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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
5ed4f2ec 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.
80d38338 65Each operator and function decides which sort of value 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
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89Extension modules can also hook into the Perl parser to define new
90kinds of keyword-headed expression. These may look like functions, but
91may also look completely different. The syntax following the keyword
92is defined entirely by the extension. If you are an implementor, see
93L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
94a module, see the module's documentation for details of the syntax that
95it defines.
96
cb1a09d0 97=head2 Perl Functions by Category
d74e8afc 98X<function>
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99
100Here are Perl's functions (including things that look like
5a964f20 101functions, like some keywords and named operators)
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102arranged by category. Some functions appear in more
103than one place.
104
13a2d996 105=over 4
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106
107=item Functions for SCALARs or strings
d74e8afc 108X<scalar> X<string> X<character>
cb1a09d0 109
22fae026 110C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
1dc8ecb8 111C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
945c54fd 112C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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113
114=item Regular expressions and pattern matching
d74e8afc 115X<regular expression> X<regex> X<regexp>
cb1a09d0 116
ab4f32c2 117C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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118
119=item Numeric functions
d74e8afc 120X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 121
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122C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
123C<sin>, C<sqrt>, C<srand>
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124
125=item Functions for real @ARRAYs
d74e8afc 126X<array>
cb1a09d0 127
22fae026 128C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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129
130=item Functions for list data
d74e8afc 131X<list>
cb1a09d0 132
1dc8ecb8 133C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
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134
135=item Functions for real %HASHes
d74e8afc 136X<hash>
cb1a09d0 137
22fae026 138C<delete>, C<each>, C<exists>, C<keys>, C<values>
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139
140=item Input and output functions
d74e8afc 141X<I/O> X<input> X<output> X<dbm>
cb1a09d0 142
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143C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
144C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
0d863452 145C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
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146C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
147C<warn>, C<write>
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148
149=item Functions for fixed length data or records
150
22fae026 151C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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152
153=item Functions for filehandles, files, or directories
d74e8afc 154X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 155
22fae026 156C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 157C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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158C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
159C<umask>, C<unlink>, C<utime>
cb1a09d0 160
cf264981 161=item Keywords related to the control flow of your Perl program
d74e8afc 162X<control flow>
cb1a09d0 163
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164C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
165C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 166
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167=item Keywords related to switch
168
36fb85f3 169C<break>, C<continue>, C<given>, C<when>, C<default>
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170
171(These are only available if you enable the "switch" feature.
172See L<feature> and L<perlsyn/"Switch statements">.)
173
54310121 174=item Keywords related to scoping
cb1a09d0 175
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176C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
177C<use>
178
179(C<state> is only available if the "state" feature is enabled. See
180L<feature>.)
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181
182=item Miscellaneous functions
183
36fb85f3 184C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
834df1c5 185C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
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186
187=item Functions for processes and process groups
d74e8afc 188X<process> X<pid> X<process id>
cb1a09d0 189
22fae026 190C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
1dc8ecb8 191C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 192C<times>, C<wait>, C<waitpid>
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193
194=item Keywords related to perl modules
d74e8afc 195X<module>
cb1a09d0 196
22fae026 197C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 198
353c6505 199=item Keywords related to classes and object-orientation
d74e8afc 200X<object> X<class> X<package>
cb1a09d0 201
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202C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
203C<untie>, C<use>
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204
205=item Low-level socket functions
d74e8afc 206X<socket> X<sock>
cb1a09d0 207
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208C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
209C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 210C<socket>, C<socketpair>
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211
212=item System V interprocess communication functions
d74e8afc 213X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 214
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215C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
216C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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217
218=item Fetching user and group info
d74e8afc 219X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 220
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221C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
222C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
223C<getpwuid>, C<setgrent>, C<setpwent>
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224
225=item Fetching network info
d74e8afc 226X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 227
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228C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
229C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
230C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
231C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
232C<setnetent>, C<setprotoent>, C<setservent>
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233
234=item Time-related functions
d74e8afc 235X<time> X<date>
cb1a09d0 236
22fae026 237C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 238
37798a01 239=item Functions new in perl5
d74e8afc 240X<perl5>
37798a01 241
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242C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
243C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
1dc8ecb8 244C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
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245C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
246C<ucfirst>, C<untie>, C<use>, C<when>
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247
248* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 249operator, which can be used in expressions.
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250
251=item Functions obsoleted in perl5
252
22fae026 253C<dbmclose>, C<dbmopen>
37798a01 254
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255=back
256
60f9f73c 257=head2 Portability
d74e8afc 258X<portability> X<Unix> X<portable>
60f9f73c 259
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260Perl was born in Unix and can therefore access all common Unix
261system calls. In non-Unix environments, the functionality of some
262Unix system calls may not be available, or details of the available
263functionality may differ slightly. The Perl functions affected
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264by this are:
265
266C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
267C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
268C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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269C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
270C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 271C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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272C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
273C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
274C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 275C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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276C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
277C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
278C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 279C<shmwrite>, C<socket>, C<socketpair>,
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280C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
281C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 282C<utime>, C<wait>, C<waitpid>
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283
284For more information about the portability of these functions, see
285L<perlport> and other available platform-specific documentation.
286
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287=head2 Alphabetical Listing of Perl Functions
288
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289=over 8
290
5b3c99c0 291=item -X FILEHANDLE
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292X<-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>
293X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 294
5b3c99c0 295=item -X EXPR
a0d0e21e 296
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297=item -X DIRHANDLE
298
5b3c99c0 299=item -X
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300
301A file test, where X is one of the letters listed below. This unary
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302operator takes one argument, either a filename, a filehandle, or a dirhandle,
303and tests the associated file to see if something is true about it. If the
7660c0ab 304argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 305Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 306the undefined value if the file doesn't exist. Despite the funny
d0821a6a 307names, precedence is the same as any other named unary operator. The
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308operator may be any of:
309
5ed4f2ec 310 -r File is readable by effective uid/gid.
311 -w File is writable by effective uid/gid.
312 -x File is executable by effective uid/gid.
313 -o File is owned by effective uid.
a0d0e21e 314
5ed4f2ec 315 -R File is readable by real uid/gid.
316 -W File is writable by real uid/gid.
317 -X File is executable by real uid/gid.
318 -O File is owned by real uid.
a0d0e21e 319
5ed4f2ec 320 -e File exists.
321 -z File has zero size (is empty).
322 -s File has nonzero size (returns size in bytes).
a0d0e21e 323
5ed4f2ec 324 -f File is a plain file.
325 -d File is a directory.
326 -l File is a symbolic link.
327 -p File is a named pipe (FIFO), or Filehandle is a pipe.
328 -S File is a socket.
329 -b File is a block special file.
330 -c File is a character special file.
331 -t Filehandle is opened to a tty.
a0d0e21e 332
5ed4f2ec 333 -u File has setuid bit set.
334 -g File has setgid bit set.
335 -k File has sticky bit set.
a0d0e21e 336
5ed4f2ec 337 -T File is an ASCII text file (heuristic guess).
338 -B File is a "binary" file (opposite of -T).
a0d0e21e 339
5ed4f2ec 340 -M Script start time minus file modification time, in days.
341 -A Same for access time.
342 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 343
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344Example:
345
346 while (<>) {
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347 chomp;
348 next unless -f $_; # ignore specials
349 #...
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350 }
351
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352The interpretation of the file permission operators C<-r>, C<-R>,
353C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
354of the file and the uids and gids of the user. There may be other
ecae030f
MO
355reasons you can't actually read, write, or execute the file: for
356example network filesystem access controls, ACLs (access control lists),
357read-only filesystems, and unrecognized executable formats. Note
358that the use of these six specific operators to verify if some operation
359is possible is usually a mistake, because it may be open to race
360conditions.
5ff3f7a4 361
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362Also note that, for the superuser on the local filesystems, the C<-r>,
363C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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GS
364if any execute bit is set in the mode. Scripts run by the superuser
365may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 366or temporarily set their effective uid to something else.
5ff3f7a4
GS
367
368If you are using ACLs, there is a pragma called C<filetest> that may
369produce more accurate results than the bare stat() mode bits.
5ff3f7a4
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370When under the C<use filetest 'access'> the above-mentioned filetests
371will test whether the permission can (not) be granted using the
468541a8 372access() family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
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373under this pragma return true even if there are no execute permission
374bits set (nor any extra execute permission ACLs). This strangeness is
ecae030f
MO
375due to the underlying system calls' definitions. Note also that, due to
376the implementation of C<use filetest 'access'>, the C<_> special
377filehandle won't cache the results of the file tests when this pragma is
378in effect. Read the documentation for the C<filetest> pragma for more
379information.
5ff3f7a4 380
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381Note that C<-s/a/b/> does not do a negated substitution. Saying
382C<-exp($foo)> still works as expected, however--only single letters
383following a minus are interpreted as file tests.
384
385The C<-T> and C<-B> switches work as follows. The first block or so of the
386file is examined for odd characters such as strange control codes or
61eff3bc 387characters with the high bit set. If too many strange characters (>30%)
cf264981 388are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
a0d0e21e 389containing null in the first block is considered a binary file. If C<-T>
9124316e 390or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 391rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 392file, or a file at EOF when testing a filehandle. Because you have to
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393read a file to do the C<-T> test, on most occasions you want to use a C<-f>
394against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 395
19799a22 396If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 397the special filehandle consisting of a solitary underline, then the stat
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398structure of the previous file test (or stat operator) is used, saving
399a system call. (This doesn't work with C<-t>, and you need to remember
400that lstat() and C<-l> will leave values in the stat structure for the
5c9aa243 401symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 402an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 403Example:
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404
405 print "Can do.\n" if -r $a || -w _ || -x _;
406
407 stat($filename);
408 print "Readable\n" if -r _;
409 print "Writable\n" if -w _;
410 print "Executable\n" if -x _;
411 print "Setuid\n" if -u _;
412 print "Setgid\n" if -g _;
413 print "Sticky\n" if -k _;
414 print "Text\n" if -T _;
415 print "Binary\n" if -B _;
416
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417As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
418test operators, in a way that C<-f -w -x $file> is equivalent to
cf264981 419C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
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420the return value of C<-f $file> as an argument to another filetest
421operator, no special magic will happen.)
422
a0d0e21e 423=item abs VALUE
d74e8afc 424X<abs> X<absolute>
a0d0e21e 425
54310121 426=item abs
bbce6d69 427
a0d0e21e 428Returns the absolute value of its argument.
7660c0ab 429If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
430
431=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 432X<accept>
a0d0e21e 433
f86cebdf 434Accepts an incoming socket connect, just as the accept(2) system call
19799a22 435does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 436See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 437
8d2a6795
GS
438On systems that support a close-on-exec flag on files, the flag will
439be set for the newly opened file descriptor, as determined by the
440value of $^F. See L<perlvar/$^F>.
441
a0d0e21e 442=item alarm SECONDS
d74e8afc
ITB
443X<alarm>
444X<SIGALRM>
445X<timer>
a0d0e21e 446
54310121 447=item alarm
bbce6d69 448
a0d0e21e 449Arranges to have a SIGALRM delivered to this process after the
cf264981 450specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
451specified, the value stored in C<$_> is used. (On some machines,
452unfortunately, the elapsed time may be up to one second less or more
453than you specified because of how seconds are counted, and process
454scheduling may delay the delivery of the signal even further.)
455
456Only one timer may be counting at once. Each call disables the
457previous timer, and an argument of C<0> may be supplied to cancel the
458previous timer without starting a new one. The returned value is the
459amount of time remaining on the previous timer.
a0d0e21e 460
2bc69794
BS
461For delays of finer granularity than one second, the Time::HiRes module
462(from CPAN, and starting from Perl 5.8 part of the standard
463distribution) provides ualarm(). You may also use Perl's four-argument
464version of select() leaving the first three arguments undefined, or you
465might be able to use the C<syscall> interface to access setitimer(2) if
466your system supports it. See L<perlfaq8> for details.
2b5ab1e7 467
80d38338
TC
468It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
469C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 470
19799a22
GS
471If you want to use C<alarm> to time out a system call you need to use an
472C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 473fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 474restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 475modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
476
477 eval {
a9a5a0dc
VP
478 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
479 alarm $timeout;
480 $nread = sysread SOCKET, $buffer, $size;
481 alarm 0;
ff68c719 482 };
ff68c719 483 if ($@) {
a9a5a0dc 484 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 485 # timed out
ff68c719
PP
486 }
487 else {
5ed4f2ec 488 # didn't
ff68c719
PP
489 }
490
91d81acc
JH
491For more information see L<perlipc>.
492
a0d0e21e 493=item atan2 Y,X
d74e8afc 494X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
495
496Returns the arctangent of Y/X in the range -PI to PI.
497
ca6e1c26 498For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
499function, or use the familiar relation:
500
501 sub tan { sin($_[0]) / cos($_[0]) }
502
a1021d57
RGS
503The return value for C<atan2(0,0)> is implementation-defined; consult
504your atan2(3) manpage for more information.
bf5f1b4c 505
a0d0e21e 506=item bind SOCKET,NAME
d74e8afc 507X<bind>
a0d0e21e
LW
508
509Binds a network address to a socket, just as the bind system call
19799a22 510does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
511packed address of the appropriate type for the socket. See the examples in
512L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 513
fae2c0fb 514=item binmode FILEHANDLE, LAYER
d74e8afc 515X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 516
a0d0e21e
LW
517=item binmode FILEHANDLE
518
1cbfc93d
NIS
519Arranges for FILEHANDLE to be read or written in "binary" or "text"
520mode on systems where the run-time libraries distinguish between
521binary and text files. If FILEHANDLE is an expression, the value is
522taken as the name of the filehandle. Returns true on success,
b5fe5ca2 523otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 524
d807c6f4
JH
525On some systems (in general, DOS and Windows-based systems) binmode()
526is necessary when you're not working with a text file. For the sake
527of portability it is a good idea to always use it when appropriate,
528and to never use it when it isn't appropriate. Also, people can
529set their I/O to be by default UTF-8 encoded Unicode, not bytes.
530
531In other words: regardless of platform, use binmode() on binary data,
532like for example images.
533
534If LAYER is present it is a single string, but may contain multiple
535directives. The directives alter the behaviour of the file handle.
920f5fe1 536When LAYER is present using binmode on a text file makes sense.
d807c6f4 537
fae2c0fb 538If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
539suitable for passing binary data. This includes turning off possible CRLF
540translation and marking it as bytes (as opposed to Unicode characters).
749683d2 541Note that, despite what may be implied in I<"Programming Perl"> (the
165a9987 542Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
80d38338 543--other layers that would affect the binary nature of the stream are
0226bbdb
NIS
544I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
545PERLIO environment variable.
01e6739c 546
d807c6f4
JH
547The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
548form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
549establish default I/O layers. See L<open>.
550
fae2c0fb
RGS
551I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
552in "Programming Perl, 3rd Edition". However, since the publishing of this
553book, by many known as "Camel III", the consensus of the naming of this
554functionality has moved from "discipline" to "layer". All documentation
555of this version of Perl therefore refers to "layers" rather than to
556"disciplines". Now back to the regularly scheduled documentation...>
557
6902c96a
T
558To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
559C<:utf8> just marks the data as UTF-8 without further checking,
560while C<:encoding(utf8)> checks the data for actually being valid
561UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 562
ed53a2bb 563In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
564is done on the filehandle. Calling binmode() will normally flush any
565pending buffered output data (and perhaps pending input data) on the
fae2c0fb 566handle. An exception to this is the C<:encoding> layer that
01e6739c 567changes the default character encoding of the handle, see L<open>.
fae2c0fb 568The C<:encoding> layer sometimes needs to be called in
3874323d
JH
569mid-stream, and it doesn't flush the stream. The C<:encoding>
570also implicitly pushes on top of itself the C<:utf8> layer because
571internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 572
19799a22 573The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
574system all work together to let the programmer treat a single
575character (C<\n>) as the line terminator, irrespective of the external
576representation. On many operating systems, the native text file
577representation matches the internal representation, but on some
578platforms the external representation of C<\n> is made up of more than
579one character.
580
68bd7414
NIS
581Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
582character to end each line in the external representation of text (even
5e12dbfa 583though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
584on Unix and most VMS files). In other systems like OS/2, DOS and the
585various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
586but what's stored in text files are the two characters C<\cM\cJ>. That
587means that, if you don't use binmode() on these systems, C<\cM\cJ>
588sequences on disk will be converted to C<\n> on input, and any C<\n> in
589your program will be converted back to C<\cM\cJ> on output. This is what
590you want for text files, but it can be disastrous for binary files.
30168b04
GS
591
592Another consequence of using binmode() (on some systems) is that
593special end-of-file markers will be seen as part of the data stream.
594For systems from the Microsoft family this means that if your binary
4375e838 595data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
596the file, unless you use binmode().
597
598binmode() is not only important for readline() and print() operations,
599but also when using read(), seek(), sysread(), syswrite() and tell()
600(see L<perlport> for more details). See the C<$/> and C<$\> variables
601in L<perlvar> for how to manually set your input and output
602line-termination sequences.
a0d0e21e 603
4633a7c4 604=item bless REF,CLASSNAME
d74e8afc 605X<bless>
a0d0e21e
LW
606
607=item bless REF
608
2b5ab1e7
TC
609This function tells the thingy referenced by REF that it is now an object
610in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 611is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 612it returns the reference for convenience. Always use the two-argument
cf264981
SP
613version if a derived class might inherit the function doing the blessing.
614See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
615of objects.
a0d0e21e 616
57668c4d 617Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 618Namespaces with all lowercase names are considered reserved for
cf264981 619Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
620confusion, you may wish to avoid such package names as well. Make sure
621that CLASSNAME is a true value.
60ad88b8
GS
622
623See L<perlmod/"Perl Modules">.
624
0d863452
RH
625=item break
626
627Break out of a C<given()> block.
628
629This keyword is enabled by the "switch" feature: see L<feature>
630for more information.
631
a0d0e21e 632=item caller EXPR
d74e8afc 633X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
634
635=item caller
636
5a964f20 637Returns the context of the current subroutine call. In scalar context,
80d38338
TC
638returns the caller's package name if there I<is> a caller (that is, if
639we're in a subroutine or C<eval> or C<require>) and the undefined value
5a964f20 640otherwise. In list context, returns
a0d0e21e 641
ee6b43cc 642 # 0 1 2
748a9306 643 ($package, $filename, $line) = caller;
a0d0e21e
LW
644
645With EXPR, it returns some extra information that the debugger uses to
646print a stack trace. The value of EXPR indicates how many call frames
647to go back before the current one.
648
ee6b43cc 649 # 0 1 2 3 4
f3aa04c2 650 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 651
652 # 5 6 7 8 9 10
b3ca2e83 653 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 654 = caller($i);
e7ea3e70 655
951ba7fe 656Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 657call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 658C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 659C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 660C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 661$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
662each C<use> statement creates a C<require> frame inside an C<eval EXPR>
663frame.) $subroutine may also be C<(unknown)> if this particular
664subroutine happens to have been deleted from the symbol table.
665C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
666C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
667compiled with. The C<$hints> and C<$bitmask> values are subject to change
668between versions of Perl, and are not meant for external use.
748a9306 669
b3ca2e83
NC
670C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
671caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
672of this hash, as they are the actual values stored in the optree.
673
748a9306 674Furthermore, when called from within the DB package, caller returns more
7660c0ab 675detailed information: it sets the list variable C<@DB::args> to be the
54310121 676arguments with which the subroutine was invoked.
748a9306 677
7660c0ab 678Be aware that the optimizer might have optimized call frames away before
19799a22 679C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 680might not return information about the call frame you expect it to, for
b76cc8ba 681C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 682previous time C<caller> was called.
7660c0ab 683
a0d0e21e 684=item chdir EXPR
d74e8afc
ITB
685X<chdir>
686X<cd>
f723aae1 687X<directory, change>
a0d0e21e 688
c4aca7d0
GA
689=item chdir FILEHANDLE
690
691=item chdir DIRHANDLE
692
ce2984c3
PF
693=item chdir
694
ffce7b87 695Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 696changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 697changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0 698variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
80d38338 699neither is set, C<chdir> does nothing. It returns true on success,
b4ad75f0 700false otherwise. See the example under C<die>.
a0d0e21e 701
80d38338 702On systems that support fchdir, you may pass a file handle or
c4aca7d0
GA
703directory handle as argument. On systems that don't support fchdir,
704passing handles produces a fatal error at run time.
705
a0d0e21e 706=item chmod LIST
d74e8afc 707X<chmod> X<permission> X<mode>
a0d0e21e
LW
708
709Changes the permissions of a list of files. The first element of the
4633a7c4 710list must be the numerical mode, which should probably be an octal
4ad40acf 711number, and which definitely should I<not> be a string of octal digits:
2f9daede 712C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 713successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
714
715 $cnt = chmod 0755, 'foo', 'bar';
716 chmod 0755, @executables;
f86cebdf
GS
717 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
718 # --w----r-T
2f9daede
TPG
719 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
720 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 721
80d38338 722On systems that support fchmod, you may pass file handles among the
c4aca7d0 723files. On systems that don't support fchmod, passing file handles
345da378
GA
724produces a fatal error at run time. The file handles must be passed
725as globs or references to be recognized. Barewords are considered
726file names.
c4aca7d0
GA
727
728 open(my $fh, "<", "foo");
729 my $perm = (stat $fh)[2] & 07777;
730 chmod($perm | 0600, $fh);
731
ca6e1c26
JH
732You can also import the symbolic C<S_I*> constants from the Fcntl
733module:
734
735 use Fcntl ':mode';
736
737 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
738 # This is identical to the chmod 0755 of the above example.
739
a0d0e21e 740=item chomp VARIABLE
d74e8afc 741X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 742
313c9f5c 743=item chomp( LIST )
a0d0e21e
LW
744
745=item chomp
746
2b5ab1e7
TC
747This safer version of L</chop> removes any trailing string
748that corresponds to the current value of C<$/> (also known as
28757baa
PP
749$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
750number of characters removed from all its arguments. It's often used to
751remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
752that the final record may be missing its newline. When in paragraph
753mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
754When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
755a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 756remove anything.
19799a22 757If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
758
759 while (<>) {
a9a5a0dc
VP
760 chomp; # avoid \n on last field
761 @array = split(/:/);
762 # ...
a0d0e21e
LW
763 }
764
4bf21a6d
RD
765If VARIABLE is a hash, it chomps the hash's values, but not its keys.
766
a0d0e21e
LW
767You can actually chomp anything that's an lvalue, including an assignment:
768
769 chomp($cwd = `pwd`);
770 chomp($answer = <STDIN>);
771
772If you chomp a list, each element is chomped, and the total number of
773characters removed is returned.
774
15e44fd8
RGS
775Note that parentheses are necessary when you're chomping anything
776that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
777is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
778C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
779C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
780as C<chomp($a, $b)>.
781
a0d0e21e 782=item chop VARIABLE
d74e8afc 783X<chop>
a0d0e21e 784
313c9f5c 785=item chop( LIST )
a0d0e21e
LW
786
787=item chop
788
789Chops off the last character of a string and returns the character
5b3eff12 790chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 791scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
792If VARIABLE is a hash, it chops the hash's values, but not its keys.
793
5b3eff12 794You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
795
796If you chop a list, each element is chopped. Only the value of the
19799a22 797last C<chop> is returned.
a0d0e21e 798
19799a22 799Note that C<chop> returns the last character. To return all but the last
748a9306
LW
800character, use C<substr($string, 0, -1)>.
801
15e44fd8
RGS
802See also L</chomp>.
803
a0d0e21e 804=item chown LIST
d74e8afc 805X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
806
807Changes the owner (and group) of a list of files. The first two
19799a22
GS
808elements of the list must be the I<numeric> uid and gid, in that
809order. A value of -1 in either position is interpreted by most
810systems to leave that value unchanged. Returns the number of files
811successfully changed.
a0d0e21e
LW
812
813 $cnt = chown $uid, $gid, 'foo', 'bar';
814 chown $uid, $gid, @filenames;
815
80d38338 816On systems that support fchown, you may pass file handles among the
c4aca7d0 817files. On systems that don't support fchown, passing file handles
345da378
GA
818produces a fatal error at run time. The file handles must be passed
819as globs or references to be recognized. Barewords are considered
820file names.
c4aca7d0 821
54310121 822Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
823
824 print "User: ";
19799a22 825 chomp($user = <STDIN>);
5a964f20 826 print "Files: ";
19799a22 827 chomp($pattern = <STDIN>);
a0d0e21e
LW
828
829 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 830 or die "$user not in passwd file";
a0d0e21e 831
5ed4f2ec 832 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
833 chown $uid, $gid, @ary;
834
54310121 835On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
836file unless you're the superuser, although you should be able to change
837the group to any of your secondary groups. On insecure systems, these
838restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
839On POSIX systems, you can detect this condition this way:
840
841 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
842 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 843
a0d0e21e 844=item chr NUMBER
d74e8afc 845X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 846
54310121 847=item chr
bbce6d69 848
a0d0e21e 849Returns the character represented by that NUMBER in the character set.
a0ed51b3 850For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 851chr(0x263a) is a Unicode smiley face.
aaa68c4a 852
8a064bd6 853Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 854except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
855(truncated to an integer) are used.
856
974da8e5
JH
857If NUMBER is omitted, uses C<$_>.
858
b76cc8ba 859For the reverse, use L</ord>.
a0d0e21e 860
2575c402
JW
861Note that characters from 128 to 255 (inclusive) are by default
862internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 863
2575c402 864See L<perlunicode> for more about Unicode.
bbce6d69 865
a0d0e21e 866=item chroot FILENAME
d74e8afc 867X<chroot> X<root>
a0d0e21e 868
54310121 869=item chroot
bbce6d69 870
5a964f20 871This function works like the system call by the same name: it makes the
4633a7c4 872named directory the new root directory for all further pathnames that
951ba7fe 873begin with a C</> by your process and all its children. (It doesn't
28757baa 874change your current working directory, which is unaffected.) For security
4633a7c4 875reasons, this call is restricted to the superuser. If FILENAME is
19799a22 876omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
877
878=item close FILEHANDLE
d74e8afc 879X<close>
a0d0e21e 880
6a518fbc
TP
881=item close
882
e0f13c26
RGS
883Closes the file or pipe associated with the file handle, flushes the IO
884buffers, and closes the system file descriptor. Returns true if those
885operations have succeeded and if no error was reported by any PerlIO
886layer. Closes the currently selected filehandle if the argument is
887omitted.
fb73857a
PP
888
889You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
890another C<open> on it, because C<open> will close it for you. (See
891C<open>.) However, an explicit C<close> on an input file resets the line
892counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 893
dede8123
RGS
894If the file handle came from a piped open, C<close> will additionally
895return false if one of the other system calls involved fails, or if the
fb73857a 896program exits with non-zero status. (If the only problem was that the
dede8123 897program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
80d38338
TC
898also waits for the process executing on the pipe to exit, in case you
899wish to look at the output of the pipe afterwards, and
e5218da5
GA
900implicitly puts the exit status value of that command into C<$?> and
901C<${^CHILD_ERROR_NATIVE}>.
5a964f20 902
80d38338
TC
903Closing the read end of a pipe before the process writing to it at the
904other end is done writing results in the writer receiving a SIGPIPE. If
905the other end can't handle that, be sure to read all the data before
906closing the pipe.
73689b13 907
fb73857a 908Example:
a0d0e21e 909
fb73857a
PP
910 open(OUTPUT, '|sort >foo') # pipe to sort
911 or die "Can't start sort: $!";
5ed4f2ec 912 #... # print stuff to output
913 close OUTPUT # wait for sort to finish
fb73857a
PP
914 or warn $! ? "Error closing sort pipe: $!"
915 : "Exit status $? from sort";
5ed4f2ec 916 open(INPUT, 'foo') # get sort's results
fb73857a 917 or die "Can't open 'foo' for input: $!";
a0d0e21e 918
5a964f20
TC
919FILEHANDLE may be an expression whose value can be used as an indirect
920filehandle, usually the real filehandle name.
a0d0e21e
LW
921
922=item closedir DIRHANDLE
d74e8afc 923X<closedir>
a0d0e21e 924
19799a22 925Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
926system call.
927
a0d0e21e 928=item connect SOCKET,NAME
d74e8afc 929X<connect>
a0d0e21e 930
80d38338
TC
931Attempts to connect to a remote socket, just like connect(2).
932Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
933packed address of the appropriate type for the socket. See the examples in
934L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 935
cb1a09d0 936=item continue BLOCK
d74e8afc 937X<continue>
cb1a09d0 938
0d863452
RH
939=item continue
940
cf264981
SP
941C<continue> is actually a flow control statement rather than a function. If
942there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
943C<foreach>), it is always executed just before the conditional is about to
944be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
945it can be used to increment a loop variable, even when the loop has been
946continued via the C<next> statement (which is similar to the C C<continue>
947statement).
948
98293880 949C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
950block. C<last> and C<redo> will behave as if they had been executed within
951the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
952block, it may be more entertaining.
953
954 while (EXPR) {
a9a5a0dc
VP
955 ### redo always comes here
956 do_something;
1d2dff63 957 } continue {
a9a5a0dc
VP
958 ### next always comes here
959 do_something_else;
960 # then back the top to re-check EXPR
1d2dff63
GS
961 }
962 ### last always comes here
963
964Omitting the C<continue> section is semantically equivalent to using an
19799a22 965empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
966to check the condition at the top of the loop.
967
0d863452
RH
968If the "switch" feature is enabled, C<continue> is also a
969function that will break out of the current C<when> or C<default>
970block, and fall through to the next case. See L<feature> and
971L<perlsyn/"Switch statements"> for more information.
972
973
a0d0e21e 974=item cos EXPR
d74e8afc 975X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 976
d6217f1e
GS
977=item cos
978
5a964f20 979Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 980takes cosine of C<$_>.
a0d0e21e 981
ca6e1c26 982For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
983function, or use this relation:
984
985 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
986
a0d0e21e 987=item crypt PLAINTEXT,SALT
d74e8afc 988X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 989X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 990
ef2e6798
MS
991Creates a digest string exactly like the crypt(3) function in the C
992library (assuming that you actually have a version there that has not
bb23f8d1 993been extirpated as a potential munition).
ef2e6798
MS
994
995crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
996into a short string, called a digest, which is returned. The same
997PLAINTEXT and SALT will always return the same string, but there is no
998(known) way to get the original PLAINTEXT from the hash. Small
999changes in the PLAINTEXT or SALT will result in large changes in the
1000digest.
1001
1002There is no decrypt function. This function isn't all that useful for
1003cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1004mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1005primarily used to check if two pieces of text are the same without
1006having to transmit or store the text itself. An example is checking
1007if a correct password is given. The digest of the password is stored,
cf264981 1008not the password itself. The user types in a password that is
ef2e6798
MS
1009crypt()'d with the same salt as the stored digest. If the two digests
1010match the password is correct.
1011
1012When verifying an existing digest string you should use the digest as
1013the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1014to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1015crypt() will hash the new string with the same salt as the digest.
1016This allows your code to work with the standard L<crypt|/crypt> and
1017with more exotic implementations. In other words, do not assume
1018anything about the returned string itself, or how many bytes in the
1019digest matter.
85c16d83
JH
1020
1021Traditionally the result is a string of 13 bytes: two first bytes of
1022the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
bb23f8d1 1023the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1024hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1025and implementations on non-Unix platforms may produce different
ef2e6798 1026strings.
85c16d83
JH
1027
1028When choosing a new salt create a random two character string whose
1029characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1030'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1031characters is just a recommendation; the characters allowed in
1032the salt depend solely on your system's crypt library, and Perl can't
1033restrict what salts C<crypt()> accepts.
e71965be 1034
a0d0e21e 1035Here's an example that makes sure that whoever runs this program knows
cf264981 1036their password:
a0d0e21e
LW
1037
1038 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1039
1040 system "stty -echo";
1041 print "Password: ";
e71965be 1042 chomp($word = <STDIN>);
a0d0e21e
LW
1043 print "\n";
1044 system "stty echo";
1045
e71965be 1046 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1047 die "Sorry...\n";
a0d0e21e 1048 } else {
a9a5a0dc 1049 print "ok\n";
54310121 1050 }
a0d0e21e 1051
9f8f0c9d 1052Of course, typing in your own password to whoever asks you
748a9306 1053for it is unwise.
a0d0e21e 1054
ef2e6798 1055The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1056of data, not least of all because you can't get the information
ef2e6798 1057back. Look at the L<Digest> module for more robust algorithms.
19799a22 1058
f2791508
JH
1059If using crypt() on a Unicode string (which I<potentially> has
1060characters with codepoints above 255), Perl tries to make sense
1061of the situation by trying to downgrade (a copy of the string)
1062the string back to an eight-bit byte string before calling crypt()
1063(on that copy). If that works, good. If not, crypt() dies with
1064C<Wide character in crypt>.
85c16d83 1065
aa689395 1066=item dbmclose HASH
d74e8afc 1067X<dbmclose>
a0d0e21e 1068
19799a22 1069[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1070
aa689395 1071Breaks the binding between a DBM file and a hash.
a0d0e21e 1072
19799a22 1073=item dbmopen HASH,DBNAME,MASK
d74e8afc 1074X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1075
19799a22 1076[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1077
7b8d334a 1078This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1079hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1080argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1081is the name of the database (without the F<.dir> or F<.pag> extension if
1082any). If the database does not exist, it is created with protection
19799a22 1083specified by MASK (as modified by the C<umask>). If your system supports
80d38338 1084only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1085program. In older versions of Perl, if your system had neither DBM nor
19799a22 1086ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1087sdbm(3).
1088
1089If you don't have write access to the DBM file, you can only read hash
1090variables, not set them. If you want to test whether you can write,
19799a22 1091either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 1092which will trap the error.
a0d0e21e 1093
19799a22
GS
1094Note that functions such as C<keys> and C<values> may return huge lists
1095when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1096function to iterate over large DBM files. Example:
1097
1098 # print out history file offsets
1099 dbmopen(%HIST,'/usr/lib/news/history',0666);
1100 while (($key,$val) = each %HIST) {
a9a5a0dc 1101 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1102 }
1103 dbmclose(%HIST);
1104
cb1a09d0 1105See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1106cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1107rich implementation.
4633a7c4 1108
2b5ab1e7
TC
1109You can control which DBM library you use by loading that library
1110before you call dbmopen():
1111
1112 use DB_File;
1113 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1114 or die "Can't open netscape history file: $!";
2b5ab1e7 1115
a0d0e21e 1116=item defined EXPR
d74e8afc 1117X<defined> X<undef> X<undefined>
a0d0e21e 1118
54310121 1119=item defined
bbce6d69 1120
2f9daede
TPG
1121Returns a Boolean value telling whether EXPR has a value other than
1122the undefined value C<undef>. If EXPR is not present, C<$_> will be
1123checked.
1124
1125Many operations return C<undef> to indicate failure, end of file,
1126system error, uninitialized variable, and other exceptional
1127conditions. This function allows you to distinguish C<undef> from
1128other values. (A simple Boolean test will not distinguish among
7660c0ab 1129C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1130false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1131doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1132returns C<undef> when its argument is an empty array, I<or> when the
1133element to return happens to be C<undef>.
1134
f10b0346
GS
1135You may also use C<defined(&func)> to check whether subroutine C<&func>
1136has ever been defined. The return value is unaffected by any forward
80d38338 1137declarations of C<&func>. A subroutine that is not defined
847c7ebe 1138may still be callable: its package may have an C<AUTOLOAD> method that
80d38338 1139makes it spring into existence the first time that it is called--see
847c7ebe 1140L<perlsub>.
f10b0346
GS
1141
1142Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1143used to report whether memory for that aggregate has ever been
1144allocated. This behavior may disappear in future versions of Perl.
1145You should instead use a simple test for size:
1146
1147 if (@an_array) { print "has array elements\n" }
1148 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1149
1150When used on a hash element, it tells you whether the value is defined,
dc848c6f 1151not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1152purpose.
a0d0e21e
LW
1153
1154Examples:
1155
1156 print if defined $switch{'D'};
1157 print "$val\n" while defined($val = pop(@ary));
1158 die "Can't readlink $sym: $!"
a9a5a0dc 1159 unless defined($value = readlink $sym);
a0d0e21e 1160 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1161 $debugging = 0 unless defined $debugging;
a0d0e21e 1162
19799a22 1163Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1164discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1165defined values. For example, if you say
a5f75d66
AD
1166
1167 "ab" =~ /a(.*)b/;
1168
80d38338 1169The pattern match succeeds and C<$1> is defined, although it
cf264981 1170matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1171matched something that happened to be zero characters long. This is all
a5f75d66 1172very above-board and honest. When a function returns an undefined value,
2f9daede 1173it's an admission that it couldn't give you an honest answer. So you
19799a22 1174should use C<defined> only when you're questioning the integrity of what
7660c0ab 1175you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1176what you want.
1177
dc848c6f 1178See also L</undef>, L</exists>, L</ref>.
2f9daede 1179
a0d0e21e 1180=item delete EXPR
d74e8afc 1181X<delete>
a0d0e21e 1182
80d38338
TC
1183Given an expression that specifies an element or slice of an aggregate (a
1184hash or an array), deletes the specified elements from that aggregate so
1185that exists() on that element no longer returns true. Setting an aggregate
1186element to the undefined value does not remove its key, but deleting it
1187does; see L</exists>.
1188
1189Returns the value or values deleted in list context, or the last such
1190element in scalar context. The return list's length always matches that of
1191the argument list: deleting non-existent elements returns the undefined
1192value in their corresponding positions.
1193
1194Deleting array elements never changes indices of existing values; use
1195shift() or splice() for that. However, if all deleted elements fall at
1196the end of an array, the array's size shrinks to the position of the
1197highest element that still tests true for exists(), or to 0 if none do.
1198
1199Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1200a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1201or array may not necessarily return anything; it depends on the implementation
1202of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1203
80d38338
TC
1204The C<delete local EXPR> construct localizes the deletion to the current
1205block at run time. Until the block exits, elements locally deleted
1206temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1207of composite types">.
eba0920a
EM
1208
1209 %hash = (foo => 11, bar => 22, baz => 33);
1210 $scalar = delete $hash{foo}; # $scalar is 11
1211 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1212 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1213
01020589 1214The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1215
5f05dabc 1216 foreach $key (keys %HASH) {
a9a5a0dc 1217 delete $HASH{$key};
a0d0e21e
LW
1218 }
1219
01020589 1220 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1221 delete $ARRAY[$index];
01020589
GS
1222 }
1223
1224And so do these:
5f05dabc 1225
01020589
GS
1226 delete @HASH{keys %HASH};
1227
9740c838 1228 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1229
80d38338
TC
1230But both are slower than assigning the empty list
1231or undefining %HASH or @ARRAY, which is the customary
1232way to empty out an aggregate:
01020589 1233
5ed4f2ec 1234 %HASH = (); # completely empty %HASH
1235 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1236
5ed4f2ec 1237 @ARRAY = (); # completely empty @ARRAY
1238 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1239
80d38338
TC
1240The EXPR can be arbitrarily complicated provided its
1241final operation is an element or slice of an aggregate:
a0d0e21e
LW
1242
1243 delete $ref->[$x][$y]{$key};
5f05dabc 1244 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1245
01020589
GS
1246 delete $ref->[$x][$y][$index];
1247 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1248
a0d0e21e 1249=item die LIST
d74e8afc 1250X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1251
19799a22
GS
1252Outside an C<eval>, prints the value of LIST to C<STDERR> and
1253exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1254exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1255status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1256an C<eval(),> the error message is stuffed into C<$@> and the
1257C<eval> is terminated with the undefined value. This makes
1258C<die> the way to raise an exception.
a0d0e21e
LW
1259
1260Equivalent examples:
1261
1262 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1263 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1264
ccac6780 1265If the last element of LIST does not end in a newline, the current
df37ec69
WW
1266script line number and input line number (if any) are also printed,
1267and a newline is supplied. Note that the "input line number" (also
1268known as "chunk") is subject to whatever notion of "line" happens to
1269be currently in effect, and is also available as the special variable
1270C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1271
1272Hint: sometimes appending C<", stopped"> to your message will cause it
1273to make better sense when the string C<"at foo line 123"> is appended.
1274Suppose you are running script "canasta".
a0d0e21e
LW
1275
1276 die "/etc/games is no good";
1277 die "/etc/games is no good, stopped";
1278
1279produce, respectively
1280
1281 /etc/games is no good at canasta line 123.
1282 /etc/games is no good, stopped at canasta line 123.
1283
2b5ab1e7 1284See also exit(), warn(), and the Carp module.
a0d0e21e 1285
a96d0188 1286If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1287previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1288This is useful for propagating exceptions:
1289
1290 eval { ... };
1291 die unless $@ =~ /Expected exception/;
1292
a96d0188 1293If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1294C<PROPAGATE> method, that method will be called with additional file
1295and line number parameters. The return value replaces the value in
80d38338 1296C<$@>. i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1297were called.
1298
7660c0ab 1299If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1300
80d38338
TC
1301You can also call C<die> with a reference argument, and if this is trapped
1302within an C<eval>, C<$@> contains that reference. This permits more
1303elaborate exception handling using objects that maintain arbitrary state
1304about the exception. Such a scheme is sometimes preferable to matching
1305particular string values of C<$@> with regular expressions. Because C<$@>
1306is a global variable and C<eval> may be used within object implementations,
1307be careful that analyzing the error object doesn't replace the reference in
1308the global variable. It's easiest to make a local copy of the reference
1309before any manipulations. Here's an example:
52531d10 1310
80d38338 1311 use Scalar::Util "blessed";
da279afe 1312
52531d10 1313 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1314 if (my $ev_err = $@) {
1315 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1316 # handle Some::Module::Exception
1317 }
1318 else {
1319 # handle all other possible exceptions
1320 }
1321 }
1322
80d38338
TC
1323Because perl stringifies uncaught exception messages before display,
1324you'll probably want to overload stringification operations on
52531d10
GS
1325exception objects. See L<overload> for details about that.
1326
19799a22
GS
1327You can arrange for a callback to be run just before the C<die>
1328does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1329handler will be called with the error text and can change the error
1330message, if it sees fit, by calling C<die> again. See
1331L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1332L<"eval BLOCK"> for some examples. Although this feature was
19799a22
GS
1333to be run only right before your program was to exit, this is not
1334currently the case--the C<$SIG{__DIE__}> hook is currently called
1335even inside eval()ed blocks/strings! If one wants the hook to do
1336nothing in such situations, put
fb73857a 1337
5ed4f2ec 1338 die @_ if $^S;
fb73857a 1339
19799a22
GS
1340as the first line of the handler (see L<perlvar/$^S>). Because
1341this promotes strange action at a distance, this counterintuitive
b76cc8ba 1342behavior may be fixed in a future release.
774d564b 1343
a0d0e21e 1344=item do BLOCK
d74e8afc 1345X<do> X<block>
a0d0e21e
LW
1346
1347Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1348sequence of commands indicated by BLOCK. When modified by the C<while> or
1349C<until> loop modifier, executes the BLOCK once before testing the loop
1350condition. (On other statements the loop modifiers test the conditional
1351first.)
a0d0e21e 1352
4968c1e4 1353C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1354C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1355See L<perlsyn> for alternative strategies.
4968c1e4 1356
a0d0e21e 1357=item do SUBROUTINE(LIST)
d74e8afc 1358X<do>
a0d0e21e 1359
cf264981 1360This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1361
1362=item do EXPR
d74e8afc 1363X<do>
a0d0e21e
LW
1364
1365Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1366file as a Perl script.
a0d0e21e
LW
1367
1368 do 'stat.pl';
1369
1370is just like
1371
986b19de 1372 eval `cat stat.pl`;
a0d0e21e 1373
2b5ab1e7 1374except that it's more efficient and concise, keeps track of the current
ea63ef19 1375filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1376C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1377variables. It also differs in that code evaluated with C<do FILENAME>
1378cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1379same, however, in that it does reparse the file every time you call it,
1380so you probably don't want to do this inside a loop.
a0d0e21e 1381
8e30cc93 1382If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1383error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1384returns undef and sets an error message in C<$@>. If the file is
1385successfully compiled, C<do> returns the value of the last expression
1386evaluated.
1387
80d38338 1388Inclusion of library modules is better done with the
19799a22 1389C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1390and raise an exception if there's a problem.
a0d0e21e 1391
5a964f20
TC
1392You might like to use C<do> to read in a program configuration
1393file. Manual error checking can be done this way:
1394
b76cc8ba 1395 # read in config files: system first, then user
f86cebdf 1396 for $file ("/share/prog/defaults.rc",
b76cc8ba 1397 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1398 {
1399 unless ($return = do $file) {
1400 warn "couldn't parse $file: $@" if $@;
1401 warn "couldn't do $file: $!" unless defined $return;
1402 warn "couldn't run $file" unless $return;
1403 }
5a964f20
TC
1404 }
1405
a0d0e21e 1406=item dump LABEL
d74e8afc 1407X<dump> X<core> X<undump>
a0d0e21e 1408
1614b0e3
JD
1409=item dump
1410
19799a22
GS
1411This function causes an immediate core dump. See also the B<-u>
1412command-line switch in L<perlrun>, which does the same thing.
1413Primarily this is so that you can use the B<undump> program (not
1414supplied) to turn your core dump into an executable binary after
1415having initialized all your variables at the beginning of the
1416program. When the new binary is executed it will begin by executing
1417a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1418Think of it as a goto with an intervening core dump and reincarnation.
1419If C<LABEL> is omitted, restarts the program from the top.
1420
1421B<WARNING>: Any files opened at the time of the dump will I<not>
1422be open any more when the program is reincarnated, with possible
80d38338 1423resulting confusion by Perl.
19799a22 1424
59f521f4
RGS
1425This function is now largely obsolete, mostly because it's very hard to
1426convert a core file into an executable. That's why you should now invoke
1427it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1428typo.
19799a22 1429
aa689395 1430=item each HASH
d74e8afc 1431X<each> X<hash, iterator>
aa689395 1432
aeedbbed
NC
1433=item each ARRAY
1434X<array, iterator>
1435
80d38338
TC
1436When called in list context, returns a 2-element list consisting of the key
1437and value for the next element of a hash, or the index and value for the
1438next element of an array, so that you can iterate over it. When called in
1439scalar context, returns only the key (not the value) in a hash, or the index
1440in an array.
2f9daede 1441
aeedbbed 1442Hash entries are returned in an apparently random order. The actual random
504f80c1
JH
1443order is subject to change in future versions of perl, but it is
1444guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1445function would produce on the same (unmodified) hash. Since Perl
22883ac5 14465.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1447for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1448
80d38338
TC
1449After C<each> has returned all entries from the hash or array, the next
1450call to C<each> returns the empty list in list context and C<undef> in
1451scalar context. The next call following that one restarts iteration. Each
1452hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1453and C<values>. The iterator is implicitly reset when C<each> has reached
1454the end as just described; it can be explicitly reset by calling C<keys> or
1455C<values> on the hash or array. If you add or delete a hash's elements
1456while iterating over it, entries may be skipped or duplicated--so don't do
1457that. Exception: It is always safe to delete the item most recently
1458returned by C<each()>, so the following code will work properly:
74fc8b5f
MJD
1459
1460 while (($key, $value) = each %hash) {
1461 print $key, "\n";
1462 delete $hash{$key}; # This is safe
1463 }
aa689395 1464
80d38338 1465This prints out your environment like the printenv(1) program,
aa689395 1466only in a different order:
a0d0e21e
LW
1467
1468 while (($key,$value) = each %ENV) {
a9a5a0dc 1469 print "$key=$value\n";
a0d0e21e
LW
1470 }
1471
19799a22 1472See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1473
1474=item eof FILEHANDLE
d74e8afc
ITB
1475X<eof>
1476X<end of file>
1477X<end-of-file>
a0d0e21e 1478
4633a7c4
LW
1479=item eof ()
1480
a0d0e21e
LW
1481=item eof
1482
1483Returns 1 if the next read on FILEHANDLE will return end of file, or if
1484FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1485gives the real filehandle. (Note that this function actually
80d38338 1486reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1487interactive context.) Do not read from a terminal file (or call
19799a22 1488C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1489as terminals may lose the end-of-file condition if you do.
1490
820475bd 1491An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1492with empty parentheses is different. It refers to the pseudo file
820475bd 1493formed from the files listed on the command line and accessed via the
61eff3bc
JH
1494C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1495as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1496used will cause C<@ARGV> to be examined to determine if input is
67408cae 1497available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1498end-of-file will assume you are processing another C<@ARGV> list,
1499and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1500see L<perlop/"I/O Operators">.
820475bd 1501
61eff3bc 1502In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1503detect the end of each file, C<eof()> will only detect the end of the
1504last file. Examples:
a0d0e21e 1505
748a9306
LW
1506 # reset line numbering on each input file
1507 while (<>) {
a9a5a0dc
VP
1508 next if /^\s*#/; # skip comments
1509 print "$.\t$_";
5a964f20 1510 } continue {
a9a5a0dc 1511 close ARGV if eof; # Not eof()!
748a9306
LW
1512 }
1513
a0d0e21e
LW
1514 # insert dashes just before last line of last file
1515 while (<>) {
a9a5a0dc
VP
1516 if (eof()) { # check for end of last file
1517 print "--------------\n";
1518 }
1519 print;
1520 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1521 }
1522
a0d0e21e 1523Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1524input operators typically return C<undef> when they run out of data, or if
1525there was an error.
a0d0e21e
LW
1526
1527=item eval EXPR
d74e8afc 1528X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1529X<error, handling> X<exception, handling>
a0d0e21e
LW
1530
1531=item eval BLOCK
1532
ce2984c3
PF
1533=item eval
1534
c7cc6f1c
GS
1535In the first form, the return value of EXPR is parsed and executed as if it
1536were a little Perl program. The value of the expression (which is itself
5a964f20 1537determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1538errors, executed in the lexical context of the current Perl program, so
1539that any variable settings or subroutine and format definitions remain
cf264981 1540afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1541If EXPR is omitted, evaluates C<$_>. This form is typically used to
1542delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1543
1544In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1545same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1546within the context of the current Perl program. This form is typically
1547used to trap exceptions more efficiently than the first (see below), while
1548also providing the benefit of checking the code within BLOCK at compile
1549time.
1550
1551The final semicolon, if any, may be omitted from the value of EXPR or within
1552the BLOCK.
1553
1554In both forms, the value returned is the value of the last expression
5a964f20 1555evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1556as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1557in void, scalar, or list context, depending on the context of the C<eval>
1558itself. See L</wantarray> for more on how the evaluation context can be
1559determined.
a0d0e21e 1560
19799a22 1561If there is a syntax error or runtime error, or a C<die> statement is
bbead3ca
BL
1562executed, C<eval> returns an undefined value in scalar context
1563or an empty list in list context, and C<$@> is set to the
a0d0e21e 1564error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1565string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1566warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1567To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1568turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1569See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1570
19799a22
GS
1571Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1572determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1573is implemented. It is also Perl's exception trapping mechanism, where
1574the die operator is used to raise exceptions.
1575
5f1da31c
NT
1576If you want to trap errors when loading an XS module, some problems with
1577the binary interface (such as Perl version skew) may be fatal even with
1578C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1579
a0d0e21e
LW
1580If the code to be executed doesn't vary, you may use the eval-BLOCK
1581form to trap run-time errors without incurring the penalty of
1582recompiling each time. The error, if any, is still returned in C<$@>.
1583Examples:
1584
54310121 1585 # make divide-by-zero nonfatal
a0d0e21e
LW
1586 eval { $answer = $a / $b; }; warn $@ if $@;
1587
1588 # same thing, but less efficient
1589 eval '$answer = $a / $b'; warn $@ if $@;
1590
1591 # a compile-time error
5ed4f2ec 1592 eval { $answer = }; # WRONG
a0d0e21e
LW
1593
1594 # a run-time error
5ed4f2ec 1595 eval '$answer ='; # sets $@
a0d0e21e 1596
cf264981
SP
1597Using the C<eval{}> form as an exception trap in libraries does have some
1598issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1599may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1600You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1601as this example shows:
774d564b 1602
80d38338 1603 # a private exception trap for divide-by-zero
f86cebdf
GS
1604 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1605 warn $@ if $@;
774d564b
PP
1606
1607This is especially significant, given that C<__DIE__> hooks can call
19799a22 1608C<die> again, which has the effect of changing their error messages:
774d564b
PP
1609
1610 # __DIE__ hooks may modify error messages
1611 {
f86cebdf
GS
1612 local $SIG{'__DIE__'} =
1613 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1614 eval { die "foo lives here" };
1615 print $@ if $@; # prints "bar lives here"
774d564b
PP
1616 }
1617
19799a22 1618Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1619may be fixed in a future release.
1620
19799a22 1621With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1622being looked at when:
1623
5ed4f2ec 1624 eval $x; # CASE 1
1625 eval "$x"; # CASE 2
a0d0e21e 1626
5ed4f2ec 1627 eval '$x'; # CASE 3
1628 eval { $x }; # CASE 4
a0d0e21e 1629
5ed4f2ec 1630 eval "\$$x++"; # CASE 5
1631 $$x++; # CASE 6
a0d0e21e 1632
2f9daede 1633Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1634the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1635the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1636and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1637does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1638purely visual reasons, but it also has the advantage of compiling at
1639compile-time instead of at run-time.) Case 5 is a place where
19799a22 1640normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1641particular situation, you can just use symbolic references instead, as
1642in case 6.
a0d0e21e 1643
8a5a710d
DN
1644The assignment to C<$@> occurs before restoration of localised variables,
1645which means a temporary is required if you want to mask some but not all
1646errors:
1647
1648 # alter $@ on nefarious repugnancy only
1649 {
1650 my $e;
1651 {
1652 local $@; # protect existing $@
1653 eval { test_repugnancy() };
1654 # $@ =~ /nefarious/ and die $@; # DOES NOT WORK
1655 $@ =~ /nefarious/ and $e = $@;
1656 }
1657 die $e if defined $e
1658 }
1659
4968c1e4 1660C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1661C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1662
80d38338 1663Note that as a special case, an C<eval ''> executed within the C<DB>
d819b83a
DM
1664package doesn't see the usual surrounding lexical scope, but rather the
1665scope of the first non-DB piece of code that called it. You don't normally
1666need to worry about this unless you are writing a Perl debugger.
1667
a0d0e21e 1668=item exec LIST
d74e8afc 1669X<exec> X<execute>
a0d0e21e 1670
8bf3b016
GS
1671=item exec PROGRAM LIST
1672
19799a22
GS
1673The C<exec> function executes a system command I<and never returns>--
1674use C<system> instead of C<exec> if you want it to return. It fails and
1675returns false only if the command does not exist I<and> it is executed
fb73857a 1676directly instead of via your system's command shell (see below).
a0d0e21e 1677
19799a22 1678Since it's a common mistake to use C<exec> instead of C<system>, Perl
80d38338
TC
1679warns you if there is a following statement that isn't C<die>, C<warn>,
1680or C<exit> (if C<-w> is set --but you always do that). If you
19799a22 1681I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1682can use one of these styles to avoid the warning:
1683
5a964f20
TC
1684 exec ('foo') or print STDERR "couldn't exec foo: $!";
1685 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1686
5a964f20 1687If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1688with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1689If there is only one scalar argument or an array with one element in it,
1690the argument is checked for shell metacharacters, and if there are any,
1691the entire argument is passed to the system's command shell for parsing
1692(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1693If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1694words and passed directly to C<execvp>, which is more efficient.
19799a22 1695Examples:
a0d0e21e 1696
19799a22
GS
1697 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1698 exec "sort $outfile | uniq";
a0d0e21e
LW
1699
1700If you don't really want to execute the first argument, but want to lie
1701to the program you are executing about its own name, you can specify
1702the program you actually want to run as an "indirect object" (without a
1703comma) in front of the LIST. (This always forces interpretation of the
54310121 1704LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1705the list.) Example:
1706
1707 $shell = '/bin/csh';
5ed4f2ec 1708 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1709
1710or, more directly,
1711
5ed4f2ec 1712 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1713
bb32b41a
GS
1714When the arguments get executed via the system shell, results will
1715be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1716for details.
1717
19799a22
GS
1718Using an indirect object with C<exec> or C<system> is also more
1719secure. This usage (which also works fine with system()) forces
1720interpretation of the arguments as a multivalued list, even if the
1721list had just one argument. That way you're safe from the shell
1722expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1723
1724 @args = ( "echo surprise" );
1725
2b5ab1e7 1726 exec @args; # subject to shell escapes
f86cebdf 1727 # if @args == 1
2b5ab1e7 1728 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1729
1730The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1731program, passing it C<"surprise"> an argument. The second version didn't;
1732it tried to run a program named I<"echo surprise">, didn't find it, and set
1733C<$?> to a non-zero value indicating failure.
5a964f20 1734
80d38338 1735Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1736output before the exec, but this may not be supported on some platforms
1737(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1738in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 1739open handles to avoid lost output.
0f897271 1740
80d38338
TC
1741Note that C<exec> will not call your C<END> blocks, nor will it invoke
1742C<DESTROY> methods on your objects.
7660c0ab 1743
a0d0e21e 1744=item exists EXPR
d74e8afc 1745X<exists> X<autovivification>
a0d0e21e 1746
01020589 1747Given an expression that specifies a hash element or array element,
8ea97a1e 1748returns true if the specified element in the hash or array has ever
95731d22 1749been initialized, even if the corresponding value is undefined.
a0d0e21e 1750
5ed4f2ec 1751 print "Exists\n" if exists $hash{$key};
1752 print "Defined\n" if defined $hash{$key};
01020589
GS
1753 print "True\n" if $hash{$key};
1754
5ed4f2ec 1755 print "Exists\n" if exists $array[$index];
1756 print "Defined\n" if defined $array[$index];
01020589 1757 print "True\n" if $array[$index];
a0d0e21e 1758
8ea97a1e 1759A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1760it exists, but the reverse doesn't necessarily hold true.
1761
afebc493
GS
1762Given an expression that specifies the name of a subroutine,
1763returns true if the specified subroutine has ever been declared, even
1764if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 1765does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
1766exist may still be callable: its package may have an C<AUTOLOAD>
1767method that makes it spring into existence the first time that it is
80d38338 1768called--see L<perlsub>.
afebc493 1769
5ed4f2ec 1770 print "Exists\n" if exists &subroutine;
1771 print "Defined\n" if defined &subroutine;
afebc493 1772
a0d0e21e 1773Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1774operation is a hash or array key lookup or subroutine name:
a0d0e21e 1775
5ed4f2ec 1776 if (exists $ref->{A}->{B}->{$key}) { }
1777 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 1778
5ed4f2ec 1779 if (exists $ref->{A}->{B}->[$ix]) { }
1780 if (exists $hash{A}{B}[$ix]) { }
01020589 1781
afebc493
GS
1782 if (exists &{$ref->{A}{B}{$key}}) { }
1783
01020589
GS
1784Although the deepest nested array or hash will not spring into existence
1785just because its existence was tested, any intervening ones will.
61eff3bc 1786Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1787into existence due to the existence test for the $key element above.
1788This happens anywhere the arrow operator is used, including even:
5a964f20 1789
2b5ab1e7 1790 undef $ref;
5ed4f2ec 1791 if (exists $ref->{"Some key"}) { }
1792 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
1793
1794This surprising autovivification in what does not at first--or even
1795second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1796release.
a0d0e21e 1797
afebc493
GS
1798Use of a subroutine call, rather than a subroutine name, as an argument
1799to exists() is an error.
1800
5ed4f2ec 1801 exists &sub; # OK
1802 exists &sub(); # Error
afebc493 1803
a0d0e21e 1804=item exit EXPR
d74e8afc 1805X<exit> X<terminate> X<abort>
a0d0e21e 1806
ce2984c3
PF
1807=item exit
1808
2b5ab1e7 1809Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1810
1811 $ans = <STDIN>;
1812 exit 0 if $ans =~ /^[Xx]/;
1813
19799a22 1814See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1815universally recognized values for EXPR are C<0> for success and C<1>
1816for error; other values are subject to interpretation depending on the
1817environment in which the Perl program is running. For example, exiting
181869 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1819the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1820
19799a22
GS
1821Don't use C<exit> to abort a subroutine if there's any chance that
1822someone might want to trap whatever error happened. Use C<die> instead,
1823which can be trapped by an C<eval>.
28757baa 1824
19799a22 1825The exit() function does not always exit immediately. It calls any
2b5ab1e7 1826defined C<END> routines first, but these C<END> routines may not
19799a22 1827themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1828be called are called before the real exit. If this is a problem, you
1829can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1830See L<perlmod> for details.
5a964f20 1831
a0d0e21e 1832=item exp EXPR
d74e8afc 1833X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1834
54310121 1835=item exp
bbce6d69 1836
b76cc8ba 1837Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1838If EXPR is omitted, gives C<exp($_)>.
1839
1840=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1841X<fcntl>
a0d0e21e 1842
f86cebdf 1843Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1844
1845 use Fcntl;
1846
0ade1984 1847first to get the correct constant definitions. Argument processing and
b76cc8ba 1848value return works just like C<ioctl> below.
a0d0e21e
LW
1849For example:
1850
1851 use Fcntl;
5a964f20 1852 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 1853 or die "can't fcntl F_GETFL: $!";
5a964f20 1854
554ad1fc 1855You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1856Like C<ioctl>, it maps a C<0> return from the system call into
1857C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1858in numeric context. It is also exempt from the normal B<-w> warnings
1859on improper numeric conversions.
5a964f20 1860
19799a22 1861Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1862doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1863manpage to learn what functions are available on your system.
a0d0e21e 1864
be2f7487 1865Here's an example of setting a filehandle named C<REMOTE> to be
1866non-blocking at the system level. You'll have to negotiate C<$|>
1867on your own, though.
1868
1869 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1870
1871 $flags = fcntl(REMOTE, F_GETFL, 0)
1872 or die "Can't get flags for the socket: $!\n";
1873
1874 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1875 or die "Can't set flags for the socket: $!\n";
1876
a0d0e21e 1877=item fileno FILEHANDLE
d74e8afc 1878X<fileno>
a0d0e21e 1879
2b5ab1e7
TC
1880Returns the file descriptor for a filehandle, or undefined if the
1881filehandle is not open. This is mainly useful for constructing
19799a22 1882bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1883If FILEHANDLE is an expression, the value is taken as an indirect
1884filehandle, generally its name.
5a964f20 1885
b76cc8ba 1886You can use this to find out whether two handles refer to the
5a964f20
TC
1887same underlying descriptor:
1888
1889 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 1890 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1891 }
1892
1893(Filehandles connected to memory objects via new features of C<open> may
1894return undefined even though they are open.)
1895
a0d0e21e
LW
1896
1897=item flock FILEHANDLE,OPERATION
d74e8afc 1898X<flock> X<lock> X<locking>
a0d0e21e 1899
19799a22
GS
1900Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1901for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1902machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1903C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1904only entire files, not records.
1905
1906Two potentially non-obvious but traditional C<flock> semantics are
1907that it waits indefinitely until the lock is granted, and that its locks
1908B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1909fewer guarantees. This means that programs that do not also use C<flock>
1910may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1911your port's specific documentation, or your system-specific local manpages
1912for details. It's best to assume traditional behavior if you're writing
1913portable programs. (But if you're not, you should as always feel perfectly
1914free to write for your own system's idiosyncrasies (sometimes called
1915"features"). Slavish adherence to portability concerns shouldn't get
1916in the way of your getting your job done.)
a3cb178b 1917
8ebc5c01
PP
1918OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1919LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1920you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1921either individually, or as a group using the ':flock' tag. LOCK_SH
1922requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1923releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1924LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1925waiting for the lock (check the return status to see if you got it).
1926
2b5ab1e7
TC
1927To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1928before locking or unlocking it.
8ebc5c01 1929
f86cebdf 1930Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1931locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1932are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1933implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1934differing semantics shouldn't bite too many people.
1935
becacb53
TM
1936Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1937be open with read intent to use LOCK_SH and requires that it be open
1938with write intent to use LOCK_EX.
1939
19799a22
GS
1940Note also that some versions of C<flock> cannot lock things over the
1941network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1942that. If you like you can force Perl to ignore your system's flock(2)
1943function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1944the switch C<-Ud_flock> to the F<Configure> program when you configure
1945perl.
4633a7c4
LW
1946
1947Here's a mailbox appender for BSD systems.
a0d0e21e 1948
7ed5353d 1949 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
1950
1951 sub lock {
a9a5a0dc
VP
1952 my ($fh) = @_;
1953 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 1954
a9a5a0dc
VP
1955 # and, in case someone appended while we were waiting...
1956 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
1957 }
1958
1959 sub unlock {
a9a5a0dc
VP
1960 my ($fh) = @_;
1961 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
1962 }
1963
b0169937 1964 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 1965 or die "Can't open mailbox: $!";
a0d0e21e 1966
7ed5353d 1967 lock($mbox);
b0169937 1968 print $mbox $msg,"\n\n";
7ed5353d 1969 unlock($mbox);
a0d0e21e 1970
2b5ab1e7
TC
1971On systems that support a real flock(), locks are inherited across fork()
1972calls, whereas those that must resort to the more capricious fcntl()
1973function lose the locks, making it harder to write servers.
1974
cb1a09d0 1975See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1976
1977=item fork
d74e8afc 1978X<fork> X<child> X<parent>
a0d0e21e 1979
2b5ab1e7
TC
1980Does a fork(2) system call to create a new process running the
1981same program at the same point. It returns the child pid to the
1982parent process, C<0> to the child process, or C<undef> if the fork is
1983unsuccessful. File descriptors (and sometimes locks on those descriptors)
1984are shared, while everything else is copied. On most systems supporting
1985fork(), great care has gone into making it extremely efficient (for
1986example, using copy-on-write technology on data pages), making it the
1987dominant paradigm for multitasking over the last few decades.
5a964f20 1988
80d38338 1989Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1990output before forking the child process, but this may not be supported
1991on some platforms (see L<perlport>). To be safe, you may need to set
1992C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 1993C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 1994
19799a22 1995If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1996accumulate zombies. On some systems, you can avoid this by setting
1997C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1998forking and reaping moribund children.
cb1a09d0 1999
28757baa
PP
2000Note that if your forked child inherits system file descriptors like
2001STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2002if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2003backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2004You should reopen those to F</dev/null> if it's any issue.
28757baa 2005
cb1a09d0 2006=item format
d74e8afc 2007X<format>
cb1a09d0 2008
19799a22 2009Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2010example:
2011
54310121 2012 format Something =
a9a5a0dc
VP
2013 Test: @<<<<<<<< @||||| @>>>>>
2014 $str, $%, '$' . int($num)
cb1a09d0
AD
2015 .
2016
2017 $str = "widget";
184e9718 2018 $num = $cost/$quantity;
cb1a09d0
AD
2019 $~ = 'Something';
2020 write;
2021
2022See L<perlform> for many details and examples.
2023
8903cb82 2024=item formline PICTURE,LIST
d74e8afc 2025X<formline>
a0d0e21e 2026
5a964f20 2027This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2028too. It formats (see L<perlform>) a list of values according to the
2029contents of PICTURE, placing the output into the format output
7660c0ab 2030accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2031Eventually, when a C<write> is done, the contents of
cf264981
SP
2032C<$^A> are written to some filehandle. You could also read C<$^A>
2033and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2034does one C<formline> per line of form, but the C<formline> function itself
748a9306 2035doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 2036that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
2037You may therefore need to use multiple formlines to implement a single
2038record format, just like the format compiler.
2039
19799a22 2040Be careful if you put double quotes around the picture, because an C<@>
748a9306 2041character may be taken to mean the beginning of an array name.
19799a22 2042C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
2043
2044=item getc FILEHANDLE
f723aae1 2045X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2046
2047=item getc
2048
2049Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
2050or the undefined value at end of file, or if there was an error (in
2051the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2052STDIN. This is not particularly efficient. However, it cannot be
2053used by itself to fetch single characters without waiting for the user
2054to hit enter. For that, try something more like:
4633a7c4
LW
2055
2056 if ($BSD_STYLE) {
a9a5a0dc 2057 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2058 }
2059 else {
a9a5a0dc 2060 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2061 }
2062
2063 $key = getc(STDIN);
2064
2065 if ($BSD_STYLE) {
a9a5a0dc 2066 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2067 }
2068 else {
a9a5a0dc 2069 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
2070 }
2071 print "\n";
2072
54310121
PP
2073Determination of whether $BSD_STYLE should be set
2074is left as an exercise to the reader.
cb1a09d0 2075
19799a22 2076The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2077systems purporting POSIX compliance. See also the C<Term::ReadKey>
2078module from your nearest CPAN site; details on CPAN can be found on
2079L<perlmodlib/CPAN>.
a0d0e21e
LW
2080
2081=item getlogin
d74e8afc 2082X<getlogin> X<login>
a0d0e21e 2083
cf264981 2084This implements the C library function of the same name, which on most
5a964f20 2085systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 2086use C<getpwuid>.
a0d0e21e 2087
f86702cc 2088 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2089
19799a22
GS
2090Do not consider C<getlogin> for authentication: it is not as
2091secure as C<getpwuid>.
4633a7c4 2092
a0d0e21e 2093=item getpeername SOCKET
d74e8afc 2094X<getpeername> X<peer>
a0d0e21e
LW
2095
2096Returns the packed sockaddr address of other end of the SOCKET connection.
2097
4633a7c4
LW
2098 use Socket;
2099 $hersockaddr = getpeername(SOCK);
19799a22 2100 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2101 $herhostname = gethostbyaddr($iaddr, AF_INET);
2102 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2103
2104=item getpgrp PID
d74e8afc 2105X<getpgrp> X<group>
a0d0e21e 2106
47e29363 2107Returns the current process group for the specified PID. Use
7660c0ab 2108a PID of C<0> to get the current process group for the
4633a7c4 2109current process. Will raise an exception if used on a machine that
f86cebdf 2110doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2111group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2112does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2113
2114=item getppid
d74e8afc 2115X<getppid> X<parent> X<pid>
a0d0e21e
LW
2116
2117Returns the process id of the parent process.
2118
4d76a344
RGS
2119Note for Linux users: on Linux, the C functions C<getpid()> and
2120C<getppid()> return different values from different threads. In order to
2121be portable, this behavior is not reflected by the perl-level function
2122C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2123to call the underlying C<getppid()>, you may use the CPAN module
2124C<Linux::Pid>.
4d76a344 2125
a0d0e21e 2126=item getpriority WHICH,WHO
d74e8afc 2127X<getpriority> X<priority> X<nice>
a0d0e21e 2128
4633a7c4 2129Returns the current priority for a process, a process group, or a user.
f4084e39 2130(See C<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2131machine that doesn't implement getpriority(2).
a0d0e21e
LW
2132
2133=item getpwnam NAME
d74e8afc
ITB
2134X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2135X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2136X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2137X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2138X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2139X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2140
2141=item getgrnam NAME
2142
2143=item gethostbyname NAME
2144
2145=item getnetbyname NAME
2146
2147=item getprotobyname NAME
2148
2149=item getpwuid UID
2150
2151=item getgrgid GID
2152
2153=item getservbyname NAME,PROTO
2154
2155=item gethostbyaddr ADDR,ADDRTYPE
2156
2157=item getnetbyaddr ADDR,ADDRTYPE
2158
2159=item getprotobynumber NUMBER
2160
2161=item getservbyport PORT,PROTO
2162
2163=item getpwent
2164
2165=item getgrent
2166
2167=item gethostent
2168
2169=item getnetent
2170
2171=item getprotoent
2172
2173=item getservent
2174
2175=item setpwent
2176
2177=item setgrent
2178
2179=item sethostent STAYOPEN
2180
2181=item setnetent STAYOPEN
2182
2183=item setprotoent STAYOPEN
2184
2185=item setservent STAYOPEN
2186
2187=item endpwent
2188
2189=item endgrent
2190
2191=item endhostent
2192
2193=item endnetent
2194
2195=item endprotoent
2196
2197=item endservent
2198
80d38338
TC
2199These routines are the same as their counterparts in the
2200system C library. In list context, the return values from the
a0d0e21e
LW
2201various get routines are as follows:
2202
2203 ($name,$passwd,$uid,$gid,
6ee623d5 2204 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2205 ($name,$passwd,$gid,$members) = getgr*
2206 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2207 ($name,$aliases,$addrtype,$net) = getnet*
2208 ($name,$aliases,$proto) = getproto*
2209 ($name,$aliases,$port,$proto) = getserv*
2210
2211(If the entry doesn't exist you get a null list.)
2212
4602f195
JH
2213The exact meaning of the $gcos field varies but it usually contains
2214the real name of the user (as opposed to the login name) and other
2215information pertaining to the user. Beware, however, that in many
2216system users are able to change this information and therefore it
106325ad 2217cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2218L<perlsec>). The $passwd and $shell, user's encrypted password and
2219login shell, are also tainted, because of the same reason.
4602f195 2220
5a964f20 2221In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2222lookup by name, in which case you get the other thing, whatever it is.
2223(If the entry doesn't exist you get the undefined value.) For example:
2224
5a964f20
TC
2225 $uid = getpwnam($name);
2226 $name = getpwuid($num);
2227 $name = getpwent();
2228 $gid = getgrnam($name);
08a33e13 2229 $name = getgrgid($num);
5a964f20
TC
2230 $name = getgrent();
2231 #etc.
a0d0e21e 2232
4602f195 2233In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2234in that they are unsupported on many systems. If the
4602f195
JH
2235$quota is unsupported, it is an empty scalar. If it is supported, it
2236usually encodes the disk quota. If the $comment field is unsupported,
2237it is an empty scalar. If it is supported it usually encodes some
2238administrative comment about the user. In some systems the $quota
2239field may be $change or $age, fields that have to do with password
2240aging. In some systems the $comment field may be $class. The $expire
2241field, if present, encodes the expiration period of the account or the
2242password. For the availability and the exact meaning of these fields
2243in your system, please consult your getpwnam(3) documentation and your
2244F<pwd.h> file. You can also find out from within Perl what your
2245$quota and $comment fields mean and whether you have the $expire field
2246by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2247C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2248files are only supported if your vendor has implemented them in the
2249intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2250shadow versions if you're running under privilege or if there exists
cf264981
SP
2251the shadow(3) functions as found in System V (this includes Solaris
2252and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2253facility are unlikely to be supported.
6ee623d5 2254
19799a22 2255The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2256the login names of the members of the group.
2257
2258For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2259C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2260C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2261addresses returned by the corresponding system library call. In the
2262Internet domain, each address is four bytes long and you can unpack it
2263by saying something like:
2264
f337b084 2265 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2266
2b5ab1e7
TC
2267The Socket library makes this slightly easier:
2268
2269 use Socket;
2270 $iaddr = inet_aton("127.1"); # or whatever address
2271 $name = gethostbyaddr($iaddr, AF_INET);
2272
2273 # or going the other way
19799a22 2274 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2275
d760c846
GS
2276In the opposite way, to resolve a hostname to the IP address
2277you can write this:
2278
2279 use Socket;
2280 $packed_ip = gethostbyname("www.perl.org");
2281 if (defined $packed_ip) {
2282 $ip_address = inet_ntoa($packed_ip);
2283 }
2284
2285Make sure <gethostbyname()> is called in SCALAR context and that
2286its return value is checked for definedness.
2287
19799a22
GS
2288If you get tired of remembering which element of the return list
2289contains which return value, by-name interfaces are provided
2290in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2291C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2292and C<User::grent>. These override the normal built-ins, supplying
2293versions that return objects with the appropriate names
2294for each field. For example:
5a964f20
TC
2295
2296 use File::stat;
2297 use User::pwent;
2298 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2299
b76cc8ba
NIS
2300Even though it looks like they're the same method calls (uid),
2301they aren't, because a C<File::stat> object is different from
19799a22 2302a C<User::pwent> object.
5a964f20 2303
a0d0e21e 2304=item getsockname SOCKET
d74e8afc 2305X<getsockname>
a0d0e21e 2306
19799a22
GS
2307Returns the packed sockaddr address of this end of the SOCKET connection,
2308in case you don't know the address because you have several different
2309IPs that the connection might have come in on.
a0d0e21e 2310
4633a7c4
LW
2311 use Socket;
2312 $mysockaddr = getsockname(SOCK);
19799a22 2313 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2314 printf "Connect to %s [%s]\n",
19799a22
GS
2315 scalar gethostbyaddr($myaddr, AF_INET),
2316 inet_ntoa($myaddr);
a0d0e21e
LW
2317
2318=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2319X<getsockopt>
a0d0e21e 2320
636e6b1f
TH
2321Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2322Options may exist at multiple protocol levels depending on the socket
2323type, but at least the uppermost socket level SOL_SOCKET (defined in the
2324C<Socket> module) will exist. To query options at another level the
2325protocol number of the appropriate protocol controlling the option
2326should be supplied. For example, to indicate that an option is to be
2327interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2328number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2329
80d38338
TC
2330The function returns a packed string representing the requested socket
2331option, or C<undef> if there is an error (the error reason will be in
2332C<$!>). What exactly is in the packed string depends on LEVEL and OPTNAME;
2333consult getsockopt(2) for details. A common case is that the option is an
2334integer, in which case the result is a packed integer, which you can decode
2335using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2336
80d38338 2337An example to test whether Nagle's algorithm is turned on on a socket:
636e6b1f 2338
4852725b 2339 use Socket qw(:all);
636e6b1f
TH
2340
2341 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2342 or die "Could not determine the protocol number for tcp";
4852725b
DD
2343 # my $tcp = IPPROTO_TCP; # Alternative
2344 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2345 or die "getsockopt TCP_NODELAY: $!";
636e6b1f
TH
2346 my $nodelay = unpack("I", $packed);
2347 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2348
a0d0e21e
LW
2349
2350=item glob EXPR
d74e8afc 2351X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2352
0a753a76
PP
2353=item glob
2354
d9a9d457
JL
2355In list context, returns a (possibly empty) list of filename expansions on
2356the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2357scalar context, glob iterates through such filename expansions, returning
2358undef when the list is exhausted. This is the internal function
2359implementing the C<< <*.c> >> operator, but you can use it directly. If
2360EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2361more detail in L<perlop/"I/O Operators">.
a0d0e21e 2362
80d38338
TC
2363Note that C<glob> splits its arguments on whitespace and treats
2364each segment as separate pattern. As such, C<glob("*.c *.h")>
2365matches all files with a F<.c> or F<.h> extension. The expression
2366C<glob(".* *")> matchs all files in the current working directory.
2367
2368If non-empty braces are the only wildcard characters used in the
2369C<glob>, no filenames are matched, but potentially many strings
2370are returned. For example, this produces nine strings, one for
2371each pairing of fruits and colors:
2372
2373 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2374
3a4b19e4 2375Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2376C<File::Glob> extension. See L<File::Glob> for details, including
2377C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2378
a0d0e21e 2379=item gmtime EXPR
d74e8afc 2380X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2381
ce2984c3
PF
2382=item gmtime
2383
435fbc73
GS
2384Works just like L<localtime> but the returned values are
2385localized for the standard Greenwich time zone.
a0d0e21e 2386
435fbc73
GS
2387Note: when called in list context, $isdst, the last value
2388returned by gmtime is always C<0>. There is no
2389Daylight Saving Time in GMT.
0a753a76 2390
62aa5637
MS
2391See L<perlport/gmtime> for portability concerns.
2392
a0d0e21e 2393=item goto LABEL
d74e8afc 2394X<goto> X<jump> X<jmp>
a0d0e21e 2395
748a9306
LW
2396=item goto EXPR
2397
a0d0e21e
LW
2398=item goto &NAME
2399
b500e03b
GG
2400The C<goto-LABEL> form finds the statement labeled with LABEL and
2401resumes execution there. It can't be used to get out of a block or
2402subroutine given to C<sort>. It can be used to go almost anywhere
2403else within the dynamic scope, including out of subroutines, but it's
2404usually better to use some other construct such as C<last> or C<die>.
2405The author of Perl has never felt the need to use this form of C<goto>
80d38338 2406(in Perl, that is--C is another matter). (The difference is that C
b500e03b
GG
2407does not offer named loops combined with loop control. Perl does, and
2408this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2409
7660c0ab
A
2410The C<goto-EXPR> form expects a label name, whose scope will be resolved
2411dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2412necessarily recommended if you're optimizing for maintainability:
2413
2414 goto ("FOO", "BAR", "GLARCH")[$i];
2415
b500e03b 2416Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2417deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2418go into any construct that requires initialization, such as a
2419subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2420construct that is optimized away.
b500e03b 2421
1b6921cb
BT
2422The C<goto-&NAME> form is quite different from the other forms of
2423C<goto>. In fact, it isn't a goto in the normal sense at all, and
2424doesn't have the stigma associated with other gotos. Instead, it
2425exits the current subroutine (losing any changes set by local()) and
2426immediately calls in its place the named subroutine using the current
2427value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2428load another subroutine and then pretend that the other subroutine had
2429been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2430in the current subroutine are propagated to the other subroutine.)
2431After the C<goto>, not even C<caller> will be able to tell that this
2432routine was called first.
2433
2434NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2435containing a code reference, or a block that evaluates to a code
6cb9131c 2436reference.
a0d0e21e
LW
2437
2438=item grep BLOCK LIST
d74e8afc 2439X<grep>
a0d0e21e
LW
2440
2441=item grep EXPR,LIST
2442
2b5ab1e7
TC
2443This is similar in spirit to, but not the same as, grep(1) and its
2444relatives. In particular, it is not limited to using regular expressions.
2f9daede 2445
a0d0e21e 2446Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2447C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2448elements for which the expression evaluated to true. In scalar
2449context, returns the number of times the expression was true.
a0d0e21e
LW
2450
2451 @foo = grep(!/^#/, @bar); # weed out comments
2452
2453or equivalently,
2454
2455 @foo = grep {!/^#/} @bar; # weed out comments
2456
be3174d2
GS
2457Note that C<$_> is an alias to the list value, so it can be used to
2458modify the elements of the LIST. While this is useful and supported,
2459it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2460Similarly, grep returns aliases into the original list, much as a for
2461loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2462element of a list returned by grep (for example, in a C<foreach>, C<map>
2463or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2464This is usually something to be avoided when writing clear code.
a0d0e21e 2465
a4fb8298 2466If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2467been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2468the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2469can't be seen from the outside, avoiding any potential side-effects.
2470
19799a22 2471See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2472
a0d0e21e 2473=item hex EXPR
d74e8afc 2474X<hex> X<hexadecimal>
a0d0e21e 2475
54310121 2476=item hex
bbce6d69 2477
2b5ab1e7 2478Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2479(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2480L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2481
2482 print hex '0xAf'; # prints '175'
2483 print hex 'aF'; # same
a0d0e21e 2484
19799a22 2485Hex strings may only represent integers. Strings that would cause
53305cf1 2486integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2487unlike oct(). To present something as hex, look into L</printf>,
2488L</sprintf>, or L</unpack>.
19799a22 2489
ce2984c3 2490=item import LIST
d74e8afc 2491X<import>
a0d0e21e 2492
19799a22 2493There is no builtin C<import> function. It is just an ordinary
4633a7c4 2494method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2495names to another module. The C<use> function calls the C<import> method
cea6626f 2496for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2497
2498=item index STR,SUBSTR,POSITION
d74e8afc 2499X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2500
2501=item index STR,SUBSTR
2502
2b5ab1e7
TC
2503The index function searches for one string within another, but without
2504the wildcard-like behavior of a full regular-expression pattern match.
2505It returns the position of the first occurrence of SUBSTR in STR at
2506or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2507beginning of the string. POSITION before the beginning of the string
2508or after its end is treated as if it were the beginning or the end,
2509respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2510you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2511is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2512
2513=item int EXPR
f723aae1 2514X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2515
54310121 2516=item int
bbce6d69 2517
7660c0ab 2518Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2519You should not use this function for rounding: one because it truncates
2520towards C<0>, and two because machine representations of floating point
2521numbers can sometimes produce counterintuitive results. For example,
2522C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2523because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2524the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2525functions will serve you better than will int().
a0d0e21e
LW
2526
2527=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2528X<ioctl>
a0d0e21e 2529
2b5ab1e7 2530Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2531
5ed4f2ec 2532 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2533
a11c483f 2534to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2535exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2536own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2537(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2538may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2539written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2540will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2541has no string value but does have a numeric value, that value will be
2542passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2543true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2544functions may be needed to manipulate the values of structures used by
b76cc8ba 2545C<ioctl>.
a0d0e21e 2546
19799a22 2547The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e 2548
5ed4f2ec 2549 if OS returns: then Perl returns:
2550 -1 undefined value
2551 0 string "0 but true"
2552 anything else that number
a0d0e21e 2553
19799a22 2554Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2555still easily determine the actual value returned by the operating
2556system:
2557
2b5ab1e7 2558 $retval = ioctl(...) || -1;
a0d0e21e
LW
2559 printf "System returned %d\n", $retval;
2560
be2f7487 2561The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2562about improper numeric conversions.
2563
a0d0e21e 2564=item join EXPR,LIST
d74e8afc 2565X<join>
a0d0e21e 2566
2b5ab1e7
TC
2567Joins the separate strings of LIST into a single string with fields
2568separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2569
2b5ab1e7 2570 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2571
eb6e2d6f
GS
2572Beware that unlike C<split>, C<join> doesn't take a pattern as its
2573first argument. Compare L</split>.
a0d0e21e 2574
aa689395 2575=item keys HASH
d74e8afc 2576X<keys> X<key>
aa689395 2577
aeedbbed
NC
2578=item keys ARRAY
2579
2580Returns a list consisting of all the keys of the named hash, or the indices
2581of an array. (In scalar context, returns the number of keys or indices.)
504f80c1 2582
aeedbbed 2583The keys of a hash are returned in an apparently random order. The actual
504f80c1
JH
2584random order is subject to change in future versions of perl, but it
2585is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2586function produces (given that the hash has not been modified). Since
2587Perl 5.8.1 the ordering is different even between different runs of
2588Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2589Attacks">).
504f80c1 2590
aeedbbed 2591As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
cf264981
SP
2592(see L</each>). In particular, calling keys() in void context resets
2593the iterator with no other overhead.
a0d0e21e 2594
aa689395 2595Here is yet another way to print your environment:
a0d0e21e
LW
2596
2597 @keys = keys %ENV;
2598 @values = values %ENV;
b76cc8ba 2599 while (@keys) {
a9a5a0dc 2600 print pop(@keys), '=', pop(@values), "\n";
a0d0e21e
LW
2601 }
2602
2603or how about sorted by key:
2604
2605 foreach $key (sort(keys %ENV)) {
a9a5a0dc 2606 print $key, '=', $ENV{$key}, "\n";
a0d0e21e
LW
2607 }
2608
8ea1e5d4
GS
2609The returned values are copies of the original keys in the hash, so
2610modifying them will not affect the original hash. Compare L</values>.
2611
19799a22 2612To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2613Here's a descending numeric sort of a hash by its values:
4633a7c4 2614
5a964f20 2615 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
a9a5a0dc 2616 printf "%4d %s\n", $hash{$key}, $key;
4633a7c4
LW
2617 }
2618
19799a22 2619As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2620allocated for the given hash. This can gain you a measure of efficiency if
2621you know the hash is going to get big. (This is similar to pre-extending
2622an array by assigning a larger number to $#array.) If you say
55497cff
PP
2623
2624 keys %hash = 200;
2625
ab192400
GS
2626then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2627in fact, since it rounds up to the next power of two. These
55497cff
PP
2628buckets will be retained even if you do C<%hash = ()>, use C<undef
2629%hash> if you want to free the storage while C<%hash> is still in scope.
2630You can't shrink the number of buckets allocated for the hash using
19799a22 2631C<keys> in this way (but you needn't worry about doing this by accident,
aeedbbed
NC
2632as trying has no effect). C<keys @array> in an lvalue context is a syntax
2633error.
55497cff 2634
19799a22 2635See also C<each>, C<values> and C<sort>.
ab192400 2636
b350dd2f 2637=item kill SIGNAL, LIST
d74e8afc 2638X<kill> X<signal>
a0d0e21e 2639
b350dd2f 2640Sends a signal to a list of processes. Returns the number of
517db077
GS
2641processes successfully signaled (which is not necessarily the
2642same as the number actually killed).
a0d0e21e
LW
2643
2644 $cnt = kill 1, $child1, $child2;
2645 kill 9, @goners;
2646
70fb64f6 2647If SIGNAL is zero, no signal is sent to the process, but the kill(2)
6cb9d3e4 2648system call will check whether it's possible to send a signal to it (that
70fb64f6
RGS
2649means, to be brief, that the process is owned by the same user, or we are
2650the super-user). This is a useful way to check that a child process is
81fd35db
DN
2651alive (even if only as a zombie) and hasn't changed its UID. See
2652L<perlport> for notes on the portability of this construct.
b350dd2f 2653
e2c0f81f
DG
2654Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2655of processes. That means you usually want to use positive not negative signals.
2656You may also use a signal name in quotes.
2657
2658The behavior of kill when a I<PROCESS> number is zero or negative depends on
2659the operating system. For example, on POSIX-conforming systems, zero will
2660signal the current process group and -1 will signal all processes.
1e9c1022
JL
2661
2662See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2663
2664=item last LABEL
d74e8afc 2665X<last> X<break>
a0d0e21e
LW
2666
2667=item last
2668
2669The C<last> command is like the C<break> statement in C (as used in
2670loops); it immediately exits the loop in question. If the LABEL is
2671omitted, the command refers to the innermost enclosing loop. The
2672C<continue> block, if any, is not executed:
2673
4633a7c4 2674 LINE: while (<STDIN>) {
a9a5a0dc
VP
2675 last LINE if /^$/; # exit when done with header
2676 #...
a0d0e21e
LW
2677 }
2678
80d38338 2679C<last> cannot be used to exit a block that returns a value such as
2b5ab1e7
TC
2680C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2681a grep() or map() operation.
4968c1e4 2682
6c1372ed
GS
2683Note that a block by itself is semantically identical to a loop
2684that executes once. Thus C<last> can be used to effect an early
2685exit out of such a block.
2686
98293880
JH
2687See also L</continue> for an illustration of how C<last>, C<next>, and
2688C<redo> work.
1d2dff63 2689
a0d0e21e 2690=item lc EXPR
d74e8afc 2691X<lc> X<lowercase>
a0d0e21e 2692
54310121 2693=item lc
bbce6d69 2694
d1be9408 2695Returns a lowercased version of EXPR. This is the internal function
e1b711da 2696implementing the C<\L> escape in double-quoted strings.
a0d0e21e 2697
7660c0ab 2698If EXPR is omitted, uses C<$_>.
bbce6d69 2699
e1b711da
KW
2700What gets returned depends on several factors:
2701
2702=over
2703
2704=item If C<use bytes> is in effect:
2705
2706=over
2707
2708=item On EBCDIC platforms
2709
2710The results are what the C language system call C<tolower()> returns.
2711
2712=item On ASCII platforms
2713
2714The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
2715respectively.
2716
2717=back
2718
2719=item Otherwise, If EXPR has the UTF8 flag set
2720
2721If the current package has a subroutine named C<ToLower>, it will be used to
2722change the case (See L<perlunicode/User-Defined Case Mappings>.)
2723Otherwise Unicode semantics are used for the case change.
2724
2725=item Otherwise, if C<use locale> is in effect
2726
2727Respects current LC_CTYPE locale. See L<perllocale>.
2728
2729=item Otherwise, if C<use feature 'unicode_strings'> is in effect:
2730
2731Unicode semantics are used for the case change. Any subroutine named
2732C<ToLower> will not be used.
2733
2734=item Otherwise:
2735
2736=over
2737
2738=item On EBCDIC platforms
2739
2740The results are what the C language system call C<tolower()> returns.
2741
2742=item On ASCII platforms
2743
2744ASCII semantics are used for the case change. The lowercase of any character
2745outside the ASCII range is the character itself.
2746
2747=back
2748
2749=back
2750
a0d0e21e 2751=item lcfirst EXPR
d74e8afc 2752X<lcfirst> X<lowercase>
a0d0e21e 2753
54310121 2754=item lcfirst
bbce6d69 2755
ad0029c4
JH
2756Returns the value of EXPR with the first character lowercased. This
2757is the internal function implementing the C<\l> escape in
e1b711da 2758double-quoted strings.
a0d0e21e 2759
7660c0ab 2760If EXPR is omitted, uses C<$_>.
bbce6d69 2761
e1b711da
KW
2762This function behaves the same way under various pragma, such as in a locale,
2763as L</lc> does.
