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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
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110C<chomp>, C<chop>, C<chr>, C<crypt>, C<fc>, C<hex>, C<index>, C<lc>,
111C<lcfirst>, C<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///>
cb1a09d0 113
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114C<fc> is available only if the C<"fc"> feature is enabled or if it is
115prefixed with C<CORE::>. The C<"fc"> feature is enabled automatically
3dd9a840 116with a C<use v5.16> (or higher) declaration in the current scope.
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117
118
cb1a09d0 119=item Regular expressions and pattern matching
d74e8afc 120X<regular expression> X<regex> X<regexp>
cb1a09d0 121
f5fa2679 122C<m//>, C<pos>, C<qr//>, C<quotemeta>, C<s///>, C<split>, C<study>
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123
124=item Numeric functions
d74e8afc 125X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 126
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127C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
128C<sin>, C<sqrt>, C<srand>
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129
130=item Functions for real @ARRAYs
d74e8afc 131X<array>
cb1a09d0 132
a5ce339c 133C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
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134
135=item Functions for list data
d74e8afc 136X<list>
cb1a09d0 137
1dc8ecb8 138C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
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139
140=item Functions for real %HASHes
d74e8afc 141X<hash>
cb1a09d0 142
22fae026 143C<delete>, C<each>, C<exists>, C<keys>, C<values>
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144
145=item Input and output functions
d74e8afc 146X<I/O> X<input> X<output> X<dbm>
cb1a09d0 147
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148C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
149C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
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150C<readdir>, C<readline> C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>,
151C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>,
152C<truncate>, C<warn>, C<write>
cb1a09d0 153
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154C<say> is available only if the C<"say"> feature is enabled or if it is
155prefixed with C<CORE::>. The C<"say"> feature is enabled automatically
156with a C<use v5.10> (or higher) declaration in the current scope.
157
5dac7880 158=item Functions for fixed-length data or records
cb1a09d0 159
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160C<pack>, C<read>, C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<unpack>,
161C<vec>
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162
163=item Functions for filehandles, files, or directories
d74e8afc 164X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 165
22fae026 166C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 167C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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168C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
169C<umask>, C<unlink>, C<utime>
cb1a09d0 170
cf264981 171=item Keywords related to the control flow of your Perl program
d74e8afc 172X<control flow>
cb1a09d0 173
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174C<caller>, C<continue>, C<die>, C<do>,
175C<dump>, C<eval>, C<evalbytes> C<exit>,
cfa52385 176C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
7c919445 177C<prototype>, C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray>
84ed0108 178
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179C<evalbytes> is only available with with the C<"evalbytes"> feature (see
180L<feature>) or if prefixed with C<CORE::>. C<__SUB__> is only available
181with with the C<"current_sub"> feature or if prefixed with C<CORE::>. Both
182the C<"evalbytes"> and C<"current_sub"> features are enabled automatically
183with a C<use v5.16> (or higher) declaration in the current scope.
cb1a09d0 184
8f1da26d 185=item Keywords related to the switch feature
0d863452 186
4a904372 187C<break>, C<continue>, C<default>, C<given>, C<when>
0d863452 188
4a904372 189Except for C<continue>, these are available only if you enable the
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190C<"switch"> feature or use the C<CORE::> prefix. See L<feature> and
191L<perlsyn/"Switch Statements">. The C<"switch"> feature is enabled
192automatically with a C<use v5.10> (or higher) declaration in the current
193scope. In Perl 5.14 and earlier, C<continue> required the C<"switch">
194feature, like the other keywords.
0d863452 195
54310121 196=item Keywords related to scoping
cb1a09d0 197
8f1da26d 198C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
36fb85f3 199
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200C<state> is available only if the C<"state"> feature is enabled or if it is
201prefixed with C<CORE::>. The C<"state"> feature is enabled automatically
202with a C<use v5.10> (or higher) declaration in the current scope.
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203
204=item Miscellaneous functions
205
7289c5e6 206C<defined>, C<dump>, C<eval>, C<evalbytes>,
7c919445 207C<formline>, C<local>, C<lock>, C<my>, C<our>, C<prototype>,
834df1c5 208C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
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209
210=item Functions for processes and process groups
d74e8afc 211X<process> X<pid> X<process id>
cb1a09d0 212
22fae026 213C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
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214C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>,
215C<setpriority>, C<sleep>, C<system>,
22fae026 216C<times>, C<wait>, C<waitpid>
cb1a09d0 217
3b10bc60 218=item Keywords related to Perl modules
d74e8afc 219X<module>
cb1a09d0 220
22fae026 221C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 222
353c6505 223=item Keywords related to classes and object-orientation
d74e8afc 224X<object> X<class> X<package>
cb1a09d0 225
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226C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
227C<untie>, C<use>
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228
229=item Low-level socket functions
d74e8afc 230X<socket> X<sock>
cb1a09d0 231
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232C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
233C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 234C<socket>, C<socketpair>
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235
236=item System V interprocess communication functions
d74e8afc 237X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 238
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239C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
240C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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241
242=item Fetching user and group info
d74e8afc 243X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 244
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245C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
246C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
247C<getpwuid>, C<setgrent>, C<setpwent>
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248
249=item Fetching network info
d74e8afc 250X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 251
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252C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
253C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
254C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
255C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
256C<setnetent>, C<setprotoent>, C<setservent>
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257
258=item Time-related functions
d74e8afc 259X<time> X<date>
cb1a09d0 260
22fae026 261C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 262
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263=item Non-function keywords
264
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265C<and>, C<AUTOLOAD>, C<BEGIN>, C<CHECK>, C<cmp>, C<CORE>, C<__DATA__>,
266C<DESTROY>, C<else>, C<elseif>, C<elsif>, C<END>, C<__END__>, C<eq>, C<for>,
267C<foreach>, C<ge>, C<gt>, C<if>, C<INIT>, C<le>, C<lt>, C<ne>, C<not>, C<or>,
268C<UNITCHECK>, C<unless>, C<until>, C<while>, C<x>, C<xor>
8f0d6a61 269
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270=back
271
60f9f73c 272=head2 Portability
d74e8afc 273X<portability> X<Unix> X<portable>
60f9f73c 274
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275Perl was born in Unix and can therefore access all common Unix
276system calls. In non-Unix environments, the functionality of some
8f1da26d 277Unix system calls may not be available or details of the available
2b5ab1e7 278functionality may differ slightly. The Perl functions affected
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279by this are:
280
281C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
282C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
283C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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284C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
285C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 286C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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287C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
288C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
289C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 290C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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291C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
292C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
293C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 294C<shmwrite>, C<socket>, C<socketpair>,
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295C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
296C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 297C<utime>, C<wait>, C<waitpid>
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298
299For more information about the portability of these functions, see
300L<perlport> and other available platform-specific documentation.
301
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302=head2 Alphabetical Listing of Perl Functions
303
3b10bc60 304=over
a0d0e21e 305
5b3c99c0 306=item -X FILEHANDLE
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307X<-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>
308X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 309
5b3c99c0 310=item -X EXPR
a0d0e21e 311
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312=item -X DIRHANDLE
313
5b3c99c0 314=item -X
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315
316A file test, where X is one of the letters listed below. This unary
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317operator takes one argument, either a filename, a filehandle, or a dirhandle,
318and tests the associated file to see if something is true about it. If the
7660c0ab 319argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 320Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 321the undefined value if the file doesn't exist. Despite the funny
d0821a6a 322names, precedence is the same as any other named unary operator. The
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323operator may be any of:
324
5ed4f2ec 325 -r File is readable by effective uid/gid.
326 -w File is writable by effective uid/gid.
327 -x File is executable by effective uid/gid.
328 -o File is owned by effective uid.
a0d0e21e 329
5ed4f2ec 330 -R File is readable by real uid/gid.
331 -W File is writable by real uid/gid.
332 -X File is executable by real uid/gid.
333 -O File is owned by real uid.
a0d0e21e 334
5ed4f2ec 335 -e File exists.
336 -z File has zero size (is empty).
337 -s File has nonzero size (returns size in bytes).
a0d0e21e 338
5ed4f2ec 339 -f File is a plain file.
340 -d File is a directory.
341 -l File is a symbolic link.
342 -p File is a named pipe (FIFO), or Filehandle is a pipe.
343 -S File is a socket.
344 -b File is a block special file.
345 -c File is a character special file.
346 -t Filehandle is opened to a tty.
a0d0e21e 347
5ed4f2ec 348 -u File has setuid bit set.
349 -g File has setgid bit set.
350 -k File has sticky bit set.
a0d0e21e 351
5ed4f2ec 352 -T File is an ASCII text file (heuristic guess).
353 -B File is a "binary" file (opposite of -T).
a0d0e21e 354
5ed4f2ec 355 -M Script start time minus file modification time, in days.
356 -A Same for access time.
357 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 358
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359Example:
360
361 while (<>) {
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362 chomp;
363 next unless -f $_; # ignore specials
364 #...
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365 }
366
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367Note that C<-s/a/b/> does not do a negated substitution. Saying
368C<-exp($foo)> still works as expected, however: only single letters
369following a minus are interpreted as file tests.
370
371These operators are exempt from the "looks like a function rule" described
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372above. That is, an opening parenthesis after the operator does not affect
373how much of the following code constitutes the argument. Put the opening
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374parentheses before the operator to separate it from code that follows (this
375applies only to operators with higher precedence than unary operators, of
376course):
377
378 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
379 (-s $file) + 1024 # correct
380
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381The interpretation of the file permission operators C<-r>, C<-R>,
382C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
383of the file and the uids and gids of the user. There may be other
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384reasons you can't actually read, write, or execute the file: for
385example network filesystem access controls, ACLs (access control lists),
386read-only filesystems, and unrecognized executable formats. Note
387that the use of these six specific operators to verify if some operation
388is possible is usually a mistake, because it may be open to race
389conditions.
5ff3f7a4 390
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391Also note that, for the superuser on the local filesystems, the C<-r>,
392C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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393if any execute bit is set in the mode. Scripts run by the superuser
394may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 395or temporarily set their effective uid to something else.
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396
397If you are using ACLs, there is a pragma called C<filetest> that may
398produce more accurate results than the bare stat() mode bits.
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399When under C<use filetest 'access'> the above-mentioned filetests
400test whether the permission can(not) be granted using the
3b10bc60 401access(2) family of system calls. Also note that the C<-x> and C<-X> may
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402under this pragma return true even if there are no execute permission
403bits set (nor any extra execute permission ACLs). This strangeness is
391b733c 404due to the underlying system calls' definitions. Note also that, due to
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405the implementation of C<use filetest 'access'>, the C<_> special
406filehandle won't cache the results of the file tests when this pragma is
407in effect. Read the documentation for the C<filetest> pragma for more
408information.
5ff3f7a4 409
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410The C<-T> and C<-B> switches work as follows. The first block or so of the
411file is examined for odd characters such as strange control codes or
61eff3bc 412characters with the high bit set. If too many strange characters (>30%)
cf264981 413are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 414containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 415or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 416rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 417file, or a file at EOF when testing a filehandle. Because you have to
4633a7c4
LW
418read a file to do the C<-T> test, on most occasions you want to use a C<-f>
419against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 420
5dac7880 421If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 422the special filehandle consisting of a solitary underline, then the stat
a0d0e21e
LW
423structure of the previous file test (or stat operator) is used, saving
424a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 425that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 426symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 427an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 428Example:
a0d0e21e
LW
429
430 print "Can do.\n" if -r $a || -w _ || -x _;
431
432 stat($filename);
433 print "Readable\n" if -r _;
434 print "Writable\n" if -w _;
435 print "Executable\n" if -x _;
436 print "Setuid\n" if -u _;
437 print "Setgid\n" if -g _;
438 print "Sticky\n" if -k _;
439 print "Text\n" if -T _;
440 print "Binary\n" if -B _;
441
fbb0b3b3
RGS
442As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
443test operators, in a way that C<-f -w -x $file> is equivalent to
391b733c 444C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
fbb0b3b3
RGS
445the return value of C<-f $file> as an argument to another filetest
446operator, no special magic will happen.)
447
bee96257 448Portability issues: L<perlport/-X>.
ea9eb35a 449
bade7fbc
TC
450To avoid confusing would-be users of your code with mysterious
451syntax errors, put something like this at the top of your script:
452
453 use 5.010; # so filetest ops can stack
454
a0d0e21e 455=item abs VALUE
d74e8afc 456X<abs> X<absolute>
a0d0e21e 457
54310121 458=item abs
bbce6d69 459
a0d0e21e 460Returns the absolute value of its argument.
7660c0ab 461If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
462
463=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 464X<accept>
a0d0e21e 465
3b10bc60 466Accepts an incoming socket connect, just as accept(2)
19799a22 467does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 468See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 469
8d2a6795
GS
470On systems that support a close-on-exec flag on files, the flag will
471be set for the newly opened file descriptor, as determined by the
472value of $^F. See L<perlvar/$^F>.
473
a0d0e21e 474=item alarm SECONDS
d74e8afc
ITB
475X<alarm>
476X<SIGALRM>
477X<timer>
a0d0e21e 478
54310121 479=item alarm
bbce6d69 480
a0d0e21e 481Arranges to have a SIGALRM delivered to this process after the
cf264981 482specified number of wallclock seconds has elapsed. If SECONDS is not
391b733c 483specified, the value stored in C<$_> is used. (On some machines,
d400eac8
JH
484unfortunately, the elapsed time may be up to one second less or more
485than you specified because of how seconds are counted, and process
486scheduling may delay the delivery of the signal even further.)
