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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
5ed4f2ec 393 -T File is an ASCII text file (heuristic guess).
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
a0d0e21e
LW
452The C<-T> and C<-B> switches work as follows. The first block or so of the
453file is examined for odd characters such as strange control codes or
61eff3bc 454characters with the high bit set. If too many strange characters (>30%)
cf264981 455are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 456containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 457or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 458rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 459file, or a file at EOF when testing a filehandle. Because you have to
4633a7c4
LW
460read a file to do the C<-T> test, on most occasions you want to use a C<-f>
461against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 462
5dac7880 463If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 464the special filehandle consisting of a solitary underline, then the stat
a0d0e21e
LW
465structure of the previous file test (or stat operator) is used, saving
466a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 467that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 468symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 469an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 470Example:
a0d0e21e
LW
471
472 print "Can do.\n" if -r $a || -w _ || -x _;
473
474 stat($filename);
475 print "Readable\n" if -r _;
476 print "Writable\n" if -w _;
477 print "Executable\n" if -x _;
478 print "Setuid\n" if -u _;
479 print "Setgid\n" if -g _;
480 print "Sticky\n" if -k _;
481 print "Text\n" if -T _;
482 print "Binary\n" if -B _;
483
e9fa405d 484As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file
fbb0b3b3 485test operators, in a way that C<-f -w -x $file> is equivalent to
a5840dee 486C<-x $file && -w _ && -f _>. (This is only fancy syntax: if you use
fbb0b3b3
RGS
487the return value of C<-f $file> as an argument to another filetest
488operator, no special magic will happen.)
489
bee96257 490Portability issues: L<perlport/-X>.
ea9eb35a 491
bade7fbc
TC
492To avoid confusing would-be users of your code with mysterious
493syntax errors, put something like this at the top of your script:
494
495 use 5.010; # so filetest ops can stack
496
a0d0e21e 497=item abs VALUE
d74e8afc 498X<abs> X<absolute>
a0d0e21e 499
54310121 500=item abs
bbce6d69 501
c17cdb72
NC
502=for Pod::Functions absolute value function
503
a0d0e21e 504Returns the absolute value of its argument.
7660c0ab 505If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
506
507=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 508X<accept>
a0d0e21e 509
c17cdb72
NC
510=for Pod::Functions accept an incoming socket connect
511
3b10bc60 512Accepts an incoming socket connect, just as accept(2)
19799a22 513does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 514See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 515
8d2a6795
GS
516On systems that support a close-on-exec flag on files, the flag will
517be set for the newly opened file descriptor, as determined by the
518value of $^F. See L<perlvar/$^F>.
519
a0d0e21e 520=item alarm SECONDS
d74e8afc
ITB
521X<alarm>
522X<SIGALRM>
523X<timer>
a0d0e21e 524
54310121 525=item alarm
bbce6d69 526
c17cdb72
NC
527=for Pod::Functions schedule a SIGALRM
528
a0d0e21e 529Arranges to have a SIGALRM delivered to this process after the
cf264981 530specified number of wallclock seconds has elapsed. If SECONDS is not
391b733c 531specified, the value stored in C<$_> is used. (On some machines,
d400eac8
JH
532unfortunately, the elapsed time may be up to one second less or more
533than you specified because of how seconds are counted, and process
534scheduling may delay the delivery of the signal even further.)
535
536Only one timer may be counting at once. Each call disables the
537previous timer, and an argument of C<0> may be supplied to cancel the
538previous timer without starting a new one. The returned value is the
539amount of time remaining on the previous timer.
a0d0e21e 540
2bc69794
BS
541For delays of finer granularity than one second, the Time::HiRes module
542(from CPAN, and starting from Perl 5.8 part of the standard
543distribution) provides ualarm(). You may also use Perl's four-argument
544version of select() leaving the first three arguments undefined, or you
545might be able to use the C<syscall> interface to access setitimer(2) if
391b733c 546your system supports it. See L<perlfaq8> for details.
2b5ab1e7 547
80d38338
TC
548It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
549C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 550
19799a22
GS
551If you want to use C<alarm> to time out a system call you need to use an
552C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 553fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 554restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 555modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
556
557 eval {
a9a5a0dc
VP
558 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
559 alarm $timeout;
560 $nread = sysread SOCKET, $buffer, $size;
561 alarm 0;
ff68c719 562 };
ff68c719 563 if ($@) {
a9a5a0dc 564 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 565 # timed out
ff68c719
PP
566 }
567 else {
5ed4f2ec 568 # didn't
ff68c719
PP
569 }
570
91d81acc
JH
571For more information see L<perlipc>.
572
ea9eb35a 573Portability issues: L<perlport/alarm>.
574
a0d0e21e 575=item atan2 Y,X
d74e8afc 576X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e 577
c17cdb72
NC
578=for Pod::Functions arctangent of Y/X in the range -PI to PI
579
a0d0e21e
LW
580Returns the arctangent of Y/X in the range -PI to PI.
581
ca6e1c26 582For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
583function, or use the familiar relation:
584
585 sub tan { sin($_[0]) / cos($_[0]) }
586
a1021d57
RGS
587The return value for C<atan2(0,0)> is implementation-defined; consult
588your atan2(3) manpage for more information.
bf5f1b4c 589
ea9eb35a 590Portability issues: L<perlport/atan2>.
591
a0d0e21e 592=item bind SOCKET,NAME
d74e8afc 593X<bind>
a0d0e21e 594
c17cdb72
NC
595=for Pod::Functions binds an address to a socket
596
3b10bc60 597Binds a network address to a socket, just as bind(2)
19799a22 598does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
599packed address of the appropriate type for the socket. See the examples in
600L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 601
fae2c0fb 602=item binmode FILEHANDLE, LAYER
d74e8afc 603X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 604
a0d0e21e
LW
605=item binmode FILEHANDLE
606
c17cdb72
NC
607=for Pod::Functions prepare binary files for I/O
608
1cbfc93d
NIS
609Arranges for FILEHANDLE to be read or written in "binary" or "text"
610mode on systems where the run-time libraries distinguish between
611binary and text files. If FILEHANDLE is an expression, the value is
612taken as the name of the filehandle. Returns true on success,
b5fe5ca2 613otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 614
8f1da26d 615On some systems (in general, DOS- and Windows-based systems) binmode()
d807c6f4 616is necessary when you're not working with a text file. For the sake
d7a0d798
FC
617of portability it is a good idea always to use it when appropriate,
618and never to use it when it isn't appropriate. Also, people can
8f1da26d 619set their I/O to be by default UTF8-encoded Unicode, not bytes.
d807c6f4
JH
620
621In other words: regardless of platform, use binmode() on binary data,
d7a0d798 622like images, for example.
d807c6f4
JH
623
624If LAYER is present it is a single string, but may contain multiple
391b733c 625directives. The directives alter the behaviour of the filehandle.
d7a0d798 626When LAYER is present, using binmode on a text file makes sense.
d807c6f4 627
fae2c0fb 628If LAYER is omitted or specified as C<:raw> the filehandle is made
391b733c 629suitable for passing binary data. This includes turning off possible CRLF
0226bbdb 630translation and marking it as bytes (as opposed to Unicode characters).
749683d2 631Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 632Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
633Other layers that would affect the binary nature of the stream are
391b733c 634I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 635PERLIO environment variable.
01e6739c 636
3b10bc60 637The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
638form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
639establish default I/O layers. See L<open>.
640
fae2c0fb
RGS
641I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
642in "Programming Perl, 3rd Edition". However, since the publishing of this
643book, by many known as "Camel III", the consensus of the naming of this
644functionality has moved from "discipline" to "layer". All documentation
645of this version of Perl therefore refers to "layers" rather than to
646"disciplines". Now back to the regularly scheduled documentation...>
647
8f1da26d 648To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
6902c96a 649C<:utf8> just marks the data as UTF-8 without further checking,
8f1da26d 650while C<:encoding(UTF-8)> checks the data for actually being valid
391b733c 651UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 652
ed53a2bb 653In general, binmode() should be called after open() but before any I/O
3b10bc60 654is done on the filehandle. Calling binmode() normally flushes any
01e6739c 655pending buffered output data (and perhaps pending input data) on the
fae2c0fb 656handle. An exception to this is the C<:encoding> layer that
d7a0d798 657changes the default character encoding of the handle; see L</open>.
fae2c0fb 658The C<:encoding> layer sometimes needs to be called in
3874323d
JH
659mid-stream, and it doesn't flush the stream. The C<:encoding>
660also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 661internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 662
19799a22 663The operating system, device drivers, C libraries, and Perl run-time
8f1da26d
TC
664system all conspire to let the programmer treat a single
665character (C<\n>) as the line terminator, irrespective of external
30168b04
GS
666representation. On many operating systems, the native text file
667representation matches the internal representation, but on some
668platforms the external representation of C<\n> is made up of more than
669one character.
670
8f1da26d
TC
671All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
672a single character to end each line in the external representation of text
673(even though that single character is CARRIAGE RETURN on old, pre-Darwin
391b733c 674flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
8f1da26d
TC
675systems like OS/2, DOS, and the various flavors of MS-Windows, your program
676sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
677two characters C<\cM\cJ>. That means that if you don't use binmode() on
678these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
679input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
680output. This is what you want for text files, but it can be disastrous for
681binary files.
30168b04
GS
682
683Another consequence of using binmode() (on some systems) is that
684special end-of-file markers will be seen as part of the data stream.
d7a0d798
FC
685For systems from the Microsoft family this means that, if your binary
686data contain C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
687the file, unless you use binmode().
688
3b10bc60 689binmode() is important not only for readline() and print() operations,
30168b04
GS
690but also when using read(), seek(), sysread(), syswrite() and tell()
691(see L<perlport> for more details). See the C<$/> and C<$\> variables
692in L<perlvar> for how to manually set your input and output
693line-termination sequences.
a0d0e21e 694
ea9eb35a 695Portability issues: L<perlport/binmode>.
696
4633a7c4 697=item bless REF,CLASSNAME
d74e8afc 698X<bless>
a0d0e21e
LW
699
700=item bless REF
701
c17cdb72
NC
702=for Pod::Functions create an object
703
2b5ab1e7
TC
704This function tells the thingy referenced by REF that it is now an object
705in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 706is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 707it returns the reference for convenience. Always use the two-argument
cf264981 708version if a derived class might inherit the function doing the blessing.
e54e4959 709See L<perlobj> for more about the blessing (and blessings) of objects.
a0d0e21e 710
57668c4d 711Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 712Namespaces with all lowercase names are considered reserved for
391b733c 713Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
714confusion, you may wish to avoid such package names as well. Make sure
715that CLASSNAME is a true value.
60ad88b8
GS
716
717See L<perlmod/"Perl Modules">.
718
0d863452
RH
719=item break
720
d9b04284 721=for Pod::Functions +switch break out of a C<given> block
c17cdb72 722
0d863452
RH
723Break out of a C<given()> block.
724
a8a26e52
JK
725This keyword is enabled by the C<"switch"> feature; see L<feature> for
726more information on C<"switch">. You can also access it by prefixing it
727with C<CORE::>. Alternatively, include a C<use v5.10> or later to the
728current scope.
0d863452 729
a0d0e21e 730=item caller EXPR
d74e8afc 731X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
732
733=item caller
734
c17cdb72
NC
735=for Pod::Functions get context of the current subroutine call
736
1d56df50
DD
737Returns the context of the current pure perl subroutine call. In scalar
738context, returns the caller's package name if there I<is> a caller (that is, if
80d38338 739we're in a subroutine or C<eval> or C<require>) and the undefined value
1d56df50
DD
740otherwise. caller never returns XS subs and they are skipped. The next pure
741perl sub will appear instead of the XS sub in caller's return values. In list
742context, caller returns
a0d0e21e 743
ee6b43cc 744 # 0 1 2
748a9306 745 ($package, $filename, $line) = caller;
a0d0e21e
LW
746
747With EXPR, it returns some extra information that the debugger uses to
748print a stack trace. The value of EXPR indicates how many call frames
749to go back before the current one.
750
ee6b43cc 751 # 0 1 2 3 4
f3aa04c2 752 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 753
754 # 5 6 7 8 9 10
b3ca2e83 755 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 756 = caller($i);
e7ea3e70 757
02729fef
DM
758Here, $subroutine is the function that the caller called (rather than the
759function containing the caller). Note that $subroutine may be C<(eval)> if
760the frame is not a subroutine call, but an C<eval>. In such a case
761additional elements $evaltext and
7660c0ab 762C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 763C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 764C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 765$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
766each C<use> statement creates a C<require> frame inside an C<eval EXPR>
767frame.) $subroutine may also be C<(unknown)> if this particular
768subroutine happens to have been deleted from the symbol table.
769C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
770C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
585d73c3 771compiled with. C<$hints> corresponds to C<$^H>, and C<$bitmask>
1adb05cd
FC
772corresponds to C<${^WARNING_BITS}>. The
773C<$hints> and C<$bitmask> values are subject
585d73c3 774to change between versions of Perl, and are not meant for external use.
748a9306 775
b3ca2e83 776C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
391b733c 777caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
b3ca2e83
NC
778of this hash, as they are the actual values stored in the optree.
779
ffe0c19d
FC
780Furthermore, when called from within the DB package in
781list context, and with an argument, caller returns more
7660c0ab 782detailed information: it sets the list variable C<@DB::args> to be the
54310121 783arguments with which the subroutine was invoked.
748a9306 784
7660c0ab 785Be aware that the optimizer might have optimized call frames away before
19799a22 786C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 787might not return information about the call frame you expect it to, for
b76cc8ba 788C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 789previous time C<caller> was called.
7660c0ab 790
8f1da26d 791Be aware that setting C<@DB::args> is I<best effort>, intended for
391b733c 792debugging or generating backtraces, and should not be relied upon. In
ca9f0cb5
NC
793particular, as C<@_> contains aliases to the caller's arguments, Perl does
794not take a copy of C<@_>, so C<@DB::args> will contain modifications the
795subroutine makes to C<@_> or its contents, not the original values at call
391b733c 796time. C<@DB::args>, like C<@_>, does not hold explicit references to its
ca9f0cb5 797elements, so under certain cases its elements may have become freed and
391b733c 798reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 799of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
800I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
801reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5 802elements), so C<@DB::args> is actually a hybrid of the current state and
391b733c 803initial state of C<@_>. Buyer beware.
ca9f0cb5 804
a0d0e21e 805=item chdir EXPR
d74e8afc
ITB
806X<chdir>
807X<cd>
f723aae1 808X<directory, change>
a0d0e21e 809
c4aca7d0
GA
810=item chdir FILEHANDLE
811
812=item chdir DIRHANDLE
813
ce2984c3
PF
814=item chdir
815
c17cdb72
NC
816=for Pod::Functions change your current working directory
817
391b733c 818Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 819changes to the directory specified by C<$ENV{HOME}>, if set; if not,
391b733c
FC
820changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
821variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
822neither is set, C<chdir> does nothing. It returns true on success,
823false otherwise. See the example under C<die>.
a0d0e21e 824
3b10bc60 825On systems that support fchdir(2), you may pass a filehandle or
34169887 826directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 827passing handles raises an exception.
c4aca7d0 828
a0d0e21e 829=item chmod LIST
d74e8afc 830X<chmod> X<permission> X<mode>
a0d0e21e 831
c17cdb72
NC
832=for Pod::Functions changes the permissions on a list of files
833
a0d0e21e 834Changes the permissions of a list of files. The first element of the
8f1da26d 835list must be the numeric mode, which should probably be an octal
4ad40acf 836number, and which definitely should I<not> be a string of octal digits:
3b10bc60 837C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 838successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 839
3b10bc60 840 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 841 chmod 0755, @executables;
3b10bc60 842 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 843 # --w----r-T
3b10bc60 844 $mode = "0644"; chmod oct($mode), "foo"; # this is better
845 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 846
3b10bc60 847On systems that support fchmod(2), you may pass filehandles among the
848files. On systems that don't support fchmod(2), passing filehandles raises
849an exception. Filehandles must be passed as globs or glob references to be
850recognized; barewords are considered filenames.
c4aca7d0
GA
851
852 open(my $fh, "<", "foo");
853 my $perm = (stat $fh)[2] & 07777;
854 chmod($perm | 0600, $fh);
855
3b10bc60 856You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
857module:
858
3b10bc60 859 use Fcntl qw( :mode );
ca6e1c26 860 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 861 # Identical to the chmod 0755 of the example above.
ca6e1c26 862
ea9eb35a 863Portability issues: L<perlport/chmod>.
864
a0d0e21e 865=item chomp VARIABLE
d74e8afc 866X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 867
313c9f5c 868=item chomp( LIST )
a0d0e21e
LW
869
870=item chomp
871
c17cdb72
NC
872=for Pod::Functions remove a trailing record separator from a string
873
2b5ab1e7
TC
874This safer version of L</chop> removes any trailing string
875that corresponds to the current value of C<$/> (also known as
28757baa
PP
876$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
877number of characters removed from all its arguments. It's often used to
878remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
879that the final record may be missing its newline. When in paragraph
880mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 881When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 882a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 883remove anything.
19799a22 884If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
885
886 while (<>) {
a9a5a0dc
VP
887 chomp; # avoid \n on last field
888 @array = split(/:/);
889 # ...
a0d0e21e
LW
890 }
891
feef49c9
FC
892If VARIABLE is a hash, it chomps the hash's values, but not its keys,
893resetting the C<each> iterator in the process.
4bf21a6d 894
a0d0e21e
LW
895You can actually chomp anything that's an lvalue, including an assignment:
896
897 chomp($cwd = `pwd`);
898 chomp($answer = <STDIN>);
899
900If you chomp a list, each element is chomped, and the total number of
901characters removed is returned.
902
15e44fd8
RGS
903Note that parentheses are necessary when you're chomping anything
904that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
905is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
906C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
907C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
908as C<chomp($a, $b)>.
909
a0d0e21e 910=item chop VARIABLE
d74e8afc 911X<chop>
a0d0e21e 912
313c9f5c 913=item chop( LIST )
a0d0e21e
LW
914
915=item chop
916
c17cdb72
NC
917=for Pod::Functions remove the last character from a string
918
a0d0e21e 919Chops off the last character of a string and returns the character
5b3eff12 920chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 921scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
feef49c9
FC
922If VARIABLE is a hash, it chops the hash's values, but not its keys,
923resetting the C<each> iterator in the process.
4bf21a6d 924
5b3eff12 925You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
926
927If you chop a list, each element is chopped. Only the value of the
19799a22 928last C<chop> is returned.
a0d0e21e 929
19799a22 930Note that C<chop> returns the last character. To return all but the last
748a9306
LW
931character, use C<substr($string, 0, -1)>.
932
15e44fd8
RGS
933See also L</chomp>.
934
a0d0e21e 935=item chown LIST
d74e8afc 936X<chown> X<owner> X<user> X<group>
a0d0e21e 937
c17cdb72
NC
938=for Pod::Functions change the ownership on a list of files
939
a0d0e21e 940Changes the owner (and group) of a list of files. The first two
19799a22
GS
941elements of the list must be the I<numeric> uid and gid, in that
942order. A value of -1 in either position is interpreted by most
943systems to leave that value unchanged. Returns the number of files
944successfully changed.
a0d0e21e
LW
945
946 $cnt = chown $uid, $gid, 'foo', 'bar';
947 chown $uid, $gid, @filenames;
948
3b10bc60 949On systems that support fchown(2), you may pass filehandles among the
950files. On systems that don't support fchown(2), passing filehandles raises
951an exception. Filehandles must be passed as globs or glob references to be
952recognized; barewords are considered filenames.
c4aca7d0 953
54310121 954Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
955
956 print "User: ";
19799a22 957 chomp($user = <STDIN>);
5a964f20 958 print "Files: ";
19799a22 959 chomp($pattern = <STDIN>);
a0d0e21e
LW
960
961 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 962 or die "$user not in passwd file";
a0d0e21e 963
5ed4f2ec 964 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
965 chown $uid, $gid, @ary;
966
54310121 967On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
968file unless you're the superuser, although you should be able to change
969the group to any of your secondary groups. On insecure systems, these
970restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
971On POSIX systems, you can detect this condition this way:
972
973 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
974 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 975
f48496b1 976Portability issues: L<perlport/chown>.
ea9eb35a 977
a0d0e21e 978=item chr NUMBER
d74e8afc 979X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 980
54310121 981=item chr
bbce6d69 982
c17cdb72
NC
983=for Pod::Functions get character this number represents
984
a0d0e21e 985Returns the character represented by that NUMBER in the character set.
a0ed51b3 986For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 987chr(0x263a) is a Unicode smiley face.
aaa68c4a 988
8a064bd6 989Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 990except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
991(truncated to an integer) are used.
992
974da8e5
JH
993If NUMBER is omitted, uses C<$_>.
994
b76cc8ba 995For the reverse, use L</ord>.
a0d0e21e 996
2575c402
JW
997Note that characters from 128 to 255 (inclusive) are by default
998internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 999
2575c402 1000See L<perlunicode> for more about Unicode.
bbce6d69 1001
a0d0e21e 1002=item chroot FILENAME
d74e8afc 1003X<chroot> X<root>
a0d0e21e 1004
54310121 1005=item chroot
bbce6d69 1006
c17cdb72
NC
1007=for Pod::Functions make directory new root for path lookups
1008
5a964f20 1009This function works like the system call by the same name: it makes the
4633a7c4 1010named directory the new root directory for all further pathnames that
951ba7fe 1011begin with a C</> by your process and all its children. (It doesn't
28757baa 1012change your current working directory, which is unaffected.) For security
4633a7c4 1013reasons, this call is restricted to the superuser. If FILENAME is
19799a22 1014omitted, does a C<chroot> to C<$_>.
a0d0e21e 1015
b00d10dc
JH
1016B<NOTE:> It is good security practice to do C<chdir("/")> (to the root
1017directory) immediately after a C<chroot()>.
1018
ea9eb35a 1019Portability issues: L<perlport/chroot>.
1020
a0d0e21e 1021=item close FILEHANDLE
d74e8afc 1022X<close>
a0d0e21e 1023
6a518fbc
TP
1024=item close
1025
c17cdb72
NC
1026=for Pod::Functions close file (or pipe or socket) handle
1027
3b10bc60 1028Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 1029buffers, and closes the system file descriptor. Returns true if those
8f1da26d 1030operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
1031layer. Closes the currently selected filehandle if the argument is
1032omitted.
fb73857a
PP
1033
1034You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 1035another C<open> on it, because C<open> closes it for you. (See
01aa884e 1036L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
19799a22 1037counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 1038
3b10bc60 1039If the filehandle came from a piped open, C<close> returns false if one of
1040the other syscalls involved fails or if its program exits with non-zero
1041status. If the only problem was that the program exited non-zero, C<$!>
1042will be set to C<0>. Closing a pipe also waits for the process executing
1043on the pipe to exit--in case you wish to look at the output of the pipe
1044afterwards--and implicitly puts the exit status value of that command into
1045C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 1046
2e0cfa16
FC
1047If there are multiple threads running, C<close> on a filehandle from a
1048piped open returns true without waiting for the child process to terminate,
1049if the filehandle is still open in another thread.
1050
80d38338
TC
1051Closing the read end of a pipe before the process writing to it at the
1052other end is done writing results in the writer receiving a SIGPIPE. If
1053the other end can't handle that, be sure to read all the data before
1054closing the pipe.
73689b13 1055
fb73857a 1056Example:
a0d0e21e 1057
fb73857a
PP
1058 open(OUTPUT, '|sort >foo') # pipe to sort
1059 or die "Can't start sort: $!";
5ed4f2ec 1060 #... # print stuff to output
1061 close OUTPUT # wait for sort to finish
fb73857a
PP
1062 or warn $! ? "Error closing sort pipe: $!"
1063 : "Exit status $? from sort";
5ed4f2ec 1064 open(INPUT, 'foo') # get sort's results
fb73857a 1065 or die "Can't open 'foo' for input: $!";
a0d0e21e 1066
5a964f20 1067FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 1068filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
1069
1070=item closedir DIRHANDLE
d74e8afc 1071X<closedir>
a0d0e21e 1072
c17cdb72
NC
1073=for Pod::Functions close directory handle
1074
19799a22 1075Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
1076system call.
1077
a0d0e21e 1078=item connect SOCKET,NAME
d74e8afc 1079X<connect>
a0d0e21e 1080
c17cdb72
NC
1081=for Pod::Functions connect to a remote socket
1082
80d38338
TC
1083Attempts to connect to a remote socket, just like connect(2).
1084Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
1085packed address of the appropriate type for the socket. See the examples in
1086L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 1087
cb1a09d0 1088=item continue BLOCK
d74e8afc 1089X<continue>
cb1a09d0 1090
0d863452
RH
1091=item continue
1092
c17cdb72
NC
1093=for Pod::Functions optional trailing block in a while or foreach
1094
4a904372
FC
1095When followed by a BLOCK, C<continue> is actually a
1096flow control statement rather than a function. If
cf264981 1097there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
1098C<foreach>), it is always executed just before the conditional is about to
1099be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
1100it can be used to increment a loop variable, even when the loop has been
1101continued via the C<next> statement (which is similar to the C C<continue>
1102statement).
1103
98293880 1104C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 1105block; C<last> and C<redo> behave as if they had been executed within
19799a22 1106the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
1107block, it may be more entertaining.
1108
1109 while (EXPR) {
a9a5a0dc
VP
1110 ### redo always comes here
1111 do_something;
1d2dff63 1112 } continue {
a9a5a0dc
VP
1113 ### next always comes here
1114 do_something_else;
1115 # then back the top to re-check EXPR
1d2dff63
GS
1116 }
1117 ### last always comes here
1118
3b10bc60 1119Omitting the C<continue> section is equivalent to using an
1120empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1121to check the condition at the top of the loop.
1122
4a904372 1123When there is no BLOCK, C<continue> is a function that
8f1da26d
TC
1124falls through the current C<when> or C<default> block instead of iterating
1125a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
4a904372
FC
1126In Perl 5.14 and earlier, this form of C<continue> was
1127only available when the C<"switch"> feature was enabled.
48238296 1128See L<feature> and L<perlsyn/"Switch Statements"> for more
8f1da26d 1129information.
0d863452 1130
a0d0e21e 1131=item cos EXPR
d74e8afc 1132X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1133
d6217f1e
GS
1134=item cos
1135
c17cdb72
NC
1136=for Pod::Functions cosine function
1137
5a964f20 1138Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1139takes the cosine of C<$_>.
a0d0e21e 1140
ca6e1c26 1141For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1142function, or use this relation:
1143
1144 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1145
a0d0e21e 1146=item crypt PLAINTEXT,SALT
d74e8afc 1147X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1148X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1149
c17cdb72
NC
1150=for Pod::Functions one-way passwd-style encryption
1151
ef2e6798
MS
1152Creates a digest string exactly like the crypt(3) function in the C
1153library (assuming that you actually have a version there that has not
bb23f8d1 1154been extirpated as a potential munition).
ef2e6798 1155
34169887 1156crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1157into a short string, called a digest, which is returned. The same
1158PLAINTEXT and SALT will always return the same string, but there is no
1159(known) way to get the original PLAINTEXT from the hash. Small
1160changes in the PLAINTEXT or SALT will result in large changes in the
1161digest.
1162
1163There is no decrypt function. This function isn't all that useful for
1164cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1165mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1166primarily used to check if two pieces of text are the same without
1167having to transmit or store the text itself. An example is checking
1168if a correct password is given. The digest of the password is stored,
cf264981 1169not the password itself. The user types in a password that is
ef2e6798 1170crypt()'d with the same salt as the stored digest. If the two digests
34169887 1171match, the password is correct.
ef2e6798
MS
1172
1173When verifying an existing digest string you should use the digest as
1174the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1175to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1176crypt() will hash the new string with the same salt as the digest.
1177This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1178with more exotic implementations. In other words, assume
1179nothing about the returned string itself nor about how many bytes
1180of SALT may matter.
85c16d83
JH
1181
1182Traditionally the result is a string of 13 bytes: two first bytes of
1183the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
391b733c 1184the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1185hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1186and implementations on non-Unix platforms may produce different
ef2e6798 1187strings.
85c16d83
JH
1188
1189When choosing a new salt create a random two character string whose
1190characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1191'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1192characters is just a recommendation; the characters allowed in
1193the salt depend solely on your system's crypt library, and Perl can't
1194restrict what salts C<crypt()> accepts.
e71965be 1195
a0d0e21e 1196Here's an example that makes sure that whoever runs this program knows
cf264981 1197their password:
a0d0e21e
LW
1198
1199 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1200
1201 system "stty -echo";
1202 print "Password: ";
e71965be 1203 chomp($word = <STDIN>);
a0d0e21e
LW
1204 print "\n";
1205 system "stty echo";
1206
e71965be 1207 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1208 die "Sorry...\n";
a0d0e21e 1209 } else {
a9a5a0dc 1210 print "ok\n";
54310121 1211 }
a0d0e21e 1212
9f8f0c9d 1213Of course, typing in your own password to whoever asks you
748a9306 1214for it is unwise.
a0d0e21e 1215
ef2e6798 1216The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1217of data, not least of all because you can't get the information
ef2e6798 1218back. Look at the L<Digest> module for more robust algorithms.
19799a22 1219
f2791508
JH
1220If using crypt() on a Unicode string (which I<potentially> has
1221characters with codepoints above 255), Perl tries to make sense
34169887 1222of the situation by trying to downgrade (a copy of)
f2791508
JH
1223the string back to an eight-bit byte string before calling crypt()
1224(on that copy). If that works, good. If not, crypt() dies with
1225C<Wide character in crypt>.
85c16d83 1226
ea9eb35a 1227Portability issues: L<perlport/crypt>.
1228
aa689395 1229=item dbmclose HASH
d74e8afc 1230X<dbmclose>
a0d0e21e 1231
c17cdb72
NC
1232=for Pod::Functions breaks binding on a tied dbm file
1233
19799a22 1234[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1235
aa689395 1236Breaks the binding between a DBM file and a hash.
a0d0e21e 1237
ea9eb35a 1238Portability issues: L<perlport/dbmclose>.
1239
19799a22 1240=item dbmopen HASH,DBNAME,MASK
d74e8afc 1241X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1242
c17cdb72
NC
1243=for Pod::Functions create binding on a tied dbm file
1244
01aa884e
KW
1245[This function has been largely superseded by the
1246L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
a0d0e21e 1247
7b8d334a 1248This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1249hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1250argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1251is the name of the database (without the F<.dir> or F<.pag> extension if
1252any). If the database does not exist, it is created with protection
1b3a6178
FC
1253specified by MASK (as modified by the C<umask>). To prevent creation of
1254the database if it doesn't exist, you may specify a MODE
1255of 0, and the function will return a false value if it
1256can't find an existing database. If your system supports
80d38338 1257only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1258program. In older versions of Perl, if your system had neither DBM nor
19799a22 1259ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1260sdbm(3).
1261
1262If you don't have write access to the DBM file, you can only read hash
1263variables, not set them. If you want to test whether you can write,
3b10bc60 1264either use file tests or try setting a dummy hash entry inside an C<eval>
1265to trap the error.
a0d0e21e 1266
19799a22
GS
1267Note that functions such as C<keys> and C<values> may return huge lists
1268when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1269function to iterate over large DBM files. Example:
1270
1271 # print out history file offsets
1272 dbmopen(%HIST,'/usr/lib/news/history',0666);
1273 while (($key,$val) = each %HIST) {
a9a5a0dc 1274 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1275 }
1276 dbmclose(%HIST);
1277
cb1a09d0 1278See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1279cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1280rich implementation.
4633a7c4 1281
2b5ab1e7
TC
1282You can control which DBM library you use by loading that library
1283before you call dbmopen():
1284
1285 use DB_File;
1286 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1287 or die "Can't open netscape history file: $!";
2b5ab1e7 1288
ea9eb35a 1289Portability issues: L<perlport/dbmopen>.
1290
a0d0e21e 1291=item defined EXPR
d74e8afc 1292X<defined> X<undef> X<undefined>
a0d0e21e 1293
54310121 1294=item defined
bbce6d69 1295
c17cdb72
NC
1296=for Pod::Functions test whether a value, variable, or function is defined
1297
2f9daede 1298Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1299the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1300checked.
1301
1302Many operations return C<undef> to indicate failure, end of file,
1303system error, uninitialized variable, and other exceptional
1304conditions. This function allows you to distinguish C<undef> from
1305other values. (A simple Boolean test will not distinguish among
7660c0ab 1306C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1307false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1308doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1309returns C<undef> when its argument is an empty array, I<or> when the
1310element to return happens to be C<undef>.
1311
f10b0346
GS
1312You may also use C<defined(&func)> to check whether subroutine C<&func>
1313has ever been defined. The return value is unaffected by any forward
80d38338 1314declarations of C<&func>. A subroutine that is not defined
847c7ebe 1315may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1316makes it spring into existence the first time that it is called; see
847c7ebe 1317L<perlsub>.
f10b0346
GS
1318
1319Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1320used to report whether memory for that aggregate had ever been
f10b0346
GS
1321allocated. This behavior may disappear in future versions of Perl.
1322You should instead use a simple test for size:
1323
1324 if (@an_array) { print "has array elements\n" }
1325 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1326
1327When used on a hash element, it tells you whether the value is defined,
dc848c6f 1328not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1329purpose.
a0d0e21e
LW
1330
1331Examples:
1332
8f1da26d 1333 print if defined $switch{D};
a0d0e21e
LW
1334 print "$val\n" while defined($val = pop(@ary));
1335 die "Can't readlink $sym: $!"
a9a5a0dc 1336 unless defined($value = readlink $sym);
a0d0e21e 1337 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1338 $debugging = 0 unless defined $debugging;
a0d0e21e 1339
8f1da26d 1340Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1341discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1342defined values. For example, if you say
a5f75d66
AD
1343
1344 "ab" =~ /a(.*)b/;
1345
80d38338 1346The pattern match succeeds and C<$1> is defined, although it
cf264981 1347matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1348matched something that happened to be zero characters long. This is all
a5f75d66 1349very above-board and honest. When a function returns an undefined value,
2f9daede 1350it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1351should use C<defined> only when questioning the integrity of what
7660c0ab 1352you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1353what you want.
1354
dc848c6f 1355See also L</undef>, L</exists>, L</ref>.
2f9daede 1356
a0d0e21e 1357=item delete EXPR
d74e8afc 1358X<delete>
a0d0e21e 1359
c17cdb72
NC
1360=for Pod::Functions deletes a value from a hash
1361
d0a76353
RS
1362Given an expression that specifies an element or slice of a hash, C<delete>
1363deletes the specified elements from that hash so that exists() on that element
1364no longer returns true. Setting a hash element to the undefined value does
1365not remove its key, but deleting it does; see L</exists>.
80d38338 1366
8f1da26d 1367In list context, returns the value or values deleted, or the last such
80d38338 1368element in scalar context. The return list's length always matches that of
d0a76353
RS
1369the argument list: deleting non-existent elements returns the undefined value
1370in their corresponding positions.
80d38338 1371
d0a76353
RS
1372delete() may also be used on arrays and array slices, but its behavior is less
1373straightforward. Although exists() will return false for deleted entries,
1374deleting array elements never changes indices of existing values; use shift()
deaf6ad3 1375or splice() for that. However, if any deleted elements fall at the end of an
d0a76353 1376array, the array's size shrinks to the position of the highest element that
deaf6ad3
AS
1377still tests true for exists(), or to 0 if none do. In other words, an
1378array won't have trailing nonexistent elements after a delete.
d0a76353 1379
8f1da26d 1380B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1381be removed in a future version of Perl.
80d38338
TC
1382
1383Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1384a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1385or array may not necessarily return anything; it depends on the implementation
1386of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1387
80d38338
TC
1388The C<delete local EXPR> construct localizes the deletion to the current
1389block at run time. Until the block exits, elements locally deleted
1390temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1391of composite types">.
eba0920a
EM
1392
1393 %hash = (foo => 11, bar => 22, baz => 33);
f7051f2c
FC
1394 $scalar = delete $hash{foo}; # $scalar is 11
1395 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1396 @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
eba0920a 1397
01020589 1398The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1399
5f05dabc 1400 foreach $key (keys %HASH) {
a9a5a0dc 1401 delete $HASH{$key};
a0d0e21e
LW
1402 }
1403
01020589 1404 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1405 delete $ARRAY[$index];
01020589
GS
1406 }
1407
1408And so do these:
5f05dabc 1409
01020589
GS
1410 delete @HASH{keys %HASH};
1411
9740c838 1412 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1413
80d38338
TC
1414But both are slower than assigning the empty list
1415or undefining %HASH or @ARRAY, which is the customary
1416way to empty out an aggregate:
01020589 1417
5ed4f2ec 1418 %HASH = (); # completely empty %HASH
1419 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1420
5ed4f2ec 1421 @ARRAY = (); # completely empty @ARRAY
1422 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1423
80d38338
TC
1424The EXPR can be arbitrarily complicated provided its
1425final operation is an element or slice of an aggregate:
a0d0e21e
LW
1426
1427 delete $ref->[$x][$y]{$key};
5f05dabc 1428 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1429
01020589
GS
1430 delete $ref->[$x][$y][$index];
1431 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1432
a0d0e21e 1433=item die LIST
d74e8afc 1434X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1435
c17cdb72
NC
1436=for Pod::Functions raise an exception or bail out
1437
391b733c 1438C<die> raises an exception. Inside an C<eval> the error message is stuffed
4c050ad5
NC
1439into C<$@> and the C<eval> is terminated with the undefined value.
1440If the exception is outside of all enclosing C<eval>s, then the uncaught
391b733c 1441exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1442need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1443
1444Equivalent examples:
1445
1446 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1447 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1448
ccac6780 1449If the last element of LIST does not end in a newline, the current
df37ec69
WW
1450script line number and input line number (if any) are also printed,
1451and a newline is supplied. Note that the "input line number" (also
1452known as "chunk") is subject to whatever notion of "line" happens to
1453be currently in effect, and is also available as the special variable
1454C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1455
1456Hint: sometimes appending C<", stopped"> to your message will cause it
1457to make better sense when the string C<"at foo line 123"> is appended.
1458Suppose you are running script "canasta".
a0d0e21e
LW
1459
1460 die "/etc/games is no good";
1461 die "/etc/games is no good, stopped";
1462
1463produce, respectively
1464
1465 /etc/games is no good at canasta line 123.
1466 /etc/games is no good, stopped at canasta line 123.
1467
a96d0188 1468If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1469previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1470This is useful for propagating exceptions:
1471
1472 eval { ... };
1473 die unless $@ =~ /Expected exception/;
1474
a96d0188 1475If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1476C<PROPAGATE> method, that method will be called with additional file
1477and line number parameters. The return value replaces the value in
34169887 1478C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1479were called.
1480
7660c0ab 1481If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1482
4c050ad5
NC
1483If an uncaught exception results in interpreter exit, the exit code is
1484determined from the values of C<$!> and C<$?> with this pseudocode:
1485
1486 exit $! if $!; # errno
1487 exit $? >> 8 if $? >> 8; # child exit status
1488 exit 255; # last resort
1489
1490The intent is to squeeze as much possible information about the likely cause
391b733c
FC
1491into the limited space of the system exit
1492code. However, as C<$!> is the value
4c050ad5
NC
1493of C's C<errno>, which can be set by any system call, this means that the value
1494of the exit code used by C<die> can be non-predictable, so should not be relied
1495upon, other than to be non-zero.
1496
80d38338
TC
1497You can also call C<die> with a reference argument, and if this is trapped
1498within an C<eval>, C<$@> contains that reference. This permits more
1499elaborate exception handling using objects that maintain arbitrary state
1500about the exception. Such a scheme is sometimes preferable to matching
1501particular string values of C<$@> with regular expressions. Because C<$@>
1502is a global variable and C<eval> may be used within object implementations,
1503be careful that analyzing the error object doesn't replace the reference in
1504the global variable. It's easiest to make a local copy of the reference
1505before any manipulations. Here's an example:
52531d10 1506
80d38338 1507 use Scalar::Util "blessed";
da279afe 1508
52531d10 1509 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7 1510 if (my $ev_err = $@) {
f7051f2c
FC
1511 if (blessed($ev_err)
1512 && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1513 # handle Some::Module::Exception
1514 }
1515 else {
1516 # handle all other possible exceptions
1517 }
1518 }
1519
3b10bc60 1520Because Perl stringifies uncaught exception messages before display,
80d38338 1521you'll probably want to overload stringification operations on
52531d10
GS
1522exception objects. See L<overload> for details about that.
1523
19799a22
GS
1524You can arrange for a callback to be run just before the C<die>
1525does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1526handler is called with the error text and can change the error
19799a22 1527message, if it sees fit, by calling C<die> again. See
96090e4f 1528L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1529L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1530to be run only right before your program was to exit, this is not
3b10bc60 1531currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1532even inside eval()ed blocks/strings! If one wants the hook to do
1533nothing in such situations, put
fb73857a 1534
5ed4f2ec 1535 die @_ if $^S;
fb73857a 1536
19799a22
GS
1537as the first line of the handler (see L<perlvar/$^S>). Because
1538this promotes strange action at a distance, this counterintuitive
b76cc8ba 1539behavior may be fixed in a future release.
774d564b 1540
4c050ad5
NC
1541See also exit(), warn(), and the Carp module.
1542
a0d0e21e 1543=item do BLOCK
d74e8afc 1544X<do> X<block>
a0d0e21e 1545
c17cdb72
NC
1546=for Pod::Functions turn a BLOCK into a TERM
1547
a0d0e21e 1548Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1549sequence of commands indicated by BLOCK. When modified by the C<while> or
1550C<until> loop modifier, executes the BLOCK once before testing the loop
391b733c 1551condition. (On other statements the loop modifiers test the conditional
6b275a1f 1552first.)
a0d0e21e 1553
4968c1e4 1554C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1555C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1556See L<perlsyn> for alternative strategies.
4968c1e4 1557
a0d0e21e 1558=item do EXPR
d74e8afc 1559X<do>
a0d0e21e
LW
1560
1561Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1562file as a Perl script.
a0d0e21e
LW
1563
1564 do 'stat.pl';
1565
c319391a 1566is largely like
a0d0e21e 1567
986b19de 1568 eval `cat stat.pl`;
a0d0e21e 1569
c319391a
AC
1570except that it's more concise, runs no external processes, keeps track of
1571the current
96090e4f
A
1572filename for error messages, searches the C<@INC> directories, and updates
1573C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1574these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1575cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1576same, however, in that it does reparse the file every time you call it,
1577so you probably don't want to do this inside a loop.
a0d0e21e 1578
8f1da26d 1579If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1580an error message in C<$@>. If C<do> cannot read the file, it returns undef
1581and sets C<$!> to the error. Always check C<$@> first, as compilation
1582could fail in a way that also sets C<$!>. If the file is successfully
1583compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1584
80d38338 1585Inclusion of library modules is better done with the
19799a22 1586C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1587and raise an exception if there's a problem.
a0d0e21e 1588
5a964f20
TC
1589You might like to use C<do> to read in a program configuration
1590file. Manual error checking can be done this way:
1591
b76cc8ba 1592 # read in config files: system first, then user
f86cebdf 1593 for $file ("/share/prog/defaults.rc",
b76cc8ba 1594 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1595 {
1596 unless ($return = do $file) {
1597 warn "couldn't parse $file: $@" if $@;
1598 warn "couldn't do $file: $!" unless defined $return;
1599 warn "couldn't run $file" unless $return;
1600 }
5a964f20
TC
1601 }
1602
a0d0e21e 1603=item dump LABEL
d74e8afc 1604X<dump> X<core> X<undump>
a0d0e21e 1605
8a7e748e
FC
1606=item dump EXPR
1607
1614b0e3
JD
1608=item dump
1609
c17cdb72
NC
1610=for Pod::Functions create an immediate core dump
1611
19799a22
GS
1612This function causes an immediate core dump. See also the B<-u>
1613command-line switch in L<perlrun>, which does the same thing.
1614Primarily this is so that you can use the B<undump> program (not
1615supplied) to turn your core dump into an executable binary after
1616having initialized all your variables at the beginning of the
1617program. When the new binary is executed it will begin by executing
1618a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1619Think of it as a goto with an intervening core dump and reincarnation.
8a7e748e
FC
1620If C<LABEL> is omitted, restarts the program from the top. The
1621C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1622computed at run time, being otherwise identical to C<dump LABEL>.
19799a22
GS
1623
1624B<WARNING>: Any files opened at the time of the dump will I<not>
1625be open any more when the program is reincarnated, with possible
80d38338 1626resulting confusion by Perl.
19799a22 1627
59f521f4 1628This function is now largely obsolete, mostly because it's very hard to
391b733c 1629convert a core file into an executable. That's why you should now invoke
59f521f4 1630it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1631typo.
19799a22 1632
2ba1f20a
FC
1633Unlike most named operators, this has the same precedence as assignment.
1634It is also exempt from the looks-like-a-function rule, so
1635C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1636C<dump>.
1637
ea9eb35a 1638Portability issues: L<perlport/dump>.
1639
532eee96 1640=item each HASH
d74e8afc 1641X<each> X<hash, iterator>
aa689395 1642
532eee96 1643=item each ARRAY
aeedbbed
NC
1644X<array, iterator>
1645
f5a93a43
TC
1646=item each EXPR
1647
c17cdb72
NC
1648=for Pod::Functions retrieve the next key/value pair from a hash
1649
bade7fbc
TC
1650When called on a hash in list context, returns a 2-element list
1651consisting of the key and value for the next element of a hash. In Perl
16525.12 and later only, it will also return the index and value for the next
1653element of an array so that you can iterate over it; older Perls consider
1654this a syntax error. When called in scalar context, returns only the key
1655(not the value) in a hash, or the index in an array.
2f9daede 1656
aeedbbed 1657Hash entries are returned in an apparently random order. The actual random
7bf59113 1658order is specific to a given hash; the exact same series of operations
7161e5c2 1659on two hashes may result in a different order for each hash. Any insertion
7bf59113
YO
1660into the hash may change the order, as will any deletion, with the exception
1661that the most recent key returned by C<each> or C<keys> may be deleted
7161e5c2 1662without changing the order. So long as a given hash is unmodified you may
7bf59113 1663rely on C<keys>, C<values> and C<each> to repeatedly return the same order
7161e5c2
FC
1664as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
1665details on why hash order is randomized. Aside from the guarantees
7bf59113
YO
1666provided here the exact details of Perl's hash algorithm and the hash
1667traversal order are subject to change in any release of Perl.
ab192400 1668
80d38338
TC
1669After C<each> has returned all entries from the hash or array, the next
1670call to C<each> returns the empty list in list context and C<undef> in
bade7fbc
TC
1671scalar context; the next call following I<that> one restarts iteration.
1672Each hash or array has its own internal iterator, accessed by C<each>,
1673C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1674reached the end as just described; it can be explicitly reset by calling
1675C<keys> or C<values> on the hash or array. If you add or delete a hash's
49daec89
DM
1676elements while iterating over it, the effect on the iterator is
1677unspecified; for example, entries may be skipped or duplicated--so don't
d8021140
PJ
1678do that. Exception: It is always safe to delete the item most recently
1679returned by C<each()>, so the following code works properly:
74fc8b5f
MJD
1680
1681 while (($key, $value) = each %hash) {
1682 print $key, "\n";
1683 delete $hash{$key}; # This is safe
1684 }
aa689395 1685
883f220b
TC
1686Tied hashes may have a different ordering behaviour to perl's hash
1687implementation.
1688
80d38338 1689This prints out your environment like the printenv(1) program,
3b10bc60 1690but in a different order:
a0d0e21e
LW
1691
1692 while (($key,$value) = each %ENV) {
a9a5a0dc 1693 print "$key=$value\n";
a0d0e21e
LW
1694 }
1695
f5a93a43
TC
1696Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1697reference to an unblessed hash or array. The argument will be dereferenced
1698automatically. This aspect of C<each> is considered highly experimental.
1699The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1700
1701 while (($key,$value) = each $hashref) { ... }
1702
e6a0db3e
FC
1703As of Perl 5.18 you can use a bare C<each> in a C<while> loop,
1704which will set C<$_> on every iteration.
1705
1706 while(each %ENV) {
1707 print "$_=$ENV{$_}\n";
1708 }
1709
bade7fbc
TC
1710To avoid confusing would-be users of your code who are running earlier
1711versions of Perl with mysterious syntax errors, put this sort of thing at
1712the top of your file to signal that your code will work I<only> on Perls of
1713a recent vintage:
1714
1715 use 5.012; # so keys/values/each work on arrays
1716 use 5.014; # so keys/values/each work on scalars (experimental)
e6a0db3e 1717 use 5.018; # so each assigns to $_ in a lone while test
bade7fbc 1718
8f1da26d 1719See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1720
1721=item eof FILEHANDLE
d74e8afc
ITB
1722X<eof>
1723X<end of file>
1724X<end-of-file>
a0d0e21e 1725
4633a7c4
LW
1726=item eof ()
1727
a0d0e21e
LW
1728=item eof
1729
c17cdb72
NC
1730=for Pod::Functions test a filehandle for its end
1731
8f1da26d 1732Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1733FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1734gives the real filehandle. (Note that this function actually
80d38338 1735reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1736interactive context.) Do not read from a terminal file (or call
19799a22 1737C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1738as terminals may lose the end-of-file condition if you do.
1739
820475bd 1740An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1741with empty parentheses is different. It refers to the pseudo file
820475bd 1742formed from the files listed on the command line and accessed via the
61eff3bc
JH
1743C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1744as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1745used will cause C<@ARGV> to be examined to determine if input is
67408cae 1746available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1747end-of-file will assume you are processing another C<@ARGV> list,
1748and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1749see L<perlop/"I/O Operators">.
820475bd 1750
61eff3bc 1751In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1752detect the end of each file, whereas C<eof()> will detect the end
1753of the very last file only. Examples:
a0d0e21e 1754
748a9306
LW
1755 # reset line numbering on each input file
1756 while (<>) {
a9a5a0dc
VP
1757 next if /^\s*#/; # skip comments
1758 print "$.\t$_";
5a964f20 1759 } continue {
a9a5a0dc 1760 close ARGV if eof; # Not eof()!
748a9306
LW
1761 }
1762
a0d0e21e
LW
1763 # insert dashes just before last line of last file
1764 while (<>) {
a9a5a0dc
VP
1765 if (eof()) { # check for end of last file
1766 print "--------------\n";
1767 }
1768 print;
f7051f2c 1769 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1770 }
1771
a0d0e21e 1772Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1773input operators typically return C<undef> when they run out of data or
1774encounter an error.
a0d0e21e
LW
1775
1776=item eval EXPR
d74e8afc 1777X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1778X<error, handling> X<exception, handling>
a0d0e21e
LW
1779
1780=item eval BLOCK
1781
ce2984c3
PF
1782=item eval
1783
c17cdb72
NC
1784=for Pod::Functions catch exceptions or compile and run code
1785
798dc914
KW
1786In the first form, often referred to as a "string eval", the return
1787value of EXPR is parsed and executed as if it
c7cc6f1c 1788were a little Perl program. The value of the expression (which is itself
8f1da26d 1789determined within scalar context) is first parsed, and if there were no
2341804c 1790errors, executed as a block within the lexical context of the current Perl
df4833a8 1791program. This means, that in particular, any outer lexical variables are
2341804c
DM
1792visible to it, and any package variable settings or subroutine and format
1793definitions remain afterwards.
1794
1795Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1796If EXPR is omitted, evaluates C<$_>. This form is typically used to
1797delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c 1798
7289c5e6
FC
1799If the C<unicode_eval> feature is enabled (which is the default under a
1800C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1801characters, so C<use utf8> declarations have no effect, and source filters
1802are forbidden. In the absence of the C<unicode_eval> feature, the string
1803will sometimes be treated as characters and sometimes as bytes, depending
1804on the internal encoding, and source filters activated within the C<eval>
1805exhibit the erratic, but historical, behaviour of affecting some outer file
1806scope that is still compiling. See also the L</evalbytes> keyword, which
1807always treats its input as a byte stream and works properly with source
1808filters, and the L<feature> pragma.
1809
798dc914
KW
1810Problems can arise if the string expands a scalar containing a floating
1811point number. That scalar can expand to letters, such as C<"NaN"> or
1812C<"Infinity">; or, within the scope of a C<use locale>, the decimal
1813point character may be something other than a dot (such as a comma).
1814None of these are likely to parse as you are likely expecting.
1815
c7cc6f1c 1816In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1817same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1818within the context of the current Perl program. This form is typically
1819used to trap exceptions more efficiently than the first (see below), while
1820also providing the benefit of checking the code within BLOCK at compile
1821time.
1822
1823The final semicolon, if any, may be omitted from the value of EXPR or within
1824the BLOCK.
1825
1826In both forms, the value returned is the value of the last expression
5a964f20 1827evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1828as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1829in void, scalar, or list context, depending on the context of the C<eval>
1830itself. See L</wantarray> for more on how the evaluation context can be
1831determined.
a0d0e21e 1832
19799a22 1833If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1834executed, C<eval> returns C<undef> in scalar context
774b80e8
FC
1835or an empty list in list context, and C<$@> is set to the error
1836message. (Prior to 5.16, a bug caused C<undef> to be returned
1837in list context for syntax errors, but not for runtime errors.)
1838If there was no error, C<$@> is set to the empty string. A
9cc672d4
FC
1839control flow operator like C<last> or C<goto> can bypass the setting of
1840C<$@>. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1841warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1842To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1843turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
44ecbbd8 1844See L</warn>, L<perlvar>, and L<warnings>.
a0d0e21e 1845
19799a22
GS
1846Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1847determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1848is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1849the die operator is used to raise exceptions.
1850
5f1da31c
NT
1851If you want to trap errors when loading an XS module, some problems with
1852the binary interface (such as Perl version skew) may be fatal even with
df4833a8 1853C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
5f1da31c 1854
a0d0e21e
LW
1855If the code to be executed doesn't vary, you may use the eval-BLOCK
1856form to trap run-time errors without incurring the penalty of
1857recompiling each time. The error, if any, is still returned in C<$@>.
1858Examples:
1859
54310121 1860 # make divide-by-zero nonfatal
a0d0e21e
LW
1861 eval { $answer = $a / $b; }; warn $@ if $@;
1862
1863 # same thing, but less efficient
1864 eval '$answer = $a / $b'; warn $@ if $@;
1865
1866 # a compile-time error
5ed4f2ec 1867 eval { $answer = }; # WRONG
a0d0e21e
LW
1868
1869 # a run-time error
5ed4f2ec 1870 eval '$answer ='; # sets $@
a0d0e21e 1871
cf264981
SP
1872Using the C<eval{}> form as an exception trap in libraries does have some
1873issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1874may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1875You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1876as this example shows:
774d564b 1877
80d38338 1878 # a private exception trap for divide-by-zero
f86cebdf
GS
1879 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1880 warn $@ if $@;
774d564b
PP
1881
1882This is especially significant, given that C<__DIE__> hooks can call
19799a22 1883C<die> again, which has the effect of changing their error messages:
774d564b
PP
1884
1885 # __DIE__ hooks may modify error messages
1886 {
f86cebdf
GS
1887 local $SIG{'__DIE__'} =
1888 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1889 eval { die "foo lives here" };
1890 print $@ if $@; # prints "bar lives here"
774d564b
PP
1891 }
1892
19799a22 1893Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1894may be fixed in a future release.
1895
19799a22 1896With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1897being looked at when:
1898
5ed4f2ec 1899 eval $x; # CASE 1
1900 eval "$x"; # CASE 2
a0d0e21e 1901
5ed4f2ec 1902 eval '$x'; # CASE 3
1903 eval { $x }; # CASE 4
a0d0e21e 1904
5ed4f2ec 1905 eval "\$$x++"; # CASE 5
1906 $$x++; # CASE 6
a0d0e21e 1907
2f9daede 1908Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1909the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1910the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1911and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1912does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1913purely visual reasons, but it also has the advantage of compiling at
1914compile-time instead of at run-time.) Case 5 is a place where
19799a22 1915normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1916particular situation, you can just use symbolic references instead, as
1917in case 6.
a0d0e21e 1918
b6538e4f 1919Before Perl 5.14, the assignment to C<$@> occurred before restoration
bade7fbc 1920of localized variables, which means that for your code to run on older
b208c909 1921versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1922errors:
1923
1924 # alter $@ on nefarious repugnancy only
1925 {
1926 my $e;
1927 {
f7051f2c
FC
1928 local $@; # protect existing $@
1929 eval { test_repugnancy() };
1930 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1931 $@ =~ /nefarious/ and $e = $@;
8a5a710d
DN
1932 }
1933 die $e if defined $e
1934 }
1935
4968c1e4 1936C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1937C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1938
4f00fc7e
FC
1939An C<eval ''> executed within a subroutine defined
1940in the C<DB> package doesn't see the usual
3b10bc60 1941surrounding lexical scope, but rather the scope of the first non-DB piece
df4833a8 1942of code that called it. You don't normally need to worry about this unless
3b10bc60 1943you are writing a Perl debugger.
d819b83a 1944
7289c5e6
FC
1945=item evalbytes EXPR
1946X<evalbytes>
1947
1948=item evalbytes
1949
d9b04284 1950=for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
c17cdb72 1951
7289c5e6
FC
1952This function is like L</eval> with a string argument, except it always
1953parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1954string containing characters whose ordinal value exceeds 255 results in an
1955error. Source filters activated within the evaluated code apply to the
1956code itself.
1957
1958This function is only available under the C<evalbytes> feature, a
1959C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1960L<feature> for more information.
1961
a0d0e21e 1962=item exec LIST
d74e8afc 1963X<exec> X<execute>
a0d0e21e 1964
8bf3b016
GS
1965=item exec PROGRAM LIST
1966
c17cdb72
NC
1967=for Pod::Functions abandon this program to run another
1968
3b10bc60 1969The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1970use C<system> instead of C<exec> if you want it to return. It fails and
1971returns false only if the command does not exist I<and> it is executed
fb73857a 1972directly instead of via your system's command shell (see below).
a0d0e21e 1973
19799a22 1974Since it's a common mistake to use C<exec> instead of C<system>, Perl
4642e50d
EB
1975warns you if C<exec> is called in void context and if there is a following
1976statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1977you always do that, right?). If you I<really> want to follow an C<exec>
1978with some other statement, you can use one of these styles to avoid the warning:
55d729e4 1979
5a964f20
TC
1980 exec ('foo') or print STDERR "couldn't exec foo: $!";
1981 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1982
667eac0c
RS
1983If there is more than one argument in LIST, this calls execvp(3) with the
1984arguments in LIST. If there is only one element in LIST, the argument is
1985checked for shell metacharacters, and if there are any, the entire
1986argument is passed to the system's command shell for parsing (this is
1987C</bin/sh -c> on Unix platforms, but varies on other platforms). If
1988there are no shell metacharacters in the argument, it is split into words
1989and passed directly to C<execvp>, which is more efficient. Examples:
a0d0e21e 1990
19799a22
GS
1991 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1992 exec "sort $outfile | uniq";
a0d0e21e
LW
1993
1994If you don't really want to execute the first argument, but want to lie
1995to the program you are executing about its own name, you can specify
1996the program you actually want to run as an "indirect object" (without a
94d4006a
TS
1997comma) in front of the LIST, as in C<exec PROGRAM LIST>. (This always
1998forces interpretation of the LIST as a multivalued list, even if there
1999is only a single scalar in the list.) Example:
a0d0e21e
LW
2000
2001 $shell = '/bin/csh';
5ed4f2ec 2002 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
2003
2004or, more directly,
2005
5ed4f2ec 2006 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 2007
3b10bc60 2008When the arguments get executed via the system shell, results are
2009subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
2010for details.
2011
19799a22
GS
2012Using an indirect object with C<exec> or C<system> is also more
2013secure. This usage (which also works fine with system()) forces
2014interpretation of the arguments as a multivalued list, even if the
2015list had just one argument. That way you're safe from the shell
2016expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
2017
2018 @args = ( "echo surprise" );
2019
2b5ab1e7 2020 exec @args; # subject to shell escapes
f86cebdf 2021 # if @args == 1
2b5ab1e7 2022 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
2023
2024The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
2025program, passing it C<"surprise"> an argument. The second version didn't;
2026it tried to run a program named I<"echo surprise">, didn't find it, and set
2027C<$?> to a non-zero value indicating failure.
5a964f20 2028
94d4006a
TS
2029On Windows, only the C<exec PROGRAM LIST> indirect object syntax will
2030reliably avoid using the shell; C<exec LIST>, even with more than one
2031element, will fall back to the shell if the first spawn fails.
2032
e9fa405d
BF
2033Perl attempts to flush all files opened for output before the exec,
2034but this may not be supported on some platforms (see L<perlport>).
2035To be safe, you may need to set C<$|> ($AUTOFLUSH in English) or
2036call the C<autoflush()> method of C<IO::Handle> on any open handles
2037to avoid lost output.
0f897271 2038
80d38338
TC
2039Note that C<exec> will not call your C<END> blocks, nor will it invoke
2040C<DESTROY> methods on your objects.
7660c0ab 2041
ea9eb35a 2042Portability issues: L<perlport/exec>.
2043
a0d0e21e 2044=item exists EXPR
d74e8afc 2045X<exists> X<autovivification>
a0d0e21e 2046
c17cdb72
NC
2047=for Pod::Functions test whether a hash key is present
2048
d0a76353
RS
2049Given an expression that specifies an element of a hash, returns true if the
2050specified element in the hash has ever been initialized, even if the
2051corresponding value is undefined.
a0d0e21e 2052
5ed4f2ec 2053 print "Exists\n" if exists $hash{$key};
2054 print "Defined\n" if defined $hash{$key};
01020589
GS
2055 print "True\n" if $hash{$key};
2056
d0a76353 2057exists may also be called on array elements, but its behavior is much less
8f1da26d 2058obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
2059that calling exists on array values is deprecated and likely to be removed in
2060a future version of Perl.
2061
5ed4f2ec 2062 print "Exists\n" if exists $array[$index];
2063 print "Defined\n" if defined $array[$index];
01020589 2064 print "True\n" if $array[$index];
a0d0e21e 2065
8f1da26d 2066A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
2067it exists, but the reverse doesn't necessarily hold true.
2068
afebc493
GS
2069Given an expression that specifies the name of a subroutine,
2070returns true if the specified subroutine has ever been declared, even
2071if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 2072does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
2073exist may still be callable: its package may have an C<AUTOLOAD>
2074method that makes it spring into existence the first time that it is
3b10bc60 2075called; see L<perlsub>.
afebc493 2076
5ed4f2ec 2077 print "Exists\n" if exists &subroutine;
2078 print "Defined\n" if defined &subroutine;
afebc493 2079
a0d0e21e 2080Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 2081operation is a hash or array key lookup or subroutine name:
a0d0e21e 2082
5ed4f2ec 2083 if (exists $ref->{A}->{B}->{$key}) { }
2084 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 2085
5ed4f2ec 2086 if (exists $ref->{A}->{B}->[$ix]) { }
2087 if (exists $hash{A}{B}[$ix]) { }
01020589 2088
afebc493
GS
2089 if (exists &{$ref->{A}{B}{$key}}) { }
2090
9590a7cd 2091Although the most deeply nested array or hash element will not spring into
3b10bc60 2092existence just because its existence was tested, any intervening ones will.
61eff3bc 2093Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 2094into existence due to the existence test for the $key element above.
3b10bc60 2095This happens anywhere the arrow operator is used, including even here:
5a964f20 2096
2b5ab1e7 2097 undef $ref;
5ed4f2ec 2098 if (exists $ref->{"Some key"}) { }
2099 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
2100
2101This surprising autovivification in what does not at first--or even
2102second--glance appear to be an lvalue context may be fixed in a future
5a964f20 2103release.
a0d0e21e 2104
afebc493
GS
2105Use of a subroutine call, rather than a subroutine name, as an argument
2106to exists() is an error.
2107
5ed4f2ec 2108 exists &sub; # OK
2109 exists &sub(); # Error
afebc493 2110
a0d0e21e 2111=item exit EXPR
d74e8afc 2112X<exit> X<terminate> X<abort>
a0d0e21e 2113
ce2984c3
PF
2114=item exit
2115
c17cdb72
NC
2116=for Pod::Functions terminate this program
2117
2b5ab1e7 2118Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
2119
2120 $ans = <STDIN>;
2121 exit 0 if $ans =~ /^[Xx]/;
2122
19799a22 2123See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
2124universally recognized values for EXPR are C<0> for success and C<1>
2125for error; other values are subject to interpretation depending on the
2126environment in which the Perl program is running. For example, exiting
212769 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2128the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 2129
19799a22
GS
2130Don't use C<exit> to abort a subroutine if there's any chance that
2131someone might want to trap whatever error happened. Use C<die> instead,
2132which can be trapped by an C<eval>.
28757baa 2133
19799a22 2134The exit() function does not always exit immediately. It calls any
2b5ab1e7 2135defined C<END> routines first, but these C<END> routines may not
19799a22 2136themselves abort the exit. Likewise any object destructors that need to
60275626 2137be called are called before the real exit. C<END> routines and destructors
391b733c 2138can change the exit status by modifying C<$?>. If this is a problem, you
fae6f8fa 2139can call C<POSIX::_exit($status)> to avoid END and destructor processing.
87275199 2140See L<perlmod> for details.
5a964f20 2141
ea9eb35a 2142Portability issues: L<perlport/exit>.
2143
a0d0e21e 2144=item exp EXPR
d74e8afc 2145X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 2146
54310121 2147=item exp
bbce6d69 2148
c17cdb72
NC
2149=for Pod::Functions raise I<e> to a power
2150
b76cc8ba 2151Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
2152If EXPR is omitted, gives C<exp($_)>.
2153
628253b8
BF
2154=item fc EXPR
2155X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2156
2157=item fc
2158
d9b04284 2159=for Pod::Functions +fc return casefolded version of a string
c17cdb72 2160
628253b8
BF
2161Returns the casefolded version of EXPR. This is the internal function
2162implementing the C<\F> escape in double-quoted strings.
2163
2164Casefolding is the process of mapping strings to a form where case
2165differences are erased; comparing two strings in their casefolded
2166form is effectively a way of asking if two strings are equal,
2167regardless of case.
2168
2169Roughly, if you ever found yourself writing this
2170
f6c6dcb6 2171 lc($this) eq lc($that) # Wrong!
628253b8 2172 # or
f6c6dcb6 2173 uc($this) eq uc($that) # Also wrong!
628253b8 2174 # or
f6c6dcb6 2175 $this =~ /^\Q$that\E\z/i # Right!
628253b8
BF
2176
2177Now you can write
2178
2179 fc($this) eq fc($that)
2180
2181And get the correct results.
2182
fc39a31f
KW
2183Perl only implements the full form of casefolding,
2184but you can access the simple folds using L<Unicode::UCD/casefold()> and
2185L<Unicode::UCD/prop_invmap()>.
628253b8
BF
2186For further information on casefolding, refer to
2187the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
21884.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2189available at L<http://www.unicode.org/versions/latest/>, as well as the
2190Case Charts available at L<http://www.unicode.org/charts/case/>.
2191
2192If EXPR is omitted, uses C<$_>.
2193
1ca267a5
KW
2194This function behaves the same way under various pragma, such as within
2195S<C<"use feature 'unicode_strings">>, as L</lc> does, with the single
2196exception of C<fc> of LATIN CAPITAL LETTER SHARP S (U+1E9E) within the
2197scope of S<C<use locale>>. The foldcase of this character would
2198normally be C<"ss">, but as explained in the L</lc> section, case
2199changes that cross the 255/256 boundary are problematic under locales,
2200and are hence prohibited. Therefore, this function under locale returns
2201instead the string C<"\x{17F}\x{17F}">, which is the LATIN SMALL LETTER
2202LONG S. Since that character itself folds to C<"s">, the string of two
2203of them together should be equivalent to a single U+1E9E when foldcased.
628253b8
BF
2204
2205While the Unicode Standard defines two additional forms of casefolding,
2206one for Turkic languages and one that never maps one character into multiple
2207characters, these are not provided by the Perl core; However, the CPAN module
2208C<Unicode::Casing> may be used to provide an implementation.
2209
2210This keyword is available only when the C<"fc"> feature is enabled,
7161e5c2 2211or when prefixed with C<CORE::>; See L<feature>. Alternately,
628253b8
BF
2212include a C<use v5.16> or later to the current scope.
2213
a0d0e21e 2214=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2215X<fcntl>
a0d0e21e 2216
c17cdb72
NC
2217=for Pod::Functions file control system call
2218
f86cebdf 2219Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
2220
2221 use Fcntl;
2222
0ade1984 2223first to get the correct constant definitions. Argument processing and
3b10bc60 2224value returned work just like C<ioctl> below.
a0d0e21e
LW
2225For example:
2226
2227 use Fcntl;
5a964f20 2228 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 2229 or die "can't fcntl F_GETFL: $!";
5a964f20 2230
554ad1fc 2231You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
2232Like C<ioctl>, it maps a C<0> return from the system call into
2233C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
2234in numeric context. It is also exempt from the normal B<-w> warnings
2235on improper numeric conversions.
5a964f20 2236
3b10bc60 2237Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
2238doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2239manpage to learn what functions are available on your system.
a0d0e21e 2240
be2f7487 2241Here's an example of setting a filehandle named C<REMOTE> to be
2242non-blocking at the system level. You'll have to negotiate C<$|>
2243on your own, though.
2244
2245 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2246
2247 $flags = fcntl(REMOTE, F_GETFL, 0)
2248 or die "Can't get flags for the socket: $!\n";
2249
2250 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2251 or die "Can't set flags for the socket: $!\n";
2252
ea9eb35a 2253Portability issues: L<perlport/fcntl>.
2254
cfa52385
FC
2255=item __FILE__
2256X<__FILE__>
2257
c17cdb72
NC
2258=for Pod::Functions the name of the current source file
2259
cfa52385
FC
2260A special token that returns the name of the file in which it occurs.
2261
a0d0e21e 2262=item fileno FILEHANDLE
d74e8afc 2263X<fileno>
a0d0e21e 2264
c17cdb72
NC
2265=for Pod::Functions return file descriptor from filehandle
2266
2b5ab1e7 2267Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
2268filehandle is not open. If there is no real file descriptor at the OS
2269level, as can happen with filehandles connected to memory objects via
2270C<open> with a reference for the third argument, -1 is returned.
2271
2272This is mainly useful for constructing
19799a22 2273bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
2274If FILEHANDLE is an expression, the value is taken as an indirect
2275filehandle, generally its name.
5a964f20 2276
b76cc8ba 2277You can use this to find out whether two handles refer to the
5a964f20
TC
2278same underlying descriptor:
2279
3231d257 2280 if (fileno(THIS) != -1 && fileno(THIS) == fileno(THAT)) {
a9a5a0dc 2281 print "THIS and THAT are dups\n";
3231d257 2282 } elsif (fileno(THIS) != -1 && fileno(THAT) != -1) {
555bd962
BG
2283 print "THIS and THAT have different " .
2284 "underlying file descriptors\n";
3231d257 2285 } else {
555bd962
BG
2286 print "At least one of THIS and THAT does " .
2287 "not have a real file descriptor\n";
b76cc8ba
NIS
2288 }
2289
a0d0e21e 2290=item flock FILEHANDLE,OPERATION
d74e8afc 2291X<flock> X<lock> X<locking>
a0d0e21e 2292
c17cdb72
NC
2293=for Pod::Functions lock an entire file with an advisory lock
2294
19799a22
GS
2295Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2296for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2297machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2298C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2299entire files only, not records.
2b5ab1e7
TC
2300
2301Two potentially non-obvious but traditional C<flock> semantics are
2302that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2303are B<merely advisory>. Such discretionary locks are more flexible, but
2304offer fewer guarantees. This means that programs that do not also use
2305C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2306your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2307for details. It's best to assume traditional behavior if you're writing
2308portable programs. (But if you're not, you should as always feel perfectly
2309free to write for your own system's idiosyncrasies (sometimes called
2310"features"). Slavish adherence to portability concerns shouldn't get
2311in the way of your getting your job done.)
a3cb178b 2312
8ebc5c01
PP
2313OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2314LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2315you can use the symbolic names if you import them from the L<Fcntl> module,
2316either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2317requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2318releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2319LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2320waiting for the lock; check the return status to see if you got it.
68dc0745 2321
2b5ab1e7
TC
2322To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2323before locking or unlocking it.
8ebc5c01 2324
f86cebdf 2325Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2326locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2327are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2328implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
2329differing semantics shouldn't bite too many people.
2330
becacb53
TM
2331Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2332be open with read intent to use LOCK_SH and requires that it be open
2333with write intent to use LOCK_EX.
2334
19799a22
GS
2335Note also that some versions of C<flock> cannot lock things over the
2336network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2337that. If you like you can force Perl to ignore your system's flock(2)
2338function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2339the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2340and build a new Perl.
4633a7c4
LW
2341
2342Here's a mailbox appender for BSD systems.
a0d0e21e 2343
f7051f2c
FC
2344 # import LOCK_* and SEEK_END constants
2345 use Fcntl qw(:flock SEEK_END);
a0d0e21e
LW
2346
2347 sub lock {
a9a5a0dc
VP
2348 my ($fh) = @_;
2349 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2350
a9a5a0dc
VP
2351 # and, in case someone appended while we were waiting...
2352 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2353 }
2354
2355 sub unlock {
a9a5a0dc
VP
2356 my ($fh) = @_;
2357 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2358 }
2359
b0169937 2360 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2361 or die "Can't open mailbox: $!";
a0d0e21e 2362
7ed5353d 2363 lock($mbox);
b0169937 2364 print $mbox $msg,"\n\n";
7ed5353d 2365 unlock($mbox);
a0d0e21e 2366
3b10bc60 2367On systems that support a real flock(2), locks are inherited across fork()
2368calls, whereas those that must resort to the more capricious fcntl(2)
2369function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2370
cb1a09d0 2371See also L<DB_File> for other flock() examples.
a0d0e21e 2372
ea9eb35a 2373Portability issues: L<perlport/flock>.
2374
a0d0e21e 2375=item fork
d74e8afc 2376X<fork> X<child> X<parent>
a0d0e21e 2377
c17cdb72
NC
2378=for Pod::Functions create a new process just like this one
2379
2b5ab1e7
TC
2380Does a fork(2) system call to create a new process running the
2381same program at the same point. It returns the child pid to the
2382parent process, C<0> to the child process, or C<undef> if the fork is
2383unsuccessful. File descriptors (and sometimes locks on those descriptors)
2384are shared, while everything else is copied. On most systems supporting
2385fork(), great care has gone into making it extremely efficient (for
2386example, using copy-on-write technology on data pages), making it the
2387dominant paradigm for multitasking over the last few decades.
5a964f20 2388
e9fa405d 2389Perl attempts to flush all files opened for
0f897271
GS
2390output before forking the child process, but this may not be supported
2391on some platforms (see L<perlport>). To be safe, you may need to set
2392C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2393C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2394
19799a22 2395If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2396accumulate zombies. On some systems, you can avoid this by setting
2397C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2398forking and reaping moribund children.
cb1a09d0 2399
28757baa
PP
2400Note that if your forked child inherits system file descriptors like
2401STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2402if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2403backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2404You should reopen those to F</dev/null> if it's any issue.
28757baa 2405
ea9eb35a 2406On some platforms such as Windows, where the fork() system call is not available,
391b733c
FC
2407Perl can be built to emulate fork() in the Perl interpreter.
2408The emulation is designed, at the level of the Perl program,
2409to be as compatible as possible with the "Unix" fork().
6d17f725 2410However it has limitations that have to be considered in code intended to be portable.
ea9eb35a 2411See L<perlfork> for more details.
2412
2413Portability issues: L<perlport/fork>.
2414
cb1a09d0 2415=item format
d74e8afc 2416X<format>
cb1a09d0 2417
c17cdb72
NC
2418=for Pod::Functions declare a picture format with use by the write() function
2419
19799a22 2420Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2421example:
2422
54310121 2423 format Something =
a9a5a0dc
VP
2424 Test: @<<<<<<<< @||||| @>>>>>
2425 $str, $%, '$' . int($num)
cb1a09d0
AD
2426 .
2427
2428 $str = "widget";
184e9718 2429 $num = $cost/$quantity;
cb1a09d0
AD
2430 $~ = 'Something';
2431 write;
2432
2433See L<perlform> for many details and examples.
2434
8903cb82 2435=item formline PICTURE,LIST
d74e8afc 2436X<formline>
a0d0e21e 2437
c17cdb72
NC
2438=for Pod::Functions internal function used for formats
2439
5a964f20 2440This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2441too. It formats (see L<perlform>) a list of values according to the
2442contents of PICTURE, placing the output into the format output
7660c0ab 2443accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2444Eventually, when a C<write> is done, the contents of
cf264981
SP
2445C<$^A> are written to some filehandle. You could also read C<$^A>
2446and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2447does one C<formline> per line of form, but the C<formline> function itself
748a9306 2448doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2449that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2450You may therefore need to use multiple formlines to implement a single
3b10bc60 2451record format, just like the C<format> compiler.
748a9306 2452
19799a22 2453Be careful if you put double quotes around the picture, because an C<@>
748a9306 2454character may be taken to mean the beginning of an array name.
19799a22 2455C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2456
445b09e5
FC
2457If you are trying to use this instead of C<write> to capture the output,
2458you may find it easier to open a filehandle to a scalar
2459(C<< open $fh, ">", \$output >>) and write to that instead.
2460
a0d0e21e 2461=item getc FILEHANDLE
f723aae1 2462X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2463
2464=item getc
2465
c17cdb72
NC
2466=for Pod::Functions get the next character from the filehandle
2467
a0d0e21e 2468Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2469or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2470the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2471STDIN. This is not particularly efficient. However, it cannot be
2472used by itself to fetch single characters without waiting for the user
2473to hit enter. For that, try something more like:
4633a7c4
LW
2474
2475 if ($BSD_STYLE) {
a9a5a0dc 2476 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2477 }
2478 else {
a9a5a0dc 2479 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2480 }
2481
2482 $key = getc(STDIN);
2483
2484 if ($BSD_STYLE) {
a9a5a0dc 2485 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2486 }
2487 else {
3b10bc60 2488 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2489 }
2490 print "\n";
2491
54310121
PP
2492Determination of whether $BSD_STYLE should be set
2493is left as an exercise to the reader.
cb1a09d0 2494
19799a22 2495The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2496systems purporting POSIX compliance. See also the C<Term::ReadKey>
3d6c5fec 2497module from your nearest L<CPAN|http://www.cpan.org> site.
a0d0e21e
LW
2498
2499=item getlogin
d74e8afc 2500X<getlogin> X<login>
a0d0e21e 2501
c17cdb72
NC
2502=for Pod::Functions return who logged in at this tty
2503
cf264981 2504This implements the C library function of the same name, which on most
3b10bc60 2505systems returns the current login from F</etc/utmp>, if any. If it
2506returns the empty string, use C<getpwuid>.
a0d0e21e 2507
f86702cc 2508 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2509
19799a22
GS
2510Do not consider C<getlogin> for authentication: it is not as
2511secure as C<getpwuid>.
4633a7c4 2512
ea9eb35a 2513Portability issues: L<perlport/getlogin>.
2514
a0d0e21e 2515=item getpeername SOCKET
d74e8afc 2516X<getpeername> X<peer>
a0d0e21e 2517
c17cdb72
NC
2518=for Pod::Functions find the other end of a socket connection
2519
a3390c9f
FC
2520Returns the packed sockaddr address of the other end of the SOCKET
2521connection.
a0d0e21e 2522
4633a7c4
LW
2523 use Socket;
2524 $hersockaddr = getpeername(SOCK);
19799a22 2525 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2526 $herhostname = gethostbyaddr($iaddr, AF_INET);
2527 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2528
2529=item getpgrp PID
d74e8afc 2530X<getpgrp> X<group>
a0d0e21e 2531
c17cdb72
NC
2532=for Pod::Functions get process group
2533
47e29363 2534Returns the current process group for the specified PID. Use
7660c0ab 2535a PID of C<0> to get the current process group for the
4633a7c4 2536current process. Will raise an exception if used on a machine that
a3390c9f
FC
2537doesn't implement getpgrp(2). If PID is omitted, returns the process
2538group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2539does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2540
ea9eb35a 2541Portability issues: L<perlport/getpgrp>.
2542
a0d0e21e 2543=item getppid
d74e8afc 2544X<getppid> X<parent> X<pid>
a0d0e21e 2545
c17cdb72
NC
2546=for Pod::Functions get parent process ID
2547
a0d0e21e
LW
2548Returns the process id of the parent process.
2549
d7c042c9
AB
2550Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2551around non-POSIX thread semantics the minority of Linux systems (and
2552Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
7161e5c2 2553has since been removed. See the documentation for L<$$|perlvar/$$> for
d7c042c9 2554details.
4d76a344 2555
ea9eb35a 2556Portability issues: L<perlport/getppid>.
2557
a0d0e21e 2558=item getpriority WHICH,WHO
d74e8afc 2559X<getpriority> X<priority> X<nice>
a0d0e21e 2560
c17cdb72
NC
2561=for Pod::Functions get current nice value
2562
4633a7c4 2563Returns the current priority for a process, a process group, or a user.
01aa884e 2564(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2565machine that doesn't implement getpriority(2).
a0d0e21e 2566
ea9eb35a 2567Portability issues: L<perlport/getpriority>.
2568
a0d0e21e 2569=item getpwnam NAME
d74e8afc
ITB
2570X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2571X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2572X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2573X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2574X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2575X<endnetent> X<endprotoent> X<endservent>
a0d0e21e 2576
c17cdb72
NC
2577=for Pod::Functions get passwd record given user login name
2578
a0d0e21e
LW
2579=item getgrnam NAME
2580
c17cdb72
NC
2581=for Pod::Functions get group record given group name
2582
a0d0e21e
LW
2583=item gethostbyname NAME
2584
c17cdb72
NC
2585=for Pod::Functions get host record given name
2586
a0d0e21e
LW
2587=item getnetbyname NAME
2588
c17cdb72
NC
2589=for Pod::Functions get networks record given name
2590
a0d0e21e
LW
2591=item getprotobyname NAME
2592
c17cdb72
NC
2593=for Pod::Functions get protocol record given name
2594
a0d0e21e
LW
2595=item getpwuid UID
2596
c17cdb72
NC
2597=for Pod::Functions get passwd record given user ID
2598
a0d0e21e
LW
2599=item getgrgid GID
2600
c17cdb72
NC
2601=for Pod::Functions get group record given group user ID
2602
a0d0e21e
LW
2603=item getservbyname NAME,PROTO
2604
c17cdb72
NC
2605=for Pod::Functions get services record given its name
2606
a0d0e21e
LW
2607=item gethostbyaddr ADDR,ADDRTYPE
2608
c17cdb72
NC
2609=for Pod::Functions get host record given its address
2610
a0d0e21e
LW
2611=item getnetbyaddr ADDR,ADDRTYPE
2612
c17cdb72
NC
2613=for Pod::Functions get network record given its address
2614
a0d0e21e
LW
2615=item getprotobynumber NUMBER
2616
c17cdb72
NC
2617=for Pod::Functions get protocol record numeric protocol
2618
a0d0e21e
LW
2619=item getservbyport PORT,PROTO
2620
c17cdb72
NC
2621=for Pod::Functions get services record given numeric port
2622
a0d0e21e
LW
2623=item getpwent
2624
c17cdb72
NC
2625=for Pod::Functions get next passwd record
2626
a0d0e21e
LW
2627=item getgrent
2628
c17cdb72
NC
2629=for Pod::Functions get next group record
2630
a0d0e21e
LW
2631=item gethostent
2632
c17cdb72
NC
2633=for Pod::Functions get next hosts record
2634
a0d0e21e
LW
2635=item getnetent
2636
c17cdb72
NC
2637=for Pod::Functions get next networks record
2638
a0d0e21e
LW
2639=item getprotoent
2640
c17cdb72
NC
2641=for Pod::Functions get next protocols record
2642
a0d0e21e
LW
2643=item getservent
2644
c17cdb72
NC
2645=for Pod::Functions get next services record
2646
a0d0e21e
LW
2647=item setpwent
2648
c17cdb72
NC
2649=for Pod::Functions prepare passwd file for use
2650
a0d0e21e
LW
2651=item setgrent
2652
c17cdb72
NC
2653=for Pod::Functions prepare group file for use
2654
a0d0e21e
LW
2655=item sethostent STAYOPEN
2656
c17cdb72
NC
2657=for Pod::Functions prepare hosts file for use
2658
a0d0e21e
LW
2659=item setnetent STAYOPEN
2660
c17cdb72
NC
2661=for Pod::Functions prepare networks file for use
2662
a0d0e21e
LW
2663=item setprotoent STAYOPEN
2664
c17cdb72
NC
2665=for Pod::Functions prepare protocols file for use
2666
a0d0e21e
LW
2667=item setservent STAYOPEN
2668
c17cdb72
NC
2669=for Pod::Functions prepare services file for use
2670
a0d0e21e
LW
2671=item endpwent
2672
c17cdb72
NC
2673=for Pod::Functions be done using passwd file
2674
a0d0e21e
LW
2675=item endgrent
2676
c17cdb72
NC
2677=for Pod::Functions be done using group file
2678
a0d0e21e
LW
2679=item endhostent
2680
c17cdb72
NC
2681=for Pod::Functions be done using hosts file
2682
a0d0e21e
LW
2683=item endnetent
2684
c17cdb72
NC
2685=for Pod::Functions be done using networks file
2686
a0d0e21e
LW
2687=item endprotoent
2688
c17cdb72
NC
2689=for Pod::Functions be done using protocols file
2690
a0d0e21e
LW
2691=item endservent
2692
c17cdb72
NC
2693=for Pod::Functions be done using services file
2694
80d38338
TC
2695These routines are the same as their counterparts in the
2696system C library. In list context, the return values from the
a0d0e21e
LW
2697various get routines are as follows:
2698
f0081f64
AP
2699 # 0 1 2 3 4
2700 ( $name, $passwd, $gid, $members ) = getgr*
2701 ( $name, $aliases, $addrtype, $net ) = getnet*
2702 ( $name, $aliases, $port, $proto ) = getserv*
2703 ( $name, $aliases, $proto ) = getproto*
2704 ( $name, $aliases, $addrtype, $length, @addrs ) = gethost*
2705 ( $name, $passwd, $uid, $gid, $quota,
2706 $comment, $gcos, $dir, $shell, $expire ) = getpw*
2707 # 5 6 7 8 9
a0d0e21e 2708
3b10bc60 2709(If the entry doesn't exist you get an empty list.)
a0d0e21e 2710
4602f195
JH
2711The exact meaning of the $gcos field varies but it usually contains
2712the real name of the user (as opposed to the login name) and other
2713information pertaining to the user. Beware, however, that in many
2714system users are able to change this information and therefore it
106325ad 2715cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2716L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2717login shell, are also tainted, for the same reason.
4602f195 2718
5a964f20 2719In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2720lookup by name, in which case you get the other thing, whatever it is.
2721(If the entry doesn't exist you get the undefined value.) For example:
2722
5a964f20
TC
2723 $uid = getpwnam($name);
2724 $name = getpwuid($num);
2725 $name = getpwent();
2726 $gid = getgrnam($name);
08a33e13 2727 $name = getgrgid($num);
5a964f20
TC
2728 $name = getgrent();
2729 #etc.
a0d0e21e 2730
4602f195 2731In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2732in that they are unsupported on many systems. If the
4602f195
JH
2733$quota is unsupported, it is an empty scalar. If it is supported, it
2734usually encodes the disk quota. If the $comment field is unsupported,
2735it is an empty scalar. If it is supported it usually encodes some
2736administrative comment about the user. In some systems the $quota
2737field may be $change or $age, fields that have to do with password
2738aging. In some systems the $comment field may be $class. The $expire
2739field, if present, encodes the expiration period of the account or the
2740password. For the availability and the exact meaning of these fields
8f1da26d 2741in your system, please consult getpwnam(3) and your system's
4602f195
JH
2742F<pwd.h> file. You can also find out from within Perl what your
2743$quota and $comment fields mean and whether you have the $expire field
2744by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2745C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2746files are supported only if your vendor has implemented them in the
4602f195 2747intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2748shadow versions if you're running under privilege or if there exists
cf264981 2749the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2750and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2751facility are unlikely to be supported.
6ee623d5 2752
a3390c9f 2753The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2754the login names of the members of the group.
2755
2756For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2757C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2758C<@addrs> value returned by a successful call is a list of raw
2759addresses returned by the corresponding library call. In the
2760Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2761by saying something like:
2762
f337b084 2763 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2764
2b5ab1e7
TC
2765The Socket library makes this slightly easier:
2766
2767 use Socket;
2768 $iaddr = inet_aton("127.1"); # or whatever address
2769 $name = gethostbyaddr($iaddr, AF_INET);
2770
2771 # or going the other way
19799a22 2772 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2773
d760c846
GS
2774In the opposite way, to resolve a hostname to the IP address
2775you can write this:
2776
2777 use Socket;
2778 $packed_ip = gethostbyname("www.perl.org");
2779 if (defined $packed_ip) {
2780 $ip_address = inet_ntoa($packed_ip);
2781 }
2782
b018eaf1 2783Make sure C<gethostbyname()> is called in SCALAR context and that
d760c846
GS
2784its return value is checked for definedness.
2785
0d043efa
FC
2786The C<getprotobynumber> function, even though it only takes one argument,
2787has the precedence of a list operator, so beware:
2788
2789 getprotobynumber $number eq 'icmp' # WRONG
2790 getprotobynumber($number eq 'icmp') # actually means this
2791 getprotobynumber($number) eq 'icmp' # better this way
2792
19799a22
GS
2793If you get tired of remembering which element of the return list
2794contains which return value, by-name interfaces are provided
2795in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2796C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2797and C<User::grent>. These override the normal built-ins, supplying
2798versions that return objects with the appropriate names
2799for each field. For example:
5a964f20
TC
2800
2801 use File::stat;
2802 use User::pwent;
2803 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2804
a3390c9f 2805Even though it looks as though they're the same method calls (uid),
b76cc8ba 2806they aren't, because a C<File::stat> object is different from
19799a22 2807a C<User::pwent> object.
5a964f20 2808
ea9eb35a 2809Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2810
a0d0e21e 2811=item getsockname SOCKET
d74e8afc 2812X<getsockname>
a0d0e21e 2813
c17cdb72
NC
2814=for Pod::Functions retrieve the sockaddr for a given socket
2815
19799a22
GS
2816Returns the packed sockaddr address of this end of the SOCKET connection,
2817in case you don't know the address because you have several different
2818IPs that the connection might have come in on.
a0d0e21e 2819
4633a7c4
LW
2820 use Socket;
2821 $mysockaddr = getsockname(SOCK);
19799a22 2822 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2823 printf "Connect to %s [%s]\n",
19799a22
GS
2824 scalar gethostbyaddr($myaddr, AF_INET),
2825 inet_ntoa($myaddr);
a0d0e21e
LW
2826
2827=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2828X<getsockopt>
a0d0e21e 2829
c17cdb72
NC
2830=for Pod::Functions get socket options on a given socket
2831
636e6b1f
TH
2832Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2833Options may exist at multiple protocol levels depending on the socket
2834type, but at least the uppermost socket level SOL_SOCKET (defined in the
391b733c 2835C<Socket> module) will exist. To query options at another level the
636e6b1f 2836protocol number of the appropriate protocol controlling the option
391b733c 2837should be supplied. For example, to indicate that an option is to be
636e6b1f 2838interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2839number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2840
80d38338 2841The function returns a packed string representing the requested socket
3b10bc60 2842option, or C<undef> on error, with the reason for the error placed in
391b733c 2843C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2844consult getsockopt(2) for details. A common case is that the option is an
2845integer, in which case the result is a packed integer, which you can decode
2846using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2847
8f1da26d 2848Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2849
4852725b 2850 use Socket qw(:all);
636e6b1f
TH
2851
2852 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2853 or die "Could not determine the protocol number for tcp";
4852725b
DD
2854 # my $tcp = IPPROTO_TCP; # Alternative
2855 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2856 or die "getsockopt TCP_NODELAY: $!";
636e6b1f 2857 my $nodelay = unpack("I", $packed);
f7051f2c
FC
2858 print "Nagle's algorithm is turned ",
2859 $nodelay ? "off\n" : "on\n";
636e6b1f 2860
ea9eb35a 2861Portability issues: L<perlport/getsockopt>.
a0d0e21e
LW
2862
2863=item glob EXPR
d74e8afc 2864X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2865
0a753a76
PP
2866=item glob
2867
c17cdb72
NC
2868=for Pod::Functions expand filenames using wildcards
2869
d9a9d457 2870In list context, returns a (possibly empty) list of filename expansions on
391b733c 2871the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
d9a9d457 2872scalar context, glob iterates through such filename expansions, returning
391b733c
FC
2873undef when the list is exhausted. This is the internal function
2874implementing the C<< <*.c> >> operator, but you can use it directly. If
d9a9d457
JL
2875EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2876more detail in L<perlop/"I/O Operators">.
a0d0e21e 2877
80d38338
TC
2878Note that C<glob> splits its arguments on whitespace and treats
2879each segment as separate pattern. As such, C<glob("*.c *.h")>
2880matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2881C<glob(".* *")> matches all files in the current working directory.
a91bb7b1
TC
2882If you want to glob filenames that might contain whitespace, you'll
2883have to use extra quotes around the spacey filename to protect it.
2884For example, to glob filenames that have an C<e> followed by a space
2885followed by an C<f>, use either of:
2886
2887 @spacies = <"*e f*">;
2888 @spacies = glob '"*e f*"';
2889 @spacies = glob q("*e f*");
2890
2891If you had to get a variable through, you could do this:
2892
2893 @spacies = glob "'*${var}e f*'";
2894 @spacies = glob qq("*${var}e f*");
80d38338
TC
2895
2896If non-empty braces are the only wildcard characters used in the
2897C<glob>, no filenames are matched, but potentially many strings
2898are returned. For example, this produces nine strings, one for
2899each pairing of fruits and colors:
2900
2901 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2902
e9fa405d 2903This operator is implemented using the standard
5c0c9249
PF
2904C<File::Glob> extension. See L<File::Glob> for details, including
2905C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2906
ea9eb35a 2907Portability issues: L<perlport/glob>.
2908
a0d0e21e 2909=item gmtime EXPR
d74e8afc 2910X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2911
ce2984c3
PF
2912=item gmtime
2913
c17cdb72
NC
2914=for Pod::Functions convert UNIX time into record or string using Greenwich time
2915
4509d391 2916Works just like L</localtime> but the returned values are
435fbc73 2917localized for the standard Greenwich time zone.
a0d0e21e 2918
a3390c9f
FC
2919Note: When called in list context, $isdst, the last value
2920returned by gmtime, is always C<0>. There is no
435fbc73 2921Daylight Saving Time in GMT.
0a753a76 2922
ea9eb35a 2923Portability issues: L<perlport/gmtime>.
62aa5637 2924
a0d0e21e 2925=item goto LABEL
d74e8afc 2926X<goto> X<jump> X<jmp>
a0d0e21e 2927
748a9306
LW
2928=item goto EXPR
2929
a0d0e21e
LW
2930=item goto &NAME
2931
c17cdb72
NC
2932=for Pod::Functions create spaghetti code
2933
5a5b79a3 2934The C<goto LABEL> form finds the statement labeled with LABEL and
391b733c 2935resumes execution there. It can't be used to get out of a block or
b500e03b
GG
2936subroutine given to C<sort>. It can be used to go almost anywhere
2937else within the dynamic scope, including out of subroutines, but it's
2938usually better to use some other construct such as C<last> or C<die>.
2939The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2940(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2941does not offer named loops combined with loop control. Perl does, and
2942this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2943
5a5b79a3 2944The C<goto EXPR> form expects to evaluate C<EXPR> to a code reference or
3e8a6370 2945a label name. If it evaluates to a code reference, it will be handled
5a5b79a3 2946like C<goto &NAME>, below. This is especially useful for implementing
3e8a6370
RS
2947tail recursion via C<goto __SUB__>.
2948
2949If the expression evaluates to a label name, its scope will be resolved
7660c0ab 2950dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2951necessarily recommended if you're optimizing for maintainability:
2952
2953 goto ("FOO", "BAR", "GLARCH")[$i];
2954
5a5b79a3 2955As shown in this example, C<goto EXPR> is exempt from the "looks like a
391b733c
FC
2956function" rule. A pair of parentheses following it does not (necessarily)
2957delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
8a7e748e
FC
2958Also, unlike most named operators, this has the same precedence as
2959assignment.
887d89fd 2960
5a5b79a3 2961Use of C<goto LABEL> or C<goto EXPR> to jump into a construct is
0b98bec9 2962deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2963go into any construct that requires initialization, such as a
2964subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2965construct that is optimized away.
b500e03b 2966
5a5b79a3 2967The C<goto &NAME> form is quite different from the other forms of
1b6921cb
BT
2968C<goto>. In fact, it isn't a goto in the normal sense at all, and
2969doesn't have the stigma associated with other gotos. Instead, it
2970exits the current subroutine (losing any changes set by local()) and
2971immediately calls in its place the named subroutine using the current
2972value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2973load another subroutine and then pretend that the other subroutine had
2974been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2975in the current subroutine are propagated to the other subroutine.)
2976After the C<goto>, not even C<caller> will be able to tell that this
2977routine was called first.
2978
2979NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2980containing a code reference or a block that evaluates to a code
6cb9131c 2981reference.
a0d0e21e
LW
2982
2983=item grep BLOCK LIST
d74e8afc 2984X<grep>
a0d0e21e
LW
2985
2986=item grep EXPR,LIST
2987
c17cdb72
NC
2988=for Pod::Functions locate elements in a list test true against a given criterion
2989
2b5ab1e7
TC
2990This is similar in spirit to, but not the same as, grep(1) and its
2991relatives. In particular, it is not limited to using regular expressions.
2f9daede 2992
a0d0e21e 2993Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2994C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2995elements for which the expression evaluated to true. In scalar
2996context, returns the number of times the expression was true.
a0d0e21e
LW
2997
2998 @foo = grep(!/^#/, @bar); # weed out comments
2999
3000or equivalently,
3001
3002 @foo = grep {!/^#/} @bar; # weed out comments
3003
be3174d2
GS
3004Note that C<$_> is an alias to the list value, so it can be used to
3005modify the elements of the LIST. While this is useful and supported,
3006it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
3007Similarly, grep returns aliases into the original list, much as a for
3008loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
3009element of a list returned by grep (for example, in a C<foreach>, C<map>
3010or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 3011This is usually something to be avoided when writing clear code.
a0d0e21e 3012
a4fb8298 3013If C<$_> is lexical in the scope where the C<grep> appears (because it has
c071e214
FC
3014been declared with the deprecated C<my $_> construct)
3015then, in addition to being locally aliased to
80d38338 3016the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
3017can't be seen from the outside, avoiding any potential side-effects.
3018
19799a22 3019See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 3020
a0d0e21e 3021=item hex EXPR
d74e8afc 3022X<hex> X<hexadecimal>
a0d0e21e 3023
54310121 3024=item hex
bbce6d69 3025
c17cdb72
NC
3026=for Pod::Functions convert a string to a hexadecimal number
3027
2b5ab1e7 3028Interprets EXPR as a hex string and returns the corresponding value.
38366c11 3029(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 3030L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
3031
3032 print hex '0xAf'; # prints '175'
3033 print hex 'aF'; # same
a0d0e21e 3034
19799a22 3035Hex strings may only represent integers. Strings that would cause
53305cf1 3036integer overflow trigger a warning. Leading whitespace is not stripped,
391b733c 3037unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 3038L</sprintf>, and L</unpack>.
19799a22 3039
ce2984c3 3040=item import LIST
d74e8afc 3041X<import>
a0d0e21e 3042
c17cdb72
NC
3043=for Pod::Functions patch a module's namespace into your own
3044
19799a22 3045There is no builtin C<import> function. It is just an ordinary
4633a7c4 3046method (subroutine) defined (or inherited) by modules that wish to export
19799a22 3047names to another module. The C<use> function calls the C<import> method
cea6626f 3048