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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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PP
40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
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45
46If you run Perl with the B<-w> switch it can warn you about this. For
47example, the third line above produces:
48
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
51
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52A few functions take no arguments at all, and therefore work as neither
53unary nor list operators. These include such functions as C<time>
54and C<endpwent>. For example, C<time+86_400> always means
55C<time() + 86_400>.
56
a0d0e21e 57For functions that can be used in either a scalar or list context,
54310121 58nonabortive failure is generally indicated in a scalar context by
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59returning the undefined value, and in a list context by returning the
60null list.
61
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62Remember the following important rule: There is B<no rule> that relates
63the behavior of an expression in list context to its behavior in scalar
64context, or vice versa. It might do two totally different things.
a0d0e21e 65Each operator and function decides which sort of value it would be most
2b5ab1e7 66appropriate to return in scalar context. Some operators return the
5a964f20 67length of the list that would have been returned in list context. Some
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68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
d74e8afc 72X<context>
a0d0e21e 73
d1be9408 74A named array in scalar context is quite different from what would at
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75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
81In general, functions in Perl that serve as wrappers for system calls
f86cebdf 82of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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83true when they succeed and C<undef> otherwise, as is usually mentioned
84in the descriptions below. This is different from the C interfaces,
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85which return C<-1> on failure. Exceptions to this rule are C<wait>,
86C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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87variable on failure. Other functions do not, except accidentally.
88
cb1a09d0 89=head2 Perl Functions by Category
d74e8afc 90X<function>
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91
92Here are Perl's functions (including things that look like
5a964f20 93functions, like some keywords and named operators)
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94arranged by category. Some functions appear in more
95than one place.
96
13a2d996 97=over 4
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98
99=item Functions for SCALARs or strings
d74e8afc 100X<scalar> X<string> X<character>
cb1a09d0 101
22fae026 102C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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103C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
104C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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105
106=item Regular expressions and pattern matching
d74e8afc 107X<regular expression> X<regex> X<regexp>
cb1a09d0 108
ab4f32c2 109C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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110
111=item Numeric functions
d74e8afc 112X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 113
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114C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
115C<sin>, C<sqrt>, C<srand>
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116
117=item Functions for real @ARRAYs
d74e8afc 118X<array>
cb1a09d0 119
22fae026 120C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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121
122=item Functions for list data
d74e8afc 123X<list>
cb1a09d0 124
ab4f32c2 125C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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126
127=item Functions for real %HASHes
d74e8afc 128X<hash>
cb1a09d0 129
22fae026 130C<delete>, C<each>, C<exists>, C<keys>, C<values>
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131
132=item Input and output functions
d74e8afc 133X<I/O> X<input> X<output> X<dbm>
cb1a09d0 134
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135C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
136C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
0d863452 137C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
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138C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
139C<warn>, C<write>
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140
141=item Functions for fixed length data or records
142
22fae026 143C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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144
145=item Functions for filehandles, files, or directories
d74e8afc 146X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 147
22fae026 148C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 149C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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150C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
151C<umask>, C<unlink>, C<utime>
cb1a09d0 152
cf264981 153=item Keywords related to the control flow of your Perl program
d74e8afc 154X<control flow>
cb1a09d0 155
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156C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
157C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 158
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159=item Keywords related to switch
160
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>,
ab4f32c2 183C<pipe>, C<qx/STRING/>, 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>,
236C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr>, C<qw>, C<qx>,
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 $_; # ignore specials
5a964f20 342 #...
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343 }
344
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345The interpretation of the file permission operators C<-r>, C<-R>,
346C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
347of the file and the uids and gids of the user. There may be other
348reasons you can't actually read, write, or execute the file. Such
349reasons may be for example network filesystem access controls, ACLs
350(access control lists), read-only filesystems, and unrecognized
351executable formats.
352
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353Also note that, for the superuser on the local filesystems, the C<-r>,
354C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
5ff3f7a4
GS
355if any execute bit is set in the mode. Scripts run by the superuser
356may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 357or temporarily set their effective uid to something else.
5ff3f7a4
GS
358
359If you are using ACLs, there is a pragma called C<filetest> that may
360produce more accurate results than the bare stat() mode bits.
5ff3f7a4
GS
361When under the C<use filetest 'access'> the above-mentioned filetests
362will test whether the permission can (not) be granted using the
468541a8 363access() family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
364under this pragma return true even if there are no execute permission
365bits set (nor any extra execute permission ACLs). This strangeness is
366due to the underlying system calls' definitions. Read the
367documentation for the C<filetest> pragma for more information.
368
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369Note that C<-s/a/b/> does not do a negated substitution. Saying
370C<-exp($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:
a0d0e21e
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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
fbb0b3b3
RGS
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
8d2a6795
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
01e6739c 545To mark FILEHANDLE as UTF-8, use C<:utf8>.
1cbfc93d 546
ed53a2bb 547In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
548is done on the filehandle. Calling binmode() will normally flush any
549pending buffered output data (and perhaps pending input data) on the
fae2c0fb 550handle. An exception to this is the C<:encoding> layer that
01e6739c 551changes the default character encoding of the handle, see L<open>.
fae2c0fb 552The C<:encoding> layer sometimes needs to be called in
3874323d
JH
553mid-stream, and it doesn't flush the stream. The C<:encoding>
554also implicitly pushes on top of itself the C<:utf8> layer because
555internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 556
19799a22 557The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
558system all work together to let the programmer treat a single
559character (C<\n>) as the line terminator, irrespective of the external
560representation. On many operating systems, the native text file
561representation matches the internal representation, but on some
562platforms the external representation of C<\n> is made up of more than
563one character.
564
68bd7414
NIS
565Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
566character to end each line in the external representation of text (even
5e12dbfa 567though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
568on Unix and most VMS files). In other systems like OS/2, DOS and the
569various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
570but what's stored in text files are the two characters C<\cM\cJ>. That
571means that, if you don't use binmode() on these systems, C<\cM\cJ>
572sequences on disk will be converted to C<\n> on input, and any C<\n> in
573your program will be converted back to C<\cM\cJ> on output. This is what
574you want for text files, but it can be disastrous for binary files.
30168b04
GS
575
576Another consequence of using binmode() (on some systems) is that
577special end-of-file markers will be seen as part of the data stream.
578For systems from the Microsoft family this means that if your binary
4375e838 579data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
580the file, unless you use binmode().
581
582binmode() is not only important for readline() and print() operations,
583but also when using read(), seek(), sysread(), syswrite() and tell()
584(see L<perlport> for more details). See the C<$/> and C<$\> variables
585in L<perlvar> for how to manually set your input and output
586line-termination sequences.
a0d0e21e 587
4633a7c4 588=item bless REF,CLASSNAME
d74e8afc 589X<bless>
a0d0e21e
LW
590
591=item bless REF
592
2b5ab1e7
TC
593This function tells the thingy referenced by REF that it is now an object
594in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 595is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 596it returns the reference for convenience. Always use the two-argument
cf264981
SP
597version if a derived class might inherit the function doing the blessing.
598See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
599of objects.
a0d0e21e 600
57668c4d 601Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 602Namespaces with all lowercase names are considered reserved for
cf264981 603Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
604confusion, you may wish to avoid such package names as well. Make sure
605that CLASSNAME is a true value.
60ad88b8
GS
606
607See L<perlmod/"Perl Modules">.
608
0d863452
RH
609=item break
610
611Break out of a C<given()> block.
612
613This keyword is enabled by the "switch" feature: see L<feature>
614for more information.
615
a0d0e21e 616=item caller EXPR
d74e8afc 617X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
618
619=item caller
620
5a964f20 621Returns the context of the current subroutine call. In scalar context,
28757baa 622returns the caller's package name if there is a caller, that is, if
19799a22 623we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 624otherwise. In list context, returns
a0d0e21e 625
ee6b43cc 626 # 0 1 2
748a9306 627 ($package, $filename, $line) = caller;
a0d0e21e
LW
628
629With EXPR, it returns some extra information that the debugger uses to
630print a stack trace. The value of EXPR indicates how many call frames
631to go back before the current one.
632
ee6b43cc 633 # 0 1 2 3 4
f3aa04c2 634 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 635
636 # 5 6 7 8 9 10
b3ca2e83 637 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 638 = caller($i);
e7ea3e70 639
951ba7fe 640Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 641call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 642C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 643C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 644C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 645$filename is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
646each C<use> statement creates a C<require> frame inside an C<eval EXPR>
647frame.) $subroutine may also be C<(unknown)> if this particular
648subroutine happens to have been deleted from the symbol table.
649C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
650C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
651compiled with. The C<$hints> and C<$bitmask> values are subject to change
652between versions of Perl, and are not meant for external use.
748a9306 653
b3ca2e83
NC
654C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
655caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
656of this hash, as they are the actual values stored in the optree.
657
748a9306 658Furthermore, when called from within the DB package, caller returns more
7660c0ab 659detailed information: it sets the list variable C<@DB::args> to be the
54310121 660arguments with which the subroutine was invoked.
748a9306 661
7660c0ab 662Be aware that the optimizer might have optimized call frames away before
19799a22 663C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 664might not return information about the call frame you expect it do, for
b76cc8ba 665C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 666previous time C<caller> was called.
7660c0ab 667
a0d0e21e 668=item chdir EXPR
d74e8afc
ITB
669X<chdir>
670X<cd>
f723aae1 671X<directory, change>
a0d0e21e 672
c4aca7d0
GA
673=item chdir FILEHANDLE
674
675=item chdir DIRHANDLE
676
ce2984c3
PF
677=item chdir
678
ffce7b87 679Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 680changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 681changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
682variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
683neither is set, C<chdir> does nothing. It returns true upon success,
684false otherwise. See the example under C<die>.
a0d0e21e 685
c4aca7d0
GA
686On systems that support fchdir, you might pass a file handle or
687directory handle as argument. On systems that don't support fchdir,
688passing handles produces a fatal error at run time.
689
a0d0e21e 690=item chmod LIST
d74e8afc 691X<chmod> X<permission> X<mode>
a0d0e21e
LW
692
693Changes the permissions of a list of files. The first element of the
4633a7c4 694list must be the numerical mode, which should probably be an octal
4ad40acf 695number, and which definitely should I<not> be a string of octal digits:
2f9daede 696C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 697successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
698
699 $cnt = chmod 0755, 'foo', 'bar';
700 chmod 0755, @executables;
f86cebdf
GS
701 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
702 # --w----r-T
2f9daede
TPG
703 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
704 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 705
c4aca7d0
GA
706On systems that support fchmod, you might pass file handles among the
707files. On systems that don't support fchmod, passing file handles
345da378
GA
708produces a fatal error at run time. The file handles must be passed
709as globs or references to be recognized. Barewords are considered
710file names.
c4aca7d0
GA
711
712 open(my $fh, "<", "foo");
713 my $perm = (stat $fh)[2] & 07777;
714 chmod($perm | 0600, $fh);
715
ca6e1c26
JH
716You can also import the symbolic C<S_I*> constants from the Fcntl
717module:
718
719 use Fcntl ':mode';
720
721 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
722 # This is identical to the chmod 0755 of the above example.
723
a0d0e21e 724=item chomp VARIABLE
d74e8afc 725X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 726
313c9f5c 727=item chomp( LIST )
a0d0e21e
LW
728
729=item chomp
730
2b5ab1e7
TC
731This safer version of L</chop> removes any trailing string
732that corresponds to the current value of C<$/> (also known as
28757baa
PP
733$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
734number of characters removed from all its arguments. It's often used to
735remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
736that the final record may be missing its newline. When in paragraph
737mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
738When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
739a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 740remove anything.
19799a22 741If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
742
743 while (<>) {
744 chomp; # avoid \n on last field
745 @array = split(/:/);
5a964f20 746 # ...
a0d0e21e
LW
747 }
748
4bf21a6d
RD
749If VARIABLE is a hash, it chomps the hash's values, but not its keys.
750
a0d0e21e
LW
751You can actually chomp anything that's an lvalue, including an assignment:
752
753 chomp($cwd = `pwd`);
754 chomp($answer = <STDIN>);
755
756If you chomp a list, each element is chomped, and the total number of
757characters removed is returned.
758
442a8c12
NC
759If the C<encoding> pragma is in scope then the lengths returned are
760calculated from the length of C<$/> in Unicode characters, which is not
761always the same as the length of C<$/> in the native encoding.
762
15e44fd8
RGS
763Note that parentheses are necessary when you're chomping anything
764that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
765is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
766C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
767C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
768as C<chomp($a, $b)>.
769
a0d0e21e 770=item chop VARIABLE
d74e8afc 771X<chop>
a0d0e21e 772
313c9f5c 773=item chop( LIST )
a0d0e21e
LW
774
775=item chop
776
777Chops off the last character of a string and returns the character
5b3eff12 778chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 779scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
780If VARIABLE is a hash, it chops the hash's values, but not its keys.
781
5b3eff12 782You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
783
784If you chop a list, each element is chopped. Only the value of the
19799a22 785last C<chop> is returned.
a0d0e21e 786
19799a22 787Note that C<chop> returns the last character. To return all but the last
748a9306
LW
788character, use C<substr($string, 0, -1)>.
789
15e44fd8
RGS
790See also L</chomp>.
791
a0d0e21e 792=item chown LIST
d74e8afc 793X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
794
795Changes the owner (and group) of a list of files. The first two
19799a22
GS
796elements of the list must be the I<numeric> uid and gid, in that
797order. A value of -1 in either position is interpreted by most
798systems to leave that value unchanged. Returns the number of files
799successfully changed.
a0d0e21e
LW
800
801 $cnt = chown $uid, $gid, 'foo', 'bar';
802 chown $uid, $gid, @filenames;
803
c4aca7d0
GA
804On systems that support fchown, you might pass file handles among the
805files. On systems that don't support fchown, passing file handles
345da378
GA
806produces a fatal error at run time. The file handles must be passed
807as globs or references to be recognized. Barewords are considered
808file names.
c4aca7d0 809
54310121 810Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
811
812 print "User: ";
19799a22 813 chomp($user = <STDIN>);
5a964f20 814 print "Files: ";
19799a22 815 chomp($pattern = <STDIN>);
a0d0e21e
LW
816
817 ($login,$pass,$uid,$gid) = getpwnam($user)
818 or die "$user not in passwd file";
819
5a964f20 820 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
821 chown $uid, $gid, @ary;
822
54310121 823On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
824file unless you're the superuser, although you should be able to change
825the group to any of your secondary groups. On insecure systems, these
826restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
827On POSIX systems, you can detect this condition this way:
828
829 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
830 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 831
a0d0e21e 832=item chr NUMBER
d74e8afc 833X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 834
54310121 835=item chr
bbce6d69 836
a0d0e21e 837Returns the character represented by that NUMBER in the character set.
a0ed51b3 838For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
1e54db1a
JH
839chr(0x263a) is a Unicode smiley face. Note that characters from 128
840to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
841backward compatibility reasons (but see L<encoding>).
aaa68c4a 842
8a064bd6 843Negative values give the Unicode replacement character (chr(0xfffd)),
5f0135eb 844except under the L<bytes> pragma, where low eight bits of the value
8a064bd6
JH
845(truncated to an integer) are used.
846
974da8e5
JH
847If NUMBER is omitted, uses C<$_>.
848
b76cc8ba 849For the reverse, use L</ord>.
a0d0e21e 850
974da8e5
JH
851Note that under the C<bytes> pragma the NUMBER is masked to
852the low eight bits.
853
854See L<perlunicode> and L<encoding> for more about Unicode.
bbce6d69 855
a0d0e21e 856=item chroot FILENAME
d74e8afc 857X<chroot> X<root>
a0d0e21e 858
54310121 859=item chroot
bbce6d69 860
5a964f20 861This function works like the system call by the same name: it makes the
4633a7c4 862named directory the new root directory for all further pathnames that
951ba7fe 863begin with a C</> by your process and all its children. (It doesn't
28757baa 864change your current working directory, which is unaffected.) For security
4633a7c4 865reasons, this call is restricted to the superuser. If FILENAME is
19799a22 866omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
867
868=item close FILEHANDLE
d74e8afc 869X<close>
a0d0e21e 870
6a518fbc
TP
871=item close
872
e0f13c26
RGS
873Closes the file or pipe associated with the file handle, flushes the IO
874buffers, and closes the system file descriptor. Returns true if those
875operations have succeeded and if no error was reported by any PerlIO
876layer. Closes the currently selected filehandle if the argument is
877omitted.
fb73857a
PP
878
879You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
880another C<open> on it, because C<open> will close it for you. (See
881C<open>.) However, an explicit C<close> on an input file resets the line
882counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 883
dede8123
RGS
884If the file handle came from a piped open, C<close> will additionally
885return false if one of the other system calls involved fails, or if the
fb73857a 886program exits with non-zero status. (If the only problem was that the
dede8123 887program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 888also waits for the process executing on the pipe to complete, in case you
b76cc8ba 889want to look at the output of the pipe afterwards, and
e5218da5
GA
890implicitly puts the exit status value of that command into C<$?> and
891C<${^CHILD_ERROR_NATIVE}>.
5a964f20 892
73689b13
GS
893Prematurely closing the read end of a pipe (i.e. before the process
894writing to it at the other end has closed it) will result in a
895SIGPIPE being delivered to the writer. If the other end can't
896handle that, be sure to read all the data before closing the pipe.
897
fb73857a 898Example:
a0d0e21e 899
fb73857a
PP
900 open(OUTPUT, '|sort >foo') # pipe to sort
901 or die "Can't start sort: $!";
5a964f20 902 #... # print stuff to output
fb73857a
PP
903 close OUTPUT # wait for sort to finish
904 or warn $! ? "Error closing sort pipe: $!"
905 : "Exit status $? from sort";
906 open(INPUT, 'foo') # get sort's results
907 or die "Can't open 'foo' for input: $!";
a0d0e21e 908
5a964f20
TC
909FILEHANDLE may be an expression whose value can be used as an indirect
910filehandle, usually the real filehandle name.
a0d0e21e
LW
911
912=item closedir DIRHANDLE
d74e8afc 913X<closedir>
a0d0e21e 914
19799a22 915Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
916system call.
917
a0d0e21e 918=item connect SOCKET,NAME
d74e8afc 919X<connect>
a0d0e21e
LW
920
921Attempts to connect to a remote socket, just as the connect system call
19799a22 922does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
923packed address of the appropriate type for the socket. See the examples in
924L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 925
cb1a09d0 926=item continue BLOCK
d74e8afc 927X<continue>
cb1a09d0 928
0d863452
RH
929=item continue
930
cf264981
SP
931C<continue> is actually a flow control statement rather than a function. If
932there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
933C<foreach>), it is always executed just before the conditional is about to
934be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
935it can be used to increment a loop variable, even when the loop has been
936continued via the C<next> statement (which is similar to the C C<continue>
937statement).
938
98293880 939C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
940block. C<last> and C<redo> will behave as if they had been executed within
941the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
942block, it may be more entertaining.
943
944 while (EXPR) {
945 ### redo always comes here
946 do_something;
947 } continue {
948 ### next always comes here
949 do_something_else;
950 # then back the top to re-check EXPR
951 }
952 ### last always comes here
953
954Omitting the C<continue> section is semantically equivalent to using an
19799a22 955empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
956to check the condition at the top of the loop.
957
0d863452
RH
958If the "switch" feature is enabled, C<continue> is also a
959function that will break out of the current C<when> or C<default>
960block, and fall through to the next case. See L<feature> and
961L<perlsyn/"Switch statements"> for more information.
962
963
a0d0e21e 964=item cos EXPR
d74e8afc 965X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 966
d6217f1e
GS
967=item cos
968
5a964f20 969Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 970takes cosine of C<$_>.
a0d0e21e 971
ca6e1c26 972For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
973function, or use this relation:
974
975 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
976
a0d0e21e 977=item crypt PLAINTEXT,SALT
d74e8afc 978X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 979X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 980
ef2e6798
MS
981Creates a digest string exactly like the crypt(3) function in the C
982library (assuming that you actually have a version there that has not
cf264981 983been extirpated as a potential munitions).
ef2e6798
MS
984
985crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
986into a short string, called a digest, which is returned. The same
987PLAINTEXT and SALT will always return the same string, but there is no
988(known) way to get the original PLAINTEXT from the hash. Small
989changes in the PLAINTEXT or SALT will result in large changes in the
990digest.
991
992There is no decrypt function. This function isn't all that useful for
993cryptography (for that, look for F<Crypt> modules on your nearby CPAN
994mirror) and the name "crypt" is a bit of a misnomer. Instead it is
995primarily used to check if two pieces of text are the same without
996having to transmit or store the text itself. An example is checking
997if a correct password is given. The digest of the password is stored,
cf264981 998not the password itself. The user types in a password that is
ef2e6798
MS
999crypt()'d with the same salt as the stored digest. If the two digests
1000match the password is correct.
1001
1002When verifying an existing digest string you should use the digest as
1003the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1004to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1005crypt() will hash the new string with the same salt as the digest.
1006This allows your code to work with the standard L<crypt|/crypt> and
1007with more exotic implementations. In other words, do not assume
1008anything about the returned string itself, or how many bytes in the
1009digest matter.
85c16d83
JH
1010
1011Traditionally the result is a string of 13 bytes: two first bytes of
1012the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
ef2e6798
MS
1013the first eight bytes of the digest string mattered, but alternative
1014hashing schemes (like MD5), higher level security schemes (like C2),
1015and implementations on non-UNIX platforms may produce different
1016strings.
85c16d83
JH
1017
1018When choosing a new salt create a random two character string whose
1019characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1020'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1021characters is just a recommendation; the characters allowed in
1022the salt depend solely on your system's crypt library, and Perl can't
1023restrict what salts C<crypt()> accepts.
e71965be 1024
a0d0e21e 1025Here's an example that makes sure that whoever runs this program knows
cf264981 1026their password:
a0d0e21e
LW
1027
1028 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1029
1030 system "stty -echo";
1031 print "Password: ";
e71965be 1032 chomp($word = <STDIN>);
a0d0e21e
LW
1033 print "\n";
1034 system "stty echo";
1035
e71965be 1036 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
1037 die "Sorry...\n";
1038 } else {
1039 print "ok\n";
54310121 1040 }
a0d0e21e 1041
9f8f0c9d 1042Of course, typing in your own password to whoever asks you
748a9306 1043for it is unwise.
a0d0e21e 1044
ef2e6798 1045The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1046of data, not least of all because you can't get the information
ef2e6798 1047back. Look at the L<Digest> module for more robust algorithms.
19799a22 1048
f2791508
JH
1049If using crypt() on a Unicode string (which I<potentially> has
1050characters with codepoints above 255), Perl tries to make sense
1051of the situation by trying to downgrade (a copy of the string)
1052the string back to an eight-bit byte string before calling crypt()
1053(on that copy). If that works, good. If not, crypt() dies with
1054C<Wide character in crypt>.
85c16d83 1055
aa689395 1056=item dbmclose HASH
d74e8afc 1057X<dbmclose>
a0d0e21e 1058
19799a22 1059[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1060
aa689395 1061Breaks the binding between a DBM file and a hash.
a0d0e21e 1062
19799a22 1063=item dbmopen HASH,DBNAME,MASK
d74e8afc 1064X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1065
19799a22 1066[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1067
7b8d334a 1068This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1069hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1070argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1071is the name of the database (without the F<.dir> or F<.pag> extension if
1072any). If the database does not exist, it is created with protection
19799a22
GS
1073specified by MASK (as modified by the C<umask>). If your system supports
1074only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 1075program. In older versions of Perl, if your system had neither DBM nor
19799a22 1076ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1077sdbm(3).
1078
1079If you don't have write access to the DBM file, you can only read hash
1080variables, not set them. If you want to test whether you can write,
19799a22 1081either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 1082which will trap the error.
a0d0e21e 1083
19799a22
GS
1084Note that functions such as C<keys> and C<values> may return huge lists
1085when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1086function to iterate over large DBM files. Example:
1087
1088 # print out history file offsets
1089 dbmopen(%HIST,'/usr/lib/news/history',0666);
1090 while (($key,$val) = each %HIST) {
1091 print $key, ' = ', unpack('L',$val), "\n";
1092 }
1093 dbmclose(%HIST);
1094
cb1a09d0 1095See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1096cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1097rich implementation.
4633a7c4 1098
2b5ab1e7
TC
1099You can control which DBM library you use by loading that library
1100before you call dbmopen():
1101
1102 use DB_File;
1103 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1104 or die "Can't open netscape history file: $!";
1105
a0d0e21e 1106=item defined EXPR
d74e8afc 1107X<defined> X<undef> X<undefined>
a0d0e21e 1108
54310121 1109=item defined
bbce6d69 1110
2f9daede
TPG
1111Returns a Boolean value telling whether EXPR has a value other than
1112the undefined value C<undef>. If EXPR is not present, C<$_> will be
1113checked.
1114
1115Many operations return C<undef> to indicate failure, end of file,
1116system error, uninitialized variable, and other exceptional
1117conditions. This function allows you to distinguish C<undef> from
1118other values. (A simple Boolean test will not distinguish among
7660c0ab 1119C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1120false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1121doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1122returns C<undef> when its argument is an empty array, I<or> when the
1123element to return happens to be C<undef>.
1124
f10b0346
GS
1125You may also use C<defined(&func)> to check whether subroutine C<&func>
1126has ever been defined. The return value is unaffected by any forward
04891299 1127declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1128may still be callable: its package may have an C<AUTOLOAD> method that
1129makes it spring into existence the first time that it is called -- see
1130L<perlsub>.
f10b0346
GS
1131
1132Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1133used to report whether memory for that aggregate has ever been
1134allocated. This behavior may disappear in future versions of Perl.
1135You should instead use a simple test for size:
1136
1137 if (@an_array) { print "has array elements\n" }
1138 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1139
1140When used on a hash element, it tells you whether the value is defined,
dc848c6f 1141not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1142purpose.
a0d0e21e
LW
1143
1144Examples:
1145
1146 print if defined $switch{'D'};
1147 print "$val\n" while defined($val = pop(@ary));
1148 die "Can't readlink $sym: $!"
1149 unless defined($value = readlink $sym);
a0d0e21e 1150 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1151 $debugging = 0 unless defined $debugging;
a0d0e21e 1152
19799a22 1153Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1154discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1155defined values. For example, if you say
a5f75d66
AD
1156
1157 "ab" =~ /a(.*)b/;
1158
7660c0ab 1159The pattern match succeeds, and C<$1> is defined, despite the fact that it
cf264981 1160matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1161matched something that happened to be zero characters long. This is all
a5f75d66 1162very above-board and honest. When a function returns an undefined value,
2f9daede 1163it's an admission that it couldn't give you an honest answer. So you
19799a22 1164should use C<defined> only when you're questioning the integrity of what
7660c0ab 1165you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1166what you want.
1167
dc848c6f 1168See also L</undef>, L</exists>, L</ref>.
2f9daede 1169
a0d0e21e 1170=item delete EXPR
d74e8afc 1171X<delete>
a0d0e21e 1172
01020589
GS
1173Given an expression that specifies a hash element, array element, hash slice,
1174or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1175In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1176the size of the array will shrink to the highest element that tests
8216c1fd 1177true for exists() (or 0 if no such element exists).
a0d0e21e 1178
eba0920a
EM
1179Returns a list with the same number of elements as the number of elements
1180for which deletion was attempted. Each element of that list consists of
1181either the value of the element deleted, or the undefined value. In scalar
1182context, this means that you get the value of the last element deleted (or
1183the undefined value if that element did not exist).
1184
1185 %hash = (foo => 11, bar => 22, baz => 33);
1186 $scalar = delete $hash{foo}; # $scalar is 11
1187 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1188 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1189
1190Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1191a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1192from a C<tie>d hash or array may not necessarily return anything.
1193
8ea97a1e
GS
1194Deleting an array element effectively returns that position of the array
1195to its initial, uninitialized state. Subsequently testing for the same
cf264981
SP
1196element with exists() will return false. Also, deleting array elements
1197in the middle of an array will not shift the index of the elements
1198after them down. Use splice() for that. See L</exists>.
8ea97a1e 1199
01020589 1200The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1201
5f05dabc
PP
1202 foreach $key (keys %HASH) {
1203 delete $HASH{$key};
a0d0e21e
LW
1204 }
1205
01020589
GS
1206 foreach $index (0 .. $#ARRAY) {
1207 delete $ARRAY[$index];
1208 }
1209
1210And so do these:
5f05dabc 1211
01020589
GS
1212 delete @HASH{keys %HASH};
1213
9740c838 1214 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1215
2b5ab1e7 1216But both of these are slower than just assigning the empty list
01020589
GS
1217or undefining %HASH or @ARRAY:
1218
1219 %HASH = (); # completely empty %HASH
1220 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1221
01020589
GS
1222 @ARRAY = (); # completely empty @ARRAY
1223 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1224
1225Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1226operation is a hash element, array element, hash slice, or array slice
1227lookup:
a0d0e21e
LW
1228
1229 delete $ref->[$x][$y]{$key};
5f05dabc 1230 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1231
01020589
GS
1232 delete $ref->[$x][$y][$index];
1233 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1234
a0d0e21e 1235=item die LIST
d74e8afc 1236X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1237
19799a22
GS
1238Outside an C<eval>, prints the value of LIST to C<STDERR> and
1239exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1240exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1241status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1242an C<eval(),> the error message is stuffed into C<$@> and the
1243C<eval> is terminated with the undefined value. This makes
1244C<die> the way to raise an exception.
a0d0e21e
LW
1245
1246Equivalent examples:
1247
1248 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1249 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1250
ccac6780 1251If the last element of LIST does not end in a newline, the current
df37ec69
WW
1252script line number and input line number (if any) are also printed,
1253and a newline is supplied. Note that the "input line number" (also
1254known as "chunk") is subject to whatever notion of "line" happens to
1255be currently in effect, and is also available as the special variable
1256C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1257
1258Hint: sometimes appending C<", stopped"> to your message will cause it
1259to make better sense when the string C<"at foo line 123"> is appended.
1260Suppose you are running script "canasta".
a0d0e21e
LW
1261
1262 die "/etc/games is no good";
1263 die "/etc/games is no good, stopped";
1264
1265produce, respectively
1266
1267 /etc/games is no good at canasta line 123.
1268 /etc/games is no good, stopped at canasta line 123.
1269
2b5ab1e7 1270See also exit(), warn(), and the Carp module.
a0d0e21e 1271
7660c0ab
A
1272If LIST is empty and C<$@> already contains a value (typically from a
1273previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1274This is useful for propagating exceptions:
1275
1276 eval { ... };
1277 die unless $@ =~ /Expected exception/;
1278
ad216e65
JH
1279If LIST is empty and C<$@> contains an object reference that has a
1280C<PROPAGATE> method, that method will be called with additional file
1281and line number parameters. The return value replaces the value in
28a5cf3b 1282C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1283were called.
1284
7660c0ab 1285If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1286
52531d10
GS
1287die() can also be called with a reference argument. If this happens to be
1288trapped within an eval(), $@ contains the reference. This behavior permits
1289a more elaborate exception handling implementation using objects that
4375e838 1290maintain arbitrary state about the nature of the exception. Such a scheme
52531d10 1291is sometimes preferable to matching particular string values of $@ using
746d7dd7
GL
1292regular expressions. Because $@ is a global variable, and eval() may be
1293used within object implementations, care must be taken that analyzing the
1294error object doesn't replace the reference in the global variable. The
1295easiest solution is to make a local copy of the reference before doing
1296other manipulations. Here's an example:
52531d10 1297
da279afe 1298 use Scalar::Util 'blessed';
1299
52531d10 1300 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1301 if (my $ev_err = $@) {
1302 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1303 # handle Some::Module::Exception
1304 }
1305 else {
1306 # handle all other possible exceptions
1307 }
1308 }
1309
19799a22 1310Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1311them, you may want to overload stringification operations on such custom
1312exception objects. See L<overload> for details about that.
1313
19799a22
GS
1314You can arrange for a callback to be run just before the C<die>
1315does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1316handler will be called with the error text and can change the error
1317message, if it sees fit, by calling C<die> again. See
1318L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1319L<"eval BLOCK"> for some examples. Although this feature was
19799a22
GS
1320to be run only right before your program was to exit, this is not
1321currently the case--the C<$SIG{__DIE__}> hook is currently called
1322even inside eval()ed blocks/strings! If one wants the hook to do
1323nothing in such situations, put
fb73857a
PP
1324
1325 die @_ if $^S;
1326
19799a22
GS
1327as the first line of the handler (see L<perlvar/$^S>). Because
1328this promotes strange action at a distance, this counterintuitive
b76cc8ba 1329behavior may be fixed in a future release.
774d564b 1330
a0d0e21e 1331=item do BLOCK
d74e8afc 1332X<do> X<block>
a0d0e21e
LW
1333
1334Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1335sequence of commands indicated by BLOCK. When modified by the C<while> or
1336C<until> loop modifier, executes the BLOCK once before testing the loop
1337condition. (On other statements the loop modifiers test the conditional
1338first.)
a0d0e21e 1339
4968c1e4 1340C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1341C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1342See L<perlsyn> for alternative strategies.
4968c1e4 1343
a0d0e21e 1344=item do SUBROUTINE(LIST)
d74e8afc 1345X<do>
a0d0e21e 1346
cf264981 1347This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1348
1349=item do EXPR
d74e8afc 1350X<do>
a0d0e21e
LW
1351
1352Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1353file as a Perl script.
a0d0e21e
LW
1354
1355 do 'stat.pl';
1356
1357is just like
1358
986b19de 1359 eval `cat stat.pl`;
a0d0e21e 1360
2b5ab1e7 1361except that it's more efficient and concise, keeps track of the current
ea63ef19 1362filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1363C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1364variables. It also differs in that code evaluated with C<do FILENAME>
1365cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1366same, however, in that it does reparse the file every time you call it,
1367so you probably don't want to do this inside a loop.
a0d0e21e 1368
8e30cc93 1369If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1370error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1371returns undef and sets an error message in C<$@>. If the file is
1372successfully compiled, C<do> returns the value of the last expression
1373evaluated.
1374
a0d0e21e 1375Note that inclusion of library modules is better done with the
19799a22 1376C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1377and raise an exception if there's a problem.
a0d0e21e 1378
5a964f20
TC
1379You might like to use C<do> to read in a program configuration
1380file. Manual error checking can be done this way:
1381
b76cc8ba 1382 # read in config files: system first, then user
f86cebdf 1383 for $file ("/share/prog/defaults.rc",
b76cc8ba 1384 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1385 {
5a964f20 1386 unless ($return = do $file) {
f86cebdf
GS
1387 warn "couldn't parse $file: $@" if $@;
1388 warn "couldn't do $file: $!" unless defined $return;
1389 warn "couldn't run $file" unless $return;
5a964f20
TC
1390 }
1391 }
1392
a0d0e21e 1393=item dump LABEL
d74e8afc 1394X<dump> X<core> X<undump>
a0d0e21e 1395
1614b0e3
JD
1396=item dump
1397
19799a22
GS
1398This function causes an immediate core dump. See also the B<-u>
1399command-line switch in L<perlrun>, which does the same thing.
1400Primarily this is so that you can use the B<undump> program (not
1401supplied) to turn your core dump into an executable binary after
1402having initialized all your variables at the beginning of the
1403program. When the new binary is executed it will begin by executing
1404a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1405Think of it as a goto with an intervening core dump and reincarnation.
1406If C<LABEL> is omitted, restarts the program from the top.
1407
1408B<WARNING>: Any files opened at the time of the dump will I<not>
1409be open any more when the program is reincarnated, with possible
b76cc8ba 1410resulting confusion on the part of Perl.
19799a22
GS
1411
1412This function is now largely obsolete, partly because it's very
1413hard to convert a core file into an executable, and because the
1414real compiler backends for generating portable bytecode and compilable
ac206dc8
RGS
1415C code have superseded it. That's why you should now invoke it as
1416C<CORE::dump()>, if you don't want to be warned against a possible
1417typo.
19799a22
GS
1418
1419If you're looking to use L<dump> to speed up your program, consider
1420generating bytecode or native C code as described in L<perlcc>. If
1421you're just trying to accelerate a CGI script, consider using the
210b36aa 1422C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
19799a22 1423You might also consider autoloading or selfloading, which at least
b76cc8ba 1424make your program I<appear> to run faster.
5a964f20 1425
aa689395 1426=item each HASH
d74e8afc 1427X<each> X<hash, iterator>
aa689395 1428
5a964f20 1429When called in list context, returns a 2-element list consisting of the
aa689395 1430key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1431it. When called in scalar context, returns only the key for the next
e902a979 1432element in the hash.
2f9daede 1433
ab192400 1434Entries are returned in an apparently random order. The actual random
504f80c1
JH
1435order is subject to change in future versions of perl, but it is
1436guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1437function would produce on the same (unmodified) hash. Since Perl
14385.8.1 the ordering is different even between different runs of Perl
1439for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1440
1441When the hash is entirely read, a null array is returned in list context
19799a22
GS
1442(which when assigned produces a false (C<0>) value), and C<undef> in
1443scalar context. The next call to C<each> after that will start iterating
1444again. There is a single iterator for each hash, shared by all C<each>,
1445C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1446reading all the elements from the hash, or by evaluating C<keys HASH> or
1447C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1448iterating over it, you may get entries skipped or duplicated, so
1449don't. Exception: It is always safe to delete the item most recently
1450returned by C<each()>, which means that the following code will work:
1451
1452 while (($key, $value) = each %hash) {
1453 print $key, "\n";
1454 delete $hash{$key}; # This is safe
1455 }
aa689395 1456
f86cebdf 1457The following prints out your environment like the printenv(1) program,
aa689395 1458only in a different order:
a0d0e21e
LW
1459
1460 while (($key,$value) = each %ENV) {
1461 print "$key=$value\n";
1462 }
1463
19799a22 1464See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1465
1466=item eof FILEHANDLE
d74e8afc
ITB
1467X<eof>
1468X<end of file>
1469X<end-of-file>
a0d0e21e 1470
4633a7c4
LW
1471=item eof ()
1472
a0d0e21e
LW
1473=item eof
1474
1475Returns 1 if the next read on FILEHANDLE will return end of file, or if
1476FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1477gives the real filehandle. (Note that this function actually
19799a22 1478reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1479interactive context.) Do not read from a terminal file (or call
19799a22 1480C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1481as terminals may lose the end-of-file condition if you do.
1482
820475bd
GS
1483An C<eof> without an argument uses the last file read. Using C<eof()>
1484with empty parentheses is very different. It refers to the pseudo file
1485formed from the files listed on the command line and accessed via the
61eff3bc
JH
1486C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1487as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1488used will cause C<@ARGV> to be examined to determine if input is
67408cae 1489available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1490end-of-file will assume you are processing another C<@ARGV> list,
1491and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1492see L<perlop/"I/O Operators">.
820475bd 1493
61eff3bc 1494In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1495detect the end of each file, C<eof()> will only detect the end of the
1496last file. Examples:
a0d0e21e 1497
748a9306
LW
1498 # reset line numbering on each input file
1499 while (<>) {
b76cc8ba 1500 next if /^\s*#/; # skip comments
748a9306 1501 print "$.\t$_";
5a964f20
TC
1502 } continue {
1503 close ARGV if eof; # Not eof()!
748a9306
LW
1504 }
1505
a0d0e21e
LW
1506 # insert dashes just before last line of last file
1507 while (<>) {
6ac88b13 1508 if (eof()) { # check for end of last file
a0d0e21e
LW
1509 print "--------------\n";
1510 }
1511 print;
6ac88b13 1512 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1513 }
1514
a0d0e21e 1515Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1516input operators typically return C<undef> when they run out of data, or if
1517there was an error.
a0d0e21e
LW
1518
1519=item eval EXPR
d74e8afc 1520X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1521X<error, handling> X<exception, handling>
a0d0e21e
LW
1522
1523=item eval BLOCK
1524
ce2984c3
PF
1525=item eval
1526
c7cc6f1c
GS
1527In the first form, the return value of EXPR is parsed and executed as if it
1528were a little Perl program. The value of the expression (which is itself
5a964f20 1529determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1530errors, executed in the lexical context of the current Perl program, so
1531that any variable settings or subroutine and format definitions remain
cf264981 1532afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1533If EXPR is omitted, evaluates C<$_>. This form is typically used to
1534delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1535
1536In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1537same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1538within the context of the current Perl program. This form is typically
1539used to trap exceptions more efficiently than the first (see below), while
1540also providing the benefit of checking the code within BLOCK at compile
1541time.
1542
1543The final semicolon, if any, may be omitted from the value of EXPR or within
1544the BLOCK.
1545
1546In both forms, the value returned is the value of the last expression
5a964f20 1547evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1548as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1549in void, scalar, or list context, depending on the context of the C<eval>
1550itself. See L</wantarray> for more on how the evaluation context can be
1551determined.
a0d0e21e 1552
19799a22
GS
1553If there is a syntax error or runtime error, or a C<die> statement is
1554executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1555error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1556string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1557warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1558To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1559turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1560See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1561
19799a22
GS
1562Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1563determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1564is implemented. It is also Perl's exception trapping mechanism, where
1565the die operator is used to raise exceptions.
1566
1567If the code to be executed doesn't vary, you may use the eval-BLOCK
1568form to trap run-time errors without incurring the penalty of
1569recompiling each time. The error, if any, is still returned in C<$@>.
1570Examples:
1571
54310121 1572 # make divide-by-zero nonfatal
a0d0e21e
LW
1573 eval { $answer = $a / $b; }; warn $@ if $@;
1574
1575 # same thing, but less efficient
1576 eval '$answer = $a / $b'; warn $@ if $@;
1577
1578 # a compile-time error
5a964f20 1579 eval { $answer = }; # WRONG
a0d0e21e
LW
1580
1581 # a run-time error
1582 eval '$answer ='; # sets $@
1583
cf264981
SP
1584Using the C<eval{}> form as an exception trap in libraries does have some
1585issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1586may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7
TC
1587You can use the C<local $SIG{__DIE__}> construct for this purpose,
1588as shown in this example:
774d564b
PP
1589
1590 # a very private exception trap for divide-by-zero
f86cebdf
GS
1591 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1592 warn $@ if $@;
774d564b
PP
1593
1594This is especially significant, given that C<__DIE__> hooks can call
19799a22 1595C<die> again, which has the effect of changing their error messages:
774d564b
PP
1596
1597 # __DIE__ hooks may modify error messages
1598 {
f86cebdf
GS
1599 local $SIG{'__DIE__'} =
1600 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1601 eval { die "foo lives here" };
1602 print $@ if $@; # prints "bar lives here"
774d564b
PP
1603 }
1604
19799a22 1605Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1606may be fixed in a future release.
1607
19799a22 1608With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1609being looked at when:
1610
1611 eval $x; # CASE 1
1612 eval "$x"; # CASE 2
1613
1614 eval '$x'; # CASE 3
1615 eval { $x }; # CASE 4
1616
5a964f20 1617 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1618 $$x++; # CASE 6
1619
2f9daede 1620Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1621the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1622the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1623and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1624does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1625purely visual reasons, but it also has the advantage of compiling at
1626compile-time instead of at run-time.) Case 5 is a place where
19799a22 1627normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1628particular situation, you can just use symbolic references instead, as
1629in case 6.
a0d0e21e 1630
4968c1e4 1631C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1632C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1633
d819b83a
DM
1634Note that as a very special case, an C<eval ''> executed within the C<DB>
1635package doesn't see the usual surrounding lexical scope, but rather the
1636scope of the first non-DB piece of code that called it. You don't normally
1637need to worry about this unless you are writing a Perl debugger.
1638
a0d0e21e 1639=item exec LIST
d74e8afc 1640X<exec> X<execute>
a0d0e21e 1641
8bf3b016
GS
1642=item exec PROGRAM LIST
1643
19799a22
GS
1644The C<exec> function executes a system command I<and never returns>--
1645use C<system> instead of C<exec> if you want it to return. It fails and
1646returns false only if the command does not exist I<and> it is executed
fb73857a 1647directly instead of via your system's command shell (see below).
a0d0e21e 1648
19799a22
GS
1649Since it's a common mistake to use C<exec> instead of C<system>, Perl
1650warns you if there is a following statement which isn't C<die>, C<warn>,
1651or C<exit> (if C<-w> is set - but you always do that). If you
1652I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1653can use one of these styles to avoid the warning:
1654
5a964f20
TC
1655 exec ('foo') or print STDERR "couldn't exec foo: $!";
1656 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1657
5a964f20 1658If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1659with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1660If there is only one scalar argument or an array with one element in it,
1661the argument is checked for shell metacharacters, and if there are any,
1662the entire argument is passed to the system's command shell for parsing
1663(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1664If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1665words and passed directly to C<execvp>, which is more efficient.
19799a22 1666Examples:
a0d0e21e 1667
19799a22
GS
1668 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1669 exec "sort $outfile | uniq";
a0d0e21e
LW
1670
1671If you don't really want to execute the first argument, but want to lie
1672to the program you are executing about its own name, you can specify
1673the program you actually want to run as an "indirect object" (without a
1674comma) in front of the LIST. (This always forces interpretation of the
54310121 1675LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1676the list.) Example:
1677
1678 $shell = '/bin/csh';
1679 exec $shell '-sh'; # pretend it's a login shell
1680
1681or, more directly,
1682
1683 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1684
bb32b41a
GS
1685When the arguments get executed via the system shell, results will
1686be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1687for details.
1688
19799a22
GS
1689Using an indirect object with C<exec> or C<system> is also more
1690secure. This usage (which also works fine with system()) forces
1691interpretation of the arguments as a multivalued list, even if the
1692list had just one argument. That way you're safe from the shell
1693expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1694
1695 @args = ( "echo surprise" );
1696
2b5ab1e7 1697 exec @args; # subject to shell escapes
f86cebdf 1698 # if @args == 1
2b5ab1e7 1699 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1700
1701The first version, the one without the indirect object, ran the I<echo>
1702program, passing it C<"surprise"> an argument. The second version
1703didn't--it tried to run a program literally called I<"echo surprise">,
1704didn't find it, and set C<$?> to a non-zero value indicating failure.
1705
0f897271
GS
1706Beginning with v5.6.0, Perl will attempt to flush all files opened for
1707output before the exec, but this may not be supported on some platforms
1708(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1709in English) or call the C<autoflush()> method of C<IO::Handle> on any
1710open handles in order to avoid lost output.
1711
19799a22 1712Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1713any C<DESTROY> methods in your objects.
1714
a0d0e21e 1715=item exists EXPR
d74e8afc 1716X<exists> X<autovivification>
a0d0e21e 1717
01020589 1718Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1719returns true if the specified element in the hash or array has ever
1720been initialized, even if the corresponding value is undefined. The
1721element is not autovivified if it doesn't exist.
a0d0e21e 1722
01020589
GS
1723 print "Exists\n" if exists $hash{$key};
1724 print "Defined\n" if defined $hash{$key};
1725 print "True\n" if $hash{$key};
1726
1727 print "Exists\n" if exists $array[$index];
1728 print "Defined\n" if defined $array[$index];
1729 print "True\n" if $array[$index];
a0d0e21e 1730
8ea97a1e 1731A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1732it exists, but the reverse doesn't necessarily hold true.
1733
afebc493
GS
1734Given an expression that specifies the name of a subroutine,
1735returns true if the specified subroutine has ever been declared, even
1736if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1737does not count as declaring it. Note that a subroutine which does not
1738exist may still be callable: its package may have an C<AUTOLOAD>
1739method that makes it spring into existence the first time that it is
1740called -- see L<perlsub>.
afebc493
GS
1741
1742 print "Exists\n" if exists &subroutine;
1743 print "Defined\n" if defined &subroutine;
1744
a0d0e21e 1745Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1746operation is a hash or array key lookup or subroutine name:
a0d0e21e 1747
2b5ab1e7
TC
1748 if (exists $ref->{A}->{B}->{$key}) { }
1749 if (exists $hash{A}{B}{$key}) { }
1750
01020589
GS
1751 if (exists $ref->{A}->{B}->[$ix]) { }
1752 if (exists $hash{A}{B}[$ix]) { }
1753
afebc493
GS
1754 if (exists &{$ref->{A}{B}{$key}}) { }
1755
01020589
GS
1756Although the deepest nested array or hash will not spring into existence
1757just because its existence was tested, any intervening ones will.
61eff3bc 1758Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1759into existence due to the existence test for the $key element above.
1760This happens anywhere the arrow operator is used, including even:
5a964f20 1761
2b5ab1e7
TC
1762 undef $ref;
1763 if (exists $ref->{"Some key"}) { }
1764 print $ref; # prints HASH(0x80d3d5c)
1765
1766This surprising autovivification in what does not at first--or even
1767second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1768release.
a0d0e21e 1769
afebc493
GS
1770Use of a subroutine call, rather than a subroutine name, as an argument
1771to exists() is an error.
