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POD tweak to 'require' in perlfunc relating to putting Perl code in @INC
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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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TC
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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TC
52A few functions take no arguments at all, and therefore work as neither
53unary nor list operators. These include such functions as C<time>
54and C<endpwent>. For example, C<time+86_400> always means
55C<time() + 86_400>.
56
a0d0e21e 57For functions that can be used in either a scalar or list context,
54310121 58nonabortive failure is generally indicated in a scalar context by
a0d0e21e 59returning the undefined value, and in a list context by returning the
3b10bc60 60empty list.
a0d0e21e 61
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62Remember the following important rule: There is B<no rule> that relates
63the behavior of an expression in list context to its behavior in scalar
64context, or vice versa. It might do two totally different things.
80d38338 65Each operator and function decides which sort of value would be most
2b5ab1e7 66appropriate to return in scalar context. Some operators return the
5a964f20 67length of the list that would have been returned in list context. Some
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68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
d74e8afc 72X<context>
a0d0e21e 73
d1be9408 74A named array in scalar context is quite different from what would at
5a964f20
TC
75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
3b10bc60 81In general, functions in Perl that serve as wrappers for system calls ("syscalls")
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
88e1f1a2
JV
89Extension modules can also hook into the Perl parser to define new
90kinds of keyword-headed expression. These may look like functions, but
91may also look completely different. The syntax following the keyword
92is defined entirely by the extension. If you are an implementor, see
93L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
94a module, see the module's documentation for details of the syntax that
95it defines.
96
cb1a09d0 97=head2 Perl Functions by Category
d74e8afc 98X<function>
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99
100Here are Perl's functions (including things that look like
5a964f20 101functions, like some keywords and named operators)
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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//>
cb1a09d0
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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
22fae026 128C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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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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AD
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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TM
143C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
144C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
0d863452 145C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
22fae026
TM
146C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
147C<warn>, C<write>
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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
22fae026
TM
202C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
203C<untie>, C<use>
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204
205=item Low-level socket functions
d74e8afc 206X<socket> X<sock>
cb1a09d0 207
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TM
208C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
209C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 210C<socket>, C<socketpair>
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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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TM
215C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
216C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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217
218=item Fetching user and group info
d74e8afc 219X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 220
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221C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
222C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
223C<getpwuid>, C<setgrent>, C<setpwent>
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224
225=item Fetching network info
d74e8afc 226X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 227
22fae026
TM
228C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
229C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
230C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
231C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
232C<setnetent>, C<setprotoent>, C<setservent>
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233
234=item Time-related functions
d74e8afc 235X<time> X<date>
cb1a09d0 236
22fae026 237C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 238
37798a01 239=item Functions new in perl5
d74e8afc 240X<perl5>
37798a01 241
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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.
37798a01
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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
JH
276C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
277C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
278C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 279C<shmwrite>, C<socket>, C<socketpair>,
80cbd5ad
JH
280C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
281C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 282C<utime>, C<wait>, C<waitpid>
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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
AD
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
a0d0e21e
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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
2b5ab1e7
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
LW
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
80d38338
TC
913Closing the read end of a pipe before the process writing to it at the
914other end is done writing results in the writer receiving a SIGPIPE. If
915the other end can't handle that, be sure to read all the data before
916closing the pipe.
73689b13 917
fb73857a 918Example:
a0d0e21e 919
fb73857a
PP
920 open(OUTPUT, '|sort >foo') # pipe to sort
921 or die "Can't start sort: $!";
5ed4f2ec 922 #... # print stuff to output
923 close OUTPUT # wait for sort to finish
fb73857a
PP
924 or warn $! ? "Error closing sort pipe: $!"
925 : "Exit status $? from sort";
5ed4f2ec 926 open(INPUT, 'foo') # get sort's results
fb73857a 927 or die "Can't open 'foo' for input: $!";
a0d0e21e 928
5a964f20
TC
929FILEHANDLE may be an expression whose value can be used as an indirect
930filehandle, usually the real filehandle name.
a0d0e21e
LW
931
932=item closedir DIRHANDLE
d74e8afc 933X<closedir>
a0d0e21e 934
19799a22 935Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
936system call.
937
a0d0e21e 938=item connect SOCKET,NAME
d74e8afc 939X<connect>
a0d0e21e 940
80d38338
TC
941Attempts to connect to a remote socket, just like connect(2).
942Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
943packed address of the appropriate type for the socket. See the examples in
944L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 945
cb1a09d0 946=item continue BLOCK
d74e8afc 947X<continue>
cb1a09d0 948
0d863452
RH
949=item continue
950
cf264981
SP
951C<continue> is actually a flow control statement rather than a function. If
952there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
953C<foreach>), it is always executed just before the conditional is about to
954be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
955it can be used to increment a loop variable, even when the loop has been
956continued via the C<next> statement (which is similar to the C C<continue>
957statement).
958
98293880 959C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 960block; C<last> and C<redo> behave as if they had been executed within
19799a22 961the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
962block, it may be more entertaining.
963
964 while (EXPR) {
a9a5a0dc
VP
965 ### redo always comes here
966 do_something;
1d2dff63 967 } continue {
a9a5a0dc
VP
968 ### next always comes here
969 do_something_else;
970 # then back the top to re-check EXPR
1d2dff63
GS
971 }
972 ### last always comes here
973
3b10bc60 974Omitting the C<continue> section is equivalent to using an
975empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
976to check the condition at the top of the loop.
977
3b10bc60 978If the C<"switch"> feature is enabled, C<continue> is also a
979function that exits the current C<when> (or C<default>) block and
980falls through to the next one. See L<feature> and
0d863452
RH
981L<perlsyn/"Switch statements"> for more information.
982
983
a0d0e21e 984=item cos EXPR
d74e8afc 985X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 986
d6217f1e
GS
987=item cos
988
5a964f20 989Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 990takes cosine of C<$_>.
a0d0e21e 991
ca6e1c26 992For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
993function, or use this relation:
994
995 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
996
a0d0e21e 997=item crypt PLAINTEXT,SALT
d74e8afc 998X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 999X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 1000
ef2e6798
MS
1001Creates a digest string exactly like the crypt(3) function in the C
1002library (assuming that you actually have a version there that has not
bb23f8d1 1003been extirpated as a potential munition).
ef2e6798
MS
1004
1005crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
1006into a short string, called a digest, which is returned. The same
1007PLAINTEXT and SALT will always return the same string, but there is no
1008(known) way to get the original PLAINTEXT from the hash. Small
1009changes in the PLAINTEXT or SALT will result in large changes in the
1010digest.
1011
1012There is no decrypt function. This function isn't all that useful for
1013cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1014mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1015primarily used to check if two pieces of text are the same without
1016having to transmit or store the text itself. An example is checking
1017if a correct password is given. The digest of the password is stored,
cf264981 1018not the password itself. The user types in a password that is
ef2e6798
MS
1019crypt()'d with the same salt as the stored digest. If the two digests
1020match the password is correct.
1021
1022When verifying an existing digest string you should use the digest as
1023the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1024to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1025crypt() will hash the new string with the same salt as the digest.
1026This allows your code to work with the standard L<crypt|/crypt> and
1027with more exotic implementations. In other words, do not assume
1028anything about the returned string itself, or how many bytes in the
1029digest matter.
85c16d83
JH
1030
1031Traditionally the result is a string of 13 bytes: two first bytes of
1032the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
bb23f8d1 1033the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1034hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1035and implementations on non-Unix platforms may produce different
ef2e6798 1036strings.
85c16d83
JH
1037
1038When choosing a new salt create a random two character string whose
1039characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1040'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1041characters is just a recommendation; the characters allowed in
1042the salt depend solely on your system's crypt library, and Perl can't
1043restrict what salts C<crypt()> accepts.
e71965be 1044
a0d0e21e 1045Here's an example that makes sure that whoever runs this program knows
cf264981 1046their password:
a0d0e21e
LW
1047
1048 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1049
1050 system "stty -echo";
1051 print "Password: ";
e71965be 1052 chomp($word = <STDIN>);
a0d0e21e
LW
1053 print "\n";
1054 system "stty echo";
1055
e71965be 1056 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1057 die "Sorry...\n";
a0d0e21e 1058 } else {
a9a5a0dc 1059 print "ok\n";
54310121 1060 }
a0d0e21e 1061
9f8f0c9d 1062Of course, typing in your own password to whoever asks you
748a9306 1063for it is unwise.
a0d0e21e 1064
ef2e6798 1065The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1066of data, not least of all because you can't get the information
ef2e6798 1067back. Look at the L<Digest> module for more robust algorithms.
19799a22 1068
f2791508
JH
1069If using crypt() on a Unicode string (which I<potentially> has
1070characters with codepoints above 255), Perl tries to make sense
1071of the situation by trying to downgrade (a copy of the string)
1072the string back to an eight-bit byte string before calling crypt()
1073(on that copy). If that works, good. If not, crypt() dies with
1074C<Wide character in crypt>.
85c16d83 1075
aa689395 1076=item dbmclose HASH
d74e8afc 1077X<dbmclose>
a0d0e21e 1078
19799a22 1079[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1080
aa689395 1081Breaks the binding between a DBM file and a hash.
a0d0e21e 1082
19799a22 1083=item dbmopen HASH,DBNAME,MASK
d74e8afc 1084X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1085
19799a22 1086[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1087
7b8d334a 1088This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1089hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1090argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1091is the name of the database (without the F<.dir> or F<.pag> extension if
1092any). If the database does not exist, it is created with protection
19799a22 1093specified by MASK (as modified by the C<umask>). If your system supports
80d38338 1094only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1095program. In older versions of Perl, if your system had neither DBM nor
19799a22 1096ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1097sdbm(3).
1098
1099If you don't have write access to the DBM file, you can only read hash
1100variables, not set them. If you want to test whether you can write,
3b10bc60 1101either use file tests or try setting a dummy hash entry inside an C<eval>
1102to trap the error.
a0d0e21e 1103
19799a22
GS
1104Note that functions such as C<keys> and C<values> may return huge lists
1105when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1106function to iterate over large DBM files. Example:
1107
1108 # print out history file offsets
1109 dbmopen(%HIST,'/usr/lib/news/history',0666);
1110 while (($key,$val) = each %HIST) {
a9a5a0dc 1111 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1112 }
1113 dbmclose(%HIST);
1114
cb1a09d0 1115See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1116cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1117rich implementation.
