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[perl #114020, #90018, #53186] Make given alias $_
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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
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3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
8f1da26d 15operator. A unary operator generally provides scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
3b10bc60 17contexts for its arguments. If it does both, scalar arguments
18come first and list argument follow, and there can only ever
19be one such list argument. For instance, splice() has three scalar
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20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
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22
23In the syntax descriptions that follow, list operators that expect a
3b10bc60 24list (and provide list context for elements of the list) are shown
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25with LIST as an argument. Such a list may consist of any combination
26of scalar arguments or list values; the list values will be included
27in the list as if each individual element were interpolated at that
28point in the list, forming a longer single-dimensional list value.
8bdbc703 29Commas should separate literal elements of the LIST.
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30
31Any function in the list below may be used either with or without
32parentheses around its arguments. (The syntax descriptions omit the
3b10bc60 33parentheses.) If you use parentheses, the simple but occasionally
34surprising rule is this: It I<looks> like a function, therefore it I<is> a
a0d0e21e 35function, and precedence doesn't matter. Otherwise it's a list
3b10bc60 36operator or unary operator, and precedence does matter. Whitespace
37between the function and left parenthesis doesn't count, so sometimes
38you need to be careful:
a0d0e21e 39
5ed4f2ec 40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
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45
46If you run Perl with the B<-w> switch it can warn you about this. For
47example, the third line above produces:
48
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
51
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52A few functions take no arguments at all, and therefore work as neither
53unary nor list operators. These include such functions as C<time>
54and C<endpwent>. For example, C<time+86_400> always means
55C<time() + 86_400>.
56
a0d0e21e 57For functions that can be used in either a scalar or list context,
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58nonabortive failure is generally indicated in scalar context by
59returning the undefined value, and in list context by returning the
3b10bc60 60empty list.
a0d0e21e 61
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62Remember the following important rule: There is B<no rule> that relates
63the behavior of an expression in list context to its behavior in scalar
64context, or vice versa. It might do two totally different things.
80d38338 65Each operator and function decides which sort of value would be most
2b5ab1e7 66appropriate to return in scalar context. Some operators return the
5a964f20 67length of the list that would have been returned in list context. Some
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68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
d74e8afc 72X<context>
a0d0e21e 73
d1be9408 74A named array in scalar context is quite different from what would at
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75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
3b10bc60 81In general, functions in Perl that serve as wrappers for system calls ("syscalls")
5dac7880 82of the same name (like chown(2), fork(2), closedir(2), etc.) return
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83true when they succeed and C<undef> otherwise, as is usually mentioned
84in the descriptions below. This is different from the C interfaces,
5dac7880 85which return C<-1> on failure. Exceptions to this rule include C<wait>,
19799a22 86C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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87variable on failure. Other functions do not, except accidentally.
88
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89Extension modules can also hook into the Perl parser to define new
90kinds of keyword-headed expression. These may look like functions, but
91may also look completely different. The syntax following the keyword
92is defined entirely by the extension. If you are an implementor, see
93L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
94a module, see the module's documentation for details of the syntax that
95it defines.
96
cb1a09d0 97=head2 Perl Functions by Category
d74e8afc 98X<function>
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99
100Here are Perl's functions (including things that look like
5a964f20 101functions, like some keywords and named operators)
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102arranged by category. Some functions appear in more
103than one place.
104
13a2d996 105=over 4
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106
107=item Functions for SCALARs or strings
d74e8afc 108X<scalar> X<string> X<character>
cb1a09d0 109
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110=for Pod::Functions =String
111
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112C<chomp>, C<chop>, C<chr>, C<crypt>, C<fc>, C<hex>, C<index>, C<lc>,
113C<lcfirst>, C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
945c54fd 114C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
cb1a09d0 115
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116C<fc> is available only if the C<"fc"> feature is enabled or if it is
117prefixed with C<CORE::>. The C<"fc"> feature is enabled automatically
3dd9a840 118with a C<use v5.16> (or higher) declaration in the current scope.
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119
120
cb1a09d0 121=item Regular expressions and pattern matching
d74e8afc 122X<regular expression> X<regex> X<regexp>
cb1a09d0 123
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124=for Pod::Functions =Regexp
125
f5fa2679 126C<m//>, C<pos>, C<qr//>, C<quotemeta>, C<s///>, C<split>, C<study>
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127
128=item Numeric functions
d74e8afc 129X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 130
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131=for Pod::Functions =Math
132
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133C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
134C<sin>, C<sqrt>, C<srand>
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135
136=item Functions for real @ARRAYs
d74e8afc 137X<array>
cb1a09d0 138
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139=for Pod::Functions =ARRAY
140
a5ce339c 141C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
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142
143=item Functions for list data
d74e8afc 144X<list>
cb1a09d0 145
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146=for Pod::Functions =LIST
147
1dc8ecb8 148C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
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149
150=item Functions for real %HASHes
d74e8afc 151X<hash>
cb1a09d0 152
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153=for Pod::Functions =HASH
154
22fae026 155C<delete>, C<each>, C<exists>, C<keys>, C<values>
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156
157=item Input and output functions
d74e8afc 158X<I/O> X<input> X<output> X<dbm>
cb1a09d0 159
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160=for Pod::Functions =I/O
161
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162C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
163C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
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164C<readdir>, C<readline> C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>,
165C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>,
166C<truncate>, C<warn>, C<write>
cb1a09d0 167
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168C<say> is available only if the C<"say"> feature is enabled or if it is
169prefixed with C<CORE::>. The C<"say"> feature is enabled automatically
170with a C<use v5.10> (or higher) declaration in the current scope.
171
5dac7880 172=item Functions for fixed-length data or records
cb1a09d0 173
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174=for Pod::Functions =Binary
175
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176C<pack>, C<read>, C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<unpack>,
177C<vec>
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178
179=item Functions for filehandles, files, or directories
d74e8afc 180X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 181
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182=for Pod::Functions =File
183
22fae026 184C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 185C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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186C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
187C<umask>, C<unlink>, C<utime>
cb1a09d0 188
cf264981 189=item Keywords related to the control flow of your Perl program
d74e8afc 190X<control flow>
cb1a09d0 191
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192=for Pod::Functions =Flow
193
dba7b065 194C<break>, C<caller>, C<continue>, C<die>, C<do>,
7289c5e6 195C<dump>, C<eval>, C<evalbytes> C<exit>,
cfa52385 196C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
17d15541 197C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray>
84ed0108 198
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199C<break> is available only if you enable the experimental C<"switch">
200feature or use the C<CORE::> prefix. The C<"switch"> feature also enables
201the C<default>, C<given> and C<when> statements, which are documented in
202L<perlsyn/"Switch Statements">. The C<"switch"> feature is enabled
203automatically with a C<use v5.10> (or higher) declaration in the current
204scope. In Perl v5.14 and earlier, C<continue> required the C<"switch">
205feature, like the other keywords.
206
e3f68f70 207C<evalbytes> is only available with the C<"evalbytes"> feature (see
4fe70ef9 208L<feature>) or if prefixed with C<CORE::>. C<__SUB__> is only available
e3f68f70 209with the C<"current_sub"> feature or if prefixed with C<CORE::>. Both
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210the C<"evalbytes"> and C<"current_sub"> features are enabled automatically
211with a C<use v5.16> (or higher) declaration in the current scope.
cb1a09d0 212
54310121 213=item Keywords related to scoping
cb1a09d0 214
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215=for Pod::Functions =Namespace
216
8f1da26d 217C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
36fb85f3 218
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219C<state> is available only if the C<"state"> feature is enabled or if it is
220prefixed with C<CORE::>. The C<"state"> feature is enabled automatically
221with a C<use v5.10> (or higher) declaration in the current scope.
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222
223=item Miscellaneous functions
224
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225=for Pod::Functions =Misc
226
17d15541 227C<defined>, C<formline>, C<lock>, C<prototype>, C<reset>, C<scalar>, C<undef>
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228
229=item Functions for processes and process groups
d74e8afc 230X<process> X<pid> X<process id>
cb1a09d0 231
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232=for Pod::Functions =Process
233
22fae026 234C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
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235C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>,
236C<setpriority>, C<sleep>, C<system>,
22fae026 237C<times>, C<wait>, C<waitpid>
cb1a09d0 238
3b10bc60 239=item Keywords related to Perl modules
d74e8afc 240X<module>
cb1a09d0 241
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242=for Pod::Functions =Modules
243
22fae026 244C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 245
353c6505 246=item Keywords related to classes and object-orientation
d74e8afc 247X<object> X<class> X<package>
cb1a09d0 248
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249=for Pod::Functions =Objects
250
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251C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
252C<untie>, C<use>
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253
254=item Low-level socket functions
d74e8afc 255X<socket> X<sock>
cb1a09d0 256
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257=for Pod::Functions =Socket
258
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259C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
260C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 261C<socket>, C<socketpair>
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262
263=item System V interprocess communication functions
d74e8afc 264X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 265
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266=for Pod::Functions =SysV
267
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268C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
269C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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270
271=item Fetching user and group info
d74e8afc 272X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 273
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274=for Pod::Functions =User
275
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276C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
277C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
278C<getpwuid>, C<setgrent>, C<setpwent>
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279
280=item Fetching network info
d74e8afc 281X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 282
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283=for Pod::Functions =Network
284
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285C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
286C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
287C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
288C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
289C<setnetent>, C<setprotoent>, C<setservent>
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290
291=item Time-related functions
d74e8afc 292X<time> X<date>
cb1a09d0 293
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294=for Pod::Functions =Time
295
22fae026 296C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 297
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298=item Non-function keywords
299
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300=for Pod::Functions =!Non-functions
301
f5fa2679 302C<and>, C<AUTOLOAD>, C<BEGIN>, C<CHECK>, C<cmp>, C<CORE>, C<__DATA__>,
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303C<default>, C<DESTROY>, C<else>, C<elseif>, C<elsif>, C<END>, C<__END__>,
304C<eq>, C<for>, C<foreach>, C<ge>, C<given>, C<gt>, C<if>, C<INIT>, C<le>,
305C<lt>, C<ne>, C<not>, C<or>, C<UNITCHECK>, C<unless>, C<until>, C<when>,
306C<while>, C<x>, C<xor>
8f0d6a61 307
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308=back
309
60f9f73c 310=head2 Portability
d74e8afc 311X<portability> X<Unix> X<portable>
60f9f73c 312
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313Perl was born in Unix and can therefore access all common Unix
314system calls. In non-Unix environments, the functionality of some
8f1da26d 315Unix system calls may not be available or details of the available
2b5ab1e7 316functionality may differ slightly. The Perl functions affected
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317by this are:
318
319C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
320C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
321C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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322C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
323C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 324C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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325C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
326C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
327C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 328C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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329C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
330C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
331C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 332C<shmwrite>, C<socket>, C<socketpair>,
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333C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
334C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 335C<utime>, C<wait>, C<waitpid>
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336
337For more information about the portability of these functions, see
338L<perlport> and other available platform-specific documentation.
339
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340=head2 Alphabetical Listing of Perl Functions
341
3b10bc60 342=over
a0d0e21e 343
5b3c99c0 344=item -X FILEHANDLE
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345X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
346X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 347
5b3c99c0 348=item -X EXPR
a0d0e21e 349
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350=item -X DIRHANDLE
351
5b3c99c0 352=item -X
a0d0e21e 353
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354=for Pod::Functions a file test (-r, -x, etc)
355
a0d0e21e 356A file test, where X is one of the letters listed below. This unary
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357operator takes one argument, either a filename, a filehandle, or a dirhandle,
358and tests the associated file to see if something is true about it. If the
7660c0ab 359argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 360Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 361the undefined value if the file doesn't exist. Despite the funny
d0821a6a 362names, precedence is the same as any other named unary operator. The
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363operator may be any of:
364
5ed4f2ec 365 -r File is readable by effective uid/gid.
366 -w File is writable by effective uid/gid.
367 -x File is executable by effective uid/gid.
368 -o File is owned by effective uid.
a0d0e21e 369
5ed4f2ec 370 -R File is readable by real uid/gid.
371 -W File is writable by real uid/gid.
372 -X File is executable by real uid/gid.
373 -O File is owned by real uid.
a0d0e21e 374
5ed4f2ec 375 -e File exists.
376 -z File has zero size (is empty).
377 -s File has nonzero size (returns size in bytes).
a0d0e21e 378
5ed4f2ec 379 -f File is a plain file.
380 -d File is a directory.
381 -l File is a symbolic link.
382 -p File is a named pipe (FIFO), or Filehandle is a pipe.
383 -S File is a socket.
384 -b File is a block special file.
385 -c File is a character special file.
386 -t Filehandle is opened to a tty.
a0d0e21e 387
5ed4f2ec 388 -u File has setuid bit set.
389 -g File has setgid bit set.
390 -k File has sticky bit set.
a0d0e21e 391
5ed4f2ec 392 -T File is an ASCII text file (heuristic guess).
393 -B File is a "binary" file (opposite of -T).
a0d0e21e 394
5ed4f2ec 395 -M Script start time minus file modification time, in days.
396 -A Same for access time.
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397 -C Same for inode change time (Unix, may differ for other
398 platforms)
a0d0e21e 399
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400Example:
401
402 while (<>) {
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403 chomp;
404 next unless -f $_; # ignore specials
405 #...
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406 }
407
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408Note that C<-s/a/b/> does not do a negated substitution. Saying
409C<-exp($foo)> still works as expected, however: only single letters
410following a minus are interpreted as file tests.
411
412These operators are exempt from the "looks like a function rule" described
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413above. That is, an opening parenthesis after the operator does not affect
414how much of the following code constitutes the argument. Put the opening
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415parentheses before the operator to separate it from code that follows (this
416applies only to operators with higher precedence than unary operators, of
417course):
418
419 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
420 (-s $file) + 1024 # correct
421
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422The interpretation of the file permission operators C<-r>, C<-R>,
423C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
424of the file and the uids and gids of the user. There may be other
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425reasons you can't actually read, write, or execute the file: for
426example network filesystem access controls, ACLs (access control lists),
427read-only filesystems, and unrecognized executable formats. Note
428that the use of these six specific operators to verify if some operation
429is possible is usually a mistake, because it may be open to race
430conditions.
5ff3f7a4 431
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432Also note that, for the superuser on the local filesystems, the C<-r>,
433C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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434if any execute bit is set in the mode. Scripts run by the superuser
435may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 436or temporarily set their effective uid to something else.
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437
438If you are using ACLs, there is a pragma called C<filetest> that may
439produce more accurate results than the bare stat() mode bits.
5dac7880
FC
440When under C<use filetest 'access'> the above-mentioned filetests
441test whether the permission can(not) be granted using the
3b10bc60 442access(2) family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
443under this pragma return true even if there are no execute permission
444bits set (nor any extra execute permission ACLs). This strangeness is
391b733c 445due to the underlying system calls' definitions. Note also that, due to
ecae030f
MO
446the implementation of C<use filetest 'access'>, the C<_> special
447filehandle won't cache the results of the file tests when this pragma is
448in effect. Read the documentation for the C<filetest> pragma for more
449information.
5ff3f7a4 450
a0d0e21e
LW
451The C<-T> and C<-B> switches work as follows. The first block or so of the
452file is examined for odd characters such as strange control codes or
61eff3bc 453characters with the high bit set. If too many strange characters (>30%)
cf264981 454are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 455containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 456or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 457rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 458file, or a file at EOF when testing a filehandle. Because you have to
4633a7c4
LW
459read a file to do the C<-T> test, on most occasions you want to use a C<-f>
460against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 461
5dac7880 462If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 463the special filehandle consisting of a solitary underline, then the stat
a0d0e21e
LW
464structure of the previous file test (or stat operator) is used, saving
465a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 466that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 467symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 468an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 469Example:
a0d0e21e
LW
470
471 print "Can do.\n" if -r $a || -w _ || -x _;
472
473 stat($filename);
474 print "Readable\n" if -r _;
475 print "Writable\n" if -w _;
476 print "Executable\n" if -x _;
477 print "Setuid\n" if -u _;
478 print "Setgid\n" if -g _;
479 print "Sticky\n" if -k _;
480 print "Text\n" if -T _;
481 print "Binary\n" if -B _;
482
e9fa405d 483As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file
fbb0b3b3 484test operators, in a way that C<-f -w -x $file> is equivalent to
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
585d73c3 766compiled with. C<$hints> corresponds to C<$^H>, and C<$bitmask>
1adb05cd
FC
767corresponds to C<${^WARNING_BITS}>. The
768C<$hints> and C<$bitmask> values are subject
585d73c3 769to change between versions of Perl, and are not meant for external use.
748a9306 770
b3ca2e83 771C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
391b733c 772caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
b3ca2e83
NC
773of this hash, as they are the actual values stored in the optree.
774
ffe0c19d
FC
775Furthermore, when called from within the DB package in
776list context, and with an argument, caller returns more
7660c0ab 777detailed information: it sets the list variable C<@DB::args> to be the
54310121 778arguments with which the subroutine was invoked.
748a9306 779
7660c0ab 780Be aware that the optimizer might have optimized call frames away before
19799a22 781C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 782might not return information about the call frame you expect it to, for
b76cc8ba 783C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 784previous time C<caller> was called.
7660c0ab 785
8f1da26d 786Be aware that setting C<@DB::args> is I<best effort>, intended for
391b733c 787debugging or generating backtraces, and should not be relied upon. In
ca9f0cb5
NC
788particular, as C<@_> contains aliases to the caller's arguments, Perl does
789not take a copy of C<@_>, so C<@DB::args> will contain modifications the
790subroutine makes to C<@_> or its contents, not the original values at call
391b733c 791time. C<@DB::args>, like C<@_>, does not hold explicit references to its
ca9f0cb5 792elements, so under certain cases its elements may have become freed and
391b733c 793reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 794of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
795I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
796reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5 797elements), so C<@DB::args> is actually a hybrid of the current state and
391b733c 798initial state of C<@_>. Buyer beware.
ca9f0cb5 799
a0d0e21e 800=item chdir EXPR
d74e8afc
ITB
801X<chdir>
802X<cd>
f723aae1 803X<directory, change>
a0d0e21e 804
c4aca7d0
GA
805=item chdir FILEHANDLE
806
807=item chdir DIRHANDLE
808
ce2984c3
PF
809=item chdir
810
c17cdb72
NC
811=for Pod::Functions change your current working directory
812
391b733c 813Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 814changes to the directory specified by C<$ENV{HOME}>, if set; if not,
391b733c
FC
815changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
816variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
817neither is set, C<chdir> does nothing. It returns true on success,
818false otherwise. See the example under C<die>.
a0d0e21e 819
3b10bc60 820On systems that support fchdir(2), you may pass a filehandle or
34169887 821directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 822passing handles raises an exception.
c4aca7d0 823
a0d0e21e 824=item chmod LIST
d74e8afc 825X<chmod> X<permission> X<mode>
a0d0e21e 826
c17cdb72
NC
827=for Pod::Functions changes the permissions on a list of files
828
a0d0e21e 829Changes the permissions of a list of files. The first element of the
8f1da26d 830list must be the numeric mode, which should probably be an octal
4ad40acf 831number, and which definitely should I<not> be a string of octal digits:
3b10bc60 832C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 833successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 834
3b10bc60 835 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 836 chmod 0755, @executables;
3b10bc60 837 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 838 # --w----r-T
3b10bc60 839 $mode = "0644"; chmod oct($mode), "foo"; # this is better
840 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 841
3b10bc60 842On systems that support fchmod(2), you may pass filehandles among the
843files. On systems that don't support fchmod(2), passing filehandles raises
844an exception. Filehandles must be passed as globs or glob references to be
845recognized; barewords are considered filenames.
c4aca7d0
GA
846
847 open(my $fh, "<", "foo");
848 my $perm = (stat $fh)[2] & 07777;
849 chmod($perm | 0600, $fh);
850
3b10bc60 851You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
852module:
853
3b10bc60 854 use Fcntl qw( :mode );
ca6e1c26 855 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 856 # Identical to the chmod 0755 of the example above.
ca6e1c26 857
ea9eb35a 858Portability issues: L<perlport/chmod>.
859
a0d0e21e 860=item chomp VARIABLE
d74e8afc 861X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 862
313c9f5c 863=item chomp( LIST )
a0d0e21e
LW
864
865=item chomp
866
c17cdb72
NC
867=for Pod::Functions remove a trailing record separator from a string
868
2b5ab1e7
TC
869This safer version of L</chop> removes any trailing string
870that corresponds to the current value of C<$/> (also known as
28757baa
PP
871$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
872number of characters removed from all its arguments. It's often used to
873remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
874that the final record may be missing its newline. When in paragraph
875mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 876When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 877a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 878remove anything.
19799a22 879If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
880
881 while (<>) {
a9a5a0dc
VP
882 chomp; # avoid \n on last field
883 @array = split(/:/);
884 # ...
a0d0e21e
LW
885 }
886
4bf21a6d
RD
887If VARIABLE is a hash, it chomps the hash's values, but not its keys.
888
a0d0e21e
LW
889You can actually chomp anything that's an lvalue, including an assignment:
890
891 chomp($cwd = `pwd`);
892 chomp($answer = <STDIN>);
893
894If you chomp a list, each element is chomped, and the total number of
895characters removed is returned.
896
15e44fd8
RGS
897Note that parentheses are necessary when you're chomping anything
898that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
899is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
900C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
901C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
902as C<chomp($a, $b)>.
903
a0d0e21e 904=item chop VARIABLE
d74e8afc 905X<chop>
a0d0e21e 906
313c9f5c 907=item chop( LIST )
a0d0e21e
LW
908
909=item chop
910
c17cdb72
NC
911=for Pod::Functions remove the last character from a string
912
a0d0e21e 913Chops off the last character of a string and returns the character
5b3eff12 914chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 915scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
916If VARIABLE is a hash, it chops the hash's values, but not its keys.
917
5b3eff12 918You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
919
920If you chop a list, each element is chopped. Only the value of the
19799a22 921last C<chop> is returned.
a0d0e21e 922
19799a22 923Note that C<chop> returns the last character. To return all but the last
748a9306
LW
924character, use C<substr($string, 0, -1)>.
925
15e44fd8
RGS
926See also L</chomp>.
927
a0d0e21e 928=item chown LIST
d74e8afc 929X<chown> X<owner> X<user> X<group>
a0d0e21e 930
c17cdb72
NC
931=for Pod::Functions change the ownership on a list of files
932
a0d0e21e 933Changes the owner (and group) of a list of files. The first two
19799a22
GS
934elements of the list must be the I<numeric> uid and gid, in that
935order. A value of -1 in either position is interpreted by most
936systems to leave that value unchanged. Returns the number of files
937successfully changed.
a0d0e21e
LW
938
939 $cnt = chown $uid, $gid, 'foo', 'bar';
940 chown $uid, $gid, @filenames;
941
3b10bc60 942On systems that support fchown(2), you may pass filehandles among the
943files. On systems that don't support fchown(2), passing filehandles raises
944an exception. Filehandles must be passed as globs or glob references to be
945recognized; barewords are considered filenames.
c4aca7d0 946
54310121 947Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
948
949 print "User: ";
19799a22 950 chomp($user = <STDIN>);
5a964f20 951 print "Files: ";
19799a22 952 chomp($pattern = <STDIN>);
a0d0e21e
LW
953
954 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 955 or die "$user not in passwd file";
a0d0e21e 956
5ed4f2ec 957 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
958 chown $uid, $gid, @ary;
959
54310121 960On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
961file unless you're the superuser, although you should be able to change
962the group to any of your secondary groups. On insecure systems, these
963restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
964On POSIX systems, you can detect this condition this way:
965
966 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
967 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 968
ea9eb35a 969Portability issues: L<perlport/chmod>.
970
a0d0e21e 971=item chr NUMBER
d74e8afc 972X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 973
54310121 974=item chr
bbce6d69 975
c17cdb72
NC
976=for Pod::Functions get character this number represents
977
a0d0e21e 978Returns the character represented by that NUMBER in the character set.
a0ed51b3 979For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 980chr(0x263a) is a Unicode smiley face.
aaa68c4a 981
8a064bd6 982Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 983except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
984(truncated to an integer) are used.
985
974da8e5
JH
986If NUMBER is omitted, uses C<$_>.
987
b76cc8ba 988For the reverse, use L</ord>.
a0d0e21e 989
2575c402
JW
990Note that characters from 128 to 255 (inclusive) are by default
991internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 992
2575c402 993See L<perlunicode> for more about Unicode.
bbce6d69 994
a0d0e21e 995=item chroot FILENAME
d74e8afc 996X<chroot> X<root>
a0d0e21e 997
54310121 998=item chroot
bbce6d69 999
c17cdb72
NC
1000=for Pod::Functions make directory new root for path lookups
1001
5a964f20 1002This function works like the system call by the same name: it makes the
4633a7c4 1003named directory the new root directory for all further pathnames that
951ba7fe 1004begin with a C</> by your process and all its children. (It doesn't
28757baa 1005change your current working directory, which is unaffected.) For security
4633a7c4 1006reasons, this call is restricted to the superuser. If FILENAME is
19799a22 1007omitted, does a C<chroot> to C<$_>.
a0d0e21e 1008
ea9eb35a 1009Portability issues: L<perlport/chroot>.
1010
a0d0e21e 1011=item close FILEHANDLE
d74e8afc 1012X<close>
a0d0e21e 1013
6a518fbc
TP
1014=item close
1015
c17cdb72
NC
1016=for Pod::Functions close file (or pipe or socket) handle
1017
3b10bc60 1018Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 1019buffers, and closes the system file descriptor. Returns true if those
8f1da26d 1020operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
1021layer. Closes the currently selected filehandle if the argument is
1022omitted.
fb73857a
PP
1023
1024You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 1025another C<open> on it, because C<open> closes it for you. (See
01aa884e 1026L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
19799a22 1027counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 1028
3b10bc60 1029If the filehandle came from a piped open, C<close> returns false if one of
1030the other syscalls involved fails or if its program exits with non-zero
1031status. If the only problem was that the program exited non-zero, C<$!>
1032will be set to C<0>. Closing a pipe also waits for the process executing
1033on the pipe to exit--in case you wish to look at the output of the pipe
1034afterwards--and implicitly puts the exit status value of that command into
1035C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 1036
2e0cfa16
FC
1037If there are multiple threads running, C<close> on a filehandle from a
1038piped open returns true without waiting for the child process to terminate,
1039if the filehandle is still open in another thread.
1040
80d38338
TC
1041Closing the read end of a pipe before the process writing to it at the
1042other end is done writing results in the writer receiving a SIGPIPE. If
1043the other end can't handle that, be sure to read all the data before
1044closing the pipe.
73689b13 1045
fb73857a 1046Example:
a0d0e21e 1047
fb73857a
PP
1048 open(OUTPUT, '|sort >foo') # pipe to sort
1049 or die "Can't start sort: $!";
5ed4f2ec 1050 #... # print stuff to output
1051 close OUTPUT # wait for sort to finish
fb73857a
PP
1052 or warn $! ? "Error closing sort pipe: $!"
1053 : "Exit status $? from sort";
5ed4f2ec 1054 open(INPUT, 'foo') # get sort's results
fb73857a 1055 or die "Can't open 'foo' for input: $!";
a0d0e21e 1056
5a964f20 1057FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 1058filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
1059
1060=item closedir DIRHANDLE
d74e8afc 1061X<closedir>
a0d0e21e 1062
c17cdb72
NC
1063=for Pod::Functions close directory handle
1064
19799a22 1065Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
1066system call.
1067
a0d0e21e 1068=item connect SOCKET,NAME
d74e8afc 1069X<connect>
a0d0e21e 1070
c17cdb72
NC
1071=for Pod::Functions connect to a remote socket
1072
80d38338
TC
1073Attempts to connect to a remote socket, just like connect(2).
1074Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
1075packed address of the appropriate type for the socket. See the examples in
1076L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 1077
cb1a09d0 1078=item continue BLOCK
d74e8afc 1079X<continue>
cb1a09d0 1080
0d863452
RH
1081=item continue
1082
c17cdb72
NC
1083=for Pod::Functions optional trailing block in a while or foreach
1084
4a904372
FC
1085When followed by a BLOCK, C<continue> is actually a
1086flow control statement rather than a function. If
cf264981 1087there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
1088C<foreach>), it is always executed just before the conditional is about to
1089be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
1090it can be used to increment a loop variable, even when the loop has been
1091continued via the C<next> statement (which is similar to the C C<continue>
1092statement).
1093
98293880 1094C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 1095block; C<last> and C<redo> behave as if they had been executed within
19799a22 1096the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
1097block, it may be more entertaining.
1098
1099 while (EXPR) {
a9a5a0dc
VP
1100 ### redo always comes here
1101 do_something;
1d2dff63 1102 } continue {
a9a5a0dc
VP
1103 ### next always comes here
1104 do_something_else;
1105 # then back the top to re-check EXPR
1d2dff63
GS
1106 }
1107 ### last always comes here
1108
3b10bc60 1109Omitting the C<continue> section is equivalent to using an
1110empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1111to check the condition at the top of the loop.
1112
4a904372 1113When there is no BLOCK, C<continue> is a function that
8f1da26d
TC
1114falls through the current C<when> or C<default> block instead of iterating
1115a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
4a904372
FC
1116In Perl 5.14 and earlier, this form of C<continue> was
1117only available when the C<"switch"> feature was enabled.
48238296 1118See L<feature> and L<perlsyn/"Switch Statements"> for more
8f1da26d 1119information.
0d863452 1120
a0d0e21e 1121=item cos EXPR
d74e8afc 1122X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1123
d6217f1e
GS
1124=item cos
1125
c17cdb72
NC
1126=for Pod::Functions cosine function
1127
5a964f20 1128Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1129takes the cosine of C<$_>.
a0d0e21e 1130
ca6e1c26 1131For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1132function, or use this relation:
1133
1134 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1135
a0d0e21e 1136=item crypt PLAINTEXT,SALT
d74e8afc 1137X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1138X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1139
c17cdb72
NC
1140=for Pod::Functions one-way passwd-style encryption
1141
ef2e6798
MS
1142Creates a digest string exactly like the crypt(3) function in the C
1143library (assuming that you actually have a version there that has not
bb23f8d1 1144been extirpated as a potential munition).
ef2e6798 1145
34169887 1146crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1147into a short string, called a digest, which is returned. The same
1148PLAINTEXT and SALT will always return the same string, but there is no
1149(known) way to get the original PLAINTEXT from the hash. Small
1150changes in the PLAINTEXT or SALT will result in large changes in the
1151digest.
1152
1153There is no decrypt function. This function isn't all that useful for
1154cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1155mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1156primarily used to check if two pieces of text are the same without
1157having to transmit or store the text itself. An example is checking
1158if a correct password is given. The digest of the password is stored,
cf264981 1159not the password itself. The user types in a password that is
ef2e6798 1160crypt()'d with the same salt as the stored digest. If the two digests
34169887 1161match, the password is correct.
ef2e6798
MS
1162
1163When verifying an existing digest string you should use the digest as
1164the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1165to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1166crypt() will hash the new string with the same salt as the digest.
1167This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1168with more exotic implementations. In other words, assume
1169nothing about the returned string itself nor about how many bytes
1170of SALT may matter.
85c16d83
JH
1171
1172Traditionally the result is a string of 13 bytes: two first bytes of
1173the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
391b733c 1174the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1175hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1176and implementations on non-Unix platforms may produce different
ef2e6798 1177strings.
85c16d83
JH
1178
1179When choosing a new salt create a random two character string whose
1180characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1181'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1182characters is just a recommendation; the characters allowed in
1183the salt depend solely on your system's crypt library, and Perl can't
1184restrict what salts C<crypt()> accepts.
e71965be 1185
a0d0e21e 1186Here's an example that makes sure that whoever runs this program knows
cf264981 1187their password:
a0d0e21e
LW
1188
1189 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1190
1191 system "stty -echo";
1192 print "Password: ";
e71965be 1193 chomp($word = <STDIN>);
a0d0e21e
LW
1194 print "\n";
1195 system "stty echo";
1196
e71965be 1197 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1198 die "Sorry...\n";
a0d0e21e 1199 } else {
a9a5a0dc 1200 print "ok\n";
54310121 1201 }
a0d0e21e 1202
9f8f0c9d 1203Of course, typing in your own password to whoever asks you
748a9306 1204for it is unwise.
a0d0e21e 1205
ef2e6798 1206The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1207of data, not least of all because you can't get the information
ef2e6798 1208back. Look at the L<Digest> module for more robust algorithms.
19799a22 1209
f2791508
JH
1210If using crypt() on a Unicode string (which I<potentially> has
1211characters with codepoints above 255), Perl tries to make sense
34169887 1212of the situation by trying to downgrade (a copy of)
f2791508
JH
1213the string back to an eight-bit byte string before calling crypt()
1214(on that copy). If that works, good. If not, crypt() dies with
1215C<Wide character in crypt>.
85c16d83 1216
ea9eb35a 1217Portability issues: L<perlport/crypt>.
1218
aa689395 1219=item dbmclose HASH
d74e8afc 1220X<dbmclose>
a0d0e21e 1221
c17cdb72
NC
1222=for Pod::Functions breaks binding on a tied dbm file
1223
19799a22 1224[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1225
aa689395 1226Breaks the binding between a DBM file and a hash.
a0d0e21e 1227
ea9eb35a 1228Portability issues: L<perlport/dbmclose>.
1229
19799a22 1230=item dbmopen HASH,DBNAME,MASK
d74e8afc 1231X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1232
c17cdb72
NC
1233=for Pod::Functions create binding on a tied dbm file
1234
01aa884e
KW
1235[This function has been largely superseded by the
1236L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
a0d0e21e 1237
7b8d334a 1238This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1239hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1240argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1241is the name of the database (without the F<.dir> or F<.pag> extension if
1242any). If the database does not exist, it is created with protection
1b3a6178
FC
1243specified by MASK (as modified by the C<umask>). To prevent creation of
1244the database if it doesn't exist, you may specify a MODE
1245of 0, and the function will return a false value if it
1246can't find an existing database. If your system supports
80d38338 1247only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1248program. In older versions of Perl, if your system had neither DBM nor
19799a22 1249ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1250sdbm(3).
1251
1252If you don't have write access to the DBM file, you can only read hash
1253variables, not set them. If you want to test whether you can write,
3b10bc60 1254either use file tests or try setting a dummy hash entry inside an C<eval>
1255to trap the error.
a0d0e21e 1256
19799a22
GS
1257Note that functions such as C<keys> and C<values> may return huge lists
1258when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1259function to iterate over large DBM files. Example:
1260
1261 # print out history file offsets
1262 dbmopen(%HIST,'/usr/lib/news/history',0666);
1263 while (($key,$val) = each %HIST) {
a9a5a0dc 1264 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1265 }
1266 dbmclose(%HIST);
1267
cb1a09d0 1268See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1269cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1270rich implementation.
4633a7c4 1271
2b5ab1e7
TC
1272You can control which DBM library you use by loading that library
1273before you call dbmopen():
1274
1275 use DB_File;
1276 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1277 or die "Can't open netscape history file: $!";
2b5ab1e7 1278
ea9eb35a 1279Portability issues: L<perlport/dbmopen>.
1280
a0d0e21e 1281=item defined EXPR
d74e8afc 1282X<defined> X<undef> X<undefined>
a0d0e21e 1283
54310121 1284=item defined
bbce6d69 1285
c17cdb72
NC
1286=for Pod::Functions test whether a value, variable, or function is defined
1287
2f9daede 1288Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1289the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1290checked.
1291
1292Many operations return C<undef> to indicate failure, end of file,
1293system error, uninitialized variable, and other exceptional
1294conditions. This function allows you to distinguish C<undef> from
1295other values. (A simple Boolean test will not distinguish among
7660c0ab 1296C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1297false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1298doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1299returns C<undef> when its argument is an empty array, I<or> when the
1300element to return happens to be C<undef>.
1301
f10b0346
GS
1302You may also use C<defined(&func)> to check whether subroutine C<&func>
1303has ever been defined. The return value is unaffected by any forward
80d38338 1304declarations of C<&func>. A subroutine that is not defined
847c7ebe 1305may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1306makes it spring into existence the first time that it is called; see
847c7ebe 1307L<perlsub>.
f10b0346
GS
1308
1309Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1310used to report whether memory for that aggregate had ever been
f10b0346
GS
1311allocated. This behavior may disappear in future versions of Perl.
1312You should instead use a simple test for size:
1313
1314 if (@an_array) { print "has array elements\n" }
1315 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1316
1317When used on a hash element, it tells you whether the value is defined,
dc848c6f 1318not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1319purpose.
a0d0e21e
LW
1320
1321Examples:
1322
8f1da26d 1323 print if defined $switch{D};
a0d0e21e
LW
1324 print "$val\n" while defined($val = pop(@ary));
1325 die "Can't readlink $sym: $!"
a9a5a0dc 1326 unless defined($value = readlink $sym);
a0d0e21e 1327 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1328 $debugging = 0 unless defined $debugging;
a0d0e21e 1329
8f1da26d 1330Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1331discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1332defined values. For example, if you say
a5f75d66
AD
1333
1334 "ab" =~ /a(.*)b/;
1335
80d38338 1336The pattern match succeeds and C<$1> is defined, although it
cf264981 1337matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1338matched something that happened to be zero characters long. This is all
a5f75d66 1339very above-board and honest. When a function returns an undefined value,
2f9daede 1340it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1341should use C<defined> only when questioning the integrity of what
7660c0ab 1342you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1343what you want.
1344
dc848c6f 1345See also L</undef>, L</exists>, L</ref>.
2f9daede 1346
a0d0e21e 1347=item delete EXPR
d74e8afc 1348X<delete>
a0d0e21e 1349
c17cdb72
NC
1350=for Pod::Functions deletes a value from a hash
1351
d0a76353
RS
1352Given an expression that specifies an element or slice of a hash, C<delete>
1353deletes the specified elements from that hash so that exists() on that element
1354no longer returns true. Setting a hash element to the undefined value does
1355not remove its key, but deleting it does; see L</exists>.
80d38338 1356
8f1da26d 1357In list context, returns the value or values deleted, or the last such
80d38338 1358element in scalar context. The return list's length always matches that of
d0a76353
RS
1359the argument list: deleting non-existent elements returns the undefined value
1360in their corresponding positions.
80d38338 1361
d0a76353
RS
1362delete() may also be used on arrays and array slices, but its behavior is less
1363straightforward. Although exists() will return false for deleted entries,
1364deleting array elements never changes indices of existing values; use shift()
1365or splice() for that. However, if all deleted elements fall at the end of an
1366array, the array's size shrinks to the position of the highest element that
1367still tests true for exists(), or to 0 if none do.
1368
8f1da26d 1369B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1370be removed in a future version of Perl.
80d38338
TC
1371
1372Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1373a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1374or array may not necessarily return anything; it depends on the implementation
1375of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1376
80d38338
TC
1377The C<delete local EXPR> construct localizes the deletion to the current
1378block at run time. Until the block exits, elements locally deleted
1379temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1380of composite types">.
eba0920a
EM
1381
1382 %hash = (foo => 11, bar => 22, baz => 33);
f7051f2c
FC
1383 $scalar = delete $hash{foo}; # $scalar is 11
1384 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1385 @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
eba0920a 1386
01020589 1387The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1388
5f05dabc 1389 foreach $key (keys %HASH) {
a9a5a0dc 1390 delete $HASH{$key};
a0d0e21e
LW
1391 }
1392
01020589 1393 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1394 delete $ARRAY[$index];
01020589
GS
1395 }
1396
1397And so do these:
5f05dabc 1398
01020589
GS
1399 delete @HASH{keys %HASH};
1400
9740c838 1401 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1402
80d38338
TC
1403But both are slower than assigning the empty list
1404or undefining %HASH or @ARRAY, which is the customary
1405way to empty out an aggregate:
01020589 1406
5ed4f2ec 1407 %HASH = (); # completely empty %HASH
1408 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1409
5ed4f2ec 1410 @ARRAY = (); # completely empty @ARRAY
1411 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1412
80d38338
TC
1413The EXPR can be arbitrarily complicated provided its
1414final operation is an element or slice of an aggregate:
a0d0e21e
LW
1415
1416 delete $ref->[$x][$y]{$key};
5f05dabc 1417 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1418
01020589
GS
1419 delete $ref->[$x][$y][$index];
1420 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1421
a0d0e21e 1422=item die LIST
d74e8afc 1423X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1424
c17cdb72
NC
1425=for Pod::Functions raise an exception or bail out
1426
391b733c 1427C<die> raises an exception. Inside an C<eval> the error message is stuffed
4c050ad5
NC
1428into C<$@> and the C<eval> is terminated with the undefined value.
1429If the exception is outside of all enclosing C<eval>s, then the uncaught
391b733c 1430exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1431need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1432
1433Equivalent examples:
1434
1435 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1436 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1437
ccac6780 1438If the last element of LIST does not end in a newline, the current
df37ec69
WW
1439script line number and input line number (if any) are also printed,
1440and a newline is supplied. Note that the "input line number" (also
1441known as "chunk") is subject to whatever notion of "line" happens to
1442be currently in effect, and is also available as the special variable
1443C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1444
1445Hint: sometimes appending C<", stopped"> to your message will cause it
1446to make better sense when the string C<"at foo line 123"> is appended.
1447Suppose you are running script "canasta".
a0d0e21e
LW
1448
1449 die "/etc/games is no good";
1450 die "/etc/games is no good, stopped";
1451
1452produce, respectively
1453
1454 /etc/games is no good at canasta line 123.
1455 /etc/games is no good, stopped at canasta line 123.
1456
a96d0188 1457If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1458previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1459This is useful for propagating exceptions:
1460
1461 eval { ... };
1462 die unless $@ =~ /Expected exception/;
1463
a96d0188 1464If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1465C<PROPAGATE> method, that method will be called with additional file
1466and line number parameters. The return value replaces the value in
34169887 1467C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1468were called.
1469
7660c0ab 1470If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1471
4c050ad5
NC
1472If an uncaught exception results in interpreter exit, the exit code is
1473determined from the values of C<$!> and C<$?> with this pseudocode:
1474
1475 exit $! if $!; # errno
1476 exit $? >> 8 if $? >> 8; # child exit status
1477 exit 255; # last resort
1478
1479The intent is to squeeze as much possible information about the likely cause
391b733c
FC
1480into the limited space of the system exit
1481code. However, as C<$!> is the value
4c050ad5
NC
1482of C's C<errno>, which can be set by any system call, this means that the value
1483of the exit code used by C<die> can be non-predictable, so should not be relied
1484upon, other than to be non-zero.
1485
80d38338
TC
1486You can also call C<die> with a reference argument, and if this is trapped
1487within an C<eval>, C<$@> contains that reference. This permits more
1488elaborate exception handling using objects that maintain arbitrary state
1489about the exception. Such a scheme is sometimes preferable to matching
1490particular string values of C<$@> with regular expressions. Because C<$@>
1491is a global variable and C<eval> may be used within object implementations,
1492be careful that analyzing the error object doesn't replace the reference in
1493the global variable. It's easiest to make a local copy of the reference
1494before any manipulations. Here's an example:
52531d10 1495
80d38338 1496 use Scalar::Util "blessed";
da279afe 1497
52531d10 1498 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7 1499 if (my $ev_err = $@) {
f7051f2c
FC
1500 if (blessed($ev_err)
1501 && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1502 # handle Some::Module::Exception
1503 }
1504 else {
1505 # handle all other possible exceptions
1506 }
1507 }
1508
3b10bc60 1509Because Perl stringifies uncaught exception messages before display,
80d38338 1510you'll probably want to overload stringification operations on
52531d10
GS
1511exception objects. See L<overload> for details about that.
1512
19799a22
GS
1513You can arrange for a callback to be run just before the C<die>
1514does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1515handler is called with the error text and can change the error
19799a22 1516message, if it sees fit, by calling C<die> again. See
96090e4f 1517L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1518L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1519to be run only right before your program was to exit, this is not
3b10bc60 1520currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1521even inside eval()ed blocks/strings! If one wants the hook to do
1522nothing in such situations, put
fb73857a 1523
5ed4f2ec 1524 die @_ if $^S;
fb73857a 1525
19799a22
GS
1526as the first line of the handler (see L<perlvar/$^S>). Because
1527this promotes strange action at a distance, this counterintuitive
b76cc8ba 1528behavior may be fixed in a future release.
774d564b 1529
4c050ad5
NC
1530See also exit(), warn(), and the Carp module.
1531
a0d0e21e 1532=item do BLOCK
d74e8afc 1533X<do> X<block>
a0d0e21e 1534
c17cdb72
NC
1535=for Pod::Functions turn a BLOCK into a TERM
1536
a0d0e21e 1537Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1538sequence of commands indicated by BLOCK. When modified by the C<while> or
1539C<until> loop modifier, executes the BLOCK once before testing the loop
391b733c 1540condition. (On other statements the loop modifiers test the conditional
6b275a1f 1541first.)
a0d0e21e 1542
4968c1e4 1543C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1544C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1545See L<perlsyn> for alternative strategies.
4968c1e4 1546
a0d0e21e 1547=item do SUBROUTINE(LIST)
d74e8afc 1548X<do>
a0d0e21e 1549
3bab73c8
FC
1550This form of subroutine call is deprecated. SUBROUTINE can be a bareword
1551or scalar variable.
a0d0e21e
LW
1552
1553=item do EXPR
d74e8afc 1554X<do>
a0d0e21e
LW
1555
1556Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1557file as a Perl script.
a0d0e21e
LW
1558
1559 do 'stat.pl';
1560
1561is just like
1562
986b19de 1563 eval `cat stat.pl`;
a0d0e21e 1564
2b5ab1e7 1565except that it's more efficient and concise, keeps track of the current
96090e4f
A
1566filename for error messages, searches the C<@INC> directories, and updates
1567C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1568these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1569cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1570same, however, in that it does reparse the file every time you call it,
1571so you probably don't want to do this inside a loop.
a0d0e21e 1572
8f1da26d 1573If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1574an error message in C<$@>. If C<do> cannot read the file, it returns undef
1575and sets C<$!> to the error. Always check C<$@> first, as compilation
1576could fail in a way that also sets C<$!>. If the file is successfully
1577compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1578
80d38338 1579Inclusion of library modules is better done with the
19799a22 1580C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1581and raise an exception if there's a problem.
a0d0e21e 1582
5a964f20
TC
1583You might like to use C<do> to read in a program configuration
1584file. Manual error checking can be done this way:
1585
b76cc8ba 1586 # read in config files: system first, then user
f86cebdf 1587 for $file ("/share/prog/defaults.rc",
b76cc8ba 1588 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1589 {
1590 unless ($return = do $file) {
1591 warn "couldn't parse $file: $@" if $@;
1592 warn "couldn't do $file: $!" unless defined $return;
1593 warn "couldn't run $file" unless $return;
1594 }
5a964f20
TC
1595 }
1596
a0d0e21e 1597=item dump LABEL
d74e8afc 1598X<dump> X<core> X<undump>
a0d0e21e 1599
8a7e748e
FC
1600=item dump EXPR
1601
1614b0e3
JD
1602=item dump
1603
c17cdb72
NC
1604=for Pod::Functions create an immediate core dump
1605
19799a22
GS
1606This function causes an immediate core dump. See also the B<-u>
1607command-line switch in L<perlrun>, which does the same thing.
1608Primarily this is so that you can use the B<undump> program (not
1609supplied) to turn your core dump into an executable binary after
1610having initialized all your variables at the beginning of the
1611program. When the new binary is executed it will begin by executing
1612a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1613Think of it as a goto with an intervening core dump and reincarnation.
8a7e748e
FC
1614If C<LABEL> is omitted, restarts the program from the top. The
1615C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1616computed at run time, being otherwise identical to C<dump LABEL>.
19799a22
GS
1617
1618B<WARNING>: Any files opened at the time of the dump will I<not>
1619be open any more when the program is reincarnated, with possible
80d38338 1620resulting confusion by Perl.
19799a22 1621
59f521f4 1622This function is now largely obsolete, mostly because it's very hard to
391b733c 1623convert a core file into an executable. That's why you should now invoke
59f521f4 1624it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1625typo.
19799a22 1626
2ba1f20a
FC
1627Unlike most named operators, this has the same precedence as assignment.
1628It is also exempt from the looks-like-a-function rule, so
1629C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1630C<dump>.
1631
ea9eb35a 1632Portability issues: L<perlport/dump>.
1633
532eee96 1634=item each HASH
d74e8afc 1635X<each> X<hash, iterator>
aa689395 1636
532eee96 1637=item each ARRAY
aeedbbed
NC
1638X<array, iterator>
1639
f5a93a43
TC
1640=item each EXPR
1641
c17cdb72
NC
1642=for Pod::Functions retrieve the next key/value pair from a hash
1643
bade7fbc
TC
1644When called on a hash in list context, returns a 2-element list
1645consisting of the key and value for the next element of a hash. In Perl
16465.12 and later only, it will also return the index and value for the next
1647element of an array so that you can iterate over it; older Perls consider
1648this a syntax error. When called in scalar context, returns only the key
1649(not the value) in a hash, or the index in an array.
2f9daede 1650
aeedbbed 1651Hash entries are returned in an apparently random order. The actual random
3b10bc60 1652order is subject to change in future versions of Perl, but it is
504f80c1 1653guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1654function would produce on the same (unmodified) hash. Since Perl
22883ac5 16555.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1656for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1657
80d38338
TC
1658After C<each> has returned all entries from the hash or array, the next
1659call to C<each> returns the empty list in list context and C<undef> in
bade7fbc
TC
1660scalar context; the next call following I<that> one restarts iteration.
1661Each hash or array has its own internal iterator, accessed by C<each>,
1662C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1663reached the end as just described; it can be explicitly reset by calling
1664C<keys> or C<values> on the hash or array. If you add or delete a hash's
1665elements while iterating over it, entries may be skipped or duplicated--so
1666don't do that. Exception: In the current implementation, it is always safe
1667to delete the item most recently returned by C<each()>, so the following
1668code works properly:
74fc8b5f
MJD
1669
1670 while (($key, $value) = each %hash) {
1671 print $key, "\n";
1672 delete $hash{$key}; # This is safe
1673 }
aa689395 1674
80d38338 1675This prints out your environment like the printenv(1) program,
3b10bc60 1676but in a different order:
a0d0e21e
LW
1677
1678 while (($key,$value) = each %ENV) {
a9a5a0dc 1679 print "$key=$value\n";
a0d0e21e
LW
1680 }
1681
f5a93a43
TC
1682Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1683reference to an unblessed hash or array. The argument will be dereferenced
1684automatically. This aspect of C<each> is considered highly experimental.
1685The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1686
1687 while (($key,$value) = each $hashref) { ... }
1688
bade7fbc
TC
1689To avoid confusing would-be users of your code who are running earlier
1690versions of Perl with mysterious syntax errors, put this sort of thing at
1691the top of your file to signal that your code will work I<only> on Perls of
1692a recent vintage:
1693
1694 use 5.012; # so keys/values/each work on arrays
1695 use 5.014; # so keys/values/each work on scalars (experimental)
1696
8f1da26d 1697See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1698
1699=item eof FILEHANDLE
d74e8afc
ITB
1700X<eof>
1701X<end of file>
1702X<end-of-file>
a0d0e21e 1703
4633a7c4
LW
1704=item eof ()
1705
a0d0e21e
LW
1706=item eof
1707
c17cdb72
NC
1708=for Pod::Functions test a filehandle for its end
1709
8f1da26d 1710Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1711FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1712gives the real filehandle. (Note that this function actually
80d38338 1713reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1714interactive context.) Do not read from a terminal file (or call
19799a22 1715C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1716as terminals may lose the end-of-file condition if you do.
1717
820475bd 1718An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1719with empty parentheses is different. It refers to the pseudo file
820475bd 1720formed from the files listed on the command line and accessed via the
61eff3bc
JH
1721C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1722as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1723used will cause C<@ARGV> to be examined to determine if input is
67408cae 1724available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1725end-of-file will assume you are processing another C<@ARGV> list,
1726and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1727see L<perlop/"I/O Operators">.
820475bd 1728
61eff3bc 1729In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1730detect the end of each file, whereas C<eof()> will detect the end
1731of the very last file only. Examples:
a0d0e21e 1732
748a9306
LW
1733 # reset line numbering on each input file
1734 while (<>) {
a9a5a0dc
VP
1735 next if /^\s*#/; # skip comments
1736 print "$.\t$_";
5a964f20 1737 } continue {
a9a5a0dc 1738 close ARGV if eof; # Not eof()!
748a9306
LW
1739 }
1740
a0d0e21e
LW
1741 # insert dashes just before last line of last file
1742 while (<>) {
a9a5a0dc
VP
1743 if (eof()) { # check for end of last file
1744 print "--------------\n";
1745 }
1746 print;
f7051f2c 1747 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1748 }
1749
a0d0e21e 1750Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1751input operators typically return C<undef> when they run out of data or
1752encounter an error.
a0d0e21e
LW
1753
1754=item eval EXPR
d74e8afc 1755X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1756X<error, handling> X<exception, handling>
a0d0e21e
LW
1757
1758=item eval BLOCK
1759
ce2984c3
PF
1760=item eval
1761
c17cdb72
NC
1762=for Pod::Functions catch exceptions or compile and run code
1763
c7cc6f1c
GS
1764In the first form, the return value of EXPR is parsed and executed as if it
1765were a little Perl program. The value of the expression (which is itself
8f1da26d 1766determined within scalar context) is first parsed, and if there were no
2341804c 1767errors, executed as a block within the lexical context of the current Perl
df4833a8 1768program. This means, that in particular, any outer lexical variables are
2341804c
DM
1769visible to it, and any package variable settings or subroutine and format
1770definitions remain afterwards.
1771
1772Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1773If EXPR is omitted, evaluates C<$_>. This form is typically used to
1774delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c 1775
7289c5e6
FC
1776If the C<unicode_eval> feature is enabled (which is the default under a
1777C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1778characters, so C<use utf8> declarations have no effect, and source filters
1779are forbidden. In the absence of the C<unicode_eval> feature, the string
1780will sometimes be treated as characters and sometimes as bytes, depending
1781on the internal encoding, and source filters activated within the C<eval>
1782exhibit the erratic, but historical, behaviour of affecting some outer file
1783scope that is still compiling. See also the L</evalbytes> keyword, which
1784always treats its input as a byte stream and works properly with source
1785filters, and the L<feature> pragma.
1786
c7cc6f1c 1787In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1788same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1789within the context of the current Perl program. This form is typically
1790used to trap exceptions more efficiently than the first (see below), while
1791also providing the benefit of checking the code within BLOCK at compile
1792time.
1793
1794The final semicolon, if any, may be omitted from the value of EXPR or within
1795the BLOCK.
1796
1797In both forms, the value returned is the value of the last expression
5a964f20 1798evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1799as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1800in void, scalar, or list context, depending on the context of the C<eval>
1801itself. See L</wantarray> for more on how the evaluation context can be
1802determined.
a0d0e21e 1803
19799a22 1804If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1805executed, C<eval> returns C<undef> in scalar context
774b80e8
FC
1806or an empty list in list context, and C<$@> is set to the error
1807message. (Prior to 5.16, a bug caused C<undef> to be returned
1808in list context for syntax errors, but not for runtime errors.)
1809If there was no error, C<$@> is set to the empty string. A
9cc672d4
FC
1810control flow operator like C<last> or C<goto> can bypass the setting of
1811C<$@>. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1812warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1813To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1814turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1815See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1816
19799a22
GS
1817Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1818determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1819is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1820the die operator is used to raise exceptions.
1821
5f1da31c
NT
1822If you want to trap errors when loading an XS module, some problems with
1823the binary interface (such as Perl version skew) may be fatal even with
df4833a8 1824C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
5f1da31c 1825
a0d0e21e
LW
1826If the code to be executed doesn't vary, you may use the eval-BLOCK
1827form to trap run-time errors without incurring the penalty of
1828recompiling each time. The error, if any, is still returned in C<$@>.
1829Examples:
1830
54310121 1831 # make divide-by-zero nonfatal
a0d0e21e
LW
1832 eval { $answer = $a / $b; }; warn $@ if $@;
1833
1834 # same thing, but less efficient
1835 eval '$answer = $a / $b'; warn $@ if $@;
1836
1837 # a compile-time error
5ed4f2ec 1838 eval { $answer = }; # WRONG
a0d0e21e
LW
1839
1840 # a run-time error
5ed4f2ec 1841 eval '$answer ='; # sets $@
a0d0e21e 1842
cf264981
SP
1843Using the C<eval{}> form as an exception trap in libraries does have some
1844issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1845may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1846You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1847as this example shows:
774d564b 1848
80d38338 1849 # a private exception trap for divide-by-zero
f86cebdf
GS
1850 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1851 warn $@ if $@;
774d564b
PP
1852
1853This is especially significant, given that C<__DIE__> hooks can call
19799a22 1854C<die> again, which has the effect of changing their error messages:
774d564b
PP
1855
1856 # __DIE__ hooks may modify error messages
1857 {
f86cebdf
GS
1858 local $SIG{'__DIE__'} =
1859 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1860 eval { die "foo lives here" };
1861 print $@ if $@; # prints "bar lives here"
774d564b
PP
1862 }
1863
19799a22 1864Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1865may be fixed in a future release.
1866
19799a22 1867With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1868being looked at when:
1869
5ed4f2ec 1870 eval $x; # CASE 1
1871 eval "$x"; # CASE 2
a0d0e21e 1872
5ed4f2ec 1873 eval '$x'; # CASE 3
1874 eval { $x }; # CASE 4
a0d0e21e 1875
5ed4f2ec 1876 eval "\$$x++"; # CASE 5
1877 $$x++; # CASE 6
a0d0e21e 1878
2f9daede 1879Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1880the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1881the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1882and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1883does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1884purely visual reasons, but it also has the advantage of compiling at
1885compile-time instead of at run-time.) Case 5 is a place where
19799a22 1886normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1887particular situation, you can just use symbolic references instead, as
1888in case 6.
a0d0e21e 1889
b6538e4f 1890Before Perl 5.14, the assignment to C<$@> occurred before restoration
bade7fbc 1891of localized variables, which means that for your code to run on older
b208c909 1892versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1893errors:
1894
1895 # alter $@ on nefarious repugnancy only
1896 {
1897 my $e;
1898 {
f7051f2c
FC
1899 local $@; # protect existing $@
1900 eval { test_repugnancy() };
1901 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1902 $@ =~ /nefarious/ and $e = $@;
8a5a710d
DN
1903 }
1904 die $e if defined $e
1905 }
1906
4968c1e4 1907C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1908C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1909
3b10bc60 1910An C<eval ''> executed within the C<DB> package doesn't see the usual
1911surrounding lexical scope, but rather the scope of the first non-DB piece
df4833a8 1912of code that called it. You don't normally need to worry about this unless
3b10bc60 1913you are writing a Perl debugger.
d819b83a 1914
7289c5e6
FC
1915=item evalbytes EXPR
1916X<evalbytes>
1917
1918=item evalbytes
1919
d9b04284 1920=for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
c17cdb72 1921
7289c5e6
FC
1922This function is like L</eval> with a string argument, except it always
1923parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1924string containing characters whose ordinal value exceeds 255 results in an
1925error. Source filters activated within the evaluated code apply to the
1926code itself.
1927
1928This function is only available under the C<evalbytes> feature, a
1929C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1930L<feature> for more information.
1931
a0d0e21e 1932=item exec LIST
d74e8afc 1933X<exec> X<execute>
a0d0e21e 1934
8bf3b016
GS
1935=item exec PROGRAM LIST
1936
c17cdb72
NC
1937=for Pod::Functions abandon this program to run another
1938
3b10bc60 1939The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1940use C<system> instead of C<exec> if you want it to return. It fails and
1941returns false only if the command does not exist I<and> it is executed
fb73857a 1942directly instead of via your system's command shell (see below).
a0d0e21e 1943
19799a22 1944Since it's a common mistake to use C<exec> instead of C<system>, Perl
4642e50d
EB
1945warns you if C<exec> is called in void context and if there is a following
1946statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1947you always do that, right?). If you I<really> want to follow an C<exec>
1948with some other statement, you can use one of these styles to avoid the warning:
55d729e4 1949
5a964f20
TC
1950 exec ('foo') or print STDERR "couldn't exec foo: $!";
1951 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1952
5a964f20 1953If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1954with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1955If there is only one scalar argument or an array with one element in it,
1956the argument is checked for shell metacharacters, and if there are any,
1957the entire argument is passed to the system's command shell for parsing
1958(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1959If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1960words and passed directly to C<execvp>, which is more efficient.
19799a22 1961Examples:
a0d0e21e 1962
19799a22
GS
1963 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1964 exec "sort $outfile | uniq";
a0d0e21e
LW
1965
1966If you don't really want to execute the first argument, but want to lie
1967to the program you are executing about its own name, you can specify
1968the program you actually want to run as an "indirect object" (without a
1969comma) in front of the LIST. (This always forces interpretation of the
54310121 1970LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1971the list.) Example:
1972
1973 $shell = '/bin/csh';
5ed4f2ec 1974 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1975
1976or, more directly,
1977
5ed4f2ec 1978 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1979
3b10bc60 1980When the arguments get executed via the system shell, results are
1981subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1982for details.
1983
19799a22
GS
1984Using an indirect object with C<exec> or C<system> is also more
1985secure. This usage (which also works fine with system()) forces
1986interpretation of the arguments as a multivalued list, even if the
1987list had just one argument. That way you're safe from the shell
1988expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1989
1990 @args = ( "echo surprise" );
1991
2b5ab1e7 1992 exec @args; # subject to shell escapes
f86cebdf 1993 # if @args == 1
2b5ab1e7 1994 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1995
1996The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1997program, passing it C<"surprise"> an argument. The second version didn't;
1998it tried to run a program named I<"echo surprise">, didn't find it, and set
1999C<$?> to a non-zero value indicating failure.
5a964f20 2000
e9fa405d
BF
2001Perl attempts to flush all files opened for output before the exec,
2002but this may not be supported on some platforms (see L<perlport>).
2003To be safe, you may need to set C<$|> ($AUTOFLUSH in English) or
2004call the C<autoflush()> method of C<IO::Handle> on any open handles
2005to avoid lost output.
0f897271 2006
80d38338
TC
2007Note that C<exec> will not call your C<END> blocks, nor will it invoke
2008C<DESTROY> methods on your objects.
7660c0ab 2009
ea9eb35a 2010Portability issues: L<perlport/exec>.
2011
a0d0e21e 2012=item exists EXPR
d74e8afc 2013X<exists> X<autovivification>
a0d0e21e 2014
c17cdb72
NC
2015=for Pod::Functions test whether a hash key is present
2016
d0a76353
RS
2017Given an expression that specifies an element of a hash, returns true if the
2018specified element in the hash has ever been initialized, even if the
2019corresponding value is undefined.
a0d0e21e 2020
5ed4f2ec 2021 print "Exists\n" if exists $hash{$key};
2022 print "Defined\n" if defined $hash{$key};
01020589
GS
2023 print "True\n" if $hash{$key};
2024
d0a76353 2025exists may also be called on array elements, but its behavior is much less
8f1da26d 2026obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
2027that calling exists on array values is deprecated and likely to be removed in
2028a future version of Perl.
2029
5ed4f2ec 2030 print "Exists\n" if exists $array[$index];
2031 print "Defined\n" if defined $array[$index];
01020589 2032 print "True\n" if $array[$index];
a0d0e21e 2033
8f1da26d 2034A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
2035it exists, but the reverse doesn't necessarily hold true.
2036
afebc493
GS
2037Given an expression that specifies the name of a subroutine,
2038returns true if the specified subroutine has ever been declared, even
2039if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 2040does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
2041exist may still be callable: its package may have an C<AUTOLOAD>
2042method that makes it spring into existence the first time that it is
3b10bc60 2043called; see L<perlsub>.
afebc493 2044
5ed4f2ec 2045 print "Exists\n" if exists &subroutine;
2046 print "Defined\n" if defined &subroutine;
afebc493 2047
a0d0e21e 2048Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 2049operation is a hash or array key lookup or subroutine name:
a0d0e21e 2050
5ed4f2ec 2051 if (exists $ref->{A}->{B}->{$key}) { }
2052 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 2053
5ed4f2ec 2054 if (exists $ref->{A}->{B}->[$ix]) { }
2055 if (exists $hash{A}{B}[$ix]) { }
01020589 2056
afebc493
GS
2057 if (exists &{$ref->{A}{B}{$key}}) { }
2058
9590a7cd 2059Although the most deeply nested array or hash element will not spring into
3b10bc60 2060existence just because its existence was tested, any intervening ones will.
61eff3bc 2061Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 2062into existence due to the existence test for the $key element above.
3b10bc60 2063This happens anywhere the arrow operator is used, including even here:
5a964f20 2064
2b5ab1e7 2065 undef $ref;
5ed4f2ec 2066 if (exists $ref->{"Some key"}) { }
2067 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
2068
2069This surprising autovivification in what does not at first--or even
2070second--glance appear to be an lvalue context may be fixed in a future
5a964f20 2071release.
a0d0e21e 2072
afebc493
GS
2073Use of a subroutine call, rather than a subroutine name, as an argument
2074to exists() is an error.
2075
5ed4f2ec 2076 exists &sub; # OK
2077 exists &sub(); # Error
afebc493 2078
a0d0e21e 2079=item exit EXPR
d74e8afc 2080X<exit> X<terminate> X<abort>
a0d0e21e 2081
ce2984c3
PF
2082=item exit
2083
c17cdb72
NC
2084=for Pod::Functions terminate this program
2085
2b5ab1e7 2086Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
2087
2088 $ans = <STDIN>;
2089 exit 0 if $ans =~ /^[Xx]/;
2090
19799a22 2091See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
2092universally recognized values for EXPR are C<0> for success and C<1>
2093for error; other values are subject to interpretation depending on the
2094environment in which the Perl program is running. For example, exiting
209569 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2096the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 2097
19799a22
GS
2098Don't use C<exit> to abort a subroutine if there's any chance that
2099someone might want to trap whatever error happened. Use C<die> instead,
2100which can be trapped by an C<eval>.
28757baa 2101
19799a22 2102The exit() function does not always exit immediately. It calls any
2b5ab1e7 2103defined C<END> routines first, but these C<END> routines may not
19799a22 2104themselves abort the exit. Likewise any object destructors that need to
60275626 2105be called are called before the real exit. C<END> routines and destructors
391b733c 2106can change the exit status by modifying C<$?>. If this is a problem, you
fae6f8fa 2107can call C<POSIX::_exit($status)> to avoid END and destructor processing.
87275199 2108See L<perlmod> for details.
5a964f20 2109
ea9eb35a 2110Portability issues: L<perlport/exit>.
2111
a0d0e21e 2112=item exp EXPR
d74e8afc 2113X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 2114
54310121 2115=item exp
bbce6d69 2116
c17cdb72
NC
2117=for Pod::Functions raise I<e> to a power
2118
b76cc8ba 2119Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
2120If EXPR is omitted, gives C<exp($_)>.
2121
628253b8
BF
2122=item fc EXPR
2123X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2124
2125=item fc
2126
d9b04284 2127=for Pod::Functions +fc return casefolded version of a string
c17cdb72 2128
628253b8
BF
2129Returns the casefolded version of EXPR. This is the internal function
2130implementing the C<\F> escape in double-quoted strings.
2131
2132Casefolding is the process of mapping strings to a form where case
2133differences are erased; comparing two strings in their casefolded
2134form is effectively a way of asking if two strings are equal,
2135regardless of case.
2136
2137Roughly, if you ever found yourself writing this
2138
f6c6dcb6 2139 lc($this) eq lc($that) # Wrong!
628253b8 2140 # or
f6c6dcb6 2141 uc($this) eq uc($that) # Also wrong!
628253b8 2142 # or
f6c6dcb6 2143 $this =~ /^\Q$that\E\z/i # Right!
628253b8
BF
2144
2145Now you can write
2146
2147 fc($this) eq fc($that)
2148
2149And get the correct results.
2150
fc39a31f
KW
2151Perl only implements the full form of casefolding,
2152but you can access the simple folds using L<Unicode::UCD/casefold()> and
2153L<Unicode::UCD/prop_invmap()>.
628253b8
BF
2154For further information on casefolding, refer to
2155the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
21564.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2157available at L<http://www.unicode.org/versions/latest/>, as well as the
2158Case Charts available at L<http://www.unicode.org/charts/case/>.
2159
2160If EXPR is omitted, uses C<$_>.
2161
2162This function behaves the same way under various pragma, such as in a locale,
2163as L</lc> does.
2164
2165While the Unicode Standard defines two additional forms of casefolding,
2166one for Turkic languages and one that never maps one character into multiple
2167characters, these are not provided by the Perl core; However, the CPAN module
2168C<Unicode::Casing> may be used to provide an implementation.
2169
2170This keyword is available only when the C<"fc"> feature is enabled,
2171or when prefixed with C<CORE::>; See L<feature>. Alternately,
2172include a C<use v5.16> or later to the current scope.
2173
a0d0e21e 2174=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2175X<fcntl>
a0d0e21e 2176
c17cdb72
NC
2177=for Pod::Functions file control system call
2178
f86cebdf 2179Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
2180
2181 use Fcntl;
2182
0ade1984 2183first to get the correct constant definitions. Argument processing and
3b10bc60 2184value returned work just like C<ioctl> below.
a0d0e21e
LW
2185For example:
2186
2187 use Fcntl;
5a964f20 2188 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 2189 or die "can't fcntl F_GETFL: $!";
5a964f20 2190
554ad1fc 2191You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
2192Like C<ioctl>, it maps a C<0> return from the system call into
2193C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
2194in numeric context. It is also exempt from the normal B<-w> warnings
2195on improper numeric conversions.
5a964f20 2196
3b10bc60 2197Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
2198doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2199manpage to learn what functions are available on your system.
a0d0e21e 2200
be2f7487 2201Here's an example of setting a filehandle named C<REMOTE> to be
2202non-blocking at the system level. You'll have to negotiate C<$|>
2203on your own, though.
2204
2205 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2206
2207 $flags = fcntl(REMOTE, F_GETFL, 0)
2208 or die "Can't get flags for the socket: $!\n";
2209
2210 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2211 or die "Can't set flags for the socket: $!\n";
2212
ea9eb35a 2213Portability issues: L<perlport/fcntl>.
2214
cfa52385
FC
2215=item __FILE__
2216X<__FILE__>
2217
c17cdb72
NC
2218=for Pod::Functions the name of the current source file
2219
cfa52385
FC
2220A special token that returns the name of the file in which it occurs.
2221
a0d0e21e 2222=item fileno FILEHANDLE
d74e8afc 2223X<fileno>
a0d0e21e 2224
c17cdb72
NC
2225=for Pod::Functions return file descriptor from filehandle
2226
2b5ab1e7 2227Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
2228filehandle is not open. If there is no real file descriptor at the OS
2229level, as can happen with filehandles connected to memory objects via
2230C<open> with a reference for the third argument, -1 is returned.
2231
2232This is mainly useful for constructing
19799a22 2233bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
2234If FILEHANDLE is an expression, the value is taken as an indirect
2235filehandle, generally its name.
5a964f20 2236
b76cc8ba 2237You can use this to find out whether two handles refer to the
5a964f20
TC
2238same underlying descriptor:
2239
2240 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 2241 print "THIS and THAT are dups\n";
b76cc8ba
NIS
2242 }
2243
a0d0e21e 2244=item flock FILEHANDLE,OPERATION
d74e8afc 2245X<flock> X<lock> X<locking>
a0d0e21e 2246
c17cdb72
NC
2247=for Pod::Functions lock an entire file with an advisory lock
2248
19799a22
GS
2249Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2250for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2251machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2252C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2253entire files only, not records.
2b5ab1e7
TC
2254
2255Two potentially non-obvious but traditional C<flock> semantics are
2256that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2257are B<merely advisory>. Such discretionary locks are more flexible, but
2258offer fewer guarantees. This means that programs that do not also use
2259C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2260your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2261for details. It's best to assume traditional behavior if you're writing
2262portable programs. (But if you're not, you should as always feel perfectly
2263free to write for your own system's idiosyncrasies (sometimes called
2264"features"). Slavish adherence to portability concerns shouldn't get
2265in the way of your getting your job done.)
a3cb178b 2266
8ebc5c01
PP
2267OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2268LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2269you can use the symbolic names if you import them from the L<Fcntl> module,
2270either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2271requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2272releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2273LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2274waiting for the lock; check the return status to see if you got it.
68dc0745 2275
2b5ab1e7
TC
2276To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2277before locking or unlocking it.
8ebc5c01 2278
f86cebdf 2279Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2280locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2281are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2282implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
2283differing semantics shouldn't bite too many people.
2284
becacb53
TM
2285Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2286be open with read intent to use LOCK_SH and requires that it be open
2287with write intent to use LOCK_EX.
2288
19799a22
GS
2289Note also that some versions of C<flock> cannot lock things over the
2290network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2291that. If you like you can force Perl to ignore your system's flock(2)
2292function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2293the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2294and build a new Perl.
4633a7c4
LW
2295
2296Here's a mailbox appender for BSD systems.
a0d0e21e 2297
f7051f2c
FC
2298 # import LOCK_* and SEEK_END constants
2299 use Fcntl qw(:flock SEEK_END);
a0d0e21e
LW
2300
2301 sub lock {
a9a5a0dc
VP
2302 my ($fh) = @_;
2303 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2304
a9a5a0dc
VP
2305 # and, in case someone appended while we were waiting...
2306 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2307 }
2308
2309 sub unlock {
a9a5a0dc
VP
2310 my ($fh) = @_;
2311 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2312 }
2313
b0169937 2314 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2315 or die "Can't open mailbox: $!";
a0d0e21e 2316
7ed5353d 2317 lock($mbox);
b0169937 2318 print $mbox $msg,"\n\n";
7ed5353d 2319 unlock($mbox);
a0d0e21e 2320
3b10bc60 2321On systems that support a real flock(2), locks are inherited across fork()
2322calls, whereas those that must resort to the more capricious fcntl(2)
2323function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2324
cb1a09d0 2325See also L<DB_File> for other flock() examples.
a0d0e21e 2326
ea9eb35a 2327Portability issues: L<perlport/flock>.
2328
a0d0e21e 2329=item fork
d74e8afc 2330X<fork> X<child> X<parent>
a0d0e21e 2331
c17cdb72
NC
2332=for Pod::Functions create a new process just like this one
2333
2b5ab1e7
TC
2334Does a fork(2) system call to create a new process running the
2335same program at the same point. It returns the child pid to the
2336parent process, C<0> to the child process, or C<undef> if the fork is
2337unsuccessful. File descriptors (and sometimes locks on those descriptors)
2338are shared, while everything else is copied. On most systems supporting
2339fork(), great care has gone into making it extremely efficient (for
2340example, using copy-on-write technology on data pages), making it the
2341dominant paradigm for multitasking over the last few decades.
5a964f20 2342
e9fa405d 2343Perl attempts to flush all files opened for
0f897271
GS
2344output before forking the child process, but this may not be supported
2345on some platforms (see L<perlport>). To be safe, you may need to set
2346C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2347C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2348
19799a22 2349If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2350accumulate zombies. On some systems, you can avoid this by setting
2351C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2352forking and reaping moribund children.
cb1a09d0 2353
28757baa
PP
2354Note that if your forked child inherits system file descriptors like
2355STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2356if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2357backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2358You should reopen those to F</dev/null> if it's any issue.
28757baa 2359
ea9eb35a 2360On some platforms such as Windows, where the fork() system call is not available,
391b733c
FC
2361Perl can be built to emulate fork() in the Perl interpreter.
2362The emulation is designed, at the level of the Perl program,
2363to be as compatible as possible with the "Unix" fork().
6d17f725 2364However it has limitations that have to be considered in code intended to be portable.
ea9eb35a 2365See L<perlfork> for more details.
2366
2367Portability issues: L<perlport/fork>.