1772
1773 exists &sub; # OK
1774 exists &sub(); # Error
1775
a0d0e21e 1776=item exit EXPR
d74e8afc 1777X<exit> X<terminate> X<abort>
a0d0e21e 1778
ce2984c3
PF
1779=item exit
1780
2b5ab1e7 1781Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1782
1783 $ans = <STDIN>;
1784 exit 0 if $ans =~ /^[Xx]/;
1785
19799a22 1786See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1787universally recognized values for EXPR are C<0> for success and C<1>
1788for error; other values are subject to interpretation depending on the
1789environment in which the Perl program is running. For example, exiting
179069 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1791the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1792
19799a22
GS
1793Don't use C<exit> to abort a subroutine if there's any chance that
1794someone might want to trap whatever error happened. Use C<die> instead,
1795which can be trapped by an C<eval>.
28757baa 1796
19799a22 1797The exit() function does not always exit immediately. It calls any
2b5ab1e7 1798defined C<END> routines first, but these C<END> routines may not
19799a22 1799themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1800be called are called before the real exit. If this is a problem, you
1801can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1802See L<perlmod> for details.
5a964f20 1803
a0d0e21e 1804=item exp EXPR
d74e8afc 1805X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1806
54310121 1807=item exp
bbce6d69 1808
b76cc8ba 1809Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1810If EXPR is omitted, gives C<exp($_)>.
1811
1812=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1813X<fcntl>
a0d0e21e 1814
f86cebdf 1815Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1816
1817 use Fcntl;
1818
0ade1984 1819first to get the correct constant definitions. Argument processing and
b76cc8ba 1820value return works just like C<ioctl> below.
a0d0e21e
LW
1821For example:
1822
1823 use Fcntl;
5a964f20
TC
1824 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1825 or die "can't fcntl F_GETFL: $!";
1826
554ad1fc 1827You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1828Like C<ioctl>, it maps a C<0> return from the system call into
1829C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1830in numeric context. It is also exempt from the normal B<-w> warnings
1831on improper numeric conversions.
5a964f20 1832
19799a22 1833Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1834doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1835manpage to learn what functions are available on your system.
a0d0e21e 1836
be2f7487 1837Here's an example of setting a filehandle named C<REMOTE> to be
1838non-blocking at the system level. You'll have to negotiate C<$|>
1839on your own, though.
1840
1841 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1842
1843 $flags = fcntl(REMOTE, F_GETFL, 0)
1844 or die "Can't get flags for the socket: $!\n";
1845
1846 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1847 or die "Can't set flags for the socket: $!\n";
1848
a0d0e21e 1849=item fileno FILEHANDLE
d74e8afc 1850X<fileno>
a0d0e21e 1851
2b5ab1e7
TC
1852Returns the file descriptor for a filehandle, or undefined if the
1853filehandle is not open. This is mainly useful for constructing
19799a22 1854bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1855If FILEHANDLE is an expression, the value is taken as an indirect
1856filehandle, generally its name.
5a964f20 1857
b76cc8ba 1858You can use this to find out whether two handles refer to the
5a964f20
TC
1859same underlying descriptor:
1860
1861 if (fileno(THIS) == fileno(THAT)) {
1862 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1863 }
1864
1865(Filehandles connected to memory objects via new features of C<open> may
1866return undefined even though they are open.)
1867
a0d0e21e
LW
1868
1869=item flock FILEHANDLE,OPERATION
d74e8afc 1870X<flock> X<lock> X<locking>
a0d0e21e 1871
19799a22
GS
1872Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1873for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1874machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1875C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1876only entire files, not records.
1877
1878Two potentially non-obvious but traditional C<flock> semantics are
1879that it waits indefinitely until the lock is granted, and that its locks
1880B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1881fewer guarantees. This means that programs that do not also use C<flock>
1882may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1883your port's specific documentation, or your system-specific local manpages
1884for details. It's best to assume traditional behavior if you're writing
1885portable programs. (But if you're not, you should as always feel perfectly
1886free to write for your own system's idiosyncrasies (sometimes called
1887"features"). Slavish adherence to portability concerns shouldn't get
1888in the way of your getting your job done.)
a3cb178b 1889
8ebc5c01
PP
1890OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1891LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1892you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1893either individually, or as a group using the ':flock' tag. LOCK_SH
1894requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1895releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1896LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1897waiting for the lock (check the return status to see if you got it).
1898
2b5ab1e7
TC
1899To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1900before locking or unlocking it.
8ebc5c01 1901
f86cebdf 1902Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1903locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1904are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1905implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1906differing semantics shouldn't bite too many people.
1907
becacb53
TM
1908Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1909be open with read intent to use LOCK_SH and requires that it be open
1910with write intent to use LOCK_EX.
1911
19799a22
GS
1912Note also that some versions of C<flock> cannot lock things over the
1913network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1914that. If you like you can force Perl to ignore your system's flock(2)
1915function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1916the switch C<-Ud_flock> to the F<Configure> program when you configure
1917perl.
4633a7c4
LW
1918
1919Here's a mailbox appender for BSD systems.
a0d0e21e 1920
7e1af8bc 1921 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1922
1923 sub lock {
7e1af8bc 1924 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1925 # and, in case someone appended
1926 # while we were waiting...
1927 seek(MBOX, 0, 2);
1928 }
1929
1930 sub unlock {
7e1af8bc 1931 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1932 }
1933
1934 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1935 or die "Can't open mailbox: $!";
1936
1937 lock();
1938 print MBOX $msg,"\n\n";
1939 unlock();
1940
2b5ab1e7
TC
1941On systems that support a real flock(), locks are inherited across fork()
1942calls, whereas those that must resort to the more capricious fcntl()
1943function lose the locks, making it harder to write servers.
1944
cb1a09d0 1945See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1946
1947=item fork
d74e8afc 1948X<fork> X<child> X<parent>
a0d0e21e 1949
2b5ab1e7
TC
1950Does a fork(2) system call to create a new process running the
1951same program at the same point. It returns the child pid to the
1952parent process, C<0> to the child process, or C<undef> if the fork is
1953unsuccessful. File descriptors (and sometimes locks on those descriptors)
1954are shared, while everything else is copied. On most systems supporting
1955fork(), great care has gone into making it extremely efficient (for
1956example, using copy-on-write technology on data pages), making it the
1957dominant paradigm for multitasking over the last few decades.
5a964f20 1958
0f897271
GS
1959Beginning with v5.6.0, Perl will attempt to flush all files opened for
1960output before forking the child process, but this may not be supported
1961on some platforms (see L<perlport>). To be safe, you may need to set
1962C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1963C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1964
19799a22 1965If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1966accumulate zombies. On some systems, you can avoid this by setting
1967C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1968forking and reaping moribund children.
cb1a09d0 1969
28757baa
PP
1970Note that if your forked child inherits system file descriptors like
1971STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1972if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1973backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1974You should reopen those to F</dev/null> if it's any issue.
28757baa 1975
cb1a09d0 1976=item format
d74e8afc 1977X<format>
cb1a09d0 1978
19799a22 1979Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1980example:
1981
54310121 1982 format Something =
cb1a09d0
AD
1983 Test: @<<<<<<<< @||||| @>>>>>
1984 $str, $%, '$' . int($num)
1985 .
1986
1987 $str = "widget";
184e9718 1988 $num = $cost/$quantity;
cb1a09d0
AD
1989 $~ = 'Something';
1990 write;
1991
1992See L<perlform> for many details and examples.
1993
8903cb82 1994=item formline PICTURE,LIST
d74e8afc 1995X<formline>
a0d0e21e 1996
5a964f20 1997This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1998too. It formats (see L<perlform>) a list of values according to the
1999contents of PICTURE, placing the output into the format output
7660c0ab 2000accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2001Eventually, when a C<write> is done, the contents of
cf264981
SP
2002C<$^A> are written to some filehandle. You could also read C<$^A>
2003and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2004does one C<formline> per line of form, but the C<formline> function itself
748a9306 2005doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 2006that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
2007You may therefore need to use multiple formlines to implement a single
2008record format, just like the format compiler.
2009
19799a22 2010Be careful if you put double quotes around the picture, because an C<@>
748a9306 2011character may be taken to mean the beginning of an array name.
19799a22 2012C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
2013
2014=item getc FILEHANDLE
f723aae1 2015X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2016
2017=item getc
2018
2019Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
2020or the undefined value at end of file, or if there was an error (in
2021the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2022STDIN. This is not particularly efficient. However, it cannot be
2023used by itself to fetch single characters without waiting for the user
2024to hit enter. For that, try something more like:
4633a7c4
LW
2025
2026 if ($BSD_STYLE) {
2027 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2028 }
2029 else {
54310121 2030 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2031 }
2032
2033 $key = getc(STDIN);
2034
2035 if ($BSD_STYLE) {
2036 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2037 }
2038 else {
5f05dabc 2039 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
2040 }
2041 print "\n";
2042
54310121
PP
2043Determination of whether $BSD_STYLE should be set
2044is left as an exercise to the reader.
cb1a09d0 2045
19799a22 2046The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2047systems purporting POSIX compliance. See also the C<Term::ReadKey>
2048module from your nearest CPAN site; details on CPAN can be found on
2049L<perlmodlib/CPAN>.
a0d0e21e
LW
2050
2051=item getlogin
d74e8afc 2052X<getlogin> X<login>
a0d0e21e 2053
cf264981 2054This implements the C library function of the same name, which on most
5a964f20 2055systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 2056use C<getpwuid>.
a0d0e21e 2057
f86702cc 2058 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2059
19799a22
GS
2060Do not consider C<getlogin> for authentication: it is not as
2061secure as C<getpwuid>.
4633a7c4 2062
a0d0e21e 2063=item getpeername SOCKET
d74e8afc 2064X<getpeername> X<peer>
a0d0e21e
LW
2065
2066Returns the packed sockaddr address of other end of the SOCKET connection.
2067
4633a7c4
LW
2068 use Socket;
2069 $hersockaddr = getpeername(SOCK);
19799a22 2070 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2071 $herhostname = gethostbyaddr($iaddr, AF_INET);
2072 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2073
2074=item getpgrp PID
d74e8afc 2075X<getpgrp> X<group>
a0d0e21e 2076
47e29363 2077Returns the current process group for the specified PID. Use
7660c0ab 2078a PID of C<0> to get the current process group for the
4633a7c4 2079current process. Will raise an exception if used on a machine that
f86cebdf 2080doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2081group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2082does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2083
2084=item getppid
d74e8afc 2085X<getppid> X<parent> X<pid>
a0d0e21e
LW
2086
2087Returns the process id of the parent process.
2088
4d76a344
RGS
2089Note for Linux users: on Linux, the C functions C<getpid()> and
2090C<getppid()> return different values from different threads. In order to
2091be portable, this behavior is not reflected by the perl-level function
2092C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2093to call the underlying C<getppid()>, you may use the CPAN module
2094C<Linux::Pid>.
4d76a344 2095
a0d0e21e 2096=item getpriority WHICH,WHO
d74e8afc 2097X<getpriority> X<priority> X<nice>
a0d0e21e 2098
4633a7c4
LW
2099Returns the current priority for a process, a process group, or a user.
2100(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2101machine that doesn't implement getpriority(2).
a0d0e21e
LW
2102
2103=item getpwnam NAME
d74e8afc
ITB
2104X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2105X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2106X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2107X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2108X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2109X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2110
2111=item getgrnam NAME
2112
2113=item gethostbyname NAME
2114
2115=item getnetbyname NAME
2116
2117=item getprotobyname NAME
2118
2119=item getpwuid UID
2120
2121=item getgrgid GID
2122
2123=item getservbyname NAME,PROTO
2124
2125=item gethostbyaddr ADDR,ADDRTYPE
2126
2127=item getnetbyaddr ADDR,ADDRTYPE
2128
2129=item getprotobynumber NUMBER
2130
2131=item getservbyport PORT,PROTO
2132
2133=item getpwent
2134
2135=item getgrent
2136
2137=item gethostent
2138
2139=item getnetent
2140
2141=item getprotoent
2142
2143=item getservent
2144
2145=item setpwent
2146
2147=item setgrent
2148
2149=item sethostent STAYOPEN
2150
2151=item setnetent STAYOPEN
2152
2153=item setprotoent STAYOPEN
2154
2155=item setservent STAYOPEN
2156
2157=item endpwent
2158
2159=item endgrent
2160
2161=item endhostent
2162
2163=item endnetent
2164
2165=item endprotoent
2166
2167=item endservent
2168
2169These routines perform the same functions as their counterparts in the
5a964f20 2170system library. In list context, the return values from the
a0d0e21e
LW
2171various get routines are as follows:
2172
2173 ($name,$passwd,$uid,$gid,
6ee623d5 2174 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2175 ($name,$passwd,$gid,$members) = getgr*
2176 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2177 ($name,$aliases,$addrtype,$net) = getnet*
2178 ($name,$aliases,$proto) = getproto*
2179 ($name,$aliases,$port,$proto) = getserv*
2180
2181(If the entry doesn't exist you get a null list.)
2182
4602f195
JH
2183The exact meaning of the $gcos field varies but it usually contains
2184the real name of the user (as opposed to the login name) and other
2185information pertaining to the user. Beware, however, that in many
2186system users are able to change this information and therefore it
106325ad 2187cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2188L<perlsec>). The $passwd and $shell, user's encrypted password and
2189login shell, are also tainted, because of the same reason.
4602f195 2190
5a964f20 2191In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2192lookup by name, in which case you get the other thing, whatever it is.
2193(If the entry doesn't exist you get the undefined value.) For example:
2194
5a964f20
TC
2195 $uid = getpwnam($name);
2196 $name = getpwuid($num);
2197 $name = getpwent();
2198 $gid = getgrnam($name);
08a33e13 2199 $name = getgrgid($num);
5a964f20
TC
2200 $name = getgrent();
2201 #etc.
a0d0e21e 2202
4602f195
JH
2203In I<getpw*()> the fields $quota, $comment, and $expire are special
2204cases in the sense that in many systems they are unsupported. If the
2205$quota is unsupported, it is an empty scalar. If it is supported, it
2206usually encodes the disk quota. If the $comment field is unsupported,
2207it is an empty scalar. If it is supported it usually encodes some
2208administrative comment about the user. In some systems the $quota
2209field may be $change or $age, fields that have to do with password
2210aging. In some systems the $comment field may be $class. The $expire
2211field, if present, encodes the expiration period of the account or the
2212password. For the availability and the exact meaning of these fields
2213in your system, please consult your getpwnam(3) documentation and your
2214F<pwd.h> file. You can also find out from within Perl what your
2215$quota and $comment fields mean and whether you have the $expire field
2216by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2217C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2218files are only supported if your vendor has implemented them in the
2219intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2220shadow versions if you're running under privilege or if there exists
cf264981
SP
2221the shadow(3) functions as found in System V (this includes Solaris
2222and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2223facility are unlikely to be supported.
6ee623d5 2224
19799a22 2225The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2226the login names of the members of the group.
2227
2228For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2229C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2230C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2231addresses returned by the corresponding system library call. In the
2232Internet domain, each address is four bytes long and you can unpack it
2233by saying something like:
2234
f337b084 2235 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2236
2b5ab1e7
TC
2237The Socket library makes this slightly easier:
2238
2239 use Socket;
2240 $iaddr = inet_aton("127.1"); # or whatever address
2241 $name = gethostbyaddr($iaddr, AF_INET);
2242
2243 # or going the other way
19799a22 2244 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2245
19799a22
GS
2246If you get tired of remembering which element of the return list
2247contains which return value, by-name interfaces are provided
2248in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2249C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2250and C<User::grent>. These override the normal built-ins, supplying
2251versions that return objects with the appropriate names
2252for each field. For example:
5a964f20
TC
2253
2254 use File::stat;
2255 use User::pwent;
2256 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2257
b76cc8ba
NIS
2258Even though it looks like they're the same method calls (uid),
2259they aren't, because a C<File::stat> object is different from
19799a22 2260a C<User::pwent> object.
5a964f20 2261
a0d0e21e 2262=item getsockname SOCKET
d74e8afc 2263X<getsockname>
a0d0e21e 2264
19799a22
GS
2265Returns the packed sockaddr address of this end of the SOCKET connection,
2266in case you don't know the address because you have several different
2267IPs that the connection might have come in on.
a0d0e21e 2268
4633a7c4
LW
2269 use Socket;
2270 $mysockaddr = getsockname(SOCK);
19799a22 2271 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2272 printf "Connect to %s [%s]\n",
19799a22
GS
2273 scalar gethostbyaddr($myaddr, AF_INET),
2274 inet_ntoa($myaddr);
a0d0e21e
LW
2275
2276=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2277X<getsockopt>
a0d0e21e 2278
636e6b1f
TH
2279Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2280Options may exist at multiple protocol levels depending on the socket
2281type, but at least the uppermost socket level SOL_SOCKET (defined in the
2282C<Socket> module) will exist. To query options at another level the
2283protocol number of the appropriate protocol controlling the option
2284should be supplied. For example, to indicate that an option is to be
2285interpreted by the TCP protocol, LEVEL should be set to the protocol
2286number of TCP, which you can get using getprotobyname.
2287
2288The call returns a packed string representing the requested socket option,
2289or C<undef> if there is an error (the error reason will be in $!). What
2290exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2291your system documentation for details. A very common case however is that
cf264981 2292the option is an integer, in which case the result will be a packed
636e6b1f
TH
2293integer which you can decode using unpack with the C<i> (or C<I>) format.
2294
2295An example testing if Nagle's algorithm is turned on on a socket:
2296
4852725b 2297 use Socket qw(:all);
636e6b1f
TH
2298
2299 defined(my $tcp = getprotobyname("tcp"))
2300 or die "Could not determine the protocol number for tcp";
4852725b
DD
2301 # my $tcp = IPPROTO_TCP; # Alternative
2302 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2303 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2304 my $nodelay = unpack("I", $packed);
2305 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2306
a0d0e21e
LW
2307
2308=item glob EXPR
d74e8afc 2309X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2310
0a753a76
PP
2311=item glob
2312
d9a9d457
JL
2313In list context, returns a (possibly empty) list of filename expansions on
2314the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2315scalar context, glob iterates through such filename expansions, returning
2316undef when the list is exhausted. This is the internal function
2317implementing the C<< <*.c> >> operator, but you can use it directly. If
2318EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2319more detail in L<perlop/"I/O Operators">.
a0d0e21e 2320
3a4b19e4
GS
2321Beginning with v5.6.0, this operator is implemented using the standard
2322C<File::Glob> extension. See L<File::Glob> for details.
2323
a0d0e21e 2324=item gmtime EXPR
d74e8afc 2325X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2326
ce2984c3
PF
2327=item gmtime
2328
435fbc73
GS
2329Works just like L<localtime> but the returned values are
2330localized for the standard Greenwich time zone.
a0d0e21e 2331
435fbc73
GS
2332Note: when called in list context, $isdst, the last value
2333returned by gmtime is always C<0>. There is no
2334Daylight Saving Time in GMT.
0a753a76 2335
62aa5637
MS
2336See L<perlport/gmtime> for portability concerns.
2337
a0d0e21e 2338=item goto LABEL
d74e8afc 2339X<goto> X<jump> X<jmp>
a0d0e21e 2340
748a9306
LW
2341=item goto EXPR
2342
a0d0e21e
LW
2343=item goto &NAME
2344
7660c0ab 2345The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2346execution there. It may not be used to go into any construct that
7660c0ab 2347requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2348also can't be used to go into a construct that is optimized away,
19799a22 2349or to get out of a block or subroutine given to C<sort>.
0a753a76 2350It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2351including out of subroutines, but it's usually better to use some other
19799a22 2352construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2353need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2354(The difference being that C does not offer named loops combined with
2355loop control. Perl does, and this replaces most structured uses of C<goto>
2356in other languages.)
a0d0e21e 2357
7660c0ab
A
2358The C<goto-EXPR> form expects a label name, whose scope will be resolved
2359dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2360necessarily recommended if you're optimizing for maintainability:
2361
2362 goto ("FOO", "BAR", "GLARCH")[$i];
2363
1b6921cb
BT
2364The C<goto-&NAME> form is quite different from the other forms of
2365C<goto>. In fact, it isn't a goto in the normal sense at all, and
2366doesn't have the stigma associated with other gotos. Instead, it
2367exits the current subroutine (losing any changes set by local()) and
2368immediately calls in its place the named subroutine using the current
2369value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2370load another subroutine and then pretend that the other subroutine had
2371been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2372in the current subroutine are propagated to the other subroutine.)
2373After the C<goto>, not even C<caller> will be able to tell that this
2374routine was called first.
2375
2376NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2377containing a code reference, or a block that evaluates to a code
6cb9131c 2378reference.
a0d0e21e
LW
2379
2380=item grep BLOCK LIST
d74e8afc 2381X<grep>
a0d0e21e
LW
2382
2383=item grep EXPR,LIST
2384
2b5ab1e7
TC
2385This is similar in spirit to, but not the same as, grep(1) and its
2386relatives. In particular, it is not limited to using regular expressions.
2f9daede 2387
a0d0e21e 2388Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2389C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2390elements for which the expression evaluated to true. In scalar
2391context, returns the number of times the expression was true.
a0d0e21e
LW
2392
2393 @foo = grep(!/^#/, @bar); # weed out comments
2394
2395or equivalently,
2396
2397 @foo = grep {!/^#/} @bar; # weed out comments
2398
be3174d2
GS
2399Note that C<$_> is an alias to the list value, so it can be used to
2400modify the elements of the LIST. While this is useful and supported,
2401it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2402Similarly, grep returns aliases into the original list, much as a for
2403loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2404element of a list returned by grep (for example, in a C<foreach>, C<map>
2405or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2406This is usually something to be avoided when writing clear code.
a0d0e21e 2407
a4fb8298 2408If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2409been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2410the list elements, C<$_> keeps being lexical inside the block; i.e. it
2411can't be seen from the outside, avoiding any potential side-effects.
2412
19799a22 2413See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2414
a0d0e21e 2415=item hex EXPR
d74e8afc 2416X<hex> X<hexadecimal>
a0d0e21e 2417
54310121 2418=item hex
bbce6d69 2419
2b5ab1e7 2420Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2421(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2422L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2423
2424 print hex '0xAf'; # prints '175'
2425 print hex 'aF'; # same
a0d0e21e 2426
19799a22 2427Hex strings may only represent integers. Strings that would cause
53305cf1 2428integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2429unlike oct(). To present something as hex, look into L</printf>,
2430L</sprintf>, or L</unpack>.
19799a22 2431
ce2984c3 2432=item import LIST
d74e8afc 2433X<import>
a0d0e21e 2434
19799a22 2435There is no builtin C<import> function. It is just an ordinary
4633a7c4 2436method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2437names to another module. The C<use> function calls the C<import> method
cea6626f 2438for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2439
2440=item index STR,SUBSTR,POSITION
d74e8afc 2441X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2442
2443=item index STR,SUBSTR
2444
2b5ab1e7
TC
2445The index function searches for one string within another, but without
2446the wildcard-like behavior of a full regular-expression pattern match.
2447It returns the position of the first occurrence of SUBSTR in STR at
2448or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2449beginning of the string. POSITION before the beginning of the string
2450or after its end is treated as if it were the beginning or the end,
2451respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2452you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2453is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2454
2455=item int EXPR
f723aae1 2456X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2457
54310121 2458=item int
bbce6d69 2459
7660c0ab 2460Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2461You should not use this function for rounding: one because it truncates
2462towards C<0>, and two because machine representations of floating point
2463numbers can sometimes produce counterintuitive results. For example,
2464C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2465because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2466the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2467functions will serve you better than will int().
a0d0e21e
LW
2468
2469=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2470X<ioctl>
a0d0e21e 2471
2b5ab1e7 2472Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2473
6c567752 2474 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2475
a11c483f 2476to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2477exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2478own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2479(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2480may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2481written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2482will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2483has no string value but does have a numeric value, that value will be
2484passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2485true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2486functions may be needed to manipulate the values of structures used by
b76cc8ba 2487C<ioctl>.
a0d0e21e 2488
19799a22 2489The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2490
2491 if OS returns: then Perl returns:
2492 -1 undefined value
2493 0 string "0 but true"
2494 anything else that number
2495
19799a22 2496Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2497still easily determine the actual value returned by the operating
2498system:
2499
2b5ab1e7 2500 $retval = ioctl(...) || -1;
a0d0e21e
LW
2501 printf "System returned %d\n", $retval;
2502
be2f7487 2503The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2504about improper numeric conversions.
2505
a0d0e21e 2506=item join EXPR,LIST
d74e8afc 2507X<join>
a0d0e21e 2508
2b5ab1e7
TC
2509Joins the separate strings of LIST into a single string with fields
2510separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2511
2b5ab1e7 2512 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2513
eb6e2d6f
GS
2514Beware that unlike C<split>, C<join> doesn't take a pattern as its
2515first argument. Compare L</split>.
a0d0e21e 2516
aa689395 2517=item keys HASH
d74e8afc 2518X<keys> X<key>
aa689395 2519
504f80c1
JH
2520Returns a list consisting of all the keys of the named hash.
2521(In scalar context, returns the number of keys.)
2522
2523The keys are returned in an apparently random order. The actual
2524random order is subject to change in future versions of perl, but it
2525is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2526function produces (given that the hash has not been modified). Since
2527Perl 5.8.1 the ordering is different even between different runs of
2528Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2529Attacks">).
504f80c1 2530
cf264981
SP
2531As a side effect, calling keys() resets the HASH's internal iterator
2532(see L</each>). In particular, calling keys() in void context resets
2533the iterator with no other overhead.
a0d0e21e 2534
aa689395 2535Here is yet another way to print your environment:
a0d0e21e
LW
2536
2537 @keys = keys %ENV;
2538 @values = values %ENV;
b76cc8ba 2539 while (@keys) {
a0d0e21e
LW
2540 print pop(@keys), '=', pop(@values), "\n";
2541 }
2542
2543or how about sorted by key:
2544
2545 foreach $key (sort(keys %ENV)) {
2546 print $key, '=', $ENV{$key}, "\n";
2547 }
2548
8ea1e5d4
GS
2549The returned values are copies of the original keys in the hash, so
2550modifying them will not affect the original hash. Compare L</values>.
2551
19799a22 2552To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2553Here's a descending numeric sort of a hash by its values:
4633a7c4 2554
5a964f20 2555 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2556 printf "%4d %s\n", $hash{$key}, $key;
2557 }
2558
19799a22 2559As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2560allocated for the given hash. This can gain you a measure of efficiency if
2561you know the hash is going to get big. (This is similar to pre-extending
2562an array by assigning a larger number to $#array.) If you say
55497cff
PP
2563
2564 keys %hash = 200;
2565
ab192400
GS
2566then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2567in fact, since it rounds up to the next power of two. These
55497cff
PP
2568buckets will be retained even if you do C<%hash = ()>, use C<undef
2569%hash> if you want to free the storage while C<%hash> is still in scope.
2570You can't shrink the number of buckets allocated for the hash using
19799a22 2571C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2572as trying has no effect).
2573
19799a22 2574See also C<each>, C<values> and C<sort>.
ab192400 2575
b350dd2f 2576=item kill SIGNAL, LIST
d74e8afc 2577X<kill> X<signal>
a0d0e21e 2578
b350dd2f 2579Sends a signal to a list of processes. Returns the number of
517db077
GS
2580processes successfully signaled (which is not necessarily the
2581same as the number actually killed).
a0d0e21e
LW
2582
2583 $cnt = kill 1, $child1, $child2;
2584 kill 9, @goners;
2585
70fb64f6 2586If SIGNAL is zero, no signal is sent to the process, but the kill(2)
6cb9d3e4 2587system call will check whether it's possible to send a signal to it (that
70fb64f6
RGS
2588means, to be brief, that the process is owned by the same user, or we are
2589the super-user). This is a useful way to check that a child process is
81fd35db
DN
2590alive (even if only as a zombie) and hasn't changed its UID. See
2591L<perlport> for notes on the portability of this construct.
b350dd2f
GS
2592
2593Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2594process groups instead of processes. (On System V, a negative I<PROCESS>
2595number will also kill process groups, but that's not portable.) That
2596means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2597use a signal name in quotes.
2598
2599See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2600
2601=item last LABEL
d74e8afc 2602X<last> X<break>
a0d0e21e
LW
2603
2604=item last
2605
2606The C<last> command is like the C<break> statement in C (as used in
2607loops); it immediately exits the loop in question. If the LABEL is
2608omitted, the command refers to the innermost enclosing loop. The
2609C<continue> block, if any, is not executed:
2610
4633a7c4
LW
2611 LINE: while (<STDIN>) {
2612 last LINE if /^$/; # exit when done with header
5a964f20 2613 #...
a0d0e21e
LW
2614 }
2615
4968c1e4 2616C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2617C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2618a grep() or map() operation.
4968c1e4 2619
6c1372ed
GS
2620Note that a block by itself is semantically identical to a loop
2621that executes once. Thus C<last> can be used to effect an early
2622exit out of such a block.
2623
98293880
JH
2624See also L</continue> for an illustration of how C<last>, C<next>, and
2625C<redo> work.
1d2dff63 2626
a0d0e21e 2627=item lc EXPR
d74e8afc 2628X<lc> X<lowercase>
a0d0e21e 2629
54310121 2630=item lc
bbce6d69 2631
d1be9408 2632Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2633implementing the C<\L> escape in double-quoted strings. Respects
2634current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2635and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2636
7660c0ab 2637If EXPR is omitted, uses C<$_>.
bbce6d69 2638
a0d0e21e 2639=item lcfirst EXPR
d74e8afc 2640X<lcfirst> X<lowercase>
a0d0e21e 2641
54310121 2642=item lcfirst
bbce6d69 2643
ad0029c4
JH
2644Returns the value of EXPR with the first character lowercased. This
2645is the internal function implementing the C<\l> escape in
2646double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2647locale> in force. See L<perllocale> and L<perlunicode> for more
2648details about locale and Unicode support.
a0d0e21e 2649
7660c0ab 2650If EXPR is omitted, uses C<$_>.
bbce6d69 2651
a0d0e21e 2652=item length EXPR
d74e8afc 2653X<length> X<size>
a0d0e21e 2654
54310121 2655=item length
bbce6d69 2656
974da8e5 2657Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2658omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2659an entire array or hash to find out how many elements these have.
2660For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2661
974da8e5
JH
2662Note the I<characters>: if the EXPR is in Unicode, you will get the
2663number of characters, not the number of bytes. To get the length
2664in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2665
a0d0e21e 2666=item link OLDFILE,NEWFILE
d74e8afc 2667X<link>
a0d0e21e 2668
19799a22 2669Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2670success, false otherwise.
a0d0e21e
LW
2671
2672=item listen SOCKET,QUEUESIZE
d74e8afc 2673X<listen>
a0d0e21e 2674
19799a22 2675Does the same thing that the listen system call does. Returns true if
b76cc8ba 2676it succeeded, false otherwise. See the example in
cea6626f 2677L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2678
2679=item local EXPR
d74e8afc 2680X<local>
a0d0e21e 2681
19799a22 2682You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2683what most people think of as "local". See
13a2d996 2684L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2685
5a964f20
TC
2686A local modifies the listed variables to be local to the enclosing
2687block, file, or eval. If more than one value is listed, the list must
2688be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2689for details, including issues with tied arrays and hashes.
a0d0e21e 2690
a0d0e21e 2691=item localtime EXPR
435fbc73 2692X<localtime> X<ctime>
a0d0e21e 2693
ba053783
AL
2694=item localtime
2695
19799a22 2696Converts a time as returned by the time function to a 9-element list
5f05dabc 2697with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2698follows:
2699
54310121 2700 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2701 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2702 localtime(time);
a0d0e21e 2703
48a26b3a 2704All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2705tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2706of the specified time.
48a26b3a 2707
ba053783
AL
2708C<$mday> is the day of the month, and C<$mon> is the month itself, in
2709the range C<0..11> with 0 indicating January and 11 indicating December.
2710This makes it easy to get a month name from a list:
54310121 2711
ba053783
AL
2712 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2713 print "$abbr[$mon] $mday";
2714 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2715
ba053783
AL
2716C<$year> is the number of years since 1900, not just the last two digits
2717of the year. That is, C<$year> is C<123> in year 2023. The proper way
2718to get a complete 4-digit year is simply:
abd75f24 2719
ba053783 2720 $year += 1900;
abd75f24 2721
435fbc73
GS
2722Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2723to do that, would you?
2724
ba053783
AL
2725To get the last two digits of the year (e.g., '01' in 2001) do:
2726
2727 $year = sprintf("%02d", $year % 100);
2728
2729C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2730Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2731(or C<0..365> in leap years.)
2732
2733C<$isdst> is true if the specified time occurs during Daylight Saving
2734Time, false otherwise.
abd75f24 2735
48a26b3a 2736If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2737
48a26b3a 2738In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2739
5f05dabc 2740 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2741
fe86afc2
NC
2742This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2743instead of local time use the L</gmtime> builtin. See also the
2744C<Time::Local> module (to convert the second, minutes, hours, ... back to
2745the integer value returned by time()), and the L<POSIX> module's strftime(3)
2746and mktime(3) functions.
2747
2748To get somewhat similar but locale dependent date strings, set up your
2749locale environment variables appropriately (please see L<perllocale>) and
2750try for example:
a3cb178b 2751
5a964f20 2752 use POSIX qw(strftime);
2b5ab1e7 2753 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2754 # or for GMT formatted appropriately for your locale:
2755 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2756
2757Note that the C<%a> and C<%b>, the short forms of the day of the week
2758and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2759
62aa5637
MS
2760See L<perlport/localtime> for portability concerns.
2761
435fbc73
GS
2762The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2763by-name access mechanism to the gmtime() and localtime() functions,
2764respectively.
2765
2766For a comprehensive date and time representation look at the
2767L<DateTime> module on CPAN.
2768
07698885 2769=item lock THING
d74e8afc 2770X<lock>
19799a22 2771
01e6739c 2772This function places an advisory lock on a shared variable, or referenced
03730085 2773object contained in I<THING> until the lock goes out of scope.
a6d5524e 2774
f3a23afb 2775lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2776by this name (before any calls to it), that function will be called
03730085
AB
2777instead. (However, if you've said C<use threads>, lock() is always a
2778keyword.) See L<threads>.
19799a22 2779
a0d0e21e 2780=item log EXPR
d74e8afc 2781X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2782
54310121 2783=item log
bbce6d69 2784
2b5ab1e7
TC
2785Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2786returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2787The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2788divided by the natural log of N. For example:
2789
2790 sub log10 {
2791 my $n = shift;
2792 return log($n)/log(10);
b76cc8ba 2793 }
2b5ab1e7
TC
2794
2795See also L</exp> for the inverse operation.
a0d0e21e 2796
a0d0e21e 2797=item lstat EXPR
d74e8afc 2798X<lstat>
a0d0e21e 2799
54310121 2800=item lstat
bbce6d69 2801
19799a22 2802Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2803special C<_> filehandle) but stats a symbolic link instead of the file
2804the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2805your system, a normal C<stat> is done. For much more detailed
2806information, please see the documentation for C<stat>.
a0d0e21e 2807
7660c0ab 2808If EXPR is omitted, stats C<$_>.
bbce6d69 2809
a0d0e21e
LW
2810=item m//
2811
2812The match operator. See L<perlop>.
2813
2814=item map BLOCK LIST
d74e8afc 2815X<map>
a0d0e21e
LW
2816
2817=item map EXPR,LIST
2818
19799a22
GS
2819Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2820C<$_> to each element) and returns the list value composed of the
2821results of each such evaluation. In scalar context, returns the
2822total number of elements so generated. Evaluates BLOCK or EXPR in
2823list context, so each element of LIST may produce zero, one, or
2824more elements in the returned value.
dd99ebda 2825
a0d0e21e
LW
2826 @chars = map(chr, @nums);
2827
2828translates a list of numbers to the corresponding characters. And
2829
4633a7c4 2830 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2831
2832is just a funny way to write
2833
2834 %hash = ();
2835 foreach $_ (@array) {
4633a7c4 2836 $hash{getkey($_)} = $_;
a0d0e21e
LW
2837 }
2838
be3174d2
GS
2839Note that C<$_> is an alias to the list value, so it can be used to
2840modify the elements of the LIST. While this is useful and supported,
2841it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2842Using a regular C<foreach> loop for this purpose would be clearer in
2843most cases. See also L</grep> for an array composed of those items of
2844the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2845
a4fb8298 2846If C<$_> is lexical in the scope where the C<map> appears (because it has
cf264981 2847been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2848the list elements, C<$_> keeps being lexical inside the block; i.e. it
2849can't be seen from the outside, avoiding any potential side-effects.
2850
205fdb4d
NC
2851C<{> starts both hash references and blocks, so C<map { ...> could be either
2852the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2853ahead for the closing C<}> it has to take a guess at which its dealing with
2854based what it finds just after the C<{>. Usually it gets it right, but if it
2855doesn't it won't realize something is wrong until it gets to the C<}> and
2856encounters the missing (or unexpected) comma. The syntax error will be
2857reported close to the C<}> but you'll need to change something near the C<{>
2858such as using a unary C<+> to give perl some help:
2859
2860 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2861 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2862 %hash = map { ("\L$_", 1) } @array # this also works
2863 %hash = map { lc($_), 1 } @array # as does this.
2864 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2865
205fdb4d
NC
2866 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2867
2868or to force an anon hash constructor use C<+{>
2869
2870 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2871
2872and you get list of anonymous hashes each with only 1 entry.
2873
19799a22 2874=item mkdir FILENAME,MASK
d74e8afc 2875X<mkdir> X<md> X<directory, create>
a0d0e21e 2876
5a211162
GS
2877=item mkdir FILENAME
2878
491873e5
RGS
2879=item mkdir
2880
0591cd52 2881Creates the directory specified by FILENAME, with permissions
19799a22
GS
2882specified by MASK (as modified by C<umask>). If it succeeds it
2883returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2884If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2885to C<$_>.
0591cd52 2886
19799a22 2887In general, it is better to create directories with permissive MASK,
0591cd52 2888and let the user modify that with their C<umask>, than it is to supply
19799a22 2889a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2890The exceptions to this rule are when the file or directory should be
2891kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2892C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2893
cc1852e8
JH
2894Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2895number of trailing slashes. Some operating and filesystems do not get
2896this right, so Perl automatically removes all trailing slashes to keep
2897everyone happy.
2898
dd184578
RGS
2899In order to recursively create a directory structure look at
2900the C<mkpath> function of the L<File::Path> module.
2901
a0d0e21e 2902=item msgctl ID,CMD,ARG
d74e8afc 2903X<msgctl>
a0d0e21e 2904
f86cebdf 2905Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2906
2907 use IPC::SysV;
2908
7660c0ab 2909first to get the correct constant definitions. If CMD is C<IPC_STAT>,
cf264981 2910then ARG must be a variable that will hold the returned C<msqid_ds>
951ba7fe
GS
2911structure. Returns like C<ioctl>: the undefined value for error,
2912C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2913L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2914
2915=item msgget KEY,FLAGS
d74e8afc 2916X<msgget>
a0d0e21e 2917
f86cebdf 2918Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2919id, or the undefined value if there is an error. See also
2920L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2921
a0d0e21e 2922=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
d74e8afc 2923X<msgrcv>
a0d0e21e
LW
2924
2925Calls the System V IPC function msgrcv to receive a message from
2926message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2927SIZE. Note that when a message is received, the message type as a
2928native long integer will be the first thing in VAR, followed by the
2929actual message. This packing may be opened with C<unpack("l! a*")>.
2930Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2931an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2932C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2933
2934=item msgsnd ID,MSG,FLAGS
d74e8afc 2935X<msgsnd>
41d6edb2
JH
2936
2937Calls the System V IPC function msgsnd to send the message MSG to the
2938message queue ID. MSG must begin with the native long integer message
2939type, and be followed by the length of the actual message, and finally
2940the message itself. This kind of packing can be achieved with
2941C<pack("l! a*", $type, $message)>. Returns true if successful,
2942or false if there is an error. See also C<IPC::SysV>
2943and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2944
2945=item my EXPR
d74e8afc 2946X<my>
a0d0e21e 2947
307ea6df
JH
2948=item my TYPE EXPR
2949
1d2de774 2950=item my EXPR : ATTRS
09bef843 2951
1d2de774 2952=item my TYPE EXPR : ATTRS
307ea6df 2953
19799a22 2954A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2955enclosing block, file, or C<eval>. If more than one value is listed,
2956the list must be placed in parentheses.
307ea6df 2957
1d2de774
JH
2958The exact semantics and interface of TYPE and ATTRS are still
2959evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2960and attributes are handled using the C<attributes> pragma, or starting
2961from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2962L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2963L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2964
a0d0e21e 2965=item next LABEL
d74e8afc 2966X<next> X<continue>
a0d0e21e
LW
2967
2968=item next
2969
2970The C<next> command is like the C<continue> statement in C; it starts
2971the next iteration of the loop:
2972
4633a7c4
LW
2973 LINE: while (<STDIN>) {
2974 next LINE if /^#/; # discard comments
5a964f20 2975 #...
a0d0e21e
LW
2976 }
2977
2978Note that if there were a C<continue> block on the above, it would get
2979executed even on discarded lines. If the LABEL is omitted, the command
2980refers to the innermost enclosing loop.
2981
4968c1e4 2982C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2983C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2984a grep() or map() operation.
4968c1e4 2985
6c1372ed
GS
2986Note that a block by itself is semantically identical to a loop
2987that executes once. Thus C<next> will exit such a block early.
2988
98293880
JH
2989See also L</continue> for an illustration of how C<last>, C<next>, and
2990C<redo> work.
1d2dff63 2991
4a66ea5a 2992=item no Module VERSION LIST
d74e8afc 2993X<no>
4a66ea5a
RGS
2994
2995=item no Module VERSION
2996
a0d0e21e
LW
2997=item no Module LIST
2998
4a66ea5a
RGS
2999=item no Module
3000
593b9c14 3001See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
3002
3003=item oct EXPR
d74e8afc 3004X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
a0d0e21e 3005
54310121 3006=item oct
bbce6d69 3007
4633a7c4 3008Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
3009value. (If EXPR happens to start off with C<0x>, interprets it as a
3010hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
3011binary string. Leading whitespace is ignored in all three cases.)
3012The following will handle decimal, binary, octal, and hex in the standard
3013Perl or C notation:
a0d0e21e
LW
3014
3015 $val = oct($val) if $val =~ /^0/;
3016
19799a22
GS
3017If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3018in octal), use sprintf() or printf():
3019
3020 $perms = (stat("filename"))[2] & 07777;
3021 $oct_perms = sprintf "%lo", $perms;
3022
3023The oct() function is commonly used when a string such as C<644> needs
3024to be converted into a file mode, for example. (Although perl will
3025automatically convert strings into numbers as needed, this automatic
3026conversion assumes base 10.)
a0d0e21e
LW
3027
3028=item open FILEHANDLE,EXPR
d74e8afc 3029X<open> X<pipe> X<file, open> X<fopen>
a0d0e21e 3030
68bd7414
NIS
3031=item open FILEHANDLE,MODE,EXPR
3032
3033=item open FILEHANDLE,MODE,EXPR,LIST
3034
ba964c95
T
3035=item open FILEHANDLE,MODE,REFERENCE
3036
a0d0e21e
LW
3037=item open FILEHANDLE
3038
3039Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
3040FILEHANDLE.
3041
3042(The following is a comprehensive reference to open(): for a gentler
3043introduction you may consider L<perlopentut>.)
3044
a28cd5c9
NT
3045If FILEHANDLE is an undefined scalar variable (or array or hash element)
3046the variable is assigned a reference to a new anonymous filehandle,
3047otherwise if FILEHANDLE is an expression, its value is used as the name of
3048the real filehandle wanted. (This is considered a symbolic reference, so
3049C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
3050
3051If EXPR is omitted, the scalar variable of the same name as the
3052FILEHANDLE contains the filename. (Note that lexical variables--those
3053declared with C<my>--will not work for this purpose; so if you're
67408cae 3054using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
3055
3056If three or more arguments are specified then the mode of opening and
3057the file name are separate. If MODE is C<< '<' >> or nothing, the file
3058is opened for input. If MODE is C<< '>' >>, the file is truncated and
3059opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 3060the file is opened for appending, again being created if necessary.
5a964f20 3061
ed53a2bb
JH
3062You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3063indicate that you want both read and write access to the file; thus
3064C<< '+<' >> is almost always preferred for read/write updates--the C<<
3065'+>' >> mode would clobber the file first. You can't usually use
3066either read-write mode for updating textfiles, since they have
3067variable length records. See the B<-i> switch in L<perlrun> for a
3068better approach. The file is created with permissions of C<0666>
3069modified by the process' C<umask> value.
3070
3071These various prefixes correspond to the fopen(3) modes of C<'r'>,
3072C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 3073
6170680b
IZ
3074In the 2-arguments (and 1-argument) form of the call the mode and
3075filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
3076spaces. It is possible to omit the mode in these forms if the mode is
3077C<< '<' >>.
6170680b 3078
7660c0ab 3079If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 3080command to which output is to be piped, and if the filename ends with a
f244e06d
GS
3081C<'|'>, the filename is interpreted as a command which pipes output to
3082us. See L<perlipc/"Using open() for IPC">
19799a22 3083for more examples of this. (You are not allowed to C<open> to a command
5a964f20 3084that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
3085and L<perlipc/"Bidirectional Communication with Another Process">
3086for alternatives.)
cb1a09d0 3087
ed53a2bb
JH
3088For three or more arguments if MODE is C<'|-'>, the filename is
3089interpreted as a command to which output is to be piped, and if MODE
3090is C<'-|'>, the filename is interpreted as a command which pipes
3091output to us. In the 2-arguments (and 1-argument) form one should
3092replace dash (C<'-'>) with the command.
3093See L<perlipc/"Using open() for IPC"> for more examples of this.
3094(You are not allowed to C<open> to a command that pipes both in I<and>
3095out, but see L<IPC::Open2>, L<IPC::Open3>, and
3096L<perlipc/"Bidirectional Communication"> for alternatives.)
3097
3098In the three-or-more argument form of pipe opens, if LIST is specified
3099(extra arguments after the command name) then LIST becomes arguments
3100to the command invoked if the platform supports it. The meaning of
3101C<open> with more than three arguments for non-pipe modes is not yet
3102specified. Experimental "layers" may give extra LIST arguments
3103meaning.
6170680b
IZ
3104
3105In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 3106and opening C<< '>-' >> opens STDOUT.
6170680b 3107
fae2c0fb
RGS
3108You may use the three-argument form of open to specify IO "layers"
3109(sometimes also referred to as "disciplines") to be applied to the handle
3110that affect how the input and output are processed (see L<open> and
3111L<PerlIO> for more details). For example
7207e29d 3112
9124316e
JH
3113 open(FH, "<:utf8", "file")
3114
3115will open the UTF-8 encoded file containing Unicode characters,
6d5e88a0
ST
3116see L<perluniintro>. Note that if layers are specified in the
3117three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3118usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
ed53a2bb
JH
3119
3120Open returns nonzero upon success, the undefined value otherwise. If
3121the C<open> involved a pipe, the return value happens to be the pid of
3122the subprocess.
cb1a09d0 3123
ed53a2bb
JH
3124If you're running Perl on a system that distinguishes between text
3125files and binary files, then you should check out L</binmode> for tips
3126for dealing with this. The key distinction between systems that need
3127C<binmode> and those that don't is their text file formats. Systems
8939ba94 3128like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
3129character, and which encode that character in C as C<"\n">, do not
3130need C<binmode>. The rest need it.
cb1a09d0 3131
fb73857a 3132When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
3133if the request failed, so C<open> is frequently used in connection with
3134C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 3135where you want to make a nicely formatted error message (but there are
5a964f20 3136modules that can help with that problem)) you should always check
19799a22 3137the return value from opening a file. The infrequent exception is when
fb73857a
PP
3138working with an unopened filehandle is actually what you want to do.
3139
cf264981 3140As a special case the 3-arg form with a read/write mode and the third
ed53a2bb 3141argument being C<undef>:
b76cc8ba
NIS
3142
3143 open(TMP, "+>", undef) or die ...
3144
f253e835
JH
3145opens a filehandle to an anonymous temporary file. Also using "+<"
3146works for symmetry, but you really should consider writing something
3147to the temporary file first. You will need to seek() to do the
3148reading.
b76cc8ba 3149
2ce64696 3150Since v5.8.0, perl has built using PerlIO by default. Unless you've