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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
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3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
15operator. A unary operator generally provides a scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
a0d0e21e 17contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 18be first, and the list argument will follow. (Note that there can ever
0f31cffe 19be only one such list argument.) For instance, splice() has three scalar
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20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
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22
23In the syntax descriptions that follow, list operators that expect a
24list (and provide list context for the elements of the list) are shown
25with LIST as an argument. Such a list may consist of any combination
26of scalar arguments or list values; the list values will be included
27in the list as if each individual element were interpolated at that
28point in the list, forming a longer single-dimensional list value.
cf264981 29Commas should separate elements of the LIST.
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30
31Any function in the list below may be used either with or without
32parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 33parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 34surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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35function, and precedence doesn't matter. Otherwise it's a list
36operator or unary operator, and precedence does matter. And whitespace
37between the function and left parenthesis doesn't count--so you need to
38be careful sometimes:
39
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>,
1dc8ecb8 103C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
945c54fd 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
1dc8ecb8 125C<grep>, C<join>, C<map>, C<qw//>, 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
36fb85f3 161C<break>, C<continue>, C<given>, C<when>, C<default>
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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
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168C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
169C<use>
170
171(C<state> is only available if the "state" feature is enabled. See
172L<feature>.)
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173
174=item Miscellaneous functions
175
36fb85f3 176C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
834df1c5 177C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
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178
179=item Functions for processes and process groups
d74e8afc 180X<process> X<pid> X<process id>
cb1a09d0 181
22fae026 182C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
1dc8ecb8 183C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 184C<times>, C<wait>, C<waitpid>
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185
186=item Keywords related to perl modules
d74e8afc 187X<module>
cb1a09d0 188
22fae026 189C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 190
353c6505 191=item Keywords related to classes and object-orientation
d74e8afc 192X<object> X<class> X<package>
cb1a09d0 193
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194C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
195C<untie>, C<use>
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196
197=item Low-level socket functions
d74e8afc 198X<socket> X<sock>
cb1a09d0 199
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200C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
201C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 202C<socket>, C<socketpair>
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203
204=item System V interprocess communication functions
d74e8afc 205X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 206
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207C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
208C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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209
210=item Fetching user and group info
d74e8afc 211X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 212
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213C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
214C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
215C<getpwuid>, C<setgrent>, C<setpwent>
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216
217=item Fetching network info
d74e8afc 218X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 219
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220C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
221C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
222C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
223C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
224C<setnetent>, C<setprotoent>, C<setservent>
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225
226=item Time-related functions
d74e8afc 227X<time> X<date>
cb1a09d0 228
22fae026 229C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 230
37798a01 231=item Functions new in perl5
d74e8afc 232X<perl5>
37798a01 233
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234C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
235C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
1dc8ecb8 236C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
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237C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
238C<ucfirst>, C<untie>, C<use>, C<when>
37798a01 239
240* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 241operator, which can be used in expressions.
37798a01 242
243=item Functions obsoleted in perl5
244
22fae026 245C<dbmclose>, C<dbmopen>
37798a01 246
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247=back
248
60f9f73c 249=head2 Portability
d74e8afc 250X<portability> X<Unix> X<portable>
60f9f73c 251
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252Perl was born in Unix and can therefore access all common Unix
253system calls. In non-Unix environments, the functionality of some
254Unix system calls may not be available, or details of the available
255functionality may differ slightly. The Perl functions affected
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256by this are:
257
258C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
259C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
260C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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261C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
262C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 263C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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264C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
265C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
266C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 267C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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268C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
269C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
270C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 271C<shmwrite>, C<socket>, C<socketpair>,
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272C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
273C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 274C<utime>, C<wait>, C<waitpid>
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275
276For more information about the portability of these functions, see
277L<perlport> and other available platform-specific documentation.
278
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279=head2 Alphabetical Listing of Perl Functions
280
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281=over 8
282
5b3c99c0 283=item -X FILEHANDLE
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284X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
285X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 286
5b3c99c0 287=item -X EXPR
a0d0e21e 288
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289=item -X DIRHANDLE
290
5b3c99c0 291=item -X
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292
293A file test, where X is one of the letters listed below. This unary
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294operator takes one argument, either a filename, a filehandle, or a dirhandle,
295and tests the associated file to see if something is true about it. If the
7660c0ab 296argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 297Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 298the undefined value if the file doesn't exist. Despite the funny
d0821a6a 299names, precedence is the same as any other named unary operator. The
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300operator may be any of:
301
302 -r File is readable by effective uid/gid.
303 -w File is writable by effective uid/gid.
304 -x File is executable by effective uid/gid.
305 -o File is owned by effective uid.
306
307 -R File is readable by real uid/gid.
308 -W File is writable by real uid/gid.
309 -X File is executable by real uid/gid.
310 -O File is owned by real uid.
311
312 -e File exists.
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313 -z File has zero size (is empty).
314 -s File has nonzero size (returns size in bytes).
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315
316 -f File is a plain file.
317 -d File is a directory.
318 -l File is a symbolic link.
9c4d0f16 319 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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320 -S File is a socket.
321 -b File is a block special file.
322 -c File is a character special file.
323 -t Filehandle is opened to a tty.
324
325 -u File has setuid bit set.
326 -g File has setgid bit set.
327 -k File has sticky bit set.
328
121910a4 329 -T File is an ASCII text file (heuristic guess).
2cdbc966 330 -B File is a "binary" file (opposite of -T).
a0d0e21e 331
95a3fe12 332 -M Script start time minus file modification time, in days.
a0d0e21e 333 -A Same for access time.
95a3fe12 334 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 335
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336Example:
337
338 while (<>) {
5b3eff12 339 chomp;
a0d0e21e 340 next unless -f $_; # ignore specials
5a964f20 341 #...
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342 }
343
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344The interpretation of the file permission operators C<-r>, C<-R>,
345C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
346of the file and the uids and gids of the user. There may be other
ecae030f
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347reasons you can't actually read, write, or execute the file: for
348example network filesystem access controls, ACLs (access control lists),
349read-only filesystems, and unrecognized executable formats. Note
350that the use of these six specific operators to verify if some operation
351is possible is usually a mistake, because it may be open to race
352conditions.
5ff3f7a4 353
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354Also note that, for the superuser on the local filesystems, the C<-r>,
355C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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356if any execute bit is set in the mode. Scripts run by the superuser
357may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 358or temporarily set their effective uid to something else.
5ff3f7a4
GS
359
360If you are using ACLs, there is a pragma called C<filetest> that may
361produce more accurate results than the bare stat() mode bits.
5ff3f7a4
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362When under the C<use filetest 'access'> the above-mentioned filetests
363will test whether the permission can (not) be granted using the
468541a8 364access() family of system calls. Also note that the C<-x> and C<-X> may
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365under this pragma return true even if there are no execute permission
366bits set (nor any extra execute permission ACLs). This strangeness is
ecae030f
MO
367due to the underlying system calls' definitions. Note also that, due to
368the implementation of C<use filetest 'access'>, the C<_> special
369filehandle won't cache the results of the file tests when this pragma is
370in effect. Read the documentation for the C<filetest> pragma for more
371information.
5ff3f7a4 372
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373Note that C<-s/a/b/> does not do a negated substitution. Saying
374C<-exp($foo)> still works as expected, however--only single letters
375following a minus are interpreted as file tests.
376
377The C<-T> and C<-B> switches work as follows. The first block or so of the
378file is examined for odd characters such as strange control codes or
61eff3bc 379characters with the high bit set. If too many strange characters (>30%)
cf264981 380are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
a0d0e21e 381containing null in the first block is considered a binary file. If C<-T>
9124316e 382or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 383rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 384file, or a file at EOF when testing a filehandle. Because you have to
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385read a file to do the C<-T> test, on most occasions you want to use a C<-f>
386against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 387
19799a22 388If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 389the special filehandle consisting of a solitary underline, then the stat
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390structure of the previous file test (or stat operator) is used, saving
391a system call. (This doesn't work with C<-t>, and you need to remember
392that lstat() and C<-l> will leave values in the stat structure for the
5c9aa243 393symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 394an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 395Example:
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396
397 print "Can do.\n" if -r $a || -w _ || -x _;
398
399 stat($filename);
400 print "Readable\n" if -r _;
401 print "Writable\n" if -w _;
402 print "Executable\n" if -x _;
403 print "Setuid\n" if -u _;
404 print "Setgid\n" if -g _;
405 print "Sticky\n" if -k _;
406 print "Text\n" if -T _;
407 print "Binary\n" if -B _;
408
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409As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
410test operators, in a way that C<-f -w -x $file> is equivalent to
cf264981 411C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
fbb0b3b3
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412the return value of C<-f $file> as an argument to another filetest
413operator, no special magic will happen.)
414
a0d0e21e 415=item abs VALUE
d74e8afc 416X<abs> X<absolute>
a0d0e21e 417
54310121 418=item abs
bbce6d69 419
a0d0e21e 420Returns the absolute value of its argument.
7660c0ab 421If VALUE is omitted, uses C<$_>.
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422
423=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 424X<accept>
a0d0e21e 425
f86cebdf 426Accepts an incoming socket connect, just as the accept(2) system call
19799a22 427does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 428See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 429
8d2a6795
GS
430On systems that support a close-on-exec flag on files, the flag will
431be set for the newly opened file descriptor, as determined by the
432value of $^F. See L<perlvar/$^F>.
433
a0d0e21e 434=item alarm SECONDS
d74e8afc
ITB
435X<alarm>
436X<SIGALRM>
437X<timer>
a0d0e21e 438
54310121 439=item alarm
bbce6d69 440
a0d0e21e 441Arranges to have a SIGALRM delivered to this process after the
cf264981 442specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
443specified, the value stored in C<$_> is used. (On some machines,
444unfortunately, the elapsed time may be up to one second less or more
445than you specified because of how seconds are counted, and process
446scheduling may delay the delivery of the signal even further.)
447
448Only one timer may be counting at once. Each call disables the
449previous timer, and an argument of C<0> may be supplied to cancel the
450previous timer without starting a new one. The returned value is the
451amount of time remaining on the previous timer.
a0d0e21e 452
2bc69794
BS
453For delays of finer granularity than one second, the Time::HiRes module
454(from CPAN, and starting from Perl 5.8 part of the standard
455distribution) provides ualarm(). You may also use Perl's four-argument
456version of select() leaving the first three arguments undefined, or you
457might be able to use the C<syscall> interface to access setitimer(2) if
458your system supports it. See L<perlfaq8> for details.
2b5ab1e7 459
68f8bed4
JH
460It is usually a mistake to intermix C<alarm> and C<sleep> calls.
461(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 462
19799a22
GS
463If you want to use C<alarm> to time out a system call you need to use an
464C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 465fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 466restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 467modulo the caveats given in L<perlipc/"Signals">.
ff68c719 468
469 eval {
f86cebdf 470 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 471 alarm $timeout;
ff68c719 472 $nread = sysread SOCKET, $buffer, $size;
36477c24 473 alarm 0;
ff68c719 474 };
ff68c719 475 if ($@) {
f86cebdf 476 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719 477 # timed out
478 }
479 else {
480 # didn't
481 }
482
91d81acc
JH
483For more information see L<perlipc>.
484
a0d0e21e 485=item atan2 Y,X
d74e8afc 486X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
487
488Returns the arctangent of Y/X in the range -PI to PI.
489
ca6e1c26 490For the tangent operation, you may use the C<Math::Trig::tan>
28757baa 491function, or use the familiar relation:
492
493 sub tan { sin($_[0]) / cos($_[0]) }
494
bf5f1b4c
JH
495Note that atan2(0, 0) is not well-defined.
496
a0d0e21e 497=item bind SOCKET,NAME
d74e8afc 498X<bind>
a0d0e21e
LW
499
500Binds a network address to a socket, just as the bind system call
19799a22 501does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
502packed address of the appropriate type for the socket. See the examples in
503L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 504
fae2c0fb 505=item binmode FILEHANDLE, LAYER
d74e8afc 506X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 507
a0d0e21e
LW
508=item binmode FILEHANDLE
509
1cbfc93d
NIS
510Arranges for FILEHANDLE to be read or written in "binary" or "text"
511mode on systems where the run-time libraries distinguish between
512binary and text files. If FILEHANDLE is an expression, the value is
513taken as the name of the filehandle. Returns true on success,
b5fe5ca2 514otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 515
d807c6f4
JH
516On some systems (in general, DOS and Windows-based systems) binmode()
517is necessary when you're not working with a text file. For the sake
518of portability it is a good idea to always use it when appropriate,
519and to never use it when it isn't appropriate. Also, people can
520set their I/O to be by default UTF-8 encoded Unicode, not bytes.
521
522In other words: regardless of platform, use binmode() on binary data,
523like for example images.
524
525If LAYER is present it is a single string, but may contain multiple
526directives. The directives alter the behaviour of the file handle.
527When LAYER is present using binmode on text file makes sense.
528
fae2c0fb 529If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
530suitable for passing binary data. This includes turning off possible CRLF
531translation and marking it as bytes (as opposed to Unicode characters).
749683d2 532Note that, despite what may be implied in I<"Programming Perl"> (the
165a9987
PJ
533Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
534-- other layers which would affect the binary nature of the stream are
0226bbdb
NIS
535I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
536PERLIO environment variable.
01e6739c 537
d807c6f4
JH
538The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
539form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
540establish default I/O layers. See L<open>.
541
fae2c0fb
RGS
542I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
543in "Programming Perl, 3rd Edition". However, since the publishing of this
544book, by many known as "Camel III", the consensus of the naming of this
545functionality has moved from "discipline" to "layer". All documentation
546of this version of Perl therefore refers to "layers" rather than to
547"disciplines". Now back to the regularly scheduled documentation...>
548
6902c96a
T
549To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
550C<:utf8> just marks the data as UTF-8 without further checking,
551while C<:encoding(utf8)> checks the data for actually being valid
552UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 553
ed53a2bb 554In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
555is done on the filehandle. Calling binmode() will normally flush any
556pending buffered output data (and perhaps pending input data) on the
fae2c0fb 557handle. An exception to this is the C<:encoding> layer that
01e6739c 558changes the default character encoding of the handle, see L<open>.
fae2c0fb 559The C<:encoding> layer sometimes needs to be called in
3874323d
JH
560mid-stream, and it doesn't flush the stream. The C<:encoding>
561also implicitly pushes on top of itself the C<:utf8> layer because
562internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 563
19799a22 564The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
565system all work together to let the programmer treat a single
566character (C<\n>) as the line terminator, irrespective of the external
567representation. On many operating systems, the native text file
568representation matches the internal representation, but on some
569platforms the external representation of C<\n> is made up of more than
570one character.
571
68bd7414
NIS
572Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
573character to end each line in the external representation of text (even
5e12dbfa 574though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
575on Unix and most VMS files). In other systems like OS/2, DOS and the
576various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
577but what's stored in text files are the two characters C<\cM\cJ>. That
578means that, if you don't use binmode() on these systems, C<\cM\cJ>
579sequences on disk will be converted to C<\n> on input, and any C<\n> in
580your program will be converted back to C<\cM\cJ> on output. This is what
581you want for text files, but it can be disastrous for binary files.
30168b04
GS
582
583Another consequence of using binmode() (on some systems) is that
584special end-of-file markers will be seen as part of the data stream.
585For systems from the Microsoft family this means that if your binary
4375e838 586data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
587the file, unless you use binmode().
588
589binmode() is not only important for readline() and print() operations,
590but also when using read(), seek(), sysread(), syswrite() and tell()
591(see L<perlport> for more details). See the C<$/> and C<$\> variables
592in L<perlvar> for how to manually set your input and output
593line-termination sequences.
a0d0e21e 594
4633a7c4 595=item bless REF,CLASSNAME
d74e8afc 596X<bless>
a0d0e21e
LW
597
598=item bless REF
599
2b5ab1e7
TC
600This function tells the thingy referenced by REF that it is now an object
601in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 602is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 603it returns the reference for convenience. Always use the two-argument
cf264981
SP
604version if a derived class might inherit the function doing the blessing.
605See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
606of objects.
a0d0e21e 607
57668c4d 608Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 609Namespaces with all lowercase names are considered reserved for
cf264981 610Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
611confusion, you may wish to avoid such package names as well. Make sure
612that CLASSNAME is a true value.
60ad88b8
GS
613
614See L<perlmod/"Perl Modules">.
615
0d863452
RH
616=item break
617
618Break out of a C<given()> block.
619
620This keyword is enabled by the "switch" feature: see L<feature>
621for more information.
622
a0d0e21e 623=item caller EXPR
d74e8afc 624X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
625
626=item caller
627
5a964f20 628Returns the context of the current subroutine call. In scalar context,
28757baa 629returns the caller's package name if there is a caller, that is, if
19799a22 630we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 631otherwise. In list context, returns
a0d0e21e 632
ee6b43cc 633 # 0 1 2
748a9306 634 ($package, $filename, $line) = caller;
a0d0e21e
LW
635
636With EXPR, it returns some extra information that the debugger uses to
637print a stack trace. The value of EXPR indicates how many call frames
638to go back before the current one.
639
ee6b43cc 640 # 0 1 2 3 4
f3aa04c2 641 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 642
643 # 5 6 7 8 9 10
b3ca2e83 644 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 645 = caller($i);
e7ea3e70 646
951ba7fe 647Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 648call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 649C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 650C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 651C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 652$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
653each C<use> statement creates a C<require> frame inside an C<eval EXPR>
654frame.) $subroutine may also be C<(unknown)> if this particular
655subroutine happens to have been deleted from the symbol table.
656C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
657C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
658compiled with. The C<$hints> and C<$bitmask> values are subject to change
659between versions of Perl, and are not meant for external use.
748a9306 660
b3ca2e83
NC
661C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
662caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
663of this hash, as they are the actual values stored in the optree.
664
748a9306 665Furthermore, when called from within the DB package, caller returns more
7660c0ab 666detailed information: it sets the list variable C<@DB::args> to be the
54310121 667arguments with which the subroutine was invoked.
748a9306 668
7660c0ab 669Be aware that the optimizer might have optimized call frames away before
19799a22 670C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 671might not return information about the call frame you expect it do, for
b76cc8ba 672C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 673previous time C<caller> was called.
7660c0ab 674
a0d0e21e 675=item chdir EXPR
d74e8afc
ITB
676X<chdir>
677X<cd>
f723aae1 678X<directory, change>
a0d0e21e 679
c4aca7d0
GA
680=item chdir FILEHANDLE
681
682=item chdir DIRHANDLE
683
ce2984c3
PF
684=item chdir
685
ffce7b87 686Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 687changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 688changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
689variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
690neither is set, C<chdir> does nothing. It returns true upon success,
691false otherwise. See the example under C<die>.
a0d0e21e 692
c4aca7d0
GA
693On systems that support fchdir, you might pass a file handle or
694directory handle as argument. On systems that don't support fchdir,
695passing handles produces a fatal error at run time.
696
a0d0e21e 697=item chmod LIST
d74e8afc 698X<chmod> X<permission> X<mode>
a0d0e21e
LW
699
700Changes the permissions of a list of files. The first element of the
4633a7c4 701list must be the numerical mode, which should probably be an octal
4ad40acf 702number, and which definitely should I<not> be a string of octal digits:
2f9daede 703C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 704successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
705
706 $cnt = chmod 0755, 'foo', 'bar';
707 chmod 0755, @executables;
f86cebdf
GS
708 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
709 # --w----r-T
2f9daede
TP
710 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
711 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 712
c4aca7d0
GA
713On systems that support fchmod, you might pass file handles among the
714files. On systems that don't support fchmod, passing file handles
345da378
GA
715produces a fatal error at run time. The file handles must be passed
716as globs or references to be recognized. Barewords are considered
717file names.
c4aca7d0
GA
718
719 open(my $fh, "<", "foo");
720 my $perm = (stat $fh)[2] & 07777;
721 chmod($perm | 0600, $fh);
722
ca6e1c26
JH
723You can also import the symbolic C<S_I*> constants from the Fcntl
724module:
725
726 use Fcntl ':mode';
727
728 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
729 # This is identical to the chmod 0755 of the above example.
730
a0d0e21e 731=item chomp VARIABLE
d74e8afc 732X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 733
313c9f5c 734=item chomp( LIST )
a0d0e21e
LW
735
736=item chomp
737
2b5ab1e7
TC
738This safer version of L</chop> removes any trailing string
739that corresponds to the current value of C<$/> (also known as
28757baa 740$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
741number of characters removed from all its arguments. It's often used to
742remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
743that the final record may be missing its newline. When in paragraph
744mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
745When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
746a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 747remove anything.
19799a22 748If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
749
750 while (<>) {
751 chomp; # avoid \n on last field
752 @array = split(/:/);
5a964f20 753 # ...
a0d0e21e
LW
754 }
755
4bf21a6d
RD
756If VARIABLE is a hash, it chomps the hash's values, but not its keys.
757
a0d0e21e
LW
758You can actually chomp anything that's an lvalue, including an assignment:
759
760 chomp($cwd = `pwd`);
761 chomp($answer = <STDIN>);
762
763If you chomp a list, each element is chomped, and the total number of
764characters removed is returned.
765
15e44fd8
RGS
766Note that parentheses are necessary when you're chomping anything
767that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
768is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
769C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
770C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
771as C<chomp($a, $b)>.
772
a0d0e21e 773=item chop VARIABLE
d74e8afc 774X<chop>
a0d0e21e 775
313c9f5c 776=item chop( LIST )
a0d0e21e
LW
777
778=item chop
779
780Chops off the last character of a string and returns the character
5b3eff12 781chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 782scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
783If VARIABLE is a hash, it chops the hash's values, but not its keys.
784
5b3eff12 785You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
786
787If you chop a list, each element is chopped. Only the value of the
19799a22 788last C<chop> is returned.
a0d0e21e 789
19799a22 790Note that C<chop> returns the last character. To return all but the last
748a9306
LW
791character, use C<substr($string, 0, -1)>.
792
15e44fd8
RGS
793See also L</chomp>.
794
a0d0e21e 795=item chown LIST
d74e8afc 796X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
797
798Changes the owner (and group) of a list of files. The first two
19799a22
GS
799elements of the list must be the I<numeric> uid and gid, in that
800order. A value of -1 in either position is interpreted by most
801systems to leave that value unchanged. Returns the number of files
802successfully changed.
a0d0e21e
LW
803
804 $cnt = chown $uid, $gid, 'foo', 'bar';
805 chown $uid, $gid, @filenames;
806
c4aca7d0
GA
807On systems that support fchown, you might pass file handles among the
808files. On systems that don't support fchown, passing file handles
345da378
GA
809produces a fatal error at run time. The file handles must be passed
810as globs or references to be recognized. Barewords are considered
811file names.
c4aca7d0 812
54310121 813Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
814
815 print "User: ";
19799a22 816 chomp($user = <STDIN>);
5a964f20 817 print "Files: ";
19799a22 818 chomp($pattern = <STDIN>);
a0d0e21e
LW
819
820 ($login,$pass,$uid,$gid) = getpwnam($user)
821 or die "$user not in passwd file";
822
5a964f20 823 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
824 chown $uid, $gid, @ary;
825
54310121 826On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
827file unless you're the superuser, although you should be able to change
828the group to any of your secondary groups. On insecure systems, these
829restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
830On POSIX systems, you can detect this condition this way:
831
832 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
833 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 834
a0d0e21e 835=item chr NUMBER
d74e8afc 836X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 837
54310121 838=item chr
bbce6d69 839
a0d0e21e 840Returns the character represented by that NUMBER in the character set.
a0ed51b3 841For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 842chr(0x263a) is a Unicode smiley face.
aaa68c4a 843
8a064bd6 844Negative values give the Unicode replacement character (chr(0xfffd)),
5f0135eb 845except under the L<bytes> pragma, where low eight bits of the value
8a064bd6
JH
846(truncated to an integer) are used.
847
974da8e5
JH
848If NUMBER is omitted, uses C<$_>.
849
b76cc8ba 850For the reverse, use L</ord>.
a0d0e21e 851
2575c402
JW
852Note that characters from 128 to 255 (inclusive) are by default
853internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 854
2575c402 855See L<perlunicode> for more about Unicode.
bbce6d69 856
a0d0e21e 857=item chroot FILENAME
d74e8afc 858X<chroot> X<root>
a0d0e21e 859
54310121 860=item chroot
bbce6d69 861
5a964f20 862This function works like the system call by the same name: it makes the
4633a7c4 863named directory the new root directory for all further pathnames that
951ba7fe 864begin with a C</> by your process and all its children. (It doesn't
28757baa 865change your current working directory, which is unaffected.) For security
4633a7c4 866reasons, this call is restricted to the superuser. If FILENAME is
19799a22 867omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
868
869=item close FILEHANDLE
d74e8afc 870X<close>
a0d0e21e 871
6a518fbc
TP
872=item close
873
e0f13c26
RGS
874Closes the file or pipe associated with the file handle, flushes the IO
875buffers, and closes the system file descriptor. Returns true if those
876operations have succeeded and if no error was reported by any PerlIO
877layer. Closes the currently selected filehandle if the argument is
878omitted.
fb73857a 879
880You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
881another C<open> on it, because C<open> will close it for you. (See
882C<open>.) However, an explicit C<close> on an input file resets the line
883counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 884
dede8123
RGS
885If the file handle came from a piped open, C<close> will additionally
886return false if one of the other system calls involved fails, or if the
fb73857a 887program exits with non-zero status. (If the only problem was that the
dede8123 888program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 889also waits for the process executing on the pipe to complete, in case you
b76cc8ba 890want to look at the output of the pipe afterwards, and
e5218da5
GA
891implicitly puts the exit status value of that command into C<$?> and
892C<${^CHILD_ERROR_NATIVE}>.
5a964f20 893
73689b13
GS
894Prematurely closing the read end of a pipe (i.e. before the process
895writing to it at the other end has closed it) will result in a
896SIGPIPE being delivered to the writer. If the other end can't
897handle that, be sure to read all the data before closing the pipe.
898
fb73857a 899Example:
a0d0e21e 900
fb73857a 901 open(OUTPUT, '|sort >foo') # pipe to sort
902 or die "Can't start sort: $!";
5a964f20 903 #... # print stuff to output
fb73857a 904 close OUTPUT # wait for sort to finish
905 or warn $! ? "Error closing sort pipe: $!"
906 : "Exit status $? from sort";
907 open(INPUT, 'foo') # get sort's results
908 or die "Can't open 'foo' for input: $!";
a0d0e21e 909
5a964f20
TC
910FILEHANDLE may be an expression whose value can be used as an indirect
911filehandle, usually the real filehandle name.
a0d0e21e
LW
912
913=item closedir DIRHANDLE
d74e8afc 914X<closedir>
a0d0e21e 915
19799a22 916Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
917system call.
918
a0d0e21e 919=item connect SOCKET,NAME
d74e8afc 920X<connect>
a0d0e21e
LW
921
922Attempts to connect to a remote socket, just as the connect system call
19799a22 923does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
924packed address of the appropriate type for the socket. See the examples in
925L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 926
cb1a09d0 927=item continue BLOCK
d74e8afc 928X<continue>
cb1a09d0 929
0d863452
RH
930=item continue
931
cf264981
SP
932C<continue> is actually a flow control statement rather than a function. If
933there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
934C<foreach>), it is always executed just before the conditional is about to
935be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
936it can be used to increment a loop variable, even when the loop has been
937continued via the C<next> statement (which is similar to the C C<continue>
938statement).
939
98293880 940C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
941block. C<last> and C<redo> will behave as if they had been executed within
942the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
943block, it may be more entertaining.
944
945 while (EXPR) {
946 ### redo always comes here
947 do_something;
948 } continue {
949 ### next always comes here
950 do_something_else;
951 # then back the top to re-check EXPR
952 }
953 ### last always comes here
954
955Omitting the C<continue> section is semantically equivalent to using an
19799a22 956empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
957to check the condition at the top of the loop.
958
0d863452
RH
959If the "switch" feature is enabled, C<continue> is also a
960function that will break out of the current C<when> or C<default>
961block, and fall through to the next case. See L<feature> and
962L<perlsyn/"Switch statements"> for more information.
963
964
a0d0e21e 965=item cos EXPR
d74e8afc 966X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 967
d6217f1e
GS
968=item cos
969
5a964f20 970Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 971takes cosine of C<$_>.
a0d0e21e 972
ca6e1c26 973For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa 974function, or use this relation:
975
976 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
977
a0d0e21e 978=item crypt PLAINTEXT,SALT
d74e8afc 979X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 980X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 981
ef2e6798
MS
982Creates a digest string exactly like the crypt(3) function in the C
983library (assuming that you actually have a version there that has not
cf264981 984been extirpated as a potential munitions).
ef2e6798
MS
985
986crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
987into a short string, called a digest, which is returned. The same
988PLAINTEXT and SALT will always return the same string, but there is no
989(known) way to get the original PLAINTEXT from the hash. Small
990changes in the PLAINTEXT or SALT will result in large changes in the
991digest.
992
993There is no decrypt function. This function isn't all that useful for
994cryptography (for that, look for F<Crypt> modules on your nearby CPAN
995mirror) and the name "crypt" is a bit of a misnomer. Instead it is
996primarily used to check if two pieces of text are the same without
997having to transmit or store the text itself. An example is checking
998if a correct password is given. The digest of the password is stored,
cf264981 999not the password itself. The user types in a password that is
ef2e6798
MS
1000crypt()'d with the same salt as the stored digest. If the two digests
1001match the password is correct.
1002
1003When verifying an existing digest string you should use the digest as
1004the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1005to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1006crypt() will hash the new string with the same salt as the digest.
1007This allows your code to work with the standard L<crypt|/crypt> and
1008with more exotic implementations. In other words, do not assume
1009anything about the returned string itself, or how many bytes in the
1010digest matter.
85c16d83
JH
1011
1012Traditionally the result is a string of 13 bytes: two first bytes of
1013the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
ef2e6798
MS
1014the first eight bytes of the digest string mattered, but alternative
1015hashing schemes (like MD5), higher level security schemes (like C2),
1016and implementations on non-UNIX platforms may produce different
1017strings.
85c16d83
JH
1018
1019When choosing a new salt create a random two character string whose
1020characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1021'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1022characters is just a recommendation; the characters allowed in
1023the salt depend solely on your system's crypt library, and Perl can't
1024restrict what salts C<crypt()> accepts.
e71965be 1025
a0d0e21e 1026Here's an example that makes sure that whoever runs this program knows
cf264981 1027their password:
a0d0e21e
LW
1028
1029 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1030
1031 system "stty -echo";
1032 print "Password: ";
e71965be 1033 chomp($word = <STDIN>);
a0d0e21e
LW
1034 print "\n";
1035 system "stty echo";
1036
e71965be 1037 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
1038 die "Sorry...\n";
1039 } else {
1040 print "ok\n";
54310121 1041 }
a0d0e21e 1042
9f8f0c9d 1043Of course, typing in your own password to whoever asks you
748a9306 1044for it is unwise.
a0d0e21e 1045
ef2e6798 1046The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1047of data, not least of all because you can't get the information
ef2e6798 1048back. Look at the L<Digest> module for more robust algorithms.
19799a22 1049
f2791508
JH
1050If using crypt() on a Unicode string (which I<potentially> has
1051characters with codepoints above 255), Perl tries to make sense
1052of the situation by trying to downgrade (a copy of the string)
1053the string back to an eight-bit byte string before calling crypt()
1054(on that copy). If that works, good. If not, crypt() dies with
1055C<Wide character in crypt>.
85c16d83 1056
aa689395 1057=item dbmclose HASH
d74e8afc 1058X<dbmclose>
a0d0e21e 1059
19799a22 1060[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1061
aa689395 1062Breaks the binding between a DBM file and a hash.
a0d0e21e 1063
19799a22 1064=item dbmopen HASH,DBNAME,MASK
d74e8afc 1065X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1066
19799a22 1067[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1068
7b8d334a 1069This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1070hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1071argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395 1072is the name of the database (without the F<.dir> or F<.pag> extension if
1073any). If the database does not exist, it is created with protection
19799a22
GS
1074specified by MASK (as modified by the C<umask>). If your system supports
1075only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 1076program. In older versions of Perl, if your system had neither DBM nor
19799a22 1077ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395 1078sdbm(3).
1079
1080If you don't have write access to the DBM file, you can only read hash
1081variables, not set them. If you want to test whether you can write,
19799a22 1082either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 1083which will trap the error.
a0d0e21e 1084
19799a22
GS
1085Note that functions such as C<keys> and C<values> may return huge lists
1086when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1087function to iterate over large DBM files. Example:
1088
1089 # print out history file offsets
1090 dbmopen(%HIST,'/usr/lib/news/history',0666);
1091 while (($key,$val) = each %HIST) {
1092 print $key, ' = ', unpack('L',$val), "\n";
1093 }
1094 dbmclose(%HIST);
1095
cb1a09d0 1096See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1097cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1098rich implementation.
4633a7c4 1099
2b5ab1e7
TC
1100You can control which DBM library you use by loading that library
1101before you call dbmopen():
1102
1103 use DB_File;
1104 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1105 or die "Can't open netscape history file: $!";
1106
a0d0e21e 1107=item defined EXPR
d74e8afc 1108X<defined> X<undef> X<undefined>
a0d0e21e 1109
54310121 1110=item defined
bbce6d69 1111
2f9daede
TP
1112Returns a Boolean value telling whether EXPR has a value other than
1113the undefined value C<undef>. If EXPR is not present, C<$_> will be
1114checked.
1115
1116Many operations return C<undef> to indicate failure, end of file,
1117system error, uninitialized variable, and other exceptional
1118conditions. This function allows you to distinguish C<undef> from
1119other values. (A simple Boolean test will not distinguish among
7660c0ab 1120C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1121false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1122doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TP
1123returns C<undef> when its argument is an empty array, I<or> when the
1124element to return happens to be C<undef>.
1125
f10b0346
GS
1126You may also use C<defined(&func)> to check whether subroutine C<&func>
1127has ever been defined. The return value is unaffected by any forward
04891299 1128declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1129may still be callable: its package may have an C<AUTOLOAD> method that
1130makes it spring into existence the first time that it is called -- see
1131L<perlsub>.
f10b0346
GS
1132
1133Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1134used to report whether memory for that aggregate has ever been
1135allocated. This behavior may disappear in future versions of Perl.
1136You should instead use a simple test for size:
1137
1138 if (@an_array) { print "has array elements\n" }
1139 if (%a_hash) { print "has hash members\n" }
2f9daede
TP
1140
1141When used on a hash element, it tells you whether the value is defined,
dc848c6f 1142not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1143purpose.
a0d0e21e
LW
1144
1145Examples:
1146
1147 print if defined $switch{'D'};
1148 print "$val\n" while defined($val = pop(@ary));
1149 die "Can't readlink $sym: $!"
1150 unless defined($value = readlink $sym);
a0d0e21e 1151 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1152 $debugging = 0 unless defined $debugging;
a0d0e21e 1153
19799a22 1154Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1155discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1156defined values. For example, if you say
a5f75d66
AD
1157
1158 "ab" =~ /a(.*)b/;
1159
7660c0ab 1160The pattern match succeeds, and C<$1> is defined, despite the fact that it
cf264981 1161matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1162matched something that happened to be zero characters long. This is all
a5f75d66 1163very above-board and honest. When a function returns an undefined value,
2f9daede 1164it's an admission that it couldn't give you an honest answer. So you
19799a22 1165should use C<defined> only when you're questioning the integrity of what
7660c0ab 1166you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TP
1167what you want.
1168
dc848c6f 1169See also L</undef>, L</exists>, L</ref>.
2f9daede 1170
a0d0e21e 1171=item delete EXPR
d74e8afc 1172X<delete>
a0d0e21e 1173
01020589
GS
1174Given an expression that specifies a hash element, array element, hash slice,
1175or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1176In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1177the size of the array will shrink to the highest element that tests
8216c1fd 1178true for exists() (or 0 if no such element exists).
a0d0e21e 1179
eba0920a
EM
1180Returns a list with the same number of elements as the number of elements
1181for which deletion was attempted. Each element of that list consists of
1182either the value of the element deleted, or the undefined value. In scalar
1183context, this means that you get the value of the last element deleted (or
1184the undefined value if that element did not exist).
1185
1186 %hash = (foo => 11, bar => 22, baz => 33);
1187 $scalar = delete $hash{foo}; # $scalar is 11
1188 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1189 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1190
1191Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1192a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1193from a C<tie>d hash or array may not necessarily return anything.
1194
8ea97a1e
GS
1195Deleting an array element effectively returns that position of the array
1196to its initial, uninitialized state. Subsequently testing for the same
cf264981
SP
1197element with exists() will return false. Also, deleting array elements
1198in the middle of an array will not shift the index of the elements
1199after them down. Use splice() for that. See L</exists>.
8ea97a1e 1200
01020589 1201The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1202
5f05dabc 1203 foreach $key (keys %HASH) {
1204 delete $HASH{$key};
a0d0e21e
LW
1205 }
1206
01020589
GS
1207 foreach $index (0 .. $#ARRAY) {
1208 delete $ARRAY[$index];
1209 }
1210
1211And so do these:
5f05dabc 1212
01020589
GS
1213 delete @HASH{keys %HASH};
1214
9740c838 1215 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1216
2b5ab1e7 1217But both of these are slower than just assigning the empty list
01020589
GS
1218or undefining %HASH or @ARRAY:
1219
1220 %HASH = (); # completely empty %HASH
1221 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1222
01020589
GS
1223 @ARRAY = (); # completely empty @ARRAY
1224 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1225
1226Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1227operation is a hash element, array element, hash slice, or array slice
1228lookup:
a0d0e21e
LW
1229
1230 delete $ref->[$x][$y]{$key};
5f05dabc 1231 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1232
01020589
GS
1233 delete $ref->[$x][$y][$index];
1234 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1235
a0d0e21e 1236=item die LIST
d74e8afc 1237X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1238
19799a22
GS
1239Outside an C<eval>, prints the value of LIST to C<STDERR> and
1240exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1241exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1242status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1243an C<eval(),> the error message is stuffed into C<$@> and the
1244C<eval> is terminated with the undefined value. This makes
1245C<die> the way to raise an exception.
a0d0e21e
LW
1246
1247Equivalent examples:
1248
1249 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1250 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1251
ccac6780 1252If the last element of LIST does not end in a newline, the current
df37ec69
WW
1253script line number and input line number (if any) are also printed,
1254and a newline is supplied. Note that the "input line number" (also
1255known as "chunk") is subject to whatever notion of "line" happens to
1256be currently in effect, and is also available as the special variable
1257C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1258
1259Hint: sometimes appending C<", stopped"> to your message will cause it
1260to make better sense when the string C<"at foo line 123"> is appended.
1261Suppose you are running script "canasta".
a0d0e21e
LW
1262
1263 die "/etc/games is no good";
1264 die "/etc/games is no good, stopped";
1265
1266produce, respectively
1267
1268 /etc/games is no good at canasta line 123.
1269 /etc/games is no good, stopped at canasta line 123.
1270
2b5ab1e7 1271See also exit(), warn(), and the Carp module.
a0d0e21e 1272
7660c0ab
A
1273If LIST is empty and C<$@> already contains a value (typically from a
1274previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a 1275This is useful for propagating exceptions:
1276
1277 eval { ... };
1278 die unless $@ =~ /Expected exception/;
1279
ad216e65
JH
1280If LIST is empty and C<$@> contains an object reference that has a
1281C<PROPAGATE> method, that method will be called with additional file
1282and line number parameters. The return value replaces the value in
28a5cf3b 1283C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1284were called.
1285
7660c0ab 1286If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1287
52531d10
GS
1288die() can also be called with a reference argument. If this happens to be
1289trapped within an eval(), $@ contains the reference. This behavior permits
1290a more elaborate exception handling implementation using objects that
4375e838 1291maintain arbitrary state about the nature of the exception. Such a scheme
52531d10 1292is sometimes preferable to matching particular string values of $@ using
746d7dd7
GL
1293regular expressions. Because $@ is a global variable, and eval() may be
1294used within object implementations, care must be taken that analyzing the
1295error object doesn't replace the reference in the global variable. The
1296easiest solution is to make a local copy of the reference before doing
1297other manipulations. Here's an example:
52531d10 1298
da279afe 1299 use Scalar::Util 'blessed';
1300
52531d10 1301 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1302 if (my $ev_err = $@) {
1303 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1304 # handle Some::Module::Exception
1305 }
1306 else {
1307 # handle all other possible exceptions
1308 }
1309 }
1310
19799a22 1311Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1312them, you may want to overload stringification operations on such custom
1313exception objects. See L<overload> for details about that.
1314
19799a22
GS
1315You can arrange for a callback to be run just before the C<die>
1316does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1317handler will be called with the error text and can change the error
1318message, if it sees fit, by calling C<die> again. See
1319L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1320L<"eval BLOCK"> for some examples. Although this feature was
19799a22
GS
1321to be run only right before your program was to exit, this is not
1322currently the case--the C<$SIG{__DIE__}> hook is currently called
1323even inside eval()ed blocks/strings! If one wants the hook to do
1324nothing in such situations, put
fb73857a 1325
1326 die @_ if $^S;
1327
19799a22
GS
1328as the first line of the handler (see L<perlvar/$^S>). Because
1329this promotes strange action at a distance, this counterintuitive
b76cc8ba 1330behavior may be fixed in a future release.
774d564b 1331
a0d0e21e 1332=item do BLOCK
d74e8afc 1333X<do> X<block>
a0d0e21e
LW
1334
1335Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1336sequence of commands indicated by BLOCK. When modified by the C<while> or
1337C<until> loop modifier, executes the BLOCK once before testing the loop
1338condition. (On other statements the loop modifiers test the conditional
1339first.)
a0d0e21e 1340
4968c1e4 1341C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1342C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1343See L<perlsyn> for alternative strategies.
4968c1e4 1344
a0d0e21e 1345=item do SUBROUTINE(LIST)
d74e8afc 1346X<do>
a0d0e21e 1347
cf264981 1348This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1349
1350=item do EXPR
d74e8afc 1351X<do>
a0d0e21e
LW
1352
1353Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1354file as a Perl script.
a0d0e21e
LW
1355
1356 do 'stat.pl';
1357
1358is just like
1359
986b19de 1360 eval `cat stat.pl`;
a0d0e21e 1361
2b5ab1e7 1362except that it's more efficient and concise, keeps track of the current
ea63ef19 1363filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1364C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1365variables. It also differs in that code evaluated with C<do FILENAME>
1366cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1367same, however, in that it does reparse the file every time you call it,
1368so you probably don't want to do this inside a loop.
a0d0e21e 1369
8e30cc93 1370If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1371error. If C<do> can read the file but cannot compile it, it
8e30cc93
MG
1372returns undef and sets an error message in C<$@>. If the file is
1373successfully compiled, C<do> returns the value of the last expression
1374evaluated.
1375
a0d0e21e 1376Note that inclusion of library modules is better done with the
19799a22 1377C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1378and raise an exception if there's a problem.
a0d0e21e 1379
5a964f20
TC
1380You might like to use C<do> to read in a program configuration
1381file. Manual error checking can be done this way:
1382
b76cc8ba 1383 # read in config files: system first, then user
f86cebdf 1384 for $file ("/share/prog/defaults.rc",
b76cc8ba 1385 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1386 {
5a964f20 1387 unless ($return = do $file) {
f86cebdf
GS
1388 warn "couldn't parse $file: $@" if $@;
1389 warn "couldn't do $file: $!" unless defined $return;
1390 warn "couldn't run $file" unless $return;
5a964f20
TC
1391 }
1392 }
1393
a0d0e21e 1394=item dump LABEL
d74e8afc 1395X<dump> X<core> X<undump>
a0d0e21e 1396
1614b0e3
JD
1397=item dump
1398
19799a22
GS
1399This function causes an immediate core dump. See also the B<-u>
1400command-line switch in L<perlrun>, which does the same thing.
1401Primarily this is so that you can use the B<undump> program (not
1402supplied) to turn your core dump into an executable binary after
1403having initialized all your variables at the beginning of the
1404program. When the new binary is executed it will begin by executing
1405a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1406Think of it as a goto with an intervening core dump and reincarnation.
1407If C<LABEL> is omitted, restarts the program from the top.
1408
1409B<WARNING>: Any files opened at the time of the dump will I<not>
1410be open any more when the program is reincarnated, with possible
b76cc8ba 1411resulting confusion on the part of Perl.
19799a22 1412
59f521f4
RGS
1413This function is now largely obsolete, mostly because it's very hard to
1414convert a core file into an executable. That's why you should now invoke
1415it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1416typo.
19799a22 1417
aa689395 1418=item each HASH
d74e8afc 1419X<each> X<hash, iterator>
aa689395 1420
aeedbbed
NC
1421=item each ARRAY
1422X<array, iterator>
1423
5a964f20 1424When called in list context, returns a 2-element list consisting of the
aeedbbed
NC
1425key and value for the next element of a hash, or the index and value for
1426the next element of an array, so that you can iterate over it. When called
1427in scalar context, returns only the key for the next element in the hash
1428(or the index for an array).
2f9daede 1429
aeedbbed 1430Hash entries are returned in an apparently random order. The actual random
504f80c1
JH
1431order is subject to change in future versions of perl, but it is
1432guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1433function would produce on the same (unmodified) hash. Since Perl
22883ac5 14345.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1435for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1436
aeedbbed
NC
1437When the hash or array is entirely read, a null array is returned in list
1438context (which when assigned produces a false (C<0>) value), and C<undef> in
19799a22 1439scalar context. The next call to C<each> after that will start iterating
aeedbbed
NC
1440again. There is a single iterator for each hash or array, shared by all
1441C<each>, C<keys>, and C<values> function calls in the program; it can be
1442reset by reading all the elements from the hash or array, or by evaluating
1443C<keys HASH>, C<values HASH>, C<keys ARRAY>, or C<values ARRAY>. If you add
1444or delete elements of a hash while you're
74fc8b5f
MJD
1445iterating over it, you may get entries skipped or duplicated, so
1446don't. Exception: It is always safe to delete the item most recently
1447returned by C<each()>, which means that the following code will work:
1448
1449 while (($key, $value) = each %hash) {
1450 print $key, "\n";
1451 delete $hash{$key}; # This is safe
1452 }
aa689395 1453
f86cebdf 1454The following prints out your environment like the printenv(1) program,
aa689395 1455only in a different order:
a0d0e21e
LW
1456
1457 while (($key,$value) = each %ENV) {
1458 print "$key=$value\n";
1459 }
1460
19799a22 1461See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1462
1463=item eof FILEHANDLE
d74e8afc
ITB
1464X<eof>
1465X<end of file>
1466X<end-of-file>
a0d0e21e 1467
4633a7c4
LW
1468=item eof ()
1469
a0d0e21e
LW
1470=item eof
1471
1472Returns 1 if the next read on FILEHANDLE will return end of file, or if
1473FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1474gives the real filehandle. (Note that this function actually
19799a22 1475reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1476interactive context.) Do not read from a terminal file (or call
19799a22 1477C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1478as terminals may lose the end-of-file condition if you do.
1479
820475bd
GS
1480An C<eof> without an argument uses the last file read. Using C<eof()>
1481with empty parentheses is very different. It refers to the pseudo file
1482formed from the files listed on the command line and accessed via the
61eff3bc
JH
1483C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1484as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1485used will cause C<@ARGV> to be examined to determine if input is
67408cae 1486available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1487end-of-file will assume you are processing another C<@ARGV> list,
1488and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1489see L<perlop/"I/O Operators">.
820475bd 1490
61eff3bc 1491In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1492detect the end of each file, C<eof()> will only detect the end of the
1493last file. Examples:
a0d0e21e 1494
748a9306
LW
1495 # reset line numbering on each input file
1496 while (<>) {
b76cc8ba 1497 next if /^\s*#/; # skip comments
748a9306 1498 print "$.\t$_";
5a964f20
TC
1499 } continue {
1500 close ARGV if eof; # Not eof()!
748a9306
LW
1501 }
1502
a0d0e21e
LW
1503 # insert dashes just before last line of last file
1504 while (<>) {
6ac88b13 1505 if (eof()) { # check for end of last file
a0d0e21e
LW
1506 print "--------------\n";
1507 }
1508 print;
6ac88b13 1509 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1510 }
1511
a0d0e21e 1512Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1513input operators typically return C<undef> when they run out of data, or if
1514there was an error.
a0d0e21e
LW
1515
1516=item eval EXPR
d74e8afc 1517X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1518X<error, handling> X<exception, handling>
a0d0e21e
LW
1519
1520=item eval BLOCK
1521
ce2984c3
PF
1522=item eval
1523
c7cc6f1c
GS
1524In the first form, the return value of EXPR is parsed and executed as if it
1525were a little Perl program. The value of the expression (which is itself
5a964f20 1526determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1527errors, executed in the lexical context of the current Perl program, so
1528that any variable settings or subroutine and format definitions remain
cf264981 1529afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1530If EXPR is omitted, evaluates C<$_>. This form is typically used to
1531delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1532
1533In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1534same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1535within the context of the current Perl program. This form is typically
1536used to trap exceptions more efficiently than the first (see below), while
1537also providing the benefit of checking the code within BLOCK at compile
1538time.
1539
1540The final semicolon, if any, may be omitted from the value of EXPR or within
1541the BLOCK.
1542
1543In both forms, the value returned is the value of the last expression
5a964f20 1544evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1545as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1546in void, scalar, or list context, depending on the context of the C<eval>
1547itself. See L</wantarray> for more on how the evaluation context can be
1548determined.
a0d0e21e 1549
19799a22
GS
1550If there is a syntax error or runtime error, or a C<die> statement is
1551executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1552error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1553string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1554warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1555To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1556turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1557See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1558
19799a22
GS
1559Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1560determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1561is implemented. It is also Perl's exception trapping mechanism, where
1562the die operator is used to raise exceptions.
1563
1564If the code to be executed doesn't vary, you may use the eval-BLOCK
1565form to trap run-time errors without incurring the penalty of
1566recompiling each time. The error, if any, is still returned in C<$@>.
1567Examples:
1568
54310121 1569 # make divide-by-zero nonfatal
a0d0e21e
LW
1570 eval { $answer = $a / $b; }; warn $@ if $@;
1571
1572 # same thing, but less efficient
1573 eval '$answer = $a / $b'; warn $@ if $@;
1574
1575 # a compile-time error
5a964f20 1576 eval { $answer = }; # WRONG
a0d0e21e
LW
1577
1578 # a run-time error
1579 eval '$answer ='; # sets $@
1580
cf264981
SP
1581Using the C<eval{}> form as an exception trap in libraries does have some
1582issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1583may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7
TC
1584You can use the C<local $SIG{__DIE__}> construct for this purpose,
1585as shown in this example:
774d564b 1586
1587 # a very private exception trap for divide-by-zero
f86cebdf
GS
1588 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1589 warn $@ if $@;
774d564b 1590
1591This is especially significant, given that C<__DIE__> hooks can call
19799a22 1592C<die> again, which has the effect of changing their error messages:
774d564b 1593
1594 # __DIE__ hooks may modify error messages
1595 {
f86cebdf
GS
1596 local $SIG{'__DIE__'} =
1597 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1598 eval { die "foo lives here" };
1599 print $@ if $@; # prints "bar lives here"
774d564b 1600 }
1601
19799a22 1602Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1603may be fixed in a future release.
1604
19799a22 1605With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1606being looked at when:
1607
1608 eval $x; # CASE 1
1609 eval "$x"; # CASE 2
1610
1611 eval '$x'; # CASE 3
1612 eval { $x }; # CASE 4
1613
5a964f20 1614 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1615 $$x++; # CASE 6
1616
2f9daede 1617Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1618the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1619the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1620and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1621does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TP
1622purely visual reasons, but it also has the advantage of compiling at
1623compile-time instead of at run-time.) Case 5 is a place where
19799a22 1624normally you I<would> like to use double quotes, except that in this
2f9daede
TP
1625particular situation, you can just use symbolic references instead, as
1626in case 6.
a0d0e21e 1627
4968c1e4 1628C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1629C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1630
d819b83a
DM
1631Note that as a very special case, an C<eval ''> executed within the C<DB>
1632package doesn't see the usual surrounding lexical scope, but rather the
1633scope of the first non-DB piece of code that called it. You don't normally
1634need to worry about this unless you are writing a Perl debugger.
1635
a0d0e21e 1636=item exec LIST
d74e8afc 1637X<exec> X<execute>
a0d0e21e 1638
8bf3b016
GS
1639=item exec PROGRAM LIST
1640
19799a22
GS
1641The C<exec> function executes a system command I<and never returns>--
1642use C<system> instead of C<exec> if you want it to return. It fails and
1643returns false only if the command does not exist I<and> it is executed
fb73857a 1644directly instead of via your system's command shell (see below).
a0d0e21e 1645
19799a22
GS
1646Since it's a common mistake to use C<exec> instead of C<system>, Perl
1647warns you if there is a following statement which isn't C<die>, C<warn>,
1648or C<exit> (if C<-w> is set - but you always do that). If you
1649I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1650can use one of these styles to avoid the warning:
1651
5a964f20
TC
1652 exec ('foo') or print STDERR "couldn't exec foo: $!";
1653 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1654
5a964f20 1655If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1656with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1657If there is only one scalar argument or an array with one element in it,
1658the argument is checked for shell metacharacters, and if there are any,
1659the entire argument is passed to the system's command shell for parsing
1660(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1661If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1662words and passed directly to C<execvp>, which is more efficient.
19799a22 1663Examples:
a0d0e21e 1664
19799a22
GS
1665 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1666 exec "sort $outfile | uniq";
a0d0e21e
LW
1667
1668If you don't really want to execute the first argument, but want to lie
1669to the program you are executing about its own name, you can specify
1670the program you actually want to run as an "indirect object" (without a
1671comma) in front of the LIST. (This always forces interpretation of the
54310121 1672LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1673the list.) Example:
1674
1675 $shell = '/bin/csh';
1676 exec $shell '-sh'; # pretend it's a login shell
1677
1678or, more directly,
1679
1680 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1681
bb32b41a
GS
1682When the arguments get executed via the system shell, results will
1683be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1684for details.
1685
19799a22
GS
1686Using an indirect object with C<exec> or C<system> is also more
1687secure. This usage (which also works fine with system()) forces
1688interpretation of the arguments as a multivalued list, even if the
1689list had just one argument. That way you're safe from the shell
1690expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1691
1692 @args = ( "echo surprise" );
1693
2b5ab1e7 1694 exec @args; # subject to shell escapes
f86cebdf 1695 # if @args == 1
2b5ab1e7 1696 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1697
1698The first version, the one without the indirect object, ran the I<echo>
1699program, passing it C<"surprise"> an argument. The second version
1700didn't--it tried to run a program literally called I<"echo surprise">,
1701didn't find it, and set C<$?> to a non-zero value indicating failure.
1702
0f897271
GS
1703Beginning with v5.6.0, Perl will attempt to flush all files opened for
1704output before the exec, but this may not be supported on some platforms
1705(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1706in English) or call the C<autoflush()> method of C<IO::Handle> on any
1707open handles in order to avoid lost output.
1708
19799a22 1709Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1710any C<DESTROY> methods in your objects.
1711
a0d0e21e 1712=item exists EXPR
d74e8afc 1713X<exists> X<autovivification>
a0d0e21e 1714
01020589 1715Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1716returns true if the specified element in the hash or array has ever
1717been initialized, even if the corresponding value is undefined. The
1718element is not autovivified if it doesn't exist.
a0d0e21e 1719
01020589
GS
1720 print "Exists\n" if exists $hash{$key};
1721 print "Defined\n" if defined $hash{$key};
1722 print "True\n" if $hash{$key};
1723
1724 print "Exists\n" if exists $array[$index];
1725 print "Defined\n" if defined $array[$index];
1726 print "True\n" if $array[$index];
a0d0e21e 1727
8ea97a1e 1728A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1729it exists, but the reverse doesn't necessarily hold true.
1730
afebc493
GS
1731Given an expression that specifies the name of a subroutine,
1732returns true if the specified subroutine has ever been declared, even
1733if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1734does not count as declaring it. Note that a subroutine which does not
1735exist may still be callable: its package may have an C<AUTOLOAD>
1736method that makes it spring into existence the first time that it is
1737called -- see L<perlsub>.
afebc493
GS
1738
1739 print "Exists\n" if exists &subroutine;
1740 print "Defined\n" if defined &subroutine;
1741
a0d0e21e 1742Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1743operation is a hash or array key lookup or subroutine name:
a0d0e21e 1744
2b5ab1e7
TC
1745 if (exists $ref->{A}->{B}->{$key}) { }
1746 if (exists $hash{A}{B}{$key}) { }
1747
01020589
GS
1748 if (exists $ref->{A}->{B}->[$ix]) { }
1749 if (exists $hash{A}{B}[$ix]) { }
1750
afebc493
GS
1751 if (exists &{$ref->{A}{B}{$key}}) { }
1752
01020589
GS
1753Although the deepest nested array or hash will not spring into existence
1754just because its existence was tested, any intervening ones will.
61eff3bc 1755Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1756into existence due to the existence test for the $key element above.
1757This happens anywhere the arrow operator is used, including even:
5a964f20 1758
2b5ab1e7
TC
1759 undef $ref;
1760 if (exists $ref->{"Some key"}) { }
1761 print $ref; # prints HASH(0x80d3d5c)
1762
1763This surprising autovivification in what does not at first--or even
1764second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1765release.
a0d0e21e 1766
afebc493
GS
1767Use of a subroutine call, rather than a subroutine name, as an argument
1768to exists() is an error.
1769
1770 exists &sub; # OK
1771 exists &sub(); # Error
1772
a0d0e21e 1773=item exit EXPR
d74e8afc 1774X<exit> X<terminate> X<abort>
a0d0e21e 1775
ce2984c3
PF
1776=item exit
1777
2b5ab1e7 1778Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1779
1780 $ans = <STDIN>;
1781 exit 0 if $ans =~ /^[Xx]/;
1782
19799a22 1783See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1784universally recognized values for EXPR are C<0> for success and C<1>
1785for error; other values are subject to interpretation depending on the
1786environment in which the Perl program is running. For example, exiting
178769 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1788the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1789
19799a22
GS
1790Don't use C<exit> to abort a subroutine if there's any chance that
1791someone might want to trap whatever error happened. Use C<die> instead,
1792which can be trapped by an C<eval>.
28757baa 1793
19799a22 1794The exit() function does not always exit immediately. It calls any
2b5ab1e7 1795defined C<END> routines first, but these C<END> routines may not
19799a22 1796themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1797be called are called before the real exit. If this is a problem, you
1798can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1799See L<perlmod> for details.
5a964f20 1800
a0d0e21e 1801=item exp EXPR
d74e8afc 1802X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1803
54310121 1804=item exp
bbce6d69 1805
b76cc8ba 1806Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1807If EXPR is omitted, gives C<exp($_)>.
1808
1809=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1810X<fcntl>
a0d0e21e 1811
f86cebdf 1812Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1813
1814 use Fcntl;
1815
0ade1984 1816first to get the correct constant definitions. Argument processing and
b76cc8ba 1817value return works just like C<ioctl> below.
a0d0e21e
LW
1818For example:
1819
1820 use Fcntl;
5a964f20
TC
1821 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1822 or die "can't fcntl F_GETFL: $!";
1823
554ad1fc 1824You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1825Like C<ioctl>, it maps a C<0> return from the system call into
1826C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1827in numeric context. It is also exempt from the normal B<-w> warnings
1828on improper numeric conversions.
5a964f20 1829
19799a22 1830Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1831doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1832manpage to learn what functions are available on your system.
a0d0e21e 1833
be2f7487
TH
1834Here's an example of setting a filehandle named C<REMOTE> to be
1835non-blocking at the system level. You'll have to negotiate C<$|>
1836on your own, though.
1837
1838 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1839
1840 $flags = fcntl(REMOTE, F_GETFL, 0)
1841 or die "Can't get flags for the socket: $!\n";
1842
1843 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1844 or die "Can't set flags for the socket: $!\n";
1845
a0d0e21e 1846=item fileno FILEHANDLE
d74e8afc 1847X<fileno>
a0d0e21e 1848
2b5ab1e7
TC
1849Returns the file descriptor for a filehandle, or undefined if the
1850filehandle is not open. This is mainly useful for constructing
19799a22 1851bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1852If FILEHANDLE is an expression, the value is taken as an indirect
1853filehandle, generally its name.
5a964f20 1854
b76cc8ba 1855You can use this to find out whether two handles refer to the
5a964f20
TC
1856same underlying descriptor:
1857
1858 if (fileno(THIS) == fileno(THAT)) {
1859 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1860 }
1861
1862(Filehandles connected to memory objects via new features of C<open> may
1863return undefined even though they are open.)
1864
a0d0e21e
LW
1865
1866=item flock FILEHANDLE,OPERATION
d74e8afc 1867X<flock> X<lock> X<locking>
a0d0e21e 1868
19799a22
GS
1869Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1870for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1871machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1872C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1873only entire files, not records.
1874
1875Two potentially non-obvious but traditional C<flock> semantics are
1876that it waits indefinitely until the lock is granted, and that its locks
1877B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1878fewer guarantees. This means that programs that do not also use C<flock>
1879may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1880your port's specific documentation, or your system-specific local manpages
1881for details. It's best to assume traditional behavior if you're writing
1882portable programs. (But if you're not, you should as always feel perfectly
1883free to write for your own system's idiosyncrasies (sometimes called
1884"features"). Slavish adherence to portability concerns shouldn't get
1885in the way of your getting your job done.)
a3cb178b 1886
8ebc5c01 1887OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1888LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1889you can use the symbolic names if you import them from the Fcntl module,
68dc0745 1890either individually, or as a group using the ':flock' tag. LOCK_SH
1891requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1892releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1893LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745 1894waiting for the lock (check the return status to see if you got it).
1895
2b5ab1e7
TC
1896To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1897before locking or unlocking it.
8ebc5c01 1898
f86cebdf 1899Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1900locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1901are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1902implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01 1903differing semantics shouldn't bite too many people.
1904
becacb53
TM
1905Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1906be open with read intent to use LOCK_SH and requires that it be open
1907with write intent to use LOCK_EX.
1908
19799a22
GS
1909Note also that some versions of C<flock> cannot lock things over the
1910network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1911that. If you like you can force Perl to ignore your system's flock(2)
1912function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 1913the switch C<-Ud_flock> to the F<Configure> program when you configure
1914perl.
4633a7c4
LW
1915
1916Here's a mailbox appender for BSD systems.
a0d0e21e 1917
7e1af8bc 1918 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1919
1920 sub lock {
7e1af8bc 1921 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1922 # and, in case someone appended
1923 # while we were waiting...
1924 seek(MBOX, 0, 2);
1925 }
1926
1927 sub unlock {
7e1af8bc 1928 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1929 }
1930
b0169937 1931 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
a0d0e21e
LW
1932 or die "Can't open mailbox: $!";
1933
1934 lock();
b0169937 1935 print $mbox $msg,"\n\n";
a0d0e21e
LW
1936 unlock();
1937
2b5ab1e7
TC
1938On systems that support a real flock(), locks are inherited across fork()
1939calls, whereas those that must resort to the more capricious fcntl()
1940function lose the locks, making it harder to write servers.
1941
cb1a09d0 1942See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1943
1944=item fork
d74e8afc 1945X<fork> X<child> X<parent>
a0d0e21e 1946
2b5ab1e7
TC
1947Does a fork(2) system call to create a new process running the
1948same program at the same point. It returns the child pid to the
1949parent process, C<0> to the child process, or C<undef> if the fork is
1950unsuccessful. File descriptors (and sometimes locks on those descriptors)
1951are shared, while everything else is copied. On most systems supporting
1952fork(), great care has gone into making it extremely efficient (for
1953example, using copy-on-write technology on data pages), making it the
1954dominant paradigm for multitasking over the last few decades.
5a964f20 1955
0f897271
GS
1956Beginning with v5.6.0, Perl will attempt to flush all files opened for
1957output before forking the child process, but this may not be supported
1958on some platforms (see L<perlport>). To be safe, you may need to set
1959C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1960C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1961
19799a22 1962If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1963accumulate zombies. On some systems, you can avoid this by setting
1964C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1965forking and reaping moribund children.
cb1a09d0 1966
28757baa 1967Note that if your forked child inherits system file descriptors like
1968STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1969if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1970backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1971You should reopen those to F</dev/null> if it's any issue.
28757baa 1972
cb1a09d0 1973=item format
d74e8afc 1974X<format>
cb1a09d0 1975
19799a22 1976Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1977example:
1978
54310121 1979 format Something =
cb1a09d0
AD
1980 Test: @<<<<<<<< @||||| @>>>>>
1981 $str, $%, '$' . int($num)
1982 .
1983
1984 $str = "widget";
184e9718 1985 $num = $cost/$quantity;
cb1a09d0
AD
1986 $~ = 'Something';
1987 write;
1988
1989See L<perlform> for many details and examples.
1990
8903cb82 1991=item formline PICTURE,LIST
d74e8afc 1992X<formline>
a0d0e21e 1993
5a964f20 1994This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1995too. It formats (see L<perlform>) a list of values according to the
1996contents of PICTURE, placing the output into the format output
7660c0ab 1997accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1998Eventually, when a C<write> is done, the contents of
cf264981
SP
1999C<$^A> are written to some filehandle. You could also read C<$^A>
2000and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2001does one C<formline> per line of form, but the C<formline> function itself
748a9306 2002doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 2003that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
2004You may therefore need to use multiple formlines to implement a single
2005record format, just like the format compiler.
2006
19799a22 2007Be careful if you put double quotes around the picture, because an C<@>
748a9306 2008character may be taken to mean the beginning of an array name.
19799a22 2009C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
2010
2011=item getc FILEHANDLE
f723aae1 2012X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2013
2014=item getc
2015
2016Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
2017or the undefined value at end of file, or if there was an error (in
2018the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2019STDIN. This is not particularly efficient. However, it cannot be
2020used by itself to fetch single characters without waiting for the user
2021to hit enter. For that, try something more like:
4633a7c4
LW
2022
2023 if ($BSD_STYLE) {
2024 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2025 }
2026 else {
54310121 2027 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2028 }
2029
2030 $key = getc(STDIN);
2031
2032 if ($BSD_STYLE) {
2033 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2034 }
2035 else {
5f05dabc 2036 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
2037 }
2038 print "\n";
2039
54310121 2040Determination of whether $BSD_STYLE should be set
2041is left as an exercise to the reader.
cb1a09d0 2042
19799a22 2043The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2044systems purporting POSIX compliance. See also the C<Term::ReadKey>
2045module from your nearest CPAN site; details on CPAN can be found on
2046L<perlmodlib/CPAN>.
a0d0e21e
LW
2047
2048=item getlogin
d74e8afc 2049X<getlogin> X<login>
a0d0e21e 2050
cf264981 2051This implements the C library function of the same name, which on most
5a964f20 2052systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 2053use C<getpwuid>.
a0d0e21e 2054
f86702cc 2055 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2056
19799a22
GS
2057Do not consider C<getlogin> for authentication: it is not as
2058secure as C<getpwuid>.
4633a7c4 2059
a0d0e21e 2060=item getpeername SOCKET
d74e8afc 2061X<getpeername> X<peer>
a0d0e21e
LW
2062
2063Returns the packed sockaddr address of other end of the SOCKET connection.
2064
4633a7c4
LW
2065 use Socket;
2066 $hersockaddr = getpeername(SOCK);
19799a22 2067 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2068 $herhostname = gethostbyaddr($iaddr, AF_INET);
2069 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2070
2071=item getpgrp PID
d74e8afc 2072X<getpgrp> X<group>
a0d0e21e 2073
47e29363 2074Returns the current process group for the specified PID. Use
7660c0ab 2075a PID of C<0> to get the current process group for the
4633a7c4 2076current process. Will raise an exception if used on a machine that
f86cebdf 2077doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2078group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2079does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2080
2081=item getppid
d74e8afc 2082X<getppid> X<parent> X<pid>
a0d0e21e
LW
2083
2084Returns the process id of the parent process.
2085
4d76a344
RGS
2086Note for Linux users: on Linux, the C functions C<getpid()> and
2087C<getppid()> return different values from different threads. In order to
2088be portable, this behavior is not reflected by the perl-level function
2089C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2090to call the underlying C<getppid()>, you may use the CPAN module
2091C<Linux::Pid>.
4d76a344 2092
a0d0e21e 2093=item getpriority WHICH,WHO
d74e8afc 2094X<getpriority> X<priority> X<nice>
a0d0e21e 2095
4633a7c4
LW
2096Returns the current priority for a process, a process group, or a user.
2097(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2098machine that doesn't implement getpriority(2).
a0d0e21e
LW
2099
2100=item getpwnam NAME
d74e8afc
ITB
2101X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2102X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2103X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2104X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2105X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2106X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2107
2108=item getgrnam NAME
2109
2110=item gethostbyname NAME
2111
2112=item getnetbyname NAME
2113
2114=item getprotobyname NAME
2115
2116=item getpwuid UID
2117
2118=item getgrgid GID
2119
2120=item getservbyname NAME,PROTO
2121
2122=item gethostbyaddr ADDR,ADDRTYPE
2123
2124=item getnetbyaddr ADDR,ADDRTYPE
2125
2126=item getprotobynumber NUMBER
2127
2128=item getservbyport PORT,PROTO
2129
2130=item getpwent
2131
2132=item getgrent
2133
2134=item gethostent
2135
2136=item getnetent
2137
2138=item getprotoent
2139
2140=item getservent
2141
2142=item setpwent
2143
2144=item setgrent
2145
2146=item sethostent STAYOPEN
2147
2148=item setnetent STAYOPEN
2149
2150=item setprotoent STAYOPEN
2151
2152=item setservent STAYOPEN
2153
2154=item endpwent
2155
2156=item endgrent
2157
2158=item endhostent
2159
2160=item endnetent
2161
2162=item endprotoent
2163
2164=item endservent
2165
2166These routines perform the same functions as their counterparts in the
5a964f20 2167system library. In list context, the return values from the
a0d0e21e
LW
2168various get routines are as follows:
2169
2170 ($name,$passwd,$uid,$gid,
6ee623d5 2171 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2172 ($name,$passwd,$gid,$members) = getgr*
2173 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2174 ($name,$aliases,$addrtype,$net) = getnet*
2175 ($name,$aliases,$proto) = getproto*
2176 ($name,$aliases,$port,$proto) = getserv*
2177
2178(If the entry doesn't exist you get a null list.)
2179
4602f195
JH
2180The exact meaning of the $gcos field varies but it usually contains
2181the real name of the user (as opposed to the login name) and other
2182information pertaining to the user. Beware, however, that in many
2183system users are able to change this information and therefore it
106325ad 2184cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2185L<perlsec>). The $passwd and $shell, user's encrypted password and
2186login shell, are also tainted, because of the same reason.
4602f195 2187
5a964f20 2188In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2189lookup by name, in which case you get the other thing, whatever it is.
2190(If the entry doesn't exist you get the undefined value.) For example:
2191
5a964f20
TC
2192 $uid = getpwnam($name);
2193 $name = getpwuid($num);
2194 $name = getpwent();
2195 $gid = getgrnam($name);
08a33e13 2196 $name = getgrgid($num);
5a964f20
TC
2197 $name = getgrent();
2198 #etc.
a0d0e21e 2199
4602f195
JH
2200In I<getpw*()> the fields $quota, $comment, and $expire are special
2201cases in the sense that in many systems they are unsupported. If the
2202$quota is unsupported, it is an empty scalar. If it is supported, it
2203usually encodes the disk quota. If the $comment field is unsupported,
2204it is an empty scalar. If it is supported it usually encodes some
2205administrative comment about the user. In some systems the $quota
2206field may be $change or $age, fields that have to do with password
2207aging. In some systems the $comment field may be $class. The $expire
2208field, if present, encodes the expiration period of the account or the
2209password. For the availability and the exact meaning of these fields
2210in your system, please consult your getpwnam(3) documentation and your
2211F<pwd.h> file. You can also find out from within Perl what your
2212$quota and $comment fields mean and whether you have the $expire field
2213by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2214C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2215files are only supported if your vendor has implemented them in the
2216intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2217shadow versions if you're running under privilege or if there exists
cf264981
SP
2218the shadow(3) functions as found in System V (this includes Solaris
2219and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2220facility are unlikely to be supported.
6ee623d5 2221
19799a22 2222The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2223the login names of the members of the group.
2224
2225For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2226C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2227C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2228addresses returned by the corresponding system library call. In the
2229Internet domain, each address is four bytes long and you can unpack it
2230by saying something like:
2231
f337b084 2232 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2233
2b5ab1e7
TC
2234The Socket library makes this slightly easier:
2235
2236 use Socket;
2237 $iaddr = inet_aton("127.1"); # or whatever address
2238 $name = gethostbyaddr($iaddr, AF_INET);
2239
2240 # or going the other way
19799a22 2241 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2242
d760c846
GS
2243In the opposite way, to resolve a hostname to the IP address
2244you can write this:
2245
2246 use Socket;
2247 $packed_ip = gethostbyname("www.perl.org");
2248 if (defined $packed_ip) {
2249 $ip_address = inet_ntoa($packed_ip);
2250 }
2251
2252Make sure <gethostbyname()> is called in SCALAR context and that
2253its return value is checked for definedness.
2254
19799a22
GS
2255If you get tired of remembering which element of the return list
2256contains which return value, by-name interfaces are provided
2257in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2258C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2259and C<User::grent>. These override the normal built-ins, supplying
2260versions that return objects with the appropriate names
2261for each field. For example:
5a964f20
TC
2262
2263 use File::stat;
2264 use User::pwent;
2265 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2266
b76cc8ba
NIS
2267Even though it looks like they're the same method calls (uid),
2268they aren't, because a C<File::stat> object is different from
19799a22 2269a C<User::pwent> object.
5a964f20 2270
a0d0e21e 2271=item getsockname SOCKET
d74e8afc 2272X<getsockname>
a0d0e21e 2273
19799a22
GS
2274Returns the packed sockaddr address of this end of the SOCKET connection,
2275in case you don't know the address because you have several different
2276IPs that the connection might have come in on.
a0d0e21e 2277
4633a7c4
LW
2278 use Socket;
2279 $mysockaddr = getsockname(SOCK);
19799a22 2280 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2281 printf "Connect to %s [%s]\n",
19799a22
GS
2282 scalar gethostbyaddr($myaddr, AF_INET),
2283 inet_ntoa($myaddr);
a0d0e21e
LW
2284
2285=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2286X<getsockopt>
a0d0e21e 2287
636e6b1f
TH
2288Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2289Options may exist at multiple protocol levels depending on the socket
2290type, but at least the uppermost socket level SOL_SOCKET (defined in the
2291C<Socket> module) will exist. To query options at another level the
2292protocol number of the appropriate protocol controlling the option
2293should be supplied. For example, to indicate that an option is to be
2294interpreted by the TCP protocol, LEVEL should be set to the protocol
2295number of TCP, which you can get using getprotobyname.
2296
2297The call returns a packed string representing the requested socket option,
2298or C<undef> if there is an error (the error reason will be in $!). What
2299exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2300your system documentation for details. A very common case however is that
cf264981 2301the option is an integer, in which case the result will be a packed
636e6b1f
TH
2302integer which you can decode using unpack with the C<i> (or C<I>) format.
2303
2304An example testing if Nagle's algorithm is turned on on a socket:
2305
4852725b 2306 use Socket qw(:all);
636e6b1f
TH
2307
2308 defined(my $tcp = getprotobyname("tcp"))
2309 or die "Could not determine the protocol number for tcp";
4852725b
DD
2310 # my $tcp = IPPROTO_TCP; # Alternative
2311 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2312 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2313 my $nodelay = unpack("I", $packed);
2314 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2315
a0d0e21e
LW
2316
2317=item glob EXPR
d74e8afc 2318X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2319
0a753a76 2320=item glob
2321
d9a9d457
JL
2322In list context, returns a (possibly empty) list of filename expansions on
2323the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2324scalar context, glob iterates through such filename expansions, returning
2325undef when the list is exhausted. This is the internal function
2326implementing the C<< <*.c> >> operator, but you can use it directly. If
2327EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2328more detail in L<perlop/"I/O Operators">.
a0d0e21e 2329
3a4b19e4
GS
2330Beginning with v5.6.0, this operator is implemented using the standard
2331C<File::Glob> extension. See L<File::Glob> for details.
2332
a0d0e21e 2333=item gmtime EXPR
d74e8afc 2334X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2335
ce2984c3
PF
2336=item gmtime
2337
435fbc73
GS
2338Works just like L<localtime> but the returned values are
2339localized for the standard Greenwich time zone.
a0d0e21e 2340
435fbc73
GS
2341Note: when called in list context, $isdst, the last value
2342returned by gmtime is always C<0>. There is no
2343Daylight Saving Time in GMT.
0a753a76 2344
62aa5637
MS
2345See L<perlport/gmtime> for portability concerns.
2346
a0d0e21e 2347=item goto LABEL
d74e8afc 2348X<goto> X<jump> X<jmp>
a0d0e21e 2349
748a9306
LW
2350=item goto EXPR
2351
a0d0e21e
LW
2352=item goto &NAME
2353
7660c0ab 2354The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2355execution there. It may not be used to go into any construct that
7660c0ab 2356requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2357also can't be used to go into a construct that is optimized away,
19799a22 2358or to get out of a block or subroutine given to C<sort>.
0a753a76 2359It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2360including out of subroutines, but it's usually better to use some other
19799a22 2361construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2362need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2363(The difference being that C does not offer named loops combined with
2364loop control. Perl does, and this replaces most structured uses of C<goto>
2365in other languages.)
a0d0e21e 2366
7660c0ab
A
2367The C<goto-EXPR> form expects a label name, whose scope will be resolved
2368dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2369necessarily recommended if you're optimizing for maintainability:
2370
2371 goto ("FOO", "BAR", "GLARCH")[$i];
2372
1b6921cb
BT
2373The C<goto-&NAME> form is quite different from the other forms of
2374C<goto>. In fact, it isn't a goto in the normal sense at all, and
2375doesn't have the stigma associated with other gotos. Instead, it
2376exits the current subroutine (losing any changes set by local()) and
2377immediately calls in its place the named subroutine using the current
2378value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2379load another subroutine and then pretend that the other subroutine had
2380been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2381in the current subroutine are propagated to the other subroutine.)
2382After the C<goto>, not even C<caller> will be able to tell that this
2383routine was called first.
2384
2385NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2386containing a code reference, or a block that evaluates to a code
6cb9131c 2387reference.
a0d0e21e
LW
2388
2389=item grep BLOCK LIST
d74e8afc 2390X<grep>
a0d0e21e
LW
2391
2392=item grep EXPR,LIST
2393
2b5ab1e7
TC
2394This is similar in spirit to, but not the same as, grep(1) and its
2395relatives. In particular, it is not limited to using regular expressions.
2f9daede 2396
a0d0e21e 2397Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2398C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2399elements for which the expression evaluated to true. In scalar
2400context, returns the number of times the expression was true.
a0d0e21e
LW
2401
2402 @foo = grep(!/^#/, @bar); # weed out comments
2403
2404or equivalently,
2405
2406 @foo = grep {!/^#/} @bar; # weed out comments
2407
be3174d2
GS
2408Note that C<$_> is an alias to the list value, so it can be used to
2409modify the elements of the LIST. While this is useful and supported,
2410it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2411Similarly, grep returns aliases into the original list, much as a for
2412loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2413element of a list returned by grep (for example, in a C<foreach>, C<map>
2414or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2415This is usually something to be avoided when writing clear code.
a0d0e21e 2416
a4fb8298 2417If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2418been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2419the list elements, C<$_> keeps being lexical inside the block; i.e. it
2420can't be seen from the outside, avoiding any potential side-effects.
2421
19799a22 2422See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2423
a0d0e21e 2424=item hex EXPR
d74e8afc 2425X<hex> X<hexadecimal>
a0d0e21e 2426
54310121 2427=item hex
bbce6d69 2428
2b5ab1e7 2429Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2430(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2431L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TP
2432
2433 print hex '0xAf'; # prints '175'
2434 print hex 'aF'; # same
a0d0e21e 2435
19799a22 2436Hex strings may only represent integers. Strings that would cause
53305cf1 2437integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2438unlike oct(). To present something as hex, look into L</printf>,
2439L</sprintf>, or L</unpack>.
19799a22 2440
ce2984c3 2441=item import LIST
d74e8afc 2442X<import>
a0d0e21e 2443
19799a22 2444There is no builtin C<import> function. It is just an ordinary
4633a7c4 2445method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2446names to another module. The C<use> function calls the C<import> method
cea6626f 2447for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2448
2449=item index STR,SUBSTR,POSITION
d74e8afc 2450X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2451
2452=item index STR,SUBSTR
2453
2b5ab1e7
TC
2454The index function searches for one string within another, but without
2455the wildcard-like behavior of a full regular-expression pattern match.
2456It returns the position of the first occurrence of SUBSTR in STR at
2457or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2458beginning of the string. POSITION before the beginning of the string
2459or after its end is treated as if it were the beginning or the end,
2460respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2461you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2462is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2463
2464=item int EXPR
f723aae1 2465X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2466
54310121 2467=item int
bbce6d69 2468
7660c0ab 2469Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2470You should not use this function for rounding: one because it truncates
2471towards C<0>, and two because machine representations of floating point
2472numbers can sometimes produce counterintuitive results. For example,
2473C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2474because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2475the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2476functions will serve you better than will int().
a0d0e21e
LW
2477
2478=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2479X<ioctl>
a0d0e21e 2480
2b5ab1e7 2481Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2482
6c567752 2483 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2484
a11c483f 2485to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2486exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2487own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2488(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2489may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2490written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2491will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2492has no string value but does have a numeric value, that value will be
2493passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2494true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2495functions may be needed to manipulate the values of structures used by
b76cc8ba 2496C<ioctl>.
a0d0e21e 2497
19799a22 2498The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2499
2500 if OS returns: then Perl returns:
2501 -1 undefined value
2502 0 string "0 but true"
2503 anything else that number
2504
19799a22 2505Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2506still easily determine the actual value returned by the operating
2507system:
2508
2b5ab1e7 2509 $retval = ioctl(...) || -1;
a0d0e21e
LW
2510 printf "System returned %d\n", $retval;
2511
be2f7487 2512The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2513about improper numeric conversions.
2514
a0d0e21e 2515=item join EXPR,LIST
d74e8afc 2516X<join>
a0d0e21e 2517
2b5ab1e7
TC
2518Joins the separate strings of LIST into a single string with fields
2519separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2520
2b5ab1e7 2521 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2522
eb6e2d6f
GS
2523Beware that unlike C<split>, C<join> doesn't take a pattern as its
2524first argument. Compare L</split>.
a0d0e21e 2525
aa689395 2526=item keys HASH
d74e8afc 2527X<keys> X<key>
aa689395 2528
aeedbbed
NC
2529=item keys ARRAY
2530
2531Returns a list consisting of all the keys of the named hash, or the indices
2532of an array. (In scalar context, returns the number of keys or indices.)
504f80c1 2533
aeedbbed 2534The keys of a hash are returned in an apparently random order. The actual
504f80c1
JH
2535random order is subject to change in future versions of perl, but it
2536is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2537function produces (given that the hash has not been modified). Since
2538Perl 5.8.1 the ordering is different even between different runs of
2539Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2540Attacks">).
504f80c1 2541
aeedbbed 2542As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
cf264981
SP
2543(see L</each>). In particular, calling keys() in void context resets
2544the iterator with no other overhead.
a0d0e21e 2545
aa689395 2546Here is yet another way to print your environment:
a0d0e21e
LW
2547
2548 @keys = keys %ENV;
2549 @values = values %ENV;
b76cc8ba 2550 while (@keys) {
a0d0e21e
LW
2551 print pop(@keys), '=', pop(@values), "\n";
2552 }
2553
2554or how about sorted by key:
2555
2556 foreach $key (sort(keys %ENV)) {
2557 print $key, '=', $ENV{$key}, "\n";
2558 }
2559
8ea1e5d4
GS
2560The returned values are copies of the original keys in the hash, so
2561modifying them will not affect the original hash. Compare L</values>.
2562
19799a22 2563To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2564Here's a descending numeric sort of a hash by its values:
4633a7c4 2565
5a964f20 2566 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2567 printf "%4d %s\n", $hash{$key}, $key;
2568 }
2569
19799a22 2570As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395 2571allocated for the given hash. This can gain you a measure of efficiency if
2572you know the hash is going to get big. (This is similar to pre-extending
2573an array by assigning a larger number to $#array.) If you say
55497cff 2574
2575 keys %hash = 200;
2576
ab192400
GS
2577then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2578in fact, since it rounds up to the next power of two. These
55497cff 2579buckets will be retained even if you do C<%hash = ()>, use C<undef
2580%hash> if you want to free the storage while C<%hash> is still in scope.
2581You can't shrink the number of buckets allocated for the hash using
19799a22 2582C<keys> in this way (but you needn't worry about doing this by accident,
aeedbbed
NC
2583as trying has no effect). C<keys @array> in an lvalue context is a syntax
2584error.
55497cff 2585
19799a22 2586See also C<each>, C<values> and C<sort>.
ab192400 2587
b350dd2f 2588=item kill SIGNAL, LIST
d74e8afc 2589X<kill> X<signal>
a0d0e21e 2590
b350dd2f 2591Sends a signal to a list of processes. Returns the number of
517db077
GS
2592processes successfully signaled (which is not necessarily the
2593same as the number actually killed).
a0d0e21e
LW
2594
2595 $cnt = kill 1, $child1, $child2;
2596 kill 9, @goners;
2597
70fb64f6 2598If SIGNAL is zero, no signal is sent to the process, but the kill(2)
6cb9d3e4 2599system call will check whether it's possible to send a signal to it (that
70fb64f6
RGS
2600means, to be brief, that the process is owned by the same user, or we are
2601the super-user). This is a useful way to check that a child process is
81fd35db
DN
2602alive (even if only as a zombie) and hasn't changed its UID. See
2603L<perlport> for notes on the portability of this construct.
b350dd2f
GS
2604
2605Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2606process groups instead of processes. (On System V, a negative I<PROCESS>
2607number will also kill process groups, but that's not portable.) That
2608means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2609use a signal name in quotes.
2610
2611See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2612
2613=item last LABEL
d74e8afc 2614X<last> X<break>
a0d0e21e
LW
2615
2616=item last
2617
2618The C<last> command is like the C<break> statement in C (as used in
2619loops); it immediately exits the loop in question. If the LABEL is
2620omitted, the command refers to the innermost enclosing loop. The
2621C<continue> block, if any, is not executed:
2622
4633a7c4
LW
2623 LINE: while (<STDIN>) {
2624 last LINE if /^$/; # exit when done with header
5a964f20 2625 #...
a0d0e21e
LW
2626 }
2627
4968c1e4 2628C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2629C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2630a grep() or map() operation.
4968c1e4 2631
6c1372ed
GS
2632Note that a block by itself is semantically identical to a loop
2633that executes once. Thus C<last> can be used to effect an early
2634exit out of such a block.
2635
98293880
JH
2636See also L</continue> for an illustration of how C<last>, C<next>, and
2637C<redo> work.
1d2dff63 2638
a0d0e21e 2639=item lc EXPR
d74e8afc 2640X<lc> X<lowercase>
a0d0e21e 2641
54310121 2642=item lc
bbce6d69 2643
d1be9408 2644Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2645implementing the C<\L> escape in double-quoted strings. Respects
2646current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2647and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2648
7660c0ab 2649If EXPR is omitted, uses C<$_>.
bbce6d69 2650
a0d0e21e 2651=item lcfirst EXPR
d74e8afc 2652X<lcfirst> X<lowercase>
a0d0e21e 2653
54310121 2654=item lcfirst
bbce6d69 2655
ad0029c4
JH
2656Returns the value of EXPR with the first character lowercased. This
2657is the internal function implementing the C<\l> escape in
2658double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2659locale> in force. See L<perllocale> and L<perlunicode> for more
2660details about locale and Unicode support.
a0d0e21e 2661
7660c0ab 2662If EXPR is omitted, uses C<$_>.
bbce6d69 2663
a0d0e21e 2664=item length EXPR
d74e8afc 2665X<length> X<size>
a0d0e21e 2666
54310121 2667=item length
bbce6d69 2668
974da8e5 2669Returns the length in I<characters> of the value of EXPR. If EXPR is
9f621bb0
NC
2670omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
2671Note that this cannot be used on an entire array or hash to find out how
2672many elements these have. For that, use C<scalar @array> and C<scalar keys
2673%hash> respectively.
a0d0e21e 2674
974da8e5
JH
2675Note the I<characters>: if the EXPR is in Unicode, you will get the
2676number of characters, not the number of bytes. To get the length
2575c402
JW
2677of the internal string in bytes, use C<bytes::length(EXPR)>, see
2678L<bytes>. Note that the internal encoding is variable, and the number
2679of bytes usually meaningless. To get the number of bytes that the
2680string would have when encoded as UTF-8, use
2681C<length(Encoding::encode_utf8(EXPR))>.
974da8e5 2682
a0d0e21e 2683=item link OLDFILE,NEWFILE
d74e8afc 2684X<link>
a0d0e21e 2685
19799a22 2686Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2687success, false otherwise.
a0d0e21e
LW
2688
2689=item listen SOCKET,QUEUESIZE
d74e8afc 2690X<listen>
a0d0e21e 2691
19799a22 2692Does the same thing that the listen system call does. Returns true if
b76cc8ba 2693it succeeded, false otherwise. See the example in
cea6626f 2694L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2695
2696=item local EXPR
d74e8afc 2697X<local>
a0d0e21e 2698
19799a22 2699You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2700what most people think of as "local". See
13a2d996 2701L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2702
5a964f20
TC
2703A local modifies the listed variables to be local to the enclosing
2704block, file, or eval. If more than one value is listed, the list must
2705be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2706for details, including issues with tied arrays and hashes.
a0d0e21e 2707
a0d0e21e 2708=item localtime EXPR
435fbc73 2709X<localtime> X<ctime>
a0d0e21e 2710
ba053783
AL
2711=item localtime
2712
19799a22 2713Converts a time as returned by the time function to a 9-element list
5f05dabc 2714with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2715follows:
2716
54310121 2717 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2718 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2719 localtime(time);
a0d0e21e 2720
48a26b3a 2721All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2722tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2723of the specified time.
48a26b3a 2724
ba053783
AL
2725C<$mday> is the day of the month, and C<$mon> is the month itself, in
2726the range C<0..11> with 0 indicating January and 11 indicating December.
2727This makes it easy to get a month name from a list:
54310121 2728
ba053783
AL
2729 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2730 print "$abbr[$mon] $mday";
2731 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2732
ba053783
AL
2733C<$year> is the number of years since 1900, not just the last two digits
2734of the year. That is, C<$year> is C<123> in year 2023. The proper way
2735to get a complete 4-digit year is simply:
abd75f24 2736
ba053783 2737 $year += 1900;
abd75f24 2738
435fbc73
GS
2739Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2740to do that, would you?
2741
ba053783
AL
2742To get the last two digits of the year (e.g., '01' in 2001) do:
2743
2744 $year = sprintf("%02d", $year % 100);
2745
2746C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2747Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2748(or C<0..365> in leap years.)
2749
2750C<$isdst> is true if the specified time occurs during Daylight Saving
2751Time, false otherwise.
abd75f24 2752
e1998452 2753If EXPR is omitted, C<localtime()> uses the current time (as returned
e3176d09 2754by time(3)).
a0d0e21e 2755
48a26b3a 2756In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2757
5f05dabc 2758 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2759
fe86afc2
NC
2760This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2761instead of local time use the L</gmtime> builtin. See also the
2762C<Time::Local> module (to convert the second, minutes, hours, ... back to
2763the integer value returned by time()), and the L<POSIX> module's strftime(3)
2764and mktime(3) functions.
2765
2766To get somewhat similar but locale dependent date strings, set up your
2767locale environment variables appropriately (please see L<perllocale>) and
2768try for example:
a3cb178b 2769
5a964f20 2770 use POSIX qw(strftime);
2b5ab1e7 2771 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2772 # or for GMT formatted appropriately for your locale:
2773 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2774
2775Note that the C<%a> and C<%b>, the short forms of the day of the week
2776and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2777
62aa5637
MS
2778See L<perlport/localtime> for portability concerns.
2779
435fbc73
GS
2780The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2781by-name access mechanism to the gmtime() and localtime() functions,
2782respectively.
2783
2784For a comprehensive date and time representation look at the
2785L<DateTime> module on CPAN.
2786
07698885 2787=item lock THING
d74e8afc 2788X<lock>
19799a22 2789
01e6739c 2790This function places an advisory lock on a shared variable, or referenced
03730085 2791object contained in I<THING> until the lock goes out of scope.
a6d5524e 2792
f3a23afb 2793lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2794by this name (before any calls to it), that function will be called
03730085
AB
2795instead. (However, if you've said C<use threads>, lock() is always a
2796keyword.) See L<threads>.
