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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
dba7b065 199C<break> is available only if you enable the experimental C<"switch">
7161e5c2 200feature or use the C<CORE::> prefix. The C<"switch"> feature also enables
dba7b065 201the C<default>, C<given> and C<when> statements, which are documented in
7161e5c2 202L<perlsyn/"Switch Statements">. The C<"switch"> feature is enabled
dba7b065 203automatically with a C<use v5.10> (or higher) declaration in the current
7161e5c2 204scope. In Perl v5.14 and earlier, C<continue> required the C<"switch">
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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
7161e5c2 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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GS
437
438If you are using ACLs, there is a pragma called C<filetest> that may
439produce more accurate results than the bare stat() mode bits.
5dac7880
FC
440When under C<use filetest 'access'> the above-mentioned filetests
441test whether the permission can(not) be granted using the
3b10bc60 442access(2) family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
443under this pragma return true even if there are no execute permission
444bits set (nor any extra execute permission ACLs). This strangeness is
391b733c 445due to the underlying system calls' definitions. Note also that, due to
ecae030f
MO
446the implementation of C<use filetest 'access'>, the C<_> special
447filehandle won't cache the results of the file tests when this pragma is
448in effect. Read the documentation for the C<filetest> pragma for more
449information.
5ff3f7a4 450
a0d0e21e
LW
451The C<-T> and C<-B> switches work as follows. The first block or so of the
452file is examined for odd characters such as strange control codes or
61eff3bc 453characters with the high bit set. If too many strange characters (>30%)
cf264981 454are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 455containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 456or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 457rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 458file, or a file at EOF when testing a filehandle. Because you have to
4633a7c4
LW
459read a file to do the C<-T> test, on most occasions you want to use a C<-f>
460against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 461
5dac7880 462If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 463the special filehandle consisting of a solitary underline, then the stat
a0d0e21e
LW
464structure of the previous file test (or stat operator) is used, saving
465a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 466that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 467symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 468an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 469Example:
a0d0e21e
LW
470
471 print "Can do.\n" if -r $a || -w _ || -x _;
472
473 stat($filename);
474 print "Readable\n" if -r _;
475 print "Writable\n" if -w _;
476 print "Executable\n" if -x _;
477 print "Setuid\n" if -u _;
478 print "Setgid\n" if -g _;
479 print "Sticky\n" if -k _;
480 print "Text\n" if -T _;
481 print "Binary\n" if -B _;
482
e9fa405d 483As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file
fbb0b3b3 484test operators, in a way that C<-f -w -x $file> is equivalent to
a5840dee 485C<-x $file && -w _ && -f _>. (This is only fancy syntax: if you use
fbb0b3b3
RGS
486the return value of C<-f $file> as an argument to another filetest
487operator, no special magic will happen.)
488
bee96257 489Portability issues: L<perlport/-X>.
ea9eb35a 490
bade7fbc
TC
491To avoid confusing would-be users of your code with mysterious
492syntax errors, put something like this at the top of your script:
493
494 use 5.010; # so filetest ops can stack
495
a0d0e21e 496=item abs VALUE
d74e8afc 497X<abs> X<absolute>
a0d0e21e 498
54310121 499=item abs
bbce6d69 500
c17cdb72
NC
501=for Pod::Functions absolute value function
502
a0d0e21e 503Returns the absolute value of its argument.
7660c0ab 504If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
505
506=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 507X<accept>
a0d0e21e 508
c17cdb72
NC
509=for Pod::Functions accept an incoming socket connect
510
3b10bc60 511Accepts an incoming socket connect, just as accept(2)
19799a22 512does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 513See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 514
8d2a6795
GS
515On systems that support a close-on-exec flag on files, the flag will
516be set for the newly opened file descriptor, as determined by the
517value of $^F. See L<perlvar/$^F>.
518
a0d0e21e 519=item alarm SECONDS
d74e8afc
ITB
520X<alarm>
521X<SIGALRM>
522X<timer>
a0d0e21e 523
54310121 524=item alarm
bbce6d69 525
c17cdb72
NC
526=for Pod::Functions schedule a SIGALRM
527
a0d0e21e 528Arranges to have a SIGALRM delivered to this process after the
cf264981 529specified number of wallclock seconds has elapsed. If SECONDS is not
391b733c 530specified, the value stored in C<$_> is used. (On some machines,
d400eac8
JH
531unfortunately, the elapsed time may be up to one second less or more
532than you specified because of how seconds are counted, and process
533scheduling may delay the delivery of the signal even further.)
534
535Only one timer may be counting at once. Each call disables the
536previous timer, and an argument of C<0> may be supplied to cancel the
537previous timer without starting a new one. The returned value is the
538amount of time remaining on the previous timer.
a0d0e21e 539
2bc69794
BS
540For delays of finer granularity than one second, the Time::HiRes module
541(from CPAN, and starting from Perl 5.8 part of the standard
542distribution) provides ualarm(). You may also use Perl's four-argument
543version of select() leaving the first three arguments undefined, or you
544might be able to use the C<syscall> interface to access setitimer(2) if
391b733c 545your system supports it. See L<perlfaq8> for details.
2b5ab1e7 546
80d38338
TC
547It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
548C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 549
19799a22
GS
550If you want to use C<alarm> to time out a system call you need to use an
551C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 552fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 553restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 554modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
555
556 eval {
a9a5a0dc
VP
557 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
558 alarm $timeout;
559 $nread = sysread SOCKET, $buffer, $size;
560 alarm 0;
ff68c719 561 };
ff68c719 562 if ($@) {
a9a5a0dc 563 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 564 # timed out
ff68c719
PP
565 }
566 else {
5ed4f2ec 567 # didn't
ff68c719
PP
568 }
569
91d81acc
JH
570For more information see L<perlipc>.
571
ea9eb35a 572Portability issues: L<perlport/alarm>.
573
a0d0e21e 574=item atan2 Y,X
d74e8afc 575X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e 576
c17cdb72
NC
577=for Pod::Functions arctangent of Y/X in the range -PI to PI
578
a0d0e21e
LW
579Returns the arctangent of Y/X in the range -PI to PI.
580
ca6e1c26 581For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
582function, or use the familiar relation:
583
584 sub tan { sin($_[0]) / cos($_[0]) }
585
a1021d57
RGS
586The return value for C<atan2(0,0)> is implementation-defined; consult
587your atan2(3) manpage for more information.
bf5f1b4c 588
ea9eb35a 589Portability issues: L<perlport/atan2>.
590
a0d0e21e 591=item bind SOCKET,NAME
d74e8afc 592X<bind>
a0d0e21e 593
c17cdb72
NC
594=for Pod::Functions binds an address to a socket
595
3b10bc60 596Binds a network address to a socket, just as bind(2)
19799a22 597does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
598packed address of the appropriate type for the socket. See the examples in
599L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 600
fae2c0fb 601=item binmode FILEHANDLE, LAYER
d74e8afc 602X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 603
a0d0e21e
LW
604=item binmode FILEHANDLE
605
c17cdb72
NC
606=for Pod::Functions prepare binary files for I/O
607
1cbfc93d
NIS
608Arranges for FILEHANDLE to be read or written in "binary" or "text"
609mode on systems where the run-time libraries distinguish between
610binary and text files. If FILEHANDLE is an expression, the value is
611taken as the name of the filehandle. Returns true on success,
b5fe5ca2 612otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 613
8f1da26d 614On some systems (in general, DOS- and Windows-based systems) binmode()
d807c6f4 615is necessary when you're not working with a text file. For the sake
d7a0d798
FC
616of portability it is a good idea always to use it when appropriate,
617and never to use it when it isn't appropriate. Also, people can
8f1da26d 618set their I/O to be by default UTF8-encoded Unicode, not bytes.
d807c6f4
JH
619
620In other words: regardless of platform, use binmode() on binary data,
d7a0d798 621like images, for example.
d807c6f4
JH
622
623If LAYER is present it is a single string, but may contain multiple
391b733c 624directives. The directives alter the behaviour of the filehandle.
d7a0d798 625When LAYER is present, using binmode on a text file makes sense.
d807c6f4 626
fae2c0fb 627If LAYER is omitted or specified as C<:raw> the filehandle is made
391b733c 628suitable for passing binary data. This includes turning off possible CRLF
0226bbdb 629translation and marking it as bytes (as opposed to Unicode characters).
749683d2 630Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 631Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
632Other layers that would affect the binary nature of the stream are
391b733c 633I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 634PERLIO environment variable.
01e6739c 635
3b10bc60 636The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
637form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
638establish default I/O layers. See L<open>.
639
fae2c0fb
RGS
640I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
641in "Programming Perl, 3rd Edition". However, since the publishing of this
642book, by many known as "Camel III", the consensus of the naming of this
643functionality has moved from "discipline" to "layer". All documentation
644of this version of Perl therefore refers to "layers" rather than to
645"disciplines". Now back to the regularly scheduled documentation...>
646
8f1da26d 647To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
6902c96a 648C<:utf8> just marks the data as UTF-8 without further checking,
8f1da26d 649while C<:encoding(UTF-8)> checks the data for actually being valid
391b733c 650UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 651
ed53a2bb 652In general, binmode() should be called after open() but before any I/O
3b10bc60 653is done on the filehandle. Calling binmode() normally flushes any
01e6739c 654pending buffered output data (and perhaps pending input data) on the
fae2c0fb 655handle. An exception to this is the C<:encoding> layer that
d7a0d798 656changes the default character encoding of the handle; see L</open>.
fae2c0fb 657The C<:encoding> layer sometimes needs to be called in
3874323d
JH
658mid-stream, and it doesn't flush the stream. The C<:encoding>
659also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 660internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 661
19799a22 662The operating system, device drivers, C libraries, and Perl run-time
8f1da26d
TC
663system all conspire to let the programmer treat a single
664character (C<\n>) as the line terminator, irrespective of external
30168b04
GS
665representation. On many operating systems, the native text file
666representation matches the internal representation, but on some
667platforms the external representation of C<\n> is made up of more than
668one character.
669
8f1da26d
TC
670All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
671a single character to end each line in the external representation of text
672(even though that single character is CARRIAGE RETURN on old, pre-Darwin
391b733c 673flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
8f1da26d
TC
674systems like OS/2, DOS, and the various flavors of MS-Windows, your program
675sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
676two characters C<\cM\cJ>. That means that if you don't use binmode() on
677these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
678input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
679output. This is what you want for text files, but it can be disastrous for
680binary files.
30168b04
GS
681
682Another consequence of using binmode() (on some systems) is that
683special end-of-file markers will be seen as part of the data stream.
d7a0d798
FC
684For systems from the Microsoft family this means that, if your binary
685data contain C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
686the file, unless you use binmode().
687
3b10bc60 688binmode() is important not only for readline() and print() operations,
30168b04
GS
689but also when using read(), seek(), sysread(), syswrite() and tell()
690(see L<perlport> for more details). See the C<$/> and C<$\> variables
691in L<perlvar> for how to manually set your input and output
692line-termination sequences.
a0d0e21e 693
ea9eb35a 694Portability issues: L<perlport/binmode>.
695
4633a7c4 696=item bless REF,CLASSNAME
d74e8afc 697X<bless>
a0d0e21e
LW
698
699=item bless REF
700
c17cdb72
NC
701=for Pod::Functions create an object
702
2b5ab1e7
TC
703This function tells the thingy referenced by REF that it is now an object
704in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 705is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 706it returns the reference for convenience. Always use the two-argument
cf264981 707version if a derived class might inherit the function doing the blessing.
e54e4959 708See L<perlobj> for more about the blessing (and blessings) of objects.
a0d0e21e 709
57668c4d 710Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 711Namespaces with all lowercase names are considered reserved for
391b733c 712Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
713confusion, you may wish to avoid such package names as well. Make sure
714that CLASSNAME is a true value.
60ad88b8
GS
715
716See L<perlmod/"Perl Modules">.
717
0d863452
RH
718=item break
719
d9b04284 720=for Pod::Functions +switch break out of a C<given> block
c17cdb72 721
0d863452
RH
722Break out of a C<given()> block.
723
a8a26e52
JK
724This keyword is enabled by the C<"switch"> feature; see L<feature> for
725more information on C<"switch">. You can also access it by prefixing it
726with C<CORE::>. Alternatively, include a C<use v5.10> or later to the
727current scope.
0d863452 728
a0d0e21e 729=item caller EXPR
d74e8afc 730X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
731
732=item caller
733
c17cdb72
NC
734=for Pod::Functions get context of the current subroutine call
735
1d56df50
DD
736Returns the context of the current pure perl subroutine call. In scalar
737context, returns the caller's package name if there I<is> a caller (that is, if
80d38338 738we're in a subroutine or C<eval> or C<require>) and the undefined value
1d56df50
DD
739otherwise. caller never returns XS subs and they are skipped. The next pure
740perl sub will appear instead of the XS sub in caller's return values. In list
741context, caller returns
a0d0e21e 742
ee6b43cc 743 # 0 1 2
748a9306 744 ($package, $filename, $line) = caller;
a0d0e21e
LW
745
746With EXPR, it returns some extra information that the debugger uses to
747print a stack trace. The value of EXPR indicates how many call frames
748to go back before the current one.