487
488Only one timer may be counting at once. Each call disables the
489previous timer, and an argument of C<0> may be supplied to cancel the
490previous timer without starting a new one. The returned value is the
491amount of time remaining on the previous timer.
a0d0e21e 492
2bc69794
BS
493For delays of finer granularity than one second, the Time::HiRes module
494(from CPAN, and starting from Perl 5.8 part of the standard
495distribution) provides ualarm(). You may also use Perl's four-argument
496version of select() leaving the first three arguments undefined, or you
497might be able to use the C<syscall> interface to access setitimer(2) if
391b733c 498your system supports it. See L<perlfaq8> for details.
2b5ab1e7 499
80d38338
TC
500It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
501C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 502
19799a22
GS
503If you want to use C<alarm> to time out a system call you need to use an
504C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 505fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 506restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 507modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
508
509 eval {
a9a5a0dc
VP
510 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
511 alarm $timeout;
512 $nread = sysread SOCKET, $buffer, $size;
513 alarm 0;
ff68c719 514 };
ff68c719 515 if ($@) {
a9a5a0dc 516 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 517 # timed out
ff68c719
PP
518 }
519 else {
5ed4f2ec 520 # didn't
ff68c719
PP
521 }
522
91d81acc
JH
523For more information see L<perlipc>.
524
ea9eb35a 525Portability issues: L<perlport/alarm>.
526
a0d0e21e 527=item atan2 Y,X
d74e8afc 528X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
529
530Returns the arctangent of Y/X in the range -PI to PI.
531
ca6e1c26 532For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
533function, or use the familiar relation:
534
535 sub tan { sin($_[0]) / cos($_[0]) }
536
a1021d57
RGS
537The return value for C<atan2(0,0)> is implementation-defined; consult
538your atan2(3) manpage for more information.
bf5f1b4c 539
ea9eb35a 540Portability issues: L<perlport/atan2>.
541
a0d0e21e 542=item bind SOCKET,NAME
d74e8afc 543X<bind>
a0d0e21e 544
3b10bc60 545Binds a network address to a socket, just as bind(2)
19799a22 546does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
547packed address of the appropriate type for the socket. See the examples in
548L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 549
fae2c0fb 550=item binmode FILEHANDLE, LAYER
d74e8afc 551X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 552
a0d0e21e
LW
553=item binmode FILEHANDLE
554
1cbfc93d
NIS
555Arranges for FILEHANDLE to be read or written in "binary" or "text"
556mode on systems where the run-time libraries distinguish between
557binary and text files. If FILEHANDLE is an expression, the value is
558taken as the name of the filehandle. Returns true on success,
b5fe5ca2 559otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 560
8f1da26d 561On some systems (in general, DOS- and Windows-based systems) binmode()
d807c6f4 562is necessary when you're not working with a text file. For the sake
d7a0d798
FC
563of portability it is a good idea always to use it when appropriate,
564and never to use it when it isn't appropriate. Also, people can
8f1da26d 565set their I/O to be by default UTF8-encoded Unicode, not bytes.
d807c6f4
JH
566
567In other words: regardless of platform, use binmode() on binary data,
d7a0d798 568like images, for example.
d807c6f4
JH
569
570If LAYER is present it is a single string, but may contain multiple
391b733c 571directives. The directives alter the behaviour of the filehandle.
d7a0d798 572When LAYER is present, using binmode on a text file makes sense.
d807c6f4 573
fae2c0fb 574If LAYER is omitted or specified as C<:raw> the filehandle is made
391b733c 575suitable for passing binary data. This includes turning off possible CRLF
0226bbdb 576translation and marking it as bytes (as opposed to Unicode characters).
749683d2 577Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 578Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
579Other layers that would affect the binary nature of the stream are
391b733c 580I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 581PERLIO environment variable.
01e6739c 582
3b10bc60 583The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
584form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
585establish default I/O layers. See L<open>.
586
fae2c0fb
RGS
587I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
588in "Programming Perl, 3rd Edition". However, since the publishing of this
589book, by many known as "Camel III", the consensus of the naming of this
590functionality has moved from "discipline" to "layer". All documentation
591of this version of Perl therefore refers to "layers" rather than to
592"disciplines". Now back to the regularly scheduled documentation...>
593
8f1da26d 594To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
6902c96a 595C<:utf8> just marks the data as UTF-8 without further checking,
8f1da26d 596while C<:encoding(UTF-8)> checks the data for actually being valid
391b733c 597UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 598
ed53a2bb 599In general, binmode() should be called after open() but before any I/O
3b10bc60 600is done on the filehandle. Calling binmode() normally flushes any
01e6739c 601pending buffered output data (and perhaps pending input data) on the
fae2c0fb 602handle. An exception to this is the C<:encoding> layer that
d7a0d798 603changes the default character encoding of the handle; see L</open>.
fae2c0fb 604The C<:encoding> layer sometimes needs to be called in
3874323d
JH
605mid-stream, and it doesn't flush the stream. The C<:encoding>
606also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 607internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 608
19799a22 609The operating system, device drivers, C libraries, and Perl run-time
8f1da26d
TC
610system all conspire to let the programmer treat a single
611character (C<\n>) as the line terminator, irrespective of external
30168b04
GS
612representation. On many operating systems, the native text file
613representation matches the internal representation, but on some
614platforms the external representation of C<\n> is made up of more than
615one character.
616
8f1da26d
TC
617All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
618a single character to end each line in the external representation of text
619(even though that single character is CARRIAGE RETURN on old, pre-Darwin
391b733c 620flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
8f1da26d
TC
621systems like OS/2, DOS, and the various flavors of MS-Windows, your program
622sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
623two characters C<\cM\cJ>. That means that if you don't use binmode() on
624these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
625input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
626output. This is what you want for text files, but it can be disastrous for
627binary files.
30168b04
GS
628
629Another consequence of using binmode() (on some systems) is that
630special end-of-file markers will be seen as part of the data stream.
d7a0d798
FC
631For systems from the Microsoft family this means that, if your binary
632data contain C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
633the file, unless you use binmode().
634
3b10bc60 635binmode() is important not only for readline() and print() operations,
30168b04
GS
636but also when using read(), seek(), sysread(), syswrite() and tell()
637(see L<perlport> for more details). See the C<$/> and C<$\> variables
638in L<perlvar> for how to manually set your input and output
639line-termination sequences.
a0d0e21e 640
ea9eb35a 641Portability issues: L<perlport/binmode>.
642
4633a7c4 643=item bless REF,CLASSNAME
d74e8afc 644X<bless>
a0d0e21e
LW
645
646=item bless REF
647
2b5ab1e7
TC
648This function tells the thingy referenced by REF that it is now an object
649in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 650is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 651it returns the reference for convenience. Always use the two-argument
cf264981 652version if a derived class might inherit the function doing the blessing.
82e1c0d9 653SeeL<perlobj> for more about the blessing (and blessings) of objects.
a0d0e21e 654
57668c4d 655Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 656Namespaces with all lowercase names are considered reserved for
391b733c 657Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
658confusion, you may wish to avoid such package names as well. Make sure
659that CLASSNAME is a true value.
60ad88b8
GS
660
661See L<perlmod/"Perl Modules">.
662
0d863452
RH
663=item break
664
665Break out of a C<given()> block.
666
8f1da26d 667This keyword is enabled by the C<"switch"> feature: see
4a904372
FC
668L<feature> for more information. You can also access it by
669prefixing it with C<CORE::>. Alternately, include a C<use
8f1da26d 670v5.10> or later to the current scope.
0d863452 671
a0d0e21e 672=item caller EXPR
d74e8afc 673X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
674
675=item caller
676
5a964f20 677Returns the context of the current subroutine call. In scalar context,
80d38338
TC
678returns the caller's package name if there I<is> a caller (that is, if
679we're in a subroutine or C<eval> or C<require>) and the undefined value
5a964f20 680otherwise. In list context, returns
a0d0e21e 681
ee6b43cc 682 # 0 1 2
748a9306 683 ($package, $filename, $line) = caller;
a0d0e21e
LW
684
685With EXPR, it returns some extra information that the debugger uses to
686print a stack trace. The value of EXPR indicates how many call frames
687to go back before the current one.
688
ee6b43cc 689 # 0 1 2 3 4
f3aa04c2 690 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 691
692 # 5 6 7 8 9 10
b3ca2e83 693 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 694 = caller($i);
e7ea3e70 695
951ba7fe 696Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 697call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 698C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 699C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 700C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 701$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
702each C<use> statement creates a C<require> frame inside an C<eval EXPR>
703frame.) $subroutine may also be C<(unknown)> if this particular
704subroutine happens to have been deleted from the symbol table.
705C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
706C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
707compiled with. The C<$hints> and C<$bitmask> values are subject to change
708between versions of Perl, and are not meant for external use.
748a9306 709
b3ca2e83 710C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
391b733c 711caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
b3ca2e83
NC
712of this hash, as they are the actual values stored in the optree.
713
ffe0c19d
FC
714Furthermore, when called from within the DB package in
715list context, and with an argument, caller returns more
7660c0ab 716detailed information: it sets the list variable C<@DB::args> to be the
54310121 717arguments with which the subroutine was invoked.
748a9306 718
7660c0ab 719Be aware that the optimizer might have optimized call frames away before
19799a22 720C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 721might not return information about the call frame you expect it to, for
b76cc8ba 722C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 723previous time C<caller> was called.
7660c0ab 724
8f1da26d 725Be aware that setting C<@DB::args> is I<best effort>, intended for
391b733c 726debugging or generating backtraces, and should not be relied upon. In
ca9f0cb5
NC
727particular, as C<@_> contains aliases to the caller's arguments, Perl does
728not take a copy of C<@_>, so C<@DB::args> will contain modifications the
729subroutine makes to C<@_> or its contents, not the original values at call
391b733c 730time. C<@DB::args>, like C<@_>, does not hold explicit references to its
ca9f0cb5 731elements, so under certain cases its elements may have become freed and
391b733c 732reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 733of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
734I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
735reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5 736elements), so C<@DB::args> is actually a hybrid of the current state and
391b733c 737initial state of C<@_>. Buyer beware.
ca9f0cb5 738
a0d0e21e 739=item chdir EXPR
d74e8afc
ITB
740X<chdir>
741X<cd>
f723aae1 742X<directory, change>
a0d0e21e 743
c4aca7d0
GA
744=item chdir FILEHANDLE
745
746=item chdir DIRHANDLE
747
ce2984c3
PF
748=item chdir
749
391b733c 750Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 751changes to the directory specified by C<$ENV{HOME}>, if set; if not,
391b733c
FC
752changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
753variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
754neither is set, C<chdir> does nothing. It returns true on success,
755false otherwise. See the example under C<die>.
a0d0e21e 756
3b10bc60 757On systems that support fchdir(2), you may pass a filehandle or
34169887 758directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 759passing handles raises an exception.
c4aca7d0 760
a0d0e21e 761=item chmod LIST
d74e8afc 762X<chmod> X<permission> X<mode>
a0d0e21e
LW
763
764Changes the permissions of a list of files. The first element of the
8f1da26d 765list must be the numeric mode, which should probably be an octal
4ad40acf 766number, and which definitely should I<not> be a string of octal digits:
3b10bc60 767C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 768successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 769
3b10bc60 770 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 771 chmod 0755, @executables;
3b10bc60 772 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 773 # --w----r-T
3b10bc60 774 $mode = "0644"; chmod oct($mode), "foo"; # this is better
775 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 776
3b10bc60 777On systems that support fchmod(2), you may pass filehandles among the
778files. On systems that don't support fchmod(2), passing filehandles raises
779an exception. Filehandles must be passed as globs or glob references to be
780recognized; barewords are considered filenames.
c4aca7d0
GA
781
782 open(my $fh, "<", "foo");
783 my $perm = (stat $fh)[2] & 07777;
784 chmod($perm | 0600, $fh);
785
3b10bc60 786You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
787module:
788
3b10bc60 789 use Fcntl qw( :mode );
ca6e1c26 790 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 791 # Identical to the chmod 0755 of the example above.
ca6e1c26 792
ea9eb35a 793Portability issues: L<perlport/chmod>.
794
a0d0e21e 795=item chomp VARIABLE
d74e8afc 796X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 797
313c9f5c 798=item chomp( LIST )
a0d0e21e
LW
799
800=item chomp
801
2b5ab1e7
TC
802This safer version of L</chop> removes any trailing string
803that corresponds to the current value of C<$/> (also known as
28757baa
PP
804$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
805number of characters removed from all its arguments. It's often used to
806remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
807that the final record may be missing its newline. When in paragraph
808mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 809When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 810a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 811remove anything.
19799a22 812If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
813
814 while (<>) {
a9a5a0dc
VP
815 chomp; # avoid \n on last field
816 @array = split(/:/);
817 # ...
a0d0e21e
LW
818 }
819
4bf21a6d
RD
820If VARIABLE is a hash, it chomps the hash's values, but not its keys.
821
a0d0e21e
LW
822You can actually chomp anything that's an lvalue, including an assignment:
823
824 chomp($cwd = `pwd`);
825 chomp($answer = <STDIN>);
826
827If you chomp a list, each element is chomped, and the total number of
828characters removed is returned.
829
15e44fd8
RGS
830Note that parentheses are necessary when you're chomping anything
831that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
832is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
833C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
834C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
835as C<chomp($a, $b)>.
836
a0d0e21e 837=item chop VARIABLE
d74e8afc 838X<chop>
a0d0e21e 839
313c9f5c 840=item chop( LIST )
a0d0e21e
LW
841
842=item chop
843
844Chops off the last character of a string and returns the character
5b3eff12 845chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 846scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
847If VARIABLE is a hash, it chops the hash's values, but not its keys.
848
5b3eff12 849You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
850
851If you chop a list, each element is chopped. Only the value of the
19799a22 852last C<chop> is returned.
a0d0e21e 853
19799a22 854Note that C<chop> returns the last character. To return all but the last
748a9306
LW
855character, use C<substr($string, 0, -1)>.
