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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
fbb0b3b3
RGS
483As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
484test operators, in a way that C<-f -w -x $file> is equivalent to
391b733c 485C<-x $file && -w _ && -f _>. (This is only fancy fancy: 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
8f1da26d 724This keyword is enabled by the C<"switch"> feature: see
4a904372
FC
725L<feature> for more information. You can also access it by
726prefixing it with C<CORE::>. Alternately, include a C<use
8f1da26d 727v5.10> or later to the current 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
766compiled with. The C<$hints> and C<$bitmask> values are subject to change
767between versions of Perl, and are not meant for external use.
748a9306 768
b3ca2e83 769C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
391b733c 770caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
b3ca2e83
NC
771of this hash, as they are the actual values stored in the optree.
772
ffe0c19d
FC
773Furthermore, when called from within the DB package in
774list context, and with an argument, caller returns more
7660c0ab 775detailed information: it sets the list variable C<@DB::args> to be the
54310121 776arguments with which the subroutine was invoked.
748a9306 777
7660c0ab 778Be aware that the optimizer might have optimized call frames away before
19799a22 779C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 780might not return information about the call frame you expect it to, for
b76cc8ba 781C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 782previous time C<caller> was called.
7660c0ab 783
8f1da26d 784Be aware that setting C<@DB::args> is I<best effort>, intended for
391b733c 785debugging or generating backtraces, and should not be relied upon. In
ca9f0cb5
NC
786particular, as C<@_> contains aliases to the caller's arguments, Perl does
787not take a copy of C<@_>, so C<@DB::args> will contain modifications the
788subroutine makes to C<@_> or its contents, not the original values at call
391b733c 789time. C<@DB::args>, like C<@_>, does not hold explicit references to its
ca9f0cb5 790elements, so under certain cases its elements may have become freed and
391b733c 791reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 792of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
793I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
794reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5 795elements), so C<@DB::args> is actually a hybrid of the current state and
391b733c 796initial state of C<@_>. Buyer beware.
ca9f0cb5 797
a0d0e21e 798=item chdir EXPR
d74e8afc
ITB
799X<chdir>
800X<cd>
f723aae1 801X<directory, change>
a0d0e21e 802
c4aca7d0
GA
803=item chdir FILEHANDLE
804
805=item chdir DIRHANDLE
806
ce2984c3
PF
807=item chdir
808
c17cdb72
NC
809=for Pod::Functions change your current working directory
810
391b733c 811Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 812changes to the directory specified by C<$ENV{HOME}>, if set; if not,
391b733c
FC
813changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
814variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
815neither is set, C<chdir> does nothing. It returns true on success,
816false otherwise. See the example under C<die>.
a0d0e21e 817
3b10bc60 818On systems that support fchdir(2), you may pass a filehandle or
34169887 819directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 820passing handles raises an exception.
c4aca7d0 821
a0d0e21e 822=item chmod LIST
d74e8afc 823X<chmod> X<permission> X<mode>
a0d0e21e 824
c17cdb72
NC
825=for Pod::Functions changes the permissions on a list of files
826
a0d0e21e 827Changes the permissions of a list of files. The first element of the
8f1da26d 828list must be the numeric mode, which should probably be an octal
4ad40acf 829number, and which definitely should I<not> be a string of octal digits:
3b10bc60 830C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 831successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 832
3b10bc60 833 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 834 chmod 0755, @executables;
3b10bc60 835 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 836 # --w----r-T
3b10bc60 837 $mode = "0644"; chmod oct($mode), "foo"; # this is better
838 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 839
3b10bc60 840On systems that support fchmod(2), you may pass filehandles among the
841files. On systems that don't support fchmod(2), passing filehandles raises
842an exception. Filehandles must be passed as globs or glob references to be
843recognized; barewords are considered filenames.
c4aca7d0
GA
844
845 open(my $fh, "<", "foo");
846 my $perm = (stat $fh)[2] & 07777;
847 chmod($perm | 0600, $fh);
848
3b10bc60 849You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
850module:
851
3b10bc60 852 use Fcntl qw( :mode );
ca6e1c26 853 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 854 # Identical to the chmod 0755 of the example above.
ca6e1c26 855
ea9eb35a 856Portability issues: L<perlport/chmod>.
857
a0d0e21e 858=item chomp VARIABLE
d74e8afc 859X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 860
313c9f5c 861=item chomp( LIST )
a0d0e21e
LW
862
863=item chomp
864
c17cdb72
NC
865=for Pod::Functions remove a trailing record separator from a string
866
2b5ab1e7
TC
867This safer version of L</chop> removes any trailing string
868that corresponds to the current value of C<$/> (also known as
28757baa
PP
869$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
870number of characters removed from all its arguments. It's often used to
871remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
872that the final record may be missing its newline. When in paragraph
873mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 874When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 875a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 876remove anything.
19799a22 877If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
878
879 while (<>) {
a9a5a0dc
VP
880 chomp; # avoid \n on last field
881 @array = split(/:/);
882 # ...
a0d0e21e
LW
883 }
884
4bf21a6d
RD
885If VARIABLE is a hash, it chomps the hash's values, but not its keys.
886
a0d0e21e
LW
887You can actually chomp anything that's an lvalue, including an assignment:
888
889 chomp($cwd = `pwd`);
890 chomp($answer = <STDIN>);
891
892If you chomp a list, each element is chomped, and the total number of
893characters removed is returned.
894
15e44fd8
RGS
895Note that parentheses are necessary when you're chomping anything
896that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
897is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
898C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
899C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
900as C<chomp($a, $b)>.
901
a0d0e21e 902=item chop VARIABLE
d74e8afc 903X<chop>
a0d0e21e 904
313c9f5c 905=item chop( LIST )
a0d0e21e
LW
906
907=item chop
908
c17cdb72
NC
909=for Pod::Functions remove the last character from a string
910
a0d0e21e 911Chops off the last character of a string and returns the character
5b3eff12 912chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 913scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
914If VARIABLE is a hash, it chops the hash's values, but not its keys.
915
5b3eff12 916You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
917
918If you chop a list, each element is chopped. Only the value of the
19799a22 919last C<chop> is returned.
a0d0e21e 920
19799a22 921Note that C<chop> returns the last character. To return all but the last
748a9306
LW
922character, use C<substr($string, 0, -1)>.
923
15e44fd8
RGS
924See also L</chomp>.
925
a0d0e21e 926=item chown LIST
d74e8afc 927X<chown> X<owner> X<user> X<group>
a0d0e21e 928
c17cdb72
NC
929=for Pod::Functions change the ownership on a list of files
930
a0d0e21e 931Changes the owner (and group) of a list of files. The first two
19799a22
GS
932elements of the list must be the I<numeric> uid and gid, in that
933order. A value of -1 in either position is interpreted by most
934systems to leave that value unchanged. Returns the number of files
935successfully changed.
a0d0e21e
LW
936
937 $cnt = chown $uid, $gid, 'foo', 'bar';
938 chown $uid, $gid, @filenames;
939
3b10bc60 940On systems that support fchown(2), you may pass filehandles among the
941files. On systems that don't support fchown(2), passing filehandles raises
942an exception. Filehandles must be passed as globs or glob references to be
943recognized; barewords are considered filenames.
c4aca7d0 944
54310121 945Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
946
947 print "User: ";
19799a22 948 chomp($user = <STDIN>);
5a964f20 949 print "Files: ";
19799a22 950 chomp($pattern = <STDIN>);
a0d0e21e
LW
951
952 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 953 or die "$user not in passwd file";
a0d0e21e 954
5ed4f2ec 955 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
956 chown $uid, $gid, @ary;
957
54310121 958On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
959file unless you're the superuser, although you should be able to change
960the group to any of your secondary groups. On insecure systems, these
961restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
962On POSIX systems, you can detect this condition this way:
963
964 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
965 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 966
ea9eb35a 967Portability issues: L<perlport/chmod>.
968
a0d0e21e 969=item chr NUMBER
d74e8afc 970X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 971
54310121 972=item chr
bbce6d69 973
c17cdb72
NC
974=for Pod::Functions get character this number represents
975
a0d0e21e 976Returns the character represented by that NUMBER in the character set.
a0ed51b3 977For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 978chr(0x263a) is a Unicode smiley face.
aaa68c4a 979
8a064bd6 980Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 981except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
982(truncated to an integer) are used.
983
974da8e5
JH
984If NUMBER is omitted, uses C<$_>.
985
b76cc8ba 986For the reverse, use L</ord>.
a0d0e21e 987
2575c402
JW
988Note that characters from 128 to 255 (inclusive) are by default
989internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 990
2575c402 991See L<perlunicode> for more about Unicode.
bbce6d69 992
a0d0e21e 993=item chroot FILENAME
d74e8afc 994X<chroot> X<root>
a0d0e21e 995
54310121 996=item chroot
bbce6d69 997
c17cdb72
NC
998=for Pod::Functions make directory new root for path lookups
999
5a964f20 1000This function works like the system call by the same name: it makes the
4633a7c4 1001named directory the new root directory for all further pathnames that
951ba7fe 1002begin with a C</> by your process and all its children. (It doesn't
28757baa 1003change your current working directory, which is unaffected.) For security
4633a7c4 1004reasons, this call is restricted to the superuser. If FILENAME is
19799a22 1005omitted, does a C<chroot> to C<$_>.
a0d0e21e 1006
ea9eb35a 1007Portability issues: L<perlport/chroot>.
1008
a0d0e21e 1009=item close FILEHANDLE
d74e8afc 1010X<close>
a0d0e21e 1011
6a518fbc
TP
1012=item close
1013
c17cdb72
NC
1014=for Pod::Functions close file (or pipe or socket) handle
1015
3b10bc60 1016Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 1017buffers, and closes the system file descriptor. Returns true if those
8f1da26d 1018operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
1019layer. Closes the currently selected filehandle if the argument is
1020omitted.
fb73857a
PP
1021
1022You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 1023another C<open> on it, because C<open> closes it for you. (See
01aa884e 1024L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
19799a22 1025counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 1026
3b10bc60 1027If the filehandle came from a piped open, C<close> returns false if one of
1028the other syscalls involved fails or if its program exits with non-zero
1029status. If the only problem was that the program exited non-zero, C<$!>
1030will be set to C<0>. Closing a pipe also waits for the process executing
1031on the pipe to exit--in case you wish to look at the output of the pipe
1032afterwards--and implicitly puts the exit status value of that command into
1033C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 1034
2e0cfa16
FC
1035If there are multiple threads running, C<close> on a filehandle from a
1036piped open returns true without waiting for the child process to terminate,
1037if the filehandle is still open in another thread.
1038
80d38338
TC
1039Closing the read end of a pipe before the process writing to it at the
1040other end is done writing results in the writer receiving a SIGPIPE. If
1041the other end can't handle that, be sure to read all the data before
1042closing the pipe.
