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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
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3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
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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20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
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22
23In the syntax descriptions that follow, list operators that expect a
3b10bc60 24list (and provide list context for elements of the list) are shown
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25with LIST as an argument. Such a list may consist of any combination
26of scalar arguments or list values; the list values will be included
27in the list as if each individual element were interpolated at that
28point in the list, forming a longer single-dimensional list value.
8bdbc703 29Commas should separate literal elements of the LIST.
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30
31Any function in the list below may be used either with or without
32parentheses around its arguments. (The syntax descriptions omit the
3b10bc60 33parentheses.) If you use parentheses, the simple but occasionally
34surprising rule is this: It I<looks> like a function, therefore it I<is> a
a0d0e21e 35function, and precedence doesn't matter. Otherwise it's a list
3b10bc60 36operator or unary operator, and precedence does matter. Whitespace
37between the function and left parenthesis doesn't count, so sometimes
38you need to be careful:
a0d0e21e 39
5ed4f2ec 40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
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45
46If you run Perl with the B<-w> switch it can warn you about this. For
47example, the third line above produces:
48
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
51
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52A few functions take no arguments at all, and therefore work as neither
53unary nor list operators. These include such functions as C<time>
54and C<endpwent>. For example, C<time+86_400> always means
55C<time() + 86_400>.
56
a0d0e21e 57For functions that can be used in either a scalar or list context,
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
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75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
3b10bc60 81In general, functions in Perl that serve as wrappers for system calls ("syscalls")
f86cebdf 82of the same name (like chown(2), fork(2), closedir(2), etc.) all 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,
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85which return C<-1> on failure. Exceptions to this rule are C<wait>,
86C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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87variable on failure. Other functions do not, except accidentally.
88
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89Extension modules can also hook into the Perl parser to define new
90kinds of keyword-headed expression. These may look like functions, but
91may also look completely different. The syntax following the keyword
92is defined entirely by the extension. If you are an implementor, see
93L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
94a module, see the module's documentation for details of the syntax that
95it defines.
96
cb1a09d0 97=head2 Perl Functions by Category
d74e8afc 98X<function>
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99
100Here are Perl's functions (including things that look like
5a964f20 101functions, like some keywords and named operators)
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102arranged by category. Some functions appear in more
103than one place.
104
13a2d996 105=over 4
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106
107=item Functions for SCALARs or strings
d74e8afc 108X<scalar> X<string> X<character>
cb1a09d0 109
22fae026 110C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
1dc8ecb8 111C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
945c54fd 112C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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113
114=item Regular expressions and pattern matching
d74e8afc 115X<regular expression> X<regex> X<regexp>
cb1a09d0 116
ab4f32c2 117C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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118
119=item Numeric functions
d74e8afc 120X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 121
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122C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
123C<sin>, C<sqrt>, C<srand>
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124
125=item Functions for real @ARRAYs
d74e8afc 126X<array>
cb1a09d0 127
a5ce339c 128C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
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129
130=item Functions for list data
d74e8afc 131X<list>
cb1a09d0 132
1dc8ecb8 133C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
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134
135=item Functions for real %HASHes
d74e8afc 136X<hash>
cb1a09d0 137
22fae026 138C<delete>, C<each>, C<exists>, C<keys>, C<values>
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139
140=item Input and output functions
d74e8afc 141X<I/O> X<input> X<output> X<dbm>
cb1a09d0 142
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143C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
144C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
0d863452 145C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
22fae026
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146C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
147C<warn>, C<write>
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148
149=item Functions for fixed length data or records
150
22fae026 151C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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152
153=item Functions for filehandles, files, or directories
d74e8afc 154X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 155
22fae026 156C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 157C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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158C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
159C<umask>, C<unlink>, C<utime>
cb1a09d0 160
cf264981 161=item Keywords related to the control flow of your Perl program
d74e8afc 162X<control flow>
cb1a09d0 163
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164C<caller>, C<continue>, C<die>, C<do>, 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.
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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
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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
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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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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
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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
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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
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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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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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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.
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250
251=item Functions obsoleted in perl5
252
22fae026 253C<dbmclose>, C<dbmopen>
37798a01 254
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255=back
256
60f9f73c 257=head2 Portability
d74e8afc 258X<portability> X<Unix> X<portable>
60f9f73c 259
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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
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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
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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>,
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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
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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>
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283
284For more information about the portability of these functions, see
285L<perlport> and other available platform-specific documentation.
286
cb1a09d0
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287=head2 Alphabetical Listing of Perl Functions
288
3b10bc60 289=over
a0d0e21e 290
5b3c99c0 291=item -X FILEHANDLE
d74e8afc
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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
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297=item -X DIRHANDLE
298
5b3c99c0 299=item -X
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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
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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
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344Example:
345
346 while (<>) {
a9a5a0dc
VP
347 chomp;
348 next unless -f $_; # ignore specials
349 #...
a0d0e21e
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350 }
351
5ff3f7a4
GS
352The interpretation of the file permission operators C<-r>, C<-R>,
353C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
354of the file and the uids and gids of the user. There may be other
ecae030f
MO
355reasons you can't actually read, write, or execute the file: for
356example network filesystem access controls, ACLs (access control lists),
357read-only filesystems, and unrecognized executable formats. Note
358that the use of these six specific operators to verify if some operation
359is possible is usually a mistake, because it may be open to race
360conditions.
5ff3f7a4 361
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TC
362Also note that, for the superuser on the local filesystems, the C<-r>,
363C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
5ff3f7a4
GS
364if any execute bit is set in the mode. Scripts run by the superuser
365may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 366or temporarily set their effective uid to something else.
5ff3f7a4
GS
367
368If you are using ACLs, there is a pragma called C<filetest> that may
369produce more accurate results than the bare stat() mode bits.
5ff3f7a4 370When under the C<use filetest 'access'> the above-mentioned filetests
3b10bc60 371test whether the permission can (not) be granted using the
372access(2) family of system calls. Also note that the C<-x> and C<-X> may
5ff3f7a4
GS
373under this pragma return true even if there are no execute permission
374bits set (nor any extra execute permission ACLs). This strangeness is
ecae030f
MO
375due to the underlying system calls' definitions. Note also that, due to
376the implementation of C<use filetest 'access'>, the C<_> special
377filehandle won't cache the results of the file tests when this pragma is
378in effect. Read the documentation for the C<filetest> pragma for more
379information.
5ff3f7a4 380
a0d0e21e 381Note that C<-s/a/b/> does not do a negated substitution. Saying
3b10bc60 382C<-exp($foo)> still works as expected, however: only single letters
a0d0e21e
LW
383following a minus are interpreted as file tests.
384
385The C<-T> and C<-B> switches work as follows. The first block or so of the
386file is examined for odd characters such as strange control codes or
61eff3bc 387characters with the high bit set. If too many strange characters (>30%)
cf264981 388are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
3b10bc60 389containing a zero byte in the first block is considered a binary file. If C<-T>
9124316e 390or C<-B> is used on a filehandle, the current IO buffer is examined
3b10bc60 391rather than the first block. Both C<-T> and C<-B> return true on an empty
54310121 392file, or a file at EOF when testing a filehandle. Because you have to
4633a7c4
LW
393read a file to do the C<-T> test, on most occasions you want to use a C<-f>
394against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 395
19799a22 396If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 397the special filehandle consisting of a solitary underline, then the stat
a0d0e21e
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398structure of the previous file test (or stat operator) is used, saving
399a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 400that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 401symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 402an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 403Example:
a0d0e21e
LW
404
405 print "Can do.\n" if -r $a || -w _ || -x _;
406
407 stat($filename);
408 print "Readable\n" if -r _;
409 print "Writable\n" if -w _;
410 print "Executable\n" if -x _;
411 print "Setuid\n" if -u _;
412 print "Setgid\n" if -g _;
413 print "Sticky\n" if -k _;
414 print "Text\n" if -T _;
415 print "Binary\n" if -B _;
416
fbb0b3b3
RGS
417As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
418test operators, in a way that C<-f -w -x $file> is equivalent to
3b10bc60 419C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
fbb0b3b3
RGS
420the return value of C<-f $file> as an argument to another filetest
421operator, no special magic will happen.)
422
a0d0e21e 423=item abs VALUE
d74e8afc 424X<abs> X<absolute>
a0d0e21e 425
54310121 426=item abs
bbce6d69 427
a0d0e21e 428Returns the absolute value of its argument.
7660c0ab 429If VALUE is omitted, uses C<$_>.
a0d0e21e
LW
430
431=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 432X<accept>
a0d0e21e 433
3b10bc60 434Accepts an incoming socket connect, just as accept(2)
19799a22 435does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 436See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 437
8d2a6795
GS
438On systems that support a close-on-exec flag on files, the flag will
439be set for the newly opened file descriptor, as determined by the
440value of $^F. See L<perlvar/$^F>.
441
a0d0e21e 442=item alarm SECONDS
d74e8afc
ITB
443X<alarm>
444X<SIGALRM>
445X<timer>
a0d0e21e 446
54310121 447=item alarm
bbce6d69 448
a0d0e21e 449Arranges to have a SIGALRM delivered to this process after the
cf264981 450specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
451specified, the value stored in C<$_> is used. (On some machines,
452unfortunately, the elapsed time may be up to one second less or more
453than you specified because of how seconds are counted, and process
454scheduling may delay the delivery of the signal even further.)
455
456Only one timer may be counting at once. Each call disables the
457previous timer, and an argument of C<0> may be supplied to cancel the
458previous timer without starting a new one. The returned value is the
459amount of time remaining on the previous timer.
a0d0e21e 460
2bc69794
BS
461For delays of finer granularity than one second, the Time::HiRes module
462(from CPAN, and starting from Perl 5.8 part of the standard
463distribution) provides ualarm(). You may also use Perl's four-argument
464version of select() leaving the first three arguments undefined, or you
465might be able to use the C<syscall> interface to access setitimer(2) if
466your system supports it. See L<perlfaq8> for details.
2b5ab1e7 467
80d38338
TC
468It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
469C<sleep> may be internally implemented on your system with C<alarm>.
a0d0e21e 470
19799a22
GS
471If you want to use C<alarm> to time out a system call you need to use an
472C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 473fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 474restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 475modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
476
477 eval {
a9a5a0dc
VP
478 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
479 alarm $timeout;
480 $nread = sysread SOCKET, $buffer, $size;
481 alarm 0;
ff68c719 482 };
ff68c719 483 if ($@) {
a9a5a0dc 484 die unless $@ eq "alarm\n"; # propagate unexpected errors
5ed4f2ec 485 # timed out
ff68c719
PP
486 }
487 else {
5ed4f2ec 488 # didn't
ff68c719
PP
489 }
490
91d81acc
JH
491For more information see L<perlipc>.
492
a0d0e21e 493=item atan2 Y,X
d74e8afc 494X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
495
496Returns the arctangent of Y/X in the range -PI to PI.
497
ca6e1c26 498For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
499function, or use the familiar relation:
500
501 sub tan { sin($_[0]) / cos($_[0]) }
502
a1021d57
RGS
503The return value for C<atan2(0,0)> is implementation-defined; consult
504your atan2(3) manpage for more information.
bf5f1b4c 505
a0d0e21e 506=item bind SOCKET,NAME
d74e8afc 507X<bind>
a0d0e21e 508
3b10bc60 509Binds a network address to a socket, just as bind(2)
19799a22 510does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
511packed address of the appropriate type for the socket. See the examples in
512L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 513
fae2c0fb 514=item binmode FILEHANDLE, LAYER
d74e8afc 515X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 516
a0d0e21e
LW
517=item binmode FILEHANDLE
518
1cbfc93d
NIS
519Arranges for FILEHANDLE to be read or written in "binary" or "text"
520mode on systems where the run-time libraries distinguish between
521binary and text files. If FILEHANDLE is an expression, the value is
522taken as the name of the filehandle. Returns true on success,
b5fe5ca2 523otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 524
d807c6f4
JH
525On some systems (in general, DOS and Windows-based systems) binmode()
526is necessary when you're not working with a text file. For the sake
527of portability it is a good idea to always use it when appropriate,
528and to never use it when it isn't appropriate. Also, people can
529set their I/O to be by default UTF-8 encoded Unicode, not bytes.
530
531In other words: regardless of platform, use binmode() on binary data,
532like for example images.
533
534If LAYER is present it is a single string, but may contain multiple
3b10bc60 535directives. The directives alter the behaviour of the filehandle.
920f5fe1 536When LAYER is present using binmode on a text file makes sense.
d807c6f4 537
fae2c0fb 538If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
539suitable for passing binary data. This includes turning off possible CRLF
540translation and marking it as bytes (as opposed to Unicode characters).
749683d2 541Note that, despite what may be implied in I<"Programming Perl"> (the
3b10bc60 542Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
543Other layers that would affect the binary nature of the stream are
544I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
0226bbdb 545PERLIO environment variable.
01e6739c 546
3b10bc60 547The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
d807c6f4
JH
548form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
549establish default I/O layers. See L<open>.