856
15e44fd8
RGS
857See also L</chomp>.
858
a0d0e21e 859=item chown LIST
d74e8afc 860X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
861
862Changes the owner (and group) of a list of files. The first two
19799a22
GS
863elements of the list must be the I<numeric> uid and gid, in that
864order. A value of -1 in either position is interpreted by most
865systems to leave that value unchanged. Returns the number of files
866successfully changed.
a0d0e21e
LW
867
868 $cnt = chown $uid, $gid, 'foo', 'bar';
869 chown $uid, $gid, @filenames;
870
3b10bc60 871On systems that support fchown(2), you may pass filehandles among the
872files. On systems that don't support fchown(2), passing filehandles raises
873an exception. Filehandles must be passed as globs or glob references to be
874recognized; barewords are considered filenames.
c4aca7d0 875
54310121 876Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
877
878 print "User: ";
19799a22 879 chomp($user = <STDIN>);
5a964f20 880 print "Files: ";
19799a22 881 chomp($pattern = <STDIN>);
a0d0e21e
LW
882
883 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 884 or die "$user not in passwd file";
a0d0e21e 885
5ed4f2ec 886 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
887 chown $uid, $gid, @ary;
888
54310121 889On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
890file unless you're the superuser, although you should be able to change
891the group to any of your secondary groups. On insecure systems, these
892restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
893On POSIX systems, you can detect this condition this way:
894
895 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
896 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 897
ea9eb35a 898Portability issues: L<perlport/chmod>.
899
a0d0e21e 900=item chr NUMBER
d74e8afc 901X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 902
54310121 903=item chr
bbce6d69 904
a0d0e21e 905Returns the character represented by that NUMBER in the character set.
a0ed51b3 906For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 907chr(0x263a) is a Unicode smiley face.
aaa68c4a 908
8a064bd6 909Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 910except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
911(truncated to an integer) are used.
912
974da8e5
JH
913If NUMBER is omitted, uses C<$_>.
914
b76cc8ba 915For the reverse, use L</ord>.
a0d0e21e 916
2575c402
JW
917Note that characters from 128 to 255 (inclusive) are by default
918internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 919
2575c402 920See L<perlunicode> for more about Unicode.
bbce6d69 921
a0d0e21e 922=item chroot FILENAME
d74e8afc 923X<chroot> X<root>
a0d0e21e 924
54310121 925=item chroot
bbce6d69 926
5a964f20 927This function works like the system call by the same name: it makes the
4633a7c4 928named directory the new root directory for all further pathnames that
951ba7fe 929begin with a C</> by your process and all its children. (It doesn't
28757baa 930change your current working directory, which is unaffected.) For security
4633a7c4 931reasons, this call is restricted to the superuser. If FILENAME is
19799a22 932omitted, does a C<chroot> to C<$_>.
a0d0e21e 933
ea9eb35a 934Portability issues: L<perlport/chroot>.
935
a0d0e21e 936=item close FILEHANDLE
d74e8afc 937X<close>
a0d0e21e 938
6a518fbc
TP
939=item close
940
3b10bc60 941Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 942buffers, and closes the system file descriptor. Returns true if those
8f1da26d 943operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
944layer. Closes the currently selected filehandle if the argument is
945omitted.
fb73857a
PP
946
947You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 948another C<open> on it, because C<open> closes it for you. (See
01aa884e 949L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
19799a22 950counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 951
3b10bc60 952If the filehandle came from a piped open, C<close> returns false if one of
953the other syscalls involved fails or if its program exits with non-zero
954status. If the only problem was that the program exited non-zero, C<$!>
955will be set to C<0>. Closing a pipe also waits for the process executing
956on the pipe to exit--in case you wish to look at the output of the pipe
957afterwards--and implicitly puts the exit status value of that command into
958C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 959
2e0cfa16
FC
960If there are multiple threads running, C<close> on a filehandle from a
961piped open returns true without waiting for the child process to terminate,
962if the filehandle is still open in another thread.
963
80d38338
TC
964Closing the read end of a pipe before the process writing to it at the
965other end is done writing results in the writer receiving a SIGPIPE. If
966the other end can't handle that, be sure to read all the data before
967closing the pipe.
73689b13 968
fb73857a 969Example:
a0d0e21e 970
fb73857a
PP
971 open(OUTPUT, '|sort >foo') # pipe to sort
972 or die "Can't start sort: $!";
5ed4f2ec 973 #... # print stuff to output
974 close OUTPUT # wait for sort to finish
fb73857a
PP
975 or warn $! ? "Error closing sort pipe: $!"
976 : "Exit status $? from sort";
5ed4f2ec 977 open(INPUT, 'foo') # get sort's results
fb73857a 978 or die "Can't open 'foo' for input: $!";
a0d0e21e 979
5a964f20 980FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 981filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
982
983=item closedir DIRHANDLE
d74e8afc 984X<closedir>
a0d0e21e 985
19799a22 986Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
987system call.
988
a0d0e21e 989=item connect SOCKET,NAME
d74e8afc 990X<connect>
a0d0e21e 991
80d38338
TC
992Attempts to connect to a remote socket, just like connect(2).
993Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
994packed address of the appropriate type for the socket. See the examples in
995L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 996
cb1a09d0 997=item continue BLOCK
d74e8afc 998X<continue>
cb1a09d0 999
0d863452
RH
1000=item continue
1001
4a904372
FC
1002When followed by a BLOCK, C<continue> is actually a
1003flow control statement rather than a function. If
cf264981 1004there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
1005C<foreach>), it is always executed just before the conditional is about to
1006be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
1007it can be used to increment a loop variable, even when the loop has been
1008continued via the C<next> statement (which is similar to the C C<continue>
1009statement).
1010
98293880 1011C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 1012block; C<last> and C<redo> behave as if they had been executed within
19799a22 1013the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
1014block, it may be more entertaining.
1015
1016 while (EXPR) {
a9a5a0dc
VP
1017 ### redo always comes here
1018 do_something;
1d2dff63 1019 } continue {
a9a5a0dc
VP
1020 ### next always comes here
1021 do_something_else;
1022 # then back the top to re-check EXPR
1d2dff63
GS
1023 }
1024 ### last always comes here
1025
3b10bc60 1026Omitting the C<continue> section is equivalent to using an
1027empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1028to check the condition at the top of the loop.
1029
4a904372 1030When there is no BLOCK, C<continue> is a function that
8f1da26d
TC
1031falls through the current C<when> or C<default> block instead of iterating
1032a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
4a904372
FC
1033In Perl 5.14 and earlier, this form of C<continue> was
1034only available when the C<"switch"> feature was enabled.
48238296 1035See L<feature> and L<perlsyn/"Switch Statements"> for more
8f1da26d 1036information.
0d863452 1037
a0d0e21e 1038=item cos EXPR
d74e8afc 1039X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1040
d6217f1e
GS
1041=item cos
1042
5a964f20 1043Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1044takes the cosine of C<$_>.
a0d0e21e 1045
ca6e1c26 1046For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1047function, or use this relation:
1048
1049 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1050
a0d0e21e 1051=item crypt PLAINTEXT,SALT
d74e8afc 1052X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1053X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1054
ef2e6798
MS
1055Creates a digest string exactly like the crypt(3) function in the C
1056library (assuming that you actually have a version there that has not
bb23f8d1 1057been extirpated as a potential munition).
ef2e6798 1058
34169887 1059crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1060into a short string, called a digest, which is returned. The same
1061PLAINTEXT and SALT will always return the same string, but there is no
1062(known) way to get the original PLAINTEXT from the hash. Small
1063changes in the PLAINTEXT or SALT will result in large changes in the
1064digest.
1065
1066There is no decrypt function. This function isn't all that useful for
1067cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1068mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1069primarily used to check if two pieces of text are the same without
1070having to transmit or store the text itself. An example is checking
1071if a correct password is given. The digest of the password is stored,
cf264981 1072not the password itself. The user types in a password that is
ef2e6798 1073crypt()'d with the same salt as the stored digest. If the two digests
34169887 1074match, the password is correct.
ef2e6798
MS
1075
1076When verifying an existing digest string you should use the digest as
1077the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1078to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1079crypt() will hash the new string with the same salt as the digest.
1080This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1081with more exotic implementations. In other words, assume
1082nothing about the returned string itself nor about how many bytes
1083of SALT may matter.
85c16d83
JH
1084
1085Traditionally the result is a string of 13 bytes: two first bytes of
1086the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
391b733c 1087the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1088hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1089and implementations on non-Unix platforms may produce different
ef2e6798 1090strings.
85c16d83
JH
1091
1092When choosing a new salt create a random two character string whose
1093characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1094'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1095characters is just a recommendation; the characters allowed in
1096the salt depend solely on your system's crypt library, and Perl can't
1097restrict what salts C<crypt()> accepts.
e71965be 1098
a0d0e21e 1099Here's an example that makes sure that whoever runs this program knows
cf264981 1100their password:
a0d0e21e
LW
1101
1102 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1103
1104 system "stty -echo";
1105 print "Password: ";
e71965be 1106 chomp($word = <STDIN>);
a0d0e21e
LW
1107 print "\n";
1108 system "stty echo";
1109
e71965be 1110 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1111 die "Sorry...\n";
a0d0e21e 1112 } else {
a9a5a0dc 1113 print "ok\n";
54310121 1114 }
a0d0e21e 1115
9f8f0c9d 1116Of course, typing in your own password to whoever asks you
748a9306 1117for it is unwise.
a0d0e21e 1118
ef2e6798 1119The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1120of data, not least of all because you can't get the information
ef2e6798 1121back. Look at the L<Digest> module for more robust algorithms.
19799a22 1122
f2791508
JH
1123If using crypt() on a Unicode string (which I<potentially> has
1124characters with codepoints above 255), Perl tries to make sense
34169887 1125of the situation by trying to downgrade (a copy of)
f2791508
JH
1126the string back to an eight-bit byte string before calling crypt()
1127(on that copy). If that works, good. If not, crypt() dies with
1128C<Wide character in crypt>.
85c16d83 1129
ea9eb35a 1130Portability issues: L<perlport/crypt>.
1131
aa689395 1132=item dbmclose HASH
d74e8afc 1133X<dbmclose>
a0d0e21e 1134
19799a22 1135[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1136
aa689395 1137Breaks the binding between a DBM file and a hash.
a0d0e21e 1138
ea9eb35a 1139Portability issues: L<perlport/dbmclose>.
1140
19799a22 1141=item dbmopen HASH,DBNAME,MASK
d74e8afc 1142X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1143
01aa884e
KW
1144[This function has been largely superseded by the
1145L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
a0d0e21e 1146
7b8d334a 1147This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1148hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1149argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1150is the name of the database (without the F<.dir> or F<.pag> extension if
1151any). If the database does not exist, it is created with protection
1b3a6178
FC
1152specified by MASK (as modified by the C<umask>). To prevent creation of
1153the database if it doesn't exist, you may specify a MODE
1154of 0, and the function will return a false value if it
1155can't find an existing database. If your system supports
80d38338 1156only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1157program. In older versions of Perl, if your system had neither DBM nor
19799a22 1158ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1159sdbm(3).
1160
1161If you don't have write access to the DBM file, you can only read hash
1162variables, not set them. If you want to test whether you can write,
3b10bc60 1163either use file tests or try setting a dummy hash entry inside an C<eval>
1164to trap the error.
a0d0e21e 1165
19799a22
GS
1166Note that functions such as C<keys> and C<values> may return huge lists
1167when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1168function to iterate over large DBM files. Example:
1169
1170 # print out history file offsets
1171 dbmopen(%HIST,'/usr/lib/news/history',0666);
1172 while (($key,$val) = each %HIST) {
a9a5a0dc 1173 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1174 }
1175 dbmclose(%HIST);
1176
cb1a09d0 1177See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1178cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1179rich implementation.
4633a7c4 1180
2b5ab1e7
TC
1181You can control which DBM library you use by loading that library
1182before you call dbmopen():
1183
1184 use DB_File;
1185 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1186 or die "Can't open netscape history file: $!";
2b5ab1e7 1187
ea9eb35a 1188Portability issues: L<perlport/dbmopen>.
1189
8f1da26d
TC
1190=item default BLOCK
1191
1192Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when>
1193that's always true. Only available after Perl 5.10, and only if the
4a904372
FC
1194C<switch> feature has been requested or if the keyword is prefixed with
1195C<CORE::>. See L</when>.
8f1da26d 1196
a0d0e21e 1197=item defined EXPR
d74e8afc 1198X<defined> X<undef> X<undefined>
a0d0e21e 1199
54310121 1200=item defined
bbce6d69 1201
2f9daede 1202Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1203the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1204checked.
1205
1206Many operations return C<undef> to indicate failure, end of file,
1207system error, uninitialized variable, and other exceptional
1208conditions. This function allows you to distinguish C<undef> from
1209other values. (A simple Boolean test will not distinguish among
7660c0ab 1210C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1211false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1212doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1213returns C<undef> when its argument is an empty array, I<or> when the
1214element to return happens to be C<undef>.
1215
f10b0346
GS
1216You may also use C<defined(&func)> to check whether subroutine C<&func>
1217has ever been defined. The return value is unaffected by any forward
80d38338 1218declarations of C<&func>. A subroutine that is not defined
847c7ebe 1219may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1220makes it spring into existence the first time that it is called; see
847c7ebe 1221L<perlsub>.
f10b0346
GS
1222
1223Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1224used to report whether memory for that aggregate had ever been
f10b0346
GS
1225allocated. This behavior may disappear in future versions of Perl.
1226You should instead use a simple test for size:
1227
1228 if (@an_array) { print "has array elements\n" }
1229 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1230
1231When used on a hash element, it tells you whether the value is defined,
dc848c6f 1232not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1233purpose.
a0d0e21e
LW
1234
1235Examples:
1236
8f1da26d 1237 print if defined $switch{D};
a0d0e21e
LW
1238 print "$val\n" while defined($val = pop(@ary));
1239 die "Can't readlink $sym: $!"
a9a5a0dc 1240 unless defined($value = readlink $sym);
a0d0e21e 1241 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1242 $debugging = 0 unless defined $debugging;
a0d0e21e 1243
8f1da26d 1244Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1245discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1246defined values. For example, if you say
a5f75d66
AD
1247
1248 "ab" =~ /a(.*)b/;
1249
80d38338 1250The pattern match succeeds and C<$1> is defined, although it
cf264981 1251matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1252matched something that happened to be zero characters long. This is all
a5f75d66 1253very above-board and honest. When a function returns an undefined value,
2f9daede 1254it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1255should use C<defined> only when questioning the integrity of what
7660c0ab 1256you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1257what you want.
1258
dc848c6f 1259See also L</undef>, L</exists>, L</ref>.
2f9daede 1260
a0d0e21e 1261=item delete EXPR
d74e8afc 1262X<delete>
a0d0e21e 1263
d0a76353
RS
1264Given an expression that specifies an element or slice of a hash, C<delete>
1265deletes the specified elements from that hash so that exists() on that element
1266no longer returns true. Setting a hash element to the undefined value does
1267not remove its key, but deleting it does; see L</exists>.
80d38338 1268
8f1da26d 1269In list context, returns the value or values deleted, or the last such
80d38338 1270element in scalar context. The return list's length always matches that of
d0a76353
RS
1271the argument list: deleting non-existent elements returns the undefined value
1272in their corresponding positions.
80d38338 1273
d0a76353
RS
1274delete() may also be used on arrays and array slices, but its behavior is less
1275straightforward. Although exists() will return false for deleted entries,
1276deleting array elements never changes indices of existing values; use shift()
1277or splice() for that. However, if all deleted elements fall at the end of an
1278array, the array's size shrinks to the position of the highest element that
1279still tests true for exists(), or to 0 if none do.
1280
8f1da26d 1281B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1282be removed in a future version of Perl.
80d38338
TC
1283
1284Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1285a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1286or array may not necessarily return anything; it depends on the implementation
1287of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1288
80d38338
TC
1289The C<delete local EXPR> construct localizes the deletion to the current
1290block at run time. Until the block exits, elements locally deleted
1291temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1292of composite types">.
eba0920a
EM
1293
1294 %hash = (foo => 11, bar => 22, baz => 33);
1295 $scalar = delete $hash{foo}; # $scalar is 11
1296 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1297 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1298
01020589 1299The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1300
5f05dabc 1301 foreach $key (keys %HASH) {
a9a5a0dc 1302 delete $HASH{$key};
a0d0e21e
LW
1303 }
1304
01020589 1305 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1306 delete $ARRAY[$index];
01020589
GS
1307 }
1308
1309And so do these:
5f05dabc 1310
01020589
GS
1311 delete @HASH{keys %HASH};
1312
9740c838 1313 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1314
80d38338
TC
1315But both are slower than assigning the empty list
1316or undefining %HASH or @ARRAY, which is the customary
1317way to empty out an aggregate:
01020589 1318
5ed4f2ec 1319 %HASH = (); # completely empty %HASH
1320 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1321
5ed4f2ec 1322 @ARRAY = (); # completely empty @ARRAY
1323 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1324
80d38338
TC
1325The EXPR can be arbitrarily complicated provided its
1326final operation is an element or slice of an aggregate:
a0d0e21e
LW
1327
1328 delete $ref->[$x][$y]{$key};
5f05dabc 1329 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1330
01020589
GS
1331 delete $ref->[$x][$y][$index];
1332 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1333
a0d0e21e 1334=item die LIST
d74e8afc 1335X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1336
391b733c 1337C<die> raises an exception. Inside an C<eval> the error message is stuffed
4c050ad5
NC
1338into C<$@> and the C<eval> is terminated with the undefined value.
1339If the exception is outside of all enclosing C<eval>s, then the uncaught
391b733c 1340exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1341need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1342
1343Equivalent examples:
1344
1345 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1346 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1347
ccac6780 1348If the last element of LIST does not end in a newline, the current
df37ec69
WW
1349script line number and input line number (if any) are also printed,
1350and a newline is supplied. Note that the "input line number" (also
1351known as "chunk") is subject to whatever notion of "line" happens to
1352be currently in effect, and is also available as the special variable
1353C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1354
1355Hint: sometimes appending C<", stopped"> to your message will cause it
1356to make better sense when the string C<"at foo line 123"> is appended.
1357Suppose you are running script "canasta".
a0d0e21e
LW
1358
1359 die "/etc/games is no good";
1360 die "/etc/games is no good, stopped";
1361
1362produce, respectively
1363
1364 /etc/games is no good at canasta line 123.
1365 /etc/games is no good, stopped at canasta line 123.
1366
a96d0188 1367If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1368previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1369This is useful for propagating exceptions:
1370
1371 eval { ... };
1372 die unless $@ =~ /Expected exception/;
1373
a96d0188 1374If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1375C<PROPAGATE> method, that method will be called with additional file
1376and line number parameters. The return value replaces the value in
34169887 1377C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1378were called.
1379
7660c0ab 1380If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1381
4c050ad5
NC
1382If an uncaught exception results in interpreter exit, the exit code is
1383determined from the values of C<$!> and C<$?> with this pseudocode:
1384
1385 exit $! if $!; # errno
1386 exit $? >> 8 if $? >> 8; # child exit status
1387 exit 255; # last resort
1388
1389The intent is to squeeze as much possible information about the likely cause
391b733c
FC
1390into the limited space of the system exit
1391code. However, as C<$!> is the value
4c050ad5
NC
1392of C's C<errno>, which can be set by any system call, this means that the value
1393of the exit code used by C<die> can be non-predictable, so should not be relied
1394upon, other than to be non-zero.
1395
80d38338
TC
1396You can also call C<die> with a reference argument, and if this is trapped
1397within an C<eval>, C<$@> contains that reference. This permits more
1398elaborate exception handling using objects that maintain arbitrary state
1399about the exception. Such a scheme is sometimes preferable to matching
1400particular string values of C<$@> with regular expressions. Because C<$@>
1401is a global variable and C<eval> may be used within object implementations,
1402be careful that analyzing the error object doesn't replace the reference in
1403the global variable. It's easiest to make a local copy of the reference
1404before any manipulations. Here's an example:
52531d10 1405
80d38338 1406 use Scalar::Util "blessed";
da279afe 1407
52531d10 1408 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1409 if (my $ev_err = $@) {
1410 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1411 # handle Some::Module::Exception
1412 }
1413 else {
1414 # handle all other possible exceptions
1415 }
1416 }
1417
3b10bc60 1418Because Perl stringifies uncaught exception messages before display,
80d38338 1419you'll probably want to overload stringification operations on
52531d10
GS
1420exception objects. See L<overload> for details about that.
1421
19799a22
GS
1422You can arrange for a callback to be run just before the C<die>
1423does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1424handler is called with the error text and can change the error
19799a22 1425message, if it sees fit, by calling C<die> again. See
96090e4f 1426L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1427L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1428to be run only right before your program was to exit, this is not
3b10bc60 1429currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1430even inside eval()ed blocks/strings! If one wants the hook to do
1431nothing in such situations, put
fb73857a 1432
5ed4f2ec 1433 die @_ if $^S;
fb73857a 1434
19799a22
GS
1435as the first line of the handler (see L<perlvar/$^S>). Because
1436this promotes strange action at a distance, this counterintuitive
b76cc8ba 1437behavior may be fixed in a future release.
774d564b 1438
4c050ad5
NC
1439See also exit(), warn(), and the Carp module.
1440
a0d0e21e 1441=item do BLOCK
d74e8afc 1442X<do> X<block>
a0d0e21e
LW
1443
1444Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1445sequence of commands indicated by BLOCK. When modified by the C<while> or
1446C<until> loop modifier, executes the BLOCK once before testing the loop
391b733c 1447condition. (On other statements the loop modifiers test the conditional
6b275a1f 1448first.)
a0d0e21e 1449
4968c1e4 1450C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1451C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1452See L<perlsyn> for alternative strategies.
4968c1e4 1453
a0d0e21e 1454=item do SUBROUTINE(LIST)
d74e8afc 1455X<do>
a0d0e21e 1456
51124b83
FC
1457This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
1458a scalar variable or a subroutine beginning with C<&>.
a0d0e21e
LW
1459
1460=item do EXPR
d74e8afc 1461X<do>
a0d0e21e
LW
1462
1463Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1464file as a Perl script.
a0d0e21e
LW
1465
1466 do 'stat.pl';
1467
1468is just like
1469
986b19de 1470 eval `cat stat.pl`;
a0d0e21e 1471
2b5ab1e7 1472except that it's more efficient and concise, keeps track of the current
96090e4f
A
1473filename for error messages, searches the C<@INC> directories, and updates
1474C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1475these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1476cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1477same, however, in that it does reparse the file every time you call it,
1478so you probably don't want to do this inside a loop.
a0d0e21e 1479
8f1da26d 1480If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1481an error message in C<$@>. If C<do> cannot read the file, it returns undef
1482and sets C<$!> to the error. Always check C<$@> first, as compilation
1483could fail in a way that also sets C<$!>. If the file is successfully
1484compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1485
80d38338 1486Inclusion of library modules is better done with the
19799a22 1487C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1488and raise an exception if there's a problem.
a0d0e21e 1489
5a964f20
TC
1490You might like to use C<do> to read in a program configuration
1491file. Manual error checking can be done this way:
1492
b76cc8ba 1493 # read in config files: system first, then user
f86cebdf 1494 for $file ("/share/prog/defaults.rc",
b76cc8ba 1495 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1496 {
1497 unless ($return = do $file) {
1498 warn "couldn't parse $file: $@" if $@;
1499 warn "couldn't do $file: $!" unless defined $return;
1500 warn "couldn't run $file" unless $return;
1501 }
5a964f20
TC
1502 }
1503
a0d0e21e 1504=item dump LABEL
d74e8afc 1505X<dump> X<core> X<undump>
a0d0e21e 1506
1614b0e3
JD
1507=item dump
1508
19799a22
GS
1509This function causes an immediate core dump. See also the B<-u>
1510command-line switch in L<perlrun>, which does the same thing.
1511Primarily this is so that you can use the B<undump> program (not
1512supplied) to turn your core dump into an executable binary after
1513having initialized all your variables at the beginning of the
1514program. When the new binary is executed it will begin by executing
1515a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1516Think of it as a goto with an intervening core dump and reincarnation.
1517If C<LABEL> is omitted, restarts the program from the top.
1518
1519B<WARNING>: Any files opened at the time of the dump will I<not>
1520be open any more when the program is reincarnated, with possible
80d38338 1521resulting confusion by Perl.
19799a22 1522
59f521f4 1523This function is now largely obsolete, mostly because it's very hard to
391b733c 1524convert a core file into an executable. That's why you should now invoke
59f521f4 1525it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1526typo.
19799a22 1527
ea9eb35a 1528Portability issues: L<perlport/dump>.
1529
532eee96 1530=item each HASH
d74e8afc 1531X<each> X<hash, iterator>
aa689395 1532
532eee96 1533=item each ARRAY
aeedbbed
NC
1534X<array, iterator>
1535
f5a93a43
TC
1536=item each EXPR
1537
bade7fbc
TC
1538When called on a hash in list context, returns a 2-element list
1539consisting of the key and value for the next element of a hash. In Perl
15405.12 and later only, it will also return the index and value for the next
1541element of an array so that you can iterate over it; older Perls consider
1542this a syntax error. When called in scalar context, returns only the key
1543(not the value) in a hash, or the index in an array.
2f9daede 1544
aeedbbed 1545Hash entries are returned in an apparently random order. The actual random
3b10bc60 1546order is subject to change in future versions of Perl, but it is
504f80c1 1547guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1548function would produce on the same (unmodified) hash. Since Perl
22883ac5 15495.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1550for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1551
80d38338
TC
1552After C<each> has returned all entries from the hash or array, the next
1553call to C<each> returns the empty list in list context and C<undef> in
bade7fbc
TC
1554scalar context; the next call following I<that> one restarts iteration.
1555Each hash or array has its own internal iterator, accessed by C<each>,
1556C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1557reached the end as just described; it can be explicitly reset by calling
1558C<keys> or C<values> on the hash or array. If you add or delete a hash's
1559elements while iterating over it, entries may be skipped or duplicated--so
1560don't do that. Exception: In the current implementation, it is always safe
1561to delete the item most recently returned by C<each()>, so the following
1562code works properly:
74fc8b5f
MJD
1563
1564 while (($key, $value) = each %hash) {
1565 print $key, "\n";
1566 delete $hash{$key}; # This is safe
1567 }
aa689395 1568
80d38338 1569This prints out your environment like the printenv(1) program,
3b10bc60 1570but in a different order:
a0d0e21e
LW
1571
1572 while (($key,$value) = each %ENV) {
a9a5a0dc 1573 print "$key=$value\n";
a0d0e21e
LW
1574 }
1575
f5a93a43
TC
1576Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1577reference to an unblessed hash or array. The argument will be dereferenced
1578automatically. This aspect of C<each> is considered highly experimental.
1579The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1580
1581 while (($key,$value) = each $hashref) { ... }
1582
bade7fbc
TC
1583To avoid confusing would-be users of your code who are running earlier
1584versions of Perl with mysterious syntax errors, put this sort of thing at
1585the top of your file to signal that your code will work I<only> on Perls of
1586a recent vintage:
1587
1588 use 5.012; # so keys/values/each work on arrays
1589 use 5.014; # so keys/values/each work on scalars (experimental)
1590
8f1da26d 1591See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1592
1593=item eof FILEHANDLE
d74e8afc
ITB
1594X<eof>
1595X<end of file>
1596X<end-of-file>
a0d0e21e 1597
4633a7c4
LW
1598=item eof ()
1599
a0d0e21e
LW
1600=item eof
1601
8f1da26d 1602Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1603FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1604gives the real filehandle. (Note that this function actually
80d38338 1605reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1606interactive context.) Do not read from a terminal file (or call
19799a22 1607C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1608as terminals may lose the end-of-file condition if you do.
1609
820475bd 1610An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1611with empty parentheses is different. It refers to the pseudo file
820475bd 1612formed from the files listed on the command line and accessed via the
61eff3bc
JH
1613C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1614as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1615used will cause C<@ARGV> to be examined to determine if input is
67408cae 1616available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1617end-of-file will assume you are processing another C<@ARGV> list,
1618and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1619see L<perlop/"I/O Operators">.
820475bd 1620
61eff3bc 1621In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1622detect the end of each file, whereas C<eof()> will detect the end
1623of the very last file only. Examples:
a0d0e21e 1624
748a9306
LW
1625 # reset line numbering on each input file
1626 while (<>) {
a9a5a0dc
VP
1627 next if /^\s*#/; # skip comments
1628 print "$.\t$_";
5a964f20 1629 } continue {
a9a5a0dc 1630 close ARGV if eof; # Not eof()!
748a9306
LW
1631 }
1632
a0d0e21e
LW
1633 # insert dashes just before last line of last file
1634 while (<>) {
a9a5a0dc
VP
1635 if (eof()) { # check for end of last file
1636 print "--------------\n";
1637 }
1638 print;
4d0444a3 1639 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1640 }
1641
a0d0e21e 1642Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1643input operators typically return C<undef> when they run out of data or
1644encounter an error.
a0d0e21e
LW
1645
1646=item eval EXPR
d74e8afc 1647X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1648X<error, handling> X<exception, handling>
a0d0e21e
LW
1649
1650=item eval BLOCK
1651
ce2984c3
PF
1652=item eval
1653
c7cc6f1c
GS
1654In the first form, the return value of EXPR is parsed and executed as if it
1655were a little Perl program. The value of the expression (which is itself
8f1da26d 1656determined within scalar context) is first parsed, and if there were no
2341804c 1657errors, executed as a block within the lexical context of the current Perl
df4833a8 1658program. This means, that in particular, any outer lexical variables are
2341804c
DM
1659visible to it, and any package variable settings or subroutine and format
1660definitions remain afterwards.
1661
1662Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1663If EXPR is omitted, evaluates C<$_>. This form is typically used to
1664delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c 1665
7289c5e6
FC
1666If the C<unicode_eval> feature is enabled (which is the default under a
1667C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1668characters, so C<use utf8> declarations have no effect, and source filters
1669are forbidden. In the absence of the C<unicode_eval> feature, the string
1670will sometimes be treated as characters and sometimes as bytes, depending
1671on the internal encoding, and source filters activated within the C<eval>
1672exhibit the erratic, but historical, behaviour of affecting some outer file
1673scope that is still compiling. See also the L</evalbytes> keyword, which
1674always treats its input as a byte stream and works properly with source
1675filters, and the L<feature> pragma.
1676
c7cc6f1c 1677In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1678same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1679within the context of the current Perl program. This form is typically
1680used to trap exceptions more efficiently than the first (see below), while
1681also providing the benefit of checking the code within BLOCK at compile
1682time.
1683
1684The final semicolon, if any, may be omitted from the value of EXPR or within
1685the BLOCK.
1686
1687In both forms, the value returned is the value of the last expression
5a964f20 1688evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1689as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1690in void, scalar, or list context, depending on the context of the C<eval>
1691itself. See L</wantarray> for more on how the evaluation context can be
1692determined.
a0d0e21e 1693
19799a22 1694If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1695executed, C<eval> returns C<undef> in scalar context
774b80e8
FC
1696or an empty list in list context, and C<$@> is set to the error
1697message. (Prior to 5.16, a bug caused C<undef> to be returned
1698in list context for syntax errors, but not for runtime errors.)
1699If there was no error, C<$@> is set to the empty string. A
9cc672d4
FC
1700control flow operator like C<last> or C<goto> can bypass the setting of
1701C<$@>. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1702warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1703To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1704turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1705See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1706
19799a22
GS
1707Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1708determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1709is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1710the die operator is used to raise exceptions.
1711
5f1da31c
NT
1712If you want to trap errors when loading an XS module, some problems with
1713the binary interface (such as Perl version skew) may be fatal even with
df4833a8 1714C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
5f1da31c 1715
a0d0e21e
LW
1716If the code to be executed doesn't vary, you may use the eval-BLOCK
1717form to trap run-time errors without incurring the penalty of
1718recompiling each time. The error, if any, is still returned in C<$@>.
1719Examples:
1720
54310121 1721 # make divide-by-zero nonfatal
a0d0e21e
LW
1722 eval { $answer = $a / $b; }; warn $@ if $@;
1723
1724 # same thing, but less efficient
1725 eval '$answer = $a / $b'; warn $@ if $@;
1726
1727 # a compile-time error
5ed4f2ec 1728 eval { $answer = }; # WRONG
a0d0e21e
LW
1729
1730 # a run-time error
5ed4f2ec 1731 eval '$answer ='; # sets $@
a0d0e21e 1732
cf264981
SP
1733Using the C<eval{}> form as an exception trap in libraries does have some
1734issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1735may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1736You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1737as this example shows:
774d564b 1738
80d38338 1739 # a private exception trap for divide-by-zero
f86cebdf
GS
1740 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1741 warn $@ if $@;
774d564b
PP
1742
1743This is especially significant, given that C<__DIE__> hooks can call
19799a22 1744C<die> again, which has the effect of changing their error messages:
774d564b
PP
1745
1746 # __DIE__ hooks may modify error messages
1747 {
f86cebdf
GS
1748 local $SIG{'__DIE__'} =
1749 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1750 eval { die "foo lives here" };
1751 print $@ if $@; # prints "bar lives here"
774d564b
PP
1752 }
1753
19799a22 1754Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1755may be fixed in a future release.
1756
19799a22 1757With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1758being looked at when:
1759
5ed4f2ec 1760 eval $x; # CASE 1
1761 eval "$x"; # CASE 2
a0d0e21e 1762
5ed4f2ec 1763 eval '$x'; # CASE 3
1764 eval { $x }; # CASE 4
a0d0e21e 1765
5ed4f2ec 1766 eval "\$$x++"; # CASE 5
1767 $$x++; # CASE 6
a0d0e21e 1768
2f9daede 1769Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1770the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1771the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1772and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1773does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1774purely visual reasons, but it also has the advantage of compiling at
1775compile-time instead of at run-time.) Case 5 is a place where
19799a22 1776normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1777particular situation, you can just use symbolic references instead, as
1778in case 6.
a0d0e21e 1779
b6538e4f 1780Before Perl 5.14, the assignment to C<$@> occurred before restoration
bade7fbc 1781of localized variables, which means that for your code to run on older
b208c909 1782versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1783errors:
1784
1785 # alter $@ on nefarious repugnancy only
1786 {
1787 my $e;
1788 {
1789 local $@; # protect existing $@
1790 eval { test_repugnancy() };
b208c909 1791 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
8a5a710d
DN
1792 $@ =~ /nefarious/ and $e = $@;
1793 }
1794 die $e if defined $e
1795 }
1796
4968c1e4 1797C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1798C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1799
3b10bc60 1800An C<eval ''> executed within the C<DB> package doesn't see the usual
1801surrounding lexical scope, but rather the scope of the first non-DB piece
df4833a8 1802of code that called it. You don't normally need to worry about this unless
3b10bc60 1803you are writing a Perl debugger.
d819b83a 1804
7289c5e6
FC
1805=item evalbytes EXPR
1806X<evalbytes>
1807
1808=item evalbytes
1809
1810This function is like L</eval> with a string argument, except it always
1811parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1812string containing characters whose ordinal value exceeds 255 results in an
1813error. Source filters activated within the evaluated code apply to the
1814code itself.
1815
1816This function is only available under the C<evalbytes> feature, a
1817C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1818L<feature> for more information.
1819
a0d0e21e 1820=item exec LIST
d74e8afc 1821X<exec> X<execute>
a0d0e21e 1822
8bf3b016
GS
1823=item exec PROGRAM LIST
1824
3b10bc60 1825The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1826use C<system> instead of C<exec> if you want it to return. It fails and
1827returns false only if the command does not exist I<and> it is executed
fb73857a 1828directly instead of via your system's command shell (see below).
a0d0e21e 1829
19799a22 1830Since it's a common mistake to use C<exec> instead of C<system>, Perl
80d38338 1831warns you if there is a following statement that isn't C<die>, C<warn>,
3b10bc60 1832or C<exit> (if C<-w> is set--but you always do that, right?). If you
19799a22 1833I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1834can use one of these styles to avoid the warning:
1835
5a964f20
TC
1836 exec ('foo') or print STDERR "couldn't exec foo: $!";
1837 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1838
5a964f20 1839If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1840with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1841If there is only one scalar argument or an array with one element in it,
1842the argument is checked for shell metacharacters, and if there are any,
1843the entire argument is passed to the system's command shell for parsing
1844(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1845If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1846words and passed directly to C<execvp>, which is more efficient.
19799a22 1847Examples:
a0d0e21e 1848
19799a22
GS
1849 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1850 exec "sort $outfile | uniq";
a0d0e21e
LW
1851
1852If you don't really want to execute the first argument, but want to lie
1853to the program you are executing about its own name, you can specify
1854the program you actually want to run as an "indirect object" (without a
1855comma) in front of the LIST. (This always forces interpretation of the
54310121 1856LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1857the list.) Example:
1858
1859 $shell = '/bin/csh';
5ed4f2ec 1860 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1861
1862or, more directly,
1863
5ed4f2ec 1864 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1865
3b10bc60 1866When the arguments get executed via the system shell, results are
1867subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1868for details.
1869
19799a22
GS
1870Using an indirect object with C<exec> or C<system> is also more
1871secure. This usage (which also works fine with system()) forces
1872interpretation of the arguments as a multivalued list, even if the
1873list had just one argument. That way you're safe from the shell
1874expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1875
1876 @args = ( "echo surprise" );
1877
2b5ab1e7 1878 exec @args; # subject to shell escapes
f86cebdf 1879 # if @args == 1
2b5ab1e7 1880 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1881
1882The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1883program, passing it C<"surprise"> an argument. The second version didn't;
1884it tried to run a program named I<"echo surprise">, didn't find it, and set
1885C<$?> to a non-zero value indicating failure.
5a964f20 1886
80d38338 1887Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1888output before the exec, but this may not be supported on some platforms
1889(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1890in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 1891open handles to avoid lost output.
0f897271 1892
80d38338
TC
1893Note that C<exec> will not call your C<END> blocks, nor will it invoke
1894C<DESTROY> methods on your objects.
7660c0ab 1895
ea9eb35a 1896Portability issues: L<perlport/exec>.
1897
a0d0e21e 1898=item exists EXPR
d74e8afc 1899X<exists> X<autovivification>
a0d0e21e 1900
d0a76353
RS
1901Given an expression that specifies an element of a hash, returns true if the
1902specified element in the hash has ever been initialized, even if the
1903corresponding value is undefined.
a0d0e21e 1904
5ed4f2ec 1905 print "Exists\n" if exists $hash{$key};
1906 print "Defined\n" if defined $hash{$key};
01020589
GS
1907 print "True\n" if $hash{$key};
1908
d0a76353 1909exists may also be called on array elements, but its behavior is much less
8f1da26d 1910obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
1911that calling exists on array values is deprecated and likely to be removed in
1912a future version of Perl.
1913
5ed4f2ec 1914 print "Exists\n" if exists $array[$index];
1915 print "Defined\n" if defined $array[$index];
01020589 1916 print "True\n" if $array[$index];
a0d0e21e 1917
8f1da26d 1918A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
1919it exists, but the reverse doesn't necessarily hold true.
1920
afebc493
GS
1921Given an expression that specifies the name of a subroutine,
1922returns true if the specified subroutine has ever been declared, even
1923if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 1924does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
1925exist may still be callable: its package may have an C<AUTOLOAD>
1926method that makes it spring into existence the first time that it is
3b10bc60 1927called; see L<perlsub>.
afebc493 1928
5ed4f2ec 1929 print "Exists\n" if exists &subroutine;
1930 print "Defined\n" if defined &subroutine;
afebc493 1931
a0d0e21e 1932Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1933operation is a hash or array key lookup or subroutine name:
a0d0e21e 1934
5ed4f2ec 1935 if (exists $ref->{A}->{B}->{$key}) { }
1936 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 1937
5ed4f2ec 1938 if (exists $ref->{A}->{B}->[$ix]) { }
1939 if (exists $hash{A}{B}[$ix]) { }
01020589 1940
afebc493
GS
1941 if (exists &{$ref->{A}{B}{$key}}) { }
1942
3b10bc60 1943Although the mostly deeply nested array or hash will not spring into
1944existence just because its existence was tested, any intervening ones will.
61eff3bc 1945Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 1946into existence due to the existence test for the $key element above.
3b10bc60 1947This happens anywhere the arrow operator is used, including even here:
5a964f20 1948
2b5ab1e7 1949 undef $ref;
5ed4f2ec 1950 if (exists $ref->{"Some key"}) { }
1951 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
1952
1953This surprising autovivification in what does not at first--or even
1954second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1955release.
a0d0e21e 1956
afebc493
GS
1957Use of a subroutine call, rather than a subroutine name, as an argument
1958to exists() is an error.
1959
5ed4f2ec 1960 exists &sub; # OK
1961 exists &sub(); # Error
afebc493 1962
a0d0e21e 1963=item exit EXPR
d74e8afc 1964X<exit> X<terminate> X<abort>
a0d0e21e 1965
ce2984c3
PF
1966=item exit
1967
2b5ab1e7 1968Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1969
1970 $ans = <STDIN>;
1971 exit 0 if $ans =~ /^[Xx]/;
1972
19799a22 1973See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1974universally recognized values for EXPR are C<0> for success and C<1>
1975for error; other values are subject to interpretation depending on the
1976environment in which the Perl program is running. For example, exiting
197769 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1978the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1979
19799a22
GS
1980Don't use C<exit> to abort a subroutine if there's any chance that
1981someone might want to trap whatever error happened. Use C<die> instead,
1982which can be trapped by an C<eval>.
28757baa 1983
19799a22 1984The exit() function does not always exit immediately. It calls any
2b5ab1e7 1985defined C<END> routines first, but these C<END> routines may not
19799a22 1986themselves abort the exit. Likewise any object destructors that need to
60275626 1987be called are called before the real exit. C<END> routines and destructors
391b733c 1988can change the exit status by modifying C<$?>. If this is a problem, you
2b5ab1e7 1989can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1990See L<perlmod> for details.
5a964f20 1991
ea9eb35a 1992Portability issues: L<perlport/exit>.
1993
a0d0e21e 1994=item exp EXPR
d74e8afc 1995X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1996
54310121 1997=item exp
bbce6d69 1998
b76cc8ba 1999Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
2000If EXPR is omitted, gives C<exp($_)>.
2001
628253b8
BF
2002=item fc EXPR
2003X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2004
2005=item fc
2006
2007Returns the casefolded version of EXPR. This is the internal function
2008implementing the C<\F> escape in double-quoted strings.
2009
2010Casefolding is the process of mapping strings to a form where case
2011differences are erased; comparing two strings in their casefolded
2012form is effectively a way of asking if two strings are equal,
2013regardless of case.
2014
2015Roughly, if you ever found yourself writing this
2016
2017 lc($this) eq lc($that) # Wrong!
2018 # or
2019 uc($this) eq uc($that) # Also wrong!
2020 # or
2021 $this =~ /\Q$that/i # Right!
2022
2023Now you can write
2024
2025 fc($this) eq fc($that)
2026
2027And get the correct results.
2028
2029Perl only implements the full form of casefolding.
2030For further information on casefolding, refer to
2031the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
20324.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2033available at L<http://www.unicode.org/versions/latest/>, as well as the
2034Case Charts available at L<http://www.unicode.org/charts/case/>.
2035
2036If EXPR is omitted, uses C<$_>.
2037
2038This function behaves the same way under various pragma, such as in a locale,
2039as L</lc> does.
2040
2041While the Unicode Standard defines two additional forms of casefolding,
2042one for Turkic languages and one that never maps one character into multiple
2043characters, these are not provided by the Perl core; However, the CPAN module
2044C<Unicode::Casing> may be used to provide an implementation.
2045
2046This keyword is available only when the C<"fc"> feature is enabled,
2047or when prefixed with C<CORE::>; See L<feature>. Alternately,
2048include a C<use v5.16> or later to the current scope.
2049
a0d0e21e 2050=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2051X<fcntl>
a0d0e21e 2052
f86cebdf 2053Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
2054
2055 use Fcntl;
2056
0ade1984 2057first to get the correct constant definitions. Argument processing and
3b10bc60 2058value returned work just like C<ioctl> below.
a0d0e21e
LW
2059For example:
2060
2061 use Fcntl;
5a964f20 2062 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 2063 or die "can't fcntl F_GETFL: $!";
5a964f20 2064
554ad1fc 2065You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
2066Like C<ioctl>, it maps a C<0> return from the system call into
2067C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
2068in numeric context. It is also exempt from the normal B<-w> warnings
2069on improper numeric conversions.
5a964f20 2070
3b10bc60 2071Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
2072doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2073manpage to learn what functions are available on your system.
a0d0e21e 2074
be2f7487 2075Here's an example of setting a filehandle named C<REMOTE> to be
2076non-blocking at the system level. You'll have to negotiate C<$|>
2077on your own, though.
2078
2079 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2080
2081 $flags = fcntl(REMOTE, F_GETFL, 0)
2082 or die "Can't get flags for the socket: $!\n";
2083
2084 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2085 or die "Can't set flags for the socket: $!\n";
2086
ea9eb35a 2087Portability issues: L<perlport/fcntl>.
2088
cfa52385
FC
2089=item __FILE__
2090X<__FILE__>
2091
2092A special token that returns the name of the file in which it occurs.
2093
a0d0e21e 2094=item fileno FILEHANDLE
d74e8afc 2095X<fileno>
a0d0e21e 2096
2b5ab1e7 2097Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
2098filehandle is not open. If there is no real file descriptor at the OS
2099level, as can happen with filehandles connected to memory objects via
2100C<open> with a reference for the third argument, -1 is returned.
2101
2102This is mainly useful for constructing
19799a22 2103bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
2104If FILEHANDLE is an expression, the value is taken as an indirect
2105filehandle, generally its name.
5a964f20 2106
b76cc8ba 2107You can use this to find out whether two handles refer to the
5a964f20
TC
2108same underlying descriptor:
2109
2110 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 2111 print "THIS and THAT are dups\n";
b76cc8ba
NIS
2112 }
2113
a0d0e21e 2114=item flock FILEHANDLE,OPERATION
d74e8afc 2115X<flock> X<lock> X<locking>
a0d0e21e 2116
19799a22
GS
2117Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2118for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2119machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2120C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2121entire files only, not records.
2b5ab1e7
TC
2122
2123Two potentially non-obvious but traditional C<flock> semantics are
2124that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2125are B<merely advisory>. Such discretionary locks are more flexible, but
2126offer fewer guarantees. This means that programs that do not also use
2127C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2128your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2129for details. It's best to assume traditional behavior if you're writing
2130portable programs. (But if you're not, you should as always feel perfectly
2131free to write for your own system's idiosyncrasies (sometimes called
2132"features"). Slavish adherence to portability concerns shouldn't get
2133in the way of your getting your job done.)
a3cb178b 2134
8ebc5c01
PP
2135OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2136LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2137you can use the symbolic names if you import them from the L<Fcntl> module,
2138either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2139requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2140releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2141LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2142waiting for the lock; check the return status to see if you got it.
68dc0745 2143
2b5ab1e7
TC
2144To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2145before locking or unlocking it.
8ebc5c01 2146
f86cebdf 2147Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2148locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2149are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2150implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
2151differing semantics shouldn't bite too many people.
2152
becacb53
TM
2153Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2154be open with read intent to use LOCK_SH and requires that it be open
2155with write intent to use LOCK_EX.
2156
19799a22
GS
2157Note also that some versions of C<flock> cannot lock things over the
2158network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2159that. If you like you can force Perl to ignore your system's flock(2)
2160function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2161the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2162and build a new Perl.
4633a7c4
LW
2163
2164Here's a mailbox appender for BSD systems.
a0d0e21e 2165
7ed5353d 2166 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
2167
2168 sub lock {
a9a5a0dc
VP
2169 my ($fh) = @_;
2170 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2171
a9a5a0dc
VP
2172 # and, in case someone appended while we were waiting...
2173 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2174 }
2175
2176 sub unlock {
a9a5a0dc
VP
2177 my ($fh) = @_;
2178 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2179 }
2180
b0169937 2181 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2182 or die "Can't open mailbox: $!";
a0d0e21e 2183
7ed5353d 2184 lock($mbox);
b0169937 2185 print $mbox $msg,"\n\n";
7ed5353d 2186 unlock($mbox);
a0d0e21e 2187
3b10bc60 2188On systems that support a real flock(2), locks are inherited across fork()
2189calls, whereas those that must resort to the more capricious fcntl(2)
2190function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2191
cb1a09d0 2192See also L<DB_File> for other flock() examples.
a0d0e21e 2193
ea9eb35a 2194Portability issues: L<perlport/flock>.
2195
a0d0e21e 2196=item fork
d74e8afc 2197X<fork> X<child> X<parent>
a0d0e21e 2198
2b5ab1e7
TC
2199Does a fork(2) system call to create a new process running the
2200same program at the same point. It returns the child pid to the
2201parent process, C<0> to the child process, or C<undef> if the fork is
2202unsuccessful. File descriptors (and sometimes locks on those descriptors)
2203are shared, while everything else is copied. On most systems supporting
2204fork(), great care has gone into making it extremely efficient (for
2205example, using copy-on-write technology on data pages), making it the
2206dominant paradigm for multitasking over the last few decades.
5a964f20 2207
80d38338 2208Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
2209output before forking the child process, but this may not be supported
2210on some platforms (see L<perlport>). To be safe, you may need to set
2211C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2212C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2213
19799a22 2214If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2215accumulate zombies. On some systems, you can avoid this by setting
2216C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2217forking and reaping moribund children.
cb1a09d0 2218
28757baa
PP
2219Note that if your forked child inherits system file descriptors like
2220STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2221if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2222backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2223You should reopen those to F</dev/null> if it's any issue.
28757baa 2224
ea9eb35a 2225On some platforms such as Windows, where the fork() system call is not available,
391b733c
FC
2226Perl can be built to emulate fork() in the Perl interpreter.
2227The emulation is designed, at the level of the Perl program,
2228to be as compatible as possible with the "Unix" fork().
6d17f725 2229However it has limitations that have to be considered in code intended to be portable.
ea9eb35a 2230See L<perlfork> for more details.
2231
2232Portability issues: L<perlport/fork>.
2233
cb1a09d0 2234=item format
d74e8afc 2235X<format>
cb1a09d0 2236
19799a22 2237Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2238example:
2239
54310121 2240 format Something =
a9a5a0dc
VP
2241 Test: @<<<<<<<< @||||| @>>>>>
2242 $str, $%, '$' . int($num)
cb1a09d0
AD
2243 .
2244
2245 $str = "widget";
184e9718 2246 $num = $cost/$quantity;
cb1a09d0
AD
2247 $~ = 'Something';
2248 write;
2249
2250See L<perlform> for many details and examples.
2251
8903cb82 2252=item formline PICTURE,LIST
d74e8afc 2253X<formline>
a0d0e21e 2254
5a964f20 2255This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2256too. It formats (see L<perlform>) a list of values according to the
2257contents of PICTURE, placing the output into the format output
7660c0ab 2258accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2259Eventually, when a C<write> is done, the contents of
cf264981
SP
2260C<$^A> are written to some filehandle. You could also read C<$^A>
2261and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2262does one C<formline> per line of form, but the C<formline> function itself
748a9306 2263doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2264that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2265You may therefore need to use multiple formlines to implement a single
3b10bc60 2266record format, just like the C<format> compiler.
748a9306 2267
19799a22 2268Be careful if you put double quotes around the picture, because an C<@>
748a9306 2269character may be taken to mean the beginning of an array name.
19799a22 2270C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2271
445b09e5
FC
2272If you are trying to use this instead of C<write> to capture the output,
2273you may find it easier to open a filehandle to a scalar
2274(C<< open $fh, ">", \$output >>) and write to that instead.
2275
a0d0e21e 2276=item getc FILEHANDLE
f723aae1 2277X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2278
2279=item getc
2280
2281Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2282or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2283the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2284STDIN. This is not particularly efficient. However, it cannot be
2285used by itself to fetch single characters without waiting for the user
2286to hit enter. For that, try something more like:
4633a7c4
LW
2287
2288 if ($BSD_STYLE) {
a9a5a0dc 2289 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2290 }
2291 else {
a9a5a0dc 2292 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2293 }
2294
2295 $key = getc(STDIN);
2296
2297 if ($BSD_STYLE) {
a9a5a0dc 2298 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2299 }
2300 else {
3b10bc60 2301 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2302 }
2303 print "\n";
2304
54310121
PP
2305Determination of whether $BSD_STYLE should be set
2306is left as an exercise to the reader.
cb1a09d0 2307
19799a22 2308The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2309systems purporting POSIX compliance. See also the C<Term::ReadKey>
a3390c9f 2310module from your nearest CPAN site; details on CPAN can be found under
2b5ab1e7 2311L<perlmodlib/CPAN>.
a0d0e21e
LW
2312
2313=item getlogin
d74e8afc 2314X<getlogin> X<login>
a0d0e21e 2315
cf264981 2316This implements the C library function of the same name, which on most
3b10bc60 2317systems returns the current login from F</etc/utmp>, if any. If it
2318returns the empty string, use C<getpwuid>.
a0d0e21e 2319
f86702cc 2320 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2321
19799a22
GS
2322Do not consider C<getlogin> for authentication: it is not as
2323secure as C<getpwuid>.
4633a7c4 2324
ea9eb35a 2325Portability issues: L<perlport/getlogin>.
2326
a0d0e21e 2327=item getpeername SOCKET
d74e8afc 2328X<getpeername> X<peer>
a0d0e21e 2329
a3390c9f
FC
2330Returns the packed sockaddr address of the other end of the SOCKET
2331connection.
a0d0e21e 2332
4633a7c4
LW
2333 use Socket;
2334 $hersockaddr = getpeername(SOCK);
19799a22 2335 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2336 $herhostname = gethostbyaddr($iaddr, AF_INET);
2337 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2338
2339=item getpgrp PID
d74e8afc 2340X<getpgrp> X<group>
a0d0e21e 2341
47e29363 2342Returns the current process group for the specified PID. Use
7660c0ab 2343a PID of C<0> to get the current process group for the
4633a7c4 2344current process. Will raise an exception if used on a machine that
a3390c9f
FC
2345doesn't implement getpgrp(2). If PID is omitted, returns the process
2346group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2347does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2348
ea9eb35a 2349Portability issues: L<perlport/getpgrp>.