73689b13 1043
fb73857a 1044Example:
a0d0e21e 1045
fb73857a
PP
1046 open(OUTPUT, '|sort >foo') # pipe to sort
1047 or die "Can't start sort: $!";
5ed4f2ec 1048 #... # print stuff to output
1049 close OUTPUT # wait for sort to finish
fb73857a
PP
1050 or warn $! ? "Error closing sort pipe: $!"
1051 : "Exit status $? from sort";
5ed4f2ec 1052 open(INPUT, 'foo') # get sort's results
fb73857a 1053 or die "Can't open 'foo' for input: $!";
a0d0e21e 1054
5a964f20 1055FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 1056filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
1057
1058=item closedir DIRHANDLE
d74e8afc 1059X<closedir>
a0d0e21e 1060
c17cdb72
NC
1061=for Pod::Functions close directory handle
1062
19799a22 1063Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
1064system call.
1065
a0d0e21e 1066=item connect SOCKET,NAME
d74e8afc 1067X<connect>
a0d0e21e 1068
c17cdb72
NC
1069=for Pod::Functions connect to a remote socket
1070
80d38338
TC
1071Attempts to connect to a remote socket, just like connect(2).
1072Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
1073packed address of the appropriate type for the socket. See the examples in
1074L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 1075
cb1a09d0 1076=item continue BLOCK
d74e8afc 1077X<continue>
cb1a09d0 1078
0d863452
RH
1079=item continue
1080
c17cdb72
NC
1081=for Pod::Functions optional trailing block in a while or foreach
1082
4a904372
FC
1083When followed by a BLOCK, C<continue> is actually a
1084flow control statement rather than a function. If
cf264981 1085there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
1086C<foreach>), it is always executed just before the conditional is about to
1087be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
1088it can be used to increment a loop variable, even when the loop has been
1089continued via the C<next> statement (which is similar to the C C<continue>
1090statement).
1091
98293880 1092C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 1093block; C<last> and C<redo> behave as if they had been executed within
19799a22 1094the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
1095block, it may be more entertaining.
1096
1097 while (EXPR) {
a9a5a0dc
VP
1098 ### redo always comes here
1099 do_something;
1d2dff63 1100 } continue {
a9a5a0dc
VP
1101 ### next always comes here
1102 do_something_else;
1103 # then back the top to re-check EXPR
1d2dff63
GS
1104 }
1105 ### last always comes here
1106
3b10bc60 1107Omitting the C<continue> section is equivalent to using an
1108empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1109to check the condition at the top of the loop.
1110
4a904372 1111When there is no BLOCK, C<continue> is a function that
8f1da26d
TC
1112falls through the current C<when> or C<default> block instead of iterating
1113a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
4a904372
FC
1114In Perl 5.14 and earlier, this form of C<continue> was
1115only available when the C<"switch"> feature was enabled.
48238296 1116See L<feature> and L<perlsyn/"Switch Statements"> for more
8f1da26d 1117information.
0d863452 1118
a0d0e21e 1119=item cos EXPR
d74e8afc 1120X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1121
d6217f1e
GS
1122=item cos
1123
c17cdb72
NC
1124=for Pod::Functions cosine function
1125
5a964f20 1126Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1127takes the cosine of C<$_>.
a0d0e21e 1128
ca6e1c26 1129For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1130function, or use this relation:
1131
1132 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1133
a0d0e21e 1134=item crypt PLAINTEXT,SALT
d74e8afc 1135X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1136X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1137
c17cdb72
NC
1138=for Pod::Functions one-way passwd-style encryption
1139
ef2e6798
MS
1140Creates a digest string exactly like the crypt(3) function in the C
1141library (assuming that you actually have a version there that has not
bb23f8d1 1142been extirpated as a potential munition).
ef2e6798 1143
34169887 1144crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1145into a short string, called a digest, which is returned. The same
1146PLAINTEXT and SALT will always return the same string, but there is no
1147(known) way to get the original PLAINTEXT from the hash. Small
1148changes in the PLAINTEXT or SALT will result in large changes in the
1149digest.
1150
1151There is no decrypt function. This function isn't all that useful for
1152cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1153mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1154primarily used to check if two pieces of text are the same without
1155having to transmit or store the text itself. An example is checking
1156if a correct password is given. The digest of the password is stored,
cf264981 1157not the password itself. The user types in a password that is
ef2e6798 1158crypt()'d with the same salt as the stored digest. If the two digests
34169887 1159match, the password is correct.
ef2e6798
MS
1160
1161When verifying an existing digest string you should use the digest as
1162the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1163to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1164crypt() will hash the new string with the same salt as the digest.
1165This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1166with more exotic implementations. In other words, assume
1167nothing about the returned string itself nor about how many bytes
1168of SALT may matter.
85c16d83
JH
1169
1170Traditionally the result is a string of 13 bytes: two first bytes of
1171the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
391b733c 1172the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1173hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1174and implementations on non-Unix platforms may produce different
ef2e6798 1175strings.
85c16d83
JH
1176
1177When choosing a new salt create a random two character string whose
1178characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1179'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1180characters is just a recommendation; the characters allowed in
1181the salt depend solely on your system's crypt library, and Perl can't
1182restrict what salts C<crypt()> accepts.
e71965be 1183
a0d0e21e 1184Here's an example that makes sure that whoever runs this program knows
cf264981 1185their password:
a0d0e21e
LW
1186
1187 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1188
1189 system "stty -echo";
1190 print "Password: ";
e71965be 1191 chomp($word = <STDIN>);
a0d0e21e
LW
1192 print "\n";
1193 system "stty echo";
1194
e71965be 1195 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1196 die "Sorry...\n";
a0d0e21e 1197 } else {
a9a5a0dc 1198 print "ok\n";
54310121 1199 }
a0d0e21e 1200
9f8f0c9d 1201Of course, typing in your own password to whoever asks you
748a9306 1202for it is unwise.
a0d0e21e 1203
ef2e6798 1204The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1205of data, not least of all because you can't get the information
ef2e6798 1206back. Look at the L<Digest> module for more robust algorithms.
19799a22 1207
f2791508
JH
1208If using crypt() on a Unicode string (which I<potentially> has
1209characters with codepoints above 255), Perl tries to make sense
34169887 1210of the situation by trying to downgrade (a copy of)
f2791508
JH
1211the string back to an eight-bit byte string before calling crypt()
1212(on that copy). If that works, good. If not, crypt() dies with
1213C<Wide character in crypt>.
85c16d83 1214
ea9eb35a 1215Portability issues: L<perlport/crypt>.
1216
aa689395 1217=item dbmclose HASH
d74e8afc 1218X<dbmclose>
a0d0e21e 1219
c17cdb72
NC
1220=for Pod::Functions breaks binding on a tied dbm file
1221
19799a22 1222[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1223
aa689395 1224Breaks the binding between a DBM file and a hash.
a0d0e21e 1225
ea9eb35a 1226Portability issues: L<perlport/dbmclose>.
1227
19799a22 1228=item dbmopen HASH,DBNAME,MASK
d74e8afc 1229X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1230
c17cdb72
NC
1231=for Pod::Functions create binding on a tied dbm file
1232
01aa884e
KW
1233[This function has been largely superseded by the
1234L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
a0d0e21e 1235
7b8d334a 1236This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1237hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1238argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1239is the name of the database (without the F<.dir> or F<.pag> extension if
1240any). If the database does not exist, it is created with protection
1b3a6178
FC
1241specified by MASK (as modified by the C<umask>). To prevent creation of
1242the database if it doesn't exist, you may specify a MODE
1243of 0, and the function will return a false value if it
1244can't find an existing database. If your system supports
80d38338 1245only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1246program. In older versions of Perl, if your system had neither DBM nor
19799a22 1247ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1248sdbm(3).
1249
1250If you don't have write access to the DBM file, you can only read hash
1251variables, not set them. If you want to test whether you can write,
3b10bc60 1252either use file tests or try setting a dummy hash entry inside an C<eval>
1253to trap the error.
a0d0e21e 1254
19799a22
GS
1255Note that functions such as C<keys> and C<values> may return huge lists
1256when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1257function to iterate over large DBM files. Example:
1258
1259 # print out history file offsets
1260 dbmopen(%HIST,'/usr/lib/news/history',0666);
1261 while (($key,$val) = each %HIST) {
a9a5a0dc 1262 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1263 }
1264 dbmclose(%HIST);
1265
cb1a09d0 1266See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1267cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1268rich implementation.
4633a7c4 1269
2b5ab1e7
TC
1270You can control which DBM library you use by loading that library
1271before you call dbmopen():
1272
1273 use DB_File;
1274 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1275 or die "Can't open netscape history file: $!";
2b5ab1e7 1276
ea9eb35a 1277Portability issues: L<perlport/dbmopen>.
1278
a0d0e21e 1279=item defined EXPR
d74e8afc 1280X<defined> X<undef> X<undefined>
a0d0e21e 1281
54310121 1282=item defined
bbce6d69 1283
c17cdb72
NC
1284=for Pod::Functions test whether a value, variable, or function is defined
1285
2f9daede 1286Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1287the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1288checked.
1289
1290Many operations return C<undef> to indicate failure, end of file,
1291system error, uninitialized variable, and other exceptional
1292conditions. This function allows you to distinguish C<undef> from
1293other values. (A simple Boolean test will not distinguish among
7660c0ab 1294C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1295false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1296doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1297returns C<undef> when its argument is an empty array, I<or> when the
1298element to return happens to be C<undef>.
1299
f10b0346
GS
1300You may also use C<defined(&func)> to check whether subroutine C<&func>
1301has ever been defined. The return value is unaffected by any forward
80d38338 1302declarations of C<&func>. A subroutine that is not defined
847c7ebe 1303may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1304makes it spring into existence the first time that it is called; see
847c7ebe 1305L<perlsub>.
f10b0346
GS
1306
1307Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1308used to report whether memory for that aggregate had ever been
f10b0346
GS
1309allocated. This behavior may disappear in future versions of Perl.
1310You should instead use a simple test for size:
1311
1312 if (@an_array) { print "has array elements\n" }
1313 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1314
1315When used on a hash element, it tells you whether the value is defined,
dc848c6f 1316not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1317purpose.
a0d0e21e
LW
1318
1319Examples:
1320
8f1da26d 1321 print if defined $switch{D};
a0d0e21e
LW
1322 print "$val\n" while defined($val = pop(@ary));
1323 die "Can't readlink $sym: $!"
a9a5a0dc 1324 unless defined($value = readlink $sym);
a0d0e21e 1325 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1326 $debugging = 0 unless defined $debugging;
a0d0e21e 1327
8f1da26d 1328Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1329discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1330defined values. For example, if you say
a5f75d66
AD
1331
1332 "ab" =~ /a(.*)b/;
1333
80d38338 1334The pattern match succeeds and C<$1> is defined, although it
cf264981 1335matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1336matched something that happened to be zero characters long. This is all
a5f75d66 1337very above-board and honest. When a function returns an undefined value,
2f9daede 1338it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1339should use C<defined> only when questioning the integrity of what
7660c0ab 1340you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1341what you want.
