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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
8f1da26d 15operator. A unary operator generally provides scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
3b10bc60 17contexts for its arguments. If it does both, scalar arguments
18come first and list argument follow, and there can only ever
19be 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
3b10bc60 24list (and provide list context for elements of the list) are shown
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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.
8bdbc703 29Commas should separate literal 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
3b10bc60 33parentheses.) If you use parentheses, the simple but occasionally
34surprising rule is this: It I<looks> like a function, therefore it I<is> a
a0d0e21e 35function, and precedence doesn't matter. Otherwise it's a list
3b10bc60 36operator or unary operator, and precedence does matter. Whitespace
37between the function and left parenthesis doesn't count, so sometimes
38you need to be careful:
a0d0e21e 39
5ed4f2ec 40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
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45
46If you run Perl with the B<-w> switch it can warn you about this. For
47example, the third line above produces:
48
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
51
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52A few functions take no arguments at all, and therefore work as neither
53unary nor list operators. These include such functions as C<time>
54and C<endpwent>. For example, C<time+86_400> always means
55C<time() + 86_400>.
56
a0d0e21e 57For functions that can be used in either a scalar or list context,
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58nonabortive failure is generally indicated in scalar context by
59returning the undefined value, and in list context by returning the
3b10bc60 60empty list.
a0d0e21e 61
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62Remember the following important rule: There is B<no rule> that relates
63the behavior of an expression in list context to its behavior in scalar
64context, or vice versa. It might do two totally different things.
80d38338 65Each operator and function decides which sort of value would be most
2b5ab1e7 66appropriate to return in scalar context. Some operators return the
5a964f20 67length of the list that would have been returned in list context. Some
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68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
d74e8afc 72X<context>
a0d0e21e 73
d1be9408 74A named array in scalar context is quite different from what would at
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75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
3b10bc60 81In general, functions in Perl that serve as wrappers for system calls ("syscalls")
5dac7880 82of the same name (like chown(2), fork(2), closedir(2), etc.) 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,
5dac7880 85which return C<-1> on failure. Exceptions to this rule include C<wait>,
19799a22 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
a5ce339c 128C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
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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>
cb1a09d0 148
5dac7880 149=item Functions for fixed-length data or records
cb1a09d0 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
8f1da26d 167=item Keywords related to the switch feature
0d863452 168
8f1da26d 169C<break>, C<continue>, C<default, >C<given>, C<when>
0d863452 170
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171These are available only if you enable the C<"switch"> feature.
172See L<feature> and L<perlsyn/"Switch statements">.
173Alternately, include a C<use v5.10> or later to the current scope.
0d863452 174
54310121 175=item Keywords related to scoping
cb1a09d0 176
8f1da26d 177C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
36fb85f3 178
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179C<state> is available only if the C<"state"> feature is enabled. See
180L<feature>. Alternately, include a C<use v5.10> or later to the current scope.
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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>
cb1a09d0 193
3b10bc60 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>
37798a01 247
3b10bc60 248* C<sub> was a keyword in Perl 4, but in Perl 5 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
8f1da26d 262Unix system calls may not be available or details of the available
2b5ab1e7 263functionality may differ slightly. The Perl functions affected
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264by this are:
265
266C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
267C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
268C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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269C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
270C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 271C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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272C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
273C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
274C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 275C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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276C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
277C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
278C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 279C<shmwrite>, C<socket>, C<socketpair>,
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280C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
281C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 282C<utime>, C<wait>, C<waitpid>
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283
284For more information about the portability of these functions, see
285L<perlport> and other available platform-specific documentation.
286
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287=head2 Alphabetical Listing of Perl Functions
288
3b10bc60 289=over
a0d0e21e 290
5b3c99c0 291=item -X FILEHANDLE
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292X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
293X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 294
5b3c99c0 295=item -X EXPR
a0d0e21e 296
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297=item -X DIRHANDLE
298
5b3c99c0 299=item -X
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300
301A file test, where X is one of the letters listed below. This unary
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302operator takes one argument, either a filename, a filehandle, or a dirhandle,
303and tests the associated file to see if something is true about it. If the
7660c0ab 304argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 305Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 306the undefined value if the file doesn't exist. Despite the funny
d0821a6a 307names, precedence is the same as any other named unary operator. The
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308operator may be any of:
309
5ed4f2ec 310 -r File is readable by effective uid/gid.
311 -w File is writable by effective uid/gid.
312 -x File is executable by effective uid/gid.
313 -o File is owned by effective uid.
a0d0e21e 314
5ed4f2ec 315 -R File is readable by real uid/gid.
316 -W File is writable by real uid/gid.
317 -X File is executable by real uid/gid.
318 -O File is owned by real uid.
a0d0e21e 319
5ed4f2ec 320 -e File exists.
321 -z File has zero size (is empty).
322 -s File has nonzero size (returns size in bytes).
a0d0e21e 323
5ed4f2ec 324 -f File is a plain file.
325 -d File is a directory.
326 -l File is a symbolic link.
327 -p File is a named pipe (FIFO), or Filehandle is a pipe.
328 -S File is a socket.
329 -b File is a block special file.
330 -c File is a character special file.
331 -t Filehandle is opened to a tty.
a0d0e21e 332
5ed4f2ec 333 -u File has setuid bit set.
334 -g File has setgid bit set.
335 -k File has sticky bit set.
a0d0e21e 336
5ed4f2ec 337 -T File is an ASCII text file (heuristic guess).
338 -B File is a "binary" file (opposite of -T).
a0d0e21e 339
5ed4f2ec 340 -M Script start time minus file modification time, in days.
341 -A Same for access time.
342 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 343
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344Example:
345
346 while (<>) {
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347 chomp;
348 next unless -f $_; # ignore specials
349 #...
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350 }
351
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352Note that C<-s/a/b/> does not do a negated substitution. Saying
353C<-exp($foo)> still works as expected, however: only single letters
354following a minus are interpreted as file tests.
355
356These operators are exempt from the "looks like a function rule" described
357above. That is, an opening parenthesis after the operator does not affect
358how much of the following code constitutes the argument. Put the opening
359parentheses before the operator to separate it from code that follows (this
360applies only to operators with higher precedence than unary operators, of
361course):
362
363 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
364 (-s $file) + 1024 # correct
365
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366The interpretation of the file permission operators C<-r>, C<-R>,
367C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
368of the file and the uids and gids of the user. There may be other
ecae030f
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369reasons you can't actually read, write, or execute the file: for
370example network filesystem access controls, ACLs (access control lists),
371read-only filesystems, and unrecognized executable formats. Note
372that the use of these six specific operators to verify if some operation
373is possible is usually a mistake, because it may be open to race
374conditions.
5ff3f7a4 375
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376Also note that, for the superuser on the local filesystems, the C<-r>,
377C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
5ff3f7a4
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378if any execute bit is set in the mode. Scripts run by the superuser
379may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 380or temporarily set their effective uid to something else.
5ff3f7a4
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381
382If you are using ACLs, there is a pragma called C<filetest> that may
383produce more accurate results than the bare stat() mode bits.
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384When under C<use filetest 'access'> the above-mentioned filetests
385test whether the permission can(not) be granted using the
3b10bc60 386access(2) family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
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387under this pragma return true even if there are no execute permission
388bits set (nor any extra execute permission ACLs). This strangeness is
ecae030f
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389due to the underlying system calls' definitions. Note also that, due to
390the implementation of C<use filetest 'access'>, the C<_> special
391filehandle won't cache the results of the file tests when this pragma is
392in effect. Read the documentation for the C<filetest> pragma for more
393information.
5ff3f7a4 394
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395The C<-T> and C<-B> switches work as follows. The first block or so of the
396file is examined for odd characters such as strange control codes or
61eff3bc 397characters with the high bit set. If too many strange characters (>30%)
cf264981 398are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 399containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 400or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 401rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 402file, or a file at EOF when testing a filehandle. Because you have to
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403read a file to do the C<-T> test, on most occasions you want to use a C<-f>
404against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 405
5dac7880 406If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 407the special filehandle consisting of a solitary underline, then the stat
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408structure of the previous file test (or stat operator) is used, saving
409a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 410that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 411symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 412an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 413Example:
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414
415 print "Can do.\n" if -r $a || -w _ || -x _;
416
417 stat($filename);
418 print "Readable\n" if -r _;
419 print "Writable\n" if -w _;
420 print "Executable\n" if -x _;
421 print "Setuid\n" if -u _;
422 print "Setgid\n" if -g _;
423 print "Sticky\n" if -k _;
424 print "Text\n" if -T _;
425 print "Binary\n" if -B _;
426
fbb0b3b3
RGS
427As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
428test operators, in a way that C<-f -w -x $file> is equivalent to
3b10bc60 429C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
fbb0b3b3
RGS
430the return value of C<-f $file> as an argument to another filetest
431operator, no special magic will happen.)
432
a0d0e21e 433=item abs VALUE
d74e8afc 434X<abs> X<absolute>
a0d0e21e 435
54310121 436=item abs
bbce6d69 437
a0d0e21e 438Returns the absolute value of its argument.
7660c0ab 439If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
440
441=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 442X<accept>
a0d0e21e 443
3b10bc60 444Accepts an incoming socket connect, just as accept(2)
19799a22 445does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 446See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 447
8d2a6795
GS
448On systems that support a close-on-exec flag on files, the flag will
449be set for the newly opened file descriptor, as determined by the
450value of $^F. See L<perlvar/$^F>.
451
a0d0e21e 452=item alarm SECONDS
d74e8afc
ITB
453X<alarm>
454X<SIGALRM>
455X<timer>
a0d0e21e 456
54310121 457=item alarm
bbce6d69 458
a0d0e21e 459Arranges to have a SIGALRM delivered to this process after the
cf264981 460specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
461specified, the value stored in C<$_> is used. (On some machines,
462unfortunately, the elapsed time may be up to one second less or more
463than you specified because of how seconds are counted, and process
464scheduling may delay the delivery of the signal even further.)
465
466Only one timer may be counting at once. Each call disables the
467previous timer, and an argument of C<0> may be supplied to cancel the
468previous timer without starting a new one. The returned value is the
469amount of time remaining on the previous timer.
a0d0e21e 470
2bc69794
BS
471For delays of finer granularity than one second, the Time::HiRes module
472(from CPAN, and starting from Perl 5.8 part of the standard
473distribution) provides ualarm(). You may also use Perl's four-argument
474version of select() leaving the first three arguments undefined, or you
475might be able to use the C<syscall> interface to access setitimer(2) if
476your system supports it. See L<perlfaq8> for details.
2b5ab1e7 477
80d38338
TC
478It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
479C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 480
19799a22
GS
481If you want to use C<alarm> to time out a system call you need to use an
482C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 483fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 484restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 485modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
486
487 eval {
a9a5a0dc
VP
488 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
489 alarm $timeout;
490 $nread = sysread SOCKET, $buffer, $size;
491 alarm 0;
ff68c719 492 };
ff68c719 493 if ($@) {
a9a5a0dc 494 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 495 # timed out
ff68c719
PP
496 }
497 else {
5ed4f2ec 498 # didn't
ff68c719
PP
499 }
500
91d81acc
JH
501For more information see L<perlipc>.
502
a0d0e21e 503=item atan2 Y,X
d74e8afc 504X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
505
506Returns the arctangent of Y/X in the range -PI to PI.
507
ca6e1c26 508For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
509function, or use the familiar relation:
510
511 sub tan { sin($_[0]) / cos($_[0]) }
512
a1021d57
RGS
513The return value for C<atan2(0,0)> is implementation-defined; consult
514your atan2(3) manpage for more information.
bf5f1b4c 515
a0d0e21e 516=item bind SOCKET,NAME
d74e8afc 517X<bind>
a0d0e21e 518
3b10bc60 519Binds a network address to a socket, just as bind(2)
19799a22 520does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
521packed address of the appropriate type for the socket. See the examples in
522L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 523
fae2c0fb 524=item binmode FILEHANDLE, LAYER
d74e8afc 525X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 526
a0d0e21e
LW
527=item binmode FILEHANDLE
528
1cbfc93d
NIS
529Arranges for FILEHANDLE to be read or written in "binary" or "text"
530mode on systems where the run-time libraries distinguish between
531binary and text files. If FILEHANDLE is an expression, the value is
532taken as the name of the filehandle. Returns true on success,
b5fe5ca2 533otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 534
8f1da26d 535On some systems (in general, DOS- and Windows-based systems) binmode()
d807c6f4 536is necessary when you're not working with a text file. For the sake
d7a0d798
FC
537of portability it is a good idea always to use it when appropriate,
538and never to use it when it isn't appropriate. Also, people can
8f1da26d 539set their I/O to be by default UTF8-encoded Unicode, not bytes.
d807c6f4
JH
540
541In other words: regardless of platform, use binmode() on binary data,
d7a0d798 542like images, for example.
d807c6f4
JH
543
544If LAYER is present it is a single string, but may contain multiple
3b10bc60 545directives. The directives alter the behaviour of the filehandle.
d7a0d798 546When LAYER is present, using binmode on a text file makes sense.
d807c6f4 547
fae2c0fb 548If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
549suitable for passing binary data. This includes turning off possible CRLF
550translation and marking it as bytes (as opposed to Unicode characters).