2350
a0d0e21e 2351=item getppid
d74e8afc 2352X<getppid> X<parent> X<pid>
a0d0e21e
LW
2353
2354Returns the process id of the parent process.
2355
d7c042c9
AB
2356Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2357around non-POSIX thread semantics the minority of Linux systems (and
2358Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2359has since been removed. See the documentation for L<$$|perlvar/$$> for
2360details.
4d76a344 2361
ea9eb35a 2362Portability issues: L<perlport/getppid>.
2363
a0d0e21e 2364=item getpriority WHICH,WHO
d74e8afc 2365X<getpriority> X<priority> X<nice>
a0d0e21e 2366
4633a7c4 2367Returns the current priority for a process, a process group, or a user.
01aa884e 2368(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2369machine that doesn't implement getpriority(2).
a0d0e21e 2370
ea9eb35a 2371Portability issues: L<perlport/getpriority>.
2372
a0d0e21e 2373=item getpwnam NAME
d74e8afc
ITB
2374X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2375X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2376X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2377X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2378X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2379X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2380
2381=item getgrnam NAME
2382
2383=item gethostbyname NAME
2384
2385=item getnetbyname NAME
2386
2387=item getprotobyname NAME
2388
2389=item getpwuid UID
2390
2391=item getgrgid GID
2392
2393=item getservbyname NAME,PROTO
2394
2395=item gethostbyaddr ADDR,ADDRTYPE
2396
2397=item getnetbyaddr ADDR,ADDRTYPE
2398
2399=item getprotobynumber NUMBER
2400
2401=item getservbyport PORT,PROTO
2402
2403=item getpwent
2404
2405=item getgrent
2406
2407=item gethostent
2408
2409=item getnetent
2410
2411=item getprotoent
2412
2413=item getservent
2414
2415=item setpwent
2416
2417=item setgrent
2418
2419=item sethostent STAYOPEN
2420
2421=item setnetent STAYOPEN
2422
2423=item setprotoent STAYOPEN
2424
2425=item setservent STAYOPEN
2426
2427=item endpwent
2428
2429=item endgrent
2430
2431=item endhostent
2432
2433=item endnetent
2434
2435=item endprotoent
2436
2437=item endservent
2438
80d38338
TC
2439These routines are the same as their counterparts in the
2440system C library. In list context, the return values from the
a0d0e21e
LW
2441various get routines are as follows:
2442
2443 ($name,$passwd,$uid,$gid,
6ee623d5 2444 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2445 ($name,$passwd,$gid,$members) = getgr*
2446 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2447 ($name,$aliases,$addrtype,$net) = getnet*
2448 ($name,$aliases,$proto) = getproto*
2449 ($name,$aliases,$port,$proto) = getserv*
2450
3b10bc60 2451(If the entry doesn't exist you get an empty list.)
a0d0e21e 2452
4602f195
JH
2453The exact meaning of the $gcos field varies but it usually contains
2454the real name of the user (as opposed to the login name) and other
2455information pertaining to the user. Beware, however, that in many
2456system users are able to change this information and therefore it
106325ad 2457cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2458L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2459login shell, are also tainted, for the same reason.
4602f195 2460
5a964f20 2461In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2462lookup by name, in which case you get the other thing, whatever it is.
2463(If the entry doesn't exist you get the undefined value.) For example:
2464
5a964f20
TC
2465 $uid = getpwnam($name);
2466 $name = getpwuid($num);
2467 $name = getpwent();
2468 $gid = getgrnam($name);
08a33e13 2469 $name = getgrgid($num);
5a964f20
TC
2470 $name = getgrent();
2471 #etc.
a0d0e21e 2472
4602f195 2473In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2474in that they are unsupported on many systems. If the
4602f195
JH
2475$quota is unsupported, it is an empty scalar. If it is supported, it
2476usually encodes the disk quota. If the $comment field is unsupported,
2477it is an empty scalar. If it is supported it usually encodes some
2478administrative comment about the user. In some systems the $quota
2479field may be $change or $age, fields that have to do with password
2480aging. In some systems the $comment field may be $class. The $expire
2481field, if present, encodes the expiration period of the account or the
2482password. For the availability and the exact meaning of these fields
8f1da26d 2483in your system, please consult getpwnam(3) and your system's
4602f195
JH
2484F<pwd.h> file. You can also find out from within Perl what your
2485$quota and $comment fields mean and whether you have the $expire field
2486by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2487C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2488files are supported only if your vendor has implemented them in the
4602f195 2489intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2490shadow versions if you're running under privilege or if there exists
cf264981 2491the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2492and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2493facility are unlikely to be supported.
6ee623d5 2494
a3390c9f 2495The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2496the login names of the members of the group.
2497
2498For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2499C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2500C<@addrs> value returned by a successful call is a list of raw
2501addresses returned by the corresponding library call. In the
2502Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2503by saying something like:
2504
f337b084 2505 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2506
2b5ab1e7
TC
2507The Socket library makes this slightly easier:
2508
2509 use Socket;
2510 $iaddr = inet_aton("127.1"); # or whatever address
2511 $name = gethostbyaddr($iaddr, AF_INET);
2512
2513 # or going the other way
19799a22 2514 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2515
d760c846
GS
2516In the opposite way, to resolve a hostname to the IP address
2517you can write this:
2518
2519 use Socket;
2520 $packed_ip = gethostbyname("www.perl.org");
2521 if (defined $packed_ip) {
2522 $ip_address = inet_ntoa($packed_ip);
2523 }
2524
b018eaf1 2525Make sure C<gethostbyname()> is called in SCALAR context and that
d760c846
GS
2526its return value is checked for definedness.
2527
0d043efa
FC
2528The C<getprotobynumber> function, even though it only takes one argument,
2529has the precedence of a list operator, so beware:
2530
2531 getprotobynumber $number eq 'icmp' # WRONG
2532 getprotobynumber($number eq 'icmp') # actually means this
2533 getprotobynumber($number) eq 'icmp' # better this way
2534
19799a22
GS
2535If you get tired of remembering which element of the return list
2536contains which return value, by-name interfaces are provided
2537in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2538C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2539and C<User::grent>. These override the normal built-ins, supplying
2540versions that return objects with the appropriate names
2541for each field. For example:
5a964f20
TC
2542
2543 use File::stat;
2544 use User::pwent;
2545 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2546
a3390c9f 2547Even though it looks as though they're the same method calls (uid),
b76cc8ba 2548they aren't, because a C<File::stat> object is different from
19799a22 2549a C<User::pwent> object.
5a964f20 2550
ea9eb35a 2551Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2552
a0d0e21e 2553=item getsockname SOCKET
d74e8afc 2554X<getsockname>
a0d0e21e 2555
19799a22
GS
2556Returns the packed sockaddr address of this end of the SOCKET connection,
2557in case you don't know the address because you have several different
2558IPs that the connection might have come in on.
a0d0e21e 2559
4633a7c4
LW
2560 use Socket;
2561 $mysockaddr = getsockname(SOCK);
19799a22 2562 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2563 printf "Connect to %s [%s]\n",
19799a22
GS
2564 scalar gethostbyaddr($myaddr, AF_INET),
2565 inet_ntoa($myaddr);
a0d0e21e
LW
2566
2567=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2568X<getsockopt>
a0d0e21e 2569
636e6b1f
TH
2570Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2571Options may exist at multiple protocol levels depending on the socket
2572type, but at least the uppermost socket level SOL_SOCKET (defined in the
391b733c 2573C<Socket> module) will exist. To query options at another level the
636e6b1f 2574protocol number of the appropriate protocol controlling the option
391b733c 2575should be supplied. For example, to indicate that an option is to be
636e6b1f 2576interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2577number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2578
80d38338 2579The function returns a packed string representing the requested socket
3b10bc60 2580option, or C<undef> on error, with the reason for the error placed in
391b733c 2581C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2582consult getsockopt(2) for details. A common case is that the option is an
2583integer, in which case the result is a packed integer, which you can decode
2584using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2585
8f1da26d 2586Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2587
4852725b 2588 use Socket qw(:all);
636e6b1f
TH
2589
2590 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2591 or die "Could not determine the protocol number for tcp";
4852725b
DD
2592 # my $tcp = IPPROTO_TCP; # Alternative
2593 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2594 or die "getsockopt TCP_NODELAY: $!";
636e6b1f
TH
2595 my $nodelay = unpack("I", $packed);
2596 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2597
ea9eb35a 2598Portability issues: L<perlport/getsockopt>.
a0d0e21e 2599
15a348aa 2600=item given EXPR BLOCK
2601X<given>
2602
2603=item given BLOCK
2604
391b733c
FC
2605C<given> is analogous to the C<switch>
2606keyword in other languages. C<given>
15a348aa 2607and C<when> are used in Perl to implement C<switch>/C<case> like statements.
8f1da26d 2608Only available after Perl 5.10. For example:
15a348aa 2609
8f1da26d 2610 use v5.10;
15a348aa 2611 given ($fruit) {
2612 when (/apples?/) {
2613 print "I like apples."
2614 }
2615 when (/oranges?/) {
2616 print "I don't like oranges."
2617 }
2618 default {
2619 print "I don't like anything"
2620 }
2621 }
2622
48238296 2623See L<perlsyn/"Switch Statements"> for detailed information.
15a348aa 2624
a0d0e21e 2625=item glob EXPR
d74e8afc 2626X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2627
0a753a76
PP
2628=item glob
2629
d9a9d457 2630In list context, returns a (possibly empty) list of filename expansions on
391b733c 2631the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
d9a9d457 2632scalar context, glob iterates through such filename expansions, returning
391b733c
FC
2633undef when the list is exhausted. This is the internal function
2634implementing the C<< <*.c> >> operator, but you can use it directly. If
d9a9d457
JL
2635EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2636more detail in L<perlop/"I/O Operators">.
a0d0e21e 2637
80d38338
TC
2638Note that C<glob> splits its arguments on whitespace and treats
2639each segment as separate pattern. As such, C<glob("*.c *.h")>
2640matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2641C<glob(".* *")> matches all files in the current working directory.
a91bb7b1
TC
2642If you want to glob filenames that might contain whitespace, you'll
2643have to use extra quotes around the spacey filename to protect it.
2644For example, to glob filenames that have an C<e> followed by a space
2645followed by an C<f>, use either of:
2646
2647 @spacies = <"*e f*">;
2648 @spacies = glob '"*e f*"';
2649 @spacies = glob q("*e f*");
2650
2651If you had to get a variable through, you could do this:
2652
2653 @spacies = glob "'*${var}e f*'";
2654 @spacies = glob qq("*${var}e f*");
80d38338
TC
2655
2656If non-empty braces are the only wildcard characters used in the
2657C<glob>, no filenames are matched, but potentially many strings
2658are returned. For example, this produces nine strings, one for
2659each pairing of fruits and colors:
2660
2661 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2662
3a4b19e4 2663Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2664C<File::Glob> extension. See L<File::Glob> for details, including
2665C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2666
ea9eb35a 2667Portability issues: L<perlport/glob>.
2668
a0d0e21e 2669=item gmtime EXPR
d74e8afc 2670X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2671
ce2984c3
PF
2672=item gmtime
2673
4509d391 2674Works just like L</localtime> but the returned values are
435fbc73 2675localized for the standard Greenwich time zone.
a0d0e21e 2676
a3390c9f
FC
2677Note: When called in list context, $isdst, the last value
2678returned by gmtime, is always C<0>. There is no
435fbc73 2679Daylight Saving Time in GMT.
0a753a76 2680
ea9eb35a 2681Portability issues: L<perlport/gmtime>.
62aa5637 2682
a0d0e21e 2683=item goto LABEL
d74e8afc 2684X<goto> X<jump> X<jmp>
a0d0e21e 2685
748a9306
LW
2686=item goto EXPR
2687
a0d0e21e
LW
2688=item goto &NAME
2689
b500e03b 2690The C<goto-LABEL> form finds the statement labeled with LABEL and
391b733c 2691resumes execution there. It can't be used to get out of a block or
b500e03b
GG
2692subroutine given to C<sort>. It can be used to go almost anywhere
2693else within the dynamic scope, including out of subroutines, but it's
2694usually better to use some other construct such as C<last> or C<die>.
2695The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2696(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2697does not offer named loops combined with loop control. Perl does, and
2698this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2699
7660c0ab
A
2700The C<goto-EXPR> form expects a label name, whose scope will be resolved
2701dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2702necessarily recommended if you're optimizing for maintainability:
2703
2704 goto ("FOO", "BAR", "GLARCH")[$i];
2705
887d89fd 2706As shown in this example, C<goto-EXPR> is exempt from the "looks like a
391b733c
FC
2707function" rule. A pair of parentheses following it does not (necessarily)
2708delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
887d89fd 2709
b500e03b 2710Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2711deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2712go into any construct that requires initialization, such as a
2713subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2714construct that is optimized away.
b500e03b 2715
1b6921cb
BT
2716The C<goto-&NAME> form is quite different from the other forms of
2717C<goto>. In fact, it isn't a goto in the normal sense at all, and
2718doesn't have the stigma associated with other gotos. Instead, it
2719exits the current subroutine (losing any changes set by local()) and
2720immediately calls in its place the named subroutine using the current
2721value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2722load another subroutine and then pretend that the other subroutine had
2723been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2724in the current subroutine are propagated to the other subroutine.)
2725After the C<goto>, not even C<caller> will be able to tell that this
2726routine was called first.
2727
2728NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2729containing a code reference or a block that evaluates to a code
6cb9131c 2730reference.
a0d0e21e
LW
2731
2732=item grep BLOCK LIST
d74e8afc 2733X<grep>
a0d0e21e
LW
2734
2735=item grep EXPR,LIST
2736
2b5ab1e7
TC
2737This is similar in spirit to, but not the same as, grep(1) and its
2738relatives. In particular, it is not limited to using regular expressions.
2f9daede 2739
a0d0e21e 2740Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2741C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2742elements for which the expression evaluated to true. In scalar
2743context, returns the number of times the expression was true.
a0d0e21e
LW
2744
2745 @foo = grep(!/^#/, @bar); # weed out comments
2746
2747or equivalently,
2748
2749 @foo = grep {!/^#/} @bar; # weed out comments
2750
be3174d2
GS
2751Note that C<$_> is an alias to the list value, so it can be used to
2752modify the elements of the LIST. While this is useful and supported,
2753it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2754Similarly, grep returns aliases into the original list, much as a for
2755loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2756element of a list returned by grep (for example, in a C<foreach>, C<map>
2757or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2758This is usually something to be avoided when writing clear code.
a0d0e21e 2759
a4fb8298 2760If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2761been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2762the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2763can't be seen from the outside, avoiding any potential side-effects.
2764
19799a22 2765See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2766
a0d0e21e 2767=item hex EXPR
d74e8afc 2768X<hex> X<hexadecimal>
a0d0e21e 2769
54310121 2770=item hex
bbce6d69 2771
2b5ab1e7 2772Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2773(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2774L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2775
2776 print hex '0xAf'; # prints '175'
2777 print hex 'aF'; # same
a0d0e21e 2778
19799a22 2779Hex strings may only represent integers. Strings that would cause
53305cf1 2780integer overflow trigger a warning. Leading whitespace is not stripped,
391b733c 2781unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 2782L</sprintf>, and L</unpack>.
19799a22 2783
ce2984c3 2784=item import LIST
d74e8afc 2785X<import>
a0d0e21e 2786
19799a22 2787There is no builtin C<import> function. It is just an ordinary
4633a7c4 2788method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2789names to another module. The C<use> function calls the C<import> method
cea6626f 2790for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2791
2792=item index STR,SUBSTR,POSITION
d74e8afc 2793X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2794
2795=item index STR,SUBSTR
2796
2b5ab1e7
TC
2797The index function searches for one string within another, but without
2798the wildcard-like behavior of a full regular-expression pattern match.
2799It returns the position of the first occurrence of SUBSTR in STR at
2800or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2801beginning of the string. POSITION before the beginning of the string
2802or after its end is treated as if it were the beginning or the end,
e1dccc0d
Z
2803respectively. POSITION and the return value are based at zero.
2804If the substring is not found, C<index> returns -1.
a0d0e21e
LW
2805
2806=item int EXPR
f723aae1 2807X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2808
54310121 2809=item int
bbce6d69 2810
7660c0ab 2811Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 2812You should not use this function for rounding: one because it truncates
3b10bc60 2813towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
2814numbers can sometimes produce counterintuitive results. For example,
2815C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2816because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2817the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2818functions will serve you better than will int().
a0d0e21e
LW
2819
2820=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2821X<ioctl>
a0d0e21e 2822
2b5ab1e7 2823Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2824
5ed4f2ec 2825 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2826
a11c483f 2827to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2828exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2829own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2830(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2831may help you in this, but it's nontrivial.) SCALAR will be read and/or
3b10bc60 2832written depending on the FUNCTION; a C pointer to the string value of SCALAR
19799a22 2833will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2834has no string value but does have a numeric value, that value will be
2835passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2836true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2837functions may be needed to manipulate the values of structures used by
b76cc8ba 2838C<ioctl>.
a0d0e21e 2839
19799a22 2840The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e 2841
5ed4f2ec 2842 if OS returns: then Perl returns:
2843 -1 undefined value
2844 0 string "0 but true"
2845 anything else that number
a0d0e21e 2846
19799a22 2847Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2848still easily determine the actual value returned by the operating
2849system:
2850
2b5ab1e7 2851 $retval = ioctl(...) || -1;
a0d0e21e
LW
2852 printf "System returned %d\n", $retval;
2853
be2f7487 2854The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2855about improper numeric conversions.
2856
ea9eb35a 2857Portability issues: L<perlport/ioctl>.
2858
a0d0e21e 2859=item join EXPR,LIST
d74e8afc 2860X<join>
a0d0e21e 2861
2b5ab1e7
TC
2862Joins the separate strings of LIST into a single string with fields
2863separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2864
2b5ab1e7 2865 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2866
eb6e2d6f
GS
2867Beware that unlike C<split>, C<join> doesn't take a pattern as its
2868first argument. Compare L</split>.
a0d0e21e 2869
532eee96 2870=item keys HASH
d74e8afc 2871X<keys> X<key>
aa689395 2872
532eee96 2873=item keys ARRAY
aeedbbed 2874
f5a93a43
TC
2875=item keys EXPR
2876
bade7fbc
TC
2877Called in list context, returns a list consisting of all the keys of the
2878named hash, or in Perl 5.12 or later only, the indices of an array. Perl
2879releases prior to 5.12 will produce a syntax error if you try to use an
2880array argument. In scalar context, returns the number of keys or indices.
504f80c1 2881
aeedbbed 2882The keys of a hash are returned in an apparently random order. The actual
3b10bc60 2883random order is subject to change in future versions of Perl, but it
504f80c1 2884is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6 2885function produces (given that the hash has not been modified). Since
c5f61d2f 2886Perl 5.8.1 the ordering can be different even between different runs of
4546b9e6 2887Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2888Attacks">).
504f80c1 2889
8f1da26d 2890As a side effect, calling keys() resets the internal interator of the HASH or ARRAY
cf264981
SP
2891(see L</each>). In particular, calling keys() in void context resets
2892the iterator with no other overhead.
a0d0e21e 2893
aa689395 2894Here is yet another way to print your environment:
a0d0e21e
LW
2895
2896 @keys = keys %ENV;
2897 @values = values %ENV;
b76cc8ba 2898 while (@keys) {
a9a5a0dc 2899 print pop(@keys), '=', pop(@values), "\n";
a0d0e21e
LW
2900 }
2901
2902or how about sorted by key:
2903
2904 foreach $key (sort(keys %ENV)) {
a9a5a0dc 2905 print $key, '=', $ENV{$key}, "\n";
a0d0e21e
LW
2906 }
2907
8ea1e5d4
GS
2908The returned values are copies of the original keys in the hash, so
2909modifying them will not affect the original hash. Compare L</values>.
2910
19799a22 2911To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2912Here's a descending numeric sort of a hash by its values:
4633a7c4 2913
5a964f20 2914 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
a9a5a0dc 2915 printf "%4d %s\n", $hash{$key}, $key;
4633a7c4
LW
2916 }
2917
3b10bc60 2918Used as an lvalue, C<keys> allows you to increase the number of hash buckets
aa689395
PP
2919allocated for the given hash. This can gain you a measure of efficiency if
2920you know the hash is going to get big. (This is similar to pre-extending
2921an array by assigning a larger number to $#array.) If you say
55497cff
PP
2922
2923 keys %hash = 200;
2924
ab192400
GS
2925then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2926in fact, since it rounds up to the next power of two. These
55497cff
PP
2927buckets will be retained even if you do C<%hash = ()>, use C<undef
2928%hash> if you want to free the storage while C<%hash> is still in scope.
2929You can't shrink the number of buckets allocated for the hash using
19799a22 2930C<keys> in this way (but you needn't worry about doing this by accident,
0d3e3823 2931as trying has no effect). C<keys @array> in an lvalue context is a syntax
aeedbbed 2932error.
55497cff 2933
f5a93a43
TC
2934Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
2935a reference to an unblessed hash or array. The argument will be
2936dereferenced automatically. This aspect of C<keys> is considered highly
2937experimental. The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
2938
2939 for (keys $hashref) { ... }
2940 for (keys $obj->get_arrayref) { ... }
2941
bade7fbc
TC
2942To avoid confusing would-be users of your code who are running earlier
2943versions of Perl with mysterious syntax errors, put this sort of thing at
2944the top of your file to signal that your code will work I<only> on Perls of
2945a recent vintage:
2946
2947 use 5.012; # so keys/values/each work on arrays
2948 use 5.014; # so keys/values/each work on scalars (experimental)
2949
8f1da26d 2950See also C<each>, C<values>, and C<sort>.
ab192400 2951
b350dd2f 2952=item kill SIGNAL, LIST
9c7e4b76
KW
2953
2954=item kill SIGNAL
d74e8afc 2955X<kill> X<signal>
a0d0e21e 2956
b350dd2f 2957Sends a signal to a list of processes. Returns the number of
517db077
GS
2958processes successfully signaled (which is not necessarily the
2959same as the number actually killed).
a0d0e21e
LW
2960
2961 $cnt = kill 1, $child1, $child2;
2962 kill 9, @goners;
2963
3b10bc60 2964If SIGNAL is zero, no signal is sent to the process, but C<kill>
2965checks whether it's I<possible> to send a signal to it (that
70fb64f6 2966means, to be brief, that the process is owned by the same user, or we are
3b10bc60 2967the super-user). This is useful to check that a child process is still
81fd35db
DN
2968alive (even if only as a zombie) and hasn't changed its UID. See
2969L<perlport> for notes on the portability of this construct.
b350dd2f 2970
e2c0f81f 2971Unlike in the shell, if SIGNAL is negative, it kills process groups instead
391b733c
FC
2972of processes. That means you usually
2973want to use positive not negative signals.
e2c0f81f
DG
2974You may also use a signal name in quotes.
2975
2976The behavior of kill when a I<PROCESS> number is zero or negative depends on
2977the operating system. For example, on POSIX-conforming systems, zero will
2978signal the current process group and -1 will signal all processes.
1e9c1022
JL
2979
2980See L<perlipc/"Signals"> for more details.
a0d0e21e 2981
ea9eb35a 2982On some platforms such as Windows where the fork() system call is not available.
2983Perl can be built to emulate fork() at the interpreter level.
6d17f725 2984This emulation has limitations related to kill that have to be considered,
ea9eb35a 2985for code running on Windows and in code intended to be portable.
2986
2987See L<perlfork> for more details.
2988
9c7e4b76
KW
2989If there is no I<LIST> of processes, no signal is sent, and the return
2990value is 0. This form is sometimes used, however, because it causes
2991tainting checks to be run. But see
2992L<perlsec/Laundering and Detecting Tainted Data>.
2993
ea9eb35a 2994Portability issues: L<perlport/kill>.
2995
a0d0e21e 2996=item last LABEL
d74e8afc 2997X<last> X<break>
a0d0e21e
LW
2998
2999=item last
3000
3001The C<last> command is like the C<break> statement in C (as used in
3002loops); it immediately exits the loop in question. If the LABEL is
3003omitted, the command refers to the innermost enclosing loop. The
3004C<continue> block, if any, is not executed:
3005
4633a7c4 3006 LINE: while (<STDIN>) {
a9a5a0dc
VP
3007 last LINE if /^$/; # exit when done with header
3008 #...
a0d0e21e
LW
3009 }
3010
80d38338 3011C<last> cannot be used to exit a block that returns a value such as
8f1da26d 3012C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
2b5ab1e7 3013a grep() or map() operation.
4968c1e4 3014
6c1372ed
GS
3015Note that a block by itself is semantically identical to a loop
3016that executes once. Thus C<last> can be used to effect an early
3017exit out of such a block.
3018
98293880
JH
3019See also L</continue> for an illustration of how C<last>, C<next>, and
3020C<redo> work.
1d2dff63 3021
a0d0e21e 3022=item lc EXPR
d74e8afc 3023X<lc> X<lowercase>
a0d0e21e 3024
54310121 3025=item lc
bbce6d69 3026
d1be9408 3027Returns a lowercased version of EXPR. This is the internal function
3980dc9c 3028implementing the C<\L> escape in double-quoted strings.
a0d0e21e 3029
7660c0ab 3030If EXPR is omitted, uses C<$_>.
bbce6d69 3031
3980dc9c
KW
3032What gets returned depends on several factors:
3033
3034=over
3035
3036=item If C<use bytes> is in effect:
3037
3038=over
3039
3040=item On EBCDIC platforms
3041
3042The results are what the C language system call C<tolower()> returns.
3043
3044=item On ASCII platforms
3045
3046The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
3047respectively.
3048
3049=back
3050
66cbab2c 3051=item Otherwise, if C<use locale> (but not C<use locale ':not_characters'>) is in effect:
3980dc9c 3052
094a2f8c
KW
3053Respects current LC_CTYPE locale for code points < 256; and uses Unicode
3054semantics for the remaining code points (this last can only happen if
3055the UTF8 flag is also set). See L<perllocale>.
3980dc9c 3056
094a2f8c
KW
3057A deficiency in this is that case changes that cross the 255/256
3058boundary are not well-defined. For example, the lower case of LATIN CAPITAL
3059LETTER SHARP S (U+1E9E) in Unicode semantics is U+00DF (on ASCII
3060platforms). But under C<use locale>, the lower case of U+1E9E is
3061itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
3062current locale, and Perl has no way of knowing if that character even
3063exists in the locale, much less what code point it is. Perl returns
3064the input character unchanged, for all instances (and there aren't
3065many) where the 255/256 boundary would otherwise be crossed.
3980dc9c 3066
66cbab2c 3067=item Otherwise, If EXPR has the UTF8 flag set:
094a2f8c
KW
3068
3069Unicode semantics are used for the case change.
3980dc9c 3070
66cbab2c 3071=item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'>) is in effect: