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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
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3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
8f1da26d 15operator. A unary operator generally provides scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
3b10bc60 17contexts for its arguments. If it does both, scalar arguments
18come first and list argument follow, and there can only ever
19be one such list argument. For instance, splice() has three scalar
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20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
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22
23In the syntax descriptions that follow, list operators that expect a
3b10bc60 24list (and provide list context for elements of the list) are shown
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25with LIST as an argument. Such a list may consist of any combination
26of scalar arguments or list values; the list values will be included
27in the list as if each individual element were interpolated at that
28point in the list, forming a longer single-dimensional list value.
8bdbc703 29Commas should separate literal elements of the LIST.
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30
31Any function in the list below may be used either with or without
32parentheses around its arguments. (The syntax descriptions omit the
3b10bc60 33parentheses.) If you use parentheses, the simple but occasionally
34surprising rule is this: It I<looks> like a function, therefore it I<is> a
a0d0e21e 35function, and precedence doesn't matter. Otherwise it's a list
3b10bc60 36operator or unary operator, and precedence does matter. Whitespace
37between the function and left parenthesis doesn't count, so sometimes
38you need to be careful:
a0d0e21e 39
5ed4f2ec 40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
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45
46If you run Perl with the B<-w> switch it can warn you about this. For
47example, the third line above produces:
48
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
51
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52A few functions take no arguments at all, and therefore work as neither
53unary nor list operators. These include such functions as C<time>
54and C<endpwent>. For example, C<time+86_400> always means
55C<time() + 86_400>.
56
a0d0e21e 57For functions that can be used in either a scalar or list context,
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58nonabortive failure is generally indicated in scalar context by
59returning the undefined value, and in list context by returning the
3b10bc60 60empty list.
a0d0e21e 61
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62Remember the following important rule: There is B<no rule> that relates
63the behavior of an expression in list context to its behavior in scalar
64context, or vice versa. It might do two totally different things.
80d38338 65Each operator and function decides which sort of value would be most
2b5ab1e7 66appropriate to return in scalar context. Some operators return the
5a964f20 67length of the list that would have been returned in list context. Some
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68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
d74e8afc 72X<context>
a0d0e21e 73
d1be9408 74A named array in scalar context is quite different from what would at
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75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
3b10bc60 81In general, functions in Perl that serve as wrappers for system calls ("syscalls")
5dac7880 82of the same name (like chown(2), fork(2), closedir(2), etc.) return
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83true when they succeed and C<undef> otherwise, as is usually mentioned
84in the descriptions below. This is different from the C interfaces,
5dac7880 85which return C<-1> on failure. Exceptions to this rule include C<wait>,
19799a22 86C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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87variable on failure. Other functions do not, except accidentally.
88
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89Extension modules can also hook into the Perl parser to define new
90kinds of keyword-headed expression. These may look like functions, but
91may also look completely different. The syntax following the keyword
92is defined entirely by the extension. If you are an implementor, see
93L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
94a module, see the module's documentation for details of the syntax that
95it defines.
96
cb1a09d0 97=head2 Perl Functions by Category
d74e8afc 98X<function>
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99
100Here are Perl's functions (including things that look like
5a964f20 101functions, like some keywords and named operators)
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102arranged by category. Some functions appear in more
103than one place.
104
13a2d996 105=over 4
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106
107=item Functions for SCALARs or strings
d74e8afc 108X<scalar> X<string> X<character>
cb1a09d0 109
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110=for Pod::Functions =String
111
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112C<chomp>, C<chop>, C<chr>, C<crypt>, C<fc>, C<hex>, C<index>, C<lc>,
113C<lcfirst>, C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
945c54fd 114C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
cb1a09d0 115
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116C<fc> is available only if the C<"fc"> feature is enabled or if it is
117prefixed with C<CORE::>. The C<"fc"> feature is enabled automatically
3dd9a840 118with a C<use v5.16> (or higher) declaration in the current scope.
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119
120
cb1a09d0 121=item Regular expressions and pattern matching
d74e8afc 122X<regular expression> X<regex> X<regexp>
cb1a09d0 123
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124=for Pod::Functions =Regexp
125
f5fa2679 126C<m//>, C<pos>, C<qr//>, C<quotemeta>, C<s///>, C<split>, C<study>
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127
128=item Numeric functions
d74e8afc 129X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 130
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131=for Pod::Functions =Math
132
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133C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
134C<sin>, C<sqrt>, C<srand>
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135
136=item Functions for real @ARRAYs
d74e8afc 137X<array>
cb1a09d0 138
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139=for Pod::Functions =ARRAY
140
a5ce339c 141C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
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142
143=item Functions for list data
d74e8afc 144X<list>
cb1a09d0 145
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146=for Pod::Functions =LIST
147
1dc8ecb8 148C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
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149
150=item Functions for real %HASHes
d74e8afc 151X<hash>
cb1a09d0 152
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153=for Pod::Functions =HASH
154
22fae026 155C<delete>, C<each>, C<exists>, C<keys>, C<values>
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156
157=item Input and output functions
d74e8afc 158X<I/O> X<input> X<output> X<dbm>
cb1a09d0 159
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160=for Pod::Functions =I/O
161
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162C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
163C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
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164C<readdir>, C<readline> C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>,
165C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>,
166C<truncate>, C<warn>, C<write>
cb1a09d0 167
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168C<say> is available only if the C<"say"> feature is enabled or if it is
169prefixed with C<CORE::>. The C<"say"> feature is enabled automatically
170with a C<use v5.10> (or higher) declaration in the current scope.
171
5dac7880 172=item Functions for fixed-length data or records
cb1a09d0 173
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174=for Pod::Functions =Binary
175
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176C<pack>, C<read>, C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<unpack>,
177C<vec>
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178
179=item Functions for filehandles, files, or directories
d74e8afc 180X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 181
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182=for Pod::Functions =File
183
22fae026 184C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 185C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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186C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
187C<umask>, C<unlink>, C<utime>
cb1a09d0 188
cf264981 189=item Keywords related to the control flow of your Perl program
d74e8afc 190X<control flow>
cb1a09d0 191
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192=for Pod::Functions =Flow
193
dba7b065 194C<break>, C<caller>, C<continue>, C<die>, C<do>,
7289c5e6 195C<dump>, C<eval>, C<evalbytes> C<exit>,
cfa52385 196C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
17d15541 197C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray>
84ed0108 198
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199C<break> is available only if you enable the experimental C<"switch">
200feature or use the C<CORE::> prefix. The C<"switch"> feature also enables
201the C<default>, C<given> and C<when> statements, which are documented in
202L<perlsyn/"Switch Statements">. The C<"switch"> feature is enabled
203automatically with a C<use v5.10> (or higher) declaration in the current
204scope. In Perl v5.14 and earlier, C<continue> required the C<"switch">
205feature, like the other keywords.
206
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207C<evalbytes> is only available with with the C<"evalbytes"> feature (see
208L<feature>) or if prefixed with C<CORE::>. C<__SUB__> is only available
209with with the C<"current_sub"> feature or if prefixed with C<CORE::>. Both
210the C<"evalbytes"> and C<"current_sub"> features are enabled automatically
211with a C<use v5.16> (or higher) declaration in the current scope.
cb1a09d0 212
54310121 213=item Keywords related to scoping
cb1a09d0 214
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215=for Pod::Functions =Namespace
216
8f1da26d 217C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
36fb85f3 218
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219C<state> is available only if the C<"state"> feature is enabled or if it is
220prefixed with C<CORE::>. The C<"state"> feature is enabled automatically
221with a C<use v5.10> (or higher) declaration in the current scope.
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222
223=item Miscellaneous functions
224
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225=for Pod::Functions =Misc
226
17d15541 227C<defined>, C<formline>, C<lock>, C<prototype>, C<reset>, C<scalar>, C<undef>
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228
229=item Functions for processes and process groups
d74e8afc 230X<process> X<pid> X<process id>
cb1a09d0 231
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232=for Pod::Functions =Process
233
22fae026 234C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
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235C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>,
236C<setpriority>, C<sleep>, C<system>,
22fae026 237C<times>, C<wait>, C<waitpid>
cb1a09d0 238
3b10bc60 239=item Keywords related to Perl modules
d74e8afc 240X<module>
cb1a09d0 241
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242=for Pod::Functions =Modules
243
22fae026 244C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 245
353c6505 246=item Keywords related to classes and object-orientation
d74e8afc 247X<object> X<class> X<package>
cb1a09d0 248
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249=for Pod::Functions =Objects
250
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251C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
252C<untie>, C<use>
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253
254=item Low-level socket functions
d74e8afc 255X<socket> X<sock>
cb1a09d0 256
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257=for Pod::Functions =Socket
258
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259C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
260C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 261C<socket>, C<socketpair>
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262
263=item System V interprocess communication functions
d74e8afc 264X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 265
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266=for Pod::Functions =SysV
267
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268C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
269C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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270
271=item Fetching user and group info
d74e8afc 272X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 273
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274=for Pod::Functions =User
275
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276C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
277C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
278C<getpwuid>, C<setgrent>, C<setpwent>
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279
280=item Fetching network info
d74e8afc 281X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 282
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283=for Pod::Functions =Network
284
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285C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
286C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
287C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
288C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
289C<setnetent>, C<setprotoent>, C<setservent>
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290
291=item Time-related functions
d74e8afc 292X<time> X<date>
cb1a09d0 293
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294=for Pod::Functions =Time
295
22fae026 296C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 297
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298=item Non-function keywords
299
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300=for Pod::Functions =!Non-functions
301
f5fa2679 302C<and>, C<AUTOLOAD>, C<BEGIN>, C<CHECK>, C<cmp>, C<CORE>, C<__DATA__>,
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303C<default>, C<DESTROY>, C<else>, C<elseif>, C<elsif>, C<END>, C<__END__>,
304C<eq>, C<for>, C<foreach>, C<ge>, C<given>, C<gt>, C<if>, C<INIT>, C<le>,
305C<lt>, C<ne>, C<not>, C<or>, C<UNITCHECK>, C<unless>, C<until>, C<when>,
306C<while>, C<x>, C<xor>
8f0d6a61 307
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308=back
309
60f9f73c 310=head2 Portability
d74e8afc 311X<portability> X<Unix> X<portable>
60f9f73c 312
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313Perl was born in Unix and can therefore access all common Unix
314system calls. In non-Unix environments, the functionality of some
8f1da26d 315Unix system calls may not be available or details of the available
2b5ab1e7 316functionality may differ slightly. The Perl functions affected
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317by this are:
318
319C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
320C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
321C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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322C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
323C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 324C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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325C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
326C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
327C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 328C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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329C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
330C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
331C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 332C<shmwrite>, C<socket>, C<socketpair>,
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333C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
334C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 335C<utime>, C<wait>, C<waitpid>
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336
337For more information about the portability of these functions, see
338L<perlport> and other available platform-specific documentation.
339
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340=head2 Alphabetical Listing of Perl Functions
341
3b10bc60 342=over
a0d0e21e 343
5b3c99c0 344=item -X FILEHANDLE
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345X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
346X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 347
5b3c99c0 348=item -X EXPR
a0d0e21e 349
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350=item -X DIRHANDLE
351
5b3c99c0 352=item -X
a0d0e21e 353
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354=for Pod::Functions a file test (-r, -x, etc)
355
a0d0e21e 356A file test, where X is one of the letters listed below. This unary
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357operator takes one argument, either a filename, a filehandle, or a dirhandle,
358and tests the associated file to see if something is true about it. If the
7660c0ab 359argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 360Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 361the undefined value if the file doesn't exist. Despite the funny
d0821a6a 362names, precedence is the same as any other named unary operator. The
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363operator may be any of:
364
5ed4f2ec 365 -r File is readable by effective uid/gid.
366 -w File is writable by effective uid/gid.
367 -x File is executable by effective uid/gid.
368 -o File is owned by effective uid.
a0d0e21e 369
5ed4f2ec 370 -R File is readable by real uid/gid.
371 -W File is writable by real uid/gid.
372 -X File is executable by real uid/gid.
373 -O File is owned by real uid.
a0d0e21e 374
5ed4f2ec 375 -e File exists.
376 -z File has zero size (is empty).
377 -s File has nonzero size (returns size in bytes).
a0d0e21e 378
5ed4f2ec 379 -f File is a plain file.
380 -d File is a directory.
381 -l File is a symbolic link.
382 -p File is a named pipe (FIFO), or Filehandle is a pipe.
383 -S File is a socket.
384 -b File is a block special file.
385 -c File is a character special file.
386 -t Filehandle is opened to a tty.
a0d0e21e 387
5ed4f2ec 388 -u File has setuid bit set.
389 -g File has setgid bit set.
390 -k File has sticky bit set.
a0d0e21e 391
5ed4f2ec 392 -T File is an ASCII text file (heuristic guess).
393 -B File is a "binary" file (opposite of -T).
a0d0e21e 394
5ed4f2ec 395 -M Script start time minus file modification time, in days.
396 -A Same for access time.
397 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 398
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399Example:
400
401 while (<>) {
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402 chomp;
403 next unless -f $_; # ignore specials
404 #...
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405 }
406
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407Note that C<-s/a/b/> does not do a negated substitution. Saying
408C<-exp($foo)> still works as expected, however: only single letters
409following a minus are interpreted as file tests.
410
411These operators are exempt from the "looks like a function rule" described
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412above. That is, an opening parenthesis after the operator does not affect
413how much of the following code constitutes the argument. Put the opening
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414parentheses before the operator to separate it from code that follows (this
415applies only to operators with higher precedence than unary operators, of
416course):
417
418 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
419 (-s $file) + 1024 # correct
420
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421The interpretation of the file permission operators C<-r>, C<-R>,
422C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
423of the file and the uids and gids of the user. There may be other
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424reasons you can't actually read, write, or execute the file: for
425example network filesystem access controls, ACLs (access control lists),
426read-only filesystems, and unrecognized executable formats. Note
427that the use of these six specific operators to verify if some operation
428is possible is usually a mistake, because it may be open to race
429conditions.
