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a0d0e21e 1=head1 NAME
d74e8afc 2X<function>
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3
4perlfunc - Perl builtin functions
5
6=head1 DESCRIPTION
7
8The functions in this section can serve as terms in an expression.
9They fall into two major categories: list operators and named unary
10operators. These differ in their precedence relationship with a
11following comma. (See the precedence table in L<perlop>.) List
12operators take more than one argument, while unary operators can never
13take more than one argument. Thus, a comma terminates the argument of
14a unary operator, but merely separates the arguments of a list
15operator. A unary operator generally provides a scalar context to its
2b5ab1e7 16argument, while a list operator may provide either scalar or list
3b10bc60 17contexts for its arguments. If it does both, scalar arguments
18come first and list argument follow, and there can only ever
19be one such list argument. For instance, splice() has three scalar
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20arguments followed by a list, whereas gethostbyname() has four scalar
21arguments.
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22
23In the syntax descriptions that follow, list operators that expect a
3b10bc60 24list (and provide list context for elements of the list) are shown
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25with LIST as an argument. Such a list may consist of any combination
26of scalar arguments or list values; the list values will be included
27in the list as if each individual element were interpolated at that
28point in the list, forming a longer single-dimensional list value.
8bdbc703 29Commas should separate literal elements of the LIST.
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30
31Any function in the list below may be used either with or without
32parentheses around its arguments. (The syntax descriptions omit the
3b10bc60 33parentheses.) If you use parentheses, the simple but occasionally
34surprising rule is this: It I<looks> like a function, therefore it I<is> a
a0d0e21e 35function, and precedence doesn't matter. Otherwise it's a list
3b10bc60 36operator or unary operator, and precedence does matter. Whitespace
37between the function and left parenthesis doesn't count, so sometimes
38you need to be careful:
a0d0e21e 39
5ed4f2ec 40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
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45
46If you run Perl with the B<-w> switch it can warn you about this. For
47example, the third line above produces:
48
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
51
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52A few functions take no arguments at all, and therefore work as neither
53unary nor list operators. These include such functions as C<time>
54and C<endpwent>. For example, C<time+86_400> always means
55C<time() + 86_400>.
56
a0d0e21e 57For functions that can be used in either a scalar or list context,
54310121 58nonabortive failure is generally indicated in a scalar context by
a0d0e21e 59returning the undefined value, and in a list context by returning the
3b10bc60 60empty list.
a0d0e21e 61
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62Remember the following important rule: There is B<no rule> that relates
63the behavior of an expression in list context to its behavior in scalar
64context, or vice versa. It might do two totally different things.
80d38338 65Each operator and function decides which sort of value would be most
2b5ab1e7 66appropriate to return in scalar context. Some operators return the
5a964f20 67length of the list that would have been returned in list context. Some
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68operators return the first value in the list. Some operators return the
69last value in the list. Some operators return a count of successful
70operations. In general, they do what you want, unless you want
71consistency.
d74e8afc 72X<context>
a0d0e21e 73
d1be9408 74A named array in scalar context is quite different from what would at
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75first glance appear to be a list in scalar context. You can't get a list
76like C<(1,2,3)> into being in scalar context, because the compiler knows
77the context at compile time. It would generate the scalar comma operator
78there, not the list construction version of the comma. That means it
79was never a list to start with.
80
3b10bc60 81In general, functions in Perl that serve as wrappers for system calls ("syscalls")
f86cebdf 82of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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83true when they succeed and C<undef> otherwise, as is usually mentioned
84in the descriptions below. This is different from the C interfaces,
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85which return C<-1> on failure. Exceptions to this rule are C<wait>,
86C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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87variable on failure. Other functions do not, except accidentally.
88
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89Extension modules can also hook into the Perl parser to define new
90kinds of keyword-headed expression. These may look like functions, but
91may also look completely different. The syntax following the keyword
92is defined entirely by the extension. If you are an implementor, see
93L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
94a module, see the module's documentation for details of the syntax that
95it defines.
96
cb1a09d0 97=head2 Perl Functions by Category
d74e8afc 98X<function>
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99
100Here are Perl's functions (including things that look like
5a964f20 101functions, like some keywords and named operators)
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102arranged by category. Some functions appear in more
103than one place.
104
13a2d996 105=over 4
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106
107=item Functions for SCALARs or strings
d74e8afc 108X<scalar> X<string> X<character>
cb1a09d0 109
22fae026 110C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
1dc8ecb8 111C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
945c54fd 112C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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113
114=item Regular expressions and pattern matching
d74e8afc 115X<regular expression> X<regex> X<regexp>
cb1a09d0 116
ab4f32c2 117C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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118
119=item Numeric functions
d74e8afc 120X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 121
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122C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
123C<sin>, C<sqrt>, C<srand>
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124
125=item Functions for real @ARRAYs
d74e8afc 126X<array>
cb1a09d0 127
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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134
135=item Functions for real %HASHes
d74e8afc 136X<hash>
cb1a09d0 137
22fae026 138C<delete>, C<each>, C<exists>, C<keys>, C<values>
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139
140=item Input and output functions
d74e8afc 141X<I/O> X<input> X<output> X<dbm>
cb1a09d0 142
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143C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
144C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
0d863452 145C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
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146C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
147C<warn>, C<write>
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148
149=item Functions for fixed length data or records
150
22fae026 151C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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152
153=item Functions for filehandles, files, or directories
d74e8afc 154X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 155
22fae026 156C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 157C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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158C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
159C<umask>, C<unlink>, C<utime>
cb1a09d0 160
cf264981 161=item Keywords related to the control flow of your Perl program
d74e8afc 162X<control flow>
cb1a09d0 163
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164C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
165C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 166
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167=item Keywords related to switch
168
36fb85f3 169C<break>, C<continue>, C<given>, C<when>, C<default>
0d863452 170
3b10bc60 171(These are available only if you enable the C<"switch"> feature.
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172See L<feature> and L<perlsyn/"Switch statements">.)
173
54310121 174=item Keywords related to scoping
cb1a09d0 175
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176C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
177C<use>
178
3b10bc60 179(C<state> is available only if the C<"state"> feature is enabled. See
36fb85f3 180L<feature>.)
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181
182=item Miscellaneous functions
183
36fb85f3 184C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
834df1c5 185C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
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186
187=item Functions for processes and process groups
d74e8afc 188X<process> X<pid> X<process id>
cb1a09d0 189
22fae026 190C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
1dc8ecb8 191C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 192C<times>, C<wait>, C<waitpid>
cb1a09d0 193
3b10bc60 194=item Keywords related to Perl modules
d74e8afc 195X<module>
cb1a09d0 196
22fae026 197C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 198
353c6505 199=item Keywords related to classes and object-orientation
d74e8afc 200X<object> X<class> X<package>
cb1a09d0 201
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202C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
203C<untie>, C<use>
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204
205=item Low-level socket functions
d74e8afc 206X<socket> X<sock>
cb1a09d0 207
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208C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
209C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 210C<socket>, C<socketpair>
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211
212=item System V interprocess communication functions
d74e8afc 213X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 214
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215C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
216C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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217
218=item Fetching user and group info
d74e8afc 219X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 220
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221C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
222C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
223C<getpwuid>, C<setgrent>, C<setpwent>
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224
225=item Fetching network info
d74e8afc 226X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 227
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228C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
229C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
230C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
231C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
232C<setnetent>, C<setprotoent>, C<setservent>
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233
234=item Time-related functions
d74e8afc 235X<time> X<date>
cb1a09d0 236
22fae026 237C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 238
37798a01 239=item Functions new in perl5
d74e8afc 240X<perl5>
37798a01 241
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242C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
243C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
1dc8ecb8 244C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
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245C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
246C<ucfirst>, C<untie>, C<use>, C<when>
37798a01 247
3b10bc60 248* C<sub> was a keyword in Perl 4, but in Perl 5 it is an
5a964f20 249operator, which can be used in expressions.
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250
251=item Functions obsoleted in perl5
252
22fae026 253C<dbmclose>, C<dbmopen>
37798a01 254
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255=back
256
60f9f73c 257=head2 Portability
d74e8afc 258X<portability> X<Unix> X<portable>
60f9f73c 259
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260Perl was born in Unix and can therefore access all common Unix
261system calls. In non-Unix environments, the functionality of some
262Unix system calls may not be available, or details of the available
263functionality may differ slightly. The Perl functions affected
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264by this are:
265
266C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
267C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
268C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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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>,
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276C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
277C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
278C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 279C<shmwrite>, C<socket>, C<socketpair>,
80cbd5ad
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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
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287=head2 Alphabetical Listing of Perl Functions
288
3b10bc60 289=over
a0d0e21e 290
5b3c99c0 291=item -X FILEHANDLE
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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
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302operator takes one argument, either a filename, a filehandle, or a dirhandle,
303and tests the associated file to see if something is true about it. If the
7660c0ab 304argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 305Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 306the undefined value if the file doesn't exist. Despite the funny
d0821a6a 307names, precedence is the same as any other named unary operator. The
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308operator may be any of:
309
5ed4f2ec 310 -r File is readable by effective uid/gid.
311 -w File is writable by effective uid/gid.
312 -x File is executable by effective uid/gid.
313 -o File is owned by effective uid.
a0d0e21e 314
5ed4f2ec 315 -R File is readable by real uid/gid.
316 -W File is writable by real uid/gid.
317 -X File is executable by real uid/gid.
318 -O File is owned by real uid.
a0d0e21e 319
5ed4f2ec 320 -e File exists.
321 -z File has zero size (is empty).
322 -s File has nonzero size (returns size in bytes).
a0d0e21e 323
5ed4f2ec 324 -f File is a plain file.
325 -d File is a directory.
326 -l File is a symbolic link.
327 -p File is a named pipe (FIFO), or Filehandle is a pipe.
328 -S File is a socket.
329 -b File is a block special file.
330 -c File is a character special file.
331 -t Filehandle is opened to a tty.
a0d0e21e 332
5ed4f2ec 333 -u File has setuid bit set.
334 -g File has setgid bit set.
335 -k File has sticky bit set.
a0d0e21e 336
5ed4f2ec 337 -T File is an ASCII text file (heuristic guess).
338 -B File is a "binary" file (opposite of -T).
a0d0e21e 339
5ed4f2ec 340 -M Script start time minus file modification time, in days.
341 -A Same for access time.
342 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 343
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344Example:
345
346 while (<>) {
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347 chomp;
348 next unless -f $_; # ignore specials
349 #...
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350 }
351
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GS
352The interpretation of the file permission operators C<-r>, C<-R>,
353C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
354of the file and the uids and gids of the user. There may be other
ecae030f
MO
355reasons you can't actually read, write, or execute the file: for
356example network filesystem access controls, ACLs (access control lists),
357read-only filesystems, and unrecognized executable formats. Note
358that the use of these six specific operators to verify if some operation
359is possible is usually a mistake, because it may be open to race
360conditions.
5ff3f7a4 361
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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
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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
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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
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398structure of the previous file test (or stat operator) is used, saving
399a system call. (This doesn't work with C<-t>, and you need to remember
3b10bc60 400that lstat() and C<-l> leave values in the stat structure for the
5c9aa243 401symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 402an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 403Example:
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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
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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
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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<$hinthash> is a reference to a hash containing the value of C<%^H> when the
671caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
672of this hash, as they are the actual values stored in the optree.
673
748a9306 674Furthermore, when called from within the DB package, caller returns more
7660c0ab 675detailed information: it sets the list variable C<@DB::args> to be the
54310121 676arguments with which the subroutine was invoked.
748a9306 677
7660c0ab 678Be aware that the optimizer might have optimized call frames away before
19799a22 679C<caller> had a chance to get the information. That means that C<caller(N)>
80d38338 680might not return information about the call frame you expect it to, for
b76cc8ba 681C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 682previous time C<caller> was called.
7660c0ab 683
a0d0e21e 684=item chdir EXPR
d74e8afc
ITB
685X<chdir>
686X<cd>
f723aae1 687X<directory, change>
a0d0e21e 688
c4aca7d0
GA
689=item chdir FILEHANDLE
690
691=item chdir DIRHANDLE
692
ce2984c3
PF
693=item chdir
694
ffce7b87 695Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 696changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 697changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0 698variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
80d38338 699neither is set, C<chdir> does nothing. It returns true on success,
b4ad75f0 700false otherwise. See the example under C<die>.
a0d0e21e 701
3b10bc60 702On systems that support fchdir(2), you may pass a filehandle or
703directory handle as argument. On systems that don't support fchdir(2),
704passing handles raises an exception.
c4aca7d0 705
a0d0e21e 706=item chmod LIST
d74e8afc 707X<chmod> X<permission> X<mode>
a0d0e21e
LW
708
709Changes the permissions of a list of files. The first element of the
4633a7c4 710list must be the numerical mode, which should probably be an octal
4ad40acf 711number, and which definitely should I<not> be a string of octal digits:
3b10bc60 712C<0644> is okay, but C<"0644"> is not. Returns the number of files
dc848c6f 713successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e 714
3b10bc60 715 $cnt = chmod 0755, "foo", "bar";
a0d0e21e 716 chmod 0755, @executables;
3b10bc60 717 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
f86cebdf 718 # --w----r-T
3b10bc60 719 $mode = "0644"; chmod oct($mode), "foo"; # this is better
720 $mode = 0644; chmod $mode, "foo"; # this is best
a0d0e21e 721
3b10bc60 722On systems that support fchmod(2), you may pass filehandles among the
723files. On systems that don't support fchmod(2), passing filehandles raises
724an exception. Filehandles must be passed as globs or glob references to be
725recognized; barewords are considered filenames.
c4aca7d0
GA
726
727 open(my $fh, "<", "foo");
728 my $perm = (stat $fh)[2] & 07777;
729 chmod($perm | 0600, $fh);
730
3b10bc60 731You can also import the symbolic C<S_I*> constants from the C<Fcntl>
ca6e1c26
JH
732module:
733
3b10bc60 734 use Fcntl qw( :mode );
ca6e1c26 735 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
3b10bc60 736 # Identical to the chmod 0755 of the example above.
ca6e1c26 737
a0d0e21e 738=item chomp VARIABLE
d74e8afc 739X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 740
313c9f5c 741=item chomp( LIST )
a0d0e21e
LW
742
743=item chomp
744
2b5ab1e7
TC
745This safer version of L</chop> removes any trailing string
746that corresponds to the current value of C<$/> (also known as
28757baa
PP
747$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
748number of characters removed from all its arguments. It's often used to
749remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
750that the final record may be missing its newline. When in paragraph
751mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
752When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
753a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 754remove anything.
19799a22 755If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
756
757 while (<>) {
a9a5a0dc
VP
758 chomp; # avoid \n on last field
759 @array = split(/:/);
760 # ...
a0d0e21e
LW
761 }
762
4bf21a6d
RD
763If VARIABLE is a hash, it chomps the hash's values, but not its keys.
764
a0d0e21e
LW
765You can actually chomp anything that's an lvalue, including an assignment:
766
767 chomp($cwd = `pwd`);
768 chomp($answer = <STDIN>);
769
770If you chomp a list, each element is chomped, and the total number of
771characters removed is returned.
772
15e44fd8
RGS
773Note that parentheses are necessary when you're chomping anything
774that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
775is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
776C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
777C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
778as C<chomp($a, $b)>.
779
a0d0e21e 780=item chop VARIABLE
d74e8afc 781X<chop>
a0d0e21e 782
313c9f5c 783=item chop( LIST )
a0d0e21e
LW
784
785=item chop
786
787Chops off the last character of a string and returns the character
5b3eff12 788chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 789scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
790If VARIABLE is a hash, it chops the hash's values, but not its keys.
791
5b3eff12 792You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
793
794If you chop a list, each element is chopped. Only the value of the
19799a22 795last C<chop> is returned.
a0d0e21e 796
19799a22 797Note that C<chop> returns the last character. To return all but the last
748a9306
LW
798character, use C<substr($string, 0, -1)>.
799
15e44fd8
RGS
800See also L</chomp>.
801
a0d0e21e 802=item chown LIST
d74e8afc 803X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
804
805Changes the owner (and group) of a list of files. The first two
19799a22
GS
806elements of the list must be the I<numeric> uid and gid, in that
807order. A value of -1 in either position is interpreted by most
808systems to leave that value unchanged. Returns the number of files
809successfully changed.
a0d0e21e
LW
810
811 $cnt = chown $uid, $gid, 'foo', 'bar';
812 chown $uid, $gid, @filenames;
813
3b10bc60 814On systems that support fchown(2), you may pass filehandles among the
815files. On systems that don't support fchown(2), passing filehandles raises
816an exception. Filehandles must be passed as globs or glob references to be
817recognized; barewords are considered filenames.
c4aca7d0 818
54310121 819Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
820
821 print "User: ";
19799a22 822 chomp($user = <STDIN>);
5a964f20 823 print "Files: ";
19799a22 824 chomp($pattern = <STDIN>);
a0d0e21e
LW
825
826 ($login,$pass,$uid,$gid) = getpwnam($user)
a9a5a0dc 827 or die "$user not in passwd file";
a0d0e21e 828
5ed4f2ec 829 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
830 chown $uid, $gid, @ary;
831
54310121 832On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
833file unless you're the superuser, although you should be able to change
834the group to any of your secondary groups. On insecure systems, these
835restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
836On POSIX systems, you can detect this condition this way:
837
838 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
839 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 840
a0d0e21e 841=item chr NUMBER
d74e8afc 842X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 843
54310121 844=item chr
bbce6d69 845
a0d0e21e 846Returns the character represented by that NUMBER in the character set.
a0ed51b3 847For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 848chr(0x263a) is a Unicode smiley face.
aaa68c4a 849
8a064bd6 850Negative values give the Unicode replacement character (chr(0xfffd)),
80d38338 851except under the L<bytes> pragma, where the low eight bits of the value
8a064bd6
JH
852(truncated to an integer) are used.
853
974da8e5
JH
854If NUMBER is omitted, uses C<$_>.
855
b76cc8ba 856For the reverse, use L</ord>.
a0d0e21e 857
2575c402
JW
858Note that characters from 128 to 255 (inclusive) are by default
859internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 860
2575c402 861See L<perlunicode> for more about Unicode.
bbce6d69 862
a0d0e21e 863=item chroot FILENAME
d74e8afc 864X<chroot> X<root>
a0d0e21e 865
54310121 866=item chroot
bbce6d69 867
5a964f20 868This function works like the system call by the same name: it makes the
4633a7c4 869named directory the new root directory for all further pathnames that
951ba7fe 870begin with a C</> by your process and all its children. (It doesn't
28757baa 871change your current working directory, which is unaffected.) For security
4633a7c4 872reasons, this call is restricted to the superuser. If FILENAME is
19799a22 873omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
874
875=item close FILEHANDLE
d74e8afc 876X<close>
a0d0e21e 877
6a518fbc
TP
878=item close
879
3b10bc60 880Closes the file or pipe associated with the filehandle, flushes the IO
e0f13c26
RGS
881buffers, and closes the system file descriptor. Returns true if those
882operations have succeeded and if no error was reported by any PerlIO
883layer. Closes the currently selected filehandle if the argument is
884omitted.
fb73857a
PP
885
886You don't have to close FILEHANDLE if you are immediately going to do
3b10bc60 887another C<open> on it, because C<open> closes it for you. (See
19799a22
GS
888C<open>.) However, an explicit C<close> on an input file resets the line
889counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 890
3b10bc60 891If the filehandle came from a piped open, C<close> returns false if one of
892the other syscalls involved fails or if its program exits with non-zero
893status. If the only problem was that the program exited non-zero, C<$!>
894will be set to C<0>. Closing a pipe also waits for the process executing
895on the pipe to exit--in case you wish to look at the output of the pipe
896afterwards--and implicitly puts the exit status value of that command into
897C<$?> and C<${^CHILD_ERROR_NATIVE}>.
5a964f20 898
80d38338
TC
899Closing the read end of a pipe before the process writing to it at the
900other end is done writing results in the writer receiving a SIGPIPE. If
901the other end can't handle that, be sure to read all the data before
902closing the pipe.
73689b13 903
fb73857a 904Example:
a0d0e21e 905
fb73857a
PP
906 open(OUTPUT, '|sort >foo') # pipe to sort
907 or die "Can't start sort: $!";
5ed4f2ec 908 #... # print stuff to output
909 close OUTPUT # wait for sort to finish
fb73857a
PP
910 or warn $! ? "Error closing sort pipe: $!"
911 : "Exit status $? from sort";
5ed4f2ec 912 open(INPUT, 'foo') # get sort's results
fb73857a 913 or die "Can't open 'foo' for input: $!";
a0d0e21e 914
5a964f20
TC
915FILEHANDLE may be an expression whose value can be used as an indirect
916filehandle, usually the real filehandle name.
a0d0e21e
LW
917
918=item closedir DIRHANDLE
d74e8afc 919X<closedir>
a0d0e21e 920
19799a22 921Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
922system call.
923
a0d0e21e 924=item connect SOCKET,NAME
d74e8afc 925X<connect>
a0d0e21e 926
80d38338
TC
927Attempts to connect to a remote socket, just like connect(2).
928Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
929packed address of the appropriate type for the socket. See the examples in
930L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 931
cb1a09d0 932=item continue BLOCK
d74e8afc 933X<continue>
cb1a09d0 934
0d863452
RH
935=item continue
936
cf264981
SP
937C<continue> is actually a flow control statement rather than a function. If
938there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
939C<foreach>), it is always executed just before the conditional is about to
940be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
941it can be used to increment a loop variable, even when the loop has been
942continued via the C<next> statement (which is similar to the C C<continue>
943statement).
944
98293880 945C<last>, C<next>, or C<redo> may appear within a C<continue>
3b10bc60 946block; C<last> and C<redo> behave as if they had been executed within
19799a22 947the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
948block, it may be more entertaining.
949
950 while (EXPR) {
a9a5a0dc
VP
951 ### redo always comes here
952 do_something;
1d2dff63 953 } continue {
a9a5a0dc
VP
954 ### next always comes here
955 do_something_else;
956 # then back the top to re-check EXPR
1d2dff63
GS
957 }
958 ### last always comes here
959
3b10bc60 960Omitting the C<continue> section is equivalent to using an
961empty one, logically enough, so C<next> goes directly back
1d2dff63
GS
962to check the condition at the top of the loop.
963
3b10bc60 964If the C<"switch"> feature is enabled, C<continue> is also a
965function that exits the current C<when> (or C<default>) block and
966falls through to the next one. See L<feature> and
0d863452
RH
967L<perlsyn/"Switch statements"> for more information.
968
969
a0d0e21e 970=item cos EXPR
d74e8afc 971X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 972
d6217f1e
GS
973=item cos
974
5a964f20 975Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 976takes cosine of C<$_>.
a0d0e21e 977
ca6e1c26 978For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
979function, or use this relation:
980
981 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
982
a0d0e21e 983=item crypt PLAINTEXT,SALT
d74e8afc 984X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 985X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 986
ef2e6798
MS
987Creates a digest string exactly like the crypt(3) function in the C
988library (assuming that you actually have a version there that has not
bb23f8d1 989been extirpated as a potential munition).
ef2e6798
MS
990
991crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
992into a short string, called a digest, which is returned. The same
993PLAINTEXT and SALT will always return the same string, but there is no
994(known) way to get the original PLAINTEXT from the hash. Small
995changes in the PLAINTEXT or SALT will result in large changes in the
996digest.
997
998There is no decrypt function. This function isn't all that useful for
999cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1000mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1001primarily used to check if two pieces of text are the same without
1002having to transmit or store the text itself. An example is checking
1003if a correct password is given. The digest of the password is stored,
cf264981 1004not the password itself. The user types in a password that is
ef2e6798
MS
1005crypt()'d with the same salt as the stored digest. If the two digests
1006match the password is correct.
1007
1008When verifying an existing digest string you should use the digest as
1009the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1010to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1011crypt() will hash the new string with the same salt as the digest.
1012This allows your code to work with the standard L<crypt|/crypt> and
1013with more exotic implementations. In other words, do not assume
1014anything about the returned string itself, or how many bytes in the
1015digest matter.
85c16d83
JH
1016
1017Traditionally the result is a string of 13 bytes: two first bytes of
1018the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
bb23f8d1 1019the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798 1020hashing schemes (like MD5), higher level security schemes (like C2),
e1020413 1021and implementations on non-Unix platforms may produce different
ef2e6798 1022strings.
85c16d83
JH
1023
1024When choosing a new salt create a random two character string whose
1025characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1026'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1027characters is just a recommendation; the characters allowed in
1028the salt depend solely on your system's crypt library, and Perl can't
1029restrict what salts C<crypt()> accepts.
e71965be 1030
a0d0e21e 1031Here's an example that makes sure that whoever runs this program knows
cf264981 1032their password:
a0d0e21e
LW
1033
1034 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1035
1036 system "stty -echo";
1037 print "Password: ";
e71965be 1038 chomp($word = <STDIN>);
a0d0e21e
LW
1039 print "\n";
1040 system "stty echo";
1041
e71965be 1042 if (crypt($word, $pwd) ne $pwd) {
a9a5a0dc 1043 die "Sorry...\n";
a0d0e21e 1044 } else {
a9a5a0dc 1045 print "ok\n";
54310121 1046 }
a0d0e21e 1047
9f8f0c9d 1048Of course, typing in your own password to whoever asks you
748a9306 1049for it is unwise.
a0d0e21e 1050
ef2e6798 1051The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1052of data, not least of all because you can't get the information
ef2e6798 1053back. Look at the L<Digest> module for more robust algorithms.
19799a22 1054
f2791508
JH
1055If using crypt() on a Unicode string (which I<potentially> has
1056characters with codepoints above 255), Perl tries to make sense
1057of the situation by trying to downgrade (a copy of the string)
1058the string back to an eight-bit byte string before calling crypt()
1059(on that copy). If that works, good. If not, crypt() dies with
1060C<Wide character in crypt>.
85c16d83 1061
aa689395 1062=item dbmclose HASH
d74e8afc 1063X<dbmclose>
a0d0e21e 1064
19799a22 1065[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1066
aa689395 1067Breaks the binding between a DBM file and a hash.
a0d0e21e 1068
19799a22 1069=item dbmopen HASH,DBNAME,MASK
d74e8afc 1070X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1071
19799a22 1072[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1073
7b8d334a 1074This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1075hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1076argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1077is the name of the database (without the F<.dir> or F<.pag> extension if
1078any). If the database does not exist, it is created with protection
19799a22 1079specified by MASK (as modified by the C<umask>). If your system supports
80d38338 1080only the older DBM functions, you may make only one C<dbmopen> call in your
aa689395 1081program. In older versions of Perl, if your system had neither DBM nor
19799a22 1082ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1083sdbm(3).
1084
1085If you don't have write access to the DBM file, you can only read hash
1086variables, not set them. If you want to test whether you can write,
3b10bc60 1087either use file tests or try setting a dummy hash entry inside an C<eval>
1088to trap the error.
a0d0e21e 1089
19799a22
GS
1090Note that functions such as C<keys> and C<values> may return huge lists
1091when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1092function to iterate over large DBM files. Example:
1093
1094 # print out history file offsets
1095 dbmopen(%HIST,'/usr/lib/news/history',0666);
1096 while (($key,$val) = each %HIST) {
a9a5a0dc 1097 print $key, ' = ', unpack('L',$val), "\n";
a0d0e21e
LW
1098 }
1099 dbmclose(%HIST);
1100
cb1a09d0 1101See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1102cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1103rich implementation.
4633a7c4 1104
2b5ab1e7
TC
1105You can control which DBM library you use by loading that library
1106before you call dbmopen():
1107
1108 use DB_File;
1109 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
a9a5a0dc 1110 or die "Can't open netscape history file: $!";
2b5ab1e7 1111
a0d0e21e 1112=item defined EXPR
d74e8afc 1113X<defined> X<undef> X<undefined>
a0d0e21e 1114
54310121 1115=item defined
bbce6d69 1116
2f9daede 1117Returns a Boolean value telling whether EXPR has a value other than
3b10bc60 1118the undefined value C<undef>. If EXPR is not present, C<$_> is
2f9daede
TPG
1119checked.
1120
1121Many operations return C<undef> to indicate failure, end of file,
1122system error, uninitialized variable, and other exceptional
1123conditions. This function allows you to distinguish C<undef> from
1124other values. (A simple Boolean test will not distinguish among
7660c0ab 1125C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1126false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1127doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1128returns C<undef> when its argument is an empty array, I<or> when the
1129element to return happens to be C<undef>.
1130
f10b0346
GS
1131You may also use C<defined(&func)> to check whether subroutine C<&func>
1132has ever been defined. The return value is unaffected by any forward
80d38338 1133declarations of C<&func>. A subroutine that is not defined
847c7ebe 1134may still be callable: its package may have an C<AUTOLOAD> method that
3b10bc60 1135makes it spring into existence the first time that it is called; see
847c7ebe 1136L<perlsub>.
f10b0346
GS
1137
1138Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1139used to report whether memory for that aggregate has ever been
1140allocated. This behavior may disappear in future versions of Perl.
1141You should instead use a simple test for size:
1142
1143 if (@an_array) { print "has array elements\n" }
1144 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1145
1146When used on a hash element, it tells you whether the value is defined,
dc848c6f 1147not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1148purpose.
a0d0e21e
LW
1149
1150Examples:
1151
1152 print if defined $switch{'D'};
1153 print "$val\n" while defined($val = pop(@ary));
1154 die "Can't readlink $sym: $!"
a9a5a0dc 1155 unless defined($value = readlink $sym);
a0d0e21e 1156 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1157 $debugging = 0 unless defined $debugging;
a0d0e21e 1158
19799a22 1159Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1160discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1161defined values. For example, if you say
a5f75d66
AD
1162
1163 "ab" =~ /a(.*)b/;
1164
80d38338 1165The pattern match succeeds and C<$1> is defined, although it
cf264981 1166matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1167matched something that happened to be zero characters long. This is all
a5f75d66 1168very above-board and honest. When a function returns an undefined value,
2f9daede 1169it's an admission that it couldn't give you an honest answer. So you
3b10bc60 1170should use C<defined> only when questioning the integrity of what
7660c0ab 1171you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1172what you want.
1173
dc848c6f 1174See also L</undef>, L</exists>, L</ref>.
2f9daede 1175
a0d0e21e 1176=item delete EXPR
d74e8afc 1177X<delete>
a0d0e21e 1178
80d38338
TC
1179Given an expression that specifies an element or slice of an aggregate (a
1180hash or an array), deletes the specified elements from that aggregate so
1181that exists() on that element no longer returns true. Setting an aggregate
1182element to the undefined value does not remove its key, but deleting it
1183does; see L</exists>.
1184
1185Returns the value or values deleted in list context, or the last such
1186element in scalar context. The return list's length always matches that of
1187the argument list: deleting non-existent elements returns the undefined
1188value in their corresponding positions.
1189
1190Deleting array elements never changes indices of existing values; use
1191shift() or splice() for that. However, if all deleted elements fall at
1192the end of an array, the array's size shrinks to the position of the
1193highest element that still tests true for exists(), or to 0 if none do.
1194
1195Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1196a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1197or array may not necessarily return anything; it depends on the implementation
1198of the C<tied> package's DELETE method, which may do whatever it pleases.
a0d0e21e 1199
80d38338
TC
1200The C<delete local EXPR> construct localizes the deletion to the current
1201block at run time. Until the block exits, elements locally deleted
1202temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1203of composite types">.
eba0920a
EM
1204
1205 %hash = (foo => 11, bar => 22, baz => 33);
1206 $scalar = delete $hash{foo}; # $scalar is 11
1207 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1208 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1209
01020589 1210The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1211
5f05dabc 1212 foreach $key (keys %HASH) {
a9a5a0dc 1213 delete $HASH{$key};
a0d0e21e
LW
1214 }
1215
01020589 1216 foreach $index (0 .. $#ARRAY) {
a9a5a0dc 1217 delete $ARRAY[$index];
01020589
GS
1218 }
1219
1220And so do these:
5f05dabc 1221
01020589
GS
1222 delete @HASH{keys %HASH};
1223
9740c838 1224 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1225
80d38338
TC
1226But both are slower than assigning the empty list
1227or undefining %HASH or @ARRAY, which is the customary
1228way to empty out an aggregate:
01020589 1229
5ed4f2ec 1230 %HASH = (); # completely empty %HASH
1231 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1232
5ed4f2ec 1233 @ARRAY = (); # completely empty @ARRAY
1234 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7 1235
80d38338
TC
1236The EXPR can be arbitrarily complicated provided its
1237final operation is an element or slice of an aggregate:
a0d0e21e
LW
1238
1239 delete $ref->[$x][$y]{$key};
5f05dabc 1240 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1241
01020589
GS
1242 delete $ref->[$x][$y][$index];
1243 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1244
a0d0e21e 1245=item die LIST
d74e8afc 1246X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1247
19799a22
GS
1248Outside an C<eval>, prints the value of LIST to C<STDERR> and
1249exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1250exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1251status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1252an C<eval(),> the error message is stuffed into C<$@> and the
1253C<eval> is terminated with the undefined value. This makes
1254C<die> the way to raise an exception.
a0d0e21e
LW
1255
1256Equivalent examples:
1257
1258 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1259 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1260
ccac6780 1261If the last element of LIST does not end in a newline, the current
df37ec69
WW
1262script line number and input line number (if any) are also printed,
1263and a newline is supplied. Note that the "input line number" (also
1264known as "chunk") is subject to whatever notion of "line" happens to
1265be currently in effect, and is also available as the special variable
1266C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1267
1268Hint: sometimes appending C<", stopped"> to your message will cause it
1269to make better sense when the string C<"at foo line 123"> is appended.
1270Suppose you are running script "canasta".
a0d0e21e
LW
1271
1272 die "/etc/games is no good";
1273 die "/etc/games is no good, stopped";
1274
1275produce, respectively
1276
1277 /etc/games is no good at canasta line 123.
1278 /etc/games is no good, stopped at canasta line 123.
1279
2b5ab1e7 1280See also exit(), warn(), and the Carp module.
a0d0e21e 1281
a96d0188 1282If the output is empty and C<$@> already contains a value (typically from a
7660c0ab 1283previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1284This is useful for propagating exceptions:
1285
1286 eval { ... };
1287 die unless $@ =~ /Expected exception/;
1288
a96d0188 1289If the output is empty and C<$@> contains an object reference that has a
ad216e65
JH
1290C<PROPAGATE> method, that method will be called with additional file
1291and line number parameters. The return value replaces the value in
80d38338 1292C<$@>. i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1293were called.
1294
7660c0ab 1295If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1296
80d38338
TC
1297You can also call C<die> with a reference argument, and if this is trapped
1298within an C<eval>, C<$@> contains that reference. This permits more
1299elaborate exception handling using objects that maintain arbitrary state
1300about the exception. Such a scheme is sometimes preferable to matching
1301particular string values of C<$@> with regular expressions. Because C<$@>
1302is a global variable and C<eval> may be used within object implementations,
1303be careful that analyzing the error object doesn't replace the reference in
1304the global variable. It's easiest to make a local copy of the reference
1305before any manipulations. Here's an example:
52531d10 1306
80d38338 1307 use Scalar::Util "blessed";
da279afe 1308
52531d10 1309 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1310 if (my $ev_err = $@) {
1311 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1312 # handle Some::Module::Exception
1313 }
1314 else {
1315 # handle all other possible exceptions
1316 }
1317 }
1318
3b10bc60 1319Because Perl stringifies uncaught exception messages before display,
80d38338 1320you'll probably want to overload stringification operations on
52531d10
GS
1321exception objects. See L<overload> for details about that.
1322
19799a22
GS
1323You can arrange for a callback to be run just before the C<die>
1324does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
3b10bc60 1325handler is called with the error text and can change the error
19799a22
GS
1326message, if it sees fit, by calling C<die> again. See
1327L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1328L<"eval BLOCK"> for some examples. Although this feature was
19799a22 1329to be run only right before your program was to exit, this is not
3b10bc60 1330currently so: the C<$SIG{__DIE__}> hook is currently called
19799a22
GS
1331even inside eval()ed blocks/strings! If one wants the hook to do
1332nothing in such situations, put
fb73857a 1333
5ed4f2ec 1334 die @_ if $^S;
fb73857a 1335
19799a22
GS
1336as the first line of the handler (see L<perlvar/$^S>). Because
1337this promotes strange action at a distance, this counterintuitive
b76cc8ba 1338behavior may be fixed in a future release.
774d564b 1339
a0d0e21e 1340=item do BLOCK
d74e8afc 1341X<do> X<block>
a0d0e21e
LW
1342
1343Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1344sequence of commands indicated by BLOCK. When modified by the C<while> or
1345C<until> loop modifier, executes the BLOCK once before testing the loop
1346condition. (On other statements the loop modifiers test the conditional
1347first.)
a0d0e21e 1348
4968c1e4 1349C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1350C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1351See L<perlsyn> for alternative strategies.
4968c1e4 1352
a0d0e21e 1353=item do SUBROUTINE(LIST)
d74e8afc 1354X<do>
a0d0e21e 1355
cf264981 1356This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1357
1358=item do EXPR
d74e8afc 1359X<do>
a0d0e21e
LW
1360
1361Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1362file as a Perl script.
a0d0e21e
LW
1363
1364 do 'stat.pl';
1365
1366is just like
1367
986b19de 1368 eval `cat stat.pl`;
a0d0e21e 1369
2b5ab1e7 1370except that it's more efficient and concise, keeps track of the current
ea63ef19 1371filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1372C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1373variables. It also differs in that code evaluated with C<do FILENAME>
1374cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1375same, however, in that it does reparse the file every time you call it,
1376so you probably don't want to do this inside a loop.
a0d0e21e 1377
8e30cc93 1378If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1379error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1380returns undef and sets an error message in C<$@>. If the file is
1381successfully compiled, C<do> returns the value of the last expression
1382evaluated.
1383
80d38338 1384Inclusion of library modules is better done with the
19799a22 1385C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1386and raise an exception if there's a problem.
a0d0e21e 1387
5a964f20
TC
1388You might like to use C<do> to read in a program configuration
1389file. Manual error checking can be done this way:
1390
b76cc8ba 1391 # read in config files: system first, then user
f86cebdf 1392 for $file ("/share/prog/defaults.rc",
b76cc8ba 1393 "$ENV{HOME}/.someprogrc")
a9a5a0dc
VP
1394 {
1395 unless ($return = do $file) {
1396 warn "couldn't parse $file: $@" if $@;
1397 warn "couldn't do $file: $!" unless defined $return;
1398 warn "couldn't run $file" unless $return;
1399 }
5a964f20
TC
1400 }
1401
a0d0e21e 1402=item dump LABEL
d74e8afc 1403X<dump> X<core> X<undump>
a0d0e21e 1404
1614b0e3
JD
1405=item dump
1406
19799a22
GS
1407This function causes an immediate core dump. See also the B<-u>
1408command-line switch in L<perlrun>, which does the same thing.
1409Primarily this is so that you can use the B<undump> program (not
1410supplied) to turn your core dump into an executable binary after
1411having initialized all your variables at the beginning of the
1412program. When the new binary is executed it will begin by executing
1413a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1414Think of it as a goto with an intervening core dump and reincarnation.
1415If C<LABEL> is omitted, restarts the program from the top.
1416
1417B<WARNING>: Any files opened at the time of the dump will I<not>
1418be open any more when the program is reincarnated, with possible
80d38338 1419resulting confusion by Perl.
19799a22 1420
59f521f4
RGS
1421This function is now largely obsolete, mostly because it's very hard to
1422convert a core file into an executable. That's why you should now invoke
1423it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1424typo.
19799a22 1425
aa689395 1426=item each HASH
d74e8afc 1427X<each> X<hash, iterator>
aa689395 1428
aeedbbed
NC
1429=item each ARRAY
1430X<array, iterator>
1431
80d38338
TC
1432When called in list context, returns a 2-element list consisting of the key
1433and value for the next element of a hash, or the index and value for the
1434next element of an array, so that you can iterate over it. When called in
1435scalar context, returns only the key (not the value) in a hash, or the index
1436in an array.
2f9daede 1437
aeedbbed 1438Hash entries are returned in an apparently random order. The actual random
3b10bc60 1439order is subject to change in future versions of Perl, but it is
504f80c1 1440guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1441function would produce on the same (unmodified) hash. Since Perl
22883ac5 14425.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1443for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1444
80d38338
TC
1445After C<each> has returned all entries from the hash or array, the next
1446call to C<each> returns the empty list in list context and C<undef> in
1447scalar context. The next call following that one restarts iteration. Each
1448hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1449and C<values>. The iterator is implicitly reset when C<each> has reached
1450the end as just described; it can be explicitly reset by calling C<keys> or
1451C<values> on the hash or array. If you add or delete a hash's elements
1452while iterating over it, entries may be skipped or duplicated--so don't do
1453that. Exception: It is always safe to delete the item most recently
3b10bc60 1454returned by C<each()>, so the following code works properly:
74fc8b5f
MJD
1455
1456 while (($key, $value) = each %hash) {
1457 print $key, "\n";
1458 delete $hash{$key}; # This is safe
1459 }
aa689395 1460
80d38338 1461This prints out your environment like the printenv(1) program,
3b10bc60 1462but in a different order:
a0d0e21e
LW
1463
1464 while (($key,$value) = each %ENV) {
a9a5a0dc 1465 print "$key=$value\n";
a0d0e21e
LW
1466 }
1467
19799a22 1468See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1469
1470=item eof FILEHANDLE
d74e8afc
ITB
1471X<eof>
1472X<end of file>
1473X<end-of-file>
a0d0e21e 1474
4633a7c4
LW
1475=item eof ()
1476
a0d0e21e
LW
1477=item eof
1478
1479Returns 1 if the next read on FILEHANDLE will return end of file, or if
1480FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1481gives the real filehandle. (Note that this function actually
80d38338 1482reads a character and then C<ungetc>s it, so isn't useful in an
748a9306 1483interactive context.) Do not read from a terminal file (or call
19799a22 1484C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1485as terminals may lose the end-of-file condition if you do.
1486
820475bd 1487An C<eof> without an argument uses the last file read. Using C<eof()>
80d38338 1488with empty parentheses is different. It refers to the pseudo file
820475bd 1489formed from the files listed on the command line and accessed via the
61eff3bc
JH
1490C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1491as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1492used will cause C<@ARGV> to be examined to determine if input is
67408cae 1493available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1494end-of-file will assume you are processing another C<@ARGV> list,
1495and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1496see L<perlop/"I/O Operators">.
820475bd 1497
61eff3bc 1498In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
3b10bc60 1499detect the end of each file, C<eof()> will detect the end of only the
820475bd 1500last file. Examples:
a0d0e21e 1501
748a9306
LW
1502 # reset line numbering on each input file
1503 while (<>) {
a9a5a0dc
VP
1504 next if /^\s*#/; # skip comments
1505 print "$.\t$_";
5a964f20 1506 } continue {
a9a5a0dc 1507 close ARGV if eof; # Not eof()!
748a9306
LW
1508 }
1509
a0d0e21e
LW
1510 # insert dashes just before last line of last file
1511 while (<>) {
a9a5a0dc
VP
1512 if (eof()) { # check for end of last file
1513 print "--------------\n";
1514 }
1515 print;
1516 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1517 }
1518
a0d0e21e 1519Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1520input operators typically return C<undef> when they run out of data, or if
1521there was an error.
a0d0e21e
LW
1522
1523=item eval EXPR
d74e8afc 1524X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1525X<error, handling> X<exception, handling>
a0d0e21e
LW
1526
1527=item eval BLOCK
1528
ce2984c3
PF
1529=item eval
1530
c7cc6f1c
GS
1531In the first form, the return value of EXPR is parsed and executed as if it
1532were a little Perl program. The value of the expression (which is itself
5a964f20 1533determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1534errors, executed in the lexical context of the current Perl program, so
1535that any variable settings or subroutine and format definitions remain
cf264981 1536afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1537If EXPR is omitted, evaluates C<$_>. This form is typically used to
1538delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1539
1540In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1541same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1542within the context of the current Perl program. This form is typically
1543used to trap exceptions more efficiently than the first (see below), while
1544also providing the benefit of checking the code within BLOCK at compile
1545time.
1546
1547The final semicolon, if any, may be omitted from the value of EXPR or within
1548the BLOCK.
1549
1550In both forms, the value returned is the value of the last expression
5a964f20 1551evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1552as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1553in void, scalar, or list context, depending on the context of the C<eval>
1554itself. See L</wantarray> for more on how the evaluation context can be
1555determined.
a0d0e21e 1556
19799a22 1557If there is a syntax error or runtime error, or a C<die> statement is
bbead3ca
BL
1558executed, C<eval> returns an undefined value in scalar context
1559or an empty list in list context, and C<$@> is set to the
3b10bc60 1560error message. If there was no error, C<$@> is guaranteed to be the empty
1561string. Beware that using C<eval> neither silences Perl from printing
c7cc6f1c 1562warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1563To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1564turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1565See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1566
19799a22
GS
1567Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1568determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1569is implemented. It is also Perl's exception trapping mechanism, where
1570the die operator is used to raise exceptions.
1571
5f1da31c
NT
1572If you want to trap errors when loading an XS module, some problems with
1573the binary interface (such as Perl version skew) may be fatal even with
1574C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1575
a0d0e21e
LW
1576If the code to be executed doesn't vary, you may use the eval-BLOCK
1577form to trap run-time errors without incurring the penalty of
1578recompiling each time. The error, if any, is still returned in C<$@>.
1579Examples:
1580
54310121 1581 # make divide-by-zero nonfatal
a0d0e21e
LW
1582 eval { $answer = $a / $b; }; warn $@ if $@;
1583
1584 # same thing, but less efficient
1585 eval '$answer = $a / $b'; warn $@ if $@;
1586
1587 # a compile-time error
5ed4f2ec 1588 eval { $answer = }; # WRONG
a0d0e21e
LW
1589
1590 # a run-time error
5ed4f2ec 1591 eval '$answer ='; # sets $@
a0d0e21e 1592
cf264981
SP
1593Using the C<eval{}> form as an exception trap in libraries does have some
1594issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1595may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7 1596You can use the C<local $SIG{__DIE__}> construct for this purpose,
80d38338 1597as this example shows:
774d564b 1598
80d38338 1599 # a private exception trap for divide-by-zero
f86cebdf
GS
1600 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1601 warn $@ if $@;
774d564b
PP
1602
1603This is especially significant, given that C<__DIE__> hooks can call
19799a22 1604C<die> again, which has the effect of changing their error messages:
774d564b
PP
1605
1606 # __DIE__ hooks may modify error messages
1607 {
f86cebdf
GS
1608 local $SIG{'__DIE__'} =
1609 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1610 eval { die "foo lives here" };
1611 print $@ if $@; # prints "bar lives here"
774d564b
PP
1612 }
1613
19799a22 1614Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1615may be fixed in a future release.
1616
19799a22 1617With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1618being looked at when:
1619
5ed4f2ec 1620 eval $x; # CASE 1
1621 eval "$x"; # CASE 2
a0d0e21e 1622
5ed4f2ec 1623 eval '$x'; # CASE 3
1624 eval { $x }; # CASE 4
a0d0e21e 1625
5ed4f2ec 1626 eval "\$$x++"; # CASE 5
1627 $$x++; # CASE 6
a0d0e21e 1628
2f9daede 1629Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1630the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1631the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1632and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1633does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1634purely visual reasons, but it also has the advantage of compiling at
1635compile-time instead of at run-time.) Case 5 is a place where
19799a22 1636normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1637particular situation, you can just use symbolic references instead, as
1638in case 6.
a0d0e21e 1639
8a5a710d
DN
1640The assignment to C<$@> occurs before restoration of localised variables,
1641which means a temporary is required if you want to mask some but not all
1642errors:
1643
1644 # alter $@ on nefarious repugnancy only
1645 {
1646 my $e;
1647 {
1648 local $@; # protect existing $@
1649 eval { test_repugnancy() };
1650 # $@ =~ /nefarious/ and die $@; # DOES NOT WORK
1651 $@ =~ /nefarious/ and $e = $@;
1652 }
1653 die $e if defined $e
1654 }
1655
4968c1e4 1656C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1657C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1658
3b10bc60 1659An C<eval ''> executed within the C<DB> package doesn't see the usual
1660surrounding lexical scope, but rather the scope of the first non-DB piece
1661of code that called it. You don't normally need to worry about this unless
1662you are writing a Perl debugger.
d819b83a 1663
a0d0e21e 1664=item exec LIST
d74e8afc 1665X<exec> X<execute>
a0d0e21e 1666
8bf3b016
GS
1667=item exec PROGRAM LIST
1668
3b10bc60 1669The C<exec> function executes a system command I<and never returns>;
19799a22
GS
1670use C<system> instead of C<exec> if you want it to return. It fails and
1671returns false only if the command does not exist I<and> it is executed
fb73857a 1672directly instead of via your system's command shell (see below).
a0d0e21e 1673
19799a22 1674Since it's a common mistake to use C<exec> instead of C<system>, Perl
80d38338 1675warns you if there is a following statement that isn't C<die>, C<warn>,
3b10bc60 1676or C<exit> (if C<-w> is set--but you always do that, right?). If you
19799a22 1677I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1678can use one of these styles to avoid the warning:
1679
5a964f20
TC
1680 exec ('foo') or print STDERR "couldn't exec foo: $!";
1681 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1682
5a964f20 1683If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1684with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1685If there is only one scalar argument or an array with one element in it,
1686the argument is checked for shell metacharacters, and if there are any,
1687the entire argument is passed to the system's command shell for parsing
1688(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1689If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1690words and passed directly to C<execvp>, which is more efficient.
19799a22 1691Examples:
a0d0e21e 1692
19799a22
GS
1693 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1694 exec "sort $outfile | uniq";
a0d0e21e
LW
1695
1696If you don't really want to execute the first argument, but want to lie
1697to the program you are executing about its own name, you can specify
1698the program you actually want to run as an "indirect object" (without a
1699comma) in front of the LIST. (This always forces interpretation of the
54310121 1700LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1701the list.) Example:
1702
1703 $shell = '/bin/csh';
5ed4f2ec 1704 exec $shell '-sh'; # pretend it's a login shell
a0d0e21e
LW
1705
1706or, more directly,
1707
5ed4f2ec 1708 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
a0d0e21e 1709
3b10bc60 1710When the arguments get executed via the system shell, results are
1711subject to its quirks and capabilities. See L<perlop/"`STRING`">
bb32b41a
GS
1712for details.
1713
19799a22
GS
1714Using an indirect object with C<exec> or C<system> is also more
1715secure. This usage (which also works fine with system()) forces
1716interpretation of the arguments as a multivalued list, even if the
1717list had just one argument. That way you're safe from the shell
1718expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1719
1720 @args = ( "echo surprise" );
1721
2b5ab1e7 1722 exec @args; # subject to shell escapes
f86cebdf 1723 # if @args == 1
2b5ab1e7 1724 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1725
1726The first version, the one without the indirect object, ran the I<echo>
80d38338
TC
1727program, passing it C<"surprise"> an argument. The second version didn't;
1728it tried to run a program named I<"echo surprise">, didn't find it, and set
1729C<$?> to a non-zero value indicating failure.
5a964f20 1730
80d38338 1731Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1732output before the exec, but this may not be supported on some platforms
1733(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1734in English) or call the C<autoflush()> method of C<IO::Handle> on any
80d38338 1735open handles to avoid lost output.
0f897271 1736
80d38338
TC
1737Note that C<exec> will not call your C<END> blocks, nor will it invoke
1738C<DESTROY> methods on your objects.
7660c0ab 1739
a0d0e21e 1740=item exists EXPR
d74e8afc 1741X<exists> X<autovivification>
a0d0e21e 1742
3b10bc60 1743Given an expression that specifies an element of a hash or array,
1744returns true if the specified element in that aggregate has ever
95731d22 1745been initialized, even if the corresponding value is undefined.
a0d0e21e 1746
5ed4f2ec 1747 print "Exists\n" if exists $hash{$key};
1748 print "Defined\n" if defined $hash{$key};
01020589
GS
1749 print "True\n" if $hash{$key};
1750
5ed4f2ec 1751 print "Exists\n" if exists $array[$index];
1752 print "Defined\n" if defined $array[$index];
01020589 1753 print "True\n" if $array[$index];
a0d0e21e 1754
8ea97a1e 1755A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1756it exists, but the reverse doesn't necessarily hold true.
1757
afebc493
GS
1758Given an expression that specifies the name of a subroutine,
1759returns true if the specified subroutine has ever been declared, even
1760if it is undefined. Mentioning a subroutine name for exists or defined
80d38338 1761does not count as declaring it. Note that a subroutine that does not
847c7ebe
DD
1762exist may still be callable: its package may have an C<AUTOLOAD>
1763method that makes it spring into existence the first time that it is
3b10bc60 1764called; see L<perlsub>.
afebc493 1765
5ed4f2ec 1766 print "Exists\n" if exists &subroutine;
1767 print "Defined\n" if defined &subroutine;
afebc493 1768
a0d0e21e 1769Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1770operation is a hash or array key lookup or subroutine name:
a0d0e21e 1771
5ed4f2ec 1772 if (exists $ref->{A}->{B}->{$key}) { }
1773 if (exists $hash{A}{B}{$key}) { }
2b5ab1e7 1774
5ed4f2ec 1775 if (exists $ref->{A}->{B}->[$ix]) { }
1776 if (exists $hash{A}{B}[$ix]) { }
01020589 1777
afebc493
GS
1778 if (exists &{$ref->{A}{B}{$key}}) { }
1779
3b10bc60 1780Although the mostly deeply nested array or hash will not spring into
1781existence just because its existence was tested, any intervening ones will.
61eff3bc 1782Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589 1783into existence due to the existence test for the $key element above.
3b10bc60 1784This happens anywhere the arrow operator is used, including even here:
5a964f20 1785
2b5ab1e7 1786 undef $ref;
5ed4f2ec 1787 if (exists $ref->{"Some key"}) { }
1788 print $ref; # prints HASH(0x80d3d5c)
2b5ab1e7
TC
1789
1790This surprising autovivification in what does not at first--or even
1791second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1792release.
a0d0e21e 1793
afebc493
GS
1794Use of a subroutine call, rather than a subroutine name, as an argument
1795to exists() is an error.
1796
5ed4f2ec 1797 exists &sub; # OK
1798 exists &sub(); # Error
afebc493 1799
a0d0e21e 1800=item exit EXPR
d74e8afc 1801X<exit> X<terminate> X<abort>
a0d0e21e 1802
ce2984c3
PF
1803=item exit
1804
2b5ab1e7 1805Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1806
1807 $ans = <STDIN>;
1808 exit 0 if $ans =~ /^[Xx]/;
1809
19799a22 1810See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1811universally recognized values for EXPR are C<0> for success and C<1>
1812for error; other values are subject to interpretation depending on the
1813environment in which the Perl program is running. For example, exiting
181469 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1815the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1816
19799a22
GS
1817Don't use C<exit> to abort a subroutine if there's any chance that
1818someone might want to trap whatever error happened. Use C<die> instead,
1819which can be trapped by an C<eval>.
28757baa 1820
19799a22 1821The exit() function does not always exit immediately. It calls any
2b5ab1e7 1822defined C<END> routines first, but these C<END> routines may not
19799a22 1823themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1824be called are called before the real exit. If this is a problem, you
1825can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1826See L<perlmod> for details.
5a964f20 1827
a0d0e21e 1828=item exp EXPR
d74e8afc 1829X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1830
54310121 1831=item exp
bbce6d69 1832
b76cc8ba 1833Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1834If EXPR is omitted, gives C<exp($_)>.
1835
1836=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1837X<fcntl>
a0d0e21e 1838
f86cebdf 1839Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1840
1841 use Fcntl;
1842
0ade1984 1843first to get the correct constant definitions. Argument processing and
3b10bc60 1844value returned work just like C<ioctl> below.
a0d0e21e
LW
1845For example:
1846
1847 use Fcntl;
5a964f20 1848 fcntl($filehandle, F_GETFL, $packed_return_buffer)
a9a5a0dc 1849 or die "can't fcntl F_GETFL: $!";
5a964f20 1850
554ad1fc 1851You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1852Like C<ioctl>, it maps a C<0> return from the system call into
1853C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1854in numeric context. It is also exempt from the normal B<-w> warnings
1855on improper numeric conversions.
5a964f20 1856
3b10bc60 1857Note that C<fcntl> raises an exception if used on a machine that
2b5ab1e7
TC
1858doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1859manpage to learn what functions are available on your system.
a0d0e21e 1860
be2f7487 1861Here's an example of setting a filehandle named C<REMOTE> to be
1862non-blocking at the system level. You'll have to negotiate C<$|>
1863on your own, though.
1864
1865 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1866
1867 $flags = fcntl(REMOTE, F_GETFL, 0)
1868 or die "Can't get flags for the socket: $!\n";
1869
1870 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1871 or die "Can't set flags for the socket: $!\n";
1872
a0d0e21e 1873=item fileno FILEHANDLE
d74e8afc 1874X<fileno>
a0d0e21e 1875
2b5ab1e7
TC
1876Returns the file descriptor for a filehandle, or undefined if the
1877filehandle is not open. This is mainly useful for constructing
19799a22 1878bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1879If FILEHANDLE is an expression, the value is taken as an indirect
1880filehandle, generally its name.
5a964f20 1881
b76cc8ba 1882You can use this to find out whether two handles refer to the
5a964f20
TC
1883same underlying descriptor:
1884
1885 if (fileno(THIS) == fileno(THAT)) {
a9a5a0dc 1886 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1887 }
1888
1889(Filehandles connected to memory objects via new features of C<open> may
1890return undefined even though they are open.)
1891
a0d0e21e
LW
1892
1893=item flock FILEHANDLE,OPERATION
d74e8afc 1894X<flock> X<lock> X<locking>
a0d0e21e 1895
19799a22
GS
1896Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1897for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1898machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1899C<flock> is Perl's portable file locking interface, although it locks
3b10bc60 1900entire files only, not records.
2b5ab1e7
TC
1901
1902Two potentially non-obvious but traditional C<flock> semantics are
1903that it waits indefinitely until the lock is granted, and that its locks
1904B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1905fewer guarantees. This means that programs that do not also use C<flock>
1906may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1907your port's specific documentation, or your system-specific local manpages
1908for details. It's best to assume traditional behavior if you're writing
1909portable programs. (But if you're not, you should as always feel perfectly
1910free to write for your own system's idiosyncrasies (sometimes called
1911"features"). Slavish adherence to portability concerns shouldn't get
1912in the way of your getting your job done.)
a3cb178b 1913
8ebc5c01
PP
1914OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1915LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1916you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1917either individually, or as a group using the ':flock' tag. LOCK_SH
1918requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be 1919releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
3b10bc60 1920LOCK_SH or LOCK_EX then C<flock> returns immediately rather than blocking
1921waiting for the lock; check the return status to see if you got it.
68dc0745 1922
2b5ab1e7
TC
1923To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1924before locking or unlocking it.
8ebc5c01 1925
f86cebdf 1926Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1927locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1928are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1929implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1930differing semantics shouldn't bite too many people.
1931
becacb53
TM
1932Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1933be open with read intent to use LOCK_SH and requires that it be open
1934with write intent to use LOCK_EX.
1935
19799a22
GS
1936Note also that some versions of C<flock> cannot lock things over the
1937network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1938that. If you like you can force Perl to ignore your system's flock(2)
1939function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01 1940the switch C<-Ud_flock> to the F<Configure> program when you configure
3b10bc60 1941Perl.
4633a7c4
LW
1942
1943Here's a mailbox appender for BSD systems.
a0d0e21e 1944
7ed5353d 1945 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
1946
1947 sub lock {
a9a5a0dc
VP
1948 my ($fh) = @_;
1949 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
7ed5353d 1950
a9a5a0dc
VP
1951 # and, in case someone appended while we were waiting...
1952 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
1953 }
1954
1955 sub unlock {
a9a5a0dc
VP
1956 my ($fh) = @_;
1957 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
1958 }
1959
b0169937 1960 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
5ed4f2ec 1961 or die "Can't open mailbox: $!";
a0d0e21e 1962
7ed5353d 1963 lock($mbox);
b0169937 1964 print $mbox $msg,"\n\n";
7ed5353d 1965 unlock($mbox);
a0d0e21e 1966
3b10bc60 1967On systems that support a real flock(2), locks are inherited across fork()
1968calls, whereas those that must resort to the more capricious fcntl(2)
1969function lose their locks, making it seriously harder to write servers.
2b5ab1e7 1970
cb1a09d0 1971See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1972
1973=item fork
d74e8afc 1974X<fork> X<child> X<parent>
a0d0e21e 1975
2b5ab1e7
TC
1976Does a fork(2) system call to create a new process running the
1977same program at the same point. It returns the child pid to the
1978parent process, C<0> to the child process, or C<undef> if the fork is
1979unsuccessful. File descriptors (and sometimes locks on those descriptors)
1980are shared, while everything else is copied. On most systems supporting
1981fork(), great care has gone into making it extremely efficient (for
1982example, using copy-on-write technology on data pages), making it the
1983dominant paradigm for multitasking over the last few decades.
5a964f20 1984
80d38338 1985Beginning with v5.6.0, Perl attempts to flush all files opened for
0f897271
GS
1986output before forking the child process, but this may not be supported
1987on some platforms (see L<perlport>). To be safe, you may need to set
1988C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
80d38338 1989C<IO::Handle> on any open handles to avoid duplicate output.
a0d0e21e 1990
19799a22 1991If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1992accumulate zombies. On some systems, you can avoid this by setting
1993C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1994forking and reaping moribund children.
cb1a09d0 1995
28757baa
PP
1996Note that if your forked child inherits system file descriptors like
1997STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1998if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1999backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 2000You should reopen those to F</dev/null> if it's any issue.
28757baa 2001
cb1a09d0 2002=item format
d74e8afc 2003X<format>
cb1a09d0 2004
19799a22 2005Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2006example:
2007
54310121 2008 format Something =
a9a5a0dc
VP
2009 Test: @<<<<<<<< @||||| @>>>>>
2010 $str, $%, '$' . int($num)
cb1a09d0
AD
2011 .
2012
2013 $str = "widget";
184e9718 2014 $num = $cost/$quantity;
cb1a09d0
AD
2015 $~ = 'Something';
2016 write;
2017
2018See L<perlform> for many details and examples.
2019
8903cb82 2020=item formline PICTURE,LIST
d74e8afc 2021X<formline>
a0d0e21e 2022
5a964f20 2023This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2024too. It formats (see L<perlform>) a list of values according to the
2025contents of PICTURE, placing the output into the format output
7660c0ab 2026accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2027Eventually, when a C<write> is done, the contents of
cf264981
SP
2028C<$^A> are written to some filehandle. You could also read C<$^A>
2029and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2030does one C<formline> per line of form, but the C<formline> function itself
748a9306 2031doesn't care how many newlines are embedded in the PICTURE. This means
3b10bc60 2032that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
748a9306 2033You may therefore need to use multiple formlines to implement a single
3b10bc60 2034record format, just like the C<format> compiler.
748a9306 2035
19799a22 2036Be careful if you put double quotes around the picture, because an C<@>
748a9306 2037character may be taken to mean the beginning of an array name.
19799a22 2038C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
2039
2040=item getc FILEHANDLE
f723aae1 2041X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2042
2043=item getc
2044
2045Returns the next character from the input file attached to FILEHANDLE,
3b10bc60 2046or the undefined value at end of file or if there was an error (in
b5fe5ca2
SR
2047the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2048STDIN. This is not particularly efficient. However, it cannot be
2049used by itself to fetch single characters without waiting for the user
2050to hit enter. For that, try something more like:
4633a7c4
LW
2051
2052 if ($BSD_STYLE) {
a9a5a0dc 2053 system "stty cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2054 }
2055 else {
a9a5a0dc 2056 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2057 }
2058
2059 $key = getc(STDIN);
2060
2061 if ($BSD_STYLE) {
a9a5a0dc 2062 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
4633a7c4
LW
2063 }
2064 else {
3b10bc60 2065 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
4633a7c4
LW
2066 }
2067 print "\n";
2068
54310121
PP
2069Determination of whether $BSD_STYLE should be set
2070is left as an exercise to the reader.
cb1a09d0 2071
19799a22 2072The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2073systems purporting POSIX compliance. See also the C<Term::ReadKey>
2074module from your nearest CPAN site; details on CPAN can be found on
2075L<perlmodlib/CPAN>.
a0d0e21e
LW
2076
2077=item getlogin
d74e8afc 2078X<getlogin> X<login>
a0d0e21e 2079
cf264981 2080This implements the C library function of the same name, which on most
3b10bc60 2081systems returns the current login from F</etc/utmp>, if any. If it
2082returns the empty string, use C<getpwuid>.
a0d0e21e 2083
f86702cc 2084 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2085
19799a22
GS
2086Do not consider C<getlogin> for authentication: it is not as
2087secure as C<getpwuid>.
4633a7c4 2088
a0d0e21e 2089=item getpeername SOCKET
d74e8afc 2090X<getpeername> X<peer>
a0d0e21e
LW
2091
2092Returns the packed sockaddr address of other end of the SOCKET connection.
2093
4633a7c4
LW
2094 use Socket;
2095 $hersockaddr = getpeername(SOCK);
19799a22 2096 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2097 $herhostname = gethostbyaddr($iaddr, AF_INET);
2098 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2099
2100=item getpgrp PID
d74e8afc 2101X<getpgrp> X<group>
a0d0e21e 2102
47e29363 2103Returns the current process group for the specified PID. Use
7660c0ab 2104a PID of C<0> to get the current process group for the
4633a7c4 2105current process. Will raise an exception if used on a machine that
f86cebdf 2106doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2107group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2108does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2109
2110=item getppid
d74e8afc 2111X<getppid> X<parent> X<pid>
a0d0e21e
LW
2112
2113Returns the process id of the parent process.
2114
4d76a344
RGS
2115Note for Linux users: on Linux, the C functions C<getpid()> and
2116C<getppid()> return different values from different threads. In order to
3b10bc60 2117be portable, this behavior is not reflected by the Perl-level function
4d76a344 2118C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2119to call the underlying C<getppid()>, you may use the CPAN module
2120C<Linux::Pid>.
4d76a344 2121
a0d0e21e 2122=item getpriority WHICH,WHO
d74e8afc 2123X<getpriority> X<priority> X<nice>
a0d0e21e 2124
4633a7c4 2125Returns the current priority for a process, a process group, or a user.
f4084e39 2126(See C<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2127machine that doesn't implement getpriority(2).
a0d0e21e
LW
2128
2129=item getpwnam NAME
d74e8afc
ITB
2130X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2131X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2132X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2133X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2134X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2135X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2136
2137=item getgrnam NAME
2138
2139=item gethostbyname NAME
2140
2141=item getnetbyname NAME
2142
2143=item getprotobyname NAME
2144
2145=item getpwuid UID
2146
2147=item getgrgid GID
2148
2149=item getservbyname NAME,PROTO
2150
2151=item gethostbyaddr ADDR,ADDRTYPE
2152
2153=item getnetbyaddr ADDR,ADDRTYPE
2154
2155=item getprotobynumber NUMBER
2156
2157=item getservbyport PORT,PROTO
2158
2159=item getpwent
2160
2161=item getgrent
2162
2163=item gethostent
2164
2165=item getnetent
2166
2167=item getprotoent
2168
2169=item getservent
2170
2171=item setpwent
2172
2173=item setgrent
2174
2175=item sethostent STAYOPEN
2176
2177=item setnetent STAYOPEN
2178
2179=item setprotoent STAYOPEN
2180
2181=item setservent STAYOPEN
2182
2183=item endpwent
2184
2185=item endgrent
2186
2187=item endhostent
2188
2189=item endnetent
2190
2191=item endprotoent
2192
2193=item endservent
2194
80d38338
TC
2195These routines are the same as their counterparts in the
2196system C library. In list context, the return values from the
a0d0e21e
LW
2197various get routines are as follows:
2198
2199 ($name,$passwd,$uid,$gid,
6ee623d5 2200 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2201 ($name,$passwd,$gid,$members) = getgr*
2202 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2203 ($name,$aliases,$addrtype,$net) = getnet*
2204 ($name,$aliases,$proto) = getproto*
2205 ($name,$aliases,$port,$proto) = getserv*
2206
3b10bc60 2207(If the entry doesn't exist you get an empty list.)
a0d0e21e 2208
4602f195
JH
2209The exact meaning of the $gcos field varies but it usually contains
2210the real name of the user (as opposed to the login name) and other
2211information pertaining to the user. Beware, however, that in many
2212system users are able to change this information and therefore it
106325ad 2213cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2214L<perlsec>). The $passwd and $shell, user's encrypted password and
2215login shell, are also tainted, because of the same reason.
4602f195 2216
5a964f20 2217In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2218lookup by name, in which case you get the other thing, whatever it is.
2219(If the entry doesn't exist you get the undefined value.) For example:
2220
5a964f20
TC
2221 $uid = getpwnam($name);
2222 $name = getpwuid($num);
2223 $name = getpwent();
2224 $gid = getgrnam($name);
08a33e13 2225 $name = getgrgid($num);
5a964f20
TC
2226 $name = getgrent();
2227 #etc.
a0d0e21e 2228
4602f195 2229In I<getpw*()> the fields $quota, $comment, and $expire are special
80d38338 2230in that they are unsupported on many systems. If the
4602f195
JH
2231$quota is unsupported, it is an empty scalar. If it is supported, it
2232usually encodes the disk quota. If the $comment field is unsupported,
2233it is an empty scalar. If it is supported it usually encodes some
2234administrative comment about the user. In some systems the $quota
2235field may be $change or $age, fields that have to do with password
2236aging. In some systems the $comment field may be $class. The $expire
2237field, if present, encodes the expiration period of the account or the
2238password. For the availability and the exact meaning of these fields
2239in your system, please consult your getpwnam(3) documentation and your
2240F<pwd.h> file. You can also find out from within Perl what your
2241$quota and $comment fields mean and whether you have the $expire field
2242by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2243C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
3b10bc60 2244files are supported only if your vendor has implemented them in the
4602f195 2245intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2246shadow versions if you're running under privilege or if there exists
cf264981
SP
2247the shadow(3) functions as found in System V (this includes Solaris
2248and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2249facility are unlikely to be supported.
6ee623d5 2250
19799a22 2251The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2252the login names of the members of the group.
2253
2254For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2255C, it will be returned to you via C<$?> if the function call fails. The
3b10bc60 2256C<@addrs> value returned by a successful call is a list of raw
2257addresses returned by the corresponding library call. In the
2258Internet domain, each address is four bytes long; you can unpack it
a0d0e21e
LW
2259by saying something like:
2260
f337b084 2261 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2262
2b5ab1e7
TC
2263The Socket library makes this slightly easier:
2264
2265 use Socket;
2266 $iaddr = inet_aton("127.1"); # or whatever address
2267 $name = gethostbyaddr($iaddr, AF_INET);
2268
2269 # or going the other way
19799a22 2270 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2271
d760c846
GS
2272In the opposite way, to resolve a hostname to the IP address
2273you can write this:
2274
2275 use Socket;
2276 $packed_ip = gethostbyname("www.perl.org");
2277 if (defined $packed_ip) {
2278 $ip_address = inet_ntoa($packed_ip);
2279 }
2280
2281Make sure <gethostbyname()> is called in SCALAR context and that
2282its return value is checked for definedness.
2283
19799a22
GS
2284If you get tired of remembering which element of the return list
2285contains which return value, by-name interfaces are provided
2286in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2287C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2288and C<User::grent>. These override the normal built-ins, supplying
2289versions that return objects with the appropriate names
2290for each field. For example:
5a964f20
TC
2291
2292 use File::stat;
2293 use User::pwent;
2294 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2295
b76cc8ba
NIS
2296Even though it looks like they're the same method calls (uid),
2297they aren't, because a C<File::stat> object is different from
19799a22 2298a C<User::pwent> object.
5a964f20 2299
a0d0e21e 2300=item getsockname SOCKET
d74e8afc 2301X<getsockname>
a0d0e21e 2302
19799a22
GS
2303Returns the packed sockaddr address of this end of the SOCKET connection,
2304in case you don't know the address because you have several different
2305IPs that the connection might have come in on.
a0d0e21e 2306
4633a7c4
LW
2307 use Socket;
2308 $mysockaddr = getsockname(SOCK);
19799a22 2309 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2310 printf "Connect to %s [%s]\n",
19799a22
GS
2311 scalar gethostbyaddr($myaddr, AF_INET),
2312 inet_ntoa($myaddr);
a0d0e21e
LW
2313
2314=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2315X<getsockopt>
a0d0e21e 2316
636e6b1f
TH
2317Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2318Options may exist at multiple protocol levels depending on the socket
2319type, but at least the uppermost socket level SOL_SOCKET (defined in the
2320C<Socket> module) will exist. To query options at another level the
2321protocol number of the appropriate protocol controlling the option
2322should be supplied. For example, to indicate that an option is to be
2323interpreted by the TCP protocol, LEVEL should be set to the protocol
80d38338 2324number of TCP, which you can get using C<getprotobyname>.
636e6b1f 2325
80d38338 2326The function returns a packed string representing the requested socket
3b10bc60 2327option, or C<undef> on error, with the reason for the error placed in
2328C<$!>). Just what is in the packed string depends on LEVEL and OPTNAME;
80d38338
TC
2329consult getsockopt(2) for details. A common case is that the option is an
2330integer, in which case the result is a packed integer, which you can decode
2331using C<unpack> with the C<i> (or C<I>) format.
636e6b1f 2332
80d38338 2333An example to test whether Nagle's algorithm is turned on on a socket:
636e6b1f 2334
4852725b 2335 use Socket qw(:all);
636e6b1f
TH
2336
2337 defined(my $tcp = getprotobyname("tcp"))
a9a5a0dc 2338 or die "Could not determine the protocol number for tcp";
4852725b
DD
2339 # my $tcp = IPPROTO_TCP; # Alternative
2340 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
80d38338 2341 or die "getsockopt TCP_NODELAY: $!";
636e6b1f
TH
2342 my $nodelay = unpack("I", $packed);
2343 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2344
a0d0e21e
LW
2345
2346=item glob EXPR
d74e8afc 2347X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2348
0a753a76
PP
2349=item glob
2350
d9a9d457
JL
2351In list context, returns a (possibly empty) list of filename expansions on
2352the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2353scalar context, glob iterates through such filename expansions, returning
2354undef when the list is exhausted. This is the internal function
2355implementing the C<< <*.c> >> operator, but you can use it directly. If
2356EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2357more detail in L<perlop/"I/O Operators">.
a0d0e21e 2358
80d38338
TC
2359Note that C<glob> splits its arguments on whitespace and treats
2360each segment as separate pattern. As such, C<glob("*.c *.h")>
2361matches all files with a F<.c> or F<.h> extension. The expression
2362C<glob(".* *")> matchs all files in the current working directory.
2363
2364If non-empty braces are the only wildcard characters used in the
2365C<glob>, no filenames are matched, but potentially many strings
2366are returned. For example, this produces nine strings, one for
2367each pairing of fruits and colors:
2368
2369 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
5c0c9249 2370
3a4b19e4 2371Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2372C<File::Glob> extension. See L<File::Glob> for details, including
2373C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2374
a0d0e21e 2375=item gmtime EXPR
d74e8afc 2376X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2377
ce2984c3
PF
2378=item gmtime
2379
435fbc73
GS
2380Works just like L<localtime> but the returned values are
2381localized for the standard Greenwich time zone.
a0d0e21e 2382
435fbc73
GS
2383Note: when called in list context, $isdst, the last value
2384returned by gmtime is always C<0>. There is no
2385Daylight Saving Time in GMT.
0a753a76 2386
62aa5637
MS
2387See L<perlport/gmtime> for portability concerns.
2388
a0d0e21e 2389=item goto LABEL
d74e8afc 2390X<goto> X<jump> X<jmp>
a0d0e21e 2391
748a9306
LW
2392=item goto EXPR
2393
a0d0e21e
LW
2394=item goto &NAME
2395
b500e03b
GG
2396The C<goto-LABEL> form finds the statement labeled with LABEL and
2397resumes execution there. It can't be used to get out of a block or
2398subroutine given to C<sort>. It can be used to go almost anywhere
2399else within the dynamic scope, including out of subroutines, but it's
2400usually better to use some other construct such as C<last> or C<die>.
2401The author of Perl has never felt the need to use this form of C<goto>
3b10bc60 2402(in Perl, that is; C is another matter). (The difference is that C
b500e03b
GG
2403does not offer named loops combined with loop control. Perl does, and
2404this replaces most structured uses of C<goto> in other languages.)
a0d0e21e 2405
7660c0ab
A
2406The C<goto-EXPR> form expects a label name, whose scope will be resolved
2407dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2408necessarily recommended if you're optimizing for maintainability:
2409
2410 goto ("FOO", "BAR", "GLARCH")[$i];
2411
b500e03b 2412Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
0b98bec9 2413deprecated and will issue a warning. Even then, it may not be used to
b500e03b
GG
2414go into any construct that requires initialization, such as a
2415subroutine or a C<foreach> loop. It also can't be used to go into a
0b98bec9 2416construct that is optimized away.
b500e03b 2417
1b6921cb
BT
2418The C<goto-&NAME> form is quite different from the other forms of
2419C<goto>. In fact, it isn't a goto in the normal sense at all, and
2420doesn't have the stigma associated with other gotos. Instead, it
2421exits the current subroutine (losing any changes set by local()) and
2422immediately calls in its place the named subroutine using the current
2423value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2424load another subroutine and then pretend that the other subroutine had
2425been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2426in the current subroutine are propagated to the other subroutine.)
2427After the C<goto>, not even C<caller> will be able to tell that this
2428routine was called first.
2429
2430NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2431containing a code reference, or a block that evaluates to a code
6cb9131c 2432reference.
a0d0e21e
LW
2433
2434=item grep BLOCK LIST
d74e8afc 2435X<grep>
a0d0e21e
LW
2436
2437=item grep EXPR,LIST
2438
2b5ab1e7
TC
2439This is similar in spirit to, but not the same as, grep(1) and its
2440relatives. In particular, it is not limited to using regular expressions.
2f9daede 2441
a0d0e21e 2442Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2443C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2444elements for which the expression evaluated to true. In scalar
2445context, returns the number of times the expression was true.
a0d0e21e
LW
2446
2447 @foo = grep(!/^#/, @bar); # weed out comments
2448
2449or equivalently,
2450
2451 @foo = grep {!/^#/} @bar; # weed out comments
2452
be3174d2
GS
2453Note that C<$_> is an alias to the list value, so it can be used to
2454modify the elements of the LIST. While this is useful and supported,
2455it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2456Similarly, grep returns aliases into the original list, much as a for
2457loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2458element of a list returned by grep (for example, in a C<foreach>, C<map>
2459or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2460This is usually something to be avoided when writing clear code.
a0d0e21e 2461
a4fb8298 2462If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2463been declared with C<my $_>) then, in addition to being locally aliased to
80d38338 2464the list elements, C<$_> keeps being lexical inside the block; i.e., it
a4fb8298
RGS
2465can't be seen from the outside, avoiding any potential side-effects.
2466
19799a22 2467See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2468
a0d0e21e 2469=item hex EXPR
d74e8afc 2470X<hex> X<hexadecimal>
a0d0e21e 2471
54310121 2472=item hex
bbce6d69 2473
2b5ab1e7 2474Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2475(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2476L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2477
2478 print hex '0xAf'; # prints '175'
2479 print hex 'aF'; # same
a0d0e21e 2480
19799a22 2481Hex strings may only represent integers. Strings that would cause
53305cf1 2482integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2483unlike oct(). To present something as hex, look into L</printf>,
2484L</sprintf>, or L</unpack>.
19799a22 2485
ce2984c3 2486=item import LIST
d74e8afc 2487X<import>
a0d0e21e 2488
19799a22 2489There is no builtin C<import> function. It is just an ordinary
4633a7c4 2490method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2491names to another module. The C<use> function calls the C<import> method
cea6626f 2492for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2493
2494=item index STR,SUBSTR,POSITION
d74e8afc 2495X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2496
2497=item index STR,SUBSTR
2498
2b5ab1e7
TC
2499The index function searches for one string within another, but without
2500the wildcard-like behavior of a full regular-expression pattern match.
2501It returns the position of the first occurrence of SUBSTR in STR at
2502or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2503beginning of the string. POSITION before the beginning of the string
2504or after its end is treated as if it were the beginning or the end,
2505respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2506you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2507is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2508
2509=item int EXPR
f723aae1 2510X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2511
54310121 2512=item int
bbce6d69 2513
7660c0ab 2514Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7 2515You should not use this function for rounding: one because it truncates
3b10bc60 2516towards C<0>, and two because machine representations of floating-point
2b5ab1e7
TC
2517numbers can sometimes produce counterintuitive results. For example,
2518C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2519because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2520the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2521functions will serve you better than will int().
a0d0e21e
LW
2522
2523=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2524X<ioctl>
a0d0e21e 2525
2b5ab1e7 2526Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2527
5ed4f2ec 2528 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2529
a11c483f 2530to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2531exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2532own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2533(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2534may help you in this, but it's nontrivial.) SCALAR will be read and/or
3b10bc60 2535written depending on the FUNCTION; a C pointer to the string value of SCALAR
19799a22 2536will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2537has no string value but does have a numeric value, that value will be
2538passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2539true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2540functions may be needed to manipulate the values of structures used by
b76cc8ba 2541C<ioctl>.
a0d0e21e 2542
19799a22 2543The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e 2544
5ed4f2ec 2545 if OS returns: then Perl returns:
2546 -1 undefined value
2547 0 string "0 but true"
2548 anything else that number
a0d0e21e 2549
19799a22 2550Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2551still easily determine the actual value returned by the operating
2552system:
2553
2b5ab1e7 2554 $retval = ioctl(...) || -1;
a0d0e21e
LW
2555 printf "System returned %d\n", $retval;
2556
be2f7487 2557The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2558about improper numeric conversions.
2559
a0d0e21e 2560=item join EXPR,LIST
d74e8afc 2561X<join>
a0d0e21e 2562
2b5ab1e7
TC
2563Joins the separate strings of LIST into a single string with fields
2564separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2565
2b5ab1e7 2566 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2567
eb6e2d6f
GS
2568Beware that unlike C<split>, C<join> doesn't take a pattern as its
2569first argument. Compare L</split>.
a0d0e21e 2570
aa689395 2571=item keys HASH
d74e8afc 2572X<keys> X<key>
aa689395 2573
aeedbbed
NC
2574=item keys ARRAY
2575
2576Returns a list consisting of all the keys of the named hash, or the indices
2577of an array. (In scalar context, returns the number of keys or indices.)
504f80c1 2578
aeedbbed 2579The keys of a hash are returned in an apparently random order. The actual
3b10bc60 2580random order is subject to change in future versions of Perl, but it
504f80c1 2581is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2582function produces (given that the hash has not been modified). Since
2583Perl 5.8.1 the ordering is different even between different runs of
2584Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2585Attacks">).
504f80c1 2586
aeedbbed 2587As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
cf264981
SP
2588(see L</each>). In particular, calling keys() in void context resets
2589the iterator with no other overhead.
a0d0e21e 2590
aa689395 2591Here is yet another way to print your environment:
a0d0e21e
LW
2592
2593 @keys = keys %ENV;
2594 @values = values %ENV;
b76cc8ba 2595 while (@keys) {
a9a5a0dc 2596 print pop(@keys), '=', pop(@values), "\n";
a0d0e21e
LW
2597 }
2598
2599or how about sorted by key:
2600
2601 foreach $key (sort(keys %ENV)) {
a9a5a0dc 2602 print $key, '=', $ENV{$key}, "\n";
a0d0e21e
LW
2603 }
2604
8ea1e5d4
GS
2605The returned values are copies of the original keys in the hash, so
2606modifying them will not affect the original hash. Compare L</values>.
2607
19799a22 2608To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2609Here's a descending numeric sort of a hash by its values:
4633a7c4 2610
5a964f20 2611 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
a9a5a0dc 2612 printf "%4d %s\n", $hash{$key}, $key;
4633a7c4
LW
2613 }
2614
3b10bc60 2615Used as an lvalue, C<keys> allows you to increase the number of hash buckets
aa689395
PP
2616allocated for the given hash. This can gain you a measure of efficiency if
2617you know the hash is going to get big. (This is similar to pre-extending
2618an array by assigning a larger number to $#array.) If you say
55497cff
PP
2619
2620 keys %hash = 200;
2621
ab192400
GS
2622then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2623in fact, since it rounds up to the next power of two. These
55497cff
PP
2624buckets will be retained even if you do C<%hash = ()>, use C<undef
2625%hash> if you want to free the storage while C<%hash> is still in scope.
2626You can't shrink the number of buckets allocated for the hash using
19799a22 2627C<keys> in this way (but you needn't worry about doing this by accident,
aeedbbed
NC
2628as trying has no effect). C<keys @array> in an lvalue context is a syntax
2629error.
55497cff 2630
19799a22 2631See also C<each>, C<values> and C<sort>.
ab192400 2632
b350dd2f 2633=item kill SIGNAL, LIST
d74e8afc 2634X<kill> X<signal>
a0d0e21e 2635
b350dd2f 2636Sends a signal to a list of processes. Returns the number of
517db077
GS
2637processes successfully signaled (which is not necessarily the
2638same as the number actually killed).
a0d0e21e
LW
2639
2640 $cnt = kill 1, $child1, $child2;
2641 kill 9, @goners;
2642
3b10bc60 2643If SIGNAL is zero, no signal is sent to the process, but C<kill>
2644checks whether it's I<possible> to send a signal to it (that
70fb64f6 2645means, to be brief, that the process is owned by the same user, or we are
3b10bc60 2646the super-user). This is useful to check that a child process is still
81fd35db
DN
2647alive (even if only as a zombie) and hasn't changed its UID. See
2648L<perlport> for notes on the portability of this construct.
b350dd2f 2649
e2c0f81f
DG
2650Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2651of processes. That means you usually want to use positive not negative signals.
2652You may also use a signal name in quotes.
2653
2654The behavior of kill when a I<PROCESS> number is zero or negative depends on
2655the operating system. For example, on POSIX-conforming systems, zero will
2656signal the current process group and -1 will signal all processes.
1e9c1022
JL
2657
2658See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2659
2660=item last LABEL
d74e8afc 2661X<last> X<break>
a0d0e21e
LW
2662
2663=item last
2664
2665The C<last> command is like the C<break> statement in C (as used in
2666loops); it immediately exits the loop in question. If the LABEL is
2667omitted, the command refers to the innermost enclosing loop. The
2668C<continue> block, if any, is not executed:
2669
4633a7c4 2670 LINE: while (<STDIN>) {
a9a5a0dc
VP
2671 last LINE if /^$/; # exit when done with header
2672 #...
a0d0e21e
LW
2673 }
2674
80d38338 2675C<last> cannot be used to exit a block that returns a value such as
2b5ab1e7
TC
2676C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2677a grep() or map() operation.
4968c1e4 2678
6c1372ed
GS
2679Note that a block by itself is semantically identical to a loop
2680that executes once. Thus C<last> can be used to effect an early
2681exit out of such a block.
2682
98293880
JH
2683See also L</continue> for an illustration of how C<last>, C<next>, and
2684C<redo> work.
1d2dff63 2685
a0d0e21e 2686=item lc EXPR
d74e8afc 2687X<lc> X<lowercase>
a0d0e21e 2688
54310121 2689=item lc
bbce6d69 2690
d1be9408 2691Returns a lowercased version of EXPR. This is the internal function
8bdbc703 2692implementing the C<\L> escape in double-quoted strings. Respects
2693current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2694and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2695
7660c0ab 2696If EXPR is omitted, uses C<$_>.
bbce6d69 2697
a0d0e21e 2698=item lcfirst EXPR
d74e8afc 2699X<lcfirst> X<lowercase>
a0d0e21e 2700
54310121 2701=item lcfirst
bbce6d69 2702
ad0029c4
JH
2703Returns the value of EXPR with the first character lowercased. This
2704is the internal function implementing the C<\l> escape in
8bdbc703 2705double-quoted strings. Respects current LC_CTYPE locale if C<use
2706locale> in force. See L<perllocale> and L<perlunicode> for more
2707details about locale and Unicode support.
a0d0e21e 2708
7660c0ab 2709If EXPR is omitted, uses C<$_>.
bbce6d69 2710
a0d0e21e 2711=item length EXPR
d74e8afc 2712X<length> X<size>
a0d0e21e 2713
54310121 2714=item length
bbce6d69 2715
974da8e5 2716Returns the length in I<characters> of the value of EXPR. If EXPR is
9f621bb0 2717omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
3b10bc60 2718
2719This function cannot be used on an entire array or hash to find out how
2720many elements these have. For that, use C<scalar @array> and C<scalar keys
2721%hash>, respectively.
2722
2723Like all Perl character operations, length() normally deals in logical
2724characters, not physical bytes. For how many bytes a string encoded as
2725UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
2726to C<use Encode> first). See L<Encode> and L<perlunicode>.
974da8e5 2727
a0d0e21e 2728=item link OLDFILE,NEWFILE
d74e8afc 2729X<link>
a0d0e21e 2730
19799a22 2731Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2732success, false otherwise.
a0d0e21e
LW
2733
2734=item listen SOCKET,QUEUESIZE
d74e8afc 2735X<listen>
a0d0e21e 2736
3b10bc60 2737Does the same thing that the listen(2) system call does. Returns true if
b76cc8ba 2738it succeeded, false otherwise. See the example in
cea6626f 2739L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2740
2741=item local EXPR
d74e8afc 2742X<local>
a0d0e21e 2743
19799a22 2744You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2745what most people think of as "local". See
13a2d996 2746L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2747
5a964f20
TC
2748A local modifies the listed variables to be local to the enclosing
2749block, file, or eval. If more than one value is listed, the list must
2750be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2751for details, including issues with tied arrays and hashes.
a0d0e21e 2752
d361fafa
VP
2753The C<delete local EXPR> construct can also be used to localize the deletion
2754of array/hash elements to the current block.
2755See L<perlsub/"Localized deletion of elements of composite types">.
2756
a0d0e21e 2757=item localtime EXPR
435fbc73 2758X<localtime> X<ctime>
a0d0e21e 2759
ba053783
AL
2760=item localtime
2761
19799a22 2762Converts a time as returned by the time function to a 9-element list
5f05dabc 2763with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2764follows:
2765
54310121 2766 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2767 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2768 localtime(time);
a0d0e21e 2769
48a26b3a 2770All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2771tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2772of the specified time.
48a26b3a 2773
ba053783
AL
2774C<$mday> is the day of the month, and C<$mon> is the month itself, in
2775the range C<0..11> with 0 indicating January and 11 indicating December.
2776This makes it easy to get a month name from a list:
54310121 2777
ba053783
AL
2778 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2779 print "$abbr[$mon] $mday";
2780 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2781
ba053783
AL
2782C<$year> is the number of years since 1900, not just the last two digits
2783of the year. That is, C<$year> is C<123> in year 2023. The proper way
80d38338 2784to get a 4-digit year is simply:
abd75f24 2785
ba053783 2786 $year += 1900;
abd75f24 2787
435fbc73
GS
2788Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2789to do that, would you?
2790
ba053783
AL
2791To get the last two digits of the year (e.g., '01' in 2001) do:
2792
2793 $year = sprintf("%02d", $year % 100);
2794
2795C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2796Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2797(or C<0..365> in leap years.)
2798
2799C<$isdst> is true if the specified time occurs during Daylight Saving
2800Time, false otherwise.
abd75f24 2801
e1998452 2802If EXPR is omitted, C<localtime()> uses the current time (as returned
e3176d09 2803by time(3)).
a0d0e21e 2804
48a26b3a 2805In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2806
5f05dabc 2807 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2808
fe86afc2
NC
2809This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2810instead of local time use the L</gmtime> builtin. See also the
2811C<Time::Local> module (to convert the second, minutes, hours, ... back to
2812the integer value returned by time()), and the L<POSIX> module's strftime(3)
2813and mktime(3) functions.
2814
2815To get somewhat similar but locale dependent date strings, set up your
2816locale environment variables appropriately (please see L<perllocale>) and
2817try for example:
a3cb178b 2818
5a964f20 2819 use POSIX qw(strftime);
2b5ab1e7 2820 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2821 # or for GMT formatted appropriately for your locale:
2822 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2823
2824Note that the C<%a> and C<%b>, the short forms of the day of the week
2825and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2826
62aa5637
MS
2827See L<perlport/localtime> for portability concerns.
2828
435fbc73
GS
2829The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2830by-name access mechanism to the gmtime() and localtime() functions,
2831respectively.
2832
2833For a comprehensive date and time representation look at the
2834L<DateTime> module on CPAN.
2835
07698885 2836=item lock THING
d74e8afc 2837X<lock>
19799a22 2838
01e6739c 2839This function places an advisory lock on a shared variable, or referenced
03730085 2840object contained in I<THING> until the lock goes out of scope.
a6d5524e 2841
f3a23afb 2842lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2843by this name (before any calls to it), that function will be called
03730085
AB
2844instead. (However, if you've said C<use threads>, lock() is always a
2845keyword.) See L<threads>.
19799a22 2846
a0d0e21e 2847=item log EXPR
d74e8afc 2848X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2849
54310121 2850=item log
bbce6d69 2851
2b5ab1e7
TC
2852Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2853returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2854The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2855divided by the natural log of N. For example:
2856
2857 sub log10 {
a9a5a0dc
VP
2858 my $n = shift;
2859 return log($n)/log(10);
b76cc8ba 2860 }
2b5ab1e7
TC
2861
2862See also L</exp> for the inverse operation.
a0d0e21e 2863
a0d0e21e 2864=item lstat EXPR
d74e8afc 2865X<lstat>
a0d0e21e 2866
54310121 2867=item lstat
bbce6d69 2868
19799a22 2869Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2870special C<_> filehandle) but stats a symbolic link instead of the file
2871the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2872your system, a normal C<stat> is done. For much more detailed
2873information, please see the documentation for C<stat>.
a0d0e21e 2874
7660c0ab 2875If EXPR is omitted, stats C<$_>.
bbce6d69 2876
a0d0e21e
LW
2877=item m//
2878
9f4b9cd0 2879The match operator. See L<perlop/"Regexp Quote-Like Operators">.
a0d0e21e
LW
2880
2881=item map BLOCK LIST
d74e8afc 2882X<map>
a0d0e21e
LW
2883
2884=item map EXPR,LIST
2885
19799a22
GS
2886Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2887C<$_> to each element) and returns the list value composed of the
2888results of each such evaluation. In scalar context, returns the
2889total number of elements so generated. Evaluates BLOCK or EXPR in
2890list context, so each element of LIST may produce zero, one, or
2891more elements in the returned value.
dd99ebda 2892
a0d0e21e
LW
2893 @chars = map(chr, @nums);
2894
2895translates a list of numbers to the corresponding characters. And
2896
d8216f19 2897 %hash = map { get_a_key_for($_) => $_ } @array;
a0d0e21e
LW
2898
2899is just a funny way to write
2900
2901 %hash = ();
d8216f19 2902 foreach (@array) {
a9a5a0dc 2903 $hash{get_a_key_for($_)} = $_;
a0d0e21e
LW
2904 }
2905
be3174d2
GS
2906Note that C<$_> is an alias to the list value, so it can be used to
2907modify the elements of the LIST. While this is useful and supported,
2908it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2909Using a regular C<foreach> loop for this purpose would be clearer in
2910most cases. See also L</grep> for an array composed of those items of
2911the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2912
a4fb8298 2913If C<$_> is lexical in the scope where the C<map> appears (because it has
d8216f19
RGS
2914been declared with C<my $_>), then, in addition to being locally aliased to
2915the list elements, C<$_> keeps being lexical inside the block; that is, it
a4fb8298
RGS
2916can't be seen from the outside, avoiding any potential side-effects.
2917
205fdb4d 2918C<{> starts both hash references and blocks, so C<map { ...> could be either
3b10bc60 2919the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
80d38338
TC
2920ahead for the closing C<}> it has to take a guess at which it's dealing with
2921based on what it finds just after the C<{>. Usually it gets it right, but if it
205fdb4d
NC
2922doesn't it won't realize something is wrong until it gets to the C<}> and
2923encounters the missing (or unexpected) comma. The syntax error will be
80d38338 2924reported close to the C<}>, but you'll need to change something near the C<{>
3b10bc60 2925such as using a unary C<+> to give Perl some help:
205fdb4d 2926
3b10bc60 2927 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
2928 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
2929 %hash = map { ("\L$_" => 1) } @array # this also works
2930 %hash = map { lc($_) => 1 } @array # as does this.
2931 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
cea6626f 2932
3b10bc60 2933 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
205fdb4d 2934
d8216f19 2935or to force an anon hash constructor use C<+{>:
205fdb4d 2936
3b10bc60 2937 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end
205fdb4d 2938
3b10bc60 2939to get a list of anonymous hashes each with only one entry apiece.
205fdb4d 2940
19799a22 2941=item mkdir FILENAME,MASK
d74e8afc 2942X<mkdir> X<md> X<directory, create>
a0d0e21e 2943
5a211162
GS
2944=item mkdir FILENAME
2945
491873e5
RGS
2946=item mkdir
2947
0591cd52 2948Creates the directory specified by FILENAME, with permissions
19799a22
GS
2949specified by MASK (as modified by C<umask>). If it succeeds it
2950returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2951If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2952to C<$_>.
0591cd52 2953
19799a22 2954In general, it is better to create directories with permissive MASK,
0591cd52 2955and let the user modify that with their C<umask>, than it is to supply
19799a22 2956a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2957The exceptions to this rule are when the file or directory should be
2958kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2959C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2960
cc1852e8
JH
2961Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2962number of trailing slashes. Some operating and filesystems do not get
2963this right, so Perl automatically removes all trailing slashes to keep
2964everyone happy.
2965
80d38338 2966To recursively create a directory structure, look at
dd184578
RGS
2967the C<mkpath> function of the L<File::Path> module.
2968
a0d0e21e 2969=item msgctl ID,CMD,ARG
d74e8afc 2970X<msgctl>
a0d0e21e 2971
f86cebdf 2972Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2973
2974 use IPC::SysV;
2975
7660c0ab 2976first to get the correct constant definitions. If CMD is C<IPC_STAT>,
cf264981 2977then ARG must be a variable that will hold the returned C<msqid_ds>
951ba7fe
GS
2978structure. Returns like C<ioctl>: the undefined value for error,
2979C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2980L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2981
2982=item msgget KEY,FLAGS
d74e8afc 2983X<msgget>
a0d0e21e 2984
f86cebdf 2985Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2986id, or the undefined value if there is an error. See also
2987L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2988
a0d0e21e 2989=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
d74e8afc 2990X<msgrcv>
a0d0e21e
LW
2991
2992Calls the System V IPC function msgrcv to receive a message from
2993message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2994SIZE. Note that when a message is received, the message type as a
2995native long integer will be the first thing in VAR, followed by the
2996actual message. This packing may be opened with C<unpack("l! a*")>.
2997Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2998an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2999C<IPC::SysV::Msg> documentation.
41d6edb2
JH
3000
3001=item msgsnd ID,MSG,FLAGS
d74e8afc 3002X<msgsnd>
41d6edb2
JH
3003
3004Calls the System V IPC function msgsnd to send the message MSG to the
3005message queue ID. MSG must begin with the native long integer message
3006type, and be followed by the length of the actual message, and finally
3007the message itself. This kind of packing can be achieved with
3008C<pack("l! a*", $type, $message)>. Returns true if successful,
3009or false if there is an error. See also C<IPC::SysV>
3010and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
3011
3012=item my EXPR
d74e8afc 3013X<my>
a0d0e21e 3014
307ea6df
JH
3015=item my TYPE EXPR
3016
1d2de774 3017=item my EXPR : ATTRS
09bef843 3018
1d2de774 3019=item my TYPE EXPR : ATTRS
307ea6df 3020
19799a22 3021A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
3022enclosing block, file, or C<eval>. If more than one value is listed,
3023the list must be placed in parentheses.
307ea6df 3024
1d2de774
JH
3025The exact semantics and interface of TYPE and ATTRS are still
3026evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
3027and attributes are handled using the C<attributes> pragma, or starting
3028from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3029L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3030L<attributes>, and L<Attribute::Handlers>.
4633a7c4 3031
a0d0e21e 3032=item next LABEL
d74e8afc 3033X<next> X<continue>
a0d0e21e
LW
3034
3035=item next
3036
3037The C<next> command is like the C<continue> statement in C; it starts
3038the next iteration of the loop:
3039
4633a7c4 3040 LINE: while (<STDIN>) {
a9a5a0dc
VP
3041 next LINE if /^#/; # discard comments
3042 #...
a0d0e21e
LW
3043 }
3044
3045Note that if there were a C<continue> block on the above, it would get
3b10bc60 3046executed even on discarded lines. If LABEL is omitted, the command
a0d0e21e
LW
3047refers to the innermost enclosing loop.
3048
4968c1e4 3049C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
3050C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3051a grep() or map() operation.
4968c1e4 3052
6c1372ed
GS
3053Note that a block by itself is semantically identical to a loop
3054that executes once. Thus C<next> will exit such a block early.
3055
98293880
JH
3056See also L</continue> for an illustration of how C<last>, C<next>, and
3057C<redo> work.
1d2dff63 3058
3b10bc60 3059=item no MODULE VERSION LIST
3060X<no declarations>
3061X<unimporting>
4a66ea5a 3062
3b10bc60 3063=item no MODULE VERSION
4a66ea5a 3064
3b10bc60 3065=item no MODULE LIST
a0d0e21e