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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>,
165C<dump>, C<eval>, C<evalbytes> C<exit>,
cfa52385 166C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
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167C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray>
168
169C<__SUB__> is only available with a C<use v5.16> (or higher) declaration or
170with the C<"current_sub"> feature (see L<feature>).
cb1a09d0 171
8f1da26d 172=item Keywords related to the switch feature
0d863452 173
4a904372 174C<break>, C<continue>, C<default>, C<given>, C<when>
0d863452 175
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176Except for C<continue>, these are available only if you enable the
177C<"switch"> feature or use the C<CORE::> prefix.
8f1da26d 178See L<feature> and L<perlsyn/"Switch statements">.
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179Alternately, include a C<use v5.10> or later to the current scope. In Perl
1805.14 and earlier, C<continue> required the C<"switch"> feature, like the
181other keywords.
0d863452 182
54310121 183=item Keywords related to scoping
cb1a09d0 184
8f1da26d 185C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
36fb85f3 186
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187C<state> is available only if the C<"state"> feature
188is enabled or if it is prefixed with C<CORE::>. See
8f1da26d 189L<feature>. Alternately, include a C<use v5.10> or later to the current scope.
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190
191=item Miscellaneous functions
192
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193C<defined>, C<dump>, C<eval>, C<evalbytes>,
194C<formline>, C<local>, C<my>, C<our>,
834df1c5 195C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
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196
197=item Functions for processes and process groups
d74e8afc 198X<process> X<pid> X<process id>
cb1a09d0 199
22fae026 200C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
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201C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>,
202C<setpriority>, C<sleep>, C<system>,
22fae026 203C<times>, C<wait>, C<waitpid>
cb1a09d0 204
3b10bc60 205=item Keywords related to Perl modules
d74e8afc 206X<module>
cb1a09d0 207
22fae026 208C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 209
353c6505 210=item Keywords related to classes and object-orientation
d74e8afc 211X<object> X<class> X<package>
cb1a09d0 212
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213C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
214C<untie>, C<use>
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215
216=item Low-level socket functions
d74e8afc 217X<socket> X<sock>
cb1a09d0 218
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219C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
220C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 221C<socket>, C<socketpair>
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222
223=item System V interprocess communication functions
d74e8afc 224X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 225
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226C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
227C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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228
229=item Fetching user and group info
d74e8afc 230X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 231
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232C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
233C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
234C<getpwuid>, C<setgrent>, C<setpwent>
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235
236=item Fetching network info
d74e8afc 237X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 238
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239C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
240C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
241C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
242C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
243C<setnetent>, C<setprotoent>, C<setservent>
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244
245=item Time-related functions
d74e8afc 246X<time> X<date>
cb1a09d0 247
22fae026 248C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 249
37798a01 250=item Functions new in perl5
d74e8afc 251X<perl5>
37798a01 252
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253C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
254C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
1dc8ecb8 255C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
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256C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
257C<ucfirst>, C<untie>, C<use>, C<when>
37798a01 258
3b10bc60 259* C<sub> was a keyword in Perl 4, but in Perl 5 it is an
5a964f20 260operator, which can be used in expressions.
37798a01 261
262=item Functions obsoleted in perl5
263
22fae026 264C<dbmclose>, C<dbmopen>
37798a01 265
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266=back
267
60f9f73c 268=head2 Portability
d74e8afc 269X<portability> X<Unix> X<portable>
60f9f73c 270
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271Perl was born in Unix and can therefore access all common Unix
272system calls. In non-Unix environments, the functionality of some
8f1da26d 273Unix system calls may not be available or details of the available
2b5ab1e7 274functionality may differ slightly. The Perl functions affected
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275by this are:
276
277C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
278C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
279C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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280C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
281C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 282C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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283C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
284C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
285C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 286C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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287C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
288C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
289C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 290C<shmwrite>, C<socket>, C<socketpair>,
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291C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
292C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 293C<utime>, C<wait>, C<waitpid>
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294
295For more information about the portability of these functions, see
296L<perlport> and other available platform-specific documentation.
297
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298=head2 Alphabetical Listing of Perl Functions
299
3b10bc60 300=over
a0d0e21e 301
5b3c99c0 302=item -X FILEHANDLE
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303X<-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>
304X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 305
5b3c99c0 306=item -X EXPR
a0d0e21e 307
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308=item -X DIRHANDLE
309
5b3c99c0 310=item -X
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311
312A file test, where X is one of the letters listed below. This unary
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313operator takes one argument, either a filename, a filehandle, or a dirhandle,
314and tests the associated file to see if something is true about it. If the
7660c0ab 315argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 316Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 317the undefined value if the file doesn't exist. Despite the funny
d0821a6a 318names, precedence is the same as any other named unary operator. The
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319operator may be any of:
320
5ed4f2ec 321 -r File is readable by effective uid/gid.
322 -w File is writable by effective uid/gid.
323 -x File is executable by effective uid/gid.
324 -o File is owned by effective uid.
a0d0e21e 325
5ed4f2ec 326 -R File is readable by real uid/gid.
327 -W File is writable by real uid/gid.
328 -X File is executable by real uid/gid.
329 -O File is owned by real uid.
a0d0e21e 330
5ed4f2ec 331 -e File exists.
332 -z File has zero size (is empty).
333 -s File has nonzero size (returns size in bytes).
a0d0e21e 334
5ed4f2ec 335 -f File is a plain file.
336 -d File is a directory.
337 -l File is a symbolic link.
338 -p File is a named pipe (FIFO), or Filehandle is a pipe.
339 -S File is a socket.
340 -b File is a block special file.
341 -c File is a character special file.
342 -t Filehandle is opened to a tty.
a0d0e21e 343
5ed4f2ec 344 -u File has setuid bit set.
345 -g File has setgid bit set.
346 -k File has sticky bit set.
a0d0e21e 347
5ed4f2ec 348 -T File is an ASCII text file (heuristic guess).
349 -B File is a "binary" file (opposite of -T).
a0d0e21e 350
5ed4f2ec 351 -M Script start time minus file modification time, in days.
352 -A Same for access time.
353 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 354
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355Example:
356
357 while (<>) {
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358 chomp;
359 next unless -f $_; # ignore specials
360 #...
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361 }
362
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363Note that C<-s/a/b/> does not do a negated substitution. Saying
364C<-exp($foo)> still works as expected, however: only single letters
365following a minus are interpreted as file tests.
366
367These operators are exempt from the "looks like a function rule" described
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368above. That is, an opening parenthesis after the operator does not affect
369how much of the following code constitutes the argument. Put the opening
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370parentheses before the operator to separate it from code that follows (this
371applies only to operators with higher precedence than unary operators, of
372course):
373
374 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
375 (-s $file) + 1024 # correct
376
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377The interpretation of the file permission operators C<-r>, C<-R>,
378C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
379of the file and the uids and gids of the user. There may be other
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380reasons you can't actually read, write, or execute the file: for
381example network filesystem access controls, ACLs (access control lists),
382read-only filesystems, and unrecognized executable formats. Note
383that the use of these six specific operators to verify if some operation
384is possible is usually a mistake, because it may be open to race
385conditions.
5ff3f7a4 386
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387Also note that, for the superuser on the local filesystems, the C<-r>,
388C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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389if any execute bit is set in the mode. Scripts run by the superuser
390may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 391or temporarily set their effective uid to something else.
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392
393If you are using ACLs, there is a pragma called C<filetest> that may
394produce more accurate results than the bare stat() mode bits.
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395When under C<use filetest 'access'> the above-mentioned filetests
396test whether the permission can(not) be granted using the
3b10bc60 397access(2) family of system calls. Also note that the C<-x> and C<-X> may
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398under this pragma return true even if there are no execute permission
399bits set (nor any extra execute permission ACLs). This strangeness is
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400due to the underlying system calls' definitions. Note also that, due to
401the implementation of C<use filetest 'access'>, the C<_> special
402filehandle won't cache the results of the file tests when this pragma is
403in effect. Read the documentation for the C<filetest> pragma for more
404information.
5ff3f7a4 405
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406The C<-T> and C<-B> switches work as follows. The first block or so of the
407file is examined for odd characters such as strange control codes or
61eff3bc 408characters with the high bit set. If too many strange characters (>30%)
cf264981 409are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 410containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 411or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 412rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 413file, or a file at EOF when testing a filehandle. Because you have to
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414read a file to do the C<-T> test, on most occasions you want to use a C<-f>
415against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 416
5dac7880 417If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 418the special filehandle consisting of a solitary underline, then the stat
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419structure of the previous file test (or stat operator) is used, saving
420a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 421that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 422symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 423an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 424Example:
a0d0e21e
LW
425
426 print "Can do.\n" if -r $a || -w _ || -x _;
427
428 stat($filename);
429 print "Readable\n" if -r _;
430 print "Writable\n" if -w _;
431 print "Executable\n" if -x _;
432 print "Setuid\n" if -u _;
433 print "Setgid\n" if -g _;
434 print "Sticky\n" if -k _;
435 print "Text\n" if -T _;
436 print "Binary\n" if -B _;
437
fbb0b3b3
RGS
438As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
439test operators, in a way that C<-f -w -x $file> is equivalent to
3b10bc60 440C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
fbb0b3b3
RGS
441the return value of C<-f $file> as an argument to another filetest
442operator, no special magic will happen.)
443
bee96257 444Portability issues: L<perlport/-X>.
ea9eb35a 445
a0d0e21e 446=item abs VALUE
d74e8afc 447X<abs> X<absolute>
a0d0e21e 448
54310121 449=item abs
bbce6d69 450
a0d0e21e 451Returns the absolute value of its argument.
7660c0ab 452If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
453
454=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 455X<accept>
a0d0e21e 456
3b10bc60 457Accepts an incoming socket connect, just as accept(2)
19799a22 458does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 459See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 460
8d2a6795
GS
461On systems that support a close-on-exec flag on files, the flag will
462be set for the newly opened file descriptor, as determined by the
463value of $^F. See L<perlvar/$^F>.
464
a0d0e21e 465=item alarm SECONDS
d74e8afc
ITB
466X<alarm>
467X<SIGALRM>
468X<timer>
a0d0e21e 469
54310121 470=item alarm
bbce6d69 471
a0d0e21e 472Arranges to have a SIGALRM delivered to this process after the
cf264981 473specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
474specified, the value stored in C<$_> is used. (On some machines,
475unfortunately, the elapsed time may be up to one second less or more
476than you specified because of how seconds are counted, and process
477scheduling may delay the delivery of the signal even further.)
478
479Only one timer may be counting at once. Each call disables the
480previous timer, and an argument of C<0> may be supplied to cancel the
481previous timer without starting a new one. The returned value is the
482amount of time remaining on the previous timer.
a0d0e21e 483
2bc69794
BS
484For delays of finer granularity than one second, the Time::HiRes module
485(from CPAN, and starting from Perl 5.8 part of the standard
486distribution) provides ualarm(). You may also use Perl's four-argument
487version of select() leaving the first three arguments undefined, or you
488might be able to use the C<syscall> interface to access setitimer(2) if
489your system supports it. See L<perlfaq8> for details.
2b5ab1e7 490
80d38338
TC
491It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
492C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 493
19799a22
GS
494If you want to use C<alarm> to time out a system call you need to use an
495C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 496fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 497restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 498modulo the caveats given in L<perlipc/"Signals">.
ff68c719 499
500 eval {
a9a5a0dc
VP
501 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
502 alarm $timeout;
503 $nread = sysread SOCKET, $buffer, $size;
504 alarm 0;
ff68c719 505 };
ff68c719 506 if ($@) {
a9a5a0dc 507 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 508 # timed out
ff68c719 509 }
510 else {
5ed4f2ec 511 # didn't
ff68c719 512 }
513
91d81acc
JH
514For more information see L<perlipc>.
515
ea9eb35a
BJ
516Portability issues: L<perlport/alarm>.
517
a0d0e21e 518=item atan2 Y,X
d74e8afc 519X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
520
521Returns the arctangent of Y/X in the range -PI to PI.
522
ca6e1c26 523For the tangent operation, you may use the C<Math::Trig::tan>
28757baa 524function, or use the familiar relation:
525
526 sub tan { sin($_[0]) / cos($_[0]) }
527
a1021d57
RGS
528The return value for C<atan2(0,0)> is implementation-defined; consult
529your atan2(3) manpage for more information.
bf5f1b4c 530
ea9eb35a
BJ
531Portability issues: L<perlport/atan2>.
532
a0d0e21e 533=item bind SOCKET,NAME
d74e8afc 534X<bind>
a0d0e21e 535
3b10bc60 536Binds a network address to a socket, just as bind(2)
19799a22 537does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
538packed address of the appropriate type for the socket. See the examples in
539L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 540
fae2c0fb 541=item binmode FILEHANDLE, LAYER
d74e8afc 542X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 543
a0d0e21e
LW
544=item binmode FILEHANDLE
545
1cbfc93d
NIS
546Arranges for FILEHANDLE to be read or written in "binary" or "text"
547mode on systems where the run-time libraries distinguish between
548binary and text files. If FILEHANDLE is an expression, the value is
549taken as the name of the filehandle. Returns true on success,
b5fe5ca2 550otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 551
8f1da26d 552On some systems (in general, DOS- and Windows-based systems) binmode()
d807c6f4 553is necessary when you're not working with a text file. For the sake
d7a0d798
FC
554of portability it is a good idea always to use it when appropriate,
555and never to use it when it isn't appropriate. Also, people can
8f1da26d 556set their I/O to be by default UTF8-encoded Unicode, not bytes.
d807c6f4
JH
557
558In other words: regardless of platform, use binmode() on binary data,
d7a0d798 559like images, for example.
d807c6f4
JH
560
561If LAYER is present it is a single string, but may contain multiple
3b10bc60 562directives. The directives alter the behaviour of the filehandle.
d7a0d798 563When LAYER is present, using binmode on a text file makes sense.
d807c6f4 564
fae2c0fb 565If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
566suitable for passing binary data. This includes turning off possible CRLF
567translation and marking it as bytes (as opposed to Unicode characters).