749683d2 551Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 552Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
553Other layers that would affect the binary nature of the stream are
554I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 555PERLIO environment variable.
01e6739c 556
3b10bc60 557The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
558form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
559establish default I/O layers. See L<open>.
560
fae2c0fb
RGS
561I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
562in "Programming Perl, 3rd Edition". However, since the publishing of this
563book, by many known as "Camel III", the consensus of the naming of this
564functionality has moved from "discipline" to "layer". All documentation
565of this version of Perl therefore refers to "layers" rather than to
566"disciplines". Now back to the regularly scheduled documentation...>
567
8f1da26d 568To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
6902c96a 569C<:utf8> just marks the data as UTF-8 without further checking,
8f1da26d 570while C<:encoding(UTF-8)> checks the data for actually being valid
6902c96a 571UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 572
ed53a2bb 573In general, binmode() should be called after open() but before any I/O
3b10bc60 574is done on the filehandle. Calling binmode() normally flushes any
01e6739c 575pending buffered output data (and perhaps pending input data) on the
fae2c0fb 576handle. An exception to this is the C<:encoding> layer that
d7a0d798 577changes the default character encoding of the handle; see L</open>.
fae2c0fb 578The C<:encoding> layer sometimes needs to be called in
3874323d
JH
579mid-stream, and it doesn't flush the stream. The C<:encoding>
580also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 581internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 582
19799a22 583The operating system, device drivers, C libraries, and Perl run-time
8f1da26d
TC
584system all conspire to let the programmer treat a single
585character (C<\n>) as the line terminator, irrespective of external
30168b04
GS
586representation. On many operating systems, the native text file
587representation matches the internal representation, but on some
588platforms the external representation of C<\n> is made up of more than
589one character.
590
8f1da26d
TC
591All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
592a single character to end each line in the external representation of text
593(even though that single character is CARRIAGE RETURN on old, pre-Darwin
594flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
595systems like OS/2, DOS, and the various flavors of MS-Windows, your program
596sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
597two characters C<\cM\cJ>. That means that if you don't use binmode() on
598these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
599input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
600output. This is what you want for text files, but it can be disastrous for
601binary files.
30168b04
GS
602
603Another consequence of using binmode() (on some systems) is that
604special end-of-file markers will be seen as part of the data stream.
d7a0d798
FC
605For systems from the Microsoft family this means that, if your binary
606data contain C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
607the file, unless you use binmode().
608
3b10bc60 609binmode() is important not only for readline() and print() operations,
30168b04
GS
610but also when using read(), seek(), sysread(), syswrite() and tell()
611(see L<perlport> for more details). See the C<$/> and C<$\> variables
612in L<perlvar> for how to manually set your input and output
613line-termination sequences.
a0d0e21e 614
4633a7c4 615=item bless REF,CLASSNAME
d74e8afc 616X<bless>
a0d0e21e
LW
617
618=item bless REF
619
2b5ab1e7
TC
620This function tells the thingy referenced by REF that it is now an object
621in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 622is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 623it returns the reference for convenience. Always use the two-argument
cf264981
SP
624version if a derived class might inherit the function doing the blessing.
625See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
626of objects.
a0d0e21e 627
57668c4d 628Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 629Namespaces with all lowercase names are considered reserved for
cf264981 630Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
631confusion, you may wish to avoid such package names as well. Make sure
632that CLASSNAME is a true value.
60ad88b8
GS
633
634See L<perlmod/"Perl Modules">.
635
0d863452
RH
636=item break
637
638Break out of a C<given()> block.
639
8f1da26d
TC
640This keyword is enabled by the C<"switch"> feature: see
641L<feature> for more information. Alternately, include a C<use
642v5.10> or later to the current scope.
0d863452 643
a0d0e21e 644=item caller EXPR
d74e8afc 645X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
646
647=item caller
648
5a964f20 649Returns the context of the current subroutine call. In scalar context,
80d38338
TC
650returns the caller's package name if there I<is> a caller (that is, if
651we're in a subroutine or C<eval> or C<require>) and the undefined value
5a964f20 652otherwise. In list context, returns
a0d0e21e 653
ee6b43cc 654 # 0 1 2
748a9306 655 ($package, $filename, $line) = caller;
a0d0e21e
LW
656
657With EXPR, it returns some extra information that the debugger uses to
658print a stack trace. The value of EXPR indicates how many call frames
659to go back before the current one.
660
ee6b43cc 661 # 0 1 2 3 4
f3aa04c2 662 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 663
664 # 5 6 7 8 9 10
b3ca2e83 665 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 666 = caller($i);
e7ea3e70 667
951ba7fe 668Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 669call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 670C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 671C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 672C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 673$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
674each C<use> statement creates a C<require> frame inside an C<eval EXPR>
675frame.) $subroutine may also be C<(unknown)> if this particular
676subroutine happens to have been deleted from the symbol table.
677C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
678C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
679compiled with. The C<$hints> and C<$bitmask> values are subject to change
680between versions of Perl, and are not meant for external use.
748a9306 681
b3ca2e83
NC
682C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
683caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
684of this hash, as they are the actual values stored in the optree.
685
748a9306 686Furthermore, when called from within the DB package, caller returns more
7660c0ab 687detailed information: it sets the list variable C<@DB::args> to be the
54310121 688arguments with which the subroutine was invoked.
748a9306 689
7660c0ab 690Be aware that the optimizer might have optimized call frames away before
19799a22 691C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 692might not return information about the call frame you expect it to, for
b76cc8ba 693C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 694previous time C<caller> was called.
7660c0ab 695
8f1da26d 696Be aware that setting C<@DB::args> is I<best effort>, intended for
ca9f0cb5
NC
697debugging or generating backtraces, and should not be relied upon. In
698particular, as C<@_> contains aliases to the caller's arguments, Perl does
699not take a copy of C<@_>, so C<@DB::args> will contain modifications the
700subroutine makes to C<@_> or its contents, not the original values at call
701time. C<@DB::args>, like C<@_>, does not hold explicit references to its
702elements, so under certain cases its elements may have become freed and
703reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 704of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
705I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
706reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5
NC
707elements), so C<@DB::args> is actually a hybrid of the current state and
708initial state of C<@_>. Buyer beware.
709
a0d0e21e 710=item chdir EXPR
d74e8afc
ITB
711X<chdir>
712X<cd>
f723aae1 713X<directory, change>
a0d0e21e 714
c4aca7d0
GA
715=item chdir FILEHANDLE
716
717=item chdir DIRHANDLE
718
ce2984c3
PF
719=item chdir
720
ffce7b87 721Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 722changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 723changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0 724variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
80d38338 725neither is set, C<chdir> does nothing. It returns true on success,
b4ad75f0 726false otherwise. See the example under C<die>.
a0d0e21e 727
3b10bc60 728On systems that support fchdir(2), you may pass a filehandle or
34169887 729directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 730passing handles raises an exception.
c4aca7d0 731
a0d0e21e 732=item chmod LIST
d74e8afc 733X<chmod> X<permission> X<mode>
a0d0e21e
LW
734
735Changes the permissions of a list of files. The first element of the
8f1da26d 736list must be the numeric mode, which should probably be an octal
4ad40acf 737number, and which definitely should I<not> be a string of octal digits:
3b10bc60 738C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 739successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 740
3b10bc60 741 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 742 chmod 0755, @executables;
3b10bc60 743 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 744 # --w----r-T
3b10bc60 745 $mode = "0644"; chmod oct($mode), "foo"; # this is better
746 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 747
3b10bc60 748On systems that support fchmod(2), you may pass filehandles among the
749files. On systems that don't support fchmod(2), passing filehandles raises
750an exception. Filehandles must be passed as globs or glob references to be
751recognized; barewords are considered filenames.
c4aca7d0
GA
752
753 open(my $fh, "<", "foo");
754 my $perm = (stat $fh)[2] & 07777;
755 chmod($perm | 0600, $fh);
756
3b10bc60 757You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
758module:
759
3b10bc60 760 use Fcntl qw( :mode );
ca6e1c26 761 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 762 # Identical to the chmod 0755 of the example above.
ca6e1c26 763
a0d0e21e 764=item chomp VARIABLE
d74e8afc 765X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 766
313c9f5c 767=item chomp( LIST )
a0d0e21e
LW
768
769=item chomp
770
2b5ab1e7
TC
771This safer version of L</chop> removes any trailing string
772that corresponds to the current value of C<$/> (also known as
28757baa
PP
773$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
774number of characters removed from all its arguments. It's often used to
775remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
776that the final record may be missing its newline. When in paragraph
777mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 778When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 779a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 780remove anything.
19799a22 781If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
782
783 while (<>) {
a9a5a0dc
VP
784 chomp; # avoid \n on last field
785 @array = split(/:/);
786 # ...
a0d0e21e
LW
787 }
788
4bf21a6d
RD
789If VARIABLE is a hash, it chomps the hash's values, but not its keys.
790
a0d0e21e
LW
791You can actually chomp anything that's an lvalue, including an assignment:
792
793 chomp($cwd = `pwd`);
794 chomp($answer = <STDIN>);
795
796If you chomp a list, each element is chomped, and the total number of
797characters removed is returned.
798
15e44fd8
RGS
799Note that parentheses are necessary when you're chomping anything
800that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
801is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
802C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
803C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
804as C<chomp($a, $b)>.
805
a0d0e21e 806=item chop VARIABLE
d74e8afc 807X<chop>
a0d0e21e 808
313c9f5c 809=item chop( LIST )
a0d0e21e
LW
810
811=item chop
812
813Chops off the last character of a string and returns the character
5b3eff12 814chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 815scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
816If VARIABLE is a hash, it chops the hash's values, but not its keys.
817
5b3eff12 818You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
819
820If you chop a list, each element is chopped. Only the value of the
19799a22 821last C<chop> is returned.
a0d0e21e 822
19799a22 823Note that C<chop> returns the last character. To return all but the last
748a9306
LW
824character, use C<substr($string, 0, -1)>.
825
15e44fd8
RGS
826See also L</chomp>.
827
a0d0e21e 828=item chown LIST
d74e8afc 829X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
830
831Changes the owner (and group) of a list of files. The first two
19799a22
GS
832elements of the list must be the I<numeric> uid and gid, in that
833order. A value of -1 in either position is interpreted by most
834systems to leave that value unchanged. Returns the number of files
835successfully changed.
a0d0e21e
LW
836
837 $cnt = chown $uid, $gid, 'foo', 'bar';
838 chown $uid, $gid, @filenames;
839
3b10bc60 840On systems that support fchown(2), you may pass filehandles among the
841files. On systems that don't support fchown(2), passing filehandles raises
842an exception. Filehandles must be passed as globs or glob references to be
843recognized; barewords are considered filenames.
c4aca7d0 844
54310121 845Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
846
847 print "User: ";
19799a22 848 chomp($user = <STDIN>);
5a964f20 849 print "Files: ";
19799a22 850 chomp($pattern = <STDIN>);
a0d0e21e
LW
851
852 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 853 or die "$user not in passwd file";
a0d0e21e 854
5ed4f2ec 855 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
856 chown $uid, $gid, @ary;
857
54310121 858On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
859file unless you're the superuser, although you should be able to change
860the group to any of your secondary groups. On insecure systems, these
861restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
862On POSIX systems, you can detect this condition this way:
863
864 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
865 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 866
a0d0e21e 867=item chr NUMBER
d74e8afc 868X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 869
54310121 870=item chr
bbce6d69 871
a0d0e21e 872Returns the character represented by that NUMBER in the character set.
a0ed51b3 873For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 874chr(0x263a) is a Unicode smiley face.
aaa68c4a 875
8a064bd6 876Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 877except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
878(truncated to an integer) are used.
879
974da8e5
JH
880If NUMBER is omitted, uses C<$_>.
881
b76cc8ba 882For the reverse, use L</ord>.
a0d0e21e 883
2575c402
JW
884Note that characters from 128 to 255 (inclusive) are by default
885internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 886
2575c402 887See L<perlunicode> for more about Unicode.
bbce6d69 888
a0d0e21e 889=item chroot FILENAME
d74e8afc 890X<chroot> X<root>
a0d0e21e 891
54310121 892=item chroot
bbce6d69 893
5a964f20 894This function works like the system call by the same name: it makes the
4633a7c4 895named directory the new root directory for all further pathnames that
951ba7fe 896begin with a C</> by your process and all its children. (It doesn't
28757baa 897change your current working directory, which is unaffected.) For security
4633a7c4 898reasons, this call is restricted to the superuser. If FILENAME is
19799a22 899omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
900
901=item close FILEHANDLE
d74e8afc 902X<close>
a0d0e21e 903
6a518fbc
TP
904=item close
905
3b10bc60 906Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 907buffers, and closes the system file descriptor. Returns true if those
8f1da26d 908operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
909layer. Closes the currently selected filehandle if the argument is
910omitted.
fb73857a
PP
911
912You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 913another C<open> on it, because C<open> closes it for you. (See
19799a22
GS
914C<open>.) However, an explicit C<close> on an input file resets the line
915counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 916
3b10bc60 917If the filehandle came from a piped open, C<close> returns false if one of
918the other syscalls involved fails or if its program exits with non-zero
919status. If the only problem was that the program exited non-zero, C<$!>
920will be set to C<0>. Closing a pipe also waits for the process executing
921on the pipe to exit--in case you wish to look at the output of the pipe
922afterwards--and implicitly puts the exit status value of that command into
923C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 924
2e0cfa16
FC
925If there are multiple threads running, C<close> on a filehandle from a
926piped open returns true without waiting for the child process to terminate,
927if the filehandle is still open in another thread.
928
80d38338
TC
929Closing the read end of a pipe before the process writing to it at the
930other end is done writing results in the writer receiving a SIGPIPE. If
931the other end can't handle that, be sure to read all the data before
932closing the pipe.
73689b13 933
fb73857a 934Example:
a0d0e21e 935
fb73857a
PP
936 open(OUTPUT, '|sort >foo') # pipe to sort
937 or die "Can't start sort: $!";
5ed4f2ec 938 #... # print stuff to output
939 close OUTPUT # wait for sort to finish
fb73857a
PP
940 or warn $! ? "Error closing sort pipe: $!"
941 : "Exit status $? from sort";
5ed4f2ec 942 open(INPUT, 'foo') # get sort's results
fb73857a 943 or die "Can't open 'foo' for input: $!";
a0d0e21e 944
5a964f20 945FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 946filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
947
948=item closedir DIRHANDLE
d74e8afc 949X<closedir>
a0d0e21e 950
19799a22 951Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
952system call.
953
a0d0e21e 954=item connect SOCKET,NAME
d74e8afc 955X<connect>
a0d0e21e 956
80d38338
TC
957Attempts to connect to a remote socket, just like connect(2).
958Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
959packed address of the appropriate type for the socket. See the examples in
960L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 961
cb1a09d0 962=item continue BLOCK
d74e8afc 963X<continue>
cb1a09d0 964
0d863452
RH
965=item continue
966
cf264981
SP
967C<continue> is actually a flow control statement rather than a function. If
968there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
969C<foreach>), it is always executed just before the conditional is about to
970be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
971it can be used to increment a loop variable, even when the loop has been
972continued via the C<next> statement (which is similar to the C C<continue>
973statement).
974
98293880 975C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 976block; C<last> and C<redo> behave as if they had been executed within
19799a22 977the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
978block, it may be more entertaining.
979
980 while (EXPR) {
a9a5a0dc
VP
981 ### redo always comes here
982 do_something;
1d2dff63 983 } continue {
a9a5a0dc
VP
984 ### next always comes here
985 do_something_else;
986 # then back the top to re-check EXPR
1d2dff63
GS
987 }
988 ### last always comes here
989
3b10bc60 990Omitting the C<continue> section is equivalent to using an
991empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
992to check the condition at the top of the loop.
993
8f1da26d
TC
994If the C<"switch"> feature is enabled, C<continue> is also a function that
995falls through the current C<when> or C<default> block instead of iterating
996a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
997See L<feature> and L<perlsyn/"Switch statements"> for more
998information.
0d863452 999
a0d0e21e 1000=item cos EXPR
d74e8afc 1001X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1002
d6217f1e
GS
1003=item cos
1004
5a964f20 1005Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1006takes the cosine of C<$_>.
a0d0e21e 1007
ca6e1c26 1008For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1009function, or use this relation:
1010
1011 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1012
a0d0e21e 1013=item crypt PLAINTEXT,SALT
d74e8afc 1014X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1015X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1016
ef2e6798
MS
1017Creates a digest string exactly like the crypt(3) function in the C
1018library (assuming that you actually have a version there that has not
bb23f8d1 1019been extirpated as a potential munition).
ef2e6798 1020
34169887 1021crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1022into a short string, called a digest, which is returned. The same
1023PLAINTEXT and SALT will always return the same string, but there is no
1024(known) way to get the original PLAINTEXT from the hash. Small
1025changes in the PLAINTEXT or SALT will result in large changes in the
1026digest.
1027
1028There is no decrypt function. This function isn't all that useful for
1029cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1030mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1031primarily used to check if two pieces of text are the same without
1032having to transmit or store the text itself. An example is checking
1033if a correct password is given. The digest of the password is stored,
cf264981 1034not the password itself. The user types in a password that is
ef2e6798 1035crypt()'d with the same salt as the stored digest. If the two digests
34169887 1036match, the password is correct.
ef2e6798
MS
1037
1038When verifying an existing digest string you should use the digest as
1039the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1040to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1041crypt() will hash the new string with the same salt as the digest.
1042This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1043with more exotic implementations. In other words, assume
1044nothing about the returned string itself nor about how many bytes
1045of SALT may matter.
85c16d83
JH
1046
1047Traditionally the result is a string of 13 bytes: two first bytes of
1048the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
bb23f8d1 1049the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1050hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1051and implementations on non-Unix platforms may produce different
ef2e6798 1052strings.
85c16d83
JH
1053
1054When choosing a new salt create a random two character string whose
1055characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1056'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1057characters is just a recommendation; the characters allowed in
1058the salt depend solely on your system's crypt library, and Perl can't
1059restrict what salts C<crypt()> accepts.
e71965be 1060
a0d0e21e 1061Here's an example that makes sure that whoever runs this program knows
cf264981 1062their password:
a0d0e21e
LW
1063
1064 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1065
1066 system "stty -echo";
1067 print "Password: ";
e71965be 1068 chomp($word = <STDIN>);
a0d0e21e
LW
1069 print "\n";
1070 system "stty echo";
1071
e71965be 1072 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1073 die "Sorry...\n";
a0d0e21e 1074 } else {
a9a5a0dc 1075 print "ok\n";
54310121 1076 }
a0d0e21e 1077
9f8f0c9d 1078Of course, typing in your own password to whoever asks you
748a9306 1079for it is unwise.
a0d0e21e 1080
ef2e6798 1081The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1082of data, not least of all because you can't get the information
ef2e6798 1083back. Look at the L<Digest> module for more robust algorithms.
19799a22 1084
f2791508
JH
1085If using crypt() on a Unicode string (which I<potentially> has
1086characters with codepoints above 255), Perl tries to make sense
34169887 1087of the situation by trying to downgrade (a copy of)
f2791508
JH
1088the string back to an eight-bit byte string before calling crypt()
1089(on that copy). If that works, good. If not, crypt() dies with
1090C<Wide character in crypt>.
85c16d83 1091
aa689395 1092=item dbmclose HASH
d74e8afc 1093X<dbmclose>
a0d0e21e 1094
19799a22 1095[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1096
aa689395 1097Breaks the binding between a DBM file and a hash.
a0d0e21e 1098
19799a22 1099=item dbmopen HASH,DBNAME,MASK
d74e8afc 1100X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1101
19799a22 1102[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1103
7b8d334a 1104This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1105hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1106argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1107is the name of the database (without the F<.dir> or F<.pag> extension if
1108any). If the database does not exist, it is created with protection
19799a22 1109specified by MASK (as modified by the C<umask>). If your system supports
80d38338 1110only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1111program. In older versions of Perl, if your system had neither DBM nor
19799a22 1112ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1113sdbm(3).
1114
1115If you don't have write access to the DBM file, you can only read hash
1116variables, not set them. If you want to test whether you can write,
3b10bc60 1117either use file tests or try setting a dummy hash entry inside an C<eval>
1118to trap the error.
a0d0e21e 1119
19799a22
GS
1120Note that functions such as C<keys> and C<values> may return huge lists
1121when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1122function to iterate over large DBM files. Example:
1123
1124 # print out history file offsets
1125 dbmopen(%HIST,'/usr/lib/news/history',0666);
1126 while (($key,$val) = each %HIST) {
a9a5a0dc 1127 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1128 }
1129 dbmclose(%HIST);
1130
cb1a09d0 1131See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1132cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1133rich implementation.
4633a7c4 1134
2b5ab1e7
TC
1135You can control which DBM library you use by loading that library
1136before you call dbmopen():
1137
1138 use DB_File;
1139 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1140 or die "Can't open netscape history file: $!";
2b5ab1e7 1141
8f1da26d
TC
1142=item default BLOCK
1143
1144Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when>
1145that's always true. Only available after Perl 5.10, and only if the
1146C<switch> feature has been requested. See L</when>.
1147
a0d0e21e 1148=item defined EXPR
d74e8afc 1149X<defined> X<undef> X<undefined>
a0d0e21e 1150
54310121 1151=item defined
bbce6d69 1152
2f9daede 1153Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1154the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1155checked.
1156
1157Many operations return C<undef> to indicate failure, end of file,
1158system error, uninitialized variable, and other exceptional
1159conditions. This function allows you to distinguish C<undef> from
1160other values. (A simple Boolean test will not distinguish among
7660c0ab 1161C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1162false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1163doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1164returns C<undef> when its argument is an empty array, I<or> when the
1165element to return happens to be C<undef>.
1166
f10b0346
GS
1167You may also use C<defined(&func)> to check whether subroutine C<&func>
1168has ever been defined. The return value is unaffected by any forward
80d38338 1169declarations of C<&func>. A subroutine that is not defined
847c7ebe 1170may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1171makes it spring into existence the first time that it is called; see
847c7ebe 1172L<perlsub>.
f10b0346
GS
1173
1174Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1175used to report whether memory for that aggregate had ever been
f10b0346
GS
1176allocated. This behavior may disappear in future versions of Perl.
1177You should instead use a simple test for size:
1178
1179 if (@an_array) { print "has array elements\n" }
1180 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1181
1182When used on a hash element, it tells you whether the value is defined,
dc848c6f 1183not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1184purpose.
a0d0e21e
LW
1185
1186Examples:
1187
8f1da26d 1188 print if defined $switch{D};
a0d0e21e
LW
1189 print "$val\n" while defined($val = pop(@ary));
1190 die "Can't readlink $sym: $!"
a9a5a0dc 1191 unless defined($value = readlink $sym);
a0d0e21e 1192 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1193 $debugging = 0 unless defined $debugging;
a0d0e21e 1194
8f1da26d 1195Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1196discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1197defined values. For example, if you say
a5f75d66
AD
1198
1199 "ab" =~ /a(.*)b/;
1200
80d38338 1201The pattern match succeeds and C<$1> is defined, although it
cf264981 1202matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1203matched something that happened to be zero characters long. This is all
a5f75d66 1204very above-board and honest. When a function returns an undefined value,
2f9daede 1205it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1206should use C<defined> only when questioning the integrity of what
7660c0ab 1207you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1208what you want.
1209
dc848c6f 1210See also L</undef>, L</exists>, L</ref>.
2f9daede 1211
a0d0e21e 1212=item delete EXPR
d74e8afc 1213X<delete>
a0d0e21e 1214
d0a76353
RS
1215Given an expression that specifies an element or slice of a hash, C<delete>
1216deletes the specified elements from that hash so that exists() on that element
1217no longer returns true. Setting a hash element to the undefined value does
1218not remove its key, but deleting it does; see L</exists>.
80d38338 1219
8f1da26d 1220In list context, returns the value or values deleted, or the last such
80d38338 1221element in scalar context. The return list's length always matches that of
d0a76353
RS
1222the argument list: deleting non-existent elements returns the undefined value
1223in their corresponding positions.
80d38338 1224
d0a76353
RS
1225delete() may also be used on arrays and array slices, but its behavior is less
1226straightforward. Although exists() will return false for deleted entries,
1227deleting array elements never changes indices of existing values; use shift()
1228or splice() for that. However, if all deleted elements fall at the end of an
1229array, the array's size shrinks to the position of the highest element that
1230still tests true for exists(), or to 0 if none do.
1231
8f1da26d 1232B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1233be removed in a future version of Perl.
80d38338
TC
1234
1235Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1236a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1237or array may not necessarily return anything; it depends on the implementation
1238of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1239
80d38338
TC
1240The C<delete local EXPR> construct localizes the deletion to the current
1241block at run time. Until the block exits, elements locally deleted
1242temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1243of composite types">.
eba0920a
EM
1244
1245 %hash = (foo => 11, bar => 22, baz => 33);
1246 $scalar = delete $hash{foo}; # $scalar is 11
1247 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1248 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1249
01020589 1250The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1251
5f05dabc 1252 foreach $key (keys %HASH) {
a9a5a0dc 1253 delete $HASH{$key};
a0d0e21e
LW
1254 }
1255
01020589 1256 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1257 delete $ARRAY[$index];
01020589
GS
1258 }
1259
1260And so do these:
5f05dabc 1261
01020589
GS
1262 delete @HASH{keys %HASH};
1263
9740c838 1264 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1265
80d38338
TC
1266But both are slower than assigning the empty list
1267or undefining %HASH or @ARRAY, which is the customary
1268way to empty out an aggregate:
01020589 1269
5ed4f2ec 1270 %HASH = (); # completely empty %HASH
1271 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1272
5ed4f2ec 1273 @ARRAY = (); # completely empty @ARRAY
1274 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1275
80d38338
TC
1276The EXPR can be arbitrarily complicated provided its
1277final operation is an element or slice of an aggregate:
a0d0e21e
LW
1278
1279 delete $ref->[$x][$y]{$key};
5f05dabc 1280 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1281
01020589
GS
1282 delete $ref->[$x][$y][$index];
1283 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1284
a0d0e21e 1285=item die LIST
d74e8afc 1286X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1287
4c050ad5
NC
1288C<die> raises an exception. Inside an C<eval> the error message is stuffed
1289into C<$@> and the C<eval> is terminated with the undefined value.
