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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
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194C<caller>, C<continue>, C<die>, C<do>,
195C<dump>, C<eval>, C<evalbytes> C<exit>,
cfa52385 196C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
7c919445 197C<prototype>, C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray>
84ed0108 198
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199C<evalbytes> is only available with with the C<"evalbytes"> feature (see
200L<feature>) or if prefixed with C<CORE::>. C<__SUB__> is only available
201with with the C<"current_sub"> feature or if prefixed with C<CORE::>. Both
202the C<"evalbytes"> and C<"current_sub"> features are enabled automatically
203with a C<use v5.16> (or higher) declaration in the current scope.
cb1a09d0 204
8f1da26d 205=item Keywords related to the switch feature
0d863452 206
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207=for Pod::Functions =Switch
208
4a904372 209C<break>, C<continue>, C<default>, C<given>, C<when>
0d863452 210
4a904372 211Except for C<continue>, these are available only if you enable the
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212C<"switch"> feature or use the C<CORE::> prefix. See L<feature> and
213L<perlsyn/"Switch Statements">. The C<"switch"> feature is enabled
214automatically with a C<use v5.10> (or higher) declaration in the current
215scope. In Perl 5.14 and earlier, C<continue> required the C<"switch">
216feature, like the other keywords.
0d863452 217
54310121 218=item Keywords related to scoping
cb1a09d0 219
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220=for Pod::Functions =Namespace
221
8f1da26d 222C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
36fb85f3 223
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224C<state> is available only if the C<"state"> feature is enabled or if it is
225prefixed with C<CORE::>. The C<"state"> feature is enabled automatically
226with a C<use v5.10> (or higher) declaration in the current scope.
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227
228=item Miscellaneous functions
229
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230=for Pod::Functions =Misc
231
7289c5e6 232C<defined>, C<dump>, C<eval>, C<evalbytes>,
7c919445 233C<formline>, C<local>, C<lock>, C<my>, C<our>, C<prototype>,
834df1c5 234C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
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235
236=item Functions for processes and process groups
d74e8afc 237X<process> X<pid> X<process id>
cb1a09d0 238
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239=for Pod::Functions =Process
240
22fae026 241C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
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242C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>,
243C<setpriority>, C<sleep>, C<system>,
22fae026 244C<times>, C<wait>, C<waitpid>
cb1a09d0 245
3b10bc60 246=item Keywords related to Perl modules
d74e8afc 247X<module>
cb1a09d0 248
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249=for Pod::Functions =Modules
250
22fae026 251C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 252
353c6505 253=item Keywords related to classes and object-orientation
d74e8afc 254X<object> X<class> X<package>
cb1a09d0 255
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256=for Pod::Functions =Objects
257
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258C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
259C<untie>, C<use>
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260
261=item Low-level socket functions
d74e8afc 262X<socket> X<sock>
cb1a09d0 263
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264=for Pod::Functions =Socket
265
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266C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
267C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 268C<socket>, C<socketpair>
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269
270=item System V interprocess communication functions
d74e8afc 271X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 272
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273=for Pod::Functions =SysV
274
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275C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
276C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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277
278=item Fetching user and group info
d74e8afc 279X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 280
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281=for Pod::Functions =User
282
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283C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
284C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
285C<getpwuid>, C<setgrent>, C<setpwent>
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286
287=item Fetching network info
d74e8afc 288X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 289
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290=for Pod::Functions =Network
291
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292C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
293C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
294C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
295C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
296C<setnetent>, C<setprotoent>, C<setservent>
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297
298=item Time-related functions
d74e8afc 299X<time> X<date>
cb1a09d0 300
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301=for Pod::Functions =Time
302
22fae026 303C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 304
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305=item Non-function keywords
306
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307=for Pod::Functions =!Non-functions
308
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309C<and>, C<AUTOLOAD>, C<BEGIN>, C<CHECK>, C<cmp>, C<CORE>, C<__DATA__>,
310C<DESTROY>, C<else>, C<elseif>, C<elsif>, C<END>, C<__END__>, C<eq>, C<for>,
311C<foreach>, C<ge>, C<gt>, C<if>, C<INIT>, C<le>, C<lt>, C<ne>, C<not>, C<or>,
312C<UNITCHECK>, C<unless>, C<until>, C<while>, C<x>, C<xor>
8f0d6a61 313
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314=back
315
60f9f73c 316=head2 Portability
d74e8afc 317X<portability> X<Unix> X<portable>
60f9f73c 318
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319Perl was born in Unix and can therefore access all common Unix
320system calls. In non-Unix environments, the functionality of some
8f1da26d 321Unix system calls may not be available or details of the available
2b5ab1e7 322functionality may differ slightly. The Perl functions affected
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323by this are:
324
325C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
326C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
327C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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328C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
329C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 330C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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331C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
332C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
333C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 334C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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335C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
336C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
337C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 338C<shmwrite>, C<socket>, C<socketpair>,
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339C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
340C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 341C<utime>, C<wait>, C<waitpid>
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342
343For more information about the portability of these functions, see
344L<perlport> and other available platform-specific documentation.
345
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346=head2 Alphabetical Listing of Perl Functions
347
3b10bc60 348=over
a0d0e21e 349
5b3c99c0 350=item -X FILEHANDLE
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351X<-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>
352X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 353
5b3c99c0 354=item -X EXPR
a0d0e21e 355
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356=item -X DIRHANDLE
357
5b3c99c0 358=item -X
a0d0e21e 359
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360=for Pod::Functions a file test (-r, -x, etc)
361
a0d0e21e 362A file test, where X is one of the letters listed below. This unary
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363operator takes one argument, either a filename, a filehandle, or a dirhandle,
364and tests the associated file to see if something is true about it. If the
7660c0ab 365argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 366Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 367the undefined value if the file doesn't exist. Despite the funny
d0821a6a 368names, precedence is the same as any other named unary operator. The
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369operator may be any of:
370
5ed4f2ec 371 -r File is readable by effective uid/gid.
372 -w File is writable by effective uid/gid.
373 -x File is executable by effective uid/gid.
374 -o File is owned by effective uid.
a0d0e21e 375
5ed4f2ec 376 -R File is readable by real uid/gid.
377 -W File is writable by real uid/gid.
378 -X File is executable by real uid/gid.
379 -O File is owned by real uid.
a0d0e21e 380
5ed4f2ec 381 -e File exists.
382 -z File has zero size (is empty).
383 -s File has nonzero size (returns size in bytes).
a0d0e21e 384
5ed4f2ec 385 -f File is a plain file.
386 -d File is a directory.
387 -l File is a symbolic link.
388 -p File is a named pipe (FIFO), or Filehandle is a pipe.
389 -S File is a socket.
390 -b File is a block special file.
391 -c File is a character special file.
392 -t Filehandle is opened to a tty.
a0d0e21e 393
5ed4f2ec 394 -u File has setuid bit set.
395 -g File has setgid bit set.
396 -k File has sticky bit set.
a0d0e21e 397
5ed4f2ec 398 -T File is an ASCII text file (heuristic guess).
399 -B File is a "binary" file (opposite of -T).
a0d0e21e 400
5ed4f2ec 401 -M Script start time minus file modification time, in days.
402 -A Same for access time.
403 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 404
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405Example:
406
407 while (<>) {
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408 chomp;
409 next unless -f $_; # ignore specials
410 #...
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411 }
412
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413Note that C<-s/a/b/> does not do a negated substitution. Saying
414C<-exp($foo)> still works as expected, however: only single letters
415following a minus are interpreted as file tests.
416
417These operators are exempt from the "looks like a function rule" described
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418above. That is, an opening parenthesis after the operator does not affect
419how much of the following code constitutes the argument. Put the opening
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420parentheses before the operator to separate it from code that follows (this
421applies only to operators with higher precedence than unary operators, of
422course):
423
424 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
425 (-s $file) + 1024 # correct
426
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427The interpretation of the file permission operators C<-r>, C<-R>,
428C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
429of the file and the uids and gids of the user. There may be other
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430reasons you can't actually read, write, or execute the file: for
431example network filesystem access controls, ACLs (access control lists),
432read-only filesystems, and unrecognized executable formats. Note
433that the use of these six specific operators to verify if some operation
434is possible is usually a mistake, because it may be open to race
435conditions.
5ff3f7a4 436
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437Also note that, for the superuser on the local filesystems, the C<-r>,
438C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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GS
439if any execute bit is set in the mode. Scripts run by the superuser
440may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 441or temporarily set their effective uid to something else.
5ff3f7a4
GS
442
443If you are using ACLs, there is a pragma called C<filetest> that may
444produce more accurate results than the bare stat() mode bits.
5dac7880
FC
445When under C<use filetest 'access'> the above-mentioned filetests
446test whether the permission can(not) be granted using the
3b10bc60 447access(2) family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
448under this pragma return true even if there are no execute permission
449bits set (nor any extra execute permission ACLs). This strangeness is
391b733c 450due to the underlying system calls' definitions. Note also that, due to
ecae030f
MO
451the implementation of C<use filetest 'access'>, the C<_> special
452filehandle won't cache the results of the file tests when this pragma is
453in effect. Read the documentation for the C<filetest> pragma for more
454information.
5ff3f7a4 455
a0d0e21e
LW
456The C<-T> and C<-B> switches work as follows. The first block or so of the
457file is examined for odd characters such as strange control codes or
61eff3bc 458characters with the high bit set. If too many strange characters (>30%)
cf264981 459are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 460containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 461or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 462rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 463file, or a file at EOF when testing a filehandle. Because you have to
4633a7c4
LW
464read a file to do the C<-T> test, on most occasions you want to use a C<-f>
465against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 466
5dac7880 467If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 468the special filehandle consisting of a solitary underline, then the stat
a0d0e21e
LW
469structure of the previous file test (or stat operator) is used, saving
470a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 471that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 472symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 473an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 474Example:
a0d0e21e
LW
475
476 print "Can do.\n" if -r $a || -w _ || -x _;
477
478 stat($filename);
479 print "Readable\n" if -r _;
480 print "Writable\n" if -w _;
481 print "Executable\n" if -x _;
482 print "Setuid\n" if -u _;
483 print "Setgid\n" if -g _;
484 print "Sticky\n" if -k _;
485 print "Text\n" if -T _;
486 print "Binary\n" if -B _;
487
fbb0b3b3
RGS
488As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
489test operators, in a way that C<-f -w -x $file> is equivalent to
391b733c 490C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
fbb0b3b3
RGS
491the return value of C<-f $file> as an argument to another filetest
492operator, no special magic will happen.)
493
bee96257 494Portability issues: L<perlport/-X>.
ea9eb35a 495
bade7fbc
TC
496To avoid confusing would-be users of your code with mysterious
497syntax errors, put something like this at the top of your script:
498
499 use 5.010; # so filetest ops can stack
500
a0d0e21e 501=item abs VALUE
d74e8afc 502X<abs> X<absolute>
a0d0e21e 503
54310121 504=item abs
bbce6d69 505
c17cdb72
NC
506=for Pod::Functions absolute value function
507
a0d0e21e 508Returns the absolute value of its argument.
7660c0ab 509If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
510
511=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 512X<accept>
a0d0e21e 513
c17cdb72
NC
514=for Pod::Functions accept an incoming socket connect
515
3b10bc60 516Accepts an incoming socket connect, just as accept(2)
19799a22 517does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 518See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 519
8d2a6795
GS
520On systems that support a close-on-exec flag on files, the flag will
521be set for the newly opened file descriptor, as determined by the
522value of $^F. See L<perlvar/$^F>.
523
a0d0e21e 524=item alarm SECONDS
d74e8afc
ITB
525X<alarm>
526X<SIGALRM>
527X<timer>
a0d0e21e 528
54310121 529=item alarm
bbce6d69 530
c17cdb72
NC
531=for Pod::Functions schedule a SIGALRM
532
a0d0e21e 533Arranges to have a SIGALRM delivered to this process after the
cf264981 534specified number of wallclock seconds has elapsed. If SECONDS is not
391b733c 535specified, the value stored in C<$_> is used. (On some machines,
d400eac8
JH
536unfortunately, the elapsed time may be up to one second less or more
537than you specified because of how seconds are counted, and process
538scheduling may delay the delivery of the signal even further.)
539
540Only one timer may be counting at once. Each call disables the
541previous timer, and an argument of C<0> may be supplied to cancel the
542previous timer without starting a new one. The returned value is the
543amount of time remaining on the previous timer.
a0d0e21e 544
2bc69794
BS
545For delays of finer granularity than one second, the Time::HiRes module
546(from CPAN, and starting from Perl 5.8 part of the standard
547distribution) provides ualarm(). You may also use Perl's four-argument
548version of select() leaving the first three arguments undefined, or you
549might be able to use the C<syscall> interface to access setitimer(2) if
391b733c 550your system supports it. See L<perlfaq8> for details.
2b5ab1e7 551
80d38338
TC
552It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
553C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 554
19799a22
GS
555If you want to use C<alarm> to time out a system call you need to use an
556C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 557fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 558restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 559modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
560
561 eval {
a9a5a0dc
VP
562 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
563 alarm $timeout;
564 $nread = sysread SOCKET, $buffer, $size;
565 alarm 0;
ff68c719 566 };
ff68c719 567 if ($@) {
a9a5a0dc 568 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 569 # timed out
ff68c719
PP
570 }
571 else {
5ed4f2ec 572 # didn't
ff68c719
PP
573 }
574
91d81acc
JH
575For more information see L<perlipc>.
576
ea9eb35a 577Portability issues: L<perlport/alarm>.
578
a0d0e21e 579=item atan2 Y,X
d74e8afc 580X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e 581
c17cdb72
NC
582=for Pod::Functions arctangent of Y/X in the range -PI to PI
583
a0d0e21e
LW
584Returns the arctangent of Y/X in the range -PI to PI.
585
ca6e1c26 586For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
587function, or use the familiar relation:
588
589 sub tan { sin($_[0]) / cos($_[0]) }
590
a1021d57
RGS
591The return value for C<atan2(0,0)> is implementation-defined; consult
592your atan2(3) manpage for more information.
bf5f1b4c 593
ea9eb35a 594Portability issues: L<perlport/atan2>.
595
a0d0e21e 596=item bind SOCKET,NAME
d74e8afc 597X<bind>
a0d0e21e 598
c17cdb72
NC
599=for Pod::Functions binds an address to a socket
600
3b10bc60 601Binds a network address to a socket, just as bind(2)
19799a22 602does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
603packed address of the appropriate type for the socket. See the examples in
604L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 605
fae2c0fb 606=item binmode FILEHANDLE, LAYER
d74e8afc 607X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 608
a0d0e21e
LW
609=item binmode FILEHANDLE
610
c17cdb72
NC
611=for Pod::Functions prepare binary files for I/O
612
1cbfc93d
NIS
613Arranges for FILEHANDLE to be read or written in "binary" or "text"
614mode on systems where the run-time libraries distinguish between
615binary and text files. If FILEHANDLE is an expression, the value is
616taken as the name of the filehandle. Returns true on success,
b5fe5ca2 617otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 618
8f1da26d 619On some systems (in general, DOS- and Windows-based systems) binmode()
d807c6f4 620is necessary when you're not working with a text file. For the sake
d7a0d798
FC
621of portability it is a good idea always to use it when appropriate,
622and never to use it when it isn't appropriate. Also, people can
8f1da26d 623set their I/O to be by default UTF8-encoded Unicode, not bytes.
d807c6f4
JH
624
625In other words: regardless of platform, use binmode() on binary data,
d7a0d798 626like images, for example.
d807c6f4
JH
627
628If LAYER is present it is a single string, but may contain multiple
391b733c 629directives. The directives alter the behaviour of the filehandle.
d7a0d798 630When LAYER is present, using binmode on a text file makes sense.
d807c6f4 631
fae2c0fb 632If LAYER is omitted or specified as C<:raw> the filehandle is made
391b733c 633suitable for passing binary data. This includes turning off possible CRLF
0226bbdb 634translation and marking it as bytes (as opposed to Unicode characters).
749683d2 635Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 636Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
637Other layers that would affect the binary nature of the stream are
391b733c 638I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 639PERLIO environment variable.
01e6739c 640
3b10bc60 641The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
642form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
643establish default I/O layers. See L<open>.
644
fae2c0fb
RGS
645I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
646in "Programming Perl, 3rd Edition". However, since the publishing of this
647book, by many known as "Camel III", the consensus of the naming of this
648functionality has moved from "discipline" to "layer". All documentation
649of this version of Perl therefore refers to "layers" rather than to
650"disciplines". Now back to the regularly scheduled documentation...>
651
8f1da26d 652To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
6902c96a 653C<:utf8> just marks the data as UTF-8 without further checking,
8f1da26d 654while C<:encoding(UTF-8)> checks the data for actually being valid
391b733c 655UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 656
ed53a2bb 657In general, binmode() should be called after open() but before any I/O
3b10bc60 658is done on the filehandle. Calling binmode() normally flushes any
01e6739c 659pending buffered output data (and perhaps pending input data) on the
fae2c0fb 660handle. An exception to this is the C<:encoding> layer that
d7a0d798 661changes the default character encoding of the handle; see L</open>.
fae2c0fb 662The C<:encoding> layer sometimes needs to be called in
3874323d
JH
663mid-stream, and it doesn't flush the stream. The C<:encoding>
664also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 665internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 666
19799a22 667The operating system, device drivers, C libraries, and Perl run-time
8f1da26d
TC
668system all conspire to let the programmer treat a single
669character (C<\n>) as the line terminator, irrespective of external
30168b04
GS
670representation. On many operating systems, the native text file
671representation matches the internal representation, but on some
672platforms the external representation of C<\n> is made up of more than
673one character.
674
8f1da26d
TC
675All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
676a single character to end each line in the external representation of text
677(even though that single character is CARRIAGE RETURN on old, pre-Darwin
391b733c 678flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
8f1da26d
TC
679systems like OS/2, DOS, and the various flavors of MS-Windows, your program
680sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
681two characters C<\cM\cJ>. That means that if you don't use binmode() on
682these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
683input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
684output. This is what you want for text files, but it can be disastrous for
685binary files.
30168b04
GS
686
687Another consequence of using binmode() (on some systems) is that
688special end-of-file markers will be seen as part of the data stream.
d7a0d798
FC
689For systems from the Microsoft family this means that, if your binary
690data contain C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
691the file, unless you use binmode().
692
3b10bc60 693binmode() is important not only for readline() and print() operations,
30168b04
GS
694but also when using read(), seek(), sysread(), syswrite() and tell()
695(see L<perlport> for more details). See the C<$/> and C<$\> variables
696in L<perlvar> for how to manually set your input and output
697line-termination sequences.
a0d0e21e 698
ea9eb35a 699Portability issues: L<perlport/binmode>.
700
4633a7c4 701=item bless REF,CLASSNAME
d74e8afc 702X<bless>
a0d0e21e
LW
703
704=item bless REF
705
c17cdb72
NC
706=for Pod::Functions create an object
707
2b5ab1e7
TC
708This function tells the thingy referenced by REF that it is now an object
709in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 710is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 711it returns the reference for convenience. Always use the two-argument
cf264981 712version if a derived class might inherit the function doing the blessing.
82e1c0d9 713SeeL<perlobj> for more about the blessing (and blessings) of objects.
a0d0e21e 714
57668c4d 715Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 716Namespaces with all lowercase names are considered reserved for
391b733c 717Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
718confusion, you may wish to avoid such package names as well. Make sure
719that CLASSNAME is a true value.
60ad88b8
GS
720
721See L<perlmod/"Perl Modules">.
722
0d863452
RH
723=item break
724
d9b04284 725=for Pod::Functions +switch break out of a C<given> block
c17cdb72 726
0d863452
RH
727Break out of a C<given()> block.
728
8f1da26d 729This keyword is enabled by the C<"switch"> feature: see
4a904372
FC
730L<feature> for more information. You can also access it by
731prefixing it with C<CORE::>. Alternately, include a C<use
8f1da26d 732v5.10> or later to the current scope.
0d863452 733
a0d0e21e 734=item caller EXPR
d74e8afc 735X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
736
737=item caller
738
c17cdb72
NC
739=for Pod::Functions get context of the current subroutine call
740
5a964f20 741Returns the context of the current subroutine call. In scalar context,
80d38338
TC
742returns the caller's package name if there I<is> a caller (that is, if
743we're in a subroutine or C<eval> or C<require>) and the undefined value
5a964f20 744otherwise. In list context, returns
a0d0e21e 745
ee6b43cc 746 # 0 1 2
748a9306 747 ($package, $filename, $line) = caller;
a0d0e21e
LW
748
749With EXPR, it returns some extra information that the debugger uses to
750print a stack trace. The value of EXPR indicates how many call frames
751to go back before the current one.
752
ee6b43cc 753 # 0 1 2 3 4
f3aa04c2 754 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 755
756 # 5 6 7 8 9 10
b3ca2e83 757 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 758 = caller($i);
e7ea3e70 759
951ba7fe 760Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 761call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 762C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 763C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 764C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 765$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
766each C<use> statement creates a C<require> frame inside an C<eval EXPR>
767frame.) $subroutine may also be C<(unknown)> if this particular
768subroutine happens to have been deleted from the symbol table.
769C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
770C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
771compiled with. The C<$hints> and C<$bitmask> values are subject to change
772between versions of Perl, and are not meant for external use.
748a9306 773
b3ca2e83 774C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
391b733c 775caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
b3ca2e83
NC
776of this hash, as they are the actual values stored in the optree.
777
ffe0c19d
FC
778Furthermore, when called from within the DB package in
779list context, and with an argument, caller returns more
7660c0ab 780detailed information: it sets the list variable C<@DB::args> to be the
54310121 781arguments with which the subroutine was invoked.
748a9306 782
7660c0ab 783Be aware that the optimizer might have optimized call frames away before
19799a22 784C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 785might not return information about the call frame you expect it to, for
b76cc8ba 786C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 787previous time C<caller> was called.
7660c0ab 788
8f1da26d 789Be aware that setting C<@DB::args> is I<best effort>, intended for
391b733c 790debugging or generating backtraces, and should not be relied upon. In
ca9f0cb5
NC
791particular, as C<@_> contains aliases to the caller's arguments, Perl does
792not take a copy of C<@_>, so C<@DB::args> will contain modifications the
793subroutine makes to C<@_> or its contents, not the original values at call
391b733c 794time. C<@DB::args>, like C<@_>, does not hold explicit references to its
ca9f0cb5 795elements, so under certain cases its elements may have become freed and
391b733c 796reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 797of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
798I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
799reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5 800elements), so C<@DB::args> is actually a hybrid of the current state and
391b733c 801initial state of C<@_>. Buyer beware.
ca9f0cb5 802
a0d0e21e 803=item chdir EXPR
d74e8afc
ITB
804X<chdir>
805X<cd>
f723aae1 806X<directory, change>
a0d0e21e 807
c4aca7d0
GA
808=item chdir FILEHANDLE
809
810=item chdir DIRHANDLE
811
ce2984c3
PF
812=item chdir
813
c17cdb72
NC
814=for Pod::Functions change your current working directory
815
391b733c 816Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 817changes to the directory specified by C<$ENV{HOME}>, if set; if not,
391b733c
FC
818changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
819variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
820neither is set, C<chdir> does nothing. It returns true on success,
821false otherwise. See the example under C<die>.
a0d0e21e 822
3b10bc60 823On systems that support fchdir(2), you may pass a filehandle or
34169887 824directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 825passing handles raises an exception.
c4aca7d0 826
a0d0e21e 827=item chmod LIST
d74e8afc 828X<chmod> X<permission> X<mode>
a0d0e21e 829
c17cdb72
NC
830=for Pod::Functions changes the permissions on a list of files
831
a0d0e21e 832Changes the permissions of a list of files. The first element of the
8f1da26d 833list must be the numeric mode, which should probably be an octal
4ad40acf 834number, and which definitely should I<not> be a string of octal digits:
3b10bc60 835C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 836successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 837
3b10bc60 838 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 839 chmod 0755, @executables;
3b10bc60 840 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 841 # --w----r-T
3b10bc60 842 $mode = "0644"; chmod oct($mode), "foo"; # this is better
843 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 844
3b10bc60 845On systems that support fchmod(2), you may pass filehandles among the
846files. On systems that don't support fchmod(2), passing filehandles raises
847an exception. Filehandles must be passed as globs or glob references to be
848recognized; barewords are considered filenames.
c4aca7d0
GA
849
850 open(my $fh, "<", "foo");
851 my $perm = (stat $fh)[2] & 07777;
852 chmod($perm | 0600, $fh);
853
3b10bc60 854You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
855module:
856
3b10bc60 857 use Fcntl qw( :mode );
ca6e1c26 858 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 859 # Identical to the chmod 0755 of the example above.
ca6e1c26 860
ea9eb35a 861Portability issues: L<perlport/chmod>.
862
a0d0e21e 863=item chomp VARIABLE
d74e8afc 864X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 865
313c9f5c 866=item chomp( LIST )
a0d0e21e
LW
867
868=item chomp
869
c17cdb72
NC
870=for Pod::Functions remove a trailing record separator from a string
871
2b5ab1e7
TC
872This safer version of L</chop> removes any trailing string
873that corresponds to the current value of C<$/> (also known as
28757baa
PP
874$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
875number of characters removed from all its arguments. It's often used to
876remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
877that the final record may be missing its newline. When in paragraph
878mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 879When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 880a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 881remove anything.
19799a22 882If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
883
884 while (<>) {
a9a5a0dc
VP
885 chomp; # avoid \n on last field
886 @array = split(/:/);
887 # ...
a0d0e21e
LW
888 }
889
4bf21a6d
RD
890If VARIABLE is a hash, it chomps the hash's values, but not its keys.
891
a0d0e21e
LW
892You can actually chomp anything that's an lvalue, including an assignment:
893
894 chomp($cwd = `pwd`);
895 chomp($answer = <STDIN>);
896
897If you chomp a list, each element is chomped, and the total number of
898characters removed is returned.
899
15e44fd8
RGS
900Note that parentheses are necessary when you're chomping anything
901that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
902is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
903C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
904C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
905as C<chomp($a, $b)>.
906
a0d0e21e 907=item chop VARIABLE
d74e8afc 908X<chop>
a0d0e21e 909
313c9f5c 910=item chop( LIST )
a0d0e21e
LW
911
912=item chop
913
c17cdb72
NC
914=for Pod::Functions remove the last character from a string
915
a0d0e21e 916Chops off the last character of a string and returns the character
5b3eff12 917chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 918scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
919If VARIABLE is a hash, it chops the hash's values, but not its keys.
