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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
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3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
8f1da26d 15operator. A unary operator generally provides scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
3b10bc60 17contexts for its arguments. If it does both, scalar arguments
18come first and list argument follow, and there can only ever
19be one such list argument. For instance, splice() has three scalar
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20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
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22
23In the syntax descriptions that follow, list operators that expect a
3b10bc60 24list (and provide list context for elements of the list) are shown
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25with LIST as an argument. Such a list may consist of any combination
26of scalar arguments or list values; the list values will be included
27in the list as if each individual element were interpolated at that
28point in the list, forming a longer single-dimensional list value.
8bdbc703 29Commas should separate literal elements of the LIST.
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30
31Any function in the list below may be used either with or without
32parentheses around its arguments. (The syntax descriptions omit the
3b10bc60 33parentheses.) If you use parentheses, the simple but occasionally
34surprising rule is this: It I<looks> like a function, therefore it I<is> a
a0d0e21e 35function, and precedence doesn't matter. Otherwise it's a list
3b10bc60 36operator or unary operator, and precedence does matter. Whitespace
37between the function and left parenthesis doesn't count, so sometimes
38you need to be careful:
a0d0e21e 39
5ed4f2ec 40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
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45
46If you run Perl with the B<-w> switch it can warn you about this. For
47example, the third line above produces:
48
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
51
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52A few functions take no arguments at all, and therefore work as neither
53unary nor list operators. These include such functions as C<time>
54and C<endpwent>. For example, C<time+86_400> always means
55C<time() + 86_400>.
56
a0d0e21e 57For functions that can be used in either a scalar or list context,
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58nonabortive failure is generally indicated in scalar context by
59returning the undefined value, and in list context by returning the
3b10bc60 60empty list.
a0d0e21e 61
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62Remember the following important rule: There is B<no rule> that relates
63the behavior of an expression in list context to its behavior in scalar
64context, or vice versa. It might do two totally different things.
80d38338 65Each operator and function decides which sort of value would be most
2b5ab1e7 66appropriate to return in scalar context. Some operators return the
5a964f20 67length of the list that would have been returned in list context. Some
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68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
d74e8afc 72X<context>
a0d0e21e 73
d1be9408 74A named array in scalar context is quite different from what would at
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75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
3b10bc60 81In general, functions in Perl that serve as wrappers for system calls ("syscalls")
5dac7880 82of the same name (like chown(2), fork(2), closedir(2), etc.) return
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83true when they succeed and C<undef> otherwise, as is usually mentioned
84in the descriptions below. This is different from the C interfaces,
5dac7880 85which return C<-1> on failure. Exceptions to this rule include C<wait>,
19799a22 86C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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87variable on failure. Other functions do not, except accidentally.
88
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89Extension modules can also hook into the Perl parser to define new
90kinds of keyword-headed expression. These may look like functions, but
91may also look completely different. The syntax following the keyword
92is defined entirely by the extension. If you are an implementor, see
93L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
94a module, see the module's documentation for details of the syntax that
95it defines.
96
cb1a09d0 97=head2 Perl Functions by Category
d74e8afc 98X<function>
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99
100Here are Perl's functions (including things that look like
5a964f20 101functions, like some keywords and named operators)
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102arranged by category. Some functions appear in more
103than one place.
104
13a2d996 105=over 4
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106
107=item Functions for SCALARs or strings
d74e8afc 108X<scalar> X<string> X<character>
cb1a09d0 109
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110C<chomp>, C<chop>, C<chr>, C<crypt>, C<fc>, C<hex>, C<index>, C<lc>,
111C<lcfirst>, C<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.
48238296 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
4a904372 187C<state> is available only if the C<"state"> feature
391b733c 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>
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249
250=back
251
60f9f73c 252=head2 Portability
d74e8afc 253X<portability> X<Unix> X<portable>
60f9f73c 254
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255Perl was born in Unix and can therefore access all common Unix
256system calls. In non-Unix environments, the functionality of some
8f1da26d 257Unix system calls may not be available or details of the available
2b5ab1e7 258functionality may differ slightly. The Perl functions affected
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259by this are:
260
261C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
262C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
263C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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264C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
265C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 266C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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267C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
268C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
269C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 270C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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271C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
272C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
273C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 274C<shmwrite>, C<socket>, C<socketpair>,
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275C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
276C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 277C<utime>, C<wait>, C<waitpid>
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278
279For more information about the portability of these functions, see
280L<perlport> and other available platform-specific documentation.
281
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282=head2 Alphabetical Listing of Perl Functions
283
3b10bc60 284=over
a0d0e21e 285
5b3c99c0 286=item -X FILEHANDLE
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287X<-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>
288X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 289
5b3c99c0 290=item -X EXPR
a0d0e21e 291
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292=item -X DIRHANDLE
293
5b3c99c0 294=item -X
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295
296A file test, where X is one of the letters listed below. This unary
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297operator takes one argument, either a filename, a filehandle, or a dirhandle,
298and tests the associated file to see if something is true about it. If the
7660c0ab 299argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 300Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 301the undefined value if the file doesn't exist. Despite the funny
d0821a6a 302names, precedence is the same as any other named unary operator. The
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303operator may be any of:
304
5ed4f2ec 305 -r File is readable by effective uid/gid.
306 -w File is writable by effective uid/gid.
307 -x File is executable by effective uid/gid.
308 -o File is owned by effective uid.
a0d0e21e 309
5ed4f2ec 310 -R File is readable by real uid/gid.
311 -W File is writable by real uid/gid.
312 -X File is executable by real uid/gid.
313 -O File is owned by real uid.
a0d0e21e 314
5ed4f2ec 315 -e File exists.
316 -z File has zero size (is empty).
317 -s File has nonzero size (returns size in bytes).
a0d0e21e 318
5ed4f2ec 319 -f File is a plain file.
320 -d File is a directory.
321 -l File is a symbolic link.
322 -p File is a named pipe (FIFO), or Filehandle is a pipe.
323 -S File is a socket.
324 -b File is a block special file.
325 -c File is a character special file.
326 -t Filehandle is opened to a tty.
a0d0e21e 327
5ed4f2ec 328 -u File has setuid bit set.
329 -g File has setgid bit set.
330 -k File has sticky bit set.
a0d0e21e 331
5ed4f2ec 332 -T File is an ASCII text file (heuristic guess).
333 -B File is a "binary" file (opposite of -T).
a0d0e21e 334
5ed4f2ec 335 -M Script start time minus file modification time, in days.
336 -A Same for access time.
337 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 338
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339Example:
340
341 while (<>) {
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342 chomp;
343 next unless -f $_; # ignore specials
344 #...
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345 }
346
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347Note that C<-s/a/b/> does not do a negated substitution. Saying
348C<-exp($foo)> still works as expected, however: only single letters
349following a minus are interpreted as file tests.
350
351These operators are exempt from the "looks like a function rule" described
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FC
352above. That is, an opening parenthesis after the operator does not affect
353how much of the following code constitutes the argument. Put the opening
4fb67938
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354parentheses before the operator to separate it from code that follows (this
355applies only to operators with higher precedence than unary operators, of
356course):
357
358 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
359 (-s $file) + 1024 # correct
360
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361The interpretation of the file permission operators C<-r>, C<-R>,
362C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
363of the file and the uids and gids of the user. There may be other
ecae030f
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364reasons you can't actually read, write, or execute the file: for
365example network filesystem access controls, ACLs (access control lists),
366read-only filesystems, and unrecognized executable formats. Note
367that the use of these six specific operators to verify if some operation
368is possible is usually a mistake, because it may be open to race
369conditions.
5ff3f7a4 370
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371Also note that, for the superuser on the local filesystems, the C<-r>,
372C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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373if any execute bit is set in the mode. Scripts run by the superuser
374may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 375or temporarily set their effective uid to something else.
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376
377If you are using ACLs, there is a pragma called C<filetest> that may
378produce more accurate results than the bare stat() mode bits.
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379When under C<use filetest 'access'> the above-mentioned filetests
380test whether the permission can(not) be granted using the
3b10bc60 381access(2) family of system calls. Also note that the C<-x> and C<-X> may
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382under this pragma return true even if there are no execute permission
383bits set (nor any extra execute permission ACLs). This strangeness is
391b733c 384due to the underlying system calls' definitions. Note also that, due to
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385the implementation of C<use filetest 'access'>, the C<_> special
386filehandle won't cache the results of the file tests when this pragma is
387in effect. Read the documentation for the C<filetest> pragma for more
388information.
5ff3f7a4 389
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390The C<-T> and C<-B> switches work as follows. The first block or so of the
391file is examined for odd characters such as strange control codes or
61eff3bc 392characters with the high bit set. If too many strange characters (>30%)
cf264981 393are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 394containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 395or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 396rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 397file, or a file at EOF when testing a filehandle. Because you have to
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398read a file to do the C<-T> test, on most occasions you want to use a C<-f>
399against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 400
5dac7880 401If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 402the special filehandle consisting of a solitary underline, then the stat
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403structure of the previous file test (or stat operator) is used, saving
404a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 405that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 406symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 407an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 408Example:
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409
410 print "Can do.\n" if -r $a || -w _ || -x _;
411
412 stat($filename);
413 print "Readable\n" if -r _;
414 print "Writable\n" if -w _;
415 print "Executable\n" if -x _;
416 print "Setuid\n" if -u _;
417 print "Setgid\n" if -g _;
418 print "Sticky\n" if -k _;
419 print "Text\n" if -T _;
420 print "Binary\n" if -B _;
421
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422As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
423test operators, in a way that C<-f -w -x $file> is equivalent to
391b733c 424C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
fbb0b3b3
RGS
425the return value of C<-f $file> as an argument to another filetest
426operator, no special magic will happen.)
427
bee96257 428Portability issues: L<perlport/-X>.
ea9eb35a 429
bade7fbc
TC
430To avoid confusing would-be users of your code with mysterious
431syntax errors, put something like this at the top of your script:
432
433 use 5.010; # so filetest ops can stack
434
a0d0e21e 435=item abs VALUE
d74e8afc 436X<abs> X<absolute>
a0d0e21e 437
54310121 438=item abs
bbce6d69 439
a0d0e21e 440Returns the absolute value of its argument.
7660c0ab 441If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
442
443=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 444X<accept>
a0d0e21e 445
3b10bc60 446Accepts an incoming socket connect, just as accept(2)
19799a22 447does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 448See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 449
8d2a6795
GS
450On systems that support a close-on-exec flag on files, the flag will
451be set for the newly opened file descriptor, as determined by the
452value of $^F. See L<perlvar/$^F>.
453
a0d0e21e 454=item alarm SECONDS
d74e8afc
ITB
455X<alarm>
456X<SIGALRM>
457X<timer>
a0d0e21e 458
54310121 459=item alarm
bbce6d69 460
a0d0e21e 461Arranges to have a SIGALRM delivered to this process after the
cf264981 462specified number of wallclock seconds has elapsed. If SECONDS is not
391b733c 463specified, the value stored in C<$_> is used. (On some machines,
d400eac8
JH
464unfortunately, the elapsed time may be up to one second less or more
465than you specified because of how seconds are counted, and process
466scheduling may delay the delivery of the signal even further.)
467
468Only one timer may be counting at once. Each call disables the
469previous timer, and an argument of C<0> may be supplied to cancel the
470previous timer without starting a new one. The returned value is the
471amount of time remaining on the previous timer.
a0d0e21e 472
2bc69794
BS
473For delays of finer granularity than one second, the Time::HiRes module
474(from CPAN, and starting from Perl 5.8 part of the standard
475distribution) provides ualarm(). You may also use Perl's four-argument
476version of select() leaving the first three arguments undefined, or you
477might be able to use the C<syscall> interface to access setitimer(2) if
391b733c 478your system supports it. See L<perlfaq8> for details.
2b5ab1e7 479
80d38338
TC
480It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
481C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 482
19799a22
GS
483If you want to use C<alarm> to time out a system call you need to use an
484C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 485fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 486restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 487modulo the caveats given in L<perlipc/"Signals">.
ff68c719 488
489 eval {
a9a5a0dc
VP
490 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
491 alarm $timeout;
492 $nread = sysread SOCKET, $buffer, $size;
493 alarm 0;
ff68c719 494 };
ff68c719 495 if ($@) {
a9a5a0dc 496 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 497 # timed out
ff68c719 498 }
499 else {
5ed4f2ec 500 # didn't
ff68c719 501 }
502
91d81acc
JH
503For more information see L<perlipc>.
504
ea9eb35a
BJ
505Portability issues: L<perlport/alarm>.
506
a0d0e21e 507=item atan2 Y,X
d74e8afc 508X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
509
510Returns the arctangent of Y/X in the range -PI to PI.
511
ca6e1c26 512For the tangent operation, you may use the C<Math::Trig::tan>
28757baa 513function, or use the familiar relation:
514
515 sub tan { sin($_[0]) / cos($_[0]) }
516
a1021d57
RGS
517The return value for C<atan2(0,0)> is implementation-defined; consult
518your atan2(3) manpage for more information.
bf5f1b4c 519
ea9eb35a
BJ
520Portability issues: L<perlport/atan2>.
521
a0d0e21e 522=item bind SOCKET,NAME
d74e8afc 523X<bind>
a0d0e21e 524
3b10bc60 525Binds a network address to a socket, just as bind(2)
19799a22 526does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
527packed address of the appropriate type for the socket. See the examples in
528L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 529
fae2c0fb 530=item binmode FILEHANDLE, LAYER
d74e8afc 531X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 532
a0d0e21e
LW
533=item binmode FILEHANDLE
534
1cbfc93d
NIS
535Arranges for FILEHANDLE to be read or written in "binary" or "text"
536mode on systems where the run-time libraries distinguish between
537binary and text files. If FILEHANDLE is an expression, the value is
538taken as the name of the filehandle. Returns true on success,
b5fe5ca2 539otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 540
8f1da26d 541On some systems (in general, DOS- and Windows-based systems) binmode()
d807c6f4 542is necessary when you're not working with a text file. For the sake
d7a0d798
FC
543of portability it is a good idea always to use it when appropriate,
544and never to use it when it isn't appropriate. Also, people can
8f1da26d 545set their I/O to be by default UTF8-encoded Unicode, not bytes.
d807c6f4
JH
546
547In other words: regardless of platform, use binmode() on binary data,
d7a0d798 548like images, for example.
d807c6f4
JH
549
550If LAYER is present it is a single string, but may contain multiple
391b733c 551directives. The directives alter the behaviour of the filehandle.
d7a0d798 552When LAYER is present, using binmode on a text file makes sense.
d807c6f4 553
fae2c0fb 554If LAYER is omitted or specified as C<:raw> the filehandle is made
391b733c 555suitable for passing binary data. This includes turning off possible CRLF
0226bbdb 556translation and marking it as bytes (as opposed to Unicode characters).
749683d2 557Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 558Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
559Other layers that would affect the binary nature of the stream are
391b733c 560I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 561PERLIO environment variable.
01e6739c 562
3b10bc60 563The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
564form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
565establish default I/O layers. See L<open>.
566
fae2c0fb
RGS
567I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
568in "Programming Perl, 3rd Edition". However, since the publishing of this
569book, by many known as "Camel III", the consensus of the naming of this
570functionality has moved from "discipline" to "layer". All documentation
571of this version of Perl therefore refers to "layers" rather than to
572"disciplines". Now back to the regularly scheduled documentation...>
573
8f1da26d 574To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
6902c96a 575C<:utf8> just marks the data as UTF-8 without further checking,
8f1da26d 576while C<:encoding(UTF-8)> checks the data for actually being valid
391b733c 577UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 578
ed53a2bb 579In general, binmode() should be called after open() but before any I/O
3b10bc60 580is done on the filehandle. Calling binmode() normally flushes any
01e6739c 581pending buffered output data (and perhaps pending input data) on the
fae2c0fb 582handle. An exception to this is the C<:encoding> layer that
d7a0d798 583changes the default character encoding of the handle; see L</open>.
fae2c0fb 584The C<:encoding> layer sometimes needs to be called in
3874323d
JH
585mid-stream, and it doesn't flush the stream. The C<:encoding>
586also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 587internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 588
19799a22 589The operating system, device drivers, C libraries, and Perl run-time
8f1da26d
TC
590system all conspire to let the programmer treat a single
591character (C<\n>) as the line terminator, irrespective of external
30168b04
GS
592representation. On many operating systems, the native text file
593representation matches the internal representation, but on some
594platforms the external representation of C<\n> is made up of more than
595one character.
596
8f1da26d
TC
597All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
598a single character to end each line in the external representation of text
599(even though that single character is CARRIAGE RETURN on old, pre-Darwin
391b733c 600flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
8f1da26d
TC
601systems like OS/2, DOS, and the various flavors of MS-Windows, your program
602sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
603two characters C<\cM\cJ>. That means that if you don't use binmode() on
604these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
605input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
606output. This is what you want for text files, but it can be disastrous for
607binary files.
30168b04
GS
608
609Another consequence of using binmode() (on some systems) is that
610special end-of-file markers will be seen as part of the data stream.
d7a0d798
FC
611For systems from the Microsoft family this means that, if your binary
612data contain C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
613the file, unless you use binmode().
614
3b10bc60 615binmode() is important not only for readline() and print() operations,
30168b04
GS
616but also when using read(), seek(), sysread(), syswrite() and tell()
617(see L<perlport> for more details). See the C<$/> and C<$\> variables
618in L<perlvar> for how to manually set your input and output
619line-termination sequences.
a0d0e21e 620
ea9eb35a
BJ
621Portability issues: L<perlport/binmode>.
622
4633a7c4 623=item bless REF,CLASSNAME
d74e8afc 624X<bless>
a0d0e21e
LW
625
626=item bless REF
627
2b5ab1e7
TC
628This function tells the thingy referenced by REF that it is now an object
629in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 630is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 631it returns the reference for convenience. Always use the two-argument
cf264981 632version if a derived class might inherit the function doing the blessing.
82e1c0d9 633SeeL<perlobj> for more about the blessing (and blessings) of objects.
a0d0e21e 634
57668c4d 635Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 636Namespaces with all lowercase names are considered reserved for
391b733c 637Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
638confusion, you may wish to avoid such package names as well. Make sure
639that CLASSNAME is a true value.
60ad88b8
GS
640
641See L<perlmod/"Perl Modules">.
642
0d863452
RH
643=item break
644
645Break out of a C<given()> block.
646
8f1da26d 647This keyword is enabled by the C<"switch"> feature: see
4a904372
FC
648L<feature> for more information. You can also access it by
649prefixing it with C<CORE::>. Alternately, include a C<use
8f1da26d 650v5.10> or later to the current scope.
0d863452 651
a0d0e21e 652=item caller EXPR
d74e8afc 653X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
654
655=item caller
656
5a964f20 657Returns the context of the current subroutine call. In scalar context,
80d38338
TC
658returns the caller's package name if there I<is> a caller (that is, if
659we're in a subroutine or C<eval> or C<require>) and the undefined value
5a964f20 660otherwise. In list context, returns
a0d0e21e 661
ee6b43cc 662 # 0 1 2
748a9306 663 ($package, $filename, $line) = caller;
a0d0e21e
LW
664
665With EXPR, it returns some extra information that the debugger uses to
666print a stack trace. The value of EXPR indicates how many call frames
667to go back before the current one.
668
ee6b43cc 669 # 0 1 2 3 4
f3aa04c2 670 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 671
672 # 5 6 7 8 9 10
b3ca2e83 673 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 674 = caller($i);
e7ea3e70 675
951ba7fe 676Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 677call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 678C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 679C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 680C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 681$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
682each C<use> statement creates a C<require> frame inside an C<eval EXPR>
683frame.) $subroutine may also be C<(unknown)> if this particular
684subroutine happens to have been deleted from the symbol table.
685C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
686C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
687compiled with. The C<$hints> and C<$bitmask> values are subject to change
688between versions of Perl, and are not meant for external use.
748a9306 689
b3ca2e83 690C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
391b733c 691caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
b3ca2e83
NC
692of this hash, as they are the actual values stored in the optree.
693
ffe0c19d
FC
694Furthermore, when called from within the DB package in
695list context, and with an argument, caller returns more
7660c0ab 696detailed information: it sets the list variable C<@DB::args> to be the
54310121 697arguments with which the subroutine was invoked.
748a9306 698
7660c0ab 699Be aware that the optimizer might have optimized call frames away before
19799a22 700C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 701might not return information about the call frame you expect it to, for
b76cc8ba 702C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 703previous time C<caller> was called.
7660c0ab 704
8f1da26d 705Be aware that setting C<@DB::args> is I<best effort>, intended for
391b733c 706debugging or generating backtraces, and should not be relied upon. In
ca9f0cb5
NC
707particular, as C<@_> contains aliases to the caller's arguments, Perl does
708not take a copy of C<@_>, so C<@DB::args> will contain modifications the
709subroutine makes to C<@_> or its contents, not the original values at call
391b733c 710time. C<@DB::args>, like C<@_>, does not hold explicit references to its
ca9f0cb5 711elements, so under certain cases its elements may have become freed and
391b733c 712reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 713of the current implementation is that the effects of C<shift @_> can
8f1da26d
TC
714I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
715reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
ca9f0cb5 716elements), so C<@DB::args> is actually a hybrid of the current state and
391b733c 717initial state of C<@_>. Buyer beware.
ca9f0cb5 718
a0d0e21e 719=item chdir EXPR
d74e8afc
ITB
720X<chdir>
721X<cd>
f723aae1 722X<directory, change>
a0d0e21e 723
c4aca7d0
GA
724=item chdir FILEHANDLE
725
726=item chdir DIRHANDLE
727
ce2984c3
PF
728=item chdir
729
391b733c 730Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 731changes to the directory specified by C<$ENV{HOME}>, if set; if not,
391b733c
FC
732changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
733variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
734neither is set, C<chdir> does nothing. It returns true on success,
735false otherwise. See the example under C<die>.
a0d0e21e 736
3b10bc60 737On systems that support fchdir(2), you may pass a filehandle or
34169887 738directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 739passing handles raises an exception.
c4aca7d0 740
a0d0e21e 741=item chmod LIST
d74e8afc 742X<chmod> X<permission> X<mode>
a0d0e21e
LW
743
744Changes the permissions of a list of files. The first element of the
8f1da26d 745list must be the numeric mode, which should probably be an octal
4ad40acf 746number, and which definitely should I<not> be a string of octal digits:
3b10bc60 747C<0644> is okay, but C<"0644"> is not. Returns the number of files
8f1da26d 748successfully changed. See also L</oct> if all you have is a string.
a0d0e21e 749
3b10bc60 750 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 751 chmod 0755, @executables;
3b10bc60 752 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 753 # --w----r-T
3b10bc60 754 $mode = "0644"; chmod oct($mode), "foo"; # this is better
755 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 756
3b10bc60 757On systems that support fchmod(2), you may pass filehandles among the
758files. On systems that don't support fchmod(2), passing filehandles raises
759an exception. Filehandles must be passed as globs or glob references to be
760recognized; barewords are considered filenames.
c4aca7d0
GA
761
762 open(my $fh, "<", "foo");
763 my $perm = (stat $fh)[2] & 07777;
764 chmod($perm | 0600, $fh);
765
3b10bc60 766You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
767module:
768
3b10bc60 769 use Fcntl qw( :mode );
ca6e1c26 770 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 771 # Identical to the chmod 0755 of the example above.
ca6e1c26 772
ea9eb35a
BJ
773Portability issues: L<perlport/chmod>.
774
a0d0e21e 775=item chomp VARIABLE
d74e8afc 776X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 777
313c9f5c 778=item chomp( LIST )
a0d0e21e
LW
779
780=item chomp
781
2b5ab1e7
TC
782This safer version of L</chop> removes any trailing string
783that corresponds to the current value of C<$/> (also known as
28757baa 784$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
785number of characters removed from all its arguments. It's often used to
786remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
787that the final record may be missing its newline. When in paragraph
788mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 789When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 790a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 791remove anything.
19799a22 792If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
793
794 while (<>) {
a9a5a0dc
VP
795 chomp; # avoid \n on last field
796 @array = split(/:/);
797 # ...
a0d0e21e
LW
798 }
799
4bf21a6d
RD
800If VARIABLE is a hash, it chomps the hash's values, but not its keys.
801
a0d0e21e
LW
802You can actually chomp anything that's an lvalue, including an assignment:
803
804 chomp($cwd = `pwd`);
805 chomp($answer = <STDIN>);
806
807If you chomp a list, each element is chomped, and the total number of
808characters removed is returned.
809
15e44fd8
RGS
810Note that parentheses are necessary when you're chomping anything
811that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
812is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
813C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
814C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
815as C<chomp($a, $b)>.
816
a0d0e21e 817=item chop VARIABLE
d74e8afc 818X<chop>
a0d0e21e 819
313c9f5c 820=item chop( LIST )
a0d0e21e
LW
821
822=item chop
823
824Chops off the last character of a string and returns the character
5b3eff12 825chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 826scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
827If VARIABLE is a hash, it chops the hash's values, but not its keys.
828
5b3eff12 829You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
830
831If you chop a list, each element is chopped. Only the value of the
19799a22 832last C<chop> is returned.
a0d0e21e 833
19799a22 834Note that C<chop> returns the last character. To return all but the last
748a9306
LW
835character, use C<substr($string, 0, -1)>.
836
15e44fd8
RGS
837See also L</chomp>.
838
a0d0e21e 839=item chown LIST
d74e8afc 840X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
841
842Changes the owner (and group) of a list of files. The first two
19799a22
GS
843elements of the list must be the I<numeric> uid and gid, in that
844order. A value of -1 in either position is interpreted by most
845systems to leave that value unchanged. Returns the number of files
846successfully changed.
a0d0e21e
LW
847
848 $cnt = chown $uid, $gid, 'foo', 'bar';
849 chown $uid, $gid, @filenames;
850
3b10bc60 851On systems that support fchown(2), you may pass filehandles among the
852files. On systems that don't support fchown(2), passing filehandles raises
853an exception. Filehandles must be passed as globs or glob references to be
854recognized; barewords are considered filenames.
c4aca7d0 855
54310121 856Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
857
858 print "User: ";
19799a22 859 chomp($user = <STDIN>);
5a964f20 860 print "Files: ";
19799a22 861 chomp($pattern = <STDIN>);
a0d0e21e
LW
862
863 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 864 or die "$user not in passwd file";
a0d0e21e 865
5ed4f2ec 866 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
867 chown $uid, $gid, @ary;
868
54310121 869On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
870file unless you're the superuser, although you should be able to change
871the group to any of your secondary groups. On insecure systems, these
872restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
873On POSIX systems, you can detect this condition this way:
874
875 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
876 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 877
ea9eb35a
BJ
878Portability issues: L<perlport/chmod>.
879
a0d0e21e 880=item chr NUMBER
d74e8afc 881X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 882
54310121 883=item chr
bbce6d69 884
a0d0e21e 885Returns the character represented by that NUMBER in the character set.
a0ed51b3 886For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 887chr(0x263a) is a Unicode smiley face.
aaa68c4a 888
8a064bd6 889Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 890except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
891(truncated to an integer) are used.
892
974da8e5
JH
893If NUMBER is omitted, uses C<$_>.
894
b76cc8ba 895For the reverse, use L</ord>.
a0d0e21e 896
2575c402
JW
897Note that characters from 128 to 255 (inclusive) are by default
898internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 899
2575c402 900See L<perlunicode> for more about Unicode.
bbce6d69 901
a0d0e21e 902=item chroot FILENAME
d74e8afc 903X<chroot> X<root>
a0d0e21e 904
54310121 905=item chroot
bbce6d69 906
5a964f20 907This function works like the system call by the same name: it makes the
4633a7c4 908named directory the new root directory for all further pathnames that
951ba7fe 909begin with a C</> by your process and all its children. (It doesn't
28757baa 910change your current working directory, which is unaffected.) For security
4633a7c4 911reasons, this call is restricted to the superuser. If FILENAME is
19799a22 912omitted, does a C<chroot> to C<$_>.
a0d0e21e 913
ea9eb35a
BJ
914Portability issues: L<perlport/chroot>.
915
a0d0e21e 916=item close FILEHANDLE
d74e8afc 917X<close>
a0d0e21e 918
6a518fbc
TP
919=item close
920
3b10bc60 921Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26 922buffers, and closes the system file descriptor. Returns true if those
8f1da26d 923operations succeed and if no error was reported by any PerlIO
e0f13c26
RGS
924layer. Closes the currently selected filehandle if the argument is
925omitted.
fb73857a 926
927You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 928another C<open> on it, because C<open> closes it for you. (See
01aa884e 929L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
19799a22 930counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 931
3b10bc60 932If the filehandle came from a piped open, C<close> returns false if one of
933the other syscalls involved fails or if its program exits with non-zero
934status. If the only problem was that the program exited non-zero, C<$!>
935will be set to C<0>. Closing a pipe also waits for the process executing
936on the pipe to exit--in case you wish to look at the output of the pipe
937afterwards--and implicitly puts the exit status value of that command into
938C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 939
2e0cfa16
FC
940If there are multiple threads running, C<close> on a filehandle from a
941piped open returns true without waiting for the child process to terminate,
942if the filehandle is still open in another thread.
943
80d38338
TC
944Closing the read end of a pipe before the process writing to it at the
945other end is done writing results in the writer receiving a SIGPIPE. If
946the other end can't handle that, be sure to read all the data before
947closing the pipe.
73689b13 948
fb73857a 949Example:
a0d0e21e 950
fb73857a 951 open(OUTPUT, '|sort >foo') # pipe to sort
952 or die "Can't start sort: $!";
5ed4f2ec 953 #... # print stuff to output
954 close OUTPUT # wait for sort to finish
fb73857a 955 or warn $! ? "Error closing sort pipe: $!"
956 : "Exit status $? from sort";
5ed4f2ec 957 open(INPUT, 'foo') # get sort's results
fb73857a 958 or die "Can't open 'foo' for input: $!";
a0d0e21e 959
5a964f20 960FILEHANDLE may be an expression whose value can be used as an indirect
8f1da26d 961filehandle, usually the real filehandle name or an autovivified handle.
a0d0e21e
LW
962
963=item closedir DIRHANDLE
d74e8afc 964X<closedir>
a0d0e21e 965
19799a22 966Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
967system call.
968
a0d0e21e 969=item connect SOCKET,NAME
d74e8afc 970X<connect>
a0d0e21e 971
80d38338
TC
972Attempts to connect to a remote socket, just like connect(2).
973Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
974packed address of the appropriate type for the socket. See the examples in
975L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 976
cb1a09d0 977=item continue BLOCK
d74e8afc 978X<continue>
cb1a09d0 979
0d863452
RH
980=item continue
981
4a904372
FC
982When followed by a BLOCK, C<continue> is actually a
983flow control statement rather than a function. If
cf264981 984there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
985C<foreach>), it is always executed just before the conditional is about to
986be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
987it can be used to increment a loop variable, even when the loop has been
988continued via the C<next> statement (which is similar to the C C<continue>
989statement).
990
98293880 991C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 992block; C<last> and C<redo> behave as if they had been executed within
19799a22 993the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
994block, it may be more entertaining.
995
996 while (EXPR) {
a9a5a0dc
VP
997 ### redo always comes here
998 do_something;
1d2dff63 999 } continue {
a9a5a0dc
VP
1000 ### next always comes here
1001 do_something_else;
1002 # then back the top to re-check EXPR
1d2dff63
GS
1003 }
1004 ### last always comes here
1005
3b10bc60 1006Omitting the C<continue> section is equivalent to using an
1007empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
1008to check the condition at the top of the loop.
1009
4a904372 1010When there is no BLOCK, C<continue> is a function that
8f1da26d
TC
1011falls through the current C<when> or C<default> block instead of iterating
1012a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
4a904372
FC
1013In Perl 5.14 and earlier, this form of C<continue> was
1014only available when the C<"switch"> feature was enabled.
48238296 1015See L<feature> and L<perlsyn/"Switch Statements"> for more
8f1da26d 1016information.
0d863452 1017
a0d0e21e 1018=item cos EXPR
d74e8afc 1019X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1020
d6217f1e
GS
1021=item cos
1022
5a964f20 1023Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1024takes the cosine of C<$_>.
a0d0e21e 1025
ca6e1c26 1026For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa 1027function, or use this relation:
1028
1029 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1030
a0d0e21e 1031=item crypt PLAINTEXT,SALT
d74e8afc 1032X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1033X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1034
ef2e6798
MS
1035Creates a digest string exactly like the crypt(3) function in the C
1036library (assuming that you actually have a version there that has not
bb23f8d1 1037been extirpated as a potential munition).
ef2e6798 1038
34169887 1039crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1040into a short string, called a digest, which is returned. The same
1041PLAINTEXT and SALT will always return the same string, but there is no
1042(known) way to get the original PLAINTEXT from the hash. Small
1043changes in the PLAINTEXT or SALT will result in large changes in the
1044digest.
1045
1046There is no decrypt function. This function isn't all that useful for
1047cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1048mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1049primarily used to check if two pieces of text are the same without
1050having to transmit or store the text itself. An example is checking
1051if a correct password is given. The digest of the password is stored,
cf264981 1052not the password itself. The user types in a password that is
ef2e6798 1053crypt()'d with the same salt as the stored digest. If the two digests
34169887 1054match, the password is correct.
ef2e6798
MS
1055
1056When verifying an existing digest string you should use the digest as
1057the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1058to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1059crypt() will hash the new string with the same salt as the digest.
1060This allows your code to work with the standard L<crypt|/crypt> and
8f1da26d
TC
1061with more exotic implementations. In other words, assume
1062nothing about the returned string itself nor about how many bytes
1063of SALT may matter.
85c16d83
JH
1064
1065Traditionally the result is a string of 13 bytes: two first bytes of
1066the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
391b733c 1067the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1068hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1069and implementations on non-Unix platforms may produce different
ef2e6798 1070strings.
85c16d83
JH
1071
1072When choosing a new salt create a random two character string whose
1073characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1074'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1075characters is just a recommendation; the characters allowed in
1076the salt depend solely on your system's crypt library, and Perl can't
1077restrict what salts C<crypt()> accepts.
e71965be 1078
a0d0e21e 1079Here's an example that makes sure that whoever runs this program knows
cf264981 1080their password:
a0d0e21e
LW
1081
1082 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1083
1084 system "stty -echo";
1085 print "Password: ";
e71965be 1086 chomp($word = <STDIN>);
a0d0e21e
LW
1087 print "\n";
1088 system "stty echo";
1089
e71965be 1090 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1091 die "Sorry...\n";
a0d0e21e 1092 } else {
a9a5a0dc 1093 print "ok\n";
54310121 1094 }
a0d0e21e 1095
9f8f0c9d 1096Of course, typing in your own password to whoever asks you
748a9306 1097for it is unwise.
a0d0e21e 1098
ef2e6798 1099The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1100of data, not least of all because you can't get the information
ef2e6798 1101back. Look at the L<Digest> module for more robust algorithms.
19799a22 1102
f2791508
JH
1103If using crypt() on a Unicode string (which I<potentially> has
1104characters with codepoints above 255), Perl tries to make sense
34169887 1105of the situation by trying to downgrade (a copy of)
f2791508
JH
1106the string back to an eight-bit byte string before calling crypt()
1107(on that copy). If that works, good. If not, crypt() dies with
1108C<Wide character in crypt>.
85c16d83 1109
ea9eb35a
BJ
1110Portability issues: L<perlport/crypt>.
1111
aa689395 1112=item dbmclose HASH
d74e8afc 1113X<dbmclose>
a0d0e21e 1114
19799a22 1115[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1116
aa689395 1117Breaks the binding between a DBM file and a hash.
a0d0e21e 1118
ea9eb35a
BJ
1119Portability issues: L<perlport/dbmclose>.
1120
19799a22 1121=item dbmopen HASH,DBNAME,MASK
d74e8afc 1122X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1123
01aa884e
KW
1124[This function has been largely superseded by the
1125L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
a0d0e21e 1126
7b8d334a 1127This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1128hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1129argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395 1130is the name of the database (without the F<.dir> or F<.pag> extension if
1131any). If the database does not exist, it is created with protection
1b3a6178
FC
1132specified by MASK (as modified by the C<umask>). To prevent creation of
1133the database if it doesn't exist, you may specify a MODE
1134of 0, and the function will return a false value if it
1135can't find an existing database. If your system supports
80d38338 1136only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1137program. In older versions of Perl, if your system had neither DBM nor
19799a22 1138ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395 1139sdbm(3).
1140
1141If you don't have write access to the DBM file, you can only read hash
1142variables, not set them. If you want to test whether you can write,
3b10bc60 1143either use file tests or try setting a dummy hash entry inside an C<eval>
1144to trap the error.
a0d0e21e 1145
19799a22
GS
1146Note that functions such as C<keys> and C<values> may return huge lists
1147when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1148function to iterate over large DBM files. Example:
1149
1150 # print out history file offsets
1151 dbmopen(%HIST,'/usr/lib/news/history',0666);
1152 while (($key,$val) = each %HIST) {
a9a5a0dc 1153 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1154 }
1155 dbmclose(%HIST);
1156
cb1a09d0 1157See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1158cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1159rich implementation.
4633a7c4 1160
2b5ab1e7
TC
1161You can control which DBM library you use by loading that library
1162before you call dbmopen():
1163
1164 use DB_File;
1165 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1166 or die "Can't open netscape history file: $!";
2b5ab1e7 1167
ea9eb35a
BJ
1168Portability issues: L<perlport/dbmopen>.
1169
8f1da26d
TC
1170=item default BLOCK
1171
1172Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when>
1173that's always true. Only available after Perl 5.10, and only if the
4a904372
FC
1174C<switch> feature has been requested or if the keyword is prefixed with
1175C<CORE::>. See L</when>.
8f1da26d 1176
a0d0e21e 1177=item defined EXPR
d74e8afc 1178X<defined> X<undef> X<undefined>
a0d0e21e 1179
54310121 1180=item defined
bbce6d69 1181
2f9daede 1182Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1183the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TP
1184checked.
1185
1186Many operations return C<undef> to indicate failure, end of file,
1187system error, uninitialized variable, and other exceptional
1188conditions. This function allows you to distinguish C<undef> from
1189other values. (A simple Boolean test will not distinguish among
7660c0ab 1190C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1191false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1192doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TP
1193returns C<undef> when its argument is an empty array, I<or> when the
1194element to return happens to be C<undef>.
1195
f10b0346
GS
1196You may also use C<defined(&func)> to check whether subroutine C<&func>
1197has ever been defined. The return value is unaffected by any forward
80d38338 1198declarations of C<&func>. A subroutine that is not defined
847c7ebe 1199may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1200makes it spring into existence the first time that it is called; see
847c7ebe 1201L<perlsub>.
f10b0346
GS
1202
1203Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1204used to report whether memory for that aggregate had ever been
f10b0346
GS
1205allocated. This behavior may disappear in future versions of Perl.
1206You should instead use a simple test for size:
1207
1208 if (@an_array) { print "has array elements\n" }
1209 if (%a_hash) { print "has hash members\n" }
2f9daede
TP
1210
1211When used on a hash element, it tells you whether the value is defined,
dc848c6f 1212not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1213purpose.
a0d0e21e
LW
1214
1215Examples:
1216
8f1da26d 1217 print if defined $switch{D};
a0d0e21e
LW
1218 print "$val\n" while defined($val = pop(@ary));
1219 die "Can't readlink $sym: $!"
a9a5a0dc 1220 unless defined($value = readlink $sym);
a0d0e21e 1221 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1222 $debugging = 0 unless defined $debugging;
a0d0e21e 1223
8f1da26d 1224Note: Many folks tend to overuse C<defined> and are then surprised to
7660c0ab 1225discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1226defined values. For example, if you say
a5f75d66
AD
1227
1228 "ab" =~ /a(.*)b/;
1229
80d38338 1230The pattern match succeeds and C<$1> is defined, although it
cf264981 1231matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1232matched something that happened to be zero characters long. This is all
a5f75d66 1233very above-board and honest. When a function returns an undefined value,
2f9daede 1234it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1235should use C<defined> only when questioning the integrity of what
7660c0ab 1236you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TP
1237what you want.
1238
dc848c6f 1239See also L</undef>, L</exists>, L</ref>.
2f9daede 1240
a0d0e21e 1241=item delete EXPR
d74e8afc 1242X<delete>
a0d0e21e 1243
d0a76353
RS
1244Given an expression that specifies an element or slice of a hash, C<delete>
1245deletes the specified elements from that hash so that exists() on that element
1246no longer returns true. Setting a hash element to the undefined value does
1247not remove its key, but deleting it does; see L</exists>.
80d38338 1248
8f1da26d 1249In list context, returns the value or values deleted, or the last such
80d38338 1250element in scalar context. The return list's length always matches that of
d0a76353
RS
1251the argument list: deleting non-existent elements returns the undefined value
1252in their corresponding positions.
80d38338 1253
d0a76353
RS
1254delete() may also be used on arrays and array slices, but its behavior is less
1255straightforward. Although exists() will return false for deleted entries,
1256deleting array elements never changes indices of existing values; use shift()
1257or splice() for that. However, if all deleted elements fall at the end of an
1258array, the array's size shrinks to the position of the highest element that
1259still tests true for exists(), or to 0 if none do.
1260
8f1da26d 1261B<WARNING:> Calling delete on array values is deprecated and likely to
d0a76353 1262be removed in a future version of Perl.
80d38338
TC
1263
1264Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1265a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1266or array may not necessarily return anything; it depends on the implementation
1267of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1268
80d38338
TC
1269The C<delete local EXPR> construct localizes the deletion to the current
1270block at run time. Until the block exits, elements locally deleted
1271temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1272of composite types">.
eba0920a
EM
1273
1274 %hash = (foo => 11, bar => 22, baz => 33);
1275 $scalar = delete $hash{foo}; # $scalar is 11
1276 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1277 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1278
01020589 1279The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1280
5f05dabc 1281 foreach $key (keys %HASH) {
a9a5a0dc 1282 delete $HASH{$key};
a0d0e21e
LW
1283 }
1284
01020589 1285 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1286 delete $ARRAY[$index];
01020589
GS
1287 }
1288
1289And so do these:
5f05dabc 1290
01020589
GS
1291 delete @HASH{keys %HASH};
1292
9740c838 1293 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1294
80d38338
TC
1295But both are slower than assigning the empty list
1296or undefining %HASH or @ARRAY, which is the customary
1297way to empty out an aggregate:
01020589 1298
5ed4f2ec 1299 %HASH = (); # completely empty %HASH
1300 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1301
5ed4f2ec 1302 @ARRAY = (); # completely empty @ARRAY
1303 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1304
80d38338
TC
1305The EXPR can be arbitrarily complicated provided its
1306final operation is an element or slice of an aggregate:
a0d0e21e
LW
1307
1308 delete $ref->[$x][$y]{$key};
5f05dabc 1309 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1310
01020589
GS
1311 delete $ref->[$x][$y][$index];
1312 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1313
a0d0e21e 1314=item die LIST
d74e8afc 1315X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1316
391b733c 1317C<die> raises an exception. Inside an C<eval> the error message is stuffed
4c050ad5
NC
1318into C<$@> and the C<eval> is terminated with the undefined value.
1319If the exception is outside of all enclosing C<eval>s, then the uncaught
391b733c 1320exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1321need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1322
1323Equivalent examples:
1324
1325 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1326 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1327
ccac6780 1328If the last element of LIST does not end in a newline, the current
df37ec69
WW
1329script line number and input line number (if any) are also printed,
1330and a newline is supplied. Note that the "input line number" (also
1331known as "chunk") is subject to whatever notion of "line" happens to
1332be currently in effect, and is also available as the special variable
1333C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1334
1335Hint: sometimes appending C<", stopped"> to your message will cause it
1336to make better sense when the string C<"at foo line 123"> is appended.
1337Suppose you are running script "canasta".
a0d0e21e
LW
1338
1339 die "/etc/games is no good";
1340 die "/etc/games is no good, stopped";
1341
1342produce, respectively
1343
1344 /etc/games is no good at canasta line 123.
1345 /etc/games is no good, stopped at canasta line 123.
1346
a96d0188 1347If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1348previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a 1349This is useful for propagating exceptions:
1350
1351 eval { ... };
1352 die unless $@ =~ /Expected exception/;
1353
a96d0188 1354If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1355C<PROPAGATE> method, that method will be called with additional file
1356and line number parameters. The return value replaces the value in
34169887 1357C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1358were called.
1359
7660c0ab 1360If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1361
4c050ad5
NC
1362If an uncaught exception results in interpreter exit, the exit code is
1363determined from the values of C<$!> and C<$?> with this pseudocode:
1364
1365 exit $! if $!; # errno
1366 exit $? >> 8 if $? >> 8; # child exit status
1367 exit 255; # last resort
1368
1369The intent is to squeeze as much possible information about the likely cause
391b733c
FC
1370into the limited space of the system exit
1371code. However, as C<$!> is the value
4c050ad5
NC
1372of C's C<errno>, which can be set by any system call, this means that the value
1373of the exit code used by C<die> can be non-predictable, so should not be relied
1374upon, other than to be non-zero.
1375
80d38338
TC
1376You can also call C<die> with a reference argument, and if this is trapped
1377within an C<eval>, C<$@> contains that reference. This permits more
1378elaborate exception handling using objects that maintain arbitrary state
1379about the exception. Such a scheme is sometimes preferable to matching
1380particular string values of C<$@> with regular expressions. Because C<$@>
1381is a global variable and C<eval> may be used within object implementations,
1382be careful that analyzing the error object doesn't replace the reference in
1383the global variable. It's easiest to make a local copy of the reference
1384before any manipulations. Here's an example:
52531d10 1385
80d38338 1386 use Scalar::Util "blessed";
da279afe 1387
52531d10 1388 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1389 if (my $ev_err = $@) {
1390 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1391 # handle Some::Module::Exception
1392 }
1393 else {
1394 # handle all other possible exceptions
1395 }
1396 }
1397
3b10bc60 1398Because Perl stringifies uncaught exception messages before display,
80d38338 1399you'll probably want to overload stringification operations on
52531d10
GS
1400exception objects. See L<overload> for details about that.
1401
19799a22
GS
1402You can arrange for a callback to be run just before the C<die>
1403does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1404handler is called with the error text and can change the error
19799a22 1405message, if it sees fit, by calling C<die> again. See
96090e4f 1406L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1407L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1408to be run only right before your program was to exit, this is not
3b10bc60 1409currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1410even inside eval()ed blocks/strings! If one wants the hook to do
1411nothing in such situations, put
fb73857a 1412
5ed4f2ec 1413 die @_ if $^S;
fb73857a 1414
19799a22
GS
1415as the first line of the handler (see L<perlvar/$^S>). Because
1416this promotes strange action at a distance, this counterintuitive
b76cc8ba 1417behavior may be fixed in a future release.
774d564b 1418
4c050ad5
NC
1419See also exit(), warn(), and the Carp module.
1420
a0d0e21e 1421=item do BLOCK
d74e8afc 1422X<do> X<block>
a0d0e21e
LW
1423
1424Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1425sequence of commands indicated by BLOCK. When modified by the C<while> or
1426C<until> loop modifier, executes the BLOCK once before testing the loop
391b733c 1427condition. (On other statements the loop modifiers test the conditional
6b275a1f 1428first.)
a0d0e21e 1429
4968c1e4 1430C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1431C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1432See L<perlsyn> for alternative strategies.
4968c1e4 1433
a0d0e21e 1434=item do SUBROUTINE(LIST)
d74e8afc 1435X<do>
a0d0e21e 1436
51124b83
FC
1437This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
1438a scalar variable or a subroutine beginning with C<&>.
a0d0e21e
LW
1439
1440=item do EXPR
d74e8afc 1441X<do>
a0d0e21e
LW
1442
1443Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1444file as a Perl script.
a0d0e21e
LW
1445
1446 do 'stat.pl';
1447
1448is just like
1449
986b19de 1450 eval `cat stat.pl`;
a0d0e21e 1451
2b5ab1e7 1452except that it's more efficient and concise, keeps track of the current
96090e4f
LB
1453filename for error messages, searches the C<@INC> directories, and updates
1454C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1455these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1456cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1457same, however, in that it does reparse the file every time you call it,
1458so you probably don't want to do this inside a loop.
a0d0e21e 1459
8f1da26d 1460If C<do> can read the file but cannot compile it, it returns C<undef> and sets
9dc513c5
DG
1461an error message in C<$@>. If C<do> cannot read the file, it returns undef
1462and sets C<$!> to the error. Always check C<$@> first, as compilation
1463could fail in a way that also sets C<$!>. If the file is successfully
1464compiled, C<do> returns the value of the last expression evaluated.
8e30cc93 1465
80d38338 1466Inclusion of library modules is better done with the
19799a22 1467C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1468and raise an exception if there's a problem.
a0d0e21e 1469
5a964f20
TC
1470You might like to use C<do> to read in a program configuration
1471file. Manual error checking can be done this way:
1472
b76cc8ba 1473 # read in config files: system first, then user
f86cebdf 1474 for $file ("/share/prog/defaults.rc",
b76cc8ba 1475 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1476 {
1477 unless ($return = do $file) {
1478 warn "couldn't parse $file: $@" if $@;
1479 warn "couldn't do $file: $!" unless defined $return;
1480 warn "couldn't run $file" unless $return;
1481 }
5a964f20
TC
1482 }
1483
a0d0e21e 1484=item dump LABEL
d74e8afc 1485X<dump> X<core> X<undump>
a0d0e21e 1486
1614b0e3
JD
1487=item dump
1488
19799a22
GS
1489This function causes an immediate core dump. See also the B<-u>
1490command-line switch in L<perlrun>, which does the same thing.
1491Primarily this is so that you can use the B<undump> program (not
1492supplied) to turn your core dump into an executable binary after
1493having initialized all your variables at the beginning of the
1494program. When the new binary is executed it will begin by executing
1495a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1496Think of it as a goto with an intervening core dump and reincarnation.
1497If C<LABEL> is omitted, restarts the program from the top.
1498
1499B<WARNING>: Any files opened at the time of the dump will I<not>
1500be open any more when the program is reincarnated, with possible
80d38338 1501resulting confusion by Perl.
19799a22 1502
59f521f4 1503This function is now largely obsolete, mostly because it's very hard to
391b733c 1504convert a core file into an executable. That's why you should now invoke
59f521f4 1505it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1506typo.
19799a22 1507
ea9eb35a
BJ
1508Portability issues: L<perlport/dump>.
1509
532eee96 1510=item each HASH
d74e8afc 1511X<each> X<hash, iterator>
aa689395 1512
532eee96 1513=item each ARRAY
aeedbbed
NC
1514X<array, iterator>
1515
f5a93a43
TC
1516=item each EXPR
1517
bade7fbc
TC
1518When called on a hash in list context, returns a 2-element list
1519consisting of the key and value for the next element of a hash. In Perl
15205.12 and later only, it will also return the index and value for the next
1521element of an array so that you can iterate over it; older Perls consider
1522this a syntax error. When called in scalar context, returns only the key
1523(not the value) in a hash, or the index in an array.
2f9daede 1524
aeedbbed 1525Hash entries are returned in an apparently random order. The actual random
3b10bc60 1526order is subject to change in future versions of Perl, but it is
504f80c1 1527guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1528function would produce on the same (unmodified) hash. Since Perl
22883ac5 15295.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1530for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1531
80d38338
TC
1532After C<each> has returned all entries from the hash or array, the next
1533call to C<each> returns the empty list in list context and C<undef> in
bade7fbc
TC
1534scalar context; the next call following I<that> one restarts iteration.
1535Each hash or array has its own internal iterator, accessed by C<each>,
1536C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1537reached the end as just described; it can be explicitly reset by calling
1538C<keys> or C<values> on the hash or array. If you add or delete a hash's
1539elements while iterating over it, entries may be skipped or duplicated--so
1540don't do that. Exception: In the current implementation, it is always safe
1541to delete the item most recently returned by C<each()>, so the following
1542code works properly:
74fc8b5f
MJD
1543
1544 while (($key, $value) = each %hash) {
1545 print $key, "\n";
1546 delete $hash{$key}; # This is safe
1547 }
aa689395 1548
80d38338 1549This prints out your environment like the printenv(1) program,
3b10bc60 1550but in a different order:
a0d0e21e
LW
1551
1552 while (($key,$value) = each %ENV) {
a9a5a0dc 1553 print "$key=$value\n";
a0d0e21e
LW
1554 }
1555
f5a93a43
TC
1556Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1557reference to an unblessed hash or array. The argument will be dereferenced
1558automatically. This aspect of C<each> is considered highly experimental.
1559The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
1560
1561 while (($key,$value) = each $hashref) { ... }
1562
bade7fbc
TC
1563To avoid confusing would-be users of your code who are running earlier
1564versions of Perl with mysterious syntax errors, put this sort of thing at
1565the top of your file to signal that your code will work I<only> on Perls of
1566a recent vintage:
1567
1568 use 5.012; # so keys/values/each work on arrays
1569 use 5.014; # so keys/values/each work on scalars (experimental)
1570
8f1da26d 1571See also C<keys>, C<values>, and C<sort>.
a0d0e21e
LW
1572
1573=item eof FILEHANDLE
d74e8afc
ITB
1574X<eof>
1575X<end of file>
1576X<end-of-file>
a0d0e21e 1577
4633a7c4
LW
1578=item eof ()
1579
a0d0e21e
LW
1580=item eof
1581
8f1da26d 1582Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
a0d0e21e 1583FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1584gives the real filehandle. (Note that this function actually
80d38338 1585reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1586interactive context.) Do not read from a terminal file (or call
19799a22 1587C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1588as terminals may lose the end-of-file condition if you do.
1589
820475bd 1590An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1591with empty parentheses is different. It refers to the pseudo file
820475bd 1592formed from the files listed on the command line and accessed via the
61eff3bc
JH
1593C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1594as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1595used will cause C<@ARGV> to be examined to determine if input is
67408cae 1596available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1597end-of-file will assume you are processing another C<@ARGV> list,
1598and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1599see L<perlop/"I/O Operators">.
820475bd 1600
61eff3bc 1601In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
8f1da26d
TC
1602detect the end of each file, whereas C<eof()> will detect the end
1603of the very last file only. Examples:
a0d0e21e 1604
748a9306
LW
1605 # reset line numbering on each input file
1606 while (<>) {
a9a5a0dc
VP
1607 next if /^\s*#/; # skip comments
1608 print "$.\t$_";
5a964f20 1609 } continue {
a9a5a0dc 1610 close ARGV if eof; # Not eof()!
748a9306
LW
1611 }
1612
a0d0e21e
LW
1613 # insert dashes just before last line of last file
1614 while (<>) {
a9a5a0dc
VP
1615 if (eof()) { # check for end of last file
1616 print "--------------\n";
1617 }
1618 print;
4d0444a3 1619 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1620 }
1621
a0d0e21e 1622Practical hint: you almost never need to use C<eof> in Perl, because the
8f1da26d
TC
1623input operators typically return C<undef> when they run out of data or
1624encounter an error.
a0d0e21e
LW
1625
1626=item eval EXPR
d74e8afc 1627X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1628X<error, handling> X<exception, handling>
a0d0e21e
LW
1629
1630=item eval BLOCK
1631
ce2984c3
PF
1632=item eval
1633
c7cc6f1c
GS
1634In the first form, the return value of EXPR is parsed and executed as if it
1635were a little Perl program. The value of the expression (which is itself
8f1da26d 1636determined within scalar context) is first parsed, and if there were no
2341804c 1637errors, executed as a block within the lexical context of the current Perl
df4833a8 1638program. This means, that in particular, any outer lexical variables are
2341804c
DM
1639visible to it, and any package variable settings or subroutine and format
1640definitions remain afterwards.
1641
1642Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1643If EXPR is omitted, evaluates C<$_>. This form is typically used to
1644delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c 1645
7289c5e6
FC
1646If the C<unicode_eval> feature is enabled (which is the default under a
1647C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1648characters, so C<use utf8> declarations have no effect, and source filters
1649are forbidden. In the absence of the C<unicode_eval> feature, the string
1650will sometimes be treated as characters and sometimes as bytes, depending
1651on the internal encoding, and source filters activated within the C<eval>
1652exhibit the erratic, but historical, behaviour of affecting some outer file
1653scope that is still compiling. See also the L</evalbytes> keyword, which
1654always treats its input as a byte stream and works properly with source
1655filters, and the L<feature> pragma.
1656
c7cc6f1c 1657In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1658same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1659within the context of the current Perl program. This form is typically
1660used to trap exceptions more efficiently than the first (see below), while
1661also providing the benefit of checking the code within BLOCK at compile
1662time.
1663
1664The final semicolon, if any, may be omitted from the value of EXPR or within
1665the BLOCK.
1666
1667In both forms, the value returned is the value of the last expression
5a964f20 1668evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1669as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1670in void, scalar, or list context, depending on the context of the C<eval>
1671itself. See L</wantarray> for more on how the evaluation context can be
1672determined.
a0d0e21e 1673
19799a22 1674If there is a syntax error or runtime error, or a C<die> statement is
8f1da26d 1675executed, C<eval> returns C<undef> in scalar context
774b80e8
FC
1676or an empty list in list context, and C<$@> is set to the error
1677message. (Prior to 5.16, a bug caused C<undef> to be returned
1678in list context for syntax errors, but not for runtime errors.)
1679If there was no error, C<$@> is set to the empty string. A
9cc672d4
FC
1680control flow operator like C<last> or C<goto> can bypass the setting of
1681C<$@>. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1682warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1683To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1684turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1685See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1686
19799a22
GS
1687Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1688determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1689is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1690the die operator is used to raise exceptions.
1691
5f1da31c
NT
1692If you want to trap errors when loading an XS module, some problems with
1693the binary interface (such as Perl version skew) may be fatal even with
df4833a8 1694C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
5f1da31c 1695
a0d0e21e
LW
1696If the code to be executed doesn't vary, you may use the eval-BLOCK
1697form to trap run-time errors without incurring the penalty of
1698recompiling each time. The error, if any, is still returned in C<$@>.
1699Examples:
1700
54310121 1701 # make divide-by-zero nonfatal
a0d0e21e
LW
1702 eval { $answer = $a / $b; }; warn $@ if $@;
1703
1704 # same thing, but less efficient
1705 eval '$answer = $a / $b'; warn $@ if $@;
1706
1707 # a compile-time error
5ed4f2ec 1708 eval { $answer = }; # WRONG
a0d0e21e
LW
1709
1710 # a run-time error
5ed4f2ec 1711 eval '$answer ='; # sets $@
a0d0e21e 1712
cf264981
SP
1713Using the C<eval{}> form as an exception trap in libraries does have some
1714issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1715may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1716You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1717as this example shows:
774d564b 1718
80d38338 1719 # a private exception trap for divide-by-zero
f86cebdf
GS
1720 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1721 warn $@ if $@;
774d564b 1722
1723This is especially significant, given that C<__DIE__> hooks can call
19799a22 1724C<die> again, which has the effect of changing their error messages:
774d564b 1725
1726 # __DIE__ hooks may modify error messages
1727 {
f86cebdf
GS
1728 local $SIG{'__DIE__'} =
1729 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1730 eval { die "foo lives here" };
1731 print $@ if $@; # prints "bar lives here"
774d564b 1732 }
1733
19799a22 1734Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1735may be fixed in a future release.
1736
19799a22 1737With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1738being looked at when:
1739
5ed4f2ec 1740 eval $x; # CASE 1
1741 eval "$x"; # CASE 2
a0d0e21e 1742
5ed4f2ec 1743 eval '$x'; # CASE 3
1744 eval { $x }; # CASE 4
a0d0e21e 1745
5ed4f2ec 1746 eval "\$$x++"; # CASE 5
1747 $$x++; # CASE 6
a0d0e21e 1748
2f9daede 1749Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1750the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1751the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1752and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1753does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TP
1754purely visual reasons, but it also has the advantage of compiling at
1755compile-time instead of at run-time.) Case 5 is a place where
19799a22 1756normally you I<would> like to use double quotes, except that in this
2f9daede
TP
1757particular situation, you can just use symbolic references instead, as
1758in case 6.
a0d0e21e 1759
b6538e4f 1760Before Perl 5.14, the assignment to C<$@> occurred before restoration
bade7fbc 1761of localized variables, which means that for your code to run on older
b208c909 1762versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1763errors:
1764
1765 # alter $@ on nefarious repugnancy only
1766 {
1767 my $e;
1768 {
1769 local $@; # protect existing $@
1770 eval { test_repugnancy() };
b208c909 1771 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
8a5a710d
DN
1772 $@ =~ /nefarious/ and $e = $@;
1773 }
1774 die $e if defined $e
1775 }
1776
4968c1e4 1777C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1778C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1779
3b10bc60 1780An C<eval ''> executed within the C<DB> package doesn't see the usual
1781surrounding lexical scope, but rather the scope of the first non-DB piece
df4833a8 1782of code that called it. You don't normally need to worry about this unless
3b10bc60 1783you are writing a Perl debugger.
d819b83a 1784
7289c5e6
FC
1785=item evalbytes EXPR
1786X<evalbytes>
1787
1788=item evalbytes
1789
1790This function is like L</eval> with a string argument, except it always
1791parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1792string containing characters whose ordinal value exceeds 255 results in an
1793error. Source filters activated within the evaluated code apply to the
1794code itself.
1795
1796This function is only available under the C<evalbytes> feature, a
1797C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1798L<feature> for more information.
1799
a0d0e21e 1800=item exec LIST
d74e8afc 1801X<exec> X<execute>
a0d0e21e 1802
8bf3b016
GS
1803=item exec PROGRAM LIST
1804
3b10bc60 1805The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1806use C<system> instead of C<exec> if you want it to return. It fails and
1807returns false only if the command does not exist I<and> it is executed
fb73857a 1808directly instead of via your system's command shell (see below).
a0d0e21e 1809
19799a22 1810Since it's a common mistake to use C<exec> instead of C<system>, Perl
80d38338 1811warns you if there is a following statement that isn't C<die>, C<warn>,
3b10bc60 1812or C<exit> (if C<-w> is set--but you always do that, right?). If you
19799a22 1813I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1814can use one of these styles to avoid the warning:
1815
5a964f20
TC
1816 exec ('foo') or print STDERR "couldn't exec foo: $!";
1817 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1818
5a964f20 1819If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1820with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1821If there is only one scalar argument or an array with one element in it,
1822the argument is checked for shell metacharacters, and if there are any,
1823the entire argument is passed to the system's command shell for parsing
1824(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1825If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1826words and passed directly to C<execvp>, which is more efficient.
19799a22 1827Examples:
a0d0e21e 1828
19799a22
GS
1829 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1830 exec "sort $outfile | uniq";
a0d0e21e
LW
1831
1832If you don't really want to execute the first argument, but want to lie
1833to the program you are executing about its own name, you can specify
1834the program you actually want to run as an "indirect object" (without a
1835comma) in front of the LIST. (This always forces interpretation of the
54310121 1836LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1837the list.) Example:
1838
1839 $shell = '/bin/csh';
5ed4f2ec 1840 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1841
1842or, more directly,
1843
5ed4f2ec 1844 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1845
3b10bc60 1846When the arguments get executed via the system shell, results are
1847subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1848for details.
1849
19799a22
GS
1850Using an indirect object with C<exec> or C<system> is also more
1851secure. This usage (which also works fine with system()) forces
1852interpretation of the arguments as a multivalued list, even if the
1853list had just one argument. That way you're safe from the shell
1854expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1855
1856 @args = ( "echo surprise" );
1857
2b5ab1e7 1858 exec @args; # subject to shell escapes
f86cebdf 1859 # if @args == 1
2b5ab1e7 1860 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1861
1862The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1863program, passing it C<"surprise"> an argument. The second version didn't;
1864it tried to run a program named I<"echo surprise">, didn't find it, and set
1865C<$?> to a non-zero value indicating failure.
5a964f20 1866
80d38338 1867Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1868output before the exec, but this may not be supported on some platforms
1869(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1870in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 1871open handles to avoid lost output.
0f897271 1872
80d38338
TC
1873Note that C<exec> will not call your C<END> blocks, nor will it invoke
1874C<DESTROY> methods on your objects.
7660c0ab 1875
ea9eb35a
BJ
1876Portability issues: L<perlport/exec>.
1877
a0d0e21e 1878=item exists EXPR
d74e8afc 1879X<exists> X<autovivification>
a0d0e21e 1880
d0a76353
RS
1881Given an expression that specifies an element of a hash, returns true if the
1882specified element in the hash has ever been initialized, even if the
1883corresponding value is undefined.
a0d0e21e 1884
5ed4f2ec 1885 print "Exists\n" if exists $hash{$key};
1886 print "Defined\n" if defined $hash{$key};
01020589
GS
1887 print "True\n" if $hash{$key};
1888
d0a76353 1889exists may also be called on array elements, but its behavior is much less
8f1da26d 1890obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
d0a76353
RS
1891that calling exists on array values is deprecated and likely to be removed in
1892a future version of Perl.
1893
5ed4f2ec 1894 print "Exists\n" if exists $array[$index];
1895 print "Defined\n" if defined $array[$index];
01020589 1896 print "True\n" if $array[$index];
a0d0e21e 1897
8f1da26d 1898A hash or array element can be true only if it's defined and defined only if
a0d0e21e
LW
1899it exists, but the reverse doesn't necessarily hold true.
1900
afebc493
GS
1901Given an expression that specifies the name of a subroutine,
1902returns true if the specified subroutine has ever been declared, even
1903if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 1904does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
1905exist may still be callable: its package may have an C<AUTOLOAD>
1906method that makes it spring into existence the first time that it is
3b10bc60 1907called; see L<perlsub>.
afebc493 1908
5ed4f2ec 1909 print "Exists\n" if exists &subroutine;
1910 print "Defined\n" if defined &subroutine;
afebc493 1911
a0d0e21e 1912Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1913operation is a hash or array key lookup or subroutine name:
a0d0e21e 1914
5ed4f2ec 1915 if (exists $ref->{A}->{B}->{$key}) { }
1916 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 1917
5ed4f2ec 1918 if (exists $ref->{A}->{B}->[$ix]) { }
1919 if (exists $hash{A}{B}[$ix]) { }
01020589 1920
afebc493
GS
1921 if (exists &{$ref->{A}{B}{$key}}) { }
1922
3b10bc60 1923Although the mostly deeply nested array or hash will not spring into
1924existence just because its existence was tested, any intervening ones will.
61eff3bc 1925Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 1926into existence due to the existence test for the $key element above.
3b10bc60 1927This happens anywhere the arrow operator is used, including even here:
5a964f20 1928
2b5ab1e7 1929 undef $ref;
5ed4f2ec 1930 if (exists $ref->{"Some key"}) { }
1931 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
1932
1933This surprising autovivification in what does not at first--or even
1934second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1935release.
a0d0e21e 1936
afebc493
GS
1937Use of a subroutine call, rather than a subroutine name, as an argument
1938to exists() is an error.
1939
5ed4f2ec 1940 exists &sub; # OK
1941 exists &sub(); # Error
afebc493 1942
a0d0e21e 1943=item exit EXPR
d74e8afc 1944X<exit> X<terminate> X<abort>
a0d0e21e 1945
ce2984c3
PF
1946=item exit
1947
2b5ab1e7 1948Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1949
1950 $ans = <STDIN>;
1951 exit 0 if $ans =~ /^[Xx]/;
1952
19799a22 1953See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1954universally recognized values for EXPR are C<0> for success and C<1>
1955for error; other values are subject to interpretation depending on the
1956environment in which the Perl program is running. For example, exiting
195769 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1958the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1959
19799a22
GS
1960Don't use C<exit> to abort a subroutine if there's any chance that
1961someone might want to trap whatever error happened. Use C<die> instead,
1962which can be trapped by an C<eval>.
28757baa 1963
19799a22 1964The exit() function does not always exit immediately. It calls any
2b5ab1e7 1965defined C<END> routines first, but these C<END> routines may not
19799a22 1966themselves abort the exit. Likewise any object destructors that need to
60275626 1967be called are called before the real exit. C<END> routines and destructors
391b733c 1968can change the exit status by modifying C<$?>. If this is a problem, you
2b5ab1e7 1969can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1970See L<perlmod> for details.
5a964f20 1971
ea9eb35a
BJ
1972Portability issues: L<perlport/exit>.
1973
a0d0e21e 1974=item exp EXPR
d74e8afc 1975X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1976
54310121 1977=item exp
bbce6d69 1978
b76cc8ba 1979Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1980If EXPR is omitted, gives C<exp($_)>.
1981
628253b8
BF
1982=item fc EXPR
1983X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
1984
1985=item fc
1986
1987Returns the casefolded version of EXPR. This is the internal function
1988implementing the C<\F> escape in double-quoted strings.
1989
1990Casefolding is the process of mapping strings to a form where case
1991differences are erased; comparing two strings in their casefolded
1992form is effectively a way of asking if two strings are equal,
1993regardless of case.
1994
1995Roughly, if you ever found yourself writing this
1996
1997 lc($this) eq lc($that) # Wrong!
1998 # or
1999 uc($this) eq uc($that) # Also wrong!
2000 # or
2001 $this =~ /\Q$that/i # Right!
2002
2003Now you can write
2004
2005 fc($this) eq fc($that)
2006
2007And get the correct results.
2008
2009Perl only implements the full form of casefolding.
2010For further information on casefolding, refer to
2011the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
20124.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2013available at L<http://www.unicode.org/versions/latest/>, as well as the
2014Case Charts available at L<http://www.unicode.org/charts/case/>.
2015
2016If EXPR is omitted, uses C<$_>.
2017
2018This function behaves the same way under various pragma, such as in a locale,
2019as L</lc> does.
2020
2021While the Unicode Standard defines two additional forms of casefolding,
2022one for Turkic languages and one that never maps one character into multiple
2023characters, these are not provided by the Perl core; However, the CPAN module
2024C<Unicode::Casing> may be used to provide an implementation.
2025
2026This keyword is available only when the C<"fc"> feature is enabled,
2027or when prefixed with C<CORE::>; See L<feature>. Alternately,
2028include a C<use v5.16> or later to the current scope.
2029
a0d0e21e 2030=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2031X<fcntl>
a0d0e21e 2032
f86cebdf 2033Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
2034
2035 use Fcntl;
2036
0ade1984 2037first to get the correct constant definitions. Argument processing and
3b10bc60 2038value returned work just like C<ioctl> below.
a0d0e21e
LW
2039For example:
2040
2041 use Fcntl;
5a964f20 2042 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 2043 or die "can't fcntl F_GETFL: $!";
5a964f20 2044
554ad1fc 2045You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
2046Like C<ioctl>, it maps a C<0> return from the system call into
2047C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
2048in numeric context. It is also exempt from the normal B<-w> warnings
2049on improper numeric conversions.
5a964f20 2050
3b10bc60 2051Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
2052doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2053manpage to learn what functions are available on your system.
a0d0e21e 2054
be2f7487
TH
2055Here's an example of setting a filehandle named C<REMOTE> to be
2056non-blocking at the system level. You'll have to negotiate C<$|>
2057on your own, though.
2058
2059 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2060
2061 $flags = fcntl(REMOTE, F_GETFL, 0)
2062 or die "Can't get flags for the socket: $!\n";
2063
2064 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2065 or die "Can't set flags for the socket: $!\n";
2066
ea9eb35a
BJ
2067Portability issues: L<perlport/fcntl>.
2068
cfa52385
FC
2069=item __FILE__
2070X<__FILE__>
2071
2072A special token that returns the name of the file in which it occurs.
2073
a0d0e21e 2074=item fileno FILEHANDLE
d74e8afc 2075X<fileno>
a0d0e21e 2076
2b5ab1e7 2077Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
2078filehandle is not open. If there is no real file descriptor at the OS
2079level, as can happen with filehandles connected to memory objects via
2080C<open> with a reference for the third argument, -1 is returned.
2081
2082This is mainly useful for constructing
19799a22 2083bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
2084If FILEHANDLE is an expression, the value is taken as an indirect
2085filehandle, generally its name.
5a964f20 2086
b76cc8ba 2087You can use this to find out whether two handles refer to the
5a964f20
TC
2088same underlying descriptor:
2089
2090 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 2091 print "THIS and THAT are dups\n";
b76cc8ba
NIS
2092 }
2093
a0d0e21e 2094=item flock FILEHANDLE,OPERATION
d74e8afc 2095X<flock> X<lock> X<locking>
a0d0e21e 2096
19799a22
GS
2097Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2098for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 2099machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 2100C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 2101entire files only, not records.
2b5ab1e7
TC
2102
2103Two potentially non-obvious but traditional C<flock> semantics are
2104that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
2105are B<merely advisory>. Such discretionary locks are more flexible, but
2106offer fewer guarantees. This means that programs that do not also use
2107C<flock> may modify files locked with C<flock>. See L<perlport>,
8f1da26d 2108your port's specific documentation, and your system-specific local manpages
2b5ab1e7
TC
2109for details. It's best to assume traditional behavior if you're writing
2110portable programs. (But if you're not, you should as always feel perfectly
2111free to write for your own system's idiosyncrasies (sometimes called
2112"features"). Slavish adherence to portability concerns shouldn't get
2113in the way of your getting your job done.)
a3cb178b 2114
8ebc5c01 2115OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2116LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
8f1da26d
TC
2117you can use the symbolic names if you import them from the L<Fcntl> module,
2118either individually, or as a group using the C<:flock> tag. LOCK_SH
68dc0745 2119requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 2120releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
8f1da26d 2121LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
3b10bc60 2122waiting for the lock; check the return status to see if you got it.
68dc0745 2123
2b5ab1e7
TC
2124To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2125before locking or unlocking it.
8ebc5c01 2126
f86cebdf 2127Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 2128locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 2129are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 2130implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01 2131differing semantics shouldn't bite too many people.
2132
becacb53
TM
2133Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2134be open with read intent to use LOCK_SH and requires that it be open
2135with write intent to use LOCK_EX.
2136
19799a22
GS
2137Note also that some versions of C<flock> cannot lock things over the
2138network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2139that. If you like you can force Perl to ignore your system's flock(2)
2140function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2141the switch C<-Ud_flock> to the F<Configure> program when you configure
8f1da26d 2142and build a new Perl.
4633a7c4
LW
2143
2144Here's a mailbox appender for BSD systems.
a0d0e21e 2145
7ed5353d 2146 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
2147
2148 sub lock {
a9a5a0dc
VP
2149 my ($fh) = @_;
2150 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2151
a9a5a0dc
VP
2152 # and, in case someone appended while we were waiting...
2153 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2154 }
2155
2156 sub unlock {
a9a5a0dc
VP
2157 my ($fh) = @_;
2158 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2159 }
2160
b0169937 2161 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2162 or die "Can't open mailbox: $!";
a0d0e21e 2163
7ed5353d 2164 lock($mbox);
b0169937 2165 print $mbox $msg,"\n\n";
7ed5353d 2166 unlock($mbox);
a0d0e21e 2167
3b10bc60 2168On systems that support a real flock(2), locks are inherited across fork()
2169calls, whereas those that must resort to the more capricious fcntl(2)
2170function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2171
cb1a09d0 2172See also L<DB_File> for other flock() examples.
a0d0e21e 2173
ea9eb35a
BJ
2174Portability issues: L<perlport/flock>.
2175
a0d0e21e 2176=item fork
d74e8afc 2177X<fork> X<child> X<parent>
a0d0e21e 2178
2b5ab1e7
TC
2179Does a fork(2) system call to create a new process running the
2180same program at the same point. It returns the child pid to the
2181parent process, C<0> to the child process, or C<undef> if the fork is
2182unsuccessful. File descriptors (and sometimes locks on those descriptors)
2183are shared, while everything else is copied. On most systems supporting
2184fork(), great care has gone into making it extremely efficient (for
2185example, using copy-on-write technology on data pages), making it the
2186dominant paradigm for multitasking over the last few decades.
5a964f20 2187
80d38338 2188Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
2189output before forking the child process, but this may not be supported
2190on some platforms (see L<perlport>). To be safe, you may need to set
2191C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2192C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2193
19799a22 2194If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2195accumulate zombies. On some systems, you can avoid this by setting
2196C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2197forking and reaping moribund children.
cb1a09d0 2198
28757baa 2199Note that if your forked child inherits system file descriptors like
2200STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2201if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2202backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2203You should reopen those to F</dev/null> if it's any issue.
28757baa 2204
ea9eb35a 2205On some platforms such as Windows, where the fork() system call is not available,
391b733c
FC
2206Perl can be built to emulate fork() in the Perl interpreter.
2207The emulation is designed, at the level of the Perl program,
2208to be as compatible as possible with the "Unix" fork().
6d17f725 2209However it has limitations that have to be considered in code intended to be portable.
ea9eb35a
BJ
2210See L<perlfork> for more details.
2211
2212Portability issues: L<perlport/fork>.
2213
cb1a09d0 2214=item format
d74e8afc 2215X<format>
cb1a09d0 2216
19799a22 2217Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2218example:
2219
54310121 2220 format Something =
a9a5a0dc
VP
2221 Test: @<<<<<<<< @||||| @>>>>>
2222 $str, $%, '$' . int($num)
cb1a09d0
AD
2223 .
2224
2225 $str = "widget";
184e9718 2226 $num = $cost/$quantity;
cb1a09d0
AD
2227 $~ = 'Something';
2228 write;
2229
2230See L<perlform> for many details and examples.
2231
8903cb82 2232=item formline PICTURE,LIST
d74e8afc 2233X<formline>
a0d0e21e 2234
5a964f20 2235This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2236too. It formats (see L<perlform>) a list of values according to the
2237contents of PICTURE, placing the output into the format output
7660c0ab 2238accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2239Eventually, when a C<write> is done, the contents of
cf264981
SP
2240C<$^A> are written to some filehandle. You could also read C<$^A>
2241and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2242does one C<formline> per line of form, but the C<formline> function itself
748a9306 2243doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2244that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2245You may therefore need to use multiple formlines to implement a single
3b10bc60 2246record format, just like the C<format> compiler.
748a9306 2247
19799a22 2248Be careful if you put double quotes around the picture, because an C<@>
748a9306 2249character may be taken to mean the beginning of an array name.
19799a22 2250C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2251
445b09e5
FC
2252If you are trying to use this instead of C<write> to capture the output,
2253you may find it easier to open a filehandle to a scalar
2254(C<< open $fh, ">", \$output >>) and write to that instead.
2255
a0d0e21e 2256=item getc FILEHANDLE
f723aae1 2257X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2258
2259=item getc
2260
2261Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2262or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2263the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2264STDIN. This is not particularly efficient. However, it cannot be
2265used by itself to fetch single characters without waiting for the user
2266to hit enter. For that, try something more like:
4633a7c4
LW
2267
2268 if ($BSD_STYLE) {
a9a5a0dc 2269 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2270 }
2271 else {
a9a5a0dc 2272 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2273 }
2274
2275 $key = getc(STDIN);
2276
2277 if ($BSD_STYLE) {
a9a5a0dc 2278 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2279 }
2280 else {
3b10bc60 2281 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2282 }
2283 print "\n";
2284
54310121 2285Determination of whether $BSD_STYLE should be set
2286is left as an exercise to the reader.
cb1a09d0 2287
19799a22 2288The C<POSIX::getattr> function can do this more portably on
2b5ab1e7 2289systems purporting POSIX compliance. See also the C<Term::ReadKey>
a3390c9f 2290module from your nearest CPAN site; details on CPAN can be found under
2b5ab1e7 2291L<perlmodlib/CPAN>.
a0d0e21e
LW
2292
2293=item getlogin
d74e8afc 2294X<getlogin> X<login>
a0d0e21e 2295
cf264981 2296This implements the C library function of the same name, which on most
3b10bc60 2297systems returns the current login from F</etc/utmp>, if any. If it
2298returns the empty string, use C<getpwuid>.
a0d0e21e 2299
f86702cc 2300 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2301
19799a22
GS
2302Do not consider C<getlogin> for authentication: it is not as
2303secure as C<getpwuid>.
4633a7c4 2304
ea9eb35a
BJ
2305Portability issues: L<perlport/getlogin>.
2306
a0d0e21e 2307=item getpeername SOCKET
d74e8afc 2308X<getpeername> X<peer>
a0d0e21e 2309
a3390c9f
FC
2310Returns the packed sockaddr address of the other end of the SOCKET
2311connection.
a0d0e21e 2312
4633a7c4
LW
2313 use Socket;
2314 $hersockaddr = getpeername(SOCK);
19799a22 2315 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2316 $herhostname = gethostbyaddr($iaddr, AF_INET);
2317 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2318
2319=item getpgrp PID
d74e8afc 2320X<getpgrp> X<group>
a0d0e21e 2321
47e29363 2322Returns the current process group for the specified PID. Use
7660c0ab 2323a PID of C<0> to get the current process group for the
4633a7c4 2324current process. Will raise an exception if used on a machine that
a3390c9f
FC
2325doesn't implement getpgrp(2). If PID is omitted, returns the process
2326group of the current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2327does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e 2328
ea9eb35a
BJ
2329Portability issues: L<perlport/getpgrp>.
2330
a0d0e21e 2331=item getppid
d74e8afc 2332X<getppid> X<parent> X<pid>
a0d0e21e
LW
2333
2334Returns the process id of the parent process.
2335
4d76a344 2336Note for Linux users: on Linux, the C functions C<getpid()> and
391b733c 2337C<getppid()> return different values from different threads. In order to
3b10bc60 2338be portable, this behavior is not reflected by the Perl-level function
391b733c 2339C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2340to call the underlying C<getppid()>, you may use the CPAN module
2341C<Linux::Pid>.
4d76a344 2342
ea9eb35a
BJ
2343Portability issues: L<perlport/getppid>.
2344
a0d0e21e 2345=item getpriority WHICH,WHO
d74e8afc 2346X<getpriority> X<priority> X<nice>
a0d0e21e 2347
4633a7c4 2348Returns the current priority for a process, a process group, or a user.
