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Re: [ID 20011114.118] chop and chomp bind too tightly
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1=head1 NAME
2
3perlfunc - Perl builtin functions
4
5=head1 DESCRIPTION
6
7The functions in this section can serve as terms in an expression.
8They fall into two major categories: list operators and named unary
9operators. These differ in their precedence relationship with a
10following comma. (See the precedence table in L<perlop>.) List
11operators take more than one argument, while unary operators can never
12take more than one argument. Thus, a comma terminates the argument of
13a unary operator, but merely separates the arguments of a list
14operator. A unary operator generally provides a scalar context to its
2b5ab1e7 15argument, while a list operator may provide either scalar or list
a0d0e21e 16contexts for its arguments. If it does both, the scalar arguments will
5f05dabc 17be first, and the list argument will follow. (Note that there can ever
0f31cffe 18be only one such list argument.) For instance, splice() has three scalar
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19arguments followed by a list, whereas gethostbyname() has four scalar
20arguments.
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21
22In the syntax descriptions that follow, list operators that expect a
23list (and provide list context for the elements of the list) are shown
24with LIST as an argument. Such a list may consist of any combination
25of scalar arguments or list values; the list values will be included
26in the list as if each individual element were interpolated at that
27point in the list, forming a longer single-dimensional list value.
28Elements of the LIST should be separated by commas.
29
30Any function in the list below may be used either with or without
31parentheses around its arguments. (The syntax descriptions omit the
5f05dabc 32parentheses.) If you use the parentheses, the simple (but occasionally
19799a22 33surprising) rule is this: It I<looks> like a function, therefore it I<is> a
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34function, and precedence doesn't matter. Otherwise it's a list
35operator or unary operator, and precedence does matter. And whitespace
36between the function and left parenthesis doesn't count--so you need to
37be careful sometimes:
38
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39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
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44
45If you run Perl with the B<-w> switch it can warn you about this. For
46example, the third line above produces:
47
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
50
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51A few functions take no arguments at all, and therefore work as neither
52unary nor list operators. These include such functions as C<time>
53and C<endpwent>. For example, C<time+86_400> always means
54C<time() + 86_400>.
55
a0d0e21e 56For functions that can be used in either a scalar or list context,
54310121 57nonabortive failure is generally indicated in a scalar context by
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58returning the undefined value, and in a list context by returning the
59null list.
60
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61Remember the following important rule: There is B<no rule> that relates
62the behavior of an expression in list context to its behavior in scalar
63context, or vice versa. It might do two totally different things.
a0d0e21e 64Each operator and function decides which sort of value it would be most
2b5ab1e7 65appropriate to return in scalar context. Some operators return the
5a964f20 66length of the list that would have been returned in list context. Some
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67operators return the first value in the list. Some operators return the
68last value in the list. Some operators return a count of successful
69operations. In general, they do what you want, unless you want
70consistency.
71
d1be9408 72A named array in scalar context is quite different from what would at
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73first glance appear to be a list in scalar context. You can't get a list
74like C<(1,2,3)> into being in scalar context, because the compiler knows
75the context at compile time. It would generate the scalar comma operator
76there, not the list construction version of the comma. That means it
77was never a list to start with.
78
79In general, functions in Perl that serve as wrappers for system calls
f86cebdf 80of the same name (like chown(2), fork(2), closedir(2), etc.) all return
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81true when they succeed and C<undef> otherwise, as is usually mentioned
82in the descriptions below. This is different from the C interfaces,
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83which return C<-1> on failure. Exceptions to this rule are C<wait>,
84C<waitpid>, and C<syscall>. System calls also set the special C<$!>
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85variable on failure. Other functions do not, except accidentally.
86
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87=head2 Perl Functions by Category
88
89Here are Perl's functions (including things that look like
5a964f20 90functions, like some keywords and named operators)
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91arranged by category. Some functions appear in more
92than one place.
93
13a2d996 94=over 4
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95
96=item Functions for SCALARs or strings
97
22fae026 98C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
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99C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
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101
102=item Regular expressions and pattern matching
103
ab4f32c2 104C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
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105
106=item Numeric functions
107
22fae026
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108C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109C<sin>, C<sqrt>, C<srand>
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110
111=item Functions for real @ARRAYs
112
22fae026 113C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
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114
115=item Functions for list data
116
ab4f32c2 117C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
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118
119=item Functions for real %HASHes
120
22fae026 121C<delete>, C<each>, C<exists>, C<keys>, C<values>
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122
123=item Input and output functions
124
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125C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
129C<warn>, C<write>
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130
131=item Functions for fixed length data or records
132
22fae026 133C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
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134
135=item Functions for filehandles, files, or directories
136
22fae026 137C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
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138C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
140C<unlink>, C<utime>
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141
142=item Keywords related to the control flow of your perl program
143
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144C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
cb1a09d0 146
54310121 147=item Keywords related to scoping
cb1a09d0 148
4375e838 149C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
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150
151=item Miscellaneous functions
152
4375e838 153C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
22fae026 154C<scalar>, C<undef>, C<wantarray>
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155
156=item Functions for processes and process groups
157
22fae026 158C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
ab4f32c2 159C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
22fae026 160C<times>, C<wait>, C<waitpid>
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161
162=item Keywords related to perl modules
163
22fae026 164C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
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165
166=item Keywords related to classes and object-orientedness
167
22fae026
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168C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
169C<untie>, C<use>
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170
171=item Low-level socket functions
172
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173C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
80cbd5ad 175C<sockatmark>, C<socket>, C<socketpair>
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176
177=item System V interprocess communication functions
178
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179C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
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181
182=item Fetching user and group info
183
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184C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186C<getpwuid>, C<setgrent>, C<setpwent>
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187
188=item Fetching network info
189
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190C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194C<setnetent>, C<setprotoent>, C<setservent>
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195
196=item Time-related functions
197
22fae026 198C<gmtime>, C<localtime>, C<time>, C<times>
cb1a09d0 199
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200=item Functions new in perl5
201
22fae026 202C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
b76cc8ba 203C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
4375e838 204C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
22fae026 205C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
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206
207* - C<sub> was a keyword in perl4, but in perl5 it is an
5a964f20 208operator, which can be used in expressions.
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209
210=item Functions obsoleted in perl5
211
22fae026 212C<dbmclose>, C<dbmopen>
37798a01 213
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214=back
215
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216=head2 Portability
217
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218Perl was born in Unix and can therefore access all common Unix
219system calls. In non-Unix environments, the functionality of some
220Unix system calls may not be available, or details of the available
221functionality may differ slightly. The Perl functions affected
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222by this are:
223
224C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
228C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
2b5ab1e7 233C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
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234C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
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237C<shmwrite>, C<sockatmark>, C<socket>, C<socketpair>,
238C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239C<times>, C<truncate>, C<umask>, C<unlink>,
2b5ab1e7 240C<utime>, C<wait>, C<waitpid>
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241
242For more information about the portability of these functions, see
243L<perlport> and other available platform-specific documentation.
244
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245=head2 Alphabetical Listing of Perl Functions
246
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247=over 8
248
22fae026 249=item I<-X> FILEHANDLE
a0d0e21e 250
22fae026 251=item I<-X> EXPR
a0d0e21e 252
22fae026 253=item I<-X>
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254
255A file test, where X is one of the letters listed below. This unary
256operator takes one argument, either a filename or a filehandle, and
257tests the associated file to see if something is true about it. If the
7660c0ab 258argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
19799a22 259Unless otherwise documented, it returns C<1> for true and C<''> for false, or
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260the undefined value if the file doesn't exist. Despite the funny
261names, precedence is the same as any other named unary operator, and
262the argument may be parenthesized like any other unary operator. The
263operator may be any of:
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264X<-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>
265X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
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266
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
271
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
276
277 -e File exists.
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278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
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280
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
9c4d0f16 284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
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285 -S File is a socket.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
289
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
293
121910a4 294 -T File is an ASCII text file (heuristic guess).
2cdbc966 295 -B File is a "binary" file (opposite of -T).
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296
297 -M Age of file in days when script started.
298 -A Same for access time.
299 -C Same for inode change time.
300
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301Example:
302
303 while (<>) {
5b3eff12 304 chomp;
a0d0e21e 305 next unless -f $_; # ignore specials
5a964f20 306 #...
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307 }
308
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309The interpretation of the file permission operators C<-r>, C<-R>,
310C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311of the file and the uids and gids of the user. There may be other
312reasons you can't actually read, write, or execute the file. Such
313reasons may be for example network filesystem access controls, ACLs
314(access control lists), read-only filesystems, and unrecognized
315executable formats.
316
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317Also note that, for the superuser on the local filesystems, the C<-r>,
318C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
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319if any execute bit is set in the mode. Scripts run by the superuser
320may thus need to do a stat() to determine the actual mode of the file,
2b5ab1e7 321or temporarily set their effective uid to something else.
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322
323If you are using ACLs, there is a pragma called C<filetest> that may
324produce more accurate results than the bare stat() mode bits.
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325When under the C<use filetest 'access'> the above-mentioned filetests
326will test whether the permission can (not) be granted using the
468541a8 327access() family of system calls. Also note that the C<-x> and C<-X> may
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328under this pragma return true even if there are no execute permission
329bits set (nor any extra execute permission ACLs). This strangeness is
330due to the underlying system calls' definitions. Read the
331documentation for the C<filetest> pragma for more information.
332
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333Note that C<-s/a/b/> does not do a negated substitution. Saying
334C<-exp($foo)> still works as expected, however--only single letters
335following a minus are interpreted as file tests.
336
337The C<-T> and C<-B> switches work as follows. The first block or so of the
338file is examined for odd characters such as strange control codes or
61eff3bc 339characters with the high bit set. If too many strange characters (>30%)
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340are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341containing null in the first block is considered a binary file. If C<-T>
342or C<-B> is used on a filehandle, the current stdio buffer is examined
19799a22 343rather than the first block. Both C<-T> and C<-B> return true on a null
54310121 344file, or a file at EOF when testing a filehandle. Because you have to
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345read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346against the file first, as in C<next unless -f $file && -T $file>.
a0d0e21e 347
19799a22 348If any of the file tests (or either the C<stat> or C<lstat> operators) are given
28757baa 349the special filehandle consisting of a solitary underline, then the stat
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350structure of the previous file test (or stat operator) is used, saving
351a system call. (This doesn't work with C<-t>, and you need to remember
352that lstat() and C<-l> will leave values in the stat structure for the
353symbolic link, not the real file.) Example:
354
355 print "Can do.\n" if -r $a || -w _ || -x _;
356
357 stat($filename);
358 print "Readable\n" if -r _;
359 print "Writable\n" if -w _;
360 print "Executable\n" if -x _;
361 print "Setuid\n" if -u _;
362 print "Setgid\n" if -g _;
363 print "Sticky\n" if -k _;
364 print "Text\n" if -T _;
365 print "Binary\n" if -B _;
366
367=item abs VALUE
368
54310121 369=item abs
bbce6d69 370
a0d0e21e 371Returns the absolute value of its argument.
7660c0ab 372If VALUE is omitted, uses C<$_>.
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373
374=item accept NEWSOCKET,GENERICSOCKET
375
f86cebdf 376Accepts an incoming socket connect, just as the accept(2) system call
19799a22 377does. Returns the packed address if it succeeded, false otherwise.
2b5ab1e7 378See the example in L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 379
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380On systems that support a close-on-exec flag on files, the flag will
381be set for the newly opened file descriptor, as determined by the
382value of $^F. See L<perlvar/$^F>.
383
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384=item alarm SECONDS
385
54310121 386=item alarm
bbce6d69 387
a0d0e21e 388Arranges to have a SIGALRM delivered to this process after the
bbce6d69 389specified number of seconds have elapsed. If SECONDS is not specified,
7660c0ab 390the value stored in C<$_> is used. (On some machines,
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391unfortunately, the elapsed time may be up to one second less than you
392specified because of how seconds are counted.) Only one timer may be
393counting at once. Each call disables the previous timer, and an
7660c0ab 394argument of C<0> may be supplied to cancel the previous timer without
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395starting a new one. The returned value is the amount of time remaining
396on the previous timer.
397
4633a7c4 398For delays of finer granularity than one second, you may use Perl's
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399four-argument version of select() leaving the first three arguments
400undefined, or you might be able to use the C<syscall> interface to
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401access setitimer(2) if your system supports it. The Time::HiRes
402module (from CPAN, and starting from Perl 5.8 part of the standard
403distribution) may also prove useful.
2b5ab1e7 404
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405It is usually a mistake to intermix C<alarm> and C<sleep> calls.
406(C<sleep> may be internally implemented in your system with C<alarm>)
a0d0e21e 407
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408If you want to use C<alarm> to time out a system call you need to use an
409C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
f86cebdf 410fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
19799a22 411restart system calls on some systems. Using C<eval>/C<die> always works,
5a964f20 412modulo the caveats given in L<perlipc/"Signals">.
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413
414 eval {
f86cebdf 415 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
36477c24 416 alarm $timeout;
ff68c719 417 $nread = sysread SOCKET, $buffer, $size;
36477c24 418 alarm 0;
ff68c719 419 };
ff68c719 420 if ($@) {
f86cebdf 421 die unless $@ eq "alarm\n"; # propagate unexpected errors
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422 # timed out
423 }
424 else {
425 # didn't
426 }
427
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428=item atan2 Y,X
429
430Returns the arctangent of Y/X in the range -PI to PI.
431
ca6e1c26 432For the tangent operation, you may use the C<Math::Trig::tan>
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433function, or use the familiar relation:
434
435 sub tan { sin($_[0]) / cos($_[0]) }
436
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437=item bind SOCKET,NAME
438
439Binds a network address to a socket, just as the bind system call
19799a22 440does. Returns true if it succeeded, false otherwise. NAME should be a
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441packed address of the appropriate type for the socket. See the examples in
442L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 443
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444=item binmode FILEHANDLE, DISCIPLINE
445
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446=item binmode FILEHANDLE
447
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448Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
449on systems where the run-time libraries distinguish between binary and
30168b04 450text files. If FILEHANDLE is an expression, the value is taken as the
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451name of the filehandle. DISCIPLINE can be either of C<":raw"> for
452binary mode or C<":crlf"> for "text" mode. If the DISCIPLINE is
e940ddbb
NC
453omitted, it defaults to C<":raw">. Returns true on success, C<undef> on
454failure.
30168b04 455
16fe6d59
GS
456binmode() should be called after open() but before any I/O is done on
457the filehandle.
458
16fe6d59
GS
459On some systems binmode() is necessary when you're not working with a
460text file. For the sake of portability it is a good idea to always use
461it when appropriate, and to never use it when it isn't appropriate.
30168b04
GS
462
463In other words: Regardless of platform, use binmode() on binary
464files, and do not use binmode() on text files.
19799a22 465
16fe6d59
GS
466The C<open> pragma can be used to establish default disciplines.
467See L<open>.
468
19799a22 469The operating system, device drivers, C libraries, and Perl run-time
30168b04
GS
470system all work together to let the programmer treat a single
471character (C<\n>) as the line terminator, irrespective of the external
472representation. On many operating systems, the native text file
473representation matches the internal representation, but on some
474platforms the external representation of C<\n> is made up of more than
475one character.
476
68bd7414
NIS
477Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
478character to end each line in the external representation of text (even
5e12dbfa 479though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
68bd7414
NIS
480on Unix and most VMS files). Consequently binmode() has no effect on
481these operating systems. In other systems like OS/2, DOS and the various
482flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, but
483what's stored in text files are the two characters C<\cM\cJ>. That means
484that, if you don't use binmode() on these systems, C<\cM\cJ> sequences on
485disk will be converted to C<\n> on input, and any C<\n> in your program
5e12dbfa
PP
486will be converted back to C<\cM\cJ> on output. This is what you want for
487text files, but it can be disastrous for binary files.
30168b04
GS
488
489Another consequence of using binmode() (on some systems) is that
490special end-of-file markers will be seen as part of the data stream.
491For systems from the Microsoft family this means that if your binary
4375e838 492data contains C<\cZ>, the I/O subsystem will regard it as the end of
30168b04
GS
493the file, unless you use binmode().
494
495binmode() is not only important for readline() and print() operations,
496but also when using read(), seek(), sysread(), syswrite() and tell()
497(see L<perlport> for more details). See the C<$/> and C<$\> variables
498in L<perlvar> for how to manually set your input and output
499line-termination sequences.
a0d0e21e 500
4633a7c4 501=item bless REF,CLASSNAME
a0d0e21e
LW
502
503=item bless REF
504
2b5ab1e7
TC
505This function tells the thingy referenced by REF that it is now an object
506in the CLASSNAME package. If CLASSNAME is omitted, the current package
19799a22 507is used. Because a C<bless> is often the last thing in a constructor,
2b5ab1e7
TC
508it returns the reference for convenience. Always use the two-argument
509version if the function doing the blessing might be inherited by a
510derived class. See L<perltoot> and L<perlobj> for more about the blessing
511(and blessings) of objects.
a0d0e21e 512
57668c4d 513Consider always blessing objects in CLASSNAMEs that are mixed case.
2b5ab1e7
TC
514Namespaces with all lowercase names are considered reserved for
515Perl pragmata. Builtin types have all uppercase names, so to prevent
516confusion, you may wish to avoid such package names as well. Make sure
517that CLASSNAME is a true value.
60ad88b8
GS
518
519See L<perlmod/"Perl Modules">.
520
a0d0e21e
LW
521=item caller EXPR
522
523=item caller
524
5a964f20 525Returns the context of the current subroutine call. In scalar context,
28757baa 526returns the caller's package name if there is a caller, that is, if
19799a22 527we're in a subroutine or C<eval> or C<require>, and the undefined value
5a964f20 528otherwise. In list context, returns
a0d0e21e 529
748a9306 530 ($package, $filename, $line) = caller;
a0d0e21e
LW
531
532With EXPR, it returns some extra information that the debugger uses to
533print a stack trace. The value of EXPR indicates how many call frames
534to go back before the current one.
535
f3aa04c2 536 ($package, $filename, $line, $subroutine, $hasargs,
e476b1b5 537 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
e7ea3e70 538
951ba7fe 539Here $subroutine may be C<(eval)> if the frame is not a subroutine
19799a22 540call, but an C<eval>. In such a case additional elements $evaltext and
7660c0ab 541C<$is_require> are set: C<$is_require> is true if the frame is created by a
19799a22 542C<require> or C<use> statement, $evaltext contains the text of the
277ddfaf 543C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
951ba7fe 544$filename is C<(eval)>, but $evaltext is undefined. (Note also that
dc848c6f 545each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
277ddfaf 546frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
e476b1b5
GS
547frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
548was compiled with. The C<$hints> and C<$bitmask> values are subject to
549change between versions of Perl, and are not meant for external use.
748a9306
LW
550
551Furthermore, when called from within the DB package, caller returns more
7660c0ab 552detailed information: it sets the list variable C<@DB::args> to be the
54310121 553arguments with which the subroutine was invoked.
748a9306 554
7660c0ab 555Be aware that the optimizer might have optimized call frames away before
19799a22 556C<caller> had a chance to get the information. That means that C<caller(N)>
7660c0ab 557might not return information about the call frame you expect it do, for
b76cc8ba 558C<< N > 1 >>. In particular, C<@DB::args> might have information from the
19799a22 559previous time C<caller> was called.
7660c0ab 560
a0d0e21e
LW
561=item chdir EXPR
562
ffce7b87 563Changes the working directory to EXPR, if possible. If EXPR is omitted,
0bfc1ec4 564changes to the directory specified by C<$ENV{HOME}>, if set; if not,
ffce7b87 565changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
b4ad75f0
AMS
566variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
567neither is set, C<chdir> does nothing. It returns true upon success,
568false otherwise. See the example under C<die>.
a0d0e21e
LW
569
570=item chmod LIST
571
572Changes the permissions of a list of files. The first element of the
4633a7c4 573list must be the numerical mode, which should probably be an octal
2f9daede
TPG
574number, and which definitely should I<not> a string of octal digits:
575C<0644> is okay, C<'0644'> is not. Returns the number of files
dc848c6f 576successfully changed. See also L</oct>, if all you have is a string.
a0d0e21e
LW
577
578 $cnt = chmod 0755, 'foo', 'bar';
579 chmod 0755, @executables;
f86cebdf
GS
580 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
581 # --w----r-T
2f9daede
TPG
582 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
583 $mode = 0644; chmod $mode, 'foo'; # this is best
a0d0e21e 584
ca6e1c26
JH
585You can also import the symbolic C<S_I*> constants from the Fcntl
586module:
587
588 use Fcntl ':mode';
589
590 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
591 # This is identical to the chmod 0755 of the above example.
592
a0d0e21e
LW
593=item chomp VARIABLE
594
313c9f5c 595=item chomp( LIST )
a0d0e21e
LW
596
597=item chomp
598
2b5ab1e7
TC
599This safer version of L</chop> removes any trailing string
600that corresponds to the current value of C<$/> (also known as
28757baa
PP
601$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
602number of characters removed from all its arguments. It's often used to
603remove the newline from the end of an input record when you're worried
2b5ab1e7
TC
604that the final record may be missing its newline. When in paragraph
605mode (C<$/ = "">), it removes all trailing newlines from the string.
4c5a6083
GS
606When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
607a reference to an integer or the like, see L<perlvar>) chomp() won't
b76cc8ba 608remove anything.
19799a22 609If VARIABLE is omitted, it chomps C<$_>. Example:
a0d0e21e
LW
610
611 while (<>) {
612 chomp; # avoid \n on last field
613 @array = split(/:/);
5a964f20 614 # ...
a0d0e21e
LW
615 }
616
4bf21a6d
RD
617If VARIABLE is a hash, it chomps the hash's values, but not its keys.
618
a0d0e21e
LW
619You can actually chomp anything that's an lvalue, including an assignment:
620
621 chomp($cwd = `pwd`);
622 chomp($answer = <STDIN>);
623
624If you chomp a list, each element is chomped, and the total number of
625characters removed is returned.
626
627=item chop VARIABLE
628
313c9f5c 629=item chop( LIST )
a0d0e21e
LW
630
631=item chop
632
633Chops off the last character of a string and returns the character
5b3eff12 634chopped. It is much more efficient than C<s/.$//s> because it neither
7660c0ab 635scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
4bf21a6d
RD
636If VARIABLE is a hash, it chops the hash's values, but not its keys.
637
5b3eff12 638You can actually chop anything that's an lvalue, including an assignment.
a0d0e21e
LW
639
640If you chop a list, each element is chopped. Only the value of the
19799a22 641last C<chop> is returned.
a0d0e21e 642
19799a22 643Note that C<chop> returns the last character. To return all but the last
748a9306
LW
644character, use C<substr($string, 0, -1)>.
645
a0d0e21e
LW
646=item chown LIST
647
648Changes the owner (and group) of a list of files. The first two
19799a22
GS
649elements of the list must be the I<numeric> uid and gid, in that
650order. A value of -1 in either position is interpreted by most
651systems to leave that value unchanged. Returns the number of files
652successfully changed.
a0d0e21e
LW
653
654 $cnt = chown $uid, $gid, 'foo', 'bar';
655 chown $uid, $gid, @filenames;
656
54310121 657Here's an example that looks up nonnumeric uids in the passwd file:
a0d0e21e
LW
658
659 print "User: ";
19799a22 660 chomp($user = <STDIN>);
5a964f20 661 print "Files: ";
19799a22 662 chomp($pattern = <STDIN>);
a0d0e21e
LW
663
664 ($login,$pass,$uid,$gid) = getpwnam($user)
665 or die "$user not in passwd file";
666
5a964f20 667 @ary = glob($pattern); # expand filenames
a0d0e21e
LW
668 chown $uid, $gid, @ary;
669
54310121 670On most systems, you are not allowed to change the ownership of the
4633a7c4
LW
671file unless you're the superuser, although you should be able to change
672the group to any of your secondary groups. On insecure systems, these
673restrictions may be relaxed, but this is not a portable assumption.
19799a22
GS
674On POSIX systems, you can detect this condition this way:
675
676 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
677 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
4633a7c4 678
a0d0e21e
LW
679=item chr NUMBER
680
54310121 681=item chr
bbce6d69 682
a0d0e21e 683Returns the character represented by that NUMBER in the character set.
a0ed51b3 684For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
121910a4
JH
685chr(0x263a) is a Unicode smiley face. Note that characters from 127
686to 255 (inclusive) are by default not encoded in Unicode for backward
687compatibility reasons (but see L<encoding>).
aaa68c4a 688
b76cc8ba 689For the reverse, use L</ord>.
121910a4 690See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 691
7660c0ab 692If NUMBER is omitted, uses C<$_>.
bbce6d69 693
a0d0e21e
LW
694=item chroot FILENAME
695
54310121 696=item chroot
bbce6d69 697
5a964f20 698This function works like the system call by the same name: it makes the
4633a7c4 699named directory the new root directory for all further pathnames that
951ba7fe 700begin with a C</> by your process and all its children. (It doesn't
28757baa 701change your current working directory, which is unaffected.) For security
4633a7c4 702reasons, this call is restricted to the superuser. If FILENAME is
19799a22 703omitted, does a C<chroot> to C<$_>.
a0d0e21e
LW
704
705=item close FILEHANDLE
706
6a518fbc
TP
707=item close
708
19799a22 709Closes the file or pipe associated with the file handle, returning true
a0d0e21e 710only if stdio successfully flushes buffers and closes the system file
19799a22 711descriptor. Closes the currently selected filehandle if the argument
6a518fbc 712is omitted.
fb73857a
PP
713
714You don't have to close FILEHANDLE if you are immediately going to do
19799a22
GS
715another C<open> on it, because C<open> will close it for you. (See
716C<open>.) However, an explicit C<close> on an input file resets the line
717counter (C<$.>), while the implicit close done by C<open> does not.
fb73857a 718
19799a22
GS
719If the file handle came from a piped open C<close> will additionally
720return false if one of the other system calls involved fails or if the
fb73857a 721program exits with non-zero status. (If the only problem was that the
b76cc8ba 722program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
2b5ab1e7 723also waits for the process executing on the pipe to complete, in case you
b76cc8ba 724want to look at the output of the pipe afterwards, and
2b5ab1e7 725implicitly puts the exit status value of that command into C<$?>.
5a964f20 726
73689b13
GS
727Prematurely closing the read end of a pipe (i.e. before the process
728writing to it at the other end has closed it) will result in a
729SIGPIPE being delivered to the writer. If the other end can't
730handle that, be sure to read all the data before closing the pipe.
731
fb73857a 732Example:
a0d0e21e 733
fb73857a
PP
734 open(OUTPUT, '|sort >foo') # pipe to sort
735 or die "Can't start sort: $!";
5a964f20 736 #... # print stuff to output
fb73857a
PP
737 close OUTPUT # wait for sort to finish
738 or warn $! ? "Error closing sort pipe: $!"
739 : "Exit status $? from sort";
740 open(INPUT, 'foo') # get sort's results
741 or die "Can't open 'foo' for input: $!";
a0d0e21e 742
5a964f20
TC
743FILEHANDLE may be an expression whose value can be used as an indirect
744filehandle, usually the real filehandle name.
a0d0e21e
LW
745
746=item closedir DIRHANDLE
747
19799a22 748Closes a directory opened by C<opendir> and returns the success of that
5a964f20
TC
749system call.
750
751DIRHANDLE may be an expression whose value can be used as an indirect
752dirhandle, usually the real dirhandle name.
a0d0e21e
LW
753
754=item connect SOCKET,NAME
755
756Attempts to connect to a remote socket, just as the connect system call
19799a22 757does. Returns true if it succeeded, false otherwise. NAME should be a
4633a7c4
LW
758packed address of the appropriate type for the socket. See the examples in
759L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e 760
cb1a09d0
AD
761=item continue BLOCK
762
763Actually a flow control statement rather than a function. If there is a
98293880
JH
764C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
765C<foreach>), it is always executed just before the conditional is about to
766be evaluated again, just like the third part of a C<for> loop in C. Thus
cb1a09d0
AD
767it can be used to increment a loop variable, even when the loop has been
768continued via the C<next> statement (which is similar to the C C<continue>
769statement).
770
98293880 771C<last>, C<next>, or C<redo> may appear within a C<continue>
19799a22
GS
772block. C<last> and C<redo> will behave as if they had been executed within
773the main block. So will C<next>, but since it will execute a C<continue>
1d2dff63
GS
774block, it may be more entertaining.
775
776 while (EXPR) {
777 ### redo always comes here
778 do_something;
779 } continue {
780 ### next always comes here
781 do_something_else;
782 # then back the top to re-check EXPR
783 }
784 ### last always comes here
785
786Omitting the C<continue> section is semantically equivalent to using an
19799a22 787empty one, logically enough. In that case, C<next> goes directly back
1d2dff63
GS
788to check the condition at the top of the loop.
789
a0d0e21e
LW
790=item cos EXPR
791
d6217f1e
GS
792=item cos
793
5a964f20 794Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
7660c0ab 795takes cosine of C<$_>.
a0d0e21e 796
ca6e1c26 797For the inverse cosine operation, you may use the C<Math::Trig::acos()>
28757baa
PP
798function, or use this relation:
799
800 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
801
a0d0e21e
LW
802=item crypt PLAINTEXT,SALT
803
f86cebdf 804Encrypts a string exactly like the crypt(3) function in the C library
4633a7c4
LW
805(assuming that you actually have a version there that has not been
806extirpated as a potential munition). This can prove useful for checking
807the password file for lousy passwords, amongst other things. Only the
808guys wearing white hats should do this.
a0d0e21e 809
85c16d83
JH
810Note that C<crypt> is intended to be a one-way function, much like
811breaking eggs to make an omelette. There is no (known) corresponding
812decrypt function (in other words, the crypt() is a one-way hash
813function). As a result, this function isn't all that useful for
11155c91 814cryptography. (For that, see your nearby CPAN mirror.)
2f9daede 815
85c16d83
JH
816When verifying an existing encrypted string you should use the
817encrypted text as the salt (like C<crypt($plain, $crypted) eq
818$crypted>). This allows your code to work with the standard C<crypt>
819and with more exotic implementations. In other words, do not assume
820anything about the returned string itself, or how many bytes in
821the encrypted string matter.
822
823Traditionally the result is a string of 13 bytes: two first bytes of
824the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
825the first eight bytes of the encrypted string mattered, but
826alternative hashing schemes (like MD5), higher level security schemes
827(like C2), and implementations on non-UNIX platforms may produce
828different strings.
829
830When choosing a new salt create a random two character string whose
831characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
832'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
e71965be 833
a0d0e21e
LW
834Here's an example that makes sure that whoever runs this program knows
835their own password:
836
837 $pwd = (getpwuid($<))[1];
a0d0e21e
LW
838
839 system "stty -echo";
840 print "Password: ";
e71965be 841 chomp($word = <STDIN>);
a0d0e21e
LW
842 print "\n";
843 system "stty echo";
844
e71965be 845 if (crypt($word, $pwd) ne $pwd) {
a0d0e21e
LW
846 die "Sorry...\n";
847 } else {
848 print "ok\n";
54310121 849 }
a0d0e21e 850
9f8f0c9d 851Of course, typing in your own password to whoever asks you
748a9306 852for it is unwise.
a0d0e21e 853
19799a22
GS
854The L<crypt> function is unsuitable for encrypting large quantities
855of data, not least of all because you can't get the information
856back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
857on your favorite CPAN mirror for a slew of potentially useful
858modules.
859
d1be9408 860If using crypt() on a Unicode string (which potentially has
85c16d83
JH
861characters with codepoints above 255), Perl tries to make sense of
862the situation by using only the low eight bits of the characters when
863calling crypt().
864
aa689395 865=item dbmclose HASH
a0d0e21e 866
19799a22 867[This function has been largely superseded by the C<untie> function.]
a0d0e21e 868
aa689395 869Breaks the binding between a DBM file and a hash.
a0d0e21e 870
19799a22 871=item dbmopen HASH,DBNAME,MASK
a0d0e21e 872
19799a22 873[This function has been largely superseded by the C<tie> function.]
a0d0e21e 874
7b8d334a 875This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
19799a22
GS
876hash. HASH is the name of the hash. (Unlike normal C<open>, the first
877argument is I<not> a filehandle, even though it looks like one). DBNAME
aa689395
PP
878is the name of the database (without the F<.dir> or F<.pag> extension if
879any). If the database does not exist, it is created with protection
19799a22
GS
880specified by MASK (as modified by the C<umask>). If your system supports
881only the older DBM functions, you may perform only one C<dbmopen> in your
aa689395 882program. In older versions of Perl, if your system had neither DBM nor
19799a22 883ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
aa689395
PP
884sdbm(3).
885
886If you don't have write access to the DBM file, you can only read hash
887variables, not set them. If you want to test whether you can write,
19799a22 888either use file tests or try setting a dummy hash entry inside an C<eval>,
aa689395 889which will trap the error.
a0d0e21e 890
19799a22
GS
891Note that functions such as C<keys> and C<values> may return huge lists
892when used on large DBM files. You may prefer to use the C<each>
a0d0e21e
LW
893function to iterate over large DBM files. Example:
894
895 # print out history file offsets
896 dbmopen(%HIST,'/usr/lib/news/history',0666);
897 while (($key,$val) = each %HIST) {
898 print $key, ' = ', unpack('L',$val), "\n";
899 }
900 dbmclose(%HIST);
901
cb1a09d0 902See also L<AnyDBM_File> for a more general description of the pros and
184e9718 903cons of the various dbm approaches, as well as L<DB_File> for a particularly
cb1a09d0 904rich implementation.
4633a7c4 905
2b5ab1e7
TC
906You can control which DBM library you use by loading that library
907before you call dbmopen():
908
909 use DB_File;
910 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
911 or die "Can't open netscape history file: $!";
912
a0d0e21e
LW
913=item defined EXPR
914
54310121 915=item defined
bbce6d69 916
2f9daede
TPG
917Returns a Boolean value telling whether EXPR has a value other than
918the undefined value C<undef>. If EXPR is not present, C<$_> will be
919checked.
920
921Many operations return C<undef> to indicate failure, end of file,
922system error, uninitialized variable, and other exceptional
923conditions. This function allows you to distinguish C<undef> from
924other values. (A simple Boolean test will not distinguish among
7660c0ab 925C<undef>, zero, the empty string, and C<"0">, which are all equally
2f9daede 926false.) Note that since C<undef> is a valid scalar, its presence
19799a22 927doesn't I<necessarily> indicate an exceptional condition: C<pop>
2f9daede
TPG
928returns C<undef> when its argument is an empty array, I<or> when the
929element to return happens to be C<undef>.
930
f10b0346
GS
931You may also use C<defined(&func)> to check whether subroutine C<&func>
932has ever been defined. The return value is unaffected by any forward
847c7ebe
DD
933declarations of C<&foo>. Note that a subroutine which is not defined
934may still be callable: its package may have an C<AUTOLOAD> method that
935makes it spring into existence the first time that it is called -- see
936L<perlsub>.
f10b0346
GS
937
938Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
939used to report whether memory for that aggregate has ever been
940allocated. This behavior may disappear in future versions of Perl.
941You should instead use a simple test for size:
942
943 if (@an_array) { print "has array elements\n" }
944 if (%a_hash) { print "has hash members\n" }
2f9daede
TPG
945
946When used on a hash element, it tells you whether the value is defined,
dc848c6f 947not whether the key exists in the hash. Use L</exists> for the latter
2f9daede 948purpose.
a0d0e21e
LW
949
950Examples:
951
952 print if defined $switch{'D'};
953 print "$val\n" while defined($val = pop(@ary));
954 die "Can't readlink $sym: $!"
955 unless defined($value = readlink $sym);
a0d0e21e 956 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
2f9daede 957 $debugging = 0 unless defined $debugging;
a0d0e21e 958
19799a22 959Note: Many folks tend to overuse C<defined>, and then are surprised to
7660c0ab 960discover that the number C<0> and C<""> (the zero-length string) are, in fact,
2f9daede 961defined values. For example, if you say
a5f75d66
AD
962
963 "ab" =~ /a(.*)b/;
964
7660c0ab 965The pattern match succeeds, and C<$1> is defined, despite the fact that it
a5f75d66 966matched "nothing". But it didn't really match nothing--rather, it
2b5ab1e7 967matched something that happened to be zero characters long. This is all
a5f75d66 968very above-board and honest. When a function returns an undefined value,
2f9daede 969it's an admission that it couldn't give you an honest answer. So you
19799a22 970should use C<defined> only when you're questioning the integrity of what
7660c0ab 971you're trying to do. At other times, a simple comparison to C<0> or C<""> is
2f9daede
TPG
972what you want.
973
dc848c6f 974See also L</undef>, L</exists>, L</ref>.
2f9daede 975
a0d0e21e
LW
976=item delete EXPR
977
01020589
GS
978Given an expression that specifies a hash element, array element, hash slice,
979or array slice, deletes the specified element(s) from the hash or array.
8216c1fd 980In the case of an array, if the array elements happen to be at the end,
b76cc8ba 981the size of the array will shrink to the highest element that tests
8216c1fd 982true for exists() (or 0 if no such element exists).
a0d0e21e 983
01020589
GS
984Returns each element so deleted or the undefined value if there was no such
985element. Deleting from C<$ENV{}> modifies the environment. Deleting from
986a hash tied to a DBM file deletes the entry from the DBM file. Deleting
987from a C<tie>d hash or array may not necessarily return anything.
988
8ea97a1e
GS
989Deleting an array element effectively returns that position of the array
990to its initial, uninitialized state. Subsequently testing for the same
8216c1fd
GS
991element with exists() will return false. Note that deleting array
992elements in the middle of an array will not shift the index of the ones
993after them down--use splice() for that. See L</exists>.
8ea97a1e 994
01020589 995The following (inefficiently) deletes all the values of %HASH and @ARRAY:
a0d0e21e 996
5f05dabc
PP
997 foreach $key (keys %HASH) {
998 delete $HASH{$key};
a0d0e21e
LW
999 }
1000
01020589
GS
1001 foreach $index (0 .. $#ARRAY) {
1002 delete $ARRAY[$index];
1003 }
1004
1005And so do these:
5f05dabc 1006
01020589
GS
1007 delete @HASH{keys %HASH};
1008
9740c838 1009 delete @ARRAY[0 .. $#ARRAY];
5f05dabc 1010
2b5ab1e7 1011But both of these are slower than just assigning the empty list
01020589
GS
1012or undefining %HASH or @ARRAY:
1013
1014 %HASH = (); # completely empty %HASH
1015 undef %HASH; # forget %HASH ever existed
2b5ab1e7 1016
01020589
GS
1017 @ARRAY = (); # completely empty @ARRAY
1018 undef @ARRAY; # forget @ARRAY ever existed
2b5ab1e7
TC
1019
1020Note that the EXPR can be arbitrarily complicated as long as the final
01020589
GS
1021operation is a hash element, array element, hash slice, or array slice
1022lookup:
a0d0e21e
LW
1023
1024 delete $ref->[$x][$y]{$key};
5f05dabc 1025 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
a0d0e21e 1026
01020589
GS
1027 delete $ref->[$x][$y][$index];
1028 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1029
a0d0e21e
LW
1030=item die LIST
1031
19799a22
GS
1032Outside an C<eval>, prints the value of LIST to C<STDERR> and
1033exits with the current value of C<$!> (errno). If C<$!> is C<0>,
61eff3bc
JH
1034exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1035status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
19799a22
GS
1036an C<eval(),> the error message is stuffed into C<$@> and the
1037C<eval> is terminated with the undefined value. This makes
1038C<die> the way to raise an exception.
a0d0e21e
LW
1039
1040Equivalent examples:
1041
1042 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
54310121 1043 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
a0d0e21e
LW
1044
1045If the value of EXPR does not end in a newline, the current script line
1046number and input line number (if any) are also printed, and a newline
883faa13
GS
1047is supplied. Note that the "input line number" (also known as "chunk")
1048is subject to whatever notion of "line" happens to be currently in
1049effect, and is also available as the special variable C<$.>.
1050See L<perlvar/"$/"> and L<perlvar/"$.">.
1051
1052Hint: sometimes appending C<", stopped"> to your message
7660c0ab 1053will cause it to make better sense when the string C<"at foo line 123"> is
a0d0e21e
LW
1054appended. Suppose you are running script "canasta".
1055
1056 die "/etc/games is no good";
1057 die "/etc/games is no good, stopped";
1058
1059produce, respectively
1060
1061 /etc/games is no good at canasta line 123.
1062 /etc/games is no good, stopped at canasta line 123.
1063
2b5ab1e7 1064See also exit(), warn(), and the Carp module.
a0d0e21e 1065
7660c0ab
A
1066If LIST is empty and C<$@> already contains a value (typically from a
1067previous eval) that value is reused after appending C<"\t...propagated">.
fb73857a
PP
1068This is useful for propagating exceptions:
1069
1070 eval { ... };
1071 die unless $@ =~ /Expected exception/;
1072
7660c0ab 1073If C<$@> is empty then the string C<"Died"> is used.
fb73857a 1074
52531d10
GS
1075die() can also be called with a reference argument. If this happens to be
1076trapped within an eval(), $@ contains the reference. This behavior permits
1077a more elaborate exception handling implementation using objects that
4375e838 1078maintain arbitrary state about the nature of the exception. Such a scheme
52531d10
GS
1079is sometimes preferable to matching particular string values of $@ using
1080regular expressions. Here's an example:
1081
1082 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1083 if ($@) {
1084 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1085 # handle Some::Module::Exception
1086 }
1087 else {
1088 # handle all other possible exceptions
1089 }
1090 }
1091
19799a22 1092Because perl will stringify uncaught exception messages before displaying
52531d10
GS
1093them, you may want to overload stringification operations on such custom
1094exception objects. See L<overload> for details about that.
1095
19799a22
GS
1096You can arrange for a callback to be run just before the C<die>
1097does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1098handler will be called with the error text and can change the error
1099message, if it sees fit, by calling C<die> again. See
1100L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1101L<"eval BLOCK"> for some examples. Although this feature was meant
1102to be run only right before your program was to exit, this is not
1103currently the case--the C<$SIG{__DIE__}> hook is currently called
1104even inside eval()ed blocks/strings! If one wants the hook to do
1105nothing in such situations, put
fb73857a
PP
1106
1107 die @_ if $^S;
1108
19799a22
GS
1109as the first line of the handler (see L<perlvar/$^S>). Because
1110this promotes strange action at a distance, this counterintuitive
b76cc8ba 1111behavior may be fixed in a future release.
774d564b 1112
a0d0e21e
LW
1113=item do BLOCK
1114
1115Not really a function. Returns the value of the last command in the
1116sequence of commands indicated by BLOCK. When modified by a loop
98293880
JH
1117modifier, executes the BLOCK once before testing the loop condition.
1118(On other statements the loop modifiers test the conditional first.)
a0d0e21e 1119
4968c1e4 1120C<do BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7
TC
1121C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1122See L<perlsyn> for alternative strategies.
4968c1e4 1123
a0d0e21e
LW
1124=item do SUBROUTINE(LIST)
1125
1126A deprecated form of subroutine call. See L<perlsub>.
1127
1128=item do EXPR
1129
1130Uses the value of EXPR as a filename and executes the contents of the
1131file as a Perl script. Its primary use is to include subroutines
1132from a Perl subroutine library.
1133
1134 do 'stat.pl';
1135
1136is just like
1137
986b19de 1138 eval `cat stat.pl`;
a0d0e21e 1139
2b5ab1e7
TC
1140except that it's more efficient and concise, keeps track of the current
1141filename for error messages, searches the @INC libraries, and updates
1142C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1143variables. It also differs in that code evaluated with C<do FILENAME>
1144cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1145same, however, in that it does reparse the file every time you call it,
1146so you probably don't want to do this inside a loop.
a0d0e21e 1147
8e30cc93 1148If C<do> cannot read the file, it returns undef and sets C<$!> to the
2b5ab1e7 1149error. If C<do> can read the file but cannot compile it, it
8e30cc93
G
1150returns undef and sets an error message in C<$@>. If the file is
1151successfully compiled, C<do> returns the value of the last expression
1152evaluated.
1153
a0d0e21e 1154Note that inclusion of library modules is better done with the
19799a22 1155C<use> and C<require> operators, which also do automatic error checking
4633a7c4 1156and raise an exception if there's a problem.
a0d0e21e 1157
5a964f20
TC
1158You might like to use C<do> to read in a program configuration
1159file. Manual error checking can be done this way:
1160
b76cc8ba 1161 # read in config files: system first, then user
f86cebdf 1162 for $file ("/share/prog/defaults.rc",
b76cc8ba 1163 "$ENV{HOME}/.someprogrc")
2b5ab1e7 1164 {
5a964f20 1165 unless ($return = do $file) {
f86cebdf
GS
1166 warn "couldn't parse $file: $@" if $@;
1167 warn "couldn't do $file: $!" unless defined $return;
1168 warn "couldn't run $file" unless $return;
5a964f20
TC
1169 }
1170 }
1171
a0d0e21e
LW
1172=item dump LABEL
1173
1614b0e3
JD
1174=item dump
1175
19799a22
GS
1176This function causes an immediate core dump. See also the B<-u>
1177command-line switch in L<perlrun>, which does the same thing.
1178Primarily this is so that you can use the B<undump> program (not
1179supplied) to turn your core dump into an executable binary after
1180having initialized all your variables at the beginning of the
1181program. When the new binary is executed it will begin by executing
1182a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1183Think of it as a goto with an intervening core dump and reincarnation.
1184If C<LABEL> is omitted, restarts the program from the top.
1185
1186B<WARNING>: Any files opened at the time of the dump will I<not>
1187be open any more when the program is reincarnated, with possible
b76cc8ba 1188resulting confusion on the part of Perl.
19799a22
GS
1189
1190This function is now largely obsolete, partly because it's very
1191hard to convert a core file into an executable, and because the
1192real compiler backends for generating portable bytecode and compilable
1193C code have superseded it.
1194
1195If you're looking to use L<dump> to speed up your program, consider
1196generating bytecode or native C code as described in L<perlcc>. If
1197you're just trying to accelerate a CGI script, consider using the
1198C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1199You might also consider autoloading or selfloading, which at least
b76cc8ba 1200make your program I<appear> to run faster.
5a964f20 1201
aa689395
PP
1202=item each HASH
1203
5a964f20 1204When called in list context, returns a 2-element list consisting of the
aa689395 1205key and value for the next element of a hash, so that you can iterate over
74fc8b5f 1206it. When called in scalar context, returns only the key for the next
e902a979 1207element in the hash.
2f9daede 1208
ab192400
GS
1209Entries are returned in an apparently random order. The actual random
1210order is subject to change in future versions of perl, but it is guaranteed
19799a22 1211to be in the same order as either the C<keys> or C<values> function
ab192400
GS
1212would produce on the same (unmodified) hash.
1213
1214When the hash is entirely read, a null array is returned in list context
19799a22
GS
1215(which when assigned produces a false (C<0>) value), and C<undef> in
1216scalar context. The next call to C<each> after that will start iterating
1217again. There is a single iterator for each hash, shared by all C<each>,
1218C<keys>, and C<values> function calls in the program; it can be reset by
2f9daede
TPG
1219reading all the elements from the hash, or by evaluating C<keys HASH> or
1220C<values HASH>. If you add or delete elements of a hash while you're
74fc8b5f
MJD
1221iterating over it, you may get entries skipped or duplicated, so
1222don't. Exception: It is always safe to delete the item most recently
1223returned by C<each()>, which means that the following code will work:
1224
1225 while (($key, $value) = each %hash) {
1226 print $key, "\n";
1227 delete $hash{$key}; # This is safe
1228 }
aa689395 1229
f86cebdf 1230The following prints out your environment like the printenv(1) program,
aa689395 1231only in a different order:
a0d0e21e
LW
1232
1233 while (($key,$value) = each %ENV) {
1234 print "$key=$value\n";
1235 }
1236
19799a22 1237See also C<keys>, C<values> and C<sort>.
a0d0e21e
LW
1238
1239=item eof FILEHANDLE
1240
4633a7c4
LW
1241=item eof ()
1242
a0d0e21e
LW
1243=item eof
1244
1245Returns 1 if the next read on FILEHANDLE will return end of file, or if
1246FILEHANDLE is not open. FILEHANDLE may be an expression whose value
5a964f20 1247gives the real filehandle. (Note that this function actually
19799a22 1248reads a character and then C<ungetc>s it, so isn't very useful in an
748a9306 1249interactive context.) Do not read from a terminal file (or call
19799a22 1250C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
748a9306
LW
1251as terminals may lose the end-of-file condition if you do.
1252
820475bd
GS
1253An C<eof> without an argument uses the last file read. Using C<eof()>
1254with empty parentheses is very different. It refers to the pseudo file
1255formed from the files listed on the command line and accessed via the
61eff3bc
JH
1256C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1257as a normal filehandle is, an C<eof()> before C<< <> >> has been
820475bd
GS
1258used will cause C<@ARGV> to be examined to determine if input is
1259available.
1260
61eff3bc 1261In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
820475bd
GS
1262detect the end of each file, C<eof()> will only detect the end of the
1263last file. Examples:
a0d0e21e 1264
748a9306
LW
1265 # reset line numbering on each input file
1266 while (<>) {
b76cc8ba 1267 next if /^\s*#/; # skip comments
748a9306 1268 print "$.\t$_";
5a964f20
TC
1269 } continue {
1270 close ARGV if eof; # Not eof()!
748a9306
LW
1271 }
1272
a0d0e21e
LW
1273 # insert dashes just before last line of last file
1274 while (<>) {
5a964f20 1275 if (eof()) { # check for end of current file
a0d0e21e 1276 print "--------------\n";
2b5ab1e7 1277 close(ARGV); # close or last; is needed if we
748a9306 1278 # are reading from the terminal
a0d0e21e
LW
1279 }
1280 print;
1281 }
1282
a0d0e21e 1283Practical hint: you almost never need to use C<eof> in Perl, because the
3ce0d271
GS
1284input operators typically return C<undef> when they run out of data, or if
1285there was an error.
a0d0e21e
LW
1286
1287=item eval EXPR
1288
1289=item eval BLOCK
1290
c7cc6f1c
GS
1291In the first form, the return value of EXPR is parsed and executed as if it
1292were a little Perl program. The value of the expression (which is itself
5a964f20 1293determined within scalar context) is first parsed, and if there weren't any
be3174d2
GS
1294errors, executed in the lexical context of the current Perl program, so
1295that any variable settings or subroutine and format definitions remain
1296afterwards. Note that the value is parsed every time the eval executes.
1297If EXPR is omitted, evaluates C<$_>. This form is typically used to
1298delay parsing and subsequent execution of the text of EXPR until run time.
c7cc6f1c
GS
1299
1300In the second form, the code within the BLOCK is parsed only once--at the
1301same time the code surrounding the eval itself was parsed--and executed
1302within the context of the current Perl program. This form is typically
1303used to trap exceptions more efficiently than the first (see below), while
1304also providing the benefit of checking the code within BLOCK at compile
1305time.
1306
1307The final semicolon, if any, may be omitted from the value of EXPR or within
1308the BLOCK.
1309
1310In both forms, the value returned is the value of the last expression
5a964f20 1311evaluated inside the mini-program; a return statement may be also used, just
c7cc6f1c 1312as with subroutines. The expression providing the return value is evaluated
5a964f20 1313in void, scalar, or list context, depending on the context of the eval itself.
c7cc6f1c 1314See L</wantarray> for more on how the evaluation context can be determined.
a0d0e21e 1315
19799a22
GS
1316If there is a syntax error or runtime error, or a C<die> statement is
1317executed, an undefined value is returned by C<eval>, and C<$@> is set to the
a0d0e21e 1318error message. If there was no error, C<$@> is guaranteed to be a null
19799a22 1319string. Beware that using C<eval> neither silences perl from printing
c7cc6f1c
GS
1320warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1321To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1322L</warn> and L<perlvar>.
a0d0e21e 1323
19799a22
GS
1324Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1325determining whether a particular feature (such as C<socket> or C<symlink>)
a0d0e21e
LW
1326is implemented. It is also Perl's exception trapping mechanism, where
1327the die operator is used to raise exceptions.
1328
1329If the code to be executed doesn't vary, you may use the eval-BLOCK
1330form to trap run-time errors without incurring the penalty of
1331recompiling each time. The error, if any, is still returned in C<$@>.
1332Examples:
1333
54310121 1334 # make divide-by-zero nonfatal
a0d0e21e
LW
1335 eval { $answer = $a / $b; }; warn $@ if $@;
1336
1337 # same thing, but less efficient
1338 eval '$answer = $a / $b'; warn $@ if $@;
1339
1340 # a compile-time error
5a964f20 1341 eval { $answer = }; # WRONG
a0d0e21e
LW
1342
1343 # a run-time error
1344 eval '$answer ='; # sets $@
1345
2b5ab1e7
TC
1346Due to the current arguably broken state of C<__DIE__> hooks, when using
1347the C<eval{}> form as an exception trap in libraries, you may wish not
1348to trigger any C<__DIE__> hooks that user code may have installed.
1349You can use the C<local $SIG{__DIE__}> construct for this purpose,
1350as shown in this example:
774d564b
PP
1351
1352 # a very private exception trap for divide-by-zero
f86cebdf
GS
1353 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1354 warn $@ if $@;
774d564b
PP
1355
1356This is especially significant, given that C<__DIE__> hooks can call
19799a22 1357C<die> again, which has the effect of changing their error messages:
774d564b
PP
1358
1359 # __DIE__ hooks may modify error messages
1360 {
f86cebdf
GS
1361 local $SIG{'__DIE__'} =
1362 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
c7cc6f1c
GS
1363 eval { die "foo lives here" };
1364 print $@ if $@; # prints "bar lives here"
774d564b
PP
1365 }
1366
19799a22 1367Because this promotes action at a distance, this counterintuitive behavior
2b5ab1e7
TC
1368may be fixed in a future release.
1369
19799a22 1370With an C<eval>, you should be especially careful to remember what's
a0d0e21e
LW
1371being looked at when:
1372
1373 eval $x; # CASE 1
1374 eval "$x"; # CASE 2
1375
1376 eval '$x'; # CASE 3
1377 eval { $x }; # CASE 4
1378
5a964f20 1379 eval "\$$x++"; # CASE 5
a0d0e21e
LW
1380 $$x++; # CASE 6
1381
2f9daede 1382Cases 1 and 2 above behave identically: they run the code contained in
19799a22 1383the variable $x. (Although case 2 has misleading double quotes making
2f9daede 1384the reader wonder what else might be happening (nothing is).) Cases 3
7660c0ab 1385and 4 likewise behave in the same way: they run the code C<'$x'>, which
19799a22 1386does nothing but return the value of $x. (Case 4 is preferred for
2f9daede
TPG
1387purely visual reasons, but it also has the advantage of compiling at
1388compile-time instead of at run-time.) Case 5 is a place where
19799a22 1389normally you I<would> like to use double quotes, except that in this
2f9daede
TPG
1390particular situation, you can just use symbolic references instead, as
1391in case 6.
a0d0e21e 1392
4968c1e4 1393C<eval BLOCK> does I<not> count as a loop, so the loop control statements
2b5ab1e7 1394C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
4968c1e4 1395
a0d0e21e
LW
1396=item exec LIST
1397
8bf3b016
GS
1398=item exec PROGRAM LIST
1399
19799a22
GS
1400The C<exec> function executes a system command I<and never returns>--
1401use C<system> instead of C<exec> if you want it to return. It fails and
1402returns false only if the command does not exist I<and> it is executed
fb73857a 1403directly instead of via your system's command shell (see below).
a0d0e21e 1404
19799a22
GS
1405Since it's a common mistake to use C<exec> instead of C<system>, Perl
1406warns you if there is a following statement which isn't C<die>, C<warn>,
1407or C<exit> (if C<-w> is set - but you always do that). If you
1408I<really> want to follow an C<exec> with some other statement, you
55d729e4
GS
1409can use one of these styles to avoid the warning:
1410
5a964f20
TC
1411 exec ('foo') or print STDERR "couldn't exec foo: $!";
1412 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
55d729e4 1413
5a964f20 1414If there is more than one argument in LIST, or if LIST is an array
f86cebdf 1415with more than one value, calls execvp(3) with the arguments in LIST.
5a964f20
TC
1416If there is only one scalar argument or an array with one element in it,
1417the argument is checked for shell metacharacters, and if there are any,
1418the entire argument is passed to the system's command shell for parsing
1419(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1420If there are no shell metacharacters in the argument, it is split into
b76cc8ba 1421words and passed directly to C<execvp>, which is more efficient.
19799a22 1422Examples:
a0d0e21e 1423
19799a22
GS
1424 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1425 exec "sort $outfile | uniq";
a0d0e21e
LW
1426
1427If you don't really want to execute the first argument, but want to lie
1428to the program you are executing about its own name, you can specify
1429the program you actually want to run as an "indirect object" (without a
1430comma) in front of the LIST. (This always forces interpretation of the
54310121 1431LIST as a multivalued list, even if there is only a single scalar in
a0d0e21e
LW
1432the list.) Example:
1433
1434 $shell = '/bin/csh';
1435 exec $shell '-sh'; # pretend it's a login shell
1436
1437or, more directly,
1438
1439 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1440
bb32b41a
GS
1441When the arguments get executed via the system shell, results will
1442be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1443for details.
1444
19799a22
GS
1445Using an indirect object with C<exec> or C<system> is also more
1446secure. This usage (which also works fine with system()) forces
1447interpretation of the arguments as a multivalued list, even if the
1448list had just one argument. That way you're safe from the shell
1449expanding wildcards or splitting up words with whitespace in them.
5a964f20
TC
1450
1451 @args = ( "echo surprise" );
1452
2b5ab1e7 1453 exec @args; # subject to shell escapes
f86cebdf 1454 # if @args == 1
2b5ab1e7 1455 exec { $args[0] } @args; # safe even with one-arg list
5a964f20
TC
1456
1457The first version, the one without the indirect object, ran the I<echo>
1458program, passing it C<"surprise"> an argument. The second version
1459didn't--it tried to run a program literally called I<"echo surprise">,
1460didn't find it, and set C<$?> to a non-zero value indicating failure.
1461
0f897271
GS
1462Beginning with v5.6.0, Perl will attempt to flush all files opened for
1463output before the exec, but this may not be supported on some platforms
1464(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1465in English) or call the C<autoflush()> method of C<IO::Handle> on any
1466open handles in order to avoid lost output.
1467
19799a22 1468Note that C<exec> will not call your C<END> blocks, nor will it call
7660c0ab
A
1469any C<DESTROY> methods in your objects.
1470
a0d0e21e
LW
1471=item exists EXPR
1472
01020589 1473Given an expression that specifies a hash element or array element,
8ea97a1e
GS
1474returns true if the specified element in the hash or array has ever
1475been initialized, even if the corresponding value is undefined. The
1476element is not autovivified if it doesn't exist.
a0d0e21e 1477
01020589
GS
1478 print "Exists\n" if exists $hash{$key};
1479 print "Defined\n" if defined $hash{$key};
1480 print "True\n" if $hash{$key};
1481
1482 print "Exists\n" if exists $array[$index];
1483 print "Defined\n" if defined $array[$index];
1484 print "True\n" if $array[$index];
a0d0e21e 1485
8ea97a1e 1486A hash or array element can be true only if it's defined, and defined if
a0d0e21e
LW
1487it exists, but the reverse doesn't necessarily hold true.
1488
afebc493
GS
1489Given an expression that specifies the name of a subroutine,
1490returns true if the specified subroutine has ever been declared, even
1491if it is undefined. Mentioning a subroutine name for exists or defined
847c7ebe
DD
1492does not count as declaring it. Note that a subroutine which does not
1493exist may still be callable: its package may have an C<AUTOLOAD>
1494method that makes it spring into existence the first time that it is
1495called -- see L<perlsub>.
afebc493
GS
1496
1497 print "Exists\n" if exists &subroutine;
1498 print "Defined\n" if defined &subroutine;
1499
a0d0e21e 1500Note that the EXPR can be arbitrarily complicated as long as the final
afebc493 1501operation is a hash or array key lookup or subroutine name:
a0d0e21e 1502
2b5ab1e7
TC
1503 if (exists $ref->{A}->{B}->{$key}) { }
1504 if (exists $hash{A}{B}{$key}) { }
1505
01020589
GS
1506 if (exists $ref->{A}->{B}->[$ix]) { }
1507 if (exists $hash{A}{B}[$ix]) { }
1508
afebc493
GS
1509 if (exists &{$ref->{A}{B}{$key}}) { }
1510
01020589
GS
1511Although the deepest nested array or hash will not spring into existence
1512just because its existence was tested, any intervening ones will.
61eff3bc 1513Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
01020589
GS
1514into existence due to the existence test for the $key element above.
1515This happens anywhere the arrow operator is used, including even:
5a964f20 1516
2b5ab1e7
TC
1517 undef $ref;
1518 if (exists $ref->{"Some key"}) { }
1519 print $ref; # prints HASH(0x80d3d5c)
1520
1521This surprising autovivification in what does not at first--or even
1522second--glance appear to be an lvalue context may be fixed in a future
5a964f20 1523release.
a0d0e21e 1524
479ba383
GS
1525See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1526on how exists() acts when used on a pseudo-hash.
e0478e5a 1527
afebc493
GS
1528Use of a subroutine call, rather than a subroutine name, as an argument
1529to exists() is an error.
1530
1531 exists &sub; # OK
1532 exists &sub(); # Error
1533
a0d0e21e
LW
1534=item exit EXPR
1535
2b5ab1e7 1536Evaluates EXPR and exits immediately with that value. Example:
a0d0e21e
LW
1537
1538 $ans = <STDIN>;
1539 exit 0 if $ans =~ /^[Xx]/;
1540
19799a22 1541See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2b5ab1e7
TC
1542universally recognized values for EXPR are C<0> for success and C<1>
1543for error; other values are subject to interpretation depending on the
1544environment in which the Perl program is running. For example, exiting
154569 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1546the mailer to return the item undelivered, but that's not true everywhere.
a0d0e21e 1547
19799a22
GS
1548Don't use C<exit> to abort a subroutine if there's any chance that
1549someone might want to trap whatever error happened. Use C<die> instead,
1550which can be trapped by an C<eval>.
28757baa 1551
19799a22 1552The exit() function does not always exit immediately. It calls any
2b5ab1e7 1553defined C<END> routines first, but these C<END> routines may not
19799a22 1554themselves abort the exit. Likewise any object destructors that need to
2b5ab1e7
TC
1555be called are called before the real exit. If this is a problem, you
1556can call C<POSIX:_exit($status)> to avoid END and destructor processing.
87275199 1557See L<perlmod> for details.
5a964f20 1558
a0d0e21e
LW
1559=item exp EXPR
1560
54310121 1561=item exp
bbce6d69 1562
b76cc8ba 1563Returns I<e> (the natural logarithm base) to the power of EXPR.
a0d0e21e
LW
1564If EXPR is omitted, gives C<exp($_)>.
1565
1566=item fcntl FILEHANDLE,FUNCTION,SCALAR
1567
f86cebdf 1568Implements the fcntl(2) function. You'll probably have to say
a0d0e21e
LW
1569
1570 use Fcntl;
1571
0ade1984 1572first to get the correct constant definitions. Argument processing and
b76cc8ba 1573value return works just like C<ioctl> below.
a0d0e21e
LW
1574For example:
1575
1576 use Fcntl;
5a964f20
TC
1577 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1578 or die "can't fcntl F_GETFL: $!";
1579
19799a22 1580You don't have to check for C<defined> on the return from C<fnctl>.
951ba7fe
GS
1581Like C<ioctl>, it maps a C<0> return from the system call into
1582C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2b5ab1e7
TC
1583in numeric context. It is also exempt from the normal B<-w> warnings
1584on improper numeric conversions.
5a964f20 1585
19799a22 1586Note that C<fcntl> will produce a fatal error if used on a machine that
2b5ab1e7
TC
1587doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1588manpage to learn what functions are available on your system.
a0d0e21e
LW
1589
1590=item fileno FILEHANDLE
1591
2b5ab1e7
TC
1592Returns the file descriptor for a filehandle, or undefined if the
1593filehandle is not open. This is mainly useful for constructing
19799a22 1594bitmaps for C<select> and low-level POSIX tty-handling operations.
2b5ab1e7
TC
1595If FILEHANDLE is an expression, the value is taken as an indirect
1596filehandle, generally its name.
5a964f20 1597
b76cc8ba 1598You can use this to find out whether two handles refer to the
5a964f20
TC
1599same underlying descriptor:
1600
1601 if (fileno(THIS) == fileno(THAT)) {
1602 print "THIS and THAT are dups\n";
b76cc8ba
NIS
1603 }
1604
1605(Filehandles connected to memory objects via new features of C<open> may
1606return undefined even though they are open.)
1607
a0d0e21e
LW
1608
1609=item flock FILEHANDLE,OPERATION
1610
19799a22
GS
1611Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1612for success, false on failure. Produces a fatal error if used on a
2b5ab1e7 1613machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
19799a22 1614C<flock> is Perl's portable file locking interface, although it locks
2b5ab1e7
TC
1615only entire files, not records.
1616
1617Two potentially non-obvious but traditional C<flock> semantics are
1618that it waits indefinitely until the lock is granted, and that its locks
1619B<merely advisory>. Such discretionary locks are more flexible, but offer
19799a22
GS
1620fewer guarantees. This means that files locked with C<flock> may be
1621modified by programs that do not also use C<flock>. See L<perlport>,
2b5ab1e7
TC
1622your port's specific documentation, or your system-specific local manpages
1623for details. It's best to assume traditional behavior if you're writing
1624portable programs. (But if you're not, you should as always feel perfectly
1625free to write for your own system's idiosyncrasies (sometimes called
1626"features"). Slavish adherence to portability concerns shouldn't get
1627in the way of your getting your job done.)
a3cb178b 1628
8ebc5c01
PP
1629OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1630LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
ea3105be 1631you can use the symbolic names if you import them from the Fcntl module,
68dc0745
PP
1632either individually, or as a group using the ':flock' tag. LOCK_SH
1633requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
ea3105be
GS
1634releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1635LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
68dc0745
PP
1636waiting for the lock (check the return status to see if you got it).
1637
2b5ab1e7
TC
1638To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1639before locking or unlocking it.
8ebc5c01 1640
f86cebdf 1641Note that the emulation built with lockf(3) doesn't provide shared
8ebc5c01 1642locks, and it requires that FILEHANDLE be open with write intent. These
2b5ab1e7 1643are the semantics that lockf(3) implements. Most if not all systems
f86cebdf 1644implement lockf(3) in terms of fcntl(2) locking, though, so the
8ebc5c01
PP
1645differing semantics shouldn't bite too many people.
1646
19799a22
GS
1647Note also that some versions of C<flock> cannot lock things over the
1648network; you would need to use the more system-specific C<fcntl> for
f86cebdf
GS
1649that. If you like you can force Perl to ignore your system's flock(2)
1650function, and so provide its own fcntl(2)-based emulation, by passing
8ebc5c01
PP
1651the switch C<-Ud_flock> to the F<Configure> program when you configure
1652perl.
4633a7c4
LW
1653
1654Here's a mailbox appender for BSD systems.
a0d0e21e 1655
7e1af8bc 1656 use Fcntl ':flock'; # import LOCK_* constants
a0d0e21e
LW
1657
1658 sub lock {
7e1af8bc 1659 flock(MBOX,LOCK_EX);
a0d0e21e
LW
1660 # and, in case someone appended
1661 # while we were waiting...
1662 seek(MBOX, 0, 2);
1663 }
1664
1665 sub unlock {
7e1af8bc 1666 flock(MBOX,LOCK_UN);
a0d0e21e
LW
1667 }
1668
1669 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1670 or die "Can't open mailbox: $!";
1671
1672 lock();
1673 print MBOX $msg,"\n\n";
1674 unlock();
1675
2b5ab1e7
TC
1676On systems that support a real flock(), locks are inherited across fork()
1677calls, whereas those that must resort to the more capricious fcntl()
1678function lose the locks, making it harder to write servers.
1679
cb1a09d0 1680See also L<DB_File> for other flock() examples.
a0d0e21e
LW
1681
1682=item fork
1683
2b5ab1e7
TC
1684Does a fork(2) system call to create a new process running the
1685same program at the same point. It returns the child pid to the
1686parent process, C<0> to the child process, or C<undef> if the fork is
1687unsuccessful. File descriptors (and sometimes locks on those descriptors)
1688are shared, while everything else is copied. On most systems supporting
1689fork(), great care has gone into making it extremely efficient (for
1690example, using copy-on-write technology on data pages), making it the
1691dominant paradigm for multitasking over the last few decades.
5a964f20 1692
0f897271
GS
1693Beginning with v5.6.0, Perl will attempt to flush all files opened for
1694output before forking the child process, but this may not be supported
1695on some platforms (see L<perlport>). To be safe, you may need to set
1696C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1697C<IO::Handle> on any open handles in order to avoid duplicate output.
a0d0e21e 1698
19799a22 1699If you C<fork> without ever waiting on your children, you will
2b5ab1e7
TC
1700accumulate zombies. On some systems, you can avoid this by setting
1701C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1702forking and reaping moribund children.
cb1a09d0 1703
28757baa
PP
1704Note that if your forked child inherits system file descriptors like
1705STDIN and STDOUT that are actually connected by a pipe or socket, even
2b5ab1e7 1706if you exit, then the remote server (such as, say, a CGI script or a
19799a22 1707backgrounded job launched from a remote shell) won't think you're done.
2b5ab1e7 1708You should reopen those to F</dev/null> if it's any issue.
28757baa 1709
cb1a09d0
AD
1710=item format
1711
19799a22 1712Declare a picture format for use by the C<write> function. For
cb1a09d0
AD
1713example:
1714
54310121 1715 format Something =
cb1a09d0
AD
1716 Test: @<<<<<<<< @||||| @>>>>>
1717 $str, $%, '$' . int($num)
1718 .
1719
1720 $str = "widget";
184e9718 1721 $num = $cost/$quantity;
cb1a09d0
AD
1722 $~ = 'Something';
1723 write;
1724
1725See L<perlform> for many details and examples.
1726
8903cb82 1727=item formline PICTURE,LIST
a0d0e21e 1728
5a964f20 1729This is an internal function used by C<format>s, though you may call it,
a0d0e21e
LW
1730too. It formats (see L<perlform>) a list of values according to the
1731contents of PICTURE, placing the output into the format output
7660c0ab 1732accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
19799a22 1733Eventually, when a C<write> is done, the contents of
a0d0e21e 1734C<$^A> are written to some filehandle, but you could also read C<$^A>
7660c0ab 1735yourself and then set C<$^A> back to C<"">. Note that a format typically
19799a22 1736does one C<formline> per line of form, but the C<formline> function itself
748a9306 1737doesn't care how many newlines are embedded in the PICTURE. This means
4633a7c4 1738that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
748a9306
LW
1739You may therefore need to use multiple formlines to implement a single
1740record format, just like the format compiler.
1741
19799a22 1742Be careful if you put double quotes around the picture, because an C<@>
748a9306 1743character may be taken to mean the beginning of an array name.
19799a22 1744C<formline> always returns true. See L<perlform> for other examples.
a0d0e21e
LW
1745
1746=item getc FILEHANDLE
1747
1748=item getc
1749
1750Returns the next character from the input file attached to FILEHANDLE,
2b5ab1e7
TC
1751or the undefined value at end of file, or if there was an error.
1752If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1753efficient. However, it cannot be used by itself to fetch single
1754characters without waiting for the user to hit enter. For that, try
1755something more like:
4633a7c4
LW
1756
1757 if ($BSD_STYLE) {
1758 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1759 }
1760 else {
54310121 1761 system "stty", '-icanon', 'eol', "\001";
4633a7c4
LW
1762 }
1763
1764 $key = getc(STDIN);
1765
1766 if ($BSD_STYLE) {
1767 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1768 }
1769 else {
5f05dabc 1770 system "stty", 'icanon', 'eol', '^@'; # ASCII null
4633a7c4
LW
1771 }
1772 print "\n";
1773
54310121
PP
1774Determination of whether $BSD_STYLE should be set
1775is left as an exercise to the reader.
cb1a09d0 1776
19799a22 1777The C<POSIX::getattr> function can do this more portably on
2b5ab1e7
TC
1778systems purporting POSIX compliance. See also the C<Term::ReadKey>
1779module from your nearest CPAN site; details on CPAN can be found on
1780L<perlmodlib/CPAN>.
a0d0e21e
LW
1781
1782=item getlogin
1783
5a964f20
TC
1784Implements the C library function of the same name, which on most
1785systems returns the current login from F</etc/utmp>, if any. If null,
19799a22 1786use C<getpwuid>.
a0d0e21e 1787
f86702cc 1788 $login = getlogin || getpwuid($<) || "Kilroy";
a0d0e21e 1789
19799a22
GS
1790Do not consider C<getlogin> for authentication: it is not as
1791secure as C<getpwuid>.
4633a7c4 1792
a0d0e21e
LW
1793=item getpeername SOCKET
1794
1795Returns the packed sockaddr address of other end of the SOCKET connection.
1796
4633a7c4
LW
1797 use Socket;
1798 $hersockaddr = getpeername(SOCK);
19799a22 1799 ($port, $iaddr) = sockaddr_in($hersockaddr);
4633a7c4
LW
1800 $herhostname = gethostbyaddr($iaddr, AF_INET);
1801 $herstraddr = inet_ntoa($iaddr);
a0d0e21e
LW
1802
1803=item getpgrp PID
1804
47e29363 1805Returns the current process group for the specified PID. Use
7660c0ab 1806a PID of C<0> to get the current process group for the
4633a7c4 1807current process. Will raise an exception if used on a machine that
f86cebdf 1808doesn't implement getpgrp(2). If PID is omitted, returns process
19799a22 1809group of current process. Note that the POSIX version of C<getpgrp>
7660c0ab 1810does not accept a PID argument, so only C<PID==0> is truly portable.
a0d0e21e
LW
1811
1812=item getppid
1813
1814Returns the process id of the parent process.
1815
1816=item getpriority WHICH,WHO
1817
4633a7c4
LW
1818Returns the current priority for a process, a process group, or a user.
1819(See L<getpriority(2)>.) Will raise a fatal exception if used on a
f86cebdf 1820machine that doesn't implement getpriority(2).
a0d0e21e
LW
1821
1822=item getpwnam NAME
1823
1824=item getgrnam NAME
1825
1826=item gethostbyname NAME
1827
1828=item getnetbyname NAME
1829
1830=item getprotobyname NAME
1831
1832=item getpwuid UID
1833
1834=item getgrgid GID
1835
1836=item getservbyname NAME,PROTO
1837
1838=item gethostbyaddr ADDR,ADDRTYPE
1839
1840=item getnetbyaddr ADDR,ADDRTYPE
1841
1842=item getprotobynumber NUMBER
1843
1844=item getservbyport PORT,PROTO
1845
1846=item getpwent
1847
1848=item getgrent
1849
1850=item gethostent
1851
1852=item getnetent
1853
1854=item getprotoent
1855
1856=item getservent
1857
1858=item setpwent
1859
1860=item setgrent
1861
1862=item sethostent STAYOPEN
1863
1864=item setnetent STAYOPEN
1865
1866=item setprotoent STAYOPEN
1867
1868=item setservent STAYOPEN
1869
1870=item endpwent
1871
1872=item endgrent
1873
1874=item endhostent
1875
1876=item endnetent
1877
1878=item endprotoent
1879
1880=item endservent
1881
1882These routines perform the same functions as their counterparts in the
5a964f20 1883system library. In list context, the return values from the
a0d0e21e
LW
1884various get routines are as follows:
1885
1886 ($name,$passwd,$uid,$gid,
6ee623d5 1887 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
a0d0e21e
LW
1888 ($name,$passwd,$gid,$members) = getgr*
1889 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1890 ($name,$aliases,$addrtype,$net) = getnet*
1891 ($name,$aliases,$proto) = getproto*
1892 ($name,$aliases,$port,$proto) = getserv*
1893
1894(If the entry doesn't exist you get a null list.)
1895
4602f195
JH
1896The exact meaning of the $gcos field varies but it usually contains
1897the real name of the user (as opposed to the login name) and other
1898information pertaining to the user. Beware, however, that in many
1899system users are able to change this information and therefore it
106325ad 1900cannot be trusted and therefore the $gcos is tainted (see
2959b6e3
JH
1901L<perlsec>). The $passwd and $shell, user's encrypted password and
1902login shell, are also tainted, because of the same reason.
4602f195 1903
5a964f20 1904In scalar context, you get the name, unless the function was a
a0d0e21e
LW
1905lookup by name, in which case you get the other thing, whatever it is.
1906(If the entry doesn't exist you get the undefined value.) For example:
1907
5a964f20
TC
1908 $uid = getpwnam($name);
1909 $name = getpwuid($num);
1910 $name = getpwent();
1911 $gid = getgrnam($name);
1912 $name = getgrgid($num;
1913 $name = getgrent();
1914 #etc.
a0d0e21e 1915
4602f195
JH
1916In I<getpw*()> the fields $quota, $comment, and $expire are special
1917cases in the sense that in many systems they are unsupported. If the
1918$quota is unsupported, it is an empty scalar. If it is supported, it
1919usually encodes the disk quota. If the $comment field is unsupported,
1920it is an empty scalar. If it is supported it usually encodes some
1921administrative comment about the user. In some systems the $quota
1922field may be $change or $age, fields that have to do with password
1923aging. In some systems the $comment field may be $class. The $expire
1924field, if present, encodes the expiration period of the account or the
1925password. For the availability and the exact meaning of these fields
1926in your system, please consult your getpwnam(3) documentation and your
1927F<pwd.h> file. You can also find out from within Perl what your
1928$quota and $comment fields mean and whether you have the $expire field
1929by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1930C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1931files are only supported if your vendor has implemented them in the
1932intuitive fashion that calling the regular C library routines gets the
5d3a0a3b
GS
1933shadow versions if you're running under privilege or if there exists
1934the shadow(3) functions as found in System V ( this includes Solaris
1935and Linux.) Those systems which implement a proprietary shadow password
1936facility are unlikely to be supported.
6ee623d5 1937
19799a22 1938The $members value returned by I<getgr*()> is a space separated list of
a0d0e21e
LW
1939the login names of the members of the group.
1940
1941For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1942C, it will be returned to you via C<$?> if the function call fails. The
7660c0ab 1943C<@addrs> value returned by a successful call is a list of the raw
a0d0e21e
LW
1944addresses returned by the corresponding system library call. In the
1945Internet domain, each address is four bytes long and you can unpack it
1946by saying something like:
1947
1948 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1949
2b5ab1e7
TC
1950The Socket library makes this slightly easier:
1951
1952 use Socket;
1953 $iaddr = inet_aton("127.1"); # or whatever address
1954 $name = gethostbyaddr($iaddr, AF_INET);
1955
1956 # or going the other way
19799a22 1957 $straddr = inet_ntoa($iaddr);
2b5ab1e7 1958
19799a22
GS
1959If you get tired of remembering which element of the return list
1960contains which return value, by-name interfaces are provided
1961in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1962C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1963and C<User::grent>. These override the normal built-ins, supplying
1964versions that return objects with the appropriate names
1965for each field. For example:
5a964f20
TC
1966
1967 use File::stat;
1968 use User::pwent;
1969 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1970
b76cc8ba
NIS
1971Even though it looks like they're the same method calls (uid),
1972they aren't, because a C<File::stat> object is different from
19799a22 1973a C<User::pwent> object.
5a964f20 1974
a0d0e21e
LW
1975=item getsockname SOCKET
1976
19799a22
GS
1977Returns the packed sockaddr address of this end of the SOCKET connection,
1978in case you don't know the address because you have several different
1979IPs that the connection might have come in on.
a0d0e21e 1980
4633a7c4
LW
1981 use Socket;
1982 $mysockaddr = getsockname(SOCK);
19799a22 1983 ($port, $myaddr) = sockaddr_in($mysockaddr);
b76cc8ba 1984 printf "Connect to %s [%s]\n",
19799a22
GS
1985 scalar gethostbyaddr($myaddr, AF_INET),
1986 inet_ntoa($myaddr);
a0d0e21e
LW
1987
1988=item getsockopt SOCKET,LEVEL,OPTNAME
1989
5a964f20 1990Returns the socket option requested, or undef if there is an error.
a0d0e21e
LW
1991
1992=item glob EXPR
1993
0a753a76
PP
1994=item glob
1995
2b5ab1e7
TC
1996Returns the value of EXPR with filename expansions such as the
1997standard Unix shell F</bin/csh> would do. This is the internal function
61eff3bc
JH
1998implementing the C<< <*.c> >> operator, but you can use it directly.
1999If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
2b5ab1e7 2000discussed in more detail in L<perlop/"I/O Operators">.
a0d0e21e 2001
3a4b19e4
GS
2002Beginning with v5.6.0, this operator is implemented using the standard
2003C<File::Glob> extension. See L<File::Glob> for details.
2004
a0d0e21e
LW
2005=item gmtime EXPR
2006
d1be9408 2007Converts a time as returned by the time function to an 8-element list
54310121 2008with the time localized for the standard Greenwich time zone.
4633a7c4 2009Typically used as follows:
a0d0e21e 2010
b76cc8ba 2011 # 0 1 2 3 4 5 6 7
48a26b3a 2012 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
a0d0e21e
LW
2013 gmtime(time);
2014
48a26b3a
GS
2015All list elements are numeric, and come straight out of the C `struct
2016tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2017specified time. $mday is the day of the month, and $mon is the month
2018itself, in the range C<0..11> with 0 indicating January and 11
2019indicating December. $year is the number of years since 1900. That
2020is, $year is C<123> in year 2023. $wday is the day of the week, with
20210 indicating Sunday and 3 indicating Wednesday. $yday is the day of
b76cc8ba 2022the year, in the range C<0..364> (or C<0..365> in leap years.)
48a26b3a
GS
2023
2024Note that the $year element is I<not> simply the last two digits of
2025the year. If you assume it is, then you create non-Y2K-compliant
2026programs--and you wouldn't want to do that, would you?
2f9daede 2027
abd75f24
GS
2028The proper way to get a complete 4-digit year is simply:
2029
2030 $year += 1900;
2031
2032And to get the last two digits of the year (e.g., '01' in 2001) do:
2033
2034 $year = sprintf("%02d", $year % 100);
2035
48a26b3a 2036If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
a0d0e21e 2037
48a26b3a 2038In scalar context, C<gmtime()> returns the ctime(3) value:
0a753a76
PP
2039
2040 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2041
19799a22 2042Also see the C<timegm> function provided by the C<Time::Local> module,
f86cebdf 2043and the strftime(3) function available via the POSIX module.
7660c0ab 2044
2b5ab1e7
TC
2045This scalar value is B<not> locale dependent (see L<perllocale>), but
2046is instead a Perl builtin. Also see the C<Time::Local> module, and the
2047strftime(3) and mktime(3) functions available via the POSIX module. To
7660c0ab
A
2048get somewhat similar but locale dependent date strings, set up your
2049locale environment variables appropriately (please see L<perllocale>)
2050and try for example:
2051
2052 use POSIX qw(strftime);
2b5ab1e7 2053 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
7660c0ab 2054
2b5ab1e7
TC
2055Note that the C<%a> and C<%b> escapes, which represent the short forms
2056of the day of the week and the month of the year, may not necessarily
2057be three characters wide in all locales.
0a753a76 2058
a0d0e21e
LW
2059=item goto LABEL
2060
748a9306
LW
2061=item goto EXPR
2062
a0d0e21e
LW
2063=item goto &NAME
2064
7660c0ab 2065The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
a0d0e21e 2066execution there. It may not be used to go into any construct that
7660c0ab 2067requires initialization, such as a subroutine or a C<foreach> loop. It
0a753a76 2068also can't be used to go into a construct that is optimized away,
19799a22 2069or to get out of a block or subroutine given to C<sort>.
0a753a76 2070It can be used to go almost anywhere else within the dynamic scope,
a0d0e21e 2071including out of subroutines, but it's usually better to use some other
19799a22 2072construct such as C<last> or C<die>. The author of Perl has never felt the
7660c0ab 2073need to use this form of C<goto> (in Perl, that is--C is another matter).
a0d0e21e 2074
7660c0ab
A
2075The C<goto-EXPR> form expects a label name, whose scope will be resolved
2076dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
748a9306
LW
2077necessarily recommended if you're optimizing for maintainability:
2078
2079 goto ("FOO", "BAR", "GLARCH")[$i];
2080
6cb9131c
GS
2081The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2082In fact, it isn't a goto in the normal sense at all, and doesn't have
2083the stigma associated with other gotos. Instead, it
2084substitutes a call to the named subroutine for the currently running
2085subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2086another subroutine and then pretend that the other subroutine had been
2087called in the first place (except that any modifications to C<@_>
2088in the current subroutine are propagated to the other subroutine.)
2089After the C<goto>, not even C<caller> will be able to tell that this
2090routine was called first.
2091
2092NAME needn't be the name of a subroutine; it can be a scalar variable
2093containing a code reference, or a block which evaluates to a code
2094reference.
a0d0e21e
LW
2095
2096=item grep BLOCK LIST
2097
2098=item grep EXPR,LIST
2099
2b5ab1e7
TC
2100This is similar in spirit to, but not the same as, grep(1) and its
2101relatives. In particular, it is not limited to using regular expressions.
2f9daede 2102
a0d0e21e 2103Evaluates the BLOCK or EXPR for each element of LIST (locally setting
7660c0ab 2104C<$_> to each element) and returns the list value consisting of those
19799a22
GS
2105elements for which the expression evaluated to true. In scalar
2106context, returns the number of times the expression was true.
a0d0e21e
LW
2107
2108 @foo = grep(!/^#/, @bar); # weed out comments
2109
2110or equivalently,
2111
2112 @foo = grep {!/^#/} @bar; # weed out comments
2113
be3174d2
GS
2114Note that C<$_> is an alias to the list value, so it can be used to
2115modify the elements of the LIST. While this is useful and supported,
2116it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2117Similarly, grep returns aliases into the original list, much as a for
2118loop's index variable aliases the list elements. That is, modifying an
19799a22
GS
2119element of a list returned by grep (for example, in a C<foreach>, C<map>
2120or another C<grep>) actually modifies the element in the original list.
2b5ab1e7 2121This is usually something to be avoided when writing clear code.
a0d0e21e 2122
19799a22 2123See also L</map> for a list composed of the results of the BLOCK or EXPR.
38325410 2124
a0d0e21e
LW
2125=item hex EXPR
2126
54310121 2127=item hex
bbce6d69 2128
2b5ab1e7
TC
2129Interprets EXPR as a hex string and returns the corresponding value.
2130(To convert strings that might start with either 0, 0x, or 0b, see
2131L</oct>.) If EXPR is omitted, uses C<$_>.
2f9daede
TPG
2132
2133 print hex '0xAf'; # prints '175'
2134 print hex 'aF'; # same
a0d0e21e 2135
19799a22 2136Hex strings may only represent integers. Strings that would cause
53305cf1
NC
2137integer overflow trigger a warning. Leading whitespace is not stripped,
2138unlike oct().
19799a22 2139
a0d0e21e
LW
2140=item import
2141
19799a22 2142There is no builtin C<import> function. It is just an ordinary
4633a7c4 2143method (subroutine) defined (or inherited) by modules that wish to export
19799a22 2144names to another module. The C<use> function calls the C<import> method
cea6626f 2145for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
a0d0e21e
LW
2146
2147=item index STR,SUBSTR,POSITION
2148
2149=item index STR,SUBSTR
2150
2b5ab1e7
TC
2151The index function searches for one string within another, but without
2152the wildcard-like behavior of a full regular-expression pattern match.
2153It returns the position of the first occurrence of SUBSTR in STR at
2154or after POSITION. If POSITION is omitted, starts searching from the
2155beginning of the string. The return value is based at C<0> (or whatever
2156you've set the C<$[> variable to--but don't do that). If the substring
2157is not found, returns one less than the base, ordinarily C<-1>.
a0d0e21e
LW
2158
2159=item int EXPR
2160
54310121 2161=item int
bbce6d69 2162
7660c0ab 2163Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2b5ab1e7
TC
2164You should not use this function for rounding: one because it truncates
2165towards C<0>, and two because machine representations of floating point
2166numbers can sometimes produce counterintuitive results. For example,
2167C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2168because it's really more like -268.99999999999994315658 instead. Usually,
19799a22 2169the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2b5ab1e7 2170functions will serve you better than will int().
a0d0e21e
LW
2171
2172=item ioctl FILEHANDLE,FUNCTION,SCALAR
2173
2b5ab1e7 2174Implements the ioctl(2) function. You'll probably first have to say
a0d0e21e 2175
4633a7c4 2176 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
a0d0e21e 2177
2b5ab1e7 2178to get the correct function definitions. If F<ioctl.ph> doesn't
a0d0e21e 2179exist or doesn't have the correct definitions you'll have to roll your
61eff3bc 2180own, based on your C header files such as F<< <sys/ioctl.h> >>.
5a964f20 2181(There is a Perl script called B<h2ph> that comes with the Perl kit that
54310121 2182may help you in this, but it's nontrivial.) SCALAR will be read and/or
4633a7c4 2183written depending on the FUNCTION--a pointer to the string value of SCALAR
19799a22 2184will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
4633a7c4
LW
2185has no string value but does have a numeric value, that value will be
2186passed rather than a pointer to the string value. To guarantee this to be
19799a22
GS
2187true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2188functions may be needed to manipulate the values of structures used by
b76cc8ba 2189C<ioctl>.
a0d0e21e 2190
19799a22 2191The return value of C<ioctl> (and C<fcntl>) is as follows:
a0d0e21e
LW
2192
2193 if OS returns: then Perl returns:
2194 -1 undefined value
2195 0 string "0 but true"
2196 anything else that number
2197
19799a22 2198Thus Perl returns true on success and false on failure, yet you can
a0d0e21e
LW
2199still easily determine the actual value returned by the operating
2200system:
2201
2b5ab1e7 2202 $retval = ioctl(...) || -1;
a0d0e21e
LW
2203 printf "System returned %d\n", $retval;
2204
c2611fb3 2205The special string "C<0> but true" is exempt from B<-w> complaints
5a964f20
TC
2206about improper numeric conversions.
2207
19799a22
GS
2208Here's an example of setting a filehandle named C<REMOTE> to be
2209non-blocking at the system level. You'll have to negotiate C<$|>
2210on your own, though.
2211
2212 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2213
2214 $flags = fcntl(REMOTE, F_GETFL, 0)
2215 or die "Can't get flags for the socket: $!\n";
2216
2217 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2218 or die "Can't set flags for the socket: $!\n";
2219
a0d0e21e
LW
2220=item join EXPR,LIST
2221
2b5ab1e7
TC
2222Joins the separate strings of LIST into a single string with fields
2223separated by the value of EXPR, and returns that new string. Example:
a0d0e21e 2224
2b5ab1e7 2225 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
a0d0e21e 2226
eb6e2d6f
GS
2227Beware that unlike C<split>, C<join> doesn't take a pattern as its
2228first argument. Compare L</split>.
a0d0e21e 2229
aa689395
PP
2230=item keys HASH
2231
19799a22 2232Returns a list consisting of all the keys of the named hash. (In
1d2dff63 2233scalar context, returns the number of keys.) The keys are returned in
ab192400
GS
2234an apparently random order. The actual random order is subject to
2235change in future versions of perl, but it is guaranteed to be the same
19799a22 2236order as either the C<values> or C<each> function produces (given
ab192400
GS
2237that the hash has not been modified). As a side effect, it resets
2238HASH's iterator.
a0d0e21e 2239
aa689395 2240Here is yet another way to print your environment:
a0d0e21e
LW
2241
2242 @keys = keys %ENV;
2243 @values = values %ENV;
b76cc8ba 2244 while (@keys) {
a0d0e21e
LW
2245 print pop(@keys), '=', pop(@values), "\n";
2246 }
2247
2248or how about sorted by key:
2249
2250 foreach $key (sort(keys %ENV)) {
2251 print $key, '=', $ENV{$key}, "\n";
2252 }
2253
8ea1e5d4
GS
2254The returned values are copies of the original keys in the hash, so
2255modifying them will not affect the original hash. Compare L</values>.
2256
19799a22 2257To sort a hash by value, you'll need to use a C<sort> function.
aa689395 2258Here's a descending numeric sort of a hash by its values:
4633a7c4 2259
5a964f20 2260 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
4633a7c4
LW
2261 printf "%4d %s\n", $hash{$key}, $key;
2262 }
2263
19799a22 2264As an lvalue C<keys> allows you to increase the number of hash buckets
aa689395
PP
2265allocated for the given hash. This can gain you a measure of efficiency if
2266you know the hash is going to get big. (This is similar to pre-extending
2267an array by assigning a larger number to $#array.) If you say
55497cff
PP
2268
2269 keys %hash = 200;
2270
ab192400
GS
2271then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2272in fact, since it rounds up to the next power of two. These
55497cff
PP
2273buckets will be retained even if you do C<%hash = ()>, use C<undef
2274%hash> if you want to free the storage while C<%hash> is still in scope.
2275You can't shrink the number of buckets allocated for the hash using
19799a22 2276C<keys> in this way (but you needn't worry about doing this by accident,
55497cff
PP
2277as trying has no effect).
2278
19799a22 2279See also C<each>, C<values> and C<sort>.
ab192400 2280
b350dd2f 2281=item kill SIGNAL, LIST
a0d0e21e 2282
b350dd2f 2283Sends a signal to a list of processes. Returns the number of
517db077
GS
2284processes successfully signaled (which is not necessarily the
2285same as the number actually killed).
a0d0e21e
LW
2286
2287 $cnt = kill 1, $child1, $child2;
2288 kill 9, @goners;
2289
b350dd2f
GS
2290If SIGNAL is zero, no signal is sent to the process. This is a
2291useful way to check that the process is alive and hasn't changed
2292its UID. See L<perlport> for notes on the portability of this
2293construct.
2294
2295Unlike in the shell, if SIGNAL is negative, it kills
4633a7c4
LW
2296process groups instead of processes. (On System V, a negative I<PROCESS>
2297number will also kill process groups, but that's not portable.) That
2298means you usually want to use positive not negative signals. You may also
da0045b7 2299use a signal name in quotes. See L<perlipc/"Signals"> for details.
a0d0e21e
LW
2300
2301=item last LABEL
2302
2303=item last
2304
2305The C<last> command is like the C<break> statement in C (as used in
2306loops); it immediately exits the loop in question. If the LABEL is
2307omitted, the command refers to the innermost enclosing loop. The
2308C<continue> block, if any, is not executed:
2309
4633a7c4
LW
2310 LINE: while (<STDIN>) {
2311 last LINE if /^$/; # exit when done with header
5a964f20 2312 #...
a0d0e21e
LW
2313 }
2314
4968c1e4 2315C<last> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2316C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2317a grep() or map() operation.
4968c1e4 2318
6c1372ed
GS
2319Note that a block by itself is semantically identical to a loop
2320that executes once. Thus C<last> can be used to effect an early
2321exit out of such a block.
2322
98293880
JH
2323See also L</continue> for an illustration of how C<last>, C<next>, and
2324C<redo> work.
1d2dff63 2325
a0d0e21e
LW
2326=item lc EXPR
2327
54310121 2328=item lc
bbce6d69 2329
d1be9408 2330Returns a lowercased version of EXPR. This is the internal function
ad0029c4
JH
2331implementing the C<\L> escape in double-quoted strings. Respects
2332current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
983ffd37 2333and L<perlunicode> for more details about locale and Unicode support.
a0d0e21e 2334
7660c0ab 2335If EXPR is omitted, uses C<$_>.
bbce6d69 2336
a0d0e21e
LW
2337=item lcfirst EXPR
2338
54310121 2339=item lcfirst
bbce6d69 2340
ad0029c4
JH
2341Returns the value of EXPR with the first character lowercased. This
2342is the internal function implementing the C<\l> escape in
2343double-quoted strings. Respects current LC_CTYPE locale if C<use
983ffd37
JH
2344locale> in force. See L<perllocale> and L<perlunicode> for more
2345details about locale and Unicode support.
a0d0e21e 2346
7660c0ab 2347If EXPR is omitted, uses C<$_>.
bbce6d69 2348
a0d0e21e
LW
2349=item length EXPR
2350
54310121 2351=item length
bbce6d69 2352
a0ed51b3 2353Returns the length in characters of the value of EXPR. If EXPR is
b76cc8ba 2354omitted, returns length of C<$_>. Note that this cannot be used on
2b5ab1e7
TC
2355an entire array or hash to find out how many elements these have.
2356For that, use C<scalar @array> and C<scalar keys %hash> respectively.
a0d0e21e
LW
2357
2358=item link OLDFILE,NEWFILE
2359
19799a22 2360Creates a new filename linked to the old filename. Returns true for
b76cc8ba 2361success, false otherwise.
a0d0e21e
LW
2362
2363=item listen SOCKET,QUEUESIZE
2364
19799a22 2365Does the same thing that the listen system call does. Returns true if
b76cc8ba 2366it succeeded, false otherwise. See the example in
cea6626f 2367L<perlipc/"Sockets: Client/Server Communication">.
a0d0e21e
LW
2368
2369=item local EXPR
2370
19799a22 2371You really probably want to be using C<my> instead, because C<local> isn't
b76cc8ba 2372what most people think of as "local". See
13a2d996 2373L<perlsub/"Private Variables via my()"> for details.
2b5ab1e7 2374
5a964f20
TC
2375A local modifies the listed variables to be local to the enclosing
2376block, file, or eval. If more than one value is listed, the list must
2377be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2378for details, including issues with tied arrays and hashes.
a0d0e21e 2379
a0d0e21e
LW
2380=item localtime EXPR
2381
19799a22 2382Converts a time as returned by the time function to a 9-element list
5f05dabc 2383with the time analyzed for the local time zone. Typically used as
a0d0e21e
LW
2384follows:
2385
54310121 2386 # 0 1 2 3 4 5 6 7 8
a0d0e21e
LW
2387 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2388 localtime(time);
2389
48a26b3a
GS
2390All list elements are numeric, and come straight out of the C `struct
2391tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2392specified time. $mday is the day of the month, and $mon is the month
2393itself, in the range C<0..11> with 0 indicating January and 11
2394indicating December. $year is the number of years since 1900. That
2395is, $year is C<123> in year 2023. $wday is the day of the week, with
23960 indicating Sunday and 3 indicating Wednesday. $yday is the day of
874b1813 2397the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
48a26b3a
GS
2398is true if the specified time occurs during daylight savings time,
2399false otherwise.
2400
2401Note that the $year element is I<not> simply the last two digits of
2402the year. If you assume it is, then you create non-Y2K-compliant
2403programs--and you wouldn't want to do that, would you?
54310121 2404
abd75f24
GS
2405The proper way to get a complete 4-digit year is simply:
2406
2407 $year += 1900;
2408
2409And to get the last two digits of the year (e.g., '01' in 2001) do:
2410
2411 $year = sprintf("%02d", $year % 100);
2412
48a26b3a 2413If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
a0d0e21e 2414
48a26b3a 2415In scalar context, C<localtime()> returns the ctime(3) value:
a0d0e21e 2416
5f05dabc 2417 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
a0d0e21e 2418
a3cb178b 2419This scalar value is B<not> locale dependent, see L<perllocale>, but
68f8bed4
JH
2420instead a Perl builtin. Also see the C<Time::Local> module
2421(to convert the second, minutes, hours, ... back to seconds since the
2422stroke of midnight the 1st of January 1970, the value returned by
ca6e1c26 2423time()), and the strftime(3) and mktime(3) functions available via the
68f8bed4
JH
2424POSIX module. To get somewhat similar but locale dependent date
2425strings, set up your locale environment variables appropriately
2426(please see L<perllocale>) and try for example:
a3cb178b 2427
5a964f20 2428 use POSIX qw(strftime);
2b5ab1e7 2429 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
a3cb178b
GS
2430
2431Note that the C<%a> and C<%b>, the short forms of the day of the week
2432and the month of the year, may not necessarily be three characters wide.
a0d0e21e 2433
19799a22
GS
2434=item lock
2435
2436 lock I<THING>
2437
2438This function places an advisory lock on a variable, subroutine,
2439or referenced object contained in I<THING> until the lock goes out
2440of scope. This is a built-in function only if your version of Perl
2441was built with threading enabled, and if you've said C<use Threads>.
2442Otherwise a user-defined function by this name will be called. See
2443L<Thread>.
2444
a0d0e21e
LW
2445=item log EXPR
2446
54310121 2447=item log
bbce6d69 2448
2b5ab1e7
TC
2449Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2450returns log of C<$_>. To get the log of another base, use basic algebra:
19799a22 2451The base-N log of a number is equal to the natural log of that number
2b5ab1e7
TC
2452divided by the natural log of N. For example:
2453
2454 sub log10 {
2455 my $n = shift;
2456 return log($n)/log(10);
b76cc8ba 2457 }
2b5ab1e7
TC
2458
2459See also L</exp> for the inverse operation.
a0d0e21e 2460
a0d0e21e
LW
2461=item lstat EXPR
2462
54310121 2463=item lstat
bbce6d69 2464
19799a22 2465Does the same thing as the C<stat> function (including setting the
5a964f20
TC
2466special C<_> filehandle) but stats a symbolic link instead of the file
2467the symbolic link points to. If symbolic links are unimplemented on
19799a22 2468your system, a normal C<stat> is done.
a0d0e21e 2469
7660c0ab 2470If EXPR is omitted, stats C<$_>.
bbce6d69 2471
a0d0e21e
LW
2472=item m//
2473
2474The match operator. See L<perlop>.
2475
2476=item map BLOCK LIST
2477
2478=item map EXPR,LIST
2479
19799a22
GS
2480Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2481C<$_> to each element) and returns the list value composed of the
2482results of each such evaluation. In scalar context, returns the
2483total number of elements so generated. Evaluates BLOCK or EXPR in
2484list context, so each element of LIST may produce zero, one, or
2485more elements in the returned value.
dd99ebda 2486
a0d0e21e
LW
2487 @chars = map(chr, @nums);
2488
2489translates a list of numbers to the corresponding characters. And
2490
4633a7c4 2491 %hash = map { getkey($_) => $_ } @array;
a0d0e21e
LW
2492
2493is just a funny way to write
2494
2495 %hash = ();
2496 foreach $_ (@array) {
4633a7c4 2497 $hash{getkey($_)} = $_;
a0d0e21e
LW
2498 }
2499
be3174d2
GS
2500Note that C<$_> is an alias to the list value, so it can be used to
2501modify the elements of the LIST. While this is useful and supported,
2502it can cause bizarre results if the elements of LIST are not variables.
2b5ab1e7
TC
2503Using a regular C<foreach> loop for this purpose would be clearer in
2504most cases. See also L</grep> for an array composed of those items of
2505the original list for which the BLOCK or EXPR evaluates to true.
fb73857a 2506
205fdb4d
NC
2507C<{> starts both hash references and blocks, so C<map { ...> could be either
2508the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2509ahead for the closing C<}> it has to take a guess at which its dealing with
2510based what it finds just after the C<{>. Usually it gets it right, but if it
2511doesn't it won't realize something is wrong until it gets to the C<}> and
2512encounters the missing (or unexpected) comma. The syntax error will be
2513reported close to the C<}> but you'll need to change something near the C<{>
2514such as using a unary C<+> to give perl some help:
2515
2516 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2517 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2518 %hash = map { ("\L$_", 1) } @array # this also works
2519 %hash = map { lc($_), 1 } @array # as does this.
2520 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
cea6626f 2521
205fdb4d
NC
2522 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2523
2524or to force an anon hash constructor use C<+{>
2525
2526 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2527
2528and you get list of anonymous hashes each with only 1 entry.
2529
19799a22 2530=item mkdir FILENAME,MASK
a0d0e21e 2531
5a211162
GS
2532=item mkdir FILENAME
2533
0591cd52 2534Creates the directory specified by FILENAME, with permissions
19799a22
GS
2535specified by MASK (as modified by C<umask>). If it succeeds it
2536returns true, otherwise it returns false and sets C<$!> (errno).
5a211162 2537If omitted, MASK defaults to 0777.
0591cd52 2538
19799a22 2539In general, it is better to create directories with permissive MASK,
0591cd52 2540and let the user modify that with their C<umask>, than it is to supply
19799a22 2541a restrictive MASK and give the user no way to be more permissive.
0591cd52
NT
2542The exceptions to this rule are when the file or directory should be
2543kept private (mail files, for instance). The perlfunc(1) entry on
19799a22 2544C<umask> discusses the choice of MASK in more detail.
a0d0e21e 2545
cc1852e8
JH
2546Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2547number of trailing slashes. Some operating and filesystems do not get
2548this right, so Perl automatically removes all trailing slashes to keep
2549everyone happy.
2550
a0d0e21e
LW
2551=item msgctl ID,CMD,ARG
2552
f86cebdf 2553Calls the System V IPC function msgctl(2). You'll probably have to say
0ade1984
JH
2554
2555 use IPC::SysV;
2556
7660c0ab
A
2557first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2558then ARG must be a variable which will hold the returned C<msqid_ds>
951ba7fe
GS
2559structure. Returns like C<ioctl>: the undefined value for error,
2560C<"0 but true"> for zero, or the actual return value otherwise. See also
4755096e 2561L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
a0d0e21e
LW
2562
2563=item msgget KEY,FLAGS
2564
f86cebdf 2565Calls the System V IPC function msgget(2). Returns the message queue
4755096e
GS
2566id, or the undefined value if there is an error. See also
2567L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
a0d0e21e 2568
a0d0e21e
LW
2569=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2570
2571Calls the System V IPC function msgrcv to receive a message from
2572message queue ID into variable VAR with a maximum message size of
41d6edb2
JH
2573SIZE. Note that when a message is received, the message type as a
2574native long integer will be the first thing in VAR, followed by the
2575actual message. This packing may be opened with C<unpack("l! a*")>.
2576Taints the variable. Returns true if successful, or false if there is
4755096e
GS
2577an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2578C<IPC::SysV::Msg> documentation.
41d6edb2
JH
2579
2580=item msgsnd ID,MSG,FLAGS
2581
2582Calls the System V IPC function msgsnd to send the message MSG to the
2583message queue ID. MSG must begin with the native long integer message
2584type, and be followed by the length of the actual message, and finally
2585the message itself. This kind of packing can be achieved with
2586C<pack("l! a*", $type, $message)>. Returns true if successful,
2587or false if there is an error. See also C<IPC::SysV>
2588and C<IPC::SysV::Msg> documentation.
a0d0e21e
LW
2589
2590=item my EXPR
2591
09bef843
SB
2592=item my EXPR : ATTRIBUTES
2593
19799a22
GS
2594A C<my> declares the listed variables to be local (lexically) to the
2595enclosing block, file, or C<eval>. If
5f05dabc 2596more than one value is listed, the list must be placed in parentheses. See
cb1a09d0 2597L<perlsub/"Private Variables via my()"> for details.
4633a7c4 2598
a0d0e21e
LW
2599=item next LABEL
2600
2601=item next
2602
2603The C<next> command is like the C<continue> statement in C; it starts
2604the next iteration of the loop:
2605
4633a7c4
LW
2606 LINE: while (<STDIN>) {
2607 next LINE if /^#/; # discard comments
5a964f20 2608 #...
a0d0e21e
LW
2609 }
2610
2611Note that if there were a C<continue> block on the above, it would get
2612executed even on discarded lines. If the LABEL is omitted, the command
2613refers to the innermost enclosing loop.
2614
4968c1e4 2615C<next> cannot be used to exit a block which returns a value such as
2b5ab1e7
TC
2616C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2617a grep() or map() operation.
4968c1e4 2618
6c1372ed
GS
2619Note that a block by itself is semantically identical to a loop
2620that executes once. Thus C<next> will exit such a block early.
2621
98293880
JH
2622See also L</continue> for an illustration of how C<last>, C<next>, and
2623C<redo> work.
1d2dff63 2624
a0d0e21e
LW
2625=item no Module LIST
2626
7660c0ab 2627See the L</use> function, which C<no> is the opposite of.
a0d0e21e
LW
2628
2629=item oct EXPR
2630
54310121 2631=item oct
bbce6d69 2632
4633a7c4 2633Interprets EXPR as an octal string and returns the corresponding
4f19785b
WSI
2634value. (If EXPR happens to start off with C<0x>, interprets it as a
2635hex string. If EXPR starts off with C<0b>, it is interpreted as a
53305cf1
NC
2636binary string. Leading whitespace is ignored in all three cases.)
2637The following will handle decimal, binary, octal, and hex in the standard
2638Perl or C notation:
a0d0e21e
LW
2639
2640 $val = oct($val) if $val =~ /^0/;
2641
19799a22
GS
2642If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2643in octal), use sprintf() or printf():
2644
2645 $perms = (stat("filename"))[2] & 07777;
2646 $oct_perms = sprintf "%lo", $perms;
2647
2648The oct() function is commonly used when a string such as C<644> needs
2649to be converted into a file mode, for example. (Although perl will
2650automatically convert strings into numbers as needed, this automatic
2651conversion assumes base 10.)
a0d0e21e
LW
2652
2653=item open FILEHANDLE,EXPR
2654
68bd7414
NIS
2655=item open FILEHANDLE,MODE,EXPR
2656
2657=item open FILEHANDLE,MODE,EXPR,LIST
2658
a0d0e21e
LW
2659=item open FILEHANDLE
2660
2661Opens the file whose filename is given by EXPR, and associates it with
68bd7414
NIS
2662FILEHANDLE. If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2663assigned a reference to a new anonymous filehandle, otherwise if FILEHANDLE is an expression,
2664its value is used as the name of the real filehandle wanted. (This is considered a symbolic
d6fd2b02
GS
2665reference, so C<use strict 'refs'> should I<not> be in effect.)
2666
2667If EXPR is omitted, the scalar
5f05dabc 2668variable of the same name as the FILEHANDLE contains the filename.
19799a22
GS
2669(Note that lexical variables--those declared with C<my>--will not work
2670for this purpose; so if you're using C<my>, specify EXPR in your call
2b5ab1e7
TC
2671to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2672files.
5f05dabc 2673
68bd7414
NIS
2674If three or more arguments are specified then the mode of opening and the file name
2675are separate. If MODE is C<< '<' >> or nothing, the file is opened for input.
61eff3bc
JH
2676If MODE is C<< '>' >>, the file is truncated and opened for
2677output, being created if necessary. If MODE is C<<< '>>' >>>,
b76cc8ba 2678the file is opened for appending, again being created if necessary.
61eff3bc
JH
2679You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2680you want both read and write access to the file; thus C<< '+<' >> is almost
2681always preferred for read/write updates--the C<< '+>' >> mode would clobber the
5a964f20
TC
2682file first. You can't usually use either read-write mode for updating
2683textfiles, since they have variable length records. See the B<-i>
0591cd52
NT
2684switch in L<perlrun> for a better approach. The file is created with
2685permissions of C<0666> modified by the process' C<umask> value.
5a964f20 2686
61eff3bc
JH
2687These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2688C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
5f05dabc 2689
6170680b
IZ
2690In the 2-arguments (and 1-argument) form of the call the mode and
2691filename should be concatenated (in this order), possibly separated by
68bd7414
NIS
2692spaces. It is possible to omit the mode in these forms if the mode is
2693C<< '<' >>.
6170680b 2694
7660c0ab 2695If the filename begins with C<'|'>, the filename is interpreted as a
5a964f20 2696command to which output is to be piped, and if the filename ends with a
f244e06d
GS
2697C<'|'>, the filename is interpreted as a command which pipes output to
2698us. See L<perlipc/"Using open() for IPC">
19799a22 2699for more examples of this. (You are not allowed to C<open> to a command
5a964f20 2700that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
4a4eefd0
GS
2701and L<perlipc/"Bidirectional Communication with Another Process">
2702for alternatives.)
cb1a09d0 2703
68bd7414 2704For three or more arguments if MODE is C<'|-'>, the filename is interpreted as a
6170680b
IZ
2705command to which output is to be piped, and if MODE is
2706C<'-|'>, the filename is interpreted as a command which pipes output to
2707us. In the 2-arguments (and 1-argument) form one should replace dash
2708(C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2709for more examples of this. (You are not allowed to C<open> to a command
2710that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
68bd7414
NIS
2711and L<perlipc/"Bidirectional Communication"> for alternatives.) In 3+ arg form of
2712pipe opens then if LIST is specified (extra arguments after the command name) then
2713LIST becomes arguments to the command invoked if the platform supports it.
2714The meaning of C<open> with more than three arguments for non-pipe modes
2715is not yet specified. Experimental "layers" may give extra LIST arguments meaning.
6170680b
IZ
2716
2717In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
b76cc8ba 2718and opening C<< '>-' >> opens STDOUT.
6170680b
IZ
2719
2720Open returns
19799a22 2721nonzero upon success, the undefined value otherwise. If the C<open>
4633a7c4 2722involved a pipe, the return value happens to be the pid of the
54310121 2723subprocess.
cb1a09d0
AD
2724
2725If you're unfortunate enough to be running Perl on a system that
2726distinguishes between text files and binary files (modern operating
2727systems don't care), then you should check out L</binmode> for tips for
19799a22 2728dealing with this. The key distinction between systems that need C<binmode>
5a964f20
TC
2729and those that don't is their text file formats. Systems like Unix, MacOS, and
2730Plan9, which delimit lines with a single character, and which encode that
19799a22 2731character in C as C<"\n">, do not need C<binmode>. The rest need it.
cb1a09d0 2732
68bd7414
NIS
2733In the three argument form MODE may also contain a list of IO "layers" (see L<open> and
2734L<PerlIO> for more details) to be applied to the handle. This can be used to achieve the
2735effect of C<binmode> as well as more complex behaviours.
2736
fb73857a 2737When opening a file, it's usually a bad idea to continue normal execution
19799a22
GS
2738if the request failed, so C<open> is frequently used in connection with
2739C<die>. Even if C<die> won't do what you want (say, in a CGI script,
fb73857a 2740where you want to make a nicely formatted error message (but there are
5a964f20 2741modules that can help with that problem)) you should always check
19799a22 2742the return value from opening a file. The infrequent exception is when
fb73857a
PP
2743working with an unopened filehandle is actually what you want to do.
2744
b76cc8ba
NIS
2745As a special case the 3 arg form with a read/write mode and the third argument
2746being C<undef>:
2747
2748 open(TMP, "+>", undef) or die ...
2749
2750opens a filehandle to an anonymous temporary file.
2751
68bd7414 2752
cb1a09d0 2753Examples:
a0d0e21e
LW
2754
2755 $ARTICLE = 100;
2756 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2757 while (<ARTICLE>) {...
2758
6170680b 2759 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
fb73857a 2760 # if the open fails, output is discarded
a0d0e21e 2761
6170680b 2762 open(DBASE, '+<', 'dbase.mine') # open for update
fb73857a 2763 or die "Can't open 'dbase.mine' for update: $!";
cb1a09d0 2764
6170680b
IZ
2765 open(DBASE, '+<dbase.mine') # ditto
2766 or die "Can't open 'dbase.mine' for update: $!";
2767
2768 open(ARTICLE, '-|', "caesar <$article") # decrypt article
fb73857a 2769 or die "Can't start caesar: $!";
a0d0e21e 2770
6170680b
IZ
2771 open(ARTICLE, "caesar <$article |") # ditto
2772 or die "Can't start caesar: $!";
2773
2774 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
fb73857a 2775 or die "Can't start sort: $!";
a0d0e21e
LW
2776
2777 # process argument list of files along with any includes
2778
2779 foreach $file (@ARGV) {
2780 process($file, 'fh00');
2781 }
2782
2783 sub process {
5a964f20 2784 my($filename, $input) = @_;
a0d0e21e
LW
2785 $input++; # this is a string increment
2786 unless (open($input, $filename)) {
2787 print STDERR "Can't open $filename: $!\n";
2788 return;
2789 }
2790
5a964f20 2791 local $_;
a0d0e21e
LW
2792 while (<$input>) { # note use of indirection
2793 if (/^#include "(.*)"/) {
2794 process($1, $input);
2795 next;
2796 }
5a964f20 2797 #... # whatever
a0d0e21e
LW
2798 }
2799 }
2800
2801You may also, in the Bourne shell tradition, specify an EXPR beginning
61eff3bc 2802with C<< '>&' >>, in which case the rest of the string is interpreted as the
5a964f20 2803name of a filehandle (or file descriptor, if numeric) to be
61eff3bc
JH
2804duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2805C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
a0d0e21e 2806mode you specify should match the mode of the original filehandle.
184e9718 2807(Duping a filehandle does not take into account any existing contents of
b76cc8ba
NIS
2808stdio buffers.) If you use the 3 arg form then you can pass either a number,
2809the name of a filehandle or the normal "reference to a glob".
6170680b 2810
a0d0e21e
LW
2811Here is a script that saves, redirects, and restores STDOUT and
2812STDERR:
2813
2814 #!/usr/bin/perl
b76cc8ba 2815 open(my $oldout, ">&", \*STDOUT);
5a964f20 2816 open(OLDERR, ">&STDERR");
a0d0e21e 2817
6170680b
IZ
2818 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2819 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
a0d0e21e
LW
2820
2821 select(STDERR); $| = 1; # make unbuffered
2822 select(STDOUT); $| = 1; # make unbuffered
2823
2824 print STDOUT "stdout 1\n"; # this works for
2825 print STDERR "stderr 1\n"; # subprocesses too
2826
2827 close(STDOUT);
2828 close(STDERR);
2829
5a964f20
TC
2830 open(STDOUT, ">&OLDOUT");
2831 open(STDERR, ">&OLDERR");
a0d0e21e
LW
2832
2833 print STDOUT "stdout 2\n";
2834 print STDERR "stderr 2\n";
2835
df632fdf
JH
2836If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2837do an equivalent of C's C<fdopen> of that file descriptor; this is
2838more parsimonious of file descriptors. For example:
a0d0e21e
LW
2839
2840 open(FILEHANDLE, "<&=$fd")
df632fdf 2841
b76cc8ba 2842or
df632fdf 2843
b76cc8ba 2844 open(FILEHANDLE, "<&=", $fd)
a0d0e21e 2845
df632fdf
JH
2846Note that if Perl is using the standard C libraries' fdopen() then on
2847many UNIX systems, fdopen() is known to fail when file descriptors
4af147f6 2848exceed a certain value, typically 255. If you need more file
b76cc8ba 2849descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
4af147f6 2850
df632fdf
JH
2851You can see whether Perl has been compiled with PerlIO or not by
2852running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2853is C<define>, you have PerlIO, otherwise you don't.
2854
6170680b
IZ
2855If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2856with 2-arguments (or 1-argument) form of open(), then
a0d0e21e 2857there is an implicit fork done, and the return value of open is the pid
7660c0ab 2858of the child within the parent process, and C<0> within the child
184e9718 2859process. (Use C<defined($pid)> to determine whether the open was successful.)
a0d0e21e
LW
2860The filehandle behaves normally for the parent, but i/o to that
2861filehandle is piped from/to the STDOUT/STDIN of the child process.
2862In the child process the filehandle isn't opened--i/o happens from/to
2863the new STDOUT or STDIN. Typically this is used like the normal
2864piped open when you want to exercise more control over just how the
2865pipe command gets executed, such as when you are running setuid, and
54310121 2866don't want to have to scan shell commands for metacharacters.
6170680b 2867The following triples are more or less equivalent:
a0d0e21e
LW
2868
2869 open(FOO, "|tr '[a-z]' '[A-Z]'");
6170680b
IZ
2870 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2871 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
b76cc8ba 2872 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
a0d0e21e
LW
2873
2874 open(FOO, "cat -n '$file'|");
6170680b
IZ
2875 open(FOO, '-|', "cat -n '$file'");
2876 open(FOO, '-|') || exec 'cat', '-n', $file;
b76cc8ba
NIS
2877 open(FOO, '-|', "cat", '-n', $file);
2878
2879The last example in each block shows the pipe as "list form", which is
2880not yet supported on all platforms.
a0d0e21e 2881
4633a7c4
LW
2882See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2883
0f897271
GS
2884Beginning with v5.6.0, Perl will attempt to flush all files opened for
2885output before any operation that may do a fork, but this may not be
2886supported on some platforms (see L<perlport>). To be safe, you may need
2887to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2888of C<IO::Handle> on any open handles.
2889
2890On systems that support a
45bc9206
GS
2891close-on-exec flag on files, the flag will be set for the newly opened
2892file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
a0d0e21e 2893
0dccf244
CS
2894Closing any piped filehandle causes the parent process to wait for the
2895child to finish, and returns the status value in C<$?>.
2896
6170680b
IZ
2897The filename passed to 2-argument (or 1-argument) form of open()
2898will have leading and trailing
f86cebdf 2899whitespace deleted, and the normal redirection characters
b76cc8ba 2900honored. This property, known as "magic open",
5a964f20 2901can often be used to good effect. A user could specify a filename of
7660c0ab 2902F<"rsh cat file |">, or you could change certain filenames as needed:
5a964f20
TC
2903
2904 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2905 open(FH, $filename) or die "Can't open $filename: $!";
2906
6170680b
IZ
2907Use 3-argument form to open a file with arbitrary weird characters in it,
2908
2909 open(FOO, '<', $file);
2910
2911otherwise it's necessary to protect any leading and trailing whitespace:
5a964f20
TC
2912
2913 $file =~ s#^(\s)#./$1#;
2914 open(FOO, "< $file\0");
2915
a31a806a 2916(this may not work on some bizarre filesystems). One should
106325ad 2917conscientiously choose between the I<magic> and 3-arguments form
6170680b
IZ
2918of open():
2919
2920 open IN, $ARGV[0];
2921
2922will allow the user to specify an argument of the form C<"rsh cat file |">,
2923but will not work on a filename which happens to have a trailing space, while
2924
2925 open IN, '<', $ARGV[0];
2926
2927will have exactly the opposite restrictions.
2928
19799a22 2929If you want a "real" C C<open> (see L<open(2)> on your system), then you
6170680b
IZ
2930should use the C<sysopen> function, which involves no such magic (but
2931may use subtly different filemodes than Perl open(), which is mapped
2932to C fopen()). This is
5a964f20
TC
2933another way to protect your filenames from interpretation. For example:
2934
2935 use IO::Handle;
2936 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2937 or die "sysopen $path: $!";
2938 $oldfh = select(HANDLE); $| = 1; select($oldfh);
38762f02 2939 print HANDLE "stuff $$\n";
5a964f20
TC
2940 seek(HANDLE, 0, 0);
2941 print "File contains: ", <HANDLE>;
2942
7660c0ab
A
2943Using the constructor from the C<IO::Handle> package (or one of its
2944subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
5a964f20
TC
2945filehandles that have the scope of whatever variables hold references to
2946them, and automatically close whenever and however you leave that scope:
c07a80fd 2947
5f05dabc 2948 use IO::File;
5a964f20 2949 #...
c07a80fd
PP
2950 sub read_myfile_munged {
2951 my $ALL = shift;
5f05dabc 2952 my $handle = new IO::File;
c07a80fd
PP
2953 open($handle, "myfile") or die "myfile: $!";
2954 $first = <$handle>
2955 or return (); # Automatically closed here.
2956 mung $first or die "mung failed"; # Or here.
2957 return $first, <$handle> if $ALL; # Or here.
2958 $first; # Or here.
2959 }
2960
b687b08b 2961See L</seek> for some details about mixing reading and writing.
a0d0e21e
LW
2962
2963=item opendir DIRHANDLE,EXPR
2964
19799a22
GS
2965Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2966C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
a0d0e21e
LW
2967DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2968
2969=item ord EXPR
2970
54310121 2971=item ord
bbce6d69 2972
121910a4
JH
2973Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
2974or Unicode) value of the first character of EXPR. If EXPR is omitted,
2975uses C<$_>.
2976
2977For the reverse, see L</chr>.
2978See L<perlunicode> and L<encoding> for more about Unicode.
a0d0e21e 2979
77ca0c92
LW
2980=item our EXPR
2981
9969eac4
BS
2982=item our EXPR : ATTRIBUTES
2983
77ca0c92
LW
2984An C<our> declares the listed variables to be valid globals within
2985the enclosing block, file, or C<eval>. That is, it has the same
2986scoping rules as a "my" declaration, but does not create a local
2987variable. If more than one value is listed, the list must be placed
2988in parentheses. The C<our> declaration has no semantic effect unless
2989"use strict vars" is in effect, in which case it lets you use the
2990declared global variable without qualifying it with a package name.
2991(But only within the lexical scope of the C<our> declaration. In this
2992it differs from "use vars", which is package scoped.)
2993
f472eb5c
GS
2994An C<our> declaration declares a global variable that will be visible
2995across its entire lexical scope, even across package boundaries. The
2996package in which the variable is entered is determined at the point
2997of the declaration, not at the point of use. This means the following
2998behavior holds:
2999
3000 package Foo;
3001 our $bar; # declares $Foo::bar for rest of lexical scope
3002 $bar = 20;
3003
3004 package Bar;
3005 print $bar; # prints 20
3006
3007Multiple C<our> declarations in the same lexical scope are allowed
3008if they are in different packages. If they happened to be in the same
3009package, Perl will emit warnings if you have asked for them.
3010
3011 use warnings;
3012 package Foo;
3013 our $bar; # declares $Foo::bar for rest of lexical scope
3014 $bar = 20;
3015
3016 package Bar;
3017 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3018 print $bar; # prints 30
3019
3020 our $bar; # emits warning
3021
9969eac4
BS
3022An C<our> declaration may also have a list of attributes associated
3023with it. B<WARNING>: This is an experimental feature that may be
3024changed or removed in future releases of Perl. It should not be
3025relied upon.
3026
51d2bbcc 3027The only currently recognized attribute is C<unique> which indicates
9969eac4
BS
3028that a single copy of the global is to be used by all interpreters
3029should the program happen to be running in a multi-interpreter
3030environment. (The default behaviour would be for each interpreter to
3031have its own copy of the global.) In such an environment, this
3032attribute also has the effect of making the global readonly.
3033Examples:
3034
51d2bbcc
JH
3035 our @EXPORT : unique = qw(foo);
3036 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3037 our $VERSION : unique = "1.00";
9969eac4
BS
3038
3039Multi-interpreter environments can come to being either through the
3040fork() emulation on Windows platforms, or by embedding perl in a
51d2bbcc 3041multi-threaded application. The C<unique> attribute does nothing in
9969eac4
BS
3042all other environments.
3043
a0d0e21e
LW
3044=item pack TEMPLATE,LIST
3045
2b6c5635
GS
3046Takes a LIST of values and converts it into a string using the rules
3047given by the TEMPLATE. The resulting string is the concatenation of
3048the converted values. Typically, each converted value looks
3049like its machine-level representation. For example, on 32-bit machines
3050a converted integer may be represented by a sequence of 4 bytes.
3051
3052The TEMPLATE is a
a0d0e21e
LW
3053sequence of characters that give the order and type of values, as
3054follows:
3055
5a929a98 3056 a A string with arbitrary binary data, will be null padded.
121910a4
JH
3057 A A text (ASCII) string, will be space padded.
3058 Z A null terminated (ASCIZ) string, will be null padded.
5a929a98 3059
2b6c5635
GS
3060 b A bit string (ascending bit order inside each byte, like vec()).
3061 B A bit string (descending bit order inside each byte).
a0d0e21e
LW
3062 h A hex string (low nybble first).
3063 H A hex string (high nybble first).
3064
3065 c A signed char value.
a0ed51b3 3066 C An unsigned char value. Only does bytes. See U for Unicode.
96e4d5b1 3067
a0d0e21e
LW
3068 s A signed short value.
3069 S An unsigned short value.
96e4d5b1 3070 (This 'short' is _exactly_ 16 bits, which may differ from
851646ae
JH
3071 what a local C compiler calls 'short'. If you want
3072 native-length shorts, use the '!' suffix.)
96e4d5b1 3073
a0d0e21e
LW
3074 i A signed integer value.
3075 I An unsigned integer value.
19799a22 3076 (This 'integer' is _at_least_ 32 bits wide. Its exact
f86cebdf
GS
3077 size depends on what a local C compiler calls 'int',
3078 and may even be larger than the 'long' described in
3079 the next item.)
96e4d5b1 3080
a0d0e21e
LW
3081 l A signed long value.
3082 L An unsigned long value.
96e4d5b1 3083 (This 'long' is _exactly_ 32 bits, which may differ from
851646ae
JH
3084 what a local C compiler calls 'long'. If you want
3085 native-length longs, use the '!' suffix.)
a0d0e21e 3086
5d11dd56
G
3087 n An unsigned short in "network" (big-endian) order.
3088 N An unsigned long in "network" (big-endian) order.
3089 v An unsigned short in "VAX" (little-endian) order.
3090 V An unsigned long in "VAX" (little-endian) order.
96e4d5b1
PP
3091 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3092 _exactly_ 32 bits, respectively.)
a0d0e21e 3093
dae0da7a
JH
3094 q A signed quad (64-bit) value.
3095 Q An unsigned quad value.
851646ae
JH
3096 (Quads are available only if your system supports 64-bit
3097 integer values _and_ if Perl has been compiled to support those.
dae0da7a
JH
3098 Causes a fatal error otherwise.)
3099
a0d0e21e
LW
3100 f A single-precision float in the native format.
3101 d A double-precision float in the native format.
3102
3103 p A pointer to a null-terminated string.
3104 P A pointer to a structure (fixed-length string).
3105
3106 u A uuencoded string.
ad0029c4
JH
3107 U A Unicode character number. Encodes to UTF-8 internally
3108 (or UTF-EBCDIC in EBCDIC platforms).
a0d0e21e 3109
96e4d5b1 3110 w A BER compressed integer. Its bytes represent an unsigned
f86cebdf
GS
3111 integer in base 128, most significant digit first, with as
3112 few digits as possible. Bit eight (the high bit) is set
3113 on each byte except the last.
def98dd4 3114
a0d0e21e
LW
3115 x A null byte.
3116 X Back up a byte.
3117 @ Null fill to absolute position.
3118
5a929a98
VU
3119The following rules apply:
3120
3121=over 8
3122
3123=item *
3124
5a964f20 3125Each letter may optionally be followed by a number giving a repeat
951ba7fe
GS
3126count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3127C<H>, and C<P> the pack function will gobble up that many values from
5a929a98 3128the LIST. A C<*> for the repeat count means to use however many items are
951ba7fe
GS
3129left, except for C<@>, C<x>, C<X>, where it is equivalent
3130to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2b6c5635
GS
3131same).
3132
951ba7fe 3133When used with C<Z>, C<*> results in the addition of a trailing null
2b6c5635
GS
3134byte (so the packed result will be one longer than the byte C<length>
3135of the item).
3136
951ba7fe 3137The repeat count for C<u> is interpreted as the maximal number of bytes
2b6c5635 3138to encode per line of output, with 0 and 1 replaced by 45.
5a929a98
VU
3139
3140=item *
3141
951ba7fe 3142The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
5a929a98 3143string of length count, padding with nulls or spaces as necessary. When
951ba7fe
GS
3144unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3145after the first null, and C<a> returns data verbatim. When packing,
3146C<a>, and C<Z> are equivalent.
2b6c5635
GS
3147
3148If the value-to-pack is too long, it is truncated. If too long and an
951ba7fe
GS
3149explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3150by a null byte. Thus C<Z> always packs a trailing null byte under
2b6c5635 3151all circumstances.
5a929a98
VU
3152
3153=item *
3154
951ba7fe 3155Likewise, the C<b> and C<B> fields pack a string that many bits long.
c73032f5
IZ
3156Each byte of the input field of pack() generates 1 bit of the result.
3157Each result bit is based on the least-significant bit of the corresponding
3158input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3159C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3160
3161Starting from the beginning of the input string of pack(), each 8-tuple
951ba7fe 3162of bytes is converted to 1 byte of output. With format C<b>
c73032f5 3163the first byte of the 8-tuple determines the least-significant bit of a
951ba7fe 3164byte, and with format C<B> it determines the most-significant bit of
c73032f5
IZ
3165a byte.
3166
3167If the length of the input string is not exactly divisible by 8, the
3168remainder is packed as if the input string were padded by null bytes
3169at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3170
3171If the input string of pack() is longer than needed, extra bytes are ignored.
2b6c5635
GS
3172A C<*> for the repeat count of pack() means to use all the bytes of
3173the input field. On unpack()ing the bits are converted to a string
3174of C<"0">s and C<"1">s.
5a929a98
VU
3175
3176=item *
3177
951ba7fe 3178The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
851646ae 3179representable as hexadecimal digits, 0-9a-f) long.
5a929a98 3180
c73032f5
IZ
3181Each byte of the input field of pack() generates 4 bits of the result.
3182For non-alphabetical bytes the result is based on the 4 least-significant
3183bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3184bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3185C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3186is compatible with the usual hexadecimal digits, so that C<"a"> and
3187C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3188C<"g".."z"> and C<"G".."Z"> is not well-defined.
3189
3190Starting from the beginning of the input string of pack(), each pair
951ba7fe 3191of bytes is converted to 1 byte of output. With format C<h> the
c73032f5 3192first byte of the pair determines the least-significant nybble of the
951ba7fe 3193output byte, and with format C<H> it determines the most-significant
c73032f5
IZ
3194nybble.
3195
3196If the length of the input string is not even, it behaves as if padded
3197by a null byte at the end. Similarly, during unpack()ing the "extra"
3198nybbles are ignored.
3199
3200If the input string of pack() is longer than needed, extra bytes are ignored.
3201A C<*> for the repeat count of pack() means to use all the bytes of
3202the input field. On unpack()ing the bits are converted to a string
3203of hexadecimal digits.
3204
5a929a98
VU
3205=item *
3206
951ba7fe 3207The C<p> type packs a pointer to a null-terminated string. You are
5a929a98
VU
3208responsible for ensuring the string is not a temporary value (which can
3209potentially get deallocated before you get around to using the packed result).
951ba7fe
GS
3210The C<P> type packs a pointer to a structure of the size indicated by the
3211length. A NULL pointer is created if the corresponding value for C<p> or
3212C<P> is C<undef>, similarly for unpack().
5a929a98
VU
3213
3214=item *
3215
951ba7fe
GS
3216The C</> template character allows packing and unpacking of strings where
3217the packed structure contains a byte count followed by the string itself.
17f4a12d 3218You write I<length-item>C</>I<string-item>.
43192e07
IP
3219
3220The I<length-item> can be any C<pack> template letter,
3221and describes how the length value is packed.
3222The ones likely to be of most use are integer-packing ones like
951ba7fe
GS
3223C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3224and C<N> (for Sun XDR).
43192e07
IP
3225
3226The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3227For C<unpack> the length of the string is obtained from the I<length-item>,
3228but if you put in the '*' it will be ignored.
3229
17f4a12d
IZ
3230 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3231 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3232 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
43192e07
IP
3233
3234The I<length-item> is not returned explicitly from C<unpack>.
3235
951ba7fe
GS
3236Adding a count to the I<length-item> letter is unlikely to do anything
3237useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3238I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
43192e07
IP
3239which Perl does not regard as legal in numeric strings.
3240
3241=item *
3242
951ba7fe
GS
3243The integer types C<s>, C<S>, C<l>, and C<L> may be
3244immediately followed by a C<!> suffix to signify native shorts or
3245longs--as you can see from above for example a bare C<l> does mean
851646ae
JH
3246exactly 32 bits, the native C<long> (as seen by the local C compiler)
3247may be larger. This is an issue mainly in 64-bit platforms. You can
951ba7fe 3248see whether using C<!> makes any difference by
726ea183 3249
4d0c1c44
GS
3250 print length(pack("s")), " ", length(pack("s!")), "\n";
3251 print length(pack("l")), " ", length(pack("l!")), "\n";
ef54e1a4 3252
951ba7fe
GS
3253C<i!> and C<I!> also work but only because of completeness;
3254they are identical to C<i> and C<I>.
ef54e1a4 3255
19799a22
GS
3256The actual sizes (in bytes) of native shorts, ints, longs, and long
3257longs on the platform where Perl was built are also available via
3258L<Config>:
3259
3260 use Config;
3261 print $Config{shortsize}, "\n";
3262 print $Co