19799a22 2797
a0d0e21e 2798=item log EXPR
d74e8afc 2799X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2800
54310121 2801=item log
bbce6d69 2802
2b5ab1e7
TC
2803Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2804returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2805The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2806divided by the natural log of N. For example:
2807
2808 sub log10 {
2809 my $n = shift;
2810 return log($n)/log(10);
b76cc8ba 2811 }
2b5ab1e7
TC
2812
2813See also L</exp> for the inverse operation.
a0d0e21e 2814
a0d0e21e 2815=item lstat EXPR
d74e8afc 2816X<lstat>
a0d0e21e 2817
54310121 2818=item lstat
bbce6d69 2819
19799a22 2820Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2821special C<_> filehandle) but stats a symbolic link instead of the file
2822the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2823your system, a normal C<stat> is done. For much more detailed
2824information, please see the documentation for C<stat>.
a0d0e21e 2825
7660c0ab 2826If EXPR is omitted, stats C<$_>.
bbce6d69 2827
a0d0e21e
LW
2828=item m//
2829
2830The match operator. See L<perlop>.
2831
2832=item map BLOCK LIST
d74e8afc 2833X<map>
a0d0e21e
LW
2834
2835=item map EXPR,LIST
2836
19799a22
GS
2837Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2838C<$_> to each element) and returns the list value composed of the
2839results of each such evaluation. In scalar context, returns the
2840total number of elements so generated. Evaluates BLOCK or EXPR in
2841list context, so each element of LIST may produce zero, one, or
2842more elements in the returned value.
dd99ebda 2843
a0d0e21e
LW
2844 @chars = map(chr, @nums);
2845
2846translates a list of numbers to the corresponding characters. And
2847
d8216f19 2848 %hash = map { get_a_key_for($_) => $_ } @array;
a0d0e21e
LW
2849
2850is just a funny way to write
2851
2852 %hash = ();
d8216f19
RGS
2853 foreach (@array) {
2854 $hash{get_a_key_for($_)} = $_;
a0d0e21e
LW
2855 }
2856
be3174d2
GS
2857Note that C<$_> is an alias to the list value, so it can be used to
2858modify the elements of the LIST. While this is useful and supported,
2859it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2860Using a regular C<foreach> loop for this purpose would be clearer in
2861most cases. See also L</grep> for an array composed of those items of
2862the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2863
a4fb8298 2864If C<$_> is lexical in the scope where the C<map> appears (because it has
d8216f19
RGS
2865been declared with C<my $_>), then, in addition to being locally aliased to
2866the list elements, C<$_> keeps being lexical inside the block; that is, it
a4fb8298
RGS
2867can't be seen from the outside, avoiding any potential side-effects.
2868
205fdb4d
NC
2869C<{> starts both hash references and blocks, so C<map { ...> could be either
2870the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2871ahead for the closing C<}> it has to take a guess at which its dealing with
2872based what it finds just after the C<{>. Usually it gets it right, but if it
2873doesn't it won't realize something is wrong until it gets to the C<}> and
2874encounters the missing (or unexpected) comma. The syntax error will be
2875reported close to the C<}> but you'll need to change something near the C<{>
2876such as using a unary C<+> to give perl some help:
2877
2878 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2879 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2880 %hash = map { ("\L$_", 1) } @array # this also works
2881 %hash = map { lc($_), 1 } @array # as does this.
2882 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2883
205fdb4d
NC
2884 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2885
d8216f19 2886or to force an anon hash constructor use C<+{>:
205fdb4d
NC
2887
2888 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2889
2890and you get list of anonymous hashes each with only 1 entry.
2891
19799a22 2892=item mkdir FILENAME,MASK
d74e8afc 2893X<mkdir> X<md> X<directory, create>
a0d0e21e 2894
5a211162
GS
2895=item mkdir FILENAME
2896
491873e5
RGS
2897=item mkdir
2898
0591cd52 2899Creates the directory specified by FILENAME, with permissions
19799a22
GS
2900specified by MASK (as modified by C<umask>). If it succeeds it
2901returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2902If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2903to C<$_>.
0591cd52 2904
19799a22 2905In general, it is better to create directories with permissive MASK,
0591cd52 2906and let the user modify that with their C<umask>, than it is to supply
19799a22 2907a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2908The exceptions to this rule are when the file or directory should be
2909kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2910C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2911
cc1852e8
JH
2912Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2913number of trailing slashes. Some operating and filesystems do not get
2914this right, so Perl automatically removes all trailing slashes to keep
2915everyone happy.
2916
dd184578
RGS
2917In order to recursively create a directory structure look at
2918the C<mkpath> function of the L<File::Path> module.
2919
a0d0e21e 2920=item msgctl ID,CMD,ARG
d74e8afc 2921X<msgctl>
a0d0e21e 2922
f86cebdf 2923Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2924
2925 use IPC::SysV;
2926
7660c0ab 2927first to get the correct constant definitions. If CMD is C<IPC_STAT>,
cf264981 2928then ARG must be a variable that will hold the returned C<msqid_ds>
951ba7fe
GS
2929structure. Returns like C<ioctl>: the undefined value for error,
2930C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2931L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2932
2933=item msgget KEY,FLAGS
d74e8afc 2934X<msgget>
a0d0e21e 2935
f86cebdf 2936Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2937id, or the undefined value if there is an error. See also
2938L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2939
a0d0e21e 2940=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
d74e8afc 2941X<msgrcv>
a0d0e21e
LW
2942
2943Calls the System V IPC function msgrcv to receive a message from
2944message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2945SIZE. Note that when a message is received, the message type as a
2946native long integer will be the first thing in VAR, followed by the
2947actual message. This packing may be opened with C<unpack("l! a*")>.
2948Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2949an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2950C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2951
2952=item msgsnd ID,MSG,FLAGS
d74e8afc 2953X<msgsnd>
41d6edb2
JH
2954
2955Calls the System V IPC function msgsnd to send the message MSG to the
2956message queue ID. MSG must begin with the native long integer message
2957type, and be followed by the length of the actual message, and finally
2958the message itself. This kind of packing can be achieved with
2959C<pack("l! a*", $type, $message)>. Returns true if successful,
2960or false if there is an error. See also C<IPC::SysV>
2961and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2962
2963=item my EXPR
d74e8afc 2964X<my>
a0d0e21e 2965
307ea6df
JH
2966=item my TYPE EXPR
2967
1d2de774 2968=item my EXPR : ATTRS
09bef843 2969
1d2de774 2970=item my TYPE EXPR : ATTRS
307ea6df 2971
19799a22 2972A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2973enclosing block, file, or C<eval>. If more than one value is listed,
2974the list must be placed in parentheses.
307ea6df 2975
1d2de774
JH
2976The exact semantics and interface of TYPE and ATTRS are still
2977evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2978and attributes are handled using the C<attributes> pragma, or starting
2979from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2980L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2981L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2982
a0d0e21e 2983=item next LABEL
d74e8afc 2984X<next> X<continue>
a0d0e21e
LW
2985
2986=item next
2987
2988The C<next> command is like the C<continue> statement in C; it starts
2989the next iteration of the loop:
2990
4633a7c4
LW
2991 LINE: while (<STDIN>) {
2992 next LINE if /^#/; # discard comments
5a964f20 2993 #...
a0d0e21e
LW
2994 }
2995
2996Note that if there were a C<continue> block on the above, it would get
2997executed even on discarded lines. If the LABEL is omitted, the command
2998refers to the innermost enclosing loop.
2999
4968c1e4 3000C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
3001C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3002a grep() or map() operation.
4968c1e4 3003
6c1372ed
GS
3004Note that a block by itself is semantically identical to a loop
3005that executes once. Thus C<next> will exit such a block early.
3006
98293880
JH
3007See also L</continue> for an illustration of how C<last>, C<next>, and
3008C<redo> work.
1d2dff63 3009
4a66ea5a 3010=item no Module VERSION LIST
d74e8afc 3011X<no>
4a66ea5a
RGS
3012
3013=item no Module VERSION
3014
a0d0e21e
LW
3015=item no Module LIST
3016
4a66ea5a
RGS
3017=item no Module
3018
c986422f
RGS
3019=item no VERSION
3020
593b9c14 3021See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
3022
3023=item oct EXPR
d74e8afc 3024X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
a0d0e21e 3025
54310121 3026=item oct
bbce6d69 3027
4633a7c4 3028Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
3029value. (If EXPR happens to start off with C<0x>, interprets it as a
3030hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
3031binary string. Leading whitespace is ignored in all three cases.)
3032The following will handle decimal, binary, octal, and hex in the standard
3033Perl or C notation:
a0d0e21e
LW
3034
3035 $val = oct($val) if $val =~ /^0/;
3036
19799a22
GS
3037If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3038in octal), use sprintf() or printf():
3039
3040 $perms = (stat("filename"))[2] & 07777;
3041 $oct_perms = sprintf "%lo", $perms;
3042
3043The oct() function is commonly used when a string such as C<644> needs
3044to be converted into a file mode, for example. (Although perl will
3045automatically convert strings into numbers as needed, this automatic
3046conversion assumes base 10.)
a0d0e21e
LW
3047
3048=item open FILEHANDLE,EXPR
d74e8afc 3049X<open> X<pipe> X<file, open> X<fopen>
a0d0e21e 3050
68bd7414
NIS
3051=item open FILEHANDLE,MODE,EXPR
3052
3053=item open FILEHANDLE,MODE,EXPR,LIST
3054
ba964c95
T
3055=item open FILEHANDLE,MODE,REFERENCE
3056
a0d0e21e
LW
3057=item open FILEHANDLE
3058
3059Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
3060FILEHANDLE.
3061
460b70c2
GS
3062Simple examples to open a file for reading:
3063
3064 open(my $fh, '<', "input.txt") or die $!;
3065
3066and for writing:
3067
3068 open(my $fh, '>', "output.txt") or die $!;
3069
ed53a2bb
JH
3070(The following is a comprehensive reference to open(): for a gentler
3071introduction you may consider L<perlopentut>.)
3072
a28cd5c9
NT
3073If FILEHANDLE is an undefined scalar variable (or array or hash element)
3074the variable is assigned a reference to a new anonymous filehandle,
3075otherwise if FILEHANDLE is an expression, its value is used as the name of
3076the real filehandle wanted. (This is considered a symbolic reference, so
3077C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
3078
3079If EXPR is omitted, the scalar variable of the same name as the
3080FILEHANDLE contains the filename. (Note that lexical variables--those
3081declared with C<my>--will not work for this purpose; so if you're
67408cae 3082using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
3083
3084If three or more arguments are specified then the mode of opening and
3085the file name are separate. If MODE is C<< '<' >> or nothing, the file
3086is opened for input. If MODE is C<< '>' >>, the file is truncated and
3087opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 3088the file is opened for appending, again being created if necessary.
5a964f20 3089
ed53a2bb
JH
3090You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3091indicate that you want both read and write access to the file; thus
3092C<< '+<' >> is almost always preferred for read/write updates--the C<<
3093'+>' >> mode would clobber the file first. You can't usually use
3094either read-write mode for updating textfiles, since they have
3095variable length records. See the B<-i> switch in L<perlrun> for a
3096better approach. The file is created with permissions of C<0666>
3097modified by the process' C<umask> value.
3098
3099These various prefixes correspond to the fopen(3) modes of C<'r'>,
3100C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 3101
6170680b
IZ
3102In the 2-arguments (and 1-argument) form of the call the mode and
3103filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
3104spaces. It is possible to omit the mode in these forms if the mode is
3105C<< '<' >>.
6170680b 3106
7660c0ab 3107If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 3108command to which output is to be piped, and if the filename ends with a
f244e06d
GS
3109C<'|'>, the filename is interpreted as a command which pipes output to
3110us. See L<perlipc/"Using open() for IPC">
19799a22 3111for more examples of this. (You are not allowed to C<open> to a command
5a964f20 3112that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
3113and L<perlipc/"Bidirectional Communication with Another Process">
3114for alternatives.)
cb1a09d0 3115
ed53a2bb
JH
3116For three or more arguments if MODE is C<'|-'>, the filename is
3117interpreted as a command to which output is to be piped, and if MODE
3118is C<'-|'>, the filename is interpreted as a command which pipes
3119output to us. In the 2-arguments (and 1-argument) form one should
3120replace dash (C<'-'>) with the command.
3121See L<perlipc/"Using open() for IPC"> for more examples of this.
3122(You are not allowed to C<open> to a command that pipes both in I<and>
3123out, but see L<IPC::Open2>, L<IPC::Open3>, and
3124L<perlipc/"Bidirectional Communication"> for alternatives.)
3125
3126In the three-or-more argument form of pipe opens, if LIST is specified
3127(extra arguments after the command name) then LIST becomes arguments
3128to the command invoked if the platform supports it. The meaning of
3129C<open> with more than three arguments for non-pipe modes is not yet
3130specified. Experimental "layers" may give extra LIST arguments
3131meaning.
6170680b
IZ
3132
3133In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 3134and opening C<< '>-' >> opens STDOUT.
6170680b 3135
fae2c0fb
RGS
3136You may use the three-argument form of open to specify IO "layers"
3137(sometimes also referred to as "disciplines") to be applied to the handle
3138that affect how the input and output are processed (see L<open> and
3139L<PerlIO> for more details). For example
7207e29d 3140
460b70c2 3141 open(my $fh, "<:encoding(UTF-8)", "file")
9124316e
JH
3142
3143will open the UTF-8 encoded file containing Unicode characters,
6d5e88a0
TS
3144see L<perluniintro>. Note that if layers are specified in the
3145three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3146usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
ed53a2bb
JH
3147
3148Open returns nonzero upon success, the undefined value otherwise. If
3149the C<open> involved a pipe, the return value happens to be the pid of
3150the subprocess.
cb1a09d0 3151
ed53a2bb
JH
3152If you're running Perl on a system that distinguishes between text