5ff3f7a4 430
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431Also note that, for the superuser on the local filesystems, the C<-r>,
432C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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433if any execute bit is set in the mode. Scripts run by the superuser
434may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 435or temporarily set their effective uid to something else.
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436
437If you are using ACLs, there is a pragma called C<filetest> that may
438produce more accurate results than the bare stat() mode bits.
5dac7880
FC
439When under C<use filetest 'access'> the above-mentioned filetests
440test whether the permission can(not) be granted using the
3b10bc60 441access(2) family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
442under this pragma return true even if there are no execute permission
443bits set (nor any extra execute permission ACLs). This strangeness is
391b733c 444due to the underlying system calls' definitions. Note also that, due to
ecae030f
MO
445the implementation of C<use filetest 'access'>, the C<_> special
446filehandle won't cache the results of the file tests when this pragma is
447in effect. Read the documentation for the C<filetest> pragma for more
448information.
5ff3f7a4 449
a0d0e21e
LW
450The C<-T> and C<-B> switches work as follows. The first block or so of the
451file is examined for odd characters such as strange control codes or
61eff3bc 452characters with the high bit set. If too many strange characters (>30%)
cf264981 453are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 454containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 455or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 456rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 457file, or a file at EOF when testing a filehandle. Because you have to
4633a7c4
LW
458read a file to do the C<-T> test, on most occasions you want to use a C<-f>
459against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 460
5dac7880 461If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 462the special filehandle consisting of a solitary underline, then the stat
a0d0e21e
LW
463structure of the previous file test (or stat operator) is used, saving
464a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 465that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 466symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 467an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 468Example:
a0d0e21e
LW
469
470 print "Can do.\n" if -r $a || -w _ || -x _;
471
472 stat($filename);
473 print "Readable\n" if -r _;
474 print "Writable\n" if -w _;
475 print "Executable\n" if -x _;
476 print "Setuid\n" if -u _;
477 print "Setgid\n" if -g _;
478 print "Sticky\n" if -k _;
479 print "Text\n" if -T _;
480 print "Binary\n" if -B _;
481
fbb0b3b3
RGS
482As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
483test operators, in a way that C<-f -w -x $file> is equivalent to
391b733c 484C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
fbb0b3b3
RGS
485the return value of C<-f $file> as an argument to another filetest
486operator, no special magic will happen.)
487
bee96257 488Portability issues: L<perlport/-X>.
ea9eb35a 489
bade7fbc
TC
490To avoid confusing would-be users of your code with mysterious
491syntax errors, put something like this at the top of your script:
492
493 use 5.010; # so filetest ops can stack
494
a0d0e21e 495=item abs VALUE
d74e8afc 496X<abs> X<absolute>
a0d0e21e 497
54310121 498=item abs
bbce6d69 499
c17cdb72
NC
500=for Pod::Functions absolute value function
501
a0d0e21e 502Returns the absolute value of its argument.
7660c0ab 503If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
504
505=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 506X<accept>
a0d0e21e 507
c17cdb72
NC
508=for Pod::Functions accept an incoming socket connect
509
3b10bc60 510Accepts an incoming socket connect, just as accept(2)
19799a22 511does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 512See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 513
8d2a6795
GS
514On systems that support a close-on-exec flag on files, the flag will
515be set for the newly opened file descriptor, as determined by the
516value of $^F. See L<perlvar/$^F>.
517
a0d0e21e 518=item alarm SECONDS
d74e8afc
ITB
519X<alarm>
520X<SIGALRM>
521X<timer>
a0d0e21e 522
54310121 523=item alarm
bbce6d69 524
c17cdb72
NC
525=for Pod::Functions schedule a SIGALRM
526
a0d0e21e 527Arranges to have a SIGALRM delivered to this process after the
cf264981 528specified number of wallclock seconds has elapsed. If SECONDS is not
391b733c 529specified, the value stored in C<$_> is used. (On some machines,
d400eac8
JH
530unfortunately, the elapsed time may be up to one second less or more
531than you specified because of how seconds are counted, and process
532scheduling may delay the delivery of the signal even further.)
533
534Only one timer may be counting at once. Each call disables the
535previous timer, and an argument of C<0> may be supplied to cancel the
536previous timer without starting a new one. The returned value is the
537amount of time remaining on the previous timer.
a0d0e21e 538
2bc69794
BS
539For delays of finer granularity than one second, the Time::HiRes module
540(from CPAN, and starting from Perl 5.8 part of the standard
541distribution) provides ualarm(). You may also use Perl's four-argument
542version of select() leaving the first three arguments undefined, or you
543might be able to use the C<syscall> interface to access setitimer(2) if
391b733c 544your system supports it. See L<perlfaq8> for details.
2b5ab1e7 545
80d38338
TC
546It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
547C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 548
19799a22
GS
549If you want to use C<alarm> to time out a system call you need to use an
550C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 551fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 552restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 553modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
554
555 eval {
a9a5a0dc
VP
556 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
557 alarm $timeout;
558 $nread = sysread SOCKET, $buffer, $size;
559 alarm 0;
ff68c719 560 };
ff68c719 561 if ($@) {
a9a5a0dc 562 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 563 # timed out
ff68c719
PP
564 }
565 else {
5ed4f2ec 566 # didn't
ff68c719
PP
567 }
568
91d81acc
JH
569For more information see L<perlipc>.
570
ea9eb35a 571Portability issues: L<perlport/alarm>.
572
a0d0e21e 573=item atan2 Y,X
d74e8afc 574X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e 575
c17cdb72
NC
576=for Pod::Functions arctangent of Y/X in the range -PI to PI
577
a0d0e21e
LW
578Returns the arctangent of Y/X in the range -PI to PI.
579
ca6e1c26 580For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
581function, or use the familiar relation:
582
583 sub tan { sin($_[0]) / cos($_[0]) }
584
a1021d57
RGS
585The return value for C<atan2(0,0)> is implementation-defined; consult
586your atan2(3) manpage for more information.
bf5f1b4c 587
ea9eb35a 588Portability issues: L<perlport/atan2>.
589
a0d0e21e 590=item bind SOCKET,NAME
d74e8afc 591X<bind>
a0d0e21e 592
c17cdb72
NC
593=for Pod::Functions binds an address to a socket
594
3b10bc60 595Binds a network address to a socket, just as bind(2)
19799a22 596does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
597packed address of the appropriate type for the socket. See the examples in
598L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 599
fae2c0fb 600=item binmode FILEHANDLE, LAYER
d74e8afc 601X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 602
a0d0e21e
LW
603=item binmode FILEHANDLE
604
c17cdb72
NC
605=for Pod::Functions prepare binary files for I/O
606
1cbfc93d
NIS
607Arranges for FILEHANDLE to be read or written in "binary" or "text"
608mode on systems where the run-time libraries distinguish between
609binary and text files. If FILEHANDLE is an expression, the value is
610taken as the name of the filehandle. Returns true on success,
b5fe5ca2 611otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 612
8f1da26d 613On some systems (in general, DOS- and Windows-based systems) binmode()
d807c6f4 614is necessary when you're not working with a text file. For the sake
d7a0d798
FC
615of portability it is a good idea always to use it when appropriate,
616and never to use it when it isn't appropriate. Also, people can
8f1da26d 617set their I/O to be by default UTF8-encoded Unicode, not bytes.
d807c6f4
JH
618
619In other words: regardless of platform, use binmode() on binary data,
d7a0d798 620like images, for example.
d807c6f4
JH
621
622If LAYER is present it is a single string, but may contain multiple
391b733c 623directives. The directives alter the behaviour of the filehandle.
d7a0d798 624When LAYER is present, using binmode on a text file makes sense.
d807c6f4 625
fae2c0fb 626If LAYER is omitted or specified as C<:raw> the filehandle is made
391b733c 627suitable for passing binary data. This includes turning off possible CRLF
0226bbdb 628translation and marking it as bytes (as opposed to Unicode characters).
749683d2 629Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 630Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
631Other layers that would affect the binary nature of the stream are
391b733c 632I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 633PERLIO environment variable.
01e6739c 634
3b10bc60 635The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
636form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
637establish default I/O layers. See L<open>.
638
fae2c0fb
RGS
639I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
640in "Programming Perl, 3rd Edition". However, since the publishing of this
641book, by many known as "Camel III", the consensus of the naming of this
642functionality has moved from "discipline" to "layer". All documentation
643of this version of Perl therefore refers to "layers" rather than to
644"disciplines". Now back to the regularly scheduled documentation...>
645
8f1da26d 646To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
6902c96a 647C<:utf8> just marks the data as UTF-8 without further checking,
8f1da26d 648while C<:encoding(UTF-8)> checks the data for actually being valid
391b733c 649UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 650
ed53a2bb 651In general, binmode() should be called after open() but before any I/O
3b10bc60 652is done on the filehandle. Calling binmode() normally flushes any
01e6739c 653pending buffered output data (and perhaps pending input data) on the
fae2c0fb 654handle. An exception to this is the C<:encoding> layer that
d7a0d798 655changes the default character encoding of the handle; see L</open>.
fae2c0fb 656The C<:encoding> layer sometimes needs to be called in
3874323d
JH
657mid-stream, and it doesn't flush the stream. The C<:encoding>
658also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 659internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 660
19799a22 661The operating system, device drivers, C libraries, and Perl run-time
8f1da26d
TC
662system all conspire to let the programmer treat a single
663character (C<\n>) as the line terminator, irrespective of external
30168b04
GS
664representation. On many operating systems, the native text file
665representation matches the internal representation, but on some
666platforms the external representation of C<\n> is made up of more than
667one character.
668
8f1da26d
TC
669All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
670a single character to end each line in the external representation of text
671(even though that single character is CARRIAGE RETURN on old, pre-Darwin
391b733c 672flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
8f1da26d
TC
673systems like OS/2, DOS, and the various flavors of MS-Windows, your program
674sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
675two characters C<\cM\cJ>. That means that if you don't use binmode() on
676these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
677input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
678output. This is what you want for text files, but it can be disastrous for
679binary files.
30168b04
GS
680
681Another consequence of using binmode() (on some systems) is that
682special end-of-file markers will be seen as part of the data stream.
d7a0d798
FC
683For systems from the Microsoft family this means that, if your binary
684data contain C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
685the file, unless you use binmode().
686
3b10bc60 687binmode() is important not only for readline() and print() operations,
30168b04
GS
688but also when using read(), seek(), sysread(), syswrite() and tell()
689(see L<perlport> for more details). See the C<$/> and C<$\> variables
690in L<perlvar> for how to manually set your input and output
691line-termination sequences.
a0d0e21e 692
ea9eb35a 693Portability issues: L<perlport/binmode>.
694
4633a7c4 695=item bless REF,CLASSNAME
d74e8afc 696X<bless>
a0d0e21e
LW
697
698=item bless REF
699
c17cdb72
NC
700=for Pod::Functions create an object
701
2b5ab1e7
TC
702This function tells the thingy referenced by REF that it is now an object
703in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 704is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 705it returns the reference for convenience. Always use the two-argument
cf264981 706version if a derived class might inherit the function doing the blessing.
82e1c0d9 707SeeL<perlobj> for more about the blessing (and blessings) of objects.
a0d0e21e 708
57668c4d 709Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 710Namespaces with all lowercase names are considered reserved for
391b733c 711Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
712confusion, you may wish to avoid such package names as well. Make sure
713that CLASSNAME is a true value.
60ad88b8
GS
714
715See L<perlmod/"Perl Modules">.
716
0d863452
RH
717=item break
718
d9b04284 719=for Pod::Functions +switch break out of a C<given> block
c17cdb72 720
0d863452
RH
721Break out of a C<given()> block.
722
8f1da26d 723This keyword is enabled by the C<"switch"> feature: see
4a904372
FC
724L<feature> for more information. You can also access it by
725prefixing it with C<CORE::>. Alternately, include a C<use
8f1da26d 726v5.10> or later to the current scope.
0d863452 727
a0d0e21e 728=item caller EXPR
d74e8afc 729X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
730
731=item caller
732
c17cdb72
NC
733=for Pod::Functions get context of the current subroutine call
734
5a964f20 735Returns the context of the current subroutine call. In scalar context,
80d38338
TC
736returns the caller's package name if there I<is> a caller (that is, if
737we're in a subroutine or C<eval> or C<require>) and the undefined value
5a964f20 738otherwise. In list context, returns
a0d0e21e 739
ee6b43cc 740 # 0 1 2
748a9306 741 ($package, $filename, $line) = caller;
a0d0e21e
LW
742
743With EXPR, it returns some extra information that the debugger uses to
744print a stack trace. The value of EXPR indicates how many call frames
745to go back before the current one.
746
ee6b43cc 747 # 0 1 2 3 4
f3aa04c2 748 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 749
750 # 5 6 7 8 9 10
b3ca2e83 751 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 752 = caller($i);
e7ea3e70 753
951ba7fe 754Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 755call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 756C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 757C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 758C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 759$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
760each C<use> statement creates a C<require> frame inside an C<eval EXPR>
761frame.) $subroutine may also be C<(unknown)> if this particular
762subroutine happens to have been deleted from the symbol table.
763C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
764C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
765compiled with. The C<$hints> and C<$bitmask> values are subject to change
766between versions of Perl, and are not meant for external use.
748a9306 767
b3ca2e83 768C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
391b733c 769caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
b3ca2e83
NC
770of this hash, as they are the actual values stored in the optree.
771
ffe0c19d
FC
772Furthermore, when called from within the DB package in
773list context, and with an argument, caller returns more
7660c0ab 774detailed information: it sets the list variable C<@DB::args> to be the
54310121 775arguments with which the subroutine was invoked.
748a9306 776
7660c0ab 777Be aware that the optimizer might have optimized call frames away before
19799a22 778C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 779might not return information about the call frame you expect it to, for
b76cc8ba 780C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 781previous time C<caller> was called.
7660c0ab 782
8f1da26d 783Be aware that setting C<@DB::args> is I<best effort>, intended for
391b733c 784debugging or generating backtraces, and should not be relied upon. In
ca9f0cb5
NC
785particular, as C<@_> contains aliases to the caller's arguments, Perl does
786not take a copy of C<@_>, so C<@DB::args> will contain modifications the
787subroutine makes to C<@_> or its contents, not the original values at call
391b733c 788time. C<@DB::args>, like C<@_>, does not hold explicit references to its
ca9f0cb5 789elements, so under certain cases its elements may have become freed and
391b733c 790reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 791of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
792I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
793reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5 794elements), so C<@DB::args> is actually a hybrid of the current state and
391b733c 795initial state of C<@_>. Buyer beware.
ca9f0cb5 796
a0d0e21e 797=item chdir EXPR
d74e8afc
ITB
798X<chdir>
799X<cd>
f723aae1 800X<directory, change>
a0d0e21e 801
c4aca7d0
GA
802=item chdir FILEHANDLE
803
804=item chdir DIRHANDLE
805
ce2984c3
PF
806=item chdir
807
c17cdb72
NC
808=for Pod::Functions change your current working directory
809
391b733c 810Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 811changes to the directory specified by C<$ENV{HOME}>, if set; if not,
391b733c
FC
812changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
813variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
814neither is set, C<chdir> does nothing. It returns true on success,
815false otherwise. See the example under C<die>.
a0d0e21e 816
3b10bc60 817On systems that support fchdir(2), you may pass a filehandle or
34169887 818directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 819passing handles raises an exception.
c4aca7d0 820
a0d0e21e 821=item chmod LIST
d74e8afc 822X<chmod> X<permission> X<mode>
a0d0e21e 823
c17cdb72
NC
824=for Pod::Functions changes the permissions on a list of files
825
a0d0e21e 826Changes the permissions of a list of files. The first element of the
8f1da26d 827list must be the numeric mode, which should probably be an octal
4ad40acf 828number, and which definitely should I<not> be a string of octal digits:
3b10bc60 829C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 830successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 831
3b10bc60 832 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 833 chmod 0755, @executables;
3b10bc60 834 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 835 # --w----r-T
3b10bc60 836 $mode = "0644"; chmod oct($mode), "foo"; # this is better
837 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 838
3b10bc60 839On systems that support fchmod(2), you may pass filehandles among the
840files. On systems that don't support fchmod(2), passing filehandles raises
841an exception. Filehandles must be passed as globs or glob references to be
842recognized; barewords are considered filenames.
c4aca7d0
GA
843
844 open(my $fh, "<", "foo");
845 my $perm = (stat $fh)[2] & 07777;
846 chmod($perm | 0600, $fh);
847
3b10bc60 848You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
849module:
850
3b10bc60 851 use Fcntl qw( :mode );
ca6e1c26 852 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 853 # Identical to the chmod 0755 of the example above.
ca6e1c26 854
ea9eb35a 855Portability issues: L<perlport/chmod>.
856
a0d0e21e 857=item chomp VARIABLE
d74e8afc 858X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 859
313c9f5c 860=item chomp( LIST )
a0d0e21e
LW
861
862=item chomp
863
c17cdb72
NC
864=for Pod::Functions remove a trailing record separator from a string
865
2b5ab1e7
TC
866This safer version of L</chop> removes any trailing string
867that corresponds to the current value of C<$/> (also known as
28757baa
PP
868$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
869number of characters removed from all its arguments. It's often used to
870remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
871that the final record may be missing its newline. When in paragraph
872mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 873When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 874a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 875remove anything.
19799a22 876If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
877
878 while (<>) {
a9a5a0dc
VP
879 chomp; # avoid \n on last field
880 @array = split(/:/);
881 # ...
a0d0e21e
LW
882 }
883
4bf21a6d
RD
884If VARIABLE is a hash, it chomps the hash's values, but not its keys.
885
a0d0e21e
LW
886You can actually chomp anything that's an lvalue, including an assignment:
887
888 chomp($cwd = `pwd`);
889 chomp($answer = <STDIN>);
890
891If you chomp a list, each element is chomped, and the total number of
892characters removed is returned.
893
15e44fd8
RGS
894Note that parentheses are necessary when you're chomping anything
895that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
896is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
897C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
898C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
899as C<chomp($a, $b)>.
900
a0d0e21e 901=item chop VARIABLE
d74e8afc 902X<chop>
a0d0e21e 903
313c9f5c 904=item chop( LIST )
a0d0e21e
LW
905
906=item chop
907
c17cdb72
NC
908=for Pod::Functions remove the last character from a string
909
a0d0e21e 910Chops off the last character of a string and returns the character
5b3eff12 911chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 912scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
913If VARIABLE is a hash, it chops the hash's values, but not its keys.
914
5b3eff12 915You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
916
917If you chop a list, each element is chopped. Only the value of the
19799a22 918last C<chop> is returned.
a0d0e21e 919
19799a22 920Note that C<chop> returns the last character. To return all but the last
748a9306
LW
921character, use C<substr($string, 0, -1)>.
922
15e44fd8
RGS
923See also L</chomp>.
924
a0d0e21e 925=item chown LIST
d74e8afc 926X<chown> X<owner> X<user> X<group>
a0d0e21e 927
c17cdb72
NC
928=for Pod::Functions change the ownership on a list of files
929
a0d0e21e 930Changes the owner (and group) of a list of files. The first two
19799a22
GS
931elements of the list must be the I<numeric> uid and gid, in that
932order. A value of -1 in either position is interpreted by most
933systems to leave that value unchanged. Returns the number of files
934successfully changed.
a0d0e21e
LW
935
936 $cnt = chown $uid, $gid, 'foo', 'bar';
937 chown $uid, $gid, @filenames;
938
3b10bc60 939On systems that support fchown(2), you may pass filehandles among the
940files. On systems that don't support fchown(2), passing filehandles raises
941an exception. Filehandles must be passed as globs or glob references to be
942recognized; barewords are considered filenames.
c4aca7d0 943
54310121 944Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
945
946 print "User: ";
19799a22 947 chomp($user = <STDIN>);
5a964f20 948 print "Files: ";
19799a22 949 chomp($pattern = <STDIN>);
a0d0e21e
LW
950
951 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 952 or die "$user not in passwd file";
a0d0e21e 953
5ed4f2ec 954 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
955 chown $uid, $gid, @ary;
956
54310121 957On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
958file unless you're the superuser, although you should be able to change
959the group to any of your secondary groups. On insecure systems, these
960restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
961On POSIX systems, you can detect this condition this way:
962
963 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
964 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 965
ea9eb35a 966Portability issues: L<perlport/chmod>.
967
a0d0e21e 968=item chr NUMBER
d74e8afc 969X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 970
54310121 971=item chr
bbce6d69 972
c17cdb72
NC
973=for Pod::Functions get character this number represents
974
a0d0e21e 975Returns the character represented by that NUMBER in the character set.
a0ed51b3 976For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 977chr(0x263a) is a Unicode smiley face.
aaa68c4a 978
8a064bd6 979Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 980except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
981(truncated to an integer) are used.
982
974da8e5
JH
983If NUMBER is omitted, uses C<$_>.
984
b76cc8ba 985For the reverse, use L</ord>.
a0d0e21e 986
2575c402
JW
987Note that characters from 128 to 255 (inclusive) are by default
988internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 989
2575c402 990See L<perlunicode> for more about Unicode.
bbce6d69 991
a0d0e21e 992=item chroot FILENAME
d74e8afc 993X<chroot> X<root>
a0d0e21e 994
54310121 995=item chroot
bbce6d69 996
c17cdb72
NC
997=for Pod::Functions make directory new root for path lookups
998
5a964f20 999This function works like the system call by the same name: it makes the
4633a7c4 1000named directory the new root directory for all further pathnames that
951ba7fe 1001begin with a C</> by your process and all its children. (It doesn't
28757baa 1002change your current working directory, which is unaffected.) For security
4633a7c4 1003reasons, this call is restricted to the superuser. If FILENAME is
19799a22 1004omitted, does a C<chroot> to C<$_>.
a0d0e21e 1005
ea9eb35a 1006Portability issues: L<perlport/chroot>.
1007
a0d0e21e 1008=item close FILEHANDLE
d74e8afc 1009X<close>
a0d0e21e 1010
6a518fbc
TP
1011=item close
1012
c17cdb72
NC
1013=for Pod::Functions close file (or pipe or socket) handle
1014
3b10bc60 1015Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 1016buffers, and closes the system file descriptor. Returns true if those
8f1da26d 1017operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
1018layer. Closes the currently selected filehandle if the argument is
1019omitted.
fb73857a
PP
1020
1021You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 1022another C<open> on it, because C<open> closes it for you. (See
01aa884e 1023L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
19799a22 1024counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 1025
3b10bc60 1026If the filehandle came from a piped open, C<close> returns false if one of
1027the other syscalls involved fails or if its program exits with non-zero
1028status. If the only problem was that the program exited non-zero, C<$!>
1029will be set to C<0>. Closing a pipe also waits for the process executing
1030on the pipe to exit--in case you wish to look at the output of the pipe
1031afterwards--and implicitly puts the exit status value of that command into
1032C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 1033
2e0cfa16
FC
1034If there are multiple threads running, C<close> on a filehandle from a
1035piped open returns true without waiting for the child process to terminate,
1036if the filehandle is still open in another thread.
1037
80d38338
TC
1038Closing the read end of a pipe before the process writing to it at the
1039other end is done writing results in the writer receiving a SIGPIPE. If
1040the other end can't handle that, be sure to read all the data before
1041closing the pipe.
73689b13 1042
fb73857a 1043Example:
a0d0e21e 1044
fb73857a
PP
1045 open(OUTPUT, '|sort >foo') # pipe to sort
1046 or die "Can't start sort: $!";
5ed4f2ec 1047 #... # print stuff to output
1048 close OUTPUT # wait for sort to finish
fb73857a
PP
1049 or warn $! ? "Error closing sort pipe: $!"
1050 : "Exit status $? from sort";
5ed4f2ec 1051 open(INPUT, 'foo') # get sort's results
fb73857a 1052 or die "Can't open 'foo' for input: $!";
a0d0e21e 1053
5a964f20 1054FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 1055filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
1056
1057=item closedir DIRHANDLE
d74e8afc 1058X<closedir>
a0d0e21e 1059
c17cdb72
NC
1060=for Pod::Functions close directory handle
1061
19799a22 1062Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
1063system call.
1064
a0d0e21e 1065=item connect SOCKET,NAME
d74e8afc 1066X<connect>
a0d0e21e 1067
c17cdb72
NC
1068=for Pod::Functions connect to a remote socket
1069
80d38338
TC
1070Attempts to connect to a remote socket, just like connect(2).
1071Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
1072packed address of the appropriate type for the socket. See the examples in
1073L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 1074
cb1a09d0 1075=item continue BLOCK
d74e8afc 1076X<continue>
cb1a09d0 1077
0d863452
RH
1078=item continue
1079
c17cdb72
NC
1080=for Pod::Functions optional trailing block in a while or foreach
1081
4a904372
FC
1082When followed by a BLOCK, C<continue> is actually a
1083flow control statement rather than a function. If
cf264981 1084there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
1085C<foreach>), it is always executed just before the conditional is about to
1086be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
1087it can be used to increment a loop variable, even when the loop has been
1088continued via the C<next> statement (which is similar to the C C<continue>
1089statement).
1090
98293880 1091C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 1092block; C<last> and C<redo> behave as if they had been executed within
19799a22 1093the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
1094block, it may be more entertaining.
1095
1096 while (EXPR) {
a9a5a0dc
VP
1097 ### redo always comes here
1098 do_something;
1d2dff63 1099 } continue {
a9a5a0dc
VP
1100 ### next always comes here
1101 do_something_else;
1102 # then back the top to re-check EXPR
1d2dff63
GS
1103 }
1104 ### last always comes here
1105
3b10bc60 1106Omitting the C<continue> section is equivalent to using an
1107empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1108to check the condition at the top of the loop.
1109
4a904372 1110When there is no BLOCK, C<continue> is a function that
8f1da26d
TC
1111falls through the current C<when> or C<default> block instead of iterating
1112a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
4a904372
FC
1113In Perl 5.14 and earlier, this form of C<continue> was
1114only available when the C<"switch"> feature was enabled.
48238296 1115See L<feature> and L<perlsyn/"Switch Statements"> for more
8f1da26d 1116information.
0d863452 1117
a0d0e21e 1118=item cos EXPR
d74e8afc 1119X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1120
d6217f1e
GS
1121=item cos
1122
c17cdb72
NC
1123=for Pod::Functions cosine function
1124
5a964f20 1125Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1126takes the cosine of C<$_>.
a0d0e21e 1127
ca6e1c26 1128For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1129function, or use this relation:
1130
1131 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1132
a0d0e21e 1133=item crypt PLAINTEXT,SALT
d74e8afc 1134X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1135X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1136
c17cdb72
NC
1137=for Pod::Functions one-way passwd-style encryption
1138
ef2e6798
MS
1139Creates a digest string exactly like the crypt(3) function in the C
1140library (assuming that you actually have a version there that has not
bb23f8d1 1141been extirpated as a potential munition).
ef2e6798 1142
34169887 1143crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1144into a short string, called a digest, which is returned. The same
1145PLAINTEXT and SALT will always return the same string, but there is no
1146(known) way to get the original PLAINTEXT from the hash. Small
1147changes in the PLAINTEXT or SALT will result in large changes in the
1148digest.
1149
1150There is no decrypt function. This function isn't all that useful for
1151cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1152mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1153primarily used to check if two pieces of text are the same without
1154having to transmit or store the text itself. An example is checking
1155if a correct password is given. The digest of the password is stored,
cf264981 1156not the password itself. The user types in a password that is
ef2e6798 1157crypt()'d with the same salt as the stored digest. If the two digests
34169887 1158match, the password is correct.
ef2e6798
MS
1159
1160When verifying an existing digest string you should use the digest as
1161the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1162to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1163crypt() will hash the new string with the same salt as the digest.
1164This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1165with more exotic implementations. In other words, assume
1166nothing about the returned string itself nor about how many bytes
1167of SALT may matter.
85c16d83
JH
1168
1169Traditionally the result is a string of 13 bytes: two first bytes of
1170the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
391b733c 1171the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1172hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1173and implementations on non-Unix platforms may produce different
ef2e6798 1174strings.
85c16d83
JH
1175
1176When choosing a new salt create a random two character string whose
1177characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1178'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1179characters is just a recommendation; the characters allowed in
1180the salt depend solely on your system's crypt library, and Perl can't
1181restrict what salts C<crypt()> accepts.
e71965be 1182
a0d0e21e 1183Here's an example that makes sure that whoever runs this program knows
cf264981 1184their password:
a0d0e21e
LW
1185
1186 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1187
1188 system "stty -echo";
1189 print "Password: ";
e71965be 1190 chomp($word = <STDIN>);
a0d0e21e
LW
1191 print "\n";
1192 system "stty echo";
1193
e71965be 1194 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1195 die "Sorry...\n";
a0d0e21e 1196 } else {
a9a5a0dc 1197 print "ok\n";
54310121 1198 }
a0d0e21e 1199
9f8f0c9d 1200Of course, typing in your own password to whoever asks you
748a9306 1201for it is unwise.
a0d0e21e 1202
ef2e6798 1203The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1204of data, not least of all because you can't get the information
ef2e6798 1205back. Look at the L<Digest> module for more robust algorithms.
19799a22 1206
f2791508
JH
1207If using crypt() on a Unicode string (which I<potentially> has
1208characters with codepoints above 255), Perl tries to make sense
34169887 1209of the situation by trying to downgrade (a copy of)
f2791508
JH
1210the string back to an eight-bit byte string before calling crypt()
1211(on that copy). If that works, good. If not, crypt() dies with
1212C<Wide character in crypt>.
85c16d83 1213
ea9eb35a 1214Portability issues: L<perlport/crypt>.
1215
aa689395 1216=item dbmclose HASH
d74e8afc 1217X<dbmclose>
a0d0e21e 1218
c17cdb72
NC
1219=for Pod::Functions breaks binding on a tied dbm file
1220
19799a22 1221[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1222
aa689395 1223Breaks the binding between a DBM file and a hash.
a0d0e21e 1224
ea9eb35a 1225Portability issues: L<perlport/dbmclose>.
1226
19799a22 1227=item dbmopen HASH,DBNAME,MASK
d74e8afc 1228X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1229
c17cdb72
NC
1230=for Pod::Functions create binding on a tied dbm file
1231
01aa884e
KW
1232[This function has been largely superseded by the
1233L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
a0d0e21e 1234
7b8d334a 1235This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1236hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1237argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1238is the name of the database (without the F<.dir> or F<.pag> extension if
1239any). If the database does not exist, it is created with protection
1b3a6178
FC
1240specified by MASK (as modified by the C<umask>). To prevent creation of
1241the database if it doesn't exist, you may specify a MODE
1242of 0, and the function will return a false value if it
1243can't find an existing database. If your system supports
80d38338 1244only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1245program. In older versions of Perl, if your system had neither DBM nor
19799a22 1246ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1247sdbm(3).
1248
1249If you don't have write access to the DBM file, you can only read hash
1250variables, not set them. If you want to test whether you can write,
3b10bc60 1251either use file tests or try setting a dummy hash entry inside an C<eval>
1252to trap the error.
a0d0e21e 1253
19799a22
GS
1254Note that functions such as C<keys> and C<values> may return huge lists
1255when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1256function to iterate over large DBM files. Example:
1257
1258 # print out history file offsets
1259 dbmopen(%HIST,'/usr/lib/news/history',0666);
1260 while (($key,$val) = each %HIST) {
a9a5a0dc 1261 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1262 }
1263 dbmclose(%HIST);
1264
cb1a09d0 1265See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1266cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1267rich implementation.
4633a7c4 1268
2b5ab1e7
TC
1269You can control which DBM library you use by loading that library
1270before you call dbmopen():
1271
1272 use DB_File;
1273 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1274 or die "Can't open netscape history file: $!";
2b5ab1e7 1275
ea9eb35a 1276Portability issues: L<perlport/dbmopen>.
1277
a0d0e21e 1278=item defined EXPR
d74e8afc 1279X<defined> X<undef> X<undefined>
a0d0e21e 1280
54310121 1281=item defined
bbce6d69 1282
c17cdb72
NC
1283=for Pod::Functions test whether a value, variable, or function is defined
1284
2f9daede 1285Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1286the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1287checked.
1288
1289Many operations return C<undef> to indicate failure, end of file,
1290system error, uninitialized variable, and other exceptional
1291conditions. This function allows you to distinguish C<undef> from
1292other values. (A simple Boolean test will not distinguish among
7660c0ab 1293C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1294false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1295doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1296returns C<undef> when its argument is an empty array, I<or> when the
1297element to return happens to be C<undef>.
1298
f10b0346
GS
1299You may also use C<defined(&func)> to check whether subroutine C<&func>
1300has ever been defined. The return value is unaffected by any forward
80d38338 1301declarations of C<&func>. A subroutine that is not defined
847c7ebe 1302may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1303makes it spring into existence the first time that it is called; see
847c7ebe 1304L<perlsub>.
f10b0346
GS
1305
1306Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1307used to report whether memory for that aggregate had ever been
f10b0346
GS
1308allocated. This behavior may disappear in future versions of Perl.
1309You should instead use a simple test for size:
1310
1311 if (@an_array) { print "has array elements\n" }
1312 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1313
1314When used on a hash element, it tells you whether the value is defined,
dc848c6f 1315not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1316purpose.
a0d0e21e
LW
1317
1318Examples:
1319
8f1da26d 1320 print if defined $switch{D};
a0d0e21e
LW
1321 print "$val\n" while defined($val = pop(@ary));
1322 die "Can't readlink $sym: $!"
a9a5a0dc 1323 unless defined($value = readlink $sym);
a0d0e21e 1324 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1325 $debugging = 0 unless defined $debugging;
a0d0e21e 1326
8f1da26d 1327Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1328discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1329defined values. For example, if you say
a5f75d66
AD
1330
1331 "ab" =~ /a(.*)b/;
1332
80d38338 1333The pattern match succeeds and C<$1> is defined, although it
cf264981 1334matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1335matched something that happened to be zero characters long. This is all
a5f75d66 1336very above-board and honest. When a function returns an undefined value,
2f9daede 1337it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1338should use C<defined> only when questioning the integrity of what
7660c0ab 1339you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1340what you want.
1341
dc848c6f 1342See also L</undef>, L</exists>, L</ref>.
2f9daede 1343
a0d0e21e 1344=item delete EXPR
d74e8afc 1345X<delete>
a0d0e21e 1346
c17cdb72
NC
1347=for Pod::Functions deletes a value from a hash
1348
d0a76353
RS
1349Given an expression that specifies an element or slice of a hash, C<delete>
1350deletes the specified elements from that hash so that exists() on that element
1351no longer returns true. Setting a hash element to the undefined value does
1352not remove its key, but deleting it does; see L</exists>.
80d38338 1353
8f1da26d 1354In list context, returns the value or values deleted, or the last such
80d38338 1355element in scalar context. The return list's length always matches that of
d0a76353
RS
1356the argument list: deleting non-existent elements returns the undefined value
1357in their corresponding positions.
80d38338 1358
d0a76353
RS
1359delete() may also be used on arrays and array slices, but its behavior is less
1360straightforward. Although exists() will return false for deleted entries,
1361deleting array elements never changes indices of existing values; use shift()
1362or splice() for that. However, if all deleted elements fall at the end of an
1363array, the array's size shrinks to the position of the highest element that
1364still tests true for exists(), or to 0 if none do.
1365
8f1da26d 1366B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1367be removed in a future version of Perl.
80d38338
TC
1368
1369Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1370a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1371or array may not necessarily return anything; it depends on the implementation
1372of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1373
80d38338
TC
1374The C<delete local EXPR> construct localizes the deletion to the current
1375block at run time. Until the block exits, elements locally deleted
1376temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1377of composite types">.
eba0920a
EM
1378
1379 %hash = (foo => 11, bar => 22, baz => 33);
1380 $scalar = delete $hash{foo}; # $scalar is 11
1381 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1382 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1383
01020589 1384The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1385
5f05dabc 1386 foreach $key (keys %HASH) {
a9a5a0dc 1387 delete $HASH{$key};
a0d0e21e
LW
1388 }
1389
01020589 1390 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1391 delete $ARRAY[$index];
01020589
GS
1392 }
1393
1394And so do these:
5f05dabc 1395
01020589
GS
1396 delete @HASH{keys %HASH};
1397
9740c838 1398 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1399
80d38338
TC
1400But both are slower than assigning the empty list
1401or undefining %HASH or @ARRAY, which is the customary
1402way to empty out an aggregate:
01020589 1403
5ed4f2ec 1404 %HASH = (); # completely empty %HASH
1405 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1406
5ed4f2ec 1407 @ARRAY = (); # completely empty @ARRAY
1408 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1409
80d38338
TC
1410The EXPR can be arbitrarily complicated provided its
1411final operation is an element or slice of an aggregate:
a0d0e21e
LW
1412
1413 delete $ref->[$x][$y]{$key};
5f05dabc 1414 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1415
01020589
GS
1416 delete $ref->[$x][$y][$index];
1417 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1418
a0d0e21e 1419=item die LIST
d74e8afc 1420X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1421
c17cdb72
NC
1422=for Pod::Functions raise an exception or bail out
1423
391b733c 1424C<die> raises an exception. Inside an C<eval> the error message is stuffed
4c050ad5
NC
1425into C<$@> and the C<eval> is terminated with the undefined value.
1426If the exception is outside of all enclosing C<eval>s, then the uncaught
391b733c 1427exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1428need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1429
1430Equivalent examples:
1431
1432 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1433 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1434
ccac6780 1435If the last element of LIST does not end in a newline, the current
df37ec69
WW
1436script line number and input line number (if any) are also printed,
1437and a newline is supplied. Note that the "input line number" (also
1438known as "chunk") is subject to whatever notion of "line" happens to
1439be currently in effect, and is also available as the special variable
1440C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1441
1442Hint: sometimes appending C<", stopped"> to your message will cause it
1443to make better sense when the string C<"at foo line 123"> is appended.
1444Suppose you are running script "canasta".
a0d0e21e
LW
1445
1446 die "/etc/games is no good";
1447 die "/etc/games is no good, stopped";
1448
1449produce, respectively
1450
1451 /etc/games is no good at canasta line 123.
1452 /etc/games is no good, stopped at canasta line 123.
1453
a96d0188 1454If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1455previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1456This is useful for propagating exceptions:
1457
1458 eval { ... };
1459 die unless $@ =~ /Expected exception/;
1460
a96d0188 1461If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1462C<PROPAGATE> method, that method will be called with additional file
1463and line number parameters. The return value replaces the value in
34169887 1464C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1465were called.
1466
7660c0ab 1467If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1468
4c050ad5
NC
1469If an uncaught exception results in interpreter exit, the exit code is
1470determined from the values of C<$!> and C<$?> with this pseudocode:
1471
1472 exit $! if $!; # errno
1473 exit $? >> 8 if $? >> 8; # child exit status
1474 exit 255; # last resort
1475
1476The intent is to squeeze as much possible information about the likely cause
391b733c
FC
1477into the limited space of the system exit
1478code. However, as C<$!> is the value
4c050ad5
NC
1479of C's C<errno>, which can be set by any system call, this means that the value
1480of the exit code used by C<die> can be non-predictable, so should not be relied
1481upon, other than to be non-zero.
1482
80d38338
TC
1483You can also call C<die> with a reference argument, and if this is trapped
1484within an C<eval>, C<$@> contains that reference. This permits more
1485elaborate exception handling using objects that maintain arbitrary state
1486about the exception. Such a scheme is sometimes preferable to matching
1487particular string values of C<$@> with regular expressions. Because C<$@>
1488is a global variable and C<eval> may be used within object implementations,
1489be careful that analyzing the error object doesn't replace the reference in
1490the global variable. It's easiest to make a local copy of the reference
1491before any manipulations. Here's an example:
52531d10 1492
80d38338 1493 use Scalar::Util "blessed";
da279afe 1494
52531d10 1495 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1496 if (my $ev_err = $@) {
1497 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1498 # handle Some::Module::Exception
1499 }
1500 else {
1501 # handle all other possible exceptions
1502 }
1503 }
1504
3b10bc60 1505Because Perl stringifies uncaught exception messages before display,
80d38338 1506you'll probably want to overload stringification operations on
52531d10
GS
1507exception objects. See L<overload> for details about that.
1508
19799a22
GS
1509You can arrange for a callback to be run just before the C<die>
1510does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1511handler is called with the error text and can change the error
19799a22 1512message, if it sees fit, by calling C<die> again. See
96090e4f 1513L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1514L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1515to be run only right before your program was to exit, this is not
3b10bc60 1516currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1517even inside eval()ed blocks/strings! If one wants the hook to do
1518nothing in such situations, put
fb73857a 1519
5ed4f2ec 1520 die @_ if $^S;
fb73857a 1521
19799a22
GS
1522as the first line of the handler (see L<perlvar/$^S>). Because
1523this promotes strange action at a distance, this counterintuitive
b76cc8ba 1524behavior may be fixed in a future release.
774d564b 1525
4c050ad5
NC
1526See also exit(), warn(), and the Carp module.
1527
a0d0e21e 1528=item do BLOCK
d74e8afc 1529X<do> X<block>
a0d0e21e 1530
c17cdb72
NC
1531=for Pod::Functions turn a BLOCK into a TERM
1532
a0d0e21e 1533Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1534sequence of commands indicated by BLOCK. When modified by the C<while> or
1535C<until> loop modifier, executes the BLOCK once before testing the loop
391b733c 1536condition. (On other statements the loop modifiers test the conditional
6b275a1f 1537first.)
a0d0e21e 1538
4968c1e4 1539C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1540C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1541See L<perlsyn> for alternative strategies.
4968c1e4 1542
a0d0e21e 1543=item do SUBROUTINE(LIST)
d74e8afc 1544X<do>
a0d0e21e 1545
51124b83
FC
1546This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
1547a scalar variable or a subroutine beginning with C<&>.
a0d0e21e
LW
1548
1549=item do EXPR
d74e8afc 1550X<do>
a0d0e21e
LW
1551
1552Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1553file as a Perl script.
a0d0e21e
LW
1554
1555 do 'stat.pl';
1556
1557is just like
1558
986b19de 1559 eval `cat stat.pl`;
a0d0e21e 1560
2b5ab1e7 1561except that it's more efficient and concise, keeps track of the current
96090e4f
A
1562filename for error messages, searches the C<@INC> directories, and updates
1563C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1564these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1565cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1566same, however, in that it does reparse the file every time you call it,
1567so you probably don't want to do this inside a loop.
a0d0e21e 1568
8f1da26d 1569If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1570an error message in C<$@>. If C<do> cannot read the file, it returns undef
1571and sets C<$!> to the error. Always check C<$@> first, as compilation
1572could fail in a way that also sets C<$!>. If the file is successfully
1573compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1574
80d38338 1575Inclusion of library modules is better done with the
19799a22 1576C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1577and raise an exception if there's a problem.
a0d0e21e 1578
5a964f20
TC
1579You might like to use C<do> to read in a program configuration
1580file. Manual error checking can be done this way:
1581
b76cc8ba 1582 # read in config files: system first, then user
f86cebdf 1583 for $file ("/share/prog/defaults.rc",
b76cc8ba 1584 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1585 {
1586 unless ($return = do $file) {
1587 warn "couldn't parse $file: $@" if $@;
1588 warn "couldn't do $file: $!" unless defined $return;
1589 warn "couldn't run $file" unless $return;
1590 }
5a964f20
TC
1591 }
1592
a0d0e21e 1593=item dump LABEL
d74e8afc 1594X<dump> X<core> X<undump>
a0d0e21e 1595
1614b0e3
JD
1596=item dump
1597
c17cdb72
NC
1598=for Pod::Functions create an immediate core dump
1599
19799a22
GS
1600This function causes an immediate core dump. See also the B<-u>
1601command-line switch in L<perlrun>, which does the same thing.
1602Primarily this is so that you can use the B<undump> program (not
1603supplied) to turn your core dump into an executable binary after
1604having initialized all your variables at the beginning of the
1605program. When the new binary is executed it will begin by executing
1606a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1607Think of it as a goto with an intervening core dump and reincarnation.
1608If C<LABEL> is omitted, restarts the program from the top.
1609
1610B<WARNING>: Any files opened at the time of the dump will I<not>
1611be open any more when the program is reincarnated, with possible
80d38338 1612resulting confusion by Perl.
19799a22 1613
59f521f4 1614This function is now largely obsolete, mostly because it's very hard to
391b733c 1615convert a core file into an executable. That's why you should now invoke
59f521f4 1616it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1617typo.
19799a22 1618
ea9eb35a 1619Portability issues: L<perlport/dump>.
1620
532eee96 1621=item each HASH
d74e8afc 1622X<each> X<hash, iterator>
aa689395 1623
532eee96 1624=item each ARRAY
aeedbbed
NC
1625X<array, iterator>
1626
f5a93a43
TC
1627=item each EXPR
1628
c17cdb72
NC
1629=for Pod::Functions retrieve the next key/value pair from a hash
1630
bade7fbc
TC
1631When called on a hash in list context, returns a 2-element list
1632consisting of the key and value for the next element of a hash. In Perl
16335.12 and later only, it will also return the index and value for the next
1634element of an array so that you can iterate over it; older Perls consider
1635this a syntax error. When called in scalar context, returns only the key
1636(not the value) in a hash, or the index in an array.
2f9daede 1637
aeedbbed 1638Hash entries are returned in an apparently random order. The actual random
3b10bc60 1639order is subject to change in future versions of Perl, but it is
504f80c1 1640guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1641function would produce on the same (unmodified) hash. Since Perl
22883ac5 16425.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1643for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1644
80d38338
TC
1645After C<each> has returned all entries from the hash or array, the next
1646call to C<each> returns the empty list in list context and C<undef> in
bade7fbc
TC
1647scalar context; the next call following I<that> one restarts iteration.
1648Each hash or array has its own internal iterator, accessed by C<each>,
1649C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1650reached the end as just described; it can be explicitly reset by calling
1651C<keys> or C<values> on the hash or array. If you add or delete a hash's
1652elements while iterating over it, entries may be skipped or duplicated--so
1653don't do that. Exception: In the current implementation, it is always safe
1654to delete the item most recently returned by C<each()>, so the following
1655code works properly:
74fc8b5f
MJD
1656
1657 while (($key, $value) = each %hash) {
1658 print $key, "\n";
1659 delete $hash{$key}; # This is safe
1660 }
aa689395 1661
80d38338 1662This prints out your environment like the printenv(1) program,
3b10bc60 1663but in a different order:
a0d0e21e
LW
1664
1665 while (($key,$value) = each %ENV) {
a9a5a0dc 1666 print "$key=$value\n";
a0d0e21e
LW
1667 }
1668
f5a93a43
TC
1669Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1670reference to an unblessed hash or array. The argument will be dereferenced
1671automatically. This aspect of C<each> is considered highly experimental.
1672The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1673
1674 while (($key,$value) = each $hashref) { ... }
1675
bade7fbc
TC
1676To avoid confusing would-be users of your code who are running earlier
1677versions of Perl with mysterious syntax errors, put this sort of thing at
1678the top of your file to signal that your code will work I<only> on Perls of
1679a recent vintage:
1680
1681 use 5.012; # so keys/values/each work on arrays
1682 use 5.014; # so keys/values/each work on scalars (experimental)
1683
8f1da26d 1684See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1685
1686=item eof FILEHANDLE
d74e8afc
ITB
1687X<eof>
1688X<end of file>
1689X<end-of-file>
a0d0e21e 1690
4633a7c4
LW
1691=item eof ()
1692
a0d0e21e
LW
1693=item eof
1694
c17cdb72
NC
1695=for Pod::Functions test a filehandle for its end
1696
8f1da26d 1697Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1698FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1699gives the real filehandle. (Note that this function actually
80d38338 1700reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1701interactive context.) Do not read from a terminal file (or call
19799a22 1702C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1703as terminals may lose the end-of-file condition if you do.
1704
820475bd 1705An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1706with empty parentheses is different. It refers to the pseudo file
820475bd 1707formed from the files listed on the command line and accessed via the
61eff3bc
JH
1708C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1709as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1710used will cause C<@ARGV> to be examined to determine if input is
67408cae 1711available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1712end-of-file will assume you are processing another C<@ARGV> list,
1713and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1714see L<perlop/"I/O Operators">.
820475bd 1715
61eff3bc 1716In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1717detect the end of each file, whereas C<eof()> will detect the end
1718of the very last file only. Examples:
a0d0e21e 1719
748a9306
LW
1720 # reset line numbering on each input file
1721 while (<>) {
a9a5a0dc
VP
1722 next if /^\s*#/; # skip comments
1723 print "$.\t$_";
5a964f20 1724 } continue {
a9a5a0dc 1725 close ARGV if eof; # Not eof()!
748a9306
LW
1726 }
1727
a0d0e21e
LW
1728 # insert dashes just before last line of last file
1729 while (<>) {
a9a5a0dc
VP
1730 if (eof()) { # check for end of last file
1731 print "--------------\n";
1732 }
1733 print;
4d0444a3 1734 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1735 }
1736
a0d0e21e 1737Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1738input operators typically return C<undef> when they run out of data or
1739encounter an error.
a0d0e21e
LW
1740
1741=item eval EXPR
d74e8afc 1742X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1743X<error, handling> X<exception, handling>
a0d0e21e
LW
1744
1745=item eval BLOCK
1746
ce2984c3
PF
1747=item eval
1748
c17cdb72
NC
1749=for Pod::Functions catch exceptions or compile and run code
1750
c7cc6f1c
GS
1751In the first form, the return value of EXPR is parsed and executed as if it
1752were a little Perl program. The value of the expression (which is itself
8f1da26d 1753determined within scalar context) is first parsed, and if there were no
2341804c 1754errors, executed as a block within the lexical context of the current Perl
df4833a8 1755program. This means, that in particular, any outer lexical variables are
2341804c
DM
1756visible to it, and any package variable settings or subroutine and format
1757definitions remain afterwards.
1758
1759Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1760If EXPR is omitted, evaluates C<$_>. This form is typically used to
1761delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c 1762
7289c5e6
FC
1763If the C<unicode_eval> feature is enabled (which is the default under a
1764C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1765characters, so C<use utf8> declarations have no effect, and source filters
1766are forbidden. In the absence of the C<unicode_eval> feature, the string
1767will sometimes be treated as characters and sometimes as bytes, depending
1768on the internal encoding, and source filters activated within the C<eval>
1769exhibit the erratic, but historical, behaviour of affecting some outer file
1770scope that is still compiling. See also the L</evalbytes> keyword, which
1771always treats its input as a byte stream and works properly with source
1772filters, and the L<feature> pragma.
1773
c7cc6f1c 1774In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1775same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1776within the context of the current Perl program. This form is typically
1777used to trap exceptions more efficiently than the first (see below), while
1778also providing the benefit of checking the code within BLOCK at compile
1779time.
1780
1781The final semicolon, if any, may be omitted from the value of EXPR or within
1782the BLOCK.
1783
1784In both forms, the value returned is the value of the last expression
5a964f20 1785evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1786as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1787in void, scalar, or list context, depending on the context of the C<eval>
1788itself. See L</wantarray> for more on how the evaluation context can be
1789determined.
a0d0e21e 1790
19799a22 1791If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1792executed, C<eval> returns C<undef> in scalar context
774b80e8
FC
1793or an empty list in list context, and C<$@> is set to the error
1794message. (Prior to 5.16, a bug caused C<undef> to be returned
1795in list context for syntax errors, but not for runtime errors.)
1796If there was no error, C<$@> is set to the empty string. A
9cc672d4
FC
1797control flow operator like C<last> or C<goto> can bypass the setting of
1798C<$@>. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1799warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1800To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1801turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1802See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1803
19799a22
GS
1804Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1805determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1806is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1807the die operator is used to raise exceptions.
1808
5f1da31c
NT
1809If you want to trap errors when loading an XS module, some problems with
1810the binary interface (such as Perl version skew) may be fatal even with
df4833a8 1811C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
5f1da31c 1812
a0d0e21e
LW
1813If the code to be executed doesn't vary, you may use the eval-BLOCK
1814form to trap run-time errors without incurring the penalty of
1815recompiling each time. The error, if any, is still returned in C<$@>.
1816Examples:
1817
54310121 1818 # make divide-by-zero nonfatal
a0d0e21e
LW
1819 eval { $answer = $a / $b; }; warn $@ if $@;
1820
1821 # same thing, but less efficient
1822 eval '$answer = $a / $b'; warn $@ if $@;
1823
1824 # a compile-time error
5ed4f2ec 1825 eval { $answer = }; # WRONG
a0d0e21e
LW
1826
1827 # a run-time error
5ed4f2ec 1828 eval '$answer ='; # sets $@
a0d0e21e 1829
cf264981
SP
1830Using the C<eval{}> form as an exception trap in libraries does have some
1831issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1832may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1833You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1834as this example shows:
774d564b 1835
80d38338 1836 # a private exception trap for divide-by-zero
f86cebdf
GS
1837 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1838 warn $@ if $@;
774d564b
PP
1839
1840This is especially significant, given that C<__DIE__> hooks can call
19799a22 1841C<die> again, which has the effect of changing their error messages:
774d564b
PP
1842
1843 # __DIE__ hooks may modify error messages
1844 {
f86cebdf
GS
1845 local $SIG{'__DIE__'} =
1846 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1847 eval { die "foo lives here" };
1848 print $@ if $@; # prints "bar lives here"
774d564b
PP
1849 }
1850
19799a22 1851Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1852may be fixed in a future release.
1853
19799a22 1854With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1855being looked at when:
1856
5ed4f2ec 1857 eval $x; # CASE 1
1858 eval "$x"; # CASE 2
a0d0e21e 1859
5ed4f2ec 1860 eval '$x'; # CASE 3
1861 eval { $x }; # CASE 4
a0d0e21e 1862
5ed4f2ec 1863 eval "\$$x++"; # CASE 5
1864 $$x++; # CASE 6
a0d0e21e 1865
2f9daede 1866Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1867the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1868the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1869and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1870does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1871purely visual reasons, but it also has the advantage of compiling at
1872compile-time instead of at run-time.) Case 5 is a place where
19799a22 1873normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1874particular situation, you can just use symbolic references instead, as
1875in case 6.
a0d0e21e 1876
b6538e4f 1877Before Perl 5.14, the assignment to C<$@> occurred before restoration
bade7fbc 1878of localized variables, which means that for your code to run on older
b208c909 1879versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1880errors:
1881
1882 # alter $@ on nefarious repugnancy only
1883 {
1884 my $e;
1885 {
1886 local $@; # protect existing $@
1887 eval { test_repugnancy() };
b208c909 1888 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
8a5a710d
DN
1889 $@ =~ /nefarious/ and $e = $@;
1890 }
1891 die $e if defined $e
1892 }
1893
4968c1e4 1894C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1895C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1896
3b10bc60 1897An C<eval ''> executed within the C<DB> package doesn't see the usual
1898surrounding lexical scope, but rather the scope of the first non-DB piece
df4833a8 1899of code that called it. You don't normally need to worry about this unless
3b10bc60 1900you are writing a Perl debugger.
d819b83a 1901
7289c5e6
FC
1902=item evalbytes EXPR
1903X<evalbytes>
1904
1905=item evalbytes
1906
d9b04284 1907=for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
c17cdb72 1908
7289c5e6
FC
1909This function is like L</eval> with a string argument, except it always
1910parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1911string containing characters whose ordinal value exceeds 255 results in an
1912error. Source filters activated within the evaluated code apply to the
1913code itself.
1914
1915This function is only available under the C<evalbytes> feature, a
1916C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1917L<feature> for more information.
1918
a0d0e21e 1919=item exec LIST
d74e8afc 1920X<exec> X<execute>
a0d0e21e 1921
8bf3b016
GS
1922=item exec PROGRAM LIST
1923
c17cdb72
NC
1924=for Pod::Functions abandon this program to run another
1925
3b10bc60 1926The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1927use C<system> instead of C<exec> if you want it to return. It fails and
1928returns false only if the command does not exist I<and> it is executed
fb73857a 1929directly instead of via your system's command shell (see below).
a0d0e21e 1930
19799a22 1931Since it's a common mistake to use C<exec> instead of C<system>, Perl
4642e50d
EB
1932warns you if C<exec> is called in void context and if there is a following
1933statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1934you always do that, right?). If you I<really> want to follow an C<exec>
1935with some other statement, you can use one of these styles to avoid the warning:
55d729e4 1936
5a964f20
TC
1937 exec ('foo') or print STDERR "couldn't exec foo: $!";
1938 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1939
5a964f20 1940If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1941with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1942If there is only one scalar argument or an array with one element in it,
1943the argument is checked for shell metacharacters, and if there are any,
1944the entire argument is passed to the system's command shell for parsing
1945(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1946If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1947words and passed directly to C<execvp>, which is more efficient.
19799a22 1948Examples:
a0d0e21e 1949
19799a22
GS
1950 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1951 exec "sort $outfile | uniq";
a0d0e21e
LW
1952
1953If you don't really want to execute the first argument, but want to lie
1954to the program you are executing about its own name, you can specify
1955the program you actually want to run as an "indirect object" (without a
1956comma) in front of the LIST. (This always forces interpretation of the
54310121 1957LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1958the list.) Example:
1959
1960 $shell = '/bin/csh';
5ed4f2ec 1961 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1962
1963or, more directly,
1964
5ed4f2ec 1965 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1966
3b10bc60 1967When the arguments get executed via the system shell, results are
1968subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1969for details.
1970
19799a22
GS
1971Using an indirect object with C<exec> or C<system> is also more
1972secure. This usage (which also works fine with system()) forces
1973interpretation of the arguments as a multivalued list, even if the
1974list had just one argument. That way you're safe from the shell
1975expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1976
1977 @args = ( "echo surprise" );
1978
2b5ab1e7 1979 exec @args; # subject to shell escapes
f86cebdf 1980 # if @args == 1
2b5ab1e7 1981 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1982
1983The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1984program, passing it C<"surprise"> an argument. The second version didn't;
1985it tried to run a program named I<"echo surprise">, didn't find it, and set
1986C<$?> to a non-zero value indicating failure.
5a964f20 1987
80d38338 1988Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1989output before the exec, but this may not be supported on some platforms
1990(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1991in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 1992open handles to avoid lost output.
0f897271 1993
80d38338
TC
1994Note that C<exec> will not call your C<END> blocks, nor will it invoke
1995C<DESTROY> methods on your objects.
7660c0ab 1996
ea9eb35a 1997Portability issues: L<perlport/exec>.
1998
a0d0e21e 1999=item exists EXPR
d74e8afc 2000X<exists> X<autovivification>
a0d0e21e 2001
c17cdb72
NC
2002=for Pod::Functions test whether a hash key is present
2003
d0a76353
RS
2004Given an expression that specifies an element of a hash, returns true if the
2005specified element in the hash has ever been initialized, even if the
2006corresponding value is undefined.
a0d0e21e 2007
5ed4f2ec 2008 print "Exists\n" if exists $hash{$key};
2009 print "Defined\n" if defined $hash{$key};
01020589
GS
2010 print "True\n" if $hash{$key};
2011
d0a76353 2012exists may also be called on array elements, but its behavior is much less
8f1da26d 2013obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
2014that calling exists on array values is deprecated and likely to be removed in
2015a future version of Perl.
2016
5ed4f2ec 2017 print "Exists\n" if exists $array[$index];
2018 print "Defined\n" if defined $array[$index];
01020589 2019 print "True\n" if $array[$index];
a0d0e21e 2020
8f1da26d 2021A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
2022it exists, but the reverse doesn't necessarily hold true.
2023
afebc493
GS
2024Given an expression that specifies the name of a subroutine,
2025returns true if the specified subroutine has ever been declared, even
2026if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 2027does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
2028exist may still be callable: its package may have an C<AUTOLOAD>
2029method that makes it spring into existence the first time that it is
3b10bc60 2030called; see L<perlsub>.
afebc493 2031
5ed4f2ec 2032 print "Exists\n" if exists &subroutine;
2033 print "Defined\n" if defined &subroutine;
afebc493 2034
a0d0e21e 2035Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 2036operation is a hash or array key lookup or subroutine name:
a0d0e21e 2037
5ed4f2ec 2038 if (exists $ref->{A}->{B}->{$key}) { }
2039 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 2040
5ed4f2ec 2041 if (exists $ref->{A}->{B}->[$ix]) { }
2042 if (exists $hash{A}{B}[$ix]) { }
01020589 2043
afebc493
GS
2044 if (exists &{$ref->{A}{B}{$key}}) { }
2045
9590a7cd 2046Although the most deeply nested array or hash element will not spring into
3b10bc60 2047existence just because its existence was tested, any intervening ones will.
61eff3bc 2048Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 2049into existence due to the existence test for the $key element above.
3b10bc60 2050This happens anywhere the arrow operator is used, including even here:
5a964f20 2051
2b5ab1e7 2052 undef $ref;
5ed4f2ec 2053 if (exists $ref->{"Some key"}) { }
2054 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
2055
2056This surprising autovivification in what does not at first--or even
2057second--glance appear to be an lvalue context may be fixed in a future
5a964f20 2058release.
a0d0e21e 2059
afebc493
GS
2060Use of a subroutine call, rather than a subroutine name, as an argument
2061to exists() is an error.
2062
5ed4f2ec 2063 exists &sub; # OK
2064 exists &sub(); # Error
afebc493 2065
a0d0e21e 2066=item exit EXPR
d74e8afc 2067X<exit> X<terminate> X<abort>
a0d0e21e 2068
ce2984c3
PF
2069=item exit
2070
c17cdb72
NC
2071=for Pod::Functions terminate this program
2072
2b5ab1e7 2073Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
2074
2075 $ans = <STDIN>;
2076 exit 0 if $ans =~ /^[Xx]/;
2077
19799a22 2078See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
2079universally recognized values for EXPR are C<0> for success and C<1>
2080for error; other values are subject to interpretation depending on the
2081environment in which the Perl program is running. For example, exiting
208269 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2083the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 2084
19799a22
GS
2085Don't use C<exit> to abort a subroutine if there's any chance that
2086someone might want to trap whatever error happened. Use C<die> instead,
2087which can be trapped by an C<eval>.
28757baa 2088
19799a22 2089The exit() function does not always exit immediately. It calls any
2b5ab1e7 2090defined C<END> routines first, but these C<END> routines may not
19799a22 2091themselves abort the exit. Likewise any object destructors that need to
60275626 2092be called are called before the real exit. C<END> routines and destructors
391b733c 2093can change the exit status by modifying C<$?>. If this is a problem, you
fae6f8fa 2094can call C<POSIX::_exit($status)> to avoid END and destructor processing.
87275199 2095See L<perlmod> for details.
5a964f20 2096
ea9eb35a 2097Portability issues: L<perlport/exit>.
2098
a0d0e21e 2099=item exp EXPR
d74e8afc 2100X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 2101
54310121 2102=item exp
bbce6d69 2103
c17cdb72
NC
2104=for Pod::Functions raise I<e> to a power
2105
b76cc8ba 2106Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
2107If EXPR is omitted, gives C<exp($_)>.
2108
628253b8
BF
2109=item fc EXPR
2110X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2111
2112=item fc
2113
d9b04284 2114=for Pod::Functions +fc return casefolded version of a string
c17cdb72 2115
628253b8
BF
2116Returns the casefolded version of EXPR. This is the internal function
2117implementing the C<\F> escape in double-quoted strings.
2118
2119Casefolding is the process of mapping strings to a form where case
2120differences are erased; comparing two strings in their casefolded
2121form is effectively a way of asking if two strings are equal,
2122regardless of case.
2123
2124Roughly, if you ever found yourself writing this
2125
f6c6dcb6 2126 lc($this) eq lc($that) # Wrong!
628253b8 2127 # or
f6c6dcb6 2128 uc($this) eq uc($that) # Also wrong!
628253b8 2129 # or
f6c6dcb6 2130 $this =~ /^\Q$that\E\z/i # Right!
628253b8
BF
2131
2132Now you can write
2133
2134 fc($this) eq fc($that)
2135
2136And get the correct results.
2137
fc39a31f
KW
2138Perl only implements the full form of casefolding,
2139but you can access the simple folds using L<Unicode::UCD/casefold()> and
2140L<Unicode::UCD/prop_invmap()>.
628253b8
BF
2141For further information on casefolding, refer to
2142the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
21434.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2144available at L<http://www.unicode.org/versions/latest/>, as well as the
2145Case Charts available at L<http://www.unicode.org/charts/case/>.
2146
2147If EXPR is omitted, uses C<$_>.
2148
2149This function behaves the same way under various pragma, such as in a locale,
2150as L</lc> does.
2151
2152While the Unicode Standard defines two additional forms of casefolding,
2153one for Turkic languages and one that never maps one character into multiple
2154characters, these are not provided by the Perl core; However, the CPAN module
2155C<Unicode::Casing> may be used to provide an implementation.
2156
2157This keyword is available only when the C<"fc"> feature is enabled,
2158or when prefixed with C<CORE::>; See L<feature>. Alternately,
2159include a C<use v5.16> or later to the current scope.
2160
a0d0e21e 2161=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2162X<fcntl>
a0d0e21e 2163
c17cdb72
NC
2164=for Pod::Functions file control system call
2165
f86cebdf 2166Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
2167
2168 use Fcntl;
2169
0ade1984 2170first to get the correct constant definitions. Argument processing and
3b10bc60 2171value returned work just like C<ioctl> below.
a0d0e21e
LW
2172For example:
2173
2174 use Fcntl;
5a964f20 2175 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 2176 or die "can't fcntl F_GETFL: $!";
5a964f20 2177
554ad1fc 2178You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
2179Like C<ioctl>, it maps a C<0> return from the system call into
2180C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
2181in numeric context. It is also exempt from the normal B<-w> warnings
2182on improper numeric conversions.
5a964f20 2183
3b10bc60 2184Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
2185doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2186manpage to learn what functions are available on your system.
a0d0e21e 2187
be2f7487 2188Here's an example of setting a filehandle named C<REMOTE> to be
2189non-blocking at the system level. You'll have to negotiate C<$|>
2190on your own, though.
2191
2192 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2193
2194 $flags = fcntl(REMOTE, F_GETFL, 0)
2195 or die "Can't get flags for the socket: $!\n";
2196
2197 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2198 or die "Can't set flags for the socket: $!\n";
2199
ea9eb35a 2200Portability issues: L<perlport/fcntl>.
2201
cfa52385
FC
2202=item __FILE__
2203X<__FILE__>
2204
c17cdb72
NC
2205=for Pod::Functions the name of the current source file
2206
cfa52385
FC
2207A special token that returns the name of the file in which it occurs.
2208
a0d0e21e 2209=item fileno FILEHANDLE
d74e8afc 2210X<fileno>
a0d0e21e 2211
c17cdb72
NC
2212=for Pod::Functions return file descriptor from filehandle
2213
2b5ab1e7 2214Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
2215filehandle is not open. If there is no real file descriptor at the OS
2216level, as can happen with filehandles connected to memory objects via
2217C<open> with a reference for the third argument, -1 is returned.
2218
2219This is mainly useful for constructing
19799a22 2220bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
2221If FILEHANDLE is an expression, the value is taken as an indirect
2222filehandle, generally its name.
5a964f20 2223
b76cc8ba 2224You can use this to find out whether two handles refer to the
5a964f20
TC
2225same underlying descriptor:
2226
2227 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 2228 print "THIS and THAT are dups\n";
b76cc8ba
NIS
2229 }
2230
a0d0e21e 2231=item flock FILEHANDLE,OPERATION
d74e8afc 2232X<flock> X<lock> X<locking>
a0d0e21e 2233
c17cdb72
NC
2234=for Pod::Functions lock an entire file with an advisory lock
2235
19799a22
GS
2236Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2237for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2238machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2239C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2240entire files only, not records.
2b5ab1e7
TC
2241
2242Two potentially non-obvious but traditional C<flock> semantics are
2243that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2244are B<merely advisory>. Such discretionary locks are more flexible, but
2245offer fewer guarantees. This means that programs that do not also use
2246C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2247your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2248for details. It's best to assume traditional behavior if you're writing
2249portable programs. (But if you're not, you should as always feel perfectly
2250free to write for your own system's idiosyncrasies (sometimes called
2251"features"). Slavish adherence to portability concerns shouldn't get
2252in the way of your getting your job done.)
a3cb178b 2253
8ebc5c01
PP
2254OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2255LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2256you can use the symbolic names if you import them from the L<Fcntl> module,
2257either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2258requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2259releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2260LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2261waiting for the lock; check the return status to see if you got it.
68dc0745 2262
2b5ab1e7
TC
2263To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2264before locking or unlocking it.
8ebc5c01 2265
f86cebdf 2266Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2267locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2268are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2269implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
2270differing semantics shouldn't bite too many people.
2271
becacb53
TM
2272Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2273be open with read intent to use LOCK_SH and requires that it be open
2274with write intent to use LOCK_EX.
2275
19799a22
GS
2276Note also that some versions of C<flock> cannot lock things over the
2277network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2278that. If you like you can force Perl to ignore your system's flock(2)
2279function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2280the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2281and build a new Perl.
4633a7c4
LW
2282
2283Here's a mailbox appender for BSD systems.
a0d0e21e 2284
7ed5353d 2285 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
2286
2287 sub lock {
a9a5a0dc
VP
2288 my ($fh) = @_;
2289 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2290
a9a5a0dc
VP
2291 # and, in case someone appended while we were waiting...
2292 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2293 }
2294
2295 sub unlock {
a9a5a0dc
VP
2296 my ($fh) = @_;
2297 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2298 }
2299
b0169937 2300 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2301 or die "Can't open mailbox: $!";
a0d0e21e 2302
7ed5353d 2303 lock($mbox);
b0169937 2304 print $mbox $msg,"\n\n";
7ed5353d 2305 unlock($mbox);
a0d0e21e 2306
3b10bc60 2307On systems that support a real flock(2), locks are inherited across fork()
2308calls, whereas those that must resort to the more capricious fcntl(2)
2309function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2310
cb1a09d0 2311See also L<DB_File> for other flock() examples.
a0d0e21e 2312
ea9eb35a 2313Portability issues: L<perlport/flock>.
2314
a0d0e21e 2315=item fork
d74e8afc 2316X<fork> X<child> X<parent>
a0d0e21e 2317
c17cdb72
NC
2318=for Pod::Functions create a new process just like this one
2319
2b5ab1e7
TC
2320Does a fork(2) system call to create a new process running the
2321same program at the same point. It returns the child pid to the
2322parent process, C<0> to the child process, or C<undef> if the fork is
2323unsuccessful. File descriptors (and sometimes locks on those descriptors)
2324are shared, while everything else is copied. On most systems supporting
2325fork(), great care has gone into making it extremely efficient (for
2326example, using copy-on-write technology on data pages), making it the
2327dominant paradigm for multitasking over the last few decades.
5a964f20 2328
80d38338 2329Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
2330output before forking the child process, but this may not be supported
2331on some platforms (see L<perlport>). To be safe, you may need to set
2332C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2333C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2334
19799a22 2335If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2336accumulate zombies. On some systems, you can avoid this by setting
2337C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2338forking and reaping moribund children.
cb1a09d0 2339
28757baa
PP
2340Note that if your forked child inherits system file descriptors like
2341STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2342if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2343backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2344You should reopen those to F</dev/null> if it's any issue.
28757baa 2345
ea9eb35a 2346On some platforms such as Windows, where the fork() system call is not available,
391b733c
FC
2347Perl can be built to emulate fork() in the Perl interpreter.
2348The emulation is designed, at the level of the Perl program,
2349to be as compatible as possible with the "Unix" fork().
6d17f725 2350However it has limitations that have to be considered in code intended to be portable.
ea9eb35a 2351See L<perlfork> for more details.
2352
2353Portability issues: L<perlport/fork>.
2354
cb1a09d0 2355=item format
d74e8afc 2356X<format>
cb1a09d0 2357
c17cdb72
NC
2358=for Pod::Functions declare a picture format with use by the write() function
2359
19799a22 2360Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2361example:
2362
54310121 2363 format Something =
a9a5a0dc
VP
2364 Test: @<<<<<<<< @||||| @>>>>>
2365 $str, $%, '$' . int($num)
cb1a09d0
AD
2366 .
2367
2368 $str = "widget";
184e9718 2369 $num = $cost/$quantity;
cb1a09d0
AD
2370 $~ = 'Something';
2371 write;
2372
2373See L<perlform> for many details and examples.
2374
8903cb82 2375=item formline PICTURE,LIST
d74e8afc 2376X<formline>
a0d0e21e 2377
c17cdb72
NC
2378=for Pod::Functions internal function used for formats
2379
5a964f20 2380This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2381too. It formats (see L<perlform>) a list of values according to the
2382contents of PICTURE, placing the output into the format output
7660c0ab 2383accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2384Eventually, when a C<write> is done, the contents of
cf264981
SP
2385C<$^A> are written to some filehandle. You could also read C<$^A>
2386and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2387does one C<formline> per line of form, but the C<formline> function itself
748a9306 2388doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2389that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2390You may therefore need to use multiple formlines to implement a single
3b10bc60 2391record format, just like the C<format> compiler.
748a9306 2392
19799a22 2393Be careful if you put double quotes around the picture, because an C<@>
748a9306 2394character may be taken to mean the beginning of an array name.
19799a22 2395C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2396
445b09e5
FC
2397If you are trying to use this instead of C<write> to capture the output,
2398you may find it easier to open a filehandle to a scalar
2399(C<< open $fh, ">", \$output >>) and write to that instead.
2400
a0d0e21e 2401=item getc FILEHANDLE
f723aae1 2402X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2403
2404=item getc
2405
c17cdb72
NC
2406=for Pod::Functions get the next character from the filehandle
2407
a0d0e21e 2408Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2409or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2410the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2411STDIN. This is not particularly efficient. However, it cannot be
2412used by itself to fetch single characters without waiting for the user
2413to hit enter. For that, try something more like:
4633a7c4
LW
2414
2415 if ($BSD_STYLE) {
a9a5a0dc 2416 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2417 }
2418 else {
a9a5a0dc 2419 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2420 }
2421
2422 $key = getc(STDIN);
2423
2424 if ($BSD_STYLE) {
a9a5a0dc 2425 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2426 }
2427 else {
3b10bc60 2428 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2429 }
2430 print "\n";
2431
54310121
PP
2432Determination of whether $BSD_STYLE should be set
2433is left as an exercise to the reader.
cb1a09d0 2434
19799a22 2435The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2436systems purporting POSIX compliance. See also the C<Term::ReadKey>
a3390c9f 2437module from your nearest CPAN site; details on CPAN can be found under
2b5ab1e7 2438L<perlmodlib/CPAN>.
a0d0e21e
LW
2439
2440=item getlogin
d74e8afc 2441X<getlogin> X<login>
a0d0e21e 2442
c17cdb72
NC
2443=for Pod::Functions return who logged in at this tty
2444
cf264981 2445This implements the C library function of the same name, which on most
3b10bc60 2446systems returns the current login from F</etc/utmp>, if any. If it
2447returns the empty string, use C<getpwuid>.
a0d0e21e 2448
f86702cc 2449 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2450
19799a22
GS
2451Do not consider C<getlogin> for authentication: it is not as
2452secure as C<getpwuid>.
4633a7c4 2453
ea9eb35a 2454Portability issues: L<perlport/getlogin>.
2455
a0d0e21e 2456=item getpeername SOCKET
d74e8afc 2457X<getpeername> X<peer>
a0d0e21e 2458
c17cdb72
NC
2459=for Pod::Functions find the other end of a socket connection
2460
a3390c9f
FC
2461Returns the packed sockaddr address of the other end of the SOCKET
2462connection.
a0d0e21e 2463
4633a7c4
LW
2464 use Socket;
2465 $hersockaddr = getpeername(SOCK);
19799a22 2466 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2467 $herhostname = gethostbyaddr($iaddr, AF_INET);
2468 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2469
2470=item getpgrp PID
d74e8afc 2471X<getpgrp> X<group>
a0d0e21e 2472
c17cdb72
NC
2473=for Pod::Functions get process group
2474
47e29363 2475Returns the current process group for the specified PID. Use
7660c0ab 2476a PID of C<0> to get the current process group for the
4633a7c4 2477current process. Will raise an exception if used on a machine that
a3390c9f
FC
2478doesn't implement getpgrp(2). If PID is omitted, returns the process
2479group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2480does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2481
ea9eb35a 2482Portability issues: L<perlport/getpgrp>.
2483
a0d0e21e 2484=item getppid
d74e8afc 2485X<getppid> X<parent> X<pid>
a0d0e21e 2486
c17cdb72
NC
2487=for Pod::Functions get parent process ID
2488
a0d0e21e
LW
2489Returns the process id of the parent process.
2490
d7c042c9
AB
2491Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2492around non-POSIX thread semantics the minority of Linux systems (and
2493Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2494has since been removed. See the documentation for L<$$|perlvar/$$> for
2495details.
4d76a344 2496
ea9eb35a 2497Portability issues: L<perlport/getppid>.
2498
a0d0e21e 2499=item getpriority WHICH,WHO
d74e8afc 2500X<getpriority> X<priority> X<nice>
a0d0e21e 2501
c17cdb72
NC
2502=for Pod::Functions get current nice value
2503
4633a7c4 2504Returns the current priority for a process, a process group, or a user.
01aa884e 2505(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2506machine that doesn't implement getpriority(2).
a0d0e21e 2507
ea9eb35a 2508Portability issues: L<perlport/getpriority>.
2509
a0d0e21e 2510=item getpwnam NAME
d74e8afc
ITB
2511X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2512X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2513X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2514X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2515X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2516X<endnetent> X<endprotoent> X<endservent>
a0d0e21e 2517
c17cdb72
NC
2518=for Pod::Functions get passwd record given user login name
2519
a0d0e21e
LW
2520=item getgrnam NAME
2521
c17cdb72
NC
2522=for Pod::Functions get group record given group name
2523
a0d0e21e
LW
2524=item gethostbyname NAME
2525
c17cdb72
NC
2526=for Pod::Functions get host record given name
2527
a0d0e21e
LW
2528=item getnetbyname NAME
2529
c17cdb72
NC
2530=for Pod::Functions get networks record given name
2531
a0d0e21e
LW
2532=item getprotobyname NAME
2533
c17cdb72
NC
2534=for Pod::Functions get protocol record given name
2535
a0d0e21e
LW
2536=item getpwuid UID
2537
c17cdb72
NC
2538=for Pod::Functions get passwd record given user ID
2539
a0d0e21e
LW
2540=item getgrgid GID
2541
c17cdb72
NC
2542=for Pod::Functions get group record given group user ID
2543
a0d0e21e
LW
2544=item getservbyname NAME,PROTO
2545
c17cdb72
NC
2546=for Pod::Functions get services record given its name
2547
a0d0e21e
LW
2548=item gethostbyaddr ADDR,ADDRTYPE
2549
c17cdb72
NC
2550=for Pod::Functions get host record given its address
2551
a0d0e21e
LW
2552=item getnetbyaddr ADDR,ADDRTYPE
2553
c17cdb72
NC
2554=for Pod::Functions get network record given its address
2555
a0d0e21e
LW
2556=item getprotobynumber NUMBER
2557
c17cdb72
NC
2558=for Pod::Functions get protocol record numeric protocol
2559
a0d0e21e
LW
2560=item getservbyport PORT,PROTO
2561
c17cdb72
NC
2562=for Pod::Functions get services record given numeric port
2563
a0d0e21e
LW
2564=item getpwent
2565
c17cdb72
NC
2566=for Pod::Functions get next passwd record
2567
a0d0e21e
LW
2568=item getgrent
2569
c17cdb72
NC
2570=for Pod::Functions get next group record
2571
a0d0e21e
LW
2572=item gethostent
2573
c17cdb72
NC
2574=for Pod::Functions get next hosts record
2575
a0d0e21e
LW
2576=item getnetent
2577
c17cdb72
NC
2578=for Pod::Functions get next networks record
2579
a0d0e21e
LW
2580=item getprotoent
2581
c17cdb72
NC
2582=for Pod::Functions get next protocols record
2583
a0d0e21e
LW
2584=item getservent
2585
c17cdb72
NC
2586=for Pod::Functions get next services record
2587
a0d0e21e
LW
2588=item setpwent
2589
c17cdb72
NC
2590=for Pod::Functions prepare passwd file for use
2591
a0d0e21e
LW
2592=item setgrent
2593
c17cdb72
NC
2594=for Pod::Functions prepare group file for use
2595
a0d0e21e
LW
2596=item sethostent STAYOPEN
2597
c17cdb72
NC
2598=for Pod::Functions prepare hosts file for use
2599
a0d0e21e
LW
2600=item setnetent STAYOPEN
2601
c17cdb72
NC
2602=for Pod::Functions prepare networks file for use
2603
a0d0e21e
LW
2604=item setprotoent STAYOPEN
2605
c17cdb72
NC
2606=for Pod::Functions prepare protocols file for use
2607
a0d0e21e
LW
2608=item setservent STAYOPEN
2609
c17cdb72
NC
2610=for Pod::Functions prepare services file for use
2611
a0d0e21e
LW
2612=item endpwent
2613
c17cdb72
NC
2614=for Pod::Functions be done using passwd file
2615
a0d0e21e
LW
2616=item endgrent
2617
c17cdb72
NC
2618=for Pod::Functions be done using group file
2619
a0d0e21e
LW
2620=item endhostent
2621
c17cdb72
NC
2622=for Pod::Functions be done using hosts file
2623
a0d0e21e
LW
2624=item endnetent
2625
c17cdb72
NC
2626=for Pod::Functions be done using networks file
2627
a0d0e21e
LW
2628=item endprotoent
2629
c17cdb72
NC
2630=for Pod::Functions be done using protocols file
2631
a0d0e21e
LW
2632=item endservent
2633
c17cdb72
NC
2634=for Pod::Functions be done using services file
2635
80d38338
TC
2636These routines are the same as their counterparts in the
2637system C library. In list context, the return values from the
a0d0e21e
LW
2638various get routines are as follows:
2639
2640 ($name,$passwd,$uid,$gid,
6ee623d5 2641 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2642 ($name,$passwd,$gid,$members) = getgr*
2643 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2644 ($name,$aliases,$addrtype,$net) = getnet*
2645 ($name,$aliases,$proto) = getproto*
2646 ($name,$aliases,$port,$proto) = getserv*
2647
3b10bc60 2648(If the entry doesn't exist you get an empty list.)
a0d0e21e 2649
4602f195
JH
2650The exact meaning of the $gcos field varies but it usually contains
2651the real name of the user (as opposed to the login name) and other
2652information pertaining to the user. Beware, however, that in many
2653system users are able to change this information and therefore it
106325ad 2654cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2655L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2656login shell, are also tainted, for the same reason.
4602f195 2657
5a964f20 2658In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2659lookup by name, in which case you get the other thing, whatever it is.
2660(If the entry doesn't exist you get the undefined value.) For example:
2661
5a964f20
TC
2662 $uid = getpwnam($name);
2663 $name = getpwuid($num);
2664 $name = getpwent();
2665 $gid = getgrnam($name);
08a33e13 2666 $name = getgrgid($num);
5a964f20
TC
2667 $name = getgrent();
2668 #etc.
a0d0e21e 2669
4602f195 2670In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2671in that they are unsupported on many systems. If the
4602f195
JH
2672$quota is unsupported, it is an empty scalar. If it is supported, it
2673usually encodes the disk quota. If the $comment field is unsupported,
2674it is an empty scalar. If it is supported it usually encodes some
2675administrative comment about the user. In some systems the $quota
2676field may be $change or $age, fields that have to do with password
2677aging. In some systems the $comment field may be $class. The $expire
2678field, if present, encodes the expiration period of the account or the
2679password. For the availability and the exact meaning of these fields
8f1da26d 2680in your system, please consult getpwnam(3) and your system's
4602f195
JH
2681F<pwd.h> file. You can also find out from within Perl what your
2682$quota and $comment fields mean and whether you have the $expire field
2683by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2684C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2685files are supported only if your vendor has implemented them in the
4602f195 2686intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2687shadow versions if you're running under privilege or if there exists
cf264981 2688the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2689and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2690facility are unlikely to be supported.
6ee623d5 2691
a3390c9f 2692The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2693the login names of the members of the group.
2694
2695For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2696C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2697C<@addrs> value returned by a successful call is a list of raw
2698addresses returned by the corresponding library call. In the
2699Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2700by saying something like:
2701
f337b084 2702 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2703
2b5ab1e7
TC
2704The Socket library makes this slightly easier:
2705
2706 use Socket;
2707 $iaddr = inet_aton("127.1"); # or whatever address
2708 $name = gethostbyaddr($iaddr, AF_INET);
2709
2710 # or going the other way
19799a22 2711 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2712
d760c846
GS
2713In the opposite way, to resolve a hostname to the IP address
2714you can write this:
2715
2716 use Socket;
2717 $packed_ip = gethostbyname("www.perl.org");
2718 if (defined $packed_ip) {
2719 $ip_address = inet_ntoa($packed_ip);
2720 }
2721
b018eaf1 2722Make sure C<gethostbyname()> is called in SCALAR context and that
d760c846
GS
2723its return value is checked for definedness.
2724
0d043efa
FC
2725The C<getprotobynumber> function, even though it only takes one argument,
2726has the precedence of a list operator, so beware:
2727
2728 getprotobynumber $number eq 'icmp' # WRONG
2729 getprotobynumber($number eq 'icmp') # actually means this
2730 getprotobynumber($number) eq 'icmp' # better this way
2731
19799a22
GS
2732If you get tired of remembering which element of the return list
2733contains which return value, by-name interfaces are provided
2734in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2735C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2736and C<User::grent>. These override the normal built-ins, supplying
2737versions that return objects with the appropriate names
2738for each field. For example:
5a964f20
TC
2739
2740 use File::stat;
2741 use User::pwent;
2742 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2743
a3390c9f 2744Even though it looks as though they're the same method calls (uid),
b76cc8ba 2745they aren't, because a C<File::stat> object is different from
19799a22 2746a C<User::pwent> object.
5a964f20 2747
ea9eb35a 2748Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2749
a0d0e21e 2750=item getsockname SOCKET
d74e8afc 2751X<getsockname>
a0d0e21e 2752
c17cdb72
NC
2753=for Pod::Functions retrieve the sockaddr for a given socket
2754
19799a22
GS
2755Returns the packed sockaddr address of this end of the SOCKET connection,
2756in case you don't know the address because you have several different
2757IPs that the connection might have come in on.
a0d0e21e 2758
4633a7c4
LW
2759 use Socket;
2760 $mysockaddr = getsockname(SOCK);
19799a22 2761 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2762 printf "Connect to %s [%s]\n",
19799a22
GS
2763 scalar gethostbyaddr($myaddr, AF_INET),
2764 inet_ntoa($myaddr);
a0d0e21e
LW
2765
2766=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2767X<getsockopt>
a0d0e21e 2768
c17cdb72
NC
2769=for Pod::Functions get socket options on a given socket
2770
636e6b1f
TH
2771Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2772Options may exist at multiple protocol levels depending on the socket
2773type, but at least the uppermost socket level SOL_SOCKET (defined in the
391b733c 2774C<Socket> module) will exist. To query options at another level the
636e6b1f 2775protocol number of the appropriate protocol controlling the option
391b733c 2776should be supplied. For example, to indicate that an option is to be
636e6b1f 2777interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2778number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2779
80d38338 2780The function returns a packed string representing the requested socket
3b10bc60 2781option, or C<undef> on error, with the reason for the error placed in
391b733c 2782C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2783consult getsockopt(2) for details. A common case is that the option is an
2784integer, in which case the result is a packed integer, which you can decode
2785using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2786
8f1da26d 2787Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2788
4852725b 2789 use Socket qw(:all);
636e6b1f
TH
2790
2791 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2792 or die "Could not determine the protocol number for tcp";
4852725b
DD
2793 # my $tcp = IPPROTO_TCP; # Alternative
2794 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2795 or die "getsockopt TCP_NODELAY: $!";
636e6b1f
TH
2796 my $nodelay = unpack("I", $packed);
2797 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2798
ea9eb35a 2799Portability issues: L<perlport/getsockopt>.
a0d0e21e
LW
2800
2801=item glob EXPR
d74e8afc 2802X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2803
0a753a76
PP
2804=item glob
2805
c17cdb72
NC
2806=for Pod::Functions expand filenames using wildcards
2807
d9a9d457 2808In list context, returns a (possibly empty) list of filename expansions on
391b733c 2809the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
d9a9d457 2810scalar context, glob iterates through such filename expansions, returning
391b733c
FC
2811undef when the list is exhausted. This is the internal function
2812implementing the C<< <*.c> >> operator, but you can use it directly. If
d9a9d457
JL
2813EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2814more detail in L<perlop/"I/O Operators">.
a0d0e21e 2815
80d38338
TC
2816Note that C<glob> splits its arguments on whitespace and treats
2817each segment as separate pattern. As such, C<glob("*.c *.h")>
2818matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2819C<glob(".* *")> matches all files in the current working directory.
a91bb7b1
TC
2820If you want to glob filenames that might contain whitespace, you'll
2821have to use extra quotes around the spacey filename to protect it.
2822For example, to glob filenames that have an C<e> followed by a space
2823followed by an C<f>, use either of:
2824
2825 @spacies = <"*e f*">;
2826 @spacies = glob '"*e f*"';
2827 @spacies = glob q("*e f*");
2828
2829If you had to get a variable through, you could do this:
2830
2831 @spacies = glob "'*${var}e f*'";
2832 @spacies = glob qq("*${var}e f*");
80d38338
TC
2833
2834If non-empty braces are the only wildcard characters used in the
2835C<glob>, no filenames are matched, but potentially many strings
2836are returned. For example, this produces nine strings, one for
2837each pairing of fruits and colors:
2838
2839 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2840
3a4b19e4 2841Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2842C<File::Glob> extension. See L<File::Glob> for details, including
2843C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2844
ea9eb35a 2845Portability issues: L<perlport/glob>.
2846
a0d0e21e 2847=item gmtime EXPR
d74e8afc 2848X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2849
ce2984c3
PF
2850=item gmtime
2851
c17cdb72
NC
2852=for Pod::Functions convert UNIX time into record or string using Greenwich time
2853
4509d391 2854Works just like L</localtime> but the returned values are
435fbc73 2855localized for the standard Greenwich time zone.
a0d0e21e 2856
a3390c9f
FC
2857Note: When called in list context, $isdst, the last value
2858returned by gmtime, is always C<0>. There is no
435fbc73 2859Daylight Saving Time in GMT.
0a753a76 2860
ea9eb35a 2861Portability issues: L<perlport/gmtime>.
62aa5637 2862
a0d0e21e 2863=item goto LABEL
d74e8afc 2864X<goto> X<jump> X<jmp>
a0d0e21e 2865
748a9306
LW
2866=item goto EXPR
2867
a0d0e21e
LW
2868=item goto &NAME
2869
c17cdb72
NC
2870=for Pod::Functions create spaghetti code
2871
b500e03b 2872The C<goto-LABEL> form finds the statement labeled with LABEL and
391b733c 2873resumes execution there. It can't be used to get out of a block or
b500e03b
GG
2874subroutine given to C<sort>. It can be used to go almost anywhere
2875else within the dynamic scope, including out of subroutines, but it's
2876usually better to use some other construct such as C<last> or C<die>.
2877The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2878(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2879does not offer named loops combined with loop control. Perl does, and
2880this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2881
7660c0ab
A
2882The C<goto-EXPR> form expects a label name, whose scope will be resolved
2883dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2884necessarily recommended if you're optimizing for maintainability:
2885
2886 goto ("FOO", "BAR", "GLARCH")[$i];
2887
887d89fd 2888As shown in this example, C<goto-EXPR> is exempt from the "looks like a
391b733c
FC
2889function" rule. A pair of parentheses following it does not (necessarily)
2890delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
887d89fd 2891
b500e03b 2892Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2893deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2894go into any construct that requires initialization, such as a
2895subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2896construct that is optimized away.
b500e03b 2897
1b6921cb
BT
2898The C<goto-&NAME> form is quite different from the other forms of
2899C<goto>. In fact, it isn't a goto in the normal sense at all, and
2900doesn't have the stigma associated with other gotos. Instead, it
2901exits the current subroutine (losing any changes set by local()) and
2902immediately calls in its place the named subroutine using the current
2903value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2904load another subroutine and then pretend that the other subroutine had
2905been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2906in the current subroutine are propagated to the other subroutine.)
2907After the C<goto>, not even C<caller> will be able to tell that this
2908routine was called first.
2909
2910NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2911containing a code reference or a block that evaluates to a code
6cb9131c 2912reference.
a0d0e21e
LW
2913
2914=item grep BLOCK LIST
d74e8afc 2915X<grep>
a0d0e21e
LW
2916
2917=item grep EXPR,LIST
2918
c17cdb72
NC
2919=for Pod::Functions locate elements in a list test true against a given criterion
2920
2b5ab1e7
TC
2921This is similar in spirit to, but not the same as, grep(1) and its
2922relatives. In particular, it is not limited to using regular expressions.
2f9daede 2923
a0d0e21e 2924Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2925C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2926elements for which the expression evaluated to true. In scalar
2927context, returns the number of times the expression was true.
a0d0e21e
LW
2928
2929 @foo = grep(!/^#/, @bar); # weed out comments
2930
2931or equivalently,
2932
2933 @foo = grep {!/^#/} @bar; # weed out comments
2934
be3174d2
GS
2935Note that C<$_> is an alias to the list value, so it can be used to
2936modify the elements of the LIST. While this is useful and supported,
2937it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2938Similarly, grep returns aliases into the original list, much as a for
2939loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2940element of a list returned by grep (for example, in a C<foreach>, C<map>
2941or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2942This is usually something to be avoided when writing clear code.
a0d0e21e 2943
a4fb8298 2944If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2945been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2946the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2947can't be seen from the outside, avoiding any potential side-effects.
2948
19799a22 2949See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2950
a0d0e21e 2951=item hex EXPR
d74e8afc 2952X<hex> X<hexadecimal>
a0d0e21e 2953
54310121 2954=item hex
bbce6d69 2955
c17cdb72
NC
2956=for Pod::Functions convert a string to a hexadecimal number
2957
2b5ab1e7 2958Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2959(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2960L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2961
2962 print hex '0xAf'; # prints '175'
2963 print hex 'aF'; # same
a0d0e21e 2964
19799a22 2965Hex strings may only represent integers. Strings that would cause
53305cf1 2966integer overflow trigger a warning. Leading whitespace is not stripped,
391b733c 2967unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 2968L</sprintf>, and L</unpack>.
19799a22 2969
ce2984c3 2970=item import LIST
d74e8afc 2971X<import>
a0d0e21e 2972
c17cdb72
NC
2973=for Pod::Functions patch a module's namespace into your own
2974
19799a22 2975There is no builtin C<import> function. It is just an ordinary
4633a7c4 2976method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2977names to another module. The C<use> function calls the C<import> method
cea6626f 2978for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2979
2980=item index STR,SUBSTR,POSITION
d74e8afc 2981X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2982
2983=item index STR,SUBSTR
2984
c17cdb72
NC
2985=for Pod::Functions find a substring within a string
2986
2b5ab1e7
TC
2987The index function searches for one string within another, but without
2988the wildcard-like behavior of a full regular-expression pattern match.
2989It returns the position of the first occurrence of SUBSTR in STR at
2990or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2991beginning of the string. POSITION before the beginning of the string
2992or after its end is treated as if it were the beginning or the end,
e1dccc0d
Z
2993respectively. POSITION and the return value are based at zero.
2994If the substring is not found, C<index> returns -1.
a0d0e21e
LW
2995
2996=item int EXPR
f723aae1 2997X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2998
54310121 2999=item int
bbce6d69 3000
c17cdb72
NC
3001=for Pod::Functions get the integer portion of a number
3002
7660c0ab 3003Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 3004You should not use this function for rounding: one because it truncates
3b10bc60 3005towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
3006numbers can sometimes produce counterintuitive results. For example,
3007C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3008because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 3009the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 3010functions will serve you better than will int().
a0d0e21e
LW
3011
3012=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 3013X<ioctl>
a0d0e21e 3014
c17cdb72
NC
3015=for Pod::Functions system-dependent device control system call
3016
2b5ab1e7 3017Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 3018
5ed4f2ec 3019 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 3020
a11c483f 3021to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 3022exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 3023own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 3024(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 3025may help you in this, but it's nontrivial.) SCALAR will be read and/or
3b10bc60 3026written depending on the FUNCTION; a C pointer to the string value of SCALAR
19799a22 3027will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
3028has no string value but does have a numeric value, that value will be
3029passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
3030true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
3031functions may be needed to manipulate the values of structures used by
b76cc8ba 3032C<ioctl>.
a0d0e21e 3033
19799a22 3034The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e 3035
5ed4f2ec 3036 if OS returns: then Perl returns:
3037 -1 undefined value
3038 0 string "0 but true"
3039 anything else that number
a0d0e21e 3040
19799a22 3041Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
3042still easily determine the actual value returned by the operating
3043system:
3044
2b5ab1e7 3045 $retval = ioctl(...) || -1;
a0d0e21e
LW
3046 printf "System returned %d\n", $retval;
3047
be2f7487 3048The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
3049about improper numeric conversions.
3050
ea9eb35a 3051Portability issues: L<perlport/ioctl>.