1342
dc848c6f 1343See also L</undef>, L</exists>, L</ref>.
2f9daede 1344
a0d0e21e 1345=item delete EXPR
d74e8afc 1346X<delete>
a0d0e21e 1347
c17cdb72
NC
1348=for Pod::Functions deletes a value from a hash
1349
d0a76353
RS
1350Given an expression that specifies an element or slice of a hash, C<delete>
1351deletes the specified elements from that hash so that exists() on that element
1352no longer returns true. Setting a hash element to the undefined value does
1353not remove its key, but deleting it does; see L</exists>.
80d38338 1354
8f1da26d 1355In list context, returns the value or values deleted, or the last such
80d38338 1356element in scalar context. The return list's length always matches that of
d0a76353
RS
1357the argument list: deleting non-existent elements returns the undefined value
1358in their corresponding positions.
80d38338 1359
d0a76353
RS
1360delete() may also be used on arrays and array slices, but its behavior is less
1361straightforward. Although exists() will return false for deleted entries,
1362deleting array elements never changes indices of existing values; use shift()
1363or splice() for that. However, if all deleted elements fall at the end of an
1364array, the array's size shrinks to the position of the highest element that
1365still tests true for exists(), or to 0 if none do.
1366
8f1da26d 1367B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1368be removed in a future version of Perl.
80d38338
TC
1369
1370Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1371a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1372or array may not necessarily return anything; it depends on the implementation
1373of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1374
80d38338
TC
1375The C<delete local EXPR> construct localizes the deletion to the current
1376block at run time. Until the block exits, elements locally deleted
1377temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1378of composite types">.
eba0920a
EM
1379
1380 %hash = (foo => 11, bar => 22, baz => 33);
f7051f2c
FC
1381 $scalar = delete $hash{foo}; # $scalar is 11
1382 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1383 @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
eba0920a 1384
01020589 1385The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1386
5f05dabc 1387 foreach $key (keys %HASH) {
a9a5a0dc 1388 delete $HASH{$key};
a0d0e21e
LW
1389 }
1390
01020589 1391 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1392 delete $ARRAY[$index];
01020589
GS
1393 }
1394
1395And so do these:
5f05dabc 1396
01020589
GS
1397 delete @HASH{keys %HASH};
1398
9740c838 1399 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1400
80d38338
TC
1401But both are slower than assigning the empty list
1402or undefining %HASH or @ARRAY, which is the customary
1403way to empty out an aggregate:
01020589 1404
5ed4f2ec 1405 %HASH = (); # completely empty %HASH
1406 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1407
5ed4f2ec 1408 @ARRAY = (); # completely empty @ARRAY
1409 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1410
80d38338
TC
1411The EXPR can be arbitrarily complicated provided its
1412final operation is an element or slice of an aggregate:
a0d0e21e
LW
1413
1414 delete $ref->[$x][$y]{$key};
5f05dabc 1415 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1416
01020589
GS
1417 delete $ref->[$x][$y][$index];
1418 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1419
a0d0e21e 1420=item die LIST
d74e8afc 1421X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1422
c17cdb72
NC
1423=for Pod::Functions raise an exception or bail out
1424
391b733c 1425C<die> raises an exception. Inside an C<eval> the error message is stuffed
4c050ad5
NC
1426into C<$@> and the C<eval> is terminated with the undefined value.
1427If the exception is outside of all enclosing C<eval>s, then the uncaught
391b733c 1428exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1429need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1430
1431Equivalent examples:
1432
1433 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1434 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1435
ccac6780 1436If the last element of LIST does not end in a newline, the current
df37ec69
WW
1437script line number and input line number (if any) are also printed,
1438and a newline is supplied. Note that the "input line number" (also
1439known as "chunk") is subject to whatever notion of "line" happens to
1440be currently in effect, and is also available as the special variable
1441C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1442
1443Hint: sometimes appending C<", stopped"> to your message will cause it
1444to make better sense when the string C<"at foo line 123"> is appended.
1445Suppose you are running script "canasta".
a0d0e21e
LW
1446
1447 die "/etc/games is no good";
1448 die "/etc/games is no good, stopped";
1449
1450produce, respectively
1451
1452 /etc/games is no good at canasta line 123.
1453 /etc/games is no good, stopped at canasta line 123.
1454
a96d0188 1455If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1456previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1457This is useful for propagating exceptions:
1458
1459 eval { ... };
1460 die unless $@ =~ /Expected exception/;
1461
a96d0188 1462If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1463C<PROPAGATE> method, that method will be called with additional file
1464and line number parameters. The return value replaces the value in
34169887 1465C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1466were called.
1467
7660c0ab 1468If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1469
4c050ad5
NC
1470If an uncaught exception results in interpreter exit, the exit code is
1471determined from the values of C<$!> and C<$?> with this pseudocode:
1472
1473 exit $! if $!; # errno
1474 exit $? >> 8 if $? >> 8; # child exit status
1475 exit 255; # last resort
1476
1477The intent is to squeeze as much possible information about the likely cause
391b733c
FC
1478into the limited space of the system exit
1479code. However, as C<$!> is the value
4c050ad5
NC
1480of C's C<errno>, which can be set by any system call, this means that the value
1481of the exit code used by C<die> can be non-predictable, so should not be relied
1482upon, other than to be non-zero.
1483
80d38338
TC
1484You can also call C<die> with a reference argument, and if this is trapped
1485within an C<eval>, C<$@> contains that reference. This permits more
1486elaborate exception handling using objects that maintain arbitrary state
1487about the exception. Such a scheme is sometimes preferable to matching
1488particular string values of C<$@> with regular expressions. Because C<$@>
1489is a global variable and C<eval> may be used within object implementations,
1490be careful that analyzing the error object doesn't replace the reference in
1491the global variable. It's easiest to make a local copy of the reference
1492before any manipulations. Here's an example:
52531d10 1493
80d38338 1494 use Scalar::Util "blessed";
da279afe 1495
52531d10 1496 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7 1497 if (my $ev_err = $@) {
f7051f2c
FC
1498 if (blessed($ev_err)
1499 && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1500 # handle Some::Module::Exception
1501 }
1502 else {
1503 # handle all other possible exceptions
1504 }
1505 }
1506
3b10bc60 1507Because Perl stringifies uncaught exception messages before display,
80d38338 1508you'll probably want to overload stringification operations on
52531d10
GS
1509exception objects. See L<overload> for details about that.
1510
19799a22
GS
1511You can arrange for a callback to be run just before the C<die>
1512does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1513handler is called with the error text and can change the error
19799a22 1514message, if it sees fit, by calling C<die> again. See
96090e4f 1515L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1516L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1517to be run only right before your program was to exit, this is not
3b10bc60 1518currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1519even inside eval()ed blocks/strings! If one wants the hook to do
1520nothing in such situations, put
fb73857a 1521
5ed4f2ec 1522 die @_ if $^S;
fb73857a 1523
19799a22
GS
1524as the first line of the handler (see L<perlvar/$^S>). Because
1525this promotes strange action at a distance, this counterintuitive
b76cc8ba 1526behavior may be fixed in a future release.
774d564b 1527
4c050ad5
NC
1528See also exit(), warn(), and the Carp module.
1529
a0d0e21e 1530=item do BLOCK
d74e8afc 1531X<do> X<block>
a0d0e21e 1532
c17cdb72
NC
1533=for Pod::Functions turn a BLOCK into a TERM
1534
a0d0e21e 1535Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1536sequence of commands indicated by BLOCK. When modified by the C<while> or
1537C<until> loop modifier, executes the BLOCK once before testing the loop
391b733c 1538condition. (On other statements the loop modifiers test the conditional
6b275a1f 1539first.)
a0d0e21e 1540
4968c1e4 1541C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1542C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1543See L<perlsyn> for alternative strategies.
4968c1e4 1544
a0d0e21e 1545=item do SUBROUTINE(LIST)
d74e8afc 1546X<do>
a0d0e21e 1547
51124b83
FC
1548This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
1549a scalar variable or a subroutine beginning with C<&>.
a0d0e21e
LW
1550
1551=item do EXPR
d74e8afc 1552X<do>
a0d0e21e
LW
1553
1554Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1555file as a Perl script.
a0d0e21e
LW
1556
1557 do 'stat.pl';
1558
1559is just like
1560
986b19de 1561 eval `cat stat.pl`;
a0d0e21e 1562
2b5ab1e7 1563except that it's more efficient and concise, keeps track of the current
96090e4f
A
1564filename for error messages, searches the C<@INC> directories, and updates
1565C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1566these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1567cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1568same, however, in that it does reparse the file every time you call it,
1569so you probably don't want to do this inside a loop.
a0d0e21e 1570
8f1da26d 1571If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1572an error message in C<$@>. If C<do> cannot read the file, it returns undef
1573and sets C<$!> to the error. Always check C<$@> first, as compilation
1574could fail in a way that also sets C<$!>. If the file is successfully
1575compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1576
80d38338 1577Inclusion of library modules is better done with the
19799a22 1578C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1579and raise an exception if there's a problem.
a0d0e21e 1580
5a964f20
TC
1581You might like to use C<do> to read in a program configuration
1582file. Manual error checking can be done this way:
1583
b76cc8ba 1584 # read in config files: system first, then user
f86cebdf 1585 for $file ("/share/prog/defaults.rc",
b76cc8ba 1586 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1587 {
1588 unless ($return = do $file) {
1589 warn "couldn't parse $file: $@" if $@;
1590 warn "couldn't do $file: $!" unless defined $return;
1591 warn "couldn't run $file" unless $return;
1592 }
5a964f20
TC
1593 }
1594
a0d0e21e 1595=item dump LABEL
d74e8afc 1596X<dump> X<core> X<undump>
a0d0e21e 1597
1614b0e3
JD
1598=item dump
1599
c17cdb72
NC
1600=for Pod::Functions create an immediate core dump
1601
19799a22
GS
1602This function causes an immediate core dump. See also the B<-u>
1603command-line switch in L<perlrun>, which does the same thing.
1604Primarily this is so that you can use the B<undump> program (not
1605supplied) to turn your core dump into an executable binary after
1606having initialized all your variables at the beginning of the
1607program. When the new binary is executed it will begin by executing
1608a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1609Think of it as a goto with an intervening core dump and reincarnation.
1610If C<LABEL> is omitted, restarts the program from the top.
1611
1612B<WARNING>: Any files opened at the time of the dump will I<not>
1613be open any more when the program is reincarnated, with possible
80d38338 1614resulting confusion by Perl.
19799a22 1615
59f521f4 1616This function is now largely obsolete, mostly because it's very hard to
391b733c 1617convert a core file into an executable. That's why you should now invoke
59f521f4 1618it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1619typo.
19799a22 1620
ea9eb35a 1621Portability issues: L<perlport/dump>.
1622
532eee96 1623=item each HASH
d74e8afc 1624X<each> X<hash, iterator>
aa689395 1625
532eee96 1626=item each ARRAY
aeedbbed
NC
1627X<array, iterator>
1628
f5a93a43
TC
1629=item each EXPR
1630
c17cdb72
NC
1631=for Pod::Functions retrieve the next key/value pair from a hash
1632
bade7fbc
TC
1633When called on a hash in list context, returns a 2-element list
1634consisting of the key and value for the next element of a hash. In Perl
16355.12 and later only, it will also return the index and value for the next
1636element of an array so that you can iterate over it; older Perls consider
1637this a syntax error. When called in scalar context, returns only the key
1638(not the value) in a hash, or the index in an array.
2f9daede 1639
aeedbbed 1640Hash entries are returned in an apparently random order. The actual random
3b10bc60 1641order is subject to change in future versions of Perl, but it is
504f80c1 1642guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1643function would produce on the same (unmodified) hash. Since Perl
22883ac5 16445.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1645for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1646
80d38338
TC
1647After C<each> has returned all entries from the hash or array, the next
1648call to C<each> returns the empty list in list context and C<undef> in
bade7fbc
TC
1649scalar context; the next call following I<that> one restarts iteration.
1650Each hash or array has its own internal iterator, accessed by C<each>,
1651C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1652reached the end as just described; it can be explicitly reset by calling
1653C<keys> or C<values> on the hash or array. If you add or delete a hash's
1654elements while iterating over it, entries may be skipped or duplicated--so
1655don't do that. Exception: In the current implementation, it is always safe
1656to delete the item most recently returned by C<each()>, so the following
1657code works properly:
74fc8b5f
MJD
1658
1659 while (($key, $value) = each %hash) {
1660 print $key, "\n";
1661 delete $hash{$key}; # This is safe
1662 }
aa689395 1663
80d38338 1664This prints out your environment like the printenv(1) program,
3b10bc60 1665but in a different order:
a0d0e21e
LW
1666
1667 while (($key,$value) = each %ENV) {
a9a5a0dc 1668 print "$key=$value\n";
a0d0e21e
LW
1669 }
1670
f5a93a43
TC
1671Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1672reference to an unblessed hash or array. The argument will be dereferenced
1673automatically. This aspect of C<each> is considered highly experimental.
1674The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1675
1676 while (($key,$value) = each $hashref) { ... }
1677
bade7fbc
TC
1678To avoid confusing would-be users of your code who are running earlier
1679versions of Perl with mysterious syntax errors, put this sort of thing at
1680the top of your file to signal that your code will work I<only> on Perls of
1681a recent vintage:
1682
1683 use 5.012; # so keys/values/each work on arrays
1684 use 5.014; # so keys/values/each work on scalars (experimental)
1685
8f1da26d 1686See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1687
1688=item eof FILEHANDLE
d74e8afc
ITB
1689X<eof>
1690X<end of file>
1691X<end-of-file>
a0d0e21e 1692
4633a7c4
LW
1693=item eof ()
1694
a0d0e21e
LW
1695=item eof
1696
c17cdb72
NC
1697=for Pod::Functions test a filehandle for its end
1698
8f1da26d 1699Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1700FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1701gives the real filehandle. (Note that this function actually
80d38338 1702reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1703interactive context.) Do not read from a terminal file (or call
19799a22 1704C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1705as terminals may lose the end-of-file condition if you do.
1706
820475bd 1707An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1708with empty parentheses is different. It refers to the pseudo file
820475bd 1709formed from the files listed on the command line and accessed via the
61eff3bc
JH
1710C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1711as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1712used will cause C<@ARGV> to be examined to determine if input is
67408cae 1713available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1714end-of-file will assume you are processing another C<@ARGV> list,
1715and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1716see L<perlop/"I/O Operators">.
820475bd 1717
61eff3bc 1718In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1719detect the end of each file, whereas C<eof()> will detect the end
1720of the very last file only. Examples:
a0d0e21e 1721
748a9306
LW
1722 # reset line numbering on each input file
1723 while (<>) {
a9a5a0dc
VP
1724 next if /^\s*#/; # skip comments
1725 print "$.\t$_";
5a964f20 1726 } continue {
a9a5a0dc 1727 close ARGV if eof; # Not eof()!
748a9306
LW
1728 }
1729
a0d0e21e
LW
1730 # insert dashes just before last line of last file
1731 while (<>) {
a9a5a0dc
VP
1732 if (eof()) { # check for end of last file
1733 print "--------------\n";
1734 }
1735 print;
f7051f2c 1736 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1737 }
1738
a0d0e21e 1739Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1740input operators typically return C<undef> when they run out of data or
1741encounter an error.
a0d0e21e
LW
1742
1743=item eval EXPR
d74e8afc 1744X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1745X<error, handling> X<exception, handling>
a0d0e21e
LW
1746
1747=item eval BLOCK
1748
ce2984c3
PF
1749=item eval
1750
c17cdb72
NC
1751=for Pod::Functions catch exceptions or compile and run code
1752
c7cc6f1c
GS
1753In the first form, the return value of EXPR is parsed and executed as if it
1754were a little Perl program. The value of the expression (which is itself
8f1da26d 1755determined within scalar context) is first parsed, and if there were no
2341804c 1756errors, executed as a block within the lexical context of the current Perl
df4833a8 1757program. This means, that in particular, any outer lexical variables are
2341804c
DM
1758visible to it, and any package variable settings or subroutine and format
1759definitions remain afterwards.
1760
1761Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1762If EXPR is omitted, evaluates C<$_>. This form is typically used to
1763delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c 1764
7289c5e6
FC
1765If the C<unicode_eval> feature is enabled (which is the default under a
1766C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1767characters, so C<use utf8> declarations have no effect, and source filters
1768are forbidden. In the absence of the C<unicode_eval> feature, the string
1769will sometimes be treated as characters and sometimes as bytes, depending
1770on the internal encoding, and source filters activated within the C<eval>
1771exhibit the erratic, but historical, behaviour of affecting some outer file
1772scope that is still compiling. See also the L</evalbytes> keyword, which
1773always treats its input as a byte stream and works properly with source
1774filters, and the L<feature> pragma.
1775
c7cc6f1c 1776In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1777same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1778within the context of the current Perl program. This form is typically
1779used to trap exceptions more efficiently than the first (see below), while
1780also providing the benefit of checking the code within BLOCK at compile
1781time.
1782
1783The final semicolon, if any, may be omitted from the value of EXPR or within
1784the BLOCK.
1785
1786In both forms, the value returned is the value of the last expression
5a964f20 1787evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1788as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1789in void, scalar, or list context, depending on the context of the C<eval>
1790itself. See L</wantarray> for more on how the evaluation context can be
1791determined.
a0d0e21e 1792
19799a22 1793If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1794executed, C<eval> returns C<undef> in scalar context
774b80e8
FC
1795or an empty list in list context, and C<$@> is set to the error
1796message. (Prior to 5.16, a bug caused C<undef> to be returned
1797in list context for syntax errors, but not for runtime errors.)
1798If there was no error, C<$@> is set to the empty string. A
9cc672d4
FC
1799control flow operator like C<last> or C<goto> can bypass the setting of
1800C<$@>. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1801warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1802To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1803turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1804See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1805
19799a22
GS
1806Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1807determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1808is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1809the die operator is used to raise exceptions.
1810
5f1da31c
NT
1811If you want to trap errors when loading an XS module, some problems with
1812the binary interface (such as Perl version skew) may be fatal even with
df4833a8 1813C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
5f1da31c 1814
a0d0e21e
LW
1815If the code to be executed doesn't vary, you may use the eval-BLOCK
1816form to trap run-time errors without incurring the penalty of
1817recompiling each time. The error, if any, is still returned in C<$@>.
1818Examples:
1819
54310121 1820 # make divide-by-zero nonfatal
a0d0e21e
LW
1821 eval { $answer = $a / $b; }; warn $@ if $@;
1822
1823 # same thing, but less efficient
1824 eval '$answer = $a / $b'; warn $@ if $@;
1825
1826 # a compile-time error
5ed4f2ec 1827 eval { $answer = }; # WRONG
a0d0e21e
LW
1828
1829 # a run-time error
5ed4f2ec 1830 eval '$answer ='; # sets $@
a0d0e21e 1831
cf264981
SP
1832Using the C<eval{}> form as an exception trap in libraries does have some
1833issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1834may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1835You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1836as this example shows:
774d564b 1837
80d38338 1838 # a private exception trap for divide-by-zero
f86cebdf
GS
1839 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1840 warn $@ if $@;
774d564b
PP
1841
1842This is especially significant, given that C<__DIE__> hooks can call
19799a22 1843C<die> again, which has the effect of changing their error messages:
774d564b
PP
1844
1845 # __DIE__ hooks may modify error messages
1846 {
f86cebdf
GS
1847 local $SIG{'__DIE__'} =
1848 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1849 eval { die "foo lives here" };
1850 print $@ if $@; # prints "bar lives here"
774d564b
PP
1851 }
1852
19799a22 1853Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1854may be fixed in a future release.
1855
19799a22 1856With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1857being looked at when:
1858
5ed4f2ec 1859 eval $x; # CASE 1
1860 eval "$x"; # CASE 2
a0d0e21e 1861
5ed4f2ec 1862 eval '$x'; # CASE 3
1863 eval { $x }; # CASE 4
a0d0e21e 1864
5ed4f2ec 1865 eval "\$$x++"; # CASE 5
1866 $$x++; # CASE 6
a0d0e21e 1867
2f9daede 1868Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1869the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1870the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1871and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1872does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1873purely visual reasons, but it also has the advantage of compiling at
1874compile-time instead of at run-time.) Case 5 is a place where
19799a22 1875normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1876particular situation, you can just use symbolic references instead, as
1877in case 6.
a0d0e21e 1878
b6538e4f 1879Before Perl 5.14, the assignment to C<$@> occurred before restoration
bade7fbc 1880of localized variables, which means that for your code to run on older
b208c909 1881versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1882errors:
1883
1884 # alter $@ on nefarious repugnancy only
1885 {
1886 my $e;
1887 {
f7051f2c
FC
1888 local $@; # protect existing $@
1889 eval { test_repugnancy() };
1890 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1891 $@ =~ /nefarious/ and $e = $@;
8a5a710d
DN
1892 }
1893 die $e if defined $e
1894 }
1895
4968c1e4 1896C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1897C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1898
3b10bc60 1899An C<eval ''> executed within the C<DB> package doesn't see the usual
1900surrounding lexical scope, but rather the scope of the first non-DB piece
df4833a8 1901of code that called it. You don't normally need to worry about this unless
3b10bc60 1902you are writing a Perl debugger.
d819b83a 1903
7289c5e6
FC
1904=item evalbytes EXPR
1905X<evalbytes>
1906
1907=item evalbytes
1908
d9b04284 1909=for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
c17cdb72 1910
7289c5e6
FC
1911This function is like L</eval> with a string argument, except it always
1912parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1913string containing characters whose ordinal value exceeds 255 results in an
1914error. Source filters activated within the evaluated code apply to the
1915code itself.
1916
1917This function is only available under the C<evalbytes> feature, a
1918C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1919L<feature> for more information.
1920
a0d0e21e 1921=item exec LIST
d74e8afc 1922X<exec> X<execute>
a0d0e21e 1923
8bf3b016
GS
1924=item exec PROGRAM LIST
1925
c17cdb72
NC
1926=for Pod::Functions abandon this program to run another
1927
3b10bc60 1928The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1929use C<system> instead of C<exec> if you want it to return. It fails and
1930returns false only if the command does not exist I<and> it is executed
fb73857a 1931directly instead of via your system's command shell (see below).
a0d0e21e 1932
19799a22 1933Since it's a common mistake to use C<exec> instead of C<system>, Perl
4642e50d
EB
1934warns you if C<exec> is called in void context and if there is a following
1935statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1936you always do that, right?). If you I<really> want to follow an C<exec>
1937with some other statement, you can use one of these styles to avoid the warning:
55d729e4 1938
5a964f20
TC
1939 exec ('foo') or print STDERR "couldn't exec foo: $!";
1940 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1941
5a964f20 1942If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1943with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1944If there is only one scalar argument or an array with one element in it,
1945the argument is checked for shell metacharacters, and if there are any,
1946the entire argument is passed to the system's command shell for parsing
1947(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1948If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1949words and passed directly to C<execvp>, which is more efficient.
19799a22 1950Examples:
a0d0e21e 1951
19799a22
GS
1952 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1953 exec "sort $outfile | uniq";
a0d0e21e
LW
1954
1955If you don't really want to execute the first argument, but want to lie
1956to the program you are executing about its own name, you can specify
1957the program you actually want to run as an "indirect object" (without a
1958comma) in front of the LIST. (This always forces interpretation of the
54310121 1959LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1960the list.) Example:
1961
1962 $shell = '/bin/csh';
5ed4f2ec 1963 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1964
1965or, more directly,
1966
5ed4f2ec 1967 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1968
3b10bc60 1969When the arguments get executed via the system shell, results are
1970subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1971for details.
1972
19799a22
GS
1973Using an indirect object with C<exec> or C<system> is also more
1974secure. This usage (which also works fine with system()) forces
1975interpretation of the arguments as a multivalued list, even if the
1976list had just one argument. That way you're safe from the shell
1977expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1978
1979 @args = ( "echo surprise" );
1980
2b5ab1e7 1981 exec @args; # subject to shell escapes
f86cebdf 1982 # if @args == 1
2b5ab1e7 1983 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1984
1985The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1986program, passing it C<"surprise"> an argument. The second version didn't;
1987it tried to run a program named I<"echo surprise">, didn't find it, and set
1988C<$?> to a non-zero value indicating failure.
5a964f20 1989
80d38338 1990Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1991output before the exec, but this may not be supported on some platforms
1992(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1993in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 1994open handles to avoid lost output.
0f897271 1995
80d38338
TC
1996Note that C<exec> will not call your C<END> blocks, nor will it invoke
1997C<DESTROY> methods on your objects.
7660c0ab 1998
ea9eb35a 1999Portability issues: L<perlport/exec>.
2000
a0d0e21e 2001=item exists EXPR
d74e8afc 2002X<exists> X<autovivification>
a0d0e21e 2003
c17cdb72
NC
2004=for Pod::Functions test whether a hash key is present
2005
d0a76353
RS
2006Given an expression that specifies an element of a hash, returns true if the
2007specified element in the hash has ever been initialized, even if the
2008corresponding value is undefined.
a0d0e21e 2009
5ed4f2ec 2010 print "Exists\n" if exists $hash{$key};
2011 print "Defined\n" if defined $hash{$key};
01020589
GS
2012 print "True\n" if $hash{$key};
2013
d0a76353 2014exists may also be called on array elements, but its behavior is much less
8f1da26d 2015obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
2016that calling exists on array values is deprecated and likely to be removed in
2017a future version of Perl.
2018
5ed4f2ec 2019 print "Exists\n" if exists $array[$index];
2020 print "Defined\n" if defined $array[$index];
01020589 2021 print "True\n" if $array[$index];
a0d0e21e 2022
8f1da26d 2023A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
2024it exists, but the reverse doesn't necessarily hold true.
2025
afebc493
GS
2026Given an expression that specifies the name of a subroutine,
2027returns true if the specified subroutine has ever been declared, even
2028if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 2029does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
2030exist may still be callable: its package may have an C<AUTOLOAD>
2031method that makes it spring into existence the first time that it is
3b10bc60 2032called; see L<perlsub>.
afebc493 2033
5ed4f2ec 2034 print "Exists\n" if exists &subroutine;
2035 print "Defined\n" if defined &subroutine;
afebc493 2036
a0d0e21e 2037Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 2038operation is a hash or array key lookup or subroutine name:
a0d0e21e 2039
5ed4f2ec 2040 if (exists $ref->{A}->{B}->{$key}) { }
2041 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 2042
5ed4f2ec 2043 if (exists $ref->{A}->{B}->[$ix]) { }
2044 if (exists $hash{A}{B}[$ix]) { }
01020589 2045
afebc493
GS
2046 if (exists &{$ref->{A}{B}{$key}}) { }
2047
9590a7cd 2048Although the most deeply nested array or hash element will not spring into
3b10bc60 2049existence just because its existence was tested, any intervening ones will.
61eff3bc 2050Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 2051into existence due to the existence test for the $key element above.
3b10bc60 2052This happens anywhere the arrow operator is used, including even here:
5a964f20 2053
2b5ab1e7 2054 undef $ref;
5ed4f2ec 2055 if (exists $ref->{"Some key"}) { }
2056 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
2057
2058This surprising autovivification in what does not at first--or even
2059second--glance appear to be an lvalue context may be fixed in a future
5a964f20 2060release.
a0d0e21e 2061
afebc493
GS
2062Use of a subroutine call, rather than a subroutine name, as an argument
2063to exists() is an error.
2064
5ed4f2ec 2065 exists &sub; # OK
2066 exists &sub(); # Error
afebc493 2067
a0d0e21e 2068=item exit EXPR
d74e8afc 2069X<exit> X<terminate> X<abort>
a0d0e21e 2070
ce2984c3
PF
2071=item exit
2072
c17cdb72
NC
2073=for Pod::Functions terminate this program
2074
2b5ab1e7 2075Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
2076
2077 $ans = <STDIN>;
2078 exit 0 if $ans =~ /^[Xx]/;
2079
19799a22 2080See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
2081universally recognized values for EXPR are C<0> for success and C<1>
2082for error; other values are subject to interpretation depending on the
2083environment in which the Perl program is running. For example, exiting
208469 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2085the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 2086
19799a22
GS
2087Don't use C<exit> to abort a subroutine if there's any chance that
2088someone might want to trap whatever error happened. Use C<die> instead,
2089which can be trapped by an C<eval>.
28757baa 2090
19799a22 2091The exit() function does not always exit immediately. It calls any
2b5ab1e7 2092defined C<END> routines first, but these C<END> routines may not
19799a22 2093themselves abort the exit. Likewise any object destructors that need to
60275626 2094be called are called before the real exit. C<END> routines and destructors
391b733c 2095can change the exit status by modifying C<$?>. If this is a problem, you
fae6f8fa 2096can call C<POSIX::_exit($status)> to avoid END and destructor processing.
87275199 2097See L<perlmod> for details.
5a964f20 2098
ea9eb35a 2099Portability issues: L<perlport/exit>.
2100
a0d0e21e 2101=item exp EXPR
d74e8afc 2102X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 2103
54310121 2104=item exp
bbce6d69 2105
c17cdb72
NC
2106=for Pod::Functions raise I<e> to a power
2107
b76cc8ba 2108Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
2109If EXPR is omitted, gives C<exp($_)>.
2110
628253b8
BF
2111=item fc EXPR
2112X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2113
2114=item fc
2115
d9b04284 2116=for Pod::Functions +fc return casefolded version of a string
c17cdb72 2117
628253b8
BF
2118Returns the casefolded version of EXPR. This is the internal function
2119implementing the C<\F> escape in double-quoted strings.
2120
2121Casefolding is the process of mapping strings to a form where case
2122differences are erased; comparing two strings in their casefolded
2123form is effectively a way of asking if two strings are equal,
2124regardless of case.
2125
2126Roughly, if you ever found yourself writing this
2127
f6c6dcb6 2128 lc($this) eq lc($that) # Wrong!
628253b8 2129 # or
f6c6dcb6 2130 uc($this) eq uc($that) # Also wrong!
628253b8 2131 # or
f6c6dcb6 2132 $this =~ /^\Q$that\E\z/i # Right!
628253b8
BF
2133
2134Now you can write
2135
2136 fc($this) eq fc($that)
2137
2138And get the correct results.
2139
fc39a31f
KW
2140Perl only implements the full form of casefolding,
2141but you can access the simple folds using L<Unicode::UCD/casefold()> and
2142L<Unicode::UCD/prop_invmap()>.
628253b8
BF
2143For further information on casefolding, refer to
2144the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
21454.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2146available at L<http://www.unicode.org/versions/latest/>, as well as the
2147Case Charts available at L<http://www.unicode.org/charts/case/>.
2148
2149If EXPR is omitted, uses C<$_>.
2150
2151This function behaves the same way under various pragma, such as in a locale,
2152as L</lc> does.
2153
2154While the Unicode Standard defines two additional forms of casefolding,
2155one for Turkic languages and one that never maps one character into multiple
2156characters, these are not provided by the Perl core; However, the CPAN module
2157C<Unicode::Casing> may be used to provide an implementation.
2158
2159This keyword is available only when the C<"fc"> feature is enabled,
2160or when prefixed with C<CORE::>; See L<feature>. Alternately,
2161include a C<use v5.16> or later to the current scope.
2162
a0d0e21e 2163=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2164X<fcntl>
a0d0e21e 2165
c17cdb72
NC
2166=for Pod::Functions file control system call
2167
f86cebdf 2168Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
2169
2170 use Fcntl;
2171
0ade1984 2172first to get the correct constant definitions. Argument processing and
3b10bc60 2173value returned work just like C<ioctl> below.
a0d0e21e
LW
2174For example:
2175
2176 use Fcntl;
5a964f20 2177 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 2178 or die "can't fcntl F_GETFL: $!";
5a964f20 2179
554ad1fc 2180You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
2181Like C<ioctl>, it maps a C<0> return from the system call into
2182C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
2183in numeric context. It is also exempt from the normal B<-w> warnings
2184on improper numeric conversions.