2764
a0d0e21e 2765=item length EXPR
d74e8afc 2766X<length> X<size>
a0d0e21e 2767
54310121 2768=item length
bbce6d69 2769
974da8e5 2770Returns the length in I<characters> of the value of EXPR. If EXPR is
9f621bb0
NC
2771omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
2772Note that this cannot be used on an entire array or hash to find out how
2773many elements these have. For that, use C<scalar @array> and C<scalar keys
2774%hash> respectively.
a0d0e21e 2775
974da8e5
JH
2776Note the I<characters>: if the EXPR is in Unicode, you will get the
2777number of characters, not the number of bytes. To get the length
2575c402
JW
2778of the internal string in bytes, use C<bytes::length(EXPR)>, see
2779L<bytes>. Note that the internal encoding is variable, and the number
2780of bytes usually meaningless. To get the number of bytes that the
2781string would have when encoded as UTF-8, use
2782C<length(Encoding::encode_utf8(EXPR))>.
974da8e5 2783
a0d0e21e 2784=item link OLDFILE,NEWFILE
d74e8afc 2785X<link>
a0d0e21e 2786
19799a22 2787Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2788success, false otherwise.
a0d0e21e
LW
2789
2790=item listen SOCKET,QUEUESIZE
d74e8afc 2791X<listen>
a0d0e21e 2792
19799a22 2793Does the same thing that the listen system call does. Returns true if
b76cc8ba 2794it succeeded, false otherwise. See the example in
cea6626f 2795L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2796
2797=item local EXPR
d74e8afc 2798X<local>
a0d0e21e 2799
19799a22 2800You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2801what most people think of as "local". See
13a2d996 2802L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2803
5a964f20
TC
2804A local modifies the listed variables to be local to the enclosing
2805block, file, or eval. If more than one value is listed, the list must
2806be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2807for details, including issues with tied arrays and hashes.
a0d0e21e 2808
d361fafa
VP
2809The C<delete local EXPR> construct can also be used to localize the deletion
2810of array/hash elements to the current block.
2811See L<perlsub/"Localized deletion of elements of composite types">.
2812
a0d0e21e 2813=item localtime EXPR
435fbc73 2814X<localtime> X<ctime>
a0d0e21e 2815
ba053783
AL
2816=item localtime
2817
19799a22 2818Converts a time as returned by the time function to a 9-element list
5f05dabc 2819with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2820follows:
2821
54310121 2822 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2823 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2824 localtime(time);
a0d0e21e 2825
48a26b3a 2826All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2827tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2828of the specified time.
48a26b3a 2829
ba053783
AL
2830C<$mday> is the day of the month, and C<$mon> is the month itself, in
2831the range C<0..11> with 0 indicating January and 11 indicating December.
2832This makes it easy to get a month name from a list:
54310121 2833
ba053783
AL
2834 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2835 print "$abbr[$mon] $mday";
2836 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2837
ba053783
AL
2838C<$year> is the number of years since 1900, not just the last two digits
2839of the year. That is, C<$year> is C<123> in year 2023. The proper way
80d38338 2840to get a 4-digit year is simply:
abd75f24 2841
ba053783 2842 $year += 1900;
abd75f24 2843
435fbc73
GS
2844Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2845to do that, would you?
2846
ba053783
AL
2847To get the last two digits of the year (e.g., '01' in 2001) do:
2848
2849 $year = sprintf("%02d", $year % 100);
2850
2851C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2852Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2853(or C<0..365> in leap years.)
2854
2855C<$isdst> is true if the specified time occurs during Daylight Saving
2856Time, false otherwise.
abd75f24 2857
e1998452 2858If EXPR is omitted, C<localtime()> uses the current time (as returned
e3176d09 2859by time(3)).
a0d0e21e 2860
48a26b3a 2861In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2862
5f05dabc 2863 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2864
fe86afc2
NC
2865This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2866instead of local time use the L</gmtime> builtin. See also the
2867C<Time::Local> module (to convert the second, minutes, hours, ... back to
2868the integer value returned by time()), and the L<POSIX> module's strftime(3)
2869and mktime(3) functions.
2870
2871To get somewhat similar but locale dependent date strings, set up your
2872locale environment variables appropriately (please see L<perllocale>) and
2873try for example:
a3cb178b 2874
5a964f20 2875 use POSIX qw(strftime);
2b5ab1e7 2876 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2877 # or for GMT formatted appropriately for your locale:
2878 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2879
2880Note that the C<%a> and C<%b>, the short forms of the day of the week
2881and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2882
62aa5637
MS
2883See L<perlport/localtime> for portability concerns.
2884
435fbc73
GS
2885The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2886by-name access mechanism to the gmtime() and localtime() functions,
2887respectively.
2888
2889For a comprehensive date and time representation look at the
2890L<DateTime> module on CPAN.
2891
07698885 2892=item lock THING
d74e8afc 2893X<lock>
19799a22 2894
01e6739c 2895This function places an advisory lock on a shared variable, or referenced
03730085 2896object contained in I<THING> until the lock goes out of scope.
a6d5524e 2897
f3a23afb 2898lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2899by this name (before any calls to it), that function will be called
03730085
AB
2900instead. (However, if you've said C<use threads>, lock() is always a
2901keyword.) See L<threads>.
19799a22 2902
a0d0e21e 2903=item log EXPR
d74e8afc 2904X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2905
54310121 2906=item log
bbce6d69 2907
2b5ab1e7
TC
2908Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2909returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2910The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2911divided by the natural log of N. For example:
2912
2913 sub log10 {
a9a5a0dc
VP
2914 my $n = shift;
2915 return log($n)/log(10);
b76cc8ba 2916 }
2b5ab1e7
TC
2917
2918See also L</exp> for the inverse operation.
a0d0e21e 2919
a0d0e21e 2920=item lstat EXPR
d74e8afc 2921X<lstat>
a0d0e21e 2922
54310121 2923=item lstat
bbce6d69 2924
19799a22 2925Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2926special C<_> filehandle) but stats a symbolic link instead of the file
2927the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2928your system, a normal C<stat> is done. For much more detailed
2929information, please see the documentation for C<stat>.
a0d0e21e 2930
7660c0ab 2931If EXPR is omitted, stats C<$_>.
bbce6d69 2932
a0d0e21e
LW
2933=item m//
2934
9f4b9cd0 2935The match operator. See L<perlop/"Regexp Quote-Like Operators">.
a0d0e21e
LW
2936
2937=item map BLOCK LIST
d74e8afc 2938X<map>
a0d0e21e
LW
2939
2940=item map EXPR,LIST
2941
19799a22
GS
2942Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2943C<$_> to each element) and returns the list value composed of the
2944results of each such evaluation. In scalar context, returns the
2945total number of elements so generated. Evaluates BLOCK or EXPR in
2946list context, so each element of LIST may produce zero, one, or
2947more elements in the returned value.
dd99ebda 2948
a0d0e21e
LW
2949 @chars = map(chr, @nums);
2950
2951translates a list of numbers to the corresponding characters. And
2952
d8216f19 2953 %hash = map { get_a_key_for($_) => $_ } @array;
a0d0e21e
LW
2954
2955is just a funny way to write
2956
2957 %hash = ();
d8216f19 2958 foreach (@array) {
a9a5a0dc 2959 $hash{get_a_key_for($_)} = $_;
a0d0e21e
LW
2960 }
2961
be3174d2
GS
2962Note that C<$_> is an alias to the list value, so it can be used to
2963modify the elements of the LIST. While this is useful and supported,
2964it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2965Using a regular C<foreach> loop for this purpose would be clearer in
2966most cases. See also L</grep> for an array composed of those items of
2967the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2968
a4fb8298 2969If C<$_> is lexical in the scope where the C<map> appears (because it has
d8216f19
RGS
2970been declared with C<my $_>), then, in addition to being locally aliased to
2971the list elements, C<$_> keeps being lexical inside the block; that is, it
a4fb8298
RGS
2972can't be seen from the outside, avoiding any potential side-effects.
2973
205fdb4d
NC
2974C<{> starts both hash references and blocks, so C<map { ...> could be either
2975the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
80d38338
TC
2976ahead for the closing C<}> it has to take a guess at which it's dealing with
2977based on what it finds just after the C<{>. Usually it gets it right, but if it
205fdb4d
NC
2978doesn't it won't realize something is wrong until it gets to the C<}> and
2979encounters the missing (or unexpected) comma. The syntax error will be
80d38338 2980reported close to the C<}>, but you'll need to change something near the C<{>
205fdb4d
NC
2981such as using a unary C<+> to give perl some help:
2982
2983 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2984 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2985 %hash = map { ("\L$_", 1) } @array # this also works
2986 %hash = map { lc($_), 1 } @array # as does this.
2987 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2988
205fdb4d
NC
2989 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2990
d8216f19 2991or to force an anon hash constructor use C<+{>:
205fdb4d
NC
2992
2993 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2994
2995and you get list of anonymous hashes each with only 1 entry.
2996
19799a22 2997=item mkdir FILENAME,MASK
d74e8afc 2998X<mkdir> X<md> X<directory, create>
a0d0e21e 2999
5a211162
GS
3000=item mkdir FILENAME
3001
491873e5
RGS
3002=item mkdir
3003
0591cd52 3004Creates the directory specified by FILENAME, with permissions
19799a22
GS
3005specified by MASK (as modified by C<umask>). If it succeeds it
3006returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
3007If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
3008to C<$_>.
0591cd52 3009
19799a22 3010In general, it is better to create directories with permissive MASK,
0591cd52 3011and let the user modify that with their C<umask>, than it is to supply
19799a22 3012a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
3013The exceptions to this rule are when the file or directory should be
3014kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 3015C<umask> discusses the choice of MASK in more detail.
a0d0e21e 3016
cc1852e8
JH
3017Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3018number of trailing slashes. Some operating and filesystems do not get
3019this right, so Perl automatically removes all trailing slashes to keep
3020everyone happy.
3021
80d38338 3022To recursively create a directory structure, look at
dd184578
RGS
3023the C<mkpath> function of the L<File::Path> module.
3024
a0d0e21e 3025=item msgctl ID,CMD,ARG
d74e8afc 3026X<msgctl>
a0d0e21e 3027
f86cebdf 3028Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
3029
3030 use IPC::SysV;
3031
7660c0ab 3032first to get the correct constant definitions. If CMD is C<IPC_STAT>,
cf264981 3033then ARG must be a variable that will hold the returned C<msqid_ds>
951ba7fe
GS
3034structure. Returns like C<ioctl>: the undefined value for error,
3035C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 3036L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
3037
3038=item msgget KEY,FLAGS
d74e8afc 3039X<msgget>
a0d0e21e 3040
f86cebdf 3041Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
3042id, or the undefined value if there is an error. See also
3043L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 3044
a0d0e21e 3045=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
d74e8afc 3046X<msgrcv>
a0d0e21e
LW
3047
3048Calls the System V IPC function msgrcv to receive a message from
3049message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
3050SIZE. Note that when a message is received, the message type as a
3051native long integer will be the first thing in VAR, followed by the
3052actual message. This packing may be opened with C<unpack("l! a*")>.
3053Taints the variable. Returns true if successful, or false if there is
4755096e
GS
3054an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
3055C<IPC::SysV::Msg> documentation.
41d6edb2
JH
3056
3057=item msgsnd ID,MSG,FLAGS
d74e8afc 3058X<msgsnd>
41d6edb2
JH
3059
3060Calls the System V IPC function msgsnd to send the message MSG to the
3061message queue ID. MSG must begin with the native long integer message
3062type, and be followed by the length of the actual message, and finally
3063the message itself. This kind of packing can be achieved with
3064C<pack("l! a*", $type, $message)>. Returns true if successful,
3065or false if there is an error. See also C<IPC::SysV>
3066and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
3067
3068=item my EXPR
d74e8afc 3069X<my>
a0d0e21e 3070
307ea6df
JH
3071=item my TYPE EXPR
3072
1d2de774 3073=item my EXPR : ATTRS
09bef843 3074
1d2de774 3075=item my TYPE EXPR : ATTRS
307ea6df 3076
19799a22 3077A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
3078enclosing block, file, or C<eval>. If more than one value is listed,
3079the list must be placed in parentheses.
307ea6df 3080
1d2de774
JH
3081The exact semantics and interface of TYPE and ATTRS are still
3082evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
3083and attributes are handled using the C<attributes> pragma, or starting
3084from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3085L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3086L<attributes>, and L<Attribute::Handlers>.
4633a7c4 3087
a0d0e21e 3088=item next LABEL
d74e8afc 3089X<next> X<continue>
a0d0e21e
LW
3090
3091=item next
3092
3093The C<next> command is like the C<continue> statement in C; it starts
3094the next iteration of the loop:
3095
4633a7c4 3096 LINE: while (<STDIN>) {
a9a5a0dc
VP
3097 next LINE if /^#/; # discard comments
3098 #...
a0d0e21e
LW
3099 }
3100
3101Note that if there were a C<continue> block on the above, it would get
3102executed even on discarded lines. If the LABEL is omitted, the command
3103refers to the innermost enclosing loop.
3104
4968c1e4 3105C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
3106C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3107a grep() or map() operation.
4968c1e4 3108
6c1372ed
GS
3109Note that a block by itself is semantically identical to a loop
3110that executes once. Thus C<next> will exit such a block early.
3111
98293880
JH
3112See also L</continue> for an illustration of how C<last>, C<next>, and
3113C<redo> work.
1d2dff63