| 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 |
| 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 |
| 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 |
| 31 | parentheses.) If you use the parentheses, the simple (but occasionally |
| 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 | |
| 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. |
| 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, |
| 51 | nonabortive failure is generally indicated in a scalar context by |
| 52 | returning the undefined value, and in a list context by returning the |
| 53 | null list. |
| 54 | |
| 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. |
| 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 |
| 60 | length of the list that would have been returned in list context. Some |
| 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 | |
| 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 |
| 74 | of the same name (like chown(2), fork(2), closedir(2), etc.) all return |
| 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, |
| 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<$!> |
| 79 | variable on failure. Other functions do not, except accidentally. |
| 80 | |
| 81 | =head2 Perl Functions by Category |
| 82 | |
| 83 | Here are Perl's functions (including things that look like |
| 84 | functions, like some keywords and named operators) |
| 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 | |
| 92 | C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>, |
| 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///> |
| 95 | |
| 96 | =item Regular expressions and pattern matching |
| 97 | |
| 98 | C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//> |
| 99 | |
| 100 | =item Numeric functions |
| 101 | |
| 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> |
| 104 | |
| 105 | =item Functions for real @ARRAYs |
| 106 | |
| 107 | C<pop>, C<push>, C<shift>, C<splice>, C<unshift> |
| 108 | |
| 109 | =item Functions for list data |
| 110 | |
| 111 | C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack> |
| 112 | |
| 113 | =item Functions for real %HASHes |
| 114 | |
| 115 | C<delete>, C<each>, C<exists>, C<keys>, C<values> |
| 116 | |
| 117 | =item Input and output functions |
| 118 | |
| 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> |
| 124 | |
| 125 | =item Functions for fixed length data or records |
| 126 | |
| 127 | C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec> |
| 128 | |
| 129 | =item Functions for filehandles, files, or directories |
| 130 | |
| 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>, |
| 133 | C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, |
| 134 | C<unlink>, C<utime> |
| 135 | |
| 136 | =item Keywords related to the control flow of your perl program |
| 137 | |
| 138 | C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<else>, C<elsif>, |
| 139 | C<eval>, C<exit>, C<for>, C<foreach>, C<goto>, C<if>, C<last>, |
| 140 | C<next>, C<redo>, C<return>, C<sub>, C<unless>, C<wantarray>, |
| 141 | C<while>, C<until> |
| 142 | |
| 143 | =item Keywords related to scoping |
| 144 | |
| 145 | C<caller>, C<import>, C<local>, C<my>, C<package>, C<use> |
| 146 | |
| 147 | =item Miscellaneous functions |
| 148 | |
| 149 | C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>, |
| 150 | C<scalar>, C<undef>, C<wantarray> |
| 151 | |
| 152 | =item Functions for processes and process groups |
| 153 | |
| 154 | C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>, |
| 155 | C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>, |
| 156 | C<times>, C<wait>, C<waitpid> |
| 157 | |
| 158 | =item Keywords related to perl modules |
| 159 | |
| 160 | C<do>, C<import>, C<no>, C<package>, C<require>, C<use> |
| 161 | |
| 162 | =item Keywords related to classes and object-orientedness |
| 163 | |
| 164 | C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>, |
| 165 | C<untie>, C<use> |
| 166 | |
| 167 | =item Low-level socket functions |
| 168 | |
| 169 | C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>, |
| 170 | C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>, |
| 171 | C<socket>, C<socketpair> |
| 172 | |
| 173 | =item System V interprocess communication functions |
| 174 | |
| 175 | C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>, |
| 176 | C<shmctl>, C<shmget>, C<shmread>, C<shmwrite> |
| 177 | |
| 178 | =item Fetching user and group info |
| 179 | |
| 180 | C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>, |
| 181 | C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>, |
| 182 | C<getpwuid>, C<setgrent>, C<setpwent> |
| 183 | |
| 184 | =item Fetching network info |
| 185 | |
| 186 | C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>, |
| 187 | C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, |
| 188 | C<getprotobyname>, C<getprotobynumber>, C<getprotoent>, |
| 189 | C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>, |
| 190 | C<setnetent>, C<setprotoent>, C<setservent> |
| 191 | |
| 192 | =item Time-related functions |
| 193 | |
| 194 | C<gmtime>, C<localtime>, C<time>, C<times> |
| 195 | |
| 196 | =item Functions new in perl5 |
| 197 | |
| 198 | C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>, |
| 199 | C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>, |
| 200 | C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>, |
| 201 | C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use> |
| 202 | |
| 203 | * - C<sub> was a keyword in perl4, but in perl5 it is an |
| 204 | operator, which can be used in expressions. |
| 205 | |
| 206 | =item Functions obsoleted in perl5 |
| 207 | |
| 208 | C<dbmclose>, C<dbmopen> |
| 209 | |
| 210 | =back |
| 211 | |
| 212 | =head2 Portability |
| 213 | |
| 214 | Perl was born in UNIX and therefore it can access all the common UNIX |
| 215 | system calls. In non-UNIX environments the functionality of many |
| 216 | UNIX system calls may not be available or the details of the available |
| 217 | functionality may be slightly different. The Perl functions affected |
| 218 | by this are: |
| 219 | |
| 220 | C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>, |
| 221 | C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>, |
| 222 | C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>, |
| 223 | C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>, |
| 224 | C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, |
| 225 | C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>, |
| 226 | C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>, |
| 227 | C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>, |
| 228 | C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>, |
| 229 | C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<select>, C<semctl>, |
| 230 | C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>, |
| 231 | C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>, |
| 232 | C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>, |
| 233 | C<shmwrite>, C<socketpair>, C<stat>, C<symlink>, C<syscall>, |
| 234 | C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<utime>, |
| 235 | C<wait>, C<waitpid> |
| 236 | |
| 237 | For more information about the portability of these functions, see |
| 238 | L<perlport> and other available platform-specific documentation. |
| 239 | |
| 240 | =head2 Alphabetical Listing of Perl Functions |
| 241 | |
| 242 | =over 8 |
| 243 | |
| 244 | =item I<-X> FILEHANDLE |
| 245 | |
| 246 | =item I<-X> EXPR |
| 247 | |
| 248 | =item I<-X> |
| 249 | |
| 250 | A file test, where X is one of the letters listed below. This unary |
| 251 | operator takes one argument, either a filename or a filehandle, and |
| 252 | tests the associated file to see if something is true about it. If the |
| 253 | argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN. |
| 254 | Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or |
| 255 | the undefined value if the file doesn't exist. Despite the funny |
| 256 | names, precedence is the same as any other named unary operator, and |
| 257 | the argument may be parenthesized like any other unary operator. The |
| 258 | operator may be any of: |
| 259 | X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p> |
| 260 | X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C> |
| 261 | |
| 262 | -r File is readable by effective uid/gid. |
| 263 | -w File is writable by effective uid/gid. |
| 264 | -x File is executable by effective uid/gid. |
| 265 | -o File is owned by effective uid. |
| 266 | |
| 267 | -R File is readable by real uid/gid. |
| 268 | -W File is writable by real uid/gid. |
| 269 | -X File is executable by real uid/gid. |
| 270 | -O File is owned by real uid. |
| 271 | |
| 272 | -e File exists. |
| 273 | -z File has zero size. |
| 274 | -s File has nonzero size (returns size). |
| 275 | |
| 276 | -f File is a plain file. |
| 277 | -d File is a directory. |
| 278 | -l File is a symbolic link. |
| 279 | -p File is a named pipe (FIFO), or Filehandle is a pipe. |
| 280 | -S File is a socket. |
| 281 | -b File is a block special file. |
| 282 | -c File is a character special file. |
| 283 | -t Filehandle is opened to a tty. |
| 284 | |
| 285 | -u File has setuid bit set. |
| 286 | -g File has setgid bit set. |
| 287 | -k File has sticky bit set. |
| 288 | |
| 289 | -T File is a text file. |
| 290 | -B File is a binary file (opposite of -T). |
| 291 | |
| 292 | -M Age of file in days when script started. |
| 293 | -A Same for access time. |
| 294 | -C Same for inode change time. |
| 295 | |
| 296 | Example: |
| 297 | |
| 298 | while (<>) { |
| 299 | chop; |
| 300 | next unless -f $_; # ignore specials |
| 301 | #... |
| 302 | } |
| 303 | |
| 304 | The interpretation of the file permission operators C<-r>, C<-R>, |
| 305 | C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode |
| 306 | of the file and the uids and gids of the user. There may be other |
| 307 | reasons you can't actually read, write, or execute the file. Such |
| 308 | reasons may be for example network filesystem access controls, ACLs |
| 309 | (access control lists), read-only filesystems, and unrecognized |
| 310 | executable formats. |
| 311 | |
| 312 | Also note that, for the superuser on the local filesystems, C<-r>, |
| 313 | C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return 1 |
| 314 | if any execute bit is set in the mode. Scripts run by the superuser |
| 315 | may thus need to do a stat() to determine the actual mode of the file, |
| 316 | or temporarily set the uid to something else. |
| 317 | |
| 318 | If you are using ACLs, there is a pragma called C<filetest> that may |
| 319 | produce more accurate results than the bare stat() mode bits. |
| 320 | |
| 321 | When under the C<use filetest 'access'> the above-mentioned filetests |
| 322 | will test whether the permission can (not) be granted using the |
| 323 | access() family of system calls. Also note that the C<-x> and C<-X> may |
| 324 | under this pragma return true even if there are no execute permission |
| 325 | bits set (nor any extra execute permission ACLs). This strangeness is |
| 326 | due to the underlying system calls' definitions. Read the |
| 327 | documentation for the C<filetest> pragma for more information. |
| 328 | |
| 329 | Note that C<-s/a/b/> does not do a negated substitution. Saying |
| 330 | C<-exp($foo)> still works as expected, however--only single letters |
| 331 | following a minus are interpreted as file tests. |
| 332 | |
| 333 | The C<-T> and C<-B> switches work as follows. The first block or so of the |
| 334 | file is examined for odd characters such as strange control codes or |
| 335 | characters with the high bit set. If too many strange characters (E<gt>30%) |
| 336 | are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file |
| 337 | containing null in the first block is considered a binary file. If C<-T> |
| 338 | or C<-B> is used on a filehandle, the current stdio buffer is examined |
| 339 | rather than the first block. Both C<-T> and C<-B> return TRUE on a null |
| 340 | file, or a file at EOF when testing a filehandle. Because you have to |
| 341 | read a file to do the C<-T> test, on most occasions you want to use a C<-f> |
| 342 | against the file first, as in C<next unless -f $file && -T $file>. |
| 343 | |
| 344 | If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given |
| 345 | the special filehandle consisting of a solitary underline, then the stat |
| 346 | structure of the previous file test (or stat operator) is used, saving |
| 347 | a system call. (This doesn't work with C<-t>, and you need to remember |
| 348 | that lstat() and C<-l> will leave values in the stat structure for the |
| 349 | symbolic link, not the real file.) Example: |
| 350 | |
| 351 | print "Can do.\n" if -r $a || -w _ || -x _; |
| 352 | |
| 353 | stat($filename); |
| 354 | print "Readable\n" if -r _; |
| 355 | print "Writable\n" if -w _; |
| 356 | print "Executable\n" if -x _; |
| 357 | print "Setuid\n" if -u _; |
| 358 | print "Setgid\n" if -g _; |
| 359 | print "Sticky\n" if -k _; |
| 360 | print "Text\n" if -T _; |
| 361 | print "Binary\n" if -B _; |
| 362 | |
| 363 | =item abs VALUE |
| 364 | |
| 365 | =item abs |
| 366 | |
| 367 | Returns the absolute value of its argument. |
| 368 | If VALUE is omitted, uses C<$_>. |
| 369 | |
| 370 | =item accept NEWSOCKET,GENERICSOCKET |
| 371 | |
| 372 | Accepts an incoming socket connect, just as the accept(2) system call |
| 373 | does. Returns the packed address if it succeeded, FALSE otherwise. |
| 374 | See example in L<perlipc/"Sockets: Client/Server Communication">. |
| 375 | |
| 376 | =item alarm SECONDS |
| 377 | |
| 378 | =item alarm |
| 379 | |
| 380 | Arranges to have a SIGALRM delivered to this process after the |
| 381 | specified number of seconds have elapsed. If SECONDS is not specified, |
| 382 | the value stored in C<$_> is used. (On some machines, |
| 383 | unfortunately, the elapsed time may be up to one second less than you |
| 384 | specified because of how seconds are counted.) Only one timer may be |
| 385 | counting at once. Each call disables the previous timer, and an |
| 386 | argument of C<0> may be supplied to cancel the previous timer without |
| 387 | starting a new one. The returned value is the amount of time remaining |
| 388 | on the previous timer. |
| 389 | |
| 390 | For delays of finer granularity than one second, you may use Perl's |
| 391 | C<syscall()> interface to access setitimer(2) if your system supports it, |
| 392 | or else see L</select()>. It is usually a mistake to intermix C<alarm()> |
| 393 | and C<sleep()> calls. |
| 394 | |
| 395 | If you want to use C<alarm()> to time out a system call you need to use an |
| 396 | C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to |
| 397 | fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to |
| 398 | restart system calls on some systems. Using C<eval()>/C<die()> always works, |
| 399 | modulo the caveats given in L<perlipc/"Signals">. |
| 400 | |
| 401 | eval { |
| 402 | local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required |
| 403 | alarm $timeout; |
| 404 | $nread = sysread SOCKET, $buffer, $size; |
| 405 | alarm 0; |
| 406 | }; |
| 407 | if ($@) { |
| 408 | die unless $@ eq "alarm\n"; # propagate unexpected errors |
| 409 | # timed out |
| 410 | } |
| 411 | else { |
| 412 | # didn't |
| 413 | } |
| 414 | |
| 415 | =item atan2 Y,X |
| 416 | |
| 417 | Returns the arctangent of Y/X in the range -PI to PI. |
| 418 | |
| 419 | For the tangent operation, you may use the C<POSIX::tan()> |
| 420 | function, or use the familiar relation: |
| 421 | |
| 422 | sub tan { sin($_[0]) / cos($_[0]) } |
| 423 | |
| 424 | =item bind SOCKET,NAME |
| 425 | |
| 426 | Binds a network address to a socket, just as the bind system call |
| 427 | does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a |
| 428 | packed address of the appropriate type for the socket. See the examples in |
| 429 | L<perlipc/"Sockets: Client/Server Communication">. |
| 430 | |
| 431 | =item binmode FILEHANDLE |
| 432 | |
| 433 | Arranges for the file to be read or written in "binary" mode in operating |
| 434 | systems that distinguish between binary and text files. Files that are |
| 435 | not in binary mode have CR LF sequences translated to LF on input and LF |
| 436 | translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS |
| 437 | and similarly archaic systems, it may be imperative--otherwise your |
| 438 | MS-DOS-damaged C library may mangle your file. The key distinction between |
| 439 | systems that need C<binmode()> and those that don't is their text file |
| 440 | formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single |
| 441 | character, and that encode that character in C as C<"\n">, do not need |
| 442 | C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value |
| 443 | is taken as the name of the filehandle. |
| 444 | |
| 445 | =item bless REF,CLASSNAME |
| 446 | |
| 447 | =item bless REF |
| 448 | |
| 449 | This function tells the thingy referenced by REF that it is now |
| 450 | an object in the CLASSNAME package--or the current package if no CLASSNAME |
| 451 | is specified, which is often the case. It returns the reference for |
| 452 | convenience, because a C<bless()> is often the last thing in a constructor. |
| 453 | Always use the two-argument version if the function doing the blessing |
| 454 | might be inherited by a derived class. See L<perltoot> and L<perlobj> |
| 455 | for more about the blessing (and blessings) of objects. |
| 456 | |
| 457 | Consider always blessing objects in CLASSNAMEs that are mixed case. |
| 458 | Namespaces with all lowercase names are considered reserved for Perl |
| 459 | pragmata. Builtin types have all uppercase names, so to prevent confusion, |
| 460 | it is best to avoid such package names as well. |
| 461 | |
| 462 | See L<perlmod/"Perl Modules">. |
| 463 | |
| 464 | =item caller EXPR |
| 465 | |
| 466 | =item caller |
| 467 | |
| 468 | Returns the context of the current subroutine call. In scalar context, |
| 469 | returns the caller's package name if there is a caller, that is, if |
| 470 | we're in a subroutine or C<eval()> or C<require()>, and the undefined value |
| 471 | otherwise. In list context, returns |
| 472 | |
| 473 | ($package, $filename, $line) = caller; |
| 474 | |
| 475 | With EXPR, it returns some extra information that the debugger uses to |
| 476 | print a stack trace. The value of EXPR indicates how many call frames |
| 477 | to go back before the current one. |
| 478 | |
| 479 | ($package, $filename, $line, $subroutine, |
| 480 | $hasargs, $wantarray, $evaltext, $is_require) = caller($i); |
| 481 | |
| 482 | Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine |
| 483 | call, but an C<eval()>. In such a case additional elements C<$evaltext> and |
| 484 | C<$is_require> are set: C<$is_require> is true if the frame is created by a |
| 485 | C<require> or C<use> statement, C<$evaltext> contains the text of the |
| 486 | C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement, |
| 487 | C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that |
| 488 | each C<use> statement creates a C<require> frame inside an C<eval EXPR>) |
| 489 | frame. |
| 490 | |
| 491 | Furthermore, when called from within the DB package, caller returns more |
| 492 | detailed information: it sets the list variable C<@DB::args> to be the |
| 493 | arguments with which the subroutine was invoked. |
| 494 | |
| 495 | Be aware that the optimizer might have optimized call frames away before |
| 496 | C<caller()> had a chance to get the information. That means that C<caller(N)> |
| 497 | might not return information about the call frame you expect it do, for |
| 498 | C<N E<gt> 1>. In particular, C<@DB::args> might have information from the |
| 499 | previous time C<caller()> was called. |
| 500 | |
| 501 | =item chdir EXPR |
| 502 | |
| 503 | Changes the working directory to EXPR, if possible. If EXPR is |
| 504 | omitted, changes to home directory. Returns TRUE upon success, FALSE |
| 505 | otherwise. See example under C<die()>. |
| 506 | |
| 507 | =item chmod LIST |
| 508 | |
| 509 | Changes the permissions of a list of files. The first element of the |
| 510 | list must be the numerical mode, which should probably be an octal |
| 511 | number, and which definitely should I<not> a string of octal digits: |
| 512 | C<0644> is okay, C<'0644'> is not. Returns the number of files |
| 513 | successfully changed. See also L</oct>, if all you have is a string. |
| 514 | |
| 515 | $cnt = chmod 0755, 'foo', 'bar'; |
| 516 | chmod 0755, @executables; |
| 517 | $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to |
| 518 | # --w----r-T |
| 519 | $mode = '0644'; chmod oct($mode), 'foo'; # this is better |
| 520 | $mode = 0644; chmod $mode, 'foo'; # this is best |
| 521 | |
| 522 | =item chomp VARIABLE |
| 523 | |
| 524 | =item chomp LIST |
| 525 | |
| 526 | =item chomp |
| 527 | |
| 528 | This is a slightly safer version of L</chop>. It removes any |
| 529 | line ending that corresponds to the current value of C<$/> (also known as |
| 530 | $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total |
| 531 | number of characters removed from all its arguments. It's often used to |
| 532 | remove the newline from the end of an input record when you're worried |
| 533 | that the final record may be missing its newline. When in paragraph mode |
| 534 | (C<$/ = "">), it removes all trailing newlines from the string. If |
| 535 | VARIABLE is omitted, it chomps C<$_>. Example: |
| 536 | |
| 537 | while (<>) { |
| 538 | chomp; # avoid \n on last field |
| 539 | @array = split(/:/); |
| 540 | # ... |
| 541 | } |
| 542 | |
| 543 | You can actually chomp anything that's an lvalue, including an assignment: |
| 544 | |
| 545 | chomp($cwd = `pwd`); |
| 546 | chomp($answer = <STDIN>); |
| 547 | |
| 548 | If you chomp a list, each element is chomped, and the total number of |
| 549 | characters removed is returned. |
| 550 | |
| 551 | =item chop VARIABLE |
| 552 | |
| 553 | =item chop LIST |
| 554 | |
| 555 | =item chop |
| 556 | |
| 557 | Chops off the last character of a string and returns the character |
| 558 | chopped. It's used primarily to remove the newline from the end of an |
| 559 | input record, but is much more efficient than C<s/\n//> because it neither |
| 560 | scans nor copies the string. If VARIABLE is omitted, chops C<$_>. |
| 561 | Example: |
| 562 | |
| 563 | while (<>) { |
| 564 | chop; # avoid \n on last field |
| 565 | @array = split(/:/); |
| 566 | #... |
| 567 | } |
| 568 | |
| 569 | You can actually chop anything that's an lvalue, including an assignment: |
| 570 | |
| 571 | chop($cwd = `pwd`); |
| 572 | chop($answer = <STDIN>); |
| 573 | |
| 574 | If you chop a list, each element is chopped. Only the value of the |
| 575 | last C<chop()> is returned. |
| 576 | |
| 577 | Note that C<chop()> returns the last character. To return all but the last |
| 578 | character, use C<substr($string, 0, -1)>. |
| 579 | |
| 580 | =item chown LIST |
| 581 | |
| 582 | Changes the owner (and group) of a list of files. The first two |
| 583 | elements of the list must be the I<NUMERICAL> uid and gid, in that order. |
| 584 | Returns the number of files successfully changed. |
| 585 | |
| 586 | $cnt = chown $uid, $gid, 'foo', 'bar'; |
| 587 | chown $uid, $gid, @filenames; |
| 588 | |
| 589 | Here's an example that looks up nonnumeric uids in the passwd file: |
| 590 | |
| 591 | print "User: "; |
| 592 | chop($user = <STDIN>); |
| 593 | print "Files: "; |
| 594 | chop($pattern = <STDIN>); |
| 595 | |
| 596 | ($login,$pass,$uid,$gid) = getpwnam($user) |
| 597 | or die "$user not in passwd file"; |
| 598 | |
| 599 | @ary = glob($pattern); # expand filenames |
| 600 | chown $uid, $gid, @ary; |
| 601 | |
| 602 | On most systems, you are not allowed to change the ownership of the |
| 603 | file unless you're the superuser, although you should be able to change |
| 604 | the group to any of your secondary groups. On insecure systems, these |
| 605 | restrictions may be relaxed, but this is not a portable assumption. |
| 606 | |
| 607 | =item chr NUMBER |
| 608 | |
| 609 | =item chr |
| 610 | |
| 611 | Returns the character represented by that NUMBER in the character set. |
| 612 | For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and |
| 613 | chr(0x263a) is a Unicode smiley face (but only within the scope of a |
| 614 | C<use utf8>). For the reverse, use L</ord>. |
| 615 | |
| 616 | If NUMBER is omitted, uses C<$_>. |
| 617 | |
| 618 | =item chroot FILENAME |
| 619 | |
| 620 | =item chroot |
| 621 | |
| 622 | This function works like the system call by the same name: it makes the |
| 623 | named directory the new root directory for all further pathnames that |
| 624 | begin with a C<"/"> by your process and all its children. (It doesn't |
| 625 | change your current working directory, which is unaffected.) For security |
| 626 | reasons, this call is restricted to the superuser. If FILENAME is |
| 627 | omitted, does a C<chroot()> to C<$_>. |
| 628 | |
| 629 | =item close FILEHANDLE |
| 630 | |
| 631 | =item close |
| 632 | |
| 633 | Closes the file or pipe associated with the file handle, returning TRUE |
| 634 | only if stdio successfully flushes buffers and closes the system file |
| 635 | descriptor. Closes the currently selected filehandle if the argument |
| 636 | is omitted. |
| 637 | |
| 638 | You don't have to close FILEHANDLE if you are immediately going to do |
| 639 | another C<open()> on it, because C<open()> will close it for you. (See |
| 640 | C<open()>.) However, an explicit C<close()> on an input file resets the line |
| 641 | counter (C<$.>), while the implicit close done by C<open()> does not. |
| 642 | |
| 643 | If the file handle came from a piped open C<close()> will additionally |
| 644 | return FALSE if one of the other system calls involved fails or if the |
| 645 | program exits with non-zero status. (If the only problem was that the |
| 646 | program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe |
| 647 | waits for the process executing on the pipe to complete, in case you |
| 648 | want to look at the output of the pipe afterwards. Closing a pipe |
| 649 | explicitly also puts the exit status value of the command into C<$?>. |
| 650 | |
| 651 | Example: |
| 652 | |
| 653 | open(OUTPUT, '|sort >foo') # pipe to sort |
| 654 | or die "Can't start sort: $!"; |
| 655 | #... # print stuff to output |
| 656 | close OUTPUT # wait for sort to finish |
| 657 | or warn $! ? "Error closing sort pipe: $!" |
| 658 | : "Exit status $? from sort"; |
| 659 | open(INPUT, 'foo') # get sort's results |
| 660 | or die "Can't open 'foo' for input: $!"; |
| 661 | |
| 662 | FILEHANDLE may be an expression whose value can be used as an indirect |
| 663 | filehandle, usually the real filehandle name. |
| 664 | |
| 665 | =item closedir DIRHANDLE |
| 666 | |
| 667 | Closes a directory opened by C<opendir()> and returns the success of that |
| 668 | system call. |
| 669 | |
| 670 | DIRHANDLE may be an expression whose value can be used as an indirect |
| 671 | dirhandle, usually the real dirhandle name. |
| 672 | |
| 673 | =item connect SOCKET,NAME |
| 674 | |
| 675 | Attempts to connect to a remote socket, just as the connect system call |
| 676 | does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a |
| 677 | packed address of the appropriate type for the socket. See the examples in |
| 678 | L<perlipc/"Sockets: Client/Server Communication">. |
| 679 | |
| 680 | =item continue BLOCK |
| 681 | |
| 682 | Actually a flow control statement rather than a function. If there is a |
| 683 | C<continue> BLOCK attached to a BLOCK (typically in a L</while> or |
| 684 | L</foreach>), it is always executed just before the conditional is about to |
| 685 | be evaluated again, just like the third part of a L</for> loop in C. Thus |
| 686 | it can be used to increment a loop variable, even when the loop has been |
| 687 | continued via the C<next> statement (which is similar to the C C<continue> |
| 688 | statement). |
| 689 | |
| 690 | L</last>, L</next>, or L</redo> may appear within a C<continue> |
| 691 | block. C<last> and C<redo> will behave as if they had been executed within |
| 692 | the main block. So will C<next>, but since it will execute a C<continue> |
| 693 | block, it may be more entertaining. |
| 694 | |
| 695 | while (EXPR) { |
| 696 | ### redo always comes here |
| 697 | do_something; |
| 698 | } continue { |
| 699 | ### next always comes here |
| 700 | do_something_else; |
| 701 | # then back the top to re-check EXPR |
| 702 | } |
| 703 | ### last always comes here |
| 704 | |
| 705 | Omitting the C<continue> section is semantically equivalent to using an |
| 706 | empty one, logically enough. In that case, C<next> goes directly back |
| 707 | to check the condition at the top of the loop. |
| 708 | |
| 709 | See also L<perlsyn>. |
| 710 | |
| 711 | =item cos EXPR |
| 712 | |
| 713 | Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, |
| 714 | takes cosine of C<$_>. |
| 715 | |
| 716 | For the inverse cosine operation, you may use the C<POSIX::acos()> |
| 717 | function, or use this relation: |
| 718 | |
| 719 | sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } |
| 720 | |
| 721 | =item crypt PLAINTEXT,SALT |
| 722 | |
| 723 | Encrypts a string exactly like the crypt(3) function in the C library |
| 724 | (assuming that you actually have a version there that has not been |
| 725 | extirpated as a potential munition). This can prove useful for checking |
| 726 | the password file for lousy passwords, amongst other things. Only the |
| 727 | guys wearing white hats should do this. |
| 728 | |
| 729 | Note that C<crypt()> is intended to be a one-way function, much like breaking |
| 730 | eggs to make an omelette. There is no (known) corresponding decrypt |
| 731 | function. As a result, this function isn't all that useful for |
| 732 | cryptography. (For that, see your nearby CPAN mirror.) |
| 733 | |
| 734 | When verifying an existing encrypted string you should use the encrypted |
| 735 | text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This |
| 736 | allows your code to work with the standard C<crypt()> and with more |
| 737 | exotic implementations. When choosing a new salt create a random two |
| 738 | character string whose characters come from the set C<[./0-9A-Za-z]> |
| 739 | (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). |
| 740 | |
| 741 | Here's an example that makes sure that whoever runs this program knows |
| 742 | their own password: |
| 743 | |
| 744 | $pwd = (getpwuid($<))[1]; |
| 745 | |
| 746 | system "stty -echo"; |
| 747 | print "Password: "; |
| 748 | chomp($word = <STDIN>); |
| 749 | print "\n"; |
| 750 | system "stty echo"; |
| 751 | |
| 752 | if (crypt($word, $pwd) ne $pwd) { |
| 753 | die "Sorry...\n"; |
| 754 | } else { |
| 755 | print "ok\n"; |
| 756 | } |
| 757 | |
| 758 | Of course, typing in your own password to whoever asks you |
| 759 | for it is unwise. |
| 760 | |
| 761 | =item dbmclose HASH |
| 762 | |
| 763 | [This function has been superseded by the C<untie()> function.] |
| 764 | |
| 765 | Breaks the binding between a DBM file and a hash. |
| 766 | |
| 767 | =item dbmopen HASH,DBNAME,MODE |
| 768 | |
| 769 | [This function has been superseded by the C<tie()> function.] |
| 770 | |
| 771 | This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a |
| 772 | hash. HASH is the name of the hash. (Unlike normal C<open()>, the first |
| 773 | argument is I<NOT> a filehandle, even though it looks like one). DBNAME |
| 774 | is the name of the database (without the F<.dir> or F<.pag> extension if |
| 775 | any). If the database does not exist, it is created with protection |
| 776 | specified by MODE (as modified by the C<umask()>). If your system supports |
| 777 | only the older DBM functions, you may perform only one C<dbmopen()> in your |
| 778 | program. In older versions of Perl, if your system had neither DBM nor |
| 779 | ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to |
| 780 | sdbm(3). |
| 781 | |
| 782 | If you don't have write access to the DBM file, you can only read hash |
| 783 | variables, not set them. If you want to test whether you can write, |
| 784 | either use file tests or try setting a dummy hash entry inside an C<eval()>, |
| 785 | which will trap the error. |
| 786 | |
| 787 | Note that functions such as C<keys()> and C<values()> may return huge lists |
| 788 | when used on large DBM files. You may prefer to use the C<each()> |
| 789 | function to iterate over large DBM files. Example: |
| 790 | |
| 791 | # print out history file offsets |
| 792 | dbmopen(%HIST,'/usr/lib/news/history',0666); |
| 793 | while (($key,$val) = each %HIST) { |
| 794 | print $key, ' = ', unpack('L',$val), "\n"; |
| 795 | } |
| 796 | dbmclose(%HIST); |
| 797 | |
| 798 | See also L<AnyDBM_File> for a more general description of the pros and |
| 799 | cons of the various dbm approaches, as well as L<DB_File> for a particularly |
| 800 | rich implementation. |
| 801 | |
| 802 | =item defined EXPR |
| 803 | |
| 804 | =item defined |
| 805 | |
| 806 | Returns a Boolean value telling whether EXPR has a value other than |
| 807 | the undefined value C<undef>. If EXPR is not present, C<$_> will be |
| 808 | checked. |
| 809 | |
| 810 | Many operations return C<undef> to indicate failure, end of file, |
| 811 | system error, uninitialized variable, and other exceptional |
| 812 | conditions. This function allows you to distinguish C<undef> from |
| 813 | other values. (A simple Boolean test will not distinguish among |
| 814 | C<undef>, zero, the empty string, and C<"0">, which are all equally |
| 815 | false.) Note that since C<undef> is a valid scalar, its presence |
| 816 | doesn't I<necessarily> indicate an exceptional condition: C<pop()> |
| 817 | returns C<undef> when its argument is an empty array, I<or> when the |
| 818 | element to return happens to be C<undef>. |
| 819 | |
| 820 | You may also use C<defined()> to check whether a subroutine exists, by |
| 821 | saying C<defined &func> without parentheses. On the other hand, use |
| 822 | of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to |
| 823 | produce intuitive results, and should probably be avoided. |
| 824 | |
| 825 | When used on a hash element, it tells you whether the value is defined, |
| 826 | not whether the key exists in the hash. Use L</exists> for the latter |
| 827 | purpose. |
| 828 | |
| 829 | Examples: |
| 830 | |
| 831 | print if defined $switch{'D'}; |
| 832 | print "$val\n" while defined($val = pop(@ary)); |
| 833 | die "Can't readlink $sym: $!" |
| 834 | unless defined($value = readlink $sym); |
| 835 | sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } |
| 836 | $debugging = 0 unless defined $debugging; |
| 837 | |
| 838 | Note: Many folks tend to overuse C<defined()>, and then are surprised to |
| 839 | discover that the number C<0> and C<""> (the zero-length string) are, in fact, |
| 840 | defined values. For example, if you say |
| 841 | |
| 842 | "ab" =~ /a(.*)b/; |
| 843 | |
| 844 | The pattern match succeeds, and C<$1> is defined, despite the fact that it |
| 845 | matched "nothing". But it didn't really match nothing--rather, it |
| 846 | matched something that happened to be C<0> characters long. This is all |
| 847 | very above-board and honest. When a function returns an undefined value, |
| 848 | it's an admission that it couldn't give you an honest answer. So you |
| 849 | should use C<defined()> only when you're questioning the integrity of what |
| 850 | you're trying to do. At other times, a simple comparison to C<0> or C<""> is |
| 851 | what you want. |
| 852 | |
| 853 | Currently, using C<defined()> on an entire array or hash reports whether |
| 854 | memory for that aggregate has ever been allocated. So an array you set |
| 855 | to the empty list appears undefined initially, and one that once was full |
| 856 | and that you then set to the empty list still appears defined. You |
| 857 | should instead use a simple test for size: |
| 858 | |
| 859 | if (@an_array) { print "has array elements\n" } |
| 860 | if (%a_hash) { print "has hash members\n" } |
| 861 | |
| 862 | Using C<undef()> on these, however, does clear their memory and then report |
| 863 | them as not defined anymore, but you shouldn't do that unless you don't |
| 864 | plan to use them again, because it saves time when you load them up |
| 865 | again to have memory already ready to be filled. The normal way to |
| 866 | free up space used by an aggregate is to assign the empty list. |
| 867 | |
| 868 | This counterintuitive behavior of C<defined()> on aggregates may be |
| 869 | changed, fixed, or broken in a future release of Perl. |
| 870 | |
| 871 | See also L</undef>, L</exists>, L</ref>. |
| 872 | |
| 873 | =item delete EXPR |
| 874 | |
| 875 | Deletes the specified key(s) and their associated values from a hash. |
| 876 | For each key, returns the deleted value associated with that key, or |
| 877 | the undefined value if there was no such key. Deleting from C<$ENV{}> |
| 878 | modifies the environment. Deleting from a hash tied to a DBM file |
| 879 | deletes the entry from the DBM file. (But deleting from a C<tie()>d hash |
| 880 | doesn't necessarily return anything.) |
| 881 | |
| 882 | The following deletes all the values of a hash: |
| 883 | |
| 884 | foreach $key (keys %HASH) { |
| 885 | delete $HASH{$key}; |
| 886 | } |
| 887 | |
| 888 | And so does this: |
| 889 | |
| 890 | delete @HASH{keys %HASH} |
| 891 | |
| 892 | (But both of these are slower than just assigning the empty list, or |
| 893 | using C<undef()>.) Note that the EXPR can be arbitrarily complicated as |
| 894 | long as the final operation is a hash element lookup or hash slice: |
| 895 | |
| 896 | delete $ref->[$x][$y]{$key}; |
| 897 | delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; |
| 898 | |
| 899 | =item die LIST |
| 900 | |
| 901 | Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with |
| 902 | the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of |
| 903 | C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)> |
| 904 | is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into |
| 905 | C<$@> and the C<eval()> is terminated with the undefined value. This makes |
| 906 | C<die()> the way to raise an exception. |
| 907 | |
| 908 | Equivalent examples: |
| 909 | |
| 910 | die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; |
| 911 | chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" |
| 912 | |
| 913 | If the value of EXPR does not end in a newline, the current script line |
| 914 | number and input line number (if any) are also printed, and a newline |
| 915 | is supplied. Hint: sometimes appending C<", stopped"> to your message |
| 916 | will cause it to make better sense when the string C<"at foo line 123"> is |
| 917 | appended. Suppose you are running script "canasta". |
| 918 | |
| 919 | die "/etc/games is no good"; |
| 920 | die "/etc/games is no good, stopped"; |
| 921 | |
| 922 | produce, respectively |
| 923 | |
| 924 | /etc/games is no good at canasta line 123. |
| 925 | /etc/games is no good, stopped at canasta line 123. |
| 926 | |
| 927 | See also C<exit()> and C<warn()>. |
| 928 | |
| 929 | If LIST is empty and C<$@> already contains a value (typically from a |
| 930 | previous eval) that value is reused after appending C<"\t...propagated">. |
| 931 | This is useful for propagating exceptions: |
| 932 | |
| 933 | eval { ... }; |
| 934 | die unless $@ =~ /Expected exception/; |
| 935 | |
| 936 | If C<$@> is empty then the string C<"Died"> is used. |
| 937 | |
| 938 | You can arrange for a callback to be run just before the C<die()> does |
| 939 | its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler |
| 940 | will be called with the error text and can change the error message, if |
| 941 | it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on |
| 942 | setting C<%SIG> entries, and L<"eval BLOCK"> for some examples. |
| 943 | |
| 944 | Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed |
| 945 | blocks/strings. If one wants the hook to do nothing in such |
| 946 | situations, put |
| 947 | |
| 948 | die @_ if $^S; |
| 949 | |
| 950 | as the first line of the handler (see L<perlvar/$^S>). |
| 951 | |
| 952 | =item do BLOCK |
| 953 | |
| 954 | Not really a function. Returns the value of the last command in the |
| 955 | sequence of commands indicated by BLOCK. When modified by a loop |
| 956 | modifier such as L</while> or L</until>, executes the BLOCK once |
| 957 | before testing the loop condition. (On other statements the loop |
| 958 | modifiers test the conditional first.) |
| 959 | |
| 960 | C<do BLOCK> does I<not> count as a loop, so the loop control statements |
| 961 | L</next>, L</last> or L</redo> cannot be used to leave or restart the block. |
| 962 | |
| 963 | =item do SUBROUTINE(LIST) |
| 964 | |
| 965 | A deprecated form of subroutine call. See L<perlsub>. |
| 966 | |
| 967 | =item do EXPR |
| 968 | |
| 969 | Uses the value of EXPR as a filename and executes the contents of the |
| 970 | file as a Perl script. Its primary use is to include subroutines |
| 971 | from a Perl subroutine library. |
| 972 | |
| 973 | do 'stat.pl'; |
| 974 | |
| 975 | is just like |
| 976 | |
| 977 | scalar eval `cat stat.pl`; |
| 978 | |
| 979 | except that it's more efficient and concise, keeps track of the |
| 980 | current filename for error messages, and searches all the B<-I> |
| 981 | libraries if the file isn't in the current directory (see also the @INC |
| 982 | array in L<perlvar/Predefined Names>). It is also different in how |
| 983 | code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing |
| 984 | scope like C<eval STRING> does. It's the same, however, in that it does |
| 985 | reparse the file every time you call it, so you probably don't want to |
| 986 | do this inside a loop. |
| 987 | |
| 988 | If C<do> cannot read the file, it returns undef and sets C<$!> to the |
| 989 | error. If C<do> can read the file but cannot compile it, it |
| 990 | returns undef and sets an error message in C<$@>. If the file is |
| 991 | successfully compiled, C<do> returns the value of the last expression |
| 992 | evaluated. |
| 993 | |
| 994 | Note that inclusion of library modules is better done with the |
| 995 | C<use()> and C<require()> operators, which also do automatic error checking |
| 996 | and raise an exception if there's a problem. |
| 997 | |
| 998 | You might like to use C<do> to read in a program configuration |
| 999 | file. Manual error checking can be done this way: |
| 1000 | |
| 1001 | # read in config files: system first, then user |
| 1002 | for $file ("/share/prog/defaults.rc", |
| 1003 | "$ENV{HOME}/.someprogrc") { |
| 1004 | unless ($return = do $file) { |
| 1005 | warn "couldn't parse $file: $@" if $@; |
| 1006 | warn "couldn't do $file: $!" unless defined $return; |
| 1007 | warn "couldn't run $file" unless $return; |
| 1008 | } |
| 1009 | } |
| 1010 | |
| 1011 | =item dump LABEL |
| 1012 | |
| 1013 | This causes an immediate core dump. Primarily this is so that you can |
| 1014 | use the B<undump> program to turn your core dump into an executable binary |
| 1015 | after having initialized all your variables at the beginning of the |
| 1016 | program. When the new binary is executed it will begin by executing a |
| 1017 | C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of |
| 1018 | it as a goto with an intervening core dump and reincarnation. If C<LABEL> |
| 1019 | is omitted, restarts the program from the top. WARNING: Any files |
| 1020 | opened at the time of the dump will NOT be open any more when the |
| 1021 | program is reincarnated, with possible resulting confusion on the part |
| 1022 | of Perl. See also B<-u> option in L<perlrun>. |
| 1023 | |
| 1024 | Example: |
| 1025 | |
| 1026 | #!/usr/bin/perl |
| 1027 | require 'getopt.pl'; |
| 1028 | require 'stat.pl'; |
| 1029 | %days = ( |
| 1030 | 'Sun' => 1, |
| 1031 | 'Mon' => 2, |
| 1032 | 'Tue' => 3, |
| 1033 | 'Wed' => 4, |
| 1034 | 'Thu' => 5, |
| 1035 | 'Fri' => 6, |
| 1036 | 'Sat' => 7, |
| 1037 | ); |
| 1038 | |
| 1039 | dump QUICKSTART if $ARGV[0] eq '-d'; |
| 1040 | |
| 1041 | QUICKSTART: |
| 1042 | Getopt('f'); |
| 1043 | |
| 1044 | This operator is largely obsolete, partly because it's very hard to |
| 1045 | convert a core file into an executable, and because the real perl-to-C |
| 1046 | compiler has superseded it. |
| 1047 | |
| 1048 | =item each HASH |
| 1049 | |
| 1050 | When called in list context, returns a 2-element list consisting of the |
| 1051 | key and value for the next element of a hash, so that you can iterate over |
| 1052 | it. When called in scalar context, returns the key for only the "next" |
| 1053 | element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically |
| 1054 | false; you may wish to avoid constructs like C<while ($k = each %foo) {}> |
| 1055 | for this reason.) |
| 1056 | |
| 1057 | Entries are returned in an apparently random order. The actual random |
| 1058 | order is subject to change in future versions of perl, but it is guaranteed |
| 1059 | to be in the same order as either the C<keys()> or C<values()> function |
| 1060 | would produce on the same (unmodified) hash. |
| 1061 | |
| 1062 | When the hash is entirely read, a null array is returned in list context |
| 1063 | (which when assigned produces a FALSE (C<0>) value), and C<undef> in |
| 1064 | scalar context. The next call to C<each()> after that will start iterating |
| 1065 | again. There is a single iterator for each hash, shared by all C<each()>, |
| 1066 | C<keys()>, and C<values()> function calls in the program; it can be reset by |
| 1067 | reading all the elements from the hash, or by evaluating C<keys HASH> or |
| 1068 | C<values HASH>. If you add or delete elements of a hash while you're |
| 1069 | iterating over it, you may get entries skipped or duplicated, so don't. |
| 1070 | |
| 1071 | The following prints out your environment like the printenv(1) program, |
| 1072 | only in a different order: |
| 1073 | |
| 1074 | while (($key,$value) = each %ENV) { |
| 1075 | print "$key=$value\n"; |
| 1076 | } |
| 1077 | |
| 1078 | See also C<keys()>, C<values()> and C<sort()>. |
| 1079 | |
| 1080 | =item else BLOCK |
| 1081 | |
| 1082 | =item elsif (EXPR) BLOCK |
| 1083 | |
| 1084 | See L</if>. |
| 1085 | |
| 1086 | =item eof FILEHANDLE |
| 1087 | |
| 1088 | =item eof () |
| 1089 | |
| 1090 | =item eof |
| 1091 | |
| 1092 | Returns 1 if the next read on FILEHANDLE will return end of file, or if |
| 1093 | FILEHANDLE is not open. FILEHANDLE may be an expression whose value |
| 1094 | gives the real filehandle. (Note that this function actually |
| 1095 | reads a character and then C<ungetc()>s it, so isn't very useful in an |
| 1096 | interactive context.) Do not read from a terminal file (or call |
| 1097 | C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such |
| 1098 | as terminals may lose the end-of-file condition if you do. |
| 1099 | |
| 1100 | An C<eof> without an argument uses the last file read as argument. |
| 1101 | Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of |
| 1102 | the files listed on the command line, i.e., C<eof()> is reasonable to |
| 1103 | use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the |
| 1104 | last file. Use C<eof(ARGV)> or eof without the parentheses to test |
| 1105 | I<EACH> file in a while (E<lt>E<gt>) loop. Examples: |
| 1106 | |
| 1107 | # reset line numbering on each input file |
| 1108 | while (<>) { |
| 1109 | next if /^\s*#/; # skip comments |
| 1110 | print "$.\t$_"; |
| 1111 | } continue { |
| 1112 | close ARGV if eof; # Not eof()! |
| 1113 | } |
| 1114 | |
| 1115 | # insert dashes just before last line of last file |
| 1116 | while (<>) { |
| 1117 | if (eof()) { # check for end of current file |
| 1118 | print "--------------\n"; |
| 1119 | close(ARGV); # close or break; is needed if we |
| 1120 | # are reading from the terminal |
| 1121 | } |
| 1122 | print; |
| 1123 | } |
| 1124 | |
| 1125 | Practical hint: you almost never need to use C<eof> in Perl, because the |
| 1126 | input operators return false values when they run out of data, or if there |
| 1127 | was an error. |
| 1128 | |
| 1129 | =item eval EXPR |
| 1130 | |
| 1131 | =item eval BLOCK |
| 1132 | |
| 1133 | In the first form, the return value of EXPR is parsed and executed as if it |
| 1134 | were a little Perl program. The value of the expression (which is itself |
| 1135 | determined within scalar context) is first parsed, and if there weren't any |
| 1136 | errors, executed in the context of the current Perl program, so that any |
| 1137 | variable settings or subroutine and format definitions remain afterwards. |
| 1138 | Note that the value is parsed every time the eval executes. If EXPR is |
| 1139 | omitted, evaluates C<$_>. This form is typically used to delay parsing |
| 1140 | and subsequent execution of the text of EXPR until run time. |
| 1141 | |
| 1142 | In the second form, the code within the BLOCK is parsed only once--at the |
| 1143 | same time the code surrounding the eval itself was parsed--and executed |
| 1144 | within the context of the current Perl program. This form is typically |
| 1145 | used to trap exceptions more efficiently than the first (see below), while |
| 1146 | also providing the benefit of checking the code within BLOCK at compile |
| 1147 | time. |
| 1148 | |
| 1149 | The final semicolon, if any, may be omitted from the value of EXPR or within |
| 1150 | the BLOCK. |
| 1151 | |
| 1152 | In both forms, the value returned is the value of the last expression |
| 1153 | evaluated inside the mini-program; a return statement may be also used, just |
| 1154 | as with subroutines. The expression providing the return value is evaluated |
| 1155 | in void, scalar, or list context, depending on the context of the eval itself. |
| 1156 | See L</wantarray> for more on how the evaluation context can be determined. |
| 1157 | |
| 1158 | If there is a syntax error or runtime error, or a C<die()> statement is |
| 1159 | executed, an undefined value is returned by C<eval()>, and C<$@> is set to the |
| 1160 | error message. If there was no error, C<$@> is guaranteed to be a null |
| 1161 | string. Beware that using C<eval()> neither silences perl from printing |
| 1162 | warnings to STDERR, nor does it stuff the text of warning messages into C<$@>. |
| 1163 | To do either of those, you have to use the C<$SIG{__WARN__}> facility. See |
| 1164 | L</warn> and L<perlvar>. |
| 1165 | |
| 1166 | Note that, because C<eval()> traps otherwise-fatal errors, it is useful for |
| 1167 | determining whether a particular feature (such as C<socket()> or C<symlink()>) |
| 1168 | is implemented. It is also Perl's exception trapping mechanism, where |
| 1169 | the die operator is used to raise exceptions. |
| 1170 | |
| 1171 | If the code to be executed doesn't vary, you may use the eval-BLOCK |
| 1172 | form to trap run-time errors without incurring the penalty of |
| 1173 | recompiling each time. The error, if any, is still returned in C<$@>. |
| 1174 | Examples: |
| 1175 | |
| 1176 | # make divide-by-zero nonfatal |
| 1177 | eval { $answer = $a / $b; }; warn $@ if $@; |
| 1178 | |
| 1179 | # same thing, but less efficient |
| 1180 | eval '$answer = $a / $b'; warn $@ if $@; |
| 1181 | |
| 1182 | # a compile-time error |
| 1183 | eval { $answer = }; # WRONG |
| 1184 | |
| 1185 | # a run-time error |
| 1186 | eval '$answer ='; # sets $@ |
| 1187 | |
| 1188 | When using the C<eval{}> form as an exception trap in libraries, you may |
| 1189 | wish not to trigger any C<__DIE__> hooks that user code may have |
| 1190 | installed. You can use the C<local $SIG{__DIE__}> construct for this |
| 1191 | purpose, as shown in this example: |
| 1192 | |
| 1193 | # a very private exception trap for divide-by-zero |
| 1194 | eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; |
| 1195 | warn $@ if $@; |
| 1196 | |
| 1197 | This is especially significant, given that C<__DIE__> hooks can call |
| 1198 | C<die()> again, which has the effect of changing their error messages: |
| 1199 | |
| 1200 | # __DIE__ hooks may modify error messages |
| 1201 | { |
| 1202 | local $SIG{'__DIE__'} = |
| 1203 | sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; |
| 1204 | eval { die "foo lives here" }; |
| 1205 | print $@ if $@; # prints "bar lives here" |
| 1206 | } |
| 1207 | |
| 1208 | With an C<eval()>, you should be especially careful to remember what's |
| 1209 | being looked at when: |
| 1210 | |
| 1211 | eval $x; # CASE 1 |
| 1212 | eval "$x"; # CASE 2 |
| 1213 | |
| 1214 | eval '$x'; # CASE 3 |
| 1215 | eval { $x }; # CASE 4 |
| 1216 | |
| 1217 | eval "\$$x++"; # CASE 5 |
| 1218 | $$x++; # CASE 6 |
| 1219 | |
| 1220 | Cases 1 and 2 above behave identically: they run the code contained in |
| 1221 | the variable C<$x>. (Although case 2 has misleading double quotes making |
| 1222 | the reader wonder what else might be happening (nothing is).) Cases 3 |
| 1223 | and 4 likewise behave in the same way: they run the code C<'$x'>, which |
| 1224 | does nothing but return the value of C<$x>. (Case 4 is preferred for |
| 1225 | purely visual reasons, but it also has the advantage of compiling at |
| 1226 | compile-time instead of at run-time.) Case 5 is a place where |
| 1227 | normally you I<WOULD> like to use double quotes, except that in this |
| 1228 | particular situation, you can just use symbolic references instead, as |
| 1229 | in case 6. |
| 1230 | |
| 1231 | C<eval BLOCK> does I<not> count as a loop, so the loop control statements |
| 1232 | C<next>, C<last> or C<redo> cannot be used to leave or restart the block. |
| 1233 | |
| 1234 | |
| 1235 | =item exec LIST |
| 1236 | |
| 1237 | =item exec PROGRAM LIST |
| 1238 | |
| 1239 | The C<exec()> function executes a system command I<AND NEVER RETURNS> - |
| 1240 | use C<system()> instead of C<exec()> if you want it to return. It fails and |
| 1241 | returns FALSE only if the command does not exist I<and> it is executed |
| 1242 | directly instead of via your system's command shell (see below). |
| 1243 | |
| 1244 | Since it's a common mistake to use C<exec()> instead of C<system()>, Perl |
| 1245 | warns you if there is a following statement which isn't C<die()>, C<warn()>, |
| 1246 | or C<exit()> (if C<-w> is set - but you always do that). If you |
| 1247 | I<really> want to follow an C<exec()> with some other statement, you |
| 1248 | can use one of these styles to avoid the warning: |
| 1249 | |
| 1250 | exec ('foo') or print STDERR "couldn't exec foo: $!"; |
| 1251 | { exec ('foo') }; print STDERR "couldn't exec foo: $!"; |
| 1252 | |
| 1253 | If there is more than one argument in LIST, or if LIST is an array |
| 1254 | with more than one value, calls execvp(3) with the arguments in LIST. |
| 1255 | If there is only one scalar argument or an array with one element in it, |
| 1256 | the argument is checked for shell metacharacters, and if there are any, |
| 1257 | the entire argument is passed to the system's command shell for parsing |
| 1258 | (this is C</bin/sh -c> on Unix platforms, but varies on other platforms). |
| 1259 | If there are no shell metacharacters in the argument, it is split into |
| 1260 | words and passed directly to C<execvp()>, which is more efficient. Note: |
| 1261 | C<exec()> and C<system()> do not flush your output buffer, so you may need to |
| 1262 | set C<$|> to avoid lost output. Examples: |
| 1263 | |
| 1264 | exec '/bin/echo', 'Your arguments are: ', @ARGV; |
| 1265 | exec "sort $outfile | uniq"; |
| 1266 | |
| 1267 | If you don't really want to execute the first argument, but want to lie |
| 1268 | to the program you are executing about its own name, you can specify |
| 1269 | the program you actually want to run as an "indirect object" (without a |
| 1270 | comma) in front of the LIST. (This always forces interpretation of the |
| 1271 | LIST as a multivalued list, even if there is only a single scalar in |
| 1272 | the list.) Example: |
| 1273 | |
| 1274 | $shell = '/bin/csh'; |
| 1275 | exec $shell '-sh'; # pretend it's a login shell |
| 1276 | |
| 1277 | or, more directly, |
| 1278 | |
| 1279 | exec {'/bin/csh'} '-sh'; # pretend it's a login shell |
| 1280 | |
| 1281 | When the arguments get executed via the system shell, results will |
| 1282 | be subject to its quirks and capabilities. See L<perlop/"`STRING`"> |
| 1283 | for details. |
| 1284 | |
| 1285 | Using an indirect object with C<exec()> or C<system()> is also more secure. |
| 1286 | This usage forces interpretation of the arguments as a multivalued list, |
| 1287 | even if the list had just one argument. That way you're safe from the |
| 1288 | shell expanding wildcards or splitting up words with whitespace in them. |
| 1289 | |
| 1290 | @args = ( "echo surprise" ); |
| 1291 | |
| 1292 | system @args; # subject to shell escapes |
| 1293 | # if @args == 1 |
| 1294 | system { $args[0] } @args; # safe even with one-arg list |
| 1295 | |
| 1296 | The first version, the one without the indirect object, ran the I<echo> |
| 1297 | program, passing it C<"surprise"> an argument. The second version |
| 1298 | didn't--it tried to run a program literally called I<"echo surprise">, |
| 1299 | didn't find it, and set C<$?> to a non-zero value indicating failure. |
| 1300 | |
| 1301 | Note that C<exec()> will not call your C<END> blocks, nor will it call |
| 1302 | any C<DESTROY> methods in your objects. |
| 1303 | |
| 1304 | =item exists EXPR |
| 1305 | |
| 1306 | Returns TRUE if the specified hash key exists in its hash array, even |
| 1307 | if the corresponding value is undefined. |
| 1308 | |
| 1309 | print "Exists\n" if exists $array{$key}; |
| 1310 | print "Defined\n" if defined $array{$key}; |
| 1311 | print "True\n" if $array{$key}; |
| 1312 | |
| 1313 | A hash element can be TRUE only if it's defined, and defined if |
| 1314 | it exists, but the reverse doesn't necessarily hold true. |
| 1315 | |
| 1316 | Note that the EXPR can be arbitrarily complicated as long as the final |
| 1317 | operation is a hash key lookup: |
| 1318 | |
| 1319 | if (exists $ref->{"A"}{"B"}{$key}) { ... } |
| 1320 | |
| 1321 | Although the last element will not spring into existence just because its |
| 1322 | existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}> |
| 1323 | C<$ref-E<gt>{"B"}> will spring into existence due to the existence |
| 1324 | test for a $key element. This autovivification may be fixed in a later |
| 1325 | release. |
| 1326 | |
| 1327 | =item exit EXPR |
| 1328 | |
| 1329 | Evaluates EXPR and exits immediately with that value. (Actually, it |
| 1330 | calls any defined C<END> routines first, but the C<END> routines may not |
| 1331 | abort the exit. Likewise any object destructors that need to be called |
| 1332 | are called before exit.) Example: |
| 1333 | |
| 1334 | $ans = <STDIN>; |
| 1335 | exit 0 if $ans =~ /^[Xx]/; |
| 1336 | |
| 1337 | See also C<die()>. If EXPR is omitted, exits with C<0> status. The only |
| 1338 | universally portable values for EXPR are C<0> for success and C<1> for error; |
| 1339 | all other values are subject to unpredictable interpretation depending |
| 1340 | on the environment in which the Perl program is running. |
| 1341 | |
| 1342 | You shouldn't use C<exit()> to abort a subroutine if there's any chance that |
| 1343 | someone might want to trap whatever error happened. Use C<die()> instead, |
| 1344 | which can be trapped by an C<eval()>. |
| 1345 | |
| 1346 | All C<END{}> blocks are run at exit time. See L<perlsub> for details. |
| 1347 | |
| 1348 | =item exp EXPR |
| 1349 | |
| 1350 | =item exp |
| 1351 | |
| 1352 | Returns I<e> (the natural logarithm base) to the power of EXPR. |
| 1353 | If EXPR is omitted, gives C<exp($_)>. |
| 1354 | |
| 1355 | =item fcntl FILEHANDLE,FUNCTION,SCALAR |
| 1356 | |
| 1357 | Implements the fcntl(2) function. You'll probably have to say |
| 1358 | |
| 1359 | use Fcntl; |
| 1360 | |
| 1361 | first to get the correct constant definitions. Argument processing and |
| 1362 | value return works just like C<ioctl()> below. |
| 1363 | For example: |
| 1364 | |
| 1365 | use Fcntl; |
| 1366 | fcntl($filehandle, F_GETFL, $packed_return_buffer) |
| 1367 | or die "can't fcntl F_GETFL: $!"; |
| 1368 | |
| 1369 | You don't have to check for C<defined()> on the return from |
| 1370 | C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system |
| 1371 | call into "C<0> but true" in Perl. This string is true in |
| 1372 | boolean context and C<0> in numeric context. It is also |
| 1373 | exempt from the normal B<-w> warnings on improper numeric |
| 1374 | conversions. |
| 1375 | |
| 1376 | Note that C<fcntl()> will produce a fatal error if used on a machine that |
| 1377 | doesn't implement fcntl(2). |
| 1378 | |
| 1379 | =item fileno FILEHANDLE |
| 1380 | |
| 1381 | Returns the file descriptor for a filehandle. This is useful for |
| 1382 | constructing bitmaps for C<select()> and low-level POSIX tty-handling |
| 1383 | operations. If FILEHANDLE is an expression, the value is taken as |
| 1384 | an indirect filehandle, generally its name. |
| 1385 | |
| 1386 | You can use this to find out whether two handles refer to the |
| 1387 | same underlying descriptor: |
| 1388 | |
| 1389 | if (fileno(THIS) == fileno(THAT)) { |
| 1390 | print "THIS and THAT are dups\n"; |
| 1391 | } |
| 1392 | |
| 1393 | =item flock FILEHANDLE,OPERATION |
| 1394 | |
| 1395 | Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for |
| 1396 | success, FALSE on failure. Produces a fatal error if used on a machine |
| 1397 | that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()> |
| 1398 | is Perl's portable file locking interface, although it locks only entire |
| 1399 | files, not records. |
| 1400 | |
| 1401 | On many platforms (including most versions or clones of Unix), locks |
| 1402 | established by C<flock()> are B<merely advisory>. Such discretionary locks |
| 1403 | are more flexible, but offer fewer guarantees. This means that files |
| 1404 | locked with C<flock()> may be modified by programs that do not also use |
| 1405 | C<flock()>. Windows NT and OS/2 are among the platforms which |
| 1406 | enforce mandatory locking. See your local documentation for details. |
| 1407 | |
| 1408 | OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with |
| 1409 | LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but |
| 1410 | you can use the symbolic names if import them from the Fcntl module, |
| 1411 | either individually, or as a group using the ':flock' tag. LOCK_SH |
| 1412 | requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN |
| 1413 | releases a previously requested lock. If LOCK_NB is added to LOCK_SH or |
| 1414 | LOCK_EX then C<flock()> will return immediately rather than blocking |
| 1415 | waiting for the lock (check the return status to see if you got it). |
| 1416 | |
| 1417 | To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE |
| 1418 | before (un)locking it. |
| 1419 | |
| 1420 | Note that the emulation built with lockf(3) doesn't provide shared |
| 1421 | locks, and it requires that FILEHANDLE be open with write intent. These |
| 1422 | are the semantics that lockf(3) implements. Most (all?) systems |
| 1423 | implement lockf(3) in terms of fcntl(2) locking, though, so the |
| 1424 | differing semantics shouldn't bite too many people. |
| 1425 | |
| 1426 | Note also that some versions of C<flock()> cannot lock things over the |
| 1427 | network; you would need to use the more system-specific C<fcntl()> for |
| 1428 | that. If you like you can force Perl to ignore your system's flock(2) |
| 1429 | function, and so provide its own fcntl(2)-based emulation, by passing |
| 1430 | the switch C<-Ud_flock> to the F<Configure> program when you configure |
| 1431 | perl. |
| 1432 | |
| 1433 | Here's a mailbox appender for BSD systems. |
| 1434 | |
| 1435 | use Fcntl ':flock'; # import LOCK_* constants |
| 1436 | |
| 1437 | sub lock { |
| 1438 | flock(MBOX,LOCK_EX); |
| 1439 | # and, in case someone appended |
| 1440 | # while we were waiting... |
| 1441 | seek(MBOX, 0, 2); |
| 1442 | } |
| 1443 | |
| 1444 | sub unlock { |
| 1445 | flock(MBOX,LOCK_UN); |
| 1446 | } |
| 1447 | |
| 1448 | open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}") |
| 1449 | or die "Can't open mailbox: $!"; |
| 1450 | |
| 1451 | lock(); |
| 1452 | print MBOX $msg,"\n\n"; |
| 1453 | unlock(); |
| 1454 | |
| 1455 | See also L<DB_File> for other flock() examples. |
| 1456 | |
| 1457 | =item for (INITIAL; WHILE; EACH) BLOCK |
| 1458 | |
| 1459 | Do INITIAL, enter BLOCK while EXPR is true, at the end of each round |
| 1460 | do EACH. For example: |
| 1461 | |
| 1462 | for ($i = 0, $j = 0; $i < 10; $i++) { |
| 1463 | if ($i % 3 == 0) { $j++ } |
| 1464 | print "i = $i, j = $j\n"; |
| 1465 | } |
| 1466 | |
| 1467 | See L<perlsyn> for more details. See also L</foreach>, a twin of |
| 1468 | C<for>, L</while> and L</until>, close cousins of L<for>, and |
| 1469 | L</last>, L</next>, and L</redo> for additional control flow. |
| 1470 | |
| 1471 | =item foreach LOOPVAR (LIST) BLOCK |
| 1472 | |
| 1473 | Enter BLOCK as LOOPVAR set in turn to each element of LIST. |
| 1474 | For example: |
| 1475 | |
| 1476 | foreach $rolling (@stones) { print "rolling $stone\n" } |
| 1477 | |
| 1478 | foreach my $file (@files) { print "file $file\n" } |
| 1479 | |
| 1480 | The LOOPVAR is optional and defaults to C<$_>. If the elements are |
| 1481 | modifiable (as opposed to constants or tied variables) you can modify them. |
| 1482 | |
| 1483 | foreach (@words) { tr/abc/xyz/ } |
| 1484 | |
| 1485 | See L<perlsyn> for more details. See also L</for>, a twin of |
| 1486 | C<foreach>, L</while> and L</until>, close cousins of L<for>, and |
| 1487 | L</last>, L</next>, and L</redo> for additional control flow. |
| 1488 | |
| 1489 | =item fork |
| 1490 | |
| 1491 | Does a fork(2) system call. Returns the child pid to the parent process, |
| 1492 | C<0> to the child process, or C<undef> if the fork is unsuccessful. |
| 1493 | |
| 1494 | Note: unflushed buffers remain unflushed in both processes, which means |
| 1495 | you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> |
| 1496 | method of C<IO::Handle> to avoid duplicate output. |
| 1497 | |
| 1498 | If you C<fork()> without ever waiting on your children, you will accumulate |
| 1499 | zombies: |
| 1500 | |
| 1501 | $SIG{CHLD} = sub { wait }; |
| 1502 | |
| 1503 | There's also the double-fork trick (error checking on |
| 1504 | C<fork()> returns omitted); |
| 1505 | |
| 1506 | unless ($pid = fork) { |
| 1507 | unless (fork) { |
| 1508 | exec "what you really wanna do"; |
| 1509 | die "no exec"; |
| 1510 | # ... or ... |
| 1511 | ## (some_perl_code_here) |
| 1512 | exit 0; |
| 1513 | } |
| 1514 | exit 0; |
| 1515 | } |
| 1516 | waitpid($pid,0); |
| 1517 | |
| 1518 | See also L<perlipc> for more examples of forking and reaping |
| 1519 | moribund children. |
| 1520 | |
| 1521 | Note that if your forked child inherits system file descriptors like |
| 1522 | STDIN and STDOUT that are actually connected by a pipe or socket, even |
| 1523 | if you exit, then the remote server (such as, say, httpd or rsh) won't think |
| 1524 | you're done. You should reopen those to F</dev/null> if it's any issue. |
| 1525 | |
| 1526 | =item format |
| 1527 | |
| 1528 | Declare a picture format for use by the C<write()> function. For |
| 1529 | example: |
| 1530 | |
| 1531 | format Something = |
| 1532 | Test: @<<<<<<<< @||||| @>>>>> |
| 1533 | $str, $%, '$' . int($num) |
| 1534 | . |
| 1535 | |
| 1536 | $str = "widget"; |
| 1537 | $num = $cost/$quantity; |
| 1538 | $~ = 'Something'; |
| 1539 | write; |
| 1540 | |
| 1541 | See L<perlform> for many details and examples. |
| 1542 | |
| 1543 | =item formline PICTURE,LIST |
| 1544 | |
| 1545 | This is an internal function used by C<format>s, though you may call it, |
| 1546 | too. It formats (see L<perlform>) a list of values according to the |
| 1547 | contents of PICTURE, placing the output into the format output |
| 1548 | accumulator, C<$^A> (or C<$ACCUMULATOR> in English). |
| 1549 | Eventually, when a C<write()> is done, the contents of |
| 1550 | C<$^A> are written to some filehandle, but you could also read C<$^A> |
| 1551 | yourself and then set C<$^A> back to C<"">. Note that a format typically |
| 1552 | does one C<formline()> per line of form, but the C<formline()> function itself |
| 1553 | doesn't care how many newlines are embedded in the PICTURE. This means |
| 1554 | that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line. |
| 1555 | You may therefore need to use multiple formlines to implement a single |
| 1556 | record format, just like the format compiler. |
| 1557 | |
| 1558 | Be careful if you put double quotes around the picture, because an "C<@>" |
| 1559 | character may be taken to mean the beginning of an array name. |
| 1560 | C<formline()> always returns TRUE. See L<perlform> for other examples. |
| 1561 | |
| 1562 | =item getc FILEHANDLE |
| 1563 | |
| 1564 | =item getc |
| 1565 | |
| 1566 | Returns the next character from the input file attached to FILEHANDLE, |
| 1567 | or the undefined value at end of file, or if there was an error. If |
| 1568 | FILEHANDLE is omitted, reads from STDIN. This is not particularly |
| 1569 | efficient. It cannot be used to get unbuffered single-characters, |
| 1570 | however. For that, try something more like: |
| 1571 | |
| 1572 | if ($BSD_STYLE) { |
| 1573 | system "stty cbreak </dev/tty >/dev/tty 2>&1"; |
| 1574 | } |
| 1575 | else { |
| 1576 | system "stty", '-icanon', 'eol', "\001"; |
| 1577 | } |
| 1578 | |
| 1579 | $key = getc(STDIN); |
| 1580 | |
| 1581 | if ($BSD_STYLE) { |
| 1582 | system "stty -cbreak </dev/tty >/dev/tty 2>&1"; |
| 1583 | } |
| 1584 | else { |
| 1585 | system "stty", 'icanon', 'eol', '^@'; # ASCII null |
| 1586 | } |
| 1587 | print "\n"; |
| 1588 | |
| 1589 | Determination of whether $BSD_STYLE should be set |
| 1590 | is left as an exercise to the reader. |
| 1591 | |
| 1592 | The C<POSIX::getattr()> function can do this more portably on systems |
| 1593 | purporting POSIX compliance. |
| 1594 | See also the C<Term::ReadKey> module from your nearest CPAN site; |
| 1595 | details on CPAN can be found on L<perlmodlib/CPAN>. |
| 1596 | |
| 1597 | =item getlogin |
| 1598 | |
| 1599 | Implements the C library function of the same name, which on most |
| 1600 | systems returns the current login from F</etc/utmp>, if any. If null, |
| 1601 | use C<getpwuid()>. |
| 1602 | |
| 1603 | $login = getlogin || getpwuid($<) || "Kilroy"; |
| 1604 | |
| 1605 | Do not consider C<getlogin()> for authentication: it is not as |
| 1606 | secure as C<getpwuid()>. |
| 1607 | |
| 1608 | =item getpeername SOCKET |
| 1609 | |
| 1610 | Returns the packed sockaddr address of other end of the SOCKET connection. |
| 1611 | |
| 1612 | use Socket; |
| 1613 | $hersockaddr = getpeername(SOCK); |
| 1614 | ($port, $iaddr) = unpack_sockaddr_in($hersockaddr); |
| 1615 | $herhostname = gethostbyaddr($iaddr, AF_INET); |
| 1616 | $herstraddr = inet_ntoa($iaddr); |
| 1617 | |
| 1618 | =item getpgrp PID |
| 1619 | |
| 1620 | Returns the current process group for the specified PID. Use |
| 1621 | a PID of C<0> to get the current process group for the |
| 1622 | current process. Will raise an exception if used on a machine that |
| 1623 | doesn't implement getpgrp(2). If PID is omitted, returns process |
| 1624 | group of current process. Note that the POSIX version of C<getpgrp()> |
| 1625 | does not accept a PID argument, so only C<PID==0> is truly portable. |
| 1626 | |
| 1627 | =item getppid |
| 1628 | |
| 1629 | Returns the process id of the parent process. |
| 1630 | |
| 1631 | =item getpriority WHICH,WHO |
| 1632 | |
| 1633 | Returns the current priority for a process, a process group, or a user. |
| 1634 | (See L<getpriority(2)>.) Will raise a fatal exception if used on a |
| 1635 | machine that doesn't implement getpriority(2). |
| 1636 | |
| 1637 | =item getpwnam NAME |
| 1638 | |
| 1639 | =item getgrnam NAME |
| 1640 | |
| 1641 | =item gethostbyname NAME |
| 1642 | |
| 1643 | =item getnetbyname NAME |
| 1644 | |
| 1645 | =item getprotobyname NAME |
| 1646 | |
| 1647 | =item getpwuid UID |
| 1648 | |
| 1649 | =item getgrgid GID |
| 1650 | |
| 1651 | =item getservbyname NAME,PROTO |
| 1652 | |
| 1653 | =item gethostbyaddr ADDR,ADDRTYPE |
| 1654 | |
| 1655 | =item getnetbyaddr ADDR,ADDRTYPE |
| 1656 | |
| 1657 | =item getprotobynumber NUMBER |
| 1658 | |
| 1659 | =item getservbyport PORT,PROTO |
| 1660 | |
| 1661 | =item getpwent |
| 1662 | |
| 1663 | =item getgrent |
| 1664 | |
| 1665 | =item gethostent |
| 1666 | |
| 1667 | =item getnetent |
| 1668 | |
| 1669 | =item getprotoent |
| 1670 | |
| 1671 | =item getservent |
| 1672 | |
| 1673 | =item setpwent |
| 1674 | |
| 1675 | =item setgrent |
| 1676 | |
| 1677 | =item sethostent STAYOPEN |
| 1678 | |
| 1679 | =item setnetent STAYOPEN |
| 1680 | |
| 1681 | =item setprotoent STAYOPEN |
| 1682 | |
| 1683 | =item setservent STAYOPEN |
| 1684 | |
| 1685 | =item endpwent |
| 1686 | |
| 1687 | =item endgrent |
| 1688 | |
| 1689 | =item endhostent |
| 1690 | |
| 1691 | =item endnetent |
| 1692 | |
| 1693 | =item endprotoent |
| 1694 | |
| 1695 | =item endservent |
| 1696 | |
| 1697 | These routines perform the same functions as their counterparts in the |
| 1698 | system library. In list context, the return values from the |
| 1699 | various get routines are as follows: |
| 1700 | |
| 1701 | ($name,$passwd,$uid,$gid, |
| 1702 | $quota,$comment,$gcos,$dir,$shell,$expire) = getpw* |
| 1703 | ($name,$passwd,$gid,$members) = getgr* |
| 1704 | ($name,$aliases,$addrtype,$length,@addrs) = gethost* |
| 1705 | ($name,$aliases,$addrtype,$net) = getnet* |
| 1706 | ($name,$aliases,$proto) = getproto* |
| 1707 | ($name,$aliases,$port,$proto) = getserv* |
| 1708 | |
| 1709 | (If the entry doesn't exist you get a null list.) |
| 1710 | |
| 1711 | In scalar context, you get the name, unless the function was a |
| 1712 | lookup by name, in which case you get the other thing, whatever it is. |
| 1713 | (If the entry doesn't exist you get the undefined value.) For example: |
| 1714 | |
| 1715 | $uid = getpwnam($name); |
| 1716 | $name = getpwuid($num); |
| 1717 | $name = getpwent(); |
| 1718 | $gid = getgrnam($name); |
| 1719 | $name = getgrgid($num; |
| 1720 | $name = getgrent(); |
| 1721 | #etc. |
| 1722 | |
| 1723 | In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special |
| 1724 | cases in the sense that in many systems they are unsupported. If the |