1290If the exception is outside of all enclosing C<eval>s, then the uncaught
1291exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1292need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1293
1294Equivalent examples:
1295
1296 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1297 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1298
ccac6780 1299If the last element of LIST does not end in a newline, the current
df37ec69
WW
1300script line number and input line number (if any) are also printed,
1301and a newline is supplied. Note that the "input line number" (also
1302known as "chunk") is subject to whatever notion of "line" happens to
1303be currently in effect, and is also available as the special variable
1304C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1305
1306Hint: sometimes appending C<", stopped"> to your message will cause it
1307to make better sense when the string C<"at foo line 123"> is appended.
1308Suppose you are running script "canasta".
a0d0e21e
LW
1309
1310 die "/etc/games is no good";
1311 die "/etc/games is no good, stopped";
1312
1313produce, respectively
1314
1315 /etc/games is no good at canasta line 123.
1316 /etc/games is no good, stopped at canasta line 123.
1317
a96d0188 1318If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1319previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1320This is useful for propagating exceptions:
1321
1322 eval { ... };
1323 die unless $@ =~ /Expected exception/;
1324
a96d0188 1325If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1326C<PROPAGATE> method, that method will be called with additional file
1327and line number parameters. The return value replaces the value in
34169887 1328C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1329were called.
1330
7660c0ab 1331If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1332
4c050ad5
NC
1333If an uncaught exception results in interpreter exit, the exit code is
1334determined from the values of C<$!> and C<$?> with this pseudocode:
1335
1336 exit $! if $!; # errno
1337 exit $? >> 8 if $? >> 8; # child exit status
1338 exit 255; # last resort
1339
1340The intent is to squeeze as much possible information about the likely cause
1341into the limited space of the system exit code. However, as C<$!> is the value
1342of C's C<errno>, which can be set by any system call, this means that the value
1343of the exit code used by C<die> can be non-predictable, so should not be relied
1344upon, other than to be non-zero.
1345
80d38338
TC
1346You can also call C<die> with a reference argument, and if this is trapped
1347within an C<eval>, C<$@> contains that reference. This permits more
1348elaborate exception handling using objects that maintain arbitrary state
1349about the exception. Such a scheme is sometimes preferable to matching
1350particular string values of C<$@> with regular expressions. Because C<$@>
1351is a global variable and C<eval> may be used within object implementations,
1352be careful that analyzing the error object doesn't replace the reference in
1353the global variable. It's easiest to make a local copy of the reference
1354before any manipulations. Here's an example:
52531d10 1355
80d38338 1356 use Scalar::Util "blessed";
da279afe 1357
52531d10 1358 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1359 if (my $ev_err = $@) {
1360 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1361 # handle Some::Module::Exception
1362 }
1363 else {
1364 # handle all other possible exceptions
1365 }
1366 }
1367
3b10bc60 1368Because Perl stringifies uncaught exception messages before display,
80d38338 1369you'll probably want to overload stringification operations on
52531d10
GS
1370exception objects. See L<overload> for details about that.
1371
19799a22
GS
1372You can arrange for a callback to be run just before the C<die>
1373does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1374handler is called with the error text and can change the error
19799a22 1375message, if it sees fit, by calling C<die> again. See
96090e4f 1376L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1377L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1378to be run only right before your program was to exit, this is not
3b10bc60 1379currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1380even inside eval()ed blocks/strings! If one wants the hook to do
1381nothing in such situations, put
fb73857a 1382
5ed4f2ec 1383 die @_ if $^S;
fb73857a 1384
19799a22
GS
1385as the first line of the handler (see L<perlvar/$^S>). Because
1386this promotes strange action at a distance, this counterintuitive
b76cc8ba 1387behavior may be fixed in a future release.
774d564b 1388
4c050ad5
NC
1389See also exit(), warn(), and the Carp module.
1390
a0d0e21e 1391=item do BLOCK
d74e8afc 1392X<do> X<block>
a0d0e21e
LW
1393
1394Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1395sequence of commands indicated by BLOCK. When modified by the C<while> or
1396C<until> loop modifier, executes the BLOCK once before testing the loop
1397condition. (On other statements the loop modifiers test the conditional
1398first.)
a0d0e21e 1399
4968c1e4 1400C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1401C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1402See L<perlsyn> for alternative strategies.
4968c1e4 1403
a0d0e21e 1404=item do SUBROUTINE(LIST)
d74e8afc 1405X<do>
a0d0e21e 1406
51124b83
FC
1407This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
1408a scalar variable or a subroutine beginning with C<&>.
a0d0e21e
LW
1409
1410=item do EXPR
d74e8afc 1411X<do>
a0d0e21e
LW
1412
1413Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1414file as a Perl script.
a0d0e21e
LW
1415
1416 do 'stat.pl';
1417
1418is just like
1419
986b19de 1420 eval `cat stat.pl`;
a0d0e21e 1421
2b5ab1e7 1422except that it's more efficient and concise, keeps track of the current
96090e4f
A
1423filename for error messages, searches the C<@INC> directories, and updates
1424C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1425these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1426cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1427same, however, in that it does reparse the file every time you call it,
1428so you probably don't want to do this inside a loop.
a0d0e21e 1429
8f1da26d 1430If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1431an error message in C<$@>. If C<do> cannot read the file, it returns undef
1432and sets C<$!> to the error. Always check C<$@> first, as compilation
1433could fail in a way that also sets C<$!>. If the file is successfully
1434compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1435
80d38338 1436Inclusion of library modules is better done with the
19799a22 1437C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1438and raise an exception if there's a problem.
a0d0e21e 1439
5a964f20
TC
1440You might like to use C<do> to read in a program configuration
1441file. Manual error checking can be done this way:
1442
b76cc8ba 1443 # read in config files: system first, then user
f86cebdf 1444 for $file ("/share/prog/defaults.rc",
b76cc8ba 1445 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1446 {
1447 unless ($return = do $file) {
1448 warn "couldn't parse $file: $@" if $@;
1449 warn "couldn't do $file: $!" unless defined $return;
1450 warn "couldn't run $file" unless $return;
1451 }
5a964f20
TC
1452 }
1453
a0d0e21e 1454=item dump LABEL
d74e8afc 1455X<dump> X<core> X<undump>
a0d0e21e 1456
1614b0e3
JD
1457=item dump
1458
19799a22
GS
1459This function causes an immediate core dump. See also the B<-u>
1460command-line switch in L<perlrun>, which does the same thing.
1461Primarily this is so that you can use the B<undump> program (not
1462supplied) to turn your core dump into an executable binary after
1463having initialized all your variables at the beginning of the
1464program. When the new binary is executed it will begin by executing
1465a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1466Think of it as a goto with an intervening core dump and reincarnation.
1467If C<LABEL> is omitted, restarts the program from the top.
1468
1469B<WARNING>: Any files opened at the time of the dump will I<not>
1470be open any more when the program is reincarnated, with possible
80d38338 1471resulting confusion by Perl.
19799a22 1472
59f521f4
RGS
1473This function is now largely obsolete, mostly because it's very hard to
1474convert a core file into an executable. That's why you should now invoke
1475it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1476typo.
19799a22 1477
532eee96 1478=item each HASH
d74e8afc 1479X<each> X<hash, iterator>
aa689395 1480
532eee96 1481=item each ARRAY
aeedbbed
NC
1482X<array, iterator>
1483
f5a93a43
TC
1484=item each EXPR
1485
80d38338
TC
1486When called in list context, returns a 2-element list consisting of the key
1487and value for the next element of a hash, or the index and value for the
1488next element of an array, so that you can iterate over it. When called in
1489scalar context, returns only the key (not the value) in a hash, or the index
1490in an array.
2f9daede 1491
aeedbbed 1492Hash entries are returned in an apparently random order. The actual random
3b10bc60 1493order is subject to change in future versions of Perl, but it is
504f80c1 1494guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1495function would produce on the same (unmodified) hash. Since Perl
22883ac5 14965.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1497for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1498
80d38338
TC
1499After C<each> has returned all entries from the hash or array, the next
1500call to C<each> returns the empty list in list context and C<undef> in
1501scalar context. The next call following that one restarts iteration. Each
1502hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1503and C<values>. The iterator is implicitly reset when C<each> has reached
1504the end as just described; it can be explicitly reset by calling C<keys> or
1505C<values> on the hash or array. If you add or delete a hash's elements
1506while iterating over it, entries may be skipped or duplicated--so don't do
1507that. Exception: It is always safe to delete the item most recently
3b10bc60 1508returned by C<each()>, so the following code works properly:
74fc8b5f
MJD
1509
1510 while (($key, $value) = each %hash) {
1511 print $key, "\n";
1512 delete $hash{$key}; # This is safe
1513 }
aa689395 1514
80d38338 1515This prints out your environment like the printenv(1) program,
3b10bc60 1516but in a different order:
a0d0e21e
LW
1517
1518 while (($key,$value) = each %ENV) {
a9a5a0dc 1519 print "$key=$value\n";
a0d0e21e
LW
1520 }
1521
f5a93a43
TC
1522Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1523reference to an unblessed hash or array. The argument will be dereferenced
1524automatically. This aspect of C<each> is considered highly experimental.
1525The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1526
1527 while (($key,$value) = each $hashref) { ... }
1528
8f1da26d 1529See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1530
1531=item eof FILEHANDLE
d74e8afc
ITB
1532X<eof>
1533X<end of file>
1534X<end-of-file>
a0d0e21e 1535
4633a7c4
LW
1536=item eof ()
1537
a0d0e21e
LW
1538=item eof
1539
8f1da26d 1540Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1541FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1542gives the real filehandle. (Note that this function actually
80d38338 1543reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1544interactive context.) Do not read from a terminal file (or call
19799a22 1545C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1546as terminals may lose the end-of-file condition if you do.
1547
820475bd 1548An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1549with empty parentheses is different. It refers to the pseudo file
820475bd 1550formed from the files listed on the command line and accessed via the
61eff3bc
JH
1551C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1552as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1553used will cause C<@ARGV> to be examined to determine if input is
67408cae 1554available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1555end-of-file will assume you are processing another C<@ARGV> list,
1556and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1557see L<perlop/"I/O Operators">.
820475bd 1558
61eff3bc 1559In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1560detect the end of each file, whereas C<eof()> will detect the end
1561of the very last file only. Examples:
a0d0e21e 1562
748a9306
LW
1563 # reset line numbering on each input file
1564 while (<>) {
a9a5a0dc
VP
1565 next if /^\s*#/; # skip comments
1566 print "$.\t$_";
5a964f20 1567 } continue {
a9a5a0dc 1568 close ARGV if eof; # Not eof()!
748a9306
LW
1569 }
1570
a0d0e21e
LW
1571 # insert dashes just before last line of last file
1572 while (<>) {
a9a5a0dc
VP
1573 if (eof()) { # check for end of last file
1574 print "--------------\n";
1575 }
1576 print;
1577 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1578 }
1579
a0d0e21e 1580Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1581input operators typically return C<undef> when they run out of data or
1582encounter an error.
a0d0e21e
LW
1583
1584=item eval EXPR
d74e8afc 1585X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1586X<error, handling> X<exception, handling>
a0d0e21e
LW
1587
1588=item eval BLOCK
1589
ce2984c3
PF
1590=item eval
1591
c7cc6f1c
GS
1592In the first form, the return value of EXPR is parsed and executed as if it
1593were a little Perl program. The value of the expression (which is itself
8f1da26d 1594determined within scalar context) is first parsed, and if there were no
2341804c
DM
1595errors, executed as a block within the lexical context of the current Perl
1596program. This means, that in particular, any outer lexical variables are
1597visible to it, and any package variable settings or subroutine and format
1598definitions remain afterwards.
1599
1600Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1601If EXPR is omitted, evaluates C<$_>. This form is typically used to
1602delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1603
1604In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1605same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1606within the context of the current Perl program. This form is typically
1607used to trap exceptions more efficiently than the first (see below), while
1608also providing the benefit of checking the code within BLOCK at compile
1609time.
1610
1611The final semicolon, if any, may be omitted from the value of EXPR or within
1612the BLOCK.
1613
1614In both forms, the value returned is the value of the last expression
5a964f20 1615evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1616as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1617in void, scalar, or list context, depending on the context of the C<eval>
1618itself. See L</wantarray> for more on how the evaluation context can be
1619determined.
a0d0e21e 1620
19799a22 1621If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1622executed, C<eval> returns C<undef> in scalar context
1f0d8f98
FC
1623or an empty list--or, for syntax errors, a list containing a single
1624undefined value--in list context, and C<$@> is set to the error
1625message. The discrepancy in the return values in list context is
1626considered a bug by some, and will probably be fixed in a future
1627release. If there was no error, C<$@> is guaranteed to be the empty
3b10bc60 1628string. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1629warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1630To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1631turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1632See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1633
19799a22
GS
1634Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1635determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1636is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1637the die operator is used to raise exceptions.
1638
5f1da31c
NT
1639If you want to trap errors when loading an XS module, some problems with
1640the binary interface (such as Perl version skew) may be fatal even with
1641C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1642
a0d0e21e
LW
1643If the code to be executed doesn't vary, you may use the eval-BLOCK
1644form to trap run-time errors without incurring the penalty of
1645recompiling each time. The error, if any, is still returned in C<$@>.
1646Examples:
1647
54310121 1648 # make divide-by-zero nonfatal
a0d0e21e
LW
1649 eval { $answer = $a / $b; }; warn $@ if $@;
1650
1651 # same thing, but less efficient
1652 eval '$answer = $a / $b'; warn $@ if $@;
1653
1654 # a compile-time error
5ed4f2ec 1655 eval { $answer = }; # WRONG
a0d0e21e
LW
1656
1657 # a run-time error
5ed4f2ec 1658 eval '$answer ='; # sets $@
a0d0e21e 1659
cf264981
SP
1660Using the C<eval{}> form as an exception trap in libraries does have some
1661issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1662may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1663You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1664as this example shows:
774d564b 1665
80d38338 1666 # a private exception trap for divide-by-zero
f86cebdf
GS
1667 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1668 warn $@ if $@;
774d564b
PP
1669
1670This is especially significant, given that C<__DIE__> hooks can call
19799a22 1671C<die> again, which has the effect of changing their error messages:
774d564b
PP
1672
1673 # __DIE__ hooks may modify error messages
1674 {
f86cebdf
GS
1675 local $SIG{'__DIE__'} =
1676 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1677 eval { die "foo lives here" };
1678 print $@ if $@; # prints "bar lives here"
774d564b
PP
1679 }
1680
19799a22 1681Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1682may be fixed in a future release.
1683
19799a22 1684With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1685being looked at when:
1686
5ed4f2ec 1687 eval $x; # CASE 1
1688 eval "$x"; # CASE 2
a0d0e21e 1689
5ed4f2ec 1690 eval '$x'; # CASE 3
1691 eval { $x }; # CASE 4
a0d0e21e 1692
5ed4f2ec 1693 eval "\$$x++"; # CASE 5
1694 $$x++; # CASE 6
a0d0e21e 1695
2f9daede 1696Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1697the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1698the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1699and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1700does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1701purely visual reasons, but it also has the advantage of compiling at
1702compile-time instead of at run-time.) Case 5 is a place where
19799a22 1703normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1704particular situation, you can just use symbolic references instead, as
1705in case 6.
a0d0e21e 1706
b6538e4f
TC
1707Before Perl 5.14, the assignment to C<$@> occurred before restoration
1708of localised variables, which means that for your code to run on older
b208c909 1709versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1710errors:
1711
1712 # alter $@ on nefarious repugnancy only
1713 {
1714 my $e;
1715 {
1716 local $@; # protect existing $@
1717 eval { test_repugnancy() };
b208c909 1718 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
8a5a710d
DN
1719 $@ =~ /nefarious/ and $e = $@;
1720 }
1721 die $e if defined $e
1722 }
1723
4968c1e4 1724C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1725C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1726
3b10bc60 1727An C<eval ''> executed within the C<DB> package doesn't see the usual
1728surrounding lexical scope, but rather the scope of the first non-DB piece
1729of code that called it. You don't normally need to worry about this unless
1730you are writing a Perl debugger.
d819b83a 1731
a0d0e21e 1732=item exec LIST
d74e8afc 1733X<exec> X<execute>
a0d0e21e 1734
8bf3b016
GS
1735=item exec PROGRAM LIST
1736
3b10bc60 1737The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1738use C<system> instead of C<exec> if you want it to return. It fails and
1739returns false only if the command does not exist I<and> it is executed
fb73857a 1740directly instead of via your system's command shell (see below).
a0d0e21e 1741
19799a22 1742Since it's a common mistake to use C<exec> instead of C<system>, Perl
80d38338 1743warns you if there is a following statement that isn't C<die>, C<warn>,
3b10bc60 1744or C<exit> (if C<-w> is set--but you always do that, right?). If you
19799a22 1745I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1746can use one of these styles to avoid the warning:
1747
5a964f20
TC
1748 exec ('foo') or print STDERR "couldn't exec foo: $!";
1749 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1750
5a964f20 1751If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1752with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1753If there is only one scalar argument or an array with one element in it,
1754the argument is checked for shell metacharacters, and if there are any,
1755the entire argument is passed to the system's command shell for parsing
1756(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1757If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1758words and passed directly to C<execvp>, which is more efficient.
19799a22 1759Examples:
a0d0e21e 1760
19799a22
GS
1761 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1762 exec "sort $outfile | uniq";
a0d0e21e
LW
1763
1764If you don't really want to execute the first argument, but want to lie
1765to the program you are executing about its own name, you can specify
1766the program you actually want to run as an "indirect object" (without a
1767comma) in front of the LIST. (This always forces interpretation of the
54310121 1768LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1769the list.) Example:
1770
1771 $shell = '/bin/csh';
5ed4f2ec 1772 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1773
1774or, more directly,
1775
5ed4f2ec 1776 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1777
3b10bc60 1778When the arguments get executed via the system shell, results are
1779subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1780for details.
1781
19799a22
GS
1782Using an indirect object with C<exec> or C<system> is also more
1783secure. This usage (which also works fine with system()) forces
1784interpretation of the arguments as a multivalued list, even if the
1785list had just one argument. That way you're safe from the shell
1786expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1787
1788 @args = ( "echo surprise" );
1789
2b5ab1e7 1790 exec @args; # subject to shell escapes
f86cebdf 1791 # if @args == 1
2b5ab1e7 1792 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1793
1794The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1795program, passing it C<"surprise"> an argument. The second version didn't;
1796it tried to run a program named I<"echo surprise">, didn't find it, and set
1797C<$?> to a non-zero value indicating failure.
5a964f20 1798
80d38338 1799Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1800output before the exec, but this may not be supported on some platforms
1801(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1802in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 1803open handles to avoid lost output.
0f897271 1804
80d38338
TC
1805Note that C<exec> will not call your C<END> blocks, nor will it invoke
1806C<DESTROY> methods on your objects.
7660c0ab 1807
a0d0e21e 1808=item exists EXPR
d74e8afc 1809X<exists> X<autovivification>
a0d0e21e 1810
d0a76353
RS
1811Given an expression that specifies an element of a hash, returns true if the
1812specified element in the hash has ever been initialized, even if the
1813corresponding value is undefined.
a0d0e21e 1814
5ed4f2ec 1815 print "Exists\n" if exists $hash{$key};
1816 print "Defined\n" if defined $hash{$key};
01020589
GS
1817 print "True\n" if $hash{$key};
1818
d0a76353 1819exists may also be called on array elements, but its behavior is much less
8f1da26d 1820obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
1821that calling exists on array values is deprecated and likely to be removed in
1822a future version of Perl.
1823
5ed4f2ec 1824 print "Exists\n" if exists $array[$index];
1825 print "Defined\n" if defined $array[$index];
01020589 1826 print "True\n" if $array[$index];
a0d0e21e 1827
8f1da26d 1828A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
1829it exists, but the reverse doesn't necessarily hold true.
1830
afebc493
GS
1831Given an expression that specifies the name of a subroutine,
1832returns true if the specified subroutine has ever been declared, even
1833if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 1834does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
1835exist may still be callable: its package may have an C<AUTOLOAD>
1836method that makes it spring into existence the first time that it is
3b10bc60 1837called; see L<perlsub>.
afebc493 1838
5ed4f2ec 1839 print "Exists\n" if exists &subroutine;
1840 print "Defined\n" if defined &subroutine;
afebc493 1841
a0d0e21e 1842Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1843operation is a hash or array key lookup or subroutine name:
a0d0e21e 1844
5ed4f2ec 1845 if (exists $ref->{A}->{B}->{$key}) { }
1846 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 1847
5ed4f2ec 1848 if (exists $ref->{A}->{B}->[$ix]) { }
1849 if (exists $hash{A}{B}[$ix]) { }
01020589 1850
afebc493
GS
1851 if (exists &{$ref->{A}{B}{$key}}) { }
1852
3b10bc60 1853Although the mostly deeply nested array or hash will not spring into
1854existence just because its existence was tested, any intervening ones will.
61eff3bc 1855Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 1856into existence due to the existence test for the $key element above.
3b10bc60 1857This happens anywhere the arrow operator is used, including even here:
5a964f20 1858
2b5ab1e7 1859 undef $ref;
5ed4f2ec 1860 if (exists $ref->{"Some key"}) { }
1861 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
1862
1863This surprising autovivification in what does not at first--or even
1864second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1865release.
a0d0e21e 1866
afebc493
GS
1867Use of a subroutine call, rather than a subroutine name, as an argument
1868to exists() is an error.
1869
5ed4f2ec 1870 exists &sub; # OK
1871 exists &sub(); # Error
afebc493 1872
a0d0e21e 1873=item exit EXPR
d74e8afc 1874X<exit> X<terminate> X<abort>
a0d0e21e 1875
ce2984c3
PF
1876=item exit
1877
2b5ab1e7 1878Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1879
1880 $ans = <STDIN>;
1881 exit 0 if $ans =~ /^[Xx]/;
1882
19799a22 1883See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1884universally recognized values for EXPR are C<0> for success and C<1>
1885for error; other values are subject to interpretation depending on the
1886environment in which the Perl program is running. For example, exiting
188769 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1888the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1889
19799a22
GS
1890Don't use C<exit> to abort a subroutine if there's any chance that
1891someone might want to trap whatever error happened. Use C<die> instead,
1892which can be trapped by an C<eval>.
28757baa 1893
19799a22 1894The exit() function does not always exit immediately. It calls any
2b5ab1e7 1895defined C<END> routines first, but these C<END> routines may not
19799a22 1896themselves abort the exit. Likewise any object destructors that need to
60275626
FC
1897be called are called before the real exit. C<END> routines and destructors
1898can change the exit status by modifying C<$?>. If this is a problem, you
2b5ab1e7 1899can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1900See L<perlmod> for details.
5a964f20 1901
a0d0e21e 1902=item exp EXPR
d74e8afc 1903X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1904
54310121 1905=item exp
bbce6d69 1906
b76cc8ba 1907Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1908If EXPR is omitted, gives C<exp($_)>.
1909
1910=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1911X<fcntl>
a0d0e21e 1912
f86cebdf 1913Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1914
1915 use Fcntl;
1916
0ade1984 1917first to get the correct constant definitions. Argument processing and
3b10bc60 1918value returned work just like C<ioctl> below.
a0d0e21e
LW
1919For example:
1920
1921 use Fcntl;
5a964f20 1922 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 1923 or die "can't fcntl F_GETFL: $!";
5a964f20 1924
554ad1fc 1925You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1926Like C<ioctl>, it maps a C<0> return from the system call into
1927C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1928in numeric context. It is also exempt from the normal B<-w> warnings
1929on improper numeric conversions.
5a964f20 1930
3b10bc60 1931Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
1932doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1933manpage to learn what functions are available on your system.
a0d0e21e 1934
be2f7487 1935Here's an example of setting a filehandle named C<REMOTE> to be
1936non-blocking at the system level. You'll have to negotiate C<$|>
1937on your own, though.
1938
1939 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1940
1941 $flags = fcntl(REMOTE, F_GETFL, 0)
1942 or die "Can't get flags for the socket: $!\n";
1943
1944 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1945 or die "Can't set flags for the socket: $!\n";
1946
a0d0e21e 1947=item fileno FILEHANDLE
d74e8afc 1948X<fileno>
a0d0e21e 1949
2b5ab1e7 1950Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
1951filehandle is not open. If there is no real file descriptor at the OS
1952level, as can happen with filehandles connected to memory objects via
1953C<open> with a reference for the third argument, -1 is returned.
1954
1955This is mainly useful for constructing
19799a22 1956bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1957If FILEHANDLE is an expression, the value is taken as an indirect
1958filehandle, generally its name.
5a964f20 1959
b76cc8ba 1960You can use this to find out whether two handles refer to the
5a964f20
TC
1961same underlying descriptor:
1962
1963 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 1964 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1965 }
1966
a0d0e21e 1967=item flock FILEHANDLE,OPERATION
d74e8afc 1968X<flock> X<lock> X<locking>
a0d0e21e 1969
19799a22
GS
1970Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1971for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1972machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 1973C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 1974entire files only, not records.
2b5ab1e7
TC
1975
1976Two potentially non-obvious but traditional C<flock> semantics are
1977that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
1978are B<merely advisory>. Such discretionary locks are more flexible, but
1979offer fewer guarantees. This means that programs that do not also use
1980C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 1981your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
1982for details. It's best to assume traditional behavior if you're writing
1983portable programs. (But if you're not, you should as always feel perfectly
1984free to write for your own system's idiosyncrasies (sometimes called
1985"features"). Slavish adherence to portability concerns shouldn't get
1986in the way of your getting your job done.)
a3cb178b 1987
8ebc5c01
PP
1988OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1989LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
1990you can use the symbolic names if you import them from the L<Fcntl> module,
1991either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 1992requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 1993releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 1994LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 1995waiting for the lock; check the return status to see if you got it.
68dc0745 1996
2b5ab1e7
TC
1997To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1998before locking or unlocking it.
8ebc5c01 1999
f86cebdf 2000Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2001locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2002are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2003implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
2004differing semantics shouldn't bite too many people.
2005
becacb53
TM
2006Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2007be open with read intent to use LOCK_SH and requires that it be open
2008with write intent to use LOCK_EX.
2009
19799a22
GS
2010Note also that some versions of C<flock> cannot lock things over the
2011network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2012that. If you like you can force Perl to ignore your system's flock(2)
2013function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2014the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2015and build a new Perl.
4633a7c4
LW
2016
2017Here's a mailbox appender for BSD systems.
a0d0e21e 2018
7ed5353d 2019 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
2020
2021 sub lock {
a9a5a0dc
VP
2022 my ($fh) = @_;
2023 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2024
a9a5a0dc
VP
2025 # and, in case someone appended while we were waiting...
2026 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2027 }
2028
2029 sub unlock {
a9a5a0dc
VP
2030 my ($fh) = @_;
2031 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2032 }
2033
b0169937 2034 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2035 or die "Can't open mailbox: $!";
a0d0e21e 2036
7ed5353d 2037 lock($mbox);
b0169937 2038 print $mbox $msg,"\n\n";
7ed5353d 2039 unlock($mbox);
a0d0e21e 2040
3b10bc60 2041On systems that support a real flock(2), locks are inherited across fork()
2042calls, whereas those that must resort to the more capricious fcntl(2)
2043function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2044
cb1a09d0 2045See also L<DB_File> for other flock() examples.
a0d0e21e
LW
2046
2047=item fork
d74e8afc 2048X<fork> X<child> X<parent>
a0d0e21e 2049
2b5ab1e7
TC
2050Does a fork(2) system call to create a new process running the
2051same program at the same point. It returns the child pid to the
2052parent process, C<0> to the child process, or C<undef> if the fork is
2053unsuccessful. File descriptors (and sometimes locks on those descriptors)
2054are shared, while everything else is copied. On most systems supporting
2055fork(), great care has gone into making it extremely efficient (for
2056example, using copy-on-write technology on data pages), making it the
2057dominant paradigm for multitasking over the last few decades.
5a964f20 2058
80d38338 2059Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
2060output before forking the child process, but this may not be supported
2061on some platforms (see L<perlport>). To be safe, you may need to set
2062C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2063C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2064
19799a22 2065If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2066accumulate zombies. On some systems, you can avoid this by setting
2067C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2068forking and reaping moribund children.
cb1a09d0 2069
28757baa
PP
2070Note that if your forked child inherits system file descriptors like
2071STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2072if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2073backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2074You should reopen those to F</dev/null> if it's any issue.
28757baa 2075
cb1a09d0 2076=item format
d74e8afc 2077X<format>
cb1a09d0 2078
19799a22 2079Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2080example:
2081
54310121 2082 format Something =
a9a5a0dc
VP
2083 Test: @<<<<<<<< @||||| @>>>>>
2084 $str, $%, '$' . int($num)
cb1a09d0
AD
2085 .
2086
2087 $str = "widget";
184e9718 2088 $num = $cost/$quantity;
cb1a09d0
AD
2089 $~ = 'Something';
2090 write;
2091
2092See L<perlform> for many details and examples.
2093
8903cb82 2094=item formline PICTURE,LIST
d74e8afc 2095X<formline>
a0d0e21e 2096
5a964f20 2097This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2098too. It formats (see L<perlform>) a list of values according to the
2099contents of PICTURE, placing the output into the format output
7660c0ab 2100accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2101Eventually, when a C<write> is done, the contents of
cf264981
SP
2102C<$^A> are written to some filehandle. You could also read C<$^A>
2103and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2104does one C<formline> per line of form, but the C<formline> function itself
748a9306 2105doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2106that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2107You may therefore need to use multiple formlines to implement a single
3b10bc60 2108record format, just like the C<format> compiler.
748a9306 2109
19799a22 2110Be careful if you put double quotes around the picture, because an C<@>
748a9306 2111character may be taken to mean the beginning of an array name.
19799a22 2112C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2113
445b09e5
FC
2114If you are trying to use this instead of C<write> to capture the output,
2115you may find it easier to open a filehandle to a scalar
2116(C<< open $fh, ">", \$output >>) and write to that instead.
2117
a0d0e21e 2118=item getc FILEHANDLE
f723aae1 2119X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2120
2121=item getc
2122
2123Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2124or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2125the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2126STDIN. This is not particularly efficient. However, it cannot be
2127used by itself to fetch single characters without waiting for the user
2128to hit enter. For that, try something more like:
4633a7c4
LW
2129
2130 if ($BSD_STYLE) {
a9a5a0dc 2131 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2132 }
2133 else {
a9a5a0dc 2134 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2135 }
2136
2137 $key = getc(STDIN);
2138
2139 if ($BSD_STYLE) {
a9a5a0dc 2140 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2141 }
2142 else {
3b10bc60 2143 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2144 }
2145 print "\n";
2146
54310121
PP
2147Determination of whether $BSD_STYLE should be set
2148is left as an exercise to the reader.
cb1a09d0 2149
19799a22 2150The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2151systems purporting POSIX compliance. See also the C<Term::ReadKey>
a3390c9f 2152module from your nearest CPAN site; details on CPAN can be found under
2b5ab1e7 2153L<perlmodlib/CPAN>.
a0d0e21e
LW
2154
2155=item getlogin
d74e8afc 2156X<getlogin> X<login>
a0d0e21e 2157
cf264981 2158This implements the C library function of the same name, which on most
3b10bc60 2159systems returns the current login from F</etc/utmp>, if any. If it
2160returns the empty string, use C<getpwuid>.
a0d0e21e 2161
f86702cc 2162 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2163
19799a22
GS
2164Do not consider C<getlogin> for authentication: it is not as
2165secure as C<getpwuid>.
4633a7c4 2166
a0d0e21e 2167=item getpeername SOCKET
d74e8afc 2168X<getpeername> X<peer>
a0d0e21e 2169
a3390c9f
FC
2170Returns the packed sockaddr address of the other end of the SOCKET
2171connection.
a0d0e21e 2172
4633a7c4
LW
2173 use Socket;
2174 $hersockaddr = getpeername(SOCK);
19799a22 2175 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2176 $herhostname = gethostbyaddr($iaddr, AF_INET);
2177 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2178
2179=item getpgrp PID
d74e8afc 2180X<getpgrp> X<group>
a0d0e21e 2181
47e29363 2182Returns the current process group for the specified PID. Use
7660c0ab 2183a PID of C<0> to get the current process group for the
4633a7c4 2184current process. Will raise an exception if used on a machine that
a3390c9f
FC
2185doesn't implement getpgrp(2). If PID is omitted, returns the process
2186group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2187does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2188
2189=item getppid
d74e8afc 2190X<getppid> X<parent> X<pid>
a0d0e21e
LW
2191
2192Returns the process id of the parent process.
2193
4d76a344
RGS
2194Note for Linux users: on Linux, the C functions C<getpid()> and
2195C<getppid()> return different values from different threads. In order to
3b10bc60 2196be portable, this behavior is not reflected by the Perl-level function
4d76a344 2197C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2198to call the underlying C<getppid()>, you may use the CPAN module
2199C<Linux::Pid>.
4d76a344 2200
a0d0e21e 2201=item getpriority WHICH,WHO
d74e8afc 2202X<getpriority> X<priority> X<nice>
a0d0e21e 2203
4633a7c4 2204Returns the current priority for a process, a process group, or a user.
f4084e39 2205(See C<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2206machine that doesn't implement getpriority(2).
a0d0e21e
LW
2207
2208=item getpwnam NAME
d74e8afc
ITB
2209X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2210X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2211X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2212X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2213X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2214X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2215
2216=item getgrnam NAME
2217
2218=item gethostbyname NAME
2219
2220=item getnetbyname NAME
2221
2222=item getprotobyname NAME
2223
2224=item getpwuid UID
2225
2226=item getgrgid GID
2227
2228=item getservbyname NAME,PROTO
2229
2230=item gethostbyaddr ADDR,ADDRTYPE
2231
2232=item getnetbyaddr ADDR,ADDRTYPE
2233
2234=item getprotobynumber NUMBER
2235
2236=item getservbyport PORT,PROTO
2237
2238=item getpwent
2239
2240=item getgrent
2241
2242=item gethostent
2243
2244=item getnetent
2245
2246=item getprotoent
2247
2248=item getservent
2249
2250=item setpwent
2251
2252=item setgrent
2253
2254=item sethostent STAYOPEN
2255
2256=item setnetent STAYOPEN
2257
2258=item setprotoent STAYOPEN
2259
2260=item setservent STAYOPEN
2261
2262=item endpwent
2263
2264=item endgrent
2265
2266=item endhostent
2267
2268=item endnetent
2269
2270=item endprotoent
2271
2272=item endservent
2273
80d38338
TC
2274These routines are the same as their counterparts in the
2275system C library. In list context, the return values from the
a0d0e21e
LW
2276various get routines are as follows:
2277
2278 ($name,$passwd,$uid,$gid,
6ee623d5 2279 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2280 ($name,$passwd,$gid,$members) = getgr*
2281 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2282 ($name,$aliases,$addrtype,$net) = getnet*
2283 ($name,$aliases,$proto) = getproto*
2284 ($name,$aliases,$port,$proto) = getserv*
2285
3b10bc60 2286(If the entry doesn't exist you get an empty list.)
a0d0e21e 2287
4602f195
JH
2288The exact meaning of the $gcos field varies but it usually contains
2289the real name of the user (as opposed to the login name) and other
2290information pertaining to the user. Beware, however, that in many
2291system users are able to change this information and therefore it
106325ad 2292cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2293L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2294login shell, are also tainted, for the same reason.
4602f195 2295
5a964f20 2296In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2297lookup by name, in which case you get the other thing, whatever it is.
2298(If the entry doesn't exist you get the undefined value.) For example:
2299
5a964f20
TC
2300 $uid = getpwnam($name);
2301 $name = getpwuid($num);
2302 $name = getpwent();
2303 $gid = getgrnam($name);
08a33e13 2304 $name = getgrgid($num);
5a964f20
TC
2305 $name = getgrent();
2306 #etc.
a0d0e21e 2307
4602f195 2308In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2309in that they are unsupported on many systems. If the
4602f195
JH
2310$quota is unsupported, it is an empty scalar. If it is supported, it
2311usually encodes the disk quota. If the $comment field is unsupported,
2312it is an empty scalar. If it is supported it usually encodes some
2313administrative comment about the user. In some systems the $quota
2314field may be $change or $age, fields that have to do with password
2315aging. In some systems the $comment field may be $class. The $expire
2316field, if present, encodes the expiration period of the account or the
2317password. For the availability and the exact meaning of these fields
8f1da26d 2318in your system, please consult getpwnam(3) and your system's
4602f195
JH
2319F<pwd.h> file. You can also find out from within Perl what your
2320$quota and $comment fields mean and whether you have the $expire field
2321by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2322C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2323files are supported only if your vendor has implemented them in the
4602f195 2324intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2325shadow versions if you're running under privilege or if there exists
cf264981 2326the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2327and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2328facility are unlikely to be supported.
6ee623d5 2329
a3390c9f 2330The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2331the login names of the members of the group.
2332
2333For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2334C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2335C<@addrs> value returned by a successful call is a list of raw
2336addresses returned by the corresponding library call. In the
2337Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2338by saying something like:
2339
f337b084 2340 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2341
2b5ab1e7
TC
2342The Socket library makes this slightly easier:
2343
2344 use Socket;
2345 $iaddr = inet_aton("127.1"); # or whatever address
2346 $name = gethostbyaddr($iaddr, AF_INET);
2347
2348 # or going the other way
19799a22 2349 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2350
d760c846
GS
2351In the opposite way, to resolve a hostname to the IP address
2352you can write this:
2353
2354 use Socket;
2355 $packed_ip = gethostbyname("www.perl.org");
2356 if (defined $packed_ip) {
2357 $ip_address = inet_ntoa($packed_ip);
2358 }
2359
2360Make sure <gethostbyname()> is called in SCALAR context and that
2361its return value is checked for definedness.
2362
19799a22
GS
2363If you get tired of remembering which element of the return list
2364contains which return value, by-name interfaces are provided
2365in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2366C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2367and C<User::grent>. These override the normal built-ins, supplying
2368versions that return objects with the appropriate names
2369for each field. For example:
5a964f20
TC
2370
2371 use File::stat;
2372 use User::pwent;
2373 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2374
a3390c9f 2375Even though it looks as though they're the same method calls (uid),
b76cc8ba 2376they aren't, because a C<File::stat> object is different from
19799a22 2377a C<User::pwent> object.
5a964f20 2378
a0d0e21e 2379=item getsockname SOCKET
d74e8afc 2380X<getsockname>
a0d0e21e 2381
19799a22
GS
2382Returns the packed sockaddr address of this end of the SOCKET connection,
2383in case you don't know the address because you have several different
2384IPs that the connection might have come in on.
a0d0e21e 2385
4633a7c4
LW
2386 use Socket;
2387 $mysockaddr = getsockname(SOCK);
19799a22 2388 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2389 printf "Connect to %s [%s]\n",
19799a22
GS
2390 scalar gethostbyaddr($myaddr, AF_INET),
2391 inet_ntoa($myaddr);
a0d0e21e
LW
2392
2393=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2394X<getsockopt>
a0d0e21e 2395
636e6b1f
TH
2396Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2397Options may exist at multiple protocol levels depending on the socket
2398type, but at least the uppermost socket level SOL_SOCKET (defined in the
2399C<Socket> module) will exist. To query options at another level the
2400protocol number of the appropriate protocol controlling the option
2401should be supplied. For example, to indicate that an option is to be
2402interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2403number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2404
80d38338 2405The function returns a packed string representing the requested socket
3b10bc60 2406option, or C<undef> on error, with the reason for the error placed in
a3390c9f 2407C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2408consult getsockopt(2) for details. A common case is that the option is an
2409integer, in which case the result is a packed integer, which you can decode
2410using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2411
8f1da26d 2412Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2413
4852725b 2414 use Socket qw(:all);
636e6b1f
TH
2415
2416 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2417 or die "Could not determine the protocol number for tcp";
4852725b
DD
2418 # my $tcp = IPPROTO_TCP; # Alternative
2419 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2420 or die "getsockopt TCP_NODELAY: $!";
636e6b1f
TH
2421 my $nodelay = unpack("I", $packed);
2422 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2423
a0d0e21e 2424
15a348aa 2425=item given EXPR BLOCK
2426X<given>
2427
2428=item given BLOCK
2429
2430C<given> is analogous to the C<switch> keyword in other languages. C<given>
2431and C<when> are used in Perl to implement C<switch>/C<case> like statements.
8f1da26d 2432Only available after Perl 5.10. For example:
15a348aa 2433
8f1da26d 2434 use v5.10;
15a348aa 2435 given ($fruit) {
2436 when (/apples?/) {
2437 print "I like apples."
2438 }
2439 when (/oranges?/) {
2440 print "I don't like oranges."
2441 }
2442 default {
2443 print "I don't like anything"
2444 }
2445 }
2446
2447See L<perlsyn/"Switch statements"> for detailed information.
2448
a0d0e21e 2449=item glob EXPR
d74e8afc 2450X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2451
0a753a76
PP
2452=item glob
2453
d9a9d457
JL
2454In list context, returns a (possibly empty) list of filename expansions on
2455the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2456scalar context, glob iterates through such filename expansions, returning
2457undef when the list is exhausted. This is the internal function
2458implementing the C<< <*.c> >> operator, but you can use it directly. If
2459EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2460more detail in L<perlop/"I/O Operators">.
a0d0e21e 2461
80d38338
TC
2462Note that C<glob> splits its arguments on whitespace and treats
2463each segment as separate pattern. As such, C<glob("*.c *.h")>
2464matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2465C<glob(".* *")> matches all files in the current working directory.
80d38338
TC
2466
2467If non-empty braces are the only wildcard characters used in the
2468C<glob>, no filenames are matched, but potentially many strings
2469are returned. For example, this produces nine strings, one for
2470each pairing of fruits and colors:
2471
2472 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2473
3a4b19e4 2474Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2475C<File::Glob> extension. See L<File::Glob> for details, including
2476C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2477
a0d0e21e 2478=item gmtime EXPR
d74e8afc 2479X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2480
ce2984c3
PF
2481=item gmtime
2482
4509d391 2483Works just like L</localtime> but the returned values are
435fbc73 2484localized for the standard Greenwich time zone.
a0d0e21e 2485
a3390c9f
FC
2486Note: When called in list context, $isdst, the last value
2487returned by gmtime, is always C<0>. There is no
435fbc73 2488Daylight Saving Time in GMT.
0a753a76 2489
62aa5637
MS
2490See L<perlport/gmtime> for portability concerns.
2491
a0d0e21e 2492=item goto LABEL
d74e8afc 2493X<goto> X<jump> X<jmp>
a0d0e21e 2494
748a9306
LW
2495=item goto EXPR
2496
a0d0e21e
LW
2497=item goto &NAME
2498
b500e03b
GG
2499The C<goto-LABEL> form finds the statement labeled with LABEL and
2500resumes execution there. It can't be used to get out of a block or
2501subroutine given to C<sort>. It can be used to go almost anywhere
2502else within the dynamic scope, including out of subroutines, but it's
2503usually better to use some other construct such as C<last> or C<die>.
2504The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2505(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2506does not offer named loops combined with loop control. Perl does, and
2507this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2508
7660c0ab
A
2509The C<goto-EXPR> form expects a label name, whose scope will be resolved
2510dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2511necessarily recommended if you're optimizing for maintainability:
2512
2513 goto ("FOO", "BAR", "GLARCH")[$i];
2514
887d89fd
FC
2515As shown in this example, C<goto-EXPR> is exempt from the "looks like a
2516function" rule. A pair of parentheses following it does not (necessarily)
2517delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2518
b500e03b 2519Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2520deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2521go into any construct that requires initialization, such as a
2522subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2523construct that is optimized away.
b500e03b 2524
1b6921cb
BT
2525The C<goto-&NAME> form is quite different from the other forms of
2526C<goto>. In fact, it isn't a goto in the normal sense at all, and
2527doesn't have the stigma associated with other gotos. Instead, it
2528exits the current subroutine (losing any changes set by local()) and
2529immediately calls in its place the named subroutine using the current
2530value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2531load another subroutine and then pretend that the other subroutine had
2532been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2533in the current subroutine are propagated to the other subroutine.)
2534After the C<goto>, not even C<caller> will be able to tell that this
2535routine was called first.
2536
2537NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2538containing a code reference or a block that evaluates to a code
6cb9131c 2539reference.
a0d0e21e
LW
2540
2541=item grep BLOCK LIST
d74e8afc 2542X<grep>
a0d0e21e
LW
2543
2544=item grep EXPR,LIST
2545
2b5ab1e7
TC
2546This is similar in spirit to, but not the same as, grep(1) and its
2547relatives. In particular, it is not limited to using regular expressions.
2f9daede 2548
a0d0e21e 2549Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2550C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2551elements for which the expression evaluated to true. In scalar
2552context, returns the number of times the expression was true.
a0d0e21e
LW
2553
2554 @foo = grep(!/^#/, @bar); # weed out comments
2555
2556or equivalently,
2557
2558 @foo = grep {!/^#/} @bar; # weed out comments
2559
be3174d2
GS
2560Note that C<$_> is an alias to the list value, so it can be used to
2561modify the elements of the LIST. While this is useful and supported,
2562it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2563Similarly, grep returns aliases into the original list, much as a for
2564loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2565element of a list returned by grep (for example, in a C<foreach>, C<map>
2566or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2567This is usually something to be avoided when writing clear code.
a0d0e21e 2568
a4fb8298 2569If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2570been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2571the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2572can't be seen from the outside, avoiding any potential side-effects.
2573
19799a22 2574See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2575
a0d0e21e 2576=item hex EXPR
d74e8afc 2577X<hex> X<hexadecimal>
a0d0e21e 2578
54310121 2579=item hex
bbce6d69 2580
2b5ab1e7 2581Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2582(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2583L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2584
2585 print hex '0xAf'; # prints '175'
2586 print hex 'aF'; # same
a0d0e21e 2587
19799a22 2588Hex strings may only represent integers. Strings that would cause
53305cf1 2589integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11 2590unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 2591L</sprintf>, and L</unpack>.
19799a22 2592
ce2984c3 2593=item import LIST
d74e8afc 2594X<import>
a0d0e21e 2595
19799a22 2596There is no builtin C<import> function. It is just an ordinary
4633a7c4 2597method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2598names to another module. The C<use> function calls the C<import> method
cea6626f 2599for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2600
2601=item index STR,SUBSTR,POSITION
d74e8afc 2602X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2603
2604=item index STR,SUBSTR
2605
2b5ab1e7
TC
2606The index function searches for one string within another, but without
2607the wildcard-like behavior of a full regular-expression pattern match.
2608It returns the position of the first occurrence of SUBSTR in STR at
2609or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2610beginning of the string. POSITION before the beginning of the string
2611or after its end is treated as if it were the beginning or the end,
2612respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2613you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2614is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2615
2616=item int EXPR
f723aae1 2617X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2618
54310121 2619=item int
bbce6d69 2620
7660c0ab 2621Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 2622You should not use this function for rounding: one because it truncates
3b10bc60 2623towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
2624numbers can sometimes produce counterintuitive results. For example,
2625C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2626because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2627the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2628functions will serve you better than will int().
a0d0e21e
LW
2629
2630=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2631X<ioctl>
a0d0e21e 2632
2b5ab1e7 2633Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2634
5ed4f2ec 2635 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2636
a11c483f 2637to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2638exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2639own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2640(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2641may help you in this, but it's nontrivial.) SCALAR will be read and/or
3b10bc60 2642written depending on the FUNCTION; a C pointer to the string value of SCALAR
19799a22 2643will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2644has no string value but does have a numeric value, that value will be
2645passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2646true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2647functions may be needed to manipulate the values of structures used by
b76cc8ba 2648C<ioctl>.
a0d0e21e 2649
19799a22 2650The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e 2651
5ed4f2ec 2652 if OS returns: then Perl returns:
2653 -1 undefined value
2654 0 string "0 but true"
2655 anything else that number
a0d0e21e 2656
19799a22 2657Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2658still easily determine the actual value returned by the operating
2659system:
2660
2b5ab1e7 2661 $retval = ioctl(...) || -1;
a0d0e21e
LW
2662 printf "System returned %d\n", $retval;
2663
be2f7487 2664The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2665about improper numeric conversions.
2666
a0d0e21e 2667=item join EXPR,LIST
d74e8afc 2668X<join>
a0d0e21e 2669
2b5ab1e7
TC
2670Joins the separate strings of LIST into a single string with fields
2671separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2672
2b5ab1e7 2673 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2674
eb6e2d6f
GS
2675Beware that unlike C<split>, C<join> doesn't take a pattern as its
2676first argument. Compare L</split>.
a0d0e21e 2677
532eee96 2678=item keys HASH
d74e8afc 2679X<keys> X<key>
aa689395 2680
532eee96 2681=item keys ARRAY
aeedbbed 2682
f5a93a43
TC
2683=item keys EXPR
2684
aeedbbed
NC
2685Returns a list consisting of all the keys of the named hash, or the indices
2686of an array. (In scalar context, returns the number of keys or indices.)
504f80c1 2687
aeedbbed 2688The keys of a hash are returned in an apparently random order. The actual
3b10bc60 2689random order is subject to change in future versions of Perl, but it
504f80c1 2690is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6 2691function produces (given that the hash has not been modified). Since
c5f61d2f 2692Perl 5.8.1 the ordering can be different even between different runs of
4546b9e6 2693Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2694Attacks">).
504f80c1 2695
8f1da26d 2696As a side effect, calling keys() resets the internal interator of the HASH or ARRAY
cf264981
SP
2697(see L</each>). In particular, calling keys() in void context resets
2698the iterator with no other overhead.
a0d0e21e 2699
aa689395 2700Here is yet another way to print your environment:
a0d0e21e
LW
2701
2702 @keys = keys %ENV;
2703 @values = values %ENV;
b76cc8ba 2704 while (@keys) {
a9a5a0dc 2705 print pop(@keys), '=', pop(@values), "\n";
a0d0e21e
LW
2706 }
2707
2708or how about sorted by key:
2709
2710 foreach $key (sort(keys %ENV)) {
a9a5a0dc 2711 print $key, '=', $ENV{$key}, "\n";
a0d0e21e
LW
2712 }
2713
8ea1e5d4
GS
2714The returned values are copies of the original keys in the hash, so
2715modifying them will not affect the original hash. Compare L</values>.
