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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
4a904372 169C<break>, C<continue>, C<default>, C<given>, C<when>
0d863452 170
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171Except for C<continue>, these are available only if you enable the
172C<"switch"> feature or use the C<CORE::> prefix.
8f1da26d 173See L<feature> and L<perlsyn/"Switch statements">.
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174Alternately, include a C<use v5.10> or later to the current scope. In Perl
1755.14 and earlier, C<continue> required the C<"switch"> feature, like the
176other keywords.
0d863452 177
54310121 178=item Keywords related to scoping
cb1a09d0 179
8f1da26d 180C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
36fb85f3 181
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182C<state> is available only if the C<"state"> feature
183is enabled or if it is prefixed with C<CORE::>. See
8f1da26d 184L<feature>. Alternately, include a C<use v5.10> or later to the current scope.
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185
186=item Miscellaneous functions
187
36fb85f3 188C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
834df1c5 189C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
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190
191=item Functions for processes and process groups
d74e8afc 192X<process> X<pid> X<process id>
cb1a09d0 193
22fae026 194C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
1dc8ecb8 195C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 196C<times>, C<wait>, C<waitpid>
cb1a09d0 197
3b10bc60 198=item Keywords related to Perl modules
d74e8afc 199X<module>
cb1a09d0 200
22fae026 201C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 202
353c6505 203=item Keywords related to classes and object-orientation
d74e8afc 204X<object> X<class> X<package>
cb1a09d0 205
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206C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
207C<untie>, C<use>
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208
209=item Low-level socket functions
d74e8afc 210X<socket> X<sock>
cb1a09d0 211
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212C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
213C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 214C<socket>, C<socketpair>
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215
216=item System V interprocess communication functions
d74e8afc 217X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 218
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219C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
220C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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221
222=item Fetching user and group info
d74e8afc 223X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 224
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225C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
226C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
227C<getpwuid>, C<setgrent>, C<setpwent>
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228
229=item Fetching network info
d74e8afc 230X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 231
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232C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
233C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
234C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
235C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
236C<setnetent>, C<setprotoent>, C<setservent>
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237
238=item Time-related functions
d74e8afc 239X<time> X<date>
cb1a09d0 240
22fae026 241C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 242
37798a01 243=item Functions new in perl5
d74e8afc 244X<perl5>
37798a01 245
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246C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
247C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
1dc8ecb8 248C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
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249C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
250C<ucfirst>, C<untie>, C<use>, C<when>
37798a01 251
3b10bc60 252* C<sub> was a keyword in Perl 4, but in Perl 5 it is an
5a964f20 253operator, which can be used in expressions.
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254
255=item Functions obsoleted in perl5
256
22fae026 257C<dbmclose>, C<dbmopen>
37798a01 258
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259=back
260
60f9f73c 261=head2 Portability
d74e8afc 262X<portability> X<Unix> X<portable>
60f9f73c 263
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264Perl was born in Unix and can therefore access all common Unix
265system calls. In non-Unix environments, the functionality of some
8f1da26d 266Unix system calls may not be available or details of the available
2b5ab1e7 267functionality may differ slightly. The Perl functions affected
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268by this are:
269
270C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
271C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
272C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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273C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
274C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 275C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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276C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
277C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
278C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 279C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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280C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
281C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
282C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 283C<shmwrite>, C<socket>, C<socketpair>,
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284C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
285C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 286C<utime>, C<wait>, C<waitpid>
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287
288For more information about the portability of these functions, see
289L<perlport> and other available platform-specific documentation.
290
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291=head2 Alphabetical Listing of Perl Functions
292
3b10bc60 293=over
a0d0e21e 294
5b3c99c0 295=item -X FILEHANDLE
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296X<-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>
297X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 298
5b3c99c0 299=item -X EXPR
a0d0e21e 300
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301=item -X DIRHANDLE
302
5b3c99c0 303=item -X
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304
305A file test, where X is one of the letters listed below. This unary
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306operator takes one argument, either a filename, a filehandle, or a dirhandle,
307and tests the associated file to see if something is true about it. If the
7660c0ab 308argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 309Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 310the undefined value if the file doesn't exist. Despite the funny
d0821a6a 311names, precedence is the same as any other named unary operator. The
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312operator may be any of:
313
5ed4f2ec 314 -r File is readable by effective uid/gid.
315 -w File is writable by effective uid/gid.
316 -x File is executable by effective uid/gid.
317 -o File is owned by effective uid.
a0d0e21e 318
5ed4f2ec 319 -R File is readable by real uid/gid.
320 -W File is writable by real uid/gid.
321 -X File is executable by real uid/gid.
322 -O File is owned by real uid.
a0d0e21e 323
5ed4f2ec 324 -e File exists.
325 -z File has zero size (is empty).
326 -s File has nonzero size (returns size in bytes).
a0d0e21e 327
5ed4f2ec 328 -f File is a plain file.
329 -d File is a directory.
330 -l File is a symbolic link.
331 -p File is a named pipe (FIFO), or Filehandle is a pipe.
332 -S File is a socket.
333 -b File is a block special file.
334 -c File is a character special file.
335 -t Filehandle is opened to a tty.
a0d0e21e 336
5ed4f2ec 337 -u File has setuid bit set.
338 -g File has setgid bit set.
339 -k File has sticky bit set.
a0d0e21e 340
5ed4f2ec 341 -T File is an ASCII text file (heuristic guess).
342 -B File is a "binary" file (opposite of -T).
a0d0e21e 343
5ed4f2ec 344 -M Script start time minus file modification time, in days.
345 -A Same for access time.
346 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 347
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348Example:
349
350 while (<>) {
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351 chomp;
352 next unless -f $_; # ignore specials
353 #...
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354 }
355
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356Note that C<-s/a/b/> does not do a negated substitution. Saying
357C<-exp($foo)> still works as expected, however: only single letters
358following a minus are interpreted as file tests.
359
360These operators are exempt from the "looks like a function rule" described
361above. That is, an opening parenthesis after the operator does not affect
362how much of the following code constitutes the argument. Put the opening
363parentheses before the operator to separate it from code that follows (this
364applies only to operators with higher precedence than unary operators, of
365course):
366
367 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
368 (-s $file) + 1024 # correct
369
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370The interpretation of the file permission operators C<-r>, C<-R>,
371C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
372of the file and the uids and gids of the user. There may be other
ecae030f
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373reasons you can't actually read, write, or execute the file: for
374example network filesystem access controls, ACLs (access control lists),
375read-only filesystems, and unrecognized executable formats. Note
376that the use of these six specific operators to verify if some operation
377is possible is usually a mistake, because it may be open to race
378conditions.
5ff3f7a4 379
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380Also note that, for the superuser on the local filesystems, the C<-r>,
381C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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382if any execute bit is set in the mode. Scripts run by the superuser
383may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 384or temporarily set their effective uid to something else.
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385
386If you are using ACLs, there is a pragma called C<filetest> that may
387produce more accurate results than the bare stat() mode bits.
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388When under C<use filetest 'access'> the above-mentioned filetests
389test whether the permission can(not) be granted using the
3b10bc60 390access(2) family of system calls. Also note that the C<-x> and C<-X> may
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391under this pragma return true even if there are no execute permission
392bits set (nor any extra execute permission ACLs). This strangeness is
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393due to the underlying system calls' definitions. Note also that, due to
394the implementation of C<use filetest 'access'>, the C<_> special
395filehandle won't cache the results of the file tests when this pragma is
396in effect. Read the documentation for the C<filetest> pragma for more
397information.
5ff3f7a4 398
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399The C<-T> and C<-B> switches work as follows. The first block or so of the
400file is examined for odd characters such as strange control codes or
61eff3bc 401characters with the high bit set. If too many strange characters (>30%)
cf264981 402are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 403containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 404or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 405rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 406file, or a file at EOF when testing a filehandle. Because you have to
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407read a file to do the C<-T> test, on most occasions you want to use a C<-f>
408against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 409
5dac7880 410If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 411the special filehandle consisting of a solitary underline, then the stat
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412structure of the previous file test (or stat operator) is used, saving
413a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 414that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 415symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 416an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 417Example:
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418
419 print "Can do.\n" if -r $a || -w _ || -x _;
420
421 stat($filename);
422 print "Readable\n" if -r _;
423 print "Writable\n" if -w _;
424 print "Executable\n" if -x _;
425 print "Setuid\n" if -u _;
426 print "Setgid\n" if -g _;
427 print "Sticky\n" if -k _;
428 print "Text\n" if -T _;
429 print "Binary\n" if -B _;
430
fbb0b3b3
RGS
431As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
432test operators, in a way that C<-f -w -x $file> is equivalent to
3b10bc60 433C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
fbb0b3b3
RGS
434the return value of C<-f $file> as an argument to another filetest
435operator, no special magic will happen.)
436
bee96257 437Portability issues: L<perlport/-X>.
ea9eb35a 438
a0d0e21e 439=item abs VALUE
d74e8afc 440X<abs> X<absolute>
a0d0e21e 441
54310121 442=item abs
bbce6d69 443
a0d0e21e 444Returns the absolute value of its argument.
7660c0ab 445If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
446
447=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 448X<accept>
a0d0e21e 449
3b10bc60 450Accepts an incoming socket connect, just as accept(2)
19799a22 451does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 452See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 453
8d2a6795
GS
454On systems that support a close-on-exec flag on files, the flag will
455be set for the newly opened file descriptor, as determined by the
456value of $^F. See L<perlvar/$^F>.
457
a0d0e21e 458=item alarm SECONDS
d74e8afc
ITB
459X<alarm>
460X<SIGALRM>
461X<timer>
a0d0e21e 462
54310121 463=item alarm
bbce6d69 464
a0d0e21e 465Arranges to have a SIGALRM delivered to this process after the
cf264981 466specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
467specified, the value stored in C<$_> is used. (On some machines,
468unfortunately, the elapsed time may be up to one second less or more
469than you specified because of how seconds are counted, and process
470scheduling may delay the delivery of the signal even further.)
471
472Only one timer may be counting at once. Each call disables the
473previous timer, and an argument of C<0> may be supplied to cancel the
474previous timer without starting a new one. The returned value is the
475amount of time remaining on the previous timer.
a0d0e21e 476
2bc69794
BS
477For delays of finer granularity than one second, the Time::HiRes module
478(from CPAN, and starting from Perl 5.8 part of the standard
479distribution) provides ualarm(). You may also use Perl's four-argument
480version of select() leaving the first three arguments undefined, or you
481might be able to use the C<syscall> interface to access setitimer(2) if
482your system supports it. See L<perlfaq8> for details.
2b5ab1e7 483
80d38338
TC
484It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
485C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 486
19799a22
GS
487If you want to use C<alarm> to time out a system call you need to use an
488C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 489fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 490restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 491modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
492
493 eval {
a9a5a0dc
VP
494 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
495 alarm $timeout;
496 $nread = sysread SOCKET, $buffer, $size;
497 alarm 0;
ff68c719 498 };
ff68c719 499 if ($@) {
a9a5a0dc 500 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 501 # timed out
ff68c719
PP
502 }
503 else {
5ed4f2ec 504 # didn't
ff68c719
PP
505 }
506
91d81acc
JH
507For more information see L<perlipc>.
508
ea9eb35a 509Portability issues: L<perlport/alarm>.
510
a0d0e21e 511=item atan2 Y,X
d74e8afc 512X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
513
514Returns the arctangent of Y/X in the range -PI to PI.
515
ca6e1c26 516For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
517function, or use the familiar relation:
518
519 sub tan { sin($_[0]) / cos($_[0]) }
520
a1021d57
RGS
521The return value for C<atan2(0,0)> is implementation-defined; consult
522your atan2(3) manpage for more information.
bf5f1b4c 523
ea9eb35a 524Portability issues: L<perlport/atan2>.
525
a0d0e21e 526=item bind SOCKET,NAME
d74e8afc 527X<bind>
a0d0e21e 528
3b10bc60 529Binds a network address to a socket, just as bind(2)
19799a22 530does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
531packed address of the appropriate type for the socket. See the examples in
532L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 533
fae2c0fb 534=item binmode FILEHANDLE, LAYER
d74e8afc 535X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 536
a0d0e21e
LW
537=item binmode FILEHANDLE
538
1cbfc93d
NIS
539Arranges for FILEHANDLE to be read or written in "binary" or "text"
540mode on systems where the run-time libraries distinguish between
541binary and text files. If FILEHANDLE is an expression, the value is
542taken as the name of the filehandle. Returns true on success,
b5fe5ca2 543otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 544
8f1da26d 545On some systems (in general, DOS- and Windows-based systems) binmode()
d807c6f4 546is necessary when you're not working with a text file. For the sake
d7a0d798
FC
547of portability it is a good idea always to use it when appropriate,
548and never to use it when it isn't appropriate. Also, people can
8f1da26d 549set their I/O to be by default UTF8-encoded Unicode, not bytes.
d807c6f4
JH
550
551In other words: regardless of platform, use binmode() on binary data,
d7a0d798 552like images, for example.
d807c6f4
JH
553
554If LAYER is present it is a single string, but may contain multiple
3b10bc60 555directives. The directives alter the behaviour of the filehandle.
d7a0d798 556When LAYER is present, using binmode on a text file makes sense.
d807c6f4 557
fae2c0fb 558If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
559suitable for passing binary data. This includes turning off possible CRLF
560translation and marking it as bytes (as opposed to Unicode characters).
749683d2 561Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 562Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
563Other layers that would affect the binary nature of the stream are
564I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 565PERLIO environment variable.
01e6739c 566
3b10bc60 567The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
568form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
569establish default I/O layers. See L<open>.
570
fae2c0fb
RGS
571I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
572in "Programming Perl, 3rd Edition". However, since the publishing of this
573book, by many known as "Camel III", the consensus of the naming of this
574functionality has moved from "discipline" to "layer". All documentation
575of this version of Perl therefore refers to "layers" rather than to
576"disciplines". Now back to the regularly scheduled documentation...>
577
8f1da26d 578To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
6902c96a 579C<:utf8> just marks the data as UTF-8 without further checking,
8f1da26d 580while C<:encoding(UTF-8)> checks the data for actually being valid
6902c96a 581UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 582
ed53a2bb 583In general, binmode() should be called after open() but before any I/O
3b10bc60 584is done on the filehandle. Calling binmode() normally flushes any
01e6739c 585pending buffered output data (and perhaps pending input data) on the
fae2c0fb 586handle. An exception to this is the C<:encoding> layer that
d7a0d798 587changes the default character encoding of the handle; see L</open>.
fae2c0fb 588The C<:encoding> layer sometimes needs to be called in
3874323d
JH
589mid-stream, and it doesn't flush the stream. The C<:encoding>
590also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 591internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 592
19799a22 593The operating system, device drivers, C libraries, and Perl run-time
8f1da26d
TC
594system all conspire to let the programmer treat a single
595character (C<\n>) as the line terminator, irrespective of external
30168b04
GS
596representation. On many operating systems, the native text file
597representation matches the internal representation, but on some
598platforms the external representation of C<\n> is made up of more than
599one character.
600
8f1da26d
TC
601All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
602a single character to end each line in the external representation of text
603(even though that single character is CARRIAGE RETURN on old, pre-Darwin
604flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
605systems like OS/2, DOS, and the various flavors of MS-Windows, your program
606sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
607two characters C<\cM\cJ>. That means that if you don't use binmode() on
608these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
609input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
610output. This is what you want for text files, but it can be disastrous for
611binary files.
30168b04
GS
612
613Another consequence of using binmode() (on some systems) is that
614special end-of-file markers will be seen as part of the data stream.
d7a0d798
FC
615For systems from the Microsoft family this means that, if your binary
616data contain C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
617the file, unless you use binmode().
618
3b10bc60 619binmode() is important not only for readline() and print() operations,
30168b04
GS
620but also when using read(), seek(), sysread(), syswrite() and tell()
621(see L<perlport> for more details). See the C<$/> and C<$\> variables
622in L<perlvar> for how to manually set your input and output
623line-termination sequences.
a0d0e21e 624
ea9eb35a 625Portability issues: L<perlport/binmode>.
626
4633a7c4 627=item bless REF,CLASSNAME
d74e8afc 628X<bless>
a0d0e21e
LW
629
630=item bless REF
631
2b5ab1e7
TC
632This function tells the thingy referenced by REF that it is now an object
633in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 634is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 635it returns the reference for convenience. Always use the two-argument
cf264981
SP
636version if a derived class might inherit the function doing the blessing.
637See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
638of objects.
a0d0e21e 639
57668c4d 640Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 641Namespaces with all lowercase names are considered reserved for
cf264981 642Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
643confusion, you may wish to avoid such package names as well. Make sure
644that CLASSNAME is a true value.
60ad88b8
GS
645
646See L<perlmod/"Perl Modules">.
647
0d863452
RH
648=item break
649
650Break out of a C<given()> block.
651
8f1da26d 652This keyword is enabled by the C<"switch"> feature: see
4a904372
FC
653L<feature> for more information. You can also access it by
654prefixing it with C<CORE::>. Alternately, include a C<use
8f1da26d 655v5.10> or later to the current scope.
0d863452 656
a0d0e21e 657=item caller EXPR
d74e8afc 658X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
659
660=item caller
661
5a964f20 662Returns the context of the current subroutine call. In scalar context,
80d38338
TC
663returns the caller's package name if there I<is> a caller (that is, if
664we're in a subroutine or C<eval> or C<require>) and the undefined value
5a964f20 665otherwise. In list context, returns
a0d0e21e 666
ee6b43cc 667 # 0 1 2
748a9306 668 ($package, $filename, $line) = caller;
a0d0e21e
LW
669
670With EXPR, it returns some extra information that the debugger uses to
671print a stack trace. The value of EXPR indicates how many call frames
672to go back before the current one.
673
ee6b43cc 674 # 0 1 2 3 4
f3aa04c2 675 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 676
677 # 5 6 7 8 9 10
b3ca2e83 678 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 679 = caller($i);
e7ea3e70 680
951ba7fe 681Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 682call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 683C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 684C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 685C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 686$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
687each C<use> statement creates a C<require> frame inside an C<eval EXPR>
688frame.) $subroutine may also be C<(unknown)> if this particular
689subroutine happens to have been deleted from the symbol table.
690C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
691C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
692compiled with. The C<$hints> and C<$bitmask> values are subject to change
693between versions of Perl, and are not meant for external use.
748a9306 694
b3ca2e83
NC
695C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
696caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
697of this hash, as they are the actual values stored in the optree.
698
748a9306 699Furthermore, when called from within the DB package, caller returns more
7660c0ab 700detailed information: it sets the list variable C<@DB::args> to be the
54310121 701arguments with which the subroutine was invoked.
748a9306 702
7660c0ab 703Be aware that the optimizer might have optimized call frames away before
19799a22 704C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 705might not return information about the call frame you expect it to, for
b76cc8ba 706C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 707previous time C<caller> was called.
7660c0ab 708
8f1da26d 709Be aware that setting C<@DB::args> is I<best effort>, intended for
ca9f0cb5
NC
710debugging or generating backtraces, and should not be relied upon. In
711particular, as C<@_> contains aliases to the caller's arguments, Perl does
712not take a copy of C<@_>, so C<@DB::args> will contain modifications the
713subroutine makes to C<@_> or its contents, not the original values at call
714time. C<@DB::args>, like C<@_>, does not hold explicit references to its
715elements, so under certain cases its elements may have become freed and
716reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 717of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
718I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
719reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5
NC
720elements), so C<@DB::args> is actually a hybrid of the current state and
721initial state of C<@_>. Buyer beware.
722
a0d0e21e 723=item chdir EXPR
d74e8afc
ITB
724X<chdir>
725X<cd>
f723aae1 726X<directory, change>
a0d0e21e 727
c4aca7d0
GA
728=item chdir FILEHANDLE
729
730=item chdir DIRHANDLE
731
ce2984c3
PF
732=item chdir
733
ffce7b87 734Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 735changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 736changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0 737variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
80d38338 738neither is set, C<chdir> does nothing. It returns true on success,
b4ad75f0 739false otherwise. See the example under C<die>.
a0d0e21e 740
3b10bc60 741On systems that support fchdir(2), you may pass a filehandle or
34169887 742directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 743passing handles raises an exception.
c4aca7d0 744
a0d0e21e 745=item chmod LIST
d74e8afc 746X<chmod> X<permission> X<mode>
a0d0e21e
LW
747
748Changes the permissions of a list of files. The first element of the
8f1da26d 749list must be the numeric mode, which should probably be an octal
4ad40acf 750number, and which definitely should I<not> be a string of octal digits:
3b10bc60 751C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 752successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 753
3b10bc60 754 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 755 chmod 0755, @executables;
3b10bc60 756 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 757 # --w----r-T
3b10bc60 758 $mode = "0644"; chmod oct($mode), "foo"; # this is better
759 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 760
3b10bc60 761On systems that support fchmod(2), you may pass filehandles among the
762files. On systems that don't support fchmod(2), passing filehandles raises
763an exception. Filehandles must be passed as globs or glob references to be
764recognized; barewords are considered filenames.
c4aca7d0
GA
765
766 open(my $fh, "<", "foo");
767 my $perm = (stat $fh)[2] & 07777;
768 chmod($perm | 0600, $fh);
769
3b10bc60 770You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
771module:
772
3b10bc60 773 use Fcntl qw( :mode );
ca6e1c26 774 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 775 # Identical to the chmod 0755 of the example above.
ca6e1c26 776
ea9eb35a 777Portability issues: L<perlport/chmod>.
778
a0d0e21e 779=item chomp VARIABLE
d74e8afc 780X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 781
313c9f5c 782=item chomp( LIST )
a0d0e21e
LW
783
784=item chomp
785
2b5ab1e7
TC
786This safer version of L</chop> removes any trailing string
787that corresponds to the current value of C<$/> (also known as
28757baa
PP
788$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
789number of characters removed from all its arguments. It's often used to
790remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
791that the final record may be missing its newline. When in paragraph
792mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 793When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 794a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 795remove anything.
19799a22 796If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
797
798 while (<>) {
a9a5a0dc
VP
799 chomp; # avoid \n on last field
800 @array = split(/:/);
801 # ...
a0d0e21e
LW
802 }
803
4bf21a6d
RD
804If VARIABLE is a hash, it chomps the hash's values, but not its keys.
805
a0d0e21e
LW
806You can actually chomp anything that's an lvalue, including an assignment:
807
808 chomp($cwd = `pwd`);
809 chomp($answer = <STDIN>);
810
811If you chomp a list, each element is chomped, and the total number of
812characters removed is returned.
813
15e44fd8
RGS
814Note that parentheses are necessary when you're chomping anything
815that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
816is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
817C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
818C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
819as C<chomp($a, $b)>.
820
a0d0e21e 821=item chop VARIABLE
d74e8afc 822X<chop>
a0d0e21e 823
313c9f5c 824=item chop( LIST )
a0d0e21e
LW
825
826=item chop
827
828Chops off the last character of a string and returns the character
5b3eff12 829chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 830scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
831If VARIABLE is a hash, it chops the hash's values, but not its keys.
832
5b3eff12 833You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
834
835If you chop a list, each element is chopped. Only the value of the
19799a22 836last C<chop> is returned.
a0d0e21e 837
19799a22 838Note that C<chop> returns the last character. To return all but the last
748a9306
LW
839character, use C<substr($string, 0, -1)>.
840
15e44fd8
RGS
841See also L</chomp>.
842
a0d0e21e 843=item chown LIST
d74e8afc 844X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
845
846Changes the owner (and group) of a list of files. The first two
19799a22
GS
847elements of the list must be the I<numeric> uid and gid, in that
848order. A value of -1 in either position is interpreted by most
849systems to leave that value unchanged. Returns the number of files
850successfully changed.
a0d0e21e
LW
851
852 $cnt = chown $uid, $gid, 'foo', 'bar';
853 chown $uid, $gid, @filenames;
854
3b10bc60 855On systems that support fchown(2), you may pass filehandles among the
856files. On systems that don't support fchown(2), passing filehandles raises
857an exception. Filehandles must be passed as globs or glob references to be
858recognized; barewords are considered filenames.
c4aca7d0 859
54310121 860Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
861
862 print "User: ";
19799a22 863 chomp($user = <STDIN>);
5a964f20 864 print "Files: ";
19799a22 865 chomp($pattern = <STDIN>);
a0d0e21e
LW
866
867 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 868 or die "$user not in passwd file";
a0d0e21e 869
5ed4f2ec 870 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
871 chown $uid, $gid, @ary;
872
54310121 873On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
874file unless you're the superuser, although you should be able to change
875the group to any of your secondary groups. On insecure systems, these
876restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
877On POSIX systems, you can detect this condition this way:
878
879 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
880 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 881
ea9eb35a 882Portability issues: L<perlport/chmod>.
883
a0d0e21e 884=item chr NUMBER
d74e8afc 885X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 886
54310121 887=item chr
bbce6d69 888
a0d0e21e 889Returns the character represented by that NUMBER in the character set.
a0ed51b3 890For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 891chr(0x263a) is a Unicode smiley face.
aaa68c4a 892
8a064bd6 893Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 894except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
895(truncated to an integer) are used.
896
974da8e5
JH
897If NUMBER is omitted, uses C<$_>.
898
b76cc8ba 899For the reverse, use L</ord>.
a0d0e21e 900
2575c402
JW
901Note that characters from 128 to 255 (inclusive) are by default
902internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 903
2575c402 904See L<perlunicode> for more about Unicode.
bbce6d69 905
a0d0e21e 906=item chroot FILENAME
d74e8afc 907X<chroot> X<root>
a0d0e21e 908
54310121 909=item chroot
bbce6d69 910
5a964f20 911This function works like the system call by the same name: it makes the
4633a7c4 912named directory the new root directory for all further pathnames that
951ba7fe 913begin with a C</> by your process and all its children. (It doesn't
28757baa 914change your current working directory, which is unaffected.) For security
4633a7c4 915reasons, this call is restricted to the superuser. If FILENAME is
19799a22 916omitted, does a C<chroot> to C<$_>.
a0d0e21e 917
ea9eb35a 918Portability issues: L<perlport/chroot>.
919
a0d0e21e 920=item close FILEHANDLE
d74e8afc 921X<close>
a0d0e21e 922
6a518fbc
TP
923=item close
924
3b10bc60 925Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 926buffers, and closes the system file descriptor. Returns true if those
8f1da26d 927operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
928layer. Closes the currently selected filehandle if the argument is
929omitted.
fb73857a
PP
930
931You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 932another C<open> on it, because C<open> closes it for you. (See
01aa884e 933L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
19799a22 934counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 935
3b10bc60 936If the filehandle came from a piped open, C<close> returns false if one of
937the other syscalls involved fails or if its program exits with non-zero
938status. If the only problem was that the program exited non-zero, C<$!>
939will be set to C<0>. Closing a pipe also waits for the process executing
940on the pipe to exit--in case you wish to look at the output of the pipe
941afterwards--and implicitly puts the exit status value of that command into
942C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 943
2e0cfa16
FC
944If there are multiple threads running, C<close> on a filehandle from a
945piped open returns true without waiting for the child process to terminate,
946if the filehandle is still open in another thread.
947
80d38338
TC
948Closing the read end of a pipe before the process writing to it at the
949other end is done writing results in the writer receiving a SIGPIPE. If
950the other end can't handle that, be sure to read all the data before
951closing the pipe.
73689b13 952
fb73857a 953Example:
a0d0e21e 954
fb73857a
PP
955 open(OUTPUT, '|sort >foo') # pipe to sort
956 or die "Can't start sort: $!";
5ed4f2ec 957 #... # print stuff to output
958 close OUTPUT # wait for sort to finish
fb73857a
PP
959 or warn $! ? "Error closing sort pipe: $!"
960 : "Exit status $? from sort";
5ed4f2ec 961 open(INPUT, 'foo') # get sort's results
fb73857a 962 or die "Can't open 'foo' for input: $!";
a0d0e21e 963
5a964f20 964FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 965filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
966
967=item closedir DIRHANDLE
d74e8afc 968X<closedir>
a0d0e21e 969
19799a22 970Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
971system call.
972
a0d0e21e 973=item connect SOCKET,NAME
d74e8afc 974X<connect>
a0d0e21e 975
80d38338
TC
976Attempts to connect to a remote socket, just like connect(2).
977Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
978packed address of the appropriate type for the socket. See the examples in
979L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 980
cb1a09d0 981=item continue BLOCK
d74e8afc 982X<continue>
cb1a09d0 983
0d863452
RH
984=item continue
985
4a904372
FC
986When followed by a BLOCK, C<continue> is actually a
987flow control statement rather than a function. If
cf264981 988there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
989C<foreach>), it is always executed just before the conditional is about to
990be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
991it can be used to increment a loop variable, even when the loop has been
992continued via the C<next> statement (which is similar to the C C<continue>
993statement).
994
98293880 995C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 996block; C<last> and C<redo> behave as if they had been executed within
19799a22 997the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
998block, it may be more entertaining.
999
1000 while (EXPR) {
a9a5a0dc
VP
1001 ### redo always comes here
1002 do_something;
1d2dff63 1003 } continue {
a9a5a0dc
VP
1004 ### next always comes here
1005 do_something_else;
1006 # then back the top to re-check EXPR
1d2dff63
GS
1007 }
1008 ### last always comes here
1009
3b10bc60 1010Omitting the C<continue> section is equivalent to using an
1011empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1012to check the condition at the top of the loop.
1013
4a904372 1014When there is no BLOCK, C<continue> is a function that
8f1da26d
TC
1015falls through the current C<when> or C<default> block instead of iterating
1016a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
4a904372
FC
1017In Perl 5.14 and earlier, this form of C<continue> was
1018only available when the C<"switch"> feature was enabled.
8f1da26d
TC
1019See L<feature> and L<perlsyn/"Switch statements"> for more
1020information.
0d863452 1021
a0d0e21e 1022=item cos EXPR
d74e8afc 1023X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1024
d6217f1e
GS
1025=item cos
1026
5a964f20 1027Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1028takes the cosine of C<$_>.
a0d0e21e 1029
ca6e1c26 1030For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1031function, or use this relation:
1032
1033 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1034
a0d0e21e 1035=item crypt PLAINTEXT,SALT
d74e8afc 1036X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1037X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1038
ef2e6798
MS
1039Creates a digest string exactly like the crypt(3) function in the C
1040library (assuming that you actually have a version there that has not
bb23f8d1 1041been extirpated as a potential munition).
ef2e6798 1042
34169887 1043crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1044into a short string, called a digest, which is returned. The same
1045PLAINTEXT and SALT will always return the same string, but there is no
1046(known) way to get the original PLAINTEXT from the hash. Small
1047changes in the PLAINTEXT or SALT will result in large changes in the
1048digest.
1049
1050There is no decrypt function. This function isn't all that useful for
1051cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1052mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1053primarily used to check if two pieces of text are the same without
1054having to transmit or store the text itself. An example is checking
1055if a correct password is given. The digest of the password is stored,
cf264981 1056not the password itself. The user types in a password that is
ef2e6798 1057crypt()'d with the same salt as the stored digest. If the two digests
34169887 1058match, the password is correct.
ef2e6798
MS
1059
1060When verifying an existing digest string you should use the digest as
1061the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1062to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1063crypt() will hash the new string with the same salt as the digest.
1064This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1065with more exotic implementations. In other words, assume
1066nothing about the returned string itself nor about how many bytes
1067of SALT may matter.
85c16d83
JH
1068
1069Traditionally the result is a string of 13 bytes: two first bytes of
1070the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
bb23f8d1 1071the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1072hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1073and implementations on non-Unix platforms may produce different
ef2e6798 1074strings.
85c16d83
JH
1075
1076When choosing a new salt create a random two character string whose
1077characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1078'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1079characters is just a recommendation; the characters allowed in
1080the salt depend solely on your system's crypt library, and Perl can't
1081restrict what salts C<crypt()> accepts.
e71965be 1082
a0d0e21e 1083Here's an example that makes sure that whoever runs this program knows
cf264981 1084their password:
a0d0e21e
LW
1085
1086 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1087
1088 system "stty -echo";
1089 print "Password: ";
e71965be 1090 chomp($word = <STDIN>);
a0d0e21e
LW
1091 print "\n";
1092 system "stty echo";
1093
e71965be 1094 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1095 die "Sorry...\n";
a0d0e21e 1096 } else {
a9a5a0dc 1097 print "ok\n";
54310121 1098 }
a0d0e21e 1099
9f8f0c9d 1100Of course, typing in your own password to whoever asks you
748a9306 1101for it is unwise.
a0d0e21e 1102
ef2e6798 1103The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1104of data, not least of all because you can't get the information
ef2e6798 1105back. Look at the L<Digest> module for more robust algorithms.
19799a22 1106
f2791508
JH
1107If using crypt() on a Unicode string (which I<potentially> has
1108characters with codepoints above 255), Perl tries to make sense
34169887 1109of the situation by trying to downgrade (a copy of)
f2791508
JH
1110the string back to an eight-bit byte string before calling crypt()
1111(on that copy). If that works, good. If not, crypt() dies with
1112C<Wide character in crypt>.
85c16d83 1113
ea9eb35a 1114Portability issues: L<perlport/crypt>.
1115
aa689395 1116=item dbmclose HASH
d74e8afc 1117X<dbmclose>
a0d0e21e 1118
19799a22 1119[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1120
aa689395 1121Breaks the binding between a DBM file and a hash.
a0d0e21e 1122
ea9eb35a 1123Portability issues: L<perlport/dbmclose>.
1124
19799a22 1125=item dbmopen HASH,DBNAME,MASK
d74e8afc 1126X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1127
01aa884e
KW
1128[This function has been largely superseded by the
1129L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
a0d0e21e 1130
7b8d334a 1131This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1132hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1133argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1134is the name of the database (without the F<.dir> or F<.pag> extension if
1135any). If the database does not exist, it is created with protection
19799a22 1136specified by MASK (as modified by the C<umask>). If your system supports
80d38338 1137only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1138program. In older versions of Perl, if your system had neither DBM nor
19799a22 1139ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1140sdbm(3).
1141
1142If you don't have write access to the DBM file, you can only read hash
1143variables, not set them. If you want to test whether you can write,
3b10bc60 1144either use file tests or try setting a dummy hash entry inside an C<eval>
1145to trap the error.
a0d0e21e 1146
19799a22
GS
1147Note that functions such as C<keys> and C<values> may return huge lists
1148when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1149function to iterate over large DBM files. Example:
1150
1151 # print out history file offsets
1152 dbmopen(%HIST,'/usr/lib/news/history',0666);
1153 while (($key,$val) = each %HIST) {
a9a5a0dc 1154 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1155 }
1156 dbmclose(%HIST);
1157
cb1a09d0 1158See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1159cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1160rich implementation.
4633a7c4 1161
2b5ab1e7
TC
1162You can control which DBM library you use by loading that library
1163before you call dbmopen():
1164
1165 use DB_File;
1166 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1167 or die "Can't open netscape history file: $!";
2b5ab1e7 1168
ea9eb35a 1169Portability issues: L<perlport/dbmopen>.
1170
8f1da26d
TC
1171=item default BLOCK
1172
1173Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when>
1174that's always true. Only available after Perl 5.10, and only if the
4a904372
FC
1175C<switch> feature has been requested or if the keyword is prefixed with
1176C<CORE::>. See L</when>.
8f1da26d 1177
a0d0e21e 1178=item defined EXPR
d74e8afc 1179X<defined> X<undef> X<undefined>
a0d0e21e 1180
54310121 1181=item defined
bbce6d69 1182
2f9daede 1183Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1184the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1185checked.
1186
1187Many operations return C<undef> to indicate failure, end of file,
1188system error, uninitialized variable, and other exceptional
1189conditions. This function allows you to distinguish C<undef> from
1190other values. (A simple Boolean test will not distinguish among
7660c0ab 1191C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1192false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1193doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1194returns C<undef> when its argument is an empty array, I<or> when the
1195element to return happens to be C<undef>.
1196
f10b0346
GS
1197You may also use C<defined(&func)> to check whether subroutine C<&func>
1198has ever been defined. The return value is unaffected by any forward
80d38338 1199declarations of C<&func>. A subroutine that is not defined
847c7ebe 1200may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1201makes it spring into existence the first time that it is called; see
847c7ebe 1202L<perlsub>.
f10b0346
GS
1203
1204Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1205used to report whether memory for that aggregate had ever been
f10b0346
GS
1206allocated. This behavior may disappear in future versions of Perl.
1207You should instead use a simple test for size:
1208
1209 if (@an_array) { print "has array elements\n" }
1210 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1211
1212When used on a hash element, it tells you whether the value is defined,
dc848c6f 1213not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1214purpose.
a0d0e21e
LW
1215
1216Examples:
1217
8f1da26d 1218 print if defined $switch{D};
a0d0e21e
LW
1219 print "$val\n" while defined($val = pop(@ary));
1220 die "Can't readlink $sym: $!"
a9a5a0dc 1221 unless defined($value = readlink $sym);
a0d0e21e 1222 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1223 $debugging = 0 unless defined $debugging;
a0d0e21e 1224
8f1da26d 1225Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1226discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1227defined values. For example, if you say
a5f75d66
AD
1228
1229 "ab" =~ /a(.*)b/;
1230
80d38338 1231The pattern match succeeds and C<$1> is defined, although it
cf264981 1232matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1233matched something that happened to be zero characters long. This is all
a5f75d66 1234very above-board and honest. When a function returns an undefined value,
2f9daede 1235it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1236should use C<defined> only when questioning the integrity of what
7660c0ab 1237you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1238what you want.
1239
dc848c6f 1240See also L</undef>, L</exists>, L</ref>.
2f9daede 1241
a0d0e21e 1242=item delete EXPR
d74e8afc 1243X<delete>
a0d0e21e 1244
d0a76353
RS
1245Given an expression that specifies an element or slice of a hash, C<delete>
1246deletes the specified elements from that hash so that exists() on that element
1247no longer returns true. Setting a hash element to the undefined value does
1248not remove its key, but deleting it does; see L</exists>.
80d38338 1249
8f1da26d 1250In list context, returns the value or values deleted, or the last such
80d38338 1251element in scalar context. The return list's length always matches that of
d0a76353
RS
1252the argument list: deleting non-existent elements returns the undefined value
1253in their corresponding positions.
80d38338 1254
d0a76353
RS
1255delete() may also be used on arrays and array slices, but its behavior is less
1256straightforward. Although exists() will return false for deleted entries,
1257deleting array elements never changes indices of existing values; use shift()
1258or splice() for that. However, if all deleted elements fall at the end of an
1259array, the array's size shrinks to the position of the highest element that
1260still tests true for exists(), or to 0 if none do.
1261
8f1da26d 1262B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1263be removed in a future version of Perl.
80d38338
TC
1264
1265Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1266a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1267or array may not necessarily return anything; it depends on the implementation
1268of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1269
80d38338
TC
1270The C<delete local EXPR> construct localizes the deletion to the current
1271block at run time. Until the block exits, elements locally deleted
1272temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1273of composite types">.
eba0920a
EM
1274
1275 %hash = (foo => 11, bar => 22, baz => 33);
1276 $scalar = delete $hash{foo}; # $scalar is 11
1277 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1278 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1279
01020589 1280The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1281
5f05dabc 1282 foreach $key (keys %HASH) {
a9a5a0dc 1283 delete $HASH{$key};
a0d0e21e
LW
1284 }
1285
01020589 1286 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1287 delete $ARRAY[$index];
01020589
GS
1288 }
1289
1290And so do these:
5f05dabc 1291
01020589
GS
1292 delete @HASH{keys %HASH};
1293
9740c838 1294 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1295
80d38338
TC
1296But both are slower than assigning the empty list
1297or undefining %HASH or @ARRAY, which is the customary
1298way to empty out an aggregate:
01020589 1299
5ed4f2ec 1300 %HASH = (); # completely empty %HASH
1301 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1302
5ed4f2ec 1303 @ARRAY = (); # completely empty @ARRAY
1304 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1305
80d38338
TC
1306The EXPR can be arbitrarily complicated provided its
1307final operation is an element or slice of an aggregate:
a0d0e21e
LW
1308
1309 delete $ref->[$x][$y]{$key};
5f05dabc 1310 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1311
01020589
GS
1312 delete $ref->[$x][$y][$index];
1313 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1314
a0d0e21e 1315=item die LIST
d74e8afc 1316X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1317
4c050ad5
NC
1318C<die> raises an exception. Inside an C<eval> the error message is stuffed
1319into C<$@> and the C<eval> is terminated with the undefined value.
1320If the exception is outside of all enclosing C<eval>s, then the uncaught
1321exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1322need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1323
1324Equivalent examples:
1325
1326 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1327 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1328
ccac6780 1329If the last element of LIST does not end in a newline, the current
df37ec69
WW
1330script line number and input line number (if any) are also printed,
1331and a newline is supplied. Note that the "input line number" (also
1332known as "chunk") is subject to whatever notion of "line" happens to
1333be currently in effect, and is also available as the special variable
1334C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1335
1336Hint: sometimes appending C<", stopped"> to your message will cause it
1337to make better sense when the string C<"at foo line 123"> is appended.
1338Suppose you are running script "canasta".
a0d0e21e
LW
1339
1340 die "/etc/games is no good";
1341 die "/etc/games is no good, stopped";
1342
1343produce, respectively
1344
1345 /etc/games is no good at canasta line 123.
1346 /etc/games is no good, stopped at canasta line 123.
1347
a96d0188 1348If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1349previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1350This is useful for propagating exceptions:
1351
1352 eval { ... };
1353 die unless $@ =~ /Expected exception/;
1354
a96d0188 1355If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1356C<PROPAGATE> method, that method will be called with additional file
1357and line number parameters. The return value replaces the value in
34169887 1358C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1359were called.
1360
7660c0ab 1361If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1362
4c050ad5
NC
1363If an uncaught exception results in interpreter exit, the exit code is
1364determined from the values of C<$!> and C<$?> with this pseudocode:
1365
1366 exit $! if $!; # errno
1367 exit $? >> 8 if $? >> 8; # child exit status
1368 exit 255; # last resort
1369
1370The intent is to squeeze as much possible information about the likely cause
1371into the limited space of the system exit code. However, as C<$!> is the value
1372of C's C<errno>, which can be set by any system call, this means that the value
1373of the exit code used by C<die> can be non-predictable, so should not be relied
1374upon, other than to be non-zero.
1375
80d38338
TC
1376You can also call C<die> with a reference argument, and if this is trapped
1377within an C<eval>, C<$@> contains that reference. This permits more
1378elaborate exception handling using objects that maintain arbitrary state
1379about the exception. Such a scheme is sometimes preferable to matching
1380particular string values of C<$@> with regular expressions. Because C<$@>
1381is a global variable and C<eval> may be used within object implementations,
1382be careful that analyzing the error object doesn't replace the reference in
1383the global variable. It's easiest to make a local copy of the reference
1384before any manipulations. Here's an example:
52531d10 1385
80d38338 1386 use Scalar::Util "blessed";
da279afe 1387
52531d10 1388 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1389 if (my $ev_err = $@) {
1390 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1391 # handle Some::Module::Exception
1392 }
1393 else {
1394 # handle all other possible exceptions
1395 }
1396 }
1397
3b10bc60 1398Because Perl stringifies uncaught exception messages before display,
80d38338 1399you'll probably want to overload stringification operations on
52531d10
GS
1400exception objects. See L<overload> for details about that.
1401
19799a22
GS
1402You can arrange for a callback to be run just before the C<die>
1403does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1404handler is called with the error text and can change the error
19799a22 1405message, if it sees fit, by calling C<die> again. See
96090e4f 1406L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1407L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1408to be run only right before your program was to exit, this is not
3b10bc60 1409currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1410even inside eval()ed blocks/strings! If one wants the hook to do
1411nothing in such situations, put
fb73857a 1412
5ed4f2ec 1413 die @_ if $^S;
fb73857a 1414
19799a22
GS
1415as the first line of the handler (see L<perlvar/$^S>). Because
1416this promotes strange action at a distance, this counterintuitive
b76cc8ba 1417behavior may be fixed in a future release.
774d564b 1418
4c050ad5
NC
1419See also exit(), warn(), and the Carp module.
1420
a0d0e21e 1421=item do BLOCK
d74e8afc 1422X<do> X<block>
a0d0e21e
LW
1423
1424Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1425sequence of commands indicated by BLOCK. When modified by the C<while> or
1426C<until> loop modifier, executes the BLOCK once before testing the loop
1427condition. (On other statements the loop modifiers test the conditional
1428first.)
a0d0e21e 1429
4968c1e4 1430C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1431C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1432See L<perlsyn> for alternative strategies.
4968c1e4 1433
a0d0e21e 1434=item do SUBROUTINE(LIST)
d74e8afc 1435X<do>
a0d0e21e 1436
51124b83
FC
1437This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
1438a scalar variable or a subroutine beginning with C<&>.
a0d0e21e
LW
1439
1440=item do EXPR
d74e8afc 1441X<do>
a0d0e21e
LW
1442
1443Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1444file as a Perl script.
a0d0e21e
LW
1445
1446 do 'stat.pl';
1447
1448is just like
1449
986b19de 1450 eval `cat stat.pl`;
a0d0e21e 1451
2b5ab1e7 1452except that it's more efficient and concise, keeps track of the current
96090e4f
A
1453filename for error messages, searches the C<@INC> directories, and updates
1454C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1455these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1456cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1457same, however, in that it does reparse the file every time you call it,
1458so you probably don't want to do this inside a loop.
a0d0e21e 1459
8f1da26d 1460If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1461an error message in C<$@>. If C<do> cannot read the file, it returns undef
1462and sets C<$!> to the error. Always check C<$@> first, as compilation
1463could fail in a way that also sets C<$!>. If the file is successfully
1464compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1465
80d38338 1466Inclusion of library modules is better done with the
19799a22 1467C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1468and raise an exception if there's a problem.
a0d0e21e 1469
5a964f20
TC
1470You might like to use C<do> to read in a program configuration
1471file. Manual error checking can be done this way:
1472
b76cc8ba 1473 # read in config files: system first, then user
f86cebdf 1474 for $file ("/share/prog/defaults.rc",
b76cc8ba 1475 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1476 {
1477 unless ($return = do $file) {
1478 warn "couldn't parse $file: $@" if $@;
1479 warn "couldn't do $file: $!" unless defined $return;
1480 warn "couldn't run $file" unless $return;
1481 }
5a964f20
TC
1482 }
1483
a0d0e21e 1484=item dump LABEL
d74e8afc 1485X<dump> X<core> X<undump>
a0d0e21e 1486
1614b0e3
JD
1487=item dump
1488
19799a22
GS
1489This function causes an immediate core dump. See also the B<-u>
1490command-line switch in L<perlrun>, which does the same thing.
1491Primarily this is so that you can use the B<undump> program (not
1492supplied) to turn your core dump into an executable binary after
1493having initialized all your variables at the beginning of the
1494program. When the new binary is executed it will begin by executing
1495a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1496Think of it as a goto with an intervening core dump and reincarnation.
1497If C<LABEL> is omitted, restarts the program from the top.
1498
1499B<WARNING>: Any files opened at the time of the dump will I<not>
1500be open any more when the program is reincarnated, with possible
80d38338 1501resulting confusion by Perl.
19799a22 1502
59f521f4
RGS
1503This function is now largely obsolete, mostly because it's very hard to
1504convert a core file into an executable. That's why you should now invoke
1505it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1506typo.
19799a22 1507
ea9eb35a 1508Portability issues: L<perlport/dump>.
1509
532eee96 1510=item each HASH
d74e8afc 1511X<each> X<hash, iterator>
aa689395 1512
532eee96 1513=item each ARRAY
aeedbbed
NC
1514X<array, iterator>
1515
f5a93a43
TC
1516=item each EXPR
1517
80d38338
TC
1518When called in list context, returns a 2-element list consisting of the key
1519and value for the next element of a hash, or the index and value for the
1520next element of an array, so that you can iterate over it. When called in
1521scalar context, returns only the key (not the value) in a hash, or the index
1522in an array.
2f9daede 1523
aeedbbed 1524Hash entries are returned in an apparently random order. The actual random
3b10bc60 1525order is subject to change in future versions of Perl, but it is
504f80c1 1526guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1527function would produce on the same (unmodified) hash. Since Perl
22883ac5 15285.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1529for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1530
80d38338
TC
1531After C<each> has returned all entries from the hash or array, the next
1532call to C<each> returns the empty list in list context and C<undef> in
1533scalar context. The next call following that one restarts iteration. Each
1534hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1535and C<values>. The iterator is implicitly reset when C<each> has reached
1536the end as just described; it can be explicitly reset by calling C<keys> or
1537C<values> on the hash or array. If you add or delete a hash's elements
1538while iterating over it, entries may be skipped or duplicated--so don't do
1539that. Exception: It is always safe to delete the item most recently
3b10bc60 1540returned by C<each()>, so the following code works properly:
74fc8b5f
MJD
1541
1542 while (($key, $value) = each %hash) {
1543 print $key, "\n";
1544 delete $hash{$key}; # This is safe
1545 }
aa689395 1546
80d38338 1547This prints out your environment like the printenv(1) program,
3b10bc60 1548but in a different order:
a0d0e21e
LW
1549
1550 while (($key,$value) = each %ENV) {
a9a5a0dc 1551 print "$key=$value\n";
a0d0e21e
LW
1552 }
1553
f5a93a43
TC
1554Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1555reference to an unblessed hash or array. The argument will be dereferenced
1556automatically. This aspect of C<each> is considered highly experimental.
1557The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1558
1559 while (($key,$value) = each $hashref) { ... }
1560
8f1da26d 1561See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1562
1563=item eof FILEHANDLE
d74e8afc
ITB
1564X<eof>
1565X<end of file>
1566X<end-of-file>
a0d0e21e 1567
4633a7c4
LW
1568=item eof ()
1569
a0d0e21e
LW
1570=item eof
1571
8f1da26d 1572Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1573FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1574gives the real filehandle. (Note that this function actually
80d38338 1575reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1576interactive context.) Do not read from a terminal file (or call
19799a22 1577C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1578as terminals may lose the end-of-file condition if you do.
1579
820475bd 1580An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1581with empty parentheses is different. It refers to the pseudo file
820475bd 1582formed from the files listed on the command line and accessed via the
61eff3bc
JH
1583C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1584as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1585used will cause C<@ARGV> to be examined to determine if input is
67408cae 1586available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1587end-of-file will assume you are processing another C<@ARGV> list,
1588and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1589see L<perlop/"I/O Operators">.
820475bd 1590
61eff3bc 1591In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1592detect the end of each file, whereas C<eof()> will detect the end
1593of the very last file only. Examples:
a0d0e21e 1594
748a9306
LW
1595 # reset line numbering on each input file
1596 while (<>) {
a9a5a0dc
VP
1597 next if /^\s*#/; # skip comments
1598 print "$.\t$_";
5a964f20 1599 } continue {
a9a5a0dc 1600 close ARGV if eof; # Not eof()!
748a9306
LW
1601 }
1602
a0d0e21e
LW
1603 # insert dashes just before last line of last file
1604 while (<>) {
a9a5a0dc
VP
1605 if (eof()) { # check for end of last file
1606 print "--------------\n";
1607 }
1608 print;
1609 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1610 }
1611
a0d0e21e 1612Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1613input operators typically return C<undef> when they run out of data or
1614encounter an error.
a0d0e21e
LW
1615
1616=item eval EXPR
d74e8afc 1617X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1618X<error, handling> X<exception, handling>
a0d0e21e
LW
1619
1620=item eval BLOCK
1621
ce2984c3
PF
1622=item eval
1623
c7cc6f1c
GS
1624In the first form, the return value of EXPR is parsed and executed as if it
1625were a little Perl program. The value of the expression (which is itself
8f1da26d 1626determined within scalar context) is first parsed, and if there were no
2341804c
DM
1627errors, executed as a block within the lexical context of the current Perl
1628program. This means, that in particular, any outer lexical variables are
1629visible to it, and any package variable settings or subroutine and format
1630definitions remain afterwards.
1631
1632Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1633If EXPR is omitted, evaluates C<$_>. This form is typically used to
1634delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1635
1636In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1637same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1638within the context of the current Perl program. This form is typically
1639used to trap exceptions more efficiently than the first (see below), while
1640also providing the benefit of checking the code within BLOCK at compile
1641time.
1642
1643The final semicolon, if any, may be omitted from the value of EXPR or within
1644the BLOCK.
1645
1646In both forms, the value returned is the value of the last expression
5a964f20 1647evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1648as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1649in void, scalar, or list context, depending on the context of the C<eval>
1650itself. See L</wantarray> for more on how the evaluation context can be
1651determined.
a0d0e21e 1652
19799a22 1653If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1654executed, C<eval> returns C<undef> in scalar context
1f0d8f98
FC
1655or an empty list--or, for syntax errors, a list containing a single
1656undefined value--in list context, and C<$@> is set to the error
1657message. The discrepancy in the return values in list context is
1658considered a bug by some, and will probably be fixed in a future
1659release. If there was no error, C<$@> is guaranteed to be the empty
3b10bc60 1660string. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1661warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1662To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1663turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1664See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1665
19799a22
GS
1666Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1667determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1668is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1669the die operator is used to raise exceptions.
1670
5f1da31c
NT
1671If you want to trap errors when loading an XS module, some problems with
1672the binary interface (such as Perl version skew) may be fatal even with
1673C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1674
a0d0e21e
LW
1675If the code to be executed doesn't vary, you may use the eval-BLOCK
1676form to trap run-time errors without incurring the penalty of
1677recompiling each time. The error, if any, is still returned in C<$@>.
1678Examples:
1679
54310121 1680 # make divide-by-zero nonfatal
a0d0e21e
LW
1681 eval { $answer = $a / $b; }; warn $@ if $@;
1682
1683 # same thing, but less efficient
1684 eval '$answer = $a / $b'; warn $@ if $@;
1685
1686 # a compile-time error
5ed4f2ec 1687 eval { $answer = }; # WRONG
a0d0e21e
LW
1688
1689 # a run-time error
5ed4f2ec 1690 eval '$answer ='; # sets $@
a0d0e21e 1691
cf264981
SP
1692Using the C<eval{}> form as an exception trap in libraries does have some
1693issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1694may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1695You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1696as this example shows:
774d564b 1697
80d38338 1698 # a private exception trap for divide-by-zero
f86cebdf
GS
1699 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1700 warn $@ if $@;
774d564b
PP
1701
1702This is especially significant, given that C<__DIE__> hooks can call
19799a22 1703C<die> again, which has the effect of changing their error messages:
774d564b
PP
1704
1705 # __DIE__ hooks may modify error messages
1706 {
f86cebdf
GS
1707 local $SIG{'__DIE__'} =
1708 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1709 eval { die "foo lives here" };
1710 print $@ if $@; # prints "bar lives here"
774d564b
PP
1711 }
1712
19799a22 1713Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1714may be fixed in a future release.
1715
19799a22 1716With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1717being looked at when:
1718
5ed4f2ec 1719 eval $x; # CASE 1
1720 eval "$x"; # CASE 2
a0d0e21e 1721
5ed4f2ec 1722 eval '$x'; # CASE 3
1723 eval { $x }; # CASE 4
a0d0e21e 1724
5ed4f2ec 1725 eval "\$$x++"; # CASE 5
1726 $$x++; # CASE 6
a0d0e21e 1727
2f9daede 1728Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1729the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1730the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1731and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1732does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1733purely visual reasons, but it also has the advantage of compiling at
1734compile-time instead of at run-time.) Case 5 is a place where
19799a22 1735normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1736particular situation, you can just use symbolic references instead, as
1737in case 6.
a0d0e21e 1738
b6538e4f
TC
1739Before Perl 5.14, the assignment to C<$@> occurred before restoration
1740of localised variables, which means that for your code to run on older
b208c909 1741versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1742errors:
1743
1744 # alter $@ on nefarious repugnancy only
1745 {
1746 my $e;
1747 {
1748 local $@; # protect existing $@
1749 eval { test_repugnancy() };
b208c909 1750 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
8a5a710d
DN
1751 $@ =~ /nefarious/ and $e = $@;
1752 }
1753 die $e if defined $e
1754 }
1755
4968c1e4 1756C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1757C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1758
3b10bc60 1759An C<eval ''> executed within the C<DB> package doesn't see the usual
1760surrounding lexical scope, but rather the scope of the first non-DB piece
1761of code that called it. You don't normally need to worry about this unless
1762you are writing a Perl debugger.
d819b83a 1763
a0d0e21e 1764=item exec LIST
d74e8afc 1765X<exec> X<execute>
a0d0e21e 1766
8bf3b016
GS
1767=item exec PROGRAM LIST
1768
3b10bc60 1769The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1770use C<system> instead of C<exec> if you want it to return. It fails and
1771returns false only if the command does not exist I<and> it is executed
fb73857a 1772directly instead of via your system's command shell (see below).
a0d0e21e 1773
19799a22 1774Since it's a common mistake to use C<exec> instead of C<system>, Perl
80d38338 1775warns you if there is a following statement that isn't C<die>, C<warn>,
3b10bc60 1776or C<exit> (if C<-w> is set--but you always do that, right?). If you
19799a22 1777I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1778can use one of these styles to avoid the warning:
1779
5a964f20
TC
1780 exec ('foo') or print STDERR "couldn't exec foo: $!";
1781 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1782
5a964f20 1783If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1784with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1785If there is only one scalar argument or an array with one element in it,
1786the argument is checked for shell metacharacters, and if there are any,
1787the entire argument is passed to the system's command shell for parsing
1788(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1789If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1790words and passed directly to C<execvp>, which is more efficient.
19799a22 1791Examples:
a0d0e21e 1792
19799a22
GS
1793 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1794 exec "sort $outfile | uniq";
a0d0e21e
LW
1795
1796If you don't really want to execute the first argument, but want to lie
1797to the program you are executing about its own name, you can specify
1798the program you actually want to run as an "indirect object" (without a
1799comma) in front of the LIST. (This always forces interpretation of the
54310121 1800LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1801the list.) Example:
1802
1803 $shell = '/bin/csh';
5ed4f2ec 1804 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1805
1806or, more directly,
1807
5ed4f2ec 1808 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1809
3b10bc60 1810When the arguments get executed via the system shell, results are
1811subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1812for details.
1813
19799a22
GS
1814Using an indirect object with C<exec> or C<system> is also more
1815secure. This usage (which also works fine with system()) forces
1816interpretation of the arguments as a multivalued list, even if the
1817list had just one argument. That way you're safe from the shell
1818expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1819
1820 @args = ( "echo surprise" );
1821
2b5ab1e7 1822 exec @args; # subject to shell escapes
f86cebdf 1823 # if @args == 1
2b5ab1e7 1824 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1825
1826The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1827program, passing it C<"surprise"> an argument. The second version didn't;
1828it tried to run a program named I<"echo surprise">, didn't find it, and set
1829C<$?> to a non-zero value indicating failure.
5a964f20 1830
80d38338 1831Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1832output before the exec, but this may not be supported on some platforms
1833(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1834in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 1835open handles to avoid lost output.
0f897271 1836
80d38338
TC
1837Note that C<exec> will not call your C<END> blocks, nor will it invoke
1838C<DESTROY> methods on your objects.
7660c0ab 1839
ea9eb35a 1840Portability issues: L<perlport/exec>.
1841
a0d0e21e 1842=item exists EXPR
d74e8afc 1843X<exists> X<autovivification>
a0d0e21e 1844
d0a76353
RS
1845Given an expression that specifies an element of a hash, returns true if the
1846specified element in the hash has ever been initialized, even if the
1847corresponding value is undefined.
a0d0e21e 1848
5ed4f2ec 1849 print "Exists\n" if exists $hash{$key};
1850 print "Defined\n" if defined $hash{$key};
01020589
GS
1851 print "True\n" if $hash{$key};
1852
d0a76353 1853exists may also be called on array elements, but its behavior is much less
8f1da26d 1854obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
1855that calling exists on array values is deprecated and likely to be removed in
1856a future version of Perl.
1857
5ed4f2ec 1858 print "Exists\n" if exists $array[$index];
1859 print "Defined\n" if defined $array[$index];
01020589 1860 print "True\n" if $array[$index];
a0d0e21e 1861
8f1da26d 1862A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
1863it exists, but the reverse doesn't necessarily hold true.
1864
afebc493
GS
1865Given an expression that specifies the name of a subroutine,
1866returns true if the specified subroutine has ever been declared, even
1867if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 1868does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
1869exist may still be callable: its package may have an C<AUTOLOAD>
1870method that makes it spring into existence the first time that it is
3b10bc60 1871called; see L<perlsub>.
afebc493 1872
5ed4f2ec 1873 print "Exists\n" if exists &subroutine;
1874 print "Defined\n" if defined &subroutine;
afebc493 1875
a0d0e21e 1876Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1877operation is a hash or array key lookup or subroutine name:
a0d0e21e 1878
5ed4f2ec 1879 if (exists $ref->{A}->{B}->{$key}) { }
1880 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 1881
5ed4f2ec 1882 if (exists $ref->{A}->{B}->[$ix]) { }
1883 if (exists $hash{A}{B}[$ix]) { }
01020589 1884
afebc493
GS
1885 if (exists &{$ref->{A}{B}{$key}}) { }
1886
3b10bc60 1887Although the mostly deeply nested array or hash will not spring into
1888existence just because its existence was tested, any intervening ones will.
61eff3bc 1889Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 1890into existence due to the existence test for the $key element above.
3b10bc60 1891This happens anywhere the arrow operator is used, including even here:
5a964f20 1892
2b5ab1e7 1893 undef $ref;
5ed4f2ec 1894 if (exists $ref->{"Some key"}) { }
1895 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
1896
1897This surprising autovivification in what does not at first--or even
1898second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1899release.
a0d0e21e 1900
afebc493
GS
1901Use of a subroutine call, rather than a subroutine name, as an argument
1902to exists() is an error.
1903
5ed4f2ec 1904 exists &sub; # OK
1905 exists &sub(); # Error
afebc493 1906
a0d0e21e 1907=item exit EXPR
d74e8afc 1908X<exit> X<terminate> X<abort>
a0d0e21e 1909
ce2984c3
PF
1910=item exit
1911
2b5ab1e7 1912Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1913
1914 $ans = <STDIN>;
1915 exit 0 if $ans =~ /^[Xx]/;
1916
19799a22 1917See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1918universally recognized values for EXPR are C<0> for success and C<1>
1919for error; other values are subject to interpretation depending on the
1920environment in which the Perl program is running. For example, exiting
192169 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1922the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1923
19799a22
GS
1924Don't use C<exit> to abort a subroutine if there's any chance that
1925someone might want to trap whatever error happened. Use C<die> instead,
1926which can be trapped by an C<eval>.
28757baa 1927
19799a22 1928The exit() function does not always exit immediately. It calls any
2b5ab1e7 1929defined C<END> routines first, but these C<END> routines may not
19799a22 1930themselves abort the exit. Likewise any object destructors that need to
60275626
FC
1931be called are called before the real exit. C<END> routines and destructors
1932can change the exit status by modifying C<$?>. If this is a problem, you
2b5ab1e7 1933can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1934See L<perlmod> for details.
5a964f20 1935
ea9eb35a 1936Portability issues: L<perlport/exit>.
1937
a0d0e21e 1938=item exp EXPR
d74e8afc 1939X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1940
54310121 1941=item exp
bbce6d69 1942
b76cc8ba 1943Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1944If EXPR is omitted, gives C<exp($_)>.
1945
1946=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1947X<fcntl>
a0d0e21e 1948
f86cebdf 1949Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1950
1951 use Fcntl;
1952
0ade1984 1953first to get the correct constant definitions. Argument processing and
3b10bc60 1954value returned work just like C<ioctl> below.
a0d0e21e
LW
1955For example:
1956
1957 use Fcntl;
5a964f20 1958 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 1959 or die "can't fcntl F_GETFL: $!";
5a964f20 1960
554ad1fc 1961You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1962Like C<ioctl>, it maps a C<0> return from the system call into
1963C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1964in numeric context. It is also exempt from the normal B<-w> warnings
1965on improper numeric conversions.
5a964f20 1966
3b10bc60 1967Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
1968doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1969manpage to learn what functions are available on your system.
a0d0e21e 1970
be2f7487 1971Here's an example of setting a filehandle named C<REMOTE> to be
1972non-blocking at the system level. You'll have to negotiate C<$|>
1973on your own, though.
1974
1975 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1976
1977 $flags = fcntl(REMOTE, F_GETFL, 0)
1978 or die "Can't get flags for the socket: $!\n";
1979
1980 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1981 or die "Can't set flags for the socket: $!\n";
1982
ea9eb35a 1983Portability issues: L<perlport/fcntl>.
1984
a0d0e21e 1985=item fileno FILEHANDLE
d74e8afc 1986X<fileno>
a0d0e21e 1987
2b5ab1e7 1988Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
1989filehandle is not open. If there is no real file descriptor at the OS
1990level, as can happen with filehandles connected to memory objects via
1991C<open> with a reference for the third argument, -1 is returned.
1992
1993This is mainly useful for constructing
19799a22 1994bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1995If FILEHANDLE is an expression, the value is taken as an indirect
1996filehandle, generally its name.
5a964f20 1997
b76cc8ba 1998You can use this to find out whether two handles refer to the
5a964f20
TC
1999same underlying descriptor:
2000
2001 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 2002 print "THIS and THAT are dups\n";
b76cc8ba
NIS
2003 }
2004
a0d0e21e 2005=item flock FILEHANDLE,OPERATION
d74e8afc 2006X<flock> X<lock> X<locking>
a0d0e21e 2007
19799a22
GS
2008Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2009for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2010machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2011C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2012entire files only, not records.
2b5ab1e7
TC
2013
2014Two potentially non-obvious but traditional C<flock> semantics are
2015that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2016are B<merely advisory>. Such discretionary locks are more flexible, but
2017offer fewer guarantees. This means that programs that do not also use
2018C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2019your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2020for details. It's best to assume traditional behavior if you're writing
2021portable programs. (But if you're not, you should as always feel perfectly
2022free to write for your own system's idiosyncrasies (sometimes called
2023"features"). Slavish adherence to portability concerns shouldn't get
2024in the way of your getting your job done.)
a3cb178b 2025
8ebc5c01
PP
2026OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2027LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2028you can use the symbolic names if you import them from the L<Fcntl> module,
2029either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2030requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2031releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2032LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2033waiting for the lock; check the return status to see if you got it.
68dc0745 2034
2b5ab1e7
TC
2035To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2036before locking or unlocking it.
8ebc5c01 2037
f86cebdf 2038Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2039locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2040are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2041implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
2042differing semantics shouldn't bite too many people.
2043
becacb53
TM
2044Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2045be open with read intent to use LOCK_SH and requires that it be open
2046with write intent to use LOCK_EX.
2047
19799a22
GS
2048Note also that some versions of C<flock> cannot lock things over the
2049network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2050that. If you like you can force Perl to ignore your system's flock(2)
2051function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2052the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2053and build a new Perl.
4633a7c4
LW
2054
2055Here's a mailbox appender for BSD systems.
a0d0e21e 2056
7ed5353d 2057 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
2058
2059 sub lock {
a9a5a0dc
VP
2060 my ($fh) = @_;
2061 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2062
a9a5a0dc
VP
2063 # and, in case someone appended while we were waiting...
2064 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2065 }
2066
2067 sub unlock {
a9a5a0dc
VP
2068 my ($fh) = @_;
2069 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2070 }
2071
b0169937 2072 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2073 or die "Can't open mailbox: $!";
a0d0e21e 2074
7ed5353d 2075 lock($mbox);
b0169937 2076 print $mbox $msg,"\n\n";
7ed5353d 2077 unlock($mbox);
a0d0e21e 2078
3b10bc60 2079On systems that support a real flock(2), locks are inherited across fork()
2080calls, whereas those that must resort to the more capricious fcntl(2)
2081function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2082
cb1a09d0 2083See also L<DB_File> for other flock() examples.
a0d0e21e 2084
ea9eb35a 2085Portability issues: L<perlport/flock>.
2086
a0d0e21e 2087=item fork
d74e8afc 2088X<fork> X<child> X<parent>
a0d0e21e 2089
2b5ab1e7
TC
2090Does a fork(2) system call to create a new process running the
2091same program at the same point. It returns the child pid to the
2092parent process, C<0> to the child process, or C<undef> if the fork is
2093unsuccessful. File descriptors (and sometimes locks on those descriptors)
2094are shared, while everything else is copied. On most systems supporting
2095fork(), great care has gone into making it extremely efficient (for
2096example, using copy-on-write technology on data pages), making it the
2097dominant paradigm for multitasking over the last few decades.
5a964f20 2098
80d38338 2099Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
2100output before forking the child process, but this may not be supported
2101on some platforms (see L<perlport>). To be safe, you may need to set
2102C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2103C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2104
19799a22 2105If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2106accumulate zombies. On some systems, you can avoid this by setting
2107C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2108forking and reaping moribund children.
cb1a09d0 2109
28757baa
PP
2110Note that if your forked child inherits system file descriptors like
2111STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2112if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2113backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2114You should reopen those to F</dev/null> if it's any issue.
28757baa 2115
ea9eb35a 2116On some platforms such as Windows, where the fork() system call is not available,
2117Perl can be built to emulate fork() in the Perl interpreter. The emulation is designed to,
2118at the level of the Perl program, be as compatible as possible with the "Unix" fork().
6d17f725 2119However it has limitations that have to be considered in code intended to be portable.
ea9eb35a 2120See L<perlfork> for more details.
2121
2122Portability issues: L<perlport/fork>.
2123
cb1a09d0 2124=item format
d74e8afc 2125X<format>
cb1a09d0 2126
19799a22 2127Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2128example:
2129
54310121 2130 format Something =
a9a5a0dc
VP
2131 Test: @<<<<<<<< @||||| @>>>>>
2132 $str, $%, '$' . int($num)
cb1a09d0
AD
2133 .
2134
2135 $str = "widget";
184e9718 2136 $num = $cost/$quantity;
cb1a09d0
AD
2137 $~ = 'Something';
2138 write;
2139
2140See L<perlform> for many details and examples.
2141
8903cb82 2142=item formline PICTURE,LIST
d74e8afc 2143X<formline>
a0d0e21e 2144
5a964f20 2145This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2146too. It formats (see L<perlform>) a list of values according to the
2147contents of PICTURE, placing the output into the format output
7660c0ab 2148accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2149Eventually, when a C<write> is done, the contents of
cf264981
SP
2150C<$^A> are written to some filehandle. You could also read C<$^A>
2151and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2152does one C<formline> per line of form, but the C<formline> function itself
748a9306 2153doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2154that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2155You may therefore need to use multiple formlines to implement a single
3b10bc60 2156record format, just like the C<format> compiler.
748a9306 2157
19799a22 2158Be careful if you put double quotes around the picture, because an C<@>
748a9306 2159character may be taken to mean the beginning of an array name.
19799a22 2160C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2161
445b09e5
FC
2162If you are trying to use this instead of C<write> to capture the output,
2163you may find it easier to open a filehandle to a scalar
2164(C<< open $fh, ">", \$output >>) and write to that instead.
2165
a0d0e21e 2166=item getc FILEHANDLE
f723aae1 2167X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2168
2169=item getc
2170
2171Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2172or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2173the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2174STDIN. This is not particularly efficient. However, it cannot be
2175used by itself to fetch single characters without waiting for the user
2176to hit enter. For that, try something more like:
4633a7c4
LW
2177
2178 if ($BSD_STYLE) {
a9a5a0dc 2179 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2180 }
2181 else {
a9a5a0dc 2182 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2183 }
2184
2185 $key = getc(STDIN);
2186
2187 if ($BSD_STYLE) {
a9a5a0dc 2188 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2189 }
2190 else {
3b10bc60 2191 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2192 }
2193 print "\n";
2194
54310121
PP
2195Determination of whether $BSD_STYLE should be set
2196is left as an exercise to the reader.
cb1a09d0 2197
19799a22 2198The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2199systems purporting POSIX compliance. See also the C<Term::ReadKey>
a3390c9f 2200module from your nearest CPAN site; details on CPAN can be found under
2b5ab1e7 2201L<perlmodlib/CPAN>.
a0d0e21e
LW
2202
2203=item getlogin
d74e8afc 2204X<getlogin> X<login>
a0d0e21e 2205
cf264981 2206This implements the C library function of the same name, which on most
3b10bc60 2207systems returns the current login from F</etc/utmp>, if any. If it
2208returns the empty string, use C<getpwuid>.
a0d0e21e 2209
f86702cc 2210 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2211
19799a22
GS
2212Do not consider C<getlogin> for authentication: it is not as
2213secure as C<getpwuid>.
4633a7c4 2214
ea9eb35a 2215Portability issues: L<perlport/getlogin>.
2216
a0d0e21e 2217=item getpeername SOCKET
d74e8afc 2218X<getpeername> X<peer>
a0d0e21e 2219
a3390c9f
FC
2220Returns the packed sockaddr address of the other end of the SOCKET
2221connection.
a0d0e21e 2222
4633a7c4
LW
2223 use Socket;
2224 $hersockaddr = getpeername(SOCK);
19799a22 2225 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2226 $herhostname = gethostbyaddr($iaddr, AF_INET);
2227 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2228
2229=item getpgrp PID
d74e8afc 2230X<getpgrp> X<group>
a0d0e21e 2231
47e29363 2232Returns the current process group for the specified PID. Use
7660c0ab 2233a PID of C<0> to get the current process group for the
4633a7c4 2234current process. Will raise an exception if used on a machine that
a3390c9f
FC
2235doesn't implement getpgrp(2). If PID is omitted, returns the process
2236group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2237does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2238
ea9eb35a 2239Portability issues: L<perlport/getpgrp>.
2240
a0d0e21e 2241=item getppid
d74e8afc 2242X<getppid> X<parent> X<pid>
a0d0e21e
LW
2243
2244Returns the process id of the parent process.
2245
4d76a344
RGS
2246Note for Linux users: on Linux, the C functions C<getpid()> and
2247C<getppid()> return different values from different threads. In order to
3b10bc60 2248be portable, this behavior is not reflected by the Perl-level function
4d76a344 2249C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2250to call the underlying C<getppid()>, you may use the CPAN module
2251C<Linux::Pid>.
4d76a344 2252
ea9eb35a 2253Portability issues: L<perlport/getppid>.
2254
a0d0e21e 2255=item getpriority WHICH,WHO
d74e8afc 2256X<getpriority> X<priority> X<nice>
a0d0e21e 2257
4633a7c4 2258Returns the current priority for a process, a process group, or a user.
01aa884e 2259(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2260machine that doesn't implement getpriority(2).
a0d0e21e 2261
ea9eb35a 2262Portability issues: L<perlport/getpriority>.
2263
a0d0e21e 2264=item getpwnam NAME
d74e8afc
ITB
2265X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2266X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2267X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2268X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2269X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2270X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2271
2272=item getgrnam NAME
2273
2274=item gethostbyname NAME
2275
2276=item getnetbyname NAME
2277
2278=item getprotobyname NAME
2279
2280=item getpwuid UID
2281
2282=item getgrgid GID
2283
2284=item getservbyname NAME,PROTO
2285
2286=item gethostbyaddr ADDR,ADDRTYPE
2287
2288=item getnetbyaddr ADDR,ADDRTYPE
2289
2290=item getprotobynumber NUMBER
2291
2292=item getservbyport PORT,PROTO
2293
2294=item getpwent
2295
2296=item getgrent
2297
2298=item gethostent
2299
2300=item getnetent
2301
2302=item getprotoent
2303
2304=item getservent
2305
2306=item setpwent
2307
2308=item setgrent
2309
2310=item sethostent STAYOPEN
2311
2312=item setnetent STAYOPEN
2313
2314=item setprotoent STAYOPEN
2315
2316=item setservent STAYOPEN
2317
2318=item endpwent
2319
2320=item endgrent
2321
2322=item endhostent
2323
2324=item endnetent
2325
2326=item endprotoent
2327
2328=item endservent
2329
80d38338
TC
2330These routines are the same as their counterparts in the
2331system C library. In list context, the return values from the
a0d0e21e
LW
2332various get routines are as follows:
2333
2334 ($name,$passwd,$uid,$gid,
6ee623d5 2335 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2336 ($name,$passwd,$gid,$members) = getgr*
2337 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2338 ($name,$aliases,$addrtype,$net) = getnet*
2339 ($name,$aliases,$proto) = getproto*
2340 ($name,$aliases,$port,$proto) = getserv*
2341
3b10bc60 2342(If the entry doesn't exist you get an empty list.)
a0d0e21e 2343
4602f195
JH
2344The exact meaning of the $gcos field varies but it usually contains
2345the real name of the user (as opposed to the login name) and other
2346information pertaining to the user. Beware, however, that in many
2347system users are able to change this information and therefore it
106325ad 2348cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2349L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2350login shell, are also tainted, for the same reason.
4602f195 2351
5a964f20 2352In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2353lookup by name, in which case you get the other thing, whatever it is.
2354(If the entry doesn't exist you get the undefined value.) For example:
2355
5a964f20
TC
2356 $uid = getpwnam($name);
2357 $name = getpwuid($num);
2358 $name = getpwent();
2359 $gid = getgrnam($name);
08a33e13 2360 $name = getgrgid($num);
5a964f20
TC
2361 $name = getgrent();
2362 #etc.
a0d0e21e 2363
4602f195 2364In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2365in that they are unsupported on many systems. If the
4602f195
JH
2366$quota is unsupported, it is an empty scalar. If it is supported, it
2367usually encodes the disk quota. If the $comment field is unsupported,
2368it is an empty scalar. If it is supported it usually encodes some
2369administrative comment about the user. In some systems the $quota
2370field may be $change or $age, fields that have to do with password
2371aging. In some systems the $comment field may be $class. The $expire
2372field, if present, encodes the expiration period of the account or the
2373password. For the availability and the exact meaning of these fields
8f1da26d 2374in your system, please consult getpwnam(3) and your system's
4602f195
JH
2375F<pwd.h> file. You can also find out from within Perl what your
2376$quota and $comment fields mean and whether you have the $expire field
2377by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2378C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2379files are supported only if your vendor has implemented them in the
4602f195 2380intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2381shadow versions if you're running under privilege or if there exists
cf264981 2382the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2383and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2384facility are unlikely to be supported.
6ee623d5 2385
a3390c9f 2386The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2387the login names of the members of the group.
2388
2389For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2390C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2391C<@addrs> value returned by a successful call is a list of raw
2392addresses returned by the corresponding library call. In the
2393Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2394by saying something like:
2395
f337b084 2396 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2397
2b5ab1e7
TC
2398The Socket library makes this slightly easier:
2399
2400 use Socket;
2401 $iaddr = inet_aton("127.1"); # or whatever address
2402 $name = gethostbyaddr($iaddr, AF_INET);
2403
2404 # or going the other way
19799a22 2405 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2406
d760c846
GS
2407In the opposite way, to resolve a hostname to the IP address
2408you can write this:
2409
2410 use Socket;
2411 $packed_ip = gethostbyname("www.perl.org");
2412 if (defined $packed_ip) {
2413 $ip_address = inet_ntoa($packed_ip);
2414 }
2415
2416Make sure <gethostbyname()> is called in SCALAR context and that
2417its return value is checked for definedness.
2418
19799a22
GS
2419If you get tired of remembering which element of the return list
2420contains which return value, by-name interfaces are provided
2421in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2422C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2423and C<User::grent>. These override the normal built-ins, supplying
2424versions that return objects with the appropriate names
2425for each field. For example:
5a964f20
TC
2426
2427 use File::stat;
2428 use User::pwent;
2429 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2430
a3390c9f 2431Even though it looks as though they're the same method calls (uid),
b76cc8ba 2432they aren't, because a C<File::stat> object is different from
19799a22 2433a C<User::pwent> object.
5a964f20 2434
ea9eb35a 2435Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2436
a0d0e21e 2437=item getsockname SOCKET
d74e8afc 2438X<getsockname>
a0d0e21e 2439
19799a22
GS
2440Returns the packed sockaddr address of this end of the SOCKET connection,
2441in case you don't know the address because you have several different
2442IPs that the connection might have come in on.
a0d0e21e 2443
4633a7c4
LW
2444 use Socket;
2445 $mysockaddr = getsockname(SOCK);
19799a22 2446 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2447 printf "Connect to %s [%s]\n",
19799a22
GS
2448 scalar gethostbyaddr($myaddr, AF_INET),
2449 inet_ntoa($myaddr);
a0d0e21e
LW
2450
2451=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2452X<getsockopt>
a0d0e21e 2453
636e6b1f
TH
2454Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2455Options may exist at multiple protocol levels depending on the socket
2456type, but at least the uppermost socket level SOL_SOCKET (defined in the
2457C<Socket> module) will exist. To query options at another level the
2458protocol number of the appropriate protocol controlling the option
2459should be supplied. For example, to indicate that an option is to be
2460interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2461number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2462
80d38338 2463The function returns a packed string representing the requested socket
3b10bc60 2464option, or C<undef> on error, with the reason for the error placed in
a3390c9f 2465C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2466consult getsockopt(2) for details. A common case is that the option is an
2467integer, in which case the result is a packed integer, which you can decode
2468using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2469
8f1da26d 2470Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2471
4852725b 2472 use Socket qw(:all);
636e6b1f
TH
2473
2474 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2475 or die "Could not determine the protocol number for tcp";
4852725b
DD
2476 # my $tcp = IPPROTO_TCP; # Alternative
2477 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2478 or die "getsockopt TCP_NODELAY: $!";
636e6b1f
TH
2479 my $nodelay = unpack("I", $packed);
2480 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2481
ea9eb35a 2482Portability issues: L<perlport/getsockopt>.
a0d0e21e 2483
15a348aa 2484=item given EXPR BLOCK
2485X<given>
2486
2487=item given BLOCK
2488
2489C<given> is analogous to the C<switch> keyword in other languages. C<given>
2490and C<when> are used in Perl to implement C<switch>/C<case> like statements.
8f1da26d 2491Only available after Perl 5.10. For example:
15a348aa 2492
8f1da26d 2493 use v5.10;
15a348aa 2494 given ($fruit) {
2495 when (/apples?/) {
2496 print "I like apples."
2497 }
2498 when (/oranges?/) {
2499 print "I don't like oranges."
2500 }
2501 default {
2502 print "I don't like anything"
2503 }
2504 }
2505
2506See L<perlsyn/"Switch statements"> for detailed information.
2507
a0d0e21e 2508=item glob EXPR
d74e8afc 2509X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2510
0a753a76
PP
2511=item glob
2512
d9a9d457
JL
2513In list context, returns a (possibly empty) list of filename expansions on
2514the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2515scalar context, glob iterates through such filename expansions, returning
2516undef when the list is exhausted. This is the internal function
2517implementing the C<< <*.c> >> operator, but you can use it directly. If
2518EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2519more detail in L<perlop/"I/O Operators">.
a0d0e21e 2520
80d38338
TC
2521Note that C<glob> splits its arguments on whitespace and treats
2522each segment as separate pattern. As such, C<glob("*.c *.h")>
2523matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2524C<glob(".* *")> matches all files in the current working directory.
80d38338
TC
2525
2526If non-empty braces are the only wildcard characters used in the
2527C<glob>, no filenames are matched, but potentially many strings
2528are returned. For example, this produces nine strings, one for
2529each pairing of fruits and colors:
2530
2531 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2532
3a4b19e4 2533Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2534C<File::Glob> extension. See L<File::Glob> for details, including
2535C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2536
ea9eb35a 2537Portability issues: L<perlport/glob>.
2538
a0d0e21e 2539=item gmtime EXPR
d74e8afc 2540X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2541
ce2984c3
PF
2542=item gmtime
2543
4509d391 2544Works just like L</localtime> but the returned values are
435fbc73 2545localized for the standard Greenwich time zone.
a0d0e21e 2546
a3390c9f
FC
2547Note: When called in list context, $isdst, the last value
2548returned by gmtime, is always C<0>. There is no
435fbc73 2549Daylight Saving Time in GMT.
0a753a76 2550
ea9eb35a 2551Portability issues: L<perlport/gmtime>.
62aa5637 2552
a0d0e21e 2553=item goto LABEL
d74e8afc 2554X<goto> X<jump> X<jmp>
a0d0e21e 2555
748a9306
LW
2556=item goto EXPR
2557
a0d0e21e
LW
2558=item goto &NAME
2559
b500e03b
GG
2560The C<goto-LABEL> form finds the statement labeled with LABEL and
2561resumes execution there. It can't be used to get out of a block or
2562subroutine given to C<sort>. It can be used to go almost anywhere
2563else within the dynamic scope, including out of subroutines, but it's
2564usually better to use some other construct such as C<last> or C<die>.
2565The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2566(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2567does not offer named loops combined with loop control. Perl does, and
2568this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2569
7660c0ab
A
2570The C<goto-EXPR> form expects a label name, whose scope will be resolved
2571dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2572necessarily recommended if you're optimizing for maintainability:
2573
2574 goto ("FOO", "BAR", "GLARCH")[$i];
2575
887d89fd
FC
2576As shown in this example, C<goto-EXPR> is exempt from the "looks like a
2577function" rule. A pair of parentheses following it does not (necessarily)
2578delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2579
b500e03b 2580Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2581deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2582go into any construct that requires initialization, such as a
2583subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2584construct that is optimized away.
b500e03b 2585
1b6921cb
BT
2586The C<goto-&NAME> form is quite different from the other forms of
2587C<goto>. In fact, it isn't a goto in the normal sense at all, and
2588doesn't have the stigma associated with other gotos. Instead, it
2589exits the current subroutine (losing any changes set by local()) and
2590immediately calls in its place the named subroutine using the current
2591value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2592load another subroutine and then pretend that the other subroutine had
2593been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2594in the current subroutine are propagated to the other subroutine.)
2595After the C<goto>, not even C<caller> will be able to tell that this
2596routine was called first.
2597
2598NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2599containing a code reference or a block that evaluates to a code
6cb9131c 2600reference.
a0d0e21e
LW
2601
2602=item grep BLOCK LIST
d74e8afc 2603X<grep>
a0d0e21e
LW
2604
2605=item grep EXPR,LIST
2606
2b5ab1e7
TC
2607This is similar in spirit to, but not the same as, grep(1) and its
2608relatives. In particular, it is not limited to using regular expressions.
2f9daede 2609
a0d0e21e 2610Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2611C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2612elements for which the expression evaluated to true. In scalar
2613context, returns the number of times the expression was true.
a0d0e21e
LW
2614
2615 @foo = grep(!/^#/, @bar); # weed out comments
2616
2617or equivalently,
2618
2619 @foo = grep {!/^#/} @bar; # weed out comments
2620
be3174d2
GS
2621Note that C<$_> is an alias to the list value, so it can be used to
2622modify the elements of the LIST. While this is useful and supported,
2623it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2624Similarly, grep returns aliases into the original list, much as a for
2625loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2626element of a list returned by grep (for example, in a C<foreach>, C<map>
2627or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2628This is usually something to be avoided when writing clear code.
a0d0e21e 2629
a4fb8298 2630If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2631been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2632the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2633can't be seen from the outside, avoiding any potential side-effects.
2634
19799a22 2635See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2636
a0d0e21e 2637=item hex EXPR
d74e8afc 2638X<hex> X<hexadecimal>
a0d0e21e 2639
54310121 2640=item hex
bbce6d69 2641
2b5ab1e7 2642Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2643(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2644L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2645
2646 print hex '0xAf'; # prints '175'
2647 print hex 'aF'; # same
a0d0e21e 2648
19799a22 2649Hex strings may only represent integers. Strings that would cause
53305cf1 2650integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11 2651unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 2652L</sprintf>, and L</unpack>.
19799a22 2653
ce2984c3 2654=item import LIST
d74e8afc 2655X<import>
a0d0e21e 2656
19799a22 2657There is no builtin C<import> function. It is just an ordinary
4633a7c4 2658method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2659names to another module. The C<use> function calls the C<import> method
cea6626f 2660for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2661
2662=item index STR,SUBSTR,POSITION
d74e8afc 2663X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2664
2665=item index STR,SUBSTR
2666
2b5ab1e7
TC
2667The index function searches for one string within another, but without
2668the wildcard-like behavior of a full regular-expression pattern match.
2669It returns the position of the first occurrence of SUBSTR in STR at
2670or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2671beginning of the string. POSITION before the beginning of the string
2672or after its end is treated as if it were the beginning or the end,
2673respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2674you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2675is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2676
2677=item int EXPR
f723aae1 2678X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2679
54310121 2680=item int
bbce6d69 2681
7660c0ab 2682Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 2683You should not use this function for rounding: one because it truncates
3b10bc60 2684towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
2685numbers can sometimes produce counterintuitive results. For example,
2686C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2687because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2688the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2689functions will serve you better than will int().
a0d0e21e
LW
2690
2691=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2692X<ioctl>
a0d0e21e 2693
2b5ab1e7 2694Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2695
5ed4f2ec 2696 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2697
a11c483f 2698to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2699exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2700own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2701(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2702may help you in this, but it's nontrivial.) SCALAR will be read and/or
3b10bc60 2703written depending on the FUNCTION; a C pointer to the string value of SCALAR
19799a22 2704will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2705has no string value but does have a numeric value, that value will be
2706passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2707true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2708functions may be needed to manipulate the values of structures used by
b76cc8ba 2709C<ioctl>.
a0d0e21e 2710
19799a22 2711The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e 2712
5ed4f2ec 2713 if OS returns: then Perl returns:
2714 -1 undefined value
2715 0 string "0 but true"
2716 anything else that number
a0d0e21e 2717
19799a22 2718Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2719still easily determine the actual value returned by the operating
2720system:
2721
2b5ab1e7 2722 $retval = ioctl(...) || -1;
a0d0e21e
LW
2723 printf "System returned %d\n", $retval;
2724
be2f7487 2725The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2726about improper numeric conversions.
2727
ea9eb35a 2728Portability issues: L<perlport/ioctl>.
