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perlfunc: Document implicit $_ in while(each)
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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
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3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
8f1da26d 15operator. A unary operator generally provides scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
3b10bc60 17contexts for its arguments. If it does both, scalar arguments
18come first and list argument follow, and there can only ever
19be one such list argument. For instance, splice() has three scalar
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20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
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22
23In the syntax descriptions that follow, list operators that expect a
3b10bc60 24list (and provide list context for elements of the list) are shown
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25with LIST as an argument. Such a list may consist of any combination
26of scalar arguments or list values; the list values will be included
27in the list as if each individual element were interpolated at that
28point in the list, forming a longer single-dimensional list value.
8bdbc703 29Commas should separate literal elements of the LIST.
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30
31Any function in the list below may be used either with or without
32parentheses around its arguments. (The syntax descriptions omit the
3b10bc60 33parentheses.) If you use parentheses, the simple but occasionally
34surprising rule is this: It I<looks> like a function, therefore it I<is> a
a0d0e21e 35function, and precedence doesn't matter. Otherwise it's a list
3b10bc60 36operator or unary operator, and precedence does matter. Whitespace
37between the function and left parenthesis doesn't count, so sometimes
38you need to be careful:
a0d0e21e 39
5ed4f2ec 40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
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45
46If you run Perl with the B<-w> switch it can warn you about this. For
47example, the third line above produces:
48
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
51
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52A few functions take no arguments at all, and therefore work as neither
53unary nor list operators. These include such functions as C<time>
54and C<endpwent>. For example, C<time+86_400> always means
55C<time() + 86_400>.
56
a0d0e21e 57For functions that can be used in either a scalar or list context,
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58nonabortive failure is generally indicated in scalar context by
59returning the undefined value, and in list context by returning the
3b10bc60 60empty list.
a0d0e21e 61
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62Remember the following important rule: There is B<no rule> that relates
63the behavior of an expression in list context to its behavior in scalar
64context, or vice versa. It might do two totally different things.
80d38338 65Each operator and function decides which sort of value would be most
2b5ab1e7 66appropriate to return in scalar context. Some operators return the
5a964f20 67length of the list that would have been returned in list context. Some
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68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
d74e8afc 72X<context>
a0d0e21e 73
d1be9408 74A named array in scalar context is quite different from what would at
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75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
3b10bc60 81In general, functions in Perl that serve as wrappers for system calls ("syscalls")
5dac7880 82of the same name (like chown(2), fork(2), closedir(2), etc.) return
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83true when they succeed and C<undef> otherwise, as is usually mentioned
84in the descriptions below. This is different from the C interfaces,
5dac7880 85which return C<-1> on failure. Exceptions to this rule include C<wait>,
19799a22 86C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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87variable on failure. Other functions do not, except accidentally.
88
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89Extension modules can also hook into the Perl parser to define new
90kinds of keyword-headed expression. These may look like functions, but
91may also look completely different. The syntax following the keyword
92is defined entirely by the extension. If you are an implementor, see
93L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
94a module, see the module's documentation for details of the syntax that
95it defines.
96
cb1a09d0 97=head2 Perl Functions by Category
d74e8afc 98X<function>
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99
100Here are Perl's functions (including things that look like
5a964f20 101functions, like some keywords and named operators)
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102arranged by category. Some functions appear in more
103than one place.
104
13a2d996 105=over 4
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106
107=item Functions for SCALARs or strings
d74e8afc 108X<scalar> X<string> X<character>
cb1a09d0 109
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110=for Pod::Functions =String
111
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112C<chomp>, C<chop>, C<chr>, C<crypt>, C<fc>, C<hex>, C<index>, C<lc>,
113C<lcfirst>, C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
945c54fd 114C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
cb1a09d0 115
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116C<fc> is available only if the C<"fc"> feature is enabled or if it is
117prefixed with C<CORE::>. The C<"fc"> feature is enabled automatically
3dd9a840 118with a C<use v5.16> (or higher) declaration in the current scope.
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119
120
cb1a09d0 121=item Regular expressions and pattern matching
d74e8afc 122X<regular expression> X<regex> X<regexp>
cb1a09d0 123
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124=for Pod::Functions =Regexp
125
f5fa2679 126C<m//>, C<pos>, C<qr//>, C<quotemeta>, C<s///>, C<split>, C<study>
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127
128=item Numeric functions
d74e8afc 129X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 130
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131=for Pod::Functions =Math
132
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133C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
134C<sin>, C<sqrt>, C<srand>
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135
136=item Functions for real @ARRAYs
d74e8afc 137X<array>
cb1a09d0 138
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139=for Pod::Functions =ARRAY
140
a5ce339c 141C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
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142
143=item Functions for list data
d74e8afc 144X<list>
cb1a09d0 145
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146=for Pod::Functions =LIST
147
1dc8ecb8 148C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
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149
150=item Functions for real %HASHes
d74e8afc 151X<hash>
cb1a09d0 152
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153=for Pod::Functions =HASH
154
22fae026 155C<delete>, C<each>, C<exists>, C<keys>, C<values>
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156
157=item Input and output functions
d74e8afc 158X<I/O> X<input> X<output> X<dbm>
cb1a09d0 159
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160=for Pod::Functions =I/O
161
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162C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
163C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
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164C<readdir>, C<readline> C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>,
165C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>,
166C<truncate>, C<warn>, C<write>
cb1a09d0 167
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168C<say> is available only if the C<"say"> feature is enabled or if it is
169prefixed with C<CORE::>. The C<"say"> feature is enabled automatically
170with a C<use v5.10> (or higher) declaration in the current scope.
171
5dac7880 172=item Functions for fixed-length data or records
cb1a09d0 173
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174=for Pod::Functions =Binary
175
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176C<pack>, C<read>, C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<unpack>,
177C<vec>
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178
179=item Functions for filehandles, files, or directories
d74e8afc 180X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 181
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182=for Pod::Functions =File
183
22fae026 184C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 185C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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186C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
187C<umask>, C<unlink>, C<utime>
cb1a09d0 188
cf264981 189=item Keywords related to the control flow of your Perl program
d74e8afc 190X<control flow>
cb1a09d0 191
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192=for Pod::Functions =Flow
193
dba7b065 194C<break>, C<caller>, C<continue>, C<die>, C<do>,
7289c5e6 195C<dump>, C<eval>, C<evalbytes> C<exit>,
cfa52385 196C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
17d15541 197C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray>
84ed0108 198
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199C<break> is available only if you enable the experimental C<"switch">
200feature or use the C<CORE::> prefix. The C<"switch"> feature also enables
201the C<default>, C<given> and C<when> statements, which are documented in
202L<perlsyn/"Switch Statements">. The C<"switch"> feature is enabled
203automatically with a C<use v5.10> (or higher) declaration in the current
204scope. In Perl v5.14 and earlier, C<continue> required the C<"switch">
205feature, like the other keywords.
206
e3f68f70 207C<evalbytes> is only available with the C<"evalbytes"> feature (see
4fe70ef9 208L<feature>) or if prefixed with C<CORE::>. C<__SUB__> is only available
e3f68f70 209with the C<"current_sub"> feature or if prefixed with C<CORE::>. Both
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210the C<"evalbytes"> and C<"current_sub"> features are enabled automatically
211with a C<use v5.16> (or higher) declaration in the current scope.
cb1a09d0 212
54310121 213=item Keywords related to scoping
cb1a09d0 214
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215=for Pod::Functions =Namespace
216
8f1da26d 217C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
36fb85f3 218
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219C<state> is available only if the C<"state"> feature is enabled or if it is
220prefixed with C<CORE::>. The C<"state"> feature is enabled automatically
221with a C<use v5.10> (or higher) declaration in the current scope.
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222
223=item Miscellaneous functions
224
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225=for Pod::Functions =Misc
226
17d15541 227C<defined>, C<formline>, C<lock>, C<prototype>, C<reset>, C<scalar>, C<undef>
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228
229=item Functions for processes and process groups
d74e8afc 230X<process> X<pid> X<process id>
cb1a09d0 231
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232=for Pod::Functions =Process
233
22fae026 234C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
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235C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>,
236C<setpriority>, C<sleep>, C<system>,
22fae026 237C<times>, C<wait>, C<waitpid>
cb1a09d0 238
3b10bc60 239=item Keywords related to Perl modules
d74e8afc 240X<module>
cb1a09d0 241
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242=for Pod::Functions =Modules
243
22fae026 244C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 245
353c6505 246=item Keywords related to classes and object-orientation
d74e8afc 247X<object> X<class> X<package>
cb1a09d0 248
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249=for Pod::Functions =Objects
250
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251C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
252C<untie>, C<use>
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253
254=item Low-level socket functions
d74e8afc 255X<socket> X<sock>
cb1a09d0 256
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257=for Pod::Functions =Socket
258
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259C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
260C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 261C<socket>, C<socketpair>
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262
263=item System V interprocess communication functions
d74e8afc 264X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 265
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266=for Pod::Functions =SysV
267
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268C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
269C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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270
271=item Fetching user and group info
d74e8afc 272X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 273
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274=for Pod::Functions =User
275
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276C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
277C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
278C<getpwuid>, C<setgrent>, C<setpwent>
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279
280=item Fetching network info
d74e8afc 281X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 282
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283=for Pod::Functions =Network
284
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285C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
286C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
287C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
288C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
289C<setnetent>, C<setprotoent>, C<setservent>
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290
291=item Time-related functions
d74e8afc 292X<time> X<date>
cb1a09d0 293
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294=for Pod::Functions =Time
295
22fae026 296C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 297
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298=item Non-function keywords
299
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300=for Pod::Functions =!Non-functions
301
f5fa2679 302C<and>, C<AUTOLOAD>, C<BEGIN>, C<CHECK>, C<cmp>, C<CORE>, C<__DATA__>,
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303C<default>, C<DESTROY>, C<else>, C<elseif>, C<elsif>, C<END>, C<__END__>,
304C<eq>, C<for>, C<foreach>, C<ge>, C<given>, C<gt>, C<if>, C<INIT>, C<le>,
305C<lt>, C<ne>, C<not>, C<or>, C<UNITCHECK>, C<unless>, C<until>, C<when>,
306C<while>, C<x>, C<xor>
8f0d6a61 307
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308=back
309
60f9f73c 310=head2 Portability
d74e8afc 311X<portability> X<Unix> X<portable>
60f9f73c 312
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313Perl was born in Unix and can therefore access all common Unix
314system calls. In non-Unix environments, the functionality of some
8f1da26d 315Unix system calls may not be available or details of the available
2b5ab1e7 316functionality may differ slightly. The Perl functions affected
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317by this are:
318
319C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
320C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
321C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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322C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
323C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 324C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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325C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
326C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
327C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 328C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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329C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
330C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
331C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 332C<shmwrite>, C<socket>, C<socketpair>,
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333C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
334C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 335C<utime>, C<wait>, C<waitpid>
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336
337For more information about the portability of these functions, see
338L<perlport> and other available platform-specific documentation.
339
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340=head2 Alphabetical Listing of Perl Functions
341
3b10bc60 342=over
a0d0e21e 343
5b3c99c0 344=item -X FILEHANDLE
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345X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
346X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 347
5b3c99c0 348=item -X EXPR
a0d0e21e 349
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350=item -X DIRHANDLE
351
5b3c99c0 352=item -X
a0d0e21e 353
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354=for Pod::Functions a file test (-r, -x, etc)
355
a0d0e21e 356A file test, where X is one of the letters listed below. This unary
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357operator takes one argument, either a filename, a filehandle, or a dirhandle,
358and tests the associated file to see if something is true about it. If the
7660c0ab 359argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 360Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 361the undefined value if the file doesn't exist. Despite the funny
d0821a6a 362names, precedence is the same as any other named unary operator. The
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363operator may be any of:
364
5ed4f2ec 365 -r File is readable by effective uid/gid.
366 -w File is writable by effective uid/gid.
367 -x File is executable by effective uid/gid.
368 -o File is owned by effective uid.
a0d0e21e 369
5ed4f2ec 370 -R File is readable by real uid/gid.
371 -W File is writable by real uid/gid.
372 -X File is executable by real uid/gid.
373 -O File is owned by real uid.
a0d0e21e 374
5ed4f2ec 375 -e File exists.
376 -z File has zero size (is empty).
377 -s File has nonzero size (returns size in bytes).
a0d0e21e 378
5ed4f2ec 379 -f File is a plain file.
380 -d File is a directory.
381 -l File is a symbolic link.
382 -p File is a named pipe (FIFO), or Filehandle is a pipe.
383 -S File is a socket.
384 -b File is a block special file.
385 -c File is a character special file.
386 -t Filehandle is opened to a tty.
a0d0e21e 387
5ed4f2ec 388 -u File has setuid bit set.
389 -g File has setgid bit set.
390 -k File has sticky bit set.
a0d0e21e 391
5ed4f2ec 392 -T File is an ASCII text file (heuristic guess).
393 -B File is a "binary" file (opposite of -T).
a0d0e21e 394
5ed4f2ec 395 -M Script start time minus file modification time, in days.
396 -A Same for access time.
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397 -C Same for inode change time (Unix, may differ for other
398 platforms)
a0d0e21e 399
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400Example:
401
402 while (<>) {
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403 chomp;
404 next unless -f $_; # ignore specials
405 #...
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406 }
407
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408Note that C<-s/a/b/> does not do a negated substitution. Saying
409C<-exp($foo)> still works as expected, however: only single letters
410following a minus are interpreted as file tests.
411
412These operators are exempt from the "looks like a function rule" described
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413above. That is, an opening parenthesis after the operator does not affect
414how much of the following code constitutes the argument. Put the opening
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415parentheses before the operator to separate it from code that follows (this
416applies only to operators with higher precedence than unary operators, of
417course):
418
419 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
420 (-s $file) + 1024 # correct
421
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422The interpretation of the file permission operators C<-r>, C<-R>,
423C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
424of the file and the uids and gids of the user. There may be other
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425reasons you can't actually read, write, or execute the file: for
426example network filesystem access controls, ACLs (access control lists),
427read-only filesystems, and unrecognized executable formats. Note
428that the use of these six specific operators to verify if some operation
429is possible is usually a mistake, because it may be open to race
430conditions.
5ff3f7a4 431
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432Also note that, for the superuser on the local filesystems, the C<-r>,
433C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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434if any execute bit is set in the mode. Scripts run by the superuser
435may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 436or temporarily set their effective uid to something else.
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437
438If you are using ACLs, there is a pragma called C<filetest> that may
439produce more accurate results than the bare stat() mode bits.
5dac7880
FC
440When under C<use filetest 'access'> the above-mentioned filetests
441test whether the permission can(not) be granted using the
3b10bc60 442access(2) family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
443under this pragma return true even if there are no execute permission
444bits set (nor any extra execute permission ACLs). This strangeness is
391b733c 445due to the underlying system calls' definitions. Note also that, due to
ecae030f
MO
446the implementation of C<use filetest 'access'>, the C<_> special
447filehandle won't cache the results of the file tests when this pragma is
448in effect. Read the documentation for the C<filetest> pragma for more
449information.
5ff3f7a4 450
a0d0e21e
LW
451The C<-T> and C<-B> switches work as follows. The first block or so of the
452file is examined for odd characters such as strange control codes or
61eff3bc 453characters with the high bit set. If too many strange characters (>30%)
cf264981 454are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 455containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 456or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 457rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 458file, or a file at EOF when testing a filehandle. Because you have to
4633a7c4
LW
459read a file to do the C<-T> test, on most occasions you want to use a C<-f>
460against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 461
5dac7880 462If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 463the special filehandle consisting of a solitary underline, then the stat
a0d0e21e
LW
464structure of the previous file test (or stat operator) is used, saving
465a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 466that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 467symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 468an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 469Example:
a0d0e21e
LW
470
471 print "Can do.\n" if -r $a || -w _ || -x _;
472
473 stat($filename);
474 print "Readable\n" if -r _;
475 print "Writable\n" if -w _;
476 print "Executable\n" if -x _;
477 print "Setuid\n" if -u _;
478 print "Setgid\n" if -g _;
479 print "Sticky\n" if -k _;
480 print "Text\n" if -T _;
481 print "Binary\n" if -B _;
482
e9fa405d 483As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file
fbb0b3b3 484test operators, in a way that C<-f -w -x $file> is equivalent to
391b733c 485C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
fbb0b3b3
RGS
486the return value of C<-f $file> as an argument to another filetest
487operator, no special magic will happen.)
488
bee96257 489Portability issues: L<perlport/-X>.
ea9eb35a 490
bade7fbc
TC
491To avoid confusing would-be users of your code with mysterious
492syntax errors, put something like this at the top of your script:
493
494 use 5.010; # so filetest ops can stack
495
a0d0e21e 496=item abs VALUE
d74e8afc 497X<abs> X<absolute>
a0d0e21e 498
54310121 499=item abs
bbce6d69 500
c17cdb72
NC
501=for Pod::Functions absolute value function
502
a0d0e21e 503Returns the absolute value of its argument.
7660c0ab 504If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
505
506=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 507X<accept>
a0d0e21e 508
c17cdb72
NC
509=for Pod::Functions accept an incoming socket connect
510
3b10bc60 511Accepts an incoming socket connect, just as accept(2)
19799a22 512does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 513See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 514
8d2a6795
GS
515On systems that support a close-on-exec flag on files, the flag will
516be set for the newly opened file descriptor, as determined by the
517value of $^F. See L<perlvar/$^F>.
518
a0d0e21e 519=item alarm SECONDS
d74e8afc
ITB
520X<alarm>
521X<SIGALRM>
522X<timer>
a0d0e21e 523
54310121 524=item alarm
bbce6d69 525
c17cdb72
NC
526=for Pod::Functions schedule a SIGALRM
527
a0d0e21e 528Arranges to have a SIGALRM delivered to this process after the
cf264981 529specified number of wallclock seconds has elapsed. If SECONDS is not
391b733c 530specified, the value stored in C<$_> is used. (On some machines,
d400eac8
JH
531unfortunately, the elapsed time may be up to one second less or more
532than you specified because of how seconds are counted, and process
533scheduling may delay the delivery of the signal even further.)
534
535Only one timer may be counting at once. Each call disables the
536previous timer, and an argument of C<0> may be supplied to cancel the
537previous timer without starting a new one. The returned value is the
538amount of time remaining on the previous timer.
a0d0e21e 539
2bc69794
BS
540For delays of finer granularity than one second, the Time::HiRes module
541(from CPAN, and starting from Perl 5.8 part of the standard
542distribution) provides ualarm(). You may also use Perl's four-argument
543version of select() leaving the first three arguments undefined, or you
544might be able to use the C<syscall> interface to access setitimer(2) if
391b733c 545your system supports it. See L<perlfaq8> for details.
2b5ab1e7 546
80d38338
TC
547It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
548C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 549
19799a22
GS
550If you want to use C<alarm> to time out a system call you need to use an
551C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 552fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 553restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 554modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
555
556 eval {
a9a5a0dc
VP
557 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
558 alarm $timeout;
559 $nread = sysread SOCKET, $buffer, $size;
560 alarm 0;
ff68c719 561 };
ff68c719 562 if ($@) {
a9a5a0dc 563 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 564 # timed out
ff68c719
PP
565 }
566 else {
5ed4f2ec 567 # didn't
ff68c719
PP
568 }
569
91d81acc
JH
570For more information see L<perlipc>.
571
ea9eb35a 572Portability issues: L<perlport/alarm>.
573
a0d0e21e 574=item atan2 Y,X
d74e8afc 575X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e 576
c17cdb72
NC
577=for Pod::Functions arctangent of Y/X in the range -PI to PI
578
a0d0e21e
LW
579Returns the arctangent of Y/X in the range -PI to PI.
580
ca6e1c26 581For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
582function, or use the familiar relation:
583
584 sub tan { sin($_[0]) / cos($_[0]) }
585
a1021d57
RGS
586The return value for C<atan2(0,0)> is implementation-defined; consult
587your atan2(3) manpage for more information.
bf5f1b4c 588
ea9eb35a 589Portability issues: L<perlport/atan2>.
590
a0d0e21e 591=item bind SOCKET,NAME
d74e8afc 592X<bind>
a0d0e21e 593
c17cdb72
NC
594=for Pod::Functions binds an address to a socket
595
3b10bc60 596Binds a network address to a socket, just as bind(2)
19799a22 597does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
598packed address of the appropriate type for the socket. See the examples in
599L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 600
fae2c0fb 601=item binmode FILEHANDLE, LAYER
d74e8afc 602X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 603
a0d0e21e
LW
604=item binmode FILEHANDLE
605
c17cdb72
NC
606=for Pod::Functions prepare binary files for I/O
607
1cbfc93d
NIS
608Arranges for FILEHANDLE to be read or written in "binary" or "text"
609mode on systems where the run-time libraries distinguish between
610binary and text files. If FILEHANDLE is an expression, the value is
611taken as the name of the filehandle. Returns true on success,
b5fe5ca2 612otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 613
8f1da26d 614On some systems (in general, DOS- and Windows-based systems) binmode()
d807c6f4 615is necessary when you're not working with a text file. For the sake
d7a0d798
FC
616of portability it is a good idea always to use it when appropriate,
617and never to use it when it isn't appropriate. Also, people can
8f1da26d 618set their I/O to be by default UTF8-encoded Unicode, not bytes.
d807c6f4
JH
619
620In other words: regardless of platform, use binmode() on binary data,
d7a0d798 621like images, for example.
d807c6f4
JH
622
623If LAYER is present it is a single string, but may contain multiple
391b733c 624directives. The directives alter the behaviour of the filehandle.
d7a0d798 625When LAYER is present, using binmode on a text file makes sense.
d807c6f4 626
fae2c0fb 627If LAYER is omitted or specified as C<:raw> the filehandle is made
391b733c 628suitable for passing binary data. This includes turning off possible CRLF
0226bbdb 629translation and marking it as bytes (as opposed to Unicode characters).
749683d2 630Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 631Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
632Other layers that would affect the binary nature of the stream are
391b733c 633I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 634PERLIO environment variable.
01e6739c 635
3b10bc60 636The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
637form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
638establish default I/O layers. See L<open>.
639
fae2c0fb
RGS
640I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
641in "Programming Perl, 3rd Edition". However, since the publishing of this
642book, by many known as "Camel III", the consensus of the naming of this
643functionality has moved from "discipline" to "layer". All documentation
644of this version of Perl therefore refers to "layers" rather than to
645"disciplines". Now back to the regularly scheduled documentation...>
646
8f1da26d 647To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
6902c96a 648C<:utf8> just marks the data as UTF-8 without further checking,
8f1da26d 649while C<:encoding(UTF-8)> checks the data for actually being valid
391b733c 650UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 651
ed53a2bb 652In general, binmode() should be called after open() but before any I/O
3b10bc60 653is done on the filehandle. Calling binmode() normally flushes any
01e6739c 654pending buffered output data (and perhaps pending input data) on the
fae2c0fb 655handle. An exception to this is the C<:encoding> layer that
d7a0d798 656changes the default character encoding of the handle; see L</open>.
fae2c0fb 657The C<:encoding> layer sometimes needs to be called in
3874323d
JH
658mid-stream, and it doesn't flush the stream. The C<:encoding>
659also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 660internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 661
19799a22 662The operating system, device drivers, C libraries, and Perl run-time
8f1da26d
TC
663system all conspire to let the programmer treat a single
664character (C<\n>) as the line terminator, irrespective of external
30168b04
GS
665representation. On many operating systems, the native text file
666representation matches the internal representation, but on some
667platforms the external representation of C<\n> is made up of more than
668one character.
669
8f1da26d
TC
670All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
671a single character to end each line in the external representation of text
672(even though that single character is CARRIAGE RETURN on old, pre-Darwin
391b733c 673flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
8f1da26d
TC
674systems like OS/2, DOS, and the various flavors of MS-Windows, your program
675sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
676two characters C<\cM\cJ>. That means that if you don't use binmode() on
677these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
678input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
679output. This is what you want for text files, but it can be disastrous for
680binary files.
30168b04
GS
681
682Another consequence of using binmode() (on some systems) is that
683special end-of-file markers will be seen as part of the data stream.
d7a0d798
FC
684For systems from the Microsoft family this means that, if your binary
685data contain C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
686the file, unless you use binmode().
687
3b10bc60 688binmode() is important not only for readline() and print() operations,
30168b04
GS
689but also when using read(), seek(), sysread(), syswrite() and tell()
690(see L<perlport> for more details). See the C<$/> and C<$\> variables
691in L<perlvar> for how to manually set your input and output
692line-termination sequences.
a0d0e21e 693
ea9eb35a 694Portability issues: L<perlport/binmode>.
695
4633a7c4 696=item bless REF,CLASSNAME
d74e8afc 697X<bless>
a0d0e21e
LW
698
699=item bless REF
700
c17cdb72
NC
701=for Pod::Functions create an object
702
2b5ab1e7
TC
703This function tells the thingy referenced by REF that it is now an object
704in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 705is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 706it returns the reference for convenience. Always use the two-argument
cf264981 707version if a derived class might inherit the function doing the blessing.
e54e4959 708See L<perlobj> for more about the blessing (and blessings) of objects.
a0d0e21e 709
57668c4d 710Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 711Namespaces with all lowercase names are considered reserved for
391b733c 712Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
713confusion, you may wish to avoid such package names as well. Make sure
714that CLASSNAME is a true value.
60ad88b8
GS
715
716See L<perlmod/"Perl Modules">.
717
0d863452
RH
718=item break
719
d9b04284 720=for Pod::Functions +switch break out of a C<given> block
c17cdb72 721
0d863452
RH
722Break out of a C<given()> block.
723
8f1da26d 724This keyword is enabled by the C<"switch"> feature: see
4a904372
FC
725L<feature> for more information. You can also access it by
726prefixing it with C<CORE::>. Alternately, include a C<use
8f1da26d 727v5.10> or later to the current scope.
0d863452 728
a0d0e21e 729=item caller EXPR
d74e8afc 730X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
731
732=item caller
733
c17cdb72
NC
734=for Pod::Functions get context of the current subroutine call
735
5a964f20 736Returns the context of the current subroutine call. In scalar context,
80d38338
TC
737returns the caller's package name if there I<is> a caller (that is, if
738we're in a subroutine or C<eval> or C<require>) and the undefined value
5a964f20 739otherwise. In list context, returns
a0d0e21e 740
ee6b43cc 741 # 0 1 2
748a9306 742 ($package, $filename, $line) = caller;
a0d0e21e
LW
743
744With EXPR, it returns some extra information that the debugger uses to
745print a stack trace. The value of EXPR indicates how many call frames
746to go back before the current one.
747
ee6b43cc 748 # 0 1 2 3 4
f3aa04c2 749 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 750
751 # 5 6 7 8 9 10
b3ca2e83 752 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 753 = caller($i);
e7ea3e70 754
951ba7fe 755Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 756call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 757C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 758C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 759C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 760$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
761each C<use> statement creates a C<require> frame inside an C<eval EXPR>
762frame.) $subroutine may also be C<(unknown)> if this particular
763subroutine happens to have been deleted from the symbol table.
764C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
765C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
585d73c3 766compiled with. C<$hints> corresponds to C<$^H>, and C<$bitmask>
1adb05cd
FC
767corresponds to C<${^WARNING_BITS}>. The
768C<$hints> and C<$bitmask> values are subject
585d73c3 769to change between versions of Perl, and are not meant for external use.
748a9306 770
b3ca2e83 771C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
391b733c 772caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
b3ca2e83
NC
773of this hash, as they are the actual values stored in the optree.
774
ffe0c19d
FC
775Furthermore, when called from within the DB package in
776list context, and with an argument, caller returns more
7660c0ab 777detailed information: it sets the list variable C<@DB::args> to be the
54310121 778arguments with which the subroutine was invoked.
748a9306 779
7660c0ab 780Be aware that the optimizer might have optimized call frames away before
19799a22 781C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 782might not return information about the call frame you expect it to, for
b76cc8ba 783C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 784previous time C<caller> was called.
7660c0ab 785
8f1da26d 786Be aware that setting C<@DB::args> is I<best effort>, intended for
391b733c 787debugging or generating backtraces, and should not be relied upon. In
ca9f0cb5
NC
788particular, as C<@_> contains aliases to the caller's arguments, Perl does
789not take a copy of C<@_>, so C<@DB::args> will contain modifications the
790subroutine makes to C<@_> or its contents, not the original values at call
391b733c 791time. C<@DB::args>, like C<@_>, does not hold explicit references to its
ca9f0cb5 792elements, so under certain cases its elements may have become freed and
391b733c 793reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 794of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
795I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
796reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5 797elements), so C<@DB::args> is actually a hybrid of the current state and
391b733c 798initial state of C<@_>. Buyer beware.
ca9f0cb5 799
a0d0e21e 800=item chdir EXPR
d74e8afc
ITB
801X<chdir>
802X<cd>
f723aae1 803X<directory, change>
a0d0e21e 804
c4aca7d0
GA
805=item chdir FILEHANDLE
806
807=item chdir DIRHANDLE
808
ce2984c3
PF
809=item chdir
810
c17cdb72
NC
811=for Pod::Functions change your current working directory
812
391b733c 813Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 814changes to the directory specified by C<$ENV{HOME}>, if set; if not,
391b733c
FC
815changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
816variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
817neither is set, C<chdir> does nothing. It returns true on success,
818false otherwise. See the example under C<die>.
a0d0e21e 819
3b10bc60 820On systems that support fchdir(2), you may pass a filehandle or
34169887 821directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 822passing handles raises an exception.
c4aca7d0 823
a0d0e21e 824=item chmod LIST
d74e8afc 825X<chmod> X<permission> X<mode>
a0d0e21e 826
c17cdb72
NC
827=for Pod::Functions changes the permissions on a list of files
828
a0d0e21e 829Changes the permissions of a list of files. The first element of the
8f1da26d 830list must be the numeric mode, which should probably be an octal
4ad40acf 831number, and which definitely should I<not> be a string of octal digits:
3b10bc60 832C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 833successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 834
3b10bc60 835 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 836 chmod 0755, @executables;
3b10bc60 837 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 838 # --w----r-T
3b10bc60 839 $mode = "0644"; chmod oct($mode), "foo"; # this is better
840 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 841
3b10bc60 842On systems that support fchmod(2), you may pass filehandles among the
843files. On systems that don't support fchmod(2), passing filehandles raises
844an exception. Filehandles must be passed as globs or glob references to be
845recognized; barewords are considered filenames.
c4aca7d0
GA
846
847 open(my $fh, "<", "foo");
848 my $perm = (stat $fh)[2] & 07777;
849 chmod($perm | 0600, $fh);
850
3b10bc60 851You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
852module:
853
3b10bc60 854 use Fcntl qw( :mode );
ca6e1c26 855 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 856 # Identical to the chmod 0755 of the example above.
ca6e1c26 857
ea9eb35a 858Portability issues: L<perlport/chmod>.
859
a0d0e21e 860=item chomp VARIABLE
d74e8afc 861X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 862
313c9f5c 863=item chomp( LIST )
a0d0e21e
LW
864
865=item chomp
866
c17cdb72
NC
867=for Pod::Functions remove a trailing record separator from a string
868
2b5ab1e7
TC
869This safer version of L</chop> removes any trailing string
870that corresponds to the current value of C<$/> (also known as
28757baa
PP
871$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
872number of characters removed from all its arguments. It's often used to
873remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
874that the final record may be missing its newline. When in paragraph
875mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 876When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 877a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 878remove anything.
19799a22 879If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
880
881 while (<>) {
a9a5a0dc
VP
882 chomp; # avoid \n on last field
883 @array = split(/:/);
884 # ...
a0d0e21e
LW
885 }
886
4bf21a6d
RD
887If VARIABLE is a hash, it chomps the hash's values, but not its keys.
888
a0d0e21e
LW
889You can actually chomp anything that's an lvalue, including an assignment:
890
891 chomp($cwd = `pwd`);
892 chomp($answer = <STDIN>);
893
894If you chomp a list, each element is chomped, and the total number of
895characters removed is returned.
896
15e44fd8
RGS
897Note that parentheses are necessary when you're chomping anything
898that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
899is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
900C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
901C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
902as C<chomp($a, $b)>.
903
a0d0e21e 904=item chop VARIABLE
d74e8afc 905X<chop>
a0d0e21e 906
313c9f5c 907=item chop( LIST )
a0d0e21e
LW
908
909=item chop
910
c17cdb72
NC
911=for Pod::Functions remove the last character from a string
912
a0d0e21e 913Chops off the last character of a string and returns the character
5b3eff12 914chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 915scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
916If VARIABLE is a hash, it chops the hash's values, but not its keys.
917
5b3eff12 918You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
919
920If you chop a list, each element is chopped. Only the value of the
19799a22 921last C<chop> is returned.
a0d0e21e 922
19799a22 923Note that C<chop> returns the last character. To return all but the last
748a9306
LW
924character, use C<substr($string, 0, -1)>.
925
15e44fd8
RGS
926See also L</chomp>.
927
a0d0e21e 928=item chown LIST
d74e8afc 929X<chown> X<owner> X<user> X<group>
a0d0e21e 930
c17cdb72
NC
931=for Pod::Functions change the ownership on a list of files
932
a0d0e21e 933Changes the owner (and group) of a list of files. The first two
19799a22
GS
934elements of the list must be the I<numeric> uid and gid, in that
935order. A value of -1 in either position is interpreted by most
936systems to leave that value unchanged. Returns the number of files
937successfully changed.
a0d0e21e
LW
938
939 $cnt = chown $uid, $gid, 'foo', 'bar';
940 chown $uid, $gid, @filenames;
941
3b10bc60 942On systems that support fchown(2), you may pass filehandles among the
943files. On systems that don't support fchown(2), passing filehandles raises
944an exception. Filehandles must be passed as globs or glob references to be
945recognized; barewords are considered filenames.
c4aca7d0 946
54310121 947Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
948
949 print "User: ";
19799a22 950 chomp($user = <STDIN>);
5a964f20 951 print "Files: ";
19799a22 952 chomp($pattern = <STDIN>);
a0d0e21e
LW
953
954 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 955 or die "$user not in passwd file";
a0d0e21e 956
5ed4f2ec 957 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
958 chown $uid, $gid, @ary;
959
54310121 960On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
961file unless you're the superuser, although you should be able to change
962the group to any of your secondary groups. On insecure systems, these
963restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
964On POSIX systems, you can detect this condition this way:
965
966 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
967 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 968
ea9eb35a 969Portability issues: L<perlport/chmod>.
970
a0d0e21e 971=item chr NUMBER
d74e8afc 972X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 973
54310121 974=item chr
bbce6d69 975
c17cdb72
NC
976=for Pod::Functions get character this number represents
977
a0d0e21e 978Returns the character represented by that NUMBER in the character set.
a0ed51b3 979For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 980chr(0x263a) is a Unicode smiley face.
aaa68c4a 981
8a064bd6 982Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 983except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
984(truncated to an integer) are used.
985
974da8e5
JH
986If NUMBER is omitted, uses C<$_>.
987
b76cc8ba 988For the reverse, use L</ord>.
a0d0e21e 989
2575c402
JW
990Note that characters from 128 to 255 (inclusive) are by default
991internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 992
2575c402 993See L<perlunicode> for more about Unicode.
bbce6d69 994
a0d0e21e 995=item chroot FILENAME
d74e8afc 996X<chroot> X<root>
a0d0e21e 997
54310121 998=item chroot
bbce6d69 999
c17cdb72
NC
1000=for Pod::Functions make directory new root for path lookups
1001
5a964f20 1002This function works like the system call by the same name: it makes the
4633a7c4 1003named directory the new root directory for all further pathnames that
951ba7fe 1004begin with a C</> by your process and all its children. (It doesn't
28757baa 1005change your current working directory, which is unaffected.) For security
4633a7c4 1006reasons, this call is restricted to the superuser. If FILENAME is
19799a22 1007omitted, does a C<chroot> to C<$_>.
a0d0e21e 1008
ea9eb35a 1009Portability issues: L<perlport/chroot>.
1010
a0d0e21e 1011=item close FILEHANDLE
d74e8afc 1012X<close>
a0d0e21e 1013
6a518fbc
TP
1014=item close
1015
c17cdb72
NC
1016=for Pod::Functions close file (or pipe or socket) handle
1017
3b10bc60 1018Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 1019buffers, and closes the system file descriptor. Returns true if those
8f1da26d 1020operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
1021layer. Closes the currently selected filehandle if the argument is
1022omitted.
fb73857a
PP
1023
1024You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 1025another C<open> on it, because C<open> closes it for you. (See
01aa884e 1026L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
19799a22 1027counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 1028
3b10bc60 1029If the filehandle came from a piped open, C<close> returns false if one of
1030the other syscalls involved fails or if its program exits with non-zero
1031status. If the only problem was that the program exited non-zero, C<$!>
1032will be set to C<0>. Closing a pipe also waits for the process executing
1033on the pipe to exit--in case you wish to look at the output of the pipe
1034afterwards--and implicitly puts the exit status value of that command into
1035C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 1036
2e0cfa16
FC
1037If there are multiple threads running, C<close> on a filehandle from a
1038piped open returns true without waiting for the child process to terminate,
1039if the filehandle is still open in another thread.
1040
80d38338
TC
1041Closing the read end of a pipe before the process writing to it at the
1042other end is done writing results in the writer receiving a SIGPIPE. If
1043the other end can't handle that, be sure to read all the data before
1044closing the pipe.
73689b13 1045
fb73857a 1046Example:
a0d0e21e 1047
fb73857a
PP
1048 open(OUTPUT, '|sort >foo') # pipe to sort
1049 or die "Can't start sort: $!";
5ed4f2ec 1050 #... # print stuff to output
1051 close OUTPUT # wait for sort to finish
fb73857a
PP
1052 or warn $! ? "Error closing sort pipe: $!"
1053 : "Exit status $? from sort";
5ed4f2ec 1054 open(INPUT, 'foo') # get sort's results
fb73857a 1055 or die "Can't open 'foo' for input: $!";
a0d0e21e 1056
5a964f20 1057FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 1058filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
1059
1060=item closedir DIRHANDLE
d74e8afc 1061X<closedir>
a0d0e21e 1062
c17cdb72
NC
1063=for Pod::Functions close directory handle
1064
19799a22 1065Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
1066system call.
1067
a0d0e21e 1068=item connect SOCKET,NAME
d74e8afc 1069X<connect>
a0d0e21e 1070
c17cdb72
NC
1071=for Pod::Functions connect to a remote socket
1072
80d38338
TC
1073Attempts to connect to a remote socket, just like connect(2).
1074Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
1075packed address of the appropriate type for the socket. See the examples in
1076L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 1077
cb1a09d0 1078=item continue BLOCK
d74e8afc 1079X<continue>
cb1a09d0 1080
0d863452
RH
1081=item continue
1082
c17cdb72
NC
1083=for Pod::Functions optional trailing block in a while or foreach
1084
4a904372
FC
1085When followed by a BLOCK, C<continue> is actually a
1086flow control statement rather than a function. If
cf264981 1087there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
1088C<foreach>), it is always executed just before the conditional is about to
1089be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
1090it can be used to increment a loop variable, even when the loop has been
1091continued via the C<next> statement (which is similar to the C C<continue>
1092statement).
1093
98293880 1094C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 1095block; C<last> and C<redo> behave as if they had been executed within
19799a22 1096the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
1097block, it may be more entertaining.
1098
1099 while (EXPR) {
a9a5a0dc
VP
1100 ### redo always comes here
1101 do_something;
1d2dff63 1102 } continue {
a9a5a0dc
VP
1103 ### next always comes here
1104 do_something_else;
1105 # then back the top to re-check EXPR
1d2dff63
GS
1106 }
1107 ### last always comes here
1108
3b10bc60 1109Omitting the C<continue> section is equivalent to using an
1110empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1111to check the condition at the top of the loop.
1112
4a904372 1113When there is no BLOCK, C<continue> is a function that
8f1da26d
TC
1114falls through the current C<when> or C<default> block instead of iterating
1115a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
4a904372
FC
1116In Perl 5.14 and earlier, this form of C<continue> was
1117only available when the C<"switch"> feature was enabled.
48238296 1118See L<feature> and L<perlsyn/"Switch Statements"> for more
8f1da26d 1119information.
0d863452 1120
a0d0e21e 1121=item cos EXPR
d74e8afc 1122X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1123
d6217f1e
GS
1124=item cos
1125
c17cdb72
NC
1126=for Pod::Functions cosine function
1127
5a964f20 1128Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1129takes the cosine of C<$_>.
a0d0e21e 1130
ca6e1c26 1131For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1132function, or use this relation:
1133
1134 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1135
a0d0e21e 1136=item crypt PLAINTEXT,SALT
d74e8afc 1137X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1138X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1139
c17cdb72
NC
1140=for Pod::Functions one-way passwd-style encryption
1141
ef2e6798
MS
1142Creates a digest string exactly like the crypt(3) function in the C
1143library (assuming that you actually have a version there that has not
bb23f8d1 1144been extirpated as a potential munition).
ef2e6798 1145
34169887 1146crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1147into a short string, called a digest, which is returned. The same
1148PLAINTEXT and SALT will always return the same string, but there is no
1149(known) way to get the original PLAINTEXT from the hash. Small
1150changes in the PLAINTEXT or SALT will result in large changes in the
1151digest.
1152
1153There is no decrypt function. This function isn't all that useful for
1154cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1155mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1156primarily used to check if two pieces of text are the same without
1157having to transmit or store the text itself. An example is checking
1158if a correct password is given. The digest of the password is stored,
cf264981 1159not the password itself. The user types in a password that is
ef2e6798 1160crypt()'d with the same salt as the stored digest. If the two digests
34169887 1161match, the password is correct.
ef2e6798
MS
1162
1163When verifying an existing digest string you should use the digest as
1164the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1165to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1166crypt() will hash the new string with the same salt as the digest.
1167This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1168with more exotic implementations. In other words, assume
1169nothing about the returned string itself nor about how many bytes
1170of SALT may matter.
85c16d83
JH
1171
1172Traditionally the result is a string of 13 bytes: two first bytes of
1173the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
391b733c 1174the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1175hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1176and implementations on non-Unix platforms may produce different
ef2e6798 1177strings.
85c16d83
JH
1178
1179When choosing a new salt create a random two character string whose
1180characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1181'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1182characters is just a recommendation; the characters allowed in
1183the salt depend solely on your system's crypt library, and Perl can't
1184restrict what salts C<crypt()> accepts.
e71965be 1185
a0d0e21e 1186Here's an example that makes sure that whoever runs this program knows
cf264981 1187their password:
a0d0e21e
LW
1188
1189 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1190
1191 system "stty -echo";
1192 print "Password: ";
e71965be 1193 chomp($word = <STDIN>);
a0d0e21e
LW
1194 print "\n";
1195 system "stty echo";
1196
e71965be 1197 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1198 die "Sorry...\n";
a0d0e21e 1199 } else {
a9a5a0dc 1200 print "ok\n";
54310121 1201 }
a0d0e21e 1202
9f8f0c9d 1203Of course, typing in your own password to whoever asks you
748a9306 1204for it is unwise.
a0d0e21e 1205
ef2e6798 1206The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1207of data, not least of all because you can't get the information
ef2e6798 1208back. Look at the L<Digest> module for more robust algorithms.
19799a22 1209
f2791508
JH
1210If using crypt() on a Unicode string (which I<potentially> has
1211characters with codepoints above 255), Perl tries to make sense
34169887 1212of the situation by trying to downgrade (a copy of)
f2791508
JH
1213the string back to an eight-bit byte string before calling crypt()
1214(on that copy). If that works, good. If not, crypt() dies with
1215C<Wide character in crypt>.
85c16d83 1216
ea9eb35a 1217Portability issues: L<perlport/crypt>.
1218
aa689395 1219=item dbmclose HASH
d74e8afc 1220X<dbmclose>
a0d0e21e 1221
c17cdb72
NC
1222=for Pod::Functions breaks binding on a tied dbm file
1223
19799a22 1224[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1225
aa689395 1226Breaks the binding between a DBM file and a hash.
a0d0e21e 1227
ea9eb35a 1228Portability issues: L<perlport/dbmclose>.
1229
19799a22 1230=item dbmopen HASH,DBNAME,MASK
d74e8afc 1231X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1232
c17cdb72
NC
1233=for Pod::Functions create binding on a tied dbm file
1234
01aa884e
KW
1235[This function has been largely superseded by the
1236L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
a0d0e21e 1237
7b8d334a 1238This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1239hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1240argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1241is the name of the database (without the F<.dir> or F<.pag> extension if
1242any). If the database does not exist, it is created with protection
1b3a6178
FC
1243specified by MASK (as modified by the C<umask>). To prevent creation of
1244the database if it doesn't exist, you may specify a MODE
1245of 0, and the function will return a false value if it
1246can't find an existing database. If your system supports
80d38338 1247only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1248program. In older versions of Perl, if your system had neither DBM nor
19799a22 1249ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1250sdbm(3).
1251
1252If you don't have write access to the DBM file, you can only read hash
1253variables, not set them. If you want to test whether you can write,
3b10bc60 1254either use file tests or try setting a dummy hash entry inside an C<eval>
1255to trap the error.
a0d0e21e 1256
19799a22
GS
1257Note that functions such as C<keys> and C<values> may return huge lists
1258when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1259function to iterate over large DBM files. Example:
1260
1261 # print out history file offsets
1262 dbmopen(%HIST,'/usr/lib/news/history',0666);
1263 while (($key,$val) = each %HIST) {
a9a5a0dc 1264 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1265 }
1266 dbmclose(%HIST);
1267
cb1a09d0 1268See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1269cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1270rich implementation.
4633a7c4 1271
2b5ab1e7
TC
1272You can control which DBM library you use by loading that library
1273before you call dbmopen():
1274
1275 use DB_File;
1276 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1277 or die "Can't open netscape history file: $!";
2b5ab1e7 1278
ea9eb35a 1279Portability issues: L<perlport/dbmopen>.
1280
a0d0e21e 1281=item defined EXPR
d74e8afc 1282X<defined> X<undef> X<undefined>
a0d0e21e 1283
54310121 1284=item defined
bbce6d69 1285
c17cdb72
NC
1286=for Pod::Functions test whether a value, variable, or function is defined
1287
2f9daede 1288Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1289the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1290checked.
1291
1292Many operations return C<undef> to indicate failure, end of file,
1293system error, uninitialized variable, and other exceptional
1294conditions. This function allows you to distinguish C<undef> from
1295other values. (A simple Boolean test will not distinguish among
7660c0ab 1296C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1297false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1298doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1299returns C<undef> when its argument is an empty array, I<or> when the
1300element to return happens to be C<undef>.
1301
f10b0346
GS
1302You may also use C<defined(&func)> to check whether subroutine C<&func>
1303has ever been defined. The return value is unaffected by any forward
80d38338 1304declarations of C<&func>. A subroutine that is not defined
847c7ebe 1305may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1306makes it spring into existence the first time that it is called; see
847c7ebe 1307L<perlsub>.
f10b0346
GS
1308
1309Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1310used to report whether memory for that aggregate had ever been
f10b0346
GS
1311allocated. This behavior may disappear in future versions of Perl.
1312You should instead use a simple test for size:
1313
1314 if (@an_array) { print "has array elements\n" }
1315 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1316
1317When used on a hash element, it tells you whether the value is defined,
dc848c6f 1318not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1319purpose.
a0d0e21e
LW
1320
1321Examples:
1322
8f1da26d 1323 print if defined $switch{D};
a0d0e21e
LW
1324 print "$val\n" while defined($val = pop(@ary));
1325 die "Can't readlink $sym: $!"
a9a5a0dc 1326 unless defined($value = readlink $sym);
a0d0e21e 1327 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1328 $debugging = 0 unless defined $debugging;
a0d0e21e 1329
8f1da26d 1330Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1331discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1332defined values. For example, if you say
a5f75d66
AD
1333
1334 "ab" =~ /a(.*)b/;
1335
80d38338 1336The pattern match succeeds and C<$1> is defined, although it
cf264981 1337matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1338matched something that happened to be zero characters long. This is all
a5f75d66 1339very above-board and honest. When a function returns an undefined value,
2f9daede 1340it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1341should use C<defined> only when questioning the integrity of what
7660c0ab 1342you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1343what you want.
1344
dc848c6f 1345See also L</undef>, L</exists>, L</ref>.
2f9daede 1346
a0d0e21e 1347=item delete EXPR
d74e8afc 1348X<delete>
a0d0e21e 1349
c17cdb72
NC
1350=for Pod::Functions deletes a value from a hash
1351
d0a76353
RS
1352Given an expression that specifies an element or slice of a hash, C<delete>
1353deletes the specified elements from that hash so that exists() on that element
1354no longer returns true. Setting a hash element to the undefined value does
1355not remove its key, but deleting it does; see L</exists>.
80d38338 1356
8f1da26d 1357In list context, returns the value or values deleted, or the last such
80d38338 1358element in scalar context. The return list's length always matches that of
d0a76353
RS
1359the argument list: deleting non-existent elements returns the undefined value
1360in their corresponding positions.
80d38338 1361
d0a76353
RS
1362delete() may also be used on arrays and array slices, but its behavior is less
1363straightforward. Although exists() will return false for deleted entries,
1364deleting array elements never changes indices of existing values; use shift()
1365or splice() for that. However, if all deleted elements fall at the end of an
1366array, the array's size shrinks to the position of the highest element that
1367still tests true for exists(), or to 0 if none do.
1368
8f1da26d 1369B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1370be removed in a future version of Perl.
80d38338
TC
1371
1372Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1373a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1374or array may not necessarily return anything; it depends on the implementation
1375of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1376
80d38338
TC
1377The C<delete local EXPR> construct localizes the deletion to the current
1378block at run time. Until the block exits, elements locally deleted
1379temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1380of composite types">.
eba0920a
EM
1381
1382 %hash = (foo => 11, bar => 22, baz => 33);
f7051f2c
FC
1383 $scalar = delete $hash{foo}; # $scalar is 11
1384 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1385 @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
eba0920a 1386
01020589 1387The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1388
5f05dabc 1389 foreach $key (keys %HASH) {
a9a5a0dc 1390 delete $HASH{$key};
a0d0e21e
LW
1391 }
1392
01020589 1393 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1394 delete $ARRAY[$index];
01020589
GS
1395 }
1396
1397And so do these:
5f05dabc 1398
01020589
GS
1399 delete @HASH{keys %HASH};
1400
9740c838 1401 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1402
80d38338
TC
1403But both are slower than assigning the empty list
1404or undefining %HASH or @ARRAY, which is the customary
1405way to empty out an aggregate:
01020589 1406
5ed4f2ec 1407 %HASH = (); # completely empty %HASH
1408 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1409
5ed4f2ec 1410 @ARRAY = (); # completely empty @ARRAY
1411 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1412
80d38338
TC
1413The EXPR can be arbitrarily complicated provided its
1414final operation is an element or slice of an aggregate:
a0d0e21e
LW
1415
1416 delete $ref->[$x][$y]{$key};
5f05dabc 1417 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1418
01020589
GS
1419 delete $ref->[$x][$y][$index];
1420 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1421
a0d0e21e 1422=item die LIST
d74e8afc 1423X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1424
c17cdb72
NC
1425=for Pod::Functions raise an exception or bail out
1426
391b733c 1427C<die> raises an exception. Inside an C<eval> the error message is stuffed
4c050ad5
NC
1428into C<$@> and the C<eval> is terminated with the undefined value.
1429If the exception is outside of all enclosing C<eval>s, then the uncaught
391b733c 1430exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1431need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1432
1433Equivalent examples:
1434
1435 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1436 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1437
ccac6780 1438If the last element of LIST does not end in a newline, the current
df37ec69
WW
1439script line number and input line number (if any) are also printed,
1440and a newline is supplied. Note that the "input line number" (also
1441known as "chunk") is subject to whatever notion of "line" happens to
1442be currently in effect, and is also available as the special variable
1443C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1444
1445Hint: sometimes appending C<", stopped"> to your message will cause it
1446to make better sense when the string C<"at foo line 123"> is appended.
1447Suppose you are running script "canasta".
a0d0e21e
LW
1448
1449 die "/etc/games is no good";
1450 die "/etc/games is no good, stopped";
1451
1452produce, respectively
1453
1454 /etc/games is no good at canasta line 123.
1455 /etc/games is no good, stopped at canasta line 123.
1456
a96d0188 1457If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1458previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1459This is useful for propagating exceptions:
1460
1461 eval { ... };
1462 die unless $@ =~ /Expected exception/;
1463
a96d0188 1464If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1465C<PROPAGATE> method, that method will be called with additional file
1466and line number parameters. The return value replaces the value in
34169887 1467C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1468were called.
1469
7660c0ab 1470If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1471
4c050ad5
NC
1472If an uncaught exception results in interpreter exit, the exit code is
1473determined from the values of C<$!> and C<$?> with this pseudocode:
1474
1475 exit $! if $!; # errno
1476 exit $? >> 8 if $? >> 8; # child exit status
1477 exit 255; # last resort
1478
1479The intent is to squeeze as much possible information about the likely cause
391b733c
FC
1480into the limited space of the system exit
1481code. However, as C<$!> is the value
4c050ad5
NC
1482of C's C<errno>, which can be set by any system call, this means that the value
1483of the exit code used by C<die> can be non-predictable, so should not be relied
1484upon, other than to be non-zero.
1485
80d38338
TC
1486You can also call C<die> with a reference argument, and if this is trapped
1487within an C<eval>, C<$@> contains that reference. This permits more
1488elaborate exception handling using objects that maintain arbitrary state
1489about the exception. Such a scheme is sometimes preferable to matching
1490particular string values of C<$@> with regular expressions. Because C<$@>
1491is a global variable and C<eval> may be used within object implementations,
1492be careful that analyzing the error object doesn't replace the reference in
1493the global variable. It's easiest to make a local copy of the reference
1494before any manipulations. Here's an example:
52531d10 1495
80d38338 1496 use Scalar::Util "blessed";
da279afe 1497
52531d10 1498 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7 1499 if (my $ev_err = $@) {
f7051f2c
FC
1500 if (blessed($ev_err)
1501 && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1502 # handle Some::Module::Exception
1503 }
1504 else {
1505 # handle all other possible exceptions
1506 }
1507 }
1508
3b10bc60 1509Because Perl stringifies uncaught exception messages before display,
80d38338 1510you'll probably want to overload stringification operations on
52531d10
GS
1511exception objects. See L<overload> for details about that.
1512
19799a22
GS
1513You can arrange for a callback to be run just before the C<die>
1514does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1515handler is called with the error text and can change the error
19799a22 1516message, if it sees fit, by calling C<die> again. See
96090e4f 1517L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1518L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1519to be run only right before your program was to exit, this is not
3b10bc60 1520currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1521even inside eval()ed blocks/strings! If one wants the hook to do
1522nothing in such situations, put
fb73857a 1523
5ed4f2ec 1524 die @_ if $^S;
fb73857a 1525
19799a22
GS
1526as the first line of the handler (see L<perlvar/$^S>). Because
1527this promotes strange action at a distance, this counterintuitive
b76cc8ba 1528behavior may be fixed in a future release.
774d564b 1529
4c050ad5
NC
1530See also exit(), warn(), and the Carp module.
1531
a0d0e21e 1532=item do BLOCK
d74e8afc 1533X<do> X<block>
a0d0e21e 1534
c17cdb72
NC
1535=for Pod::Functions turn a BLOCK into a TERM
1536
a0d0e21e 1537Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1538sequence of commands indicated by BLOCK. When modified by the C<while> or
1539C<until> loop modifier, executes the BLOCK once before testing the loop
391b733c 1540condition. (On other statements the loop modifiers test the conditional
6b275a1f 1541first.)
a0d0e21e 1542
4968c1e4 1543C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1544C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1545See L<perlsyn> for alternative strategies.
4968c1e4 1546
a0d0e21e 1547=item do SUBROUTINE(LIST)
d74e8afc 1548X<do>
a0d0e21e 1549
3bab73c8
FC
1550This form of subroutine call is deprecated. SUBROUTINE can be a bareword
1551or scalar variable.
a0d0e21e
LW
1552
1553=item do EXPR
d74e8afc 1554X<do>
a0d0e21e
LW
1555
1556Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1557file as a Perl script.
a0d0e21e
LW
1558
1559 do 'stat.pl';
1560
1561is just like
1562
986b19de 1563 eval `cat stat.pl`;
a0d0e21e 1564
2b5ab1e7 1565except that it's more efficient and concise, keeps track of the current
96090e4f
A
1566filename for error messages, searches the C<@INC> directories, and updates
1567C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1568these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1569cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1570same, however, in that it does reparse the file every time you call it,
1571so you probably don't want to do this inside a loop.
a0d0e21e 1572
8f1da26d 1573If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1574an error message in C<$@>. If C<do> cannot read the file, it returns undef
1575and sets C<$!> to the error. Always check C<$@> first, as compilation
1576could fail in a way that also sets C<$!>. If the file is successfully
1577compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1578
80d38338 1579Inclusion of library modules is better done with the
19799a22 1580C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1581and raise an exception if there's a problem.
a0d0e21e 1582
5a964f20
TC
1583You might like to use C<do> to read in a program configuration
1584file. Manual error checking can be done this way:
1585
b76cc8ba 1586 # read in config files: system first, then user
f86cebdf 1587 for $file ("/share/prog/defaults.rc",
b76cc8ba 1588 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1589 {
1590 unless ($return = do $file) {
1591 warn "couldn't parse $file: $@" if $@;
1592 warn "couldn't do $file: $!" unless defined $return;
1593 warn "couldn't run $file" unless $return;
1594 }
5a964f20
TC
1595 }
1596
a0d0e21e 1597=item dump LABEL
d74e8afc 1598X<dump> X<core> X<undump>
a0d0e21e 1599
8a7e748e
FC
1600=item dump EXPR
1601
1614b0e3
JD
1602=item dump
1603
c17cdb72
NC
1604=for Pod::Functions create an immediate core dump
1605
19799a22
GS
1606This function causes an immediate core dump. See also the B<-u>
1607command-line switch in L<perlrun>, which does the same thing.
1608Primarily this is so that you can use the B<undump> program (not
1609supplied) to turn your core dump into an executable binary after
1610having initialized all your variables at the beginning of the
1611program. When the new binary is executed it will begin by executing
1612a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1613Think of it as a goto with an intervening core dump and reincarnation.
8a7e748e
FC
1614If C<LABEL> is omitted, restarts the program from the top. The
1615C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1616computed at run time, being otherwise identical to C<dump LABEL>.
19799a22
GS
1617
1618B<WARNING>: Any files opened at the time of the dump will I<not>
1619be open any more when the program is reincarnated, with possible
80d38338 1620resulting confusion by Perl.
19799a22 1621
59f521f4 1622This function is now largely obsolete, mostly because it's very hard to
391b733c 1623convert a core file into an executable. That's why you should now invoke
59f521f4 1624it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1625typo.
19799a22 1626
2ba1f20a
FC
1627Unlike most named operators, this has the same precedence as assignment.
1628It is also exempt from the looks-like-a-function rule, so
1629C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1630C<dump>.
1631
ea9eb35a 1632Portability issues: L<perlport/dump>.
1633
532eee96 1634=item each HASH
d74e8afc 1635X<each> X<hash, iterator>
aa689395 1636
532eee96 1637=item each ARRAY
aeedbbed
NC
1638X<array, iterator>
1639
f5a93a43
TC
1640=item each EXPR
1641
c17cdb72
NC
1642=for Pod::Functions retrieve the next key/value pair from a hash
1643
bade7fbc
TC
1644When called on a hash in list context, returns a 2-element list
1645consisting of the key and value for the next element of a hash. In Perl
16465.12 and later only, it will also return the index and value for the next
1647element of an array so that you can iterate over it; older Perls consider
1648this a syntax error. When called in scalar context, returns only the key
1649(not the value) in a hash, or the index in an array.
2f9daede 1650
aeedbbed 1651Hash entries are returned in an apparently random order. The actual random
3b10bc60 1652order is subject to change in future versions of Perl, but it is
504f80c1 1653guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1654function would produce on the same (unmodified) hash. Since Perl
22883ac5 16555.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1656for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1657
80d38338
TC
1658After C<each> has returned all entries from the hash or array, the next
1659call to C<each> returns the empty list in list context and C<undef> in
bade7fbc
TC
1660scalar context; the next call following I<that> one restarts iteration.
1661Each hash or array has its own internal iterator, accessed by C<each>,
1662C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1663reached the end as just described; it can be explicitly reset by calling
1664C<keys> or C<values> on the hash or array. If you add or delete a hash's
1665elements while iterating over it, entries may be skipped or duplicated--so
1666don't do that. Exception: In the current implementation, it is always safe
1667to delete the item most recently returned by C<each()>, so the following
1668code works properly:
74fc8b5f
MJD
1669
1670 while (($key, $value) = each %hash) {
1671 print $key, "\n";
1672 delete $hash{$key}; # This is safe
1673 }
aa689395 1674
80d38338 1675This prints out your environment like the printenv(1) program,
3b10bc60 1676but in a different order:
a0d0e21e
LW
1677
1678 while (($key,$value) = each %ENV) {
a9a5a0dc 1679 print "$key=$value\n";
a0d0e21e
LW
1680 }
1681
f5a93a43
TC
1682Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1683reference to an unblessed hash or array. The argument will be dereferenced
1684automatically. This aspect of C<each> is considered highly experimental.
1685The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1686
1687 while (($key,$value) = each $hashref) { ... }
1688
e6a0db3e
FC
1689As of Perl 5.18 you can use a bare C<each> in a C<while> loop,
1690which will set C<$_> on every iteration.
1691
1692 while(each %ENV) {
1693 print "$_=$ENV{$_}\n";
1694 }
1695
bade7fbc
TC
1696To avoid confusing would-be users of your code who are running earlier
1697versions of Perl with mysterious syntax errors, put this sort of thing at
1698the top of your file to signal that your code will work I<only> on Perls of
1699a recent vintage:
1700
1701 use 5.012; # so keys/values/each work on arrays
1702 use 5.014; # so keys/values/each work on scalars (experimental)
e6a0db3e 1703 use 5.018; # so each assigns to $_ in a lone while test
bade7fbc 1704
8f1da26d 1705See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1706
1707=item eof FILEHANDLE
d74e8afc
ITB
1708X<eof>
1709X<end of file>
1710X<end-of-file>
a0d0e21e 1711
4633a7c4
LW
1712=item eof ()
1713
a0d0e21e
LW
1714=item eof
1715
c17cdb72
NC
1716=for Pod::Functions test a filehandle for its end
1717
8f1da26d 1718Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1719FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1720gives the real filehandle. (Note that this function actually
80d38338 1721reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1722interactive context.) Do not read from a terminal file (or call
19799a22 1723C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1724as terminals may lose the end-of-file condition if you do.
1725
820475bd 1726An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1727with empty parentheses is different. It refers to the pseudo file
820475bd 1728formed from the files listed on the command line and accessed via the
61eff3bc
JH
1729C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1730as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1731used will cause C<@ARGV> to be examined to determine if input is
67408cae 1732available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1733end-of-file will assume you are processing another C<@ARGV> list,
1734and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1735see L<perlop/"I/O Operators">.
820475bd 1736
61eff3bc 1737In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1738detect the end of each file, whereas C<eof()> will detect the end
1739of the very last file only. Examples:
a0d0e21e 1740
748a9306
LW
1741 # reset line numbering on each input file
1742 while (<>) {
a9a5a0dc
VP
1743 next if /^\s*#/; # skip comments
1744 print "$.\t$_";
5a964f20 1745 } continue {
a9a5a0dc 1746 close ARGV if eof; # Not eof()!
748a9306
LW
1747 }
1748
a0d0e21e
LW
1749 # insert dashes just before last line of last file
1750 while (<>) {
a9a5a0dc
VP
1751 if (eof()) { # check for end of last file
1752 print "--------------\n";
1753 }
1754 print;
f7051f2c 1755 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1756 }
1757
a0d0e21e 1758Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1759input operators typically return C<undef> when they run out of data or
1760encounter an error.
a0d0e21e
LW
1761
1762=item eval EXPR
d74e8afc 1763X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1764X<error, handling> X<exception, handling>
a0d0e21e
LW
1765
1766=item eval BLOCK
1767
ce2984c3
PF
1768=item eval
1769
c17cdb72
NC
1770=for Pod::Functions catch exceptions or compile and run code
1771
c7cc6f1c
GS
1772In the first form, the return value of EXPR is parsed and executed as if it
1773were a little Perl program. The value of the expression (which is itself
8f1da26d 1774determined within scalar context) is first parsed, and if there were no
2341804c 1775errors, executed as a block within the lexical context of the current Perl
df4833a8 1776program. This means, that in particular, any outer lexical variables are
2341804c
DM
1777visible to it, and any package variable settings or subroutine and format
1778definitions remain afterwards.
1779
1780Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1781If EXPR is omitted, evaluates C<$_>. This form is typically used to
1782delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c 1783
7289c5e6
FC
1784If the C<unicode_eval> feature is enabled (which is the default under a
1785C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1786characters, so C<use utf8> declarations have no effect, and source filters
1787are forbidden. In the absence of the C<unicode_eval> feature, the string
1788will sometimes be treated as characters and sometimes as bytes, depending
1789on the internal encoding, and source filters activated within the C<eval>
1790exhibit the erratic, but historical, behaviour of affecting some outer file
1791scope that is still compiling. See also the L</evalbytes> keyword, which
1792always treats its input as a byte stream and works properly with source
1793filters, and the L<feature> pragma.
1794
c7cc6f1c 1795In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1796same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1797within the context of the current Perl program. This form is typically
1798used to trap exceptions more efficiently than the first (see below), while
1799also providing the benefit of checking the code within BLOCK at compile
1800time.
1801
1802The final semicolon, if any, may be omitted from the value of EXPR or within
1803the BLOCK.
1804
1805In both forms, the value returned is the value of the last expression
5a964f20 1806evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1807as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1808in void, scalar, or list context, depending on the context of the C<eval>
1809itself. See L</wantarray> for more on how the evaluation context can be
1810determined.
a0d0e21e 1811
19799a22 1812If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1813executed, C<eval> returns C<undef> in scalar context
774b80e8
FC
1814or an empty list in list context, and C<$@> is set to the error
1815message. (Prior to 5.16, a bug caused C<undef> to be returned
1816in list context for syntax errors, but not for runtime errors.)
1817If there was no error, C<$@> is set to the empty string. A
9cc672d4
FC
1818control flow operator like C<last> or C<goto> can bypass the setting of
1819C<$@>. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1820warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1821To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1822turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1823See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1824
19799a22
GS
1825Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1826determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1827is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1828the die operator is used to raise exceptions.
1829
5f1da31c
NT
1830If you want to trap errors when loading an XS module, some problems with
1831the binary interface (such as Perl version skew) may be fatal even with
df4833a8 1832C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
5f1da31c 1833
a0d0e21e
LW
1834If the code to be executed doesn't vary, you may use the eval-BLOCK
1835form to trap run-time errors without incurring the penalty of
1836recompiling each time. The error, if any, is still returned in C<$@>.
1837Examples:
1838
54310121 1839 # make divide-by-zero nonfatal
a0d0e21e
LW
1840 eval { $answer = $a / $b; }; warn $@ if $@;
1841
1842 # same thing, but less efficient
1843 eval '$answer = $a / $b'; warn $@ if $@;
1844
1845 # a compile-time error
5ed4f2ec 1846 eval { $answer = }; # WRONG
a0d0e21e
LW
1847
1848 # a run-time error
5ed4f2ec 1849 eval '$answer ='; # sets $@
a0d0e21e 1850
cf264981
SP
1851Using the C<eval{}> form as an exception trap in libraries does have some
1852issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1853may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1854You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1855as this example shows:
774d564b 1856
80d38338 1857 # a private exception trap for divide-by-zero
f86cebdf
GS
1858 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1859 warn $@ if $@;
774d564b
PP
1860
1861This is especially significant, given that C<__DIE__> hooks can call
19799a22 1862C<die> again, which has the effect of changing their error messages:
774d564b
PP
1863
1864 # __DIE__ hooks may modify error messages
1865 {
f86cebdf
GS
1866 local $SIG{'__DIE__'} =
1867 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1868 eval { die "foo lives here" };
1869 print $@ if $@; # prints "bar lives here"
774d564b
PP
1870 }
1871
19799a22 1872Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1873may be fixed in a future release.
1874
19799a22 1875With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1876being looked at when:
1877
5ed4f2ec 1878 eval $x; # CASE 1
1879 eval "$x"; # CASE 2
a0d0e21e 1880
5ed4f2ec 1881 eval '$x'; # CASE 3
1882 eval { $x }; # CASE 4
a0d0e21e 1883
5ed4f2ec 1884 eval "\$$x++"; # CASE 5
1885 $$x++; # CASE 6
a0d0e21e 1886
2f9daede 1887Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1888the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1889the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1890and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1891does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1892purely visual reasons, but it also has the advantage of compiling at
1893compile-time instead of at run-time.) Case 5 is a place where
19799a22 1894normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1895particular situation, you can just use symbolic references instead, as
1896in case 6.
a0d0e21e 1897
b6538e4f 1898Before Perl 5.14, the assignment to C<$@> occurred before restoration
bade7fbc 1899of localized variables, which means that for your code to run on older
b208c909 1900versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1901errors:
1902
1903 # alter $@ on nefarious repugnancy only
1904 {
1905 my $e;
1906 {
f7051f2c
FC
1907 local $@; # protect existing $@
1908 eval { test_repugnancy() };
1909 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1910 $@ =~ /nefarious/ and $e = $@;
8a5a710d
DN
1911 }
1912 die $e if defined $e
1913 }
1914
4968c1e4 1915C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1916C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1917
3b10bc60 1918An C<eval ''> executed within the C<DB> package doesn't see the usual
1919surrounding lexical scope, but rather the scope of the first non-DB piece
df4833a8 1920of code that called it. You don't normally need to worry about this unless
3b10bc60 1921you are writing a Perl debugger.
d819b83a 1922
7289c5e6
FC
1923=item evalbytes EXPR
1924X<evalbytes>
1925
1926=item evalbytes
1927
d9b04284 1928=for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
c17cdb72 1929
7289c5e6
FC
1930This function is like L</eval> with a string argument, except it always
1931parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1932string containing characters whose ordinal value exceeds 255 results in an
1933error. Source filters activated within the evaluated code apply to the
1934code itself.
1935
1936This function is only available under the C<evalbytes> feature, a
1937C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1938L<feature> for more information.
1939
a0d0e21e 1940=item exec LIST
d74e8afc 1941X<exec> X<execute>
a0d0e21e 1942
8bf3b016
GS
1943=item exec PROGRAM LIST
1944
c17cdb72
NC
1945=for Pod::Functions abandon this program to run another
1946
3b10bc60 1947The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1948use C<system> instead of C<exec> if you want it to return. It fails and
1949returns false only if the command does not exist I<and> it is executed
fb73857a 1950directly instead of via your system's command shell (see below).
a0d0e21e 1951
19799a22 1952Since it's a common mistake to use C<exec> instead of C<system>, Perl
4642e50d
EB
1953warns you if C<exec> is called in void context and if there is a following
1954statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1955you always do that, right?). If you I<really> want to follow an C<exec>
1956with some other statement, you can use one of these styles to avoid the warning:
55d729e4 1957
5a964f20
TC
1958 exec ('foo') or print STDERR "couldn't exec foo: $!";
1959 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1960
5a964f20 1961If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1962with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1963If there is only one scalar argument or an array with one element in it,
1964the argument is checked for shell metacharacters, and if there are any,
1965the entire argument is passed to the system's command shell for parsing
1966(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1967If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1968words and passed directly to C<execvp>, which is more efficient.
19799a22 1969Examples:
a0d0e21e 1970
19799a22
GS
1971 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1972 exec "sort $outfile | uniq";
a0d0e21e
LW
1973
1974If you don't really want to execute the first argument, but want to lie
1975to the program you are executing about its own name, you can specify
1976the program you actually want to run as an "indirect object" (without a
1977comma) in front of the LIST. (This always forces interpretation of the
54310121 1978LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1979the list.) Example:
1980
1981 $shell = '/bin/csh';
5ed4f2ec 1982 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1983
1984or, more directly,
1985
5ed4f2ec 1986 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1987
3b10bc60 1988When the arguments get executed via the system shell, results are
1989subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1990for details.
1991
19799a22
GS
1992Using an indirect object with C<exec> or C<system> is also more
1993secure. This usage (which also works fine with system()) forces
1994interpretation of the arguments as a multivalued list, even if the
1995list had just one argument. That way you're safe from the shell
1996expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1997
1998 @args = ( "echo surprise" );
1999
2b5ab1e7 2000 exec @args; # subject to shell escapes
f86cebdf 2001 # if @args == 1
2b5ab1e7 2002 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
2003
2004The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
2005program, passing it C<"surprise"> an argument. The second version didn't;
2006it tried to run a program named I<"echo surprise">, didn't find it, and set
2007C<$?> to a non-zero value indicating failure.
5a964f20 2008
e9fa405d
BF
2009Perl attempts to flush all files opened for output before the exec,
2010but this may not be supported on some platforms (see L<perlport>).
2011To be safe, you may need to set C<$|> ($AUTOFLUSH in English) or
2012call the C<autoflush()> method of C<IO::Handle> on any open handles
2013to avoid lost output.
0f897271 2014
80d38338
TC
2015Note that C<exec> will not call your C<END> blocks, nor will it invoke
2016C<DESTROY> methods on your objects.
7660c0ab 2017
ea9eb35a 2018Portability issues: L<perlport/exec>.
2019
a0d0e21e 2020=item exists EXPR
d74e8afc 2021X<exists> X<autovivification>
a0d0e21e 2022
c17cdb72
NC
2023=for Pod::Functions test whether a hash key is present
2024
d0a76353
RS
2025Given an expression that specifies an element of a hash, returns true if the
2026specified element in the hash has ever been initialized, even if the
2027corresponding value is undefined.
a0d0e21e 2028
5ed4f2ec 2029 print "Exists\n" if exists $hash{$key};
2030 print "Defined\n" if defined $hash{$key};
01020589
GS
2031 print "True\n" if $hash{$key};
2032
d0a76353 2033exists may also be called on array elements, but its behavior is much less
8f1da26d 2034obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
2035that calling exists on array values is deprecated and likely to be removed in
2036a future version of Perl.
2037
5ed4f2ec 2038 print "Exists\n" if exists $array[$index];
2039 print "Defined\n" if defined $array[$index];
01020589 2040 print "True\n" if $array[$index];
a0d0e21e 2041
8f1da26d 2042A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
2043it exists, but the reverse doesn't necessarily hold true.
2044
afebc493
GS
2045Given an expression that specifies the name of a subroutine,
2046returns true if the specified subroutine has ever been declared, even
2047if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 2048does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
2049exist may still be callable: its package may have an C<AUTOLOAD>
2050method that makes it spring into existence the first time that it is
3b10bc60 2051called; see L<perlsub>.
afebc493 2052
5ed4f2ec 2053 print "Exists\n" if exists &subroutine;
2054 print "Defined\n" if defined &subroutine;
afebc493 2055
a0d0e21e 2056Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 2057operation is a hash or array key lookup or subroutine name:
a0d0e21e 2058
5ed4f2ec 2059 if (exists $ref->{A}->{B}->{$key}) { }
2060 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 2061
5ed4f2ec 2062 if (exists $ref->{A}->{B}->[$ix]) { }
2063 if (exists $hash{A}{B}[$ix]) { }
01020589 2064
afebc493
GS
2065 if (exists &{$ref->{A}{B}{$key}}) { }
2066
9590a7cd 2067Although the most deeply nested array or hash element will not spring into
3b10bc60 2068existence just because its existence was tested, any intervening ones will.
61eff3bc 2069Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 2070into existence due to the existence test for the $key element above.
3b10bc60 2071This happens anywhere the arrow operator is used, including even here:
5a964f20 2072
2b5ab1e7 2073 undef $ref;
5ed4f2ec 2074 if (exists $ref->{"Some key"}) { }
2075 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
2076
2077This surprising autovivification in what does not at first--or even
2078second--glance appear to be an lvalue context may be fixed in a future
5a964f20 2079release.
a0d0e21e 2080
afebc493
GS
2081Use of a subroutine call, rather than a subroutine name, as an argument
2082to exists() is an error.
2083
5ed4f2ec 2084 exists &sub; # OK
2085 exists &sub(); # Error
afebc493 2086
a0d0e21e 2087=item exit EXPR
d74e8afc 2088X<exit> X<terminate> X<abort>
a0d0e21e 2089
ce2984c3
PF
2090=item exit
2091
c17cdb72
NC
2092=for Pod::Functions terminate this program
2093
2b5ab1e7 2094Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
2095
2096 $ans = <STDIN>;
2097 exit 0 if $ans =~ /^[Xx]/;
2098
19799a22 2099See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
2100universally recognized values for EXPR are C<0> for success and C<1>
2101for error; other values are subject to interpretation depending on the
2102environment in which the Perl program is running. For example, exiting
210369 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2104the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 2105
19799a22
GS
2106Don't use C<exit> to abort a subroutine if there's any chance that
2107someone might want to trap whatever error happened. Use C<die> instead,
2108which can be trapped by an C<eval>.
28757baa 2109
19799a22 2110The exit() function does not always exit immediately. It calls any
2b5ab1e7 2111defined C<END> routines first, but these C<END> routines may not
19799a22 2112themselves abort the exit. Likewise any object destructors that need to
60275626 2113be called are called before the real exit. C<END> routines and destructors
391b733c 2114can change the exit status by modifying C<$?>. If this is a problem, you
fae6f8fa 2115can call C<POSIX::_exit($status)> to avoid END and destructor processing.
87275199 2116See L<perlmod> for details.
5a964f20 2117
ea9eb35a 2118Portability issues: L<perlport/exit>.
2119
a0d0e21e 2120=item exp EXPR
d74e8afc 2121X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 2122
54310121 2123=item exp
bbce6d69 2124
c17cdb72
NC
2125=for Pod::Functions raise I<e> to a power
2126
b76cc8ba 2127Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
2128If EXPR is omitted, gives C<exp($_)>.
2129
628253b8
BF
2130=item fc EXPR
2131X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2132
2133=item fc
2134
d9b04284 2135=for Pod::Functions +fc return casefolded version of a string
c17cdb72 2136
628253b8
BF
2137Returns the casefolded version of EXPR. This is the internal function
2138implementing the C<\F> escape in double-quoted strings.
2139
2140Casefolding is the process of mapping strings to a form where case
2141differences are erased; comparing two strings in their casefolded
2142form is effectively a way of asking if two strings are equal,
2143regardless of case.
2144
2145Roughly, if you ever found yourself writing this
2146
f6c6dcb6 2147 lc($this) eq lc($that) # Wrong!
628253b8 2148 # or
f6c6dcb6 2149 uc($this) eq uc($that) # Also wrong!
628253b8 2150 # or
f6c6dcb6 2151 $this =~ /^\Q$that\E\z/i # Right!
628253b8
BF
2152
2153Now you can write
2154
2155 fc($this) eq fc($that)
2156
2157And get the correct results.
2158
fc39a31f
KW
2159Perl only implements the full form of casefolding,
2160but you can access the simple folds using L<Unicode::UCD/casefold()> and
2161L<Unicode::UCD/prop_invmap()>.
628253b8
BF
2162For further information on casefolding, refer to
2163the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
21644.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2165available at L<http://www.unicode.org/versions/latest/>, as well as the
2166Case Charts available at L<http://www.unicode.org/charts/case/>.
2167
2168If EXPR is omitted, uses C<$_>.
2169
2170This function behaves the same way under various pragma, such as in a locale,
2171as L</lc> does.
2172
2173While the Unicode Standard defines two additional forms of casefolding,
2174one for Turkic languages and one that never maps one character into multiple
2175characters, these are not provided by the Perl core; However, the CPAN module
2176C<Unicode::Casing> may be used to provide an implementation.
2177
2178This keyword is available only when the C<"fc"> feature is enabled,
2179or when prefixed with C<CORE::>; See L<feature>. Alternately,
2180include a C<use v5.16> or later to the current scope.
2181
a0d0e21e 2182=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2183X<fcntl>
a0d0e21e 2184
c17cdb72
NC
2185=for Pod::Functions file control system call
2186
f86cebdf 2187Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
2188
2189 use Fcntl;
2190
0ade1984 2191first to get the correct constant definitions. Argument processing and
3b10bc60 2192value returned work just like C<ioctl> below.
a0d0e21e
LW
2193For example:
2194
2195 use Fcntl;
5a964f20 2196 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 2197 or die "can't fcntl F_GETFL: $!";
5a964f20 2198
554ad1fc 2199You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
2200Like C<ioctl>, it maps a C<0> return from the system call into
2201C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
2202in numeric context. It is also exempt from the normal B<-w> warnings
2203on improper numeric conversions.
5a964f20 2204
3b10bc60 2205Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
2206doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2207manpage to learn what functions are available on your system.
a0d0e21e 2208
be2f7487 2209Here's an example of setting a filehandle named C<REMOTE> to be
2210non-blocking at the system level. You'll have to negotiate C<$|>
2211on your own, though.
2212
2213 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2214
2215 $flags = fcntl(REMOTE, F_GETFL, 0)
2216 or die "Can't get flags for the socket: $!\n";
2217
2218 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2219 or die "Can't set flags for the socket: $!\n";
2220
ea9eb35a 2221Portability issues: L<perlport/fcntl>.
2222
cfa52385
FC
2223=item __FILE__
2224X<__FILE__>
2225
c17cdb72
NC
2226=for Pod::Functions the name of the current source file
2227
cfa52385
FC
2228A special token that returns the name of the file in which it occurs.
2229
a0d0e21e 2230=item fileno FILEHANDLE
d74e8afc 2231X<fileno>
a0d0e21e 2232
c17cdb72
NC
2233=for Pod::Functions return file descriptor from filehandle
2234
2b5ab1e7 2235Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
2236filehandle is not open. If there is no real file descriptor at the OS
2237level, as can happen with filehandles connected to memory objects via
2238C<open> with a reference for the third argument, -1 is returned.
2239
2240This is mainly useful for constructing
19799a22 2241bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
2242If FILEHANDLE is an expression, the value is taken as an indirect
2243filehandle, generally its name.
5a964f20 2244
b76cc8ba 2245You can use this to find out whether two handles refer to the
5a964f20
TC
2246same underlying descriptor:
2247
2248 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 2249 print "THIS and THAT are dups\n";
b76cc8ba
NIS
2250 }
2251
a0d0e21e 2252=item flock FILEHANDLE,OPERATION
d74e8afc 2253X<flock> X<lock> X<locking>
a0d0e21e 2254
c17cdb72
NC
2255=for Pod::Functions lock an entire file with an advisory lock
2256
19799a22
GS
2257Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2258for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2259machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2260C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2261entire files only, not records.
2b5ab1e7
TC
2262
2263Two potentially non-obvious but traditional C<flock> semantics are
2264that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2265are B<merely advisory>. Such discretionary locks are more flexible, but
2266offer fewer guarantees. This means that programs that do not also use
2267C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2268your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2269for details. It's best to assume traditional behavior if you're writing
2270portable programs. (But if you're not, you should as always feel perfectly
2271free to write for your own system's idiosyncrasies (sometimes called
2272"features"). Slavish adherence to portability concerns shouldn't get
2273in the way of your getting your job done.)
a3cb178b 2274
8ebc5c01
PP
2275OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2276LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2277you can use the symbolic names if you import them from the L<Fcntl> module,
2278either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2279requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2280releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2281LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2282waiting for the lock; check the return status to see if you got it.
68dc0745 2283
2b5ab1e7
TC
2284To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2285before locking or unlocking it.
8ebc5c01 2286
f86cebdf 2287Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2288locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2289are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2290implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
2291differing semantics shouldn't bite too many people.
2292
becacb53
TM
2293Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2294be open with read intent to use LOCK_SH and requires that it be open
2295with write intent to use LOCK_EX.
2296
19799a22
GS
2297Note also that some versions of C<flock> cannot lock things over the
2298network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2299that. If you like you can force Perl to ignore your system's flock(2)
2300function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2301the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2302and build a new Perl.
4633a7c4
LW
2303
2304Here's a mailbox appender for BSD systems.
a0d0e21e 2305
f7051f2c
FC
2306 # import LOCK_* and SEEK_END constants
2307 use Fcntl qw(:flock SEEK_END);
a0d0e21e
LW
2308
2309 sub lock {
a9a5a0dc
VP
2310 my ($fh) = @_;
2311 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2312
a9a5a0dc
VP
2313 # and, in case someone appended while we were waiting...
2314 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2315 }
2316
2317 sub unlock {
a9a5a0dc
VP
2318 my ($fh) = @_;
2319 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2320 }
2321
b0169937 2322 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2323 or die "Can't open mailbox: $!";
a0d0e21e 2324
7ed5353d 2325 lock($mbox);
b0169937 2326 print $mbox $msg,"\n\n";
7ed5353d 2327 unlock($mbox);
a0d0e21e 2328
3b10bc60 2329On systems that support a real flock(2), locks are inherited across fork()
2330calls, whereas those that must resort to the more capricious fcntl(2)
2331function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2332
cb1a09d0 2333See also L<DB_File> for other flock() examples.
a0d0e21e 2334
ea9eb35a 2335Portability issues: L<perlport/flock>.
2336
a0d0e21e 2337=item fork
d74e8afc 2338X<fork> X<child> X<parent>
a0d0e21e 2339
c17cdb72
NC
2340=for Pod::Functions create a new process just like this one
2341
2b5ab1e7
TC
2342Does a fork(2) system call to create a new process running the
2343same program at the same point. It returns the child pid to the
2344parent process, C<0> to the child process, or C<undef> if the fork is
2345unsuccessful. File descriptors (and sometimes locks on those descriptors)
2346are shared, while everything else is copied. On most systems supporting
2347fork(), great care has gone into making it extremely efficient (for
2348example, using copy-on-write technology on data pages), making it the
2349dominant paradigm for multitasking over the last few decades.
5a964f20 2350
e9fa405d 2351Perl attempts to flush all files opened for
0f897271
GS
2352output before forking the child process, but this may not be supported
2353on some platforms (see L<perlport>). To be safe, you may need to set
2354C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2355C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2356
19799a22 2357If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2358accumulate zombies. On some systems, you can avoid this by setting
2359C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2360forking and reaping moribund children.
cb1a09d0 2361
28757baa
PP
2362Note that if your forked child inherits system file descriptors like
2363STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2364if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2365backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2366You should reopen those to F</dev/null> if it's any issue.
28757baa 2367
ea9eb35a 2368On some platforms such as Windows, where the fork() system call is not available,
391b733c
FC
2369Perl can be built to emulate fork() in the Perl interpreter.
2370The emulation is designed, at the level of the Perl program,
2371to be as compatible as possible with the "Unix" fork().
6d17f725 2372However it has limitations that have to be considered in code intended to be portable.
ea9eb35a 2373See L<perlfork> for more details.
2374
2375Portability issues: L<perlport/fork>.
2376
cb1a09d0 2377=item format
d74e8afc 2378X<format>
cb1a09d0 2379
c17cdb72
NC
2380=for Pod::Functions declare a picture format with use by the write() function
2381
19799a22 2382Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2383example:
2384
54310121 2385 format Something =
a9a5a0dc
VP
2386 Test: @<<<<<<<< @||||| @>>>>>
2387 $str, $%, '$' . int($num)
cb1a09d0
AD
2388 .
2389
2390 $str = "widget";
184e9718 2391 $num = $cost/$quantity;
cb1a09d0
AD
2392 $~ = 'Something';
2393 write;
2394
2395See L<perlform> for many details and examples.
2396
8903cb82 2397=item formline PICTURE,LIST
d74e8afc 2398X<formline>
a0d0e21e 2399
c17cdb72
NC
2400=for Pod::Functions internal function used for formats
2401
5a964f20 2402This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2403too. It formats (see L<perlform>) a list of values according to the
2404contents of PICTURE, placing the output into the format output
7660c0ab 2405accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2406Eventually, when a C<write> is done, the contents of
cf264981
SP
2407C<$^A> are written to some filehandle. You could also read C<$^A>
2408and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2409does one C<formline> per line of form, but the C<formline> function itself
748a9306 2410doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2411that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2412You may therefore need to use multiple formlines to implement a single
3b10bc60 2413record format, just like the C<format> compiler.
748a9306 2414
19799a22 2415Be careful if you put double quotes around the picture, because an C<@>
748a9306 2416character may be taken to mean the beginning of an array name.
19799a22 2417C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2418
445b09e5
FC
2419If you are trying to use this instead of C<write> to capture the output,
2420you may find it easier to open a filehandle to a scalar
2421(C<< open $fh, ">", \$output >>) and write to that instead.
2422
a0d0e21e 2423=item getc FILEHANDLE
f723aae1 2424X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2425
2426=item getc
2427
c17cdb72
NC
2428=for Pod::Functions get the next character from the filehandle
2429
a0d0e21e 2430Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2431or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2432the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2433STDIN. This is not particularly efficient. However, it cannot be
2434used by itself to fetch single characters without waiting for the user
2435to hit enter. For that, try something more like:
4633a7c4
LW
2436
2437 if ($BSD_STYLE) {
a9a5a0dc 2438 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2439 }
2440 else {
a9a5a0dc 2441 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2442 }
2443
2444 $key = getc(STDIN);
2445
2446 if ($BSD_STYLE) {
a9a5a0dc 2447 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2448 }
2449 else {
3b10bc60 2450 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2451 }
2452 print "\n";
2453
54310121
PP
2454Determination of whether $BSD_STYLE should be set
2455is left as an exercise to the reader.
cb1a09d0 2456
19799a22 2457The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2458systems purporting POSIX compliance. See also the C<Term::ReadKey>
a3390c9f 2459module from your nearest CPAN site; details on CPAN can be found under
2b5ab1e7 2460L<perlmodlib/CPAN>.
a0d0e21e
LW
2461
2462=item getlogin
d74e8afc 2463X<getlogin> X<login>
a0d0e21e 2464
c17cdb72
NC
2465=for Pod::Functions return who logged in at this tty
2466
cf264981 2467This implements the C library function of the same name, which on most
3b10bc60 2468systems returns the current login from F</etc/utmp>, if any. If it
2469returns the empty string, use C<getpwuid>.
a0d0e21e 2470
f86702cc 2471 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2472
19799a22
GS
2473Do not consider C<getlogin> for authentication: it is not as
2474secure as C<getpwuid>.
4633a7c4 2475
ea9eb35a 2476Portability issues: L<perlport/getlogin>.
2477
a0d0e21e 2478=item getpeername SOCKET
d74e8afc 2479X<getpeername> X<peer>
a0d0e21e 2480
c17cdb72
NC
2481=for Pod::Functions find the other end of a socket connection
2482
a3390c9f
FC
2483Returns the packed sockaddr address of the other end of the SOCKET
2484connection.
a0d0e21e 2485
4633a7c4
LW
2486 use Socket;
2487 $hersockaddr = getpeername(SOCK);
19799a22 2488 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2489 $herhostname = gethostbyaddr($iaddr, AF_INET);
2490 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2491
2492=item getpgrp PID
d74e8afc 2493X<getpgrp> X<group>
a0d0e21e 2494
c17cdb72
NC
2495=for Pod::Functions get process group
2496
47e29363 2497Returns the current process group for the specified PID. Use
7660c0ab 2498a PID of C<0> to get the current process group for the
4633a7c4 2499current process. Will raise an exception if used on a machine that
a3390c9f
FC
2500doesn't implement getpgrp(2). If PID is omitted, returns the process
2501group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2502does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2503
ea9eb35a 2504Portability issues: L<perlport/getpgrp>.
2505
a0d0e21e 2506=item getppid
d74e8afc 2507X<getppid> X<parent> X<pid>
a0d0e21e 2508
c17cdb72
NC
2509=for Pod::Functions get parent process ID
2510
a0d0e21e
LW
2511Returns the process id of the parent process.
2512
d7c042c9
AB
2513Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2514around non-POSIX thread semantics the minority of Linux systems (and
2515Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2516has since been removed. See the documentation for L<$$|perlvar/$$> for
2517details.
4d76a344 2518
ea9eb35a 2519Portability issues: L<perlport/getppid>.
2520
a0d0e21e 2521=item getpriority WHICH,WHO
d74e8afc 2522X<getpriority> X<priority> X<nice>
a0d0e21e 2523
c17cdb72
NC
2524=for Pod::Functions get current nice value
2525
4633a7c4 2526Returns the current priority for a process, a process group, or a user.
01aa884e 2527(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2528machine that doesn't implement getpriority(2).
a0d0e21e 2529
ea9eb35a 2530Portability issues: L<perlport/getpriority>.
2531
a0d0e21e 2532=item getpwnam NAME
d74e8afc
ITB
2533X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2534X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2535X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2536X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2537X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2538X<endnetent> X<endprotoent> X<endservent>
a0d0e21e 2539
c17cdb72
NC
2540=for Pod::Functions get passwd record given user login name
2541
a0d0e21e
LW
2542=item getgrnam NAME
2543
c17cdb72
NC
2544=for Pod::Functions get group record given group name
2545
a0d0e21e
LW
2546=item gethostbyname NAME
2547
c17cdb72
NC
2548=for Pod::Functions get host record given name
2549
a0d0e21e
LW
2550=item getnetbyname NAME
2551
c17cdb72
NC
2552=for Pod::Functions get networks record given name
2553
a0d0e21e
LW
2554=item getprotobyname NAME
2555
c17cdb72
NC
2556=for Pod::Functions get protocol record given name
2557
a0d0e21e
LW
2558=item getpwuid UID
2559
c17cdb72
NC
2560=for Pod::Functions get passwd record given user ID
2561
a0d0e21e
LW
2562=item getgrgid GID
2563
c17cdb72
NC
2564=for Pod::Functions get group record given group user ID
2565
a0d0e21e
LW
2566=item getservbyname NAME,PROTO
2567
c17cdb72
NC
2568=for Pod::Functions get services record given its name
2569
a0d0e21e
LW
2570=item gethostbyaddr ADDR,ADDRTYPE
2571
c17cdb72
NC
2572=for Pod::Functions get host record given its address
2573
a0d0e21e
LW
2574=item getnetbyaddr ADDR,ADDRTYPE
2575
c17cdb72
NC
2576=for Pod::Functions get network record given its address
2577
a0d0e21e
LW
2578=item getprotobynumber NUMBER
2579
c17cdb72
NC
2580=for Pod::Functions get protocol record numeric protocol
2581
a0d0e21e
LW
2582=item getservbyport PORT,PROTO
2583
c17cdb72
NC
2584=for Pod::Functions get services record given numeric port
2585
a0d0e21e
LW
2586=item getpwent
2587
c17cdb72
NC
2588=for Pod::Functions get next passwd record
2589
a0d0e21e
LW
2590=item getgrent
2591
c17cdb72
NC
2592=for Pod::Functions get next group record
2593
a0d0e21e
LW
2594=item gethostent
2595
c17cdb72
NC
2596=for Pod::Functions get next hosts record
2597
a0d0e21e
LW
2598=item getnetent
2599
c17cdb72
NC
2600=for Pod::Functions get next networks record
2601
a0d0e21e
LW
2602=item getprotoent
2603
c17cdb72
NC
2604=for Pod::Functions get next protocols record
2605
a0d0e21e
LW
2606=item getservent
2607
c17cdb72
NC
2608=for Pod::Functions get next services record
2609
a0d0e21e
LW
2610=item setpwent
2611
c17cdb72
NC
2612=for Pod::Functions prepare passwd file for use
2613
a0d0e21e
LW
2614=item setgrent
2615
c17cdb72
NC
2616=for Pod::Functions prepare group file for use
2617
a0d0e21e
LW
2618=item sethostent STAYOPEN
2619
c17cdb72
NC
2620=for Pod::Functions prepare hosts file for use
2621
a0d0e21e
LW
2622=item setnetent STAYOPEN
2623
c17cdb72
NC
2624=for Pod::Functions prepare networks file for use
2625
a0d0e21e
LW
2626=item setprotoent STAYOPEN
2627
c17cdb72
NC
2628=for Pod::Functions prepare protocols file for use
2629
a0d0e21e
LW
2630=item setservent STAYOPEN
2631
c17cdb72
NC
2632=for Pod::Functions prepare services file for use
2633
a0d0e21e
LW
2634=item endpwent
2635
c17cdb72
NC
2636=for Pod::Functions be done using passwd file
2637
a0d0e21e
LW
2638=item endgrent
2639
c17cdb72
NC
2640=for Pod::Functions be done using group file
2641
a0d0e21e
LW
2642=item endhostent
2643
c17cdb72
NC
2644=for Pod::Functions be done using hosts file
2645
a0d0e21e
LW
2646=item endnetent
2647
c17cdb72
NC
2648=for Pod::Functions be done using networks file
2649
a0d0e21e
LW
2650=item endprotoent
2651
c17cdb72
NC
2652=for Pod::Functions be done using protocols file
2653
a0d0e21e
LW
2654=item endservent
2655
c17cdb72
NC
2656=for Pod::Functions be done using services file
2657
80d38338
TC
2658These routines are the same as their counterparts in the
2659system C library. In list context, the return values from the
a0d0e21e
LW
2660various get routines are as follows:
2661
2662 ($name,$passwd,$uid,$gid,
6ee623d5 2663 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2664 ($name,$passwd,$gid,$members) = getgr*
2665 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2666 ($name,$aliases,$addrtype,$net) = getnet*
2667 ($name,$aliases,$proto) = getproto*
2668 ($name,$aliases,$port,$proto) = getserv*
2669
3b10bc60 2670(If the entry doesn't exist you get an empty list.)
a0d0e21e 2671
4602f195
JH
2672The exact meaning of the $gcos field varies but it usually contains
2673the real name of the user (as opposed to the login name) and other
2674information pertaining to the user. Beware, however, that in many
2675system users are able to change this information and therefore it
106325ad 2676cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2677L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2678login shell, are also tainted, for the same reason.
4602f195 2679
5a964f20 2680In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2681lookup by name, in which case you get the other thing, whatever it is.
2682(If the entry doesn't exist you get the undefined value.) For example:
2683
5a964f20
TC
2684 $uid = getpwnam($name);
2685 $name = getpwuid($num);
2686 $name = getpwent();
2687 $gid = getgrnam($name);
08a33e13 2688 $name = getgrgid($num);
5a964f20
TC
2689 $name = getgrent();
2690 #etc.
a0d0e21e 2691
4602f195 2692In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2693in that they are unsupported on many systems. If the
4602f195
JH
2694$quota is unsupported, it is an empty scalar. If it is supported, it
2695usually encodes the disk quota. If the $comment field is unsupported,
2696it is an empty scalar. If it is supported it usually encodes some
2697administrative comment about the user. In some systems the $quota
2698field may be $change or $age, fields that have to do with password
2699aging. In some systems the $comment field may be $class. The $expire
2700field, if present, encodes the expiration period of the account or the
2701password. For the availability and the exact meaning of these fields
8f1da26d 2702in your system, please consult getpwnam(3) and your system's
4602f195
JH
2703F<pwd.h> file. You can also find out from within Perl what your
2704$quota and $comment fields mean and whether you have the $expire field
2705by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2706C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2707files are supported only if your vendor has implemented them in the
4602f195 2708intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2709shadow versions if you're running under privilege or if there exists
cf264981 2710the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2711and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2712facility are unlikely to be supported.
6ee623d5 2713
a3390c9f 2714The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2715the login names of the members of the group.
2716
2717For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2718C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2719C<@addrs> value returned by a successful call is a list of raw
2720addresses returned by the corresponding library call. In the
2721Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2722by saying something like:
2723
f337b084 2724 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2725
2b5ab1e7
TC
2726The Socket library makes this slightly easier:
2727
2728 use Socket;
2729 $iaddr = inet_aton("127.1"); # or whatever address
2730 $name = gethostbyaddr($iaddr, AF_INET);
2731
2732 # or going the other way
19799a22 2733 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2734
d760c846
GS
2735In the opposite way, to resolve a hostname to the IP address
2736you can write this:
2737
2738 use Socket;
2739 $packed_ip = gethostbyname("www.perl.org");
2740 if (defined $packed_ip) {
2741 $ip_address = inet_ntoa($packed_ip);
2742 }
2743
b018eaf1 2744Make sure C<gethostbyname()> is called in SCALAR context and that
d760c846
GS
2745its return value is checked for definedness.
2746
0d043efa
FC
2747The C<getprotobynumber> function, even though it only takes one argument,
2748has the precedence of a list operator, so beware:
2749
2750 getprotobynumber $number eq 'icmp' # WRONG
2751 getprotobynumber($number eq 'icmp') # actually means this
2752 getprotobynumber($number) eq 'icmp' # better this way
2753
19799a22
GS
2754If you get tired of remembering which element of the return list
2755contains which return value, by-name interfaces are provided
2756in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2757C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2758and C<User::grent>. These override the normal built-ins, supplying
2759versions that return objects with the appropriate names
2760for each field. For example:
5a964f20
TC
2761
2762 use File::stat;
2763 use User::pwent;
2764 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2765
a3390c9f 2766Even though it looks as though they're the same method calls (uid),
b76cc8ba 2767they aren't, because a C<File::stat> object is different from
19799a22 2768a C<User::pwent> object.
5a964f20 2769
ea9eb35a 2770Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2771
a0d0e21e 2772=item getsockname SOCKET
d74e8afc 2773X<getsockname>
a0d0e21e 2774
c17cdb72
NC
2775=for Pod::Functions retrieve the sockaddr for a given socket
2776
19799a22
GS
2777Returns the packed sockaddr address of this end of the SOCKET connection,
2778in case you don't know the address because you have several different
2779IPs that the connection might have come in on.
a0d0e21e 2780
4633a7c4
LW
2781 use Socket;
2782 $mysockaddr = getsockname(SOCK);
19799a22 2783 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2784 printf "Connect to %s [%s]\n",
19799a22
GS
2785 scalar gethostbyaddr($myaddr, AF_INET),
2786 inet_ntoa($myaddr);
a0d0e21e
LW
2787
2788=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2789X<getsockopt>
a0d0e21e 2790
c17cdb72
NC
2791=for Pod::Functions get socket options on a given socket
2792
636e6b1f
TH
2793Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2794Options may exist at multiple protocol levels depending on the socket
2795type, but at least the uppermost socket level SOL_SOCKET (defined in the
391b733c 2796C<Socket> module) will exist. To query options at another level the
636e6b1f 2797protocol number of the appropriate protocol controlling the option
391b733c 2798should be supplied. For example, to indicate that an option is to be
636e6b1f 2799interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2800number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2801
80d38338 2802The function returns a packed string representing the requested socket
3b10bc60 2803option, or C<undef> on error, with the reason for the error placed in
391b733c 2804C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2805consult getsockopt(2) for details. A common case is that the option is an
2806integer, in which case the result is a packed integer, which you can decode
2807using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2808
8f1da26d 2809Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2810
4852725b 2811 use Socket qw(:all);
636e6b1f
TH
2812
2813 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2814 or die "Could not determine the protocol number for tcp";
4852725b
DD
2815 # my $tcp = IPPROTO_TCP; # Alternative
2816 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2817 or die "getsockopt TCP_NODELAY: $!";
636e6b1f 2818 my $nodelay = unpack("I", $packed);
f7051f2c
FC
2819 print "Nagle's algorithm is turned ",
2820 $nodelay ? "off\n" : "on\n";
636e6b1f 2821
ea9eb35a 2822Portability issues: L<perlport/getsockopt>.
a0d0e21e
LW
2823
2824=item glob EXPR
d74e8afc 2825X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2826
0a753a76
PP
2827=item glob
2828
c17cdb72
NC
2829=for Pod::Functions expand filenames using wildcards
2830
d9a9d457 2831In list context, returns a (possibly empty) list of filename expansions on
391b733c 2832the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
d9a9d457 2833scalar context, glob iterates through such filename expansions, returning
391b733c
FC
2834undef when the list is exhausted. This is the internal function
2835implementing the C<< <*.c> >> operator, but you can use it directly. If
d9a9d457
JL
2836EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2837more detail in L<perlop/"I/O Operators">.
a0d0e21e 2838
80d38338
TC
2839Note that C<glob> splits its arguments on whitespace and treats
2840each segment as separate pattern. As such, C<glob("*.c *.h")>
2841matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2842C<glob(".* *")> matches all files in the current working directory.
a91bb7b1
TC
2843If you want to glob filenames that might contain whitespace, you'll
2844have to use extra quotes around the spacey filename to protect it.
2845For example, to glob filenames that have an C<e> followed by a space
2846followed by an C<f>, use either of:
2847
2848 @spacies = <"*e f*">;
2849 @spacies = glob '"*e f*"';
2850 @spacies = glob q("*e f*");
2851
2852If you had to get a variable through, you could do this:
2853
2854 @spacies = glob "'*${var}e f*'";
2855 @spacies = glob qq("*${var}e f*");
80d38338
TC
2856
2857If non-empty braces are the only wildcard characters used in the
2858C<glob>, no filenames are matched, but potentially many strings
2859are returned. For example, this produces nine strings, one for
2860each pairing of fruits and colors:
2861
2862 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2863
e9fa405d 2864This operator is implemented using the standard
5c0c9249
PF
2865C<File::Glob> extension. See L<File::Glob> for details, including
2866C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2867
ea9eb35a 2868Portability issues: L<perlport/glob>.
2869
a0d0e21e 2870=item gmtime EXPR
d74e8afc 2871X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2872
ce2984c3
PF
2873=item gmtime
2874
c17cdb72
NC
2875=for Pod::Functions convert UNIX time into record or string using Greenwich time
2876
4509d391 2877Works just like L</localtime> but the returned values are
435fbc73 2878localized for the standard Greenwich time zone.
a0d0e21e 2879
a3390c9f
FC
2880Note: When called in list context, $isdst, the last value
2881returned by gmtime, is always C<0>. There is no
435fbc73 2882Daylight Saving Time in GMT.
0a753a76 2883
ea9eb35a 2884Portability issues: L<perlport/gmtime>.
62aa5637 2885
a0d0e21e 2886=item goto LABEL
d74e8afc 2887X<goto> X<jump> X<jmp>
a0d0e21e 2888
748a9306
LW
2889=item goto EXPR
2890
a0d0e21e
LW
2891=item goto &NAME
2892
c17cdb72
NC
2893=for Pod::Functions create spaghetti code
2894
b500e03b 2895The C<goto-LABEL> form finds the statement labeled with LABEL and
391b733c 2896resumes execution there. It can't be used to get out of a block or
b500e03b
GG
2897subroutine given to C<sort>. It can be used to go almost anywhere
2898else within the dynamic scope, including out of subroutines, but it's
2899usually better to use some other construct such as C<last> or C<die>.
2900The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2901(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2902does not offer named loops combined with loop control. Perl does, and
2903this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2904
7660c0ab
A
2905The C<goto-EXPR> form expects a label name, whose scope will be resolved
2906dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2907necessarily recommended if you're optimizing for maintainability:
2908
2909 goto ("FOO", "BAR", "GLARCH")[$i];
2910
887d89fd 2911As shown in this example, C<goto-EXPR> is exempt from the "looks like a
391b733c
FC
2912function" rule. A pair of parentheses following it does not (necessarily)
2913delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
8a7e748e
FC
2914Also, unlike most named operators, this has the same precedence as
2915assignment.
887d89fd 2916
b500e03b 2917Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2918deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2919go into any construct that requires initialization, such as a
2920subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2921construct that is optimized away.
b500e03b 2922
1b6921cb
BT
2923The C<goto-&NAME> form is quite different from the other forms of
2924C<goto>. In fact, it isn't a goto in the normal sense at all, and
2925doesn't have the stigma associated with other gotos. Instead, it
2926exits the current subroutine (losing any changes set by local()) and
2927immediately calls in its place the named subroutine using the current
2928value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2929load another subroutine and then pretend that the other subroutine had
2930been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2931in the current subroutine are propagated to the other subroutine.)
2932After the C<goto>, not even C<caller> will be able to tell that this
2933routine was called first.
2934
2935NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2936containing a code reference or a block that evaluates to a code
6cb9131c 2937reference.
a0d0e21e
LW
2938
2939=item grep BLOCK LIST
d74e8afc 2940X<grep>
a0d0e21e
LW
2941
2942=item grep EXPR,LIST
2943
c17cdb72
NC
2944=for Pod::Functions locate elements in a list test true against a given criterion
2945
2b5ab1e7
TC
2946This is similar in spirit to, but not the same as, grep(1) and its
2947relatives. In particular, it is not limited to using regular expressions.
2f9daede 2948
a0d0e21e 2949Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2950C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2951elements for which the expression evaluated to true. In scalar
2952context, returns the number of times the expression was true.
a0d0e21e
LW
2953
2954 @foo = grep(!/^#/, @bar); # weed out comments
2955
2956or equivalently,
2957
2958 @foo = grep {!/^#/} @bar; # weed out comments
2959
be3174d2
GS
2960Note that C<$_> is an alias to the list value, so it can be used to
2961modify the elements of the LIST. While this is useful and supported,
2962it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2963Similarly, grep returns aliases into the original list, much as a for
2964loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2965element of a list returned by grep (for example, in a C<foreach>, C<map>
2966or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2967This is usually something to be avoided when writing clear code.
a0d0e21e 2968
a4fb8298 2969If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2970been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2971the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2972can't be seen from the outside, avoiding any potential side-effects.
2973
19799a22 2974See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2975
a0d0e21e 2976=item hex EXPR
d74e8afc 2977X<hex> X<hexadecimal>
a0d0e21e 2978
54310121 2979=item hex
bbce6d69 2980
c17cdb72
NC
2981=for Pod::Functions convert a string to a hexadecimal number
2982
2b5ab1e7 2983Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2984(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2985L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2986
2987 print hex '0xAf'; # prints '175'
2988 print hex 'aF'; # same
a0d0e21e 2989
19799a22 2990Hex strings may only represent integers. Strings that would cause
53305cf1 2991integer overflow trigger a warning. Leading whitespace is not stripped,
391b733c 2992unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 2993L</sprintf>, and L</unpack>.
19799a22 2994
ce2984c3 2995=item import LIST
d74e8afc 2996X<import>
a0d0e21e 2997
c17cdb72
NC
2998=for Pod::Functions patch a module's namespace into your own
2999
19799a22 3000There is no builtin C<import> function. It is just an ordinary
4633a7c4 3001method (subroutine) defined (or inherited) by modules that wish to export
19799a22 3002names to another module. The C<use> function calls the C<import> method
cea6626f 3003for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
3004
3005=item index STR,SUBSTR,POSITION
d74e8afc 3006X<index> X<indexOf> X<InStr>
a0d0e21e
LW
3007
3008=item index STR,SUBSTR
3009
c17cdb72
NC
3010=for Pod::Functions find a substring within a string
3011
2b5ab1e7
TC
3012The index function searches for one string within another, but without
3013the wildcard-like behavior of a full regular-expression pattern match.
3014It returns the position of the first occurrence of SUBSTR in STR at
3015or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
3016beginning of the string. POSITION before the beginning of the string
3017or after its end is treated as if it were the beginning or the end,
e1dccc0d
Z
3018respectively. POSITION and the return value are based at zero.
3019If the substring is not found, C<index> returns -1.
a0d0e21e
LW
3020
3021=item int EXPR
f723aae1 3022X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 3023
54310121 3024=item int
bbce6d69 3025
c17cdb72
NC
3026=for Pod::Functions get the integer portion of a number
3027
7660c0ab 3028Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 3029You should not use this function for rounding: one because it truncates
3b10bc60 3030towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
3031numbers can sometimes produce counterintuitive results. For example,
3032C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3033because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 3034the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 3035functions will serve you better than will int().
a0d0e21e
LW
3036
3037=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 3038X<ioctl>
a0d0e21e 3039
c17cdb72
NC
3040=for Pod::Functions system-dependent device control system call
3041
2b5ab1e7 3042Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 3043
f7051f2c
FC
3044 require "sys/ioctl.ph"; # probably in
3045 # $Config{archlib}/sys/ioctl.ph
a0d0e21e 3046
a11c483f 3047to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 3048exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 3049own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 3050(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 3051