01aa884e 2349(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2350machine that doesn't implement getpriority(2).
a0d0e21e 2351
ea9eb35a
BJ
2352Portability issues: L<perlport/getpriority>.
2353
a0d0e21e 2354=item getpwnam NAME
d74e8afc
ITB
2355X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2356X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2357X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2358X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2359X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2360X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2361
2362=item getgrnam NAME
2363
2364=item gethostbyname NAME
2365
2366=item getnetbyname NAME
2367
2368=item getprotobyname NAME
2369
2370=item getpwuid UID
2371
2372=item getgrgid GID
2373
2374=item getservbyname NAME,PROTO
2375
2376=item gethostbyaddr ADDR,ADDRTYPE
2377
2378=item getnetbyaddr ADDR,ADDRTYPE
2379
2380=item getprotobynumber NUMBER
2381
2382=item getservbyport PORT,PROTO
2383
2384=item getpwent
2385
2386=item getgrent
2387
2388=item gethostent
2389
2390=item getnetent
2391
2392=item getprotoent
2393
2394=item getservent
2395
2396=item setpwent
2397
2398=item setgrent
2399
2400=item sethostent STAYOPEN
2401
2402=item setnetent STAYOPEN
2403
2404=item setprotoent STAYOPEN
2405
2406=item setservent STAYOPEN
2407
2408=item endpwent
2409
2410=item endgrent
2411
2412=item endhostent
2413
2414=item endnetent
2415
2416=item endprotoent
2417
2418=item endservent
2419
80d38338
TC
2420These routines are the same as their counterparts in the
2421system C library. In list context, the return values from the
a0d0e21e
LW
2422various get routines are as follows:
2423
2424 ($name,$passwd,$uid,$gid,
6ee623d5 2425 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2426 ($name,$passwd,$gid,$members) = getgr*
2427 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2428 ($name,$aliases,$addrtype,$net) = getnet*
2429 ($name,$aliases,$proto) = getproto*
2430 ($name,$aliases,$port,$proto) = getserv*
2431
3b10bc60 2432(If the entry doesn't exist you get an empty list.)
a0d0e21e 2433
4602f195
JH
2434The exact meaning of the $gcos field varies but it usually contains
2435the real name of the user (as opposed to the login name) and other
2436information pertaining to the user. Beware, however, that in many
2437system users are able to change this information and therefore it
106325ad 2438cannot be trusted and therefore the $gcos is tainted (see
2959b6e3 2439L<perlsec>). The $passwd and $shell, user's encrypted password and
a3390c9f 2440login shell, are also tainted, for the same reason.
4602f195 2441
5a964f20 2442In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2443lookup by name, in which case you get the other thing, whatever it is.
2444(If the entry doesn't exist you get the undefined value.) For example:
2445
5a964f20
TC
2446 $uid = getpwnam($name);
2447 $name = getpwuid($num);
2448 $name = getpwent();
2449 $gid = getgrnam($name);
08a33e13 2450 $name = getgrgid($num);
5a964f20
TC
2451 $name = getgrent();
2452 #etc.
a0d0e21e 2453
4602f195 2454In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2455in that they are unsupported on many systems. If the
4602f195
JH
2456$quota is unsupported, it is an empty scalar. If it is supported, it
2457usually encodes the disk quota. If the $comment field is unsupported,
2458it is an empty scalar. If it is supported it usually encodes some
2459administrative comment about the user. In some systems the $quota
2460field may be $change or $age, fields that have to do with password
2461aging. In some systems the $comment field may be $class. The $expire
2462field, if present, encodes the expiration period of the account or the
2463password. For the availability and the exact meaning of these fields
8f1da26d 2464in your system, please consult getpwnam(3) and your system's
4602f195
JH
2465F<pwd.h> file. You can also find out from within Perl what your
2466$quota and $comment fields mean and whether you have the $expire field
2467by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2468C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2469files are supported only if your vendor has implemented them in the
4602f195 2470intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2471shadow versions if you're running under privilege or if there exists
cf264981 2472the shadow(3) functions as found in System V (this includes Solaris
a3390c9f 2473and Linux). Those systems that implement a proprietary shadow password
5d3a0a3b 2474facility are unlikely to be supported.
6ee623d5 2475
a3390c9f 2476The $members value returned by I<getgr*()> is a space-separated list of
a0d0e21e
LW
2477the login names of the members of the group.
2478
2479For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2480C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2481C<@addrs> value returned by a successful call is a list of raw
2482addresses returned by the corresponding library call. In the
2483Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2484by saying something like:
2485
f337b084 2486 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2487
2b5ab1e7
TC
2488The Socket library makes this slightly easier:
2489
2490 use Socket;
2491 $iaddr = inet_aton("127.1"); # or whatever address
2492 $name = gethostbyaddr($iaddr, AF_INET);
2493
2494 # or going the other way
19799a22 2495 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2496
d760c846
GS
2497In the opposite way, to resolve a hostname to the IP address
2498you can write this:
2499
2500 use Socket;
2501 $packed_ip = gethostbyname("www.perl.org");
2502 if (defined $packed_ip) {
2503 $ip_address = inet_ntoa($packed_ip);
2504 }
2505
b018eaf1 2506Make sure C<gethostbyname()> is called in SCALAR context and that
d760c846
GS
2507its return value is checked for definedness.
2508
0d043efa
FC
2509The C<getprotobynumber> function, even though it only takes one argument,
2510has the precedence of a list operator, so beware:
2511
2512 getprotobynumber $number eq 'icmp' # WRONG
2513 getprotobynumber($number eq 'icmp') # actually means this
2514 getprotobynumber($number) eq 'icmp' # better this way
2515
19799a22
GS
2516If you get tired of remembering which element of the return list
2517contains which return value, by-name interfaces are provided
2518in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2519C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2520and C<User::grent>. These override the normal built-ins, supplying
2521versions that return objects with the appropriate names
2522for each field. For example:
5a964f20
TC
2523
2524 use File::stat;
2525 use User::pwent;
2526 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2527
a3390c9f 2528Even though it looks as though they're the same method calls (uid),
b76cc8ba 2529they aren't, because a C<File::stat> object is different from
19799a22 2530a C<User::pwent> object.
5a964f20 2531
ea9eb35a
BJ
2532Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2533
a0d0e21e 2534=item getsockname SOCKET
d74e8afc 2535X<getsockname>
a0d0e21e 2536
19799a22
GS
2537Returns the packed sockaddr address of this end of the SOCKET connection,
2538in case you don't know the address because you have several different
2539IPs that the connection might have come in on.
a0d0e21e 2540
4633a7c4
LW
2541 use Socket;
2542 $mysockaddr = getsockname(SOCK);
19799a22 2543 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2544 printf "Connect to %s [%s]\n",
19799a22
GS
2545 scalar gethostbyaddr($myaddr, AF_INET),
2546 inet_ntoa($myaddr);
a0d0e21e
LW
2547
2548=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2549X<getsockopt>
a0d0e21e 2550
636e6b1f
TH
2551Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2552Options may exist at multiple protocol levels depending on the socket
2553type, but at least the uppermost socket level SOL_SOCKET (defined in the
391b733c 2554C<Socket> module) will exist. To query options at another level the
636e6b1f 2555protocol number of the appropriate protocol controlling the option
391b733c 2556should be supplied. For example, to indicate that an option is to be
636e6b1f 2557interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2558number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2559
80d38338 2560The function returns a packed string representing the requested socket
3b10bc60 2561option, or C<undef> on error, with the reason for the error placed in
391b733c 2562C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2563consult getsockopt(2) for details. A common case is that the option is an
2564integer, in which case the result is a packed integer, which you can decode
2565using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2566
8f1da26d 2567Here's an example to test whether Nagle's algorithm is enabled on a socket:
636e6b1f 2568
4852725b 2569 use Socket qw(:all);
636e6b1f
TH
2570
2571 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2572 or die "Could not determine the protocol number for tcp";
4852725b
DD
2573 # my $tcp = IPPROTO_TCP; # Alternative
2574 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2575 or die "getsockopt TCP_NODELAY: $!";
636e6b1f
TH
2576 my $nodelay = unpack("I", $packed);
2577 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2578
ea9eb35a 2579Portability issues: L<perlport/getsockopt>.
a0d0e21e 2580
15a348aa
NC
2581=item given EXPR BLOCK
2582X<given>
2583
2584=item given BLOCK
2585
391b733c
FC
2586C<given> is analogous to the C<switch>
2587keyword in other languages. C<given>
15a348aa 2588and C<when> are used in Perl to implement C<switch>/C<case> like statements.
8f1da26d 2589Only available after Perl 5.10. For example:
15a348aa 2590
8f1da26d 2591 use v5.10;
15a348aa
NC
2592 given ($fruit) {
2593 when (/apples?/) {
2594 print "I like apples."
2595 }
2596 when (/oranges?/) {
2597 print "I don't like oranges."
2598 }
2599 default {
2600 print "I don't like anything"
2601 }
2602 }
2603
48238296 2604See L<perlsyn/"Switch Statements"> for detailed information.
15a348aa 2605
a0d0e21e 2606=item glob EXPR
d74e8afc 2607X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2608
0a753a76 2609=item glob
2610
d9a9d457 2611In list context, returns a (possibly empty) list of filename expansions on
391b733c 2612the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
d9a9d457 2613scalar context, glob iterates through such filename expansions, returning
391b733c
FC
2614undef when the list is exhausted. This is the internal function
2615implementing the C<< <*.c> >> operator, but you can use it directly. If
d9a9d457
JL
2616EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2617more detail in L<perlop/"I/O Operators">.
a0d0e21e 2618
80d38338
TC
2619Note that C<glob> splits its arguments on whitespace and treats
2620each segment as separate pattern. As such, C<glob("*.c *.h")>
2621matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2622C<glob(".* *")> matches all files in the current working directory.
a91bb7b1
TC
2623If you want to glob filenames that might contain whitespace, you'll
2624have to use extra quotes around the spacey filename to protect it.
2625For example, to glob filenames that have an C<e> followed by a space
2626followed by an C<f>, use either of:
2627
2628 @spacies = <"*e f*">;
2629 @spacies = glob '"*e f*"';
2630 @spacies = glob q("*e f*");
2631
2632If you had to get a variable through, you could do this:
2633
2634 @spacies = glob "'*${var}e f*'";
2635 @spacies = glob qq("*${var}e f*");
80d38338
TC
2636
2637If non-empty braces are the only wildcard characters used in the
2638C<glob>, no filenames are matched, but potentially many strings
2639are returned. For example, this produces nine strings, one for
2640each pairing of fruits and colors:
2641
2642 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2643
3a4b19e4 2644Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2645C<File::Glob> extension. See L<File::Glob> for details, including
2646C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2647
ea9eb35a
BJ
2648Portability issues: L<perlport/glob>.
2649
a0d0e21e 2650=item gmtime EXPR
d74e8afc 2651X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2652
ce2984c3
PF
2653=item gmtime
2654
4509d391 2655Works just like L</localtime> but the returned values are
435fbc73 2656localized for the standard Greenwich time zone.
a0d0e21e 2657
a3390c9f
FC
2658Note: When called in list context, $isdst, the last value
2659returned by gmtime, is always C<0>. There is no
435fbc73 2660Daylight Saving Time in GMT.
0a753a76 2661
ea9eb35a 2662Portability issues: L<perlport/gmtime>.
62aa5637 2663
a0d0e21e 2664=item goto LABEL
d74e8afc 2665X<goto> X<jump> X<jmp>
a0d0e21e 2666
748a9306
LW
2667=item goto EXPR
2668
a0d0e21e
LW
2669=item goto &NAME
2670
b500e03b 2671The C<goto-LABEL> form finds the statement labeled with LABEL and
391b733c 2672resumes execution there. It can't be used to get out of a block or
b500e03b
GG
2673subroutine given to C<sort>. It can be used to go almost anywhere
2674else within the dynamic scope, including out of subroutines, but it's
2675usually better to use some other construct such as C<last> or C<die>.
2676The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2677(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2678does not offer named loops combined with loop control. Perl does, and
2679this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2680
7660c0ab
A
2681The C<goto-EXPR> form expects a label name, whose scope will be resolved
2682dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2683necessarily recommended if you're optimizing for maintainability:
2684
2685 goto ("FOO", "BAR", "GLARCH")[$i];
2686
887d89fd 2687As shown in this example, C<goto-EXPR> is exempt from the "looks like a
391b733c
FC
2688function" rule. A pair of parentheses following it does not (necessarily)
2689delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
887d89fd 2690
b500e03b 2691Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2692deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2693go into any construct that requires initialization, such as a
2694subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2695construct that is optimized away.
b500e03b 2696
1b6921cb
BT
2697The C<goto-&NAME> form is quite different from the other forms of
2698C<goto>. In fact, it isn't a goto in the normal sense at all, and
2699doesn't have the stigma associated with other gotos. Instead, it
2700exits the current subroutine (losing any changes set by local()) and
2701immediately calls in its place the named subroutine using the current
2702value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2703load another subroutine and then pretend that the other subroutine had
2704been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2705in the current subroutine are propagated to the other subroutine.)
2706After the C<goto>, not even C<caller> will be able to tell that this
2707routine was called first.
2708
2709NAME needn't be the name of a subroutine; it can be a scalar variable
8f1da26d 2710containing a code reference or a block that evaluates to a code
6cb9131c 2711reference.
a0d0e21e
LW
2712
2713=item grep BLOCK LIST
d74e8afc 2714X<grep>
a0d0e21e
LW
2715
2716=item grep EXPR,LIST
2717
2b5ab1e7
TC
2718This is similar in spirit to, but not the same as, grep(1) and its
2719relatives. In particular, it is not limited to using regular expressions.
2f9daede 2720
a0d0e21e 2721Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2722C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2723elements for which the expression evaluated to true. In scalar
2724context, returns the number of times the expression was true.
a0d0e21e
LW
2725
2726 @foo = grep(!/^#/, @bar); # weed out comments
2727
2728or equivalently,
2729
2730 @foo = grep {!/^#/} @bar; # weed out comments
2731
be3174d2
GS
2732Note that C<$_> is an alias to the list value, so it can be used to
2733modify the elements of the LIST. While this is useful and supported,
2734it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2735Similarly, grep returns aliases into the original list, much as a for
2736loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2737element of a list returned by grep (for example, in a C<foreach>, C<map>
2738or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2739This is usually something to be avoided when writing clear code.
a0d0e21e 2740
a4fb8298 2741If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2742been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2743the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2744can't be seen from the outside, avoiding any potential side-effects.
2745
19799a22 2746See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2747
a0d0e21e 2748=item hex EXPR
d74e8afc 2749X<hex> X<hexadecimal>
a0d0e21e 2750
54310121 2751=item hex
bbce6d69 2752
2b5ab1e7 2753Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2754(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2755L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TP
2756
2757 print hex '0xAf'; # prints '175'
2758 print hex 'aF'; # same
a0d0e21e 2759
19799a22 2760Hex strings may only represent integers. Strings that would cause
53305cf1 2761integer overflow trigger a warning. Leading whitespace is not stripped,
391b733c 2762unlike oct(). To present something as hex, look into L</printf>,
8f1da26d 2763L</sprintf>, and L</unpack>.
19799a22 2764
ce2984c3 2765=item import LIST
d74e8afc 2766X<import>
a0d0e21e 2767
19799a22 2768There is no builtin C<import> function. It is just an ordinary
4633a7c4 2769method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2770names to another module. The C<use> function calls the C<import> method
cea6626f 2771for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2772
2773=item index STR,SUBSTR,POSITION
d74e8afc 2774X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2775
2776=item index STR,SUBSTR
2777
2b5ab1e7
TC
2778The index function searches for one string within another, but without
2779the wildcard-like behavior of a full regular-expression pattern match.
2780It returns the position of the first occurrence of SUBSTR in STR at
2781or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2782beginning of the string. POSITION before the beginning of the string
2783or after its end is treated as if it were the beginning or the end,
e1dccc0d
Z
2784respectively. POSITION and the return value are based at zero.
2785If the substring is not found, C<index> returns -1.
a0d0e21e
LW
2786
2787=item int EXPR
f723aae1 2788X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2789
54310121 2790=item int
bbce6d69 2791
7660c0ab 2792Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 2793You should not use this function for rounding: one because it truncates
3b10bc60 2794towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
2795numbers can sometimes produce counterintuitive results. For example,
2796C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2797because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2798the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2799functions will serve you better than will int().
a0d0e21e
LW
2800
2801=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2802X<ioctl>
a0d0e21e 2803
2b5ab1e7 2804Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2805
5ed4f2ec 2806 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2807
a11c483f 2808to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2809exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2810own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2811(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2812may help you in this, but it's nontrivial.) SCALAR will be read and/or
3b10bc60 2813written depending on the FUNCTION; a C pointer to the string value of SCALAR
19799a22 2814will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2815has no string value but does have a numeric value, that value will be
2816passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2817true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2818functions may be needed to manipulate the values of structures used by
b76cc8ba 2819C<ioctl>.
a0d0e21e 2820
19799a22 2821The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e 2822
5ed4f2ec 2823 if OS returns: then Perl returns:
2824 -1 undefined value
2825 0 string "0 but true"
2826 anything else that number
a0d0e21e 2827
19799a22 2828Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2829still easily determine the actual value returned by the operating
2830system:
2831
2b5ab1e7 2832 $retval = ioctl(...) || -1;
a0d0e21e
LW
2833 printf "System returned %d\n", $retval;
2834
be2f7487 2835The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2836about improper numeric conversions.
2837
ea9eb35a
BJ
2838Portability issues: L<perlport/ioctl>.
2839
a0d0e21e 2840=item join EXPR,LIST
d74e8afc 2841X<join>
a0d0e21e 2842
2b5ab1e7
TC
2843Joins the separate strings of LIST into a single string with fields
2844separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2845
2b5ab1e7 2846 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2847
eb6e2d6f
GS
2848Beware that unlike C<split>, C<join> doesn't take a pattern as its
2849first argument. Compare L</split>.
a0d0e21e 2850
532eee96 2851=item keys HASH
d74e8afc 2852X<keys> X<key>
aa689395 2853
532eee96 2854=item keys ARRAY
aeedbbed 2855
f5a93a43
TC
2856=item keys EXPR
2857
bade7fbc
TC
2858Called in list context, returns a list consisting of all the keys of the
2859named hash, or in Perl 5.12 or later only, the indices of an array. Perl
2860releases prior to 5.12 will produce a syntax error if you try to use an
2861array argument. In scalar context, returns the number of keys or indices.
504f80c1 2862
aeedbbed 2863The keys of a hash are returned in an apparently random order. The actual
3b10bc60 2864random order is subject to change in future versions of Perl, but it
504f80c1 2865is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6 2866function produces (given that the hash has not been modified). Since
c5f61d2f 2867Perl 5.8.1 the ordering can be different even between different runs of
4546b9e6 2868Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2869Attacks">).
504f80c1 2870
8f1da26d 2871As a side effect, calling keys() resets the internal interator of the HASH or ARRAY
cf264981
SP
2872(see L</each>). In particular, calling keys() in void context resets
2873the iterator with no other overhead.
a0d0e21e 2874
aa689395 2875Here is yet another way to print your environment:
a0d0e21e
LW
2876
2877 @keys = keys %ENV;
2878 @values = values %ENV;
b76cc8ba 2879 while (@keys) {
a9a5a0dc 2880 print pop(@keys), '=', pop(@values), "\n";
a0d0e21e
LW
2881 }
2882
2883or how about sorted by key:
2884
2885 foreach $key (sort(keys %ENV)) {
a9a5a0dc 2886 print $key, '=', $ENV{$key}, "\n";
a0d0e21e
LW
2887 }
2888
8ea1e5d4
GS
2889The returned values are copies of the original keys in the hash, so
2890modifying them will not affect the original hash. Compare L</values>.
2891
19799a22 2892To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2893Here's a descending numeric sort of a hash by its values:
4633a7c4 2894
5a964f20 2895 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
a9a5a0dc 2896 printf "%4d %s\n", $hash{$key}, $key;
4633a7c4
LW
2897 }
2898
3b10bc60 2899Used as an lvalue, C<keys> allows you to increase the number of hash buckets
aa689395 2900allocated for the given hash. This can gain you a measure of efficiency if
2901you know the hash is going to get big. (This is similar to pre-extending
2902an array by assigning a larger number to $#array.) If you say
55497cff 2903
2904 keys %hash = 200;
2905
ab192400
GS
2906then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2907in fact, since it rounds up to the next power of two. These
55497cff 2908buckets will be retained even if you do C<%hash = ()>, use C<undef
2909%hash> if you want to free the storage while C<%hash> is still in scope.
2910You can't shrink the number of buckets allocated for the hash using
19799a22 2911C<keys> in this way (but you needn't worry about doing this by accident,
0d3e3823 2912as trying has no effect). C<keys @array> in an lvalue context is a syntax
aeedbbed 2913error.
55497cff 2914
f5a93a43
TC
2915Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
2916a reference to an unblessed hash or array. The argument will be
2917dereferenced automatically. This aspect of C<keys> is considered highly
2918experimental. The exact behaviour may change in a future version of Perl.
cba5a3b0
DG
2919
2920 for (keys $hashref) { ... }
2921 for (keys $obj->get_arrayref) { ... }
2922
bade7fbc
TC
2923To avoid confusing would-be users of your code who are running earlier
2924versions of Perl with mysterious syntax errors, put this sort of thing at
2925the top of your file to signal that your code will work I<only> on Perls of
2926a recent vintage:
2927
2928 use 5.012; # so keys/values/each work on arrays
2929 use 5.014; # so keys/values/each work on scalars (experimental)
2930
8f1da26d 2931See also C<each>, C<values>, and C<sort>.
ab192400 2932
b350dd2f 2933=item kill SIGNAL, LIST
9c7e4b76
KW
2934
2935=item kill SIGNAL
d74e8afc 2936X<kill> X<signal>
a0d0e21e 2937
b350dd2f 2938Sends a signal to a list of processes. Returns the number of
517db077
GS
2939processes successfully signaled (which is not necessarily the
2940same as the number actually killed).
a0d0e21e
LW
2941
2942 $cnt = kill 1, $child1, $child2;
2943 kill 9, @goners;
2944
3b10bc60 2945If SIGNAL is zero, no signal is sent to the process, but C<kill>
2946checks whether it's I<possible> to send a signal to it (that
70fb64f6 2947means, to be brief, that the process is owned by the same user, or we are
3b10bc60 2948the super-user). This is useful to check that a child process is still
81fd35db
DN
2949alive (even if only as a zombie) and hasn't changed its UID. See
2950L<perlport> for notes on the portability of this construct.
b350dd2f 2951
e2c0f81f 2952Unlike in the shell, if SIGNAL is negative, it kills process groups instead
391b733c
FC
2953of processes. That means you usually
2954want to use positive not negative signals.
e2c0f81f
DG
2955You may also use a signal name in quotes.
2956
2957The behavior of kill when a I<PROCESS> number is zero or negative depends on
2958the operating system. For example, on POSIX-conforming systems, zero will
2959signal the current process group and -1 will signal all processes.
1e9c1022
JL
2960
2961See L<perlipc/"Signals"> for more details.
a0d0e21e 2962
ea9eb35a
BJ
2963On some platforms such as Windows where the fork() system call is not available.
2964Perl can be built to emulate fork() at the interpreter level.
6d17f725 2965This emulation has limitations related to kill that have to be considered,
ea9eb35a
BJ
2966for code running on Windows and in code intended to be portable.
2967
2968See L<perlfork> for more details.
2969
9c7e4b76
KW
2970If there is no I<LIST> of processes, no signal is sent, and the return
2971value is 0. This form is sometimes used, however, because it causes
2972tainting checks to be run. But see
2973L<perlsec/Laundering and Detecting Tainted Data>.
2974
ea9eb35a
BJ
2975Portability issues: L<perlport/kill>.
2976
a0d0e21e 2977=item last LABEL
d74e8afc 2978X<last> X<break>
a0d0e21e
LW
2979
2980=item last
2981
2982The C<last> command is like the C<break> statement in C (as used in
2983loops); it immediately exits the loop in question. If the LABEL is
2984omitted, the command refers to the innermost enclosing loop. The
2985C<continue> block, if any, is not executed:
2986
4633a7c4 2987 LINE: while (<STDIN>) {
a9a5a0dc
VP
2988 last LINE if /^$/; # exit when done with header
2989 #...
a0d0e21e
LW
2990 }
2991
80d38338 2992C<last> cannot be used to exit a block that returns a value such as
8f1da26d 2993C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
2b5ab1e7 2994a grep() or map() operation.
4968c1e4 2995
6c1372ed
GS
2996Note that a block by itself is semantically identical to a loop
2997that executes once. Thus C<last> can be used to effect an early
2998exit out of such a block.
2999
98293880
JH
3000See also L</continue> for an illustration of how C<last>, C<next>, and
3001C<redo> work.
1d2dff63 3002
a0d0e21e 3003=item lc EXPR
d74e8afc 3004X<lc> X<lowercase>
a0d0e21e 3005
54310121 3006=item lc
bbce6d69 3007
d1be9408 3008Returns a lowercased version of EXPR. This is the internal function
3980dc9c 3009implementing the C<\L> escape in double-quoted strings.
a0d0e21e 3010
7660c0ab 3011If EXPR is omitted, uses C<$_>.
bbce6d69 3012
3980dc9c
KW
3013What gets returned depends on several factors:
3014
3015=over
3016
3017=item If C<use bytes> is in effect:
3018
3019=over
3020
3021=item On EBCDIC platforms
3022
3023The results are what the C language system call C<tolower()> returns.
3024
3025=item On ASCII platforms
3026
3027The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
3028respectively.
3029
3030=back
3031
66cbab2c 3032=item Otherwise, if C<use locale> (but not C<use locale ':not_characters'>) is in effect:
3980dc9c 3033
094a2f8c
KW
3034Respects current LC_CTYPE locale for code points < 256; and uses Unicode
3035semantics for the remaining code points (this last can only happen if
3036the UTF8 flag is also set). See L<perllocale>.
3980dc9c 3037
094a2f8c
KW
3038A deficiency in this is that case changes that cross the 255/256
3039boundary are not well-defined. For example, the lower case of LATIN CAPITAL
3040LETTER SHARP S (U+1E9E) in Unicode semantics is U+00DF (on ASCII
3041platforms). But under C<use locale>, the lower case of U+1E9E is
3042itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
3043current locale, and Perl has no way of knowing if that character even
3044exists in the locale, much less what code point it is. Perl returns
3045the input character unchanged, for all instances (and there aren't
3046many) where the 255/256 boundary would otherwise be crossed.
3980dc9c 3047
66cbab2c 3048=item Otherwise, If EXPR has the UTF8 flag set:
094a2f8c
KW
3049
3050Unicode semantics are used for the case change.
3980dc9c 3051
66cbab2c 3052=item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'>) is in effect:
3980dc9c 3053
5d1892be 3054Unicode semantics are used for the case change.
3980dc9c
KW
3055
3056=item Otherwise:
3057
3058=over
3059
3060=item On EBCDIC platforms
3061
3062The results are what the C language system call C<tolower()> returns.
3063
3064=item On ASCII platforms
3065
3066ASCII semantics are used for the case change. The lowercase of any character
3067outside the ASCII range is the character itself.
3068
3069=back
3070
3071=back
3072
a0d0e21e 3073=item lcfirst EXPR
d74e8afc 3074X<lcfirst> X<lowercase>
a0d0e21e 3075
54310121 3076=item lcfirst
bbce6d69 3077