749
ee6b43cc 750 # 0 1 2 3 4
f3aa04c2 751 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 752
753 # 5 6 7 8 9 10
b3ca2e83 754 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 755 = caller($i);
e7ea3e70 756
951ba7fe 757Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 758call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 759C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 760C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 761C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 762$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
763each C<use> statement creates a C<require> frame inside an C<eval EXPR>
764frame.) $subroutine may also be C<(unknown)> if this particular
765subroutine happens to have been deleted from the symbol table.
766C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
767C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
585d73c3 768compiled with. C<$hints> corresponds to C<$^H>, and C<$bitmask>
1adb05cd
FC
769corresponds to C<${^WARNING_BITS}>. The
770C<$hints> and C<$bitmask> values are subject
585d73c3 771to change between versions of Perl, and are not meant for external use.
748a9306 772
b3ca2e83 773C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
391b733c 774caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
b3ca2e83
NC
775of this hash, as they are the actual values stored in the optree.
776
ffe0c19d
FC
777Furthermore, when called from within the DB package in
778list context, and with an argument, caller returns more
7660c0ab 779detailed information: it sets the list variable C<@DB::args> to be the
54310121 780arguments with which the subroutine was invoked.
748a9306 781
7660c0ab 782Be aware that the optimizer might have optimized call frames away before
19799a22 783C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 784might not return information about the call frame you expect it to, for
b76cc8ba 785C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 786previous time C<caller> was called.
7660c0ab 787
8f1da26d 788Be aware that setting C<@DB::args> is I<best effort>, intended for
391b733c 789debugging or generating backtraces, and should not be relied upon. In
ca9f0cb5
NC
790particular, as C<@_> contains aliases to the caller's arguments, Perl does
791not take a copy of C<@_>, so C<@DB::args> will contain modifications the
792subroutine makes to C<@_> or its contents, not the original values at call
391b733c 793time. C<@DB::args>, like C<@_>, does not hold explicit references to its
ca9f0cb5 794elements, so under certain cases its elements may have become freed and
391b733c 795reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 796of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
797I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
798reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5 799elements), so C<@DB::args> is actually a hybrid of the current state and
391b733c 800initial state of C<@_>. Buyer beware.
ca9f0cb5 801
a0d0e21e 802=item chdir EXPR
d74e8afc
ITB
803X<chdir>
804X<cd>
f723aae1 805X<directory, change>
a0d0e21e 806
c4aca7d0
GA
807=item chdir FILEHANDLE
808
809=item chdir DIRHANDLE
810
ce2984c3
PF
811=item chdir
812
c17cdb72
NC
813=for Pod::Functions change your current working directory
814
391b733c 815Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 816changes to the directory specified by C<$ENV{HOME}>, if set; if not,
391b733c
FC
817changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
818variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
819neither is set, C<chdir> does nothing. It returns true on success,
820false otherwise. See the example under C<die>.
a0d0e21e 821
3b10bc60 822On systems that support fchdir(2), you may pass a filehandle or
34169887 823directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 824passing handles raises an exception.
c4aca7d0 825
a0d0e21e 826=item chmod LIST
d74e8afc 827X<chmod> X<permission> X<mode>
a0d0e21e 828
c17cdb72
NC
829=for Pod::Functions changes the permissions on a list of files
830
a0d0e21e 831Changes the permissions of a list of files. The first element of the
8f1da26d 832list must be the numeric mode, which should probably be an octal
4ad40acf 833number, and which definitely should I<not> be a string of octal digits:
3b10bc60 834C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 835successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 836
3b10bc60 837 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 838 chmod 0755, @executables;
3b10bc60 839 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 840 # --w----r-T
3b10bc60 841 $mode = "0644"; chmod oct($mode), "foo"; # this is better
842 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 843
3b10bc60 844On systems that support fchmod(2), you may pass filehandles among the
845files. On systems that don't support fchmod(2), passing filehandles raises
846an exception. Filehandles must be passed as globs or glob references to be
847recognized; barewords are considered filenames.
c4aca7d0
GA
848
849 open(my $fh, "<", "foo");
850 my $perm = (stat $fh)[2] & 07777;
851 chmod($perm | 0600, $fh);
852
3b10bc60 853You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
854module:
855
3b10bc60 856 use Fcntl qw( :mode );
ca6e1c26 857 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 858 # Identical to the chmod 0755 of the example above.
ca6e1c26 859
ea9eb35a 860Portability issues: L<perlport/chmod>.
861
a0d0e21e 862=item chomp VARIABLE
d74e8afc 863X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 864
313c9f5c 865=item chomp( LIST )
a0d0e21e
LW
866
867=item chomp
868
c17cdb72
NC
869=for Pod::Functions remove a trailing record separator from a string
870
2b5ab1e7
TC
871This safer version of L</chop> removes any trailing string
872that corresponds to the current value of C<$/> (also known as
28757baa
PP
873$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
874number of characters removed from all its arguments. It's often used to
875remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
876that the final record may be missing its newline. When in paragraph
877mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 878When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 879a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 880remove anything.
19799a22 881If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
882
883 while (<>) {
a9a5a0dc
VP
884 chomp; # avoid \n on last field
885 @array = split(/:/);
886 # ...
a0d0e21e
LW
887 }
888
feef49c9
FC
889If VARIABLE is a hash, it chomps the hash's values, but not its keys,
890resetting the C<each> iterator in the process.
4bf21a6d 891
a0d0e21e
LW
892You can actually chomp anything that's an lvalue, including an assignment:
893
894 chomp($cwd = `pwd`);
895 chomp($answer = <STDIN>);
896
897If you chomp a list, each element is chomped, and the total number of
898characters removed is returned.
899
15e44fd8
RGS
900Note that parentheses are necessary when you're chomping anything
901that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
902is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
903C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
904C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
905as C<chomp($a, $b)>.
906
a0d0e21e 907=item chop VARIABLE
d74e8afc 908X<chop>
a0d0e21e 909
313c9f5c 910=item chop( LIST )
a0d0e21e
LW
911
912=item chop
913
c17cdb72
NC
914=for Pod::Functions remove the last character from a string
915
a0d0e21e 916Chops off the last character of a string and returns the character
5b3eff12 917chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 918scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
feef49c9
FC
919If VARIABLE is a hash, it chops the hash's values, but not its keys,
920resetting the C<each> iterator in the process.
4bf21a6d 921
5b3eff12 922You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
923
924If you chop a list, each element is chopped. Only the value of the
19799a22 925last C<chop> is returned.
a0d0e21e 926
19799a22 927Note that C<chop> returns the last character. To return all but the last
748a9306
LW
928character, use C<substr($string, 0, -1)>.
929
15e44fd8
RGS
930See also L</chomp>.
931
a0d0e21e 932=item chown LIST
d74e8afc 933X<chown> X<owner> X<user> X<group>
a0d0e21e 934
c17cdb72
NC
935=for Pod::Functions change the ownership on a list of files
936
a0d0e21e 937Changes the owner (and group) of a list of files. The first two
19799a22
GS
938elements of the list must be the I<numeric> uid and gid, in that
939order. A value of -1 in either position is interpreted by most
940systems to leave that value unchanged. Returns the number of files
941successfully changed.
a0d0e21e
LW
942
943 $cnt = chown $uid, $gid, 'foo', 'bar';
944 chown $uid, $gid, @filenames;
945
3b10bc60 946On systems that support fchown(2), you may pass filehandles among the
947files. On systems that don't support fchown(2), passing filehandles raises
948an exception. Filehandles must be passed as globs or glob references to be
949recognized; barewords are considered filenames.
c4aca7d0 950
54310121 951Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
952
953 print "User: ";
19799a22 954 chomp($user = <STDIN>);
5a964f20 955 print "Files: ";
19799a22 956 chomp($pattern = <STDIN>);
a0d0e21e
LW
957
958 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 959 or die "$user not in passwd file";
a0d0e21e 960
5ed4f2ec 961 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
962 chown $uid, $gid, @ary;
963
54310121 964On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
965file unless you're the superuser, although you should be able to change
966the group to any of your secondary groups. On insecure systems, these
967restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
968On POSIX systems, you can detect this condition this way:
969
970 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
971 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 972
ea9eb35a 973Portability issues: L<perlport/chmod>.
974
a0d0e21e 975=item chr NUMBER
d74e8afc 976X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 977
54310121 978=item chr
bbce6d69 979
c17cdb72
NC
980=for Pod::Functions get character this number represents
981
a0d0e21e 982Returns the character represented by that NUMBER in the character set.
a0ed51b3 983For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 984chr(0x263a) is a Unicode smiley face.
aaa68c4a 985
8a064bd6 986Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 987except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
988(truncated to an integer) are used.
989
974da8e5
JH
990If NUMBER is omitted, uses C<$_>.
991
b76cc8ba 992For the reverse, use L</ord>.
a0d0e21e 993
2575c402
JW
994Note that characters from 128 to 255 (inclusive) are by default
995internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 996
2575c402 997See L<perlunicode> for more about Unicode.
bbce6d69 998
a0d0e21e 999=item chroot FILENAME
d74e8afc 1000X<chroot> X<root>
a0d0e21e 1001
54310121 1002=item chroot
bbce6d69 1003
c17cdb72
NC
1004=for Pod::Functions make directory new root for path lookups
1005
5a964f20 1006This function works like the system call by the same name: it makes the
4633a7c4 1007named directory the new root directory for all further pathnames that
951ba7fe 1008begin with a C</> by your process and all its children. (It doesn't
28757baa 1009change your current working directory, which is unaffected.) For security
4633a7c4 1010reasons, this call is restricted to the superuser. If FILENAME is
19799a22 1011omitted, does a C<chroot> to C<$_>.
a0d0e21e 1012
ea9eb35a 1013Portability issues: L<perlport/chroot>.
1014
a0d0e21e 1015=item close FILEHANDLE
d74e8afc 1016X<close>
a0d0e21e 1017
6a518fbc
TP
1018=item close
1019
c17cdb72
NC
1020=for Pod::Functions close file (or pipe or socket) handle
1021
3b10bc60 1022Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 1023buffers, and closes the system file descriptor. Returns true if those
8f1da26d 1024operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
1025layer. Closes the currently selected filehandle if the argument is
1026omitted.
fb73857a
PP
1027
1028You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 1029another C<open> on it, because C<open> closes it for you. (See
01aa884e 1030L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
19799a22 1031counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 1032
3b10bc60 1033If the filehandle came from a piped open, C<close> returns false if one of
1034the other syscalls involved fails or if its program exits with non-zero
1035status. If the only problem was that the program exited non-zero, C<$!>
1036will be set to C<0>. Closing a pipe also waits for the process executing
1037on the pipe to exit--in case you wish to look at the output of the pipe
1038afterwards--and implicitly puts the exit status value of that command into
1039C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 1040
2e0cfa16
FC
1041If there are multiple threads running, C<close> on a filehandle from a
1042piped open returns true without waiting for the child process to terminate,
1043if the filehandle is still open in another thread.
1044
80d38338
TC
1045Closing the read end of a pipe before the process writing to it at the
1046other end is done writing results in the writer receiving a SIGPIPE. If
1047the other end can't handle that, be sure to read all the data before
1048closing the pipe.
73689b13 1049
fb73857a 1050Example:
a0d0e21e 1051
fb73857a
PP
1052 open(OUTPUT, '|sort >foo') # pipe to sort
1053 or die "Can't start sort: $!";
5ed4f2ec 1054 #... # print stuff to output
1055 close OUTPUT # wait for sort to finish
fb73857a
PP
1056 or warn $! ? "Error closing sort pipe: $!"
1057 : "Exit status $? from sort";
5ed4f2ec 1058 open(INPUT, 'foo') # get sort's results
fb73857a 1059 or die "Can't open 'foo' for input: $!";
a0d0e21e 1060
5a964f20 1061FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 1062filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
1063
1064=item closedir DIRHANDLE
d74e8afc 1065X<closedir>
a0d0e21e 1066
c17cdb72
NC
1067=for Pod::Functions close directory handle
1068
19799a22 1069Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
1070system call.
1071
a0d0e21e 1072=item connect SOCKET,NAME
d74e8afc 1073X<connect>
a0d0e21e 1074
c17cdb72
NC
1075=for Pod::Functions connect to a remote socket
1076
80d38338
TC
1077Attempts to connect to a remote socket, just like connect(2).
1078Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
1079packed address of the appropriate type for the socket. See the examples in
1080L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 1081
cb1a09d0 1082=item continue BLOCK
d74e8afc 1083X<continue>
cb1a09d0 1084
0d863452
RH
1085=item continue
1086
c17cdb72
NC
1087=for Pod::Functions optional trailing block in a while or foreach
1088
4a904372
FC
1089When followed by a BLOCK, C<continue> is actually a
1090flow control statement rather than a function. If
cf264981 1091there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
1092C<foreach>), it is always executed just before the conditional is about to
1093be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
1094it can be used to increment a loop variable, even when the loop has been
1095continued via the C<next> statement (which is similar to the C C<continue>
1096statement).
1097
98293880 1098C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 1099block; C<last> and C<redo> behave as if they had been executed within
19799a22 1100the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
1101block, it may be more entertaining.
1102
1103 while (EXPR) {
a9a5a0dc
VP
1104 ### redo always comes here
1105 do_something;
1d2dff63 1106 } continue {
a9a5a0dc
VP
1107 ### next always comes here
1108 do_something_else;
1109 # then back the top to re-check EXPR
1d2dff63
GS
1110 }
1111 ### last always comes here
1112
3b10bc60 1113Omitting the C<continue> section is equivalent to using an
1114empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1115to check the condition at the top of the loop.
1116
4a904372 1117When there is no BLOCK, C<continue> is a function that
8f1da26d
TC
1118falls through the current C<when> or C<default> block instead of iterating
1119a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
4a904372
FC
1120In Perl 5.14 and earlier, this form of C<continue> was
1121only available when the C<"switch"> feature was enabled.
48238296 1122See L<feature> and L<perlsyn/"Switch Statements"> for more
8f1da26d 1123information.
0d863452 1124
a0d0e21e 1125=item cos EXPR
d74e8afc 1126X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1127
d6217f1e
GS
1128=item cos
1129
c17cdb72
NC
1130=for Pod::Functions cosine function
1131
5a964f20 1132Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1133takes the cosine of C<$_>.
a0d0e21e 1134
ca6e1c26 1135For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1136function, or use this relation:
1137
1138 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1139
a0d0e21e 1140=item crypt PLAINTEXT,SALT
d74e8afc 1141X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1142X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1143
c17cdb72
NC
1144=for Pod::Functions one-way passwd-style encryption
1145
ef2e6798
MS
1146Creates a digest string exactly like the crypt(3) function in the C
1147library (assuming that you actually have a version there that has not
bb23f8d1 1148been extirpated as a potential munition).
ef2e6798 1149
34169887 1150crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1151into a short string, called a digest, which is returned. The same
1152PLAINTEXT and SALT will always return the same string, but there is no
1153(known) way to get the original PLAINTEXT from the hash. Small
1154changes in the PLAINTEXT or SALT will result in large changes in the
1155digest.
1156
1157There is no decrypt function. This function isn't all that useful for
1158cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1159mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1160primarily used to check if two pieces of text are the same without
1161having to transmit or store the text itself. An example is checking
1162if a correct password is given. The digest of the password is stored,
cf264981 1163not the password itself. The user types in a password that is
ef2e6798 1164crypt()'d with the same salt as the stored digest. If the two digests
34169887 1165match, the password is correct.
ef2e6798
MS
1166
1167When verifying an existing digest string you should use the digest as
1168the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1169to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1170crypt() will hash the new string with the same salt as the digest.
1171This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1172with more exotic implementations. In other words, assume
1173nothing about the returned string itself nor about how many bytes
1174of SALT may matter.
85c16d83
JH
1175
1176Traditionally the result is a string of 13 bytes: two first bytes of
1177the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
391b733c 1178the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1179hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1180and implementations on non-Unix platforms may produce different
ef2e6798 1181strings.
85c16d83
JH
1182
1183When choosing a new salt create a random two character string whose
1184characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1185'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1186characters is just a recommendation; the characters allowed in
1187the salt depend solely on your system's crypt library, and Perl can't
1188restrict what salts C<crypt()> accepts.
e71965be 1189
a0d0e21e 1190Here's an example that makes sure that whoever runs this program knows
cf264981 1191their password:
a0d0e21e
LW
1192
1193 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1194
1195 system "stty -echo";
1196 print "Password: ";
e71965be 1197 chomp($word = <STDIN>);
a0d0e21e
LW
1198 print "\n";
1199 system "stty echo";
1200
e71965be 1201 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1202 die "Sorry...\n";
a0d0e21e 1203 } else {
a9a5a0dc 1204 print "ok\n";
54310121 1205 }
a0d0e21e 1206
9f8f0c9d 1207Of course, typing in your own password to whoever asks you
748a9306 1208for it is unwise.
a0d0e21e 1209
ef2e6798 1210The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1211of data, not least of all because you can't get the information
ef2e6798 1212back. Look at the L<Digest> module for more robust algorithms.
19799a22 1213
f2791508
JH
1214If using crypt() on a Unicode string (which I<potentially> has
1215characters with codepoints above 255), Perl tries to make sense
34169887 1216of the situation by trying to downgrade (a copy of)
f2791508
JH
1217the string back to an eight-bit byte string before calling crypt()
1218(on that copy). If that works, good. If not, crypt() dies with
1219C<Wide character in crypt>.
85c16d83 1220
ea9eb35a 1221Portability issues: L<perlport/crypt>.
1222
aa689395 1223=item dbmclose HASH
d74e8afc 1224X<dbmclose>
a0d0e21e 1225
c17cdb72
NC
1226=for Pod::Functions breaks binding on a tied dbm file
1227
19799a22 1228[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1229
aa689395 1230Breaks the binding between a DBM file and a hash.
a0d0e21e 1231
ea9eb35a 1232Portability issues: L<perlport/dbmclose>.
1233
19799a22 1234=item dbmopen HASH,DBNAME,MASK
d74e8afc 1235X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1236
c17cdb72
NC
1237=for Pod::Functions create binding on a tied dbm file
1238
01aa884e
KW
1239[This function has been largely superseded by the
1240L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
a0d0e21e 1241
7b8d334a 1242This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1243hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1244argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1245is the name of the database (without the F<.dir> or F<.pag> extension if
1246any). If the database does not exist, it is created with protection
1b3a6178
FC
1247specified by MASK (as modified by the C<umask>). To prevent creation of
1248the database if it doesn't exist, you may specify a MODE
1249of 0, and the function will return a false value if it
1250can't find an existing database. If your system supports
80d38338 1251only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1252program. In older versions of Perl, if your system had neither DBM nor
19799a22 1253ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1254sdbm(3).
1255
1256If you don't have write access to the DBM file, you can only read hash
1257variables, not set them. If you want to test whether you can write,
3b10bc60 1258either use file tests or try setting a dummy hash entry inside an C<eval>
1259to trap the error.
a0d0e21e 1260
19799a22
GS
1261Note that functions such as C<keys> and C<values> may return huge lists
1262when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1263function to iterate over large DBM files. Example:
1264
1265 # print out history file offsets
1266 dbmopen(%HIST,'/usr/lib/news/history',0666);
1267 while (($key,$val) = each %HIST) {
a9a5a0dc 1268 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1269 }
1270 dbmclose(%HIST);
1271
cb1a09d0 1272See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1273cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1274rich implementation.
4633a7c4 1275
2b5ab1e7
TC
1276You can control which DBM library you use by loading that library
1277before you call dbmopen():
1278
1279 use DB_File;
1280 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1281 or die "Can't open netscape history file: $!";
2b5ab1e7 1282
ea9eb35a 1283Portability issues: L<perlport/dbmopen>.
1284
a0d0e21e 1285=item defined EXPR
d74e8afc 1286X<defined> X<undef> X<undefined>
a0d0e21e 1287
54310121 1288=item defined
bbce6d69 1289
c17cdb72
NC
1290=for Pod::Functions test whether a value, variable, or function is defined
1291
2f9daede 1292Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1293the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1294checked.
1295
1296Many operations return C<undef> to indicate failure, end of file,
1297system error, uninitialized variable, and other exceptional
1298conditions. This function allows you to distinguish C<undef> from
1299other values. (A simple Boolean test will not distinguish among
7660c0ab 1300C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1301false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1302doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1303returns C<undef> when its argument is an empty array, I<or> when the
1304element to return happens to be C<undef>.
1305
f10b0346
GS
1306You may also use C<defined(&func)> to check whether subroutine C<&func>
1307has ever been defined. The return value is unaffected by any forward
80d38338 1308declarations of C<&func>. A subroutine that is not defined
847c7ebe 1309may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1310makes it spring into existence the first time that it is called; see
847c7ebe 1311L<perlsub>.
f10b0346
GS
1312
1313Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1314used to report whether memory for that aggregate had ever been
f10b0346
GS
1315allocated. This behavior may disappear in future versions of Perl.
1316You should instead use a simple test for size:
1317
1318 if (@an_array) { print "has array elements\n" }
1319 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1320
1321When used on a hash element, it tells you whether the value is defined,
dc848c6f 1322not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1323purpose.
a0d0e21e
LW
1324
1325Examples:
1326
8f1da26d 1327 print if defined $switch{D};
a0d0e21e
LW
1328 print "$val\n" while defined($val = pop(@ary));
1329 die "Can't readlink $sym: $!"
a9a5a0dc 1330 unless defined($value = readlink $sym);
a0d0e21e 1331 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1332 $debugging = 0 unless defined $debugging;
a0d0e21e 1333
8f1da26d 1334Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1335discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1336defined values. For example, if you say
a5f75d66
AD
1337
1338 "ab" =~ /a(.*)b/;
1339
80d38338 1340The pattern match succeeds and C<$1> is defined, although it
cf264981 1341matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1342matched something that happened to be zero characters long. This is all
a5f75d66 1343very above-board and honest. When a function returns an undefined value,
2f9daede 1344it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1345should use C<defined> only when questioning the integrity of what
7660c0ab 1346you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1347what you want.
1348
dc848c6f 1349See also L</undef>, L</exists>, L</ref>.
2f9daede 1350
a0d0e21e 1351=item delete EXPR
d74e8afc 1352X<delete>
a0d0e21e 1353
c17cdb72
NC
1354=for Pod::Functions deletes a value from a hash
1355
d0a76353
RS
1356Given an expression that specifies an element or slice of a hash, C<delete>
1357deletes the specified elements from that hash so that exists() on that element
1358no longer returns true. Setting a hash element to the undefined value does
1359not remove its key, but deleting it does; see L</exists>.
80d38338 1360
8f1da26d 1361In list context, returns the value or values deleted, or the last such
80d38338 1362element in scalar context. The return list's length always matches that of
d0a76353
RS
1363the argument list: deleting non-existent elements returns the undefined value
1364in their corresponding positions.
80d38338 1365
d0a76353
RS
1366delete() may also be used on arrays and array slices, but its behavior is less
1367straightforward. Although exists() will return false for deleted entries,
1368deleting array elements never changes indices of existing values; use shift()
1369or splice() for that. However, if all deleted elements fall at the end of an
1370array, the array's size shrinks to the position of the highest element that
1371still tests true for exists(), or to 0 if none do.
1372
8f1da26d 1373B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1374be removed in a future version of Perl.
80d38338
TC
1375
1376Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1377a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1378or array may not necessarily return anything; it depends on the implementation
1379of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1380
80d38338
TC
1381The C<delete local EXPR> construct localizes the deletion to the current
1382block at run time. Until the block exits, elements locally deleted
1383temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1384of composite types">.
eba0920a
EM
1385
1386 %hash = (foo => 11, bar => 22, baz => 33);
f7051f2c
FC
1387 $scalar = delete $hash{foo}; # $scalar is 11
1388 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1389 @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
eba0920a 1390
01020589 1391The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1392
5f05dabc 1393 foreach $key (keys %HASH) {
a9a5a0dc 1394 delete $HASH{$key};
a0d0e21e
LW
1395 }
1396
01020589 1397 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1398 delete $ARRAY[$index];
01020589
GS
1399 }
1400
1401And so do these:
5f05dabc 1402
01020589
GS
1403 delete @HASH{keys %HASH};
1404
9740c838 1405 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1406
80d38338
TC
1407But both are slower than assigning the empty list
1408or undefining %HASH or @ARRAY, which is the customary
1409way to empty out an aggregate:
01020589 1410
5ed4f2ec 1411 %HASH = (); # completely empty %HASH
1412 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1413
5ed4f2ec 1414 @ARRAY = (); # completely empty @ARRAY
1415 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1416
80d38338
TC
1417The EXPR can be arbitrarily complicated provided its
1418final operation is an element or slice of an aggregate:
a0d0e21e
LW
1419
1420 delete $ref->[$x][$y]{$key};
5f05dabc 1421 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1422
01020589
GS
1423 delete $ref->[$x][$y][$index];
1424 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1425
a0d0e21e 1426=item die LIST
d74e8afc 1427X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1428
c17cdb72
NC
1429=for Pod::Functions raise an exception or bail out
1430
391b733c 1431C<die> raises an exception. Inside an C<eval> the error message is stuffed
4c050ad5
NC
1432into C<$@> and the C<eval> is terminated with the undefined value.
1433If the exception is outside of all enclosing C<eval>s, then the uncaught
391b733c 1434exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1435need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1436
1437Equivalent examples:
1438
1439 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1440 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1441
ccac6780 1442If the last element of LIST does not end in a newline, the current
df37ec69
WW
1443script line number and input line number (if any) are also printed,
1444and a newline is supplied. Note that the "input line number" (also
1445known as "chunk") is subject to whatever notion of "line" happens to
1446be currently in effect, and is also available as the special variable
1447C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1448
1449Hint: sometimes appending C<", stopped"> to your message will cause it
1450to make better sense when the string C<"at foo line 123"> is appended.
1451Suppose you are running script "canasta".
a0d0e21e
LW
1452
1453 die "/etc/games is no good";
1454 die "/etc/games is no good, stopped";
1455
1456produce, respectively
1457
1458 /etc/games is no good at canasta line 123.
1459 /etc/games is no good, stopped at canasta line 123.
1460
a96d0188 1461If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1462previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1463This is useful for propagating exceptions:
1464
1465 eval { ... };
1466 die unless $@ =~ /Expected exception/;
1467
a96d0188 1468If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1469C<PROPAGATE> method, that method will be called with additional file
1470and line number parameters. The return value replaces the value in
34169887 1471C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1472were called.
1473
7660c0ab 1474If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1475
4c050ad5
NC
1476If an uncaught exception results in interpreter exit, the exit code is
1477determined from the values of C<$!> and C<$?> with this pseudocode:
1478
1479 exit $! if $!; # errno
1480 exit $? >> 8 if $? >> 8; # child exit status
1481 exit 255; # last resort
1482
1483The intent is to squeeze as much possible information about the likely cause
391b733c
FC
1484into the limited space of the system exit
1485code. However, as C<$!> is the value
4c050ad5
NC
1486of C's C<errno>, which can be set by any system call, this means that the value
1487of the exit code used by C<die> can be non-predictable, so should not be relied
1488upon, other than to be non-zero.
1489
80d38338
TC
1490You can also call C<die> with a reference argument, and if this is trapped
1491within an C<eval>, C<$@> contains that reference. This permits more
1492elaborate exception handling using objects that maintain arbitrary state
1493about the exception. Such a scheme is sometimes preferable to matching
1494particular string values of C<$@> with regular expressions. Because C<$@>
1495is a global variable and C<eval> may be used within object implementations,
1496be careful that analyzing the error object doesn't replace the reference in
1497the global variable. It's easiest to make a local copy of the reference
1498before any manipulations. Here's an example:
52531d10 1499
80d38338 1500 use Scalar::Util "blessed";
da279afe 1501
52531d10 1502 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7 1503 if (my $ev_err = $@) {
f7051f2c
FC
1504 if (blessed($ev_err)
1505 && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1506 # handle Some::Module::Exception
1507 }
1508 else {
1509 # handle all other possible exceptions
1510 }
1511 }
1512
3b10bc60 1513Because Perl stringifies uncaught exception messages before display,
80d38338 1514you'll probably want to overload stringification operations on
52531d10
GS
1515exception objects. See L<overload> for details about that.
1516
19799a22
GS
1517You can arrange for a callback to be run just before the C<die>
1518does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1519handler is called with the error text and can change the error
19799a22 1520message, if it sees fit, by calling C<die> again. See
96090e4f 1521L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1522L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1523to be run only right before your program was to exit, this is not
3b10bc60 1524currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1525even inside eval()ed blocks/strings! If one wants the hook to do
1526nothing in such situations, put
fb73857a 1527
5ed4f2ec 1528 die @_ if $^S;
fb73857a 1529
19799a22
GS
1530as the first line of the handler (see L<perlvar/$^S>). Because
1531this promotes strange action at a distance, this counterintuitive
b76cc8ba 1532behavior may be fixed in a future release.
774d564b 1533
4c050ad5
NC
1534See also exit(), warn(), and the Carp module.
1535
a0d0e21e 1536=item do BLOCK
d74e8afc 1537X<do> X<block>
a0d0e21e 1538
c17cdb72
NC
1539=for Pod::Functions turn a BLOCK into a TERM
1540
a0d0e21e 1541Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1542sequence of commands indicated by BLOCK. When modified by the C<while> or
1543C<until> loop modifier, executes the BLOCK once before testing the loop
391b733c 1544condition. (On other statements the loop modifiers test the conditional
6b275a1f 1545first.)
a0d0e21e 1546
4968c1e4 1547C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1548C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1549See L<perlsyn> for alternative strategies.
4968c1e4 1550
a0d0e21e 1551=item do SUBROUTINE(LIST)
d74e8afc 1552X<do>
a0d0e21e 1553
3bab73c8
FC
1554This form of subroutine call is deprecated. SUBROUTINE can be a bareword
1555or scalar variable.
a0d0e21e
LW
1556
1557=item do EXPR
d74e8afc 1558X<do>
a0d0e21e
LW
1559
1560Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1561file as a Perl script.
a0d0e21e
LW
1562
1563 do 'stat.pl';
1564
c319391a 1565is largely like
a0d0e21e 1566
986b19de 1567 eval `cat stat.pl`;
a0d0e21e 1568
c319391a
AC
1569except that it's more concise, runs no external processes, keeps track of
1570the current
96090e4f
A
1571filename for error messages, searches the C<@INC> directories, and updates
1572C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1573these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1574cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1575same, however, in that it does reparse the file every time you call it,
1576so you probably don't want to do this inside a loop.
a0d0e21e 1577
8f1da26d 1578If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1579an error message in C<$@>. If C<do> cannot read the file, it returns undef
1580and sets C<$!> to the error. Always check C<$@> first, as compilation
1581could fail in a way that also sets C<$!>. If the file is successfully
1582compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1583
80d38338 1584Inclusion of library modules is better done with the
19799a22 1585C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1586and raise an exception if there's a problem.
a0d0e21e 1587
5a964f20
TC
1588You might like to use C<do> to read in a program configuration
1589file. Manual error checking can be done this way:
1590
b76cc8ba 1591 # read in config files: system first, then user
f86cebdf 1592 for $file ("/share/prog/defaults.rc",
b76cc8ba 1593 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1594 {
1595 unless ($return = do $file) {
1596 warn "couldn't parse $file: $@" if $@;
1597 warn "couldn't do $file: $!" unless defined $return;
1598 warn "couldn't run $file" unless $return;
1599 }
5a964f20
TC
1600 }
1601
a0d0e21e 1602=item dump LABEL
d74e8afc 1603X<dump> X<core> X<undump>
a0d0e21e 1604
8a7e748e
FC
1605=item dump EXPR
1606
1614b0e3
JD
1607=item dump
1608
c17cdb72
NC
1609=for Pod::Functions create an immediate core dump
1610
19799a22
GS
1611This function causes an immediate core dump. See also the B<-u>
1612command-line switch in L<perlrun>, which does the same thing.
1613Primarily this is so that you can use the B<undump> program (not
1614supplied) to turn your core dump into an executable binary after
1615having initialized all your variables at the beginning of the
1616program. When the new binary is executed it will begin by executing
1617a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1618Think of it as a goto with an intervening core dump and reincarnation.
8a7e748e
FC
1619If C<LABEL> is omitted, restarts the program from the top. The
1620C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1621computed at run time, being otherwise identical to C<dump LABEL>.
19799a22
GS
1622
1623B<WARNING>: Any files opened at the time of the dump will I<not>
1624be open any more when the program is reincarnated, with possible
80d38338 1625resulting confusion by Perl.
19799a22 1626
59f521f4 1627This function is now largely obsolete, mostly because it's very hard to
391b733c 1628convert a core file into an executable. That's why you should now invoke
59f521f4 1629it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1630typo.
19799a22 1631
2ba1f20a
FC
1632Unlike most named operators, this has the same precedence as assignment.
1633It is also exempt from the looks-like-a-function rule, so
1634C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1635C<dump>.
1636
ea9eb35a 1637Portability issues: L<perlport/dump>.
1638
532eee96 1639=item each HASH
d74e8afc 1640X<each> X<hash, iterator>
aa689395 1641
532eee96 1642=item each ARRAY
aeedbbed
NC
1643X<array, iterator>
1644
f5a93a43
TC
1645=item each EXPR
1646
c17cdb72
NC
1647=for Pod::Functions retrieve the next key/value pair from a hash
1648
bade7fbc
TC
1649When called on a hash in list context, returns a 2-element list
1650consisting of the key and value for the next element of a hash. In Perl
16515.12 and later only, it will also return the index and value for the next
1652element of an array so that you can iterate over it; older Perls consider
1653this a syntax error. When called in scalar context, returns only the key
1654(not the value) in a hash, or the index in an array.
2f9daede 1655
aeedbbed 1656Hash entries are returned in an apparently random order. The actual random
7bf59113 1657order is specific to a given hash; the exact same series of operations
7161e5c2 1658on two hashes may result in a different order for each hash. Any insertion
7bf59113
YO
1659into the hash may change the order, as will any deletion, with the exception
1660that the most recent key returned by C<each> or C<keys> may be deleted
7161e5c2 1661without changing the order. So long as a given hash is unmodified you may
7bf59113 1662rely on C<keys>, C<values> and C<each> to repeatedly return the same order
7161e5c2
FC
1663as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
1664details on why hash order is randomized. Aside from the guarantees
7bf59113
YO
1665provided here the exact details of Perl's hash algorithm and the hash
1666traversal order are subject to change in any release of Perl.
ab192400 1667
80d38338
TC
1668After C<each> has returned all entries from the hash or array, the next
1669call to C<each> returns the empty list in list context and C<undef> in
bade7fbc
TC
1670scalar context; the next call following I<that> one restarts iteration.
1671Each hash or array has its own internal iterator, accessed by C<each>,
1672C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1673reached the end as just described; it can be explicitly reset by calling
1674C<keys> or C<values> on the hash or array. If you add or delete a hash's
1675elements while iterating over it, entries may be skipped or duplicated--so
1676don't do that. Exception: In the current implementation, it is always safe
1677to delete the item most recently returned by C<each()>, so the following
1678code works properly:
74fc8b5f
MJD
1679
1680 while (($key, $value) = each %hash) {
1681 print $key, "\n";
1682 delete $hash{$key}; # This is safe
1683 }
aa689395 1684
80d38338 1685This prints out your environment like the printenv(1) program,
3b10bc60 1686but in a different order:
a0d0e21e
LW
1687
1688 while (($key,$value) = each %ENV) {
a9a5a0dc 1689 print "$key=$value\n";
a0d0e21e
LW
1690 }
1691
f5a93a43
TC
1692Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1693reference to an unblessed hash or array. The argument will be dereferenced
1694automatically. This aspect of C<each> is considered highly experimental.
1695The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1696
1697 while (($key,$value) = each $hashref) { ... }
1698
e6a0db3e
FC
1699As of Perl 5.18 you can use a bare C<each> in a C<while> loop,
1700which will set C<$_> on every iteration.
1701
1702 while(each %ENV) {
1703 print "$_=$ENV{$_}\n";
1704 }
1705
bade7fbc
TC
1706To avoid confusing would-be users of your code who are running earlier
1707versions of Perl with mysterious syntax errors, put this sort of thing at
1708the top of your file to signal that your code will work I<only> on Perls of
1709a recent vintage:
1710
1711 use 5.012; # so keys/values/each work on arrays
1712 use 5.014; # so keys/values/each work on scalars (experimental)
e6a0db3e 1713 use 5.018; # so each assigns to $_ in a lone while test
bade7fbc 1714
8f1da26d 1715See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1716
1717=item eof FILEHANDLE
d74e8afc
ITB
1718X<eof>
1719X<end of file>
1720X<end-of-file>
a0d0e21e 1721
4633a7c4
LW
1722=item eof ()
1723
a0d0e21e
LW
1724=item eof
1725
c17cdb72
NC
1726=for Pod::Functions test a filehandle for its end
1727
8f1da26d 1728Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1729FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1730gives the real filehandle. (Note that this function actually
80d38338 1731reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1732interactive context.) Do not read from a terminal file (or call
19799a22 1733C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1734as terminals may lose the end-of-file condition if you do.
1735
820475bd 1736An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1737with empty parentheses is different. It refers to the pseudo file
820475bd 1738formed from the files listed on the command line and accessed via the
61eff3bc
JH
1739C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1740as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1741used will cause C<@ARGV> to be examined to determine if input is
67408cae 1742available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1743end-of-file will assume you are processing another C<@ARGV> list,
1744and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1745see L<perlop/"I/O Operators">.
820475bd 1746
61eff3bc 1747In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1748detect the end of each file, whereas C<eof()> will detect the end
1749of the very last file only. Examples:
a0d0e21e 1750
748a9306
LW
1751 # reset line numbering on each input file
1752 while (<>) {
a9a5a0dc
VP
1753 next if /^\s*#/; # skip comments
1754 print "$.\t$_";
5a964f20 1755 } continue {
a9a5a0dc 1756 close ARGV if eof; # Not eof()!
748a9306
LW
1757 }
1758
a0d0e21e
LW
1759 # insert dashes just before last line of last file
1760 while (<>) {
a9a5a0dc
VP
1761 if (eof()) { # check for end of last file
1762 print "--------------\n";
1763 }
1764 print;
f7051f2c 1765 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1766 }
1767
a0d0e21e 1768Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1769input operators typically return C<undef> when they run out of data or
1770encounter an error.
a0d0e21e
LW
1771
1772=item eval EXPR
d74e8afc 1773X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1774X<error, handling> X<exception, handling>
a0d0e21e
LW
1775
1776=item eval BLOCK
1777
ce2984c3
PF
1778=item eval
1779
c17cdb72
NC
1780=for Pod::Functions catch exceptions or compile and run code
1781
798dc914
KW
1782In the first form, often referred to as a "string eval", the return
1783value of EXPR is parsed and executed as if it
c7cc6f1c 1784were a little Perl program. The value of the expression (which is itself
8f1da26d 1785determined within scalar context) is first parsed, and if there were no
2341804c 1786errors, executed as a block within the lexical context of the current Perl
df4833a8 1787program. This means, that in particular, any outer lexical variables are
2341804c
DM
1788visible to it, and any package variable settings or subroutine and format
1789definitions remain afterwards.
1790
1791Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1792If EXPR is omitted, evaluates C<$_>. This form is typically used to
1793delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c 1794
7289c5e6
FC
1795If the C<unicode_eval> feature is enabled (which is the default under a
1796C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1797characters, so C<use utf8> declarations have no effect, and source filters
1798are forbidden. In the absence of the C<unicode_eval> feature, the string
1799will sometimes be treated as characters and sometimes as bytes, depending
1800on the internal encoding, and source filters activated within the C<eval>
1801exhibit the erratic, but historical, behaviour of affecting some outer file
1802scope that is still compiling. See also the L</evalbytes> keyword, which
1803always treats its input as a byte stream and works properly with source
1804filters, and the L<feature> pragma.
1805
798dc914
KW
1806Problems can arise if the string expands a scalar containing a floating
1807point number. That scalar can expand to letters, such as C<"NaN"> or
1808C<"Infinity">; or, within the scope of a C<use locale>, the decimal
1809point character may be something other than a dot (such as a comma).
1810None of these are likely to parse as you are likely expecting.
1811
c7cc6f1c 1812In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1813same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1814within the context of the current Perl program. This form is typically
1815used to trap exceptions more efficiently than the first (see below), while
1816also providing the benefit of checking the code within BLOCK at compile
1817time.
1818
1819The final semicolon, if any, may be omitted from the value of EXPR or within
1820the BLOCK.
1821
1822In both forms, the value returned is the value of the last expression
5a964f20 1823evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1824as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1825in void, scalar, or list context, depending on the context of the C<eval>
1826itself. See L</wantarray> for more on how the evaluation context can be
1827determined.
a0d0e21e 1828
19799a22 1829If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1830executed, C<eval> returns C<undef> in scalar context
774b80e8
FC
1831or an empty list in list context, and C<$@> is set to the error
1832message. (Prior to 5.16, a bug caused C<undef> to be returned
1833in list context for syntax errors, but not for runtime errors.)
1834If there was no error, C<$@> is set to the empty string. A
9cc672d4
FC
1835control flow operator like C<last> or C<goto> can bypass the setting of
1836C<$@>. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1837warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1838To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1839turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1840See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1841
19799a22
GS
1842Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1843determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1844is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1845the die operator is used to raise exceptions.
1846
5f1da31c
NT
1847If you want to trap errors when loading an XS module, some problems with
1848the binary interface (such as Perl version skew) may be fatal even with
df4833a8 1849C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
5f1da31c 1850
a0d0e21e
LW
1851If the code to be executed doesn't vary, you may use the eval-BLOCK
1852form to trap run-time errors without incurring the penalty of
1853recompiling each time. The error, if any, is still returned in C<$@>.
1854Examples:
1855
54310121 1856 # make divide-by-zero nonfatal
a0d0e21e
LW
1857 eval { $answer = $a / $b; }; warn $@ if $@;
1858
1859 # same thing, but less efficient
1860 eval '$answer = $a / $b'; warn $@ if $@;
1861
1862 # a compile-time error
5ed4f2ec 1863 eval { $answer = }; # WRONG
a0d0e21e
LW
1864
1865 # a run-time error
5ed4f2ec 1866 eval '$answer ='; # sets $@
a0d0e21e 1867
cf264981
SP
1868Using the C<eval{}> form as an exception trap in libraries does have some
1869issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1870may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1871You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1872as this example shows:
774d564b 1873
80d38338 1874 # a private exception trap for divide-by-zero
f86cebdf
GS
1875 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1876 warn $@ if $@;
774d564b
PP
1877
1878This is especially significant, given that C<__DIE__> hooks can call
19799a22 1879C<die> again, which has the effect of changing their error messages:
774d564b
PP
1880
1881 # __DIE__ hooks may modify error messages
1882 {
f86cebdf
GS
1883 local $SIG{'__DIE__'} =
1884 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1885 eval { die "foo lives here" };
1886 print $@ if $@; # prints "bar lives here"
774d564b
PP
1887 }
1888
19799a22 1889Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1890may be fixed in a future release.
1891
19799a22 1892With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1893being looked at when:
1894
5ed4f2ec 1895 eval $x; # CASE 1
1896 eval "$x"; # CASE 2
a0d0e21e 1897
5ed4f2ec 1898 eval '$x'; # CASE 3
1899 eval { $x }; # CASE 4
a0d0e21e 1900
5ed4f2ec 1901 eval "\$$x++"; # CASE 5
1902 $$x++; # CASE 6
a0d0e21e 1903
2f9daede 1904Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1905the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1906the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1907and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1908does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1909purely visual reasons, but it also has the advantage of compiling at
1910compile-time instead of at run-time.) Case 5 is a place where
19799a22 1911normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1912particular situation, you can just use symbolic references instead, as
1913in case 6.
a0d0e21e 1914
b6538e4f 1915Before Perl 5.14, the assignment to C<$@> occurred before restoration
bade7fbc 1916of localized variables, which means that for your code to run on older
b208c909 1917versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1918errors:
1919
1920 # alter $@ on nefarious repugnancy only
1921 {
1922 my $e;
1923 {
f7051f2c
FC
1924 local $@; # protect existing $@
1925 eval { test_repugnancy() };
1926 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1927 $@ =~ /nefarious/ and $e = $@;
8a5a710d
DN
1928 }
1929 die $e if defined $e
1930 }
1931
4968c1e4 1932C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1933C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1934
4f00fc7e
FC
1935An C<eval ''> executed within a subroutine defined
1936in the C<DB> package doesn't see the usual
3b10bc60 1937surrounding lexical scope, but rather the scope of the first non-DB piece
df4833a8 1938of code that called it. You don't normally need to worry about this unless
3b10bc60 1939you are writing a Perl debugger.
d819b83a 1940
7289c5e6
FC
1941=item evalbytes EXPR
1942X<evalbytes>
1943
1944=item evalbytes
1945
d9b04284 1946=for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
c17cdb72 1947
7289c5e6
FC
1948This function is like L</eval> with a string argument, except it always
1949parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1950string containing characters whose ordinal value exceeds 255 results in an
1951error. Source filters activated within the evaluated code apply to the
1952code itself.
1953
1954This function is only available under the C<evalbytes> feature, a
1955C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1956L<feature> for more information.
1957
a0d0e21e 1958=item exec LIST
d74e8afc 1959X<exec> X<execute>
a0d0e21e 1960
8bf3b016
GS
1961=item exec PROGRAM LIST
1962
c17cdb72
NC
1963=for Pod::Functions abandon this program to run another
1964
3b10bc60 1965The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1966use C<system> instead of C<exec> if you want it to return. It fails and
1967returns false only if the command does not exist I<and> it is executed
fb73857a 1968directly instead of via your system's command shell (see below).
a0d0e21e 1969
19799a22 1970Since it's a common mistake to use C<exec> instead of C<system>, Perl
4642e50d
EB
1971warns you if C<exec> is called in void context and if there is a following
1972statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1973you always do that, right?). If you I<really> want to follow an C<exec>
1974with some other statement, you can use one of these styles to avoid the warning:
55d729e4 1975
5a964f20
TC
1976 exec ('foo') or print STDERR "couldn't exec foo: $!";
1977 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1978
5a964f20 1979If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1980with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1981If there is only one scalar argument or an array with one element in it,
1982the argument is checked for shell metacharacters, and if there are any,
1983the entire argument is passed to the system's command shell for parsing
1984(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1985If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1986words and passed directly to C<execvp>, which is more efficient.
19799a22 1987Examples:
a0d0e21e 1988
19799a22
GS
1989 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1990 exec "sort $outfile | uniq";
a0d0e21e
LW
1991
1992If you don't really want to execute the first argument, but want to lie
1993to the program you are executing about its own name, you can specify
1994the program you actually want to run as an "indirect object" (without a
1995comma) in front of the LIST. (This always forces interpretation of the
54310121 1996LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1997the list.) Example:
1998
1999 $shell = '/bin/csh';
5ed4f2ec 2000 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
2001
2002or, more directly,
2003
5ed4f2ec 2004 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 2005
3b10bc60 2006When the arguments get executed via the system shell, results are
2007subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
2008for details.
2009
19799a22
GS
2010Using an indirect object with C<exec> or C<system> is also more
2011secure. This usage (which also works fine with system()) forces
2012interpretation of the arguments as a multivalued list, even if the
2013list had just one argument. That way you're safe from the shell
2014expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
2015
2016 @args = ( "echo surprise" );
2017
2b5ab1e7 2018 exec @args; # subject to shell escapes
f86cebdf 2019 # if @args == 1
2b5ab1e7 2020 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
2021
2022The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
2023program, passing it C<"surprise"> an argument. The second version didn't;
2024it tried to run a program named I<"echo surprise">, didn't find it, and set
2025C<$?> to a non-zero value indicating failure.
5a964f20 2026
e9fa405d
BF
2027Perl attempts to flush all files opened for output before the exec,
2028but this may not be supported on some platforms (see L<perlport>).
2029To be safe, you may need to set C<$|> ($AUTOFLUSH in English) or
2030call the C<autoflush()> method of C<IO::Handle> on any open handles
2031to avoid lost output.
0f897271 2032
80d38338
TC
2033Note that C<exec> will not call your C<END> blocks, nor will it invoke
2034C<DESTROY> methods on your objects.
7660c0ab 2035
ea9eb35a 2036Portability issues: L<perlport/exec>.
2037
a0d0e21e 2038=item exists EXPR
d74e8afc 2039X<exists> X<autovivification>
a0d0e21e 2040
c17cdb72
NC
2041=for Pod::Functions test whether a hash key is present
2042
d0a76353
RS
2043Given an expression that specifies an element of a hash, returns true if the
2044specified element in the hash has ever been initialized, even if the
2045corresponding value is undefined.
a0d0e21e 2046
5ed4f2ec 2047 print "Exists\n" if exists $hash{$key};
2048 print "Defined\n" if defined $hash{$key};
01020589
GS
2049 print "True\n" if $hash{$key};
2050
d0a76353 2051exists may also be called on array elements, but its behavior is much less
8f1da26d 2052obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
2053that calling exists on array values is deprecated and likely to be removed in
2054a future version of Perl.
2055
5ed4f2ec 2056 print "Exists\n" if exists $array[$index];
2057 print "Defined\n" if defined $array[$index];
01020589 2058 print "True\n" if $array[$index];
a0d0e21e 2059
8f1da26d 2060A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
2061it exists, but the reverse doesn't necessarily hold true.
2062
afebc493
GS
2063Given an expression that specifies the name of a subroutine,
2064returns true if the specified subroutine has ever been declared, even
2065if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 2066does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
2067exist may still be callable: its package may have an C<AUTOLOAD>
2068method that makes it spring into existence the first time that it is
3b10bc60 2069called; see L<perlsub>.
afebc493 2070
5ed4f2ec 2071 print "Exists\n" if exists &subroutine;
2072 print "Defined\n" if defined &subroutine;
afebc493 2073
a0d0e21e 2074Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 2075operation is a hash or array key lookup or subroutine name:
a0d0e21e 2076
5ed4f2ec 2077 if (exists $ref->{A}->{B}->{$key}) { }
2078 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 2079
5ed4f2ec 2080 if (exists $ref->{A}->{B}->[$ix]) { }
2081 if (exists $hash{A}{B}[$ix]) { }
01020589 2082
afebc493
GS
2083 if (exists &{$ref->{A}{B}{$key}}) { }
2084
9590a7cd 2085Although the most deeply nested array or hash element will not spring into
3b10bc60 2086existence just because its existence was tested, any intervening ones will.
61eff3bc 2087Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 2088into existence due to the existence test for the $key element above.
3b10bc60 2089This happens anywhere the arrow operator is used, including even here:
5a964f20 2090
2b5ab1e7 2091 undef $ref;
5ed4f2ec 2092 if (exists $ref->{"Some key"}) { }
2093 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
2094
2095This surprising autovivification in what does not at first--or even
2096second--glance appear to be an lvalue context may be fixed in a future
5a964f20 2097release.
a0d0e21e 2098
afebc493
GS
2099Use of a subroutine call, rather than a subroutine name, as an argument
2100to exists() is an error.
2101
5ed4f2ec 2102 exists &sub; # OK
2103 exists &sub(); # Error
afebc493 2104
a0d0e21e 2105=item exit EXPR
d74e8afc 2106X<exit> X<terminate> X<abort>
a0d0e21e 2107
ce2984c3
PF
2108=item exit
2109
c17cdb72
NC
2110=for Pod::Functions terminate this program
2111
2b5ab1e7 2112Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
2113
2114 $ans = <STDIN>;
2115 exit 0 if $ans =~ /^[Xx]/;
2116
19799a22 2117See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
2118universally recognized values for EXPR are C<0> for success and C<1>
2119for error; other values are subject to interpretation depending on the
2120environment in which the Perl program is running. For example, exiting
212169 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2122the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 2123
19799a22
GS
2124Don't use C<exit> to abort a subroutine if there's any chance that
2125someone might want to trap whatever error happened. Use C<die> instead,
2126which can be trapped by an C<eval>.
28757baa 2127
19799a22 2128The exit() function does not always exit immediately. It calls any
2b5ab1e7 2129defined C<END> routines first, but these C<END> routines may not
19799a22 2130themselves abort the exit. Likewise any object destructors that need to
60275626 2131be called are called before the real exit. C<END> routines and destructors
391b733c 2132can change the exit status by modifying C<$?>. If this is a problem, you
fae6f8fa 2133can call C<POSIX::_exit($status)> to avoid END and destructor processing.
87275199 2134See L<perlmod> for details.
5a964f20 2135
ea9eb35a 2136Portability issues: L<perlport/exit>.
2137
a0d0e21e 2138=item exp EXPR
d74e8afc 2139X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 2140
54310121 2141=item exp
bbce6d69 2142
c17cdb72
NC
2143=for Pod::Functions raise I<e> to a power
2144
b76cc8ba 2145Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
2146If EXPR is omitted, gives C<exp($_)>.
2147
628253b8
BF
2148=item fc EXPR
2149X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2150
2151=item fc
2152
d9b04284 2153=for Pod::Functions +fc return casefolded version of a string
c17cdb72 2154
628253b8
BF
2155Returns the casefolded version of EXPR. This is the internal function
2156implementing the C<\F> escape in double-quoted strings.
2157
2158Casefolding is the process of mapping strings to a form where case
2159differences are erased; comparing two strings in their casefolded
2160form is effectively a way of asking if two strings are equal,
2161regardless of case.
2162
2163Roughly, if you ever found yourself writing this
2164
f6c6dcb6 2165 lc($this) eq lc($that) # Wrong!
628253b8 2166 # or
f6c6dcb6 2167 uc($this) eq uc($that) # Also wrong!
628253b8 2168 # or
f6c6dcb6 2169 $this =~ /^\Q$that\E\z/i # Right!
628253b8
BF
2170
2171Now you can write
2172
2173 fc($this) eq fc($that)
2174
2175And get the correct results.
2176
fc39a31f
KW
2177Perl only implements the full form of casefolding,
2178but you can access the simple folds using L<Unicode::UCD/casefold()> and
2179L<Unicode::UCD/prop_invmap()>.
628253b8
BF
2180For further information on casefolding, refer to
2181the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
21824.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2183available at L<http://www.unicode.org/versions/latest/>, as well as the
2184Case Charts available at L<http://www.unicode.org/charts/case/>.
2185
2186If EXPR is omitted, uses C<$_>.
2187
1ca267a5
KW
2188This function behaves the same way under various pragma, such as within
2189S<C<"use feature 'unicode_strings">>, as L</lc> does, with the single
2190exception of C<fc> of LATIN CAPITAL LETTER SHARP S (U+1E9E) within the
2191scope of S<C<use locale>>. The foldcase of this character would
2192normally be C<"ss">, but as explained in the L</lc> section, case
2193changes that cross the 255/256 boundary are problematic under locales,
2194and are hence prohibited. Therefore, this function under locale returns
2195instead the string C<"\x{17F}\x{17F}">, which is the LATIN SMALL LETTER
2196LONG S. Since that character itself folds to C<"s">, the string of two
2197of them together should be equivalent to a single U+1E9E when foldcased.
628253b8
BF
2198
2199While the Unicode Standard defines two additional forms of casefolding,
2200one for Turkic languages and one that never maps one character into multiple
2201characters, these are not provided by the Perl core; However, the CPAN module
2202C<Unicode::Casing> may be used to provide an implementation.
2203
2204This keyword is available only when the C<"fc"> feature is enabled,
7161e5c2 2205or when prefixed with C<CORE::>; See L<feature>. Alternately,
628253b8
BF
2206include a C<use v5.16> or later to the current scope.
2207
a0d0e21e 2208=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2209X<fcntl>
a0d0e21e 2210
c17cdb72
NC
2211=for Pod::Functions file control system call
2212
f86cebdf 2213Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
2214
2215 use Fcntl;
2216
0ade1984 2217first to get the correct constant definitions. Argument processing and
3b10bc60 2218value returned work just like C<ioctl> below.
a0d0e21e
LW
2219For example:
2220
2221 use Fcntl;
5a964f20 2222 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 2223 or die "can't fcntl F_GETFL: $!";
5a964f20 2224
554ad1fc 2225You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
2226Like C<ioctl>, it maps a C<0> return from the system call into
2227C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
2228in numeric context. It is also exempt from the normal B<-w> warnings
2229on improper numeric conversions.
5a964f20 2230
3b10bc60 2231Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
2232doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2233manpage to learn what functions are available on your system.
a0d0e21e 2234
be2f7487 2235Here's an example of setting a filehandle named C<REMOTE> to be
2236non-blocking at the system level. You'll have to negotiate C<$|>
2237on your own, though.
2238
2239 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2240
2241 $flags = fcntl(REMOTE, F_GETFL, 0)
2242 or die "Can't get flags for the socket: $!\n";
2243
2244 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2245 or die "Can't set flags for the socket: $!\n";
2246
ea9eb35a 2247Portability issues: L<perlport/fcntl>.
2248
cfa52385
FC
2249=item __FILE__
2250X<__FILE__>
2251
c17cdb72
NC
2252=for Pod::Functions the name of the current source file
2253
cfa52385
FC
2254A special token that returns the name of the file in which it occurs.
2255
a0d0e21e 2256=item fileno FILEHANDLE
d74e8afc 2257X<fileno>
a0d0e21e 2258
c17cdb72
NC
2259=for Pod::Functions return file descriptor from filehandle
2260
2b5ab1e7 2261Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
2262filehandle is not open. If there is no real file descriptor at the OS
2263level, as can happen with filehandles connected to memory objects via
2264C<open> with a reference for the third argument, -1 is returned.
2265
2266This is mainly useful for constructing
19799a22 2267bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
2268If FILEHANDLE is an expression, the value is taken as an indirect
2269filehandle, generally its name.
5a964f20 2270
b76cc8ba 2271You can use this to find out whether two handles refer to the
5a964f20
TC
2272same underlying descriptor:
2273
3231d257 2274 if (fileno(THIS) != -1 && fileno(THIS) == fileno(THAT)) {
a9a5a0dc 2275 print "THIS and THAT are dups\n";
3231d257 2276 } elsif (fileno(THIS) != -1 && fileno(THAT) != -1) {
555bd962
BG
2277 print "THIS and THAT have different " .
2278 "underlying file descriptors\n";
3231d257 2279 } else {
555bd962
BG
2280 print "At least one of THIS and THAT does " .
2281 "not have a real file descriptor\n";
b76cc8ba
NIS
2282 }
2283
a0d0e21e 2284=item flock FILEHANDLE,OPERATION
d74e8afc 2285X<flock> X<lock> X<locking>
a0d0e21e 2286
c17cdb72
NC
2287=for Pod::Functions lock an entire file with an advisory lock
2288
19799a22
GS
2289Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2290for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2291machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2292C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2293entire files only, not records.
2b5ab1e7
TC
2294
2295Two potentially non-obvious but traditional C<flock> semantics are
2296that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2297are B<merely advisory>. Such discretionary locks are more flexible, but
2298offer fewer guarantees. This means that programs that do not also use
2299C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2300your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2301for details. It's best to assume traditional behavior if you're writing
2302portable programs. (But if you're not, you should as always feel perfectly
2303free to write for your own system's idiosyncrasies (sometimes called
2304"features"). Slavish adherence to portability concerns shouldn't get
2305in the way of your getting your job done.)
a3cb178b 2306
8ebc5c01
PP
2307OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2308LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2309you can use the symbolic names if you import them from the L<Fcntl> module,
2310either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2311requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2312releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2313LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2314waiting for the lock; check the return status to see if you got it.
68dc0745 2315
2b5ab1e7
TC
2316To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2317before locking or unlocking it.
8ebc5c01 2318
f86cebdf 2319Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2320locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2321are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2322implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
2323differing semantics shouldn't bite too many people.
2324
becacb53
TM
2325Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2326be open with read intent to use LOCK_SH and requires that it be open
2327with write intent to use LOCK_EX.
2328
19799a22
GS
2329Note also that some versions of C<flock> cannot lock things over the
2330network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2331that. If you like you can force Perl to ignore your system's flock(2)
2332function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2333the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2334and build a new Perl.
4633a7c4
LW
2335
2336Here's a mailbox appender for BSD systems.
a0d0e21e 2337
f7051f2c
FC
2338 # import LOCK_* and SEEK_END constants
2339 use Fcntl qw(:flock SEEK_END);
a0d0e21e
LW
2340
2341 sub lock {
a9a5a0dc
VP
2342 my ($fh) = @_;
2343 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2344
a9a5a0dc
VP
2345 # and, in case someone appended while we were waiting...
2346 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2347 }
2348
2349 sub unlock {
a9a5a0dc
VP
2350 my ($fh) = @_;
2351 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2352 }
2353
b0169937 2354 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2355 or die "Can't open mailbox: $!";
a0d0e21e 2356
7ed5353d 2357 lock($mbox);
b0169937 2358 print $mbox $msg,"\n\n";
7ed5353d 2359 unlock($mbox);
a0d0e21e 2360
3b10bc60 2361On systems that support a real flock(2), locks are inherited across fork()
2362calls, whereas those that must resort to the more capricious fcntl(2)
2363function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2364
cb1a09d0 2365See also L<DB_File> for other flock() examples.
a0d0e21e 2366
ea9eb35a 2367Portability issues: L<perlport/flock>.
2368
a0d0e21e 2369=item fork
d74e8afc 2370X<fork> X<child> X<parent>
a0d0e21e 2371
c17cdb72
NC
2372=for Pod::Functions create a new process just like this one
2373
2b5ab1e7
TC
2374Does a fork(2) system call to create a new process running the
2375same program at the same point. It returns the child pid to the
2376parent process, C<0> to the child process, or C<undef> if the fork is
2377unsuccessful. File descriptors (and sometimes locks on those descriptors)
2378are shared, while everything else is copied. On most systems supporting
2379fork(), great care has gone into making it extremely efficient (for
2380example, using copy-on-write technology on data pages), making it the
2381dominant paradigm for multitasking over the last few decades.
5a964f20 2382
e9fa405d 2383Perl attempts to flush all files opened for
0f897271
GS
2384output before forking the child process, but this may not be supported
2385on some platforms (see L<perlport>). To be safe, you may need to set
2386C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2387C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2388
19799a22 2389If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2390accumulate zombies. On some systems, you can avoid this by setting
2391C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2392forking and reaping moribund children.
cb1a09d0 2393
28757baa
PP
2394Note that if your forked child inherits system file descriptors like
2395STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2396if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2397backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2398You should reopen those to F</dev/null> if it's any issue.
28757baa 2399
ea9eb35a 2400On some platforms such as Windows, where the fork() system call is not available,
391b733c
FC
2401Perl can be built to emulate fork() in the Perl interpreter.
2402The emulation is designed, at the level of the Perl program,
2403to be as compatible as possible with the "Unix" fork().
6d17f725 2404However it has limitations that have to be considered in code intended to be portable.
ea9eb35a 2405See L<perlfork> for more details.
2406
2407Portability issues: L<perlport/fork>.
2408
cb1a09d0 2409=item format
d74e8afc 2410X<format>
cb1a09d0 2411
c17cdb72
NC
2412=for Pod::Functions declare a picture format with use by the write() function
2413
19799a22 2414Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2415example:
2416
54310121 2417 format Something =
a9a5a0dc
VP
2418 Test: @<<<<<<<< @||||| @>>>>>
2419 $str, $%, '$' . int($num)
cb1a09d0
AD
2420 .
2421
2422 $str = "widget";
184e9718 2423 $num = $cost/$quantity;
cb1a09d0
AD
2424 $~ = 'Something';
2425 write;
2426
2427See L<perlform> for many details and examples.
2428
8903cb82 2429=item formline PICTURE,LIST
d74e8afc 2430X<formline>
a0d0e21e 2431
c17cdb72
NC
2432=for Pod::Functions internal function used for formats
2433
5a964f20 2434This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2435too. It formats (see L<perlform>) a list of values according to the
2436contents of PICTURE, placing the output into the format output
7660c0ab 2437accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2438Eventually, when a C<write> is done, the contents of
cf264981
SP
2439C<$^A> are written to some filehandle. You could also read C<$^A>
2440and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2441does one C<formline> per line of form, but the C<formline> function itself
748a9306 2442doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2443that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2444You may therefore need to use multiple formlines to implement a single
3b10bc60 2445record format, just like the C<format> compiler.
748a9306 2446
19799a22 2447Be careful if you put double quotes around the picture, because an C<@>
748a9306 2448character may be taken to mean the beginning of an array name.
19799a22 2449C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2450
445b09e5
FC
2451If you are trying to use this instead of C<write> to capture the output,
2452you may find it easier to open a filehandle to a scalar
2453(C<< open $fh, ">", \$output >>) and write to that instead.
2454
a0d0e21e 2455=item getc FILEHANDLE
f723aae1 2456X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2457
2458=item getc
2459
c17cdb72
NC
2460=for Pod::Functions get the next character from the filehandle
2461
a0d0e21e 2462Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2463or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2464the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2465STDIN. This is not particularly efficient. However, it cannot be
2466used by itself to fetch single characters without waiting for the user
2467to hit enter. For that, try something more like:
4633a7c4
LW
2468
2469 if ($BSD_STYLE) {
a9a5a0dc 2470 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2471 }
2472 else {
a9a5a0dc 2473 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2474 }
2475
2476 $key = getc(STDIN);
2477
2478 if ($BSD_STYLE) {
a9a5a0dc 2479 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2480 }
2481 else {
3b10bc60 2482 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2483 }
2484 print "\n";
2485
54310121
PP
2486Determination of whether $BSD_STYLE should be set
2487is left as an exercise to the reader.
cb1a09d0 2488
19799a22 2489The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2490systems purporting POSIX compliance. See also the C<Term::ReadKey>
3d6c5fec 2491module from your nearest L<CPAN|http://www.cpan.org> site.
a0d0e21e
LW
2492
2493=item getlogin
d74e8afc 2494X<getlogin> X<login>
a0d0e21e 2495
c17cdb72
NC
2496=for Pod::Functions return who logged in at this tty
2497
cf264981 2498This implements the C library function of the same name, which on most
3b10bc60 2499systems returns the current login from F</etc/utmp>, if any. If it
2500returns the empty string, use C<getpwuid>.
a0d0e21e 2501
f86702cc 2502 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2503
19799a22
GS
2504Do not consider C<getlogin> for authentication: it is not as
2505secure as C<getpwuid>.
4633a7c4 2506
ea9eb35a 2507Portability issues: L<perlport/getlogin>.
2508
a0d0e21e 2509=item getpeername SOCKET
d74e8afc 2510X<getpeername> X<peer>
a0d0e21e 2511
c17cdb72
NC
2512=for Pod::Functions find the other end of a socket connection
2513
a3390c9f
FC
2514Returns the packed sockaddr address of the other end of the SOCKET
2515connection.
a0d0e21e 2516
4633a7c4
LW
2517 use Socket;
2518 $hersockaddr = getpeername(SOCK);
19799a22 2519 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2520 $herhostname = gethostbyaddr($iaddr, AF_INET);
2521 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2522
2523=item getpgrp PID
d74e8afc 2524X<getpgrp> X<group>
a0d0e21e 2525
c17cdb72
NC
2526=for Pod::Functions get process group
2527
47e29363 2528Returns the current process group for the specified PID. Use
7660c0ab 2529a PID of C<0> to get the current process group for the
4633a7c4 2530current process. Will raise an exception if used on a machine that
a3390c9f
FC
2531doesn't implement getpgrp(2). If PID is omitted, returns the process
2532group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2533does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2534
ea9eb35a 2535Portability issues: L<perlport/getpgrp>.
2536
a0d0e21e 2537=item getppid
d74e8afc 2538X<getppid> X<parent> X<pid>
a0d0e21e 2539
c17cdb72
NC
2540=for Pod::Functions get parent process ID
2541
a0d0e21e
LW
2542Returns the process id of the parent process.
2543
d7c042c9
AB
2544Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2545around non-POSIX thread semantics the minority of Linux systems (and
2546Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
7161e5c2 2547has since been removed. See the documentation for L<$$|perlvar/$$> for
d7c042c9 2548details.
4d76a344 2549
ea9eb35a 2550Portability issues: L<perlport/getppid>.
2551
a0d0e21e 2552=item getpriority WHICH,WHO
d74e8afc 2553X<getpriority> X<priority> X<nice>
a0d0e21e 2554
c17cdb72
NC
2555=for Pod::Functions get current nice value
2556
4633a7c4 2557Returns the current priority for a process, a process group, or a user.
01aa884e 2558(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2559machine that doesn't implement getpriority(2).
a0d0e21e 2560
ea9eb35a 2561Portability issues: L<perlport/getpriority>.
2562
a0d0e21e 2563=item getpwnam NAME
d74e8afc
ITB
2564X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2565X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2566X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2567X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2568X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2569X<endnetent> X<endprotoent> X<endservent>
a0d0e21e 2570
c17cdb72
NC
2571=for Pod::Functions get passwd record given user login name
2572
a0d0e21e
LW
2573=item getgrnam NAME
2574
c17cdb72
NC
2575=for Pod::Functions get group record given group name
2576
a0d0e21e
LW
2577=item gethostbyname NAME
2578
c17cdb72
NC
2579=for Pod::Functions get host record given name
2580
a0d0e21e
LW
2581=item getnetbyname NAME
2582
c17cdb72
NC
2583=for Pod::Functions get networks record given name
2584
a0d0e21e
LW
2585=item getprotobyname NAME
2586
c17cdb72
NC
2587=for Pod::Functions get protocol record given name
2588
a0d0e21e
LW
2589=item getpwuid UID
2590
c17cdb72
NC
2591=for Pod::Functions get passwd record given user ID
2592
a0d0e21e
LW
2593=item getgrgid GID
2594
c17cdb72
NC
2595=for Pod::Functions get group record given group user ID
2596
a0d0e21e
LW
2597=item getservbyname NAME,PROTO
2598
c17cdb72
NC
2599=for Pod::Functions get services record given its name
2600
a0d0e21e
LW
2601=item gethostbyaddr ADDR,ADDRTYPE
2602
c17cdb72
NC
2603=for Pod::Functions get host record given its address
2604
a0d0e21e
LW
2605=item getnetbyaddr ADDR,ADDRTYPE
2606
c17cdb72
NC
2607=for Pod::Functions get network record given its address
2608
a0d0e21e
LW
2609=item getprotobynumber NUMBER
2610
c17cdb72
NC
2611=for Pod::Functions get protocol record numeric protocol
2612
a0d0e21e
LW
2613=item getservbyport PORT,PROTO
2614
c17cdb72
NC
2615=for Pod::Functions get services record given numeric port
2616
a0d0e21e
LW
2617=item getpwent
2618
c17cdb72
NC
2619=for Pod::Functions get next passwd record
2620
a0d0e21e
LW
2621=item getgrent
2622
c17cdb72
NC
2623=for Pod::Functions get next group record
2624
a0d0e21e
LW
2625=item gethostent
2626
c17cdb72
NC
2627=for Pod::Functions get next hosts record
2628
a0d0e21e
LW
2629=item getnetent
2630
c17cdb72
NC
2631=for Pod::Functions get next networks record
2632
a0d0e21e
LW
2633=item getprotoent
2634
c17cdb72
NC
2635=for Pod::Functions get next protocols record
2636
a0d0e21e
LW
2637=item getservent
2638
c17cdb72
NC
2639=for Pod::Functions get next services record
2640
a0d0e21e
LW
2641=item setpwent
2642
c17cdb72
NC
2643=for Pod::Functions prepare passwd file for use
2644
a0d0e21e
LW
2645=item setgrent
2646
c17cdb72
NC
2647=for Pod::Functions prepare group file for use
2648
a0d0e21e
LW
2649=item sethostent STAYOPEN
2650
c17cdb72
NC
2651=for Pod::Functions prepare hosts file for use
2652
a0d0e21e
LW
2653=item setnetent STAYOPEN
2654
c17cdb72
NC
2655=for Pod::Functions prepare networks file for use
2656
a0d0e21e
LW
2657=item setprotoent STAYOPEN
2658
c17cdb72
NC
2659=for Pod::Functions prepare protocols file for use
2660
a0d0e21e
LW
2661=item setservent STAYOPEN
2662
c17cdb72
NC
2663=for Pod::Functions prepare services file for use
2664
a0d0e21e
LW
2665=item endpwent
2666
c17cdb72
NC
2667=for Pod::Functions be done using passwd file
2668
a0d0e21e
LW
2669=item endgrent
2670
c17cdb72
NC
2671=for Pod::Functions be done using group file
2672
a0d0e21e
LW
2673=item endhostent
2674
c17cdb72
NC
2675=for Pod::Functions be done using hosts file
2676
a0d0e21e
LW
2677=item endnetent
2678
c17cdb72
NC
2679=for Pod::Functions be done using networks file
2680
a0d0e21e
LW
2681=item endprotoent
2682
c17cdb72
NC
2683=for Pod::Functions be done using protocols file
2684
a0d0e21e
LW
2685=item endservent
2686
c17cdb72
NC
2687=for Pod::Functions be done using services file
2688
80d38338
TC
2689These routines are the same as their counterparts in the
2690system C library. In list context, the return values from the
a0d0e21e
LW
2691various get routines are as follows:
2692
2693 ($name,$passwd,$uid,$gid,
6ee623d5 2694 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2695 ($name,$passwd,$gid,$members) = getgr*
2696 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2697 ($name,$aliases,$addrtype,$net) = getnet*
2698 ($name,$aliases,$proto) = getproto*
2699 ($name,$aliases,$port,$proto) = getserv*
2700
3b10bc60 2701(If the entry doesn't exist you get an empty list.)
a0d0e21e 2702
4602f195
JH
2703The exact meaning of the $gcos field varies but it usually contains
2704the real name of the user (as opposed to the login name) and other
2705information pertaining to the user. Beware, however, that in many
2706system users are able to change this information and therefore it
106325ad 2707cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2708L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2709login shell, are also tainted, for the same reason.
4602f195 2710
5a964f20 2711In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2712lookup by name, in which case you get the other thing, whatever it is.
2713(If the entry doesn't exist you get the undefined value.) For example:
2714
5a964f20
TC
2715 $uid = getpwnam($name);
2716 $name = getpwuid($num);
2717 $name = getpwent();
2718 $gid = getgrnam($name);
08a33e13 2719 $name = getgrgid($num);
5a964f20
TC
2720 $name = getgrent();
2721 #etc.
a0d0e21e 2722
4602f195 2723In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2724in that they are unsupported on many systems. If the
4602f195
JH
2725$quota is unsupported, it is an empty scalar. If it is supported, it
2726usually encodes the disk quota. If the $comment field is unsupported,
2727it is an empty scalar. If it is supported it usually encodes some
2728administrative comment about the user. In some systems the $quota
2729field may be $change or $age, fields that have to do with password
2730aging. In some systems the $comment field may be $class. The $expire
2731field, if present, encodes the expiration period of the account or the
2732password. For the availability and the exact meaning of these fields
8f1da26d 2733in your system, please consult getpwnam(3) and your system's
4602f195
JH
2734F<pwd.h> file. You can also find out from within Perl what your
2735$quota and $comment fields mean and whether you have the $expire field
2736by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2737C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2738files are supported only if your vendor has implemented them in the
4602f195 2739intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2740shadow versions if you're running under privilege or if there exists
cf264981 2741the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2742and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2743facility are unlikely to be supported.
6ee623d5 2744
a3390c9f 2745The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2746the login names of the members of the group.
2747
2748For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2749C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2750C<@addrs> value returned by a successful call is a list of raw
2751addresses returned by the corresponding library call. In the
2752Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2753by saying something like:
2754
f337b084 2755 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2756
2b5ab1e7
TC
2757The Socket library makes this slightly easier:
2758
2759 use Socket;
2760 $iaddr = inet_aton("127.1"); # or whatever address
2761 $name = gethostbyaddr($iaddr, AF_INET);
2762
2763 # or going the other way
19799a22 2764 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2765
d760c846
GS
2766In the opposite way, to resolve a hostname to the IP address
2767you can write this:
2768
2769 use Socket;
2770 $packed_ip = gethostbyname("www.perl.org");
2771 if (defined $packed_ip) {
2772 $ip_address = inet_ntoa($packed_ip);
2773 }
2774
b018eaf1 2775Make sure C<gethostbyname()> is called in SCALAR context and that
d760c846
GS
2776its return value is checked for definedness.
2777
0d043efa
FC
2778The C<getprotobynumber> function, even though it only takes one argument,
2779has the precedence of a list operator, so beware:
2780
2781 getprotobynumber $number eq 'icmp' # WRONG
2782 getprotobynumber($number eq 'icmp') # actually means this
2783 getprotobynumber($number) eq 'icmp' # better this way
2784
19799a22
GS
2785If you get tired of remembering which element of the return list
2786contains which return value, by-name interfaces are provided
2787in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2788C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2789and C<User::grent>. These override the normal built-ins, supplying
2790versions that return objects with the appropriate names
2791for each field. For example:
5a964f20
TC
2792
2793 use File::stat;
2794 use User::pwent;
2795 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2796
a3390c9f 2797Even though it looks as though they're the same method calls (uid),
b76cc8ba 2798they aren't, because a C<File::stat> object is different from
19799a22 2799a C<User::pwent> object.
5a964f20 2800
ea9eb35a 2801Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2802
a0d0e21e 2803=item getsockname SOCKET
d74e8afc 2804X<getsockname>
a0d0e21e 2805
c17cdb72
NC
2806=for Pod::Functions retrieve the sockaddr for a given socket
2807
19799a22
GS
2808Returns the packed sockaddr address of this end of the SOCKET connection,
2809in case you don't know the address because you have several different
2810IPs that the connection might have come in on.
a0d0e21e 2811
4633a7c4
LW
2812 use Socket;
2813 $mysockaddr = getsockname(SOCK);
19799a22 2814 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2815 printf "Connect to %s [%s]\n",
19799a22
GS
2816 scalar gethostbyaddr($myaddr, AF_INET),
2817 inet_ntoa($myaddr);
a0d0e21e
LW
2818
2819=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2820X<getsockopt>
a0d0e21e 2821
c17cdb72
NC
2822=for Pod::Functions get socket options on a given socket
2823
636e6b1f
TH
2824Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2825Options may exist at multiple protocol levels depending on the socket
2826type, but at least the uppermost socket level SOL_SOCKET (defined in the
391b733c 2827C<Socket> module) will exist. To query options at another level the
636e6b1f 2828protocol number of the appropriate protocol controlling the option
391b733c 2829should be supplied. For example, to indicate that an option is to be
636e6b1f 2830interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2831number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2832
80d38338 2833The function returns a packed string representing the requested socket
3b10bc60 2834option, or C<undef> on error, with the reason for the error placed in
391b733c 2835C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2836consult getsockopt(2) for details. A common case is that the option is an
2837integer, in which case the result is a packed integer, which you can decode
2838using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2839
8f1da26d 2840Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2841
4852725b 2842 use Socket qw(:all);
636e6b1f
TH
2843
2844 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2845 or die "Could not determine the protocol number for tcp";
4852725b
DD
2846 # my $tcp = IPPROTO_TCP; # Alternative
2847 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2848 or die "getsockopt TCP_NODELAY: $!";
636e6b1f 2849 my $nodelay = unpack("I", $packed);
f7051f2c
FC
2850 print "Nagle's algorithm is turned ",
2851 $nodelay ? "off\n" : "on\n";
636e6b1f 2852
ea9eb35a 2853Portability issues: L<perlport/getsockopt>.
a0d0e21e
LW
2854
2855=item glob EXPR
d74e8afc 2856X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2857
0a753a76
PP
2858=item glob
2859
c17cdb72
NC
2860=for Pod::Functions expand filenames using wildcards
2861
d9a9d457 2862In list context, returns a (possibly empty) list of filename expansions on
391b733c 2863the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
d9a9d457 2864scalar context, glob iterates through such filename expansions, returning
391b733c
FC
2865undef when the list is exhausted. This is the internal function
2866implementing the C<< <*.c> >> operator, but you can use it directly. If
d9a9d457
JL
2867EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2868more detail in L<perlop/"I/O Operators">.
a0d0e21e 2869
80d38338
TC
2870Note that C<glob> splits its arguments on whitespace and treats
2871each segment as separate pattern. As such, C<glob("*.c *.h")>
2872matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2873C<glob(".* *")> matches all files in the current working directory.
a91bb7b1
TC
2874If you want to glob filenames that might contain whitespace, you'll
2875have to use extra quotes around the spacey filename to protect it.
2876For example, to glob filenames that have an C<e> followed by a space
2877followed by an C<f>, use either of:
2878
2879 @spacies = <"*e f*">;
2880 @spacies = glob '"*e f*"';
2881 @spacies = glob q("*e f*");
2882
2883If you had to get a variable through, you could do this:
2884
2885 @spacies = glob "'*${var}e f*'";
2886 @spacies = glob qq("*${var}e f*");
80d38338
TC
2887
2888If non-empty braces are the only wildcard characters used in the
2889C<glob>, no filenames are matched, but potentially many strings
2890are returned. For example, this produces nine strings, one for
2891each pairing of fruits and colors:
2892
2893 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2894
e9fa405d 2895This operator is implemented using the standard
5c0c9249
PF
2896C<File::Glob> extension. See L<File::Glob> for details, including
2897C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2898
ea9eb35a 2899Portability issues: L<perlport/glob>.
2900
a0d0e21e 2901=item gmtime EXPR
d74e8afc 2902X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2903
ce2984c3
PF
2904=item gmtime
2905
c17cdb72
NC
2906=for Pod::Functions convert UNIX time into record or string using Greenwich time
2907
4509d391 2908Works just like L</localtime> but the returned values are
435fbc73 2909localized for the standard Greenwich time zone.
a0d0e21e 2910
a3390c9f
FC
2911Note: When called in list context, $isdst, the last value
2912returned by gmtime, is always C<0>. There is no
435fbc73 2913Daylight Saving Time in GMT.
0a753a76 2914
ea9eb35a 2915Portability issues: L<perlport/gmtime>.
62aa5637 2916
a0d0e21e 2917=item goto LABEL
d74e8afc 2918X<goto> X<jump> X<jmp>
a0d0e21e 2919
748a9306
LW
2920=item goto EXPR
2921
a0d0e21e
LW
2922=item goto &NAME
2923
c17cdb72
NC
2924=for Pod::Functions create spaghetti code
2925
5a5b79a3 2926The C<goto LABEL> form finds the statement labeled with LABEL and
391b733c 2927resumes execution there. It can't be used to get out of a block or
b500e03b
GG
2928subroutine given to C<sort>. It can be used to go almost anywhere
2929else within the dynamic scope, including out of subroutines, but it's
2930usually better to use some other construct such as C<last> or C<die>.
2931The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2932(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2933does not offer named loops combined with loop control. Perl does, and
2934this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2935
5a5b79a3 2936The C<goto EXPR> form expects to evaluate C<EXPR> to a code reference or
3e8a6370 2937a label name. If it evaluates to a code reference, it will be handled
5a5b79a3 2938like C<goto &NAME>, below. This is especially useful for implementing
3e8a6370
RS
2939tail recursion via C<goto __SUB__>.
2940
2941If the expression evaluates to a label name, its scope will be resolved
7660c0ab 2942dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2943necessarily recommended if you're optimizing for maintainability:
2944
2945 goto ("FOO", "BAR", "GLARCH")[$i];
2946
5a5b79a3 2947As shown in this example, C<goto EXPR> is exempt from the "looks like a
391b733c
FC
2948function" rule. A pair of parentheses following it does not (necessarily)
2949delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
8a7e748e
FC
2950Also, unlike most named operators, this has the same precedence as
2951assignment.
887d89fd 2952
5a5b79a3 2953Use of C<goto LABEL> or C<goto EXPR> to jump into a construct is
0b98bec9 2954deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2955go into any construct that requires initialization, such as a
2956subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2957construct that is optimized away.
b500e03b 2958
5a5b79a3 2959The C<goto &NAME> form is quite different from the other forms of
1b6921cb
BT
2960C<goto>. In fact, it isn't a goto in the normal sense at all, and
2961doesn't have the stigma associated with other gotos. Instead, it
2962exits the current subroutine (losing any changes set by local()) and
2963immediately calls in its place the named subroutine using the current
2964value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2965load another subroutine and then pretend that the other subroutine had
2966been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2967in the current subroutine are propagated to the other subroutine.)
2968After the C<goto>, not even C<caller> will be able to tell that this
2969routine was called first.
2970
2971NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2972containing a code reference or a block that evaluates to a code
6cb9131c 2973reference.
a0d0e21e
LW
2974
2975=item grep BLOCK LIST
d74e8afc 2976X<grep>
a0d0e21e
LW
2977
2978=item grep EXPR,LIST
2979
c17cdb72
NC
2980=for Pod::Functions locate elements in a list test true against a given criterion
2981
2b5ab1e7
TC
2982This is similar in spirit to, but not the same as, grep(1) and its
2983relatives. In particular, it is not limited to using regular expressions.
2f9daede 2984
a0d0e21e 2985Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2986C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2987elements for which the expression evaluated to true. In scalar
2988context, returns the number of times the expression was true.
a0d0e21e
LW
2989
2990 @foo = grep(!/^#/, @bar); # weed out comments
2991
2992or equivalently,
2993
2994 @foo = grep {!/^#/} @bar; # weed out comments
2995
be3174d2
GS
2996Note that C<$_> is an alias to the list value, so it can be used to
2997modify the elements of the LIST. While this is useful and supported,
2998it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2999Similarly, grep returns aliases into the original list, much as a for
3000loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
3001element of a list returned by grep (for example, in a C<foreach>, C<map>
3002or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 3003This is usually something to be avoided when writing clear code.
a0d0e21e 3004
a4fb8298 3005If C<$_> is lexical in the scope where the C<grep> appears (because it has
c071e214
FC
3006been declared with the deprecated C<my $_> construct)
3007then, in addition to being locally aliased to
80d38338 3008the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
3009can't be seen from the outside, avoiding any potential side-effects.
3010
19799a22 3011See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 3012
a0d0e21e 3013=item hex EXPR
d74e8afc 3014X<hex> X<hexadecimal>
a0d0e21e 3015
54310121 3016=item hex
bbce6d69 3017
c17cdb72
NC
3018=for Pod::Functions convert a string to a hexadecimal number
3019
2b5ab1e7 3020Interprets EXPR as a hex string and returns the corresponding value.
38366c11 3021(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 3022L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
3023
3024 print hex '0xAf'; # prints '175'
3025 print hex 'aF'; # same
a0d0e21e 3026
19799a22 3027Hex strings may only represent integers. Strings that would cause
53305cf1 3028integer overflow trigger a warning. Leading whitespace is not stripped,
391b733c 3029unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 3030L</sprintf>, and L</unpack>.
19799a22 3031
ce2984c3 3032=item import LIST
d74e8afc 3033X<import>
a0d0e21e 3034
c17cdb72
NC
3035=for Pod::Functions patch a module's namespace into your own
3036
19799a22 3037There is no builtin C<import> function. It is just an ordinary
4633a7c4 3038method (subroutine) defined (or inherited) by modules that wish to export
19799a22 3039names to another module. The C<use> function calls the C<import> method
cea6626f 3040for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
3041
3042=item index STR,SUBSTR,POSITION
d74e8afc 3043X<index> X<indexOf> X<InStr>
a0d0e21e
LW
3044
3045=item index STR,SUBSTR
3046
c17cdb72
NC
3047=for Pod::Functions find a substring within a string
3048