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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
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3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
15operator. A unary operator generally provides a scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
a0d0e21e 17contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 18be first, and the list argument will follow. (Note that there can ever
0f31cffe 19be only one such list argument.) For instance, splice() has three scalar
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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>,
0d863452 137C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
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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
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159=item Keywords related to switch
160
161C<break>, C<continue>
162
163(These are only available if you enable the "switch" feature.
164See L<feature> and L<perlsyn/"Switch statements">.)
165
54310121 166=item Keywords related to scoping
cb1a09d0 167
4375e838 168C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
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169
170=item Miscellaneous functions
171
4375e838 172C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
22fae026 173C<scalar>, C<undef>, C<wantarray>
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174
175=item Functions for processes and process groups
d74e8afc 176X<process> X<pid> X<process id>
cb1a09d0 177
22fae026 178C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 179C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 180C<times>, C<wait>, C<waitpid>
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181
182=item Keywords related to perl modules
d74e8afc 183X<module>
cb1a09d0 184
22fae026 185C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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186
187=item Keywords related to classes and object-orientedness
d74e8afc 188X<object> X<class> X<package>
cb1a09d0 189
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190C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
191C<untie>, C<use>
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192
193=item Low-level socket functions
d74e8afc 194X<socket> X<sock>
cb1a09d0 195
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196C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
197C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 198C<socket>, C<socketpair>
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199
200=item System V interprocess communication functions
d74e8afc 201X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 202
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203C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
204C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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205
206=item Fetching user and group info
d74e8afc 207X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 208
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209C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
210C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
211C<getpwuid>, C<setgrent>, C<setpwent>
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212
213=item Fetching network info
d74e8afc 214X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 215
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216C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
217C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
218C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
219C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
220C<setnetent>, C<setprotoent>, C<setservent>
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221
222=item Time-related functions
d74e8afc 223X<time> X<date>
cb1a09d0 224
22fae026 225C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 226
37798a01 227=item Functions new in perl5
d74e8afc 228X<perl5>
37798a01 229
22fae026 230C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
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231C<import>, C<lc>, C<lcfirst>, C<lock>, C<map>, C<my>, C<no>, C<our>,
232C<prototype>, C<qr>, C<qw>, C<qx>, C<readline>, C<readpipe>, C<ref>,
233C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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234
235* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 236operator, which can be used in expressions.
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237
238=item Functions obsoleted in perl5
239
22fae026 240C<dbmclose>, C<dbmopen>
37798a01 241
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242=back
243
60f9f73c 244=head2 Portability
d74e8afc 245X<portability> X<Unix> X<portable>
60f9f73c 246
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247Perl was born in Unix and can therefore access all common Unix
248system calls. In non-Unix environments, the functionality of some
249Unix system calls may not be available, or details of the available
250functionality may differ slightly. The Perl functions affected
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251by this are:
252
253C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
254C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
255C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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256C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
257C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 258C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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259C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
260C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
261C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 262C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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263C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
264C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
265C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 266C<shmwrite>, C<socket>, C<socketpair>,
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267C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
268C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 269C<utime>, C<wait>, C<waitpid>
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270
271For more information about the portability of these functions, see
272L<perlport> and other available platform-specific documentation.
273
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274=head2 Alphabetical Listing of Perl Functions
275
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276=over 8
277
5b3c99c0 278=item -X FILEHANDLE
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279X<-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>
280X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 281
5b3c99c0 282=item -X EXPR
a0d0e21e 283
5b3c99c0 284=item -X
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285
286A file test, where X is one of the letters listed below. This unary
287operator takes one argument, either a filename or a filehandle, and
288tests the associated file to see if something is true about it. If the
7660c0ab 289argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 290Unless otherwise documented, it returns C<1> for true and C<''> for false, or
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291the undefined value if the file doesn't exist. Despite the funny
292names, precedence is the same as any other named unary operator, and
293the argument may be parenthesized like any other unary operator. The
294operator may be any of:
295
296 -r File is readable by effective uid/gid.
297 -w File is writable by effective uid/gid.
298 -x File is executable by effective uid/gid.
299 -o File is owned by effective uid.
300
301 -R File is readable by real uid/gid.
302 -W File is writable by real uid/gid.
303 -X File is executable by real uid/gid.
304 -O File is owned by real uid.
305
306 -e File exists.
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307 -z File has zero size (is empty).
308 -s File has nonzero size (returns size in bytes).
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309
310 -f File is a plain file.
311 -d File is a directory.
312 -l File is a symbolic link.
9c4d0f16 313 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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314 -S File is a socket.
315 -b File is a block special file.
316 -c File is a character special file.
317 -t Filehandle is opened to a tty.
318
319 -u File has setuid bit set.
320 -g File has setgid bit set.
321 -k File has sticky bit set.
322
121910a4 323 -T File is an ASCII text file (heuristic guess).
2cdbc966 324 -B File is a "binary" file (opposite of -T).
a0d0e21e 325
95a3fe12 326 -M Script start time minus file modification time, in days.
a0d0e21e 327 -A Same for access time.
95a3fe12 328 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 329
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330Example:
331
332 while (<>) {
5b3eff12 333 chomp;
a0d0e21e 334 next unless -f $_; # ignore specials
5a964f20 335 #...
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336 }
337
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338The interpretation of the file permission operators C<-r>, C<-R>,
339C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
340of the file and the uids and gids of the user. There may be other
341reasons you can't actually read, write, or execute the file. Such
342reasons may be for example network filesystem access controls, ACLs
343(access control lists), read-only filesystems, and unrecognized
344executable formats.
345
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346Also note that, for the superuser on the local filesystems, the C<-r>,
347C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
5ff3f7a4
GS
348if any execute bit is set in the mode. Scripts run by the superuser
349may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 350or temporarily set their effective uid to something else.
5ff3f7a4
GS
351
352If you are using ACLs, there is a pragma called C<filetest> that may
353produce more accurate results than the bare stat() mode bits.
5ff3f7a4
GS
354When under the C<use filetest 'access'> the above-mentioned filetests
355will test whether the permission can (not) be granted using the
468541a8 356access() family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
357under this pragma return true even if there are no execute permission
358bits set (nor any extra execute permission ACLs). This strangeness is
359due to the underlying system calls' definitions. Read the
360documentation for the C<filetest> pragma for more information.
361
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362Note that C<-s/a/b/> does not do a negated substitution. Saying
363C<-exp($foo)> still works as expected, however--only single letters
364following a minus are interpreted as file tests.
365
366The C<-T> and C<-B> switches work as follows. The first block or so of the
367file is examined for odd characters such as strange control codes or
61eff3bc 368characters with the high bit set. If too many strange characters (>30%)
cf264981 369are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
a0d0e21e 370containing null in the first block is considered a binary file. If C<-T>
9124316e 371or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 372rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 373file, or a file at EOF when testing a filehandle. Because you have to
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374read a file to do the C<-T> test, on most occasions you want to use a C<-f>
375against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 376
19799a22 377If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 378the special filehandle consisting of a solitary underline, then the stat
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379structure of the previous file test (or stat operator) is used, saving
380a system call. (This doesn't work with C<-t>, and you need to remember
381that lstat() and C<-l> will leave values in the stat structure for the
5c9aa243 382symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 383an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 384Example:
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385
386 print "Can do.\n" if -r $a || -w _ || -x _;
387
388 stat($filename);
389 print "Readable\n" if -r _;
390 print "Writable\n" if -w _;
391 print "Executable\n" if -x _;
392 print "Setuid\n" if -u _;
393 print "Setgid\n" if -g _;
394 print "Sticky\n" if -k _;
395 print "Text\n" if -T _;
396 print "Binary\n" if -B _;
397
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398As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
399test operators, in a way that C<-f -w -x $file> is equivalent to
cf264981 400C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
fbb0b3b3
RGS
401the return value of C<-f $file> as an argument to another filetest
402operator, no special magic will happen.)
403
a0d0e21e 404=item abs VALUE
d74e8afc 405X<abs> X<absolute>
a0d0e21e 406
54310121 407=item abs
bbce6d69 408
a0d0e21e 409Returns the absolute value of its argument.
7660c0ab 410If VALUE is omitted, uses C<$_>.
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411
412=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 413X<accept>
a0d0e21e 414
f86cebdf 415Accepts an incoming socket connect, just as the accept(2) system call
19799a22 416does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 417See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 418
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419On systems that support a close-on-exec flag on files, the flag will
420be set for the newly opened file descriptor, as determined by the
421value of $^F. See L<perlvar/$^F>.
422
a0d0e21e 423=item alarm SECONDS
d74e8afc
ITB
424X<alarm>
425X<SIGALRM>
426X<timer>
a0d0e21e 427
54310121 428=item alarm
bbce6d69 429
a0d0e21e 430Arranges to have a SIGALRM delivered to this process after the
cf264981 431specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
432specified, the value stored in C<$_> is used. (On some machines,
433unfortunately, the elapsed time may be up to one second less or more
434than you specified because of how seconds are counted, and process
435scheduling may delay the delivery of the signal even further.)
436
437Only one timer may be counting at once. Each call disables the
438previous timer, and an argument of C<0> may be supplied to cancel the
439previous timer without starting a new one. The returned value is the
440amount of time remaining on the previous timer.
a0d0e21e 441
4633a7c4 442For delays of finer granularity than one second, you may use Perl's
19799a22
GS
443four-argument version of select() leaving the first three arguments
444undefined, or you might be able to use the C<syscall> interface to
83df6a1d
JH
445access setitimer(2) if your system supports it. The Time::HiRes
446module (from CPAN, and starting from Perl 5.8 part of the standard
447distribution) may also prove useful.
2b5ab1e7 448
68f8bed4
JH
449It is usually a mistake to intermix C<alarm> and C<sleep> calls.
450(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 451
19799a22
GS
452If you want to use C<alarm> to time out a system call you need to use an
453C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 454fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 455restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 456modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
457
458 eval {
f86cebdf 459 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 460 alarm $timeout;
ff68c719 461 $nread = sysread SOCKET, $buffer, $size;
36477c24 462 alarm 0;
ff68c719 463 };
ff68c719 464 if ($@) {
f86cebdf 465 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719
PP
466 # timed out
467 }
468 else {
469 # didn't
470 }
471
91d81acc
JH
472For more information see L<perlipc>.
473
a0d0e21e 474=item atan2 Y,X
d74e8afc 475X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
476
477Returns the arctangent of Y/X in the range -PI to PI.
478
ca6e1c26 479For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
480function, or use the familiar relation:
481
482 sub tan { sin($_[0]) / cos($_[0]) }
483
bf5f1b4c
JH
484Note that atan2(0, 0) is not well-defined.
485
a0d0e21e 486=item bind SOCKET,NAME
d74e8afc 487X<bind>
a0d0e21e
LW
488
489Binds a network address to a socket, just as the bind system call
19799a22 490does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
491packed address of the appropriate type for the socket. See the examples in
492L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 493
fae2c0fb 494=item binmode FILEHANDLE, LAYER
d74e8afc 495X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 496
a0d0e21e
LW
497=item binmode FILEHANDLE
498
1cbfc93d
NIS
499Arranges for FILEHANDLE to be read or written in "binary" or "text"
500mode on systems where the run-time libraries distinguish between
501binary and text files. If FILEHANDLE is an expression, the value is
502taken as the name of the filehandle. Returns true on success,
b5fe5ca2 503otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 504
d807c6f4
JH
505On some systems (in general, DOS and Windows-based systems) binmode()
506is necessary when you're not working with a text file. For the sake
507of portability it is a good idea to always use it when appropriate,
508and to never use it when it isn't appropriate. Also, people can
509set their I/O to be by default UTF-8 encoded Unicode, not bytes.
510
511In other words: regardless of platform, use binmode() on binary data,
512like for example images.
513
514If LAYER is present it is a single string, but may contain multiple
515directives. The directives alter the behaviour of the file handle.
516When LAYER is present using binmode on text file makes sense.
517
fae2c0fb 518If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
519suitable for passing binary data. This includes turning off possible CRLF
520translation and marking it as bytes (as opposed to Unicode characters).
749683d2
YST
521Note that, despite what may be implied in I<"Programming Perl"> (the
522Camel) or elsewhere, C<:raw> is I<not> the simply inverse of C<:crlf>
fae2c0fb 523-- other layers which would affect binary nature of the stream are
0226bbdb
NIS
524I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
525PERLIO environment variable.
01e6739c 526
d807c6f4
JH
527The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
528form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
529establish default I/O layers. See L<open>.
530
fae2c0fb
RGS
531I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
532in "Programming Perl, 3rd Edition". However, since the publishing of this
533book, by many known as "Camel III", the consensus of the naming of this
534functionality has moved from "discipline" to "layer". All documentation
535of this version of Perl therefore refers to "layers" rather than to
536"disciplines". Now back to the regularly scheduled documentation...>
537
01e6739c 538To mark FILEHANDLE as UTF-8, use C<:utf8>.
1cbfc93d 539
ed53a2bb 540In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
541is done on the filehandle. Calling binmode() will normally flush any
542pending buffered output data (and perhaps pending input data) on the
fae2c0fb 543handle. An exception to this is the C<:encoding> layer that
01e6739c 544changes the default character encoding of the handle, see L<open>.
fae2c0fb 545The C<:encoding> layer sometimes needs to be called in
3874323d
JH
546mid-stream, and it doesn't flush the stream. The C<:encoding>
547also implicitly pushes on top of itself the C<:utf8> layer because
548internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 549
19799a22 550The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
551system all work together to let the programmer treat a single
552character (C<\n>) as the line terminator, irrespective of the external
553representation. On many operating systems, the native text file
554representation matches the internal representation, but on some
555platforms the external representation of C<\n> is made up of more than
556one character.
557
68bd7414
NIS
558Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
559character to end each line in the external representation of text (even
5e12dbfa 560though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
561on Unix and most VMS files). In other systems like OS/2, DOS and the
562various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
563but what's stored in text files are the two characters C<\cM\cJ>. That
564means that, if you don't use binmode() on these systems, C<\cM\cJ>
565sequences on disk will be converted to C<\n> on input, and any C<\n> in
566your program will be converted back to C<\cM\cJ> on output. This is what
567you want for text files, but it can be disastrous for binary files.
30168b04
GS
568
569Another consequence of using binmode() (on some systems) is that
570special end-of-file markers will be seen as part of the data stream.
571For systems from the Microsoft family this means that if your binary
4375e838 572data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
573the file, unless you use binmode().
574
575binmode() is not only important for readline() and print() operations,
576but also when using read(), seek(), sysread(), syswrite() and tell()
577(see L<perlport> for more details). See the C<$/> and C<$\> variables
578in L<perlvar> for how to manually set your input and output
579line-termination sequences.
a0d0e21e 580
4633a7c4 581=item bless REF,CLASSNAME
d74e8afc 582X<bless>
a0d0e21e
LW
583
584=item bless REF
585
2b5ab1e7
TC
586This function tells the thingy referenced by REF that it is now an object
587in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 588is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 589it returns the reference for convenience. Always use the two-argument
cf264981
SP
590version if a derived class might inherit the function doing the blessing.
591See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
592of objects.
a0d0e21e 593
57668c4d 594Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 595Namespaces with all lowercase names are considered reserved for
cf264981 596Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
597confusion, you may wish to avoid such package names as well. Make sure
598that CLASSNAME is a true value.
60ad88b8
GS
599
600See L<perlmod/"Perl Modules">.
601
0d863452
RH
602=item break
603
604Break out of a C<given()> block.
605
606This keyword is enabled by the "switch" feature: see L<feature>
607for more information.
608
a0d0e21e 609=item caller EXPR
d74e8afc 610X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
611
612=item caller
613
5a964f20 614Returns the context of the current subroutine call. In scalar context,
28757baa 615returns the caller's package name if there is a caller, that is, if
19799a22 616we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 617otherwise. In list context, returns
a0d0e21e 618
748a9306 619 ($package, $filename, $line) = caller;
a0d0e21e
LW
620
621With EXPR, it returns some extra information that the debugger uses to
622print a stack trace. The value of EXPR indicates how many call frames
623to go back before the current one.
624
f3aa04c2 625 ($package, $filename, $line, $subroutine, $hasargs,
b3ca2e83
NC
626 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
627 = caller($i);
e7ea3e70 628
951ba7fe 629Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 630call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 631C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 632C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 633C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 634$filename is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
635each C<use> statement creates a C<require> frame inside an C<eval EXPR>
636frame.) $subroutine may also be C<(unknown)> if this particular
637subroutine happens to have been deleted from the symbol table.
638C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
639C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
640compiled with. The C<$hints> and C<$bitmask> values are subject to change
641between versions of Perl, and are not meant for external use.
748a9306 642
b3ca2e83
NC
643C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
644caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
645of this hash, as they are the actual values stored in the optree.
646
748a9306 647Furthermore, when called from within the DB package, caller returns more
7660c0ab 648detailed information: it sets the list variable C<@DB::args> to be the
54310121 649arguments with which the subroutine was invoked.
748a9306 650
7660c0ab 651Be aware that the optimizer might have optimized call frames away before
19799a22 652C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 653might not return information about the call frame you expect it do, for
b76cc8ba 654C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 655previous time C<caller> was called.
7660c0ab 656
a0d0e21e 657=item chdir EXPR
d74e8afc
ITB
658X<chdir>
659X<cd>
a0d0e21e 660
c4aca7d0
GA
661=item chdir FILEHANDLE
662
663=item chdir DIRHANDLE
664
ce2984c3
PF
665=item chdir
666
ffce7b87 667Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 668changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 669changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
670variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
671neither is set, C<chdir> does nothing. It returns true upon success,
672false otherwise. See the example under C<die>.
a0d0e21e 673
c4aca7d0
GA
674On systems that support fchdir, you might pass a file handle or
675directory handle as argument. On systems that don't support fchdir,
676passing handles produces a fatal error at run time.
677
a0d0e21e 678=item chmod LIST
d74e8afc 679X<chmod> X<permission> X<mode>
a0d0e21e
LW
680
681Changes the permissions of a list of files. The first element of the
4633a7c4 682list must be the numerical mode, which should probably be an octal
4ad40acf 683number, and which definitely should I<not> be a string of octal digits:
2f9daede 684C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 685successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
686
687 $cnt = chmod 0755, 'foo', 'bar';
688 chmod 0755, @executables;
f86cebdf
GS
689 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
690 # --w----r-T
2f9daede
TPG
691 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
692 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 693
c4aca7d0
GA
694On systems that support fchmod, you might pass file handles among the
695files. On systems that don't support fchmod, passing file handles
345da378
GA
696produces a fatal error at run time. The file handles must be passed
697as globs or references to be recognized. Barewords are considered
698file names.
c4aca7d0
GA
699
700 open(my $fh, "<", "foo");
701 my $perm = (stat $fh)[2] & 07777;
702 chmod($perm | 0600, $fh);
703
ca6e1c26
JH
704You can also import the symbolic C<S_I*> constants from the Fcntl
705module:
706
707 use Fcntl ':mode';
708
709 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
710 # This is identical to the chmod 0755 of the above example.
711
a0d0e21e 712=item chomp VARIABLE
d74e8afc 713X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 714
313c9f5c 715=item chomp( LIST )
a0d0e21e
LW
716
717=item chomp
718
2b5ab1e7
TC
719This safer version of L</chop> removes any trailing string
720that corresponds to the current value of C<$/> (also known as
28757baa
PP
721$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
722number of characters removed from all its arguments. It's often used to
723remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
724that the final record may be missing its newline. When in paragraph
725mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
726When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
727a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 728remove anything.
19799a22 729If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
730
731 while (<>) {
732 chomp; # avoid \n on last field
733 @array = split(/:/);
5a964f20 734 # ...
a0d0e21e
LW
735 }
736
4bf21a6d
RD
737If VARIABLE is a hash, it chomps the hash's values, but not its keys.
738
a0d0e21e
LW
739You can actually chomp anything that's an lvalue, including an assignment:
740
741 chomp($cwd = `pwd`);
742 chomp($answer = <STDIN>);
743
744If you chomp a list, each element is chomped, and the total number of
745characters removed is returned.
746
442a8c12
NC
747If the C<encoding> pragma is in scope then the lengths returned are
748calculated from the length of C<$/> in Unicode characters, which is not
749always the same as the length of C<$/> in the native encoding.
750
15e44fd8
RGS
751Note that parentheses are necessary when you're chomping anything
752that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
753is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
754C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
755C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
756as C<chomp($a, $b)>.
757
a0d0e21e 758=item chop VARIABLE
d74e8afc 759X<chop>
a0d0e21e 760
313c9f5c 761=item chop( LIST )
a0d0e21e
LW
762
763=item chop
764
765Chops off the last character of a string and returns the character
5b3eff12 766chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 767scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
768If VARIABLE is a hash, it chops the hash's values, but not its keys.
769
5b3eff12 770You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
771
772If you chop a list, each element is chopped. Only the value of the
19799a22 773last C<chop> is returned.
a0d0e21e 774
19799a22 775Note that C<chop> returns the last character. To return all but the last
748a9306
LW
776character, use C<substr($string, 0, -1)>.
777
15e44fd8
RGS
778See also L</chomp>.
779
a0d0e21e 780=item chown LIST
d74e8afc 781X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
782
783Changes the owner (and group) of a list of files. The first two
19799a22
GS
784elements of the list must be the I<numeric> uid and gid, in that
785order. A value of -1 in either position is interpreted by most
786systems to leave that value unchanged. Returns the number of files
787successfully changed.
a0d0e21e
LW
788
789 $cnt = chown $uid, $gid, 'foo', 'bar';
790 chown $uid, $gid, @filenames;
791
c4aca7d0
GA
792On systems that support fchown, you might pass file handles among the
793files. On systems that don't support fchown, passing file handles
345da378
GA
794produces a fatal error at run time. The file handles must be passed
795as globs or references to be recognized. Barewords are considered
796file names.
c4aca7d0 797
54310121 798Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
799
800 print "User: ";
19799a22 801 chomp($user = <STDIN>);
5a964f20 802 print "Files: ";
19799a22 803 chomp($pattern = <STDIN>);
a0d0e21e
LW
804
805 ($login,$pass,$uid,$gid) = getpwnam($user)
806 or die "$user not in passwd file";
807
5a964f20 808 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
809 chown $uid, $gid, @ary;
810
54310121 811On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
812file unless you're the superuser, although you should be able to change
813the group to any of your secondary groups. On insecure systems, these
814restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
815On POSIX systems, you can detect this condition this way:
816
817 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
818 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 819
a0d0e21e 820=item chr NUMBER
d74e8afc 821X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 822
54310121 823=item chr
bbce6d69 824
a0d0e21e 825Returns the character represented by that NUMBER in the character set.
a0ed51b3 826For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1e54db1a
JH
827chr(0x263a) is a Unicode smiley face. Note that characters from 128
828to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
829backward compatibility reasons (but see L<encoding>).
aaa68c4a 830
8a064bd6 831Negative values give the Unicode replacement character (chr(0xfffd)),
5f0135eb 832except under the L<bytes> pragma, where low eight bits of the value
8a064bd6
JH
833(truncated to an integer) are used.
834
974da8e5
JH
835If NUMBER is omitted, uses C<$_>.
836
b76cc8ba 837For the reverse, use L</ord>.
a0d0e21e 838
974da8e5
JH
839Note that under the C<bytes> pragma the NUMBER is masked to
840the low eight bits.
841
842See L<perlunicode> and L<encoding> for more about Unicode.
bbce6d69 843
a0d0e21e 844=item chroot FILENAME
d74e8afc 845X<chroot> X<root>
a0d0e21e 846
54310121 847=item chroot
bbce6d69 848
5a964f20 849This function works like the system call by the same name: it makes the
4633a7c4 850named directory the new root directory for all further pathnames that
951ba7fe 851begin with a C</> by your process and all its children. (It doesn't
28757baa 852change your current working directory, which is unaffected.) For security
4633a7c4 853reasons, this call is restricted to the superuser. If FILENAME is
19799a22 854omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
855
856=item close FILEHANDLE
d74e8afc 857X<close>
a0d0e21e 858
6a518fbc
TP
859=item close
860
9124316e
JH
861Closes the file or pipe associated with the file handle, returning
862true only if IO buffers are successfully flushed and closes the system
863file descriptor. Closes the currently selected filehandle if the
864argument is omitted.
fb73857a
PP
865
866You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
867another C<open> on it, because C<open> will close it for you. (See
868C<open>.) However, an explicit C<close> on an input file resets the line
869counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 870
dede8123
RGS
871If the file handle came from a piped open, C<close> will additionally
872return false if one of the other system calls involved fails, or if the
fb73857a 873program exits with non-zero status. (If the only problem was that the
dede8123 874program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 875also waits for the process executing on the pipe to complete, in case you
b76cc8ba 876want to look at the output of the pipe afterwards, and
e5218da5
GA
877implicitly puts the exit status value of that command into C<$?> and
878C<${^CHILD_ERROR_NATIVE}>.
5a964f20 879
73689b13
GS
880Prematurely closing the read end of a pipe (i.e. before the process
881writing to it at the other end has closed it) will result in a
882SIGPIPE being delivered to the writer. If the other end can't
883handle that, be sure to read all the data before closing the pipe.
884
fb73857a 885Example:
a0d0e21e 886
fb73857a
PP
887 open(OUTPUT, '|sort >foo') # pipe to sort
888 or die "Can't start sort: $!";
5a964f20 889 #... # print stuff to output
fb73857a
PP
890 close OUTPUT # wait for sort to finish
891 or warn $! ? "Error closing sort pipe: $!"
892 : "Exit status $? from sort";
893 open(INPUT, 'foo') # get sort's results
894 or die "Can't open 'foo' for input: $!";
a0d0e21e 895
5a964f20
TC
896FILEHANDLE may be an expression whose value can be used as an indirect
897filehandle, usually the real filehandle name.
a0d0e21e
LW
898
899=item closedir DIRHANDLE
d74e8afc 900X<closedir>
a0d0e21e 901
19799a22 902Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
903system call.
904
a0d0e21e 905=item connect SOCKET,NAME
d74e8afc 906X<connect>
a0d0e21e
LW
907
908Attempts to connect to a remote socket, just as the connect system call
19799a22 909does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
910packed address of the appropriate type for the socket. See the examples in
911L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 912
cb1a09d0 913=item continue BLOCK
d74e8afc 914X<continue>
cb1a09d0 915
0d863452
RH
916=item continue
917
cf264981
SP
918C<continue> is actually a flow control statement rather than a function. If
919there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
920C<foreach>), it is always executed just before the conditional is about to
921be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
922it can be used to increment a loop variable, even when the loop has been
923continued via the C<next> statement (which is similar to the C C<continue>
924statement).
925
98293880 926C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
927block. C<last> and C<redo> will behave as if they had been executed within
928the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
929block, it may be more entertaining.
930
931 while (EXPR) {
932 ### redo always comes here
933 do_something;
934 } continue {
935 ### next always comes here
936 do_something_else;
937 # then back the top to re-check EXPR
938 }
939 ### last always comes here
940
941Omitting the C<continue> section is semantically equivalent to using an
19799a22 942empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
943to check the condition at the top of the loop.
944
0d863452
RH
945If the "switch" feature is enabled, C<continue> is also a
946function that will break out of the current C<when> or C<default>
947block, and fall through to the next case. See L<feature> and
948L<perlsyn/"Switch statements"> for more information.
949
950
a0d0e21e 951=item cos EXPR
d74e8afc 952X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 953
d6217f1e
GS
954=item cos
955
5a964f20 956Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 957takes cosine of C<$_>.
a0d0e21e 958
ca6e1c26 959For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
960function, or use this relation:
961
962 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
963
a0d0e21e 964=item crypt PLAINTEXT,SALT
d74e8afc
ITB
965X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
966X<decrypt> X<cryptography> X<passwd>
a0d0e21e 967
ef2e6798
MS
968Creates a digest string exactly like the crypt(3) function in the C
969library (assuming that you actually have a version there that has not
cf264981 970been extirpated as a potential munitions).
ef2e6798
MS
971
972crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
973into a short string, called a digest, which is returned. The same
974PLAINTEXT and SALT will always return the same string, but there is no
975(known) way to get the original PLAINTEXT from the hash. Small
976changes in the PLAINTEXT or SALT will result in large changes in the
977digest.
978
979There is no decrypt function. This function isn't all that useful for
980cryptography (for that, look for F<Crypt> modules on your nearby CPAN
981mirror) and the name "crypt" is a bit of a misnomer. Instead it is
982primarily used to check if two pieces of text are the same without
983having to transmit or store the text itself. An example is checking
984if a correct password is given. The digest of the password is stored,
cf264981 985not the password itself. The user types in a password that is
ef2e6798
MS
986crypt()'d with the same salt as the stored digest. If the two digests
987match the password is correct.
988
989When verifying an existing digest string you should use the digest as
990the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 991to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
992crypt() will hash the new string with the same salt as the digest.
993This allows your code to work with the standard L<crypt|/crypt> and
994with more exotic implementations. In other words, do not assume
995anything about the returned string itself, or how many bytes in the
996digest matter.
85c16d83
JH
997
998Traditionally the result is a string of 13 bytes: two first bytes of
999the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
ef2e6798
MS
1000the first eight bytes of the digest string mattered, but alternative
1001hashing schemes (like MD5), higher level security schemes (like C2),
1002and implementations on non-UNIX platforms may produce different
1003strings.
85c16d83
JH
1004
1005When choosing a new salt create a random two character string whose
1006characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1007'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1008characters is just a recommendation; the characters allowed in
1009the salt depend solely on your system's crypt library, and Perl can't
1010restrict what salts C<crypt()> accepts.
e71965be 1011
a0d0e21e 1012Here's an example that makes sure that whoever runs this program knows
cf264981 1013their password:
a0d0e21e
LW
1014
1015 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1016
1017 system "stty -echo";
1018 print "Password: ";
e71965be 1019 chomp($word = <STDIN>);
a0d0e21e
LW
1020 print "\n";
1021 system "stty echo";
1022
e71965be 1023 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
1024 die "Sorry...\n";
1025 } else {
1026 print "ok\n";
54310121 1027 }
a0d0e21e 1028
9f8f0c9d 1029Of course, typing in your own password to whoever asks you
748a9306 1030for it is unwise.
a0d0e21e 1031
ef2e6798 1032The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1033of data, not least of all because you can't get the information
ef2e6798 1034back. Look at the L<Digest> module for more robust algorithms.
19799a22 1035
f2791508
JH
1036If using crypt() on a Unicode string (which I<potentially> has
1037characters with codepoints above 255), Perl tries to make sense
1038of the situation by trying to downgrade (a copy of the string)
1039the string back to an eight-bit byte string before calling crypt()
1040(on that copy). If that works, good. If not, crypt() dies with
1041C<Wide character in crypt>.
85c16d83 1042
aa689395 1043=item dbmclose HASH
d74e8afc 1044X<dbmclose>
a0d0e21e 1045
19799a22 1046[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1047
aa689395 1048Breaks the binding between a DBM file and a hash.
a0d0e21e 1049
19799a22 1050=item dbmopen HASH,DBNAME,MASK
d74e8afc 1051X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1052
19799a22 1053[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1054
7b8d334a 1055This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1056hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1057argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1058is the name of the database (without the F<.dir> or F<.pag> extension if
1059any). If the database does not exist, it is created with protection
19799a22
GS
1060specified by MASK (as modified by the C<umask>). If your system supports
1061only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 1062program. In older versions of Perl, if your system had neither DBM nor
19799a22 1063ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1064sdbm(3).
1065
1066If you don't have write access to the DBM file, you can only read hash
1067variables, not set them. If you want to test whether you can write,
19799a22 1068either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 1069which will trap the error.
a0d0e21e 1070
19799a22
GS
1071Note that functions such as C<keys> and C<values> may return huge lists
1072when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1073function to iterate over large DBM files. Example:
1074
1075 # print out history file offsets
1076 dbmopen(%HIST,'/usr/lib/news/history',0666);
1077 while (($key,$val) = each %HIST) {
1078 print $key, ' = ', unpack('L',$val), "\n";
1079 }
1080 dbmclose(%HIST);
1081
cb1a09d0 1082See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1083cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1084rich implementation.
4633a7c4 1085
2b5ab1e7
TC
1086You can control which DBM library you use by loading that library
1087before you call dbmopen():
1088
1089 use DB_File;
1090 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1091 or die "Can't open netscape history file: $!";
1092
a0d0e21e 1093=item defined EXPR
d74e8afc 1094X<defined> X<undef> X<undefined>
a0d0e21e 1095
54310121 1096=item defined
bbce6d69 1097
2f9daede
TPG
1098Returns a Boolean value telling whether EXPR has a value other than
1099the undefined value C<undef>. If EXPR is not present, C<$_> will be
1100checked.
1101
1102Many operations return C<undef> to indicate failure, end of file,
1103system error, uninitialized variable, and other exceptional
1104conditions. This function allows you to distinguish C<undef> from
1105other values. (A simple Boolean test will not distinguish among
7660c0ab 1106C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1107false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1108doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1109returns C<undef> when its argument is an empty array, I<or> when the
1110element to return happens to be C<undef>.
1111
f10b0346
GS
1112You may also use C<defined(&func)> to check whether subroutine C<&func>
1113has ever been defined. The return value is unaffected by any forward
04891299 1114declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1115may still be callable: its package may have an C<AUTOLOAD> method that
1116makes it spring into existence the first time that it is called -- see
1117L<perlsub>.
f10b0346
GS
1118
1119Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1120used to report whether memory for that aggregate has ever been
1121allocated. This behavior may disappear in future versions of Perl.
1122You should instead use a simple test for size:
1123
1124 if (@an_array) { print "has array elements\n" }
1125 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1126
1127When used on a hash element, it tells you whether the value is defined,
dc848c6f 1128not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1129purpose.
a0d0e21e
LW
1130
1131Examples:
1132
1133 print if defined $switch{'D'};
1134 print "$val\n" while defined($val = pop(@ary));
1135 die "Can't readlink $sym: $!"
1136 unless defined($value = readlink $sym);
a0d0e21e 1137 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1138 $debugging = 0 unless defined $debugging;
a0d0e21e 1139
19799a22 1140Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1141discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1142defined values. For example, if you say
a5f75d66
AD
1143
1144 "ab" =~ /a(.*)b/;
1145
7660c0ab 1146The pattern match succeeds, and C<$1> is defined, despite the fact that it
cf264981 1147matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1148matched something that happened to be zero characters long. This is all
a5f75d66 1149very above-board and honest. When a function returns an undefined value,
2f9daede 1150it's an admission that it couldn't give you an honest answer. So you
19799a22 1151should use C<defined> only when you're questioning the integrity of what
7660c0ab 1152you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1153what you want.
1154
dc848c6f 1155See also L</undef>, L</exists>, L</ref>.
2f9daede 1156
a0d0e21e 1157=item delete EXPR
d74e8afc 1158X<delete>
a0d0e21e 1159
01020589
GS
1160Given an expression that specifies a hash element, array element, hash slice,
1161or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1162In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1163the size of the array will shrink to the highest element that tests
8216c1fd 1164true for exists() (or 0 if no such element exists).
a0d0e21e 1165
eba0920a
EM
1166Returns a list with the same number of elements as the number of elements
1167for which deletion was attempted. Each element of that list consists of
1168either the value of the element deleted, or the undefined value. In scalar
1169context, this means that you get the value of the last element deleted (or
1170the undefined value if that element did not exist).
1171
1172 %hash = (foo => 11, bar => 22, baz => 33);
1173 $scalar = delete $hash{foo}; # $scalar is 11
1174 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1175 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1176
1177Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1178a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1179from a C<tie>d hash or array may not necessarily return anything.
1180
8ea97a1e
GS
1181Deleting an array element effectively returns that position of the array
1182to its initial, uninitialized state. Subsequently testing for the same
cf264981
SP
1183element with exists() will return false. Also, deleting array elements
1184in the middle of an array will not shift the index of the elements
1185after them down. Use splice() for that. See L</exists>.
8ea97a1e 1186
01020589 1187The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1188
5f05dabc
PP
1189 foreach $key (keys %HASH) {
1190 delete $HASH{$key};
a0d0e21e
LW
1191 }
1192
01020589
GS
1193 foreach $index (0 .. $#ARRAY) {
1194 delete $ARRAY[$index];
1195 }
1196
1197And so do these:
5f05dabc 1198
01020589
GS
1199 delete @HASH{keys %HASH};
1200
9740c838 1201 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1202
2b5ab1e7 1203But both of these are slower than just assigning the empty list
01020589
GS
1204or undefining %HASH or @ARRAY:
1205
1206 %HASH = (); # completely empty %HASH
1207 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1208
01020589
GS
1209 @ARRAY = (); # completely empty @ARRAY
1210 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1211
1212Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1213operation is a hash element, array element, hash slice, or array slice
1214lookup:
a0d0e21e
LW
1215
1216 delete $ref->[$x][$y]{$key};
5f05dabc 1217 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1218
01020589
GS
1219 delete $ref->[$x][$y][$index];
1220 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1221
a0d0e21e 1222=item die LIST
d74e8afc 1223X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1224
19799a22
GS
1225Outside an C<eval>, prints the value of LIST to C<STDERR> and
1226exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1227exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1228status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1229an C<eval(),> the error message is stuffed into C<$@> and the
1230C<eval> is terminated with the undefined value. This makes
1231C<die> the way to raise an exception.
a0d0e21e
LW
1232
1233Equivalent examples:
1234
1235 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1236 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1237
ccac6780 1238If the last element of LIST does not end in a newline, the current
df37ec69
WW
1239script line number and input line number (if any) are also printed,
1240and a newline is supplied. Note that the "input line number" (also
1241known as "chunk") is subject to whatever notion of "line" happens to
1242be currently in effect, and is also available as the special variable
1243C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1244
1245Hint: sometimes appending C<", stopped"> to your message will cause it
1246to make better sense when the string C<"at foo line 123"> is appended.
1247Suppose you are running script "canasta".
a0d0e21e
LW
1248
1249 die "/etc/games is no good";
1250 die "/etc/games is no good, stopped";
1251
1252produce, respectively
1253
1254 /etc/games is no good at canasta line 123.
1255 /etc/games is no good, stopped at canasta line 123.
1256
2b5ab1e7 1257See also exit(), warn(), and the Carp module.
a0d0e21e 1258
7660c0ab
A
1259If LIST is empty and C<$@> already contains a value (typically from a
1260previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1261This is useful for propagating exceptions:
1262
1263 eval { ... };
1264 die unless $@ =~ /Expected exception/;
1265
ad216e65
JH
1266If LIST is empty and C<$@> contains an object reference that has a
1267C<PROPAGATE> method, that method will be called with additional file
1268and line number parameters. The return value replaces the value in
28a5cf3b 1269C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1270were called.
1271
7660c0ab 1272If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1273
52531d10
GS
1274die() can also be called with a reference argument. If this happens to be
1275trapped within an eval(), $@ contains the reference. This behavior permits
1276a more elaborate exception handling implementation using objects that
4375e838 1277maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1278is sometimes preferable to matching particular string values of $@ using
1279regular expressions. Here's an example:
1280
da279afe 1281 use Scalar::Util 'blessed';
1282
52531d10
GS
1283 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1284 if ($@) {
da279afe 1285 if (blessed($@) && $@->isa("Some::Module::Exception")) {
52531d10
GS
1286 # handle Some::Module::Exception
1287 }
1288 else {
1289 # handle all other possible exceptions
1290 }
1291 }
1292
19799a22 1293Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1294them, you may want to overload stringification operations on such custom
1295exception objects. See L<overload> for details about that.
1296
19799a22
GS
1297You can arrange for a callback to be run just before the C<die>
1298does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1299handler will be called with the error text and can change the error
1300message, if it sees fit, by calling C<die> again. See
1301L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1302L<"eval BLOCK"> for some examples. Although this feature was
19799a22
GS
1303to be run only right before your program was to exit, this is not
1304currently the case--the C<$SIG{__DIE__}> hook is currently called
1305even inside eval()ed blocks/strings! If one wants the hook to do
1306nothing in such situations, put
fb73857a
PP
1307
1308 die @_ if $^S;
1309
19799a22
GS
1310as the first line of the handler (see L<perlvar/$^S>). Because
1311this promotes strange action at a distance, this counterintuitive
b76cc8ba 1312behavior may be fixed in a future release.
774d564b 1313
a0d0e21e 1314=item do BLOCK
d74e8afc 1315X<do> X<block>
a0d0e21e
LW
1316
1317Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1318sequence of commands indicated by BLOCK. When modified by the C<while> or
1319C<until> loop modifier, executes the BLOCK once before testing the loop
1320condition. (On other statements the loop modifiers test the conditional
1321first.)
a0d0e21e 1322
4968c1e4 1323C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1324C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1325See L<perlsyn> for alternative strategies.
4968c1e4 1326
a0d0e21e 1327=item do SUBROUTINE(LIST)
d74e8afc 1328X<do>
a0d0e21e 1329
cf264981 1330This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1331
1332=item do EXPR
d74e8afc 1333X<do>
a0d0e21e
LW
1334
1335Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1336file as a Perl script.
a0d0e21e
LW
1337
1338 do 'stat.pl';
1339
1340is just like
1341
986b19de 1342 eval `cat stat.pl`;
a0d0e21e 1343
2b5ab1e7 1344except that it's more efficient and concise, keeps track of the current
ea63ef19 1345filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1346C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1347variables. It also differs in that code evaluated with C<do FILENAME>
1348cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1349same, however, in that it does reparse the file every time you call it,
1350so you probably don't want to do this inside a loop.
a0d0e21e 1351
8e30cc93 1352If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1353error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1354returns undef and sets an error message in C<$@>. If the file is
1355successfully compiled, C<do> returns the value of the last expression
1356evaluated.
1357
a0d0e21e 1358Note that inclusion of library modules is better done with the
19799a22 1359C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1360and raise an exception if there's a problem.
a0d0e21e 1361
5a964f20
TC
1362You might like to use C<do> to read in a program configuration
1363file. Manual error checking can be done this way:
1364
b76cc8ba 1365 # read in config files: system first, then user
f86cebdf 1366 for $file ("/share/prog/defaults.rc",
b76cc8ba 1367 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1368 {
5a964f20 1369 unless ($return = do $file) {
f86cebdf
GS
1370 warn "couldn't parse $file: $@" if $@;
1371 warn "couldn't do $file: $!" unless defined $return;
1372 warn "couldn't run $file" unless $return;
5a964f20
TC
1373 }
1374 }
1375
a0d0e21e 1376=item dump LABEL
d74e8afc 1377X<dump> X<core> X<undump>
a0d0e21e 1378
1614b0e3
JD
1379=item dump
1380
19799a22
GS
1381This function causes an immediate core dump. See also the B<-u>
1382command-line switch in L<perlrun>, which does the same thing.
1383Primarily this is so that you can use the B<undump> program (not
1384supplied) to turn your core dump into an executable binary after
1385having initialized all your variables at the beginning of the
1386program. When the new binary is executed it will begin by executing
1387a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1388Think of it as a goto with an intervening core dump and reincarnation.
1389If C<LABEL> is omitted, restarts the program from the top.
1390
1391B<WARNING>: Any files opened at the time of the dump will I<not>
1392be open any more when the program is reincarnated, with possible
b76cc8ba 1393resulting confusion on the part of Perl.
19799a22
GS
1394
1395This function is now largely obsolete, partly because it's very
1396hard to convert a core file into an executable, and because the
1397real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1398C code have superseded it. That's why you should now invoke it as
1399C<CORE::dump()>, if you don't want to be warned against a possible
1400typo.
19799a22
GS
1401
1402If you're looking to use L<dump> to speed up your program, consider
1403generating bytecode or native C code as described in L<perlcc>. If
1404you're just trying to accelerate a CGI script, consider using the
210b36aa 1405C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1406You might also consider autoloading or selfloading, which at least
b76cc8ba 1407make your program I<appear> to run faster.
5a964f20 1408
aa689395 1409=item each HASH
d74e8afc 1410X<each> X<hash, iterator>
aa689395 1411
5a964f20 1412When called in list context, returns a 2-element list consisting of the
aa689395 1413key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1414it. When called in scalar context, returns only the key for the next
e902a979 1415element in the hash.
2f9daede 1416
ab192400 1417Entries are returned in an apparently random order. The actual random
504f80c1
JH
1418order is subject to change in future versions of perl, but it is
1419guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1420function would produce on the same (unmodified) hash. Since Perl
14215.8.1 the ordering is different even between different runs of Perl
1422for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1423
1424When the hash is entirely read, a null array is returned in list context
19799a22
GS
1425(which when assigned produces a false (C<0>) value), and C<undef> in
1426scalar context. The next call to C<each> after that will start iterating
1427again. There is a single iterator for each hash, shared by all C<each>,
1428C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1429reading all the elements from the hash, or by evaluating C<keys HASH> or
1430C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1431iterating over it, you may get entries skipped or duplicated, so
1432don't. Exception: It is always safe to delete the item most recently
1433returned by C<each()>, which means that the following code will work:
1434
1435 while (($key, $value) = each %hash) {
1436 print $key, "\n";
1437 delete $hash{$key}; # This is safe
1438 }
aa689395 1439
f86cebdf 1440The following prints out your environment like the printenv(1) program,
aa689395 1441only in a different order:
a0d0e21e
LW
1442
1443 while (($key,$value) = each %ENV) {
1444 print "$key=$value\n";
1445 }
1446
19799a22 1447See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1448
1449=item eof FILEHANDLE
d74e8afc
ITB
1450X<eof>
1451X<end of file>
1452X<end-of-file>
a0d0e21e 1453
4633a7c4
LW
1454=item eof ()
1455
a0d0e21e
LW
1456=item eof
1457
1458Returns 1 if the next read on FILEHANDLE will return end of file, or if
1459FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1460gives the real filehandle. (Note that this function actually
19799a22 1461reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1462interactive context.) Do not read from a terminal file (or call
19799a22 1463C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1464as terminals may lose the end-of-file condition if you do.
1465
820475bd
GS
1466An C<eof> without an argument uses the last file read. Using C<eof()>
1467with empty parentheses is very different. It refers to the pseudo file
1468formed from the files listed on the command line and accessed via the
61eff3bc
JH
1469C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1470as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1471used will cause C<@ARGV> to be examined to determine if input is
67408cae 1472available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1473end-of-file will assume you are processing another C<@ARGV> list,
1474and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1475see L<perlop/"I/O Operators">.
820475bd 1476
61eff3bc 1477In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1478detect the end of each file, C<eof()> will only detect the end of the
1479last file. Examples:
a0d0e21e 1480
748a9306
LW
1481 # reset line numbering on each input file
1482 while (<>) {
b76cc8ba 1483 next if /^\s*#/; # skip comments
748a9306 1484 print "$.\t$_";
5a964f20
TC
1485 } continue {
1486 close ARGV if eof; # Not eof()!
748a9306
LW
1487 }
1488
a0d0e21e
LW
1489 # insert dashes just before last line of last file
1490 while (<>) {
6ac88b13 1491 if (eof()) { # check for end of last file
a0d0e21e
LW
1492 print "--------------\n";
1493 }
1494 print;
6ac88b13 1495 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1496 }
1497
a0d0e21e 1498Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1499input operators typically return C<undef> when they run out of data, or if
1500there was an error.
a0d0e21e
LW
1501
1502=item eval EXPR
d74e8afc 1503X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
a0d0e21e
LW
1504
1505=item eval BLOCK
1506
ce2984c3
PF
1507=item eval
1508
c7cc6f1c
GS
1509In the first form, the return value of EXPR is parsed and executed as if it
1510were a little Perl program. The value of the expression (which is itself
5a964f20 1511determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1512errors, executed in the lexical context of the current Perl program, so
1513that any variable settings or subroutine and format definitions remain
cf264981 1514afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1515If EXPR is omitted, evaluates C<$_>. This form is typically used to
1516delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1517
1518In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1519same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1520within the context of the current Perl program. This form is typically
1521used to trap exceptions more efficiently than the first (see below), while
1522also providing the benefit of checking the code within BLOCK at compile
1523time.
1524
1525The final semicolon, if any, may be omitted from the value of EXPR or within
1526the BLOCK.
1527
1528In both forms, the value returned is the value of the last expression
5a964f20 1529evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1530as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1531in void, scalar, or list context, depending on the context of the C<eval>
1532itself. See L</wantarray> for more on how the evaluation context can be
1533determined.
a0d0e21e 1534
19799a22
GS
1535If there is a syntax error or runtime error, or a C<die> statement is
1536executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1537error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1538string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1539warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1540To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1541turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1542See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1543
19799a22
GS
1544Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1545determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1546is implemented. It is also Perl's exception trapping mechanism, where
1547the die operator is used to raise exceptions.
1548
1549If the code to be executed doesn't vary, you may use the eval-BLOCK
1550form to trap run-time errors without incurring the penalty of
1551recompiling each time. The error, if any, is still returned in C<$@>.
1552Examples:
1553
54310121 1554 # make divide-by-zero nonfatal
a0d0e21e
LW
1555 eval { $answer = $a / $b; }; warn $@ if $@;
1556
1557 # same thing, but less efficient
1558 eval '$answer = $a / $b'; warn $@ if $@;
1559
1560 # a compile-time error
5a964f20 1561 eval { $answer = }; # WRONG
a0d0e21e
LW
1562
1563 # a run-time error
1564 eval '$answer ='; # sets $@
1565
cf264981
SP
1566Using the C<eval{}> form as an exception trap in libraries does have some
1567issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1568may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7
TC
1569You can use the C<local $SIG{__DIE__}> construct for this purpose,
1570as shown in this example:
774d564b
PP
1571
1572 # a very private exception trap for divide-by-zero
f86cebdf
GS
1573 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1574 warn $@ if $@;
774d564b
PP
1575
1576This is especially significant, given that C<__DIE__> hooks can call
19799a22 1577C<die> again, which has the effect of changing their error messages:
774d564b
PP
1578
1579 # __DIE__ hooks may modify error messages
1580 {
f86cebdf
GS
1581 local $SIG{'__DIE__'} =
1582 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1583 eval { die "foo lives here" };
1584 print $@ if $@; # prints "bar lives here"
774d564b
PP
1585 }
1586
19799a22 1587Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1588may be fixed in a future release.
1589
19799a22 1590With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1591being looked at when:
1592
1593 eval $x; # CASE 1
1594 eval "$x"; # CASE 2
1595
1596 eval '$x'; # CASE 3
1597 eval { $x }; # CASE 4
1598
5a964f20 1599 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1600 $$x++; # CASE 6
1601
2f9daede 1602Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1603the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1604the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1605and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1606does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1607purely visual reasons, but it also has the advantage of compiling at
1608compile-time instead of at run-time.) Case 5 is a place where
19799a22 1609normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1610particular situation, you can just use symbolic references instead, as
1611in case 6.
a0d0e21e 1612
4968c1e4 1613C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1614C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1615
d819b83a
DM
1616Note that as a very special case, an C<eval ''> executed within the C<DB>
1617package doesn't see the usual surrounding lexical scope, but rather the
1618scope of the first non-DB piece of code that called it. You don't normally
1619need to worry about this unless you are writing a Perl debugger.
1620
a0d0e21e 1621=item exec LIST
d74e8afc 1622X<exec> X<execute>
a0d0e21e 1623
8bf3b016
GS
1624=item exec PROGRAM LIST
1625
19799a22
GS
1626The C<exec> function executes a system command I<and never returns>--
1627use C<system> instead of C<exec> if you want it to return. It fails and
1628returns false only if the command does not exist I<and> it is executed
fb73857a 1629directly instead of via your system's command shell (see below).
a0d0e21e 1630
19799a22
GS
1631Since it's a common mistake to use C<exec> instead of C<system>, Perl
1632warns you if there is a following statement which isn't C<die>, C<warn>,
1633or C<exit> (if C<-w> is set - but you always do that). If you
1634I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1635can use one of these styles to avoid the warning:
1636
5a964f20
TC
1637 exec ('foo') or print STDERR "couldn't exec foo: $!";
1638 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1639
5a964f20 1640If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1641with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1642If there is only one scalar argument or an array with one element in it,
1643the argument is checked for shell metacharacters, and if there are any,
1644the entire argument is passed to the system's command shell for parsing
1645(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1646If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1647words and passed directly to C<execvp>, which is more efficient.
19799a22 1648Examples:
a0d0e21e 1649
19799a22
GS
1650 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1651 exec "sort $outfile | uniq";
a0d0e21e
LW
1652
1653If you don't really want to execute the first argument, but want to lie
1654to the program you are executing about its own name, you can specify
1655the program you actually want to run as an "indirect object" (without a
1656comma) in front of the LIST. (This always forces interpretation of the
54310121 1657LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1658the list.) Example:
1659
1660 $shell = '/bin/csh';
1661 exec $shell '-sh'; # pretend it's a login shell
1662
1663or, more directly,
1664
1665 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1666
bb32b41a
GS
1667When the arguments get executed via the system shell, results will
1668be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1669for details.
1670
19799a22
GS
1671Using an indirect object with C<exec> or C<system> is also more
1672secure. This usage (which also works fine with system()) forces
1673interpretation of the arguments as a multivalued list, even if the
1674list had just one argument. That way you're safe from the shell
1675expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1676
1677 @args = ( "echo surprise" );
1678
2b5ab1e7 1679 exec @args; # subject to shell escapes
f86cebdf 1680 # if @args == 1
2b5ab1e7 1681 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1682
1683The first version, the one without the indirect object, ran the I<echo>
1684program, passing it C<"surprise"> an argument. The second version
1685didn't--it tried to run a program literally called I<"echo surprise">,
1686didn't find it, and set C<$?> to a non-zero value indicating failure.
1687
0f897271
GS
1688Beginning with v5.6.0, Perl will attempt to flush all files opened for
1689output before the exec, but this may not be supported on some platforms
1690(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1691in English) or call the C<autoflush()> method of C<IO::Handle> on any
1692open handles in order to avoid lost output.
1693
19799a22 1694Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1695any C<DESTROY> methods in your objects.
1696
a0d0e21e 1697=item exists EXPR
d74e8afc 1698X<exists> X<autovivification>
a0d0e21e 1699
01020589 1700Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1701returns true if the specified element in the hash or array has ever
1702been initialized, even if the corresponding value is undefined. The
1703element is not autovivified if it doesn't exist.
a0d0e21e 1704
01020589
GS
1705 print "Exists\n" if exists $hash{$key};
1706 print "Defined\n" if defined $hash{$key};
1707 print "True\n" if $hash{$key};
1708
1709 print "Exists\n" if exists $array[$index];
1710 print "Defined\n" if defined $array[$index];
1711 print "True\n" if $array[$index];
a0d0e21e 1712
8ea97a1e 1713A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1714it exists, but the reverse doesn't necessarily hold true.
1715
afebc493
GS
1716Given an expression that specifies the name of a subroutine,
1717returns true if the specified subroutine has ever been declared, even
1718if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1719does not count as declaring it. Note that a subroutine which does not
1720exist may still be callable: its package may have an C<AUTOLOAD>
1721method that makes it spring into existence the first time that it is
1722called -- see L<perlsub>.
afebc493
GS
1723
1724 print "Exists\n" if exists &subroutine;
1725 print "Defined\n" if defined &subroutine;
1726
a0d0e21e 1727Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1728operation is a hash or array key lookup or subroutine name:
a0d0e21e 1729
2b5ab1e7
TC
1730 if (exists $ref->{A}->{B}->{$key}) { }
1731 if (exists $hash{A}{B}{$key}) { }
1732
01020589
GS
1733 if (exists $ref->{A}->{B}->[$ix]) { }
1734 if (exists $hash{A}{B}[$ix]) { }
1735
afebc493
GS
1736 if (exists &{$ref->{A}{B}{$key}}) { }
1737
01020589
GS
1738Although the deepest nested array or hash will not spring into existence
1739just because its existence was tested, any intervening ones will.
61eff3bc 1740Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1741into existence due to the existence test for the $key element above.
1742This happens anywhere the arrow operator is used, including even:
5a964f20 1743
2b5ab1e7
TC
1744 undef $ref;
1745 if (exists $ref->{"Some key"}) { }
1746 print $ref; # prints HASH(0x80d3d5c)
1747
1748This surprising autovivification in what does not at first--or even
1749second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1750release.
a0d0e21e 1751
afebc493
GS
1752Use of a subroutine call, rather than a subroutine name, as an argument
1753to exists() is an error.
1754
1755 exists &sub; # OK
1756 exists &sub(); # Error
1757
a0d0e21e 1758=item exit EXPR
d74e8afc 1759X<exit> X<terminate> X<abort>
a0d0e21e 1760
ce2984c3
PF
1761=item exit
1762
2b5ab1e7 1763Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1764
1765 $ans = <STDIN>;
1766 exit 0 if $ans =~ /^[Xx]/;
1767
19799a22 1768See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1769universally recognized values for EXPR are C<0> for success and C<1>
1770for error; other values are subject to interpretation depending on the
1771environment in which the Perl program is running. For example, exiting
177269 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1773the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1774
19799a22
GS
1775Don't use C<exit> to abort a subroutine if there's any chance that
1776someone might want to trap whatever error happened. Use C<die> instead,
1777which can be trapped by an C<eval>.
28757baa 1778
19799a22 1779The exit() function does not always exit immediately. It calls any
2b5ab1e7 1780defined C<END> routines first, but these C<END> routines may not
19799a22 1781themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1782be called are called before the real exit. If this is a problem, you
1783can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1784See L<perlmod> for details.
5a964f20 1785
a0d0e21e 1786=item exp EXPR
d74e8afc 1787X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1788
54310121 1789=item exp
bbce6d69 1790
b76cc8ba 1791Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1792If EXPR is omitted, gives C<exp($_)>.
1793
1794=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1795X<fcntl>
a0d0e21e 1796
f86cebdf 1797Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1798
1799 use Fcntl;
1800
0ade1984 1801first to get the correct constant definitions. Argument processing and
b76cc8ba 1802value return works just like C<ioctl> below.
a0d0e21e
LW
1803For example:
1804
1805 use Fcntl;
5a964f20
TC
1806 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1807 or die "can't fcntl F_GETFL: $!";
1808
554ad1fc 1809You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1810Like C<ioctl>, it maps a C<0> return from the system call into
1811C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1812in numeric context. It is also exempt from the normal B<-w> warnings
1813on improper numeric conversions.
5a964f20 1814
19799a22 1815Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1816doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1817manpage to learn what functions are available on your system.
a0d0e21e 1818
be2f7487 1819Here's an example of setting a filehandle named C<REMOTE> to be
1820non-blocking at the system level. You'll have to negotiate C<$|>
1821on your own, though.
1822
1823 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1824
1825 $flags = fcntl(REMOTE, F_GETFL, 0)
1826 or die "Can't get flags for the socket: $!\n";
1827
1828 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1829 or die "Can't set flags for the socket: $!\n";
1830
a0d0e21e 1831=item fileno FILEHANDLE
d74e8afc 1832X<fileno>
a0d0e21e 1833
2b5ab1e7
TC
1834Returns the file descriptor for a filehandle, or undefined if the
1835filehandle is not open. This is mainly useful for constructing
19799a22 1836bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1837If FILEHANDLE is an expression, the value is taken as an indirect
1838filehandle, generally its name.
5a964f20 1839
b76cc8ba 1840You can use this to find out whether two handles refer to the
5a964f20
TC
1841same underlying descriptor:
1842
1843 if (fileno(THIS) == fileno(THAT)) {
1844 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1845 }
1846
1847(Filehandles connected to memory objects via new features of C<open> may
1848return undefined even though they are open.)
1849
a0d0e21e
LW
1850
1851=item flock FILEHANDLE,OPERATION
d74e8afc 1852X<flock> X<lock> X<locking>
a0d0e21e 1853
19799a22
GS
1854Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1855for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1856machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1857C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1858only entire files, not records.
1859
1860Two potentially non-obvious but traditional C<flock> semantics are
1861that it waits indefinitely until the lock is granted, and that its locks
1862B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1863fewer guarantees. This means that programs that do not also use C<flock>
1864may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1865your port's specific documentation, or your system-specific local manpages
1866for details. It's best to assume traditional behavior if you're writing
1867portable programs. (But if you're not, you should as always feel perfectly
1868free to write for your own system's idiosyncrasies (sometimes called
1869"features"). Slavish adherence to portability concerns shouldn't get
1870in the way of your getting your job done.)
a3cb178b 1871
8ebc5c01
PP
1872OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1873LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1874you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1875either individually, or as a group using the ':flock' tag. LOCK_SH
1876requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1877releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1878LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1879waiting for the lock (check the return status to see if you got it).
1880
2b5ab1e7
TC
1881To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1882before locking or unlocking it.
8ebc5c01 1883
f86cebdf 1884Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1885locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1886are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1887implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1888differing semantics shouldn't bite too many people.
1889
becacb53
TM
1890Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1891be open with read intent to use LOCK_SH and requires that it be open
1892with write intent to use LOCK_EX.
1893
19799a22
GS
1894Note also that some versions of C<flock> cannot lock things over the
1895network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1896that. If you like you can force Perl to ignore your system's flock(2)
1897function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1898the switch C<-Ud_flock> to the F<Configure> program when you configure
1899perl.
4633a7c4
LW
1900
1901Here's a mailbox appender for BSD systems.
a0d0e21e 1902
7e1af8bc 1903 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1904
1905 sub lock {
7e1af8bc 1906 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1907 # and, in case someone appended
1908 # while we were waiting...
1909 seek(MBOX, 0, 2);
1910 }
1911
1912 sub unlock {
7e1af8bc 1913 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1914 }
1915
1916 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1917 or die "Can't open mailbox: $!";
1918
1919 lock();
1920 print MBOX $msg,"\n\n";
1921 unlock();
1922
2b5ab1e7
TC
1923On systems that support a real flock(), locks are inherited across fork()
1924calls, whereas those that must resort to the more capricious fcntl()
1925function lose the locks, making it harder to write servers.
1926
cb1a09d0 1927See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1928
1929=item fork
d74e8afc 1930X<fork> X<child> X<parent>
a0d0e21e 1931
2b5ab1e7
TC
1932Does a fork(2) system call to create a new process running the
1933same program at the same point. It returns the child pid to the
1934parent process, C<0> to the child process, or C<undef> if the fork is
1935unsuccessful. File descriptors (and sometimes locks on those descriptors)
1936are shared, while everything else is copied. On most systems supporting
1937fork(), great care has gone into making it extremely efficient (for
1938example, using copy-on-write technology on data pages), making it the
1939dominant paradigm for multitasking over the last few decades.
5a964f20 1940
0f897271
GS
1941Beginning with v5.6.0, Perl will attempt to flush all files opened for
1942output before forking the child process, but this may not be supported
1943on some platforms (see L<perlport>). To be safe, you may need to set
1944C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1945C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1946
19799a22 1947If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1948accumulate zombies. On some systems, you can avoid this by setting
1949C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1950forking and reaping moribund children.
cb1a09d0 1951
28757baa
PP
1952Note that if your forked child inherits system file descriptors like
1953STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1954if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1955backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1956You should reopen those to F</dev/null> if it's any issue.
28757baa 1957
cb1a09d0 1958=item format
d74e8afc 1959X<format>
cb1a09d0 1960
19799a22 1961Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1962example:
1963
54310121 1964 format Something =
cb1a09d0
AD
1965 Test: @<<<<<<<< @||||| @>>>>>
1966 $str, $%, '$' . int($num)
1967 .
1968
1969 $str = "widget";
184e9718 1970 $num = $cost/$quantity;
cb1a09d0
AD
1971 $~ = 'Something';
1972 write;
1973
1974See L<perlform> for many details and examples.
1975
8903cb82 1976=item formline PICTURE,LIST
d74e8afc 1977X<formline>
a0d0e21e 1978
5a964f20 1979This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1980too. It formats (see L<perlform>) a list of values according to the
1981contents of PICTURE, placing the output into the format output
7660c0ab 1982accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1983Eventually, when a C<write> is done, the contents of
cf264981
SP
1984C<$^A> are written to some filehandle. You could also read C<$^A>
1985and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1986does one C<formline> per line of form, but the C<formline> function itself
748a9306 1987doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1988that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1989You may therefore need to use multiple formlines to implement a single
1990record format, just like the format compiler.
1991
19799a22 1992Be careful if you put double quotes around the picture, because an C<@>
748a9306 1993character may be taken to mean the beginning of an array name.
19799a22 1994C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1995
1996=item getc FILEHANDLE
d74e8afc 1997X<getc> X<getchar>
a0d0e21e
LW
1998
1999=item getc
2000
2001Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
2002or the undefined value at end of file, or if there was an error (in
2003the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2004STDIN. This is not particularly efficient. However, it cannot be
2005used by itself to fetch single characters without waiting for the user
2006to hit enter. For that, try something more like:
4633a7c4
LW
2007
2008 if ($BSD_STYLE) {
2009 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2010 }
2011 else {
54310121 2012 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2013 }
2014
2015 $key = getc(STDIN);
2016
2017 if ($BSD_STYLE) {
2018 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2019 }
2020 else {
5f05dabc 2021 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
2022 }
2023 print "\n";
2024
54310121
PP
2025Determination of whether $BSD_STYLE should be set
2026is left as an exercise to the reader.
cb1a09d0 2027
19799a22 2028The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2029systems purporting POSIX compliance. See also the C<Term::ReadKey>
2030module from your nearest CPAN site; details on CPAN can be found on
2031L<perlmodlib/CPAN>.
a0d0e21e
LW
2032
2033=item getlogin
d74e8afc 2034X<getlogin> X<login>
a0d0e21e 2035
cf264981 2036This implements the C library function of the same name, which on most
5a964f20 2037systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 2038use C<getpwuid>.
a0d0e21e 2039
f86702cc 2040 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2041
19799a22
GS
2042Do not consider C<getlogin> for authentication: it is not as
2043secure as C<getpwuid>.
4633a7c4 2044
a0d0e21e 2045=item getpeername SOCKET
d74e8afc 2046X<getpeername> X<peer>
a0d0e21e
LW
2047
2048Returns the packed sockaddr address of other end of the SOCKET connection.
