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[perl #97464] Document actual case when @DB::args is set
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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
22fae026 110C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
1dc8ecb8 111C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
945c54fd 112C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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113
114=item Regular expressions and pattern matching
d74e8afc 115X<regular expression> X<regex> X<regexp>
cb1a09d0 116
ab4f32c2 117C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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118
119=item Numeric functions
d74e8afc 120X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 121
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122C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
123C<sin>, C<sqrt>, C<srand>
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124
125=item Functions for real @ARRAYs
d74e8afc 126X<array>
cb1a09d0 127
a5ce339c 128C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
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129
130=item Functions for list data
d74e8afc 131X<list>
cb1a09d0 132
1dc8ecb8 133C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
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134
135=item Functions for real %HASHes
d74e8afc 136X<hash>
cb1a09d0 137
22fae026 138C<delete>, C<each>, C<exists>, C<keys>, C<values>
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139
140=item Input and output functions
d74e8afc 141X<I/O> X<input> X<output> X<dbm>
cb1a09d0 142
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143C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
144C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
0d863452 145C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
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146C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
147C<warn>, C<write>
cb1a09d0 148
5dac7880 149=item Functions for fixed-length data or records
cb1a09d0 150
22fae026 151C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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152
153=item Functions for filehandles, files, or directories
d74e8afc 154X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 155
22fae026 156C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 157C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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158C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
159C<umask>, C<unlink>, C<utime>
cb1a09d0 160
cf264981 161=item Keywords related to the control flow of your Perl program
d74e8afc 162X<control flow>
cb1a09d0 163
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164C<caller>, C<continue>, C<die>, C<do>,
165C<dump>, C<eval>, C<evalbytes> C<exit>,
cfa52385 166C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
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167C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray>
168
169C<__SUB__> is only available with a C<use v5.16> (or higher) declaration or
170with the C<"current_sub"> feature (see L<feature>).
cb1a09d0 171
8f1da26d 172=item Keywords related to the switch feature
0d863452 173
4a904372 174C<break>, C<continue>, C<default>, C<given>, C<when>
0d863452 175
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176Except for C<continue>, these are available only if you enable the
177C<"switch"> feature or use the C<CORE::> prefix.
8f1da26d 178See L<feature> and L<perlsyn/"Switch statements">.
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179Alternately, include a C<use v5.10> or later to the current scope. In Perl
1805.14 and earlier, C<continue> required the C<"switch"> feature, like the
181other keywords.
0d863452 182
54310121 183=item Keywords related to scoping
cb1a09d0 184
8f1da26d 185C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
36fb85f3 186
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187C<state> is available only if the C<"state"> feature
188is enabled or if it is prefixed with C<CORE::>. See
8f1da26d 189L<feature>. Alternately, include a C<use v5.10> or later to the current scope.
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190
191=item Miscellaneous functions
192
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193C<defined>, C<dump>, C<eval>, C<evalbytes>,
194C<formline>, C<local>, C<my>, C<our>,
834df1c5 195C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
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196
197=item Functions for processes and process groups
d74e8afc 198X<process> X<pid> X<process id>
cb1a09d0 199
22fae026 200C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
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201C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>,
202C<setpriority>, C<sleep>, C<system>,
22fae026 203C<times>, C<wait>, C<waitpid>
cb1a09d0 204
3b10bc60 205=item Keywords related to Perl modules
d74e8afc 206X<module>
cb1a09d0 207
22fae026 208C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 209
353c6505 210=item Keywords related to classes and object-orientation
d74e8afc 211X<object> X<class> X<package>
cb1a09d0 212
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213C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
214C<untie>, C<use>
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215
216=item Low-level socket functions
d74e8afc 217X<socket> X<sock>
cb1a09d0 218
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219C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
220C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 221C<socket>, C<socketpair>
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222
223=item System V interprocess communication functions
d74e8afc 224X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 225
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226C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
227C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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228
229=item Fetching user and group info
d74e8afc 230X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 231
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232C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
233C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
234C<getpwuid>, C<setgrent>, C<setpwent>
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235
236=item Fetching network info
d74e8afc 237X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 238
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239C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
240C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
241C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
242C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
243C<setnetent>, C<setprotoent>, C<setservent>
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244
245=item Time-related functions
d74e8afc 246X<time> X<date>
cb1a09d0 247
22fae026 248C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 249
37798a01 250=item Functions new in perl5
d74e8afc 251X<perl5>
37798a01 252
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253C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
254C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
1dc8ecb8 255C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
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256C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
257C<ucfirst>, C<untie>, C<use>, C<when>
37798a01 258
3b10bc60 259* C<sub> was a keyword in Perl 4, but in Perl 5 it is an
5a964f20 260operator, which can be used in expressions.
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261
262=item Functions obsoleted in perl5
263
22fae026 264C<dbmclose>, C<dbmopen>
37798a01 265
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266=back
267
60f9f73c 268=head2 Portability
d74e8afc 269X<portability> X<Unix> X<portable>
60f9f73c 270
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271Perl was born in Unix and can therefore access all common Unix
272system calls. In non-Unix environments, the functionality of some
8f1da26d 273Unix system calls may not be available or details of the available
2b5ab1e7 274functionality may differ slightly. The Perl functions affected
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275by this are:
276
277C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
278C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
279C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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280C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
281C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 282C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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283C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
284C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
285C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 286C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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287C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
288C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
289C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 290C<shmwrite>, C<socket>, C<socketpair>,
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291C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
292C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 293C<utime>, C<wait>, C<waitpid>
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294
295For more information about the portability of these functions, see
296L<perlport> and other available platform-specific documentation.
297
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298=head2 Alphabetical Listing of Perl Functions
299
3b10bc60 300=over
a0d0e21e 301
5b3c99c0 302=item -X FILEHANDLE
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303X<-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>
304X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 305
5b3c99c0 306=item -X EXPR
a0d0e21e 307
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308=item -X DIRHANDLE
309
5b3c99c0 310=item -X
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311
312A file test, where X is one of the letters listed below. This unary
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313operator takes one argument, either a filename, a filehandle, or a dirhandle,
314and tests the associated file to see if something is true about it. If the
7660c0ab 315argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 316Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 317the undefined value if the file doesn't exist. Despite the funny
d0821a6a 318names, precedence is the same as any other named unary operator. The
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319operator may be any of:
320
5ed4f2ec 321 -r File is readable by effective uid/gid.
322 -w File is writable by effective uid/gid.
323 -x File is executable by effective uid/gid.
324 -o File is owned by effective uid.
a0d0e21e 325
5ed4f2ec 326 -R File is readable by real uid/gid.
327 -W File is writable by real uid/gid.
328 -X File is executable by real uid/gid.
329 -O File is owned by real uid.
a0d0e21e 330
5ed4f2ec 331 -e File exists.
332 -z File has zero size (is empty).
333 -s File has nonzero size (returns size in bytes).
a0d0e21e 334
5ed4f2ec 335 -f File is a plain file.
336 -d File is a directory.
337 -l File is a symbolic link.
338 -p File is a named pipe (FIFO), or Filehandle is a pipe.
339 -S File is a socket.
340 -b File is a block special file.
341 -c File is a character special file.
342 -t Filehandle is opened to a tty.
a0d0e21e 343
5ed4f2ec 344 -u File has setuid bit set.
345 -g File has setgid bit set.
346 -k File has sticky bit set.
a0d0e21e 347
5ed4f2ec 348 -T File is an ASCII text file (heuristic guess).
349 -B File is a "binary" file (opposite of -T).
a0d0e21e 350
5ed4f2ec 351 -M Script start time minus file modification time, in days.
352 -A Same for access time.
353 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 354
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355Example:
356
357 while (<>) {
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358 chomp;
359 next unless -f $_; # ignore specials
360 #...
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361 }
362
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363Note that C<-s/a/b/> does not do a negated substitution. Saying
364C<-exp($foo)> still works as expected, however: only single letters
365following a minus are interpreted as file tests.
366
367These operators are exempt from the "looks like a function rule" described
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368above. That is, an opening parenthesis after the operator does not affect
369how much of the following code constitutes the argument. Put the opening
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370parentheses before the operator to separate it from code that follows (this
371applies only to operators with higher precedence than unary operators, of
372course):
373
374 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
375 (-s $file) + 1024 # correct
376
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377The interpretation of the file permission operators C<-r>, C<-R>,
378C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
379of the file and the uids and gids of the user. There may be other
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380reasons you can't actually read, write, or execute the file: for
381example network filesystem access controls, ACLs (access control lists),
382read-only filesystems, and unrecognized executable formats. Note
383that the use of these six specific operators to verify if some operation
384is possible is usually a mistake, because it may be open to race
385conditions.
5ff3f7a4 386
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387Also note that, for the superuser on the local filesystems, the C<-r>,
388C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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389if any execute bit is set in the mode. Scripts run by the superuser
390may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 391or temporarily set their effective uid to something else.
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392
393If you are using ACLs, there is a pragma called C<filetest> that may
394produce more accurate results than the bare stat() mode bits.
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395When under C<use filetest 'access'> the above-mentioned filetests
396test whether the permission can(not) be granted using the
3b10bc60 397access(2) family of system calls. Also note that the C<-x> and C<-X> may
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398under this pragma return true even if there are no execute permission
399bits set (nor any extra execute permission ACLs). This strangeness is
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400due to the underlying system calls' definitions. Note also that, due to
401the implementation of C<use filetest 'access'>, the C<_> special
402filehandle won't cache the results of the file tests when this pragma is
403in effect. Read the documentation for the C<filetest> pragma for more
404information.
5ff3f7a4 405
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406The C<-T> and C<-B> switches work as follows. The first block or so of the
407file is examined for odd characters such as strange control codes or
61eff3bc 408characters with the high bit set. If too many strange characters (>30%)
cf264981 409are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 410containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 411or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 412rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 413file, or a file at EOF when testing a filehandle. Because you have to
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414read a file to do the C<-T> test, on most occasions you want to use a C<-f>
415against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 416
5dac7880 417If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 418the special filehandle consisting of a solitary underline, then the stat
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LW
419structure of the previous file test (or stat operator) is used, saving
420a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 421that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 422symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 423an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 424Example:
a0d0e21e
LW
425
426 print "Can do.\n" if -r $a || -w _ || -x _;
427
428 stat($filename);
429 print "Readable\n" if -r _;
430 print "Writable\n" if -w _;
431 print "Executable\n" if -x _;
432 print "Setuid\n" if -u _;
433 print "Setgid\n" if -g _;
434 print "Sticky\n" if -k _;
435 print "Text\n" if -T _;
436 print "Binary\n" if -B _;
437
fbb0b3b3
RGS
438As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
439test operators, in a way that C<-f -w -x $file> is equivalent to
3b10bc60 440C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
fbb0b3b3
RGS
441the return value of C<-f $file> as an argument to another filetest
442operator, no special magic will happen.)
443
bee96257 444Portability issues: L<perlport/-X>.
ea9eb35a 445
a0d0e21e 446=item abs VALUE
d74e8afc 447X<abs> X<absolute>
a0d0e21e 448
54310121 449=item abs
bbce6d69 450
a0d0e21e 451Returns the absolute value of its argument.
7660c0ab 452If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
453
454=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 455X<accept>
a0d0e21e 456
3b10bc60 457Accepts an incoming socket connect, just as accept(2)
19799a22 458does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 459See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 460
8d2a6795
GS
461On systems that support a close-on-exec flag on files, the flag will
462be set for the newly opened file descriptor, as determined by the
463value of $^F. See L<perlvar/$^F>.
464
a0d0e21e 465=item alarm SECONDS
d74e8afc
ITB
466X<alarm>
467X<SIGALRM>
468X<timer>
a0d0e21e 469
54310121 470=item alarm
bbce6d69 471
a0d0e21e 472Arranges to have a SIGALRM delivered to this process after the
cf264981 473specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
474specified, the value stored in C<$_> is used. (On some machines,
475unfortunately, the elapsed time may be up to one second less or more
476than you specified because of how seconds are counted, and process
477scheduling may delay the delivery of the signal even further.)
478
479Only one timer may be counting at once. Each call disables the
480previous timer, and an argument of C<0> may be supplied to cancel the
481previous timer without starting a new one. The returned value is the
482amount of time remaining on the previous timer.
a0d0e21e 483
2bc69794
BS
484For delays of finer granularity than one second, the Time::HiRes module
485(from CPAN, and starting from Perl 5.8 part of the standard
486distribution) provides ualarm(). You may also use Perl's four-argument
487version of select() leaving the first three arguments undefined, or you
488might be able to use the C<syscall> interface to access setitimer(2) if
489your system supports it. See L<perlfaq8> for details.
2b5ab1e7 490
80d38338
TC
491It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
492C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 493
19799a22
GS
494If you want to use C<alarm> to time out a system call you need to use an
495C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 496fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 497restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 498modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
499
500 eval {
a9a5a0dc
VP
501 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
502 alarm $timeout;
503 $nread = sysread SOCKET, $buffer, $size;
504 alarm 0;
ff68c719 505 };
ff68c719 506 if ($@) {
a9a5a0dc 507 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 508 # timed out
ff68c719
PP
509 }
510 else {
5ed4f2ec 511 # didn't
ff68c719
PP
512 }
513
91d81acc
JH
514For more information see L<perlipc>.
515
ea9eb35a 516Portability issues: L<perlport/alarm>.
517
a0d0e21e 518=item atan2 Y,X
d74e8afc 519X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
520
521Returns the arctangent of Y/X in the range -PI to PI.
522
ca6e1c26 523For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
524function, or use the familiar relation:
525
526 sub tan { sin($_[0]) / cos($_[0]) }
527
a1021d57
RGS
528The return value for C<atan2(0,0)> is implementation-defined; consult
529your atan2(3) manpage for more information.
bf5f1b4c 530
ea9eb35a 531Portability issues: L<perlport/atan2>.
532
a0d0e21e 533=item bind SOCKET,NAME
d74e8afc 534X<bind>
a0d0e21e 535
3b10bc60 536Binds a network address to a socket, just as bind(2)
19799a22 537does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
538packed address of the appropriate type for the socket. See the examples in
539L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 540
fae2c0fb 541=item binmode FILEHANDLE, LAYER
d74e8afc 542X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 543
a0d0e21e
LW
544=item binmode FILEHANDLE
545
1cbfc93d
NIS
546Arranges for FILEHANDLE to be read or written in "binary" or "text"
547mode on systems where the run-time libraries distinguish between
548binary and text files. If FILEHANDLE is an expression, the value is
549taken as the name of the filehandle. Returns true on success,
b5fe5ca2 550otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 551
8f1da26d 552On some systems (in general, DOS- and Windows-based systems) binmode()
d807c6f4 553is necessary when you're not working with a text file. For the sake
d7a0d798
FC
554of portability it is a good idea always to use it when appropriate,
555and never to use it when it isn't appropriate. Also, people can
8f1da26d 556set their I/O to be by default UTF8-encoded Unicode, not bytes.
d807c6f4
JH
557
558In other words: regardless of platform, use binmode() on binary data,
d7a0d798 559like images, for example.
d807c6f4
JH
560
561If LAYER is present it is a single string, but may contain multiple
3b10bc60 562directives. The directives alter the behaviour of the filehandle.
d7a0d798 563When LAYER is present, using binmode on a text file makes sense.
d807c6f4 564
fae2c0fb 565If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
566suitable for passing binary data. This includes turning off possible CRLF
567translation and marking it as bytes (as opposed to Unicode characters).
749683d2 568Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 569Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
570Other layers that would affect the binary nature of the stream are
571I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 572PERLIO environment variable.
01e6739c 573
3b10bc60 574The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
575form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
576establish default I/O layers. See L<open>.
577
fae2c0fb
RGS
578I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
579in "Programming Perl, 3rd Edition". However, since the publishing of this
580book, by many known as "Camel III", the consensus of the naming of this
581functionality has moved from "discipline" to "layer". All documentation
582of this version of Perl therefore refers to "layers" rather than to
583"disciplines". Now back to the regularly scheduled documentation...>
584
8f1da26d 585To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
6902c96a 586C<:utf8> just marks the data as UTF-8 without further checking,
8f1da26d 587while C<:encoding(UTF-8)> checks the data for actually being valid
6902c96a 588UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 589
ed53a2bb 590In general, binmode() should be called after open() but before any I/O
3b10bc60 591is done on the filehandle. Calling binmode() normally flushes any
01e6739c 592pending buffered output data (and perhaps pending input data) on the
fae2c0fb 593handle. An exception to this is the C<:encoding> layer that
d7a0d798 594changes the default character encoding of the handle; see L</open>.
fae2c0fb 595The C<:encoding> layer sometimes needs to be called in
3874323d
JH
596mid-stream, and it doesn't flush the stream. The C<:encoding>
597also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 598internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 599
19799a22 600The operating system, device drivers, C libraries, and Perl run-time
8f1da26d
TC
601system all conspire to let the programmer treat a single
602character (C<\n>) as the line terminator, irrespective of external
30168b04
GS
603representation. On many operating systems, the native text file
604representation matches the internal representation, but on some
605platforms the external representation of C<\n> is made up of more than
606one character.
607
8f1da26d
TC
608All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
609a single character to end each line in the external representation of text
610(even though that single character is CARRIAGE RETURN on old, pre-Darwin
611flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
612systems like OS/2, DOS, and the various flavors of MS-Windows, your program
613sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
614two characters C<\cM\cJ>. That means that if you don't use binmode() on
615these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
616input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
617output. This is what you want for text files, but it can be disastrous for
618binary files.
30168b04
GS
619
620Another consequence of using binmode() (on some systems) is that
621special end-of-file markers will be seen as part of the data stream.
d7a0d798
FC
622For systems from the Microsoft family this means that, if your binary
623data contain C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
624the file, unless you use binmode().
625
3b10bc60 626binmode() is important not only for readline() and print() operations,
30168b04
GS
627but also when using read(), seek(), sysread(), syswrite() and tell()
628(see L<perlport> for more details). See the C<$/> and C<$\> variables
629in L<perlvar> for how to manually set your input and output
630line-termination sequences.
a0d0e21e 631
ea9eb35a 632Portability issues: L<perlport/binmode>.
633
4633a7c4 634=item bless REF,CLASSNAME
d74e8afc 635X<bless>
a0d0e21e
LW
636
637=item bless REF
638
2b5ab1e7
TC
639This function tells the thingy referenced by REF that it is now an object
640in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 641is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 642it returns the reference for convenience. Always use the two-argument
cf264981 643version if a derived class might inherit the function doing the blessing.
82e1c0d9 644SeeL<perlobj> for more about the blessing (and blessings) of objects.
a0d0e21e 645
57668c4d 646Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 647Namespaces with all lowercase names are considered reserved for
cf264981 648Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
649confusion, you may wish to avoid such package names as well. Make sure
650that CLASSNAME is a true value.
60ad88b8
GS
651
652See L<perlmod/"Perl Modules">.
653
0d863452
RH
654=item break
655
656Break out of a C<given()> block.
657
8f1da26d 658This keyword is enabled by the C<"switch"> feature: see
4a904372
FC
659L<feature> for more information. You can also access it by
660prefixing it with C<CORE::>. Alternately, include a C<use
8f1da26d 661v5.10> or later to the current scope.
0d863452 662
a0d0e21e 663=item caller EXPR
d74e8afc 664X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
665
666=item caller
667
5a964f20 668Returns the context of the current subroutine call. In scalar context,
80d38338
TC
669returns the caller's package name if there I<is> a caller (that is, if
670we're in a subroutine or C<eval> or C<require>) and the undefined value
5a964f20 671otherwise. In list context, returns
a0d0e21e 672
ee6b43cc 673 # 0 1 2
748a9306 674 ($package, $filename, $line) = caller;
a0d0e21e
LW
675
676With EXPR, it returns some extra information that the debugger uses to
677print a stack trace. The value of EXPR indicates how many call frames
678to go back before the current one.
679
ee6b43cc 680 # 0 1 2 3 4
f3aa04c2 681 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 682
683 # 5 6 7 8 9 10
b3ca2e83 684 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 685 = caller($i);
e7ea3e70 686
951ba7fe 687Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 688call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 689C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 690C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 691C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 692$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
693each C<use> statement creates a C<require> frame inside an C<eval EXPR>
694frame.) $subroutine may also be C<(unknown)> if this particular
695subroutine happens to have been deleted from the symbol table.
696C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
697C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
698compiled with. The C<$hints> and C<$bitmask> values are subject to change
699between versions of Perl, and are not meant for external use.
748a9306 700
b3ca2e83
NC
701C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
702caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
703of this hash, as they are the actual values stored in the optree.
704
ffe0c19d
FC
705Furthermore, when called from within the DB package in
706list context, and with an argument, caller returns more
7660c0ab 707detailed information: it sets the list variable C<@DB::args> to be the
54310121 708arguments with which the subroutine was invoked.
748a9306 709
7660c0ab 710Be aware that the optimizer might have optimized call frames away before
19799a22 711C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 712might not return information about the call frame you expect it to, for
b76cc8ba 713C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 714previous time C<caller> was called.
7660c0ab 715
8f1da26d 716Be aware that setting C<@DB::args> is I<best effort>, intended for
ca9f0cb5
NC
717debugging or generating backtraces, and should not be relied upon. In
718particular, as C<@_> contains aliases to the caller's arguments, Perl does
719not take a copy of C<@_>, so C<@DB::args> will contain modifications the
720subroutine makes to C<@_> or its contents, not the original values at call
721time. C<@DB::args>, like C<@_>, does not hold explicit references to its
722elements, so under certain cases its elements may have become freed and
723reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 724of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
725I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
726reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5
NC
727elements), so C<@DB::args> is actually a hybrid of the current state and
728initial state of C<@_>. Buyer beware.
729
a0d0e21e 730=item chdir EXPR
d74e8afc
ITB
731X<chdir>
732X<cd>
f723aae1 733X<directory, change>
a0d0e21e 734
c4aca7d0
GA
735=item chdir FILEHANDLE
736
737=item chdir DIRHANDLE
738
ce2984c3
PF
739=item chdir
740
ffce7b87 741Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 742changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 743changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0 744variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
80d38338 745neither is set, C<chdir> does nothing. It returns true on success,
b4ad75f0 746false otherwise. See the example under C<die>.
a0d0e21e 747
3b10bc60 748On systems that support fchdir(2), you may pass a filehandle or
34169887 749directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 750passing handles raises an exception.
c4aca7d0 751
a0d0e21e 752=item chmod LIST
d74e8afc 753X<chmod> X<permission> X<mode>
a0d0e21e
LW
754
755Changes the permissions of a list of files. The first element of the
8f1da26d 756list must be the numeric mode, which should probably be an octal
4ad40acf 757number, and which definitely should I<not> be a string of octal digits:
3b10bc60 758C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 759successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 760
3b10bc60 761 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 762 chmod 0755, @executables;
3b10bc60 763 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 764 # --w----r-T
3b10bc60 765 $mode = "0644"; chmod oct($mode), "foo"; # this is better
766 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 767
3b10bc60 768On systems that support fchmod(2), you may pass filehandles among the
769files. On systems that don't support fchmod(2), passing filehandles raises
770an exception. Filehandles must be passed as globs or glob references to be
771recognized; barewords are considered filenames.
c4aca7d0
GA
772
773 open(my $fh, "<", "foo");
774 my $perm = (stat $fh)[2] & 07777;
775 chmod($perm | 0600, $fh);
776
3b10bc60 777You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
778module:
779
3b10bc60 780 use Fcntl qw( :mode );
ca6e1c26 781 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 782 # Identical to the chmod 0755 of the example above.
ca6e1c26 783
ea9eb35a 784Portability issues: L<perlport/chmod>.
785
a0d0e21e 786=item chomp VARIABLE
d74e8afc 787X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 788
313c9f5c 789=item chomp( LIST )
a0d0e21e
LW
790
791=item chomp
792
2b5ab1e7
TC
793This safer version of L</chop> removes any trailing string
794that corresponds to the current value of C<$/> (also known as
28757baa
PP
795$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
796number of characters removed from all its arguments. It's often used to
797remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
798that the final record may be missing its newline. When in paragraph
799mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 800When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 801a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 802remove anything.
19799a22 803If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
804
805 while (<>) {
a9a5a0dc
VP
806 chomp; # avoid \n on last field
807 @array = split(/:/);
808 # ...
a0d0e21e
LW
809 }
810
4bf21a6d
RD
811If VARIABLE is a hash, it chomps the hash's values, but not its keys.
812
a0d0e21e
LW
813You can actually chomp anything that's an lvalue, including an assignment:
814
815 chomp($cwd = `pwd`);
816 chomp($answer = <STDIN>);
817
818If you chomp a list, each element is chomped, and the total number of
819characters removed is returned.
820
15e44fd8
RGS
821Note that parentheses are necessary when you're chomping anything
822that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
823is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
824C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
825C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
826as C<chomp($a, $b)>.
827
a0d0e21e 828=item chop VARIABLE
d74e8afc 829X<chop>
a0d0e21e 830
313c9f5c 831=item chop( LIST )
a0d0e21e
LW
832
833=item chop
834
835Chops off the last character of a string and returns the character
5b3eff12 836chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 837scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
838If VARIABLE is a hash, it chops the hash's values, but not its keys.
839
5b3eff12 840You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
841
842If you chop a list, each element is chopped. Only the value of the
19799a22 843last C<chop> is returned.
a0d0e21e 844
19799a22 845Note that C<chop> returns the last character. To return all but the last
748a9306
LW
846character, use C<substr($string, 0, -1)>.
847
15e44fd8
RGS
848See also L</chomp>.
849
a0d0e21e 850=item chown LIST
d74e8afc 851X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
852
853Changes the owner (and group) of a list of files. The first two
19799a22
GS
854elements of the list must be the I<numeric> uid and gid, in that
855order. A value of -1 in either position is interpreted by most
856systems to leave that value unchanged. Returns the number of files
857successfully changed.
a0d0e21e
LW
858
859 $cnt = chown $uid, $gid, 'foo', 'bar';
860 chown $uid, $gid, @filenames;
861
3b10bc60 862On systems that support fchown(2), you may pass filehandles among the
863files. On systems that don't support fchown(2), passing filehandles raises
864an exception. Filehandles must be passed as globs or glob references to be
865recognized; barewords are considered filenames.
c4aca7d0 866
54310121 867Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
868
869 print "User: ";
19799a22 870 chomp($user = <STDIN>);
5a964f20 871 print "Files: ";
19799a22 872 chomp($pattern = <STDIN>);
a0d0e21e
LW
873
874 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 875 or die "$user not in passwd file";
a0d0e21e 876
5ed4f2ec 877 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
878 chown $uid, $gid, @ary;
879
54310121 880On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
881file unless you're the superuser, although you should be able to change
882the group to any of your secondary groups. On insecure systems, these
883restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
884On POSIX systems, you can detect this condition this way:
885
886 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
887 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 888
ea9eb35a 889Portability issues: L<perlport/chmod>.
890
a0d0e21e 891=item chr NUMBER
d74e8afc 892X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 893
54310121 894=item chr
bbce6d69 895
a0d0e21e 896Returns the character represented by that NUMBER in the character set.
a0ed51b3 897For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 898chr(0x263a) is a Unicode smiley face.
aaa68c4a 899
8a064bd6 900Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 901except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
902(truncated to an integer) are used.
903
974da8e5
JH
904If NUMBER is omitted, uses C<$_>.
905
b76cc8ba 906For the reverse, use L</ord>.
a0d0e21e 907
2575c402
JW
908Note that characters from 128 to 255 (inclusive) are by default
909internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 910
2575c402 911See L<perlunicode> for more about Unicode.
bbce6d69 912
a0d0e21e 913=item chroot FILENAME
d74e8afc 914X<chroot> X<root>
a0d0e21e 915
54310121 916=item chroot
bbce6d69 917
5a964f20 918This function works like the system call by the same name: it makes the
4633a7c4 919named directory the new root directory for all further pathnames that
951ba7fe 920begin with a C</> by your process and all its children. (It doesn't
28757baa 921change your current working directory, which is unaffected.) For security
4633a7c4 922reasons, this call is restricted to the superuser. If FILENAME is
19799a22 923omitted, does a C<chroot> to C<$_>.
a0d0e21e 924
ea9eb35a 925Portability issues: L<perlport/chroot>.
926
a0d0e21e 927=item close FILEHANDLE
d74e8afc 928X<close>
a0d0e21e 929
6a518fbc
TP
930=item close
931
3b10bc60 932Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 933buffers, and closes the system file descriptor. Returns true if those
8f1da26d 934operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
935layer. Closes the currently selected filehandle if the argument is
936omitted.
fb73857a
PP
937
938You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 939another C<open> on it, because C<open> closes it for you. (See
01aa884e 940L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
19799a22 941counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 942
3b10bc60 943If the filehandle came from a piped open, C<close> returns false if one of
944the other syscalls involved fails or if its program exits with non-zero
945status. If the only problem was that the program exited non-zero, C<$!>
946will be set to C<0>. Closing a pipe also waits for the process executing
947on the pipe to exit--in case you wish to look at the output of the pipe
948afterwards--and implicitly puts the exit status value of that command into
949C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 950
2e0cfa16
FC
951If there are multiple threads running, C<close> on a filehandle from a
952piped open returns true without waiting for the child process to terminate,
953if the filehandle is still open in another thread.
954
80d38338
TC
955Closing the read end of a pipe before the process writing to it at the
956other end is done writing results in the writer receiving a SIGPIPE. If
957the other end can't handle that, be sure to read all the data before
958closing the pipe.
73689b13 959
fb73857a 960Example:
a0d0e21e 961
fb73857a
PP
962 open(OUTPUT, '|sort >foo') # pipe to sort
963 or die "Can't start sort: $!";
5ed4f2ec 964 #... # print stuff to output
965 close OUTPUT # wait for sort to finish
fb73857a
PP
966 or warn $! ? "Error closing sort pipe: $!"
967 : "Exit status $? from sort";
5ed4f2ec 968 open(INPUT, 'foo') # get sort's results
fb73857a 969 or die "Can't open 'foo' for input: $!";
a0d0e21e 970
5a964f20 971FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 972filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
973
974=item closedir DIRHANDLE
d74e8afc 975X<closedir>
a0d0e21e 976
19799a22 977Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
978system call.
979
a0d0e21e 980=item connect SOCKET,NAME
d74e8afc 981X<connect>
a0d0e21e 982
80d38338
TC
983Attempts to connect to a remote socket, just like connect(2).
984Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
985packed address of the appropriate type for the socket. See the examples in
986L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 987
cb1a09d0 988=item continue BLOCK
d74e8afc 989X<continue>
cb1a09d0 990
0d863452
RH
991=item continue
992
4a904372
FC
993When followed by a BLOCK, C<continue> is actually a
994flow control statement rather than a function. If
cf264981 995there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
996C<foreach>), it is always executed just before the conditional is about to
997be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
998it can be used to increment a loop variable, even when the loop has been
999continued via the C<next> statement (which is similar to the C C<continue>
1000statement).
1001
98293880 1002C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 1003block; C<last> and C<redo> behave as if they had been executed within
19799a22 1004the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
1005block, it may be more entertaining.
1006
1007 while (EXPR) {
a9a5a0dc
VP
1008 ### redo always comes here
1009 do_something;
1d2dff63 1010 } continue {
a9a5a0dc
VP
1011 ### next always comes here
1012 do_something_else;
1013 # then back the top to re-check EXPR
1d2dff63
GS
1014 }
1015 ### last always comes here
1016
3b10bc60 1017Omitting the C<continue> section is equivalent to using an
1018empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1019to check the condition at the top of the loop.
1020
4a904372 1021When there is no BLOCK, C<continue> is a function that
8f1da26d
TC
1022falls through the current C<when> or C<default> block instead of iterating
1023a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
4a904372
FC
1024In Perl 5.14 and earlier, this form of C<continue> was
1025only available when the C<"switch"> feature was enabled.
8f1da26d
TC
1026See L<feature> and L<perlsyn/"Switch statements"> for more
1027information.
0d863452 1028
a0d0e21e 1029=item cos EXPR
d74e8afc 1030X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1031
d6217f1e
GS
1032=item cos
1033
5a964f20 1034Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1035takes the cosine of C<$_>.
a0d0e21e 1036
ca6e1c26 1037For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1038function, or use this relation:
1039
1040 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1041
a0d0e21e 1042=item crypt PLAINTEXT,SALT
d74e8afc 1043X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1044X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1045
ef2e6798
MS
1046Creates a digest string exactly like the crypt(3) function in the C
1047library (assuming that you actually have a version there that has not
bb23f8d1 1048been extirpated as a potential munition).
ef2e6798 1049
34169887 1050crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1051into a short string, called a digest, which is returned. The same
1052PLAINTEXT and SALT will always return the same string, but there is no
1053(known) way to get the original PLAINTEXT from the hash. Small
1054changes in the PLAINTEXT or SALT will result in large changes in the
1055digest.
1056
1057There is no decrypt function. This function isn't all that useful for
1058cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1059mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1060primarily used to check if two pieces of text are the same without
1061having to transmit or store the text itself. An example is checking
1062if a correct password is given. The digest of the password is stored,
cf264981 1063not the password itself. The user types in a password that is
ef2e6798 1064crypt()'d with the same salt as the stored digest. If the two digests
34169887 1065match, the password is correct.
ef2e6798
MS
1066
1067When verifying an existing digest string you should use the digest as
1068the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1069to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1070crypt() will hash the new string with the same salt as the digest.
1071This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1072with more exotic implementations. In other words, assume
1073nothing about the returned string itself nor about how many bytes
1074of SALT may matter.
85c16d83
JH
1075
1076Traditionally the result is a string of 13 bytes: two first bytes of
1077the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
bb23f8d1 1078the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1079hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1080and implementations on non-Unix platforms may produce different
ef2e6798 1081strings.
85c16d83
JH
1082
1083When choosing a new salt create a random two character string whose
1084characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1085'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1086characters is just a recommendation; the characters allowed in
1087the salt depend solely on your system's crypt library, and Perl can't
1088restrict what salts C<crypt()> accepts.
e71965be 1089
a0d0e21e 1090Here's an example that makes sure that whoever runs this program knows
cf264981 1091their password:
a0d0e21e
LW
1092
1093 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1094
1095 system "stty -echo";
1096 print "Password: ";
e71965be 1097 chomp($word = <STDIN>);
a0d0e21e
LW
1098 print "\n";
1099 system "stty echo";
1100
e71965be 1101 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1102 die "Sorry...\n";
a0d0e21e 1103 } else {
a9a5a0dc 1104 print "ok\n";
54310121 1105 }
a0d0e21e 1106
9f8f0c9d 1107Of course, typing in your own password to whoever asks you
748a9306 1108for it is unwise.
a0d0e21e 1109
ef2e6798 1110The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1111of data, not least of all because you can't get the information
ef2e6798 1112back. Look at the L<Digest> module for more robust algorithms.
19799a22 1113
f2791508
JH
1114If using crypt() on a Unicode string (which I<potentially> has
1115characters with codepoints above 255), Perl tries to make sense
34169887 1116of the situation by trying to downgrade (a copy of)
f2791508
JH
1117the string back to an eight-bit byte string before calling crypt()
1118(on that copy). If that works, good. If not, crypt() dies with
1119C<Wide character in crypt>.
85c16d83 1120
ea9eb35a 1121Portability issues: L<perlport/crypt>.
1122
aa689395 1123=item dbmclose HASH
d74e8afc 1124X<dbmclose>
a0d0e21e 1125
19799a22 1126[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1127
aa689395 1128Breaks the binding between a DBM file and a hash.
a0d0e21e 1129
ea9eb35a 1130Portability issues: L<perlport/dbmclose>.
1131
19799a22 1132=item dbmopen HASH,DBNAME,MASK
d74e8afc 1133X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1134
01aa884e
KW
1135[This function has been largely superseded by the
1136L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
a0d0e21e 1137
7b8d334a 1138This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1139hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1140argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1141is the name of the database (without the F<.dir> or F<.pag> extension if
1142any). If the database does not exist, it is created with protection
19799a22 1143specified by MASK (as modified by the C<umask>). If your system supports
80d38338 1144only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1145program. In older versions of Perl, if your system had neither DBM nor
19799a22 1146ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1147sdbm(3).
1148
1149If you don't have write access to the DBM file, you can only read hash
1150variables, not set them. If you want to test whether you can write,
3b10bc60 1151either use file tests or try setting a dummy hash entry inside an C<eval>
1152to trap the error.
a0d0e21e 1153
19799a22
GS
1154Note that functions such as C<keys> and C<values> may return huge lists
1155when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1156function to iterate over large DBM files. Example:
1157
1158 # print out history file offsets
1159 dbmopen(%HIST,'/usr/lib/news/history',0666);
1160 while (($key,$val) = each %HIST) {
a9a5a0dc 1161 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1162 }
1163 dbmclose(%HIST);
1164
cb1a09d0 1165See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1166cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1167rich implementation.
4633a7c4 1168
2b5ab1e7
TC
1169You can control which DBM library you use by loading that library
1170before you call dbmopen():
1171
1172 use DB_File;
1173 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1174 or die "Can't open netscape history file: $!";
2b5ab1e7 1175
ea9eb35a 1176Portability issues: L<perlport/dbmopen>.
1177
8f1da26d
TC
1178=item default BLOCK
1179
1180Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when>
1181that's always true. Only available after Perl 5.10, and only if the
4a904372
FC
1182C<switch> feature has been requested or if the keyword is prefixed with
1183C<CORE::>. See L</when>.
8f1da26d 1184
a0d0e21e 1185=item defined EXPR
d74e8afc 1186X<defined> X<undef> X<undefined>
a0d0e21e 1187
54310121 1188=item defined
bbce6d69 1189
2f9daede 1190Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1191the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1192checked.
1193
1194Many operations return C<undef> to indicate failure, end of file,
1195system error, uninitialized variable, and other exceptional
1196conditions. This function allows you to distinguish C<undef> from
1197other values. (A simple Boolean test will not distinguish among
7660c0ab 1198C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1199false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1200doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1201returns C<undef> when its argument is an empty array, I<or> when the
1202element to return happens to be C<undef>.
1203
f10b0346
GS
1204You may also use C<defined(&func)> to check whether subroutine C<&func>
1205has ever been defined. The return value is unaffected by any forward
80d38338 1206declarations of C<&func>. A subroutine that is not defined
847c7ebe 1207may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1208makes it spring into existence the first time that it is called; see
847c7ebe 1209L<perlsub>.
f10b0346
GS
1210
1211Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1212used to report whether memory for that aggregate had ever been
f10b0346
GS
1213allocated. This behavior may disappear in future versions of Perl.
1214You should instead use a simple test for size:
1215
1216 if (@an_array) { print "has array elements\n" }
1217 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1218
1219When used on a hash element, it tells you whether the value is defined,
dc848c6f 1220not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1221purpose.
a0d0e21e
LW
1222
1223Examples:
1224
8f1da26d 1225 print if defined $switch{D};
a0d0e21e
LW
1226 print "$val\n" while defined($val = pop(@ary));
1227 die "Can't readlink $sym: $!"
a9a5a0dc 1228 unless defined($value = readlink $sym);
a0d0e21e 1229 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1230 $debugging = 0 unless defined $debugging;
a0d0e21e 1231
8f1da26d 1232Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1233discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1234defined values. For example, if you say
a5f75d66
AD
1235
1236 "ab" =~ /a(.*)b/;
1237
80d38338 1238The pattern match succeeds and C<$1> is defined, although it
cf264981 1239matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1240matched something that happened to be zero characters long. This is all
a5f75d66 1241very above-board and honest. When a function returns an undefined value,
2f9daede 1242it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1243should use C<defined> only when questioning the integrity of what
7660c0ab 1244you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1245what you want.
1246
dc848c6f 1247See also L</undef>, L</exists>, L</ref>.
2f9daede 1248
a0d0e21e 1249=item delete EXPR
d74e8afc 1250X<delete>
a0d0e21e 1251
d0a76353
RS
1252Given an expression that specifies an element or slice of a hash, C<delete>
1253deletes the specified elements from that hash so that exists() on that element
1254no longer returns true. Setting a hash element to the undefined value does
1255not remove its key, but deleting it does; see L</exists>.
80d38338 1256
8f1da26d 1257In list context, returns the value or values deleted, or the last such
80d38338 1258element in scalar context. The return list's length always matches that of
d0a76353
RS
1259the argument list: deleting non-existent elements returns the undefined value
1260in their corresponding positions.
80d38338 1261
d0a76353
RS
1262delete() may also be used on arrays and array slices, but its behavior is less
1263straightforward. Although exists() will return false for deleted entries,
1264deleting array elements never changes indices of existing values; use shift()
1265or splice() for that. However, if all deleted elements fall at the end of an
1266array, the array's size shrinks to the position of the highest element that
1267still tests true for exists(), or to 0 if none do.
1268
8f1da26d 1269B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1270be removed in a future version of Perl.
80d38338
TC
1271
1272Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1273a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1274or array may not necessarily return anything; it depends on the implementation
1275of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1276
80d38338
TC
1277The C<delete local EXPR> construct localizes the deletion to the current
1278block at run time. Until the block exits, elements locally deleted
1279temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1280of composite types">.
eba0920a
EM
1281
1282 %hash = (foo => 11, bar => 22, baz => 33);
1283 $scalar = delete $hash{foo}; # $scalar is 11
1284 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1285 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1286
01020589 1287The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1288
5f05dabc 1289 foreach $key (keys %HASH) {
a9a5a0dc 1290 delete $HASH{$key};
a0d0e21e
LW
1291 }
1292
01020589 1293 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1294 delete $ARRAY[$index];
01020589
GS
1295 }
1296
1297And so do these:
5f05dabc 1298
01020589
GS
1299 delete @HASH{keys %HASH};
1300
9740c838 1301 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1302
80d38338
TC
1303But both are slower than assigning the empty list
1304or undefining %HASH or @ARRAY, which is the customary
1305way to empty out an aggregate:
01020589 1306
5ed4f2ec 1307 %HASH = (); # completely empty %HASH
1308 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1309
5ed4f2ec 1310 @ARRAY = (); # completely empty @ARRAY
1311 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1312
80d38338
TC
1313The EXPR can be arbitrarily complicated provided its
1314final operation is an element or slice of an aggregate:
a0d0e21e
LW
1315
1316 delete $ref->[$x][$y]{$key};
5f05dabc 1317 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1318
01020589
GS
1319 delete $ref->[$x][$y][$index];
1320 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1321
a0d0e21e 1322=item die LIST
d74e8afc 1323X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1324
4c050ad5
NC
1325C<die> raises an exception. Inside an C<eval> the error message is stuffed
1326into C<$@> and the C<eval> is terminated with the undefined value.
1327If the exception is outside of all enclosing C<eval>s, then the uncaught
1328exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1329need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1330
1331Equivalent examples:
1332
1333 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1334 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1335
ccac6780 1336If the last element of LIST does not end in a newline, the current
df37ec69
WW
1337script line number and input line number (if any) are also printed,
1338and a newline is supplied. Note that the "input line number" (also
1339known as "chunk") is subject to whatever notion of "line" happens to
1340be currently in effect, and is also available as the special variable
1341C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1342
1343Hint: sometimes appending C<", stopped"> to your message will cause it
1344to make better sense when the string C<"at foo line 123"> is appended.
1345Suppose you are running script "canasta".
a0d0e21e
LW
1346
1347 die "/etc/games is no good";
1348 die "/etc/games is no good, stopped";
1349
1350produce, respectively
1351
1352 /etc/games is no good at canasta line 123.
1353 /etc/games is no good, stopped at canasta line 123.
1354
a96d0188 1355If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1356previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1357This is useful for propagating exceptions:
1358
1359 eval { ... };
1360 die unless $@ =~ /Expected exception/;
1361
a96d0188 1362If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1363C<PROPAGATE> method, that method will be called with additional file
1364and line number parameters. The return value replaces the value in
34169887 1365C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1366were called.
1367
7660c0ab 1368If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1369
4c050ad5
NC
1370If an uncaught exception results in interpreter exit, the exit code is
1371determined from the values of C<$!> and C<$?> with this pseudocode:
1372
1373 exit $! if $!; # errno
1374 exit $? >> 8 if $? >> 8; # child exit status
1375 exit 255; # last resort
1376
1377The intent is to squeeze as much possible information about the likely cause
1378into the limited space of the system exit code. However, as C<$!> is the value
1379of C's C<errno>, which can be set by any system call, this means that the value
1380of the exit code used by C<die> can be non-predictable, so should not be relied
1381upon, other than to be non-zero.
1382
80d38338
TC
1383You can also call C<die> with a reference argument, and if this is trapped
1384within an C<eval>, C<$@> contains that reference. This permits more
1385elaborate exception handling using objects that maintain arbitrary state
1386about the exception. Such a scheme is sometimes preferable to matching
1387particular string values of C<$@> with regular expressions. Because C<$@>
1388is a global variable and C<eval> may be used within object implementations,
1389be careful that analyzing the error object doesn't replace the reference in
1390the global variable. It's easiest to make a local copy of the reference
1391before any manipulations. Here's an example:
52531d10 1392
80d38338 1393 use Scalar::Util "blessed";
da279afe 1394
52531d10 1395 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1396 if (my $ev_err = $@) {
1397 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1398 # handle Some::Module::Exception
1399 }
1400 else {
1401 # handle all other possible exceptions
1402 }
1403 }
1404
3b10bc60 1405Because Perl stringifies uncaught exception messages before display,
80d38338 1406you'll probably want to overload stringification operations on
52531d10
GS
1407exception objects. See L<overload> for details about that.
1408
19799a22
GS
1409You can arrange for a callback to be run just before the C<die>
1410does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1411handler is called with the error text and can change the error
19799a22 1412message, if it sees fit, by calling C<die> again. See
96090e4f 1413L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1414L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1415to be run only right before your program was to exit, this is not
3b10bc60 1416currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1417even inside eval()ed blocks/strings! If one wants the hook to do
1418nothing in such situations, put
fb73857a 1419
5ed4f2ec 1420 die @_ if $^S;
fb73857a 1421
19799a22
GS
1422as the first line of the handler (see L<perlvar/$^S>). Because
1423this promotes strange action at a distance, this counterintuitive
b76cc8ba 1424behavior may be fixed in a future release.
774d564b 1425
4c050ad5
NC
1426See also exit(), warn(), and the Carp module.
1427
a0d0e21e 1428=item do BLOCK
d74e8afc 1429X<do> X<block>
a0d0e21e
LW
1430
1431Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1432sequence of commands indicated by BLOCK. When modified by the C<while> or
1433C<until> loop modifier, executes the BLOCK once before testing the loop
1434condition. (On other statements the loop modifiers test the conditional
1435first.)
a0d0e21e 1436
4968c1e4 1437C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1438C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1439See L<perlsyn> for alternative strategies.
4968c1e4 1440
a0d0e21e 1441=item do SUBROUTINE(LIST)
d74e8afc 1442X<do>
a0d0e21e 1443
51124b83
FC
1444This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
1445a scalar variable or a subroutine beginning with C<&>.
a0d0e21e
LW
1446
1447=item do EXPR
d74e8afc 1448X<do>
a0d0e21e
LW
1449
1450Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1451file as a Perl script.
a0d0e21e
LW
1452
1453 do 'stat.pl';
1454
1455is just like
1456
986b19de 1457 eval `cat stat.pl`;
a0d0e21e 1458
2b5ab1e7 1459except that it's more efficient and concise, keeps track of the current
96090e4f
A
1460filename for error messages, searches the C<@INC> directories, and updates
1461C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1462these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1463cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1464same, however, in that it does reparse the file every time you call it,
1465so you probably don't want to do this inside a loop.
a0d0e21e 1466
8f1da26d 1467If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1468an error message in C<$@>. If C<do> cannot read the file, it returns undef
1469and sets C<$!> to the error. Always check C<$@> first, as compilation
1470could fail in a way that also sets C<$!>. If the file is successfully
1471compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1472
80d38338 1473Inclusion of library modules is better done with the
19799a22 1474C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1475and raise an exception if there's a problem.
a0d0e21e 1476
5a964f20
TC
1477You might like to use C<do> to read in a program configuration
1478file. Manual error checking can be done this way:
1479
b76cc8ba 1480 # read in config files: system first, then user
f86cebdf 1481 for $file ("/share/prog/defaults.rc",
b76cc8ba 1482 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1483 {
1484 unless ($return = do $file) {
1485 warn "couldn't parse $file: $@" if $@;
1486 warn "couldn't do $file: $!" unless defined $return;
1487 warn "couldn't run $file" unless $return;
1488 }
5a964f20
TC
1489 }
1490
a0d0e21e 1491=item dump LABEL
d74e8afc 1492X<dump> X<core> X<undump>
a0d0e21e 1493
1614b0e3
JD
1494=item dump
1495
19799a22
GS
1496This function causes an immediate core dump. See also the B<-u>
1497command-line switch in L<perlrun>, which does the same thing.
1498Primarily this is so that you can use the B<undump> program (not
1499supplied) to turn your core dump into an executable binary after
1500having initialized all your variables at the beginning of the
1501program. When the new binary is executed it will begin by executing
1502a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1503Think of it as a goto with an intervening core dump and reincarnation.
1504If C<LABEL> is omitted, restarts the program from the top.
1505
1506B<WARNING>: Any files opened at the time of the dump will I<not>
1507be open any more when the program is reincarnated, with possible
80d38338 1508resulting confusion by Perl.
19799a22 1509
59f521f4
RGS
1510This function is now largely obsolete, mostly because it's very hard to
1511convert a core file into an executable. That's why you should now invoke
1512it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1513typo.
19799a22 1514
ea9eb35a 1515Portability issues: L<perlport/dump>.
1516
532eee96 1517=item each HASH
d74e8afc 1518X<each> X<hash, iterator>
aa689395 1519
532eee96 1520=item each ARRAY
aeedbbed
NC
1521X<array, iterator>
1522
f5a93a43
TC
1523=item each EXPR
1524
80d38338
TC
1525When called in list context, returns a 2-element list consisting of the key
1526and value for the next element of a hash, or the index and value for the
1527next element of an array, so that you can iterate over it. When called in
1528scalar context, returns only the key (not the value) in a hash, or the index
1529in an array.
2f9daede 1530
aeedbbed 1531Hash entries are returned in an apparently random order. The actual random
3b10bc60 1532order is subject to change in future versions of Perl, but it is
504f80c1 1533guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1534function would produce on the same (unmodified) hash. Since Perl
22883ac5 15355.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1536for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1537
80d38338
TC
1538After C<each> has returned all entries from the hash or array, the next
1539call to C<each> returns the empty list in list context and C<undef> in
1540scalar context. The next call following that one restarts iteration. Each
1541hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1542and C<values>. The iterator is implicitly reset when C<each> has reached
1543the end as just described; it can be explicitly reset by calling C<keys> or
1544C<values> on the hash or array. If you add or delete a hash's elements
1545while iterating over it, entries may be skipped or duplicated--so don't do
1546that. Exception: It is always safe to delete the item most recently
3b10bc60 1547returned by C<each()>, so the following code works properly:
74fc8b5f
MJD
1548
1549 while (($key, $value) = each %hash) {
1550 print $key, "\n";
1551 delete $hash{$key}; # This is safe
1552 }
aa689395 1553
80d38338 1554This prints out your environment like the printenv(1) program,
3b10bc60 1555but in a different order:
a0d0e21e
LW
1556
1557 while (($key,$value) = each %ENV) {
a9a5a0dc 1558 print "$key=$value\n";
a0d0e21e
LW
1559 }
1560
f5a93a43
TC
1561Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1562reference to an unblessed hash or array. The argument will be dereferenced
1563automatically. This aspect of C<each> is considered highly experimental.
1564The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1565
1566 while (($key,$value) = each $hashref) { ... }
1567
8f1da26d 1568See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1569
1570=item eof FILEHANDLE
d74e8afc
ITB
1571X<eof>
1572X<end of file>
1573X<end-of-file>
a0d0e21e 1574
4633a7c4
LW
1575=item eof ()
1576
a0d0e21e
LW
1577=item eof
1578
8f1da26d 1579Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1580FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1581gives the real filehandle. (Note that this function actually
80d38338 1582reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1583interactive context.) Do not read from a terminal file (or call
19799a22 1584C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1585as terminals may lose the end-of-file condition if you do.
1586
820475bd 1587An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1588with empty parentheses is different. It refers to the pseudo file
820475bd 1589formed from the files listed on the command line and accessed via the
61eff3bc
JH
1590C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1591as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1592used will cause C<@ARGV> to be examined to determine if input is
67408cae 1593available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1594end-of-file will assume you are processing another C<@ARGV> list,
1595and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1596see L<perlop/"I/O Operators">.
820475bd 1597
61eff3bc 1598In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1599detect the end of each file, whereas C<eof()> will detect the end
1600of the very last file only. Examples:
a0d0e21e 1601
748a9306
LW
1602 # reset line numbering on each input file
1603 while (<>) {
a9a5a0dc
VP
1604 next if /^\s*#/; # skip comments
1605 print "$.\t$_";
5a964f20 1606 } continue {
a9a5a0dc 1607 close ARGV if eof; # Not eof()!
748a9306
LW
1608 }
1609
a0d0e21e
LW
1610 # insert dashes just before last line of last file
1611 while (<>) {
a9a5a0dc
VP
1612 if (eof()) { # check for end of last file
1613 print "--------------\n";
1614 }
1615 print;
4d0444a3 1616 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1617 }
1618
a0d0e21e 1619Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1620input operators typically return C<undef> when they run out of data or
1621encounter an error.
a0d0e21e
LW
1622
1623=item eval EXPR
d74e8afc 1624X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1625X<error, handling> X<exception, handling>
a0d0e21e
LW
1626
1627=item eval BLOCK
1628
ce2984c3
PF
1629=item eval
1630
c7cc6f1c
GS
1631In the first form, the return value of EXPR is parsed and executed as if it
1632were a little Perl program. The value of the expression (which is itself
8f1da26d 1633determined within scalar context) is first parsed, and if there were no
2341804c 1634errors, executed as a block within the lexical context of the current Perl
df4833a8 1635program. This means, that in particular, any outer lexical variables are
2341804c
DM
1636visible to it, and any package variable settings or subroutine and format
1637definitions remain afterwards.
1638
1639Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1640If EXPR is omitted, evaluates C<$_>. This form is typically used to
1641delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c 1642
7289c5e6
FC
1643If the C<unicode_eval> feature is enabled (which is the default under a
1644C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1645characters, so C<use utf8> declarations have no effect, and source filters
1646are forbidden. In the absence of the C<unicode_eval> feature, the string
1647will sometimes be treated as characters and sometimes as bytes, depending
1648on the internal encoding, and source filters activated within the C<eval>
1649exhibit the erratic, but historical, behaviour of affecting some outer file
1650scope that is still compiling. See also the L</evalbytes> keyword, which
1651always treats its input as a byte stream and works properly with source
1652filters, and the L<feature> pragma.
1653
c7cc6f1c 1654In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1655same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1656within the context of the current Perl program. This form is typically
1657used to trap exceptions more efficiently than the first (see below), while
1658also providing the benefit of checking the code within BLOCK at compile
1659time.
1660
1661The final semicolon, if any, may be omitted from the value of EXPR or within
1662the BLOCK.
1663
1664In both forms, the value returned is the value of the last expression
5a964f20 1665evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1666as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1667in void, scalar, or list context, depending on the context of the C<eval>
1668itself. See L</wantarray> for more on how the evaluation context can be
1669determined.
a0d0e21e 1670
19799a22 1671If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1672executed, C<eval> returns C<undef> in scalar context
774b80e8
FC
1673or an empty list in list context, and C<$@> is set to the error
1674message. (Prior to 5.16, a bug caused C<undef> to be returned
1675in list context for syntax errors, but not for runtime errors.)
1676If there was no error, C<$@> is set to the empty string. A
9cc672d4
FC
1677control flow operator like C<last> or C<goto> can bypass the setting of
1678C<$@>. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1679warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1680To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1681turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1682See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1683
19799a22
GS
1684Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1685determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1686is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1687the die operator is used to raise exceptions.
1688
5f1da31c
NT
1689If you want to trap errors when loading an XS module, some problems with
1690the binary interface (such as Perl version skew) may be fatal even with
df4833a8 1691C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
5f1da31c 1692
a0d0e21e
LW
1693If the code to be executed doesn't vary, you may use the eval-BLOCK
1694form to trap run-time errors without incurring the penalty of
1695recompiling each time. The error, if any, is still returned in C<$@>.
1696Examples:
1697
54310121 1698 # make divide-by-zero nonfatal
a0d0e21e
LW
1699 eval { $answer = $a / $b; }; warn $@ if $@;
1700
1701 # same thing, but less efficient
1702 eval '$answer = $a / $b'; warn $@ if $@;
1703
1704 # a compile-time error
5ed4f2ec 1705 eval { $answer = }; # WRONG
a0d0e21e
LW
1706
1707 # a run-time error
5ed4f2ec 1708 eval '$answer ='; # sets $@
a0d0e21e 1709
cf264981
SP
1710Using the C<eval{}> form as an exception trap in libraries does have some
1711issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1712may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1713You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1714as this example shows:
774d564b 1715
80d38338 1716 # a private exception trap for divide-by-zero
f86cebdf
GS
1717 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1718 warn $@ if $@;
774d564b
PP
1719
1720This is especially significant, given that C<__DIE__> hooks can call
19799a22 1721C<die> again, which has the effect of changing their error messages:
774d564b
PP
1722
1723 # __DIE__ hooks may modify error messages
1724 {
f86cebdf
GS
1725 local $SIG{'__DIE__'} =
1726 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1727 eval { die "foo lives here" };
1728 print $@ if $@; # prints "bar lives here"
774d564b
PP
1729 }
1730
19799a22 1731Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1732may be fixed in a future release.
1733
19799a22 1734With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1735being looked at when:
1736
5ed4f2ec 1737 eval $x; # CASE 1
1738 eval "$x"; # CASE 2
a0d0e21e 1739
5ed4f2ec 1740 eval '$x'; # CASE 3
1741 eval { $x }; # CASE 4
a0d0e21e 1742
5ed4f2ec 1743 eval "\$$x++"; # CASE 5
1744 $$x++; # CASE 6
a0d0e21e 1745
2f9daede 1746Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1747the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1748the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1749and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1750does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1751purely visual reasons, but it also has the advantage of compiling at
1752compile-time instead of at run-time.) Case 5 is a place where
19799a22 1753normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1754particular situation, you can just use symbolic references instead, as
1755in case 6.
a0d0e21e 1756
b6538e4f
TC
1757Before Perl 5.14, the assignment to C<$@> occurred before restoration
1758of localised variables, which means that for your code to run on older
b208c909 1759versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1760errors:
1761
1762 # alter $@ on nefarious repugnancy only
1763 {
1764 my $e;
1765 {
1766 local $@; # protect existing $@
1767 eval { test_repugnancy() };
b208c909 1768 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
8a5a710d
DN
1769 $@ =~ /nefarious/ and $e = $@;
1770 }
1771 die $e if defined $e
1772 }
1773
4968c1e4 1774C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1775C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1776
3b10bc60 1777An C<eval ''> executed within the C<DB> package doesn't see the usual
1778surrounding lexical scope, but rather the scope of the first non-DB piece
df4833a8 1779of code that called it. You don't normally need to worry about this unless
3b10bc60 1780you are writing a Perl debugger.
d819b83a 1781
7289c5e6
FC
1782=item evalbytes EXPR
1783X<evalbytes>
1784
1785=item evalbytes
1786
1787This function is like L</eval> with a string argument, except it always
1788parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1789string containing characters whose ordinal value exceeds 255 results in an
1790error. Source filters activated within the evaluated code apply to the
1791code itself.
1792
1793This function is only available under the C<evalbytes> feature, a
1794C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1795L<feature> for more information.
1796
a0d0e21e 1797=item exec LIST
d74e8afc 1798X<exec> X<execute>
a0d0e21e 1799
8bf3b016
GS
1800=item exec PROGRAM LIST
1801
3b10bc60 1802The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1803use C<system> instead of C<exec> if you want it to return. It fails and
1804returns false only if the command does not exist I<and> it is executed
fb73857a 1805directly instead of via your system's command shell (see below).
a0d0e21e 1806
19799a22 1807Since it's a common mistake to use C<exec> instead of C<system>, Perl
80d38338 1808warns you if there is a following statement that isn't C<die>, C<warn>,
3b10bc60 1809or C<exit> (if C<-w> is set--but you always do that, right?). If you
19799a22 1810I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1811can use one of these styles to avoid the warning:
1812
5a964f20
TC
1813 exec ('foo') or print STDERR "couldn't exec foo: $!";
1814 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1815
5a964f20 1816If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1817with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1818If there is only one scalar argument or an array with one element in it,
1819the argument is checked for shell metacharacters, and if there are any,
1820the entire argument is passed to the system's command shell for parsing
1821(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1822If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1823words and passed directly to C<execvp>, which is more efficient.
19799a22 1824Examples:
a0d0e21e 1825
19799a22
GS
1826 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1827 exec "sort $outfile | uniq";
a0d0e21e
LW
1828
1829If you don't really want to execute the first argument, but want to lie
1830to the program you are executing about its own name, you can specify
1831the program you actually want to run as an "indirect object" (without a
1832comma) in front of the LIST. (This always forces interpretation of the
54310121 1833LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1834the list.) Example:
1835
1836 $shell = '/bin/csh';
5ed4f2ec 1837 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1838
1839or, more directly,
1840
5ed4f2ec 1841 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1842
3b10bc60 1843When the arguments get executed via the system shell, results are
1844subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1845for details.
1846
19799a22
GS
1847Using an indirect object with C<exec> or C<system> is also more
1848secure. This usage (which also works fine with system()) forces
1849interpretation of the arguments as a multivalued list, even if the
1850list had just one argument. That way you're safe from the shell
1851expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1852
1853 @args = ( "echo surprise" );
1854
2b5ab1e7 1855 exec @args; # subject to shell escapes
f86cebdf 1856 # if @args == 1
2b5ab1e7 1857 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1858
1859The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1860program, passing it C<"surprise"> an argument. The second version didn't;
1861it tried to run a program named I<"echo surprise">, didn't find it, and set
1862C<$?> to a non-zero value indicating failure.
5a964f20 1863
80d38338 1864Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1865output before the exec, but this may not be supported on some platforms
1866(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1867in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 1868open handles to avoid lost output.
0f897271 1869
80d38338
TC
1870Note that C<exec> will not call your C<END> blocks, nor will it invoke
1871C<DESTROY> methods on your objects.
7660c0ab 1872
ea9eb35a 1873Portability issues: L<perlport/exec>.
1874
a0d0e21e 1875=item exists EXPR
d74e8afc 1876X<exists> X<autovivification>
a0d0e21e 1877
d0a76353
RS
1878Given an expression that specifies an element of a hash, returns true if the
1879specified element in the hash has ever been initialized, even if the
1880corresponding value is undefined.
a0d0e21e 1881
5ed4f2ec 1882 print "Exists\n" if exists $hash{$key};
1883 print "Defined\n" if defined $hash{$key};
01020589
GS
1884 print "True\n" if $hash{$key};
1885
d0a76353 1886exists may also be called on array elements, but its behavior is much less
8f1da26d 1887obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
1888that calling exists on array values is deprecated and likely to be removed in
1889a future version of Perl.
1890
5ed4f2ec 1891 print "Exists\n" if exists $array[$index];
1892 print "Defined\n" if defined $array[$index];
01020589 1893 print "True\n" if $array[$index];
a0d0e21e 1894
8f1da26d 1895A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
1896it exists, but the reverse doesn't necessarily hold true.
1897
afebc493
GS
1898Given an expression that specifies the name of a subroutine,
1899returns true if the specified subroutine has ever been declared, even
1900if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 1901does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
1902exist may still be callable: its package may have an C<AUTOLOAD>
1903method that makes it spring into existence the first time that it is
3b10bc60 1904called; see L<perlsub>.
afebc493 1905
5ed4f2ec 1906 print "Exists\n" if exists &subroutine;
1907 print "Defined\n" if defined &subroutine;
afebc493 1908
a0d0e21e 1909Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1910operation is a hash or array key lookup or subroutine name:
a0d0e21e 1911
5ed4f2ec 1912 if (exists $ref->{A}->{B}->{$key}) { }
1913 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 1914
5ed4f2ec 1915 if (exists $ref->{A}->{B}->[$ix]) { }
1916 if (exists $hash{A}{B}[$ix]) { }
01020589 1917
afebc493
GS
1918 if (exists &{$ref->{A}{B}{$key}}) { }
1919
3b10bc60 1920Although the mostly deeply nested array or hash will not spring into
1921existence just because its existence was tested, any intervening ones will.
61eff3bc 1922Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 1923into existence due to the existence test for the $key element above.
3b10bc60 1924This happens anywhere the arrow operator is used, including even here:
5a964f20 1925
2b5ab1e7 1926 undef $ref;
5ed4f2ec 1927 if (exists $ref->{"Some key"}) { }
1928 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
1929
1930This surprising autovivification in what does not at first--or even
1931second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1932release.
a0d0e21e 1933
afebc493
GS
1934Use of a subroutine call, rather than a subroutine name, as an argument
1935to exists() is an error.
1936
5ed4f2ec 1937 exists &sub; # OK
1938 exists &sub(); # Error
afebc493 1939
a0d0e21e 1940=item exit EXPR
d74e8afc 1941X<exit> X<terminate> X<abort>
a0d0e21e 1942
ce2984c3
PF
1943=item exit
1944
2b5ab1e7 1945Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1946
1947 $ans = <STDIN>;
1948 exit 0 if $ans =~ /^[Xx]/;
1949
19799a22 1950See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1951universally recognized values for EXPR are C<0> for success and C<1>
1952for error; other values are subject to interpretation depending on the
1953environment in which the Perl program is running. For example, exiting
195469 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1955the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1956
19799a22
GS
1957Don't use C<exit> to abort a subroutine if there's any chance that
1958someone might want to trap whatever error happened. Use C<die> instead,
1959which can be trapped by an C<eval>.
28757baa 1960
19799a22 1961The exit() function does not always exit immediately. It calls any
2b5ab1e7 1962defined C<END> routines first, but these C<END> routines may not
19799a22 1963themselves abort the exit. Likewise any object destructors that need to
60275626
FC
1964be called are called before the real exit. C<END> routines and destructors
1965can change the exit status by modifying C<$?>. If this is a problem, you
2b5ab1e7 1966can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1967See L<perlmod> for details.
5a964f20 1968
ea9eb35a 1969Portability issues: L<perlport/exit>.
1970
a0d0e21e 1971=item exp EXPR
d74e8afc 1972X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1973
54310121 1974=item exp
bbce6d69 1975
b76cc8ba 1976Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1977If EXPR is omitted, gives C<exp($_)>.
1978
1979=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1980X<fcntl>
a0d0e21e 1981
f86cebdf 1982Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1983
1984 use Fcntl;
1985
0ade1984 1986first to get the correct constant definitions. Argument processing and
3b10bc60 1987value returned work just like C<ioctl> below.
a0d0e21e
LW
1988For example:
1989
1990 use Fcntl;
5a964f20 1991 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 1992 or die "can't fcntl F_GETFL: $!";
5a964f20 1993
554ad1fc 1994You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1995Like C<ioctl>, it maps a C<0> return from the system call into
1996C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1997in numeric context. It is also exempt from the normal B<-w> warnings
1998on improper numeric conversions.
5a964f20 1999
3b10bc60 2000Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
2001doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2002manpage to learn what functions are available on your system.
a0d0e21e 2003
be2f7487 2004Here's an example of setting a filehandle named C<REMOTE> to be
2005non-blocking at the system level. You'll have to negotiate C<$|>
2006on your own, though.
2007
2008 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2009
2010 $flags = fcntl(REMOTE, F_GETFL, 0)
2011 or die "Can't get flags for the socket: $!\n";
2012
2013 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2014 or die "Can't set flags for the socket: $!\n";
2015
ea9eb35a 2016Portability issues: L<perlport/fcntl>.
2017
cfa52385
FC
2018=item __FILE__
2019X<__FILE__>
2020
2021A special token that returns the name of the file in which it occurs.
2022
a0d0e21e 2023=item fileno FILEHANDLE
d74e8afc 2024X<fileno>
a0d0e21e 2025
2b5ab1e7 2026Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
2027filehandle is not open. If there is no real file descriptor at the OS
2028level, as can happen with filehandles connected to memory objects via
2029C<open> with a reference for the third argument, -1 is returned.
2030
2031This is mainly useful for constructing
19799a22 2032bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
2033If FILEHANDLE is an expression, the value is taken as an indirect
2034filehandle, generally its name.
5a964f20 2035
b76cc8ba 2036You can use this to find out whether two handles refer to the
5a964f20
TC
2037same underlying descriptor:
2038
2039 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 2040 print "THIS and THAT are dups\n";
b76cc8ba
NIS
2041 }
2042
a0d0e21e 2043=item flock FILEHANDLE,OPERATION
d74e8afc 2044X<flock> X<lock> X<locking>
a0d0e21e 2045
19799a22
GS
2046Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2047for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2048machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2049C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2050entire files only, not records.
2b5ab1e7
TC
2051
2052Two potentially non-obvious but traditional C<flock> semantics are
2053that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2054are B<merely advisory>. Such discretionary locks are more flexible, but
2055offer fewer guarantees. This means that programs that do not also use
2056C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2057your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2058for details. It's best to assume traditional behavior if you're writing
2059portable programs. (But if you're not, you should as always feel perfectly
2060free to write for your own system's idiosyncrasies (sometimes called
2061"features"). Slavish adherence to portability concerns shouldn't get
2062in the way of your getting your job done.)
a3cb178b 2063
8ebc5c01
PP
2064OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2065LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2066you can use the symbolic names if you import them from the L<Fcntl> module,
2067either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2068requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2069releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2070LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2071waiting for the lock; check the return status to see if you got it.
68dc0745 2072
2b5ab1e7
TC
2073To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2074before locking or unlocking it.
8ebc5c01 2075
f86cebdf 2076Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2077locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2078are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2079implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
2080differing semantics shouldn't bite too many people.
2081
becacb53
TM
2082Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2083be open with read intent to use LOCK_SH and requires that it be open
2084with write intent to use LOCK_EX.
2085
19799a22
GS
2086Note also that some versions of C<flock> cannot lock things over the
2087network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2088that. If you like you can force Perl to ignore your system's flock(2)
2089function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2090the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2091and build a new Perl.
4633a7c4
LW
2092
2093Here's a mailbox appender for BSD systems.
a0d0e21e 2094
7ed5353d 2095 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
2096
2097 sub lock {
a9a5a0dc
VP
2098 my ($fh) = @_;
2099 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2100
a9a5a0dc
VP
2101 # and, in case someone appended while we were waiting...
2102 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2103 }
2104
2105 sub unlock {
a9a5a0dc
VP
2106 my ($fh) = @_;
2107 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2108 }
2109
b0169937 2110 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2111 or die "Can't open mailbox: $!";
a0d0e21e 2112
7ed5353d 2113 lock($mbox);
b0169937 2114 print $mbox $msg,"\n\n";
7ed5353d 2115 unlock($mbox);
a0d0e21e 2116
3b10bc60 2117On systems that support a real flock(2), locks are inherited across fork()
2118calls, whereas those that must resort to the more capricious fcntl(2)
2119function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2120
cb1a09d0 2121See also L<DB_File> for other flock() examples.
a0d0e21e 2122
ea9eb35a 2123Portability issues: L<perlport/flock>.
2124
a0d0e21e 2125=item fork
d74e8afc 2126X<fork> X<child> X<parent>
a0d0e21e 2127
2b5ab1e7
TC
2128Does a fork(2) system call to create a new process running the
2129same program at the same point. It returns the child pid to the
2130parent process, C<0> to the child process, or C<undef> if the fork is
2131unsuccessful. File descriptors (and sometimes locks on those descriptors)
2132are shared, while everything else is copied. On most systems supporting
2133fork(), great care has gone into making it extremely efficient (for
2134example, using copy-on-write technology on data pages), making it the
2135dominant paradigm for multitasking over the last few decades.
5a964f20 2136
80d38338 2137Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
2138output before forking the child process, but this may not be supported
2139on some platforms (see L<perlport>). To be safe, you may need to set
2140C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2141C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2142
19799a22 2143If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2144accumulate zombies. On some systems, you can avoid this by setting
2145C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2146forking and reaping moribund children.
cb1a09d0 2147
28757baa
PP
2148Note that if your forked child inherits system file descriptors like
2149STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2150if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2151backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2152You should reopen those to F</dev/null> if it's any issue.
28757baa 2153
ea9eb35a 2154On some platforms such as Windows, where the fork() system call is not available,
2155Perl can be built to emulate fork() in the Perl interpreter. The emulation is designed to,
2156at the level of the Perl program, be as compatible as possible with the "Unix" fork().
6d17f725 2157However it has limitations that have to be considered in code intended to be portable.
ea9eb35a 2158See L<perlfork> for more details.
2159
2160Portability issues: L<perlport/fork>.
2161
cb1a09d0 2162=item format
d74e8afc 2163X<format>
cb1a09d0 2164
19799a22 2165Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2166example:
2167
54310121 2168 format Something =
a9a5a0dc
VP
2169 Test: @<<<<<<<< @||||| @>>>>>
2170 $str, $%, '$' . int($num)
cb1a09d0
AD
2171 .
2172
2173 $str = "widget";
184e9718 2174 $num = $cost/$quantity;
cb1a09d0
AD
2175 $~ = 'Something';
2176 write;
2177
2178See L<perlform> for many details and examples.
2179
8903cb82 2180=item formline PICTURE,LIST
d74e8afc 2181X<formline>
a0d0e21e 2182
5a964f20 2183This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2184too. It formats (see L<perlform>) a list of values according to the
2185contents of PICTURE, placing the output into the format output
7660c0ab 2186accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2187Eventually, when a C<write> is done, the contents of
cf264981
SP
2188C<$^A> are written to some filehandle. You could also read C<$^A>
2189and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2190does one C<formline> per line of form, but the C<formline> function itself
748a9306 2191doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2192that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2193You may therefore need to use multiple formlines to implement a single
3b10bc60 2194record format, just like the C<format> compiler.
748a9306 2195
19799a22 2196Be careful if you put double quotes around the picture, because an C<@>
748a9306 2197character may be taken to mean the beginning of an array name.
19799a22 2198C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2199
445b09e5
FC
2200If you are trying to use this instead of C<write> to capture the output,
2201you may find it easier to open a filehandle to a scalar
2202(C<< open $fh, ">", \$output >>) and write to that instead.
2203
a0d0e21e 2204=item getc FILEHANDLE
f723aae1 2205X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2206
2207=item getc
2208
2209Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2210or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2211the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2212STDIN. This is not particularly efficient. However, it cannot be
2213used by itself to fetch single characters without waiting for the user
2214to hit enter. For that, try something more like:
4633a7c4
LW
2215
2216 if ($BSD_STYLE) {
a9a5a0dc 2217 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2218 }
2219 else {
a9a5a0dc 2220 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2221 }
2222
2223 $key = getc(STDIN);
2224
2225 if ($BSD_STYLE) {
a9a5a0dc 2226 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2227 }
2228 else {
3b10bc60 2229 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2230 }
2231 print "\n";
2232
54310121
PP
2233Determination of whether $BSD_STYLE should be set
2234is left as an exercise to the reader.
cb1a09d0 2235
19799a22 2236The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2237systems purporting POSIX compliance. See also the C<Term::ReadKey>
a3390c9f 2238module from your nearest CPAN site; details on CPAN can be found under
2b5ab1e7 2239L<perlmodlib/CPAN>.
a0d0e21e
LW
2240
2241=item getlogin
d74e8afc 2242X<getlogin> X<login>
a0d0e21e 2243
cf264981 2244This implements the C library function of the same name, which on most
3b10bc60 2245systems returns the current login from F</etc/utmp>, if any. If it
2246returns the empty string, use C<getpwuid>.
a0d0e21e 2247
f86702cc 2248 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2249
19799a22
GS
2250Do not consider C<getlogin> for authentication: it is not as
2251secure as C<getpwuid>.
4633a7c4 2252
ea9eb35a 2253Portability issues: L<perlport/getlogin>.
2254
a0d0e21e 2255=item getpeername SOCKET
d74e8afc 2256X<getpeername> X<peer>
a0d0e21e 2257
a3390c9f
FC
2258Returns the packed sockaddr address of the other end of the SOCKET
2259connection.
a0d0e21e 2260
4633a7c4
LW
2261 use Socket;
2262 $hersockaddr = getpeername(SOCK);
19799a22 2263 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2264 $herhostname = gethostbyaddr($iaddr, AF_INET);
2265 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2266
2267=item getpgrp PID
d74e8afc 2268X<getpgrp> X<group>
a0d0e21e 2269
47e29363 2270Returns the current process group for the specified PID. Use
7660c0ab 2271a PID of C<0> to get the current process group for the
4633a7c4 2272current process. Will raise an exception if used on a machine that
a3390c9f
FC
2273doesn't implement getpgrp(2). If PID is omitted, returns the process
2274group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2275does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2276
ea9eb35a 2277Portability issues: L<perlport/getpgrp>.
2278
a0d0e21e 2279=item getppid
d74e8afc 2280X<getppid> X<parent> X<pid>
a0d0e21e
LW
2281
2282Returns the process id of the parent process.
2283
4d76a344
RGS
2284Note for Linux users: on Linux, the C functions C<getpid()> and
2285C<getppid()> return different values from different threads. In order to
3b10bc60 2286be portable, this behavior is not reflected by the Perl-level function
4d76a344 2287C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2288to call the underlying C<getppid()>, you may use the CPAN module
2289C<Linux::Pid>.
4d76a344 2290
ea9eb35a 2291Portability issues: L<perlport/getppid>.
2292
a0d0e21e 2293=item getpriority WHICH,WHO
d74e8afc 2294X<getpriority> X<priority> X<nice>
a0d0e21e 2295
4633a7c4 2296Returns the current priority for a process, a process group, or a user.
01aa884e 2297(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2298machine that doesn't implement getpriority(2).
a0d0e21e 2299
ea9eb35a 2300Portability issues: L<perlport/getpriority>.
2301
a0d0e21e 2302=item getpwnam NAME
d74e8afc
ITB
2303X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2304X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2305X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2306X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2307X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2308X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2309
2310=item getgrnam NAME
2311
2312=item gethostbyname NAME
2313
2314=item getnetbyname NAME
2315
2316=item getprotobyname NAME
2317
2318=item getpwuid UID
2319
2320=item getgrgid GID
2321
2322=item getservbyname NAME,PROTO
2323
2324=item gethostbyaddr ADDR,ADDRTYPE
2325
2326=item getnetbyaddr ADDR,ADDRTYPE
2327
2328=item getprotobynumber NUMBER
2329
2330=item getservbyport PORT,PROTO
2331
2332=item getpwent
2333
2334=item getgrent
2335
2336=item gethostent
2337
2338=item getnetent
2339
2340=item getprotoent
2341
2342=item getservent
2343
2344=item setpwent
2345
2346=item setgrent
2347
2348=item sethostent STAYOPEN
2349
2350=item setnetent STAYOPEN
2351
2352=item setprotoent STAYOPEN
2353
2354=item setservent STAYOPEN
2355
2356=item endpwent
2357
2358=item endgrent
2359
2360=item endhostent
2361
2362=item endnetent
2363
2364=item endprotoent
2365
2366=item endservent
2367
80d38338
TC
2368These routines are the same as their counterparts in the
2369system C library. In list context, the return values from the
a0d0e21e
LW
2370various get routines are as follows:
2371
2372 ($name,$passwd,$uid,$gid,
6ee623d5 2373 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2374 ($name,$passwd,$gid,$members) = getgr*
2375 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2376 ($name,$aliases,$addrtype,$net) = getnet*
2377 ($name,$aliases,$proto) = getproto*
2378 ($name,$aliases,$port,$proto) = getserv*
2379
3b10bc60 2380(If the entry doesn't exist you get an empty list.)
a0d0e21e 2381
4602f195
JH
2382The exact meaning of the $gcos field varies but it usually contains
2383the real name of the user (as opposed to the login name) and other
2384information pertaining to the user. Beware, however, that in many
2385system users are able to change this information and therefore it
106325ad 2386cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2387L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2388login shell, are also tainted, for the same reason.
4602f195 2389
5a964f20 2390In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2391lookup by name, in which case you get the other thing, whatever it is.
2392(If the entry doesn't exist you get the undefined value.) For example:
2393
5a964f20
TC
2394 $uid = getpwnam($name);
2395 $name = getpwuid($num);
2396 $name = getpwent();
2397 $gid = getgrnam($name);
08a33e13 2398 $name = getgrgid($num);
5a964f20
TC
2399 $name = getgrent();
2400 #etc.
a0d0e21e 2401
4602f195 2402In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2403in that they are unsupported on many systems. If the
4602f195
JH
2404$quota is unsupported, it is an empty scalar. If it is supported, it
2405usually encodes the disk quota. If the $comment field is unsupported,
2406it is an empty scalar. If it is supported it usually encodes some
2407administrative comment about the user. In some systems the $quota
2408field may be $change or $age, fields that have to do with password
2409aging. In some systems the $comment field may be $class. The $expire
2410field, if present, encodes the expiration period of the account or the
2411password. For the availability and the exact meaning of these fields
8f1da26d 2412in your system, please consult getpwnam(3) and your system's
4602f195
JH
2413F<pwd.h> file. You can also find out from within Perl what your
2414$quota and $comment fields mean and whether you have the $expire field
2415by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2416C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2417files are supported only if your vendor has implemented them in the
4602f195 2418intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2419shadow versions if you're running under privilege or if there exists
cf264981 2420the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2421and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2422facility are unlikely to be supported.
6ee623d5 2423
a3390c9f 2424The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2425the login names of the members of the group.
2426
2427For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2428C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2429C<@addrs> value returned by a successful call is a list of raw
2430addresses returned by the corresponding library call. In the
2431Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2432by saying something like:
2433
f337b084 2434 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2435
2b5ab1e7
TC
2436The Socket library makes this slightly easier:
2437
2438 use Socket;
2439 $iaddr = inet_aton("127.1"); # or whatever address
2440 $name = gethostbyaddr($iaddr, AF_INET);
2441
2442 # or going the other way
19799a22 2443 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2444
d760c846
GS
2445In the opposite way, to resolve a hostname to the IP address
2446you can write this:
2447
2448 use Socket;
2449 $packed_ip = gethostbyname("www.perl.org");
2450 if (defined $packed_ip) {
2451 $ip_address = inet_ntoa($packed_ip);
2452 }
2453
b018eaf1 2454Make sure C<gethostbyname()> is called in SCALAR context and that
d760c846
GS
2455its return value is checked for definedness.
2456
0d043efa
FC
2457The C<getprotobynumber> function, even though it only takes one argument,
2458has the precedence of a list operator, so beware:
2459
2460 getprotobynumber $number eq 'icmp' # WRONG
2461 getprotobynumber($number eq 'icmp') # actually means this
2462 getprotobynumber($number) eq 'icmp' # better this way
2463
19799a22
GS
2464If you get tired of remembering which element of the return list
2465contains which return value, by-name interfaces are provided
2466in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2467C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2468and C<User::grent>. These override the normal built-ins, supplying
2469versions that return objects with the appropriate names
2470for each field. For example:
5a964f20
TC
2471
2472 use File::stat;
2473 use User::pwent;
2474 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2475
a3390c9f 2476Even though it looks as though they're the same method calls (uid),
b76cc8ba 2477they aren't, because a C<File::stat> object is different from
19799a22 2478a C<User::pwent> object.
5a964f20 2479
ea9eb35a 2480Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2481
a0d0e21e 2482=item getsockname SOCKET
d74e8afc 2483X<getsockname>
a0d0e21e 2484
19799a22
GS
2485Returns the packed sockaddr address of this end of the SOCKET connection,
2486in case you don't know the address because you have several different
2487IPs that the connection might have come in on.
a0d0e21e 2488
4633a7c4
LW
2489 use Socket;
2490 $mysockaddr = getsockname(SOCK);
19799a22 2491 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2492 printf "Connect to %s [%s]\n",
19799a22
GS
2493 scalar gethostbyaddr($myaddr, AF_INET),
2494 inet_ntoa($myaddr);
a0d0e21e
LW
2495
2496=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2497X<getsockopt>
a0d0e21e 2498
636e6b1f
TH
2499Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2500Options may exist at multiple protocol levels depending on the socket
2501type, but at least the uppermost socket level SOL_SOCKET (defined in the
2502C<Socket> module) will exist. To query options at another level the
2503protocol number of the appropriate protocol controlling the option
2504should be supplied. For example, to indicate that an option is to be
2505interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2506number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2507
80d38338 2508The function returns a packed string representing the requested socket
3b10bc60 2509option, or C<undef> on error, with the reason for the error placed in
a3390c9f 2510C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2511consult getsockopt(2) for details. A common case is that the option is an
2512integer, in which case the result is a packed integer, which you can decode
2513using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2514
8f1da26d 2515Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2516
4852725b 2517 use Socket qw(:all);
636e6b1f
TH
2518
2519 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2520 or die "Could not determine the protocol number for tcp";
4852725b
DD
2521 # my $tcp = IPPROTO_TCP; # Alternative
2522 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2523 or die "getsockopt TCP_NODELAY: $!";
636e6b1f
TH
2524 my $nodelay = unpack("I", $packed);
2525 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2526
ea9eb35a 2527Portability issues: L<perlport/getsockopt>.
a0d0e21e 2528
15a348aa 2529=item given EXPR BLOCK
2530X<given>
2531
2532=item given BLOCK
2533
2534C<given> is analogous to the C<switch> keyword in other languages. C<given>
2535and C<when> are used in Perl to implement C<switch>/C<case> like statements.
8f1da26d 2536Only available after Perl 5.10. For example:
15a348aa 2537
8f1da26d 2538 use v5.10;
15a348aa 2539 given ($fruit) {
2540 when (/apples?/) {
2541 print "I like apples."
2542 }
2543 when (/oranges?/) {
2544 print "I don't like oranges."
2545 }
2546 default {
2547 print "I don't like anything"
2548 }
2549 }
2550
2551See L<perlsyn/"Switch statements"> for detailed information.
2552
a0d0e21e 2553=item glob EXPR
d74e8afc 2554X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2555
0a753a76
PP
2556=item glob
2557
d9a9d457
JL
2558In list context, returns a (possibly empty) list of filename expansions on
2559the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2560scalar context, glob iterates through such filename expansions, returning
2561undef when the list is exhausted. This is the internal function
2562implementing the C<< <*.c> >> operator, but you can use it directly. If
2563EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2564more detail in L<perlop/"I/O Operators">.
a0d0e21e 2565
80d38338
TC
2566Note that C<glob> splits its arguments on whitespace and treats
2567each segment as separate pattern. As such, C<glob("*.c *.h")>
2568matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2569C<glob(".* *")> matches all files in the current working directory.
a91bb7b1
TC
2570If you want to glob filenames that might contain whitespace, you'll
2571have to use extra quotes around the spacey filename to protect it.
2572For example, to glob filenames that have an C<e> followed by a space
2573followed by an C<f>, use either of:
2574
2575 @spacies = <"*e f*">;
2576 @spacies = glob '"*e f*"';
2577 @spacies = glob q("*e f*");
2578
2579If you had to get a variable through, you could do this:
2580
2581 @spacies = glob "'*${var}e f*'";
2582 @spacies = glob qq("*${var}e f*");
80d38338
TC
2583
2584If non-empty braces are the only wildcard characters used in the
2585C<glob>, no filenames are matched, but potentially many strings
2586are returned. For example, this produces nine strings, one for
2587each pairing of fruits and colors:
2588
2589 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2590
3a4b19e4 2591Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2592C<File::Glob> extension. See L<File::Glob> for details, including
2593C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2594
ea9eb35a 2595Portability issues: L<perlport/glob>.
2596
a0d0e21e 2597=item gmtime EXPR
d74e8afc 2598X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2599
ce2984c3
PF
2600=item gmtime
2601
4509d391 2602Works just like L</localtime> but the returned values are
435fbc73 2603localized for the standard Greenwich time zone.
a0d0e21e 2604
a3390c9f
FC
2605Note: When called in list context, $isdst, the last value
2606returned by gmtime, is always C<0>. There is no
435fbc73 2607Daylight Saving Time in GMT.
0a753a76 2608
ea9eb35a 2609Portability issues: L<perlport/gmtime>.
62aa5637 2610
a0d0e21e 2611=item goto LABEL
d74e8afc 2612X<goto> X<jump> X<jmp>
a0d0e21e 2613
748a9306
LW
2614=item goto EXPR
2615
a0d0e21e
LW
2616=item goto &NAME
2617
b500e03b
GG
2618The C<goto-LABEL> form finds the statement labeled with LABEL and
2619resumes execution there. It can't be used to get out of a block or
2620subroutine given to C<sort>. It can be used to go almost anywhere
2621else within the dynamic scope, including out of subroutines, but it's
2622usually better to use some other construct such as C<last> or C<die>.
2623The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2624(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2625does not offer named loops combined with loop control. Perl does, and
2626this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2627
7660c0ab
A
2628The C<goto-EXPR> form expects a label name, whose scope will be resolved
2629dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2630necessarily recommended if you're optimizing for maintainability:
2631
2632 goto ("FOO", "BAR", "GLARCH")[$i];
2633
887d89fd
FC
2634As shown in this example, C<goto-EXPR> is exempt from the "looks like a
2635function" rule. A pair of parentheses following it does not (necessarily)
2636delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2637
b500e03b 2638Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2639deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2640go into any construct that requires initialization, such as a
2641subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2642construct that is optimized away.
b500e03b 2643
1b6921cb
BT
2644The C<goto-&NAME> form is quite different from the other forms of
2645C<goto>. In fact, it isn't a goto in the normal sense at all, and
2646doesn't have the stigma associated with other gotos. Instead, it
2647exits the current subroutine (losing any changes set by local()) and
2648immediately calls in its place the named subroutine using the current
2649value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2650load another subroutine and then pretend that the other subroutine had
2651been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2652in the current subroutine are propagated to the other subroutine.)
2653After the C<goto>, not even C<caller> will be able to tell that this
2654routine was called first.
2655
2656NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2657containing a code reference or a block that evaluates to a code
6cb9131c 2658reference.
a0d0e21e
LW
2659
2660=item grep BLOCK LIST
d74e8afc 2661X<grep>
a0d0e21e
LW
2662
2663=item grep EXPR,LIST
2664
2b5ab1e7
TC
2665This is similar in spirit to, but not the same as, grep(1) and its
2666relatives. In particular, it is not limited to using regular expressions.
2f9daede 2667
a0d0e21e 2668Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2669C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2670elements for which the expression evaluated to true. In scalar
2671context, returns the number of times the expression was true.
a0d0e21e
LW
2672
2673 @foo = grep(!/^#/, @bar); # weed out comments
2674
2675or equivalently,
2676
2677 @foo = grep {!/^#/} @bar; # weed out comments
2678
be3174d2
GS
2679Note that C<$_> is an alias to the list value, so it can be used to
2680modify the elements of the LIST. While this is useful and supported,
2681it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2682Similarly, grep returns aliases into the original list, much as a for
2683loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2684element of a list returned by grep (for example, in a C<foreach>, C<map>
2685or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2686This is usually something to be avoided when writing clear code.
a0d0e21e 2687
a4fb8298 2688If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2689been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2690the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2691can't be seen from the outside, avoiding any potential side-effects.
2692
19799a22 2693See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2694
a0d0e21e 2695=item hex EXPR
d74e8afc 2696X<hex> X<hexadecimal>
a0d0e21e 2697
54310121 2698=item hex
bbce6d69 2699
2b5ab1e7 2700Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2701(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2702L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2703
2704 print hex '0xAf'; # prints '175'
2705 print hex 'aF'; # same
a0d0e21e 2706
19799a22 2707Hex strings may only represent integers. Strings that would cause
53305cf1 2708integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11 2709unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 2710L</sprintf>, and L</unpack>.
19799a22 2711
ce2984c3 2712=item import LIST
d74e8afc 2713X<import>
a0d0e21e 2714
19799a22 2715There is no builtin C<import> function. It is just an ordinary
4633a7c4 2716method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2717names to another module. The C<use> function calls the C<import> method
cea6626f 2718for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2719
2720=item index STR,SUBSTR,POSITION
d74e8afc 2721X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2722
2723=item index STR,SUBSTR
2724
2b5ab1e7
TC
2725The index function searches for one string within another, but without
2726the wildcard-like behavior of a full regular-expression pattern match.
2727It returns the position of the first occurrence of SUBSTR in STR at
2728or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2729beginning of the string. POSITION before the beginning of the string
2730or after its end is treated as if it were the beginning or the end,
e1dccc0d
Z
2731respectively. POSITION and the return value are based at zero.
2732If the substring is not found, C<index> returns -1.
a0d0e21e
LW
2733
2734=item int EXPR
f723aae1 2735X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2736
54310121 2737=item int
bbce6d69 2738
7660c0ab 2739Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 2740You should not use this function for rounding: one because it truncates
3b10bc60 2741towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
2742numbers can sometimes produce counterintuitive results. For example,
2743C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2744because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2745the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2746functions will serve you better than will int().
a0d0e21e
LW
2747
2748=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2749X<ioctl>
a0d0e21e 2750
2b5ab1e7 2751Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2752
5ed4f2ec 2753 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2754
a11c483f 2755to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2756exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2757own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2758(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2759may help you in this, but it's nontrivial.) SCALAR will be read and/or
3b10bc60 2760written depending on the FUNCTION; a C pointer to the string value of SCALAR
19799a22 2761will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2762has no string value but does have a numeric value, that value will be
2763passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2764true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2765functions may be needed to manipulate the values of structures used by
b76cc8ba 2766C<ioctl>.
a0d0e21e 2767
19799a22 2768The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e 2769
5ed4f2ec 2770 if OS returns: then Perl returns:
2771 -1 undefined value
2772 0 string "0 but true"
2773 anything else that number
a0d0e21e 2774
19799a22 2775Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2776still easily determine the actual value returned by the operating
2777system:
2778
2b5ab1e7 2779 $retval = ioctl(...) || -1;
a0d0e21e
LW
2780 printf "System returned %d\n", $retval;
2781
be2f7487 2782The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2783about improper numeric conversions.
2784
ea9eb35a 2785Portability issues: L<perlport/ioctl>.
2786
a0d0e21e 2787=item join EXPR,LIST
d74e8afc 2788X<join>
a0d0e21e 2789
2b5ab1e7
TC
2790Joins the separate strings of LIST into a single string with fields
2791separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2792
2b5ab1e7 2793 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2794
eb6e2d6f
GS
2795Beware that unlike C<split>, C<join> doesn't take a pattern as its
2796first argument. Compare L</split>.
a0d0e21e 2797
532eee96 2798=item keys HASH
d74e8afc 2799X<keys> X<key>
aa689395 2800
532eee96 2801=item keys ARRAY
aeedbbed 2802
f5a93a43
TC
2803=item keys EXPR
2804
aeedbbed
NC
2805Returns a list consisting of all the keys of the named hash, or the indices
2806of an array. (In scalar context, returns the number of keys or indices.)
504f80c1 2807
aeedbbed 2808The keys of a hash are returned in an apparently random order. The actual
3b10bc60 2809random order is subject to change in future versions of Perl, but it
504f80c1 2810is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6 2811function produces (given that the hash has not been modified). Since
c5f61d2f 2812Perl 5.8.1 the ordering can be different even between different runs of
4546b9e6 2813Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2814Attacks">).
504f80c1 2815
8f1da26d 2816As a side effect, calling keys() resets the internal interator of the HASH or ARRAY
cf264981
SP
2817(see L</each>). In particular, calling keys() in void context resets
2818the iterator with no other overhead.
a0d0e21e 2819
aa689395 2820Here is yet another way to print your environment:
a0d0e21e
LW
2821
2822 @keys = keys %ENV;
2823 @values = values %ENV;
b76cc8ba 2824 while (@keys) {
a9a5a0dc 2825 print pop(@keys), '=', pop(@values), "\n";
a0d0e21e
LW
2826 }
2827
2828or how about sorted by key:
2829
2830 foreach $key (sort(keys %ENV)) {
a9a5a0dc 2831 print $key, '=', $ENV{$key}, "\n";
a0d0e21e
LW
2832 }
2833
8ea1e5d4
GS
2834The returned values are copies of the original keys in the hash, so
2835modifying them will not affect the original hash. Compare L</values>.
2836
19799a22 2837To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2838Here's a descending numeric sort of a hash by its values:
4633a7c4 2839
5a964f20 2840 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
a9a5a0dc 2841 printf "%4d %s\n", $hash{$key}, $key;
4633a7c4
LW
2842 }
2843
3b10bc60 2844Used as an lvalue, C<keys> allows you to increase the number of hash buckets
aa689395
PP
2845allocated for the given hash. This can gain you a measure of efficiency if
2846you know the hash is going to get big. (This is similar to pre-extending
2847an array by assigning a larger number to $#array.) If you say
55497cff
PP
2848
2849 keys %hash = 200;
2850
ab192400
GS
2851then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2852in fact, since it rounds up to the next power of two. These
55497cff
PP
2853buckets will be retained even if you do C<%hash = ()>, use C<undef
2854%hash> if you want to free the storage while C<%hash> is still in scope.
2855You can't shrink the number of buckets allocated for the hash using
19799a22 2856C<keys> in this way (but you needn't worry about doing this by accident,
aeedbbed
NC
2857as trying has no effect). C<keys @array> in an lvalue context is a syntax
2858error.
55497cff 2859
f5a93a43
TC
2860Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
2861a reference to an unblessed hash or array. The argument will be
2862dereferenced automatically. This aspect of C<keys> is considered highly
2863experimental. The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
2864
2865 for (keys $hashref) { ... }
2866 for (keys $obj->get_arrayref) { ... }
2867
8f1da26d 2868See also C<each>, C<values>, and C<sort>.
ab192400 2869
b350dd2f 2870=item kill SIGNAL, LIST
9c7e4b76
KW
2871
2872=item kill SIGNAL
d74e8afc 2873X<kill> X<signal>
a0d0e21e 2874
b350dd2f 2875Sends a signal to a list of processes. Returns the number of
517db077
GS
2876processes successfully signaled (which is not necessarily the
2877same as the number actually killed).
a0d0e21e
LW
2878
2879 $cnt = kill 1, $child1, $child2;
2880 kill 9, @goners;
2881
3b10bc60 2882If SIGNAL is zero, no signal is sent to the process, but C<kill>
2883checks whether it's I<possible> to send a signal to it (that
70fb64f6 2884means, to be brief, that the process is owned by the same user, or we are
3b10bc60 2885the super-user). This is useful to check that a child process is still
81fd35db
DN
2886alive (even if only as a zombie) and hasn't changed its UID. See
2887L<perlport> for notes on the portability of this construct.
b350dd2f 2888
e2c0f81f
DG
2889Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2890of processes. That means you usually want to use positive not negative signals.
2891You may also use a signal name in quotes.
2892
2893The behavior of kill when a I<PROCESS> number is zero or negative depends on
2894the operating system. For example, on POSIX-conforming systems, zero will
2895signal the current process group and -1 will signal all processes.
1e9c1022
JL
2896
2897See L<perlipc/"Signals"> for more details.
a0d0e21e 2898
ea9eb35a 2899On some platforms such as Windows where the fork() system call is not available.
2900Perl can be built to emulate fork() at the interpreter level.
6d17f725 2901This emulation has limitations related to kill that have to be considered,
ea9eb35a 2902for code running on Windows and in code intended to be portable.
2903
2904See L<perlfork> for more details.
2905
9c7e4b76
KW
2906If there is no I<LIST> of processes, no signal is sent, and the return
2907value is 0. This form is sometimes used, however, because it causes
2908tainting checks to be run. But see
2909L<perlsec/Laundering and Detecting Tainted Data>.
2910
ea9eb35a 2911Portability issues: L<perlport/kill>.
2912
a0d0e21e 2913=item last LABEL
d74e8afc 2914X<last> X<break>
a0d0e21e
LW
2915
2916=item last
2917
2918The C<last> command is like the C<break> statement in C (as used in
2919loops); it immediately exits the loop in question. If the LABEL is
2920omitted, the command refers to the innermost enclosing loop. The
2921C<continue> block, if any, is not executed:
2922
4633a7c4 2923 LINE: while (<STDIN>) {
a9a5a0dc
VP
2924 last LINE if /^$/; # exit when done with header
2925 #...
a0d0e21e
LW
2926 }
2927
80d38338 2928C<last> cannot be used to exit a block that returns a value such as
8f1da26d 2929C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
2b5ab1e7 2930a grep() or map() operation.
4968c1e4 2931
6c1372ed
GS
2932Note that a block by itself is semantically identical to a loop
2933that executes once. Thus C<last> can be used to effect an early
2934exit out of such a block.
2935
98293880
JH
2936See also L</continue> for an illustration of how C<last>, C<next>, and
2937C<redo> work.
1d2dff63 2938
a0d0e21e 2939=item lc EXPR
d74e8afc 2940X<lc> X<lowercase>
a0d0e21e 2941
54310121 2942=item lc
bbce6d69 2943
d1be9408 2944Returns a lowercased version of EXPR. This is the internal function
3980dc9c 2945implementing the C<\L> escape in double-quoted strings.
a0d0e21e 2946
7660c0ab 2947If EXPR is omitted, uses C<$_>.
bbce6d69 2948
3980dc9c
KW
2949What gets returned depends on several factors:
2950
2951=over
2952
2953=item If C<use bytes> is in effect:
2954
2955=over
2956
2957=item On EBCDIC platforms
2958
2959The results are what the C language system call C<tolower()> returns.
2960
2961=item On ASCII platforms
2962
2963The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
2964respectively.
2965
2966=back
2967
094a2f8c 2968=item Otherwise, if C<use locale> is in effect
3980dc9c 2969
094a2f8c
KW
2970Respects current LC_CTYPE locale for code points < 256; and uses Unicode
2971semantics for the remaining code points (this last can only happen if
2972the UTF8 flag is also set). See L<perllocale>.
3980dc9c 2973
094a2f8c
KW
2974A deficiency in this is that case changes that cross the 255/256
2975boundary are not well-defined. For example, the lower case of LATIN CAPITAL
2976LETTER SHARP S (U+1E9E) in Unicode semantics is U+00DF (on ASCII
2977platforms). But under C<use locale>, the lower case of U+1E9E is
2978itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
2979current locale, and Perl has no way of knowing if that character even
2980exists in the locale, much less what code point it is. Perl returns
2981the input character unchanged, for all instances (and there aren't
2982many) where the 255/256 boundary would otherwise be crossed.
3980dc9c 2983
094a2f8c
KW
2984=item Otherwise, If EXPR has the UTF8 flag set
2985
2986Unicode semantics are used for the case change.
3980dc9c
KW
2987
2988=item Otherwise, if C<use feature 'unicode_strings'> is in effect:
2989
5d1892be 2990Unicode semantics are used for the case change.
3980dc9c
KW
2991
2992=item Otherwise:
2993
2994=over
2995
2996=item On EBCDIC platforms
2997
2998The results are what the C language system call C<tolower()> returns.
2999
3000=item On ASCII platforms
3001
3002ASCII semantics are used for the case change. The lowercase of any character
3003outside the ASCII range is the character itself.
3004
3005=back
3006
3007=back
3008
a0d0e21e 3009=item lcfirst EXPR
d74e8afc 3010X<lcfirst> X<lowercase>
a0d0e21e 3011
54310121 3012=item lcfirst
bbce6d69 3013
ad0029c4
JH
3014Returns the value of EXPR with the first character lowercased. This
3015is the internal function implementing the C<\l> escape in
3980dc9c 3016double-quoted strings.
a0d0e21e 3017
7660c0ab 3018If EXPR is omitted, uses C<$_>.
bbce6d69 3019
15dbbbab 3020This function behaves the same way under various pragmata, such as in a locale,
3980dc9c
KW
3021as L</lc> does.
3022
a0d0e21e 3023=item length EXPR
d74e8afc 3024X<length> X<size>
a0d0e21e 3025
54310121 3026=item length
bbce6d69 3027
974da8e5 3028Returns the length in I<characters> of the value of EXPR. If EXPR is
15dbbbab
FC
3029omitted, returns the length of C<$_>. If EXPR is undefined, returns
3030C<undef>.
3b10bc60 3031
3032This function cannot be used on an entire array or hash to find out how
3033many elements these have. For that, use C<scalar @array> and C<scalar keys
3034%hash>, respectively.
3035
3036Like all Perl character operations, length() normally deals in logical
3037characters, not physical bytes. For how many bytes a string encoded as
3038UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
3039to C<use Encode> first). See L<Encode> and L<perlunicode>.
974da8e5 3040
cfa52385
FC
3041=item __LINE__
3042X<__LINE__>
3043
3044A special token that compiles to the current line number.
3045
a0d0e21e 3046=item link OLDFILE,NEWFILE
d74e8afc 3047X<link>
a0d0e21e 3048
19799a22 3049Creates a new filename linked to the old filename. Returns true for
b76cc8ba 3050success, false otherwise.
a0d0e21e 3051
ea9eb35a 3052Portability issues: L<perlport/link>.
3053
a0d0e21e 3054=item listen SOCKET,QUEUESIZE
d74e8afc 3055X<listen>
a0d0e21e 3056
3b10bc60 3057Does the same thing that the listen(2) system call does. Returns true if
b76cc8ba 3058it succeeded, false otherwise. See the example in
cea6626f 3059L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
3060
3061=item local EXPR
d74e8afc 3062X<local>
a0d0e21e 3063
19799a22 3064You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 3065what most people think of as "local". See
13a2d996 3066L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 3067
5a964f20
TC
3068A local modifies the listed variables to be local to the enclosing
3069block, file, or eval. If more than one value is listed, the list must
3070be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3071for details, including issues with tied arrays and hashes.
a0d0e21e 3072
d361fafa
VP
3073The C<delete local EXPR> construct can also be used to localize the deletion