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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
a0d0e21e
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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
15operator. A unary operator generally provides a 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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TC
20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
a0d0e21e
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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,
54310121 58nonabortive failure is generally indicated in a scalar context by
a0d0e21e 59returning the undefined value, and in a 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
5a964f20
TC
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<$!>
5a964f20
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87variable on failure. Other functions do not, except accidentally.
88
88e1f1a2
JV
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)
cb1a09d0
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102arranged by category. Some functions appear in more
103than one place.
104
13a2d996 105=over 4
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106
107=item Functions for SCALARs or strings
d74e8afc 108X<scalar> X<string> X<character>
cb1a09d0 109
22fae026 110C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
1dc8ecb8 111C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
945c54fd 112C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
cb1a09d0
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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//>
cb1a09d0
AD
118
119=item Numeric functions
d74e8afc 120X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 121
22fae026
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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>
cb1a09d0
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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>
cb1a09d0
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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>
cb1a09d0
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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
22fae026
TM
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>,
22fae026
TM
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>
cb1a09d0
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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>,
1e278fd9
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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>, C<dump>, C<eval>, C<exit>,
165C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 166
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167=item Keywords related to switch
168
36fb85f3 169C<break>, C<continue>, C<given>, C<when>, C<default>
0d863452 170
3b10bc60 171(These are available only if you enable the C<"switch"> feature.
0d863452
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172See L<feature> and L<perlsyn/"Switch statements">.)
173
54310121 174=item Keywords related to scoping
cb1a09d0 175
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176C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
177C<use>
178
3b10bc60 179(C<state> is available only if the C<"state"> feature is enabled. See
36fb85f3 180L<feature>.)
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181
182=item Miscellaneous functions
183
36fb85f3 184C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
834df1c5 185C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
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186
187=item Functions for processes and process groups
d74e8afc 188X<process> X<pid> X<process id>
cb1a09d0 189
22fae026 190C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
1dc8ecb8 191C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 192C<times>, C<wait>, C<waitpid>
cb1a09d0 193
3b10bc60 194=item Keywords related to Perl modules
d74e8afc 195X<module>
cb1a09d0 196
22fae026 197C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 198
353c6505 199=item Keywords related to classes and object-orientation
d74e8afc 200X<object> X<class> X<package>
cb1a09d0 201
22fae026
TM
202C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
203C<untie>, C<use>
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204
205=item Low-level socket functions
d74e8afc 206X<socket> X<sock>
cb1a09d0 207
22fae026
TM
208C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
209C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 210C<socket>, C<socketpair>
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AD
211
212=item System V interprocess communication functions
d74e8afc 213X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 214
22fae026
TM
215C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
216C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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217
218=item Fetching user and group info
d74e8afc 219X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 220
22fae026
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221C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
222C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
223C<getpwuid>, C<setgrent>, C<setpwent>
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224
225=item Fetching network info
d74e8afc 226X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 227
22fae026
TM
228C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
229C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
230C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
231C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
232C<setnetent>, C<setprotoent>, C<setservent>
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233
234=item Time-related functions
d74e8afc 235X<time> X<date>
cb1a09d0 236
22fae026 237C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 238
37798a01 239=item Functions new in perl5
d74e8afc 240X<perl5>
37798a01 241
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SP
242C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
243C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
1dc8ecb8 244C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
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SP
245C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
246C<ucfirst>, C<untie>, C<use>, C<when>
37798a01 247
3b10bc60 248* C<sub> was a keyword in Perl 4, but in Perl 5 it is an
5a964f20 249operator, which can be used in expressions.
37798a01
PP
250
251=item Functions obsoleted in perl5
252
22fae026 253C<dbmclose>, C<dbmopen>
37798a01 254
cb1a09d0
AD
255=back
256
60f9f73c 257=head2 Portability
d74e8afc 258X<portability> X<Unix> X<portable>
60f9f73c 259
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TC
260Perl was born in Unix and can therefore access all common Unix
261system calls. In non-Unix environments, the functionality of some
262Unix system calls may not be available, or details of the available
263functionality may differ slightly. The Perl functions affected
60f9f73c
JH
264by this are:
265
266C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
267C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
268C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
ef5a6dd7
JH
269C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
270C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 271C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
60f9f73c
JH
272C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
273C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
274C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 275C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
60f9f73c
JH
276C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
277C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
278C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 279C<shmwrite>, C<socket>, C<socketpair>,
80cbd5ad
JH
280C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
281C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 282C<utime>, C<wait>, C<waitpid>
60f9f73c
JH
283
284For more information about the portability of these functions, see
285L<perlport> and other available platform-specific documentation.
286
cb1a09d0
AD
287=head2 Alphabetical Listing of Perl Functions
288
3b10bc60 289=over
a0d0e21e 290
5b3c99c0 291=item -X FILEHANDLE
d74e8afc
ITB
292X<-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>
293X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 294
5b3c99c0 295=item -X EXPR
a0d0e21e 296
5228a96c
SP
297=item -X DIRHANDLE
298
5b3c99c0 299=item -X
a0d0e21e
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300
301A file test, where X is one of the letters listed below. This unary
5228a96c
SP
302operator takes one argument, either a filename, a filehandle, or a dirhandle,
303and tests the associated file to see if something is true about it. If the
7660c0ab 304argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 305Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 306the undefined value if the file doesn't exist. Despite the funny
d0821a6a 307names, precedence is the same as any other named unary operator. The
a0d0e21e
LW
308operator may be any of:
309
5ed4f2ec 310 -r File is readable by effective uid/gid.
311 -w File is writable by effective uid/gid.
312 -x File is executable by effective uid/gid.
313 -o File is owned by effective uid.
a0d0e21e 314
5ed4f2ec 315 -R File is readable by real uid/gid.
316 -W File is writable by real uid/gid.
317 -X File is executable by real uid/gid.
318 -O File is owned by real uid.
a0d0e21e 319
5ed4f2ec 320 -e File exists.
321 -z File has zero size (is empty).
322 -s File has nonzero size (returns size in bytes).
a0d0e21e 323
5ed4f2ec 324 -f File is a plain file.
325 -d File is a directory.
326 -l File is a symbolic link.
327 -p File is a named pipe (FIFO), or Filehandle is a pipe.
328 -S File is a socket.
329 -b File is a block special file.
330 -c File is a character special file.
331 -t Filehandle is opened to a tty.
a0d0e21e 332
5ed4f2ec 333 -u File has setuid bit set.
334 -g File has setgid bit set.
335 -k File has sticky bit set.
a0d0e21e 336
5ed4f2ec 337 -T File is an ASCII text file (heuristic guess).
338 -B File is a "binary" file (opposite of -T).
a0d0e21e 339
5ed4f2ec 340 -M Script start time minus file modification time, in days.
341 -A Same for access time.
342 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 343
a0d0e21e
LW
344Example:
345
346 while (<>) {
a9a5a0dc
VP
347 chomp;
348 next unless -f $_; # ignore specials
349 #...
a0d0e21e
LW
350 }
351
4fb67938
FC
352Note that C<-s/a/b/> does not do a negated substitution. Saying
353C<-exp($foo)> still works as expected, however: only single letters
354following a minus are interpreted as file tests.
355
356These operators are exempt from the "looks like a function rule" described
357above. That is, an opening parenthesis after the operator does not affect
358how much of the following code constitutes the argument. Put the opening
359parentheses before the operator to separate it from code that follows (this
360applies only to operators with higher precedence than unary operators, of
361course):
362
363 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
364 (-s $file) + 1024 # correct
365
5ff3f7a4
GS
366The interpretation of the file permission operators C<-r>, C<-R>,
367C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
368of the file and the uids and gids of the user. There may be other
ecae030f
MO
369reasons you can't actually read, write, or execute the file: for
370example network filesystem access controls, ACLs (access control lists),
371read-only filesystems, and unrecognized executable formats. Note
372that the use of these six specific operators to verify if some operation
373is possible is usually a mistake, because it may be open to race
374conditions.
5ff3f7a4 375
2b5ab1e7
TC
376Also note that, for the superuser on the local filesystems, the C<-r>,
377C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
5ff3f7a4
GS
378if any execute bit is set in the mode. Scripts run by the superuser
379may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 380or temporarily set their effective uid to something else.
5ff3f7a4
GS
381
382If you are using ACLs, there is a pragma called C<filetest> that may
383produce more accurate results than the bare stat() mode bits.
5dac7880
FC
384When under C<use filetest 'access'> the above-mentioned filetests
385test whether the permission can(not) be granted using the
3b10bc60 386access(2) family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
387under this pragma return true even if there are no execute permission
388bits set (nor any extra execute permission ACLs). This strangeness is
ecae030f
MO
389due to the underlying system calls' definitions. Note also that, due to
390the implementation of C<use filetest 'access'>, the C<_> special
391filehandle won't cache the results of the file tests when this pragma is
392in effect. Read the documentation for the C<filetest> pragma for more
393information.
5ff3f7a4 394
a0d0e21e
LW
395The C<-T> and C<-B> switches work as follows. The first block or so of the
396file is examined for odd characters such as strange control codes or
61eff3bc 397characters with the high bit set. If too many strange characters (>30%)
cf264981 398are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 399containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 400or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 401rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 402file, or a file at EOF when testing a filehandle. Because you have to
4633a7c4
LW
403read a file to do the C<-T> test, on most occasions you want to use a C<-f>
404against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 405
5dac7880 406If any of the file tests (or either the C<stat> or C<lstat> operator) is given
28757baa 407the special filehandle consisting of a solitary underline, then the stat
a0d0e21e
LW
408structure of the previous file test (or stat operator) is used, saving
409a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 410that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 411symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 412an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 413Example:
a0d0e21e
LW
414
415 print "Can do.\n" if -r $a || -w _ || -x _;
416
417 stat($filename);
418 print "Readable\n" if -r _;
419 print "Writable\n" if -w _;
420 print "Executable\n" if -x _;
421 print "Setuid\n" if -u _;
422 print "Setgid\n" if -g _;
423 print "Sticky\n" if -k _;
424 print "Text\n" if -T _;
425 print "Binary\n" if -B _;
426
fbb0b3b3
RGS
427As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
428test operators, in a way that C<-f -w -x $file> is equivalent to
3b10bc60 429C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
fbb0b3b3
RGS
430the return value of C<-f $file> as an argument to another filetest
431operator, no special magic will happen.)
432
a0d0e21e 433=item abs VALUE
d74e8afc 434X<abs> X<absolute>
a0d0e21e 435
54310121 436=item abs
bbce6d69 437
a0d0e21e 438Returns the absolute value of its argument.
7660c0ab 439If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
440
441=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 442X<accept>
a0d0e21e 443
3b10bc60 444Accepts an incoming socket connect, just as accept(2)
19799a22 445does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 446See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 447
8d2a6795
GS
448On systems that support a close-on-exec flag on files, the flag will
449be set for the newly opened file descriptor, as determined by the
450value of $^F. See L<perlvar/$^F>.
451
a0d0e21e 452=item alarm SECONDS
d74e8afc
ITB
453X<alarm>
454X<SIGALRM>
455X<timer>
a0d0e21e 456
54310121 457=item alarm
bbce6d69 458
a0d0e21e 459Arranges to have a SIGALRM delivered to this process after the
cf264981 460specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
461specified, the value stored in C<$_> is used. (On some machines,
462unfortunately, the elapsed time may be up to one second less or more
463than you specified because of how seconds are counted, and process
464scheduling may delay the delivery of the signal even further.)
465
466Only one timer may be counting at once. Each call disables the
467previous timer, and an argument of C<0> may be supplied to cancel the
468previous timer without starting a new one. The returned value is the
469amount of time remaining on the previous timer.
a0d0e21e 470
2bc69794
BS
471For delays of finer granularity than one second, the Time::HiRes module
472(from CPAN, and starting from Perl 5.8 part of the standard
473distribution) provides ualarm(). You may also use Perl's four-argument
474version of select() leaving the first three arguments undefined, or you
475might be able to use the C<syscall> interface to access setitimer(2) if
476your system supports it. See L<perlfaq8> for details.
2b5ab1e7 477
80d38338
TC
478It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
479C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 480
19799a22
GS
481If you want to use C<alarm> to time out a system call you need to use an
482C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 483fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 484restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 485modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
486
487 eval {
a9a5a0dc
VP
488 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
489 alarm $timeout;
490 $nread = sysread SOCKET, $buffer, $size;
491 alarm 0;
ff68c719 492 };
ff68c719 493 if ($@) {
a9a5a0dc 494 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 495 # timed out
ff68c719
PP
496 }
497 else {
5ed4f2ec 498 # didn't
ff68c719
PP
499 }
500
91d81acc
JH
501For more information see L<perlipc>.
502
a0d0e21e 503=item atan2 Y,X
d74e8afc 504X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
505
506Returns the arctangent of Y/X in the range -PI to PI.
507
ca6e1c26 508For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
509function, or use the familiar relation:
510
511 sub tan { sin($_[0]) / cos($_[0]) }
512
a1021d57
RGS
513The return value for C<atan2(0,0)> is implementation-defined; consult
514your atan2(3) manpage for more information.
bf5f1b4c 515
a0d0e21e 516=item bind SOCKET,NAME
d74e8afc 517X<bind>
a0d0e21e 518
3b10bc60 519Binds a network address to a socket, just as bind(2)
19799a22 520does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
521packed address of the appropriate type for the socket. See the examples in
522L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 523
fae2c0fb 524=item binmode FILEHANDLE, LAYER
d74e8afc 525X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 526
a0d0e21e
LW
527=item binmode FILEHANDLE
528
1cbfc93d
NIS
529Arranges for FILEHANDLE to be read or written in "binary" or "text"
530mode on systems where the run-time libraries distinguish between
531binary and text files. If FILEHANDLE is an expression, the value is
532taken as the name of the filehandle. Returns true on success,
b5fe5ca2 533otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 534
d807c6f4
JH
535On some systems (in general, DOS and Windows-based systems) binmode()
536is necessary when you're not working with a text file. For the sake
d7a0d798
FC
537of portability it is a good idea always to use it when appropriate,
538and never to use it when it isn't appropriate. Also, people can
d807c6f4
JH
539set their I/O to be by default UTF-8 encoded Unicode, not bytes.
540
541In other words: regardless of platform, use binmode() on binary data,
d7a0d798 542like images, for example.
d807c6f4
JH
543
544If LAYER is present it is a single string, but may contain multiple
3b10bc60 545directives. The directives alter the behaviour of the filehandle.
d7a0d798 546When LAYER is present, using binmode on a text file makes sense.
d807c6f4 547
fae2c0fb 548If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
549suitable for passing binary data. This includes turning off possible CRLF
550translation and marking it as bytes (as opposed to Unicode characters).
749683d2 551Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 552Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
553Other layers that would affect the binary nature of the stream are
554I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 555PERLIO environment variable.
01e6739c 556
3b10bc60 557The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
558form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
559establish default I/O layers. See L<open>.
560
fae2c0fb
RGS
561I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
562in "Programming Perl, 3rd Edition". However, since the publishing of this
563book, by many known as "Camel III", the consensus of the naming of this
564functionality has moved from "discipline" to "layer". All documentation
565of this version of Perl therefore refers to "layers" rather than to
566"disciplines". Now back to the regularly scheduled documentation...>
567
6902c96a
T
568To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
569C<:utf8> just marks the data as UTF-8 without further checking,
570while C<:encoding(utf8)> checks the data for actually being valid
571UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 572
ed53a2bb 573In general, binmode() should be called after open() but before any I/O
3b10bc60 574is done on the filehandle. Calling binmode() normally flushes any
01e6739c 575pending buffered output data (and perhaps pending input data) on the
fae2c0fb 576handle. An exception to this is the C<:encoding> layer that
d7a0d798 577changes the default character encoding of the handle; see L</open>.
fae2c0fb 578The C<:encoding> layer sometimes needs to be called in
3874323d
JH
579mid-stream, and it doesn't flush the stream. The C<:encoding>
580also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 581internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 582
19799a22 583The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
584system all work together to let the programmer treat a single
585character (C<\n>) as the line terminator, irrespective of the external
586representation. On many operating systems, the native text file
587representation matches the internal representation, but on some
588platforms the external representation of C<\n> is made up of more than
589one character.
590
68bd7414
NIS
591Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
592character to end each line in the external representation of text (even
5e12dbfa 593though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
594on Unix and most VMS files). In other systems like OS/2, DOS and the
595various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
596but what's stored in text files are the two characters C<\cM\cJ>. That
597means that, if you don't use binmode() on these systems, C<\cM\cJ>
598sequences on disk will be converted to C<\n> on input, and any C<\n> in
599your program will be converted back to C<\cM\cJ> on output. This is what
600you want for text files, but it can be disastrous for binary files.
30168b04
GS
601
602Another consequence of using binmode() (on some systems) is that
603special end-of-file markers will be seen as part of the data stream.
d7a0d798
FC
604For systems from the Microsoft family this means that, if your binary
605data contain C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
606the file, unless you use binmode().
607
3b10bc60 608binmode() is important not only for readline() and print() operations,
30168b04
GS
609but also when using read(), seek(), sysread(), syswrite() and tell()
610(see L<perlport> for more details). See the C<$/> and C<$\> variables
611in L<perlvar> for how to manually set your input and output
612line-termination sequences.
a0d0e21e 613
4633a7c4 614=item bless REF,CLASSNAME
d74e8afc 615X<bless>
a0d0e21e
LW
616
617=item bless REF
618
2b5ab1e7
TC
619This function tells the thingy referenced by REF that it is now an object
620in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 621is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 622it returns the reference for convenience. Always use the two-argument
cf264981
SP
623version if a derived class might inherit the function doing the blessing.
624See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
625of objects.
a0d0e21e 626
57668c4d 627Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 628Namespaces with all lowercase names are considered reserved for
cf264981 629Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
630confusion, you may wish to avoid such package names as well. Make sure
631that CLASSNAME is a true value.
60ad88b8
GS
632
633See L<perlmod/"Perl Modules">.
634
0d863452
RH
635=item break
636
637Break out of a C<given()> block.
638
3b10bc60 639This keyword is enabled by the C<"switch"> feature: see L<feature>
0d863452
RH
640for more information.
641
a0d0e21e 642=item caller EXPR
d74e8afc 643X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
644
645=item caller
646
5a964f20 647Returns the context of the current subroutine call. In scalar context,
80d38338
TC
648returns the caller's package name if there I<is> a caller (that is, if
649we're in a subroutine or C<eval> or C<require>) and the undefined value
5a964f20 650otherwise. In list context, returns
a0d0e21e 651
ee6b43cc 652 # 0 1 2
748a9306 653 ($package, $filename, $line) = caller;
a0d0e21e
LW
654
655With EXPR, it returns some extra information that the debugger uses to
656print a stack trace. The value of EXPR indicates how many call frames
657to go back before the current one.
658
ee6b43cc 659 # 0 1 2 3 4
f3aa04c2 660 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 661
662 # 5 6 7 8 9 10
b3ca2e83 663 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 664 = caller($i);
e7ea3e70 665
951ba7fe 666Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 667call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 668C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 669C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 670C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 671$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
672each C<use> statement creates a C<require> frame inside an C<eval EXPR>
673frame.) $subroutine may also be C<(unknown)> if this particular
674subroutine happens to have been deleted from the symbol table.
675C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
676C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
677compiled with. The C<$hints> and C<$bitmask> values are subject to change
678between versions of Perl, and are not meant for external use.
748a9306 679
b3ca2e83
NC
680C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
681caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
682of this hash, as they are the actual values stored in the optree.
683
748a9306 684Furthermore, when called from within the DB package, caller returns more
7660c0ab 685detailed information: it sets the list variable C<@DB::args> to be the
54310121 686arguments with which the subroutine was invoked.
748a9306 687
7660c0ab 688Be aware that the optimizer might have optimized call frames away before
19799a22 689C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 690might not return information about the call frame you expect it to, for
b76cc8ba 691C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 692previous time C<caller> was called.
7660c0ab 693
ca9f0cb5
NC
694Also be aware that setting C<@DB::args> is I<best effort>, intended for
695debugging or generating backtraces, and should not be relied upon. In
696particular, as C<@_> contains aliases to the caller's arguments, Perl does
697not take a copy of C<@_>, so C<@DB::args> will contain modifications the
698subroutine makes to C<@_> or its contents, not the original values at call
699time. C<@DB::args>, like C<@_>, does not hold explicit references to its
700elements, so under certain cases its elements may have become freed and
701reallocated for other variables or temporary values. Finally, a side effect
d7a0d798 702of the current implementation is that the effects of C<shift @_> can
ca9f0cb5
NC
703I<normally> be undone (but not C<pop @_> or other splicing, and not if a
704reference to C<@_> has been taken, and subject to the caveat about reallocated
705elements), so C<@DB::args> is actually a hybrid of the current state and
706initial state of C<@_>. Buyer beware.
707
a0d0e21e 708=item chdir EXPR
d74e8afc
ITB
709X<chdir>
710X<cd>
f723aae1 711X<directory, change>
a0d0e21e 712
c4aca7d0
GA
713=item chdir FILEHANDLE
714
715=item chdir DIRHANDLE
716
ce2984c3
PF
717=item chdir
718
ffce7b87 719Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 720changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 721changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0 722variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
80d38338 723neither is set, C<chdir> does nothing. It returns true on success,
b4ad75f0 724false otherwise. See the example under C<die>.
a0d0e21e 725
3b10bc60 726On systems that support fchdir(2), you may pass a filehandle or
34169887 727directory handle as the argument. On systems that don't support fchdir(2),
3b10bc60 728passing handles raises an exception.
c4aca7d0 729
a0d0e21e 730=item chmod LIST
d74e8afc 731X<chmod> X<permission> X<mode>
a0d0e21e
LW
732
733Changes the permissions of a list of files. The first element of the
4633a7c4 734list must be the numerical mode, which should probably be an octal
4ad40acf 735number, and which definitely should I<not> be a string of octal digits:
3b10bc60 736C<0644> is okay, but C<"0644"> is not. Returns the number of files
dc848c6f 737successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e 738
3b10bc60 739 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 740 chmod 0755, @executables;
3b10bc60 741 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 742 # --w----r-T
3b10bc60 743 $mode = "0644"; chmod oct($mode), "foo"; # this is better
744 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 745
3b10bc60 746On systems that support fchmod(2), you may pass filehandles among the
747files. On systems that don't support fchmod(2), passing filehandles raises
748an exception. Filehandles must be passed as globs or glob references to be
749recognized; barewords are considered filenames.
c4aca7d0
GA
750
751 open(my $fh, "<", "foo");
752 my $perm = (stat $fh)[2] & 07777;
753 chmod($perm | 0600, $fh);
754
3b10bc60 755You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
756module:
757
3b10bc60 758 use Fcntl qw( :mode );
ca6e1c26 759 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 760 # Identical to the chmod 0755 of the example above.
ca6e1c26 761
a0d0e21e 762=item chomp VARIABLE
d74e8afc 763X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 764
313c9f5c 765=item chomp( LIST )
a0d0e21e
LW
766
767=item chomp
768
2b5ab1e7
TC
769This safer version of L</chop> removes any trailing string
770that corresponds to the current value of C<$/> (also known as
28757baa
PP
771$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
772number of characters removed from all its arguments. It's often used to
773remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
774that the final record may be missing its newline. When in paragraph
775mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083 776When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
34169887 777a reference to an integer or the like; see L<perlvar>) chomp() won't
b76cc8ba 778remove anything.
19799a22 779If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
780
781 while (<>) {
a9a5a0dc
VP
782 chomp; # avoid \n on last field
783 @array = split(/:/);
784 # ...
a0d0e21e
LW
785 }
786
4bf21a6d
RD
787If VARIABLE is a hash, it chomps the hash's values, but not its keys.
788
a0d0e21e
LW
789You can actually chomp anything that's an lvalue, including an assignment:
790
791 chomp($cwd = `pwd`);
792 chomp($answer = <STDIN>);
793
794If you chomp a list, each element is chomped, and the total number of
795characters removed is returned.
796
15e44fd8
RGS
797Note that parentheses are necessary when you're chomping anything
798that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
799is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
800C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
801C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
802as C<chomp($a, $b)>.
803
a0d0e21e 804=item chop VARIABLE
d74e8afc 805X<chop>
a0d0e21e 806
313c9f5c 807=item chop( LIST )
a0d0e21e
LW
808
809=item chop
810
811Chops off the last character of a string and returns the character
5b3eff12 812chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 813scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
814If VARIABLE is a hash, it chops the hash's values, but not its keys.
815
5b3eff12 816You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
817
818If you chop a list, each element is chopped. Only the value of the
19799a22 819last C<chop> is returned.
a0d0e21e 820
19799a22 821Note that C<chop> returns the last character. To return all but the last
748a9306
LW
822character, use C<substr($string, 0, -1)>.
823
15e44fd8
RGS
824See also L</chomp>.
825
a0d0e21e 826=item chown LIST
d74e8afc 827X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
828
829Changes the owner (and group) of a list of files. The first two
19799a22
GS
830elements of the list must be the I<numeric> uid and gid, in that
831order. A value of -1 in either position is interpreted by most
832systems to leave that value unchanged. Returns the number of files
833successfully changed.
a0d0e21e
LW
834
835 $cnt = chown $uid, $gid, 'foo', 'bar';
836 chown $uid, $gid, @filenames;
837
3b10bc60 838On systems that support fchown(2), you may pass filehandles among the
839files. On systems that don't support fchown(2), passing filehandles raises
840an exception. Filehandles must be passed as globs or glob references to be
841recognized; barewords are considered filenames.
c4aca7d0 842
54310121 843Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
844
845 print "User: ";
19799a22 846 chomp($user = <STDIN>);
5a964f20 847 print "Files: ";
19799a22 848 chomp($pattern = <STDIN>);
a0d0e21e
LW
849
850 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 851 or die "$user not in passwd file";
a0d0e21e 852
5ed4f2ec 853 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
854 chown $uid, $gid, @ary;
855
54310121 856On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
857file unless you're the superuser, although you should be able to change
858the group to any of your secondary groups. On insecure systems, these
859restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
860On POSIX systems, you can detect this condition this way:
861
862 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
863 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 864
a0d0e21e 865=item chr NUMBER
d74e8afc 866X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 867
54310121 868=item chr
bbce6d69 869
a0d0e21e 870Returns the character represented by that NUMBER in the character set.
a0ed51b3 871For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 872chr(0x263a) is a Unicode smiley face.
aaa68c4a 873
8a064bd6 874Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 875except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
876(truncated to an integer) are used.
877
974da8e5
JH
878If NUMBER is omitted, uses C<$_>.
879
b76cc8ba 880For the reverse, use L</ord>.
a0d0e21e 881
2575c402
JW
882Note that characters from 128 to 255 (inclusive) are by default
883internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 884
2575c402 885See L<perlunicode> for more about Unicode.
bbce6d69 886
a0d0e21e 887=item chroot FILENAME
d74e8afc 888X<chroot> X<root>
a0d0e21e 889
54310121 890=item chroot
bbce6d69 891
5a964f20 892This function works like the system call by the same name: it makes the
4633a7c4 893named directory the new root directory for all further pathnames that
951ba7fe 894begin with a C</> by your process and all its children. (It doesn't
28757baa 895change your current working directory, which is unaffected.) For security
4633a7c4 896reasons, this call is restricted to the superuser. If FILENAME is
19799a22 897omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
898
899=item close FILEHANDLE
d74e8afc 900X<close>
a0d0e21e 901
6a518fbc
TP
902=item close
903
3b10bc60 904Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26
RGS
905buffers, and closes the system file descriptor. Returns true if those
906operations have succeeded and if no error was reported by any PerlIO
907layer. Closes the currently selected filehandle if the argument is
908omitted.
fb73857a
PP
909
910You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 911another C<open> on it, because C<open> closes it for you. (See
19799a22
GS
912C<open>.) However, an explicit C<close> on an input file resets the line
913counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 914
3b10bc60 915If the filehandle came from a piped open, C<close> returns false if one of
916the other syscalls involved fails or if its program exits with non-zero
917status. If the only problem was that the program exited non-zero, C<$!>
918will be set to C<0>. Closing a pipe also waits for the process executing
919on the pipe to exit--in case you wish to look at the output of the pipe
920afterwards--and implicitly puts the exit status value of that command into
921C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 922
2e0cfa16
FC
923If there are multiple threads running, C<close> on a filehandle from a
924piped open returns true without waiting for the child process to terminate,
925if the filehandle is still open in another thread.
926
80d38338
TC
927Closing the read end of a pipe before the process writing to it at the
928other end is done writing results in the writer receiving a SIGPIPE. If
929the other end can't handle that, be sure to read all the data before
930closing the pipe.
73689b13 931
fb73857a 932Example:
a0d0e21e 933
fb73857a
PP
934 open(OUTPUT, '|sort >foo') # pipe to sort
935 or die "Can't start sort: $!";
5ed4f2ec 936 #... # print stuff to output
937 close OUTPUT # wait for sort to finish
fb73857a
PP
938 or warn $! ? "Error closing sort pipe: $!"
939 : "Exit status $? from sort";
5ed4f2ec 940 open(INPUT, 'foo') # get sort's results
fb73857a 941 or die "Can't open 'foo' for input: $!";
a0d0e21e 942
5a964f20
TC
943FILEHANDLE may be an expression whose value can be used as an indirect
944filehandle, usually the real filehandle name.
a0d0e21e
LW
945
946=item closedir DIRHANDLE
d74e8afc 947X<closedir>
a0d0e21e 948
19799a22 949Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
950system call.
951
a0d0e21e 952=item connect SOCKET,NAME
d74e8afc 953X<connect>
a0d0e21e 954
80d38338
TC
955Attempts to connect to a remote socket, just like connect(2).
956Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
957packed address of the appropriate type for the socket. See the examples in
958L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 959
cb1a09d0 960=item continue BLOCK
d74e8afc 961X<continue>
cb1a09d0 962
0d863452
RH
963=item continue
964
cf264981
SP
965C<continue> is actually a flow control statement rather than a function. If
966there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
967C<foreach>), it is always executed just before the conditional is about to
968be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
969it can be used to increment a loop variable, even when the loop has been
970continued via the C<next> statement (which is similar to the C C<continue>
971statement).
972
98293880 973C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 974block; C<last> and C<redo> behave as if they had been executed within
19799a22 975the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
976block, it may be more entertaining.
977
978 while (EXPR) {
a9a5a0dc
VP
979 ### redo always comes here
980 do_something;
1d2dff63 981 } continue {
a9a5a0dc
VP
982 ### next always comes here
983 do_something_else;
984 # then back the top to re-check EXPR
1d2dff63
GS
985 }
986 ### last always comes here
987
3b10bc60 988Omitting the C<continue> section is equivalent to using an
989empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
990to check the condition at the top of the loop.
991
3b10bc60 992If the C<"switch"> feature is enabled, C<continue> is also a
993function that exits the current C<when> (or C<default>) block and
994falls through to the next one. See L<feature> and
0d863452
RH
995L<perlsyn/"Switch statements"> for more information.
996
997
a0d0e21e 998=item cos EXPR
d74e8afc 999X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 1000
d6217f1e
GS
1001=item cos
1002
5a964f20 1003Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
34169887 1004takes the cosine of C<$_>.
a0d0e21e 1005
ca6e1c26 1006For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
1007function, or use this relation:
1008
1009 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1010
a0d0e21e 1011=item crypt PLAINTEXT,SALT
d74e8afc 1012X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1013X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1014
ef2e6798
MS
1015Creates a digest string exactly like the crypt(3) function in the C
1016library (assuming that you actually have a version there that has not
bb23f8d1 1017been extirpated as a potential munition).
ef2e6798 1018
34169887 1019crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
ef2e6798
MS
1020into a short string, called a digest, which is returned. The same
1021PLAINTEXT and SALT will always return the same string, but there is no
1022(known) way to get the original PLAINTEXT from the hash. Small
1023changes in the PLAINTEXT or SALT will result in large changes in the
1024digest.
1025
1026There is no decrypt function. This function isn't all that useful for
1027cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1028mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1029primarily used to check if two pieces of text are the same without
1030having to transmit or store the text itself. An example is checking
1031if a correct password is given. The digest of the password is stored,
cf264981 1032not the password itself. The user types in a password that is
ef2e6798 1033crypt()'d with the same salt as the stored digest. If the two digests
34169887 1034match, the password is correct.
ef2e6798
MS
1035
1036When verifying an existing digest string you should use the digest as
1037the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1038to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1039crypt() will hash the new string with the same salt as the digest.
1040This allows your code to work with the standard L<crypt|/crypt> and
1041with more exotic implementations. In other words, do not assume
1042anything about the returned string itself, or how many bytes in the
1043digest matter.
85c16d83
JH
1044
1045Traditionally the result is a string of 13 bytes: two first bytes of
1046the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
bb23f8d1 1047the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1048hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1049and implementations on non-Unix platforms may produce different
ef2e6798 1050strings.
85c16d83
JH
1051
1052When choosing a new salt create a random two character string whose
1053characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1054'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1055characters is just a recommendation; the characters allowed in
1056the salt depend solely on your system's crypt library, and Perl can't
1057restrict what salts C<crypt()> accepts.
e71965be 1058
a0d0e21e 1059Here's an example that makes sure that whoever runs this program knows
cf264981 1060their password:
a0d0e21e
LW
1061
1062 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1063
1064 system "stty -echo";
1065 print "Password: ";
e71965be 1066 chomp($word = <STDIN>);
a0d0e21e
LW
1067 print "\n";
1068 system "stty echo";
1069
e71965be 1070 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1071 die "Sorry...\n";
a0d0e21e 1072 } else {
a9a5a0dc 1073 print "ok\n";
54310121 1074 }
a0d0e21e 1075
9f8f0c9d 1076Of course, typing in your own password to whoever asks you
748a9306 1077for it is unwise.
a0d0e21e 1078
ef2e6798 1079The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1080of data, not least of all because you can't get the information
ef2e6798 1081back. Look at the L<Digest> module for more robust algorithms.
19799a22 1082
f2791508
JH
1083If using crypt() on a Unicode string (which I<potentially> has
1084characters with codepoints above 255), Perl tries to make sense
34169887 1085of the situation by trying to downgrade (a copy of)
f2791508
JH
1086the string back to an eight-bit byte string before calling crypt()
1087(on that copy). If that works, good. If not, crypt() dies with
1088C<Wide character in crypt>.
85c16d83 1089
aa689395 1090=item dbmclose HASH
d74e8afc 1091X<dbmclose>
a0d0e21e 1092
19799a22 1093[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1094
aa689395 1095Breaks the binding between a DBM file and a hash.
a0d0e21e 1096
19799a22 1097=item dbmopen HASH,DBNAME,MASK
d74e8afc 1098X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1099
19799a22 1100[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1101
7b8d334a 1102This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1103hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1104argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1105is the name of the database (without the F<.dir> or F<.pag> extension if
1106any). If the database does not exist, it is created with protection
19799a22 1107specified by MASK (as modified by the C<umask>). If your system supports
80d38338 1108only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1109program. In older versions of Perl, if your system had neither DBM nor
19799a22 1110ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1111sdbm(3).
1112
1113If you don't have write access to the DBM file, you can only read hash
1114variables, not set them. If you want to test whether you can write,
3b10bc60 1115either use file tests or try setting a dummy hash entry inside an C<eval>
1116to trap the error.
a0d0e21e 1117
19799a22
GS
1118Note that functions such as C<keys> and C<values> may return huge lists
1119when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1120function to iterate over large DBM files. Example:
1121
1122 # print out history file offsets
1123 dbmopen(%HIST,'/usr/lib/news/history',0666);
1124 while (($key,$val) = each %HIST) {
a9a5a0dc 1125 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1126 }
1127 dbmclose(%HIST);
1128
cb1a09d0 1129See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1130cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1131rich implementation.
4633a7c4 1132
2b5ab1e7
TC
1133You can control which DBM library you use by loading that library
1134before you call dbmopen():
1135
1136 use DB_File;
1137 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1138 or die "Can't open netscape history file: $!";
2b5ab1e7 1139
a0d0e21e 1140=item defined EXPR
d74e8afc 1141X<defined> X<undef> X<undefined>
a0d0e21e 1142
54310121 1143=item defined
bbce6d69 1144
2f9daede 1145Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1146the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1147checked.
1148
1149Many operations return C<undef> to indicate failure, end of file,
1150system error, uninitialized variable, and other exceptional
1151conditions. This function allows you to distinguish C<undef> from
1152other values. (A simple Boolean test will not distinguish among
7660c0ab 1153C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1154false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1155doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1156returns C<undef> when its argument is an empty array, I<or> when the
1157element to return happens to be C<undef>.
1158
f10b0346
GS
1159You may also use C<defined(&func)> to check whether subroutine C<&func>
1160has ever been defined. The return value is unaffected by any forward
80d38338 1161declarations of C<&func>. A subroutine that is not defined
847c7ebe 1162may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1163makes it spring into existence the first time that it is called; see
847c7ebe 1164L<perlsub>.
f10b0346
GS
1165
1166Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
34169887 1167used to report whether memory for that aggregate had ever been
f10b0346
GS
1168allocated. This behavior may disappear in future versions of Perl.
1169You should instead use a simple test for size:
1170
1171 if (@an_array) { print "has array elements\n" }
1172 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1173
1174When used on a hash element, it tells you whether the value is defined,
dc848c6f 1175not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1176purpose.
a0d0e21e
LW
1177
1178Examples:
1179
1180 print if defined $switch{'D'};
1181 print "$val\n" while defined($val = pop(@ary));
1182 die "Can't readlink $sym: $!"
a9a5a0dc 1183 unless defined($value = readlink $sym);
a0d0e21e 1184 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1185 $debugging = 0 unless defined $debugging;
a0d0e21e 1186
19799a22 1187Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1188discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1189defined values. For example, if you say
a5f75d66
AD
1190
1191 "ab" =~ /a(.*)b/;
1192
80d38338 1193The pattern match succeeds and C<$1> is defined, although it
cf264981 1194matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1195matched something that happened to be zero characters long. This is all
a5f75d66 1196very above-board and honest. When a function returns an undefined value,
2f9daede 1197it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1198should use C<defined> only when questioning the integrity of what
7660c0ab 1199you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1200what you want.
1201
dc848c6f 1202See also L</undef>, L</exists>, L</ref>.
2f9daede 1203
a0d0e21e 1204=item delete EXPR
d74e8afc 1205X<delete>
a0d0e21e 1206
d0a76353
RS
1207Given an expression that specifies an element or slice of a hash, C<delete>
1208deletes the specified elements from that hash so that exists() on that element
1209no longer returns true. Setting a hash element to the undefined value does
1210not remove its key, but deleting it does; see L</exists>.
80d38338 1211
d0a76353 1212It returns the value or values deleted in list context, or the last such
80d38338 1213element in scalar context. The return list's length always matches that of
d0a76353
RS
1214the argument list: deleting non-existent elements returns the undefined value
1215in their corresponding positions.
80d38338 1216
d0a76353
RS
1217delete() may also be used on arrays and array slices, but its behavior is less
1218straightforward. Although exists() will return false for deleted entries,
1219deleting array elements never changes indices of existing values; use shift()
1220or splice() for that. However, if all deleted elements fall at the end of an
1221array, the array's size shrinks to the position of the highest element that
1222still tests true for exists(), or to 0 if none do.
1223
1224B<Be aware> that calling delete on array values is deprecated and likely to
1225be removed in a future version of Perl.
80d38338
TC
1226
1227Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1228a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1229or array may not necessarily return anything; it depends on the implementation
1230of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1231
80d38338
TC
1232The C<delete local EXPR> construct localizes the deletion to the current
1233block at run time. Until the block exits, elements locally deleted
1234temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1235of composite types">.
eba0920a
EM
1236
1237 %hash = (foo => 11, bar => 22, baz => 33);
1238 $scalar = delete $hash{foo}; # $scalar is 11
1239 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1240 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1241
01020589 1242The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1243
5f05dabc 1244 foreach $key (keys %HASH) {
a9a5a0dc 1245 delete $HASH{$key};
a0d0e21e
LW
1246 }
1247
01020589 1248 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1249 delete $ARRAY[$index];
01020589
GS
1250 }
1251
1252And so do these:
5f05dabc 1253
01020589
GS
1254 delete @HASH{keys %HASH};
1255
9740c838 1256 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1257
80d38338
TC
1258But both are slower than assigning the empty list
1259or undefining %HASH or @ARRAY, which is the customary
1260way to empty out an aggregate:
01020589 1261
5ed4f2ec 1262 %HASH = (); # completely empty %HASH
1263 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1264
5ed4f2ec 1265 @ARRAY = (); # completely empty @ARRAY
1266 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1267
80d38338
TC
1268The EXPR can be arbitrarily complicated provided its
1269final operation is an element or slice of an aggregate:
a0d0e21e
LW
1270
1271 delete $ref->[$x][$y]{$key};
5f05dabc 1272 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1273
01020589
GS
1274 delete $ref->[$x][$y][$index];
1275 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1276
a0d0e21e 1277=item die LIST
d74e8afc 1278X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1279
4c050ad5
NC
1280C<die> raises an exception. Inside an C<eval> the error message is stuffed
1281into C<$@> and the C<eval> is terminated with the undefined value.
1282If the exception is outside of all enclosing C<eval>s, then the uncaught
1283exception prints LIST to C<STDERR> and exits with a non-zero value. If you
96090e4f 1284need to exit the process with a specific exit code, see L</exit>.
a0d0e21e
LW
1285
1286Equivalent examples:
1287
1288 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1289 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1290
ccac6780 1291If the last element of LIST does not end in a newline, the current
df37ec69
WW
1292script line number and input line number (if any) are also printed,
1293and a newline is supplied. Note that the "input line number" (also
1294known as "chunk") is subject to whatever notion of "line" happens to
1295be currently in effect, and is also available as the special variable
1296C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1297
1298Hint: sometimes appending C<", stopped"> to your message will cause it
1299to make better sense when the string C<"at foo line 123"> is appended.
1300Suppose you are running script "canasta".
a0d0e21e
LW
1301
1302 die "/etc/games is no good";
1303 die "/etc/games is no good, stopped";
1304
1305produce, respectively
1306
1307 /etc/games is no good at canasta line 123.
1308 /etc/games is no good, stopped at canasta line 123.
1309
a96d0188 1310If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1311previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1312This is useful for propagating exceptions:
1313
1314 eval { ... };
1315 die unless $@ =~ /Expected exception/;
1316
a96d0188 1317If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1318C<PROPAGATE> method, that method will be called with additional file
1319and line number parameters. The return value replaces the value in
34169887 1320C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1321were called.
1322
7660c0ab 1323If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1324
4c050ad5
NC
1325If an uncaught exception results in interpreter exit, the exit code is
1326determined from the values of C<$!> and C<$?> with this pseudocode:
1327
1328 exit $! if $!; # errno
1329 exit $? >> 8 if $? >> 8; # child exit status
1330 exit 255; # last resort
1331
1332The intent is to squeeze as much possible information about the likely cause
1333into the limited space of the system exit code. However, as C<$!> is the value
1334of C's C<errno>, which can be set by any system call, this means that the value
1335of the exit code used by C<die> can be non-predictable, so should not be relied
1336upon, other than to be non-zero.
1337
80d38338
TC
1338You can also call C<die> with a reference argument, and if this is trapped
1339within an C<eval>, C<$@> contains that reference. This permits more
1340elaborate exception handling using objects that maintain arbitrary state
1341about the exception. Such a scheme is sometimes preferable to matching
1342particular string values of C<$@> with regular expressions. Because C<$@>
1343is a global variable and C<eval> may be used within object implementations,
1344be careful that analyzing the error object doesn't replace the reference in
1345the global variable. It's easiest to make a local copy of the reference
1346before any manipulations. Here's an example:
52531d10 1347
80d38338 1348 use Scalar::Util "blessed";
da279afe 1349
52531d10 1350 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1351 if (my $ev_err = $@) {
1352 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1353 # handle Some::Module::Exception
1354 }
1355 else {
1356 # handle all other possible exceptions
1357 }
1358 }
1359
3b10bc60 1360Because Perl stringifies uncaught exception messages before display,
80d38338 1361you'll probably want to overload stringification operations on
52531d10
GS
1362exception objects. See L<overload> for details about that.
1363
19799a22
GS
1364You can arrange for a callback to be run just before the C<die>
1365does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1366handler is called with the error text and can change the error
19799a22 1367message, if it sees fit, by calling C<die> again. See
96090e4f 1368L<perlvar/%SIG> for details on setting C<%SIG> entries, and
cf264981 1369L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1370to be run only right before your program was to exit, this is not
3b10bc60 1371currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1372even inside eval()ed blocks/strings! If one wants the hook to do
1373nothing in such situations, put
fb73857a 1374
5ed4f2ec 1375 die @_ if $^S;
fb73857a 1376
19799a22
GS
1377as the first line of the handler (see L<perlvar/$^S>). Because
1378this promotes strange action at a distance, this counterintuitive
b76cc8ba 1379behavior may be fixed in a future release.
774d564b 1380
4c050ad5
NC
1381See also exit(), warn(), and the Carp module.
1382
a0d0e21e 1383=item do BLOCK
d74e8afc 1384X<do> X<block>
a0d0e21e
LW
1385
1386Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1387sequence of commands indicated by BLOCK. When modified by the C<while> or
1388C<until> loop modifier, executes the BLOCK once before testing the loop
1389condition. (On other statements the loop modifiers test the conditional
1390first.)
a0d0e21e 1391
4968c1e4 1392C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1393C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1394See L<perlsyn> for alternative strategies.
4968c1e4 1395
a0d0e21e 1396=item do SUBROUTINE(LIST)
d74e8afc 1397X<do>
a0d0e21e 1398
51124b83
FC
1399This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
1400a scalar variable or a subroutine beginning with C<&>.
a0d0e21e
LW
1401
1402=item do EXPR
d74e8afc 1403X<do>
a0d0e21e
LW
1404
1405Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1406file as a Perl script.
a0d0e21e
LW
1407
1408 do 'stat.pl';
1409
1410is just like
1411
986b19de 1412 eval `cat stat.pl`;
a0d0e21e 1413
2b5ab1e7 1414except that it's more efficient and concise, keeps track of the current
96090e4f
A
1415filename for error messages, searches the C<@INC> directories, and updates
1416C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1417these variables. It also differs in that code evaluated with C<do FILENAME>
2b5ab1e7
TC
1418cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1419same, however, in that it does reparse the file every time you call it,
1420so you probably don't want to do this inside a loop.
a0d0e21e 1421
8e30cc93 1422If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1423error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1424returns undef and sets an error message in C<$@>. If the file is
1425successfully compiled, C<do> returns the value of the last expression
1426evaluated.
1427
80d38338 1428Inclusion of library modules is better done with the
19799a22 1429C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1430and raise an exception if there's a problem.
a0d0e21e 1431
5a964f20
TC
1432You might like to use C<do> to read in a program configuration
1433file. Manual error checking can be done this way:
1434
b76cc8ba 1435 # read in config files: system first, then user
f86cebdf 1436 for $file ("/share/prog/defaults.rc",
b76cc8ba 1437 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1438 {
1439 unless ($return = do $file) {
1440 warn "couldn't parse $file: $@" if $@;
1441 warn "couldn't do $file: $!" unless defined $return;
1442 warn "couldn't run $file" unless $return;
1443 }
5a964f20
TC
1444 }
1445
a0d0e21e 1446=item dump LABEL
d74e8afc 1447X<dump> X<core> X<undump>
a0d0e21e 1448
1614b0e3
JD
1449=item dump
1450
19799a22
GS
1451This function causes an immediate core dump. See also the B<-u>
1452command-line switch in L<perlrun>, which does the same thing.
1453Primarily this is so that you can use the B<undump> program (not
1454supplied) to turn your core dump into an executable binary after
1455having initialized all your variables at the beginning of the
1456program. When the new binary is executed it will begin by executing
1457a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1458Think of it as a goto with an intervening core dump and reincarnation.
1459If C<LABEL> is omitted, restarts the program from the top.
1460
1461B<WARNING>: Any files opened at the time of the dump will I<not>
1462be open any more when the program is reincarnated, with possible
80d38338 1463resulting confusion by Perl.
19799a22 1464
59f521f4
RGS
1465This function is now largely obsolete, mostly because it's very hard to
1466convert a core file into an executable. That's why you should now invoke
1467it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1468typo.
19799a22 1469
cba5a3b0 1470=item each HASH (or HASHREF)
d74e8afc 1471X<each> X<hash, iterator>
aa689395 1472
cba5a3b0 1473=item each ARRAY (or ARRAYREF)
aeedbbed
NC
1474X<array, iterator>
1475
80d38338
TC
1476When called in list context, returns a 2-element list consisting of the key
1477and value for the next element of a hash, or the index and value for the
1478next element of an array, so that you can iterate over it. When called in
1479scalar context, returns only the key (not the value) in a hash, or the index
1480in an array.
2f9daede 1481
aeedbbed 1482Hash entries are returned in an apparently random order. The actual random
3b10bc60 1483order is subject to change in future versions of Perl, but it is
504f80c1 1484guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1485function would produce on the same (unmodified) hash. Since Perl
22883ac5 14865.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1487for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1488
80d38338
TC
1489After C<each> has returned all entries from the hash or array, the next
1490call to C<each> returns the empty list in list context and C<undef> in
1491scalar context. The next call following that one restarts iteration. Each
1492hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1493and C<values>. The iterator is implicitly reset when C<each> has reached
1494the end as just described; it can be explicitly reset by calling C<keys> or
1495C<values> on the hash or array. If you add or delete a hash's elements
1496while iterating over it, entries may be skipped or duplicated--so don't do
1497that. Exception: It is always safe to delete the item most recently
3b10bc60 1498returned by C<each()>, so the following code works properly:
74fc8b5f
MJD
1499
1500 while (($key, $value) = each %hash) {
1501 print $key, "\n";
1502 delete $hash{$key}; # This is safe
1503 }
aa689395 1504
80d38338 1505This prints out your environment like the printenv(1) program,
3b10bc60 1506but in a different order:
a0d0e21e
LW
1507
1508 while (($key,$value) = each %ENV) {
a9a5a0dc 1509 print "$key=$value\n";
a0d0e21e
LW
1510 }
1511
cba5a3b0
DG
1512When given a reference to a hash or array, the argument will be
1513dereferenced automatically.
1514
1515 while (($key,$value) = each $hashref) { ... }
1516
1517If the reference is a blessed object that overrides either C<%{}> or
1518C<@{}>, the override will be used instead of dereferencing the underlying
1519variable type. If both overrides are provided, C<%{}> will be the default.
1520If this is not desired, you must dereference the argument yourself.
1521
19799a22 1522See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1523
1524=item eof FILEHANDLE
d74e8afc
ITB
1525X<eof>
1526X<end of file>
1527X<end-of-file>
a0d0e21e 1528
4633a7c4
LW
1529=item eof ()
1530
a0d0e21e
LW
1531=item eof
1532
1533Returns 1 if the next read on FILEHANDLE will return end of file, or if
1534FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1535gives the real filehandle. (Note that this function actually
80d38338 1536reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1537interactive context.) Do not read from a terminal file (or call
19799a22 1538C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1539as terminals may lose the end-of-file condition if you do.
1540
820475bd 1541An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1542with empty parentheses is different. It refers to the pseudo file
820475bd 1543formed from the files listed on the command line and accessed via the
61eff3bc
JH
1544C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1545as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1546used will cause C<@ARGV> to be examined to determine if input is
67408cae 1547available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1548end-of-file will assume you are processing another C<@ARGV> list,
1549and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1550see L<perlop/"I/O Operators">.
820475bd 1551
61eff3bc 1552In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
3b10bc60 1553detect the end of each file, C<eof()> will detect the end of only the
820475bd 1554last file. Examples:
a0d0e21e 1555
748a9306
LW
1556 # reset line numbering on each input file
1557 while (<>) {
a9a5a0dc
VP
1558 next if /^\s*#/; # skip comments
1559 print "$.\t$_";
5a964f20 1560 } continue {
a9a5a0dc 1561 close ARGV if eof; # Not eof()!
748a9306
LW
1562 }
1563
a0d0e21e
LW
1564 # insert dashes just before last line of last file
1565 while (<>) {
a9a5a0dc
VP
1566 if (eof()) { # check for end of last file
1567 print "--------------\n";
1568 }
1569 print;
1570 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1571 }
1572
a0d0e21e 1573Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1574input operators typically return C<undef> when they run out of data, or if
1575there was an error.
a0d0e21e
LW
1576
1577=item eval EXPR
d74e8afc 1578X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1579X<error, handling> X<exception, handling>
a0d0e21e
LW
1580
1581=item eval BLOCK
1582
ce2984c3
PF
1583=item eval
1584
c7cc6f1c
GS
1585In the first form, the return value of EXPR is parsed and executed as if it
1586were a little Perl program. The value of the expression (which is itself
5a964f20 1587determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1588errors, executed in the lexical context of the current Perl program, so
1589that any variable settings or subroutine and format definitions remain
cf264981 1590afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1591If EXPR is omitted, evaluates C<$_>. This form is typically used to
1592delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1593
1594In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1595same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1596within the context of the current Perl program. This form is typically
1597used to trap exceptions more efficiently than the first (see below), while
1598also providing the benefit of checking the code within BLOCK at compile
1599time.
1600
1601The final semicolon, if any, may be omitted from the value of EXPR or within
1602the BLOCK.
1603
1604In both forms, the value returned is the value of the last expression
5a964f20 1605evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1606as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1607in void, scalar, or list context, depending on the context of the C<eval>
1608itself. See L</wantarray> for more on how the evaluation context can be
1609determined.
a0d0e21e 1610
19799a22 1611If there is a syntax error or runtime error, or a C<die> statement is
bbead3ca 1612executed, C<eval> returns an undefined value in scalar context
1f0d8f98
FC
1613or an empty list--or, for syntax errors, a list containing a single
1614undefined value--in list context, and C<$@> is set to the error
1615message. The discrepancy in the return values in list context is
1616considered a bug by some, and will probably be fixed in a future
1617release. If there was no error, C<$@> is guaranteed to be the empty
3b10bc60 1618string. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1619warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1620To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1621turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1622See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1623
19799a22
GS
1624Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1625determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1626is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1627the die operator is used to raise exceptions.
1628
5f1da31c
NT
1629If you want to trap errors when loading an XS module, some problems with
1630the binary interface (such as Perl version skew) may be fatal even with
1631C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1632
a0d0e21e
LW
1633If the code to be executed doesn't vary, you may use the eval-BLOCK
1634form to trap run-time errors without incurring the penalty of
1635recompiling each time. The error, if any, is still returned in C<$@>.
1636Examples:
1637
54310121 1638 # make divide-by-zero nonfatal
a0d0e21e
LW
1639 eval { $answer = $a / $b; }; warn $@ if $@;
1640
1641 # same thing, but less efficient
1642 eval '$answer = $a / $b'; warn $@ if $@;
1643
1644 # a compile-time error
5ed4f2ec 1645 eval { $answer = }; # WRONG
a0d0e21e
LW
1646
1647 # a run-time error
5ed4f2ec 1648 eval '$answer ='; # sets $@
a0d0e21e 1649
cf264981
SP
1650Using the C<eval{}> form as an exception trap in libraries does have some
1651issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1652may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1653You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1654as this example shows:
774d564b 1655
80d38338 1656 # a private exception trap for divide-by-zero
f86cebdf
GS
1657 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1658 warn $@ if $@;
774d564b
PP
1659
1660This is especially significant, given that C<__DIE__> hooks can call
19799a22 1661C<die> again, which has the effect of changing their error messages:
774d564b
PP
1662
1663 # __DIE__ hooks may modify error messages
1664 {
f86cebdf
GS
1665 local $SIG{'__DIE__'} =
1666 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1667 eval { die "foo lives here" };
1668 print $@ if $@; # prints "bar lives here"
774d564b
PP
1669 }
1670
19799a22 1671Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1672may be fixed in a future release.
1673
19799a22 1674With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1675being looked at when:
1676
5ed4f2ec 1677 eval $x; # CASE 1
1678 eval "$x"; # CASE 2
a0d0e21e 1679
5ed4f2ec 1680 eval '$x'; # CASE 3
1681 eval { $x }; # CASE 4
a0d0e21e 1682
5ed4f2ec 1683 eval "\$$x++"; # CASE 5
1684 $$x++; # CASE 6
a0d0e21e 1685
2f9daede 1686Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1687the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1688the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1689and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1690does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1691purely visual reasons, but it also has the advantage of compiling at
1692compile-time instead of at run-time.) Case 5 is a place where
19799a22 1693normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1694particular situation, you can just use symbolic references instead, as
1695in case 6.
a0d0e21e 1696
b6538e4f
TC
1697Before Perl 5.14, the assignment to C<$@> occurred before restoration
1698of localised variables, which means that for your code to run on older
b208c909 1699versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1700errors:
1701
1702 # alter $@ on nefarious repugnancy only
1703 {
1704 my $e;
1705 {
1706 local $@; # protect existing $@
1707 eval { test_repugnancy() };
b208c909 1708 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
8a5a710d
DN
1709 $@ =~ /nefarious/ and $e = $@;
1710 }
1711 die $e if defined $e
1712 }
1713
4968c1e4 1714C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1715C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1716
3b10bc60 1717An C<eval ''> executed within the C<DB> package doesn't see the usual
1718surrounding lexical scope, but rather the scope of the first non-DB piece
1719of code that called it. You don't normally need to worry about this unless
1720you are writing a Perl debugger.
d819b83a 1721
a0d0e21e 1722=item exec LIST
d74e8afc 1723X<exec> X<execute>
a0d0e21e 1724
8bf3b016
GS
1725=item exec PROGRAM LIST
1726
3b10bc60 1727The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1728use C<system> instead of C<exec> if you want it to return. It fails and
1729returns false only if the command does not exist I<and> it is executed
fb73857a 1730directly instead of via your system's command shell (see below).
a0d0e21e 1731
19799a22 1732Since it's a common mistake to use C<exec> instead of C<system>, Perl
80d38338 1733warns you if there is a following statement that isn't C<die>, C<warn>,
3b10bc60 1734or C<exit> (if C<-w> is set--but you always do that, right?). If you
19799a22 1735I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1736can use one of these styles to avoid the warning:
1737
5a964f20
TC
1738 exec ('foo') or print STDERR "couldn't exec foo: $!";
1739 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1740
5a964f20 1741If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1742with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1743If there is only one scalar argument or an array with one element in it,
1744the argument is checked for shell metacharacters, and if there are any,
1745the entire argument is passed to the system's command shell for parsing
1746(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1747If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1748words and passed directly to C<execvp>, which is more efficient.
19799a22 1749Examples:
a0d0e21e 1750
19799a22
GS
1751 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1752 exec "sort $outfile | uniq";
a0d0e21e
LW
1753
1754If you don't really want to execute the first argument, but want to lie
1755to the program you are executing about its own name, you can specify
1756the program you actually want to run as an "indirect object" (without a
1757comma) in front of the LIST. (This always forces interpretation of the
54310121 1758LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1759the list.) Example:
1760
1761 $shell = '/bin/csh';
5ed4f2ec 1762 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1763
1764or, more directly,
1765
5ed4f2ec 1766 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1767
3b10bc60 1768When the arguments get executed via the system shell, results are
1769subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1770for details.
1771
19799a22
GS
1772Using an indirect object with C<exec> or C<system> is also more
1773secure. This usage (which also works fine with system()) forces
1774interpretation of the arguments as a multivalued list, even if the
1775list had just one argument. That way you're safe from the shell
1776expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1777
1778 @args = ( "echo surprise" );
1779
2b5ab1e7 1780 exec @args; # subject to shell escapes
f86cebdf 1781 # if @args == 1
2b5ab1e7 1782 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1783
1784The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1785program, passing it C<"surprise"> an argument. The second version didn't;
1786it tried to run a program named I<"echo surprise">, didn't find it, and set
1787C<$?> to a non-zero value indicating failure.
5a964f20 1788
80d38338 1789Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1790output before the exec, but this may not be supported on some platforms
1791(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1792in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 1793open handles to avoid lost output.
0f897271 1794
80d38338
TC
1795Note that C<exec> will not call your C<END> blocks, nor will it invoke
1796C<DESTROY> methods on your objects.
7660c0ab 1797
a0d0e21e 1798=item exists EXPR
d74e8afc 1799X<exists> X<autovivification>
a0d0e21e 1800
d0a76353
RS
1801Given an expression that specifies an element of a hash, returns true if the
1802specified element in the hash has ever been initialized, even if the
1803corresponding value is undefined.
a0d0e21e 1804
5ed4f2ec 1805 print "Exists\n" if exists $hash{$key};
1806 print "Defined\n" if defined $hash{$key};
01020589
GS
1807 print "True\n" if $hash{$key};
1808
d0a76353
RS
1809exists may also be called on array elements, but its behavior is much less
1810obvious, and is strongly tied to the use of L</delete> on arrays. B<Be aware>
1811that calling exists on array values is deprecated and likely to be removed in
1812a future version of Perl.
1813
5ed4f2ec 1814 print "Exists\n" if exists $array[$index];
1815 print "Defined\n" if defined $array[$index];
01020589 1816 print "True\n" if $array[$index];
a0d0e21e 1817
8ea97a1e 1818A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1819it exists, but the reverse doesn't necessarily hold true.
1820
afebc493
GS
1821Given an expression that specifies the name of a subroutine,
1822returns true if the specified subroutine has ever been declared, even
1823if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 1824does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
1825exist may still be callable: its package may have an C<AUTOLOAD>
1826method that makes it spring into existence the first time that it is
3b10bc60 1827called; see L<perlsub>.
afebc493 1828
5ed4f2ec 1829 print "Exists\n" if exists &subroutine;
1830 print "Defined\n" if defined &subroutine;
afebc493 1831
a0d0e21e 1832Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1833operation is a hash or array key lookup or subroutine name:
a0d0e21e 1834
5ed4f2ec 1835 if (exists $ref->{A}->{B}->{$key}) { }
1836 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 1837
5ed4f2ec 1838 if (exists $ref->{A}->{B}->[$ix]) { }
1839 if (exists $hash{A}{B}[$ix]) { }
01020589 1840
afebc493
GS
1841 if (exists &{$ref->{A}{B}{$key}}) { }
1842
3b10bc60 1843Although the mostly deeply nested array or hash will not spring into
1844existence just because its existence was tested, any intervening ones will.
61eff3bc 1845Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 1846into existence due to the existence test for the $key element above.
3b10bc60 1847This happens anywhere the arrow operator is used, including even here:
5a964f20 1848
2b5ab1e7 1849 undef $ref;
5ed4f2ec 1850 if (exists $ref->{"Some key"}) { }
1851 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
1852
1853This surprising autovivification in what does not at first--or even
1854second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1855release.
a0d0e21e 1856
afebc493
GS
1857Use of a subroutine call, rather than a subroutine name, as an argument
1858to exists() is an error.
1859
5ed4f2ec 1860 exists &sub; # OK
1861 exists &sub(); # Error
afebc493 1862
a0d0e21e 1863=item exit EXPR
d74e8afc 1864X<exit> X<terminate> X<abort>
a0d0e21e 1865
ce2984c3
PF
1866=item exit
1867
2b5ab1e7 1868Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1869
1870 $ans = <STDIN>;
1871 exit 0 if $ans =~ /^[Xx]/;
1872
19799a22 1873See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1874universally recognized values for EXPR are C<0> for success and C<1>
1875for error; other values are subject to interpretation depending on the
1876environment in which the Perl program is running. For example, exiting
187769 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1878the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1879
19799a22
GS
1880Don't use C<exit> to abort a subroutine if there's any chance that
1881someone might want to trap whatever error happened. Use C<die> instead,
1882which can be trapped by an C<eval>.
28757baa 1883
19799a22 1884The exit() function does not always exit immediately. It calls any
2b5ab1e7 1885defined C<END> routines first, but these C<END> routines may not
19799a22 1886themselves abort the exit. Likewise any object destructors that need to
60275626
FC
1887be called are called before the real exit. C<END> routines and destructors
1888can change the exit status by modifying C<$?>. If this is a problem, you
2b5ab1e7 1889can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1890See L<perlmod> for details.
5a964f20 1891
a0d0e21e 1892=item exp EXPR
d74e8afc 1893X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1894
54310121 1895=item exp
bbce6d69 1896
b76cc8ba 1897Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1898If EXPR is omitted, gives C<exp($_)>.
1899
1900=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1901X<fcntl>
a0d0e21e 1902
f86cebdf 1903Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1904
1905 use Fcntl;
1906
0ade1984 1907first to get the correct constant definitions. Argument processing and
3b10bc60 1908value returned work just like C<ioctl> below.
a0d0e21e
LW
1909For example:
1910
1911 use Fcntl;
5a964f20 1912 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 1913 or die "can't fcntl F_GETFL: $!";
5a964f20 1914
554ad1fc 1915You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1916Like C<ioctl>, it maps a C<0> return from the system call into
1917C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1918in numeric context. It is also exempt from the normal B<-w> warnings
1919on improper numeric conversions.
5a964f20 1920
3b10bc60 1921Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
1922doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1923manpage to learn what functions are available on your system.
a0d0e21e 1924
be2f7487 1925Here's an example of setting a filehandle named C<REMOTE> to be
1926non-blocking at the system level. You'll have to negotiate C<$|>
1927on your own, though.
1928
1929 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1930
1931 $flags = fcntl(REMOTE, F_GETFL, 0)
1932 or die "Can't get flags for the socket: $!\n";
1933
1934 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1935 or die "Can't set flags for the socket: $!\n";
1936
a0d0e21e 1937=item fileno FILEHANDLE
d74e8afc 1938X<fileno>
a0d0e21e 1939
2b5ab1e7 1940Returns the file descriptor for a filehandle, or undefined if the
a7c1632d
FC
1941filehandle is not open. If there is no real file descriptor at the OS
1942level, as can happen with filehandles connected to memory objects via
1943C<open> with a reference for the third argument, -1 is returned.
1944
1945This is mainly useful for constructing
19799a22 1946bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1947If FILEHANDLE is an expression, the value is taken as an indirect
1948filehandle, generally its name.
5a964f20 1949
b76cc8ba 1950You can use this to find out whether two handles refer to the
5a964f20
TC
1951same underlying descriptor:
1952
1953 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 1954 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1955 }
1956
a0d0e21e 1957=item flock FILEHANDLE,OPERATION
d74e8afc 1958X<flock> X<lock> X<locking>
a0d0e21e 1959
19799a22
GS
1960Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1961for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1962machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
dbfe1e81 1963C<flock> is Perl's portable file-locking interface, although it locks
3b10bc60 1964entire files only, not records.
2b5ab1e7
TC
1965
1966Two potentially non-obvious but traditional C<flock> semantics are
1967that it waits indefinitely until the lock is granted, and that its locks
dbfe1e81
FC
1968are B<merely advisory>. Such discretionary locks are more flexible, but
1969offer fewer guarantees. This means that programs that do not also use
1970C<flock> may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1971your port's specific documentation, or your system-specific local manpages
1972for details. It's best to assume traditional behavior if you're writing
1973portable programs. (But if you're not, you should as always feel perfectly
1974free to write for your own system's idiosyncrasies (sometimes called
1975"features"). Slavish adherence to portability concerns shouldn't get
1976in the way of your getting your job done.)
a3cb178b 1977
8ebc5c01
PP
1978OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1979LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1980you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1981either individually, or as a group using the ':flock' tag. LOCK_SH
1982requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 1983releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
3b10bc60 1984LOCK_SH or LOCK_EX then C<flock> returns immediately rather than blocking
1985waiting for the lock; check the return status to see if you got it.
68dc0745 1986
2b5ab1e7
TC
1987To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1988before locking or unlocking it.
8ebc5c01 1989
f86cebdf 1990Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1991locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1992are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1993implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1994differing semantics shouldn't bite too many people.
1995
becacb53
TM
1996Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1997be open with read intent to use LOCK_SH and requires that it be open
1998with write intent to use LOCK_EX.
1999
19799a22
GS
2000Note also that some versions of C<flock> cannot lock things over the
2001network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
2002that. If you like you can force Perl to ignore your system's flock(2)
2003function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 2004the switch C<-Ud_flock> to the F<Configure> program when you configure
3b10bc60 2005Perl.
4633a7c4
LW
2006
2007Here's a mailbox appender for BSD systems.
a0d0e21e 2008
7ed5353d 2009 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
2010
2011 sub lock {
a9a5a0dc
VP
2012 my ($fh) = @_;
2013 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 2014
a9a5a0dc
VP
2015 # and, in case someone appended while we were waiting...
2016 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2017 }
2018
2019 sub unlock {
a9a5a0dc
VP
2020 my ($fh) = @_;
2021 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2022 }
2023
b0169937 2024 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2025 or die "Can't open mailbox: $!";
a0d0e21e 2026
7ed5353d 2027 lock($mbox);
b0169937 2028 print $mbox $msg,"\n\n";
7ed5353d 2029 unlock($mbox);
a0d0e21e 2030
3b10bc60 2031On systems that support a real flock(2), locks are inherited across fork()
2032calls, whereas those that must resort to the more capricious fcntl(2)
2033function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2034
cb1a09d0 2035See also L<DB_File> for other flock() examples.
a0d0e21e
LW
2036
2037=item fork
d74e8afc 2038X<fork> X<child> X<parent>
a0d0e21e 2039
2b5ab1e7
TC
2040Does a fork(2) system call to create a new process running the
2041same program at the same point. It returns the child pid to the
2042parent process, C<0> to the child process, or C<undef> if the fork is
2043unsuccessful. File descriptors (and sometimes locks on those descriptors)
2044are shared, while everything else is copied. On most systems supporting
2045fork(), great care has gone into making it extremely efficient (for
2046example, using copy-on-write technology on data pages), making it the
2047dominant paradigm for multitasking over the last few decades.
5a964f20 2048
80d38338 2049Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
2050output before forking the child process, but this may not be supported
2051on some platforms (see L<perlport>). To be safe, you may need to set
2052C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2053C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2054
19799a22 2055If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2056accumulate zombies. On some systems, you can avoid this by setting
2057C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2058forking and reaping moribund children.
cb1a09d0 2059
28757baa
PP
2060Note that if your forked child inherits system file descriptors like
2061STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2062if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2063backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2064You should reopen those to F</dev/null> if it's any issue.
28757baa 2065
cb1a09d0 2066=item format
d74e8afc 2067X<format>
cb1a09d0 2068
19799a22 2069Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2070example:
2071
54310121 2072 format Something =
a9a5a0dc
VP
2073 Test: @<<<<<<<< @||||| @>>>>>
2074 $str, $%, '$' . int($num)
cb1a09d0
AD
2075 .
2076
2077 $str = "widget";
184e9718 2078 $num = $cost/$quantity;
cb1a09d0
AD
2079 $~ = 'Something';
2080 write;
2081
2082See L<perlform> for many details and examples.
2083
8903cb82 2084=item formline PICTURE,LIST
d74e8afc 2085X<formline>
a0d0e21e 2086
5a964f20 2087This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2088too. It formats (see L<perlform>) a list of values according to the
2089contents of PICTURE, placing the output into the format output
7660c0ab 2090accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2091Eventually, when a C<write> is done, the contents of
cf264981
SP
2092C<$^A> are written to some filehandle. You could also read C<$^A>
2093and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2094does one C<formline> per line of form, but the C<formline> function itself
748a9306 2095doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2096that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2097You may therefore need to use multiple formlines to implement a single
3b10bc60 2098record format, just like the C<format> compiler.
748a9306 2099
19799a22 2100Be careful if you put double quotes around the picture, because an C<@>
748a9306 2101character may be taken to mean the beginning of an array name.
19799a22 2102C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e 2103
445b09e5
FC
2104If you are trying to use this instead of C<write> to capture the output,
2105you may find it easier to open a filehandle to a scalar
2106(C<< open $fh, ">", \$output >>) and write to that instead.
2107
a0d0e21e 2108=item getc FILEHANDLE
f723aae1 2109X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2110
2111=item getc
2112
2113Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2114or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2115the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2116STDIN. This is not particularly efficient. However, it cannot be
2117used by itself to fetch single characters without waiting for the user
2118to hit enter. For that, try something more like:
4633a7c4
LW
2119
2120 if ($BSD_STYLE) {
a9a5a0dc 2121 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2122 }
2123 else {
a9a5a0dc 2124 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2125 }
2126
2127 $key = getc(STDIN);
2128
2129 if ($BSD_STYLE) {
a9a5a0dc 2130 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2131 }
2132 else {
3b10bc60 2133 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2134 }
2135 print "\n";
2136
54310121
PP
2137Determination of whether $BSD_STYLE should be set
2138is left as an exercise to the reader.
cb1a09d0 2139
19799a22 2140The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2141systems purporting POSIX compliance. See also the C<Term::ReadKey>
2142module from your nearest CPAN site; details on CPAN can be found on
2143L<perlmodlib/CPAN>.
a0d0e21e
LW
2144
2145=item getlogin
d74e8afc 2146X<getlogin> X<login>
a0d0e21e 2147
cf264981 2148This implements the C library function of the same name, which on most
3b10bc60 2149systems returns the current login from F</etc/utmp>, if any. If it
2150returns the empty string, use C<getpwuid>.
a0d0e21e 2151
f86702cc 2152 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2153
19799a22
GS
2154Do not consider C<getlogin> for authentication: it is not as
2155secure as C<getpwuid>.
4633a7c4 2156
a0d0e21e 2157=item getpeername SOCKET
d74e8afc 2158X<getpeername> X<peer>
a0d0e21e
LW
2159
2160Returns the packed sockaddr address of other end of the SOCKET connection.
2161
4633a7c4
LW
2162 use Socket;
2163 $hersockaddr = getpeername(SOCK);
19799a22 2164 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2165 $herhostname = gethostbyaddr($iaddr, AF_INET);
2166 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2167
2168=item getpgrp PID
d74e8afc 2169X<getpgrp> X<group>
a0d0e21e 2170
47e29363 2171Returns the current process group for the specified PID. Use
7660c0ab 2172a PID of C<0> to get the current process group for the
4633a7c4 2173current process. Will raise an exception if used on a machine that
f86cebdf 2174doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2175group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2176does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2177
2178=item getppid
d74e8afc 2179X<getppid> X<parent> X<pid>
a0d0e21e
LW
2180
2181Returns the process id of the parent process.
2182
4d76a344
RGS
2183Note for Linux users: on Linux, the C functions C<getpid()> and
2184C<getppid()> return different values from different threads. In order to
3b10bc60 2185be portable, this behavior is not reflected by the Perl-level function
4d76a344 2186C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2187to call the underlying C<getppid()>, you may use the CPAN module
2188C<Linux::Pid>.
4d76a344 2189
a0d0e21e 2190=item getpriority WHICH,WHO
d74e8afc 2191X<getpriority> X<priority> X<nice>
a0d0e21e 2192
4633a7c4 2193Returns the current priority for a process, a process group, or a user.
f4084e39 2194(See C<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2195machine that doesn't implement getpriority(2).
a0d0e21e
LW
2196
2197=item getpwnam NAME
d74e8afc
ITB
2198X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2199X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2200X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2201X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2202X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2203X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2204
2205=item getgrnam NAME
2206
2207=item gethostbyname NAME
2208
2209=item getnetbyname NAME
2210
2211=item getprotobyname NAME
2212
2213=item getpwuid UID
2214
2215=item getgrgid GID
2216
2217=item getservbyname NAME,PROTO
2218
2219=item gethostbyaddr ADDR,ADDRTYPE
2220
2221=item getnetbyaddr ADDR,ADDRTYPE
2222
2223=item getprotobynumber NUMBER
2224
2225=item getservbyport PORT,PROTO
2226
2227=item getpwent
2228
2229=item getgrent
2230
2231=item gethostent
2232
2233=item getnetent
2234
2235=item getprotoent
2236
2237=item getservent
2238
2239=item setpwent
2240
2241=item setgrent
2242
2243=item sethostent STAYOPEN
2244
2245=item setnetent STAYOPEN
2246
2247=item setprotoent STAYOPEN
2248
2249=item setservent STAYOPEN
2250
2251=item endpwent
2252
2253=item endgrent
2254
2255=item endhostent
2256
2257=item endnetent
2258
2259=item endprotoent
2260
2261=item endservent
2262
80d38338
TC
2263These routines are the same as their counterparts in the
2264system C library. In list context, the return values from the
a0d0e21e
LW
2265various get routines are as follows:
2266
2267 ($name,$passwd,$uid,$gid,
6ee623d5 2268 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2269 ($name,$passwd,$gid,$members) = getgr*
2270 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2271 ($name,$aliases,$addrtype,$net) = getnet*
2272 ($name,$aliases,$proto) = getproto*
2273 ($name,$aliases,$port,$proto) = getserv*
2274
3b10bc60 2275(If the entry doesn't exist you get an empty list.)
a0d0e21e 2276
4602f195
JH
2277The exact meaning of the $gcos field varies but it usually contains
2278the real name of the user (as opposed to the login name) and other
2279information pertaining to the user. Beware, however, that in many
2280system users are able to change this information and therefore it
106325ad 2281cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2282L<perlsec>). The $passwd and $shell, user's encrypted password and
2283login shell, are also tainted, because of the same reason.
4602f195 2284
5a964f20 2285In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2286lookup by name, in which case you get the other thing, whatever it is.
2287(If the entry doesn't exist you get the undefined value.) For example:
2288
5a964f20
TC
2289 $uid = getpwnam($name);
2290 $name = getpwuid($num);
2291 $name = getpwent();
2292 $gid = getgrnam($name);
08a33e13 2293 $name = getgrgid($num);
5a964f20
TC
2294 $name = getgrent();
2295 #etc.
a0d0e21e 2296
4602f195 2297In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2298in that they are unsupported on many systems. If the
4602f195
JH
2299$quota is unsupported, it is an empty scalar. If it is supported, it
2300usually encodes the disk quota. If the $comment field is unsupported,
2301it is an empty scalar. If it is supported it usually encodes some
2302administrative comment about the user. In some systems the $quota
2303field may be $change or $age, fields that have to do with password
2304aging. In some systems the $comment field may be $class. The $expire
2305field, if present, encodes the expiration period of the account or the
2306password. For the availability and the exact meaning of these fields
2307in your system, please consult your getpwnam(3) documentation and your
2308F<pwd.h> file. You can also find out from within Perl what your
2309$quota and $comment fields mean and whether you have the $expire field
2310by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2311C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2312files are supported only if your vendor has implemented them in the
4602f195 2313intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2314shadow versions if you're running under privilege or if there exists
cf264981
SP
2315the shadow(3) functions as found in System V (this includes Solaris
2316and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2317facility are unlikely to be supported.
6ee623d5 2318
19799a22 2319The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2320the login names of the members of the group.
2321
2322For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2323C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2324C<@addrs> value returned by a successful call is a list of raw
2325addresses returned by the corresponding library call. In the
2326Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2327by saying something like:
2328
f337b084 2329 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2330
2b5ab1e7
TC
2331The Socket library makes this slightly easier:
2332
2333 use Socket;
2334 $iaddr = inet_aton("127.1"); # or whatever address
2335 $name = gethostbyaddr($iaddr, AF_INET);
2336
2337 # or going the other way
19799a22 2338 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2339
d760c846
GS
2340In the opposite way, to resolve a hostname to the IP address
2341you can write this:
2342
2343 use Socket;
2344 $packed_ip = gethostbyname("www.perl.org");
2345 if (defined $packed_ip) {
2346 $ip_address = inet_ntoa($packed_ip);
2347 }
2348
2349Make sure <gethostbyname()> is called in SCALAR context and that
2350its return value is checked for definedness.
2351
19799a22
GS
2352If you get tired of remembering which element of the return list
2353contains which return value, by-name interfaces are provided
2354in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2355C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2356and C<User::grent>. These override the normal built-ins, supplying
2357versions that return objects with the appropriate names
2358for each field. For example:
5a964f20
TC
2359
2360 use File::stat;
2361 use User::pwent;
2362 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2363
b76cc8ba
NIS
2364Even though it looks like they're the same method calls (uid),
2365they aren't, because a C<File::stat> object is different from
19799a22 2366a C<User::pwent> object.
5a964f20 2367
a0d0e21e 2368=item getsockname SOCKET
d74e8afc 2369X<getsockname>
a0d0e21e 2370
19799a22
GS
2371Returns the packed sockaddr address of this end of the SOCKET connection,
2372in case you don't know the address because you have several different
2373IPs that the connection might have come in on.
a0d0e21e 2374
4633a7c4
LW
2375 use Socket;
2376 $mysockaddr = getsockname(SOCK);
19799a22 2377 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2378 printf "Connect to %s [%s]\n",
19799a22
GS
2379 scalar gethostbyaddr($myaddr, AF_INET),
2380 inet_ntoa($myaddr);
a0d0e21e
LW
2381
2382=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2383X<getsockopt>
a0d0e21e 2384
636e6b1f
TH
2385Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2386Options may exist at multiple protocol levels depending on the socket
2387type, but at least the uppermost socket level SOL_SOCKET (defined in the
2388C<Socket> module) will exist. To query options at another level the
2389protocol number of the appropriate protocol controlling the option
2390should be supplied. For example, to indicate that an option is to be
2391interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2392number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2393
80d38338 2394The function returns a packed string representing the requested socket
3b10bc60 2395option, or C<undef> on error, with the reason for the error placed in
2396C<$!>). Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2397consult getsockopt(2) for details. A common case is that the option is an
2398integer, in which case the result is a packed integer, which you can decode
2399using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2400
80d38338 2401An example to test whether Nagle's algorithm is turned on on a socket:
636e6b1f 2402
4852725b 2403 use Socket qw(:all);
636e6b1f
TH
2404
2405 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2406 or die "Could not determine the protocol number for tcp";
4852725b
DD
2407 # my $tcp = IPPROTO_TCP; # Alternative
2408 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2409 or die "getsockopt TCP_NODELAY: $!";
636e6b1f
TH
2410 my $nodelay = unpack("I", $packed);
2411 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2412
a0d0e21e
LW
2413
2414=item glob EXPR
d74e8afc 2415X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2416
0a753a76
PP
2417=item glob
2418
d9a9d457
JL
2419In list context, returns a (possibly empty) list of filename expansions on
2420the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2421scalar context, glob iterates through such filename expansions, returning
2422undef when the list is exhausted. This is the internal function
2423implementing the C<< <*.c> >> operator, but you can use it directly. If
2424EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2425more detail in L<perlop/"I/O Operators">.
a0d0e21e 2426
80d38338
TC
2427Note that C<glob> splits its arguments on whitespace and treats
2428each segment as separate pattern. As such, C<glob("*.c *.h")>
2429matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2430C<glob(".* *")> matches all files in the current working directory.
80d38338
TC
2431
2432If non-empty braces are the only wildcard characters used in the
2433C<glob>, no filenames are matched, but potentially many strings
2434are returned. For example, this produces nine strings, one for
2435each pairing of fruits and colors:
2436
2437 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2438
3a4b19e4 2439Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2440C<File::Glob> extension. See L<File::Glob> for details, including
2441C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2442
a0d0e21e 2443=item gmtime EXPR
d74e8afc 2444X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2445
ce2984c3
PF
2446=item gmtime
2447
435fbc73
GS
2448Works just like L<localtime> but the returned values are
2449localized for the standard Greenwich time zone.
a0d0e21e 2450
435fbc73
GS
2451Note: when called in list context, $isdst, the last value
2452returned by gmtime is always C<0>. There is no
2453Daylight Saving Time in GMT.
0a753a76 2454
62aa5637
MS
2455See L<perlport/gmtime> for portability concerns.
2456
a0d0e21e 2457=item goto LABEL
d74e8afc 2458X<goto> X<jump> X<jmp>
a0d0e21e 2459
748a9306
LW
2460=item goto EXPR
2461
a0d0e21e
LW
2462=item goto &NAME
2463
b500e03b
GG
2464The C<goto-LABEL> form finds the statement labeled with LABEL and
2465resumes execution there. It can't be used to get out of a block or
2466subroutine given to C<sort>. It can be used to go almost anywhere
2467else within the dynamic scope, including out of subroutines, but it's
2468usually better to use some other construct such as C<last> or C<die>.
2469The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2470(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2471does not offer named loops combined with loop control. Perl does, and
2472this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2473
7660c0ab
A
2474The C<goto-EXPR> form expects a label name, whose scope will be resolved
2475dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2476necessarily recommended if you're optimizing for maintainability:
2477
2478 goto ("FOO", "BAR", "GLARCH")[$i];
2479
b500e03b 2480Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2481deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2482go into any construct that requires initialization, such as a
2483subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2484construct that is optimized away.
b500e03b 2485
1b6921cb
BT
2486The C<goto-&NAME> form is quite different from the other forms of
2487C<goto>. In fact, it isn't a goto in the normal sense at all, and
2488doesn't have the stigma associated with other gotos. Instead, it
2489exits the current subroutine (losing any changes set by local()) and
2490immediately calls in its place the named subroutine using the current
2491value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2492load another subroutine and then pretend that the other subroutine had
2493been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2494in the current subroutine are propagated to the other subroutine.)
2495After the C<goto>, not even C<caller> will be able to tell that this
2496routine was called first.
2497
2498NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2499containing a code reference, or a block that evaluates to a code
6cb9131c 2500reference.
a0d0e21e
LW
2501
2502=item grep BLOCK LIST
d74e8afc 2503X<grep>
a0d0e21e
LW
2504
2505=item grep EXPR,LIST
2506
2b5ab1e7
TC
2507This is similar in spirit to, but not the same as, grep(1) and its
2508relatives. In particular, it is not limited to using regular expressions.
2f9daede 2509
a0d0e21e 2510Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2511C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2512elements for which the expression evaluated to true. In scalar
2513context, returns the number of times the expression was true.
a0d0e21e
LW
2514
2515 @foo = grep(!/^#/, @bar); # weed out comments
2516
2517or equivalently,
2518
2519 @foo = grep {!/^#/} @bar; # weed out comments
2520
be3174d2
GS
2521Note that C<$_> is an alias to the list value, so it can be used to
2522modify the elements of the LIST. While this is useful and supported,
2523it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2524Similarly, grep returns aliases into the original list, much as a for
2525loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2526element of a list returned by grep (for example, in a C<foreach>, C<map>
2527or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2528This is usually something to be avoided when writing clear code.
a0d0e21e 2529
a4fb8298 2530If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2531been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2532the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2533can't be seen from the outside, avoiding any potential side-effects.
2534
19799a22 2535See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2536
a0d0e21e 2537=item hex EXPR
d74e8afc 2538X<hex> X<hexadecimal>
a0d0e21e 2539
54310121 2540=item hex
bbce6d69 2541
2b5ab1e7 2542Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2543(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2544L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2545
2546 print hex '0xAf'; # prints '175'
2547 print hex 'aF'; # same
a0d0e21e 2548
19799a22 2549Hex strings may only represent integers. Strings that would cause
53305cf1 2550integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2551unlike oct(). To present something as hex, look into L</printf>,
2552L</sprintf>, or L</unpack>.
19799a22 2553
ce2984c3 2554=item import LIST
d74e8afc 2555X<import>
a0d0e21e 2556
19799a22 2557There is no builtin C<import> function. It is just an ordinary
4633a7c4 2558method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2559names to another module. The C<use> function calls the C<import> method
cea6626f 2560for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2561
2562=item index STR,SUBSTR,POSITION
d74e8afc 2563X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2564
2565=item index STR,SUBSTR
2566
2b5ab1e7
TC
2567The index function searches for one string within another, but without
2568the wildcard-like behavior of a full regular-expression pattern match.
2569It returns the position of the first occurrence of SUBSTR in STR at
2570or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2571beginning of the string. POSITION before the beginning of the string
2572or after its end is treated as if it were the beginning or the end,
2573respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2574you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2575is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2576
2577=item int EXPR
f723aae1 2578X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2579
54310121 2580=item int
bbce6d69 2581
7660c0ab 2582Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 2583You should not use this function for rounding: one because it truncates
3b10bc60 2584towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
2585numbers can sometimes produce counterintuitive results. For example,
2586C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2587because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2588the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2589functions will serve you better than will int().
a0d0e21e
LW
2590
2591=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2592X<ioctl>
a0d0e21e 2593
2b5ab1e7 2594Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2595
5ed4f2ec 2596 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2597
a11c483f 2598to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2599exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2600own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2601(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2602may help you in this, but it's nontrivial.) SCALAR will be read and/or
3b10bc60 2603written depending on the FUNCTION; a C pointer to the string value of SCALAR
19799a22 2604will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2605has no string value but does have a numeric value, that value will be
2606passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2607true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2608functions may be needed to manipulate the values of structures used by
b76cc8ba 2609C<ioctl>.
a0d0e21e 2610
19799a22 2611The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e 2612
5ed4f2ec 2613 if OS returns: then Perl returns:
2614 -1 undefined value
2615 0 string "0 but true"
2616 anything else that number
a0d0e21e 2617
19799a22 2618Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2619still easily determine the actual value returned by the operating
2620system:
2621
2b5ab1e7 2622 $retval = ioctl(...) || -1;
a0d0e21e
LW
2623 printf "System returned %d\n", $retval;
2624
be2f7487 2625The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2626about improper numeric conversions.
2627
a0d0e21e 2628=item join EXPR,LIST
d74e8afc 2629X<join>
a0d0e21e 2630
2b5ab1e7
TC
2631Joins the separate strings of LIST into a single string with fields
2632separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2633
2b5ab1e7 2634 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2635
eb6e2d6f
GS
2636Beware that unlike C<split>, C<join> doesn't take a pattern as its
2637first argument. Compare L</split>.
a0d0e21e 2638
cba5a3b0 2639=item keys HASH (or HASHREF)
d74e8afc 2640X<keys> X<key>
aa689395 2641
cba5a3b0 2642=item keys ARRAY (or ARRAYREF)
aeedbbed
NC
2643
2644Returns a list consisting of all the keys of the named hash, or the indices
2645of an array. (In scalar context, returns the number of keys or indices.)
504f80c1 2646
aeedbbed 2647The keys of a hash are returned in an apparently random order. The actual
3b10bc60 2648random order is subject to change in future versions of Perl, but it
504f80c1 2649is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6 2650function produces (given that the hash has not been modified). Since
c5f61d2f 2651Perl 5.8.1 the ordering can be different even between different runs of
4546b9e6 2652Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2653Attacks">).
504f80c1 2654
aeedbbed 2655As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
cf264981
SP
2656(see L</each>). In particular, calling keys() in void context resets
2657the iterator with no other overhead.
a0d0e21e 2658
aa689395 2659Here is yet another way to print your environment:
a0d0e21e
LW
2660
2661 @keys = keys %ENV;
2662 @values = values %ENV;
b76cc8ba 2663 while (@keys) {
a9a5a0dc 2664 print pop(@keys), '=', pop(@values), "\n";
a0d0e21e
LW
2665 }
2666
2667or how about sorted by key:
2668
2669 foreach $key (sort(keys %ENV)) {
a9a5a0dc 2670 print $key, '=', $ENV{$key}, "\n";
a0d0e21e
LW
2671 }
2672
8ea1e5d4
GS
2673The returned values are copies of the original keys in the hash, so
2674modifying them will not affect the original hash. Compare L</values>.
2675
19799a22 2676To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2677Here's a descending numeric sort of a hash by its values:
4633a7c4 2678
5a964f20 2679 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
a9a5a0dc 2680 printf "%4d %s\n", $hash{$key}, $key;
4633a7c4
LW
2681 }
2682
3b10bc60 2683Used as an lvalue, C<keys> allows you to increase the number of hash buckets
aa689395
PP
2684allocated for the given hash. This can gain you a measure of efficiency if
2685you know the hash is going to get big. (This is similar to pre-extending
2686an array by assigning a larger number to $#array.) If you say
55497cff
PP
2687
2688 keys %hash = 200;
2689
ab192400
GS
2690then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2691in fact, since it rounds up to the next power of two. These
55497cff
PP
2692buckets will be retained even if you do C<%hash = ()>, use C<undef
2693%hash> if you want to free the storage while C<%hash> is still in scope.
2694You can't shrink the number of buckets allocated for the hash using
19799a22 2695C<keys> in this way (but you needn't worry about doing this by accident,
aeedbbed
NC
2696as trying has no effect). C<keys @array> in an lvalue context is a syntax
2697error.
55497cff 2698
cba5a3b0
DG
2699When given a reference to a hash or array, the argument will be
2700dereferenced automatically.
2701
2702 for (keys $hashref) { ... }
2703 for (keys $obj->get_arrayref) { ... }
2704
2705If the reference is a blessed object that overrides either C<%{}> or
2706C<@{}>, the override will be used instead of dereferencing the underlying
2707variable type. If both overrides are provided, C<%{}> will be the default.
2708If this is not desired, you must dereference the argument yourself.
2709
19799a22 2710See also C<each>, C<values> and C<sort>.
ab192400 2711
b350dd2f 2712=item kill SIGNAL, LIST
d74e8afc 2713X<kill> X<signal>
a0d0e21e 2714
b350dd2f 2715Sends a signal to a list of processes. Returns the number of
517db077
GS
2716processes successfully signaled (which is not necessarily the
2717same as the number actually killed).
a0d0e21e
LW
2718
2719 $cnt = kill 1, $child1, $child2;
2720 kill 9, @goners;
2721
3b10bc60 2722If SIGNAL is zero, no signal is sent to the process, but C<kill>
2723checks whether it's I<possible> to send a signal to it (that
70fb64f6 2724means, to be brief, that the process is owned by the same user, or we are
3b10bc60 2725the super-user). This is useful to check that a child process is still
81fd35db
DN
2726alive (even if only as a zombie) and hasn't changed its UID. See
2727L<perlport> for notes on the portability of this construct.
b350dd2f 2728
e2c0f81f
DG
2729Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2730of processes. That means you usually want to use positive not negative signals.
2731You may also use a signal name in quotes.
2732
2733The behavior of kill when a I<PROCESS> number is zero or negative depends on
2734the operating system. For example, on POSIX-conforming systems, zero will
2735signal the current process group and -1 will signal all processes.
1e9c1022
JL
2736
2737See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2738
2739=item last LABEL
d74e8afc 2740X<last> X<break>
a0d0e21e
LW
2741
2742=item last
2743
2744The C<last> command is like the C<break> statement in C (as used in
2745loops); it immediately exits the loop in question. If the LABEL is
2746omitted, the command refers to the innermost enclosing loop. The
2747C<continue> block, if any, is not executed:
2748
4633a7c4 2749 LINE: while (<STDIN>) {
a9a5a0dc
VP
2750 last LINE if /^$/; # exit when done with header
2751 #...
a0d0e21e
LW
2752 }
2753
80d38338 2754C<last> cannot be used to exit a block that returns a value such as
2b5ab1e7
TC
2755C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2756a grep() or map() operation.
4968c1e4 2757
6c1372ed
GS
2758Note that a block by itself is semantically identical to a loop
2759that executes once. Thus C<last> can be used to effect an early
2760exit out of such a block.
2761
98293880
JH
2762See also L</continue> for an illustration of how C<last>, C<next>, and
2763C<redo> work.
1d2dff63 2764
a0d0e21e 2765=item lc EXPR
d74e8afc 2766X<lc> X<lowercase>
a0d0e21e 2767
54310121 2768=item lc
bbce6d69 2769
d1be9408 2770Returns a lowercased version of EXPR. This is the internal function
3980dc9c 2771implementing the C<\L> escape in double-quoted strings.
a0d0e21e 2772
7660c0ab 2773If EXPR is omitted, uses C<$_>.
bbce6d69 2774
3980dc9c
KW
2775What gets returned depends on several factors:
2776
2777=over
2778
2779=item If C<use bytes> is in effect:
2780
2781=over
2782
2783=item On EBCDIC platforms
2784
2785The results are what the C language system call C<tolower()> returns.
2786
2787=item On ASCII platforms
2788
2789The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
2790respectively.
2791
2792=back
2793
2794=item Otherwise, If EXPR has the UTF8 flag set
2795
2796If the current package has a subroutine named C<ToLower>, it will be used to
96090e4f
A
2797change the case
2798(See L<perlunicode/"User-Defined Case Mappings (for serious hackers only)">.)
3980dc9c
KW
2799Otherwise Unicode semantics are used for the case change.
2800
2801=item Otherwise, if C<use locale> is in effect
2802
2803Respects current LC_CTYPE locale. See L<perllocale>.
2804
2805=item Otherwise, if C<use feature 'unicode_strings'> is in effect:
2806
2807Unicode semantics are used for the case change. Any subroutine named
2808C<ToLower> will not be used.
2809
2810=item Otherwise:
2811
2812=over
2813
2814=item On EBCDIC platforms
2815
2816The results are what the C language system call C<tolower()> returns.
2817
2818=item On ASCII platforms
2819
2820ASCII semantics are used for the case change. The lowercase of any character
2821outside the ASCII range is the character itself.
2822
2823=back
2824
2825=back
2826
a0d0e21e 2827=item lcfirst EXPR
d74e8afc 2828X<lcfirst> X<lowercase>
a0d0e21e 2829
54310121 2830=item lcfirst
bbce6d69 2831
ad0029c4
JH
2832Returns the value of EXPR with the first character lowercased. This
2833is the internal function implementing the C<\l> escape in
3980dc9c 2834double-quoted strings.
a0d0e21e 2835
7660c0ab 2836If EXPR is omitted, uses C<$_>.
bbce6d69 2837
3980dc9c
KW
2838This function behaves the same way under various pragma, such as in a locale,
2839as L</lc> does.
2840
a0d0e21e 2841=item length EXPR
d74e8afc 2842X<length> X<size>
a0d0e21e 2843
54310121 2844=item length
bbce6d69 2845
974da8e5 2846Returns the length in I<characters> of the value of EXPR. If EXPR is
9f621bb0 2847omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
3b10bc60 2848
2849This function cannot be used on an entire array or hash to find out how
2850many elements these have. For that, use C<scalar @array> and C<scalar keys
2851%hash>, respectively.
2852
2853Like all Perl character operations, length() normally deals in logical
2854characters, not physical bytes. For how many bytes a string encoded as
2855UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
2856to C<use Encode> first). See L<Encode> and L<perlunicode>.
974da8e5 2857
a0d0e21e 2858=item link OLDFILE,NEWFILE
d74e8afc 2859X<link>
a0d0e21e 2860
19799a22 2861Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2862success, false otherwise.
a0d0e21e
LW
2863
2864=item listen SOCKET,QUEUESIZE
d74e8afc 2865X<listen>
a0d0e21e 2866
3b10bc60 2867Does the same thing that the listen(2) system call does. Returns true if
b76cc8ba 2868it succeeded, false otherwise. See the example in
cea6626f 2869L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2870
2871=item local EXPR
d74e8afc 2872X<local>
a0d0e21e 2873
19799a22 2874You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2875what most people think of as "local". See
13a2d996 2876L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2877
5a964f20
TC
2878A local modifies the listed variables to be local to the enclosing
2879block, file, or eval. If more than one value is listed, the list must
2880be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2881for details, including issues with tied arrays and hashes.
a0d0e21e 2882
d361fafa
VP
2883The C<delete local EXPR> construct can also be used to localize the deletion
2884of array/hash elements to the current block.
2885See L<perlsub/"Localized deletion of elements of composite types">.
2886
a0d0e21e 2887=item localtime EXPR
435fbc73 2888X<localtime> X<ctime>
a0d0e21e 2889
ba053783
AL
2890=item localtime
2891
19799a22 2892Converts a time as returned by the time function to a 9-element list
5f05dabc 2893with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2894follows:
2895
54310121 2896 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2897 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2898 localtime(time);
a0d0e21e 2899
48a26b3a 2900All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2901tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2902of the specified time.
48a26b3a 2903
ba053783
AL
2904C<$mday> is the day of the month, and C<$mon> is the month itself, in
2905the range C<0..11> with 0 indicating January and 11 indicating December.
2906This makes it easy to get a month name from a list:
54310121 2907
ba053783
AL
2908 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2909 print "$abbr[$mon] $mday";
2910 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2911
ba053783
AL
2912C<$year> is the number of years since 1900, not just the last two digits
2913of the year. That is, C<$year> is C<123> in year 2023. The proper way
80d38338 2914to get a 4-digit year is simply:
abd75f24 2915
ba053783 2916 $year += 1900;
abd75f24 2917
435fbc73
GS
2918Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2919to do that, would you?
2920
ba053783
AL
2921To get the last two digits of the year (e.g., '01' in 2001) do:
2922
2923 $year = sprintf("%02d", $year % 100);
2924
2925C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2926Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2927(or C<0..365> in leap years.)
2928
2929C<$isdst> is true if the specified time occurs during Daylight Saving
2930Time, false otherwise.
abd75f24 2931
e1998452 2932If EXPR is omitted, C<localtime()> uses the current time (as returned
e3176d09 2933by time(3)).
a0d0e21e 2934
48a26b3a 2935In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2936
5f05dabc 2937 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2938
fe86afc2
NC
2939This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2940instead of local time use the L</gmtime> builtin. See also the
2941C<Time::Local> module (to convert the second, minutes, hours, ... back to
2942the integer value returned by time()), and the L<POSIX> module's strftime(3)
2943and mktime(3) functions.
2944
2945To get somewhat similar but locale dependent date strings, set up your
2946locale environment variables appropriately (please see L<perllocale>) and
2947try for example:
a3cb178b 2948
5a964f20 2949 use POSIX qw(strftime);
2b5ab1e7 2950 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2951 # or for GMT formatted appropriately for your locale:
2952 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2953
2954Note that the C<%a> and C<%b>, the short forms of the day of the week
2955and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2956
62aa5637
MS
2957See L<perlport/localtime> for portability concerns.
2958
435fbc73
GS
2959The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2960by-name access mechanism to the gmtime() and localtime() functions,
2961respectively.
2962
2963For a comprehensive date and time representation look at the
2964L<DateTime> module on CPAN.
2965
07698885 2966=item lock THING
d74e8afc 2967X<lock>
19799a22 2968
01e6739c 2969This function places an advisory lock on a shared variable, or referenced
03730085 2970object contained in I<THING> until the lock goes out of scope.
a6d5524e 2971
f3a23afb 2972lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2973by this name (before any calls to it), that function will be called
7b043ca5
RGS
2974instead. If you are not under C<use threads::shared> this does nothing.
2975See L<threads::shared>.
19799a22 2976
a0d0e21e 2977=item log EXPR
d74e8afc 2978X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2979
54310121 2980=item log
bbce6d69 2981
2b5ab1e7
TC
2982Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2983returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2984The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2985divided by the natural log of N. For example:
2986
2987 sub log10 {
a9a5a0dc
VP
2988 my $n = shift;
2989 return log($n)/log(10);
b76cc8ba 2990 }
2b5ab1e7
TC
2991
2992See also L</exp> for the inverse operation.
a0d0e21e 2993
a0d0e21e 2994=item lstat EXPR
d74e8afc 2995X<lstat>
a0d0e21e 2996
54310121 2997=item lstat
bbce6d69 2998
19799a22 2999Does the same thing as the C<stat> function (including setting the
5a964f20
TC
3000special C<_> filehandle) but stats a symbolic link instead of the file
3001the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
3002your system, a normal C<stat> is done. For much more detailed
3003information, please see the documentation for C<stat>.
a0d0e21e 3004
7660c0ab 3005If EXPR is omitted, stats C<$_>.
bbce6d69 3006
a0d0e21e
LW
3007=item m//
3008
9f4b9cd0 3009The match operator. See L<perlop/"Regexp Quote-Like Operators">.
a0d0e21e
LW
3010
3011=item map BLOCK LIST
d74e8afc 3012X<map>
a0d0e21e
LW
3013
3014=item map EXPR,LIST
3015
19799a22
GS
3016Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3017C<$_> to each element) and returns the list value composed of the
3018results of each such evaluation. In scalar context, returns the
3019total number of elements so generated. Evaluates BLOCK or EXPR in
3020list context, so each element of LIST may produce zero, one, or
3021more elements in the returned value.
dd99ebda 3022
f9476272 3023 @chars = map(chr, @numbers);
a0d0e21e 3024
f9476272
AH
3025translates a list of numbers to the corresponding characters.
3026
3027 my @squares = map { $_ * $_ } @numbers;
3028
3029translates a list of numbers to their squared values.
3030
3031 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3032
3033shows that number of returned elements can differ from the number of
3034input elements. To omit an element, return an empty list ().
3035This could also be achieved by writing
3036
3037 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3038
3039which makes the intention more clear.
3040
3041Map always returns a list which can be assigned to a hash where the elements
3042become key/value pairs. See L<perldata> for more details.
a0d0e21e 3043
d8216f19 3044 %hash = map { get_a_key_for($_) => $_ } @array;
a0d0e21e
LW
3045
3046is just a funny way to write
3047
3048 %hash = ();
d8216f19 3049 foreach (@array) {
a9a5a0dc 3050 $hash{get_a_key_for($_)} = $_;
a0d0e21e
LW
3051 }
3052
be3174d2
GS
3053Note that C<$_> is an alias to the list value, so it can be used to
3054modify the elements of the LIST. While this is useful and supported,
3055it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
3056Using a regular C<foreach> loop for this purpose would be clearer in
3057most cases. See also L</grep> for an array composed of those items of
3058the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 3059
a4fb8298 3060If C<$_> is lexical in the scope where the C<map> appears (because it has
d8216f19
RGS
3061been declared with C<my $_>), then, in addition to being locally aliased to
3062the list elements, C<$_> keeps being lexical inside the block; that is, it
a4fb8298
RGS
3063can't be seen from the outside, avoiding any potential side-effects.
3064
205fdb4d 3065C<{> starts both hash references and blocks, so C<map { ...> could be either
3b10bc60 3066the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
80d38338
TC
3067ahead for the closing C<}> it has to take a guess at which it's dealing with
3068based on what it finds just after the C<{>. Usually it gets it right, but if it
205fdb4d
NC
3069doesn't it won't realize something is wrong until it gets to the C<}> and
3070encounters the missing (or unexpected) comma. The syntax error will be
80d38338 3071reported close to the C<}>, but you'll need to change something near the C<{>
3b10bc60 3072such as using a unary C<+> to give Perl some help:
205fdb4d 3073
3b10bc60 3074 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3075 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3076 %hash = map { ("\L$_" => 1) } @array # this also works
3077 %hash = map { lc($_) => 1 } @array # as does this.
3078 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
cea6626f 3079
3b10bc60 3080 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
205fdb4d 3081
d8216f19 3082or to force an anon hash constructor use C<+{>:
205fdb4d 3083
3b10bc60 3084 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at