749683d2 568Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 569Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
570Other layers that would affect the binary nature of the stream are
571I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 572PERLIO environment variable.
01e6739c 573
3b10bc60 574The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
575form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
576establish default I/O layers. See L<open>.
577
fae2c0fb
RGS
578I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
579in "Programming Perl, 3rd Edition". However, since the publishing of this
580book, by many known as "Camel III", the consensus of the naming of this
581functionality has moved from "discipline" to "layer". All documentation
582of this version of Perl therefore refers to "layers" rather than to
583"disciplines". Now back to the regularly scheduled documentation...>
584
8f1da26d 585To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
6902c96a 586C<:utf8> just marks the data as UTF-8 without further checking,
8f1da26d 587while C<:encoding(UTF-8)> checks the data for actually being valid
6902c96a 588UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 589
ed53a2bb 590In general, binmode() should be called after open() but before any I/O
3b10bc60 591is done on the filehandle. Calling binmode() normally flushes any
01e6739c 592pending buffered output data (and perhaps pending input data) on the
fae2c0fb 593handle. An exception to this is the C<:encoding> layer that
d7a0d798 594changes the default character encoding of the handle; see L</open>.
fae2c0fb 595The C<:encoding> layer sometimes needs to be called in
3874323d
JH
596mid-stream, and it doesn't flush the stream. The C<:encoding>
597also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 598internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 599
19799a22 600The operating system, device drivers, C libraries, and Perl run-time
8f1da26d
TC
601system all conspire to let the programmer treat a single
602character (C<\n>) as the line terminator, irrespective of external
30168b04
GS
603representation. On many operating systems, the native text file
604representation matches the internal representation, but on some
605platforms the external representation of C<\n> is made up of more than
606one character.
607
8f1da26d
TC
608All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
609a single character to end each line in the external representation of text
610(even though that single character is CARRIAGE RETURN on old, pre-Darwin
611flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
612systems like OS/2, DOS, and the various flavors of MS-Windows, your program
613sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
614two characters C<\cM\cJ>. That means that if you don't use binmode() on
615these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
616input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
617output. This is what you want for text files, but it can be disastrous for
618binary files.
30168b04
GS
619
620Another consequence of using binmode() (on some systems) is that
621special end-of-file markers will be seen as part of the data stream.
d7a0d798
FC
622For systems from the Microsoft family this means that, if your binary
623data contain C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
624the file, unless you use binmode().
625
3b10bc60 626binmode() is important not only for readline() and print() operations,
30168b04
GS
627but also when using read(), seek(), sysread(), syswrite() and tell()
628(see L<perlport> for more details). See the C<$/> and C<$\> variables
629in L<perlvar> for how to manually set your input and output
630line-termination sequences.
a0d0e21e 631
ea9eb35a
BJ
632Portability issues: L<perlport/binmode>.
633
4633a7c4 634=item bless REF,CLASSNAME
d74e8afc 635X<bless>
a0d0e21e
LW
636
637=item bless REF
638
2b5ab1e7
TC
639This function tells the thingy referenced by REF that it is now an object
640in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 641is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 642it returns the reference for convenience. Always use the two-argument
cf264981 643version if a derived class might inherit the function doing the blessing.
82e1c0d9 644SeeL<perlobj> for more about the blessing (and blessings) of objects.
a0d0e21e 645
57668c4d 646Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 647Namespaces with all lowercase names are considered reserved for
cf264981 648Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
649confusion, you may wish to avoid such package names as well. Make sure
650that CLASSNAME is a true value.
60ad88b8
GS
651
652See L<perlmod/"Perl Modules">.
653
0d863452
RH
654=item break
655
656Break out of a C<given()> block.
657
8f1da26d 658This keyword is enabled by the C<"switch"> feature: see
4a904372
FC
659L<feature> for more information. You can also access it by
660prefixing it with C<CORE::>. Alternately, include a C<use
8f1da26d 661v5.10> or later to the current scope.
0d863452 662
a0d0e21e 663=item caller EXPR
d74e8afc 664X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
665
666=item caller
667
5a964f20 668Returns the context of the current subroutine call. In scalar context,
80d38338
TC
669returns the caller's package name if there I<is> a caller (that is, if
670we're in a subroutine or C<eval> or C<require>) and the undefined value
5a964f20 671otherwise. In list context, returns
a0d0e21e 672
ee6b43cc 673 # 0 1 2
748a9306 674 ($package, $filename, $line) = caller;
a0d0e21e
LW
675
676With EXPR, it returns some extra information that the debugger uses to
677print a stack trace. The value of EXPR indicates how many call frames
678to go back before the current one.
679
ee6b43cc 680 # 0 1 2 3 4
f3aa04c2 681 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 682
683 # 5 6 7 8 9 10
b3ca2e83 684 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 685 = caller($i);
e7ea3e70 686
951ba7fe 687Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 688call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 689C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 690C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 691C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 692$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
693each C<use> statement creates a C<require> frame inside an C<eval EXPR>
694frame.) $subroutine may also be C<(unknown)> if this particular
695subroutine happens to have been deleted from the symbol table.
696C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
697C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
698compiled with. The C<$hints> and C<$bitmask> values are subject to change
699between versions of Perl, and are not meant for external use.
748a9306 700
b3ca2e83
NC
701C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
702caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
703of this hash, as they are the actual values stored in the optree.
704
748a9306 705Furthermore, when called from within the DB package, caller returns more
7660c0ab 706detailed information: it sets the list variable C<@DB::args> to be the
54310121 707arguments with which the subroutine was invoked.
748a9306 708
7660c0ab 709Be aware that the optimizer might have optimized call frames away before
19799a22 710C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 711might not return information about the call frame you expect it to, for
b76cc8ba 712C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 713previous time C<caller> was called.
7660c0ab 714
8f1da26d 715Be aware that setting C<@DB::args> is I<best effort>, intended for
ca9f0cb5
NC
716debugging or generating backtraces, and should not be relied upon. In
717particular, as C<@_> contains aliases to the caller's arguments, Perl does
718not take a copy of C<@_>, so C<@DB::args> will contain modifications the
719subroutine makes to C<@_> or its contents, not the original values at call
720time. C<@DB::args>, like C<@_>, does not hold explicit references to its
721elements, so under certain cases its elements may have become freed and
722reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 723of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
724I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
725reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5
NC
726elements), so C<@DB::args> is actually a hybrid of the current state and
727initial state of C<@_>. Buyer beware.
728
a0d0e21e 729=item chdir EXPR
d74e8afc
ITB
730X<chdir>
731X<cd>
f723aae1 732X<directory, change>
a0d0e21e 733
c4aca7d0
GA
734=item chdir FILEHANDLE
735
736=item chdir DIRHANDLE
737
ce2984c3
PF
738=item chdir
739
ffce7b87 740Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 741changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 742changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0 743variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
80d38338 744neither is set, C<chdir> does nothing. It returns true on success,
b4ad75f0 745false otherwise. See the example under C<die>.
a0d0e21e 746
3b10bc60 747On systems that support fchdir(2), you may pass a filehandle or
34169887 748directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 749passing handles raises an exception.
c4aca7d0 750
a0d0e21e 751=item chmod LIST
d74e8afc 752X<chmod> X<permission> X<mode>
a0d0e21e
LW
753
754Changes the permissions of a list of files. The first element of the
8f1da26d 755list must be the numeric mode, which should probably be an octal
4ad40acf 756number, and which definitely should I<not> be a string of octal digits:
3b10bc60 757C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 758successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 759
3b10bc60 760 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 761 chmod 0755, @executables;
3b10bc60 762 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 763 # --w----r-T
3b10bc60 764 $mode = "0644"; chmod oct($mode), "foo"; # this is better
765 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 766
3b10bc60 767On systems that support fchmod(2), you may pass filehandles among the
768files. On systems that don't support fchmod(2), passing filehandles raises
769an exception. Filehandles must be passed as globs or glob references to be
770recognized; barewords are considered filenames.
c4aca7d0
GA
771
772 open(my $fh, "<", "foo");
773 my $perm = (stat $fh)[2] & 07777;
774 chmod($perm | 0600, $fh);
775
3b10bc60 776You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
777module:
778
3b10bc60 779 use Fcntl qw( :mode );
ca6e1c26 780 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 781 # Identical to the chmod 0755 of the example above.
ca6e1c26 782
ea9eb35a
BJ
783Portability issues: L<perlport/chmod>.
784
a0d0e21e 785=item chomp VARIABLE
d74e8afc 786X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 787
313c9f5c 788=item chomp( LIST )
a0d0e21e
LW
789
790=item chomp
791
2b5ab1e7
TC
792This safer version of L</chop> removes any trailing string
793that corresponds to the current value of C<$/> (also known as
28757baa 794$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
795number of characters removed from all its arguments. It's often used to
796remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
797that the final record may be missing its newline. When in paragraph
798mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 799When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 800a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 801remove anything.
19799a22 802If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
803
804 while (<>) {
a9a5a0dc
VP
805 chomp; # avoid \n on last field
806 @array = split(/:/);
807 # ...
a0d0e21e
LW
808 }
809
4bf21a6d
RD
810If VARIABLE is a hash, it chomps the hash's values, but not its keys.
811
a0d0e21e
LW
812You can actually chomp anything that's an lvalue, including an assignment:
813
814 chomp($cwd = `pwd`);
815 chomp($answer = <STDIN>);
816
817If you chomp a list, each element is chomped, and the total number of
818characters removed is returned.
819
15e44fd8
RGS
820Note that parentheses are necessary when you're chomping anything
821that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
822is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
823C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
824C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
825as C<chomp($a, $b)>.
826
a0d0e21e 827=item chop VARIABLE
d74e8afc 828X<chop>
a0d0e21e 829
313c9f5c 830=item chop( LIST )
a0d0e21e
LW
831
832=item chop
833
834Chops off the last character of a string and returns the character
5b3eff12 835chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 836scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
837If VARIABLE is a hash, it chops the hash's values, but not its keys.
838
5b3eff12 839You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
840
841If you chop a list, each element is chopped. Only the value of the
19799a22 842last C<chop> is returned.
a0d0e21e 843
19799a22 844Note that C<chop> returns the last character. To return all but the last
748a9306
LW
845character, use C<substr($string, 0, -1)>.
846
15e44fd8
RGS
847See also L</chomp>.
848
a0d0e21e 849=item chown LIST
d74e8afc 850X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
851
852Changes the owner (and group) of a list of files. The first two
19799a22
GS
853elements of the list must be the I<numeric> uid and gid, in that
854order. A value of -1 in either position is interpreted by most
855systems to leave that value unchanged. Returns the number of files
856successfully changed.
a0d0e21e
LW
857
858 $cnt = chown $uid, $gid, 'foo', 'bar';
859 chown $uid, $gid, @filenames;
860
3b10bc60 861On systems that support fchown(2), you may pass filehandles among the
862files. On systems that don't support fchown(2), passing filehandles raises
863an exception. Filehandles must be passed as globs or glob references to be
864recognized; barewords are considered filenames.
c4aca7d0 865
54310121 866Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
867
868 print "User: ";
19799a22 869 chomp($user = <STDIN>);
5a964f20 870 print "Files: ";
19799a22 871 chomp($pattern = <STDIN>);
a0d0e21e
LW
872
873 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 874 or die "$user not in passwd file";
a0d0e21e 875
5ed4f2ec 876 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
877 chown $uid, $gid, @ary;
878
54310121 879On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
880file unless you're the superuser, although you should be able to change
881the group to any of your secondary groups. On insecure systems, these
882restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
883On POSIX systems, you can detect this condition this way:
884
885 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
886 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 887
ea9eb35a
BJ
888Portability issues: L<perlport/chmod>.
889
a0d0e21e 890=item chr NUMBER
d74e8afc 891X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 892
54310121 893=item chr
bbce6d69 894
a0d0e21e 895Returns the character represented by that NUMBER in the character set.
a0ed51b3 896For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 897chr(0x263a) is a Unicode smiley face.
aaa68c4a 898
8a064bd6 899Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 900except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
901(truncated to an integer) are used.
902
974da8e5
JH
903If NUMBER is omitted, uses C<$_>.
904
b76cc8ba 905For the reverse, use L</ord>.
a0d0e21e 906
2575c402
JW
907Note that characters from 128 to 255 (inclusive) are by default
908internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 909
2575c402 910See L<perlunicode> for more about Unicode.
bbce6d69 911
a0d0e21e 912=item chroot FILENAME
d74e8afc 913X<chroot> X<root>
a0d0e21e 914
54310121 915=item chroot
bbce6d69 916
5a964f20 917This function works like the system call by the same name: it makes the
4633a7c4 918named directory the new root directory for all further pathnames that
951ba7fe 919begin with a C</> by your process and all its children. (It doesn't
28757baa 920change your current working directory, which is unaffected.) For security
4633a7c4 921reasons, this call is restricted to the superuser. If FILENAME is
19799a22 922omitted, does a C<chroot> to C<$_>.
a0d0e21e 923
ea9eb35a
BJ
924Portability issues: L<perlport/chroot>.
925
a0d0e21e 926=item close FILEHANDLE
d74e8afc 927X<close>
a0d0e21e 928
6a518fbc
TP
929=item close
930
3b10bc60 931Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 932buffers, and closes the system file descriptor. Returns true if those
8f1da26d 933operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
934layer. Closes the currently selected filehandle if the argument is
935omitted.
fb73857a 936
937You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 938another C<open> on it, because C<open> closes it for you. (See
01aa884e 939L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
19799a22 940counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 941
3b10bc60 942If the filehandle came from a piped open, C<close> returns false if one of
943the other syscalls involved fails or if its program exits with non-zero
944status. If the only problem was that the program exited non-zero, C<$!>
945will be set to C<0>. Closing a pipe also waits for the process executing
946on the pipe to exit--in case you wish to look at the output of the pipe
947afterwards--and implicitly puts the exit status value of that command into
948C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 949
2e0cfa16
FC
950If there are multiple threads running, C<close> on a filehandle from a
951piped open returns true without waiting for the child process to terminate,
952if the filehandle is still open in another thread.
953
80d38338
TC
954Closing the read end of a pipe before the process writing to it at the
955other end is done writing results in the writer receiving a SIGPIPE. If
956the other end can't handle that, be sure to read all the data before
957closing the pipe.
73689b13 958
fb73857a 959Example:
a0d0e21e 960
fb73857a 961 open(OUTPUT, '|sort >foo') # pipe to sort
962 or die "Can't start sort: $!";
5ed4f2ec 963 #... # print stuff to output
964 close OUTPUT # wait for sort to finish
fb73857a 965 or warn $! ? "Error closing sort pipe: $!"
966 : "Exit status $? from sort";
5ed4f2ec 967 open(INPUT, 'foo') # get sort's results
fb73857a 968 or die "Can't open 'foo' for input: $!";
a0d0e21e 969
5a964f20 970FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 971filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
972
973=item closedir DIRHANDLE
d74e8afc 974X<closedir>
a0d0e21e 975
19799a22 976Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
977system call.
978
a0d0e21e 979=item connect SOCKET,NAME
d74e8afc 980X<connect>
a0d0e21e 981
80d38338
TC
982Attempts to connect to a remote socket, just like connect(2).
983Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
984packed address of the appropriate type for the socket. See the examples in
985L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 986
cb1a09d0 987=item continue BLOCK
d74e8afc 988X<continue>
cb1a09d0 989
0d863452
RH
990=item continue
991
4a904372
FC
992When followed by a BLOCK, C<continue> is actually a
993flow control statement rather than a function. If
cf264981 994there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
995C<foreach>), it is always executed just before the conditional is about to
996be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
997it can be used to increment a loop variable, even when the loop has been
998continued via the C<next> statement (which is similar to the C C<continue>
999statement).
1000
98293880 1001C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 1002block; C<last> and C<redo> behave as if they had been executed within
19799a22 1003the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
1004block, it may be more entertaining.
1005
1006 while (EXPR) {
a9a5a0dc
VP
1007 ### redo always comes here
1008 do_something;
1d2dff63 1009 } continue {
a9a5a0dc
VP
1010 ### next always comes here
1011 do_something_else;
1012 # then back the top to re-check EXPR
1d2dff63
GS
1013 }
1014 ### last always comes here
1015
3b10bc60 1016Omitting the C<continue> section is equivalent to using an
1017empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1018to check the condition at the top of the loop.
1019
4a904372 1020When there is no BLOCK, C<continue> is a function that
8f1da26d
TC
1021falls through the current C<when> or C<default> block instead of iterating
1022a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
4a904372
FC
1023In Perl 5.14 and earlier, this form of C<continue> was
1024only available when the C<"switch"> feature was enabled.
8f1da26d
TC
1025See L<feature> and L<perlsyn/"Switch statements"> for more
1026information.
0d863452 1027
a0d0e21e 1028=item cos EXPR
d74e8afc 1029X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1030
d6217f1e
GS
1031=item cos
1032
5a964f20 1033Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1034takes the cosine of C<$_>.
a0d0e21e 1035
ca6e1c26 1036For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa 1037function, or use this relation:
1038
1039 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1040
a0d0e21e 1041=item crypt PLAINTEXT,SALT
d74e8afc 1042X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1043X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1044
ef2e6798
MS
1045Creates a digest string exactly like the crypt(3) function in the C
1046library (assuming that you actually have a version there that has not
bb23f8d1 1047been extirpated as a potential munition).
ef2e6798 1048
34169887 1049crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1050into a short string, called a digest, which is returned. The same
1051PLAINTEXT and SALT will always return the same string, but there is no
1052(known) way to get the original PLAINTEXT from the hash. Small
1053changes in the PLAINTEXT or SALT will result in large changes in the
1054digest.
1055
1056There is no decrypt function. This function isn't all that useful for
1057cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1058mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1059primarily used to check if two pieces of text are the same without
1060having to transmit or store the text itself. An example is checking
1061if a correct password is given. The digest of the password is stored,
cf264981 1062not the password itself. The user types in a password that is
ef2e6798 1063crypt()'d with the same salt as the stored digest. If the two digests
34169887 1064match, the password is correct.
ef2e6798
MS
1065
1066When verifying an existing digest string you should use the digest as
1067the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1068to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1069crypt() will hash the new string with the same salt as the digest.
1070This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1071with more exotic implementations. In other words, assume
1072nothing about the returned string itself nor about how many bytes
1073of SALT may matter.
85c16d83
JH
1074
1075Traditionally the result is a string of 13 bytes: two first bytes of
1076the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
bb23f8d1 1077the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1078hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1079and implementations on non-Unix platforms may produce different
ef2e6798 1080strings.
85c16d83
JH
1081
1082When choosing a new salt create a random two character string whose
1083characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1084'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1085characters is just a recommendation; the characters allowed in
1086the salt depend solely on your system's crypt library, and Perl can't
1087restrict what salts C<crypt()> accepts.
e71965be 1088
a0d0e21e 1089Here's an example that makes sure that whoever runs this program knows
cf264981 1090their password:
a0d0e21e
LW
1091
1092 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1093
1094 system "stty -echo";
1095 print "Password: ";
e71965be 1096 chomp($word = <STDIN>);
a0d0e21e
LW
1097 print "\n";
1098 system "stty echo";
1099
e71965be 1100 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1101 die "Sorry...\n";
a0d0e21e 1102 } else {
a9a5a0dc 1103 print "ok\n";
54310121 1104 }
a0d0e21e 1105
9f8f0c9d 1106Of course, typing in your own password to whoever asks you
748a9306 1107for it is unwise.
a0d0e21e 1108
ef2e6798 1109The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1110of data, not least of all because you can't get the information
ef2e6798 1111back. Look at the L<Digest> module for more robust algorithms.
19799a22 1112
f2791508
JH
1113If using crypt() on a Unicode string (which I<potentially> has
1114characters with codepoints above 255), Perl tries to make sense
34169887 1115of the situation by trying to downgrade (a copy of)
f2791508
JH
1116the string back to an eight-bit byte string before calling crypt()
1117(on that copy). If that works, good. If not, crypt() dies with
1118C<Wide character in crypt>.
85c16d83 1119
ea9eb35a
BJ
1120Portability issues: L<perlport/crypt>.
1121
aa689395 1122=item dbmclose HASH
d74e8afc 1123X<dbmclose>
a0d0e21e 1124
19799a22 1125[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1126
aa689395 1127Breaks the binding between a DBM file and a hash.
a0d0e21e 1128
ea9eb35a
BJ
1129Portability issues: L<perlport/dbmclose>.
1130
19799a22 1131=item dbmopen HASH,DBNAME,MASK
d74e8afc 1132X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1133
01aa884e
KW
1134[This function has been largely superseded by the
1135L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
a0d0e21e 1136
7b8d334a 1137This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1138hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1139argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395 1140is the name of the database (without the F<.dir> or F<.pag> extension if
1141any). If the database does not exist, it is created with protection
19799a22 1142specified by MASK (as modified by the C<umask>). If your system supports
80d38338 1143only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1144program. In older versions of Perl, if your system had neither DBM nor
19799a22 1145ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395 1146sdbm(3).
1147
1148If you don't have write access to the DBM file, you can only read hash
1149variables, not set them. If you want to test whether you can write,
3b10bc60 1150either use file tests or try setting a dummy hash entry inside an C<eval>
1151to trap the error.
a0d0e21e 1152
19799a22
GS
1153Note that functions such as C<keys> and C<values> may return huge lists
1154when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1155function to iterate over large DBM files. Example:
1156
1157 # print out history file offsets
1158 dbmopen(%HIST,'/usr/lib/news/history',0666);
1159 while (($key,$val) = each %HIST) {
a9a5a0dc 1160 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1161 }
1162 dbmclose(%HIST);
1163
cb1a09d0 1164See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1165cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1166rich implementation.
4633a7c4 1167
2b5ab1e7
TC
1168You can control which DBM library you use by loading that library
1169before you call dbmopen():
1170
1171 use DB_File;
1172 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1173 or die "Can't open netscape history file: $!";
2b5ab1e7 1174
ea9eb35a
BJ
1175Portability issues: L<perlport/dbmopen>.
1176
8f1da26d
TC
1177=item default BLOCK
1178
1179Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when>
1180that's always true. Only available after Perl 5.10, and only if the
4a904372
FC
1181C<switch> feature has been requested or if the keyword is prefixed with
1182C<CORE::>. See L</when>.
8f1da26d 1183
a0d0e21e 1184=item defined EXPR
d74e8afc 1185X<defined> X<undef> X<undefined>
a0d0e21e 1186
54310121 1187=item defined
bbce6d69 1188
2f9daede 1189Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1190the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TP
1191checked.
1192
1193Many operations return C<undef> to indicate failure, end of file,
1194system error, uninitialized variable, and other exceptional
1195conditions. This function allows you to distinguish C<undef> from
1196other values. (A simple Boolean test will not distinguish among
7660c0ab 1197C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1198false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1199doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TP
1200returns C<undef> when its argument is an empty array, I<or> when the
1201element to return happens to be C<undef>.
1202
f10b0346
GS
1203You may also use C<defined(&func)> to check whether subroutine C<&func>
1204has ever been defined. The return value is unaffected by any forward
80d38338 1205declarations of C<&func>. A subroutine that is not defined
847c7ebe 1206may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1207makes it spring into existence the first time that it is called; see
847c7ebe 1208L<perlsub>.
f10b0346
GS
1209
1210Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1211used to report whether memory for that aggregate had ever been
f10b0346
GS
1212allocated. This behavior may disappear in future versions of Perl.
1213You should instead use a simple test for size:
1214
1215 if (@an_array) { print "has array elements\n" }
1216 if (%a_hash) { print "has hash members\n" }
2f9daede
TP
1217
1218When used on a hash element, it tells you whether the value is defined,
dc848c6f 1219not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1220purpose.
a0d0e21e
LW
1221
1222Examples:
1223
8f1da26d 1224 print if defined $switch{D};
a0d0e21e
LW
1225 print "$val\n" while defined($val = pop(@ary));
1226 die "Can't readlink $sym: $!"
a9a5a0dc 1227 unless defined($value = readlink $sym);
a0d0e21e 1228 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1229 $debugging = 0 unless defined $debugging;
a0d0e21e 1230
8f1da26d 1231Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1232discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1233defined values. For example, if you say
a5f75d66
AD
1234
1235 "ab" =~ /a(.*)b/;
1236
80d38338 1237The pattern match succeeds and C<$1> is defined, although it
cf264981 1238matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1239matched something that happened to be zero characters long. This is all
a5f75d66 1240very above-board and honest. When a function returns an undefined value,
2f9daede 1241it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1242should use C<defined> only when questioning the integrity of what
7660c0ab 1243you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TP
1244what you want.
1245
dc848c6f 1246See also L</undef>, L</exists>, L</ref>.
2f9daede 1247
a0d0e21e 1248=item delete EXPR
d74e8afc 1249X<delete>
a0d0e21e 1250
d0a76353
RS
1251Given an expression that specifies an element or slice of a hash, C<delete>
1252deletes the specified elements from that hash so that exists() on that element
1253no longer returns true. Setting a hash element to the undefined value does
1254not remove its key, but deleting it does; see L</exists>.
80d38338 1255
8f1da26d 1256In list context, returns the value or values deleted, or the last such
80d38338 1257element in scalar context. The return list's length always matches that of
d0a76353
RS
1258the argument list: deleting non-existent elements returns the undefined value
1259in their corresponding positions.
80d38338 1260
d0a76353
RS
1261delete() may also be used on arrays and array slices, but its behavior is less
1262straightforward. Although exists() will return false for deleted entries,
1263deleting array elements never changes indices of existing values; use shift()
1264or splice() for that. However, if all deleted elements fall at the end of an
1265array, the array's size shrinks to the position of the highest element that
1266still tests true for exists(), or to 0 if none do.
1267
8f1da26d 1268B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1269be removed in a future version of Perl.
80d38338
TC
1270
1271Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1272a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1273or array may not necessarily return anything; it depends on the implementation
1274of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1275
80d38338
TC
1276The C<delete local EXPR> construct localizes the deletion to the current
1277block at run time. Until the block exits, elements locally deleted
1278temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1279of composite types">.
eba0920a
EM
1280
1281 %hash = (foo => 11, bar => 22, baz => 33);
1282 $scalar = delete $hash{foo}; # $scalar is 11
1283 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1284 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1285
01020589 1286The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1287
5f05dabc 1288 foreach $key (keys %HASH) {
a9a5a0dc 1289 delete $HASH{$key};
a0d0e21e
LW
1290 }
1291
01020589 1292 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1293 delete $ARRAY[$index];
01020589
GS
1294 }
1295
1296And so do these:
5f05dabc 1297
01020589
GS
1298 delete @HASH{keys %HASH};
1299
9740c838 1300 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1301
80d38338
TC
1302But both are slower than assigning the empty list
1303or undefining %HASH or @ARRAY, which is the customary
1304way to empty out an aggregate:
01020589 1305
5ed4f2ec 1306 %HASH = (); # completely empty %HASH
1307 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1308
5ed4f2ec 1309 @ARRAY = (); # completely empty @ARRAY
1310 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1311
80d38338
TC
1312The EXPR can be arbitrarily complicated provided its
1313final operation is an element or slice of an aggregate:
a0d0e21e
LW
1314
1315 delete $ref->[$x][$y]{$key};
5f05dabc 1316 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1317
01020589
GS
1318 delete $ref->[$x][$y][$index];
1319 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1320
a0d0e21e 1321=item die LIST
d74e8afc 1322X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1323
4c050ad5
NC
1324C<die> raises an exception. Inside an C<eval> the error message is stuffed
1325into C<$@> and the C<eval> is terminated with the undefined value.
1326If the exception is outside of all enclosing C<eval>s, then the uncaught
1327exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1328need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1329
1330Equivalent examples:
1331
1332 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1333 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1334
ccac6780 1335If the last element of LIST does not end in a newline, the current
df37ec69
WW
1336script line number and input line number (if any) are also printed,
1337and a newline is supplied. Note that the "input line number" (also
1338known as "chunk") is subject to whatever notion of "line" happens to
1339be currently in effect, and is also available as the special variable
1340C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1341
1342Hint: sometimes appending C<", stopped"> to your message will cause it
1343to make better sense when the string C<"at foo line 123"> is appended.
1344Suppose you are running script "canasta".
a0d0e21e
LW
1345
1346 die "/etc/games is no good";
1347 die "/etc/games is no good, stopped";
1348
1349produce, respectively
1350
1351 /etc/games is no good at canasta line 123.
1352 /etc/games is no good, stopped at canasta line 123.
1353
a96d0188 1354If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1355previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a 1356This is useful for propagating exceptions:
1357
1358 eval { ... };
1359 die unless $@ =~ /Expected exception/;
1360
a96d0188 1361If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1362C<PROPAGATE> method, that method will be called with additional file
1363and line number parameters. The return value replaces the value in
34169887 1364C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1365were called.
1366
7660c0ab 1367If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1368
4c050ad5
NC
1369If an uncaught exception results in interpreter exit, the exit code is
1370determined from the values of C<$!> and C<$?> with this pseudocode:
1371
1372 exit $! if $!; # errno
1373 exit $? >> 8 if $? >> 8; # child exit status
1374 exit 255; # last resort
1375
1376The intent is to squeeze as much possible information about the likely cause
1377into the limited space of the system exit code. However, as C<$!> is the value
1378of C's C<errno>, which can be set by any system call, this means that the value
1379of the exit code used by C<die> can be non-predictable, so should not be relied
1380upon, other than to be non-zero.
1381
80d38338
TC
1382You can also call C<die> with a reference argument, and if this is trapped
1383within an C<eval>, C<$@> contains that reference. This permits more
1384elaborate exception handling using objects that maintain arbitrary state
1385about the exception. Such a scheme is sometimes preferable to matching
1386particular string values of C<$@> with regular expressions. Because C<$@>
1387is a global variable and C<eval> may be used within object implementations,
1388be careful that analyzing the error object doesn't replace the reference in
1389the global variable. It's easiest to make a local copy of the reference
1390before any manipulations. Here's an example:
52531d10 1391
80d38338 1392 use Scalar::Util "blessed";
da279afe 1393
52531d10 1394 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1395 if (my $ev_err = $@) {
1396 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1397 # handle Some::Module::Exception
1398 }
1399 else {
1400 # handle all other possible exceptions
1401 }
1402 }
1403
3b10bc60 1404Because Perl stringifies uncaught exception messages before display,
80d38338 1405you'll probably want to overload stringification operations on
52531d10
GS
1406exception objects. See L<overload> for details about that.
1407
19799a22
GS
1408You can arrange for a callback to be run just before the C<die>
1409does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1410handler is called with the error text and can change the error
19799a22 1411message, if it sees fit, by calling C<die> again. See
96090e4f 1412L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1413L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1414to be run only right before your program was to exit, this is not
3b10bc60 1415currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1416even inside eval()ed blocks/strings! If one wants the hook to do
1417nothing in such situations, put
fb73857a 1418
5ed4f2ec 1419 die @_ if $^S;
fb73857a 1420
19799a22
GS
1421as the first line of the handler (see L<perlvar/$^S>). Because
1422this promotes strange action at a distance, this counterintuitive
b76cc8ba 1423behavior may be fixed in a future release.
774d564b 1424
4c050ad5
NC
1425See also exit(), warn(), and the Carp module.
1426
a0d0e21e 1427=item do BLOCK
d74e8afc 1428X<do> X<block>
a0d0e21e
LW
1429
1430Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1431sequence of commands indicated by BLOCK. When modified by the C<while> or
1432C<until> loop modifier, executes the BLOCK once before testing the loop
1433condition. (On other statements the loop modifiers test the conditional
1434first.)
a0d0e21e 1435
4968c1e4 1436C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1437C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1438See L<perlsyn> for alternative strategies.
4968c1e4 1439
a0d0e21e 1440=item do SUBROUTINE(LIST)
d74e8afc 1441X<do>
a0d0e21e 1442
51124b83
FC
1443This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
1444a scalar variable or a subroutine beginning with C<&>.
a0d0e21e
LW
1445
1446=item do EXPR
d74e8afc 1447X<do>
a0d0e21e
LW
1448
1449Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1450file as a Perl script.
a0d0e21e
LW
1451
1452 do 'stat.pl';
1453
1454is just like
1455
986b19de 1456 eval `cat stat.pl`;
a0d0e21e 1457
2b5ab1e7 1458except that it's more efficient and concise, keeps track of the current
96090e4f
LB
1459filename for error messages, searches the C<@INC> directories, and updates
1460C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1461these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1462cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1463same, however, in that it does reparse the file every time you call it,
1464so you probably don't want to do this inside a loop.
a0d0e21e 1465
8f1da26d 1466If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1467an error message in C<$@>. If C<do> cannot read the file, it returns undef
1468and sets C<$!> to the error. Always check C<$@> first, as compilation
1469could fail in a way that also sets C<$!>. If the file is successfully
1470compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1471
80d38338 1472Inclusion of library modules is better done with the
19799a22 1473C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1474and raise an exception if there's a problem.
a0d0e21e 1475
5a964f20
TC
1476You might like to use C<do> to read in a program configuration
1477file. Manual error checking can be done this way:
1478
b76cc8ba 1479 # read in config files: system first, then user
f86cebdf 1480 for $file ("/share/prog/defaults.rc",
b76cc8ba 1481 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1482 {
1483 unless ($return = do $file) {
1484 warn "couldn't parse $file: $@" if $@;
1485 warn "couldn't do $file: $!" unless defined $return;
1486 warn "couldn't run $file" unless $return;
1487 }
5a964f20
TC
1488 }
1489
a0d0e21e 1490=item dump LABEL
d74e8afc 1491X<dump> X<core> X<undump>
a0d0e21e 1492
1614b0e3
JD
1493=item dump
1494
19799a22
GS
1495This function causes an immediate core dump. See also the B<-u>
1496command-line switch in L<perlrun>, which does the same thing.
1497Primarily this is so that you can use the B<undump> program (not
1498supplied) to turn your core dump into an executable binary after
1499having initialized all your variables at the beginning of the
1500program. When the new binary is executed it will begin by executing
1501a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1502Think of it as a goto with an intervening core dump and reincarnation.
1503If C<LABEL> is omitted, restarts the program from the top.
1504
1505B<WARNING>: Any files opened at the time of the dump will I<not>
1506be open any more when the program is reincarnated, with possible
80d38338 1507resulting confusion by Perl.
19799a22 1508
59f521f4
RGS
1509This function is now largely obsolete, mostly because it's very hard to
1510convert a core file into an executable. That's why you should now invoke
1511it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1512typo.
19799a22 1513
ea9eb35a
BJ
1514Portability issues: L<perlport/dump>.
1515
532eee96 1516=item each HASH
d74e8afc 1517X<each> X<hash, iterator>
aa689395 1518
532eee96 1519=item each ARRAY
aeedbbed
NC
1520X<array, iterator>
1521
f5a93a43
TC
1522=item each EXPR
1523
80d38338
TC
1524When called in list context, returns a 2-element list consisting of the key
1525and value for the next element of a hash, or the index and value for the
1526next element of an array, so that you can iterate over it. When called in
1527scalar context, returns only the key (not the value) in a hash, or the index
1528in an array.
2f9daede 1529
aeedbbed 1530Hash entries are returned in an apparently random order. The actual random
3b10bc60 1531order is subject to change in future versions of Perl, but it is
504f80c1 1532guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1533function would produce on the same (unmodified) hash. Since Perl
22883ac5 15345.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1535for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1536
80d38338
TC
1537After C<each> has returned all entries from the hash or array, the next
1538call to C<each> returns the empty list in list context and C<undef> in
1539scalar context. The next call following that one restarts iteration. Each
1540hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1541and C<values>. The iterator is implicitly reset when C<each> has reached
1542the end as just described; it can be explicitly reset by calling C<keys> or
1543C<values> on the hash or array. If you add or delete a hash's elements
1544while iterating over it, entries may be skipped or duplicated--so don't do
1545that. Exception: It is always safe to delete the item most recently
3b10bc60 1546returned by C<each()>, so the following code works properly:
74fc8b5f
MJD
1547
1548 while (($key, $value) = each %hash) {
1549 print $key, "\n";
1550 delete $hash{$key}; # This is safe
1551 }
aa689395 1552
80d38338 1553This prints out your environment like the printenv(1) program,
3b10bc60 1554but in a different order:
a0d0e21e
LW
1555
1556 while (($key,$value) = each %ENV) {
a9a5a0dc 1557 print "$key=$value\n";
a0d0e21e
LW
1558 }
1559
f5a93a43
TC
1560Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1561reference to an unblessed hash or array. The argument will be dereferenced
1562automatically. This aspect of C<each> is considered highly experimental.
1563The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1564
1565 while (($key,$value) = each $hashref) { ... }
1566
8f1da26d 1567See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1568
1569=item eof FILEHANDLE
d74e8afc
ITB
1570X<eof>
1571X<end of file>
1572X<end-of-file>
a0d0e21e 1573
4633a7c4
LW
1574=item eof ()
1575
a0d0e21e
LW
1576=item eof
1577
8f1da26d 1578Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1579FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1580gives the real filehandle. (Note that this function actually
80d38338 1581reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1582interactive context.) Do not read from a terminal file (or call
19799a22 1583C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1584as terminals may lose the end-of-file condition if you do.
1585
820475bd 1586An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1587with empty parentheses is different. It refers to the pseudo file
820475bd 1588formed from the files listed on the command line and accessed via the
61eff3bc
JH
1589C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1590as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1591used will cause C<@ARGV> to be examined to determine if input is
67408cae 1592available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1593end-of-file will assume you are processing another C<@ARGV> list,
1594and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1595see L<perlop/"I/O Operators">.
820475bd 1596
61eff3bc 1597In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1598detect the end of each file, whereas C<eof()> will detect the end
1599of the very last file only. Examples:
a0d0e21e 1600
748a9306
LW
1601 # reset line numbering on each input file
1602 while (<>) {
a9a5a0dc
VP
1603 next if /^\s*#/; # skip comments
1604 print "$.\t$_";
5a964f20 1605 } continue {
a9a5a0dc 1606 close ARGV if eof; # Not eof()!
748a9306
LW
1607 }
1608
a0d0e21e
LW
1609 # insert dashes just before last line of last file
1610 while (<>) {
a9a5a0dc
VP
1611 if (eof()) { # check for end of last file
1612 print "--------------\n";
1613 }
1614 print;
4d0444a3 1615 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1616 }
1617
a0d0e21e 1618Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1619input operators typically return C<undef> when they run out of data or
1620encounter an error.
a0d0e21e
LW
1621
1622=item eval EXPR
d74e8afc 1623X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1624X<error, handling> X<exception, handling>
a0d0e21e
LW
1625
1626=item eval BLOCK
1627
ce2984c3
PF
1628=item eval
1629
c7cc6f1c
GS
1630In the first form, the return value of EXPR is parsed and executed as if it
1631were a little Perl program. The value of the expression (which is itself
8f1da26d 1632determined within scalar context) is first parsed, and if there were no
2341804c 1633errors, executed as a block within the lexical context of the current Perl
df4833a8 1634program. This means, that in particular, any outer lexical variables are
2341804c
DM
1635visible to it, and any package variable settings or subroutine and format
1636definitions remain afterwards.