5a964f20 2185
3b10bc60 2186Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
2187doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2188manpage to learn what functions are available on your system.
a0d0e21e 2189
be2f7487 2190Here's an example of setting a filehandle named C<REMOTE> to be
2191non-blocking at the system level. You'll have to negotiate C<$|>
2192on your own, though.
2193
2194 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2195
2196 $flags = fcntl(REMOTE, F_GETFL, 0)
2197 or die "Can't get flags for the socket: $!\n";
2198
2199 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2200 or die "Can't set flags for the socket: $!\n";
2201
ea9eb35a 2202Portability issues: L<perlport/fcntl>.
2203
cfa52385
FC
2204=item __FILE__
2205X<__FILE__>
2206
c17cdb72
NC
2207=for Pod::Functions the name of the current source file
2208
cfa52385
FC
2209A special token that returns the name of the file in which it occurs.
2210
a0d0e21e 2211=item fileno FILEHANDLE
d74e8afc 2212X<fileno>
a0d0e21e 2213
c17cdb72
NC
2214=for Pod::Functions return file descriptor from filehandle
2215
2b5ab1e7 2216Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
2217filehandle is not open. If there is no real file descriptor at the OS
2218level, as can happen with filehandles connected to memory objects via
2219C<open> with a reference for the third argument, -1 is returned.
2220
2221This is mainly useful for constructing
19799a22 2222bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
2223If FILEHANDLE is an expression, the value is taken as an indirect
2224filehandle, generally its name.
5a964f20 2225
b76cc8ba 2226You can use this to find out whether two handles refer to the
5a964f20
TC
2227same underlying descriptor:
2228
2229 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 2230 print "THIS and THAT are dups\n";
b76cc8ba
NIS
2231 }
2232
a0d0e21e 2233=item flock FILEHANDLE,OPERATION
d74e8afc 2234X<flock> X<lock> X<locking>
a0d0e21e 2235
c17cdb72
NC
2236=for Pod::Functions lock an entire file with an advisory lock
2237
19799a22
GS
2238Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2239for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2240machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2241C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2242entire files only, not records.
2b5ab1e7
TC
2243
2244Two potentially non-obvious but traditional C<flock> semantics are
2245that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2246are B<merely advisory>. Such discretionary locks are more flexible, but
2247offer fewer guarantees. This means that programs that do not also use
2248C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2249your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2250for details. It's best to assume traditional behavior if you're writing
2251portable programs. (But if you're not, you should as always feel perfectly
2252free to write for your own system's idiosyncrasies (sometimes called
2253"features"). Slavish adherence to portability concerns shouldn't get
2254in the way of your getting your job done.)
a3cb178b 2255
8ebc5c01
PP
2256OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2257LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2258you can use the symbolic names if you import them from the L<Fcntl> module,
2259either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2260requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2261releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2262LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2263waiting for the lock; check the return status to see if you got it.
68dc0745 2264
2b5ab1e7
TC
2265To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2266before locking or unlocking it.
8ebc5c01 2267
f86cebdf 2268Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2269locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2270are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2271implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
2272differing semantics shouldn't bite too many people.
2273
becacb53
TM
2274Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2275be open with read intent to use LOCK_SH and requires that it be open
2276with write intent to use LOCK_EX.
2277
19799a22
GS
2278Note also that some versions of C<flock> cannot lock things over the
2279network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2280that. If you like you can force Perl to ignore your system's flock(2)
2281function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2282the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2283and build a new Perl.
4633a7c4
LW
2284
2285Here's a mailbox appender for BSD systems.
a0d0e21e 2286
f7051f2c
FC
2287 # import LOCK_* and SEEK_END constants
2288 use Fcntl qw(:flock SEEK_END);
a0d0e21e
LW
2289
2290 sub lock {
a9a5a0dc
VP
2291 my ($fh) = @_;
2292 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2293
a9a5a0dc
VP
2294 # and, in case someone appended while we were waiting...
2295 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2296 }
2297
2298 sub unlock {
a9a5a0dc
VP
2299 my ($fh) = @_;
2300 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2301 }
2302
b0169937 2303 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2304 or die "Can't open mailbox: $!";
a0d0e21e 2305
7ed5353d 2306 lock($mbox);
b0169937 2307 print $mbox $msg,"\n\n";
7ed5353d 2308 unlock($mbox);
a0d0e21e 2309
3b10bc60 2310On systems that support a real flock(2), locks are inherited across fork()
2311calls, whereas those that must resort to the more capricious fcntl(2)
2312function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2313
cb1a09d0 2314See also L<DB_File> for other flock() examples.
a0d0e21e 2315
ea9eb35a 2316Portability issues: L<perlport/flock>.
2317
a0d0e21e 2318=item fork
d74e8afc 2319X<fork> X<child> X<parent>
a0d0e21e 2320
c17cdb72
NC
2321=for Pod::Functions create a new process just like this one
2322
2b5ab1e7
TC
2323Does a fork(2) system call to create a new process running the
2324same program at the same point. It returns the child pid to the
2325parent process, C<0> to the child process, or C<undef> if the fork is
2326unsuccessful. File descriptors (and sometimes locks on those descriptors)
2327are shared, while everything else is copied. On most systems supporting
2328fork(), great care has gone into making it extremely efficient (for
2329example, using copy-on-write technology on data pages), making it the
2330dominant paradigm for multitasking over the last few decades.
5a964f20 2331
80d38338 2332Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
2333output before forking the child process, but this may not be supported
2334on some platforms (see L<perlport>). To be safe, you may need to set
2335C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2336C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2337
19799a22 2338If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2339accumulate zombies. On some systems, you can avoid this by setting
2340C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2341forking and reaping moribund children.
cb1a09d0 2342
28757baa
PP
2343Note that if your forked child inherits system file descriptors like
2344STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2345if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2346backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2347You should reopen those to F</dev/null> if it's any issue.
28757baa 2348
ea9eb35a 2349On some platforms such as Windows, where the fork() system call is not available,
391b733c
FC
2350Perl can be built to emulate fork() in the Perl interpreter.
2351The emulation is designed, at the level of the Perl program,
2352to be as compatible as possible with the "Unix" fork().
6d17f725 2353However it has limitations that have to be considered in code intended to be portable.
ea9eb35a 2354See L<perlfork> for more details.
2355
2356Portability issues: L<perlport/fork>.
2357
cb1a09d0 2358=item format
d74e8afc 2359X<format>
cb1a09d0 2360
c17cdb72
NC
2361=for Pod::Functions declare a picture format with use by the write() function
2362
19799a22 2363Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2364example:
2365
54310121 2366 format Something =
a9a5a0dc
VP
2367 Test: @<<<<<<<< @||||| @>>>>>
2368 $str, $%, '$' . int($num)
cb1a09d0
AD
2369 .
2370
2371 $str = "widget";
184e9718 2372 $num = $cost/$quantity;
cb1a09d0
AD
2373 $~ = 'Something';
2374 write;
2375
2376See L<perlform> for many details and examples.
2377
8903cb82 2378=item formline PICTURE,LIST
d74e8afc 2379X<formline>
a0d0e21e 2380
c17cdb72
NC
2381=for Pod::Functions internal function used for formats
2382
5a964f20 2383This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2384too. It formats (see L<perlform>) a list of values according to the
2385contents of PICTURE, placing the output into the format output
7660c0ab 2386accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2387Eventually, when a C<write> is done, the contents of
cf264981
SP
2388C<$^A> are written to some filehandle. You could also read C<$^A>
2389and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2390does one C<formline> per line of form, but the C<formline> function itself
748a9306 2391doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2392that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2393You may therefore need to use multiple formlines to implement a single
3b10bc60 2394record format, just like the C<format> compiler.
748a9306 2395
19799a22 2396Be careful if you put double quotes around the picture, because an C<@>
748a9306 2397character may be taken to mean the beginning of an array name.
19799a22 2398C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2399
445b09e5
FC
2400If you are trying to use this instead of C<write> to capture the output,
2401you may find it easier to open a filehandle to a scalar
2402(C<< open $fh, ">", \$output >>) and write to that instead.
2403
a0d0e21e 2404=item getc FILEHANDLE
f723aae1 2405X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2406
2407=item getc
2408
c17cdb72
NC
2409=for Pod::Functions get the next character from the filehandle
2410
a0d0e21e 2411Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2412or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2413the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2414STDIN. This is not particularly efficient. However, it cannot be
2415used by itself to fetch single characters without waiting for the user
2416to hit enter. For that, try something more like:
4633a7c4
LW
2417
2418 if ($BSD_STYLE) {
a9a5a0dc 2419 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2420 }
2421 else {
a9a5a0dc 2422 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2423 }
2424
2425 $key = getc(STDIN);
2426
2427 if ($BSD_STYLE) {
a9a5a0dc 2428 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2429 }
2430 else {
3b10bc60 2431 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2432 }
2433 print "\n";
2434
54310121
PP
2435Determination of whether $BSD_STYLE should be set
2436is left as an exercise to the reader.
cb1a09d0 2437
19799a22 2438The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2439systems purporting POSIX compliance. See also the C<Term::ReadKey>
a3390c9f 2440module from your nearest CPAN site; details on CPAN can be found under
2b5ab1e7 2441L<perlmodlib/CPAN>.
a0d0e21e
LW
2442
2443=item getlogin
d74e8afc 2444X<getlogin> X<login>
a0d0e21e 2445
c17cdb72
NC
2446=for Pod::Functions return who logged in at this tty
2447
cf264981 2448This implements the C library function of the same name, which on most
3b10bc60 2449systems returns the current login from F</etc/utmp>, if any. If it
2450returns the empty string, use C<getpwuid>.
a0d0e21e 2451
f86702cc 2452 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2453
19799a22
GS
2454Do not consider C<getlogin> for authentication: it is not as
2455secure as C<getpwuid>.
4633a7c4 2456
ea9eb35a 2457Portability issues: L<perlport/getlogin>.
2458
a0d0e21e 2459=item getpeername SOCKET
d74e8afc 2460X<getpeername> X<peer>
a0d0e21e 2461
c17cdb72
NC
2462=for Pod::Functions find the other end of a socket connection
2463
a3390c9f
FC
2464Returns the packed sockaddr address of the other end of the SOCKET
2465connection.
a0d0e21e 2466
4633a7c4
LW
2467 use Socket;
2468 $hersockaddr = getpeername(SOCK);
19799a22 2469 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2470 $herhostname = gethostbyaddr($iaddr, AF_INET);
2471 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2472
2473=item getpgrp PID
d74e8afc 2474X<getpgrp> X<group>
a0d0e21e 2475
c17cdb72
NC
2476=for Pod::Functions get process group
2477
47e29363 2478Returns the current process group for the specified PID. Use
7660c0ab 2479a PID of C<0> to get the current process group for the
4633a7c4 2480current process. Will raise an exception if used on a machine that
a3390c9f
FC
2481doesn't implement getpgrp(2). If PID is omitted, returns the process
2482group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2483does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2484
ea9eb35a 2485Portability issues: L<perlport/getpgrp>.
2486
a0d0e21e 2487=item getppid
d74e8afc 2488X<getppid> X<parent> X<pid>
a0d0e21e 2489
c17cdb72
NC
2490=for Pod::Functions get parent process ID
2491
a0d0e21e
LW
2492Returns the process id of the parent process.
2493
d7c042c9
AB
2494Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2495around non-POSIX thread semantics the minority of Linux systems (and
2496Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2497has since been removed. See the documentation for L<$$|perlvar/$$> for
2498details.
4d76a344 2499
ea9eb35a 2500Portability issues: L<perlport/getppid>.
2501
a0d0e21e 2502=item getpriority WHICH,WHO
d74e8afc 2503X<getpriority> X<priority> X<nice>
a0d0e21e 2504
c17cdb72
NC
2505=for Pod::Functions get current nice value
2506
4633a7c4 2507Returns the current priority for a process, a process group, or a user.
01aa884e 2508(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2509machine that doesn't implement getpriority(2).
a0d0e21e 2510
ea9eb35a 2511Portability issues: L<perlport/getpriority>.
2512
a0d0e21e 2513=item getpwnam NAME
d74e8afc
ITB
2514X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2515X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2516X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2517X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2518X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2519X<endnetent> X<endprotoent> X<endservent>
a0d0e21e 2520
c17cdb72
NC
2521=for Pod::Functions get passwd record given user login name
2522
a0d0e21e
LW
2523=item getgrnam NAME
2524
c17cdb72
NC
2525=for Pod::Functions get group record given group name
2526
a0d0e21e
LW
2527=item gethostbyname NAME
2528
c17cdb72
NC
2529=for Pod::Functions get host record given name
2530
a0d0e21e
LW
2531=item getnetbyname NAME
2532
c17cdb72
NC
2533=for Pod::Functions get networks record given name
2534
a0d0e21e
LW
2535=item getprotobyname NAME
2536
c17cdb72
NC
2537=for Pod::Functions get protocol record given name
2538
a0d0e21e
LW
2539=item getpwuid UID
2540
c17cdb72
NC
2541=for Pod::Functions get passwd record given user ID
2542
a0d0e21e
LW
2543=item getgrgid GID
2544
c17cdb72
NC
2545=for Pod::Functions get group record given group user ID
2546
a0d0e21e
LW
2547=item getservbyname NAME,PROTO
2548
c17cdb72
NC
2549=for Pod::Functions get services record given its name
2550
a0d0e21e
LW
2551=item gethostbyaddr ADDR,ADDRTYPE
2552
c17cdb72
NC
2553=for Pod::Functions get host record given its address
2554
a0d0e21e
LW
2555=item getnetbyaddr ADDR,ADDRTYPE
2556
c17cdb72
NC
2557=for Pod::Functions get network record given its address
2558
a0d0e21e
LW
2559=item getprotobynumber NUMBER
2560
c17cdb72
NC
2561=for Pod::Functions get protocol record numeric protocol
2562
a0d0e21e
LW
2563=item getservbyport PORT,PROTO
2564
c17cdb72
NC
2565=for Pod::Functions get services record given numeric port
2566
a0d0e21e
LW
2567=item getpwent
2568
c17cdb72
NC
2569=for Pod::Functions get next passwd record
2570
a0d0e21e
LW
2571=item getgrent
2572
c17cdb72
NC
2573=for Pod::Functions get next group record
2574
a0d0e21e
LW
2575=item gethostent
2576
c17cdb72
NC
2577=for Pod::Functions get next hosts record
2578
a0d0e21e
LW
2579=item getnetent
2580
c17cdb72
NC
2581=for Pod::Functions get next networks record
2582
a0d0e21e
LW
2583=item getprotoent
2584
c17cdb72
NC
2585=for Pod::Functions get next protocols record
2586
a0d0e21e
LW
2587=item getservent
2588
c17cdb72
NC
2589=for Pod::Functions get next services record
2590
a0d0e21e
LW
2591=item setpwent
2592
c17cdb72
NC
2593=for Pod::Functions prepare passwd file for use
2594
a0d0e21e
LW
2595=item setgrent
2596
c17cdb72
NC
2597=for Pod::Functions prepare group file for use
2598
a0d0e21e
LW
2599=item sethostent STAYOPEN
2600
c17cdb72
NC
2601=for Pod::Functions prepare hosts file for use
2602
a0d0e21e
LW
2603=item setnetent STAYOPEN
2604
c17cdb72
NC
2605=for Pod::Functions prepare networks file for use
2606
a0d0e21e
LW
2607=item setprotoent STAYOPEN
2608
c17cdb72
NC
2609=for Pod::Functions prepare protocols file for use
2610
a0d0e21e
LW
2611=item setservent STAYOPEN
2612
c17cdb72
NC
2613=for Pod::Functions prepare services file for use
2614
a0d0e21e
LW
2615=item endpwent
2616
c17cdb72
NC
2617=for Pod::Functions be done using passwd file
2618
a0d0e21e
LW
2619=item endgrent
2620
c17cdb72
NC
2621=for Pod::Functions be done using group file
2622
a0d0e21e
LW
2623=item endhostent
2624
c17cdb72
NC
2625=for Pod::Functions be done using hosts file
2626
a0d0e21e
LW
2627=item endnetent
2628
c17cdb72
NC
2629=for Pod::Functions be done using networks file
2630
a0d0e21e
LW
2631=item endprotoent
2632
c17cdb72
NC
2633=for Pod::Functions be done using protocols file
2634
a0d0e21e
LW
2635=item endservent
2636
c17cdb72
NC
2637=for Pod::Functions be done using services file
2638
80d38338
TC
2639These routines are the same as their counterparts in the
2640system C library. In list context, the return values from the
a0d0e21e
LW
2641various get routines are as follows:
2642
2643 ($name,$passwd,$uid,$gid,
6ee623d5 2644 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2645 ($name,$passwd,$gid,$members) = getgr*
2646 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2647 ($name,$aliases,$addrtype,$net) = getnet*
2648 ($name,$aliases,$proto) = getproto*
2649 ($name,$aliases,$port,$proto) = getserv*
2650
3b10bc60 2651(If the entry doesn't exist you get an empty list.)
a0d0e21e 2652
4602f195
JH
2653The exact meaning of the $gcos field varies but it usually contains
2654the real name of the user (as opposed to the login name) and other
2655information pertaining to the user. Beware, however, that in many
2656system users are able to change this information and therefore it
106325ad 2657cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2658L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2659login shell, are also tainted, for the same reason.
4602f195 2660
5a964f20 2661In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2662lookup by name, in which case you get the other thing, whatever it is.
2663(If the entry doesn't exist you get the undefined value.) For example:
2664
5a964f20
TC
2665 $uid = getpwnam($name);
2666 $name = getpwuid($num);
2667 $name = getpwent();
2668 $gid = getgrnam($name);
08a33e13 2669 $name = getgrgid($num);
5a964f20
TC
2670 $name = getgrent();
2671 #etc.
a0d0e21e 2672
4602f195 2673In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2674in that they are unsupported on many systems. If the
4602f195
JH
2675$quota is unsupported, it is an empty scalar. If it is supported, it
2676usually encodes the disk quota. If the $comment field is unsupported,
2677it is an empty scalar. If it is supported it usually encodes some
2678administrative comment about the user. In some systems the $quota
2679field may be $change or $age, fields that have to do with password
2680aging. In some systems the $comment field may be $class. The $expire
2681field, if present, encodes the expiration period of the account or the
2682password. For the availability and the exact meaning of these fields
8f1da26d 2683in your system, please consult getpwnam(3) and your system's
4602f195
JH
2684F<pwd.h> file. You can also find out from within Perl what your
2685$quota and $comment fields mean and whether you have the $expire field
2686by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2687C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2688files are supported only if your vendor has implemented them in the
4602f195 2689intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2690shadow versions if you're running under privilege or if there exists
cf264981 2691the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2692and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2693facility are unlikely to be supported.
6ee623d5 2694
a3390c9f 2695The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2696the login names of the members of the group.
2697
2698For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2699C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2700C<@addrs> value returned by a successful call is a list of raw
2701addresses returned by the corresponding library call. In the
2702Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2703by saying something like:
2704
f337b084 2705 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2706
2b5ab1e7
TC
2707The Socket library makes this slightly easier:
2708
2709 use Socket;
2710 $iaddr = inet_aton("127.1"); # or whatever address
2711 $name = gethostbyaddr($iaddr, AF_INET);
2712
2713 # or going the other way
19799a22 2714 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2715
d760c846
GS
2716In the opposite way, to resolve a hostname to the IP address
2717you can write this:
2718
2719 use Socket;
2720 $packed_ip = gethostbyname("www.perl.org");
2721 if (defined $packed_ip) {
2722 $ip_address = inet_ntoa($packed_ip);
2723 }
2724
b018eaf1 2725Make sure C<gethostbyname()> is called in SCALAR context and that
d760c846
GS
2726its return value is checked for definedness.
2727
0d043efa
FC
2728The C<getprotobynumber> function, even though it only takes one argument,
2729has the precedence of a list operator, so beware:
2730
2731 getprotobynumber $number eq 'icmp' # WRONG
2732 getprotobynumber($number eq 'icmp') # actually means this
2733 getprotobynumber($number) eq 'icmp' # better this way
2734
19799a22
GS
2735If you get tired of remembering which element of the return list
2736contains which return value, by-name interfaces are provided
2737in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2738C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2739and C<User::grent>. These override the normal built-ins, supplying
2740versions that return objects with the appropriate names
2741for each field. For example:
5a964f20
TC
2742
2743 use File::stat;
2744 use User::pwent;
2745 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2746
a3390c9f 2747Even though it looks as though they're the same method calls (uid),
b76cc8ba 2748they aren't, because a C<File::stat> object is different from
19799a22 2749a C<User::pwent> object.
5a964f20 2750
ea9eb35a 2751Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2752
a0d0e21e 2753=item getsockname SOCKET
d74e8afc 2754X<getsockname>
a0d0e21e 2755
c17cdb72
NC
2756=for Pod::Functions retrieve the sockaddr for a given socket
2757
19799a22
GS
2758Returns the packed sockaddr address of this end of the SOCKET connection,
2759in case you don't know the address because you have several different
2760IPs that the connection might have come in on.
a0d0e21e 2761
4633a7c4
LW
2762 use Socket;
2763 $mysockaddr = getsockname(SOCK);
19799a22 2764 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2765 printf "Connect to %s [%s]\n",
19799a22
GS
2766 scalar gethostbyaddr($myaddr, AF_INET),
2767 inet_ntoa($myaddr);
a0d0e21e
LW
2768
2769=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2770X<getsockopt>
a0d0e21e 2771
c17cdb72
NC
2772=for Pod::Functions get socket options on a given socket
2773
636e6b1f
TH
2774Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2775Options may exist at multiple protocol levels depending on the socket
2776type, but at least the uppermost socket level SOL_SOCKET (defined in the
391b733c 2777C<Socket> module) will exist. To query options at another level the
636e6b1f 2778protocol number of the appropriate protocol controlling the option
391b733c 2779should be supplied. For example, to indicate that an option is to be
636e6b1f 2780interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2781number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2782
80d38338 2783The function returns a packed string representing the requested socket
3b10bc60 2784option, or C<undef> on error, with the reason for the error placed in
391b733c 2785C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2786consult getsockopt(2) for details. A common case is that the option is an
2787integer, in which case the result is a packed integer, which you can decode
2788using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2789
8f1da26d 2790Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2791
4852725b 2792 use Socket qw(:all);
636e6b1f
TH
2793
2794 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2795 or die "Could not determine the protocol number for tcp";
4852725b
DD
2796 # my $tcp = IPPROTO_TCP; # Alternative
2797 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2798 or die "getsockopt TCP_NODELAY: $!";
636e6b1f 2799 my $nodelay = unpack("I", $packed);
f7051f2c
FC
2800 print "Nagle's algorithm is turned ",
2801 $nodelay ? "off\n" : "on\n";
636e6b1f 2802
ea9eb35a 2803Portability issues: L<perlport/getsockopt>.
a0d0e21e
LW
2804
2805=item glob EXPR
d74e8afc 2806X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2807
0a753a76
PP
2808=item glob
2809
c17cdb72
NC
2810=for Pod::Functions expand filenames using wildcards
2811
d9a9d457 2812In list context, returns a (possibly empty) list of filename expansions on
391b733c 2813the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
d9a9d457 2814scalar context, glob iterates through such filename expansions, returning
391b733c
FC
2815undef when the list is exhausted. This is the internal function
2816implementing the C<< <*.c> >> operator, but you can use it directly. If
d9a9d457
JL
2817EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2818more detail in L<perlop/"I/O Operators">.
a0d0e21e 2819
80d38338
TC
2820Note that C<glob> splits its arguments on whitespace and treats
2821each segment as separate pattern. As such, C<glob("*.c *.h")>
2822matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2823C<glob(".* *")> matches all files in the current working directory.
a91bb7b1
TC
2824If you want to glob filenames that might contain whitespace, you'll
2825have to use extra quotes around the spacey filename to protect it.
2826For example, to glob filenames that have an C<e> followed by a space
2827followed by an C<f>, use either of:
2828
2829 @spacies = <"*e f*">;
2830 @spacies = glob '"*e f*"';
2831 @spacies = glob q("*e f*");
2832
2833If you had to get a variable through, you could do this:
2834
2835 @spacies = glob "'*${var}e f*'";
2836 @spacies = glob qq("*${var}e f*");
80d38338
TC
2837
2838If non-empty braces are the only wildcard characters used in the
2839C<glob>, no filenames are matched, but potentially many strings
2840are returned. For example, this produces nine strings, one for
2841each pairing of fruits and colors:
2842
2843 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2844
3a4b19e4 2845Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2846C<File::Glob> extension. See L<File::Glob> for details, including
2847C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2848
ea9eb35a 2849Portability issues: L<perlport/glob>.
2850
a0d0e21e 2851=item gmtime EXPR
d74e8afc 2852X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2853
ce2984c3
PF
2854=item gmtime
2855
c17cdb72
NC
2856=for Pod::Functions convert UNIX time into record or string using Greenwich time
2857
4509d391 2858Works just like L</localtime> but the returned values are
435fbc73 2859localized for the standard Greenwich time zone.
a0d0e21e 2860
a3390c9f
FC
2861Note: When called in list context, $isdst, the last value
2862returned by gmtime, is always C<0>. There is no
435fbc73 2863Daylight Saving Time in GMT.
0a753a76 2864
ea9eb35a 2865Portability issues: L<perlport/gmtime>.
62aa5637 2866
a0d0e21e 2867=item goto LABEL
d74e8afc 2868X<goto> X<jump> X<jmp>
a0d0e21e 2869
748a9306
LW
2870=item goto EXPR
2871
a0d0e21e
LW
2872=item goto &NAME
2873
c17cdb72
NC
2874=for Pod::Functions create spaghetti code
2875
b500e03b 2876The C<goto-LABEL> form finds the statement labeled with LABEL and
391b733c 2877resumes execution there. It can't be used to get out of a block or
b500e03b
GG
2878subroutine given to C<sort>. It can be used to go almost anywhere
2879else within the dynamic scope, including out of subroutines, but it's
2880usually better to use some other construct such as C<last> or C<die>.
2881The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2882(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2883does not offer named loops combined with loop control. Perl does, and
2884this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2885
7660c0ab
A
2886The C<goto-EXPR> form expects a label name, whose scope will be resolved
2887dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2888necessarily recommended if you're optimizing for maintainability:
2889
2890 goto ("FOO", "BAR", "GLARCH")[$i];
2891
887d89fd 2892As shown in this example, C<goto-EXPR> is exempt from the "looks like a
391b733c
FC
2893function" rule. A pair of parentheses following it does not (necessarily)
2894delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
887d89fd 2895
b500e03b 2896Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2897deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2898go into any construct that requires initialization, such as a
2899subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2900construct that is optimized away.
b500e03b 2901
1b6921cb
BT
2902The C<goto-&NAME> form is quite different from the other forms of
2903C<goto>. In fact, it isn't a goto in the normal sense at all, and
2904doesn't have the stigma associated with other gotos. Instead, it
2905exits the current subroutine (losing any changes set by local()) and
2906immediately calls in its place the named subroutine using the current
2907value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2908load another subroutine and then pretend that the other subroutine had
2909been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2910in the current subroutine are propagated to the other subroutine.)
2911After the C<goto>, not even C<caller> will be able to tell that this
2912routine was called first.
2913
2914NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2915containing a code reference or a block that evaluates to a code
6cb9131c 2916reference.
a0d0e21e
LW
2917
2918=item grep BLOCK LIST
d74e8afc 2919X<grep>
a0d0e21e
LW
2920
2921=item grep EXPR,LIST
2922
c17cdb72
NC
2923=for Pod::Functions locate elements in a list test true against a given criterion
2924
2b5ab1e7
TC
2925This is similar in spirit to, but not the same as, grep(1) and its
2926relatives. In particular, it is not limited to using regular expressions.
2f9daede 2927
a0d0e21e 2928Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2929C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2930elements for which the expression evaluated to true. In scalar
2931context, returns the number of times the expression was true.
a0d0e21e
LW
2932
2933 @foo = grep(!/^#/, @bar); # weed out comments
2934
2935or equivalently,
2936
2937 @foo = grep {!/^#/} @bar; # weed out comments
2938
be3174d2
GS
2939Note that C<$_> is an alias to the list value, so it can be used to
2940modify the elements of the LIST. While this is useful and supported,
2941it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2942Similarly, grep returns aliases into the original list, much as a for
2943loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2944element of a list returned by grep (for example, in a C<foreach>, C<map>
2945or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2946This is usually something to be avoided when writing clear code.
a0d0e21e 2947
a4fb8298 2948If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2949been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2950the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2951can't be seen from the outside, avoiding any potential side-effects.
2952
19799a22 2953See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2954
a0d0e21e 2955=item hex EXPR
d74e8afc 2956X<hex> X<hexadecimal>
a0d0e21e 2957
54310121 2958=item hex
bbce6d69 2959
c17cdb72
NC
2960=for Pod::Functions convert a string to a hexadecimal number
2961
2b5ab1e7 2962Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2963(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2964L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2965
2966 print hex '0xAf'; # prints '175'
2967 print hex 'aF'; # same
a0d0e21e 2968
19799a22 2969Hex strings may only represent integers. Strings that would cause
53305cf1 2970integer overflow trigger a warning. Leading whitespace is not stripped,
391b733c 2971unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 2972L</sprintf>, and L</unpack>.
19799a22 2973
ce2984c3 2974=item import LIST
d74e8afc 2975X<import>
a0d0e21e 2976
c17cdb72
NC
2977=for Pod::Functions patch a module's namespace into your own
2978
19799a22 2979There is no builtin C<import> function. It is just an ordinary
4633a7c4 2980method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2981names to another module. The C<use> function calls the C<import> method
cea6626f 2982for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2983
2984=item index STR,SUBSTR,POSITION
d74e8afc 2985X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2986
2987=item index STR,SUBSTR
2988
c17cdb72
NC
2989=for Pod::Functions find a substring within a string
2990
2b5ab1e7
TC
2991The index function searches for one string within another, but without
2992the wildcard-like behavior of a full regular-expression pattern match.
2993It returns the position of the first occurrence of SUBSTR in STR at
2994or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2995beginning of the string. POSITION before the beginning of the string
2996or after its end is treated as if it were the beginning or the end,
e1dccc0d
Z
2997respectively. POSITION and the return value are based at zero.
2998If the substring is not found, C<index> returns -1.
a0d0e21e
LW
2999
3000=item int EXPR
f723aae1 3001X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 3002
54310121 3003=item int
bbce6d69 3004
c17cdb72
NC
3005=for Pod::Functions get the integer portion of a number
3006
7660c0ab 3007Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 3008You should not use this function for rounding: one because it truncates
3b10bc60 3009towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
3010numbers can sometimes produce counterintuitive results. For example,
3011C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3012because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 3013the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 3014functions will serve you better than will int().
a0d0e21e
LW
3015
3016=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 3017X<ioctl>
a0d0e21e 3018
c17cdb72
NC
3019=for Pod::Functions system-dependent device control system call
3020
2b5ab1e7 3021Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 3022
f7051f2c
FC
3023 require "sys/ioctl.ph"; # probably in
3024 # $Config{archlib}/sys/ioctl.ph
a0d0e21e 3025
a11c483f 3026to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 3027exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 3028own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 3029(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 3030may help you in this, but it's nontrivial.) SCALAR will be read and/or
3b10bc60 3031written depending on the FUNCTION; a C pointer to the string value of SCALAR
19799a22 3032will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
3033has no string value but does have a numeric value, that value will be
3034passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
3035true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
3036functions may be needed to manipulate the values of structures used by
b76cc8ba 3037C<ioctl>.
a0d0e21e 3038
19799a22 3039The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e 3040
5ed4f2ec 3041 if OS returns: then Perl returns:
3042 -1 undefined value
3043 0 string "0 but true"
3044 anything else that number
a0d0e21e 3045
19799a22 3046Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
3047still easily determine the actual value returned by the operating
3048system: