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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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110=for Pod::Functions =String
111
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112C<chomp>, C<chop>, C<chr>, C<crypt>, C<fc>, C<hex>, C<index>, C<lc>,
113C<lcfirst>, C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
945c54fd 114C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
cb1a09d0 115
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116C<fc> is available only if the C<"fc"> feature is enabled or if it is
117prefixed with C<CORE::>. The C<"fc"> feature is enabled automatically
3dd9a840 118with a C<use v5.16> (or higher) declaration in the current scope.
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119
120
cb1a09d0 121=item Regular expressions and pattern matching
d74e8afc 122X<regular expression> X<regex> X<regexp>
cb1a09d0 123
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124=for Pod::Functions =Regexp
125
f5fa2679 126C<m//>, C<pos>, C<qr//>, C<quotemeta>, C<s///>, C<split>, C<study>
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127
128=item Numeric functions
d74e8afc 129X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 130
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131=for Pod::Functions =Math
132
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133C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
134C<sin>, C<sqrt>, C<srand>
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135
136=item Functions for real @ARRAYs
d74e8afc 137X<array>
cb1a09d0 138
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139=for Pod::Functions =ARRAY
140
a5ce339c 141C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
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142
143=item Functions for list data
d74e8afc 144X<list>
cb1a09d0 145
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146=for Pod::Functions =LIST
147
1dc8ecb8 148C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
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149
150=item Functions for real %HASHes
d74e8afc 151X<hash>
cb1a09d0 152
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153=for Pod::Functions =HASH
154
22fae026 155C<delete>, C<each>, C<exists>, C<keys>, C<values>
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156
157=item Input and output functions
d74e8afc 158X<I/O> X<input> X<output> X<dbm>
cb1a09d0 159
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160=for Pod::Functions =I/O
161
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162C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
163C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
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164C<readdir>, C<readline> C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>,
165C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>,
166C<truncate>, C<warn>, C<write>
cb1a09d0 167
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168C<say> is available only if the C<"say"> feature is enabled or if it is
169prefixed with C<CORE::>. The C<"say"> feature is enabled automatically
170with a C<use v5.10> (or higher) declaration in the current scope.
171
5dac7880 172=item Functions for fixed-length data or records
cb1a09d0 173
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174=for Pod::Functions =Binary
175
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176C<pack>, C<read>, C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<unpack>,
177C<vec>
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178
179=item Functions for filehandles, files, or directories
d74e8afc 180X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 181
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182=for Pod::Functions =File
183
22fae026 184C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 185C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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186C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
187C<umask>, C<unlink>, C<utime>
cb1a09d0 188
cf264981 189=item Keywords related to the control flow of your Perl program
d74e8afc 190X<control flow>
cb1a09d0 191
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192=for Pod::Functions =Flow
193
dba7b065 194C<break>, C<caller>, C<continue>, C<die>, C<do>,
7289c5e6 195C<dump>, C<eval>, C<evalbytes> C<exit>,
cfa52385 196C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
17d15541 197C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray>
84ed0108 198
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199C<break> is available only if you enable the experimental C<"switch">
200feature or use the C<CORE::> prefix. The C<"switch"> feature also enables
201the C<default>, C<given> and C<when> statements, which are documented in
202L<perlsyn/"Switch Statements">. The C<"switch"> feature is enabled
203automatically with a C<use v5.10> (or higher) declaration in the current
204scope. In Perl v5.14 and earlier, C<continue> required the C<"switch">
205feature, like the other keywords.
206
e3f68f70 207C<evalbytes> is only available with the C<"evalbytes"> feature (see
4fe70ef9 208L<feature>) or if prefixed with C<CORE::>. C<__SUB__> is only available
e3f68f70 209with the C<"current_sub"> feature or if prefixed with C<CORE::>. Both
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210the C<"evalbytes"> and C<"current_sub"> features are enabled automatically
211with a C<use v5.16> (or higher) declaration in the current scope.
cb1a09d0 212
54310121 213=item Keywords related to scoping
cb1a09d0 214
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215=for Pod::Functions =Namespace
216
8f1da26d 217C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
36fb85f3 218
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219C<state> is available only if the C<"state"> feature is enabled or if it is
220prefixed with C<CORE::>. The C<"state"> feature is enabled automatically
221with a C<use v5.10> (or higher) declaration in the current scope.
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222
223=item Miscellaneous functions
224
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225=for Pod::Functions =Misc
226
17d15541 227C<defined>, C<formline>, C<lock>, C<prototype>, C<reset>, C<scalar>, C<undef>
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228
229=item Functions for processes and process groups
d74e8afc 230X<process> X<pid> X<process id>
cb1a09d0 231
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232=for Pod::Functions =Process
233
22fae026 234C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
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235C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>,
236C<setpriority>, C<sleep>, C<system>,
22fae026 237C<times>, C<wait>, C<waitpid>
cb1a09d0 238
3b10bc60 239=item Keywords related to Perl modules
d74e8afc 240X<module>
cb1a09d0 241
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242=for Pod::Functions =Modules
243
22fae026 244C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 245
353c6505 246=item Keywords related to classes and object-orientation
d74e8afc 247X<object> X<class> X<package>
cb1a09d0 248
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249=for Pod::Functions =Objects
250
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251C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
252C<untie>, C<use>
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253
254=item Low-level socket functions
d74e8afc 255X<socket> X<sock>
cb1a09d0 256
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257=for Pod::Functions =Socket
258
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259C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
260C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 261C<socket>, C<socketpair>
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262
263=item System V interprocess communication functions
d74e8afc 264X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 265
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266=for Pod::Functions =SysV
267
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268C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
269C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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270
271=item Fetching user and group info
d74e8afc 272X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 273
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274=for Pod::Functions =User
275
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276C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
277C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
278C<getpwuid>, C<setgrent>, C<setpwent>
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279
280=item Fetching network info
d74e8afc 281X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 282
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283=for Pod::Functions =Network
284
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285C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
286C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
287C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
288C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
289C<setnetent>, C<setprotoent>, C<setservent>
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290
291=item Time-related functions
d74e8afc 292X<time> X<date>
cb1a09d0 293
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294=for Pod::Functions =Time
295
22fae026 296C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 297
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298=item Non-function keywords
299
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300=for Pod::Functions =!Non-functions
301
f5fa2679 302C<and>, C<AUTOLOAD>, C<BEGIN>, C<CHECK>, C<cmp>, C<CORE>, C<__DATA__>,
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303C<default>, C<DESTROY>, C<else>, C<elseif>, C<elsif>, C<END>, C<__END__>,
304C<eq>, C<for>, C<foreach>, C<ge>, C<given>, C<gt>, C<if>, C<INIT>, C<le>,
305C<lt>, C<ne>, C<not>, C<or>, C<UNITCHECK>, C<unless>, C<until>, C<when>,
306C<while>, C<x>, C<xor>
8f0d6a61 307
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308=back
309
60f9f73c 310=head2 Portability
d74e8afc 311X<portability> X<Unix> X<portable>
60f9f73c 312
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313Perl was born in Unix and can therefore access all common Unix
314system calls. In non-Unix environments, the functionality of some
8f1da26d 315Unix system calls may not be available or details of the available
2b5ab1e7 316functionality may differ slightly. The Perl functions affected
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317by this are:
318
319C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
320C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
321C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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322C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
323C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 324C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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325C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
326C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
327C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 328C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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329C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
330C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
331C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 332C<shmwrite>, C<socket>, C<socketpair>,
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333C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
334C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 335C<utime>, C<wait>, C<waitpid>
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336
337For more information about the portability of these functions, see
338L<perlport> and other available platform-specific documentation.
339
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340=head2 Alphabetical Listing of Perl Functions
341
3b10bc60 342=over
a0d0e21e 343
5b3c99c0 344=item -X FILEHANDLE
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345X<-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>
346X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 347
5b3c99c0 348=item -X EXPR
a0d0e21e 349
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350=item -X DIRHANDLE
351
5b3c99c0 352=item -X
a0d0e21e 353
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354=for Pod::Functions a file test (-r, -x, etc)
355
a0d0e21e 356A file test, where X is one of the letters listed below. This unary
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357operator takes one argument, either a filename, a filehandle, or a dirhandle,
358and tests the associated file to see if something is true about it. If the
7660c0ab 359argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 360Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 361the undefined value if the file doesn't exist. Despite the funny
d0821a6a 362names, precedence is the same as any other named unary operator. The
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363operator may be any of:
364
5ed4f2ec 365 -r File is readable by effective uid/gid.
366 -w File is writable by effective uid/gid.
367 -x File is executable by effective uid/gid.
368 -o File is owned by effective uid.
a0d0e21e 369
5ed4f2ec 370 -R File is readable by real uid/gid.
371 -W File is writable by real uid/gid.
372 -X File is executable by real uid/gid.
373 -O File is owned by real uid.
a0d0e21e 374
5ed4f2ec 375 -e File exists.
376 -z File has zero size (is empty).
377 -s File has nonzero size (returns size in bytes).
a0d0e21e 378
5ed4f2ec 379 -f File is a plain file.
380 -d File is a directory.
381 -l File is a symbolic link.
382 -p File is a named pipe (FIFO), or Filehandle is a pipe.
383 -S File is a socket.
384 -b File is a block special file.
385 -c File is a character special file.
386 -t Filehandle is opened to a tty.
a0d0e21e 387
5ed4f2ec 388 -u File has setuid bit set.
389 -g File has setgid bit set.
390 -k File has sticky bit set.
a0d0e21e 391
5ed4f2ec 392 -T File is an ASCII text file (heuristic guess).
393 -B File is a "binary" file (opposite of -T).
a0d0e21e 394
5ed4f2ec 395 -M Script start time minus file modification time, in days.
396 -A Same for access time.
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397 -C Same for inode change time (Unix, may differ for other
398 platforms)
a0d0e21e 399
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400Example:
401
402 while (<>) {
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403 chomp;
404 next unless -f $_; # ignore specials
405 #...
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406 }
407
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408Note that C<-s/a/b/> does not do a negated substitution. Saying
409C<-exp($foo)> still works as expected, however: only single letters
410following a minus are interpreted as file tests.
411
412These operators are exempt from the "looks like a function rule" described
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413above. That is, an opening parenthesis after the operator does not affect
414how much of the following code constitutes the argument. Put the opening
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415parentheses before the operator to separate it from code that follows (this
416applies only to operators with higher precedence than unary operators, of
417course):
418
419 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
420 (-s $file) + 1024 # correct
421
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422The interpretation of the file permission operators C<-r>, C<-R>,
423C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
424of the file and the uids and gids of the user. There may be other
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425reasons you can't actually read, write, or execute the file: for
426example network filesystem access controls, ACLs (access control lists),
427read-only filesystems, and unrecognized executable formats. Note
428that the use of these six specific operators to verify if some operation
429is possible is usually a mistake, because it may be open to race
430conditions.
5ff3f7a4 431
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432Also note that, for the superuser on the local filesystems, the C<-r>,
433C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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434if any execute bit is set in the mode. Scripts run by the superuser
435may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 436or temporarily set their effective uid to something else.
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437
438If you are using ACLs, there is a pragma called C<filetest> that may
439produce more accurate results than the bare stat() mode bits.
5dac7880
FC
440When under C<use filetest 'access'> the above-mentioned filetests
441test whether the permission can(not) be granted using the
3b10bc60 442access(2) family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
443under this pragma return true even if there are no execute permission
444bits set (nor any extra execute permission ACLs). This strangeness is
391b733c 445due to the underlying system calls' definitions. Note also that, due to
ecae030f
MO
446the implementation of C<use filetest 'access'>, the C<_> special
447filehandle won't cache the results of the file tests when this pragma is
448in effect. Read the documentation for the C<filetest> pragma for more
449information.
5ff3f7a4 450
a0d0e21e
LW
451The C<-T> and C<-B> switches work as follows. The first block or so of the
452file is examined for odd characters such as strange control codes or
61eff3bc 453characters with the high bit set. If too many strange characters (>30%)
cf264981 454are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 455containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 456or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 457rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 458file, or a file at EOF when testing a filehandle. Because you have to
4633a7c4
LW
459read a file to do the C<-T> test, on most occasions you want to use a C<-f>
460against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 461
5dac7880 462If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 463the special filehandle consisting of a solitary underline, then the stat
a0d0e21e
LW
464structure of the previous file test (or stat operator) is used, saving
465a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 466that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 467symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 468an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 469Example:
a0d0e21e
LW
470
471 print "Can do.\n" if -r $a || -w _ || -x _;
472
473 stat($filename);
474 print "Readable\n" if -r _;
475 print "Writable\n" if -w _;
476 print "Executable\n" if -x _;
477 print "Setuid\n" if -u _;
478 print "Setgid\n" if -g _;
479 print "Sticky\n" if -k _;
480 print "Text\n" if -T _;
481 print "Binary\n" if -B _;
482
e9fa405d 483As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file
fbb0b3b3 484test operators, in a way that C<-f -w -x $file> is equivalent to
a5840dee 485C<-x $file && -w _ && -f _>. (This is only fancy syntax: if you use
fbb0b3b3
RGS
486the return value of C<-f $file> as an argument to another filetest
487operator, no special magic will happen.)
488
bee96257 489Portability issues: L<perlport/-X>.
ea9eb35a 490
bade7fbc
TC
491To avoid confusing would-be users of your code with mysterious
492syntax errors, put something like this at the top of your script:
493
494 use 5.010; # so filetest ops can stack
495
a0d0e21e 496=item abs VALUE
d74e8afc 497X<abs> X<absolute>
a0d0e21e 498
54310121 499=item abs
bbce6d69 500
c17cdb72
NC
501=for Pod::Functions absolute value function
502
a0d0e21e 503Returns the absolute value of its argument.
7660c0ab 504If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
505
506=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 507X<accept>
a0d0e21e 508
c17cdb72
NC
509=for Pod::Functions accept an incoming socket connect
510
3b10bc60 511Accepts an incoming socket connect, just as accept(2)
19799a22 512does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 513See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 514
8d2a6795
GS
515On systems that support a close-on-exec flag on files, the flag will
516be set for the newly opened file descriptor, as determined by the
517value of $^F. See L<perlvar/$^F>.
518
a0d0e21e 519=item alarm SECONDS
d74e8afc
ITB
520X<alarm>
521X<SIGALRM>
522X<timer>
a0d0e21e 523
54310121 524=item alarm
bbce6d69 525
c17cdb72
NC
526=for Pod::Functions schedule a SIGALRM
527
a0d0e21e 528Arranges to have a SIGALRM delivered to this process after the
cf264981 529specified number of wallclock seconds has elapsed. If SECONDS is not
391b733c 530specified, the value stored in C<$_> is used. (On some machines,
d400eac8
JH
531unfortunately, the elapsed time may be up to one second less or more
532than you specified because of how seconds are counted, and process
533scheduling may delay the delivery of the signal even further.)
534
535Only one timer may be counting at once. Each call disables the
536previous timer, and an argument of C<0> may be supplied to cancel the
537previous timer without starting a new one. The returned value is the
538amount of time remaining on the previous timer.
a0d0e21e 539
2bc69794
BS
540For delays of finer granularity than one second, the Time::HiRes module
541(from CPAN, and starting from Perl 5.8 part of the standard
542distribution) provides ualarm(). You may also use Perl's four-argument
543version of select() leaving the first three arguments undefined, or you
544might be able to use the C<syscall> interface to access setitimer(2) if
391b733c 545your system supports it. See L<perlfaq8> for details.
2b5ab1e7 546
80d38338
TC
547It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
548C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 549
19799a22
GS
550If you want to use C<alarm> to time out a system call you need to use an
551C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 552fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 553restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 554modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
555
556 eval {
a9a5a0dc
VP
557 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
558 alarm $timeout;
559 $nread = sysread SOCKET, $buffer, $size;
560 alarm 0;
ff68c719 561 };
ff68c719 562 if ($@) {
a9a5a0dc 563 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 564 # timed out
ff68c719
PP
565 }
566 else {
5ed4f2ec 567 # didn't
ff68c719
PP
568 }
569
91d81acc
JH
570For more information see L<perlipc>.
571
ea9eb35a 572Portability issues: L<perlport/alarm>.
573
a0d0e21e 574=item atan2 Y,X
d74e8afc 575X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e 576
c17cdb72
NC
577=for Pod::Functions arctangent of Y/X in the range -PI to PI
578
a0d0e21e
LW
579Returns the arctangent of Y/X in the range -PI to PI.
580
ca6e1c26 581For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
582function, or use the familiar relation:
583
584 sub tan { sin($_[0]) / cos($_[0]) }
585
a1021d57
RGS
586The return value for C<atan2(0,0)> is implementation-defined; consult
587your atan2(3) manpage for more information.
bf5f1b4c 588
ea9eb35a 589Portability issues: L<perlport/atan2>.
590
a0d0e21e 591=item bind SOCKET,NAME
d74e8afc 592X<bind>
a0d0e21e 593
c17cdb72
NC
594=for Pod::Functions binds an address to a socket
595
3b10bc60 596Binds a network address to a socket, just as bind(2)
19799a22 597does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
598packed address of the appropriate type for the socket. See the examples in
599L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 600
fae2c0fb 601=item binmode FILEHANDLE, LAYER
d74e8afc 602X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 603
a0d0e21e
LW
604=item binmode FILEHANDLE
605
c17cdb72
NC
606=for Pod::Functions prepare binary files for I/O
607
1cbfc93d
NIS
608Arranges for FILEHANDLE to be read or written in "binary" or "text"
609mode on systems where the run-time libraries distinguish between
610binary and text files. If FILEHANDLE is an expression, the value is
611taken as the name of the filehandle. Returns true on success,
b5fe5ca2 612otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 613
8f1da26d 614On some systems (in general, DOS- and Windows-based systems) binmode()
d807c6f4 615is necessary when you're not working with a text file. For the sake
d7a0d798
FC
616of portability it is a good idea always to use it when appropriate,
617and never to use it when it isn't appropriate. Also, people can
8f1da26d 618set their I/O to be by default UTF8-encoded Unicode, not bytes.
d807c6f4
JH
619
620In other words: regardless of platform, use binmode() on binary data,
d7a0d798 621like images, for example.
d807c6f4
JH
622
623If LAYER is present it is a single string, but may contain multiple
391b733c 624directives. The directives alter the behaviour of the filehandle.
d7a0d798 625When LAYER is present, using binmode on a text file makes sense.
d807c6f4 626
fae2c0fb 627If LAYER is omitted or specified as C<:raw> the filehandle is made
391b733c 628suitable for passing binary data. This includes turning off possible CRLF
0226bbdb 629translation and marking it as bytes (as opposed to Unicode characters).
749683d2 630Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 631Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
632Other layers that would affect the binary nature of the stream are
391b733c 633I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 634PERLIO environment variable.
01e6739c 635
3b10bc60 636The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
637form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
638establish default I/O layers. See L<open>.
639
fae2c0fb
RGS
640I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
641in "Programming Perl, 3rd Edition". However, since the publishing of this
642book, by many known as "Camel III", the consensus of the naming of this
643functionality has moved from "discipline" to "layer". All documentation
644of this version of Perl therefore refers to "layers" rather than to
645"disciplines". Now back to the regularly scheduled documentation...>
646
8f1da26d 647To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
6902c96a 648C<:utf8> just marks the data as UTF-8 without further checking,
8f1da26d 649while C<:encoding(UTF-8)> checks the data for actually being valid
391b733c 650UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 651
ed53a2bb 652In general, binmode() should be called after open() but before any I/O
3b10bc60 653is done on the filehandle. Calling binmode() normally flushes any
01e6739c 654pending buffered output data (and perhaps pending input data) on the
fae2c0fb 655handle. An exception to this is the C<:encoding> layer that
d7a0d798 656changes the default character encoding of the handle; see L</open>.
fae2c0fb 657The C<:encoding> layer sometimes needs to be called in
3874323d
JH
658mid-stream, and it doesn't flush the stream. The C<:encoding>
659also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 660internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 661
19799a22 662The operating system, device drivers, C libraries, and Perl run-time
8f1da26d
TC
663system all conspire to let the programmer treat a single
664character (C<\n>) as the line terminator, irrespective of external
30168b04
GS
665representation. On many operating systems, the native text file
666representation matches the internal representation, but on some
667platforms the external representation of C<\n> is made up of more than
668one character.
669
8f1da26d
TC
670All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
671a single character to end each line in the external representation of text
672(even though that single character is CARRIAGE RETURN on old, pre-Darwin
391b733c 673flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
8f1da26d
TC
674systems like OS/2, DOS, and the various flavors of MS-Windows, your program
675sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
676two characters C<\cM\cJ>. That means that if you don't use binmode() on
677these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
678input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
679output. This is what you want for text files, but it can be disastrous for
680binary files.
30168b04
GS
681
682Another consequence of using binmode() (on some systems) is that
683special end-of-file markers will be seen as part of the data stream.
d7a0d798
FC
684For systems from the Microsoft family this means that, if your binary
685data contain C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
686the file, unless you use binmode().
687
3b10bc60 688binmode() is important not only for readline() and print() operations,
30168b04
GS
689but also when using read(), seek(), sysread(), syswrite() and tell()
690(see L<perlport> for more details). See the C<$/> and C<$\> variables
691in L<perlvar> for how to manually set your input and output
692line-termination sequences.
a0d0e21e 693
ea9eb35a 694Portability issues: L<perlport/binmode>.
695
4633a7c4 696=item bless REF,CLASSNAME
d74e8afc 697X<bless>
a0d0e21e
LW
698
699=item bless REF
700
c17cdb72
NC
701=for Pod::Functions create an object
702
2b5ab1e7
TC
703This function tells the thingy referenced by REF that it is now an object
704in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 705is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 706it returns the reference for convenience. Always use the two-argument
cf264981 707version if a derived class might inherit the function doing the blessing.
e54e4959 708See L<perlobj> for more about the blessing (and blessings) of objects.
a0d0e21e 709
57668c4d 710Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 711Namespaces with all lowercase names are considered reserved for
391b733c 712Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
713confusion, you may wish to avoid such package names as well. Make sure
714that CLASSNAME is a true value.
60ad88b8
GS
715
716See L<perlmod/"Perl Modules">.
717
0d863452
RH
718=item break
719
d9b04284 720=for Pod::Functions +switch break out of a C<given> block
c17cdb72 721
0d863452
RH
722Break out of a C<given()> block.
723
a8a26e52
JK
724This keyword is enabled by the C<"switch"> feature; see L<feature> for
725more information on C<"switch">. You can also access it by prefixing it
726with C<CORE::>. Alternatively, include a C<use v5.10> or later to the
727current scope.
0d863452 728
a0d0e21e 729=item caller EXPR
d74e8afc 730X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
731
732=item caller
733
c17cdb72
NC
734=for Pod::Functions get context of the current subroutine call
735
5a964f20 736Returns the context of the current subroutine call. In scalar context,
80d38338
TC
737returns the caller's package name if there I<is> a caller (that is, if
738we're in a subroutine or C<eval> or C<require>) and the undefined value
5a964f20 739otherwise. In list context, returns
a0d0e21e 740
ee6b43cc 741 # 0 1 2
748a9306 742 ($package, $filename, $line) = caller;
a0d0e21e
LW
743
744With EXPR, it returns some extra information that the debugger uses to
745print a stack trace. The value of EXPR indicates how many call frames
746to go back before the current one.
747
ee6b43cc 748 # 0 1 2 3 4
f3aa04c2 749 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 750
751 # 5 6 7 8 9 10
b3ca2e83 752 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 753 = caller($i);
e7ea3e70 754
951ba7fe 755Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 756call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 757C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 758C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 759C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 760$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
761each C<use> statement creates a C<require> frame inside an C<eval EXPR>
762frame.) $subroutine may also be C<(unknown)> if this particular
763subroutine happens to have been deleted from the symbol table.
764C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
765C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
585d73c3 766compiled with. C<$hints> corresponds to C<$^H>, and C<$bitmask>
1adb05cd
FC
767corresponds to C<${^WARNING_BITS}>. The
768C<$hints> and C<$bitmask> values are subject
585d73c3 769to change between versions of Perl, and are not meant for external use.
748a9306 770
b3ca2e83 771C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
391b733c 772caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
b3ca2e83
NC
773of this hash, as they are the actual values stored in the optree.
774
ffe0c19d
FC
775Furthermore, when called from within the DB package in
776list context, and with an argument, caller returns more
7660c0ab 777detailed information: it sets the list variable C<@DB::args> to be the
54310121 778arguments with which the subroutine was invoked.
748a9306 779
7660c0ab 780Be aware that the optimizer might have optimized call frames away before
19799a22 781C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 782might not return information about the call frame you expect it to, for
b76cc8ba 783C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 784previous time C<caller> was called.
7660c0ab 785
8f1da26d 786Be aware that setting C<@DB::args> is I<best effort>, intended for
391b733c 787debugging or generating backtraces, and should not be relied upon. In
ca9f0cb5
NC
788particular, as C<@_> contains aliases to the caller's arguments, Perl does
789not take a copy of C<@_>, so C<@DB::args> will contain modifications the
790subroutine makes to C<@_> or its contents, not the original values at call
391b733c 791time. C<@DB::args>, like C<@_>, does not hold explicit references to its
ca9f0cb5 792elements, so under certain cases its elements may have become freed and
391b733c 793reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 794of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
795I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
796reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5 797elements), so C<@DB::args> is actually a hybrid of the current state and
391b733c 798initial state of C<@_>. Buyer beware.
ca9f0cb5 799
a0d0e21e 800=item chdir EXPR
d74e8afc
ITB
801X<chdir>
802X<cd>
f723aae1 803X<directory, change>
a0d0e21e 804
c4aca7d0
GA
805=item chdir FILEHANDLE
806
807=item chdir DIRHANDLE
808
ce2984c3
PF
809=item chdir
810
c17cdb72
NC
811=for Pod::Functions change your current working directory
812
391b733c 813Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 814changes to the directory specified by C<$ENV{HOME}>, if set; if not,
391b733c
FC
815changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
816variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
817neither is set, C<chdir> does nothing. It returns true on success,
818false otherwise. See the example under C<die>.
a0d0e21e 819
3b10bc60 820On systems that support fchdir(2), you may pass a filehandle or
34169887 821directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 822passing handles raises an exception.
c4aca7d0 823
a0d0e21e 824=item chmod LIST
d74e8afc 825X<chmod> X<permission> X<mode>
a0d0e21e 826
c17cdb72
NC
827=for Pod::Functions changes the permissions on a list of files
828
a0d0e21e 829Changes the permissions of a list of files. The first element of the
8f1da26d 830list must be the numeric mode, which should probably be an octal
4ad40acf 831number, and which definitely should I<not> be a string of octal digits:
3b10bc60 832C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 833successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 834
3b10bc60 835 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 836 chmod 0755, @executables;
3b10bc60 837 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 838 # --w----r-T
3b10bc60 839 $mode = "0644"; chmod oct($mode), "foo"; # this is better
840 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 841
3b10bc60 842On systems that support fchmod(2), you may pass filehandles among the
843files. On systems that don't support fchmod(2), passing filehandles raises
844an exception. Filehandles must be passed as globs or glob references to be
845recognized; barewords are considered filenames.
c4aca7d0
GA
846
847 open(my $fh, "<", "foo");
848 my $perm = (stat $fh)[2] & 07777;
849 chmod($perm | 0600, $fh);
850
3b10bc60 851You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
852module:
853
3b10bc60 854 use Fcntl qw( :mode );
ca6e1c26 855 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 856 # Identical to the chmod 0755 of the example above.
ca6e1c26 857
ea9eb35a 858Portability issues: L<perlport/chmod>.
859
a0d0e21e 860=item chomp VARIABLE
d74e8afc 861X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 862
313c9f5c 863=item chomp( LIST )
a0d0e21e
LW
864
865=item chomp
866
c17cdb72
NC
867=for Pod::Functions remove a trailing record separator from a string
868
2b5ab1e7
TC
869This safer version of L</chop> removes any trailing string
870that corresponds to the current value of C<$/> (also known as
28757baa
PP
871$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
872number of characters removed from all its arguments. It's often used to
873remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
874that the final record may be missing its newline. When in paragraph
875mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 876When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 877a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 878remove anything.
19799a22 879If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
880
881 while (<>) {
a9a5a0dc
VP
882 chomp; # avoid \n on last field
883 @array = split(/:/);
884 # ...
a0d0e21e
LW
885 }
886
feef49c9
FC
887If VARIABLE is a hash, it chomps the hash's values, but not its keys,
888resetting the C<each> iterator in the process.
4bf21a6d 889
a0d0e21e
LW
890You can actually chomp anything that's an lvalue, including an assignment:
891
892 chomp($cwd = `pwd`);
893 chomp($answer = <STDIN>);
894
895If you chomp a list, each element is chomped, and the total number of
896characters removed is returned.
897
15e44fd8
RGS
898Note that parentheses are necessary when you're chomping anything
899that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
900is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
901C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
902C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
903as C<chomp($a, $b)>.
904
a0d0e21e 905=item chop VARIABLE
d74e8afc 906X<chop>
a0d0e21e 907
313c9f5c 908=item chop( LIST )
a0d0e21e
LW
909
910=item chop
911
c17cdb72
NC
912=for Pod::Functions remove the last character from a string
913
a0d0e21e 914Chops off the last character of a string and returns the character
5b3eff12 915chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 916scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
feef49c9
FC
917If VARIABLE is a hash, it chops the hash's values, but not its keys,
918resetting the C<each> iterator in the process.
4bf21a6d 919
5b3eff12 920You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
921
922If you chop a list, each element is chopped. Only the value of the
19799a22 923last C<chop> is returned.
a0d0e21e 924
19799a22 925Note that C<chop> returns the last character. To return all but the last
748a9306
LW
926character, use C<substr($string, 0, -1)>.
927
15e44fd8
RGS
928See also L</chomp>.
929
a0d0e21e 930=item chown LIST
d74e8afc 931X<chown> X<owner> X<user> X<group>
a0d0e21e 932
c17cdb72
NC
933=for Pod::Functions change the ownership on a list of files
934
a0d0e21e 935Changes the owner (and group) of a list of files. The first two
19799a22
GS
936elements of the list must be the I<numeric> uid and gid, in that
937order. A value of -1 in either position is interpreted by most
938systems to leave that value unchanged. Returns the number of files
939successfully changed.
a0d0e21e
LW
940
941 $cnt = chown $uid, $gid, 'foo', 'bar';
942 chown $uid, $gid, @filenames;
943
3b10bc60 944On systems that support fchown(2), you may pass filehandles among the
945files. On systems that don't support fchown(2), passing filehandles raises
946an exception. Filehandles must be passed as globs or glob references to be
947recognized; barewords are considered filenames.
c4aca7d0 948
54310121 949Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
950
951 print "User: ";
19799a22 952 chomp($user = <STDIN>);
5a964f20 953 print "Files: ";
19799a22 954 chomp($pattern = <STDIN>);
a0d0e21e
LW
955
956 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 957 or die "$user not in passwd file";
a0d0e21e 958
5ed4f2ec 959 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
960 chown $uid, $gid, @ary;
961
54310121 962On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
963file unless you're the superuser, although you should be able to change
964the group to any of your secondary groups. On insecure systems, these
965restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
966On POSIX systems, you can detect this condition this way:
967
968 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
969 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 970
ea9eb35a 971Portability issues: L<perlport/chmod>.
972
a0d0e21e 973=item chr NUMBER
d74e8afc 974X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 975
54310121 976=item chr
bbce6d69 977
c17cdb72
NC
978=for Pod::Functions get character this number represents
979
a0d0e21e 980Returns the character represented by that NUMBER in the character set.
a0ed51b3 981For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 982chr(0x263a) is a Unicode smiley face.
aaa68c4a 983
8a064bd6 984Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 985except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
986(truncated to an integer) are used.
987
974da8e5
JH
988If NUMBER is omitted, uses C<$_>.
989
b76cc8ba 990For the reverse, use L</ord>.
a0d0e21e 991
2575c402
JW
992Note that characters from 128 to 255 (inclusive) are by default
993internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 994
2575c402 995See L<perlunicode> for more about Unicode.
bbce6d69 996
a0d0e21e 997=item chroot FILENAME
d74e8afc 998X<chroot> X<root>
a0d0e21e 999
54310121 1000=item chroot
bbce6d69 1001
c17cdb72
NC
1002=for Pod::Functions make directory new root for path lookups
1003
5a964f20 1004This function works like the system call by the same name: it makes the
4633a7c4 1005named directory the new root directory for all further pathnames that
951ba7fe 1006begin with a C</> by your process and all its children. (It doesn't
28757baa 1007change your current working directory, which is unaffected.) For security
4633a7c4 1008reasons, this call is restricted to the superuser. If FILENAME is
19799a22 1009omitted, does a C<chroot> to C<$_>.
a0d0e21e 1010
ea9eb35a 1011Portability issues: L<perlport/chroot>.
1012
a0d0e21e 1013=item close FILEHANDLE
d74e8afc 1014X<close>
a0d0e21e 1015
6a518fbc
TP
1016=item close
1017
c17cdb72
NC
1018=for Pod::Functions close file (or pipe or socket) handle
1019
3b10bc60 1020Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 1021buffers, and closes the system file descriptor. Returns true if those
8f1da26d 1022operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
1023layer. Closes the currently selected filehandle if the argument is
1024omitted.
fb73857a
PP
1025
1026You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 1027another C<open> on it, because C<open> closes it for you. (See
01aa884e 1028L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
19799a22 1029counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 1030
3b10bc60 1031If the filehandle came from a piped open, C<close> returns false if one of
1032the other syscalls involved fails or if its program exits with non-zero
1033status. If the only problem was that the program exited non-zero, C<$!>
1034will be set to C<0>. Closing a pipe also waits for the process executing
1035on the pipe to exit--in case you wish to look at the output of the pipe
1036afterwards--and implicitly puts the exit status value of that command into
1037C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 1038
2e0cfa16
FC
1039If there are multiple threads running, C<close> on a filehandle from a
1040piped open returns true without waiting for the child process to terminate,
1041if the filehandle is still open in another thread.
1042
80d38338
TC
1043Closing the read end of a pipe before the process writing to it at the
1044other end is done writing results in the writer receiving a SIGPIPE. If
1045the other end can't handle that, be sure to read all the data before
1046closing the pipe.
73689b13 1047
fb73857a 1048Example:
a0d0e21e 1049
fb73857a
PP
1050 open(OUTPUT, '|sort >foo') # pipe to sort
1051 or die "Can't start sort: $!";
5ed4f2ec 1052 #... # print stuff to output
1053 close OUTPUT # wait for sort to finish
fb73857a
PP
1054 or warn $! ? "Error closing sort pipe: $!"
1055 : "Exit status $? from sort";
5ed4f2ec 1056 open(INPUT, 'foo') # get sort's results
fb73857a 1057 or die "Can't open 'foo' for input: $!";
a0d0e21e 1058
5a964f20 1059FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 1060filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
1061
1062=item closedir DIRHANDLE
d74e8afc 1063X<closedir>
a0d0e21e 1064
c17cdb72
NC
1065=for Pod::Functions close directory handle
1066
19799a22 1067Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
1068system call.
1069
a0d0e21e 1070=item connect SOCKET,NAME
d74e8afc 1071X<connect>
a0d0e21e 1072
c17cdb72
NC
1073=for Pod::Functions connect to a remote socket
1074
80d38338
TC
1075Attempts to connect to a remote socket, just like connect(2).
1076Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
1077packed address of the appropriate type for the socket. See the examples in
1078L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 1079
cb1a09d0 1080=item continue BLOCK
d74e8afc 1081X<continue>
cb1a09d0 1082
0d863452
RH
1083=item continue
1084
c17cdb72
NC
1085=for Pod::Functions optional trailing block in a while or foreach
1086
4a904372
FC
1087When followed by a BLOCK, C<continue> is actually a
1088flow control statement rather than a function. If
cf264981 1089there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
1090C<foreach>), it is always executed just before the conditional is about to
1091be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
1092it can be used to increment a loop variable, even when the loop has been
1093continued via the C<next> statement (which is similar to the C C<continue>
1094statement).
1095
98293880 1096C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 1097block; C<last> and C<redo> behave as if they had been executed within
19799a22 1098the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
1099block, it may be more entertaining.
1100
1101 while (EXPR) {
a9a5a0dc
VP
1102 ### redo always comes here
1103 do_something;
1d2dff63 1104 } continue {
a9a5a0dc
VP
1105 ### next always comes here
1106 do_something_else;
1107 # then back the top to re-check EXPR
1d2dff63
GS
1108 }
1109 ### last always comes here
1110
3b10bc60 1111Omitting the C<continue> section is equivalent to using an
1112empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1113to check the condition at the top of the loop.
1114
4a904372 1115When there is no BLOCK, C<continue> is a function that
8f1da26d
TC
1116falls through the current C<when> or C<default> block instead of iterating
1117a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
4a904372
FC
1118In Perl 5.14 and earlier, this form of C<continue> was
1119only available when the C<"switch"> feature was enabled.
48238296 1120See L<feature> and L<perlsyn/"Switch Statements"> for more
8f1da26d 1121information.
0d863452 1122
a0d0e21e 1123=item cos EXPR
d74e8afc 1124X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1125
d6217f1e
GS
1126=item cos
1127
c17cdb72
NC
1128=for Pod::Functions cosine function
1129
5a964f20 1130Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1131takes the cosine of C<$_>.
a0d0e21e 1132
ca6e1c26 1133For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1134function, or use this relation:
1135
1136 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1137
a0d0e21e 1138=item crypt PLAINTEXT,SALT
d74e8afc 1139X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1140X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1141
c17cdb72
NC
1142=for Pod::Functions one-way passwd-style encryption
1143
ef2e6798
MS
1144Creates a digest string exactly like the crypt(3) function in the C
1145library (assuming that you actually have a version there that has not
bb23f8d1 1146been extirpated as a potential munition).
ef2e6798 1147
34169887 1148crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1149into a short string, called a digest, which is returned. The same
1150PLAINTEXT and SALT will always return the same string, but there is no
1151(known) way to get the original PLAINTEXT from the hash. Small
1152changes in the PLAINTEXT or SALT will result in large changes in the
1153digest.
1154
1155There is no decrypt function. This function isn't all that useful for
1156cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1157mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1158primarily used to check if two pieces of text are the same without
1159having to transmit or store the text itself. An example is checking
1160if a correct password is given. The digest of the password is stored,
cf264981 1161not the password itself. The user types in a password that is
ef2e6798 1162crypt()'d with the same salt as the stored digest. If the two digests
34169887 1163match, the password is correct.
ef2e6798
MS
1164
1165When verifying an existing digest string you should use the digest as
1166the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1167to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1168crypt() will hash the new string with the same salt as the digest.
1169This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1170with more exotic implementations. In other words, assume
1171nothing about the returned string itself nor about how many bytes
1172of SALT may matter.
85c16d83
JH
1173
1174Traditionally the result is a string of 13 bytes: two first bytes of
1175the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
391b733c 1176the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1177hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1178and implementations on non-Unix platforms may produce different
ef2e6798 1179strings.
85c16d83
JH
1180
1181When choosing a new salt create a random two character string whose
1182characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1183'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1184characters is just a recommendation; the characters allowed in
1185the salt depend solely on your system's crypt library, and Perl can't
1186restrict what salts C<crypt()> accepts.
e71965be 1187
a0d0e21e 1188Here's an example that makes sure that whoever runs this program knows
cf264981 1189their password:
a0d0e21e
LW
1190
1191 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1192
1193 system "stty -echo";
1194 print "Password: ";
e71965be 1195 chomp($word = <STDIN>);
a0d0e21e
LW
1196 print "\n";
1197 system "stty echo";
1198
e71965be 1199 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1200 die "Sorry...\n";
a0d0e21e 1201 } else {
a9a5a0dc 1202 print "ok\n";
54310121 1203 }
a0d0e21e 1204
9f8f0c9d 1205Of course, typing in your own password to whoever asks you
748a9306 1206for it is unwise.
a0d0e21e 1207
ef2e6798 1208The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1209of data, not least of all because you can't get the information
ef2e6798 1210back. Look at the L<Digest> module for more robust algorithms.
19799a22 1211
f2791508
JH
1212If using crypt() on a Unicode string (which I<potentially> has
1213characters with codepoints above 255), Perl tries to make sense
34169887 1214of the situation by trying to downgrade (a copy of)
f2791508
JH
1215the string back to an eight-bit byte string before calling crypt()
1216(on that copy). If that works, good. If not, crypt() dies with
1217C<Wide character in crypt>.
85c16d83 1218
ea9eb35a 1219Portability issues: L<perlport/crypt>.
1220
aa689395 1221=item dbmclose HASH
d74e8afc 1222X<dbmclose>
a0d0e21e 1223
c17cdb72
NC
1224=for Pod::Functions breaks binding on a tied dbm file
1225
19799a22 1226[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1227
aa689395 1228Breaks the binding between a DBM file and a hash.
a0d0e21e 1229
ea9eb35a 1230Portability issues: L<perlport/dbmclose>.
1231
19799a22 1232=item dbmopen HASH,DBNAME,MASK
d74e8afc 1233X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1234
c17cdb72
NC
1235=for Pod::Functions create binding on a tied dbm file
1236
01aa884e
KW
1237[This function has been largely superseded by the
1238L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
a0d0e21e 1239
7b8d334a 1240This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1241hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1242argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1243is the name of the database (without the F<.dir> or F<.pag> extension if
1244any). If the database does not exist, it is created with protection
1b3a6178
FC
1245specified by MASK (as modified by the C<umask>). To prevent creation of
1246the database if it doesn't exist, you may specify a MODE
1247of 0, and the function will return a false value if it
1248can't find an existing database. If your system supports
80d38338 1249only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1250program. In older versions of Perl, if your system had neither DBM nor
19799a22 1251ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1252sdbm(3).
1253
1254If you don't have write access to the DBM file, you can only read hash
1255variables, not set them. If you want to test whether you can write,
3b10bc60 1256either use file tests or try setting a dummy hash entry inside an C<eval>
1257to trap the error.
a0d0e21e 1258
19799a22
GS
1259Note that functions such as C<keys> and C<values> may return huge lists
1260when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1261function to iterate over large DBM files. Example:
1262
1263 # print out history file offsets
1264 dbmopen(%HIST,'/usr/lib/news/history',0666);
1265 while (($key,$val) = each %HIST) {
a9a5a0dc 1266 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1267 }
1268 dbmclose(%HIST);
1269
cb1a09d0 1270See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1271cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1272rich implementation.
4633a7c4 1273
2b5ab1e7
TC
1274You can control which DBM library you use by loading that library
1275before you call dbmopen():
1276
1277 use DB_File;
1278 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1279 or die "Can't open netscape history file: $!";
2b5ab1e7 1280
ea9eb35a 1281Portability issues: L<perlport/dbmopen>.
1282
a0d0e21e 1283=item defined EXPR
d74e8afc 1284X<defined> X<undef> X<undefined>
a0d0e21e 1285
54310121 1286=item defined
bbce6d69 1287
c17cdb72
NC
1288=for Pod::Functions test whether a value, variable, or function is defined
1289
2f9daede 1290Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1291the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1292checked.
1293
1294Many operations return C<undef> to indicate failure, end of file,
1295system error, uninitialized variable, and other exceptional
1296conditions. This function allows you to distinguish C<undef> from
1297other values. (A simple Boolean test will not distinguish among
7660c0ab 1298C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1299false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1300doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1301returns C<undef> when its argument is an empty array, I<or> when the
1302element to return happens to be C<undef>.
1303
f10b0346
GS
1304You may also use C<defined(&func)> to check whether subroutine C<&func>
1305has ever been defined. The return value is unaffected by any forward
80d38338 1306declarations of C<&func>. A subroutine that is not defined
847c7ebe 1307may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1308makes it spring into existence the first time that it is called; see
847c7ebe 1309L<perlsub>.
f10b0346
GS
1310
1311Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1312used to report whether memory for that aggregate had ever been
f10b0346
GS
1313allocated. This behavior may disappear in future versions of Perl.
1314You should instead use a simple test for size:
1315
1316 if (@an_array) { print "has array elements\n" }
1317 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1318
1319When used on a hash element, it tells you whether the value is defined,
dc848c6f 1320not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1321purpose.
a0d0e21e
LW
1322
1323Examples:
1324
8f1da26d 1325 print if defined $switch{D};
a0d0e21e
LW
1326 print "$val\n" while defined($val = pop(@ary));
1327 die "Can't readlink $sym: $!"
a9a5a0dc 1328 unless defined($value = readlink $sym);
a0d0e21e 1329 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1330 $debugging = 0 unless defined $debugging;
a0d0e21e 1331
8f1da26d 1332Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1333discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1334defined values. For example, if you say
a5f75d66
AD
1335
1336 "ab" =~ /a(.*)b/;
1337
80d38338 1338The pattern match succeeds and C<$1> is defined, although it
cf264981 1339matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1340matched something that happened to be zero characters long. This is all
a5f75d66 1341very above-board and honest. When a function returns an undefined value,
2f9daede 1342it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1343should use C<defined> only when questioning the integrity of what
7660c0ab 1344you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1345what you want.
1346
dc848c6f 1347See also L</undef>, L</exists>, L</ref>.
2f9daede 1348
a0d0e21e 1349=item delete EXPR
d74e8afc 1350X<delete>
a0d0e21e 1351
c17cdb72
NC
1352=for Pod::Functions deletes a value from a hash
1353
d0a76353
RS
1354Given an expression that specifies an element or slice of a hash, C<delete>
1355deletes the specified elements from that hash so that exists() on that element
1356no longer returns true. Setting a hash element to the undefined value does
1357not remove its key, but deleting it does; see L</exists>.
80d38338 1358
8f1da26d 1359In list context, returns the value or values deleted, or the last such
80d38338 1360element in scalar context. The return list's length always matches that of
d0a76353
RS
1361the argument list: deleting non-existent elements returns the undefined value
1362in their corresponding positions.
80d38338 1363
d0a76353
RS
1364delete() may also be used on arrays and array slices, but its behavior is less
1365straightforward. Although exists() will return false for deleted entries,
1366deleting array elements never changes indices of existing values; use shift()
1367or splice() for that. However, if all deleted elements fall at the end of an
1368array, the array's size shrinks to the position of the highest element that
1369still tests true for exists(), or to 0 if none do.
1370
8f1da26d 1371B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1372be removed in a future version of Perl.
80d38338
TC
1373
1374Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1375a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1376or array may not necessarily return anything; it depends on the implementation
1377of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1378
80d38338
TC
1379The C<delete local EXPR> construct localizes the deletion to the current
1380block at run time. Until the block exits, elements locally deleted
1381temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1382of composite types">.
eba0920a
EM
1383
1384 %hash = (foo => 11, bar => 22, baz => 33);
f7051f2c
FC
1385 $scalar = delete $hash{foo}; # $scalar is 11
1386 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1387 @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
eba0920a 1388
01020589 1389The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1390
5f05dabc 1391 foreach $key (keys %HASH) {
a9a5a0dc 1392 delete $HASH{$key};
a0d0e21e
LW
1393 }
1394
01020589 1395 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1396 delete $ARRAY[$index];
01020589
GS
1397 }
1398
1399And so do these:
5f05dabc 1400
01020589
GS
1401 delete @HASH{keys %HASH};
1402
9740c838 1403 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1404
80d38338
TC
1405But both are slower than assigning the empty list
1406or undefining %HASH or @ARRAY, which is the customary
1407way to empty out an aggregate:
01020589 1408
5ed4f2ec 1409 %HASH = (); # completely empty %HASH
1410 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1411
5ed4f2ec 1412 @ARRAY = (); # completely empty @ARRAY
1413 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1414
80d38338
TC
1415The EXPR can be arbitrarily complicated provided its
1416final operation is an element or slice of an aggregate:
a0d0e21e
LW
1417
1418 delete $ref->[$x][$y]{$key};
5f05dabc 1419 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1420
01020589
GS
1421 delete $ref->[$x][$y][$index];
1422 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1423
a0d0e21e 1424=item die LIST
d74e8afc 1425X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1426
c17cdb72
NC
1427=for Pod::Functions raise an exception or bail out
1428
391b733c 1429C<die> raises an exception. Inside an C<eval> the error message is stuffed
4c050ad5
NC
1430into C<$@> and the C<eval> is terminated with the undefined value.
1431If the exception is outside of all enclosing C<eval>s, then the uncaught
391b733c 1432exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1433need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1434
1435Equivalent examples:
1436
1437 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1438 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1439
ccac6780 1440If the last element of LIST does not end in a newline, the current
df37ec69
WW
1441script line number and input line number (if any) are also printed,
1442and a newline is supplied. Note that the "input line number" (also
1443known as "chunk") is subject to whatever notion of "line" happens to
1444be currently in effect, and is also available as the special variable
1445C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1446
1447Hint: sometimes appending C<", stopped"> to your message will cause it
1448to make better sense when the string C<"at foo line 123"> is appended.
1449Suppose you are running script "canasta".
a0d0e21e
LW
1450
1451 die "/etc/games is no good";
1452 die "/etc/games is no good, stopped";
1453
1454produce, respectively
1455
1456 /etc/games is no good at canasta line 123.
1457 /etc/games is no good, stopped at canasta line 123.
1458
a96d0188 1459If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1460previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1461This is useful for propagating exceptions:
1462
1463 eval { ... };
1464 die unless $@ =~ /Expected exception/;
1465
a96d0188 1466If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1467C<PROPAGATE> method, that method will be called with additional file
1468and line number parameters. The return value replaces the value in
34169887 1469C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1470were called.
1471
7660c0ab 1472If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1473
4c050ad5
NC
1474If an uncaught exception results in interpreter exit, the exit code is
1475determined from the values of C<$!> and C<$?> with this pseudocode:
1476
1477 exit $! if $!; # errno
1478 exit $? >> 8 if $? >> 8; # child exit status
1479 exit 255; # last resort
1480
1481The intent is to squeeze as much possible information about the likely cause
391b733c
FC
1482into the limited space of the system exit
1483code. However, as C<$!> is the value
4c050ad5
NC
1484of C's C<errno>, which can be set by any system call, this means that the value
1485of the exit code used by C<die> can be non-predictable, so should not be relied
1486upon, other than to be non-zero.
1487
80d38338
TC
1488You can also call C<die> with a reference argument, and if this is trapped
1489within an C<eval>, C<$@> contains that reference. This permits more
1490elaborate exception handling using objects that maintain arbitrary state
1491about the exception. Such a scheme is sometimes preferable to matching
1492particular string values of C<$@> with regular expressions. Because C<$@>
1493is a global variable and C<eval> may be used within object implementations,
1494be careful that analyzing the error object doesn't replace the reference in
1495the global variable. It's easiest to make a local copy of the reference
1496before any manipulations. Here's an example:
52531d10 1497
80d38338 1498 use Scalar::Util "blessed";
da279afe 1499
52531d10 1500 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7 1501 if (my $ev_err = $@) {
f7051f2c
FC
1502 if (blessed($ev_err)
1503 && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1504 # handle Some::Module::Exception
1505 }
1506 else {
1507 # handle all other possible exceptions
1508 }
1509 }
1510
3b10bc60 1511Because Perl stringifies uncaught exception messages before display,
80d38338 1512you'll probably want to overload stringification operations on
52531d10
GS
1513exception objects. See L<overload> for details about that.
1514
19799a22
GS
1515You can arrange for a callback to be run just before the C<die>
1516does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1517handler is called with the error text and can change the error
19799a22 1518message, if it sees fit, by calling C<die> again. See
96090e4f 1519L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1520L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1521to be run only right before your program was to exit, this is not
3b10bc60 1522currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1523even inside eval()ed blocks/strings! If one wants the hook to do
1524nothing in such situations, put
fb73857a 1525
5ed4f2ec 1526 die @_ if $^S;
fb73857a 1527
19799a22
GS
1528as the first line of the handler (see L<perlvar/$^S>). Because
1529this promotes strange action at a distance, this counterintuitive
b76cc8ba 1530behavior may be fixed in a future release.
774d564b 1531
4c050ad5
NC
1532See also exit(), warn(), and the Carp module.
1533
a0d0e21e 1534=item do BLOCK
d74e8afc 1535X<do> X<block>
a0d0e21e 1536
c17cdb72
NC
1537=for Pod::Functions turn a BLOCK into a TERM
1538
a0d0e21e 1539Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1540sequence of commands indicated by BLOCK. When modified by the C<while> or
1541C<until> loop modifier, executes the BLOCK once before testing the loop
391b733c 1542condition. (On other statements the loop modifiers test the conditional
6b275a1f 1543first.)
a0d0e21e 1544
4968c1e4 1545C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1546C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1547See L<perlsyn> for alternative strategies.
4968c1e4 1548
a0d0e21e 1549=item do SUBROUTINE(LIST)
d74e8afc 1550X<do>
a0d0e21e 1551
3bab73c8
FC
1552This form of subroutine call is deprecated. SUBROUTINE can be a bareword
1553or scalar variable.
a0d0e21e
LW
1554
1555=item do EXPR
d74e8afc 1556X<do>
a0d0e21e
LW
1557
1558Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1559file as a Perl script.
a0d0e21e
LW
1560
1561 do 'stat.pl';
1562
c319391a 1563is largely like
a0d0e21e 1564
986b19de 1565 eval `cat stat.pl`;
a0d0e21e 1566
c319391a
AC
1567except that it's more concise, runs no external processes, keeps track of
1568the current
96090e4f
A
1569filename for error messages, searches the C<@INC> directories, and updates
1570C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1571these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1572cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1573same, however, in that it does reparse the file every time you call it,
1574so you probably don't want to do this inside a loop.
a0d0e21e 1575
8f1da26d 1576If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1577an error message in C<$@>. If C<do> cannot read the file, it returns undef
1578and sets C<$!> to the error. Always check C<$@> first, as compilation
1579could fail in a way that also sets C<$!>. If the file is successfully
1580compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1581
80d38338 1582Inclusion of library modules is better done with the
19799a22 1583C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1584and raise an exception if there's a problem.
a0d0e21e 1585
5a964f20
TC
1586You might like to use C<do> to read in a program configuration
1587file. Manual error checking can be done this way:
1588
b76cc8ba 1589 # read in config files: system first, then user
f86cebdf 1590 for $file ("/share/prog/defaults.rc",
b76cc8ba 1591 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1592 {
1593 unless ($return = do $file) {
1594 warn "couldn't parse $file: $@" if $@;
1595 warn "couldn't do $file: $!" unless defined $return;
1596 warn "couldn't run $file" unless $return;
1597 }
5a964f20
TC
1598 }
1599
a0d0e21e 1600=item dump LABEL
d74e8afc 1601X<dump> X<core> X<undump>
a0d0e21e 1602
8a7e748e
FC
1603=item dump EXPR
1604
1614b0e3
JD
1605=item dump
1606
c17cdb72
NC
1607=for Pod::Functions create an immediate core dump
1608
19799a22
GS
1609This function causes an immediate core dump. See also the B<-u>
1610command-line switch in L<perlrun>, which does the same thing.
1611Primarily this is so that you can use the B<undump> program (not
1612supplied) to turn your core dump into an executable binary after
1613having initialized all your variables at the beginning of the
1614program. When the new binary is executed it will begin by executing
1615a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1616Think of it as a goto with an intervening core dump and reincarnation.
8a7e748e
FC
1617If C<LABEL> is omitted, restarts the program from the top. The
1618C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1619computed at run time, being otherwise identical to C<dump LABEL>.
19799a22
GS
1620
1621B<WARNING>: Any files opened at the time of the dump will I<not>
1622be open any more when the program is reincarnated, with possible
80d38338 1623resulting confusion by Perl.
19799a22 1624
59f521f4 1625This function is now largely obsolete, mostly because it's very hard to
391b733c 1626convert a core file into an executable. That's why you should now invoke
59f521f4 1627it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1628typo.
19799a22 1629
2ba1f20a
FC
1630Unlike most named operators, this has the same precedence as assignment.
1631It is also exempt from the looks-like-a-function rule, so
1632C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1633C<dump>.
1634
ea9eb35a 1635Portability issues: L<perlport/dump>.
1636
532eee96 1637=item each HASH
d74e8afc 1638X<each> X<hash, iterator>
aa689395 1639
532eee96 1640=item each ARRAY
aeedbbed
NC
1641X<array, iterator>
1642
f5a93a43
TC
1643=item each EXPR
1644
c17cdb72
NC
1645=for Pod::Functions retrieve the next key/value pair from a hash
1646
bade7fbc
TC
1647When called on a hash in list context, returns a 2-element list
1648consisting of the key and value for the next element of a hash. In Perl
16495.12 and later only, it will also return the index and value for the next
1650element of an array so that you can iterate over it; older Perls consider
1651this a syntax error. When called in scalar context, returns only the key
1652(not the value) in a hash, or the index in an array.
2f9daede 1653
aeedbbed 1654Hash entries are returned in an apparently random order. The actual random
7bf59113
YO
1655order is specific to a given hash; the exact same series of operations
1656on two hashes may result in a different order for each hash. Any insertion
1657into the hash may change the order, as will any deletion, with the exception
1658that the most recent key returned by C<each> or C<keys> may be deleted
1659without changing the order. So long as a given hash is unmodified you may
1660rely on C<keys>, C<values> and C<each> to repeatedly return the same order
1661as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
1662details on why hash order is randomized. Aside from the guarantees
1663provided here the exact details of Perl's hash algorithm and the hash
1664traversal order are subject to change in any release of Perl.
ab192400 1665
80d38338
TC
1666After C<each> has returned all entries from the hash or array, the next
1667call to C<each> returns the empty list in list context and C<undef> in
bade7fbc
TC
1668scalar context; the next call following I<that> one restarts iteration.
1669Each hash or array has its own internal iterator, accessed by C<each>,
1670C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1671reached the end as just described; it can be explicitly reset by calling
1672C<keys> or C<values> on the hash or array. If you add or delete a hash's
1673elements while iterating over it, entries may be skipped or duplicated--so
1674don't do that. Exception: In the current implementation, it is always safe
1675to delete the item most recently returned by C<each()>, so the following
1676code works properly:
74fc8b5f
MJD
1677
1678 while (($key, $value) = each %hash) {
1679 print $key, "\n";
1680 delete $hash{$key}; # This is safe
1681 }
aa689395 1682
80d38338 1683This prints out your environment like the printenv(1) program,
3b10bc60 1684but in a different order:
a0d0e21e
LW
1685
1686 while (($key,$value) = each %ENV) {
a9a5a0dc 1687 print "$key=$value\n";
a0d0e21e
LW
1688 }
1689
f5a93a43
TC
1690Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1691reference to an unblessed hash or array. The argument will be dereferenced
1692automatically. This aspect of C<each> is considered highly experimental.
1693The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1694
1695 while (($key,$value) = each $hashref) { ... }
1696
e6a0db3e
FC
1697As of Perl 5.18 you can use a bare C<each> in a C<while> loop,
1698which will set C<$_> on every iteration.
1699
1700 while(each %ENV) {
1701 print "$_=$ENV{$_}\n";
1702 }
1703
bade7fbc
TC
1704To avoid confusing would-be users of your code who are running earlier
1705versions of Perl with mysterious syntax errors, put this sort of thing at
1706the top of your file to signal that your code will work I<only> on Perls of
1707a recent vintage:
1708
1709 use 5.012; # so keys/values/each work on arrays
1710 use 5.014; # so keys/values/each work on scalars (experimental)
e6a0db3e 1711 use 5.018; # so each assigns to $_ in a lone while test
bade7fbc 1712
8f1da26d 1713See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1714
1715=item eof FILEHANDLE
d74e8afc
ITB
1716X<eof>
1717X<end of file>
1718X<end-of-file>
a0d0e21e 1719
4633a7c4
LW
1720=item eof ()
1721
a0d0e21e
LW
1722=item eof
1723
c17cdb72
NC
1724=for Pod::Functions test a filehandle for its end
1725
8f1da26d 1726Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1727FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1728gives the real filehandle. (Note that this function actually
80d38338 1729reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1730interactive context.) Do not read from a terminal file (or call
19799a22 1731C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1732as terminals may lose the end-of-file condition if you do.
1733
820475bd 1734An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1735with empty parentheses is different. It refers to the pseudo file
820475bd 1736formed from the files listed on the command line and accessed via the
61eff3bc
JH
1737C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1738as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1739used will cause C<@ARGV> to be examined to determine if input is
67408cae 1740available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1741end-of-file will assume you are processing another C<@ARGV> list,
1742and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1743see L<perlop/"I/O Operators">.
820475bd 1744
61eff3bc 1745In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1746detect the end of each file, whereas C<eof()> will detect the end
1747of the very last file only. Examples:
a0d0e21e 1748
748a9306
LW
1749 # reset line numbering on each input file
1750 while (<>) {
a9a5a0dc
VP
1751 next if /^\s*#/; # skip comments
1752 print "$.\t$_";
5a964f20 1753 } continue {
a9a5a0dc 1754 close ARGV if eof; # Not eof()!
748a9306
LW
1755 }
1756
a0d0e21e
LW
1757 # insert dashes just before last line of last file
1758 while (<>) {
a9a5a0dc
VP
1759 if (eof()) { # check for end of last file
1760 print "--------------\n";
1761 }
1762 print;
f7051f2c 1763 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1764 }
1765
a0d0e21e 1766Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1767input operators typically return C<undef> when they run out of data or
1768encounter an error.
a0d0e21e
LW
1769
1770=item eval EXPR
d74e8afc 1771X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1772X<error, handling> X<exception, handling>
a0d0e21e
LW
1773
1774=item eval BLOCK
1775
ce2984c3
PF
1776=item eval
1777
c17cdb72
NC
1778=for Pod::Functions catch exceptions or compile and run code
1779
798dc914
KW
1780In the first form, often referred to as a "string eval", the return
1781value of EXPR is parsed and executed as if it
c7cc6f1c 1782were a little Perl program. The value of the expression (which is itself
8f1da26d 1783determined within scalar context) is first parsed, and if there were no
2341804c 1784errors, executed as a block within the lexical context of the current Perl
df4833a8 1785program. This means, that in particular, any outer lexical variables are
2341804c
DM
1786visible to it, and any package variable settings or subroutine and format
1787definitions remain afterwards.
1788
1789Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1790If EXPR is omitted, evaluates C<$_>. This form is typically used to
1791delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c 1792
7289c5e6
FC
1793If the C<unicode_eval> feature is enabled (which is the default under a
1794C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1795characters, so C<use utf8> declarations have no effect, and source filters
1796are forbidden. In the absence of the C<unicode_eval> feature, the string
1797will sometimes be treated as characters and sometimes as bytes, depending
1798on the internal encoding, and source filters activated within the C<eval>
1799exhibit the erratic, but historical, behaviour of affecting some outer file
1800scope that is still compiling. See also the L</evalbytes> keyword, which
1801always treats its input as a byte stream and works properly with source
1802filters, and the L<feature> pragma.
1803
798dc914
KW
1804Problems can arise if the string expands a scalar containing a floating
1805point number. That scalar can expand to letters, such as C<"NaN"> or
1806C<"Infinity">; or, within the scope of a C<use locale>, the decimal
1807point character may be something other than a dot (such as a comma).
1808None of these are likely to parse as you are likely expecting.
1809
c7cc6f1c 1810In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1811same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1812within the context of the current Perl program. This form is typically
1813used to trap exceptions more efficiently than the first (see below), while
1814also providing the benefit of checking the code within BLOCK at compile
1815time.
1816
1817The final semicolon, if any, may be omitted from the value of EXPR or within
1818the BLOCK.
1819
1820In both forms, the value returned is the value of the last expression
5a964f20 1821evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1822as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1823in void, scalar, or list context, depending on the context of the C<eval>
1824itself. See L</wantarray> for more on how the evaluation context can be
1825determined.
a0d0e21e 1826
19799a22 1827If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1828executed, C<eval> returns C<undef> in scalar context
774b80e8
FC
1829or an empty list in list context, and C<$@> is set to the error
1830message. (Prior to 5.16, a bug caused C<undef> to be returned
1831in list context for syntax errors, but not for runtime errors.)
1832If there was no error, C<$@> is set to the empty string. A
9cc672d4
FC
1833control flow operator like C<last> or C<goto> can bypass the setting of
1834C<$@>. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1835warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1836To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1837turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1838See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1839
19799a22
GS
1840Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1841determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1842is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1843the die operator is used to raise exceptions.
1844
5f1da31c
NT
1845If you want to trap errors when loading an XS module, some problems with
1846the binary interface (such as Perl version skew) may be fatal even with
df4833a8 1847C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
5f1da31c 1848
a0d0e21e
LW
1849If the code to be executed doesn't vary, you may use the eval-BLOCK
1850form to trap run-time errors without incurring the penalty of
1851recompiling each time. The error, if any, is still returned in C<$@>.
1852Examples:
1853
54310121 1854 # make divide-by-zero nonfatal
a0d0e21e
LW
1855 eval { $answer = $a / $b; }; warn $@ if $@;
1856
1857 # same thing, but less efficient
1858 eval '$answer = $a / $b'; warn $@ if $@;
1859
1860 # a compile-time error
5ed4f2ec 1861 eval { $answer = }; # WRONG
a0d0e21e
LW
1862
1863 # a run-time error
5ed4f2ec 1864 eval '$answer ='; # sets $@
a0d0e21e 1865
cf264981
SP
1866Using the C<eval{}> form as an exception trap in libraries does have some
1867issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1868may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1869You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1870as this example shows:
774d564b 1871
80d38338 1872 # a private exception trap for divide-by-zero
f86cebdf
GS
1873 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1874 warn $@ if $@;
774d564b
PP
1875
1876This is especially significant, given that C<__DIE__> hooks can call
19799a22 1877C<die> again, which has the effect of changing their error messages:
774d564b
PP
1878
1879 # __DIE__ hooks may modify error messages
1880 {
f86cebdf
GS
1881 local $SIG{'__DIE__'} =
1882 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1883 eval { die "foo lives here" };
1884 print $@ if $@; # prints "bar lives here"
774d564b
PP
1885 }
1886
19799a22 1887Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1888may be fixed in a future release.
1889
19799a22 1890With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1891being looked at when:
1892
5ed4f2ec 1893 eval $x; # CASE 1
1894 eval "$x"; # CASE 2
a0d0e21e 1895
5ed4f2ec 1896 eval '$x'; # CASE 3
1897 eval { $x }; # CASE 4
a0d0e21e 1898
5ed4f2ec 1899 eval "\$$x++"; # CASE 5
1900 $$x++; # CASE 6
a0d0e21e 1901
2f9daede 1902Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1903the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1904the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1905and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1906does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1907purely visual reasons, but it also has the advantage of compiling at
1908compile-time instead of at run-time.) Case 5 is a place where
19799a22 1909normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1910particular situation, you can just use symbolic references instead, as
1911in case 6.
a0d0e21e 1912
b6538e4f 1913Before Perl 5.14, the assignment to C<$@> occurred before restoration
bade7fbc 1914of localized variables, which means that for your code to run on older
b208c909 1915versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1916errors:
1917
1918 # alter $@ on nefarious repugnancy only
1919 {
1920 my $e;
1921 {
f7051f2c
FC
1922 local $@; # protect existing $@
1923 eval { test_repugnancy() };
1924 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1925 $@ =~ /nefarious/ and $e = $@;
8a5a710d
DN
1926 }
1927 die $e if defined $e
1928 }
1929
4968c1e4 1930C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1931C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1932
4f00fc7e
FC
1933An C<eval ''> executed within a subroutine defined
1934in the C<DB> package doesn't see the usual
3b10bc60 1935surrounding lexical scope, but rather the scope of the first non-DB piece
df4833a8 1936of code that called it. You don't normally need to worry about this unless
3b10bc60 1937you are writing a Perl debugger.
d819b83a 1938
7289c5e6
FC
1939=item evalbytes EXPR
1940X<evalbytes>
1941
1942=item evalbytes
1943
d9b04284 1944=for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
c17cdb72 1945
7289c5e6
FC
1946This function is like L</eval> with a string argument, except it always
1947parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1948string containing characters whose ordinal value exceeds 255 results in an
1949error. Source filters activated within the evaluated code apply to the
1950code itself.
1951
1952This function is only available under the C<evalbytes> feature, a
1953C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1954L<feature> for more information.
1955
a0d0e21e 1956=item exec LIST
d74e8afc 1957X<exec> X<execute>
a0d0e21e 1958
8bf3b016
GS
1959=item exec PROGRAM LIST
1960
c17cdb72
NC
1961=for Pod::Functions abandon this program to run another
1962
3b10bc60 1963The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1964use C<system> instead of C<exec> if you want it to return. It fails and
1965returns false only if the command does not exist I<and> it is executed
fb73857a 1966directly instead of via your system's command shell (see below).
a0d0e21e 1967
19799a22 1968Since it's a common mistake to use C<exec> instead of C<system>, Perl
4642e50d
EB
1969warns you if C<exec> is called in void context and if there is a following
1970statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1971you always do that, right?). If you I<really> want to follow an C<exec>
1972with some other statement, you can use one of these styles to avoid the warning:
55d729e4 1973
5a964f20
TC
1974 exec ('foo') or print STDERR "couldn't exec foo: $!";
1975 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1976
5a964f20 1977If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1978with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1979If there is only one scalar argument or an array with one element in it,
1980the argument is checked for shell metacharacters, and if there are any,
1981the entire argument is passed to the system's command shell for parsing
1982(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1983If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1984words and passed directly to C<execvp>, which is more efficient.
19799a22 1985Examples:
a0d0e21e 1986
19799a22
GS
1987 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1988 exec "sort $outfile | uniq";
a0d0e21e
LW
1989
1990If you don't really want to execute the first argument, but want to lie
1991to the program you are executing about its own name, you can specify
1992the program you actually want to run as an "indirect object" (without a
1993comma) in front of the LIST. (This always forces interpretation of the
54310121 1994LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1995the list.) Example:
1996
1997 $shell = '/bin/csh';
5ed4f2ec 1998 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1999
2000or, more directly,
2001
5ed4f2ec 2002 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 2003
3b10bc60 2004When the arguments get executed via the system shell, results are
2005subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
2006for details.
2007
19799a22
GS
2008Using an indirect object with C<exec> or C<system> is also more
2009secure. This usage (which also works fine with system()) forces
2010interpretation of the arguments as a multivalued list, even if the
2011list had just one argument. That way you're safe from the shell
2012expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
2013
2014 @args = ( "echo surprise" );
2015
2b5ab1e7 2016 exec @args; # subject to shell escapes
f86cebdf 2017 # if @args == 1
2b5ab1e7 2018 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
2019
2020The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
2021program, passing it C<"surprise"> an argument. The second version didn't;
2022it tried to run a program named I<"echo surprise">, didn't find it, and set
2023C<$?> to a non-zero value indicating failure.
5a964f20 2024
e9fa405d
BF
2025Perl attempts to flush all files opened for output before the exec,
2026but this may not be supported on some platforms (see L<perlport>).
2027To be safe, you may need to set C<$|> ($AUTOFLUSH in English) or
2028call the C<autoflush()> method of C<IO::Handle> on any open handles
2029to avoid lost output.
0f897271 2030
80d38338
TC
2031Note that C<exec> will not call your C<END> blocks, nor will it invoke
2032C<DESTROY> methods on your objects.
7660c0ab 2033
ea9eb35a 2034Portability issues: L<perlport/exec>.
2035
a0d0e21e 2036=item exists EXPR
d74e8afc 2037X<exists> X<autovivification>
a0d0e21e 2038
c17cdb72
NC
2039=for Pod::Functions test whether a hash key is present
2040
d0a76353
RS
2041Given an expression that specifies an element of a hash, returns true if the
2042specified element in the hash has ever been initialized, even if the
2043corresponding value is undefined.
a0d0e21e 2044
5ed4f2ec 2045 print "Exists\n" if exists $hash{$key};
2046 print "Defined\n" if defined $hash{$key};
01020589
GS
2047 print "True\n" if $hash{$key};
2048
d0a76353 2049exists may also be called on array elements, but its behavior is much less
8f1da26d 2050obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
2051that calling exists on array values is deprecated and likely to be removed in
2052a future version of Perl.
2053
5ed4f2ec 2054 print "Exists\n" if exists $array[$index];
2055 print "Defined\n" if defined $array[$index];
01020589 2056 print "True\n" if $array[$index];
a0d0e21e 2057
8f1da26d 2058A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
2059it exists, but the reverse doesn't necessarily hold true.
2060
afebc493
GS
2061Given an expression that specifies the name of a subroutine,
2062returns true if the specified subroutine has ever been declared, even
2063if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 2064does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
2065exist may still be callable: its package may have an C<AUTOLOAD>
2066method that makes it spring into existence the first time that it is
3b10bc60 2067called; see L<perlsub>.
afebc493 2068
5ed4f2ec 2069 print "Exists\n" if exists &subroutine;
2070 print "Defined\n" if defined &subroutine;
afebc493 2071
a0d0e21e 2072Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 2073operation is a hash or array key lookup or subroutine name:
a0d0e21e 2074
5ed4f2ec 2075 if (exists $ref->{A}->{B}->{$key}) { }
2076 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 2077
5ed4f2ec 2078 if (exists $ref->{A}->{B}->[$ix]) { }
2079 if (exists $hash{A}{B}[$ix]) { }
01020589 2080
afebc493
GS
2081 if (exists &{$ref->{A}{B}{$key}}) { }
2082
9590a7cd 2083Although the most deeply nested array or hash element will not spring into
3b10bc60 2084existence just because its existence was tested, any intervening ones will.
61eff3bc 2085Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 2086into existence due to the existence test for the $key element above.
3b10bc60 2087This happens anywhere the arrow operator is used, including even here:
5a964f20 2088
2b5ab1e7 2089 undef $ref;
5ed4f2ec 2090 if (exists $ref->{"Some key"}) { }
2091 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
2092
2093This surprising autovivification in what does not at first--or even
2094second--glance appear to be an lvalue context may be fixed in a future
5a964f20 2095release.
a0d0e21e 2096
afebc493
GS
2097Use of a subroutine call, rather than a subroutine name, as an argument
2098to exists() is an error.
2099
5ed4f2ec 2100 exists &sub; # OK
2101 exists &sub(); # Error
afebc493 2102
a0d0e21e 2103=item exit EXPR
d74e8afc 2104X<exit> X<terminate> X<abort>
a0d0e21e 2105
ce2984c3
PF
2106=item exit
2107
c17cdb72
NC
2108=for Pod::Functions terminate this program
2109
2b5ab1e7 2110Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
2111
2112 $ans = <STDIN>;
2113 exit 0 if $ans =~ /^[Xx]/;
2114
19799a22 2115See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
2116universally recognized values for EXPR are C<0> for success and C<1>
2117for error; other values are subject to interpretation depending on the
2118environment in which the Perl program is running. For example, exiting
211969 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2120the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 2121
19799a22
GS
2122Don't use C<exit> to abort a subroutine if there's any chance that
2123someone might want to trap whatever error happened. Use C<die> instead,
2124which can be trapped by an C<eval>.
28757baa 2125
19799a22 2126The exit() function does not always exit immediately. It calls any
2b5ab1e7 2127defined C<END> routines first, but these C<END> routines may not
19799a22 2128themselves abort the exit. Likewise any object destructors that need to
60275626 2129be called are called before the real exit. C<END> routines and destructors
391b733c 2130can change the exit status by modifying C<$?>. If this is a problem, you
fae6f8fa 2131can call C<POSIX::_exit($status)> to avoid END and destructor processing.
87275199 2132See L<perlmod> for details.
5a964f20 2133
ea9eb35a 2134Portability issues: L<perlport/exit>.
2135
a0d0e21e 2136=item exp EXPR
d74e8afc 2137X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 2138
54310121 2139=item exp
bbce6d69 2140
c17cdb72
NC
2141=for Pod::Functions raise I<e> to a power
2142
b76cc8ba 2143Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
2144If EXPR is omitted, gives C<exp($_)>.
2145
628253b8
BF
2146=item fc EXPR
2147X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2148
2149=item fc
2150
d9b04284 2151=for Pod::Functions +fc return casefolded version of a string
c17cdb72 2152
628253b8
BF
2153Returns the casefolded version of EXPR. This is the internal function
2154implementing the C<\F> escape in double-quoted strings.
2155
2156Casefolding is the process of mapping strings to a form where case
2157differences are erased; comparing two strings in their casefolded
2158form is effectively a way of asking if two strings are equal,
2159regardless of case.
2160
2161Roughly, if you ever found yourself writing this
2162
f6c6dcb6 2163 lc($this) eq lc($that) # Wrong!
628253b8 2164 # or
f6c6dcb6 2165 uc($this) eq uc($that) # Also wrong!
628253b8 2166 # or
f6c6dcb6 2167 $this =~ /^\Q$that\E\z/i # Right!
628253b8
BF
2168
2169Now you can write
2170
2171 fc($this) eq fc($that)
2172
2173And get the correct results.
2174
fc39a31f
KW
2175Perl only implements the full form of casefolding,
2176but you can access the simple folds using L<Unicode::UCD/casefold()> and
2177L<Unicode::UCD/prop_invmap()>.
628253b8
BF
2178For further information on casefolding, refer to
2179the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
21804.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2181available at L<http://www.unicode.org/versions/latest/>, as well as the
2182Case Charts available at L<http://www.unicode.org/charts/case/>.
2183
2184If EXPR is omitted, uses C<$_>.
2185
1ca267a5
KW
2186This function behaves the same way under various pragma, such as within
2187S<C<"use feature 'unicode_strings">>, as L</lc> does, with the single
2188exception of C<fc> of LATIN CAPITAL LETTER SHARP S (U+1E9E) within the
2189scope of S<C<use locale>>. The foldcase of this character would
2190normally be C<"ss">, but as explained in the L</lc> section, case
2191changes that cross the 255/256 boundary are problematic under locales,
2192and are hence prohibited. Therefore, this function under locale returns
2193instead the string C<"\x{17F}\x{17F}">, which is the LATIN SMALL LETTER
2194LONG S. Since that character itself folds to C<"s">, the string of two
2195of them together should be equivalent to a single U+1E9E when foldcased.
628253b8
BF
2196
2197While the Unicode Standard defines two additional forms of casefolding,
2198one for Turkic languages and one that never maps one character into multiple
2199characters, these are not provided by the Perl core; However, the CPAN module
2200C<Unicode::Casing> may be used to provide an implementation.
2201
2202This keyword is available only when the C<"fc"> feature is enabled,
2203or when prefixed with C<CORE::>; See L<feature>. Alternately,
2204include a C<use v5.16> or later to the current scope.
2205
a0d0e21e 2206=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2207X<fcntl>
a0d0e21e 2208
c17cdb72
NC
2209=for Pod::Functions file control system call
2210
f86cebdf 2211Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
2212
2213 use Fcntl;
2214
0ade1984 2215first to get the correct constant definitions. Argument processing and
3b10bc60 2216value returned work just like C<ioctl> below.
a0d0e21e
LW
2217For example:
2218
2219 use Fcntl;
5a964f20 2220 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 2221 or die "can't fcntl F_GETFL: $!";
5a964f20 2222
554ad1fc 2223You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
2224Like C<ioctl>, it maps a C<0> return from the system call into
2225C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
2226in numeric context. It is also exempt from the normal B<-w> warnings
2227on improper numeric conversions.
5a964f20 2228
3b10bc60 2229Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
2230doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2231manpage to learn what functions are available on your system.
a0d0e21e 2232
be2f7487 2233Here's an example of setting a filehandle named C<REMOTE> to be
2234non-blocking at the system level. You'll have to negotiate C<$|>
2235on your own, though.
2236
2237 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2238
2239 $flags = fcntl(REMOTE, F_GETFL, 0)
2240 or die "Can't get flags for the socket: $!\n";
2241
2242 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2243 or die "Can't set flags for the socket: $!\n";
2244
ea9eb35a 2245Portability issues: L<perlport/fcntl>.
2246
cfa52385
FC
2247=item __FILE__
2248X<__FILE__>
2249
c17cdb72
NC
2250=for Pod::Functions the name of the current source file
2251
cfa52385
FC
2252A special token that returns the name of the file in which it occurs.
2253
a0d0e21e 2254=item fileno FILEHANDLE
d74e8afc 2255X<fileno>
a0d0e21e 2256
c17cdb72
NC
2257=for Pod::Functions return file descriptor from filehandle
2258
2b5ab1e7 2259Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
2260filehandle is not open. If there is no real file descriptor at the OS
2261level, as can happen with filehandles connected to memory objects via
2262C<open> with a reference for the third argument, -1 is returned.
2263
2264This is mainly useful for constructing
19799a22 2265bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
2266If FILEHANDLE is an expression, the value is taken as an indirect
2267filehandle, generally its name.
5a964f20 2268
b76cc8ba 2269You can use this to find out whether two handles refer to the
5a964f20
TC
2270same underlying descriptor:
2271
3231d257 2272 if (fileno(THIS) != -1 && fileno(THIS) == fileno(THAT)) {
a9a5a0dc 2273 print "THIS and THAT are dups\n";
3231d257
DH
2274 } elsif (fileno(THIS) != -1 && fileno(THAT) != -1) {
2275 print "THIS and THAT have different underlying file descriptors\n";
2276 } else {
2277 print "At least one of THIS and THAT does not have a real file descriptor\n";
b76cc8ba
NIS
2278 }
2279
a0d0e21e 2280=item flock FILEHANDLE,OPERATION
d74e8afc 2281X<flock> X<lock> X<locking>
a0d0e21e 2282
c17cdb72
NC
2283=for Pod::Functions lock an entire file with an advisory lock
2284
19799a22
GS
2285Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2286for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2287machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2288C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2289entire files only, not records.
2b5ab1e7
TC
2290
2291Two potentially non-obvious but traditional C<flock> semantics are
2292that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2293are B<merely advisory>. Such discretionary locks are more flexible, but
2294offer fewer guarantees. This means that programs that do not also use
2295C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2296your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2297for details. It's best to assume traditional behavior if you're writing
2298portable programs. (But if you're not, you should as always feel perfectly
2299free to write for your own system's idiosyncrasies (sometimes called
2300"features"). Slavish adherence to portability concerns shouldn't get
2301in the way of your getting your job done.)
a3cb178b 2302
8ebc5c01
PP
2303OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2304LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2305you can use the symbolic names if you import them from the L<Fcntl> module,
2306either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2307requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2308releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2309LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2310waiting for the lock; check the return status to see if you got it.
68dc0745 2311
2b5ab1e7
TC
2312To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2313before locking or unlocking it.
8ebc5c01 2314
f86cebdf 2315Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2316locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2317are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2318implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
2319differing semantics shouldn't bite too many people.
2320
becacb53
TM
2321Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2322be open with read intent to use LOCK_SH and requires that it be open
2323with write intent to use LOCK_EX.
2324
19799a22
GS
2325Note also that some versions of C<flock> cannot lock things over the
2326network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2327that. If you like you can force Perl to ignore your system's flock(2)
2328function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2329the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2330and build a new Perl.
4633a7c4
LW
2331
2332Here's a mailbox appender for BSD systems.
a0d0e21e 2333
f7051f2c
FC
2334 # import LOCK_* and SEEK_END constants
2335 use Fcntl qw(:flock SEEK_END);
a0d0e21e
LW
2336
2337 sub lock {
a9a5a0dc
VP
2338 my ($fh) = @_;
2339 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2340
a9a5a0dc
VP
2341 # and, in case someone appended while we were waiting...
2342 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2343 }
2344
2345 sub unlock {
a9a5a0dc
VP
2346 my ($fh) = @_;
2347 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2348 }
2349
b0169937 2350 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2351 or die "Can't open mailbox: $!";
a0d0e21e 2352
7ed5353d 2353 lock($mbox);
b0169937 2354 print $mbox $msg,"\n\n";
7ed5353d 2355 unlock($mbox);
a0d0e21e 2356
3b10bc60 2357On systems that support a real flock(2), locks are inherited across fork()
2358calls, whereas those that must resort to the more capricious fcntl(2)
2359function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2360
cb1a09d0 2361See also L<DB_File> for other flock() examples.
a0d0e21e 2362
ea9eb35a 2363Portability issues: L<perlport/flock>.
2364
a0d0e21e 2365=item fork
d74e8afc 2366X<fork> X<child> X<parent>
a0d0e21e 2367
c17cdb72
NC
2368=for Pod::Functions create a new process just like this one
2369
2b5ab1e7
TC
2370Does a fork(2) system call to create a new process running the
2371same program at the same point. It returns the child pid to the
2372parent process, C<0> to the child process, or C<undef> if the fork is
2373unsuccessful. File descriptors (and sometimes locks on those descriptors)
2374are shared, while everything else is copied. On most systems supporting
2375fork(), great care has gone into making it extremely efficient (for
2376example, using copy-on-write technology on data pages), making it the
2377dominant paradigm for multitasking over the last few decades.
5a964f20 2378
e9fa405d 2379Perl attempts to flush all files opened for
0f897271
GS
2380output before forking the child process, but this may not be supported
2381on some platforms (see L<perlport>). To be safe, you may need to set
2382C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2383C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2384
19799a22 2385If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2386accumulate zombies. On some systems, you can avoid this by setting
2387C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2388forking and reaping moribund children.
cb1a09d0 2389
28757baa
PP
2390Note that if your forked child inherits system file descriptors like
2391STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2392if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2393backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2394You should reopen those to F</dev/null> if it's any issue.
28757baa 2395
ea9eb35a 2396On some platforms such as Windows, where the fork() system call is not available,
391b733c
FC
2397Perl can be built to emulate fork() in the Perl interpreter.
2398The emulation is designed, at the level of the Perl program,
2399to be as compatible as possible with the "Unix" fork().
6d17f725 2400However it has limitations that have to be considered in code intended to be portable.
ea9eb35a 2401See L<perlfork> for more details.
2402
2403Portability issues: L<perlport/fork>.
2404
cb1a09d0 2405=item format
d74e8afc 2406X<format>
cb1a09d0 2407
c17cdb72
NC
2408=for Pod::Functions declare a picture format with use by the write() function
2409
19799a22 2410Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2411example:
2412
54310121 2413 format Something =
a9a5a0dc
VP
2414 Test: @<<<<<<<< @||||| @>>>>>
2415 $str, $%, '$' . int($num)
cb1a09d0
AD
2416 .
2417
2418 $str = "widget";
184e9718 2419 $num = $cost/$quantity;
cb1a09d0
AD
2420 $~ = 'Something';
2421 write;
2422
2423See L<perlform> for many details and examples.
2424
8903cb82 2425=item formline PICTURE,LIST
d74e8afc 2426X<formline>
a0d0e21e 2427
c17cdb72
NC
2428=for Pod::Functions internal function used for formats
2429
5a964f20 2430This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2431too. It formats (see L<perlform>) a list of values according to the
2432contents of PICTURE, placing the output into the format output
7660c0ab 2433accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2434Eventually, when a C<write> is done, the contents of
cf264981
SP
2435C<$^A> are written to some filehandle. You could also read C<$^A>
2436and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2437does one C<formline> per line of form, but the C<formline> function itself
748a9306 2438doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2439that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2440You may therefore need to use multiple formlines to implement a single
3b10bc60 2441record format, just like the C<format> compiler.
748a9306 2442
19799a22 2443Be careful if you put double quotes around the picture, because an C<@>
748a9306 2444character may be taken to mean the beginning of an array name.
19799a22 2445C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2446
445b09e5
FC
2447If you are trying to use this instead of C<write> to capture the output,
2448you may find it easier to open a filehandle to a scalar
2449(C<< open $fh, ">", \$output >>) and write to that instead.
2450
a0d0e21e 2451=item getc FILEHANDLE
f723aae1 2452X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2453
2454=item getc
2455
c17cdb72
NC
2456=for Pod::Functions get the next character from the filehandle
2457
a0d0e21e 2458Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2459or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2460the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2461STDIN. This is not particularly efficient. However, it cannot be
2462used by itself to fetch single characters without waiting for the user
2463to hit enter. For that, try something more like:
4633a7c4
LW
2464
2465 if ($BSD_STYLE) {
a9a5a0dc 2466 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2467 }
2468 else {
a9a5a0dc 2469 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2470 }
2471
2472 $key = getc(STDIN);
2473
2474 if ($BSD_STYLE) {
a9a5a0dc 2475 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2476 }
2477 else {
3b10bc60 2478 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2479 }
2480 print "\n";
2481
54310121
PP
2482Determination of whether $BSD_STYLE should be set
2483is left as an exercise to the reader.
cb1a09d0 2484
19799a22 2485The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2486systems purporting POSIX compliance. See also the C<Term::ReadKey>
3d6c5fec 2487module from your nearest L<CPAN|http://www.cpan.org> site.
a0d0e21e
LW
2488
2489=item getlogin
d74e8afc 2490X<getlogin> X<login>
a0d0e21e 2491
c17cdb72
NC
2492=for Pod::Functions return who logged in at this tty
2493
cf264981 2494This implements the C library function of the same name, which on most
3b10bc60 2495systems returns the current login from F</etc/utmp>, if any. If it
2496returns the empty string, use C<getpwuid>.
a0d0e21e 2497
f86702cc 2498 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2499
19799a22
GS
2500Do not consider C<getlogin> for authentication: it is not as
2501secure as C<getpwuid>.
4633a7c4 2502
ea9eb35a 2503Portability issues: L<perlport/getlogin>.
2504
a0d0e21e 2505=item getpeername SOCKET
d74e8afc 2506X<getpeername> X<peer>
a0d0e21e 2507
c17cdb72
NC
2508=for Pod::Functions find the other end of a socket connection
2509
a3390c9f
FC
2510Returns the packed sockaddr address of the other end of the SOCKET
2511connection.
a0d0e21e 2512
4633a7c4
LW
2513 use Socket;
2514 $hersockaddr = getpeername(SOCK);
19799a22 2515 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2516 $herhostname = gethostbyaddr($iaddr, AF_INET);
2517 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2518
2519=item getpgrp PID
d74e8afc 2520X<getpgrp> X<group>
a0d0e21e 2521
c17cdb72
NC
2522=for Pod::Functions get process group
2523
47e29363 2524Returns the current process group for the specified PID. Use
7660c0ab 2525a PID of C<0> to get the current process group for the
4633a7c4 2526current process. Will raise an exception if used on a machine that
a3390c9f
FC
2527doesn't implement getpgrp(2). If PID is omitted, returns the process
2528group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2529does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2530
ea9eb35a 2531Portability issues: L<perlport/getpgrp>.
2532
a0d0e21e 2533=item getppid
d74e8afc 2534X<getppid> X<parent> X<pid>
a0d0e21e 2535
c17cdb72
NC
2536=for Pod::Functions get parent process ID
2537
a0d0e21e
LW
2538Returns the process id of the parent process.
2539
d7c042c9
AB
2540Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2541around non-POSIX thread semantics the minority of Linux systems (and
2542Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2543has since been removed. See the documentation for L<$$|perlvar/$$> for
2544details.
4d76a344 2545
ea9eb35a 2546Portability issues: L<perlport/getppid>.
2547
a0d0e21e 2548=item getpriority WHICH,WHO
d74e8afc 2549X<getpriority> X<priority> X<nice>
a0d0e21e 2550
c17cdb72
NC
2551=for Pod::Functions get current nice value
2552
4633a7c4 2553Returns the current priority for a process, a process group, or a user.
01aa884e 2554(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2555machine that doesn't implement getpriority(2).
a0d0e21e 2556
ea9eb35a 2557Portability issues: L<perlport/getpriority>.
2558
a0d0e21e 2559=item getpwnam NAME
d74e8afc
ITB
2560X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2561X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2562X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2563X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2564X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2565X<endnetent> X<endprotoent> X<endservent>
a0d0e21e 2566
c17cdb72
NC
2567=for Pod::Functions get passwd record given user login name
2568
a0d0e21e
LW
2569=item getgrnam NAME
2570
c17cdb72
NC
2571=for Pod::Functions get group record given group name
2572
a0d0e21e
LW
2573=item gethostbyname NAME
2574
c17cdb72
NC
2575=for Pod::Functions get host record given name
2576
a0d0e21e
LW
2577=item getnetbyname NAME
2578
c17cdb72
NC
2579=for Pod::Functions get networks record given name
2580
a0d0e21e
LW
2581=item getprotobyname NAME
2582
c17cdb72
NC
2583=for Pod::Functions get protocol record given name
2584
a0d0e21e
LW
2585=item getpwuid UID
2586
c17cdb72
NC
2587=for Pod::Functions get passwd record given user ID
2588
a0d0e21e
LW
2589=item getgrgid GID
2590
c17cdb72
NC
2591=for Pod::Functions get group record given group user ID
2592
a0d0e21e
LW
2593=item getservbyname NAME,PROTO
2594
c17cdb72
NC
2595=for Pod::Functions get services record given its name
2596
a0d0e21e
LW
2597=item gethostbyaddr ADDR,ADDRTYPE
2598
c17cdb72
NC
2599=for Pod::Functions get host record given its address
2600
a0d0e21e
LW
2601=item getnetbyaddr ADDR,ADDRTYPE
2602
c17cdb72
NC
2603=for Pod::Functions get network record given its address
2604
a0d0e21e
LW
2605=item getprotobynumber NUMBER
2606
c17cdb72
NC
2607=for Pod::Functions get protocol record numeric protocol
2608
a0d0e21e
LW
2609=item getservbyport PORT,PROTO
2610
c17cdb72
NC
2611=for Pod::Functions get services record given numeric port
2612
a0d0e21e
LW
2613=item getpwent
2614
c17cdb72
NC
2615=for Pod::Functions get next passwd record
2616
a0d0e21e
LW
2617=item getgrent
2618
c17cdb72
NC
2619=for Pod::Functions get next group record
2620
a0d0e21e
LW
2621=item gethostent
2622
c17cdb72
NC
2623=for Pod::Functions get next hosts record
2624
a0d0e21e
LW
2625=item getnetent
2626
c17cdb72
NC
2627=for Pod::Functions get next networks record
2628
a0d0e21e
LW
2629=item getprotoent
2630
c17cdb72
NC
2631=for Pod::Functions get next protocols record
2632
a0d0e21e
LW
2633=item getservent
2634
c17cdb72
NC
2635=for Pod::Functions get next services record
2636
a0d0e21e
LW
2637=item setpwent
2638
c17cdb72
NC
2639=for Pod::Functions prepare passwd file for use
2640
a0d0e21e
LW
2641=item setgrent
2642
c17cdb72
NC
2643=for Pod::Functions prepare group file for use
2644
a0d0e21e
LW
2645=item sethostent STAYOPEN
2646
c17cdb72
NC
2647=for Pod::Functions prepare hosts file for use
2648
a0d0e21e
LW
2649=item setnetent STAYOPEN
2650
c17cdb72
NC
2651=for Pod::Functions prepare networks file for use
2652
a0d0e21e
LW
2653=item setprotoent STAYOPEN
2654
c17cdb72
NC
2655=for Pod::Functions prepare protocols file for use
2656
a0d0e21e
LW
2657=item setservent STAYOPEN
2658
c17cdb72
NC
2659=for Pod::Functions prepare services file for use
2660
a0d0e21e
LW
2661=item endpwent
2662
c17cdb72
NC
2663=for Pod::Functions be done using passwd file
2664
a0d0e21e
LW
2665=item endgrent
2666
c17cdb72
NC
2667=for Pod::Functions be done using group file
2668
a0d0e21e
LW
2669=item endhostent
2670
c17cdb72
NC
2671=for Pod::Functions be done using hosts file
2672
a0d0e21e
LW
2673=item endnetent
2674
c17cdb72
NC
2675=for Pod::Functions be done using networks file
2676
a0d0e21e
LW
2677=item endprotoent
2678
c17cdb72
NC
2679=for Pod::Functions be done using protocols file
2680
a0d0e21e
LW
2681=item endservent
2682
c17cdb72
NC
2683=for Pod::Functions be done using services file
2684
80d38338
TC
2685These routines are the same as their counterparts in the
2686system C library. In list context, the return values from the
a0d0e21e
LW
2687various get routines are as follows:
2688
2689 ($name,$passwd,$uid,$gid,
6ee623d5 2690 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2691 ($name,$passwd,$gid,$members) = getgr*
2692 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2693 ($name,$aliases,$addrtype,$net) = getnet*
2694 ($name,$aliases,$proto) = getproto*
2695 ($name,$aliases,$port,$proto) = getserv*
2696
3b10bc60 2697(If the entry doesn't exist you get an empty list.)
a0d0e21e 2698
4602f195
JH
2699The exact meaning of the $gcos field varies but it usually contains
2700the real name of the user (as opposed to the login name) and other
2701information pertaining to the user. Beware, however, that in many
2702system users are able to change this information and therefore it
106325ad 2703cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2704L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2705login shell, are also tainted, for the same reason.
4602f195 2706
5a964f20 2707In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2708lookup by name, in which case you get the other thing, whatever it is.
2709(If the entry doesn't exist you get the undefined value.) For example:
2710
5a964f20
TC
2711 $uid = getpwnam($name);
2712 $name = getpwuid($num);
2713 $name = getpwent();
2714 $gid = getgrnam($name);
08a33e13 2715 $name = getgrgid($num);
5a964f20
TC
2716 $name = getgrent();
2717 #etc.
a0d0e21e 2718
4602f195 2719In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2720in that they are unsupported on many systems. If the
4602f195
JH
2721$quota is unsupported, it is an empty scalar. If it is supported, it
2722usually encodes the disk quota. If the $comment field is unsupported,
2723it is an empty scalar. If it is supported it usually encodes some
2724administrative comment about the user. In some systems the $quota
2725field may be $change or $age, fields that have to do with password
2726aging. In some systems the $comment field may be $class. The $expire
2727field, if present, encodes the expiration period of the account or the
2728password. For the availability and the exact meaning of these fields
8f1da26d 2729in your system, please consult getpwnam(3) and your system's
4602f195
JH
2730F<pwd.h> file. You can also find out from within Perl what your
2731$quota and $comment fields mean and whether you have the $expire field
2732by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2733C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2734files are supported only if your vendor has implemented them in the
4602f195 2735intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2736shadow versions if you're running under privilege or if there exists
cf264981 2737the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2738and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2739facility are unlikely to be supported.
6ee623d5 2740
a3390c9f 2741The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2742the login names of the members of the group.
2743
2744For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2745C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2746C<@addrs> value returned by a successful call is a list of raw
2747addresses returned by the corresponding library call. In the
2748Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2749by saying something like:
2750
f337b084 2751 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2752
2b5ab1e7
TC
2753The Socket library makes this slightly easier:
2754
2755 use Socket;
2756 $iaddr = inet_aton("127.1"); # or whatever address
2757 $name = gethostbyaddr($iaddr, AF_INET);
2758
2759 # or going the other way
19799a22 2760 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2761
d760c846
GS
2762In the opposite way, to resolve a hostname to the IP address
2763you can write this:
2764
2765 use Socket;
2766 $packed_ip = gethostbyname("www.perl.org");
2767 if (defined $packed_ip) {
2768 $ip_address = inet_ntoa($packed_ip);
2769 }
2770
b018eaf1 2771Make sure C<gethostbyname()> is called in SCALAR context and that
d760c846
GS
2772its return value is checked for definedness.
2773
0d043efa
FC
2774The C<getprotobynumber> function, even though it only takes one argument,
2775has the precedence of a list operator, so beware:
2776
2777 getprotobynumber $number eq 'icmp' # WRONG
2778 getprotobynumber($number eq 'icmp') # actually means this
2779 getprotobynumber($number) eq 'icmp' # better this way
2780
19799a22
GS
2781If you get tired of remembering which element of the return list
2782contains which return value, by-name interfaces are provided
2783in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2784C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2785and C<User::grent>. These override the normal built-ins, supplying
2786versions that return objects with the appropriate names
2787for each field. For example:
5a964f20
TC
2788
2789 use File::stat;
2790 use User::pwent;
2791 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2792
a3390c9f 2793Even though it looks as though they're the same method calls (uid),
b76cc8ba 2794they aren't, because a C<File::stat> object is different from
19799a22 2795a C<User::pwent> object.
5a964f20 2796
ea9eb35a 2797Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2798
a0d0e21e 2799=item getsockname SOCKET
d74e8afc 2800X<getsockname>
a0d0e21e 2801
c17cdb72
NC
2802=for Pod::Functions retrieve the sockaddr for a given socket
2803
19799a22
GS
2804Returns the packed sockaddr address of this end of the SOCKET connection,
2805in case you don't know the address because you have several different
2806IPs that the connection might have come in on.
a0d0e21e 2807
4633a7c4
LW
2808 use Socket;
2809 $mysockaddr = getsockname(SOCK);
19799a22 2810 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2811 printf "Connect to %s [%s]\n",
19799a22
GS
2812 scalar gethostbyaddr($myaddr, AF_INET),
2813 inet_ntoa($myaddr);
a0d0e21e
LW
2814
2815=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2816X<getsockopt>
a0d0e21e 2817
c17cdb72
NC
2818=for Pod::Functions get socket options on a given socket
2819
636e6b1f
TH
2820Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2821Options may exist at multiple protocol levels depending on the socket
2822type, but at least the uppermost socket level SOL_SOCKET (defined in the
391b733c 2823C<Socket> module) will exist. To query options at another level the
636e6b1f 2824protocol number of the appropriate protocol controlling the option
391b733c 2825should be supplied. For example, to indicate that an option is to be
636e6b1f 2826interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2827number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2828
80d38338 2829The function returns a packed string representing the requested socket
3b10bc60 2830option, or C<undef> on error, with the reason for the error placed in
391b733c 2831C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2832consult getsockopt(2) for details. A common case is that the option is an
2833integer, in which case the result is a packed integer, which you can decode
2834using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2835
8f1da26d 2836Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2837
4852725b 2838 use Socket qw(:all);
636e6b1f
TH
2839
2840 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2841 or die "Could not determine the protocol number for tcp";
4852725b
DD
2842 # my $tcp = IPPROTO_TCP; # Alternative
2843 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2844 or die "getsockopt TCP_NODELAY: $!";
636e6b1f 2845 my $nodelay = unpack("I", $packed);
f7051f2c
FC
2846 print "Nagle's algorithm is turned ",
2847 $nodelay ? "off\n" : "on\n";
636e6b1f 2848
ea9eb35a 2849Portability issues: L<perlport/getsockopt>.
a0d0e21e
LW
2850
2851=item glob EXPR
d74e8afc 2852X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2853
0a753a76
PP
2854=item glob
2855
c17cdb72
NC
2856=for Pod::Functions expand filenames using wildcards
2857
d9a9d457 2858In list context, returns a (possibly empty) list of filename expansions on
391b733c 2859the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
d9a9d457 2860scalar context, glob iterates through such filename expansions, returning
391b733c
FC
2861undef when the list is exhausted. This is the internal function
2862implementing the C<< <*.c> >> operator, but you can use it directly. If
d9a9d457
JL
2863EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2864more detail in L<perlop/"I/O Operators">.
a0d0e21e 2865
80d38338
TC
2866Note that C<glob> splits its arguments on whitespace and treats
2867each segment as separate pattern. As such, C<glob("*.c *.h")>
2868matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2869C<glob(".* *")> matches all files in the current working directory.
a91bb7b1
TC
2870If you want to glob filenames that might contain whitespace, you'll
2871have to use extra quotes around the spacey filename to protect it.
2872For example, to glob filenames that have an C<e> followed by a space
2873followed by an C<f>, use either of:
2874
2875 @spacies = <"*e f*">;
2876 @spacies = glob '"*e f*"';
2877 @spacies = glob q("*e f*");
2878
2879If you had to get a variable through, you could do this:
2880
2881 @spacies = glob "'*${var}e f*'";
2882 @spacies = glob qq("*${var}e f*");
80d38338
TC
2883
2884If non-empty braces are the only wildcard characters used in the
2885C<glob>, no filenames are matched, but potentially many strings
2886are returned. For example, this produces nine strings, one for
2887each pairing of fruits and colors:
2888
2889 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2890
e9fa405d 2891This operator is implemented using the standard
5c0c9249
PF
2892C<File::Glob> extension. See L<File::Glob> for details, including
2893C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2894
ea9eb35a 2895Portability issues: L<perlport/glob>.
2896
a0d0e21e 2897=item gmtime EXPR
d74e8afc 2898X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2899
ce2984c3
PF
2900=item gmtime
2901
c17cdb72
NC
2902=for Pod::Functions convert UNIX time into record or string using Greenwich time
2903
4509d391 2904Works just like L</localtime> but the returned values are
435fbc73 2905localized for the standard Greenwich time zone.
a0d0e21e 2906
a3390c9f
FC
2907Note: When called in list context, $isdst, the last value
2908returned by gmtime, is always C<0>. There is no
435fbc73 2909Daylight Saving Time in GMT.
0a753a76 2910
ea9eb35a 2911Portability issues: L<perlport/gmtime>.
62aa5637 2912
a0d0e21e 2913=item goto LABEL
d74e8afc 2914X<goto> X<jump> X<jmp>
a0d0e21e 2915
748a9306
LW
2916=item goto EXPR
2917
a0d0e21e
LW
2918=item goto &NAME
2919
c17cdb72
NC
2920=for Pod::Functions create spaghetti code
2921
5a5b79a3 2922The C<goto LABEL> form finds the statement labeled with LABEL and
391b733c 2923resumes execution there. It can't be used to get out of a block or
b500e03b
GG
2924subroutine given to C<sort>. It can be used to go almost anywhere
2925else within the dynamic scope, including out of subroutines, but it's
2926usually better to use some other construct such as C<last> or C<die>.
2927The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2928(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2929does not offer named loops combined with loop control. Perl does, and
2930this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2931
5a5b79a3 2932The C<goto EXPR> form expects to evaluate C<EXPR> to a code reference or
3e8a6370 2933a label name. If it evaluates to a code reference, it will be handled
5a5b79a3 2934like C<goto &NAME>, below. This is especially useful for implementing
3e8a6370
RS
2935tail recursion via C<goto __SUB__>.
2936
2937If the expression evaluates to a label name, its scope will be resolved
7660c0ab 2938dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2939necessarily recommended if you're optimizing for maintainability:
2940
2941 goto ("FOO", "BAR", "GLARCH")[$i];
2942
5a5b79a3 2943As shown in this example, C<goto EXPR> is exempt from the "looks like a
391b733c
FC
2944function" rule. A pair of parentheses following it does not (necessarily)
2945delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
8a7e748e
FC
2946Also, unlike most named operators, this has the same precedence as
2947assignment.
887d89fd 2948
5a5b79a3 2949Use of C<goto LABEL> or C<goto EXPR> to jump into a construct is
0b98bec9 2950deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2951go into any construct that requires initialization, such as a
2952subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2953construct that is optimized away.
b500e03b 2954
5a5b79a3 2955The C<goto &NAME> form is quite different from the other forms of
1b6921cb
BT
2956C<goto>. In fact, it isn't a goto in the normal sense at all, and
2957doesn't have the stigma associated with other gotos. Instead, it
2958exits the current subroutine (losing any changes set by local()) and
2959immediately calls in its place the named subroutine using the current
2960value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2961load another subroutine and then pretend that the other subroutine had
2962been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2963in the current subroutine are propagated to the other subroutine.)
2964After the C<goto>, not even C<caller> will be able to tell that this
2965routine was called first.
2966
2967NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2968containing a code reference or a block that evaluates to a code
6cb9131c 2969reference.
a0d0e21e
LW
2970
2971=item grep BLOCK LIST
d74e8afc 2972X<grep>
a0d0e21e
LW
2973
2974=item grep EXPR,LIST
2975
c17cdb72
NC
2976=for Pod::Functions locate elements in a list test true against a given criterion
2977
2b5ab1e7
TC
2978This is similar in spirit to, but not the same as, grep(1) and its
2979relatives. In particular, it is not limited to using regular expressions.
2f9daede 2980
a0d0e21e 2981Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2982C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2983elements for which the expression evaluated to true. In scalar
2984context, returns the number of times the expression was true.
a0d0e21e
LW
2985
2986 @foo = grep(!/^#/, @bar); # weed out comments
2987
2988or equivalently,
2989
2990 @foo = grep {!/^#/} @bar; # weed out comments
2991
be3174d2
GS
2992Note that C<$_> is an alias to the list value, so it can be used to
2993modify the elements of the LIST. While this is useful and supported,
2994it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2995Similarly, grep returns aliases into the original list, much as a for
2996loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2997element of a list returned by grep (for example, in a C<foreach>, C<map>
2998or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2999This is usually something to be avoided when writing clear code.
a0d0e21e 3000
a4fb8298 3001If C<$_> is lexical in the scope where the C<grep> appears (because it has
c071e214
FC
3002been declared with the deprecated C<my $_> construct)
3003then, in addition to being locally aliased to
80d38338 3004the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
3005can't be seen from the outside, avoiding any potential side-effects.
3006
19799a22 3007See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 3008
a0d0e21e 3009=item hex EXPR
d74e8afc 3010X<hex> X<hexadecimal>
a0d0e21e 3011
54310121 3012=item hex
bbce6d69 3013
c17cdb72
NC
3014=for Pod::Functions convert a string to a hexadecimal number
3015
2b5ab1e7 3016Interprets EXPR as a hex string and returns the corresponding value.
38366c11 3017(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 3018L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
3019
3020 print hex '0xAf'; # prints '175'
3021 print hex 'aF'; # same
a0d0e21e 3022
19799a22 3023Hex strings may only represent integers. Strings that would cause
53305cf1 3024integer overflow trigger a warning. Leading whitespace is not stripped,
391b733c 3025unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 3026L</sprintf>, and L</unpack>.
19799a22 3027
ce2984c3 3028=item import LIST
d74e8afc 3029X<import>
a0d0e21e 3030
c17cdb72
NC
3031=for Pod::Functions patch a module's namespace into your own
3032
19799a22 3033There is no builtin C<import> function. It is just an ordinary
4633a7c4 3034method (subroutine) defined (or inherited) by modules that wish to export
19799a22 3035names to another module. The C<use> function calls the C<import> method
cea6626f 3036for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
3037
3038=item index STR,SUBSTR,POSITION
d74e8afc 3039X<index> X<indexOf> X<InStr>
a0d0e21e
LW
3040
3041=item index STR,SUBSTR
3042
c17cdb72
NC
3043=for Pod::Functions find a substring within a string
3044
2b5ab1e7
TC
3045The index function searches for one string within another, but without
3046the wildcard-like behavior of a full regular-expression pattern match.
3047It returns the position of the first occurrence of SUBSTR in STR at
3048or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
3049beginning of the string. POSITION before the beginning of the string
3050or after its end is treated as if it were the beginning or the end,
e1dccc0d
Z
3051respectively. POSITION and the return value are based at zero.
3052If the substring is not found, C<index> returns -1.
a0d0e21e
LW
3053
3054=item int EXPR
f723aae1 3055X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 3056
54310121