() for IPC"> for more examples of this.
3067(You are not allowed to C<open> to a command that pipes both in I<and>
3068out, but see L<IPC::Open2>, L<IPC::Open3>, and
3069L<perlipc/"Bidirectional Communication"> for alternatives.)
3070
3071In the three-or-more argument form of pipe opens, if LIST is specified
3072(extra arguments after the command name) then LIST becomes arguments
3073to the command invoked if the platform supports it. The meaning of
3074C<open> with more than three arguments for non-pipe modes is not yet
3075specified. Experimental "layers" may give extra LIST arguments
3076meaning.
6170680b
IZ
3077
3078In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 3079and opening C<< '>-' >> opens STDOUT.
6170680b 3080
fae2c0fb
RGS
3081You may use the three-argument form of open to specify IO "layers"
3082(sometimes also referred to as "disciplines") to be applied to the handle
3083that affect how the input and output are processed (see L<open> and
3084L<PerlIO> for more details). For example
7207e29d 3085
9124316e
JH
3086 open(FH, "<:utf8", "file")
3087
3088will open the UTF-8 encoded file containing Unicode characters,
fae2c0fb
RGS
3089see L<perluniintro>. (Note that if layers are specified in the
3090three-arg form then default layers set by the C<open> pragma are
01e6739c 3091ignored.)
ed53a2bb
JH
3092
3093Open returns nonzero upon success, the undefined value otherwise. If
3094the C<open> involved a pipe, the return value happens to be the pid of
3095the subprocess.
cb1a09d0 3096
ed53a2bb
JH
3097If you're running Perl on a system that distinguishes between text
3098files and binary files, then you should check out L</binmode> for tips
3099for dealing with this. The key distinction between systems that need
3100C<binmode> and those that don't is their text file formats. Systems
8939ba94 3101like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
3102character, and which encode that character in C as C<"\n">, do not
3103need C<binmode>. The rest need it.
cb1a09d0 3104
fb73857a 3105When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
3106if the request failed, so C<open> is frequently used in connection with
3107C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 3108where you want to make a nicely formatted error message (but there are
5a964f20 3109modules that can help with that problem)) you should always check
19799a22 3110the return value from opening a file. The infrequent exception is when
fb73857a
PP
3111working with an unopened filehandle is actually what you want to do.
3112
cf264981 3113As a special case the 3-arg form with a read/write mode and the third
ed53a2bb 3114argument being C<undef>:
b76cc8ba
NIS
3115
3116 open(TMP, "+>", undef) or die ...
3117
f253e835
JH
3118opens a filehandle to an anonymous temporary file. Also using "+<"
3119works for symmetry, but you really should consider writing something
3120to the temporary file first. You will need to seek() to do the
3121reading.
b76cc8ba 3122
2ce64696 3123Since v5.8.0, perl has built using PerlIO by default. Unless you've
28a5cf3b 3124changed this (i.e. Configure -Uuseperlio), you can open file handles to
2ce64696 3125"in memory" files held in Perl scalars via:
ba964c95 3126
b996200f
SB
3127 open($fh, '>', \$variable) || ..
3128
3129Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3130file, you have to close it first:
3131
3132 close STDOUT;
3133 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
ba964c95 3134
cb1a09d0 3135Examples:
a0d0e21e
LW
3136
3137 $ARTICLE = 100;
3138 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3139 while (<ARTICLE>) {...
3140
6170680b 3141 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 3142 # if the open fails, output is discarded
a0d0e21e 3143
6170680b 3144 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 3145 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 3146
6170680b
IZ
3147 open(DBASE, '+<dbase.mine') # ditto
3148 or die "