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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
3 | perlfunc - Perl builtin functions | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | The functions in this section can serve as terms in an expression. | |
8 | They fall into two major categories: list operators and named unary | |
9 | operators. These differ in their precedence relationship with a | |
10 | following comma. (See the precedence table in L<perlop>.) List | |
11 | operators take more than one argument, while unary operators can never | |
12 | take more than one argument. Thus, a comma terminates the argument of | |
13 | a unary operator, but merely separates the arguments of a list | |
14 | operator. A unary operator generally provides a scalar context to its | |
15 | argument, while a list operator may provide either scalar and list | |
16 | contexts for its arguments. If it does both, the scalar arguments will | |
5f05dabc | 17 | be first, and the list argument will follow. (Note that there can ever |
18 | be only one list argument.) For instance, splice() has three scalar | |
a0d0e21e LW |
19 | arguments followed by a list. |
20 | ||
21 | In the syntax descriptions that follow, list operators that expect a | |
22 | list (and provide list context for the elements of the list) are shown | |
23 | with LIST as an argument. Such a list may consist of any combination | |
24 | of scalar arguments or list values; the list values will be included | |
25 | in the list as if each individual element were interpolated at that | |
26 | point in the list, forming a longer single-dimensional list value. | |
27 | Elements of the LIST should be separated by commas. | |
28 | ||
29 | Any function in the list below may be used either with or without | |
30 | parentheses around its arguments. (The syntax descriptions omit the | |
5f05dabc | 31 | parentheses.) If you use the parentheses, the simple (but occasionally |
a0d0e21e LW |
32 | surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a |
33 | function, and precedence doesn't matter. Otherwise it's a list | |
34 | operator or unary operator, and precedence does matter. And whitespace | |
35 | between the function and left parenthesis doesn't count--so you need to | |
36 | be careful sometimes: | |
37 | ||
68dc0745 | 38 | print 1+2+4; # Prints 7. |
39 | print(1+2) + 4; # Prints 3. | |
40 | print (1+2)+4; # Also prints 3! | |
41 | print +(1+2)+4; # Prints 7. | |
42 | print ((1+2)+4); # Prints 7. | |
a0d0e21e LW |
43 | |
44 | If you run Perl with the B<-w> switch it can warn you about this. For | |
45 | example, the third line above produces: | |
46 | ||
47 | print (...) interpreted as function at - line 1. | |
48 | Useless use of integer addition in void context at - line 1. | |
49 | ||
50 | For functions that can be used in either a scalar or list context, | |
54310121 | 51 | nonabortive failure is generally indicated in a scalar context by |
a0d0e21e LW |
52 | returning the undefined value, and in a list context by returning the |
53 | null list. | |
54 | ||
5a964f20 TC |
55 | Remember the following important rule: There is B<no rule> that relates |
56 | the behavior of an expression in list context to its behavior in scalar | |
57 | context, or vice versa. It might do two totally different things. | |
a0d0e21e LW |
58 | Each operator and function decides which sort of value it would be most |
59 | appropriate to return in a scalar context. Some operators return the | |
5a964f20 | 60 | length of the list that would have been returned in list context. Some |
a0d0e21e LW |
61 | operators return the first value in the list. Some operators return the |
62 | last value in the list. Some operators return a count of successful | |
63 | operations. In general, they do what you want, unless you want | |
64 | consistency. | |
65 | ||
5a964f20 TC |
66 | An named array in scalar context is quite different from what would at |
67 | first glance appear to be a list in scalar context. You can't get a list | |
68 | like C<(1,2,3)> into being in scalar context, because the compiler knows | |
69 | the context at compile time. It would generate the scalar comma operator | |
70 | there, not the list construction version of the comma. That means it | |
71 | was never a list to start with. | |
72 | ||
73 | In general, functions in Perl that serve as wrappers for system calls | |
7660c0ab | 74 | of the same name (like C<chown(2)>, C<fork(2)>, C<closedir(2)>, etc.) all return |
5a964f20 TC |
75 | true when they succeed and C<undef> otherwise, as is usually mentioned |
76 | in the descriptions below. This is different from the C interfaces, | |
7660c0ab A |
77 | which return C<-1> on failure. Exceptions to this rule are C<wait()>, |
78 | C<waitpid()>, and C<syscall()>. System calls also set the special C<$!> | |
5a964f20 TC |
79 | variable on failure. Other functions do not, except accidentally. |
80 | ||
cb1a09d0 AD |
81 | =head2 Perl Functions by Category |
82 | ||
83 | Here are Perl's functions (including things that look like | |
5a964f20 | 84 | functions, like some keywords and named operators) |
cb1a09d0 AD |
85 | arranged by category. Some functions appear in more |
86 | than one place. | |
87 | ||
88 | =over | |
89 | ||
90 | =item Functions for SCALARs or strings | |
91 | ||
22fae026 | 92 | C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>, |
ab4f32c2 A |
93 | C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>, |
94 | C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///> | |
cb1a09d0 AD |
95 | |
96 | =item Regular expressions and pattern matching | |
97 | ||
ab4f32c2 | 98 | C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//> |
cb1a09d0 AD |
99 | |
100 | =item Numeric functions | |
101 | ||
22fae026 TM |
102 | C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>, |
103 | C<sin>, C<sqrt>, C<srand> | |
cb1a09d0 AD |
104 | |
105 | =item Functions for real @ARRAYs | |
106 | ||
22fae026 | 107 | C<pop>, C<push>, C<shift>, C<splice>, C<unshift> |
cb1a09d0 AD |
108 | |
109 | =item Functions for list data | |
110 | ||
ab4f32c2 | 111 | C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack> |
cb1a09d0 AD |
112 | |
113 | =item Functions for real %HASHes | |
114 | ||
22fae026 | 115 | C<delete>, C<each>, C<exists>, C<keys>, C<values> |
cb1a09d0 AD |
116 | |
117 | =item Input and output functions | |
118 | ||
22fae026 TM |
119 | C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>, |
120 | C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>, | |
121 | C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>, | |
122 | C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>, | |
123 | C<warn>, C<write> | |
cb1a09d0 AD |
124 | |
125 | =item Functions for fixed length data or records | |
126 | ||
22fae026 | 127 | C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec> |
cb1a09d0 AD |
128 | |
129 | =item Functions for filehandles, files, or directories | |
130 | ||
22fae026 TM |
131 | C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>, |
132 | C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, C<readlink>, | |
133 | C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, C<unlink>, C<utime> | |
cb1a09d0 AD |
134 | |
135 | =item Keywords related to the control flow of your perl program | |
136 | ||
22fae026 TM |
137 | C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>, |
138 | C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray> | |
cb1a09d0 | 139 | |
54310121 | 140 | =item Keywords related to scoping |
cb1a09d0 | 141 | |
22fae026 | 142 | C<caller>, C<import>, C<local>, C<my>, C<package>, C<use> |
cb1a09d0 AD |
143 | |
144 | =item Miscellaneous functions | |
145 | ||
22fae026 TM |
146 | C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>, |
147 | C<scalar>, C<undef>, C<wantarray> | |
cb1a09d0 AD |
148 | |
149 | =item Functions for processes and process groups | |
150 | ||
22fae026 | 151 | C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>, |
ab4f32c2 | 152 | C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>, |
22fae026 | 153 | C<times>, C<wait>, C<waitpid> |
cb1a09d0 AD |
154 | |
155 | =item Keywords related to perl modules | |
156 | ||
22fae026 | 157 | C<do>, C<import>, C<no>, C<package>, C<require>, C<use> |
cb1a09d0 AD |
158 | |
159 | =item Keywords related to classes and object-orientedness | |
160 | ||
22fae026 TM |
161 | C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>, |
162 | C<untie>, C<use> | |
cb1a09d0 AD |
163 | |
164 | =item Low-level socket functions | |
165 | ||
22fae026 TM |
166 | C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>, |
167 | C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>, | |
168 | C<socket>, C<socketpair> | |
cb1a09d0 AD |
169 | |
170 | =item System V interprocess communication functions | |
171 | ||
22fae026 TM |
172 | C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>, |
173 | C<shmctl>, C<shmget>, C<shmread>, C<shmwrite> | |
cb1a09d0 AD |
174 | |
175 | =item Fetching user and group info | |
176 | ||
22fae026 TM |
177 | C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>, |
178 | C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>, | |
179 | C<getpwuid>, C<setgrent>, C<setpwent> | |
cb1a09d0 AD |
180 | |
181 | =item Fetching network info | |
182 | ||
22fae026 TM |
183 | C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>, |
184 | C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, | |
185 | C<getprotobyname>, C<getprotobynumber>, C<getprotoent>, | |
186 | C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>, | |
187 | C<setnetent>, C<setprotoent>, C<setservent> | |
cb1a09d0 AD |
188 | |
189 | =item Time-related functions | |
190 | ||
22fae026 | 191 | C<gmtime>, C<localtime>, C<time>, C<times> |
cb1a09d0 | 192 | |
37798a01 | 193 | =item Functions new in perl5 |
194 | ||
22fae026 TM |
195 | C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>, |
196 | C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>, | |
197 | C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>, | |
198 | C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use> | |
37798a01 | 199 | |
200 | * - C<sub> was a keyword in perl4, but in perl5 it is an | |
5a964f20 | 201 | operator, which can be used in expressions. |
37798a01 | 202 | |
203 | =item Functions obsoleted in perl5 | |
204 | ||
22fae026 | 205 | C<dbmclose>, C<dbmopen> |
37798a01 | 206 | |
cb1a09d0 AD |
207 | =back |
208 | ||
209 | =head2 Alphabetical Listing of Perl Functions | |
210 | ||
a0d0e21e LW |
211 | =over 8 |
212 | ||
22fae026 | 213 | =item I<-X> FILEHANDLE |
a0d0e21e | 214 | |
22fae026 | 215 | =item I<-X> EXPR |
a0d0e21e | 216 | |
22fae026 | 217 | =item I<-X> |
a0d0e21e LW |
218 | |
219 | A file test, where X is one of the letters listed below. This unary | |
220 | operator takes one argument, either a filename or a filehandle, and | |
221 | tests the associated file to see if something is true about it. If the | |
7660c0ab | 222 | argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN. |
a0d0e21e LW |
223 | Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or |
224 | the undefined value if the file doesn't exist. Despite the funny | |
225 | names, precedence is the same as any other named unary operator, and | |
226 | the argument may be parenthesized like any other unary operator. The | |
227 | operator may be any of: | |
228 | ||
229 | -r File is readable by effective uid/gid. | |
230 | -w File is writable by effective uid/gid. | |
231 | -x File is executable by effective uid/gid. | |
232 | -o File is owned by effective uid. | |
233 | ||
234 | -R File is readable by real uid/gid. | |
235 | -W File is writable by real uid/gid. | |
236 | -X File is executable by real uid/gid. | |
237 | -O File is owned by real uid. | |
238 | ||
239 | -e File exists. | |
240 | -z File has zero size. | |
54310121 | 241 | -s File has nonzero size (returns size). |
a0d0e21e LW |
242 | |
243 | -f File is a plain file. | |
244 | -d File is a directory. | |
245 | -l File is a symbolic link. | |
246 | -p File is a named pipe (FIFO). | |
247 | -S File is a socket. | |
248 | -b File is a block special file. | |
249 | -c File is a character special file. | |
250 | -t Filehandle is opened to a tty. | |
251 | ||
252 | -u File has setuid bit set. | |
253 | -g File has setgid bit set. | |
254 | -k File has sticky bit set. | |
255 | ||
256 | -T File is a text file. | |
257 | -B File is a binary file (opposite of -T). | |
258 | ||
259 | -M Age of file in days when script started. | |
260 | -A Same for access time. | |
261 | -C Same for inode change time. | |
262 | ||
263 | The interpretation of the file permission operators C<-r>, C<-R>, C<-w>, | |
5f05dabc | 264 | C<-W>, C<-x>, and C<-X> is based solely on the mode of the file and the |
a0d0e21e | 265 | uids and gids of the user. There may be other reasons you can't actually |
5a964f20 | 266 | read, write, or execute the file, such as AFS access control lists. Also note that, for the superuser, |
7660c0ab A |
267 | C<-r>, C<-R>, C<-w>, and C<-W> always return C<1>, and C<-x> and C<-X> return |
268 | C<1> if any execute bit is set in the mode. Scripts run by the superuser may | |
269 | thus need to do a C<stat()> to determine the actual mode of the | |
a0d0e21e LW |
270 | file, or temporarily set the uid to something else. |
271 | ||
272 | Example: | |
273 | ||
274 | while (<>) { | |
275 | chop; | |
276 | next unless -f $_; # ignore specials | |
5a964f20 | 277 | #... |
a0d0e21e LW |
278 | } |
279 | ||
280 | Note that C<-s/a/b/> does not do a negated substitution. Saying | |
281 | C<-exp($foo)> still works as expected, however--only single letters | |
282 | following a minus are interpreted as file tests. | |
283 | ||
284 | The C<-T> and C<-B> switches work as follows. The first block or so of the | |
285 | file is examined for odd characters such as strange control codes or | |
5a964f20 | 286 | characters with the high bit set. If too many strange characters (E<gt>30%) |
a0d0e21e LW |
287 | are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file |
288 | containing null in the first block is considered a binary file. If C<-T> | |
289 | or C<-B> is used on a filehandle, the current stdio buffer is examined | |
290 | rather than the first block. Both C<-T> and C<-B> return TRUE on a null | |
54310121 | 291 | file, or a file at EOF when testing a filehandle. Because you have to |
4633a7c4 LW |
292 | read a file to do the C<-T> test, on most occasions you want to use a C<-f> |
293 | against the file first, as in C<next unless -f $file && -T $file>. | |
a0d0e21e | 294 | |
7660c0ab | 295 | If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given |
28757baa | 296 | the special filehandle consisting of a solitary underline, then the stat |
a0d0e21e LW |
297 | structure of the previous file test (or stat operator) is used, saving |
298 | a system call. (This doesn't work with C<-t>, and you need to remember | |
299 | that lstat() and C<-l> will leave values in the stat structure for the | |
300 | symbolic link, not the real file.) Example: | |
301 | ||
302 | print "Can do.\n" if -r $a || -w _ || -x _; | |
303 | ||
304 | stat($filename); | |
305 | print "Readable\n" if -r _; | |
306 | print "Writable\n" if -w _; | |
307 | print "Executable\n" if -x _; | |
308 | print "Setuid\n" if -u _; | |
309 | print "Setgid\n" if -g _; | |
310 | print "Sticky\n" if -k _; | |
311 | print "Text\n" if -T _; | |
312 | print "Binary\n" if -B _; | |
313 | ||
314 | =item abs VALUE | |
315 | ||
54310121 | 316 | =item abs |
bbce6d69 | 317 | |
a0d0e21e | 318 | Returns the absolute value of its argument. |
7660c0ab | 319 | If VALUE is omitted, uses C<$_>. |
a0d0e21e LW |
320 | |
321 | =item accept NEWSOCKET,GENERICSOCKET | |
322 | ||
7660c0ab | 323 | Accepts an incoming socket connect, just as the C<accept(2)> system call |
a0d0e21e | 324 | does. Returns the packed address if it succeeded, FALSE otherwise. |
4633a7c4 | 325 | See example in L<perlipc/"Sockets: Client/Server Communication">. |
a0d0e21e LW |
326 | |
327 | =item alarm SECONDS | |
328 | ||
54310121 | 329 | =item alarm |
bbce6d69 | 330 | |
a0d0e21e | 331 | Arranges to have a SIGALRM delivered to this process after the |
bbce6d69 | 332 | specified number of seconds have elapsed. If SECONDS is not specified, |
7660c0ab | 333 | the value stored in C<$_> is used. (On some machines, |
a0d0e21e LW |
334 | unfortunately, the elapsed time may be up to one second less than you |
335 | specified because of how seconds are counted.) Only one timer may be | |
336 | counting at once. Each call disables the previous timer, and an | |
7660c0ab | 337 | argument of C<0> may be supplied to cancel the previous timer without |
a0d0e21e LW |
338 | starting a new one. The returned value is the amount of time remaining |
339 | on the previous timer. | |
340 | ||
4633a7c4 | 341 | For delays of finer granularity than one second, you may use Perl's |
7660c0ab A |
342 | C<syscall()> interface to access C<setitimer(2)> if your system supports it, |
343 | or else see L</select()>. It is usually a mistake to intermix C<alarm()> | |
344 | and C<sleep()> calls. | |
a0d0e21e | 345 | |
7660c0ab | 346 | If you want to use C<alarm()> to time out a system call you need to use an |
ab4f32c2 | 347 | C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to |
5a964f20 | 348 | fail with C<$!> set to EINTR because Perl sets up signal handlers to |
ab4f32c2 | 349 | restart system calls on some systems. Using C<eval()>/C<die()> always works, |
5a964f20 | 350 | modulo the caveats given in L<perlipc/"Signals">. |
ff68c719 | 351 | |
352 | eval { | |
5a964f20 | 353 | local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required |
36477c24 | 354 | alarm $timeout; |
ff68c719 | 355 | $nread = sysread SOCKET, $buffer, $size; |
36477c24 | 356 | alarm 0; |
ff68c719 | 357 | }; |
ff68c719 | 358 | if ($@) { |
5a964f20 | 359 | die unless $@ eq "alarm\n"; # propagate unexpected errors |
ff68c719 | 360 | # timed out |
361 | } | |
362 | else { | |
363 | # didn't | |
364 | } | |
365 | ||
a0d0e21e LW |
366 | =item atan2 Y,X |
367 | ||
368 | Returns the arctangent of Y/X in the range -PI to PI. | |
369 | ||
7660c0ab | 370 | For the tangent operation, you may use the C<POSIX::tan()> |
28757baa | 371 | function, or use the familiar relation: |
372 | ||
373 | sub tan { sin($_[0]) / cos($_[0]) } | |
374 | ||
a0d0e21e LW |
375 | =item bind SOCKET,NAME |
376 | ||
377 | Binds a network address to a socket, just as the bind system call | |
378 | does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a | |
4633a7c4 LW |
379 | packed address of the appropriate type for the socket. See the examples in |
380 | L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e LW |
381 | |
382 | =item binmode FILEHANDLE | |
383 | ||
cb1a09d0 AD |
384 | Arranges for the file to be read or written in "binary" mode in operating |
385 | systems that distinguish between binary and text files. Files that are | |
386 | not in binary mode have CR LF sequences translated to LF on input and LF | |
54310121 | 387 | translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS |
cb1a09d0 | 388 | and similarly archaic systems, it may be imperative--otherwise your |
54310121 | 389 | MS-DOS-damaged C library may mangle your file. The key distinction between |
ab4f32c2 | 390 | systems that need C<binmode()> and those that don't is their text file |
5a964f20 | 391 | formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single |
7660c0ab | 392 | character, and that encode that character in C as C<"\n">, do not need |
ab4f32c2 | 393 | C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value |
cb1a09d0 | 394 | is taken as the name of the filehandle. |
a0d0e21e | 395 | |
4633a7c4 | 396 | =item bless REF,CLASSNAME |
a0d0e21e LW |
397 | |
398 | =item bless REF | |
399 | ||
28757baa | 400 | This function tells the thingy referenced by REF that it is now |
4633a7c4 LW |
401 | an object in the CLASSNAME package--or the current package if no CLASSNAME |
402 | is specified, which is often the case. It returns the reference for | |
7660c0ab | 403 | convenience, because a C<bless()> is often the last thing in a constructor. |
4633a7c4 | 404 | Always use the two-argument version if the function doing the blessing |
5a964f20 TC |
405 | might be inherited by a derived class. See L<perltoot> and L<perlobj> |
406 | for more about the blessing (and blessings) of objects. | |
a0d0e21e LW |
407 | |
408 | =item caller EXPR | |
409 | ||
410 | =item caller | |
411 | ||
5a964f20 | 412 | Returns the context of the current subroutine call. In scalar context, |
28757baa | 413 | returns the caller's package name if there is a caller, that is, if |
7660c0ab | 414 | we're in a subroutine or C<eval()> or C<require()>, and the undefined value |
5a964f20 | 415 | otherwise. In list context, returns |
a0d0e21e | 416 | |
748a9306 | 417 | ($package, $filename, $line) = caller; |
a0d0e21e LW |
418 | |
419 | With EXPR, it returns some extra information that the debugger uses to | |
420 | print a stack trace. The value of EXPR indicates how many call frames | |
421 | to go back before the current one. | |
422 | ||
54310121 | 423 | ($package, $filename, $line, $subroutine, |
e7ea3e70 IZ |
424 | $hasargs, $wantarray, $evaltext, $is_require) = caller($i); |
425 | ||
7660c0ab | 426 | Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine |
ab4f32c2 | 427 | call, but an C<eval()>. In such a case additional elements C<$evaltext> and |
7660c0ab A |
428 | C<$is_require> are set: C<$is_require> is true if the frame is created by a |
429 | C<require> or C<use> statement, C<$evaltext> contains the text of the | |
dc848c6f | 430 | C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement, |
7660c0ab | 431 | C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that |
dc848c6f | 432 | each C<use> statement creates a C<require> frame inside an C<eval EXPR>) |
433 | frame. | |
748a9306 LW |
434 | |
435 | Furthermore, when called from within the DB package, caller returns more | |
7660c0ab | 436 | detailed information: it sets the list variable C<@DB::args> to be the |
54310121 | 437 | arguments with which the subroutine was invoked. |
748a9306 | 438 | |
7660c0ab | 439 | Be aware that the optimizer might have optimized call frames away before |
ab4f32c2 | 440 | C<caller()> had a chance to get the information. That means that C<caller(N)> |
7660c0ab A |
441 | might not return information about the call frame you expect it do, for |
442 | C<N E<gt> 1>. In particular, C<@DB::args> might have information from the | |
443 | previous time C<caller()> was called. | |
444 | ||
a0d0e21e LW |
445 | =item chdir EXPR |
446 | ||
447 | Changes the working directory to EXPR, if possible. If EXPR is | |
448 | omitted, changes to home directory. Returns TRUE upon success, FALSE | |
7660c0ab | 449 | otherwise. See example under C<die()>. |
a0d0e21e LW |
450 | |
451 | =item chmod LIST | |
452 | ||
453 | Changes the permissions of a list of files. The first element of the | |
4633a7c4 | 454 | list must be the numerical mode, which should probably be an octal |
2f9daede TP |
455 | number, and which definitely should I<not> a string of octal digits: |
456 | C<0644> is okay, C<'0644'> is not. Returns the number of files | |
dc848c6f | 457 | successfully changed. See also L</oct>, if all you have is a string. |
a0d0e21e LW |
458 | |
459 | $cnt = chmod 0755, 'foo', 'bar'; | |
460 | chmod 0755, @executables; | |
2f9daede TP |
461 | $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to --w----r-T |
462 | $mode = '0644'; chmod oct($mode), 'foo'; # this is better | |
463 | $mode = 0644; chmod $mode, 'foo'; # this is best | |
a0d0e21e LW |
464 | |
465 | =item chomp VARIABLE | |
466 | ||
467 | =item chomp LIST | |
468 | ||
469 | =item chomp | |
470 | ||
3e3baf6d | 471 | This is a slightly safer version of L</chop>. It removes any |
a0d0e21e | 472 | line ending that corresponds to the current value of C<$/> (also known as |
28757baa | 473 | $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total |
474 | number of characters removed from all its arguments. It's often used to | |
475 | remove the newline from the end of an input record when you're worried | |
476 | that the final record may be missing its newline. When in paragraph mode | |
477 | (C<$/ = "">), it removes all trailing newlines from the string. If | |
7660c0ab | 478 | VARIABLE is omitted, it chomps C<$_>. Example: |
a0d0e21e LW |
479 | |
480 | while (<>) { | |
481 | chomp; # avoid \n on last field | |
482 | @array = split(/:/); | |
5a964f20 | 483 | # ... |
a0d0e21e LW |
484 | } |
485 | ||
486 | You can actually chomp anything that's an lvalue, including an assignment: | |
487 | ||
488 | chomp($cwd = `pwd`); | |
489 | chomp($answer = <STDIN>); | |
490 | ||
491 | If you chomp a list, each element is chomped, and the total number of | |
492 | characters removed is returned. | |
493 | ||
494 | =item chop VARIABLE | |
495 | ||
496 | =item chop LIST | |
497 | ||
498 | =item chop | |
499 | ||
500 | Chops off the last character of a string and returns the character | |
501 | chopped. It's used primarily to remove the newline from the end of an | |
502 | input record, but is much more efficient than C<s/\n//> because it neither | |
7660c0ab | 503 | scans nor copies the string. If VARIABLE is omitted, chops C<$_>. |
a0d0e21e LW |
504 | Example: |
505 | ||
506 | while (<>) { | |
507 | chop; # avoid \n on last field | |
508 | @array = split(/:/); | |
5a964f20 | 509 | #... |
a0d0e21e LW |
510 | } |
511 | ||
512 | You can actually chop anything that's an lvalue, including an assignment: | |
513 | ||
514 | chop($cwd = `pwd`); | |
515 | chop($answer = <STDIN>); | |
516 | ||
517 | If you chop a list, each element is chopped. Only the value of the | |
ab4f32c2 | 518 | last C<chop()> is returned. |
a0d0e21e | 519 | |
ab4f32c2 | 520 | Note that C<chop()> returns the last character. To return all but the last |
748a9306 LW |
521 | character, use C<substr($string, 0, -1)>. |
522 | ||
a0d0e21e LW |
523 | =item chown LIST |
524 | ||
525 | Changes the owner (and group) of a list of files. The first two | |
526 | elements of the list must be the I<NUMERICAL> uid and gid, in that order. | |
527 | Returns the number of files successfully changed. | |
528 | ||
529 | $cnt = chown $uid, $gid, 'foo', 'bar'; | |
530 | chown $uid, $gid, @filenames; | |
531 | ||
54310121 | 532 | Here's an example that looks up nonnumeric uids in the passwd file: |
a0d0e21e LW |
533 | |
534 | print "User: "; | |
535 | chop($user = <STDIN>); | |
5a964f20 | 536 | print "Files: "; |
a0d0e21e LW |
537 | chop($pattern = <STDIN>); |
538 | ||
539 | ($login,$pass,$uid,$gid) = getpwnam($user) | |
540 | or die "$user not in passwd file"; | |
541 | ||
5a964f20 | 542 | @ary = glob($pattern); # expand filenames |
a0d0e21e LW |
543 | chown $uid, $gid, @ary; |
544 | ||
54310121 | 545 | On most systems, you are not allowed to change the ownership of the |
4633a7c4 LW |
546 | file unless you're the superuser, although you should be able to change |
547 | the group to any of your secondary groups. On insecure systems, these | |
548 | restrictions may be relaxed, but this is not a portable assumption. | |
549 | ||
a0d0e21e LW |
550 | =item chr NUMBER |
551 | ||
54310121 | 552 | =item chr |
bbce6d69 | 553 | |
a0d0e21e | 554 | Returns the character represented by that NUMBER in the character set. |
7660c0ab | 555 | For example, C<chr(65)> is C<"A"> in ASCII. For the reverse, use L</ord>. |
a0d0e21e | 556 | |
7660c0ab | 557 | If NUMBER is omitted, uses C<$_>. |
bbce6d69 | 558 | |
a0d0e21e LW |
559 | =item chroot FILENAME |
560 | ||
54310121 | 561 | =item chroot |
bbce6d69 | 562 | |
5a964f20 | 563 | This function works like the system call by the same name: it makes the |
4633a7c4 | 564 | named directory the new root directory for all further pathnames that |
7660c0ab | 565 | begin with a C<"/"> by your process and all its children. (It doesn't |
28757baa | 566 | change your current working directory, which is unaffected.) For security |
4633a7c4 | 567 | reasons, this call is restricted to the superuser. If FILENAME is |
ab4f32c2 | 568 | omitted, does a C<chroot()> to C<$_>. |
a0d0e21e LW |
569 | |
570 | =item close FILEHANDLE | |
571 | ||
6a518fbc TP |
572 | =item close |
573 | ||
a0d0e21e LW |
574 | Closes the file or pipe associated with the file handle, returning TRUE |
575 | only if stdio successfully flushes buffers and closes the system file | |
6a518fbc TP |
576 | descriptor. Closes the currently selected filehandle if the argument |
577 | is omitted. | |
fb73857a | 578 | |
579 | You don't have to close FILEHANDLE if you are immediately going to do | |
7660c0ab | 580 | another C<open()> on it, because C<open()> will close it for you. (See |
ab4f32c2 | 581 | C<open()>.) However, an explicit C<close()> on an input file resets the line |
7660c0ab | 582 | counter (C<$.>), while the implicit close done by C<open()> does not. |
fb73857a | 583 | |
ab4f32c2 | 584 | If the file handle came from a piped open C<close()> will additionally |
fb73857a | 585 | return FALSE if one of the other system calls involved fails or if the |
586 | program exits with non-zero status. (If the only problem was that the | |
7660c0ab | 587 | program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe |
5a964f20 | 588 | waits for the process executing on the pipe to complete, in case you |
fb73857a | 589 | want to look at the output of the pipe afterwards. Closing a pipe |
590 | explicitly also puts the exit status value of the command into C<$?>. | |
5a964f20 | 591 | |
fb73857a | 592 | Example: |
a0d0e21e | 593 | |
fb73857a | 594 | open(OUTPUT, '|sort >foo') # pipe to sort |
595 | or die "Can't start sort: $!"; | |
5a964f20 | 596 | #... # print stuff to output |
fb73857a | 597 | close OUTPUT # wait for sort to finish |
598 | or warn $! ? "Error closing sort pipe: $!" | |
599 | : "Exit status $? from sort"; | |
600 | open(INPUT, 'foo') # get sort's results | |
601 | or die "Can't open 'foo' for input: $!"; | |
a0d0e21e | 602 | |
5a964f20 TC |
603 | FILEHANDLE may be an expression whose value can be used as an indirect |
604 | filehandle, usually the real filehandle name. | |
a0d0e21e LW |
605 | |
606 | =item closedir DIRHANDLE | |
607 | ||
7660c0ab | 608 | Closes a directory opened by C<opendir()> and returns the success of that |
5a964f20 TC |
609 | system call. |
610 | ||
611 | DIRHANDLE may be an expression whose value can be used as an indirect | |
612 | dirhandle, usually the real dirhandle name. | |
a0d0e21e LW |
613 | |
614 | =item connect SOCKET,NAME | |
615 | ||
616 | Attempts to connect to a remote socket, just as the connect system call | |
617 | does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a | |
4633a7c4 LW |
618 | packed address of the appropriate type for the socket. See the examples in |
619 | L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e | 620 | |
cb1a09d0 AD |
621 | =item continue BLOCK |
622 | ||
623 | Actually a flow control statement rather than a function. If there is a | |
624 | C<continue> BLOCK attached to a BLOCK (typically in a C<while> or | |
625 | C<foreach>), it is always executed just before the conditional is about to | |
626 | be evaluated again, just like the third part of a C<for> loop in C. Thus | |
627 | it can be used to increment a loop variable, even when the loop has been | |
628 | continued via the C<next> statement (which is similar to the C C<continue> | |
629 | statement). | |
630 | ||
1d2dff63 GS |
631 | C<last>, C<next>, or C<redo> may appear within a C<continue> |
632 | block. C<last> and C<redo> will behave as if they had been executed within | |
633 | the main block. So will C<next>, but since it will execute a C<continue> | |
634 | block, it may be more entertaining. | |
635 | ||
636 | while (EXPR) { | |
637 | ### redo always comes here | |
638 | do_something; | |
639 | } continue { | |
640 | ### next always comes here | |
641 | do_something_else; | |
642 | # then back the top to re-check EXPR | |
643 | } | |
644 | ### last always comes here | |
645 | ||
646 | Omitting the C<continue> section is semantically equivalent to using an | |
647 | empty one, logically enough. In that case, C<next> goes directly back | |
648 | to check the condition at the top of the loop. | |
649 | ||
a0d0e21e LW |
650 | =item cos EXPR |
651 | ||
5a964f20 | 652 | Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, |
7660c0ab | 653 | takes cosine of C<$_>. |
a0d0e21e | 654 | |
7660c0ab | 655 | For the inverse cosine operation, you may use the C<POSIX::acos()> |
28757baa | 656 | function, or use this relation: |
657 | ||
658 | sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } | |
659 | ||
a0d0e21e LW |
660 | =item crypt PLAINTEXT,SALT |
661 | ||
7660c0ab | 662 | Encrypts a string exactly like the C<crypt(3)> function in the C library |
4633a7c4 LW |
663 | (assuming that you actually have a version there that has not been |
664 | extirpated as a potential munition). This can prove useful for checking | |
665 | the password file for lousy passwords, amongst other things. Only the | |
666 | guys wearing white hats should do this. | |
a0d0e21e | 667 | |
7660c0ab | 668 | Note that C<crypt()> is intended to be a one-way function, much like breaking |
11155c91 CS |
669 | eggs to make an omelette. There is no (known) corresponding decrypt |
670 | function. As a result, this function isn't all that useful for | |
671 | cryptography. (For that, see your nearby CPAN mirror.) | |
2f9daede | 672 | |
a0d0e21e LW |
673 | Here's an example that makes sure that whoever runs this program knows |
674 | their own password: | |
675 | ||
676 | $pwd = (getpwuid($<))[1]; | |
677 | $salt = substr($pwd, 0, 2); | |
678 | ||
679 | system "stty -echo"; | |
680 | print "Password: "; | |
681 | chop($word = <STDIN>); | |
682 | print "\n"; | |
683 | system "stty echo"; | |
684 | ||
685 | if (crypt($word, $salt) ne $pwd) { | |
686 | die "Sorry...\n"; | |
687 | } else { | |
688 | print "ok\n"; | |
54310121 | 689 | } |
a0d0e21e | 690 | |
9f8f0c9d | 691 | Of course, typing in your own password to whoever asks you |
748a9306 | 692 | for it is unwise. |
a0d0e21e | 693 | |
aa689395 | 694 | =item dbmclose HASH |
a0d0e21e | 695 | |
7660c0ab | 696 | [This function has been superseded by the C<untie()> function.] |
a0d0e21e | 697 | |
aa689395 | 698 | Breaks the binding between a DBM file and a hash. |
a0d0e21e | 699 | |
aa689395 | 700 | =item dbmopen HASH,DBNAME,MODE |
a0d0e21e | 701 | |
7660c0ab | 702 | [This function has been superseded by the C<tie()> function.] |
a0d0e21e | 703 | |
7b8d334a | 704 | This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a |
ab4f32c2 | 705 | hash. HASH is the name of the hash. (Unlike normal C<open()>, the first |
aa689395 | 706 | argument is I<NOT> a filehandle, even though it looks like one). DBNAME |
707 | is the name of the database (without the F<.dir> or F<.pag> extension if | |
708 | any). If the database does not exist, it is created with protection | |
7660c0ab A |
709 | specified by MODE (as modified by the C<umask()>). If your system supports |
710 | only the older DBM functions, you may perform only one C<dbmopen()> in your | |
aa689395 | 711 | program. In older versions of Perl, if your system had neither DBM nor |
7660c0ab | 712 | ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to |
aa689395 | 713 | sdbm(3). |
714 | ||
715 | If you don't have write access to the DBM file, you can only read hash | |
716 | variables, not set them. If you want to test whether you can write, | |
7660c0ab | 717 | either use file tests or try setting a dummy hash entry inside an C<eval()>, |
aa689395 | 718 | which will trap the error. |
a0d0e21e | 719 | |
7660c0ab A |
720 | Note that functions such as C<keys()> and C<values()> may return huge lists |
721 | when used on large DBM files. You may prefer to use the C<each()> | |
a0d0e21e LW |
722 | function to iterate over large DBM files. Example: |
723 | ||
724 | # print out history file offsets | |
725 | dbmopen(%HIST,'/usr/lib/news/history',0666); | |
726 | while (($key,$val) = each %HIST) { | |
727 | print $key, ' = ', unpack('L',$val), "\n"; | |
728 | } | |
729 | dbmclose(%HIST); | |
730 | ||
cb1a09d0 | 731 | See also L<AnyDBM_File> for a more general description of the pros and |
184e9718 | 732 | cons of the various dbm approaches, as well as L<DB_File> for a particularly |
cb1a09d0 | 733 | rich implementation. |
4633a7c4 | 734 | |
a0d0e21e LW |
735 | =item defined EXPR |
736 | ||
54310121 | 737 | =item defined |
bbce6d69 | 738 | |
2f9daede TP |
739 | Returns a Boolean value telling whether EXPR has a value other than |
740 | the undefined value C<undef>. If EXPR is not present, C<$_> will be | |
741 | checked. | |
742 | ||
743 | Many operations return C<undef> to indicate failure, end of file, | |
744 | system error, uninitialized variable, and other exceptional | |
745 | conditions. This function allows you to distinguish C<undef> from | |
746 | other values. (A simple Boolean test will not distinguish among | |
7660c0ab | 747 | C<undef>, zero, the empty string, and C<"0">, which are all equally |
2f9daede | 748 | false.) Note that since C<undef> is a valid scalar, its presence |
7660c0ab | 749 | doesn't I<necessarily> indicate an exceptional condition: C<pop()> |
2f9daede TP |
750 | returns C<undef> when its argument is an empty array, I<or> when the |
751 | element to return happens to be C<undef>. | |
752 | ||
7660c0ab | 753 | You may also use C<defined()> to check whether a subroutine exists, by |
5a964f20 | 754 | saying C<defined &func> without parentheses. On the other hand, use |
7660c0ab | 755 | of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to |
5a964f20 | 756 | produce intuitive results, and should probably be avoided. |
2f9daede TP |
757 | |
758 | When used on a hash element, it tells you whether the value is defined, | |
dc848c6f | 759 | not whether the key exists in the hash. Use L</exists> for the latter |
2f9daede | 760 | purpose. |
a0d0e21e LW |
761 | |
762 | Examples: | |
763 | ||
764 | print if defined $switch{'D'}; | |
765 | print "$val\n" while defined($val = pop(@ary)); | |
766 | die "Can't readlink $sym: $!" | |
767 | unless defined($value = readlink $sym); | |
a0d0e21e | 768 | sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } |
2f9daede | 769 | $debugging = 0 unless defined $debugging; |
a0d0e21e | 770 | |
7660c0ab A |
771 | Note: Many folks tend to overuse C<defined()>, and then are surprised to |
772 | discover that the number C<0> and C<""> (the zero-length string) are, in fact, | |
2f9daede | 773 | defined values. For example, if you say |
a5f75d66 AD |
774 | |
775 | "ab" =~ /a(.*)b/; | |
776 | ||
7660c0ab | 777 | The pattern match succeeds, and C<$1> is defined, despite the fact that it |
a5f75d66 | 778 | matched "nothing". But it didn't really match nothing--rather, it |
7660c0ab | 779 | matched something that happened to be C<0> characters long. This is all |
a5f75d66 | 780 | very above-board and honest. When a function returns an undefined value, |
2f9daede | 781 | it's an admission that it couldn't give you an honest answer. So you |
7660c0ab A |
782 | should use C<defined()> only when you're questioning the integrity of what |
783 | you're trying to do. At other times, a simple comparison to C<0> or C<""> is | |
2f9daede TP |
784 | what you want. |
785 | ||
7660c0ab | 786 | Currently, using C<defined()> on an entire array or hash reports whether |
2f9daede TP |
787 | memory for that aggregate has ever been allocated. So an array you set |
788 | to the empty list appears undefined initially, and one that once was full | |
789 | and that you then set to the empty list still appears defined. You | |
790 | should instead use a simple test for size: | |
28757baa | 791 | |
792 | if (@an_array) { print "has array elements\n" } | |
793 | if (%a_hash) { print "has hash members\n" } | |
794 | ||
7660c0ab | 795 | Using C<undef()> on these, however, does clear their memory and then report |
5a964f20 | 796 | them as not defined anymore, but you shouldn't do that unless you don't |
28757baa | 797 | plan to use them again, because it saves time when you load them up |
5a964f20 TC |
798 | again to have memory already ready to be filled. The normal way to |
799 | free up space used by an aggregate is to assign the empty list. | |
28757baa | 800 | |
7660c0ab | 801 | This counterintuitive behavior of C<defined()> on aggregates may be |
28757baa | 802 | changed, fixed, or broken in a future release of Perl. |
803 | ||
dc848c6f | 804 | See also L</undef>, L</exists>, L</ref>. |
2f9daede | 805 | |
a0d0e21e LW |
806 | =item delete EXPR |
807 | ||
aa689395 | 808 | Deletes the specified key(s) and their associated values from a hash. |
809 | For each key, returns the deleted value associated with that key, or | |
810 | the undefined value if there was no such key. Deleting from C<$ENV{}> | |
811 | modifies the environment. Deleting from a hash tied to a DBM file | |
7660c0ab | 812 | deletes the entry from the DBM file. (But deleting from a C<tie()>d hash |
5f05dabc | 813 | doesn't necessarily return anything.) |
a0d0e21e | 814 | |
aa689395 | 815 | The following deletes all the values of a hash: |
a0d0e21e | 816 | |
5f05dabc | 817 | foreach $key (keys %HASH) { |
818 | delete $HASH{$key}; | |
a0d0e21e LW |
819 | } |
820 | ||
5f05dabc | 821 | And so does this: |
822 | ||
823 | delete @HASH{keys %HASH} | |
824 | ||
5a964f20 | 825 | (But both of these are slower than just assigning the empty list, or |
7660c0ab | 826 | using C<undef()>.) Note that the EXPR can be arbitrarily complicated as |
5a964f20 | 827 | long as the final operation is a hash element lookup or hash slice: |
a0d0e21e LW |
828 | |
829 | delete $ref->[$x][$y]{$key}; | |
5f05dabc | 830 | delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; |
a0d0e21e LW |
831 | |
832 | =item die LIST | |
833 | ||
7660c0ab A |
834 | Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with |
835 | the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of | |
54310121 | 836 | C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)> |
7660c0ab A |
837 | is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into |
838 | C<$@> and the C<eval()> is terminated with the undefined value. This makes | |
839 | C<die()> the way to raise an exception. | |
a0d0e21e LW |
840 | |
841 | Equivalent examples: | |
842 | ||
843 | die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; | |
54310121 | 844 | chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" |
a0d0e21e LW |
845 | |
846 | If the value of EXPR does not end in a newline, the current script line | |
847 | number and input line number (if any) are also printed, and a newline | |
7660c0ab A |
848 | is supplied. Hint: sometimes appending C<", stopped"> to your message |
849 | will cause it to make better sense when the string C<"at foo line 123"> is | |
a0d0e21e LW |
850 | appended. Suppose you are running script "canasta". |
851 | ||
852 | die "/etc/games is no good"; | |
853 | die "/etc/games is no good, stopped"; | |
854 | ||
855 | produce, respectively | |
856 | ||
857 | /etc/games is no good at canasta line 123. | |
858 | /etc/games is no good, stopped at canasta line 123. | |
859 | ||
7660c0ab | 860 | See also C<exit()> and C<warn()>. |
a0d0e21e | 861 | |
7660c0ab A |
862 | If LIST is empty and C<$@> already contains a value (typically from a |
863 | previous eval) that value is reused after appending C<"\t...propagated">. | |
fb73857a | 864 | This is useful for propagating exceptions: |
865 | ||
866 | eval { ... }; | |
867 | die unless $@ =~ /Expected exception/; | |
868 | ||
7660c0ab | 869 | If C<$@> is empty then the string C<"Died"> is used. |
fb73857a | 870 | |
7660c0ab | 871 | You can arrange for a callback to be run just before the C<die()> does |
774d564b | 872 | its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler |
873 | will be called with the error text and can change the error message, if | |
7660c0ab | 874 | it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on |
fb73857a | 875 | setting C<%SIG> entries, and L<"eval BLOCK"> for some examples. |
876 | ||
877 | Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed | |
878 | blocks/strings. If one wants the hook to do nothing in such | |
879 | situations, put | |
880 | ||
881 | die @_ if $^S; | |
882 | ||
883 | as the first line of the handler (see L<perlvar/$^S>). | |
774d564b | 884 | |
a0d0e21e LW |
885 | =item do BLOCK |
886 | ||
887 | Not really a function. Returns the value of the last command in the | |
888 | sequence of commands indicated by BLOCK. When modified by a loop | |
889 | modifier, executes the BLOCK once before testing the loop condition. | |
890 | (On other statements the loop modifiers test the conditional first.) | |
891 | ||
892 | =item do SUBROUTINE(LIST) | |
893 | ||
894 | A deprecated form of subroutine call. See L<perlsub>. | |
895 | ||
896 | =item do EXPR | |
897 | ||
898 | Uses the value of EXPR as a filename and executes the contents of the | |
899 | file as a Perl script. Its primary use is to include subroutines | |
900 | from a Perl subroutine library. | |
901 | ||
902 | do 'stat.pl'; | |
903 | ||
904 | is just like | |
905 | ||
fb73857a | 906 | scalar eval `cat stat.pl`; |
a0d0e21e | 907 | |
5a964f20 | 908 | except that it's more efficient and concise, keeps track of the |
a0d0e21e LW |
909 | current filename for error messages, and searches all the B<-I> |
910 | libraries if the file isn't in the current directory (see also the @INC | |
dc1be6b5 GS |
911 | array in L<perlvar/Predefined Names>). It is also different in how |
912 | code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing | |
913 | scope like C<eval STRING> does. It's the same, however, in that it does | |
54310121 | 914 | reparse the file every time you call it, so you probably don't want to |
a0d0e21e LW |
915 | do this inside a loop. |
916 | ||
917 | Note that inclusion of library modules is better done with the | |
7660c0ab | 918 | C<use()> and C<require()> operators, which also do automatic error checking |
4633a7c4 | 919 | and raise an exception if there's a problem. |
a0d0e21e | 920 | |
5a964f20 TC |
921 | You might like to use C<do> to read in a program configuration |
922 | file. Manual error checking can be done this way: | |
923 | ||
924 | # read in config files: system first, then user | |
925 | for $file ('/share/prog/defaults.rc", "$ENV{HOME}/.someprogrc") { | |
926 | unless ($return = do $file) { | |
927 | warn "couldn't parse $file: $@" if $@; | |
928 | warn "couldn't do $file: $!" unless defined $return; | |
929 | warn "couldn't run $file" unless $return; | |
930 | } | |
931 | } | |
932 | ||
a0d0e21e LW |
933 | =item dump LABEL |
934 | ||
935 | This causes an immediate core dump. Primarily this is so that you can | |
936 | use the B<undump> program to turn your core dump into an executable binary | |
937 | after having initialized all your variables at the beginning of the | |
938 | program. When the new binary is executed it will begin by executing a | |
939 | C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of | |
940 | it as a goto with an intervening core dump and reincarnation. If LABEL | |
5a964f20 | 941 | is omitted, restarts the program from the top. WARNING: Any files |
a0d0e21e LW |
942 | opened at the time of the dump will NOT be open any more when the |
943 | program is reincarnated, with possible resulting confusion on the part | |
944 | of Perl. See also B<-u> option in L<perlrun>. | |
945 | ||
946 | Example: | |
947 | ||
948 | #!/usr/bin/perl | |
949 | require 'getopt.pl'; | |
950 | require 'stat.pl'; | |
951 | %days = ( | |
952 | 'Sun' => 1, | |
953 | 'Mon' => 2, | |
954 | 'Tue' => 3, | |
955 | 'Wed' => 4, | |
956 | 'Thu' => 5, | |
957 | 'Fri' => 6, | |
958 | 'Sat' => 7, | |
959 | ); | |
960 | ||
961 | dump QUICKSTART if $ARGV[0] eq '-d'; | |
962 | ||
963 | QUICKSTART: | |
964 | Getopt('f'); | |
965 | ||
5a964f20 TC |
966 | This operator is largely obsolete, partly because it's very hard to |
967 | convert a core file into an executable, and because the real perl-to-C | |
968 | compiler has superseded it. | |
969 | ||
aa689395 | 970 | =item each HASH |
971 | ||
5a964f20 | 972 | When called in list context, returns a 2-element list consisting of the |
aa689395 | 973 | key and value for the next element of a hash, so that you can iterate over |
5a964f20 | 974 | it. When called in scalar context, returns the key for only the "next" |
7660c0ab | 975 | element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically |
2f9daede TP |
976 | false; you may wish to avoid constructs like C<while ($k = each %foo) {}> |
977 | for this reason.) | |
978 | ||
979 | Entries are returned in an apparently random order. When the hash is | |
980 | entirely read, a null array is returned in list context (which when | |
7660c0ab A |
981 | assigned produces a FALSE (C<0>) value), and C<undef> in |
982 | scalar context. The next call to C<each()> after that will start iterating | |
983 | again. There is a single iterator for each hash, shared by all C<each()>, | |
984 | C<keys()>, and C<values()> function calls in the program; it can be reset by | |
2f9daede TP |
985 | reading all the elements from the hash, or by evaluating C<keys HASH> or |
986 | C<values HASH>. If you add or delete elements of a hash while you're | |
987 | iterating over it, you may get entries skipped or duplicated, so don't. | |
aa689395 | 988 | |
7660c0ab | 989 | The following prints out your environment like the C<printenv(1)> program, |
aa689395 | 990 | only in a different order: |
a0d0e21e LW |
991 | |
992 | while (($key,$value) = each %ENV) { | |
993 | print "$key=$value\n"; | |
994 | } | |
995 | ||
7660c0ab | 996 | See also C<keys()> and C<values()>. |
a0d0e21e LW |
997 | |
998 | =item eof FILEHANDLE | |
999 | ||
4633a7c4 LW |
1000 | =item eof () |
1001 | ||
a0d0e21e LW |
1002 | =item eof |
1003 | ||
1004 | Returns 1 if the next read on FILEHANDLE will return end of file, or if | |
1005 | FILEHANDLE is not open. FILEHANDLE may be an expression whose value | |
5a964f20 | 1006 | gives the real filehandle. (Note that this function actually |
7660c0ab | 1007 | reads a character and then C<ungetc()>s it, so isn't very useful in an |
748a9306 LW |
1008 | interactive context.) Do not read from a terminal file (or call |
1009 | C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such | |
1010 | as terminals may lose the end-of-file condition if you do. | |
1011 | ||
1012 | An C<eof> without an argument uses the last file read as argument. | |
5a964f20 | 1013 | Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of |
2f9daede TP |
1014 | the files listed on the command line, i.e., C<eof()> is reasonable to |
1015 | use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the | |
1016 | last file. Use C<eof(ARGV)> or eof without the parentheses to test | |
1017 | I<EACH> file in a while (E<lt>E<gt>) loop. Examples: | |
a0d0e21e | 1018 | |
748a9306 LW |
1019 | # reset line numbering on each input file |
1020 | while (<>) { | |
5a964f20 | 1021 | next if /^\s*#/; # skip comments |
748a9306 | 1022 | print "$.\t$_"; |
5a964f20 TC |
1023 | } continue { |
1024 | close ARGV if eof; # Not eof()! | |
748a9306 LW |
1025 | } |
1026 | ||
a0d0e21e LW |
1027 | # insert dashes just before last line of last file |
1028 | while (<>) { | |
5a964f20 | 1029 | if (eof()) { # check for end of current file |
a0d0e21e | 1030 | print "--------------\n"; |
748a9306 LW |
1031 | close(ARGV); # close or break; is needed if we |
1032 | # are reading from the terminal | |
a0d0e21e LW |
1033 | } |
1034 | print; | |
1035 | } | |
1036 | ||
a0d0e21e | 1037 | Practical hint: you almost never need to use C<eof> in Perl, because the |
3b02c43c GS |
1038 | input operators return false values when they run out of data, or if there |
1039 | was an error. | |
a0d0e21e LW |
1040 | |
1041 | =item eval EXPR | |
1042 | ||
1043 | =item eval BLOCK | |
1044 | ||
c7cc6f1c GS |
1045 | In the first form, the return value of EXPR is parsed and executed as if it |
1046 | were a little Perl program. The value of the expression (which is itself | |
5a964f20 | 1047 | determined within scalar context) is first parsed, and if there weren't any |
c7cc6f1c | 1048 | errors, executed in the context of the current Perl program, so that any |
5f05dabc | 1049 | variable settings or subroutine and format definitions remain afterwards. |
c7cc6f1c GS |
1050 | Note that the value is parsed every time the eval executes. If EXPR is |
1051 | omitted, evaluates C<$_>. This form is typically used to delay parsing | |
1052 | and subsequent execution of the text of EXPR until run time. | |
1053 | ||
1054 | In the second form, the code within the BLOCK is parsed only once--at the | |
1055 | same time the code surrounding the eval itself was parsed--and executed | |
1056 | within the context of the current Perl program. This form is typically | |
1057 | used to trap exceptions more efficiently than the first (see below), while | |
1058 | also providing the benefit of checking the code within BLOCK at compile | |
1059 | time. | |
1060 | ||
1061 | The final semicolon, if any, may be omitted from the value of EXPR or within | |
1062 | the BLOCK. | |
1063 | ||
1064 | In both forms, the value returned is the value of the last expression | |
5a964f20 | 1065 | evaluated inside the mini-program; a return statement may be also used, just |
c7cc6f1c | 1066 | as with subroutines. The expression providing the return value is evaluated |
5a964f20 | 1067 | in void, scalar, or list context, depending on the context of the eval itself. |
c7cc6f1c | 1068 | See L</wantarray> for more on how the evaluation context can be determined. |
a0d0e21e | 1069 | |
7660c0ab A |
1070 | If there is a syntax error or runtime error, or a C<die()> statement is |
1071 | executed, an undefined value is returned by C<eval()>, and C<$@> is set to the | |
a0d0e21e | 1072 | error message. If there was no error, C<$@> is guaranteed to be a null |
7660c0ab | 1073 | string. Beware that using C<eval()> neither silences perl from printing |
c7cc6f1c GS |
1074 | warnings to STDERR, nor does it stuff the text of warning messages into C<$@>. |
1075 | To do either of those, you have to use the C<$SIG{__WARN__}> facility. See | |
1076 | L</warn> and L<perlvar>. | |
a0d0e21e | 1077 | |
7660c0ab A |
1078 | Note that, because C<eval()> traps otherwise-fatal errors, it is useful for |
1079 | determining whether a particular feature (such as C<socket()> or C<symlink()>) | |
a0d0e21e LW |
1080 | is implemented. It is also Perl's exception trapping mechanism, where |
1081 | the die operator is used to raise exceptions. | |
1082 | ||
1083 | If the code to be executed doesn't vary, you may use the eval-BLOCK | |
1084 | form to trap run-time errors without incurring the penalty of | |
1085 | recompiling each time. The error, if any, is still returned in C<$@>. | |
1086 | Examples: | |
1087 | ||
54310121 | 1088 | # make divide-by-zero nonfatal |
a0d0e21e LW |
1089 | eval { $answer = $a / $b; }; warn $@ if $@; |
1090 | ||
1091 | # same thing, but less efficient | |
1092 | eval '$answer = $a / $b'; warn $@ if $@; | |
1093 | ||
1094 | # a compile-time error | |
5a964f20 | 1095 | eval { $answer = }; # WRONG |
a0d0e21e LW |
1096 | |
1097 | # a run-time error | |
1098 | eval '$answer ='; # sets $@ | |
1099 | ||
7660c0ab | 1100 | When using the C<eval{}> form as an exception trap in libraries, you may |
774d564b | 1101 | wish not to trigger any C<__DIE__> hooks that user code may have |
1102 | installed. You can use the C<local $SIG{__DIE__}> construct for this | |
1103 | purpose, as shown in this example: | |
1104 | ||
1105 | # a very private exception trap for divide-by-zero | |
