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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
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3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
15operator. A unary operator generally provides a scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
a0d0e21e 17contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 18be first, and the list argument will follow. (Note that there can ever
0f31cffe 19be only one such list argument.) For instance, splice() has three scalar
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20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
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22
23In the syntax descriptions that follow, list operators that expect a
24list (and provide list context for the elements of the list) are shown
25with LIST as an argument. Such a list may consist of any combination
26of scalar arguments or list values; the list values will be included
27in the list as if each individual element were interpolated at that
28point in the list, forming a longer single-dimensional list value.
cf264981 29Commas should separate elements of the LIST.
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30
31Any function in the list below may be used either with or without
32parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 33parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 34surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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35function, and precedence doesn't matter. Otherwise it's a list
36operator or unary operator, and precedence does matter. And whitespace
37between the function and left parenthesis doesn't count--so you need to
38be careful sometimes:
39
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PP
40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
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45
46If you run Perl with the B<-w> switch it can warn you about this. For
47example, the third line above produces:
48
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
51
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52A few functions take no arguments at all, and therefore work as neither
53unary nor list operators. These include such functions as C<time>
54and C<endpwent>. For example, C<time+86_400> always means
55C<time() + 86_400>.
56
a0d0e21e 57For functions that can be used in either a scalar or list context,
54310121 58nonabortive failure is generally indicated in a scalar context by
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59returning the undefined value, and in a list context by returning the
60null list.
61
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62Remember the following important rule: There is B<no rule> that relates
63the behavior of an expression in list context to its behavior in scalar
64context, or vice versa. It might do two totally different things.
a0d0e21e 65Each operator and function decides which sort of value it would be most
2b5ab1e7 66appropriate to return in scalar context. Some operators return the
5a964f20 67length of the list that would have been returned in list context. Some
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68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
d74e8afc 72X<context>
a0d0e21e 73
d1be9408 74A named array in scalar context is quite different from what would at
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75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
81In general, functions in Perl that serve as wrappers for system calls
f86cebdf 82of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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83true when they succeed and C<undef> otherwise, as is usually mentioned
84in the descriptions below. This is different from the C interfaces,
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85which return C<-1> on failure. Exceptions to this rule are C<wait>,
86C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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87variable on failure. Other functions do not, except accidentally.
88
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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>,
80cbd5ad
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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
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.
314
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.
319
320 -e File exists.
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MJD
321 -z File has zero size (is empty).
322 -s File has nonzero size (returns size in bytes).
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323
324 -f File is a plain file.
325 -d File is a directory.
326 -l File is a symbolic link.
9c4d0f16 327 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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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.
332
333 -u File has setuid bit set.
334 -g File has setgid bit set.
335 -k File has sticky bit set.
336
121910a4 337 -T File is an ASCII text file (heuristic guess).
2cdbc966 338 -B File is a "binary" file (opposite of -T).
a0d0e21e 339
95a3fe12 340 -M Script start time minus file modification time, in days.
a0d0e21e 341 -A Same for access time.
95a3fe12 342 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 343
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344Example:
345
346 while (<>) {
5b3eff12 347 chomp;
a0d0e21e 348 next unless -f $_; # ignore specials
5a964f20 349 #...
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350 }
351
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GS
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
5ff3f7a4
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
GS
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
GS
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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LW
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
fbb0b3b3
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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
fbb0b3b3
RGS
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<$_> is used. (On some machines,
452unfortunately, the elapsed time may be up to one second less or more
453than you specified because of how seconds are counted, and process
454scheduling may delay the delivery of the signal even further.)
455
456Only one timer may be counting at once. Each call disables the
457previous timer, and an argument of C<0> may be supplied to cancel the
458previous timer without starting a new one. The returned value is the
459amount of time remaining on the previous timer.
a0d0e21e 460
2bc69794
BS
461For delays of finer granularity than one second, the Time::HiRes module
462(from CPAN, and starting from Perl 5.8 part of the standard
463distribution) provides ualarm(). You may also use Perl's four-argument
464version of select() leaving the first three arguments undefined, or you
465might be able to use the C<syscall> interface to access setitimer(2) if
466your system supports it. See L<perlfaq8> for details.
2b5ab1e7 467
68f8bed4
JH
468It is usually a mistake to intermix C<alarm> and C<sleep> calls.
469(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 470
19799a22
GS
471If you want to use C<alarm> to time out a system call you need to use an
472C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 473fail with C<$!> 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 {
f86cebdf 478 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 479 alarm $timeout;
ff68c719 480 $nread = sysread SOCKET, $buffer, $size;
36477c24 481 alarm 0;
ff68c719 482 };
ff68c719 483 if ($@) {
f86cebdf 484 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719
PP
485 # timed out
486 }
487 else {
488 # didn't
489 }
490
91d81acc
JH
491For more information see L<perlipc>.
492
a0d0e21e 493=item atan2 Y,X
d74e8afc 494X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
495
496Returns the arctangent of Y/X in the range -PI to PI.
497
ca6e1c26 498For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
499function, or use the familiar relation:
500
501 sub tan { sin($_[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
PJ
542Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
543-- other layers which 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,
28757baa 638returns the caller's package name if there is a caller, that is, if
19799a22 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<$hinthash> is a reference to a hash containing the value of C<%^H> when the
671caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
672of this hash, as they are the actual values stored in the optree.
673
748a9306 674Furthermore, when called from within the DB package, caller returns more
7660c0ab 675detailed information: it sets the list variable C<@DB::args> to be the
54310121 676arguments with which the subroutine was invoked.
748a9306 677
7660c0ab 678Be aware that the optimizer might have optimized call frames away before
19799a22 679C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 680might not return information about the call frame you expect it do, for
b76cc8ba 681C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 682previous time C<caller> was called.
7660c0ab 683
a0d0e21e 684=item chdir EXPR
d74e8afc
ITB
685X<chdir>
686X<cd>
f723aae1 687X<directory, change>
a0d0e21e 688
c4aca7d0
GA
689=item chdir FILEHANDLE
690
691=item chdir DIRHANDLE
692
ce2984c3
PF
693=item chdir
694
ffce7b87 695Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 696changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 697changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
698variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
699neither is set, C<chdir> does nothing. It returns true upon success,
700false otherwise. See the example under C<die>.
a0d0e21e 701
c4aca7d0
GA
702On systems that support fchdir, you might pass a file handle or
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
c4aca7d0
GA
722On systems that support fchmod, you might pass file handles among the
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 (<>) {
760 chomp; # avoid \n on last field
761 @array = split(/:/);
5a964f20 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, $gid, @filenames;
815
c4aca7d0
GA
816On systems that support fchown, you might pass file handles among the
817files. On systems that don't support fchown, passing file handles
345da378
GA
818produces a fatal error at run time. The file handles must be passed
819as globs or references to be recognized. Barewords are considered
820file names.
c4aca7d0 821
54310121 822Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
823
824 print "User: ";
19799a22 825 chomp($user = <STDIN>);
5a964f20 826 print "Files: ";
19799a22 827 chomp($pattern = <STDIN>);
a0d0e21e
LW
828
829 ($login,$pass,$uid,$gid) = getpwnam($user)
830 or die "$user not in passwd file";
831
5a964f20 832 @ary = glob($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 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 843
a0d0e21e 844=item chr NUMBER
d74e8afc 845X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 846
54310121 847=item chr
bbce6d69 848
a0d0e21e 849Returns the character represented by that NUMBER in the character set.
a0ed51b3 850For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 851chr(0x263a) is a Unicode smiley face.
aaa68c4a 852
8a064bd6 853Negative values give the Unicode replacement character (chr(0xfffd)),
5f0135eb 854except under the L<bytes> pragma, where low eight bits of the value
8a064bd6
JH
855(truncated to an integer) are used.
856
974da8e5
JH
857If NUMBER is omitted, uses C<$_>.
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<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 898also waits for the process executing on the pipe to complete, in case you
b76cc8ba 899want to look at the output of the pipe afterwards, and
e5218da5
GA
900implicitly puts the exit status value of that command into C<$?> and
901C<${^CHILD_ERROR_NATIVE}>.
5a964f20 902
73689b13
GS
903Prematurely closing the read end of a pipe (i.e. before the process
904writing to it at the other end has closed it) will result in a
905SIGPIPE being delivered to the writer. If the other end can't
906handle that, be sure to read all the data before closing the pipe.
907
fb73857a 908Example:
a0d0e21e 909
fb73857a
PP
910 open(OUTPUT, '|sort >foo') # pipe to sort
911 or die "Can't start sort: $!";
5a964f20 912 #... # print stuff to output
fb73857a
PP
913 close OUTPUT # wait for sort to finish
914 or warn $! ? "Error closing sort pipe: $!"
915 : "Exit status $? from sort";
916 open(INPUT, 'foo') # get sort's results
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
LW
930
931Attempts to connect to a remote socket, just as the connect system call
19799a22 932does. Returns 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) {
955 ### redo always comes here
956 do_something;
957 } continue {
958 ### next always comes here
959 do_something_else;
960 # then back the top to re-check EXPR
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
MS
1024hashing schemes (like MD5), higher level security schemes (like C2),
1025and implementations on non-UNIX platforms may produce different
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) {
a0d0e21e
LW
1047 die "Sorry...\n";
1048 } else {
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
GS
1083specified by MASK (as modified by the C<umask>). If your system supports
1084only the older DBM functions, you may perform only one C<dbmopen> 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) {
1101 print $key, ' = ', unpack('L',$val), "\n";
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")
1114 or die "Can't open netscape history file: $!";
1115
a0d0e21e 1116=item defined EXPR
d74e8afc 1117X<defined> X<undef> X<undefined>
a0d0e21e 1118
54310121 1119=item defined
bbce6d69 1120
2f9daede
TPG
1121Returns a Boolean value telling whether EXPR has a value other than
1122the undefined value C<undef>. If EXPR is not present, C<$_> 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
04891299 1137declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1138may still be callable: its package may have an C<AUTOLOAD> method that
1139makes it spring into existence the first time that it is called -- see
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: $!"