920
5b3eff12 921You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
922
923If you chop a list, each element is chopped. Only the value of the
19799a22 924last C<chop> is returned.
a0d0e21e 925
19799a22 926Note that C<chop> returns the last character. To return all but the last
748a9306
LW
927character, use C<substr($string, 0, -1)>.
928
15e44fd8
RGS
929See also L</chomp>.
930
a0d0e21e 931=item chown LIST
d74e8afc 932X<chown> X<owner> X<user> X<group>
a0d0e21e 933
c17cdb72
NC
934=for Pod::Functions change the ownership on a list of files
935
a0d0e21e 936Changes the owner (and group) of a list of files. The first two
19799a22
GS
937elements of the list must be the I<numeric> uid and gid, in that
938order. A value of -1 in either position is interpreted by most
939systems to leave that value unchanged. Returns the number of files
940successfully changed.
a0d0e21e
LW
941
942 $cnt = chown $uid, $gid, 'foo', 'bar';
943 chown $uid, $gid, @filenames;
944
3b10bc60 945On systems that support fchown(2), you may pass filehandles among the
946files. On systems that don't support fchown(2), passing filehandles raises
947an exception. Filehandles must be passed as globs or glob references to be
948recognized; barewords are considered filenames.
c4aca7d0 949
54310121 950Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
951
952 print "User: ";
19799a22 953 chomp($user = <STDIN>);
5a964f20 954 print "Files: ";
19799a22 955 chomp($pattern = <STDIN>);
a0d0e21e
LW
956
957 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 958 or die "$user not in passwd file";
a0d0e21e 959
5ed4f2ec 960 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
961 chown $uid, $gid, @ary;
962
54310121 963On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
964file unless you're the superuser, although you should be able to change
965the group to any of your secondary groups. On insecure systems, these
966restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
967On POSIX systems, you can detect this condition this way:
968
969 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
970 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 971
ea9eb35a 972Portability issues: L<perlport/chmod>.
973
a0d0e21e 974=item chr NUMBER
d74e8afc 975X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 976
54310121 977=item chr
bbce6d69 978
c17cdb72
NC
979=for Pod::Functions get character this number represents
980
a0d0e21e 981Returns the character represented by that NUMBER in the character set.
a0ed51b3 982For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 983chr(0x263a) is a Unicode smiley face.
aaa68c4a 984
8a064bd6 985Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 986except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
987(truncated to an integer) are used.
988
974da8e5
JH
989If NUMBER is omitted, uses C<$_>.
990
b76cc8ba 991For the reverse, use L</ord>.
a0d0e21e 992
2575c402
JW
993Note that characters from 128 to 255 (inclusive) are by default
994internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 995
2575c402 996See L<perlunicode> for more about Unicode.
bbce6d69 997
a0d0e21e 998=item chroot FILENAME
d74e8afc 999X<chroot> X<root>
a0d0e21e 1000
54310121 1001=item chroot
bbce6d69 1002
c17cdb72
NC
1003=for Pod::Functions make directory new root for path lookups
1004
5a964f20 1005This function works like the system call by the same name: it makes the
4633a7c4 1006named directory the new root directory for all further pathnames that
951ba7fe 1007begin with a C</> by your process and all its children. (It doesn't
28757baa 1008change your current working directory, which is unaffected.) For security
4633a7c4 1009reasons, this call is restricted to the superuser. If FILENAME is
19799a22 1010omitted, does a C<chroot> to C<$_>.
a0d0e21e 1011
ea9eb35a 1012Portability issues: L<perlport/chroot>.
1013
a0d0e21e 1014=item close FILEHANDLE
d74e8afc 1015X<close>
a0d0e21e 1016
6a518fbc
TP
1017=item close
1018
c17cdb72
NC
1019=for Pod::Functions close file (or pipe or socket) handle
1020
3b10bc60 1021Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 1022buffers, and closes the system file descriptor. Returns true if those
8f1da26d 1023operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
1024layer. Closes the currently selected filehandle if the argument is
1025omitted.
fb73857a
PP
1026
1027You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 1028another C<open> on it, because C<open> closes it for you. (See
01aa884e 1029L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
19799a22 1030counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 1031
3b10bc60 1032If the filehandle came from a piped open, C<close> returns false if one of
1033the other syscalls involved fails or if its program exits with non-zero
1034status. If the only problem was that the program exited non-zero, C<$!>
1035will be set to C<0>. Closing a pipe also waits for the process executing
1036on the pipe to exit--in case you wish to look at the output of the pipe
1037afterwards--and implicitly puts the exit status value of that command into
1038C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 1039
2e0cfa16
FC
1040If there are multiple threads running, C<close> on a filehandle from a
1041piped open returns true without waiting for the child process to terminate,
1042if the filehandle is still open in another thread.
1043
80d38338
TC
1044Closing the read end of a pipe before the process writing to it at the
1045other end is done writing results in the writer receiving a SIGPIPE. If
1046the other end can't handle that, be sure to read all the data before
1047closing the pipe.
73689b13 1048
fb73857a 1049Example:
a0d0e21e 1050
fb73857a
PP
1051 open(OUTPUT, '|sort >foo') # pipe to sort
1052 or die "Can't start sort: $!";
5ed4f2ec 1053 #... # print stuff to output
1054 close OUTPUT # wait for sort to finish
fb73857a
PP
1055 or warn $! ? "Error closing sort pipe: $!"
1056 : "Exit status $? from sort";
5ed4f2ec 1057 open(INPUT, 'foo') # get sort's results
fb73857a 1058 or die "Can't open 'foo' for input: $!";
a0d0e21e 1059
5a964f20 1060FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 1061filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
1062
1063=item closedir DIRHANDLE
d74e8afc 1064X<closedir>
a0d0e21e 1065
c17cdb72
NC
1066=for Pod::Functions close directory handle
1067
19799a22 1068Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
1069system call.
1070
a0d0e21e 1071=item connect SOCKET,NAME
d74e8afc 1072X<connect>
a0d0e21e 1073
c17cdb72
NC
1074=for Pod::Functions connect to a remote socket
1075
80d38338
TC
1076Attempts to connect to a remote socket, just like connect(2).
1077Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
1078packed address of the appropriate type for the socket. See the examples in
1079L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 1080
cb1a09d0 1081=item continue BLOCK
d74e8afc 1082X<continue>
cb1a09d0 1083
0d863452
RH
1084=item continue
1085
c17cdb72
NC
1086=for Pod::Functions optional trailing block in a while or foreach
1087
4a904372
FC
1088When followed by a BLOCK, C<continue> is actually a
1089flow control statement rather than a function. If
cf264981 1090there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
1091C<foreach>), it is always executed just before the conditional is about to
1092be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
1093it can be used to increment a loop variable, even when the loop has been
1094continued via the C<next> statement (which is similar to the C C<continue>
1095statement).
1096
98293880 1097C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 1098block; C<last> and C<redo> behave as if they had been executed within
19799a22 1099the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
1100block, it may be more entertaining.
1101
1102 while (EXPR) {
a9a5a0dc
VP
1103 ### redo always comes here
1104 do_something;
1d2dff63 1105 } continue {
a9a5a0dc
VP
1106 ### next always comes here
1107 do_something_else;
1108 # then back the top to re-check EXPR
1d2dff63
GS
1109 }
1110 ### last always comes here
1111
3b10bc60 1112Omitting the C<continue> section is equivalent to using an
1113empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1114to check the condition at the top of the loop.
1115
4a904372 1116When there is no BLOCK, C<continue> is a function that
8f1da26d
TC
1117falls through the current C<when> or C<default> block instead of iterating
1118a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
4a904372
FC
1119In Perl 5.14 and earlier, this form of C<continue> was
1120only available when the C<"switch"> feature was enabled.
48238296 1121See L<feature> and L<perlsyn/"Switch Statements"> for more
8f1da26d 1122information.
0d863452 1123
a0d0e21e 1124=item cos EXPR
d74e8afc 1125X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1126
d6217f1e
GS
1127=item cos
1128
c17cdb72
NC
1129=for Pod::Functions cosine function
1130
5a964f20 1131Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1132takes the cosine of C<$_>.
a0d0e21e 1133
ca6e1c26 1134For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1135function, or use this relation:
1136
1137 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1138
a0d0e21e 1139=item crypt PLAINTEXT,SALT
d74e8afc 1140X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1141X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1142
c17cdb72
NC
1143=for Pod::Functions one-way passwd-style encryption
1144
ef2e6798
MS
1145Creates a digest string exactly like the crypt(3) function in the C
1146library (assuming that you actually have a version there that has not
bb23f8d1 1147been extirpated as a potential munition).
ef2e6798 1148
34169887 1149crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1150into a short string, called a digest, which is returned. The same
1151PLAINTEXT and SALT will always return the same string, but there is no
1152(known) way to get the original PLAINTEXT from the hash. Small
1153changes in the PLAINTEXT or SALT will result in large changes in the
1154digest.
1155
1156There is no decrypt function. This function isn't all that useful for
1157cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1158mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1159primarily used to check if two pieces of text are the same without
1160having to transmit or store the text itself. An example is checking
1161if a correct password is given. The digest of the password is stored,
cf264981 1162not the password itself. The user types in a password that is
ef2e6798 1163crypt()'d with the same salt as the stored digest. If the two digests
34169887 1164match, the password is correct.
ef2e6798
MS
1165
1166When verifying an existing digest string you should use the digest as
1167the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1168to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1169crypt() will hash the new string with the same salt as the digest.
1170This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1171with more exotic implementations. In other words, assume
1172nothing about the returned string itself nor about how many bytes
1173of SALT may matter.
85c16d83
JH
1174
1175Traditionally the result is a string of 13 bytes: two first bytes of
1176the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
391b733c 1177the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1178hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1179and implementations on non-Unix platforms may produce different
ef2e6798 1180strings.
85c16d83
JH
1181
1182When choosing a new salt create a random two character string whose
1183characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1184'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1185characters is just a recommendation; the characters allowed in
1186the salt depend solely on your system's crypt library, and Perl can't
1187restrict what salts C<crypt()> accepts.
e71965be 1188
a0d0e21e 1189Here's an example that makes sure that whoever runs this program knows
cf264981 1190their password:
a0d0e21e
LW
1191
1192 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1193
1194 system "stty -echo";
1195 print "Password: ";
e71965be 1196 chomp($word = <STDIN>);
a0d0e21e
LW
1197 print "\n";
1198 system "stty echo";
1199
e71965be 1200 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1201 die "Sorry...\n";
a0d0e21e 1202 } else {
a9a5a0dc 1203 print "ok\n";
54310121 1204 }
a0d0e21e 1205
9f8f0c9d 1206Of course, typing in your own password to whoever asks you
748a9306 1207for it is unwise.
a0d0e21e 1208
ef2e6798 1209The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1210of data, not least of all because you can't get the information
ef2e6798 1211back. Look at the L<Digest> module for more robust algorithms.
19799a22 1212
f2791508
JH
1213If using crypt() on a Unicode string (which I<potentially> has
1214characters with codepoints above 255), Perl tries to make sense
34169887 1215of the situation by trying to downgrade (a copy of)
f2791508
JH
1216the string back to an eight-bit byte string before calling crypt()
1217(on that copy). If that works, good. If not, crypt() dies with
1218C<Wide character in crypt>.
85c16d83 1219
ea9eb35a 1220Portability issues: L<perlport/crypt>.
1221
aa689395 1222=item dbmclose HASH
d74e8afc 1223X<dbmclose>
a0d0e21e 1224
c17cdb72
NC
1225=for Pod::Functions breaks binding on a tied dbm file
1226
19799a22 1227[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1228
aa689395 1229Breaks the binding between a DBM file and a hash.
a0d0e21e 1230
ea9eb35a 1231Portability issues: L<perlport/dbmclose>.
1232
19799a22 1233=item dbmopen HASH,DBNAME,MASK
d74e8afc 1234X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1235
c17cdb72
NC
1236=for Pod::Functions create binding on a tied dbm file
1237
01aa884e
KW
1238[This function has been largely superseded by the
1239L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
a0d0e21e 1240
7b8d334a 1241This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1242hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1243argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1244is the name of the database (without the F<.dir> or F<.pag> extension if
1245any). If the database does not exist, it is created with protection
1b3a6178
FC
1246specified by MASK (as modified by the C<umask>). To prevent creation of
1247the database if it doesn't exist, you may specify a MODE
1248of 0, and the function will return a false value if it
1249can't find an existing database. If your system supports
80d38338 1250only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1251program. In older versions of Perl, if your system had neither DBM nor
19799a22 1252ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1253sdbm(3).
1254
1255If you don't have write access to the DBM file, you can only read hash
1256variables, not set them. If you want to test whether you can write,
3b10bc60 1257either use file tests or try setting a dummy hash entry inside an C<eval>
1258to trap the error.
a0d0e21e 1259
19799a22
GS
1260Note that functions such as C<keys> and C<values> may return huge lists
1261when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1262function to iterate over large DBM files. Example:
1263
1264 # print out history file offsets
1265 dbmopen(%HIST,'/usr/lib/news/history',0666);
1266 while (($key,$val) = each %HIST) {
a9a5a0dc 1267 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1268 }
1269 dbmclose(%HIST);
1270
cb1a09d0 1271See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1272cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1273rich implementation.
4633a7c4 1274
2b5ab1e7
TC
1275You can control which DBM library you use by loading that library
1276before you call dbmopen():
1277
1278 use DB_File;
1279 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1280 or die "Can't open netscape history file: $!";
2b5ab1e7 1281
ea9eb35a 1282Portability issues: L<perlport/dbmopen>.
1283
8f1da26d
TC
1284=item default BLOCK
1285
c17cdb72
NC
1286=for Pod::Functions !RT #108848
1287
8f1da26d
TC
1288Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when>
1289that's always true. Only available after Perl 5.10, and only if the
4a904372
FC
1290C<switch> feature has been requested or if the keyword is prefixed with
1291C<CORE::>. See L</when>.
8f1da26d 1292
a0d0e21e 1293=item defined EXPR
d74e8afc 1294X<defined> X<undef> X<undefined>
a0d0e21e 1295
54310121 1296=item defined
bbce6d69 1297
c17cdb72
NC
1298=for Pod::Functions test whether a value, variable, or function is defined
1299
2f9daede 1300Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1301the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1302checked.
1303
1304Many operations return C<undef> to indicate failure, end of file,
1305system error, uninitialized variable, and other exceptional
1306conditions. This function allows you to distinguish C<undef> from
1307other values. (A simple Boolean test will not distinguish among
7660c0ab 1308C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1309false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1310doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1311returns C<undef> when its argument is an empty array, I<or> when the
1312element to return happens to be C<undef>.
1313
f10b0346
GS
1314You may also use C<defined(&func)> to check whether subroutine C<&func>
1315has ever been defined. The return value is unaffected by any forward
80d38338 1316declarations of C<&func>. A subroutine that is not defined
847c7ebe 1317may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1318makes it spring into existence the first time that it is called; see
847c7ebe 1319L<perlsub>.
f10b0346
GS
1320
1321Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1322used to report whether memory for that aggregate had ever been
f10b0346
GS
1323allocated. This behavior may disappear in future versions of Perl.
1324You should instead use a simple test for size:
1325
1326 if (@an_array) { print "has array elements\n" }
1327 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1328
1329When used on a hash element, it tells you whether the value is defined,
dc848c6f 1330not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1331purpose.
a0d0e21e
LW
1332
1333Examples:
1334
8f1da26d 1335 print if defined $switch{D};
a0d0e21e
LW
1336 print "$val\n" while defined($val = pop(@ary));
1337 die "Can't readlink $sym: $!"
a9a5a0dc 1338 unless defined($value = readlink $sym);
a0d0e21e 1339 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1340 $debugging = 0 unless defined $debugging;
a0d0e21e 1341
8f1da26d 1342Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1343discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1344defined values. For example, if you say
a5f75d66
AD
1345
1346 "ab" =~ /a(.*)b/;
1347
80d38338 1348The pattern match succeeds and C<$1> is defined, although it
cf264981 1349matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1350matched something that happened to be zero characters long. This is all
a5f75d66 1351very above-board and honest. When a function returns an undefined value,
2f9daede 1352it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1353should use C<defined> only when questioning the integrity of what
7660c0ab 1354you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1355what you want.
1356
dc848c6f 1357See also L</undef>, L</exists>, L</ref>.
2f9daede 1358
a0d0e21e 1359=item delete EXPR
d74e8afc 1360X<delete>
a0d0e21e 1361
c17cdb72
NC
1362=for Pod::Functions deletes a value from a hash
1363
d0a76353
RS
1364Given an expression that specifies an element or slice of a hash, C<delete>
1365deletes the specified elements from that hash so that exists() on that element
1366no longer returns true. Setting a hash element to the undefined value does
1367not remove its key, but deleting it does; see L</exists>.
80d38338 1368
8f1da26d 1369In list context, returns the value or values deleted, or the last such
80d38338 1370element in scalar context. The return list's length always matches that of
d0a76353
RS
1371the argument list: deleting non-existent elements returns the undefined value
1372in their corresponding positions.
80d38338 1373
d0a76353
RS
1374delete() may also be used on arrays and array slices, but its behavior is less
1375straightforward. Although exists() will return false for deleted entries,
1376deleting array elements never changes indices of existing values; use shift()
1377or splice() for that. However, if all deleted elements fall at the end of an
1378array, the array's size shrinks to the position of the highest element that
1379still tests true for exists(), or to 0 if none do.
1380
8f1da26d 1381B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1382be removed in a future version of Perl.
80d38338
TC
1383
1384Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1385a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1386or array may not necessarily return anything; it depends on the implementation
1387of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1388
80d38338
TC
1389The C<delete local EXPR> construct localizes the deletion to the current
1390block at run time. Until the block exits, elements locally deleted
1391temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1392of composite types">.
eba0920a
EM
1393
1394 %hash = (foo => 11, bar => 22, baz => 33);
1395 $scalar = delete $hash{foo}; # $scalar is 11
1396 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1397 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1398
01020589 1399The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1400
5f05dabc 1401 foreach $key (keys %HASH) {
a9a5a0dc 1402 delete $HASH{$key};
a0d0e21e
LW
1403 }
1404
01020589 1405 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1406 delete $ARRAY[$index];
01020589
GS
1407 }
1408
1409And so do these:
5f05dabc 1410
01020589
GS
1411 delete @HASH{keys %HASH};
1412
9740c838 1413 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1414
80d38338
TC
1415But both are slower than assigning the empty list
1416or undefining %HASH or @ARRAY, which is the customary
1417way to empty out an aggregate:
01020589 1418
5ed4f2ec 1419 %HASH = (); # completely empty %HASH
1420 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1421
5ed4f2ec 1422 @ARRAY = (); # completely empty @ARRAY
1423 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1424
80d38338
TC
1425The EXPR can be arbitrarily complicated provided its
1426final operation is an element or slice of an aggregate:
a0d0e21e
LW
1427
1428 delete $ref->[$x][$y]{$key};
5f05dabc 1429 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1430
01020589
GS
1431 delete $ref->[$x][$y][$index];
1432 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1433
a0d0e21e 1434=item die LIST
d74e8afc 1435X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1436
c17cdb72
NC
1437=for Pod::Functions raise an exception or bail out
1438
391b733c 1439C<die> raises an exception. Inside an C<eval> the error message is stuffed
4c050ad5
NC
1440into C<$@> and the C<eval> is terminated with the undefined value.
1441If the exception is outside of all enclosing C<eval>s, then the uncaught
391b733c 1442exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1443need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1444
1445Equivalent examples:
1446
1447 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1448 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1449
ccac6780 1450If the last element of LIST does not end in a newline, the current
df37ec69
WW
1451script line number and input line number (if any) are also printed,
1452and a newline is supplied. Note that the "input line number" (also
1453known as "chunk") is subject to whatever notion of "line" happens to
1454be currently in effect, and is also available as the special variable
1455C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1456
1457Hint: sometimes appending C<", stopped"> to your message will cause it
1458to make better sense when the string C<"at foo line 123"> is appended.
1459Suppose you are running script "canasta".
a0d0e21e
LW
1460
1461 die "/etc/games is no good";
1462 die "/etc/games is no good, stopped";
1463
1464produce, respectively
1465
1466 /etc/games is no good at canasta line 123.
1467 /etc/games is no good, stopped at canasta line 123.
1468
a96d0188 1469If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1470previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1471This is useful for propagating exceptions:
1472
1473 eval { ... };
1474 die unless $@ =~ /Expected exception/;
1475
a96d0188 1476If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1477C<PROPAGATE> method, that method will be called with additional file
1478and line number parameters. The return value replaces the value in
34169887 1479C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1480were called.
1481
7660c0ab 1482If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1483
4c050ad5
NC
1484If an uncaught exception results in interpreter exit, the exit code is
1485determined from the values of C<$!> and C<$?> with this pseudocode:
1486
1487 exit $! if $!; # errno
1488 exit $? >> 8 if $? >> 8; # child exit status
1489 exit 255; # last resort
1490
1491The intent is to squeeze as much possible information about the likely cause
391b733c
FC
1492into the limited space of the system exit
1493code. However, as C<$!> is the value
4c050ad5
NC
1494of C's C<errno>, which can be set by any system call, this means that the value
1495of the exit code used by C<die> can be non-predictable, so should not be relied
1496upon, other than to be non-zero.
1497
80d38338
TC
1498You can also call C<die> with a reference argument, and if this is trapped
1499within an C<eval>, C<$@> contains that reference. This permits more
1500elaborate exception handling using objects that maintain arbitrary state
1501about the exception. Such a scheme is sometimes preferable to matching
1502particular string values of C<$@> with regular expressions. Because C<$@>
1503is a global variable and C<eval> may be used within object implementations,
1504be careful that analyzing the error object doesn't replace the reference in
1505the global variable. It's easiest to make a local copy of the reference
1506before any manipulations. Here's an example:
52531d10 1507
80d38338 1508 use Scalar::Util "blessed";
da279afe 1509
52531d10 1510 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1511 if (my $ev_err = $@) {
1512 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1513 # handle Some::Module::Exception
1514 }
1515 else {
1516 # handle all other possible exceptions
1517 }
1518 }
1519
3b10bc60 1520Because Perl stringifies uncaught exception messages before display,
80d38338 1521you'll probably want to overload stringification operations on
52531d10
GS
1522exception objects. See L<overload> for details about that.
1523
19799a22
GS
1524You can arrange for a callback to be run just before the C<die>
1525does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1526handler is called with the error text and can change the error
19799a22 1527message, if it sees fit, by calling C<die> again. See
96090e4f 1528L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1529L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1530to be run only right before your program was to exit, this is not
3b10bc60 1531currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1532even inside eval()ed blocks/strings! If one wants the hook to do
1533nothing in such situations, put
fb73857a 1534
5ed4f2ec 1535 die @_ if $^S;
fb73857a 1536
19799a22
GS
1537as the first line of the handler (see L<perlvar/$^S>). Because
1538this promotes strange action at a distance, this counterintuitive
b76cc8ba 1539behavior may be fixed in a future release.
774d564b 1540
4c050ad5
NC
1541See also exit(), warn(), and the Carp module.
1542
a0d0e21e 1543=item do BLOCK
d74e8afc 1544X<do> X<block>
a0d0e21e 1545
c17cdb72
NC
1546=for Pod::Functions turn a BLOCK into a TERM
1547
a0d0e21e 1548Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1549sequence of commands indicated by BLOCK. When modified by the C<while> or
1550C<until> loop modifier, executes the BLOCK once before testing the loop
391b733c 1551condition. (On other statements the loop modifiers test the conditional
6b275a1f 1552first.)
a0d0e21e 1553
4968c1e4 1554C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1555C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1556See L<perlsyn> for alternative strategies.
4968c1e4 1557
a0d0e21e 1558=item do SUBROUTINE(LIST)
d74e8afc 1559X<do>
a0d0e21e 1560
51124b83
FC
1561This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
1562a scalar variable or a subroutine beginning with C<&>.
a0d0e21e
LW
1563
1564=item do EXPR
d74e8afc 1565X<do>
a0d0e21e
LW
1566
1567Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1568file as a Perl script.
a0d0e21e
LW
1569
1570 do 'stat.pl';
1571
1572is just like
1573
986b19de 1574 eval `cat stat.pl`;
a0d0e21e 1575
2b5ab1e7 1576except that it's more efficient and concise, keeps track of the current
96090e4f
A
1577filename for error messages, searches the C<@INC> directories, and updates
1578C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1579these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1580cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1581same, however, in that it does reparse the file every time you call it,
1582so you probably don't want to do this inside a loop.
a0d0e21e 1583
8f1da26d 1584If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1585an error message in C<$@>. If C<do> cannot read the file, it returns undef
1586and sets C<$!> to the error. Always check C<$@> first, as compilation
1587could fail in a way that also sets C<$!>. If the file is successfully
1588compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1589
80d38338 1590Inclusion of library modules is better done with the
19799a22 1591C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1592and raise an exception if there's a problem.
a0d0e21e 1593
5a964f20
TC
1594You might like to use C<do> to read in a program configuration
1595file. Manual error checking can be done this way:
1596
b76cc8ba 1597 # read in config files: system first, then user
f86cebdf 1598 for $file ("/share/prog/defaults.rc",
b76cc8ba 1599 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1600 {
1601 unless ($return = do $file) {
1602 warn "couldn't parse $file: $@" if $@;
1603 warn "couldn't do $file: $!" unless defined $return;
1604 warn "couldn't run $file" unless $return;
1605 }
5a964f20
TC
1606 }
1607
a0d0e21e 1608=item dump LABEL
d74e8afc 1609X<dump> X<core> X<undump>
a0d0e21e 1610
1614b0e3
JD
1611=item dump
1612
c17cdb72
NC
1613=for Pod::Functions create an immediate core dump
1614
19799a22
GS
1615This function causes an immediate core dump. See also the B<-u>
1616command-line switch in L<perlrun>, which does the same thing.
1617Primarily this is so that you can use the B<undump> program (not
1618supplied) to turn your core dump into an executable binary after
1619having initialized all your variables at the beginning of the
1620program. When the new binary is executed it will begin by executing
1621a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1622Think of it as a goto with an intervening core dump and reincarnation.
1623If C<LABEL> is omitted, restarts the program from the top.
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
ea9eb35a 1634Portability issues: L<perlport/dump>.
1635
532eee96 1636=item each HASH
d74e8afc 1637X<each> X<hash, iterator>
aa689395 1638
532eee96 1639=item each ARRAY
aeedbbed
NC
1640X<array, iterator>
1641
f5a93a43
TC
1642=item each EXPR
1643
c17cdb72
NC
1644=for Pod::Functions retrieve the next key/value pair from a hash
1645
bade7fbc
TC
1646When called on a hash in list context, returns a 2-element list
1647consisting of the key and value for the next element of a hash. In Perl
16485.12 and later only, it will also return the index and value for the next
1649element of an array so that you can iterate over it; older Perls consider
1650this a syntax error. When called in scalar context, returns only the key
1651(not the value) in a hash, or the index in an array.
2f9daede 1652
aeedbbed 1653Hash entries are returned in an apparently random order. The actual random
3b10bc60 1654order is subject to change in future versions of Perl, but it is
504f80c1 1655guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1656function would produce on the same (unmodified) hash. Since Perl
22883ac5 16575.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1658for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1659
80d38338
TC
1660After C<each> has returned all entries from the hash or array, the next
1661call to C<each> returns the empty list in list context and C<undef> in
bade7fbc
TC
1662scalar context; the next call following I<that> one restarts iteration.
1663Each hash or array has its own internal iterator, accessed by C<each>,
1664C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1665reached the end as just described; it can be explicitly reset by calling
1666C<keys> or C<values> on the hash or array. If you add or delete a hash's
1667elements while iterating over it, entries may be skipped or duplicated--so
1668don't do that. Exception: In the current implementation, it is always safe
1669to delete the item most recently returned by C<each()>, so the following
1670code works properly:
74fc8b5f
MJD
1671
1672 while (($key, $value) = each %hash) {
1673 print $key, "\n";
1674 delete $hash{$key}; # This is safe
1675 }
aa689395 1676
80d38338 1677This prints out your environment like the printenv(1) program,
3b10bc60 1678but in a different order:
a0d0e21e
LW
1679
1680 while (($key,$value) = each %ENV) {
a9a5a0dc 1681 print "$key=$value\n";
a0d0e21e
LW
1682 }
1683
f5a93a43
TC
1684Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1685reference to an unblessed hash or array. The argument will be dereferenced
1686automatically. This aspect of C<each> is considered highly experimental.
1687The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1688
1689 while (($key,$value) = each $hashref) { ... }
1690
bade7fbc
TC
1691To avoid confusing would-be users of your code who are running earlier
1692versions of Perl with mysterious syntax errors, put this sort of thing at
1693the top of your file to signal that your code will work I<only> on Perls of
1694a recent vintage:
1695
1696 use 5.012; # so keys/values/each work on arrays
1697 use 5.014; # so keys/values/each work on scalars (experimental)
1698
8f1da26d 1699See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1700
1701=item eof FILEHANDLE
d74e8afc
ITB
1702X<eof>
1703X<end of file>
1704X<end-of-file>
a0d0e21e 1705
4633a7c4
LW
1706=item eof ()
1707
a0d0e21e
LW
1708=item eof
1709
c17cdb72
NC
1710=for Pod::Functions test a filehandle for its end
1711
8f1da26d 1712Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1713FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1714gives the real filehandle. (Note that this function actually
80d38338 1715reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1716interactive context.) Do not read from a terminal file (or call
19799a22 1717C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1718as terminals may lose the end-of-file condition if you do.
1719
820475bd 1720An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1721with empty parentheses is different. It refers to the pseudo file
820475bd 1722formed from the files listed on the command line and accessed via the
61eff3bc
JH
1723C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1724as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1725used will cause C<@ARGV> to be examined to determine if input is
67408cae 1726available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1727end-of-file will assume you are processing another C<@ARGV> list,
1728and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1729see L<perlop/"I/O Operators">.
820475bd 1730
61eff3bc 1731In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1732detect the end of each file, whereas C<eof()> will detect the end
1733of the very last file only. Examples:
a0d0e21e 1734
748a9306
LW
1735 # reset line numbering on each input file
1736 while (<>) {
a9a5a0dc
VP
1737 next if /^\s*#/; # skip comments
1738 print "$.\t$_";
5a964f20 1739 } continue {
a9a5a0dc 1740 close ARGV if eof; # Not eof()!
748a9306
LW
1741 }
1742
a0d0e21e
LW
1743 # insert dashes just before last line of last file
1744 while (<>) {
a9a5a0dc
VP
1745 if (eof()) { # check for end of last file
1746 print "--------------\n";
1747 }
1748 print;
4d0444a3 1749 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1750 }
1751
a0d0e21e 1752Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1753input operators typically return C<undef> when they run out of data or
1754encounter an error.
a0d0e21e
LW
1755
1756=item eval EXPR
d74e8afc 1757X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1758X<error, handling> X<exception, handling>
a0d0e21e
LW
1759
1760=item eval BLOCK
1761
ce2984c3
PF
1762=item eval
1763
c17cdb72
NC
1764=for Pod::Functions catch exceptions or compile and run code
1765
c7cc6f1c
GS
1766In the first form, the return value of EXPR is parsed and executed as if it
1767were a little Perl program. The value of the expression (which is itself
8f1da26d 1768determined within scalar context) is first parsed, and if there were no
2341804c 1769errors, executed as a block within the lexical context of the current Perl
df4833a8 1770program. This means, that in particular, any outer lexical variables are
2341804c
DM
1771visible to it, and any package variable settings or subroutine and format
1772definitions remain afterwards.
1773
1774Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1775If EXPR is omitted, evaluates C<$_>. This form is typically used to
1776delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c 1777
7289c5e6
FC
1778If the C<unicode_eval> feature is enabled (which is the default under a
1779C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1780characters, so C<use utf8> declarations have no effect, and source filters
1781are forbidden. In the absence of the C<unicode_eval> feature, the string
1782will sometimes be treated as characters and sometimes as bytes, depending
1783on the internal encoding, and source filters activated within the C<eval>
1784exhibit the erratic, but historical, behaviour of affecting some outer file
1785scope that is still compiling. See also the L</evalbytes> keyword, which
1786always treats its input as a byte stream and works properly with source
1787filters, and the L<feature> pragma.
1788
c7cc6f1c 1789In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1790same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1791within the context of the current Perl program. This form is typically
1792used to trap exceptions more efficiently than the first (see below), while
1793also providing the benefit of checking the code within BLOCK at compile
1794time.
1795
1796The final semicolon, if any, may be omitted from the value of EXPR or within
1797the BLOCK.
1798
1799In both forms, the value returned is the value of the last expression
5a964f20 1800evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1801as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1802in void, scalar, or list context, depending on the context of the C<eval>
1803itself. See L</wantarray> for more on how the evaluation context can be
1804determined.
a0d0e21e 1805
19799a22 1806If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1807executed, C<eval> returns C<undef> in scalar context
774b80e8
FC
1808or an empty list in list context, and C<$@> is set to the error
1809message. (Prior to 5.16, a bug caused C<undef> to be returned
1810in list context for syntax errors, but not for runtime errors.)
1811If there was no error, C<$@> is set to the empty string. A
9cc672d4
FC
1812control flow operator like C<last> or C<goto> can bypass the setting of
1813C<$@>. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1814warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1815To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1816turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1817See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1818
19799a22
GS
1819Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1820determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1821is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1822the die operator is used to raise exceptions.
1823
5f1da31c
NT
1824If you want to trap errors when loading an XS module, some problems with
1825the binary interface (such as Perl version skew) may be fatal even with
df4833a8 1826C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
5f1da31c 1827
a0d0e21e
LW
1828If the code to be executed doesn't vary, you may use the eval-BLOCK
1829form to trap run-time errors without incurring the penalty of
1830recompiling each time. The error, if any, is still returned in C<$@>.
1831Examples:
1832
54310121 1833 # make divide-by-zero nonfatal
a0d0e21e
LW
1834 eval { $answer = $a / $b; }; warn $@ if $@;
1835
1836 # same thing, but less efficient
1837 eval '$answer = $a / $b'; warn $@ if $@;
1838
1839 # a compile-time error
5ed4f2ec 1840 eval { $answer = }; # WRONG
a0d0e21e
LW
1841
1842 # a run-time error
5ed4f2ec 1843 eval '$answer ='; # sets $@
a0d0e21e 1844
cf264981
SP
1845Using the C<eval{}> form as an exception trap in libraries does have some
1846issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1847may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1848You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1849as this example shows:
774d564b 1850
80d38338 1851 # a private exception trap for divide-by-zero
f86cebdf
GS
1852 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1853 warn $@ if $@;
774d564b
PP
1854
1855This is especially significant, given that C<__DIE__> hooks can call
19799a22 1856C<die> again, which has the effect of changing their error messages:
774d564b
PP
1857
1858 # __DIE__ hooks may modify error messages
1859 {
f86cebdf
GS
1860 local $SIG{'__DIE__'} =
1861 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1862 eval { die "foo lives here" };
1863 print $@ if $@; # prints "bar lives here"
774d564b
PP
1864 }
1865
19799a22 1866Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1867may be fixed in a future release.
1868
19799a22 1869With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1870being looked at when:
1871
5ed4f2ec 1872 eval $x; # CASE 1
1873 eval "$x"; # CASE 2
a0d0e21e 1874
5ed4f2ec 1875 eval '$x'; # CASE 3
1876 eval { $x }; # CASE 4
a0d0e21e 1877
5ed4f2ec 1878 eval "\$$x++"; # CASE 5
1879 $$x++; # CASE 6
a0d0e21e 1880
2f9daede 1881Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1882the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1883the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1884and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1885does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1886purely visual reasons, but it also has the advantage of compiling at
1887compile-time instead of at run-time.) Case 5 is a place where
19799a22 1888normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1889particular situation, you can just use symbolic references instead, as
1890in case 6.
a0d0e21e 1891
b6538e4f 1892Before Perl 5.14, the assignment to C<$@> occurred before restoration
bade7fbc 1893of localized variables, which means that for your code to run on older
b208c909 1894versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1895errors:
1896
1897 # alter $@ on nefarious repugnancy only
1898 {
1899 my $e;
1900 {
1901 local $@; # protect existing $@
1902 eval { test_repugnancy() };
b208c909 1903 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
8a5a710d
DN
1904 $@ =~ /nefarious/ and $e = $@;
1905 }
1906 die $e if defined $e
1907 }
1908
4968c1e4 1909C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1910C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1911
3b10bc60 1912An C<eval ''> executed within the C<DB> package doesn't see the usual
1913surrounding lexical scope, but rather the scope of the first non-DB piece
df4833a8 1914of code that called it. You don't normally need to worry about this unless
3b10bc60 1915you are writing a Perl debugger.
d819b83a 1916
7289c5e6
FC
1917=item evalbytes EXPR
1918X<evalbytes>
1919
1920=item evalbytes
1921
d9b04284 1922=for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
c17cdb72 1923
7289c5e6
FC
1924This function is like L</eval> with a string argument, except it always
1925parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1926string containing characters whose ordinal value exceeds 255 results in an
1927error. Source filters activated within the evaluated code apply to the
1928code itself.
1929
1930This function is only available under the C<evalbytes> feature, a
1931C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1932L<feature> for more information.
1933
a0d0e21e 1934=item exec LIST
d74e8afc 1935X<exec> X<execute>
a0d0e21e 1936
8bf3b016
GS
1937=item exec PROGRAM LIST
1938
c17cdb72
NC
1939=for Pod::Functions abandon this program to run another
1940
3b10bc60 1941The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1942use C<system> instead of C<exec> if you want it to return. It fails and
1943returns false only if the command does not exist I<and> it is executed
fb73857a 1944directly instead of via your system's command shell (see below).
a0d0e21e 1945
19799a22 1946Since it's a common mistake to use C<exec> instead of C<system>, Perl
4642e50d
EB
1947warns you if C<exec> is called in void context and if there is a following
1948statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1949you always do that, right?). If you I<really> want to follow an C<exec>
1950with some other statement, you can use one of these styles to avoid the warning:
55d729e4 1951
5a964f20
TC
1952 exec ('foo') or print STDERR "couldn't exec foo: $!";
1953 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1954
5a964f20 1955If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1956with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1957If there is only one scalar argument or an array with one element in it,
1958the argument is checked for shell metacharacters, and if there are any,
1959the entire argument is passed to the system's command shell for parsing
1960(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1961If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1962words and passed directly to C<execvp>, which is more efficient.
19799a22 1963Examples:
a0d0e21e 1964
19799a22
GS
1965 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1966 exec "sort $outfile | uniq";
a0d0e21e
LW
1967
1968If you don't really want to execute the first argument, but want to lie
1969to the program you are executing about its own name, you can specify
1970the program you actually want to run as an "indirect object" (without a
1971comma) in front of the LIST. (This always forces interpretation of the
54310121 1972LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1973the list.) Example:
1974
1975 $shell = '/bin/csh';
5ed4f2ec 1976 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1977
1978or, more directly,
1979
5ed4f2ec 1980 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1981
3b10bc60 1982When the arguments get executed via the system shell, results are
1983subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1984for details.
1985
19799a22
GS
1986Using an indirect object with C<exec> or C<system> is also more
1987secure. This usage (which also works fine with system()) forces
1988interpretation of the arguments as a multivalued list, even if the
1989list had just one argument. That way you're safe from the shell
1990expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1991
1992 @args = ( "echo surprise" );
1993
2b5ab1e7 1994 exec @args; # subject to shell escapes
f86cebdf 1995 # if @args == 1
2b5ab1e7 1996 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1997
1998The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1999program, passing it C<"surprise"> an argument. The second version didn't;
2000it tried to run a program named I<"echo surprise">, didn't find it, and set
2001C<$?> to a non-zero value indicating failure.
5a964f20 2002
80d38338 2003Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
2004output before the exec, but this may not be supported on some platforms
2005(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
2006in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 2007open handles to avoid lost output.
0f897271 2008
80d38338
TC
2009Note that C<exec> will not call your C<END> blocks, nor will it invoke
2010C<DESTROY> methods on your objects.
7660c0ab 2011
ea9eb35a 2012Portability issues: L<perlport/exec>.
2013
a0d0e21e 2014=item exists EXPR
d74e8afc 2015X<exists> X<autovivification>
a0d0e21e 2016
c17cdb72
NC
2017=for Pod::Functions test whether a hash key is present
2018
d0a76353
RS
2019Given an expression that specifies an element of a hash, returns true if the
2020specified element in the hash has ever been initialized, even if the
2021corresponding value is undefined.
a0d0e21e 2022
5ed4f2ec 2023 print "Exists\n" if exists $hash{$key};
2024 print "Defined\n" if defined $hash{$key};
01020589
GS
2025 print "True\n" if $hash{$key};
2026
d0a76353 2027exists may also be called on array elements, but its behavior is much less
8f1da26d 2028obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
2029that calling exists on array values is deprecated and likely to be removed in
2030a future version of Perl.
2031
5ed4f2ec 2032 print "Exists\n" if exists $array[$index];
2033 print "Defined\n" if defined $array[$index];
01020589 2034 print "True\n" if $array[$index];
a0d0e21e 2035
8f1da26d 2036A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
2037it exists, but the reverse doesn't necessarily hold true.
2038
afebc493
GS
2039Given an expression that specifies the name of a subroutine,
2040returns true if the specified subroutine has ever been declared, even
2041if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 2042does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
2043exist may still be callable: its package may have an C<AUTOLOAD>
2044method that makes it spring into existence the first time that it is
3b10bc60 2045called; see L<perlsub>.
afebc493 2046
5ed4f2ec 2047 print "Exists\n" if exists &subroutine;
2048 print "Defined\n" if defined &subroutine;
afebc493 2049
a0d0e21e 2050Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 2051operation is a hash or array key lookup or subroutine name:
a0d0e21e 2052
5ed4f2ec 2053 if (exists $ref->{A}->{B}->{$key}) { }
2054 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 2055
5ed4f2ec 2056 if (exists $ref->{A}->{B}->[$ix]) { }
2057 if (exists $hash{A}{B}[$ix]) { }
01020589 2058
afebc493
GS
2059 if (exists &{$ref->{A}{B}{$key}}) { }
2060
9590a7cd 2061Although the most deeply nested array or hash element will not spring into
3b10bc60 2062existence just because its existence was tested, any intervening ones will.
61eff3bc 2063Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 2064into existence due to the existence test for the $key element above.
3b10bc60 2065This happens anywhere the arrow operator is used, including even here:
5a964f20 2066
2b5ab1e7 2067 undef $ref;
5ed4f2ec 2068 if (exists $ref->{"Some key"}) { }
2069 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
2070
2071This surprising autovivification in what does not at first--or even
2072second--glance appear to be an lvalue context may be fixed in a future
5a964f20 2073release.
a0d0e21e 2074
afebc493
GS
2075Use of a subroutine call, rather than a subroutine name, as an argument
2076to exists() is an error.
2077
5ed4f2ec 2078 exists &sub; # OK
2079 exists &sub(); # Error
afebc493 2080
a0d0e21e 2081=item exit EXPR
d74e8afc 2082X<exit> X<terminate> X<abort>
a0d0e21e 2083
ce2984c3
PF
2084=item exit
2085
c17cdb72
NC
2086=for Pod::Functions terminate this program
2087
2b5ab1e7 2088Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
2089
2090 $ans = <STDIN>;
2091 exit 0 if $ans =~ /^[Xx]/;
2092
19799a22 2093See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
2094universally recognized values for EXPR are C<0> for success and C<1>
2095for error; other values are subject to interpretation depending on the
2096environment in which the Perl program is running. For example, exiting
209769 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2098the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 2099
19799a22
GS
2100Don't use C<exit> to abort a subroutine if there's any chance that
2101someone might want to trap whatever error happened. Use C<die> instead,
2102which can be trapped by an C<eval>.
28757baa 2103
19799a22 2104The exit() function does not always exit immediately. It calls any
2b5ab1e7 2105defined C<END> routines first, but these C<END> routines may not
19799a22 2106themselves abort the exit. Likewise any object destructors that need to
60275626 2107be called are called before the real exit. C<END> routines and destructors
391b733c 2108can change the exit status by modifying C<$?>. If this is a problem, you
2b5ab1e7 2109can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 2110See L<perlmod> for details.
5a964f20 2111
ea9eb35a 2112Portability issues: L<perlport/exit>.
2113
a0d0e21e 2114=item exp EXPR
d74e8afc 2115X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 2116
54310121 2117=item exp
bbce6d69 2118
c17cdb72
NC
2119=for Pod::Functions raise I<e> to a power
2120
b76cc8ba 2121Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
2122If EXPR is omitted, gives C<exp($_)>.
2123
628253b8
BF
2124=item fc EXPR
2125X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2126
2127=item fc
2128
d9b04284 2129=for Pod::Functions +fc return casefolded version of a string
c17cdb72 2130
628253b8
BF
2131Returns the casefolded version of EXPR. This is the internal function
2132implementing the C<\F> escape in double-quoted strings.
2133
2134Casefolding is the process of mapping strings to a form where case
2135differences are erased; comparing two strings in their casefolded
2136form is effectively a way of asking if two strings are equal,
2137regardless of case.
2138
2139Roughly, if you ever found yourself writing this
2140
2141 lc($this) eq lc($that) # Wrong!
2142 # or
2143 uc($this) eq uc($that) # Also wrong!
2144 # or
2145 $this =~ /\Q$that/i # Right!
2146
2147Now you can write
2148
2149 fc($this) eq fc($that)
2150
2151And get the correct results.
2152
2153Perl only implements the full form of casefolding.
2154For further information on casefolding, refer to
2155the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
21564.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2157available at L<http://www.unicode.org/versions/latest/>, as well as the
2158Case Charts available at L<http://www.unicode.org/charts/case/>.
2159
2160If EXPR is omitted, uses C<$_>.
2161
2162This function behaves the same way under various pragma, such as in a locale,
2163as L</lc> does.
2164
2165While the Unicode Standard defines two additional forms of casefolding,
2166one for Turkic languages and one that never maps one character into multiple
2167characters, these are not provided by the Perl core; However, the CPAN module
2168C<Unicode::Casing> may be used to provide an implementation.
2169
2170This keyword is available only when the C<"fc"> feature is enabled,
2171or when prefixed with C<CORE::>; See L<feature>. Alternately,
2172include a C<use v5.16> or later to the current scope.
2173
a0d0e21e 2174=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2175X<fcntl>
a0d0e21e 2176
c17cdb72
NC
2177=for Pod::Functions file control system call
2178
f86cebdf 2179Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
2180
2181 use Fcntl;
2182
0ade1984 2183first to get the correct constant definitions. Argument processing and
3b10bc60 2184value returned work just like C<ioctl> below.
a0d0e21e
LW
2185For example:
2186
2187 use Fcntl;
5a964f20 2188 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 2189 or die "can't fcntl F_GETFL: $!";
5a964f20 2190
554ad1fc 2191You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
2192Like C<ioctl>, it maps a C<0> return from the system call into
2193C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
2194in numeric context. It is also exempt from the normal B<-w> warnings
2195on improper numeric conversions.
5a964f20 2196
3b10bc60 2197Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
2198doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2199manpage to learn what functions are available on your system.
a0d0e21e 2200
be2f7487 2201Here's an example of setting a filehandle named C<REMOTE> to be
2202non-blocking at the system level. You'll have to negotiate C<$|>
2203on your own, though.
2204
2205 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2206
2207 $flags = fcntl(REMOTE, F_GETFL, 0)
2208 or die "Can't get flags for the socket: $!\n";
2209
2210 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2211 or die "Can't set flags for the socket: $!\n";
2212
ea9eb35a 2213Portability issues: L<perlport/fcntl>.
2214
cfa52385
FC
2215=item __FILE__
2216X<__FILE__>
2217
c17cdb72
NC
2218=for Pod::Functions the name of the current source file
2219
cfa52385
FC
2220A special token that returns the name of the file in which it occurs.
2221
a0d0e21e 2222=item fileno FILEHANDLE
d74e8afc 2223X<fileno>
a0d0e21e 2224
c17cdb72
NC
2225=for Pod::Functions return file descriptor from filehandle
2226
2b5ab1e7 2227Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
2228filehandle is not open. If there is no real file descriptor at the OS
2229level, as can happen with filehandles connected to memory objects via
2230C<open> with a reference for the third argument, -1 is returned.
2231
2232This is mainly useful for constructing
19799a22 2233bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
2234If FILEHANDLE is an expression, the value is taken as an indirect
2235filehandle, generally its name.
5a964f20 2236
b76cc8ba 2237You can use this to find out whether two handles refer to the
5a964f20
TC
2238same underlying descriptor:
2239
2240 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 2241 print "THIS and THAT are dups\n";
b76cc8ba
NIS
2242 }
2243
a0d0e21e 2244=item flock FILEHANDLE,OPERATION
d74e8afc 2245X<flock> X<lock> X<locking>
a0d0e21e 2246
c17cdb72
NC
2247=for Pod::Functions lock an entire file with an advisory lock
2248
19799a22
GS
2249Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2250for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2251machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2252C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2253entire files only, not records.
2b5ab1e7
TC
2254
2255Two potentially non-obvious but traditional C<flock> semantics are
2256that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2257are B<merely advisory>. Such discretionary locks are more flexible, but
2258offer fewer guarantees. This means that programs that do not also use
2259C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2260your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2261for details. It's best to assume traditional behavior if you're writing
2262portable programs. (But if you're not, you should as always feel perfectly
2263free to write for your own system's idiosyncrasies (sometimes called
2264"features"). Slavish adherence to portability concerns shouldn't get
2265in the way of your getting your job done.)
a3cb178b 2266
8ebc5c01
PP
2267OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2268LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2269you can use the symbolic names if you import them from the L<Fcntl> module,
2270either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2271requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2272releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2273LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2274waiting for the lock; check the return status to see if you got it.
68dc0745 2275
2b5ab1e7
TC
2276To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2277before locking or unlocking it.
8ebc5c01 2278
f86cebdf 2279Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2280locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2281are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2282implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
2283differing semantics shouldn't bite too many people.
2284
becacb53
TM
2285Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2286be open with read intent to use LOCK_SH and requires that it be open
2287with write intent to use LOCK_EX.
2288
19799a22
GS
2289Note also that some versions of C<flock> cannot lock things over the
2290network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2291that. If you like you can force Perl to ignore your system's flock(2)
2292function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2293the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2294and build a new Perl.
4633a7c4
LW
2295
2296Here's a mailbox appender for BSD systems.
a0d0e21e 2297
7ed5353d 2298 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
2299
2300 sub lock {
a9a5a0dc
VP
2301 my ($fh) = @_;
2302 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2303
a9a5a0dc
VP
2304 # and, in case someone appended while we were waiting...
2305 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2306 }
2307
2308 sub unlock {
a9a5a0dc
VP
2309 my ($fh) = @_;
2310 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2311 }
2312
b0169937 2313 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2314 or die "Can't open mailbox: $!";
a0d0e21e 2315
7ed5353d 2316 lock($mbox);
b0169937 2317 print $mbox $msg,"\n\n";
7ed5353d 2318 unlock($mbox);
a0d0e21e 2319
3b10bc60 2320On systems that support a real flock(2), locks are inherited across fork()
2321calls, whereas those that must resort to the more capricious fcntl(2)
2322function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2323
cb1a09d0 2324See also L<DB_File> for other flock() examples.
a0d0e21e 2325
ea9eb35a 2326Portability issues: L<perlport/flock>.
2327
a0d0e21e 2328=item fork
d74e8afc 2329X<fork> X<child> X<parent>
a0d0e21e 2330
c17cdb72
NC
2331=for Pod::Functions create a new process just like this one
2332
2b5ab1e7
TC
2333Does a fork(2) system call to create a new process running the
2334same program at the same point. It returns the child pid to the
2335parent process, C<0> to the child process, or C<undef> if the fork is
2336unsuccessful. File descriptors (and sometimes locks on those descriptors)
2337are shared, while everything else is copied. On most systems supporting
2338fork(), great care has gone into making it extremely efficient (for
2339example, using copy-on-write technology on data pages), making it the
2340dominant paradigm for multitasking over the last few decades.
5a964f20 2341
80d38338 2342Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
2343output before forking the child process, but this may not be supported
2344on some platforms (see L<perlport>). To be safe, you may need to set
2345C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2346C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2347
19799a22 2348If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2349accumulate zombies. On some systems, you can avoid this by setting
2350C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2351forking and reaping moribund children.
cb1a09d0 2352
28757baa
PP
2353Note that if your forked child inherits system file descriptors like
2354STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2355if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2356backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2357You should reopen those to F</dev/null> if it's any issue.
28757baa 2358
ea9eb35a 2359On some platforms such as Windows, where the fork() system call is not available,
391b733c
FC
2360Perl can be built to emulate fork() in the Perl interpreter.
2361The emulation is designed, at the level of the Perl program,
2362to be as compatible as possible with the "Unix" fork().
6d17f725 2363However it has limitations that have to be considered in code intended to be portable.
ea9eb35a 2364See L<perlfork> for more details.
2365
2366Portability issues: L<perlport/fork>.
2367
cb1a09d0 2368=item format
d74e8afc 2369X<format>
cb1a09d0 2370
c17cdb72
NC
2371=for Pod::Functions declare a picture format with use by the write() function
2372
19799a22 2373Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2374example:
2375
54310121 2376 format Something =
a9a5a0dc
VP
2377 Test: @<<<<<<<< @||||| @>>>>>
2378 $str, $%, '$' . int($num)
cb1a09d0
AD
2379 .
2380
2381 $str = "widget";
184e9718 2382 $num = $cost/$quantity;
cb1a09d0
AD
2383 $~ = 'Something';
2384 write;
2385
2386See L<perlform> for many details and examples.
2387
8903cb82 2388=item formline PICTURE,LIST
d74e8afc 2389X<formline>
a0d0e21e 2390
c17cdb72
NC
2391=for Pod::Functions internal function used for formats
2392
5a964f20 2393This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2394too. It formats (see L<perlform>) a list of values according to the
2395contents of PICTURE, placing the output into the format output
7660c0ab 2396accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2397Eventually, when a C<write> is done, the contents of
cf264981
SP
2398C<$^A> are written to some filehandle. You could also read C<$^A>
2399and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2400does one C<formline> per line of form, but the C<formline> function itself
748a9306 2401doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2402that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2403You may therefore need to use multiple formlines to implement a single
3b10bc60 2404record format, just like the C<format> compiler.
748a9306 2405
19799a22 2406Be careful if you put double quotes around the picture, because an C<@>
748a9306 2407character may be taken to mean the beginning of an array name.
19799a22 2408C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2409
445b09e5
FC
2410If you are trying to use this instead of C<write> to capture the output,
2411you may find it easier to open a filehandle to a scalar
2412(C<< open $fh, ">", \$output >>) and write to that instead.
2413
a0d0e21e 2414=item getc FILEHANDLE
f723aae1 2415X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2416
2417=item getc
2418
c17cdb72
NC
2419=for Pod::Functions get the next character from the filehandle
2420
a0d0e21e 2421Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2422or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2423the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2424STDIN. This is not particularly efficient. However, it cannot be
2425used by itself to fetch single characters without waiting for the user
2426to hit enter. For that, try something more like:
4633a7c4
LW
2427
2428 if ($BSD_STYLE) {
a9a5a0dc 2429 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2430 }
2431 else {
a9a5a0dc 2432 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2433 }
2434
2435 $key = getc(STDIN);
2436
2437 if ($BSD_STYLE) {
a9a5a0dc 2438 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2439 }
2440 else {
3b10bc60 2441 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2442 }
2443 print "\n";
2444
54310121
PP
2445Determination of whether $BSD_STYLE should be set
2446is left as an exercise to the reader.
cb1a09d0 2447
19799a22 2448The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2449systems purporting POSIX compliance. See also the C<Term::ReadKey>
a3390c9f 2450module from your nearest CPAN site; details on CPAN can be found under
2b5ab1e7 2451L<perlmodlib/CPAN>.
a0d0e21e
LW
2452
2453=item getlogin
d74e8afc 2454X<getlogin> X<login>
a0d0e21e 2455
c17cdb72
NC
2456=for Pod::Functions return who logged in at this tty
2457
cf264981 2458This implements the C library function of the same name, which on most
3b10bc60 2459systems returns the current login from F</etc/utmp>, if any. If it
2460returns the empty string, use C<getpwuid>.
a0d0e21e 2461
f86702cc 2462 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2463
19799a22
GS
2464Do not consider C<getlogin> for authentication: it is not as
2465secure as C<getpwuid>.
4633a7c4 2466
ea9eb35a 2467Portability issues: L<perlport/getlogin>.
2468
a0d0e21e 2469=item getpeername SOCKET
d74e8afc 2470X<getpeername> X<peer>
a0d0e21e 2471
c17cdb72
NC
2472=for Pod::Functions find the other end of a socket connection
2473
a3390c9f
FC
2474Returns the packed sockaddr address of the other end of the SOCKET
2475connection.
a0d0e21e 2476
4633a7c4
LW
2477 use Socket;
2478 $hersockaddr = getpeername(SOCK);
19799a22 2479 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2480 $herhostname = gethostbyaddr($iaddr, AF_INET);
2481 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2482
2483=item getpgrp PID
d74e8afc 2484X<getpgrp> X<group>
a0d0e21e 2485
c17cdb72
NC
2486=for Pod::Functions get process group
2487
47e29363 2488Returns the current process group for the specified PID. Use
7660c0ab 2489a PID of C<0> to get the current process group for the
4633a7c4 2490current process. Will raise an exception if used on a machine that
a3390c9f
FC
2491doesn't implement getpgrp(2). If PID is omitted, returns the process
2492group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2493does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2494
ea9eb35a 2495Portability issues: L<perlport/getpgrp>.
2496
a0d0e21e 2497=item getppid
d74e8afc 2498X<getppid> X<parent> X<pid>
a0d0e21e 2499
c17cdb72
NC
2500=for Pod::Functions get parent process ID
2501
a0d0e21e
LW
2502Returns the process id of the parent process.
2503
d7c042c9
AB
2504Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2505around non-POSIX thread semantics the minority of Linux systems (and
2506Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2507has since been removed. See the documentation for L<$$|perlvar/$$> for
2508details.
4d76a344 2509
ea9eb35a 2510Portability issues: L<perlport/getppid>.
2511
a0d0e21e 2512=item getpriority WHICH,WHO
d74e8afc 2513X<getpriority> X<priority> X<nice>
a0d0e21e 2514
c17cdb72
NC
2515=for Pod::Functions get current nice value
2516
4633a7c4 2517Returns the current priority for a process, a process group, or a user.
01aa884e 2518(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2519machine that doesn't implement getpriority(2).
a0d0e21e 2520
ea9eb35a 2521Portability issues: L<perlport/getpriority>.
2522
a0d0e21e 2523=item getpwnam NAME
d74e8afc
ITB
2524X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2525X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2526X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2527X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2528X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2529X<endnetent> X<endprotoent> X<endservent>
a0d0e21e 2530
c17cdb72
NC
2531=for Pod::Functions get passwd record given user login name
2532
a0d0e21e
LW
2533=item getgrnam NAME
2534
c17cdb72
NC
2535=for Pod::Functions get group record given group name
2536
a0d0e21e
LW
2537=item gethostbyname NAME
2538
c17cdb72
NC
2539=for Pod::Functions get host record given name
2540
a0d0e21e
LW
2541=item getnetbyname NAME
2542
c17cdb72
NC
2543=for Pod::Functions get networks record given name
2544
a0d0e21e
LW
2545=item getprotobyname NAME
2546
c17cdb72
NC
2547=for Pod::Functions get protocol record given name
2548
a0d0e21e
LW
2549=item getpwuid UID
2550
c17cdb72
NC
2551=for Pod::Functions get passwd record given user ID
2552
a0d0e21e
LW
2553=item getgrgid GID
2554
c17cdb72
NC
2555=for Pod::Functions get group record given group user ID
2556
a0d0e21e
LW
2557=item getservbyname NAME,PROTO
2558
c17cdb72
NC
2559=for Pod::Functions get services record given its name
2560
a0d0e21e
LW
2561=item gethostbyaddr ADDR,ADDRTYPE
2562
c17cdb72
NC
2563=for Pod::Functions get host record given its address
2564
a0d0e21e
LW
2565=item getnetbyaddr ADDR,ADDRTYPE
2566
c17cdb72
NC
2567=for Pod::Functions get network record given its address
2568
a0d0e21e
LW
2569=item getprotobynumber NUMBER
2570
c17cdb72
NC
2571=for Pod::Functions get protocol record numeric protocol
2572
a0d0e21e
LW
2573=item getservbyport PORT,PROTO
2574
c17cdb72
NC
2575=for Pod::Functions get services record given numeric port
2576
a0d0e21e
LW
2577=item getpwent
2578
c17cdb72
NC
2579=for Pod::Functions get next passwd record
2580
a0d0e21e
LW
2581=item getgrent
2582
c17cdb72
NC
2583=for Pod::Functions get next group record
2584
a0d0e21e
LW
2585=item gethostent
2586
c17cdb72
NC
2587=for Pod::Functions get next hosts record
2588
a0d0e21e
LW
2589=item getnetent
2590
c17cdb72
NC
2591=for Pod::Functions get next networks record
2592
a0d0e21e
LW
2593=item getprotoent
2594
c17cdb72
NC
2595=for Pod::Functions get next protocols record
2596
a0d0e21e
LW
2597=item getservent
2598
c17cdb72
NC
2599=for Pod::Functions get next services record
2600
a0d0e21e
LW
2601=item setpwent
2602
c17cdb72
NC
2603=for Pod::Functions prepare passwd file for use
2604
a0d0e21e
LW
2605=item setgrent
2606
c17cdb72
NC
2607=for Pod::Functions prepare group file for use
2608
a0d0e21e
LW
2609=item sethostent STAYOPEN
2610
c17cdb72
NC
2611=for Pod::Functions prepare hosts file for use
2612
a0d0e21e
LW
2613=item setnetent STAYOPEN
2614
c17cdb72
NC
2615=for Pod::Functions prepare networks file for use
2616
a0d0e21e
LW
2617=item setprotoent STAYOPEN
2618
c17cdb72
NC
2619=for Pod::Functions prepare protocols file for use
2620
a0d0e21e
LW
2621=item setservent STAYOPEN
2622
c17cdb72
NC
2623=for Pod::Functions prepare services file for use
2624
a0d0e21e
LW
2625=item endpwent
2626
c17cdb72
NC
2627=for Pod::Functions be done using passwd file
2628
a0d0e21e
LW
2629=item endgrent
2630
c17cdb72
NC
2631=for Pod::Functions be done using group file
2632
a0d0e21e
LW
2633=item endhostent
2634
c17cdb72
NC
2635=for Pod::Functions be done using hosts file
2636
a0d0e21e
LW
2637=item endnetent
2638
c17cdb72
NC
2639=for Pod::Functions be done using networks file
2640
a0d0e21e
LW
2641=item endprotoent
2642
c17cdb72
NC
2643=for Pod::Functions be done using protocols file
2644
a0d0e21e
LW
2645=item endservent
2646
c17cdb72
NC
2647=for Pod::Functions be done using services file
2648
80d38338
TC
2649These routines are the same as their counterparts in the
2650system C library. In list context, the return values from the
a0d0e21e
LW
2651various get routines are as follows:
2652
2653 ($name,$passwd,$uid,$gid,
6ee623d5 2654 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2655 ($name,$passwd,$gid,$members) = getgr*
2656 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2657 ($name,$aliases,$addrtype,$net) = getnet*
2658 ($name,$aliases,$proto) = getproto*
2659 ($name,$aliases,$port,$proto) = getserv*
2660
3b10bc60 2661(If the entry doesn't exist you get an empty list.)
a0d0e21e 2662
4602f195
JH
2663The exact meaning of the $gcos field varies but it usually contains
2664the real name of the user (as opposed to the login name) and other
2665information pertaining to the user. Beware, however, that in many
2666system users are able to change this information and therefore it
106325ad 2667cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2668L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2669login shell, are also tainted, for the same reason.
4602f195 2670
5a964f20 2671In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2672lookup by name, in which case you get the other thing, whatever it is.
2673(If the entry doesn't exist you get the undefined value.) For example:
2674
5a964f20
TC
2675 $uid = getpwnam($name);
2676 $name = getpwuid($num);
2677 $name = getpwent();
2678 $gid = getgrnam($name);
08a33e13 2679 $name = getgrgid($num);
5a964f20
TC
2680 $name = getgrent();
2681 #etc.
a0d0e21e 2682
4602f195 2683In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2684in that they are unsupported on many systems. If the
4602f195
JH
2685$quota is unsupported, it is an empty scalar. If it is supported, it
2686usually encodes the disk quota. If the $comment field is unsupported,
2687it is an empty scalar. If it is supported it usually encodes some
2688administrative comment about the user. In some systems the $quota
2689field may be $change or $age, fields that have to do with password
2690aging. In some systems the $comment field may be $class. The $expire
2691field, if present, encodes the expiration period of the account or the
2692password. For the availability and the exact meaning of these fields
8f1da26d 2693in your system, please consult getpwnam(3) and your system's
4602f195
JH
2694F<pwd.h> file. You can also find out from within Perl what your
2695$quota and $comment fields mean and whether you have the $expire field
2696by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2697C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2698files are supported only if your vendor has implemented them in the
4602f195 2699intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2700shadow versions if you're running under privilege or if there exists
cf264981 2701the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2702and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2703facility are unlikely to be supported.
6ee623d5 2704
a3390c9f 2705The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2706the login names of the members of the group.
2707
2708For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2709C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2710C<@addrs> value returned by a successful call is a list of raw
2711addresses returned by the corresponding library call. In the
2712Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2713by saying something like:
2714
f337b084 2715 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2716
2b5ab1e7
TC
2717The Socket library makes this slightly easier:
2718
2719 use Socket;
2720 $iaddr = inet_aton("127.1"); # or whatever address
2721 $name = gethostbyaddr($iaddr, AF_INET);
2722
2723 # or going the other way
19799a22 2724 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2725
d760c846
GS
2726In the opposite way, to resolve a hostname to the IP address
2727you can write this:
2728
2729 use Socket;
2730 $packed_ip = gethostbyname("www.perl.org");
2731 if (defined $packed_ip) {
2732 $ip_address = inet_ntoa($packed_ip);
2733 }
2734
b018eaf1 2735Make sure C<gethostbyname()> is called in SCALAR context and that
d760c846
GS
2736its return value is checked for definedness.
2737
0d043efa
FC
2738The C<getprotobynumber> function, even though it only takes one argument,
2739has the precedence of a list operator, so beware:
2740
2741 getprotobynumber $number eq 'icmp' # WRONG
2742 getprotobynumber($number eq 'icmp') # actually means this
2743 getprotobynumber($number) eq 'icmp' # better this way
2744
19799a22
GS
2745If you get tired of remembering which element of the return list
2746contains which return value, by-name interfaces are provided
2747in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2748C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2749and C<User::grent>. These override the normal built-ins, supplying
2750versions that return objects with the appropriate names
2751for each field. For example:
5a964f20
TC
2752
2753 use File::stat;
2754 use User::pwent;
2755 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2756
a3390c9f 2757Even though it looks as though they're the same method calls (uid),
b76cc8ba 2758they aren't, because a C<File::stat> object is different from
19799a22 2759a C<User::pwent> object.
5a964f20 2760
ea9eb35a 2761Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2762
a0d0e21e 2763=item getsockname SOCKET
d74e8afc 2764X<getsockname>
a0d0e21e 2765
c17cdb72
NC
2766=for Pod::Functions retrieve the sockaddr for a given socket
2767
19799a22
GS
2768Returns the packed sockaddr address of this end of the SOCKET connection,
2769in case you don't know the address because you have several different
2770IPs that the connection might have come in on.
a0d0e21e 2771
4633a7c4
LW
2772 use Socket;
2773 $mysockaddr = getsockname(SOCK);
19799a22 2774 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2775 printf "Connect to %s [%s]\n",
19799a22
GS
2776 scalar gethostbyaddr($myaddr, AF_INET),
2777 inet_ntoa($myaddr);
a0d0e21e
LW
2778
2779=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2780X<getsockopt>
a0d0e21e 2781
c17cdb72
NC
2782=for Pod::Functions get socket options on a given socket
2783
636e6b1f
TH
2784Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2785Options may exist at multiple protocol levels depending on the socket
2786type, but at least the uppermost socket level SOL_SOCKET (defined in the
391b733c 2787C<Socket> module) will exist. To query options at another level the
636e6b1f 2788protocol number of the appropriate protocol controlling the option
391b733c 2789should be supplied. For example, to indicate that an option is to be
636e6b1f 2790interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2791number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2792
80d38338 2793The function returns a packed string representing the requested socket
3b10bc60 2794option, or C<undef> on error, with the reason for the error placed in
391b733c 2795C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2796consult getsockopt(2) for details. A common case is that the option is an
2797integer, in which case the result is a packed integer, which you can decode
2798using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2799
8f1da26d 2800Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2801
4852725b 2802 use Socket qw(:all);
636e6b1f
TH
2803
2804 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2805 or die "Could not determine the protocol number for tcp";
4852725b
DD
2806 # my $tcp = IPPROTO_TCP; # Alternative
2807 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2808 or die "getsockopt TCP_NODELAY: $!";
636e6b1f
TH
2809 my $nodelay = unpack("I", $packed);
2810 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2811
ea9eb35a 2812Portability issues: L<perlport/getsockopt>.
a0d0e21e 2813
15a348aa 2814=item given EXPR BLOCK
2815X<given>
2816
2817=item given BLOCK
2818
c17cdb72
NC
2819=for Pod::Functions !RT #108848
2820
391b733c
FC
2821C<given> is analogous to the C<switch>
2822keyword in other languages. C<given>
15a348aa 2823and C<when> are used in Perl to implement C<switch>/C<case> like statements.
8f1da26d 2824Only available after Perl 5.10. For example:
15a348aa 2825
8f1da26d 2826 use v5.10;
15a348aa 2827 given ($fruit) {
2828 when (/apples?/) {
2829 print "I like apples."
2830 }
2831 when (/oranges?/) {
2832 print "I don't like oranges."
2833 }
2834 default {
2835 print "I don't like anything"
2836 }
2837 }
2838
48238296 2839See L<perlsyn/"Switch Statements"> for detailed information.
15a348aa 2840
a0d0e21e 2841=item glob EXPR
d74e8afc 2842X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2843
0a753a76
PP
2844=item glob
2845
c17cdb72
NC
2846=for Pod::Functions expand filenames using wildcards
2847
d9a9d457 2848In list context, returns a (possibly empty) list of filename expansions on
391b733c 2849the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
d9a9d457 2850scalar context, glob iterates through such filename expansions, returning
391b733c
FC
2851undef when the list is exhausted. This is the internal function
2852implementing the C<< <*.c> >> operator, but you can use it directly. If
d9a9d457
JL
2853EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2854more detail in L<perlop/"I/O Operators">.
a0d0e21e 2855
80d38338
TC
2856Note that C<glob> splits its arguments on whitespace and treats
2857each segment as separate pattern. As such, C<glob("*.c *.h")>
2858matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2859C<glob(".* *")> matches all files in the current working directory.
a91bb7b1
TC
2860If you want to glob filenames that might contain whitespace, you'll
2861have to use extra quotes around the spacey filename to protect it.
2862For example, to glob filenames that have an C<e> followed by a space
2863followed by an C<f>, use either of:
2864
2865 @spacies = <"*e f*">;
2866 @spacies = glob '"*e f*"';
2867 @spacies = glob q("*e f*");
2868
2869If you had to get a variable through, you could do this:
2870
2871 @spacies = glob "'*${var}e f*'";
2872 @spacies = glob qq("*${var}e f*");
80d38338
TC
2873
2874If non-empty braces are the only wildcard characters used in the
2875C<glob>, no filenames are matched, but potentially many strings
2876are returned. For example, this produces nine strings, one for
2877each pairing of fruits and colors:
2878
2879 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2880
3a4b19e4 2881Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2882C<File::Glob> extension. See L<File::Glob> for details, including
2883C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2884
ea9eb35a 2885Portability issues: L<perlport/glob>.
2886
a0d0e21e 2887=item gmtime EXPR
d74e8afc 2888X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2889
ce2984c3
PF
2890=item gmtime
2891
c17cdb72
NC
2892=for Pod::Functions convert UNIX time into record or string using Greenwich time
2893
4509d391 2894Works just like L</localtime> but the returned values are
435fbc73 2895localized for the standard Greenwich time zone.
a0d0e21e 2896
a3390c9f
FC
2897Note: When called in list context, $isdst, the last value
2898returned by gmtime, is always C<0>. There is no
435fbc73 2899Daylight Saving Time in GMT.
0a753a76 2900
ea9eb35a 2901Portability issues: L<perlport/gmtime>.
62aa5637 2902
a0d0e21e 2903=item goto LABEL
d74e8afc 2904X<goto> X<jump> X<jmp>
a0d0e21e 2905
748a9306
LW
2906=item goto EXPR
2907
a0d0e21e
LW
2908=item goto &NAME
2909
c17cdb72
NC
2910=for Pod::Functions create spaghetti code
2911
b500e03b 2912The C<goto-LABEL> form finds the statement labeled with LABEL and
391b733c 2913resumes execution there. It can't be used to get out of a block or
b500e03b
GG
2914subroutine given to C<sort>. It can be used to go almost anywhere
2915else within the dynamic scope, including out of subroutines, but it's
2916usually better to use some other construct such as C<last> or C<die>.
2917The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2918(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2919does not offer named loops combined with loop control. Perl does, and
2920this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2921
7660c0ab
A
2922The C<goto-EXPR> form expects a label name, whose scope will be resolved
2923dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2924necessarily recommended if you're optimizing for maintainability:
2925
2926 goto ("FOO", "BAR", "GLARCH")[$i];
2927
887d89fd 2928As shown in this example, C<goto-EXPR> is exempt from the "looks like a
391b733c
FC
2929function" rule. A pair of parentheses following it does not (necessarily)
2930delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
887d89fd 2931
b500e03b 2932Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2933deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2934go into any construct that requires initialization, such as a
2935subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2936construct that is optimized away.
b500e03b 2937
1b6921cb
BT
2938The C<goto-&NAME> form is quite different from the other forms of
2939C<goto>. In fact, it isn't a goto in the normal sense at all, and
2940doesn't have the stigma associated with other gotos. Instead, it
2941exits the current subroutine (losing any changes set by local()) and
2942immediately calls in its place the named subroutine using the current
2943value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2944load another subroutine and then pretend that the other subroutine had
2945been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2946in the current subroutine are propagated to the other subroutine.)
2947After the C<goto>, not even C<caller> will be able to tell that this
2948routine was called first.
2949
2950NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2951containing a code reference or a block that evaluates to a code
6cb9131c 2952reference.
a0d0e21e
LW
2953
2954=item grep BLOCK LIST
d74e8afc 2955X<grep>
a0d0e21e
LW
2956
2957=item grep EXPR,LIST
2958
c17cdb72
NC
2959=for Pod::Functions locate elements in a list test true against a given criterion
2960
2b5ab1e7
TC
2961This is similar in spirit to, but not the same as, grep(1) and its
2962relatives. In particular, it is not limited to using regular expressions.
2f9daede 2963
a0d0e21e 2964Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2965C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2966elements for which the expression evaluated to true. In scalar
2967context, returns the number of times the expression was true.
a0d0e21e
LW
2968
2969 @foo = grep(!/^#/, @bar); # weed out comments
2970
2971or equivalently,
2972
2973 @foo = grep {!/^#/} @bar; # weed out comments
2974
be3174d2
GS
2975Note that C<$_> is an alias to the list value, so it can be used to
2976modify the elements of the LIST. While this is useful and supported,
2977it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2978Similarly, grep returns aliases into the original list, much as a for
2979loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2980element of a list returned by grep (for example, in a C<foreach>, C<map>
2981or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2982This is usually something to be avoided when writing clear code.
a0d0e21e 2983
a4fb8298 2984If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2985been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2986the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2987can't be seen from the outside, avoiding any potential side-effects.
2988
19799a22 2989See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2990
a0d0e21e 2991=item hex EXPR
d74e8afc 2992X<hex> X<hexadecimal>
a0d0e21e 2993
54310121 2994=item hex
bbce6d69 2995
c17cdb72
NC
2996=for Pod::Functions convert a string to a hexadecimal number
2997
2b5ab1e7 2998Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2999(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 3000L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
3001
3002 print hex '0xAf'; # prints '175'
3003 print hex 'aF'; # same
a0d0e21e 3004
19799a22 3005Hex strings may only represent integers. Strings that would cause
53305cf1 3006integer overflow trigger a warning. Leading whitespace is not stripped,
391b733c 3007unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 3008L</sprintf>, and L</unpack>.
19799a22 3009
ce2984c3 3010=item import LIST
d74e8afc 3011X<import>
a0d0e21e 3012
c17cdb72
NC
3013=for Pod::Functions patch a module's namespace into your own
3014
19799a22 3015There is no builtin C<import> function. It is just an ordinary
4633a7c4 3016method (subroutine) defined (or inherited) by modules that wish to export
19799a22 3017names to another module. The C<use> function calls the C<import> method
cea6626f 3018for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
3019
3020=item index STR,SUBSTR,POSITION
d74e8afc 3021X<index> X<indexOf> X<InStr>
a0d0e21e
LW
3022
3023=item index STR,SUBSTR
3024
c17cdb72
NC
3025=for Pod::Functions find a substring within a string
3026
2b5ab1e7
TC
3027The index function searches for one string within another, but without
3028the wildcard-like behavior of a full regular-expression pattern match.
3029It returns the position of the first occurrence of SUBSTR in STR at
3030or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
3031beginning of the string. POSITION before the beginning of the string
3032or after its end is treated as if it were the beginning or the end,
e1dccc0d
Z
3033respectively. POSITION and the return value are based at zero.
3034If the substring is not found, C<index> returns -1.
a0d0e21e
LW
3035
3036=item int EXPR
f723aae1 3037X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 3038
54310121 3039=item int
bbce6d69 3040
c17cdb72
NC
3041=for Pod::Functions get the integer portion of a number
3042
7660c0ab 3043Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 3044You should not use this function for rounding: one because it truncates
3b10bc60 3045towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
3046numbers can sometimes produce counterintuitive results. For example,
3047C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3048because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 3049the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 3050functions will serve you better than will int().
a0d0e21e
LW
3051
3052=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 3053X<ioctl>
a0d0e21e 3054
c17cdb72
NC
3055=for Pod::Functions system-dependent device control system call
3056
2b5ab1e7 3057Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 3058
5ed4f2ec 3059 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 3060