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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
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3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
15operator. A unary operator generally provides a scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
a0d0e21e 17contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 18be first, and the list argument will follow. (Note that there can ever
0f31cffe 19be only one such list argument.) For instance, splice() has three scalar
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20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
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22
23In the syntax descriptions that follow, list operators that expect a
24list (and provide list context for the elements of the list) are shown
25with LIST as an argument. Such a list may consist of any combination
26of scalar arguments or list values; the list values will be included
27in the list as if each individual element were interpolated at that
28point in the list, forming a longer single-dimensional list value.
cf264981 29Commas should separate elements of the LIST.
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30
31Any function in the list below may be used either with or without
32parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 33parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 34surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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35function, and precedence doesn't matter. Otherwise it's a list
36operator or unary operator, and precedence does matter. And whitespace
37between the function and left parenthesis doesn't count--so you need to
38be careful sometimes:
39
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40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
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45
46If you run Perl with the B<-w> switch it can warn you about this. For
47example, the third line above produces:
48
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
51
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52A few functions take no arguments at all, and therefore work as neither
53unary nor list operators. These include such functions as C<time>
54and C<endpwent>. For example, C<time+86_400> always means
55C<time() + 86_400>.
56
a0d0e21e 57For functions that can be used in either a scalar or list context,
54310121 58nonabortive failure is generally indicated in a scalar context by
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59returning the undefined value, and in a list context by returning the
60null list.
61
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62Remember the following important rule: There is B<no rule> that relates
63the behavior of an expression in list context to its behavior in scalar
64context, or vice versa. It might do two totally different things.
a0d0e21e 65Each operator and function decides which sort of value it would be most
2b5ab1e7 66appropriate to return in scalar context. Some operators return the
5a964f20 67length of the list that would have been returned in list context. Some
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68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
d74e8afc 72X<context>
a0d0e21e 73
d1be9408 74A named array in scalar context is quite different from what would at
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75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
81In general, functions in Perl that serve as wrappers for system calls
f86cebdf 82of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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83true when they succeed and C<undef> otherwise, as is usually mentioned
84in the descriptions below. This is different from the C interfaces,
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85which return C<-1> on failure. Exceptions to this rule are C<wait>,
86C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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87variable on failure. Other functions do not, except accidentally.
88
cb1a09d0 89=head2 Perl Functions by Category
d74e8afc 90X<function>
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91
92Here are Perl's functions (including things that look like
5a964f20 93functions, like some keywords and named operators)
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94arranged by category. Some functions appear in more
95than one place.
96
13a2d996 97=over 4
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98
99=item Functions for SCALARs or strings
d74e8afc 100X<scalar> X<string> X<character>
cb1a09d0 101
22fae026 102C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
1dc8ecb8 103C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
945c54fd 104C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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105
106=item Regular expressions and pattern matching
d74e8afc 107X<regular expression> X<regex> X<regexp>
cb1a09d0 108
ab4f32c2 109C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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110
111=item Numeric functions
d74e8afc 112X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 113
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114C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
115C<sin>, C<sqrt>, C<srand>
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116
117=item Functions for real @ARRAYs
d74e8afc 118X<array>
cb1a09d0 119
22fae026 120C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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121
122=item Functions for list data
d74e8afc 123X<list>
cb1a09d0 124
1dc8ecb8 125C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
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126
127=item Functions for real %HASHes
d74e8afc 128X<hash>
cb1a09d0 129
22fae026 130C<delete>, C<each>, C<exists>, C<keys>, C<values>
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131
132=item Input and output functions
d74e8afc 133X<I/O> X<input> X<output> X<dbm>
cb1a09d0 134
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135C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
136C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
0d863452 137C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
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138C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
139C<warn>, C<write>
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140
141=item Functions for fixed length data or records
142
22fae026 143C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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144
145=item Functions for filehandles, files, or directories
d74e8afc 146X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 147
22fae026 148C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 149C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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150C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
151C<umask>, C<unlink>, C<utime>
cb1a09d0 152
cf264981 153=item Keywords related to the control flow of your Perl program
d74e8afc 154X<control flow>
cb1a09d0 155
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156C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
157C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 158
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159=item Keywords related to switch
160
36fb85f3 161C<break>, C<continue>, C<given>, C<when>, C<default>
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162
163(These are only available if you enable the "switch" feature.
164See L<feature> and L<perlsyn/"Switch statements">.)
165
54310121 166=item Keywords related to scoping
cb1a09d0 167
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168C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
169C<use>
170
171(C<state> is only available if the "state" feature is enabled. See
172L<feature>.)
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173
174=item Miscellaneous functions
175
36fb85f3 176C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
834df1c5 177C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
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178
179=item Functions for processes and process groups
d74e8afc 180X<process> X<pid> X<process id>
cb1a09d0 181
22fae026 182C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
1dc8ecb8 183C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 184C<times>, C<wait>, C<waitpid>
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185
186=item Keywords related to perl modules
d74e8afc 187X<module>
cb1a09d0 188
22fae026 189C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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190
191=item Keywords related to classes and object-orientedness
d74e8afc 192X<object> X<class> X<package>
cb1a09d0 193
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194C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
195C<untie>, C<use>
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196
197=item Low-level socket functions
d74e8afc 198X<socket> X<sock>
cb1a09d0 199
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200C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
201C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 202C<socket>, C<socketpair>
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203
204=item System V interprocess communication functions
d74e8afc 205X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 206
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207C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
208C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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209
210=item Fetching user and group info
d74e8afc 211X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 212
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213C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
214C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
215C<getpwuid>, C<setgrent>, C<setpwent>
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216
217=item Fetching network info
d74e8afc 218X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 219
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220C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
221C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
222C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
223C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
224C<setnetent>, C<setprotoent>, C<setservent>
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225
226=item Time-related functions
d74e8afc 227X<time> X<date>
cb1a09d0 228
22fae026 229C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 230
37798a01 231=item Functions new in perl5
d74e8afc 232X<perl5>
37798a01 233
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234C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
235C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
1dc8ecb8 236C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
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237C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
238C<ucfirst>, C<untie>, C<use>, C<when>
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239
240* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 241operator, which can be used in expressions.
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242
243=item Functions obsoleted in perl5
244
22fae026 245C<dbmclose>, C<dbmopen>
37798a01 246
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247=back
248
60f9f73c 249=head2 Portability
d74e8afc 250X<portability> X<Unix> X<portable>
60f9f73c 251
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252Perl was born in Unix and can therefore access all common Unix
253system calls. In non-Unix environments, the functionality of some
254Unix system calls may not be available, or details of the available
255functionality may differ slightly. The Perl functions affected
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256by this are:
257
258C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
259C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
260C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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261C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
262C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 263C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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264C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
265C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
266C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 267C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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268C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
269C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
270C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 271C<shmwrite>, C<socket>, C<socketpair>,
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272C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
273C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 274C<utime>, C<wait>, C<waitpid>
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275
276For more information about the portability of these functions, see
277L<perlport> and other available platform-specific documentation.
278
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279=head2 Alphabetical Listing of Perl Functions
280
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281=over 8
282
5b3c99c0 283=item -X FILEHANDLE
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284X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
285X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 286
5b3c99c0 287=item -X EXPR
a0d0e21e 288
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289=item -X DIRHANDLE
290
5b3c99c0 291=item -X
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292
293A file test, where X is one of the letters listed below. This unary
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294operator takes one argument, either a filename, a filehandle, or a dirhandle,
295and tests the associated file to see if something is true about it. If the
7660c0ab 296argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 297Unless otherwise documented, it returns C<1> for true and C<''> for false, or
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298the undefined value if the file doesn't exist. Despite the funny
299names, precedence is the same as any other named unary operator, and
300the argument may be parenthesized like any other unary operator. The
301operator may be any of:
302
303 -r File is readable by effective uid/gid.
304 -w File is writable by effective uid/gid.
305 -x File is executable by effective uid/gid.
306 -o File is owned by effective uid.
307
308 -R File is readable by real uid/gid.
309 -W File is writable by real uid/gid.
310 -X File is executable by real uid/gid.
311 -O File is owned by real uid.
312
313 -e File exists.
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314 -z File has zero size (is empty).
315 -s File has nonzero size (returns size in bytes).
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316
317 -f File is a plain file.
318 -d File is a directory.
319 -l File is a symbolic link.
9c4d0f16 320 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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321 -S File is a socket.
322 -b File is a block special file.
323 -c File is a character special file.
324 -t Filehandle is opened to a tty.
325
326 -u File has setuid bit set.
327 -g File has setgid bit set.
328 -k File has sticky bit set.
329
121910a4 330 -T File is an ASCII text file (heuristic guess).
2cdbc966 331 -B File is a "binary" file (opposite of -T).
a0d0e21e 332
95a3fe12 333 -M Script start time minus file modification time, in days.
a0d0e21e 334 -A Same for access time.
95a3fe12 335 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 336
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337Example:
338
339 while (<>) {
5b3eff12 340 chomp;
a0d0e21e 341 next unless -f $_; # ignore specials
5a964f20 342 #...
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343 }
344
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345The interpretation of the file permission operators C<-r>, C<-R>,
346C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
347of the file and the uids and gids of the user. There may be other
348reasons you can't actually read, write, or execute the file. Such
349reasons may be for example network filesystem access controls, ACLs
350(access control lists), read-only filesystems, and unrecognized
351executable formats.
352
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353Also note that, for the superuser on the local filesystems, the C<-r>,
354C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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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.
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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
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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
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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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381read a file to do the C<-T> test, on most occasions you want to use a C<-f>
382against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 383
19799a22 384If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 385the special filehandle consisting of a solitary underline, then the stat
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386structure of the previous file test (or stat operator) is used, saving
387a system call. (This doesn't work with C<-t>, and you need to remember
388that lstat() and C<-l> will leave values in the stat structure for the
5c9aa243 389symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 390an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 391Example:
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392
393 print "Can do.\n" if -r $a || -w _ || -x _;
394
395 stat($filename);
396 print "Readable\n" if -r _;
397 print "Writable\n" if -w _;
398 print "Executable\n" if -x _;
399 print "Setuid\n" if -u _;
400 print "Setgid\n" if -g _;
401 print "Sticky\n" if -k _;
402 print "Text\n" if -T _;
403 print "Binary\n" if -B _;
404
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405As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
406test operators, in a way that C<-f -w -x $file> is equivalent to
cf264981 407C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
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RGS
408the return value of C<-f $file> as an argument to another filetest
409operator, no special magic will happen.)
410
a0d0e21e 411=item abs VALUE
d74e8afc 412X<abs> X<absolute>
a0d0e21e 413
54310121 414=item abs
bbce6d69 415
a0d0e21e 416Returns the absolute value of its argument.
7660c0ab 417If VALUE is omitted, uses C<$_>.
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418
419=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 420X<accept>
a0d0e21e 421
f86cebdf 422Accepts an incoming socket connect, just as the accept(2) system call
19799a22 423does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 424See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 425
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426On systems that support a close-on-exec flag on files, the flag will
427be set for the newly opened file descriptor, as determined by the
428value of $^F. See L<perlvar/$^F>.
429
a0d0e21e 430=item alarm SECONDS
d74e8afc
ITB
431X<alarm>
432X<SIGALRM>
433X<timer>
a0d0e21e 434
54310121 435=item alarm
bbce6d69 436
a0d0e21e 437Arranges to have a SIGALRM delivered to this process after the
cf264981 438specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
439specified, the value stored in C<$_> is used. (On some machines,
440unfortunately, the elapsed time may be up to one second less or more
441than you specified because of how seconds are counted, and process
442scheduling may delay the delivery of the signal even further.)
443
444Only one timer may be counting at once. Each call disables the
445previous timer, and an argument of C<0> may be supplied to cancel the
446previous timer without starting a new one. The returned value is the
447amount of time remaining on the previous timer.
a0d0e21e 448
4633a7c4 449For delays of finer granularity than one second, you may use Perl's
19799a22
GS
450four-argument version of select() leaving the first three arguments
451undefined, or you might be able to use the C<syscall> interface to
83df6a1d
JH
452access setitimer(2) if your system supports it. The Time::HiRes
453module (from CPAN, and starting from Perl 5.8 part of the standard
454distribution) may also prove useful.
2b5ab1e7 455
68f8bed4
JH
456It is usually a mistake to intermix C<alarm> and C<sleep> calls.
457(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 458
19799a22
GS
459If you want to use C<alarm> to time out a system call you need to use an
460C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 461fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 462restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 463modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
464
465 eval {
f86cebdf 466 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 467 alarm $timeout;
ff68c719 468 $nread = sysread SOCKET, $buffer, $size;
36477c24 469 alarm 0;
ff68c719 470 };
ff68c719 471 if ($@) {
f86cebdf 472 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719
PP
473 # timed out
474 }
475 else {
476 # didn't
477 }
478
91d81acc
JH
479For more information see L<perlipc>.
480
a0d0e21e 481=item atan2 Y,X
d74e8afc 482X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
483
484Returns the arctangent of Y/X in the range -PI to PI.
485
ca6e1c26 486For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
487function, or use the familiar relation:
488
489 sub tan { sin($_[0]) / cos($_[0]) }
490
bf5f1b4c
JH
491Note that atan2(0, 0) is not well-defined.
492
a0d0e21e 493=item bind SOCKET,NAME
d74e8afc 494X<bind>
a0d0e21e
LW
495
496Binds a network address to a socket, just as the bind system call
19799a22 497does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
498packed address of the appropriate type for the socket. See the examples in
499L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 500
fae2c0fb 501=item binmode FILEHANDLE, LAYER
d74e8afc 502X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 503
a0d0e21e
LW
504=item binmode FILEHANDLE
505
1cbfc93d
NIS
506Arranges for FILEHANDLE to be read or written in "binary" or "text"
507mode on systems where the run-time libraries distinguish between
508binary and text files. If FILEHANDLE is an expression, the value is
509taken as the name of the filehandle. Returns true on success,
b5fe5ca2 510otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 511
d807c6f4
JH
512On some systems (in general, DOS and Windows-based systems) binmode()
513is necessary when you're not working with a text file. For the sake
514of portability it is a good idea to always use it when appropriate,
515and to never use it when it isn't appropriate. Also, people can
516set their I/O to be by default UTF-8 encoded Unicode, not bytes.
517
518In other words: regardless of platform, use binmode() on binary data,
519like for example images.
520
521If LAYER is present it is a single string, but may contain multiple
522directives. The directives alter the behaviour of the file handle.
523When LAYER is present using binmode on text file makes sense.
524
fae2c0fb 525If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
526suitable for passing binary data. This includes turning off possible CRLF
527translation and marking it as bytes (as opposed to Unicode characters).
749683d2 528Note that, despite what may be implied in I<"Programming Perl"> (the
165a9987
PJ
529Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
530-- other layers which would affect the binary nature of the stream are
0226bbdb
NIS
531I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
532PERLIO environment variable.
01e6739c 533
d807c6f4
JH
534The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
535form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
536establish default I/O layers. See L<open>.
537
fae2c0fb
RGS
538I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
539in "Programming Perl, 3rd Edition". However, since the publishing of this
540book, by many known as "Camel III", the consensus of the naming of this
541functionality has moved from "discipline" to "layer". All documentation
542of this version of Perl therefore refers to "layers" rather than to
543"disciplines". Now back to the regularly scheduled documentation...>
544
6902c96a
T
545To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
546C<:utf8> just marks the data as UTF-8 without further checking,
547while C<:encoding(utf8)> checks the data for actually being valid
548UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 549
ed53a2bb 550In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
551is done on the filehandle. Calling binmode() will normally flush any
552pending buffered output data (and perhaps pending input data) on the
fae2c0fb 553handle. An exception to this is the C<:encoding> layer that
01e6739c 554changes the default character encoding of the handle, see L<open>.
fae2c0fb 555The C<:encoding> layer sometimes needs to be called in
3874323d
JH
556mid-stream, and it doesn't flush the stream. The C<:encoding>
557also implicitly pushes on top of itself the C<:utf8> layer because
558internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 559
19799a22 560The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
561system all work together to let the programmer treat a single
562character (C<\n>) as the line terminator, irrespective of the external
563representation. On many operating systems, the native text file
564representation matches the internal representation, but on some
565platforms the external representation of C<\n> is made up of more than
566one character.
567
68bd7414
NIS
568Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
569character to end each line in the external representation of text (even
5e12dbfa 570though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
571on Unix and most VMS files). In other systems like OS/2, DOS and the
572various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
573but what's stored in text files are the two characters C<\cM\cJ>. That
574means that, if you don't use binmode() on these systems, C<\cM\cJ>
575sequences on disk will be converted to C<\n> on input, and any C<\n> in
576your program will be converted back to C<\cM\cJ> on output. This is what
577you want for text files, but it can be disastrous for binary files.
30168b04
GS
578
579Another consequence of using binmode() (on some systems) is that
580special end-of-file markers will be seen as part of the data stream.
581For systems from the Microsoft family this means that if your binary
4375e838 582data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
583the file, unless you use binmode().
584
585binmode() is not only important for readline() and print() operations,
586but also when using read(), seek(), sysread(), syswrite() and tell()
587(see L<perlport> for more details). See the C<$/> and C<$\> variables
588in L<perlvar> for how to manually set your input and output
589line-termination sequences.
a0d0e21e 590
4633a7c4 591=item bless REF,CLASSNAME
d74e8afc 592X<bless>
a0d0e21e
LW
593
594=item bless REF
595
2b5ab1e7
TC
596This function tells the thingy referenced by REF that it is now an object
597in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 598is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 599it returns the reference for convenience. Always use the two-argument
cf264981
SP
600version if a derived class might inherit the function doing the blessing.
601See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
602of objects.
a0d0e21e 603
57668c4d 604Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 605Namespaces with all lowercase names are considered reserved for
cf264981 606Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
607confusion, you may wish to avoid such package names as well. Make sure
608that CLASSNAME is a true value.
60ad88b8
GS
609
610See L<perlmod/"Perl Modules">.
611
0d863452
RH
612=item break
613
614Break out of a C<given()> block.
615
616This keyword is enabled by the "switch" feature: see L<feature>
617for more information.
618
a0d0e21e 619=item caller EXPR
d74e8afc 620X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
621
622=item caller
623
5a964f20 624Returns the context of the current subroutine call. In scalar context,
28757baa 625returns the caller's package name if there is a caller, that is, if
19799a22 626we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 627otherwise. In list context, returns
a0d0e21e 628
ee6b43cc 629 # 0 1 2
748a9306 630 ($package, $filename, $line) = caller;
a0d0e21e
LW
631
632With EXPR, it returns some extra information that the debugger uses to
633print a stack trace. The value of EXPR indicates how many call frames
634to go back before the current one.
635
ee6b43cc 636 # 0 1 2 3 4
f3aa04c2 637 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 638
639 # 5 6 7 8 9 10
b3ca2e83 640 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 641 = caller($i);
e7ea3e70 642
951ba7fe 643Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 644call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 645C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 646C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 647C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 648$filename is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
649each C<use> statement creates a C<require> frame inside an C<eval EXPR>
650frame.) $subroutine may also be C<(unknown)> if this particular
651subroutine happens to have been deleted from the symbol table.
652C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
653C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
654compiled with. The C<$hints> and C<$bitmask> values are subject to change
655between versions of Perl, and are not meant for external use.
748a9306 656
b3ca2e83
NC
657C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
658caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
659of this hash, as they are the actual values stored in the optree.
660
748a9306 661Furthermore, when called from within the DB package, caller returns more
7660c0ab 662detailed information: it sets the list variable C<@DB::args> to be the
54310121 663arguments with which the subroutine was invoked.
748a9306 664
7660c0ab 665Be aware that the optimizer might have optimized call frames away before
19799a22 666C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 667might not return information about the call frame you expect it do, for
b76cc8ba 668C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 669previous time C<caller> was called.
7660c0ab 670
a0d0e21e 671=item chdir EXPR
d74e8afc
ITB
672X<chdir>
673X<cd>
f723aae1 674X<directory, change>
a0d0e21e 675
c4aca7d0
GA
676=item chdir FILEHANDLE
677
678=item chdir DIRHANDLE
679
ce2984c3
PF
680=item chdir
681
ffce7b87 682Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 683changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 684changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
685variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
686neither is set, C<chdir> does nothing. It returns true upon success,
687false otherwise. See the example under C<die>.
a0d0e21e 688
c4aca7d0
GA
689On systems that support fchdir, you might pass a file handle or
690directory handle as argument. On systems that don't support fchdir,
691passing handles produces a fatal error at run time.
692
a0d0e21e 693=item chmod LIST
d74e8afc 694X<chmod> X<permission> X<mode>
a0d0e21e
LW
695
696Changes the permissions of a list of files. The first element of the
4633a7c4 697list must be the numerical mode, which should probably be an octal
4ad40acf 698number, and which definitely should I<not> be a string of octal digits:
2f9daede 699C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 700successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
701
702 $cnt = chmod 0755, 'foo', 'bar';
703 chmod 0755, @executables;
f86cebdf
GS
704 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
705 # --w----r-T
2f9daede
TPG
706 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
707 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 708
c4aca7d0
GA
709On systems that support fchmod, you might pass file handles among the
710files. On systems that don't support fchmod, passing file handles
345da378
GA
711produces a fatal error at run time. The file handles must be passed
712as globs or references to be recognized. Barewords are considered
713file names.
c4aca7d0
GA
714
715 open(my $fh, "<", "foo");
716 my $perm = (stat $fh)[2] & 07777;
717 chmod($perm | 0600, $fh);
718
ca6e1c26
JH
719You can also import the symbolic C<S_I*> constants from the Fcntl
720module:
721
722 use Fcntl ':mode';
723
724 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
725 # This is identical to the chmod 0755 of the above example.
726
a0d0e21e 727=item chomp VARIABLE
d74e8afc 728X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 729
313c9f5c 730=item chomp( LIST )
a0d0e21e
LW
731
732=item chomp
733
2b5ab1e7
TC
734This safer version of L</chop> removes any trailing string
735that corresponds to the current value of C<$/> (also known as
28757baa
PP
736$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
737number of characters removed from all its arguments. It's often used to
738remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
739that the final record may be missing its newline. When in paragraph
740mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
741When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
742a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 743remove anything.
19799a22 744If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
745
746 while (<>) {
747 chomp; # avoid \n on last field
748 @array = split(/:/);
5a964f20 749 # ...
a0d0e21e
LW
750 }
751
4bf21a6d
RD
752If VARIABLE is a hash, it chomps the hash's values, but not its keys.
753
a0d0e21e
LW
754You can actually chomp anything that's an lvalue, including an assignment:
755
756 chomp($cwd = `pwd`);
757 chomp($answer = <STDIN>);
758
759If you chomp a list, each element is chomped, and the total number of
760characters removed is returned.
761
15e44fd8
RGS
762Note that parentheses are necessary when you're chomping anything
763that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
764is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
765C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
766C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
767as C<chomp($a, $b)>.
768
a0d0e21e 769=item chop VARIABLE
d74e8afc 770X<chop>
a0d0e21e 771
313c9f5c 772=item chop( LIST )
a0d0e21e
LW
773
774=item chop
775
776Chops off the last character of a string and returns the character
5b3eff12 777chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 778scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
779If VARIABLE is a hash, it chops the hash's values, but not its keys.
780
5b3eff12 781You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
782
783If you chop a list, each element is chopped. Only the value of the
19799a22 784last C<chop> is returned.
a0d0e21e 785
19799a22 786Note that C<chop> returns the last character. To return all but the last
748a9306
LW
787character, use C<substr($string, 0, -1)>.
788
15e44fd8
RGS
789See also L</chomp>.
790
a0d0e21e 791=item chown LIST
d74e8afc 792X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
793
794Changes the owner (and group) of a list of files. The first two
19799a22
GS
795elements of the list must be the I<numeric> uid and gid, in that
796order. A value of -1 in either position is interpreted by most
797systems to leave that value unchanged. Returns the number of files
798successfully changed.
a0d0e21e
LW
799
800 $cnt = chown $uid, $gid, 'foo', 'bar';
801 chown $uid, $gid, @filenames;
802
c4aca7d0
GA
803On systems that support fchown, you might pass file handles among the
804files. On systems that don't support fchown, passing file handles
345da378
GA
805produces a fatal error at run time. The file handles must be passed
806as globs or references to be recognized. Barewords are considered
807file names.
c4aca7d0 808
54310121 809Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
810
811 print "User: ";
19799a22 812 chomp($user = <STDIN>);
5a964f20 813 print "Files: ";
19799a22 814 chomp($pattern = <STDIN>);
a0d0e21e
LW
815
816 ($login,$pass,$uid,$gid) = getpwnam($user)
817 or die "$user not in passwd file";
818
5a964f20 819 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
820 chown $uid, $gid, @ary;
821
54310121 822On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
823file unless you're the superuser, although you should be able to change
824the group to any of your secondary groups. On insecure systems, these
825restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
826On POSIX systems, you can detect this condition this way:
827
828 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
829 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 830
a0d0e21e 831=item chr NUMBER
d74e8afc 832X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 833
54310121 834=item chr
bbce6d69 835
a0d0e21e 836Returns the character represented by that NUMBER in the character set.
a0ed51b3 837For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 838chr(0x263a) is a Unicode smiley face.
aaa68c4a 839
8a064bd6 840Negative values give the Unicode replacement character (chr(0xfffd)),
5f0135eb 841except under the L<bytes> pragma, where low eight bits of the value
8a064bd6
JH
842(truncated to an integer) are used.
843
974da8e5
JH
844If NUMBER is omitted, uses C<$_>.
845
b76cc8ba 846For the reverse, use L</ord>.
a0d0e21e 847
2575c402
JW
848Note that characters from 128 to 255 (inclusive) are by default
849internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 850
2575c402 851See L<perlunicode> for more about Unicode.
bbce6d69 852
a0d0e21e 853=item chroot FILENAME
d74e8afc 854X<chroot> X<root>
a0d0e21e 855
54310121 856=item chroot
bbce6d69 857
5a964f20 858This function works like the system call by the same name: it makes the
4633a7c4 859named directory the new root directory for all further pathnames that
951ba7fe 860begin with a C</> by your process and all its children. (It doesn't
28757baa 861change your current working directory, which is unaffected.) For security
4633a7c4 862reasons, this call is restricted to the superuser. If FILENAME is
19799a22 863omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
864
865=item close FILEHANDLE
d74e8afc 866X<close>
a0d0e21e 867
6a518fbc
TP
868=item close
869
e0f13c26
RGS
870Closes the file or pipe associated with the file handle, flushes the IO
871buffers, and closes the system file descriptor. Returns true if those
872operations have succeeded and if no error was reported by any PerlIO
873layer. Closes the currently selected filehandle if the argument is
874omitted.
fb73857a
PP
875
876You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
877another C<open> on it, because C<open> will close it for you. (See
878C<open>.) However, an explicit C<close> on an input file resets the line
879counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 880
dede8123
RGS
881If the file handle came from a piped open, C<close> will additionally
882return false if one of the other system calls involved fails, or if the
fb73857a 883program exits with non-zero status. (If the only problem was that the
dede8123 884program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 885also waits for the process executing on the pipe to complete, in case you
b76cc8ba 886want to look at the output of the pipe afterwards, and
e5218da5
GA
887implicitly puts the exit status value of that command into C<$?> and
888C<${^CHILD_ERROR_NATIVE}>.
5a964f20 889
73689b13
GS
890Prematurely closing the read end of a pipe (i.e. before the process
891writing to it at the other end has closed it) will result in a
892SIGPIPE being delivered to the writer. If the other end can't
893handle that, be sure to read all the data before closing the pipe.
894
fb73857a 895Example:
a0d0e21e 896
fb73857a
PP
897 open(OUTPUT, '|sort >foo') # pipe to sort
898 or die "Can't start sort: $!";
5a964f20 899 #... # print stuff to output
fb73857a
PP
900 close OUTPUT # wait for sort to finish
901 or warn $! ? "Error closing sort pipe: $!"
902 : "Exit status $? from sort";
903 open(INPUT, 'foo') # get sort's results
904 or die "Can't open 'foo' for input: $!";
a0d0e21e 905
5a964f20
TC
906FILEHANDLE may be an expression whose value can be used as an indirect
907filehandle, usually the real filehandle name.
a0d0e21e
LW
908
909=item closedir DIRHANDLE
d74e8afc 910X<closedir>
a0d0e21e 911
19799a22 912Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
913system call.
914
a0d0e21e 915=item connect SOCKET,NAME
d74e8afc 916X<connect>
a0d0e21e
LW
917
918Attempts to connect to a remote socket, just as the connect system call
19799a22 919does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
920packed address of the appropriate type for the socket. See the examples in
921L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 922
cb1a09d0 923=item continue BLOCK
d74e8afc 924X<continue>
cb1a09d0 925
0d863452
RH
926=item continue
927
cf264981
SP
928C<continue> is actually a flow control statement rather than a function. If
929there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
930C<foreach>), it is always executed just before the conditional is about to
931be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
932it can be used to increment a loop variable, even when the loop has been
933continued via the C<next> statement (which is similar to the C C<continue>
934statement).
935
98293880 936C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
937block. C<last> and C<redo> will behave as if they had been executed within
938the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
939block, it may be more entertaining.
940
941 while (EXPR) {
942 ### redo always comes here
943 do_something;
944 } continue {
945 ### next always comes here
946 do_something_else;
947 # then back the top to re-check EXPR
948 }
949 ### last always comes here
950
951Omitting the C<continue> section is semantically equivalent to using an
19799a22 952empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
953to check the condition at the top of the loop.
954
0d863452
RH
955If the "switch" feature is enabled, C<continue> is also a
956function that will break out of the current C<when> or C<default>
957block, and fall through to the next case. See L<feature> and
958L<perlsyn/"Switch statements"> for more information.
959
960
a0d0e21e 961=item cos EXPR
d74e8afc 962X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 963
d6217f1e
GS
964=item cos
965
5a964f20 966Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 967takes cosine of C<$_>.
a0d0e21e 968
ca6e1c26 969For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
970function, or use this relation:
971
972 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
973
a0d0e21e 974=item crypt PLAINTEXT,SALT
d74e8afc 975X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 976X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 977
ef2e6798
MS
978Creates a digest string exactly like the crypt(3) function in the C
979library (assuming that you actually have a version there that has not
cf264981 980been extirpated as a potential munitions).
ef2e6798
MS
981
982crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
983into a short string, called a digest, which is returned. The same
984PLAINTEXT and SALT will always return the same string, but there is no
985(known) way to get the original PLAINTEXT from the hash. Small
986changes in the PLAINTEXT or SALT will result in large changes in the
987digest.
988
989There is no decrypt function. This function isn't all that useful for
990cryptography (for that, look for F<Crypt> modules on your nearby CPAN
991mirror) and the name "crypt" is a bit of a misnomer. Instead it is
992primarily used to check if two pieces of text are the same without
993having to transmit or store the text itself. An example is checking
994if a correct password is given. The digest of the password is stored,
cf264981 995not the password itself. The user types in a password that is
ef2e6798
MS
996crypt()'d with the same salt as the stored digest. If the two digests
997match the password is correct.
998
999When verifying an existing digest string you should use the digest as
1000the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1001to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1002crypt() will hash the new string with the same salt as the digest.
1003This allows your code to work with the standard L<crypt|/crypt> and
1004with more exotic implementations. In other words, do not assume
1005anything about the returned string itself, or how many bytes in the
1006digest matter.
85c16d83
JH
1007
1008Traditionally the result is a string of 13 bytes: two first bytes of
1009the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
ef2e6798
MS
1010the first eight bytes of the digest string mattered, but alternative
1011hashing schemes (like MD5), higher level security schemes (like C2),
1012and implementations on non-UNIX platforms may produce different
1013strings.
85c16d83
JH
1014
1015When choosing a new salt create a random two character string whose
1016characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1017'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1018characters is just a recommendation; the characters allowed in
1019the salt depend solely on your system's crypt library, and Perl can't
1020restrict what salts C<crypt()> accepts.
e71965be 1021
a0d0e21e 1022Here's an example that makes sure that whoever runs this program knows
cf264981 1023their password:
a0d0e21e
LW
1024
1025 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1026
1027 system "stty -echo";
1028 print "Password: ";
e71965be 1029 chomp($word = <STDIN>);
a0d0e21e
LW
1030 print "\n";
1031 system "stty echo";
1032
e71965be 1033 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
1034 die "Sorry...\n";
1035 } else {
1036 print "ok\n";
54310121 1037 }
a0d0e21e 1038
9f8f0c9d 1039Of course, typing in your own password to whoever asks you
748a9306 1040for it is unwise.
a0d0e21e 1041
ef2e6798 1042The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1043of data, not least of all because you can't get the information
ef2e6798 1044back. Look at the L<Digest> module for more robust algorithms.
19799a22 1045
f2791508
JH
1046If using crypt() on a Unicode string (which I<potentially> has
1047characters with codepoints above 255), Perl tries to make sense
1048of the situation by trying to downgrade (a copy of the string)
1049the string back to an eight-bit byte string before calling crypt()
1050(on that copy). If that works, good. If not, crypt() dies with
1051C<Wide character in crypt>.
85c16d83 1052
aa689395 1053=item dbmclose HASH
d74e8afc 1054X<dbmclose>
a0d0e21e 1055
19799a22 1056[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1057
aa689395 1058Breaks the binding between a DBM file and a hash.
a0d0e21e 1059
19799a22 1060=item dbmopen HASH,DBNAME,MASK
d74e8afc 1061X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1062
19799a22 1063[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1064
7b8d334a 1065This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1066hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1067argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1068is the name of the database (without the F<.dir> or F<.pag> extension if
1069any). If the database does not exist, it is created with protection
19799a22
GS
1070specified by MASK (as modified by the C<umask>). If your system supports
1071only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 1072program. In older versions of Perl, if your system had neither DBM nor
19799a22 1073ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1074sdbm(3).
1075
1076If you don't have write access to the DBM file, you can only read hash
1077variables, not set them. If you want to test whether you can write,
19799a22 1078either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 1079which will trap the error.
a0d0e21e 1080
19799a22
GS
1081Note that functions such as C<keys> and C<values> may return huge lists
1082when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1083function to iterate over large DBM files. Example:
1084
1085 # print out history file offsets
1086 dbmopen(%HIST,'/usr/lib/news/history',0666);
1087 while (($key,$val) = each %HIST) {
1088 print $key, ' = ', unpack('L',$val), "\n";
1089 }
1090 dbmclose(%HIST);
1091
cb1a09d0 1092See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1093cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1094rich implementation.
4633a7c4 1095
2b5ab1e7
TC
1096You can control which DBM library you use by loading that library
1097before you call dbmopen():
1098
1099 use DB_File;
1100 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1101 or die "Can't open netscape history file: $!";
1102
a0d0e21e 1103=item defined EXPR
d74e8afc 1104X<defined> X<undef> X<undefined>
a0d0e21e 1105
54310121 1106=item defined
bbce6d69 1107
2f9daede
TPG
1108Returns a Boolean value telling whether EXPR has a value other than
1109the undefined value C<undef>. If EXPR is not present, C<$_> will be
1110checked.
1111
1112Many operations return C<undef> to indicate failure, end of file,
1113system error, uninitialized variable, and other exceptional
1114conditions. This function allows you to distinguish C<undef> from
1115other values. (A simple Boolean test will not distinguish among
7660c0ab 1116C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1117false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1118doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1119returns C<undef> when its argument is an empty array, I<or> when the
1120element to return happens to be C<undef>.
1121
f10b0346
GS
1122You may also use C<defined(&func)> to check whether subroutine C<&func>
1123has ever been defined. The return value is unaffected by any forward
04891299 1124declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1125may still be callable: its package may have an C<AUTOLOAD> method that
1126makes it spring into existence the first time that it is called -- see
1127L<perlsub>.
f10b0346
GS
1128
1129Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1130used to report whether memory for that aggregate has ever been
1131allocated. This behavior may disappear in future versions of Perl.
1132You should instead use a simple test for size:
1133
1134 if (@an_array) { print "has array elements\n" }
1135 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1136
1137When used on a hash element, it tells you whether the value is defined,
dc848c6f 1138not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1139purpose.
a0d0e21e
LW
1140
1141Examples:
1142
1143 print if defined $switch{'D'};
1144 print "$val\n" while defined($val = pop(@ary));
1145 die "Can't readlink $sym: $!"
1146 unless defined($value = readlink $sym);
a0d0e21e 1147 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1148 $debugging = 0 unless defined $debugging;
a0d0e21e 1149
19799a22 1150Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1151discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1152defined values. For example, if you say
a5f75d66
AD
1153
1154 "ab" =~ /a(.*)b/;
1155
7660c0ab 1156The pattern match succeeds, and C<$1> is defined, despite the fact that it
cf264981 1157matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1158matched something that happened to be zero characters long. This is all
a5f75d66 1159very above-board and honest. When a function returns an undefined value,
2f9daede 1160it's an admission that it couldn't give you an honest answer. So you
19799a22 1161should use C<defined> only when you're questioning the integrity of what
7660c0ab 1162you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1163what you want.
1164
dc848c6f 1165See also L</undef>, L</exists>, L</ref>.
2f9daede 1166
a0d0e21e 1167=item delete EXPR
d74e8afc 1168X<delete>
a0d0e21e 1169
01020589
GS
1170Given an expression that specifies a hash element, array element, hash slice,
1171or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1172In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1173the size of the array will shrink to the highest element that tests
8216c1fd 1174true for exists() (or 0 if no such element exists).
a0d0e21e 1175
eba0920a
EM
1176Returns a list with the same number of elements as the number of elements
1177for which deletion was attempted. Each element of that list consists of
1178either the value of the element deleted, or the undefined value. In scalar
1179context, this means that you get the value of the last element deleted (or
1180the undefined value if that element did not exist).
1181
1182 %hash = (foo => 11, bar => 22, baz => 33);
1183 $scalar = delete $hash{foo}; # $scalar is 11
1184 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1185 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1186
1187Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1188a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1189from a C<tie>d hash or array may not necessarily return anything.
1190
8ea97a1e
GS
1191Deleting an array element effectively returns that position of the array
1192to its initial, uninitialized state. Subsequently testing for the same
cf264981
SP
1193element with exists() will return false. Also, deleting array elements
1194in the middle of an array will not shift the index of the elements
1195after them down. Use splice() for that. See L</exists>.
8ea97a1e 1196
01020589 1197The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1198
5f05dabc
PP
1199 foreach $key (keys %HASH) {
1200 delete $HASH{$key};
a0d0e21e
LW
1201 }
1202
01020589
GS
1203 foreach $index (0 .. $#ARRAY) {
1204 delete $ARRAY[$index];
1205 }
1206
1207And so do these:
5f05dabc 1208
01020589
GS
1209 delete @HASH{keys %HASH};
1210
9740c838 1211 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1212
2b5ab1e7 1213But both of these are slower than just assigning the empty list
01020589
GS
1214or undefining %HASH or @ARRAY:
1215
1216 %HASH = (); # completely empty %HASH
1217 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1218
01020589
GS
1219 @ARRAY = (); # completely empty @ARRAY
1220 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1221
1222Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1223operation is a hash element, array element, hash slice, or array slice
1224lookup:
a0d0e21e
LW
1225
1226 delete $ref->[$x][$y]{$key};
5f05dabc 1227 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1228
01020589
GS
1229 delete $ref->[$x][$y][$index];
1230 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1231
a0d0e21e 1232=item die LIST
d74e8afc 1233X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1234
19799a22
GS
1235Outside an C<eval>, prints the value of LIST to C<STDERR> and
1236exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1237exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1238status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1239an C<eval(),> the error message is stuffed into C<$@> and the
1240C<eval> is terminated with the undefined value. This makes
1241C<die> the way to raise an exception.
a0d0e21e
LW
1242
1243Equivalent examples:
1244
1245 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1246 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1247
ccac6780 1248If the last element of LIST does not end in a newline, the current
df37ec69
WW
1249script line number and input line number (if any) are also printed,
1250and a newline is supplied. Note that the "input line number" (also
1251known as "chunk") is subject to whatever notion of "line" happens to
1252be currently in effect, and is also available as the special variable
1253C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1254
1255Hint: sometimes appending C<", stopped"> to your message will cause it
1256to make better sense when the string C<"at foo line 123"> is appended.
1257Suppose you are running script "canasta".
a0d0e21e
LW
1258
1259 die "/etc/games is no good";
1260 die "/etc/games is no good, stopped";
1261
1262produce, respectively
1263
1264 /etc/games is no good at canasta line 123.
1265 /etc/games is no good, stopped at canasta line 123.
1266
2b5ab1e7 1267See also exit(), warn(), and the Carp module.
a0d0e21e 1268
7660c0ab
A
1269If LIST is empty and C<$@> already contains a value (typically from a
1270previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1271This is useful for propagating exceptions:
1272
1273 eval { ... };
1274 die unless $@ =~ /Expected exception/;
1275
ad216e65
JH
1276If LIST is empty and C<$@> contains an object reference that has a
1277C<PROPAGATE> method, that method will be called with additional file
1278and line number parameters. The return value replaces the value in
28a5cf3b 1279C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1280were called.
1281
7660c0ab 1282If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1283
52531d10
GS
1284die() can also be called with a reference argument. If this happens to be
1285trapped within an eval(), $@ contains the reference. This behavior permits
1286a more elaborate exception handling implementation using objects that
4375e838 1287maintain arbitrary state about the nature of the exception. Such a scheme
52531d10 1288is sometimes preferable to matching particular string values of $@ using
746d7dd7
GL
1289regular expressions. Because $@ is a global variable, and eval() may be
1290used within object implementations, care must be taken that analyzing the
1291error object doesn't replace the reference in the global variable. The
1292easiest solution is to make a local copy of the reference before doing
1293other manipulations. Here's an example:
52531d10 1294
da279afe 1295 use Scalar::Util 'blessed';
1296
52531d10 1297 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1298 if (my $ev_err = $@) {
1299 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1300 # handle Some::Module::Exception
1301 }
1302 else {
1303 # handle all other possible exceptions
1304 }
1305 }
1306
19799a22 1307Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1308them, you may want to overload stringification operations on such custom
1309exception objects. See L<overload> for details about that.
1310
19799a22
GS
1311You can arrange for a callback to be run just before the C<die>
1312does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1313handler will be called with the error text and can change the error
1314message, if it sees fit, by calling C<die> again. See
1315L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1316L<"eval BLOCK"> for some examples. Although this feature was
19799a22
GS
1317to be run only right before your program was to exit, this is not
1318currently the case--the C<$SIG{__DIE__}> hook is currently called
1319even inside eval()ed blocks/strings! If one wants the hook to do
1320nothing in such situations, put
fb73857a
PP
1321
1322 die @_ if $^S;
1323
19799a22
GS
1324as the first line of the handler (see L<perlvar/$^S>). Because
1325this promotes strange action at a distance, this counterintuitive
b76cc8ba 1326behavior may be fixed in a future release.
774d564b 1327
a0d0e21e 1328=item do BLOCK
d74e8afc 1329X<do> X<block>
a0d0e21e
LW
1330
1331Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1332sequence of commands indicated by BLOCK. When modified by the C<while> or
1333C<until> loop modifier, executes the BLOCK once before testing the loop
1334condition. (On other statements the loop modifiers test the conditional
1335first.)
a0d0e21e 1336
4968c1e4 1337C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1338C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1339See L<perlsyn> for alternative strategies.
4968c1e4 1340
a0d0e21e 1341=item do SUBROUTINE(LIST)
d74e8afc 1342X<do>
a0d0e21e 1343
cf264981 1344This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1345
1346=item do EXPR
d74e8afc 1347X<do>
a0d0e21e
LW
1348
1349Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1350file as a Perl script.
a0d0e21e
LW
1351
1352 do 'stat.pl';
1353
1354is just like
1355
986b19de 1356 eval `cat stat.pl`;
a0d0e21e 1357
2b5ab1e7 1358except that it's more efficient and concise, keeps track of the current
ea63ef19 1359filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1360C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1361variables. It also differs in that code evaluated with C<do FILENAME>
1362cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1363same, however, in that it does reparse the file every time you call it,
1364so you probably don't want to do this inside a loop.
a0d0e21e 1365
8e30cc93 1366If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1367error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1368returns undef and sets an error message in C<$@>. If the file is
1369successfully compiled, C<do> returns the value of the last expression
1370evaluated.
1371
a0d0e21e 1372Note that inclusion of library modules is better done with the
19799a22 1373C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1374and raise an exception if there's a problem.
a0d0e21e 1375
5a964f20
TC
1376You might like to use C<do> to read in a program configuration
1377file. Manual error checking can be done this way:
1378
b76cc8ba 1379 # read in config files: system first, then user
f86cebdf 1380 for $file ("/share/prog/defaults.rc",
b76cc8ba 1381 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1382 {
5a964f20 1383 unless ($return = do $file) {
f86cebdf
GS
1384 warn "couldn't parse $file: $@" if $@;
1385 warn "couldn't do $file: $!" unless defined $return;
1386 warn "couldn't run $file" unless $return;
5a964f20
TC
1387 }
1388 }
1389
a0d0e21e 1390=item dump LABEL
d74e8afc 1391X<dump> X<core> X<undump>
a0d0e21e 1392
1614b0e3
JD
1393=item dump
1394
19799a22
GS
1395This function causes an immediate core dump. See also the B<-u>
1396command-line switch in L<perlrun>, which does the same thing.
1397Primarily this is so that you can use the B<undump> program (not
1398supplied) to turn your core dump into an executable binary after
1399having initialized all your variables at the beginning of the
1400program. When the new binary is executed it will begin by executing
1401a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1402Think of it as a goto with an intervening core dump and reincarnation.
1403If C<LABEL> is omitted, restarts the program from the top.
1404
1405B<WARNING>: Any files opened at the time of the dump will I<not>
1406be open any more when the program is reincarnated, with possible
b76cc8ba 1407resulting confusion on the part of Perl.
19799a22 1408
59f521f4
RGS
1409This function is now largely obsolete, mostly because it's very hard to
1410convert a core file into an executable. That's why you should now invoke
1411it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1412typo.
19799a22 1413
aa689395 1414=item each HASH
d74e8afc 1415X<each> X<hash, iterator>
aa689395 1416
5a964f20 1417When called in list context, returns a 2-element list consisting of the
aa689395 1418key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1419it. When called in scalar context, returns only the key for the next
e902a979 1420element in the hash.
2f9daede 1421
ab192400 1422Entries are returned in an apparently random order. The actual random
504f80c1
JH
1423order is subject to change in future versions of perl, but it is
1424guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6
JH
1425function would produce on the same (unmodified) hash. Since Perl
14265.8.1 the ordering is different even between different runs of Perl
1427for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400
GS
1428
1429When the hash is entirely read, a null array is returned in list context
19799a22
GS
1430(which when assigned produces a false (C<0>) value), and C<undef> in
1431scalar context. The next call to C<each> after that will start iterating
1432again. There is a single iterator for each hash, shared by all C<each>,
1433C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1434reading all the elements from the hash, or by evaluating C<keys HASH> or
1435C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1436iterating over it, you may get entries skipped or duplicated, so
1437don't. Exception: It is always safe to delete the item most recently
1438returned by C<each()>, which means that the following code will work:
1439
1440 while (($key, $value) = each %hash) {
1441 print $key, "\n";
1442 delete $hash{$key}; # This is safe
1443 }
aa689395 1444
f86cebdf 1445The following prints out your environment like the printenv(1) program,
aa689395 1446only in a different order:
a0d0e21e
LW
1447
1448 while (($key,$value) = each %ENV) {
1449 print "$key=$value\n";
1450 }
1451
19799a22 1452See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1453
1454=item eof FILEHANDLE
d74e8afc
ITB
1455X<eof>
1456X<end of file>
1457X<end-of-file>
a0d0e21e 1458
4633a7c4
LW
1459=item eof ()
1460
a0d0e21e
LW
1461=item eof
1462
1463Returns 1 if the next read on FILEHANDLE will return end of file, or if
1464FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1465gives the real filehandle. (Note that this function actually
19799a22 1466reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1467interactive context.) Do not read from a terminal file (or call
19799a22 1468C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1469as terminals may lose the end-of-file condition if you do.
1470
820475bd
GS
1471An C<eof> without an argument uses the last file read. Using C<eof()>
1472with empty parentheses is very different. It refers to the pseudo file
1473formed from the files listed on the command line and accessed via the
61eff3bc
JH
1474C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1475as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1476used will cause C<@ARGV> to be examined to determine if input is
67408cae 1477available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1478end-of-file will assume you are processing another C<@ARGV> list,
1479and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1480see L<perlop/"I/O Operators">.
820475bd 1481
61eff3bc 1482In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1483detect the end of each file, C<eof()> will only detect the end of the
1484last file. Examples:
a0d0e21e 1485
748a9306
LW
1486 # reset line numbering on each input file
1487 while (<>) {
b76cc8ba 1488 next if /^\s*#/; # skip comments
748a9306 1489 print "$.\t$_";
5a964f20
TC
1490 } continue {
1491 close ARGV if eof; # Not eof()!
748a9306
LW
1492 }
1493
a0d0e21e
LW
1494 # insert dashes just before last line of last file
1495 while (<>) {
6ac88b13 1496 if (eof()) { # check for end of last file
a0d0e21e
LW
1497 print "--------------\n";
1498 }
1499 print;
6ac88b13 1500 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1501 }
1502
a0d0e21e 1503Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1504input operators typically return C<undef> when they run out of data, or if
1505there was an error.
a0d0e21e
LW
1506
1507=item eval EXPR
d74e8afc 1508X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1509X<error, handling> X<exception, handling>
a0d0e21e
LW
1510
1511=item eval BLOCK
1512
ce2984c3
PF
1513=item eval
1514
c7cc6f1c
GS
1515In the first form, the return value of EXPR is parsed and executed as if it
1516were a little Perl program. The value of the expression (which is itself
5a964f20 1517determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1518errors, executed in the lexical context of the current Perl program, so
1519that any variable settings or subroutine and format definitions remain
cf264981 1520afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1521If EXPR is omitted, evaluates C<$_>. This form is typically used to
1522delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1523
1524In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1525same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1526within the context of the current Perl program. This form is typically
1527used to trap exceptions more efficiently than the first (see below), while
1528also providing the benefit of checking the code within BLOCK at compile
1529time.
1530
1531The final semicolon, if any, may be omitted from the value of EXPR or within
1532the BLOCK.
1533
1534In both forms, the value returned is the value of the last expression
5a964f20 1535evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1536as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1537in void, scalar, or list context, depending on the context of the C<eval>
1538itself. See L</wantarray> for more on how the evaluation context can be
1539determined.
a0d0e21e 1540
19799a22
GS
1541If there is a syntax error or runtime error, or a C<die> statement is
1542executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1543error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1544string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1545warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1546To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1547turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1548See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1549
19799a22
GS
1550Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1551determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1552is implemented. It is also Perl's exception trapping mechanism, where
1553the die operator is used to raise exceptions.
1554
1555If the code to be executed doesn't vary, you may use the eval-BLOCK
1556form to trap run-time errors without incurring the penalty of
1557recompiling each time. The error, if any, is still returned in C<$@>.
1558Examples:
1559
54310121 1560 # make divide-by-zero nonfatal
a0d0e21e
LW
1561 eval { $answer = $a / $b; }; warn $@ if $@;
1562
1563 # same thing, but less efficient
1564 eval '$answer = $a / $b'; warn $@ if $@;
1565
1566 # a compile-time error
5a964f20 1567 eval { $answer = }; # WRONG
a0d0e21e
LW
1568
1569 # a run-time error
1570 eval '$answer ='; # sets $@
1571
cf264981
SP
1572Using the C<eval{}> form as an exception trap in libraries does have some
1573issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1574may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7
TC
1575You can use the C<local $SIG{__DIE__}> construct for this purpose,
1576as shown in this example:
774d564b
PP
1577
1578 # a very private exception trap for divide-by-zero
f86cebdf
GS
1579 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1580 warn $@ if $@;
774d564b
PP
1581
1582This is especially significant, given that C<__DIE__> hooks can call
19799a22 1583C<die> again, which has the effect of changing their error messages:
774d564b
PP
1584
1585 # __DIE__ hooks may modify error messages
1586 {
f86cebdf
GS
1587 local $SIG{'__DIE__'} =
1588 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1589 eval { die "foo lives here" };
1590 print $@ if $@; # prints "bar lives here"
774d564b
PP
1591 }
1592
19799a22 1593Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1594may be fixed in a future release.
1595
19799a22 1596With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1597being looked at when:
1598
1599 eval $x; # CASE 1
1600 eval "$x"; # CASE 2
1601
1602 eval '$x'; # CASE 3
1603 eval { $x }; # CASE 4
1604
5a964f20 1605 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1606 $$x++; # CASE 6
1607
2f9daede 1608Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1609the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1610the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1611and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1612does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1613purely visual reasons, but it also has the advantage of compiling at
1614compile-time instead of at run-time.) Case 5 is a place where
19799a22 1615normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1616particular situation, you can just use symbolic references instead, as
1617in case 6.
a0d0e21e 1618
4968c1e4 1619C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1620C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1621
d819b83a
DM
1622Note that as a very special case, an C<eval ''> executed within the C<DB>
1623package doesn't see the usual surrounding lexical scope, but rather the
1624scope of the first non-DB piece of code that called it. You don't normally
1625need to worry about this unless you are writing a Perl debugger.
1626
a0d0e21e 1627=item exec LIST
d74e8afc 1628X<exec> X<execute>
a0d0e21e 1629
8bf3b016
GS
1630=item exec PROGRAM LIST
1631
19799a22
GS
1632The C<exec> function executes a system command I<and never returns>--
1633use C<system> instead of C<exec> if you want it to return. It fails and
1634returns false only if the command does not exist I<and> it is executed
fb73857a 1635directly instead of via your system's command shell (see below).
a0d0e21e 1636
19799a22
GS
1637Since it's a common mistake to use C<exec> instead of C<system>, Perl
1638warns you if there is a following statement which isn't C<die>, C<warn>,
1639or C<exit> (if C<-w> is set - but you always do that). If you
1640I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1641can use one of these styles to avoid the warning:
1642
5a964f20
TC
1643 exec ('foo') or print STDERR "couldn't exec foo: $!";
1644 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1645
5a964f20 1646If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1647with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1648If there is only one scalar argument or an array with one element in it,
1649the argument is checked for shell metacharacters, and if there are any,
1650the entire argument is passed to the system's command shell for parsing
1651(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1652If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1653words and passed directly to C<execvp>, which is more efficient.
19799a22 1654Examples:
a0d0e21e 1655
19799a22
GS
1656 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1657 exec "sort $outfile | uniq";
a0d0e21e
LW
1658
1659If you don't really want to execute the first argument, but want to lie
1660to the program you are executing about its own name, you can specify
1661the program you actually want to run as an "indirect object" (without a
1662comma) in front of the LIST. (This always forces interpretation of the
54310121 1663LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1664the list.) Example:
1665
1666 $shell = '/bin/csh';
1667 exec $shell '-sh'; # pretend it's a login shell
1668
1669or, more directly,
1670
1671 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1672
bb32b41a
GS
1673When the arguments get executed via the system shell, results will
1674be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1675for details.
1676
19799a22
GS
1677Using an indirect object with C<exec> or C<system> is also more
1678secure. This usage (which also works fine with system()) forces
1679interpretation of the arguments as a multivalued list, even if the
1680list had just one argument. That way you're safe from the shell
1681expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1682
1683 @args = ( "echo surprise" );
1684
2b5ab1e7 1685 exec @args; # subject to shell escapes
f86cebdf 1686 # if @args == 1
2b5ab1e7 1687 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1688
1689The first version, the one without the indirect object, ran the I<echo>
1690program, passing it C<"surprise"> an argument. The second version
1691didn't--it tried to run a program literally called I<"echo surprise">,
1692didn't find it, and set C<$?> to a non-zero value indicating failure.
1693
0f897271
GS
1694Beginning with v5.6.0, Perl will attempt to flush all files opened for
1695output before the exec, but this may not be supported on some platforms
1696(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1697in English) or call the C<autoflush()> method of C<IO::Handle> on any
1698open handles in order to avoid lost output.
1699
19799a22 1700Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1701any C<DESTROY> methods in your objects.
1702
a0d0e21e 1703=item exists EXPR
d74e8afc 1704X<exists> X<autovivification>
a0d0e21e 1705
01020589 1706Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1707returns true if the specified element in the hash or array has ever
1708been initialized, even if the corresponding value is undefined. The
1709element is not autovivified if it doesn't exist.
a0d0e21e 1710
01020589
GS
1711 print "Exists\n" if exists $hash{$key};
1712 print "Defined\n" if defined $hash{$key};
1713 print "True\n" if $hash{$key};
1714
1715 print "Exists\n" if exists $array[$index];
1716 print "Defined\n" if defined $array[$index];
1717 print "True\n" if $array[$index];
a0d0e21e 1718
8ea97a1e 1719A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1720it exists, but the reverse doesn't necessarily hold true.
1721
afebc493
GS
1722Given an expression that specifies the name of a subroutine,
1723returns true if the specified subroutine has ever been declared, even
1724if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1725does not count as declaring it. Note that a subroutine which does not
1726exist may still be callable: its package may have an C<AUTOLOAD>
1727method that makes it spring into existence the first time that it is
1728called -- see L<perlsub>.
afebc493
GS
1729
1730 print "Exists\n" if exists &subroutine;
1731 print "Defined\n" if defined &subroutine;
1732
a0d0e21e 1733Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1734operation is a hash or array key lookup or subroutine name:
a0d0e21e 1735
2b5ab1e7
TC
1736 if (exists $ref->{A}->{B}->{$key}) { }
1737 if (exists $hash{A}{B}{$key}) { }
1738
01020589
GS
1739 if (exists $ref->{A}->{B}->[$ix]) { }
1740 if (exists $hash{A}{B}[$ix]) { }
1741
afebc493
GS
1742 if (exists &{$ref->{A}{B}{$key}}) { }
1743
01020589
GS
1744Although the deepest nested array or hash will not spring into existence
1745just because its existence was tested, any intervening ones will.
61eff3bc 1746Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1747into existence due to the existence test for the $key element above.
1748This happens anywhere the arrow operator is used, including even:
5a964f20 1749
2b5ab1e7
TC
1750 undef $ref;
1751 if (exists $ref->{"Some key"}) { }
1752 print $ref; # prints HASH(0x80d3d5c)
1753
1754This surprising autovivification in what does not at first--or even
1755second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1756release.
a0d0e21e 1757
afebc493
GS
1758Use of a subroutine call, rather than a subroutine name, as an argument
1759to exists() is an error.
1760
1761 exists &sub; # OK
1762 exists &sub(); # Error
1763
a0d0e21e 1764=item exit EXPR
d74e8afc 1765X<exit> X<terminate> X<abort>
a0d0e21e 1766
ce2984c3
PF
1767=item exit
1768
2b5ab1e7 1769Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1770
1771 $ans = <STDIN>;
1772 exit 0 if $ans =~ /^[Xx]/;
1773
19799a22 1774See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1775universally recognized values for EXPR are C<0> for success and C<1>
1776for error; other values are subject to interpretation depending on the
1777environment in which the Perl program is running. For example, exiting
177869 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1779the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1780
19799a22
GS
1781Don't use C<exit> to abort a subroutine if there's any chance that
1782someone might want to trap whatever error happened. Use C<die> instead,
1783which can be trapped by an C<eval>.
28757baa 1784
19799a22 1785The exit() function does not always exit immediately. It calls any
2b5ab1e7 1786defined C<END> routines first, but these C<END> routines may not
19799a22 1787themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1788be called are called before the real exit. If this is a problem, you
1789can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1790See L<perlmod> for details.
5a964f20 1791
a0d0e21e 1792=item exp EXPR
d74e8afc 1793X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1794
54310121 1795=item exp
bbce6d69 1796
b76cc8ba 1797Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1798If EXPR is omitted, gives C<exp($_)>.
1799
1800=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1801X<fcntl>
a0d0e21e 1802
f86cebdf 1803Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1804
1805 use Fcntl;
1806
0ade1984 1807first to get the correct constant definitions. Argument processing and
b76cc8ba 1808value return works just like C<ioctl> below.
a0d0e21e
LW
1809For example:
1810
1811 use Fcntl;
5a964f20
TC
1812 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1813 or die "can't fcntl F_GETFL: $!";
1814
554ad1fc 1815You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1816Like C<ioctl>, it maps a C<0> return from the system call into
1817C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1818in numeric context. It is also exempt from the normal B<-w> warnings
1819on improper numeric conversions.
5a964f20 1820
19799a22 1821Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1822doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1823manpage to learn what functions are available on your system.
a0d0e21e 1824
be2f7487 1825Here's an example of setting a filehandle named C<REMOTE> to be
1826non-blocking at the system level. You'll have to negotiate C<$|>
1827on your own, though.
1828
1829 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1830
1831 $flags = fcntl(REMOTE, F_GETFL, 0)
1832 or die "Can't get flags for the socket: $!\n";
1833
1834 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1835 or die "Can't set flags for the socket: $!\n";
1836
a0d0e21e 1837=item fileno FILEHANDLE
d74e8afc 1838X<fileno>
a0d0e21e 1839
2b5ab1e7
TC
1840Returns the file descriptor for a filehandle, or undefined if the
1841filehandle is not open. This is mainly useful for constructing
19799a22 1842bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1843If FILEHANDLE is an expression, the value is taken as an indirect
1844filehandle, generally its name.
5a964f20 1845
b76cc8ba 1846You can use this to find out whether two handles refer to the
5a964f20
TC
1847same underlying descriptor:
1848
1849 if (fileno(THIS) == fileno(THAT)) {
1850 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1851 }
1852
1853(Filehandles connected to memory objects via new features of C<open> may
1854return undefined even though they are open.)
1855
a0d0e21e
LW
1856
1857=item flock FILEHANDLE,OPERATION
d74e8afc 1858X<flock> X<lock> X<locking>
a0d0e21e 1859
19799a22
GS
1860Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1861for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1862machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1863C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1864only entire files, not records.
1865
1866Two potentially non-obvious but traditional C<flock> semantics are
1867that it waits indefinitely until the lock is granted, and that its locks
1868B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1869fewer guarantees. This means that programs that do not also use C<flock>
1870may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1871your port's specific documentation, or your system-specific local manpages
1872for details. It's best to assume traditional behavior if you're writing
1873portable programs. (But if you're not, you should as always feel perfectly
1874free to write for your own system's idiosyncrasies (sometimes called
1875"features"). Slavish adherence to portability concerns shouldn't get
1876in the way of your getting your job done.)
a3cb178b 1877
8ebc5c01
PP
1878OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1879LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1880you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1881either individually, or as a group using the ':flock' tag. LOCK_SH
1882requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1883releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1884LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1885waiting for the lock (check the return status to see if you got it).
1886
2b5ab1e7
TC
1887To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1888before locking or unlocking it.
8ebc5c01 1889
f86cebdf 1890Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1891locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1892are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1893implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1894differing semantics shouldn't bite too many people.
1895
becacb53
TM
1896Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1897be open with read intent to use LOCK_SH and requires that it be open
1898with write intent to use LOCK_EX.
1899
19799a22
GS
1900Note also that some versions of C<flock> cannot lock things over the
1901network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1902that. If you like you can force Perl to ignore your system's flock(2)
1903function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1904the switch C<-Ud_flock> to the F<Configure> program when you configure
1905perl.
4633a7c4
LW
1906
1907Here's a mailbox appender for BSD systems.
a0d0e21e 1908
7e1af8bc 1909 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1910
1911 sub lock {
7e1af8bc 1912 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1913 # and, in case someone appended
1914 # while we were waiting...
1915 seek(MBOX, 0, 2);
1916 }
1917
1918 sub unlock {
7e1af8bc 1919 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1920 }
1921
1922 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1923 or die "Can't open mailbox: $!";
1924
1925 lock();
1926 print MBOX $msg,"\n\n";
1927 unlock();
1928
2b5ab1e7
TC
1929On systems that support a real flock(), locks are inherited across fork()
1930calls, whereas those that must resort to the more capricious fcntl()
1931function lose the locks, making it harder to write servers.
1932
cb1a09d0 1933See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1934
1935=item fork
d74e8afc 1936X<fork> X<child> X<parent>
a0d0e21e 1937
2b5ab1e7
TC
1938Does a fork(2) system call to create a new process running the
1939same program at the same point. It returns the child pid to the
1940parent process, C<0> to the child process, or C<undef> if the fork is
1941unsuccessful. File descriptors (and sometimes locks on those descriptors)
1942are shared, while everything else is copied. On most systems supporting
1943fork(), great care has gone into making it extremely efficient (for
1944example, using copy-on-write technology on data pages), making it the
1945dominant paradigm for multitasking over the last few decades.
5a964f20 1946
0f897271
GS
1947Beginning with v5.6.0, Perl will attempt to flush all files opened for
1948output before forking the child process, but this may not be supported
1949on some platforms (see L<perlport>). To be safe, you may need to set
1950C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1951C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1952
19799a22 1953If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1954accumulate zombies. On some systems, you can avoid this by setting
1955C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1956forking and reaping moribund children.
cb1a09d0 1957
28757baa
PP
1958Note that if your forked child inherits system file descriptors like
1959STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1960if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1961backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1962You should reopen those to F</dev/null> if it's any issue.
28757baa 1963
cb1a09d0 1964=item format
d74e8afc 1965X<format>
cb1a09d0 1966
19799a22 1967Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1968example:
1969
54310121 1970 format Something =
cb1a09d0
AD
1971 Test: @<<<<<<<< @||||| @>>>>>
1972 $str, $%, '$' . int($num)
1973 .
1974
1975 $str = "widget";
184e9718 1976 $num = $cost/$quantity;
cb1a09d0
AD
1977 $~ = 'Something';
1978 write;
1979
1980See L<perlform> for many details and examples.
1981
8903cb82 1982=item formline PICTURE,LIST
d74e8afc 1983X<formline>
a0d0e21e 1984
5a964f20 1985This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1986too. It formats (see L<perlform>) a list of values according to the
1987contents of PICTURE, placing the output into the format output
7660c0ab 1988accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1989Eventually, when a C<write> is done, the contents of
cf264981
SP
1990C<$^A> are written to some filehandle. You could also read C<$^A>
1991and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1992does one C<formline> per line of form, but the C<formline> function itself
748a9306 1993doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1994that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1995You may therefore need to use multiple formlines to implement a single
1996record format, just like the format compiler.
1997
19799a22 1998Be careful if you put double quotes around the picture, because an C<@>
748a9306 1999character may be taken to mean the beginning of an array name.
19799a22 2000C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
2001
2002=item getc FILEHANDLE
f723aae1 2003X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2004
2005=item getc
2006
2007Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
2008or the undefined value at end of file, or if there was an error (in
2009the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2010STDIN. This is not particularly efficient. However, it cannot be
2011used by itself to fetch single characters without waiting for the user
2012to hit enter. For that, try something more like:
4633a7c4
LW
2013
2014 if ($BSD_STYLE) {
2015 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2016 }
2017 else {
54310121 2018 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2019 }
2020
2021 $key = getc(STDIN);
2022
2023 if ($BSD_STYLE) {
2024 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2025 }
2026 else {
5f05dabc 2027 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
2028 }
2029 print "\n";
2030
54310121
PP
2031Determination of whether $BSD_STYLE should be set
2032is left as an exercise to the reader.
cb1a09d0 2033
19799a22 2034The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2035systems purporting POSIX compliance. See also the C<Term::ReadKey>
2036module from your nearest CPAN site; details on CPAN can be found on
2037L<perlmodlib/CPAN>.
a0d0e21e
LW
2038
2039=item getlogin
d74e8afc 2040X<getlogin> X<login>
a0d0e21e 2041
cf264981 2042This implements the C library function of the same name, which on most
5a964f20 2043systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 2044use C<getpwuid>.
a0d0e21e 2045
f86702cc 2046 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2047
19799a22
GS
2048Do not consider C<getlogin> for authentication: it is not as
2049secure as C<getpwuid>.
4633a7c4 2050
a0d0e21e 2051=item getpeername SOCKET
d74e8afc 2052X<getpeername> X<peer>
a0d0e21e
LW
2053
2054Returns the packed sockaddr address of other end of the SOCKET connection.
2055
4633a7c4
LW
2056 use Socket;
2057 $hersockaddr = getpeername(SOCK);
19799a22 2058 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2059 $herhostname = gethostbyaddr($iaddr, AF_INET);
2060 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2061
2062=item getpgrp PID
d74e8afc 2063X<getpgrp> X<group>
a0d0e21e 2064
47e29363 2065Returns the current process group for the specified PID. Use
7660c0ab 2066a PID of C<0> to get the current process group for the
4633a7c4 2067current process. Will raise an exception if used on a machine that
f86cebdf 2068doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2069group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2070does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2071
2072=item getppid
d74e8afc 2073X<getppid> X<parent> X<pid>
a0d0e21e
LW
2074
2075Returns the process id of the parent process.
2076
4d76a344
RGS
2077Note for Linux users: on Linux, the C functions C<getpid()> and
2078C<getppid()> return different values from different threads. In order to
2079be portable, this behavior is not reflected by the perl-level function
2080C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2081to call the underlying C<getppid()>, you may use the CPAN module
2082C<Linux::Pid>.
4d76a344 2083
a0d0e21e 2084=item getpriority WHICH,WHO
d74e8afc 2085X<getpriority> X<priority> X<nice>
a0d0e21e 2086
4633a7c4
LW
2087Returns the current priority for a process, a process group, or a user.
2088(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2089machine that doesn't implement getpriority(2).
a0d0e21e
LW
2090
2091=item getpwnam NAME
d74e8afc
ITB
2092X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2093X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2094X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2095X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2096X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2097X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2098
2099=item getgrnam NAME
2100
2101=item gethostbyname NAME
2102
2103=item getnetbyname NAME
2104
2105=item getprotobyname NAME
2106
2107=item getpwuid UID
2108
2109=item getgrgid GID
2110
2111=item getservbyname NAME,PROTO
2112
2113=item gethostbyaddr ADDR,ADDRTYPE
2114
2115=item getnetbyaddr ADDR,ADDRTYPE
2116
2117=item getprotobynumber NUMBER
2118
2119=item getservbyport PORT,PROTO
2120
2121=item getpwent
2122
2123=item getgrent
2124
2125=item gethostent
2126
2127=item getnetent
2128
2129=item getprotoent
2130
2131=item getservent
2132
2133=item setpwent
2134
2135=item setgrent
2136
2137=item sethostent STAYOPEN
2138
2139=item setnetent STAYOPEN
2140
2141=item setprotoent STAYOPEN
2142
2143=item setservent STAYOPEN
2144
2145=item endpwent
2146
2147=item endgrent
2148
2149=item endhostent
2150
2151=item endnetent
2152
2153=item endprotoent
2154
2155=item endservent
2156
2157These routines perform the same functions as their counterparts in the
5a964f20 2158system library. In list context, the return values from the
a0d0e21e
LW
2159various get routines are as follows:
2160
2161 ($name,$passwd,$uid,$gid,
6ee623d5 2162 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2163 ($name,$passwd,$gid,$members) = getgr*
2164 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2165 ($name,$aliases,$addrtype,$net) = getnet*
2166 ($name,$aliases,$proto) = getproto*
2167 ($name,$aliases,$port,$proto) = getserv*
2168
2169(If the entry doesn't exist you get a null list.)
2170
4602f195
JH
2171The exact meaning of the $gcos field varies but it usually contains
2172the real name of the user (as opposed to the login name) and other
2173information pertaining to the user. Beware, however, that in many
2174system users are able to change this information and therefore it
106325ad 2175cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2176L<perlsec>). The $passwd and $shell, user's encrypted password and
2177login shell, are also tainted, because of the same reason.
4602f195 2178
5a964f20 2179In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2180lookup by name, in which case you get the other thing, whatever it is.
2181(If the entry doesn't exist you get the undefined value.) For example:
2182
5a964f20
TC
2183 $uid = getpwnam($name);
2184 $name = getpwuid($num);
2185 $name = getpwent();
2186 $gid = getgrnam($name);
08a33e13 2187 $name = getgrgid($num);
5a964f20
TC
2188 $name = getgrent();
2189 #etc.
a0d0e21e 2190
4602f195
JH
2191In I<getpw*()> the fields $quota, $comment, and $expire are special
2192cases in the sense that in many systems they are unsupported. If the
2193$quota is unsupported, it is an empty scalar. If it is supported, it
2194usually encodes the disk quota. If the $comment field is unsupported,
2195it is an empty scalar. If it is supported it usually encodes some
2196administrative comment about the user. In some systems the $quota
2197field may be $change or $age, fields that have to do with password
2198aging. In some systems the $comment field may be $class. The $expire
2199field, if present, encodes the expiration period of the account or the
2200password. For the availability and the exact meaning of these fields
2201in your system, please consult your getpwnam(3) documentation and your
2202F<pwd.h> file. You can also find out from within Perl what your
2203$quota and $comment fields mean and whether you have the $expire field
2204by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2205C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2206files are only supported if your vendor has implemented them in the
2207intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2208shadow versions if you're running under privilege or if there exists
cf264981
SP
2209the shadow(3) functions as found in System V (this includes Solaris
2210and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2211facility are unlikely to be supported.
6ee623d5 2212
19799a22 2213The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2214the login names of the members of the group.
2215
2216For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2217C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2218C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2219addresses returned by the corresponding system library call. In the
2220Internet domain, each address is four bytes long and you can unpack it
2221by saying something like:
2222
f337b084 2223 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2224
2b5ab1e7
TC
2225The Socket library makes this slightly easier:
2226
2227 use Socket;
2228 $iaddr = inet_aton("127.1"); # or whatever address
2229 $name = gethostbyaddr($iaddr, AF_INET);
2230
2231 # or going the other way
19799a22 2232 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2233
19799a22
GS
2234If you get tired of remembering which element of the return list
2235contains which return value, by-name interfaces are provided
2236in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2237C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2238and C<User::grent>. These override the normal built-ins, supplying
2239versions that return objects with the appropriate names
2240for each field. For example:
5a964f20
TC
2241
2242 use File::stat;
2243 use User::pwent;
2244 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2245
b76cc8ba
NIS
2246Even though it looks like they're the same method calls (uid),
2247they aren't, because a C<File::stat> object is different from
19799a22 2248a C<User::pwent> object.
5a964f20 2249
a0d0e21e 2250=item getsockname SOCKET
d74e8afc 2251X<getsockname>
a0d0e21e 2252
19799a22
GS
2253Returns the packed sockaddr address of this end of the SOCKET connection,
2254in case you don't know the address because you have several different
2255IPs that the connection might have come in on.
a0d0e21e 2256
4633a7c4
LW
2257 use Socket;
2258 $mysockaddr = getsockname(SOCK);
19799a22 2259 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2260 printf "Connect to %s [%s]\n",
19799a22
GS
2261 scalar gethostbyaddr($myaddr, AF_INET),
2262 inet_ntoa($myaddr);
a0d0e21e
LW
2263
2264=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2265X<getsockopt>
a0d0e21e 2266
636e6b1f
TH
2267Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2268Options may exist at multiple protocol levels depending on the socket
2269type, but at least the uppermost socket level SOL_SOCKET (defined in the
2270C<Socket> module) will exist. To query options at another level the
2271protocol number of the appropriate protocol controlling the option
2272should be supplied. For example, to indicate that an option is to be
2273interpreted by the TCP protocol, LEVEL should be set to the protocol
2274number of TCP, which you can get using getprotobyname.
2275
2276The call returns a packed string representing the requested socket option,
2277or C<undef> if there is an error (the error reason will be in $!). What
2278exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2279your system documentation for details. A very common case however is that
cf264981 2280the option is an integer, in which case the result will be a packed
636e6b1f
TH
2281integer which you can decode using unpack with the C<i> (or C<I>) format.
2282
2283An example testing if Nagle's algorithm is turned on on a socket:
2284
4852725b 2285 use Socket qw(:all);
636e6b1f
TH
2286
2287 defined(my $tcp = getprotobyname("tcp"))
2288 or die "Could not determine the protocol number for tcp";
4852725b
DD
2289 # my $tcp = IPPROTO_TCP; # Alternative
2290 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2291 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2292 my $nodelay = unpack("I", $packed);
2293 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2294
a0d0e21e
LW
2295
2296=item glob EXPR
d74e8afc 2297X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2298
0a753a76
PP
2299=item glob
2300
d9a9d457
JL
2301In list context, returns a (possibly empty) list of filename expansions on
2302the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2303scalar context, glob iterates through such filename expansions, returning
2304undef when the list is exhausted. This is the internal function
2305implementing the C<< <*.c> >> operator, but you can use it directly. If
2306EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2307more detail in L<perlop/"I/O Operators">.
a0d0e21e 2308
3a4b19e4
GS
2309Beginning with v5.6.0, this operator is implemented using the standard
2310C<File::Glob> extension. See L<File::Glob> for details.
2311
a0d0e21e 2312=item gmtime EXPR
d74e8afc 2313X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2314
ce2984c3
PF
2315=item gmtime
2316
435fbc73
GS
2317Works just like L<localtime> but the returned values are
2318localized for the standard Greenwich time zone.
a0d0e21e 2319
435fbc73
GS
2320Note: when called in list context, $isdst, the last value
2321returned by gmtime is always C<0>. There is no
2322Daylight Saving Time in GMT.
0a753a76 2323
62aa5637
MS
2324See L<perlport/gmtime> for portability concerns.
2325
a0d0e21e 2326=item goto LABEL
d74e8afc 2327X<goto> X<jump> X<jmp>
a0d0e21e 2328
748a9306
LW
2329=item goto EXPR
2330
a0d0e21e
LW
2331=item goto &NAME
2332
7660c0ab 2333The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2334execution there. It may not be used to go into any construct that
7660c0ab 2335requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2336also can't be used to go into a construct that is optimized away,
19799a22 2337or to get out of a block or subroutine given to C<sort>.
0a753a76 2338It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2339including out of subroutines, but it's usually better to use some other
19799a22 2340construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2341need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2342(The difference being that C does not offer named loops combined with
2343loop control. Perl does, and this replaces most structured uses of C<goto>
2344in other languages.)
a0d0e21e 2345
7660c0ab
A
2346The C<goto-EXPR> form expects a label name, whose scope will be resolved
2347dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2348necessarily recommended if you're optimizing for maintainability:
2349
2350 goto ("FOO", "BAR", "GLARCH")[$i];
2351
1b6921cb
BT
2352The C<goto-&NAME> form is quite different from the other forms of
2353C<goto>. In fact, it isn't a goto in the normal sense at all, and
2354doesn't have the stigma associated with other gotos. Instead, it
2355exits the current subroutine (losing any changes set by local()) and
2356immediately calls in its place the named subroutine using the current
2357value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2358load another subroutine and then pretend that the other subroutine had
2359been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2360in the current subroutine are propagated to the other subroutine.)
2361After the C<goto>, not even C<caller> will be able to tell that this
2362routine was called first.
2363
2364NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2365containing a code reference, or a block that evaluates to a code
6cb9131c 2366reference.
a0d0e21e
LW
2367
2368=item grep BLOCK LIST
d74e8afc 2369X<grep>
a0d0e21e
LW
2370
2371=item grep EXPR,LIST
2372
2b5ab1e7
TC
2373This is similar in spirit to, but not the same as, grep(1) and its
2374relatives. In particular, it is not limited to using regular expressions.
2f9daede 2375
a0d0e21e 2376Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2377C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2378elements for which the expression evaluated to true. In scalar
2379context, returns the number of times the expression was true.
a0d0e21e
LW
2380
2381 @foo = grep(!/^#/, @bar); # weed out comments
2382
2383or equivalently,
2384
2385 @foo = grep {!/^#/} @bar; # weed out comments
2386
be3174d2
GS
2387Note that C<$_> is an alias to the list value, so it can be used to
2388modify the elements of the LIST. While this is useful and supported,
2389it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2390Similarly, grep returns aliases into the original list, much as a for
2391loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2392element of a list returned by grep (for example, in a C<foreach>, C<map>
2393or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2394This is usually something to be avoided when writing clear code.
a0d0e21e 2395
a4fb8298 2396If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2397been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2398the list elements, C<$_> keeps being lexical inside the block; i.e. it
2399can't be seen from the outside, avoiding any potential side-effects.
2400
19799a22 2401See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2402
a0d0e21e 2403=item hex EXPR
d74e8afc 2404X<hex> X<hexadecimal>
a0d0e21e 2405
54310121 2406=item hex
bbce6d69 2407
2b5ab1e7 2408Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2409(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2410L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2411
2412 print hex '0xAf'; # prints '175'
2413 print hex 'aF'; # same
a0d0e21e 2414
19799a22 2415Hex strings may only represent integers. Strings that would cause
53305cf1 2416integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2417unlike oct(). To present something as hex, look into L</printf>,
2418L</sprintf>, or L</unpack>.
19799a22 2419
ce2984c3 2420=item import LIST
d74e8afc 2421X<import>
a0d0e21e 2422
19799a22 2423There is no builtin C<import> function. It is just an ordinary
4633a7c4 2424method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2425names to another module. The C<use> function calls the C<import> method
cea6626f 2426for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2427
2428=item index STR,SUBSTR,POSITION
d74e8afc 2429X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2430
2431=item index STR,SUBSTR
2432
2b5ab1e7
TC
2433The index function searches for one string within another, but without
2434the wildcard-like behavior of a full regular-expression pattern match.
2435It returns the position of the first occurrence of SUBSTR in STR at
2436or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2437beginning of the string. POSITION before the beginning of the string
2438or after its end is treated as if it were the beginning or the end,
2439respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2440you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2441is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2442
2443=item int EXPR
f723aae1 2444X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2445
54310121 2446=item int
bbce6d69 2447
7660c0ab 2448Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2449You should not use this function for rounding: one because it truncates
2450towards C<0>, and two because machine representations of floating point
2451numbers can sometimes produce counterintuitive results. For example,
2452C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2453because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2454the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2455functions will serve you better than will int().
a0d0e21e
LW
2456
2457=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2458X<ioctl>
a0d0e21e 2459
2b5ab1e7 2460Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2461
6c567752 2462 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2463
a11c483f 2464to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2465exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2466own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2467(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2468may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2469written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2470will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2471has no string value but does have a numeric value, that value will be
2472passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2473true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2474functions may be needed to manipulate the values of structures used by
b76cc8ba 2475C<ioctl>.
a0d0e21e 2476
19799a22 2477The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2478
2479 if OS returns: then Perl returns:
2480 -1 undefined value
2481 0 string "0 but true"
2482 anything else that number
2483
19799a22 2484Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2485still easily determine the actual value returned by the operating
2486system:
2487
2b5ab1e7 2488 $retval = ioctl(...) || -1;
a0d0e21e
LW
2489 printf "System returned %d\n", $retval;
2490
be2f7487 2491The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2492about improper numeric conversions.
2493
a0d0e21e 2494=item join EXPR,LIST
d74e8afc 2495X<join>
a0d0e21e 2496
2b5ab1e7
TC
2497Joins the separate strings of LIST into a single string with fields
2498separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2499
2b5ab1e7 2500 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2501
eb6e2d6f
GS
2502Beware that unlike C<split>, C<join> doesn't take a pattern as its
2503first argument. Compare L</split>.
a0d0e21e 2504
aa689395 2505=item keys HASH
d74e8afc 2506X<keys> X<key>
aa689395 2507
504f80c1
JH
2508Returns a list consisting of all the keys of the named hash.
2509(In scalar context, returns the number of keys.)
2510
2511The keys are returned in an apparently random order. The actual
2512random order is subject to change in future versions of perl, but it
2513is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2514function produces (given that the hash has not been modified). Since
2515Perl 5.8.1 the ordering is different even between different runs of
2516Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2517Attacks">).
504f80c1 2518
cf264981
SP
2519As a side effect, calling keys() resets the HASH's internal iterator
2520(see L</each>). In particular, calling keys() in void context resets
2521the iterator with no other overhead.
a0d0e21e 2522
aa689395 2523Here is yet another way to print your environment:
a0d0e21e
LW
2524
2525 @keys = keys %ENV;
2526 @values = values %ENV;
b76cc8ba 2527 while (@keys) {
a0d0e21e
LW
2528 print pop(@keys), '=', pop(@values), "\n";
2529 }
2530
2531or how about sorted by key:
2532
2533 foreach $key (sort(keys %ENV)) {
2534 print $key, '=', $ENV{$key}, "\n";
2535 }
2536
8ea1e5d4
GS
2537The returned values are copies of the original keys in the hash, so
2538modifying them will not affect the original hash. Compare L</values>.
2539
19799a22 2540To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2541Here's a descending numeric sort of a hash by its values:
4633a7c4 2542
5a964f20 2543 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2544 printf "%4d %s\n", $hash{$key}, $key;
2545 }
2546
19799a22 2547As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2548allocated for the given hash. This can gain you a measure of efficiency if
2549you know the hash is going to get big. (This is similar to pre-extending
2550an array by assigning a larger number to $#array.) If you say
55497cff
PP
2551
2552 keys %hash = 200;
2553
ab192400
GS
2554then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2555in fact, since it rounds up to the next power of two. These
55497cff
PP
2556buckets will be retained even if you do C<%hash = ()>, use C<undef
2557%hash> if you want to free the storage while C<%hash> is still in scope.
2558You can't shrink the number of buckets allocated for the hash using
19799a22 2559C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2560as trying has no effect).
2561
19799a22 2562See also C<each>, C<values> and C<sort>.
ab192400 2563
b350dd2f 2564=item kill SIGNAL, LIST
d74e8afc 2565X<kill> X<signal>
a0d0e21e 2566
b350dd2f 2567Sends a signal to a list of processes. Returns the number of
517db077
GS
2568processes successfully signaled (which is not necessarily the
2569same as the number actually killed).
a0d0e21e
LW
2570
2571 $cnt = kill 1, $child1, $child2;
2572 kill 9, @goners;
2573
70fb64f6 2574If SIGNAL is zero, no signal is sent to the process, but the kill(2)
6cb9d3e4 2575system call will check whether it's possible to send a signal to it (that
70fb64f6
RGS
2576means, to be brief, that the process is owned by the same user, or we are
2577the super-user). This is a useful way to check that a child process is
81fd35db
DN
2578alive (even if only as a zombie) and hasn't changed its UID. See
2579L<perlport> for notes on the portability of this construct.
b350dd2f
GS
2580
2581Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2582process groups instead of processes. (On System V, a negative I<PROCESS>
2583number will also kill process groups, but that's not portable.) That
2584means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2585use a signal name in quotes.
2586
2587See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2588
2589=item last LABEL
d74e8afc 2590X<last> X<break>
a0d0e21e
LW
2591
2592=item last
2593
2594The C<last> command is like the C<break> statement in C (as used in
2595loops); it immediately exits the loop in question. If the LABEL is
2596omitted, the command refers to the innermost enclosing loop. The
2597C<continue> block, if any, is not executed:
2598
4633a7c4
LW
2599 LINE: while (<STDIN>) {
2600 last LINE if /^$/; # exit when done with header
5a964f20 2601 #...
a0d0e21e
LW
2602 }
2603
4968c1e4 2604C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2605C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2606a grep() or map() operation.
4968c1e4 2607
6c1372ed
GS
2608Note that a block by itself is semantically identical to a loop
2609that executes once. Thus C<last> can be used to effect an early
2610exit out of such a block.
2611
98293880
JH
2612See also L</continue> for an illustration of how C<last>, C<next>, and
2613C<redo> work.
1d2dff63 2614
a0d0e21e 2615=item lc EXPR
d74e8afc 2616X<lc> X<lowercase>
a0d0e21e 2617
54310121 2618=item lc
bbce6d69 2619
d1be9408 2620Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2621implementing the C<\L> escape in double-quoted strings. Respects
2622current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2623and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2624
7660c0ab 2625If EXPR is omitted, uses C<$_>.
bbce6d69 2626
a0d0e21e 2627=item lcfirst EXPR
d74e8afc 2628X<lcfirst> X<lowercase>
a0d0e21e 2629
54310121 2630=item lcfirst
bbce6d69 2631
ad0029c4
JH
2632Returns the value of EXPR with the first character lowercased. This
2633is the internal function implementing the C<\l> escape in
2634double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2635locale> in force. See L<perllocale> and L<perlunicode> for more
2636details about locale and Unicode support.
a0d0e21e 2637
7660c0ab 2638If EXPR is omitted, uses C<$_>.
bbce6d69 2639
a0d0e21e 2640=item length EXPR
d74e8afc 2641X<length> X<size>
a0d0e21e 2642
54310121 2643=item length
bbce6d69 2644
974da8e5 2645Returns the length in I<characters> of the value of EXPR. If EXPR is
b76cc8ba 2646omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2647an entire array or hash to find out how many elements these have.
2648For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e 2649
974da8e5
JH
2650Note the I<characters>: if the EXPR is in Unicode, you will get the
2651number of characters, not the number of bytes. To get the length
2575c402
JW
2652of the internal string in bytes, use C<bytes::length(EXPR)>, see
2653L<bytes>. Note that the internal encoding is variable, and the number
2654of bytes usually meaningless. To get the number of bytes that the
2655string would have when encoded as UTF-8, use
2656C<length(Encoding::encode_utf8(EXPR))>.
974da8e5 2657
a0d0e21e 2658=item link OLDFILE,NEWFILE
d74e8afc 2659X<link>
a0d0e21e 2660
19799a22 2661Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2662success, false otherwise.
a0d0e21e
LW
2663
2664=item listen SOCKET,QUEUESIZE
d74e8afc 2665X<listen>
a0d0e21e 2666
19799a22 2667Does the same thing that the listen system call does. Returns true if
b76cc8ba 2668it succeeded, false otherwise. See the example in
cea6626f 2669L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2670
2671=item local EXPR
d74e8afc 2672X<local>
a0d0e21e 2673
19799a22 2674You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2675what most people think of as "local". See
13a2d996 2676L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2677
5a964f20
TC
2678A local modifies the listed variables to be local to the enclosing
2679block, file, or eval. If more than one value is listed, the list must
2680be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2681for details, including issues with tied arrays and hashes.
a0d0e21e 2682
a0d0e21e 2683=item localtime EXPR
435fbc73 2684X<localtime> X<ctime>
a0d0e21e 2685
ba053783
AL
2686=item localtime
2687
19799a22 2688Converts a time as returned by the time function to a 9-element list
5f05dabc 2689with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2690follows:
2691
54310121 2692 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2693 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2694 localtime(time);
a0d0e21e 2695
48a26b3a 2696All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2697tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2698of the specified time.
48a26b3a 2699
ba053783
AL
2700C<$mday> is the day of the month, and C<$mon> is the month itself, in
2701the range C<0..11> with 0 indicating January and 11 indicating December.
2702This makes it easy to get a month name from a list:
54310121 2703
ba053783
AL
2704 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2705 print "$abbr[$mon] $mday";
2706 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2707
ba053783
AL
2708C<$year> is the number of years since 1900, not just the last two digits
2709of the year. That is, C<$year> is C<123> in year 2023. The proper way
2710to get a complete 4-digit year is simply:
abd75f24 2711
ba053783 2712 $year += 1900;
abd75f24 2713
435fbc73
GS
2714Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2715to do that, would you?
2716
ba053783
AL
2717To get the last two digits of the year (e.g., '01' in 2001) do:
2718
2719 $year = sprintf("%02d", $year % 100);
2720
2721C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2722Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2723(or C<0..365> in leap years.)
2724
2725C<$isdst> is true if the specified time occurs during Daylight Saving
2726Time, false otherwise.
abd75f24 2727
48a26b3a 2728If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2729
48a26b3a 2730In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2731
5f05dabc 2732 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2733
fe86afc2
NC
2734This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2735instead of local time use the L</gmtime> builtin. See also the
2736C<Time::Local> module (to convert the second, minutes, hours, ... back to
2737the integer value returned by time()), and the L<POSIX> module's strftime(3)
2738and mktime(3) functions.
2739
2740To get somewhat similar but locale dependent date strings, set up your
2741locale environment variables appropriately (please see L<perllocale>) and
2742try for example:
a3cb178b 2743
5a964f20 2744 use POSIX qw(strftime);
2b5ab1e7 2745 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2746 # or for GMT formatted appropriately for your locale:
2747 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2748
2749Note that the C<%a> and C<%b>, the short forms of the day of the week
2750and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2751
62aa5637
MS
2752See L<perlport/localtime> for portability concerns.
2753
435fbc73
GS
2754The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2755by-name access mechanism to the gmtime() and localtime() functions,
2756respectively.
2757
2758For a comprehensive date and time representation look at the
2759L<DateTime> module on CPAN.
2760
07698885 2761=item lock THING
d74e8afc 2762X<lock>
19799a22 2763
01e6739c 2764This function places an advisory lock on a shared variable, or referenced
03730085 2765object contained in I<THING> until the lock goes out of scope.
a6d5524e 2766
f3a23afb 2767lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2768by this name (before any calls to it), that function will be called
03730085
AB
2769instead. (However, if you've said C<use threads>, lock() is always a
2770keyword.) See L<threads>.
19799a22 2771
a0d0e21e 2772=item log EXPR
d74e8afc 2773X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2774
54310121 2775=item log
bbce6d69 2776
2b5ab1e7
TC
2777Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2778returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2779The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2780divided by the natural log of N. For example:
2781
2782 sub log10 {
2783 my $n = shift;
2784 return log($n)/log(10);
b76cc8ba 2785 }
2b5ab1e7
TC
2786
2787See also L</exp> for the inverse operation.
a0d0e21e 2788
a0d0e21e 2789=item lstat EXPR
d74e8afc 2790X<lstat>
a0d0e21e 2791
54310121 2792=item lstat
bbce6d69 2793
19799a22 2794Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2795special C<_> filehandle) but stats a symbolic link instead of the file
2796the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2797your system, a normal C<stat> is done. For much more detailed
2798information, please see the documentation for C<stat>.
a0d0e21e 2799
7660c0ab 2800If EXPR is omitted, stats C<$_>.
bbce6d69 2801
a0d0e21e
LW
2802=item m//
2803
2804The match operator. See L<perlop>.
2805
2806=item map BLOCK LIST
d74e8afc 2807X<map>
a0d0e21e
LW
2808
2809=item map EXPR,LIST
2810
19799a22
GS
2811Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2812C<$_> to each element) and returns the list value composed of the
2813results of each such evaluation. In scalar context, returns the
2814total number of elements so generated. Evaluates BLOCK or EXPR in
2815list context, so each element of LIST may produce zero, one, or
2816more elements in the returned value.
dd99ebda 2817
a0d0e21e
LW
2818 @chars = map(chr, @nums);
2819
2820translates a list of numbers to the corresponding characters. And
2821
d8216f19 2822 %hash = map { get_a_key_for($_) => $_ } @array;
a0d0e21e
LW
2823
2824is just a funny way to write
2825
2826 %hash = ();
d8216f19
RGS
2827 foreach (@array) {
2828 $hash{get_a_key_for($_)} = $_;
a0d0e21e
LW
2829 }
2830
be3174d2
GS
2831Note that C<$_> is an alias to the list value, so it can be used to
2832modify the elements of the LIST. While this is useful and supported,
2833it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2834Using a regular C<foreach> loop for this purpose would be clearer in
2835most cases. See also L</grep> for an array composed of those items of
2836the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2837
a4fb8298 2838If C<$_> is lexical in the scope where the C<map> appears (because it has
d8216f19
RGS
2839been declared with C<my $_>), then, in addition to being locally aliased to
2840the list elements, C<$_> keeps being lexical inside the block; that is, it
a4fb8298
RGS
2841can't be seen from the outside, avoiding any potential side-effects.
2842
205fdb4d
NC
2843C<{> starts both hash references and blocks, so C<map { ...> could be either
2844the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2845ahead for the closing C<}> it has to take a guess at which its dealing with
2846based what it finds just after the C<{>. Usually it gets it right, but if it
2847doesn't it won't realize something is wrong until it gets to the C<}> and
2848encounters the missing (or unexpected) comma. The syntax error will be
2849reported close to the C<}> but you'll need to change something near the C<{>
2850such as using a unary C<+> to give perl some help:
2851
2852 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2853 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2854 %hash = map { ("\L$_", 1) } @array # this also works
2855 %hash = map { lc($_), 1 } @array # as does this.
2856 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2857
205fdb4d
NC
2858 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2859
d8216f19 2860or to force an anon hash constructor use C<+{>:
205fdb4d
NC
2861
2862 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2863
2864and you get list of anonymous hashes each with only 1 entry.
2865
19799a22 2866=item mkdir FILENAME,MASK
d74e8afc 2867X<mkdir> X<md> X<directory, create>
a0d0e21e 2868
5a211162
GS
2869=item mkdir FILENAME
2870
491873e5
RGS
2871=item mkdir
2872
0591cd52 2873Creates the directory specified by FILENAME, with permissions
19799a22
GS
2874specified by MASK (as modified by C<umask>). If it succeeds it
2875returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2876If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2877to C<$_>.
0591cd52 2878
19799a22 2879In general, it is better to create directories with permissive MASK,
0591cd52 2880and let the user modify that with their C<umask>, than it is to supply
19799a22 2881a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2882The exceptions to this rule are when the file or directory should be
2883kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2884C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2885
cc1852e8
JH
2886Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2887number of trailing slashes. Some operating and filesystems do not get
2888this right, so Perl automatically removes all trailing slashes to keep
2889everyone happy.
2890
dd184578
RGS
2891In order to recursively create a directory structure look at
2892the C<mkpath> function of the L<File::Path> module.
2893
a0d0e21e 2894=item msgctl ID,CMD,ARG
d74e8afc 2895X<msgctl>
a0d0e21e 2896
f86cebdf 2897Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2898
2899 use IPC::SysV;
2900
7660c0ab 2901first to get the correct constant definitions. If CMD is C<IPC_STAT>,
cf264981 2902then ARG must be a variable that will hold the returned C<msqid_ds>
951ba7fe
GS
2903structure. Returns like C<ioctl>: the undefined value for error,
2904C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2905L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2906
2907=item msgget KEY,FLAGS
d74e8afc 2908X<msgget>
a0d0e21e 2909
f86cebdf 2910Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2911id, or the undefined value if there is an error. See also
2912L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2913
a0d0e21e 2914=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
d74e8afc 2915X<msgrcv>
a0d0e21e
LW
2916
2917Calls the System V IPC function msgrcv to receive a message from
2918message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2919SIZE. Note that when a message is received, the message type as a
2920native long integer will be the first thing in VAR, followed by the
2921actual message. This packing may be opened with C<unpack("l! a*")>.
2922Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2923an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2924C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2925
2926=item msgsnd ID,MSG,FLAGS
d74e8afc 2927X<msgsnd>
41d6edb2
JH
2928
2929Calls the System V IPC function msgsnd to send the message MSG to the
2930message queue ID. MSG must begin with the native long integer message
2931type, and be followed by the length of the actual message, and finally
2932the message itself. This kind of packing can be achieved with
2933C<pack("l! a*", $type, $message)>. Returns true if successful,
2934or false if there is an error. See also C<IPC::SysV>
2935and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2936
2937=item my EXPR
d74e8afc 2938X<my>
a0d0e21e 2939
307ea6df
JH
2940=item my TYPE EXPR
2941
1d2de774 2942=item my EXPR : ATTRS
09bef843 2943
1d2de774 2944=item my TYPE EXPR : ATTRS
307ea6df 2945
19799a22 2946A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2947enclosing block, file, or C<eval>. If more than one value is listed,
2948the list must be placed in parentheses.
307ea6df 2949
1d2de774
JH
2950The exact semantics and interface of TYPE and ATTRS are still
2951evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
2952and attributes are handled using the C<attributes> pragma, or starting
2953from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2954L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2955L<attributes>, and L<Attribute::Handlers>.
4633a7c4 2956
a0d0e21e 2957=item next LABEL
d74e8afc 2958X<next> X<continue>
a0d0e21e
LW
2959
2960=item next
2961
2962The C<next> command is like the C<continue> statement in C; it starts
2963the next iteration of the loop:
2964
4633a7c4
LW
2965 LINE: while (<STDIN>) {
2966 next LINE if /^#/; # discard comments
5a964f20 2967 #...
a0d0e21e
LW
2968 }
2969
2970Note that if there were a C<continue> block on the above, it would get
2971executed even on discarded lines. If the LABEL is omitted, the command
2972refers to the innermost enclosing loop.
2973
4968c1e4 2974C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2975C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2976a grep() or map() operation.
4968c1e4 2977
6c1372ed
GS
2978Note that a block by itself is semantically identical to a loop
2979that executes once. Thus C<next> will exit such a block early.
2980
98293880
JH
2981See also L</continue> for an illustration of how C<last>, C<next>, and
2982C<redo> work.
1d2dff63 2983
4a66ea5a 2984=item no Module VERSION LIST
d74e8afc 2985X<no>
4a66ea5a
RGS
2986
2987=item no Module VERSION
2988
a0d0e21e
LW
2989=item no Module LIST
2990
4a66ea5a
RGS
2991=item no Module
2992
c986422f
RGS
2993=item no VERSION
2994
593b9c14 2995See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
2996
2997=item oct EXPR
d74e8afc 2998X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
a0d0e21e 2999
54310121 3000=item oct
bbce6d69 3001
4633a7c4 3002Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
3003value. (If EXPR happens to start off with C<0x>, interprets it as a
3004hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
3005binary string. Leading whitespace is ignored in all three cases.)
3006The following will handle decimal, binary, octal, and hex in the standard
3007Perl or C notation:
a0d0e21e
LW
3008
3009 $val = oct($val) if $val =~ /^0/;
3010
19799a22
GS
3011If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3012in octal), use sprintf() or printf():
3013
3014 $perms = (stat("filename"))[2] & 07777;
3015 $oct_perms = sprintf "%lo", $perms;
3016
3017The oct() function is commonly used when a string such as C<644> needs
3018to be converted into a file mode, for example. (Although perl will
3019automatically convert strings into numbers as needed, this automatic
3020conversion assumes base 10.)
a0d0e21e
LW
3021
3022=item open FILEHANDLE,EXPR
d74e8afc 3023X<open> X<pipe> X<file, open> X<fopen>
a0d0e21e 3024
68bd7414
NIS
3025=item open FILEHANDLE,MODE,EXPR
3026
3027=item open FILEHANDLE,MODE,EXPR,LIST
3028
ba964c95
T
3029=item open FILEHANDLE,MODE,REFERENCE
3030
a0d0e21e
LW
3031=item open FILEHANDLE
3032
3033Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
3034FILEHANDLE.
3035
3036(The following is a comprehensive reference to open(): for a gentler
3037introduction you may consider L<perlopentut>.)
3038
a28cd5c9
NT
3039If FILEHANDLE is an undefined scalar variable (or array or hash element)
3040the variable is assigned a reference to a new anonymous filehandle,
3041otherwise if FILEHANDLE is an expression, its value is used as the name of
3042the real filehandle wanted. (This is considered a symbolic reference, so
3043C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
3044
3045If EXPR is omitted, the scalar variable of the same name as the
3046FILEHANDLE contains the filename. (Note that lexical variables--those
3047declared with C<my>--will not work for this purpose; so if you're
67408cae 3048using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
3049
3050If three or more arguments are specified then the mode of opening and
3051the file name are separate. If MODE is C<< '<' >> or nothing, the file
3052is opened for input. If MODE is C<< '>' >>, the file is truncated and
3053opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 3054the file is opened for appending, again being created if necessary.
5a964f20 3055
ed53a2bb
JH
3056You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3057indicate that you want both read and write access to the file; thus
3058C<< '+<' >> is almost always preferred for read/write updates--the C<<
3059'+>' >> mode would clobber the file first. You can't usually use
3060either read-write mode for updating textfiles, since they have
3061variable length records. See the B<-i> switch in L<perlrun> for a
3062better approach. The file is created with permissions of C<0666>
3063modified by the process' C<umask> value.
3064
3065These various prefixes correspond to the fopen(3) modes of C<'r'>,
3066C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 3067
6170680b
IZ
3068In the 2-arguments (and 1-argument) form of the call the mode and
3069filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
3070spaces. It is possible to omit the mode in these forms if the mode is
3071C<< '<' >>.
6170680b 3072
7660c0ab 3073If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 3074command to which output is to be piped, and if the filename ends with a
f244e06d
GS
3075C<'|'>, the filename is interpreted as a command which pipes output to
3076us. See L<perlipc/"Using open() for IPC">
19799a22 3077for more examples of this. (You are not allowed to C<open> to a command
5a964f20 3078that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
3079and L<perlipc/"Bidirectional Communication with Another Process">
3080for alternatives.)
cb1a09d0 3081
ed53a2bb
JH
3082For three or more arguments if MODE is C<'|-'>, the filename is
3083interpreted as a command to which output is to be piped, and if MODE
3084is C<'-|'>, the filename is interpreted as a command which pipes
3085output to us. In the 2-arguments (and 1-argument) form one should
3086replace dash (C<'-'>) with the command.
3087See L<perlipc/"Using open() for IPC"> for more examples of this.
3088(You are not allowed to C<open> to a command that pipes both in I<and>
3089out, but see L<IPC::Open2>, L<IPC::Open3>, and
3090L<perlipc/"Bidirectional Communication"> for alternatives.)
3091
3092In the three-or-more argument form of pipe opens, if LIST is specified
3093(extra arguments after the command name) then LIST becomes arguments
3094to the command invoked if the platform supports it. The meaning of
3095C<open> with more than three arguments for non-pipe modes is not yet
3096specified. Experimental "layers" may give extra LIST arguments
3097meaning.
6170680b
IZ
3098
3099In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 3100and opening C<< '>-' >> opens STDOUT.
6170680b 3101
fae2c0fb
RGS
3102You may use the three-argument form of open to specify IO "layers"
3103(sometimes also referred to as "disciplines") to be applied to the handle
3104that affect how the input and output are processed (see L<open> and
3105L<PerlIO> for more details). For example
7207e29d 3106
2575c402 3107 open(FH, "<:encoding(UTF-8)", "file")
9124316e
JH
3108
3109will open the UTF-8 encoded file containing Unicode characters,
6d5e88a0
ST
3110see L<perluniintro>. Note that if layers are specified in the
3111three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3112usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
ed53a2bb
JH
3113
3114Open returns nonzero upon success, the undefined value otherwise. If
3115the C<open> involved a pipe, the return value happens to be the pid of
3116the subprocess.
cb1a09d0 3117
ed53a2bb
JH
3118If you're running Perl on a system that distinguishes between text
3119files and binary files, then you should check out L</binmode> for tips
3120for dealing with this. The key distinction between systems that need
3121C<binmode> and those that don't is their text file formats. Systems
8939ba94 3122like Unix, Mac OS, and Plan 9, which delimit lines with a single
ed53a2bb
JH
3123character, and which encode that character in C as C<"\n">, do not
3124need C<binmode>. The rest need it.
cb1a09d0 3125
fb73857a 3126When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
3127if the request failed, so C<open> is frequently used in connection with
3128C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 3129where you want to make a nicely formatted error message (but there are
5a964f20 3130modules that can help with that problem)) you should always check
19799a22 3131the return value from opening a file. The infrequent exception is when
fb73857a
PP
3132working with an unopened filehandle is actually what you want to do.
3133
cf264981 3134As a special case the 3-arg form with a read/write mode and the third
ed53a2bb 3135argument being C<undef>:
b76cc8ba
NIS
3136
3137 open(TMP, "+>", undef) or die ...
3138
f253e835
JH
3139opens a filehandle to an anonymous temporary file. Also using "+<"
3140works for symmetry, but you really should consider writing something
3141to the temporary file first. You will need to seek() to do the
3142reading.
b76cc8ba 3143
2ce64696 3144Since v5.8.0, perl has built using PerlIO by default. Unless you've
28a5cf3b 3145changed this (i.e. Configure -Uuseperlio), you can open file handles to