1159 unless defined($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
7660c0ab 1169The pattern match succeeds, and C<$1> is defined, despite the fact that 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
01020589
GS
1183Given an expression that specifies a hash element, array element, hash slice,
1184or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1185In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1186the size of the array will shrink to the highest element that tests
8216c1fd 1187true for exists() (or 0 if no such element exists).
a0d0e21e 1188
eba0920a
EM
1189Returns a list with the same number of elements as the number of elements
1190for which deletion was attempted. Each element of that list consists of
1191either the value of the element deleted, or the undefined value. In scalar
1192context, this means that you get the value of the last element deleted (or
1193the undefined value if that element did not exist).
1194
1195 %hash = (foo => 11, bar => 22, baz => 33);
1196 $scalar = delete $hash{foo}; # $scalar is 11
1197 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1198 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1199
1200Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1201a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1202from a C<tie>d hash or array may not necessarily return anything.
1203
8ea97a1e
GS
1204Deleting an array element effectively returns that position of the array
1205to its initial, uninitialized state. Subsequently testing for the same
cf264981
SP
1206element with exists() will return false. Also, deleting array elements
1207in the middle of an array will not shift the index of the elements
1208after them down. Use splice() for that. See L</exists>.
8ea97a1e 1209
01020589 1210The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1211
5f05dabc
PP
1212 foreach $key (keys %HASH) {
1213 delete $HASH{$key};
a0d0e21e
LW
1214 }
1215
01020589
GS
1216 foreach $index (0 .. $#ARRAY) {
1217 delete $ARRAY[$index];
1218 }
1219
1220And so do these:
5f05dabc 1221
01020589
GS
1222 delete @HASH{keys %HASH};
1223
9740c838 1224 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1225
2b5ab1e7 1226But both of these are slower than just assigning the empty list
01020589
GS
1227or undefining %HASH or @ARRAY:
1228
1229 %HASH = (); # completely empty %HASH
1230 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1231
01020589
GS
1232 @ARRAY = (); # completely empty @ARRAY
1233 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1234
1235Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1236operation is a hash element, array element, hash slice, or array slice
1237lookup:
a0d0e21e
LW
1238
1239 delete $ref->[$x][$y]{$key};
5f05dabc 1240 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1241
01020589
GS
1242 delete $ref->[$x][$y][$index];
1243 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1244
d361fafa
VP
1245The C<delete local EXPR> construct can also be used to localize the deletion
1246of array/hash elements to the current block.
1247See L<perlsub/"Localized deletion of elements of composite types">.
1248
a0d0e21e 1249=item die LIST
d74e8afc 1250X<die> X<throw> X<exception> X<raise> X<$@> 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
28a5cf3b 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
52531d10
GS
1301die() can also be called with a reference argument. If this happens to be
1302trapped within an eval(), $@ contains the reference. This behavior permits
1303a more elaborate exception handling implementation using objects that
4375e838 1304maintain arbitrary state about the nature of the exception. Such a scheme
52531d10 1305is sometimes preferable to matching particular string values of $@ using
746d7dd7
GL
1306regular expressions. Because $@ is a global variable, and eval() may be
1307used within object implementations, care must be taken that analyzing the
1308error object doesn't replace the reference in the global variable. The
1309easiest solution is to make a local copy of the reference before doing
1310other manipulations. Here's an example:
52531d10 1311
da279afe 1312 use Scalar::Util 'blessed';
1313
52531d10 1314 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1315 if (my $ev_err = $@) {
1316 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1317 # handle Some::Module::Exception
1318 }
1319 else {
1320 # handle all other possible exceptions
1321 }
1322 }
1323
19799a22 1324Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1325them, you may want to overload stringification operations on such custom
1326exception objects. See L<overload> for details about that.
1327
19799a22
GS
1328You can arrange for a callback to be run just before the C<die>
1329does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1330handler will be called with the error text and can change the error
1331message, if it sees fit, by calling C<die> again. See
1332L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1333L<"eval BLOCK"> for some examples. Although this feature was
19799a22
GS
1334to be run only right before your program was to exit, this is not
1335currently the case--the C<$SIG{__DIE__}> hook is currently called
1336even inside eval()ed blocks/strings! If one wants the hook to do
1337nothing in such situations, put
fb73857a
PP
1338
1339 die @_ if $^S;
1340
19799a22
GS
1341as the first line of the handler (see L<perlvar/$^S>). Because
1342this promotes strange action at a distance, this counterintuitive
b76cc8ba 1343behavior may be fixed in a future release.
774d564b 1344
a0d0e21e 1345=item do BLOCK
d74e8afc 1346X<do> X<block>
a0d0e21e
LW
1347
1348Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1349sequence of commands indicated by BLOCK. When modified by the C<while> or
1350C<until> loop modifier, executes the BLOCK once before testing the loop
1351condition. (On other statements the loop modifiers test the conditional
1352first.)
a0d0e21e 1353
4968c1e4 1354C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1355C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1356See L<perlsyn> for alternative strategies.
4968c1e4 1357
a0d0e21e 1358=item do SUBROUTINE(LIST)
d74e8afc 1359X<do>
a0d0e21e 1360
cf264981 1361This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1362
1363=item do EXPR
d74e8afc 1364X<do>
a0d0e21e
LW
1365
1366Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1367file as a Perl script.
a0d0e21e
LW
1368
1369 do 'stat.pl';
1370
1371is just like
1372
986b19de 1373 eval `cat stat.pl`;
a0d0e21e 1374
2b5ab1e7 1375except that it's more efficient and concise, keeps track of the current
ea63ef19 1376filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1377C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1378variables. It also differs in that code evaluated with C<do FILENAME>
1379cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1380same, however, in that it does reparse the file every time you call it,
1381so you probably don't want to do this inside a loop.
a0d0e21e 1382
8e30cc93 1383If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1384error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1385returns undef and sets an error message in C<$@>. If the file is
1386successfully compiled, C<do> returns the value of the last expression
1387evaluated.
1388
a0d0e21e 1389Note that inclusion of library modules is better done with the
19799a22 1390C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1391and raise an exception if there's a problem.
a0d0e21e 1392
5a964f20
TC
1393You might like to use C<do> to read in a program configuration
1394file. Manual error checking can be done this way:
1395
b76cc8ba 1396 # read in config files: system first, then user
f86cebdf 1397 for $file ("/share/prog/defaults.rc",
b76cc8ba 1398 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1399 {
5a964f20 1400 unless ($return = do $file) {
f86cebdf
GS
1401 warn "couldn't parse $file: $@" if $@;
1402 warn "couldn't do $file: $!" unless defined $return;
1403 warn "couldn't run $file" unless $return;
5a964f20
TC
1404 }
1405 }
1406
a0d0e21e 1407=item dump LABEL
d74e8afc 1408X<dump> X<core> X<undump>
a0d0e21e 1409
1614b0e3
JD
1410=item dump
1411
19799a22
GS
1412This function causes an immediate core dump. See also the B<-u>
1413command-line switch in L<perlrun>, which does the same thing.
1414Primarily this is so that you can use the B<undump> program (not
1415supplied) to turn your core dump into an executable binary after
1416having initialized all your variables at the beginning of the
1417program. When the new binary is executed it will begin by executing
1418a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1419Think of it as a goto with an intervening core dump and reincarnation.
1420If C<LABEL> is omitted, restarts the program from the top.
1421
1422B<WARNING>: Any files opened at the time of the dump will I<not>
1423be open any more when the program is reincarnated, with possible
b76cc8ba 1424resulting confusion on the part of Perl.
19799a22 1425
59f521f4
RGS
1426This function is now largely obsolete, mostly because it's very hard to
1427convert a core file into an executable. That's why you should now invoke
1428it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1429typo.
19799a22 1430
aa689395 1431=item each HASH
d74e8afc 1432X<each> X<hash, iterator>
aa689395 1433
aeedbbed
NC
1434=item each ARRAY
1435X<array, iterator>
1436
5a964f20 1437When called in list context, returns a 2-element list consisting of the
aeedbbed
NC
1438key and value for the next element of a hash, or the index and value for
1439the next element of an array, so that you can iterate over it. When called
1440in scalar context, returns only the key for the next element in the hash
1441(or the index for an array).
2f9daede 1442
aeedbbed 1443Hash entries are returned in an apparently random order. The actual random
504f80c1
JH
1444order is subject to change in future versions of perl, but it is
1445guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1446function would produce on the same (unmodified) hash. Since Perl
22883ac5 14475.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1448for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1449
aeedbbed
NC
1450When the hash or array is entirely read, a null array is returned in list
1451context (which when assigned produces a false (C<0>) value), and C<undef> in
19799a22 1452scalar context. The next call to C<each> after that will start iterating
aeedbbed
NC
1453again. There is a single iterator for each hash or array, shared by all
1454C<each>, C<keys>, and C<values> function calls in the program; it can be
1455reset by reading all the elements from the hash or array, or by evaluating
1456C<keys HASH>, C<values HASH>, C<keys ARRAY>, or C<values ARRAY>. If you add
1457or delete elements of a hash while you're
74fc8b5f
MJD
1458iterating over it, you may get entries skipped or duplicated, so
1459don't. Exception: It is always safe to delete the item most recently
1460returned by C<each()>, which means that the following code will work:
1461
1462 while (($key, $value) = each %hash) {
1463 print $key, "\n";
1464 delete $hash{$key}; # This is safe
1465 }
aa689395 1466
f86cebdf 1467The following prints out your environment like the printenv(1) program,
aa689395 1468only in a different order:
a0d0e21e
LW
1469
1470 while (($key,$value) = each %ENV) {
1471 print "$key=$value\n";
1472 }
1473
19799a22 1474See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1475
1476=item eof FILEHANDLE
d74e8afc
ITB
1477X<eof>
1478X<end of file>
1479X<end-of-file>
a0d0e21e 1480
4633a7c4
LW
1481=item eof ()
1482
a0d0e21e
LW
1483=item eof
1484
1485Returns 1 if the next read on FILEHANDLE will return end of file, or if
1486FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1487gives the real filehandle. (Note that this function actually
19799a22 1488reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1489interactive context.) Do not read from a terminal file (or call
19799a22 1490C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1491as terminals may lose the end-of-file condition if you do.