2716
19799a22 2717To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2718Here's a descending numeric sort of a hash by its values:
4633a7c4 2719
5a964f20 2720 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
a9a5a0dc 2721 printf "%4d %s\n", $hash{$key}, $key;
4633a7c4
LW
2722 }
2723
3b10bc60 2724Used as an lvalue, C<keys> allows you to increase the number of hash buckets
aa689395
PP
2725allocated for the given hash. This can gain you a measure of efficiency if
2726you know the hash is going to get big. (This is similar to pre-extending
2727an array by assigning a larger number to $#array.) If you say
55497cff
PP
2728
2729 keys %hash = 200;
2730
ab192400
GS
2731then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2732in fact, since it rounds up to the next power of two. These
55497cff
PP
2733buckets will be retained even if you do C<%hash = ()>, use C<undef
2734%hash> if you want to free the storage while C<%hash> is still in scope.
2735You can't shrink the number of buckets allocated for the hash using
19799a22 2736C<keys> in this way (but you needn't worry about doing this by accident,
aeedbbed
NC
2737as trying has no effect). C<keys @array> in an lvalue context is a syntax
2738error.
55497cff 2739
f5a93a43
TC
2740Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
2741a reference to an unblessed hash or array. The argument will be
2742dereferenced automatically. This aspect of C<keys> is considered highly
2743experimental. The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
2744
2745 for (keys $hashref) { ... }
2746 for (keys $obj->get_arrayref) { ... }
2747
8f1da26d 2748See also C<each>, C<values>, and C<sort>.
ab192400 2749
b350dd2f 2750=item kill SIGNAL, LIST
d74e8afc 2751X<kill> X<signal>
a0d0e21e 2752
b350dd2f 2753Sends a signal to a list of processes. Returns the number of
517db077
GS
2754processes successfully signaled (which is not necessarily the
2755same as the number actually killed).
a0d0e21e
LW
2756
2757 $cnt = kill 1, $child1, $child2;
2758 kill 9, @goners;
2759
3b10bc60 2760If SIGNAL is zero, no signal is sent to the process, but C<kill>
2761checks whether it's I<possible> to send a signal to it (that
70fb64f6 2762means, to be brief, that the process is owned by the same user, or we are
3b10bc60 2763the super-user). This is useful to check that a child process is still
81fd35db
DN
2764alive (even if only as a zombie) and hasn't changed its UID. See
2765L<perlport> for notes on the portability of this construct.
b350dd2f 2766
e2c0f81f
DG
2767Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2768of processes. That means you usually want to use positive not negative signals.
2769You may also use a signal name in quotes.
2770
2771The behavior of kill when a I<PROCESS> number is zero or negative depends on
2772the operating system. For example, on POSIX-conforming systems, zero will
2773signal the current process group and -1 will signal all processes.
1e9c1022
JL
2774
2775See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2776
2777=item last LABEL
d74e8afc 2778X<last> X<break>
a0d0e21e
LW
2779
2780=item last
2781
2782The C<last> command is like the C<break> statement in C (as used in
2783loops); it immediately exits the loop in question. If the LABEL is
2784omitted, the command refers to the innermost enclosing loop. The
2785C<continue> block, if any, is not executed:
2786
4633a7c4 2787 LINE: while (<STDIN>) {
a9a5a0dc
VP
2788 last LINE if /^$/; # exit when done with header
2789 #...
a0d0e21e
LW
2790 }
2791
80d38338 2792C<last> cannot be used to exit a block that returns a value such as
8f1da26d 2793C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
2b5ab1e7 2794a grep() or map() operation.
4968c1e4 2795
6c1372ed
GS
2796Note that a block by itself is semantically identical to a loop
2797that executes once. Thus C<last> can be used to effect an early
2798exit out of such a block.
2799
98293880
JH
2800See also L</continue> for an illustration of how C<last>, C<next>, and
2801C<redo> work.
1d2dff63 2802
a0d0e21e 2803=item lc EXPR
d74e8afc 2804X<lc> X<lowercase>
a0d0e21e 2805
54310121 2806=item lc
bbce6d69 2807
d1be9408 2808Returns a lowercased version of EXPR. This is the internal function
3980dc9c 2809implementing the C<\L> escape in double-quoted strings.
a0d0e21e 2810
7660c0ab 2811If EXPR is omitted, uses C<$_>.
bbce6d69 2812
3980dc9c
KW
2813What gets returned depends on several factors:
2814
2815=over
2816
2817=item If C<use bytes> is in effect:
2818
2819=over
2820
2821=item On EBCDIC platforms
2822
2823The results are what the C language system call C<tolower()> returns.
2824
2825=item On ASCII platforms
2826
2827The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
2828respectively.
2829
2830=back
2831
2832=item Otherwise, If EXPR has the UTF8 flag set
2833
2834If the current package has a subroutine named C<ToLower>, it will be used to
96090e4f
A
2835change the case
2836(See L<perlunicode/"User-Defined Case Mappings (for serious hackers only)">.)
3980dc9c
KW
2837Otherwise Unicode semantics are used for the case change.
2838
2839=item Otherwise, if C<use locale> is in effect
2840
2841Respects current LC_CTYPE locale. See L<perllocale>.
2842
2843=item Otherwise, if C<use feature 'unicode_strings'> is in effect:
2844
2845Unicode semantics are used for the case change. Any subroutine named
15dbbbab 2846C<ToLower> will be ignored.
3980dc9c
KW
2847
2848=item Otherwise:
2849
2850=over
2851
2852=item On EBCDIC platforms
2853
2854The results are what the C language system call C<tolower()> returns.
2855
2856=item On ASCII platforms
2857
2858ASCII semantics are used for the case change. The lowercase of any character
2859outside the ASCII range is the character itself.
2860
2861=back
2862
2863=back
2864
a0d0e21e 2865=item lcfirst EXPR
d74e8afc 2866X<lcfirst> X<lowercase>
a0d0e21e 2867
54310121 2868=item lcfirst
bbce6d69 2869
ad0029c4
JH
2870Returns the value of EXPR with the first character lowercased. This
2871is the internal function implementing the C<\l> escape in
3980dc9c 2872double-quoted strings.
a0d0e21e 2873
7660c0ab 2874If EXPR is omitted, uses C<$_>.
bbce6d69 2875
15dbbbab 2876This function behaves the same way under various pragmata, such as in a locale,
3980dc9c
KW
2877as L</lc> does.
2878
a0d0e21e 2879=item length EXPR
d74e8afc 2880X<length> X<size>
a0d0e21e 2881
54310121 2882=item length
bbce6d69 2883
974da8e5 2884Returns the length in I<characters> of the value of EXPR. If EXPR is
15dbbbab
FC
2885omitted, returns the length of C<$_>. If EXPR is undefined, returns
2886C<undef>.
3b10bc60 2887
2888This function cannot be used on an entire array or hash to find out how
2889many elements these have. For that, use C<scalar @array> and C<scalar keys
2890%hash>, respectively.
2891
2892Like all Perl character operations, length() normally deals in logical
2893characters, not physical bytes. For how many bytes a string encoded as
2894UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
2895to C<use Encode> first). See L<Encode> and L<perlunicode>.
974da8e5 2896
a0d0e21e 2897=item link OLDFILE,NEWFILE
d74e8afc 2898X<link>
a0d0e21e 2899
19799a22 2900Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2901success, false otherwise.
a0d0e21e
LW
2902
2903=item listen SOCKET,QUEUESIZE
d74e8afc 2904X<listen>
a0d0e21e 2905
3b10bc60 2906Does the same thing that the listen(2) system call does. Returns true if
b76cc8ba 2907it succeeded, false otherwise. See the example in
cea6626f 2908L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2909
2910=item local EXPR
d74e8afc 2911X<local>
a0d0e21e 2912
19799a22 2913You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2914what most people think of as "local". See
13a2d996 2915L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2916
5a964f20
TC
2917A local modifies the listed variables to be local to the enclosing
2918block, file, or eval. If more than one value is listed, the list must
2919be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2920for details, including issues with tied arrays and hashes.
a0d0e21e 2921
d361fafa
VP
2922The C<delete local EXPR> construct can also be used to localize the deletion
2923of array/hash elements to the current block.
2924See L<perlsub/"Localized deletion of elements of composite types">.
2925
a0d0e21e 2926=item localtime EXPR
435fbc73 2927X<localtime> X<ctime>
a0d0e21e 2928
ba053783
AL
2929=item localtime
2930
19799a22 2931Converts a time as returned by the time function to a 9-element list
5f05dabc 2932with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2933follows:
2934
54310121 2935 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2936 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2937 localtime(time);
a0d0e21e 2938
8f1da26d 2939All list elements are numeric and come straight out of the C `struct
ba053783
AL
2940tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2941of the specified time.
48a26b3a 2942
8f1da26d
TC
2943C<$mday> is the day of the month and C<$mon> the month in
2944the range C<0..11>, with 0 indicating January and 11 indicating December.
ba053783 2945This makes it easy to get a month name from a list:
54310121 2946
ba053783
AL
2947 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2948 print "$abbr[$mon] $mday";
2949 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2950
8f1da26d 2951C<$year> is the number of years since 1900, B<not> just the last two digits
ba053783 2952of the year. That is, C<$year> is C<123> in year 2023. The proper way
80d38338 2953to get a 4-digit year is simply:
abd75f24 2954
ba053783 2955 $year += 1900;
abd75f24 2956
435fbc73
GS
2957Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2958to do that, would you?
2959
8f1da26d 2960To get the last two digits of the year (e.g., "01" in 2001) do:
ba053783
AL
2961
2962 $year = sprintf("%02d", $year % 100);
2963
2964C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2965Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2966(or C<0..365> in leap years.)
2967
2968C<$isdst> is true if the specified time occurs during Daylight Saving
2969Time, false otherwise.
abd75f24 2970
e1998452 2971If EXPR is omitted, C<localtime()> uses the current time (as returned
e3176d09 2972by time(3)).
a0d0e21e 2973
48a26b3a 2974In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2975
5f05dabc 2976 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2977
15dbbbab 2978This scalar value is B<not> locale-dependent but is a Perl builtin. For GMT
fe86afc2 2979instead of local time use the L</gmtime> builtin. See also the
8f1da26d 2980C<Time::Local> module (for converting seconds, minutes, hours, and such back to
fe86afc2
NC
2981the integer value returned by time()), and the L<POSIX> module's strftime(3)
2982and mktime(3) functions.
2983
15dbbbab 2984To get somewhat similar but locale-dependent date strings, set up your
fe86afc2
NC
2985locale environment variables appropriately (please see L<perllocale>) and
2986try for example:
a3cb178b 2987
5a964f20 2988 use POSIX qw(strftime);
2b5ab1e7 2989 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2990 # or for GMT formatted appropriately for your locale:
2991 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2992
2993Note that the C<%a> and C<%b>, the short forms of the day of the week
2994and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2995
62aa5637
MS
2996See L<perlport/localtime> for portability concerns.
2997
15dbbbab 2998The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
435fbc73
GS
2999by-name access mechanism to the gmtime() and localtime() functions,
3000respectively.
3001
3002For a comprehensive date and time representation look at the
3003L<DateTime> module on CPAN.
3004
07698885 3005=item lock THING
d74e8afc 3006X<lock>
19799a22 3007
15dbbbab 3008This function places an advisory lock on a shared variable or referenced
03730085 3009object contained in I<THING> until the lock goes out of scope.
a6d5524e 3010
f3a23afb 3011lock() is a "weak keyword" : this means that if you've defined a function
67408cae 3012by this name (before any calls to it), that function will be called
7b043ca5
RGS
3013instead. If you are not under C<use threads::shared> this does nothing.
3014See L<threads::shared>.
19799a22 3015
a0d0e21e 3016=item log EXPR
d74e8afc 3017X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 3018
54310121 3019=item log
bbce6d69 3020
2b5ab1e7 3021Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
15dbbbab
FC
3022returns the log of C<$_>. To get the
3023log of another base, use basic algebra:
19799a22 3024The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
3025divided by the natural log of N. For example:
3026
3027 sub log10 {
a9a5a0dc
VP
3028 my $n = shift;
3029 return log($n)/log(10);
b76cc8ba 3030 }
2b5ab1e7
TC
3031
3032See also L</exp> for the inverse operation.
a0d0e21e 3033
a0d0e21e 3034=item lstat EXPR
d74e8afc 3035X<lstat>
a0d0e21e 3036
54310121 3037=item lstat
bbce6d69 3038
19799a22 3039Does the same thing as the C<stat> function (including setting the
5a964f20
TC
3040special C<_> filehandle) but stats a symbolic link instead of the file
3041the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
3042your system, a normal C<stat> is done. For much more detailed
3043information, please see the documentation for C<stat>.
a0d0e21e 3044
7660c0ab 3045If EXPR is omitted, stats C<$_>.
bbce6d69 3046
a0d0e21e
LW
3047=item m//
3048
9f4b9cd0 3049The match operator. See L<perlop/"Regexp Quote-Like Operators">.
a0d0e21e
LW
3050
3051=item map BLOCK LIST
d74e8afc 3052X<map>
a0d0e21e
LW
3053
3054=item map EXPR,LIST
3055
19799a22
GS
3056Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3057C<$_> to each element) and returns the list value composed of the
3058results of each such evaluation. In scalar context, returns the
3059total number of elements so generated. Evaluates BLOCK or EXPR in
3060list context, so each element of LIST may produce zero, one, or
3061more elements in the returned value.
dd99ebda 3062
f9476272