2049
4633a7c4
LW
2050 use Socket;
2051 $hersockaddr = getpeername(SOCK);
19799a22 2052 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2053 $herhostname = gethostbyaddr($iaddr, AF_INET);
2054 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2055
2056=item getpgrp PID
d74e8afc 2057X<getpgrp> X<group>
a0d0e21e 2058
47e29363 2059Returns the current process group for the specified PID. Use
7660c0ab 2060a PID of C<0> to get the current process group for the
4633a7c4 2061current process. Will raise an exception if used on a machine that
f86cebdf 2062doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2063group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2064does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2065
2066=item getppid
d74e8afc 2067X<getppid> X<parent> X<pid>
a0d0e21e
LW
2068
2069Returns the process id of the parent process.
2070
4d76a344
RGS
2071Note for Linux users: on Linux, the C functions C<getpid()> and
2072C<getppid()> return different values from different threads. In order to
2073be portable, this behavior is not reflected by the perl-level function
2074C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2075to call the underlying C<getppid()>, you may use the CPAN module
2076C<Linux::Pid>.
4d76a344 2077
a0d0e21e 2078=item getpriority WHICH,WHO
d74e8afc 2079X<getpriority> X<priority> X<nice>
a0d0e21e 2080
4633a7c4
LW
2081Returns the current priority for a process, a process group, or a user.
2082(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2083machine that doesn't implement getpriority(2).
a0d0e21e
LW
2084
2085=item getpwnam NAME
d74e8afc
ITB
2086X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2087X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2088X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2089X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2090X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2091X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2092
2093=item getgrnam NAME
2094
2095=item gethostbyname NAME
2096
2097=item getnetbyname NAME
2098
2099=item getprotobyname NAME
2100
2101=item getpwuid UID
2102
2103=item getgrgid GID
2104
2105=item getservbyname NAME,PROTO
2106
2107=item gethostbyaddr ADDR,ADDRTYPE
2108
2109=item getnetbyaddr ADDR,ADDRTYPE
2110
2111=item getprotobynumber NUMBER
2112
2113=item getservbyport PORT,PROTO
2114
2115=item getpwent
2116
2117=item getgrent
2118
2119=item gethostent
2120
2121=item getnetent
2122
2123=item getprotoent
2124
2125=item getservent
2126
2127=item setpwent
2128
2129=item setgrent
2130
2131=item sethostent STAYOPEN
2132
2133=item setnetent STAYOPEN
2134
2135=item setprotoent STAYOPEN
2136
2137=item setservent STAYOPEN
2138
2139=item endpwent
2140
2141=item endgrent
2142
2143=item endhostent
2144
2145=item endnetent
2146
2147=item endprotoent
2148
2149=item endservent
2150
2151These routines perform the same functions as their counterparts in the
5a964f20 2152system library. In list context, the return values from the
a0d0e21e
LW
2153various get routines are as follows:
2154
2155 ($name,$passwd,$uid,$gid,
6ee623d5 2156 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2157 ($name,$passwd,$gid,$members) = getgr*
2158 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2159 ($name,$aliases,$addrtype,$net) = getnet*
2160 ($name,$aliases,$proto) = getproto*
2161 ($name,$aliases,$port,$proto) = getserv*
2162
2163(If the entry doesn't exist you get a null list.)
2164
4602f195
JH
2165The exact meaning of the $gcos field varies but it usually contains
2166the real name of the user (as opposed to the login name) and other
2167information pertaining to the user. Beware, however, that in many
2168system users are able to change this information and therefore it
106325ad 2169cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2170L<perlsec>). The $passwd and $shell, user's encrypted password and
2171login shell, are also tainted, because of the same reason.
4602f195 2172
5a964f20 2173In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2174lookup by name, in which case you get the other thing, whatever it is.
2175(If the entry doesn't exist you get the undefined value.) For example:
2176
5a964f20
TC
2177 $uid = getpwnam($name);
2178 $name = getpwuid($num);
2179 $name = getpwent();
2180 $gid = getgrnam($name);
08a33e13 2181 $name = getgrgid($num);
5a964f20
TC
2182 $name = getgrent();
2183 #etc.
a0d0e21e 2184
4602f195
JH
2185In I<getpw*()> the fields $quota, $comment, and $expire are special
2186cases in the sense that in many systems they are unsupported. If the
2187$quota is unsupported, it is an empty scalar. If it is supported, it
2188usually encodes the disk quota. If the $comment field is unsupported,
2189it is an empty scalar. If it is supported it usually encodes some
2190administrative comment about the user. In some systems the $quota
2191field may be $change or $age, fields that have to do with password
2192aging. In some systems the $comment field may be $class. The $expire
2193field, if present, encodes the expiration period of the account or the
2194password. For the availability and the exact meaning of these fields
2195in your system, please consult your getpwnam(3) documentation and your
2196F<pwd.h> file. You can also find out from within Perl what your
2197$quota and $comment fields mean and whether you have the $expire field
2198by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2199C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2200files are only supported if your vendor has implemented them in the
2201intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2202shadow versions if you're running under privilege or if there exists
cf264981
SP
2203the shadow(3) functions as found in System V (this includes Solaris
2204and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2205facility are unlikely to be supported.
6ee623d5 2206
19799a22 2207The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2208the login names of the members of the group.
2209
2210For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2211C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2212C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2213addresses returned by the corresponding system library call. In the
2214Internet domain, each address is four bytes long and you can unpack it
2215by saying something like:
2216
f337b084 2217 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2218
2b5ab1e7
TC
2219The Socket library makes this slightly easier:
2220
2221 use Socket;
2222 $iaddr = inet_aton("127.1"); # or whatever address
2223 $name = gethostbyaddr($iaddr, AF_INET);
2224
2225 # or going the other way
19799a22 2226 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2227
19799a22
GS
2228If you get tired of remembering which element of the return list
2229contains which return value, by-name interfaces are provided
2230in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2231C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2232and C<User::grent>. These override the normal built-ins, supplying
2233versions that return objects with the appropriate names
2234for each field. For example:
5a964f20
TC
2235
2236 use File::stat;
2237 use User::pwent;
2238 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2239
b76cc8ba
NIS
2240Even though it looks like they're the same method calls (uid),
2241they aren't, because a C<File::stat> object is different from
19799a22 2242a C<User::pwent> object.
5a964f20 2243
a0d0e21e 2244=item getsockname SOCKET
d74e8afc 2245X<getsockname>
a0d0e21e 2246
19799a22
GS
2247Returns the packed sockaddr address of this end of the SOCKET connection,
2248in case you don't know the address because you have several different
2249IPs that the connection might have come in on.
a0d0e21e 2250
4633a7c4
LW
2251 use Socket;
2252 $mysockaddr = getsockname(SOCK);
19799a22 2253 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2254 printf "Connect to %s [%s]\n",
19799a22
GS
2255 scalar gethostbyaddr($myaddr, AF_INET),
2256 inet_ntoa($myaddr);
a0d0e21e
LW
2257
2258=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2259X<getsockopt>
a0d0e21e 2260
636e6b1f
TH
2261Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2262Options may exist at multiple protocol levels depending on the socket
2263type, but at least the uppermost socket level SOL_SOCKET (defined in the
2264C<Socket> module) will exist. To query options at another level the
2265protocol number of the appropriate protocol controlling the option
2266should be supplied. For example, to indicate that an option is to be
2267interpreted by the TCP protocol, LEVEL should be set to the protocol
2268number of TCP, which you can get using getprotobyname.
2269
2270The call returns a packed string representing the requested socket option,
2271or C<undef> if there is an error (the error reason will be in $!). What
2272exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2273your system documentation for details. A very common case however is that
cf264981 2274the option is an integer, in which case the result will be a packed
636e6b1f
TH
2275integer which you can decode using unpack with the C<i> (or C<I>) format.
2276
2277An example testing if Nagle's algorithm is turned on on a socket:
2278
4852725b 2279 use Socket qw(:all);
636e6b1f
TH
2280
2281 defined(my $tcp = getprotobyname("tcp"))
2282 or die "Could not determine the protocol number for tcp";
4852725b
DD
2283 # my $tcp = IPPROTO_TCP; # Alternative
2284 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2285 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2286 my $nodelay = unpack("I", $packed);
2287 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2288
a0d0e21e
LW
2289
2290=item glob EXPR
d74e8afc 2291X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2292
0a753a76
PP
2293=item glob
2294
d9a9d457
JL
2295In list context, returns a (possibly empty) list of filename expansions on
2296the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2297scalar context, glob iterates through such filename expansions, returning
2298undef when the list is exhausted. This is the internal function
2299implementing the C<< <*.c> >> operator, but you can use it directly. If
2300EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2301more detail in L<perlop/"I/O Operators">.
a0d0e21e 2302
3a4b19e4
GS
2303Beginning with v5.6.0, this operator is implemented using the standard
2304C<File::Glob> extension. See L<File::Glob> for details.
2305
a0d0e21e 2306=item gmtime EXPR
d74e8afc 2307X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2308
ce2984c3
PF
2309=item gmtime
2310
82dd32fe 2311Converts a time as returned by the time function to an 9-element list
54310121 2312with the time localized for the standard Greenwich time zone.
4633a7c4 2313Typically used as follows:
a0d0e21e 2314
82dd32fe
SP
2315 # 0 1 2 3 4 5 6 7 8
2316 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
a0d0e21e
LW
2317 gmtime(time);
2318
48a26b3a
GS
2319All list elements are numeric, and come straight out of the C `struct
2320tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2321specified time. $mday is the day of the month, and $mon is the month
2322itself, in the range C<0..11> with 0 indicating January and 11
2323indicating December. $year is the number of years since 1900. That
2324is, $year is C<123> in year 2023. $wday is the day of the week, with
23250 indicating Sunday and 3 indicating Wednesday. $yday is the day of
82dd32fe
SP
2326the year, in the range C<0..364> (or C<0..365> in leap years). $isdst
2327is always C<0>.
48a26b3a
GS
2328
2329Note that the $year element is I<not> simply the last two digits of
cf264981 2330the year. If you assume it is then you create non-Y2K-compliant
48a26b3a 2331programs--and you wouldn't want to do that, would you?
2f9daede 2332
abd75f24
GS
2333The proper way to get a complete 4-digit year is simply:
2334
2335 $year += 1900;
2336
2337And to get the last two digits of the year (e.g., '01' in 2001) do:
2338
2339 $year = sprintf("%02d", $year % 100);
2340
48a26b3a 2341If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2342
48a26b3a 2343In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2344
2345 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2346
fe86afc2
NC
2347If you need local time instead of GMT use the L</localtime> builtin.
2348See also the C<timegm> function provided by the C<Time::Local> module,
2349and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
7660c0ab 2350
fe86afc2
NC
2351This scalar value is B<not> locale dependent (see L<perllocale>), but is
2352instead a Perl builtin. To get somewhat similar but locale dependent date
2353strings, see the example in L</localtime>.
0a753a76 2354
62aa5637
MS
2355See L<perlport/gmtime> for portability concerns.
2356
a0d0e21e 2357=item goto LABEL
d74e8afc 2358X<goto> X<jump> X<jmp>
a0d0e21e 2359
748a9306
LW
2360=item goto EXPR
2361
a0d0e21e
LW
2362=item goto &NAME
2363
7660c0ab 2364The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2365execution there. It may not be used to go into any construct that
7660c0ab 2366requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2367also can't be used to go into a construct that is optimized away,
19799a22 2368or to get out of a block or subroutine given to C<sort>.
0a753a76 2369It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2370including out of subroutines, but it's usually better to use some other
19799a22 2371construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2372need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2373(The difference being that C does not offer named loops combined with
2374loop control. Perl does, and this replaces most structured uses of C<goto>
2375in other languages.)
a0d0e21e 2376
7660c0ab
A
2377The C<goto-EXPR> form expects a label name, whose scope will be resolved
2378dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2379necessarily recommended if you're optimizing for maintainability:
2380
2381 goto ("FOO", "BAR", "GLARCH")[$i];
2382
1b6921cb
BT
2383The C<goto-&NAME> form is quite different from the other forms of
2384C<goto>. In fact, it isn't a goto in the normal sense at all, and
2385doesn't have the stigma associated with other gotos. Instead, it
2386exits the current subroutine (losing any changes set by local()) and
2387immediately calls in its place the named subroutine using the current
2388value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2389load another subroutine and then pretend that the other subroutine had
2390been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2391in the current subroutine are propagated to the other subroutine.)
2392After the C<goto>, not even C<caller> will be able to tell that this
2393routine was called first.
2394
2395NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2396containing a code reference, or a block that evaluates to a code
6cb9131c 2397reference.
a0d0e21e
LW
2398
2399=item grep BLOCK LIST
d74e8afc 2400X<grep>
a0d0e21e
LW
2401
2402=item grep EXPR,LIST
2403
2b5ab1e7
TC
2404This is similar in spirit to, but not the same as, grep(1) and its
2405relatives. In particular, it is not limited to using regular expressions.
2f9daede 2406
a0d0e21e 2407Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2408C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2409elements for which the expression evaluated to true. In scalar
2410context, returns the number of times the expression was true.
a0d0e21e
LW
2411
2412 @foo = grep(!/^#/, @bar); # weed out comments
2413
2414or equivalently,
2415
2416 @foo = grep {!/^#/} @bar; # weed out comments
2417
be3174d2
GS
2418Note that C<$_> is an alias to the list value, so it can be used to
2419modify the elements of the LIST. While this is useful and supported,
2420it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2421Similarly, grep returns aliases into the original list, much as a for
2422loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2423element of a list returned by grep (for example, in a C<foreach>, C<map>
2424or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2425This is usually something to be avoided when writing clear code.
a0d0e21e 2426
a4fb8298 2427If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2428been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2429the list elements, C<$_> keeps being lexical inside the block; i.e. it
2430can't be seen from the outside, avoiding any potential side-effects.
2431
19799a22 2432See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2433
a0d0e21e 2434=item hex EXPR
d74e8afc 2435X<hex> X<hexadecimal>
a0d0e21e 2436
54310121 2437=item hex
bbce6d69 2438
2b5ab1e7 2439Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2440(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2441L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2442
2443 print hex '0xAf'; # prints '175'
2444 print hex 'aF'; # same
a0d0e21e 2445
19799a22 2446Hex strings may only represent integers. Strings that would cause
53305cf1 2447integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2448unlike oct(). To present something as hex, look into L</printf>,
2449L</sprintf>, or L</unpack>.
19799a22 2450
ce2984c3 2451=item import LIST
d74e8afc 2452X<import>
a0d0e21e 2453
19799a22 2454There is no builtin C<import> function. It is just an ordinary
4633a7c4 2455method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2456names to another module. The C<use> function calls the C<import> method
cea6626f 2457for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2458
2459=item index STR,SUBSTR,POSITION
d74e8afc 2460X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2461
2462=item index STR,SUBSTR
2463
2b5ab1e7
TC
2464The index function searches for one string within another, but without
2465the wildcard-like behavior of a full regular-expression pattern match.
2466It returns the position of the first occurrence of SUBSTR in STR at
2467or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2468beginning of the string. POSITION before the beginning of the string
2469or after its end is treated as if it were the beginning or the end,
2470respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2471you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2472is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2473
2474=item int EXPR
d74e8afc 2475X<int> X<integer> X<truncate> X<trunc>
a0d0e21e 2476
54310121 2477=item int
bbce6d69 2478
7660c0ab 2479Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2480You should not use this function for rounding: one because it truncates
2481towards C<0>, and two because machine representations of floating point
2482numbers can sometimes produce counterintuitive results. For example,
2483C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2484because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2485the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2486functions will serve you better than will int().
a0d0e21e
LW
2487
2488=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2489X<ioctl>
a0d0e21e 2490
2b5ab1e7 2491Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2492
6c567752 2493 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2494
a11c483f 2495to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2496exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2497own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2498(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2499may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2500written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2501will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2502has no string value but does have a numeric value, that value will be
2503passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2504true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2505functions may be needed to manipulate the values of structures used by
b76cc8ba 2506C<ioctl>.
a0d0e21e 2507
19799a22 2508The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2509
2510 if OS returns: then Perl returns:
2511 -1 undefined value
2512 0 string "0 but true"
2513 anything else that number
2514
19799a22 2515Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2516still easily determine the actual value returned by the operating
2517system:
2518
2b5ab1e7 2519 $retval = ioctl(...) || -1;
a0d0e21e
LW
2520 printf "System returned %d\n", $retval;
2521
be2f7487 2522The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2523about improper numeric conversions.
2524
a0d0e21e 2525=item join EXPR,LIST
d74e8afc 2526X<join>
a0d0e21e 2527
2b5ab1e7
TC
2528Joins the separate strings of LIST into a single string with fields
2529separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2530
2b5ab1e7 2531 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2532
eb6e2d6f
GS
2533Beware that unlike C<split>, C<join> doesn't take a pattern as its
2534first argument. Compare L</split>.
a0d0e21e 2535
aa689395 2536=item keys HASH
d74e8afc 2537X<keys> X<key>
aa689395 2538
504f80c1
JH
2539Returns a list consisting of all the keys of the named hash.
2540(In scalar context, returns the number of keys.)
2541
2542The keys are returned in an apparently random order. The actual
2543random order is subject to change in future versions of perl, but it
2544is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2545function produces (given that the hash has not been modified). Since
2546Perl 5.8.1 the ordering is different even between different runs of
2547Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2548Attacks">).
504f80c1 2549
cf264981
SP
2550As a side effect, calling keys() resets the HASH's internal iterator
2551(see L</each>). In particular, calling keys() in void context resets
2552the iterator with no other overhead.
a0d0e21e 2553
aa689395 2554Here is yet another way to print your environment:
a0d0e21e
LW
2555
2556 @keys = keys %ENV;
2557 @values = values %ENV;
b76cc8ba 2558 while (@keys) {
a0d0e21e
LW
2559 print pop(@keys), '=', pop(@values), "\n";
2560 }
2561
2562or how about sorted by key:
2563
2564 foreach $key (sort(keys %ENV)) {
2565 print $key, '=', $ENV{$key}, "\n";
2566 }
2567
8ea1e5d4
GS
2568The returned values are copies of the original keys in the hash, so
2569modifying them will not affect the original hash. Compare L</values>.
2570
19799a22 2571To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2572Here's a descending numeric sort of a hash by its values:
4633a7c4 2573
5a964f20 2574 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2575 printf "%4d %s\n", $hash{$key}, $key;
2576 }
2577
19799a22 2578As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2579allocated for the given hash. This can gain you a measure of efficiency if
2580you know the hash is going to get big. (This is similar to pre-extending
2581an array by assigning a larger number to $#array.) If you say
55497cff
PP
2582
2583 keys %hash = 200;
2584
ab192400
GS
2585then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2586in fact, since it rounds up to the next power of two. These
55497cff
PP
2587buckets will be retained even if you do C<%hash = ()>, use C<undef
2588%hash> if you want to free the storage while C<%hash> is still in scope.
2589You can't shrink the number of buckets allocated for the hash using
19799a22 2590C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2591as trying has no effect).
2592
19799a22 2593See also C<each>, C<values> and C<sort>.
ab192400 2594
b350dd2f 2595=item kill SIGNAL, LIST
d74e8afc 2596X<kill> X<signal>
a0d0e21e 2597
b350dd2f 2598Sends a signal to a list of processes. Returns the number of
517db077
GS
2599processes successfully signaled (which is not necessarily the
2600same as the number actually killed).
a0d0e21e
LW
2601
2602 $cnt = kill 1, $child1, $child2;
2603 kill 9, @goners;
2604
70fb64f6 2605If SIGNAL is zero, no signal is sent to the process, but the kill(2)
6cb9d3e4 2606system call will check whether it's possible to send a signal to it (that
70fb64f6
RGS
2607means, to be brief, that the process is owned by the same user, or we are
2608the super-user). This is a useful way to check that a child process is
2609alive and hasn't changed its UID. See L<perlport> for notes on the
2610portability of this construct.
b350dd2f
GS
2611
2612Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2613process groups instead of processes. (On System V, a negative I<PROCESS>
2614number will also kill process groups, but that's not portable.) That
2615means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2616use a signal name in quotes.
2617
2618See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2619
2620=item last LABEL
d74e8afc 2621X<last> X<break>
a0d0e21e
LW
2622
2623=item last
2624
2625The C<last> command is like the C<break> statement in C (as used in
2626loops); it immediately exits the loop in question. If the LABEL is
2627omitted, the command refers to the innermost enclosing loop. The
2628C<continue> block, if any, is not executed:
2629
4633a7c4
LW
2630 LINE: while (<STDIN>) {
2631 last LINE if /^$/; # exit when done with header
5a964f20 2632 #...
a0d0e21e
LW
2633 }
2634
4968c1e4 2635C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2636C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2637a grep() or map() operation.
4968c1e4 2638
6c1372ed
GS
2639Note that a block by itself is semantically identical to a loop
2640that executes once. Thus C<last> can be used to effect an early
2641exit out of such a block.
2642
98293880
JH
2643See also L</continue> for an illustration of how C<last>, C<next>, and
2644C<redo> work.
1d2dff63 2645
a0d0e21e 2646=item lc EXPR
d74e8afc 2647X<lc> X<lowercase>
a0d0e21e 2648
54310121 2649=item lc
bbce6d69 2650
d1be9408 2651Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2652implementing the C<\L> escape in double-quoted strings. Respects
2653current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2654and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2655
7660c0ab 2656If EXPR is omitted, uses C<$_>.
bbce6d69 2657
a0d0e21e 2658=item lcfirst EXPR
d74e8afc 2659X<lcfirst> X<lowercase>
a0d0e21e 2660
54310121 2661=item lcfirst
bbce6d69 2662
ad0029c4
JH
2663Returns the value of EXPR with the first character lowercased. This
2664is the internal function implementing the C<\l> escape in
2665double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2666locale> in force. See L<perllocale> and L<perlunicode> for more
2667details about locale and Unicode support.
a0d0e21e 2668
7660c0ab 2669If EXPR is omitted, uses C<$_>.
bbce6d69 2670
a0d0e21e 2671=item length EXPR
d74e8afc 2672X<length> X<size>
a0d0e21e 2673
54310121 2674=item length
bbce6d69 2675
974da8e5 2676Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2677omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2678an entire array or hash to find out how many elements these have.
2679For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2680
974da8e5
JH
2681Note the I<characters>: if the EXPR is in Unicode, you will get the
2682number of characters, not the number of bytes. To get the length
2683in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2684
a0d0e21e 2685=item link OLDFILE,NEWFILE
d74e8afc 2686X<link>
a0d0e21e 2687
19799a22 2688Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2689success, false otherwise.
a0d0e21e
LW
2690
2691=item listen SOCKET,QUEUESIZE
d74e8afc 2692X<listen>
a0d0e21e 2693
19799a22 2694Does the same thing that the listen system call does. Returns true if
b76cc8ba 2695it succeeded, false otherwise. See the example in
cea6626f 2696L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2697
2698=item local EXPR
d74e8afc 2699X<local>
a0d0e21e 2700
19799a22 2701You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2702what most people think of as "local". See
13a2d996 2703L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2704
5a964f20
TC
2705A local modifies the listed variables to be local to the enclosing
2706block, file, or eval. If more than one value is listed, the list must
2707be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2708for details, including issues with tied arrays and hashes.
a0d0e21e 2709
a0d0e21e 2710=item localtime EXPR
d74e8afc 2711X<localtime>
a0d0e21e 2712
ba053783
AL
2713=item localtime
2714
19799a22 2715Converts a time as returned by the time function to a 9-element list
5f05dabc 2716with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2717follows:
2718
54310121 2719 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2720 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2721 localtime(time);
a0d0e21e 2722
48a26b3a 2723All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2724tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2725of the specified time.
48a26b3a 2726
ba053783
AL
2727C<$mday> is the day of the month, and C<$mon> is the month itself, in
2728the range C<0..11> with 0 indicating January and 11 indicating December.
2729This makes it easy to get a month name from a list:
54310121 2730
ba053783
AL
2731 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2732 print "$abbr[$mon] $mday";
2733 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2734
ba053783
AL
2735C<$year> is the number of years since 1900, not just the last two digits
2736of the year. That is, C<$year> is C<123> in year 2023. The proper way
2737to get a complete 4-digit year is simply:
abd75f24 2738
ba053783 2739 $year += 1900;
abd75f24 2740
ba053783
AL
2741To get the last two digits of the year (e.g., '01' in 2001) do:
2742
2743 $year = sprintf("%02d", $year % 100);
2744
2745C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2746Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2747(or C<0..365> in leap years.)
2748
2749C<$isdst> is true if the specified time occurs during Daylight Saving
2750Time, false otherwise.
abd75f24 2751
48a26b3a 2752If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2753
48a26b3a 2754In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2755
5f05dabc 2756 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2757
fe86afc2
NC
2758This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2759instead of local time use the L</gmtime> builtin. See also the
2760C<Time::Local> module (to convert the second, minutes, hours, ... back to
2761the integer value returned by time()), and the L<POSIX> module's strftime(3)
2762and mktime(3) functions.
2763
2764To get somewhat similar but locale dependent date strings, set up your
2765locale environment variables appropriately (please see L<perllocale>) and
2766try for example:
a3cb178b 2767
5a964f20 2768 use POSIX qw(strftime);
2b5ab1e7 2769 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2770 # or for GMT formatted appropriately for your locale:
2771 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2772
2773Note that the C<%a> and C<%b>, the short forms of the day of the week
2774and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2775
62aa5637
MS
2776See L<perlport/localtime> for portability concerns.
2777
07698885 2778=item lock THING
d74e8afc 2779X<lock>
19799a22 2780
01e6739c 2781This function places an advisory lock on a shared variable, or referenced
03730085 2782object contained in I<THING> until the lock goes out of scope.
a6d5524e 2783
f3a23afb 2784lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2785by this name (before any calls to it), that function will be called
03730085
AB
2786instead. (However, if you've said C<use threads>, lock() is always a
2787keyword.) See L<threads>.
19799a22 2788
a0d0e21e 2789=item log EXPR
d74e8afc 2790X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2791
54310121 2792=item log
bbce6d69 2793
2b5ab1e7
TC
2794Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2795returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2796The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2797divided by the natural log of N. For example:
2798
2799 sub log10 {
2800 my $n = shift;
2801 return log($n)/log(10);
b76cc8ba 2802 }
2b5ab1e7
TC
2803
2804See also L</exp> for the inverse operation.
a0d0e21e 2805
a0d0e21e 2806=item lstat EXPR
d74e8afc 2807X<lstat>
a0d0e21e 2808
54310121 2809=item lstat
bbce6d69 2810
19799a22 2811Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2812special C<_> filehandle) but stats a symbolic link instead of the file
2813the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2814your system, a normal C<stat> is done. For much more detailed
2815information, please see the documentation for C<stat>.
a0d0e21e 2816
7660c0ab 2817If EXPR is omitted, stats C<$_>.
bbce6d69 2818
a0d0e21e
LW
2819=item m//
2820
2821The match operator. See L<perlop>.
2822
2823=item map BLOCK LIST
d74e8afc 2824X<map>
a0d0e21e
LW
2825
2826=item map EXPR,LIST
2827
19799a22
GS
2828Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2829C<$_> to each element) and returns the list value composed of the
2830results of each such evaluation. In scalar context, returns the
2831total number of elements so generated. Evaluates BLOCK or EXPR in
2832list context, so each element of LIST may produce zero, one, or
2833more elements in the returned value.
dd99ebda 2834
a0d0e21e
LW
2835 @chars = map(chr, @nums);
2836
2837translates a list of numbers to the corresponding characters. And
2838
4633a7c4 2839 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2840
2841is just a funny way to write
2842
2843 %hash = ();
2844 foreach $_ (@array) {
4633a7c4 2845 $hash{getkey($_)} = $_;
a0d0e21e
LW
2846 }
2847
be3174d2
GS
2848Note that C<$_> is an alias to the list value, so it can be used to
2849modify the elements of the LIST. While this is useful and supported,
2850it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2851Using a regular C<foreach> loop for this purpose would be clearer in
2852most cases. See also L</grep> for an array composed of those items of
2853the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2854
a4fb8298 2855If C<$_> is lexical in the scope where the C<map> appears (because it has
cf264981 2856been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2857the list elements, C<$_> keeps being lexical inside the block; i.e. it
2858can't be seen from the outside, avoiding any potential side-effects.
2859
205fdb4d
NC
2860C<{> starts both hash references and blocks, so C<map { ...> could be either
2861the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2862ahead for the closing C<}> it has to take a guess at which its dealing with
2863based what it finds just after the C<{>. Usually it gets it right, but if it
2864doesn't it won't realize something is wrong until it gets to the C<}> and
2865encounters the missing (or unexpected) comma. The syntax error will be
2866reported close to the C<}> but you'll need to change something near the C<{>
2867such as using a unary C<+> to give perl some help:
2868
2869 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2870 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2871 %hash = map { ("\L$_", 1) } @array # this also works
2872 %hash = map { lc($_), 1 } @array # as does this.
2873 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2874
205fdb4d
NC
2875 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2876
2877or to force an anon hash constructor use C<+{>
2878
2879 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2880
2881and you get list of anonymous hashes each with only 1 entry.
2882
19799a22 2883=item mkdir FILENAME,MASK
d74e8afc 2884X<mkdir> X<md> X<directory, create>
a0d0e21e 2885
5a211162
GS
2886=item mkdir FILENAME
2887
491873e5
RGS
2888=item mkdir
2889
0591cd52 2890Creates the directory specified by FILENAME, with permissions
19799a22
GS
2891specified by MASK (as modified by C<umask>). If it succeeds it
2892returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2893If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2894to C<$_>.
0591cd52 2895
19799a22 2896In general, it is better to create directories with permissive MASK,
0591cd52 2897and let the user modify that with their C<umask>, than it is to supply
19799a22 2898a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2899The exceptions to this rule are when the file or directory should be
2900kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2901C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2902
cc1852e8
JH
2903Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2904number of trailing slashes. Some operating and filesystems do not get
2905this right, so Perl automatically removes all trailing slashes to keep
2906everyone happy.
2907
a0d0e21e 2908=item msgctl ID,CMD,ARG
d74e8afc 2909X<msgctl>
a0d0e21e 2910
f86cebdf 2911Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2912
2913 use IPC::SysV;
2914
7660c0ab 2915first to get the correct constant definitions. If CMD is C<IPC_STAT>,
cf264981 2916then ARG must be a variable that will hold the returned C<msqid_ds>
951ba7fe
GS
2917structure. Returns like C<ioctl>: the undefined value for error,
2918C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2919L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2920
2921=item msgget KEY,FLAGS
d74e8afc 2922X<msgget>
a0d0e21e 2923
f86cebdf 2924Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2925id, or the undefined value if there is an error. See also
2926L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2927
a0d0e21e 2928=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
d74e8afc 2929X<msgrcv>
a0d0e21e
LW
2930
2931Calls the System V IPC function msgrcv to receive a message from
2932message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2933SIZE. Note that when a message is received, the message type as a
2934native long integer will be the first thing in VAR, followed by the
2935actual message. This packing may be opened with C<unpack("l! a*")>.
2936Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2937an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2938C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2939
2940=item msgsnd ID,MSG,FLAGS
d74e8afc 2941X<msgsnd>
41d6edb2
JH
2942
2943Calls the System V IPC function msgsnd to send the message MSG to the
2944message queue ID. MSG must begin with the native long integer message
2945type, and be followed by the length of the actual message, and finally
2946the message itself. This kind of packing can be achieved with
2947C<pack("l! a*", $type, $message)>. Returns true if successful,
2948or false if there is an error. See also C<IPC::SysV>
2949and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2950
2951=item my EXPR
d74e8afc 2952X<my>
a0d0e21e 2953
307ea6df
JH
2954=item my TYPE EXPR
2955
1d2de774 2956=item my EXPR : ATTRS
09bef843 2957
1d2de774 2958=item my TYPE EXPR : ATTRS
307ea6df 2959
19799a22 2960A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2961enclosing block, file, or C<eval>. If more than one value is listed,
2962the list must be placed in parentheses.
307ea6df 2963
1d2de774
JH
2964The exact semantics and interface of TYPE and ATTRS are still
2965evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2966and attributes are handled using the C<attributes> pragma, or starting
2967from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2968L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2969L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2970
a0d0e21e 2971=item next LABEL
d74e8afc 2972X<next> X<continue>
a0d0e21e
LW
2973
2974=item next
2975
2976The C<next> command is like the C<continue> statement in C; it starts
2977the next iteration of the loop:
2978
4633a7c4
LW
2979 LINE: while (<STDIN>) {
2980 next LINE if /^#/; # discard comments
5a964f20 2981 #...
a0d0e21e
LW
2982 }
2983
2984Note that if there were a C<continue> block on the above, it would get
2985executed even on discarded lines. If the LABEL is omitted, the command
2986refers to the innermost enclosing loop.
2987
4968c1e4 2988C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2989C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2990a grep() or map() operation.
4968c1e4 2991
6c1372ed
GS
2992Note that a block by itself is semantically identical to a loop
2993that executes once. Thus C<next> will exit such a block early.
2994
98293880
JH
2995See also L</continue> for an illustration of how C<last>, C<next>, and
2996C<redo> work.
1d2dff63 2997
4a66ea5a 2998=item no Module VERSION LIST
d74e8afc 2999X<no>
4a66ea5a
RGS
3000
3001=item no Module VERSION
3002
a0d0e21e
LW
3003=item no Module LIST
3004
4a66ea5a
RGS
3005=item no Module
3006
593b9c14 3007See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
3008
3009=item oct EXPR
d74e8afc 3010X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
a0d0e21e 3011
54310121 3012=item oct
bbce6d69 3013
4633a7c4 3014Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
3015value. (If EXPR happens to start off with C<0x>, interprets it as a
3016hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
3017binary string. Leading whitespace is ignored in all three cases.)
3018The following will handle decimal, binary, octal, and hex in the standard
3019Perl or C notation:
a0d0e21e
LW
3020
3021 $val = oct($val) if $val =~ /^0/;
3022
19799a22
GS
3023If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3024in octal), use sprintf() or printf():
3025
3026 $perms = (stat("filename"))[2] & 07777;
3027 $oct_perms = sprintf "%lo", $perms;
3028
3029The oct() function is commonly used when a string such as C<644> needs
3030to be converted into a file mode, for example. (Although perl will
3031automatically convert strings into numbers as needed, this automatic
3032conversion assumes base 10.)
a0d0e21e
LW
3033
3034=item open FILEHANDLE,EXPR
d74e8afc 3035X<open> X<pipe> X<file, open> X<fopen>
a0d0e21e 3036
68bd7414
NIS
3037=item open FILEHANDLE,MODE,EXPR
3038
3039=item open FILEHANDLE,MODE,EXPR,LIST
3040
ba964c95
T
3041=item open FILEHANDLE,MODE,REFERENCE
3042
a0d0e21e
LW
3043=item open FILEHANDLE
3044
3045Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
3046FILEHANDLE.
3047
3048(The following is a comprehensive reference to open(): for a gentler
3049introduction you may consider L<perlopentut>.)
3050
a28cd5c9
NT
3051If FILEHANDLE is an undefined scalar variable (or array or hash element)
3052the variable is assigned a reference to a new anonymous filehandle,
3053otherwise if FILEHANDLE is an expression, its value is used as the name of
3054the real filehandle wanted. (This is considered a symbolic reference, so
3055C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
3056
3057If EXPR is omitted, the scalar variable of the same name as the
3058FILEHANDLE contains the filename. (Note that lexical variables--those
3059declared with C<my>--will not work for this purpose; so if you're
67408cae 3060using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
3061
3062If three or more arguments are specified then the mode of opening and
3063the file name are separate. If MODE is C<< '<' >> or nothing, the file
3064is opened for input. If MODE is C<< '>' >>, the file is truncated and
3065opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 3066the file is opened for appending, again being created if necessary.
5a964f20 3067
ed53a2bb
JH
3068You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3069indicate that you want both read and write access to the file; thus
3070C<< '+<' >> is almost always preferred for read/write updates--the C<<
3071'+>' >> mode would clobber the file first. You can't usually use
3072either read-write mode for updating textfiles, since they have
3073variable length records. See the B<-i> switch in L<perlrun> for a
3074better approach. The file is created with permissions of C<0666>
3075modified by the process' C<umask> value.
3076
3077These various prefixes correspond to the fopen(3) modes of C<'r'>,
3078C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 3079
6170680b
IZ
3080In the 2-arguments (and 1-argument) form of the call the mode and
3081filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
3082spaces. It is possible to omit the mode in these forms if the mode is
3083C<< '<' >>.
6170680b 3084
7660c0ab 3085If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 3086command to which output is to be piped, and if the filename ends with a
f244e06d
GS
3087C<'|'>, the filename is interpreted as a command which pipes output to
3088us. See L<perlipc/"Using open() for IPC">
19799a22 3089for more examples of this. (You are not allowed to C<open> to a command
5a964f20 3090that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
3091and L<perlipc/"Bidirectional Communication with Another Process">
3092for alternatives.)
cb1a09d0 3093
ed53a2bb
JH
3094For three or more arguments if MODE is C<'|-'>, the filename is
3095interpreted as a command to which output is to be piped, and if MODE
3096is C<'-|'>, the filename is interpreted as a command which pipes
3097output to us. In the 2-arguments (and 1-argument) form one should
3098replace dash (C<'-'>) with the command.
3099See L<perlipc/"Using open() for IPC"> for more examples of this.
3100(You are not allowed to C<open> to a command that pipes both in I<and>
3101out, but see L<IPC::Open2>, L<IPC::Open3>, and
3102L<perlipc/"Bidirectional Communication"> for alternatives.)
3103
3104In the three-or-more argument form of pipe opens, if LIST is specified
3105(extra arguments after the command name) then LIST becomes arguments
3106to the command invoked if the platform supports it. The meaning of
3107C<open> with more than three arguments for non-pipe modes is not yet
3108specified. Experimental "layers" may give extra LIST arguments
3109meaning.
6170680b
IZ
3110
3111In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 3112and opening C<< '>-' >> opens STDOUT.
6170680b 3113
fae2c0fb
RGS
3114You may use the three-argument form of open to specify IO "layers"
3115(sometimes also referred to as "disciplines") to be applied to the handle
3116that affect how the input and output are processed (see L<open> and
3117L<PerlIO> for more details). For example
7207e29d 3118
9124316e
JH
3119 open(FH, "<:utf8", "file")
3120
3121will open the UTF-8 encoded file containing Unicode characters,
6d5e88a0
ST
3122see L<perluniintro>. Note that if layers are specified in the
3123three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3124usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
ed53a2bb
JH
3125
3126Open returns nonzero upon success, the undefined value otherwise. If
3127the C<open> involved a pipe, the return value happens to be the pid of
3128the subprocess.
cb1a09d0 3129
ed53a2bb
JH
3130If you're running Perl on a system that distinguishes between text
3131files and binary files, then you should check out L</binmode> for tips
3132for dealing with this. The key distinction between systems that need
3133C<binmode> and those that don't is their text file formats. Systems
8939ba94 3134like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
3135character, and which encode that character in C as C<"\n">, do not
3136need C<binmode>. The rest need it.
cb1a09d0 3137
fb73857a 3138When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
3139if the request failed, so C<open> is frequently used in connection with
3140C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 3141where you want to make a nicely formatted error message (but there are
5a964f20 3142modules that can help with that problem)) you should always check
19799a22 3143the return value from opening a file. The infrequent exception is when
fb73857a
PP
3144working with an unopened filehandle is actually what you want to do.
3145
cf264981 3146As a special case the 3-arg form with a read/write mode and the third
ed53a2bb 3147argument being C<undef>:
b76cc8ba
NIS
3148
3149 open(TMP, "+>", undef) or die ...
3150
f253e835
JH