1492
820475bd
GS
1493An C<eof> without an argument uses the last file read. Using C<eof()>
1494with empty parentheses is very different. It refers to the pseudo file
1495formed from the files listed on the command line and accessed via the
61eff3bc
JH
1496C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1497as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1498used will cause C<@ARGV> to be examined to determine if input is
67408cae 1499available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1500end-of-file will assume you are processing another C<@ARGV> list,
1501and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1502see L<perlop/"I/O Operators">.
820475bd 1503
61eff3bc 1504In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1505detect the end of each file, C<eof()> will only detect the end of the
1506last file. Examples:
a0d0e21e 1507
748a9306
LW
1508 # reset line numbering on each input file
1509 while (<>) {
b76cc8ba 1510 next if /^\s*#/; # skip comments
748a9306 1511 print "$.\t$_";
5a964f20
TC
1512 } continue {
1513 close ARGV if eof; # Not eof()!
748a9306
LW
1514 }
1515
a0d0e21e
LW
1516 # insert dashes just before last line of last file
1517 while (<>) {
6ac88b13 1518 if (eof()) { # check for end of last file
a0d0e21e
LW
1519 print "--------------\n";
1520 }
1521 print;
6ac88b13 1522 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1523 }
1524
a0d0e21e 1525Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1526input operators typically return C<undef> when they run out of data, or if
1527there was an error.
a0d0e21e
LW
1528
1529=item eval EXPR
d74e8afc 1530X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1531X<error, handling> X<exception, handling>
a0d0e21e
LW
1532
1533=item eval BLOCK
1534
ce2984c3
PF
1535=item eval
1536
c7cc6f1c
GS
1537In the first form, the return value of EXPR is parsed and executed as if it
1538were a little Perl program. The value of the expression (which is itself
5a964f20 1539determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1540errors, executed in the lexical context of the current Perl program, so
1541that any variable settings or subroutine and format definitions remain
cf264981 1542afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1543If EXPR is omitted, evaluates C<$_>. This form is typically used to
1544delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1545
1546In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1547same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1548within the context of the current Perl program. This form is typically
1549used to trap exceptions more efficiently than the first (see below), while
1550also providing the benefit of checking the code within BLOCK at compile
1551time.
1552
1553The final semicolon, if any, may be omitted from the value of EXPR or within
1554the BLOCK.
1555
1556In both forms, the value returned is the value of the last expression
5a964f20 1557evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1558as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1559in void, scalar, or list context, depending on the context of the C<eval>
1560itself. See L</wantarray> for more on how the evaluation context can be
1561determined.
a0d0e21e 1562
19799a22 1563If there is a syntax error or runtime error, or a C<die> statement is
bbead3ca
BL
1564executed, C<eval> returns an undefined value in scalar context
1565or an empty list in list context, and C<$@> is set to the
a0d0e21e 1566error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1567string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1568warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1569To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1570turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1571See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1572
19799a22
GS
1573Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1574determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1575is implemented. It is also Perl's exception trapping mechanism, where
1576the die operator is used to raise exceptions.
1577
5f1da31c
NT
1578If you want to trap errors when loading an XS module, some problems with
1579the binary interface (such as Perl version skew) may be fatal even with
1580C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1581
a0d0e21e
LW
1582If the code to be executed doesn't vary, you may use the eval-BLOCK
1583form to trap run-time errors without incurring the penalty of
1584recompiling each time. The error, if any, is still returned in C<$@>.
1585Examples:
1586
54310121 1587 # make divide-by-zero nonfatal
a0d0e21e
LW
1588 eval { $answer = $a / $b; }; warn $@ if $@;
1589
1590 # same thing, but less efficient
1591 eval '$answer = $a / $b'; warn $@ if $@;
1592
1593 # a compile-time error
5a964f20 1594 eval { $answer = }; # WRONG
a0d0e21e
LW
1595
1596 # a run-time error
1597 eval '$answer ='; # sets $@
1598
cf264981
SP
1599Using the C<eval{}> form as an exception trap in libraries does have some
1600issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1601may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7
TC
1602You can use the C<local $SIG{__DIE__}> construct for this purpose,
1603as shown in this example:
774d564b
PP
1604
1605 # a very private exception trap for divide-by-zero
f86cebdf
GS
1606 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1607 warn $@ if $@;
774d564b
PP
1608
1609This is especially significant, given that C<__DIE__> hooks can call
19799a22 1610C<die> again, which has the effect of changing their error messages:
774d564b
PP
1611
1612 # __DIE__ hooks may modify error messages
1613 {
f86cebdf
GS
1614 local $SIG{'__DIE__'} =
1615 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1616 eval { die "foo lives here" };
1617 print $@ if $@; # prints "bar lives here"
774d564b
PP
1618 }
1619
19799a22 1620Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1621may be fixed in a future release.
1622
19799a22 1623With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1624being looked at when:
1625
1626 eval $x; # CASE 1
1627 eval "$x"; # CASE 2
1628
1629 eval '$x'; # CASE 3
1630 eval { $x }; # CASE 4
1631
5a964f20 1632 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1633 $$x++; # CASE 6
1634
2f9daede 1635Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1636the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1637the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1638and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1639does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1640purely visual reasons, but it also has the advantage of compiling at
1641compile-time instead of at run-time.) Case 5 is a place where
19799a22 1642normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1643particular situation, you can just use symbolic references instead, as
1644in case 6.
a0d0e21e 1645
8a5a710d
DN
1646The assignment to C<$@> occurs before restoration of localised variables,
1647which means a temporary is required if you want to mask some but not all
1648errors:
1649
1650 # alter $@ on nefarious repugnancy only
1651 {
1652 my $e;
1653 {
1654 local $@; # protect existing $@
1655 eval { test_repugnancy() };
1656 # $@ =~ /nefarious/ and die $@; # DOES NOT WORK
1657 $@ =~ /nefarious/ and $e = $@;
1658 }
1659 die $e if defined $e
1660 }
1661
4968c1e4 1662C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1663C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1664
d819b83a
DM
1665Note that as a very special case, an C<eval ''> executed within the C<DB>
1666package doesn't see the usual surrounding lexical scope, but rather the
1667scope of the first non-DB piece of code that called it. You don't normally
1668need to worry about this unless you are writing a Perl debugger.
1669
a0d0e21e 1670=item exec LIST
d74e8afc 1671X<exec> X<execute>
a0d0e21e 1672
8bf3b016
GS
1673=item exec PROGRAM LIST
1674
19799a22
GS
1675The C<exec> function executes a system command I<and never returns>--
1676use C<system> instead of C<exec> if you want it to return. It fails and
1677returns false only if the command does not exist I<and> it is executed
fb73857a 1678directly instead of via your system's command shell (see below).
a0d0e21e 1679
19799a22
GS
1680Since it's a common mistake to use C<exec> instead of C<system>, Perl
1681warns you if there is a following statement which isn't C<die>, C<warn>,
1682or C<exit> (if C<-w> is set - but you always do that). If you
1683I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1684can use one of these styles to avoid the warning:
1685
5a964f20
TC
1686 exec ('foo') or print STDERR "couldn't exec foo: $!";
1687 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1688
5a964f20 1689If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1690with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1691If there is only one scalar argument or an array with one element in it,
1692the argument is checked for shell metacharacters, and if there are any,
1693the entire argument is passed to the system's command shell for parsing
1694(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1695If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1696words and passed directly to C<execvp>, which is more efficient.
19799a22 1697Examples:
a0d0e21e 1698
19799a22
GS
1699 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1700 exec "sort $outfile | uniq";
a0d0e21e
LW
1701
1702If you don't really want to execute the first argument, but want to lie
1703to the program you are executing about its own name, you can specify
1704the program you actually want to run as an "indirect object" (without a
1705comma) in front of the LIST. (This always forces interpretation of the
54310121 1706LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1707the list.) Example:
1708
1709 $shell = '/bin/csh';
1710 exec $shell '-sh'; # pretend it's a login shell
1711
1712or, more directly,
1713
1714 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1715
bb32b41a
GS
1716When the arguments get executed via the system shell, results will
1717be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1718for details.
1719
19799a22
GS
1720Using an indirect object with C<exec> or C<system> is also more
1721secure. This usage (which also works fine with system()) forces
1722interpretation of the arguments as a multivalued list, even if the
1723list had just one argument. That way you're safe from the shell
1724expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1725
1726 @args = ( "echo surprise" );
1727
2b5ab1e7 1728 exec @args; # subject to shell escapes
f86cebdf 1729 # if @args == 1
2b5ab1e7 1730 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1731
1732The first version, the one without the indirect object, ran the I<echo>
1733program, passing it C<"surprise"> an argument. The second version
1734didn't--it tried to run a program literally called I<"echo surprise">,
1735didn't find it, and set C<$?> to a non-zero value indicating failure.
1736
0f897271
GS
1737Beginning with v5.6.0, Perl will attempt to flush all files opened for
1738output before the exec, but this may not be supported on some platforms
1739(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1740in English) or call the C<autoflush()> method of C<IO::Handle> on any
1741open handles in order to avoid lost output.
1742
19799a22 1743Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1744any C<DESTROY> methods in your objects.
1745
a0d0e21e 1746=item exists EXPR
d74e8afc 1747X<exists> X<autovivification>
a0d0e21e 1748
01020589 1749Given an expression that specifies a hash element or array element,
8ea97a1e 1750returns true if the specified element in the hash or array has ever
95731d22 1751been initialized, even if the corresponding value is undefined.
a0d0e21e 1752
01020589
GS
1753 print "Exists\n" if exists $hash{$key};
1754 print "Defined\n" if defined $hash{$key};
1755 print "True\n" if $hash{$key};
1756
1757 print "Exists\n" if exists $array[$index];
1758 print "Defined\n" if defined $array[$index];
1759 print "True\n" if $array[$index];
a0d0e21e 1760
8ea97a1e 1761A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1762it exists, but the reverse doesn't necessarily hold true.
1763
afebc493
GS
1764Given an expression that specifies the name of a subroutine,
1765returns true if the specified subroutine has ever been declared, even
1766if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1767does not count as declaring it. Note that a subroutine which does not
1768exist may still be callable: its package may have an C<AUTOLOAD>
1769method that makes it spring into existence the first time that it is
1770called -- see L<perlsub>.
afebc493
GS
1771
1772 print "Exists\n" if exists &subroutine;
1773 print "Defined\n" if defined &subroutine;
1774
a0d0e21e 1775Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1776operation is a hash or array key lookup or subroutine name:
a0d0e21e 1777
2b5ab1e7
TC
1778 if (exists $ref->{A}->{B}->{$key}) { }
1779 if (exists $hash{A}{B}{$key}) { }
1780
01020589
GS
1781 if (exists $ref->{A}->{B}->[$ix]) { }
1782 if (exists $hash{A}{B}[$ix]) { }
1783
afebc493
GS
1784 if (exists &{$ref->{A}{B}{$key}}) { }
1785
01020589
GS
1786Although the deepest nested array or hash will not spring into existence
1787just because its existence was tested, any intervening ones will.
61eff3bc 1788Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1789into existence due to the existence test for the $key element above.
1790This happens anywhere the arrow operator is used, including even:
5a964f20 1791
2b5ab1e7
TC
1792 undef $ref;
1793 if (exists $ref->{"Some key"}) { }
1794 print $ref; # prints HASH(0x80d3d5c)
1795
1796This surprising autovivification in what does not at first--or even
1797second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1798release.
a0d0e21e 1799
afebc493
GS
1800Use of a subroutine call, rather than a subroutine name, as an argument
1801to exists() is an error.
1802
1803 exists &sub; # OK
1804 exists &sub(); # Error
1805
a0d0e21e 1806=item exit EXPR
d74e8afc 1807X<exit> X<terminate> X<abort>
a0d0e21e 1808
ce2984c3
PF
1809=item exit
1810
2b5ab1e7 1811Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1812
1813 $ans = <STDIN>;
1814 exit 0 if $ans =~ /^[Xx]/;
1815
19799a22 1816See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1817universally recognized values for EXPR are C<0> for success and C<1>
1818for error; other values are subject to interpretation depending on the
1819environment in which the Perl program is running. For example, exiting
182069 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1821the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1822
19799a22
GS
1823Don't use C<exit> to abort a subroutine if there's any chance that
1824someone might want to trap whatever error happened. Use C<die> instead,
1825which can be trapped by an C<eval>.
28757baa 1826
19799a22 1827The exit() function does not always exit immediately. It calls any
2b5ab1e7 1828defined C<END> routines first, but these C<END> routines may not
19799a22 1829themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1830be called are called before the real exit. If this is a problem, you
1831can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1832See L<perlmod> for details.
5a964f20 1833
a0d0e21e 1834=item exp EXPR
d74e8afc 1835X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1836
54310121 1837=item exp
bbce6d69 1838
b76cc8ba 1839Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1840If EXPR is omitted, gives C<exp($_)>.
1841
1842=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1843X<fcntl>
a0d0e21e 1844
f86cebdf 1845Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1846
1847 use Fcntl;
1848
0ade1984 1849first to get the correct constant definitions. Argument processing and
b76cc8ba 1850value return works just like C<ioctl> below.
a0d0e21e
LW
1851For example:
1852
1853 use Fcntl;
5a964f20
TC
1854 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1855 or die "can't fcntl F_GETFL: $!";
1856
554ad1fc 1857You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1858Like C<ioctl>, it maps a C<0> return from the system call into
1859C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1860in numeric context. It is also exempt from the normal B<-w> warnings
1861on improper numeric conversions.
5a964f20 1862
19799a22 1863Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1864doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1865manpage to learn what functions are available on your system.
a0d0e21e 1866
be2f7487 1867Here's an example of setting a filehandle named C<REMOTE> to be
1868non-blocking at the system level. You'll have to negotiate C<$|>
1869on your own, though.
1870
1871 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1872
1873 $flags = fcntl(REMOTE, F_GETFL, 0)
1874 or die "Can't get flags for the socket: $!\n";
1875
1876 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1877 or die "Can't set flags for the socket: $!\n";
1878
a0d0e21e 1879=item fileno FILEHANDLE
d74e8afc 1880X<fileno>
a0d0e21e 1881
2b5ab1e7
TC
1882Returns the file descriptor for a filehandle, or undefined if the
1883filehandle is not open. This is mainly useful for constructing
19799a22 1884bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1885If FILEHANDLE is an expression, the value is taken as an indirect
1886filehandle, generally its name.
5a964f20 1887
b76cc8ba 1888You can use this to find out whether two handles refer to the
5a964f20
TC
1889same underlying descriptor:
1890
1891 if (fileno(THIS) == fileno(THAT)) {
1892 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1893 }
1894
1895(Filehandles connected to memory objects via new features of C<open> may
1896return undefined even though they are open.)
1897
a0d0e21e
LW
1898
1899=item flock FILEHANDLE,OPERATION
d74e8afc 1900X<flock> X<lock> X<locking>
a0d0e21e 1901
19799a22
GS
1902Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1903for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1904machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1905C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1906only entire files, not records.
1907
1908Two potentially non-obvious but traditional C<flock> semantics are
1909that it waits indefinitely until the lock is granted, and that its locks
1910B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1911fewer guarantees. This means that programs that do not also use C<flock>
1912may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1913your port's specific documentation, or your system-specific local manpages
1914for details. It's best to assume traditional behavior if you're writing
1915portable programs. (But if you're not, you should as always feel perfectly
1916free to write for your own system's idiosyncrasies (sometimes called
1917"features"). Slavish adherence to portability concerns shouldn't get
1918in the way of your getting your job done.)
a3cb178b 1919
8ebc5c01
PP
1920OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1921LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1922you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1923either individually, or as a group using the ':flock' tag. LOCK_SH
1924requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1925releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1926LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1927waiting for the lock (check the return status to see if you got it).
1928
2b5ab1e7
TC
1929To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1930before locking or unlocking it.
8ebc5c01 1931
f86cebdf 1932Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1933locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1934are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1935implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1936differing semantics shouldn't bite too many people.
1937
becacb53
TM
1938Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1939be open with read intent to use LOCK_SH and requires that it be open
1940with write intent to use LOCK_EX.
1941
19799a22
GS
1942Note also that some versions of C<flock> cannot lock things over the
1943network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1944that. If you like you can force Perl to ignore your system's flock(2)
1945function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1946the switch C<-Ud_flock> to the F<Configure> program when you configure
1947perl.
4633a7c4
LW
1948
1949Here's a mailbox appender for BSD systems.
a0d0e21e 1950
7ed5353d 1951 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
1952
1953 sub lock {
7ed5353d
PF
1954 my ($fh) = @_;
1955 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
1956
1957 # and, in case someone appended while we were waiting...
1958 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
1959 }
1960
1961 sub unlock {
7ed5353d
PF
1962 my ($fh) = @_;
1963 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
1964 }
1965
b0169937 1966 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
a0d0e21e
LW
1967 or die "Can't open mailbox: $!";
1968
7ed5353d 1969 lock($mbox);
b0169937 1970 print $mbox $msg,"\n\n";
7ed5353d 1971 unlock($mbox);
a0d0e21e 1972
2b5ab1e7
TC
1973On systems that support a real flock(), locks are inherited across fork()
1974calls, whereas those that must resort to the more capricious fcntl()
1975function lose the locks, making it harder to write servers.
1976
cb1a09d0 1977See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1978
1979=item fork
d74e8afc 1980X<fork> X<child> X<parent>
a0d0e21e 1981
2b5ab1e7
TC
1982Does a fork(2) system call to create a new process running the
1983same program at the same point. It returns the child pid to the
1984parent process, C<0> to the child process, or C<undef> if the fork is
1985unsuccessful. File descriptors (and sometimes locks on those descriptors)
1986are shared, while everything else is copied. On most systems supporting
1987fork(), great care has gone into making it extremely efficient (for
1988example, using copy-on-write technology on data pages), making it the
1989dominant paradigm for multitasking over the last few decades.
5a964f20 1990
0f897271
GS
1991Beginning with v5.6.0, Perl will attempt to flush all files opened for
1992output before forking the child process, but this may not be supported
1993on some platforms (see L<perlport>). To be safe, you may need to set
1994C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1995C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1996
19799a22 1997If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1998accumulate zombies. On some systems, you can avoid this by setting
1999C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2000forking and reaping moribund children.
cb1a09d0 2001
28757baa
PP
2002Note that if your forked child inherits system file descriptors like
2003STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2004if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2005backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2006You should reopen those to F</dev/null> if it's any issue.
28757baa 2007
cb1a09d0 2008=item format
d74e8afc 2009X<format>
cb1a09d0 2010
19799a22 2011Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2012example:
2013
54310121 2014 format Something =
cb1a09d0
AD
2015 Test: @<<<<<<<< @||||| @>>>>>
2016 $str, $%, '$' . int($num)
2017 .
2018
2019 $str = "widget";
184e9718 2020 $num = $cost/$quantity;
cb1a09d0
AD
2021 $~ = 'Something';
2022 write;
2023
2024See L<perlform> for many details and examples.
2025
8903cb82 2026=item formline PICTURE,LIST
d74e8afc 2027X<formline>
a0d0e21e 2028
5a964f20 2029This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2030too. It formats (see L<perlform>) a list of values according to the
2031contents of PICTURE, placing the output into the format output
7660c0ab 2032accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2033Eventually, when a C<write> is done, the contents of
cf264981
SP
2034C<$^A> are written to some filehandle. You could also read C<$^A>
2035and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2036does one C<formline> per line of form, but the C<formline> function itself
748a9306 2037doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 2038that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
2039You may therefore need to use multiple formlines to implement a single
2040record format, just like the format compiler.
2041
19799a22 2042Be careful if you put double quotes around the picture, because an C<@>
748a9306 2043character may be taken to mean the beginning of an array name.
19799a22 2044C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
2045
2046=item getc FILEHANDLE
f723aae1 2047X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2048
2049=item getc
2050
2051Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
2052or the undefined value at end of file, or if there was an error (in
2053the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2054STDIN. This is not particularly efficient. However, it cannot be
2055used by itself to fetch single characters without waiting for the user
2056to hit enter. For that, try something more like:
4633a7c4
LW
2057
2058 if ($BSD_STYLE) {
2059 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2060 }
2061 else {
54310121 2062 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2063 }
2064
2065 $key = getc(STDIN);
2066
2067 if ($BSD_STYLE) {
2068 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2069 }
2070 else {
5f05dabc 2071 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
2072 }
2073 print "\n";
2074
54310121
PP
2075Determination of whether $BSD_STYLE should be set
2076is left as an exercise to the reader.
cb1a09d0 2077
19799a22 2078The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2079systems purporting POSIX compliance. See also the C<Term::ReadKey>
2080module from your nearest CPAN site; details on CPAN can be found on
2081L<perlmodlib/CPAN>.
a0d0e21e
LW
2082
2083=item getlogin
d74e8afc 2084X<getlogin> X<login>
a0d0e21e 2085
cf264981 2086This implements the C library function of the same name, which on most
5a964f20 2087systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 2088use C<getpwuid>.
a0d0e21e 2089
f86702cc 2090 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2091
19799a22
GS
2092Do not consider C<getlogin> for authentication: it is not as
2093secure as C<getpwuid>.
4633a7c4 2094
a0d0e21e 2095=item getpeername SOCKET
d74e8afc 2096X<getpeername> X<peer>
a0d0e21e
LW
2097
2098Returns the packed sockaddr address of other end of the SOCKET connection.
2099
4633a7c4
LW
2100 use Socket;
2101 $hersockaddr = getpeername(SOCK);
19799a22 2102 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2103 $herhostname = gethostbyaddr($iaddr, AF_INET);
2104 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2105
2106=item getpgrp PID
d74e8afc 2107X<getpgrp> X<group>
a0d0e21e 2108
47e29363 2109Returns the current process group for the specified PID. Use
7660c0ab 2110a PID of C<0> to get the current process group for the
4633a7c4 2111current process. Will raise an exception if used on a machine that
f86cebdf 2112doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2113group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2114does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2115
2116=item getppid
d74e8afc 2117X<getppid> X<parent> X<pid>
a0d0e21e
LW
2118
2119Returns the process id of the parent process.
2120
4d76a344
RGS
2121Note for Linux users: on Linux, the C functions C<getpid()> and
2122C<getppid()> return different values from different threads. In order to
2123be portable, this behavior is not reflected by the perl-level function
2124C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2125to call the underlying C<getppid()>, you may use the CPAN module
2126C<Linux::Pid>.
4d76a344 2127
a0d0e21e 2128=item getpriority WHICH,WHO
d74e8afc 2129X<getpriority> X<priority> X<nice>
a0d0e21e 2130
4633a7c4 2131Returns the current priority for a process, a process group, or a user.
f4084e39 2132(See C<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2133machine that doesn't implement getpriority(2).
a0d0e21e
LW
2134
2135=item getpwnam NAME
d74e8afc
ITB
2136X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2137X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2138X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2139X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2140X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2141X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2142
2143=item getgrnam NAME
2144
2145=item gethostbyname NAME
2146
2147=item getnetbyname NAME
2148
2149=item getprotobyname NAME
2150
2151=item getpwuid UID
2152
2153=item getgrgid GID
2154
2155=item getservbyname NAME,PROTO
2156
2157=item gethostbyaddr ADDR,ADDRTYPE
2158
2159=item getnetbyaddr ADDR,ADDRTYPE
2160
2161=item getprotobynumber NUMBER
2162
2163=item getservbyport PORT,PROTO
2164
2165=item getpwent
2166
2167=item getgrent
2168
2169=item gethostent
2170
2171=item getnetent
2172
2173=item getprotoent
2174
2175=item getservent
2176
2177=item setpwent
2178
2179=item setgrent
2180
2181=item sethostent STAYOPEN
2182
2183=item setnetent STAYOPEN
2184
2185=item setprotoent STAYOPEN
2186
2187=item setservent STAYOPEN
2188
2189=item endpwent
2190
2191=item endgrent
2192
2193=item endhostent
2194
2195=item endnetent
2196
2197=item endprotoent
2198
2199=item endservent
2200
2201These routines perform the same functions as their counterparts in the
5a964f20 2202system library. In list context, the return values from the
a0d0e21e
LW
2203various get routines are as follows:
2204
2205 ($name,$passwd,$uid,$gid,
6ee623d5 2206 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2207 ($name,$passwd,$gid,$members) = getgr*
2208 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2209 ($name,$aliases,$addrtype,$net) = getnet*
2210 ($name,$aliases,$proto) = getproto*
2211 ($name,$aliases,$port,$proto) = getserv*
2212
2213(If the entry doesn't exist you get a null list.)
2214
4602f195
JH
2215The exact meaning of the $gcos field varies but it usually contains
2216the real name of the user (as opposed to the login name) and other
2217information pertaining to the user. Beware, however, that in many
2218system users are able to change this information and therefore it
106325ad 2219cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2220L<perlsec>). The $passwd and $shell, user's encrypted password and
2221login shell, are also tainted, because of the same reason.
4602f195 2222
5a964f20 2223In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2224lookup by name, in which case you get the other thing, whatever it is.
2225(If the entry doesn't exist you get the undefined value.) For example:
2226
5a964f20
TC
2227 $uid = getpwnam($name);
2228 $name = getpwuid($num);
2229 $name = getpwent();
2230 $gid = getgrnam($name);
08a33e13 2231 $name = getgrgid($num);
5a964f20
TC
2232 $name = getgrent();
2233 #etc.
a0d0e21e 2234
4602f195
JH
2235In I<getpw*()> the fields $quota, $comment, and $expire are special
2236cases in the sense that in many systems they are unsupported. If the
2237$quota is unsupported, it is an empty scalar. If it is supported, it
2238usually encodes the disk quota. If the $comment field is unsupported,
2239it is an empty scalar. If it is supported it usually encodes some
2240administrative comment about the user. In some systems the $quota
2241field may be $change or $age, fields that have to do with password
2242aging. In some systems the $comment field may be $class. The $expire
2243field, if present, encodes the expiration period of the account or the
2244password. For the availability and the exact meaning of these fields
2245in your system, please consult your getpwnam(3) documentation and your
2246F<pwd.h> file. You can also find out from within Perl what your
2247$quota and $comment fields mean and whether you have the $expire field
2248by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2249C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2250files are only supported if your vendor has implemented them in the
2251intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2252shadow versions if you're running under privilege or if there exists
cf264981
SP
2253the shadow(3) functions as found in System V (this includes Solaris
2254and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2255facility are unlikely to be supported.
6ee623d5 2256
19799a22 2257The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2258the login names of the members of the group.
2259
2260For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2261C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2262C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2263addresses returned by the corresponding system library call. In the
2264Internet domain, each address is four bytes long and you can unpack it
2265by saying something like:
2266
f337b084 2267 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2268
2b5ab1e7
TC
2269The Socket library makes this slightly easier:
2270
2271 use Socket;
2272 $iaddr = inet_aton("127.1"); # or whatever address
2273 $name = gethostbyaddr($iaddr, AF_INET);
2274
2275 # or going the other way
19799a22 2276 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2277
d760c846
GS
2278In the opposite way, to resolve a hostname to the IP address
2279you can write this:
2280
2281 use Socket;
2282 $packed_ip = gethostbyname("www.perl.org");
2283 if (defined $packed_ip) {
2284 $ip_address = inet_ntoa($packed_ip);
2285 }
2286
2287Make sure <gethostbyname()> is called in SCALAR context and that
2288its return value is checked for definedness.
2289
19799a22
GS
2290If you get tired of remembering which element of the return list
2291contains which return value, by-name interfaces are provided
2292in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2293C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2294and C<User::grent>. These override the normal built-ins, supplying
2295versions that return objects with the appropriate names
2296for each field. For example:
5a964f20
TC
2297
2298 use File::stat;
2299 use User::pwent;
2300 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2301
b76cc8ba
NIS
2302Even though it looks like they're the same method calls (uid),
2303they aren't, because a C<File::stat> object is different from
19799a22 2304a C<User::pwent> object.
5a964f20 2305
a0d0e21e 2306=item getsockname SOCKET
d74e8afc 2307X<getsockname>
a0d0e21e 2308
19799a22
GS
2309Returns the packed sockaddr address of this end of the SOCKET connection,
2310in case you don't know the address because you have several different
2311IPs that the connection might have come in on.
a0d0e21e 2312
4633a7c4
LW
2313 use Socket;
2314 $mysockaddr = getsockname(SOCK);
19799a22 2315 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2316 printf "Connect to %s [%s]\n",
19799a22
GS
2317 scalar gethostbyaddr($myaddr, AF_INET),
2318 inet_ntoa($myaddr);
a0d0e21e
LW
2319
2320=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2321X<getsockopt>
a0d0e21e 2322
636e6b1f
TH
2323Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2324Options may exist at multiple protocol levels depending on the socket
2325type, but at least the uppermost socket level SOL_SOCKET (defined in the
2326C<Socket> module) will exist. To query options at another level the
2327protocol number of the appropriate protocol controlling the option
2328should be supplied. For example, to indicate that an option is to be
2329interpreted by the TCP protocol, LEVEL should be set to the protocol
2330number of TCP, which you can get using getprotobyname.
2331
2332The call returns a packed string representing the requested socket option,
2333or C<undef> if there is an error (the error reason will be in $!). What
2334exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2335your system documentation for details. A very common case however is that
cf264981 2336the option is an integer, in which case the result will be a packed
636e6b1f
TH
2337integer which you can decode using unpack with the C<i> (or C<I>) format.
2338
2339An example testing if Nagle's algorithm is turned on on a socket:
2340
4852725b 2341 use Socket qw(:all);
636e6b1f
TH
2342
2343 defined(my $tcp = getprotobyname("tcp"))
2344 or die "Could not determine the protocol number for tcp";
4852725b
DD
2345 # my $tcp = IPPROTO_TCP; # Alternative
2346 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2347 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2348 my $nodelay = unpack("I", $packed);
2349 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2350
a0d0e21e
LW
2351
2352=item glob EXPR
d74e8afc 2353X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2354
0a753a76
PP
2355=item glob
2356
d9a9d457
JL
2357In list context, returns a (possibly empty) list of filename expansions on
2358the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2359scalar context, glob iterates through such filename expansions, returning
2360undef when the list is exhausted. This is the internal function
2361implementing the C<< <*.c> >> operator, but you can use it directly. If
2362EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2363more detail in L<perlop/"I/O Operators">.
a0d0e21e 2364
5c0c9249
PF
2365Note that C<glob> will split its arguments on whitespace, treating
2366each segment as separate pattern. As such, C<glob('*.c *.h')> would
2367match all files with a F<.c> or F<.h> extension. The expression
2368C<glob('.* *')> would match all files in the current working directory.
2369
3a4b19e4 2370Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2371C<File::Glob> extension. See L<File::Glob> for details, including
2372C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2373
a0d0e21e 2374=item gmtime EXPR
d74e8afc 2375X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2376
ce2984c3
PF
2377=item gmtime
2378
435fbc73
GS
2379Works just like L<localtime> but the returned values are
2380localized for the standard Greenwich time zone.
a0d0e21e 2381
435fbc73
GS
2382Note: when called in list context, $isdst, the last value
2383returned by gmtime is always C<0>. There is no
2384Daylight Saving Time in GMT.
0a753a76 2385
62aa5637
MS
2386See L<perlport/gmtime> for portability concerns.
2387
a0d0e21e 2388=item goto LABEL
d74e8afc 2389X<goto> X<jump> X<jmp>
a0d0e21e 2390
748a9306
LW
2391=item goto EXPR
2392
a0d0e21e
LW
2393=item goto &NAME
2394
7660c0ab 2395The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2396execution there. It may not be used to go into any construct that
7660c0ab 2397requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2398also can't be used to go into a construct that is optimized away,
19799a22 2399or to get out of a block or subroutine given to C<sort>.
0a753a76 2400It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2401including out of subroutines, but it's usually better to use some other
19799a22 2402construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2403need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2404(The difference being that C does not offer named loops combined with
2405loop control. Perl does, and this replaces most structured uses of C<goto>
2406in other languages.)
a0d0e21e 2407
7660c0ab
A
2408The C<goto-EXPR> form expects a label name, whose scope will be resolved
2409dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2410necessarily recommended if you're optimizing for maintainability:
2411
2412 goto ("FOO", "BAR", "GLARCH")[$i];
2413
1b6921cb
BT
2414The C<goto-&NAME> form is quite different from the other forms of
2415C<goto>. In fact, it isn't a goto in the normal sense at all, and
2416doesn't have the stigma associated with other gotos. Instead, it
2417exits the current subroutine (losing any changes set by local()) and
2418immediately calls in its place the named subroutine using the current
2419value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2420load another subroutine and then pretend that the other subroutine had
2421been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2422in the current subroutine are propagated to the other subroutine.)
2423After the C<goto>, not even C<caller> will be able to tell that this
2424routine was called first.
2425
2426NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2427containing a code reference, or a block that evaluates to a code
6cb9131c 2428reference.
a0d0e21e
LW
2429
2430=item grep BLOCK LIST
d74e8afc 2431X<grep>
a0d0e21e
LW
2432
2433=item grep EXPR,LIST
2434
2b5ab1e7
TC
2435This is similar in spirit to, but not the same as, grep(1) and its
2436relatives. In particular, it is not limited to using regular expressions.
2f9daede 2437
a0d0e21e 2438Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2439C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2440elements for which the expression evaluated to true. In scalar
2441context, returns the number of times the expression was true.
a0d0e21e
LW
2442
2443 @foo = grep(!/^#/, @bar); # weed out comments
2444
2445or equivalently,
2446
2447 @foo = grep {!/^#/} @bar; # weed out comments
2448
be3174d2
GS
2449Note that C<$_> is an alias to the list value, so it can be used to
2450modify the elements of the LIST. While this is useful and supported,
2451it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2452Similarly, grep returns aliases into the original list, much as a for
2453loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2454element of a list returned by grep (for example, in a C<foreach>, C<map>
2455or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2456This is usually something to be avoided when writing clear code.
a0d0e21e 2457
a4fb8298 2458If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2459been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2460the list elements, C<$_> keeps being lexical inside the block; i.e. it
2461can't be seen from the outside, avoiding any potential side-effects.
2462
19799a22 2463See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2464
a0d0e21e 2465=item hex EXPR
d74e8afc 2466X<hex> X<hexadecimal>
a0d0e21e 2467
54310121 2468=item hex
bbce6d69 2469
2b5ab1e7 2470Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2471(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2472L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2473
2474 print hex '0xAf'; # prints '175'
2475 print hex 'aF'; # same
a0d0e21e 2476
19799a22 2477Hex strings may only represent integers. Strings that would cause
53305cf1 2478integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2479unlike oct(). To present something as hex, look into L</printf>,
2480L</sprintf>, or L</unpack>.
19799a22 2481
ce2984c3 2482=item import LIST
d74e8afc 2483X<import>
a0d0e21e 2484
19799a22 2485There is no builtin C<import> function. It is just an ordinary
4633a7c4 2486method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2487names to another module. The C<use> function calls the C<import> method
cea6626f 2488for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2489
2490=item index STR,SUBSTR,POSITION
d74e8afc 2491X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2492
2493=item index STR,SUBSTR
2494
2b5ab1e7
TC
2495The index function searches for one string within another, but without
2496the wildcard-like behavior of a full regular-expression pattern match.
2497It returns the position of the first occurrence of SUBSTR in STR at
2498or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2499beginning of the string. POSITION before the beginning of the string
2500or after its end is treated as if it were the beginning or the end,
2501respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2502you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2503is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2504
2505=item int EXPR
f723aae1 2506X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2507
54310121 2508=item int
bbce6d69 2509
7660c0ab 2510Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2511You should not use this function for rounding: one because it truncates
2512towards C<0>, and two because machine representations of floating point
2513numbers can sometimes produce counterintuitive results. For example,
2514C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2515because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2516the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2517functions will serve you better than will int().
a0d0e21e
LW
2518
2519=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2520X<ioctl>
a0d0e21e 2521
2b5ab1e7 2522Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2523
6c567752 2524 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2525
a11c483f 2526to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2527exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2528own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2529(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2530may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2531written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2532will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2533has no string value but does have a numeric value, that value will be
2534passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2535true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2536functions may be needed to manipulate the values of structures used by
b76cc8ba 2537C<ioctl>.
a0d0e21e 2538
19799a22 2539The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2540
2541 if OS returns: then Perl returns:
2542 -1 undefined value
2543 0 string "0 but true"
2544 anything else that number
2545
19799a22 2546Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2547still easily determine the actual value returned by the operating
2548system:
2549
2b5ab1e7 2550 $retval = ioctl(...) || -1;
a0d0e21e
LW
2551 printf "System returned %d\n", $retval;
2552
be2f7487 2553The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2554about improper numeric conversions.
2555
a0d0e21e 2556=item join EXPR,LIST
d74e8afc 2557X<join>
a0d0e21e 2558
2b5ab1e7
TC
2559Joins the separate strings of LIST into a single string with fields
2560separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2561
2b5ab1e7 2562 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2563
eb6e2d6f
GS
2564Beware that unlike C<split>, C<join> doesn't take a pattern as its
2565first argument. Compare L</split>.
a0d0e21e 2566
aa689395 2567=item keys HASH
d74e8afc 2568X<keys> X<key>
aa689395 2569
aeedbbed
NC
2570=item keys ARRAY
2571
2572Returns a list consisting of all the keys of the named hash, or the indices
2573of an array. (In scalar context, returns the number of keys or indices.)
504f80c1 2574
aeedbbed 2575The keys of a hash are returned in an apparently random order. The actual
504f80c1
JH
2576random order is subject to change in future versions of perl, but it
2577is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2578function produces (given that the hash has not been modified). Since
2579Perl 5.8.1 the ordering is different even between different runs of
2580Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2581Attacks">).
504f80c1 2582
aeedbbed 2583As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
cf264981
SP
2584(see L</each>). In particular, calling keys() in void context resets
2585the iterator with no other overhead.
a0d0e21e 2586
aa689395 2587Here is yet another way to print your environment:
a0d0e21e
LW
2588
2589 @keys = keys %ENV;
2590 @values = values %ENV;
b76cc8ba 2591 while (@keys) {
a0d0e21e
LW
2592 print pop(@keys), '=', pop(@values), "\n";
2593 }
2594
2595or how about sorted by key:
2596
2597 foreach $key (sort(keys %ENV)) {
2598 print $key, '=', $ENV{$key}, "\n";
2599 }
2600
8ea1e5d4
GS
2601The returned values are copies of the original keys in the hash, so
2602modifying them will not affect the original hash. Compare L</values>.
2603
19799a22 2604To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2605Here's a descending numeric sort of a hash by its values:
4633a7c4 2606
5a964f20 2607 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2608 printf "%4d %s\n", $hash{$key}, $key;
2609 }
2610
19799a22 2611As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2612allocated for the given hash. This can gain you a measure of efficiency if
2613you know the hash is going to get big. (This is similar to pre-extending
2614an array by assigning a larger number to $#array.) If you say
55497cff
PP
2615
2616 keys %hash = 200;
2617
ab192400
GS
2618then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2619in fact, since it rounds up to the next power of two. These
55497cff
PP
2620buckets will be retained even if you do C<%hash = ()>, use C<undef
2621%hash> if you want to free the storage while C<%hash> is still in scope.
2622You can't shrink the number of buckets allocated for the hash using
19799a22 2623C<keys> in this way (but you needn't worry about doing this by accident,
aeedbbed
NC
2624as trying has no effect). C<keys @array> in an lvalue context is a syntax
2625error.
55497cff 2626
19799a22 2627See also C<each>, C<values> and C<sort>.
ab192400 2628
b350dd2f 2629=item kill SIGNAL, LIST
d74e8afc 2630X<kill> X<signal>
a0d0e21e 2631
b350dd2f 2632Sends a signal to a list of processes. Returns the number of
517db077
GS
2633processes successfully signaled (which is not necessarily the
2634same as the number actually killed).
a0d0e21e
LW
2635
2636 $cnt = kill 1, $child1, $child2;
2637 kill 9, @goners;
2638
70fb64f6 2639If SIGNAL is zero, no signal is sent to the process, but the kill(2)
6cb9d3e4 2640system call will check whether it's possible to send a signal to it (that
70fb64f6
RGS
2641means, to be brief, that the process is owned by the same user, or we are
2642the super-user). This is a useful way to check that a child process is
81fd35db
DN
2643alive (even if only as a zombie) and hasn't changed its UID. See
2644L<perlport> for notes on the portability of this construct.
b350dd2f 2645
e2c0f81f
DG
2646Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2647of processes. That means you usually want to use positive not negative signals.
2648You may also use a signal name in quotes.
2649
2650The behavior of kill when a I<PROCESS> number is zero or negative depends on
2651the operating system. For example, on POSIX-conforming systems, zero will
2652signal the current process group and -1 will signal all processes.
1e9c1022
JL
2653
2654See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2655
2656=item last LABEL
d74e8afc 2657X<last> X<break>
a0d0e21e
LW
2658
2659=item last
2660
2661The C<last> command is like the C<break> statement in C (as used in
2662loops); it immediately exits the loop in question. If the LABEL is
2663omitted, the command refers to the innermost enclosing loop. The
2664C<continue> block, if any, is not executed:
2665
4633a7c4
LW
2666 LINE: while (<STDIN>) {
2667 last LINE if /^$/; # exit when done with header
5a964f20 2668 #...
a0d0e21e
LW
2669 }
2670
4968c1e4 2671C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2672C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2673a grep() or map() operation.
4968c1e4 2674
6c1372ed
GS
2675Note that a block by itself is semantically identical to a loop
2676that executes once. Thus C<last> can be used to effect an early
2677exit out of such a block.
2678
98293880
JH
2679See also L</continue> for an illustration of how C<last>, C<next>, and
2680C<redo> work.
1d2dff63 2681
a0d0e21e 2682=item lc EXPR
d74e8afc 2683X<lc> X<lowercase>
a0d0e21e 2684
54310121 2685=item lc
bbce6d69 2686
d1be9408 2687Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2688implementing the C<\L> escape in double-quoted strings. Respects
2689current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2690and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2691
7660c0ab 2692If EXPR is omitted, uses C<$_>.
bbce6d69 2693
a0d0e21e 2694=item lcfirst EXPR
d74e8afc 2695X<lcfirst> X<lowercase>
a0d0e21e 2696
54310121 2697=item lcfirst
bbce6d69 2698
ad0029c4
JH
2699Returns the value of EXPR with the first character lowercased. This
2700is the internal function implementing the C<\l> escape in
2701double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2702locale> in force. See L<perllocale> and L<perlunicode> for more
2703details about locale and Unicode support.
a0d0e21e 2704
7660c0ab 2705If EXPR is omitted, uses C<$_>.
bbce6d69 2706
a0d0e21e 2707=item length EXPR
d74e8afc 2708X<length> X<size>
a0d0e21e 2709
54310121 2710=item length
bbce6d69 2711
974da8e5 2712Returns the length in I<characters> of the value of EXPR. If EXPR is
9f621bb0
NC
2713omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
2714Note that this cannot be used on an entire array or hash to find out how
2715many elements these have. For that, use C<scalar @array> and C<scalar keys
2716%hash> respectively.
a0d0e21e 2717
974da8e5
JH
2718Note the I<characters>: if the EXPR is in Unicode, you will get the
2719number of characters, not the number of bytes. To get the length
2575c402
JW
2720of the internal string in bytes, use C<bytes::length(EXPR)>, see
2721L<bytes>. Note that the internal encoding is variable, and the number
2722of bytes usually meaningless. To get the number of bytes that the
2723string would have when encoded as UTF-8, use
2724C<length(Encoding::encode_utf8(EXPR))>.
974da8e5 2725
a0d0e21e 2726=item link OLDFILE,NEWFILE
d74e8afc 2727X<link>
a0d0e21e 2728
19799a22 2729Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2730success, false otherwise.
a0d0e21e
LW
2731
2732=item listen SOCKET,QUEUESIZE
d74e8afc 2733X<listen>
a0d0e21e 2734
19799a22 2735Does the same thing that the listen system call does. Returns true if
b76cc8ba 2736it succeeded, false otherwise. See the example in
cea6626f 2737L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2738
2739=item local EXPR
d74e8afc 2740X<local>
a0d0e21e 2741
19799a22 2742You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2743what most people think of as "local". See
13a2d996 2744L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2745
5a964f20
TC
2746A local modifies the listed variables to be local to the enclosing
2747block, file, or eval. If more than one value is listed, the list must
2748be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2749for details, including issues with tied arrays and hashes.
a0d0e21e 2750
d361fafa
VP
2751The C<delete local EXPR> construct can also be used to localize the deletion
2752of array/hash elements to the current block.
2753See L<perlsub/"Localized deletion of elements of composite types">.
2754
a0d0e21e 2755=item localtime EXPR
435fbc73 2756X<localtime> X<ctime>
a0d0e21e 2757
ba053783
AL
2758=item localtime
2759
19799a22 2760Converts a time as returned by the time function to a 9-element list
5f05dabc 2761with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2762follows:
2763
54310121 2764 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2765 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2766 localtime(time);
a0d0e21e 2767
48a26b3a 2768All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2769tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2770of the specified time.
48a26b3a 2771
ba053783
AL
2772C<$mday> is the day of the month, and C<$mon> is the month itself, in
2773the range C<0..11> with 0 indicating January and 11 indicating December.
2774This makes it easy to get a month name from a list:
54310121 2775
ba053783
AL
2776 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2777 print "$abbr[$mon] $mday";
2778 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2779
ba053783
AL
2780C<$year> is the number of years since 1900, not just the last two digits
2781of the year. That is, C<$year> is C<123> in year 2023. The proper way
2782to get a complete 4-digit year is simply:
abd75f24 2783
ba053783 2784 $year += 1900;
abd75f24 2785
435fbc73
GS
2786Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2787to do that, would you?
2788
ba053783
AL
2789To get the last two digits of the year (e.g., '01' in 2001) do:
2790
2791 $year = sprintf("%02d", $year % 100);
2792
2793C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2794Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2795(or C<0..365> in leap years.)
2796
2797C<$isdst> is true if the specified time occurs during Daylight Saving
2798Time, false otherwise.
abd75f24 2799
e1998452 2800If EXPR is omitted, C<localtime()> uses the current time (as returned
e3176d09 2801by time(3)).
a0d0e21e 2802
48a26b3a 2803In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2804
5f05dabc 2805 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2806
fe86afc2
NC
2807This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2808instead of local time use the L</gmtime> builtin. See also the
2809C<Time::Local> module (to convert the second, minutes, hours, ... back to
2810the integer value returned by time()), and the L<POSIX> module's strftime(3)
2811and mktime(3) functions.
2812
2813To get somewhat similar but locale dependent date strings, set up your
2814locale environment variables appropriately (please see L<perllocale>) and
2815try for example:
a3cb178b 2816
5a964f20 2817 use POSIX qw(strftime);
2b5ab1e7 2818 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2819 # or for GMT formatted appropriately for your locale:
2820 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2821
2822Note that the C<%a> and C<%b>, the short forms of the day of the week
2823and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2824
62aa5637
MS
2825See L<perlport/localtime> for portability concerns.
2826
435fbc73
GS
2827The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2828by-name access mechanism to the gmtime() and localtime() functions,
2829respectively.
2830
2831For a comprehensive date and time representation look at the
2832L<DateTime> module on CPAN.
2833
07698885 2834=item lock THING
d74e8afc 2835X<lock>
19799a22 2836
01e6739c 2837This function places an advisory lock on a shared variable, or referenced
03730085 2838object contained in I<THING> until the lock goes out of scope.
a6d5524e 2839
f3a23afb 2840lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2841by this name (before any calls to it), that function will be called
03730085
AB
2842instead. (However, if you've said C<use threads>, lock() is always a
2843keyword.) See L<threads>.
19799a22 2844
a0d0e21e 2845=item log EXPR
d74e8afc 2846X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2847
54310121 2848=item log
bbce6d69 2849
2b5ab1e7
TC
2850Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2851returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2852The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2853divided by the natural log of N. For example:
2854
2855 sub log10 {
2856 my $n = shift;
2857 return log($n)/log(10);
b76cc8ba 2858 }
2b5ab1e7
TC
2859
2860See also L</exp> for the inverse operation.
a0d0e21e 2861
a0d0e21e 2862=item lstat EXPR
d74e8afc 2863X<lstat>
a0d0e21e 2864
54310121 2865=item lstat
bbce6d69 2866
19799a22 2867Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2868special C<_> filehandle) but stats a symbolic link instead of the file
2869the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2870your system, a normal C<stat> is done. For much more detailed
2871information, please see the documentation for C<stat>.
a0d0e21e 2872
7660c0ab 2873If EXPR is omitted, stats C<$_>.
bbce6d69 2874
a0d0e21e
LW
2875=item m//
2876
9f4b9cd0 2877The match operator. See L<perlop/"Regexp Quote-Like Operators">.
a0d0e21e
LW
2878
2879=item map BLOCK LIST
d74e8afc 2880X<map>
a0d0e21e
LW
2881
2882=item map EXPR,LIST
2883
19799a22
GS
2884Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2885C<$_> to each element) and returns the list value composed of the
2886results of each such evaluation. In scalar context, returns the
2887total number of elements so generated. Evaluates BLOCK or EXPR in
2888list context, so each element of LIST may produce zero, one, or
2889more elements in the returned value.
dd99ebda 2890
a0d0e21e
LW
2891 @chars = map(chr, @nums);
2892
2893translates a list of numbers to the corresponding characters. And
2894
d8216f19 2895 %hash = map { get_a_key_for($_) => $_ } @array;
a0d0e21e
LW
2896
2897is just a funny way to write
2898
2899 %hash = ();
d8216f19
RGS
2900 foreach (@array) {
2901 $hash{get_a_key_for($_)} = $_;
a0d0e21e
LW
2902 }
2903
be3174d2
GS
2904Note that C<$_> is an alias to the list value, so it can be used to
2905modify the elements of the LIST. While this is useful and supported,
2906it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2907Using a regular C<foreach> loop for this purpose would be clearer in
2908most cases. See also L</grep> for an array composed of those items of
2909the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2910
a4fb8298 2911If C<$_> is lexical in the scope where the C<map> appears (because it has
d8216f19
RGS
2912been declared with C<my $_>), then, in addition to being locally aliased to
2913the list elements, C<$_> keeps being lexical inside the block; that is, it
a4fb8298
RGS
2914can't be seen from the outside, avoiding any potential side-effects.
2915
205fdb4d
NC
2916C<{> starts both hash references and blocks, so C<map { ...> could be either
2917the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2918ahead for the closing C<}> it has to take a guess at which its dealing with
2919based what it finds just after the C<{>. Usually it gets it right, but if it
2920doesn't it won't realize something is wrong until it gets to the C<}> and
2921encounters the missing (or unexpected) comma. The syntax error will be
2922reported close to the C<}> but you'll need to change something near the C<{>
2923such as using a unary C<+> to give perl some help:
2924
2925 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2926 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2927 %hash = map { ("\L$_", 1) } @array # this also works
2928 %hash = map { lc($_), 1 } @array # as does this.
2929 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2930
205fdb4d
NC
2931 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2932
d8216f19 2933or to force an anon hash constructor use C<+{>:
205fdb4d
NC
2934
2935 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2936
2937and you get list of anonymous hashes each with only 1 entry.
2938
19799a22 2939=item mkdir FILENAME,MASK
d74e8afc 2940X<mkdir> X<md> X<directory, create>
a0d0e21e 2941
5a211162
GS
2942=item mkdir FILENAME
2943
491873e5
RGS
2944=item mkdir
2945
0591cd52 2946Creates the directory specified by FILENAME, with permissions
19799a22
GS
2947specified by MASK (as modified by C<umask>). If it succeeds it
2948returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2949If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2950to C<$_>.
0591cd52 2951
19799a22 2952In general, it is better to create directories with permissive MASK,
0591cd52 2953and let the user modify that with their C<umask>, than it is to supply
19799a22 2954a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2955The exceptions to this rule are when the file or directory should be
2956kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2957C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2958
cc1852e8
JH
2959Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2960number of trailing slashes. Some operating and filesystems do not get
2961this right, so Perl automatically removes all trailing slashes to keep
2962everyone happy.
2963
dd184578
RGS
2964In order to recursively create a directory structure look at
2965the C<mkpath> function of the L<File::Path> module.
2966
a0d0e21e 2967=item msgctl ID,CMD,ARG
d74e8afc 2968X<msgctl>
a0d0e21e 2969
f86cebdf 2970Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2971
2972 use IPC::SysV;
2973
7660c0ab 2974first to get the correct constant definitions. If CMD is C<IPC_STAT>,
cf264981 2975then ARG must be a variable that will hold the returned C<msqid_ds>
951ba7fe
GS
2976structure. Returns like C<ioctl>: the undefined value for error,
2977C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2978L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2979
2980=item msgget KEY,FLAGS
d74e8afc 2981X<msgget>
a0d0e21e 2982
f86cebdf 2983Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2984id, or the undefined value if there is an error. See also
2985L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2986
a0d0e21e 2987=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
d74e8afc 2988X<msgrcv>
a0d0e21e
LW
2989
2990Calls the System V IPC function msgrcv to receive a message from
2991message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2992SIZE. Note that when a message is received, the message type as a
2993native long integer will be the first thing in VAR, followed by the
2994actual message. This packing may be opened with C<unpack("l! a*")>.
2995Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2996an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2997C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2998
2999=item msgsnd ID,MSG,FLAGS
d74e8afc 3000X<msgsnd>
41d6edb2
JH
3001
3002Calls the System V IPC function msgsnd to send the message MSG to the
3003message queue ID. MSG must begin with the native long integer message
3004type, and be followed by the length of the actual message, and finally
3005the message itself. This kind of packing can be achieved with
3006C<pack("l! a*", $type, $message)>. Returns true if successful,
3007or false if there is an error. See also C<IPC::SysV>
3008and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
3009
3010=item my EXPR
d74e8afc 3011X<my>
a0d0e21e 3012
307ea6df
JH
3013=item my TYPE EXPR
3014
1d2de774 3015=item my EXPR : ATTRS
09bef843 3016
1d2de774 3017=item my TYPE EXPR : ATTRS
307ea6df 3018
19799a22 3019A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
3020enclosing block, file, or C<eval>. If more than one value is listed,
3021the list must be placed in parentheses.
307ea6df 3022
1d2de774
JH
3023The exact semantics and interface of TYPE and ATTRS are still
3024evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
3025and attributes are handled using the C<attributes> pragma, or starting
3026from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3027L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3028L<attributes>, and L<Attribute::Handlers>.
4633a7c4 3029
a0d0e21e 3030=item next LABEL
d74e8afc 3031X<next> X<continue>
a0d0e21e
LW
3032
3033=item next
3034
3035The C<next> command is like the C<continue> statement in C; it starts
3036the next iteration of the loop:
3037
4633a7c4
LW
3038 LINE: while (<STDIN>) {
3039 next LINE if /^#/; # discard comments
5a964f20 3040 #...
a0d0e21e
LW
3041 }
3042
3043Note that if there were a C<continue> block on the above, it would get
3044executed even on discarded lines. If the LABEL is omitted, the command
3045refers to the innermost enclosing loop.
3046
4968c1e4 3047C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
3048C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3049a grep() or map() operation.
4968c1e4 3050
6c1372ed
GS
3051Note that a block by itself is semantically identical to a loop
3052that executes once. Thus C<next> will exit such a block early.
3053
98293880
JH
3054See also L</continue> for an illustration of how C<last>, C<next>, and
3055C<redo> work.
1d2dff63 3056
4a66ea5a 3057=item no Module VERSION LIST
d74e8afc 3058X<no>
4a66ea5a
RGS
3059
3060=item no Module VERSION
3061
a0d0e21e
LW
3062=item no Module LIST
3063
4a66ea5a
RGS
3064=item no Module
3065
c986422f
RGS
3066=item no VERSION
3067
593b9c14 3068See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
3069
3070=item oct EXPR
d74e8afc 3071X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
a0d0e21e 3072
54310121 3073=item oct
bbce6d69 3074
4633a7c4 3075Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
3076value. (If EXPR happens to start off with C<0x>, interprets it as a
3077hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
3078binary string. Leading whitespace is ignored in all three cases.)
3079The following will handle decimal, binary, octal, and hex in the standard
3080Perl or C notation:
a0d0e21e
LW
3081
3082 $val = oct($val) if $val =~ /^0/;
3083
19799a22
GS
3084If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3085in octal), use sprintf() or printf():
3086
3087 $perms = (stat("filename"))[2] & 07777;
3088 $oct_perms = sprintf "%lo", $perms;
3089
3090The oct() function is commonly used when a string such as C<644> needs
3091to be converted into a file mode, for example. (Although perl will
3092automatically convert strings into numbers as needed, this automatic
3093conversion assumes base 10.)
a0d0e21e
LW
3094
3095=item open FILEHANDLE,EXPR
d74e8afc 3096X<open> X<pipe> X<file, open> X<fopen>
a0d0e21e 3097
68bd7414
NIS
3098=item open FILEHANDLE,MODE,EXPR
3099
3100=item open FILEHANDLE,MODE,EXPR,LIST
3101
ba964c95
T
3102=item open FILEHANDLE,MODE,REFERENCE
3103
a0d0e21e
LW
3104=item open FILEHANDLE
3105
3106Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
3107FILEHANDLE.
3108
460b70c2
GS
3109Simple examples to open a file for reading:
3110
3111 open(my $fh, '<', "input.txt") or die $!;
3112
3113and for writing:
3114
3115 open(my $fh, '>', "output.txt") or die $!;
3116
ed53a2bb
JH
3117(The following is a comprehensive reference to open(): for a gentler
3118introduction you may consider L<perlopentut>.)
3119
a28cd5c9
NT
3120If FILEHANDLE is an undefined scalar variable (or array or hash element)
3121the variable is assigned a reference to a new anonymous filehandle,
3122otherwise if FILEHANDLE is an expression, its value is used as the name of
3123the real filehandle wanted. (This is considered a symbolic reference, so
3124C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
3125
3126If EXPR is omitted, the scalar variable of the same name as the
3127FILEHANDLE contains the filename. (Note that lexical variables--those
3128declared with C<my>--will not work for this purpose; so if you're
67408cae 3129using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
3130
3131If three or more arguments are specified then the mode of opening and
3132the file name are separate. If MODE is C<< '<' >> or nothing, the file
3133is opened for input. If MODE is C<< '>' >>, the file is truncated and
3134opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 3135the file is opened for appending, again being created if necessary.
5a964f20 3136
ed53a2bb
JH
3137You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3138indicate that you want both read and write access to the file; thus
3139C<< '+<' >> is almost always preferred for read/write updates--the C<<
3140'+>' >> mode would clobber the file first. You can't usually use
3141either read-write mode for updating textfiles, since they have
3142variable length records. See the B<-i> switch in L<perlrun> for a
3143better approach. The file is created with permissions of C<0666>
3144modified by the process' C<umask> value.
3145
3146These various prefixes correspond to the fopen(3) modes of C<'r'>,
3147C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.