2368
cb1a09d0 2369=item format
d74e8afc 2370X<format>
cb1a09d0 2371
c17cdb72
NC
2372=for Pod::Functions declare a picture format with use by the write() function
2373
19799a22 2374Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2375example:
2376
54310121 2377 format Something =
a9a5a0dc
VP
2378 Test: @<<<<<<<< @||||| @>>>>>
2379 $str, $%, '$' . int($num)
cb1a09d0
AD
2380 .
2381
2382 $str = "widget";
184e9718 2383 $num = $cost/$quantity;
cb1a09d0
AD
2384 $~ = 'Something';
2385 write;
2386
2387See L<perlform> for many details and examples.
2388
8903cb82 2389=item formline PICTURE,LIST
d74e8afc 2390X<formline>
a0d0e21e 2391
c17cdb72
NC
2392=for Pod::Functions internal function used for formats
2393
5a964f20 2394This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2395too. It formats (see L<perlform>) a list of values according to the
2396contents of PICTURE, placing the output into the format output
7660c0ab 2397accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2398Eventually, when a C<write> is done, the contents of
cf264981
SP
2399C<$^A> are written to some filehandle. You could also read C<$^A>
2400and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2401does one C<formline> per line of form, but the C<formline> function itself
748a9306 2402doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2403that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2404You may therefore need to use multiple formlines to implement a single
3b10bc60 2405record format, just like the C<format> compiler.
748a9306 2406
19799a22 2407Be careful if you put double quotes around the picture, because an C<@>
748a9306 2408character may be taken to mean the beginning of an array name.
19799a22 2409C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2410
445b09e5
FC
2411If you are trying to use this instead of C<write> to capture the output,
2412you may find it easier to open a filehandle to a scalar
2413(C<< open $fh, ">", \$output >>) and write to that instead.
2414
a0d0e21e 2415=item getc FILEHANDLE
f723aae1 2416X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2417
2418=item getc
2419
c17cdb72
NC
2420=for Pod::Functions get the next character from the filehandle
2421
a0d0e21e 2422Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2423or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2424the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2425STDIN. This is not particularly efficient. However, it cannot be
2426used by itself to fetch single characters without waiting for the user
2427to hit enter. For that, try something more like:
4633a7c4
LW
2428
2429 if ($BSD_STYLE) {
a9a5a0dc 2430 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2431 }
2432 else {
a9a5a0dc 2433 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2434 }
2435
2436 $key = getc(STDIN);
2437
2438 if ($BSD_STYLE) {
a9a5a0dc 2439 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2440 }
2441 else {
3b10bc60 2442 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2443 }
2444 print "\n";
2445
54310121
PP
2446Determination of whether $BSD_STYLE should be set
2447is left as an exercise to the reader.
cb1a09d0 2448
19799a22 2449The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2450systems purporting POSIX compliance. See also the C<Term::ReadKey>
a3390c9f 2451module from your nearest CPAN site; details on CPAN can be found under
2b5ab1e7 2452L<perlmodlib/CPAN>.
a0d0e21e
LW
2453
2454=item getlogin
d74e8afc 2455X<getlogin> X<login>
a0d0e21e 2456
c17cdb72
NC
2457=for Pod::Functions return who logged in at this tty
2458
cf264981 2459This implements the C library function of the same name, which on most
3b10bc60 2460systems returns the current login from F</etc/utmp>, if any. If it
2461returns the empty string, use C<getpwuid>.
a0d0e21e 2462
f86702cc 2463 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2464
19799a22
GS
2465Do not consider C<getlogin> for authentication: it is not as
2466secure as C<getpwuid>.
4633a7c4 2467
ea9eb35a 2468Portability issues: L<perlport/getlogin>.
2469
a0d0e21e 2470=item getpeername SOCKET
d74e8afc 2471X<getpeername> X<peer>
a0d0e21e 2472
c17cdb72
NC
2473=for Pod::Functions find the other end of a socket connection
2474
a3390c9f
FC
2475Returns the packed sockaddr address of the other end of the SOCKET
2476connection.
a0d0e21e 2477
4633a7c4
LW
2478 use Socket;
2479 $hersockaddr = getpeername(SOCK);
19799a22 2480 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2481 $herhostname = gethostbyaddr($iaddr, AF_INET);
2482 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2483
2484=item getpgrp PID
d74e8afc 2485X<getpgrp> X<group>
a0d0e21e 2486
c17cdb72
NC
2487=for Pod::Functions get process group
2488
47e29363 2489Returns the current process group for the specified PID. Use
7660c0ab 2490a PID of C<0> to get the current process group for the
4633a7c4 2491current process. Will raise an exception if used on a machine that
a3390c9f
FC
2492doesn't implement getpgrp(2). If PID is omitted, returns the process
2493group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2494does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2495
ea9eb35a 2496Portability issues: L<perlport/getpgrp>.
2497
a0d0e21e 2498=item getppid
d74e8afc 2499X<getppid> X<parent> X<pid>
a0d0e21e 2500
c17cdb72
NC
2501=for Pod::Functions get parent process ID
2502
a0d0e21e
LW
2503Returns the process id of the parent process.
2504
d7c042c9
AB
2505Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2506around non-POSIX thread semantics the minority of Linux systems (and
2507Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2508has since been removed. See the documentation for L<$$|perlvar/$$> for
2509details.
4d76a344 2510
ea9eb35a 2511Portability issues: L<perlport/getppid>.
2512
a0d0e21e 2513=item getpriority WHICH,WHO
d74e8afc 2514X<getpriority> X<priority> X<nice>
a0d0e21e 2515
c17cdb72
NC
2516=for Pod::Functions get current nice value
2517
4633a7c4 2518Returns the current priority for a process, a process group, or a user.
01aa884e 2519(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2520machine that doesn't implement getpriority(2).
a0d0e21e 2521
ea9eb35a 2522Portability issues: L<perlport/getpriority>.
2523
a0d0e21e 2524=item getpwnam NAME
d74e8afc
ITB
2525X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2526X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2527X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2528X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2529X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2530X<endnetent> X<endprotoent> X<endservent>
a0d0e21e 2531
c17cdb72
NC
2532=for Pod::Functions get passwd record given user login name
2533
a0d0e21e
LW
2534=item getgrnam NAME
2535
c17cdb72
NC
2536=for Pod::Functions get group record given group name
2537
a0d0e21e
LW
2538=item gethostbyname NAME
2539
c17cdb72
NC
2540=for Pod::Functions get host record given name
2541
a0d0e21e
LW
2542=item getnetbyname NAME
2543
c17cdb72
NC
2544=for Pod::Functions get networks record given name
2545
a0d0e21e
LW
2546=item getprotobyname NAME
2547
c17cdb72
NC
2548=for Pod::Functions get protocol record given name
2549
a0d0e21e
LW
2550=item getpwuid UID
2551
c17cdb72
NC
2552=for Pod::Functions get passwd record given user ID
2553
a0d0e21e
LW
2554=item getgrgid GID
2555
c17cdb72
NC
2556=for Pod::Functions get group record given group user ID
2557
a0d0e21e
LW
2558=item getservbyname NAME,PROTO
2559
c17cdb72
NC
2560=for Pod::Functions get services record given its name
2561
a0d0e21e
LW
2562=item gethostbyaddr ADDR,ADDRTYPE
2563
c17cdb72
NC
2564=for Pod::Functions get host record given its address
2565
a0d0e21e
LW
2566=item getnetbyaddr ADDR,ADDRTYPE
2567
c17cdb72
NC
2568=for Pod::Functions get network record given its address
2569
a0d0e21e
LW
2570=item getprotobynumber NUMBER
2571
c17cdb72
NC
2572=for Pod::Functions get protocol record numeric protocol
2573
a0d0e21e
LW
2574=item getservbyport PORT,PROTO
2575
c17cdb72
NC
2576=for Pod::Functions get services record given numeric port
2577
a0d0e21e
LW
2578=item getpwent
2579
c17cdb72
NC
2580=for Pod::Functions get next passwd record
2581
a0d0e21e
LW
2582=item getgrent
2583
c17cdb72
NC
2584=for Pod::Functions get next group record
2585
a0d0e21e
LW
2586=item gethostent
2587
c17cdb72
NC
2588=for Pod::Functions get next hosts record
2589
a0d0e21e
LW
2590=item getnetent
2591
c17cdb72
NC
2592=for Pod::Functions get next networks record
2593
a0d0e21e
LW
2594=item getprotoent
2595
c17cdb72
NC
2596=for Pod::Functions get next protocols record
2597
a0d0e21e
LW
2598=item getservent
2599
c17cdb72
NC
2600=for Pod::Functions get next services record
2601
a0d0e21e
LW
2602=item setpwent
2603
c17cdb72
NC
2604=for Pod::Functions prepare passwd file for use
2605
a0d0e21e
LW
2606=item setgrent
2607
c17cdb72
NC
2608=for Pod::Functions prepare group file for use
2609
a0d0e21e
LW
2610=item sethostent STAYOPEN
2611
c17cdb72
NC
2612=for Pod::Functions prepare hosts file for use
2613
a0d0e21e
LW
2614=item setnetent STAYOPEN
2615
c17cdb72
NC
2616=for Pod::Functions prepare networks file for use
2617
a0d0e21e
LW
2618=item setprotoent STAYOPEN
2619
c17cdb72
NC
2620=for Pod::Functions prepare protocols file for use
2621
a0d0e21e
LW
2622=item setservent STAYOPEN
2623
c17cdb72
NC
2624=for Pod::Functions prepare services file for use
2625
a0d0e21e
LW
2626=item endpwent
2627
c17cdb72
NC
2628=for Pod::Functions be done using passwd file
2629
a0d0e21e
LW
2630=item endgrent
2631
c17cdb72
NC
2632=for Pod::Functions be done using group file
2633
a0d0e21e
LW
2634=item endhostent
2635
c17cdb72
NC
2636=for Pod::Functions be done using hosts file
2637
a0d0e21e
LW
2638=item endnetent
2639
c17cdb72
NC
2640=for Pod::Functions be done using networks file
2641
a0d0e21e
LW
2642=item endprotoent
2643
c17cdb72
NC
2644=for Pod::Functions be done using protocols file
2645
a0d0e21e
LW
2646=item endservent
2647
c17cdb72
NC
2648=for Pod::Functions be done using services file
2649
80d38338
TC
2650These routines are the same as their counterparts in the
2651system C library. In list context, the return values from the
a0d0e21e
LW
2652various get routines are as follows:
2653
2654 ($name,$passwd,$uid,$gid,
6ee623d5 2655 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2656 ($name,$passwd,$gid,$members) = getgr*
2657 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2658 ($name,$aliases,$addrtype,$net) = getnet*
2659 ($name,$aliases,$proto) = getproto*
2660 ($name,$aliases,$port,$proto) = getserv*
2661
3b10bc60 2662(If the entry doesn't exist you get an empty list.)
a0d0e21e 2663
4602f195
JH
2664The exact meaning of the $gcos field varies but it usually contains
2665the real name of the user (as opposed to the login name) and other
2666information pertaining to the user. Beware, however, that in many
2667system users are able to change this information and therefore it
106325ad 2668cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2669L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2670login shell, are also tainted, for the same reason.
4602f195 2671
5a964f20 2672In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2673lookup by name, in which case you get the other thing, whatever it is.
2674(If the entry doesn't exist you get the undefined value.) For example:
2675
5a964f20
TC
2676 $uid = getpwnam($name);
2677 $name = getpwuid($num);
2678 $name = getpwent();
2679 $gid = getgrnam($name);
08a33e13 2680 $name = getgrgid($num);
5a964f20
TC
2681 $name = getgrent();
2682 #etc.
a0d0e21e 2683
4602f195 2684In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2685in that they are unsupported on many systems. If the
4602f195
JH
2686$quota is unsupported, it is an empty scalar. If it is supported, it
2687usually encodes the disk quota. If the $comment field is unsupported,
2688it is an empty scalar. If it is supported it usually encodes some
2689administrative comment about the user. In some systems the $quota
2690field may be $change or $age, fields that have to do with password
2691aging. In some systems the $comment field may be $class. The $expire
2692field, if present, encodes the expiration period of the account or the
2693password. For the availability and the exact meaning of these fields
8f1da26d 2694in your system, please consult getpwnam(3) and your system's
4602f195
JH
2695F<pwd.h> file. You can also find out from within Perl what your
2696$quota and $comment fields mean and whether you have the $expire field
2697by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2698C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2699files are supported only if your vendor has implemented them in the
4602f195 2700intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2701shadow versions if you're running under privilege or if there exists
cf264981 2702the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2703and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2704facility are unlikely to be supported.
6ee623d5 2705
a3390c9f 2706The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2707the login names of the members of the group.
2708
2709For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2710C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2711C<@addrs> value returned by a successful call is a list of raw
2712addresses returned by the corresponding library call. In the
2713Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2714by saying something like:
2715
f337b084 2716 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2717
2b5ab1e7
TC
2718The Socket library makes this slightly easier:
2719
2720 use Socket;
2721 $iaddr = inet_aton("127.1"); # or whatever address
2722 $name = gethostbyaddr($iaddr, AF_INET);
2723
2724 # or going the other way
19799a22 2725 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2726
d760c846
GS
2727In the opposite way, to resolve a hostname to the IP address
2728you can write this:
2729
2730 use Socket;
2731 $packed_ip = gethostbyname("www.perl.org");
2732 if (defined $packed_ip) {
2733 $ip_address = inet_ntoa($packed_ip);
2734 }
2735
b018eaf1 2736Make sure C<gethostbyname()> is called in SCALAR context and that
d760c846
GS
2737its return value is checked for definedness.
2738
0d043efa
FC
2739The C<getprotobynumber> function, even though it only takes one argument,
2740has the precedence of a list operator, so beware:
2741
2742 getprotobynumber $number eq 'icmp' # WRONG
2743 getprotobynumber($number eq 'icmp') # actually means this
2744 getprotobynumber($number) eq 'icmp' # better this way
2745
19799a22
GS
2746If you get tired of remembering which element of the return list
2747contains which return value, by-name interfaces are provided
2748in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2749C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2750and C<User::grent>. These override the normal built-ins, supplying
2751versions that return objects with the appropriate names
2752for each field. For example:
5a964f20
TC
2753
2754 use File::stat;
2755 use User::pwent;
2756 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2757
a3390c9f 2758Even though it looks as though they're the same method calls (uid),
b76cc8ba 2759they aren't, because a C<File::stat> object is different from
19799a22 2760a C<User::pwent> object.
5a964f20 2761
ea9eb35a 2762Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2763
a0d0e21e 2764=item getsockname SOCKET
d74e8afc 2765X<getsockname>
a0d0e21e 2766
c17cdb72
NC
2767=for Pod::Functions retrieve the sockaddr for a given socket
2768
19799a22
GS
2769Returns the packed sockaddr address of this end of the SOCKET connection,
2770in case you don't know the address because you have several different
2771IPs that the connection might have come in on.
a0d0e21e 2772
4633a7c4
LW
2773 use Socket;
2774 $mysockaddr = getsockname(SOCK);
19799a22 2775 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2776 printf "Connect to %s [%s]\n",
19799a22
GS
2777 scalar gethostbyaddr($myaddr, AF_INET),
2778 inet_ntoa($myaddr);
a0d0e21e
LW
2779
2780=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2781X<getsockopt>
a0d0e21e 2782
c17cdb72
NC
2783=for Pod::Functions get socket options on a given socket
2784
636e6b1f
TH
2785Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2786Options may exist at multiple protocol levels depending on the socket
2787type, but at least the uppermost socket level SOL_SOCKET (defined in the
391b733c 2788C<Socket> module) will exist. To query options at another level the
636e6b1f 2789protocol number of the appropriate protocol controlling the option
391b733c 2790should be supplied. For example, to indicate that an option is to be
636e6b1f 2791interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2792number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2793
80d38338 2794The function returns a packed string representing the requested socket
3b10bc60 2795option, or C<undef> on error, with the reason for the error placed in
391b733c 2796C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2797consult getsockopt(2) for details. A common case is that the option is an
2798integer, in which case the result is a packed integer, which you can decode
2799using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2800
8f1da26d 2801Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2802
4852725b 2803 use Socket qw(:all);
636e6b1f
TH
2804
2805 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2806 or die "Could not determine the protocol number for tcp";
4852725b
DD
2807 # my $tcp = IPPROTO_TCP; # Alternative
2808 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2809 or die "getsockopt TCP_NODELAY: $!";
636e6b1f 2810 my $nodelay = unpack("I", $packed);
f7051f2c
FC
2811 print "Nagle's algorithm is turned ",
2812 $nodelay ? "off\n" : "on\n";
636e6b1f 2813
ea9eb35a 2814Portability issues: L<perlport/getsockopt>.
a0d0e21e
LW
2815
2816=item glob EXPR
d74e8afc 2817X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2818
0a753a76
PP
2819=item glob
2820
c17cdb72
NC
2821=for Pod::Functions expand filenames using wildcards
2822
d9a9d457 2823In list context, returns a (possibly empty) list of filename expansions on
391b733c 2824the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
d9a9d457 2825scalar context, glob iterates through such filename expansions, returning
391b733c
FC
2826undef when the list is exhausted. This is the internal function
2827implementing the C<< <*.c> >> operator, but you can use it directly. If
d9a9d457
JL
2828EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2829more detail in L<perlop/"I/O Operators">.
a0d0e21e 2830
80d38338
TC
2831Note that C<glob> splits its arguments on whitespace and treats
2832each segment as separate pattern. As such, C<glob("*.c *.h")>
2833matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2834C<glob(".* *")> matches all files in the current working directory.
a91bb7b1
TC
2835If you want to glob filenames that might contain whitespace, you'll
2836have to use extra quotes around the spacey filename to protect it.
2837For example, to glob filenames that have an C<e> followed by a space
2838followed by an C<f>, use either of:
2839
2840 @spacies = <"*e f*">;
2841 @spacies = glob '"*e f*"';
2842 @spacies = glob q("*e f*");
2843
2844If you had to get a variable through, you could do this:
2845
2846 @spacies = glob "'*${var}e f*'";
2847 @spacies = glob qq("*${var}e f*");
80d38338
TC
2848
2849If non-empty braces are the only wildcard characters used in the
2850C<glob>, no filenames are matched, but potentially many strings
2851are returned. For example, this produces nine strings, one for
2852each pairing of fruits and colors:
2853
2854 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2855
e9fa405d 2856This operator is implemented using the standard
5c0c9249
PF
2857C<File::Glob> extension. See L<File::Glob> for details, including
2858C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2859
ea9eb35a 2860Portability issues: L<perlport/glob>.
2861
a0d0e21e 2862=item gmtime EXPR
d74e8afc 2863X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2864
ce2984c3
PF
2865=item gmtime
2866
c17cdb72
NC
2867=for Pod::Functions convert UNIX time into record or string using Greenwich time
2868
4509d391 2869Works just like L</localtime> but the returned values are
435fbc73 2870localized for the standard Greenwich time zone.
a0d0e21e 2871
a3390c9f
FC
2872Note: When called in list context, $isdst, the last value
2873returned by gmtime, is always C<0>. There is no
435fbc73 2874Daylight Saving Time in GMT.
0a753a76 2875
ea9eb35a 2876Portability issues: L<perlport/gmtime>.
62aa5637 2877
a0d0e21e 2878=item goto LABEL
d74e8afc 2879X<goto> X<jump> X<jmp>
a0d0e21e 2880
748a9306
LW
2881=item goto EXPR
2882
a0d0e21e
LW
2883=item goto &NAME
2884
c17cdb72
NC
2885=for Pod::Functions create spaghetti code
2886
b500e03b 2887The C<goto-LABEL> form finds the statement labeled with LABEL and
391b733c 2888resumes execution there. It can't be used to get out of a block or
b500e03b
GG
2889subroutine given to C<sort>. It can be used to go almost anywhere
2890else within the dynamic scope, including out of subroutines, but it's
2891usually better to use some other construct such as C<last> or C<die>.
2892The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2893(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2894does not offer named loops combined with loop control. Perl does, and
2895this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2896
7660c0ab
A
2897The C<goto-EXPR> form expects a label name, whose scope will be resolved
2898dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2899necessarily recommended if you're optimizing for maintainability:
2900
2901 goto ("FOO", "BAR", "GLARCH")[$i];
2902
887d89fd 2903As shown in this example, C<goto-EXPR> is exempt from the "looks like a
391b733c
FC
2904function" rule. A pair of parentheses following it does not (necessarily)
2905delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
8a7e748e
FC
2906Also, unlike most named operators, this has the same precedence as
2907assignment.
887d89fd 2908
b500e03b 2909Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2910deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2911go into any construct that requires initialization, such as a
2912subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2913construct that is optimized away.
b500e03b 2914
1b6921cb
BT
2915The C<goto-&NAME> form is quite different from the other forms of
2916C<goto>. In fact, it isn't a goto in the normal sense at all, and
2917doesn't have the stigma associated with other gotos. Instead, it
2918exits the current subroutine (losing any changes set by local()) and
2919immediately calls in its place the named subroutine using the current
2920value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2921load another subroutine and then pretend that the other subroutine had
2922been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2923in the current subroutine are propagated to the other subroutine.)
2924After the C<goto>, not even C<caller> will be able to tell that this
2925routine was called first.
2926
2927NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2928containing a code reference or a block that evaluates to a code
6cb9131c 2929reference.
a0d0e21e
LW
2930
2931=item grep BLOCK LIST
d74e8afc 2932X<grep>
a0d0e21e
LW
2933
2934=item grep EXPR,LIST
2935
c17cdb72
NC
2936=for Pod::Functions locate elements in a list test true against a given criterion
2937
2b5ab1e7
TC
2938This is similar in spirit to, but not the same as, grep(1) and its
2939relatives. In particular, it is not limited to using regular expressions.
2f9daede 2940
a0d0e21e 2941Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2942C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2943elements for which the expression evaluated to true. In scalar
2944context, returns the number of times the expression was true.
a0d0e21e
LW
2945
2946 @foo = grep(!/^#/, @bar); # weed out comments
2947
2948or equivalently,
2949
2950 @foo = grep {!/^#/} @bar; # weed out comments
2951
be3174d2
GS
2952Note that C<$_> is an alias to the list value, so it can be used to
2953modify the elements of the LIST. While this is useful and supported,
2954it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2955Similarly, grep returns aliases into the original list, much as a for
2956loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2957element of a list returned by grep (for example, in a C<foreach>, C<map>
2958or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2959This is usually something to be avoided when writing clear code.
a0d0e21e 2960
a4fb8298 2961If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2962been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2963the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2964can't be seen from the outside, avoiding any potential side-effects.
2965
19799a22 2966See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2967
a0d0e21e 2968=item hex EXPR
d74e8afc 2969X<hex> X<hexadecimal>
a0d0e21e 2970
54310121 2971=item hex
bbce6d69 2972
c17cdb72
NC
2973=for Pod::Functions convert a string to a hexadecimal number
2974
2b5ab1e7 2975Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2976(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2977L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2978
2979 print hex '0xAf'; # prints '175'
2980 print hex 'aF'; # same
a0d0e21e 2981
19799a22 2982Hex strings may only represent integers. Strings that would cause
53305cf1 2983integer overflow trigger a warning. Leading whitespace is not stripped,
391b733c 2984unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 2985L</sprintf>, and L</unpack>.
19799a22 2986
ce2984c3 2987=item import LIST
d74e8afc 2988X<import>
a0d0e21e 2989
c17cdb72
NC
2990=for Pod::Functions patch a module's namespace into your own
2991
19799a22 2992There is no builtin C<import> function. It is just an ordinary
4633a7c4 2993method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2994names to another module. The C<use> function calls the C<import> method
cea6626f 2995for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2996
2997=item index STR,SUBSTR,POSITION
d74e8afc 2998X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2999
3000=item index STR,SUBSTR
3001
c17cdb72
NC
3002=for Pod::Functions find a substring within a string
3003
2b5ab1e7
TC
3004The index function searches for one string within another, but without
3005the wildcard-like behavior of a full regular-expression pattern match.
3006It returns the position of the first occurrence of SUBSTR in STR at
3007or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
3008beginning of the string. POSITION before the beginning of the string
3009or after its end is treated as if it were the beginning or the end,
e1dccc0d
Z
3010respectively. POSITION and the return value are based at zero.
3011If the substring is not found, C<index> returns -1.
a0d0e21e
LW
3012
3013=item int EXPR
f723aae1 3014X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 3015
54310121 3016=item int
bbce6d69 3017
c17cdb72
NC
3018=for Pod::Functions get the integer portion of a number
3019
7660c0ab 3020Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 3021You should not use this function for rounding: one because it truncates
3b10bc60 3022towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
3023numbers can sometimes produce counterintuitive results. For example,
3024C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3025because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 3026the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 3027functions will serve you better than will int().
a0d0e21e
LW
3028
3029=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 3030X<ioctl>
a0d0e21e 3031
c17cdb72
NC
3032=for Pod::Functions system-dependent device control system call
3033
2b5ab1e7 3034Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 3035
f7051f2c
FC
3036 require "sys/ioctl.ph"; # probably in
3037 # $Config{archlib}/sys/ioctl.ph
a0d0e21e 3038
a11c483f 3039to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 3040exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 3041own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 3042(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 3043may help you in this, but it's nontrivial.) SCALAR will be read and/or
3b10bc60 3044written depending on the FUNCTION; a C pointer to the string value of SCALAR
19799a22 3045will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
3046has no string value but does have a numeric value, that value will be
3047passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
3048true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>