2729
a0d0e21e 2730=item join EXPR,LIST
d74e8afc 2731X<join>
a0d0e21e 2732
2b5ab1e7
TC
2733Joins the separate strings of LIST into a single string with fields
2734separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2735
2b5ab1e7 2736 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2737
eb6e2d6f
GS
2738Beware that unlike C<split>, C<join> doesn't take a pattern as its
2739first argument. Compare L</split>.
a0d0e21e 2740
532eee96 2741=item keys HASH
d74e8afc 2742X<keys> X<key>
aa689395 2743
532eee96 2744=item keys ARRAY
aeedbbed 2745
f5a93a43
TC
2746=item keys EXPR
2747
aeedbbed
NC
2748Returns a list consisting of all the keys of the named hash, or the indices
2749of an array. (In scalar context, returns the number of keys or indices.)
504f80c1 2750
aeedbbed 2751The keys of a hash are returned in an apparently random order. The actual
3b10bc60 2752random order is subject to change in future versions of Perl, but it
504f80c1 2753is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6 2754function produces (given that the hash has not been modified). Since
c5f61d2f 2755Perl 5.8.1 the ordering can be different even between different runs of
4546b9e6 2756Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2757Attacks">).
504f80c1 2758
8f1da26d 2759As a side effect, calling keys() resets the internal interator of the HASH or ARRAY
cf264981
SP
2760(see L</each>). In particular, calling keys() in void context resets
2761the iterator with no other overhead.
a0d0e21e 2762
aa689395 2763Here is yet another way to print your environment:
a0d0e21e
LW
2764
2765 @keys = keys %ENV;
2766 @values = values %ENV;
b76cc8ba 2767 while (@keys) {
a9a5a0dc 2768 print pop(@keys), '=', pop(@values), "\n";
a0d0e21e
LW
2769 }
2770
2771or how about sorted by key:
2772
2773 foreach $key (sort(keys %ENV)) {
a9a5a0dc 2774 print $key, '=', $ENV{$key}, "\n";
a0d0e21e
LW
2775 }
2776
8ea1e5d4
GS
2777The returned values are copies of the original keys in the hash, so
2778modifying them will not affect the original hash. Compare L</values>.
2779
19799a22 2780To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2781Here's a descending numeric sort of a hash by its values:
4633a7c4 2782
5a964f20 2783 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
a9a5a0dc 2784 printf "%4d %s\n", $hash{$key}, $key;
4633a7c4
LW
2785 }
2786
3b10bc60 2787Used as an lvalue, C<keys> allows you to increase the number of hash buckets
aa689395
PP
2788allocated for the given hash. This can gain you a measure of efficiency if
2789you know the hash is going to get big. (This is similar to pre-extending
2790an array by assigning a larger number to $#array.) If you say
55497cff
PP
2791
2792 keys %hash = 200;
2793
ab192400
GS
2794then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2795in fact, since it rounds up to the next power of two. These
55497cff
PP
2796buckets will be retained even if you do C<%hash = ()>, use C<undef
2797%hash> if you want to free the storage while C<%hash> is still in scope.
2798You can't shrink the number of buckets allocated for the hash using
19799a22 2799C<keys> in this way (but you needn't worry about doing this by accident,
aeedbbed
NC
2800as trying has no effect). C<keys @array> in an lvalue context is a syntax
2801error.
55497cff 2802
f5a93a43
TC
2803Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
2804a reference to an unblessed hash or array. The argument will be
2805dereferenced automatically. This aspect of C<keys> is considered highly
2806experimental. The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
2807
2808 for (keys $hashref) { ... }
2809 for (keys $obj->get_arrayref) { ... }
2810
8f1da26d 2811See also C<each>, C<values>, and C<sort>.
ab192400 2812
b350dd2f 2813=item kill SIGNAL, LIST
d74e8afc 2814X<kill> X<signal>
a0d0e21e 2815
b350dd2f 2816Sends a signal to a list of processes. Returns the number of
517db077
GS
2817processes successfully signaled (which is not necessarily the
2818same as the number actually killed).
a0d0e21e
LW
2819
2820 $cnt = kill 1, $child1, $child2;
2821 kill 9, @goners;
2822
3b10bc60 2823If SIGNAL is zero, no signal is sent to the process, but C<kill>
2824checks whether it's I<possible> to send a signal to it (that
70fb64f6 2825means, to be brief, that the process is owned by the same user, or we are
3b10bc60 2826the super-user). This is useful to check that a child process is still
81fd35db
DN
2827alive (even if only as a zombie) and hasn't changed its UID. See
2828L<perlport> for notes on the portability of this construct.
b350dd2f 2829
e2c0f81f
DG
2830Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2831of processes. That means you usually want to use positive not negative signals.
2832You may also use a signal name in quotes.
2833
2834The behavior of kill when a I<PROCESS> number is zero or negative depends on
2835the operating system. For example, on POSIX-conforming systems, zero will
2836signal the current process group and -1 will signal all processes.
1e9c1022
JL
2837
2838See L<perlipc/"Signals"> for more details.
a0d0e21e 2839
ea9eb35a 2840On some platforms such as Windows where the fork() system call is not available.
2841Perl can be built to emulate fork() at the interpreter level.
6d17f725 2842This emulation has limitations related to kill that have to be considered,
ea9eb35a 2843for code running on Windows and in code intended to be portable.
2844
2845See L<perlfork> for more details.
2846
2847Portability issues: L<perlport/kill>.
2848
a0d0e21e 2849=item last LABEL
d74e8afc 2850X<last> X<break>
a0d0e21e
LW
2851
2852=item last
2853
2854The C<last> command is like the C<break> statement in C (as used in
2855loops); it immediately exits the loop in question. If the LABEL is
2856omitted, the command refers to the innermost enclosing loop. The
2857C<continue> block, if any, is not executed:
2858
4633a7c4 2859 LINE: while (<STDIN>) {
a9a5a0dc
VP
2860 last LINE if /^$/; # exit when done with header
2861 #...
a0d0e21e
LW
2862 }
2863
80d38338 2864C<last> cannot be used to exit a block that returns a value such as
8f1da26d 2865C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
2b5ab1e7 2866a grep() or map() operation.
4968c1e4 2867
6c1372ed
GS
2868Note that a block by itself is semantically identical to a loop
2869that executes once. Thus C<last> can be used to effect an early
2870exit out of such a block.
2871
98293880
JH
2872See also L</continue> for an illustration of how C<last>, C<next>, and
2873C<redo> work.
1d2dff63 2874
a0d0e21e 2875=item lc EXPR
d74e8afc 2876X<lc> X<lowercase>
a0d0e21e 2877
54310121 2878=item lc
bbce6d69 2879
d1be9408 2880Returns a lowercased version of EXPR. This is the internal function
3980dc9c 2881implementing the C<\L> escape in double-quoted strings.
a0d0e21e 2882
7660c0ab 2883If EXPR is omitted, uses C<$_>.
bbce6d69 2884
3980dc9c
KW
2885What gets returned depends on several factors:
2886
2887=over
2888
2889=item If C<use bytes> is in effect:
2890
2891=over
2892
2893=item On EBCDIC platforms
2894
2895The results are what the C language system call C<tolower()> returns.
2896
2897=item On ASCII platforms
2898
2899The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
2900respectively.
2901
2902=back
2903
2904=item Otherwise, If EXPR has the UTF8 flag set
2905
2906If the current package has a subroutine named C<ToLower>, it will be used to
96090e4f
A
2907change the case
2908(See L<perlunicode/"User-Defined Case Mappings (for serious hackers only)">.)
3980dc9c
KW
2909Otherwise Unicode semantics are used for the case change.
2910
2911=item Otherwise, if C<use locale> is in effect
2912
2913Respects current LC_CTYPE locale. See L<perllocale>.
2914
2915=item Otherwise, if C<use feature 'unicode_strings'> is in effect:
2916
2917Unicode semantics are used for the case change. Any subroutine named
15dbbbab 2918C<ToLower> will be ignored.
3980dc9c
KW
2919
2920=item Otherwise:
2921
2922=over
2923
2924=item On EBCDIC platforms
2925
2926The results are what the C language system call C<tolower()> returns.
2927
2928=item On ASCII platforms
2929
2930ASCII semantics are used for the case change. The lowercase of any character
2931outside the ASCII range is the character itself.
2932
2933=back
2934
2935=back
2936
a0d0e21e 2937=item lcfirst EXPR
d74e8afc 2938X<lcfirst> X<lowercase>
a0d0e21e 2939
54310121 2940=item lcfirst
bbce6d69 2941
ad0029c4
JH
2942Returns the value of EXPR with the first character lowercased. This
2943is the internal function implementing the C<\l> escape in
3980dc9c 2944double-quoted strings.
a0d0e21e 2945
7660c0ab 2946If EXPR is omitted, uses C<$_>.
bbce6d69 2947
15dbbbab 2948This function behaves the same way under various pragmata, such as in a locale,
3980dc9c
KW
2949as L</lc> does.
2950
a0d0e21e 2951=item length EXPR
d74e8afc 2952X<length> X<size>
a0d0e21e 2953
54310121 2954=item length
bbce6d69 2955
974da8e5 2956Returns the length in I<characters> of the value of EXPR. If EXPR is
15dbbbab
FC
2957omitted, returns the length of C<$_>. If EXPR is undefined, returns
2958C<undef>.
3b10bc60 2959
2960This function cannot be used on an entire array or hash to find out how
2961many elements these have. For that, use C<scalar @array> and C<scalar keys
2962%hash>, respectively.
2963
2964Like all Perl character operations, length() normally deals in logical
2965characters, not physical bytes. For how many bytes a string encoded as
2966UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
2967to C<use Encode> first). See L<Encode> and L<perlunicode>.
974da8e5 2968
a0d0e21e 2969=item link OLDFILE,NEWFILE
d74e8afc 2970X<link>
a0d0e21e 2971
19799a22 2972Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2973success, false otherwise.
a0d0e21e 2974
ea9eb35a 2975Portability issues: L<perlport/link>.
2976
a0d0e21e 2977=item listen SOCKET,QUEUESIZE
d74e8afc 2978X<listen>
a0d0e21e 2979
3b10bc60 2980Does the same thing that the listen(2) system call does. Returns true if
b76cc8ba 2981it succeeded, false otherwise. See the example in
cea6626f 2982L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2983
2984=item local EXPR
d74e8afc 2985X<local>
a0d0e21e 2986
19799a22 2987You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2988what most people think of as "local". See
13a2d996 2989L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2990
5a964f20
TC
2991A local modifies the listed variables to be local to the enclosing
2992block, file, or eval. If more than one value is listed, the list must
2993be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2994for details, including issues with tied arrays and hashes.
a0d0e21e 2995
d361fafa
VP
2996The C<delete local EXPR> construct can also be used to localize the deletion
2997of array/hash elements to the current block.
2998See L<perlsub/"Localized deletion of elements of composite types">.
2999
a0d0e21e 3000=item localtime EXPR
435fbc73 3001X<localtime> X<ctime>
a0d0e21e 3002
ba053783
AL
3003=item localtime
3004
19799a22 3005Converts a time as returned by the time function to a 9-element list
5f05dabc 3006with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
3007follows:
3008
54310121 3009 # 0 1 2 3 4 5 6 7 8
a0d0e21e 3010 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 3011 localtime(time);
a0d0e21e 3012
8f1da26d 3013All list elements are numeric and come straight out of the C `struct
ba053783
AL
3014tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3015of the specified time.
48a26b3a 3016
8f1da26d
TC
3017C<$mday> is the day of the month and C<$mon> the month in
3018the range C<0..11>, with 0 indicating January and 11 indicating December.
ba053783 3019This makes it easy to get a month name from a list:
54310121 3020
ba053783
AL
3021 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
3022 print "$abbr[$mon] $mday";
3023 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 3024
8f1da26d 3025C<$year> is the number of years since 1900, B<not> just the last two digits
ba053783 3026of the year. That is, C<$year> is C<123> in year 2023. The proper way
80d38338 3027to get a 4-digit year is simply:
abd75f24 3028
ba053783 3029 $year += 1900;
abd75f24 3030
435fbc73
GS
3031Otherwise you create non-Y2K-compliant programs--and you wouldn't want
3032to do that, would you?
3033
8f1da26d 3034To get the last two digits of the year (e.g., "01" in 2001) do:
ba053783
AL
3035
3036 $year = sprintf("%02d", $year % 100);
3037
3038C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3039Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3040(or C<0..365> in leap years.)
3041
3042C<$isdst> is true if the specified time occurs during Daylight Saving
3043Time, false otherwise.
abd75f24 3044
e1998452 3045If EXPR is omitted, C<localtime()> uses the current time (as returned
e3176d09 3046by time(3)).
a0d0e21e 3047
48a26b3a 3048In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 3049
5f05dabc 3050 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 3051
15dbbbab 3052This scalar value is B<not> locale-dependent but is a Perl builtin. For GMT
fe86afc2 3053instead of local time use the L</gmtime> builtin. See also the
8f1da26d 3054C<Time::Local> module (for converting seconds, minutes, hours, and such back to
fe86afc2
NC
3055the integer value returned by time()), and the L<POSIX> module's strftime(3)
3056and mktime(3) functions.
3057
15dbbbab 3058To get somewhat similar but locale-dependent date strings, set up your
fe86afc2
NC
3059locale environment variables appropriately (please see L<perllocale>) and
3060try for example:
a3cb178b 3061
5a964f20 3062 use POSIX qw(strftime);
2b5ab1e7 3063 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;