550
fae2c0fb
RGS
551I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
552in "Programming Perl, 3rd Edition". However, since the publishing of this
553book, by many known as "Camel III", the consensus of the naming of this
554functionality has moved from "discipline" to "layer". All documentation
555of this version of Perl therefore refers to "layers" rather than to
556"disciplines". Now back to the regularly scheduled documentation...>
557
6902c96a
T
558To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
559C<:utf8> just marks the data as UTF-8 without further checking,
560while C<:encoding(utf8)> checks the data for actually being valid
561UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 562
ed53a2bb 563In general, binmode() should be called after open() but before any I/O
3b10bc60 564is done on the filehandle. Calling binmode() normally flushes any
01e6739c 565pending buffered output data (and perhaps pending input data) on the
fae2c0fb 566handle. An exception to this is the C<:encoding> layer that
01e6739c 567changes the default character encoding of the handle, see L<open>.
fae2c0fb 568The C<:encoding> layer sometimes needs to be called in
3874323d
JH
569mid-stream, and it doesn't flush the stream. The C<:encoding>
570also implicitly pushes on top of itself the C<:utf8> layer because
3b10bc60 571internally Perl operates on UTF8-encoded Unicode characters.
16fe6d59 572
19799a22 573The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
574system all work together to let the programmer treat a single
575character (C<\n>) as the line terminator, irrespective of the external
576representation. On many operating systems, the native text file
577representation matches the internal representation, but on some
578platforms the external representation of C<\n> is made up of more than
579one character.
580
68bd7414
NIS
581Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
582character to end each line in the external representation of text (even
5e12dbfa 583though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
584on Unix and most VMS files). In other systems like OS/2, DOS and the
585various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
586but what's stored in text files are the two characters C<\cM\cJ>. That
587means that, if you don't use binmode() on these systems, C<\cM\cJ>
588sequences on disk will be converted to C<\n> on input, and any C<\n> in
589your program will be converted back to C<\cM\cJ> on output. This is what
590you want for text files, but it can be disastrous for binary files.
30168b04
GS
591
592Another consequence of using binmode() (on some systems) is that
593special end-of-file markers will be seen as part of the data stream.
594For systems from the Microsoft family this means that if your binary
4375e838 595data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
596the file, unless you use binmode().
597
3b10bc60 598binmode() is important not only for readline() and print() operations,
30168b04
GS
599but also when using read(), seek(), sysread(), syswrite() and tell()
600(see L<perlport> for more details). See the C<$/> and C<$\> variables
601in L<perlvar> for how to manually set your input and output
602line-termination sequences.
a0d0e21e 603
4633a7c4 604=item bless REF,CLASSNAME
d74e8afc 605X<bless>
a0d0e21e
LW
606
607=item bless REF
608
2b5ab1e7
TC
609This function tells the thingy referenced by REF that it is now an object
610in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 611is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 612it returns the reference for convenience. Always use the two-argument
cf264981
SP
613version if a derived class might inherit the function doing the blessing.
614See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
615of objects.
a0d0e21e 616
57668c4d 617Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 618Namespaces with all lowercase names are considered reserved for
cf264981 619Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
620confusion, you may wish to avoid such package names as well. Make sure
621that CLASSNAME is a true value.
60ad88b8
GS
622
623See L<perlmod/"Perl Modules">.
624
0d863452
RH
625=item break
626
627Break out of a C<given()> block.
628
3b10bc60 629This keyword is enabled by the C<"switch"> feature: see L<feature>
0d863452
RH
630for more information.
631
a0d0e21e 632=item caller EXPR
d74e8afc 633X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
634
635=item caller
636
5a964f20 637Returns the context of the current subroutine call. In scalar context,
80d38338
TC
638returns the caller's package name if there I<is> a caller (that is, if
639we're in a subroutine or C<eval> or C<require>) and the undefined value
5a964f20 640otherwise. In list context, returns
a0d0e21e 641
ee6b43cc 642 # 0 1 2
748a9306 643 ($package, $filename, $line) = caller;
a0d0e21e
LW
644
645With EXPR, it returns some extra information that the debugger uses to
646print a stack trace. The value of EXPR indicates how many call frames
647to go back before the current one.
648
ee6b43cc 649 # 0 1 2 3 4
f3aa04c2 650 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 651
652 # 5 6 7 8 9 10
b3ca2e83 653 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 654 = caller($i);
e7ea3e70 655
951ba7fe 656Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 657call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 658C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 659C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 660C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 661$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
662each C<use> statement creates a C<require> frame inside an C<eval EXPR>
663frame.) $subroutine may also be C<(unknown)> if this particular
664subroutine happens to have been deleted from the symbol table.
665C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
666C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
667compiled with. The C<$hints> and C<$bitmask> values are subject to change
668between versions of Perl, and are not meant for external use.
748a9306 669
b3ca2e83
NC
670C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
671caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
672of this hash, as they are the actual values stored in the optree.
673
748a9306 674Furthermore, when called from within the DB package, caller returns more
7660c0ab 675detailed information: it sets the list variable C<@DB::args> to be the
54310121 676arguments with which the subroutine was invoked.
748a9306 677
7660c0ab 678Be aware that the optimizer might have optimized call frames away before
19799a22 679C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 680might not return information about the call frame you expect it to, for
b76cc8ba 681C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 682previous time C<caller> was called.
7660c0ab 683
ca9f0cb5
NC
684Also be aware that setting C<@DB::args> is I<best effort>, intended for
685debugging or generating backtraces, and should not be relied upon. In
686particular, as C<@_> contains aliases to the caller's arguments, Perl does
687not take a copy of C<@_>, so C<@DB::args> will contain modifications the
688subroutine makes to C<@_> or its contents, not the original values at call
689time. C<@DB::args>, like C<@_>, does not hold explicit references to its
690elements, so under certain cases its elements may have become freed and
691reallocated for other variables or temporary values. Finally, a side effect
692of the current implementation means that the effects of C<shift @_> can
693I<normally> be undone (but not C<pop @_> or other splicing, and not if a
694reference to C<@_> has been taken, and subject to the caveat about reallocated
695elements), so C<@DB::args> is actually a hybrid of the current state and
696initial state of C<@_>. Buyer beware.
697
a0d0e21e 698=item chdir EXPR
d74e8afc
ITB
699X<chdir>
700X<cd>
f723aae1 701X<directory, change>
a0d0e21e 702
c4aca7d0
GA
703=item chdir FILEHANDLE
704
705=item chdir DIRHANDLE
706
ce2984c3
PF
707=item chdir
708
ffce7b87 709Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 710changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 711changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0 712variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
80d38338 713neither is set, C<chdir> does nothing. It returns true on success,
b4ad75f0 714false otherwise. See the example under C<die>.
a0d0e21e 715
3b10bc60 716On systems that support fchdir(2), you may pass a filehandle or
717directory handle as argument. On systems that don't support fchdir(2),
718passing handles raises an exception.
c4aca7d0 719
a0d0e21e 720=item chmod LIST
d74e8afc 721X<chmod> X<permission> X<mode>
a0d0e21e
LW
722
723Changes the permissions of a list of files. The first element of the
4633a7c4 724list must be the numerical mode, which should probably be an octal
4ad40acf 725number, and which definitely should I<not> be a string of octal digits:
3b10bc60 726C<0644> is okay, but C<"0644"> is not. Returns the number of files
dc848c6f 727successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e 728
3b10bc60 729 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 730 chmod 0755, @executables;
3b10bc60 731 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 732 # --w----r-T
3b10bc60 733 $mode = "0644"; chmod oct($mode), "foo"; # this is better
734 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 735
3b10bc60 736On systems that support fchmod(2), you may pass filehandles among the
737files. On systems that don't support fchmod(2), passing filehandles raises
738an exception. Filehandles must be passed as globs or glob references to be
739recognized; barewords are considered filenames.
c4aca7d0
GA
740
741 open(my $fh, "<", "foo");
742 my $perm = (stat $fh)[2] & 07777;
743 chmod($perm | 0600, $fh);
744
3b10bc60 745You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
746module:
747
3b10bc60 748 use Fcntl qw( :mode );
ca6e1c26 749 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 750 # Identical to the chmod 0755 of the example above.
ca6e1c26 751
a0d0e21e 752=item chomp VARIABLE
d74e8afc 753X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 754
313c9f5c 755=item chomp( LIST )
a0d0e21e
LW
756
757=item chomp
758
2b5ab1e7
TC
759This safer version of L</chop> removes any trailing string
760that corresponds to the current value of C<$/> (also known as
28757baa
PP
761$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
762number of characters removed from all its arguments. It's often used to
763remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
764that the final record may be missing its newline. When in paragraph
765mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
766When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
767a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 768remove anything.
19799a22 769If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
770
771 while (<>) {
a9a5a0dc
VP
772 chomp; # avoid \n on last field
773 @array = split(/:/);
774 # ...
a0d0e21e
LW
775 }
776
4bf21a6d
RD
777If VARIABLE is a hash, it chomps the hash's values, but not its keys.
778
a0d0e21e
LW
779You can actually chomp anything that's an lvalue, including an assignment:
780
781 chomp($cwd = `pwd`);
782 chomp($answer = <STDIN>);
783
784If you chomp a list, each element is chomped, and the total number of
785characters removed is returned.
786
15e44fd8
RGS
787Note that parentheses are necessary when you're chomping anything
788that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
789is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
790C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
791C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
792as C<chomp($a, $b)>.
793
a0d0e21e 794=item chop VARIABLE
d74e8afc 795X<chop>
a0d0e21e 796
313c9f5c 797=item chop( LIST )
a0d0e21e
LW
798
799=item chop
800
801Chops off the last character of a string and returns the character
5b3eff12 802chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 803scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
804If VARIABLE is a hash, it chops the hash's values, but not its keys.
805
5b3eff12 806You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
807
808If you chop a list, each element is chopped. Only the value of the
19799a22 809last C<chop> is returned.
a0d0e21e 810
19799a22 811Note that C<chop> returns the last character. To return all but the last
748a9306
LW
812character, use C<substr($string, 0, -1)>.
813
15e44fd8
RGS
814See also L</chomp>.
815
a0d0e21e 816=item chown LIST
d74e8afc 817X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
818
819Changes the owner (and group) of a list of files. The first two
19799a22
GS
820elements of the list must be the I<numeric> uid and gid, in that
821order. A value of -1 in either position is interpreted by most
822systems to leave that value unchanged. Returns the number of files
823successfully changed.
a0d0e21e
LW
824
825 $cnt = chown $uid, $gid, 'foo', 'bar';
826 chown $uid, $gid, @filenames;
827
3b10bc60 828On systems that support fchown(2), you may pass filehandles among the
829files. On systems that don't support fchown(2), passing filehandles raises
830an exception. Filehandles must be passed as globs or glob references to be
831recognized; barewords are considered filenames.
c4aca7d0 832
54310121 833Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
834
835 print "User: ";
19799a22 836 chomp($user = <STDIN>);
5a964f20 837 print "Files: ";
19799a22 838 chomp($pattern = <STDIN>);
a0d0e21e
LW
839
840 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 841 or die "$user not in passwd file";
a0d0e21e 842
5ed4f2ec 843 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
844 chown $uid, $gid, @ary;
845
54310121 846On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
847file unless you're the superuser, although you should be able to change
848the group to any of your secondary groups. On insecure systems, these
849restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
850On POSIX systems, you can detect this condition this way:
851
852 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
853 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 854
a0d0e21e 855=item chr NUMBER
d74e8afc 856X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 857
54310121 858=item chr
bbce6d69 859
a0d0e21e 860Returns the character represented by that NUMBER in the character set.
a0ed51b3 861For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 862chr(0x263a) is a Unicode smiley face.
aaa68c4a 863
8a064bd6 864Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 865except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
866(truncated to an integer) are used.
867
974da8e5
JH
868If NUMBER is omitted, uses C<$_>.
869
b76cc8ba 870For the reverse, use L</ord>.
a0d0e21e 871
2575c402
JW
872Note that characters from 128 to 255 (inclusive) are by default
873internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 874
2575c402 875See L<perlunicode> for more about Unicode.
bbce6d69 876
a0d0e21e 877=item chroot FILENAME
d74e8afc 878X<chroot> X<root>
a0d0e21e 879
54310121 880=item chroot
bbce6d69 881
5a964f20 882This function works like the system call by the same name: it makes the
4633a7c4 883named directory the new root directory for all further pathnames that
951ba7fe 884begin with a C</> by your process and all its children. (It doesn't
28757baa 885change your current working directory, which is unaffected.) For security
4633a7c4 886reasons, this call is restricted to the superuser. If FILENAME is
19799a22 887omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
888
889=item close FILEHANDLE
d74e8afc 890X<close>
a0d0e21e 891
6a518fbc
TP
892=item close
893
3b10bc60 894Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26
RGS
895buffers, and closes the system file descriptor. Returns true if those
896operations have succeeded and if no error was reported by any PerlIO
897layer. Closes the currently selected filehandle if the argument is
898omitted.
fb73857a
PP
899
900You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 901another C<open> on it, because C<open> closes it for you. (See
19799a22
GS
902C<open>.) However, an explicit C<close> on an input file resets the line
903counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 904
3b10bc60 905If the filehandle came from a piped open, C<close> returns false if one of
906the other syscalls involved fails or if its program exits with non-zero
907status. If the only problem was that the program exited non-zero, C<$!>
908will be set to C<0>. Closing a pipe also waits for the process executing
909on the pipe to exit--in case you wish to look at the output of the pipe
910afterwards--and implicitly puts the exit status value of that command into
911C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 912
2e0cfa16
FC
913If there are multiple threads running, C<close> on a filehandle from a
914piped open returns true without waiting for the child process to terminate,
915if the filehandle is still open in another thread.
916
80d38338
TC
917Closing the read end of a pipe before the process writing to it at the
918other end is done writing results in the writer receiving a SIGPIPE. If
919the other end can't handle that, be sure to read all the data before
920closing the pipe.
73689b13 921
fb73857a 922Example:
a0d0e21e 923
fb73857a
PP
924 open(OUTPUT, '|sort >foo') # pipe to sort
925 or die "Can't start sort: $!";
5ed4f2ec 926 #... # print stuff to output
927 close OUTPUT # wait for sort to finish
fb73857a
PP
928 or warn $! ? "Error closing sort pipe: $!"
929 : "Exit status $? from sort";
5ed4f2ec 930 open(INPUT, 'foo') # get sort's results
fb73857a 931 or die "Can't open 'foo' for input: $!";
a0d0e21e 932
5a964f20
TC
933FILEHANDLE may be an expression whose value can be used as an indirect
934filehandle, usually the real filehandle name.
a0d0e21e
LW
935
936=item closedir DIRHANDLE
d74e8afc 937X<closedir>
a0d0e21e 938
19799a22 939Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
940system call.
941
a0d0e21e 942=item connect SOCKET,NAME
d74e8afc 943X<connect>
a0d0e21e 944
80d38338
TC
945Attempts to connect to a remote socket, just like connect(2).
946Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
947packed address of the appropriate type for the socket. See the examples in
948L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 949
cb1a09d0 950=item continue BLOCK
d74e8afc 951X<continue>
cb1a09d0 952
0d863452
RH
953=item continue
954
cf264981
SP
955C<continue> is actually a flow control statement rather than a function. If
956there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
957C<foreach>), it is always executed just before the conditional is about to
958be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
959it can be used to increment a loop variable, even when the loop has been
960continued via the C<next> statement (which is similar to the C C<continue>
961statement).
962
98293880 963C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 964block; C<last> and C<redo> behave as if they had been executed within
19799a22 965the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
966block, it may be more entertaining.
967
968 while (EXPR) {
a9a5a0dc
VP
969 ### redo always comes here
970 do_something;
1d2dff63 971 } continue {
a9a5a0dc
VP
972 ### next always comes here
973 do_something_else;
974 # then back the top to re-check EXPR
1d2dff63
GS
975 }
976 ### last always comes here
977
3b10bc60 978Omitting the C<continue> section is equivalent to using an
979empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
980to check the condition at the top of the loop.
981
3b10bc60 982If the C<"switch"> feature is enabled, C<continue> is also a
983function that exits the current C<when> (or C<default>) block and
984falls through to the next one. See L<feature> and
0d863452
RH
985L<perlsyn/"Switch statements"> for more information.
986
987
a0d0e21e 988=item cos EXPR
d74e8afc 989X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 990
d6217f1e
GS
991=item cos
992
5a964f20 993Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 994takes cosine of C<$_>.
a0d0e21e 995
ca6e1c26 996For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
997function, or use this relation:
998
999 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1000
a0d0e21e 1001=item crypt PLAINTEXT,SALT
d74e8afc 1002X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 1003X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1004
ef2e6798
MS
1005Creates a digest string exactly like the crypt(3) function in the C
1006library (assuming that you actually have a version there that has not
bb23f8d1 1007been extirpated as a potential munition).
ef2e6798
MS
1008
1009crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
1010into a short string, called a digest, which is returned. The same
1011PLAINTEXT and SALT will always return the same string, but there is no
1012(known) way to get the original PLAINTEXT from the hash. Small
1013changes in the PLAINTEXT or SALT will result in large changes in the
1014digest.
1015
1016There is no decrypt function. This function isn't all that useful for
1017cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1018mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1019primarily used to check if two pieces of text are the same without
1020having to transmit or store the text itself. An example is checking
1021if a correct password is given. The digest of the password is stored,
cf264981 1022not the password itself. The user types in a password that is
ef2e6798
MS
1023crypt()'d with the same salt as the stored digest. If the two digests
1024match the password is correct.
1025
1026When verifying an existing digest string you should use the digest as
1027the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1028to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1029crypt() will hash the new string with the same salt as the digest.
1030This allows your code to work with the standard L<crypt|/crypt> and
1031with more exotic implementations. In other words, do not assume
1032anything about the returned string itself, or how many bytes in the
1033digest matter.
85c16d83
JH
1034
1035Traditionally the result is a string of 13 bytes: two first bytes of
1036the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
bb23f8d1 1037the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1038hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1039and implementations on non-Unix platforms may produce different
ef2e6798 1040strings.
85c16d83
JH
1041
1042When choosing a new salt create a random two character string whose
1043characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1044'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1045characters is just a recommendation; the characters allowed in
1046the salt depend solely on your system's crypt library, and Perl can't
1047restrict what salts C<crypt()> accepts.
e71965be 1048
a0d0e21e 1049Here's an example that makes sure that whoever runs this program knows
cf264981 1050their password:
a0d0e21e
LW
1051
1052 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1053
1054 system "stty -echo";
1055 print "Password: ";
e71965be 1056 chomp($word = <STDIN>);
a0d0e21e
LW
1057 print "\n";
1058 system "stty echo";
1059
e71965be 1060 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1061 die "Sorry...\n";
a0d0e21e 1062 } else {
a9a5a0dc 1063 print "ok\n";
54310121 1064 }
a0d0e21e 1065
9f8f0c9d 1066Of course, typing in your own password to whoever asks you
748a9306 1067for it is unwise.
a0d0e21e 1068
ef2e6798 1069The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1070of data, not least of all because you can't get the information
ef2e6798 1071back. Look at the L<Digest> module for more robust algorithms.
19799a22 1072
f2791508
JH
1073If using crypt() on a Unicode string (which I<potentially> has
1074characters with codepoints above 255), Perl tries to make sense
1075of the situation by trying to downgrade (a copy of the string)
1076the string back to an eight-bit byte string before calling crypt()
1077(on that copy). If that works, good. If not, crypt() dies with
1078C<Wide character in crypt>.
85c16d83 1079
aa689395 1080=item dbmclose HASH
d74e8afc 1081X<dbmclose>
a0d0e21e 1082
19799a22 1083[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1084
aa689395 1085Breaks the binding between a DBM file and a hash.
a0d0e21e 1086
19799a22 1087=item dbmopen HASH,DBNAME,MASK
d74e8afc 1088X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1089
19799a22 1090[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1091
7b8d334a 1092This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1093hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1094argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1095is the name of the database (without the F<.dir> or F<.pag> extension if
1096any). If the database does not exist, it is created with protection
19799a22 1097specified by MASK (as modified by the C<umask>). If your system supports
80d38338 1098only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1099program. In older versions of Perl, if your system had neither DBM nor
19799a22 1100ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1101sdbm(3).
1102
1103If you don't have write access to the DBM file, you can only read hash
1104variables, not set them. If you want to test whether you can write,
3b10bc60 1105either use file tests or try setting a dummy hash entry inside an C<eval>
1106to trap the error.
a0d0e21e 1107
19799a22
GS
1108Note that functions such as C<keys> and C<values> may return huge lists
1109when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1110function to iterate over large DBM files. Example:
1111
1112 # print out history file offsets
1113 dbmopen(%HIST,'/usr/lib/news/history',0666);
1114 while (($key,$val) = each %HIST) {
a9a5a0dc 1115 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1116 }
1117 dbmclose(%HIST);
1118
cb1a09d0 1119See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1120cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1121rich implementation.
4633a7c4 1122
2b5ab1e7
TC
1123You can control which DBM library you use by loading that library
1124before you call dbmopen():
1125
1126 use DB_File;
1127 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1128 or die "Can't open netscape history file: $!";
2b5ab1e7 1129
a0d0e21e 1130=item defined EXPR
d74e8afc 1131X<defined> X<undef> X<undefined>
a0d0e21e 1132
54310121 1133=item defined
bbce6d69 1134
2f9daede 1135Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1136the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1137checked.
1138
1139Many operations return C<undef> to indicate failure, end of file,
1140system error, uninitialized variable, and other exceptional
1141conditions. This function allows you to distinguish C<undef> from
1142other values. (A simple Boolean test will not distinguish among
7660c0ab 1143C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1144false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1145doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1146returns C<undef> when its argument is an empty array, I<or> when the
1147element to return happens to be C<undef>.
1148
f10b0346
GS
1149You may also use C<defined(&func)> to check whether subroutine C<&func>
1150has ever been defined. The return value is unaffected by any forward
80d38338 1151declarations of C<&func>. A subroutine that is not defined
847c7ebe 1152may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1153makes it spring into existence the first time that it is called; see
847c7ebe 1154L<perlsub>.
f10b0346
GS
1155
1156Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1157used to report whether memory for that aggregate has ever been
1158allocated. This behavior may disappear in future versions of Perl.
1159You should instead use a simple test for size:
1160
1161 if (@an_array) { print "has array elements\n" }
1162 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1163
1164When used on a hash element, it tells you whether the value is defined,
dc848c6f 1165not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1166purpose.
a0d0e21e
LW
1167
1168Examples:
1169
1170 print if defined $switch{'D'};
1171 print "$val\n" while defined($val = pop(@ary));
1172 die "Can't readlink $sym: $!"
a9a5a0dc 1173 unless defined($value = readlink $sym);
a0d0e21e 1174 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1175 $debugging = 0 unless defined $debugging;
a0d0e21e 1176
19799a22 1177Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1178discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1179defined values. For example, if you say
a5f75d66
AD
1180
1181 "ab" =~ /a(.*)b/;
1182
80d38338 1183The pattern match succeeds and C<$1> is defined, although it
cf264981 1184matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1185matched something that happened to be zero characters long. This is all
a5f75d66 1186very above-board and honest. When a function returns an undefined value,
2f9daede 1187it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1188should use C<defined> only when questioning the integrity of what
7660c0ab 1189you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1190what you want.
1191
dc848c6f 1192See also L</undef>, L</exists>, L</ref>.
2f9daede 1193
a0d0e21e 1194=item delete EXPR
d74e8afc 1195X<delete>
a0d0e21e 1196
d0a76353
RS
1197Given an expression that specifies an element or slice of a hash, C<delete>
1198deletes the specified elements from that hash so that exists() on that element
1199no longer returns true. Setting a hash element to the undefined value does
1200not remove its key, but deleting it does; see L</exists>.
80d38338 1201
d0a76353 1202It returns the value or values deleted in list context, or the last such
80d38338 1203element in scalar context. The return list's length always matches that of
d0a76353
RS
1204the argument list: deleting non-existent elements returns the undefined value
1205in their corresponding positions.
80d38338 1206
d0a76353
RS
1207delete() may also be used on arrays and array slices, but its behavior is less
1208straightforward. Although exists() will return false for deleted entries,
1209deleting array elements never changes indices of existing values; use shift()
1210or splice() for that. However, if all deleted elements fall at the end of an
1211array, the array's size shrinks to the position of the highest element that
1212still tests true for exists(), or to 0 if none do.
1213
1214B<Be aware> that calling delete on array values is deprecated and likely to
1215be removed in a future version of Perl.
80d38338
TC
1216
1217Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1218a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1219or array may not necessarily return anything; it depends on the implementation
1220of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1221
80d38338
TC
1222The C<delete local EXPR> construct localizes the deletion to the current
1223block at run time. Until the block exits, elements locally deleted
1224temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1225of composite types">.
eba0920a
EM
1226
1227 %hash = (foo => 11, bar => 22, baz => 33);
1228 $scalar = delete $hash{foo}; # $scalar is 11
1229 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1230 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1231
01020589 1232The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1233
5f05dabc 1234 foreach $key (keys %HASH) {
a9a5a0dc 1235 delete $HASH{$key};
a0d0e21e
LW
1236 }
1237
01020589 1238 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1239 delete $ARRAY[$index];
01020589
GS
1240 }
1241
1242And so do these:
5f05dabc 1243
01020589
GS
1244 delete @HASH{keys %HASH};
1245
9740c838 1246 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1247
80d38338
TC
1248But both are slower than assigning the empty list
1249or undefining %HASH or @ARRAY, which is the customary
1250way to empty out an aggregate:
01020589 1251
5ed4f2ec 1252 %HASH = (); # completely empty %HASH
1253 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1254
5ed4f2ec 1255 @ARRAY = (); # completely empty @ARRAY
1256 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1257
80d38338
TC
1258The EXPR can be arbitrarily complicated provided its
1259final operation is an element or slice of an aggregate:
a0d0e21e
LW
1260
1261 delete $ref->[$x][$y]{$key};
5f05dabc 1262 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1263
01020589
GS
1264 delete $ref->[$x][$y][$index];
1265 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1266
a0d0e21e 1267=item die LIST
d74e8afc 1268X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1269
4c050ad5
NC
1270C<die> raises an exception. Inside an C<eval> the error message is stuffed
1271into C<$@> and the C<eval> is terminated with the undefined value.
1272If the exception is outside of all enclosing C<eval>s, then the uncaught
1273exception prints LIST to C<STDERR> and exits with a non-zero value. If you
1274need to exit the process with a specific exit code, see L<exit>.
a0d0e21e
LW
1275
1276Equivalent examples:
1277
1278 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1279 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1280
ccac6780 1281If the last element of LIST does not end in a newline, the current
df37ec69
WW
1282script line number and input line number (if any) are also printed,
1283and a newline is supplied. Note that the "input line number" (also
1284known as "chunk") is subject to whatever notion of "line" happens to
1285be currently in effect, and is also available as the special variable
1286C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1287
1288Hint: sometimes appending C<", stopped"> to your message will cause it
1289to make better sense when the string C<"at foo line 123"> is appended.
1290Suppose you are running script "canasta".
a0d0e21e
LW
1291
1292 die "/etc/games is no good";
1293 die "/etc/games is no good, stopped";
1294
1295produce, respectively
1296
1297 /etc/games is no good at canasta line 123.
1298 /etc/games is no good, stopped at canasta line 123.
1299
a96d0188 1300If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1301previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1302This is useful for propagating exceptions:
1303
1304 eval { ... };
1305 die unless $@ =~ /Expected exception/;
1306
a96d0188 1307If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1308C<PROPAGATE> method, that method will be called with additional file
1309and line number parameters. The return value replaces the value in
80d38338 1310C<$@>. i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1311were called.
1312
7660c0ab 1313If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1314
4c050ad5
NC
1315If an uncaught exception results in interpreter exit, the exit code is
1316determined from the values of C<$!> and C<$?> with this pseudocode:
1317
1318 exit $! if $!; # errno
1319 exit $? >> 8 if $? >> 8; # child exit status
1320 exit 255; # last resort
1321
1322The intent is to squeeze as much possible information about the likely cause
1323into the limited space of the system exit code. However, as C<$!> is the value
1324of C's C<errno>, which can be set by any system call, this means that the value
1325of the exit code used by C<die> can be non-predictable, so should not be relied
1326upon, other than to be non-zero.
1327
80d38338
TC
1328You can also call C<die> with a reference argument, and if this is trapped
1329within an C<eval>, C<$@> contains that reference. This permits more
1330elaborate exception handling using objects that maintain arbitrary state
1331about the exception. Such a scheme is sometimes preferable to matching
1332particular string values of C<$@> with regular expressions. Because C<$@>
1333is a global variable and C<eval> may be used within object implementations,
1334be careful that analyzing the error object doesn't replace the reference in
1335the global variable. It's easiest to make a local copy of the reference
1336before any manipulations. Here's an example:
52531d10 1337
80d38338 1338 use Scalar::Util "blessed";
da279afe 1339
52531d10 1340 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1341 if (my $ev_err = $@) {
1342 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1343 # handle Some::Module::Exception
1344 }
1345 else {
1346 # handle all other possible exceptions
1347 }
1348 }
1349
3b10bc60 1350Because Perl stringifies uncaught exception messages before display,
80d38338 1351you'll probably want to overload stringification operations on
52531d10
GS
1352exception objects. See L<overload> for details about that.
1353
19799a22
GS
1354You can arrange for a callback to be run just before the C<die>
1355does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1356handler is called with the error text and can change the error
19799a22
GS
1357message, if it sees fit, by calling C<die> again. See
1358L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1359L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1360to be run only right before your program was to exit, this is not
3b10bc60 1361currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1362even inside eval()ed blocks/strings! If one wants the hook to do
1363nothing in such situations, put
fb73857a 1364
5ed4f2ec 1365 die @_ if $^S;
fb73857a 1366
19799a22
GS
1367as the first line of the handler (see L<perlvar/$^S>). Because
1368this promotes strange action at a distance, this counterintuitive
b76cc8ba 1369behavior may be fixed in a future release.
774d564b 1370
4c050ad5
NC
1371See also exit(), warn(), and the Carp module.
1372
a0d0e21e 1373=item do BLOCK
d74e8afc 1374X<do> X<block>
a0d0e21e
LW
1375
1376Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1377sequence of commands indicated by BLOCK. When modified by the C<while> or
1378C<until> loop modifier, executes the BLOCK once before testing the loop
1379condition. (On other statements the loop modifiers test the conditional
1380first.)
a0d0e21e 1381
4968c1e4 1382C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1383C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1384See L<perlsyn> for alternative strategies.
4968c1e4 1385
a0d0e21e 1386=item do SUBROUTINE(LIST)
d74e8afc 1387X<do>
a0d0e21e 1388
cf264981 1389This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1390
1391=item do EXPR
d74e8afc 1392X<do>
a0d0e21e
LW
1393
1394Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1395file as a Perl script.
a0d0e21e
LW
1396
1397 do 'stat.pl';
1398
1399is just like
1400
986b19de 1401 eval `cat stat.pl`;
a0d0e21e 1402
2b5ab1e7 1403except that it's more efficient and concise, keeps track of the current
ea63ef19 1404filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1405C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1406variables. It also differs in that code evaluated with C<do FILENAME>
1407cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1408same, however, in that it does reparse the file every time you call it,
1409so you probably don't want to do this inside a loop.
a0d0e21e 1410
8e30cc93 1411If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1412error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1413returns undef and sets an error message in C<$@>. If the file is
1414successfully compiled, C<do> returns the value of the last expression
1415evaluated.
1416
80d38338 1417Inclusion of library modules is better done with the
19799a22 1418C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1419and raise an exception if there's a problem.
a0d0e21e 1420
5a964f20
TC
1421You might like to use C<do> to read in a program configuration
1422file. Manual error checking can be done this way:
1423
b76cc8ba 1424 # read in config files: system first, then user
f86cebdf 1425 for $file ("/share/prog/defaults.rc",
b76cc8ba 1426 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1427 {
1428 unless ($return = do $file) {
1429 warn "couldn't parse $file: $@" if $@;
1430 warn "couldn't do $file: $!" unless defined $return;
1431 warn "couldn't run $file" unless $return;
1432 }
5a964f20
TC
1433 }
1434
a0d0e21e 1435=item dump LABEL
d74e8afc 1436X<dump> X<core> X<undump>
a0d0e21e 1437
1614b0e3
JD
1438=item dump
1439
19799a22
GS
1440This function causes an immediate core dump. See also the B<-u>
1441command-line switch in L<perlrun>, which does the same thing.
1442Primarily this is so that you can use the B<undump> program (not
1443supplied) to turn your core dump into an executable binary after
1444having initialized all your variables at the beginning of the
1445program. When the new binary is executed it will begin by executing
1446a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1447Think of it as a goto with an intervening core dump and reincarnation.
1448If C<LABEL> is omitted, restarts the program from the top.
1449
1450B<WARNING>: Any files opened at the time of the dump will I<not>
1451be open any more when the program is reincarnated, with possible
80d38338 1452resulting confusion by Perl.
19799a22 1453
59f521f4
RGS
1454This function is now largely obsolete, mostly because it's very hard to
1455convert a core file into an executable. That's why you should now invoke
1456it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1457typo.
19799a22 1458
cba5a3b0 1459=item each HASH (or HASHREF)
d74e8afc 1460X<each> X<hash, iterator>
aa689395 1461
cba5a3b0 1462=item each ARRAY (or ARRAYREF)
aeedbbed
NC
1463X<array, iterator>
1464
80d38338
TC
1465When called in list context, returns a 2-element list consisting of the key
1466and value for the next element of a hash, or the index and value for the
1467next element of an array, so that you can iterate over it. When called in
1468scalar context, returns only the key (not the value) in a hash, or the index
1469in an array.
2f9daede 1470
aeedbbed 1471Hash entries are returned in an apparently random order. The actual random
3b10bc60 1472order is subject to change in future versions of Perl, but it is
504f80c1 1473guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1474function would produce on the same (unmodified) hash. Since Perl
22883ac5 14755.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1476for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1477
80d38338
TC
1478After C<each> has returned all entries from the hash or array, the next
1479call to C<each> returns the empty list in list context and C<undef> in
1480scalar context. The next call following that one restarts iteration. Each
1481hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1482and C<values>. The iterator is implicitly reset when C<each> has reached
1483the end as just described; it can be explicitly reset by calling C<keys> or
1484C<values> on the hash or array. If you add or delete a hash's elements
1485while iterating over it, entries may be skipped or duplicated--so don't do
1486that. Exception: It is always safe to delete the item most recently
3b10bc60 1487returned by C<each()>, so the following code works properly:
74fc8b5f
MJD
1488
1489 while (($key, $value) = each %hash) {
1490 print $key, "\n";
1491 delete $hash{$key}; # This is safe
1492 }
aa689395 1493
80d38338 1494This prints out your environment like the printenv(1) program,
3b10bc60 1495but in a different order:
a0d0e21e
LW
1496
1497 while (($key,$value) = each %ENV) {
a9a5a0dc 1498 print "$key=$value\n";
a0d0e21e
LW
1499 }
1500
cba5a3b0
DG
1501When given a reference to a hash or array, the argument will be
1502dereferenced automatically.
1503
1504 while (($key,$value) = each $hashref) { ... }
1505
1506If the reference is a blessed object that overrides either C<%{}> or
1507C<@{}>, the override will be used instead of dereferencing the underlying
1508variable type. If both overrides are provided, C<%{}> will be the default.
1509If this is not desired, you must dereference the argument yourself.
1510
19799a22 1511See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1512
1513=item eof FILEHANDLE
d74e8afc
ITB
1514X<eof>
1515X<end of file>
1516X<end-of-file>
a0d0e21e 1517
4633a7c4
LW
1518=item eof ()
1519
a0d0e21e
LW
1520=item eof
1521
1522Returns 1 if the next read on FILEHANDLE will return end of file, or if
1523FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1524gives the real filehandle. (Note that this function actually
80d38338 1525reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1526interactive context.) Do not read from a terminal file (or call
19799a22 1527C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1528as terminals may lose the end-of-file condition if you do.
1529
820475bd 1530An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1531with empty parentheses is different. It refers to the pseudo file
820475bd 1532formed from the files listed on the command line and accessed via the
61eff3bc
JH
1533C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1534as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1535used will cause C<@ARGV> to be examined to determine if input is
67408cae 1536available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1537end-of-file will assume you are processing another C<@ARGV> list,
1538and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1539see L<perlop/"I/O Operators">.
820475bd 1540
61eff3bc 1541In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
3b10bc60 1542detect the end of each file, C<eof()> will detect the end of only the
820475bd 1543last file. Examples:
a0d0e21e 1544
748a9306
LW
1545 # reset line numbering on each input file
1546 while (<>) {
a9a5a0dc
VP
1547 next if /^\s*#/; # skip comments
1548 print "$.\t$_";
5a964f20 1549 } continue {
a9a5a0dc 1550 close ARGV if eof; # Not eof()!
748a9306
LW
1551 }
1552
a0d0e21e
LW
1553 # insert dashes just before last line of last file
1554 while (<>) {
a9a5a0dc
VP
1555 if (eof()) { # check for end of last file
1556 print "--------------\n";
1557 }
1558 print;
1559 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1560 }
1561
a0d0e21e 1562Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1563input operators typically return C<undef> when they run out of data, or if
1564there was an error.
a0d0e21e
LW
1565
1566=item eval EXPR
d74e8afc 1567X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1568X<error, handling> X<exception, handling>
a0d0e21e
LW
1569
1570=item eval BLOCK
1571
ce2984c3
PF
1572=item eval
1573
c7cc6f1c
GS
1574In the first form, the return value of EXPR is parsed and executed as if it
1575were a little Perl program. The value of the expression (which is itself
5a964f20 1576determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1577errors, executed in the lexical context of the current Perl program, so
1578that any variable settings or subroutine and format definitions remain
cf264981 1579afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1580If EXPR is omitted, evaluates C<$_>. This form is typically used to
1581delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1582
1583In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1584same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1585within the context of the current Perl program. This form is typically
1586used to trap exceptions more efficiently than the first (see below), while
1587also providing the benefit of checking the code within BLOCK at compile
1588time.
1589
1590The final semicolon, if any, may be omitted from the value of EXPR or within
1591the BLOCK.
1592
1593In both forms, the value returned is the value of the last expression
5a964f20 1594evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1595as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1596in void, scalar, or list context, depending on the context of the C<eval>
1597itself. See L</wantarray> for more on how the evaluation context can be
1598determined.
a0d0e21e 1599
19799a22 1600If there is a syntax error or runtime error, or a C<die> statement is
bbead3ca
BL
1601executed, C<eval> returns an undefined value in scalar context
1602or an empty list in list context, and C<$@> is set to the
3b10bc60 1603error message. If there was no error, C<$@> is guaranteed to be the empty
1604string. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1605warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1606To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1607turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1608See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1609
19799a22
GS
1610Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1611determining whether a particular feature (such as C<socket> or C<symlink>)
82bcec1b 1612is implemented. It is also Perl's exception-trapping mechanism, where
a0d0e21e
LW
1613the die operator is used to raise exceptions.
1614
5f1da31c
NT
1615If you want to trap errors when loading an XS module, some problems with
1616the binary interface (such as Perl version skew) may be fatal even with
1617C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1618
a0d0e21e
LW
1619If the code to be executed doesn't vary, you may use the eval-BLOCK
1620form to trap run-time errors without incurring the penalty of
1621recompiling each time. The error, if any, is still returned in C<$@>.
1622Examples:
1623
54310121 1624 # make divide-by-zero nonfatal
a0d0e21e
LW
1625 eval { $answer = $a / $b; }; warn $@ if $@;
1626
1627 # same thing, but less efficient
1628 eval '$answer = $a / $b'; warn $@ if $@;
1629
1630 # a compile-time error
5ed4f2ec 1631 eval { $answer = }; # WRONG
a0d0e21e
LW
1632
1633 # a run-time error
5ed4f2ec 1634 eval '$answer ='; # sets $@
a0d0e21e 1635
cf264981
SP
1636Using the C<eval{}> form as an exception trap in libraries does have some
1637issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1638may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1639You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1640as this example shows:
774d564b 1641
80d38338 1642 # a private exception trap for divide-by-zero
f86cebdf
GS
1643 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1644 warn $@ if $@;
774d564b
PP
1645
1646This is especially significant, given that C<__DIE__> hooks can call
19799a22 1647C<die> again, which has the effect of changing their error messages:
774d564b
PP
1648
1649 # __DIE__ hooks may modify error messages
1650 {
f86cebdf
GS
1651 local $SIG{'__DIE__'} =
1652 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1653 eval { die "foo lives here" };
1654 print $@ if $@; # prints "bar lives here"
774d564b
PP
1655 }
1656
19799a22 1657Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1658may be fixed in a future release.
1659
19799a22 1660With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1661being looked at when:
1662
5ed4f2ec 1663 eval $x; # CASE 1
1664 eval "$x"; # CASE 2
a0d0e21e 1665
5ed4f2ec 1666 eval '$x'; # CASE 3
1667 eval { $x }; # CASE 4
a0d0e21e 1668
5ed4f2ec 1669 eval "\$$x++"; # CASE 5
1670 $$x++; # CASE 6
a0d0e21e 1671
2f9daede 1672Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1673the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1674the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1675and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1676does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1677purely visual reasons, but it also has the advantage of compiling at
1678compile-time instead of at run-time.) Case 5 is a place where
19799a22 1679normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1680particular situation, you can just use symbolic references instead, as
1681in case 6.
a0d0e21e 1682
b208c909
FC
1683Before Perl 5.14, the assignment to C<$@> occured before restoration of localised variables, which means that, if your code is to run on older
1684versions, a temporary is required if you want to mask some but not all
8a5a710d
DN
1685errors:
1686
1687 # alter $@ on nefarious repugnancy only
1688 {
1689 my $e;
1690 {
1691 local $@; # protect existing $@
1692 eval { test_repugnancy() };
b208c909 1693 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
8a5a710d
DN
1694 $@ =~ /nefarious/ and $e = $@;
1695 }
1696 die $e if defined $e
1697 }
1698
4968c1e4 1699C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1700C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1701
3b10bc60 1702An C<eval ''> executed within the C<DB> package doesn't see the usual
1703surrounding lexical scope, but rather the scope of the first non-DB piece
1704of code that called it. You don't normally need to worry about this unless
1705you are writing a Perl debugger.
d819b83a 1706
a0d0e21e 1707=item exec LIST
d74e8afc 1708X<exec> X<execute>
a0d0e21e 1709
8bf3b016
GS
1710=item exec PROGRAM LIST
1711
3b10bc60 1712The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1713use C<system> instead of C<exec> if you want it to return. It fails and
1714returns false only if the command does not exist I<and> it is executed
fb73857a 1715directly instead of via your system's command shell (see below).
a0d0e21e 1716
19799a22 1717Since it's a common mistake to use C<exec> instead of C<system>, Perl
80d38338 1718warns you if there is a following statement that isn't C<die>, C<warn>,
3b10bc60 1719or C<exit> (if C<-w> is set--but you always do that, right?). If you
19799a22 1720I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1721can use one of these styles to avoid the warning:
1722
5a964f20
TC
1723 exec ('foo') or print STDERR "couldn't exec foo: $!";
1724 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1725
5a964f20 1726If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1727with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1728If there is only one scalar argument or an array with one element in it,
1729the argument is checked for shell metacharacters, and if there are any,
1730the entire argument is passed to the system's command shell for parsing
1731(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1732If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1733words and passed directly to C<execvp>, which is more efficient.
19799a22 1734Examples:
a0d0e21e 1735
19799a22
GS
1736 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1737 exec "sort $outfile | uniq";
a0d0e21e
LW
1738
1739If you don't really want to execute the first argument, but want to lie
1740to the program you are executing about its own name, you can specify
1741the program you actually want to run as an "indirect object" (without a
1742comma) in front of the LIST. (This always forces interpretation of the
54310121 1743LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1744the list.) Example:
1745
1746 $shell = '/bin/csh';
5ed4f2ec 1747 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1748
1749or, more directly,
1750
5ed4f2ec 1751 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1752
3b10bc60 1753When the arguments get executed via the system shell, results are
1754subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1755for details.
1756
19799a22
GS
1757Using an indirect object with C<exec> or C<system> is also more
1758secure. This usage (which also works fine with system()) forces
1759interpretation of the arguments as a multivalued list, even if the
1760list had just one argument. That way you're safe from the shell
1761expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1762
1763 @args = ( "echo surprise" );
1764
2b5ab1e7 1765 exec @args; # subject to shell escapes
f86cebdf 1766 # if @args == 1
2b5ab1e7 1767 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1768
1769The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1770program, passing it C<"surprise"> an argument. The second version didn't;
1771it tried to run a program named I<"echo surprise">, didn't find it, and set
1772C<$?> to a non-zero value indicating failure.
5a964f20 1773
80d38338 1774Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1775output before the exec, but this may not be supported on some platforms
1776(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1777in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 1778open handles to avoid lost output.
0f897271 1779
80d38338
TC
1780Note that C<exec> will not call your C<END> blocks, nor will it invoke
1781C<DESTROY> methods on your objects.
7660c0ab 1782
a0d0e21e 1783=item exists EXPR
d74e8afc 1784X<exists> X<autovivification>
a0d0e21e 1785
d0a76353
RS
1786Given an expression that specifies an element of a hash, returns true if the
1787specified element in the hash has ever been initialized, even if the
1788corresponding value is undefined.
a0d0e21e 1789
5ed4f2ec 1790 print "Exists\n" if exists $hash{$key};
1791 print "Defined\n" if defined $hash{$key};
01020589
GS
1792 print "True\n" if $hash{$key};
1793
d0a76353
RS
1794exists may also be called on array elements, but its behavior is much less
1795obvious, and is strongly tied to the use of L</delete> on arrays. B<Be aware>
1796that calling exists on array values is deprecated and likely to be removed in
1797a future version of Perl.
1798
5ed4f2ec 1799 print "Exists\n" if exists $array[$index];
1800 print "Defined\n" if defined $array[$index];
01020589 1801 print "True\n" if $array[$index];
a0d0e21e 1802
8ea97a1e 1803A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1804it exists, but the reverse doesn't necessarily hold true.
1805
afebc493
GS
1806Given an expression that specifies the name of a subroutine,
1807returns true if the specified subroutine has ever been declared, even
1808if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 1809does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
1810exist may still be callable: its package may have an C<AUTOLOAD>
1811method that makes it spring into existence the first time that it is
3b10bc60 1812called; see L<perlsub>.
afebc493 1813
5ed4f2ec 1814 print "Exists\n" if exists &subroutine;
1815 print "Defined\n" if defined &subroutine;
afebc493 1816
a0d0e21e 1817Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1818operation is a hash or array key lookup or subroutine name:
a0d0e21e 1819
5ed4f2ec 1820 if (exists $ref->{A}->{B}->{$key}) { }
1821 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 1822
5ed4f2ec 1823 if (exists $ref->{A}->{B}->[$ix]) { }
1824 if (exists $hash{A}{B}[$ix]) { }
01020589 1825
afebc493
GS
1826 if (exists &{$ref->{A}{B}{$key}}) { }
1827
3b10bc60 1828Although the mostly deeply nested array or hash will not spring into
1829existence just because its existence was tested, any intervening ones will.
61eff3bc 1830Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 1831into existence due to the existence test for the $key element above.
3b10bc60 1832This happens anywhere the arrow operator is used, including even here:
5a964f20 1833
2b5ab1e7 1834 undef $ref;
5ed4f2ec 1835 if (exists $ref->{"Some key"}) { }
1836 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
1837
1838This surprising autovivification in what does not at first--or even
1839second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1840release.
a0d0e21e 1841
afebc493
GS
1842Use of a subroutine call, rather than a subroutine name, as an argument
1843to exists() is an error.
1844
5ed4f2ec 1845 exists &sub; # OK
1846 exists &sub(); # Error
afebc493 1847
a0d0e21e 1848=item exit EXPR
d74e8afc 1849X<exit> X<terminate> X<abort>
a0d0e21e 1850
ce2984c3
PF
1851=item exit
1852
2b5ab1e7 1853Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1854
1855 $ans = <STDIN>;
1856 exit 0 if $ans =~ /^[Xx]/;
1857
19799a22 1858See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1859universally recognized values for EXPR are C<0> for success and C<1>
1860for error; other values are subject to interpretation depending on the
1861environment in which the Perl program is running. For example, exiting
186269 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1863the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1864
19799a22
GS
1865Don't use C<exit> to abort a subroutine if there's any chance that
1866someone might want to trap whatever error happened. Use C<die> instead,
1867which can be trapped by an C<eval>.
28757baa 1868
19799a22 1869The exit() function does not always exit immediately. It calls any
2b5ab1e7 1870defined C<END> routines first, but these C<END> routines may not
19799a22 1871themselves abort the exit. Likewise any object destructors that need to
60275626
FC
1872be called are called before the real exit. C<END> routines and destructors
1873can change the exit status by modifying C<$?>. If this is a problem, you
2b5ab1e7 1874can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1875See L<perlmod> for details.
5a964f20 1876
a0d0e21e 1877=item exp EXPR
d74e8afc 1878X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1879
54310121 1880=item exp
bbce6d69 1881
b76cc8ba 1882Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1883If EXPR is omitted, gives C<exp($_)>.
1884
1885=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1886X<fcntl>
a0d0e21e 1887
f86cebdf 1888Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1889
1890 use Fcntl;
1891
0ade1984 1892first to get the correct constant definitions. Argument processing and
3b10bc60 1893value returned work just like C<ioctl> below.
a0d0e21e
LW
1894For example:
1895
1896 use Fcntl;
5a964f20 1897 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 1898 or die "can't fcntl F_GETFL: $!";
5a964f20 1899
554ad1fc 1900You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1901Like C<ioctl>, it maps a C<0> return from the system call into
1902C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1903in numeric context. It is also exempt from the normal B<-w> warnings
1904on improper numeric conversions.
5a964f20 1905
3b10bc60 1906Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
1907doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1908manpage to learn what functions are available on your system.
a0d0e21e 1909
be2f7487 1910Here's an example of setting a filehandle named C<REMOTE> to be
1911non-blocking at the system level. You'll have to negotiate C<$|>
1912on your own, though.
1913
1914 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1915
1916 $flags = fcntl(REMOTE, F_GETFL, 0)
1917 or die "Can't get flags for the socket: $!\n";
1918
1919 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1920 or die "Can't set flags for the socket: $!\n";
1921
a0d0e21e 1922=item fileno FILEHANDLE
d74e8afc 1923X<fileno>
a0d0e21e 1924
2b5ab1e7
TC
1925Returns the file descriptor for a filehandle, or undefined if the
1926filehandle is not open. This is mainly useful for constructing
19799a22 1927bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1928If FILEHANDLE is an expression, the value is taken as an indirect
1929filehandle, generally its name.
5a964f20 1930
b76cc8ba 1931You can use this to find out whether two handles refer to the
5a964f20
TC
1932same underlying descriptor:
1933
1934 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 1935 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1936 }
1937
1938(Filehandles connected to memory objects via new features of C<open> may
1939return undefined even though they are open.)
1940
a0d0e21e
LW
1941
1942=item flock FILEHANDLE,OPERATION
d74e8afc 1943X<flock> X<lock> X<locking>
a0d0e21e 1944
19799a22
GS
1945Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1946for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1947machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1948C<flock> is Perl's portable file locking interface, although it locks
3b10bc60 1949entire files only, not records.
2b5ab1e7
TC
1950
1951Two potentially non-obvious but traditional C<flock> semantics are
1952that it waits indefinitely until the lock is granted, and that its locks
1953B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1954fewer guarantees. This means that programs that do not also use C<flock>
1955may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1956your port's specific documentation, or your system-specific local manpages
1957for details. It's best to assume traditional behavior if you're writing
1958portable programs. (But if you're not, you should as always feel perfectly
1959free to write for your own system's idiosyncrasies (sometimes called
1960"features"). Slavish adherence to portability concerns shouldn't get
1961in the way of your getting your job done.)
a3cb178b 1962
8ebc5c01
PP
1963OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1964LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1965you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1966either individually, or as a group using the ':flock' tag. LOCK_SH
1967requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 1968releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
3b10bc60 1969LOCK_SH or LOCK_EX then C<flock> returns immediately rather than blocking
1970waiting for the lock; check the return status to see if you got it.
68dc0745 1971
2b5ab1e7
TC
1972To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1973before locking or unlocking it.
8ebc5c01 1974
f86cebdf 1975Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1976locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1977are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1978implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1979differing semantics shouldn't bite too many people.
1980
becacb53
TM
1981Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1982be open with read intent to use LOCK_SH and requires that it be open
1983with write intent to use LOCK_EX.
1984
19799a22
GS
1985Note also that some versions of C<flock> cannot lock things over the
1986network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1987that. If you like you can force Perl to ignore your system's flock(2)
1988function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 1989the switch C<-Ud_flock> to the F<Configure> program when you configure
3b10bc60 1990Perl.
4633a7c4
LW
1991
1992Here's a mailbox appender for BSD systems.
a0d0e21e 1993
7ed5353d 1994 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
1995
1996 sub lock {
a9a5a0dc
VP
1997 my ($fh) = @_;
1998 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 1999
a9a5a0dc
VP
2000 # and, in case someone appended while we were waiting...
2001 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
2002 }
2003
2004 sub unlock {
a9a5a0dc
VP
2005 my ($fh) = @_;
2006 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
2007 }
2008
b0169937 2009 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 2010 or die "Can't open mailbox: $!";
a0d0e21e 2011
7ed5353d 2012 lock($mbox);
b0169937 2013 print $mbox $msg,"\n\n";
7ed5353d 2014 unlock($mbox);
a0d0e21e 2015
3b10bc60 2016On systems that support a real flock(2), locks are inherited across fork()
2017calls, whereas those that must resort to the more capricious fcntl(2)
2018function lose their locks, making it seriously harder to write servers.
2b5ab1e7 2019
cb1a09d0 2020See also L<DB_File> for other flock() examples.
a0d0e21e
LW
2021
2022=item fork
d74e8afc 2023X<fork> X<child> X<parent>
a0d0e21e 2024
2b5ab1e7
TC
2025Does a fork(2) system call to create a new process running the
2026same program at the same point. It returns the child pid to the
2027parent process, C<0> to the child process, or C<undef> if the fork is
2028unsuccessful. File descriptors (and sometimes locks on those descriptors)
2029are shared, while everything else is copied. On most systems supporting
2030fork(), great care has gone into making it extremely efficient (for
2031example, using copy-on-write technology on data pages), making it the
2032dominant paradigm for multitasking over the last few decades.
5a964f20 2033
80d38338 2034Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
2035output before forking the child process, but this may not be supported
2036on some platforms (see L<perlport>). To be safe, you may need to set
2037C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2038C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2039
19799a22 2040If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2041accumulate zombies. On some systems, you can avoid this by setting
2042C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2043forking and reaping moribund children.
cb1a09d0 2044
28757baa
PP
2045Note that if your forked child inherits system file descriptors like
2046STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2047if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2048backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2049You should reopen those to F</dev/null> if it's any issue.
28757baa 2050
cb1a09d0 2051=item format
d74e8afc 2052X<format>
cb1a09d0 2053
19799a22 2054Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2055example:
2056
54310121 2057 format Something =
a9a5a0dc
VP
2058 Test: @<<<<<<<< @||||| @>>>>>
2059 $str, $%, '$' . int($num)
cb1a09d0
AD
2060 .
2061
2062 $str = "widget";
184e9718 2063 $num = $cost/$quantity;
cb1a09d0
AD
2064 $~ = 'Something';
2065 write;
2066
2067See L<perlform> for many details and examples.
2068
8903cb82 2069=item formline PICTURE,LIST
d74e8afc 2070X<formline>
a0d0e21e 2071
5a964f20 2072This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2073too. It formats (see L<perlform>) a list of values according to the
2074contents of PICTURE, placing the output into the format output
7660c0ab 2075accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2076Eventually, when a C<write> is done, the contents of
cf264981
SP
2077C<$^A> are written to some filehandle. You could also read C<$^A>
2078and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2079does one C<formline> per line of form, but the C<formline> function itself
748a9306 2080doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2081that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2082You may therefore need to use multiple formlines to implement a single
3b10bc60 2083record format, just like the C<format> compiler.
748a9306 2084
19799a22 2085Be careful if you put double quotes around the picture, because an C<@>
748a9306 2086character may be taken to mean the beginning of an array name.
19799a22 2087C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
2088
2089=item getc FILEHANDLE
f723aae1 2090X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2091
2092=item getc
2093
2094Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2095or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2096the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2097STDIN. This is not particularly efficient. However, it cannot be
2098used by itself to fetch single characters without waiting for the user
2099to hit enter. For that, try something more like:
4633a7c4
LW
2100
2101 if ($BSD_STYLE) {
a9a5a0dc 2102 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2103 }
2104 else {
a9a5a0dc 2105 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2106 }
2107
2108 $key = getc(STDIN);
2109
2110 if ($BSD_STYLE) {
a9a5a0dc 2111 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2112 }
2113 else {
3b10bc60 2114 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2115 }
2116 print "\n";
2117
54310121
PP
2118Determination of whether $BSD_STYLE should be set
2119is left as an exercise to the reader.
cb1a09d0 2120
19799a22 2121The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2122systems purporting POSIX compliance. See also the C<Term::ReadKey>
2123module from your nearest CPAN site; details on CPAN can be found on
2124L<perlmodlib/CPAN>.
a0d0e21e
LW
2125
2126=item getlogin
d74e8afc 2127X<getlogin> X<login>
a0d0e21e 2128
cf264981 2129This implements the C library function of the same name, which on most
3b10bc60 2130systems returns the current login from F</etc/utmp>, if any. If it
2131returns the empty string, use C<getpwuid>.
a0d0e21e 2132
f86702cc 2133 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2134
19799a22
GS
2135Do not consider C<getlogin> for authentication: it is not as
2136secure as C<getpwuid>.
4633a7c4 2137
a0d0e21e 2138=item getpeername SOCKET
d74e8afc 2139X<getpeername> X<peer>
a0d0e21e
LW
2140
2141Returns the packed sockaddr address of other end of the SOCKET connection.
2142
4633a7c4
LW
2143 use Socket;
2144 $hersockaddr = getpeername(SOCK);
19799a22 2145 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2146 $herhostname = gethostbyaddr($iaddr, AF_INET);
2147 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2148
2149=item getpgrp PID
d74e8afc 2150X<getpgrp> X<group>
a0d0e21e 2151
47e29363 2152Returns the current process group for the specified PID. Use
7660c0ab 2153a PID of C<0> to get the current process group for the
4633a7c4 2154current process. Will raise an exception if used on a machine that
f86cebdf 2155doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2156group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2157does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2158
2159=item getppid
d74e8afc 2160X<getppid> X<parent> X<pid>
a0d0e21e
LW
2161
2162Returns the process id of the parent process.
2163
4d76a344
RGS
2164Note for Linux users: on Linux, the C functions C<getpid()> and
2165C<getppid()> return different values from different threads. In order to
3b10bc60 2166be portable, this behavior is not reflected by the Perl-level function
4d76a344 2167C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2168to call the underlying C<getppid()>, you may use the CPAN module
2169C<Linux::Pid>.
4d76a344 2170
a0d0e21e 2171=item getpriority WHICH,WHO
d74e8afc 2172X<getpriority> X<priority> X<nice>
a0d0e21e 2173
4633a7c4 2174Returns the current priority for a process, a process group, or a user.
f4084e39 2175(See C<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2176machine that doesn't implement getpriority(2).
a0d0e21e
LW
2177
2178=item getpwnam NAME
d74e8afc
ITB
2179X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2180X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2181X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2182X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2183X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2184X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2185
2186=item getgrnam NAME
2187
2188=item gethostbyname NAME
2189
2190=item getnetbyname NAME
2191
2192=item getprotobyname NAME
2193
2194=item getpwuid UID
2195
2196=item getgrgid GID
2197
2198=item getservbyname NAME,PROTO
2199
2200=item gethostbyaddr ADDR,ADDRTYPE
2201
2202=item getnetbyaddr ADDR,ADDRTYPE
2203
2204=item getprotobynumber NUMBER
2205
2206=item getservbyport PORT,PROTO
2207
2208=item getpwent
2209
2210=item getgrent
2211
2212=item gethostent
2213
2214=item getnetent
2215
2216=item getprotoent
2217
2218=item getservent
2219
2220=item setpwent
2221
2222=item setgrent
2223
2224=item sethostent STAYOPEN
2225
2226=item setnetent STAYOPEN
2227
2228=item setprotoent STAYOPEN
2229
2230=item setservent STAYOPEN
2231
2232=item endpwent
2233
2234=item endgrent
2235
2236=item endhostent
2237
2238=item endnetent
2239
2240=item endprotoent
2241
2242=item endservent
2243
80d38338
TC
2244These routines are the same as their counterparts in the
2245system C library. In list context, the return values from the
a0d0e21e
LW
2246various get routines are as follows:
2247
2248 ($name,$passwd,$uid,$gid,
6ee623d5 2249 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2250 ($name,$passwd,$gid,$members) = getgr*
2251 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2252 ($name,$aliases,$addrtype,$net) = getnet*
2253 ($name,$aliases,$proto) = getproto*
2254 ($name,$aliases,$port,$proto) = getserv*
2255
3b10bc60 2256(If the entry doesn't exist you get an empty list.)
a0d0e21e 2257
4602f195
JH
2258The exact meaning of the $gcos field varies but it usually contains
2259the real name of the user (as opposed to the login name) and other
2260information pertaining to the user. Beware, however, that in many
2261system users are able to change this information and therefore it
106325ad 2262cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2263L<perlsec>). The $passwd and $shell, user's encrypted password and
2264login shell, are also tainted, because of the same reason.
4602f195 2265
5a964f20 2266In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2267lookup by name, in which case you get the other thing, whatever it is.
2268(If the entry doesn't exist you get the undefined value.) For example:
2269
5a964f20
TC
2270 $uid = getpwnam($name);
2271 $name = getpwuid($num);
2272 $name = getpwent();
2273 $gid = getgrnam($name);
08a33e13 2274 $name = getgrgid($num);
5a964f20
TC
2275 $name = getgrent();
2276 #etc.
a0d0e21e 2277
4602f195 2278In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2279in that they are unsupported on many systems. If the
4602f195
JH
2280$quota is unsupported, it is an empty scalar. If it is supported, it
2281usually encodes the disk quota. If the $comment field is unsupported,
2282it is an empty scalar. If it is supported it usually encodes some
2283administrative comment about the user. In some systems the $quota
2284field may be $change or $age, fields that have to do with password
2285aging. In some systems the $comment field may be $class. The $expire
2286field, if present, encodes the expiration period of the account or the
2287password. For the availability and the exact meaning of these fields
2288in your system, please consult your getpwnam(3) documentation and your
2289F<pwd.h> file. You can also find out from within Perl what your
2290$quota and $comment fields mean and whether you have the $expire field
2291by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2292C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2293files are supported only if your vendor has implemented them in the
4602f195 2294intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2295shadow versions if you're running under privilege or if there exists
cf264981
SP
2296the shadow(3) functions as found in System V (this includes Solaris
2297and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2298facility are unlikely to be supported.
6ee623d5 2299
19799a22 2300The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2301the login names of the members of the group.
2302
2303For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2304C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2305C<@addrs> value returned by a successful call is a list of raw
2306addresses returned by the corresponding library call. In the
2307Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2308by saying something like:
2309
f337b084 2310 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2311
2b5ab1e7
TC
2312The Socket library makes this slightly easier:
2313
2314 use Socket;
2315 $iaddr = inet_aton("127.1"); # or whatever address
2316 $name = gethostbyaddr($iaddr, AF_INET);
2317
2318 # or going the other way
19799a22 2319 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2320
d760c846
GS
2321In the opposite way, to resolve a hostname to the IP address
2322you can write this:
2323
2324 use Socket;
2325 $packed_ip = gethostbyname("www.perl.org");
2326 if (defined $packed_ip) {
2327 $ip_address = inet_ntoa($packed_ip);
2328 }
2329
2330Make sure <gethostbyname()> is called in SCALAR context and that
2331its return value is checked for definedness.
2332
19799a22
GS
2333If you get tired of remembering which element of the return list
2334contains which return value, by-name interfaces are provided
2335in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2336C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2337and C<User::grent>. These override the normal built-ins, supplying
2338versions that return objects with the appropriate names
2339for each field. For example:
5a964f20
TC
2340
2341 use File::stat;
2342 use User::pwent;
2343 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2344
b76cc8ba
NIS
2345Even though it looks like they're the same method calls (uid),
2346they aren't, because a C<File::stat> object is different from
19799a22 2347a C<User::pwent> object.
5a964f20 2348
a0d0e21e 2349=item getsockname SOCKET
d74e8afc 2350X<getsockname>
a0d0e21e 2351
19799a22
GS
2352Returns the packed sockaddr address of this end of the SOCKET connection,
2353in case you don't know the address because you have several different
2354IPs that the connection might have come in on.
a0d0e21e 2355
4633a7c4
LW
2356 use Socket;
2357 $mysockaddr = getsockname(SOCK);
19799a22 2358 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2359 printf "Connect to %s [%s]\n",
19799a22
GS
2360 scalar gethostbyaddr($myaddr, AF_INET),
2361 inet_ntoa($myaddr);
a0d0e21e
LW
2362
2363=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2364X<getsockopt>
a0d0e21e 2365
636e6b1f
TH
2366Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2367Options may exist at multiple protocol levels depending on the socket
2368type, but at least the uppermost socket level SOL_SOCKET (defined in the
2369C<Socket> module) will exist. To query options at another level the
2370protocol number of the appropriate protocol controlling the option
2371should be supplied. For example, to indicate that an option is to be
2372interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2373number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2374
80d38338 2375The function returns a packed string representing the requested socket
3b10bc60 2376option, or C<undef> on error, with the reason for the error placed in
2377C<$!>). Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2378consult getsockopt(2) for details. A common case is that the option is an
2379integer, in which case the result is a packed integer, which you can decode
2380using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2381
80d38338 2382An example to test whether Nagle's algorithm is turned on on a socket:
636e6b1f 2383
4852725b 2384 use Socket qw(:all);
636e6b1f
TH
2385
2386 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2387 or die "Could not determine the protocol number for tcp";
4852725b
DD
2388 # my $tcp = IPPROTO_TCP; # Alternative
2389 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2390 or die "getsockopt TCP_NODELAY: $!";
636e6b1f
TH
2391 my $nodelay = unpack("I", $packed);
2392 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2393
a0d0e21e
LW
2394
2395=item glob EXPR
d74e8afc 2396X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2397
0a753a76
PP
2398=item glob
2399
d9a9d457
JL
2400In list context, returns a (possibly empty) list of filename expansions on
2401the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2402scalar context, glob iterates through such filename expansions, returning
2403undef when the list is exhausted. This is the internal function
2404implementing the C<< <*.c> >> operator, but you can use it directly. If
2405EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2406more detail in L<perlop/"I/O Operators">.
a0d0e21e 2407
80d38338
TC
2408Note that C<glob> splits its arguments on whitespace and treats
2409each segment as separate pattern. As such, C<glob("*.c *.h")>
2410matches all files with a F<.c> or F<.h> extension. The expression
b474a1b1 2411C<glob(".* *")> matches all files in the current working directory.
80d38338
TC
2412
2413If non-empty braces are the only wildcard characters used in the
2414C<glob>, no filenames are matched, but potentially many strings
2415are returned. For example, this produces nine strings, one for
2416each pairing of fruits and colors:
2417
2418 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2419
3a4b19e4 2420Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2421C<File::Glob> extension. See L<File::Glob> for details, including
2422C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2423
a0d0e21e 2424=item gmtime EXPR
d74e8afc 2425X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2426
ce2984c3
PF
2427=item gmtime
2428
435fbc73
GS
2429Works just like L<localtime> but the returned values are
2430localized for the standard Greenwich time zone.
a0d0e21e 2431
435fbc73
GS
2432Note: when called in list context, $isdst, the last value
2433returned by gmtime is always C<0>. There is no
2434Daylight Saving Time in GMT.
0a753a76 2435
62aa5637
MS
2436See L<perlport/gmtime> for portability concerns.
2437
a0d0e21e 2438=item goto LABEL
d74e8afc 2439X<goto> X<jump> X<jmp>
a0d0e21e 2440
748a9306
LW
2441=item goto EXPR
2442
a0d0e21e
LW
2443=item goto &NAME
2444
b500e03b
GG
2445The C<goto-LABEL> form finds the statement labeled with LABEL and
2446resumes execution there. It can't be used to get out of a block or
2447subroutine given to C<sort>. It can be used to go almost anywhere
2448else within the dynamic scope, including out of subroutines, but it's
2449usually better to use some other construct such as C<last> or C<die>.
2450The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2451(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2452does not offer named loops combined with loop control. Perl does, and
2453this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2454
7660c0ab
A
2455The C<goto-EXPR> form expects a label name, whose scope will be resolved
2456dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2457necessarily recommended if you're optimizing for maintainability:
2458
2459 goto ("FOO", "BAR", "GLARCH")[$i];
2460
b500e03b 2461Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2462deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2463go into any construct that requires initialization, such as a
2464subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2465construct that is optimized away.
b500e03b 2466
1b6921cb
BT
2467The C<goto-&NAME> form is quite different from the other forms of
2468C<goto>. In fact, it isn't a goto in the normal sense at all, and
2469doesn't have the stigma associated with other gotos. Instead, it
2470exits the current subroutine (losing any changes set by local()) and
2471immediately calls in its place the named subroutine using the current
2472value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2473load another subroutine and then pretend that the other subroutine had
2474been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2475in the current subroutine are propagated to the other subroutine.)
2476After the C<goto>, not even C<caller> will be able to tell that this
2477routine was called first.
2478
2479NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2480containing a code reference, or a block that evaluates to a code
6cb9131c 2481reference.
a0d0e21e
LW
2482
2483=item grep BLOCK LIST
d74e8afc 2484X<grep>
a0d0e21e
LW
2485
2486=item grep EXPR,LIST
2487
2b5ab1e7
TC
2488This is similar in spirit to, but not the same as, grep(1) and its
2489relatives. In particular, it is not limited to using regular expressions.
2f9daede 2490
a0d0e21e 2491Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2492C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2493elements for which the expression evaluated to true. In scalar
2494context, returns the number of times the expression was true.
a0d0e21e
LW
2495
2496 @foo = grep(!/^#/, @bar); # weed out comments
2497
2498or equivalently,
2499
2500 @foo = grep {!/^#/} @bar; # weed out comments
2501
be3174d2
GS
2502Note that C<$_> is an alias to the list value, so it can be used to
2503modify the elements of the LIST. While this is useful and supported,
2504it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2505Similarly, grep returns aliases into the original list, much as a for
2506loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2507element of a list returned by grep (for example, in a C<foreach>, C<map>
2508or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2509This is usually something to be avoided when writing clear code.
a0d0e21e 2510
a4fb8298 2511If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2512been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2513the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2514can't be seen from the outside, avoiding any potential side-effects.
2515
19799a22 2516See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2517
a0d0e21e 2518=item hex EXPR
d74e8afc 2519X<hex> X<hexadecimal>
a0d0e21e 2520
54310121 2521=item hex
bbce6d69 2522
2b5ab1e7 2523Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2524(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2525L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2526
2527 print hex '0xAf'; # prints '175'
2528 print hex 'aF'; # same
a0d0e21e 2529
19799a22 2530Hex strings may only represent integers. Strings that would cause
53305cf1 2531integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2532unlike oct(). To present something as hex, look into L</printf>,
2533L</sprintf>, or L</unpack>.
19799a22 2534
ce2984c3 2535=item import LIST
d74e8afc 2536X<import>
a0d0e21e 2537
19799a22 2538There is no builtin C<import> function. It is just an ordinary
4633a7c4 2539method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2540names to another module. The C<use> function calls the C<import> method
cea6626f 2541for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2542
2543=item index STR,SUBSTR,POSITION
d74e8afc 2544X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2545
2546=item index STR,SUBSTR
2547
2b5ab1e7
TC
2548The index function searches for one string within another, but without
2549the wildcard-like behavior of a full regular-expression pattern match.
2550It returns the position of the first occurrence of SUBSTR in STR at
2551or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2552beginning of the string. POSITION before the beginning of the string
2553or after its end is treated as if it were the beginning or the end,
2554respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2555you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2556is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2557
2558=item int EXPR
f723aae1 2559X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2560
54310121 2561=item int
bbce6d69 2562
7660c0ab 2563Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 2564You should not use this function for rounding: one because it truncates
3b10bc60 2565towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
2566numbers can sometimes produce counterintuitive results. For example,
2567C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2568because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2569the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2570functions will serve you better than will int().
a0d0e21e
LW
2571
2572=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2573X<ioctl>
a0d0e21e 2574
2b5ab1e7 2575Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2576
5ed4f2ec 2577 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2578
a11c483f 2579to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2580exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2581own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2582(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2583may help you in this, but it's nontrivial.) SCALAR will be read and/or
3b10bc60 2584written depending on the FUNCTION; a C pointer to the string value of SCALAR
19799a22 2585will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2586has no string value but does have a numeric value, that value will be
2587passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2588true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2589functions may be needed to manipulate the values of structures used by
b76cc8ba 2590C<ioctl>.
a0d0e21e 2591
19799a22 2592The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e 2593
5ed4f2ec 2594 if OS returns: then Perl returns:
2595 -1 undefined value
2596 0 string "0 but true"
2597 anything else that number
a0d0e21e 2598
19799a22 2599Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2600still easily determine the actual value returned by the operating
2601system:
2602
2b5ab1e7 2603 $retval = ioctl(...) || -1;
a0d0e21e
LW
2604 printf "System returned %d\n", $retval;
2605
be2f7487 2606The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2607about improper numeric conversions.
2608
a0d0e21e 2609=item join EXPR,LIST
d74e8afc 2610X<join>
a0d0e21e 2611
2b5ab1e7
TC
2612Joins the separate strings of LIST into a single string with fields
2613separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2614
2b5ab1e7 2615 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2616
eb6e2d6f
GS
2617Beware that unlike C<split>, C<join> doesn't take a pattern as its
2618first argument. Compare L</split>.
a0d0e21e 2619
cba5a3b0 2620=item keys HASH (or HASHREF)
d74e8afc 2621X<keys> X<key>
aa689395 2622
cba5a3b0 2623=item keys ARRAY (or ARRAYREF)
aeedbbed
NC
2624
2625Returns a list consisting of all the keys of the named hash, or the indices
2626of an array. (In scalar context, returns the number of keys or indices.)
504f80c1 2627
aeedbbed 2628The keys of a hash are returned in an apparently random order. The actual
3b10bc60 2629random order is subject to change in future versions of Perl, but it
504f80c1 2630is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6 2631function produces (given that the hash has not been modified). Since
c5f61d2f 2632Perl 5.8.1 the ordering can be different even between different runs of
4546b9e6 2633Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2634Attacks">).
504f80c1 2635
aeedbbed 2636As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
cf264981
SP
2637(see L</each>). In particular, calling keys() in void context resets
2638the iterator with no other overhead.
a0d0e21e 2639
aa689395 2640Here is yet another way to print your environment:
a0d0e21e
LW
2641
2642 @keys = keys %ENV;
2643 @values = values %ENV;
b76cc8ba 2644 while (@keys) {
a9a5a0dc 2645 print pop(@keys), '=', pop(@values), "\n";
a0d0e21e
LW
2646 }
2647
2648or how about sorted by key:
2649
2650 foreach $key (sort(keys %ENV)) {
a9a5a0dc 2651 print $key, '=', $ENV{$key}, "\n";
a0d0e21e
LW
2652 }
2653
8ea1e5d4
GS
2654The returned values are copies of the original keys in the hash, so
2655modifying them will not affect the original hash. Compare L</values>.
2656
19799a22 2657To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2658Here's a descending numeric sort of a hash by its values:
4633a7c4 2659
5a964f20 2660 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
a9a5a0dc 2661 printf "%4d %s\n", $hash{$key}, $key;
4633a7c4
LW
2662 }
2663
3b10bc60 2664Used as an lvalue, C<keys> allows you to increase the number of hash buckets
aa689395
PP
2665allocated for the given hash. This can gain you a measure of efficiency if
2666you know the hash is going to get big. (This is similar to pre-extending
2667an array by assigning a larger number to $#array.) If you say
55497cff
PP
2668
2669 keys %hash = 200;
2670
ab192400
GS
2671then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2672in fact, since it rounds up to the next power of two. These
55497cff
PP
2673buckets will be retained even if you do C<%hash = ()>, use C<undef
2674%hash> if you want to free the storage while C<%hash> is still in scope.
2675You can't shrink the number of buckets allocated for the hash using
19799a22 2676C<keys> in this way (but you needn't worry about doing this by accident,
aeedbbed
NC
2677as trying has no effect). C<keys @array> in an lvalue context is a syntax
2678error.
55497cff 2679
cba5a3b0
DG
2680When given a reference to a hash or array, the argument will be
2681dereferenced automatically.
2682
2683 for (keys $hashref) { ... }
2684 for (keys $obj->get_arrayref) { ... }
2685
2686If the reference is a blessed object that overrides either C<%{}> or
2687C<@{}>, the override will be used instead of dereferencing the underlying
2688variable type. If both overrides are provided, C<%{}> will be the default.
2689If this is not desired, you must dereference the argument yourself.
2690
19799a22 2691See also C<each>, C<values> and C<sort>.
ab192400 2692
b350dd2f 2693=item kill SIGNAL, LIST
d74e8afc 2694X<kill> X<signal>
a0d0e21e 2695
b350dd2f 2696Sends a signal to a list of processes. Returns the number of
517db077
GS
2697processes successfully signaled (which is not necessarily the
2698same as the number actually killed).
a0d0e21e
LW
2699
2700 $cnt = kill 1, $child1, $child2;
2701 kill 9, @goners;
2702
3b10bc60 2703If SIGNAL is zero, no signal is sent to the process, but C<kill>
2704checks whether it's I<possible> to send a signal to it (that
70fb64f6 2705means, to be brief, that the process is owned by the same user, or we are
3b10bc60 2706the super-user). This is useful to check that a child process is still
81fd35db
DN
2707alive (even if only as a zombie) and hasn't changed its UID. See
2708L<perlport> for notes on the portability of this construct.
b350dd2f 2709
e2c0f81f
DG
2710Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2711of processes. That means you usually want to use positive not negative signals.
2712You may also use a signal name in quotes.
2713
2714The behavior of kill when a I<PROCESS> number is zero or negative depends on
2715the operating system. For example, on POSIX-conforming systems, zero will
2716signal the current process group and -1 will signal all processes.
1e9c1022
JL
2717
2718See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2719
2720=item last LABEL
d74e8afc 2721X<last> X<break>
a0d0e21e
LW
2722
2723=item last
2724
2725The C<last> command is like the C<break> statement in C (as used in
2726loops); it immediately exits the loop in question. If the LABEL is
2727omitted, the command refers to the innermost enclosing loop. The
2728C<continue> block, if any, is not executed:
2729
4633a7c4 2730 LINE: while (<STDIN>) {
a9a5a0dc
VP
2731 last LINE if /^$/; # exit when done with header
2732 #...
a0d0e21e
LW
2733 }
2734
80d38338 2735C<last> cannot be used to exit a block that returns a value such as
2b5ab1e7
TC
2736C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2737a grep() or map() operation.
4968c1e4 2738
6c1372ed
GS
2739Note that a block by itself is semantically identical to a loop
2740that executes once. Thus C<last> can be used to effect an early
2741exit out of such a block.
2742
98293880
JH
2743See also L</continue> for an illustration of how C<last>, C<next>, and
2744C<redo> work.
1d2dff63 2745
a0d0e21e 2746=item lc EXPR
d74e8afc 2747X<lc> X<lowercase>
a0d0e21e 2748
54310121 2749=item lc
bbce6d69 2750
d1be9408 2751Returns a lowercased version of EXPR. This is the internal function
3980dc9c 2752implementing the C<\L> escape in double-quoted strings.
a0d0e21e 2753
7660c0ab 2754If EXPR is omitted, uses C<$_>.
bbce6d69 2755
3980dc9c
KW
2756What gets returned depends on several factors:
2757
2758=over
2759
2760=item If C<use bytes> is in effect:
2761
2762=over
2763
2764=item On EBCDIC platforms
2765
2766The results are what the C language system call C<tolower()> returns.
2767
2768=item On ASCII platforms
2769
2770The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
2771respectively.
2772
2773=back
2774
2775=item Otherwise, If EXPR has the UTF8 flag set
2776
2777If the current package has a subroutine named C<ToLower>, it will be used to
2778change the case (See L<perlunicode/User-Defined Case Mappings>.)
2779Otherwise Unicode semantics are used for the case change.
2780
2781=item Otherwise, if C<use locale> is in effect
2782
2783Respects current LC_CTYPE locale. See L<perllocale>.
2784
2785=item Otherwise, if C<use feature 'unicode_strings'> is in effect:
2786
2787Unicode semantics are used for the case change. Any subroutine named
2788C<ToLower> will not be used.
2789
2790=item Otherwise:
2791
2792=over
2793
2794=item On EBCDIC platforms
2795
2796The results are what the C language system call C<tolower()> returns.
2797
2798=item On ASCII platforms
2799
2800ASCII semantics are used for the case change. The lowercase of any character
2801outside the ASCII range is the character itself.
2802
2803=back
2804
2805=back
2806
a0d0e21e 2807=item lcfirst EXPR
d74e8afc 2808X<lcfirst> X<lowercase>
a0d0e21e 2809
54310121 2810=item lcfirst
bbce6d69 2811
ad0029c4
JH
2812Returns the value of EXPR with the first character lowercased. This
2813is the internal function implementing the C<\l> escape in
3980dc9c 2814double-quoted strings.
a0d0e21e 2815
7660c0ab 2816If EXPR is omitted, uses C<$_>.
bbce6d69 2817
3980dc9c
KW
2818This function behaves the same way under various pragma, such as in a locale,
2819as L</lc> does.
2820
a0d0e21e 2821=item length EXPR
d74e8afc 2822X<length> X<size>
a0d0e21e 2823
54310121 2824=item length
bbce6d69 2825
974da8e5 2826Returns the length in I<characters> of the value of EXPR. If EXPR is
9f621bb0 2827omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
3b10bc60 2828
2829This function cannot be used on an entire array or hash to find out how
2830many elements these have. For that, use C<scalar @array> and C<scalar keys
2831%hash>, respectively.
2832
2833Like all Perl character operations, length() normally deals in logical
2834characters, not physical bytes. For how many bytes a string encoded as
2835UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
2836to C<use Encode> first). See L<Encode> and L<perlunicode>.
974da8e5 2837
a0d0e21e 2838=item link OLDFILE,NEWFILE
d74e8afc 2839X<link>
a0d0e21e 2840
19799a22 2841Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2842success, false otherwise.
a0d0e21e
LW
2843
2844=item listen SOCKET,QUEUESIZE
d74e8afc 2845X<listen>
a0d0e21e 2846
3b10bc60 2847Does the same thing that the listen(2) system call does. Returns true if
b76cc8ba 2848it succeeded, false otherwise. See the example in
cea6626f 2849L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2850
2851=item local EXPR
d74e8afc 2852X<local>
a0d0e21e 2853
19799a22 2854You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2855what most people think of as "local". See
13a2d996 2856L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2857
5a964f20
TC
2858A local modifies the listed variables to be local to the enclosing
2859block, file, or eval. If more than one value is listed, the list must
2860be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2861for details, including issues with tied arrays and hashes.
a0d0e21e 2862
d361fafa
VP
2863The C<delete local EXPR> construct can also be used to localize the deletion
2864of array/hash elements to the current block.
2865See L<perlsub/"Localized deletion of elements of composite types">.
2866
a0d0e21e 2867=item localtime EXPR
435fbc73 2868X<localtime> X<ctime>
a0d0e21e 2869
ba053783
AL
2870=item localtime
2871
19799a22 2872Converts a time as returned by the time function to a 9-element list
5f05dabc 2873with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2874follows:
2875
54310121 2876 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2877 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2878 localtime(time);
a0d0e21e 2879
48a26b3a 2880All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2881tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2882of the specified time.
48a26b3a 2883
ba053783
AL
2884C<$mday> is the day of the month, and C<$mon> is the month itself, in
2885the range C<0..11> with 0 indicating January and 11 indicating December.
2886This makes it easy to get a month name from a list:
54310121 2887
ba053783
AL
2888 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2889 print "$abbr[$mon] $mday";
2890 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2891
ba053783
AL
2892C<$year> is the number of years since 1900, not just the last two digits
2893of the year. That is, C<$year> is C<123> in year 2023. The proper way
80d38338 2894to get a 4-digit year is simply:
abd75f24 2895
ba053783 2896 $year += 1900;
abd75f24 2897
435fbc73
GS
2898Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2899to do that, would you?
2900
ba053783
AL
2901To get the last two digits of the year (e.g., '01' in 2001) do:
2902
2903 $year = sprintf("%02d", $year % 100);
2904
2905C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2906Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2907(or C<0..365> in leap years.)
2908
2909C<$isdst> is true if the specified time occurs during Daylight Saving
2910Time, false otherwise.
abd75f24 2911
e1998452 2912If EXPR is omitted, C<localtime()> uses the current time (as returned
e3176d09 2913by time(3)).
a0d0e21e 2914
48a26b3a 2915In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2916
5f05dabc 2917 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2918
fe86afc2
NC
2919This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2920instead of local time use the L</gmtime> builtin. See also the
2921C<Time::Local> module (to convert the second, minutes, hours, ... back to
2922the integer value returned by time()), and the L<POSIX> module's strftime(3)
2923and mktime(3) functions.
2924
2925To get somewhat similar but locale dependent date strings, set up your
2926locale environment variables appropriately (please see L<perllocale>) and
2927try for example:
a3cb178b 2928
5a964f20 2929 use POSIX qw(strftime);
2b5ab1e7 2930 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2931 # or for GMT formatted appropriately for your locale:
2932 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2933
2934Note that the C<%a> and C<%b>, the short forms of the day of the week
2935and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2936
62aa5637
MS
2937See L<perlport/localtime> for portability concerns.
2938
435fbc73
GS
2939The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2940by-name access mechanism to the gmtime() and localtime() functions,
2941respectively.
2942
2943For a comprehensive date and time representation look at the
2944L<DateTime> module on CPAN.
2945
07698885 2946=item lock THING
d74e8afc 2947X<lock>
19799a22 2948
01e6739c 2949This function places an advisory lock on a shared variable, or referenced
03730085 2950object contained in I<THING> until the lock goes out of scope.
a6d5524e 2951
f3a23afb 2952lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2953by this name (before any calls to it), that function will be called
7b043ca5
RGS
2954instead. If you are not under C<use threads::shared> this does nothing.
2955See L<threads::shared>.
19799a22 2956
a0d0e21e 2957=item log EXPR
d74e8afc 2958X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2959
54310121 2960=item log
bbce6d69 2961
2b5ab1e7
TC
2962Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2963returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2964The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2965divided by the natural log of N. For example:
2966
2967 sub log10 {
a9a5a0dc
VP
2968 my $n = shift;
2969 return log($n)/log(10);
b76cc8ba 2970 }
2b5ab1e7
TC
2971
2972See also L</exp> for the inverse operation.
a0d0e21e 2973
a0d0e21e 2974=item lstat EXPR
d74e8afc 2975X<lstat>
a0d0e21e 2976
54310121 2977=item lstat
bbce6d69 2978
19799a22 2979Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2980special C<_> filehandle) but stats a symbolic link instead of the file
2981the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2982your system, a normal C<stat> is done. For much more detailed
2983information, please see the documentation for C<stat>.
a0d0e21e 2984
7660c0ab 2985If EXPR is omitted, stats C<$_>.
bbce6d69 2986
a0d0e21e
LW
2987=item m//
2988
9f4b9cd0 2989The match operator. See L<perlop/"Regexp Quote-Like Operators">.
a0d0e21e
LW
2990
2991=item map BLOCK LIST
d74e8afc 2992X<map>
a0d0e21e
LW
2993
2994=item map EXPR,LIST
2995
19799a22
GS
2996Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2997C<$_> to each element) and returns the list value composed of the
2998results of each such evaluation. In scalar context, returns the
2999total number of elements so generated. Evaluates BLOCK or EXPR in
3000list context, so each element of LIST may produce zero, one, or
3001more elements in the returned value.
dd99ebda 3002
f9476272 3003 @chars = map(chr, @numbers);
a0d0e21e 3004
f9476272
AH
3005translates a list of numbers to the corresponding characters.
3006
3007 my @squares = map { $_ * $_ } @numbers;
3008
3009translates a list of numbers to their squared values.
3010
3011 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3012
3013shows that number of returned elements can differ from the number of
3014input elements. To omit an element, return an empty list ().
3015This could also be achieved by writing
3016
3017 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3018
3019which makes the intention more clear.
3020
3021Map always returns a list which can be assigned to a hash where the elements
3022become key/value pairs. See L<perldata> for more details.
a0d0e21e 3023
d8216f19 3024 %hash = map { get_a_key_for($_) => $_ } @array;
a0d0e21e
LW
3025
3026is just a funny way to write
3027
3028 %hash = ();
d8216f19 3029 foreach (@array) {
a9a5a0dc 3030 $hash{get_a_key_for($_)} = $_;
a0d0e21e
LW
3031 }
3032
be3174d2
GS
3033Note that C<$_> is an alias to the list value, so it can be used to
3034modify the elements of the LIST. While this is useful and supported,
3035it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
3036Using a regular C<foreach> loop for this purpose would be clearer in
3037most cases. See also L</grep> for an array composed of those items of
3038the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 3039
a4fb8298 3040If C<$_> is lexical in the scope where the C<map> appears (because it has
d8216f19
RGS
3041been declared with C<my $_>), then, in addition to being locally aliased to
3042the list elements, C<$_> keeps being lexical inside the block; that is, it
a4fb8298
RGS
3043can't be seen from the outside, avoiding any potential side-effects.
3044
205fdb4d 3045C<{> starts both hash references and blocks, so C<map { ...> could be either
3b10bc60 3046the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
80d38338
TC
3047ahead for the closing C<}> it has to take a guess at which it's dealing with
3048based on what it finds just after the C<{>. Usually it gets it right, but if it
205fdb4d
NC
3049doesn't it won't realize something is wrong until it gets to the C<}> and
3050encounters the missing (or unexpected) comma. The syntax error will be
80d38338 3051reported close to the C<}>, but you'll need to change something near the C<{>
3b10bc60 3052such as using a unary C<+> to give Perl some help:
205fdb4d 3053
3b10bc60 3054 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3055 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3056 %hash = map { ("\L$_" => 1) } @array # this also works
3057 %hash = map { lc($_) => 1 } @array # as does this.
3058 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
cea6626f 3059
3b10bc60 3060 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
205fdb4d 3061
d8216f19 3062or to force an anon hash constructor use C<+{>:
205fdb4d 3063
3b10bc60 3064 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end
205fdb4d 3065
3b10bc60 3066to get a list of anonymous hashes each with only one entry apiece.
205fdb4d 3067
19799a22 3068=item mkdir FILENAME,MASK
d74e8afc 3069X<mkdir> X<md> X<directory, create>
a0d0e21e 3070
5a211162
GS
3071=item mkdir FILENAME
3072
491873e5
RGS
3073=item mkdir
3074
0591cd52 3075Creates the directory specified by FILENAME, with permissions
19799a22
GS
3076specified by MASK (as modified by C<umask>). If it succeeds it
3077returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
3078If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
3079to C<$_>.
0591cd52 3080
19799a22 3081In general, it is better to create directories with permissive MASK,
0591cd52 3082and let the user modify that with their C<umask>, than it is to supply
19799a22 3083a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
3084The exceptions to this rule are when the file or directory should be
3085kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 3086C<umask> discusses the choice of MASK in more detail.
a0d0e21e 3087
cc1852e8
JH
3088Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3089number of trailing slashes. Some operating and filesystems do not get
3090this right, so Perl automatically removes all trailing slashes to keep
3091everyone happy.