4633a7c4 1118
2b5ab1e7
TC
1119You can control which DBM library you use by loading that library
1120before you call dbmopen():
1121
1122 use DB_File;
1123 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1124 or die "Can't open netscape history file: $!";
2b5ab1e7 1125
a0d0e21e 1126=item defined EXPR
d74e8afc 1127X<defined> X<undef> X<undefined>
a0d0e21e 1128
54310121 1129=item defined
bbce6d69 1130
2f9daede 1131Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1132the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1133checked.
1134
1135Many operations return C<undef> to indicate failure, end of file,
1136system error, uninitialized variable, and other exceptional
1137conditions. This function allows you to distinguish C<undef> from
1138other values. (A simple Boolean test will not distinguish among
7660c0ab 1139C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1140false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1141doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1142returns C<undef> when its argument is an empty array, I<or> when the
1143element to return happens to be C<undef>.
1144
f10b0346
GS
1145You may also use C<defined(&func)> to check whether subroutine C<&func>
1146has ever been defined. The return value is unaffected by any forward
80d38338 1147declarations of C<&func>. A subroutine that is not defined
847c7ebe 1148may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1149makes it spring into existence the first time that it is called; see
847c7ebe 1150L<perlsub>.
f10b0346
GS
1151
1152Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1153used to report whether memory for that aggregate has ever been
1154allocated. This behavior may disappear in future versions of Perl.
1155You should instead use a simple test for size:
1156
1157 if (@an_array) { print "has array elements\n" }
1158 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1159
1160When used on a hash element, it tells you whether the value is defined,
dc848c6f 1161not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1162purpose.
a0d0e21e
LW
1163
1164Examples:
1165
1166 print if defined $switch{'D'};
1167 print "$val\n" while defined($val = pop(@ary));
1168 die "Can't readlink $sym: $!"
a9a5a0dc 1169 unless defined($value = readlink $sym);
a0d0e21e 1170 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1171 $debugging = 0 unless defined $debugging;
a0d0e21e 1172
19799a22 1173Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1174discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1175defined values. For example, if you say
a5f75d66
AD
1176
1177 "ab" =~ /a(.*)b/;
1178
80d38338 1179The pattern match succeeds and C<$1> is defined, although it
cf264981 1180matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1181matched something that happened to be zero characters long. This is all
a5f75d66 1182very above-board and honest. When a function returns an undefined value,
2f9daede 1183it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1184should use C<defined> only when questioning the integrity of what
7660c0ab 1185you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1186what you want.
1187
dc848c6f 1188See also L</undef>, L</exists>, L</ref>.
2f9daede 1189
a0d0e21e 1190=item delete EXPR
d74e8afc 1191X<delete>
a0d0e21e 1192
d0a76353
RS
1193Given an expression that specifies an element or slice of a hash, C<delete>
1194deletes the specified elements from that hash so that exists() on that element
1195no longer returns true. Setting a hash element to the undefined value does
1196not remove its key, but deleting it does; see L</exists>.
80d38338 1197
d0a76353 1198It returns the value or values deleted in list context, or the last such
80d38338 1199element in scalar context. The return list's length always matches that of
d0a76353
RS
1200the argument list: deleting non-existent elements returns the undefined value
1201in their corresponding positions.
80d38338 1202
d0a76353
RS
1203delete() may also be used on arrays and array slices, but its behavior is less
1204straightforward. Although exists() will return false for deleted entries,
1205deleting array elements never changes indices of existing values; use shift()
1206or splice() for that. However, if all deleted elements fall at the end of an
1207array, the array's size shrinks to the position of the highest element that
1208still tests true for exists(), or to 0 if none do.
1209
1210B<Be aware> that calling delete on array values is deprecated and likely to
1211be removed in a future version of Perl.
80d38338
TC
1212
1213Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1214a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1215or array may not necessarily return anything; it depends on the implementation
1216of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1217
80d38338
TC
1218The C<delete local EXPR> construct localizes the deletion to the current
1219block at run time. Until the block exits, elements locally deleted
1220temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1221of composite types">.
eba0920a
EM
1222
1223 %hash = (foo => 11, bar => 22, baz => 33);
1224 $scalar = delete $hash{foo}; # $scalar is 11
1225 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1226 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1227
01020589 1228The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1229
5f05dabc 1230 foreach $key (keys %HASH) {
a9a5a0dc 1231 delete $HASH{$key};
a0d0e21e
LW
1232 }
1233
01020589 1234 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1235 delete $ARRAY[$index];
01020589
GS
1236 }
1237
1238And so do these:
5f05dabc 1239
01020589
GS
1240 delete @HASH{keys %HASH};
1241
9740c838 1242 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1243
80d38338
TC
1244But both are slower than assigning the empty list
1245or undefining %HASH or @ARRAY, which is the customary
1246way to empty out an aggregate:
01020589 1247
5ed4f2ec 1248 %HASH = (); # completely empty %HASH
1249 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1250
5ed4f2ec 1251 @ARRAY = (); # completely empty @ARRAY
1252 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1253
80d38338
TC
1254The EXPR can be arbitrarily complicated provided its
1255final operation is an element or slice of an aggregate:
a0d0e21e
LW
1256
1257 delete $ref->[$x][$y]{$key};
5f05dabc 1258 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1259
01020589
GS
1260 delete $ref->[$x][$y][$index];
1261 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1262
a0d0e21e 1263=item die LIST
d74e8afc 1264X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1265
19799a22
GS
1266Outside an C<eval>, prints the value of LIST to C<STDERR> and
1267exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1268exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1269status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1270an C<eval(),> the error message is stuffed into C<$@> and the
1271C<eval> is terminated with the undefined value. This makes
1272C<die> the way to raise an exception.
a0d0e21e
LW
1273
1274Equivalent examples:
1275
1276 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1277 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1278
ccac6780 1279If the last element of LIST does not end in a newline, the current
df37ec69
WW
1280script line number and input line number (if any) are also printed,
1281and a newline is supplied. Note that the "input line number" (also
1282known as "chunk") is subject to whatever notion of "line" happens to
1283be currently in effect, and is also available as the special variable
1284C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1285
1286Hint: sometimes appending C<", stopped"> to your message will cause it
1287to make better sense when the string C<"at foo line 123"> is appended.
1288Suppose you are running script "canasta".
a0d0e21e
LW
1289
1290 die "/etc/games is no good";
1291 die "/etc/games is no good, stopped";
1292
1293produce, respectively
1294
1295 /etc/games is no good at canasta line 123.
1296 /etc/games is no good, stopped at canasta line 123.
1297
2b5ab1e7 1298See also exit(), warn(), and the Carp module.
a0d0e21e 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
80d38338
TC
1315You can also call C<die> with a reference argument, and if this is trapped
1316within an C<eval>, C<$@> contains that reference. This permits more
1317elaborate exception handling using objects that maintain arbitrary state
1318about the exception. Such a scheme is sometimes preferable to matching
1319particular string values of C<$@> with regular expressions. Because C<$@>
1320is a global variable and C<eval> may be used within object implementations,
1321be careful that analyzing the error object doesn't replace the reference in
1322the global variable. It's easiest to make a local copy of the reference
1323before any manipulations. Here's an example:
52531d10 1324
80d38338 1325 use Scalar::Util "blessed";
da279afe 1326
52531d10 1327 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1328 if (my $ev_err = $@) {
1329 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1330 # handle Some::Module::Exception
1331 }
1332 else {
1333 # handle all other possible exceptions
1334 }
1335 }
1336
3b10bc60 1337Because Perl stringifies uncaught exception messages before display,
80d38338 1338you'll probably want to overload stringification operations on
52531d10
GS
1339exception objects. See L<overload> for details about that.
1340
19799a22
GS
1341You can arrange for a callback to be run just before the C<die>
1342does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1343handler is called with the error text and can change the error
19799a22
GS
1344message, if it sees fit, by calling C<die> again. See
1345L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1346L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1347to be run only right before your program was to exit, this is not
3b10bc60 1348currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1349even inside eval()ed blocks/strings! If one wants the hook to do
1350nothing in such situations, put
fb73857a 1351
5ed4f2ec 1352 die @_ if $^S;
fb73857a 1353
19799a22
GS
1354as the first line of the handler (see L<perlvar/$^S>). Because
1355this promotes strange action at a distance, this counterintuitive
b76cc8ba 1356behavior may be fixed in a future release.
774d564b 1357
a0d0e21e 1358=item do BLOCK
d74e8afc 1359X<do> X<block>
a0d0e21e
LW
1360
1361Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1362sequence of commands indicated by BLOCK. When modified by the C<while> or
1363C<until> loop modifier, executes the BLOCK once before testing the loop
1364condition. (On other statements the loop modifiers test the conditional
1365first.)
a0d0e21e 1366
4968c1e4 1367C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1368C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1369See L<perlsyn> for alternative strategies.
4968c1e4 1370
a0d0e21e 1371=item do SUBROUTINE(LIST)
d74e8afc 1372X<do>
a0d0e21e 1373
cf264981 1374This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1375
1376=item do EXPR
d74e8afc 1377X<do>
a0d0e21e
LW
1378
1379Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1380file as a Perl script.
a0d0e21e
LW
1381
1382 do 'stat.pl';
1383
1384is just like
1385
986b19de 1386 eval `cat stat.pl`;
a0d0e21e 1387
2b5ab1e7 1388except that it's more efficient and concise, keeps track of the current
ea63ef19 1389filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1390C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1391variables. It also differs in that code evaluated with C<do FILENAME>
1392cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1393same, however, in that it does reparse the file every time you call it,
1394so you probably don't want to do this inside a loop.
a0d0e21e 1395
8e30cc93 1396If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1397error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1398returns undef and sets an error message in C<$@>. If the file is
1399successfully compiled, C<do> returns the value of the last expression
1400evaluated.