1106 | eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@; | |
1107 | ||
1108 | This is especially significant, given that C<__DIE__> hooks can call | |
7660c0ab | 1109 | C<die()> again, which has the effect of changing their error messages: |
774d564b | 1110 | |
1111 | # __DIE__ hooks may modify error messages | |
1112 | { | |
1113 | local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; | |
c7cc6f1c GS |
1114 | eval { die "foo lives here" }; |
1115 | print $@ if $@; # prints "bar lives here" | |
774d564b | 1116 | } |
1117 | ||
7660c0ab | 1118 | With an C<eval()>, you should be especially careful to remember what's |
a0d0e21e LW |
1119 | being looked at when: |
1120 | ||
1121 | eval $x; # CASE 1 | |
1122 | eval "$x"; # CASE 2 | |
1123 | ||
1124 | eval '$x'; # CASE 3 | |
1125 | eval { $x }; # CASE 4 | |
1126 | ||
5a964f20 | 1127 | eval "\$$x++"; # CASE 5 |
a0d0e21e LW |
1128 | $$x++; # CASE 6 |
1129 | ||
2f9daede | 1130 | Cases 1 and 2 above behave identically: they run the code contained in |
7660c0ab | 1131 | the variable C<$x>. (Although case 2 has misleading double quotes making |
2f9daede | 1132 | the reader wonder what else might be happening (nothing is).) Cases 3 |
7660c0ab | 1133 | and 4 likewise behave in the same way: they run the code C<'$x'>, which |
2f9daede TP |
1134 | does nothing but return the value of C<$x>. (Case 4 is preferred for |
1135 | purely visual reasons, but it also has the advantage of compiling at | |
1136 | compile-time instead of at run-time.) Case 5 is a place where | |
54310121 | 1137 | normally you I<WOULD> like to use double quotes, except that in this |
2f9daede TP |
1138 | particular situation, you can just use symbolic references instead, as |
1139 | in case 6. | |
a0d0e21e LW |
1140 | |
1141 | =item exec LIST | |
1142 | ||
8bf3b016 GS |
1143 | =item exec PROGRAM LIST |
1144 | ||
7660c0ab A |
1145 | The C<exec()> function executes a system command I<AND NEVER RETURNS> - |
1146 | use C<system()> instead of C<exec()> if you want it to return. It fails and | |
fb73857a | 1147 | returns FALSE only if the command does not exist I<and> it is executed |
1148 | directly instead of via your system's command shell (see below). | |
a0d0e21e | 1149 | |
7660c0ab A |
1150 | Since it's a common mistake to use C<exec()> instead of C<system()>, Perl |
1151 | warns you if there is a following statement which isn't C<die()>, C<warn()>, | |
1152 | or C<exit()> (if C<-w> is set - but you always do that). If you | |
1153 | I<really> want to follow an C<exec()> with some other statement, you | |
55d729e4 GS |
1154 | can use one of these styles to avoid the warning: |
1155 | ||
5a964f20 TC |
1156 | exec ('foo') or print STDERR "couldn't exec foo: $!"; |
1157 | { exec ('foo') }; print STDERR "couldn't exec foo: $!"; | |
55d729e4 | 1158 | |
5a964f20 | 1159 | If there is more than one argument in LIST, or if LIST is an array |
7660c0ab | 1160 | with more than one value, calls C<execvp(3)> with the arguments in LIST. |
5a964f20 TC |
1161 | If there is only one scalar argument or an array with one element in it, |
1162 | the argument is checked for shell metacharacters, and if there are any, | |
1163 | the entire argument is passed to the system's command shell for parsing | |
1164 | (this is C</bin/sh -c> on Unix platforms, but varies on other platforms). | |
1165 | If there are no shell metacharacters in the argument, it is split into | |
7660c0ab A |
1166 | words and passed directly to C<execvp()>, which is more efficient. Note: |
1167 | C<exec()> and C<system()> do not flush your output buffer, so you may need to | |
5a964f20 | 1168 | set C<$|> to avoid lost output. Examples: |
a0d0e21e LW |
1169 | |
1170 | exec '/bin/echo', 'Your arguments are: ', @ARGV; | |
1171 | exec "sort $outfile | uniq"; | |
1172 | ||
1173 | If you don't really want to execute the first argument, but want to lie | |
1174 | to the program you are executing about its own name, you can specify | |
1175 | the program you actually want to run as an "indirect object" (without a | |
1176 | comma) in front of the LIST. (This always forces interpretation of the | |
54310121 | 1177 | LIST as a multivalued list, even if there is only a single scalar in |
a0d0e21e LW |
1178 | the list.) Example: |
1179 | ||
1180 | $shell = '/bin/csh'; | |
1181 | exec $shell '-sh'; # pretend it's a login shell | |
1182 | ||
1183 | or, more directly, | |
1184 | ||
1185 | exec {'/bin/csh'} '-sh'; # pretend it's a login shell | |
1186 | ||
bb32b41a GS |
1187 | When the arguments get executed via the system shell, results will |
1188 | be subject to its quirks and capabilities. See L<perlop/"`STRING`"> | |
1189 | for details. | |
1190 | ||
ab4f32c2 | 1191 | Using an indirect object with C<exec()> or C<system()> is also more secure. |
5a964f20 TC |
1192 | This usage forces interpretation of the arguments as a multivalued list, |
1193 | even if the list had just one argument. That way you're safe from the | |
1194 | shell expanding wildcards or splitting up words with whitespace in them. | |
1195 | ||
1196 | @args = ( "echo surprise" ); | |
1197 | ||
1198 | system @args; # subject to shell escapes if @args == 1 | |
1199 | system { $args[0] } @args; # safe even with one-arg list | |
1200 | ||
1201 | The first version, the one without the indirect object, ran the I<echo> | |
1202 | program, passing it C<"surprise"> an argument. The second version | |
1203 | didn't--it tried to run a program literally called I<"echo surprise">, | |
1204 | didn't find it, and set C<$?> to a non-zero value indicating failure. | |
1205 | ||
ab4f32c2 | 1206 | Note that C<exec()> will not call your C<END> blocks, nor will it call |
7660c0ab A |
1207 | any C<DESTROY> methods in your objects. |
1208 | ||
a0d0e21e LW |
1209 | =item exists EXPR |
1210 | ||
1211 | Returns TRUE if the specified hash key exists in its hash array, even | |
1212 | if the corresponding value is undefined. | |
1213 | ||
1214 | print "Exists\n" if exists $array{$key}; | |
1215 | print "Defined\n" if defined $array{$key}; | |
1216 | print "True\n" if $array{$key}; | |
1217 | ||
5f05dabc | 1218 | A hash element can be TRUE only if it's defined, and defined if |
a0d0e21e LW |
1219 | it exists, but the reverse doesn't necessarily hold true. |
1220 | ||
1221 | Note that the EXPR can be arbitrarily complicated as long as the final | |
1222 | operation is a hash key lookup: | |
1223 | ||
5a964f20 TC |
1224 | if (exists $ref->{"A"}{"B"}{$key}) { ... } |
1225 | ||
1226 | Although the last element will not spring into existence just because its | |
1227 | existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}> | |
1228 | C<$ref-E<gt>{"B"}> will spring into existence due to the existence | |
1229 | test for a $key element. This autovivification may be fixed in a later | |
1230 | release. | |
a0d0e21e LW |
1231 | |
1232 | =item exit EXPR | |
1233 | ||
1234 | Evaluates EXPR and exits immediately with that value. (Actually, it | |
1235 | calls any defined C<END> routines first, but the C<END> routines may not | |
1236 | abort the exit. Likewise any object destructors that need to be called | |
1237 | are called before exit.) Example: | |
1238 | ||
1239 | $ans = <STDIN>; | |
1240 | exit 0 if $ans =~ /^[Xx]/; | |
1241 | ||
7660c0ab A |
1242 | See also C<die()>. If EXPR is omitted, exits with C<0> status. The only |
1243 | universally portable values for EXPR are C<0> for success and C<1> for error; | |
f86702cc | 1244 | all other values are subject to unpredictable interpretation depending |
1245 | on the environment in which the Perl program is running. | |
a0d0e21e | 1246 | |
7660c0ab A |
1247 | You shouldn't use C<exit()> to abort a subroutine if there's any chance that |
1248 | someone might want to trap whatever error happened. Use C<die()> instead, | |
1249 | which can be trapped by an C<eval()>. | |
28757baa | 1250 | |
5a964f20 TC |
1251 | All C<END{}> blocks are run at exit time. See L<perlsub> for details. |
1252 | ||
a0d0e21e LW |
1253 | =item exp EXPR |
1254 | ||
54310121 | 1255 | =item exp |
bbce6d69 | 1256 | |
54310121 | 1257 | Returns I<e> (the natural logarithm base) to the power of EXPR. |
a0d0e21e LW |
1258 | If EXPR is omitted, gives C<exp($_)>. |
1259 | ||
1260 | =item fcntl FILEHANDLE,FUNCTION,SCALAR | |
1261 | ||
7660c0ab | 1262 | Implements the C<fcntl(2)> function. You'll probably have to say |
a0d0e21e LW |
1263 | |
1264 | use Fcntl; | |
1265 | ||
0ade1984 | 1266 | first to get the correct constant definitions. Argument processing and |
7660c0ab | 1267 | value return works just like C<ioctl()> below. |
a0d0e21e LW |
1268 | For example: |
1269 | ||
1270 | use Fcntl; | |
5a964f20 TC |
1271 | fcntl($filehandle, F_GETFL, $packed_return_buffer) |
1272 | or die "can't fcntl F_GETFL: $!"; | |
1273 | ||
ab4f32c2 A |
1274 | You don't have to check for C<defined()> on the return from |
1275 | C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system | |
7660c0ab A |
1276 | call into "C<0> but true" in Perl. This string is true in |
1277 | boolean context and C<0> in numeric context. It is also | |
5a964f20 TC |
1278 | exempt from the normal B<-w> warnings on improper numeric |
1279 | conversions. | |
1280 | ||
7660c0ab A |
1281 | Note that C<fcntl()> will produce a fatal error if used on a machine that |
1282 | doesn't implement C<fcntl(2)>. | |
a0d0e21e LW |
1283 | |
1284 | =item fileno FILEHANDLE | |
1285 | ||
1286 | Returns the file descriptor for a filehandle. This is useful for | |
7660c0ab | 1287 | constructing bitmaps for C<select()> and low-level POSIX tty-handling |
5a964f20 TC |
1288 | operations. If FILEHANDLE is an expression, the value is taken as |
1289 | an indirect filehandle, generally its name. | |
1290 | ||
1291 | You can use this to find out whether two handles refer to the | |
1292 | same underlying descriptor: | |
1293 | ||
1294 | if (fileno(THIS) == fileno(THAT)) { | |
1295 | print "THIS and THAT are dups\n"; | |
1296 | } | |
a0d0e21e LW |
1297 | |
1298 | =item flock FILEHANDLE,OPERATION | |
1299 | ||
7660c0ab | 1300 | Calls C<flock(2)>, or an emulation of it, on FILEHANDLE. Returns TRUE for |
68dc0745 | 1301 | success, FALSE on failure. Produces a fatal error if used on a machine |
7660c0ab | 1302 | that doesn't implement C<flock(2)>, C<fcntl(2)> locking, or C<lockf(3)>. C<flock()> |
68dc0745 | 1303 | is Perl's portable file locking interface, although it locks only entire |
1304 | files, not records. | |
8ebc5c01 | 1305 | |
a3cb178b | 1306 | On many platforms (including most versions or clones of Unix), locks |
7660c0ab | 1307 | established by C<flock()> are B<merely advisory>. Such discretionary locks |
5a964f20 | 1308 | are more flexible, but offer fewer guarantees. This means that files |
7660c0ab A |
1309 | locked with C<flock()> may be modified by programs that do not also use |
1310 | C<flock()>. Windows NT and OS/2 are among the platforms which | |
5a964f20 | 1311 | enforce mandatory locking. See your local documentation for details. |
a3cb178b | 1312 | |
8ebc5c01 | 1313 | OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with |
1314 | LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but | |
68dc0745 | 1315 | you can use the symbolic names if import them from the Fcntl module, |
1316 | either individually, or as a group using the ':flock' tag. LOCK_SH | |
1317 | requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN | |
1318 | releases a previously requested lock. If LOCK_NB is added to LOCK_SH or | |
7660c0ab | 1319 | LOCK_EX then C<flock()> will return immediately rather than blocking |
68dc0745 | 1320 | waiting for the lock (check the return status to see if you got it). |
1321 | ||
1322 | To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE | |
1323 | before (un)locking it. | |
8ebc5c01 | 1324 | |
7660c0ab | 1325 | Note that the emulation built with C<lockf(3)> doesn't provide shared |
8ebc5c01 | 1326 | locks, and it requires that FILEHANDLE be open with write intent. These |
7660c0ab A |
1327 | are the semantics that C<lockf(3)> implements. Most (all?) systems |
1328 | implement C<lockf(3)> in terms of C<fcntl(2)> locking, though, so the | |
8ebc5c01 | 1329 | differing semantics shouldn't bite too many people. |
1330 | ||
7660c0ab A |
1331 | Note also that some versions of C<flock()> cannot lock things over the |
1332 | network; you would need to use the more system-specific C<fcntl()> for | |
1333 | that. If you like you can force Perl to ignore your system's C<flock(2)> | |
1334 | function, and so provide its own C<fcntl(2)>-based emulation, by passing | |
8ebc5c01 | 1335 | the switch C<-Ud_flock> to the F<Configure> program when you configure |
1336 | perl. | |
4633a7c4 LW |
1337 | |
1338 | Here's a mailbox appender for BSD systems. | |
a0d0e21e | 1339 | |
7e1af8bc | 1340 | use Fcntl ':flock'; # import LOCK_* constants |
a0d0e21e LW |
1341 | |
1342 | sub lock { | |
7e1af8bc | 1343 | flock(MBOX,LOCK_EX); |
a0d0e21e LW |
1344 | # and, in case someone appended |
1345 | # while we were waiting... | |
1346 | seek(MBOX, 0, 2); | |
1347 | } | |
1348 | ||
1349 | sub unlock { | |
7e1af8bc | 1350 | flock(MBOX,LOCK_UN); |
a0d0e21e LW |
1351 | } |
1352 | ||
1353 | open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}") | |
1354 | or die "Can't open mailbox: $!"; | |
1355 | ||
1356 | lock(); | |
1357 | print MBOX $msg,"\n\n"; | |
1358 | unlock(); | |
1359 | ||
cb1a09d0 | 1360 | See also L<DB_File> for other flock() examples. |
a0d0e21e LW |
1361 | |
1362 | =item fork | |
1363 | ||
7660c0ab A |
1364 | Does a C<fork(2)> system call. Returns the child pid to the parent process, |
1365 | C<0> to the child process, or C<undef> if the fork is unsuccessful. | |
5a964f20 | 1366 | |
a0d0e21e | 1367 | Note: unflushed buffers remain unflushed in both processes, which means |
7660c0ab A |
1368 | you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> |
1369 | method of C<IO::Handle> to avoid duplicate output. | |
a0d0e21e | 1370 | |
7660c0ab | 1371 | If you C<fork()> without ever waiting on your children, you will accumulate |
a0d0e21e LW |
1372 | zombies: |
1373 | ||
4633a7c4 | 1374 | $SIG{CHLD} = sub { wait }; |
a0d0e21e | 1375 | |
54310121 | 1376 | There's also the double-fork trick (error checking on |
7660c0ab | 1377 | C<fork()> returns omitted); |
a0d0e21e LW |
1378 | |
1379 | unless ($pid = fork) { | |
1380 | unless (fork) { | |
1381 | exec "what you really wanna do"; | |
1382 | die "no exec"; | |
1383 | # ... or ... | |
4633a7c4 | 1384 | ## (some_perl_code_here) |
a0d0e21e LW |
1385 | exit 0; |
1386 | } | |
1387 | exit 0; | |
1388 | } | |
1389 | waitpid($pid,0); | |
1390 | ||
cb1a09d0 AD |
1391 | See also L<perlipc> for more examples of forking and reaping |
1392 | moribund children. | |
1393 | ||
28757baa | 1394 | Note that if your forked child inherits system file descriptors like |
1395 | STDIN and STDOUT that are actually connected by a pipe or socket, even | |
5a964f20 | 1396 | if you exit, then the remote server (such as, say, httpd or rsh) won't think |
7660c0ab | 1397 | you're done. You should reopen those to F</dev/null> if it's any issue. |
28757baa | 1398 | |
cb1a09d0 AD |
1399 | =item format |
1400 | ||
7660c0ab | 1401 | Declare a picture format for use by the C<write()> function. For |
cb1a09d0 AD |
1402 | example: |
1403 | ||
54310121 | 1404 | format Something = |
cb1a09d0 AD |
1405 | Test: @<<<<<<<< @||||| @>>>>> |
1406 | $str, $%, '$' . int($num) | |
1407 | . | |
1408 | ||
1409 | $str = "widget"; | |
184e9718 | 1410 | $num = $cost/$quantity; |
cb1a09d0 AD |
1411 | $~ = 'Something'; |
1412 | write; | |
1413 | ||
1414 | See L<perlform> for many details and examples. | |
1415 | ||
8903cb82 | 1416 | =item formline PICTURE,LIST |
a0d0e21e | 1417 | |
5a964f20 | 1418 | This is an internal function used by C<format>s, though you may call it, |
a0d0e21e LW |
1419 | too. It formats (see L<perlform>) a list of values according to the |
1420 | contents of PICTURE, placing the output into the format output | |
7660c0ab A |
1421 | accumulator, C<$^A> (or C<$ACCUMULATOR> in English). |
1422 | Eventually, when a C<write()> is done, the contents of | |
a0d0e21e | 1423 | C<$^A> are written to some filehandle, but you could also read C<$^A> |
7660c0ab A |
1424 | yourself and then set C<$^A> back to C<"">. Note that a format typically |
1425 | does one C<formline()> per line of form, but the C<formline()> function itself | |
748a9306 | 1426 | doesn't care how many newlines are embedded in the PICTURE. This means |
4633a7c4 | 1427 | that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line. |
748a9306 LW |
1428 | You may therefore need to use multiple formlines to implement a single |
1429 | record format, just like the format compiler. | |
1430 | ||
5f05dabc | 1431 | Be careful if you put double quotes around the picture, because an "C<@>" |
748a9306 | 1432 | character may be taken to mean the beginning of an array name. |
7660c0ab | 1433 | C<formline()> always returns TRUE. See L<perlform> for other examples. |
a0d0e21e LW |
1434 | |
1435 | =item getc FILEHANDLE | |
1436 | ||
1437 | =item getc | |
1438 | ||
1439 | Returns the next character from the input file attached to FILEHANDLE, | |
9bc64814 | 1440 | or the undefined value at end of file, or if there was an error. If |
3b02c43c GS |
1441 | FILEHANDLE is omitted, reads from STDIN. This is not particularly |
1442 | efficient. It cannot be used to get unbuffered single-characters, | |
1443 | however. For that, try something more like: | |
4633a7c4 LW |
1444 | |
1445 | if ($BSD_STYLE) { | |
1446 | system "stty cbreak </dev/tty >/dev/tty 2>&1"; | |
1447 | } | |
1448 | else { | |
54310121 | 1449 | system "stty", '-icanon', 'eol', "\001"; |
4633a7c4 LW |
1450 | } |
1451 | ||
1452 | $key = getc(STDIN); | |
1453 | ||
1454 | if ($BSD_STYLE) { | |
1455 | system "stty -cbreak </dev/tty >/dev/tty 2>&1"; | |
1456 | } | |
1457 | else { | |
5f05dabc | 1458 | system "stty", 'icanon', 'eol', '^@'; # ASCII null |
4633a7c4 LW |
1459 | } |
1460 | print "\n"; | |
1461 | ||
54310121 | 1462 | Determination of whether $BSD_STYLE should be set |
1463 | is left as an exercise to the reader. | |
cb1a09d0 | 1464 | |
7660c0ab | 1465 | The C<POSIX::getattr()> function can do this more portably on systems |
5a964f20 | 1466 | purporting POSIX compliance. |
cb1a09d0 | 1467 | See also the C<Term::ReadKey> module from your nearest CPAN site; |
54310121 | 1468 | details on CPAN can be found on L<perlmod/CPAN>. |
a0d0e21e LW |
1469 | |
1470 | =item getlogin | |
1471 | ||
5a964f20 TC |
1472 | Implements the C library function of the same name, which on most |
1473 | systems returns the current login from F</etc/utmp>, if any. If null, | |
7660c0ab | 1474 | use C<getpwuid()>. |
a0d0e21e | 1475 | |
f86702cc | 1476 | $login = getlogin || getpwuid($<) || "Kilroy"; |
a0d0e21e | 1477 | |
7660c0ab A |
1478 | Do not consider C<getlogin()> for authentication: it is not as |
1479 | secure as C<getpwuid()>. | |
4633a7c4 | 1480 | |
a0d0e21e LW |
1481 | =item getpeername SOCKET |
1482 | ||
1483 | Returns the packed sockaddr address of other end of the SOCKET connection. | |
1484 | ||
4633a7c4 LW |
1485 | use Socket; |
1486 | $hersockaddr = getpeername(SOCK); | |
1487 | ($port, $iaddr) = unpack_sockaddr_in($hersockaddr); | |
1488 | $herhostname = gethostbyaddr($iaddr, AF_INET); | |
1489 | $herstraddr = inet_ntoa($iaddr); | |
a0d0e21e LW |
1490 | |
1491 | =item getpgrp PID | |
1492 | ||
47e29363 | 1493 | Returns the current process group for the specified PID. Use |
7660c0ab | 1494 | a PID of C<0> to get the current process group for the |
4633a7c4 | 1495 | current process. Will raise an exception if used on a machine that |
7660c0ab A |
1496 | doesn't implement C<getpgrp(2)>. If PID is omitted, returns process |
1497 | group of current process. Note that the POSIX version of C<getpgrp()> | |
1498 | does not accept a PID argument, so only C<PID==0> is truly portable. | |
a0d0e21e LW |
1499 | |
1500 | =item getppid | |
1501 | ||
1502 | Returns the process id of the parent process. | |
1503 | ||
1504 | =item getpriority WHICH,WHO | |
1505 | ||
4633a7c4 LW |
1506 | Returns the current priority for a process, a process group, or a user. |
1507 | (See L<getpriority(2)>.) Will raise a fatal exception if used on a | |
7660c0ab | 1508 | machine that doesn't implement C<getpriority(2)>. |
a0d0e21e LW |
1509 | |
1510 | =item getpwnam NAME | |
1511 | ||
1512 | =item getgrnam NAME | |
1513 | ||
1514 | =item gethostbyname NAME | |
1515 | ||
1516 | =item getnetbyname NAME | |
1517 | ||
1518 | =item getprotobyname NAME | |
1519 | ||
1520 | =item getpwuid UID | |
1521 | ||
1522 | =item getgrgid GID | |
1523 | ||
1524 | =item getservbyname NAME,PROTO | |
1525 | ||
1526 | =item gethostbyaddr ADDR,ADDRTYPE | |
1527 | ||
1528 | =item getnetbyaddr ADDR,ADDRTYPE | |
1529 | ||
1530 | =item getprotobynumber NUMBER | |
1531 | ||
1532 | =item getservbyport PORT,PROTO | |
1533 | ||
1534 | =item getpwent | |
1535 | ||
1536 | =item getgrent | |
1537 | ||
1538 | =item gethostent | |
1539 | ||
1540 | =item getnetent | |
1541 | ||
1542 | =item getprotoent | |
1543 | ||
1544 | =item getservent | |
1545 | ||
1546 | =item setpwent | |
1547 | ||
1548 | =item setgrent | |
1549 | ||
1550 | =item sethostent STAYOPEN | |
1551 | ||
1552 | =item setnetent STAYOPEN | |
1553 | ||
1554 | =item setprotoent STAYOPEN | |
1555 | ||
1556 | =item setservent STAYOPEN | |
1557 | ||
1558 | =item endpwent | |
1559 | ||
1560 | =item endgrent | |
1561 | ||
1562 | =item endhostent | |
1563 | ||
1564 | =item endnetent | |
1565 | ||
1566 | =item endprotoent | |
1567 | ||
1568 | =item endservent | |
1569 | ||
1570 | These routines perform the same functions as their counterparts in the | |
5a964f20 | 1571 | system library. In list context, the return values from the |
a0d0e21e LW |
1572 | various get routines are as follows: |
1573 | ||
1574 | ($name,$passwd,$uid,$gid, | |
6ee623d5 | 1575 | $quota,$comment,$gcos,$dir,$shell,$expire) = getpw* |
a0d0e21e LW |
1576 | ($name,$passwd,$gid,$members) = getgr* |
1577 | ($name,$aliases,$addrtype,$length,@addrs) = gethost* | |
1578 | ($name,$aliases,$addrtype,$net) = getnet* | |
1579 | ($name,$aliases,$proto) = getproto* | |
1580 | ($name,$aliases,$port,$proto) = getserv* | |
1581 | ||
1582 | (If the entry doesn't exist you get a null list.) | |
1583 | ||
5a964f20 | 1584 | In scalar context, you get the name, unless the function was a |
a0d0e21e LW |
1585 | lookup by name, in which case you get the other thing, whatever it is. |
1586 | (If the entry doesn't exist you get the undefined value.) For example: | |
1587 | ||
5a964f20 TC |
1588 | $uid = getpwnam($name); |
1589 | $name = getpwuid($num); | |
1590 | $name = getpwent(); | |
1591 | $gid = getgrnam($name); | |
1592 | $name = getgrgid($num; | |
1593 | $name = getgrent(); | |
1594 | #etc. | |
a0d0e21e | 1595 | |
7660c0ab | 1596 | In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special |
6ee623d5 | 1597 | cases in the sense that in many systems they are unsupported. If the |
7660c0ab A |
1598 | C<$quota> is unsupported, it is an empty scalar. If it is supported, it |
1599 | usually encodes the disk quota. If the C<$comment> field is unsupported, | |
6ee623d5 GS |
1600 | it is an empty scalar. If it is supported it usually encodes some |
1601 | administrative comment about the user. In some systems the $quota | |
7660c0ab A |
1602 | field may be C<$change> or C<$age>, fields that have to do with password |
1603 | aging. In some systems the C<$comment> field may be C<$class>. The C<$expire> | |
6ee623d5 GS |
1604 | field, if present, encodes the expiration period of the account or the |
1605 | password. For the availability and the exact meaning of these fields | |
7660c0ab A |
1606 | in your system, please consult your C<getpwnam(3)> documentation and your |
1607 | F<pwd.h> file. You can also find out from within Perl which meaning | |
1608 | your C<$quota> and C<$comment> fields have and whether you have the C<$expire> | |
1609 | field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>, | |
1610 | C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. | |
6ee623d5 | 1611 | |
7660c0ab | 1612 | The C<$members> value returned by I<getgr*()> is a space separated list of |
a0d0e21e LW |
1613 | the login names of the members of the group. |
1614 | ||
1615 | For the I<gethost*()> functions, if the C<h_errno> variable is supported in | |
1616 | C, it will be returned to you via C<$?> if the function call fails. The | |
7660c0ab | 1617 | C<@addrs> value returned by a successful call is a list of the raw |
a0d0e21e LW |
1618 | addresses returned by the corresponding system library call. In the |
1619 | Internet domain, each address is four bytes long and you can unpack it | |
1620 | by saying something like: | |
1621 | ||
1622 | ($a,$b,$c,$d) = unpack('C4',$addr[0]); | |
1623 | ||
5a964f20 TC |
1624 | If you get tired of remembering which element of the return list contains |
1625 | which return value, by-name interfaces are also provided in modules: | |
7660c0ab A |
1626 | C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>, |
1627 | C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the | |
5a964f20 TC |
1628 | normal built-in, replacing them with versions that return objects with |
1629 | the appropriate names for each field. For example: | |
1630 | ||
1631 | use File::stat; | |
1632 | use User::pwent; | |
1633 | $is_his = (stat($filename)->uid == pwent($whoever)->uid); | |
1634 | ||
1635 | Even though it looks like they're the same method calls (uid), | |
7660c0ab | 1636 | they aren't, because a C<File::stat> object is different from a C<User::pwent> object. |
5a964f20 | 1637 | |
a0d0e21e LW |
1638 | =item getsockname SOCKET |
1639 | ||
1640 | Returns the packed sockaddr address of this end of the SOCKET connection. | |
1641 | ||
4633a7c4 LW |
1642 | use Socket; |
1643 | $mysockaddr = getsockname(SOCK); | |
1644 | ($port, $myaddr) = unpack_sockaddr_in($mysockaddr); | |
a0d0e21e LW |
1645 | |
1646 | =item getsockopt SOCKET,LEVEL,OPTNAME | |
1647 | ||
5a964f20 | 1648 | Returns the socket option requested, or undef if there is an error. |
a0d0e21e LW |
1649 | |
1650 | =item glob EXPR | |
1651 | ||
0a753a76 | 1652 | =item glob |
1653 | ||
7660c0ab | 1654 | Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would |
68dc0745 | 1655 | do. This is the internal function implementing the C<E<lt>*.cE<gt>> |
7660c0ab | 1656 | operator, but you can use it directly. If EXPR is omitted, C<$_> is used. |
68dc0745 | 1657 | The C<E<lt>*.cE<gt>> operator is discussed in more detail in |
1658 | L<perlop/"I/O Operators">. | |
a0d0e21e LW |
1659 | |
1660 | =item gmtime EXPR | |
1661 | ||
1662 | Converts a time as returned by the time function to a 9-element array | |
54310121 | 1663 | with the time localized for the standard Greenwich time zone. |
4633a7c4 | 1664 | Typically used as follows: |
a0d0e21e | 1665 | |
54310121 | 1666 | # 0 1 2 3 4 5 6 7 8 |
a0d0e21e LW |
1667 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = |
1668 | gmtime(time); | |
1669 | ||
1670 | All array elements are numeric, and come straight out of a struct tm. | |
7660c0ab A |
1671 | In particular this means that C<$mon> has the range C<0..11> and C<$wday> has |
1672 | the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of | |
1673 | years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year. | |
2f9daede TP |
1674 | |
1675 | If EXPR is omitted, does C<gmtime(time())>. | |
a0d0e21e | 1676 | |
7660c0ab | 1677 | In scalar context, returns the C<ctime(3)> value: |
0a753a76 | 1678 | |
1679 | $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994" | |
1680 | ||
7660c0ab A |
1681 | Also see the C<timegm()> function provided by the C<Time::Local> module, |
1682 | and the C<strftime(3)> function available via the POSIX module. | |
1683 | ||
1684 | This scalar value is B<not> locale dependent, see L<perllocale>, but | |
1685 | instead a Perl builtin. Also see the C<Time::Local> module, and the | |
1686 | C<strftime(3)> and C<mktime(3)> function available via the POSIX module. To | |
1687 | get somewhat similar but locale dependent date strings, set up your | |
1688 | locale environment variables appropriately (please see L<perllocale>) | |
1689 | and try for example: | |
1690 | ||
1691 | use POSIX qw(strftime); | |
1692 | $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; | |
1693 | ||
1694 | Note that the C<%a> and C<%b>, the short forms of the day of the week | |
1695 | and the month of the year, may not necessarily be three characters wide. | |
0a753a76 | 1696 | |
a0d0e21e LW |
1697 | =item goto LABEL |
1698 | ||
748a9306 LW |
1699 | =item goto EXPR |
1700 | ||
a0d0e21e LW |
1701 | =item goto &NAME |
1702 | ||
7660c0ab | 1703 | The C<goto-LABEL> form finds the statement labeled with LABEL and resumes |
a0d0e21e | 1704 | execution there. It may not be used to go into any construct that |
7660c0ab | 1705 | requires initialization, such as a subroutine or a C<foreach> loop. It |
0a753a76 | 1706 | also can't be used to go into a construct that is optimized away, |
7660c0ab | 1707 | or to get out of a block or subroutine given to C<sort()>. |
0a753a76 | 1708 | It can be used to go almost anywhere else within the dynamic scope, |
a0d0e21e | 1709 | including out of subroutines, but it's usually better to use some other |
ab4f32c2 | 1710 | construct such as C<last> or C<die()>. The author of Perl has never felt the |
7660c0ab | 1711 | need to use this form of C<goto> (in Perl, that is--C is another matter). |
a0d0e21e | 1712 | |
7660c0ab A |
1713 | The C<goto-EXPR> form expects a label name, whose scope will be resolved |
1714 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
748a9306 LW |
1715 | necessarily recommended if you're optimizing for maintainability: |
1716 | ||
1717 | goto ("FOO", "BAR", "GLARCH")[$i]; | |
1718 | ||
7660c0ab | 1719 | The C<goto-&NAME> form is highly magical, and substitutes a call to the |
a0d0e21e | 1720 | named subroutine for the currently running subroutine. This is used by |
7660c0ab | 1721 | C<AUTOLOAD> subroutines that wish to load another subroutine and then |
a0d0e21e | 1722 | pretend that the other subroutine had been called in the first place |
7660c0ab A |
1723 | (except that any modifications to C<@_> in the current subroutine are |
1724 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> | |
a0d0e21e LW |
1725 | will be able to tell that this routine was called first. |
1726 | ||
1727 | =item grep BLOCK LIST | |
1728 | ||
1729 | =item grep EXPR,LIST | |
1730 | ||
7660c0ab | 1731 | This is similar in spirit to, but not the same as, C<grep(1)> |
2f9daede TP |
1732 | and its relatives. In particular, it is not limited to using |
1733 | regular expressions. | |
1734 | ||
a0d0e21e | 1735 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting |
7660c0ab | 1736 | C<$_> to each element) and returns the list value consisting of those |
a0d0e21e LW |
1737 | elements for which the expression evaluated to TRUE. In a scalar |
1738 | context, returns the number of times the expression was TRUE. | |
1739 | ||
1740 | @foo = grep(!/^#/, @bar); # weed out comments | |
1741 | ||
1742 | or equivalently, | |
1743 | ||
1744 | @foo = grep {!/^#/} @bar; # weed out comments | |
1745 | ||
7660c0ab | 1746 | Note that, because C<$_> is a reference into the list value, it can be used |
a0d0e21e LW |
1747 | to modify the elements of the array. While this is useful and |
1748 | supported, it can cause bizarre results if the LIST is not a named | |
2f9daede | 1749 | array. Similarly, grep returns aliases into the original list, |
7b8d334a | 1750 | much like the way that a for loops's index variable aliases the list |
2f9daede | 1751 | elements. That is, modifying an element of a list returned by grep |
ab4f32c2 | 1752 | (for example, in a C<foreach>, C<map()> or another C<grep()>) |
2f9daede | 1753 | actually modifies the element in the original list. |
a0d0e21e | 1754 | |
fb73857a | 1755 | See also L</map> for an array composed of the results of the BLOCK or EXPR. |
38325410 | 1756 | |
a0d0e21e LW |
1757 | =item hex EXPR |
1758 | ||
54310121 | 1759 | =item hex |
bbce6d69 | 1760 | |
54310121 | 1761 | Interprets EXPR as a hex string and returns the corresponding |
2f9daede | 1762 | value. (To convert strings that might start with either 0 or 0x |
7660c0ab | 1763 | see L</oct>.) If EXPR is omitted, uses C<$_>. |
2f9daede TP |
1764 | |
1765 | print hex '0xAf'; # prints '175' | |
1766 | print hex 'aF'; # same | |
a0d0e21e LW |
1767 | |
1768 | =item import | |
1769 | ||
7660c0ab | 1770 | There is no builtin C<import()> function. It is just an ordinary |
4633a7c4 | 1771 | method (subroutine) defined (or inherited) by modules that wish to export |
7660c0ab | 1772 | names to another module. The C<use()> function calls the C<import()> method |
54310121 | 1773 | for the package used. See also L</use()>, L<perlmod>, and L<Exporter>. |
a0d0e21e LW |
1774 | |
1775 | =item index STR,SUBSTR,POSITION | |
1776 | ||
1777 | =item index STR,SUBSTR | |
1778 | ||
4633a7c4 LW |
1779 | Returns the position of the first occurrence of SUBSTR in STR at or after |
1780 | POSITION. If POSITION is omitted, starts searching from the beginning of | |
7660c0ab | 1781 | the string. The return value is based at C<0> (or whatever you've set the C<$[> |
4633a7c4 | 1782 | variable to--but don't do that). If the substring is not found, returns |
7660c0ab | 1783 | one less than the base, ordinarily C<-1>. |
a0d0e21e LW |
1784 | |
1785 | =item int EXPR | |
1786 | ||
54310121 | 1787 | =item int |
bbce6d69 | 1788 | |
7660c0ab | 1789 | Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>. |
5a964f20 | 1790 | You should not use this for rounding, because it truncates |
7660c0ab A |
1791 | towards C<0>, and because machine representations of floating point |
1792 | numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>, | |
1793 | or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better. | |
a0d0e21e LW |
1794 | |
1795 | =item ioctl FILEHANDLE,FUNCTION,SCALAR | |
1796 | ||
7660c0ab | 1797 | Implements the C<ioctl(2)> function. You'll probably have to say |
a0d0e21e | 1798 | |
4633a7c4 | 1799 | require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph |
a0d0e21e | 1800 | |
4633a7c4 | 1801 | first to get the correct function definitions. If F<ioctl.ph> doesn't |
a0d0e21e | 1802 | exist or doesn't have the correct definitions you'll have to roll your |
4633a7c4 | 1803 | own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>. |
5a964f20 | 1804 | (There is a Perl script called B<h2ph> that comes with the Perl kit that |
54310121 | 1805 | may help you in this, but it's nontrivial.) SCALAR will be read and/or |
4633a7c4 | 1806 | written depending on the FUNCTION--a pointer to the string value of SCALAR |
ab4f32c2 | 1807 | will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR |
4633a7c4 LW |
1808 | has no string value but does have a numeric value, that value will be |
1809 | passed rather than a pointer to the string value. To guarantee this to be | |
7660c0ab | 1810 | TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()> |
4633a7c4 | 1811 | functions are useful for manipulating the values of structures used by |
7660c0ab | 1812 | C<ioctl()>. The following example sets the erase character to DEL. |
a0d0e21e LW |
1813 | |
1814 | require 'ioctl.ph'; | |
4633a7c4 LW |
1815 | $getp = &TIOCGETP; |
1816 | die "NO TIOCGETP" if $@ || !$getp; | |
a0d0e21e | 1817 | $sgttyb_t = "ccccs"; # 4 chars and a short |
4633a7c4 | 1818 | if (ioctl(STDIN,$getp,$sgttyb)) { |
a0d0e21e LW |
1819 | @ary = unpack($sgttyb_t,$sgttyb); |
1820 | $ary[2] = 127; | |
1821 | $sgttyb = pack($sgttyb_t,@ary); | |
4633a7c4 | 1822 | ioctl(STDIN,&TIOCSETP,$sgttyb) |
a0d0e21e LW |
1823 | || die "Can't ioctl: $!"; |
1824 | } | |
1825 | ||
ab4f32c2 | 1826 | The return value of C<ioctl()> (and C<fcntl()>) is as follows: |
a0d0e21e LW |
1827 | |
1828 | if OS returns: then Perl returns: | |
1829 | -1 undefined value | |
1830 | 0 string "0 but true" | |
1831 | anything else that number | |
1832 | ||
1833 | Thus Perl returns TRUE on success and FALSE on failure, yet you can | |
1834 | still easily determine the actual value returned by the operating | |
1835 | system: | |
1836 | ||
1837 | ($retval = ioctl(...)) || ($retval = -1); | |
1838 | printf "System returned %d\n", $retval; | |
1839 | ||
7660c0ab | 1840 | The special string "C<0> but true" is excempt from B<-w> complaints |
5a964f20 TC |
1841 | about improper numeric conversions. |
1842 | ||
a0d0e21e LW |
1843 | =item join EXPR,LIST |
1844 | ||
54310121 | 1845 | Joins the separate strings of LIST into a single string with |
a0d0e21e LW |
1846 | fields separated by the value of EXPR, and returns the string. |
1847 | Example: | |
1848 | ||
1849 | $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); | |
1850 | ||
7660c0ab | 1851 | See L</split>. |
a0d0e21e | 1852 | |
aa689395 | 1853 | =item keys HASH |
1854 | ||
1d2dff63 GS |
1855 | Returns a list consisting of all the keys of the named hash. (In a |
1856 | scalar context, returns the number of keys.) The keys are returned in | |
aa689395 | 1857 | an apparently random order, but it is the same order as either the |
7660c0ab | 1858 | C<values()> or C<each()> function produces (given that the hash has not been |
aa689395 | 1859 | modified). As a side effect, it resets HASH's iterator. |
a0d0e21e | 1860 | |
aa689395 | 1861 | Here is yet another way to print your environment: |
a0d0e21e LW |
1862 | |
1863 | @keys = keys %ENV; | |
1864 | @values = values %ENV; | |
1865 | while ($#keys >= 0) { | |
1866 | print pop(@keys), '=', pop(@values), "\n"; | |
1867 | } | |
1868 | ||
1869 | or how about sorted by key: | |
1870 | ||
1871 | foreach $key (sort(keys %ENV)) { | |
1872 | print $key, '=', $ENV{$key}, "\n"; | |
1873 | } | |
1874 | ||
ab4f32c2 | 1875 | To sort an array by value, you'll need to use a C<sort()> function. |
aa689395 | 1876 | Here's a descending numeric sort of a hash by its values: |
4633a7c4 | 1877 | |
5a964f20 | 1878 | foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { |
4633a7c4 LW |
1879 | printf "%4d %s\n", $hash{$key}, $key; |
1880 | } | |
1881 | ||
ab4f32c2 | 1882 | As an lvalue C<keys()> allows you to increase the number of hash buckets |
aa689395 | 1883 | allocated for the given hash. This can gain you a measure of efficiency if |
1884 | you know the hash is going to get big. (This is similar to pre-extending | |
1885 | an array by assigning a larger number to $#array.) If you say | |
55497cff | 1886 | |
1887 | keys %hash = 200; | |
1888 | ||
5a964f20 TC |
1889 | then C<%hash> will have at least 200 buckets allocated for it--256 of them, in fact, since |
1890 | it rounds up to the next power of two. These | |
55497cff | 1891 | buckets will be retained even if you do C<%hash = ()>, use C<undef |
1892 | %hash> if you want to free the storage while C<%hash> is still in scope. | |
1893 | You can't shrink the number of buckets allocated for the hash using | |
ab4f32c2 | 1894 | C<keys()> in this way (but you needn't worry about doing this by accident, |
55497cff | 1895 | as trying has no effect). |
1896 | ||
a0d0e21e LW |
1897 | =item kill LIST |
1898 | ||
54310121 | 1899 | Sends a signal to a list of processes. The first element of |
1900 | the list must be the signal to send. Returns the number of | |
4633a7c4 | 1901 | processes successfully signaled. |
a0d0e21e LW |
1902 | |
1903 | $cnt = kill 1, $child1, $child2; | |
1904 | kill 9, @goners; | |
1905 | ||
4633a7c4 LW |
1906 | Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills |
1907 | process groups instead of processes. (On System V, a negative I<PROCESS> | |
1908 | number will also kill process groups, but that's not portable.) That | |
1909 | means you usually want to use positive not negative signals. You may also | |
da0045b7 | 1910 | use a signal name in quotes. See L<perlipc/"Signals"> for details. |
a0d0e21e LW |
1911 | |
1912 | =item last LABEL | |
1913 | ||
1914 | =item last | |
1915 | ||
1916 | The C<last> command is like the C<break> statement in C (as used in | |
1917 | loops); it immediately exits the loop in question. If the LABEL is | |
1918 | omitted, the command refers to the innermost enclosing loop. The | |
1919 | C<continue> block, if any, is not executed: | |
1920 | ||
4633a7c4 LW |
1921 | LINE: while (<STDIN>) { |
1922 | last LINE if /^$/; # exit when done with header | |
5a964f20 | 1923 | #... |
a0d0e21e LW |
1924 | } |
1925 | ||
1d2dff63 GS |
1926 | See also L</continue> for an illustration of how C<last>, C<next>, and |
1927 | C<redo> work. | |
1928 | ||
a0d0e21e LW |
1929 | =item lc EXPR |
1930 | ||
54310121 | 1931 | =item lc |
bbce6d69 | 1932 | |
a0d0e21e | 1933 | Returns an lowercased version of EXPR. This is the internal function |
7660c0ab A |
1934 | implementing the C<\L> escape in double-quoted strings. |
1935 | Respects current C<LC_CTYPE> locale if C<use locale> in force. See L<perllocale>. | |
a0d0e21e | 1936 | |
7660c0ab | 1937 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 1938 | |
a0d0e21e LW |
1939 | =item lcfirst EXPR |
1940 | ||
54310121 | 1941 | =item lcfirst |
bbce6d69 | 1942 | |
a0d0e21e | 1943 | Returns the value of EXPR with the first character lowercased. This is |
7660c0ab A |
1944 | the internal function implementing the C<\l> escape in double-quoted strings. |
1945 | Respects current C<LC_CTYPE> locale if C<use locale> in force. See L<perllocale>. | |
a0d0e21e | 1946 | |
7660c0ab | 1947 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 1948 | |
a0d0e21e LW |
1949 | =item length EXPR |
1950 | ||
54310121 | 1951 | =item length |
bbce6d69 | 1952 | |
5a964f20 | 1953 | Returns the length in bytes of the value of EXPR. If EXPR is |
7660c0ab | 1954 | omitted, returns length of C<$_>. |
a0d0e21e LW |
1955 | |
1956 | =item link OLDFILE,NEWFILE | |
1957 | ||
5a964f20 TC |
1958 | Creates a new filename linked to the old filename. Returns TRUE for |
1959 | success, FALSE otherwise. | |
a0d0e21e LW |
1960 | |
1961 | =item listen SOCKET,QUEUESIZE | |
1962 | ||
1963 | Does the same thing that the listen system call does. Returns TRUE if | |
4633a7c4 | 1964 | it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">. |
a0d0e21e LW |
1965 | |
1966 | =item local EXPR | |
1967 | ||
5a964f20 TC |
1968 | A local modifies the listed variables to be local to the enclosing |
1969 | block, file, or eval. If more than one value is listed, the list must | |
1970 | be placed in parentheses. See L<perlsub/"Temporary Values via local()"> | |
1971 | for details, including issues with tied arrays and hashes. | |
a0d0e21e | 1972 | |
7660c0ab | 1973 | You really probably want to be using C<my()> instead, because C<local()> isn't |
7b8d334a | 1974 | what most people think of as "local". See L<perlsub/"Private Variables |
cb1a09d0 | 1975 | via my()"> for details. |
a0d0e21e LW |
1976 | |
1977 | =item localtime EXPR | |
1978 | ||
1979 | Converts a time as returned by the time function to a 9-element array | |
5f05dabc | 1980 | with the time analyzed for the local time zone. Typically used as |
a0d0e21e LW |
1981 | follows: |
1982 | ||
54310121 | 1983 | # 0 1 2 3 4 5 6 7 8 |
a0d0e21e LW |
1984 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = |
1985 | localtime(time); | |
1986 | ||
1987 | All array elements are numeric, and come straight out of a struct tm. | |
7660c0ab A |
1988 | In particular this means that C<$mon> has the range C<0..11> and C<$wday> has |
1989 | the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of | |
1990 | years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year. | |
54310121 | 1991 | |
1992 | If EXPR is omitted, uses the current time (C<localtime(time)>). | |
a0d0e21e | 1993 | |
7660c0ab | 1994 | In scalar context, returns the C<ctime(3)> value: |
a0d0e21e | 1995 | |
5f05dabc | 1996 | $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" |
a0d0e21e | 1997 | |
a3cb178b | 1998 | This scalar value is B<not> locale dependent, see L<perllocale>, but |
7660c0ab A |
1999 | instead a Perl builtin. Also see the C<Time::Local> module, and the |
2000 | C<strftime(3)> and C<mktime(3)> function available via the POSIX module. To | |
a3cb178b GS |
2001 | get somewhat similar but locale dependent date strings, set up your |
2002 | locale environment variables appropriately (please see L<perllocale>) | |
5a964f20 | 2003 | and try for example: |
a3cb178b | 2004 | |
5a964f20 | 2005 | use POSIX qw(strftime); |
a3cb178b GS |
2006 | $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; |
2007 | ||
2008 | Note that the C<%a> and C<%b>, the short forms of the day of the week | |
2009 | and the month of the year, may not necessarily be three characters wide. | |
a0d0e21e LW |
2010 | |
2011 | =item log EXPR | |
2012 | ||
54310121 | 2013 | =item log |
bbce6d69 | 2014 | |
5a964f20 | 2015 | Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log |
7660c0ab | 2016 | of C<$_>. |
a0d0e21e LW |
2017 | |
2018 | =item lstat FILEHANDLE | |
2019 | ||
2020 | =item lstat EXPR | |
2021 | ||
54310121 | 2022 | =item lstat |
bbce6d69 | 2023 | |
7660c0ab | 2024 | Does the same thing as the C<stat()> function (including setting the |
5a964f20 TC |
2025 | special C<_> filehandle) but stats a symbolic link instead of the file |
2026 | the symbolic link points to. If symbolic links are unimplemented on | |
7660c0ab | 2027 | your system, a normal C<stat()> is done. |
a0d0e21e | 2028 | |
7660c0ab | 2029 | If EXPR is omitted, stats C<$_>. |
bbce6d69 | 2030 | |
a0d0e21e LW |
2031 | =item m// |
2032 | ||
2033 | The match operator. See L<perlop>. | |
2034 | ||
2035 | =item map BLOCK LIST | |
2036 | ||
2037 | =item map EXPR,LIST | |
2038 | ||
7660c0ab | 2039 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each |
a0d0e21e LW |
2040 | element) and returns the list value composed of the results of each such |
2041 | evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST | |
2042 | may produce zero, one, or more elements in the returned value. | |
2043 | ||
2044 | @chars = map(chr, @nums); | |
2045 | ||
2046 | translates a list of numbers to the corresponding characters. And | |
2047 | ||
4633a7c4 | 2048 | %hash = map { getkey($_) => $_ } @array; |
a0d0e21e LW |
2049 | |
2050 | is just a funny way to write | |
2051 | ||
2052 | %hash = (); | |
2053 | foreach $_ (@array) { | |
4633a7c4 | 2054 | $hash{getkey($_)} = $_; |
a0d0e21e LW |
2055 | } |
2056 | ||
7660c0ab | 2057 | Note that, because C<$_> is a reference into the list value, it can be used |
fb73857a | 2058 | to modify the elements of the array. While this is useful and |
2059 | supported, it can cause bizarre results if the LIST is not a named | |
2060 | array. See also L</grep> for an array composed of those items of the | |
2061 | original list for which the BLOCK or EXPR evaluates to true. | |
2062 | ||
a0d0e21e LW |
2063 | =item mkdir FILENAME,MODE |
2064 | ||
2065 | Creates the directory specified by FILENAME, with permissions specified | |
5a964f20 TC |
2066 | by MODE (as modified by umask). If it succeeds it returns TRUE, otherwise |
2067 | it returns FALSE and sets C<$!> (errno). | |
a0d0e21e LW |
2068 | |
2069 | =item msgctl ID,CMD,ARG | |
2070 | ||
7660c0ab | 2071 | Calls the System V IPC function C<msgctl(2)>. You'll probably have to say |
0ade1984 JH |
2072 | |
2073 | use IPC::SysV; | |
2074 | ||
7660c0ab A |
2075 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, |
2076 | then ARG must be a variable which will hold the returned C<msqid_ds> | |
ab4f32c2 | 2077 | structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but |
0ade1984 | 2078 | true" for zero, or the actual return value otherwise. See also |
7660c0ab | 2079 | C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation. |
a0d0e21e LW |
2080 | |
2081 | =item msgget KEY,FLAGS | |
2082 | ||
7660c0ab A |
2083 | Calls the System V IPC function C<msgget(2)>. Returns the message queue |
2084 | id, or the undefined value if there is an error. See also C<IPC::SysV> | |
2085 | and C<IPC::SysV::Msg> documentation. | |
a0d0e21e LW |
2086 | |
2087 | =item msgsnd ID,MSG,FLAGS | |
2088 | ||
2089 | Calls the System V IPC function msgsnd to send the message MSG to the | |
2090 | message queue ID. MSG must begin with the long integer message type, | |
c07a80fd | 2091 | which may be created with C<pack("l", $type)>. Returns TRUE if |
7660c0ab A |
2092 | successful, or FALSE if there is an error. See also C<IPC::SysV> |
2093 | and C<IPC::SysV::Msg> documentation. | |
a0d0e21e LW |
2094 | |
2095 | =item msgrcv ID,VAR,SIZE,TYPE,FLAGS | |
2096 | ||
2097 | Calls the System V IPC function msgrcv to receive a message from | |
2098 | message queue ID into variable VAR with a maximum message size of | |
0ade1984 JH |
2099 | SIZE. Note that if a message is received, the message type will be |
2100 | the first thing in VAR, and the maximum length of VAR is SIZE plus the | |
2101 | size of the message type. Returns TRUE if successful, or FALSE if | |
7660c0ab | 2102 | there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation. |
a0d0e21e LW |
2103 | |
2104 | =item my EXPR | |
2105 | ||
ab4f32c2 A |
2106 | A C<my()> declares the listed variables to be local (lexically) to the |
2107 | enclosing block, file, or C<eval()>. If | |
5f05dabc | 2108 | more than one value is listed, the list must be placed in parentheses. See |
cb1a09d0 | 2109 | L<perlsub/"Private Variables via my()"> for details. |
4633a7c4 | 2110 | |
a0d0e21e LW |
2111 | =item next LABEL |
2112 | ||
2113 | =item next | |
2114 | ||
2115 | The C<next> command is like the C<continue> statement in C; it starts | |
2116 | the next iteration of the loop: | |
2117 | ||
4633a7c4 LW |
2118 | LINE: while (<STDIN>) { |
2119 | next LINE if /^#/; # discard comments | |
5a964f20 | 2120 | #... |
a0d0e21e LW |
2121 | } |
2122 | ||
2123 | Note that if there were a C<continue> block on the above, it would get | |
2124 | executed even on discarded lines. If the LABEL is omitted, the command | |
2125 | refers to the innermost enclosing loop. | |
2126 | ||
1d2dff63 GS |
2127 | See also L</continue> for an illustration of how C<last>, C<next>, and |
2128 | C<redo> work. | |
2129 | ||
a0d0e21e LW |
2130 | =item no Module LIST |
2131 | ||
7660c0ab | 2132 | See the L</use> function, which C<no> is the opposite of. |
a0d0e21e LW |
2133 | |
2134 | =item oct EXPR | |
2135 | ||
54310121 | 2136 | =item oct |
bbce6d69 | 2137 | |
4633a7c4 | 2138 | Interprets EXPR as an octal string and returns the corresponding |
7660c0ab | 2139 | value. (If EXPR happens to start off with C<0x>, interprets it as |
4633a7c4 LW |
2140 | a hex string instead.) The following will handle decimal, octal, and |
2141 | hex in the standard Perl or C notation: | |
a0d0e21e LW |
2142 | |
2143 | $val = oct($val) if $val =~ /^0/; | |
2144 | ||
7660c0ab A |
2145 | If EXPR is omitted, uses C<$_>. This function is commonly used when |
2146 | a string such as C<644> needs to be converted into a file mode, for | |
2f9daede TP |
2147 | example. (Although perl will automatically convert strings into |
2148 | numbers as needed, this automatic conversion assumes base 10.) | |
a0d0e21e LW |
2149 | |
2150 | =item open FILEHANDLE,EXPR | |
2151 | ||
2152 | =item open FILEHANDLE | |
2153 | ||
2154 | Opens the file whose filename is given by EXPR, and associates it with | |
5f05dabc | 2155 | FILEHANDLE. If FILEHANDLE is an expression, its value is used as the |
2156 | name of the real filehandle wanted. If EXPR is omitted, the scalar | |
2157 | variable of the same name as the FILEHANDLE contains the filename. | |
ab4f32c2 A |
2158 | (Note that lexical variables--those declared with C<my()>--will not work |
2159 | for this purpose; so if you're using C<my()>, specify EXPR in your call | |
5f05dabc | 2160 | to open.) |
2161 | ||
7660c0ab A |
2162 | If the filename begins with C<'E<lt>'> or nothing, the file is opened for input. |
2163 | If the filename begins with C<'E<gt>'>, the file is truncated and opened for | |
2164 | output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>, | |
fbb426e4 | 2165 | the file is opened for appending, again being created if necessary. |
7660c0ab A |
2166 | You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that |
2167 | you want both read and write access to the file; thus C<'+E<lt>'> is almost | |
2168 | always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the | |
5a964f20 TC |
2169 | file first. You can't usually use either read-write mode for updating |
2170 | textfiles, since they have variable length records. See the B<-i> | |
2171 | switch in L<perlrun> for a better approach. | |
2172 | ||
2173 | The prefix and the filename may be separated with spaces. | |
7660c0ab A |
2174 | These various prefixes correspond to the C<fopen(3)> modes of C<'r'>, C<'r+'>, C<'w'>, |
2175 | C<'w+'>, C<'a'>, and C<'a+'>. | |
5f05dabc | 2176 | |
7660c0ab | 2177 | If the filename begins with C<'|'>, the filename is interpreted as a |
5a964f20 | 2178 | command to which output is to be piped, and if the filename ends with a |
7660c0ab A |
2179 | C<'|'>, the filename is interpreted See L<perlipc/"Using open() for IPC"> |
2180 | for more examples of this. (You are not allowed to C<open()> to a command | |
5a964f20 TC |
2181 | that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, |
2182 | and L<perlipc/"Bidirectional Communication"> for alternatives.) | |
cb1a09d0 | 2183 | |
7660c0ab | 2184 | Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns |
ab4f32c2 | 2185 | nonzero upon success, the undefined value otherwise. If the C<open()> |
4633a7c4 | 2186 | involved a pipe, the return value happens to be the pid of the |
54310121 | 2187 | subprocess. |
cb1a09d0 AD |
2188 | |
2189 | If you're unfortunate enough to be running Perl on a system that | |
2190 | distinguishes between text files and binary files (modern operating | |
2191 | systems don't care), then you should check out L</binmode> for tips for | |
ab4f32c2 | 2192 | dealing with this. The key distinction between systems that need C<binmode()> |
5a964f20 TC |
2193 | and those that don't is their text file formats. Systems like Unix, MacOS, and |
2194 | Plan9, which delimit lines with a single character, and which encode that | |
ab4f32c2 | 2195 | character in C as C<"\n">, do not need C<binmode()>. The rest need it. |
cb1a09d0 | 2196 | |
fb73857a | 2197 | When opening a file, it's usually a bad idea to continue normal execution |
ab4f32c2 A |
2198 | if the request failed, so C<open()> is frequently used in connection with |
2199 | C<die()>. Even if C<die()> won't do what you want (say, in a CGI script, | |
fb73857a | 2200 | where you want to make a nicely formatted error message (but there are |
5a964f20 | 2201 | modules that can help with that problem)) you should always check |
fb73857a | 2202 | the return value from opening a file. The infrequent exception is when |
2203 | working with an unopened filehandle is actually what you want to do. | |
2204 | ||
cb1a09d0 | 2205 | Examples: |
a0d0e21e LW |
2206 | |
2207 | $ARTICLE = 100; | |
2208 | open ARTICLE or die "Can't find article $ARTICLE: $!\n"; | |
2209 | while (<ARTICLE>) {... | |
2210 | ||
2211 | open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) | |
fb73857a | 2212 | # if the open fails, output is discarded |
a0d0e21e | 2213 | |
fb73857a | 2214 | open(DBASE, '+<dbase.mine') # open for update |
2215 | or die "Can't open 'dbase.mine' for update: $!"; | |
cb1a09d0 | 2216 | |
fb73857a | 2217 | open(ARTICLE, "caesar <$article |") # decrypt article |
2218 | or die "Can't start caesar: $!"; | |
a0d0e21e | 2219 | |
fb73857a | 2220 | open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id |
2221 | or die "Can't start sort: $!"; | |
a0d0e21e LW |
2222 | |
2223 | # process argument list of files along with any includes | |
2224 | ||
2225 | foreach $file (@ARGV) { | |
2226 | process($file, 'fh00'); | |
2227 | } | |
2228 | ||
2229 | sub process { | |
5a964f20 | 2230 | my($filename, $input) = @_; |
a0d0e21e LW |
2231 | $input++; # this is a string increment |
2232 | unless (open($input, $filename)) { | |
2233 | print STDERR "Can't open $filename: $!\n"; | |
2234 | return; | |
2235 | } | |
2236 | ||
5a964f20 | 2237 | local $_; |
a0d0e21e LW |
2238 | while (<$input>) { # note use of indirection |
2239 | if (/^#include "(.*)"/) { | |
2240 | process($1, $input); | |
2241 | next; | |
2242 | } | |
5a964f20 | 2243 | #... # whatever |
a0d0e21e LW |
2244 | } |
2245 | } | |
2246 | ||
2247 | You may also, in the Bourne shell tradition, specify an EXPR beginning | |
7660c0ab | 2248 | with C<'E<gt>&'>, in which case the rest of the string is interpreted as the |
5a964f20 | 2249 | name of a filehandle (or file descriptor, if numeric) to be |
7660c0ab A |
2250 | duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>, |
2251 | C<+E<gt>E<gt>>, and C<+E<lt>>. The | |
a0d0e21e | 2252 | mode you specify should match the mode of the original filehandle. |
184e9718 | 2253 | (Duping a filehandle does not take into account any existing contents of |
cb1a09d0 | 2254 | stdio buffers.) |
a0d0e21e LW |
2255 | Here is a script that saves, redirects, and restores STDOUT and |
2256 | STDERR: | |
2257 | ||
2258 | #!/usr/bin/perl | |
5a964f20 TC |
2259 | open(OLDOUT, ">&STDOUT"); |
2260 | open(OLDERR, ">&STDERR"); | |
a0d0e21e LW |
2261 | |
2262 | open(STDOUT, ">foo.out") || die "Can't redirect stdout"; | |
2263 | open(STDERR, ">&STDOUT") || die "Can't dup stdout"; | |
2264 | ||
2265 | select(STDERR); $| = 1; # make unbuffered | |
2266 | select(STDOUT); $| = 1; # make unbuffered | |
2267 | ||
2268 | print STDOUT "stdout 1\n"; # this works for | |
2269 | print STDERR "stderr 1\n"; # subprocesses too | |
2270 | ||
2271 | close(STDOUT); | |
2272 | close(STDERR); | |
2273 | ||
5a964f20 TC |
2274 | open(STDOUT, ">&OLDOUT"); |
2275 | open(STDERR, ">&OLDERR"); | |
a0d0e21e LW |
2276 | |
2277 | print STDOUT "stdout 2\n"; | |
2278 | print STDERR "stderr 2\n"; | |
2279 | ||
2280 | ||
7660c0ab A |
2281 | If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an |
2282 | equivalent of C's C<fdopen()> of that file descriptor; this is more | |
4633a7c4 | 2283 | parsimonious of file descriptors. For example: |
a0d0e21e LW |
2284 | |
2285 | open(FILEHANDLE, "<&=$fd") | |
2286 | ||
7660c0ab | 2287 | If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then |
a0d0e21e | 2288 | there is an implicit fork done, and the return value of open is the pid |
7660c0ab | 2289 | of the child within the parent process, and C<0> within the child |
184e9718 | 2290 | process. (Use C<defined($pid)> to determine whether the open was successful.) |
a0d0e21e LW |
2291 | The filehandle behaves normally for the parent, but i/o to that |
2292 | filehandle is piped from/to the STDOUT/STDIN of the child process. | |
2293 | In the child process the filehandle isn't opened--i/o happens from/to | |
2294 | the new STDOUT or STDIN. Typically this is used like the normal | |
2295 | piped open when you want to exercise more control over just how the | |
2296 | pipe command gets executed, such as when you are running setuid, and | |
54310121 | 2297 | don't want to have to scan shell commands for metacharacters. |
4633a7c4 | 2298 | The following pairs are more or less equivalent: |
a0d0e21e LW |
2299 | |
2300 | open(FOO, "|tr '[a-z]' '[A-Z]'"); | |
2301 | open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]'; | |
2302 | ||
2303 | open(FOO, "cat -n '$file'|"); | |
2304 | open(FOO, "-|") || exec 'cat', '-n', $file; | |
2305 | ||
4633a7c4 LW |
2306 | See L<perlipc/"Safe Pipe Opens"> for more examples of this. |
2307 | ||
5a964f20 | 2308 | NOTE: On any operation that may do a fork, any unflushed buffers remain |
184e9718 | 2309 | unflushed in both processes, which means you may need to set C<$|> to |
a0d0e21e LW |
2310 | avoid duplicate output. |
2311 | ||
0dccf244 CS |
2312 | Closing any piped filehandle causes the parent process to wait for the |
2313 | child to finish, and returns the status value in C<$?>. | |
2314 | ||
5a964f20 TC |
2315 | The filename passed to open will have leading and trailing |
2316 | whitespace deleted, and the normal redirection chararacters | |
2317 | honored. This property, known as "magic open", | |
2318 | can often be used to good effect. A user could specify a filename of | |
7660c0ab | 2319 | F<"rsh cat file |">, or you could change certain filenames as needed: |
5a964f20 TC |
2320 | |
2321 | $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; | |
2322 | open(FH, $filename) or die "Can't open $filename: $!"; | |
2323 | ||
2324 | However, to open a file with arbitrary weird characters in it, it's | |
2325 | necessary to protect any leading and trailing whitespace: | |
2326 | ||
2327 | $file =~ s#^(\s)#./$1#; | |
2328 | open(FOO, "< $file\0"); | |
2329 | ||
7660c0ab A |
2330 | If you want a "real" C C<open()> (see L<open(2)> on your system), then you |
2331 | should use the C<sysopen()> function, which involves no such magic. This is | |
5a964f20 TC |
2332 | another way to protect your filenames from interpretation. For example: |
2333 | ||
2334 | use IO::Handle; | |
2335 | sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) | |
2336 | or die "sysopen $path: $!"; | |
2337 | $oldfh = select(HANDLE); $| = 1; select($oldfh); | |
2338 | print HANDLE "stuff $$\n"); | |
2339 | seek(HANDLE, 0, 0); | |
2340 | print "File contains: ", <HANDLE>; | |
2341 | ||
7660c0ab A |
2342 | Using the constructor from the C<IO::Handle> package (or one of its |
2343 | subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous | |
5a964f20 TC |
2344 | filehandles that have the scope of whatever variables hold references to |
2345 | them, and automatically close whenever and however you leave that scope: | |
c07a80fd | 2346 | |
5f05dabc | 2347 | use IO::File; |
5a964f20 | 2348 | #... |
c07a80fd | 2349 | sub read_myfile_munged { |
2350 | my $ALL = shift; | |
5f05dabc | 2351 | my $handle = new IO::File; |
c07a80fd | 2352 | open($handle, "myfile") or die "myfile: $!"; |
2353 | $first = <$handle> | |
2354 | or return (); # Automatically closed here. | |
2355 | mung $first or die "mung failed"; # Or here. | |
2356 | return $first, <$handle> if $ALL; # Or here. | |
2357 | $first; # Or here. | |
2358 | } | |
2359 | ||
cb1a09d0 | 2360 | See L</seek()> for some details about mixing reading and writing. |
a0d0e21e LW |
2361 | |
2362 | =item opendir DIRHANDLE,EXPR | |
2363 | ||
7660c0ab A |
2364 | Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>, |
2365 | C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful. | |
a0d0e21e LW |
2366 | DIRHANDLEs have their own namespace separate from FILEHANDLEs. |
2367 | ||
2368 | =item ord EXPR | |
2369 | ||
54310121 | 2370 | =item ord |
bbce6d69 | 2371 | |
a0d0e21e | 2372 | Returns the numeric ascii value of the first character of EXPR. If |
7660c0ab | 2373 | EXPR is omitted, uses C<$_>. For the reverse, see L</chr>. |
a0d0e21e LW |
2374 | |
2375 | =item pack TEMPLATE,LIST | |
2376 | ||
2377 | Takes an array or list of values and packs it into a binary structure, | |
2378 | returning the string containing the structure. The TEMPLATE is a | |
2379 | sequence of characters that give the order and type of values, as | |
2380 | follows: | |
2381 | ||
2382 | A An ascii string, will be space padded. | |
2383 | a An ascii string, will be null padded. | |
2384 | b A bit string (ascending bit order, like vec()). | |
2385 | B A bit string (descending bit order). | |
2386 | h A hex string (low nybble first). | |
2387 | H A hex string (high nybble first). | |
2388 | ||
2389 | c A signed char value. | |
2390 | C An unsigned char value. | |
96e4d5b1 | 2391 | |
a0d0e21e LW |
2392 | s A signed short value. |
2393 | S An unsigned short value. | |
96e4d5b1 | 2394 | (This 'short' is _exactly_ 16 bits, which may differ from |
2395 | what a local C compiler calls 'short'.) | |
2396 | ||
a0d0e21e LW |
2397 | i A signed integer value. |
2398 | I An unsigned integer value. | |
96e4d5b1 | 2399 | (This 'integer' is _at_least_ 32 bits wide. Its exact size |
2400 | depends on what a local C compiler calls 'int', and may | |
2401 | even be larger than the 'long' described in the next item.) | |
2402 | ||
a0d0e21e LW |
2403 | l A signed long value. |
2404 | L An unsigned long value. | |
96e4d5b1 | 2405 | (This 'long' is _exactly_ 32 bits, which may differ from |
2406 | what a local C compiler calls 'long'.) | |
a0d0e21e | 2407 | |
96e4d5b1 | 2408 | n A short in "network" (big-endian) order. |
2409 | N A long in "network" (big-endian) order. | |
a0d0e21e LW |
2410 | v A short in "VAX" (little-endian) order. |
2411 | V A long in "VAX" (little-endian) order. | |
96e4d5b1 | 2412 | (These 'shorts' and 'longs' are _exactly_ 16 bits and |
2413 | _exactly_ 32 bits, respectively.) | |
a0d0e21e LW |
2414 | |
2415 | f A single-precision float in the native format. | |
2416 | d A double-precision float in the native format. | |
2417 | ||
2418 | p A pointer to a null-terminated string. | |
2419 | P A pointer to a structure (fixed-length string). | |
2420 | ||
2421 | u A uuencoded string. | |
2422 | ||
96e4d5b1 | 2423 | w A BER compressed integer. Its bytes represent an unsigned |
2424 | integer in base 128, most significant digit first, with as few | |
2425 | digits as possible. Bit eight (the high bit) is set on each | |
2426 | byte except the last. | |
def98dd4 | 2427 | |
a0d0e21e LW |
2428 | x A null byte. |
2429 | X Back up a byte. | |
2430 | @ Null fill to absolute position. | |
2431 | ||
5a964f20 | 2432 | Each letter may optionally be followed by a number giving a repeat |
7660c0ab A |
2433 | count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the |
2434 | pack function will gobble up that many values from the LIST. A C<*> for the | |
2435 | repeat count means to use however many items are left. The C<"a"> and C<"A"> | |
a0d0e21e | 2436 | types gobble just one value, but pack it as a string of length count, |
7660c0ab A |
2437 | padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips |
2438 | trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B"> | |
2439 | fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a | |
2440 | string that many nybbles long. The C<"p"> type packs a pointer to a null- | |
84902520 TB |
2441 | terminated string. You are responsible for ensuring the string is not a |
2442 | temporary value (which can potentially get deallocated before you get | |
7660c0ab | 2443 | around to using the packed result). The C<"P"> packs a pointer to a structure |
61167c6f | 2444 | of the size indicated by the length. A NULL pointer is created if the |
7660c0ab | 2445 | corresponding value for C<"p"> or C<"P"> is C<undef>. |
61167c6f | 2446 | Real numbers (floats and doubles) are |
a0d0e21e LW |
2447 | in the native machine format only; due to the multiplicity of floating |
2448 | formats around, and the lack of a standard "network" representation, no | |
2449 | facility for interchange has been made. This means that packed floating | |
2450 | point data written on one machine may not be readable on another - even if | |
2451 | both use IEEE floating point arithmetic (as the endian-ness of the memory | |
2452 | representation is not part of the IEEE spec). Note that Perl uses doubles | |
2453 | internally for all numeric calculation, and converting from double into | |
5f05dabc | 2454 | float and thence back to double again will lose precision (i.e., |
7660c0ab | 2455 | C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>). |
a0d0e21e LW |
2456 | |
2457 | Examples: | |
2458 | ||
2459 | $foo = pack("cccc",65,66,67,68); | |
2460 | # foo eq "ABCD" | |
2461 | $foo = pack("c4",65,66,67,68); | |
2462 | # same thing | |
2463 | ||
2464 | $foo = pack("ccxxcc",65,66,67,68); | |
2465 | # foo eq "AB\0\0CD" | |
2466 | ||
2467 | $foo = pack("s2",1,2); | |
2468 | # "\1\0\2\0" on little-endian | |
2469 | # "\0\1\0\2" on big-endian | |
2470 | ||
2471 | $foo = pack("a4","abcd","x","y","z"); | |
2472 | # "abcd" | |
2473 | ||
2474 | $foo = pack("aaaa","abcd","x","y","z"); | |
2475 | # "axyz" | |
2476 | ||
2477 | $foo = pack("a14","abcdefg"); | |
2478 | # "abcdefg\0\0\0\0\0\0\0" | |
2479 | ||
2480 | $foo = pack("i9pl", gmtime); | |
2481 | # a real struct tm (on my system anyway) | |
2482 | ||
2483 | sub bintodec { | |
2484 | unpack("N", pack("B32", substr("0" x 32 . shift, -32))); | |
2485 | } | |
2486 | ||
2487 | The same template may generally also be used in the unpack function. | |
2488 | ||
5a964f20 TC |
2489 | =item package |
2490 | ||
cb1a09d0 AD |
2491 | =item package NAMESPACE |
2492 | ||
2493 | Declares the compilation unit as being in the given namespace. The scope | |
2494 | of the package declaration is from the declaration itself through the end of | |
7660c0ab | 2495 | the enclosing block (the same scope as the C<local()> operator). All further |
cb1a09d0 | 2496 | unqualified dynamic identifiers will be in this namespace. A package |
5f05dabc | 2497 | statement affects only dynamic variables--including those you've used |
7660c0ab | 2498 | C<local()> on--but I<not> lexical variables created with C<my()>. Typically it |
cb1a09d0 AD |
2499 | would be the first declaration in a file to be included by the C<require> |
2500 | or C<use> operator. You can switch into a package in more than one place; | |
5a964f20 | 2501 | it merely influences which symbol table is used by the compiler for the |
cb1a09d0 AD |
2502 | rest of that block. You can refer to variables and filehandles in other |
2503 | packages by prefixing the identifier with the package name and a double | |
2504 | colon: C<$Package::Variable>. If the package name is null, the C<main> | |
2505 | package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>. | |
2506 | ||
5a964f20 TC |
2507 | If NAMESPACE is omitted, then there is no current package, and all |
2508 | identifiers must be fully qualified or lexicals. This is stricter | |
2509 | than C<use strict>, since it also extends to function names. | |
2510 | ||
cb1a09d0 AD |
2511 | See L<perlmod/"Packages"> for more information about packages, modules, |
2512 | and classes. See L<perlsub> for other scoping issues. | |
2513 | ||
a0d0e21e LW |
2514 | =item pipe READHANDLE,WRITEHANDLE |
2515 | ||
2516 | Opens a pair of connected pipes like the corresponding system call. | |
2517 | Note that if you set up a loop of piped processes, deadlock can occur | |
2518 | unless you are very careful. In addition, note that Perl's pipes use | |
184e9718 | 2519 | stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE |
a0d0e21e LW |
2520 | after each command, depending on the application. |
2521 | ||
7e1af8bc | 2522 | See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication"> |
4633a7c4 LW |
2523 | for examples of such things. |
2524 | ||
a0d0e21e LW |
2525 | =item pop ARRAY |
2526 | ||
54310121 | 2527 | =item pop |
28757baa | 2528 | |
a0d0e21e LW |
2529 | Pops and returns the last value of the array, shortening the array by |
2530 | 1. Has a similar effect to | |
2531 | ||
2532 | $tmp = $ARRAY[$#ARRAY--]; | |
2533 | ||
2534 | If there are no elements in the array, returns the undefined value. | |
cb1a09d0 | 2535 | If ARRAY is omitted, pops the |
7660c0ab A |
2536 | C<@ARGV> array in the main program, and the C<@_> array in subroutines, just |
2537 | like C<shift()>. | |
a0d0e21e LW |
2538 | |
2539 | =item pos SCALAR | |
2540 | ||
54310121 | 2541 | =item pos |
bbce6d69 | 2542 | |
4633a7c4 | 2543 | Returns the offset of where the last C<m//g> search left off for the variable |
7660c0ab | 2544 | is in question (C<$_> is used when the variable is not specified). May be |
44a8e56a | 2545 | modified to change that offset. Such modification will also influence |
2546 | the C<\G> zero-width assertion in regular expressions. See L<perlre> and | |
2547 | L<perlop>. | |
a0d0e21e LW |
2548 | |
2549 | =item print FILEHANDLE LIST | |
2550 | ||
2551 | =item print LIST | |
2552 | ||
2553 | =item print | |
2554 | ||
cb1a09d0 | 2555 | Prints a string or a comma-separated list of strings. Returns TRUE |
a0d0e21e | 2556 | if successful. FILEHANDLE may be a scalar variable name, in which case |
cb1a09d0 | 2557 | the variable contains the name of or a reference to the filehandle, thus introducing one |
a0d0e21e LW |
2558 | level of indirection. (NOTE: If FILEHANDLE is a variable and the next |
2559 | token is a term, it may be misinterpreted as an operator unless you | |
7660c0ab | 2560 | interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is |
a0d0e21e | 2561 | omitted, prints by default to standard output (or to the last selected |
7660c0ab | 2562 | output channel--see L</select>). If LIST is also omitted, prints C<$_> to |
5a964f20 | 2563 | the currently selected output channel. To set the default output channel to something other than |
a0d0e21e | 2564 | STDOUT use the select operation. Note that, because print takes a |
5a964f20 | 2565 | LIST, anything in the LIST is evaluated in list context, and any |
a0d0e21e | 2566 | subroutine that you call will have one or more of its expressions |
5a964f20 | 2567 | evaluated in list context. Also be careful not to follow the print |
a0d0e21e | 2568 | keyword with a left parenthesis unless you want the corresponding right |
7660c0ab | 2569 | parenthesis to terminate the arguments to the print--interpose a C<+> or |
5f05dabc | 2570 | put parentheses around all the arguments. |
a0d0e21e | 2571 | |
4633a7c4 | 2572 | Note that if you're storing FILEHANDLES in an array or other expression, |
da0045b7 | 2573 | you will have to use a block returning its value instead: |
4633a7c4 LW |
2574 | |
2575 | print { $files[$i] } "stuff\n"; | |
2576 | print { $OK ? STDOUT : STDERR } "stuff\n"; | |
2577 | ||
5f05dabc | 2578 | =item printf FILEHANDLE FORMAT, LIST |
a0d0e21e | 2579 | |
5f05dabc | 2580 | =item printf FORMAT, LIST |
a0d0e21e | 2581 | |
7660c0ab | 2582 | Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\> |
a3cb178b | 2583 | (the output record separator) is not appended. The first argument |
ab4f32c2 | 2584 | of the list will be interpreted as the C<printf()> format. If C<use locale> is |
a034a98d DD |
2585 | in effect, the character used for the decimal point in formatted real numbers |
2586 | is affected by the LC_NUMERIC locale. See L<perllocale>. | |
a0d0e21e | 2587 | |
7660c0ab A |
2588 | Don't fall into the trap of using a C<printf()> when a simple |
2589 | C<print()> would do. The C<print()> is more efficient and less | |
28757baa | 2590 | error prone. |
2591 | ||
da0045b7 | 2592 | =item prototype FUNCTION |
2593 | ||
2594 | Returns the prototype of a function as a string (or C<undef> if the | |
5f05dabc | 2595 | function has no prototype). FUNCTION is a reference to, or the name of, |
2596 | the function whose prototype you want to retrieve. | |
da0045b7 | 2597 | |
b6c543e3 IZ |
2598 | If FUNCTION is a string starting with C<CORE::>, the rest is taken as |
2599 | a name for Perl builtin. If builtin is not I<overridable> (such as | |
ab4f32c2 A |
2600 | C<qw//>) or its arguments cannot be expressed by a prototype (such as |
2601 | C<system()>) - in other words, the builtin does not behave like a Perl | |
b6c543e3 IZ |
2602 | function - returns C<undef>. Otherwise, the string describing the |
2603 | equivalent prototype is returned. | |
2604 | ||
a0d0e21e LW |
2605 | =item push ARRAY,LIST |
2606 | ||
2607 | Treats ARRAY as a stack, and pushes the values of LIST | |
2608 | onto the end of ARRAY. The length of ARRAY increases by the length of | |
2609 | LIST. Has the same effect as | |
2610 | ||
2611 | for $value (LIST) { | |
2612 | $ARRAY[++$#ARRAY] = $value; | |
2613 | } | |
2614 | ||
2615 | but is more efficient. Returns the new number of elements in the array. | |
2616 | ||
2617 | =item q/STRING/ | |
2618 | ||
2619 | =item qq/STRING/ | |
2620 | ||
8782bef2 GB |
2621 | =item qr/STRING/ |
2622 | ||
a0d0e21e LW |
2623 | =item qx/STRING/ |
2624 | ||
2625 | =item qw/STRING/ | |
2626 | ||
2627 | Generalized quotes. See L<perlop>. | |
2628 | ||
2629 | =item quotemeta EXPR | |
2630 | ||
54310121 | 2631 | =item quotemeta |
bbce6d69 | 2632 | |
68dc0745 | 2633 | Returns the value of EXPR with all non-alphanumeric |
a034a98d DD |
2634 | characters backslashed. (That is, all characters not matching |
2635 | C</[A-Za-z_0-9]/> will be preceded by a backslash in the | |
2636 | returned string, regardless of any locale settings.) | |
2637 | This is the internal function implementing | |
7660c0ab | 2638 | the C<\Q> escape in double-quoted strings. |
a0d0e21e | 2639 | |
7660c0ab | 2640 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 2641 | |
a0d0e21e LW |
2642 | =item rand EXPR |
2643 | ||
2644 | =item rand | |
2645 | ||
7660c0ab | 2646 | Returns a random fractional number greater than or equal to C<0> and less |
3e3baf6d | 2647 | than the value of EXPR. (EXPR should be positive.) If EXPR is |
7660c0ab A |
2648 | omitted, the value C<1> is used. Automatically calls C<srand()> unless |
2649 | C<srand()> has already been called. See also C<srand()>. | |
a0d0e21e | 2650 | |
2f9daede | 2651 | (Note: If your rand function consistently returns numbers that are too |
a0d0e21e | 2652 | large or too small, then your version of Perl was probably compiled |
2f9daede | 2653 | with the wrong number of RANDBITS.) |
a0d0e21e LW |
2654 | |
2655 | =item read FILEHANDLE,SCALAR,LENGTH,OFFSET | |
2656 | ||
2657 | =item read FILEHANDLE,SCALAR,LENGTH | |
2658 | ||
2659 | Attempts to read LENGTH bytes of data into variable SCALAR from the | |
3b02c43c GS |
2660 | specified FILEHANDLE. Returns the number of bytes actually read, |
2661 | C<0> at end of file, or undef if there was an error. SCALAR will be grown | |
2662 | or shrunk to the length actually read. An OFFSET may be specified to | |
2663 | place the read data at some other place than the beginning of the | |
7660c0ab A |
2664 | string. This call is actually implemented in terms of stdio's C<fread(3)> |
2665 | call. To get a true C<read(2)> system call, see C<sysread()>. | |
a0d0e21e LW |
2666 | |
2667 | =item readdir DIRHANDLE | |
2668 | ||
7660c0ab | 2669 | Returns the next directory entry for a directory opened by C<opendir()>. |
5a964f20 | 2670 | If used in list context, returns all the rest of the entries in the |
a0d0e21e | 2671 | directory. If there are no more entries, returns an undefined value in |
5a964f20 | 2672 | scalar context or a null list in list context. |
a0d0e21e | 2673 | |
7660c0ab | 2674 | If you're planning to filetest the return values out of a C<readdir()>, you'd |
5f05dabc | 2675 | better prepend the directory in question. Otherwise, because we didn't |
7660c0ab | 2676 | C<chdir()> there, it would have been testing the wrong file. |
cb1a09d0 AD |
2677 | |
2678 | opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!"; | |
2679 | @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR); | |
2680 | closedir DIR; | |
2681 | ||
84902520 TB |
2682 | =item readline EXPR |
2683 | ||
5a964f20 | 2684 | Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line |
84902520 TB |
2685 | is read and returned. In list context, reads until end-of-file is |
2686 | reached and returns a list of lines (however you've defined lines | |
7660c0ab | 2687 | with C<$/> or C<$INPUT_RECORD_SEPARATOR>). |
84902520 TB |
2688 | This is the internal function implementing the C<E<lt>EXPRE<gt>> |
2689 | operator, but you can use it directly. The C<E<lt>EXPRE<gt>> | |
2690 | operator is discussed in more detail in L<perlop/"I/O Operators">. | |
2691 | ||
5a964f20 TC |
2692 | $line = <STDIN>; |
2693 | $line = readline(*STDIN); # same thing | |
2694 | ||
a0d0e21e LW |
2695 | =item readlink EXPR |
2696 | ||
54310121 | 2697 | =item readlink |
bbce6d69 | 2698 | |
a0d0e21e LW |
2699 | Returns the value of a symbolic link, if symbolic links are |
2700 | implemented. If not, gives a fatal error. If there is some system | |
184e9718 | 2701 | error, returns the undefined value and sets C<$!> (errno). If EXPR is |
7660c0ab | 2702 | omitted, uses C<$_>. |
a0d0e21e | 2703 | |
84902520 TB |
2704 | =item readpipe EXPR |
2705 | ||
5a964f20 | 2706 | EXPR is executed as a system command. |
84902520 TB |
2707 | The collected standard output of the command is returned. |
2708 | In scalar context, it comes back as a single (potentially | |
2709 | multi-line) string. In list context, returns a list of lines | |
7660c0ab | 2710 | (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>). |
84902520 TB |
2711 | This is the internal function implementing the C<qx/EXPR/> |
2712 | operator, but you can use it directly. The C<qx/EXPR/> | |
2713 | operator is discussed in more detail in L<perlop/"I/O Operators">. | |
2714 | ||
a0d0e21e LW |
2715 | =item recv SOCKET,SCALAR,LEN,FLAGS |
2716 | ||
2717 | Receives a message on a socket. Attempts to receive LENGTH bytes of | |
2718 | data into variable SCALAR from the specified SOCKET filehandle. | |
7660c0ab | 2719 | Actually does a C C<recvfrom()>, so that it can return the address of the |
a0d0e21e LW |
2720 | sender. Returns the undefined value if there's an error. SCALAR will |
2721 | be grown or shrunk to the length actually read. Takes the same flags | |
54310121 | 2722 | as the system call of the same name. |
4633a7c4 | 2723 | See L<perlipc/"UDP: Message Passing"> for examples. |
a0d0e21e LW |
2724 | |
2725 | =item redo LABEL | |
2726 | ||
2727 | =item redo | |
2728 | ||
2729 | The C<redo> command restarts the loop block without evaluating the | |
2730 | conditional again. The C<continue> block, if any, is not executed. If | |
2731 | the LABEL is omitted, the command refers to the innermost enclosing | |
2732 | loop. This command is normally used by programs that want to lie to | |
2733 | themselves about what was just input: | |
2734 | ||
2735 | # a simpleminded Pascal comment stripper | |
2736 | # (warning: assumes no { or } in strings) | |
4633a7c4 | 2737 | LINE: while (<STDIN>) { |
a0d0e21e LW |
2738 | while (s|({.*}.*){.*}|$1 |) {} |
2739 | s|{.*}| |; | |
2740 | if (s|{.*| |) { | |
2741 | $front = $_; | |
2742 | while (<STDIN>) { | |
2743 | if (/}/) { # end of comment? | |
5a964f20 | 2744 | s|^|$front\{|; |
4633a7c4 | 2745 | redo LINE; |
a0d0e21e LW |
2746 | } |
2747 | } | |
2748 | } | |
2749 | print; | |
2750 | } | |
2751 | ||
1d2dff63 GS |
2752 | See also L</continue> for an illustration of how C<last>, C<next>, and |
2753 | C<redo> work. | |
2754 | ||
a0d0e21e LW |
2755 | =item ref EXPR |
2756 | ||
54310121 | 2757 | =item ref |
bbce6d69 | 2758 | |
2f9daede | 2759 | Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR |
7660c0ab | 2760 | is not specified, C<$_> will be used. The value returned depends on the |
bbce6d69 | 2761 | type of thing the reference is a reference to. |
a0d0e21e LW |
2762 | Builtin types include: |
2763 | ||
2764 | REF | |
2765 | SCALAR | |
2766 | ARRAY | |
2767 | HASH | |
2768 | CODE | |
2769 | GLOB | |
2770 | ||
54310121 | 2771 | If the referenced object has been blessed into a package, then that package |
7660c0ab | 2772 | name is returned instead. You can think of C<ref()> as a C<typeof()> operator. |
a0d0e21e LW |
2773 | |
2774 | if (ref($r) eq "HASH") { | |
aa689395 | 2775 | print "r is a reference to a hash.\n"; |
54310121 | 2776 | } |
5a964f20 | 2777 | if (!ref($r)) { |
a0d0e21e | 2778 | print "r is not a reference at all.\n"; |
54310121 | 2779 | } |
a0d0e21e LW |
2780 | |
2781 | See also L<perlref>. | |
2782 | ||
2783 | =item rename OLDNAME,NEWNAME | |
2784 | ||
7660c0ab | 2785 | Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will |
5f05dabc | 2786 | not work across file system boundaries. |
a0d0e21e LW |
2787 | |
2788 | =item require EXPR | |
2789 | ||
2790 | =item require | |
2791 | ||
7660c0ab | 2792 | Demands some semantics specified by EXPR, or by C<$_> if EXPR is not |
a0d0e21e | 2793 | supplied. If EXPR is numeric, demands that the current version of Perl |
184e9718 | 2794 | (C<$]> or $PERL_VERSION) be equal or greater than EXPR. |
a0d0e21e LW |
2795 | |
2796 | Otherwise, demands that a library file be included if it hasn't already | |
2797 | been included. The file is included via the do-FILE mechanism, which is | |
7660c0ab | 2798 | essentially just a variety of C<eval()>. Has semantics similar to the following |
a0d0e21e LW |
2799 | subroutine: |
2800 | ||
2801 | sub require { | |
5a964f20 | 2802 | my($filename) = @_; |
a0d0e21e | 2803 | return 1 if $INC{$filename}; |
5a964f20 | 2804 | my($realfilename,$result); |
a0d0e21e LW |
2805 | ITER: { |
2806 | foreach $prefix (@INC) { | |
2807 | $realfilename = "$prefix/$filename"; | |
2808 | if (-f $realfilename) { | |
2809 | $result = do $realfilename; | |
2810 | last ITER; | |
2811 | } | |
2812 | } | |
2813 | die "Can't find $filename in \@INC"; | |
2814 | } | |
2815 | die $@ if $@; | |
2816 | die "$filename did not return true value" unless $result; | |
2817 | $INC{$filename} = $realfilename; | |
5a964f20 | 2818 | return $result; |
a0d0e21e LW |
2819 | } |
2820 | ||
2821 | Note that the file will not be included twice under the same specified | |
2822 | name. The file must return TRUE as the last statement to indicate | |
2823 | successful execution of any initialization code, so it's customary to | |
7660c0ab | 2824 | end such a file with "C<1;>" unless you're sure it'll return TRUE |
a0d0e21e LW |
2825 | otherwise. But it's better just to put the "C<1;>", in case you add more |
2826 | statements. | |
2827 | ||
54310121 | 2828 | If EXPR is a bareword, the require assumes a "F<.pm>" extension and |
da0045b7 | 2829 | replaces "F<::>" with "F</>" in the filename for you, |
54310121 | 2830 | to make it easy to load standard modules. This form of loading of |
a0d0e21e LW |
2831 | modules does not risk altering your namespace. |
2832 | ||
ee580363 GS |
2833 | In other words, if you try this: |
2834 | ||
5a964f20 | 2835 | require Foo::Bar; # a splendid bareword |
ee580363 | 2836 | |
7660c0ab A |
2837 | The require function will actually look for the "F<Foo/Bar.pm>" file in the |
2838 | directories specified in the C<@INC> array. | |
ee580363 | 2839 | |
5a964f20 | 2840 | But if you try this: |
ee580363 GS |
2841 | |
2842 | $class = 'Foo::Bar'; | |
5a964f20 TC |
2843 | require $class; # $class is not a bareword |
2844 | #or | |
2845 | require "Foo::Bar"; # not a bareword because of the "" | |
ee580363 | 2846 | |
7660c0ab A |
2847 | The require function will look for the "F<Foo::Bar>" file in the @INC array and |
2848 | will complain about not finding "F<Foo::Bar>" there. In this case you can do: | |
ee580363 GS |
2849 | |
2850 | eval "require $class"; | |
2851 | ||
2852 | For a yet-more-powerful import facility, see L</use> and L<perlmod>. | |
a0d0e21e LW |
2853 | |
2854 | =item reset EXPR | |
2855 | ||
2856 | =item reset | |
2857 | ||
2858 | Generally used in a C<continue> block at the end of a loop to clear | |
7660c0ab | 2859 | variables and reset C<??> searches so that they work again. The |
a0d0e21e LW |
2860 | expression is interpreted as a list of single characters (hyphens |
2861 | allowed for ranges). All variables and arrays beginning with one of | |
2862 | those letters are reset to their pristine state. If the expression is | |
7660c0ab | 2863 | omitted, one-match searches (C<?pattern?>) are reset to match again. Resets |
5f05dabc | 2864 | only variables or searches in the current package. Always returns |
a0d0e21e LW |
2865 | 1. Examples: |
2866 | ||
2867 | reset 'X'; # reset all X variables | |
2868 | reset 'a-z'; # reset lower case variables | |
2869 | reset; # just reset ?? searches | |
2870 | ||
7660c0ab A |
2871 | Resetting C<"A-Z"> is not recommended because you'll wipe out your |
2872 | C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables | |
a0d0e21e | 2873 | are unaffected, but they clean themselves up on scope exit anyway, |
da0045b7 | 2874 | so you'll probably want to use them instead. See L</my>. |
a0d0e21e | 2875 | |
54310121 | 2876 | =item return EXPR |
2877 | ||
2878 | =item return | |
2879 | ||
7660c0ab | 2880 | Returns from a subroutine, C<eval()>, or C<do FILE> with the value |
5a964f20 | 2881 | given in EXPR. Evaluation of EXPR may be in list, scalar, or void |
54310121 | 2882 | context, depending on how the return value will be used, and the context |
7660c0ab | 2883 | may vary from one execution to the next (see C<wantarray()>). If no EXPR |
5a964f20 TC |
2884 | is given, returns an empty list in list context, an undefined value in |
2885 | scalar context, or nothing in a void context. | |
a0d0e21e | 2886 | |
68dc0745 | 2887 | (Note that in the absence of a return, a subroutine, eval, or do FILE |
2888 | will automatically return the value of the last expression evaluated.) | |
a0d0e21e LW |
2889 | |
2890 | =item reverse LIST | |
2891 | ||
5a964f20 TC |
2892 | In list context, returns a list value consisting of the elements |
2893 | of LIST in the opposite order. In scalar context, concatenates the | |
2f9daede TP |
2894 | elements of LIST, and returns a string value consisting of those bytes, |
2895 | but in the opposite order. | |
4633a7c4 | 2896 | |
2f9daede | 2897 | print reverse <>; # line tac, last line first |
4633a7c4 | 2898 | |
2f9daede TP |
2899 | undef $/; # for efficiency of <> |
2900 | print scalar reverse <>; # byte tac, last line tsrif | |
2901 | ||
2902 | This operator is also handy for inverting a hash, although there are some | |
2903 | caveats. If a value is duplicated in the original hash, only one of those | |
2904 | can be represented as a key in the inverted hash. Also, this has to | |
2905 | unwind one hash and build a whole new one, which may take some time | |
2906 | on a large hash. | |
2907 | ||
2908 | %by_name = reverse %by_address; # Invert the hash | |
a0d0e21e LW |
2909 | |
2910 | =item rewinddir DIRHANDLE | |
2911 | ||
2912 | Sets the current position to the beginning of the directory for the | |
7660c0ab | 2913 | C<readdir()> routine on DIRHANDLE. |
a0d0e21e LW |
2914 | |
2915 | =item rindex STR,SUBSTR,POSITION | |
2916 | ||
2917 | =item rindex STR,SUBSTR | |
2918 | ||
2919 | Works just like index except that it returns the position of the LAST | |
2920 | occurrence of SUBSTR in STR. If POSITION is specified, returns the | |
2921 | last occurrence at or before that position. | |
2922 | ||
2923 | =item rmdir FILENAME | |
2924 | ||
54310121 | 2925 | =item rmdir |
bbce6d69 | 2926 | |
5a964f20 TC |
2927 | Deletes the directory specified by FILENAME if that directory is empty. If it |
2928 | succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If | |
7660c0ab | 2929 | FILENAME is omitted, uses C<$_>. |
a0d0e21e LW |
2930 | |
2931 | =item s/// | |
2932 | ||
2933 | The substitution operator. See L<perlop>. | |
2934 | ||
2935 | =item scalar EXPR | |
2936 | ||
5a964f20 | 2937 | Forces EXPR to be interpreted in scalar context and returns the value |
54310121 | 2938 | of EXPR. |
cb1a09d0 AD |
2939 | |
2940 | @counts = ( scalar @a, scalar @b, scalar @c ); | |
2941 | ||
54310121 | 2942 | There is no equivalent operator to force an expression to |
5a964f20 | 2943 | be interpolated in list context because it's in practice never |
cb1a09d0 AD |
2944 | needed. If you really wanted to do so, however, you could use |
2945 | the construction C<@{[ (some expression) ]}>, but usually a simple | |
2946 | C<(some expression)> suffices. | |
a0d0e21e LW |
2947 | |
2948 | =item seek FILEHANDLE,POSITION,WHENCE | |
2949 | ||
ab4f32c2 | 2950 | Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>. |
8903cb82 | 2951 | FILEHANDLE may be an expression whose value gives the name of the |
7660c0ab A |
2952 | filehandle. The values for WHENCE are C<0> to set the new position to |
2953 | POSITION, C<1> to set it to the current position plus POSITION, and C<2> to | |
8903cb82 | 2954 | set it to EOF plus POSITION (typically negative). For WHENCE you may |
7660c0ab A |
2955 | use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the |
2956 | C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise. | |
8903cb82 | 2957 | |
7660c0ab A |
2958 | If you want to position file for C<sysread()> or C<syswrite()>, don't use |
2959 | C<seek()> -- buffering makes its effect on the file's system position | |
2960 | unpredictable and non-portable. Use C<sysseek()> instead. | |
a0d0e21e | 2961 | |
cb1a09d0 AD |
2962 | On some systems you have to do a seek whenever you switch between reading |
2963 | and writing. Amongst other things, this may have the effect of calling | |
7660c0ab | 2964 | stdio's C<clearerr(3)>. A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving |
8903cb82 | 2965 | the file position: |
cb1a09d0 AD |
2966 | |
2967 | seek(TEST,0,1); | |
2968 | ||
2969 | This is also useful for applications emulating C<tail -f>. Once you hit | |
2970 | EOF on your read, and then sleep for a while, you might have to stick in a | |
7660c0ab | 2971 | seek() to reset things. The C<seek()> doesn't change the current position, |
8903cb82 | 2972 | but it I<does> clear the end-of-file condition on the handle, so that the |
2973 | next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope. | |
cb1a09d0 AD |
2974 | |
2975 | If that doesn't work (some stdios are particularly cantankerous), then | |
2976 | you may need something more like this: | |
2977 | ||
2978 | for (;;) { | |
2979 | for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) { | |
2980 | # search for some stuff and put it into files | |
2981 | } | |
2982 | sleep($for_a_while); | |
2983 | seek(FILE, $curpos, 0); | |
2984 | } | |
2985 | ||
a0d0e21e LW |
2986 | =item seekdir DIRHANDLE,POS |
2987 | ||
7660c0ab A |
2988 | Sets the current position for the C<readdir()> routine on DIRHANDLE. POS |
2989 | must be a value returned by C<telldir()>. Has the same caveats about | |
a0d0e21e LW |
2990 | possible directory compaction as the corresponding system library |
2991 | routine. | |
2992 | ||
2993 | =item select FILEHANDLE | |
2994 | ||
2995 | =item select | |
2996 | ||
2997 | Returns the currently selected filehandle. Sets the current default | |
2998 | filehandle for output, if FILEHANDLE is supplied. This has two | |
ab4f32c2 | 2999 | effects: first, a C<write()> or a C<print()> without a filehandle will |
a0d0e21e LW |
3000 | default to this FILEHANDLE. Second, references to variables related to |
3001 | output will refer to this output channel. For example, if you have to | |
3002 | set the top of form format for more than one output channel, you might | |
3003 | do the following: | |
3004 | ||
3005 | select(REPORT1); | |
3006 | $^ = 'report1_top'; | |
3007 | select(REPORT2); | |
3008 | $^ = 'report2_top'; | |
3009 | ||
3010 | FILEHANDLE may be an expression whose value gives the name of the | |
3011 | actual filehandle. Thus: | |
3012 | ||
3013 | $oldfh = select(STDERR); $| = 1; select($oldfh); | |
3014 | ||
4633a7c4 LW |
3015 | Some programmers may prefer to think of filehandles as objects with |
3016 | methods, preferring to write the last example as: | |
a0d0e21e | 3017 | |
28757baa | 3018 | use IO::Handle; |
a0d0e21e LW |
3019 | STDERR->autoflush(1); |
3020 | ||
3021 | =item select RBITS,WBITS,EBITS,TIMEOUT | |
3022 | ||
7660c0ab A |
3023 | This calls the C<select(2)> system call with the bit masks specified, which |
3024 | can be constructed using C<fileno()> and C<vec()>, along these lines: | |
a0d0e21e LW |
3025 | |
3026 | $rin = $win = $ein = ''; | |
3027 | vec($rin,fileno(STDIN),1) = 1; | |
3028 | vec($win,fileno(STDOUT),1) = 1; | |
3029 | $ein = $rin | $win; | |
3030 | ||
3031 | If you want to select on many filehandles you might wish to write a | |
3032 | subroutine: | |
3033 | ||
3034 | sub fhbits { | |
5a964f20 TC |
3035 | my(@fhlist) = split(' ',$_[0]); |
3036 | my($bits); | |
a0d0e21e LW |
3037 | for (@fhlist) { |
3038 | vec($bits,fileno($_),1) = 1; | |
3039 | } | |
3040 | $bits; | |
3041 | } | |
4633a7c4 | 3042 | $rin = fhbits('STDIN TTY SOCK'); |
a0d0e21e LW |
3043 | |
3044 | The usual idiom is: | |
3045 | ||
3046 | ($nfound,$timeleft) = | |
3047 | select($rout=$rin, $wout=$win, $eout=$ein, $timeout); | |
3048 | ||
54310121 | 3049 | or to block until something becomes ready just do this |
a0d0e21e LW |
3050 | |
3051 | $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); | |
3052 | ||
7660c0ab A |
3053 | Most systems do not bother to return anything useful in C<$timeleft>, so |
3054 | calling select() in scalar context just returns C<$nfound>. | |
c07a80fd | 3055 | |
5f05dabc | 3056 | Any of the bit masks can also be undef. The timeout, if specified, is |
a0d0e21e | 3057 | in seconds, which may be fractional. Note: not all implementations are |
7660c0ab A |
3058 | capable of returning theC<$timeleft>. If not, they always return |
3059 | C<$timeleft> equal to the supplied C<$timeout>. | |
a0d0e21e | 3060 | |
ff68c719 | 3061 | You can effect a sleep of 250 milliseconds this way: |
a0d0e21e LW |
3062 | |
3063 | select(undef, undef, undef, 0.25); | |
3064 | ||
7660c0ab A |
3065 | B<WARNING>: One should not attempt to mix buffered I/O (like C<read()> |
3066 | or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even | |
3067 | then only on POSIX systems. You have to use C<sysread()> instead. | |
a0d0e21e LW |
3068 | |
3069 | =item semctl ID,SEMNUM,CMD,ARG | |
3070 | ||
ab4f32c2 | 3071 | Calls the System V IPC function C<semctl()>. You'll probably have to say |
0ade1984 JH |
3072 | |
3073 | use IPC::SysV; | |
3074 | ||
3075 | first to get the correct constant definitions. If CMD is IPC_STAT or | |
3076 | GETALL, then ARG must be a variable which will hold the returned | |
ab4f32c2 | 3077 | semid_ds structure or semaphore value array. Returns like C<ioctl()>: the |
7660c0ab A |
3078 | undefined value for error, "C<0> but true" for zero, or the actual return |
3079 | value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation. | |
a0d0e21e LW |
3080 | |
3081 | =item semget KEY,NSEMS,FLAGS | |
3082 | ||
3083 | Calls the System V IPC function semget. Returns the semaphore id, or | |
7660c0ab A |
3084 | the undefined value if there is an error. See also C<IPC::SysV> and |
3085 | C<IPC::SysV::Semaphore> documentation. | |
a0d0e21e LW |
3086 | |
3087 | =item semop KEY,OPSTRING | |
3088 | ||
3089 | Calls the System V IPC function semop to perform semaphore operations | |
3090 | such as signaling and waiting. OPSTRING must be a packed array of | |
3091 | semop structures. Each semop structure can be generated with | |
3092 | C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore | |
3093 | operations is implied by the length of OPSTRING. Returns TRUE if | |
3094 | successful, or FALSE if there is an error. As an example, the | |
7660c0ab | 3095 | following code waits on semaphore C<$semnum> of semaphore id C<$semid>: |
a0d0e21e LW |
3096 | |
3097 | $semop = pack("sss", $semnum, -1, 0); | |
3098 | die "Semaphore trouble: $!\n" unless semop($semid, $semop); | |
3099 | ||
7660c0ab A |
3100 | To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV> |
3101 | and C<IPC::SysV::Semaphore> documentation. | |
a0d0e21e LW |
3102 | |
3103 | =item send SOCKET,MSG,FLAGS,TO | |
3104 | ||
3105 | =item send SOCKET,MSG,FLAGS | |
3106 | ||
3107 | Sends a message on a socket. Takes the same flags as the system call | |
3108 | of the same name. On unconnected sockets you must specify a | |
7660c0ab | 3109 | destination to send TO, in which case it does a C C<sendto()>. Returns |
a0d0e21e LW |
3110 | the number of characters sent, or the undefined value if there is an |
3111 | error. | |
4633a7c4 | 3112 | See L<perlipc/"UDP: Message Passing"> for examples. |
a0d0e21e LW |
3113 | |
3114 | =item setpgrp PID,PGRP | |
3115 | ||
7660c0ab | 3116 | Sets the current process group for the specified PID, C<0> for the current |
a0d0e21e | 3117 | process. Will produce a fatal error if used on a machine that doesn't |
7660c0ab A |
3118 | implement C<setpgrp(2)>. If the arguments are omitted, it defaults to |
3119 | C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any | |
3120 | arguments, so only setpgrp C<0,0> is portable. | |
a0d0e21e LW |
3121 | |
3122 | =item setpriority WHICH,WHO,PRIORITY | |
3123 | ||
3124 | Sets the current priority for a process, a process group, or a user. | |
7660c0ab A |
3125 | (See C<setpriority(2)>.) Will produce a fatal error if used on a machine |
3126 | that doesn't implement C<setpriority(2)>. | |
a0d0e21e LW |
3127 | |
3128 | =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL | |
3129 | ||
3130 | Sets the socket option requested. Returns undefined if there is an | |
7660c0ab | 3131 | error. OPTVAL may be specified as C<undef> if you don't want to pass an |
a0d0e21e LW |
3132 | argument. |
3133 | ||
3134 | =item shift ARRAY | |
3135 | ||
3136 | =item shift | |
3137 | ||
3138 | Shifts the first value of the array off and returns it, shortening the | |
3139 | array by 1 and moving everything down. If there are no elements in the | |
3140 | array, returns the undefined value. If ARRAY is omitted, shifts the | |
7660c0ab A |
3141 | C<@_> array within the lexical scope of subroutines and formats, and the |
3142 | C<@ARGV> array at file scopes or within the lexical scopes established by | |
977336f5 | 3143 | the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs. |
7660c0ab A |
3144 | See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the |
3145 | same thing to the left end of an array that C<pop()> and C<push()> do to the | |
977336f5 | 3146 | right end. |
a0d0e21e LW |
3147 | |
3148 | =item shmctl ID,CMD,ARG | |
3149 | ||
0ade1984 JH |
3150 | Calls the System V IPC function shmctl. You'll probably have to say |
3151 | ||
3152 | use IPC::SysV; | |
3153 | ||
7660c0ab A |
3154 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, |
3155 | then ARG must be a variable which will hold the returned C<shmid_ds> | |
3156 | structure. Returns like ioctl: the undefined value for error, "C<0> but | |
0ade1984 | 3157 | true" for zero, or the actual return value otherwise. |
7660c0ab | 3158 | See also C<IPC::SysV> documentation. |
a0d0e21e LW |
3159 | |
3160 | =item shmget KEY,SIZE,FLAGS | |
3161 | ||
3162 | Calls the System V IPC function shmget. Returns the shared memory | |
3163 | segment id, or the undefined value if there is an error. | |
7660c0ab | 3164 | See also C<IPC::SysV> documentation. |
a0d0e21e LW |
3165 | |
3166 | =item shmread ID,VAR,POS,SIZE | |
3167 | ||
3168 | =item shmwrite ID,STRING,POS,SIZE | |
3169 | ||
3170 | Reads or writes the System V shared memory segment ID starting at | |
3171 | position POS for size SIZE by attaching to it, copying in/out, and | |
5a964f20 | 3172 | detaching from it. When reading, VAR must be a variable that will |
a0d0e21e LW |
3173 | hold the data read. When writing, if STRING is too long, only SIZE |
3174 | bytes are used; if STRING is too short, nulls are written to fill out | |
3175 | SIZE bytes. Return TRUE if successful, or FALSE if there is an error. | |
7660c0ab | 3176 | See also C<IPC::SysV> documentation. |
a0d0e21e LW |
3177 | |
3178 | =item shutdown SOCKET,HOW | |
3179 | ||
3180 | Shuts down a socket connection in the manner indicated by HOW, which | |
3181 | has the same interpretation as in the system call of the same name. | |
3182 | ||
5a964f20 TC |
3183 | shutdown(SOCKET, 0); # I/we have stopped reading data |
3184 | shutdown(SOCKET, 1); # I/we have stopped writing data | |
3185 | shutdown(SOCKET, 2); # I/we have stopped using this socket | |
3186 | ||
3187 | This is useful with sockets when you want to tell the other | |
3188 | side you're done writing but not done reading, or vice versa. | |
3189 | It's also a more insistent form of close because it also | |
3190 | disables the filedescriptor in any forked copies in other | |
3191 | processes. | |
3192 | ||
a0d0e21e LW |
3193 | =item sin EXPR |
3194 | ||
54310121 | 3195 | =item sin |
bbce6d69 | 3196 | |
a0d0e21e | 3197 | Returns the sine of EXPR (expressed in radians). If EXPR is omitted, |
7660c0ab | 3198 | returns sine of C<$_>. |
a0d0e21e | 3199 | |
7660c0ab | 3200 | For the inverse sine operation, you may use the C<POSIX::asin()> |
28757baa | 3201 | function, or use this relation: |
3202 | ||
3203 | sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } | |
3204 | ||
a0d0e21e LW |
3205 | =item sleep EXPR |
3206 | ||
3207 | =item sleep | |
3208 | ||
3209 | Causes the script to sleep for EXPR seconds, or forever if no EXPR. | |
7660c0ab | 3210 | May be interrupted if the process receives a signal such as C<SIGALRM>. |
1d3434b8 | 3211 | Returns the number of seconds actually slept. You probably cannot |
7660c0ab A |
3212 | mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented |
3213 | using C<alarm()>. | |
a0d0e21e LW |
3214 | |
3215 | On some older systems, it may sleep up to a full second less than what | |
3216 | you requested, depending on how it counts seconds. Most modern systems | |
5a964f20 TC |
3217 | always sleep the full amount. They may appear to sleep longer than that, |
3218 | however, because your process might not be scheduled right away in a | |
3219 | busy multitasking system. | |
a0d0e21e | 3220 | |
cb1a09d0 | 3221 | For delays of finer granularity than one second, you may use Perl's |
7660c0ab | 3222 | C<syscall()> interface to access C<setitimer(2)> if your system supports it, |
7b8d334a | 3223 | or else see L</select()> above. |
cb1a09d0 | 3224 | |
7660c0ab | 3225 | See also the POSIX module's C<sigpause()> function. |
5f05dabc | 3226 | |
a0d0e21e LW |
3227 | =item socket SOCKET,DOMAIN,TYPE,PROTOCOL |
3228 | ||
3229 | Opens a socket of the specified kind and attaches it to filehandle | |
5f05dabc | 3230 | SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the |
7660c0ab | 3231 | system call of the same name. You should "C<use Socket;>" first to get |
4633a7c4 | 3232 | the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">. |
a0d0e21e LW |
3233 | |
3234 | =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL | |
3235 | ||
3236 | Creates an unnamed pair of sockets in the specified domain, of the | |
5f05dabc | 3237 | specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as |
a0d0e21e LW |
3238 | for the system call of the same name. If unimplemented, yields a fatal |
3239 | error. Returns TRUE if successful. | |
3240 | ||
ab4f32c2 | 3241 | Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call |
5a964f20 TC |
3242 | to C<pipe(Rdr, Wtr)> is essentially: |
3243 | ||
3244 | use Socket; | |
3245 | socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); | |
3246 | shutdown(Rdr, 1); # no more writing for reader | |
3247 | shutdown(Wtr, 0); # no more reading for writer | |
3248 | ||
3249 | See L<perlipc> for an example of socketpair use. | |
3250 | ||
a0d0e21e LW |
3251 | =item sort SUBNAME LIST |
3252 | ||
3253 | =item sort BLOCK LIST | |
3254 | ||
3255 | =item sort LIST | |
3256 | ||
2f9daede | 3257 | Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK |
ab4f32c2 | 3258 | is omitted, C<sort()>s in standard string comparison order. If SUBNAME is |
2f9daede | 3259 | specified, it gives the name of a subroutine that returns an integer |
7660c0ab | 3260 | less than, equal to, or greater than C<0>, depending on how the elements |
2f9daede TP |
3261 | of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp> |
3262 | operators are extremely useful in such routines.) SUBNAME may be a | |
1d3434b8 GS |
3263 | scalar variable name (unsubscripted), in which case the value provides |
3264 | the name of (or a reference to) the actual subroutine to use. In place | |
3265 | of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort | |
3266 | subroutine. | |
a0d0e21e | 3267 | |
cb1a09d0 AD |
3268 | In the interests of efficiency the normal calling code for subroutines is |
3269 | bypassed, with the following effects: the subroutine may not be a | |
3270 | recursive subroutine, and the two elements to be compared are passed into | |
7660c0ab A |
3271 | the subroutine not via C<@_> but as the package global variables C<$a> and |
3272 | C<$b> (see example below). They are passed by reference, so don't | |
3273 | modify C<$a> and C<$b>. And don't try to declare them as lexicals either. | |
a0d0e21e | 3274 | |
0a753a76 | 3275 | You also cannot exit out of the sort block or subroutine using any of the |
7660c0ab | 3276 | loop control operators described in L<perlsyn> or with C<goto()>. |
0a753a76 | 3277 | |
a034a98d DD |
3278 | When C<use locale> is in effect, C<sort LIST> sorts LIST according to the |
3279 | current collation locale. See L<perllocale>. | |
3280 | ||
a0d0e21e LW |
3281 | Examples: |
3282 | ||
3283 | # sort lexically | |
3284 | @articles = sort @files; | |
3285 | ||
3286 | # same thing, but with explicit sort routine | |
3287 | @articles = sort {$a cmp $b} @files; | |
3288 | ||
cb1a09d0 | 3289 | # now case-insensitively |
54310121 | 3290 | @articles = sort {uc($a) cmp uc($b)} @files; |
cb1a09d0 | 3291 | |
a0d0e21e LW |
3292 | # same thing in reversed order |
3293 | @articles = sort {$b cmp $a} @files; | |
3294 | ||
3295 | # sort numerically ascending | |
3296 | @articles = sort {$a <=> $b} @files; | |
3297 | ||
3298 | # sort numerically descending | |
3299 | @articles = sort {$b <=> $a} @files; | |
3300 | ||
3301 | # sort using explicit subroutine name | |
3302 | sub byage { | |
2f9daede | 3303 | $age{$a} <=> $age{$b}; # presuming numeric |
a0d0e21e LW |
3304 | } |
3305 | @sortedclass = sort byage @class; | |
3306 | ||
aa689395 | 3307 | # this sorts the %age hash by value instead of key |
3308 | # using an in-line function | |
c07a80fd | 3309 | @eldest = sort { $age{$b} <=> $age{$a} } keys %age; |
3310 | ||
a0d0e21e LW |
3311 | sub backwards { $b cmp $a; } |
3312 | @harry = ('dog','cat','x','Cain','Abel'); | |
3313 | @george = ('gone','chased','yz','Punished','Axed'); | |
3314 | print sort @harry; | |
3315 | # prints AbelCaincatdogx | |
3316 | print sort backwards @harry; | |
3317 | # prints xdogcatCainAbel | |
3318 | print sort @george, 'to', @harry; | |
3319 | # prints AbelAxedCainPunishedcatchaseddoggonetoxyz | |
3320 | ||
54310121 | 3321 | # inefficiently sort by descending numeric compare using |
3322 | # the first integer after the first = sign, or the | |
cb1a09d0 AD |
3323 | # whole record case-insensitively otherwise |
3324 | ||
3325 | @new = sort { | |
3326 | ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] | |
3327 | || | |
3328 | uc($a) cmp uc($b) | |
3329 | } @old; | |
3330 | ||
3331 | # same thing, but much more efficiently; | |
3332 | # we'll build auxiliary indices instead | |
3333 | # for speed | |
3334 | @nums = @caps = (); | |
54310121 | 3335 | for (@old) { |
cb1a09d0 AD |
3336 | push @nums, /=(\d+)/; |
3337 | push @caps, uc($_); | |
54310121 | 3338 | } |
cb1a09d0 AD |
3339 | |
3340 | @new = @old[ sort { | |
3341 | $nums[$b] <=> $nums[$a] | |
3342 | || | |
3343 | $caps[$a] cmp $caps[$b] | |
3344 | } 0..$#old | |
3345 | ]; | |
3346 | ||
3347 | # same thing using a Schwartzian Transform (no temps) | |
3348 | @new = map { $_->[0] } | |
3349 | sort { $b->[1] <=> $a->[1] | |
3350 | || | |
3351 | $a->[2] cmp $b->[2] | |
3352 | } map { [$_, /=(\d+)/, uc($_)] } @old; | |
3353 | ||
7660c0ab A |
3354 | If you're using strict, you I<MUST NOT> declare C<$a> |
3355 | and C<$b> as lexicals. They are package globals. That means | |
cb1a09d0 AD |
3356 | if you're in the C<main> package, it's |
3357 | ||
3358 | @articles = sort {$main::b <=> $main::a} @files; | |
3359 | ||
3360 | or just | |
3361 | ||
3362 | @articles = sort {$::b <=> $::a} @files; | |
3363 | ||
3364 | but if you're in the C<FooPack> package, it's | |
3365 | ||
3366 | @articles = sort {$FooPack::b <=> $FooPack::a} @files; | |
3367 | ||
55497cff | 3368 | The comparison function is required to behave. If it returns |
7660c0ab A |
3369 | inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and |
3370 | sometimes saying the opposite, for example) the results are not | |
3371 | well-defined. | |
55497cff | 3372 | |
a0d0e21e LW |
3373 | =item splice ARRAY,OFFSET,LENGTH,LIST |
3374 | ||
3375 | =item splice ARRAY,OFFSET,LENGTH | |
3376 | ||
3377 | =item splice ARRAY,OFFSET | |
3378 | ||
3379 | Removes the elements designated by OFFSET and LENGTH from an array, and | |
5a964f20 TC |
3380 | replaces them with the elements of LIST, if any. In list context, |
3381 | returns the elements removed from the array. In scalar context, | |
43051805 | 3382 | returns the last element removed, or C<undef> if no elements are |
48cdf507 GA |
3383 | removed. The array grows or shrinks as necessary. |
3384 | If OFFSET is negative then it start that far from the end of the array. | |
3385 | If LENGTH is omitted, removes everything from OFFSET onward. | |
3386 | If LENGTH is negative, leave that many elements off the end of the array. | |
3387 | The following equivalences hold (assuming C<$[ == 0>): | |
a0d0e21e | 3388 | |
48cdf507 | 3389 | push(@a,$x,$y) splice(@a,@a,0,$x,$y) |
a0d0e21e LW |
3390 | pop(@a) splice(@a,-1) |
3391 | shift(@a) splice(@a,0,1) | |
3392 | unshift(@a,$x,$y) splice(@a,0,0,$x,$y) | |
5a964f20 | 3393 | $a[$x] = $y splice(@a,$x,1,$y) |
a0d0e21e LW |
3394 | |
3395 | Example, assuming array lengths are passed before arrays: | |
3396 | ||
3397 | sub aeq { # compare two list values | |
5a964f20 TC |
3398 | my(@a) = splice(@_,0,shift); |
3399 | my(@b) = splice(@_,0,shift); | |
a0d0e21e LW |
3400 | return 0 unless @a == @b; # same len? |
3401 | while (@a) { | |
3402 | return 0 if pop(@a) ne pop(@b); | |
3403 | } | |
3404 | return 1; | |
3405 | } | |
3406 | if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } | |
3407 | ||
3408 | =item split /PATTERN/,EXPR,LIMIT | |
3409 | ||
3410 | =item split /PATTERN/,EXPR | |
3411 | ||
3412 | =item split /PATTERN/ | |
3413 | ||
3414 | =item split | |
3415 | ||
5a964f20 TC |
3416 | Splits a string into an array of strings, and returns it. By default, |
3417 | empty leading fields are preserved, and empty trailing ones are deleted. | |
a0d0e21e | 3418 | |
5a964f20 | 3419 | If not in list context, returns the number of fields found and splits into |
7660c0ab | 3420 | the C<@_> array. (In list context, you can force the split into C<@_> by |
1d2dff63 | 3421 | using C<??> as the pattern delimiters, but it still returns the list |
7660c0ab | 3422 | value.) The use of implicit split to C<@_> is deprecated, however, because |
5a964f20 | 3423 | it clobbers your subroutine arguments. |
a0d0e21e | 3424 | |
7660c0ab | 3425 | If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted, |
4633a7c4 LW |
3426 | splits on whitespace (after skipping any leading whitespace). Anything |
3427 | matching PATTERN is taken to be a delimiter separating the fields. (Note | |
fb73857a | 3428 | that the delimiter may be longer than one character.) |
3429 | ||
5a964f20 | 3430 | If LIMIT is specified and positive, splits into no more than that |
7b8d334a GS |
3431 | many fields (though it may split into fewer). If LIMIT is unspecified |
3432 | or zero, trailing null fields are stripped (which potential users | |
7660c0ab | 3433 | of C<pop()> would do well to remember). If LIMIT is negative, it is |
fb73857a | 3434 | treated as if an arbitrarily large LIMIT had been specified. |
a0d0e21e LW |
3435 | |
3436 | A pattern matching the null string (not to be confused with | |
748a9306 | 3437 | a null pattern C<//>, which is just one member of the set of patterns |
a0d0e21e LW |
3438 | matching a null string) will split the value of EXPR into separate |
3439 | characters at each point it matches that way. For example: | |
3440 | ||
3441 | print join(':', split(/ */, 'hi there')); | |
3442 | ||
3443 | produces the output 'h:i:t:h:e:r:e'. | |
3444 | ||
5f05dabc | 3445 | The LIMIT parameter can be used to split a line partially |
a0d0e21e LW |
3446 | |
3447 | ($login, $passwd, $remainder) = split(/:/, $_, 3); | |
3448 | ||
3449 | When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT | |
3450 | one larger than the number of variables in the list, to avoid | |
3451 | unnecessary work. For the list above LIMIT would have been 4 by | |
3452 | default. In time critical applications it behooves you not to split | |
3453 | into more fields than you really need. | |
3454 | ||
3455 | If the PATTERN contains parentheses, additional array elements are | |
3456 | created from each matching substring in the delimiter. | |
3457 | ||
da0045b7 | 3458 | split(/([,-])/, "1-10,20", 3); |
a0d0e21e LW |
3459 | |
3460 | produces the list value | |
3461 | ||
3462 | (1, '-', 10, ',', 20) | |
3463 | ||
7660c0ab | 3464 | If you had the entire header of a normal Unix email message in C<$header>, |
4633a7c4 LW |
3465 | you could split it up into fields and their values this way: |
3466 | ||
3467 | $header =~ s/\n\s+/ /g; # fix continuation lines | |
fb73857a | 3468 | %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header); |
4633a7c4 | 3469 | |
a0d0e21e LW |
3470 | The pattern C</PATTERN/> may be replaced with an expression to specify |
3471 | patterns that vary at runtime. (To do runtime compilation only once, | |
748a9306 LW |
3472 | use C</$variable/o>.) |
3473 | ||
3474 | As a special case, specifying a PATTERN of space (C<' '>) will split on | |
ab4f32c2 | 3475 | white space just as C<split()> with no arguments does. Thus, C<split(' ')> can |
748a9306 LW |
3476 | be used to emulate B<awk>'s default behavior, whereas C<split(/ /)> |
3477 | will give you as many null initial fields as there are leading spaces. | |
ab4f32c2 A |
3478 | A C<split()> on C</\s+/> is like a C<split(' ')> except that any leading |
3479 | whitespace produces a null first field. A C<split()> with no arguments | |
748a9306 | 3480 | really does a C<split(' ', $_)> internally. |
a0d0e21e LW |
3481 | |
3482 | Example: | |
3483 | ||
5a964f20 TC |
3484 | open(PASSWD, '/etc/passwd'); |
3485 | while (<PASSWD>) { | |
3486 | ($login, $passwd, $uid, $gid, $gcos,$home, $shell) = split(/:/); | |
3487 | #... | |
a0d0e21e LW |
3488 | } |
3489 | ||
7660c0ab | 3490 | (Note that C<$shell> above will still have a newline on it. See L</chop>, |
a0d0e21e LW |
3491 | L</chomp>, and L</join>.) |
3492 | ||
5f05dabc | 3493 | =item sprintf FORMAT, LIST |
a0d0e21e | 3494 | |
ab4f32c2 | 3495 | Returns a string formatted by the usual C<printf()> conventions of the |
7660c0ab | 3496 | C library function C<sprintf()>. See L<sprintf(3)> or L<printf(3)> |
74a77017 CS |
3497 | on your system for an explanation of the general principles. |
3498 | ||
7660c0ab A |
3499 | Perl does its own C<sprintf()> formatting -- it emulates the C |
3500 | function C<sprintf()>, but it doesn't use it (except for floating-point | |
74a77017 | 3501 | numbers, and even then only the standard modifiers are allowed). As a |
7660c0ab | 3502 | result, any non-standard extensions in your local C<sprintf()> are not |
74a77017 CS |
3503 | available from Perl. |
3504 | ||
7660c0ab | 3505 | Perl's C<sprintf()> permits the following universally-known conversions: |
74a77017 CS |
3506 | |
3507 | %% a percent sign | |
3508 | %c a character with the given number | |
3509 | %s a string | |
3510 | %d a signed integer, in decimal | |
3511 | %u an unsigned integer, in decimal | |
3512 | %o an unsigned integer, in octal | |
3513 | %x an unsigned integer, in hexadecimal | |
3514 | %e a floating-point number, in scientific notation | |
3515 | %f a floating-point number, in fixed decimal notation | |
3516 | %g a floating-point number, in %e or %f notation | |
3517 | ||
1b3f7d21 | 3518 | In addition, Perl permits the following widely-supported conversions: |
74a77017 | 3519 | |
74a77017 CS |
3520 | %X like %x, but using upper-case letters |
3521 | %E like %e, but using an upper-case "E" | |
3522 | %G like %g, but with an upper-case "E" (if applicable) | |
3523 | %p a pointer (outputs the Perl value's address in hexadecimal) | |
1b3f7d21 CS |
3524 | %n special: *stores* the number of characters output so far |
3525 | into the next variable in the parameter list | |
74a77017 | 3526 | |
1b3f7d21 CS |
3527 | Finally, for backward (and we do mean "backward") compatibility, Perl |
3528 | permits these unnecessary but widely-supported conversions: | |
74a77017 | 3529 | |
1b3f7d21 | 3530 | %i a synonym for %d |
74a77017 CS |
3531 | %D a synonym for %ld |
3532 | %U a synonym for %lu | |
3533 | %O a synonym for %lo | |
3534 | %F a synonym for %f | |
3535 | ||
3536 | Perl permits the following universally-known flags between the C<%> | |
3537 | and the conversion letter: | |
3538 | ||
3539 | space prefix positive number with a space | |
3540 | + prefix positive number with a plus sign | |
3541 | - left-justify within the field | |
3542 | 0 use zeros, not spaces, to right-justify | |
a3cb178b | 3543 | # prefix non-zero octal with "0", non-zero hex with "0x" |
74a77017 CS |
3544 | number minimum field width |
3545 | .number "precision": digits after decimal point for floating-point, | |
3546 | max length for string, minimum length for integer | |
3547 | l interpret integer as C type "long" or "unsigned long" | |
74a77017 CS |
3548 | h interpret integer as C type "short" or "unsigned short" |
3549 | ||
1b3f7d21 | 3550 | There is also one Perl-specific flag: |
74a77017 CS |
3551 | |
3552 | V interpret integer as Perl's standard integer type | |
3553 | ||
7660c0ab | 3554 | Where a number would appear in the flags, an asterisk ("C<*>") may be |
74a77017 CS |
3555 | used instead, in which case Perl uses the next item in the parameter |
3556 | list as the given number (that is, as the field width or precision). | |
7660c0ab A |
3557 | If a field width obtained through "C<*>" is negative, it has the same |
3558 | effect as the "C<->" flag: left-justification. | |
74a77017 CS |
3559 | |
3560 | If C<use locale> is in effect, the character used for the decimal | |
3561 | point in formatted real numbers is affected by the LC_NUMERIC locale. | |
3562 | See L<perllocale>. | |
a0d0e21e LW |
3563 | |
3564 | =item sqrt EXPR | |
3565 | ||
54310121 | 3566 | =item sqrt |
bbce6d69 | 3567 | |
a0d0e21e | 3568 | Return the square root of EXPR. If EXPR is omitted, returns square |
7660c0ab | 3569 | root of C<$_>. |
a0d0e21e LW |
3570 | |
3571 | =item srand EXPR | |
3572 | ||
93dc8474 CS |
3573 | =item srand |
3574 | ||
ab4f32c2 | 3575 | Sets the random number seed for the C<rand()> operator. If EXPR is |
93dc8474 CS |
3576 | omitted, uses a semi-random value based on the current time and process |
3577 | ID, among other things. In versions of Perl prior to 5.004 the default | |
7660c0ab | 3578 | seed was just the current C<time()>. This isn't a particularly good seed, |
93dc8474 | 3579 | so many old programs supply their own seed value (often C<time ^ $$> or |
7b8d334a | 3580 | C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more. |
93dc8474 | 3581 | |
7660c0ab | 3582 | In fact, it's usually not necessary to call C<srand()> at all, because if |
93dc8474 | 3583 | it is not called explicitly, it is called implicitly at the first use of |
ab4f32c2 | 3584 | the C<rand()> operator. However, this was not the case in version of Perl |
2f9daede | 3585 | before 5.004, so if your script will run under older Perl versions, it |
7660c0ab | 3586 | should call C<srand()>. |
93dc8474 | 3587 | |
2f9daede TP |
3588 | Note that you need something much more random than the default seed for |
3589 | cryptographic purposes. Checksumming the compressed output of one or more | |
3590 | rapidly changing operating system status programs is the usual method. For | |
3591 | example: | |
28757baa | 3592 | |
3593 | srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`); | |
3594 | ||
7660c0ab | 3595 | If you're particularly concerned with this, see the C<Math::TrulyRandom> |
0078ec44 RS |
3596 | module in CPAN. |
3597 | ||
7660c0ab | 3598 | Do I<not> call C<srand()> multiple times in your program unless you know |
28757baa | 3599 | exactly what you're doing and why you're doing it. The point of the |
7660c0ab | 3600 | function is to "seed" the C<rand()> function so that C<rand()> can produce |
28757baa | 3601 | a different sequence each time you run your program. Just do it once at the |
7660c0ab | 3602 | top of your program, or you I<won't> get random numbers out of C<rand()>! |
28757baa | 3603 | |
54310121 | 3604 | Frequently called programs (like CGI scripts) that simply use |
28757baa | 3605 | |
3606 | time ^ $$ | |
3607 | ||
54310121 | 3608 | for a seed can fall prey to the mathematical property that |
28757baa | 3609 | |
3610 | a^b == (a+1)^(b+1) | |
3611 | ||
0078ec44 | 3612 | one-third of the time. So don't do that. |
f86702cc | 3613 | |
a0d0e21e LW |
3614 | =item stat FILEHANDLE |
3615 | ||
3616 | =item stat EXPR | |
3617 | ||
54310121 | 3618 | =item stat |
bbce6d69 | 3619 | |
1d2dff63 GS |
3620 | Returns a 13-element list giving the status info for a file, either |
3621 | the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, | |
7660c0ab | 3622 | it stats C<$_>. Returns a null list if the stat fails. Typically used |
1d2dff63 | 3623 | as follows: |
a0d0e21e LW |
3624 | |
3625 | ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, | |
3626 | $atime,$mtime,$ctime,$blksize,$blocks) | |
3627 | = stat($filename); | |
3628 | ||
54310121 | 3629 | Not all fields are supported on all filesystem types. Here are the |
c07a80fd | 3630 | meaning of the fields: |
3631 | ||
54310121 | 3632 | 0 dev device number of filesystem |
3633 | 1 ino inode number | |
3634 | 2 mode file mode (type and permissions) | |
3635 | 3 nlink number of (hard) links to the file | |
3636 | 4 uid numeric user ID of file's owner | |
3637 | 5 gid numeric group ID of file's owner | |
3638 | 6 rdev the device identifier (special files only) | |
3639 | 7 size total size of file, in bytes | |
3640 | 8 atime last access time since the epoch | |
3641 | 9 mtime last modify time since the epoch | |
3642 | 10 ctime inode change time (NOT creation time!) since the epoch | |
3643 | 11 blksize preferred block size for file system I/O | |
3644 | 12 blocks actual number of blocks allocated | |
c07a80fd | 3645 | |
3646 | (The epoch was at 00:00 January 1, 1970 GMT.) | |
3647 | ||
a0d0e21e LW |
3648 | If stat is passed the special filehandle consisting of an underline, no |
3649 | stat is done, but the current contents of the stat structure from the | |
3650 | last stat or filetest are returned. Example: | |
3651 | ||
3652 | if (-x $file && (($d) = stat(_)) && $d < 0) { | |
3653 | print "$file is executable NFS file\n"; | |
3654 | } | |
3655 | ||
5f05dabc | 3656 | (This works on machines only for which the device number is negative under NFS.) |
a0d0e21e | 3657 | |
ab4f32c2 | 3658 | In scalar context, C<stat()> returns a boolean value indicating success |
1d2dff63 GS |
3659 | or failure, and, if successful, sets the information associated with |
3660 | the special filehandle C<_>. | |
3661 | ||
a0d0e21e LW |
3662 | =item study SCALAR |
3663 | ||
3664 | =item study | |
3665 | ||
184e9718 | 3666 | Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of |
a0d0e21e LW |
3667 | doing many pattern matches on the string before it is next modified. |
3668 | This may or may not save time, depending on the nature and number of | |
3669 | patterns you are searching on, and on the distribution of character | |
54310121 | 3670 | frequencies in the string to be searched -- you probably want to compare |
5f05dabc | 3671 | run times with and without it to see which runs faster. Those loops |
a0d0e21e LW |
3672 | which scan for many short constant strings (including the constant |
3673 | parts of more complex patterns) will benefit most. You may have only | |
ab4f32c2 A |
3674 | one C<study()> active at a time -- if you study a different scalar the first |
3675 | is "unstudied". (The way C<study()> works is this: a linked list of every | |
a0d0e21e | 3676 | character in the string to be searched is made, so we know, for |
7660c0ab | 3677 | example, where all the C<'k'> characters are. From each search string, |
a0d0e21e LW |
3678 | the rarest character is selected, based on some static frequency tables |
3679 | constructed from some C programs and English text. Only those places | |
3680 | that contain this "rarest" character are examined.) | |
3681 | ||
5a964f20 | 3682 | For example, here is a loop that inserts index producing entries |
a0d0e21e LW |
3683 | before any line containing a certain pattern: |
3684 | ||
3685 | while (<>) { | |
3686 | study; | |
3687 | print ".IX foo\n" if /\bfoo\b/; | |
3688 | print ".IX bar\n" if /\bbar\b/; | |
3689 | print ".IX blurfl\n" if /\bblurfl\b/; | |
5a964f20 | 3690 | # ... |
a0d0e21e LW |
3691 | print; |
3692 | } | |
3693 | ||
7660c0ab A |
3694 | In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f"> |
3695 | will be looked at, because C<"f"> is rarer than C<"o">. In general, this is | |
a0d0e21e LW |
3696 | a big win except in pathological cases. The only question is whether |
3697 | it saves you more time than it took to build the linked list in the | |
3698 | first place. | |
3699 | ||
3700 | Note that if you have to look for strings that you don't know till | |
ab4f32c2 | 3701 | runtime, you can build an entire loop as a string and C<eval()> that to |
a0d0e21e | 3702 | avoid recompiling all your patterns all the time. Together with |
7660c0ab A |
3703 | undefining C<$/> to input entire files as one record, this can be very |
3704 | fast, often faster than specialized programs like C<fgrep(1)>. The following | |
184e9718 | 3705 | scans a list of files (C<@files>) for a list of words (C<@words>), and prints |
a0d0e21e LW |
3706 | out the names of those files that contain a match: |
3707 | ||
3708 | $search = 'while (<>) { study;'; | |
3709 | foreach $word (@words) { | |
3710 | $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; | |
3711 | } | |
3712 | $search .= "}"; | |
3713 | @ARGV = @files; | |
3714 | undef $/; | |
3715 | eval $search; # this screams | |
5f05dabc | 3716 | $/ = "\n"; # put back to normal input delimiter |
a0d0e21e LW |
3717 | foreach $file (sort keys(%seen)) { |
3718 | print $file, "\n"; | |
3719 | } | |
3720 | ||
cb1a09d0 AD |
3721 | =item sub BLOCK |
3722 | ||
3723 | =item sub NAME | |
3724 | ||
3725 | =item sub NAME BLOCK | |
3726 | ||
3727 | This is subroutine definition, not a real function I<per se>. With just a | |
3728 | NAME (and possibly prototypes), it's just a forward declaration. Without | |
3729 | a NAME, it's an anonymous function declaration, and does actually return a | |
2f9daede | 3730 | value: the CODE ref of the closure you just created. See L<perlsub> and |
cb1a09d0 AD |
3731 | L<perlref> for details. |
3732 | ||
7b8d334a GS |
3733 | =item substr EXPR,OFFSET,LEN,REPLACEMENT |
3734 | ||
a0d0e21e LW |
3735 | =item substr EXPR,OFFSET,LEN |
3736 | ||
3737 | =item substr EXPR,OFFSET | |
3738 | ||
3739 | Extracts a substring out of EXPR and returns it. First character is at | |
7660c0ab | 3740 | offset C<0>, or whatever you've set C<$[> to (but don't do that). |
84902520 | 3741 | If OFFSET is negative (or more precisely, less than C<$[>), starts |
a0d0e21e | 3742 | that far from the end of the string. If LEN is omitted, returns |
748a9306 LW |
3743 | everything to the end of the string. If LEN is negative, leaves that |
3744 | many characters off the end of the string. | |
3745 | ||
5a964f20 | 3746 | If you specify a substring that is partly outside the string, the part |
84902520 TB |
3747 | within the string is returned. If the substring is totally outside |
3748 | the string a warning is produced. | |
3749 | ||
7660c0ab | 3750 | You can use the C<substr()> function |
a0d0e21e LW |
3751 | as an lvalue, in which case EXPR must be an lvalue. If you assign |
3752 | something shorter than LEN, the string will shrink, and if you assign | |
3753 | something longer than LEN, the string will grow to accommodate it. To | |
3754 | keep the string the same length you may need to pad or chop your value | |
7660c0ab | 3755 | using C<sprintf()>. |
a0d0e21e | 3756 | |
7660c0ab | 3757 | An alternative to using C<substr()> as an lvalue is to specify the |
7b8d334a GS |
3758 | replacement string as the 4th argument. This allows you to replace |
3759 | parts of the EXPR and return what was there before in one operation. | |
7b8d334a | 3760 | |
a0d0e21e LW |
3761 | =item symlink OLDFILE,NEWFILE |
3762 | ||
3763 | Creates a new filename symbolically linked to the old filename. | |
7660c0ab | 3764 | Returns C<1> for success, C<0> otherwise. On systems that don't support |
a0d0e21e LW |
3765 | symbolic links, produces a fatal error at run time. To check for that, |
3766 | use eval: | |
3767 | ||
5a964f20 | 3768 | $symlink_exists = eval { symlink("",""); 1 }; |
a0d0e21e LW |
3769 | |
3770 | =item syscall LIST | |
3771 | ||
3772 | Calls the system call specified as the first element of the list, | |
3773 | passing the remaining elements as arguments to the system call. If | |
3774 | unimplemented, produces a fatal error. The arguments are interpreted | |
3775 | as follows: if a given argument is numeric, the argument is passed as | |
3776 | an int. If not, the pointer to the string value is passed. You are | |
3777 | responsible to make sure a string is pre-extended long enough to | |
a3cb178b | 3778 | receive any result that might be written into a string. You can't use a |
7660c0ab | 3779 | string literal (or other read-only string) as an argument to C<syscall()> |
a3cb178b GS |
3780 | because Perl has to assume that any string pointer might be written |
3781 | through. If your | |
a0d0e21e | 3782 | integer arguments are not literals and have never been interpreted in a |
7660c0ab A |
3783 | numeric context, you may need to add C<0> to them to force them to look |
3784 | like numbers. This emulates the C<syswrite()> function (or vice versa): | |
a0d0e21e LW |
3785 | |
3786 | require 'syscall.ph'; # may need to run h2ph | |
a3cb178b GS |
3787 | $s = "hi there\n"; |
3788 | syscall(&SYS_write, fileno(STDOUT), $s, length $s); | |
a0d0e21e | 3789 | |
5f05dabc | 3790 | Note that Perl supports passing of up to only 14 arguments to your system call, |
a0d0e21e LW |
3791 | which in practice should usually suffice. |
3792 | ||
fb73857a | 3793 | Syscall returns whatever value returned by the system call it calls. |
ab4f32c2 | 3794 | If the system call fails, C<syscall()> returns C<-1> and sets C<$!> (errno). |
7660c0ab | 3795 | Note that some system calls can legitimately return C<-1>. The proper |
fb73857a | 3796 | way to handle such calls is to assign C<$!=0;> before the call and |
7660c0ab | 3797 | check the value of C<$!> if syscall returns C<-1>. |
fb73857a | 3798 | |
3799 | There's a problem with C<syscall(&SYS_pipe)>: it returns the file | |
3800 | number of the read end of the pipe it creates. There is no way | |
3801 | to retrieve the file number of the other end. You can avoid this | |
ab4f32c2 | 3802 | problem by using C<pipe()> instead. |
fb73857a | 3803 | |
c07a80fd | 3804 | =item sysopen FILEHANDLE,FILENAME,MODE |
3805 | ||
3806 | =item sysopen FILEHANDLE,FILENAME,MODE,PERMS | |
3807 | ||
3808 | Opens the file whose filename is given by FILENAME, and associates it | |
3809 | with FILEHANDLE. If FILEHANDLE is an expression, its value is used as | |
3810 | the name of the real filehandle wanted. This function calls the | |
ab4f32c2 | 3811 | underlying operating system's C<open()> function with the parameters |
c07a80fd | 3812 | FILENAME, MODE, PERMS. |
3813 | ||
3814 | The possible values and flag bits of the MODE parameter are | |
3815 | system-dependent; they are available via the standard module C<Fcntl>. | |
3816 | However, for historical reasons, some values are universal: zero means | |
3817 | read-only, one means write-only, and two means read/write. | |
3818 | ||
ab4f32c2 | 3819 | If the file named by FILENAME does not exist and the C<open()> call creates |
7660c0ab | 3820 | it (typically because MODE includes the C<O_CREAT> flag), then the value of |
5a964f20 | 3821 | PERMS specifies the permissions of the newly created file. If you omit |
ab4f32c2 | 3822 | the PERMS argument to C<sysopen()>, Perl uses the octal value C<0666>. |
5a964f20 TC |
3823 | These permission values need to be in octal, and are modified by your |
3824 | process's current C<umask>. The C<umask> value is a number representing | |
7660c0ab | 3825 | disabled permissions bits--if your C<umask> were C<027> (group can't write; |
ab4f32c2 | 3826 | others can't read, write, or execute), then passing C<sysopen()> C<0666> would |
7660c0ab | 3827 | create a file with mode C<0640> (C<0666 &~ 027> is C<0640>). |
5a964f20 | 3828 | |
ab4f32c2 | 3829 | If you find this C<umask()> talk confusing, here's some advice: supply a |
7660c0ab | 3830 | creation mode of C<0666> for regular files and one of C<0777> for directories |
ab4f32c2 | 3831 | (in C<mkdir()>) and executable files. This gives users the freedom of |
5a964f20 | 3832 | choice: if they want protected files, they might choose process umasks |
7660c0ab | 3833 | of C<022>, C<027>, or even the particularly antisocial mask of C<077>. Programs |
5a964f20 TC |
3834 | should rarely if ever make policy decisions better left to the user. |
3835 | The exception to this is when writing files that should be kept private: | |
3836 | mail files, web browser cookies, I<.rhosts> files, and so on. In short, | |
ab4f32c2 | 3837 | seldom if ever use C<0644> as argument to C<sysopen()> because that takes |
5a964f20 | 3838 | away the user's option to have a more permissive umask. Better to omit it. |
c07a80fd | 3839 | |
7660c0ab | 3840 | The C<IO::File> module provides a more object-oriented approach, if you're |
28757baa | 3841 | into that kind of thing. |
3842 | ||
a0d0e21e LW |
3843 | =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET |
3844 | ||
3845 | =item sysread FILEHANDLE,SCALAR,LENGTH | |
3846 | ||
3847 | Attempts to read LENGTH bytes of data into variable SCALAR from the | |
7660c0ab A |
3848 | specified FILEHANDLE, using the system call C<read(2)>. It bypasses |
3849 | stdio, so mixing this with other kinds of reads, C<print()>, C<write()>, | |
3850 | C<seek()>, or C<tell()> can cause confusion because stdio usually buffers | |
3b02c43c GS |
3851 | data. Returns the number of bytes actually read, C<0> at end of file, |
3852 | or undef if there was an error. SCALAR will be grown or shrunk so that | |
3853 | the last byte actually read is the last byte of the scalar after the read. | |
ff68c719 | 3854 | |
3855 | An OFFSET may be specified to place the read data at some place in the | |
3856 | string other than the beginning. A negative OFFSET specifies | |
3857 | placement at that many bytes counting backwards from the end of the | |
3858 | string. A positive OFFSET greater than the length of SCALAR results | |
7660c0ab | 3859 | in the string being padded to the required size with C<"\0"> bytes before |
ff68c719 | 3860 | the result of the read is appended. |
a0d0e21e | 3861 | |
137443ea | 3862 | =item sysseek FILEHANDLE,POSITION,WHENCE |
3863 | ||
7660c0ab A |
3864 | Sets FILEHANDLE's system position using the system call C<lseek(2)>. It |
3865 | bypasses stdio, so mixing this with reads (other than C<sysread()>), | |
3866 | C<print()>, C<write()>, C<seek()>, or C<tell()> may cause confusion. FILEHANDLE may | |
8903cb82 | 3867 | be an expression whose value gives the name of the filehandle. The |
7660c0ab A |
3868 | values for WHENCE are C<0> to set the new position to POSITION, C<1> to set |
3869 | the it to the current position plus POSITION, and C<2> to set it to EOF | |
8903cb82 | 3870 | plus POSITION (typically negative). For WHENCE, you may use the |
7660c0ab | 3871 | constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the C<IO::Seekable> |
8903cb82 | 3872 | or the POSIX module. |
3873 | ||
3874 | Returns the new position, or the undefined value on failure. A position | |
7660c0ab | 3875 | of zero is returned as the string "C<0> but true"; thus C<sysseek()> returns |
8903cb82 | 3876 | TRUE on success and FALSE on failure, yet you can still easily determine |
3877 | the new position. | |
137443ea | 3878 | |
a0d0e21e LW |
3879 | =item system LIST |
3880 | ||
8bf3b016 GS |
3881 | =item system PROGRAM LIST |
3882 | ||
7660c0ab | 3883 | Does exactly the same thing as "C<exec LIST>" except that a fork is done |
a0d0e21e LW |
3884 | first, and the parent process waits for the child process to complete. |
3885 | Note that argument processing varies depending on the number of | |
a2008d6d GS |
3886 | arguments. If there is more than one argument in LIST, or if LIST is |
3887 | an array with more than one value, starts the program given by the | |
3888 | first element of the list with arguments given by the rest of the list. | |
3889 | If there is only one scalar argument, the argument is | |
3890 | checked for shell metacharacters, and if there are any, the entire | |
3891 | argument is passed to the system's command shell for parsing (this is | |
3892 | C</bin/sh -c> on Unix platforms, but varies on other platforms). If | |
3893 | there are no shell metacharacters in the argument, it is split into | |
7660c0ab | 3894 | words and passed directly to C<execvp()>, which is more efficient. |
a2008d6d GS |
3895 | |
3896 | The return value is the exit status of the program as | |
7660c0ab | 3897 | returned by the C<wait()> call. To get the actual exit value divide by |
54310121 | 3898 | 256. See also L</exec>. This is I<NOT> what you want to use to capture |
3899 | the output from a command, for that you should use merely backticks or | |
7660c0ab | 3900 | C<qx//>, as described in L<perlop/"`STRING`">. |
a0d0e21e | 3901 | |
7660c0ab A |
3902 | Like C<exec()>, C<system()> allows you to lie to a program about its name if |
3903 | you use the "C<system PROGRAM LIST>" syntax. Again, see L</exec>. | |
8bf3b016 | 3904 | |
7660c0ab | 3905 | Because C<system()> and backticks block C<SIGINT> and C<SIGQUIT>, killing the |
28757baa | 3906 | program they're running doesn't actually interrupt your program. |
3907 | ||
3908 | @args = ("command", "arg1", "arg2"); | |
54310121 | 3909 | system(@args) == 0 |
3910 | or die "system @args failed: $?" | |
28757baa | 3911 | |
5a964f20 TC |
3912 | You can check all the failure possibilities by inspecting |
3913 | C<$?> like this: | |
28757baa | 3914 | |
5a964f20 TC |
3915 | $exit_value = $? >> 8; |
3916 | $signal_num = $? & 127; | |
3917 | $dumped_core = $? & 128; | |
f86702cc | 3918 | |
c8db1d39 TC |
3919 | When the arguments get executed via the system shell, results |
3920 | and return codes will be subject to its quirks and capabilities. | |
3921 | See L<perlop/"`STRING`"> and L</exec> for details. | |
bb32b41a | 3922 | |
a0d0e21e LW |
3923 | =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET |
3924 | ||
3925 | =item syswrite FILEHANDLE,SCALAR,LENGTH | |
3926 | ||
3927 | Attempts to write LENGTH bytes of data from variable SCALAR to the | |
7660c0ab A |
3928 | specified FILEHANDLE, using the system call C<write(2)>. It bypasses |
3929 | stdio, so mixing this with reads (other than C<sysread())>, C<print()>, | |
3930 | C<write()>, C<seek()>, or C<tell()> may cause confusion because stdio usually | |
3931 | buffers data. Returns the number of bytes actually written, or C<undef> | |
fb73857a | 3932 | if there was an error. If the LENGTH is greater than the available |
3933 | data in the SCALAR after the OFFSET, only as much data as is available | |
96e4d5b1 | 3934 | will be written. |
ff68c719 | 3935 | |
3936 | An OFFSET may be specified to write the data from some part of the | |
3937 | string other than the beginning. A negative OFFSET specifies writing | |
fb73857a | 3938 | that many bytes counting backwards from the end of the string. In the |
3939 | case the SCALAR is empty you can use OFFSET but only zero offset. | |
a0d0e21e LW |
3940 | |
3941 | =item tell FILEHANDLE | |
3942 | ||
3943 | =item tell | |
3944 | ||
8903cb82 | 3945 | Returns the current position for FILEHANDLE. FILEHANDLE may be an |
a0d0e21e LW |
3946 | expression whose value gives the name of the actual filehandle. If |
3947 | FILEHANDLE is omitted, assumes the file last read. | |
3948 | ||
3949 | =item telldir DIRHANDLE | |
3950 | ||
7660c0ab A |
3951 | Returns the current position of the C<readdir()> routines on DIRHANDLE. |
3952 | Value may be given to C<seekdir()> to access a particular location in a | |
a0d0e21e LW |
3953 | directory. Has the same caveats about possible directory compaction as |
3954 | the corresponding system library routine. | |
3955 | ||
4633a7c4 | 3956 | =item tie VARIABLE,CLASSNAME,LIST |
a0d0e21e | 3957 | |
4633a7c4 LW |
3958 | This function binds a variable to a package class that will provide the |
3959 | implementation for the variable. VARIABLE is the name of the variable | |
3960 | to be enchanted. CLASSNAME is the name of a class implementing objects | |
ab4f32c2 | 3961 | of correct type. Any additional arguments are passed to the "C<new()>" |
7660c0ab A |
3962 | method of the class (meaning C<TIESCALAR>, C<TIEARRAY>, or C<TIEHASH>). |
3963 | Typically these are arguments such as might be passed to the C<dbm_open()> | |
ab4f32c2 | 3964 | function of C. The object returned by the "C<new()>" method is also |
7660c0ab | 3965 | returned by the C<tie()> function, which would be useful if you want to |
4633a7c4 | 3966 | access other methods in CLASSNAME. |
a0d0e21e | 3967 | |
7660c0ab | 3968 | Note that functions such as C<keys()> and C<values()> may return huge lists |
1d2dff63 | 3969 | when used on large objects, like DBM files. You may prefer to use the |
7660c0ab | 3970 | C<each()> function to iterate over such. Example: |
a0d0e21e LW |
3971 | |
3972 | # print out history file offsets | |
4633a7c4 | 3973 | use NDBM_File; |
da0045b7 | 3974 | tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); |
a0d0e21e LW |
3975 | while (($key,$val) = each %HIST) { |
3976 | print $key, ' = ', unpack('L',$val), "\n"; | |
3977 | } | |
3978 | untie(%HIST); | |
3979 | ||
aa689395 | 3980 | A class implementing a hash should have the following methods: |
a0d0e21e | 3981 | |
4633a7c4 | 3982 | TIEHASH classname, LIST |
a0d0e21e LW |
3983 | DESTROY this |
3984 | FETCH this, key | |
3985 | STORE this, key, value | |
3986 | DELETE this, key | |
3987 | EXISTS this, key | |
3988 | FIRSTKEY this | |
3989 | NEXTKEY this, lastkey | |
3990 | ||
4633a7c4 | 3991 | A class implementing an ordinary array should have the following methods: |
a0d0e21e | 3992 | |
4633a7c4 | 3993 | TIEARRAY classname, LIST |
a0d0e21e LW |
3994 | DESTROY this |
3995 | FETCH this, key | |
3996 | STORE this, key, value | |
3997 | [others TBD] | |
3998 | ||
4633a7c4 | 3999 | A class implementing a scalar should have the following methods: |
a0d0e21e | 4000 | |
4633a7c4 | 4001 | TIESCALAR classname, LIST |
a0d0e21e | 4002 | DESTROY this |
54310121 | 4003 | FETCH this, |
a0d0e21e LW |
4004 | STORE this, value |
4005 | ||
7660c0ab | 4006 | Unlike C<dbmopen()>, the C<tie()> function will not use or require a module |
4633a7c4 | 4007 | for you--you need to do that explicitly yourself. See L<DB_File> |
7660c0ab | 4008 | or the F<Config> module for interesting C<tie()> implementations. |
4633a7c4 | 4009 | |
cc6b7395 IZ |
4010 | For further details see L<perltie>, L<tied VARIABLE>. |
4011 | ||
f3cbc334 RS |
4012 | =item tied VARIABLE |
4013 | ||
4014 | Returns a reference to the object underlying VARIABLE (the same value | |
7660c0ab | 4015 | that was originally returned by the C<tie()> call that bound the variable |
f3cbc334 RS |
4016 | to a package.) Returns the undefined value if VARIABLE isn't tied to a |
4017 | package. | |
4018 | ||
a0d0e21e LW |
4019 | =item time |
4020 | ||
da0045b7 | 4021 | Returns the number of non-leap seconds since whatever time the system |
4022 | considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS, | |
4023 | and 00:00:00 UTC, January 1, 1970 for most other systems). | |
7660c0ab | 4024 | Suitable for feeding to C<gmtime()> and C<localtime()>. |
a0d0e21e LW |
4025 | |
4026 | =item times | |
4027 | ||
1d2dff63 | 4028 | Returns a four-element list giving the user and system times, in |
a0d0e21e LW |
4029 | seconds, for this process and the children of this process. |
4030 | ||
4031 | ($user,$system,$cuser,$csystem) = times; | |
4032 | ||
4033 | =item tr/// | |
4034 | ||
7660c0ab | 4035 | The transliteration operator. Same as C<y///>. See L<perlop>. |
a0d0e21e LW |
4036 | |
4037 | =item truncate FILEHANDLE,LENGTH | |
4038 | ||
4039 | =item truncate EXPR,LENGTH | |
4040 | ||
4041 | Truncates the file opened on FILEHANDLE, or named by EXPR, to the | |
4042 | specified length. Produces a fatal error if truncate isn't implemented | |
a3cb178b GS |
4043 | on your system. Returns TRUE if successful, the undefined value |
4044 | otherwise. | |
a0d0e21e LW |
4045 | |
4046 | =item uc EXPR | |
4047 | ||
54310121 | 4048 | =item uc |
bbce6d69 | 4049 | |
a0d0e21e | 4050 | Returns an uppercased version of EXPR. This is the internal function |
7660c0ab | 4051 | implementing the C<\U> escape in double-quoted strings. |
a034a98d | 4052 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. |
a0d0e21e | 4053 | |
7660c0ab | 4054 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 4055 | |
a0d0e21e LW |
4056 | =item ucfirst EXPR |
4057 | ||
54310121 | 4058 | =item ucfirst |
bbce6d69 | 4059 | |
a0d0e21e | 4060 | Returns the value of EXPR with the first character uppercased. This is |
7660c0ab | 4061 | the internal function implementing the C<\u> escape in double-quoted strings. |
a034a98d | 4062 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. |
a0d0e21e | 4063 | |
7660c0ab | 4064 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 4065 | |
a0d0e21e LW |
4066 | =item umask EXPR |
4067 | ||
4068 | =item umask | |
4069 | ||
2f9daede | 4070 | Sets the umask for the process to EXPR and returns the previous value. |
7660c0ab | 4071 | If EXPR is omitted, merely returns the current umask. If C<umask(2)> is |
7b8d334a | 4072 | not implemented on your system, returns C<undef>. Remember that a |
2f9daede | 4073 | umask is a number, usually given in octal; it is I<not> a string of octal |
dc848c6f | 4074 | digits. See also L</oct>, if all you have is a string. |
a0d0e21e LW |
4075 | |
4076 | =item undef EXPR | |
4077 | ||
4078 | =item undef | |
4079 | ||
54310121 | 4080 | Undefines the value of EXPR, which must be an lvalue. Use only on a |
7660c0ab A |
4081 | scalar value, an array (using "C<@>"), a hash (using "C<%>"), a subroutine |
4082 | (using "C<&>"), or a typeglob (using "<*>"). (Saying C<undef $hash{$key}> | |
20408e3c GS |
4083 | will probably not do what you expect on most predefined variables or |
4084 | DBM list values, so don't do that; see L<delete>.) Always returns the | |
4085 | undefined value. You can omit the EXPR, in which case nothing is | |
4086 | undefined, but you still get an undefined value that you could, for | |
4087 | instance, return from a subroutine, assign to a variable or pass as a | |
4088 | parameter. Examples: | |
a0d0e21e LW |
4089 | |
4090 | undef $foo; | |
2f9daede | 4091 | undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; |
a0d0e21e | 4092 | undef @ary; |
aa689395 | 4093 | undef %hash; |
a0d0e21e | 4094 | undef &mysub; |
20408e3c | 4095 | undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. |
54310121 | 4096 | return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; |
2f9daede TP |
4097 | select undef, undef, undef, 0.25; |
4098 | ($a, $b, undef, $c) = &foo; # Ignore third value returned | |
a0d0e21e | 4099 | |
5a964f20 TC |
4100 | Note that this is a unary operator, not a list operator. |
4101 | ||
a0d0e21e LW |
4102 | =item unlink LIST |
4103 | ||
54310121 | 4104 | =item unlink |
bbce6d69 | 4105 | |
a0d0e21e LW |
4106 | Deletes a list of files. Returns the number of files successfully |
4107 | deleted. | |
4108 | ||
4109 | $cnt = unlink 'a', 'b', 'c'; | |
4110 | unlink @goners; | |
4111 | unlink <*.bak>; | |
4112 | ||
ab4f32c2 | 4113 | Note: C<unlink()> will not delete directories unless you are superuser and |
a0d0e21e LW |
4114 | the B<-U> flag is supplied to Perl. Even if these conditions are |
4115 | met, be warned that unlinking a directory can inflict damage on your | |
ab4f32c2 | 4116 | filesystem. Use C<rmdir()> instead. |
a0d0e21e | 4117 | |
7660c0ab | 4118 | If LIST is omitted, uses C<$_>. |
bbce6d69 | 4119 | |
a0d0e21e LW |
4120 | =item unpack TEMPLATE,EXPR |
4121 | ||
ab4f32c2 | 4122 | C<Unpack()> does the reverse of C<pack()>: it takes a string representing a |
a0d0e21e | 4123 | structure and expands it out into a list value, returning the array |
5a964f20 | 4124 | value. (In scalar context, it returns merely the first value |
ab4f32c2 | 4125 | produced.) The TEMPLATE has the same format as in the C<pack()> function. |
a0d0e21e LW |
4126 | Here's a subroutine that does substring: |
4127 | ||
4128 | sub substr { | |
5a964f20 | 4129 | my($what,$where,$howmuch) = @_; |
a0d0e21e LW |
4130 | unpack("x$where a$howmuch", $what); |
4131 | } | |
4132 | ||
4133 | and then there's | |
4134 | ||
4135 | sub ordinal { unpack("c",$_[0]); } # same as ord() | |
4136 | ||
184e9718 | 4137 | In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that |
4138 | you want a E<lt>numberE<gt>-bit checksum of the items instead of the items | |
a0d0e21e LW |
4139 | themselves. Default is a 16-bit checksum. For example, the following |
4140 | computes the same number as the System V sum program: | |
4141 | ||
4142 | while (<>) { | |
4143 | $checksum += unpack("%16C*", $_); | |
4144 | } | |
4145 | $checksum %= 65536; | |
4146 | ||
4147 | The following efficiently counts the number of set bits in a bit vector: | |
4148 | ||
4149 | $setbits = unpack("%32b*", $selectmask); | |
4150 | ||
4151 | =item untie VARIABLE | |
4152 | ||
7660c0ab | 4153 | Breaks the binding between a variable and a package. (See C<tie()>.) |
a0d0e21e LW |
4154 | |
4155 | =item unshift ARRAY,LIST | |
4156 | ||
ab4f32c2 | 4157 | Does the opposite of a C<shift()>. Or the opposite of a C<push()>, |
a0d0e21e LW |
4158 | depending on how you look at it. Prepends list to the front of the |
4159 | array, and returns the new number of elements in the array. | |
4160 | ||
4161 | unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/; | |
4162 | ||
4163 | Note the LIST is prepended whole, not one element at a time, so the | |
ab4f32c2 | 4164 | prepended elements stay in the same order. Use C<reverse()> to do the |
a0d0e21e LW |
4165 | reverse. |
4166 | ||
4167 | =item use Module LIST | |
4168 | ||
4169 | =item use Module | |
4170 | ||
da0045b7 | 4171 | =item use Module VERSION LIST |
4172 | ||
4173 | =item use VERSION | |
4174 | ||
a0d0e21e LW |
4175 | Imports some semantics into the current package from the named module, |
4176 | generally by aliasing certain subroutine or variable names into your | |
4177 | package. It is exactly equivalent to | |
4178 | ||
4179 | BEGIN { require Module; import Module LIST; } | |
4180 | ||
54310121 | 4181 | except that Module I<must> be a bareword. |
da0045b7 | 4182 | |
4183 | If the first argument to C<use> is a number, it is treated as a version | |
4184 | number instead of a module name. If the version of the Perl interpreter | |
4185 | is less than VERSION, then an error message is printed and Perl exits | |
4186 | immediately. This is often useful if you need to check the current | |
5a964f20 | 4187 | Perl version before C<use>ing library modules that have changed in |
da0045b7 | 4188 | incompatible ways from older versions of Perl. (We try not to do |
4189 | this more than we have to.) | |
4190 | ||
ab4f32c2 | 4191 | The C<BEGIN> forces the C<require> and C<import()> to happen at compile time. The |
7660c0ab | 4192 | C<require> makes sure the module is loaded into memory if it hasn't been |
ab4f32c2 | 4193 | yet. The C<import()> is not a builtin--it's just an ordinary static method |
7660c0ab | 4194 | call into the "C<Module>" package to tell the module to import the list of |
a0d0e21e | 4195 | features back into the current package. The module can implement its |
ab4f32c2 A |
4196 | C<import()> method any way it likes, though most modules just choose to |
4197 | derive their C<import()> method via inheritance from the C<Exporter> class that | |
4198 | is defined in the C<Exporter> module. See L<Exporter>. If no C<import()> | |
2f9daede | 4199 | method can be found then the error is currently silently ignored. This |
55497cff | 4200 | may change to a fatal error in a future version. |
cb1a09d0 AD |
4201 | |
4202 | If you don't want your namespace altered, explicitly supply an empty list: | |
4203 | ||
4204 | use Module (); | |
4205 | ||
4206 | That is exactly equivalent to | |
4207 | ||
5a964f20 | 4208 | BEGIN { require Module } |
a0d0e21e | 4209 | |
da0045b7 | 4210 | If the VERSION argument is present between Module and LIST, then the |
71be2cbc | 4211 | C<use> will call the VERSION method in class Module with the given |
4212 | version as an argument. The default VERSION method, inherited from | |
4213 | the Universal class, croaks if the given version is larger than the | |
7660c0ab | 4214 | value of the variable C<$Module::VERSION>. (Note that there is not a |
71be2cbc | 4215 | comma after VERSION!) |
da0045b7 | 4216 | |
a0d0e21e LW |
4217 | Because this is a wide-open interface, pragmas (compiler directives) |
4218 | are also implemented this way. Currently implemented pragmas are: | |
4219 | ||
4220 | use integer; | |
4633a7c4 | 4221 | use diagnostics; |
a0d0e21e LW |
4222 | use sigtrap qw(SEGV BUS); |
4223 | use strict qw(subs vars refs); | |
4224 | use subs qw(afunc blurfl); | |
4225 | ||
5a964f20 TC |
4226 | Some of these these pseudo-modules import semantics into the current |
4227 | block scope (like C<strict> or C<integer>, unlike ordinary modules, | |
4228 | which import symbols into the current package (which are effective | |
4229 | through the end of the file). | |
a0d0e21e | 4230 | |
7660c0ab | 4231 | There's a corresponding "C<no>" command that unimports meanings imported |
ab4f32c2 | 4232 | by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import()>. |
a0d0e21e LW |
4233 | |
4234 | no integer; | |
4235 | no strict 'refs'; | |
4236 | ||
ab4f32c2 | 4237 | If no C<unimport()> method can be found the call fails with a fatal error. |
55497cff | 4238 | |
a0d0e21e LW |
4239 | See L<perlmod> for a list of standard modules and pragmas. |
4240 | ||
4241 | =item utime LIST | |
4242 | ||
4243 | Changes the access and modification times on each file of a list of | |
4244 | files. The first two elements of the list must be the NUMERICAL access | |
4245 | and modification times, in that order. Returns the number of files | |
4246 | successfully changed. The inode modification time of each file is set | |
7660c0ab | 4247 | to the current time. This code has the same effect as the "C<touch>" |
a3cb178b | 4248 | command if the files already exist: |
a0d0e21e LW |
4249 | |
4250 | #!/usr/bin/perl | |
4251 | $now = time; | |
4252 | utime $now, $now, @ARGV; | |
4253 | ||
aa689395 | 4254 | =item values HASH |
a0d0e21e | 4255 | |
1d2dff63 GS |
4256 | Returns a list consisting of all the values of the named hash. (In a |
4257 | scalar context, returns the number of values.) The values are | |
4258 | returned in an apparently random order, but it is the same order as | |
7660c0ab A |
4259 | either the C<keys()> or C<each()> function would produce on the same hash. |
4260 | As a side effect, it resets HASH's iterator. See also C<keys()>, C<each()>, | |
4261 | and C<sort()>. | |
a0d0e21e LW |
4262 | |
4263 | =item vec EXPR,OFFSET,BITS | |
4264 | ||
22dc801b | 4265 | Treats the string in EXPR as a vector of unsigned integers, and |
5f05dabc | 4266 | returns the value of the bit field specified by OFFSET. BITS specifies |
22dc801b | 4267 | the number of bits that are reserved for each entry in the bit |
7660c0ab | 4268 | vector. This must be a power of two from 1 to 32. C<vec()> may also be |
5f05dabc | 4269 | assigned to, in which case parentheses are needed to give the expression |
22dc801b | 4270 | the correct precedence as in |
4271 | ||
4272 | vec($image, $max_x * $x + $y, 8) = 3; | |
a0d0e21e | 4273 | |
7660c0ab A |
4274 | Vectors created with C<vec()> can also be manipulated with the logical |
4275 | operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is | |
a0d0e21e LW |
4276 | desired when both operands are strings. |
4277 | ||
7660c0ab | 4278 | The following code will build up an ASCII string saying C<'PerlPerlPerl'>. |
cca87523 GS |
4279 | The comments show the string after each step. Note that this code works |
4280 | in the same way on big-endian or little-endian machines. | |
4281 | ||
4282 | my $foo = ''; | |
4283 | vec($foo, 0, 32) = 0x5065726C; # 'Perl' | |
4284 | vec($foo, 2, 16) = 0x5065; # 'PerlPe' | |
4285 | vec($foo, 3, 16) = 0x726C; # 'PerlPerl' | |
4286 | vec($foo, 8, 8) = 0x50; # 'PerlPerlP' | |
4287 | vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' | |
4288 | vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" | |
4289 | vec($foo, 21, 4) = 7; # 'PerlPerlPer' # 'r' is "\x72" | |
4290 | vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" | |
4291 | vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" | |
4292 | vec($foo, 94, 1) = 1; # 'PerlPerlPerl' # 'l' is "\x6c" | |
4293 | ||
a0d0e21e LW |
4294 | To transform a bit vector into a string or array of 0's and 1's, use these: |
4295 | ||
4296 | $bits = unpack("b*", $vector); | |
4297 | @bits = split(//, unpack("b*", $vector)); | |
4298 | ||
7660c0ab | 4299 | If you know the exact length in bits, it can be used in place of the C<*>. |
a0d0e21e LW |
4300 | |
4301 | =item wait | |
4302 | ||
4303 | Waits for a child process to terminate and returns the pid of the | |
7660c0ab | 4304 | deceased process, or C<-1> if there are no child processes. The status is |
184e9718 | 4305 | returned in C<$?>. |
a0d0e21e LW |
4306 | |
4307 | =item waitpid PID,FLAGS | |
4308 | ||
4309 | Waits for a particular child process to terminate and returns the pid | |
7660c0ab | 4310 | of the deceased process, or C<-1> if there is no such child process. The |
184e9718 | 4311 | status is returned in C<$?>. If you say |
a0d0e21e | 4312 | |
5f05dabc | 4313 | use POSIX ":sys_wait_h"; |
5a964f20 | 4314 | #... |
a0d0e21e LW |
4315 | waitpid(-1,&WNOHANG); |
4316 | ||
4317 | then you can do a non-blocking wait for any process. Non-blocking wait | |
7660c0ab A |
4318 | is available on machines supporting either the C<waitpid(2)> or |
4319 | C<wait4(2)> system calls. However, waiting for a particular pid with | |
4320 | FLAGS of C<0> is implemented everywhere. (Perl emulates the system call | |
a0d0e21e LW |
4321 | by remembering the status values of processes that have exited but have |
4322 | not been harvested by the Perl script yet.) | |
4323 | ||
5a964f20 TC |
4324 | See L<perlipc> for other examples. |
4325 | ||
a0d0e21e LW |
4326 | =item wantarray |
4327 | ||
4328 | Returns TRUE if the context of the currently executing subroutine is | |
4329 | looking for a list value. Returns FALSE if the context is looking | |
54310121 | 4330 | for a scalar. Returns the undefined value if the context is looking |
4331 | for no value (void context). | |
a0d0e21e | 4332 | |
54310121 | 4333 | return unless defined wantarray; # don't bother doing more |
4334 | my @a = complex_calculation(); | |
4335 | return wantarray ? @a : "@a"; | |
a0d0e21e LW |
4336 | |
4337 | =item warn LIST | |
4338 | ||
7660c0ab | 4339 | Produces a message on STDERR just like C<die()>, but doesn't exit or throw |
774d564b | 4340 | an exception. |
4341 | ||
7660c0ab A |
4342 | If LIST is empty and C<$@> already contains a value (typically from a |
4343 | previous eval) that value is used after appending C<"\t...caught"> | |
4344 | to C<$@>. This is useful for staying almost, but not entirely similar to | |
4345 | C<die()>. | |
43051805 | 4346 | |
7660c0ab | 4347 | If C<$@> is empty then the string C<"Warning: Something's wrong"> is used. |
43051805 | 4348 | |
774d564b | 4349 | No message is printed if there is a C<$SIG{__WARN__}> handler |
4350 | installed. It is the handler's responsibility to deal with the message | |
7660c0ab | 4351 | as it sees fit (like, for instance, converting it into a C<die()>). Most |
774d564b | 4352 | handlers must therefore make arrangements to actually display the |
7660c0ab | 4353 | warnings that they are not prepared to deal with, by calling C<warn()> |
774d564b | 4354 | again in the handler. Note that this is quite safe and will not |
4355 | produce an endless loop, since C<__WARN__> hooks are not called from | |
4356 | inside one. | |
4357 | ||
4358 | You will find this behavior is slightly different from that of | |
4359 | C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can | |
7660c0ab | 4360 | instead call C<die()> again to change it). |
774d564b | 4361 | |
4362 | Using a C<__WARN__> handler provides a powerful way to silence all | |
4363 | warnings (even the so-called mandatory ones). An example: | |
4364 | ||
4365 | # wipe out *all* compile-time warnings | |
4366 | BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } | |
4367 | my $foo = 10; | |
4368 | my $foo = 20; # no warning about duplicate my $foo, | |
4369 | # but hey, you asked for it! | |
4370 | # no compile-time or run-time warnings before here | |
4371 | $DOWARN = 1; | |
4372 | ||
4373 | # run-time warnings enabled after here | |
4374 | warn "\$foo is alive and $foo!"; # does show up | |
4375 | ||
4376 | See L<perlvar> for details on setting C<%SIG> entries, and for more | |
4377 | examples. | |
a0d0e21e LW |
4378 | |
4379 | =item write FILEHANDLE | |
4380 | ||
4381 | =item write EXPR | |
4382 | ||
4383 | =item write | |
4384 | ||
5a964f20 | 4385 | Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, |
a0d0e21e | 4386 | using the format associated with that file. By default the format for |
54310121 | 4387 | a file is the one having the same name as the filehandle, but the |
7660c0ab | 4388 | format for the current output channel (see the C<select()> function) may be set |
184e9718 | 4389 | explicitly by assigning the name of the format to the C<$~> variable. |
a0d0e21e LW |
4390 | |
4391 | Top of form processing is handled automatically: if there is | |
4392 | insufficient room on the current page for the formatted record, the | |
4393 | page is advanced by writing a form feed, a special top-of-page format | |
4394 | is used to format the new page header, and then the record is written. | |
4395 | By default the top-of-page format is the name of the filehandle with | |
4396 | "_TOP" appended, but it may be dynamically set to the format of your | |
184e9718 | 4397 | choice by assigning the name to the C<$^> variable while the filehandle is |
a0d0e21e | 4398 | selected. The number of lines remaining on the current page is in |
7660c0ab | 4399 | variable C<$->, which can be set to C<0> to force a new page. |
a0d0e21e LW |
4400 | |
4401 | If FILEHANDLE is unspecified, output goes to the current default output | |
4402 | channel, which starts out as STDOUT but may be changed by the | |
ab4f32c2 | 4403 | C<select()> operator. If the FILEHANDLE is an EXPR, then the expression |
a0d0e21e LW |
4404 | is evaluated and the resulting string is used to look up the name of |
4405 | the FILEHANDLE at run time. For more on formats, see L<perlform>. | |
4406 | ||
ab4f32c2 | 4407 | Note that write is I<NOT> the opposite of C<read()>. Unfortunately. |
a0d0e21e LW |
4408 | |
4409 | =item y/// | |
4410 | ||
7660c0ab | 4411 | The transliteration operator. Same as C<tr///>. See L<perlop>. |
a0d0e21e LW |
4412 | |
4413 | =back |