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Fix readline example in perlfunc
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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
a0d0e21e 17contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 18be first, and the list argument will follow. (Note that there can ever
0f31cffe 19be only 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
24list (and provide list context for the elements of the list) are shown
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.
cf264981 29Commas should separate 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
5f05dabc 33parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 34surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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35function, and precedence doesn't matter. Otherwise it's a list
36operator or unary operator, and precedence does matter. And whitespace
37between the function and left parenthesis doesn't count--so you need to
38be careful sometimes:
39
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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
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59returning the undefined value, and in a list context by returning the
60null list.
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.
a0d0e21e 65Each operator and function decides which sort of value it 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
81In general, functions in Perl that serve as wrappers for system calls
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
cb1a09d0 89=head2 Perl Functions by Category
d74e8afc 90X<function>
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91
92Here are Perl's functions (including things that look like
5a964f20 93functions, like some keywords and named operators)
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94arranged by category. Some functions appear in more
95than one place.
96
13a2d996 97=over 4
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98
99=item Functions for SCALARs or strings
d74e8afc 100X<scalar> X<string> X<character>
cb1a09d0 101
22fae026 102C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
1dc8ecb8 103C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
945c54fd 104C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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105
106=item Regular expressions and pattern matching
d74e8afc 107X<regular expression> X<regex> X<regexp>
cb1a09d0 108
ab4f32c2 109C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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110
111=item Numeric functions
d74e8afc 112X<numeric> X<number> X<trigonometric> X<trigonometry>
cb1a09d0 113
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114C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
115C<sin>, C<sqrt>, C<srand>
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116
117=item Functions for real @ARRAYs
d74e8afc 118X<array>
cb1a09d0 119
22fae026 120C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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121
122=item Functions for list data
d74e8afc 123X<list>
cb1a09d0 124
1dc8ecb8 125C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
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126
127=item Functions for real %HASHes
d74e8afc 128X<hash>
cb1a09d0 129
22fae026 130C<delete>, C<each>, C<exists>, C<keys>, C<values>
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131
132=item Input and output functions
d74e8afc 133X<I/O> X<input> X<output> X<dbm>
cb1a09d0 134
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135C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
136C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
0d863452 137C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
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138C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
139C<warn>, C<write>
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140
141=item Functions for fixed length data or records
142
22fae026 143C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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144
145=item Functions for filehandles, files, or directories
d74e8afc 146X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
cb1a09d0 147
22fae026 148C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
5ff3f7a4 149C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
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150C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
151C<umask>, C<unlink>, C<utime>
cb1a09d0 152
cf264981 153=item Keywords related to the control flow of your Perl program
d74e8afc 154X<control flow>
cb1a09d0 155
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156C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
157C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 158
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159=item Keywords related to switch
160
36fb85f3 161C<break>, C<continue>, C<given>, C<when>, C<default>
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162
163(These are only available if you enable the "switch" feature.
164See L<feature> and L<perlsyn/"Switch statements">.)
165
54310121 166=item Keywords related to scoping
cb1a09d0 167
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168C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
169C<use>
170
171(C<state> is only available if the "state" feature is enabled. See
172L<feature>.)
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173
174=item Miscellaneous functions
175
36fb85f3 176C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
834df1c5 177C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
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178
179=item Functions for processes and process groups
d74e8afc 180X<process> X<pid> X<process id>
cb1a09d0 181
22fae026 182C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
1dc8ecb8 183C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 184C<times>, C<wait>, C<waitpid>
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185
186=item Keywords related to perl modules
d74e8afc 187X<module>
cb1a09d0 188
22fae026 189C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
cb1a09d0 190
353c6505 191=item Keywords related to classes and object-orientation
d74e8afc 192X<object> X<class> X<package>
cb1a09d0 193
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194C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
195C<untie>, C<use>
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196
197=item Low-level socket functions
d74e8afc 198X<socket> X<sock>
cb1a09d0 199
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200C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
201C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
737dd4b4 202C<socket>, C<socketpair>
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203
204=item System V interprocess communication functions
d74e8afc 205X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
cb1a09d0 206
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207C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
208C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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209
210=item Fetching user and group info
d74e8afc 211X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
cb1a09d0 212
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213C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
214C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
215C<getpwuid>, C<setgrent>, C<setpwent>
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216
217=item Fetching network info
d74e8afc 218X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
cb1a09d0 219
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220C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
221C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
222C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
223C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
224C<setnetent>, C<setprotoent>, C<setservent>
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225
226=item Time-related functions
d74e8afc 227X<time> X<date>
cb1a09d0 228
22fae026 229C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 230
37798a01 231=item Functions new in perl5
d74e8afc 232X<perl5>
37798a01 233
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234C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
235C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
1dc8ecb8 236C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
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237C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
238C<ucfirst>, C<untie>, C<use>, C<when>
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239
240* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 241operator, which can be used in expressions.
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242
243=item Functions obsoleted in perl5
244
22fae026 245C<dbmclose>, C<dbmopen>
37798a01 246
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247=back
248
60f9f73c 249=head2 Portability
d74e8afc 250X<portability> X<Unix> X<portable>
60f9f73c 251
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252Perl was born in Unix and can therefore access all common Unix
253system calls. In non-Unix environments, the functionality of some
254Unix system calls may not be available, or details of the available
255functionality may differ slightly. The Perl functions affected
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256by this are:
257
258C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
259C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
260C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
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261C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
262C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
54d7b083 263C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
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264C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
265C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
266C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 267C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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268C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
269C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
270C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
737dd4b4 271C<shmwrite>, C<socket>, C<socketpair>,
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272C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
273C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 274C<utime>, C<wait>, C<waitpid>
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275
276For more information about the portability of these functions, see
277L<perlport> and other available platform-specific documentation.
278
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279=head2 Alphabetical Listing of Perl Functions
280
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281=over 8
282
5b3c99c0 283=item -X FILEHANDLE
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284X<-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>
285X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
a0d0e21e 286
5b3c99c0 287=item -X EXPR
a0d0e21e 288
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289=item -X DIRHANDLE
290
5b3c99c0 291=item -X
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292
293A file test, where X is one of the letters listed below. This unary
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294operator takes one argument, either a filename, a filehandle, or a dirhandle,
295and tests the associated file to see if something is true about it. If the
7660c0ab 296argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 297Unless otherwise documented, it returns C<1> for true and C<''> for false, or
a0d0e21e 298the undefined value if the file doesn't exist. Despite the funny
d0821a6a 299names, precedence is the same as any other named unary operator. The
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300operator may be any of:
301
302 -r File is readable by effective uid/gid.
303 -w File is writable by effective uid/gid.
304 -x File is executable by effective uid/gid.
305 -o File is owned by effective uid.
306
307 -R File is readable by real uid/gid.
308 -W File is writable by real uid/gid.
309 -X File is executable by real uid/gid.
310 -O File is owned by real uid.
311
312 -e File exists.
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313 -z File has zero size (is empty).
314 -s File has nonzero size (returns size in bytes).
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315
316 -f File is a plain file.
317 -d File is a directory.
318 -l File is a symbolic link.
9c4d0f16 319 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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320 -S File is a socket.
321 -b File is a block special file.
322 -c File is a character special file.
323 -t Filehandle is opened to a tty.
324
325 -u File has setuid bit set.
326 -g File has setgid bit set.
327 -k File has sticky bit set.
328
121910a4 329 -T File is an ASCII text file (heuristic guess).
2cdbc966 330 -B File is a "binary" file (opposite of -T).
a0d0e21e 331
95a3fe12 332 -M Script start time minus file modification time, in days.
a0d0e21e 333 -A Same for access time.
95a3fe12 334 -C Same for inode change time (Unix, may differ for other platforms)
a0d0e21e 335
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336Example:
337
338 while (<>) {
5b3eff12 339 chomp;
a0d0e21e 340 next unless -f $_; # ignore specials
5a964f20 341 #...
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342 }
343
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344The interpretation of the file permission operators C<-r>, C<-R>,
345C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
346of the file and the uids and gids of the user. There may be other
ecae030f
MO
347reasons you can't actually read, write, or execute the file: for
348example network filesystem access controls, ACLs (access control lists),
349read-only filesystems, and unrecognized executable formats. Note
350that the use of these six specific operators to verify if some operation
351is possible is usually a mistake, because it may be open to race
352conditions.
5ff3f7a4 353
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354Also note that, for the superuser on the local filesystems, the C<-r>,
355C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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356if any execute bit is set in the mode. Scripts run by the superuser
357may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 358or temporarily set their effective uid to something else.
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GS
359
360If you are using ACLs, there is a pragma called C<filetest> that may
361produce more accurate results than the bare stat() mode bits.
5ff3f7a4
GS
362When under the C<use filetest 'access'> the above-mentioned filetests
363will test whether the permission can (not) be granted using the
468541a8 364access() family of system calls. Also note that the C<-x> and C<-X> may
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365under this pragma return true even if there are no execute permission
366bits set (nor any extra execute permission ACLs). This strangeness is
ecae030f
MO
367due to the underlying system calls' definitions. Note also that, due to
368the implementation of C<use filetest 'access'>, the C<_> special
369filehandle won't cache the results of the file tests when this pragma is
370in effect. Read the documentation for the C<filetest> pragma for more
371information.
5ff3f7a4 372
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373Note that C<-s/a/b/> does not do a negated substitution. Saying
374C<-exp($foo)> still works as expected, however--only single letters
375following a minus are interpreted as file tests.
376
377The C<-T> and C<-B> switches work as follows. The first block or so of the
378file is examined for odd characters such as strange control codes or
61eff3bc 379characters with the high bit set. If too many strange characters (>30%)
cf264981 380are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
a0d0e21e 381containing null in the first block is considered a binary file. If C<-T>
9124316e 382or C<-B> is used on a filehandle, the current IO buffer is examined
19799a22 383rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 384file, or a file at EOF when testing a filehandle. Because you have to
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385read a file to do the C<-T> test, on most occasions you want to use a C<-f>
386against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 387
19799a22 388If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 389the special filehandle consisting of a solitary underline, then the stat
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390structure of the previous file test (or stat operator) is used, saving
391a system call. (This doesn't work with C<-t>, and you need to remember
392that lstat() and C<-l> will leave values in the stat structure for the
5c9aa243 393symbolic link, not the real file.) (Also, if the stat buffer was filled by
cf264981 394an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
5c9aa243 395Example:
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396
397 print "Can do.\n" if -r $a || -w _ || -x _;
398
399 stat($filename);
400 print "Readable\n" if -r _;
401 print "Writable\n" if -w _;
402 print "Executable\n" if -x _;
403 print "Setuid\n" if -u _;
404 print "Setgid\n" if -g _;
405 print "Sticky\n" if -k _;
406 print "Text\n" if -T _;
407 print "Binary\n" if -B _;
408
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409As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
410test operators, in a way that C<-f -w -x $file> is equivalent to
cf264981 411C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
fbb0b3b3
RGS
412the return value of C<-f $file> as an argument to another filetest
413operator, no special magic will happen.)
414
a0d0e21e 415=item abs VALUE
d74e8afc 416X<abs> X<absolute>
a0d0e21e 417
54310121 418=item abs
bbce6d69 419
a0d0e21e 420Returns the absolute value of its argument.
7660c0ab 421If VALUE is omitted, uses C<$_>.
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422
423=item accept NEWSOCKET,GENERICSOCKET
d74e8afc 424X<accept>
a0d0e21e 425
f86cebdf 426Accepts an incoming socket connect, just as the accept(2) system call
19799a22 427does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 428See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 429
8d2a6795
GS
430On systems that support a close-on-exec flag on files, the flag will
431be set for the newly opened file descriptor, as determined by the
432value of $^F. See L<perlvar/$^F>.
433
a0d0e21e 434=item alarm SECONDS
d74e8afc
ITB
435X<alarm>
436X<SIGALRM>
437X<timer>
a0d0e21e 438
54310121 439=item alarm
bbce6d69 440
a0d0e21e 441Arranges to have a SIGALRM delivered to this process after the
cf264981 442specified number of wallclock seconds has elapsed. If SECONDS is not
d400eac8
JH
443specified, the value stored in C<$_> is used. (On some machines,
444unfortunately, the elapsed time may be up to one second less or more
445than you specified because of how seconds are counted, and process
446scheduling may delay the delivery of the signal even further.)
447
448Only one timer may be counting at once. Each call disables the
449previous timer, and an argument of C<0> may be supplied to cancel the
450previous timer without starting a new one. The returned value is the
451amount of time remaining on the previous timer.
a0d0e21e 452
2bc69794
BS
453For delays of finer granularity than one second, the Time::HiRes module
454(from CPAN, and starting from Perl 5.8 part of the standard
455distribution) provides ualarm(). You may also use Perl's four-argument
456version of select() leaving the first three arguments undefined, or you
457might be able to use the C<syscall> interface to access setitimer(2) if
458your system supports it. See L<perlfaq8> for details.
2b5ab1e7 459
68f8bed4
JH
460It is usually a mistake to intermix C<alarm> and C<sleep> calls.
461(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 462
19799a22
GS
463If you want to use C<alarm> to time out a system call you need to use an
464C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 465fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 466restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 467modulo the caveats given in L<perlipc/"Signals">.
ff68c719
PP
468
469 eval {
f86cebdf 470 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 471 alarm $timeout;
ff68c719 472 $nread = sysread SOCKET, $buffer, $size;
36477c24 473 alarm 0;
ff68c719 474 };
ff68c719 475 if ($@) {
f86cebdf 476 die unless $@ eq "alarm\n"; # propagate unexpected errors
ff68c719
PP
477 # timed out
478 }
479 else {
480 # didn't
481 }
482
91d81acc
JH
483For more information see L<perlipc>.
484
a0d0e21e 485=item atan2 Y,X
d74e8afc 486X<atan2> X<arctangent> X<tan> X<tangent>
a0d0e21e
LW
487
488Returns the arctangent of Y/X in the range -PI to PI.
489
ca6e1c26 490For the tangent operation, you may use the C<Math::Trig::tan>
28757baa
PP
491function, or use the familiar relation:
492
493 sub tan { sin($_[0]) / cos($_[0]) }
494
a1021d57
RGS
495The return value for C<atan2(0,0)> is implementation-defined; consult
496your atan2(3) manpage for more information.
bf5f1b4c 497
a0d0e21e 498=item bind SOCKET,NAME
d74e8afc 499X<bind>
a0d0e21e
LW
500
501Binds a network address to a socket, just as the bind system call
19799a22 502does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
503packed address of the appropriate type for the socket. See the examples in
504L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 505
fae2c0fb 506=item binmode FILEHANDLE, LAYER
d74e8afc 507X<binmode> X<binary> X<text> X<DOS> X<Windows>
1c1fc3ea 508
a0d0e21e
LW
509=item binmode FILEHANDLE
510
1cbfc93d
NIS
511Arranges for FILEHANDLE to be read or written in "binary" or "text"
512mode on systems where the run-time libraries distinguish between
513binary and text files. If FILEHANDLE is an expression, the value is
514taken as the name of the filehandle. Returns true on success,
b5fe5ca2 515otherwise it returns C<undef> and sets C<$!> (errno).
1cbfc93d 516
d807c6f4
JH
517On some systems (in general, DOS and Windows-based systems) binmode()
518is necessary when you're not working with a text file. For the sake
519of portability it is a good idea to always use it when appropriate,
520and to never use it when it isn't appropriate. Also, people can
521set their I/O to be by default UTF-8 encoded Unicode, not bytes.
522
523In other words: regardless of platform, use binmode() on binary data,
524like for example images.
525
526If LAYER is present it is a single string, but may contain multiple
527directives. The directives alter the behaviour of the file handle.
920f5fe1 528When LAYER is present using binmode on a text file makes sense.
d807c6f4 529
fae2c0fb 530If LAYER is omitted or specified as C<:raw> the filehandle is made
0226bbdb
NIS
531suitable for passing binary data. This includes turning off possible CRLF
532translation and marking it as bytes (as opposed to Unicode characters).
749683d2 533Note that, despite what may be implied in I<"Programming Perl"> (the
165a9987
PJ
534Camel) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>
535-- other layers which would affect the binary nature of the stream are
0226bbdb
NIS
536I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
537PERLIO environment variable.
01e6739c 538
d807c6f4
JH
539The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
540form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
541establish default I/O layers. See L<open>.
542
fae2c0fb
RGS
543I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
544in "Programming Perl, 3rd Edition". However, since the publishing of this
545book, by many known as "Camel III", the consensus of the naming of this
546functionality has moved from "discipline" to "layer". All documentation
547of this version of Perl therefore refers to "layers" rather than to
548"disciplines". Now back to the regularly scheduled documentation...>
549
6902c96a
T
550To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
551C<:utf8> just marks the data as UTF-8 without further checking,
552while C<:encoding(utf8)> checks the data for actually being valid
553UTF-8. More details can be found in L<PerlIO::encoding>.
1cbfc93d 554
ed53a2bb 555In general, binmode() should be called after open() but before any I/O
01e6739c
NIS
556is done on the filehandle. Calling binmode() will normally flush any
557pending buffered output data (and perhaps pending input data) on the
fae2c0fb 558handle. An exception to this is the C<:encoding> layer that
01e6739c 559changes the default character encoding of the handle, see L<open>.
fae2c0fb 560The C<:encoding> layer sometimes needs to be called in
3874323d
JH
561mid-stream, and it doesn't flush the stream. The C<:encoding>
562also implicitly pushes on top of itself the C<:utf8> layer because
563internally Perl will operate on UTF-8 encoded Unicode characters.
16fe6d59 564
19799a22 565The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
566system all work together to let the programmer treat a single
567character (C<\n>) as the line terminator, irrespective of the external
568representation. On many operating systems, the native text file
569representation matches the internal representation, but on some
570platforms the external representation of C<\n> is made up of more than
571one character.
572
68bd7414
NIS
573Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
574character to end each line in the external representation of text (even
5e12dbfa 575though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
01e6739c
NIS
576on Unix and most VMS files). In other systems like OS/2, DOS and the
577various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
578but what's stored in text files are the two characters C<\cM\cJ>. That
579means that, if you don't use binmode() on these systems, C<\cM\cJ>
580sequences on disk will be converted to C<\n> on input, and any C<\n> in
581your program will be converted back to C<\cM\cJ> on output. This is what
582you want for text files, but it can be disastrous for binary files.
30168b04
GS
583
584Another consequence of using binmode() (on some systems) is that
585special end-of-file markers will be seen as part of the data stream.
586For systems from the Microsoft family this means that if your binary
4375e838 587data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
588the file, unless you use binmode().
589
590binmode() is not only important for readline() and print() operations,
591but also when using read(), seek(), sysread(), syswrite() and tell()
592(see L<perlport> for more details). See the C<$/> and C<$\> variables
593in L<perlvar> for how to manually set your input and output
594line-termination sequences.
a0d0e21e 595
4633a7c4 596=item bless REF,CLASSNAME
d74e8afc 597X<bless>
a0d0e21e
LW
598
599=item bless REF
600
2b5ab1e7
TC
601This function tells the thingy referenced by REF that it is now an object
602in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 603is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7 604it returns the reference for convenience. Always use the two-argument
cf264981
SP
605version if a derived class might inherit the function doing the blessing.
606See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
607of objects.
a0d0e21e 608
57668c4d 609Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7 610Namespaces with all lowercase names are considered reserved for
cf264981 611Perl pragmata. Builtin types have all uppercase names. To prevent
2b5ab1e7
TC
612confusion, you may wish to avoid such package names as well. Make sure
613that CLASSNAME is a true value.
60ad88b8
GS
614
615See L<perlmod/"Perl Modules">.
616
0d863452
RH
617=item break
618
619Break out of a C<given()> block.
620
621This keyword is enabled by the "switch" feature: see L<feature>
622for more information.
623
a0d0e21e 624=item caller EXPR
d74e8afc 625X<caller> X<call stack> X<stack> X<stack trace>
a0d0e21e
LW
626
627=item caller
628
5a964f20 629Returns the context of the current subroutine call. In scalar context,
28757baa 630returns the caller's package name if there is a caller, that is, if
19799a22 631we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 632otherwise. In list context, returns
a0d0e21e 633
ee6b43cc 634 # 0 1 2
748a9306 635 ($package, $filename, $line) = caller;
a0d0e21e
LW
636
637With EXPR, it returns some extra information that the debugger uses to
638print a stack trace. The value of EXPR indicates how many call frames
639to go back before the current one.
640
ee6b43cc 641 # 0 1 2 3 4
f3aa04c2 642 ($package, $filename, $line, $subroutine, $hasargs,
ee6b43cc 643
644 # 5 6 7 8 9 10
b3ca2e83 645 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
ee6b43cc 646 = caller($i);
e7ea3e70 647
951ba7fe 648Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 649call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 650C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 651C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 652C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
cc1c2e42 653$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
0fc9dec4
RGS
654each C<use> statement creates a C<require> frame inside an C<eval EXPR>
655frame.) $subroutine may also be C<(unknown)> if this particular
656subroutine happens to have been deleted from the symbol table.
657C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
658C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
659compiled with. The C<$hints> and C<$bitmask> values are subject to change
660between versions of Perl, and are not meant for external use.
748a9306 661
b3ca2e83
NC
662C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
663caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
664of this hash, as they are the actual values stored in the optree.
665
748a9306 666Furthermore, when called from within the DB package, caller returns more
7660c0ab 667detailed information: it sets the list variable C<@DB::args> to be the
54310121 668arguments with which the subroutine was invoked.
748a9306 669
7660c0ab 670Be aware that the optimizer might have optimized call frames away before
19799a22 671C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 672might not return information about the call frame you expect it do, for
b76cc8ba 673C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 674previous time C<caller> was called.
7660c0ab 675
a0d0e21e 676=item chdir EXPR
d74e8afc
ITB
677X<chdir>
678X<cd>
f723aae1 679X<directory, change>
a0d0e21e 680
c4aca7d0
GA
681=item chdir FILEHANDLE
682
683=item chdir DIRHANDLE
684
ce2984c3
PF
685=item chdir
686
ffce7b87 687Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 688changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 689changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
690variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
691neither is set, C<chdir> does nothing. It returns true upon success,
692false otherwise. See the example under C<die>.
a0d0e21e 693
c4aca7d0
GA
694On systems that support fchdir, you might pass a file handle or
695directory handle as argument. On systems that don't support fchdir,
696passing handles produces a fatal error at run time.
697
a0d0e21e 698=item chmod LIST
d74e8afc 699X<chmod> X<permission> X<mode>
a0d0e21e
LW
700
701Changes the permissions of a list of files. The first element of the
4633a7c4 702list must be the numerical mode, which should probably be an octal
4ad40acf 703number, and which definitely should I<not> be a string of octal digits:
2f9daede 704C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 705successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
706
707 $cnt = chmod 0755, 'foo', 'bar';
708 chmod 0755, @executables;
f86cebdf
GS
709 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
710 # --w----r-T
2f9daede
TPG
711 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
712 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 713
c4aca7d0
GA
714On systems that support fchmod, you might pass file handles among the
715files. On systems that don't support fchmod, passing file handles
345da378
GA
716produces a fatal error at run time. The file handles must be passed
717as globs or references to be recognized. Barewords are considered
718file names.
c4aca7d0
GA
719
720 open(my $fh, "<", "foo");
721 my $perm = (stat $fh)[2] & 07777;
722 chmod($perm | 0600, $fh);
723
ca6e1c26
JH
724You can also import the symbolic C<S_I*> constants from the Fcntl
725module:
726
727 use Fcntl ':mode';
728
729 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
730 # This is identical to the chmod 0755 of the above example.
731
a0d0e21e 732=item chomp VARIABLE
d74e8afc 733X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
a0d0e21e 734
313c9f5c 735=item chomp( LIST )
a0d0e21e
LW
736
737=item chomp
738
2b5ab1e7
TC
739This safer version of L</chop> removes any trailing string
740that corresponds to the current value of C<$/> (also known as
28757baa
PP
741$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
742number of characters removed from all its arguments. It's often used to
743remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
744that the final record may be missing its newline. When in paragraph
745mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
746When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
747a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 748remove anything.
19799a22 749If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
750
751 while (<>) {
752 chomp; # avoid \n on last field
753 @array = split(/:/);
5a964f20 754 # ...
a0d0e21e
LW
755 }
756
4bf21a6d
RD
757If VARIABLE is a hash, it chomps the hash's values, but not its keys.
758
a0d0e21e
LW
759You can actually chomp anything that's an lvalue, including an assignment:
760
761 chomp($cwd = `pwd`);
762 chomp($answer = <STDIN>);
763
764If you chomp a list, each element is chomped, and the total number of
765characters removed is returned.
766
15e44fd8
RGS
767Note that parentheses are necessary when you're chomping anything
768that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
769is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
770C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
771C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
772as C<chomp($a, $b)>.
773
a0d0e21e 774=item chop VARIABLE
d74e8afc 775X<chop>
a0d0e21e 776
313c9f5c 777=item chop( LIST )
a0d0e21e
LW
778
779=item chop
780
781Chops off the last character of a string and returns the character
5b3eff12 782chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 783scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
784If VARIABLE is a hash, it chops the hash's values, but not its keys.
785
5b3eff12 786You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
787
788If you chop a list, each element is chopped. Only the value of the
19799a22 789last C<chop> is returned.
a0d0e21e 790
19799a22 791Note that C<chop> returns the last character. To return all but the last
748a9306
LW
792character, use C<substr($string, 0, -1)>.
793
15e44fd8
RGS
794See also L</chomp>.
795
a0d0e21e 796=item chown LIST
d74e8afc 797X<chown> X<owner> X<user> X<group>
a0d0e21e
LW
798
799Changes the owner (and group) of a list of files. The first two
19799a22
GS
800elements of the list must be the I<numeric> uid and gid, in that
801order. A value of -1 in either position is interpreted by most
802systems to leave that value unchanged. Returns the number of files
803successfully changed.
a0d0e21e
LW
804
805 $cnt = chown $uid, $gid, 'foo', 'bar';
806 chown $uid, $gid, @filenames;
807
c4aca7d0
GA
808On systems that support fchown, you might pass file handles among the
809files. On systems that don't support fchown, passing file handles
345da378
GA
810produces a fatal error at run time. The file handles must be passed
811as globs or references to be recognized. Barewords are considered
812file names.
c4aca7d0 813
54310121 814Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
815
816 print "User: ";
19799a22 817 chomp($user = <STDIN>);
5a964f20 818 print "Files: ";
19799a22 819 chomp($pattern = <STDIN>);
a0d0e21e
LW
820
821 ($login,$pass,$uid,$gid) = getpwnam($user)
822 or die "$user not in passwd file";
823
5a964f20 824 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
825 chown $uid, $gid, @ary;
826
54310121 827On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
828file unless you're the superuser, although you should be able to change
829the group to any of your secondary groups. On insecure systems, these
830restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
831On POSIX systems, you can detect this condition this way:
832
833 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
834 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 835
a0d0e21e 836=item chr NUMBER
d74e8afc 837X<chr> X<character> X<ASCII> X<Unicode>
a0d0e21e 838
54310121 839=item chr
bbce6d69 840
a0d0e21e 841Returns the character represented by that NUMBER in the character set.
a0ed51b3 842For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
2575c402 843chr(0x263a) is a Unicode smiley face.
aaa68c4a 844
8a064bd6 845Negative values give the Unicode replacement character (chr(0xfffd)),
5f0135eb 846except under the L<bytes> pragma, where low eight bits of the value
8a064bd6
JH
847(truncated to an integer) are used.
848
974da8e5
JH
849If NUMBER is omitted, uses C<$_>.
850
b76cc8ba 851For the reverse, use L</ord>.
a0d0e21e 852
2575c402
JW
853Note that characters from 128 to 255 (inclusive) are by default
854internally not encoded as UTF-8 for backward compatibility reasons.
974da8e5 855
2575c402 856See L<perlunicode> for more about Unicode.
bbce6d69 857
a0d0e21e 858=item chroot FILENAME
d74e8afc 859X<chroot> X<root>
a0d0e21e 860
54310121 861=item chroot
bbce6d69 862
5a964f20 863This function works like the system call by the same name: it makes the
4633a7c4 864named directory the new root directory for all further pathnames that
951ba7fe 865begin with a C</> by your process and all its children. (It doesn't
28757baa 866change your current working directory, which is unaffected.) For security
4633a7c4 867reasons, this call is restricted to the superuser. If FILENAME is
19799a22 868omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
869
870=item close FILEHANDLE
d74e8afc 871X<close>
a0d0e21e 872
6a518fbc
TP
873=item close
874
e0f13c26
RGS
875Closes the file or pipe associated with the file handle, flushes the IO
876buffers, and closes the system file descriptor. Returns true if those
877operations have succeeded and if no error was reported by any PerlIO
878layer. Closes the currently selected filehandle if the argument is
879omitted.
fb73857a
PP
880
881You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
882another C<open> on it, because C<open> will close it for you. (See
883C<open>.) However, an explicit C<close> on an input file resets the line
884counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 885
dede8123
RGS
886If the file handle came from a piped open, C<close> will additionally
887return false if one of the other system calls involved fails, or if the
fb73857a 888program exits with non-zero status. (If the only problem was that the
dede8123 889program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 890also waits for the process executing on the pipe to complete, in case you
b76cc8ba 891want to look at the output of the pipe afterwards, and
e5218da5
GA
892implicitly puts the exit status value of that command into C<$?> and
893C<${^CHILD_ERROR_NATIVE}>.
5a964f20 894
73689b13
GS
895Prematurely closing the read end of a pipe (i.e. before the process
896writing to it at the other end has closed it) will result in a
897SIGPIPE being delivered to the writer. If the other end can't
898handle that, be sure to read all the data before closing the pipe.
899
fb73857a 900Example:
a0d0e21e 901
fb73857a
PP
902 open(OUTPUT, '|sort >foo') # pipe to sort
903 or die "Can't start sort: $!";
5a964f20 904 #... # print stuff to output
fb73857a
PP
905 close OUTPUT # wait for sort to finish
906 or warn $! ? "Error closing sort pipe: $!"
907 : "Exit status $? from sort";
908 open(INPUT, 'foo') # get sort's results
909 or die "Can't open 'foo' for input: $!";
a0d0e21e 910
5a964f20
TC
911FILEHANDLE may be an expression whose value can be used as an indirect
912filehandle, usually the real filehandle name.
a0d0e21e
LW
913
914=item closedir DIRHANDLE
d74e8afc 915X<closedir>
a0d0e21e 916
19799a22 917Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
918system call.
919
a0d0e21e 920=item connect SOCKET,NAME
d74e8afc 921X<connect>
a0d0e21e
LW
922
923Attempts to connect to a remote socket, just as the connect system call
19799a22 924does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
925packed address of the appropriate type for the socket. See the examples in
926L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 927
cb1a09d0 928=item continue BLOCK
d74e8afc 929X<continue>
cb1a09d0 930
0d863452
RH
931=item continue
932
cf264981
SP
933C<continue> is actually a flow control statement rather than a function. If
934there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
98293880
JH
935C<foreach>), it is always executed just before the conditional is about to
936be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
937it can be used to increment a loop variable, even when the loop has been
938continued via the C<next> statement (which is similar to the C C<continue>
939statement).
940
98293880 941C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
942block. C<last> and C<redo> will behave as if they had been executed within
943the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
944block, it may be more entertaining.
945
946 while (EXPR) {
947 ### redo always comes here
948 do_something;
949 } continue {
950 ### next always comes here
951 do_something_else;
952 # then back the top to re-check EXPR
953 }
954 ### last always comes here
955
956Omitting the C<continue> section is semantically equivalent to using an
19799a22 957empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
958to check the condition at the top of the loop.
959
0d863452
RH
960If the "switch" feature is enabled, C<continue> is also a
961function that will break out of the current C<when> or C<default>
962block, and fall through to the next case. See L<feature> and
963L<perlsyn/"Switch statements"> for more information.
964
965
a0d0e21e 966=item cos EXPR
d74e8afc 967X<cos> X<cosine> X<acos> X<arccosine>
a0d0e21e 968
d6217f1e
GS
969=item cos
970
5a964f20 971Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 972takes cosine of C<$_>.
a0d0e21e 973
ca6e1c26 974For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
975function, or use this relation:
976
977 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
978
a0d0e21e 979=item crypt PLAINTEXT,SALT
d74e8afc 980X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
f723aae1 981X<decrypt> X<cryptography> X<passwd> X<encrypt>
a0d0e21e 982
ef2e6798
MS
983Creates a digest string exactly like the crypt(3) function in the C
984library (assuming that you actually have a version there that has not
bb23f8d1 985been extirpated as a potential munition).
ef2e6798
MS
986
987crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
988into a short string, called a digest, which is returned. The same
989PLAINTEXT and SALT will always return the same string, but there is no
990(known) way to get the original PLAINTEXT from the hash. Small
991changes in the PLAINTEXT or SALT will result in large changes in the
992digest.
993
994There is no decrypt function. This function isn't all that useful for
995cryptography (for that, look for F<Crypt> modules on your nearby CPAN
996mirror) and the name "crypt" is a bit of a misnomer. Instead it is
997primarily used to check if two pieces of text are the same without
998having to transmit or store the text itself. An example is checking
999if a correct password is given. The digest of the password is stored,
cf264981 1000not the password itself. The user types in a password that is
ef2e6798
MS
1001crypt()'d with the same salt as the stored digest. If the two digests
1002match the password is correct.
1003
1004When verifying an existing digest string you should use the digest as
1005the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
cf264981 1006to create the digest is visible as part of the digest. This ensures
ef2e6798
MS
1007crypt() will hash the new string with the same salt as the digest.
1008This allows your code to work with the standard L<crypt|/crypt> and
1009with more exotic implementations. In other words, do not assume
1010anything about the returned string itself, or how many bytes in the
1011digest matter.
85c16d83
JH
1012
1013Traditionally the result is a string of 13 bytes: two first bytes of
1014the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
bb23f8d1 1015the first eight bytes of PLAINTEXT mattered. But alternative
ef2e6798
MS
1016hashing schemes (like MD5), higher level security schemes (like C2),
1017and implementations on non-UNIX platforms may produce different
1018strings.
85c16d83
JH
1019
1020When choosing a new salt create a random two character string whose
1021characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
d3989d75
CW
1022'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1023characters is just a recommendation; the characters allowed in
1024the salt depend solely on your system's crypt library, and Perl can't
1025restrict what salts C<crypt()> accepts.
e71965be 1026
a0d0e21e 1027Here's an example that makes sure that whoever runs this program knows
cf264981 1028their password:
a0d0e21e
LW
1029
1030 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
1031
1032 system "stty -echo";
1033 print "Password: ";
e71965be 1034 chomp($word = <STDIN>);
a0d0e21e
LW
1035 print "\n";
1036 system "stty echo";
1037
e71965be 1038 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
1039 die "Sorry...\n";
1040 } else {
1041 print "ok\n";
54310121 1042 }
a0d0e21e 1043
9f8f0c9d 1044Of course, typing in your own password to whoever asks you
748a9306 1045for it is unwise.
a0d0e21e 1046
ef2e6798 1047The L<crypt|/crypt> function is unsuitable for hashing large quantities
19799a22 1048of data, not least of all because you can't get the information
ef2e6798 1049back. Look at the L<Digest> module for more robust algorithms.
19799a22 1050
f2791508
JH
1051If using crypt() on a Unicode string (which I<potentially> has
1052characters with codepoints above 255), Perl tries to make sense
1053of the situation by trying to downgrade (a copy of the string)
1054the string back to an eight-bit byte string before calling crypt()
1055(on that copy). If that works, good. If not, crypt() dies with
1056C<Wide character in crypt>.
85c16d83 1057
aa689395 1058=item dbmclose HASH
d74e8afc 1059X<dbmclose>
a0d0e21e 1060
19799a22 1061[This function has been largely superseded by the C<untie> function.]
a0d0e21e 1062
aa689395 1063Breaks the binding between a DBM file and a hash.
a0d0e21e 1064
19799a22 1065=item dbmopen HASH,DBNAME,MASK
d74e8afc 1066X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
a0d0e21e 1067
19799a22 1068[This function has been largely superseded by the C<tie> function.]
a0d0e21e 1069
7b8d334a 1070This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
1071hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1072argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
1073is the name of the database (without the F<.dir> or F<.pag> extension if
1074any). If the database does not exist, it is created with protection
19799a22
GS
1075specified by MASK (as modified by the C<umask>). If your system supports
1076only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 1077program. In older versions of Perl, if your system had neither DBM nor
19799a22 1078ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
1079sdbm(3).
1080
1081If you don't have write access to the DBM file, you can only read hash
1082variables, not set them. If you want to test whether you can write,
19799a22 1083either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 1084which will trap the error.
a0d0e21e 1085
19799a22
GS
1086Note that functions such as C<keys> and C<values> may return huge lists
1087when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
1088function to iterate over large DBM files. Example:
1089
1090 # print out history file offsets
1091 dbmopen(%HIST,'/usr/lib/news/history',0666);
1092 while (($key,$val) = each %HIST) {
1093 print $key, ' = ', unpack('L',$val), "\n";
1094 }
1095 dbmclose(%HIST);
1096
cb1a09d0 1097See also L<AnyDBM_File> for a more general description of the pros and
184e9718 1098cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 1099rich implementation.
4633a7c4 1100
2b5ab1e7
TC
1101You can control which DBM library you use by loading that library
1102before you call dbmopen():
1103
1104 use DB_File;
1105 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1106 or die "Can't open netscape history file: $!";
1107
a0d0e21e 1108=item defined EXPR
d74e8afc 1109X<defined> X<undef> X<undefined>
a0d0e21e 1110
54310121 1111=item defined
bbce6d69 1112
2f9daede
TPG
1113Returns a Boolean value telling whether EXPR has a value other than
1114the undefined value C<undef>. If EXPR is not present, C<$_> will be
1115checked.
1116
1117Many operations return C<undef> to indicate failure, end of file,
1118system error, uninitialized variable, and other exceptional
1119conditions. This function allows you to distinguish C<undef> from
1120other values. (A simple Boolean test will not distinguish among
7660c0ab 1121C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 1122false.) Note that since C<undef> is a valid scalar, its presence
19799a22 1123doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
1124returns C<undef> when its argument is an empty array, I<or> when the
1125element to return happens to be C<undef>.
1126
f10b0346
GS
1127You may also use C<defined(&func)> to check whether subroutine C<&func>
1128has ever been defined. The return value is unaffected by any forward
04891299 1129declarations of C<&func>. Note that a subroutine which is not defined
847c7ebe
DD
1130may still be callable: its package may have an C<AUTOLOAD> method that
1131makes it spring into existence the first time that it is called -- see
1132L<perlsub>.
f10b0346
GS
1133
1134Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1135used to report whether memory for that aggregate has ever been
1136allocated. This behavior may disappear in future versions of Perl.
1137You should instead use a simple test for size:
1138
1139 if (@an_array) { print "has array elements\n" }
1140 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
1141
1142When used on a hash element, it tells you whether the value is defined,
dc848c6f 1143not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 1144purpose.
a0d0e21e
LW
1145
1146Examples:
1147
1148 print if defined $switch{'D'};
1149 print "$val\n" while defined($val = pop(@ary));
1150 die "Can't readlink $sym: $!"
1151 unless defined($value = readlink $sym);
a0d0e21e 1152 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 1153 $debugging = 0 unless defined $debugging;
a0d0e21e 1154
19799a22 1155Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 1156discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 1157defined values. For example, if you say
a5f75d66
AD
1158
1159 "ab" =~ /a(.*)b/;
1160
7660c0ab 1161The pattern match succeeds, and C<$1> is defined, despite the fact that it
cf264981 1162matched "nothing". It didn't really fail to match anything. Rather, it
2b5ab1e7 1163matched something that happened to be zero characters long. This is all
a5f75d66 1164very above-board and honest. When a function returns an undefined value,
2f9daede 1165it's an admission that it couldn't give you an honest answer. So you
19799a22 1166should use C<defined> only when you're questioning the integrity of what
7660c0ab 1167you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
1168what you want.
1169
dc848c6f 1170See also L</undef>, L</exists>, L</ref>.
2f9daede 1171
a0d0e21e 1172=item delete EXPR
d74e8afc 1173X<delete>
a0d0e21e 1174
01020589
GS
1175Given an expression that specifies a hash element, array element, hash slice,
1176or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 1177In the case of an array, if the array elements happen to be at the end,
b76cc8ba 1178the size of the array will shrink to the highest element that tests
8216c1fd 1179true for exists() (or 0 if no such element exists).
a0d0e21e 1180
eba0920a
EM
1181Returns a list with the same number of elements as the number of elements
1182for which deletion was attempted. Each element of that list consists of
1183either the value of the element deleted, or the undefined value. In scalar
1184context, this means that you get the value of the last element deleted (or
1185the undefined value if that element did not exist).
1186
1187 %hash = (foo => 11, bar => 22, baz => 33);
1188 $scalar = delete $hash{foo}; # $scalar is 11
1189 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1190 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1191
1192Deleting from C<%ENV> modifies the environment. Deleting from
01020589
GS
1193a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1194from a C<tie>d hash or array may not necessarily return anything.
1195
8ea97a1e
GS
1196Deleting an array element effectively returns that position of the array
1197to its initial, uninitialized state. Subsequently testing for the same
cf264981
SP
1198element with exists() will return false. Also, deleting array elements
1199in the middle of an array will not shift the index of the elements
1200after them down. Use splice() for that. See L</exists>.
8ea97a1e 1201
01020589 1202The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 1203
5f05dabc
PP
1204 foreach $key (keys %HASH) {
1205 delete $HASH{$key};
a0d0e21e
LW
1206 }
1207
01020589
GS
1208 foreach $index (0 .. $#ARRAY) {
1209 delete $ARRAY[$index];
1210 }
1211
1212And so do these:
5f05dabc 1213
01020589
GS
1214 delete @HASH{keys %HASH};
1215
9740c838 1216 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1217
2b5ab1e7 1218But both of these are slower than just assigning the empty list
01020589
GS
1219or undefining %HASH or @ARRAY:
1220
1221 %HASH = (); # completely empty %HASH
1222 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1223
01020589
GS
1224 @ARRAY = (); # completely empty @ARRAY
1225 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1226
1227Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1228operation is a hash element, array element, hash slice, or array slice
1229lookup:
a0d0e21e
LW
1230
1231 delete $ref->[$x][$y]{$key};
5f05dabc 1232 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1233
01020589
GS
1234 delete $ref->[$x][$y][$index];
1235 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1236
a0d0e21e 1237=item die LIST
d74e8afc 1238X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
a0d0e21e 1239
19799a22
GS
1240Outside an C<eval>, prints the value of LIST to C<STDERR> and
1241exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1242exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1243status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1244an C<eval(),> the error message is stuffed into C<$@> and the
1245C<eval> is terminated with the undefined value. This makes
1246C<die> the way to raise an exception.
a0d0e21e
LW
1247
1248Equivalent examples:
1249
1250 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1251 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e 1252
ccac6780 1253If the last element of LIST does not end in a newline, the current
df37ec69
WW
1254script line number and input line number (if any) are also printed,
1255and a newline is supplied. Note that the "input line number" (also
1256known as "chunk") is subject to whatever notion of "line" happens to
1257be currently in effect, and is also available as the special variable
1258C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1259
1260Hint: sometimes appending C<", stopped"> to your message will cause it
1261to make better sense when the string C<"at foo line 123"> is appended.
1262Suppose you are running script "canasta".
a0d0e21e
LW
1263
1264 die "/etc/games is no good";
1265 die "/etc/games is no good, stopped";
1266
1267produce, respectively
1268
1269 /etc/games is no good at canasta line 123.
1270 /etc/games is no good, stopped at canasta line 123.
1271
2b5ab1e7 1272See also exit(), warn(), and the Carp module.
a0d0e21e 1273
7660c0ab
A
1274If LIST is empty and C<$@> already contains a value (typically from a
1275previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1276This is useful for propagating exceptions:
1277
1278 eval { ... };
1279 die unless $@ =~ /Expected exception/;
1280
ad216e65
JH
1281If LIST is empty and C<$@> contains an object reference that has a
1282C<PROPAGATE> method, that method will be called with additional file
1283and line number parameters. The return value replaces the value in
28a5cf3b 1284C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
ad216e65
JH
1285were called.
1286
7660c0ab 1287If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1288
52531d10
GS
1289die() can also be called with a reference argument. If this happens to be
1290trapped within an eval(), $@ contains the reference. This behavior permits
1291a more elaborate exception handling implementation using objects that
4375e838 1292maintain arbitrary state about the nature of the exception. Such a scheme
52531d10 1293is sometimes preferable to matching particular string values of $@ using
746d7dd7
GL
1294regular expressions. Because $@ is a global variable, and eval() may be
1295used within object implementations, care must be taken that analyzing the
1296error object doesn't replace the reference in the global variable. The
1297easiest solution is to make a local copy of the reference before doing
1298other manipulations. Here's an example:
52531d10 1299
da279afe 1300 use Scalar::Util 'blessed';
1301
52531d10 1302 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
746d7dd7
GL
1303 if (my $ev_err = $@) {
1304 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
52531d10
GS
1305 # handle Some::Module::Exception
1306 }
1307 else {
1308 # handle all other possible exceptions
1309 }
1310 }
1311
19799a22 1312Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1313them, you may want to overload stringification operations on such custom
1314exception objects. See L<overload> for details about that.
1315
19799a22
GS
1316You can arrange for a callback to be run just before the C<die>
1317does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1318handler will be called with the error text and can change the error
1319message, if it sees fit, by calling C<die> again. See
1320L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
cf264981 1321L<"eval BLOCK"> for some examples. Although this feature was
19799a22
GS
1322to be run only right before your program was to exit, this is not
1323currently the case--the C<$SIG{__DIE__}> hook is currently called
1324even inside eval()ed blocks/strings! If one wants the hook to do
1325nothing in such situations, put
fb73857a
PP
1326
1327 die @_ if $^S;
1328
19799a22
GS
1329as the first line of the handler (see L<perlvar/$^S>). Because
1330this promotes strange action at a distance, this counterintuitive
b76cc8ba 1331behavior may be fixed in a future release.
774d564b 1332
a0d0e21e 1333=item do BLOCK
d74e8afc 1334X<do> X<block>
a0d0e21e
LW
1335
1336Not really a function. Returns the value of the last command in the
6b275a1f
RGS
1337sequence of commands indicated by BLOCK. When modified by the C<while> or
1338C<until> loop modifier, executes the BLOCK once before testing the loop
1339condition. (On other statements the loop modifiers test the conditional
1340first.)
a0d0e21e 1341
4968c1e4 1342C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1343C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1344See L<perlsyn> for alternative strategies.
4968c1e4 1345
a0d0e21e 1346=item do SUBROUTINE(LIST)
d74e8afc 1347X<do>
a0d0e21e 1348
cf264981 1349This form of subroutine call is deprecated. See L<perlsub>.
a0d0e21e
LW
1350
1351=item do EXPR
d74e8afc 1352X<do>
a0d0e21e
LW
1353
1354Uses the value of EXPR as a filename and executes the contents of the
ea63ef19 1355file as a Perl script.
a0d0e21e
LW
1356
1357 do 'stat.pl';
1358
1359is just like
1360
986b19de 1361 eval `cat stat.pl`;
a0d0e21e 1362
2b5ab1e7 1363except that it's more efficient and concise, keeps track of the current
ea63ef19 1364filename for error messages, searches the @INC directories, and updates
2b5ab1e7
TC
1365C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1366variables. It also differs in that code evaluated with C<do FILENAME>
1367cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1368same, however, in that it does reparse the file every time you call it,
1369so you probably don't want to do this inside a loop.
a0d0e21e 1370
8e30cc93 1371If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1372error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1373returns undef and sets an error message in C<$@>. If the file is
1374successfully compiled, C<do> returns the value of the last expression
1375evaluated.
1376
a0d0e21e 1377Note that inclusion of library modules is better done with the
19799a22 1378C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1379and raise an exception if there's a problem.
a0d0e21e 1380
5a964f20
TC
1381You might like to use C<do> to read in a program configuration
1382file. Manual error checking can be done this way:
1383
b76cc8ba 1384 # read in config files: system first, then user
f86cebdf 1385 for $file ("/share/prog/defaults.rc",
b76cc8ba 1386 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1387 {
5a964f20 1388 unless ($return = do $file) {
f86cebdf
GS
1389 warn "couldn't parse $file: $@" if $@;
1390 warn "couldn't do $file: $!" unless defined $return;
1391 warn "couldn't run $file" unless $return;
5a964f20
TC
1392 }
1393 }
1394
a0d0e21e 1395=item dump LABEL
d74e8afc 1396X<dump> X<core> X<undump>
a0d0e21e 1397
1614b0e3
JD
1398=item dump
1399
19799a22
GS
1400This function causes an immediate core dump. See also the B<-u>
1401command-line switch in L<perlrun>, which does the same thing.
1402Primarily this is so that you can use the B<undump> program (not
1403supplied) to turn your core dump into an executable binary after
1404having initialized all your variables at the beginning of the
1405program. When the new binary is executed it will begin by executing
1406a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1407Think of it as a goto with an intervening core dump and reincarnation.
1408If C<LABEL> is omitted, restarts the program from the top.
1409
1410B<WARNING>: Any files opened at the time of the dump will I<not>
1411be open any more when the program is reincarnated, with possible
b76cc8ba 1412resulting confusion on the part of Perl.
19799a22 1413
59f521f4
RGS
1414This function is now largely obsolete, mostly because it's very hard to
1415convert a core file into an executable. That's why you should now invoke
1416it as C<CORE::dump()>, if you don't want to be warned against a possible
ac206dc8 1417typo.
19799a22 1418
aa689395 1419=item each HASH
d74e8afc 1420X<each> X<hash, iterator>
aa689395 1421
aeedbbed
NC
1422=item each ARRAY
1423X<array, iterator>
1424
5a964f20 1425When called in list context, returns a 2-element list consisting of the
aeedbbed
NC
1426key and value for the next element of a hash, or the index and value for
1427the next element of an array, so that you can iterate over it. When called
1428in scalar context, returns only the key for the next element in the hash
1429(or the index for an array).
2f9daede 1430
aeedbbed 1431Hash entries are returned in an apparently random order. The actual random
504f80c1
JH
1432order is subject to change in future versions of perl, but it is
1433guaranteed to be in the same order as either the C<keys> or C<values>
4546b9e6 1434function would produce on the same (unmodified) hash. Since Perl
22883ac5 14355.8.2 the ordering can be different even between different runs of Perl
4546b9e6 1436for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
ab192400 1437
aeedbbed
NC
1438When the hash or array is entirely read, a null array is returned in list
1439context (which when assigned produces a false (C<0>) value), and C<undef> in
19799a22 1440scalar context. The next call to C<each> after that will start iterating
aeedbbed
NC
1441again. There is a single iterator for each hash or array, shared by all
1442C<each>, C<keys>, and C<values> function calls in the program; it can be
1443reset by reading all the elements from the hash or array, or by evaluating
1444C<keys HASH>, C<values HASH>, C<keys ARRAY>, or C<values ARRAY>. If you add
1445or delete elements of a hash while you're
74fc8b5f
MJD
1446iterating over it, you may get entries skipped or duplicated, so
1447don't. Exception: It is always safe to delete the item most recently
1448returned by C<each()>, which means that the following code will work:
1449
1450 while (($key, $value) = each %hash) {
1451 print $key, "\n";
1452 delete $hash{$key}; # This is safe
1453 }
aa689395 1454
f86cebdf 1455The following prints out your environment like the printenv(1) program,
aa689395 1456only in a different order:
a0d0e21e
LW
1457
1458 while (($key,$value) = each %ENV) {
1459 print "$key=$value\n";
1460 }
1461
19799a22 1462See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1463
1464=item eof FILEHANDLE
d74e8afc
ITB
1465X<eof>
1466X<end of file>
1467X<end-of-file>
a0d0e21e 1468
4633a7c4
LW
1469=item eof ()
1470
a0d0e21e
LW
1471=item eof
1472
1473Returns 1 if the next read on FILEHANDLE will return end of file, or if
1474FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1475gives the real filehandle. (Note that this function actually
19799a22 1476reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1477interactive context.) Do not read from a terminal file (or call
19799a22 1478C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1479as terminals may lose the end-of-file condition if you do.
1480
820475bd
GS
1481An C<eof> without an argument uses the last file read. Using C<eof()>
1482with empty parentheses is very different. It refers to the pseudo file
1483formed from the files listed on the command line and accessed via the
61eff3bc
JH
1484C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1485as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd 1486used will cause C<@ARGV> to be examined to determine if input is
67408cae 1487available. Similarly, an C<eof()> after C<< <> >> has returned
efdd0218
RB
1488end-of-file will assume you are processing another C<@ARGV> list,
1489and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1490see L<perlop/"I/O Operators">.
820475bd 1491
61eff3bc 1492In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1493detect the end of each file, C<eof()> will only detect the end of the
1494last file. Examples:
a0d0e21e 1495
748a9306
LW
1496 # reset line numbering on each input file
1497 while (<>) {
b76cc8ba 1498 next if /^\s*#/; # skip comments
748a9306 1499 print "$.\t$_";
5a964f20
TC
1500 } continue {
1501 close ARGV if eof; # Not eof()!
748a9306
LW
1502 }
1503
a0d0e21e
LW
1504 # insert dashes just before last line of last file
1505 while (<>) {
6ac88b13 1506 if (eof()) { # check for end of last file
a0d0e21e
LW
1507 print "--------------\n";
1508 }
1509 print;
6ac88b13 1510 last if eof(); # needed if we're reading from a terminal
a0d0e21e
LW
1511 }
1512
a0d0e21e 1513Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1514input operators typically return C<undef> when they run out of data, or if
1515there was an error.
a0d0e21e
LW
1516
1517=item eval EXPR
d74e8afc 1518X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
f723aae1 1519X<error, handling> X<exception, handling>
a0d0e21e
LW
1520
1521=item eval BLOCK
1522
ce2984c3
PF
1523=item eval
1524
c7cc6f1c
GS
1525In the first form, the return value of EXPR is parsed and executed as if it
1526were a little Perl program. The value of the expression (which is itself
5a964f20 1527determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1528errors, executed in the lexical context of the current Perl program, so
1529that any variable settings or subroutine and format definitions remain
cf264981 1530afterwards. Note that the value is parsed every time the C<eval> executes.
be3174d2
GS
1531If EXPR is omitted, evaluates C<$_>. This form is typically used to
1532delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1533
1534In the second form, the code within the BLOCK is parsed only once--at the
cf264981 1535same time the code surrounding the C<eval> itself was parsed--and executed
c7cc6f1c
GS
1536within the context of the current Perl program. This form is typically
1537used to trap exceptions more efficiently than the first (see below), while
1538also providing the benefit of checking the code within BLOCK at compile
1539time.
1540
1541The final semicolon, if any, may be omitted from the value of EXPR or within
1542the BLOCK.
1543
1544In both forms, the value returned is the value of the last expression
5a964f20 1545evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1546as with subroutines. The expression providing the return value is evaluated
cf264981
SP
1547in void, scalar, or list context, depending on the context of the C<eval>
1548itself. See L</wantarray> for more on how the evaluation context can be
1549determined.
a0d0e21e 1550
19799a22 1551If there is a syntax error or runtime error, or a C<die> statement is
bbead3ca
BL
1552executed, C<eval> returns an undefined value in scalar context
1553or an empty list in list context, and C<$@> is set to the
a0d0e21e 1554error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1555string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c 1556warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
d9984052
A
1557To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1558turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1559See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
a0d0e21e 1560
19799a22
GS
1561Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1562determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1563is implemented. It is also Perl's exception trapping mechanism, where
1564the die operator is used to raise exceptions.
1565
5f1da31c
NT
1566If you want to trap errors when loading an XS module, some problems with
1567the binary interface (such as Perl version skew) may be fatal even with
1568C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1569
a0d0e21e
LW
1570If the code to be executed doesn't vary, you may use the eval-BLOCK
1571form to trap run-time errors without incurring the penalty of
1572recompiling each time. The error, if any, is still returned in C<$@>.
1573Examples:
1574
54310121 1575 # make divide-by-zero nonfatal
a0d0e21e
LW
1576 eval { $answer = $a / $b; }; warn $@ if $@;
1577
1578 # same thing, but less efficient
1579 eval '$answer = $a / $b'; warn $@ if $@;
1580
1581 # a compile-time error
5a964f20 1582 eval { $answer = }; # WRONG
a0d0e21e
LW
1583
1584 # a run-time error
1585 eval '$answer ='; # sets $@
1586
cf264981
SP
1587Using the C<eval{}> form as an exception trap in libraries does have some
1588issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1589may wish not to trigger any C<__DIE__> hooks that user code may have installed.
2b5ab1e7
TC
1590You can use the C<local $SIG{__DIE__}> construct for this purpose,
1591as shown in this example:
774d564b
PP
1592
1593 # a very private exception trap for divide-by-zero
f86cebdf
GS
1594 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1595 warn $@ if $@;
774d564b
PP
1596
1597This is especially significant, given that C<__DIE__> hooks can call
19799a22 1598C<die> again, which has the effect of changing their error messages:
774d564b
PP
1599
1600 # __DIE__ hooks may modify error messages
1601 {
f86cebdf
GS
1602 local $SIG{'__DIE__'} =
1603 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1604 eval { die "foo lives here" };
1605 print $@ if $@; # prints "bar lives here"
774d564b
PP
1606 }
1607
19799a22 1608Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1609may be fixed in a future release.
1610
19799a22 1611With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1612being looked at when:
1613
1614 eval $x; # CASE 1
1615 eval "$x"; # CASE 2
1616
1617 eval '$x'; # CASE 3
1618 eval { $x }; # CASE 4
1619
5a964f20 1620 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1621 $$x++; # CASE 6
1622
2f9daede 1623Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1624the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1625the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1626and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1627does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1628purely visual reasons, but it also has the advantage of compiling at
1629compile-time instead of at run-time.) Case 5 is a place where
19799a22 1630normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1631particular situation, you can just use symbolic references instead, as
1632in case 6.
a0d0e21e 1633
8a5a710d
DN
1634The assignment to C<$@> occurs before restoration of localised variables,
1635which means a temporary is required if you want to mask some but not all
1636errors:
1637
1638 # alter $@ on nefarious repugnancy only
1639 {
1640 my $e;
1641 {
1642 local $@; # protect existing $@
1643 eval { test_repugnancy() };
1644 # $@ =~ /nefarious/ and die $@; # DOES NOT WORK
1645 $@ =~ /nefarious/ and $e = $@;
1646 }
1647 die $e if defined $e
1648 }
1649
4968c1e4 1650C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1651C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1652
d819b83a
DM
1653Note that as a very special case, an C<eval ''> executed within the C<DB>
1654package doesn't see the usual surrounding lexical scope, but rather the
1655scope of the first non-DB piece of code that called it. You don't normally
1656need to worry about this unless you are writing a Perl debugger.
1657
a0d0e21e 1658=item exec LIST
d74e8afc 1659X<exec> X<execute>
a0d0e21e 1660
8bf3b016
GS
1661=item exec PROGRAM LIST
1662
19799a22
GS
1663The C<exec> function executes a system command I<and never returns>--
1664use C<system> instead of C<exec> if you want it to return. It fails and
1665returns false only if the command does not exist I<and> it is executed
fb73857a 1666directly instead of via your system's command shell (see below).
a0d0e21e 1667
19799a22
GS
1668Since it's a common mistake to use C<exec> instead of C<system>, Perl
1669warns you if there is a following statement which isn't C<die>, C<warn>,
1670or C<exit> (if C<-w> is set - but you always do that). If you
1671I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1672can use one of these styles to avoid the warning:
1673
5a964f20
TC
1674 exec ('foo') or print STDERR "couldn't exec foo: $!";
1675 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1676
5a964f20 1677If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1678with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1679If there is only one scalar argument or an array with one element in it,
1680the argument is checked for shell metacharacters, and if there are any,
1681the entire argument is passed to the system's command shell for parsing
1682(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1683If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1684words and passed directly to C<execvp>, which is more efficient.
19799a22 1685Examples:
a0d0e21e 1686
19799a22
GS
1687 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1688 exec "sort $outfile | uniq";
a0d0e21e
LW
1689
1690If you don't really want to execute the first argument, but want to lie
1691to the program you are executing about its own name, you can specify
1692the program you actually want to run as an "indirect object" (without a
1693comma) in front of the LIST. (This always forces interpretation of the
54310121 1694LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1695the list.) Example:
1696
1697 $shell = '/bin/csh';
1698 exec $shell '-sh'; # pretend it's a login shell
1699
1700or, more directly,
1701
1702 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1703
bb32b41a
GS
1704When the arguments get executed via the system shell, results will
1705be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1706for details.
1707
19799a22
GS
1708Using an indirect object with C<exec> or C<system> is also more
1709secure. This usage (which also works fine with system()) forces
1710interpretation of the arguments as a multivalued list, even if the
1711list had just one argument. That way you're safe from the shell
1712expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1713
1714 @args = ( "echo surprise" );
1715
2b5ab1e7 1716 exec @args; # subject to shell escapes
f86cebdf 1717 # if @args == 1
2b5ab1e7 1718 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1719
1720The first version, the one without the indirect object, ran the I<echo>
1721program, passing it C<"surprise"> an argument. The second version
1722didn't--it tried to run a program literally called I<"echo surprise">,
1723didn't find it, and set C<$?> to a non-zero value indicating failure.
1724
0f897271
GS
1725Beginning with v5.6.0, Perl will attempt to flush all files opened for
1726output before the exec, but this may not be supported on some platforms
1727(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1728in English) or call the C<autoflush()> method of C<IO::Handle> on any
1729open handles in order to avoid lost output.
1730
19799a22 1731Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1732any C<DESTROY> methods in your objects.
1733
a0d0e21e 1734=item exists EXPR
d74e8afc 1735X<exists> X<autovivification>
a0d0e21e 1736
01020589 1737Given an expression that specifies a hash element or array element,
8ea97a1e 1738returns true if the specified element in the hash or array has ever
95731d22 1739been initialized, even if the corresponding value is undefined.
a0d0e21e 1740
01020589
GS
1741 print "Exists\n" if exists $hash{$key};
1742 print "Defined\n" if defined $hash{$key};
1743 print "True\n" if $hash{$key};
1744
1745 print "Exists\n" if exists $array[$index];
1746 print "Defined\n" if defined $array[$index];
1747 print "True\n" if $array[$index];
a0d0e21e 1748
8ea97a1e 1749A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1750it exists, but the reverse doesn't necessarily hold true.
1751
afebc493
GS
1752Given an expression that specifies the name of a subroutine,
1753returns true if the specified subroutine has ever been declared, even
1754if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1755does not count as declaring it. Note that a subroutine which does not
1756exist may still be callable: its package may have an C<AUTOLOAD>
1757method that makes it spring into existence the first time that it is
1758called -- see L<perlsub>.
afebc493
GS
1759
1760 print "Exists\n" if exists &subroutine;
1761 print "Defined\n" if defined &subroutine;
1762
a0d0e21e 1763Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1764operation is a hash or array key lookup or subroutine name:
a0d0e21e 1765
2b5ab1e7
TC
1766 if (exists $ref->{A}->{B}->{$key}) { }
1767 if (exists $hash{A}{B}{$key}) { }
1768
01020589
GS
1769 if (exists $ref->{A}->{B}->[$ix]) { }
1770 if (exists $hash{A}{B}[$ix]) { }
1771
afebc493
GS
1772 if (exists &{$ref->{A}{B}{$key}}) { }
1773
01020589
GS
1774Although the deepest nested array or hash will not spring into existence
1775just because its existence was tested, any intervening ones will.
61eff3bc 1776Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1777into existence due to the existence test for the $key element above.
1778This happens anywhere the arrow operator is used, including even:
5a964f20 1779
2b5ab1e7
TC
1780 undef $ref;
1781 if (exists $ref->{"Some key"}) { }
1782 print $ref; # prints HASH(0x80d3d5c)
1783
1784This surprising autovivification in what does not at first--or even
1785second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1786release.
a0d0e21e 1787
afebc493
GS
1788Use of a subroutine call, rather than a subroutine name, as an argument
1789to exists() is an error.
1790
1791 exists &sub; # OK
1792 exists &sub(); # Error
1793
a0d0e21e 1794=item exit EXPR
d74e8afc 1795X<exit> X<terminate> X<abort>
a0d0e21e 1796
ce2984c3
PF
1797=item exit
1798
2b5ab1e7 1799Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1800
1801 $ans = <STDIN>;
1802 exit 0 if $ans =~ /^[Xx]/;
1803
19799a22 1804See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1805universally recognized values for EXPR are C<0> for success and C<1>
1806for error; other values are subject to interpretation depending on the
1807environment in which the Perl program is running. For example, exiting
180869 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1809the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1810
19799a22
GS
1811Don't use C<exit> to abort a subroutine if there's any chance that
1812someone might want to trap whatever error happened. Use C<die> instead,
1813which can be trapped by an C<eval>.
28757baa 1814
19799a22 1815The exit() function does not always exit immediately. It calls any
2b5ab1e7 1816defined C<END> routines first, but these C<END> routines may not
19799a22 1817themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1818be called are called before the real exit. If this is a problem, you
1819can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1820See L<perlmod> for details.
5a964f20 1821
a0d0e21e 1822=item exp EXPR
d74e8afc 1823X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
a0d0e21e 1824
54310121 1825=item exp
bbce6d69 1826
b76cc8ba 1827Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1828If EXPR is omitted, gives C<exp($_)>.
1829
1830=item fcntl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 1831X<fcntl>
a0d0e21e 1832
f86cebdf 1833Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1834
1835 use Fcntl;
1836
0ade1984 1837first to get the correct constant definitions. Argument processing and
b76cc8ba 1838value return works just like C<ioctl> below.
a0d0e21e
LW
1839For example:
1840
1841 use Fcntl;
5a964f20
TC
1842 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1843 or die "can't fcntl F_GETFL: $!";
1844
554ad1fc 1845You don't have to check for C<defined> on the return from C<fcntl>.
951ba7fe
GS
1846Like C<ioctl>, it maps a C<0> return from the system call into
1847C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1848in numeric context. It is also exempt from the normal B<-w> warnings
1849on improper numeric conversions.
5a964f20 1850
19799a22 1851Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1852doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1853manpage to learn what functions are available on your system.
a0d0e21e 1854
be2f7487 1855Here's an example of setting a filehandle named C<REMOTE> to be
1856non-blocking at the system level. You'll have to negotiate C<$|>
1857on your own, though.
1858
1859 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1860
1861 $flags = fcntl(REMOTE, F_GETFL, 0)
1862 or die "Can't get flags for the socket: $!\n";
1863
1864 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1865 or die "Can't set flags for the socket: $!\n";
1866
a0d0e21e 1867=item fileno FILEHANDLE
d74e8afc 1868X<fileno>
a0d0e21e 1869
2b5ab1e7
TC
1870Returns the file descriptor for a filehandle, or undefined if the
1871filehandle is not open. This is mainly useful for constructing
19799a22 1872bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1873If FILEHANDLE is an expression, the value is taken as an indirect
1874filehandle, generally its name.
5a964f20 1875
b76cc8ba 1876You can use this to find out whether two handles refer to the
5a964f20
TC
1877same underlying descriptor:
1878
1879 if (fileno(THIS) == fileno(THAT)) {
1880 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1881 }
1882
1883(Filehandles connected to memory objects via new features of C<open> may
1884return undefined even though they are open.)
1885
a0d0e21e
LW
1886
1887=item flock FILEHANDLE,OPERATION
d74e8afc 1888X<flock> X<lock> X<locking>
a0d0e21e 1889
19799a22
GS
1890Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1891for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1892machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1893C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1894only entire files, not records.
1895
1896Two potentially non-obvious but traditional C<flock> semantics are
1897that it waits indefinitely until the lock is granted, and that its locks
1898B<merely advisory>. Such discretionary locks are more flexible, but offer
cf264981
SP
1899fewer guarantees. This means that programs that do not also use C<flock>
1900may modify files locked with C<flock>. See L<perlport>,
2b5ab1e7
TC
1901your port's specific documentation, or your system-specific local manpages
1902for details. It's best to assume traditional behavior if you're writing
1903portable programs. (But if you're not, you should as always feel perfectly
1904free to write for your own system's idiosyncrasies (sometimes called
1905"features"). Slavish adherence to portability concerns shouldn't get
1906in the way of your getting your job done.)
a3cb178b 1907
8ebc5c01
PP
1908OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1909LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1910you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1911either individually, or as a group using the ':flock' tag. LOCK_SH
1912requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1913releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1914LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1915waiting for the lock (check the return status to see if you got it).
1916
2b5ab1e7
TC
1917To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1918before locking or unlocking it.
8ebc5c01 1919
f86cebdf 1920Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1921locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1922are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1923implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1924differing semantics shouldn't bite too many people.
1925
becacb53
TM
1926Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1927be open with read intent to use LOCK_SH and requires that it be open
1928with write intent to use LOCK_EX.
1929
19799a22
GS
1930Note also that some versions of C<flock> cannot lock things over the
1931network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1932that. If you like you can force Perl to ignore your system's flock(2)
1933function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1934the switch C<-Ud_flock> to the F<Configure> program when you configure
1935perl.
4633a7c4
LW
1936
1937Here's a mailbox appender for BSD systems.
a0d0e21e 1938
7ed5353d 1939 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
a0d0e21e
LW
1940
1941 sub lock {
7ed5353d
PF
1942 my ($fh) = @_;
1943 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
1944
1945 # and, in case someone appended while we were waiting...
1946 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
a0d0e21e
LW
1947 }
1948
1949 sub unlock {
7ed5353d
PF
1950 my ($fh) = @_;
1951 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
a0d0e21e
LW
1952 }
1953
b0169937 1954 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
a0d0e21e
LW
1955 or die "Can't open mailbox: $!";
1956
7ed5353d 1957 lock($mbox);
b0169937 1958 print $mbox $msg,"\n\n";
7ed5353d 1959 unlock($mbox);
a0d0e21e 1960
2b5ab1e7
TC
1961On systems that support a real flock(), locks are inherited across fork()
1962calls, whereas those that must resort to the more capricious fcntl()
1963function lose the locks, making it harder to write servers.
1964
cb1a09d0 1965See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1966
1967=item fork
d74e8afc 1968X<fork> X<child> X<parent>
a0d0e21e 1969
2b5ab1e7
TC
1970Does a fork(2) system call to create a new process running the
1971same program at the same point. It returns the child pid to the
1972parent process, C<0> to the child process, or C<undef> if the fork is
1973unsuccessful. File descriptors (and sometimes locks on those descriptors)
1974are shared, while everything else is copied. On most systems supporting
1975fork(), great care has gone into making it extremely efficient (for
1976example, using copy-on-write technology on data pages), making it the
1977dominant paradigm for multitasking over the last few decades.
5a964f20 1978
0f897271
GS
1979Beginning with v5.6.0, Perl will attempt to flush all files opened for
1980output before forking the child process, but this may not be supported
1981on some platforms (see L<perlport>). To be safe, you may need to set
1982C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1983C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1984
19799a22 1985If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1986accumulate zombies. On some systems, you can avoid this by setting
1987C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1988forking and reaping moribund children.
cb1a09d0 1989
28757baa
PP
1990Note that if your forked child inherits system file descriptors like
1991STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1992if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1993backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1994You should reopen those to F</dev/null> if it's any issue.
28757baa 1995
cb1a09d0 1996=item format
d74e8afc 1997X<format>
cb1a09d0 1998
19799a22 1999Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
2000example:
2001
54310121 2002 format Something =
cb1a09d0
AD
2003 Test: @<<<<<<<< @||||| @>>>>>
2004 $str, $%, '$' . int($num)
2005 .
2006
2007 $str = "widget";
184e9718 2008 $num = $cost/$quantity;
cb1a09d0
AD
2009 $~ = 'Something';
2010 write;
2011
2012See L<perlform> for many details and examples.
2013
8903cb82 2014=item formline PICTURE,LIST
d74e8afc 2015X<formline>
a0d0e21e 2016
5a964f20 2017This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
2018too. It formats (see L<perlform>) a list of values according to the
2019contents of PICTURE, placing the output into the format output
7660c0ab 2020accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 2021Eventually, when a C<write> is done, the contents of
cf264981
SP
2022C<$^A> are written to some filehandle. You could also read C<$^A>
2023and then set C<$^A> back to C<"">. Note that a format typically
19799a22 2024does one C<formline> per line of form, but the C<formline> function itself
748a9306 2025doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 2026that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
2027You may therefore need to use multiple formlines to implement a single
2028record format, just like the format compiler.
2029
19799a22 2030Be careful if you put double quotes around the picture, because an C<@>
748a9306 2031character may be taken to mean the beginning of an array name.
19799a22 2032C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
2033
2034=item getc FILEHANDLE
f723aae1 2035X<getc> X<getchar> X<character> X<file, read>
a0d0e21e
LW
2036
2037=item getc
2038
2039Returns the next character from the input file attached to FILEHANDLE,
b5fe5ca2
SR
2040or the undefined value at end of file, or if there was an error (in
2041the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2042STDIN. This is not particularly efficient. However, it cannot be
2043used by itself to fetch single characters without waiting for the user
2044to hit enter. For that, try something more like:
4633a7c4
LW
2045
2046 if ($BSD_STYLE) {
2047 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2048 }
2049 else {
54310121 2050 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
2051 }
2052
2053 $key = getc(STDIN);
2054
2055 if ($BSD_STYLE) {
2056 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2057 }
2058 else {
5f05dabc 2059 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
2060 }
2061 print "\n";
2062
54310121
PP
2063Determination of whether $BSD_STYLE should be set
2064is left as an exercise to the reader.
cb1a09d0 2065
19799a22 2066The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
2067systems purporting POSIX compliance. See also the C<Term::ReadKey>
2068module from your nearest CPAN site; details on CPAN can be found on
2069L<perlmodlib/CPAN>.
a0d0e21e
LW
2070
2071=item getlogin
d74e8afc 2072X<getlogin> X<login>
a0d0e21e 2073
cf264981 2074This implements the C library function of the same name, which on most
5a964f20 2075systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 2076use C<getpwuid>.
a0d0e21e 2077
f86702cc 2078 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 2079
19799a22
GS
2080Do not consider C<getlogin> for authentication: it is not as
2081secure as C<getpwuid>.
4633a7c4 2082
a0d0e21e 2083=item getpeername SOCKET
d74e8afc 2084X<getpeername> X<peer>
a0d0e21e
LW
2085
2086Returns the packed sockaddr address of other end of the SOCKET connection.
2087
4633a7c4
LW
2088 use Socket;
2089 $hersockaddr = getpeername(SOCK);
19799a22 2090 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
2091 $herhostname = gethostbyaddr($iaddr, AF_INET);
2092 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
2093
2094=item getpgrp PID
d74e8afc 2095X<getpgrp> X<group>
a0d0e21e 2096
47e29363 2097Returns the current process group for the specified PID. Use
7660c0ab 2098a PID of C<0> to get the current process group for the
4633a7c4 2099current process. Will raise an exception if used on a machine that
f86cebdf 2100doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 2101group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 2102does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
2103
2104=item getppid
d74e8afc 2105X<getppid> X<parent> X<pid>
a0d0e21e
LW
2106
2107Returns the process id of the parent process.
2108
4d76a344
RGS
2109Note for Linux users: on Linux, the C functions C<getpid()> and
2110C<getppid()> return different values from different threads. In order to
2111be portable, this behavior is not reflected by the perl-level function
2112C<getppid()>, that returns a consistent value across threads. If you want
e3256f86
RGS
2113to call the underlying C<getppid()>, you may use the CPAN module
2114C<Linux::Pid>.
4d76a344 2115
a0d0e21e 2116=item getpriority WHICH,WHO
d74e8afc 2117X<getpriority> X<priority> X<nice>
a0d0e21e 2118
4633a7c4 2119Returns the current priority for a process, a process group, or a user.
f4084e39 2120(See C<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 2121machine that doesn't implement getpriority(2).
a0d0e21e
LW
2122
2123=item getpwnam NAME
d74e8afc
ITB
2124X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2125X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2126X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2127X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2128X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2129X<endnetent> X<endprotoent> X<endservent>
a0d0e21e
LW
2130
2131=item getgrnam NAME
2132
2133=item gethostbyname NAME
2134
2135=item getnetbyname NAME
2136
2137=item getprotobyname NAME
2138
2139=item getpwuid UID
2140
2141=item getgrgid GID
2142
2143=item getservbyname NAME,PROTO
2144
2145=item gethostbyaddr ADDR,ADDRTYPE
2146
2147=item getnetbyaddr ADDR,ADDRTYPE
2148
2149=item getprotobynumber NUMBER
2150
2151=item getservbyport PORT,PROTO
2152
2153=item getpwent
2154
2155=item getgrent
2156
2157=item gethostent
2158
2159=item getnetent
2160
2161=item getprotoent
2162
2163=item getservent
2164
2165=item setpwent
2166
2167=item setgrent
2168
2169=item sethostent STAYOPEN
2170
2171=item setnetent STAYOPEN
2172
2173=item setprotoent STAYOPEN
2174
2175=item setservent STAYOPEN
2176
2177=item endpwent
2178
2179=item endgrent
2180
2181=item endhostent
2182
2183=item endnetent
2184
2185=item endprotoent
2186
2187=item endservent
2188
2189These routines perform the same functions as their counterparts in the
5a964f20 2190system library. In list context, the return values from the
a0d0e21e
LW
2191various get routines are as follows:
2192
2193 ($name,$passwd,$uid,$gid,
6ee623d5 2194 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
2195 ($name,$passwd,$gid,$members) = getgr*
2196 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2197 ($name,$aliases,$addrtype,$net) = getnet*
2198 ($name,$aliases,$proto) = getproto*
2199 ($name,$aliases,$port,$proto) = getserv*
2200
2201(If the entry doesn't exist you get a null list.)
2202
4602f195
JH
2203The exact meaning of the $gcos field varies but it usually contains
2204the real name of the user (as opposed to the login name) and other
2205information pertaining to the user. Beware, however, that in many
2206system users are able to change this information and therefore it
106325ad 2207cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
2208L<perlsec>). The $passwd and $shell, user's encrypted password and
2209login shell, are also tainted, because of the same reason.
4602f195 2210
5a964f20 2211In scalar context, you get the name, unless the function was a
a0d0e21e
LW
2212lookup by name, in which case you get the other thing, whatever it is.
2213(If the entry doesn't exist you get the undefined value.) For example:
2214
5a964f20
TC
2215 $uid = getpwnam($name);
2216 $name = getpwuid($num);
2217 $name = getpwent();
2218 $gid = getgrnam($name);
08a33e13 2219 $name = getgrgid($num);
5a964f20
TC
2220 $name = getgrent();
2221 #etc.
a0d0e21e 2222
4602f195
JH
2223In I<getpw*()> the fields $quota, $comment, and $expire are special
2224cases in the sense that in many systems they are unsupported. If the
2225$quota is unsupported, it is an empty scalar. If it is supported, it
2226usually encodes the disk quota. If the $comment field is unsupported,
2227it is an empty scalar. If it is supported it usually encodes some
2228administrative comment about the user. In some systems the $quota
2229field may be $change or $age, fields that have to do with password
2230aging. In some systems the $comment field may be $class. The $expire
2231field, if present, encodes the expiration period of the account or the
2232password. For the availability and the exact meaning of these fields
2233in your system, please consult your getpwnam(3) documentation and your
2234F<pwd.h> file. You can also find out from within Perl what your
2235$quota and $comment fields mean and whether you have the $expire field
2236by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2237C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2238files are only supported if your vendor has implemented them in the
2239intuitive fashion that calling the regular C library routines gets the
5d3a0a3b 2240shadow versions if you're running under privilege or if there exists
cf264981
SP
2241the shadow(3) functions as found in System V (this includes Solaris
2242and Linux.) Those systems that implement a proprietary shadow password
5d3a0a3b 2243facility are unlikely to be supported.
6ee623d5 2244
19799a22 2245The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
2246the login names of the members of the group.
2247
2248For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2249C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 2250C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
2251addresses returned by the corresponding system library call. In the
2252Internet domain, each address is four bytes long and you can unpack it
2253by saying something like:
2254
f337b084 2255 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
a0d0e21e 2256
2b5ab1e7
TC
2257The Socket library makes this slightly easier:
2258
2259 use Socket;
2260 $iaddr = inet_aton("127.1"); # or whatever address
2261 $name = gethostbyaddr($iaddr, AF_INET);
2262
2263 # or going the other way
19799a22 2264 $straddr = inet_ntoa($iaddr);
2b5ab1e7 2265
d760c846
GS
2266In the opposite way, to resolve a hostname to the IP address
2267you can write this:
2268
2269 use Socket;
2270 $packed_ip = gethostbyname("www.perl.org");
2271 if (defined $packed_ip) {
2272 $ip_address = inet_ntoa($packed_ip);
2273 }
2274
2275Make sure <gethostbyname()> is called in SCALAR context and that
2276its return value is checked for definedness.
2277
19799a22
GS
2278If you get tired of remembering which element of the return list
2279contains which return value, by-name interfaces are provided
2280in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2281C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2282and C<User::grent>. These override the normal built-ins, supplying
2283versions that return objects with the appropriate names
2284for each field. For example:
5a964f20
TC
2285
2286 use File::stat;
2287 use User::pwent;
2288 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2289
b76cc8ba
NIS
2290Even though it looks like they're the same method calls (uid),
2291they aren't, because a C<File::stat> object is different from
19799a22 2292a C<User::pwent> object.
5a964f20 2293
a0d0e21e 2294=item getsockname SOCKET
d74e8afc 2295X<getsockname>
a0d0e21e 2296
19799a22
GS
2297Returns the packed sockaddr address of this end of the SOCKET connection,
2298in case you don't know the address because you have several different
2299IPs that the connection might have come in on.
a0d0e21e 2300
4633a7c4
LW
2301 use Socket;
2302 $mysockaddr = getsockname(SOCK);
19799a22 2303 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 2304 printf "Connect to %s [%s]\n",
19799a22
GS
2305 scalar gethostbyaddr($myaddr, AF_INET),
2306 inet_ntoa($myaddr);
a0d0e21e
LW
2307
2308=item getsockopt SOCKET,LEVEL,OPTNAME
d74e8afc 2309X<getsockopt>
a0d0e21e 2310
636e6b1f
TH
2311Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2312Options may exist at multiple protocol levels depending on the socket
2313type, but at least the uppermost socket level SOL_SOCKET (defined in the
2314C<Socket> module) will exist. To query options at another level the
2315protocol number of the appropriate protocol controlling the option
2316should be supplied. For example, to indicate that an option is to be
2317interpreted by the TCP protocol, LEVEL should be set to the protocol
2318number of TCP, which you can get using getprotobyname.
2319
2320The call returns a packed string representing the requested socket option,
2321or C<undef> if there is an error (the error reason will be in $!). What
2322exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2323your system documentation for details. A very common case however is that
cf264981 2324the option is an integer, in which case the result will be a packed
636e6b1f
TH
2325integer which you can decode using unpack with the C<i> (or C<I>) format.
2326
2327An example testing if Nagle's algorithm is turned on on a socket:
2328
4852725b 2329 use Socket qw(:all);
636e6b1f
TH
2330
2331 defined(my $tcp = getprotobyname("tcp"))
2332 or die "Could not determine the protocol number for tcp";
4852725b
DD
2333 # my $tcp = IPPROTO_TCP; # Alternative
2334 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2335 or die "Could not query TCP_NODELAY socket option: $!";
636e6b1f
TH
2336 my $nodelay = unpack("I", $packed);
2337 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2338
a0d0e21e
LW
2339
2340=item glob EXPR
d74e8afc 2341X<glob> X<wildcard> X<filename, expansion> X<expand>
a0d0e21e 2342
0a753a76
PP
2343=item glob
2344
d9a9d457
JL
2345In list context, returns a (possibly empty) list of filename expansions on
2346the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2347scalar context, glob iterates through such filename expansions, returning
2348undef when the list is exhausted. This is the internal function
2349implementing the C<< <*.c> >> operator, but you can use it directly. If
2350EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2351more detail in L<perlop/"I/O Operators">.
a0d0e21e 2352
5c0c9249
PF
2353Note that C<glob> will split its arguments on whitespace, treating
2354each segment as separate pattern. As such, C<glob('*.c *.h')> would
2355match all files with a F<.c> or F<.h> extension. The expression
2356C<glob('.* *')> would match all files in the current working directory.
2357
3a4b19e4 2358Beginning with v5.6.0, this operator is implemented using the standard
5c0c9249
PF
2359C<File::Glob> extension. See L<File::Glob> for details, including
2360C<bsd_glob> which does not treat whitespace as a pattern separator.
3a4b19e4 2361
a0d0e21e 2362=item gmtime EXPR
d74e8afc 2363X<gmtime> X<UTC> X<Greenwich>
a0d0e21e 2364
ce2984c3
PF
2365=item gmtime
2366
435fbc73
GS
2367Works just like L<localtime> but the returned values are
2368localized for the standard Greenwich time zone.
a0d0e21e 2369
435fbc73
GS
2370Note: when called in list context, $isdst, the last value
2371returned by gmtime is always C<0>. There is no
2372Daylight Saving Time in GMT.
0a753a76 2373
62aa5637
MS
2374See L<perlport/gmtime> for portability concerns.
2375
a0d0e21e 2376=item goto LABEL
d74e8afc 2377X<goto> X<jump> X<jmp>
a0d0e21e 2378
748a9306
LW
2379=item goto EXPR
2380
a0d0e21e
LW
2381=item goto &NAME
2382
7660c0ab 2383The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2384execution there. It may not be used to go into any construct that
7660c0ab 2385requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2386also can't be used to go into a construct that is optimized away,
19799a22 2387or to get out of a block or subroutine given to C<sort>.
0a753a76 2388It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2389including out of subroutines, but it's usually better to use some other
19799a22 2390construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2391need to use this form of C<goto> (in Perl, that is--C is another matter).
1b6921cb
BT
2392(The difference being that C does not offer named loops combined with
2393loop control. Perl does, and this replaces most structured uses of C<goto>
2394in other languages.)
a0d0e21e 2395
7660c0ab
A
2396The C<goto-EXPR> form expects a label name, whose scope will be resolved
2397dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2398necessarily recommended if you're optimizing for maintainability:
2399
2400 goto ("FOO", "BAR", "GLARCH")[$i];
2401
1b6921cb
BT
2402The C<goto-&NAME> form is quite different from the other forms of
2403C<goto>. In fact, it isn't a goto in the normal sense at all, and
2404doesn't have the stigma associated with other gotos. Instead, it
2405exits the current subroutine (losing any changes set by local()) and
2406immediately calls in its place the named subroutine using the current
2407value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2408load another subroutine and then pretend that the other subroutine had
2409been called in the first place (except that any modifications to C<@_>
6cb9131c
GS
2410in the current subroutine are propagated to the other subroutine.)
2411After the C<goto>, not even C<caller> will be able to tell that this
2412routine was called first.
2413
2414NAME needn't be the name of a subroutine; it can be a scalar variable
cf264981 2415containing a code reference, or a block that evaluates to a code
6cb9131c 2416reference.
a0d0e21e
LW
2417
2418=item grep BLOCK LIST
d74e8afc 2419X<grep>
a0d0e21e
LW
2420
2421=item grep EXPR,LIST
2422
2b5ab1e7
TC
2423This is similar in spirit to, but not the same as, grep(1) and its
2424relatives. In particular, it is not limited to using regular expressions.
2f9daede 2425
a0d0e21e 2426Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2427C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2428elements for which the expression evaluated to true. In scalar
2429context, returns the number of times the expression was true.
a0d0e21e
LW
2430
2431 @foo = grep(!/^#/, @bar); # weed out comments
2432
2433or equivalently,
2434
2435 @foo = grep {!/^#/} @bar; # weed out comments
2436
be3174d2
GS
2437Note that C<$_> is an alias to the list value, so it can be used to
2438modify the elements of the LIST. While this is useful and supported,
2439it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2440Similarly, grep returns aliases into the original list, much as a for
2441loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2442element of a list returned by grep (for example, in a C<foreach>, C<map>
2443or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2444This is usually something to be avoided when writing clear code.
a0d0e21e 2445
a4fb8298 2446If C<$_> is lexical in the scope where the C<grep> appears (because it has
cf264981 2447been declared with C<my $_>) then, in addition to being locally aliased to
a4fb8298
RGS
2448the list elements, C<$_> keeps being lexical inside the block; i.e. it
2449can't be seen from the outside, avoiding any potential side-effects.
2450
19799a22 2451See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2452
a0d0e21e 2453=item hex EXPR
d74e8afc 2454X<hex> X<hexadecimal>
a0d0e21e 2455
54310121 2456=item hex
bbce6d69 2457
2b5ab1e7 2458Interprets EXPR as a hex string and returns the corresponding value.
38366c11 2459(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2b5ab1e7 2460L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2461
2462 print hex '0xAf'; # prints '175'
2463 print hex 'aF'; # same
a0d0e21e 2464
19799a22 2465Hex strings may only represent integers. Strings that would cause
53305cf1 2466integer overflow trigger a warning. Leading whitespace is not stripped,
38366c11
DN
2467unlike oct(). To present something as hex, look into L</printf>,
2468L</sprintf>, or L</unpack>.
19799a22 2469
ce2984c3 2470=item import LIST
d74e8afc 2471X<import>
a0d0e21e 2472
19799a22 2473There is no builtin C<import> function. It is just an ordinary
4633a7c4 2474method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2475names to another module. The C<use> function calls the C<import> method
cea6626f 2476for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2477
2478=item index STR,SUBSTR,POSITION
d74e8afc 2479X<index> X<indexOf> X<InStr>
a0d0e21e
LW
2480
2481=item index STR,SUBSTR
2482
2b5ab1e7
TC
2483The index function searches for one string within another, but without
2484the wildcard-like behavior of a full regular-expression pattern match.
2485It returns the position of the first occurrence of SUBSTR in STR at
2486or after POSITION. If POSITION is omitted, starts searching from the
26f149de
YST
2487beginning of the string. POSITION before the beginning of the string
2488or after its end is treated as if it were the beginning or the end,
2489respectively. POSITION and the return value are based at C<0> (or whatever
2b5ab1e7 2490you've set the C<$[> variable to--but don't do that). If the substring
cf264981 2491is not found, C<index> returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2492
2493=item int EXPR
f723aae1 2494X<int> X<integer> X<truncate> X<trunc> X<floor>
a0d0e21e 2495
54310121 2496=item int
bbce6d69 2497
7660c0ab 2498Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2499You should not use this function for rounding: one because it truncates
2500towards C<0>, and two because machine representations of floating point
2501numbers can sometimes produce counterintuitive results. For example,
2502C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2503because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2504the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2505functions will serve you better than will int().
a0d0e21e
LW
2506
2507=item ioctl FILEHANDLE,FUNCTION,SCALAR
d74e8afc 2508X<ioctl>
a0d0e21e 2509
2b5ab1e7 2510Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2511
6c567752 2512 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
a0d0e21e 2513
a11c483f 2514to get the correct function definitions. If F<sys/ioctl.ph> doesn't
a0d0e21e 2515exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2516own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2517(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2518may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2519written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2520will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2521has no string value but does have a numeric value, that value will be
2522passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2523true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2524functions may be needed to manipulate the values of structures used by
b76cc8ba 2525C<ioctl>.
a0d0e21e 2526
19799a22 2527The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2528
2529 if OS returns: then Perl returns:
2530 -1 undefined value
2531 0 string "0 but true"
2532 anything else that number
2533
19799a22 2534Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2535still easily determine the actual value returned by the operating
2536system:
2537
2b5ab1e7 2538 $retval = ioctl(...) || -1;
a0d0e21e
LW
2539 printf "System returned %d\n", $retval;
2540
be2f7487 2541The special string C<"0 but true"> is exempt from B<-w> complaints
5a964f20
TC
2542about improper numeric conversions.
2543
a0d0e21e 2544=item join EXPR,LIST
d74e8afc 2545X<join>
a0d0e21e 2546
2b5ab1e7
TC
2547Joins the separate strings of LIST into a single string with fields
2548separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2549
2b5ab1e7 2550 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2551
eb6e2d6f
GS
2552Beware that unlike C<split>, C<join> doesn't take a pattern as its
2553first argument. Compare L</split>.
a0d0e21e 2554
aa689395 2555=item keys HASH
d74e8afc 2556X<keys> X<key>
aa689395 2557
aeedbbed
NC
2558=item keys ARRAY
2559
2560Returns a list consisting of all the keys of the named hash, or the indices
2561of an array. (In scalar context, returns the number of keys or indices.)
504f80c1 2562
aeedbbed 2563The keys of a hash are returned in an apparently random order. The actual
504f80c1
JH
2564random order is subject to change in future versions of perl, but it
2565is guaranteed to be the same order as either the C<values> or C<each>
4546b9e6
JH
2566function produces (given that the hash has not been modified). Since
2567Perl 5.8.1 the ordering is different even between different runs of
2568Perl for security reasons (see L<perlsec/"Algorithmic Complexity
d6df3700 2569Attacks">).
504f80c1 2570
aeedbbed 2571As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
cf264981
SP
2572(see L</each>). In particular, calling keys() in void context resets
2573the iterator with no other overhead.
a0d0e21e 2574
aa689395 2575Here is yet another way to print your environment:
a0d0e21e
LW
2576
2577 @keys = keys %ENV;
2578 @values = values %ENV;
b76cc8ba 2579 while (@keys) {
a0d0e21e
LW
2580 print pop(@keys), '=', pop(@values), "\n";
2581 }
2582
2583or how about sorted by key:
2584
2585 foreach $key (sort(keys %ENV)) {
2586 print $key, '=', $ENV{$key}, "\n";
2587 }
2588
8ea1e5d4
GS
2589The returned values are copies of the original keys in the hash, so
2590modifying them will not affect the original hash. Compare L</values>.
2591
19799a22 2592To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2593Here's a descending numeric sort of a hash by its values:
4633a7c4 2594
5a964f20 2595 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2596 printf "%4d %s\n", $hash{$key}, $key;
2597 }
2598
19799a22 2599As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2600allocated for the given hash. This can gain you a measure of efficiency if
2601you know the hash is going to get big. (This is similar to pre-extending
2602an array by assigning a larger number to $#array.) If you say
55497cff
PP
2603
2604 keys %hash = 200;
2605
ab192400
GS
2606then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2607in fact, since it rounds up to the next power of two. These
55497cff
PP
2608buckets will be retained even if you do C<%hash = ()>, use C<undef
2609%hash> if you want to free the storage while C<%hash> is still in scope.
2610You can't shrink the number of buckets allocated for the hash using
19799a22 2611C<keys> in this way (but you needn't worry about doing this by accident,
aeedbbed
NC
2612as trying has no effect). C<keys @array> in an lvalue context is a syntax
2613error.
55497cff 2614
19799a22 2615See also C<each>, C<values> and C<sort>.
ab192400 2616
b350dd2f 2617=item kill SIGNAL, LIST
d74e8afc 2618X<kill> X<signal>
a0d0e21e 2619
b350dd2f 2620Sends a signal to a list of processes. Returns the number of
517db077
GS
2621processes successfully signaled (which is not necessarily the
2622same as the number actually killed).
a0d0e21e
LW
2623
2624 $cnt = kill 1, $child1, $child2;
2625 kill 9, @goners;
2626
70fb64f6 2627If SIGNAL is zero, no signal is sent to the process, but the kill(2)
6cb9d3e4 2628system call will check whether it's possible to send a signal to it (that
70fb64f6
RGS
2629means, to be brief, that the process is owned by the same user, or we are
2630the super-user). This is a useful way to check that a child process is
81fd35db
DN
2631alive (even if only as a zombie) and hasn't changed its UID. See
2632L<perlport> for notes on the portability of this construct.
b350dd2f
GS
2633
2634Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2635process groups instead of processes. (On System V, a negative I<PROCESS>
2636number will also kill process groups, but that's not portable.) That
2637means you usually want to use positive not negative signals. You may also
1e9c1022
JL
2638use a signal name in quotes.
2639
2640See L<perlipc/"Signals"> for more details.
a0d0e21e
LW
2641
2642=item last LABEL
d74e8afc 2643X<last> X<break>
a0d0e21e
LW
2644
2645=item last
2646
2647The C<last> command is like the C<break> statement in C (as used in
2648loops); it immediately exits the loop in question. If the LABEL is
2649omitted, the command refers to the innermost enclosing loop. The
2650C<continue> block, if any, is not executed:
2651
4633a7c4
LW
2652 LINE: while (<STDIN>) {
2653 last LINE if /^$/; # exit when done with header
5a964f20 2654 #...
a0d0e21e
LW
2655 }
2656
4968c1e4 2657C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2658C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2659a grep() or map() operation.
4968c1e4 2660
6c1372ed
GS
2661Note that a block by itself is semantically identical to a loop
2662that executes once. Thus C<last> can be used to effect an early
2663exit out of such a block.
2664
98293880
JH
2665See also L</continue> for an illustration of how C<last>, C<next>, and
2666C<redo> work.
1d2dff63 2667
a0d0e21e 2668=item lc EXPR
d74e8afc 2669X<lc> X<lowercase>
a0d0e21e 2670
54310121 2671=item lc
bbce6d69 2672
d1be9408 2673Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2674implementing the C<\L> escape in double-quoted strings. Respects
2675current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2676and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2677
7660c0ab 2678If EXPR is omitted, uses C<$_>.
bbce6d69 2679
a0d0e21e 2680=item lcfirst EXPR
d74e8afc 2681X<lcfirst> X<lowercase>
a0d0e21e 2682
54310121 2683=item lcfirst
bbce6d69 2684
ad0029c4
JH
2685Returns the value of EXPR with the first character lowercased. This
2686is the internal function implementing the C<\l> escape in
2687double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2688locale> in force. See L<perllocale> and L<perlunicode> for more
2689details about locale and Unicode support.
a0d0e21e 2690
7660c0ab 2691If EXPR is omitted, uses C<$_>.
bbce6d69 2692
a0d0e21e 2693=item length EXPR
d74e8afc 2694X<length> X<size>
a0d0e21e 2695
54310121 2696=item length
bbce6d69 2697
974da8e5 2698Returns the length in I<characters> of the value of EXPR. If EXPR is
9f621bb0
NC
2699omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>.
2700Note that this cannot be used on an entire array or hash to find out how
2701many elements these have. For that, use C<scalar @array> and C<scalar keys
2702%hash> respectively.
a0d0e21e 2703
974da8e5
JH
2704Note the I<characters>: if the EXPR is in Unicode, you will get the
2705number of characters, not the number of bytes. To get the length
2575c402
JW
2706of the internal string in bytes, use C<bytes::length(EXPR)>, see
2707L<bytes>. Note that the internal encoding is variable, and the number
2708of bytes usually meaningless. To get the number of bytes that the
2709string would have when encoded as UTF-8, use
2710C<length(Encoding::encode_utf8(EXPR))>.
974da8e5 2711
a0d0e21e 2712=item link OLDFILE,NEWFILE
d74e8afc 2713X<link>
a0d0e21e 2714
19799a22 2715Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2716success, false otherwise.
a0d0e21e
LW
2717
2718=item listen SOCKET,QUEUESIZE
d74e8afc 2719X<listen>
a0d0e21e 2720
19799a22 2721Does the same thing that the listen system call does. Returns true if
b76cc8ba 2722it succeeded, false otherwise. See the example in
cea6626f 2723L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2724
2725=item local EXPR
d74e8afc 2726X<local>
a0d0e21e 2727
19799a22 2728You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2729what most people think of as "local". See
13a2d996 2730L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2731
5a964f20
TC
2732A local modifies the listed variables to be local to the enclosing
2733block, file, or eval. If more than one value is listed, the list must
2734be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2735for details, including issues with tied arrays and hashes.
a0d0e21e 2736
a0d0e21e 2737=item localtime EXPR
435fbc73 2738X<localtime> X<ctime>
a0d0e21e 2739
ba053783
AL
2740=item localtime
2741
19799a22 2742Converts a time as returned by the time function to a 9-element list
5f05dabc 2743with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2744follows:
2745
54310121 2746 # 0 1 2 3 4 5 6 7 8
a0d0e21e 2747 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
ba053783 2748 localtime(time);
a0d0e21e 2749
48a26b3a 2750All list elements are numeric, and come straight out of the C `struct
ba053783
AL
2751tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2752of the specified time.
48a26b3a 2753
ba053783
AL
2754C<$mday> is the day of the month, and C<$mon> is the month itself, in
2755the range C<0..11> with 0 indicating January and 11 indicating December.
2756This makes it easy to get a month name from a list:
54310121 2757
ba053783
AL
2758 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2759 print "$abbr[$mon] $mday";
2760 # $mon=9, $mday=18 gives "Oct 18"
abd75f24 2761
ba053783
AL
2762C<$year> is the number of years since 1900, not just the last two digits
2763of the year. That is, C<$year> is C<123> in year 2023. The proper way
2764to get a complete 4-digit year is simply:
abd75f24 2765
ba053783 2766 $year += 1900;
abd75f24 2767
435fbc73
GS
2768Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2769to do that, would you?
2770
ba053783
AL
2771To get the last two digits of the year (e.g., '01' in 2001) do:
2772
2773 $year = sprintf("%02d", $year % 100);
2774
2775C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2776Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2777(or C<0..365> in leap years.)
2778
2779C<$isdst> is true if the specified time occurs during Daylight Saving
2780Time, false otherwise.
abd75f24 2781
e1998452 2782If EXPR is omitted, C<localtime()> uses the current time (as returned
e3176d09 2783by time(3)).
a0d0e21e 2784
48a26b3a 2785In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2786
5f05dabc 2787 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2788
fe86afc2
NC
2789This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2790instead of local time use the L</gmtime> builtin. See also the
2791C<Time::Local> module (to convert the second, minutes, hours, ... back to
2792the integer value returned by time()), and the L<POSIX> module's strftime(3)
2793and mktime(3) functions.
2794
2795To get somewhat similar but locale dependent date strings, set up your
2796locale environment variables appropriately (please see L<perllocale>) and
2797try for example:
a3cb178b 2798
5a964f20 2799 use POSIX qw(strftime);
2b5ab1e7 2800 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
fe86afc2
NC
2801 # or for GMT formatted appropriately for your locale:
2802 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
a3cb178b
GS
2803
2804Note that the C<%a> and C<%b>, the short forms of the day of the week
2805and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2806
62aa5637
MS
2807See L<perlport/localtime> for portability concerns.
2808
435fbc73
GS
2809The L<Time::gmtime> and L<Time::localtime> modules provides a convenient,
2810by-name access mechanism to the gmtime() and localtime() functions,
2811respectively.
2812
2813For a comprehensive date and time representation look at the
2814L<DateTime> module on CPAN.
2815
07698885 2816=item lock THING
d74e8afc 2817X<lock>
19799a22 2818
01e6739c 2819This function places an advisory lock on a shared variable, or referenced
03730085 2820object contained in I<THING> until the lock goes out of scope.
a6d5524e 2821
f3a23afb 2822lock() is a "weak keyword" : this means that if you've defined a function
67408cae 2823by this name (before any calls to it), that function will be called
03730085
AB
2824instead. (However, if you've said C<use threads>, lock() is always a
2825keyword.) See L<threads>.
19799a22 2826
a0d0e21e 2827=item log EXPR
d74e8afc 2828X<log> X<logarithm> X<e> X<ln> X<base>
a0d0e21e 2829
54310121 2830=item log
bbce6d69 2831
2b5ab1e7
TC
2832Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2833returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2834The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2835divided by the natural log of N. For example:
2836
2837 sub log10 {
2838 my $n = shift;
2839 return log($n)/log(10);
b76cc8ba 2840 }
2b5ab1e7
TC
2841
2842See also L</exp> for the inverse operation.
a0d0e21e 2843
a0d0e21e 2844=item lstat EXPR
d74e8afc 2845X<lstat>
a0d0e21e 2846
54310121 2847=item lstat
bbce6d69 2848
19799a22 2849Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2850special C<_> filehandle) but stats a symbolic link instead of the file
2851the symbolic link points to. If symbolic links are unimplemented on
c837d5b4
DP
2852your system, a normal C<stat> is done. For much more detailed
2853information, please see the documentation for C<stat>.
a0d0e21e 2854
7660c0ab 2855If EXPR is omitted, stats C<$_>.
bbce6d69 2856
a0d0e21e
LW
2857=item m//
2858
9f4b9cd0 2859The match operator. See L<perlop/"Regexp Quote-Like Operators">.
a0d0e21e
LW
2860
2861=item map BLOCK LIST
d74e8afc 2862X<map>
a0d0e21e
LW
2863
2864=item map EXPR,LIST
2865
19799a22
GS
2866Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2867C<$_> to each element) and returns the list value composed of the
2868results of each such evaluation. In scalar context, returns the
2869total number of elements so generated. Evaluates BLOCK or EXPR in
2870list context, so each element of LIST may produce zero, one, or
2871more elements in the returned value.
dd99ebda 2872
a0d0e21e
LW
2873 @chars = map(chr, @nums);
2874
2875translates a list of numbers to the corresponding characters. And
2876
d8216f19 2877 %hash = map { get_a_key_for($_) => $_ } @array;
a0d0e21e
LW
2878
2879is just a funny way to write
2880
2881 %hash = ();
d8216f19
RGS
2882 foreach (@array) {
2883 $hash{get_a_key_for($_)} = $_;
a0d0e21e
LW
2884 }
2885
be3174d2
GS
2886Note that C<$_> is an alias to the list value, so it can be used to
2887modify the elements of the LIST. While this is useful and supported,
2888it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2889Using a regular C<foreach> loop for this purpose would be clearer in
2890most cases. See also L</grep> for an array composed of those items of
2891the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2892
a4fb8298 2893If C<$_> is lexical in the scope where the C<map> appears (because it has
d8216f19
RGS
2894been declared with C<my $_>), then, in addition to being locally aliased to
2895the list elements, C<$_> keeps being lexical inside the block; that is, it
a4fb8298
RGS
2896can't be seen from the outside, avoiding any potential side-effects.
2897
205fdb4d
NC
2898C<{> starts both hash references and blocks, so C<map { ...> could be either
2899the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2900ahead for the closing C<}> it has to take a guess at which its dealing with
2901based what it finds just after the C<{>. Usually it gets it right, but if it
2902doesn't it won't realize something is wrong until it gets to the C<}> and
2903encounters the missing (or unexpected) comma. The syntax error will be
2904reported close to the C<}> but you'll need to change something near the C<{>
2905such as using a unary C<+> to give perl some help:
2906
2907 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2908 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2909 %hash = map { ("\L$_", 1) } @array # this also works
2910 %hash = map { lc($_), 1 } @array # as does this.
2911 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2912
205fdb4d
NC
2913 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2914
d8216f19 2915or to force an anon hash constructor use C<+{>:
205fdb4d
NC
2916
2917 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2918
2919and you get list of anonymous hashes each with only 1 entry.
2920
19799a22 2921=item mkdir FILENAME,MASK
d74e8afc 2922X<mkdir> X<md> X<directory, create>
a0d0e21e 2923
5a211162
GS
2924=item mkdir FILENAME
2925
491873e5
RGS
2926=item mkdir
2927
0591cd52 2928Creates the directory specified by FILENAME, with permissions
19799a22
GS
2929specified by MASK (as modified by C<umask>). If it succeeds it
2930returns true, otherwise it returns false and sets C<$!> (errno).
491873e5
RGS
2931If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2932to C<$_>.
0591cd52 2933
19799a22 2934In general, it is better to create directories with permissive MASK,
0591cd52 2935and let the user modify that with their C<umask>, than it is to supply
19799a22 2936a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2937The exceptions to this rule are when the file or directory should be
2938kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2939C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2940
cc1852e8
JH
2941Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2942number of trailing slashes. Some operating and filesystems do not get
2943this right, so Perl automatically removes all trailing slashes to keep
2944everyone happy.
2945
dd184578
RGS
2946In order to recursively create a directory structure look at
2947the C<mkpath> function of the L<File::Path> module.
2948
a0d0e21e 2949=item msgctl ID,CMD,ARG
d74e8afc 2950X<msgctl>
a0d0e21e 2951
f86cebdf 2952Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2953
2954 use IPC::SysV;
2955
7660c0ab 2956first to get the correct constant definitions. If CMD is C<IPC_STAT>,
cf264981 2957then ARG must be a variable that will hold the returned C<msqid_ds>
951ba7fe
GS
2958structure. Returns like C<ioctl>: the undefined value for error,
2959C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2960L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2961
2962=item msgget KEY,FLAGS
d74e8afc 2963X<msgget>
a0d0e21e 2964
f86cebdf 2965Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2966id, or the undefined value if there is an error. See also
2967L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2968
a0d0e21e 2969=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
d74e8afc 2970X<msgrcv>
a0d0e21e
LW
2971
2972Calls the System V IPC function msgrcv to receive a message from
2973message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2974SIZE. Note that when a message is received, the message type as a
2975native long integer will be the first thing in VAR, followed by the
2976actual message. This packing may be opened with C<unpack("l! a*")>.
2977Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2978an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2979C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2980
2981=item msgsnd ID,MSG,FLAGS
d74e8afc 2982X<msgsnd>
41d6edb2
JH
2983
2984Calls the System V IPC function msgsnd to send the message MSG to the
2985message queue ID. MSG must begin with the native long integer message
2986type, and be followed by the length of the actual message, and finally
2987the message itself. This kind of packing can be achieved with
2988C<pack("l! a*", $type, $message)>. Returns true if successful,
2989or false if there is an error. See also C<IPC::SysV>
2990and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2991
2992=item my EXPR
d74e8afc 2993X<my>
a0d0e21e 2994
307ea6df
JH
2995=item my TYPE EXPR
2996
1d2de774 2997=item my EXPR : ATTRS
09bef843 2998
1d2de774 2999=item my TYPE EXPR : ATTRS
307ea6df 3000
19799a22 3001A C<my> declares the listed variables to be local (lexically) to the
1d2de774
JH
3002enclosing block, file, or C<eval>. If more than one value is listed,
3003the list must be placed in parentheses.
307ea6df 3004
1d2de774
JH
3005The exact semantics and interface of TYPE and ATTRS are still
3006evolving. TYPE is currently bound to the use of C<fields> pragma,
307ea6df
JH
3007and attributes are handled using the C<attributes> pragma, or starting
3008from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3009L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3010L<attributes>, and L<Attribute::Handlers>.
4633a7c4 3011
a0d0e21e 3012=item next LABEL
d74e8afc 3013X<next> X<continue>
a0d0e21e
LW
3014
3015=item next
3016
3017The C<next> command is like the C<continue> statement in C; it starts
3018the next iteration of the loop:
3019
4633a7c4
LW
3020 LINE: while (<STDIN>) {
3021 next LINE if /^#/; # discard comments
5a964f20 3022 #...
a0d0e21e
LW
3023 }
3024
3025Note that if there were a C<continue> block on the above, it would get
3026executed even on discarded lines. If the LABEL is omitted, the command
3027refers to the innermost enclosing loop.
3028
4968c1e4 3029C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
3030C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3031a grep() or map() operation.
4968c1e4 3032
6c1372ed
GS
3033Note that a block by itself is semantically identical to a loop
3034that executes once. Thus C<next> will exit such a block early.
3035
98293880
JH
3036See also L</continue> for an illustration of how C<last>, C<next>, and
3037C<redo> work.
1d2dff63 3038
4a66ea5a 3039=item no Module VERSION LIST
d74e8afc 3040X<no>
4a66ea5a
RGS
3041
3042=item no Module VERSION
3043
a0d0e21e
LW
3044=item no Module LIST
3045
4a66ea5a
RGS
3046=item no Module
3047
c986422f
RGS
3048=item no VERSION
3049
593b9c14 3050See the C<use> function, of which C<no> is the opposite.
a0d0e21e
LW
3051
3052=item oct EXPR
d74e8afc 3053X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
a0d0e21e 3054
54310121 3055=item oct
bbce6d69 3056
4633a7c4 3057Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
3058value. (If EXPR happens to start off with C<0x>, interprets it as a
3059hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
3060binary string. Leading whitespace is ignored in all three cases.)
3061The following will handle decimal, binary, octal, and hex in the standard
3062Perl or C notation:
a0d0e21e
LW
3063
3064 $val = oct($val) if $val =~ /^0/;
3065
19799a22
GS
3066If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3067in octal), use sprintf() or printf():
3068
3069 $perms = (stat("filename"))[2] & 07777;
3070 $oct_perms = sprintf "%lo", $perms;
3071
3072The oct() function is commonly used when a string such as C<644> needs
3073to be converted into a file mode, for example. (Although perl will
3074automatically convert strings into numbers as needed, this automatic
3075conversion assumes base 10.)
a0d0e21e
LW
3076
3077=item open FILEHANDLE,EXPR
d74e8afc 3078X<open> X<pipe> X<file, open> X<fopen>
a0d0e21e 3079
68bd7414
NIS
3080=item open FILEHANDLE,MODE,EXPR
3081
3082=item open FILEHANDLE,MODE,EXPR,LIST
3083
ba964c95
T
3084=item open FILEHANDLE,MODE,REFERENCE
3085
a0d0e21e
LW
3086=item open FILEHANDLE
3087
3088Opens the file whose filename is given by EXPR, and associates it with
ed53a2bb
JH
3089FILEHANDLE.
3090
460b70c2
GS
3091Simple examples to open a file for reading:
3092
3093 open(my $fh, '<', "input.txt") or die $!;
3094
3095and for writing:
3096
3097 open(my $fh, '>', "output.txt") or die $!;
3098
ed53a2bb
JH
3099(The following is a comprehensive reference to open(): for a gentler
3100introduction you may consider L<perlopentut>.)
3101
a28cd5c9
NT
3102If FILEHANDLE is an undefined scalar variable (or array or hash element)
3103the variable is assigned a reference to a new anonymous filehandle,
3104otherwise if FILEHANDLE is an expression, its value is used as the name of
3105the real filehandle wanted. (This is considered a symbolic reference, so
3106C<use strict 'refs'> should I<not> be in effect.)
ed53a2bb
JH
3107
3108If EXPR is omitted, the scalar variable of the same name as the
3109FILEHANDLE contains the filename. (Note that lexical variables--those
3110declared with C<my>--will not work for this purpose; so if you're
67408cae 3111using C<my>, specify EXPR in your call to open.)
ed53a2bb
JH
3112
3113If three or more arguments are specified then the mode of opening and
3114the file name are separate. If MODE is C<< '<' >> or nothing, the file
3115is opened for input. If MODE is C<< '>' >>, the file is truncated and
3116opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 3117the file is opened for appending, again being created if necessary.
5a964f20 3118
ed53a2bb
JH
3119You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3120indicate that you want both read and write access to the file; thus
3121C<< '+<' >> is almost always preferred for read/write updates--the C<<
3122'+>' >> mode would clobber the file first. You can't usually use
3123either read-write mode for updating textfiles, since they have
3124variable length records. See the B<-i> switch in L<perlrun> for a
3125better approach. The file is created with permissions of C<0666>
3126modified by the process' C<umask> value.
3127
3128These various prefixes correspond to the fopen(3) modes of C<'r'>,
3129C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 3130
6170680b
IZ
3131In the 2-arguments (and 1-argument) form of the call the mode and
3132filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
3133spaces. It is possible to omit the mode in these forms if the mode is
3134C<< '<' >>.
6170680b 3135
7660c0ab 3136If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 3137command to which output is to be piped, and if the filename ends with a
f244e06d
GS
3138C<'|'>, the filename is interpreted as a command which pipes output to
3139us. See L<perlipc/"Using open() for IPC">
19799a22 3140for more examples of this. (You are not allowed to C<open> to a command
5a964f20 3141that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
3142and L<perlipc/"Bidirectional Communication with Another Process">
3143for alternatives.)
cb1a09d0