1401
80d38338 1402Inclusion of library modules is better done with the
19799a22 1403C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1404and raise an exception if there's a problem.
a0d0e21e 1405
5a964f20
TC
1406You might like to use C<do> to read in a program configuration
1407file. Manual error checking can be done this way:
1408
b76cc8ba 1409 # read in config files: system first, then user
f86cebdf 1410 for $file ("/share/prog/defaults.rc",
b76cc8ba 1411 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1412 {
1413 unless ($return = do $file) {
1414 warn "couldn't parse $file: $@" if $@;
1415 warn "couldn't do $file: $!" unless defined $return;
1416 warn "couldn't run $file" unless $return;
1417 }
5a964f20
TC
1418 }
1419
a0d0e21e 1420=item dump LABEL
d74e8afc 1421X<dump> X<core> X<undump>
a0d0e21e 1422
1614b0e3
JD
1423=item dump
1424
19799a22
GS
1425This function causes an immediate core dump. See also the B<-u>
1426command-line switch in L<perlrun>, which does the same thing.
1427Primarily this is so that you can use the B<undump> program (not
1428supplied) to turn your core dump into an executable binary after
1429having initialized all your variables at the beginning of the
1430program. When the new binary is executed it will begin by executing
1431a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1432Think of it as a goto with an intervening core dump and reincarnation.
1433If C<LABEL> is omitted, restarts the program from the top.
1434
1435B<WARNING>: Any files opened at the time of the dump will I<not>
1436be open any more when the program is reincarnated, with possible
80d38338 1437resulting confusion by Perl.
19799a22 1438
59f521f4
RGS
1439This function is now largely obsolete, mostly because it's very hard to
1440convert a core file into an executable. That's why you should now invoke
1441it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1442typo.
19799a22 1443
aa689395 1444=item each HASH
d74e8afc 1445X<each> X<hash, iterator>
aa689395 1446
aeedbbed
NC
1447=item each ARRAY
1448X<array, iterator>
1449
80d38338
TC
1450When called in list context, returns a 2-element list consisting of the key
1451and value for the next element of a hash, or the index and value for the
1452next element of an array, so that you can iterate over it. When called in
1453scalar context, returns only the key (not the value) in a hash, or the index
1454in an array.
2f9daede 1455
aeedbbed 1456Hash entries are returned in an apparently random order. The actual random
3b10bc60 1457order is subject to change in future versions of Perl, but it is
504f80c1 1458guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1459function would produce on the same (unmodified) hash. Since Perl
22883ac5 14605.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1461for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1462
80d38338
TC
1463After C<each> has returned all entries from the hash or array, the next
1464call to C<each> returns the empty list in list context and C<undef> in
1465scalar context. The next call following that one restarts iteration. Each
1466hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1467and C<values>. The iterator is implicitly reset when C<each> has reached
1468the end as just described; it can be explicitly reset by calling C<keys> or
1469C<values> on the hash or array. If you add or delete a hash's elements
1470while iterating over it, entries may be skipped or duplicated--so don't do
1471that. Exception: It is always safe to delete the item most recently
3b10bc60 1472returned by C<each()>, so the following code works properly:
74fc8b5f
MJD
1473
1474 while (($key, $value) = each %hash) {
1475 print $key, "\n";
1476 delete $hash{$key}; # This is safe
1477 }
aa689395 1478
80d38338 1479This prints out your environment like the printenv(1) program,
3b10bc60 1480but in a different order:
a0d0e21e
LW
1481
1482 while (($key,$value) = each %ENV) {
a9a5a0dc 1483 print "$key=$value\n";
a0d0e21e
LW
1484 }
1485
19799a22 1486See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1487
1488=item eof FILEHANDLE
d74e8afc
ITB
1489X<eof>
1490X<end of file>
1491X<end-of-file>
a0d0e21e 1492
4633a7c4
LW
1493=item eof ()
1494
a0d0e21e
LW
1495=item eof
1496
1497Returns 1 if the next read on FILEHANDLE will return end of file, or if
1498FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1499gives the real filehandle. (Note that this function actually
80d38338 1500reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1501interactive context.) Do not read from a terminal file (or call
19799a22 1502C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1503as terminals may lose the end-of-file condition if you do.
1504
820475bd 1505An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1506with empty parentheses is different. It refers to the pseudo file
820475bd 1507formed from the files listed on the command line and accessed via the
61eff3bc
JH
1508C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1509as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1510used will cause C<@ARGV> to be examined to determine if input is
67408cae 1511available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1512end-of-file will assume you are processing another C<@ARGV> list,
1513and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1514see L<perlop/"I/O Operators">.
820475bd 1515
61eff3bc 1516In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
3b10bc60 1517detect the end of each file, C<eof()> will detect the end of only the
820475bd 1518last file. Examples:
a0d0e21e 1519
748a9306
LW
1520 # reset line numbering on each input file
1521 while (<>) {
a9a5a0dc
VP
1522 next if /^\s*#/; # skip comments
1523 print "$.\t$_";
5a964f20 1524 } continue {
a9a5a0dc 1525 close ARGV if eof; # Not eof()!
748a9306
LW
1526 }
1527
a0d0e21e
LW
1528 # insert dashes just before last line of last file
1529 while (<>) {
a9a5a0dc
VP
1530 if (eof()) { # check for end of last file
1531 print "--------------\n";
1532 }
1533 print;
1534 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1535 }
1536
a0d0e21e 1537Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1538input operators typically return C<undef> when they run out of data, or if
1539there was an error.
a0d0e21e
LW
1540
1541=item eval EXPR
d74e8afc 1542X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1543X<error, handling> X<exception, handling>
a0d0e21e
LW
1544
1545=item eval BLOCK
1546
ce2984c3
PF
1547=item eval
1548
c7cc6f1c
GS
1549In the first form, the return value of EXPR is parsed and executed as if it
1550were a little Perl program. The value of the expression (which is itself
5a964f20 1551determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1552errors, executed in the lexical context of the current Perl program, so
1553that any variable settings or subroutine and format definitions remain
cf264981 1554afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1555If EXPR is omitted, evaluates C<$_>. This form is typically used to
1556delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1557
1558In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1559same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1560within the context of the current Perl program. This form is typically
1561used to trap exceptions more efficiently than the first (see below), while
1562also providing the benefit of checking the code within BLOCK at compile
1563time.
1564
1565The final semicolon, if any, may be omitted from the value of EXPR or within
1566the BLOCK.
1567
1568In both forms, the value returned is the value of the last expression
5a964f20 1569evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1570as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1571in void, scalar, or list context, depending on the context of the C<eval>
1572itself. See L</wantarray> for more on how the evaluation context can be
1573determined.
a0d0e21e 1574
19799a22 1575If there is a syntax error or runtime error, or a C<die> statement is
bbead3ca
BL
1576executed, C<eval> returns an undefined value in scalar context
1577or an empty list in list context, and C<$@> is set to the
3b10bc60 1578error message. If there was no error, C<$@> is guaranteed to be the empty
1579string. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1580warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1581To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1582turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1583See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1584
19799a22
GS
1585Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1586determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1587is implemented. It is also Perl's exception trapping mechanism, where
1588the die operator is used to raise exceptions.
1589
5f1da31c
NT
1590If you want to trap errors when loading an XS module, some problems with
1591the binary interface (such as Perl version skew) may be fatal even with
1592C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1593
a0d0e21e
LW
1594If the code to be executed doesn't vary, you may use the eval-BLOCK
1595form to trap run-time errors without incurring the penalty of
1596recompiling each time. The error, if any, is still returned in C<$@>.
1597Examples:
1598
54310121 1599 # make divide-by-zero nonfatal
a0d0e21e
LW
1600 eval { $answer = $a / $b; }; warn $@ if $@;
1601
1602 # same thing, but less efficient
1603 eval '$answer = $a / $b'; warn $@ if $@;
1604
1605 # a compile-time error
5ed4f2ec 1606 eval { $answer = }; # WRONG
a0d0e21e
LW
1607
1608 # a run-time error
5ed4f2ec 1609 eval '$answer ='; # sets $@
a0d0e21e 1610
cf264981
SP
1611Using the C<eval{}> form as an exception trap in libraries does have some
1612issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1613may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1614You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1615as this example shows:
774d564b 1616
80d38338 1617 # a private exception trap for divide-by-zero
f86cebdf
GS
1618 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1619 warn $@ if $@;
774d564b
PP
1620
1621This is especially significant, given that C<__DIE__> hooks can call
19799a22 1622C<die> again, which has the effect of changing their error messages:
774d564b
PP
1623
1624 # __DIE__ hooks may modify error messages
1625 {
f86cebdf
GS
1626 local $SIG{'__DIE__'} =
1627 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1628 eval { die "foo lives here" };
1629 print $@ if $@; # prints "bar lives here"
774d564b
PP
1630 }
1631
19799a22 1632Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1633may be fixed in a future release.
1634
19799a22 1635With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1636being looked at when:
1637
5ed4f2ec 1638 eval $x; # CASE 1
1639 eval "$x"; # CASE 2
a0d0e21e 1640
5ed4f2ec 1641 eval '$x'; # CASE 3
1642 eval { $x }; # CASE 4
a0d0e21e 1643
5ed4f2ec 1644 eval "\$$x++"; # CASE 5
1645 $$x++; # CASE 6
a0d0e21e 1646
2f9daede 1647Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1648the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1649the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1650and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1651does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1652purely visual reasons, but it also has the advantage of compiling at
1653compile-time instead of at run-time.) Case 5 is a place where
19799a22 1654normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1655particular situation, you can just use symbolic references instead, as
1656in case 6.
a0d0e21e 1657
8a5a710d
DN
1658The assignment to C<$@> occurs before restoration of localised variables,
1659which means a temporary is required if you want to mask some but not all
1660errors:
1661
1662 # alter $@ on nefarious repugnancy only
1663 {
1664 my $e;
1665 {
1666 local $@; # protect existing $@
1667 eval { test_repugnancy() };
1668 # $@ =~ /nefarious/ and die $@; # DOES NOT WORK
1669 $@ =~ /nefarious/ and $e = $@;
1670 }
1671 die $e if defined $e
1672 }
1673
4968c1e4 1674C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1675C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1676
3b10bc60 1677An C<eval ''> executed within the C<DB> package doesn't see the usual
1678surrounding lexical scope, but rather the scope of the first non-DB piece
1679of code that called it. You don't normally need to worry about this unless
1680you are writing a Perl debugger.
d819b83a 1681
a0d0e21e 1682=item exec LIST
d74e8afc 1683X<exec> X<execute>
a0d0e21e 1684
8bf3b016
GS
1685=item exec PROGRAM LIST
1686
3b10bc60 1687The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1688use C<system> instead of C<exec> if you want it to return. It fails and
1689returns false only if the command does not exist I<and> it is executed
fb73857a 1690directly instead of via your system's command shell (see below).
a0d0e21e 1691
19799a22 1692Since it's a common mistake to use C<exec> instead of C<system>, Perl
80d38338 1693warns you if there is a following statement that isn't C<die>, C<warn>,
3b10bc60 1694or C<exit> (if C<-w> is set--but you always do that, right?). If you
19799a22 1695I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1696can use one of these styles to avoid the warning:
1697
5a964f20
TC
1698 exec ('foo') or print STDERR "couldn't exec foo: $!";
1699 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1700
5a964f20 1701If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1702with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1703If there is only one scalar argument or an array with one element in it,
1704the argument is checked for shell metacharacters, and if there are any,
1705the entire argument is passed to the system's command shell for parsing
1706(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1707If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1708words and passed directly to C<execvp>, which is more efficient.
19799a22 1709Examples:
a0d0e21e 1710
19799a22
GS
1711 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1712 exec "sort $outfile | uniq";
a0d0e21e
LW
1713
1714If you don't really want to execute the first argument, but want to lie
1715to the program you are executing about its own name, you can specify
1716the program you actually want to run as an "indirect object" (without a
1717comma) in front of the LIST. (This always forces interpretation of the
54310121 1718LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1719the list.) Example:
1720
1721 $shell = '/bin/csh';
5ed4f2ec 1722 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1723
1724or, more directly,
1725
5ed4f2ec 1726 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1727
3b10bc60 1728When the arguments get executed via the system shell, results are
1729subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1730for details.
1731
19799a22
GS
1732Using an indirect object with C<exec> or C<system> is also more
1733secure. This usage (which also works fine with system()) forces
1734interpretation of the arguments as a multivalued list, even if the
1735list had just one argument. That way you're safe from the shell
1736expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1737
1738 @args = ( "echo surprise" );
1739
2b5ab1e7 1740 exec @args; # subject to shell escapes
f86cebdf 1741 # if @args == 1
2b5ab1e7 1742 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1743
1744The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1745program, passing it C<"surprise"> an argument. The second version didn't;
1746it tried to run a program named I<"echo surprise">, didn't find it, and set
1747C<$?> to a non-zero value indicating failure.
5a964f20 1748
80d38338 1749Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1750output before the exec, but this may not be supported on some platforms
1751(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1752in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 1753open handles to avoid lost output.
0f897271 1754
80d38338
TC
1755Note that C<exec> will not call your C<END> blocks, nor will it invoke
1756C<DESTROY> methods on your objects.
7660c0ab 1757
a0d0e21e 1758=item exists EXPR
d74e8afc 1759X<exists> X<autovivification>
a0d0e21e 1760
d0a76353
RS
1761Given an expression that specifies an element of a hash, returns true if the
1762specified element in the hash has ever been initialized, even if the
1763corresponding value is undefined.
a0d0e21e 1764
5ed4f2ec 1765 print "Exists\n" if exists $hash{$key};
1766 print "Defined\n" if defined $hash{$key};
01020589
GS
1767 print "True\n" if $hash{$key};
1768
d0a76353
RS
1769exists may also be called on array elements, but its behavior is much less
1770obvious, and is strongly tied to the use of L</delete> on arrays. B<Be aware>
1771that calling exists on array values is deprecated and likely to be removed in
1772a future version of Perl.
1773
5ed4f2ec 1774 print "Exists\n" if exists $array[$index];
1775 print "Defined\n" if defined $array[$index];
01020589 1776 print "True\n" if $array[$index];
a0d0e21e 1777
8ea97a1e 1778A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1779it exists, but the reverse doesn't necessarily hold true.
1780
afebc493
GS
1781Given an expression that specifies the name of a subroutine,
1782returns true if the specified subroutine has ever been declared, even
1783if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 1784does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
1785exist may still be callable: its package may have an C<AUTOLOAD>
1786method that makes it spring into existence the first time that it is
3b10bc60 1787called; see L<perlsub>.
afebc493 1788
5ed4f2ec 1789 print "Exists\n" if exists &subroutine;
1790 print "Defined\n" if defined &subroutine;
afebc493 1791
a0d0e21e 1792Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1793operation is a hash or array key lookup or subroutine name:
a0d0e21e 1794
5ed4f2ec 1795 if (exists $ref->{A}->{B}->{$key}) { }
1796 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 1797
5ed4f2ec 1798 if (exists $ref->{A}->{B}->[$ix]) { }
1799 if (exists $hash{A}{B}[$ix]) { }
01020589 1800
afebc493
GS
1801 if (exists &{$ref->{A}{B}{$key}}) { }
1802
3b10bc60 1803Although the mostly deeply nested array or hash will not spring into
1804existence just because its existence was tested, any intervening ones will.
61eff3bc 1805Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 1806into existence due to the existence test for the $key element above.
3b10bc60 1807This happens anywhere the arrow operator is used, including even here:
5a964f20 1808
2b5ab1e7 1809 undef $ref;
5ed4f2ec 1810 if (exists $ref->{"Some key"}) { }
1811 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
1812
1813This surprising autovivification in what does not at first--or even
1814second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1815release.
a0d0e21e 1816
afebc493
GS
1817Use of a subroutine call, rather than a subroutine name, as an argument
1818to exists() is an error.
1819
5ed4f2ec 1820 exists &sub; # OK
1821 exists &sub(); # Error
afebc493 1822
a0d0e21e 1823=item exit EXPR
d74e8afc 1824X<exit> X<terminate> X<abort>
a0d0e21e 1825
ce2984c3
PF
1826=item exit
1827
2b5ab1e7 1828Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1829
1830 $ans = <STDIN>;
1831 exit 0 if $ans =~ /^[Xx]/;
1832
19799a22 1833See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1834universally recognized values for EXPR are C<0> for success and C<1>
1835for error; other values are subject to interpretation depending on the
1836environment in which the Perl program is running. For example, exiting
183769 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1838the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1839
19799a22
GS
1840Don't use C<exit> to abort a subroutine if there's any chance that
1841someone might want to trap whatever error happened. Use C<die> instead,
1842which can be trapped by an C<eval>.
28757baa 1843
19799a22 1844The exit() function does not always exit immediately. It calls any
2b5ab1e7 1845defined C<END> routines first, but these C<END> routines may not
19799a22 1846themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1847be called are called before the real exit. If this is a problem, you
1848can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1849See L<perlmod> for details.
5a964f20 1850
a0d0e21e 1851=item exp EXPR
d74e8afc 1852X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1853
54310121 1854=item exp
bbce6d69 1855
b76cc8ba 1856Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1857If EXPR is omitted, gives C<exp($_)>.
1858
1859=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1860X<fcntl>
a0d0e21e 1861
f86cebdf 1862Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1863
1864 use Fcntl;
1865
0ade1984 1866first to get the correct constant definitions. Argument processing and
3b10bc60 1867value returned work just like C<ioctl> below.
a0d0e21e
LW
1868For example:
1869
1870 use Fcntl;
5a964f20 1871 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 1872 or die "can't fcntl F_GETFL: $!";
5a964f20 1873
554ad1fc 1874You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1875Like C<ioctl>, it maps a C<0> return from the system call into
1876C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1877in numeric context. It is also exempt from the normal B<-w> warnings
1878on improper numeric conversions.
5a964f20 1879
3b10bc60 1880Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
1881doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1882manpage to learn what functions are available on your system.
a0d0e21e 1883
be2f7487 1884Here's an example of setting a filehandle named C<REMOTE> to be
1885non-blocking at the system level. You'll have to negotiate C<$|>
1886on your own, though.
1887
1888 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1889
1890 $flags = fcntl(REMOTE, F_GETFL, 0)
1891 or die "Can't get flags for the socket: $!\n";
1892
1893 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1894 or die "Can't set flags for the socket: $!\n";
1895
a0d0e21e 1896=item fileno FILEHANDLE
d74e8afc 1897X<fileno>
a0d0e21e 1898
2b5ab1e7
TC
1899Returns the file descriptor for a filehandle, or undefined if the
1900filehandle is not open. This is mainly useful for constructing
19799a22 1901bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1902If FILEHANDLE is an expression, the value is taken as an indirect
1903filehandle, generally its name.
5a964f20 1904
b76cc8ba 1905You can use this to find out whether two handles refer to the
5a964f20
TC
1906same underlying descriptor:
1907
1908 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 1909 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1910 }
1911
1912(Filehandles connected to memory objects via new features of C<open> may
1913return undefined even though they are open.)
1914
a0d0e21e
LW
1915
1916=item flock FILEHANDLE,OPERATION
d74e8afc 1917X<flock> X<lock> X<locking>
a0d0e21e 1918
19799a22
GS
1919Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1920for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1921machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1922C<flock> is Perl's portable file locking interface, although it locks
3b10bc60 1923entire files only, not records.
2b5ab1e7
TC
1924
1925Two potentially non-obvious but traditional C<flock> semantics are
1926that it waits indefinitely until the lock is granted, and that its locks
1927B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1928fewer guarantees. This means that programs that do not also use C<flock>
1929may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1930your port's specific documentation, or your system-specific local manpages
1931for details. It's best to assume traditional behavior if you're writing
1932portable programs. (But if you're not, you should as always feel perfectly
1933free to write for your own system's idiosyncrasies (sometimes called
1934"features"). Slavish adherence to portability concerns shouldn't get
1935in the way of your getting your job done.)
a3cb178b 1936
8ebc5c01
PP
1937OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1938LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1939you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1940either individually, or as a group using the ':flock' tag. LOCK_SH
1941requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 1942releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
3b10bc60 1943LOCK_SH or LOCK_EX then C<flock> returns immediately rather than blocking
1944waiting for the lock; check the return status to see if you got it.
68dc0745 1945
2b5ab1e7
TC
1946To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1947before locking or unlocking it.
8ebc5c01 1948
f86cebdf 1949Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1950locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1951are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1952implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1953differing semantics shouldn't bite too many people.
1954
becacb53
TM
1955Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1956be open with read intent to use LOCK_SH and requires that it be open
1957with write intent to use LOCK_EX.
1958
19799a22
GS
1959Note also that some versions of C<flock> cannot lock things over the
1960network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1961that. If you like you can force Perl to ignore your system's flock(2)
1962function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 1963the switch C<-Ud_flock> to the F<Configure> program when you configure
3b10bc60 1964Perl.
4633a7c4
LW
1965
1966Here's a mailbox appender for BSD systems.
a0d0e21e 1967
7ed5353d 1968 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
1969
1970 sub lock {
a9a5a0dc
VP
1971 my ($fh) = @_;
1972 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 1973
a9a5a0dc
VP
1974 # and, in case someone appended while we were waiting...
1975 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
1976 }
1977
1978 sub unlock {
a9a5a0dc
VP
1979 my ($fh) = @_;
1980 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
1981 }
1982
b0169937 1983 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 1984 or die "Can't open mailbox: $!";
a0d0e21e 1985
7ed5353d 1986 lock($mbox);
b0169937 1987 print $mbox $msg,"\n\n";
7ed5353d 1988 unlock($mbox);
a0d0e21e 1989
3b10bc60 1990On systems that support a real flock(2), locks are inherited across fork()
1991calls, whereas those that must resort to the more capricious fcntl(2)
1992function lose their locks, making it seriously harder to write servers.
2b5ab1e7 1993
cb1a09d0 1994See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1995
1996=item fork
d74e8afc 1997X<fork> X<child> X<parent>
a0d0e21e 1998
2b5ab1e7
TC
1999Does a fork(2) system call to create a new process running the
2000same program at the same point. It returns the child pid to the
2001parent process, C<0> to the child process, or C<undef> if the fork is
2002unsuccessful. File descriptors (and sometimes locks on those descriptors)
2003are shared, while everything else is copied. On most systems supporting
2004fork(), great care has gone into making it extremely efficient (for
2005example, using copy-on-write technology on data pages), making it the
2006dominant paradigm for multitasking over the last few decades.
5a964f20 2007
80d38338 2008Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
2009output before forking the child process, but this may not be supported
2010on some platforms (see L<perlport>). To be safe, you may need to set
2011C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 2012C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 2013
19799a22 2014If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
2015accumulate zombies. On some systems, you can avoid this by setting
2016C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2017forking and reaping moribund children.
cb1a09d0 2018
28757baa
PP
2019Note that if your forked child inherits system file descriptors like
2020STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 2021if you exit, then the remote server (such as, say, a CGI script or a
19799a22 2022backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2023You should reopen those to F</dev/null> if it's any issue.
28757baa 2024
cb1a09d0 2025=item format
d74e8afc 2026X<format>
cb1a09d0 2027
19799a22 2028Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2029example:
2030
54310121 2031 format Something =
a9a5a0dc
VP
2032 Test: @<<<<<<<< @||||| @>>>>>
2033 $str, $%, '$' . int($num)
cb1a09d0
AD
2034 .
2035
2036 $str = "widget";
184e9718 2037 $num = $cost/$quantity;
cb1a09d0
AD
2038 $~ = 'Something';
2039 write;
2040
2041See L<perlform> for many details and examples.
2042
8903cb82 2043=item formline PICTURE,LIST
d74e8afc 2044X<formline>
a0d0e21e 2045
5a964f20 2046This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2047too. It formats (see L<perlform>) a list of values according to the
2048contents of PICTURE, placing the output into the format output
7660c0ab 2049accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2050Eventually, when a C<write> is done, the contents of
cf264981
SP
2051C<$^A> are written to some filehandle. You could also read C<$^A>
2052and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2053does one C<formline> per line of form, but the C<formline> function itself
748a9306 2054doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2055that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2056You may therefore need to use multiple formlines to implement a single
3b10bc60 2057record format, just like the C<format> compiler.
748a9306 2058
19799a22 2059Be careful if you put double quotes around the picture, because an C<@>
748a9306 2060character may be taken to mean the beginning of an array name.
19799a22 2061C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
2062
2063=item getc FILEHANDLE
f723aae1 2064X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2065
2066=item getc
2067
2068Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2069or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2070the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2071STDIN. This is not particularly efficient. However, it cannot be
2072used by itself to fetch single characters without waiting for the user
2073to hit enter. For that, try something more like:
4633a7c4
LW
2074
2075 if ($BSD_STYLE) {
a9a5a0dc 2076 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2077 }
2078 else {
a9a5a0dc 2079 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2080 }
2081
2082 $key = getc(STDIN);
2083
2084 if ($BSD_STYLE) {
a9a5a0dc 2085 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2086 }
2087 else {
3b10bc60 2088 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2089 }
2090 print "\n";
2091
54310121
PP
2092Determination of whether $BSD_STYLE should be set
2093is left as an exercise to the reader.
cb1a09d0 2094
19799a22 2095The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2096systems purporting POSIX compliance. See also the C<Term::ReadKey>
2097module from your nearest CPAN site; details on CPAN can be found on
2098L<perlmodlib/CPAN>.
a0d0e21e
LW
2099
2100=item getlogin
d74e8afc 2101X<getlogin> X<login>
a0d0e21e 2102
cf264981 2103This implements the C library function of the same name, which on most
3b10bc60 2104systems returns the current login from F</etc/utmp>, if any. If it
2105returns the empty string, use C<getpwuid>.
a0d0e21e 2106
f86702cc 2107 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2108
19799a22
GS
2109Do not consider C<getlogin> for authentication: it is not as
2110secure as C<getpwuid>.
4633a7c4 2111
a0d0e21e 2112=item getpeername SOCKET
d74e8afc 2113X<getpeername> X<peer>
a0d0e21e
LW
2114
2115Returns the packed sockaddr address of other end of the SOCKET connection.
2116
4633a7c4
LW
2117 use Socket;
2118 $hersockaddr = getpeername(SOCK);
19799a22 2119 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2120 $herhostname = gethostbyaddr($iaddr, AF_INET);
2121 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2122
2123=item getpgrp PID
d74e8afc 2124X<getpgrp> X<group>
a0d0e21e 2125
47e29363 2126Returns the current process group for the specified PID. Use
7660c0ab 2127a PID of C<0> to get the current process group for the
4633a7c4 2128current process. Will raise an exception if used on a machine that
f86cebdf 2129doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2130group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2131does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2132
2133=item getppid
d74e8afc 2134X<getppid> X<parent> X<pid>
a0d0e21e
LW
2135
2136Returns the process id of the parent process.
2137
4d76a344
RGS
2138Note for Linux users: on Linux, the C functions C<getpid()> and
2139C<getppid()> return different values from different threads. In order to
3b10bc60 2140be portable, this behavior is not reflected by the Perl-level function
4d76a344 2141C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2142to call the underlying C<getppid()>, you may use the CPAN module
2143C<Linux::Pid>.
4d76a344 2144
a0d0e21e 2145=item getpriority WHICH,WHO
d74e8afc 2146X<getpriority> X<priority> X<nice>
a0d0e21e 2147
4633a7c4 2148Returns the current priority for a process, a process group, or a user.
f4084e39 2149(See C<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2150machine that doesn't implement getpriority(2).
a0d0e21e
LW
2151
2152=item getpwnam NAME
d74e8afc
ITB
2153X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2154X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2155X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2156X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2157X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2158X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2159
2160=item getgrnam NAME
2161
2162=item gethostbyname NAME
2163
2164=item getnetbyname NAME
2165
2166=item getprotobyname NAME
2167
2168=item getpwuid UID
2169
2170=item getgrgid GID
2171
2172=item getservbyname NAME,PROTO
2173
2174=item gethostbyaddr ADDR,ADDRTYPE
2175
2176=item getnetbyaddr ADDR,ADDRTYPE
2177
2178=item getprotobynumber NUMBER
2179
2180=item getservbyport PORT,PROTO
2181
2182=item getpwent
2183
2184=item getgrent
2185
2186=item gethostent
2187
2188=item getnetent
2189
2190=item getprotoent
2191
2192=item getservent
2193
2194=item setpwent
2195
2196=item setgrent
2197
2198=item sethostent STAYOPEN
2199
2200=item setnetent STAYOPEN
2201
2202=item setprotoent STAYOPEN
2203
2204=item setservent STAYOPEN
2205
2206=item endpwent
2207
2208=item endgrent
2209
2210=item endhostent
2211
2212=item endnetent
2213
2214=item endprotoent
2215
2216=item endservent
2217
80d38338
TC
2218These routines are the same as their counterparts in the
2219system C library. In list context, the return values from the
a0d0e21e
LW
2220various get routines are as follows:
2221
2222 ($name,$passwd,$uid,$gid,
6ee623d5 2223 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2224 ($name,$passwd,$gid,$members) = getgr*
2225 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2226 ($name,$aliases,$addrtype,$net) = getnet*
2227 ($name,$aliases,$proto) = getproto*
2228 ($name,$aliases,$port,$proto) = getserv*
2229
3b10bc60 2230(If the entry doesn't exist you get an empty list.)
a0d0e21e 2231
4602f195
JH
2232The exact meaning of the $gcos field varies but it usually contains
2233the real name of the user (as opposed to the login name) and other
2234information pertaining to the user. Beware, however, that in many
2235system users are able to change this information and therefore it
106325ad 2236cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2237L<perlsec>). The $passwd and $shell, user's encrypted password and
2238login shell, are also tainted, because of the same reason.
4602f195 2239
5a964f20 2240In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2241lookup by name, in which case you get the other thing, whatever it is.
2242(If the entry doesn't exist you get the undefined value.) For example:
2243
5a964f20
TC
2244 $uid = getpwnam($name);
2245 $name = getpwuid($num);
2246 $name = getpwent();
2247 $gid = getgrnam($name);
08a33e13 2248 $name = getgrgid($num);
5a964f20
TC
2249 $name = getgrent();
2250 #etc.
a0d0e21e 2251
4602f195 2252In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2253in that they are unsupported on many systems. If the
4602f195
JH
2254$quota is unsupported, it is an empty scalar. If it is supported, it
2255usually encodes the disk quota. If the $comment field is unsupported,
2256it is an empty scalar. If it is supported it usually encodes some
2257administrative comment about the user. In some systems the $quota
2258field may be $change or $age, fields that have to do with password
2259aging. In some systems the $comment field may be $class. The $expire
2260field, if present, encodes the expiration period of the account or the
2261password. For the availability and the exact meaning of these fields
2262in your system, please consult your getpwnam(3) documentation and your
2263F<pwd.h> file. You can also find out from within Perl what your
2264$quota and $comment fields mean and whether you have the $expire field
2265by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2266C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2267files are supported only if your vendor has implemented them in the
4602f195 2268intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2269shadow versions if you're running under privilege or if there exists
cf264981
SP
2270the shadow(3) functions as found in System V (this includes Solaris
2271and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2272facility are unlikely to be supported.
6ee623d5 2273
19799a22 2274The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2275the login names of the members of the group.
2276
2277For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2278C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2279C<@addrs> value returned by a successful call is a list of raw
2280addresses returned by the corresponding library call. In the
2281Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2282by saying something like:
2283
f337b084 2284 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2285
2b5ab1e7
TC
2286The Socket library makes this slightly easier:
2287
2288 use Socket;
2289 $iaddr = inet_aton("127.1"); # or whatever address
2290 $name = gethostbyaddr($iaddr, AF_INET);
2291
2292 # or going the other way
19799a22 2293 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2294
d760c846
GS
2295In the opposite way, to resolve a hostname to the IP address
2296you can write this:
2297
2298 use Socket;
2299 $packed_ip = gethostbyname("www.perl.org");
2300 if (defined $packed_ip) {
2301 $ip_address = inet_ntoa($packed_ip);
2302 }
2303
2304Make sure <gethostbyname()> is called in SCALAR context and that
2305its return value is checked for definedness.
2306
19799a22
GS
2307If you get tired of remembering which element of the return list
2308contains which return value, by-name interfaces are provided
2309in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2310C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2311and C<User::grent>. These override the normal built-ins, supplying
2312versions that return objects with the appropriate names
2313for each field. For example:
5a964f20
TC
2314
2315 use File::stat;
2316 use User::pwent;
2317 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2318
b76cc8ba
NIS
2319Even though it looks like they're the same method calls (uid),
2320they aren't, because a C<File::stat> object is different from
19799a22 2321a C<User::pwent> object.
5a964f20 2322
a0d0e21e 2323=item getsockname SOCKET
d74e8afc 2324X<getsockname>
a0d0e21e 2325
19799a22
GS
2326Returns the packed sockaddr address of this end of the SOCKET connection,
2327in case you don't know the address because you have several different
2328IPs that the connection might have come in on.
a0d0e21e 2329
4633a7c4
LW
2330 use Socket;
2331 $mysockaddr = getsockname(SOCK);
19799a22 2332 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2333 printf "Connect to %s [%s]\n",
19799a22
GS
2334 scalar gethostbyaddr($myaddr, AF_INET),
2335 inet_ntoa($myaddr);
a0d0e21e
LW
2336
2337=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2338X<getsockopt>
a0d0e21e 2339
636e6b1f
TH
2340Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2341Options may exist at multiple protocol levels depending on the socket
2342type, but at least the uppermost socket level SOL_SOCKET (defined in the
2343C<Socket> module) will exist. To query options at another level the
2344protocol number of the appropriate protocol controlling the option
2345should be supplied. For example, to indicate that an option is to be
2346interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2347number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2348
80d38338 2349The function returns a packed string representing the requested socket
3b10bc60 2350option, or C<undef> on error, with the reason for the error placed in
2351C<$!>). Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2352consult getsockopt(2) for details. A common case is that the option is an
2353integer, in which case the result is a packed integer, which you can decode
2354using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2355
80d38338 2356An example to test whether Nagle's algorithm is turned on on a socket:
636e6b1f 2357
4852725b 2358 use Socket qw(:all);
636e6b1f
TH
2359
2360 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2361 or die "Could not determine the protocol number for tcp";
4852725b
DD
2362 # my $tcp = IPPROTO_TCP; # Alternative
2363 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2364 or die "getsockopt TCP_NODELAY: $!";
636e6b1f
TH
2365 my $nodelay = unpack("I", $packed);
2366 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2367
a0d0e21e
LW
2368
2369=item glob EXPR
d74e8afc 2370X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2371
0a753a76
PP
2372=item glob
2373
d9a9d457
JL
2374In list context, returns a (possibly empty) list of filename expansions on
2375the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2376scalar context, glob iterates through such filename expansions, returning
2377undef when the list is exhausted. This is the internal function
2378implementing the C<< <*.c> >> operator, but you can use it directly. If
2379EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2380more detail in L<perlop/"I/O Operators">.
a0d0e21e 2381
80d38338
TC
2382Note that C<glob> splits its arguments on whitespace and treats
2383each segment as separate pattern. As such, C<glob("*.c *.h")>
2384matches all files with a F<.c> or F<.h> extension. The expression
2385C<glob(".* *")> matchs all files in the current working directory.
2386
2387If non-empty braces are the only wildcard characters used in the
2388C<glob>, no filenames are matched, but potentially many strings
2389are returned. For example, this produces nine strings, one for
2390each pairing of fruits and colors:
2391
2392 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2393
3a4b19e4 2394Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2395C<File::Glob> extension. See L<File::Glob> for details, including
2396C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2397
a0d0e21e 2398=item gmtime EXPR
d74e8afc 2399X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2400
ce2984c3
PF
2401=item gmtime
2402
435fbc73
GS
2403Works just like L<localtime> but the returned values are
2404localized for the standard Greenwich time zone.
a0d0e21e 2405
435fbc73
GS
2406Note: when called in list context, $isdst, the last value
2407returned by gmtime is always C<0>. There is no
2408Daylight Saving Time in GMT.
0a753a76 2409
62aa5637
MS
2410See L<perlport/gmtime> for portability concerns.
2411
a0d0e21e 2412=item goto LABEL
d74e8afc 2413X<goto> X<jump> X<jmp>
a0d0e21e 2414
748a9306
LW
2415=item goto EXPR
2416
a0d0e21e
LW
2417=item goto &NAME
2418
b500e03b
GG
2419The C<goto-LABEL> form finds the statement labeled with LABEL and
2420resumes execution there. It can't be used to get out of a block or
2421subroutine given to C<sort>. It can be used to go almost anywhere
2422else within the dynamic scope, including out of subroutines, but it's
2423usually better to use some other construct such as C<last> or C<die>.
2424The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2425(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2426does not offer named loops combined with loop control. Perl does, and
2427this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2428
7660c0ab
A
2429The C<goto-EXPR> form expects a label name, whose scope will be resolved
2430dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2431necessarily recommended if you're optimizing for maintainability:
2432
2433 goto ("FOO", "BAR", "GLARCH")[$i];
2434
b500e03b 2435Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2436deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2437go into any construct that requires initialization, such as a
2438subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2439construct that is optimized away.
b500e03b 2440
1b6921cb
BT
2441The C<goto-&NAME> form is quite different from the other forms of
2442C<goto>. In fact, it isn't a goto in the normal sense at all, and
2443doesn't have the stigma associated with other gotos. Instead, it
2444exits the current subroutine (losing any changes set by local()) and
2445immediately calls in its place the named subroutine using the current
2446value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2447load another subroutine and then pretend that the other subroutine had
2448been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2449in the current subroutine are propagated to the other subroutine.)
2450After the C<goto>, not even C<caller> will be able to tell that this
2451routine was called first.
2452
2453NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2454containing a code reference, or a block that evaluates to a code
6cb9131c 2455reference.
a0d0e21e
LW
2456
2457=item grep BLOCK LIST
d74e8afc 2458X<grep>
a0d0e21e
LW
2459
2460=item grep EXPR,LIST
2461
2b5ab1e7
TC
2462This is similar in spirit to, but not the same as, grep(1) and its
2463relatives. In particular, it is not limited to using regular expressions.
2f9daede 2464
a0d0e21e 2465Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2466C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2467elements for which the expression evaluated to true. In scalar
2468context, returns the number of times the expression was true.
a0d0e21e
LW
2469
2470 @foo = grep(!/^#/, @bar); # weed out comments
2471
2472or equivalently,
2473
2474 @foo = grep {!/^#/} @bar; # weed out comments
2475
be3174d2
GS
2476Note that C<$_> is an alias to the list value, so it can be used to
2477modify the elements of the LIST. While this is useful and supported,
2478it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2479Similarly, grep returns aliases into the original list, much as a for
2480loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2481element of a list returned by grep (for example, in a C<foreach>, C<map>
2482or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2483This is usually something to be avoided when writing clear code.
a0d0e21e 2484
a4fb8298 2485If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2486been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2487the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2488can't be seen from the outside, avoiding any potential side-effects.
2489
19799a22 2490See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2491
a0d0e21e 2492=item hex EXPR
d74e8afc 2493X<hex> X<hexadecimal>
a0d0e21e 2494
54310121 2495=item hex
bbce6d69 2496
2b5ab1e7 2497Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2498(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2499L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2500
2501 print hex '0xAf'; # prints '175'
2502 print hex 'aF'; # same
a0d0e21e 2503
19799a22 2504Hex strings may only represent integers. Strings that would cause
53305cf1 2505integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2506unlike oct(). To present something as hex, look into L</printf>,
2507L</sprintf>, or L</unpack>.
19799a22 2508
ce2984c3 2509=item import LIST
d74e8afc 2510X<import>
a0d0e21e 2511
19799a22 2512There is no builtin C<import> function. It is just an ordinary
4633a7c4 2513method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2514names to another module. The C<use> function calls the C<import> method
cea6626f 2515for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2516
2517=item index STR,SUBSTR,POSITION
d74e8afc 2518X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2519
2520=item index STR,SUBSTR
2521
2b5ab1e7
TC
2522The index function searches for one string within another, but without
2523the wildcard-like behavior of a full regular-expression pattern match.
2524It returns the position of the first occurrence of SUBSTR in STR at
2525or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2526beginning of the string. POSITION before the beginning of the string
2527or after its end is treated as if it were the beginning or the end,
2528respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2529you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2530is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2531
2532=item int EXPR
f723aae1 2533X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2534
54310121 2535=item int
bbce6d69 2536
7660c0ab 2537Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 2538You should not use this function for rounding: one because it truncates
3b10bc60 2539towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
2540numbers can sometimes produce counterintuitive results. For example,
2541C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2542because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2543the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2544functions will serve you better than will int().
a0d0e21e
LW
2545
2546=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2547X<ioctl>
a0d0e21e 2548
2b5ab1e7 2549Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2550
5ed4f2ec 2551 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2552
a11c483f 2553to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2554exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2555own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2556(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2557may help you in this, but it's nontrivial.) SCALAR will be read and/or
3b10bc60 2558written depending on the FUNCTION; a C pointer to the string value of SCALAR
19799a22 2559will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2560has no string value but does have a numeric value, that value will be
2561passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2562true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2563functions may be needed to manipulate the values of structures used by
b76cc8ba 2564C<ioctl>.
a0d0e21e 2565
19799a22 2566The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e 2567
5ed4f2ec 2568 if OS returns: then Perl returns:
2569 -1 undefined value
2570 0 string "0 but true"
2571 anything else that number
a0d0e21e 2572
19799a22 2573Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2574still easily determine the actual value returned by the operating
2575system:
2576
2b5ab1e7 2577 $retval = ioctl(...) || -1;
a0d0e21e
LW
2578 printf "System returned %d\n", $retval;
2579
be2f7487 2580The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2581about improper numeric conversions.
2582
a0d0e21e 2583=item join EXPR,LIST
d74e8afc 2584X<join>
a0d0e21e 2585
2b5ab1e7
TC
2586Joins the separate strings of LIST into a single string with fields
2587separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2588
2b5ab1e7 2589 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2590
eb6e2d6f
GS
2591Beware that unlike C<split>, C<join> doesn't take a pattern as its
2592first argument. Compare L</split>.
a0d0e21e 2593
aa689395 2594=item keys HASH
d74e8afc 2595X<keys> X<key>
aa689395 2596
aeedbbed
NC
2597=item keys ARRAY
2598
2599Returns a list consisting of all the keys of the named hash, or the indices
2600of an array. (In scalar context, returns the number of keys or indices.)
504f80c1 2601
aeedbbed 2602The keys of a hash are returned in an apparently random order. The actual
3b10bc60 2603random order is subject to change in future versions of Perl, but it
504f80c1 2604is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2605function produces (given that the hash has not been modified). Since
2606Perl 5.8.1 the ordering is different even between different runs of
2607Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2608Attacks">).
504f80c1 2609
aeedbbed 2610As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
cf264981
SP
2611(see L</each>). In particular, calling keys() in void context resets
2612the iterator with no other overhead.
a0d0e21e 2613
aa689395 2614Here is yet another way to print your environment:
a0d0e21e
LW
2615
2616 @keys = keys %ENV;
2617 @values = values %ENV;
b76cc8ba 2618 while (@keys) {
a9a5a0dc 2619 print pop(@keys), '=', pop(@values), "\n";
a0d0e21e
LW
2620 }
2621
2622or how about sorted by key:
2623
2624 foreach $key (sort(keys %ENV)) {
a9a5a0dc 2625 print $key, '=', $ENV{$key}, "\n";
a0d0e21e
LW
2626 }
2627
8ea1e5d4
GS
2628The returned values are copies of the original keys in the hash, so
2629modifying them will not affect the original hash. Compare L</values>.
2630
19799a22 2631To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2632Here's a descending numeric sort of a hash by its values:
4633a7c4 2633
5a964f20 2634 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
a9a5a0dc 2635 printf "%4d %s\n", $hash{$key}, $key;
4633a7c4
LW
2636 }
2637
3b10bc60 2638Used as an lvalue, C<keys> allows you to increase the number of hash buckets
aa689395
PP
2639allocated for the given hash. This can gain you a measure of efficiency if
2640you know the hash is going to get big. (This is similar to pre-extending
2641an array by assigning a larger number to $#array.) If you say
55497cff
PP
2642
2643 keys %hash = 200;
2644
ab192400
GS
2645then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2646in fact, since it rounds up to the next power of two. These
55497cff
PP
2647buckets will be retained even if you do C<%hash = ()>, use C<undef
2648%hash> if you want to free the storage while C<%hash> is still in scope.
2649You can't shrink the number of buckets allocated for the hash using
19799a22 2650C<keys> in this way (but you needn't worry about doing this by accident,
aeedbbed
NC
2651as trying has no effect). C<keys @array> in an lvalue context is a syntax
2652error.
55497cff 2653
19799a22 2654See also C<each>, C<values> and C<sort>.
ab192400 2655
b350dd2f 2656=item kill SIGNAL, LIST
d74e8afc 2657X<kill> X<signal>
a0d0e21e 2658
b350dd2f 2659Sends a signal to a list of processes. Returns the number of
517db077
GS
2660processes successfully signaled (which is not necessarily the
2661same as the number actually killed).
a0d0e21e
LW
2662
2663 $cnt = kill 1, $child1, $child2;
2664 kill 9, @goners;
2665
3b10bc60 2666If SIGNAL is zero, no signal is sent to the process, but C<kill>
2667checks whether it's I<possible> to send a signal to it (that
70fb64f6 2668means, to be brief, that the process is owned by the same user, or we are
3b10bc60 2669the super-user). This is useful to check that a child process is still
81fd35db
DN
2670alive (even if only as a zombie) and hasn't changed its UID. See
2671L<perlport> for notes on the portability of this construct.
b350dd2f 2672
e2c0f81f
DG
2673Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2674of processes. That means you usually want to use positive not negative signals.
2675You may also use a signal name in quotes.
2676
2677The behavior of kill when a I<PROCESS> number is zero or negative depends on
2678the operating system. For example, on POSIX-conforming systems, zero will
2679signal the current process group and -1 will signal all processes.
1e9c1022
JL
2680
2681See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2682
2683=item last LABEL
d74e8afc 2684X<last> X<break>
a0d0e21e
LW
2685
2686=item last
2687
2688The C<last> command is like the C<break> statement in C (as used in
2689loops); it immediately exits the loop in question. If the LABEL is
2690omitted, the command refers to the innermost enclosing loop. The
2691C<continue> block, if any, is not executed:
2692
4633a7c4 2693 LINE: while (<STDIN>) {
a9a5a0dc
VP
2694 last LINE if /^$/; # exit when done with header
2695 #...
a0d0e21e
LW
2696 }
2697
80d38338 2698C<last> cannot be used to exit a block that returns a value such as
2b5ab1e7
TC
2699C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2700a grep() or map() operation.
4968c1e4 2701
6c1372ed
GS
2702Note that a block by itself is semantically identical to a loop
2703that executes once. Thus C<last> can be used to effect an early
2704exit out of such a block.
2705
98293880
JH
2706See also L</continue> for an illustration of how C<last>, C<next>, and
2707C<redo> work.
1d2dff63 2708
a0d0e21e 2709=item lc EXPR
d74e8afc 2710X<lc> X<lowercase>
a0d0e21e 2711
54310121 2712=item lc
bbce6d69 2713
d1be9408 2714Returns a lowercased version of EXPR. This is the internal function
3980dc9c 2715implementing the C<\L> escape in double-quoted strings.
a0d0e21e 2716
7660c0ab 2717If EXPR is omitted, uses C<$_>.
bbce6d69 2718
3980dc9c
KW
2719What gets returned depends on several factors:
2720
2721=over
2722
2723=item If C<use bytes> is in effect:
2724
2725=over
2726
2727=item On EBCDIC platforms
2728
2729The results are what the C language system call C<tolower()> returns.
2730
2731=item On ASCII platforms
2732
2733The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
2734respectively.
2735
2736=back
2737
2738=item Otherwise, If EXPR has the UTF8 flag set
2739
2740If the current package has a subroutine named C<ToLower>, it will be used to
2741change the case (See L<perlunicode/User-Defined Case Mappings>.)
2742Otherwise Unicode semantics are used for the case change.
2743
2744=item Otherwise, if C<use locale> is in effect
2745
2746Respects current LC_CTYPE locale. See L<perllocale>.
2747
2748=item Otherwise, if C<use feature 'unicode_strings'> is in effect:
2749
2750Unicode semantics are used for the case change. Any subroutine named
2751C<ToLower> will not be used.
2752
2753=item Otherwise:
2754
2755=over
2756
2757=item On EBCDIC platforms
2758
2759The results are what the C language system call C<tolower()> returns.
2760
2761=item On ASCII platforms
2762
2763ASCII semantics are used for the case change. The lowercase of any character
2764outside the ASCII range is the character itself.
2765
2766=back
2767
2768=back
2769
a0d0e21e 2770=item lcfirst EXPR
d74e8afc 2771X<lcfirst> X<lowercase>
a0d0e21e 2772
54310121 2773=item lcfirst
bbce6d69 2774
ad0029c4
JH
2775Returns the value of EXPR with the first character lowercased. This
2776is the internal function implementing the C<\l> escape in
3980dc9c 2777double-quoted strings.
a0d0e21e 2778
7660c0ab 2779If EXPR is omitted, uses C<$_>.
bbce6d69 2780
3980dc9c
KW
2781This function behaves the same way under various pragma, such as in a locale,
2782as L</lc> does.
2783
a0d0e21e 2784=item length EXPR
d74e8afc 2785X<length> X<size>
a0d0e21e 2786
54310121 2787=item length
bbce6d69 2788
974da8e5 2789Returns the length in I<characters> of the value of EXPR. If EXPR is
9f621bb0 2790omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
3b10bc60 2791
2792This function cannot be used on an entire array or hash to find out how
2793many elements these have. For that, use C<scalar @array> and C<scalar keys
2794%hash>, respectively.
2795
2796Like all Perl character operations, length() normally deals in logical
2797characters, not physical bytes. For how many bytes a string encoded as
2798UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
2799to C<use Encode> first). See L<Encode> and L<perlunicode>.
974da8e5 2800
a0d0e21e 2801=item link OLDFILE,NEWFILE
d74e8afc 2802X<link>
a0d0e21e 2803
19799a22 2804Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2805success, false otherwise.
a0d0e21e
LW
2806
2807=item listen SOCKET,QUEUESIZE
d74e8afc 2808X<listen>
a0d0e21e 2809
3b10bc60 2810Does the same thing that the listen(2) system call does. Returns true if
b76cc8ba 2811it succeeded, false otherwise. See the example in
cea6626f 2812L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2813
2814=item local EXPR
d74e8afc 2815X<local>
a0d0e21e 2816
19799a22 2817You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2818what most people think of as "local". See
13a2d996 2819L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2820
5a964f20
TC
2821A local modifies the listed variables to be local to the enclosing
2822block, file, or eval. If more than one value is listed, the list must
2823be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2824for details, including issues with tied arrays and hashes.
a0d0e21e 2825
d361fafa
VP
2826The C<delete local EXPR> construct can also be used to localize the deletion
2827of array/hash elements to the current block.
2828See L<perlsub/"Localized deletion of elements of composite types">.
2829
a0d0e21e 2830=item localtime EXPR
435fbc73 2831X<localtime> X<ctime>
a0d0e21e 2832
ba053783
AL
2833=item localtime
2834
19799a22 2835Converts a time as returned by the time function to a 9-element list
5f05dabc 2836with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2837follows:
2838
54310121 2839 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2840 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2841 localtime(time);
a0d0e21e 2842
48a26b3a 2843All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2844tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2845of the specified time.
48a26b3a 2846
ba053783
AL
2847C<$mday> is the day of the month, and C<$mon> is the month itself, in
2848the range C<0..11> with 0 indicating January and 11 indicating December.
2849This makes it easy to get a month name from a list:
54310121 2850
ba053783
AL
2851 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2852 print "$abbr[$mon] $mday";
2853 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2854
ba053783
AL
2855C<$year> is the number of years since 1900, not just the last two digits
2856of the year. That is, C<$year> is C<123> in year 2023. The proper way
80d38338 2857to get a 4-digit year is simply:
abd75f24 2858
ba053783 2859 $year += 1900;
abd75f24 2860
435fbc73
GS
2861Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2862to do that, would you?
2863
ba053783
AL
2864To get the last two digits of the year (e.g., '01' in 2001) do:
2865
2866 $year = sprintf("%02d", $year % 100);
2867
2868C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2869Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2870(or C<0..365> in leap years.)
2871
2872C<$isdst> is true if the specified time occurs during Daylight Saving
2873Time, false otherwise.
abd75f24 2874
e1998452 2875If EXPR is omitted, C<localtime()> uses the current time (as returned
e3176d09 2876by time(3)).
a0d0e21e 2877
48a26b3a 2878In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2879
5f05dabc 2880 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2881
fe86afc2
NC
2882This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2883instead of local time use the L</gmtime> builtin. See also the
2884C<Time::Local> module (to convert the second, minutes, hours, ... back to
2885the integer value returned by time()), and the L<POSIX> module's strftime(3)
2886and mktime(3) functions.
2887
2888To get somewhat similar but locale dependent date strings, set up your
2889locale environment variables appropriately (please see L<perllocale>) and
2890try for example:
a3cb178b 2891
5a964f20 2892 use POSIX qw(strftime);
2b5ab1e7 2893 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2894 # or for GMT formatted appropriately for your locale:
2895 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2896
2897Note that the C<%a> and C<%b>, the short forms of the day of the week
2898and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2899
62aa5637
MS
2900See L<perlport/localtime> for portability concerns.
2901
435fbc73
GS
2902The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2903by-name access mechanism to the gmtime() and localtime() functions,
2904respectively.
2905
2906For a comprehensive date and time representation look at the
2907L<DateTime> module on CPAN.
2908
07698885 2909=item lock THING
d74e8afc 2910X<lock>
19799a22 2911
01e6739c 2912This function places an advisory lock on a shared variable, or referenced
03730085 2913object contained in I<THING> until the lock goes out of scope.
a6d5524e 2914
f3a23afb 2915lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2916by this name (before any calls to it), that function will be called
7b043ca5
RGS
2917instead. If you are not under C<use threads::shared> this does nothing.
2918See L<threads::shared>.
19799a22 2919
a0d0e21e 2920=item log EXPR
d74e8afc 2921X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2922
54310121 2923=item log
bbce6d69 2924
2b5ab1e7
TC
2925Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2926returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2927The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2928divided by the natural log of N. For example:
2929
2930 sub log10 {
a9a5a0dc
VP
2931 my $n = shift;
2932 return log($n)/log(10);
b76cc8ba 2933 }
2b5ab1e7
TC
2934
2935See also L</exp> for the inverse operation.
a0d0e21e 2936
a0d0e21e 2937=item lstat EXPR
d74e8afc 2938X<lstat>
a0d0e21e 2939
54310121 2940=item lstat
bbce6d69 2941
19799a22 2942Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2943special C<_> filehandle) but stats a symbolic link instead of the file
2944the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2945your system, a normal C<stat> is done. For much more detailed
2946information, please see the documentation for C<stat>.
a0d0e21e 2947
7660c0ab 2948If EXPR is omitted, stats C<$_>.
bbce6d69 2949
a0d0e21e
LW
2950=item m//
2951
9f4b9cd0 2952The match operator. See L<perlop/"Regexp Quote-Like Operators">.
a0d0e21e
LW
2953
2954=item map BLOCK LIST
d74e8afc 2955X<map>
a0d0e21e
LW
2956
2957=item map EXPR,LIST
2958
19799a22
GS
2959Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2960C<$_> to each element) and returns the list value composed of the
2961results of each such evaluation. In scalar context, returns the
2962total number of elements so generated. Evaluates BLOCK or EXPR in
2963list context, so each element of LIST may produce zero, one, or
2964more elements in the returned value.
dd99ebda 2965
a0d0e21e
LW
2966 @chars = map(chr, @nums);
2967
2968translates a list of numbers to the corresponding characters. And
2969
d8216f19 2970 %hash = map { get_a_key_for($_) => $_ } @array;
a0d0e21e
LW
2971
2972is just a funny way to write
2973
2974 %hash = ();
d8216f19 2975 foreach (@array) {
a9a5a0dc 2976 $hash{get_a_key_for($_)} = $_;
a0d0e21e
LW
2977 }
2978
be3174d2
GS
2979Note that C<$_> is an alias to the list value, so it can be used to
2980modify the elements of the LIST. While this is useful and supported,
2981it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2982Using a regular C<foreach> loop for this purpose would be clearer in
2983most cases. See also L</grep> for an array composed of those items of
2984the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2985
a4fb8298 2986If C<$_> is lexical in the scope where the C<map> appears (because it has
d8216f19
RGS
2987been declared with C<my $_>), then, in addition to being locally aliased to
2988the list elements, C<$_> keeps being lexical inside the block; that is, it
a4fb8298
RGS
2989can't be seen from the outside, avoiding any potential side-effects.
2990
205fdb4d 2991C<{> starts both hash references and blocks, so C<map { ...> could be either
3b10bc60 2992the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
80d38338
TC
2993ahead for the closing C<}> it has to take a guess at which it's dealing with
2994based on what it finds just after the C<{>. Usually it gets it right, but if it
205fdb4d
NC
2995doesn't it won't realize something is wrong until it gets to the C<}> and
2996encounters the missing (or unexpected) comma. The syntax error will be
80d38338 2997reported close to the C<}>, but you'll need to change something near the C<{>
3b10bc60 2998such as using a unary C<+> to give Perl some help:
205fdb4d 2999
3b10bc60 3000 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3001 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3002 %hash = map { ("\L$_" => 1) } @array # this also works
3003 %hash = map { lc($_) => 1 } @array # as does this.
3004 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
cea6626f 3005
3b10bc60 3006 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
205fdb4d 3007
d8216f19 3008or to force an anon hash constructor use C<+{>:
205fdb4d 3009
3b10bc60 3010 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end
205fdb4d 3011
3b10bc60 3012to get a list of anonymous hashes each with only one entry apiece.
205fdb4d 3013
19799a22 3014=item mkdir FILENAME,MASK
d74e8afc 3015X<mkdir> X<md> X<directory, create>
a0d0e21e 3016
5a211162
GS
3017=item mkdir FILENAME
3018
491873e5
RGS
3019=item mkdir
3020
0591cd52 3021Creates the directory specified by FILENAME, with permissions
19799a22
GS
3022specified by MASK (as modified by C<umask>). If it succeeds it
3023returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
3024If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
3025to C<$_>.
0591cd52 3026
19799a22 3027In general, it is better to create directories with permissive MASK,
0591cd52 3028and let the user modify that with their C<umask>, than it is to supply
19799a22 3029a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
3030The exceptions to this rule are when the file or directory should be
3031kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 3032C<umask> discusses the choice of MASK in more detail.
a0d0e21e 3033
cc1852e8
JH
3034Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3035number of trailing slashes. Some operating and filesystems do not get
3036this right, so Perl automatically removes all trailing slashes to keep
3037everyone happy.
3038
80d38338 3039To recursively create a directory structure, look at
dd184578
RGS
3040the C<mkpath> function of the L<File::Path> module.
3041
a0d0e21e 3042=item msgctl ID,CMD,ARG
d74e8afc 3043X<msgctl>
a0d0e21e 3044
f86cebdf 3045Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
3046
3047 use IPC::SysV;
3048
7660c0ab 3049first to get the correct constant definitions. If CMD is C<IPC_STAT>,
cf264981 3050then ARG must be a variable that will hold the returned C<msqid_ds>
951ba7fe
GS
3051structure. Returns like C<ioctl>: the undefined value for error,
3052C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 3053L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
3054
3055=item msgget KEY,FLAGS
d74e8afc 3056X<msgget>
a0d0e21e 3057
f86cebdf 3058Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
3059id, or the undefined value if there is an error. See also
3060L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 3061
a0d0e21e 3062=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
d74e8afc 3063X<msgrcv>
a0d0e21e
LW
3064
3065Calls the System V IPC function msgrcv to receive a message from
3066message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
3067SIZE. Note that when a message is received, the message type as a
3068native long integer will be the first thing in VAR, followed by the
3069actual message. This packing may be opened with C<unpack("l! a*")>.
3070Taints the variable. Returns true if successful, or false if there is
4755096e
GS
3071an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
3072C<IPC::SysV::Msg> documentation.
41d6edb2
JH
3073
3074=item msgsnd ID,MSG,FLAGS
d74e8afc 3075X<msgsnd>
41d6edb2
JH
3076
3077Calls the System V IPC function msgsnd to send the message MSG to the
3078message queue ID. MSG must begin with the native long integer message
3079type, and be followed by the length of the actual message, and finally
3080the message itself. This kind of packing can be achieved with
3081C<pack("l! a*", $type, $message)>. Returns true if successful,
3082or false if there is an error. See also C<IPC::SysV>
3083and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW