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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
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3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
8f1da26d 15operator. A unary operator generally provides scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
3b10bc60 17contexts for its arguments. If it does both, scalar arguments
18come first and list argument follow, and there can only ever
19be one such list argument. For instance, splice() has three scalar
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20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
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22
23In the syntax descriptions that follow, list operators that expect a
3b10bc60 24list (and provide list context for elements of the list) are shown
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25with LIST as an argument. Such a list may consist of any combination
26of scalar arguments or list values; the list values will be included
27in the list as if each individual element were interpolated at that
28point in the list, forming a longer single-dimensional list value.
8bdbc703 29Commas should separate literal elements of the LIST.
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30
31Any function in the list below may be used either with or without
32parentheses around its arguments. (The syntax descriptions omit the
3b10bc60 33parentheses.) If you use parentheses, the simple but occasionally
34surprising rule is this: It I<looks> like a function, therefore it I<is> a
a0d0e21e 35function, and precedence doesn't matter. Otherwise it's a list
3b10bc60 36operator or unary operator, and precedence does matter. Whitespace
37between the function and left parenthesis doesn't count, so sometimes
38you need to be careful:
a0d0e21e 39
5ed4f2ec 40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
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45
46If you run Perl with the B<-w> switch it can warn you about this. For
47example, the third line above produces:
48
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
51
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52A few functions take no arguments at all, and therefore work as neither
53unary nor list operators. These include such functions as C<time>
54and C<endpwent>. For example, C<time+86_400> always means
55C<time() + 86_400>.
56
a0d0e21e 57For functions that can be used in either a scalar or list context,
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58nonabortive failure is generally indicated in scalar context by
59returning the undefined value, and in list context by returning the
3b10bc60 60empty list.
a0d0e21e 61
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62Remember the following important rule: There is B<no rule> that relates
63the behavior of an expression in list context to its behavior in scalar
64context, or vice versa. It might do two totally different things.
80d38338 65Each operator and function decides which sort of value would be most
2b5ab1e7 66appropriate to return in scalar context. Some operators return the
5a964f20 67length of the list that would have been returned in list context. Some
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68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
d74e8afc 72X<context>
a0d0e21e 73
d1be9408 74A named array in scalar context is quite different from what would at
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75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
3b10bc60 81In general, functions in Perl that serve as wrappers for system calls ("syscalls")
5dac7880 82of the same name (like chown(2), fork(2), closedir(2), etc.) return
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83true when they succeed and C<undef> otherwise, as is usually mentioned
84in the descriptions below. This is different from the C interfaces,
5dac7880 85which return C<-1> on failure. Exceptions to this rule include C<wait>,
19799a22 86C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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87variable on failure. Other functions do not, except accidentally.
88
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89Extension modules can also hook into the Perl parser to define new
90kinds of keyword-headed expression. These may look like functions, but
91may also look completely different. The syntax following the keyword
92is defined entirely by the extension. If you are an implementor, see
93L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
94a module, see the module's documentation for details of the syntax that
95it defines.
96
cb1a09d0 97=head2 Perl Functions by Category
d74e8afc 98X<function>
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99
100Here are Perl's functions (including things that look like
5a964f20 101functions, like some keywords and named operators)
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102arranged by category. Some functions appear in more
103than one place.
104
13a2d996 105=over 4
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106
107=item Functions for SCALARs or strings
d74e8afc 108X<scalar> X<string> X<character>
cb1a09d0 109
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110=for Pod::Functions =String
111
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112C<chomp>, C<chop>, C<chr>, C<crypt>, C<fc>, C<hex>, C<index>, C<lc>,
113C<lcfirst>, C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
945c54fd 114C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
cb1a09d0 115
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116C<fc> is available only if the C<"fc"> feature is enabled or if it is
117prefixed with C<CORE::>. The C<"fc"> feature is enabled automatically
3dd9a840 118with a C<use v5.16> (or higher) declaration in the current scope.
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119
120
cb1a09d0 121=item Regular expressions and pattern matching
d74e8afc 122X<regular expression> X<regex> X<regexp>
cb1a09d0 123
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124=for Pod::Functions =Regexp
125
f5fa2679 126C<m//>, C<pos>, C<qr//>, C<quotemeta>, C<s///>, C<split>, C<study>
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127
128=item Numeric functions
d74e8afc 129X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 130
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131=for Pod::Functions =Math
132
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133C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
134C<sin>, C<sqrt>, C<srand>
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135
136=item Functions for real @ARRAYs
d74e8afc 137X<array>
cb1a09d0 138
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139=for Pod::Functions =ARRAY
140
a5ce339c 141C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
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142
143=item Functions for list data
d74e8afc 144X<list>
cb1a09d0 145
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146=for Pod::Functions =LIST
147
1dc8ecb8 148C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
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149
150=item Functions for real %HASHes
d74e8afc 151X<hash>
cb1a09d0 152
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153=for Pod::Functions =HASH
154
22fae026 155C<delete>, C<each>, C<exists>, C<keys>, C<values>
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156
157=item Input and output functions
d74e8afc 158X<I/O> X<input> X<output> X<dbm>
cb1a09d0 159
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160=for Pod::Functions =I/O
161
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162C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
163C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
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164C<readdir>, C<readline> C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>,
165C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>,
166C<truncate>, C<warn>, C<write>
cb1a09d0 167
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168C<say> is available only if the C<"say"> feature is enabled or if it is
169prefixed with C<CORE::>. The C<"say"> feature is enabled automatically
170with a C<use v5.10> (or higher) declaration in the current scope.
171
5dac7880 172=item Functions for fixed-length data or records
cb1a09d0 173
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174=for Pod::Functions =Binary
175
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176C<pack>, C<read>, C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<unpack>,
177C<vec>
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178
179=item Functions for filehandles, files, or directories
d74e8afc 180X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 181
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182=for Pod::Functions =File
183
22fae026 184C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 185C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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186C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
187C<umask>, C<unlink>, C<utime>
cb1a09d0 188
cf264981 189=item Keywords related to the control flow of your Perl program
d74e8afc 190X<control flow>
cb1a09d0 191
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192=for Pod::Functions =Flow
193
dba7b065 194C<break>, C<caller>, C<continue>, C<die>, C<do>,
7289c5e6 195C<dump>, C<eval>, C<evalbytes> C<exit>,
cfa52385 196C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
17d15541 197C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray>
84ed0108 198
dba7b065 199C<break> is available only if you enable the experimental C<"switch">
7161e5c2 200feature or use the C<CORE::> prefix. The C<"switch"> feature also enables
dba7b065 201the C<default>, C<given> and C<when> statements, which are documented in
7161e5c2 202L<perlsyn/"Switch Statements">. The C<"switch"> feature is enabled
dba7b065 203automatically with a C<use v5.10> (or higher) declaration in the current
7161e5c2 204scope. In Perl v5.14 and earlier, C<continue> required the C<"switch">
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205feature, like the other keywords.
206
e3f68f70 207C<evalbytes> is only available with the C<"evalbytes"> feature (see
4fe70ef9 208L<feature>) or if prefixed with C<CORE::>. C<__SUB__> is only available
7161e5c2 209with the C<"current_sub"> feature or if prefixed with C<CORE::>. Both
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210the C<"evalbytes"> and C<"current_sub"> features are enabled automatically
211with a C<use v5.16> (or higher) declaration in the current scope.
cb1a09d0 212
54310121 213=item Keywords related to scoping
cb1a09d0 214
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215=for Pod::Functions =Namespace
216
8f1da26d 217C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
36fb85f3 218
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219C<state> is available only if the C<"state"> feature is enabled or if it is
220prefixed with C<CORE::>. The C<"state"> feature is enabled automatically
221with a C<use v5.10> (or higher) declaration in the current scope.
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222
223=item Miscellaneous functions
224
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225=for Pod::Functions =Misc
226
17d15541 227C<defined>, C<formline>, C<lock>, C<prototype>, C<reset>, C<scalar>, C<undef>
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228
229=item Functions for processes and process groups
d74e8afc 230X<process> X<pid> X<process id>
cb1a09d0 231
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232=for Pod::Functions =Process
233
22fae026 234C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
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235C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>,
236C<setpriority>, C<sleep>, C<system>,
22fae026 237C<times>, C<wait>, C<waitpid>
cb1a09d0 238
3b10bc60 239=item Keywords related to Perl modules
d74e8afc 240X<module>
cb1a09d0 241
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242=for Pod::Functions =Modules
243
22fae026 244C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 245
353c6505 246=item Keywords related to classes and object-orientation
d74e8afc 247X<object> X<class> X<package>
cb1a09d0 248
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249=for Pod::Functions =Objects
250
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251C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
252C<untie>, C<use>
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253
254=item Low-level socket functions
d74e8afc 255X<socket> X<sock>
cb1a09d0 256
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257=for Pod::Functions =Socket
258
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259C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
260C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 261C<socket>, C<socketpair>
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262
263=item System V interprocess communication functions
d74e8afc 264X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 265
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266=for Pod::Functions =SysV
267
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268C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
269C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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270
271=item Fetching user and group info
d74e8afc 272X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 273
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274=for Pod::Functions =User
275
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276C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
277C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
278C<getpwuid>, C<setgrent>, C<setpwent>
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279
280=item Fetching network info
d74e8afc 281X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 282
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283=for Pod::Functions =Network
284
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285C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
286C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
287C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
288C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
289C<setnetent>, C<setprotoent>, C<setservent>
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290
291=item Time-related functions
d74e8afc 292X<time> X<date>
cb1a09d0 293
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294=for Pod::Functions =Time
295
22fae026 296C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 297
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298=item Non-function keywords
299
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300=for Pod::Functions =!Non-functions
301
f5fa2679 302C<and>, C<AUTOLOAD>, C<BEGIN>, C<CHECK>, C<cmp>, C<CORE>, C<__DATA__>,
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303C<default>, C<DESTROY>, C<else>, C<elseif>, C<elsif>, C<END>, C<__END__>,
304C<eq>, C<for>, C<foreach>, C<ge>, C<given>, C<gt>, C<if>, C<INIT>, C<le>,
305C<lt>, C<ne>, C<not>, C<or>, C<UNITCHECK>, C<unless>, C<until>, C<when>,
306C<while>, C<x>, C<xor>
8f0d6a61 307
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308=back
309
60f9f73c 310=head2 Portability
d74e8afc 311X<portability> X<Unix> X<portable>
60f9f73c 312
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313Perl was born in Unix and can therefore access all common Unix
314system calls. In non-Unix environments, the functionality of some
8f1da26d 315Unix system calls may not be available or details of the available
2b5ab1e7 316functionality may differ slightly. The Perl functions affected
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317by this are:
318
319C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
320C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
321C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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322C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
323C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 324C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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325C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
326C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
327C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 328C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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329C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
330C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
331C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 332C<shmwrite>, C<socket>, C<socketpair>,
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333C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
334C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 335C<utime>, C<wait>, C<waitpid>
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336
337For more information about the portability of these functions, see
338L<perlport> and other available platform-specific documentation.
339
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340=head2 Alphabetical Listing of Perl Functions
341
3b10bc60 342=over
a0d0e21e 343
5b3c99c0 344=item -X FILEHANDLE
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345X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
346X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 347
5b3c99c0 348=item -X EXPR
a0d0e21e 349
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350=item -X DIRHANDLE
351
5b3c99c0 352=item -X
a0d0e21e 353
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354=for Pod::Functions a file test (-r, -x, etc)
355
a0d0e21e 356A file test, where X is one of the letters listed below. This unary
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357operator takes one argument, either a filename, a filehandle, or a dirhandle,
358and tests the associated file to see if something is true about it. If the
7660c0ab 359argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
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360Unless otherwise documented, it returns C<1> for true and C<''> for false.
361If the file doesn't exist or can't be examined, it returns C<undef> and
362sets C<$!> (errno). Despite the funny names, precedence is the same as any
363other named unary operator. The operator may be any of:
a0d0e21e 364
5ed4f2ec 365 -r File is readable by effective uid/gid.
366 -w File is writable by effective uid/gid.
367 -x File is executable by effective uid/gid.
368 -o File is owned by effective uid.
a0d0e21e 369
5ed4f2ec 370 -R File is readable by real uid/gid.
371 -W File is writable by real uid/gid.
372 -X File is executable by real uid/gid.
373 -O File is owned by real uid.
a0d0e21e 374
5ed4f2ec 375 -e File exists.
376 -z File has zero size (is empty).
377 -s File has nonzero size (returns size in bytes).
a0d0e21e 378
5ed4f2ec 379 -f File is a plain file.
380 -d File is a directory.
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381 -l File is a symbolic link (false if symlinks aren't
382 supported by the file system).
5ed4f2ec 383 -p File is a named pipe (FIFO), or Filehandle is a pipe.
384 -S File is a socket.
385 -b File is a block special file.
386 -c File is a character special file.
387 -t Filehandle is opened to a tty.
a0d0e21e 388
5ed4f2ec 389 -u File has setuid bit set.
390 -g File has setgid bit set.
391 -k File has sticky bit set.
a0d0e21e 392
65cc07c9 393 -T File is an ASCII or UTF-8 text file (heuristic guess).
5ed4f2ec 394 -B File is a "binary" file (opposite of -T).
a0d0e21e 395
5ed4f2ec 396 -M Script start time minus file modification time, in days.
397 -A Same for access time.
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398 -C Same for inode change time (Unix, may differ for other
399 platforms)
a0d0e21e 400
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401Example:
402
403 while (<>) {
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404 chomp;
405 next unless -f $_; # ignore specials
406 #...
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407 }
408
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409Note that C<-s/a/b/> does not do a negated substitution. Saying
410C<-exp($foo)> still works as expected, however: only single letters
411following a minus are interpreted as file tests.
412
413These operators are exempt from the "looks like a function rule" described
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414above. That is, an opening parenthesis after the operator does not affect
415how much of the following code constitutes the argument. Put the opening
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416parentheses before the operator to separate it from code that follows (this
417applies only to operators with higher precedence than unary operators, of
418course):
419
420 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
421 (-s $file) + 1024 # correct
422
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423The interpretation of the file permission operators C<-r>, C<-R>,
424C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
425of the file and the uids and gids of the user. There may be other
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426reasons you can't actually read, write, or execute the file: for
427example network filesystem access controls, ACLs (access control lists),
428read-only filesystems, and unrecognized executable formats. Note
429that the use of these six specific operators to verify if some operation
430is possible is usually a mistake, because it may be open to race
431conditions.
5ff3f7a4 432
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433Also note that, for the superuser on the local filesystems, the C<-r>,
434C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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GS
435if any execute bit is set in the mode. Scripts run by the superuser
436may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 437or temporarily set their effective uid to something else.
5ff3f7a4
GS
438
439If you are using ACLs, there is a pragma called C<filetest> that may
440produce more accurate results than the bare stat() mode bits.
5dac7880
FC
441When under C<use filetest 'access'> the above-mentioned filetests
442test whether the permission can(not) be granted using the
3b10bc60 443access(2) family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
444under this pragma return true even if there are no execute permission
445bits set (nor any extra execute permission ACLs). This strangeness is
391b733c 446due to the underlying system calls' definitions. Note also that, due to
ecae030f
MO
447the implementation of C<use filetest 'access'>, the C<_> special
448filehandle won't cache the results of the file tests when this pragma is
449in effect. Read the documentation for the C<filetest> pragma for more
450information.
5ff3f7a4 451
65cc07c9
KW
452The C<-T> and C<-B> switches work as follows. The first block or so of
453the file is examined to see if it is valid UTF-8 that includes non-ASCII
454characters. If, so it's a C<-T> file. Otherwise, that same portion of
455the file is examined for odd characters such as strange control codes or
456characters with the high bit set. If more than a third of the
457characters are strange, it's a C<-B> file; otherwise it's a C<-T> file.
458Also, any file containing a zero byte in the examined portion is
459considered a binary file. (If executed within the scope of a L<S<use
460locale>|perllocale> which includes C<LC_CTYPE>, odd characters are
444d4f5c 461anything that isn't a printable nor space in the current locale.) If
65cc07c9
KW
462C<-T> or C<-B> is used on a filehandle, the current IO buffer is
463examined
3b10bc60 464rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 465file, or a file at EOF when testing a filehandle. Because you have to
4633a7c4
LW
466read a file to do the C<-T> test, on most occasions you want to use a C<-f>
467against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 468
5dac7880 469If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 470the special filehandle consisting of a solitary underline, then the stat
a0d0e21e
LW
471structure of the previous file test (or stat operator) is used, saving
472a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 473that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 474symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 475an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 476Example:
a0d0e21e
LW
477
478 print "Can do.\n" if -r $a || -w _ || -x _;
479
480 stat($filename);
481 print "Readable\n" if -r _;
482 print "Writable\n" if -w _;
483 print "Executable\n" if -x _;
484 print "Setuid\n" if -u _;
485 print "Setgid\n" if -g _;
486 print "Sticky\n" if -k _;
487 print "Text\n" if -T _;
488 print "Binary\n" if -B _;
489
e9fa405d 490As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file
fbb0b3b3 491test operators, in a way that C<-f -w -x $file> is equivalent to
a5840dee 492C<-x $file && -w _ && -f _>. (This is only fancy syntax: if you use
fbb0b3b3
RGS
493the return value of C<-f $file> as an argument to another filetest
494operator, no special magic will happen.)
495
bee96257 496Portability issues: L<perlport/-X>.
ea9eb35a 497
bade7fbc
TC
498To avoid confusing would-be users of your code with mysterious
499syntax errors, put something like this at the top of your script:
500
501 use 5.010; # so filetest ops can stack
502
a0d0e21e 503=item abs VALUE
d74e8afc 504X<abs> X<absolute>
a0d0e21e 505
54310121 506=item abs
bbce6d69 507
c17cdb72
NC
508=for Pod::Functions absolute value function
509
a0d0e21e 510Returns the absolute value of its argument.
7660c0ab 511If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
512
513=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 514X<accept>
a0d0e21e 515
c17cdb72
NC
516=for Pod::Functions accept an incoming socket connect
517
3b10bc60 518Accepts an incoming socket connect, just as accept(2)
19799a22 519does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 520See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 521
8d2a6795
GS
522On systems that support a close-on-exec flag on files, the flag will
523be set for the newly opened file descriptor, as determined by the
524value of $^F. See L<perlvar/$^F>.
525
a0d0e21e 526=item alarm SECONDS
d74e8afc
ITB
527X<alarm>
528X<SIGALRM>
529X<timer>
a0d0e21e 530
54310121 531=item alarm
bbce6d69 532
c17cdb72
NC
533=for Pod::Functions schedule a SIGALRM
534
a0d0e21e 535Arranges to have a SIGALRM delivered to this process after the
cf264981 536specified number of wallclock seconds has elapsed. If SECONDS is not
391b733c 537specified, the value stored in C<$_> is used. (On some machines,
d400eac8
JH
538unfortunately, the elapsed time may be up to one second less or more
539than you specified because of how seconds are counted, and process
540scheduling may delay the delivery of the signal even further.)
541
542Only one timer may be counting at once. Each call disables the
543previous timer, and an argument of C<0> may be supplied to cancel the
544previous timer without starting a new one. The returned value is the
545amount of time remaining on the previous timer.
a0d0e21e 546
2bc69794
BS
547For delays of finer granularity than one second, the Time::HiRes module
548(from CPAN, and starting from Perl 5.8 part of the standard
549distribution) provides ualarm(). You may also use Perl's four-argument
550version of select() leaving the first three arguments undefined, or you
551might be able to use the C<syscall> interface to access setitimer(2) if
391b733c 552your system supports it. See L<perlfaq8> for details.
2b5ab1e7 553
80d38338
TC
554It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
555C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 556
19799a22
GS
557If you want to use C<alarm> to time out a system call you need to use an
558C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 559fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 560restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 561modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
562
563 eval {
a9a5a0dc
VP
564 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
565 alarm $timeout;
566 $nread = sysread SOCKET, $buffer, $size;
567 alarm 0;
ff68c719 568 };
ff68c719 569 if ($@) {
a9a5a0dc 570 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 571 # timed out
ff68c719
PP
572 }
573 else {
5ed4f2ec 574 # didn't
ff68c719
PP
575 }
576
91d81acc
JH
577For more information see L<perlipc>.
578
ea9eb35a 579Portability issues: L<perlport/alarm>.
580
a0d0e21e 581=item atan2 Y,X
d74e8afc 582X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e 583
c17cdb72
NC
584=for Pod::Functions arctangent of Y/X in the range -PI to PI
585
a0d0e21e
LW
586Returns the arctangent of Y/X in the range -PI to PI.
587
ca6e1c26 588For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
589function, or use the familiar relation:
590
591 sub tan { sin($_[0]) / cos($_[0]) }
592
a1021d57
RGS
593The return value for C<atan2(0,0)> is implementation-defined; consult
594your atan2(3) manpage for more information.
bf5f1b4c 595
ea9eb35a 596Portability issues: L<perlport/atan2>.
597
a0d0e21e 598=item bind SOCKET,NAME
d74e8afc 599X<bind>
a0d0e21e 600
c17cdb72
NC
601=for Pod::Functions binds an address to a socket
602
3b10bc60 603Binds a network address to a socket, just as bind(2)
19799a22 604does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
605packed address of the appropriate type for the socket. See the examples in
606L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 607
fae2c0fb 608=item binmode FILEHANDLE, LAYER
d74e8afc 609X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 610
a0d0e21e
LW
611=item binmode FILEHANDLE
612
c17cdb72
NC
613=for Pod::Functions prepare binary files for I/O
614
1cbfc93d
NIS
615Arranges for FILEHANDLE to be read or written in "binary" or "text"
616mode on systems where the run-time libraries distinguish between
617binary and text files. If FILEHANDLE is an expression, the value is
618taken as the name of the filehandle. Returns true on success,
b5fe5ca2 619otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 620
8f1da26d 621On some systems (in general, DOS- and Windows-based systems) binmode()
d807c6f4 622is necessary when you're not working with a text file. For the sake
d7a0d798
FC
623of portability it is a good idea always to use it when appropriate,
624and never to use it when it isn't appropriate. Also, people can
8f1da26d 625set their I/O to be by default UTF8-encoded Unicode, not bytes.
d807c6f4
JH
626
627In other words: regardless of platform, use binmode() on binary data,
d7a0d798 628like images, for example.
d807c6f4
JH
629
630If LAYER is present it is a single string, but may contain multiple
391b733c 631directives. The directives alter the behaviour of the filehandle.
d7a0d798 632When LAYER is present, using binmode on a text file makes sense.
d807c6f4 633
fae2c0fb 634If LAYER is omitted or specified as C<:raw> the filehandle is made
391b733c 635suitable for passing binary data. This includes turning off possible CRLF
0226bbdb 636translation and marking it as bytes (as opposed to Unicode characters).
749683d2 637Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 638Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
639Other layers that would affect the binary nature of the stream are
391b733c 640I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 641PERLIO environment variable.
01e6739c 642
3b10bc60 643The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
644form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
645establish default I/O layers. See L<open>.
646
fae2c0fb
RGS
647I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
648in "Programming Perl, 3rd Edition". However, since the publishing of this
649book, by many known as "Camel III", the consensus of the naming of this
650functionality has moved from "discipline" to "layer". All documentation
651of this version of Perl therefore refers to "layers" rather than to
652"disciplines". Now back to the regularly scheduled documentation...>
653
8f1da26d 654To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
6902c96a 655C<:utf8> just marks the data as UTF-8 without further checking,
8f1da26d 656while C<:encoding(UTF-8)> checks the data for actually being valid
391b733c 657UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 658
ed53a2bb 659In general, binmode() should be called after open() but before any I/O
3b10bc60 660is done on the filehandle. Calling binmode() normally flushes any
01e6739c 661pending buffered output data (and perhaps pending input data) on the
fae2c0fb 662handle. An exception to this is the C<:encoding> layer that
d7a0d798 663changes the default character encoding of the handle; see L</open>.
fae2c0fb 664The C<:encoding> layer sometimes needs to be called in
3874323d
JH
665mid-stream, and it doesn't flush the stream. The C<:encoding>
666also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 667internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 668
19799a22 669The operating system, device drivers, C libraries, and Perl run-time
8f1da26d
TC
670system all conspire to let the programmer treat a single
671character (C<\n>) as the line terminator, irrespective of external
30168b04
GS
672representation. On many operating systems, the native text file
673representation matches the internal representation, but on some
674platforms the external representation of C<\n> is made up of more than
675one character.
676
8f1da26d
TC
677All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
678a single character to end each line in the external representation of text
679(even though that single character is CARRIAGE RETURN on old, pre-Darwin
391b733c 680flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
8f1da26d
TC
681systems like OS/2, DOS, and the various flavors of MS-Windows, your program
682sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
683two characters C<\cM\cJ>. That means that if you don't use binmode() on
684these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
685input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
686output. This is what you want for text files, but it can be disastrous for
687binary files.
30168b04
GS
688
689Another consequence of using binmode() (on some systems) is that
690special end-of-file markers will be seen as part of the data stream.
d7a0d798
FC
691For systems from the Microsoft family this means that, if your binary
692data contain C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
693the file, unless you use binmode().
694
3b10bc60 695binmode() is important not only for readline() and print() operations,
30168b04
GS
696but also when using read(), seek(), sysread(), syswrite() and tell()
697(see L<perlport> for more details). See the C<$/> and C<$\> variables
698in L<perlvar> for how to manually set your input and output
699line-termination sequences.
a0d0e21e 700
ea9eb35a 701Portability issues: L<perlport/binmode>.
702
4633a7c4 703=item bless REF,CLASSNAME
d74e8afc 704X<bless>
a0d0e21e
LW
705
706=item bless REF
707
c17cdb72
NC
708=for Pod::Functions create an object
709
2b5ab1e7
TC
710This function tells the thingy referenced by REF that it is now an object
711in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 712is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 713it returns the reference for convenience. Always use the two-argument
cf264981 714version if a derived class might inherit the function doing the blessing.
e54e4959 715See L<perlobj> for more about the blessing (and blessings) of objects.
a0d0e21e 716
57668c4d 717Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 718Namespaces with all lowercase names are considered reserved for
391b733c 719Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
720confusion, you may wish to avoid such package names as well. Make sure
721that CLASSNAME is a true value.
60ad88b8
GS
722
723See L<perlmod/"Perl Modules">.
724
0d863452
RH
725=item break
726
d9b04284 727=for Pod::Functions +switch break out of a C<given> block
c17cdb72 728
0d863452
RH
729Break out of a C<given()> block.
730
a8a26e52
JK
731This keyword is enabled by the C<"switch"> feature; see L<feature> for
732more information on C<"switch">. You can also access it by prefixing it
733with C<CORE::>. Alternatively, include a C<use v5.10> or later to the
734current scope.
0d863452 735
a0d0e21e 736=item caller EXPR
d74e8afc 737X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
738
739=item caller
740
c17cdb72
NC
741=for Pod::Functions get context of the current subroutine call
742
1d56df50
DD
743Returns the context of the current pure perl subroutine call. In scalar
744context, returns the caller's package name if there I<is> a caller (that is, if
80d38338 745we're in a subroutine or C<eval> or C<require>) and the undefined value
1d56df50 746otherwise. caller never returns XS subs and they are skipped. The next pure
444d4f5c
FC
747perl sub will appear instead of the XS
748sub in caller's return values. In list
1d56df50 749context, caller returns
a0d0e21e 750
ee6b43cc 751 # 0 1 2
748a9306 752 ($package, $filename, $line) = caller;
a0d0e21e
LW
753
754With EXPR, it returns some extra information that the debugger uses to
755print a stack trace. The value of EXPR indicates how many call frames
756to go back before the current one.
757
ee6b43cc 758 # 0 1 2 3 4
f3aa04c2 759 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 760
761 # 5 6 7 8 9 10
b3ca2e83 762 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 763 = caller($i);
e7ea3e70 764
02729fef 765Here, $subroutine is the function that the caller called (rather than the
444d4f5c 766function containing the caller). Note that $subroutine may be C<(eval)> if
02729fef
DM
767the frame is not a subroutine call, but an C<eval>. In such a case
768additional elements $evaltext and
7660c0ab 769C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 770C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 771C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 772$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
773each C<use> statement creates a C<require> frame inside an C<eval EXPR>
774frame.) $subroutine may also be C<(unknown)> if this particular
775subroutine happens to have been deleted from the symbol table.
776C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
777C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
585d73c3 778compiled with. C<$hints> corresponds to C<$^H>, and C<$bitmask>
1adb05cd
FC
779corresponds to C<${^WARNING_BITS}>. The
780C<$hints> and C<$bitmask> values are subject
585d73c3 781to change between versions of Perl, and are not meant for external use.
748a9306 782
b3ca2e83 783C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
391b733c 784caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
b3ca2e83
NC
785of this hash, as they are the actual values stored in the optree.
786
ffe0c19d
FC
787Furthermore, when called from within the DB package in
788list context, and with an argument, caller returns more
7660c0ab 789detailed information: it sets the list variable C<@DB::args> to be the
54310121 790arguments with which the subroutine was invoked.
748a9306 791
7660c0ab 792Be aware that the optimizer might have optimized call frames away before
19799a22 793C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 794might not return information about the call frame you expect it to, for
b76cc8ba 795C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 796previous time C<caller> was called.
7660c0ab 797
8f1da26d 798Be aware that setting C<@DB::args> is I<best effort>, intended for
391b733c 799debugging or generating backtraces, and should not be relied upon. In
ca9f0cb5
NC
800particular, as C<@_> contains aliases to the caller's arguments, Perl does
801not take a copy of C<@_>, so C<@DB::args> will contain modifications the
802subroutine makes to C<@_> or its contents, not the original values at call
391b733c 803time. C<@DB::args>, like C<@_>, does not hold explicit references to its
ca9f0cb5 804elements, so under certain cases its elements may have become freed and
391b733c 805reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 806of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
807I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
808reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5 809elements), so C<@DB::args> is actually a hybrid of the current state and
391b733c 810initial state of C<@_>. Buyer beware.
ca9f0cb5 811
a0d0e21e 812=item chdir EXPR
d74e8afc
ITB
813X<chdir>
814X<cd>
f723aae1 815X<directory, change>
a0d0e21e 816
c4aca7d0
GA
817=item chdir FILEHANDLE
818
819=item chdir DIRHANDLE
820
ce2984c3
PF
821=item chdir
822
c17cdb72
NC
823=for Pod::Functions change your current working directory
824
391b733c 825Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 826changes to the directory specified by C<$ENV{HOME}>, if set; if not,
391b733c
FC
827changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
828variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
829neither is set, C<chdir> does nothing. It returns true on success,
830false otherwise. See the example under C<die>.
a0d0e21e 831
3b10bc60 832On systems that support fchdir(2), you may pass a filehandle or
34169887 833directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 834passing handles raises an exception.
c4aca7d0 835
a0d0e21e 836=item chmod LIST
d74e8afc 837X<chmod> X<permission> X<mode>
a0d0e21e 838
c17cdb72
NC
839=for Pod::Functions changes the permissions on a list of files
840
a0d0e21e 841Changes the permissions of a list of files. The first element of the
8f1da26d 842list must be the numeric mode, which should probably be an octal
4ad40acf 843number, and which definitely should I<not> be a string of octal digits:
3b10bc60 844C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 845successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 846
3b10bc60 847 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 848 chmod 0755, @executables;
3b10bc60 849 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 850 # --w----r-T
3b10bc60 851 $mode = "0644"; chmod oct($mode), "foo"; # this is better
852 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 853
3b10bc60 854On systems that support fchmod(2), you may pass filehandles among the
855files. On systems that don't support fchmod(2), passing filehandles raises
856an exception. Filehandles must be passed as globs or glob references to be
857recognized; barewords are considered filenames.
c4aca7d0
GA
858
859 open(my $fh, "<", "foo");
860 my $perm = (stat $fh)[2] & 07777;
861 chmod($perm | 0600, $fh);
862
3b10bc60 863You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
864module:
865
3b10bc60 866 use Fcntl qw( :mode );
ca6e1c26 867 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 868 # Identical to the chmod 0755 of the example above.
ca6e1c26 869
ea9eb35a 870Portability issues: L<perlport/chmod>.
871
a0d0e21e 872=item chomp VARIABLE
d74e8afc 873X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 874
313c9f5c 875=item chomp( LIST )
a0d0e21e
LW
876
877=item chomp
878
c17cdb72
NC
879=for Pod::Functions remove a trailing record separator from a string
880
2b5ab1e7
TC
881This safer version of L</chop> removes any trailing string
882that corresponds to the current value of C<$/> (also known as
28757baa
PP
883$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
884number of characters removed from all its arguments. It's often used to
885remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
886that the final record may be missing its newline. When in paragraph
887mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 888When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 889a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 890remove anything.
19799a22 891If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
892
893 while (<>) {
a9a5a0dc
VP
894 chomp; # avoid \n on last field
895 @array = split(/:/);
896 # ...
a0d0e21e
LW
897 }
898
feef49c9
FC
899If VARIABLE is a hash, it chomps the hash's values, but not its keys,
900resetting the C<each> iterator in the process.
4bf21a6d 901
a0d0e21e
LW
902You can actually chomp anything that's an lvalue, including an assignment:
903
904 chomp($cwd = `pwd`);
905 chomp($answer = <STDIN>);
906
907If you chomp a list, each element is chomped, and the total number of
908characters removed is returned.
909
15e44fd8
RGS
910Note that parentheses are necessary when you're chomping anything
911that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
912is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
913C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
914C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
915as C<chomp($a, $b)>.
916
a0d0e21e 917=item chop VARIABLE
d74e8afc 918X<chop>
a0d0e21e 919
313c9f5c 920=item chop( LIST )
a0d0e21e
LW
921
922=item chop
923
c17cdb72
NC
924=for Pod::Functions remove the last character from a string
925
a0d0e21e 926Chops off the last character of a string and returns the character
5b3eff12 927chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 928scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
feef49c9
FC
929If VARIABLE is a hash, it chops the hash's values, but not its keys,
930resetting the C<each> iterator in the process.
4bf21a6d 931
5b3eff12 932You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
933
934If you chop a list, each element is chopped. Only the value of the
19799a22 935last C<chop> is returned.
a0d0e21e 936
19799a22 937Note that C<chop> returns the last character. To return all but the last
748a9306
LW
938character, use C<substr($string, 0, -1)>.
939
15e44fd8
RGS
940See also L</chomp>.
941
a0d0e21e 942=item chown LIST
d74e8afc 943X<chown> X<owner> X<user> X<group>
a0d0e21e 944
c17cdb72
NC
945=for Pod::Functions change the ownership on a list of files
946
a0d0e21e 947Changes the owner (and group) of a list of files. The first two
19799a22
GS
948elements of the list must be the I<numeric> uid and gid, in that
949order. A value of -1 in either position is interpreted by most
950systems to leave that value unchanged. Returns the number of files
951successfully changed.
a0d0e21e
LW
952
953 $cnt = chown $uid, $gid, 'foo', 'bar';
954 chown $uid, $gid, @filenames;
955
3b10bc60 956On systems that support fchown(2), you may pass filehandles among the
957files. On systems that don't support fchown(2), passing filehandles raises
958an exception. Filehandles must be passed as globs or glob references to be
959recognized; barewords are considered filenames.
c4aca7d0 960
54310121 961Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
962
963 print "User: ";
19799a22 964 chomp($user = <STDIN>);
5a964f20 965 print "Files: ";
19799a22 966 chomp($pattern = <STDIN>);
a0d0e21e
LW
967
968 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 969 or die "$user not in passwd file";
a0d0e21e 970
5ed4f2ec 971 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
972 chown $uid, $gid, @ary;
973
54310121 974On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
975file unless you're the superuser, although you should be able to change
976the group to any of your secondary groups. On insecure systems, these
977restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
978On POSIX systems, you can detect this condition this way:
979
980 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
981 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 982
f48496b1 983Portability issues: L<perlport/chown>.
ea9eb35a 984
a0d0e21e 985=item chr NUMBER
d74e8afc 986X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 987
54310121 988=item chr
bbce6d69 989
c17cdb72
NC
990=for Pod::Functions get character this number represents
991
a0d0e21e 992Returns the character represented by that NUMBER in the character set.
a0ed51b3 993For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 994chr(0x263a) is a Unicode smiley face.
aaa68c4a 995
8a064bd6 996Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 997except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
998(truncated to an integer) are used.
999
974da8e5
JH
1000If NUMBER is omitted, uses C<$_>.
1001
b76cc8ba 1002For the reverse, use L</ord>.
a0d0e21e 1003
2575c402
JW
1004Note that characters from 128 to 255 (inclusive) are by default
1005internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 1006
2575c402 1007See L<perlunicode> for more about Unicode.
bbce6d69 1008
a0d0e21e 1009=item chroot FILENAME
d74e8afc 1010X<chroot> X<root>
a0d0e21e 1011
54310121 1012=item chroot
bbce6d69 1013
c17cdb72
NC
1014=for Pod::Functions make directory new root for path lookups
1015
5a964f20 1016This function works like the system call by the same name: it makes the
4633a7c4 1017named directory the new root directory for all further pathnames that
951ba7fe 1018begin with a C</> by your process and all its children. (It doesn't
28757baa 1019change your current working directory, which is unaffected.) For security
4633a7c4 1020reasons, this call is restricted to the superuser. If FILENAME is
19799a22 1021omitted, does a C<chroot> to C<$_>.
a0d0e21e 1022
b00d10dc
JH
1023B<NOTE:> It is good security practice to do C<chdir("/")> (to the root
1024directory) immediately after a C<chroot()>.
1025
ea9eb35a 1026Portability issues: L<perlport/chroot>.
1027
a0d0e21e 1028=item close FILEHANDLE
d74e8afc 1029X<close>
a0d0e21e 1030
6a518fbc
TP
1031=item close
1032
c17cdb72
NC
1033=for Pod::Functions close file (or pipe or socket) handle
1034
3b10bc60 1035Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 1036buffers, and closes the system file descriptor. Returns true if those
8f1da26d 1037operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
1038layer. Closes the currently selected filehandle if the argument is
1039omitted.
fb73857a
PP
1040
1041You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 1042another C<open> on it, because C<open> closes it for you. (See
01aa884e 1043L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
19799a22 1044counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 1045
3b10bc60 1046If the filehandle came from a piped open, C<close> returns false if one of
1047the other syscalls involved fails or if its program exits with non-zero
1048status. If the only problem was that the program exited non-zero, C<$!>
1049will be set to C<0>. Closing a pipe also waits for the process executing
1050on the pipe to exit--in case you wish to look at the output of the pipe
1051afterwards--and implicitly puts the exit status value of that command into
1052C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 1053
2e0cfa16
FC
1054If there are multiple threads running, C<close> on a filehandle from a
1055piped open returns true without waiting for the child process to terminate,
1056if the filehandle is still open in another thread.
1057
80d38338
TC
1058Closing the read end of a pipe before the process writing to it at the
1059other end is done writing results in the writer receiving a SIGPIPE. If
1060the other end can't handle that, be sure to read all the data before
1061closing the pipe.
73689b13 1062
fb73857a 1063Example:
a0d0e21e 1064
fb73857a
PP
1065 open(OUTPUT, '|sort >foo') # pipe to sort
1066 or die "Can't start sort: $!";
5ed4f2ec 1067 #... # print stuff to output
1068 close OUTPUT # wait for sort to finish
fb73857a
PP
1069 or warn $! ? "Error closing sort pipe: $!"
1070 : "Exit status $? from sort";
5ed4f2ec 1071 open(INPUT, 'foo') # get sort's results
fb73857a 1072 or die "Can't open 'foo' for input: $!";
a0d0e21e 1073
5a964f20 1074FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 1075filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
1076
1077=item closedir DIRHANDLE
d74e8afc 1078X<closedir>
a0d0e21e 1079
c17cdb72
NC
1080=for Pod::Functions close directory handle
1081
19799a22 1082Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
1083system call.
1084
a0d0e21e 1085=item connect SOCKET,NAME
d74e8afc 1086X<connect>
a0d0e21e 1087
c17cdb72
NC
1088=for Pod::Functions connect to a remote socket
1089
80d38338
TC
1090Attempts to connect to a remote socket, just like connect(2).
1091Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
1092packed address of the appropriate type for the socket. See the examples in
1093L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 1094
cb1a09d0 1095=item continue BLOCK
d74e8afc 1096X<continue>
cb1a09d0 1097
0d863452
RH
1098=item continue
1099
c17cdb72
NC
1100=for Pod::Functions optional trailing block in a while or foreach
1101
4a904372
FC
1102When followed by a BLOCK, C<continue> is actually a
1103flow control statement rather than a function. If
cf264981 1104there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
1105C<foreach>), it is always executed just before the conditional is about to
1106be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
1107it can be used to increment a loop variable, even when the loop has been
1108continued via the C<next> statement (which is similar to the C C<continue>
1109statement).
1110
98293880 1111C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 1112block; C<last> and C<redo> behave as if they had been executed within
19799a22 1113the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
1114block, it may be more entertaining.
1115
1116 while (EXPR) {
a9a5a0dc
VP
1117 ### redo always comes here
1118 do_something;
1d2dff63 1119 } continue {
a9a5a0dc
VP
1120 ### next always comes here
1121 do_something_else;
1122 # then back the top to re-check EXPR
1d2dff63
GS
1123 }
1124 ### last always comes here
1125
3b10bc60 1126Omitting the C<continue> section is equivalent to using an
1127empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1128to check the condition at the top of the loop.
1129
4a904372 1130When there is no BLOCK, C<continue> is a function that
8f1da26d
TC
1131falls through the current C<when> or C<default> block instead of iterating
1132a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
4a904372
FC
1133In Perl 5.14 and earlier, this form of C<continue> was
1134only available when the C<"switch"> feature was enabled.
48238296 1135See L<feature> and L<perlsyn/"Switch Statements"> for more
8f1da26d 1136information.
0d863452 1137
a0d0e21e 1138=item cos EXPR
d74e8afc 1139X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1140
d6217f1e
GS
1141=item cos
1142
c17cdb72
NC
1143=for Pod::Functions cosine function
1144
5a964f20 1145Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1146takes the cosine of C<$_>.
a0d0e21e 1147
ca6e1c26 1148For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1149function, or use this relation:
1150
1151 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1152
a0d0e21e 1153=item crypt PLAINTEXT,SALT
d74e8afc 1154X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1155X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1156
c17cdb72
NC
1157=for Pod::Functions one-way passwd-style encryption
1158
ef2e6798
MS
1159Creates a digest string exactly like the crypt(3) function in the C
1160library (assuming that you actually have a version there that has not
bb23f8d1 1161been extirpated as a potential munition).
ef2e6798 1162
34169887 1163crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1164into a short string, called a digest, which is returned. The same
1165PLAINTEXT and SALT will always return the same string, but there is no
1166(known) way to get the original PLAINTEXT from the hash. Small
1167changes in the PLAINTEXT or SALT will result in large changes in the
1168digest.
1169
1170There is no decrypt function. This function isn't all that useful for
1171cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1172mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1173primarily used to check if two pieces of text are the same without
1174having to transmit or store the text itself. An example is checking
1175if a correct password is given. The digest of the password is stored,
cf264981 1176not the password itself. The user types in a password that is
ef2e6798 1177crypt()'d with the same salt as the stored digest. If the two digests
34169887 1178match, the password is correct.
ef2e6798
MS
1179
1180When verifying an existing digest string you should use the digest as
1181the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1182to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1183crypt() will hash the new string with the same salt as the digest.
1184This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1185with more exotic implementations. In other words, assume
1186nothing about the returned string itself nor about how many bytes
1187of SALT may matter.
85c16d83
JH
1188
1189Traditionally the result is a string of 13 bytes: two first bytes of
1190the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
391b733c 1191the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1192hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1193and implementations on non-Unix platforms may produce different
ef2e6798 1194strings.
85c16d83
JH
1195
1196When choosing a new salt create a random two character string whose
1197characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1198'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1199characters is just a recommendation; the characters allowed in
1200the salt depend solely on your system's crypt library, and Perl can't
1201restrict what salts C<crypt()> accepts.
e71965be 1202
a0d0e21e 1203Here's an example that makes sure that whoever runs this program knows
cf264981 1204their password:
a0d0e21e
LW
1205
1206 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1207
1208 system "stty -echo";
1209 print "Password: ";
e71965be 1210 chomp($word = <STDIN>);
a0d0e21e
LW
1211 print "\n";
1212 system "stty echo";
1213
e71965be 1214 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1215 die "Sorry...\n";
a0d0e21e 1216 } else {
a9a5a0dc 1217 print "ok\n";
54310121 1218 }
a0d0e21e 1219
9f8f0c9d 1220Of course, typing in your own password to whoever asks you
748a9306 1221for it is unwise.
a0d0e21e 1222
ef2e6798 1223The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1224of data, not least of all because you can't get the information
ef2e6798 1225back. Look at the L<Digest> module for more robust algorithms.
19799a22 1226
f2791508
JH
1227If using crypt() on a Unicode string (which I<potentially> has
1228characters with codepoints above 255), Perl tries to make sense
34169887 1229of the situation by trying to downgrade (a copy of)
f2791508
JH
1230the string back to an eight-bit byte string before calling crypt()
1231(on that copy). If that works, good. If not, crypt() dies with
1232C<Wide character in crypt>.
85c16d83 1233
ea9eb35a 1234Portability issues: L<perlport/crypt>.
1235
aa689395 1236=item dbmclose HASH
d74e8afc 1237X<dbmclose>
a0d0e21e 1238
c17cdb72
NC
1239=for Pod::Functions breaks binding on a tied dbm file
1240
19799a22 1241[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1242
aa689395 1243Breaks the binding between a DBM file and a hash.
a0d0e21e 1244
ea9eb35a 1245Portability issues: L<perlport/dbmclose>.
1246
19799a22 1247=item dbmopen HASH,DBNAME,MASK
d74e8afc 1248X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1249
c17cdb72
NC
1250=for Pod::Functions create binding on a tied dbm file
1251
01aa884e
KW
1252[This function has been largely superseded by the
1253L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
a0d0e21e 1254
7b8d334a 1255This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1256hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1257argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1258is the name of the database (without the F<.dir> or F<.pag> extension if
1259any). If the database does not exist, it is created with protection
1b3a6178
FC
1260specified by MASK (as modified by the C<umask>). To prevent creation of
1261the database if it doesn't exist, you may specify a MODE
1262of 0, and the function will return a false value if it
1263can't find an existing database. If your system supports
80d38338 1264only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1265program. In older versions of Perl, if your system had neither DBM nor
19799a22 1266ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1267sdbm(3).
1268
1269If you don't have write access to the DBM file, you can only read hash
1270variables, not set them. If you want to test whether you can write,
3b10bc60 1271either use file tests or try setting a dummy hash entry inside an C<eval>
1272to trap the error.
a0d0e21e 1273
19799a22
GS
1274Note that functions such as C<keys> and C<values> may return huge lists
1275when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1276function to iterate over large DBM files. Example:
1277
1278 # print out history file offsets
1279 dbmopen(%HIST,'/usr/lib/news/history',0666);
1280 while (($key,$val) = each %HIST) {
a9a5a0dc 1281 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1282 }
1283 dbmclose(%HIST);
1284
cb1a09d0 1285See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1286cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1287rich implementation.
4633a7c4 1288
2b5ab1e7
TC
1289You can control which DBM library you use by loading that library
1290before you call dbmopen():
1291
1292 use DB_File;
1293 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1294 or die "Can't open netscape history file: $!";
2b5ab1e7 1295
ea9eb35a 1296Portability issues: L<perlport/dbmopen>.
1297
a0d0e21e 1298=item defined EXPR
d74e8afc 1299X<defined> X<undef> X<undefined>
a0d0e21e 1300
54310121 1301=item defined
bbce6d69 1302
c17cdb72
NC
1303=for Pod::Functions test whether a value, variable, or function is defined
1304
2f9daede 1305Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1306the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1307checked.
1308
1309Many operations return C<undef> to indicate failure, end of file,
1310system error, uninitialized variable, and other exceptional
1311conditions. This function allows you to distinguish C<undef> from
1312other values. (A simple Boolean test will not distinguish among
7660c0ab 1313C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1314false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1315doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1316returns C<undef> when its argument is an empty array, I<or> when the
1317element to return happens to be C<undef>.
1318
f10b0346
GS
1319You may also use C<defined(&func)> to check whether subroutine C<&func>
1320has ever been defined. The return value is unaffected by any forward
80d38338 1321declarations of C<&func>. A subroutine that is not defined
847c7ebe 1322may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1323makes it spring into existence the first time that it is called; see
847c7ebe 1324L<perlsub>.
f10b0346
GS
1325
1326Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1327used to report whether memory for that aggregate had ever been
f10b0346
GS
1328allocated. This behavior may disappear in future versions of Perl.
1329You should instead use a simple test for size:
1330
1331 if (@an_array) { print "has array elements\n" }
1332 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1333
1334When used on a hash element, it tells you whether the value is defined,
dc848c6f 1335not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1336purpose.
a0d0e21e
LW
1337
1338Examples:
1339
8f1da26d 1340 print if defined $switch{D};
a0d0e21e
LW
1341 print "$val\n" while defined($val = pop(@ary));
1342 die "Can't readlink $sym: $!"
a9a5a0dc 1343 unless defined($value = readlink $sym);
a0d0e21e 1344 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1345 $debugging = 0 unless defined $debugging;
a0d0e21e 1346
8f1da26d 1347Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1348discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1349defined values. For example, if you say
a5f75d66
AD
1350
1351 "ab" =~ /a(.*)b/;
1352
80d38338 1353The pattern match succeeds and C<$1> is defined, although it
cf264981 1354matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1355matched something that happened to be zero characters long. This is all
a5f75d66 1356very above-board and honest. When a function returns an undefined value,
2f9daede 1357it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1358should use C<defined> only when questioning the integrity of what
7660c0ab 1359you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1360what you want.
1361
dc848c6f 1362See also L</undef>, L</exists>, L</ref>.
2f9daede 1363
a0d0e21e 1364=item delete EXPR
d74e8afc 1365X<delete>
a0d0e21e 1366
c17cdb72
NC
1367=for Pod::Functions deletes a value from a hash
1368
d0a76353
RS
1369Given an expression that specifies an element or slice of a hash, C<delete>
1370deletes the specified elements from that hash so that exists() on that element
1371no longer returns true. Setting a hash element to the undefined value does
1372not remove its key, but deleting it does; see L</exists>.
80d38338 1373
8f1da26d 1374In list context, returns the value or values deleted, or the last such
80d38338 1375element in scalar context. The return list's length always matches that of
d0a76353
RS
1376the argument list: deleting non-existent elements returns the undefined value
1377in their corresponding positions.
80d38338 1378
d0a76353
RS
1379delete() may also be used on arrays and array slices, but its behavior is less
1380straightforward. Although exists() will return false for deleted entries,
1381deleting array elements never changes indices of existing values; use shift()
deaf6ad3 1382or splice() for that. However, if any deleted elements fall at the end of an
d0a76353 1383array, the array's size shrinks to the position of the highest element that
444d4f5c 1384still tests true for exists(), or to 0 if none do. In other words, an
deaf6ad3 1385array won't have trailing nonexistent elements after a delete.
d0a76353 1386
2fbadc08
RS
1387B<WARNING:> Calling C<delete> on array values is strongly discouraged. The
1388notion of deleting or checking the existence of Perl array elements is not
1389conceptually coherent, and can lead to surprising behavior.
80d38338
TC
1390
1391Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1392a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1393or array may not necessarily return anything; it depends on the implementation
1394of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1395
80d38338
TC
1396The C<delete local EXPR> construct localizes the deletion to the current
1397block at run time. Until the block exits, elements locally deleted
1398temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1399of composite types">.
eba0920a
EM
1400
1401 %hash = (foo => 11, bar => 22, baz => 33);
f7051f2c
FC
1402 $scalar = delete $hash{foo}; # $scalar is 11
1403 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1404 @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
eba0920a 1405
01020589 1406The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1407
5f05dabc 1408 foreach $key (keys %HASH) {
a9a5a0dc 1409 delete $HASH{$key};
a0d0e21e
LW
1410 }
1411
01020589 1412 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1413 delete $ARRAY[$index];
01020589
GS
1414 }
1415
1416And so do these:
5f05dabc 1417
01020589
GS
1418 delete @HASH{keys %HASH};
1419
9740c838 1420 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1421
80d38338
TC
1422But both are slower than assigning the empty list
1423or undefining %HASH or @ARRAY, which is the customary
1424way to empty out an aggregate:
01020589 1425
5ed4f2ec 1426 %HASH = (); # completely empty %HASH
1427 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1428
5ed4f2ec 1429 @ARRAY = (); # completely empty @ARRAY
1430 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1431
80d38338
TC
1432The EXPR can be arbitrarily complicated provided its
1433final operation is an element or slice of an aggregate:
a0d0e21e
LW
1434
1435 delete $ref->[$x][$y]{$key};
5f05dabc 1436 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1437
01020589
GS
1438 delete $ref->[$x][$y][$index];
1439 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1440
a0d0e21e 1441=item die LIST
d74e8afc 1442X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1443
c17cdb72
NC
1444=for Pod::Functions raise an exception or bail out
1445
391b733c 1446C<die> raises an exception. Inside an C<eval> the error message is stuffed
4c050ad5
NC
1447into C<$@> and the C<eval> is terminated with the undefined value.
1448If the exception is outside of all enclosing C<eval>s, then the uncaught
391b733c 1449exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1450need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1451
1452Equivalent examples:
1453
1454 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1455 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1456
ccac6780 1457If the last element of LIST does not end in a newline, the current
df37ec69
WW
1458script line number and input line number (if any) are also printed,
1459and a newline is supplied. Note that the "input line number" (also
1460known as "chunk") is subject to whatever notion of "line" happens to
1461be currently in effect, and is also available as the special variable
1462C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1463
1464Hint: sometimes appending C<", stopped"> to your message will cause it
1465to make better sense when the string C<"at foo line 123"> is appended.
1466Suppose you are running script "canasta".
a0d0e21e
LW
1467
1468 die "/etc/games is no good";
1469 die "/etc/games is no good, stopped";
1470
1471produce, respectively
1472
1473 /etc/games is no good at canasta line 123.
1474 /etc/games is no good, stopped at canasta line 123.
1475
a96d0188 1476If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1477previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1478This is useful for propagating exceptions:
1479
1480 eval { ... };
1481 die unless $@ =~ /Expected exception/;
1482
a96d0188 1483If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1484C<PROPAGATE> method, that method will be called with additional file
1485and line number parameters. The return value replaces the value in
34169887 1486C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1487were called.
1488
7660c0ab 1489If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1490
4c050ad5
NC
1491If an uncaught exception results in interpreter exit, the exit code is
1492determined from the values of C<$!> and C<$?> with this pseudocode:
1493
1494 exit $! if $!; # errno
1495 exit $? >> 8 if $? >> 8; # child exit status
1496 exit 255; # last resort
1497
1498The intent is to squeeze as much possible information about the likely cause
391b733c
FC
1499into the limited space of the system exit
1500code. However, as C<$!> is the value
4c050ad5
NC
1501of C's C<errno>, which can be set by any system call, this means that the value
1502of the exit code used by C<die> can be non-predictable, so should not be relied
1503upon, other than to be non-zero.
1504
80d38338
TC
1505You can also call C<die> with a reference argument, and if this is trapped
1506within an C<eval>, C<$@> contains that reference. This permits more
1507elaborate exception handling using objects that maintain arbitrary state
1508about the exception. Such a scheme is sometimes preferable to matching
1509particular string values of C<$@> with regular expressions. Because C<$@>
1510is a global variable and C<eval> may be used within object implementations,
1511be careful that analyzing the error object doesn't replace the reference in
1512the global variable. It's easiest to make a local copy of the reference
1513before any manipulations. Here's an example:
52531d10 1514
80d38338 1515 use Scalar::Util "blessed";
da279afe 1516
52531d10 1517 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7 1518 if (my $ev_err = $@) {
f7051f2c
FC
1519 if (blessed($ev_err)
1520 && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1521 # handle Some::Module::Exception
1522 }
1523 else {
1524 # handle all other possible exceptions
1525 }
1526 }
1527
3b10bc60 1528Because Perl stringifies uncaught exception messages before display,
80d38338 1529you'll probably want to overload stringification operations on
52531d10
GS
1530exception objects. See L<overload> for details about that.
1531
19799a22
GS
1532You can arrange for a callback to be run just before the C<die>
1533does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1534handler is called with the error text and can change the error
19799a22 1535message, if it sees fit, by calling C<die> again. See
96090e4f 1536L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1537L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1538to be run only right before your program was to exit, this is not
3b10bc60 1539currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1540even inside eval()ed blocks/strings! If one wants the hook to do
1541nothing in such situations, put
fb73857a 1542
5ed4f2ec 1543 die @_ if $^S;
fb73857a 1544
19799a22
GS
1545as the first line of the handler (see L<perlvar/$^S>). Because
1546this promotes strange action at a distance, this counterintuitive
b76cc8ba 1547behavior may be fixed in a future release.
774d564b 1548
4c050ad5
NC
1549See also exit(), warn(), and the Carp module.
1550
a0d0e21e 1551=item do BLOCK
d74e8afc 1552X<do> X<block>
a0d0e21e 1553
c17cdb72
NC
1554=for Pod::Functions turn a BLOCK into a TERM
1555
a0d0e21e 1556Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1557sequence of commands indicated by BLOCK. When modified by the C<while> or
1558C<until> loop modifier, executes the BLOCK once before testing the loop
391b733c 1559condition. (On other statements the loop modifiers test the conditional
6b275a1f 1560first.)
a0d0e21e 1561
4968c1e4 1562C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1563C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1564See L<perlsyn> for alternative strategies.
4968c1e4 1565
a0d0e21e 1566=item do EXPR
d74e8afc 1567X<do>
a0d0e21e
LW
1568
1569Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1570file as a Perl script.
a0d0e21e
LW
1571
1572 do 'stat.pl';
1573
c319391a 1574is largely like
a0d0e21e 1575
986b19de 1576 eval `cat stat.pl`;
a0d0e21e 1577
c319391a
AC
1578except that it's more concise, runs no external processes, keeps track of
1579the current
96090e4f
A
1580filename for error messages, searches the C<@INC> directories, and updates
1581C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1582these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1583cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1584same, however, in that it does reparse the file every time you call it,
1585so you probably don't want to do this inside a loop.
a0d0e21e 1586
8f1da26d 1587If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1588an error message in C<$@>. If C<do> cannot read the file, it returns undef
1589and sets C<$!> to the error. Always check C<$@> first, as compilation
1590could fail in a way that also sets C<$!>. If the file is successfully
1591compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1592
80d38338 1593Inclusion of library modules is better done with the
19799a22 1594C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1595and raise an exception if there's a problem.
a0d0e21e 1596
5a964f20
TC
1597You might like to use C<do> to read in a program configuration
1598file. Manual error checking can be done this way:
1599
b76cc8ba 1600 # read in config files: system first, then user
f86cebdf 1601 for $file ("/share/prog/defaults.rc",
b76cc8ba 1602 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1603 {
1604 unless ($return = do $file) {
1605 warn "couldn't parse $file: $@" if $@;
1606 warn "couldn't do $file: $!" unless defined $return;
1607 warn "couldn't run $file" unless $return;
1608 }
5a964f20
TC
1609 }
1610
a0d0e21e 1611=item dump LABEL
d74e8afc 1612X<dump> X<core> X<undump>
a0d0e21e 1613
8a7e748e
FC
1614=item dump EXPR
1615
1614b0e3
JD
1616=item dump
1617
c17cdb72
NC
1618=for Pod::Functions create an immediate core dump
1619
19799a22
GS
1620This function causes an immediate core dump. See also the B<-u>
1621command-line switch in L<perlrun>, which does the same thing.
1622Primarily this is so that you can use the B<undump> program (not
1623supplied) to turn your core dump into an executable binary after
1624having initialized all your variables at the beginning of the
1625program. When the new binary is executed it will begin by executing
1626a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1627Think of it as a goto with an intervening core dump and reincarnation.
8a7e748e
FC
1628If C<LABEL> is omitted, restarts the program from the top. The
1629C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1630computed at run time, being otherwise identical to C<dump LABEL>.
19799a22
GS
1631
1632B<WARNING>: Any files opened at the time of the dump will I<not>
1633be open any more when the program is reincarnated, with possible
80d38338 1634resulting confusion by Perl.
19799a22 1635
59f521f4 1636This function is now largely obsolete, mostly because it's very hard to
391b733c 1637convert a core file into an executable. That's why you should now invoke
59f521f4 1638it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1639typo.
19799a22 1640
2ba1f20a
FC
1641Unlike most named operators, this has the same precedence as assignment.
1642It is also exempt from the looks-like-a-function rule, so
1643C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1644C<dump>.
1645
ea9eb35a 1646Portability issues: L<perlport/dump>.
1647
532eee96 1648=item each HASH
d74e8afc 1649X<each> X<hash, iterator>
aa689395 1650
532eee96 1651=item each ARRAY
aeedbbed
NC
1652X<array, iterator>
1653
f5a93a43
TC
1654=item each EXPR
1655
c17cdb72
NC
1656=for Pod::Functions retrieve the next key/value pair from a hash
1657
bade7fbc
TC
1658When called on a hash in list context, returns a 2-element list
1659consisting of the key and value for the next element of a hash. In Perl
16605.12 and later only, it will also return the index and value for the next
1661element of an array so that you can iterate over it; older Perls consider
1662this a syntax error. When called in scalar context, returns only the key
1663(not the value) in a hash, or the index in an array.
2f9daede 1664
aeedbbed 1665Hash entries are returned in an apparently random order. The actual random
7bf59113 1666order is specific to a given hash; the exact same series of operations
7161e5c2 1667on two hashes may result in a different order for each hash. Any insertion
7bf59113
YO
1668into the hash may change the order, as will any deletion, with the exception
1669that the most recent key returned by C<each> or C<keys> may be deleted
7161e5c2 1670without changing the order. So long as a given hash is unmodified you may
7bf59113 1671rely on C<keys>, C<values> and C<each> to repeatedly return the same order
7161e5c2
FC
1672as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
1673details on why hash order is randomized. Aside from the guarantees
7bf59113
YO
1674provided here the exact details of Perl's hash algorithm and the hash
1675traversal order are subject to change in any release of Perl.
ab192400 1676
80d38338
TC
1677After C<each> has returned all entries from the hash or array, the next
1678call to C<each> returns the empty list in list context and C<undef> in
bade7fbc
TC
1679scalar context; the next call following I<that> one restarts iteration.
1680Each hash or array has its own internal iterator, accessed by C<each>,
1681C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1682reached the end as just described; it can be explicitly reset by calling
1683C<keys> or C<values> on the hash or array. If you add or delete a hash's
49daec89
DM
1684elements while iterating over it, the effect on the iterator is
1685unspecified; for example, entries may be skipped or duplicated--so don't
d8021140
PJ
1686do that. Exception: It is always safe to delete the item most recently
1687returned by C<each()>, so the following code works properly:
74fc8b5f
MJD
1688
1689 while (($key, $value) = each %hash) {
1690 print $key, "\n";
1691 delete $hash{$key}; # This is safe
1692 }
aa689395 1693
883f220b
TC
1694Tied hashes may have a different ordering behaviour to perl's hash
1695implementation.
1696
80d38338 1697This prints out your environment like the printenv(1) program,
3b10bc60 1698but in a different order:
a0d0e21e
LW
1699
1700 while (($key,$value) = each %ENV) {
a9a5a0dc 1701 print "$key=$value\n";
a0d0e21e
LW
1702 }
1703
f5a93a43
TC
1704Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1705reference to an unblessed hash or array. The argument will be dereferenced
1706automatically. This aspect of C<each> is considered highly experimental.
1707The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1708
1709 while (($key,$value) = each $hashref) { ... }
1710
e6a0db3e
FC
1711As of Perl 5.18 you can use a bare C<each> in a C<while> loop,
1712which will set C<$_> on every iteration.
1713
1714 while(each %ENV) {
1715 print "$_=$ENV{$_}\n";
1716 }
1717
bade7fbc
TC
1718To avoid confusing would-be users of your code who are running earlier
1719versions of Perl with mysterious syntax errors, put this sort of thing at
1720the top of your file to signal that your code will work I<only> on Perls of
1721a recent vintage:
1722
1723 use 5.012; # so keys/values/each work on arrays
1724 use 5.014; # so keys/values/each work on scalars (experimental)
e6a0db3e 1725 use 5.018; # so each assigns to $_ in a lone while test
bade7fbc 1726
8f1da26d 1727See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1728
1729=item eof FILEHANDLE
d74e8afc
ITB
1730X<eof>
1731X<end of file>
1732X<end-of-file>
a0d0e21e 1733
4633a7c4
LW
1734=item eof ()
1735
a0d0e21e
LW
1736=item eof
1737
c17cdb72
NC
1738=for Pod::Functions test a filehandle for its end
1739
8f1da26d 1740Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1741FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1742gives the real filehandle. (Note that this function actually
80d38338 1743reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1744interactive context.) Do not read from a terminal file (or call
19799a22 1745C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1746as terminals may lose the end-of-file condition if you do.
1747
820475bd 1748An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1749with empty parentheses is different. It refers to the pseudo file
820475bd 1750formed from the files listed on the command line and accessed via the
61eff3bc
JH
1751C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1752as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1753used will cause C<@ARGV> to be examined to determine if input is
67408cae 1754available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1755end-of-file will assume you are processing another C<@ARGV> list,
1756and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1757see L<perlop/"I/O Operators">.
820475bd 1758
61eff3bc 1759In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1760detect the end of each file, whereas C<eof()> will detect the end
1761of the very last file only. Examples:
a0d0e21e 1762
748a9306
LW
1763 # reset line numbering on each input file
1764 while (<>) {
a9a5a0dc
VP
1765 next if /^\s*#/; # skip comments
1766 print "$.\t$_";
5a964f20 1767 } continue {
a9a5a0dc 1768 close ARGV if eof; # Not eof()!
748a9306
LW
1769 }
1770
a0d0e21e
LW
1771 # insert dashes just before last line of last file
1772 while (<>) {
a9a5a0dc
VP
1773 if (eof()) { # check for end of last file
1774 print "--------------\n";
1775 }
1776 print;
f7051f2c 1777 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1778 }
1779
a0d0e21e 1780Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1781input operators typically return C<undef> when they run out of data or
1782encounter an error.
a0d0e21e
LW
1783
1784=item eval EXPR
d74e8afc 1785X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1786X<error, handling> X<exception, handling>
a0d0e21e
LW
1787
1788=item eval BLOCK
1789
ce2984c3
PF
1790=item eval
1791
c17cdb72
NC
1792=for Pod::Functions catch exceptions or compile and run code
1793
798dc914
KW
1794In the first form, often referred to as a "string eval", the return
1795value of EXPR is parsed and executed as if it
c7cc6f1c 1796were a little Perl program. The value of the expression (which is itself
8f1da26d 1797determined within scalar context) is first parsed, and if there were no
2341804c 1798errors, executed as a block within the lexical context of the current Perl
df4833a8 1799program. This means, that in particular, any outer lexical variables are
2341804c
DM
1800visible to it, and any package variable settings or subroutine and format
1801definitions remain afterwards.
1802
1803Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1804If EXPR is omitted, evaluates C<$_>. This form is typically used to
1805delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c 1806
7289c5e6
FC
1807If the C<unicode_eval> feature is enabled (which is the default under a
1808C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1809characters, so C<use utf8> declarations have no effect, and source filters
1810are forbidden. In the absence of the C<unicode_eval> feature, the string
1811will sometimes be treated as characters and sometimes as bytes, depending
1812on the internal encoding, and source filters activated within the C<eval>
1813exhibit the erratic, but historical, behaviour of affecting some outer file
1814scope that is still compiling. See also the L</evalbytes> keyword, which
1815always treats its input as a byte stream and works properly with source
1816filters, and the L<feature> pragma.
1817
798dc914
KW
1818Problems can arise if the string expands a scalar containing a floating
1819point number. That scalar can expand to letters, such as C<"NaN"> or
1820C<"Infinity">; or, within the scope of a C<use locale>, the decimal
1821point character may be something other than a dot (such as a comma).
1822None of these are likely to parse as you are likely expecting.
1823
c7cc6f1c 1824In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1825same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1826within the context of the current Perl program. This form is typically
1827used to trap exceptions more efficiently than the first (see below), while
1828also providing the benefit of checking the code within BLOCK at compile
1829time.
1830
1831The final semicolon, if any, may be omitted from the value of EXPR or within
1832the BLOCK.
1833
1834In both forms, the value returned is the value of the last expression
5a964f20 1835evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1836as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1837in void, scalar, or list context, depending on the context of the C<eval>
1838itself. See L</wantarray> for more on how the evaluation context can be
1839determined.
a0d0e21e 1840
19799a22 1841If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1842executed, C<eval> returns C<undef> in scalar context
774b80e8
FC
1843or an empty list in list context, and C<$@> is set to the error
1844message. (Prior to 5.16, a bug caused C<undef> to be returned
1845in list context for syntax errors, but not for runtime errors.)
1846If there was no error, C<$@> is set to the empty string. A
9cc672d4
FC
1847control flow operator like C<last> or C<goto> can bypass the setting of
1848C<$@>. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1849warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1850To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1851turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
44ecbbd8 1852See L</warn>, L<perlvar>, and L<warnings>.
a0d0e21e 1853
19799a22
GS
1854Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1855determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1856is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1857the die operator is used to raise exceptions.
1858
5f1da31c
NT
1859If you want to trap errors when loading an XS module, some problems with
1860the binary interface (such as Perl version skew) may be fatal even with
df4833a8 1861C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
5f1da31c 1862
a0d0e21e
LW
1863If the code to be executed doesn't vary, you may use the eval-BLOCK
1864form to trap run-time errors without incurring the penalty of
1865recompiling each time. The error, if any, is still returned in C<$@>.
1866Examples:
1867
54310121 1868 # make divide-by-zero nonfatal
a0d0e21e
LW
1869 eval { $answer = $a / $b; }; warn $@ if $@;
1870
1871 # same thing, but less efficient
1872 eval '$answer = $a / $b'; warn $@ if $@;
1873
1874 # a compile-time error
5ed4f2ec 1875 eval { $answer = }; # WRONG
a0d0e21e
LW
1876
1877 # a run-time error
5ed4f2ec 1878 eval '$answer ='; # sets $@
a0d0e21e 1879
cf264981
SP
1880Using the C<eval{}> form as an exception trap in libraries does have some
1881issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1882may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1883You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1884as this example shows:
774d564b 1885
80d38338 1886 # a private exception trap for divide-by-zero
f86cebdf
GS
1887 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1888 warn $@ if $@;
774d564b
PP
1889
1890This is especially significant, given that C<__DIE__> hooks can call
19799a22 1891C<die> again, which has the effect of changing their error messages:
774d564b
PP
1892
1893 # __DIE__ hooks may modify error messages
1894 {
f86cebdf
GS
1895 local $SIG{'__DIE__'} =
1896 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1897 eval { die "foo lives here" };
1898 print $@ if $@; # prints "bar lives here"
774d564b
PP
1899 }
1900
19799a22 1901Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1902may be fixed in a future release.
1903
19799a22 1904With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1905being looked at when:
1906
5ed4f2ec 1907 eval $x; # CASE 1
1908 eval "$x"; # CASE 2
a0d0e21e 1909
5ed4f2ec 1910 eval '$x'; # CASE 3
1911 eval { $x }; # CASE 4
a0d0e21e 1912
5ed4f2ec 1913 eval "\$$x++"; # CASE 5
1914 $$x++; # CASE 6
a0d0e21e 1915
2f9daede 1916Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1917the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1918the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1919and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1920does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1921purely visual reasons, but it also has the advantage of compiling at
1922compile-time instead of at run-time.) Case 5 is a place where
19799a22 1923normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1924particular situation, you can just use symbolic references instead, as
1925in case 6.
a0d0e21e 1926
b6538e4f 1927Before Perl 5.14, the assignment to C<$@> occurred before restoration
bade7fbc 1928of localized variables, which means that for your code to run on older
b208c909 1929versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1930errors:
1931
1932 # alter $@ on nefarious repugnancy only
1933 {
1934 my $e;
1935 {
f7051f2c
FC
1936 local $@; # protect existing $@
1937 eval { test_repugnancy() };
1938 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1939 $@ =~ /nefarious/ and $e = $@;
8a5a710d
DN
1940 }
1941 die $e if defined $e
1942 }
1943
4968c1e4 1944C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1945C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1946
4f00fc7e
FC
1947An C<eval ''> executed within a subroutine defined
1948in the C<DB> package doesn't see the usual
3b10bc60 1949surrounding lexical scope, but rather the scope of the first non-DB piece
df4833a8 1950of code that called it. You don't normally need to worry about this unless
3b10bc60 1951you are writing a Perl debugger.
d819b83a 1952
7289c5e6
FC
1953=item evalbytes EXPR
1954X<evalbytes>
1955
1956=item evalbytes
1957
d9b04284 1958=for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
c17cdb72 1959
7289c5e6
FC
1960This function is like L</eval> with a string argument, except it always
1961parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1962string containing characters whose ordinal value exceeds 255 results in an
1963error. Source filters activated within the evaluated code apply to the
1964code itself.
1965
1966This function is only available under the C<evalbytes> feature, a
1967C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1968L<feature> for more information.
1969
a0d0e21e 1970=item exec LIST
d74e8afc 1971X<exec> X<execute>
a0d0e21e 1972
8bf3b016
GS
1973=item exec PROGRAM LIST
1974
c17cdb72
NC
1975=for Pod::Functions abandon this program to run another
1976
3b10bc60 1977The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1978use C<system> instead of C<exec> if you want it to return. It fails and
1979returns false only if the command does not exist I<and> it is executed
fb73857a 1980directly instead of via your system's command shell (see below).
a0d0e21e 1981
19799a22 1982Since it's a common mistake to use C<exec> instead of C<system>, Perl
4642e50d
EB
1983warns you if C<exec> is called in void context and if there is a following
1984statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1985you always do that, right?). If you I<really> want to follow an C<exec>
1986with some other statement, you can use one of these styles to avoid the warning:
55d729e4 1987
5a964f20
TC
1988 exec ('foo') or print STDERR "couldn't exec foo: $!";
1989 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1990
667eac0c
RS
1991If there is more than one argument in LIST, this calls execvp(3) with the
1992arguments in LIST. If there is only one element in LIST, the argument is
1993checked for shell metacharacters, and if there are any, the entire
1994argument is passed to the system's command shell for parsing (this is
1995C</bin/sh -c> on Unix platforms, but varies on other platforms). If
1996there are no shell metacharacters in the argument, it is split into words
1997and passed directly to C<execvp>, which is more efficient. Examples:
a0d0e21e 1998
19799a22
GS
1999 exec '/bin/echo', 'Your arguments are: ', @ARGV;
2000 exec "sort $outfile | uniq";
a0d0e21e
LW
2001
2002If you don't really want to execute the first argument, but want to lie
2003to the program you are executing about its own name, you can specify
2004the program you actually want to run as an "indirect object" (without a
94d4006a
TS
2005comma) in front of the LIST, as in C<exec PROGRAM LIST>. (This always
2006forces interpretation of the LIST as a multivalued list, even if there
2007is only a single scalar in the list.) Example:
a0d0e21e
LW
2008
2009 $shell = '/bin/csh';
5ed4f2ec 2010 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
2011
2012or, more directly,
2013
5ed4f2ec 2014 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 2015
3b10bc60 2016When the arguments get executed via the system shell, results are
2017subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
2018for details.
2019
19799a22
GS
2020Using an indirect object with C<exec> or C<system> is also more
2021secure. This usage (which also works fine with system()) forces
2022interpretation of the arguments as a multivalued list, even if the
2023list had just one argument. That way you're safe from the shell
2024expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
2025
2026 @args = ( "echo surprise" );
2027
2b5ab1e7 2028 exec @args; # subject to shell escapes
f86cebdf 2029 # if @args == 1
2b5ab1e7 2030 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
2031
2032The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
2033program, passing it C<"surprise"> an argument. The second version didn't;
2034it tried to run a program named I<"echo surprise">, didn't find it, and set
2035C<$?> to a non-zero value indicating failure.
5a964f20 2036
94d4006a
TS
2037On Windows, only the C<exec PROGRAM LIST> indirect object syntax will
2038reliably avoid using the shell; C<exec LIST>, even with more than one
2039element, will fall back to the shell if the first spawn fails.
2040
e9fa405d
BF
2041Perl attempts to flush all files opened for output before the exec,
2042but this may not be supported on some platforms (see L<perlport>).
2043To be safe, you may need to set C<$|> ($AUTOFLUSH in English) or
2044call the C<autoflush()> method of C<IO::Handle> on any open handles
2045to avoid lost output.
0f897271 2046
80d38338
TC
2047Note that C<exec> will not call your C<END> blocks, nor will it invoke
2048C<DESTROY> methods on your objects.
7660c0ab 2049
ea9eb35a 2050Portability issues: L<perlport/exec>.
2051
a0d0e21e 2052=item exists EXPR
d74e8afc 2053X<exists> X<autovivification>
a0d0e21e 2054
c17cdb72
NC
2055=for Pod::Functions test whether a hash key is present
2056
d0a76353
RS
2057Given an expression that specifies an element of a hash, returns true if the
2058specified element in the hash has ever been initialized, even if the
2059corresponding value is undefined.
a0d0e21e 2060
5ed4f2ec 2061 print "Exists\n" if exists $hash{$key};
2062 print "Defined\n" if defined $hash{$key};
01020589
GS
2063 print "True\n" if $hash{$key};
2064
d0a76353 2065exists may also be called on array elements, but its behavior is much less
2fbadc08
RS
2066obvious and is strongly tied to the use of L</delete> on arrays.
2067
2068B<WARNING:> Calling C<exists> on array values is strongly discouraged. The
2069notion of deleting or checking the existence of Perl array elements is not
2070conceptually coherent, and can lead to surprising behavior.
d0a76353 2071
5ed4f2ec 2072 print "Exists\n" if exists $array[$index];
2073 print "Defined\n" if defined $array[$index];
01020589 2074 print "True\n" if $array[$index];
a0d0e21e 2075
8f1da26d 2076A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
2077it exists, but the reverse doesn't necessarily hold true.
2078
afebc493
GS
2079Given an expression that specifies the name of a subroutine,
2080returns true if the specified subroutine has ever been declared, even
2081if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 2082does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
2083exist may still be callable: its package may have an C<AUTOLOAD>
2084method that makes it spring into existence the first time that it is
3b10bc60 2085called; see L<perlsub>.
afebc493 2086
5ed4f2ec 2087 print "Exists\n" if exists &subroutine;
2088 print "Defined\n" if defined &subroutine;
afebc493 2089
a0d0e21e 2090Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 2091operation is a hash or array key lookup or subroutine name:
a0d0e21e 2092
5ed4f2ec 2093 if (exists $ref->{A}->{B}->{$key}) { }
2094 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 2095
5ed4f2ec 2096 if (exists $ref->{A}->{B}->[$ix]) { }
2097 if (exists $hash{A}{B}[$ix]) { }
01020589 2098
afebc493
GS
2099 if (exists &{$ref->{A}{B}{$key}}) { }
2100
9590a7cd 2101Although the most deeply nested array or hash element will not spring into
3b10bc60 2102existence just because its existence was tested, any intervening ones will.
61eff3bc 2103Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 2104into existence due to the existence test for the $key element above.
3b10bc60 2105This happens anywhere the arrow operator is used, including even here:
5a964f20 2106
2b5ab1e7 2107 undef $ref;
5ed4f2ec 2108 if (exists $ref->{"Some key"}) { }
2109 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
2110
2111This surprising autovivification in what does not at first--or even
2112second--glance appear to be an lvalue context may be fixed in a future
5a964f20 2113release.
a0d0e21e 2114
afebc493
GS
2115Use of a subroutine call, rather than a subroutine name, as an argument
2116to exists() is an error.
2117
5ed4f2ec 2118 exists &sub; # OK
2119 exists &sub(); # Error
afebc493 2120
a0d0e21e 2121=item exit EXPR
d74e8afc 2122X<exit> X<terminate> X<abort>
a0d0e21e 2123
ce2984c3
PF
2124=item exit
2125
c17cdb72
NC
2126=for Pod::Functions terminate this program
2127
2b5ab1e7 2128Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
2129
2130 $ans = <STDIN>;
2131 exit 0 if $ans =~ /^[Xx]/;
2132
19799a22 2133See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
2134universally recognized values for EXPR are C<0> for success and C<1>
2135for error; other values are subject to interpretation depending on the
2136environment in which the Perl program is running. For example, exiting
213769 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2138the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 2139
19799a22
GS
2140Don't use C<exit> to abort a subroutine if there's any chance that
2141someone might want to trap whatever error happened. Use C<die> instead,
2142which can be trapped by an C<eval>.
28757baa 2143
19799a22 2144The exit() function does not always exit immediately. It calls any
2b5ab1e7 2145defined C<END> routines first, but these C<END> routines may not
19799a22 2146themselves abort the exit. Likewise any object destructors that need to
60275626 2147be called are called before the real exit. C<END> routines and destructors
391b733c 2148can change the exit status by modifying C<$?>. If this is a problem, you
fae6f8fa 2149can call C<POSIX::_exit($status)> to avoid END and destructor processing.
87275199 2150See L<perlmod> for details.
5a964f20 2151
ea9eb35a 2152Portability issues: L<perlport/exit>.
2153
a0d0e21e 2154=item exp EXPR
d74e8afc 2155X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 2156
54310121 2157=item exp
bbce6d69 2158
c17cdb72
NC
2159=for Pod::Functions raise I<e> to a power
2160
b76cc8ba 2161Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
2162If EXPR is omitted, gives C<exp($_)>.
2163
628253b8
BF
2164=item fc EXPR
2165X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2166
2167=item fc
2168
d9b04284 2169=for Pod::Functions +fc return casefolded version of a string
c17cdb72 2170
628253b8
BF
2171Returns the casefolded version of EXPR. This is the internal function
2172implementing the C<\F> escape in double-quoted strings.
2173
2174Casefolding is the process of mapping strings to a form where case
2175differences are erased; comparing two strings in their casefolded
2176form is effectively a way of asking if two strings are equal,
2177regardless of case.
2178
2179Roughly, if you ever found yourself writing this
2180
f6c6dcb6 2181 lc($this) eq lc($that) # Wrong!
628253b8 2182 # or
f6c6dcb6 2183 uc($this) eq uc($that) # Also wrong!
628253b8 2184 # or
f6c6dcb6 2185 $this =~ /^\Q$that\E\z/i # Right!
628253b8
BF
2186
2187Now you can write
2188
2189 fc($this) eq fc($that)
2190
2191And get the correct results.
2192
fc39a31f
KW
2193Perl only implements the full form of casefolding,
2194but you can access the simple folds using L<Unicode::UCD/casefold()> and
2195L<Unicode::UCD/prop_invmap()>.
628253b8
BF
2196For further information on casefolding, refer to
2197the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
21984.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2199available at L<http://www.unicode.org/versions/latest/>, as well as the
2200Case Charts available at L<http://www.unicode.org/charts/case/>.
2201
2202If EXPR is omitted, uses C<$_>.
2203
1ca267a5
KW
2204This function behaves the same way under various pragma, such as within
2205S<C<"use feature 'unicode_strings">>, as L</lc> does, with the single
2206exception of C<fc> of LATIN CAPITAL LETTER SHARP S (U+1E9E) within the
2207scope of S<C<use locale>>. The foldcase of this character would
2208normally be C<"ss">, but as explained in the L</lc> section, case
2209changes that cross the 255/256 boundary are problematic under locales,
2210and are hence prohibited. Therefore, this function under locale returns
2211instead the string C<"\x{17F}\x{17F}">, which is the LATIN SMALL LETTER
2212LONG S. Since that character itself folds to C<"s">, the string of two
2213of them together should be equivalent to a single U+1E9E when foldcased.
628253b8
BF
2214
2215While the Unicode Standard defines two additional forms of casefolding,
2216one for Turkic languages and one that never maps one character into multiple
2217characters, these are not provided by the Perl core; However, the CPAN module
2218C<Unicode::Casing> may be used to provide an implementation.
2219
2220This keyword is available only when the C<"fc"> feature is enabled,
7161e5c2 2221or when prefixed with C<CORE::>; See L<feature>. Alternately,
628253b8
BF
2222include a C<use v5.16> or later to the current scope.
2223
a0d0e21e 2224=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2225X<fcntl>
a0d0e21e 2226
c17cdb72
NC
2227=for Pod::Functions file control system call
2228
f86cebdf 2229Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
2230
2231 use Fcntl;
2232
0ade1984 2233first to get the correct constant definitions. Argument processing and
3b10bc60 2234value returned work just like C<ioctl> below.
a0d0e21e
LW
2235For example:
2236
2237 use Fcntl;
5a964f20 2238 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 2239 or die "can't fcntl F_GETFL: $!";
5a964f20 2240
554ad1fc 2241You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
2242Like C<ioctl>, it maps a C<0> return from the system call into
2243C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
2244in numeric context. It is also exempt from the normal B<-w> warnings
2245on improper numeric conversions.
5a964f20 2246
3b10bc60 2247Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
2248doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2249manpage to learn what functions are available on your system.
a0d0e21e 2250
be2f7487 2251Here's an example of setting a filehandle named C<REMOTE> to be
2252non-blocking at the system level. You'll have to negotiate C<$|>
2253on your own, though.
2254
2255 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2256
2257 $flags = fcntl(REMOTE, F_GETFL, 0)
2258 or die "Can't get flags for the socket: $!\n";
2259
2260 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2261 or die "Can't set flags for the socket: $!\n";
2262
ea9eb35a 2263Portability issues: L<perlport/fcntl>.
2264
cfa52385
FC
2265=item __FILE__
2266X<__FILE__>
2267
c17cdb72
NC
2268=for Pod::Functions the name of the current source file
2269
cfa52385
FC
2270A special token that returns the name of the file in which it occurs.
2271
a0d0e21e 2272=item fileno FILEHANDLE
d74e8afc 2273X<fileno>
a0d0e21e 2274
c17cdb72
NC
2275=for Pod::Functions return file descriptor from filehandle
2276
2b5ab1e7 2277Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
2278filehandle is not open. If there is no real file descriptor at the OS
2279level, as can happen with filehandles connected to memory objects via
2280C<open> with a reference for the third argument, -1 is returned.
2281
2282This is mainly useful for constructing
19799a22 2283bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
2284If FILEHANDLE is an expression, the value is taken as an indirect
2285filehandle, generally its name.
5a964f20 2286
b76cc8ba 2287You can use this to find out whether two handles refer to the
5a964f20
TC
2288same underlying descriptor:
2289
3231d257 2290 if (fileno(THIS) != -1 && fileno(THIS) == fileno(THAT)) {
a9a5a0dc 2291 print "THIS and THAT are dups\n";
3231d257 2292 } elsif (fileno(THIS) != -1 && fileno(THAT) != -1) {
555bd962
BG
2293 print "THIS and THAT have different " .
2294 "underlying file descriptors\n";
3231d257 2295 } else {
555bd962
BG
2296 print "At least one of THIS and THAT does " .
2297 "not have a real file descriptor\n";
b76cc8ba
NIS
2298 }
2299
a0d0e21e 2300=item flock FILEHANDLE,OPERATION
d74e8afc 2301X<flock> X<lock> X<locking>
a0d0e21e 2302
c17cdb72
NC
2303=for Pod::Functions lock an entire file with an advisory lock
2304
19799a22
GS
2305Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2306for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2307machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2308C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2309entire files only, not records.
2b5ab1e7
TC
2310
2311Two potentially non-obvious but traditional C<flock> semantics are
2312that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2313are B<merely advisory>. Such discretionary locks are more flexible, but
2314offer fewer guarantees. This means that programs that do not also use
2315C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2316your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2317for details. It's best to assume traditional behavior if you're writing
2318portable programs. (But if you're not, you should as always feel perfectly
2319free to write for your own system's idiosyncrasies (sometimes called
2320"features"). Slavish adherence to portability concerns shouldn't get
2321in the way of your getting your job done.)
a3cb178b 2322
8ebc5c01
PP
2323OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2324LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2325you can use the symbolic names if you import them from the L<Fcntl> module,
2326either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2327requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2328releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2329LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2330waiting for the lock; check the return status to see if you got it.
68dc0745 2331
2b5ab1e7
TC
2332To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2333before locking or unlocking it.
8ebc5c01 2334
f86cebdf 2335Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2336locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2337are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2338implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
2339differing semantics shouldn't bite too many people.
2340
becacb53
TM
2341Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2342be open with read intent to use LOCK_SH and requires that it be open
2343with write intent to use LOCK_EX.
2344
19799a22
GS
2345Note also that some versions of C<flock> cannot lock things over the
2346network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2347that. If you like you can force Perl to ignore your system's flock(2)
2348function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2349the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2350and build a new Perl.
4633a7c4
LW
2351
2352Here's a mailbox appender for BSD systems.
a0d0e21e 2353
f7051f2c
FC
2354 # import LOCK_* and SEEK_END constants
2355 use Fcntl qw(:flock SEEK_END);
a0d0e21e
LW
2356
2357 sub lock {
a9a5a0dc
VP
2358 my ($fh) = @_;
2359 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2360
a9a5a0dc
VP
2361 # and, in case someone appended while we were waiting...
2362 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2363 }
2364
2365 sub unlock {
a9a5a0dc
VP
2366 my ($fh) = @_;
2367 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2368 }
2369
b0169937 2370 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2371 or die "Can't open mailbox: $!";
a0d0e21e 2372
7ed5353d 2373 lock($mbox);
b0169937 2374 print $mbox $msg,"\n\n";
7ed5353d 2375 unlock($mbox);
a0d0e21e 2376
3b10bc60 2377On systems that support a real flock(2), locks are inherited across fork()
2378calls, whereas those that must resort to the more capricious fcntl(2)
2379function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2380
cb1a09d0 2381See also L<DB_File> for other flock() examples.
a0d0e21e 2382
ea9eb35a 2383Portability issues: L<perlport/flock>.
2384
a0d0e21e 2385=item fork
d74e8afc 2386X<fork> X<child> X<parent>
a0d0e21e 2387
c17cdb72
NC
2388=for Pod::Functions create a new process just like this one
2389
2b5ab1e7
TC
2390Does a fork(2) system call to create a new process running the
2391same program at the same point. It returns the child pid to the
2392parent process, C<0> to the child process, or C<undef> if the fork is
2393unsuccessful. File descriptors (and sometimes locks on those descriptors)
2394are shared, while everything else is copied. On most systems supporting
2395fork(), great care has gone into making it extremely efficient (for
2396example, using copy-on-write technology on data pages), making it the
2397dominant paradigm for multitasking over the last few decades.
5a964f20 2398
e9fa405d 2399Perl attempts to flush all files opened for
0f897271
GS
2400output before forking the child process, but this may not be supported
2401on some platforms (see L<perlport>). To be safe, you may need to set
2402C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2403C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2404
19799a22 2405If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2406accumulate zombies. On some systems, you can avoid this by setting
2407C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2408forking and reaping moribund children.
cb1a09d0 2409
28757baa
PP
2410Note that if your forked child inherits system file descriptors like
2411STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2412if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2413backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2414You should reopen those to F</dev/null> if it's any issue.
28757baa 2415
ea9eb35a 2416On some platforms such as Windows, where the fork() system call is not available,
391b733c
FC
2417Perl can be built to emulate fork() in the Perl interpreter.
2418The emulation is designed, at the level of the Perl program,
2419to be as compatible as possible with the "Unix" fork().
6d17f725 2420However it has limitations that have to be considered in code intended to be portable.
ea9eb35a 2421See L<perlfork> for more details.
2422
2423Portability issues: L<perlport/fork>.
2424
cb1a09d0 2425=item format
d74e8afc 2426X<format>
cb1a09d0 2427
c17cdb72
NC
2428=for Pod::Functions declare a picture format with use by the write() function
2429
19799a22 2430Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2431example:
2432
54310121 2433 format Something =
a9a5a0dc
VP
2434 Test: @<<<<<<<< @||||| @>>>>>
2435 $str, $%, '$' . int($num)
cb1a09d0
AD
2436 .
2437
2438 $str = "widget";
184e9718 2439 $num = $cost/$quantity;
cb1a09d0
AD
2440 $~ = 'Something';
2441 write;
2442
2443See L<perlform> for many details and examples.
2444
8903cb82 2445=item formline PICTURE,LIST
d74e8afc 2446X<formline>
a0d0e21e 2447
c17cdb72
NC
2448=for Pod::Functions internal function used for formats
2449
5a964f20 2450This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2451too. It formats (see L<perlform>) a list of values according to the
2452contents of PICTURE, placing the output into the format output
7660c0ab 2453accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2454Eventually, when a C<write> is done, the contents of
cf264981
SP
2455C<$^A> are written to some filehandle. You could also read C<$^A>
2456and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2457does one C<formline> per line of form, but the C<formline> function itself
748a9306 2458doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2459that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2460You may therefore need to use multiple formlines to implement a single
3b10bc60 2461record format, just like the C<format> compiler.
748a9306 2462
19799a22 2463Be careful if you put double quotes around the picture, because an C<@>
748a9306 2464character may be taken to mean the beginning of an array name.
19799a22 2465C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2466
445b09e5
FC
2467If you are trying to use this instead of C<write> to capture the output,
2468you may find it easier to open a filehandle to a scalar
2469(C<< open $fh, ">", \$output >>) and write to that instead.
2470
a0d0e21e 2471=item getc FILEHANDLE
f723aae1 2472X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2473
2474=item getc
2475
c17cdb72
NC
2476=for Pod::Functions get the next character from the filehandle
2477
a0d0e21e 2478Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2479or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2480the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2481STDIN. This is not particularly efficient. However, it cannot be
2482used by itself to fetch single characters without waiting for the user
2483to hit enter. For that, try something more like:
4633a7c4
LW
2484
2485 if ($BSD_STYLE) {
a9a5a0dc 2486 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2487 }
2488 else {
a9a5a0dc 2489 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2490 }
2491
2492 $key = getc(STDIN);
2493
2494 if ($BSD_STYLE) {
a9a5a0dc 2495 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2496 }
2497 else {
3b10bc60 2498 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2499 }
2500 print "\n";
2501
54310121
PP
2502Determination of whether $BSD_STYLE should be set
2503is left as an exercise to the reader.
cb1a09d0 2504
19799a22 2505The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2506systems purporting POSIX compliance. See also the C<Term::ReadKey>
3d6c5fec 2507module from your nearest L<CPAN|http://www.cpan.org> site.
a0d0e21e
LW
2508
2509=item getlogin
d74e8afc 2510X<getlogin> X<login>
a0d0e21e 2511
c17cdb72
NC
2512=for Pod::Functions return who logged in at this tty
2513
cf264981 2514This implements the C library function of the same name, which on most
3b10bc60 2515systems returns the current login from F</etc/utmp>, if any. If it
2516returns the empty string, use C<getpwuid>.
a0d0e21e 2517
f86702cc 2518 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2519
19799a22
GS
2520Do not consider C<getlogin> for authentication: it is not as
2521secure as C<getpwuid>.
4633a7c4 2522
ea9eb35a 2523Portability issues: L<perlport/getlogin>.
2524
a0d0e21e 2525=item getpeername SOCKET
d74e8afc 2526X<getpeername> X<peer>
a0d0e21e 2527
c17cdb72
NC
2528=for Pod::Functions find the other end of a socket connection
2529
a3390c9f
FC
2530Returns the packed sockaddr address of the other end of the SOCKET
2531connection.
a0d0e21e 2532
4633a7c4
LW
2533 use Socket;
2534 $hersockaddr = getpeername(SOCK);
19799a22 2535 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2536 $herhostname = gethostbyaddr($iaddr, AF_INET);
2537 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2538
2539=item getpgrp PID
d74e8afc 2540X<getpgrp> X<group>
a0d0e21e 2541
c17cdb72
NC
2542=for Pod::Functions get process group
2543
47e29363 2544Returns the current process group for the specified PID. Use
7660c0ab 2545a PID of C<0> to get the current process group for the
4633a7c4 2546current process. Will raise an exception if used on a machine that
a3390c9f
FC
2547doesn't implement getpgrp(2). If PID is omitted, returns the process
2548group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2549does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2550
ea9eb35a 2551Portability issues: L<perlport/getpgrp>.
2552
a0d0e21e 2553=item getppid
d74e8afc 2554X<getppid> X<parent> X<pid>
a0d0e21e 2555
c17cdb72
NC
2556=for Pod::Functions get parent process ID
2557
a0d0e21e
LW
2558Returns the process id of the parent process.
2559
d7c042c9
AB
2560Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2561around non-POSIX thread semantics the minority of Linux systems (and
2562Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
7161e5c2 2563has since been removed. See the documentation for L<$$|perlvar/$$> for
d7c042c9 2564details.
4d76a344 2565
ea9eb35a 2566Portability issues: L<perlport/getppid>.
2567
a0d0e21e 2568=item getpriority WHICH,WHO
d74e8afc 2569X<getpriority> X<priority> X<nice>
a0d0e21e 2570
c17cdb72
NC
2571=for Pod::Functions get current nice value
2572
4633a7c4 2573Returns the current priority for a process, a process group, or a user.
01aa884e 2574(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2575machine that doesn't implement getpriority(2).
a0d0e21e 2576
ea9eb35a 2577Portability issues: L<perlport/getpriority>.
2578
a0d0e21e 2579=item getpwnam NAME
d74e8afc
ITB
2580X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2581X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2582X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2583X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2584X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2585X<endnetent> X<endprotoent> X<endservent>
a0d0e21e 2586
c17cdb72
NC
2587=for Pod::Functions get passwd record given user login name
2588
a0d0e21e
LW
2589=item getgrnam NAME
2590
c17cdb72
NC
2591=for Pod::Functions get group record given group name
2592
a0d0e21e
LW
2593=item gethostbyname NAME
2594
c17cdb72
NC
2595=for Pod::Functions get host record given name
2596
a0d0e21e
LW
2597=item getnetbyname NAME
2598
c17cdb72
NC
2599=for Pod::Functions get networks record given name
2600
a0d0e21e
LW
2601=item getprotobyname NAME
2602
c17cdb72
NC
2603=for Pod::Functions get protocol record given name
2604
a0d0e21e
LW
2605=item getpwuid UID
2606
c17cdb72
NC
2607=for Pod::Functions get passwd record given user ID
2608
a0d0e21e
LW
2609=item getgrgid GID
2610
c17cdb72
NC
2611=for Pod::Functions get group record given group user ID
2612
a0d0e21e
LW
2613=item getservbyname NAME,PROTO
2614
c17cdb72
NC
2615=for Pod::Functions get services record given its name
2616
a0d0e21e
LW
2617=item gethostbyaddr ADDR,ADDRTYPE
2618
c17cdb72
NC
2619=for Pod::Functions get host record given its address
2620
a0d0e21e
LW
2621=item getnetbyaddr ADDR,ADDRTYPE
2622
c17cdb72
NC
2623=for Pod::Functions get network record given its address
2624
a0d0e21e
LW
2625=item getprotobynumber NUMBER
2626
c17cdb72
NC
2627=for Pod::Functions get protocol record numeric protocol
2628
a0d0e21e
LW
2629=item getservbyport PORT,PROTO
2630
c17cdb72
NC
2631=for Pod::Functions get services record given numeric port
2632
a0d0e21e
LW
2633=item getpwent
2634
c17cdb72
NC
2635=for Pod::Functions get next passwd record
2636
a0d0e21e
LW
2637=item getgrent
2638
c17cdb72
NC
2639=for Pod::Functions get next group record
2640
a0d0e21e
LW
2641=item gethostent
2642
c17cdb72
NC
2643=for Pod::Functions get next hosts record
2644
a0d0e21e
LW
2645=item getnetent
2646
c17cdb72
NC
2647=for Pod::Functions get next networks record
2648
a0d0e21e
LW
2649=item getprotoent
2650
c17cdb72
NC
2651=for Pod::Functions get next protocols record
2652
a0d0e21e
LW
2653=item getservent
2654
c17cdb72
NC
2655=for Pod::Functions get next services record
2656
a0d0e21e
LW
2657=item setpwent
2658
c17cdb72
NC
2659=for Pod::Functions prepare passwd file for use
2660
a0d0e21e
LW
2661=item setgrent
2662
c17cdb72
NC
2663=for Pod::Functions prepare group file for use
2664
a0d0e21e
LW
2665=item sethostent STAYOPEN
2666
c17cdb72
NC
2667=for Pod::Functions prepare hosts file for use
2668
a0d0e21e
LW
2669=item setnetent STAYOPEN
2670
c17cdb72
NC
2671=for Pod::Functions prepare networks file for use
2672
a0d0e21e
LW
2673=item setprotoent STAYOPEN
2674
c17cdb72
NC
2675=for Pod::Functions prepare protocols file for use
2676
a0d0e21e
LW
2677=item setservent STAYOPEN
2678
c17cdb72
NC
2679=for Pod::Functions prepare services file for use
2680
a0d0e21e
LW
2681=item endpwent
2682
c17cdb72
NC
2683=for Pod::Functions be done using passwd file
2684
a0d0e21e
LW
2685=item endgrent
2686
c17cdb72
NC
2687=for Pod::Functions be done using group file
2688
a0d0e21e
LW
2689=item endhostent
2690
c17cdb72
NC
2691=for Pod::Functions be done using hosts file
2692
a0d0e21e
LW
2693=item endnetent
2694
c17cdb72
NC
2695=for Pod::Functions be done using networks file
2696
a0d0e21e
LW
2697=item endprotoent
2698
c17cdb72
NC
2699=for Pod::Functions be done using protocols file
2700
a0d0e21e
LW
2701=item endservent
2702
c17cdb72
NC
2703=for Pod::Functions be done using services file
2704
80d38338
TC
2705These routines are the same as their counterparts in the
2706system C library. In list context, the return values from the
a0d0e21e
LW
2707various get routines are as follows:
2708
f0081f64
AP
2709 # 0 1 2 3 4
2710 ( $name, $passwd, $gid, $members ) = getgr*
2711 ( $name, $aliases, $addrtype, $net ) = getnet*
2712 ( $name, $aliases, $port, $proto ) = getserv*
2713 ( $name, $aliases, $proto ) = getproto*
2714 ( $name, $aliases, $addrtype, $length, @addrs ) = gethost*
2715 ( $name, $passwd, $uid, $gid, $quota,
2716 $comment, $gcos, $dir, $shell, $expire ) = getpw*
2717 # 5 6 7 8 9
a0d0e21e 2718
3b10bc60 2719(If the entry doesn't exist you get an empty list.)
a0d0e21e 2720
4602f195
JH
2721The exact meaning of the $gcos field varies but it usually contains
2722the real name of the user (as opposed to the login name) and other
2723information pertaining to the user. Beware, however, that in many
2724system users are able to change this information and therefore it
106325ad 2725cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2726L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2727login shell, are also tainted, for the same reason.
4602f195 2728
5a964f20 2729In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2730lookup by name, in which case you get the other thing, whatever it is.
2731(If the entry doesn't exist you get the undefined value.) For example:
2732
5a964f20
TC
2733 $uid = getpwnam($name);
2734 $name = getpwuid($num);
2735 $name = getpwent();
2736 $gid = getgrnam($name);
08a33e13 2737 $name = getgrgid($num);
5a964f20
TC
2738 $name = getgrent();
2739 #etc.
a0d0e21e 2740
4602f195 2741In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2742in that they are unsupported on many systems. If the
4602f195
JH
2743$quota is unsupported, it is an empty scalar. If it is supported, it
2744usually encodes the disk quota. If the $comment field is unsupported,
2745it is an empty scalar. If it is supported it usually encodes some
2746administrative comment about the user. In some systems the $quota
2747field may be $change or $age, fields that have to do with password
2748aging. In some systems the $comment field may be $class. The $expire
2749field, if present, encodes the expiration period of the account or the
2750password. For the availability and the exact meaning of these fields
8f1da26d 2751in your system, please consult getpwnam(3) and your system's
4602f195
JH
2752F<pwd.h> file. You can also find out from within Perl what your
2753$quota and $comment fields mean and whether you have the $expire field
2754by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2755C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2756files are supported only if your vendor has implemented them in the
4602f195 2757intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2758shadow versions if you're running under privilege or if there exists
cf264981 2759the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2760and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2761facility are unlikely to be supported.
6ee623d5 2762
a3390c9f 2763The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2764the login names of the members of the group.
2765
2766For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2767C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2768C<@addrs> value returned by a successful call is a list of raw
2769addresses returned by the corresponding library call. In the
2770Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2771by saying something like:
2772
f337b084 2773 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2774
2b5ab1e7
TC
2775The Socket library makes this slightly easier:
2776
2777 use Socket;
2778 $iaddr = inet_aton("127.1"); # or whatever address
2779 $name = gethostbyaddr($iaddr, AF_INET);
2780
2781 # or going the other way
19799a22 2782 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2783
d760c846
GS
2784In the opposite way, to resolve a hostname to the IP address
2785you can write this:
2786
2787 use Socket;
2788 $packed_ip = gethostbyname("www.perl.org");
2789 if (defined $packed_ip) {
2790 $ip_address = inet_ntoa($packed_ip);
2791 }
2792
b018eaf1 2793Make sure C<gethostbyname()> is called in SCALAR context and that
d760c846
GS
2794its return value is checked for definedness.
2795
0d043efa
FC
2796The C<getprotobynumber> function, even though it only takes one argument,
2797has the precedence of a list operator, so beware:
2798
2799 getprotobynumber $number eq 'icmp' # WRONG
2800 getprotobynumber($number eq 'icmp') # actually means this
2801 getprotobynumber($number) eq 'icmp' # better this way
2802
19799a22
GS
2803If you get tired of remembering which element of the return list
2804contains which return value, by-name interfaces are provided
2805in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2806C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2807and C<User::grent>. These override the normal built-ins, supplying
2808versions that return objects with the appropriate names
2809for each field. For example:
5a964f20
TC
2810
2811 use File::stat;
2812 use User::pwent;
2813 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2814
a3390c9f 2815Even though it looks as though they're the same method calls (uid),
b76cc8ba 2816they aren't, because a C<File::stat> object is different from
19799a22 2817a C<User::pwent> object.
5a964f20 2818
ea9eb35a 2819Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2820
a0d0e21e 2821=item getsockname SOCKET
d74e8afc 2822X<getsockname>
a0d0e21e 2823
c17cdb72
NC
2824=for Pod::Functions retrieve the sockaddr for a given socket
2825
19799a22
GS
2826Returns the packed sockaddr address of this end of the SOCKET connection,
2827in case you don't know the address because you have several different
2828IPs that the connection might have come in on.
a0d0e21e 2829
4633a7c4
LW
2830 use Socket;
2831 $mysockaddr = getsockname(SOCK);
19799a22 2832 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2833 printf "Connect to %s [%s]\n",
19799a22
GS
2834 scalar gethostbyaddr($myaddr, AF_INET),
2835 inet_ntoa($myaddr);
a0d0e21e
LW
2836
2837=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2838X<getsockopt>
a0d0e21e 2839
c17cdb72
NC
2840=for Pod::Functions get socket options on a given socket
2841
636e6b1f
TH
2842Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2843Options may exist at multiple protocol levels depending on the socket
2844type, but at least the uppermost socket level SOL_SOCKET (defined in the
391b733c 2845C<Socket> module) will exist. To query options at another level the
636e6b1f 2846protocol number of the appropriate protocol controlling the option
391b733c 2847should be supplied. For example, to indicate that an option is to be
636e6b1f 2848interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2849number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2850
80d38338 2851The function returns a packed string representing the requested socket
3b10bc60 2852option, or C<undef> on error, with the reason for the error placed in
391b733c 2853C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2854consult getsockopt(2) for details. A common case is that the option is an
2855integer, in which case the result is a packed integer, which you can decode
2856using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2857
8f1da26d 2858Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2859
4852725b 2860 use Socket qw(:all);
636e6b1f
TH
2861
2862 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2863 or die "Could not determine the protocol number for tcp";
4852725b
DD
2864 # my $tcp = IPPROTO_TCP; # Alternative
2865 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2866 or die "getsockopt TCP_NODELAY: $!";
636e6b1f 2867 my $nodelay = unpack("I", $packed);
f7051f2c
FC
2868 print "Nagle's algorithm is turned ",
2869 $nodelay ? "off\n" : "on\n";
636e6b1f 2870
ea9eb35a 2871Portability issues: L<perlport/getsockopt>.
a0d0e21e
LW
2872
2873=item glob EXPR
d74e8afc 2874X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2875
0a753a76
PP
2876=item glob
2877
c17cdb72
NC
2878=for Pod::Functions expand filenames using wildcards
2879
d9a9d457 2880In list context, returns a (possibly empty) list of filename expansions on
391b733c 2881the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
d9a9d457 2882scalar context, glob iterates through such filename expansions, returning
391b733c
FC
2883undef when the list is exhausted. This is the internal function
2884implementing the C<< <*.c> >> operator, but you can use it directly. If
d9a9d457
JL
2885EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2886more detail in L<perlop/"I/O Operators">.
a0d0e21e 2887
80d38338
TC
2888Note that C<glob> splits its arguments on whitespace and treats
2889each segment as separate pattern. As such, C<glob("*.c *.h")>
2890matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2891C<glob(".* *")> matches all files in the current working directory.
a91bb7b1
TC
2892If you want to glob filenames that might contain whitespace, you'll
2893have to use extra quotes around the spacey filename to protect it.
2894For example, to glob filenames that have an C<e> followed by a space
2895followed by an C<f>, use either of:
2896
2897 @spacies = <"*e f*">;
2898 @spacies = glob '"*e f*"';
2899 @spacies = glob q("*e f*");
2900
2901If you had to get a variable through, you could do this:
2902
2903 @spacies = glob "'*${var}e f*'";
2904 @spacies = glob qq("*${var}e f*");
80d38338
TC
2905
2906If non-empty braces are the only wildcard characters used in the
2907C<glob>, no filenames are matched, but potentially many strings
2908are returned. For example, this produces nine strings, one for
2909each pairing of fruits and colors:
2910
2911 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2912
e9fa405d 2913This operator is implemented using the standard
5c0c9249
PF
2914C<File::Glob> extension. See L<File::Glob> for details, including
2915C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2916
ea9eb35a 2917Portability issues: L<perlport/glob>.
2918
a0d0e21e 2919=item gmtime EXPR
d74e8afc 2920X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2921
ce2984c3
PF
2922=item gmtime
2923
c17cdb72
NC
2924=for Pod::Functions convert UNIX time into record or string using Greenwich time
2925
4509d391 2926Works just like L</localtime> but the returned values are
435fbc73 2927localized for the standard Greenwich time zone.
a0d0e21e 2928
a3390c9f
FC
2929Note: When called in list context, $isdst, the last value
2930returned by gmtime, is always C<0>. There is no
435fbc73 2931Daylight Saving Time in GMT.
0a753a76 2932
ea9eb35a 2933Portability issues: L<perlport/gmtime>.
62aa5637 2934
a0d0e21e 2935=item goto LABEL
d74e8afc 2936X<goto> X<jump> X<jmp>
a0d0e21e 2937
748a9306
LW
2938=item goto EXPR
2939
a0d0e21e
LW
2940=item goto &NAME
2941
c17cdb72
NC
2942=for Pod::Functions create spaghetti code
2943
5a5b79a3 2944The C<goto LABEL> form finds the statement labeled with LABEL and
391b733c 2945resumes execution there. It can't be used to get out of a block or
b500e03b
GG
2946subroutine given to C<sort>. It can be used to go almost anywhere
2947else within the dynamic scope, including out of subroutines, but it's
2948usually better to use some other construct such as C<last> or C<die>.
2949The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2950(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2951does not offer named loops combined with loop control. Perl does, and
2952this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2953
5a5b79a3 2954The C<goto EXPR> form expects to evaluate C<EXPR> to a code reference or
3e8a6370 2955a label name. If it evaluates to a code reference, it will be handled
5a5b79a3 2956like C<goto &NAME>, below. This is especially useful for implementing
3e8a6370
RS
2957tail recursion via C<goto __SUB__>.
2958
2959If the expression evaluates to a label name, its scope will be resolved
7660c0ab 2960dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2961necessarily recommended if you're optimizing for maintainability:
2962
2963 goto ("FOO", "BAR", "GLARCH")[$i];
2964
5a5b79a3 2965As shown in this example, C<goto EXPR> is exempt from the "looks like a
391b733c
FC
2966function" rule. A pair of parentheses following it does not (necessarily)
2967delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
8a7e748e
FC
2968Also, unlike most named operators, this has the same precedence as
2969assignment.
887d89fd 2970
5a5b79a3 2971Use of C<goto LABEL> or C<goto EXPR> to jump into a construct is
0b98bec9 2972deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2973go into any construct that requires initialization, such as a
2974subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2975construct that is optimized away.
b500e03b 2976
5a5b79a3 2977The C<goto &NAME> form is quite different from the other forms of
1b6921cb
BT
2978C<goto>. In fact, it isn't a goto in the normal sense at all, and
2979doesn't have the stigma associated with other gotos. Instead, it
2980exits the current subroutine (losing any changes set by local()) and
2981immediately calls in its place the named subroutine using the current
2982value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2983load another subroutine and then pretend that the other subroutine had
2984been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2985in the current subroutine are propagated to the other subroutine.)
2986After the C<goto>, not even C<caller> will be able to tell that this
2987routine was called first.
2988
2989NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2990containing a code reference or a block that evaluates to a code
6cb9131c 2991reference.
a0d0e21e
LW
2992
2993=item grep BLOCK LIST
d74e8afc 2994X<grep>
a0d0e21e
LW
2995
2996=item grep EXPR,LIST
2997
c17cdb72
NC
2998=for Pod::Functions locate elements in a list test true against a given criterion
2999
2b5ab1e7
TC
3000This is similar in spirit to, but not the same as, grep(1) and its
3001relatives. In particular, it is not limited to using regular expressions.
2f9daede 3002
a0d0e21e 3003Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 3004C<$_> to each element) and returns the list value consisting of those
19799a22
GS
3005elements for which the expression evaluated to true. In scalar
3006context, returns the number of times the expression was true.
a0d0e21e
LW
3007
3008 @foo = grep(!/^#/, @bar); # weed out comments
3009
3010or equivalently,
3011
3012 @foo = grep {!/^#/} @bar; # weed out comments
3013
be3174d2
GS
3014Note that C<$_> is an alias to the list value, so it can be used to
3015modify the elements of the LIST. While this is useful and supported,
3016it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
3017Similarly, grep returns aliases into the original list, much as a for
3018loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
3019element of a list returned by grep (for example, in a C<foreach>, C<map>
3020or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 3021This is usually something to be avoided when writing clear code.
a0d0e21e 3022
a4fb8298 3023If C<$_> is lexical in the scope where the C<grep> appears (because it has
c071e214
FC
3024been declared with the deprecated C<my $_> construct)
3025then, in addition to being locally aliased to
80d38338 3026the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
3027can't be seen from the outside, avoiding any potential side-effects.
3028
19799a22 3029See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 3030
a0d0e21e 3031=item hex EXPR
d74e8afc 3032X<hex> X<hexadecimal>
a0d0e21e 3033
54310121 3034=item hex
bbce6d69 3035
c17cdb72
NC
3036=for Pod::Functions convert a string to a hexadecimal number
3037
2b5ab1e7 3038Interprets EXPR as a hex string and returns the corresponding value.
38366c11 3039(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 3040L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
3041
3042 print hex '0xAf'; # prints '175'
3043 print hex 'aF'; # same
a0d0e21e 3044
19799a22 3045Hex strings may only represent integers. Strings that would cause
53305cf1 3046integer overflow trigger a warning. Leading whitespace is not stripped,
391b733c 3047unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 3048L</sprintf>, and L</unpack>.
19799a22 3049
ce2984c3 3050=item import LIST