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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.
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360Unless otherwise documented, it returns C<1> for true and C<''> for false.
361If the file doesn't exist or can't be examined, it returns C<undef> and
362sets C<$!> (errno). Despite the funny names, precedence is the same as any
363other named unary operator. The operator may be any of:
a0d0e21e 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.
5ff3f7a4
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
02729fef
DM
757Here, $subroutine is the function that the caller called (rather than the
758function containing the caller). Note that $subroutine may be C<(eval)> if
759the frame is not a subroutine call, but an C<eval>. In such a case
760additional elements $evaltext and
7660c0ab 761C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 762C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 763C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 764$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
765each C<use> statement creates a C<require> frame inside an C<eval EXPR>
766frame.) $subroutine may also be C<(unknown)> if this particular
767subroutine happens to have been deleted from the symbol table.
768C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
769C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
585d73c3 770compiled with. C<$hints> corresponds to C<$^H>, and C<$bitmask>
1adb05cd
FC
771corresponds to C<${^WARNING_BITS}>. The
772C<$hints> and C<$bitmask> values are subject
585d73c3 773to change between versions of Perl, and are not meant for external use.
748a9306 774
b3ca2e83 775C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
391b733c 776caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
b3ca2e83
NC
777of this hash, as they are the actual values stored in the optree.
778
ffe0c19d
FC
779Furthermore, when called from within the DB package in
780list context, and with an argument, caller returns more
7660c0ab 781detailed information: it sets the list variable C<@DB::args> to be the
54310121 782arguments with which the subroutine was invoked.
748a9306 783
7660c0ab 784Be aware that the optimizer might have optimized call frames away before
19799a22 785C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 786might not return information about the call frame you expect it to, for
b76cc8ba 787C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 788previous time C<caller> was called.
7660c0ab 789
8f1da26d 790Be aware that setting C<@DB::args> is I<best effort>, intended for
391b733c 791debugging or generating backtraces, and should not be relied upon. In
ca9f0cb5
NC
792particular, as C<@_> contains aliases to the caller's arguments, Perl does
793not take a copy of C<@_>, so C<@DB::args> will contain modifications the
794subroutine makes to C<@_> or its contents, not the original values at call
391b733c 795time. C<@DB::args>, like C<@_>, does not hold explicit references to its
ca9f0cb5 796elements, so under certain cases its elements may have become freed and
391b733c 797reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 798of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
799I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
800reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5 801elements), so C<@DB::args> is actually a hybrid of the current state and
391b733c 802initial state of C<@_>. Buyer beware.
ca9f0cb5 803
a0d0e21e 804=item chdir EXPR
d74e8afc
ITB
805X<chdir>
806X<cd>
f723aae1 807X<directory, change>
a0d0e21e 808
c4aca7d0
GA
809=item chdir FILEHANDLE
810
811=item chdir DIRHANDLE
812
ce2984c3
PF
813=item chdir
814
c17cdb72
NC
815=for Pod::Functions change your current working directory
816
391b733c 817Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 818changes to the directory specified by C<$ENV{HOME}>, if set; if not,
391b733c
FC
819changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
820variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
821neither is set, C<chdir> does nothing. It returns true on success,
822false otherwise. See the example under C<die>.
a0d0e21e 823
3b10bc60 824On systems that support fchdir(2), you may pass a filehandle or
34169887 825directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 826passing handles raises an exception.
c4aca7d0 827
a0d0e21e 828=item chmod LIST
d74e8afc 829X<chmod> X<permission> X<mode>
a0d0e21e 830
c17cdb72
NC
831=for Pod::Functions changes the permissions on a list of files
832
a0d0e21e 833Changes the permissions of a list of files. The first element of the
8f1da26d 834list must be the numeric mode, which should probably be an octal
4ad40acf 835number, and which definitely should I<not> be a string of octal digits:
3b10bc60 836C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 837successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 838
3b10bc60 839 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 840 chmod 0755, @executables;
3b10bc60 841 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 842 # --w----r-T
3b10bc60 843 $mode = "0644"; chmod oct($mode), "foo"; # this is better
844 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 845
3b10bc60 846On systems that support fchmod(2), you may pass filehandles among the
847files. On systems that don't support fchmod(2), passing filehandles raises
848an exception. Filehandles must be passed as globs or glob references to be
849recognized; barewords are considered filenames.
c4aca7d0
GA
850
851 open(my $fh, "<", "foo");
852 my $perm = (stat $fh)[2] & 07777;
853 chmod($perm | 0600, $fh);
854
3b10bc60 855You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
856module:
857
3b10bc60 858 use Fcntl qw( :mode );
ca6e1c26 859 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 860 # Identical to the chmod 0755 of the example above.
ca6e1c26 861
ea9eb35a 862Portability issues: L<perlport/chmod>.
863
a0d0e21e 864=item chomp VARIABLE
d74e8afc 865X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 866
313c9f5c 867=item chomp( LIST )
a0d0e21e
LW
868
869=item chomp
870
c17cdb72
NC
871=for Pod::Functions remove a trailing record separator from a string
872
2b5ab1e7
TC
873This safer version of L</chop> removes any trailing string
874that corresponds to the current value of C<$/> (also known as
28757baa
PP
875$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
876number of characters removed from all its arguments. It's often used to
877remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
878that the final record may be missing its newline. When in paragraph
879mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 880When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 881a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 882remove anything.
19799a22 883If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
884
885 while (<>) {
a9a5a0dc
VP
886 chomp; # avoid \n on last field
887 @array = split(/:/);
888 # ...
a0d0e21e
LW
889 }
890
feef49c9
FC
891If VARIABLE is a hash, it chomps the hash's values, but not its keys,
892resetting the C<each> iterator in the process.
4bf21a6d 893
a0d0e21e
LW
894You can actually chomp anything that's an lvalue, including an assignment:
895
896 chomp($cwd = `pwd`);
897 chomp($answer = <STDIN>);
898
899If you chomp a list, each element is chomped, and the total number of
900characters removed is returned.
901
15e44fd8
RGS
902Note that parentheses are necessary when you're chomping anything
903that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
904is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
905C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
906C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
907as C<chomp($a, $b)>.
908
a0d0e21e 909=item chop VARIABLE
d74e8afc 910X<chop>
a0d0e21e 911
313c9f5c 912=item chop( LIST )
a0d0e21e
LW
913
914=item chop
915
c17cdb72
NC
916=for Pod::Functions remove the last character from a string
917
a0d0e21e 918Chops off the last character of a string and returns the character
5b3eff12 919chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 920scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
feef49c9
FC
921If VARIABLE is a hash, it chops the hash's values, but not its keys,
922resetting the C<each> iterator in the process.
4bf21a6d 923
5b3eff12 924You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
925
926If you chop a list, each element is chopped. Only the value of the
19799a22 927last C<chop> is returned.
a0d0e21e 928
19799a22 929Note that C<chop> returns the last character. To return all but the last
748a9306
LW
930character, use C<substr($string, 0, -1)>.
931
15e44fd8
RGS
932See also L</chomp>.
933
a0d0e21e 934=item chown LIST
d74e8afc 935X<chown> X<owner> X<user> X<group>
a0d0e21e 936
c17cdb72
NC
937=for Pod::Functions change the ownership on a list of files
938
a0d0e21e 939Changes the owner (and group) of a list of files. The first two
19799a22
GS
940elements of the list must be the I<numeric> uid and gid, in that
941order. A value of -1 in either position is interpreted by most
942systems to leave that value unchanged. Returns the number of files
943successfully changed.
a0d0e21e
LW
944
945 $cnt = chown $uid, $gid, 'foo', 'bar';
946 chown $uid, $gid, @filenames;
947
3b10bc60 948On systems that support fchown(2), you may pass filehandles among the
949files. On systems that don't support fchown(2), passing filehandles raises
950an exception. Filehandles must be passed as globs or glob references to be
951recognized; barewords are considered filenames.
c4aca7d0 952
54310121 953Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
954
955 print "User: ";
19799a22 956 chomp($user = <STDIN>);
5a964f20 957 print "Files: ";
19799a22 958 chomp($pattern = <STDIN>);
a0d0e21e
LW
959
960 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 961 or die "$user not in passwd file";
a0d0e21e 962
5ed4f2ec 963 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
964 chown $uid, $gid, @ary;
965
54310121 966On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
967file unless you're the superuser, although you should be able to change
968the group to any of your secondary groups. On insecure systems, these
969restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
970On POSIX systems, you can detect this condition this way:
971
972 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
973 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 974
f48496b1 975Portability issues: L<perlport/chown>.
ea9eb35a 976
a0d0e21e 977=item chr NUMBER
d74e8afc 978X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 979
54310121 980=item chr
bbce6d69 981
c17cdb72
NC
982=for Pod::Functions get character this number represents
983
a0d0e21e 984Returns the character represented by that NUMBER in the character set.
a0ed51b3 985For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 986chr(0x263a) is a Unicode smiley face.
aaa68c4a 987
8a064bd6 988Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 989except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
990(truncated to an integer) are used.
991
974da8e5
JH
992If NUMBER is omitted, uses C<$_>.
993
b76cc8ba 994For the reverse, use L</ord>.
a0d0e21e 995
2575c402
JW
996Note that characters from 128 to 255 (inclusive) are by default
997internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 998
2575c402 999See L<perlunicode> for more about Unicode.
bbce6d69 1000
a0d0e21e 1001=item chroot FILENAME
d74e8afc 1002X<chroot> X<root>
a0d0e21e 1003
54310121 1004=item chroot
bbce6d69 1005
c17cdb72
NC
1006=for Pod::Functions make directory new root for path lookups
1007
5a964f20 1008This function works like the system call by the same name: it makes the
4633a7c4 1009named directory the new root directory for all further pathnames that
951ba7fe 1010begin with a C</> by your process and all its children. (It doesn't
28757baa 1011change your current working directory, which is unaffected.) For security
4633a7c4 1012reasons, this call is restricted to the superuser. If FILENAME is
19799a22 1013omitted, does a C<chroot> to C<$_>.
a0d0e21e 1014
ea9eb35a 1015Portability issues: L<perlport/chroot>.
1016
a0d0e21e 1017=item close FILEHANDLE
d74e8afc 1018X<close>
a0d0e21e 1019
6a518fbc
TP
1020=item close
1021
c17cdb72
NC
1022=for Pod::Functions close file (or pipe or socket) handle
1023
3b10bc60 1024Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 1025buffers, and closes the system file descriptor. Returns true if those
8f1da26d 1026operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
1027layer. Closes the currently selected filehandle if the argument is
1028omitted.
fb73857a
PP
1029
1030You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 1031another C<open> on it, because C<open> closes it for you. (See
01aa884e 1032L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
19799a22 1033counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 1034
3b10bc60 1035If the filehandle came from a piped open, C<close> returns false if one of
1036the other syscalls involved fails or if its program exits with non-zero
1037status. If the only problem was that the program exited non-zero, C<$!>
1038will be set to C<0>. Closing a pipe also waits for the process executing
1039on the pipe to exit--in case you wish to look at the output of the pipe
1040afterwards--and implicitly puts the exit status value of that command into
1041C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 1042
2e0cfa16
FC
1043If there are multiple threads running, C<close> on a filehandle from a
1044piped open returns true without waiting for the child process to terminate,
1045if the filehandle is still open in another thread.
1046
80d38338
TC
1047Closing the read end of a pipe before the process writing to it at the
1048other end is done writing results in the writer receiving a SIGPIPE. If
1049the other end can't handle that, be sure to read all the data before
1050closing the pipe.
73689b13 1051
fb73857a 1052Example:
a0d0e21e 1053
fb73857a
PP
1054 open(OUTPUT, '|sort >foo') # pipe to sort
1055 or die "Can't start sort: $!";
5ed4f2ec 1056 #... # print stuff to output
1057 close OUTPUT # wait for sort to finish
fb73857a
PP
1058 or warn $! ? "Error closing sort pipe: $!"
1059 : "Exit status $? from sort";
5ed4f2ec 1060 open(INPUT, 'foo') # get sort's results
fb73857a 1061 or die "Can't open 'foo' for input: $!";
a0d0e21e 1062
5a964f20 1063FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 1064filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
1065
1066=item closedir DIRHANDLE
d74e8afc 1067X<closedir>
a0d0e21e 1068
c17cdb72
NC
1069=for Pod::Functions close directory handle
1070
19799a22 1071Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
1072system call.
1073
a0d0e21e 1074=item connect SOCKET,NAME
d74e8afc 1075X<connect>
a0d0e21e 1076
c17cdb72
NC
1077=for Pod::Functions connect to a remote socket
1078
80d38338
TC
1079Attempts to connect to a remote socket, just like connect(2).
1080Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
1081packed address of the appropriate type for the socket. See the examples in
1082L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 1083
cb1a09d0 1084=item continue BLOCK
d74e8afc 1085X<continue>
cb1a09d0 1086
0d863452
RH
1087=item continue
1088
c17cdb72
NC
1089=for Pod::Functions optional trailing block in a while or foreach
1090
4a904372
FC
1091When followed by a BLOCK, C<continue> is actually a
1092flow control statement rather than a function. If
cf264981 1093there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
1094C<foreach>), it is always executed just before the conditional is about to
1095be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
1096it can be used to increment a loop variable, even when the loop has been
1097continued via the C<next> statement (which is similar to the C C<continue>
1098statement).
1099
98293880 1100C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 1101block; C<last> and C<redo> behave as if they had been executed within
19799a22 1102the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
1103block, it may be more entertaining.
1104
1105 while (EXPR) {
a9a5a0dc
VP
1106 ### redo always comes here
1107 do_something;
1d2dff63 1108 } continue {
a9a5a0dc
VP
1109 ### next always comes here
1110 do_something_else;
1111 # then back the top to re-check EXPR
1d2dff63
GS
1112 }
1113 ### last always comes here
1114
3b10bc60 1115Omitting the C<continue> section is equivalent to using an
1116empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1117to check the condition at the top of the loop.
1118
4a904372 1119When there is no BLOCK, C<continue> is a function that
8f1da26d
TC
1120falls through the current C<when> or C<default> block instead of iterating
1121a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
4a904372
FC
1122In Perl 5.14 and earlier, this form of C<continue> was
1123only available when the C<"switch"> feature was enabled.
48238296 1124See L<feature> and L<perlsyn/"Switch Statements"> for more
8f1da26d 1125information.
0d863452 1126
a0d0e21e 1127=item cos EXPR
d74e8afc 1128X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1129
d6217f1e
GS
1130=item cos
1131
c17cdb72
NC
1132=for Pod::Functions cosine function
1133
5a964f20 1134Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1135takes the cosine of C<$_>.
a0d0e21e 1136
ca6e1c26 1137For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1138function, or use this relation:
1139
1140 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1141
a0d0e21e 1142=item crypt PLAINTEXT,SALT
d74e8afc 1143X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1144X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1145
c17cdb72
NC
1146=for Pod::Functions one-way passwd-style encryption
1147
ef2e6798
MS
1148Creates a digest string exactly like the crypt(3) function in the C
1149library (assuming that you actually have a version there that has not
bb23f8d1 1150been extirpated as a potential munition).
ef2e6798 1151
34169887 1152crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1153into a short string, called a digest, which is returned. The same
1154PLAINTEXT and SALT will always return the same string, but there is no
1155(known) way to get the original PLAINTEXT from the hash. Small
1156changes in the PLAINTEXT or SALT will result in large changes in the
1157digest.
1158
1159There is no decrypt function. This function isn't all that useful for
1160cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1161mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1162primarily used to check if two pieces of text are the same without
1163having to transmit or store the text itself. An example is checking
1164if a correct password is given. The digest of the password is stored,
cf264981 1165not the password itself. The user types in a password that is
ef2e6798 1166crypt()'d with the same salt as the stored digest. If the two digests
34169887 1167match, the password is correct.
ef2e6798
MS
1168
1169When verifying an existing digest string you should use the digest as
1170the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1171to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1172crypt() will hash the new string with the same salt as the digest.
1173This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1174with more exotic implementations. In other words, assume
1175nothing about the returned string itself nor about how many bytes
1176of SALT may matter.
85c16d83
JH
1177
1178Traditionally the result is a string of 13 bytes: two first bytes of
1179the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
391b733c 1180the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1181hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1182and implementations on non-Unix platforms may produce different
ef2e6798 1183strings.
85c16d83
JH
1184
1185When choosing a new salt create a random two character string whose
1186characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1187'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1188characters is just a recommendation; the characters allowed in
1189the salt depend solely on your system's crypt library, and Perl can't
1190restrict what salts C<crypt()> accepts.
e71965be 1191
a0d0e21e 1192Here's an example that makes sure that whoever runs this program knows
cf264981 1193their password:
a0d0e21e
LW
1194
1195 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1196
1197 system "stty -echo";
1198 print "Password: ";
e71965be 1199 chomp($word = <STDIN>);
a0d0e21e
LW
1200 print "\n";
1201 system "stty echo";
1202
e71965be 1203 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1204 die "Sorry...\n";
a0d0e21e 1205 } else {
a9a5a0dc 1206 print "ok\n";
54310121 1207 }
a0d0e21e 1208
9f8f0c9d 1209Of course, typing in your own password to whoever asks you
748a9306 1210for it is unwise.
a0d0e21e 1211
ef2e6798 1212The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1213of data, not least of all because you can't get the information
ef2e6798 1214back. Look at the L<Digest> module for more robust algorithms.
19799a22 1215
f2791508
JH
1216If using crypt() on a Unicode string (which I<potentially> has
1217characters with codepoints above 255), Perl tries to make sense
34169887 1218of the situation by trying to downgrade (a copy of)
f2791508
JH
1219the string back to an eight-bit byte string before calling crypt()
1220(on that copy). If that works, good. If not, crypt() dies with
1221C<Wide character in crypt>.
85c16d83 1222
ea9eb35a 1223Portability issues: L<perlport/crypt>.
1224
aa689395 1225=item dbmclose HASH
d74e8afc 1226X<dbmclose>
a0d0e21e 1227
c17cdb72
NC
1228=for Pod::Functions breaks binding on a tied dbm file
1229
19799a22 1230[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1231
aa689395 1232Breaks the binding between a DBM file and a hash.
a0d0e21e 1233
ea9eb35a 1234Portability issues: L<perlport/dbmclose>.
1235
19799a22 1236=item dbmopen HASH,DBNAME,MASK
d74e8afc 1237X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1238
c17cdb72
NC
1239=for Pod::Functions create binding on a tied dbm file
1240
01aa884e
KW
1241[This function has been largely superseded by the
1242L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
a0d0e21e 1243
7b8d334a 1244This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1245hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1246argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1247is the name of the database (without the F<.dir> or F<.pag> extension if
1248any). If the database does not exist, it is created with protection
1b3a6178
FC
1249specified by MASK (as modified by the C<umask>). To prevent creation of
1250the database if it doesn't exist, you may specify a MODE
1251of 0, and the function will return a false value if it
1252can't find an existing database. If your system supports
80d38338 1253only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1254program. In older versions of Perl, if your system had neither DBM nor
19799a22 1255ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1256sdbm(3).
1257
1258If you don't have write access to the DBM file, you can only read hash
1259variables, not set them. If you want to test whether you can write,
3b10bc60 1260either use file tests or try setting a dummy hash entry inside an C<eval>
1261to trap the error.
a0d0e21e 1262
19799a22
GS
1263Note that functions such as C<keys> and C<values> may return huge lists
1264when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1265function to iterate over large DBM files. Example:
1266
1267 # print out history file offsets
1268 dbmopen(%HIST,'/usr/lib/news/history',0666);
1269 while (($key,$val) = each %HIST) {
a9a5a0dc 1270 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1271 }
1272 dbmclose(%HIST);
1273
cb1a09d0 1274See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1275cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1276rich implementation.
4633a7c4 1277
2b5ab1e7
TC
1278You can control which DBM library you use by loading that library
1279before you call dbmopen():
1280
1281 use DB_File;
1282 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1283 or die "Can't open netscape history file: $!";
2b5ab1e7 1284
ea9eb35a 1285Portability issues: L<perlport/dbmopen>.
1286
a0d0e21e 1287=item defined EXPR
d74e8afc 1288X<defined> X<undef> X<undefined>
a0d0e21e 1289
54310121 1290=item defined
bbce6d69 1291
c17cdb72
NC
1292=for Pod::Functions test whether a value, variable, or function is defined
1293
2f9daede 1294Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1295the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1296checked.
1297
1298Many operations return C<undef> to indicate failure, end of file,
1299system error, uninitialized variable, and other exceptional
1300conditions. This function allows you to distinguish C<undef> from
1301other values. (A simple Boolean test will not distinguish among
7660c0ab 1302C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1303false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1304doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1305returns C<undef> when its argument is an empty array, I<or> when the
1306element to return happens to be C<undef>.
1307
f10b0346
GS
1308You may also use C<defined(&func)> to check whether subroutine C<&func>
1309has ever been defined. The return value is unaffected by any forward
80d38338 1310declarations of C<&func>. A subroutine that is not defined
847c7ebe 1311may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1312makes it spring into existence the first time that it is called; see
847c7ebe 1313L<perlsub>.
f10b0346
GS
1314
1315Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1316used to report whether memory for that aggregate had ever been
f10b0346
GS
1317allocated. This behavior may disappear in future versions of Perl.
1318You should instead use a simple test for size:
1319
1320 if (@an_array) { print "has array elements\n" }
1321 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1322
1323When used on a hash element, it tells you whether the value is defined,
dc848c6f 1324not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1325purpose.
a0d0e21e
LW
1326
1327Examples:
1328
8f1da26d 1329 print if defined $switch{D};
a0d0e21e
LW
1330 print "$val\n" while defined($val = pop(@ary));
1331 die "Can't readlink $sym: $!"
a9a5a0dc 1332 unless defined($value = readlink $sym);
a0d0e21e 1333 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1334 $debugging = 0 unless defined $debugging;
a0d0e21e 1335
8f1da26d 1336Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1337discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1338defined values. For example, if you say
a5f75d66
AD
1339
1340 "ab" =~ /a(.*)b/;
1341
80d38338 1342The pattern match succeeds and C<$1> is defined, although it
cf264981 1343matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1344matched something that happened to be zero characters long. This is all
a5f75d66 1345very above-board and honest. When a function returns an undefined value,
2f9daede 1346it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1347should use C<defined> only when questioning the integrity of what
7660c0ab 1348you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1349what you want.
1350
dc848c6f 1351See also L</undef>, L</exists>, L</ref>.
2f9daede 1352
a0d0e21e 1353=item delete EXPR
d74e8afc 1354X<delete>
a0d0e21e 1355
c17cdb72
NC
1356=for Pod::Functions deletes a value from a hash
1357
d0a76353
RS
1358Given an expression that specifies an element or slice of a hash, C<delete>
1359deletes the specified elements from that hash so that exists() on that element
1360no longer returns true. Setting a hash element to the undefined value does
1361not remove its key, but deleting it does; see L</exists>.
80d38338 1362
8f1da26d 1363In list context, returns the value or values deleted, or the last such
80d38338 1364element in scalar context. The return list's length always matches that of
d0a76353
RS
1365the argument list: deleting non-existent elements returns the undefined value
1366in their corresponding positions.
80d38338 1367
d0a76353
RS
1368delete() may also be used on arrays and array slices, but its behavior is less
1369straightforward. Although exists() will return false for deleted entries,
1370deleting array elements never changes indices of existing values; use shift()
1371or splice() for that. However, if all deleted elements fall at the end of an
1372array, the array's size shrinks to the position of the highest element that
1373still tests true for exists(), or to 0 if none do.
1374
8f1da26d 1375B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1376be removed in a future version of Perl.
80d38338
TC
1377
1378Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1379a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1380or array may not necessarily return anything; it depends on the implementation
1381of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1382
80d38338
TC
1383The C<delete local EXPR> construct localizes the deletion to the current
1384block at run time. Until the block exits, elements locally deleted
1385temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1386of composite types">.
eba0920a
EM
1387
1388 %hash = (foo => 11, bar => 22, baz => 33);
f7051f2c
FC
1389 $scalar = delete $hash{foo}; # $scalar is 11
1390 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1391 @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
eba0920a 1392
01020589 1393The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1394
5f05dabc 1395 foreach $key (keys %HASH) {
a9a5a0dc 1396 delete $HASH{$key};
a0d0e21e
LW
1397 }
1398
01020589 1399 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1400 delete $ARRAY[$index];
01020589
GS
1401 }
1402
1403And so do these:
5f05dabc 1404
01020589
GS
1405 delete @HASH{keys %HASH};
1406
9740c838 1407 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1408
80d38338
TC
1409But both are slower than assigning the empty list
1410or undefining %HASH or @ARRAY, which is the customary
1411way to empty out an aggregate:
01020589 1412
5ed4f2ec 1413 %HASH = (); # completely empty %HASH
1414 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1415
5ed4f2ec 1416 @ARRAY = (); # completely empty @ARRAY
1417 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1418
80d38338
TC
1419The EXPR can be arbitrarily complicated provided its
1420final operation is an element or slice of an aggregate:
a0d0e21e
LW
1421
1422 delete $ref->[$x][$y]{$key};
5f05dabc 1423 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1424
01020589
GS
1425 delete $ref->[$x][$y][$index];
1426 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1427
a0d0e21e 1428=item die LIST
d74e8afc 1429X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1430
c17cdb72
NC
1431=for Pod::Functions raise an exception or bail out
1432
391b733c 1433C<die> raises an exception. Inside an C<eval> the error message is stuffed
4c050ad5
NC
1434into C<$@> and the C<eval> is terminated with the undefined value.
1435If the exception is outside of all enclosing C<eval>s, then the uncaught
391b733c 1436exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1437need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1438
1439Equivalent examples:
1440
1441 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1442 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1443
ccac6780 1444If the last element of LIST does not end in a newline, the current
df37ec69
WW
1445script line number and input line number (if any) are also printed,
1446and a newline is supplied. Note that the "input line number" (also
1447known as "chunk") is subject to whatever notion of "line" happens to
1448be currently in effect, and is also available as the special variable
1449C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1450
1451Hint: sometimes appending C<", stopped"> to your message will cause it
1452to make better sense when the string C<"at foo line 123"> is appended.
1453Suppose you are running script "canasta".
a0d0e21e
LW
1454
1455 die "/etc/games is no good";
1456 die "/etc/games is no good, stopped";
1457
1458produce, respectively
1459
1460 /etc/games is no good at canasta line 123.
1461 /etc/games is no good, stopped at canasta line 123.
1462
a96d0188 1463If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1464previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1465This is useful for propagating exceptions:
1466
1467 eval { ... };
1468 die unless $@ =~ /Expected exception/;
1469
a96d0188 1470If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1471C<PROPAGATE> method, that method will be called with additional file
1472and line number parameters. The return value replaces the value in
34169887 1473C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1474were called.
1475
7660c0ab 1476If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1477
4c050ad5
NC
1478If an uncaught exception results in interpreter exit, the exit code is
1479determined from the values of C<$!> and C<$?> with this pseudocode:
1480
1481 exit $! if $!; # errno
1482 exit $? >> 8 if $? >> 8; # child exit status
1483 exit 255; # last resort
1484
1485The intent is to squeeze as much possible information about the likely cause
391b733c
FC
1486into the limited space of the system exit
1487code. However, as C<$!> is the value
4c050ad5
NC
1488of C's C<errno>, which can be set by any system call, this means that the value
1489of the exit code used by C<die> can be non-predictable, so should not be relied
1490upon, other than to be non-zero.
1491
80d38338
TC
1492You can also call C<die> with a reference argument, and if this is trapped
1493within an C<eval>, C<$@> contains that reference. This permits more
1494elaborate exception handling using objects that maintain arbitrary state
1495about the exception. Such a scheme is sometimes preferable to matching
1496particular string values of C<$@> with regular expressions. Because C<$@>
1497is a global variable and C<eval> may be used within object implementations,
1498be careful that analyzing the error object doesn't replace the reference in
1499the global variable. It's easiest to make a local copy of the reference
1500before any manipulations. Here's an example:
52531d10 1501
80d38338 1502 use Scalar::Util "blessed";
da279afe 1503
52531d10 1504 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7 1505 if (my $ev_err = $@) {
f7051f2c
FC
1506 if (blessed($ev_err)
1507 && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1508 # handle Some::Module::Exception
1509 }
1510 else {
1511 # handle all other possible exceptions
1512 }
1513 }
1514
3b10bc60 1515Because Perl stringifies uncaught exception messages before display,
80d38338 1516you'll probably want to overload stringification operations on
52531d10
GS
1517exception objects. See L<overload> for details about that.
1518
19799a22
GS
1519You can arrange for a callback to be run just before the C<die>
1520does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1521handler is called with the error text and can change the error
19799a22 1522message, if it sees fit, by calling C<die> again. See
96090e4f 1523L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1524L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1525to be run only right before your program was to exit, this is not
3b10bc60 1526currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1527even inside eval()ed blocks/strings! If one wants the hook to do
1528nothing in such situations, put
fb73857a 1529
5ed4f2ec 1530 die @_ if $^S;
fb73857a 1531
19799a22
GS
1532as the first line of the handler (see L<perlvar/$^S>). Because
1533this promotes strange action at a distance, this counterintuitive
b76cc8ba 1534behavior may be fixed in a future release.
774d564b 1535
4c050ad5
NC
1536See also exit(), warn(), and the Carp module.
1537
a0d0e21e 1538=item do BLOCK
d74e8afc 1539X<do> X<block>
a0d0e21e 1540
c17cdb72
NC
1541=for Pod::Functions turn a BLOCK into a TERM
1542
a0d0e21e 1543Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1544sequence of commands indicated by BLOCK. When modified by the C<while> or
1545C<until> loop modifier, executes the BLOCK once before testing the loop
391b733c 1546condition. (On other statements the loop modifiers test the conditional
6b275a1f 1547first.)
a0d0e21e 1548
4968c1e4 1549C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1550C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1551See L<perlsyn> for alternative strategies.
4968c1e4 1552
a0d0e21e 1553=item do SUBROUTINE(LIST)
d74e8afc 1554X<do>
a0d0e21e 1555
3bab73c8
FC
1556This form of subroutine call is deprecated. SUBROUTINE can be a bareword
1557or scalar variable.
a0d0e21e
LW
1558
1559=item do EXPR
d74e8afc 1560X<do>
a0d0e21e
LW
1561
1562Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1563file as a Perl script.
a0d0e21e
LW
1564
1565 do 'stat.pl';
1566
c319391a 1567is largely like
a0d0e21e 1568
986b19de 1569 eval `cat stat.pl`;
a0d0e21e 1570
c319391a
AC
1571except that it's more concise, runs no external processes, keeps track of
1572the current
96090e4f
A
1573filename for error messages, searches the C<@INC> directories, and updates
1574C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1575these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1576cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1577same, however, in that it does reparse the file every time you call it,
1578so you probably don't want to do this inside a loop.
a0d0e21e 1579
8f1da26d 1580If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1581an error message in C<$@>. If C<do> cannot read the file, it returns undef
1582and sets C<$!> to the error. Always check C<$@> first, as compilation
1583could fail in a way that also sets C<$!>. If the file is successfully
1584compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1585
80d38338 1586Inclusion of library modules is better done with the
19799a22 1587C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1588and raise an exception if there's a problem.
a0d0e21e 1589
5a964f20
TC
1590You might like to use C<do> to read in a program configuration
1591file. Manual error checking can be done this way:
1592
b76cc8ba 1593 # read in config files: system first, then user
f86cebdf 1594 for $file ("/share/prog/defaults.rc",
b76cc8ba 1595 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1596 {
1597 unless ($return = do $file) {
1598 warn "couldn't parse $file: $@" if $@;
1599 warn "couldn't do $file: $!" unless defined $return;
1600 warn "couldn't run $file" unless $return;
1601 }
5a964f20
TC
1602 }
1603
a0d0e21e 1604=item dump LABEL
d74e8afc 1605X<dump> X<core> X<undump>
a0d0e21e 1606
8a7e748e
FC
1607=item dump EXPR
1608
1614b0e3
JD
1609=item dump
1610
c17cdb72
NC
1611=for Pod::Functions create an immediate core dump
1612
19799a22
GS
1613This function causes an immediate core dump. See also the B<-u>
1614command-line switch in L<perlrun>, which does the same thing.
1615Primarily this is so that you can use the B<undump> program (not
1616supplied) to turn your core dump into an executable binary after
1617having initialized all your variables at the beginning of the
1618program. When the new binary is executed it will begin by executing
1619a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1620Think of it as a goto with an intervening core dump and reincarnation.
8a7e748e
FC
1621If C<LABEL> is omitted, restarts the program from the top. The
1622C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1623computed at run time, being otherwise identical to C<dump LABEL>.
19799a22
GS
1624
1625B<WARNING>: Any files opened at the time of the dump will I<not>
1626be open any more when the program is reincarnated, with possible
80d38338 1627resulting confusion by Perl.
19799a22 1628
59f521f4 1629This function is now largely obsolete, mostly because it's very hard to
391b733c 1630convert a core file into an executable. That's why you should now invoke
59f521f4 1631it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1632typo.
19799a22 1633
2ba1f20a
FC
1634Unlike most named operators, this has the same precedence as assignment.
1635It is also exempt from the looks-like-a-function rule, so
1636C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1637C<dump>.
1638
ea9eb35a 1639Portability issues: L<perlport/dump>.
1640
532eee96 1641=item each HASH
d74e8afc 1642X<each> X<hash, iterator>
aa689395 1643
532eee96 1644=item each ARRAY
aeedbbed
NC
1645X<array, iterator>
1646
f5a93a43
TC
1647=item each EXPR
1648
c17cdb72
NC
1649=for Pod::Functions retrieve the next key/value pair from a hash
1650
bade7fbc
TC
1651When called on a hash in list context, returns a 2-element list
1652consisting of the key and value for the next element of a hash. In Perl
16535.12 and later only, it will also return the index and value for the next
1654element of an array so that you can iterate over it; older Perls consider
1655this a syntax error. When called in scalar context, returns only the key
1656(not the value) in a hash, or the index in an array.
2f9daede 1657
aeedbbed 1658Hash entries are returned in an apparently random order. The actual random
7bf59113 1659order is specific to a given hash; the exact same series of operations
7161e5c2 1660on two hashes may result in a different order for each hash. Any insertion
7bf59113
YO
1661into the hash may change the order, as will any deletion, with the exception
1662that the most recent key returned by C<each> or C<keys> may be deleted
7161e5c2 1663without changing the order. So long as a given hash is unmodified you may
7bf59113 1664rely on C<keys>, C<values> and C<each> to repeatedly return the same order
7161e5c2
FC
1665as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
1666details on why hash order is randomized. Aside from the guarantees
7bf59113
YO
1667provided here the exact details of Perl's hash algorithm and the hash
1668traversal order are subject to change in any release of Perl.
ab192400 1669
80d38338
TC
1670After C<each> has returned all entries from the hash or array, the next
1671call to C<each> returns the empty list in list context and C<undef> in
bade7fbc
TC
1672scalar context; the next call following I<that> one restarts iteration.
1673Each hash or array has its own internal iterator, accessed by C<each>,
1674C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1675reached the end as just described; it can be explicitly reset by calling
1676C<keys> or C<values> on the hash or array. If you add or delete a hash's
49daec89
DM
1677elements while iterating over it, the effect on the iterator is
1678unspecified; for example, entries may be skipped or duplicated--so don't
1679do that. Exception: In the current implementation, it is always safe to
1680delete the item most recently returned by C<each()>, so the following code
1681works properly:
74fc8b5f
MJD
1682
1683 while (($key, $value) = each %hash) {
1684 print $key, "\n";
1685 delete $hash{$key}; # This is safe
1686 }
aa689395 1687
80d38338 1688This prints out your environment like the printenv(1) program,
3b10bc60 1689but in a different order:
a0d0e21e
LW
1690
1691 while (($key,$value) = each %ENV) {
a9a5a0dc 1692 print "$key=$value\n";
a0d0e21e
LW
1693 }
1694
f5a93a43
TC
1695Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1696reference to an unblessed hash or array. The argument will be dereferenced
1697automatically. This aspect of C<each> is considered highly experimental.
1698The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1699
1700 while (($key,$value) = each $hashref) { ... }
1701
e6a0db3e
FC
1702As of Perl 5.18 you can use a bare C<each> in a C<while> loop,
1703which will set C<$_> on every iteration.
1704
1705 while(each %ENV) {
1706 print "$_=$ENV{$_}\n";
1707 }
1708
bade7fbc
TC
1709To avoid confusing would-be users of your code who are running earlier
1710versions of Perl with mysterious syntax errors, put this sort of thing at
1711the top of your file to signal that your code will work I<only> on Perls of
1712a recent vintage:
1713
1714 use 5.012; # so keys/values/each work on arrays
1715 use 5.014; # so keys/values/each work on scalars (experimental)
e6a0db3e 1716 use 5.018; # so each assigns to $_ in a lone while test
bade7fbc 1717
8f1da26d 1718See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1719
1720=item eof FILEHANDLE
d74e8afc
ITB
1721X<eof>
1722X<end of file>
1723X<end-of-file>
a0d0e21e 1724
4633a7c4
LW
1725=item eof ()
1726
a0d0e21e
LW
1727=item eof
1728
c17cdb72
NC
1729=for Pod::Functions test a filehandle for its end
1730
8f1da26d 1731Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1732FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1733gives the real filehandle. (Note that this function actually
80d38338 1734reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1735interactive context.) Do not read from a terminal file (or call
19799a22 1736C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1737as terminals may lose the end-of-file condition if you do.
1738
820475bd 1739An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1740with empty parentheses is different. It refers to the pseudo file
820475bd 1741formed from the files listed on the command line and accessed via the
61eff3bc
JH
1742C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1743as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1744used will cause C<@ARGV> to be examined to determine if input is
67408cae 1745available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1746end-of-file will assume you are processing another C<@ARGV> list,
1747and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1748see L<perlop/"I/O Operators">.
820475bd 1749
61eff3bc 1750In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1751detect the end of each file, whereas C<eof()> will detect the end
1752of the very last file only. Examples:
a0d0e21e 1753
748a9306
LW
1754 # reset line numbering on each input file
1755 while (<>) {
a9a5a0dc
VP
1756 next if /^\s*#/; # skip comments
1757 print "$.\t$_";
5a964f20 1758 } continue {
a9a5a0dc 1759 close ARGV if eof; # Not eof()!
748a9306
LW
1760 }
1761
a0d0e21e
LW
1762 # insert dashes just before last line of last file
1763 while (<>) {
a9a5a0dc
VP
1764 if (eof()) { # check for end of last file
1765 print "--------------\n";
1766 }
1767 print;
f7051f2c 1768 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1769 }
1770
a0d0e21e 1771Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1772input operators typically return C<undef> when they run out of data or
1773encounter an error.
a0d0e21e
LW
1774
1775=item eval EXPR
d74e8afc 1776X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1777X<error, handling> X<exception, handling>
a0d0e21e
LW
1778
1779=item eval BLOCK
1780
ce2984c3
PF
1781=item eval
1782
c17cdb72
NC
1783=for Pod::Functions catch exceptions or compile and run code
1784
798dc914
KW
1785In the first form, often referred to as a "string eval", the return
1786value of EXPR is parsed and executed as if it
c7cc6f1c 1787were a little Perl program. The value of the expression (which is itself
8f1da26d 1788determined within scalar context) is first parsed, and if there were no
2341804c 1789errors, executed as a block within the lexical context of the current Perl
df4833a8 1790program. This means, that in particular, any outer lexical variables are
2341804c
DM
1791visible to it, and any package variable settings or subroutine and format
1792definitions remain afterwards.
1793
1794Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1795If EXPR is omitted, evaluates C<$_>. This form is typically used to
1796delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c 1797
7289c5e6
FC
1798If the C<unicode_eval> feature is enabled (which is the default under a
1799C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1800characters, so C<use utf8> declarations have no effect, and source filters
1801are forbidden. In the absence of the C<unicode_eval> feature, the string
1802will sometimes be treated as characters and sometimes as bytes, depending
1803on the internal encoding, and source filters activated within the C<eval>
1804exhibit the erratic, but historical, behaviour of affecting some outer file
1805scope that is still compiling. See also the L</evalbytes> keyword, which
1806always treats its input as a byte stream and works properly with source
1807filters, and the L<feature> pragma.
1808
798dc914
KW
1809Problems can arise if the string expands a scalar containing a floating
1810point number. That scalar can expand to letters, such as C<"NaN"> or
1811C<"Infinity">; or, within the scope of a C<use locale>, the decimal
1812point character may be something other than a dot (such as a comma).
1813None of these are likely to parse as you are likely expecting.
1814
c7cc6f1c 1815In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1816same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1817within the context of the current Perl program. This form is typically
1818used to trap exceptions more efficiently than the first (see below), while
1819also providing the benefit of checking the code within BLOCK at compile
1820time.
1821
1822The final semicolon, if any, may be omitted from the value of EXPR or within
1823the BLOCK.
1824
1825In both forms, the value returned is the value of the last expression
5a964f20 1826evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1827as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1828in void, scalar, or list context, depending on the context of the C<eval>
1829itself. See L</wantarray> for more on how the evaluation context can be
1830determined.
a0d0e21e 1831
19799a22 1832If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1833executed, C<eval> returns C<undef> in scalar context
774b80e8
FC
1834or an empty list in list context, and C<$@> is set to the error
1835message. (Prior to 5.16, a bug caused C<undef> to be returned
1836in list context for syntax errors, but not for runtime errors.)
1837If there was no error, C<$@> is set to the empty string. A
9cc672d4
FC
1838control flow operator like C<last> or C<goto> can bypass the setting of
1839C<$@>. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1840warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1841To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1842turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1843See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1844
19799a22
GS
1845Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1846determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1847is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1848the die operator is used to raise exceptions.
1849
5f1da31c
NT
1850If you want to trap errors when loading an XS module, some problems with
1851the binary interface (such as Perl version skew) may be fatal even with
df4833a8 1852C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
5f1da31c 1853
a0d0e21e
LW
1854If the code to be executed doesn't vary, you may use the eval-BLOCK
1855form to trap run-time errors without incurring the penalty of
1856recompiling each time. The error, if any, is still returned in C<$@>.
1857Examples:
1858
54310121 1859 # make divide-by-zero nonfatal
a0d0e21e
LW
1860 eval { $answer = $a / $b; }; warn $@ if $@;
1861
1862 # same thing, but less efficient
1863 eval '$answer = $a / $b'; warn $@ if $@;
1864
1865 # a compile-time error
5ed4f2ec 1866 eval { $answer = }; # WRONG
a0d0e21e
LW
1867
1868 # a run-time error
5ed4f2ec 1869 eval '$answer ='; # sets $@
a0d0e21e 1870
cf264981
SP
1871Using the C<eval{}> form as an exception trap in libraries does have some
1872issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1873may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1874You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1875as this example shows:
774d564b 1876
80d38338 1877 # a private exception trap for divide-by-zero
f86cebdf
GS
1878 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1879 warn $@ if $@;
774d564b
PP
1880
1881This is especially significant, given that C<__DIE__> hooks can call
19799a22 1882C<die> again, which has the effect of changing their error messages:
774d564b
PP
1883
1884 # __DIE__ hooks may modify error messages
1885 {
f86cebdf
GS
1886 local $SIG{'__DIE__'} =
1887 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1888 eval { die "foo lives here" };
1889 print $@ if $@; # prints "bar lives here"
774d564b
PP
1890 }
1891
19799a22 1892Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1893may be fixed in a future release.
1894
19799a22 1895With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1896being looked at when:
1897
5ed4f2ec 1898 eval $x; # CASE 1
1899 eval "$x"; # CASE 2
a0d0e21e 1900
5ed4f2ec 1901 eval '$x'; # CASE 3
1902 eval { $x }; # CASE 4
a0d0e21e 1903
5ed4f2ec 1904 eval "\$$x++"; # CASE 5
1905 $$x++; # CASE 6
a0d0e21e 1906
2f9daede 1907Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1908the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1909the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1910and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1911does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1912purely visual reasons, but it also has the advantage of compiling at
1913compile-time instead of at run-time.) Case 5 is a place where
19799a22 1914normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1915particular situation, you can just use symbolic references instead, as
1916in case 6.
a0d0e21e 1917
b6538e4f 1918Before Perl 5.14, the assignment to C<$@> occurred before restoration
bade7fbc 1919of localized variables, which means that for your code to run on older
b208c909 1920versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1921errors:
1922
1923 # alter $@ on nefarious repugnancy only
1924 {
1925 my $e;
1926 {
f7051f2c
FC
1927 local $@; # protect existing $@
1928 eval { test_repugnancy() };
1929 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1930 $@ =~ /nefarious/ and $e = $@;
8a5a710d
DN
1931 }
1932 die $e if defined $e
1933 }
1934
4968c1e4 1935C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1936C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1937
4f00fc7e
FC
1938An C<eval ''> executed within a subroutine defined
1939in the C<DB> package doesn't see the usual
3b10bc60 1940surrounding lexical scope, but rather the scope of the first non-DB piece
df4833a8 1941of code that called it. You don't normally need to worry about this unless
3b10bc60 1942you are writing a Perl debugger.
d819b83a 1943
7289c5e6
FC
1944=item evalbytes EXPR
1945X<evalbytes>
1946
1947=item evalbytes
1948
d9b04284 1949=for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
c17cdb72 1950
7289c5e6
FC
1951This function is like L</eval> with a string argument, except it always
1952parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1953string containing characters whose ordinal value exceeds 255 results in an
1954error. Source filters activated within the evaluated code apply to the
1955code itself.
1956
1957This function is only available under the C<evalbytes> feature, a
1958C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1959L<feature> for more information.
1960
a0d0e21e 1961=item exec LIST
d74e8afc 1962X<exec> X<execute>
a0d0e21e 1963
8bf3b016
GS
1964=item exec PROGRAM LIST
1965
c17cdb72
NC
1966=for Pod::Functions abandon this program to run another
1967
3b10bc60 1968The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1969use C<system> instead of C<exec> if you want it to return. It fails and
1970returns false only if the command does not exist I<and> it is executed
fb73857a 1971directly instead of via your system's command shell (see below).
a0d0e21e 1972
19799a22 1973Since it's a common mistake to use C<exec> instead of C<system>, Perl
4642e50d
EB
1974warns you if C<exec> is called in void context and if there is a following
1975statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1976you always do that, right?). If you I<really> want to follow an C<exec>
1977with some other statement, you can use one of these styles to avoid the warning:
55d729e4 1978
5a964f20
TC
1979 exec ('foo') or print STDERR "couldn't exec foo: $!";
1980 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1981
5a964f20 1982If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1983with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1984If there is only one scalar argument or an array with one element in it,
1985the argument is checked for shell metacharacters, and if there are any,
1986the entire argument is passed to the system's command shell for parsing
1987(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1988If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1989words and passed directly to C<execvp>, which is more efficient.
19799a22 1990Examples:
a0d0e21e 1991
19799a22
GS
1992 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1993 exec "sort $outfile | uniq";
a0d0e21e
LW
1994
1995If you don't really want to execute the first argument, but want to lie
1996to the program you are executing about its own name, you can specify
1997the program you actually want to run as an "indirect object" (without a
1998comma) in front of the LIST. (This always forces interpretation of the
54310121 1999LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
2000the list.) Example:
2001
2002 $shell = '/bin/csh';
5ed4f2ec 2003 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
2004
2005or, more directly,
2006
5ed4f2ec 2007 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 2008
3b10bc60 2009When the arguments get executed via the system shell, results are
2010subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
2011for details.
2012
19799a22
GS
2013Using an indirect object with C<exec> or C<system> is also more
2014secure. This usage (which also works fine with system()) forces
2015interpretation of the arguments as a multivalued list, even if the
2016list had just one argument. That way you're safe from the shell
2017expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
2018
2019 @args = ( "echo surprise" );
2020
2b5ab1e7 2021 exec @args; # subject to shell escapes
f86cebdf 2022 # if @args == 1
2b5ab1e7 2023 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
2024
2025The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
2026program, passing it C<"surprise"> an argument. The second version didn't;
2027it tried to run a program named I<"echo surprise">, didn't find it, and set
2028C<$?> to a non-zero value indicating failure.
5a964f20 2029
e9fa405d
BF
2030Perl attempts to flush all files opened for output before the exec,
2031but this may not be supported on some platforms (see L<perlport>).
2032To be safe, you may need to set C<$|> ($AUTOFLUSH in English) or
2033call the C<autoflush()> method of C<IO::Handle> on any open handles
2034to avoid lost output.
0f897271 2035
80d38338
TC
2036Note that C<exec> will not call your C<END> blocks, nor will it invoke
2037C<DESTROY> methods on your objects.
7660c0ab 2038
ea9eb35a 2039Portability issues: L<perlport/exec>.
2040
a0d0e21e 2041=item exists EXPR
d74e8afc 2042X<exists> X<autovivification>
a0d0e21e 2043
c17cdb72
NC
2044=for Pod::Functions test whether a hash key is present
2045
d0a76353
RS
2046Given an expression that specifies an element of a hash, returns true if the
2047specified element in the hash has ever been initialized, even if the
2048corresponding value is undefined.
a0d0e21e 2049
5ed4f2ec 2050 print "Exists\n" if exists $hash{$key};
2051 print "Defined\n" if defined $hash{$key};
01020589
GS
2052 print "True\n" if $hash{$key};
2053
d0a76353 2054exists may also be called on array elements, but its behavior is much less
8f1da26d 2055obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
2056that calling exists on array values is deprecated and likely to be removed in
2057a future version of Perl.
2058
5ed4f2ec 2059 print "Exists\n" if exists $array[$index];
2060 print "Defined\n" if defined $array[$index];
01020589 2061 print "True\n" if $array[$index];
a0d0e21e 2062
8f1da26d 2063A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
2064it exists, but the reverse doesn't necessarily hold true.
2065
afebc493
GS
2066Given an expression that specifies the name of a subroutine,
2067returns true if the specified subroutine has ever been declared, even
2068if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 2069does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
2070exist may still be callable: its package may have an C<AUTOLOAD>
2071method that makes it spring into existence the first time that it is
3b10bc60 2072called; see L<perlsub>.
afebc493 2073
5ed4f2ec 2074 print "Exists\n" if exists &subroutine;
2075 print "Defined\n" if defined &subroutine;
afebc493 2076
a0d0e21e 2077Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 2078operation is a hash or array key lookup or subroutine name:
a0d0e21e 2079
5ed4f2ec 2080 if (exists $ref->{A}->{B}->{$key}) { }
2081 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 2082
5ed4f2ec 2083 if (exists $ref->{A}->{B}->[$ix]) { }
2084 if (exists $hash{A}{B}[$ix]) { }
01020589 2085
afebc493
GS
2086 if (exists &{$ref->{A}{B}{$key}}) { }
2087
9590a7cd 2088Although the most deeply nested array or hash element will not spring into
3b10bc60 2089existence just because its existence was tested, any intervening ones will.
61eff3bc 2090Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 2091into existence due to the existence test for the $key element above.
3b10bc60 2092This happens anywhere the arrow operator is used, including even here:
5a964f20 2093
2b5ab1e7 2094 undef $ref;
5ed4f2ec 2095 if (exists $ref->{"Some key"}) { }
2096 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
2097
2098This surprising autovivification in what does not at first--or even
2099second--glance appear to be an lvalue context may be fixed in a future
5a964f20 2100release.
a0d0e21e 2101
afebc493
GS
2102Use of a subroutine call, rather than a subroutine name, as an argument
2103to exists() is an error.
2104
5ed4f2ec 2105 exists &sub; # OK
2106 exists &sub(); # Error
afebc493 2107
a0d0e21e 2108=item exit EXPR
d74e8afc 2109X<exit> X<terminate> X<abort>
a0d0e21e 2110
ce2984c3
PF
2111=item exit
2112
c17cdb72
NC
2113=for Pod::Functions terminate this program
2114
2b5ab1e7 2115Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
2116
2117 $ans = <STDIN>;
2118 exit 0 if $ans =~ /^[Xx]/;
2119
19799a22 2120See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
2121universally recognized values for EXPR are C<0> for success and C<1>
2122for error; other values are subject to interpretation depending on the
2123environment in which the Perl program is running. For example, exiting
212469 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2125the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 2126
19799a22
GS
2127Don't use C<exit> to abort a subroutine if there's any chance that
2128someone might want to trap whatever error happened. Use C<die> instead,
2129which can be trapped by an C<eval>.
28757baa 2130
19799a22 2131The exit() function does not always exit immediately. It calls any
2b5ab1e7 2132defined C<END> routines first, but these C<END> routines may not
19799a22 2133themselves abort the exit. Likewise any object destructors that need to
60275626 2134be called are called before the real exit. C<END> routines and destructors
391b733c 2135can change the exit status by modifying C<$?>. If this is a problem, you
fae6f8fa 2136can call C<POSIX::_exit($status)> to avoid END and destructor processing.
87275199 2137See L<perlmod> for details.
5a964f20 2138
ea9eb35a 2139Portability issues: L<perlport/exit>.
2140
a0d0e21e 2141=item exp EXPR
d74e8afc 2142X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 2143
54310121 2144=item exp
bbce6d69 2145
c17cdb72
NC
2146=for Pod::Functions raise I<e> to a power
2147
b76cc8ba 2148Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
2149If EXPR is omitted, gives C<exp($_)>.
2150
628253b8
BF
2151=item fc EXPR
2152X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2153
2154=item fc
2155
d9b04284 2156=for Pod::Functions +fc return casefolded version of a string
c17cdb72 2157
628253b8
BF
2158Returns the casefolded version of EXPR. This is the internal function
2159implementing the C<\F> escape in double-quoted strings.
2160
2161Casefolding is the process of mapping strings to a form where case
2162differences are erased; comparing two strings in their casefolded
2163form is effectively a way of asking if two strings are equal,
2164regardless of case.
2165
2166Roughly, if you ever found yourself writing this
2167
f6c6dcb6 2168 lc($this) eq lc($that) # Wrong!
628253b8 2169 # or
f6c6dcb6 2170 uc($this) eq uc($that) # Also wrong!
628253b8 2171 # or
f6c6dcb6 2172 $this =~ /^\Q$that\E\z/i # Right!
628253b8
BF
2173
2174Now you can write
2175
2176 fc($this) eq fc($that)
2177
2178And get the correct results.
2179
fc39a31f
KW
2180Perl only implements the full form of casefolding,
2181but you can access the simple folds using L<Unicode::UCD/casefold()> and
2182L<Unicode::UCD/prop_invmap()>.
628253b8
BF
2183For further information on casefolding, refer to
2184the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
21854.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2186available at L<http://www.unicode.org/versions/latest/>, as well as the
2187Case Charts available at L<http://www.unicode.org/charts/case/>.
2188
2189If EXPR is omitted, uses C<$_>.
2190
1ca267a5
KW
2191This function behaves the same way under various pragma, such as within
2192S<C<"use feature 'unicode_strings">>, as L</lc> does, with the single
2193exception of C<fc> of LATIN CAPITAL LETTER SHARP S (U+1E9E) within the
2194scope of S<C<use locale>>. The foldcase of this character would
2195normally be C<"ss">, but as explained in the L</lc> section, case
2196changes that cross the 255/256 boundary are problematic under locales,
2197and are hence prohibited. Therefore, this function under locale returns
2198instead the string C<"\x{17F}\x{17F}">, which is the LATIN SMALL LETTER
2199LONG S. Since that character itself folds to C<"s">, the string of two
2200of them together should be equivalent to a single U+1E9E when foldcased.
628253b8
BF
2201
2202While the Unicode Standard defines two additional forms of casefolding,
2203one for Turkic languages and one that never maps one character into multiple
2204characters, these are not provided by the Perl core; However, the CPAN module
2205C<Unicode::Casing> may be used to provide an implementation.
2206
2207This keyword is available only when the C<"fc"> feature is enabled,
7161e5c2 2208or when prefixed with C<CORE::>; See L<feature>. Alternately,
628253b8
BF
2209include a C<use v5.16> or later to the current scope.
2210
a0d0e21e 2211=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2212X<fcntl>
a0d0e21e 2213
c17cdb72
NC
2214=for Pod::Functions file control system call
2215
f86cebdf 2216Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
2217
2218 use Fcntl;
2219
0ade1984 2220first to get the correct constant definitions. Argument processing and
3b10bc60 2221value returned work just like C<ioctl> below.
a0d0e21e
LW
2222For example:
2223
2224 use Fcntl;
5a964f20 2225 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 2226 or die "can't fcntl F_GETFL: $!";
5a964f20 2227
554ad1fc 2228You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
2229Like C<ioctl>, it maps a C<0> return from the system call into
2230C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
2231in numeric context. It is also exempt from the normal B<-w> warnings
2232on improper numeric conversions.
5a964f20 2233
3b10bc60 2234Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
2235doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2236manpage to learn what functions are available on your system.
a0d0e21e 2237
be2f7487 2238Here's an example of setting a filehandle named C<REMOTE> to be
2239non-blocking at the system level. You'll have to negotiate C<$|>
2240on your own, though.
2241
2242 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2243
2244 $flags = fcntl(REMOTE, F_GETFL, 0)
2245 or die "Can't get flags for the socket: $!\n";
2246
2247 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2248 or die "Can't set flags for the socket: $!\n";
2249
ea9eb35a 2250Portability issues: L<perlport/fcntl>.
2251
cfa52385
FC
2252=item __FILE__
2253X<__FILE__>
2254
c17cdb72
NC
2255=for Pod::Functions the name of the current source file
2256
cfa52385
FC
2257A special token that returns the name of the file in which it occurs.
2258
a0d0e21e 2259=item fileno FILEHANDLE
d74e8afc 2260X<fileno>
a0d0e21e 2261
c17cdb72
NC
2262=for Pod::Functions return file descriptor from filehandle
2263
2b5ab1e7 2264Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
2265filehandle is not open. If there is no real file descriptor at the OS
2266level, as can happen with filehandles connected to memory objects via
2267C<open> with a reference for the third argument, -1 is returned.
2268
2269This is mainly useful for constructing
19799a22 2270bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
2271If FILEHANDLE is an expression, the value is taken as an indirect
2272filehandle, generally its name.
5a964f20 2273
b76cc8ba 2274You can use this to find out whether two handles refer to the
5a964f20
TC
2275same underlying descriptor:
2276
3231d257 2277 if (fileno(THIS) != -1 && fileno(THIS) == fileno(THAT)) {
a9a5a0dc 2278 print "THIS and THAT are dups\n";
3231d257 2279 } elsif (fileno(THIS) != -1 && fileno(THAT) != -1) {
555bd962
BG
2280 print "THIS and THAT have different " .
2281 "underlying file descriptors\n";
3231d257 2282 } else {
555bd962
BG
2283 print "At least one of THIS and THAT does " .
2284 "not have a real file descriptor\n";
b76cc8ba
NIS
2285 }
2286
a0d0e21e 2287=item flock FILEHANDLE,OPERATION
d74e8afc 2288X<flock> X<lock> X<locking>
a0d0e21e 2289
c17cdb72
NC
2290=for Pod::Functions lock an entire file with an advisory lock
2291
19799a22
GS
2292Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2293for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2294machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2295C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2296entire files only, not records.
2b5ab1e7
TC
2297
2298Two potentially non-obvious but traditional C<flock> semantics are
2299that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2300are B<merely advisory>. Such discretionary locks are more flexible, but
2301offer fewer guarantees. This means that programs that do not also use
2302C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2303your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2304for details. It's best to assume traditional behavior if you're writing
2305portable programs. (But if you're not, you should as always feel perfectly
2306free to write for your own system's idiosyncrasies (sometimes called
2307"features"). Slavish adherence to portability concerns shouldn't get
2308in the way of your getting your job done.)
a3cb178b 2309
8ebc5c01
PP
2310OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2311LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2312you can use the symbolic names if you import them from the L<Fcntl> module,
2313either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2314requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2315releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2316LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2317waiting for the lock; check the return status to see if you got it.
68dc0745 2318
2b5ab1e7
TC
2319To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2320before locking or unlocking it.
8ebc5c01 2321
f86cebdf 2322Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2323locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2324are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2325implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
2326differing semantics shouldn't bite too many people.
2327
becacb53
TM
2328Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2329be open with read intent to use LOCK_SH and requires that it be open
2330with write intent to use LOCK_EX.
2331
19799a22
GS
2332Note also that some versions of C<flock> cannot lock things over the
2333network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2334that. If you like you can force Perl to ignore your system's flock(2)
2335function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2336the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2337and build a new Perl.
4633a7c4
LW
2338
2339Here's a mailbox appender for BSD systems.
a0d0e21e 2340
f7051f2c
FC
2341 # import LOCK_* and SEEK_END constants
2342 use Fcntl qw(:flock SEEK_END);
a0d0e21e
LW
2343
2344 sub lock {
a9a5a0dc
VP
2345 my ($fh) = @_;
2346 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2347
a9a5a0dc
VP
2348 # and, in case someone appended while we were waiting...
2349 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2350 }
2351
2352 sub unlock {
a9a5a0dc
VP
2353 my ($fh) = @_;
2354 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2355 }
2356
b0169937 2357 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2358 or die "Can't open mailbox: $!";
a0d0e21e 2359
7ed5353d 2360 lock($mbox);
b0169937 2361 print $mbox $msg,"\n\n";
7ed5353d 2362 unlock($mbox);
a0d0e21e 2363
3b10bc60 2364On systems that support a real flock(2), locks are inherited across fork()
2365calls, whereas those that must resort to the more capricious fcntl(2)
2366function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2367
cb1a09d0 2368See also L<DB_File> for other flock() examples.
a0d0e21e 2369
ea9eb35a 2370Portability issues: L<perlport/flock>.
2371
a0d0e21e 2372=item fork
d74e8afc 2373X<fork> X<child> X<parent>
a0d0e21e 2374
c17cdb72
NC
2375=for Pod::Functions create a new process just like this one
2376
2b5ab1e7
TC
2377Does a fork(2) system call to create a new process running the
2378same program at the same point. It returns the child pid to the
2379parent process, C<0> to the child process, or C<undef> if the fork is
2380unsuccessful. File descriptors (and sometimes locks on those descriptors)
2381are shared, while everything else is copied. On most systems supporting
2382fork(), great care has gone into making it extremely efficient (for
2383example, using copy-on-write technology on data pages), making it the
2384dominant paradigm for multitasking over the last few decades.
5a964f20 2385
e9fa405d 2386Perl attempts to flush all files opened for
0f897271
GS
2387output before forking the child process, but this may not be supported
2388on some platforms (see L<perlport>). To be safe, you may need to set
2389C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2390C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2391
19799a22 2392If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2393accumulate zombies. On some systems, you can avoid this by setting
2394C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2395forking and reaping moribund children.
cb1a09d0 2396
28757baa
PP
2397Note that if your forked child inherits system file descriptors like
2398STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2399if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2400backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2401You should reopen those to F</dev/null> if it's any issue.
28757baa 2402
ea9eb35a 2403On some platforms such as Windows, where the fork() system call is not available,
391b733c
FC
2404Perl can be built to emulate fork() in the Perl interpreter.
2405The emulation is designed, at the level of the Perl program,
2406to be as compatible as possible with the "Unix" fork().
6d17f725 2407However it has limitations that have to be considered in code intended to be portable.
ea9eb35a 2408See L<perlfork> for more details.
2409
2410Portability issues: L<perlport/fork>.
2411
cb1a09d0 2412=item format
d74e8afc 2413X<format>
cb1a09d0 2414
c17cdb72
NC
2415=for Pod::Functions declare a picture format with use by the write() function
2416
19799a22 2417Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2418example:
2419
54310121 2420 format Something =
a9a5a0dc
VP
2421 Test: @<<<<<<<< @||||| @>>>>>
2422 $str, $%, '$' . int($num)
cb1a09d0
AD
2423 .
2424
2425 $str = "widget";
184e9718 2426 $num = $cost/$quantity;
cb1a09d0
AD
2427 $~ = 'Something';
2428 write;
2429
2430See L<perlform> for many details and examples.
2431
8903cb82 2432=item formline PICTURE,LIST
d74e8afc 2433X<formline>
a0d0e21e 2434
c17cdb72
NC
2435=for Pod::Functions internal function used for formats
2436
5a964f20 2437This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2438too. It formats (see L<perlform>) a list of values according to the
2439contents of PICTURE, placing the output into the format output
7660c0ab 2440accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2441Eventually, when a C<write> is done, the contents of
cf264981
SP
2442C<$^A> are written to some filehandle. You could also read C<$^A>
2443and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2444does one C<formline> per line of form, but the C<formline> function itself
748a9306 2445doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2446that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2447You may therefore need to use multiple formlines to implement a single
3b10bc60 2448record format, just like the C<format> compiler.
748a9306 2449
19799a22 2450Be careful if you put double quotes around the picture, because an C<@>
748a9306 2451character may be taken to mean the beginning of an array name.
19799a22 2452C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2453
445b09e5
FC
2454If you are trying to use this instead of C<write> to capture the output,
2455you may find it easier to open a filehandle to a scalar
2456(C<< open $fh, ">", \$output >>) and write to that instead.
2457
a0d0e21e 2458=item getc FILEHANDLE
f723aae1 2459X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2460
2461=item getc
2462
c17cdb72
NC
2463=for Pod::Functions get the next character from the filehandle
2464
a0d0e21e 2465Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2466or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2467the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2468STDIN. This is not particularly efficient. However, it cannot be
2469used by itself to fetch single characters without waiting for the user
2470to hit enter. For that, try something more like:
4633a7c4
LW
2471
2472 if ($BSD_STYLE) {
a9a5a0dc 2473 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2474 }
2475 else {
a9a5a0dc 2476 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2477 }
2478
2479 $key = getc(STDIN);
2480
2481 if ($BSD_STYLE) {
a9a5a0dc 2482 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2483 }
2484 else {
3b10bc60 2485 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2486 }
2487 print "\n";
2488
54310121
PP
2489Determination of whether $BSD_STYLE should be set
2490is left as an exercise to the reader.
cb1a09d0 2491
19799a22 2492The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2493systems purporting POSIX compliance. See also the C<Term::ReadKey>
3d6c5fec 2494module from your nearest L<CPAN|http://www.cpan.org> site.
a0d0e21e
LW
2495
2496=item getlogin
d74e8afc 2497X<getlogin> X<login>
a0d0e21e 2498
c17cdb72
NC
2499=for Pod::Functions return who logged in at this tty
2500
cf264981 2501This implements the C library function of the same name, which on most
3b10bc60 2502systems returns the current login from F</etc/utmp>, if any. If it
2503returns the empty string, use C<getpwuid>.
a0d0e21e 2504
f86702cc 2505 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2506
19799a22
GS
2507Do not consider C<getlogin> for authentication: it is not as
2508secure as C<getpwuid>.
4633a7c4 2509
ea9eb35a 2510Portability issues: L<perlport/getlogin>.
2511
a0d0e21e 2512=item getpeername SOCKET
d74e8afc 2513X<getpeername> X<peer>
a0d0e21e 2514
c17cdb72
NC
2515=for Pod::Functions find the other end of a socket connection
2516
a3390c9f
FC
2517Returns the packed sockaddr address of the other end of the SOCKET
2518connection.
a0d0e21e 2519
4633a7c4
LW
2520 use Socket;
2521 $hersockaddr = getpeername(SOCK);
19799a22 2522 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2523 $herhostname = gethostbyaddr($iaddr, AF_INET);
2524 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2525
2526=item getpgrp PID
d74e8afc 2527X<getpgrp> X<group>
a0d0e21e 2528
c17cdb72
NC
2529=for Pod::Functions get process group
2530
47e29363 2531Returns the current process group for the specified PID. Use
7660c0ab 2532a PID of C<0> to get the current process group for the
4633a7c4 2533current process. Will raise an exception if used on a machine that
a3390c9f
FC
2534doesn't implement getpgrp(2). If PID is omitted, returns the process
2535group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2536does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2537
ea9eb35a 2538Portability issues: L<perlport/getpgrp>.
2539
a0d0e21e 2540=item getppid
d74e8afc 2541X<getppid> X<parent> X<pid>
a0d0e21e 2542
c17cdb72
NC
2543=for Pod::Functions get parent process ID
2544
a0d0e21e
LW
2545Returns the process id of the parent process.
2546
d7c042c9
AB
2547Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2548around non-POSIX thread semantics the minority of Linux systems (and
2549Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
7161e5c2 2550has since been removed. See the documentation for L<$$|perlvar/$$> for
d7c042c9 2551details.
4d76a344 2552
ea9eb35a 2553Portability issues: L<perlport/getppid>.
2554
a0d0e21e 2555=item getpriority WHICH,WHO
d74e8afc 2556X<getpriority> X<priority> X<nice>
a0d0e21e 2557
c17cdb72
NC
2558=for Pod::Functions get current nice value
2559
4633a7c4 2560Returns the current priority for a process, a process group, or a user.
01aa884e 2561(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2562machine that doesn't implement getpriority(2).
a0d0e21e 2563
ea9eb35a 2564Portability issues: L<perlport/getpriority>.
2565
a0d0e21e 2566=item getpwnam NAME
d74e8afc
ITB
2567X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2568X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2569X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2570X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2571X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2572X<endnetent> X<endprotoent> X<endservent>
a0d0e21e 2573
c17cdb72
NC
2574=for Pod::Functions get passwd record given user login name
2575
a0d0e21e
LW
2576=item getgrnam NAME
2577
c17cdb72
NC
2578=for Pod::Functions get group record given group name
2579
a0d0e21e
LW
2580=item gethostbyname NAME
2581
c17cdb72
NC
2582=for Pod::Functions get host record given name
2583
a0d0e21e
LW
2584=item getnetbyname NAME
2585
c17cdb72
NC
2586=for Pod::Functions get networks record given name
2587
a0d0e21e
LW
2588=item getprotobyname NAME
2589
c17cdb72
NC
2590=for Pod::Functions get protocol record given name
2591
a0d0e21e
LW
2592=item getpwuid UID
2593
c17cdb72
NC
2594=for Pod::Functions get passwd record given user ID
2595
a0d0e21e
LW
2596=item getgrgid GID
2597
c17cdb72
NC
2598=for Pod::Functions get group record given group user ID
2599
a0d0e21e
LW
2600=item getservbyname NAME,PROTO
2601
c17cdb72
NC
2602=for Pod::Functions get services record given its name
2603
a0d0e21e
LW
2604=item gethostbyaddr ADDR,ADDRTYPE
2605
c17cdb72
NC
2606=for Pod::Functions get host record given its address
2607
a0d0e21e
LW
2608=item getnetbyaddr ADDR,ADDRTYPE
2609
c17cdb72
NC
2610=for Pod::Functions get network record given its address
2611
a0d0e21e
LW
2612=item getprotobynumber NUMBER
2613
c17cdb72
NC
2614=for Pod::Functions get protocol record numeric protocol
2615
a0d0e21e
LW
2616=item getservbyport PORT,PROTO
2617
c17cdb72
NC
2618=for Pod::Functions get services record given numeric port
2619
a0d0e21e
LW
2620=item getpwent
2621
c17cdb72
NC
2622=for Pod::Functions get next passwd record
2623
a0d0e21e
LW
2624=item getgrent
2625
c17cdb72
NC
2626=for Pod::Functions get next group record
2627
a0d0e21e
LW
2628=item gethostent
2629
c17cdb72
NC
2630=for Pod::Functions get next hosts record
2631
a0d0e21e
LW
2632=item getnetent
2633
c17cdb72
NC
2634=for Pod::Functions get next networks record
2635
a0d0e21e
LW
2636=item getprotoent
2637
c17cdb72
NC
2638=for Pod::Functions get next protocols record
2639
a0d0e21e
LW
2640=item getservent
2641
c17cdb72
NC
2642=for Pod::Functions get next services record
2643
a0d0e21e
LW
2644=item setpwent
2645
c17cdb72
NC
2646=for Pod::Functions prepare passwd file for use
2647
a0d0e21e
LW
2648=item setgrent
2649
c17cdb72
NC
2650=for Pod::Functions prepare group file for use
2651
a0d0e21e
LW
2652=item sethostent STAYOPEN
2653
c17cdb72
NC
2654=for Pod::Functions prepare hosts file for use
2655
a0d0e21e
LW
2656=item setnetent STAYOPEN
2657
c17cdb72
NC
2658=for Pod::Functions prepare networks file for use
2659
a0d0e21e
LW
2660=item setprotoent STAYOPEN
2661
c17cdb72
NC
2662=for Pod::Functions prepare protocols file for use
2663
a0d0e21e
LW
2664=item setservent STAYOPEN
2665
c17cdb72
NC
2666=for Pod::Functions prepare services file for use
2667
a0d0e21e
LW
2668=item endpwent
2669
c17cdb72
NC
2670=for Pod::Functions be done using passwd file
2671
a0d0e21e
LW
2672=item endgrent
2673
c17cdb72
NC
2674=for Pod::Functions be done using group file
2675
a0d0e21e
LW
2676=item endhostent
2677
c17cdb72
NC
2678=for Pod::Functions be done using hosts file
2679
a0d0e21e
LW
2680=item endnetent
2681
c17cdb72
NC
2682=for Pod::Functions be done using networks file
2683
a0d0e21e
LW
2684=item endprotoent
2685
c17cdb72
NC
2686=for Pod::Functions be done using protocols file
2687
a0d0e21e
LW
2688=item endservent
2689
c17cdb72
NC
2690=for Pod::Functions be done using services file
2691
80d38338
TC
2692These routines are the same as their counterparts in the
2693system C library. In list context, the return values from the
a0d0e21e
LW
2694various get routines are as follows:
2695
2696 ($name,$passwd,$uid,$gid,
6ee623d5 2697 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2698 ($name,$passwd,$gid,$members) = getgr*
2699 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2700 ($name,$aliases,$addrtype,$net) = getnet*
2701 ($name,$aliases,$proto) = getproto*
2702 ($name,$aliases,$port,$proto) = getserv*
2703
3b10bc60 2704(If the entry doesn't exist you get an empty list.)
a0d0e21e 2705
4602f195
JH
2706The exact meaning of the $gcos field varies but it usually contains
2707the real name of the user (as opposed to the login name) and other
2708information pertaining to the user. Beware, however, that in many
2709system users are able to change this information and therefore it
106325ad 2710cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2711L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2712login shell, are also tainted, for the same reason.
4602f195 2713
5a964f20 2714In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2715lookup by name, in which case you get the other thing, whatever it is.
2716(If the entry doesn't exist you get the undefined value.) For example:
2717
5a964f20
TC
2718 $uid = getpwnam($name);
2719 $name = getpwuid($num);
2720 $name = getpwent();
2721 $gid = getgrnam($name);
08a33e13 2722 $name = getgrgid($num);
5a964f20
TC
2723 $name = getgrent();
2724 #etc.
a0d0e21e 2725
4602f195 2726In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2727in that they are unsupported on many systems. If the
4602f195
JH
2728$quota is unsupported, it is an empty scalar. If it is supported, it
2729usually encodes the disk quota. If the $comment field is unsupported,
2730it is an empty scalar. If it is supported it usually encodes some
2731administrative comment about the user. In some systems the $quota
2732field may be $change or $age, fields that have to do with password
2733aging. In some systems the $comment field may be $class. The $expire
2734field, if present, encodes the expiration period of the account or the
2735password. For the availability and the exact meaning of these fields
8f1da26d 2736in your system, please consult getpwnam(3) and your system's
4602f195
JH
2737F<pwd.h> file. You can also find out from within Perl what your
2738$quota and $comment fields mean and whether you have the $expire field
2739by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2740C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2741files are supported only if your vendor has implemented them in the
4602f195 2742intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2743shadow versions if you're running under privilege or if there exists
cf264981 2744the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2745and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2746facility are unlikely to be supported.
6ee623d5 2747
a3390c9f 2748The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2749the login names of the members of the group.
2750
2751For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2752C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2753C<@addrs> value returned by a successful call is a list of raw
2754addresses returned by the corresponding library call. In the
2755Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2756by saying something like:
2757
f337b084 2758 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2759
2b5ab1e7
TC
2760The Socket library makes this slightly easier:
2761
2762 use Socket;
2763 $iaddr = inet_aton("127.1"); # or whatever address
2764 $name = gethostbyaddr($iaddr, AF_INET);
2765
2766 # or going the other way
19799a22 2767 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2768
d760c846
GS
2769In the opposite way, to resolve a hostname to the IP address
2770you can write this:
2771
2772 use Socket;
2773 $packed_ip = gethostbyname("www.perl.org");
2774 if (defined $packed_ip) {
2775 $ip_address = inet_ntoa($packed_ip);
2776 }
2777
b018eaf1 2778Make sure C<gethostbyname()> is called in SCALAR context and that
d760c846
GS
2779its return value is checked for definedness.
2780
0d043efa
FC
2781The C<getprotobynumber> function, even though it only takes one argument,
2782has the precedence of a list operator, so beware:
2783
2784 getprotobynumber $number eq 'icmp' # WRONG
2785 getprotobynumber($number eq 'icmp') # actually means this
2786 getprotobynumber($number) eq 'icmp' # better this way
2787
19799a22
GS
2788If you get tired of remembering which element of the return list
2789contains which return value, by-name interfaces are provided
2790in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2791C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2792and C<User::grent>. These override the normal built-ins, supplying
2793versions that return objects with the appropriate names
2794for each field. For example:
5a964f20
TC
2795
2796 use File::stat;
2797 use User::pwent;
2798 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2799
a3390c9f 2800Even though it looks as though they're the same method calls (uid),
b76cc8ba 2801they aren't, because a C<File::stat> object is different from
19799a22 2802a C<User::pwent> object.
5a964f20 2803
ea9eb35a 2804Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2805
a0d0e21e 2806=item getsockname SOCKET
d74e8afc 2807X<getsockname>
a0d0e21e 2808
c17cdb72
NC
2809=for Pod::Functions retrieve the sockaddr for a given socket
2810
19799a22
GS
2811Returns the packed sockaddr address of this end of the SOCKET connection,
2812in case you don't know the address because you have several different
2813IPs that the connection might have come in on.
a0d0e21e 2814
4633a7c4
LW
2815 use Socket;
2816 $mysockaddr = getsockname(SOCK);
19799a22 2817 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2818 printf "Connect to %s [%s]\n",
19799a22
GS
2819 scalar gethostbyaddr($myaddr, AF_INET),
2820 inet_ntoa($myaddr);
a0d0e21e
LW
2821
2822=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2823X<getsockopt>
a0d0e21e 2824
c17cdb72
NC
2825=for Pod::Functions get socket options on a given socket
2826
636e6b1f
TH
2827Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2828Options may exist at multiple protocol levels depending on the socket
2829type, but at least the uppermost socket level SOL_SOCKET (defined in the
391b733c 2830C<Socket> module) will exist. To query options at another level the
636e6b1f 2831protocol number of the appropriate protocol controlling the option
391b733c 2832should be supplied. For example, to indicate that an option is to be
636e6b1f 2833interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2834number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2835
80d38338 2836The function returns a packed string representing the requested socket
3b10bc60 2837option, or C<undef> on error, with the reason for the error placed in
391b733c 2838C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2839consult getsockopt(2) for details. A common case is that the option is an
2840integer, in which case the result is a packed integer, which you can decode
2841using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2842
8f1da26d 2843Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2844
4852725b 2845 use Socket qw(:all);
636e6b1f
TH
2846
2847 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2848 or die "Could not determine the protocol number for tcp";
4852725b
DD
2849 # my $tcp = IPPROTO_TCP; # Alternative
2850 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2851 or die "getsockopt TCP_NODELAY: $!";
636e6b1f 2852 my $nodelay = unpack("I", $packed);
f7051f2c
FC
2853 print "Nagle's algorithm is turned ",
2854 $nodelay ? "off\n" : "on\n";
636e6b1f 2855
ea9eb35a 2856Portability issues: L<perlport/getsockopt>.
a0d0e21e
LW
2857
2858=item glob EXPR
d74e8afc 2859X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2860
0a753a76
PP
2861=item glob
2862
c17cdb72
NC
2863=for Pod::Functions expand filenames using wildcards
2864
d9a9d457 2865In list context, returns a (possibly empty) list of filename expansions on
391b733c 2866the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
d9a9d457 2867scalar context, glob iterates through such filename expansions, returning
391b733c
FC
2868undef when the list is exhausted. This is the internal function
2869implementing the C<< <*.c> >> operator, but you can use it directly. If
d9a9d457
JL
2870EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2871more detail in L<perlop/"I/O Operators">.
a0d0e21e 2872
80d38338
TC
2873Note that C<glob> splits its arguments on whitespace and treats
2874each segment as separate pattern. As such, C<glob("*.c *.h")>
2875matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2876C<glob(".* *")> matches all files in the current working directory.
a91bb7b1
TC
2877If you want to glob filenames that might contain whitespace, you'll
2878have to use extra quotes around the spacey filename to protect it.
2879For example, to glob filenames that have an C<e> followed by a space
2880followed by an C<f>, use either of:
2881
2882 @spacies = <"*e f*">;
2883 @spacies = glob '"*e f*"';
2884 @spacies = glob q("*e f*");
2885
2886If you had to get a variable through, you could do this:
2887
2888 @spacies = glob "'*${var}e f*'";
2889 @spacies = glob qq("*${var}e f*");
80d38338
TC
2890
2891If non-empty braces are the only wildcard characters used in the
2892C<glob>, no filenames are matched, but potentially many strings
2893are returned. For example, this produces nine strings, one for
2894each pairing of fruits and colors:
2895
2896 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2897
e9fa405d 2898This operator is implemented using the standard
5c0c9249
PF
2899C<File::Glob> extension. See L<File::Glob> for details, including
2900C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2901
ea9eb35a 2902Portability issues: L<perlport/glob>.
2903
a0d0e21e 2904=item gmtime EXPR
d74e8afc 2905X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2906
ce2984c3
PF
2907=item gmtime
2908
c17cdb72
NC
2909=for Pod::Functions convert UNIX time into record or string using Greenwich time
2910
4509d391 2911Works just like L</localtime> but the returned values are
435fbc73 2912localized for the standard Greenwich time zone.
a0d0e21e 2913
a3390c9f
FC
2914Note: When called in list context, $isdst, the last value
2915returned by gmtime, is always C<0>. There is no
435fbc73 2916Daylight Saving Time in GMT.
0a753a76 2917
ea9eb35a 2918Portability issues: L<perlport/gmtime>.
62aa5637 2919
a0d0e21e 2920=item goto LABEL
d74e8afc 2921X<goto> X<jump> X<jmp>
a0d0e21e 2922
748a9306
LW
2923=item goto EXPR
2924
a0d0e21e
LW
2925=item goto &NAME
2926
c17cdb72
NC
2927=for Pod::Functions create spaghetti code
2928
5a5b79a3 2929The C<goto LABEL> form finds the statement labeled with LABEL and
391b733c 2930resumes execution there. It can't be used to get out of a block or
b500e03b
GG
2931subroutine given to C<sort>. It can be used to go almost anywhere
2932else within the dynamic scope, including out of subroutines, but it's
2933usually better to use some other construct such as C<last> or C<die>.
2934The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2935(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2936does not offer named loops combined with loop control. Perl does, and
2937this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2938
5a5b79a3 2939The C<goto EXPR> form expects to evaluate C<EXPR> to a code reference or
3e8a6370 2940a label name. If it evaluates to a code reference, it will be handled
5a5b79a3 2941like C<goto &NAME>, below. This is especially useful for implementing
3e8a6370
RS
2942tail recursion via C<goto __SUB__>.
2943
2944If the expression evaluates to a label name, its scope will be resolved
7660c0ab 2945dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2946necessarily recommended if you're optimizing for maintainability:
2947
2948 goto ("FOO", "BAR", "GLARCH")[$i];
2949
5a5b79a3 2950As shown in this example, C<goto EXPR> is exempt from the "looks like a
391b733c
FC
2951function" rule. A pair of parentheses following it does not (necessarily)
2952delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
8a7e748e
FC
2953Also, unlike most named operators, this has the same precedence as
2954assignment.
887d89fd 2955
5a5b79a3 2956Use of C<goto LABEL> or C<goto EXPR> to jump into a construct is
0b98bec9 2957deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2958go into any construct that requires initialization, such as a
2959subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2960construct that is optimized away.
b500e03b 2961
5a5b79a3 2962The C<goto &NAME> form is quite different from the other forms of
1b6921cb
BT
2963C<goto>. In fact, it isn't a goto in the normal sense at all, and
2964doesn't have the stigma associated with other gotos. Instead, it
2965exits the current subroutine (losing any changes set by local()) and
2966immediately calls in its place the named subroutine using the current
2967value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2968load another subroutine and then pretend that the other subroutine had
2969been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2970in the current subroutine are propagated to the other subroutine.)
2971After the C<goto>, not even C<caller> will be able to tell that this
2972routine was called first.
2973
2974NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2975containing a code reference or a block that evaluates to a code
6cb9131c 2976reference.
a0d0e21e
LW
2977
2978=item grep BLOCK LIST
d74e8afc 2979X<grep>
a0d0e21e
LW
2980
2981=item grep EXPR,LIST
2982
c17cdb72
NC
2983=for Pod::Functions locate elements in a list test true against a given criterion
2984
2b5ab1e7
TC
2985This is similar in spirit to, but not the same as, grep(1) and its
2986relatives. In particular, it is not limited to using regular expressions.
2f9daede 2987
a0d0e21e 2988Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2989C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2990elements for which the expression evaluated to true. In scalar
2991context, returns the number of times the expression was true.
a0d0e21e
LW
2992
2993 @foo = grep(!/^#/, @bar); # weed out comments
2994
2995or equivalently,
2996
2997 @foo = grep {!/^#/} @bar; # weed out comments
2998
be3174d2
GS
2999Note that C<$_> is an alias to the list value, so it can be used to
3000modify the elements of the LIST. While this is useful and supported,
3001it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
3002Similarly, grep returns aliases into the original list, much as a for
3003loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
3004element of a list returned by grep (for example, in a C<foreach>, C<map>
3005or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 3006This is usually something to be avoided when writing clear code.
a0d0e21e 3007
a4fb8298 3008If C<$_> is lexical in the scope where the C<grep> appears (because it has
c071e214
FC
3009been declared with the deprecated C<my $_> construct)
3010then, in addition to being locally aliased to
80d38338 3011the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
3012can't be seen from the outside, avoiding any potential side-effects.
3013
19799a22 3014See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 3015
a0d0e21e 3016=item hex EXPR
d74e8afc 3017X<hex> X<hexadecimal>
a0d0e21e 3018
54310121 3019=item hex
bbce6d69 3020
c17cdb72
NC
3021=for Pod::Functions convert a string to a hexadecimal number
3022
2b5ab1e7 3023Interprets EXPR as a hex string and returns the corresponding value.
38366c11 3024(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 3025L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
3026
3027 print hex '0xAf'; # prints '175'
3028 print hex 'aF'; # same
a0d0e21e 3029
19799a22 3030Hex strings may only represent integers. Strings that would cause
53305cf1 3031integer overflow trigger a warning. Leading whitespace is not stripped,
391b733c 3032unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 3033L</sprintf>, and L</unpack>.
19799a22 3034
ce2984c3 3035=item import LIST
d74e8afc 3036X<import>
a0d0e21e 3037
c17cdb72
NC
3038=for Pod::Functions patch a module's namespace into your own
3039
19799a22 3040There is no builtin C<import> function. It is just an ordinary
4633a7c4 3041method (subroutine) defined (or inherited) by modules that wish to export
19799a22 3042names to another module. The C<use> function calls the C<import> method
cea6626f 3043for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
3044
3045=item index STR,SUBSTR,POSITION
d74e8afc 3046X<index> X<indexOf> X<InStr>
a0d0e21e
LW
3047
3048=item index STR,SUBSTR
3049