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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
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3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
15operator. A unary operator generally provides a scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
a0d0e21e 17contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 18be first, and the list argument will follow. (Note that there can ever
0f31cffe 19be only one such list argument.) For instance, splice() has three scalar
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20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
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22
23In the syntax descriptions that follow, list operators that expect a
24list (and provide list context for the elements of the list) are shown
25with LIST as an argument. Such a list may consist of any combination
26of scalar arguments or list values; the list values will be included
27in the list as if each individual element were interpolated at that
28point in the list, forming a longer single-dimensional list value.
cf264981 29Commas should separate elements of the LIST.
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30
31Any function in the list below may be used either with or without
32parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 33parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 34surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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35function, and precedence doesn't matter. Otherwise it's a list
36operator or unary operator, and precedence does matter. And whitespace
37between the function and left parenthesis doesn't count--so you need to
38be careful sometimes:
39
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40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
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45
46If you run Perl with the B<-w> switch it can warn you about this. For
47example, the third line above produces:
48
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
51
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52A few functions take no arguments at all, and therefore work as neither
53unary nor list operators. These include such functions as C<time>
54and C<endpwent>. For example, C<time+86_400> always means
55C<time() + 86_400>.
56
a0d0e21e 57For functions that can be used in either a scalar or list context,
54310121 58nonabortive failure is generally indicated in a scalar context by
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59returning the undefined value, and in a list context by returning the
60null list.
61
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62Remember the following important rule: There is B<no rule> that relates
63the behavior of an expression in list context to its behavior in scalar
64context, or vice versa. It might do two totally different things.
a0d0e21e 65Each operator and function decides which sort of value it would be most
2b5ab1e7 66appropriate to return in scalar context. Some operators return the
5a964f20 67length of the list that would have been returned in list context. Some
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68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
d74e8afc 72X<context>
a0d0e21e 73
d1be9408 74A named array in scalar context is quite different from what would at
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75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
81In general, functions in Perl that serve as wrappers for system calls
f86cebdf 82of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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83true when they succeed and C<undef> otherwise, as is usually mentioned
84in the descriptions below. This is different from the C interfaces,
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85which return C<-1> on failure. Exceptions to this rule are C<wait>,
86C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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87variable on failure. Other functions do not, except accidentally.
88
cb1a09d0 89=head2 Perl Functions by Category
d74e8afc 90X<function>
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91
92Here are Perl's functions (including things that look like
5a964f20 93functions, like some keywords and named operators)
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94arranged by category. Some functions appear in more
95than one place.
96
13a2d996 97=over 4
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98
99=item Functions for SCALARs or strings
d74e8afc 100X<scalar> X<string> X<character>
cb1a09d0 101
22fae026 102C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
1dc8ecb8 103C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
945c54fd 104C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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105
106=item Regular expressions and pattern matching
d74e8afc 107X<regular expression> X<regex> X<regexp>
cb1a09d0 108
ab4f32c2 109C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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110
111=item Numeric functions
d74e8afc 112X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 113
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114C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
115C<sin>, C<sqrt>, C<srand>
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116
117=item Functions for real @ARRAYs
d74e8afc 118X<array>
cb1a09d0 119
22fae026 120C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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121
122=item Functions for list data
d74e8afc 123X<list>
cb1a09d0 124
1dc8ecb8 125C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
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126
127=item Functions for real %HASHes
d74e8afc 128X<hash>
cb1a09d0 129
22fae026 130C<delete>, C<each>, C<exists>, C<keys>, C<values>
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131
132=item Input and output functions
d74e8afc 133X<I/O> X<input> X<output> X<dbm>
cb1a09d0 134
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135C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
136C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
0d863452 137C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
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138C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
139C<warn>, C<write>
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140
141=item Functions for fixed length data or records
142
22fae026 143C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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144
145=item Functions for filehandles, files, or directories
d74e8afc 146X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 147
22fae026 148C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 149C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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150C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
151C<umask>, C<unlink>, C<utime>
cb1a09d0 152
cf264981 153=item Keywords related to the control flow of your Perl program
d74e8afc 154X<control flow>
cb1a09d0 155
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156C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
157C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 158
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159=item Keywords related to switch
160
36fb85f3 161C<break>, C<continue>, C<given>, C<when>, C<default>
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162
163(These are only available if you enable the "switch" feature.
164See L<feature> and L<perlsyn/"Switch statements">.)
165
54310121 166=item Keywords related to scoping
cb1a09d0 167
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168C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
169C<use>
170
171(C<state> is only available if the "state" feature is enabled. See
172L<feature>.)
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173
174=item Miscellaneous functions
175
36fb85f3 176C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
834df1c5 177C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
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178
179=item Functions for processes and process groups
d74e8afc 180X<process> X<pid> X<process id>
cb1a09d0 181
22fae026 182C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
1dc8ecb8 183C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 184C<times>, C<wait>, C<waitpid>
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185
186=item Keywords related to perl modules
d74e8afc 187X<module>
cb1a09d0 188
22fae026 189C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 190
353c6505 191=item Keywords related to classes and object-orientation
d74e8afc 192X<object> X<class> X<package>
cb1a09d0 193
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194C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
195C<untie>, C<use>
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196
197=item Low-level socket functions
d74e8afc 198X<socket> X<sock>
cb1a09d0 199
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200C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
201C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 202C<socket>, C<socketpair>
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203
204=item System V interprocess communication functions
d74e8afc 205X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 206
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207C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
208C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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209
210=item Fetching user and group info
d74e8afc 211X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 212
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213C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
214C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
215C<getpwuid>, C<setgrent>, C<setpwent>
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216
217=item Fetching network info
d74e8afc 218X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 219
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220C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
221C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
222C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
223C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
224C<setnetent>, C<setprotoent>, C<setservent>
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225
226=item Time-related functions
d74e8afc 227X<time> X<date>
cb1a09d0 228
22fae026 229C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 230
37798a01 231=item Functions new in perl5
d74e8afc 232X<perl5>
37798a01 233
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234C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
235C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
1dc8ecb8 236C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
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237C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
238C<ucfirst>, C<untie>, C<use>, C<when>
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239
240* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 241operator, which can be used in expressions.
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242
243=item Functions obsoleted in perl5
244
22fae026 245C<dbmclose>, C<dbmopen>
37798a01 246
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247=back
248
60f9f73c 249=head2 Portability
d74e8afc 250X<portability> X<Unix> X<portable>
60f9f73c 251
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252Perl was born in Unix and can therefore access all common Unix
253system calls. In non-Unix environments, the functionality of some
254Unix system calls may not be available, or details of the available
255functionality may differ slightly. The Perl functions affected
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256by this are:
257
258C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
259C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
260C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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261C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
262C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 263C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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264C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
265C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
266C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 267C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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268C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
269C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
270C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 271C<shmwrite>, C<socket>, C<socketpair>,
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272C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
273C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 274C<utime>, C<wait>, C<waitpid>
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275
276For more information about the portability of these functions, see
277L<perlport> and other available platform-specific documentation.
278
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279=head2 Alphabetical Listing of Perl Functions
280
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281=over 8
282
5b3c99c0 283=item -X FILEHANDLE
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284X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
285X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 286
5b3c99c0 287=item -X EXPR
a0d0e21e 288
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289=item -X DIRHANDLE
290
5b3c99c0 291=item -X
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292
293A file test, where X is one of the letters listed below. This unary
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294operator takes one argument, either a filename, a filehandle, or a dirhandle,
295and tests the associated file to see if something is true about it. If the
7660c0ab 296argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 297Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 298the undefined value if the file doesn't exist. Despite the funny
d0821a6a 299names, precedence is the same as any other named unary operator. The
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300operator may be any of:
301
302 -r File is readable by effective uid/gid.
303 -w File is writable by effective uid/gid.
304 -x File is executable by effective uid/gid.
305 -o File is owned by effective uid.
306
307 -R File is readable by real uid/gid.
308 -W File is writable by real uid/gid.
309 -X File is executable by real uid/gid.
310 -O File is owned by real uid.
311
312 -e File exists.
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313 -z File has zero size (is empty).
314 -s File has nonzero size (returns size in bytes).
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315
316 -f File is a plain file.
317 -d File is a directory.
318 -l File is a symbolic link.
9c4d0f16 319 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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320 -S File is a socket.
321 -b File is a block special file.
322 -c File is a character special file.
323 -t Filehandle is opened to a tty.
324
325 -u File has setuid bit set.
326 -g File has setgid bit set.
327 -k File has sticky bit set.
328
121910a4 329 -T File is an ASCII text file (heuristic guess).
2cdbc966 330 -B File is a "binary" file (opposite of -T).
a0d0e21e 331
95a3fe12 332 -M Script start time minus file modification time, in days.
a0d0e21e 333 -A Same for access time.
95a3fe12 334 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 335
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336Example:
337
338 while (<>) {
5b3eff12 339 chomp;
a0d0e21e 340 next unless -f $_; # ignore specials
5a964f20 341 #...
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342 }
343
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GS
344The interpretation of the file permission operators C<-r>, C<-R>,
345C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
346of the file and the uids and gids of the user. There may be other
ecae030f
MO
347reasons you can't actually read, write, or execute the file: for
348example network filesystem access controls, ACLs (access control lists),
349read-only filesystems, and unrecognized executable formats. Note
350that the use of these six specific operators to verify if some operation
351is possible is usually a mistake, because it may be open to race
352conditions.
5ff3f7a4 353
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354Also note that, for the superuser on the local filesystems, the C<-r>,
355C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
5ff3f7a4
GS
356if any execute bit is set in the mode. Scripts run by the superuser
357may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 358or temporarily set their effective uid to something else.
5ff3f7a4
GS
359
360If you are using ACLs, there is a pragma called C<filetest> that may
361produce more accurate results than the bare stat() mode bits.
5ff3f7a4
GS
362When under the C<use filetest 'access'> the above-mentioned filetests
363will test whether the permission can (not) be granted using the
468541a8 364access() family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
365under this pragma return true even if there are no execute permission
366bits set (nor any extra execute permission ACLs). This strangeness is
ecae030f
MO
367due to the underlying system calls' definitions. Note also that, due to
368the implementation of C<use filetest 'access'>, the C<_> special
369filehandle won't cache the results of the file tests when this pragma is
370in effect. Read the documentation for the C<filetest> pragma for more
371information.
5ff3f7a4 372
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373Note that C<-s/a/b/> does not do a negated substitution. Saying
374C<-exp($foo)> still works as expected, however--only single letters
375following a minus are interpreted as file tests.
376
377The C<-T> and C<-B> switches work as follows. The first block or so of the
378file is examined for odd characters such as strange control codes or
61eff3bc 379characters with the high bit set. If too many strange characters (>30%)
cf264981 380are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
a0d0e21e 381containing null in the first block is considered a binary file. If C<-T>
9124316e 382or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 383rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 384file, or a file at EOF when testing a filehandle. Because you have to
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LW
385read a file to do the C<-T> test, on most occasions you want to use a C<-f>
386against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 387
19799a22 388If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 389the special filehandle consisting of a solitary underline, then the stat
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390structure of the previous file test (or stat operator) is used, saving
391a system call. (This doesn't work with C<-t>, and you need to remember
392that lstat() and C<-l> will leave values in the stat structure for the
5c9aa243 393symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 394an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 395Example:
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396
397 print "Can do.\n" if -r $a || -w _ || -x _;
398
399 stat($filename);
400 print "Readable\n" if -r _;
401 print "Writable\n" if -w _;
402 print "Executable\n" if -x _;
403 print "Setuid\n" if -u _;
404 print "Setgid\n" if -g _;
405 print "Sticky\n" if -k _;
406 print "Text\n" if -T _;
407 print "Binary\n" if -B _;
408
fbb0b3b3
RGS
409As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
410test operators, in a way that C<-f -w -x $file> is equivalent to
cf264981 411C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
fbb0b3b3
RGS
412the return value of C<-f $file> as an argument to another filetest
413operator, no special magic will happen.)
414
a0d0e21e 415=item abs VALUE
d74e8afc 416X<abs> X<absolute>
a0d0e21e 417
54310121 418=item abs
bbce6d69 419
a0d0e21e 420Returns the absolute value of its argument.
7660c0ab 421If VALUE is omitted, uses C<$_>.
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422
423=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 424X<accept>
a0d0e21e 425
f86cebdf 426Accepts an incoming socket connect, just as the accept(2) system call
19799a22 427does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 428See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 429
8d2a6795
GS
430On systems that support a close-on-exec flag on files, the flag will
431be set for the newly opened file descriptor, as determined by the
432value of $^F. See L<perlvar/$^F>.
433
a0d0e21e 434=item alarm SECONDS
d74e8afc
ITB
435X<alarm>
436X<SIGALRM>
437X<timer>
a0d0e21e 438
54310121 439=item alarm
bbce6d69 440
a0d0e21e 441Arranges to have a SIGALRM delivered to this process after the
cf264981 442specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
443specified, the value stored in C<$_> is used. (On some machines,
444unfortunately, the elapsed time may be up to one second less or more
445than you specified because of how seconds are counted, and process
446scheduling may delay the delivery of the signal even further.)
447
448Only one timer may be counting at once. Each call disables the
449previous timer, and an argument of C<0> may be supplied to cancel the
450previous timer without starting a new one. The returned value is the
451amount of time remaining on the previous timer.
a0d0e21e 452
2bc69794
BS
453For delays of finer granularity than one second, the Time::HiRes module
454(from CPAN, and starting from Perl 5.8 part of the standard
455distribution) provides ualarm(). You may also use Perl's four-argument
456version of select() leaving the first three arguments undefined, or you
457might be able to use the C<syscall> interface to access setitimer(2) if
458your system supports it. See L<perlfaq8> for details.
2b5ab1e7 459
68f8bed4
JH
460It is usually a mistake to intermix C<alarm> and C<sleep> calls.
461(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 462
19799a22
GS
463If you want to use C<alarm> to time out a system call you need to use an
464C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 465fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 466restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 467modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
468
469 eval {
f86cebdf 470 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 471 alarm $timeout;
ff68c719 472 $nread = sysread SOCKET, $buffer, $size;
36477c24 473 alarm 0;
ff68c719 474 };
ff68c719 475 if ($@) {
f86cebdf 476 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719
PP
477 # timed out
478 }
479 else {
480 # didn't
481 }
482
91d81acc
JH
483For more information see L<perlipc>.
484
a0d0e21e 485=item atan2 Y,X
d74e8afc 486X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
487
488Returns the arctangent of Y/X in the range -PI to PI.
489
ca6e1c26 490For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
491function, or use the familiar relation:
492
493 sub tan { sin($_[0]) / cos($_[0]) }
494
a1021d57
RGS
495The return value for C<atan2(0,0)> is implementation-defined; consult
496your atan2(3) manpage for more information.
bf5f1b4c 497
a0d0e21e 498=item bind SOCKET,NAME
d74e8afc 499X<bind>
a0d0e21e
LW
500
501Binds a network address to a socket, just as the bind system call
19799a22 502does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
503packed address of the appropriate type for the socket. See the examples in
504L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 505
fae2c0fb 506=item binmode FILEHANDLE, LAYER
d74e8afc 507X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 508
a0d0e21e
LW
509=item binmode FILEHANDLE
510
1cbfc93d
NIS
511Arranges for FILEHANDLE to be read or written in "binary" or "text"
512mode on systems where the run-time libraries distinguish between
513binary and text files. If FILEHANDLE is an expression, the value is
514taken as the name of the filehandle. Returns true on success,
b5fe5ca2 515otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 516
d807c6f4
JH
517On some systems (in general, DOS and Windows-based systems) binmode()
518is necessary when you're not working with a text file. For the sake
519of portability it is a good idea to always use it when appropriate,
520and to never use it when it isn't appropriate. Also, people can
521set their I/O to be by default UTF-8 encoded Unicode, not bytes.
522
523In other words: regardless of platform, use binmode() on binary data,
524like for example images.
525
526If LAYER is present it is a single string, but may contain multiple
527directives. The directives alter the behaviour of the file handle.
920f5fe1 528When LAYER is present using binmode on a text file makes sense.
d807c6f4 529
fae2c0fb 530If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
531suitable for passing binary data. This includes turning off possible CRLF
532translation and marking it as bytes (as opposed to Unicode characters).
749683d2 533Note that, despite what may be implied in I<"Programming Perl"> (the
165a9987
PJ
534Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
535-- other layers which would affect the binary nature of the stream are
0226bbdb
NIS
536I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
537PERLIO environment variable.
01e6739c 538
d807c6f4
JH
539The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
540form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
541establish default I/O layers. See L<open>.
542
fae2c0fb
RGS
543I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
544in "Programming Perl, 3rd Edition". However, since the publishing of this
545book, by many known as "Camel III", the consensus of the naming of this
546functionality has moved from "discipline" to "layer". All documentation
547of this version of Perl therefore refers to "layers" rather than to
548"disciplines". Now back to the regularly scheduled documentation...>
549
6902c96a
T
550To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
551C<:utf8> just marks the data as UTF-8 without further checking,
552while C<:encoding(utf8)> checks the data for actually being valid
553UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 554
ed53a2bb 555In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
556is done on the filehandle. Calling binmode() will normally flush any
557pending buffered output data (and perhaps pending input data) on the
fae2c0fb 558handle. An exception to this is the C<:encoding> layer that
01e6739c 559changes the default character encoding of the handle, see L<open>.
fae2c0fb 560The C<:encoding> layer sometimes needs to be called in
3874323d
JH
561mid-stream, and it doesn't flush the stream. The C<:encoding>
562also implicitly pushes on top of itself the C<:utf8> layer because
563internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 564
19799a22 565The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
566system all work together to let the programmer treat a single
567character (C<\n>) as the line terminator, irrespective of the external
568representation. On many operating systems, the native text file
569representation matches the internal representation, but on some
570platforms the external representation of C<\n> is made up of more than
571one character.
572
68bd7414
NIS
573Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
574character to end each line in the external representation of text (even
5e12dbfa 575though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
576on Unix and most VMS files). In other systems like OS/2, DOS and the
577various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
578but what's stored in text files are the two characters C<\cM\cJ>. That
579means that, if you don't use binmode() on these systems, C<\cM\cJ>
580sequences on disk will be converted to C<\n> on input, and any C<\n> in
581your program will be converted back to C<\cM\cJ> on output. This is what
582you want for text files, but it can be disastrous for binary files.
30168b04
GS
583
584Another consequence of using binmode() (on some systems) is that
585special end-of-file markers will be seen as part of the data stream.
586For systems from the Microsoft family this means that if your binary
4375e838 587data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
588the file, unless you use binmode().
589
590binmode() is not only important for readline() and print() operations,
591but also when using read(), seek(), sysread(), syswrite() and tell()
592(see L<perlport> for more details). See the C<$/> and C<$\> variables
593in L<perlvar> for how to manually set your input and output
594line-termination sequences.
a0d0e21e 595
4633a7c4 596=item bless REF,CLASSNAME
d74e8afc 597X<bless>
a0d0e21e
LW
598
599=item bless REF
600
2b5ab1e7
TC
601This function tells the thingy referenced by REF that it is now an object
602in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 603is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 604it returns the reference for convenience. Always use the two-argument
cf264981
SP
605version if a derived class might inherit the function doing the blessing.
606See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
607of objects.
a0d0e21e 608
57668c4d 609Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 610Namespaces with all lowercase names are considered reserved for
cf264981 611Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
612confusion, you may wish to avoid such package names as well. Make sure
613that CLASSNAME is a true value.
60ad88b8
GS
614
615See L<perlmod/"Perl Modules">.
616
0d863452
RH
617=item break
618
619Break out of a C<given()> block.
620
621This keyword is enabled by the "switch" feature: see L<feature>
622for more information.
623
a0d0e21e 624=item caller EXPR
d74e8afc 625X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
626
627=item caller
628
5a964f20 629Returns the context of the current subroutine call. In scalar context,
28757baa 630returns the caller's package name if there is a caller, that is, if
19799a22 631we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 632otherwise. In list context, returns
a0d0e21e 633
ee6b43cc 634 # 0 1 2
748a9306 635 ($package, $filename, $line) = caller;
a0d0e21e
LW
636
637With EXPR, it returns some extra information that the debugger uses to
638print a stack trace. The value of EXPR indicates how many call frames
639to go back before the current one.
640
ee6b43cc 641 # 0 1 2 3 4
f3aa04c2 642 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 643
644 # 5 6 7 8 9 10
b3ca2e83 645 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 646 = caller($i);
e7ea3e70 647
951ba7fe 648Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 649call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 650C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 651C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 652C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 653$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
654each C<use> statement creates a C<require> frame inside an C<eval EXPR>
655frame.) $subroutine may also be C<(unknown)> if this particular
656subroutine happens to have been deleted from the symbol table.
657C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
658C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
659compiled with. The C<$hints> and C<$bitmask> values are subject to change
660between versions of Perl, and are not meant for external use.
748a9306 661
b3ca2e83
NC
662C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
663caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
664of this hash, as they are the actual values stored in the optree.
665
748a9306 666Furthermore, when called from within the DB package, caller returns more
7660c0ab 667detailed information: it sets the list variable C<@DB::args> to be the
54310121 668arguments with which the subroutine was invoked.
748a9306 669
7660c0ab 670Be aware that the optimizer might have optimized call frames away before
19799a22 671C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 672might not return information about the call frame you expect it do, for
b76cc8ba 673C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 674previous time C<caller> was called.
7660c0ab 675
a0d0e21e 676=item chdir EXPR
d74e8afc
ITB
677X<chdir>
678X<cd>
f723aae1 679X<directory, change>
a0d0e21e 680
c4aca7d0
GA
681=item chdir FILEHANDLE
682
683=item chdir DIRHANDLE
684
ce2984c3
PF
685=item chdir
686
ffce7b87 687Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 688changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 689changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
690variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
691neither is set, C<chdir> does nothing. It returns true upon success,
692false otherwise. See the example under C<die>.
a0d0e21e 693
c4aca7d0
GA
694On systems that support fchdir, you might pass a file handle or
695directory handle as argument. On systems that don't support fchdir,
696passing handles produces a fatal error at run time.
697
a0d0e21e 698=item chmod LIST
d74e8afc 699X<chmod> X<permission> X<mode>
a0d0e21e
LW
700
701Changes the permissions of a list of files. The first element of the
4633a7c4 702list must be the numerical mode, which should probably be an octal
4ad40acf 703number, and which definitely should I<not> be a string of octal digits:
2f9daede 704C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 705successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
706
707 $cnt = chmod 0755, 'foo', 'bar';
708 chmod 0755, @executables;
f86cebdf
GS
709 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
710 # --w----r-T
2f9daede
TPG
711 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
712 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 713
c4aca7d0
GA
714On systems that support fchmod, you might pass file handles among the
715files. On systems that don't support fchmod, passing file handles
345da378
GA
716produces a fatal error at run time. The file handles must be passed
717as globs or references to be recognized. Barewords are considered
718file names.
c4aca7d0
GA
719
720 open(my $fh, "<", "foo");
721 my $perm = (stat $fh)[2] & 07777;
722 chmod($perm | 0600, $fh);
723
ca6e1c26
JH
724You can also import the symbolic C<S_I*> constants from the Fcntl
725module:
726
727 use Fcntl ':mode';
728
729 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
730 # This is identical to the chmod 0755 of the above example.
731
a0d0e21e 732=item chomp VARIABLE
d74e8afc 733X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 734
313c9f5c 735=item chomp( LIST )
a0d0e21e
LW
736
737=item chomp
738
2b5ab1e7
TC
739This safer version of L</chop> removes any trailing string
740that corresponds to the current value of C<$/> (also known as
28757baa
PP
741$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
742number of characters removed from all its arguments. It's often used to
743remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
744that the final record may be missing its newline. When in paragraph
745mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
746When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
747a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 748remove anything.
19799a22 749If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
750
751 while (<>) {
752 chomp; # avoid \n on last field
753 @array = split(/:/);
5a964f20 754 # ...
a0d0e21e
LW
755 }
756
4bf21a6d
RD
757If VARIABLE is a hash, it chomps the hash's values, but not its keys.
758
a0d0e21e
LW
759You can actually chomp anything that's an lvalue, including an assignment:
760
761 chomp($cwd = `pwd`);
762 chomp($answer = <STDIN>);
763
764If you chomp a list, each element is chomped, and the total number of
765characters removed is returned.
766
15e44fd8
RGS
767Note that parentheses are necessary when you're chomping anything
768that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
769is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
770C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
771C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
772as C<chomp($a, $b)>.
773
a0d0e21e 774=item chop VARIABLE
d74e8afc 775X<chop>
a0d0e21e 776
313c9f5c 777=item chop( LIST )
a0d0e21e
LW
778
779=item chop
780
781Chops off the last character of a string and returns the character
5b3eff12 782chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 783scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
784If VARIABLE is a hash, it chops the hash's values, but not its keys.
785
5b3eff12 786You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
787
788If you chop a list, each element is chopped. Only the value of the
19799a22 789last C<chop> is returned.
a0d0e21e 790
19799a22 791Note that C<chop> returns the last character. To return all but the last
748a9306
LW
792character, use C<substr($string, 0, -1)>.
793
15e44fd8
RGS
794See also L</chomp>.
795
a0d0e21e 796=item chown LIST
d74e8afc 797X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
798
799Changes the owner (and group) of a list of files. The first two
19799a22
GS
800elements of the list must be the I<numeric> uid and gid, in that
801order. A value of -1 in either position is interpreted by most
802systems to leave that value unchanged. Returns the number of files
803successfully changed.
a0d0e21e
LW
804
805 $cnt = chown $uid, $gid, 'foo', 'bar';
806 chown $uid, $gid, @filenames;
807
c4aca7d0
GA
808On systems that support fchown, you might pass file handles among the
809files. On systems that don't support fchown, passing file handles
345da378
GA
810produces a fatal error at run time. The file handles must be passed
811as globs or references to be recognized. Barewords are considered
812file names.
c4aca7d0 813
54310121 814Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
815
816 print "User: ";
19799a22 817 chomp($user = <STDIN>);
5a964f20 818 print "Files: ";
19799a22 819 chomp($pattern = <STDIN>);
a0d0e21e
LW
820
821 ($login,$pass,$uid,$gid) = getpwnam($user)
822 or die "$user not in passwd file";
823
5a964f20 824 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
825 chown $uid, $gid, @ary;
826
54310121 827On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
828file unless you're the superuser, although you should be able to change
829the group to any of your secondary groups. On insecure systems, these
830restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
831On POSIX systems, you can detect this condition this way:
832
833 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
834 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 835
a0d0e21e 836=item chr NUMBER
d74e8afc 837X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 838
54310121 839=item chr
bbce6d69 840
a0d0e21e 841Returns the character represented by that NUMBER in the character set.
a0ed51b3 842For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 843chr(0x263a) is a Unicode smiley face.
aaa68c4a 844
8a064bd6 845Negative values give the Unicode replacement character (chr(0xfffd)),
5f0135eb 846except under the L<bytes> pragma, where low eight bits of the value
8a064bd6
JH
847(truncated to an integer) are used.
848
974da8e5
JH
849If NUMBER is omitted, uses C<$_>.
850
b76cc8ba 851For the reverse, use L</ord>.
a0d0e21e 852
2575c402
JW
853Note that characters from 128 to 255 (inclusive) are by default
854internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 855
2575c402 856See L<perlunicode> for more about Unicode.
bbce6d69 857
a0d0e21e 858=item chroot FILENAME
d74e8afc 859X<chroot> X<root>
a0d0e21e 860
54310121 861=item chroot
bbce6d69 862
5a964f20 863This function works like the system call by the same name: it makes the
4633a7c4 864named directory the new root directory for all further pathnames that
951ba7fe 865begin with a C</> by your process and all its children. (It doesn't
28757baa 866change your current working directory, which is unaffected.) For security
4633a7c4 867reasons, this call is restricted to the superuser. If FILENAME is
19799a22 868omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
869
870=item close FILEHANDLE
d74e8afc 871X<close>
a0d0e21e 872
6a518fbc
TP
873=item close
874
e0f13c26
RGS
875Closes the file or pipe associated with the file handle, flushes the IO
876buffers, and closes the system file descriptor. Returns true if those
877operations have succeeded and if no error was reported by any PerlIO
878layer. Closes the currently selected filehandle if the argument is
879omitted.
fb73857a
PP
880
881You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
882another C<open> on it, because C<open> will close it for you. (See
883C<open>.) However, an explicit C<close> on an input file resets the line
884counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 885
dede8123
RGS
886If the file handle came from a piped open, C<close> will additionally
887return false if one of the other system calls involved fails, or if the
fb73857a 888program exits with non-zero status. (If the only problem was that the
dede8123 889program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 890also waits for the process executing on the pipe to complete, in case you
b76cc8ba 891want to look at the output of the pipe afterwards, and
e5218da5
GA
892implicitly puts the exit status value of that command into C<$?> and
893C<${^CHILD_ERROR_NATIVE}>.
5a964f20 894
73689b13
GS
895Prematurely closing the read end of a pipe (i.e. before the process
896writing to it at the other end has closed it) will result in a
897SIGPIPE being delivered to the writer. If the other end can't
898handle that, be sure to read all the data before closing the pipe.
899
fb73857a 900Example:
a0d0e21e 901
fb73857a
PP
902 open(OUTPUT, '|sort >foo') # pipe to sort
903 or die "Can't start sort: $!";
5a964f20 904 #... # print stuff to output
fb73857a
PP
905 close OUTPUT # wait for sort to finish
906 or warn $! ? "Error closing sort pipe: $!"
907 : "Exit status $? from sort";
908 open(INPUT, 'foo') # get sort's results
909 or die "Can't open 'foo' for input: $!";
a0d0e21e 910
5a964f20
TC
911FILEHANDLE may be an expression whose value can be used as an indirect
912filehandle, usually the real filehandle name.
a0d0e21e
LW
913
914=item closedir DIRHANDLE
d74e8afc 915X<closedir>
a0d0e21e 916
19799a22 917Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
918system call.
919
a0d0e21e 920=item connect SOCKET,NAME
d74e8afc 921X<connect>
a0d0e21e
LW
922
923Attempts to connect to a remote socket, just as the connect system call
19799a22 924does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
925packed address of the appropriate type for the socket. See the examples in
926L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 927
cb1a09d0 928=item continue BLOCK
d74e8afc 929X<continue>
cb1a09d0 930
0d863452
RH
931=item continue
932
cf264981
SP
933C<continue> is actually a flow control statement rather than a function. If
934there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
935C<foreach>), it is always executed just before the conditional is about to
936be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
937it can be used to increment a loop variable, even when the loop has been
938continued via the C<next> statement (which is similar to the C C<continue>
939statement).
940
98293880 941C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
942block. C<last> and C<redo> will behave as if they had been executed within
943the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
944block, it may be more entertaining.
945
946 while (EXPR) {
947 ### redo always comes here
948 do_something;
949 } continue {
950 ### next always comes here
951 do_something_else;
952 # then back the top to re-check EXPR
953 }
954 ### last always comes here
955
956Omitting the C<continue> section is semantically equivalent to using an
19799a22 957empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
958to check the condition at the top of the loop.
959
0d863452
RH
960If the "switch" feature is enabled, C<continue> is also a
961function that will break out of the current C<when> or C<default>
962block, and fall through to the next case. See L<feature> and
963L<perlsyn/"Switch statements"> for more information.
964
965
a0d0e21e 966=item cos EXPR
d74e8afc 967X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 968
d6217f1e
GS
969=item cos
970
5a964f20 971Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 972takes cosine of C<$_>.
a0d0e21e 973
ca6e1c26 974For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
975function, or use this relation:
976
977 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
978
a0d0e21e 979=item crypt PLAINTEXT,SALT
d74e8afc 980X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 981X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 982
ef2e6798
MS
983Creates a digest string exactly like the crypt(3) function in the C
984library (assuming that you actually have a version there that has not
cf264981 985been extirpated as a potential munitions).
ef2e6798
MS
986
987crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
988into a short string, called a digest, which is returned. The same
989PLAINTEXT and SALT will always return the same string, but there is no
990(known) way to get the original PLAINTEXT from the hash. Small
991changes in the PLAINTEXT or SALT will result in large changes in the
992digest.
993
994There is no decrypt function. This function isn't all that useful for
995cryptography (for that, look for F<Crypt> modules on your nearby CPAN
996mirror) and the name "crypt" is a bit of a misnomer. Instead it is
997primarily used to check if two pieces of text are the same without
998having to transmit or store the text itself. An example is checking
999if a correct password is given. The digest of the password is stored,
cf264981 1000not the password itself. The user types in a password that is
ef2e6798
MS
1001crypt()'d with the same salt as the stored digest. If the two digests
1002match the password is correct.
1003
1004When verifying an existing digest string you should use the digest as
1005the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1006to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1007crypt() will hash the new string with the same salt as the digest.
1008This allows your code to work with the standard L<crypt|/crypt> and
1009with more exotic implementations. In other words, do not assume
1010anything about the returned string itself, or how many bytes in the
1011digest matter.
85c16d83
JH
1012
1013Traditionally the result is a string of 13 bytes: two first bytes of
1014the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
ef2e6798
MS
1015the first eight bytes of the digest string mattered, but alternative
1016hashing schemes (like MD5), higher level security schemes (like C2),
1017and implementations on non-UNIX platforms may produce different
1018strings.
85c16d83
JH
1019
1020When choosing a new salt create a random two character string whose
1021characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1022'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1023characters is just a recommendation; the characters allowed in
1024the salt depend solely on your system's crypt library, and Perl can't
1025restrict what salts C<crypt()> accepts.
e71965be 1026
a0d0e21e 1027Here's an example that makes sure that whoever runs this program knows
cf264981 1028their password:
a0d0e21e
LW
1029
1030 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1031
1032 system "stty -echo";
1033 print "Password: ";
e71965be 1034 chomp($word = <STDIN>);
a0d0e21e
LW
1035 print "\n";
1036 system "stty echo";
1037
e71965be 1038 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
1039 die "Sorry...\n";
1040 } else {
1041 print "ok\n";
54310121 1042 }
a0d0e21e 1043
9f8f0c9d 1044Of course, typing in your own password to whoever asks you
748a9306 1045for it is unwise.
a0d0e21e 1046
ef2e6798 1047The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1048of data, not least of all because you can't get the information
ef2e6798 1049back. Look at the L<Digest> module for more robust algorithms.
19799a22 1050
f2791508
JH
1051If using crypt() on a Unicode string (which I<potentially> has
1052characters with codepoints above 255), Perl tries to make sense
1053of the situation by trying to downgrade (a copy of the string)
1054the string back to an eight-bit byte string before calling crypt()
1055(on that copy). If that works, good. If not, crypt() dies with
1056C<Wide character in crypt>.
85c16d83 1057
aa689395 1058=item dbmclose HASH
d74e8afc 1059X<dbmclose>
a0d0e21e 1060
19799a22 1061[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1062
aa689395 1063Breaks the binding between a DBM file and a hash.
a0d0e21e 1064
19799a22 1065=item dbmopen HASH,DBNAME,MASK
d74e8afc 1066X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1067
19799a22 1068[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1069
7b8d334a 1070This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1071hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1072argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1073is the name of the database (without the F<.dir> or F<.pag> extension if
1074any). If the database does not exist, it is created with protection
19799a22
GS
1075specified by MASK (as modified by the C<umask>). If your system supports
1076only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 1077program. In older versions of Perl, if your system had neither DBM nor
19799a22 1078ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1079sdbm(3).
1080
1081If you don't have write access to the DBM file, you can only read hash
1082variables, not set them. If you want to test whether you can write,
19799a22 1083either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 1084which will trap the error.
a0d0e21e 1085
19799a22
GS
1086Note that functions such as C<keys> and C<values> may return huge lists
1087when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1088function to iterate over large DBM files. Example:
1089
1090 # print out history file offsets
1091 dbmopen(%HIST,'/usr/lib/news/history',0666);
1092 while (($key,$val) = each %HIST) {
1093 print $key, ' = ', unpack('L',$val), "\n";
1094 }
1095 dbmclose(%HIST);
1096
cb1a09d0 1097See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1098cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1099rich implementation.
4633a7c4 1100
2b5ab1e7
TC
1101You can control which DBM library you use by loading that library
1102before you call dbmopen():
1103
1104 use DB_File;
1105 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1106 or die "Can't open netscape history file: $!";
1107
a0d0e21e 1108=item defined EXPR
d74e8afc 1109X<defined> X<undef> X<undefined>
a0d0e21e 1110
54310121 1111=item defined
bbce6d69 1112
2f9daede
TPG
1113Returns a Boolean value telling whether EXPR has a value other than
1114the undefined value C<undef>. If EXPR is not present, C<$_> will be
1115checked.
1116
1117Many operations return C<undef> to indicate failure, end of file,
1118system error, uninitialized variable, and other exceptional
1119conditions. This function allows you to distinguish C<undef> from
1120other values. (A simple Boolean test will not distinguish among
7660c0ab 1121C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1122false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1123doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1124returns C<undef> when its argument is an empty array, I<or> when the
1125element to return happens to be C<undef>.
1126
f10b0346
GS
1127You may also use C<defined(&func)> to check whether subroutine C<&func>
1128has ever been defined. The return value is unaffected by any forward
04891299 1129declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1130may still be callable: its package may have an C<AUTOLOAD> method that
1131makes it spring into existence the first time that it is called -- see
1132L<perlsub>.
f10b0346
GS
1133
1134Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1135used to report whether memory for that aggregate has ever been
1136allocated. This behavior may disappear in future versions of Perl.
1137You should instead use a simple test for size:
1138
1139 if (@an_array) { print "has array elements\n" }
1140 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1141
1142When used on a hash element, it tells you whether the value is defined,
dc848c6f 1143not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1144purpose.
a0d0e21e
LW
1145
1146Examples:
1147
1148 print if defined $switch{'D'};
1149 print "$val\n" while defined($val = pop(@ary));
1150 die "Can't readlink $sym: $!"
1151 unless defined($value = readlink $sym);
a0d0e21e 1152 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1153 $debugging = 0 unless defined $debugging;
a0d0e21e 1154
19799a22 1155Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1156discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1157defined values. For example, if you say
a5f75d66
AD
1158
1159 "ab" =~ /a(.*)b/;
1160
7660c0ab 1161The pattern match succeeds, and C<$1> is defined, despite the fact that it
cf264981 1162matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1163matched something that happened to be zero characters long. This is all
a5f75d66 1164very above-board and honest. When a function returns an undefined value,
2f9daede 1165it's an admission that it couldn't give you an honest answer. So you
19799a22 1166should use C<defined> only when you're questioning the integrity of what
7660c0ab 1167you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1168what you want.
1169
dc848c6f 1170See also L</undef>, L</exists>, L</ref>.
2f9daede 1171
a0d0e21e 1172=item delete EXPR
d74e8afc 1173X<delete>
a0d0e21e 1174
01020589
GS
1175Given an expression that specifies a hash element, array element, hash slice,
1176or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1177In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1178the size of the array will shrink to the highest element that tests
8216c1fd 1179true for exists() (or 0 if no such element exists).
a0d0e21e 1180
eba0920a
EM
1181Returns a list with the same number of elements as the number of elements
1182for which deletion was attempted. Each element of that list consists of
1183either the value of the element deleted, or the undefined value. In scalar
1184context, this means that you get the value of the last element deleted (or
1185the undefined value if that element did not exist).
1186
1187 %hash = (foo => 11, bar => 22, baz => 33);
1188 $scalar = delete $hash{foo}; # $scalar is 11
1189 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1190 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1191
1192Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1193a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1194from a C<tie>d hash or array may not necessarily return anything.
1195
8ea97a1e
GS
1196Deleting an array element effectively returns that position of the array
1197to its initial, uninitialized state. Subsequently testing for the same
cf264981
SP
1198element with exists() will return false. Also, deleting array elements
1199in the middle of an array will not shift the index of the elements
1200after them down. Use splice() for that. See L</exists>.
8ea97a1e 1201
01020589 1202The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1203
5f05dabc
PP
1204 foreach $key (keys %HASH) {
1205 delete $HASH{$key};
a0d0e21e
LW
1206 }
1207
01020589
GS
1208 foreach $index (0 .. $#ARRAY) {
1209 delete $ARRAY[$index];
1210 }
1211
1212And so do these:
5f05dabc 1213
01020589
GS
1214 delete @HASH{keys %HASH};
1215
9740c838 1216 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1217
2b5ab1e7 1218But both of these are slower than just assigning the empty list
01020589
GS
1219or undefining %HASH or @ARRAY:
1220
1221 %HASH = (); # completely empty %HASH
1222 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1223
01020589
GS
1224 @ARRAY = (); # completely empty @ARRAY
1225 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1226
1227Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1228operation is a hash element, array element, hash slice, or array slice
1229lookup:
a0d0e21e
LW
1230
1231 delete $ref->[$x][$y]{$key};
5f05dabc 1232 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1233
01020589
GS
1234 delete $ref->[$x][$y][$index];
1235 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1236
a0d0e21e 1237=item die LIST
d74e8afc 1238X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1239
19799a22
GS
1240Outside an C<eval>, prints the value of LIST to C<STDERR> and
1241exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1242exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1243status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1244an C<eval(),> the error message is stuffed into C<$@> and the
1245C<eval> is terminated with the undefined value. This makes
1246C<die> the way to raise an exception.
a0d0e21e
LW
1247
1248Equivalent examples:
1249
1250 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1251 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1252
ccac6780 1253If the last element of LIST does not end in a newline, the current
df37ec69
WW
1254script line number and input line number (if any) are also printed,
1255and a newline is supplied. Note that the "input line number" (also
1256known as "chunk") is subject to whatever notion of "line" happens to
1257be currently in effect, and is also available as the special variable
1258C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1259
1260Hint: sometimes appending C<", stopped"> to your message will cause it
1261to make better sense when the string C<"at foo line 123"> is appended.