1637
1638Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1639If EXPR is omitted, evaluates C<$_>. This form is typically used to
1640delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c 1641
7289c5e6
FC
1642If the C<unicode_eval> feature is enabled (which is the default under a
1643C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1644characters, so C<use utf8> declarations have no effect, and source filters
1645are forbidden. In the absence of the C<unicode_eval> feature, the string
1646will sometimes be treated as characters and sometimes as bytes, depending
1647on the internal encoding, and source filters activated within the C<eval>
1648exhibit the erratic, but historical, behaviour of affecting some outer file
1649scope that is still compiling. See also the L</evalbytes> keyword, which
1650always treats its input as a byte stream and works properly with source
1651filters, and the L<feature> pragma.
1652
c7cc6f1c 1653In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1654same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1655within the context of the current Perl program. This form is typically
1656used to trap exceptions more efficiently than the first (see below), while
1657also providing the benefit of checking the code within BLOCK at compile
1658time.
1659
1660The final semicolon, if any, may be omitted from the value of EXPR or within
1661the BLOCK.
1662
1663In both forms, the value returned is the value of the last expression
5a964f20 1664evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1665as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1666in void, scalar, or list context, depending on the context of the C<eval>
1667itself. See L</wantarray> for more on how the evaluation context can be
1668determined.
a0d0e21e 1669
19799a22 1670If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1671executed, C<eval> returns C<undef> in scalar context
774b80e8
FC
1672or an empty list in list context, and C<$@> is set to the error
1673message. (Prior to 5.16, a bug caused C<undef> to be returned
1674in list context for syntax errors, but not for runtime errors.)
1675If there was no error, C<$@> is set to the empty string. A
9cc672d4
FC
1676control flow operator like C<last> or C<goto> can bypass the setting of
1677C<$@>. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1678warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1679To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1680turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1681See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1682
19799a22
GS
1683Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1684determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1685is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1686the die operator is used to raise exceptions.
1687
5f1da31c
NT
1688If you want to trap errors when loading an XS module, some problems with
1689the binary interface (such as Perl version skew) may be fatal even with
df4833a8 1690C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
5f1da31c 1691
a0d0e21e
LW
1692If the code to be executed doesn't vary, you may use the eval-BLOCK
1693form to trap run-time errors without incurring the penalty of
1694recompiling each time. The error, if any, is still returned in C<$@>.
1695Examples:
1696
54310121 1697 # make divide-by-zero nonfatal
a0d0e21e
LW
1698 eval { $answer = $a / $b; }; warn $@ if $@;
1699
1700 # same thing, but less efficient
1701 eval '$answer = $a / $b'; warn $@ if $@;
1702
1703 # a compile-time error
5ed4f2ec 1704 eval { $answer = }; # WRONG
a0d0e21e
LW
1705
1706 # a run-time error
5ed4f2ec 1707 eval '$answer ='; # sets $@
a0d0e21e 1708
cf264981
SP
1709Using the C<eval{}> form as an exception trap in libraries does have some
1710issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1711may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1712You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1713as this example shows:
774d564b 1714
80d38338 1715 # a private exception trap for divide-by-zero
f86cebdf
GS
1716 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1717 warn $@ if $@;
774d564b 1718
1719This is especially significant, given that C<__DIE__> hooks can call
19799a22 1720C<die> again, which has the effect of changing their error messages:
774d564b 1721
1722 # __DIE__ hooks may modify error messages
1723 {
f86cebdf
GS
1724 local $SIG{'__DIE__'} =
1725 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1726 eval { die "foo lives here" };
1727 print $@ if $@; # prints "bar lives here"
774d564b 1728 }
1729
19799a22 1730Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1731may be fixed in a future release.
1732
19799a22 1733With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1734being looked at when:
1735
5ed4f2ec 1736 eval $x; # CASE 1
1737 eval "$x"; # CASE 2
a0d0e21e 1738
5ed4f2ec 1739 eval '$x'; # CASE 3
1740 eval { $x }; # CASE 4
a0d0e21e 1741
5ed4f2ec 1742 eval "\$$x++"; # CASE 5
1743 $$x++; # CASE 6
a0d0e21e 1744
2f9daede 1745Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1746the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1747the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1748and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1749does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TP
1750purely visual reasons, but it also has the advantage of compiling at
1751compile-time instead of at run-time.) Case 5 is a place where
19799a22 1752normally you I<would> like to use double quotes, except that in this
2f9daede
TP
1753particular situation, you can just use symbolic references instead, as
1754in case 6.
a0d0e21e 1755
b6538e4f
TC
1756Before Perl 5.14, the assignment to C<$@> occurred before restoration
1757of localised variables, which means that for your code to run on older
b208c909 1758versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1759errors:
1760
1761 # alter $@ on nefarious repugnancy only
1762 {
1763 my $e;
1764 {
1765 local $@; # protect existing $@
1766 eval { test_repugnancy() };
b208c909 1767 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
8a5a710d
DN
1768 $@ =~ /nefarious/ and $e = $@;
1769 }
1770 die $e if defined $e
1771 }
1772
4968c1e4 1773C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1774C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1775
3b10bc60 1776An C<eval ''> executed within the C<DB> package doesn't see the usual
1777surrounding lexical scope, but rather the scope of the first non-DB piece
df4833a8 1778of code that called it. You don't normally need to worry about this unless
3b10bc60 1779you are writing a Perl debugger.
d819b83a 1780
7289c5e6
FC
1781=item evalbytes EXPR
1782X<evalbytes>
1783
1784=item evalbytes
1785
1786This function is like L</eval> with a string argument, except it always
1787parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1788string containing characters whose ordinal value exceeds 255 results in an
1789error. Source filters activated within the evaluated code apply to the
1790code itself.
1791
1792This function is only available under the C<evalbytes> feature, a
1793C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1794L<feature> for more information.
1795
a0d0e21e 1796=item exec LIST
d74e8afc 1797X<exec> X<execute>
a0d0e21e 1798
8bf3b016
GS
1799=item exec PROGRAM LIST
1800
3b10bc60 1801The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1802use C<system> instead of C<exec> if you want it to return. It fails and
1803returns false only if the command does not exist I<and> it is executed
fb73857a 1804directly instead of via your system's command shell (see below).
a0d0e21e 1805
19799a22 1806Since it's a common mistake to use C<exec> instead of C<system>, Perl
80d38338 1807warns you if there is a following statement that isn't C<die>, C<warn>,
3b10bc60 1808or C<exit> (if C<-w> is set--but you always do that, right?). If you
19799a22 1809I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1810can use one of these styles to avoid the warning:
1811
5a964f20
TC
1812 exec ('foo') or print STDERR "couldn't exec foo: $!";
1813 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1814
5a964f20 1815If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1816with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1817If there is only one scalar argument or an array with one element in it,
1818the argument is checked for shell metacharacters, and if there are any,
1819the entire argument is passed to the system's command shell for parsing
1820(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1821If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1822words and passed directly to C<execvp>, which is more efficient.
19799a22 1823Examples:
a0d0e21e 1824
19799a22
GS
1825 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1826 exec "sort $outfile | uniq";
a0d0e21e
LW
1827
1828If you don't really want to execute the first argument, but want to lie
1829to the program you are executing about its own name, you can specify
1830the program you actually want to run as an "indirect object" (without a
1831comma) in front of the LIST. (This always forces interpretation of the
54310121 1832LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1833the list.) Example:
1834
1835 $shell = '/bin/csh';
5ed4f2ec 1836 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1837
1838or, more directly,
1839
5ed4f2ec 1840 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1841
3b10bc60 1842When the arguments get executed via the system shell, results are
1843subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1844for details.
1845
19799a22
GS
1846Using an indirect object with C<exec> or C<system> is also more
1847secure. This usage (which also works fine with system()) forces
1848interpretation of the arguments as a multivalued list, even if the
1849list had just one argument. That way you're safe from the shell
1850expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1851
1852 @args = ( "echo surprise" );
1853
2b5ab1e7 1854 exec @args; # subject to shell escapes
f86cebdf 1855 # if @args == 1
2b5ab1e7 1856 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1857
1858The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1859program, passing it C<"surprise"> an argument. The second version didn't;
1860it tried to run a program named I<"echo surprise">, didn't find it, and set
1861C<$?> to a non-zero value indicating failure.
5a964f20 1862
80d38338 1863Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1864output before the exec, but this may not be supported on some platforms
1865(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1866in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 1867open handles to avoid lost output.
0f897271 1868
80d38338
TC
1869Note that C<exec> will not call your C<END> blocks, nor will it invoke
1870C<DESTROY> methods on your objects.
7660c0ab 1871
ea9eb35a
BJ
1872Portability issues: L<perlport/exec>.
1873
a0d0e21e 1874=item exists EXPR
d74e8afc 1875X<exists> X<autovivification>
a0d0e21e 1876
d0a76353
RS
1877Given an expression that specifies an element of a hash, returns true if the
1878specified element in the hash has ever been initialized, even if the
1879corresponding value is undefined.
a0d0e21e 1880
5ed4f2ec 1881 print "Exists\n" if exists $hash{$key};
1882 print "Defined\n" if defined $hash{$key};
01020589
GS
1883 print "True\n" if $hash{$key};
1884
d0a76353 1885exists may also be called on array elements, but its behavior is much less
8f1da26d 1886obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
1887that calling exists on array values is deprecated and likely to be removed in
1888a future version of Perl.
1889
5ed4f2ec 1890 print "Exists\n" if exists $array[$index];
1891 print "Defined\n" if defined $array[$index];
01020589 1892 print "True\n" if $array[$index];
a0d0e21e 1893
8f1da26d 1894A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
1895it exists, but the reverse doesn't necessarily hold true.
1896
afebc493
GS
1897Given an expression that specifies the name of a subroutine,
1898returns true if the specified subroutine has ever been declared, even
1899if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 1900does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
1901exist may still be callable: its package may have an C<AUTOLOAD>
1902method that makes it spring into existence the first time that it is
3b10bc60 1903called; see L<perlsub>.
afebc493 1904
5ed4f2ec 1905 print "Exists\n" if exists &subroutine;
1906 print "Defined\n" if defined &subroutine;
afebc493 1907
a0d0e21e 1908Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1909operation is a hash or array key lookup or subroutine name:
a0d0e21e 1910
5ed4f2ec 1911 if (exists $ref->{A}->{B}->{$key}) { }
1912 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 1913
5ed4f2ec 1914 if (exists $ref->{A}->{B}->[$ix]) { }
1915 if (exists $hash{A}{B}[$ix]) { }
01020589 1916
afebc493
GS
1917 if (exists &{$ref->{A}{B}{$key}}) { }
1918
3b10bc60 1919Although the mostly deeply nested array or hash will not spring into
1920existence just because its existence was tested, any intervening ones will.
61eff3bc 1921Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 1922into existence due to the existence test for the $key element above.
3b10bc60 1923This happens anywhere the arrow operator is used, including even here:
5a964f20 1924
2b5ab1e7 1925 undef $ref;
5ed4f2ec 1926 if (exists $ref->{"Some key"}) { }
1927 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
1928
1929This surprising autovivification in what does not at first--or even
1930second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1931release.
a0d0e21e 1932
afebc493
GS
1933Use of a subroutine call, rather than a subroutine name, as an argument
1934to exists() is an error.
1935
5ed4f2ec 1936 exists &sub; # OK
1937 exists &sub(); # Error
afebc493 1938
a0d0e21e 1939=item exit EXPR
d74e8afc 1940X<exit> X<terminate> X<abort>
a0d0e21e 1941
ce2984c3
PF
1942=item exit
1943
2b5ab1e7 1944Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1945
1946 $ans = <STDIN>;
1947 exit 0 if $ans =~ /^[Xx]/;
1948
19799a22 1949See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1950universally recognized values for EXPR are C<0> for success and C<1>
1951for error; other values are subject to interpretation depending on the
1952environment in which the Perl program is running. For example, exiting
195369 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1954the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1955
19799a22
GS
1956Don't use C<exit> to abort a subroutine if there's any chance that
1957someone might want to trap whatever error happened. Use C<die> instead,
1958which can be trapped by an C<eval>.
28757baa 1959
19799a22 1960The exit() function does not always exit immediately. It calls any
2b5ab1e7 1961defined C<END> routines first, but these C<END> routines may not
19799a22 1962themselves abort the exit. Likewise any object destructors that need to
60275626
FC
1963be called are called before the real exit. C<END> routines and destructors
1964can change the exit status by modifying C<$?>. If this is a problem, you
2b5ab1e7 1965can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1966See L<perlmod> for details.
5a964f20 1967
ea9eb35a
BJ
1968Portability issues: L<perlport/exit>.
1969
a0d0e21e 1970=item exp EXPR
d74e8afc 1971X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1972
54310121 1973=item exp
bbce6d69 1974
b76cc8ba 1975Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1976If EXPR is omitted, gives C<exp($_)>.
1977
1978=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1979X<fcntl>
a0d0e21e 1980
f86cebdf 1981Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1982
1983 use Fcntl;
1984
0ade1984 1985first to get the correct constant definitions. Argument processing and
3b10bc60 1986value returned work just like C<ioctl> below.
a0d0e21e
LW
1987For example:
1988
1989 use Fcntl;
5a964f20 1990 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 1991 or die "can't fcntl F_GETFL: $!";
5a964f20 1992
554ad1fc 1993You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1994Like C<ioctl>, it maps a C<0> return from the system call into
1995C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1996in numeric context. It is also exempt from the normal B<-w> warnings
1997on improper numeric conversions.
5a964f20 1998
3b10bc60 1999Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
2000doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2001manpage to learn what functions are available on your system.
a0d0e21e 2002
be2f7487
TH
2003Here's an example of setting a filehandle named C<REMOTE> to be
2004non-blocking at the system level. You'll have to negotiate C<$|>
2005on your own, though.
2006
2007 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2008
2009 $flags = fcntl(REMOTE, F_GETFL, 0)
2010 or die "Can't get flags for the socket: $!\n";
2011
2012 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2013 or die "Can't set flags for the socket: $!\n";
2014
ea9eb35a
BJ
2015Portability issues: L<perlport/fcntl>.
2016
cfa52385
FC
2017=item __FILE__
2018X<__FILE__>
2019
2020A special token that returns the name of the file in which it occurs.
2021
a0d0e21e 2022=item fileno FILEHANDLE
d74e8afc 2023X<fileno>
a0d0e21e 2024
2b5ab1e7 2025Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
2026filehandle is not open. If there is no real file descriptor at the OS
2027level, as can happen with filehandles connected to memory objects via
2028C<open> with a reference for the third argument, -1 is returned.
2029
2030This is mainly useful for constructing
19799a22 2031bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
2032If FILEHANDLE is an expression, the value is taken as an indirect
2033filehandle, generally its name.
5a964f20 2034
b76cc8ba 2035You can use this to find out whether two handles refer to the
5a964f20
TC
2036same underlying descriptor:
2037
2038 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 2039 print "THIS and THAT are dups\n";
b76cc8ba
NIS
2040 }
2041
a0d0e21e 2042=item flock FILEHANDLE,OPERATION
d74e8afc 2043X<flock> X<lock> X<locking>
a0d0e21e 2044
19799a22
GS
2045Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2046for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2047machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2048C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2049entire files only, not records.
2b5ab1e7
TC
2050
2051Two potentially non-obvious but traditional C<flock> semantics are
2052that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2053are B<merely advisory>. Such discretionary locks are more flexible, but
2054offer fewer guarantees. This means that programs that do not also use
2055C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2056your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2057for details. It's best to assume traditional behavior if you're writing
2058portable programs. (But if you're not, you should as always feel perfectly
2059free to write for your own system's idiosyncrasies (sometimes called
2060"features"). Slavish adherence to portability concerns shouldn't get
2061in the way of your getting your job done.)
a3cb178b 2062
8ebc5c01 2063OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2064LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2065you can use the symbolic names if you import them from the L<Fcntl> module,
2066either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2067requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2068releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2069LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2070waiting for the lock; check the return status to see if you got it.
68dc0745 2071
2b5ab1e7
TC
2072To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2073before locking or unlocking it.
8ebc5c01 2074
f86cebdf 2075Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2076locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2077are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2078implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01 2079differing semantics shouldn't bite too many people.
2080
becacb53
TM
2081Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2082be open with read intent to use LOCK_SH and requires that it be open
2083with write intent to use LOCK_EX.
2084
19799a22
GS
2085Note also that some versions of C<flock> cannot lock things over the
2086network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2087that. If you like you can force Perl to ignore your system's flock(2)
2088function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2089the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2090and build a new Perl.
4633a7c4
LW
2091
2092Here's a mailbox appender for BSD systems.
a0d0e21e 2093
7ed5353d 2094 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
2095
2096 sub lock {
a9a5a0dc
VP
2097 my ($fh) = @_;
2098 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2099
a9a5a0dc
VP
2100 # and, in case someone appended while we were waiting...
2101 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2102 }
2103
2104 sub unlock {
a9a5a0dc
VP
2105 my ($fh) = @_;
2106 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2107 }
2108
b0169937 2109 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2110 or die "Can't open mailbox: $!";
a0d0e21e 2111
7ed5353d 2112 lock($mbox);
b0169937 2113 print $mbox $msg,"\n\n";
7ed5353d 2114 unlock($mbox);
a0d0e21e 2115
3b10bc60 2116On systems that support a real flock(2), locks are inherited across fork()
2117calls, whereas those that must resort to the more capricious fcntl(2)
2118function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2119
cb1a09d0 2120See also L<DB_File> for other flock() examples.
a0d0e21e 2121
ea9eb35a
BJ
2122Portability issues: L<perlport/flock>.
2123
a0d0e21e 2124=item fork
d74e8afc 2125X<fork> X<child> X<parent>
a0d0e21e 2126
2b5ab1e7
TC
2127Does a fork(2) system call to create a new process running the
2128same program at the same point. It returns the child pid to the
2129parent process, C<0> to the child process, or C<undef> if the fork is
2130unsuccessful. File descriptors (and sometimes locks on those descriptors)
2131are shared, while everything else is copied. On most systems supporting
2132fork(), great care has gone into making it extremely efficient (for
2133example, using copy-on-write technology on data pages), making it the
2134dominant paradigm for multitasking over the last few decades.
5a964f20 2135
80d38338 2136Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
2137output before forking the child process, but this may not be supported
2138on some platforms (see L<perlport>). To be safe, you may need to set
2139C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2140C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2141
19799a22 2142If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2143accumulate zombies. On some systems, you can avoid this by setting
2144C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2145forking and reaping moribund children.
cb1a09d0 2146
28757baa 2147Note that if your forked child inherits system file descriptors like
2148STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2149if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2150backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2151You should reopen those to F</dev/null> if it's any issue.
28757baa 2152
ea9eb35a
BJ
2153On some platforms such as Windows, where the fork() system call is not available,
2154Perl can be built to emulate fork() in the Perl interpreter. The emulation is designed to,
2155at the level of the Perl program, be as compatible as possible with the "Unix" fork().
6d17f725 2156However it has limitations that have to be considered in code intended to be portable.
ea9eb35a
BJ
2157See L<perlfork> for more details.
2158
2159Portability issues: L<perlport/fork>.
2160
cb1a09d0 2161=item format
d74e8afc 2162X<format>
cb1a09d0 2163
19799a22 2164Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2165example:
2166
54310121 2167 format Something =
a9a5a0dc
VP
2168 Test: @<<<<<<<< @||||| @>>>>>
2169 $str, $%, '$' . int($num)
cb1a09d0
AD
2170 .
2171
2172 $str = "widget";
184e9718 2173 $num = $cost/$quantity;
cb1a09d0
AD
2174 $~ = 'Something';
2175 write;
2176
2177See L<perlform> for many details and examples.
2178
8903cb82 2179=item formline PICTURE,LIST
d74e8afc 2180X<formline>
a0d0e21e 2181
5a964f20 2182This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2183too. It formats (see L<perlform>) a list of values according to the
2184contents of PICTURE, placing the output into the format output
7660c0ab 2185accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2186Eventually, when a C<write> is done, the contents of
cf264981
SP
2187C<$^A> are written to some filehandle. You could also read C<$^A>
2188and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2189does one C<formline> per line of form, but the C<formline> function itself
748a9306 2190doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2191that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2192You may therefore need to use multiple formlines to implement a single
3b10bc60 2193record format, just like the C<format> compiler.
748a9306 2194
19799a22 2195Be careful if you put double quotes around the picture, because an C<@>
748a9306 2196character may be taken to mean the beginning of an array name.
19799a22 2197C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2198
445b09e5
FC
2199If you are trying to use this instead of C<write> to capture the output,
2200you may find it easier to open a filehandle to a scalar
2201(C<< open $fh, ">", \$output >>) and write to that instead.
2202
a0d0e21e 2203=item getc FILEHANDLE
f723aae1 2204X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2205
2206=item getc
2207
2208Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2209or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2210the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2211STDIN. This is not particularly efficient. However, it cannot be
2212used by itself to fetch single characters without waiting for the user
2213to hit enter. For that, try something more like:
4633a7c4
LW
2214
2215 if ($BSD_STYLE) {
a9a5a0dc 2216 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2217 }
2218 else {
a9a5a0dc 2219 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2220 }
2221
2222 $key = getc(STDIN);
2223
2224 if ($BSD_STYLE) {
a9a5a0dc 2225 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2226 }
2227 else {
3b10bc60 2228 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2229 }
2230 print "\n";
2231
54310121 2232Determination of whether $BSD_STYLE should be set
2233is left as an exercise to the reader.
cb1a09d0 2234
19799a22 2235The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2236systems purporting POSIX compliance. See also the C<Term::ReadKey>
a3390c9f 2237module from your nearest CPAN site; details on CPAN can be found under
2b5ab1e7 2238L<perlmodlib/CPAN>.
a0d0e21e
LW
2239
2240=item getlogin
d74e8afc 2241X<getlogin> X<login>
a0d0e21e 2242
cf264981 2243This implements the C library function of the same name, which on most
3b10bc60 2244systems returns the current login from F</etc/utmp>, if any. If it
2245returns the empty string, use C<getpwuid>.
a0d0e21e 2246
f86702cc 2247 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2248
19799a22
GS
2249Do not consider C<getlogin> for authentication: it is not as
2250secure as C<getpwuid>.
4633a7c4 2251
ea9eb35a
BJ
2252Portability issues: L<perlport/getlogin>.
2253
a0d0e21e 2254=item getpeername SOCKET
d74e8afc 2255X<getpeername> X<peer>
a0d0e21e 2256
a3390c9f
FC
2257Returns the packed sockaddr address of the other end of the SOCKET
2258connection.
a0d0e21e 2259
4633a7c4
LW
2260 use Socket;
2261 $hersockaddr = getpeername(SOCK);
19799a22 2262 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2263 $herhostname = gethostbyaddr($iaddr, AF_INET);
2264 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2265
2266=item getpgrp PID
d74e8afc 2267X<getpgrp> X<group>
a0d0e21e 2268
47e29363 2269Returns the current process group for the specified PID. Use
7660c0ab 2270a PID of C<0> to get the current process group for the
4633a7c4 2271current process. Will raise an exception if used on a machine that
a3390c9f
FC
2272doesn't implement getpgrp(2). If PID is omitted, returns the process
2273group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2274does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2275
ea9eb35a
BJ
2276Portability issues: L<perlport/getpgrp>.
2277
a0d0e21e 2278=item getppid
d74e8afc 2279X<getppid> X<parent> X<pid>
a0d0e21e
LW
2280
2281Returns the process id of the parent process.
2282
4d76a344
RGS
2283Note for Linux users: on Linux, the C functions C<getpid()> and
2284C<getppid()> return different values from different threads. In order to
3b10bc60 2285be portable, this behavior is not reflected by the Perl-level function
4d76a344 2286C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2287to call the underlying C<getppid()>, you may use the CPAN module
2288C<Linux::Pid>.
4d76a344 2289
ea9eb35a
BJ
2290Portability issues: L<perlport/getppid>.
2291
a0d0e21e 2292=item getpriority WHICH,WHO
d74e8afc 2293X<getpriority> X<priority> X<nice>
a0d0e21e 2294
4633a7c4 2295Returns the current priority for a process, a process group, or a user.
01aa884e 2296(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2297machine that doesn't implement getpriority(2).
a0d0e21e 2298
ea9eb35a
BJ
2299Portability issues: L<perlport/getpriority>.
2300
a0d0e21e 2301=item getpwnam NAME
d74e8afc
ITB
2302X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2303X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2304X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2305X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2306X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2307X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2308
2309=item getgrnam NAME
2310
2311=item gethostbyname NAME
2312
2313=item getnetbyname NAME
2314
2315=item getprotobyname NAME
2316
2317=item getpwuid UID
2318
2319=item getgrgid GID
2320
2321=item getservbyname NAME,PROTO
2322
2323=item gethostbyaddr ADDR,ADDRTYPE
2324
2325=item getnetbyaddr ADDR,ADDRTYPE
2326
2327=item getprotobynumber NUMBER
2328
2329=item getservbyport PORT,PROTO
2330
2331=item getpwent
2332
2333=item getgrent
2334
2335=item gethostent
2336
2337=item getnetent
2338
2339=item getprotoent
2340
2341=item getservent
2342
2343=item setpwent
2344
2345=item setgrent
2346
2347=item sethostent STAYOPEN
2348
2349=item setnetent STAYOPEN
2350
2351=item setprotoent STAYOPEN
2352
2353=item setservent STAYOPEN
2354
2355=item endpwent
2356
2357=item endgrent
2358
2359=item endhostent
2360
2361=item endnetent
2362
2363=item endprotoent
2364
2365=item endservent
2366
80d38338
TC
2367These routines are the same as their counterparts in the
2368system C library. In list context, the return values from the
a0d0e21e
LW
2369various get routines are as follows:
2370
2371 ($name,$passwd,$uid,$gid,
6ee623d5 2372 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2373 ($name,$passwd,$gid,$members) = getgr*
2374 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2375 ($name,$aliases,$addrtype,$net) = getnet*
2376 ($name,$aliases,$proto) = getproto*
2377 ($name,$aliases,$port,$proto) = getserv*
2378
3b10bc60 2379(If the entry doesn't exist you get an empty list.)
a0d0e21e 2380
4602f195
JH
2381The exact meaning of the $gcos field varies but it usually contains
2382the real name of the user (as opposed to the login name) and other
2383information pertaining to the user. Beware, however, that in many
2384system users are able to change this information and therefore it
106325ad 2385cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2386L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2387login shell, are also tainted, for the same reason.
4602f195 2388
5a964f20 2389In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2390lookup by name, in which case you get the other thing, whatever it is.
2391(If the entry doesn't exist you get the undefined value.) For example:
2392
5a964f20
TC
2393 $uid = getpwnam($name);
2394 $name = getpwuid($num);
2395 $name = getpwent();
2396 $gid = getgrnam($name);
08a33e13 2397 $name = getgrgid($num);
5a964f20
TC
2398 $name = getgrent();
2399 #etc.
a0d0e21e 2400
4602f195 2401In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2402in that they are unsupported on many systems. If the
4602f195
JH
2403$quota is unsupported, it is an empty scalar. If it is supported, it
2404usually encodes the disk quota. If the $comment field is unsupported,
2405it is an empty scalar. If it is supported it usually encodes some
2406administrative comment about the user. In some systems the $quota
2407field may be $change or $age, fields that have to do with password
2408aging. In some systems the $comment field may be $class. The $expire
2409field, if present, encodes the expiration period of the account or the
2410password. For the availability and the exact meaning of these fields
8f1da26d 2411in your system, please consult getpwnam(3) and your system's
4602f195
JH
2412F<pwd.h> file. You can also find out from within Perl what your
2413$quota and $comment fields mean and whether you have the $expire field
2414by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2415C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2416files are supported only if your vendor has implemented them in the
4602f195 2417intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2418shadow versions if you're running under privilege or if there exists
cf264981 2419the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2420and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2421facility are unlikely to be supported.
6ee623d5 2422
a3390c9f 2423The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2424the login names of the members of the group.
2425
2426For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2427C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2428C<@addrs> value returned by a successful call is a list of raw
2429addresses returned by the corresponding library call. In the
2430Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2431by saying something like:
2432
f337b084 2433 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2434
2b5ab1e7
TC
2435The Socket library makes this slightly easier:
2436
2437 use Socket;
2438 $iaddr = inet_aton("127.1"); # or whatever address
2439 $name = gethostbyaddr($iaddr, AF_INET);
2440
2441 # or going the other way
19799a22 2442 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2443
d760c846
GS
2444In the opposite way, to resolve a hostname to the IP address
2445you can write this:
2446
2447 use Socket;
2448 $packed_ip = gethostbyname("www.perl.org");
2449 if (defined $packed_ip) {
2450 $ip_address = inet_ntoa($packed_ip);
2451 }
2452
b018eaf1 2453Make sure C<gethostbyname()> is called in SCALAR context and that
d760c846
GS
2454its return value is checked for definedness.
2455
0d043efa
FC
2456The C<getprotobynumber> function, even though it only takes one argument,
2457has the precedence of a list operator, so beware:
2458
2459 getprotobynumber $number eq 'icmp' # WRONG
2460 getprotobynumber($number eq 'icmp') # actually means this
2461 getprotobynumber($number) eq 'icmp' # better this way
2462
19799a22
GS
2463If you get tired of remembering which element of the return list
2464contains which return value, by-name interfaces are provided
2465in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2466C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2467and C<User::grent>. These override the normal built-ins, supplying
2468versions that return objects with the appropriate names
2469for each field. For example:
5a964f20
TC
2470
2471 use File::stat;
2472 use User::pwent;
2473 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2474
a3390c9f 2475Even though it looks as though they're the same method calls (uid),
b76cc8ba 2476they aren't, because a C<File::stat> object is different from
19799a22 2477a C<User::pwent> object.
5a964f20 2478
ea9eb35a
BJ
2479Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2480
a0d0e21e 2481=item getsockname SOCKET
d74e8afc 2482X<getsockname>
a0d0e21e 2483
19799a22
GS
2484Returns the packed sockaddr address of this end of the SOCKET connection,
2485in case you don't know the address because you have several different
2486IPs that the connection might have come in on.
a0d0e21e 2487
4633a7c4
LW
2488 use Socket;
2489 $mysockaddr = getsockname(SOCK);
19799a22 2490 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2491 printf "Connect to %s [%s]\n",
19799a22
GS
2492 scalar gethostbyaddr($myaddr, AF_INET),
2493 inet_ntoa($myaddr);
a0d0e21e
LW
2494
2495=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2496X<getsockopt>
a0d0e21e 2497
636e6b1f
TH
2498Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2499Options may exist at multiple protocol levels depending on the socket
2500type, but at least the uppermost socket level SOL_SOCKET (defined in the
2501C<Socket> module) will exist. To query options at another level the
2502protocol number of the appropriate protocol controlling the option
2503should be supplied. For example, to indicate that an option is to be
2504interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2505number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2506
80d38338 2507The function returns a packed string representing the requested socket
3b10bc60 2508option, or C<undef> on error, with the reason for the error placed in
a3390c9f 2509C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2510consult getsockopt(2) for details. A common case is that the option is an
2511integer, in which case the result is a packed integer, which you can decode
2512using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2513
8f1da26d 2514Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2515
4852725b 2516 use Socket qw(:all);
636e6b1f
TH
2517
2518 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2519 or die "Could not determine the protocol number for tcp";
4852725b
DD
2520 # my $tcp = IPPROTO_TCP; # Alternative
2521 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2522 or die "getsockopt TCP_NODELAY: $!";
636e6b1f
TH
2523 my $nodelay = unpack("I", $packed);
2524 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2525
ea9eb35a 2526Portability issues: L<perlport/getsockopt>.
a0d0e21e 2527
15a348aa
NC
2528=item given EXPR BLOCK
2529X<given>
2530
2531=item given BLOCK
2532
2533C<given> is analogous to the C<switch> keyword in other languages. C<given>
2534and C<when> are used in Perl to implement C<switch>/C<case> like statements.
8f1da26d 2535Only available after Perl 5.10. For example:
15a348aa 2536
8f1da26d 2537 use v5.10;
15a348aa
NC
2538 given ($fruit) {
2539 when (/apples?/) {
2540 print "I like apples."
2541 }
2542 when (/oranges?/) {
2543 print "I don't like oranges."
2544 }
2545 default {
2546 print "I don't like anything"
2547 }
2548 }
2549
2550See L<perlsyn/"Switch statements"> for detailed information.
2551
a0d0e21e 2552=item glob EXPR
d74e8afc 2553X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2554
0a753a76 2555=item glob
2556
d9a9d457
JL
2557In list context, returns a (possibly empty) list of filename expansions on
2558the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2559scalar context, glob iterates through such filename expansions, returning
2560undef when the list is exhausted. This is the internal function
2561implementing the C<< <*.c> >> operator, but you can use it directly. If
2562EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2563more detail in L<perlop/"I/O Operators">.
a0d0e21e 2564
80d38338
TC
2565Note that C<glob> splits its arguments on whitespace and treats
2566each segment as separate pattern. As such, C<glob("*.c *.h")>
2567matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2568C<glob(".* *")> matches all files in the current working directory.
a91bb7b1
TC
2569If you want to glob filenames that might contain whitespace, you'll
2570have to use extra quotes around the spacey filename to protect it.
2571For example, to glob filenames that have an C<e> followed by a space
2572followed by an C<f>, use either of:
2573
2574 @spacies = <"*e f*">;
2575 @spacies = glob '"*e f*"';
2576 @spacies = glob q("*e f*");
2577
2578If you had to get a variable through, you could do this:
2579
2580 @spacies = glob "'*${var}e f*'";
2581 @spacies = glob qq("*${var}e f*");
80d38338
TC
2582
2583If non-empty braces are the only wildcard characters used in the
2584C<glob>, no filenames are matched, but potentially many strings
2585are returned. For example, this produces nine strings, one for
2586each pairing of fruits and colors:
2587
2588 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2589
3a4b19e4 2590Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2591C<File::Glob> extension. See L<File::Glob> for details, including
2592C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2593
ea9eb35a
BJ
2594Portability issues: L<perlport/glob>.
2595
a0d0e21e 2596=item gmtime EXPR
d74e8afc 2597X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2598
ce2984c3
PF
2599=item gmtime
2600
4509d391 2601Works just like L</localtime> but the returned values are
435fbc73 2602localized for the standard Greenwich time zone.
a0d0e21e 2603
a3390c9f
FC
2604Note: When called in list context, $isdst, the last value
2605returned by gmtime, is always C<0>. There is no
435fbc73 2606Daylight Saving Time in GMT.
0a753a76 2607
ea9eb35a 2608Portability issues: L<perlport/gmtime>.
62aa5637 2609
a0d0e21e 2610=item goto LABEL
d74e8afc 2611X<goto> X<jump> X<jmp>
a0d0e21e 2612
748a9306
LW
2613=item goto EXPR
2614
a0d0e21e
LW
2615=item goto &NAME
2616
b500e03b
GG
2617The C<goto-LABEL> form finds the statement labeled with LABEL and
2618resumes execution there. It can't be used to get out of a block or
2619subroutine given to C<sort>. It can be used to go almost anywhere
2620else within the dynamic scope, including out of subroutines, but it's
2621usually better to use some other construct such as C<last> or C<die>.
2622The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2623(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2624does not offer named loops combined with loop control. Perl does, and
2625this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2626
7660c0ab
A
2627The C<goto-EXPR> form expects a label name, whose scope will be resolved
2628dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2629necessarily recommended if you're optimizing for maintainability:
2630
2631 goto ("FOO", "BAR", "GLARCH")[$i];
2632
887d89fd
FC
2633As shown in this example, C<goto-EXPR> is exempt from the "looks like a
2634function" rule. A pair of parentheses following it does not (necessarily)
2635delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2636
b500e03b 2637Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2638deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2639go into any construct that requires initialization, such as a
2640subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2641construct that is optimized away.
b500e03b 2642
1b6921cb
BT
2643The C<goto-&NAME> form is quite different from the other forms of
2644C<goto>. In fact, it isn't a goto in the normal sense at all, and
2645doesn't have the stigma associated with other gotos. Instead, it
2646exits the current subroutine (losing any changes set by local()) and
2647immediately calls in its place the named subroutine using the current
2648value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2649load another subroutine and then pretend that the other subroutine had
2650been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2651in the current subroutine are propagated to the other subroutine.)
2652After the C<goto>, not even C<caller> will be able to tell that this
2653routine was called first.
2654
2655NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2656containing a code reference or a block that evaluates to a code
6cb9131c 2657reference.
a0d0e21e
LW
2658
2659=item grep BLOCK LIST
d74e8afc 2660X<grep>
a0d0e21e
LW
2661
2662=item grep EXPR,LIST
2663
2b5ab1e7
TC
2664This is similar in spirit to, but not the same as, grep(1) and its
2665relatives. In particular, it is not limited to using regular expressions.
2f9daede 2666
a0d0e21e 2667Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2668C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2669elements for which the expression evaluated to true. In scalar
2670context, returns the number of times the expression was true.
a0d0e21e
LW
2671
2672 @foo = grep(!/^#/, @bar); # weed out comments
2673
2674or equivalently,
2675
2676 @foo = grep {!/^#/} @bar; # weed out comments
2677
be3174d2
GS
2678Note that C<$_> is an alias to the list value, so it can be used to
2679modify the elements of the LIST. While this is useful and supported,
2680it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2681Similarly, grep returns aliases into the original list, much as a for
2682loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2683element of a list returned by grep (for example, in a C<foreach>, C<map>
2684or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2685This is usually something to be avoided when writing clear code.
a0d0e21e 2686
a4fb8298 2687If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2688been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2689the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2690can't be seen from the outside, avoiding any potential side-effects.
2691
19799a22 2692See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2693
a0d0e21e 2694=item hex EXPR
d74e8afc 2695X<hex> X<hexadecimal>
a0d0e21e 2696
54310121 2697=item hex
bbce6d69 2698
2b5ab1e7 2699Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2700(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2701L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TP
2702
2703 print hex '0xAf'; # prints '175'
2704 print hex 'aF'; # same
a0d0e21e 2705
19799a22 2706Hex strings may only represent integers. Strings that would cause
53305cf1 2707integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11 2708unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 2709L</sprintf>, and L</unpack>.
19799a22 2710
ce2984c3 2711=item import LIST
d74e8afc 2712X<import>
a0d0e21e 2713
19799a22 2714There is no builtin C<import> function. It is just an ordinary
4633a7c4 2715method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2716names to another module. The C<use> function calls the C<import> method
cea6626f 2717for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2718
2719=item index STR,SUBSTR,POSITION
d74e8afc 2720X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2721
2722=item index STR,SUBSTR
2723
2b5ab1e7
TC
2724The index function searches for one string within another, but without
2725the wildcard-like behavior of a full regular-expression pattern match.
2726It returns the position of the first occurrence of SUBSTR in STR at
2727or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2728beginning of the string. POSITION before the beginning of the string
2729or after its end is treated as if it were the beginning or the end,
e1dccc0d
Z
2730respectively. POSITION and the return value are based at zero.
