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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
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3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
15operator. A unary operator generally provides a scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
a0d0e21e 17contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 18be first, and the list argument will follow. (Note that there can ever
0f31cffe 19be only one such list argument.) For instance, splice() has three scalar
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20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
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22
23In the syntax descriptions that follow, list operators that expect a
24list (and provide list context for the elements of the list) are shown
25with LIST as an argument. Such a list may consist of any combination
26of scalar arguments or list values; the list values will be included
27in the list as if each individual element were interpolated at that
28point in the list, forming a longer single-dimensional list value.
cf264981 29Commas should separate elements of the LIST.
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30
31Any function in the list below may be used either with or without
32parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 33parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 34surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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35function, and precedence doesn't matter. Otherwise it's a list
36operator or unary operator, and precedence does matter. And whitespace
37between the function and left parenthesis doesn't count--so you need to
38be careful sometimes:
39
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40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
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45
46If you run Perl with the B<-w> switch it can warn you about this. For
47example, the third line above produces:
48
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
51
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52A few functions take no arguments at all, and therefore work as neither
53unary nor list operators. These include such functions as C<time>
54and C<endpwent>. For example, C<time+86_400> always means
55C<time() + 86_400>.
56
a0d0e21e 57For functions that can be used in either a scalar or list context,
54310121 58nonabortive failure is generally indicated in a scalar context by
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59returning the undefined value, and in a list context by returning the
60null list.
61
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62Remember the following important rule: There is B<no rule> that relates
63the behavior of an expression in list context to its behavior in scalar
64context, or vice versa. It might do two totally different things.
a0d0e21e 65Each operator and function decides which sort of value it would be most
2b5ab1e7 66appropriate to return in scalar context. Some operators return the
5a964f20 67length of the list that would have been returned in list context. Some
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68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
d74e8afc 72X<context>
a0d0e21e 73
d1be9408 74A named array in scalar context is quite different from what would at
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75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
81In general, functions in Perl that serve as wrappers for system calls
f86cebdf 82of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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83true when they succeed and C<undef> otherwise, as is usually mentioned
84in the descriptions below. This is different from the C interfaces,
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85which return C<-1> on failure. Exceptions to this rule are C<wait>,
86C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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87variable on failure. Other functions do not, except accidentally.
88
cb1a09d0 89=head2 Perl Functions by Category
d74e8afc 90X<function>
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91
92Here are Perl's functions (including things that look like
5a964f20 93functions, like some keywords and named operators)
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94arranged by category. Some functions appear in more
95than one place.
96
13a2d996 97=over 4
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98
99=item Functions for SCALARs or strings
d74e8afc 100X<scalar> X<string> X<character>
cb1a09d0 101
22fae026 102C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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>
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239
240* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 241operator, which can be used in expressions.
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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 $_; # ignore specials
5a964f20 341 #...
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342 }
343
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344The interpretation of the file permission operators C<-r>, C<-R>,
345C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
346of the file and the uids and gids of the user. There may be other
347reasons you can't actually read, write, or execute the file. Such
348reasons may be for example network filesystem access controls, ACLs
349(access control lists), read-only filesystems, and unrecognized
350executable formats.
351
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352Also note that, for the superuser on the local filesystems, the C<-r>,
353C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
5ff3f7a4
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354if any execute bit is set in the mode. Scripts run by the superuser
355may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 356or temporarily set their effective uid to something else.
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GS
357
358If you are using ACLs, there is a pragma called C<filetest> that may
359produce more accurate results than the bare stat() mode bits.
5ff3f7a4
GS
360When under the C<use filetest 'access'> the above-mentioned filetests
361will test whether the permission can (not) be granted using the
468541a8 362access() family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
363under this pragma return true even if there are no execute permission
364bits set (nor any extra execute permission ACLs). This strangeness is
365due to the underlying system calls' definitions. Read the
366documentation for the C<filetest> pragma for more information.
367
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368Note that C<-s/a/b/> does not do a negated substitution. Saying
369C<-exp($foo)> still works as expected, however--only single letters
370following a minus are interpreted as file tests.
371
372The C<-T> and C<-B> switches work as follows. The first block or so of the
373file is examined for odd characters such as strange control codes or
61eff3bc 374characters with the high bit set. If too many strange characters (>30%)
cf264981 375are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
a0d0e21e 376containing null in the first block is considered a binary file. If C<-T>
9124316e 377or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 378rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 379file, or a file at EOF when testing a filehandle. Because you have to
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380read a file to do the C<-T> test, on most occasions you want to use a C<-f>
381against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 382
19799a22 383If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 384the special filehandle consisting of a solitary underline, then the stat
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385structure of the previous file test (or stat operator) is used, saving
386a system call. (This doesn't work with C<-t>, and you need to remember
387that lstat() and C<-l> will leave values in the stat structure for the
5c9aa243 388symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 389an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 390Example:
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391
392 print "Can do.\n" if -r $a || -w _ || -x _;
393
394 stat($filename);
395 print "Readable\n" if -r _;
396 print "Writable\n" if -w _;
397 print "Executable\n" if -x _;
398 print "Setuid\n" if -u _;
399 print "Setgid\n" if -g _;
400 print "Sticky\n" if -k _;
401 print "Text\n" if -T _;
402 print "Binary\n" if -B _;
403
fbb0b3b3
RGS
404As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
405test operators, in a way that C<-f -w -x $file> is equivalent to
cf264981 406C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
fbb0b3b3
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407the return value of C<-f $file> as an argument to another filetest
408operator, no special magic will happen.)
409
a0d0e21e 410=item abs VALUE
d74e8afc 411X<abs> X<absolute>
a0d0e21e 412
54310121 413=item abs
bbce6d69 414
a0d0e21e 415Returns the absolute value of its argument.
7660c0ab 416If VALUE is omitted, uses C<$_>.
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417
418=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 419X<accept>
a0d0e21e 420
f86cebdf 421Accepts an incoming socket connect, just as the accept(2) system call
19799a22 422does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 423See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 424
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425On systems that support a close-on-exec flag on files, the flag will
426be set for the newly opened file descriptor, as determined by the
427value of $^F. See L<perlvar/$^F>.
428
a0d0e21e 429=item alarm SECONDS
d74e8afc
ITB
430X<alarm>
431X<SIGALRM>
432X<timer>
a0d0e21e 433
54310121 434=item alarm
bbce6d69 435
a0d0e21e 436Arranges to have a SIGALRM delivered to this process after the
cf264981 437specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
438specified, the value stored in C<$_> is used. (On some machines,
439unfortunately, the elapsed time may be up to one second less or more
440than you specified because of how seconds are counted, and process
441scheduling may delay the delivery of the signal even further.)
442
443Only one timer may be counting at once. Each call disables the
444previous timer, and an argument of C<0> may be supplied to cancel the
445previous timer without starting a new one. The returned value is the
446amount of time remaining on the previous timer.
a0d0e21e 447
2bc69794
BS
448For delays of finer granularity than one second, the Time::HiRes module
449(from CPAN, and starting from Perl 5.8 part of the standard
450distribution) provides ualarm(). You may also use Perl's four-argument
451version of select() leaving the first three arguments undefined, or you
452might be able to use the C<syscall> interface to access setitimer(2) if
453your system supports it. See L<perlfaq8> for details.
2b5ab1e7 454
68f8bed4
JH
455It is usually a mistake to intermix C<alarm> and C<sleep> calls.
456(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 457
19799a22
GS
458If you want to use C<alarm> to time out a system call you need to use an
459C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 460fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 461restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 462modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
463
464 eval {
f86cebdf 465 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 466 alarm $timeout;
ff68c719 467 $nread = sysread SOCKET, $buffer, $size;
36477c24 468 alarm 0;
ff68c719 469 };
ff68c719 470 if ($@) {
f86cebdf 471 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719
PP
472 # timed out
473 }
474 else {
475 # didn't
476 }
477
91d81acc
JH
478For more information see L<perlipc>.
479
a0d0e21e 480=item atan2 Y,X
d74e8afc 481X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
482
483Returns the arctangent of Y/X in the range -PI to PI.
484
ca6e1c26 485For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
486function, or use the familiar relation:
487
488 sub tan { sin($_[0]) / cos($_[0]) }
489
bf5f1b4c
JH
490Note that atan2(0, 0) is not well-defined.
491
a0d0e21e 492=item bind SOCKET,NAME
d74e8afc 493X<bind>
a0d0e21e
LW
494
495Binds a network address to a socket, just as the bind system call
19799a22 496does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
497packed address of the appropriate type for the socket. See the examples in
498L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 499
fae2c0fb 500=item binmode FILEHANDLE, LAYER
d74e8afc 501X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 502
a0d0e21e
LW
503=item binmode FILEHANDLE
504
1cbfc93d
NIS
505Arranges for FILEHANDLE to be read or written in "binary" or "text"
506mode on systems where the run-time libraries distinguish between
507binary and text files. If FILEHANDLE is an expression, the value is
508taken as the name of the filehandle. Returns true on success,
b5fe5ca2 509otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 510
d807c6f4
JH
511On some systems (in general, DOS and Windows-based systems) binmode()
512is necessary when you're not working with a text file. For the sake
513of portability it is a good idea to always use it when appropriate,
514and to never use it when it isn't appropriate. Also, people can
515set their I/O to be by default UTF-8 encoded Unicode, not bytes.
516
517In other words: regardless of platform, use binmode() on binary data,
518like for example images.
519
520If LAYER is present it is a single string, but may contain multiple
521directives. The directives alter the behaviour of the file handle.
522When LAYER is present using binmode on text file makes sense.
523
fae2c0fb 524If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
525suitable for passing binary data. This includes turning off possible CRLF
526translation and marking it as bytes (as opposed to Unicode characters).
749683d2 527Note that, despite what may be implied in I<"Programming Perl"> (the
165a9987
PJ
528Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
529-- other layers which would affect the binary nature of the stream are
0226bbdb
NIS
530I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
531PERLIO environment variable.
01e6739c 532
d807c6f4
JH
533The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
534form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
535establish default I/O layers. See L<open>.
536
fae2c0fb
RGS
537I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
538in "Programming Perl, 3rd Edition". However, since the publishing of this
539book, by many known as "Camel III", the consensus of the naming of this
540functionality has moved from "discipline" to "layer". All documentation
541of this version of Perl therefore refers to "layers" rather than to
542"disciplines". Now back to the regularly scheduled documentation...>
543
6902c96a
T
544To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
545C<:utf8> just marks the data as UTF-8 without further checking,
546while C<:encoding(utf8)> checks the data for actually being valid
547UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 548
ed53a2bb 549In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
550is done on the filehandle. Calling binmode() will normally flush any
551pending buffered output data (and perhaps pending input data) on the
fae2c0fb 552handle. An exception to this is the C<:encoding> layer that
01e6739c 553changes the default character encoding of the handle, see L<open>.
fae2c0fb 554The C<:encoding> layer sometimes needs to be called in
3874323d
JH
555mid-stream, and it doesn't flush the stream. The C<:encoding>
556also implicitly pushes on top of itself the C<:utf8> layer because
557internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 558
19799a22 559The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
560system all work together to let the programmer treat a single
561character (C<\n>) as the line terminator, irrespective of the external
562representation. On many operating systems, the native text file
563representation matches the internal representation, but on some
564platforms the external representation of C<\n> is made up of more than
565one character.
566
68bd7414
NIS
567Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
568character to end each line in the external representation of text (even
5e12dbfa 569though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
570on Unix and most VMS files). In other systems like OS/2, DOS and the
571various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
572but what's stored in text files are the two characters C<\cM\cJ>. That
573means that, if you don't use binmode() on these systems, C<\cM\cJ>
574sequences on disk will be converted to C<\n> on input, and any C<\n> in
575your program will be converted back to C<\cM\cJ> on output. This is what
576you want for text files, but it can be disastrous for binary files.
30168b04
GS
577
578Another consequence of using binmode() (on some systems) is that
579special end-of-file markers will be seen as part of the data stream.
580For systems from the Microsoft family this means that if your binary
4375e838 581data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
582the file, unless you use binmode().
583
584binmode() is not only important for readline() and print() operations,
585but also when using read(), seek(), sysread(), syswrite() and tell()
586(see L<perlport> for more details). See the C<$/> and C<$\> variables
587in L<perlvar> for how to manually set your input and output
588line-termination sequences.
a0d0e21e 589
4633a7c4 590=item bless REF,CLASSNAME
d74e8afc 591X<bless>
a0d0e21e
LW
592
593=item bless REF
594
2b5ab1e7
TC
595This function tells the thingy referenced by REF that it is now an object
596in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 597is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 598it returns the reference for convenience. Always use the two-argument
cf264981
SP
599version if a derived class might inherit the function doing the blessing.
600See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
601of objects.
a0d0e21e 602
57668c4d 603Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 604Namespaces with all lowercase names are considered reserved for
cf264981 605Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
606confusion, you may wish to avoid such package names as well. Make sure
607that CLASSNAME is a true value.
60ad88b8
GS
608
609See L<perlmod/"Perl Modules">.
