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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
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4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
15operator. A unary operator generally provides a scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
a0d0e21e 17contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 18be first, and the list argument will follow. (Note that there can ever
0f31cffe 19be only one such list argument.) For instance, splice() has three scalar
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20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
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22
23In the syntax descriptions that follow, list operators that expect a
24list (and provide list context for the elements of the list) are shown
25with LIST as an argument. Such a list may consist of any combination
26of scalar arguments or list values; the list values will be included
27in the list as if each individual element were interpolated at that
28point in the list, forming a longer single-dimensional list value.
cf264981 29Commas should separate elements of the LIST.
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30
31Any function in the list below may be used either with or without
32parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 33parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 34surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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35function, and precedence doesn't matter. Otherwise it's a list
36operator or unary operator, and precedence does matter. And whitespace
37between the function and left parenthesis doesn't count--so you need to
38be careful sometimes:
39
68dc0745 40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
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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>,
b76cc8ba 231C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
4375e838 232C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
22fae026 233C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
37798a01 234
235* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 236operator, which can be used in expressions.
37798a01 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
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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.
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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.
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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
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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
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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
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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
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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 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 $@ eq "alarm\n"; # propagate unexpected errors
ff68c719 466 # timed out
467 }
468 else {
469 # didn't
470 }
471
91d81acc
JH
472For more information see L<perlipc>.
473
a0d0e21e 474=item atan2 Y,X
d74e8afc 475X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
476
477Returns the arctangent of Y/X in the range -PI to PI.
478
ca6e1c26 479For the tangent operation, you may use the C<Math::Trig::tan>
28757baa 480function, or use the familiar relation:
481
482 sub tan { sin($_[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,
e476b1b5 626 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 627
951ba7fe 628Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 629call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 630C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 631C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 632C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 633$filename is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
634each C<use> statement creates a C<require> frame inside an C<eval EXPR>
635frame.) $subroutine may also be C<(unknown)> if this particular
636subroutine happens to have been deleted from the symbol table.
637C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
638C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
639compiled with. The C<$hints> and C<$bitmask> values are subject to change
640between versions of Perl, and are not meant for external use.
748a9306
LW
641
642Furthermore, when called from within the DB package, caller returns more
7660c0ab 643detailed information: it sets the list variable C<@DB::args> to be the
54310121 644arguments with which the subroutine was invoked.
748a9306 645
7660c0ab 646Be aware that the optimizer might have optimized call frames away before
19799a22 647C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 648might not return information about the call frame you expect it do, for
b76cc8ba 649C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 650previous time C<caller> was called.
7660c0ab 651
a0d0e21e 652=item chdir EXPR
d74e8afc
ITB
653X<chdir>
654X<cd>
a0d0e21e 655
c4aca7d0
GA
656=item chdir FILEHANDLE
657
658=item chdir DIRHANDLE
659
ce2984c3
PF
660=item chdir
661
ffce7b87 662Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 663changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 664changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
665variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
666neither is set, C<chdir> does nothing. It returns true upon success,
667false otherwise. See the example under C<die>.
a0d0e21e 668
c4aca7d0
GA
669On systems that support fchdir, you might pass a file handle or
670directory handle as argument. On systems that don't support fchdir,
671passing handles produces a fatal error at run time.
672
a0d0e21e 673=item chmod LIST
d74e8afc 674X<chmod> X<permission> X<mode>
a0d0e21e
LW
675
676Changes the permissions of a list of files. The first element of the
4633a7c4 677list must be the numerical mode, which should probably be an octal
4ad40acf 678number, and which definitely should I<not> be a string of octal digits:
2f9daede 679C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 680successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
681
682 $cnt = chmod 0755, 'foo', 'bar';
683 chmod 0755, @executables;
f86cebdf
GS
684 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
685 # --w----r-T
2f9daede
TP
686 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
687 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 688
c4aca7d0
GA
689On systems that support fchmod, you might pass file handles among the
690files. On systems that don't support fchmod, passing file handles
691produces a fatal error at run time.
692
693 open(my $fh, "<", "foo");
694 my $perm = (stat $fh)[2] & 07777;
695 chmod($perm | 0600, $fh);
696
ca6e1c26
JH
697You can also import the symbolic C<S_I*> constants from the Fcntl
698module:
699
700 use Fcntl ':mode';
701
702 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
703 # This is identical to the chmod 0755 of the above example.
704
a0d0e21e 705=item chomp VARIABLE
d74e8afc 706X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 707
313c9f5c 708=item chomp( LIST )
a0d0e21e
LW
709
710=item chomp
711
2b5ab1e7
TC
712This safer version of L</chop> removes any trailing string
713that corresponds to the current value of C<$/> (also known as
28757baa 714$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
715number of characters removed from all its arguments. It's often used to
716remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
717that the final record may be missing its newline. When in paragraph
718mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
719When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
720a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 721remove anything.
19799a22 722If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
723
724 while (<>) {
725 chomp; # avoid \n on last field
726 @array = split(/:/);
5a964f20 727 # ...
a0d0e21e
LW
728 }
729
4bf21a6d
RD
730If VARIABLE is a hash, it chomps the hash's values, but not its keys.
731
a0d0e21e
LW
732You can actually chomp anything that's an lvalue, including an assignment:
733
734 chomp($cwd = `pwd`);
735 chomp($answer = <STDIN>);
736
737If you chomp a list, each element is chomped, and the total number of
738characters removed is returned.
739
442a8c12
NC
740If the C<encoding> pragma is in scope then the lengths returned are
741calculated from the length of C<$/> in Unicode characters, which is not
742always the same as the length of C<$/> in the native encoding.
743
15e44fd8
RGS
744Note that parentheses are necessary when you're chomping anything
745that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
746is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
747C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
748C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
749as C<chomp($a, $b)>.
750
a0d0e21e 751=item chop VARIABLE
d74e8afc 752X<chop>
a0d0e21e 753
313c9f5c 754=item chop( LIST )
a0d0e21e
LW
755
756=item chop
757
758Chops off the last character of a string and returns the character
5b3eff12 759chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 760scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
761If VARIABLE is a hash, it chops the hash's values, but not its keys.
762
5b3eff12 763You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
764
765If you chop a list, each element is chopped. Only the value of the
19799a22 766last C<chop> is returned.
a0d0e21e 767
19799a22 768Note that C<chop> returns the last character. To return all but the last
748a9306
LW
769character, use C<substr($string, 0, -1)>.
770
15e44fd8
RGS
771See also L</chomp>.
772
a0d0e21e 773=item chown LIST
d74e8afc 774X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
775
776Changes the owner (and group) of a list of files. The first two
19799a22
GS
777elements of the list must be the I<numeric> uid and gid, in that
778order. A value of -1 in either position is interpreted by most
779systems to leave that value unchanged. Returns the number of files
780successfully changed.
a0d0e21e
LW
781
782 $cnt = chown $uid, $gid, 'foo', 'bar';
783 chown $uid, $gid, @filenames;
784
c4aca7d0
GA
785On systems that support fchown, you might pass file handles among the
786files. On systems that don't support fchown, passing file handles
787produces a fatal error at run time.
788
54310121 789Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
790
791 print "User: ";
19799a22 792 chomp($user = <STDIN>);
5a964f20 793 print "Files: ";
19799a22 794 chomp($pattern = <STDIN>);
a0d0e21e
LW
795
796 ($login,$pass,$uid,$gid) = getpwnam($user)
797 or die "$user not in passwd file";
798
5a964f20 799 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
800 chown $uid, $gid, @ary;
801
54310121 802On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
803file unless you're the superuser, although you should be able to change
804the group to any of your secondary groups. On insecure systems, these
805restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
806On POSIX systems, you can detect this condition this way:
807
808 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
809 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 810
a0d0e21e 811=item chr NUMBER
d74e8afc 812X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 813
54310121 814=item chr
bbce6d69 815
a0d0e21e 816Returns the character represented by that NUMBER in the character set.
a0ed51b3 817For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1e54db1a
JH
818chr(0x263a) is a Unicode smiley face. Note that characters from 128
819to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
820backward compatibility reasons (but see L<encoding>).
aaa68c4a 821
8a064bd6 822Negative values give the Unicode replacement character (chr(0xfffd)),
5f0135eb 823except under the L<bytes> pragma, where low eight bits of the value
8a064bd6
JH
824(truncated to an integer) are used.
825
974da8e5
JH
826If NUMBER is omitted, uses C<$_>.
827
b76cc8ba 828For the reverse, use L</ord>.
a0d0e21e 829
974da8e5
JH
830Note that under the C<bytes> pragma the NUMBER is masked to
831the low eight bits.
832
833See L<perlunicode> and L<encoding> for more about Unicode.
bbce6d69 834
a0d0e21e 835=item chroot FILENAME
d74e8afc 836X<chroot> X<root>
a0d0e21e 837
54310121 838=item chroot
bbce6d69 839
5a964f20 840This function works like the system call by the same name: it makes the
4633a7c4 841named directory the new root directory for all further pathnames that
951ba7fe 842begin with a C</> by your process and all its children. (It doesn't
28757baa 843change your current working directory, which is unaffected.) For security
4633a7c4 844reasons, this call is restricted to the superuser. If FILENAME is
19799a22 845omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
846
847=item close FILEHANDLE
d74e8afc 848X<close>
a0d0e21e 849
6a518fbc
TP
850=item close
851
9124316e
JH
852Closes the file or pipe associated with the file handle, returning
853true only if IO buffers are successfully flushed and closes the system
854file descriptor. Closes the currently selected filehandle if the
855argument is omitted.
fb73857a 856
857You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
858another C<open> on it, because C<open> will close it for you. (See
859C<open>.) However, an explicit C<close> on an input file resets the line
860counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 861
dede8123
RGS
862If the file handle came from a piped open, C<close> will additionally
863return false if one of the other system calls involved fails, or if the
fb73857a 864program exits with non-zero status. (If the only problem was that the
dede8123 865program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 866also waits for the process executing on the pipe to complete, in case you
b76cc8ba 867want to look at the output of the pipe afterwards, and
e5218da5
GA
868implicitly puts the exit status value of that command into C<$?> and
869C<${^CHILD_ERROR_NATIVE}>.
5a964f20 870
73689b13
GS
871Prematurely closing the read end of a pipe (i.e. before the process
872writing to it at the other end has closed it) will result in a
873SIGPIPE being delivered to the writer. If the other end can't
874handle that, be sure to read all the data before closing the pipe.
875
fb73857a 876Example:
a0d0e21e 877
fb73857a 878 open(OUTPUT, '|sort >foo') # pipe to sort
879 or die "Can't start sort: $!";
5a964f20 880 #... # print stuff to output
fb73857a 881 close OUTPUT # wait for sort to finish
882 or warn $! ? "Error closing sort pipe: $!"
883 : "Exit status $? from sort";
884 open(INPUT, 'foo') # get sort's results
885 or die "Can't open 'foo' for input: $!";
a0d0e21e 886
5a964f20
TC
887FILEHANDLE may be an expression whose value can be used as an indirect
888filehandle, usually the real filehandle name.
a0d0e21e
LW
889
890=item closedir DIRHANDLE
d74e8afc 891X<closedir>
a0d0e21e 892
19799a22 893Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
894system call.
895
a0d0e21e 896=item connect SOCKET,NAME
d74e8afc 897X<connect>
a0d0e21e
LW
898
899Attempts to connect to a remote socket, just as the connect system call
19799a22 900does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
901packed address of the appropriate type for the socket. See the examples in
902L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 903
cb1a09d0 904=item continue BLOCK
d74e8afc 905X<continue>
cb1a09d0 906
0d863452
RH
907=item continue
908
cf264981
SP
909C<continue> is actually a flow control statement rather than a function. If
910there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
911C<foreach>), it is always executed just before the conditional is about to
912be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
913it can be used to increment a loop variable, even when the loop has been
914continued via the C<next> statement (which is similar to the C C<continue>
915statement).
916
98293880 917C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
918block. C<last> and C<redo> will behave as if they had been executed within
919the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
920block, it may be more entertaining.
921
922 while (EXPR) {
923 ### redo always comes here
924 do_something;
925 } continue {
926 ### next always comes here
927 do_something_else;
928 # then back the top to re-check EXPR
929 }
930 ### last always comes here
931
932Omitting the C<continue> section is semantically equivalent to using an
19799a22 933empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
934to check the condition at the top of the loop.
935
0d863452
RH
936If the "switch" feature is enabled, C<continue> is also a
937function that will break out of the current C<when> or C<default>
938block, and fall through to the next case. See L<feature> and
939L<perlsyn/"Switch statements"> for more information.
940
941
a0d0e21e 942=item cos EXPR
d74e8afc 943X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 944
d6217f1e
GS
945=item cos
946
5a964f20 947Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 948takes cosine of C<$_>.
a0d0e21e 949
ca6e1c26 950For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa 951function, or use this relation:
952
953 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
954
a0d0e21e 955=item crypt PLAINTEXT,SALT
d74e8afc
ITB
956X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
957X<decrypt> X<cryptography> X<passwd>
a0d0e21e 958
ef2e6798
MS
959Creates a digest string exactly like the crypt(3) function in the C
960library (assuming that you actually have a version there that has not
cf264981 961been extirpated as a potential munitions).
ef2e6798
MS
962
963crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
964into a short string, called a digest, which is returned. The same
965PLAINTEXT and SALT will always return the same string, but there is no
966(known) way to get the original PLAINTEXT from the hash. Small
967changes in the PLAINTEXT or SALT will result in large changes in the
968digest.
969
970There is no decrypt function. This function isn't all that useful for
971cryptography (for that, look for F<Crypt> modules on your nearby CPAN
972mirror) and the name "crypt" is a bit of a misnomer. Instead it is
973primarily used to check if two pieces of text are the same without
974having to transmit or store the text itself. An example is checking
975if a correct password is given. The digest of the password is stored,
cf264981 976not the password itself. The user types in a password that is
ef2e6798
MS
977crypt()'d with the same salt as the stored digest. If the two digests
978match the password is correct.
979
980When verifying an existing digest string you should use the digest as
981the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 982to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
983crypt() will hash the new string with the same salt as the digest.
984This allows your code to work with the standard L<crypt|/crypt> and
985with more exotic implementations. In other words, do not assume
986anything about the returned string itself, or how many bytes in the
987digest matter.
85c16d83
JH
988
989Traditionally the result is a string of 13 bytes: two first bytes of
990the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
ef2e6798
MS
991the first eight bytes of the digest string mattered, but alternative
992hashing schemes (like MD5), higher level security schemes (like C2),
993and implementations on non-UNIX platforms may produce different
994strings.
85c16d83
JH
995
996When choosing a new salt create a random two character string whose
997characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
998'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
999characters is just a recommendation; the characters allowed in
1000the salt depend solely on your system's crypt library, and Perl can't
1001restrict what salts C<crypt()> accepts.
e71965be 1002
a0d0e21e 1003Here's an example that makes sure that whoever runs this program knows
cf264981 1004their password:
a0d0e21e
LW
1005
1006 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1007
1008 system "stty -echo";
1009 print "Password: ";
e71965be 1010 chomp($word = <STDIN>);
a0d0e21e
LW
1011 print "\n";
1012 system "stty echo";
1013
e71965be 1014 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
1015 die "Sorry...\n";
1016 } else {
1017 print "ok\n";
54310121 1018 }
a0d0e21e 1019
9f8f0c9d 1020Of course, typing in your own password to whoever asks you
748a9306 1021for it is unwise.
a0d0e21e 1022
ef2e6798 1023The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1024of data, not least of all because you can't get the information
ef2e6798 1025back. Look at the L<Digest> module for more robust algorithms.
19799a22 1026
f2791508
JH
1027If using crypt() on a Unicode string (which I<potentially> has
1028characters with codepoints above 255), Perl tries to make sense
1029of the situation by trying to downgrade (a copy of the string)
1030the string back to an eight-bit byte string before calling crypt()
1031(on that copy). If that works, good. If not, crypt() dies with
1032C<Wide character in crypt>.
85c16d83 1033
aa689395 1034=item dbmclose HASH
d74e8afc 1035X<dbmclose>
a0d0e21e 1036
19799a22 1037[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1038
aa689395 1039Breaks the binding between a DBM file and a hash.
a0d0e21e 1040
19799a22 1041=item dbmopen HASH,DBNAME,MASK
d74e8afc 1042X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1043
19799a22 1044[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1045
7b8d334a 1046This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1047hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1048argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395 1049is the name of the database (without the F<.dir> or F<.pag> extension if
1050any). If the database does not exist, it is created with protection
19799a22
GS
1051specified by MASK (as modified by the C<umask>). If your system supports
1052only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 1053program. In older versions of Perl, if your system had neither DBM nor
19799a22 1054ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395 1055sdbm(3).
1056
1057If you don't have write access to the DBM file, you can only read hash
1058variables, not set them. If you want to test whether you can write,
19799a22 1059either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 1060which will trap the error.
a0d0e21e 1061
19799a22
GS
1062Note that functions such as C<keys> and C<values> may return huge lists
1063when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1064function to iterate over large DBM files. Example:
1065
1066 # print out history file offsets
1067 dbmopen(%HIST,'/usr/lib/news/history',0666);
1068 while (($key,$val) = each %HIST) {
1069 print $key, ' = ', unpack('L',$val), "\n";
1070 }
1071 dbmclose(%HIST);
1072
cb1a09d0 1073See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1074cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1075rich implementation.
4633a7c4 1076
2b5ab1e7
TC
1077You can control which DBM library you use by loading that library
1078before you call dbmopen():
1079
1080 use DB_File;
1081 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1082 or die "Can't open netscape history file: $!";
1083
a0d0e21e 1084=item defined EXPR
d74e8afc 1085X<defined> X<undef> X<undefined>
a0d0e21e 1086
54310121 1087=item defined
bbce6d69 1088
2f9daede
TP
1089Returns a Boolean value telling whether EXPR has a value other than
1090the undefined value C<undef>. If EXPR is not present, C<$_> will be
1091checked.
1092
1093Many operations return C<undef> to indicate failure, end of file,
1094system error, uninitialized variable, and other exceptional
1095conditions. This function allows you to distinguish C<undef> from
1096other values. (A simple Boolean test will not distinguish among
7660c0ab 1097C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1098false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1099doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TP
1100returns C<undef> when its argument is an empty array, I<or> when the
1101element to return happens to be C<undef>.
1102
f10b0346
GS
1103You may also use C<defined(&func)> to check whether subroutine C<&func>
1104has ever been defined. The return value is unaffected by any forward
04891299 1105declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1106may still be callable: its package may have an C<AUTOLOAD> method that
1107makes it spring into existence the first time that it is called -- see
1108L<perlsub>.
f10b0346
GS
1109
1110Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1111used to report whether memory for that aggregate has ever been
1112allocated. This behavior may disappear in future versions of Perl.
1113You should instead use a simple test for size:
1114
1115 if (@an_array) { print "has array elements\n" }
1116 if (%a_hash) { print "has hash members\n" }
2f9daede
TP
1117
1118When used on a hash element, it tells you whether the value is defined,
dc848c6f 1119not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1120purpose.
a0d0e21e
LW
1121
1122Examples:
1123
1124 print if defined $switch{'D'};
1125 print "$val\n" while defined($val = pop(@ary));
1126 die "Can't readlink $sym: $!"
1127 unless defined($value = readlink $sym);
a0d0e21e 1128 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1129 $debugging = 0 unless defined $debugging;
a0d0e21e 1130
19799a22 1131Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1132discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1133defined values. For example, if you say
a5f75d66
AD
1134
1135 "ab" =~ /a(.*)b/;
1136
7660c0ab 1137The pattern match succeeds, and C<$1> is defined, despite the fact that it
cf264981 1138matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1139matched something that happened to be zero characters long. This is all
a5f75d66 1140very above-board and honest. When a function returns an undefined value,
2f9daede 1141it's an admission that it couldn't give you an honest answer. So you
19799a22 1142should use C<defined> only when you're questioning the integrity of what
7660c0ab 1143you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TP
1144what you want.
1145
dc848c6f 1146See also L</undef>, L</exists>, L</ref>.
2f9daede 1147
a0d0e21e 1148=item delete EXPR
d74e8afc 1149X<delete>
a0d0e21e 1150
01020589
GS
1151Given an expression that specifies a hash element, array element, hash slice,
1152or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1153In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1154the size of the array will shrink to the highest element that tests
8216c1fd 1155true for exists() (or 0 if no such element exists).
a0d0e21e 1156
eba0920a
EM
1157Returns a list with the same number of elements as the number of elements
1158for which deletion was attempted. Each element of that list consists of
1159either the value of the element deleted, or the undefined value. In scalar
1160context, this means that you get the value of the last element deleted (or
1161the undefined value if that element did not exist).
1162
1163 %hash = (foo => 11, bar => 22, baz => 33);
1164 $scalar = delete $hash{foo}; # $scalar is 11
1165 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1166 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1167
1168Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1169a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1170from a C<tie>d hash or array may not necessarily return anything.
1171
8ea97a1e
GS
1172Deleting an array element effectively returns that position of the array
1173to its initial, uninitialized state. Subsequently testing for the same
cf264981
SP
1174element with exists() will return false. Also, deleting array elements
1175in the middle of an array will not shift the index of the elements
1176after them down. Use splice() for that. See L</exists>.
8ea97a1e 1177
01020589 1178The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1179
5f05dabc 1180 foreach $key (keys %HASH) {
1181 delete $HASH{$key};
a0d0e21e
LW
1182 }
1183
01020589
GS
1184 foreach $index (0 .. $#ARRAY) {
1185 delete $ARRAY[$index];
1186 }
1187
1188And so do these:
5f05dabc 1189
01020589
GS
1190 delete @HASH{keys %HASH};
1191
9740c838 1192 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1193
2b5ab1e7 1194But both of these are slower than just assigning the empty list
01020589
GS
1195or undefining %HASH or @ARRAY:
1196
1197 %HASH = (); # completely empty %HASH
1198 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1199
01020589
GS
1200 @ARRAY = (); # completely empty @ARRAY
1201 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1202
1203Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1204operation is a hash element, array element, hash slice, or array slice
1205lookup:
a0d0e21e
LW
1206
1207 delete $ref->[$x][$y]{$key};
5f05dabc 1208 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1209
01020589
GS
1210 delete $ref->[$x][$y][$index];
1211 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1212
a0d0e21e 1213=item die LIST
d74e8afc 1214X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1215
19799a22
GS
1216Outside an C<eval>, prints the value of LIST to C<STDERR> and
1217exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1218exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1219status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1220an C<eval(),> the error message is stuffed into C<$@> and the
1221C<eval> is terminated with the undefined value. This makes
1222C<die> the way to raise an exception.
a0d0e21e
LW
1223
1224Equivalent examples:
1225
1226 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1227 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1228
ccac6780 1229If the last element of LIST does not end in a newline, the current
df37ec69
WW
1230script line number and input line number (if any) are also printed,
1231and a newline is supplied. Note that the "input line number" (also
1232known as "chunk") is subject to whatever notion of "line" happens to
1233be currently in effect, and is also available as the special variable
1234C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1235
1236Hint: sometimes appending C<", stopped"> to your message will cause it
1237to make better sense when the string C<"at foo line 123"> is appended.
1238Suppose you are running script "canasta".
a0d0e21e
LW
1239
1240 die "/etc/games is no good";
1241 die "/etc/games is no good, stopped";
1242
1243produce, respectively
1244
1245 /etc/games is no good at canasta line 123.
1246 /etc/games is no good, stopped at canasta line 123.
1247
2b5ab1e7 1248See also exit(), warn(), and the Carp module.
a0d0e21e 1249
7660c0ab
A
1250If LIST is empty and C<$@> already contains a value (typically from a
1251previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a 1252This is useful for propagating exceptions:
1253
1254 eval { ... };
1255 die unless $@ =~ /Expected exception/;
1256
ad216e65
JH
1257If LIST is empty and C<$@> contains an object reference that has a
1258C<PROPAGATE> method, that method will be called with additional file
1259and line number parameters. The return value replaces the value in
28a5cf3b 1260C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1261were called.
1262
7660c0ab 1263If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1264
52531d10
GS
1265die() can also be called with a reference argument. If this happens to be
1266trapped within an eval(), $@ contains the reference. This behavior permits
1267a more elaborate exception handling implementation using objects that
4375e838 1268maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1269is sometimes preferable to matching particular string values of $@ using
1270regular expressions. Here's an example:
1271
da279afe 1272 use Scalar::Util 'blessed';
1273
52531d10
GS
1274 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1275 if ($@) {
da279afe 1276 if (blessed($@) && $@->isa("Some::Module::Exception")) {
52531d10
GS
1277 # handle Some::Module::Exception
1278 }
1279 else {
1280 # handle all other possible exceptions
1281 }
1282 }
1283
19799a22 1284Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1285them, you may want to overload stringification operations on such custom
1286exception objects. See L<overload> for details about that.
1287
19799a22
GS
1288You can arrange for a callback to be run just before the C<die>
1289does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1290handler will be called with the error text and can change the error
1291message, if it sees fit, by calling C<die> again. See
1292L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1293L<"eval BLOCK"> for some examples. Although this feature was
19799a22
GS
1294to be run only right before your program was to exit, this is not
1295currently the case--the C<$SIG{__DIE__}> hook is currently called
1296even inside eval()ed blocks/strings! If one wants the hook to do
1297nothing in such situations, put
fb73857a 1298
1299 die @_ if $^S;
1300
19799a22
GS
1301as the first line of the handler (see L<perlvar/$^S>). Because
1302this promotes strange action at a distance, this counterintuitive
b76cc8ba 1303behavior may be fixed in a future release.
774d564b 1304
a0d0e21e 1305=item do BLOCK
d74e8afc 1306X<do> X<block>
a0d0e21e
LW
1307
1308Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1309sequence of commands indicated by BLOCK. When modified by the C<while> or
1310C<until> loop modifier, executes the BLOCK once before testing the loop
1311condition. (On other statements the loop modifiers test the conditional
1312first.)
a0d0e21e 1313
4968c1e4 1314C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1315C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1316See L<perlsyn> for alternative strategies.
4968c1e4 1317
a0d0e21e 1318=item do SUBROUTINE(LIST)
d74e8afc 1319X<do>
a0d0e21e 1320
cf264981 1321This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1322
1323=item do EXPR
d74e8afc 1324X<do>
a0d0e21e
LW
1325
1326Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1327file as a Perl script.
a0d0e21e
LW
1328
1329 do 'stat.pl';
1330
1331is just like
1332
986b19de 1333 eval `cat stat.pl`;
a0d0e21e 1334
2b5ab1e7 1335except that it's more efficient and concise, keeps track of the current
ea63ef19 1336filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1337C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1338variables. It also differs in that code evaluated with C<do FILENAME>
1339cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1340same, however, in that it does reparse the file every time you call it,
1341so you probably don't want to do this inside a loop.
a0d0e21e 1342
8e30cc93 1343If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1344error. If C<do> can read the file but cannot compile it, it
8e30cc93
MG
1345returns undef and sets an error message in C<$@>. If the file is
1346successfully compiled, C<do> returns the value of the last expression
1347evaluated.
1348
a0d0e21e 1349Note that inclusion of library modules is better done with the
19799a22 1350C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1351and raise an exception if there's a problem.
a0d0e21e 1352
5a964f20
TC
1353You might like to use C<do> to read in a program configuration
1354file. Manual error checking can be done this way:
1355
b76cc8ba 1356 # read in config files: system first, then user
f86cebdf 1357 for $file ("/share/prog/defaults.rc",
b76cc8ba 1358 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1359 {
5a964f20 1360 unless ($return = do $file) {
f86cebdf
GS
1361 warn "couldn't parse $file: $@" if $@;
1362 warn "couldn't do $file: $!" unless defined $return;
1363 warn "couldn't run $file" unless $return;
5a964f20
TC
1364 }
1365 }
1366
a0d0e21e 1367=item dump LABEL
d74e8afc 1368X<dump> X<core> X<undump>
a0d0e21e 1369
1614b0e3
JD
1370=item dump
1371
19799a22
GS
1372This function causes an immediate core dump. See also the B<-u>
1373command-line switch in L<perlrun>, which does the same thing.
1374Primarily this is so that you can use the B<undump> program (not
1375supplied) to turn your core dump into an executable binary after
1376having initialized all your variables at the beginning of the
1377program. When the new binary is executed it will begin by executing
1378a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1379Think of it as a goto with an intervening core dump and reincarnation.
1380If C<LABEL> is omitted, restarts the program from the top.
1381
1382B<WARNING>: Any files opened at the time of the dump will I<not>
1383be open any more when the program is reincarnated, with possible
b76cc8ba 1384resulting confusion on the part of Perl.
19799a22
GS
1385
1386This function is now largely obsolete, partly because it's very
1387hard to convert a core file into an executable, and because the
1388real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1389C code have superseded it. That's why you should now invoke it as
1390C<CORE::dump()>, if you don't want to be warned against a possible
1391typo.
19799a22
GS
1392
1393If you're looking to use L<dump> to speed up your program, consider
1394generating bytecode or native C code as described in L<perlcc>. If
1395you're just trying to accelerate a CGI script, consider using the
210b36aa 1396C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1397You might also consider autoloading or selfloading, which at least
b76cc8ba 1398make your program I<appear> to run faster.
5a964f20 1399
aa689395 1400=item each HASH
d74e8afc 1401X<each> X<hash, iterator>
aa689395 1402
5a964f20 1403When called in list context, returns a 2-element list consisting of the
aa689395 1404key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1405it. When called in scalar context, returns only the key for the next
e902a979 1406element in the hash.
2f9daede 1407
ab192400 1408Entries are returned in an apparently random order. The actual random
504f80c1
JH
1409order is subject to change in future versions of perl, but it is
1410guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1411function would produce on the same (unmodified) hash. Since Perl
14125.8.1 the ordering is different even between different runs of Perl
1413for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1414
1415When the hash is entirely read, a null array is returned in list context
19799a22
GS
1416(which when assigned produces a false (C<0>) value), and C<undef> in
1417scalar context. The next call to C<each> after that will start iterating
1418again. There is a single iterator for each hash, shared by all C<each>,
1419C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TP
1420reading all the elements from the hash, or by evaluating C<keys HASH> or
1421C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1422iterating over it, you may get entries skipped or duplicated, so
1423don't. Exception: It is always safe to delete the item most recently
1424returned by C<each()>, which means that the following code will work:
1425
1426 while (($key, $value) = each %hash) {
1427 print $key, "\n";
1428 delete $hash{$key}; # This is safe
1429 }
aa689395 1430
f86cebdf 1431The following prints out your environment like the printenv(1) program,
aa689395 1432only in a different order:
a0d0e21e
LW
1433
1434 while (($key,$value) = each %ENV) {
1435 print "$key=$value\n";
1436 }
1437
19799a22 1438See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1439
1440=item eof FILEHANDLE
d74e8afc
ITB
1441X<eof>
1442X<end of file>
1443X<end-of-file>
a0d0e21e 1444
4633a7c4
LW
1445=item eof ()
1446
a0d0e21e
LW
1447=item eof
1448
1449Returns 1 if the next read on FILEHANDLE will return end of file, or if
1450FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1451gives the real filehandle. (Note that this function actually
19799a22 1452reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1453interactive context.) Do not read from a terminal file (or call
19799a22 1454C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1455as terminals may lose the end-of-file condition if you do.
1456
820475bd
GS
1457An C<eof> without an argument uses the last file read. Using C<eof()>
1458with empty parentheses is very different. It refers to the pseudo file
1459formed from the files listed on the command line and accessed via the
61eff3bc
JH
1460C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1461as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1462used will cause C<@ARGV> to be examined to determine if input is
67408cae 1463available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1464end-of-file will assume you are processing another C<@ARGV> list,
1465and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1466see L<perlop/"I/O Operators">.
820475bd 1467
61eff3bc 1468In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1469detect the end of each file, C<eof()> will only detect the end of the
1470last file. Examples:
a0d0e21e 1471
748a9306
LW
1472 # reset line numbering on each input file
1473 while (<>) {
b76cc8ba 1474 next if /^\s*#/; # skip comments
748a9306 1475 print "$.\t$_";
5a964f20
TC
1476 } continue {
1477 close ARGV if eof; # Not eof()!
748a9306
LW
1478 }
1479
a0d0e21e
LW
1480 # insert dashes just before last line of last file
1481 while (<>) {
6ac88b13 1482 if (eof()) { # check for end of last file
a0d0e21e
LW
1483 print "--------------\n";
1484 }
1485 print;
6ac88b13 1486 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1487 }
1488
a0d0e21e 1489Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1490input operators typically return C<undef> when they run out of data, or if
1491there was an error.
a0d0e21e
LW
1492
1493=item eval EXPR
d74e8afc 1494X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
a0d0e21e
LW
1495
1496=item eval BLOCK
1497
ce2984c3
PF
1498=item eval
1499
c7cc6f1c
GS
1500In the first form, the return value of EXPR is parsed and executed as if it
1501were a little Perl program. The value of the expression (which is itself
5a964f20 1502determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1503errors, executed in the lexical context of the current Perl program, so
1504that any variable settings or subroutine and format definitions remain
cf264981 1505afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1506If EXPR is omitted, evaluates C<$_>. This form is typically used to
1507delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1508
1509In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1510same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1511within the context of the current Perl program. This form is typically
1512used to trap exceptions more efficiently than the first (see below), while
1513also providing the benefit of checking the code within BLOCK at compile
1514time.
1515
1516The final semicolon, if any, may be omitted from the value of EXPR or within
1517the BLOCK.
1518
1519In both forms, the value returned is the value of the last expression
5a964f20 1520evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1521as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1522in void, scalar, or list context, depending on the context of the C<eval>
1523itself. See L</wantarray> for more on how the evaluation context can be
1524determined.
a0d0e21e 1525
19799a22
GS
1526If there is a syntax error or runtime error, or a C<die> statement is
1527executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1528error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1529string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1530warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1531To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1532turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1533See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1534
19799a22
GS
1535Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1536determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1537is implemented. It is also Perl's exception trapping mechanism, where
1538the die operator is used to raise exceptions.
1539
1540If the code to be executed doesn't vary, you may use the eval-BLOCK
1541form to trap run-time errors without incurring the penalty of
1542recompiling each time. The error, if any, is still returned in C<$@>.
1543Examples:
1544
54310121 1545 # make divide-by-zero nonfatal
a0d0e21e
LW
1546 eval { $answer = $a / $b; }; warn $@ if $@;
1547
1548 # same thing, but less efficient
1549 eval '$answer = $a / $b'; warn $@ if $@;
1550
1551 # a compile-time error
5a964f20 1552 eval { $answer = }; # WRONG
a0d0e21e
LW
1553
1554 # a run-time error
1555 eval '$answer ='; # sets $@
1556
cf264981
SP
1557Using the C<eval{}> form as an exception trap in libraries does have some
1558issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1559may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7
TC
1560You can use the C<local $SIG{__DIE__}> construct for this purpose,
1561as shown in this example:
774d564b 1562
1563 # a very private exception trap for divide-by-zero
f86cebdf
GS
1564 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1565 warn $@ if $@;
774d564b 1566
1567This is especially significant, given that C<__DIE__> hooks can call
19799a22 1568C<die> again, which has the effect of changing their error messages:
774d564b 1569
1570 # __DIE__ hooks may modify error messages
1571 {
f86cebdf
GS
1572 local $SIG{'__DIE__'} =
1573 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1574 eval { die "foo lives here" };
1575 print $@ if $@; # prints "bar lives here"
774d564b 1576 }
1577
19799a22 1578Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1579may be fixed in a future release.
1580
19799a22 1581With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1582being looked at when:
1583
1584 eval $x; # CASE 1
1585 eval "$x"; # CASE 2
1586
1587 eval '$x'; # CASE 3
1588 eval { $x }; # CASE 4
1589
5a964f20 1590 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1591 $$x++; # CASE 6
1592
2f9daede 1593Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1594the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1595the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1596and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1597does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TP
1598purely visual reasons, but it also has the advantage of compiling at
1599compile-time instead of at run-time.) Case 5 is a place where
19799a22 1600normally you I<would> like to use double quotes, except that in this
2f9daede
TP
1601particular situation, you can just use symbolic references instead, as
1602in case 6.
a0d0e21e 1603
4968c1e4 1604C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1605C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1606
d819b83a
DM
1607Note that as a very special case, an C<eval ''> executed within the C<DB>
1608package doesn't see the usual surrounding lexical scope, but rather the
1609scope of the first non-DB piece of code that called it. You don't normally
1610need to worry about this unless you are writing a Perl debugger.
1611
a0d0e21e 1612=item exec LIST
d74e8afc 1613X<exec> X<execute>
a0d0e21e 1614
8bf3b016
GS
1615=item exec PROGRAM LIST
1616
19799a22
GS
1617The C<exec> function executes a system command I<and never returns>--
1618use C<system> instead of C<exec> if you want it to return. It fails and
1619returns false only if the command does not exist I<and> it is executed
fb73857a 1620directly instead of via your system's command shell (see below).
a0d0e21e 1621
19799a22
GS
1622Since it's a common mistake to use C<exec> instead of C<system>, Perl
1623warns you if there is a following statement which isn't C<die>, C<warn>,
1624or C<exit> (if C<-w> is set - but you always do that). If you
1625I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1626can use one of these styles to avoid the warning:
1627
5a964f20
TC
1628 exec ('foo') or print STDERR "couldn't exec foo: $!";
1629 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1630
5a964f20 1631If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1632with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1633If there is only one scalar argument or an array with one element in it,
1634the argument is checked for shell metacharacters, and if there are any,
1635the entire argument is passed to the system's command shell for parsing
1636(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1637If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1638words and passed directly to C<execvp>, which is more efficient.
19799a22 1639Examples:
a0d0e21e 1640
19799a22
GS
1641 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1642 exec "sort $outfile | uniq";
a0d0e21e
LW
1643
1644If you don't really want to execute the first argument, but want to lie
1645to the program you are executing about its own name, you can specify
1646the program you actually want to run as an "indirect object" (without a
1647comma) in front of the LIST. (This always forces interpretation of the
54310121 1648LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1649the list.) Example:
1650
1651 $shell = '/bin/csh';
1652 exec $shell '-sh'; # pretend it's a login shell
1653
1654or, more directly,
1655
1656 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1657
bb32b41a
GS
1658When the arguments get executed via the system shell, results will
1659be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1660for details.
1661
19799a22
GS
1662Using an indirect object with C<exec> or C<system> is also more
1663secure. This usage (which also works fine with system()) forces
1664interpretation of the arguments as a multivalued list, even if the
1665list had just one argument. That way you're safe from the shell
1666expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1667
1668 @args = ( "echo surprise" );
1669
2b5ab1e7 1670 exec @args; # subject to shell escapes
f86cebdf 1671 # if @args == 1
2b5ab1e7 1672 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1673
1674The first version, the one without the indirect object, ran the I<echo>
1675program, passing it C<"surprise"> an argument. The second version
1676didn't--it tried to run a program literally called I<"echo surprise">,
1677didn't find it, and set C<$?> to a non-zero value indicating failure.
1678
0f897271
GS
1679Beginning with v5.6.0, Perl will attempt to flush all files opened for
1680output before the exec, but this may not be supported on some platforms
1681(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1682in English) or call the C<autoflush()> method of C<IO::Handle> on any
1683open handles in order to avoid lost output.
1684
19799a22 1685Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1686any C<DESTROY> methods in your objects.
1687
a0d0e21e 1688=item exists EXPR
d74e8afc 1689X<exists> X<autovivification>
a0d0e21e 1690
01020589 1691Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1692returns true if the specified element in the hash or array has ever
1693been initialized, even if the corresponding value is undefined. The
1694element is not autovivified if it doesn't exist.
a0d0e21e 1695
01020589
GS
1696 print "Exists\n" if exists $hash{$key};
1697 print "Defined\n" if defined $hash{$key};
1698 print "True\n" if $hash{$key};
1699
1700 print "Exists\n" if exists $array[$index];
1701 print "Defined\n" if defined $array[$index];
1702 print "True\n" if $array[$index];
a0d0e21e 1703
8ea97a1e 1704A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1705it exists, but the reverse doesn't necessarily hold true.
1706
afebc493
GS
1707Given an expression that specifies the name of a subroutine,
1708returns true if the specified subroutine has ever been declared, even
1709if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1710does not count as declaring it. Note that a subroutine which does not
1711exist may still be callable: its package may have an C<AUTOLOAD>
1712method that makes it spring into existence the first time that it is
1713called -- see L<perlsub>.
afebc493
GS
1714
1715 print "Exists\n" if exists &subroutine;
1716 print "Defined\n" if defined &subroutine;
1717
a0d0e21e 1718Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1719operation is a hash or array key lookup or subroutine name:
a0d0e21e 1720
2b5ab1e7
TC
1721 if (exists $ref->{A}->{B}->{$key}) { }
1722 if (exists $hash{A}{B}{$key}) { }
1723
01020589
GS
1724 if (exists $ref->{A}->{B}->[$ix]) { }
1725 if (exists $hash{A}{B}[$ix]) { }
1726
afebc493
GS
1727 if (exists &{$ref->{A}{B}{$key}}) { }
1728
01020589
GS
1729Although the deepest nested array or hash will not spring into existence
1730just because its existence was tested, any intervening ones will.
61eff3bc 1731Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1732into existence due to the existence test for the $key element above.
1733This happens anywhere the arrow operator is used, including even:
5a964f20 1734
2b5ab1e7
TC
1735 undef $ref;
1736 if (exists $ref->{"Some key"}) { }
1737 print $ref; # prints HASH(0x80d3d5c)
1738
1739This surprising autovivification in what does not at first--or even
1740second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1741release.
a0d0e21e 1742
afebc493
GS
1743Use of a subroutine call, rather than a subroutine name, as an argument
1744to exists() is an error.
1745
1746 exists &sub; # OK
1747 exists &sub(); # Error
1748
a0d0e21e 1749=item exit EXPR
d74e8afc 1750X<exit> X<terminate> X<abort>
a0d0e21e 1751
ce2984c3
PF
1752=item exit
1753
2b5ab1e7 1754Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1755
1756 $ans = <STDIN>;
1757 exit 0 if $ans =~ /^[Xx]/;
1758
19799a22 1759See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1760universally recognized values for EXPR are C<0> for success and C<1>
1761for error; other values are subject to interpretation depending on the
1762environment in which the Perl program is running. For example, exiting
176369 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1764the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1765
19799a22
GS
1766Don't use C<exit> to abort a subroutine if there's any chance that
1767someone might want to trap whatever error happened. Use C<die> instead,
1768which can be trapped by an C<eval>.
28757baa 1769
19799a22 1770The exit() function does not always exit immediately. It calls any
2b5ab1e7 1771defined C<END> routines first, but these C<END> routines may not
19799a22 1772themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1773be called are called before the real exit. If this is a problem, you
1774can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1775See L<perlmod> for details.
5a964f20 1776
a0d0e21e 1777=item exp EXPR
d74e8afc 1778X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1779
54310121 1780=item exp
bbce6d69 1781
b76cc8ba 1782Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1783If EXPR is omitted, gives C<exp($_)>.
1784
1785=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1786X<fcntl>
a0d0e21e 1787
f86cebdf 1788Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1789
1790 use Fcntl;
1791
0ade1984 1792first to get the correct constant definitions. Argument processing and
b76cc8ba 1793value return works just like C<ioctl> below.
a0d0e21e
LW
1794For example:
1795
1796 use Fcntl;
5a964f20
TC
1797 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1798 or die "can't fcntl F_GETFL: $!";
1799
554ad1fc 1800You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1801Like C<ioctl>, it maps a C<0> return from the system call into
1802C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1803in numeric context. It is also exempt from the normal B<-w> warnings
1804on improper numeric conversions.
5a964f20 1805
19799a22 1806Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1807doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1808manpage to learn what functions are available on your system.
a0d0e21e 1809
be2f7487
TH
1810Here's an example of setting a filehandle named C<REMOTE> to be
1811non-blocking at the system level. You'll have to negotiate C<$|>
1812on your own, though.
1813
1814 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1815
1816 $flags = fcntl(REMOTE, F_GETFL, 0)
1817 or die "Can't get flags for the socket: $!\n";
1818
1819 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1820 or die "Can't set flags for the socket: $!\n";
1821
a0d0e21e 1822=item fileno FILEHANDLE
d74e8afc 1823X<fileno>
a0d0e21e 1824
2b5ab1e7
TC
1825Returns the file descriptor for a filehandle, or undefined if the
1826filehandle is not open. This is mainly useful for constructing
19799a22 1827bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1828If FILEHANDLE is an expression, the value is taken as an indirect
1829filehandle, generally its name.
5a964f20 1830
b76cc8ba 1831You can use this to find out whether two handles refer to the
5a964f20
TC
1832same underlying descriptor:
1833
1834 if (fileno(THIS) == fileno(THAT)) {
1835 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1836 }
1837
1838(Filehandles connected to memory objects via new features of C<open> may
1839return undefined even though they are open.)
1840
a0d0e21e
LW
1841
1842=item flock FILEHANDLE,OPERATION
d74e8afc 1843X<flock> X<lock> X<locking>
a0d0e21e 1844
19799a22
GS
1845Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1846for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1847machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1848C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1849only entire files, not records.
1850
1851Two potentially non-obvious but traditional C<flock> semantics are
1852that it waits indefinitely until the lock is granted, and that its locks
1853B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1854fewer guarantees. This means that programs that do not also use C<flock>
1855may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1856your port's specific documentation, or your system-specific local manpages
1857for details. It's best to assume traditional behavior if you're writing
1858portable programs. (But if you're not, you should as always feel perfectly
1859free to write for your own system's idiosyncrasies (sometimes called
1860"features"). Slavish adherence to portability concerns shouldn't get
1861in the way of your getting your job done.)
a3cb178b 1862
8ebc5c01 1863OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1864LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1865you can use the symbolic names if you import them from the Fcntl module,
68dc0745 1866either individually, or as a group using the ':flock' tag. LOCK_SH
1867requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1868releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1869LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745 1870waiting for the lock (check the return status to see if you got it).
1871
2b5ab1e7
TC
1872To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1873before locking or unlocking it.
8ebc5c01 1874
f86cebdf 1875Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1876locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1877are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1878implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01 1879differing semantics shouldn't bite too many people.
1880
becacb53
TM
1881Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1882be open with read intent to use LOCK_SH and requires that it be open
1883with write intent to use LOCK_EX.
1884
19799a22
GS
1885Note also that some versions of C<flock> cannot lock things over the
1886network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1887that. If you like you can force Perl to ignore your system's flock(2)
1888function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 1889the switch C<-Ud_flock> to the F<Configure> program when you configure
1890perl.
4633a7c4
LW
1891
1892Here's a mailbox appender for BSD systems.
a0d0e21e 1893
7e1af8bc 1894 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1895
1896 sub lock {
7e1af8bc 1897 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1898 # and, in case someone appended
1899 # while we were waiting...
1900 seek(MBOX, 0, 2);
1901 }
1902
1903 sub unlock {
7e1af8bc 1904 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1905 }
1906
1907 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1908 or die "Can't open mailbox: $!";
1909
1910 lock();
1911 print MBOX $msg,"\n\n";
1912 unlock();
1913
2b5ab1e7
TC
1914On systems that support a real flock(), locks are inherited across fork()
1915calls, whereas those that must resort to the more capricious fcntl()
1916function lose the locks, making it harder to write servers.
1917
cb1a09d0 1918See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1919
1920=item fork
d74e8afc 1921X<fork> X<child> X<parent>
a0d0e21e 1922
2b5ab1e7
TC
1923Does a fork(2) system call to create a new process running the
1924same program at the same point. It returns the child pid to the
1925parent process, C<0> to the child process, or C<undef> if the fork is
1926unsuccessful. File descriptors (and sometimes locks on those descriptors)
1927are shared, while everything else is copied. On most systems supporting
1928fork(), great care has gone into making it extremely efficient (for
1929example, using copy-on-write technology on data pages), making it the
1930dominant paradigm for multitasking over the last few decades.
5a964f20 1931
0f897271
GS
1932Beginning with v5.6.0, Perl will attempt to flush all files opened for
1933output before forking the child process, but this may not be supported
1934on some platforms (see L<perlport>). To be safe, you may need to set
1935C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1936C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1937
19799a22 1938If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1939accumulate zombies. On some systems, you can avoid this by setting
1940C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1941forking and reaping moribund children.
cb1a09d0 1942
28757baa 1943Note that if your forked child inherits system file descriptors like
1944STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1945if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1946backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1947You should reopen those to F</dev/null> if it's any issue.
28757baa 1948
cb1a09d0 1949=item format
d74e8afc 1950X<format>
cb1a09d0 1951
19799a22 1952Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1953example:
1954
54310121 1955 format Something =
cb1a09d0
AD
1956 Test: @<<<<<<<< @||||| @>>>>>
1957 $str, $%, '$' . int($num)
1958 .
1959
1960 $str = "widget";
184e9718 1961 $num = $cost/$quantity;
cb1a09d0
AD
1962 $~ = 'Something';
1963 write;
1964
1965See L<perlform> for many details and examples.
1966
8903cb82 1967=item formline PICTURE,LIST
d74e8afc 1968X<formline>
a0d0e21e 1969
5a964f20 1970This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1971too. It formats (see L<perlform>) a list of values according to the
1972contents of PICTURE, placing the output into the format output
7660c0ab 1973accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1974Eventually, when a C<write> is done, the contents of
cf264981
SP
1975C<$^A> are written to some filehandle. You could also read C<$^A>
1976and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1977does one C<formline> per line of form, but the C<formline> function itself
748a9306 1978doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1979that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1980You may therefore need to use multiple formlines to implement a single
1981record format, just like the format compiler.
1982
19799a22 1983Be careful if you put double quotes around the picture, because an C<@>
748a9306 1984character may be taken to mean the beginning of an array name.
19799a22 1985C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1986
1987=item getc FILEHANDLE
d74e8afc 1988X<getc> X<getchar>
a0d0e21e
LW
1989
1990=item getc
1991
1992Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
1993or the undefined value at end of file, or if there was an error (in
1994the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1995STDIN. This is not particularly efficient. However, it cannot be
1996used by itself to fetch single characters without waiting for the user
1997to hit enter. For that, try something more like:
4633a7c4
LW
1998
1999 if ($BSD_STYLE) {
2000 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2001 }
2002 else {
54310121 2003 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2004 }
2005
2006 $key = getc(STDIN);
2007
2008 if ($BSD_STYLE) {
2009 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2010 }
2011 else {
5f05dabc 2012 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
2013 }
2014 print "\n";
2015
54310121 2016Determination of whether $BSD_STYLE should be set
2017is left as an exercise to the reader.
cb1a09d0 2018
19799a22 2019The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2020systems purporting POSIX compliance. See also the C<Term::ReadKey>
2021module from your nearest CPAN site; details on CPAN can be found on
2022L<perlmodlib/CPAN>.
a0d0e21e
LW
2023
2024=item getlogin
d74e8afc 2025X<getlogin> X<login>
a0d0e21e 2026
cf264981 2027This implements the C library function of the same name, which on most
5a964f20 2028systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 2029use C<getpwuid>.
a0d0e21e 2030
f86702cc 2031 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2032
19799a22
GS
2033Do not consider C<getlogin> for authentication: it is not as
2034secure as C<getpwuid>.
4633a7c4 2035
a0d0e21e 2036=item getpeername SOCKET
d74e8afc 2037X<getpeername> X<peer>
a0d0e21e
LW
2038
2039Returns the packed sockaddr address of other end of the SOCKET connection.
2040
4633a7c4
LW
2041 use Socket;
2042 $hersockaddr = getpeername(SOCK);
19799a22 2043 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2044 $herhostname = gethostbyaddr($iaddr, AF_INET);
2045 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2046
2047=item getpgrp PID
d74e8afc 2048X<getpgrp> X<group>
a0d0e21e 2049
47e29363 2050Returns the current process group for the specified PID. Use
7660c0ab 2051a PID of C<0> to get the current process group for the
4633a7c4 2052current process. Will raise an exception if used on a machine that
f86cebdf 2053doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2054group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2055does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2056
2057=item getppid
d74e8afc 2058X<getppid> X<parent> X<pid>
a0d0e21e
LW
2059
2060Returns the process id of the parent process.
2061
4d76a344
RGS
2062Note for Linux users: on Linux, the C functions C<getpid()> and
2063C<getppid()> return different values from different threads. In order to
2064be portable, this behavior is not reflected by the perl-level function
2065C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2066to call the underlying C<getppid()>, you may use the CPAN module
2067C<Linux::Pid>.
4d76a344 2068
a0d0e21e 2069=item getpriority WHICH,WHO
d74e8afc 2070X<getpriority> X<priority> X<nice>
a0d0e21e 2071
4633a7c4
LW
2072Returns the current priority for a process, a process group, or a user.
2073(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2074machine that doesn't implement getpriority(2).
a0d0e21e
LW
2075
2076=item getpwnam NAME
d74e8afc
ITB
2077X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2078X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2079X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2080X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2081X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2082X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2083
2084=item getgrnam NAME
2085
2086=item gethostbyname NAME
2087
2088=item getnetbyname NAME
2089
2090=item getprotobyname NAME
2091
2092=item getpwuid UID
2093
2094=item getgrgid GID
2095
2096=item getservbyname NAME,PROTO
2097
2098=item gethostbyaddr ADDR,ADDRTYPE
2099
2100=item getnetbyaddr ADDR,ADDRTYPE
2101
2102=item getprotobynumber NUMBER
2103
2104=item getservbyport PORT,PROTO
2105
2106=item getpwent
2107
2108=item getgrent
2109
2110=item gethostent
2111
2112=item getnetent
2113
2114=item getprotoent
2115
2116=item getservent
2117
2118=item setpwent
2119
2120=item setgrent
2121
2122=item sethostent STAYOPEN
2123
2124=item setnetent STAYOPEN
2125
2126=item setprotoent STAYOPEN
2127
2128=item setservent STAYOPEN
2129
2130=item endpwent
2131
2132=item endgrent
2133
2134=item endhostent
2135
2136=item endnetent
2137
2138=item endprotoent
2139
2140=item endservent
2141
2142These routines perform the same functions as their counterparts in the
5a964f20 2143system library. In list context, the return values from the
a0d0e21e
LW
2144various get routines are as follows:
2145
2146 ($name,$passwd,$uid,$gid,
6ee623d5 2147 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2148 ($name,$passwd,$gid,$members) = getgr*
2149 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2150 ($name,$aliases,$addrtype,$net) = getnet*
2151 ($name,$aliases,$proto) = getproto*
2152 ($name,$aliases,$port,$proto) = getserv*
2153
2154(If the entry doesn't exist you get a null list.)
2155
4602f195
JH
2156The exact meaning of the $gcos field varies but it usually contains
2157the real name of the user (as opposed to the login name) and other
2158information pertaining to the user. Beware, however, that in many
2159system users are able to change this information and therefore it
106325ad 2160cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2161L<perlsec>). The $passwd and $shell, user's encrypted password and
2162login shell, are also tainted, because of the same reason.
4602f195 2163
5a964f20 2164In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2165lookup by name, in which case you get the other thing, whatever it is.
2166(If the entry doesn't exist you get the undefined value.) For example:
2167
5a964f20
TC
2168 $uid = getpwnam($name);
2169 $name = getpwuid($num);
2170 $name = getpwent();
2171 $gid = getgrnam($name);
08a33e13 2172 $name = getgrgid($num);
5a964f20
TC
2173 $name = getgrent();
2174 #etc.
a0d0e21e 2175
4602f195
JH
2176In I<getpw*()> the fields $quota, $comment, and $expire are special
2177cases in the sense that in many systems they are unsupported. If the
2178$quota is unsupported, it is an empty scalar. If it is supported, it
2179usually encodes the disk quota. If the $comment field is unsupported,
2180it is an empty scalar. If it is supported it usually encodes some
2181administrative comment about the user. In some systems the $quota
2182field may be $change or $age, fields that have to do with password
2183aging. In some systems the $comment field may be $class. The $expire
2184field, if present, encodes the expiration period of the account or the
2185password. For the availability and the exact meaning of these fields
2186in your system, please consult your getpwnam(3) documentation and your
2187F<pwd.h> file. You can also find out from within Perl what your
2188$quota and $comment fields mean and whether you have the $expire field
2189by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2190C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2191files are only supported if your vendor has implemented them in the
2192intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2193shadow versions if you're running under privilege or if there exists
cf264981
SP
2194the shadow(3) functions as found in System V (this includes Solaris
2195and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2196facility are unlikely to be supported.
6ee623d5 2197
19799a22 2198The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2199the login names of the members of the group.
2200
2201For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2202C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2203C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2204addresses returned by the corresponding system library call. In the
2205Internet domain, each address is four bytes long and you can unpack it
2206by saying something like:
2207
f337b084 2208 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2209
2b5ab1e7
TC
2210The Socket library makes this slightly easier:
2211
2212 use Socket;
2213 $iaddr = inet_aton("127.1"); # or whatever address
2214 $name = gethostbyaddr($iaddr, AF_INET);
2215
2216 # or going the other way
19799a22 2217 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2218
19799a22
GS
2219If you get tired of remembering which element of the return list
2220contains which return value, by-name interfaces are provided
2221in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2222C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2223and C<User::grent>. These override the normal built-ins, supplying
2224versions that return objects with the appropriate names
2225for each field. For example:
5a964f20
TC
2226
2227 use File::stat;
2228 use User::pwent;
2229 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2230
b76cc8ba
NIS
2231Even though it looks like they're the same method calls (uid),
2232they aren't, because a C<File::stat> object is different from
19799a22 2233a C<User::pwent> object.
5a964f20 2234
a0d0e21e 2235=item getsockname SOCKET
d74e8afc 2236X<getsockname>
a0d0e21e 2237
19799a22
GS
2238Returns the packed sockaddr address of this end of the SOCKET connection,
2239in case you don't know the address because you have several different
2240IPs that the connection might have come in on.
a0d0e21e 2241
4633a7c4
LW
2242 use Socket;
2243 $mysockaddr = getsockname(SOCK);
19799a22 2244 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2245 printf "Connect to %s [%s]\n",
19799a22
GS
2246 scalar gethostbyaddr($myaddr, AF_INET),
2247 inet_ntoa($myaddr);
a0d0e21e
LW
2248
2249=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2250X<getsockopt>
a0d0e21e 2251
636e6b1f
TH
2252Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2253Options may exist at multiple protocol levels depending on the socket
2254type, but at least the uppermost socket level SOL_SOCKET (defined in the
2255C<Socket> module) will exist. To query options at another level the
2256protocol number of the appropriate protocol controlling the option
2257should be supplied. For example, to indicate that an option is to be
2258interpreted by the TCP protocol, LEVEL should be set to the protocol
2259number of TCP, which you can get using getprotobyname.
2260
2261The call returns a packed string representing the requested socket option,
2262or C<undef> if there is an error (the error reason will be in $!). What
2263exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2264your system documentation for details. A very common case however is that
cf264981 2265the option is an integer, in which case the result will be a packed
636e6b1f
TH
2266integer which you can decode using unpack with the C<i> (or C<I>) format.
2267
2268An example testing if Nagle's algorithm is turned on on a socket:
2269
4852725b 2270 use Socket qw(:all);
636e6b1f
TH
2271
2272 defined(my $tcp = getprotobyname("tcp"))
2273 or die "Could not determine the protocol number for tcp";
4852725b
DD
2274 # my $tcp = IPPROTO_TCP; # Alternative
2275 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2276 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2277 my $nodelay = unpack("I", $packed);
2278 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2279
a0d0e21e
LW
2280
2281=item glob EXPR
d74e8afc 2282X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2283
0a753a76 2284=item glob
2285
d9a9d457
JL
2286In list context, returns a (possibly empty) list of filename expansions on
2287the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2288scalar context, glob iterates through such filename expansions, returning
2289undef when the list is exhausted. This is the internal function
2290implementing the C<< <*.c> >> operator, but you can use it directly. If
2291EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2292more detail in L<perlop/"I/O Operators">.
a0d0e21e 2293
3a4b19e4
GS
2294Beginning with v5.6.0, this operator is implemented using the standard
2295C<File::Glob> extension. See L<File::Glob> for details.
2296
a0d0e21e 2297=item gmtime EXPR
d74e8afc 2298X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2299
ce2984c3
PF
2300=item gmtime
2301
82dd32fe 2302Converts a time as returned by the time function to an 9-element list
54310121 2303with the time localized for the standard Greenwich time zone.
4633a7c4 2304Typically used as follows:
a0d0e21e 2305
82dd32fe
SP
2306 # 0 1 2 3 4 5 6 7 8
2307 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
a0d0e21e
LW
2308 gmtime(time);
2309
48a26b3a
GS
2310All list elements are numeric, and come straight out of the C `struct
2311tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2312specified time. $mday is the day of the month, and $mon is the month
2313itself, in the range C<0..11> with 0 indicating January and 11
2314indicating December. $year is the number of years since 1900. That
2315is, $year is C<123> in year 2023. $wday is the day of the week, with
23160 indicating Sunday and 3 indicating Wednesday. $yday is the day of
82dd32fe
SP
2317the year, in the range C<0..364> (or C<0..365> in leap years). $isdst
2318is always C<0>.
48a26b3a
GS
2319
2320Note that the $year element is I<not> simply the last two digits of
cf264981 2321the year. If you assume it is then you create non-Y2K-compliant
48a26b3a 2322programs--and you wouldn't want to do that, would you?
2f9daede 2323
abd75f24
GS
2324The proper way to get a complete 4-digit year is simply:
2325
2326 $year += 1900;
2327
2328And to get the last two digits of the year (e.g., '01' in 2001) do:
2329
2330 $year = sprintf("%02d", $year % 100);
2331
48a26b3a 2332If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2333
48a26b3a 2334In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76 2335
2336 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2337
fe86afc2
NC
2338If you need local time instead of GMT use the L</localtime> builtin.
2339See also the C<timegm> function provided by the C<Time::Local> module,
2340and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
7660c0ab 2341
fe86afc2
NC
2342This scalar value is B<not> locale dependent (see L<perllocale>), but is
2343instead a Perl builtin. To get somewhat similar but locale dependent date
2344strings, see the example in L</localtime>.
0a753a76 2345
62aa5637
MS
2346See L<perlport/gmtime> for portability concerns.
2347
a0d0e21e 2348=item goto LABEL
d74e8afc 2349X<goto> X<jump> X<jmp>
a0d0e21e 2350
748a9306
LW
2351=item goto EXPR
2352
a0d0e21e
LW
2353=item goto &NAME
2354
7660c0ab 2355The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2356execution there. It may not be used to go into any construct that
7660c0ab 2357requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2358also can't be used to go into a construct that is optimized away,
19799a22 2359or to get out of a block or subroutine given to C<sort>.
0a753a76 2360It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2361including out of subroutines, but it's usually better to use some other
19799a22 2362construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2363need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2364(The difference being that C does not offer named loops combined with
2365loop control. Perl does, and this replaces most structured uses of C<goto>
2366in other languages.)
a0d0e21e 2367
7660c0ab
A
2368The C<goto-EXPR> form expects a label name, whose scope will be resolved
2369dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2370necessarily recommended if you're optimizing for maintainability:
2371
2372 goto ("FOO", "BAR", "GLARCH")[$i];
2373
1b6921cb
BT
2374The C<goto-&NAME> form is quite different from the other forms of
2375C<goto>. In fact, it isn't a goto in the normal sense at all, and
2376doesn't have the stigma associated with other gotos. Instead, it
2377exits the current subroutine (losing any changes set by local()) and
2378immediately calls in its place the named subroutine using the current
2379value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2380load another subroutine and then pretend that the other subroutine had
2381been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2382in the current subroutine are propagated to the other subroutine.)
2383After the C<goto>, not even C<caller> will be able to tell that this
2384routine was called first.
2385
2386NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2387containing a code reference, or a block that evaluates to a code
6cb9131c 2388reference.
a0d0e21e
LW
2389
2390=item grep BLOCK LIST
d74e8afc 2391X<grep>
a0d0e21e
LW
2392
2393=item grep EXPR,LIST
2394
2b5ab1e7
TC
2395This is similar in spirit to, but not the same as, grep(1) and its
2396relatives. In particular, it is not limited to using regular expressions.
2f9daede 2397
a0d0e21e 2398Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2399C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2400elements for which the expression evaluated to true. In scalar
2401context, returns the number of times the expression was true.
a0d0e21e
LW
2402
2403 @foo = grep(!/^#/, @bar); # weed out comments
2404
2405or equivalently,
2406
2407 @foo = grep {!/^#/} @bar; # weed out comments
2408
be3174d2
GS
2409Note that C<$_> is an alias to the list value, so it can be used to
2410modify the elements of the LIST. While this is useful and supported,
2411it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2412Similarly, grep returns aliases into the original list, much as a for
2413loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2414element of a list returned by grep (for example, in a C<foreach>, C<map>
2415or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2416This is usually something to be avoided when writing clear code.
a0d0e21e 2417
a4fb8298 2418If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2419been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2420the list elements, C<$_> keeps being lexical inside the block; i.e. it
2421can't be seen from the outside, avoiding any potential side-effects.
2422
19799a22 2423See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2424
a0d0e21e 2425=item hex EXPR
d74e8afc 2426X<hex> X<hexadecimal>
a0d0e21e 2427
54310121 2428=item hex
bbce6d69 2429
2b5ab1e7 2430Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2431(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2432L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TP
2433
2434 print hex '0xAf'; # prints '175'
2435 print hex 'aF'; # same
a0d0e21e 2436
19799a22 2437Hex strings may only represent integers. Strings that would cause
53305cf1 2438integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2439unlike oct(). To present something as hex, look into L</printf>,
2440L</sprintf>, or L</unpack>.
19799a22 2441
ce2984c3 2442=item import LIST
d74e8afc 2443X<import>
a0d0e21e 2444
19799a22 2445There is no builtin C<import> function. It is just an ordinary
4633a7c4 2446method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2447names to another module. The C<use> function calls the C<import> method
cea6626f 2448for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2449
2450=item index STR,SUBSTR,POSITION
d74e8afc 2451X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2452
2453=item index STR,SUBSTR
2454
2b5ab1e7
TC
2455The index function searches for one string within another, but without
2456the wildcard-like behavior of a full regular-expression pattern match.
2457It returns the position of the first occurrence of SUBSTR in STR at
2458or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2459beginning of the string. POSITION before the beginning of the string
2460or after its end is treated as if it were the beginning or the end,
2461respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2462you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2463is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2464
2465=item int EXPR
d74e8afc 2466X<int> X<integer> X<truncate> X<trunc>
a0d0e21e 2467
54310121 2468=item int
bbce6d69 2469
7660c0ab 2470Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2471You should not use this function for rounding: one because it truncates
2472towards C<0>, and two because machine representations of floating point
2473numbers can sometimes produce counterintuitive results. For example,
2474C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2475because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2476the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2477functions will serve you better than will int().
a0d0e21e
LW
2478
2479=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2480X<ioctl>
a0d0e21e 2481
2b5ab1e7 2482Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2483
6c567752 2484 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2485
a11c483f 2486to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2487exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2488own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2489(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2490may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2491written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2492will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2493has no string value but does have a numeric value, that value will be
2494passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2495true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2496functions may be needed to manipulate the values of structures used by
b76cc8ba 2497C<ioctl>.
a0d0e21e 2498
19799a22 2499The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2500
2501 if OS returns: then Perl returns:
2502 -1 undefined value
2503 0 string "0 but true"
2504 anything else that number
2505
19799a22 2506Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2507still easily determine the actual value returned by the operating
2508system:
2509
2b5ab1e7 2510 $retval = ioctl(...) || -1;
a0d0e21e
LW
2511 printf "System returned %d\n", $retval;
2512
be2f7487 2513The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2514about improper numeric conversions.
2515
a0d0e21e 2516=item join EXPR,LIST
d74e8afc 2517X<join>
a0d0e21e 2518
2b5ab1e7
TC
2519Joins the separate strings of LIST into a single string with fields
2520separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2521
2b5ab1e7 2522 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2523
eb6e2d6f
GS
2524Beware that unlike C<split>, C<join> doesn't take a pattern as its
2525first argument. Compare L</split>.
a0d0e21e 2526
aa689395 2527=item keys HASH
d74e8afc 2528X<keys> X<key>
aa689395 2529
504f80c1
JH
2530Returns a list consisting of all the keys of the named hash.
2531(In scalar context, returns the number of keys.)
2532
2533The keys are returned in an apparently random order. The actual
2534random order is subject to change in future versions of perl, but it
2535is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2536function produces (given that the hash has not been modified). Since
2537Perl 5.8.1 the ordering is different even between different runs of
2538Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2539Attacks">).
504f80c1 2540
cf264981
SP
2541As a side effect, calling keys() resets the HASH's internal iterator
2542(see L</each>). In particular, calling keys() in void context resets
2543the iterator with no other overhead.
a0d0e21e 2544
aa689395 2545Here is yet another way to print your environment:
a0d0e21e
LW
2546
2547 @keys = keys %ENV;
2548 @values = values %ENV;
b76cc8ba 2549 while (@keys) {
a0d0e21e
LW
2550 print pop(@keys), '=', pop(@values), "\n";
2551 }
2552
2553or how about sorted by key:
2554
2555 foreach $key (sort(keys %ENV)) {
2556 print $key, '=', $ENV{$key}, "\n";
2557 }
2558
8ea1e5d4
GS
2559The returned values are copies of the original keys in the hash, so
2560modifying them will not affect the original hash. Compare L</values>.
2561
19799a22 2562To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2563Here's a descending numeric sort of a hash by its values:
4633a7c4 2564
5a964f20 2565 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2566 printf "%4d %s\n", $hash{$key}, $key;
2567 }
2568
19799a22 2569As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395 2570allocated for the given hash. This can gain you a measure of efficiency if
2571you know the hash is going to get big. (This is similar to pre-extending
2572an array by assigning a larger number to $#array.) If you say
55497cff 2573
2574 keys %hash = 200;
2575
ab192400
GS
2576then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2577in fact, since it rounds up to the next power of two. These
55497cff 2578buckets will be retained even if you do C<%hash = ()>, use C<undef
2579%hash> if you want to free the storage while C<%hash> is still in scope.
2580You can't shrink the number of buckets allocated for the hash using
19799a22 2581C<keys> in this way (but you needn't worry about doing this by accident,
55497cff 2582as trying has no effect).
2583
19799a22 2584See also C<each>, C<values> and C<sort>.
ab192400 2585
b350dd2f 2586=item kill SIGNAL, LIST
d74e8afc 2587X<kill> X<signal>
a0d0e21e 2588
b350dd2f 2589Sends a signal to a list of processes. Returns the number of
517db077
GS
2590processes successfully signaled (which is not necessarily the
2591same as the number actually killed).
a0d0e21e
LW
2592
2593 $cnt = kill 1, $child1, $child2;
2594 kill 9, @goners;
2595
70fb64f6 2596If SIGNAL is zero, no signal is sent to the process, but the kill(2)
6cb9d3e4 2597system call will check whether it's possible to send a signal to it (that
70fb64f6
RGS
2598means, to be brief, that the process is owned by the same user, or we are
2599the super-user). This is a useful way to check that a child process is
2600alive and hasn't changed its UID. See L<perlport> for notes on the
2601portability of this construct.
b350dd2f
GS
2602
2603Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2604process groups instead of processes. (On System V, a negative I<PROCESS>
2605number will also kill process groups, but that's not portable.) That
2606means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2607use a signal name in quotes.
2608
2609See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2610
2611=item last LABEL
d74e8afc 2612X<last> X<break>
a0d0e21e
LW
2613
2614=item last
2615
2616The C<last> command is like the C<break> statement in C (as used in
2617loops); it immediately exits the loop in question. If the LABEL is
2618omitted, the command refers to the innermost enclosing loop. The
2619C<continue> block, if any, is not executed:
2620
4633a7c4
LW
2621 LINE: while (<STDIN>) {
2622 last LINE if /^$/; # exit when done with header
5a964f20 2623 #...
a0d0e21e
LW
2624 }
2625
4968c1e4 2626C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2627C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2628a grep() or map() operation.
4968c1e4 2629
6c1372ed
GS
2630Note that a block by itself is semantically identical to a loop
2631that executes once. Thus C<last> can be used to effect an early
2632exit out of such a block.
2633
98293880
JH
2634See also L</continue> for an illustration of how C<last>, C<next>, and
2635C<redo> work.
1d2dff63 2636
a0d0e21e 2637=item lc EXPR
d74e8afc 2638X<lc> X<lowercase>
a0d0e21e 2639
54310121 2640=item lc
bbce6d69 2641
d1be9408 2642Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2643implementing the C<\L> escape in double-quoted strings. Respects
2644current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2645and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2646
7660c0ab 2647If EXPR is omitted, uses C<$_>.
bbce6d69 2648
a0d0e21e 2649=item lcfirst EXPR
d74e8afc 2650X<lcfirst> X<lowercase>
a0d0e21e 2651
54310121 2652=item lcfirst
bbce6d69 2653
ad0029c4
JH
2654Returns the value of EXPR with the first character lowercased. This
2655is the internal function implementing the C<\l> escape in
2656double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2657locale> in force. See L<perllocale> and L<perlunicode> for more
2658details about locale and Unicode support.
a0d0e21e 2659
7660c0ab 2660If EXPR is omitted, uses C<$_>.
bbce6d69 2661
a0d0e21e 2662=item length EXPR
d74e8afc 2663X<length> X<size>
a0d0e21e 2664
54310121 2665=item length
bbce6d69 2666
974da8e5 2667Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2668omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2669an entire array or hash to find out how many elements these have.
2670For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2671
974da8e5
JH
2672Note the I<characters>: if the EXPR is in Unicode, you will get the
2673number of characters, not the number of bytes. To get the length
2674in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2675
a0d0e21e 2676=item link OLDFILE,NEWFILE
d74e8afc 2677X<link>
a0d0e21e 2678
19799a22 2679Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2680success, false otherwise.
a0d0e21e
LW
2681
2682=item listen SOCKET,QUEUESIZE
d74e8afc 2683X<listen>
a0d0e21e 2684
19799a22 2685Does the same thing that the listen system call does. Returns true if
b76cc8ba 2686it succeeded, false otherwise. See the example in
cea6626f 2687L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2688
2689=item local EXPR
d74e8afc 2690X<local>
a0d0e21e 2691
19799a22 2692You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2693what most people think of as "local". See
13a2d996 2694L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2695
5a964f20
TC
2696A local modifies the listed variables to be local to the enclosing
2697block, file, or eval. If more than one value is listed, the list must
2698be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2699for details, including issues with tied arrays and hashes.
a0d0e21e 2700
a0d0e21e 2701=item localtime EXPR
d74e8afc 2702X<localtime>
a0d0e21e 2703
ba053783
AL
2704=item localtime
2705
19799a22 2706Converts a time as returned by the time function to a 9-element list
5f05dabc 2707with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2708follows:
2709
54310121 2710 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2711 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2712 localtime(time);
a0d0e21e 2713
48a26b3a 2714All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2715tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2716of the specified time.
48a26b3a 2717
ba053783
AL
2718C<$mday> is the day of the month, and C<$mon> is the month itself, in
2719the range C<0..11> with 0 indicating January and 11 indicating December.
2720This makes it easy to get a month name from a list:
54310121 2721
ba053783
AL
2722 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2723 print "$abbr[$mon] $mday";
2724 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2725
ba053783
AL
2726C<$year> is the number of years since 1900, not just the last two digits
2727of the year. That is, C<$year> is C<123> in year 2023. The proper way
2728to get a complete 4-digit year is simply:
abd75f24 2729
ba053783 2730 $year += 1900;
abd75f24 2731
ba053783
AL
2732To get the last two digits of the year (e.g., '01' in 2001) do:
2733
2734 $year = sprintf("%02d", $year % 100);
2735
2736C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2737Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2738(or C<0..365> in leap years.)
2739
2740C<$isdst> is true if the specified time occurs during Daylight Saving
2741Time, false otherwise.
abd75f24 2742
48a26b3a 2743If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2744
48a26b3a 2745In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2746
5f05dabc 2747 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2748
fe86afc2
NC
2749This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2750instead of local time use the L</gmtime> builtin. See also the
2751C<Time::Local> module (to convert the second, minutes, hours, ... back to
2752the integer value returned by time()), and the L<POSIX> module's strftime(3)
2753and mktime(3) functions.
2754
2755To get somewhat similar but locale dependent date strings, set up your
2756locale environment variables appropriately (please see L<perllocale>) and
2757try for example:
a3cb178b 2758
5a964f20 2759 use POSIX qw(strftime);
2b5ab1e7 2760 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2761 # or for GMT formatted appropriately for your locale:
2762 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2763
2764Note that the C<%a> and C<%b>, the short forms of the day of the week
2765and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2766
62aa5637
MS
2767See L<perlport/localtime> for portability concerns.
2768
07698885 2769=item lock THING
d74e8afc 2770X<lock>
19799a22 2771
01e6739c 2772This function places an advisory lock on a shared variable, or referenced
03730085 2773object contained in I<THING> until the lock goes out of scope.
a6d5524e 2774
f3a23afb 2775lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2776by this name (before any calls to it), that function will be called
03730085
AB
2777instead. (However, if you've said C<use threads>, lock() is always a
2778keyword.) See L<threads>.
19799a22 2779
a0d0e21e 2780=item log EXPR
d74e8afc 2781X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2782
54310121 2783=item log
bbce6d69 2784
2b5ab1e7
TC
2785Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2786returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2787The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2788divided by the natural log of N. For example:
2789
2790 sub log10 {
2791 my $n = shift;
2792 return log($n)/log(10);
b76cc8ba 2793 }
2b5ab1e7
TC
2794
2795See also L</exp> for the inverse operation.
a0d0e21e 2796
a0d0e21e 2797=item lstat EXPR
d74e8afc 2798X<lstat>
a0d0e21e 2799
54310121 2800=item lstat
bbce6d69 2801
19799a22 2802Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2803special C<_> filehandle) but stats a symbolic link instead of the file
2804the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2805your system, a normal C<stat> is done. For much more detailed
2806information, please see the documentation for C<stat>.
a0d0e21e 2807
7660c0ab 2808If EXPR is omitted, stats C<$_>.
bbce6d69 2809
a0d0e21e
LW
2810=item m//
2811
2812The match operator. See L<perlop>.
2813
2814=item map BLOCK LIST
d74e8afc 2815X<map>
a0d0e21e
LW
2816
2817=item map EXPR,LIST
2818
19799a22
GS
2819Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2820C<$_> to each element) and returns the list value composed of the
2821results of each such evaluation. In scalar context, returns the
2822total number of elements so generated. Evaluates BLOCK or EXPR in
2823list context, so each element of LIST may produce zero, one, or
2824more elements in the returned value.
dd99ebda 2825
a0d0e21e
LW
2826 @chars = map(chr, @nums);
2827
2828translates a list of numbers to the corresponding characters. And
2829
4633a7c4 2830 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2831
2832is just a funny way to write
2833
2834 %hash = ();
2835 foreach $_ (@array) {
4633a7c4 2836 $hash{getkey($_)} = $_;
a0d0e21e
LW
2837 }
2838
be3174d2
GS
2839Note that C<$_> is an alias to the list value, so it can be used to
2840modify the elements of the LIST. While this is useful and supported,
2841it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2842Using a regular C<foreach> loop for this purpose would be clearer in
2843most cases. See also L</grep> for an array composed of those items of
2844the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2845
a4fb8298 2846If C<$_> is lexical in the scope where the C<map> appears (because it has
cf264981 2847been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2848the list elements, C<$_> keeps being lexical inside the block; i.e. it
2849can't be seen from the outside, avoiding any potential side-effects.
2850
205fdb4d
NC
2851C<{> starts both hash references and blocks, so C<map { ...> could be either
2852the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2853ahead for the closing C<}> it has to take a guess at which its dealing with
2854based what it finds just after the C<{>. Usually it gets it right, but if it
2855doesn't it won't realize something is wrong until it gets to the C<}> and
2856encounters the missing (or unexpected) comma. The syntax error will be
2857reported close to the C<}> but you'll need to change something near the C<{>
2858such as using a unary C<+> to give perl some help:
2859
2860 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2861 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2862 %hash = map { ("\L$_", 1) } @array # this also works
2863 %hash = map { lc($_), 1 } @array # as does this.
2864 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2865
205fdb4d
NC
2866 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2867
2868or to force an anon hash constructor use C<+{>
2869
2870 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2871
2872and you get list of anonymous hashes each with only 1 entry.
2873
19799a22 2874=item mkdir FILENAME,MASK
d74e8afc 2875X<mkdir> X<md> X<directory, create>
a0d0e21e 2876
5a211162
GS
2877=item mkdir FILENAME
2878
491873e5
RGS
2879=item mkdir
2880
0591cd52 2881Creates the directory specified by FILENAME, with permissions
19799a22
GS
2882specified by MASK (as modified by C<umask>). If it succeeds it
2883returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2884If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2885to C<$_>.
0591cd52 2886
19799a22 2887In general, it is better to create directories with permissive MASK,
0591cd52 2888and let the user modify that with their C<umask>, than it is to supply
19799a22 2889a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2890The exceptions to this rule are when the file or directory should be
2891kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2892C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2893
cc1852e8
JH
2894Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2895number of trailing slashes. Some operating and filesystems do not get
2896this right, so Perl automatically removes all trailing slashes to keep
2897everyone happy.
2898
a0d0e21e 2899=item msgctl ID,CMD,ARG
d74e8afc 2900X<msgctl>
a0d0e21e 2901
f86cebdf 2902Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2903
2904 use IPC::SysV;
2905
7660c0ab 2906first to get the correct constant definitions. If CMD is C<IPC_STAT>,
cf264981 2907then ARG must be a variable that will hold the returned C<msqid_ds>
951ba7fe
GS
2908structure. Returns like C<ioctl>: the undefined value for error,
2909C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2910L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2911
2912=item msgget KEY,FLAGS
d74e8afc 2913X<msgget>
a0d0e21e 2914
f86cebdf 2915Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2916id, or the undefined value if there is an error. See also
2917L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2918
a0d0e21e 2919=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
d74e8afc 2920X<msgrcv>
a0d0e21e
LW
2921
2922Calls the System V IPC function msgrcv to receive a message from
2923message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2924SIZE. Note that when a message is received, the message type as a
2925native long integer will be the first thing in VAR, followed by the
2926actual message. This packing may be opened with C<unpack("l! a*")>.
2927Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2928an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2929C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2930
2931=item msgsnd ID,MSG,FLAGS
d74e8afc 2932X<msgsnd>
41d6edb2
JH
2933
2934Calls the System V IPC function msgsnd to send the message MSG to the
2935message queue ID. MSG must begin with the native long integer message
2936type, and be followed by the length of the actual message, and finally
2937the message itself. This kind of packing can be achieved with
2938C<pack("l! a*", $type, $message)>. Returns true if successful,
2939or false if there is an error. See also C<IPC::SysV>
2940and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2941
2942=item my EXPR
d74e8afc 2943X<my>
a0d0e21e 2944
307ea6df
JH
2945=item my TYPE EXPR
2946
1d2de774 2947=item my EXPR : ATTRS
09bef843 2948
1d2de774 2949=item my TYPE EXPR : ATTRS
307ea6df 2950
19799a22 2951A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2952enclosing block, file, or C<eval>. If more than one value is listed,
2953the list must be placed in parentheses.
307ea6df 2954
1d2de774
JH
2955The exact semantics and interface of TYPE and ATTRS are still
2956evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2957and attributes are handled using the C<attributes> pragma, or starting
2958from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2959L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2960L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2961
a0d0e21e 2962=item next LABEL
d74e8afc 2963X<next> X<continue>
a0d0e21e
LW
2964
2965=item next
2966
2967The C<next> command is like the C<continue> statement in C; it starts
2968the next iteration of the loop:
2969
4633a7c4
LW
2970 LINE: while (<STDIN>) {
2971 next LINE if /^#/; # discard comments
5a964f20 2972 #...
a0d0e21e
LW
2973 }
2974
2975Note that if there were a C<continue> block on the above, it would get
2976executed even on discarded lines. If the LABEL is omitted, the command
2977refers to the innermost enclosing loop.
2978
4968c1e4 2979C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2980C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2981a grep() or map() operation.
4968c1e4 2982
6c1372ed
GS
2983Note that a block by itself is semantically identical to a loop
2984that executes once. Thus C<next> will exit such a block early.
2985
98293880
JH
2986See also L</continue> for an illustration of how C<last>, C<next>, and
2987C<redo> work.
1d2dff63 2988
4a66ea5a 2989=item no Module VERSION LIST
d74e8afc 2990X<no>
4a66ea5a
RGS
2991
2992=item no Module VERSION
2993
a0d0e21e
LW
2994=item no Module LIST
2995
4a66ea5a
RGS
2996=item no Module
2997
593b9c14 2998See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
2999
3000=item oct EXPR
d74e8afc 3001X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
a0d0e21e 3002
54310121 3003=item oct
bbce6d69 3004
4633a7c4 3005Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
3006value. (If EXPR happens to start off with C<0x>, interprets it as a
3007hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
3008binary string. Leading whitespace is ignored in all three cases.)
3009The following will handle decimal, binary, octal, and hex in the standard
3010Perl or C notation:
a0d0e21e
LW
3011
3012 $val = oct($val) if $val =~ /^0/;
3013
19799a22
GS
3014If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3015in octal), use sprintf() or printf():
3016
3017 $perms = (stat("filename"))[2] & 07777;
3018 $oct_perms = sprintf "%lo", $perms;
3019
3020The oct() function is commonly used when a string such as C<644> needs
3021to be converted into a file mode, for example. (Although perl will
3022automatically convert strings into numbers as needed, this automatic
3023conversion assumes base 10.)
a0d0e21e
LW
3024
3025=item open FILEHANDLE,EXPR
d74e8afc 3026X<open> X<pipe> X<file, open> X<fopen>
a0d0e21e 3027
68bd7414
NIS
3028=item open FILEHANDLE,MODE,EXPR
3029
3030=item open FILEHANDLE,MODE,EXPR,LIST
3031
ba964c95
T
3032=item open FILEHANDLE,MODE,REFERENCE
3033
a0d0e21e
LW
3034=item open FILEHANDLE
3035
3036Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
3037FILEHANDLE.
3038
3039(The following is a comprehensive reference to open(): for a gentler
3040introduction you may consider L<perlopentut>.)
3041
a28cd5c9
NT
3042If FILEHANDLE is an undefined scalar variable (or array or hash element)
3043the variable is assigned a reference to a new anonymous filehandle,
3044otherwise if FILEHANDLE is an expression, its value is used as the name of
3045the real filehandle wanted. (This is considered a symbolic reference, so
3046C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
3047
3048If EXPR is omitted, the scalar variable of the same name as the
3049FILEHANDLE contains the filename. (Note that lexical variables--those
3050declared with C<my>--will not work for this purpose; so if you're
67408cae 3051using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
3052
3053If three or more arguments are specified then the mode of opening and
3054the file name are separate. If MODE is C<< '<' >> or nothing, the file
3055is opened for input. If MODE is C<< '>' >>, the file is truncated and
3056opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 3057the file is opened for appending, again being created if necessary.
5a964f20 3058
ed53a2bb
JH
3059You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3060indicate that you want both read and write access to the file; thus
3061C<< '+<' >> is almost always preferred for read/write updates--the C<<
3062'+>' >> mode would clobber the file first. You can't usually use
3063either read-write mode for updating textfiles, since they have
3064variable length records. See the B<-i> switch in L<perlrun> for a
3065better approach. The file is created with permissions of C<0666>
3066modified by the process' C<umask> value.
3067
3068These various prefixes correspond to the fopen(3) modes of C<'r'>,
3069C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 3070
6170680b
IZ
3071In the 2-arguments (and 1-argument) form of the call the mode and
3072filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
3073spaces. It is possible to omit the mode in these forms if the mode is
3074C<< '<' >>.
6170680b 3075
7660c0ab 3076If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 3077command to which output is to be piped, and if the filename ends with a
f244e06d
GS
3078C<'|'>, the filename is interpreted as a command which pipes output to
3079us. See L<perlipc/"Using open() for IPC">
19799a22 3080for more examples of this. (You are not allowed to C<open> to a command
5a964f20 3081that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
3082and L<perlipc/"Bidirectional Communication with Another Process">
3083for alternatives.)
cb1a09d0 3084
ed53a2bb
JH
3085For three or more arguments if MODE is C<'|-'>, the filename is
3086interpreted as a command to which output is to be piped, and if MODE
3087is C<'-|'>, the filename is interpreted as a command which pipes
3088output to us. In the 2-arguments (and 1-argument) form one should
3089replace dash (C<'-'>) with the command.
3090See L<perlipc/"Using open() for IPC"> for more examples of this.
3091(You are not allowed to C<open> to a command that pipes both in I<and>
3092out, but see L<IPC::Open2>, L<IPC::Open3>, and
3093L<perlipc/"Bidirectional Communication"> for alternatives.)
3094
3095In the three-or-more argument form of pipe opens, if LIST is specified
3096(extra arguments after the command name) then LIST becomes arguments
3097to the command invoked if the platform supports it. The meaning of
3098C<open> with more than three arguments for non-pipe modes is not yet
3099specified. Experimental "layers" may give extra LIST arguments
3100meaning.
6170680b
IZ
3101
3102In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 3103and opening C<< '>-' >> opens STDOUT.
6170680b 3104
fae2c0fb
RGS
3105You may use the three-argument form of open to specify IO "layers"
3106(sometimes also referred to as "disciplines") to be applied to the handle
3107that affect how the input and output are processed (see L<open> and
3108L<PerlIO> for more details). For example
7207e29d 3109
9124316e
JH
3110 open(FH, "<:utf8", "file")
3111
3112will open the UTF-8 encoded file containing Unicode characters,
6d5e88a0
TS
3113see L<perluniintro>. Note that if layers are specified in the
3114three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3115usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
ed53a2bb
JH
3116
3117Open returns nonzero upon success, the undefined value otherwise. If
3118the C<open> involved a pipe, the return value happens to be the pid of
3119the subprocess.
cb1a09d0 3120
ed53a2bb
JH
3121If you're running Perl on a system that distinguishes between text
3122files and binary files, then you should check out L</binmode> for tips
3123for dealing with this. The key distinction between systems that need
3124C<binmode> and those that don't is their text file formats. Systems
8939ba94 3125like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
3126character, and which encode that character in C as C<"\n">, do not
3127need C<binmode>. The rest need it.
cb1a09d0 3128
fb73857a 3129When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
3130if the request failed, so C<open> is frequently used in connection with
3131C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 3132where you want to make a nicely formatted error message (but there are
5a964f20 3133modules that can help with that problem)) you should always check
19799a22 3134the return value from opening a file. The infrequent exception is when
fb73857a 3135working with an unopened filehandle is actually what you want to do.
3136
cf264981 3137As a special case the 3-arg form with a read/write mode and the third
ed53a2bb 3138argument being C<undef>:
b76cc8ba
NIS
3139
3140 open(TMP, "+>", undef) or die ...
3141
f253e835
JH
3142opens a filehandle to an anonymous temporary file. Also using "+<"
3143works for symmetry, but you really should consider writing something
3144to the temporary file first. You will need to seek() to do the
3145reading.
b76cc8ba 3146
2ce64696 3147Since v5.8.0, perl has built using PerlIO by default. Unless you've
28a5cf3b 3148changed this (i.e. Configure -Uuseperlio), you can open file handles to
2ce64696 3149"in memory" files held in Perl scalars via:
ba964c95 3150
b996200f
SB
3151 open($fh, '>', \$variable) || ..
3152
3153Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3154file, you have to close it first:
3155
3156 close STDOUT;
3157 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
ba964c95