1262Suppose you are running script "canasta".
a0d0e21e
LW
1263
1264 die "/etc/games is no good";
1265 die "/etc/games is no good, stopped";
1266
1267produce, respectively
1268
1269 /etc/games is no good at canasta line 123.
1270 /etc/games is no good, stopped at canasta line 123.
1271
2b5ab1e7 1272See also exit(), warn(), and the Carp module.
a0d0e21e 1273
7660c0ab
A
1274If LIST is empty and C<$@> already contains a value (typically from a
1275previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1276This is useful for propagating exceptions:
1277
1278 eval { ... };
1279 die unless $@ =~ /Expected exception/;
1280
ad216e65
JH
1281If LIST is empty and C<$@> contains an object reference that has a
1282C<PROPAGATE> method, that method will be called with additional file
1283and line number parameters. The return value replaces the value in
28a5cf3b 1284C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1285were called.
1286
7660c0ab 1287If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1288
52531d10
GS
1289die() can also be called with a reference argument. If this happens to be
1290trapped within an eval(), $@ contains the reference. This behavior permits
1291a more elaborate exception handling implementation using objects that
4375e838 1292maintain arbitrary state about the nature of the exception. Such a scheme
52531d10 1293is sometimes preferable to matching particular string values of $@ using
746d7dd7
GL
1294regular expressions. Because $@ is a global variable, and eval() may be
1295used within object implementations, care must be taken that analyzing the
1296error object doesn't replace the reference in the global variable. The
1297easiest solution is to make a local copy of the reference before doing
1298other manipulations. Here's an example:
52531d10 1299
da279afe 1300 use Scalar::Util 'blessed';
1301
52531d10 1302 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1303 if (my $ev_err = $@) {
1304 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1305 # handle Some::Module::Exception
1306 }
1307 else {
1308 # handle all other possible exceptions
1309 }
1310 }
1311
19799a22 1312Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1313them, you may want to overload stringification operations on such custom
1314exception objects. See L<overload> for details about that.
1315
19799a22
GS
1316You can arrange for a callback to be run just before the C<die>
1317does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1318handler will be called with the error text and can change the error
1319message, if it sees fit, by calling C<die> again. See
1320L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1321L<"eval BLOCK"> for some examples. Although this feature was
19799a22
GS
1322to be run only right before your program was to exit, this is not
1323currently the case--the C<$SIG{__DIE__}> hook is currently called
1324even inside eval()ed blocks/strings! If one wants the hook to do
1325nothing in such situations, put
fb73857a
PP
1326
1327 die @_ if $^S;
1328
19799a22
GS
1329as the first line of the handler (see L<perlvar/$^S>). Because
1330this promotes strange action at a distance, this counterintuitive
b76cc8ba 1331behavior may be fixed in a future release.
774d564b 1332
a0d0e21e 1333=item do BLOCK
d74e8afc 1334X<do> X<block>
a0d0e21e
LW
1335
1336Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1337sequence of commands indicated by BLOCK. When modified by the C<while> or
1338C<until> loop modifier, executes the BLOCK once before testing the loop
1339condition. (On other statements the loop modifiers test the conditional
1340first.)
a0d0e21e 1341
4968c1e4 1342C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1343C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1344See L<perlsyn> for alternative strategies.
4968c1e4 1345
a0d0e21e 1346=item do SUBROUTINE(LIST)
d74e8afc 1347X<do>
a0d0e21e 1348
cf264981 1349This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1350
1351=item do EXPR
d74e8afc 1352X<do>
a0d0e21e
LW
1353
1354Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1355file as a Perl script.
a0d0e21e
LW
1356
1357 do 'stat.pl';
1358
1359is just like
1360
986b19de 1361 eval `cat stat.pl`;
a0d0e21e 1362
2b5ab1e7 1363except that it's more efficient and concise, keeps track of the current
ea63ef19 1364filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1365C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1366variables. It also differs in that code evaluated with C<do FILENAME>
1367cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1368same, however, in that it does reparse the file every time you call it,
1369so you probably don't want to do this inside a loop.
a0d0e21e 1370
8e30cc93 1371If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1372error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1373returns undef and sets an error message in C<$@>. If the file is
1374successfully compiled, C<do> returns the value of the last expression
1375evaluated.
1376
a0d0e21e 1377Note that inclusion of library modules is better done with the
19799a22 1378C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1379and raise an exception if there's a problem.
a0d0e21e 1380
5a964f20
TC
1381You might like to use C<do> to read in a program configuration
1382file. Manual error checking can be done this way:
1383
b76cc8ba 1384 # read in config files: system first, then user
f86cebdf 1385 for $file ("/share/prog/defaults.rc",
b76cc8ba 1386 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1387 {
5a964f20 1388 unless ($return = do $file) {
f86cebdf
GS
1389 warn "couldn't parse $file: $@" if $@;
1390 warn "couldn't do $file: $!" unless defined $return;
1391 warn "couldn't run $file" unless $return;
5a964f20
TC
1392 }
1393 }
1394
a0d0e21e 1395=item dump LABEL
d74e8afc 1396X<dump> X<core> X<undump>
a0d0e21e 1397
1614b0e3
JD
1398=item dump
1399
19799a22
GS
1400This function causes an immediate core dump. See also the B<-u>
1401command-line switch in L<perlrun>, which does the same thing.
1402Primarily this is so that you can use the B<undump> program (not
1403supplied) to turn your core dump into an executable binary after
1404having initialized all your variables at the beginning of the
1405program. When the new binary is executed it will begin by executing
1406a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1407Think of it as a goto with an intervening core dump and reincarnation.
1408If C<LABEL> is omitted, restarts the program from the top.
1409
1410B<WARNING>: Any files opened at the time of the dump will I<not>
1411be open any more when the program is reincarnated, with possible
b76cc8ba 1412resulting confusion on the part of Perl.
19799a22 1413
59f521f4
RGS
1414This function is now largely obsolete, mostly because it's very hard to
1415convert a core file into an executable. That's why you should now invoke
1416it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1417typo.
19799a22 1418
aa689395 1419=item each HASH
d74e8afc 1420X<each> X<hash, iterator>
aa689395 1421
aeedbbed
NC
1422=item each ARRAY
1423X<array, iterator>
1424
5a964f20 1425When called in list context, returns a 2-element list consisting of the
aeedbbed
NC
1426key and value for the next element of a hash, or the index and value for
1427the next element of an array, so that you can iterate over it. When called
1428in scalar context, returns only the key for the next element in the hash
1429(or the index for an array).
2f9daede 1430
aeedbbed 1431Hash entries are returned in an apparently random order. The actual random
504f80c1
JH
1432order is subject to change in future versions of perl, but it is
1433guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1434function would produce on the same (unmodified) hash. Since Perl
22883ac5 14355.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1436for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1437
aeedbbed
NC
1438When the hash or array is entirely read, a null array is returned in list
1439context (which when assigned produces a false (C<0>) value), and C<undef> in
19799a22 1440scalar context. The next call to C<each> after that will start iterating
aeedbbed
NC
1441again. There is a single iterator for each hash or array, shared by all
1442C<each>, C<keys>, and C<values> function calls in the program; it can be
1443reset by reading all the elements from the hash or array, or by evaluating
1444C<keys HASH>, C<values HASH>, C<keys ARRAY>, or C<values ARRAY>. If you add
1445or delete elements of a hash while you're
74fc8b5f
MJD
1446iterating over it, you may get entries skipped or duplicated, so
1447don't. Exception: It is always safe to delete the item most recently
1448returned by C<each()>, which means that the following code will work:
1449
1450 while (($key, $value) = each %hash) {
1451 print $key, "\n";
1452 delete $hash{$key}; # This is safe
1453 }
aa689395 1454
f86cebdf 1455The following prints out your environment like the printenv(1) program,
aa689395 1456only in a different order:
a0d0e21e
LW
1457
1458 while (($key,$value) = each %ENV) {
1459 print "$key=$value\n";
1460 }
1461
19799a22 1462See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1463
1464=item eof FILEHANDLE
d74e8afc
ITB
1465X<eof>
1466X<end of file>
1467X<end-of-file>
a0d0e21e 1468
4633a7c4
LW
1469=item eof ()
1470
a0d0e21e
LW
1471=item eof
1472
1473Returns 1 if the next read on FILEHANDLE will return end of file, or if
1474FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1475gives the real filehandle. (Note that this function actually
19799a22 1476reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1477interactive context.) Do not read from a terminal file (or call
19799a22 1478C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1479as terminals may lose the end-of-file condition if you do.
1480
820475bd
GS
1481An C<eof> without an argument uses the last file read. Using C<eof()>
1482with empty parentheses is very different. It refers to the pseudo file
1483formed from the files listed on the command line and accessed via the
61eff3bc
JH
1484C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1485as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1486used will cause C<@ARGV> to be examined to determine if input is
67408cae 1487available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1488end-of-file will assume you are processing another C<@ARGV> list,
1489and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1490see L<perlop/"I/O Operators">.
820475bd 1491
61eff3bc 1492In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1493detect the end of each file, C<eof()> will only detect the end of the
1494last file. Examples:
a0d0e21e 1495
748a9306
LW
1496 # reset line numbering on each input file
1497 while (<>) {
b76cc8ba 1498 next if /^\s*#/; # skip comments
748a9306 1499 print "$.\t$_";
5a964f20
TC
1500 } continue {
1501 close ARGV if eof; # Not eof()!
748a9306
LW
1502 }
1503
a0d0e21e
LW
1504 # insert dashes just before last line of last file
1505 while (<>) {
6ac88b13 1506 if (eof()) { # check for end of last file
a0d0e21e
LW
1507 print "--------------\n";
1508 }
1509 print;
6ac88b13 1510 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1511 }
1512
a0d0e21e 1513Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1514input operators typically return C<undef> when they run out of data, or if
1515there was an error.
a0d0e21e
LW
1516
1517=item eval EXPR
d74e8afc 1518X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1519X<error, handling> X<exception, handling>
a0d0e21e
LW
1520
1521=item eval BLOCK
1522
ce2984c3
PF
1523=item eval
1524
c7cc6f1c
GS
1525In the first form, the return value of EXPR is parsed and executed as if it
1526were a little Perl program. The value of the expression (which is itself
5a964f20 1527determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1528errors, executed in the lexical context of the current Perl program, so
1529that any variable settings or subroutine and format definitions remain
cf264981 1530afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1531If EXPR is omitted, evaluates C<$_>. This form is typically used to
1532delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1533
1534In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1535same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1536within the context of the current Perl program. This form is typically
1537used to trap exceptions more efficiently than the first (see below), while
1538also providing the benefit of checking the code within BLOCK at compile
1539time.
1540
1541The final semicolon, if any, may be omitted from the value of EXPR or within
1542the BLOCK.
1543
1544In both forms, the value returned is the value of the last expression
5a964f20 1545evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1546as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1547in void, scalar, or list context, depending on the context of the C<eval>
1548itself. See L</wantarray> for more on how the evaluation context can be
1549determined.
a0d0e21e 1550
19799a22
GS
1551If there is a syntax error or runtime error, or a C<die> statement is
1552executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1553error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1554string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1555warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1556To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1557turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1558See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1559
19799a22
GS
1560Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1561determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1562is implemented. It is also Perl's exception trapping mechanism, where
1563the die operator is used to raise exceptions.
1564
5f1da31c
NT
1565If you want to trap errors when loading an XS module, some problems with
1566the binary interface (such as Perl version skew) may be fatal even with
1567C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1568
a0d0e21e
LW
1569If the code to be executed doesn't vary, you may use the eval-BLOCK
1570form to trap run-time errors without incurring the penalty of
1571recompiling each time. The error, if any, is still returned in C<$@>.
1572Examples:
1573
54310121 1574 # make divide-by-zero nonfatal
a0d0e21e
LW
1575 eval { $answer = $a / $b; }; warn $@ if $@;
1576
1577 # same thing, but less efficient
1578 eval '$answer = $a / $b'; warn $@ if $@;
1579
1580 # a compile-time error
5a964f20 1581 eval { $answer = }; # WRONG
a0d0e21e
LW
1582
1583 # a run-time error
1584 eval '$answer ='; # sets $@
1585
cf264981
SP
1586Using the C<eval{}> form as an exception trap in libraries does have some
1587issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1588may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7
TC
1589You can use the C<local $SIG{__DIE__}> construct for this purpose,
1590as shown in this example:
774d564b
PP
1591
1592 # a very private exception trap for divide-by-zero
f86cebdf
GS
1593 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1594 warn $@ if $@;
774d564b
PP
1595
1596This is especially significant, given that C<__DIE__> hooks can call
19799a22 1597C<die> again, which has the effect of changing their error messages:
774d564b
PP
1598
1599 # __DIE__ hooks may modify error messages
1600 {
f86cebdf
GS
1601 local $SIG{'__DIE__'} =
1602 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1603 eval { die "foo lives here" };
1604 print $@ if $@; # prints "bar lives here"
774d564b
PP
1605 }
1606
19799a22 1607Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1608may be fixed in a future release.
1609
19799a22 1610With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1611being looked at when:
1612
1613 eval $x; # CASE 1
1614 eval "$x"; # CASE 2
1615
1616 eval '$x'; # CASE 3
1617 eval { $x }; # CASE 4
1618
5a964f20 1619 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1620 $$x++; # CASE 6
1621
2f9daede 1622Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1623the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1624the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1625and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1626does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1627purely visual reasons, but it also has the advantage of compiling at
1628compile-time instead of at run-time.) Case 5 is a place where
19799a22 1629normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1630particular situation, you can just use symbolic references instead, as
1631in case 6.
a0d0e21e 1632
8a5a710d
DN
1633The assignment to C<$@> occurs before restoration of localised variables,
1634which means a temporary is required if you want to mask some but not all
1635errors:
1636
1637 # alter $@ on nefarious repugnancy only
1638 {
1639 my $e;
1640 {
1641 local $@; # protect existing $@
1642 eval { test_repugnancy() };
1643 # $@ =~ /nefarious/ and die $@; # DOES NOT WORK
1644 $@ =~ /nefarious/ and $e = $@;
1645 }
1646 die $e if defined $e
1647 }
1648
4968c1e4 1649C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1650C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1651
d819b83a
DM
1652Note that as a very special case, an C<eval ''> executed within the C<DB>
1653package doesn't see the usual surrounding lexical scope, but rather the
1654scope of the first non-DB piece of code that called it. You don't normally
1655need to worry about this unless you are writing a Perl debugger.
1656
a0d0e21e 1657=item exec LIST
d74e8afc 1658X<exec> X<execute>
a0d0e21e 1659
8bf3b016
GS
1660=item exec PROGRAM LIST
1661
19799a22
GS
1662The C<exec> function executes a system command I<and never returns>--
1663use C<system> instead of C<exec> if you want it to return. It fails and
1664returns false only if the command does not exist I<and> it is executed
fb73857a 1665directly instead of via your system's command shell (see below).
a0d0e21e 1666
19799a22
GS
1667Since it's a common mistake to use C<exec> instead of C<system>, Perl
1668warns you if there is a following statement which isn't C<die>, C<warn>,
1669or C<exit> (if C<-w> is set - but you always do that). If you
1670I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1671can use one of these styles to avoid the warning:
1672
5a964f20
TC
1673 exec ('foo') or print STDERR "couldn't exec foo: $!";
1674 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1675
5a964f20 1676If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1677with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1678If there is only one scalar argument or an array with one element in it,
1679the argument is checked for shell metacharacters, and if there are any,
1680the entire argument is passed to the system's command shell for parsing
1681(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1682If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1683words and passed directly to C<execvp>, which is more efficient.
19799a22 1684Examples:
a0d0e21e 1685
19799a22
GS
1686 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1687 exec "sort $outfile | uniq";
a0d0e21e
LW
1688
1689If you don't really want to execute the first argument, but want to lie
1690to the program you are executing about its own name, you can specify
1691the program you actually want to run as an "indirect object" (without a
1692comma) in front of the LIST. (This always forces interpretation of the
54310121 1693LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1694the list.) Example:
1695
1696 $shell = '/bin/csh';
1697 exec $shell '-sh'; # pretend it's a login shell
1698
1699or, more directly,
1700
1701 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1702
bb32b41a
GS
1703When the arguments get executed via the system shell, results will
1704be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1705for details.
1706
19799a22
GS
1707Using an indirect object with C<exec> or C<system> is also more
1708secure. This usage (which also works fine with system()) forces
1709interpretation of the arguments as a multivalued list, even if the
1710list had just one argument. That way you're safe from the shell
1711expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1712
1713 @args = ( "echo surprise" );
1714
2b5ab1e7 1715 exec @args; # subject to shell escapes
f86cebdf 1716 # if @args == 1
2b5ab1e7 1717 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1718
1719The first version, the one without the indirect object, ran the I<echo>
1720program, passing it C<"surprise"> an argument. The second version
1721didn't--it tried to run a program literally called I<"echo surprise">,
1722didn't find it, and set C<$?> to a non-zero value indicating failure.
1723
0f897271
GS
1724Beginning with v5.6.0, Perl will attempt to flush all files opened for
1725output before the exec, but this may not be supported on some platforms
1726(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1727in English) or call the C<autoflush()> method of C<IO::Handle> on any
1728open handles in order to avoid lost output.
1729
19799a22 1730Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1731any C<DESTROY> methods in your objects.
1732
a0d0e21e 1733=item exists EXPR
d74e8afc 1734X<exists> X<autovivification>
a0d0e21e 1735
01020589 1736Given an expression that specifies a hash element or array element,
8ea97a1e 1737returns true if the specified element in the hash or array has ever
95731d22 1738been initialized, even if the corresponding value is undefined.
a0d0e21e 1739
01020589
GS
1740 print "Exists\n" if exists $hash{$key};
1741 print "Defined\n" if defined $hash{$key};
1742 print "True\n" if $hash{$key};
1743
1744 print "Exists\n" if exists $array[$index];
1745 print "Defined\n" if defined $array[$index];
1746 print "True\n" if $array[$index];
a0d0e21e 1747
8ea97a1e 1748A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1749it exists, but the reverse doesn't necessarily hold true.
1750
afebc493
GS
1751Given an expression that specifies the name of a subroutine,
1752returns true if the specified subroutine has ever been declared, even
1753if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1754does not count as declaring it. Note that a subroutine which does not
1755exist may still be callable: its package may have an C<AUTOLOAD>
1756method that makes it spring into existence the first time that it is
1757called -- see L<perlsub>.
afebc493
GS
1758
1759 print "Exists\n" if exists &subroutine;
1760 print "Defined\n" if defined &subroutine;
1761
a0d0e21e 1762Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1763operation is a hash or array key lookup or subroutine name:
a0d0e21e 1764
2b5ab1e7
TC
1765 if (exists $ref->{A}->{B}->{$key}) { }
1766 if (exists $hash{A}{B}{$key}) { }
1767
01020589
GS
1768 if (exists $ref->{A}->{B}->[$ix]) { }
1769 if (exists $hash{A}{B}[$ix]) { }
1770
afebc493
GS
1771 if (exists &{$ref->{A}{B}{$key}}) { }
1772
01020589
GS
1773Although the deepest nested array or hash will not spring into existence
1774just because its existence was tested, any intervening ones will.
61eff3bc 1775Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1776into existence due to the existence test for the $key element above.
1777This happens anywhere the arrow operator is used, including even:
5a964f20 1778
2b5ab1e7
TC
1779 undef $ref;
1780 if (exists $ref->{"Some key"}) { }
1781 print $ref; # prints HASH(0x80d3d5c)
1782
1783This surprising autovivification in what does not at first--or even
1784second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1785release.
a0d0e21e 1786
afebc493
GS
1787Use of a subroutine call, rather than a subroutine name, as an argument
1788to exists() is an error.
1789
1790 exists &sub; # OK
1791 exists &sub(); # Error
1792
a0d0e21e 1793=item exit EXPR
d74e8afc 1794X<exit> X<terminate> X<abort>
a0d0e21e 1795
ce2984c3
PF
1796=item exit
1797
2b5ab1e7 1798Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1799
1800 $ans = <STDIN>;
1801 exit 0 if $ans =~ /^[Xx]/;
1802
19799a22 1803See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1804universally recognized values for EXPR are C<0> for success and C<1>
1805for error; other values are subject to interpretation depending on the
1806environment in which the Perl program is running. For example, exiting
180769 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1808the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1809
19799a22
GS
1810Don't use C<exit> to abort a subroutine if there's any chance that
1811someone might want to trap whatever error happened. Use C<die> instead,
1812which can be trapped by an C<eval>.
28757baa 1813
19799a22 1814The exit() function does not always exit immediately. It calls any
2b5ab1e7 1815defined C<END> routines first, but these C<END> routines may not
19799a22 1816themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1817be called are called before the real exit. If this is a problem, you
1818can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1819See L<perlmod> for details.
5a964f20 1820
a0d0e21e 1821=item exp EXPR
d74e8afc 1822X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1823
54310121 1824=item exp
bbce6d69 1825
b76cc8ba 1826Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1827If EXPR is omitted, gives C<exp($_)>.
1828
1829=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1830X<fcntl>
a0d0e21e 1831
f86cebdf 1832Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1833
1834 use Fcntl;
1835
0ade1984 1836first to get the correct constant definitions. Argument processing and
b76cc8ba 1837value return works just like C<ioctl> below.
a0d0e21e
LW
1838For example:
1839
1840 use Fcntl;
5a964f20
TC
1841 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1842 or die "can't fcntl F_GETFL: $!";
1843
554ad1fc 1844You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1845Like C<ioctl>, it maps a C<0> return from the system call into
1846C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1847in numeric context. It is also exempt from the normal B<-w> warnings
1848on improper numeric conversions.
5a964f20 1849
19799a22 1850Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1851doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1852manpage to learn what functions are available on your system.
a0d0e21e 1853
be2f7487 1854Here's an example of setting a filehandle named C<REMOTE> to be
1855non-blocking at the system level. You'll have to negotiate C<$|>
1856on your own, though.
1857
1858 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1859
1860 $flags = fcntl(REMOTE, F_GETFL, 0)
1861 or die "Can't get flags for the socket: $!\n";
1862
1863 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1864 or die "Can't set flags for the socket: $!\n";
1865
a0d0e21e 1866=item fileno FILEHANDLE
d74e8afc 1867X<fileno>
a0d0e21e 1868
2b5ab1e7
TC
1869Returns the file descriptor for a filehandle, or undefined if the
1870filehandle is not open. This is mainly useful for constructing
19799a22 1871bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1872If FILEHANDLE is an expression, the value is taken as an indirect
1873filehandle, generally its name.
5a964f20 1874
b76cc8ba 1875You can use this to find out whether two handles refer to the
5a964f20
TC
1876same underlying descriptor:
1877
1878 if (fileno(THIS) == fileno(THAT)) {
1879 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1880 }
1881
1882(Filehandles connected to memory objects via new features of C<open> may
1883return undefined even though they are open.)
1884
a0d0e21e
LW
1885
1886=item flock FILEHANDLE,OPERATION
d74e8afc 1887X<flock> X<lock> X<locking>
a0d0e21e 1888
19799a22
GS
1889Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1890for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1891machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1892C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1893only entire files, not records.
1894
1895Two potentially non-obvious but traditional C<flock> semantics are
1896that it waits indefinitely until the lock is granted, and that its locks
1897B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1898fewer guarantees. This means that programs that do not also use C<flock>
1899may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1900your port's specific documentation, or your system-specific local manpages
1901for details. It's best to assume traditional behavior if you're writing
1902portable programs. (But if you're not, you should as always feel perfectly
1903free to write for your own system's idiosyncrasies (sometimes called
1904"features"). Slavish adherence to portability concerns shouldn't get
1905in the way of your getting your job done.)
a3cb178b 1906
8ebc5c01
PP
1907OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1908LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1909you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1910either individually, or as a group using the ':flock' tag. LOCK_SH
1911requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1912releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1913LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1914waiting for the lock (check the return status to see if you got it).
1915
2b5ab1e7
TC
1916To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1917before locking or unlocking it.
8ebc5c01 1918
f86cebdf 1919Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1920locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1921are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1922implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1923differing semantics shouldn't bite too many people.
1924
becacb53
TM
1925Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1926be open with read intent to use LOCK_SH and requires that it be open
1927with write intent to use LOCK_EX.
1928
19799a22
GS
1929Note also that some versions of C<flock> cannot lock things over the
1930network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1931that. If you like you can force Perl to ignore your system's flock(2)
1932function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1933the switch C<-Ud_flock> to the F<Configure> program when you configure
1934perl.
4633a7c4
LW
1935
1936Here's a mailbox appender for BSD systems.
a0d0e21e 1937
7e1af8bc 1938 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1939
1940 sub lock {
7e1af8bc 1941 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1942 # and, in case someone appended
1943 # while we were waiting...
1944 seek(MBOX, 0, 2);
1945 }
1946
1947 sub unlock {
7e1af8bc 1948 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1949 }
1950
b0169937 1951 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
a0d0e21e
LW
1952 or die "Can't open mailbox: $!";
1953
1954 lock();
b0169937 1955 print $mbox $msg,"\n\n";
a0d0e21e
LW
1956 unlock();
1957
2b5ab1e7
TC
1958On systems that support a real flock(), locks are inherited across fork()
1959calls, whereas those that must resort to the more capricious fcntl()
1960function lose the locks, making it harder to write servers.
1961
cb1a09d0 1962See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1963
1964=item fork
d74e8afc 1965X<fork> X<child> X<parent>
a0d0e21e 1966
2b5ab1e7
TC
1967Does a fork(2) system call to create a new process running the
1968same program at the same point. It returns the child pid to the
1969parent process, C<0> to the child process, or C<undef> if the fork is
1970unsuccessful. File descriptors (and sometimes locks on those descriptors)
1971are shared, while everything else is copied. On most systems supporting
1972fork(), great care has gone into making it extremely efficient (for
1973example, using copy-on-write technology on data pages), making it the
1974dominant paradigm for multitasking over the last few decades.
5a964f20 1975
0f897271
GS
1976Beginning with v5.6.0, Perl will attempt to flush all files opened for
1977output before forking the child process, but this may not be supported
1978on some platforms (see L<perlport>). To be safe, you may need to set
1979C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1980C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1981
19799a22 1982If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1983accumulate zombies. On some systems, you can avoid this by setting
1984C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1985forking and reaping moribund children.
cb1a09d0 1986
28757baa
PP
1987Note that if your forked child inherits system file descriptors like
1988STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1989if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1990backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1991You should reopen those to F</dev/null> if it's any issue.
28757baa 1992
cb1a09d0 1993=item format
d74e8afc 1994X<format>
cb1a09d0 1995
19799a22 1996Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1997example:
1998
54310121 1999 format Something =
cb1a09d0
AD
2000 Test: @<<<<<<<< @||||| @>>>>>
2001 $str, $%, '$' . int($num)
2002 .