610
0d863452
RH
611=item break
612
613Break out of a C<given()> block.
614
615This keyword is enabled by the "switch" feature: see L<feature>
616for more information.
617
a0d0e21e 618=item caller EXPR
d74e8afc 619X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
620
621=item caller
622
5a964f20 623Returns the context of the current subroutine call. In scalar context,
28757baa 624returns the caller's package name if there is a caller, that is, if
19799a22 625we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 626otherwise. In list context, returns
a0d0e21e 627
ee6b43cc 628 # 0 1 2
748a9306 629 ($package, $filename, $line) = caller;
a0d0e21e
LW
630
631With EXPR, it returns some extra information that the debugger uses to
632print a stack trace. The value of EXPR indicates how many call frames
633to go back before the current one.
634
ee6b43cc 635 # 0 1 2 3 4
f3aa04c2 636 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 637
638 # 5 6 7 8 9 10
b3ca2e83 639 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 640 = caller($i);
e7ea3e70 641
951ba7fe 642Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 643call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 644C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 645C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 646C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 647$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
648each C<use> statement creates a C<require> frame inside an C<eval EXPR>
649frame.) $subroutine may also be C<(unknown)> if this particular
650subroutine happens to have been deleted from the symbol table.
651C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
652C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
653compiled with. The C<$hints> and C<$bitmask> values are subject to change
654between versions of Perl, and are not meant for external use.
748a9306 655
b3ca2e83
NC
656C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
657caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
658of this hash, as they are the actual values stored in the optree.
659
748a9306 660Furthermore, when called from within the DB package, caller returns more
7660c0ab 661detailed information: it sets the list variable C<@DB::args> to be the
54310121 662arguments with which the subroutine was invoked.
748a9306 663
7660c0ab 664Be aware that the optimizer might have optimized call frames away before
19799a22 665C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 666might not return information about the call frame you expect it do, for
b76cc8ba 667C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 668previous time C<caller> was called.
7660c0ab 669
a0d0e21e 670=item chdir EXPR
d74e8afc
ITB
671X<chdir>
672X<cd>
f723aae1 673X<directory, change>
a0d0e21e 674
c4aca7d0
GA
675=item chdir FILEHANDLE
676
677=item chdir DIRHANDLE
678
ce2984c3
PF
679=item chdir
680
ffce7b87 681Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 682changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 683changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
684variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
685neither is set, C<chdir> does nothing. It returns true upon success,
686false otherwise. See the example under C<die>.
a0d0e21e 687
c4aca7d0
GA
688On systems that support fchdir, you might pass a file handle or
689directory handle as argument. On systems that don't support fchdir,
690passing handles produces a fatal error at run time.
691
a0d0e21e 692=item chmod LIST
d74e8afc 693X<chmod> X<permission> X<mode>
a0d0e21e
LW
694
695Changes the permissions of a list of files. The first element of the
4633a7c4 696list must be the numerical mode, which should probably be an octal
4ad40acf 697number, and which definitely should I<not> be a string of octal digits:
2f9daede 698C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 699successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
700
701 $cnt = chmod 0755, 'foo', 'bar';
702 chmod 0755, @executables;
f86cebdf
GS
703 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
704 # --w----r-T
2f9daede
TPG
705 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
706 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 707
c4aca7d0
GA
708On systems that support fchmod, you might pass file handles among the
709files. On systems that don't support fchmod, passing file handles
345da378
GA
710produces a fatal error at run time. The file handles must be passed
711as globs or references to be recognized. Barewords are considered
712file names.
c4aca7d0
GA
713
714 open(my $fh, "<", "foo");
715 my $perm = (stat $fh)[2] & 07777;
716 chmod($perm | 0600, $fh);
717
ca6e1c26
JH
718You can also import the symbolic C<S_I*> constants from the Fcntl
719module:
720
721 use Fcntl ':mode';
722
723 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
724 # This is identical to the chmod 0755 of the above example.
725
a0d0e21e 726=item chomp VARIABLE
d74e8afc 727X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 728
313c9f5c 729=item chomp( LIST )
a0d0e21e
LW
730
731=item chomp
732
2b5ab1e7
TC
733This safer version of L</chop> removes any trailing string
734that corresponds to the current value of C<$/> (also known as
28757baa
PP
735$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
736number of characters removed from all its arguments. It's often used to
737remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
738that the final record may be missing its newline. When in paragraph
739mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
740When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
741a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 742remove anything.
19799a22 743If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
744
745 while (<>) {
746 chomp; # avoid \n on last field
747 @array = split(/:/);
5a964f20 748 # ...
a0d0e21e
LW
749 }
750
4bf21a6d
RD
751If VARIABLE is a hash, it chomps the hash's values, but not its keys.
752
a0d0e21e
LW
753You can actually chomp anything that's an lvalue, including an assignment:
754
755 chomp($cwd = `pwd`);
756 chomp($answer = <STDIN>);
757
758If you chomp a list, each element is chomped, and the total number of
759characters removed is returned.
760
15e44fd8
RGS
761Note that parentheses are necessary when you're chomping anything
762that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
763is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
764C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
765C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
766as C<chomp($a, $b)>.
767
a0d0e21e 768=item chop VARIABLE
d74e8afc 769X<chop>
a0d0e21e 770
313c9f5c 771=item chop( LIST )
a0d0e21e
LW
772
773=item chop
774
775Chops off the last character of a string and returns the character
5b3eff12 776chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 777scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
778If VARIABLE is a hash, it chops the hash's values, but not its keys.
779
5b3eff12 780You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
781
782If you chop a list, each element is chopped. Only the value of the
19799a22 783last C<chop> is returned.
a0d0e21e 784
19799a22 785Note that C<chop> returns the last character. To return all but the last
748a9306
LW
786character, use C<substr($string, 0, -1)>.
787
15e44fd8
RGS
788See also L</chomp>.
789
a0d0e21e 790=item chown LIST
d74e8afc 791X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
792
793Changes the owner (and group) of a list of files. The first two
19799a22
GS
794elements of the list must be the I<numeric> uid and gid, in that
795order. A value of -1 in either position is interpreted by most
796systems to leave that value unchanged. Returns the number of files
797successfully changed.
a0d0e21e
LW
798
799 $cnt = chown $uid, $gid, 'foo', 'bar';
800 chown $uid, $gid, @filenames;
801
c4aca7d0
GA
802On systems that support fchown, you might pass file handles among the
803files. On systems that don't support fchown, passing file handles
345da378
GA
804produces a fatal error at run time. The file handles must be passed
805as globs or references to be recognized. Barewords are considered
806file names.
c4aca7d0 807
54310121 808Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
809
810 print "User: ";
19799a22 811 chomp($user = <STDIN>);
5a964f20 812 print "Files: ";
19799a22 813 chomp($pattern = <STDIN>);
a0d0e21e
LW
814
815 ($login,$pass,$uid,$gid) = getpwnam($user)
816 or die "$user not in passwd file";
817
5a964f20 818 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
819 chown $uid, $gid, @ary;
820
54310121 821On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
822file unless you're the superuser, although you should be able to change
823the group to any of your secondary groups. On insecure systems, these
824restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
825On POSIX systems, you can detect this condition this way:
826
827 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
828 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 829
a0d0e21e 830=item chr NUMBER
d74e8afc 831X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 832
54310121 833=item chr
bbce6d69 834
a0d0e21e 835Returns the character represented by that NUMBER in the character set.
a0ed51b3 836For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 837chr(0x263a) is a Unicode smiley face.
aaa68c4a 838
8a064bd6 839Negative values give the Unicode replacement character (chr(0xfffd)),
5f0135eb 840except under the L<bytes> pragma, where low eight bits of the value
8a064bd6
JH
841(truncated to an integer) are used.
842
974da8e5
JH
843If NUMBER is omitted, uses C<$_>.
844
b76cc8ba 845For the reverse, use L</ord>.
a0d0e21e 846
2575c402
JW
847Note that characters from 128 to 255 (inclusive) are by default
848internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 849
2575c402 850See L<perlunicode> for more about Unicode.
bbce6d69 851
a0d0e21e 852=item chroot FILENAME
d74e8afc 853X<chroot> X<root>
a0d0e21e 854
54310121 855=item chroot
bbce6d69 856
5a964f20 857This function works like the system call by the same name: it makes the
4633a7c4 858named directory the new root directory for all further pathnames that
951ba7fe 859begin with a C</> by your process and all its children. (It doesn't
28757baa 860change your current working directory, which is unaffected.) For security
4633a7c4 861reasons, this call is restricted to the superuser. If FILENAME is
19799a22 862omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
863
864=item close FILEHANDLE
d74e8afc 865X<close>
a0d0e21e 866
6a518fbc
TP
867=item close
868
e0f13c26
RGS
869Closes the file or pipe associated with the file handle, flushes the IO
870buffers, and closes the system file descriptor. Returns true if those
871operations have succeeded and if no error was reported by any PerlIO
872layer. Closes the currently selected filehandle if the argument is
873omitted.
fb73857a
PP
874
875You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
876another C<open> on it, because C<open> will close it for you. (See
877C<open>.) However, an explicit C<close> on an input file resets the line
878counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 879
dede8123
RGS
880If the file handle came from a piped open, C<close> will additionally
881return false if one of the other system calls involved fails, or if the
fb73857a 882program exits with non-zero status. (If the only problem was that the
dede8123 883program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 884also waits for the process executing on the pipe to complete, in case you
b76cc8ba 885want to look at the output of the pipe afterwards, and
e5218da5
GA
886implicitly puts the exit status value of that command into C<$?> and
887C<${^CHILD_ERROR_NATIVE}>.
5a964f20 888
73689b13
GS
889Prematurely closing the read end of a pipe (i.e. before the process
890writing to it at the other end has closed it) will result in a
891SIGPIPE being delivered to the writer. If the other end can't
892handle that, be sure to read all the data before closing the pipe.
893
fb73857a 894Example:
a0d0e21e 895
fb73857a
PP
896 open(OUTPUT, '|sort >foo') # pipe to sort
897 or die "Can't start sort: $!";
5a964f20 898 #... # print stuff to output
fb73857a
PP
899 close OUTPUT # wait for sort to finish
900 or warn $! ? "Error closing sort pipe: $!"
901 : "Exit status $? from sort";
902 open(INPUT, 'foo') # get sort's results
903 or die "Can't open 'foo' for input: $!";
a0d0e21e 904
5a964f20
TC
905FILEHANDLE may be an expression whose value can be used as an indirect
906filehandle, usually the real filehandle name.
a0d0e21e
LW
907
908=item closedir DIRHANDLE
d74e8afc 909X<closedir>
a0d0e21e 910
19799a22 911Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
912system call.
913
a0d0e21e 914=item connect SOCKET,NAME
d74e8afc 915X<connect>
a0d0e21e
LW
916
917Attempts to connect to a remote socket, just as the connect system call
19799a22 918does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
919packed address of the appropriate type for the socket. See the examples in
920L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 921
cb1a09d0 922=item continue BLOCK
d74e8afc 923X<continue>
cb1a09d0 924
0d863452
RH
925=item continue
926
cf264981
SP
927C<continue> is actually a flow control statement rather than a function. If
928there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
929C<foreach>), it is always executed just before the conditional is about to
930be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
931it can be used to increment a loop variable, even when the loop has been
932continued via the C<next> statement (which is similar to the C C<continue>
933statement).
934
98293880 935C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
936block. C<last> and C<redo> will behave as if they had been executed within
937the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
938block, it may be more entertaining.
939
940 while (EXPR) {
941 ### redo always comes here
942 do_something;
943 } continue {
944 ### next always comes here
945 do_something_else;
946 # then back the top to re-check EXPR
947 }
948 ### last always comes here
949
950Omitting the C<continue> section is semantically equivalent to using an
19799a22 951empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
952to check the condition at the top of the loop.
953
0d863452
RH
954If the "switch" feature is enabled, C<continue> is also a
955function that will break out of the current C<when> or C<default>
956block, and fall through to the next case. See L<feature> and
957L<perlsyn/"Switch statements"> for more information.
958
959
a0d0e21e 960=item cos EXPR
d74e8afc 961X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 962
d6217f1e
GS
963=item cos
964
5a964f20 965Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 966takes cosine of C<$_>.
a0d0e21e 967
ca6e1c26 968For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
969function, or use this relation:
970
971 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
972
a0d0e21e 973=item crypt PLAINTEXT,SALT
d74e8afc 974X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 975X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 976
ef2e6798
MS
977Creates a digest string exactly like the crypt(3) function in the C
978library (assuming that you actually have a version there that has not
cf264981 979been extirpated as a potential munitions).
ef2e6798
MS
980
981crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
982into a short string, called a digest, which is returned. The same
983PLAINTEXT and SALT will always return the same string, but there is no
984(known) way to get the original PLAINTEXT from the hash. Small
985changes in the PLAINTEXT or SALT will result in large changes in the
986digest.
987
988There is no decrypt function. This function isn't all that useful for
989cryptography (for that, look for F<Crypt> modules on your nearby CPAN
990mirror) and the name "crypt" is a bit of a misnomer. Instead it is
991primarily used to check if two pieces of text are the same without
992having to transmit or store the text itself. An example is checking
993if a correct password is given. The digest of the password is stored,
cf264981 994not the password itself. The user types in a password that is
ef2e6798
MS
995crypt()'d with the same salt as the stored digest. If the two digests
996match the password is correct.
997
998When verifying an existing digest string you should use the digest as
999the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1000to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1001crypt() will hash the new string with the same salt as the digest.
1002This allows your code to work with the standard L<crypt|/crypt> and
1003with more exotic implementations. In other words, do not assume
1004anything about the returned string itself, or how many bytes in the
1005digest matter.
85c16d83
JH
1006
1007Traditionally the result is a string of 13 bytes: two first bytes of
1008the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
ef2e6798
MS
1009the first eight bytes of the digest string mattered, but alternative
1010hashing schemes (like MD5), higher level security schemes (like C2),
1011and implementations on non-UNIX platforms may produce different
1012strings.
85c16d83
JH
1013
1014When choosing a new salt create a random two character string whose
1015characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1016'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1017characters is just a recommendation; the characters allowed in
1018the salt depend solely on your system's crypt library, and Perl can't
1019restrict what salts C<crypt()> accepts.
e71965be 1020
a0d0e21e 1021Here's an example that makes sure that whoever runs this program knows
cf264981 1022their password:
a0d0e21e
LW
1023
1024 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1025
1026 system "stty -echo";
1027 print "Password: ";
e71965be 1028 chomp($word = <STDIN>);
a0d0e21e
LW
1029 print "\n";
1030 system "stty echo";
1031
e71965be 1032 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
1033 die "Sorry...\n";
1034 } else {
1035 print "ok\n";
54310121 1036 }
a0d0e21e 1037
9f8f0c9d 1038Of course, typing in your own password to whoever asks you
748a9306 1039for it is unwise.
a0d0e21e 1040
ef2e6798 1041The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1042of data, not least of all because you can't get the information
ef2e6798 1043back. Look at the L<Digest> module for more robust algorithms.
19799a22 1044
f2791508
JH
1045If using crypt() on a Unicode string (which I<potentially> has
1046characters with codepoints above 255), Perl tries to make sense
1047of the situation by trying to downgrade (a copy of the string)
1048the string back to an eight-bit byte string before calling crypt()
1049(on that copy). If that works, good. If not, crypt() dies with
1050C<Wide character in crypt>.
85c16d83 1051
aa689395 1052=item dbmclose HASH
d74e8afc 1053X<dbmclose>
a0d0e21e 1054
19799a22 1055[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1056
aa689395 1057Breaks the binding between a DBM file and a hash.
a0d0e21e 1058
19799a22 1059=item dbmopen HASH,DBNAME,MASK
d74e8afc 1060X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1061
19799a22 1062[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1063
7b8d334a 1064This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1065hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1066argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1067is the name of the database (without the F<.dir> or F<.pag> extension if
1068any). If the database does not exist, it is created with protection
19799a22
GS
1069specified by MASK (as modified by the C<umask>). If your system supports
1070only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 1071program. In older versions of Perl, if your system had neither DBM nor
19799a22 1072ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1073sdbm(3).
1074
1075If you don't have write access to the DBM file, you can only read hash
1076variables, not set them. If you want to test whether you can write,
19799a22 1077either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 1078which will trap the error.
a0d0e21e 1079
19799a22
GS
1080Note that functions such as C<keys> and C<values> may return huge lists
1081when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1082function to iterate over large DBM files. Example:
1083
1084 # print out history file offsets
1085 dbmopen(%HIST,'/usr/lib/news/history',0666);
1086 while (($key,$val) = each %HIST) {
1087 print $key, ' = ', unpack('L',$val), "\n";
1088 }
1089 dbmclose(%HIST);
1090
cb1a09d0 1091See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1092cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1093rich implementation.
4633a7c4 1094
2b5ab1e7
TC
1095You can control which DBM library you use by loading that library
1096before you call dbmopen():
1097
1098 use DB_File;
1099 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1100 or die "Can't open netscape history file: $!";
1101
a0d0e21e 1102=item defined EXPR
d74e8afc 1103X<defined> X<undef> X<undefined>
a0d0e21e 1104
54310121 1105=item defined
bbce6d69 1106
2f9daede
TPG
1107Returns a Boolean value telling whether EXPR has a value other than
1108the undefined value C<undef>. If EXPR is not present, C<$_> will be
1109checked.
1110
1111Many operations return C<undef> to indicate failure, end of file,
1112system error, uninitialized variable, and other exceptional
1113conditions. This function allows you to distinguish C<undef> from
1114other values. (A simple Boolean test will not distinguish among
7660c0ab 1115C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1116false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1117doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1118returns C<undef> when its argument is an empty array, I<or> when the
1119element to return happens to be C<undef>.
1120
f10b0346
GS
1121You may also use C<defined(&func)> to check whether subroutine C<&func>
1122has ever been defined. The return value is unaffected by any forward
04891299 1123declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1124may still be callable: its package may have an C<AUTOLOAD> method that
1125makes it spring into existence the first time that it is called -- see
1126L<perlsub>.
f10b0346
GS
1127
1128Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1129used to report whether memory for that aggregate has ever been
1130allocated. This behavior may disappear in future versions of Perl.
1131You should instead use a simple test for size:
1132
1133 if (@an_array) { print "has array elements\n" }
1134 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1135
1136When used on a hash element, it tells you whether the value is defined,
dc848c6f 1137not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1138purpose.
a0d0e21e
LW
1139
1140Examples:
1141
1142 print if defined $switch{'D'};
1143 print "$val\n" while defined($val = pop(@ary));
1144 die "Can't readlink $sym: $!"
1145 unless defined($value = readlink $sym);
a0d0e21e 1146 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1147 $debugging = 0 unless defined $debugging;
a0d0e21e 1148
19799a22 1149Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1150discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1151defined values. For example, if you say
a5f75d66
AD
1152
1153 "ab" =~ /a(.*)b/;
1154
7660c0ab 1155The pattern match succeeds, and C<$1> is defined, despite the fact that it
cf264981 1156matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1157matched something that happened to be zero characters long. This is all
a5f75d66 1158very above-board and honest. When a function returns an undefined value,
2f9daede 1159it's an admission that it couldn't give you an honest answer. So you
19799a22 1160should use C<defined> only when you're questioning the integrity of what
7660c0ab 1161you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1162what you want.
1163
dc848c6f 1164See also L</undef>, L</exists>, L</ref>.
2f9daede 1165
a0d0e21e 1166=item delete EXPR
d74e8afc 1167X<delete>
a0d0e21e 1168
01020589
GS
1169Given an expression that specifies a hash element, array element, hash slice,
1170or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1171In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1172the size of the array will shrink to the highest element that tests
8216c1fd 1173true for exists() (or 0 if no such element exists).
a0d0e21e 1174
eba0920a
EM
1175Returns a list with the same number of elements as the number of elements
1176for which deletion was attempted. Each element of that list consists of
1177either the value of the element deleted, or the undefined value. In scalar
1178context, this means that you get the value of the last element deleted (or
1179the undefined value if that element did not exist).
1180
1181 %hash = (foo => 11, bar => 22, baz => 33);
1182 $scalar = delete $hash{foo}; # $scalar is 11
1183 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1184 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1185
1186Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1187a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1188from a C<tie>d hash or array may not necessarily return anything.
1189
8ea97a1e
GS
1190Deleting an array element effectively returns that position of the array
1191to its initial, uninitialized state. Subsequently testing for the same
cf264981
SP
1192element with exists() will return false. Also, deleting array elements
1193in the middle of an array will not shift the index of the elements
1194after them down. Use splice() for that. See L</exists>.
8ea97a1e 1195
01020589 1196The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1197
5f05dabc
PP
1198 foreach $key (keys %HASH) {
1199 delete $HASH{$key};
a0d0e21e
LW
1200 }
1201
01020589
GS
1202 foreach $index (0 .. $#ARRAY) {
1203 delete $ARRAY[$index];
1204 }
1205
1206And so do these:
5f05dabc 1207
01020589
GS
1208 delete @HASH{keys %HASH};
1209
9740c838 1210 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1211
2b5ab1e7 1212But both of these are slower than just assigning the empty list
01020589
GS
1213or undefining %HASH or @ARRAY:
1214
1215 %HASH = (); # completely empty %HASH
1216 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1217
01020589
GS
1218 @ARRAY = (); # completely empty @ARRAY
1219 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1220
1221Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1222operation is a hash element, array element, hash slice, or array slice
1223lookup:
a0d0e21e
LW
1224
1225 delete $ref->[$x][$y]{$key};
5f05dabc 1226 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1227
01020589
GS
1228 delete $ref->[$x][$y][$index];
1229 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1230
a0d0e21e 1231=item die LIST
d74e8afc 1232X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1233
19799a22
GS
1234Outside an C<eval>, prints the value of LIST to C<STDERR> and
1235exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1236exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1237status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1238an C<eval(),> the error message is stuffed into C<$@> and the
1239C<eval> is terminated with the undefined value. This makes
1240C<die> the way to raise an exception.
a0d0e21e
LW
1241
1242Equivalent examples:
1243
1244 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1245 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1246
ccac6780 1247If the last element of LIST does not end in a newline, the current
df37ec69
WW
1248script line number and input line number (if any) are also printed,
1249and a newline is supplied. Note that the "input line number" (also
1250known as "chunk") is subject to whatever notion of "line" happens to
1251be currently in effect, and is also available as the special variable
1252C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1253
1254Hint: sometimes appending C<", stopped"> to your message will cause it
1255to make better sense when the string C<"at foo line 123"> is appended.
1256Suppose you are running script "canasta".
a0d0e21e
LW
1257
1258 die "/etc/games is no good";
1259 die "/etc/games is no good, stopped";
1260
1261produce, respectively
1262
1263 /etc/games is no good at canasta line 123.
1264 /etc/games is no good, stopped at canasta line 123.
1265
2b5ab1e7 1266See also exit(), warn(), and the Carp module.
a0d0e21e 1267
7660c0ab
A
1268If LIST is empty and C<$@> already contains a value (typically from a
1269previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1270This is useful for propagating exceptions:
1271
1272 eval { ... };
1273 die unless $@ =~ /Expected exception/;
1274
ad216e65
JH
1275If LIST is empty and C<$@> contains an object reference that has a
1276C<PROPAGATE> method, that method will be called with additional file
1277and line number parameters. The return value replaces the value in
28a5cf3b 1278C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1279were called.
1280
7660c0ab 1281If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1282
52531d10
GS
1283die() can also be called with a reference argument. If this happens to be
1284trapped within an eval(), $@ contains the reference. This behavior permits
1285a more elaborate exception handling implementation using objects that
4375e838 1286maintain arbitrary state about the nature of the exception. Such a scheme
52531d10 1287is sometimes preferable to matching particular string values of $@ using
746d7dd7
GL
1288regular expressions. Because $@ is a global variable, and eval() may be
1289used within object implementations, care must be taken that analyzing the
1290error object doesn't replace the reference in the global variable. The
1291easiest solution is to make a local copy of the reference before doing
1292other manipulations. Here's an example:
52531d10 1293
da279afe 1294 use Scalar::Util 'blessed';
1295
52531d10 1296 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1297 if (my $ev_err = $@) {
1298 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1299 # handle Some::Module::Exception
1300 }
1301 else {
1302 # handle all other possible exceptions
1303 }
1304 }
1305
19799a22 1306Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1307them, you may want to overload stringification operations on such custom
1308exception objects. See L<overload> for details about that.
1309
19799a22
GS
1310You can arrange for a callback to be run just before the C<die>
1311does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1312handler will be called with the error text and can change the error
1313message, if it sees fit, by calling C<die> again. See
1314L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1315L<"eval BLOCK"> for some examples. Although this feature was
19799a22
GS
1316to be run only right before your program was to exit, this is not
1317currently the case--the C<$SIG{__DIE__}> hook is currently called
1318even inside eval()ed blocks/strings! If one wants the hook to do
1319nothing in such situations, put
fb73857a
PP
1320
1321 die @_ if $^S;
1322
19799a22
GS
1323as the first line of the handler (see L<perlvar/$^S>). Because
1324this promotes strange action at a distance, this counterintuitive
b76cc8ba 1325behavior may be fixed in a future release.
774d564b 1326
a0d0e21e 1327=item do BLOCK
d74e8afc 1328X<do> X<block>
a0d0e21e
LW
1329
1330Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1331sequence of commands indicated by BLOCK. When modified by the C<while> or
1332C<until> loop modifier, executes the BLOCK once before testing the loop
1333condition. (On other statements the loop modifiers test the conditional
1334first.)
a0d0e21e 1335
4968c1e4 1336C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1337C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1338See L<perlsyn> for alternative strategies.
4968c1e4 1339
a0d0e21e 1340=item do SUBROUTINE(LIST)
d74e8afc 1341X<do>
a0d0e21e 1342
cf264981 1343This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1344
1345=item do EXPR
d74e8afc 1346X<do>
a0d0e21e
LW
1347
1348Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1349file as a Perl script.
a0d0e21e
LW
1350
1351 do 'stat.pl';
1352
1353is just like
1354
986b19de 1355 eval `cat stat.pl`;
a0d0e21e 1356
2b5ab1e7 1357except that it's more efficient and concise, keeps track of the current
ea63ef19 1358filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1359C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1360variables. It also differs in that code evaluated with C<do FILENAME>
1361cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1362same, however, in that it does reparse the file every time you call it,
1363so you probably don't want to do this inside a loop.
a0d0e21e 1364
8e30cc93 1365If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1366error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1367returns undef and sets an error message in C<$@>. If the file is
1368successfully compiled, C<do> returns the value of the last expression
1369evaluated.
1370
a0d0e21e 1371Note that inclusion of library modules is better done with the
19799a22 1372C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1373and raise an exception if there's a problem.
a0d0e21e 1374
5a964f20
TC
1375You might like to use C<do> to read in a program configuration
1376file. Manual error checking can be done this way:
1377
b76cc8ba 1378 # read in config files: system first, then user
f86cebdf 1379 for $file ("/share/prog/defaults.rc",
b76cc8ba 1380 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1381 {
5a964f20 1382 unless ($return = do $file) {
f86cebdf
GS
1383 warn "couldn't parse $file: $@" if $@;
1384 warn "couldn't do $file: $!" unless defined $return;
1385 warn "couldn't run $file" unless $return;
5a964f20
TC
1386 }
1387 }
1388
a0d0e21e 1389=item dump LABEL
d74e8afc 1390X<dump> X<core> X<undump>
a0d0e21e 1391
1614b0e3
JD
1392=item dump
1393
19799a22
GS
1394This function causes an immediate core dump. See also the B<-u>
1395command-line switch in L<perlrun>, which does the same thing.
1396Primarily this is so that you can use the B<undump> program (not
1397supplied) to turn your core dump into an executable binary after
1398having initialized all your variables at the beginning of the
1399program. When the new binary is executed it will begin by executing
1400a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1401Think of it as a goto with an intervening core dump and reincarnation.
1402If C<LABEL> is omitted, restarts the program from the top.
1403
1404B<WARNING>: Any files opened at the time of the dump will I<not>
1405be open any more when the program is reincarnated, with possible
b76cc8ba 1406resulting confusion on the part of Perl.
19799a22 1407
59f521f4
RGS
1408This function is now largely obsolete, mostly because it's very hard to
1409convert a core file into an executable. That's why you should now invoke
1410it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1411typo.
19799a22 1412
aa689395 1413=item each HASH
d74e8afc 1414X<each> X<hash, iterator>
aa689395 1415
5a964f20 1416When called in list context, returns a 2-element list consisting of the
aa689395 1417key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1418it. When called in scalar context, returns only the key for the next
e902a979 1419element in the hash.
2f9daede 1420
ab192400 1421Entries are returned in an apparently random order. The actual random
504f80c1
JH
1422order is subject to change in future versions of perl, but it is
1423guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1424function would produce on the same (unmodified) hash. Since Perl
14255.8.1 the ordering is different even between different runs of Perl
1426for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1427
1428When the hash is entirely read, a null array is returned in list context
19799a22
GS
1429(which when assigned produces a false (C<0>) value), and C<undef> in
1430scalar context. The next call to C<each> after that will start iterating
1431again. There is a single iterator for each hash, shared by all C<each>,
1432C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1433reading all the elements from the hash, or by evaluating C<keys HASH> or
1434C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1435iterating over it, you may get entries skipped or duplicated, so
1436don't. Exception: It is always safe to delete the item most recently
1437returned by C<each()>, which means that the following code will work:
1438
1439 while (($key, $value) = each %hash) {
1440 print $key, "\n";
1441 delete $hash{$key}; # This is safe
1442 }
aa689395 1443
f86cebdf 1444The following prints out your environment like the printenv(1) program,
aa689395 1445only in a different order:
a0d0e21e
LW
1446
1447 while (($key,$value) = each %ENV) {
1448 print "$key=$value\n";
1449 }
1450
19799a22 1451See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1452
1453=item eof FILEHANDLE
d74e8afc
ITB
1454X<eof>
1455X<end of file>
1456X<end-of-file>
a0d0e21e 1457
4633a7c4
LW
1458=item eof ()
1459
a0d0e21e
LW
1460=item eof
1461
1462Returns 1 if the next read on FILEHANDLE will return end of file, or if
1463FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1464gives the real filehandle. (Note that this function actually
19799a22 1465reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1466interactive context.) Do not read from a terminal file (or call
19799a22 1467C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1468as terminals may lose the end-of-file condition if you do.
1469
820475bd
GS
1470An C<eof> without an argument uses the last file read. Using C<eof()>
1471with empty parentheses is very different. It refers to the pseudo file
1472formed from the files listed on the command line and accessed via the
61eff3bc
JH
1473C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1474as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1475used will cause C<@ARGV> to be examined to determine if input is
67408cae 1476available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1477end-of-file will assume you are processing another C<@ARGV> list,
1478and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1479see L<perlop/"I/O Operators">.
820475bd 1480
61eff3bc 1481In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1482detect the end of each file, C<eof()> will only detect the end of the
1483last file. Examples:
a0d0e21e 1484
748a9306
LW
1485 # reset line numbering on each input file
1486 while (<>) {
b76cc8ba 1487 next if /^\s*#/; # skip comments
748a9306 1488 print "$.\t$_";
5a964f20
TC
1489 } continue {
1490 close ARGV if eof; # Not eof()!
748a9306
LW
1491 }
1492
a0d0e21e
LW
1493 # insert dashes just before last line of last file
1494 while (<>) {
6ac88b13 1495 if (eof()) { # check for end of last file
a0d0e21e
LW
1496 print "--------------\n";
1497 }
1498 print;
6ac88b13 1499 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1500 }
1501
a0d0e21e 1502Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1503input operators typically return C<undef> when they run out of data, or if
1504there was an error.
a0d0e21e
LW
1505
1506=item eval EXPR
d74e8afc 1507X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1508X<error, handling> X<exception, handling>
a0d0e21e
LW
1509
1510=item eval BLOCK
1511
ce2984c3
PF
1512=item eval
1513
c7cc6f1c
GS
1514In the first form, the return value of EXPR is parsed and executed as if it
1515were a little Perl program. The value of the expression (which is itself
5a964f20 1516determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1517errors, executed in the lexical context of the current Perl program, so
1518that any variable settings or subroutine and format definitions remain
cf264981 1519afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1520If EXPR is omitted, evaluates C<$_>. This form is typically used to
1521delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1522
1523In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1524same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1525within the context of the current Perl program. This form is typically
1526used to trap exceptions more efficiently than the first (see below), while
1527also providing the benefit of checking the code within BLOCK at compile
1528time.
1529
1530The final semicolon, if any, may be omitted from the value of EXPR or within
1531the BLOCK.
1532
1533In both forms, the value returned is the value of the last expression
5a964f20 1534evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1535as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1536in void, scalar, or list context, depending on the context of the C<eval>
1537itself. See L</wantarray> for more on how the evaluation context can be
1538determined.
a0d0e21e 1539
19799a22
GS
1540If there is a syntax error or runtime error, or a C<die> statement is
1541executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1542error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1543string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1544warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1545To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1546turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1547See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1548
19799a22
GS
1549Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1550determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1551is implemented. It is also Perl's exception trapping mechanism, where
1552the die operator is used to raise exceptions.
1553
1554If the code to be executed doesn't vary, you may use the eval-BLOCK
1555form to trap run-time errors without incurring the penalty of
1556recompiling each time. The error, if any, is still returned in C<$@>.
1557Examples:
1558
54310121 1559 # make divide-by-zero nonfatal
a0d0e21e
LW
1560 eval { $answer = $a / $b; }; warn $@ if $@;
1561
1562 # same thing, but less efficient
1563 eval '$answer = $a / $b'; warn $@ if $@;
1564
1565 # a compile-time error
5a964f20 1566 eval { $answer = }; # WRONG
a0d0e21e
LW
1567
1568 # a run-time error
1569 eval '$answer ='; # sets $@
1570
cf264981
SP
1571Using the C<eval{}> form as an exception trap in libraries does have some
1572issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1573may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7
TC
1574You can use the C<local $SIG{__DIE__}> construct for this purpose,
1575as shown in this example:
774d564b
PP
1576
1577 # a very private exception trap for divide-by-zero
f86cebdf
GS
1578 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1579 warn $@ if $@;
774d564b
PP
1580
1581This is especially significant, given that C<__DIE__> hooks can call
19799a22 1582C<die> again, which has the effect of changing their error messages:
774d564b
PP
1583
1584 # __DIE__ hooks may modify error messages
1585 {
f86cebdf
GS
1586 local $SIG{'__DIE__'} =
1587 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1588 eval { die "foo lives here" };
1589 print $@ if $@; # prints "bar lives here"
774d564b
PP
1590 }
1591
19799a22 1592Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1593may be fixed in a future release.
1594
19799a22 1595With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1596being looked at when:
1597
1598 eval $x; # CASE 1
1599 eval "$x"; # CASE 2
1600
1601 eval '$x'; # CASE 3
1602 eval { $x }; # CASE 4
1603
5a964f20 1604 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1605 $$x++; # CASE 6
1606
2f9daede 1607Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1608the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1609the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1610and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1611does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1612purely visual reasons, but it also has the advantage of compiling at
1613compile-time instead of at run-time.) Case 5 is a place where
19799a22 1614normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1615particular situation, you can just use symbolic references instead, as
1616in case 6.
a0d0e21e 1617
4968c1e4 1618C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1619C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1620
d819b83a
DM
1621Note that as a very special case, an C<eval ''> executed within the C<DB>
1622package doesn't see the usual surrounding lexical scope, but rather the
1623scope of the first non-DB piece of code that called it. You don't normally
1624need to worry about this unless you are writing a Perl debugger.
1625
a0d0e21e 1626=item exec LIST
d74e8afc 1627X<exec> X<execute>
a0d0e21e 1628
8bf3b016
GS
1629=item exec PROGRAM LIST
1630
19799a22
GS
1631The C<exec> function executes a system command I<and never returns>--
1632use C<system> instead of C<exec> if you want it to return. It fails and
1633returns false only if the command does not exist I<and> it is executed
fb73857a 1634directly instead of via your system's command shell (see below).
a0d0e21e 1635
19799a22
GS
1636Since it's a common mistake to use C<exec> instead of C<system>, Perl
1637warns you if there is a following statement which isn't C<die>, C<warn>,
1638or C<exit> (if C<-w> is set - but you always do that). If you
1639I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1640can use one of these styles to avoid the warning:
1641
5a964f20
TC
1642 exec ('foo') or print STDERR "couldn't exec foo: $!";
1643 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1644
5a964f20 1645If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1646with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1647If there is only one scalar argument or an array with one element in it,
1648the argument is checked for shell metacharacters, and if there are any,
1649the entire argument is passed to the system's command shell for parsing
1650(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1651If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1652words and passed directly to C<execvp>, which is more efficient.
19799a22 1653Examples:
a0d0e21e 1654
19799a22
GS
1655 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1656 exec "sort $outfile | uniq";
a0d0e21e
LW
1657
1658If you don't really want to execute the first argument, but want to lie
1659to the program you are executing about its own name, you can specify
1660the program you actually want to run as an "indirect object" (without a
1661comma) in front of the LIST. (This always forces interpretation of the
54310121 1662LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1663the list.) Example:
1664
1665 $shell = '/bin/csh';
1666 exec $shell '-sh'; # pretend it's a login shell
1667
1668or, more directly,
1669
1670 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1671
bb32b41a
GS
1672When the arguments get executed via the system shell, results will
1673be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1674for details.
1675
19799a22
GS
1676Using an indirect object with C<exec> or C<system> is also more
1677secure. This usage (which also works fine with system()) forces
1678interpretation of the arguments as a multivalued list, even if the
1679list had just one argument. That way you're safe from the shell
1680expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1681
1682 @args = ( "echo surprise" );
1683
2b5ab1e7 1684 exec @args; # subject to shell escapes
f86cebdf 1685 # if @args == 1
2b5ab1e7 1686 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1687
1688The first version, the one without the indirect object, ran the I<echo>
1689program, passing it C<"surprise"> an argument. The second version
1690didn't--it tried to run a program literally called I<"echo surprise">,
1691didn't find it, and set C<$?> to a non-zero value indicating failure.
1692
0f897271
GS
1693Beginning with v5.6.0, Perl will attempt to flush all files opened for
1694output before the exec, but this may not be supported on some platforms
1695(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1696in English) or call the C<autoflush()> method of C<IO::Handle> on any
1697open handles in order to avoid lost output.
1698
19799a22 1699Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1700any C<DESTROY> methods in your objects.
1701
a0d0e21e 1702=item exists EXPR
d74e8afc 1703X<exists> X<autovivification>
a0d0e21e 1704
01020589 1705Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1706returns true if the specified element in the hash or array has ever
1707been initialized, even if the corresponding value is undefined. The
1708element is not autovivified if it doesn't exist.
a0d0e21e 1709
01020589
GS
1710 print "Exists\n" if exists $hash{$key};
1711 print "Defined\n" if defined $hash{$key};
1712 print "True\n" if $hash{$key};
1713
1714 print "Exists\n" if exists $array[$index];
1715 print "Defined\n" if defined $array[$index];
1716 print "True\n" if $array[$index];
a0d0e21e 1717
8ea97a1e 1718A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1719it exists, but the reverse doesn't necessarily hold true.
1720
afebc493
GS
1721Given an expression that specifies the name of a subroutine,
1722returns true if the specified subroutine has ever been declared, even
1723if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1724does not count as declaring it. Note that a subroutine which does not
1725exist may still be callable: its package may have an C<AUTOLOAD>
1726method that makes it spring into existence the first time that it is
1727called -- see L<perlsub>.
afebc493
GS
1728
1729 print "Exists\n" if exists &subroutine;
1730 print "Defined\n" if defined &subroutine;
1731
a0d0e21e 1732Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1733operation is a hash or array key lookup or subroutine name:
a0d0e21e 1734
2b5ab1e7
TC
1735 if (exists $ref->{A}->{B}->{$key}) { }
1736 if (exists $hash{A}{B}{$key}) { }
1737
01020589
GS
1738 if (exists $ref->{A}->{B}->[$ix]) { }
1739 if (exists $hash{A}{B}[$ix]) { }
1740
afebc493
GS
1741 if (exists &{$ref->{A}{B}{$key}}) { }
1742
01020589
GS
1743Although the deepest nested array or hash will not spring into existence
1744just because its existence was tested, any intervening ones will.
61eff3bc 1745Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1746into existence due to the existence test for the $key element above.
1747This happens anywhere the arrow operator is used, including even:
5a964f20 1748
2b5ab1e7
TC
1749 undef $ref;
1750 if (exists $ref->{"Some key"}) { }
1751 print $ref; # prints HASH(0x80d3d5c)
1752
1753This surprising autovivification in what does not at first--or even
1754second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1755release.
a0d0e21e 1756
afebc493
GS
1757Use of a subroutine call, rather than a subroutine name, as an argument
1758to exists() is an error.
1759
1760 exists &sub; # OK
1761 exists &sub(); # Error
1762
a0d0e21e 1763=item exit EXPR
d74e8afc 1764X<exit> X<terminate> X<abort>
a0d0e21e 1765
ce2984c3
PF
1766=item exit
1767
2b5ab1e7 1768Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1769
1770 $ans = <STDIN>;
1771 exit 0 if $ans =~ /^[Xx]/;
1772
19799a22 1773See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1774universally recognized values for EXPR are C<0> for success and C<1>
1775for error; other values are subject to interpretation depending on the
1776environment in which the Perl program is running. For example, exiting
177769 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1778the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1779
19799a22
GS
1780Don't use C<exit> to abort a subroutine if there's any chance that
1781someone might want to trap whatever error happened. Use C<die> instead,
1782which can be trapped by an C<eval>.
28757baa 1783
19799a22 1784The exit() function does not always exit immediately. It calls any
2b5ab1e7 1785defined C<END> routines first, but these C<END> routines may not
19799a22 1786themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1787be called are called before the real exit. If this is a problem, you
1788can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1789See L<perlmod> for details.
5a964f20 1790
a0d0e21e 1791=item exp EXPR
d74e8afc 1792X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1793
54310121 1794=item exp
bbce6d69 1795
b76cc8ba 1796Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1797If EXPR is omitted, gives C<exp($_)>.
1798
1799=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1800X<fcntl>
a0d0e21e 1801
f86cebdf 1802Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1803
1804 use Fcntl;
1805
0ade1984 1806first to get the correct constant definitions. Argument processing and
b76cc8ba 1807value return works just like C<ioctl> below.
a0d0e21e
LW
1808For example:
1809
1810 use Fcntl;
5a964f20
TC
1811 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1812 or die "can't fcntl F_GETFL: $!";
1813
554ad1fc 1814You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1815Like C<ioctl>, it maps a C<0> return from the system call into
1816C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1817in numeric context. It is also exempt from the normal B<-w> warnings
1818on improper numeric conversions.
5a964f20 1819
19799a22 1820Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1821doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1822manpage to learn what functions are available on your system.
a0d0e21e 1823
be2f7487 1824Here's an example of setting a filehandle named C<REMOTE> to be
1825non-blocking at the system level. You'll have to negotiate C<$|>
1826on your own, though.
1827
1828 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1829
1830 $flags = fcntl(REMOTE, F_GETFL, 0)
1831 or die "Can't get flags for the socket: $!\n";
1832
1833 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1834 or die "Can't set flags for the socket: $!\n";
1835
a0d0e21e 1836=item fileno FILEHANDLE
d74e8afc 1837X<fileno>
a0d0e21e 1838
2b5ab1e7
TC
1839Returns the file descriptor for a filehandle, or undefined if the
1840filehandle is not open. This is mainly useful for constructing
19799a22 1841bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1842If FILEHANDLE is an expression, the value is taken as an indirect
1843filehandle, generally its name.
5a964f20 1844
b76cc8ba 1845You can use this to find out whether two handles refer to the
5a964f20
TC
1846same underlying descriptor:
1847
1848 if (fileno(THIS) == fileno(THAT)) {
1849 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1850 }
1851
1852(Filehandles connected to memory objects via new features of C<open> may
1853return undefined even though they are open.)
1854
a0d0e21e
LW
1855
1856=item flock FILEHANDLE,OPERATION
d74e8afc 1857X<flock> X<lock> X<locking>
a0d0e21e 1858
19799a22
GS
1859Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1860for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1861machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1862C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1863only entire files, not records.
1864
1865Two potentially non-obvious but traditional C<flock> semantics are
1866that it waits indefinitely until the lock is granted, and that its locks
1867B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1868fewer guarantees. This means that programs that do not also use C<flock>
1869may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1870your port's specific documentation, or your system-specific local manpages
1871for details. It's best to assume traditional behavior if you're writing
1872portable programs. (But if you're not, you should as always feel perfectly
1873free to write for your own system's idiosyncrasies (sometimes called
1874"features"). Slavish adherence to portability concerns shouldn't get
1875in the way of your getting your job done.)
a3cb178b 1876
8ebc5c01
PP
1877OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1878LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1879you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1880either individually, or as a group using the ':flock' tag. LOCK_SH
1881requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1882releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1883LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1884waiting for the lock (check the return status to see if you got it).
1885
2b5ab1e7
TC
1886To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1887before locking or unlocking it.
8ebc5c01 1888
f86cebdf 1889Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1890locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1891are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1892implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1893differing semantics shouldn't bite too many people.
1894
becacb53
TM
1895Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1896be open with read intent to use LOCK_SH and requires that it be open
1897with write intent to use LOCK_EX.
1898
19799a22
GS
1899Note also that some versions of C<flock> cannot lock things over the
1900network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1901that. If you like you can force Perl to ignore your system's flock(2)
1902function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1903the switch C<-Ud_flock> to the F<Configure> program when you configure
1904perl.
4633a7c4
LW
1905
1906Here's a mailbox appender for BSD systems.
a0d0e21e 1907
7e1af8bc 1908 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1909
1910 sub lock {
7e1af8bc 1911 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1912 # and, in case someone appended
1913 # while we were waiting...
1914 seek(MBOX, 0, 2);
1915 }
1916
1917 sub unlock {
7e1af8bc 1918 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1919 }
1920
1921 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1922 or die "Can't open mailbox: $!";
1923
1924 lock();
1925 print MBOX $msg,"\n\n";
1926 unlock();
1927
2b5ab1e7
TC
1928On systems that support a real flock(), locks are inherited across fork()
1929calls, whereas those that must resort to the more capricious fcntl()
1930function lose the locks, making it harder to write servers.
1931
cb1a09d0 1932See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1933
1934=item fork
d74e8afc 1935X<fork> X<child> X<parent>
a0d0e21e 1936
2b5ab1e7
TC
1937Does a fork(2) system call to create a new process running the
1938same program at the same point. It returns the child pid to the
1939parent process, C<0> to the child process, or C<undef> if the fork is
1940unsuccessful. File descriptors (and sometimes locks on those descriptors)
1941are shared, while everything else is copied. On most systems supporting
1942fork(), great care has gone into making it extremely efficient (for
1943example, using copy-on-write technology on data pages), making it the
1944dominant paradigm for multitasking over the last few decades.
5a964f20 1945
0f897271
GS
1946Beginning with v5.6.0, Perl will attempt to flush all files opened for
1947output before forking the child process, but this may not be supported
1948on some platforms (see L<perlport>). To be safe, you may need to set
1949C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1950C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1951
19799a22 1952If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1953accumulate zombies. On some systems, you can avoid this by setting
1954C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1955forking and reaping moribund children.
cb1a09d0 1956
28757baa
PP
1957Note that if your forked child inherits system file descriptors like
1958STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1959if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1960backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1961You should reopen those to F</dev/null> if it's any issue.
28757baa 1962
cb1a09d0 1963=item format
d74e8afc 1964X<format>
cb1a09d0 1965
19799a22 1966Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1967example:
1968
54310121 1969 format Something =
cb1a09d0
AD
1970 Test: @<<<<<<<< @||||| @>>>>>
1971 $str, $%, '$' . int($num)
1972 .
1973
1974 $str = "widget";
184e9718 1975 $num = $cost/$quantity;
cb1a09d0
AD
1976 $~ = 'Something';
1977 write;
1978
1979See L<perlform> for many details and examples.
1980
8903cb82 1981=item formline PICTURE,LIST
d74e8afc 1982X<formline>
a0d0e21e 1983
5a964f20 1984This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1985too. It formats (see L<perlform>) a list of values according to the
1986contents of PICTURE, placing the output into the format output
7660c0ab 1987accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1988Eventually, when a C<write> is done, the contents of
cf264981
SP
1989C<$^A> are written to some filehandle. You could also read C<$^A>
1990and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1991does one C<formline> per line of form, but the C<formline> function itself
748a9306 1992doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1993that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1994You may therefore need to use multiple formlines to implement a single
1995record format, just like the format compiler.
1996
19799a22 1997Be careful if you put double quotes around the picture, because an C<@>
748a9306 1998character may be taken to mean the beginning of an array name.
19799a22 1999C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
2000
2001=item getc FILEHANDLE
f723aae1 2002X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2003
2004=item getc
2005
2006Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
2007or the undefined value at end of file, or if there was an error (in
2008the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2009STDIN. This is not particularly efficient. However, it cannot be
2010used by itself to fetch single characters without waiting for the user
2011to hit enter. For that, try something more like:
4633a7c4
LW
2012
2013 if ($BSD_STYLE) {
2014 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2015 }
2016 else {
54310121 2017 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2018 }
2019
2020 $key = getc(STDIN);
2021
2022 if ($BSD_STYLE) {
2023 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2024 }
2025 else {
5f05dabc 2026 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
2027 }
2028 print "\n";
2029
54310121
PP
2030Determination of whether $BSD_STYLE should be set
2031is left as an exercise to the reader.
cb1a09d0 2032
19799a22 2033The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2034systems purporting POSIX compliance. See also the C<Term::ReadKey>
2035module from your nearest CPAN site; details on CPAN can be found on
2036L<perlmodlib/CPAN>.
a0d0e21e
LW
2037
2038=item getlogin
d74e8afc 2039X<getlogin> X<login>
a0d0e21e 2040
cf264981 2041This implements the C library function of the same name, which on most
5a964f20 2042systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 2043use C<getpwuid>.
a0d0e21e 2044
f86702cc 2045 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2046
19799a22
GS
2047Do not consider C<getlogin> for authentication: it is not as
2048secure as C<getpwuid>.
4633a7c4 2049
a0d0e21e 2050=item getpeername SOCKET
d74e8afc 2051X<getpeername> X<peer>
a0d0e21e
LW
2052
2053Returns the packed sockaddr address of other end of the SOCKET connection.
2054
4633a7c4
LW
2055 use Socket;
2056 $hersockaddr = getpeername(SOCK);
19799a22 2057 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2058 $herhostname = gethostbyaddr($iaddr, AF_INET);
2059 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2060
2061=item getpgrp PID
d74e8afc 2062X<getpgrp> X<group>
a0d0e21e 2063
47e29363 2064Returns the current process group for the specified PID. Use
7660c0ab 2065a PID of C<0> to get the current process group for the
4633a7c4 2066current process. Will raise an exception if used on a machine that
f86cebdf 2067doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2068group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2069does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2070
2071=item getppid
d74e8afc 2072X<getppid> X<parent> X<pid>
a0d0e21e
LW
2073
2074Returns the process id of the parent process.
2075
4d76a344
RGS
2076Note for Linux users: on Linux, the C functions C<getpid()> and
2077C<getppid()> return different values from different threads. In order to
2078be portable, this behavior is not reflected by the perl-level function
2079C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2080to call the underlying C<getppid()>, you may use the CPAN module
2081C<Linux::Pid>.
4d76a344 2082
a0d0e21e 2083=item getpriority WHICH,WHO
d74e8afc 2084X<getpriority> X<priority> X<nice>
a0d0e21e 2085
4633a7c4
LW
2086Returns the current priority for a process, a process group, or a user.
2087(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2088machine that doesn't implement getpriority(2).
a0d0e21e
LW
2089
2090=item getpwnam NAME
d74e8afc
ITB
2091X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2092X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2093X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2094X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2095X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2096X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2097
2098=item getgrnam NAME
2099
2100=item gethostbyname NAME
2101
2102=item getnetbyname NAME
2103
2104=item getprotobyname NAME
2105
2106=item getpwuid UID
2107
2108=item getgrgid GID
2109
2110=item getservbyname NAME,PROTO
2111
2112=item gethostbyaddr ADDR,ADDRTYPE
2113
2114=item getnetbyaddr ADDR,ADDRTYPE
2115
2116=item getprotobynumber NUMBER
2117
2118=item getservbyport PORT,PROTO
2119
2120=item getpwent
2121
2122=item getgrent
2123
2124=item gethostent
2125
2126=item getnetent
2127
2128=item getprotoent
2129
2130=item getservent
2131
2132=item setpwent
2133
2134=item setgrent
2135
2136=item sethostent STAYOPEN
2137
2138=item setnetent STAYOPEN
2139
2140=item setprotoent STAYOPEN
2141
2142=item setservent STAYOPEN
2143
2144=item endpwent
2145
2146=item endgrent
2147
2148=item endhostent
2149
2150=item endnetent
2151
2152=item endprotoent
2153
2154=item endservent
2155
2156These routines perform the same functions as their counterparts in the
5a964f20 2157system library. In list context, the return values from the
a0d0e21e
LW
2158various get routines are as follows:
2159
2160 ($name,$passwd,$uid,$gid,
6ee623d5 2161 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2162 ($name,$passwd,$gid,$members) = getgr*
2163 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2164 ($name,$aliases,$addrtype,$net) = getnet*
2165 ($name,$aliases,$proto) = getproto*
2166 ($name,$aliases,$port,$proto) = getserv*
2167
2168(If the entry doesn't exist you get a null list.)
2169
4602f195
JH
2170The exact meaning of the $gcos field varies but it usually contains
2171the real name of the user (as opposed to the login name) and other
2172information pertaining to the user. Beware, however, that in many
2173system users are able to change this information and therefore it
106325ad 2174cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2175L<perlsec>). The $passwd and $shell, user's encrypted password and
2176login shell, are also tainted, because of the same reason.
4602f195 2177
5a964f20 2178In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2179lookup by name, in which case you get the other thing, whatever it is.
2180(If the entry doesn't exist you get the undefined value.) For example:
2181
5a964f20
TC
2182 $uid = getpwnam($name);
2183 $name = getpwuid($num);
2184 $name = getpwent();
2185 $gid = getgrnam($name);
08a33e13 2186 $name = getgrgid($num);
5a964f20
TC
2187 $name = getgrent();
2188 #etc.
a0d0e21e 2189
4602f195
JH
2190In I<getpw*()> the fields $quota, $comment, and $expire are special
2191cases in the sense that in many systems they are unsupported. If the
2192$quota is unsupported, it is an empty scalar. If it is supported, it
2193usually encodes the disk quota. If the $comment field is unsupported,
2194it is an empty scalar. If it is supported it usually encodes some
2195administrative comment about the user. In some systems the $quota
2196field may be $change or $age, fields that have to do with password
2197aging. In some systems the $comment field may be $class. The $expire
2198field, if present, encodes the expiration period of the account or the
2199password. For the availability and the exact meaning of these fields
2200in your system, please consult your getpwnam(3) documentation and your
2201F<pwd.h> file. You can also find out from within Perl what your
2202$quota and $comment fields mean and whether you have the $expire field
2203by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2204C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2205files are only supported if your vendor has implemented them in the
2206intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2207shadow versions if you're running under privilege or if there exists
cf264981
SP
2208the shadow(3) functions as found in System V (this includes Solaris
2209and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2210facility are unlikely to be supported.
6ee623d5 2211
19799a22 2212The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2213the login names of the members of the group.
2214
2215For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2216C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2217C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2218addresses returned by the corresponding system library call. In the
2219Internet domain, each address is four bytes long and you can unpack it
2220by saying something like:
2221
f337b084 2222 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2223
2b5ab1e7
TC
2224The Socket library makes this slightly easier:
2225
2226 use Socket;
2227 $iaddr = inet_aton("127.1"); # or whatever address
2228 $name = gethostbyaddr($iaddr, AF_INET);
2229
2230 # or going the other way
19799a22 2231 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2232
d760c846
GS
2233In the opposite way, to resolve a hostname to the IP address
2234you can write this:
2235
2236 use Socket;
2237 $packed_ip = gethostbyname("www.perl.org");
2238 if (defined $packed_ip) {
2239 $ip_address = inet_ntoa($packed_ip);
2240 }
2241
2242Make sure <gethostbyname()> is called in SCALAR context and that
2243its return value is checked for definedness.
2244
19799a22
GS
2245If you get tired of remembering which element of the return list
2246contains which return value, by-name interfaces are provided
2247in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2248C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2249and C<User::grent>. These override the normal built-ins, supplying
2250versions that return objects with the appropriate names
2251for each field. For example:
5a964f20
TC
2252
2253 use File::stat;
2254 use User::pwent;
2255 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2256
b76cc8ba
NIS
2257Even though it looks like they're the same method calls (uid),
2258they aren't, because a C<File::stat> object is different from
19799a22 2259a C<User::pwent> object.
5a964f20 2260
a0d0e21e 2261=item getsockname SOCKET
d74e8afc 2262X<getsockname>
a0d0e21e 2263
19799a22
GS
2264Returns the packed sockaddr address of this end of the SOCKET connection,
2265in case you don't know the address because you have several different
2266IPs that the connection might have come in on.
a0d0e21e 2267
4633a7c4
LW
2268 use Socket;
2269 $mysockaddr = getsockname(SOCK);
19799a22 2270 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2271 printf "Connect to %s [%s]\n",
19799a22
GS
2272 scalar gethostbyaddr($myaddr, AF_INET),
2273 inet_ntoa($myaddr);
a0d0e21e
LW
2274
2275=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2276X<getsockopt>
a0d0e21e 2277
636e6b1f
TH
2278Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2279Options may exist at multiple protocol levels depending on the socket
2280type, but at least the uppermost socket level SOL_SOCKET (defined in the
2281C<Socket> module) will exist. To query options at another level the
2282protocol number of the appropriate protocol controlling the option
2283should be supplied. For example, to indicate that an option is to be
2284interpreted by the TCP protocol, LEVEL should be set to the protocol
2285number of TCP, which you can get using getprotobyname.
2286
2287The call returns a packed string representing the requested socket option,
2288or C<undef> if there is an error (the error reason will be in $!). What
2289exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2290your system documentation for details. A very common case however is that
cf264981 2291the option is an integer, in which case the result will be a packed
636e6b1f
TH
2292integer which you can decode using unpack with the C<i> (or C<I>) format.
2293
2294An example testing if Nagle's algorithm is turned on on a socket:
2295
4852725b 2296 use Socket qw(:all);
636e6b1f
TH
2297
2298 defined(my $tcp = getprotobyname("tcp"))
2299 or die "Could not determine the protocol number for tcp";
4852725b
DD
2300 # my $tcp = IPPROTO_TCP; # Alternative
2301 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2302 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2303 my $nodelay = unpack("I", $packed);
2304 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2305
a0d0e21e
LW
2306
2307=item glob EXPR
d74e8afc 2308X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2309
0a753a76
PP
2310=item glob
2311
d9a9d457
JL
2312In list context, returns a (possibly empty) list of filename expansions on
2313the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2314scalar context, glob iterates through such filename expansions, returning
2315undef when the list is exhausted. This is the internal function
2316implementing the C<< <*.c> >> operator, but you can use it directly. If
2317EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2318more detail in L<perlop/"I/O Operators">.
a0d0e21e 2319
3a4b19e4
GS
2320Beginning with v5.6.0, this operator is implemented using the standard
2321C<File::Glob> extension. See L<File::Glob> for details.
2322
a0d0e21e 2323=item gmtime EXPR
d74e8afc 2324X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2325
ce2984c3
PF
2326=item gmtime
2327
435fbc73
GS
2328Works just like L<localtime> but the returned values are
2329localized for the standard Greenwich time zone.
a0d0e21e 2330
435fbc73
GS
2331Note: when called in list context, $isdst, the last value
2332returned by gmtime is always C<0>. There is no
2333Daylight Saving Time in GMT.
0a753a76 2334
62aa5637
MS
2335See L<perlport/gmtime> for portability concerns.
2336
a0d0e21e 2337=item goto LABEL
d74e8afc 2338X<goto> X<jump> X<jmp>
a0d0e21e 2339
748a9306
LW
2340=item goto EXPR
2341
a0d0e21e
LW
2342=item goto &NAME
2343
7660c0ab 2344The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2345execution there. It may not be used to go into any construct that
7660c0ab 2346requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2347also can't be used to go into a construct that is optimized away,
19799a22 2348or to get out of a block or subroutine given to C<sort>.
0a753a76 2349It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2350including out of subroutines, but it's usually better to use some other
19799a22 2351construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2352need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2353(The difference being that C does not offer named loops combined with
2354loop control. Perl does, and this replaces most structured uses of C<goto>
2355in other languages.)
a0d0e21e 2356
7660c0ab
A
2357The C<goto-EXPR> form expects a label name, whose scope will be resolved
2358dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2359necessarily recommended if you're optimizing for maintainability:
2360
2361 goto ("FOO", "BAR", "GLARCH")[$i];
2362
1b6921cb
BT
2363The C<goto-&NAME> form is quite different from the other forms of
2364C<goto>. In fact, it isn't a goto in the normal sense at all, and
2365doesn't have the stigma associated with other gotos. Instead, it
2366exits the current subroutine (losing any changes set by local()) and
2367immediately calls in its place the named subroutine using the current
2368value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2369load another subroutine and then pretend that the other subroutine had
2370been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2371in the current subroutine are propagated to the other subroutine.)
2372After the C<goto>, not even C<caller> will be able to tell that this
2373routine was called first.
2374
2375NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2376containing a code reference, or a block that evaluates to a code
6cb9131c 2377reference.
a0d0e21e
LW
2378
2379=item grep BLOCK LIST
d74e8afc 2380X<grep>
a0d0e21e
LW
2381
2382=item grep EXPR,LIST
2383
2b5ab1e7
TC
2384This is similar in spirit to, but not the same as, grep(1) and its
2385relatives. In particular, it is not limited to using regular expressions.
2f9daede 2386
a0d0e21e 2387Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2388C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2389elements for which the expression evaluated to true. In scalar
2390context, returns the number of times the expression was true.
a0d0e21e
LW
2391
2392 @foo = grep(!/^#/, @bar); # weed out comments
2393
2394or equivalently,
2395
2396 @foo = grep {!/^#/} @bar; # weed out comments
2397
be3174d2
GS
2398Note that C<$_> is an alias to the list value, so it can be used to
2399modify the elements of the LIST. While this is useful and supported,
2400it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2401Similarly, grep returns aliases into the original list, much as a for
2402loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2403element of a list returned by grep (for example, in a C<foreach>, C<map>
2404or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2405This is usually something to be avoided when writing clear code.
a0d0e21e 2406
a4fb8298 2407If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2408been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2409the list elements, C<$_> keeps being lexical inside the block; i.e. it
2410can't be seen from the outside, avoiding any potential side-effects.
2411
19799a22 2412See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2413
a0d0e21e 2414=item hex EXPR
d74e8afc 2415X<hex> X<hexadecimal>
a0d0e21e 2416
54310121 2417=item hex
bbce6d69 2418
2b5ab1e7 2419Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2420(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2421L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2422
2423 print hex '0xAf'; # prints '175'
2424 print hex 'aF'; # same
a0d0e21e 2425
19799a22 2426Hex strings may only represent integers. Strings that would cause
53305cf1 2427integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2428unlike oct(). To present something as hex, look into L</printf>,
2429L</sprintf>, or L</unpack>.
19799a22 2430
ce2984c3 2431=item import LIST
d74e8afc 2432X<import>
a0d0e21e 2433
19799a22 2434There is no builtin C<import> function. It is just an ordinary
4633a7c4 2435method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2436names to another module. The C<use> function calls the C<import> method
cea6626f 2437for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2438
2439=item index STR,SUBSTR,POSITION
d74e8afc 2440X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2441
2442=item index STR,SUBSTR
2443
2b5ab1e7
TC
2444The index function searches for one string within another, but without
2445the wildcard-like behavior of a full regular-expression pattern match.
2446It returns the position of the first occurrence of SUBSTR in STR at
2447or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2448beginning of the string. POSITION before the beginning of the string
2449or after its end is treated as if it were the beginning or the end,
2450respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2451you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2452is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2453
2454=item int EXPR
f723aae1 2455X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2456
54310121 2457=item int
bbce6d69 2458
7660c0ab 2459Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2460You should not use this function for rounding: one because it truncates
2461towards C<0>, and two because machine representations of floating point
2462numbers can sometimes produce counterintuitive results. For example,
2463C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2464because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2465the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2466functions will serve you better than will int().
a0d0e21e
LW
2467
2468=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2469X<ioctl>
a0d0e21e 2470
2b5ab1e7 2471Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2472
6c567752 2473 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2474
a11c483f 2475to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2476exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2477own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2478(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2479may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2480written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2481will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2482has no string value but does have a numeric value, that value will be
2483passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2484true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2485functions may be needed to manipulate the values of structures used by
b76cc8ba 2486C<ioctl>.
a0d0e21e 2487
19799a22 2488The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2489
2490 if OS returns: then Perl returns:
2491 -1 undefined value
2492 0 string "0 but true"
2493 anything else that number
2494
19799a22 2495Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2496still easily determine the actual value returned by the operating
2497system:
2498
2b5ab1e7 2499 $retval = ioctl(...) || -1;
a0d0e21e
LW
2500 printf "System returned %d\n", $retval;
2501
be2f7487 2502The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2503about improper numeric conversions.
2504
a0d0e21e 2505=item join EXPR,LIST
d74e8afc 2506X<join>
a0d0e21e 2507
2b5ab1e7
TC
2508Joins the separate strings of LIST into a single string with fields
2509separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2510
2b5ab1e7 2511 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2512
eb6e2d6f
GS
2513Beware that unlike C<split>, C<join> doesn't take a pattern as its
2514first argument. Compare L</split>.
a0d0e21e 2515
aa689395 2516=item keys HASH
d74e8afc 2517X<keys> X<key>
aa689395 2518
504f80c1
JH
2519Returns a list consisting of all the keys of the named hash.
2520(In scalar context, returns the number of keys.)
2521
2522The keys are returned in an apparently random order. The actual
2523random order is subject to change in future versions of perl, but it
2524is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2525function produces (given that the hash has not been modified). Since
2526Perl 5.8.1 the ordering is different even between different runs of
2527Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2528Attacks">).
504f80c1 2529
cf264981
SP
2530As a side effect, calling keys() resets the HASH's internal iterator
2531(see L</each>). In particular, calling keys() in void context resets
2532the iterator with no other overhead.
a0d0e21e 2533
aa689395 2534Here is yet another way to print your environment:
a0d0e21e
LW
2535
2536 @keys = keys %ENV;
2537 @values = values %ENV;
b76cc8ba 2538 while (@keys) {
a0d0e21e
LW
2539 print pop(@keys), '=', pop(@values), "\n";
2540 }
2541
2542or how about sorted by key:
2543
2544 foreach $key (sort(keys %ENV)) {
2545 print $key, '=', $ENV{$key}, "\n";
2546 }
2547
8ea1e5d4
GS
2548The returned values are copies of the original keys in the hash, so
2549modifying them will not affect the original hash. Compare L</values>.
2550
19799a22 2551To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2552Here's a descending numeric sort of a hash by its values:
4633a7c4 2553
5a964f20 2554 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2555 printf "%4d %s\n", $hash{$key}, $key;
2556 }
2557
19799a22 2558As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2559allocated for the given hash. This can gain you a measure of efficiency if
2560you know the hash is going to get big. (This is similar to pre-extending
2561an array by assigning a larger number to $#array.) If you say
55497cff
PP
2562
2563 keys %hash = 200;
2564
ab192400
GS
2565then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2566in fact, since it rounds up to the next power of two. These
55497cff
PP
2567buckets will be retained even if you do C<%hash = ()>, use C<undef
2568%hash> if you want to free the storage while C<%hash> is still in scope.
2569You can't shrink the number of buckets allocated for the hash using
19799a22 2570C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2571as trying has no effect).
2572
19799a22 2573See also C<each>, C<values> and C<sort>.
ab192400 2574
b350dd2f 2575=item kill SIGNAL, LIST
d74e8afc 2576X<kill> X<signal>
a0d0e21e 2577
b350dd2f 2578Sends a signal to a list of processes. Returns the number of
517db077
GS
2579processes successfully signaled (which is not necessarily the
2580same as the number actually killed).
a0d0e21e
LW
2581
2582 $cnt = kill 1, $child1, $child2;
2583 kill 9, @goners;
2584
70fb64f6 2585If SIGNAL is zero, no signal is sent to the process, but the kill(2)
6cb9d3e4 2586system call will check whether it's possible to send a signal to it (that
70fb64f6
RGS
2587means, to be brief, that the process is owned by the same user, or we are
2588the super-user). This is a useful way to check that a child process is
81fd35db
DN
2589alive (even if only as a zombie) and hasn't changed its UID. See
2590L<perlport> for notes on the portability of this construct.
b350dd2f
GS
2591
2592Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2593process groups instead of processes. (On System V, a negative I<PROCESS>
2594number will also kill process groups, but that's not portable.) That
2595means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2596use a signal name in quotes.
2597
2598See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2599
2600=item last LABEL
d74e8afc 2601X<last> X<break>
a0d0e21e
LW
2602
2603=item last
2604
2605The C<last> command is like the C<break> statement in C (as used in
2606loops); it immediately exits the loop in question. If the LABEL is
2607omitted, the command refers to the innermost enclosing loop. The
2608C<continue> block, if any, is not executed:
2609
4633a7c4
LW
2610 LINE: while (<STDIN>) {
2611 last LINE if /^$/; # exit when done with header
5a964f20 2612 #...
a0d0e21e
LW
2613 }
2614
4968c1e4 2615C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2616C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2617a grep() or map() operation.
4968c1e4 2618
6c1372ed
GS
2619Note that a block by itself is semantically identical to a loop
2620that executes once. Thus C<last> can be used to effect an early
2621exit out of such a block.
2622
98293880
JH
2623See also L</continue> for an illustration of how C<last>, C<next>, and
2624C<redo> work.
1d2dff63 2625
a0d0e21e 2626=item lc EXPR
d74e8afc 2627X<lc> X<lowercase>
a0d0e21e 2628
54310121 2629=item lc
bbce6d69 2630
d1be9408 2631Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2632implementing the C<\L> escape in double-quoted strings. Respects
2633current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2634and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2635
7660c0ab 2636If EXPR is omitted, uses C<$_>.
bbce6d69 2637
a0d0e21e 2638=item lcfirst EXPR
d74e8afc 2639X<lcfirst> X<lowercase>
a0d0e21e 2640
54310121 2641=item lcfirst
bbce6d69 2642
ad0029c4
JH
2643Returns the value of EXPR with the first character lowercased. This
2644is the internal function implementing the C<\l> escape in
2645double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2646locale> in force. See L<perllocale> and L<perlunicode> for more
2647details about locale and Unicode support.
a0d0e21e 2648
7660c0ab 2649If EXPR is omitted, uses C<$_>.
bbce6d69 2650
a0d0e21e 2651=item length EXPR
d74e8afc 2652X<length> X<size>
a0d0e21e 2653
54310121 2654=item length
bbce6d69 2655
974da8e5 2656Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2657omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2658an entire array or hash to find out how many elements these have.
2659For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2660
974da8e5
JH
2661Note the I<characters>: if the EXPR is in Unicode, you will get the
2662number of characters, not the number of bytes. To get the length
2575c402
JW
2663of the internal string in bytes, use C<bytes::length(EXPR)>, see
2664L<bytes>. Note that the internal encoding is variable, and the number
2665of bytes usually meaningless. To get the number of bytes that the
2666string would have when encoded as UTF-8, use
2667C<length(Encoding::encode_utf8(EXPR))>.
974da8e5 2668
a0d0e21e 2669=item link OLDFILE,NEWFILE
d74e8afc 2670X<link>
a0d0e21e 2671
19799a22 2672Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2673success, false otherwise.
a0d0e21e
LW
2674
2675=item listen SOCKET,QUEUESIZE
d74e8afc 2676X<listen>
a0d0e21e 2677
19799a22 2678Does the same thing that the listen system call does. Returns true if
b76cc8ba 2679it succeeded, false otherwise. See the example in
cea6626f 2680L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2681
2682=item local EXPR
d74e8afc 2683X<local>
a0d0e21e 2684
19799a22 2685You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2686what most people think of as "local". See
13a2d996 2687L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2688
5a964f20
TC
2689A local modifies the listed variables to be local to the enclosing
2690block, file, or eval. If more than one value is listed, the list must
2691be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2692for details, including issues with tied arrays and hashes.
a0d0e21e 2693
a0d0e21e 2694=item localtime EXPR
435fbc73 2695X<localtime> X<ctime>
a0d0e21e 2696
ba053783
AL
2697=item localtime
2698
19799a22 2699Converts a time as returned by the time function to a 9-element list
5f05dabc 2700with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2701follows:
2702
54310121 2703 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2704 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2705 localtime(time);
a0d0e21e 2706
48a26b3a 2707All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2708tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2709of the specified time.
48a26b3a 2710
ba053783
AL
2711C<$mday> is the day of the month, and C<$mon> is the month itself, in
2712the range C<0..11> with 0 indicating January and 11 indicating December.
2713This makes it easy to get a month name from a list:
54310121 2714
ba053783
AL
2715 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2716 print "$abbr[$mon] $mday";
2717 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2718
ba053783
AL
2719C<$year> is the number of years since 1900, not just the last two digits
2720of the year. That is, C<$year> is C<123> in year 2023. The proper way
2721to get a complete 4-digit year is simply:
abd75f24 2722
ba053783 2723 $year += 1900;
abd75f24 2724
435fbc73
GS
2725Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2726to do that, would you?
2727
ba053783
AL
2728To get the last two digits of the year (e.g., '01' in 2001) do:
2729
2730 $year = sprintf("%02d", $year % 100);
2731
2732C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2733Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2734(or C<0..365> in leap years.)
2735
2736C<$isdst> is true if the specified time occurs during Daylight Saving
2737Time, false otherwise.
abd75f24 2738
48a26b3a 2739If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2740
48a26b3a 2741In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2742
5f05dabc 2743 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2744
fe86afc2
NC
2745This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2746instead of local time use the L</gmtime> builtin. See also the
2747C<Time::Local> module (to convert the second, minutes, hours, ... back to
2748the integer value returned by time()), and the L<POSIX> module's strftime(3)
2749and mktime(3) functions.
2750
2751To get somewhat similar but locale dependent date strings, set up your
2752locale environment variables appropriately (please see L<perllocale>) and
2753try for example:
a3cb178b 2754
5a964f20 2755 use POSIX qw(strftime);
2b5ab1e7 2756 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2757 # or for GMT formatted appropriately for your locale:
2758 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2759
2760Note that the C<%a> and C<%b>, the short forms of the day of the week
2761and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2762
62aa5637
MS
2763See L<perlport/localtime> for portability concerns.
2764
435fbc73
GS
2765The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2766by-name access mechanism to the gmtime() and localtime() functions,
2767respectively.
2768
2769For a comprehensive date and time representation look at the
2770L<DateTime> module on CPAN.
2771
07698885 2772=item lock THING
d74e8afc 2773X<lock>
19799a22 2774
01e6739c 2775This function places an advisory lock on a shared variable, or referenced
03730085 2776object contained in I<THING> until the lock goes out of scope.
a6d5524e 2777
f3a23afb 2778lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2779by this name (before any calls to it), that function will be called
03730085
AB
2780instead. (However, if you've said C<use threads>, lock() is always a
2781keyword.) See L<threads>.
19799a22 2782
a0d0e21e 2783=item log EXPR
d74e8afc 2784X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2785
54310121 2786=item log
bbce6d69 2787
2b5ab1e7
TC
2788Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2789returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2790The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2791divided by the natural log of N. For example:
2792
2793 sub log10 {
2794 my $n = shift;
2795 return log($n)/log(10);
b76cc8ba 2796 }
2b5ab1e7
TC
2797
2798See also L</exp> for the inverse operation.
a0d0e21e 2799
a0d0e21e 2800=item lstat EXPR
d74e8afc 2801X<lstat>
a0d0e21e 2802
54310121 2803=item lstat
bbce6d69 2804
19799a22 2805Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2806special C<_> filehandle) but stats a symbolic link instead of the file
2807the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2808your system, a normal C<stat> is done. For much more detailed
2809information, please see the documentation for C<stat>.
a0d0e21e 2810
7660c0ab 2811If EXPR is omitted, stats C<$_>.
bbce6d69 2812
a0d0e21e
LW
2813=item m//
2814
2815The match operator. See L<perlop>.
2816
2817=item map BLOCK LIST
d74e8afc 2818X<map>
a0d0e21e
LW
2819
2820=item map EXPR,LIST
2821
19799a22
GS
2822Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2823C<$_> to each element) and returns the list value composed of the
2824results of each such evaluation. In scalar context, returns the
2825total number of elements so generated. Evaluates BLOCK or EXPR in
2826list context, so each element of LIST may produce zero, one, or
2827more elements in the returned value.
dd99ebda 2828
a0d0e21e
LW
2829 @chars = map(chr, @nums);
2830
2831translates a list of numbers to the corresponding characters. And
2832
d8216f19 2833 %hash = map { get_a_key_for($_) => $_ } @array;
a0d0e21e
LW
2834
2835is just a funny way to write
2836
2837 %hash = ();
d8216f19
RGS
2838 foreach (@array) {
2839 $hash{get_a_key_for($_)} = $_;
a0d0e21e
LW
2840 }
2841
be3174d2
GS
2842Note that C<$_> is an alias to the list value, so it can be used to
2843modify the elements of the LIST. While this is useful and supported,
2844it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2845Using a regular C<foreach> loop for this purpose would be clearer in
2846most cases. See also L</grep> for an array composed of those items of
2847the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2848
a4fb8298 2849If C<$_> is lexical in the scope where the C<map> appears (because it has
d8216f19
RGS
2850been declared with C<my $_>), then, in addition to being locally aliased to
2851the list elements, C<$_> keeps being lexical inside the block; that is, it
a4fb8298
RGS
2852can't be seen from the outside, avoiding any potential side-effects.
2853
205fdb4d
NC
2854C<{> starts both hash references and blocks, so C<map { ...> could be either
2855the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2856ahead for the closing C<}> it has to take a guess at which its dealing with
2857based what it finds just after the C<{>. Usually it gets it right, but if it
2858doesn't it won't realize something is wrong until it gets to the C<}> and
2859encounters the missing (or unexpected) comma. The syntax error will be
2860reported close to the C<}> but you'll need to change something near the C<{>
2861such as using a unary C<+> to give perl some help:
2862
2863 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2864 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2865 %hash = map { ("\L$_", 1) } @array # this also works
2866 %hash = map { lc($_), 1 } @array # as does this.
2867 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2868
205fdb4d
NC
2869 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2870
d8216f19 2871or to force an anon hash constructor use C<+{>:
205fdb4d
NC
2872
2873 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2874
2875and you get list of anonymous hashes each with only 1 entry.
2876
19799a22 2877=item mkdir FILENAME,MASK
d74e8afc 2878X<mkdir> X<md> X<directory, create>
a0d0e21e 2879
5a211162
GS
2880=item mkdir FILENAME
2881
491873e5
RGS
2882=item mkdir
2883
0591cd52 2884Creates the directory specified by FILENAME, with permissions
19799a22
GS
2885specified by MASK (as modified by C<umask>). If it succeeds it
2886returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2887If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2888to C<$_>.
0591cd52 2889
19799a22 2890In general, it is better to create directories with permissive MASK,
0591cd52 2891and let the user modify that with their C<umask>, than it is to supply
19799a22 2892a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2893The exceptions to this rule are when the file or directory should be
2894kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2895C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2896
cc1852e8
JH
2897Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2898number of trailing slashes. Some operating and filesystems do not get
2899this right, so Perl automatically removes all trailing slashes to keep
2900everyone happy.
2901
dd184578
RGS
2902In order to recursively create a directory structure look at
2903the C<mkpath> function of the L<File::Path> module.
2904
a0d0e21e 2905=item msgctl ID,CMD,ARG
d74e8afc 2906X<msgctl>
a0d0e21e 2907
f86cebdf 2908Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2909
2910 use IPC::SysV;
2911
7660c0ab 2912first to get the correct constant definitions. If CMD is C<IPC_STAT>,
cf264981 2913then ARG must be a variable that will hold the returned C<msqid_ds>
951ba7fe
GS
2914structure. Returns like C<ioctl>: the undefined value for error,
2915C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2916L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2917
2918=item msgget KEY,FLAGS
d74e8afc 2919X<msgget>
a0d0e21e 2920
f86cebdf 2921Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2922id, or the undefined value if there is an error. See also
2923L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2924
a0d0e21e 2925=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
d74e8afc 2926X<msgrcv>
a0d0e21e
LW
2927
2928Calls the System V IPC function msgrcv to receive a message from
2929message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2930SIZE. Note that when a message is received, the message type as a
2931native long integer will be the first thing in VAR, followed by the
2932actual message. This packing may be opened with C<unpack("l! a*")>.
2933Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2934an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2935C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2936
2937=item msgsnd ID,MSG,FLAGS
d74e8afc 2938X<msgsnd>
41d6edb2
JH
2939
2940Calls the System V IPC function msgsnd to send the message MSG to the
2941message queue ID. MSG must begin with the native long integer message
2942type, and be followed by the length of the actual message, and finally
2943the message itself. This kind of packing can be achieved with
2944C<pack("l! a*", $type, $message)>. Returns true if successful,
2945or false if there is an error. See also C<IPC::SysV>
2946and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2947
2948=item my EXPR
d74e8afc 2949X<my>
a0d0e21e 2950
307ea6df
JH
2951=item my TYPE EXPR
2952
1d2de774 2953=item my EXPR : ATTRS
09bef843 2954
1d2de774 2955=item my TYPE EXPR : ATTRS
307ea6df 2956
19799a22 2957A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2958enclosing block, file, or C<eval>. If more than one value is listed,
2959the list must be placed in parentheses.
307ea6df 2960
1d2de774
JH
2961The exact semantics and interface of TYPE and ATTRS are still
2962evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2963and attributes are handled using the C<attributes> pragma, or starting
2964from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2965L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2966L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2967
a0d0e21e 2968=item next LABEL
d74e8afc 2969X<next> X<continue>
a0d0e21e
LW
2970
2971=item next
2972
2973The C<next> command is like the C<continue> statement in C; it starts
2974the next iteration of the loop:
2975
4633a7c4
LW
2976 LINE: while (<STDIN>) {
2977 next LINE if /^#/; # discard comments
5a964f20 2978 #...
a0d0e21e
LW
2979 }
2980
2981Note that if there were a C<continue> block on the above, it would get
2982executed even on discarded lines. If the LABEL is omitted, the command
2983refers to the innermost enclosing loop.
2984
4968c1e4 2985C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2986C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2987a grep() or map() operation.
4968c1e4 2988
6c1372ed
GS
2989Note that a block by itself is semantically identical to a loop
2990that executes once. Thus C<next> will exit such a block early.
2991
98293880
JH
2992See also L</continue> for an illustration of how C<last>, C<next>, and
2993C<redo> work.
1d2dff63 2994
4a66ea5a 2995=item no Module VERSION LIST
d74e8afc 2996X<no>
4a66ea5a
RGS
2997
2998=item no Module VERSION
2999
a0d0e21e
LW
3000=item no Module LIST
3001
4a66ea5a
RGS
3002=item no Module
3003
c986422f
RGS
3004=item no VERSION
3005
593b9c14 3006See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
3007
3008=item oct EXPR
d74e8afc 3009X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
a0d0e21e 3010
54310121 3011=item oct
bbce6d69 3012
4633a7c4 3013Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
3014value. (If EXPR happens to start off with C<0x>, interprets it as a
3015hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
3016binary string. Leading whitespace is ignored in all three cases.)
3017The following will handle decimal, binary, octal, and hex in the standard
3018Perl or C notation:
a0d0e21e
LW
3019
3020 $val = oct($val) if $val =~ /^0/;
3021
19799a22
GS
3022If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3023in octal), use sprintf() or printf():
3024
3025 $perms = (stat("filename"))[2] & 07777;
3026 $oct_perms = sprintf "%lo", $perms;
3027
3028The oct() function is commonly used when a string such as C<644> needs
3029to be converted into a file mode, for example. (Although perl will
3030automatically convert strings into numbers as needed, this automatic
3031conversion assumes base 10.)
a0d0e21e
LW
3032
3033=item open FILEHANDLE,EXPR
d74e8afc 3034X<open> X<pipe> X<file, open> X<fopen>
a0d0e21e 3035
68bd7414
NIS
3036=item open FILEHANDLE,MODE,EXPR
3037
3038=item open FILEHANDLE,MODE,EXPR,LIST
3039
ba964c95
T
3040=item open FILEHANDLE,MODE,REFERENCE
3041
a0d0e21e
LW
3042=item open FILEHANDLE
3043
3044Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
3045FILEHANDLE.
3046
3047(The following is a comprehensive reference to open(): for a gentler
3048introduction you may consider L<perlopentut>.)
3049
a28cd5c9
NT
3050If FILEHANDLE is an undefined scalar variable (or array or hash element)
3051the variable is assigned a reference to a new anonymous filehandle,
3052otherwise if FILEHANDLE is an expression, its value is used as the name of
3053the real filehandle wanted. (This is considered a symbolic reference, so
3054C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
3055
3056If EXPR is omitted, the scalar variable of the same name as the
3057FILEHANDLE contains the filename. (Note that lexical variables--those
3058declared with C<my>--will not work for this purpose; so if you're
67408cae 3059using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
3060
3061If three or more arguments are specified then the mode of opening and
3062the file name are separate. If MODE is C<< '<' >> or nothing, the file
3063is opened for input. If MODE is C<< '>' >>, the file is truncated and
3064opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 3065the file is opened for appending, again being created if necessary.
5a964f20 3066
ed53a2bb
JH
3067You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3068indicate that you want both read and write access to the file; thus
3069C<< '+<' >> is almost always preferred for read/write updates--the C<<
3070'+>' >> mode would clobber the file first. You can't usually use
3071either read-write mode for updating textfiles, since they have
3072variable length records. See the B<-i> switch in L<perlrun> for a
3073better approach. The file is created with permissions of C<0666>
3074modified by the process' C<umask> value.
3075
3076These various prefixes correspond to the fopen(3) modes of C<'r'>,
3077C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 3078
6170680b
IZ
3079In the 2-arguments (and 1-argument) form of the call the mode and
3080filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
3081spaces. It is possible to omit the mode in these forms if the mode is
3082C<< '<' >>.
6170680b 3083
7660c0ab 3084If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 3085command to which output is to be piped, and if the filename ends with a
f244e06d
GS
3086C<'|'>, the filename is interpreted as a command which pipes output to
3087us. See L<perlipc/"Using open() for IPC">
19799a22 3088for more examples of this. (You are not allowed to C<open> to a command
5a964f20 3089that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
3090and L<perlipc/"Bidirectional Communication with Another Process">
3091for alternatives.)
cb1a09d0 3092
ed53a2bb
JH
3093For three or more arguments if MODE is C<'|-'>, the filename is
3094interpreted as a command to which output is to be piped, and if MODE
3095is C<'-|'>, the filename is interpreted as a command which pipes
3096output to us. In the 2-arguments (and 1-argument) form one should
3097replace dash (C<'-'>) with the command.
3098See L<perlipc/"Using open() for IPC"> for more examples of this.
3099(You are not allowed to C<open> to a command that pipes both in I<and>
3100out, but see L<IPC::Open2>, L<IPC::Open3>, and
3101L<perlipc/"Bidirectional Communication"> for alternatives.)
3102
3103In the three-or-more argument form of pipe opens, if LIST is specified
3104(extra arguments after the command name) then LIST becomes arguments
3105to the command invoked if the platform supports it. The meaning of
3106C<open> with more than three arguments for non-pipe modes is not yet
3107specified. Experimental "layers" may give extra LIST arguments
3108meaning.
6170680b
IZ
3109
3110In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 3111and opening C<< '>-' >> opens STDOUT.
6170680b 3112
fae2c0fb
RGS
3113You may use the three-argument form of open to specify IO "layers"
3114(sometimes also referred to as "disciplines") to be applied to the handle
3115that affect how the input and output are processed (see L<open> and
3116L<PerlIO> for more details). For example
7207e29d 3117
2575c402 3118 open(FH, "<:encoding(UTF-8)", "file")
9124316e
JH
3119
3120will open the UTF-8 encoded file containing Unicode characters,
6d5e88a0
ST
3121see L<perluniintro>. Note that if layers are specified in the
3122three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3123usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
ed53a2bb
JH
3124
3125Open returns nonzero upon success, the undefined value otherwise. If
3126the C<open> involved a pipe, the return value happens to be the pid of
3127the subprocess.
cb1a09d0 3128
ed53a2bb
JH
3129If you're running Perl on a system that distinguishes between text
3130files and binary files, then you should check out L</binmode> for tips
3131for dealing with this. The key distinction between systems that need
3132C<binmode> and those that don't is their text file formats. Systems
8939ba94 3133like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
3134character, and which encode that character in C as C<"\n">, do not
3135need C<binmode>. The rest need it.
cb1a09d0 3136
fb73857a 3137When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
3138if the request failed, so C<open> is frequently used in connection with
3139C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 3140where you want to make a nicely formatted error message (but there are
5a964f20 3141modules that can help with that problem)) you should always check
19799a22 3142the return value from opening a file. The infrequent exception is when
fb73857a
PP
3143working with an unopened filehandle is actually what you want to do.
3144
cf264981 3145As a special case the 3-arg form with a read/write mode and the third
ed53a2bb 3146argument being C<undef>:
b76cc8ba
NIS
3147