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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>
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190
191=item Keywords related to classes and object-orientedness
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
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298the undefined value if the file doesn't exist. Despite the funny
299names, precedence is the same as any other named unary operator, and
300the argument may be parenthesized like any other unary operator. The
301operator may be any of:
302
303 -r File is readable by effective uid/gid.
304 -w File is writable by effective uid/gid.
305 -x File is executable by effective uid/gid.
306 -o File is owned by effective uid.
307
308 -R File is readable by real uid/gid.
309 -W File is writable by real uid/gid.
310 -X File is executable by real uid/gid.
311 -O File is owned by real uid.
312
313 -e File exists.
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314 -z File has zero size (is empty).
315 -s File has nonzero size (returns size in bytes).
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316
317 -f File is a plain file.
318 -d File is a directory.
319 -l File is a symbolic link.
9c4d0f16 320 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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321 -S File is a socket.
322 -b File is a block special file.
323 -c File is a character special file.
324 -t Filehandle is opened to a tty.
325
326 -u File has setuid bit set.
327 -g File has setgid bit set.
328 -k File has sticky bit set.
329
121910a4 330 -T File is an ASCII text file (heuristic guess).
2cdbc966 331 -B File is a "binary" file (opposite of -T).
a0d0e21e 332
95a3fe12 333 -M Script start time minus file modification time, in days.
a0d0e21e 334 -A Same for access time.
95a3fe12 335 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 336
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337Example:
338
339 while (<>) {
5b3eff12 340 chomp;
a0d0e21e 341 next unless -f $_; # ignore specials
5a964f20 342 #...
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343 }
344
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345The interpretation of the file permission operators C<-r>, C<-R>,
346C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
347of the file and the uids and gids of the user. There may be other
348reasons you can't actually read, write, or execute the file. Such
349reasons may be for example network filesystem access controls, ACLs
350(access control lists), read-only filesystems, and unrecognized
351executable formats.
352
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353Also note that, for the superuser on the local filesystems, the C<-r>,
354C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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GS
355if any execute bit is set in the mode. Scripts run by the superuser
356may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 357or temporarily set their effective uid to something else.
5ff3f7a4
GS
358
359If you are using ACLs, there is a pragma called C<filetest> that may
360produce more accurate results than the bare stat() mode bits.
5ff3f7a4
GS
361When under the C<use filetest 'access'> the above-mentioned filetests
362will test whether the permission can (not) be granted using the
468541a8 363access() family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
364under this pragma return true even if there are no execute permission
365bits set (nor any extra execute permission ACLs). This strangeness is
366due to the underlying system calls' definitions. Read the
367documentation for the C<filetest> pragma for more information.
368
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369Note that C<-s/a/b/> does not do a negated substitution. Saying
370C<-exp($foo)> still works as expected, however--only single letters
371following a minus are interpreted as file tests.
372
373The C<-T> and C<-B> switches work as follows. The first block or so of the
374file is examined for odd characters such as strange control codes or
61eff3bc 375characters with the high bit set. If too many strange characters (>30%)
cf264981 376are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
a0d0e21e 377containing null in the first block is considered a binary file. If C<-T>
9124316e 378or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 379rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 380file, or a file at EOF when testing a filehandle. Because you have to
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LW
381read a file to do the C<-T> test, on most occasions you want to use a C<-f>
382against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 383
19799a22 384If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 385the special filehandle consisting of a solitary underline, then the stat
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386structure of the previous file test (or stat operator) is used, saving
387a system call. (This doesn't work with C<-t>, and you need to remember
388that lstat() and C<-l> will leave values in the stat structure for the
5c9aa243 389symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 390an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 391Example:
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392
393 print "Can do.\n" if -r $a || -w _ || -x _;
394
395 stat($filename);
396 print "Readable\n" if -r _;
397 print "Writable\n" if -w _;
398 print "Executable\n" if -x _;
399 print "Setuid\n" if -u _;
400 print "Setgid\n" if -g _;
401 print "Sticky\n" if -k _;
402 print "Text\n" if -T _;
403 print "Binary\n" if -B _;
404
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405As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
406test operators, in a way that C<-f -w -x $file> is equivalent to
cf264981 407C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
fbb0b3b3
RGS
408the return value of C<-f $file> as an argument to another filetest
409operator, no special magic will happen.)
410
a0d0e21e 411=item abs VALUE
d74e8afc 412X<abs> X<absolute>
a0d0e21e 413
54310121 414=item abs
bbce6d69 415
a0d0e21e 416Returns the absolute value of its argument.
7660c0ab 417If VALUE is omitted, uses C<$_>.
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418
419=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 420X<accept>
a0d0e21e 421
f86cebdf 422Accepts an incoming socket connect, just as the accept(2) system call
19799a22 423does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 424See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 425
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GS
426On systems that support a close-on-exec flag on files, the flag will
427be set for the newly opened file descriptor, as determined by the
428value of $^F. See L<perlvar/$^F>.
429
a0d0e21e 430=item alarm SECONDS
d74e8afc
ITB
431X<alarm>
432X<SIGALRM>
433X<timer>
a0d0e21e 434
54310121 435=item alarm
bbce6d69 436
a0d0e21e 437Arranges to have a SIGALRM delivered to this process after the
cf264981 438specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
439specified, the value stored in C<$_> is used. (On some machines,
440unfortunately, the elapsed time may be up to one second less or more
441than you specified because of how seconds are counted, and process
442scheduling may delay the delivery of the signal even further.)
443
444Only one timer may be counting at once. Each call disables the
445previous timer, and an argument of C<0> may be supplied to cancel the
446previous timer without starting a new one. The returned value is the
447amount of time remaining on the previous timer.
a0d0e21e 448
4633a7c4 449For delays of finer granularity than one second, you may use Perl's
19799a22
GS
450four-argument version of select() leaving the first three arguments
451undefined, or you might be able to use the C<syscall> interface to
83df6a1d
JH
452access setitimer(2) if your system supports it. The Time::HiRes
453module (from CPAN, and starting from Perl 5.8 part of the standard
454distribution) may also prove useful.
2b5ab1e7 455
68f8bed4
JH
456It is usually a mistake to intermix C<alarm> and C<sleep> calls.
457(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 458
19799a22
GS
459If you want to use C<alarm> to time out a system call you need to use an
460C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 461fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 462restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 463modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
464
465 eval {
f86cebdf 466 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 467 alarm $timeout;
ff68c719 468 $nread = sysread SOCKET, $buffer, $size;
36477c24 469 alarm 0;
ff68c719 470 };
ff68c719 471 if ($@) {
f86cebdf 472 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719
PP
473 # timed out
474 }
475 else {
476 # didn't
477 }
478
91d81acc
JH
479For more information see L<perlipc>.
480
a0d0e21e 481=item atan2 Y,X
d74e8afc 482X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
483
484Returns the arctangent of Y/X in the range -PI to PI.
485
ca6e1c26 486For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
487function, or use the familiar relation:
488
489 sub tan { sin($_[0]) / cos($_[0]) }
490
bf5f1b4c
JH
491Note that atan2(0, 0) is not well-defined.
492
a0d0e21e 493=item bind SOCKET,NAME
d74e8afc 494X<bind>
a0d0e21e
LW
495
496Binds a network address to a socket, just as the bind system call
19799a22 497does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
498packed address of the appropriate type for the socket. See the examples in
499L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 500
fae2c0fb 501=item binmode FILEHANDLE, LAYER
d74e8afc 502X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 503
a0d0e21e
LW
504=item binmode FILEHANDLE
505
1cbfc93d
NIS
506Arranges for FILEHANDLE to be read or written in "binary" or "text"
507mode on systems where the run-time libraries distinguish between
508binary and text files. If FILEHANDLE is an expression, the value is
509taken as the name of the filehandle. Returns true on success,
b5fe5ca2 510otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 511
d807c6f4
JH
512On some systems (in general, DOS and Windows-based systems) binmode()
513is necessary when you're not working with a text file. For the sake
514of portability it is a good idea to always use it when appropriate,
515and to never use it when it isn't appropriate. Also, people can
516set their I/O to be by default UTF-8 encoded Unicode, not bytes.
517
518In other words: regardless of platform, use binmode() on binary data,
519like for example images.
520
521If LAYER is present it is a single string, but may contain multiple
522directives. The directives alter the behaviour of the file handle.
523When LAYER is present using binmode on text file makes sense.
524
fae2c0fb 525If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
526suitable for passing binary data. This includes turning off possible CRLF
527translation and marking it as bytes (as opposed to Unicode characters).
749683d2 528Note that, despite what may be implied in I<"Programming Perl"> (the
165a9987
PJ
529Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
530-- other layers which would affect the binary nature of the stream are
0226bbdb
NIS
531I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
532PERLIO environment variable.
01e6739c 533
d807c6f4
JH
534The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
535form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
536establish default I/O layers. See L<open>.
537
fae2c0fb
RGS
538I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
539in "Programming Perl, 3rd Edition". However, since the publishing of this
540book, by many known as "Camel III", the consensus of the naming of this
541functionality has moved from "discipline" to "layer". All documentation
542of this version of Perl therefore refers to "layers" rather than to
543"disciplines". Now back to the regularly scheduled documentation...>
544
6902c96a
T
545To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
546C<:utf8> just marks the data as UTF-8 without further checking,
547while C<:encoding(utf8)> checks the data for actually being valid
548UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 549
ed53a2bb 550In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
551is done on the filehandle. Calling binmode() will normally flush any
552pending buffered output data (and perhaps pending input data) on the
fae2c0fb 553handle. An exception to this is the C<:encoding> layer that
01e6739c 554changes the default character encoding of the handle, see L<open>.
fae2c0fb 555The C<:encoding> layer sometimes needs to be called in
3874323d
JH
556mid-stream, and it doesn't flush the stream. The C<:encoding>
557also implicitly pushes on top of itself the C<:utf8> layer because
558internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 559
19799a22 560The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
561system all work together to let the programmer treat a single
562character (C<\n>) as the line terminator, irrespective of the external
563representation. On many operating systems, the native text file
564representation matches the internal representation, but on some
565platforms the external representation of C<\n> is made up of more than
566one character.
567
68bd7414
NIS
568Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
569character to end each line in the external representation of text (even
5e12dbfa 570though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
571on Unix and most VMS files). In other systems like OS/2, DOS and the
572various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
573but what's stored in text files are the two characters C<\cM\cJ>. That
574means that, if you don't use binmode() on these systems, C<\cM\cJ>
575sequences on disk will be converted to C<\n> on input, and any C<\n> in
576your program will be converted back to C<\cM\cJ> on output. This is what
577you want for text files, but it can be disastrous for binary files.
30168b04
GS
578
579Another consequence of using binmode() (on some systems) is that
580special end-of-file markers will be seen as part of the data stream.
581For systems from the Microsoft family this means that if your binary
4375e838 582data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
583the file, unless you use binmode().
584
585binmode() is not only important for readline() and print() operations,
586but also when using read(), seek(), sysread(), syswrite() and tell()
587(see L<perlport> for more details). See the C<$/> and C<$\> variables
588in L<perlvar> for how to manually set your input and output
589line-termination sequences.
a0d0e21e 590
4633a7c4 591=item bless REF,CLASSNAME
d74e8afc 592X<bless>
a0d0e21e
LW
593
594=item bless REF
595
2b5ab1e7
TC
596This function tells the thingy referenced by REF that it is now an object
597in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 598is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 599it returns the reference for convenience. Always use the two-argument
cf264981
SP
600version if a derived class might inherit the function doing the blessing.
601See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
602of objects.
a0d0e21e 603
57668c4d 604Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 605Namespaces with all lowercase names are considered reserved for
cf264981 606Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
607confusion, you may wish to avoid such package names as well. Make sure
608that CLASSNAME is a true value.
60ad88b8
GS
609
610See L<perlmod/"Perl Modules">.
611
0d863452
RH
612=item break
613
614Break out of a C<given()> block.
615
616This keyword is enabled by the "switch" feature: see L<feature>
617for more information.
618
a0d0e21e 619=item caller EXPR
d74e8afc 620X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
621
622=item caller
623
5a964f20 624Returns the context of the current subroutine call. In scalar context,
28757baa 625returns the caller's package name if there is a caller, that is, if
19799a22 626we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 627otherwise. In list context, returns
a0d0e21e 628
ee6b43cc 629 # 0 1 2
748a9306 630 ($package, $filename, $line) = caller;
a0d0e21e
LW
631
632With EXPR, it returns some extra information that the debugger uses to
633print a stack trace. The value of EXPR indicates how many call frames
634to go back before the current one.
635
ee6b43cc 636 # 0 1 2 3 4
f3aa04c2 637 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 638
639 # 5 6 7 8 9 10
b3ca2e83 640 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 641 = caller($i);
e7ea3e70 642
951ba7fe 643Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 644call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 645C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 646C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 647C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 648$filename is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
649each C<use> statement creates a C<require> frame inside an C<eval EXPR>
650frame.) $subroutine may also be C<(unknown)> if this particular
651subroutine happens to have been deleted from the symbol table.
652C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
653C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
654compiled with. The C<$hints> and C<$bitmask> values are subject to change
655between versions of Perl, and are not meant for external use.
748a9306 656
b3ca2e83
NC
657C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
658caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
659of this hash, as they are the actual values stored in the optree.
660
748a9306 661Furthermore, when called from within the DB package, caller returns more
7660c0ab 662detailed information: it sets the list variable C<@DB::args> to be the
54310121 663arguments with which the subroutine was invoked.
748a9306 664
7660c0ab 665Be aware that the optimizer might have optimized call frames away before
19799a22 666C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 667might not return information about the call frame you expect it do, for
b76cc8ba 668C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 669previous time C<caller> was called.
7660c0ab 670
a0d0e21e 671=item chdir EXPR
d74e8afc
ITB
672X<chdir>
673X<cd>
f723aae1 674X<directory, change>
a0d0e21e 675
c4aca7d0
GA
676=item chdir FILEHANDLE
677
678=item chdir DIRHANDLE
679
ce2984c3
PF
680=item chdir
681
ffce7b87 682Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 683changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 684changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
685variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
686neither is set, C<chdir> does nothing. It returns true upon success,
687false otherwise. See the example under C<die>.
a0d0e21e 688
c4aca7d0
GA
689On systems that support fchdir, you might pass a file handle or
690directory handle as argument. On systems that don't support fchdir,
691passing handles produces a fatal error at run time.
692
a0d0e21e 693=item chmod LIST
d74e8afc 694X<chmod> X<permission> X<mode>
a0d0e21e
LW
695
696Changes the permissions of a list of files. The first element of the
4633a7c4 697list must be the numerical mode, which should probably be an octal
4ad40acf 698number, and which definitely should I<not> be a string of octal digits:
2f9daede 699C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 700successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
701
702 $cnt = chmod 0755, 'foo', 'bar';
703 chmod 0755, @executables;
f86cebdf
GS
704 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
705 # --w----r-T
2f9daede
TPG
706 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
707 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 708
c4aca7d0
GA
709On systems that support fchmod, you might pass file handles among the
710files. On systems that don't support fchmod, passing file handles
345da378
GA
711produces a fatal error at run time. The file handles must be passed
712as globs or references to be recognized. Barewords are considered
713file names.
c4aca7d0
GA
714
715 open(my $fh, "<", "foo");
716 my $perm = (stat $fh)[2] & 07777;
717 chmod($perm | 0600, $fh);
718
ca6e1c26
JH
719You can also import the symbolic C<S_I*> constants from the Fcntl
720module:
721
722 use Fcntl ':mode';
723
724 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
725 # This is identical to the chmod 0755 of the above example.
726
a0d0e21e 727=item chomp VARIABLE
d74e8afc 728X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 729
313c9f5c 730=item chomp( LIST )
a0d0e21e
LW
731
732=item chomp
733
2b5ab1e7
TC
734This safer version of L</chop> removes any trailing string
735that corresponds to the current value of C<$/> (also known as
28757baa
PP
736$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
737number of characters removed from all its arguments. It's often used to
738remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
739that the final record may be missing its newline. When in paragraph
740mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
741When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
742a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 743remove anything.
19799a22 744If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
745
746 while (<>) {
747 chomp; # avoid \n on last field
748 @array = split(/:/);
5a964f20 749 # ...
a0d0e21e
LW
750 }
751
4bf21a6d
RD
752If VARIABLE is a hash, it chomps the hash's values, but not its keys.
753
a0d0e21e
LW
754You can actually chomp anything that's an lvalue, including an assignment:
755
756 chomp($cwd = `pwd`);
757 chomp($answer = <STDIN>);
758
759If you chomp a list, each element is chomped, and the total number of
760characters removed is returned.
761
442a8c12
NC
762If the C<encoding> pragma is in scope then the lengths returned are
763calculated from the length of C<$/> in Unicode characters, which is not
764always the same as the length of C<$/> in the native encoding.
765
15e44fd8
RGS
766Note that parentheses are necessary when you're chomping anything
767that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
768is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
769C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
770C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
771as C<chomp($a, $b)>.
772
a0d0e21e 773=item chop VARIABLE
d74e8afc 774X<chop>
a0d0e21e 775
313c9f5c 776=item chop( LIST )
a0d0e21e
LW
777
778=item chop
779
780Chops off the last character of a string and returns the character
5b3eff12 781chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 782scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
783If VARIABLE is a hash, it chops the hash's values, but not its keys.
784
5b3eff12 785You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
786
787If you chop a list, each element is chopped. Only the value of the
19799a22 788last C<chop> is returned.
a0d0e21e 789
19799a22 790Note that C<chop> returns the last character. To return all but the last
748a9306
LW
791character, use C<substr($string, 0, -1)>.
792
15e44fd8
RGS
793See also L</chomp>.
794
a0d0e21e 795=item chown LIST
d74e8afc 796X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
797
798Changes the owner (and group) of a list of files. The first two
19799a22
GS
799elements of the list must be the I<numeric> uid and gid, in that
800order. A value of -1 in either position is interpreted by most
801systems to leave that value unchanged. Returns the number of files
802successfully changed.
a0d0e21e
LW
803
804 $cnt = chown $uid, $gid, 'foo', 'bar';
805 chown $uid, $gid, @filenames;
806
c4aca7d0
GA
807On systems that support fchown, you might pass file handles among the
808files. On systems that don't support fchown, passing file handles
345da378
GA
809produces a fatal error at run time. The file handles must be passed
810as globs or references to be recognized. Barewords are considered
811file names.
c4aca7d0 812
54310121 813Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
814
815 print "User: ";
19799a22 816 chomp($user = <STDIN>);
5a964f20 817 print "Files: ";
19799a22 818 chomp($pattern = <STDIN>);
a0d0e21e
LW
819
820 ($login,$pass,$uid,$gid) = getpwnam($user)
821 or die "$user not in passwd file";
822
5a964f20 823 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
824 chown $uid, $gid, @ary;
825
54310121 826On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
827file unless you're the superuser, although you should be able to change
828the group to any of your secondary groups. On insecure systems, these
829restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
830On POSIX systems, you can detect this condition this way:
831
832 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
833 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 834
a0d0e21e 835=item chr NUMBER
d74e8afc 836X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 837
54310121 838=item chr
bbce6d69 839
a0d0e21e 840Returns the character represented by that NUMBER in the character set.
a0ed51b3 841For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1e54db1a
JH
842chr(0x263a) is a Unicode smiley face. Note that characters from 128
843to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
844backward compatibility reasons (but see L<encoding>).
aaa68c4a 845
8a064bd6 846Negative values give the Unicode replacement character (chr(0xfffd)),
5f0135eb 847except under the L<bytes> pragma, where low eight bits of the value
8a064bd6
JH
848(truncated to an integer) are used.
849
974da8e5
JH
850If NUMBER is omitted, uses C<$_>.
851
b76cc8ba 852For the reverse, use L</ord>.
a0d0e21e 853
974da8e5
JH
854Note that under the C<bytes> pragma the NUMBER is masked to
855the low eight bits.
856
857See L<perlunicode> and L<encoding> for more about Unicode.
bbce6d69 858
a0d0e21e 859=item chroot FILENAME
d74e8afc 860X<chroot> X<root>
a0d0e21e 861
54310121 862=item chroot
bbce6d69 863
5a964f20 864This function works like the system call by the same name: it makes the
4633a7c4 865named directory the new root directory for all further pathnames that
951ba7fe 866begin with a C</> by your process and all its children. (It doesn't
28757baa 867change your current working directory, which is unaffected.) For security
4633a7c4 868reasons, this call is restricted to the superuser. If FILENAME is
19799a22 869omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
870
871=item close FILEHANDLE
d74e8afc 872X<close>
a0d0e21e 873
6a518fbc
TP
874=item close
875
e0f13c26
RGS
876Closes the file or pipe associated with the file handle, flushes the IO
877buffers, and closes the system file descriptor. Returns true if those
878operations have succeeded and if no error was reported by any PerlIO
879layer. Closes the currently selected filehandle if the argument is
880omitted.
fb73857a
PP
881
882You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
883another C<open> on it, because C<open> will close it for you. (See
884C<open>.) However, an explicit C<close> on an input file resets the line
885counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 886
dede8123
RGS
887If the file handle came from a piped open, C<close> will additionally
888return false if one of the other system calls involved fails, or if the
fb73857a 889program exits with non-zero status. (If the only problem was that the
dede8123 890program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 891also waits for the process executing on the pipe to complete, in case you
b76cc8ba 892want to look at the output of the pipe afterwards, and
e5218da5
GA
893implicitly puts the exit status value of that command into C<$?> and
894C<${^CHILD_ERROR_NATIVE}>.
5a964f20 895
73689b13
GS
896Prematurely closing the read end of a pipe (i.e. before the process
897writing to it at the other end has closed it) will result in a
898SIGPIPE being delivered to the writer. If the other end can't
899handle that, be sure to read all the data before closing the pipe.
900
fb73857a 901Example:
a0d0e21e 902
fb73857a
PP
903 open(OUTPUT, '|sort >foo') # pipe to sort
904 or die "Can't start sort: $!";
5a964f20 905 #... # print stuff to output
fb73857a
PP
906 close OUTPUT # wait for sort to finish
907 or warn $! ? "Error closing sort pipe: $!"
908 : "Exit status $? from sort";
909 open(INPUT, 'foo') # get sort's results
910 or die "Can't open 'foo' for input: $!";
a0d0e21e 911
5a964f20
TC
912FILEHANDLE may be an expression whose value can be used as an indirect
913filehandle, usually the real filehandle name.
a0d0e21e
LW
914
915=item closedir DIRHANDLE
d74e8afc 916X<closedir>
a0d0e21e 917
19799a22 918Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
919system call.
920
a0d0e21e 921=item connect SOCKET,NAME
d74e8afc 922X<connect>
a0d0e21e
LW
923
924Attempts to connect to a remote socket, just as the connect system call
19799a22 925does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
926packed address of the appropriate type for the socket. See the examples in
927L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 928
cb1a09d0 929=item continue BLOCK
d74e8afc 930X<continue>
cb1a09d0 931
0d863452
RH
932=item continue
933
cf264981
SP
934C<continue> is actually a flow control statement rather than a function. If
935there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
936C<foreach>), it is always executed just before the conditional is about to
937be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
938it can be used to increment a loop variable, even when the loop has been
939continued via the C<next> statement (which is similar to the C C<continue>
940statement).
941
98293880 942C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
943block. C<last> and C<redo> will behave as if they had been executed within
944the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
945block, it may be more entertaining.
946
947 while (EXPR) {
948 ### redo always comes here
949 do_something;
950 } continue {
951 ### next always comes here
952 do_something_else;
953 # then back the top to re-check EXPR
954 }
955 ### last always comes here
956
957Omitting the C<continue> section is semantically equivalent to using an
19799a22 958empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
959to check the condition at the top of the loop.
960
0d863452
RH
961If the "switch" feature is enabled, C<continue> is also a
962function that will break out of the current C<when> or C<default>
963block, and fall through to the next case. See L<feature> and
964L<perlsyn/"Switch statements"> for more information.
965
966
a0d0e21e 967=item cos EXPR
d74e8afc 968X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 969
d6217f1e
GS
970=item cos
971
5a964f20 972Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 973takes cosine of C<$_>.
a0d0e21e 974
ca6e1c26 975For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
976function, or use this relation:
977
978 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
979
a0d0e21e 980=item crypt PLAINTEXT,SALT
d74e8afc 981X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 982X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 983
ef2e6798
MS
984Creates a digest string exactly like the crypt(3) function in the C
985library (assuming that you actually have a version there that has not
cf264981 986been extirpated as a potential munitions).
ef2e6798
MS
987
988crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
989into a short string, called a digest, which is returned. The same
990PLAINTEXT and SALT will always return the same string, but there is no
991(known) way to get the original PLAINTEXT from the hash. Small
992changes in the PLAINTEXT or SALT will result in large changes in the
993digest.
994
995There is no decrypt function. This function isn't all that useful for
996cryptography (for that, look for F<Crypt> modules on your nearby CPAN
997mirror) and the name "crypt" is a bit of a misnomer. Instead it is
998primarily used to check if two pieces of text are the same without
999having to transmit or store the text itself. An example is checking
1000if a correct password is given. The digest of the password is stored,
cf264981 1001not the password itself. The user types in a password that is
ef2e6798
MS
1002crypt()'d with the same salt as the stored digest. If the two digests
1003match the password is correct.
1004
1005When verifying an existing digest string you should use the digest as
1006the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1007to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1008crypt() will hash the new string with the same salt as the digest.
1009This allows your code to work with the standard L<crypt|/crypt> and
1010with more exotic implementations. In other words, do not assume
1011anything about the returned string itself, or how many bytes in the
1012digest matter.
85c16d83
JH
1013
1014Traditionally the result is a string of 13 bytes: two first bytes of
1015the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
ef2e6798
MS
1016the first eight bytes of the digest string mattered, but alternative
1017hashing schemes (like MD5), higher level security schemes (like C2),
1018and implementations on non-UNIX platforms may produce different
1019strings.
85c16d83
JH
1020
1021When choosing a new salt create a random two character string whose
1022characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1023'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1024characters is just a recommendation; the characters allowed in
1025the salt depend solely on your system's crypt library, and Perl can't
1026restrict what salts C<crypt()> accepts.
e71965be 1027
a0d0e21e 1028Here's an example that makes sure that whoever runs this program knows
cf264981 1029their password:
a0d0e21e
LW
1030
1031 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1032
1033 system "stty -echo";
1034 print "Password: ";
e71965be 1035 chomp($word = <STDIN>);
a0d0e21e
LW
1036 print "\n";
1037 system "stty echo";
1038
e71965be 1039 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
1040 die "Sorry...\n";
1041 } else {
1042 print "ok\n";
54310121 1043 }
a0d0e21e 1044
9f8f0c9d 1045Of course, typing in your own password to whoever asks you
748a9306 1046for it is unwise.
a0d0e21e 1047
ef2e6798 1048The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1049of data, not least of all because you can't get the information
ef2e6798 1050back. Look at the L<Digest> module for more robust algorithms.
19799a22 1051
f2791508
JH
1052If using crypt() on a Unicode string (which I<potentially> has
1053characters with codepoints above 255), Perl tries to make sense
1054of the situation by trying to downgrade (a copy of the string)
1055the string back to an eight-bit byte string before calling crypt()
1056(on that copy). If that works, good. If not, crypt() dies with
1057C<Wide character in crypt>.
85c16d83 1058
aa689395 1059=item dbmclose HASH
d74e8afc 1060X<dbmclose>
a0d0e21e 1061
19799a22 1062[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1063
aa689395 1064Breaks the binding between a DBM file and a hash.
a0d0e21e 1065
19799a22 1066=item dbmopen HASH,DBNAME,MASK
d74e8afc 1067X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1068
19799a22 1069[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1070
7b8d334a 1071This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1072hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1073argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1074is the name of the database (without the F<.dir> or F<.pag> extension if
1075any). If the database does not exist, it is created with protection
19799a22
GS
1076specified by MASK (as modified by the C<umask>). If your system supports
1077only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 1078program. In older versions of Perl, if your system had neither DBM nor
19799a22 1079ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1080sdbm(3).
1081
1082If you don't have write access to the DBM file, you can only read hash
1083variables, not set them. If you want to test whether you can write,
19799a22 1084either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 1085which will trap the error.
a0d0e21e 1086
19799a22
GS
1087Note that functions such as C<keys> and C<values> may return huge lists
1088when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1089function to iterate over large DBM files. Example:
1090
1091 # print out history file offsets
1092 dbmopen(%HIST,'/usr/lib/news/history',0666);
1093 while (($key,$val) = each %HIST) {
1094 print $key, ' = ', unpack('L',$val), "\n";
1095 }
1096 dbmclose(%HIST);
1097
cb1a09d0 1098See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1099cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1100rich implementation.
4633a7c4 1101
2b5ab1e7
TC
1102You can control which DBM library you use by loading that library
1103before you call dbmopen():
1104
1105 use DB_File;
1106 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1107 or die "Can't open netscape history file: $!";
1108
a0d0e21e 1109=item defined EXPR
d74e8afc 1110X<defined> X<undef> X<undefined>
a0d0e21e 1111
54310121 1112=item defined
bbce6d69 1113
2f9daede
TPG
1114Returns a Boolean value telling whether EXPR has a value other than
1115the undefined value C<undef>. If EXPR is not present, C<$_> will be
1116checked.
1117
1118Many operations return C<undef> to indicate failure, end of file,
1119system error, uninitialized variable, and other exceptional
1120conditions. This function allows you to distinguish C<undef> from
1121other values. (A simple Boolean test will not distinguish among
7660c0ab 1122C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1123false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1124doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1125returns C<undef> when its argument is an empty array, I<or> when the
1126element to return happens to be C<undef>.
1127
f10b0346
GS
1128You may also use C<defined(&func)> to check whether subroutine C<&func>
1129has ever been defined. The return value is unaffected by any forward
04891299 1130declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1131may still be callable: its package may have an C<AUTOLOAD> method that
1132makes it spring into existence the first time that it is called -- see
1133L<perlsub>.
f10b0346
GS
1134
1135Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1136used to report whether memory for that aggregate has ever been
1137allocated. This behavior may disappear in future versions of Perl.
1138You should instead use a simple test for size:
1139
1140 if (@an_array) { print "has array elements\n" }
1141 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1142
1143When used on a hash element, it tells you whether the value is defined,
dc848c6f 1144not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1145purpose.
a0d0e21e
LW
1146
1147Examples:
1148
1149 print if defined $switch{'D'};
1150 print "$val\n" while defined($val = pop(@ary));
1151 die "Can't readlink $sym: $!"
1152 unless defined($value = readlink $sym);
a0d0e21e 1153 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1154 $debugging = 0 unless defined $debugging;
a0d0e21e 1155
19799a22 1156Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1157discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1158defined values. For example, if you say
a5f75d66
AD
1159
1160 "ab" =~ /a(.*)b/;
1161
7660c0ab 1162The pattern match succeeds, and C<$1> is defined, despite the fact that it
cf264981 1163matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1164matched something that happened to be zero characters long. This is all
a5f75d66 1165very above-board and honest. When a function returns an undefined value,
2f9daede 1166it's an admission that it couldn't give you an honest answer. So you
19799a22 1167should use C<defined> only when you're questioning the integrity of what
7660c0ab 1168you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1169what you want.
1170
dc848c6f 1171See also L</undef>, L</exists>, L</ref>.
2f9daede 1172
a0d0e21e 1173=item delete EXPR
d74e8afc 1174X<delete>
a0d0e21e 1175
01020589
GS
1176Given an expression that specifies a hash element, array element, hash slice,
1177or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1178In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1179the size of the array will shrink to the highest element that tests
8216c1fd 1180true for exists() (or 0 if no such element exists).
a0d0e21e 1181
eba0920a
EM
1182Returns a list with the same number of elements as the number of elements
1183for which deletion was attempted. Each element of that list consists of
1184either the value of the element deleted, or the undefined value. In scalar
1185context, this means that you get the value of the last element deleted (or
1186the undefined value if that element did not exist).
1187
1188 %hash = (foo => 11, bar => 22, baz => 33);
1189 $scalar = delete $hash{foo}; # $scalar is 11
1190 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1191 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1192
1193Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1194a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1195from a C<tie>d hash or array may not necessarily return anything.
1196
8ea97a1e
GS
1197Deleting an array element effectively returns that position of the array
1198to its initial, uninitialized state. Subsequently testing for the same
cf264981
SP
1199element with exists() will return false. Also, deleting array elements
1200in the middle of an array will not shift the index of the elements
1201after them down. Use splice() for that. See L</exists>.
8ea97a1e 1202
01020589 1203The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1204
5f05dabc
PP
1205 foreach $key (keys %HASH) {
1206 delete $HASH{$key};
a0d0e21e
LW
1207 }
1208
01020589
GS
1209 foreach $index (0 .. $#ARRAY) {
1210 delete $ARRAY[$index];
1211 }
1212
1213And so do these:
5f05dabc 1214
01020589
GS
1215 delete @HASH{keys %HASH};
1216
9740c838 1217 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1218
2b5ab1e7 1219But both of these are slower than just assigning the empty list
01020589
GS
1220or undefining %HASH or @ARRAY:
1221
1222 %HASH = (); # completely empty %HASH
1223 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1224
01020589
GS
1225 @ARRAY = (); # completely empty @ARRAY
1226 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1227
1228Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1229operation is a hash element, array element, hash slice, or array slice
1230lookup:
a0d0e21e
LW
1231
1232 delete $ref->[$x][$y]{$key};
5f05dabc 1233 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1234
01020589
GS
1235 delete $ref->[$x][$y][$index];
1236 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1237
a0d0e21e 1238=item die LIST
d74e8afc 1239X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1240
19799a22
GS
1241Outside an C<eval>, prints the value of LIST to C<STDERR> and
1242exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1243exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1244status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1245an C<eval(),> the error message is stuffed into C<$@> and the
1246C<eval> is terminated with the undefined value. This makes
1247C<die> the way to raise an exception.
a0d0e21e
LW
1248
1249Equivalent examples:
1250
1251 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1252 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1253
ccac6780 1254If the last element of LIST does not end in a newline, the current
df37ec69
WW
1255script line number and input line number (if any) are also printed,
1256and a newline is supplied. Note that the "input line number" (also
1257known as "chunk") is subject to whatever notion of "line" happens to
1258be currently in effect, and is also available as the special variable
1259C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1260
1261Hint: sometimes appending C<", stopped"> to your message will cause it
1262to make better sense when the string C<"at foo line 123"> is appended.
1263Suppose you are running script "canasta".
a0d0e21e
LW
1264
1265 die "/etc/games is no good";
1266 die "/etc/games is no good, stopped";
1267
1268produce, respectively
1269
1270 /etc/games is no good at canasta line 123.
1271 /etc/games is no good, stopped at canasta line 123.
1272
2b5ab1e7 1273See also exit(), warn(), and the Carp module.
a0d0e21e 1274
7660c0ab
A
1275If LIST is empty and C<$@> already contains a value (typically from a
1276previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1277This is useful for propagating exceptions:
1278
1279 eval { ... };
1280 die unless $@ =~ /Expected exception/;
1281
ad216e65
JH
1282If LIST is empty and C<$@> contains an object reference that has a
1283C<PROPAGATE> method, that method will be called with additional file
1284and line number parameters. The return value replaces the value in
28a5cf3b 1285C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1286were called.
1287
7660c0ab 1288If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1289
52531d10
GS
1290die() can also be called with a reference argument. If this happens to be
1291trapped within an eval(), $@ contains the reference. This behavior permits
1292a more elaborate exception handling implementation using objects that
4375e838 1293maintain arbitrary state about the nature of the exception. Such a scheme
52531d10 1294is sometimes preferable to matching particular string values of $@ using
746d7dd7
GL
1295regular expressions. Because $@ is a global variable, and eval() may be
1296used within object implementations, care must be taken that analyzing the
1297error object doesn't replace the reference in the global variable. The
1298easiest solution is to make a local copy of the reference before doing
1299other manipulations. Here's an example:
52531d10 1300
da279afe 1301 use Scalar::Util 'blessed';
1302
52531d10 1303 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1304 if (my $ev_err = $@) {
1305 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1306 # handle Some::Module::Exception
1307 }
1308 else {
1309 # handle all other possible exceptions
1310 }
1311 }
1312
19799a22 1313Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1314them, you may want to overload stringification operations on such custom
1315exception objects. See L<overload> for details about that.
1316
19799a22
GS
1317You can arrange for a callback to be run just before the C<die>
1318does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1319handler will be called with the error text and can change the error
1320message, if it sees fit, by calling C<die> again. See
1321L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1322L<"eval BLOCK"> for some examples. Although this feature was
19799a22
GS
1323to be run only right before your program was to exit, this is not
1324currently the case--the C<$SIG{__DIE__}> hook is currently called
1325even inside eval()ed blocks/strings! If one wants the hook to do
1326nothing in such situations, put
fb73857a
PP
1327
1328 die @_ if $^S;
1329
19799a22
GS
1330as the first line of the handler (see L<perlvar/$^S>). Because
1331this promotes strange action at a distance, this counterintuitive
b76cc8ba 1332behavior may be fixed in a future release.
774d564b 1333
a0d0e21e 1334=item do BLOCK
d74e8afc 1335X<do> X<block>
a0d0e21e
LW
1336
1337Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1338sequence of commands indicated by BLOCK. When modified by the C<while> or
1339C<until> loop modifier, executes the BLOCK once before testing the loop
1340condition. (On other statements the loop modifiers test the conditional
1341first.)
a0d0e21e 1342
4968c1e4 1343C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1344C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1345See L<perlsyn> for alternative strategies.
4968c1e4 1346
a0d0e21e 1347=item do SUBROUTINE(LIST)
d74e8afc 1348X<do>
a0d0e21e 1349
cf264981 1350This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1351
1352=item do EXPR
d74e8afc 1353X<do>
a0d0e21e
LW
1354
1355Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1356file as a Perl script.
a0d0e21e
LW
1357
1358 do 'stat.pl';
1359
1360is just like
1361
986b19de 1362 eval `cat stat.pl`;
a0d0e21e 1363
2b5ab1e7 1364except that it's more efficient and concise, keeps track of the current
ea63ef19 1365filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1366C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1367variables. It also differs in that code evaluated with C<do FILENAME>
1368cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1369same, however, in that it does reparse the file every time you call it,
1370so you probably don't want to do this inside a loop.
a0d0e21e 1371
8e30cc93 1372If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1373error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1374returns undef and sets an error message in C<$@>. If the file is
1375successfully compiled, C<do> returns the value of the last expression
1376evaluated.
1377
a0d0e21e 1378Note that inclusion of library modules is better done with the
19799a22 1379C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1380and raise an exception if there's a problem.
a0d0e21e 1381
5a964f20
TC
1382You might like to use C<do> to read in a program configuration
1383file. Manual error checking can be done this way:
1384
b76cc8ba 1385 # read in config files: system first, then user
f86cebdf 1386 for $file ("/share/prog/defaults.rc",
b76cc8ba 1387 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1388 {
5a964f20 1389 unless ($return = do $file) {
f86cebdf
GS
1390 warn "couldn't parse $file: $@" if $@;
1391 warn "couldn't do $file: $!" unless defined $return;
1392 warn "couldn't run $file" unless $return;
5a964f20
TC
1393 }
1394 }
1395
a0d0e21e 1396=item dump LABEL
d74e8afc 1397X<dump> X<core> X<undump>
a0d0e21e 1398
1614b0e3
JD
1399=item dump
1400
19799a22
GS
1401This function causes an immediate core dump. See also the B<-u>
1402command-line switch in L<perlrun>, which does the same thing.
1403Primarily this is so that you can use the B<undump> program (not
1404supplied) to turn your core dump into an executable binary after
1405having initialized all your variables at the beginning of the
1406program. When the new binary is executed it will begin by executing
1407a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1408Think of it as a goto with an intervening core dump and reincarnation.
1409If C<LABEL> is omitted, restarts the program from the top.
1410
1411B<WARNING>: Any files opened at the time of the dump will I<not>
1412be open any more when the program is reincarnated, with possible
b76cc8ba 1413resulting confusion on the part of Perl.
19799a22
GS
1414
1415This function is now largely obsolete, partly because it's very
1416hard to convert a core file into an executable, and because the
1417real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1418C code have superseded it. That's why you should now invoke it as
1419C<CORE::dump()>, if you don't want to be warned against a possible
1420typo.
19799a22
GS
1421
1422If you're looking to use L<dump> to speed up your program, consider
1423generating bytecode or native C code as described in L<perlcc>. If
1424you're just trying to accelerate a CGI script, consider using the
210b36aa 1425C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1426You might also consider autoloading or selfloading, which at least
b76cc8ba 1427make your program I<appear> to run faster.
5a964f20 1428
aa689395 1429=item each HASH
d74e8afc 1430X<each> X<hash, iterator>
aa689395 1431
5a964f20 1432When called in list context, returns a 2-element list consisting of the
aa689395 1433key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1434it. When called in scalar context, returns only the key for the next
e902a979 1435element in the hash.
2f9daede 1436
ab192400 1437Entries are returned in an apparently random order. The actual random
504f80c1
JH
1438order is subject to change in future versions of perl, but it is
1439guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1440function would produce on the same (unmodified) hash. Since Perl
14415.8.1 the ordering is different even between different runs of Perl
1442for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1443
1444When the hash is entirely read, a null array is returned in list context
19799a22
GS
1445(which when assigned produces a false (C<0>) value), and C<undef> in
1446scalar context. The next call to C<each> after that will start iterating
1447again. There is a single iterator for each hash, shared by all C<each>,
1448C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1449reading all the elements from the hash, or by evaluating C<keys HASH> or
1450C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1451iterating over it, you may get entries skipped or duplicated, so
1452don't. Exception: It is always safe to delete the item most recently
1453returned by C<each()>, which means that the following code will work:
1454
1455 while (($key, $value) = each %hash) {
1456 print $key, "\n";
1457 delete $hash{$key}; # This is safe
1458 }
aa689395 1459
f86cebdf 1460The following prints out your environment like the printenv(1) program,
aa689395 1461only in a different order:
a0d0e21e
LW
1462
1463 while (($key,$value) = each %ENV) {
1464 print "$key=$value\n";
1465 }
1466
19799a22 1467See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1468
1469=item eof FILEHANDLE
d74e8afc
ITB
1470X<eof>
1471X<end of file>
1472X<end-of-file>
a0d0e21e 1473
4633a7c4
LW
1474=item eof ()
1475
a0d0e21e
LW
1476=item eof
1477
1478Returns 1 if the next read on FILEHANDLE will return end of file, or if
1479FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1480gives the real filehandle. (Note that this function actually
19799a22 1481reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1482interactive context.) Do not read from a terminal file (or call
19799a22 1483C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1484as terminals may lose the end-of-file condition if you do.
1485
820475bd
GS
1486An C<eof> without an argument uses the last file read. Using C<eof()>
1487with empty parentheses is very different. It refers to the pseudo file
1488formed from the files listed on the command line and accessed via the
61eff3bc
JH
1489C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1490as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1491used will cause C<@ARGV> to be examined to determine if input is
67408cae 1492available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1493end-of-file will assume you are processing another C<@ARGV> list,
1494and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1495see L<perlop/"I/O Operators">.
820475bd 1496
61eff3bc 1497In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1498detect the end of each file, C<eof()> will only detect the end of the
1499last file. Examples:
a0d0e21e 1500
748a9306
LW
1501 # reset line numbering on each input file
1502 while (<>) {
b76cc8ba 1503 next if /^\s*#/; # skip comments
748a9306 1504 print "$.\t$_";
5a964f20
TC
1505 } continue {
1506 close ARGV if eof; # Not eof()!
748a9306
LW
1507 }
1508
a0d0e21e
LW
1509 # insert dashes just before last line of last file
1510 while (<>) {
6ac88b13 1511 if (eof()) { # check for end of last file
a0d0e21e
LW
1512 print "--------------\n";
1513 }
1514 print;
6ac88b13 1515 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1516 }
1517
a0d0e21e 1518Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1519input operators typically return C<undef> when they run out of data, or if
1520there was an error.
a0d0e21e
LW
1521
1522=item eval EXPR
d74e8afc 1523X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1524X<error, handling> X<exception, handling>
a0d0e21e
LW
1525
1526=item eval BLOCK
1527
ce2984c3
PF
1528=item eval
1529
c7cc6f1c
GS
1530In the first form, the return value of EXPR is parsed and executed as if it
1531were a little Perl program. The value of the expression (which is itself
5a964f20 1532determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1533errors, executed in the lexical context of the current Perl program, so
1534that any variable settings or subroutine and format definitions remain
cf264981 1535afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1536If EXPR is omitted, evaluates C<$_>. This form is typically used to
1537delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1538
1539In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1540same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1541within the context of the current Perl program. This form is typically
1542used to trap exceptions more efficiently than the first (see below), while
1543also providing the benefit of checking the code within BLOCK at compile
1544time.
1545
1546The final semicolon, if any, may be omitted from the value of EXPR or within
1547the BLOCK.
1548
1549In both forms, the value returned is the value of the last expression
5a964f20 1550evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1551as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1552in void, scalar, or list context, depending on the context of the C<eval>
1553itself. See L</wantarray> for more on how the evaluation context can be
1554determined.
a0d0e21e 1555
19799a22
GS
1556If there is a syntax error or runtime error, or a C<die> statement is
1557executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1558error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1559string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1560warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1561To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1562turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1563See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1564
19799a22
GS
1565Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1566determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1567is implemented. It is also Perl's exception trapping mechanism, where
1568the die operator is used to raise exceptions.
1569
1570If the code to be executed doesn't vary, you may use the eval-BLOCK
1571form to trap run-time errors without incurring the penalty of
1572recompiling each time. The error, if any, is still returned in C<$@>.
1573Examples:
1574
54310121 1575 # make divide-by-zero nonfatal
a0d0e21e
LW
1576 eval { $answer = $a / $b; }; warn $@ if $@;
1577
1578 # same thing, but less efficient
1579 eval '$answer = $a / $b'; warn $@ if $@;
1580
1581 # a compile-time error
5a964f20 1582 eval { $answer = }; # WRONG
a0d0e21e
LW
1583
1584 # a run-time error
1585 eval '$answer ='; # sets $@
1586
cf264981
SP
1587Using the C<eval{}> form as an exception trap in libraries does have some
1588issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1589may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7
TC
1590You can use the C<local $SIG{__DIE__}> construct for this purpose,
1591as shown in this example:
774d564b
PP
1592
1593 # a very private exception trap for divide-by-zero
f86cebdf
GS
1594 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1595 warn $@ if $@;
774d564b
PP
1596
1597This is especially significant, given that C<__DIE__> hooks can call
19799a22 1598C<die> again, which has the effect of changing their error messages:
774d564b
PP
1599
1600 # __DIE__ hooks may modify error messages
1601 {
f86cebdf
GS
1602 local $SIG{'__DIE__'} =
1603 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1604 eval { die "foo lives here" };
1605 print $@ if $@; # prints "bar lives here"
774d564b
PP
1606 }
1607
19799a22 1608Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1609may be fixed in a future release.
1610
19799a22 1611With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1612being looked at when:
1613
1614 eval $x; # CASE 1
1615 eval "$x"; # CASE 2
1616
1617 eval '$x'; # CASE 3
1618 eval { $x }; # CASE 4
1619
5a964f20 1620 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1621 $$x++; # CASE 6
1622
2f9daede 1623Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1624the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1625the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1626and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1627does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1628purely visual reasons, but it also has the advantage of compiling at
1629compile-time instead of at run-time.) Case 5 is a place where
19799a22 1630normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1631particular situation, you can just use symbolic references instead, as
1632in case 6.
a0d0e21e 1633
4968c1e4 1634C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1635C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1636
d819b83a
DM
1637Note that as a very special case, an C<eval ''> executed within the C<DB>
1638package doesn't see the usual surrounding lexical scope, but rather the
1639scope of the first non-DB piece of code that called it. You don't normally
1640need to worry about this unless you are writing a Perl debugger.
1641
a0d0e21e 1642=item exec LIST
d74e8afc 1643X<exec> X<execute>
a0d0e21e 1644
8bf3b016
GS
1645=item exec PROGRAM LIST
1646
19799a22
GS
1647The C<exec> function executes a system command I<and never returns>--
1648use C<system> instead of C<exec> if you want it to return. It fails and
1649returns false only if the command does not exist I<and> it is executed
fb73857a 1650directly instead of via your system's command shell (see below).
a0d0e21e 1651
19799a22
GS
1652Since it's a common mistake to use C<exec> instead of C<system>, Perl
1653warns you if there is a following statement which isn't C<die>, C<warn>,
1654or C<exit> (if C<-w> is set - but you always do that). If you
1655I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1656can use one of these styles to avoid the warning:
1657
5a964f20
TC
1658 exec ('foo') or print STDERR "couldn't exec foo: $!";
1659 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1660
5a964f20 1661If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1662with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1663If there is only one scalar argument or an array with one element in it,
1664the argument is checked for shell metacharacters, and if there are any,
1665the entire argument is passed to the system's command shell for parsing
1666(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1667If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1668words and passed directly to C<execvp>, which is more efficient.
19799a22 1669Examples:
a0d0e21e 1670
19799a22
GS
1671 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1672 exec "sort $outfile | uniq";
a0d0e21e
LW
1673
1674If you don't really want to execute the first argument, but want to lie
1675to the program you are executing about its own name, you can specify
1676the program you actually want to run as an "indirect object" (without a
1677comma) in front of the LIST. (This always forces interpretation of the
54310121 1678LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1679the list.) Example:
1680
1681 $shell = '/bin/csh';
1682 exec $shell '-sh'; # pretend it's a login shell
1683
1684or, more directly,
1685
1686 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1687
bb32b41a
GS
1688When the arguments get executed via the system shell, results will
1689be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1690for details.
1691
19799a22
GS
1692Using an indirect object with C<exec> or C<system> is also more
1693secure. This usage (which also works fine with system()) forces
1694interpretation of the arguments as a multivalued list, even if the
1695list had just one argument. That way you're safe from the shell
1696expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1697
1698 @args = ( "echo surprise" );
1699
2b5ab1e7 1700 exec @args; # subject to shell escapes
f86cebdf 1701 # if @args == 1
2b5ab1e7 1702 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1703
1704The first version, the one without the indirect object, ran the I<echo>
1705program, passing it C<"surprise"> an argument. The second version
1706didn't--it tried to run a program literally called I<"echo surprise">,
1707didn't find it, and set C<$?> to a non-zero value indicating failure.
1708
0f897271
GS
1709Beginning with v5.6.0, Perl will attempt to flush all files opened for
1710output before the exec, but this may not be supported on some platforms
1711(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1712in English) or call the C<autoflush()> method of C<IO::Handle> on any
1713open handles in order to avoid lost output.
1714
19799a22 1715Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1716any C<DESTROY> methods in your objects.
1717
a0d0e21e 1718=item exists EXPR
d74e8afc 1719X<exists> X<autovivification>
a0d0e21e 1720
01020589 1721Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1722returns true if the specified element in the hash or array has ever
1723been initialized, even if the corresponding value is undefined. The
1724element is not autovivified if it doesn't exist.
a0d0e21e 1725
01020589
GS
1726 print "Exists\n" if exists $hash{$key};
1727 print "Defined\n" if defined $hash{$key};
1728 print "True\n" if $hash{$key};
1729
1730 print "Exists\n" if exists $array[$index];
1731 print "Defined\n" if defined $array[$index];
1732 print "True\n" if $array[$index];
a0d0e21e 1733
8ea97a1e 1734A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1735it exists, but the reverse doesn't necessarily hold true.
1736
afebc493
GS
1737Given an expression that specifies the name of a subroutine,
1738returns true if the specified subroutine has ever been declared, even
1739if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1740does not count as declaring it. Note that a subroutine which does not
1741exist may still be callable: its package may have an C<AUTOLOAD>
1742method that makes it spring into existence the first time that it is
1743called -- see L<perlsub>.
afebc493
GS
1744
1745 print "Exists\n" if exists &subroutine;
1746 print "Defined\n" if defined &subroutine;
1747
a0d0e21e 1748Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1749operation is a hash or array key lookup or subroutine name:
a0d0e21e 1750
2b5ab1e7
TC
1751 if (exists $ref->{A}->{B}->{$key}) { }
1752 if (exists $hash{A}{B}{$key}) { }
1753
01020589
GS
1754 if (exists $ref->{A}->{B}->[$ix]) { }
1755 if (exists $hash{A}{B}[$ix]) { }
1756
afebc493
GS
1757 if (exists &{$ref->{A}{B}{$key}}) { }
1758
01020589
GS
1759Although the deepest nested array or hash will not spring into existence
1760just because its existence was tested, any intervening ones will.
61eff3bc 1761Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1762into existence due to the existence test for the $key element above.
1763This happens anywhere the arrow operator is used, including even:
5a964f20 1764
2b5ab1e7
TC
1765 undef $ref;
1766 if (exists $ref->{"Some key"}) { }
1767 print $ref; # prints HASH(0x80d3d5c)
1768
1769This surprising autovivification in what does not at first--or even
1770second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1771release.
a0d0e21e 1772
afebc493
GS
1773Use of a subroutine call, rather than a subroutine name, as an argument
1774to exists() is an error.
1775
1776 exists &sub; # OK
1777 exists &sub(); # Error
1778
a0d0e21e 1779=item exit EXPR
d74e8afc 1780X<exit> X<terminate> X<abort>
a0d0e21e 1781
ce2984c3
PF
1782=item exit
1783
2b5ab1e7 1784Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1785
1786 $ans = <STDIN>;
1787 exit 0 if $ans =~ /^[Xx]/;
1788
19799a22 1789See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1790universally recognized values for EXPR are C<0> for success and C<1>
1791for error; other values are subject to interpretation depending on the
1792environment in which the Perl program is running. For example, exiting
179369 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1794the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1795
19799a22
GS
1796Don't use C<exit> to abort a subroutine if there's any chance that
1797someone might want to trap whatever error happened. Use C<die> instead,
1798which can be trapped by an C<eval>.
28757baa 1799
19799a22 1800The exit() function does not always exit immediately. It calls any
2b5ab1e7 1801defined C<END> routines first, but these C<END> routines may not
19799a22 1802themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1803be called are called before the real exit. If this is a problem, you
1804can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1805See L<perlmod> for details.
5a964f20 1806
a0d0e21e 1807=item exp EXPR
d74e8afc 1808X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1809
54310121 1810=item exp
bbce6d69 1811
b76cc8ba 1812Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1813If EXPR is omitted, gives C<exp($_)>.
1814
1815=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1816X<fcntl>
a0d0e21e 1817
f86cebdf 1818Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1819
1820 use Fcntl;
1821
0ade1984 1822first to get the correct constant definitions. Argument processing and
b76cc8ba 1823value return works just like C<ioctl> below.
a0d0e21e
LW
1824For example:
1825
1826 use Fcntl;
5a964f20
TC
1827 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1828 or die "can't fcntl F_GETFL: $!";
1829
554ad1fc 1830You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1831Like C<ioctl>, it maps a C<0> return from the system call into
1832C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1833in numeric context. It is also exempt from the normal B<-w> warnings
1834on improper numeric conversions.
5a964f20 1835
19799a22 1836Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1837doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1838manpage to learn what functions are available on your system.
a0d0e21e 1839
be2f7487 1840Here's an example of setting a filehandle named C<REMOTE> to be
1841non-blocking at the system level. You'll have to negotiate C<$|>
1842on your own, though.
1843
1844 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1845
1846 $flags = fcntl(REMOTE, F_GETFL, 0)
1847 or die "Can't get flags for the socket: $!\n";
1848
1849 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1850 or die "Can't set flags for the socket: $!\n";
1851
a0d0e21e 1852=item fileno FILEHANDLE
d74e8afc 1853X<fileno>
a0d0e21e 1854
2b5ab1e7
TC
1855Returns the file descriptor for a filehandle, or undefined if the
1856filehandle is not open. This is mainly useful for constructing
19799a22 1857bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1858If FILEHANDLE is an expression, the value is taken as an indirect
1859filehandle, generally its name.
5a964f20 1860
b76cc8ba 1861You can use this to find out whether two handles refer to the
5a964f20
TC
1862same underlying descriptor:
1863
1864 if (fileno(THIS) == fileno(THAT)) {
1865 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1866 }
1867
1868(Filehandles connected to memory objects via new features of C<open> may
1869return undefined even though they are open.)
1870
a0d0e21e
LW
1871
1872=item flock FILEHANDLE,OPERATION
d74e8afc 1873X<flock> X<lock> X<locking>
a0d0e21e 1874
19799a22
GS
1875Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1876for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1877machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1878C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1879only entire files, not records.
1880
1881Two potentially non-obvious but traditional C<flock> semantics are
1882that it waits indefinitely until the lock is granted, and that its locks
1883B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1884fewer guarantees. This means that programs that do not also use C<flock>
1885may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1886your port's specific documentation, or your system-specific local manpages
1887for details. It's best to assume traditional behavior if you're writing
1888portable programs. (But if you're not, you should as always feel perfectly
1889free to write for your own system's idiosyncrasies (sometimes called
1890"features"). Slavish adherence to portability concerns shouldn't get
1891in the way of your getting your job done.)
a3cb178b 1892
8ebc5c01
PP
1893OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1894LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1895you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1896either individually, or as a group using the ':flock' tag. LOCK_SH
1897requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1898releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1899LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1900waiting for the lock (check the return status to see if you got it).
1901
2b5ab1e7
TC
1902To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1903before locking or unlocking it.
8ebc5c01 1904
f86cebdf 1905Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1906locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1907are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1908implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1909differing semantics shouldn't bite too many people.
1910
becacb53
TM
1911Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1912be open with read intent to use LOCK_SH and requires that it be open
1913with write intent to use LOCK_EX.
1914
19799a22
GS
1915Note also that some versions of C<flock> cannot lock things over the
1916network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1917that. If you like you can force Perl to ignore your system's flock(2)
1918function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1919the switch C<-Ud_flock> to the F<Configure> program when you configure
1920perl.
4633a7c4
LW
1921
1922Here's a mailbox appender for BSD systems.
a0d0e21e 1923
7e1af8bc 1924 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1925
1926 sub lock {
7e1af8bc 1927 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1928 # and, in case someone appended
1929 # while we were waiting...
1930 seek(MBOX, 0, 2);
1931 }
1932
1933 sub unlock {
7e1af8bc 1934 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1935 }
1936
1937 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1938 or die "Can't open mailbox: $!";
1939
1940 lock();
1941 print MBOX $msg,"\n\n";
1942 unlock();
1943
2b5ab1e7
TC
1944On systems that support a real flock(), locks are inherited across fork()
1945calls, whereas those that must resort to the more capricious fcntl()
1946function lose the locks, making it harder to write servers.
1947
cb1a09d0 1948See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1949
1950=item fork
d74e8afc 1951X<fork> X<child> X<parent>
a0d0e21e 1952
2b5ab1e7
TC
1953Does a fork(2) system call to create a new process running the
1954same program at the same point. It returns the child pid to the
1955parent process, C<0> to the child process, or C<undef> if the fork is
1956unsuccessful. File descriptors (and sometimes locks on those descriptors)
1957are shared, while everything else is copied. On most systems supporting
1958fork(), great care has gone into making it extremely efficient (for
1959example, using copy-on-write technology on data pages), making it the
1960dominant paradigm for multitasking over the last few decades.
5a964f20 1961
0f897271
GS
1962Beginning with v5.6.0, Perl will attempt to flush all files opened for
1963output before forking the child process, but this may not be supported
1964on some platforms (see L<perlport>). To be safe, you may need to set
1965C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1966C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1967
19799a22 1968If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1969accumulate zombies. On some systems, you can avoid this by setting
1970C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1971forking and reaping moribund children.
cb1a09d0 1972
28757baa
PP
1973Note that if your forked child inherits system file descriptors like
1974STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1975if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1976backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1977You should reopen those to F</dev/null> if it's any issue.
28757baa 1978
cb1a09d0 1979=item format
d74e8afc 1980X<format>
cb1a09d0 1981
19799a22 1982Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1983example:
1984
54310121 1985 format Something =
cb1a09d0
AD
1986 Test: @<<<<<<<< @||||| @>>>>>
1987 $str, $%, '$' . int($num)
1988 .
1989
1990 $str = "widget";
184e9718 1991 $num = $cost/$quantity;
cb1a09d0
AD
1992 $~ = 'Something';
1993 write;
1994
1995See L<perlform> for many details and examples.
1996
8903cb82 1997=item formline PICTURE,LIST
d74e8afc 1998X<formline>
a0d0e21e 1999
5a964f20 2000This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2001too. It formats (see L<perlform>) a list of values according to the
2002contents of PICTURE, placing the output into the format output
7660c0ab 2003accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2004Eventually, when a C<write> is done, the contents of
cf264981
SP
2005C<$^A> are written to some filehandle. You could also read C<$^A>
2006and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2007does one C<formline> per line of form, but the C<formline> function itself
748a9306 2008doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 2009that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
2010You may therefore need to use multiple formlines to implement a single
2011record format, just like the format compiler.
2012
19799a22 2013Be careful if you put double quotes around the picture, because an C<@>
748a9306 2014character may be taken to mean the beginning of an array name.
19799a22 2015C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
2016
2017=item getc FILEHANDLE
f723aae1 2018X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2019
2020=item getc
2021
2022Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
2023or the undefined value at end of file, or if there was an error (in
2024the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2025STDIN. This is not particularly efficient. However, it cannot be
2026used by itself to fetch single characters without waiting for the user
2027to hit enter. For that, try something more like:
4633a7c4
LW
2028
2029 if ($BSD_STYLE) {
2030 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2031 }
2032 else {
54310121 2033 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2034 }
2035
2036 $key = getc(STDIN);
2037
2038 if ($BSD_STYLE) {
2039 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2040 }
2041 else {
5f05dabc 2042 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
2043 }
2044 print "\n";
2045
54310121
PP
2046Determination of whether $BSD_STYLE should be set
2047is left as an exercise to the reader.
cb1a09d0 2048
19799a22 2049The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2050systems purporting POSIX compliance. See also the C<Term::ReadKey>
2051module from your nearest CPAN site; details on CPAN can be found on
2052L<perlmodlib/CPAN>.
a0d0e21e
LW
2053
2054=item getlogin
d74e8afc 2055X<getlogin> X<login>
a0d0e21e 2056
cf264981 2057This implements the C library function of the same name, which on most
5a964f20 2058systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 2059use C<getpwuid>.
a0d0e21e 2060
f86702cc 2061 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2062
19799a22
GS
2063Do not consider C<getlogin> for authentication: it is not as
2064secure as C<getpwuid>.
4633a7c4 2065
a0d0e21e 2066=item getpeername SOCKET
d74e8afc 2067X<getpeername> X<peer>
a0d0e21e
LW
2068
2069Returns the packed sockaddr address of other end of the SOCKET connection.
2070
4633a7c4
LW
2071 use Socket;
2072 $hersockaddr = getpeername(SOCK);
19799a22 2073 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2074 $herhostname = gethostbyaddr($iaddr, AF_INET);
2075 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2076
2077=item getpgrp PID
d74e8afc 2078X<getpgrp> X<group>
a0d0e21e 2079
47e29363 2080Returns the current process group for the specified PID. Use
7660c0ab 2081a PID of C<0> to get the current process group for the
4633a7c4 2082current process. Will raise an exception if used on a machine that
f86cebdf 2083doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2084group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2085does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2086
2087=item getppid
d74e8afc 2088X<getppid> X<parent> X<pid>
a0d0e21e
LW
2089
2090Returns the process id of the parent process.
2091
4d76a344
RGS
2092Note for Linux users: on Linux, the C functions C<getpid()> and
2093C<getppid()> return different values from different threads. In order to
2094be portable, this behavior is not reflected by the perl-level function
2095C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2096to call the underlying C<getppid()>, you may use the CPAN module
2097C<Linux::Pid>.
4d76a344 2098
a0d0e21e 2099=item getpriority WHICH,WHO
d74e8afc 2100X<getpriority> X<priority> X<nice>
a0d0e21e 2101
4633a7c4
LW
2102Returns the current priority for a process, a process group, or a user.
2103(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2104machine that doesn't implement getpriority(2).
a0d0e21e
LW
2105
2106=item getpwnam NAME
d74e8afc
ITB
2107X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2108X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2109X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2110X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2111X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2112X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2113
2114=item getgrnam NAME
2115
2116=item gethostbyname NAME
2117
2118=item getnetbyname NAME
2119
2120=item getprotobyname NAME
2121
2122=item getpwuid UID
2123
2124=item getgrgid GID
2125
2126=item getservbyname NAME,PROTO
2127
2128=item gethostbyaddr ADDR,ADDRTYPE
2129
2130=item getnetbyaddr ADDR,ADDRTYPE
2131
2132=item getprotobynumber NUMBER
2133
2134=item getservbyport PORT,PROTO
2135
2136=item getpwent
2137
2138=item getgrent
2139
2140=item gethostent
2141
2142=item getnetent
2143
2144=item getprotoent
2145
2146=item getservent
2147
2148=item setpwent
2149
2150=item setgrent
2151
2152=item sethostent STAYOPEN
2153
2154=item setnetent STAYOPEN
2155
2156=item setprotoent STAYOPEN
2157
2158=item setservent STAYOPEN
2159
2160=item endpwent
2161
2162=item endgrent
2163
2164=item endhostent
2165
2166=item endnetent
2167
2168=item endprotoent
2169
2170=item endservent
2171
2172These routines perform the same functions as their counterparts in the
5a964f20 2173system library. In list context, the return values from the
a0d0e21e
LW
2174various get routines are as follows:
2175
2176 ($name,$passwd,$uid,$gid,
6ee623d5 2177 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2178 ($name,$passwd,$gid,$members) = getgr*
2179 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2180 ($name,$aliases,$addrtype,$net) = getnet*
2181 ($name,$aliases,$proto) = getproto*
2182 ($name,$aliases,$port,$proto) = getserv*
2183
2184(If the entry doesn't exist you get a null list.)
2185
4602f195
JH
2186The exact meaning of the $gcos field varies but it usually contains
2187the real name of the user (as opposed to the login name) and other
2188information pertaining to the user. Beware, however, that in many
2189system users are able to change this information and therefore it
106325ad 2190cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2191L<perlsec>). The $passwd and $shell, user's encrypted password and
2192login shell, are also tainted, because of the same reason.
4602f195 2193
5a964f20 2194In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2195lookup by name, in which case you get the other thing, whatever it is.
2196(If the entry doesn't exist you get the undefined value.) For example:
2197
5a964f20
TC
2198 $uid = getpwnam($name);
2199 $name = getpwuid($num);
2200 $name = getpwent();
2201 $gid = getgrnam($name);
08a33e13 2202 $name = getgrgid($num);
5a964f20
TC
2203 $name = getgrent();
2204 #etc.
a0d0e21e 2205
4602f195
JH
2206In I<getpw*()> the fields $quota, $comment, and $expire are special
2207cases in the sense that in many systems they are unsupported. If the
2208$quota is unsupported, it is an empty scalar. If it is supported, it
2209usually encodes the disk quota. If the $comment field is unsupported,
2210it is an empty scalar. If it is supported it usually encodes some
2211administrative comment about the user. In some systems the $quota
2212field may be $change or $age, fields that have to do with password
2213aging. In some systems the $comment field may be $class. The $expire
2214field, if present, encodes the expiration period of the account or the
2215password. For the availability and the exact meaning of these fields
2216in your system, please consult your getpwnam(3) documentation and your
2217F<pwd.h> file. You can also find out from within Perl what your
2218$quota and $comment fields mean and whether you have the $expire field
2219by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2220C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2221files are only supported if your vendor has implemented them in the
2222intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2223shadow versions if you're running under privilege or if there exists
cf264981
SP
2224the shadow(3) functions as found in System V (this includes Solaris
2225and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2226facility are unlikely to be supported.
6ee623d5 2227
19799a22 2228The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2229the login names of the members of the group.
2230
2231For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2232C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2233C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2234addresses returned by the corresponding system library call. In the
2235Internet domain, each address is four bytes long and you can unpack it
2236by saying something like:
2237
f337b084 2238 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2239
2b5ab1e7
TC
2240The Socket library makes this slightly easier:
2241
2242 use Socket;
2243 $iaddr = inet_aton("127.1"); # or whatever address
2244 $name = gethostbyaddr($iaddr, AF_INET);
2245
2246 # or going the other way
19799a22 2247 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2248
19799a22
GS
2249If you get tired of remembering which element of the return list
2250contains which return value, by-name interfaces are provided
2251in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2252C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2253and C<User::grent>. These override the normal built-ins, supplying
2254versions that return objects with the appropriate names
2255for each field. For example:
5a964f20
TC
2256
2257 use File::stat;
2258 use User::pwent;
2259 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2260
b76cc8ba
NIS
2261Even though it looks like they're the same method calls (uid),
2262they aren't, because a C<File::stat> object is different from
19799a22 2263a C<User::pwent> object.
5a964f20 2264
a0d0e21e 2265=item getsockname SOCKET
d74e8afc 2266X<getsockname>
a0d0e21e 2267
19799a22
GS
2268Returns the packed sockaddr address of this end of the SOCKET connection,
2269in case you don't know the address because you have several different
2270IPs that the connection might have come in on.
a0d0e21e 2271
4633a7c4
LW
2272 use Socket;
2273 $mysockaddr = getsockname(SOCK);
19799a22 2274 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2275 printf "Connect to %s [%s]\n",
19799a22
GS
2276 scalar gethostbyaddr($myaddr, AF_INET),
2277 inet_ntoa($myaddr);
a0d0e21e
LW
2278
2279=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2280X<getsockopt>
a0d0e21e 2281
636e6b1f
TH
2282Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2283Options may exist at multiple protocol levels depending on the socket
2284type, but at least the uppermost socket level SOL_SOCKET (defined in the
2285C<Socket> module) will exist. To query options at another level the
2286protocol number of the appropriate protocol controlling the option
2287should be supplied. For example, to indicate that an option is to be
2288interpreted by the TCP protocol, LEVEL should be set to the protocol
2289number of TCP, which you can get using getprotobyname.
2290
2291The call returns a packed string representing the requested socket option,
2292or C<undef> if there is an error (the error reason will be in $!). What
2293exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2294your system documentation for details. A very common case however is that
cf264981 2295the option is an integer, in which case the result will be a packed
636e6b1f
TH
2296integer which you can decode using unpack with the C<i> (or C<I>) format.
2297
2298An example testing if Nagle's algorithm is turned on on a socket:
2299
4852725b 2300 use Socket qw(:all);
636e6b1f
TH
2301
2302 defined(my $tcp = getprotobyname("tcp"))
2303 or die "Could not determine the protocol number for tcp";
4852725b
DD
2304 # my $tcp = IPPROTO_TCP; # Alternative
2305 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2306 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2307 my $nodelay = unpack("I", $packed);
2308 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2309
a0d0e21e
LW
2310
2311=item glob EXPR
d74e8afc 2312X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2313
0a753a76
PP
2314=item glob
2315
d9a9d457
JL
2316In list context, returns a (possibly empty) list of filename expansions on
2317the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2318scalar context, glob iterates through such filename expansions, returning
2319undef when the list is exhausted. This is the internal function
2320implementing the C<< <*.c> >> operator, but you can use it directly. If
2321EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2322more detail in L<perlop/"I/O Operators">.
a0d0e21e 2323
3a4b19e4
GS
2324Beginning with v5.6.0, this operator is implemented using the standard
2325C<File::Glob> extension. See L<File::Glob> for details.
2326
a0d0e21e 2327=item gmtime EXPR
d74e8afc 2328X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2329
ce2984c3
PF
2330=item gmtime
2331
435fbc73
GS
2332Works just like L<localtime> but the returned values are
2333localized for the standard Greenwich time zone.
a0d0e21e 2334
435fbc73
GS
2335Note: when called in list context, $isdst, the last value
2336returned by gmtime is always C<0>. There is no
2337Daylight Saving Time in GMT.
0a753a76 2338
62aa5637
MS
2339See L<perlport/gmtime> for portability concerns.
2340
a0d0e21e 2341=item goto LABEL
d74e8afc 2342X<goto> X<jump> X<jmp>
a0d0e21e 2343
748a9306
LW
2344=item goto EXPR
2345
a0d0e21e
LW
2346=item goto &NAME
2347
7660c0ab 2348The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2349execution there. It may not be used to go into any construct that
7660c0ab 2350requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2351also can't be used to go into a construct that is optimized away,
19799a22 2352or to get out of a block or subroutine given to C<sort>.
0a753a76 2353It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2354including out of subroutines, but it's usually better to use some other
19799a22 2355construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2356need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2357(The difference being that C does not offer named loops combined with
2358loop control. Perl does, and this replaces most structured uses of C<goto>
2359in other languages.)
a0d0e21e 2360
7660c0ab
A
2361The C<goto-EXPR> form expects a label name, whose scope will be resolved
2362dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2363necessarily recommended if you're optimizing for maintainability:
2364
2365 goto ("FOO", "BAR", "GLARCH")[$i];
2366
1b6921cb
BT
2367The C<goto-&NAME> form is quite different from the other forms of
2368C<goto>. In fact, it isn't a goto in the normal sense at all, and
2369doesn't have the stigma associated with other gotos. Instead, it
2370exits the current subroutine (losing any changes set by local()) and
2371immediately calls in its place the named subroutine using the current
2372value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2373load another subroutine and then pretend that the other subroutine had
2374been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2375in the current subroutine are propagated to the other subroutine.)
2376After the C<goto>, not even C<caller> will be able to tell that this
2377routine was called first.
2378
2379NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2380containing a code reference, or a block that evaluates to a code
6cb9131c 2381reference.
a0d0e21e
LW
2382
2383=item grep BLOCK LIST
d74e8afc 2384X<grep>
a0d0e21e
LW
2385
2386=item grep EXPR,LIST
2387
2b5ab1e7
TC
2388This is similar in spirit to, but not the same as, grep(1) and its
2389relatives. In particular, it is not limited to using regular expressions.
2f9daede 2390
a0d0e21e 2391Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2392C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2393elements for which the expression evaluated to true. In scalar
2394context, returns the number of times the expression was true.
a0d0e21e
LW
2395
2396 @foo = grep(!/^#/, @bar); # weed out comments
2397
2398or equivalently,
2399
2400 @foo = grep {!/^#/} @bar; # weed out comments
2401
be3174d2
GS
2402Note that C<$_> is an alias to the list value, so it can be used to
2403modify the elements of the LIST. While this is useful and supported,
2404it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2405Similarly, grep returns aliases into the original list, much as a for
2406loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2407element of a list returned by grep (for example, in a C<foreach>, C<map>
2408or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2409This is usually something to be avoided when writing clear code.
a0d0e21e 2410
a4fb8298 2411If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2412been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2413the list elements, C<$_> keeps being lexical inside the block; i.e. it
2414can't be seen from the outside, avoiding any potential side-effects.
2415
19799a22 2416See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2417
a0d0e21e 2418=item hex EXPR
d74e8afc 2419X<hex> X<hexadecimal>
a0d0e21e 2420
54310121 2421=item hex
bbce6d69 2422
2b5ab1e7 2423Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2424(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2425L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2426
2427 print hex '0xAf'; # prints '175'
2428 print hex 'aF'; # same
a0d0e21e 2429
19799a22 2430Hex strings may only represent integers. Strings that would cause
53305cf1 2431integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2432unlike oct(). To present something as hex, look into L</printf>,
2433L</sprintf>, or L</unpack>.
19799a22 2434
ce2984c3 2435=item import LIST
d74e8afc 2436X<import>
a0d0e21e 2437
19799a22 2438There is no builtin C<import> function. It is just an ordinary
4633a7c4 2439method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2440names to another module. The C<use> function calls the C<import> method
cea6626f 2441for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2442
2443=item index STR,SUBSTR,POSITION
d74e8afc 2444X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2445
2446=item index STR,SUBSTR
2447
2b5ab1e7
TC
2448The index function searches for one string within another, but without
2449the wildcard-like behavior of a full regular-expression pattern match.
2450It returns the position of the first occurrence of SUBSTR in STR at
2451or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2452beginning of the string. POSITION before the beginning of the string
2453or after its end is treated as if it were the beginning or the end,
2454respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2455you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2456is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2457
2458=item int EXPR
f723aae1 2459X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2460
54310121 2461=item int
bbce6d69 2462
7660c0ab 2463Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2464You should not use this function for rounding: one because it truncates
2465towards C<0>, and two because machine representations of floating point
2466numbers can sometimes produce counterintuitive results. For example,
2467C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2468because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2469the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2470functions will serve you better than will int().
a0d0e21e
LW
2471
2472=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2473X<ioctl>
a0d0e21e 2474
2b5ab1e7 2475Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2476
6c567752 2477 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2478
a11c483f 2479to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2480exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2481own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2482(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2483may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2484written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2485will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2486has no string value but does have a numeric value, that value will be
2487passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2488true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2489functions may be needed to manipulate the values of structures used by
b76cc8ba 2490C<ioctl>.
a0d0e21e 2491
19799a22 2492The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2493
2494 if OS returns: then Perl returns:
2495 -1 undefined value
2496 0 string "0 but true"
2497 anything else that number
2498
19799a22 2499Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2500still easily determine the actual value returned by the operating
2501system:
2502
2b5ab1e7 2503 $retval = ioctl(...) || -1;
a0d0e21e
LW
2504 printf "System returned %d\n", $retval;
2505
be2f7487 2506The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2507about improper numeric conversions.
2508
a0d0e21e 2509=item join EXPR,LIST
d74e8afc 2510X<join>
a0d0e21e 2511
2b5ab1e7
TC
2512Joins the separate strings of LIST into a single string with fields
2513separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2514
2b5ab1e7 2515 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2516
eb6e2d6f
GS
2517Beware that unlike C<split>, C<join> doesn't take a pattern as its
2518first argument. Compare L</split>.
a0d0e21e 2519
aa689395 2520=item keys HASH
d74e8afc 2521X<keys> X<key>
aa689395 2522
504f80c1
JH
2523Returns a list consisting of all the keys of the named hash.
2524(In scalar context, returns the number of keys.)
2525
2526The keys are returned in an apparently random order. The actual
2527random order is subject to change in future versions of perl, but it
2528is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2529function produces (given that the hash has not been modified). Since
2530Perl 5.8.1 the ordering is different even between different runs of
2531Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2532Attacks">).
504f80c1 2533
cf264981
SP
2534As a side effect, calling keys() resets the HASH's internal iterator
2535(see L</each>). In particular, calling keys() in void context resets
2536the iterator with no other overhead.
a0d0e21e 2537
aa689395 2538Here is yet another way to print your environment:
a0d0e21e
LW
2539
2540 @keys = keys %ENV;
2541 @values = values %ENV;
b76cc8ba 2542 while (@keys) {
a0d0e21e
LW
2543 print pop(@keys), '=', pop(@values), "\n";
2544 }
2545
2546or how about sorted by key:
2547
2548 foreach $key (sort(keys %ENV)) {
2549 print $key, '=', $ENV{$key}, "\n";
2550 }
2551
8ea1e5d4
GS
2552The returned values are copies of the original keys in the hash, so
2553modifying them will not affect the original hash. Compare L</values>.
2554
19799a22 2555To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2556Here's a descending numeric sort of a hash by its values:
4633a7c4 2557
5a964f20 2558 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2559 printf "%4d %s\n", $hash{$key}, $key;
2560 }
2561
19799a22 2562As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2563allocated for the given hash. This can gain you a measure of efficiency if
2564you know the hash is going to get big. (This is similar to pre-extending
2565an array by assigning a larger number to $#array.) If you say
55497cff
PP
2566
2567 keys %hash = 200;
2568
ab192400
GS
2569then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2570in fact, since it rounds up to the next power of two. These
55497cff
PP
2571buckets will be retained even if you do C<%hash = ()>, use C<undef
2572%hash> if you want to free the storage while C<%hash> is still in scope.
2573You can't shrink the number of buckets allocated for the hash using
19799a22 2574C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2575as trying has no effect).
2576
19799a22 2577See also C<each>, C<values> and C<sort>.
ab192400 2578
b350dd2f 2579=item kill SIGNAL, LIST
d74e8afc 2580X<kill> X<signal>
a0d0e21e 2581
b350dd2f 2582Sends a signal to a list of processes. Returns the number of
517db077
GS
2583processes successfully signaled (which is not necessarily the
2584same as the number actually killed).
a0d0e21e
LW
2585
2586 $cnt = kill 1, $child1, $child2;
2587 kill 9, @goners;
2588
70fb64f6 2589If SIGNAL is zero, no signal is sent to the process, but the kill(2)
6cb9d3e4 2590system call will check whether it's possible to send a signal to it (that
70fb64f6
RGS
2591means, to be brief, that the process is owned by the same user, or we are
2592the super-user). This is a useful way to check that a child process is
81fd35db
DN
2593alive (even if only as a zombie) and hasn't changed its UID. See
2594L<perlport> for notes on the portability of this construct.
b350dd2f
GS
2595
2596Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2597process groups instead of processes. (On System V, a negative I<PROCESS>
2598number will also kill process groups, but that's not portable.) That
2599means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2600use a signal name in quotes.
2601
2602See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2603
2604=item last LABEL
d74e8afc 2605X<last> X<break>
a0d0e21e
LW
2606
2607=item last
2608
2609The C<last> command is like the C<break> statement in C (as used in
2610loops); it immediately exits the loop in question. If the LABEL is
2611omitted, the command refers to the innermost enclosing loop. The
2612C<continue> block, if any, is not executed:
2613
4633a7c4
LW
2614 LINE: while (<STDIN>) {
2615 last LINE if /^$/; # exit when done with header
5a964f20 2616 #...
a0d0e21e
LW
2617 }
2618
4968c1e4 2619C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2620C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2621a grep() or map() operation.
4968c1e4 2622
6c1372ed
GS
2623Note that a block by itself is semantically identical to a loop
2624that executes once. Thus C<last> can be used to effect an early
2625exit out of such a block.
2626
98293880
JH
2627See also L</continue> for an illustration of how C<last>, C<next>, and
2628C<redo> work.
1d2dff63 2629
a0d0e21e 2630=item lc EXPR
d74e8afc 2631X<lc> X<lowercase>
a0d0e21e 2632
54310121 2633=item lc
bbce6d69 2634
d1be9408 2635Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2636implementing the C<\L> escape in double-quoted strings. Respects
2637current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2638and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2639
7660c0ab 2640If EXPR is omitted, uses C<$_>.
bbce6d69 2641
a0d0e21e 2642=item lcfirst EXPR
d74e8afc 2643X<lcfirst> X<lowercase>
a0d0e21e 2644
54310121 2645=item lcfirst
bbce6d69 2646
ad0029c4
JH
2647Returns the value of EXPR with the first character lowercased. This
2648is the internal function implementing the C<\l> escape in
2649double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2650locale> in force. See L<perllocale> and L<perlunicode> for more
2651details about locale and Unicode support.
a0d0e21e 2652
7660c0ab 2653If EXPR is omitted, uses C<$_>.
bbce6d69 2654
a0d0e21e 2655=item length EXPR
d74e8afc 2656X<length> X<size>
a0d0e21e 2657
54310121 2658=item length
bbce6d69 2659
974da8e5 2660Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2661omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2662an entire array or hash to find out how many elements these have.
2663For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2664
974da8e5
JH
2665Note the I<characters>: if the EXPR is in Unicode, you will get the
2666number of characters, not the number of bytes. To get the length
2667in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2668
a0d0e21e 2669=item link OLDFILE,NEWFILE
d74e8afc 2670X<link>
a0d0e21e 2671
19799a22 2672Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2673success, false otherwise.
a0d0e21e
LW
2674
2675=item listen SOCKET,QUEUESIZE
d74e8afc 2676X<listen>
a0d0e21e 2677
19799a22 2678Does the same thing that the listen system call does. Returns true if
b76cc8ba 2679it succeeded, false otherwise. See the example in
cea6626f 2680L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2681
2682=item local EXPR
d74e8afc 2683X<local>
a0d0e21e 2684
19799a22 2685You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2686what most people think of as "local". See
13a2d996 2687L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2688
5a964f20
TC
2689A local modifies the listed variables to be local to the enclosing
2690block, file, or eval. If more than one value is listed, the list must
2691be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2692for details, including issues with tied arrays and hashes.
a0d0e21e 2693
a0d0e21e 2694=item localtime EXPR
435fbc73 2695X<localtime> X<ctime>
a0d0e21e 2696
ba053783
AL
2697=item localtime
2698
19799a22 2699Converts a time as returned by the time function to a 9-element list
5f05dabc 2700with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2701follows:
2702
54310121 2703 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2704 ($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
593b9c14 3004See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
3005
3006=item oct EXPR
d74e8afc 3007X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
a0d0e21e 3008
54310121 3009=item oct
bbce6d69 3010
4633a7c4 3011Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
3012value. (If EXPR happens to start off with C<0x>, interprets it as a
3013hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
3014binary string. Leading whitespace is ignored in all three cases.)
3015The following will handle decimal, binary, octal, and hex in the standard
3016Perl or C notation:
a0d0e21e
LW
3017
3018 $val = oct($val) if $val =~ /^0/;
3019
19799a22
GS
3020If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3021in octal), use sprintf() or printf():
3022
3023 $perms = (stat("filename"))[2] & 07777;
3024 $oct_perms = sprintf "%lo", $perms;
3025
3026The oct() function is commonly used when a string such as C<644> needs
3027to be converted into a file mode, for example. (Although perl will
3028automatically convert strings into numbers as needed, this automatic
3029conversion assumes base 10.)
a0d0e21e
LW
3030
3031=item open FILEHANDLE,EXPR
d74e8afc 3032X<open> X<pipe> X<file, open> X<fopen>
a0d0e21e 3033
68bd7414
NIS
3034=item open FILEHANDLE,MODE,EXPR
3035
3036=item open FILEHANDLE,MODE,EXPR,LIST
3037
ba964c95
T
3038=item open FILEHANDLE,MODE,REFERENCE
3039
a0d0e21e
LW
3040=item open FILEHANDLE
3041
3042Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
3043FILEHANDLE.
3044
3045(The following is a comprehensive reference to open(): for a gentler
3046introduction you may consider L<perlopentut>.)
3047
a28cd5c9
NT
3048If FILEHANDLE is an undefined scalar variable (or array or hash element)
3049the variable is assigned a reference to a new anonymous filehandle,
3050otherwise if FILEHANDLE is an expression, its value is used as the name of
3051the real filehandle wanted. (This is considered a symbolic reference, so
3052C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
3053
3054If EXPR is omitted, the scalar variable of the same name as the
3055FILEHANDLE contains the filename. (Note that lexical variables--those
3056declared with C<my>--will not work for this purpose; so if you're
67408cae 3057using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
3058
3059If three or more arguments are specified then the mode of opening and
3060the file name are separate. If MODE is C<< '<' >> or nothing, the file
3061is opened for input. If MODE is C<< '>' >>, the file is truncated and
3062opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 3063the file is opened for appending, again being created if necessary.
5a964f20 3064
ed53a2bb
JH
3065You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3066indicate that you want both read and write access to the file; thus
3067C<< '+<' >> is almost always preferred for read/write updates--the C<<
3068'+>' >> mode would clobber the file first. You can't usually use
3069either read-write mode for updating textfiles, since they have
3070variable length records. See the B<-i> switch in L<perlrun> for a
3071better approach. The file is created with permissions of C<0666>
3072modified by the process' C<umask> value.
3073
3074These various prefixes correspond to the fopen(3) modes of C<'r'>,
3075C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 3076
6170680b
IZ
3077In the 2-arguments (and 1-argument) form of the call the mode and
3078filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
3079spaces. It is possible to omit the mode in these forms if the mode is
3080C<< '<' >>.
6170680b 3081
7660c0ab 3082If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 3083command to which output is to be piped, and if the filename ends with a
f244e06d
GS
3084C<'|'>, the filename is interpreted as a command which pipes output to
3085us. See L<perlipc/"Using open() for IPC">
19799a22 3086for more examples of this. (You are not allowed to C<open> to a command
5a964f20 3087that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
3088and L<perlipc/"Bidirectional Communication with Another Process">
3089for alternatives.)
cb1a09d0 3090
ed53a2bb
JH
3091For three or more arguments if MODE is C<'|-'>, the filename is
3092interpreted as a command to which output is to be piped, and if MODE
3093is C<'-|'>, the filename is interpreted as a command which pipes
3094output to us. In the 2-arguments (and 1-argument) form one should
3095replace dash (C<'-'>) with the command.
3096See L<perlipc/"Using open() for IPC"> for more examples of this.
3097(You are not allowed to C<open> to a command that pipes both in I<and>
3098out, but see L<IPC::Open2>, L<IPC::Open3>, and
3099L<perlipc/"Bidirectional Communication"> for alternatives.)
3100
3101In the three-or-more argument form of pipe opens, if LIST is specified
3102(extra arguments after the command name) then LIST becomes arguments
3103to the command invoked if the platform supports it. The meaning of
3104C<open> with more than three arguments for non-pipe modes is not yet
3105specified. Experimental "layers" may give extra LIST arguments
3106meaning.
6170680b
IZ
3107
3108In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 3109and opening C<< '>-' >> opens STDOUT.
6170680b 3110
fae2c0fb
RGS
3111You may use the three-argument form of open to specify IO "layers"
3112(sometimes also referred to as "disciplines") to be applied to the handle
3113that affect how the input and output are processed (see L<open> and
3114L<PerlIO> for more details). For example
7207e29d 3115
9124316e
JH
3116 open(FH, "<:utf8", "file")
3117
3118will open the UTF-8 encoded file containing Unicode characters,
6d5e88a0
ST
3119see L<perluniintro>. Note that if layers are specified in the
3120three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3121usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
ed53a2bb
JH
3122
3123Open returns nonzero upon success, the undefined value otherwise. If
3124the C<open> involved a pipe, the return value happens to be the pid of
3125the subprocess.
cb1a09d0 3126
ed53a2bb
JH
3127If you're running Perl on a system that distinguishes between text
3128files and binary files, then you should check out L</binmode> for tips
3129for dealing with this. The key distinction between systems that need
3130C<binmode> and those that don't is their text file formats. Systems
8939ba94 3131like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
3132character, and which encode that character in C as C<"\n">, do not
3133need C<binmode>. The rest need it.
cb1a09d0 3134
fb73857a 3135When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
3136if the request failed, so C<open> is frequently used in connection with
3137C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 3138where you want to make a nicely formatted error message (but there are
5a964f20 3139modules that can help with that problem)) you should always check
19799a22 3140the return value from opening a file. The infrequent exception is when
fb73857a
PP
3141working with an unopened filehandle is actually what you want to do.
3142
cf264981 3143As a special case the 3-arg form with a read/write mode and the third
ed53a2bb 3144argument being C<undef>:
b76cc8ba
NIS
3145
3146 open(TMP, "+>", undef) or die ...
3147
f253e835
JH
3148