2731If the substring is not found, C<index> returns -1.
a0d0e21e
LW
2732
2733=item int EXPR
f723aae1 2734X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2735
54310121 2736=item int
bbce6d69 2737
7660c0ab 2738Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 2739You should not use this function for rounding: one because it truncates
3b10bc60 2740towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
2741numbers can sometimes produce counterintuitive results. For example,
2742C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2743because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2744the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2745functions will serve you better than will int().
a0d0e21e
LW
2746
2747=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2748X<ioctl>
a0d0e21e 2749
2b5ab1e7 2750Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2751
5ed4f2ec 2752 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2753
a11c483f 2754to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2755exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2756own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2757(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2758may help you in this, but it's nontrivial.) SCALAR will be read and/or
3b10bc60 2759written depending on the FUNCTION; a C pointer to the string value of SCALAR
19799a22 2760will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2761has no string value but does have a numeric value, that value will be
2762passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2763true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2764functions may be needed to manipulate the values of structures used by
b76cc8ba 2765C<ioctl>.
a0d0e21e 2766
19799a22 2767The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e 2768
5ed4f2ec 2769 if OS returns: then Perl returns:
2770 -1 undefined value
2771 0 string "0 but true"
2772 anything else that number
a0d0e21e 2773
19799a22 2774Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2775still easily determine the actual value returned by the operating
2776system:
2777
2b5ab1e7 2778 $retval = ioctl(...) || -1;
a0d0e21e
LW
2779 printf "System returned %d\n", $retval;
2780
be2f7487 2781The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2782about improper numeric conversions.
2783
ea9eb35a
BJ
2784Portability issues: L<perlport/ioctl>.
2785
a0d0e21e 2786=item join EXPR,LIST
d74e8afc 2787X<join>
a0d0e21e 2788
2b5ab1e7
TC
2789Joins the separate strings of LIST into a single string with fields
2790separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2791
2b5ab1e7 2792 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2793
eb6e2d6f
GS
2794Beware that unlike C<split>, C<join> doesn't take a pattern as its
2795first argument. Compare L</split>.
a0d0e21e 2796
532eee96 2797=item keys HASH
d74e8afc 2798X<keys> X<key>
aa689395 2799
532eee96 2800=item keys ARRAY
aeedbbed 2801
f5a93a43
TC
2802=item keys EXPR
2803
aeedbbed
NC
2804Returns a list consisting of all the keys of the named hash, or the indices
2805of an array. (In scalar context, returns the number of keys or indices.)
504f80c1 2806
aeedbbed 2807The keys of a hash are returned in an apparently random order. The actual
3b10bc60 2808random order is subject to change in future versions of Perl, but it
504f80c1 2809is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6 2810function produces (given that the hash has not been modified). Since
c5f61d2f 2811Perl 5.8.1 the ordering can be different even between different runs of
4546b9e6 2812Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2813Attacks">).
504f80c1 2814
8f1da26d 2815As a side effect, calling keys() resets the internal interator of the HASH or ARRAY
cf264981
SP
2816(see L</each>). In particular, calling keys() in void context resets
2817the iterator with no other overhead.
a0d0e21e 2818
aa689395 2819Here is yet another way to print your environment:
a0d0e21e
LW
2820
2821 @keys = keys %ENV;
2822 @values = values %ENV;
b76cc8ba 2823 while (@keys) {
a9a5a0dc 2824 print pop(@keys), '=', pop(@values), "\n";
a0d0e21e
LW
2825 }
2826
2827or how about sorted by key:
2828
2829 foreach $key (sort(keys %ENV)) {
a9a5a0dc 2830 print $key, '=', $ENV{$key}, "\n";
a0d0e21e
LW
2831 }
2832
8ea1e5d4
GS
2833The returned values are copies of the original keys in the hash, so
2834modifying them will not affect the original hash. Compare L</values>.
2835
19799a22 2836To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2837Here's a descending numeric sort of a hash by its values:
4633a7c4 2838
5a964f20 2839 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
a9a5a0dc 2840 printf "%4d %s\n", $hash{$key}, $key;
4633a7c4
LW
2841 }
2842
3b10bc60 2843Used as an lvalue, C<keys> allows you to increase the number of hash buckets
aa689395 2844allocated for the given hash. This can gain you a measure of efficiency if
2845you know the hash is going to get big. (This is similar to pre-extending
2846an array by assigning a larger number to $#array.) If you say
55497cff 2847
2848 keys %hash = 200;
2849
ab192400
GS
2850then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2851in fact, since it rounds up to the next power of two. These
55497cff 2852buckets will be retained even if you do C<%hash = ()>, use C<undef
2853%hash> if you want to free the storage while C<%hash> is still in scope.
2854You can't shrink the number of buckets allocated for the hash using
19799a22 2855C<keys> in this way (but you needn't worry about doing this by accident,
aeedbbed
NC
2856as trying has no effect). C<keys @array> in an lvalue context is a syntax
2857error.
55497cff 2858
f5a93a43
TC
2859Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
2860a reference to an unblessed hash or array. The argument will be
2861dereferenced automatically. This aspect of C<keys> is considered highly
2862experimental. The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
2863
2864 for (keys $hashref) { ... }
2865 for (keys $obj->get_arrayref) { ... }
2866
8f1da26d 2867See also C<each>, C<values>, and C<sort>.
ab192400 2868
b350dd2f 2869=item kill SIGNAL, LIST
9c7e4b76
KW
2870
2871=item kill SIGNAL
d74e8afc 2872X<kill> X<signal>
a0d0e21e 2873
b350dd2f 2874Sends a signal to a list of processes. Returns the number of
517db077
GS
2875processes successfully signaled (which is not necessarily the
2876same as the number actually killed).
a0d0e21e
LW
2877
2878 $cnt = kill 1, $child1, $child2;
2879 kill 9, @goners;
2880
3b10bc60 2881If SIGNAL is zero, no signal is sent to the process, but C<kill>
2882checks whether it's I<possible> to send a signal to it (that
70fb64f6 2883means, to be brief, that the process is owned by the same user, or we are
3b10bc60 2884the super-user). This is useful to check that a child process is still
81fd35db
DN
2885alive (even if only as a zombie) and hasn't changed its UID. See
2886L<perlport> for notes on the portability of this construct.
b350dd2f 2887
e2c0f81f
DG
2888Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2889of processes. That means you usually want to use positive not negative signals.
2890You may also use a signal name in quotes.
2891
2892The behavior of kill when a I<PROCESS> number is zero or negative depends on
2893the operating system. For example, on POSIX-conforming systems, zero will
2894signal the current process group and -1 will signal all processes.
1e9c1022
JL
2895
2896See L<perlipc/"Signals"> for more details.
a0d0e21e 2897
ea9eb35a
BJ
2898On some platforms such as Windows where the fork() system call is not available.
2899Perl can be built to emulate fork() at the interpreter level.
6d17f725 2900This emulation has limitations related to kill that have to be considered,
ea9eb35a
BJ
2901for code running on Windows and in code intended to be portable.
2902
2903See L<perlfork> for more details.
2904
9c7e4b76
KW
2905If there is no I<LIST> of processes, no signal is sent, and the return
2906value is 0. This form is sometimes used, however, because it causes
2907tainting checks to be run. But see
2908L<perlsec/Laundering and Detecting Tainted Data>.
2909
ea9eb35a
BJ
2910Portability issues: L<perlport/kill>.
2911
a0d0e21e 2912=item last LABEL
d74e8afc 2913X<last> X<break>
a0d0e21e
LW
2914
2915=item last
2916
2917The C<last> command is like the C<break> statement in C (as used in
2918loops); it immediately exits the loop in question. If the LABEL is
2919omitted, the command refers to the innermost enclosing loop. The
2920C<continue> block, if any, is not executed:
2921
4633a7c4 2922 LINE: while (<STDIN>) {
a9a5a0dc
VP
2923 last LINE if /^$/; # exit when done with header
2924 #...
a0d0e21e
LW
2925 }
2926
80d38338 2927C<last> cannot be used to exit a block that returns a value such as
8f1da26d 2928C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
2b5ab1e7 2929a grep() or map() operation.
4968c1e4 2930
6c1372ed
GS
2931Note that a block by itself is semantically identical to a loop
2932that executes once. Thus C<last> can be used to effect an early
2933exit out of such a block.
2934
98293880
JH
2935See also L</continue> for an illustration of how C<last>, C<next>, and
2936C<redo> work.
1d2dff63 2937
a0d0e21e 2938=item lc EXPR
d74e8afc 2939X<lc> X<lowercase>
a0d0e21e 2940
54310121 2941=item lc
bbce6d69 2942
d1be9408 2943Returns a lowercased version of EXPR. This is the internal function
3980dc9c 2944implementing the C<\L> escape in double-quoted strings.
a0d0e21e 2945
7660c0ab 2946If EXPR is omitted, uses C<$_>.
bbce6d69 2947
3980dc9c
KW
2948What gets returned depends on several factors:
2949
2950=over
2951
2952=item If C<use bytes> is in effect:
2953
2954=over
2955
2956=item On EBCDIC platforms
2957
2958The results are what the C language system call C<tolower()> returns.
2959
2960=item On ASCII platforms
2961
2962The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
2963respectively.
2964
2965=back
2966
094a2f8c 2967=item Otherwise, if C<use locale> is in effect
3980dc9c 2968
094a2f8c
KW
2969Respects current LC_CTYPE locale for code points < 256; and uses Unicode
2970semantics for the remaining code points (this last can only happen if
2971the UTF8 flag is also set). See L<perllocale>.
3980dc9c 2972
094a2f8c
KW
2973A deficiency in this is that case changes that cross the 255/256
2974boundary are not well-defined. For example, the lower case of LATIN CAPITAL
2975LETTER SHARP S (U+1E9E) in Unicode semantics is U+00DF (on ASCII
2976platforms). But under C<use locale>, the lower case of U+1E9E is
2977itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
2978current locale, and Perl has no way of knowing if that character even
2979exists in the locale, much less what code point it is. Perl returns
2980the input character unchanged, for all instances (and there aren't
2981many) where the 255/256 boundary would otherwise be crossed.
3980dc9c 2982
094a2f8c
KW
2983=item Otherwise, If EXPR has the UTF8 flag set
2984
2985Unicode semantics are used for the case change.
3980dc9c
KW
2986
2987=item Otherwise, if C<use feature 'unicode_strings'> is in effect:
2988
5d1892be 2989Unicode semantics are used for the case change.
3980dc9c
KW
2990
2991=item Otherwise:
2992
2993=over
2994
2995=item On EBCDIC platforms
2996
2997The results are what the C language system call C<tolower()> returns.
2998
2999=item On ASCII platforms
3000
3001ASCII semantics are used for the case change. The lowercase of any character
3002outside the ASCII range is the character itself.
3003
3004=back
3005
3006=back
3007
a0d0e21e 3008=item lcfirst EXPR
d74e8afc 3009X<lcfirst> X<lowercase>
a0d0e21e 3010
54310121 3011=item lcfirst
bbce6d69 3012
ad0029c4
JH
3013Returns the value of EXPR with the first character lowercased. This
3014is the internal function implementing the C<\l> escape in
3980dc9c 3015double-quoted strings.
a0d0e21e 3016
7660c0ab 3017If EXPR is omitted, uses C<$_>.
bbce6d69 3018
15dbbbab 3019This function behaves the same way under various pragmata, such as in a locale,
3980dc9c
KW
3020as L</lc> does.
3021
a0d0e21e 3022=item length EXPR
d74e8afc 3023X<length> X<size>
a0d0e21e 3024
54310121 3025=item length
bbce6d69 3026
974da8e5 3027Returns the length in I<characters> of the value of EXPR. If EXPR is
15dbbbab
FC
3028omitted, returns the length of C<$_>. If EXPR is undefined, returns
3029C<undef>.
3b10bc60 3030
3031This function cannot be used on an entire array or hash to find out how
3032many elements these have. For that, use C<scalar @array> and C<scalar keys
3033%hash>, respectively.
3034
3035Like all Perl character operations, length() normally deals in logical
3036characters, not physical bytes. For how many bytes a string encoded as
3037UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
3038to C<use Encode> first). See L<Encode> and L<perlunicode>.
974da8e5 3039
cfa52385
FC
3040=item __LINE__
3041X<__LINE__>
3042
3043A special token that compiles to the current line number.
3044
a0d0e21e 3045=item link OLDFILE,NEWFILE
d74e8afc 3046X<link>
a0d0e21e 3047
19799a22 3048Creates a new filename linked to the old filename. Returns true for
b76cc8ba 3049success, false otherwise.
a0d0e21e 3050
ea9eb35a
BJ
3051Portability issues: L<perlport/link>.
3052
a0d0e21e 3053=item listen SOCKET,QUEUESIZE
d74e8afc 3054X<listen>
a0d0e21e 3055
3b10bc60 3056Does the same thing that the listen(2) system call does. Returns true if
b76cc8ba 3057it succeeded, false otherwise. See the example in
cea6626f 3058L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
3059
3060=item local EXPR
d74e8afc 3061X<local>
a0d0e21e 3062
19799a22 3063You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 3064what most people think of as "local". See
13a2d996 3065L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 3066
5a964f20
TC
3067A local modifies the listed variables to be local to the enclosing
3068block, file, or eval. If more than one value is listed, the list must
3069be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3070for details, including issues with tied arrays and hashes.
a0d0e21e 3071
d361fafa
VP
3072The C<delete local EXPR> construct can also be used to localize the deletion
3073of array/hash elements to the current block.
3074See L<perlsub/"Localized deletion of elements of composite types">.
3075
a0d0e21e 3076=item localtime EXPR
435fbc73