2003
2004 $str = "widget";
184e9718 2005 $num = $cost/$quantity;
cb1a09d0
AD
2006 $~ = 'Something';
2007 write;
2008
2009See L<perlform> for many details and examples.
2010
8903cb82 2011=item formline PICTURE,LIST
d74e8afc 2012X<formline>
a0d0e21e 2013
5a964f20 2014This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2015too. It formats (see L<perlform>) a list of values according to the
2016contents of PICTURE, placing the output into the format output
7660c0ab 2017accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2018Eventually, when a C<write> is done, the contents of
cf264981
SP
2019C<$^A> are written to some filehandle. You could also read C<$^A>
2020and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2021does one C<formline> per line of form, but the C<formline> function itself
748a9306 2022doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 2023that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
2024You may therefore need to use multiple formlines to implement a single
2025record format, just like the format compiler.
2026
19799a22 2027Be careful if you put double quotes around the picture, because an C<@>
748a9306 2028character may be taken to mean the beginning of an array name.
19799a22 2029C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
2030
2031=item getc FILEHANDLE
f723aae1 2032X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2033
2034=item getc
2035
2036Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
2037or the undefined value at end of file, or if there was an error (in
2038the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2039STDIN. This is not particularly efficient. However, it cannot be
2040used by itself to fetch single characters without waiting for the user
2041to hit enter. For that, try something more like:
4633a7c4
LW
2042
2043 if ($BSD_STYLE) {
2044 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2045 }
2046 else {
54310121 2047 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2048 }
2049
2050 $key = getc(STDIN);
2051
2052 if ($BSD_STYLE) {
2053 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2054 }
2055 else {
5f05dabc 2056 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
2057 }
2058 print "\n";
2059
54310121
PP
2060Determination of whether $BSD_STYLE should be set
2061is left as an exercise to the reader.
cb1a09d0 2062
19799a22 2063The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2064systems purporting POSIX compliance. See also the C<Term::ReadKey>
2065module from your nearest CPAN site; details on CPAN can be found on
2066L<perlmodlib/CPAN>.
a0d0e21e
LW
2067
2068=item getlogin
d74e8afc 2069X<getlogin> X<login>
a0d0e21e 2070
cf264981 2071This implements the C library function of the same name, which on most
5a964f20 2072systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 2073use C<getpwuid>.
a0d0e21e 2074
f86702cc 2075 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2076
19799a22
GS
2077Do not consider C<getlogin> for authentication: it is not as
2078secure as C<getpwuid>.
4633a7c4 2079
a0d0e21e 2080=item getpeername SOCKET
d74e8afc 2081X<getpeername> X<peer>
a0d0e21e
LW
2082
2083Returns the packed sockaddr address of other end of the SOCKET connection.
2084
4633a7c4
LW
2085 use Socket;
2086 $hersockaddr = getpeername(SOCK);
19799a22 2087 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2088 $herhostname = gethostbyaddr($iaddr, AF_INET);
2089 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2090
2091=item getpgrp PID
d74e8afc 2092X<getpgrp> X<group>
a0d0e21e 2093
47e29363 2094Returns the current process group for the specified PID. Use
7660c0ab 2095a PID of C<0> to get the current process group for the
4633a7c4 2096current process. Will raise an exception if used on a machine that
f86cebdf 2097doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2098group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2099does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2100
2101=item getppid
d74e8afc 2102X<getppid> X<parent> X<pid>
a0d0e21e
LW
2103
2104Returns the process id of the parent process.
2105
4d76a344
RGS
2106Note for Linux users: on Linux, the C functions C<getpid()> and
2107C<getppid()> return different values from different threads. In order to
2108be portable, this behavior is not reflected by the perl-level function
2109C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2110to call the underlying C<getppid()>, you may use the CPAN module
2111C<Linux::Pid>.
4d76a344 2112
a0d0e21e 2113=item getpriority WHICH,WHO
d74e8afc 2114X<getpriority> X<priority> X<nice>
a0d0e21e 2115
4633a7c4
LW
2116Returns the current priority for a process, a process group, or a user.
2117(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2118machine that doesn't implement getpriority(2).
a0d0e21e
LW
2119
2120=item getpwnam NAME
d74e8afc
ITB
2121X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2122X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2123X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2124X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2125X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2126X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2127
2128=item getgrnam NAME
2129
2130=item gethostbyname NAME
2131
2132=item getnetbyname NAME
2133
2134=item getprotobyname NAME
2135
2136=item getpwuid UID
2137
2138=item getgrgid GID
2139
2140=item getservbyname NAME,PROTO
2141
2142=item gethostbyaddr ADDR,ADDRTYPE
2143
2144=item getnetbyaddr ADDR,ADDRTYPE
2145
2146=item getprotobynumber NUMBER
2147
2148=item getservbyport PORT,PROTO
2149
2150=item getpwent
2151
2152=item getgrent
2153
2154=item gethostent
2155
2156=item getnetent
2157
2158=item getprotoent
2159
2160=item getservent
2161
2162=item setpwent
2163
2164=item setgrent
2165
2166=item sethostent STAYOPEN
2167
2168=item setnetent STAYOPEN
2169
2170=item setprotoent STAYOPEN
2171
2172=item setservent STAYOPEN
2173
2174=item endpwent
2175
2176=item endgrent
2177
2178=item endhostent
2179
2180=item endnetent
2181
2182=item endprotoent
2183
2184=item endservent
2185
2186These routines perform the same functions as their counterparts in the
5a964f20 2187system library. In list context, the return values from the
a0d0e21e
LW
2188various get routines are as follows:
2189
2190 ($name,$passwd,$uid,$gid,
6ee623d5 2191 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2192 ($name,$passwd,$gid,$members) = getgr*
2193 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2194 ($name,$aliases,$addrtype,$net) = getnet*
2195 ($name,$aliases,$proto) = getproto*
2196 ($name,$aliases,$port,$proto) = getserv*
2197
2198(If the entry doesn't exist you get a null list.)
2199
4602f195
JH
2200The exact meaning of the $gcos field varies but it usually contains
2201the real name of the user (as opposed to the login name) and other
2202information pertaining to the user. Beware, however, that in many
2203system users are able to change this information and therefore it
106325ad 2204cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2205L<perlsec>). The $passwd and $shell, user's encrypted password and
2206login shell, are also tainted, because of the same reason.
4602f195 2207
5a964f20 2208In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2209lookup by name, in which case you get the other thing, whatever it is.
2210(If the entry doesn't exist you get the undefined value.) For example:
2211
5a964f20
TC
2212 $uid = getpwnam($name);
2213 $name = getpwuid($num);
2214 $name = getpwent();
2215 $gid = getgrnam($name);
08a33e13 2216 $name = getgrgid($num);
5a964f20
TC
2217 $name = getgrent();
2218 #etc.
a0d0e21e 2219
4602f195
JH
2220In I<getpw*()> the fields $quota, $comment, and $expire are special
2221cases in the sense that in many systems they are unsupported. If the
2222$quota is unsupported, it is an empty scalar. If it is supported, it
2223usually encodes the disk quota. If the $comment field is unsupported,
2224it is an empty scalar. If it is supported it usually encodes some
2225administrative comment about the user. In some systems the $quota
2226field may be $change or $age, fields that have to do with password
2227aging. In some systems the $comment field may be $class. The $expire
2228field, if present, encodes the expiration period of the account or the
2229password. For the availability and the exact meaning of these fields
2230in your system, please consult your getpwnam(3) documentation and your
2231F<pwd.h> file. You can also find out from within Perl what your
2232$quota and $comment fields mean and whether you have the $expire field
2233by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2234C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2235files are only supported if your vendor has implemented them in the
2236intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2237shadow versions if you're running under privilege or if there exists
cf264981
SP
2238the shadow(3) functions as found in System V (this includes Solaris
2239and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2240facility are unlikely to be supported.
6ee623d5 2241
19799a22 2242The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2243the login names of the members of the group.
2244
2245For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2246C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2247C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2248addresses returned by the corresponding system library call. In the
2249Internet domain, each address is four bytes long and you can unpack it
2250by saying something like:
2251
f337b084 2252 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2253
2b5ab1e7
TC
2254The Socket library makes this slightly easier:
2255
2256 use Socket;
2257 $iaddr = inet_aton("127.1"); # or whatever address
2258 $name = gethostbyaddr($iaddr, AF_INET);
2259
2260 # or going the other way
19799a22 2261 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2262
d760c846
GS
2263In the opposite way, to resolve a hostname to the IP address
2264you can write this:
2265
2266 use Socket;
2267 $packed_ip = gethostbyname("www.perl.org");
2268 if (defined $packed_ip) {
2269 $ip_address = inet_ntoa($packed_ip);
2270 }
2271
2272Make sure <gethostbyname()> is called in SCALAR context and that
2273its return value is checked for definedness.
2274
19799a22
GS
2275If you get tired of remembering which element of the return list
2276contains which return value, by-name interfaces are provided
2277in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2278C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2279and C<User::grent>. These override the normal built-ins, supplying
2280versions that return objects with the appropriate names
2281for each field. For example:
5a964f20
TC
2282
2283 use File::stat;
2284 use User::pwent;
2285 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2286
b76cc8ba
NIS
2287Even though it looks like they're the same method calls (uid),
2288they aren't, because a C<File::stat> object is different from
19799a22 2289a C<User::pwent> object.
5a964f20 2290
a0d0e21e 2291=item getsockname SOCKET
d74e8afc 2292X<getsockname>
a0d0e21e 2293
19799a22
GS
2294Returns the packed sockaddr address of this end of the SOCKET connection,
2295in case you don't know the address because you have several different
2296IPs that the connection might have come in on.
a0d0e21e 2297
4633a7c4
LW
2298 use Socket;
2299 $mysockaddr = getsockname(SOCK);
19799a22 2300 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2301 printf "Connect to %s [%s]\n",
19799a22
GS
2302 scalar gethostbyaddr($myaddr, AF_INET),
2303 inet_ntoa($myaddr);
a0d0e21e
LW
2304
2305=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2306X<getsockopt>
a0d0e21e 2307
636e6b1f
TH
2308Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2309Options may exist at multiple protocol levels depending on the socket
2310type, but at least the uppermost socket level SOL_SOCKET (defined in the
2311C<Socket> module) will exist. To query options at another level the
2312protocol number of the appropriate protocol controlling the option
2313should be supplied. For example, to indicate that an option is to be
2314interpreted by the TCP protocol, LEVEL should be set to the protocol
2315number of TCP, which you can get using getprotobyname.
2316
2317The call returns a packed string representing the requested socket option,
2318or C<undef> if there is an error (the error reason will be in $!). What
2319exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2320your system documentation for details. A very common case however is that
cf264981 2321the option is an integer, in which case the result will be a packed
636e6b1f
TH
2322integer which you can decode using unpack with the C<i> (or C<I>) format.
2323
2324An example testing if Nagle's algorithm is turned on on a socket:
2325
4852725b 2326 use Socket qw(:all);
636e6b1f
TH
2327
2328 defined(my $tcp = getprotobyname("tcp"))
2329 or die "Could not determine the protocol number for tcp";
4852725b
DD
2330 # my $tcp = IPPROTO_TCP; # Alternative
2331 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2332 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2333 my $nodelay = unpack("I", $packed);
2334 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2335
a0d0e21e
LW
2336
2337=item glob EXPR
d74e8afc 2338X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2339
0a753a76
PP
2340=item glob
2341
d9a9d457
JL
2342In list context, returns a (possibly empty) list of filename expansions on
2343the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2344scalar context, glob iterates through such filename expansions, returning
2345undef when the list is exhausted. This is the internal function
2346implementing the C<< <*.c> >> operator, but you can use it directly. If
2347EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2348more detail in L<perlop/"I/O Operators">.
a0d0e21e 2349
5c0c9249
PF
2350Note that C<glob> will split its arguments on whitespace, treating
2351each segment as separate pattern. As such, C<glob('*.c *.h')> would
2352match all files with a F<.c> or F<.h> extension. The expression
2353C<glob('.* *')> would match all files in the current working directory.
2354
3a4b19e4 2355Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2356C<File::Glob> extension. See L<File::Glob> for details, including
2357C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2358
a0d0e21e 2359=item gmtime EXPR
d74e8afc 2360X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2361
ce2984c3
PF
2362=item gmtime
2363
435fbc73
GS
2364Works just like L<localtime> but the returned values are
2365localized for the standard Greenwich time zone.
a0d0e21e 2366
435fbc73
GS
2367Note: when called in list context, $isdst, the last value
2368returned by gmtime is always C<0>. There is no
2369Daylight Saving Time in GMT.
0a753a76 2370
62aa5637
MS
2371See L<perlport/gmtime> for portability concerns.
2372
a0d0e21e 2373=item goto LABEL
d74e8afc 2374X<goto> X<jump> X<jmp>
a0d0e21e 2375
748a9306
LW
2376=item goto EXPR
2377
a0d0e21e
LW
2378=item goto &NAME
2379
7660c0ab 2380The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2381execution there. It may not be used to go into any construct that
7660c0ab 2382requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2383also can't be used to go into a construct that is optimized away,
19799a22 2384or to get out of a block or subroutine given to C<sort>.
0a753a76 2385It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2386including out of subroutines, but it's usually better to use some other
19799a22 2387construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2388need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2389(The difference being that C does not offer named loops combined with
2390loop control. Perl does, and this replaces most structured uses of C<goto>
2391in other languages.)
a0d0e21e 2392
7660c0ab
A
2393The C<goto-EXPR> form expects a label name, whose scope will be resolved
2394dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2395necessarily recommended if you're optimizing for maintainability:
2396
2397 goto ("FOO", "BAR", "GLARCH")[$i];
2398
1b6921cb
BT
2399The C<goto-&NAME> form is quite different from the other forms of
2400C<goto>. In fact, it isn't a goto in the normal sense at all, and
2401doesn't have the stigma associated with other gotos. Instead, it
2402exits the current subroutine (losing any changes set by local()) and
2403immediately calls in its place the named subroutine using the current
2404value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2405load another subroutine and then pretend that the other subroutine had
2406been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2407in the current subroutine are propagated to the other subroutine.)
2408After the C<goto>, not even C<caller> will be able to tell that this
2409routine was called first.
2410
2411NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2412containing a code reference, or a block that evaluates to a code
6cb9131c 2413reference.
a0d0e21e
LW
2414
2415=item grep BLOCK LIST
d74e8afc 2416X<grep>
a0d0e21e
LW
2417
2418=item grep EXPR,LIST
2419
2b5ab1e7
TC
2420This is similar in spirit to, but not the same as, grep(1) and its
2421relatives. In particular, it is not limited to using regular expressions.
2f9daede 2422
a0d0e21e 2423Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2424C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2425elements for which the expression evaluated to true. In scalar
2426context, returns the number of times the expression was true.
a0d0e21e
LW
2427
2428 @foo = grep(!/^#/, @bar); # weed out comments
2429
2430or equivalently,
2431
2432 @foo = grep {!/^#/} @bar; # weed out comments
2433
be3174d2
GS
2434Note that C<$_> is an alias to the list value, so it can be used to
2435modify the elements of the LIST. While this is useful and supported,
2436it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2437Similarly, grep returns aliases into the original list, much as a for
2438loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2439element of a list returned by grep (for example, in a C<foreach>, C<map>
2440or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2441This is usually something to be avoided when writing clear code.
a0d0e21e 2442
a4fb8298 2443If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2444been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2445the list elements, C<$_> keeps being lexical inside the block; i.e. it
2446can't be seen from the outside, avoiding any potential side-effects.
2447
19799a22 2448See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2449
a0d0e21e 2450=item hex EXPR
d74e8afc 2451X<hex> X<hexadecimal>
a0d0e21e 2452
54310121 2453=item hex
bbce6d69 2454
2b5ab1e7 2455Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2456(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2457L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2458
2459 print hex '0xAf'; # prints '175'
2460 print hex 'aF'; # same
a0d0e21e 2461
19799a22 2462Hex strings may only represent integers. Strings that would cause
53305cf1 2463integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2464unlike oct(). To present something as hex, look into L</printf>,
2465L</sprintf>, or L</unpack>.
19799a22 2466
ce2984c3 2467=item import LIST
d74e8afc 2468X<import>
a0d0e21e 2469
19799a22 2470There is no builtin C<import> function. It is just an ordinary
4633a7c4 2471method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2472names to another module. The C<use> function calls the C<import> method
cea6626f 2473for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2474
2475=item index STR,SUBSTR,POSITION
d74e8afc 2476X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2477
2478=item index STR,SUBSTR
2479
2b5ab1e7
TC
2480The index function searches for one string within another, but without
2481the wildcard-like behavior of a full regular-expression pattern match.
2482It returns the position of the first occurrence of SUBSTR in STR at
2483or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2484beginning of the string. POSITION before the beginning of the string
2485or after its end is treated as if it were the beginning or the end,
2486respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2487you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2488is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2489
2490=item int EXPR
f723aae1 2491X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2492
54310121 2493=item int
bbce6d69 2494
7660c0ab 2495Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2496You should not use this function for rounding: one because it truncates
2497towards C<0>, and two because machine representations of floating point
2498numbers can sometimes produce counterintuitive results. For example,
2499C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2500because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2501the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2502functions will serve you better than will int().
a0d0e21e
LW
2503
2504=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2505X<ioctl>
a0d0e21e 2506
2b5ab1e7 2507Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2508
6c567752 2509 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2510
a11c483f 2511to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2512exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2513own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2514(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2515may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2516written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2517will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2518has no string value but does have a numeric value, that value will be
2519passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2520true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2521functions may be needed to manipulate the values of structures used by
b76cc8ba 2522C<ioctl>.
a0d0e21e 2523
19799a22 2524The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2525
2526 if OS returns: then Perl returns:
2527 -1 undefined value
2528 0 string "0 but true"
2529 anything else that number
2530
19799a22 2531Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2532still easily determine the actual value returned by the operating
2533system:
2534
2b5ab1e7 2535 $retval = ioctl(...) || -1;
a0d0e21e
LW
2536 printf "System returned %d\n", $retval;
2537
be2f7487 2538The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2539about improper numeric conversions.
2540
a0d0e21e 2541=item join EXPR,LIST
d74e8afc 2542X<join>
a0d0e21e 2543
2b5ab1e7
TC
2544Joins the separate strings of LIST into a single string with fields
2545separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2546
2b5ab1e7 2547 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2548
eb6e2d6f
GS
2549Beware that unlike C<split>, C<join> doesn't take a pattern as its
2550first argument. Compare L</split>.
a0d0e21e 2551
aa689395 2552=item keys HASH
d74e8afc 2553X<keys> X<key>
aa689395 2554
aeedbbed
NC
2555=item keys ARRAY
2556
2557Returns a list consisting of all the keys of the named hash, or the indices
2558of an array. (In scalar context, returns the number of keys or indices.)
504f80c1 2559
aeedbbed 2560The keys of a hash are returned in an apparently random order. The actual
504f80c1
JH
2561random order is subject to change in future versions of perl, but it
2562is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2563function produces (given that the hash has not been modified). Since
2564Perl 5.8.1 the ordering is different even between different runs of
2565Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2566Attacks">).
504f80c1 2567
aeedbbed 2568As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
cf264981
SP
2569(see L</each>). In particular, calling keys() in void context resets
2570the iterator with no other overhead.
a0d0e21e 2571
aa689395 2572Here is yet another way to print your environment:
a0d0e21e
LW
2573
2574 @keys = keys %ENV;
2575 @values = values %ENV;
b76cc8ba 2576 while (@keys) {
a0d0e21e
LW
2577 print pop(@keys), '=', pop(@values), "\n";
2578 }
2579
2580or how about sorted by key:
2581
2582 foreach $key (sort(keys %ENV)) {
2583 print $key, '=', $ENV{$key}, "\n";
2584 }
2585
8ea1e5d4
GS
2586The returned values are copies of the original keys in the hash, so
2587modifying them will not affect the original hash. Compare L</values>.
2588
19799a22 2589To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2590Here's a descending numeric sort of a hash by its values:
4633a7c4 2591
5a964f20 2592 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2593 printf "%4d %s\n", $hash{$key}, $key;
2594 }
2595
19799a22 2596As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2597allocated for the given hash. This can gain you a measure of efficiency if
2598you know the hash is going to get big. (This is similar to pre-extending
2599an array by assigning a larger number to $#array.) If you say
55497cff
PP
2600
2601 keys %hash = 200;
2602
ab192400
GS
2603then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2604in fact, since it rounds up to the next power of two. These
55497cff
PP
2605buckets will be retained even if you do C<%hash = ()>, use C<undef
2606%hash> if you want to free the storage while C<%hash> is still in scope.
2607You can't shrink the number of buckets allocated for the hash using
19799a22 2608C<keys> in this way (but you needn't worry about doing this by accident,
aeedbbed
NC
2609as trying has no effect). C<keys @array> in an lvalue context is a syntax
2610error.
55497cff 2611
19799a22 2612See also C<each>, C<values> and C<sort>.
ab192400 2613
b350dd2f 2614=item kill SIGNAL, LIST
d74e8afc 2615X<kill> X<signal>
a0d0e21e 2616
b350dd2f 2617Sends a signal to a list of processes. Returns the number of
517db077
GS
2618processes successfully signaled (which is not necessarily the
2619same as the number actually killed).
a0d0e21e
LW
2620
2621 $cnt = kill 1, $child1, $child2;
2622 kill 9, @goners;
2623
70fb64f6 2624If SIGNAL is zero, no signal is sent to the process, but the kill(2)
6cb9d3e4 2625system call will check whether it's possible to send a signal to it (that
70fb64f6
RGS
2626means, to be brief, that the process is owned by the same user, or we are
2627the super-user). This is a useful way to check that a child process is
81fd35db
DN
2628alive (even if only as a zombie) and hasn't changed its UID. See
2629L<perlport> for notes on the portability of this construct.
b350dd2f
GS
2630
2631Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2632process groups instead of processes. (On System V, a negative I<PROCESS>
2633number will also kill process groups, but that's not portable.) That
2634means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2635use a signal name in quotes.
2636
2637See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2638
2639=item last LABEL
d74e8afc 2640X<last> X<break>
a0d0e21e
LW
2641
2642=item last
2643
2644The C<last> command is like the C<break> statement in C (as used in
2645loops); it immediately exits the loop in question. If the LABEL is
2646omitted, the command refers to the innermost enclosing loop. The
2647C<continue> block, if any, is not executed:
2648
4633a7c4
LW
2649 LINE: while (<STDIN>) {
2650 last LINE if /^$/; # exit when done with header
5a964f20 2651 #...
a0d0e21e
LW
2652 }
2653
4968c1e4 2654C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2655C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2656a grep() or map() operation.
4968c1e4 2657
6c1372ed
GS
2658Note that a block by itself is semantically identical to a loop
2659that executes once. Thus C<last> can be used to effect an early
2660exit out of such a block.
2661
98293880
JH
2662See also L</continue> for an illustration of how C<last>, C<next>, and
2663C<redo> work.
1d2dff63 2664
a0d0e21e 2665=item lc EXPR
d74e8afc 2666X<lc> X<lowercase>
a0d0e21e 2667
54310121 2668=item lc
bbce6d69 2669
d1be9408 2670Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2671implementing the C<\L> escape in double-quoted strings. Respects
2672current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2673and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2674
7660c0ab 2675If EXPR is omitted, uses C<$_>.
bbce6d69 2676
a0d0e21e 2677=item lcfirst EXPR
d74e8afc 2678X<lcfirst> X<lowercase>
a0d0e21e 2679
54310121 2680=item lcfirst
bbce6d69 2681
ad0029c4
JH
2682Returns the value of EXPR with the first character lowercased. This
2683is the internal function implementing the C<\l> escape in
2684double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2685locale> in force. See L<perllocale> and L<perlunicode> for more
2686details about locale and Unicode support.
a0d0e21e 2687
7660c0ab 2688If EXPR is omitted, uses C<$_>.
bbce6d69 2689
a0d0e21e 2690=item length EXPR
d74e8afc 2691X<length> X<size>
a0d0e21e 2692
54310121 2693=item length
bbce6d69 2694
974da8e5 2695Returns the length in I<characters> of the value of EXPR. If EXPR is
9f621bb0
NC
2696omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
2697Note that this cannot be used on an entire array or hash to find out how
2698many elements these have. For that, use C<scalar @array> and C<scalar keys
2699%hash> respectively.
a0d0e21e 2700
974da8e5
JH
2701Note the I<characters>: if the EXPR is in Unicode, you will get the
2702number of characters, not the number of bytes. To get the length
2575c402
JW
2703of the internal string in bytes, use C<bytes::length(EXPR)>, see
2704L<bytes>. Note that the internal encoding is variable, and the number
2705of bytes usually meaningless. To get the number of bytes that the
2706string would have when encoded as UTF-8, use
2707C<length(Encoding::encode_utf8(EXPR))>.
974da8e5 2708
a0d0e21e 2709=item link OLDFILE,NEWFILE
d74e8afc 2710X<link>
a0d0e21e 2711
19799a22 2712Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2713success, false otherwise.
a0d0e21e
LW
2714
2715=item listen SOCKET,QUEUESIZE
d74e8afc 2716X<listen>
a0d0e21e 2717
19799a22 2718Does the same thing that the listen system call does. Returns true if
b76cc8ba 2719it succeeded, false otherwise. See the example in
cea6626f 2720L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2721
2722=item local EXPR
d74e8afc 2723X<local>
a0d0e21e 2724
19799a22 2725You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2726what most people think of as "local". See
13a2d996 2727L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2728
5a964f20
TC
2729A local modifies the listed variables to be local to the enclosing
2730block, file, or eval. If more than one value is listed, the list must
2731be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2732for details, including issues with tied arrays and hashes.
a0d0e21e 2733
a0d0e21e 2734=item localtime EXPR
435fbc73 2735X<localtime> X<ctime>
a0d0e21e 2736
ba053783
AL
2737=item localtime
2738
19799a22 2739Converts a time as returned by the time function to a 9-element list
5f05dabc 2740with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2741follows:
2742
54310121 2743 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2744 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2745 localtime(time);
a0d0e21e 2746
48a26b3a 2747All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2748tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2749of the specified time.
48a26b3a 2750
ba053783
AL
2751C<$mday> is the day of the month, and C<$mon> is the month itself, in
2752the range C<0..11> with 0 indicating January and 11 indicating December.
2753This makes it easy to get a month name from a list:
54310121 2754
ba053783
AL
2755 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2756 print "$abbr[$mon] $mday";
2757 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2758
ba053783
AL
2759C<$year> is the number of years since 1900, not just the last two digits
2760of the year. That is, C<$year> is C<123> in year 2023. The proper way
2761to get a complete 4-digit year is simply:
abd75f24 2762
ba053783 2763 $year += 1900;
abd75f24 2764
435fbc73
GS
2765Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2766to do that, would you?
2767
ba053783
AL
2768To get the last two digits of the year (e.g., '01' in 2001) do:
2769
2770 $year = sprintf("%02d", $year % 100);
2771
2772C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2773Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2774(or C<0..365> in leap years.)
2775
2776C<$isdst> is true if the specified time occurs during Daylight Saving
2777Time, false otherwise.
abd75f24 2778
e1998452 2779If EXPR is omitted, C<localtime()> uses the current time (as returned
e3176d09 2780by time(3)).
a0d0e21e 2781
48a26b3a 2782In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2783
5f05dabc 2784 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2785
fe86afc2
NC
2786This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2787instead of local time use the L</gmtime> builtin. See also the
2788C<Time::Local> module (to convert the second, minutes, hours, ... back to
2789the integer value returned by time()), and the L<POSIX> module's strftime(3)
2790and mktime(3) functions.
2791
2792To get somewhat similar but locale dependent date strings, set up your
2793locale environment variables appropriately (please see L<perllocale>) and
2794try for example:
a3cb178b 2795
5a964f20 2796 use POSIX qw(strftime);
2b5ab1e7 2797 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2798 # or for GMT formatted appropriately for your locale:
2799 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2800
2801Note that the C<%a> and C<%b>, the short forms of the day of the week
2802and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2803
62aa5637
MS
2804See L<perlport/localtime> for portability concerns.
2805
435fbc73
GS
2806The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2807by-name access mechanism to the gmtime() and localtime() functions,
2808respectively.
2809
2810For a comprehensive date and time representation look at the
2811L<DateTime> module on CPAN.
2812
07698885 2813=item lock THING
d74e8afc 2814X<lock>
19799a22 2815
01e6739c 2816This function places an advisory lock on a shared variable, or referenced
03730085 2817object contained in I<THING> until the lock goes out of scope.
a6d5524e 2818
f3a23afb 2819lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2820by this name (before any calls to it), that function will be called
03730085
AB
2821instead. (However, if you've said C<use threads>, lock() is always a
2822keyword.) See L<threads>.
19799a22 2823
a0d0e21e 2824=item log EXPR
d74e8afc 2825X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2826
54310121 2827=item log
bbce6d69 2828
2b5ab1e7
TC
2829Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2830returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2831The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2832divided by the natural log of N. For example:
2833
2834 sub log10 {
2835 my $n = shift;
2836 return log($n)/log(10);
b76cc8ba 2837 }
2b5ab1e7
TC
2838
2839See also L</exp> for the inverse operation.
a0d0e21e 2840
a0d0e21e 2841=item lstat EXPR
d74e8afc 2842X<lstat>
a0d0e21e 2843
54310121 2844=item lstat
bbce6d69 2845
19799a22 2846Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2847special C<_> filehandle) but stats a symbolic link instead of the file
2848the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2849your system, a normal C<stat> is done. For much more detailed
2850information, please see the documentation for C<stat>.
a0d0e21e 2851
7660c0ab 2852If EXPR is omitted, stats C<$_>.
bbce6d69 2853
a0d0e21e
LW
2854=item m//
2855
2856The match operator. See L<perlop>.
2857
2858=item map BLOCK LIST
d74e8afc 2859X<map>
a0d0e21e
LW
2860
2861=item map EXPR,LIST
2862
19799a22
GS
2863Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2864C<$_> to each element) and returns the list value composed of the
2865results of each such evaluation. In scalar context, returns the
2866total number of elements so generated. Evaluates BLOCK or EXPR in
2867list context, so each element of LIST may produce zero, one, or
2868more elements in the returned value.
dd99ebda 2869
a0d0e21e
LW
2870 @chars = map(chr, @nums);
2871
2872translates a list of numbers to the corresponding characters. And
2873
d8216f19 2874 %hash = map { get_a_key_for($_) => $_ } @array;
a0d0e21e
LW
2875
2876is just a funny way to write
2877
2878 %hash = ();
d8216f19
RGS
2879 foreach (@array) {
2880 $hash{get_a_key_for($_)} = $_;
a0d0e21e
LW
2881 }
2882
be3174d2
GS
2883Note that C<$_> is an alias to the list value, so it can be used to
2884modify the elements of the LIST. While this is useful and supported,
2885it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2886Using a regular C<foreach> loop for this purpose would be clearer in
2887most cases. See also L</grep> for an array composed of those items of
2888the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2889
a4fb8298 2890If C<$_> is lexical in the scope where the C<map> appears (because it has
d8216f19
RGS
2891been declared with C<my $_>), then, in addition to being locally aliased to
2892the list elements, C<$_> keeps being lexical inside the block; that is, it
a4fb8298
RGS
2893can't be seen from the outside, avoiding any potential side-effects.
2894
205fdb4d
NC
2895C<{> starts both hash references and blocks, so C<map { ...> could be either
2896the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2897ahead for the closing C<}> it has to take a guess at which its dealing with
2898based what it finds just after the C<{>. Usually it gets it right, but if it
2899doesn't it won't realize something is wrong until it gets to the C<}> and
2900encounters the missing (or unexpected) comma. The syntax error will be
2901reported close to the C<}> but you'll need to change something near the C<{>
2902such as using a unary C<+> to give perl some help:
2903
2904 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2905 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2906 %hash = map { ("\L$_", 1) } @array # this also works
2907 %hash = map { lc($_), 1 } @array # as does this.
2908 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2909
205fdb4d
NC
2910 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2911
d8216f19 2912or to force an anon hash constructor use C<+{>:
205fdb4d
NC
2913
2914 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2915
2916and you get list of anonymous hashes each with only 1 entry.
2917
19799a22 2918=item mkdir FILENAME,MASK
d74e8afc 2919X<mkdir> X<md> X<directory, create>
a0d0e21e 2920
5a211162
GS
2921=item mkdir FILENAME
2922
491873e5
RGS
2923=item mkdir
2924
0591cd52 2925Creates the directory specified by FILENAME, with permissions
19799a22
GS
2926specified by MASK (as modified by C<umask>). If it succeeds it
2927returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2928If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2929to C<$_>.
0591cd52 2930
19799a22 2931In general, it is better to create directories with permissive MASK,
0591cd52 2932and let the user modify that with their C<umask>, than it is to supply
19799a22 2933a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2934The exceptions to this rule are when the file or directory should be
2935kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2936C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2937
cc1852e8
JH
2938Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2939number of trailing slashes. Some operating and filesystems do not get
2940this right, so Perl automatically removes all trailing slashes to keep
2941everyone happy.
2942
dd184578
RGS
2943In order to recursively create a directory structure look at
2944the C<mkpath> function of the L<File::Path> module.
2945
a0d0e21e 2946=item msgctl ID,CMD,ARG
d74e8afc 2947X<msgctl>
a0d0e21e 2948
f86cebdf 2949Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2950
2951 use IPC::SysV;
2952
7660c0ab 2953first to get the correct constant definitions. If CMD is C<IPC_STAT>,
cf264981 2954then ARG must be a variable that will hold the returned C<msqid_ds>
951ba7fe
GS
2955structure. Returns like C<ioctl>: the undefined value for error,
2956C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2957L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2958
2959=item msgget KEY,FLAGS
d74e8afc 2960X<msgget>
a0d0e21e 2961
f86cebdf 2962Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2963id, or the undefined value if there is an error. See also
2964L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2965
a0d0e21e 2966=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
d74e8afc 2967X<msgrcv>
a0d0e21e
LW
2968
2969Calls the System V IPC function msgrcv to receive a message from
2970message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2971SIZE. Note that when a message is received, the message type as a
2972native long integer will be the first thing in VAR, followed by the
2973actual message. This packing may be opened with C<unpack("l! a*")>.
2974Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2975an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2976C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2977
2978=item msgsnd ID,MSG,FLAGS
d74e8afc 2979X<msgsnd>
41d6edb2
JH
2980
2981Calls the System V IPC function msgsnd to send the message MSG to the
2982message queue ID. MSG must begin with the native long integer message
2983type, and be followed by the length of the actual message, and finally
2984the message itself. This kind of packing can be achieved with
2985C<pack("l! a*", $type, $message)>. Returns true if successful,
2986or false if there is an error. See also C<IPC::SysV>
2987and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2988
2989=item my EXPR
d74e8afc 2990X<my>
a0d0e21e 2991
307ea6df
JH
2992=item my TYPE EXPR
2993
1d2de774 2994=item my EXPR : ATTRS
09bef843 2995
1d2de774 2996=item my TYPE EXPR : ATTRS
307ea6df 2997
19799a22 2998A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
2999enclosing block, file, or C<eval>. If more than one value is listed,
3000the list must be placed in parentheses.
307ea6df 3001
1d2de774
JH
3002The exact semantics and interface of TYPE and ATTRS are still
3003evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
3004and attributes are handled using the C<attributes> pragma, or starting
3005from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3006L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3007L<attributes>, and L<Attribute::Handlers>.
4633a7c4 3008
a0d0e21e 3009=item next LABEL
d74e8afc 3010X<next> X<continue>
a0d0e21e
LW
3011
3012=item next
3013
3014The C<next> command is like the C<continue> statement in C; it starts
3015the next iteration of the loop:
3016
4633a7c4
LW
3017 LINE: while (<STDIN>) {
3018 next LINE if /^#/; # discard comments
5a964f20 3019 #...
a0d0e21e
LW
3020 }
3021
3022Note that if there were a C<continue> block on the above, it would get
3023executed even on discarded lines. If the LABEL is omitted, the command
3024refers to the innermost enclosing loop.
3025
4968c1e4 3026C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
3027C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3028a grep() or map() operation.
4968c1e4 3029
6c1372ed
GS
3030Note that a block by itself is semantically identical to a loop
3031that executes once. Thus C<next> will exit such a block early.
3032
98293880
JH
3033See also L</continue> for an illustration of how C<last>, C<next>, and
3034C<redo> work.
1d2dff63 3035
4a66ea5a 3036=item no Module VERSION LIST
d74e8afc 3037X<no>
4a66ea5a
RGS
3038
3039=item no Module VERSION
3040
a0d0e21e
LW
3041=item no Module LIST
3042
4a66ea5a
RGS
3043=item no Module
3044
c986422f
RGS
3045=item no VERSION
3046
593b9c14 3047See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
3048
3049=item oct EXPR
d74e8afc 3050X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
a0d0e21e 3051
54310121 3052=item oct
bbce6d69 3053
4633a7c4 3054Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
3055value. (If EXPR happens to start off with C<0x>, interprets it as a
3056hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
3057binary string. Leading whitespace is ignored in all three cases.)
3058The following will handle decimal, binary, octal, and hex in the standard
3059Perl or C notation:
a0d0e21e
LW
3060
3061 $val = oct($val) if $val =~ /^0/;
3062
19799a22
GS
3063If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3064in octal), use sprintf() or printf():
3065
3066 $perms = (stat("filename"))[2] & 07777;
3067 $oct_perms = sprintf "%lo", $perms;
3068
3069The oct() function is commonly used when a string such as C<644> needs
3070to be converted into a file mode, for example. (Although perl will
3071automatically convert strings into numbers as needed, this automatic
3072conversion assumes base 10.)
a0d0e21e
LW
3073
3074=item open FILEHANDLE,EXPR
d74e8afc 3075X<open> X<pipe> X<file, open> X<fopen>
a0d0e21e 3076
68bd7414
NIS
3077=item open FILEHANDLE,MODE,EXPR
3078
3079=item open FILEHANDLE,MODE,EXPR,LIST
3080
ba964c95
T
3081=item open FILEHANDLE,MODE,REFERENCE
3082
a0d0e21e
LW
3083=item open FILEHANDLE
3084
3085Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
3086FILEHANDLE.
3087
460b70c2
GS
3088Simple examples to open a file for reading:
3089
3090 open(my $fh, '<', "input.txt") or die $!;
3091
3092and for writing:
3093
3094 open(my $fh, '>', "output.txt") or die $!;
3095
ed53a2bb
JH
3096(The following is a comprehensive reference to open(): for a gentler
3097introduction you may consider L<perlopentut>.)
3098
a28cd5c9
NT
3099If FILEHANDLE is an undefined scalar variable (or array or hash element)
3100the variable is assigned a reference to a new anonymous filehandle,
3101otherwise if FILEHANDLE is an expression, its value is used as the name of
3102the real filehandle wanted. (This is considered a symbolic reference, so
3103C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
3104
3105If EXPR is omitted, the scalar variable of the same name as the
3106FILEHANDLE contains the filename. (Note that lexical variables--those
3107declared with C<my>--will not work for this purpose; so if you're
67408cae 3108using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
3109
3110If three or more arguments are specified then the mode of opening and
3111the file name are separate. If MODE is C<< '<' >> or nothing, the file
3112is opened for input. If MODE is C<< '>' >>, the file is truncated and
3113opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 3114the file is opened for appending, again being created if necessary.
5a964f20 3115
ed53a2bb
JH
3116You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3117indicate that you want both read and write access to the file; thus
3118C<< '+<' >> is almost always preferred for read/write updates--the C<<
3119'+>' >> mode would clobber the file first. You can't usually use
3120either read-write mode for updating textfiles, since they have
3121variable length records. See the B<-i> switch in L<perlrun> for a
3122better approach. The file is created with permissions of C<0666>
3123modified by the process' C<umask> value.
3124
3125These various prefixes correspond to the fopen(3) modes of C<'r'>,
3126C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 3127
6170680b
IZ
3128In the 2-arguments (and 1-argument) form of the call the mode and
3129filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
3130spaces. It is possible to omit the mode in these forms if the mode is
3131C<< '<' >>.
6170680b 3132
7660c0ab 3133If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 3134command to which output is to be piped, and if the filename ends with a
f244e06d
GS
3135C<'|'>, the filename is interpreted as a command which pipes output to
3136us. See L<perlipc/"Using open() for IPC">
19799a22 3137for more examples of this. (You are not allowed to C<open> to a command
5a964f20 3138that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
3139and L<perlipc/"Bidirectional Communication with Another Process">
3140for alternatives.)
cb1a09d0 3141
ed53a2bb
JH
3142For three or more arguments if MODE is C<'|-'>, the filename is
3143interpreted as a command to which output is to be piped, and if MODE
3144is C<'-|'>, the filename is interpreted as a command which pipes
3145output to us. In the 2-arguments (and 1-argument) form one should
3146replace dash (C<'-'>) with the command.
3147See L<perlipc/"Using open() for IPC"> for more examples of this.