| 1725 | C<$quota> is unsupported, it is an empty scalar. If it is supported, it |
| 1726 | usually encodes the disk quota. If the C<$comment> field is unsupported, |
| 1727 | it is an empty scalar. If it is supported it usually encodes some |
| 1728 | administrative comment about the user. In some systems the $quota |
| 1729 | field may be C<$change> or C<$age>, fields that have to do with password |
| 1730 | aging. In some systems the C<$comment> field may be C<$class>. The C<$expire> |
| 1731 | field, if present, encodes the expiration period of the account or the |
| 1732 | password. For the availability and the exact meaning of these fields |
| 1733 | in your system, please consult your getpwnam(3) documentation and your |
| 1734 | F<pwd.h> file. You can also find out from within Perl which meaning |
| 1735 | your C<$quota> and C<$comment> fields have and whether you have the C<$expire> |
| 1736 | field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>, |
| 1737 | C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. |
| 1738 | |
| 1739 | The C<$members> value returned by I<getgr*()> is a space separated list of |
| 1740 | the login names of the members of the group. |
| 1741 | |
| 1742 | For the I<gethost*()> functions, if the C<h_errno> variable is supported in |
| 1743 | C, it will be returned to you via C<$?> if the function call fails. The |
| 1744 | C<@addrs> value returned by a successful call is a list of the raw |
| 1745 | addresses returned by the corresponding system library call. In the |
| 1746 | Internet domain, each address is four bytes long and you can unpack it |
| 1747 | by saying something like: |
| 1748 | |
| 1749 | ($a,$b,$c,$d) = unpack('C4',$addr[0]); |
| 1750 | |
| 1751 | If you get tired of remembering which element of the return list contains |
| 1752 | which return value, by-name interfaces are also provided in modules: |
| 1753 | C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>, |
| 1754 | C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the |
| 1755 | normal built-in, replacing them with versions that return objects with |
| 1756 | the appropriate names for each field. For example: |
| 1757 | |
| 1758 | use File::stat; |
| 1759 | use User::pwent; |
| 1760 | $is_his = (stat($filename)->uid == pwent($whoever)->uid); |
| 1761 | |
| 1762 | Even though it looks like they're the same method calls (uid), |
| 1763 | they aren't, because a C<File::stat> object is different from a C<User::pwent> object. |
| 1764 | |
| 1765 | =item getsockname SOCKET |
| 1766 | |
| 1767 | Returns the packed sockaddr address of this end of the SOCKET connection. |
| 1768 | |
| 1769 | use Socket; |
| 1770 | $mysockaddr = getsockname(SOCK); |
| 1771 | ($port, $myaddr) = unpack_sockaddr_in($mysockaddr); |
| 1772 | |
| 1773 | =item getsockopt SOCKET,LEVEL,OPTNAME |
| 1774 | |
| 1775 | Returns the socket option requested, or undef if there is an error. |
| 1776 | |
| 1777 | =item glob EXPR |
| 1778 | |
| 1779 | =item glob |
| 1780 | |
| 1781 | Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would |
| 1782 | do. This is the internal function implementing the C<E<lt>*.cE<gt>> |
| 1783 | operator, but you can use it directly. If EXPR is omitted, C<$_> is used. |
| 1784 | The C<E<lt>*.cE<gt>> operator is discussed in more detail in |
| 1785 | L<perlop/"I/O Operators">. |
| 1786 | |
| 1787 | =item gmtime EXPR |
| 1788 | |
| 1789 | Converts a time as returned by the time function to a 9-element array |
| 1790 | with the time localized for the standard Greenwich time zone. |
| 1791 | Typically used as follows: |
| 1792 | |
| 1793 | # 0 1 2 3 4 5 6 7 8 |
| 1794 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = |
| 1795 | gmtime(time); |
| 1796 | |
| 1797 | All array elements are numeric, and come straight out of a struct tm. |
| 1798 | In particular this means that C<$mon> has the range C<0..11> and C<$wday> has |
| 1799 | the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of |
| 1800 | years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year. |
| 1801 | |
| 1802 | If EXPR is omitted, does C<gmtime(time())>. |
| 1803 | |
| 1804 | In scalar context, returns the ctime(3) value: |
| 1805 | |
| 1806 | $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994" |
| 1807 | |
| 1808 | Also see the C<timegm()> function provided by the C<Time::Local> module, |
| 1809 | and the strftime(3) function available via the POSIX module. |
| 1810 | |
| 1811 | This scalar value is B<not> locale dependent, see L<perllocale>, but |
| 1812 | instead a Perl builtin. Also see the C<Time::Local> module, and the |
| 1813 | strftime(3) and mktime(3) function available via the POSIX module. To |
| 1814 | get somewhat similar but locale dependent date strings, set up your |
| 1815 | locale environment variables appropriately (please see L<perllocale>) |
| 1816 | and try for example: |
| 1817 | |
| 1818 | use POSIX qw(strftime); |
| 1819 | $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; |
| 1820 | |
| 1821 | Note that the C<%a> and C<%b>, the short forms of the day of the week |
| 1822 | and the month of the year, may not necessarily be three characters wide. |
| 1823 | |
| 1824 | =item goto LABEL |
| 1825 | |
| 1826 | =item goto EXPR |
| 1827 | |
| 1828 | =item goto &NAME |
| 1829 | |
| 1830 | The C<goto-LABEL> form finds the statement labeled with LABEL and resumes |
| 1831 | execution there. It may not be used to go into any construct that |
| 1832 | requires initialization, such as a subroutine or a C<foreach> loop. It |
| 1833 | also can't be used to go into a construct that is optimized away, |
| 1834 | or to get out of a block or subroutine given to C<sort()>. |
| 1835 | It can be used to go almost anywhere else within the dynamic scope, |
| 1836 | including out of subroutines, but it's usually better to use some other |
| 1837 | construct such as C<last> or C<die()>. The author of Perl has never felt the |
| 1838 | need to use this form of C<goto> (in Perl, that is--C is another matter). |
| 1839 | |
| 1840 | The C<goto-EXPR> form expects a label name, whose scope will be resolved |
| 1841 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't |
| 1842 | necessarily recommended if you're optimizing for maintainability: |
| 1843 | |
| 1844 | goto ("FOO", "BAR", "GLARCH")[$i]; |
| 1845 | |
| 1846 | The C<goto-&NAME> form is highly magical, and substitutes a call to the |
| 1847 | named subroutine for the currently running subroutine. This is used by |
| 1848 | C<AUTOLOAD> subroutines that wish to load another subroutine and then |
| 1849 | pretend that the other subroutine had been called in the first place |
| 1850 | (except that any modifications to C<@_> in the current subroutine are |
| 1851 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> |
| 1852 | will be able to tell that this routine was called first. |
| 1853 | |
| 1854 | =item grep BLOCK LIST |
| 1855 | |
| 1856 | =item grep EXPR,LIST |
| 1857 | |
| 1858 | This is similar in spirit to, but not the same as, grep(1) |
| 1859 | and its relatives. In particular, it is not limited to using |
| 1860 | regular expressions. |
| 1861 | |
| 1862 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting |
| 1863 | C<$_> to each element) and returns the list value consisting of those |
| 1864 | elements for which the expression evaluated to TRUE. In a scalar |
| 1865 | context, returns the number of times the expression was TRUE. |
| 1866 | |
| 1867 | @foo = grep(!/^#/, @bar); # weed out comments |
| 1868 | |
| 1869 | or equivalently, |
| 1870 | |
| 1871 | @foo = grep {!/^#/} @bar; # weed out comments |
| 1872 | |
| 1873 | Note that, because C<$_> is a reference into the list value, it can be used |
| 1874 | to modify the elements of the array. While this is useful and |
| 1875 | supported, it can cause bizarre results if the LIST is not a named |
| 1876 | array. Similarly, grep returns aliases into the original list, |
| 1877 | much like the way that a for loop's index variable aliases the list |
| 1878 | elements. That is, modifying an element of a list returned by grep |
| 1879 | (for example, in a C<foreach>, C<map()> or another C<grep()>) |
| 1880 | actually modifies the element in the original list. |
| 1881 | |
| 1882 | See also L</map> for an array composed of the results of the BLOCK or EXPR. |
| 1883 | |
| 1884 | =item hex EXPR |
| 1885 | |
| 1886 | =item hex |
| 1887 | |
| 1888 | Interprets EXPR as a hex string and returns the corresponding |
| 1889 | value. (To convert strings that might start with either 0 or 0x |
| 1890 | see L</oct>.) If EXPR is omitted, uses C<$_>. |
| 1891 | |
| 1892 | print hex '0xAf'; # prints '175' |
| 1893 | print hex 'aF'; # same |
| 1894 | |
| 1895 | =item if (EXPR) BLOCK |
| 1896 | |
| 1897 | =item if (EXPR) BLOCK else BLOCK2 |
| 1898 | |
| 1899 | =item if (EXPR) BLOCK elsif (EXPR2) BLOCK2 |
| 1900 | |
| 1901 | Enter BLOCKs conditionally. The first EXPR to return true |
| 1902 | causes the corresponding BLOCK to be entered, or, in the case |
| 1903 | of C<else>, the fall-through default BLOCK. |
| 1904 | |
| 1905 | Note 1: Perl wants BLOCKS, expressions won't do (like they do |
| 1906 | e.g. in C, C++, Java, Pascal). |
| 1907 | |
| 1908 | Note 2: It's C<elsif>, not C<elseif>. You can have as many |
| 1909 | C<elsif>s as you want. |
| 1910 | |
| 1911 | See L<perlsyn> for more details. See also C<unless>. |
| 1912 | |
| 1913 | =item import |
| 1914 | |
| 1915 | There is no builtin C<import()> function. It is just an ordinary |
| 1916 | method (subroutine) defined (or inherited) by modules that wish to export |
| 1917 | names to another module. The C<use()> function calls the C<import()> method |
| 1918 | for the package used. See also L</use()>, L<perlmod>, and L<Exporter>. |
| 1919 | |
| 1920 | =item index STR,SUBSTR,POSITION |
| 1921 | |
| 1922 | =item index STR,SUBSTR |
| 1923 | |
| 1924 | Returns the position of the first occurrence of SUBSTR in STR at or after |
| 1925 | POSITION. If POSITION is omitted, starts searching from the beginning of |
| 1926 | the string. The return value is based at C<0> (or whatever you've set the C<$[> |
| 1927 | variable to--but don't do that). If the substring is not found, returns |
| 1928 | one less than the base, ordinarily C<-1>. |
| 1929 | |
| 1930 | =item int EXPR |
| 1931 | |
| 1932 | =item int |
| 1933 | |
| 1934 | Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>. |
| 1935 | You should not use this for rounding, because it truncates |
| 1936 | towards C<0>, and because machine representations of floating point |
| 1937 | numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>, |
| 1938 | or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better. |
| 1939 | |
| 1940 | =item ioctl FILEHANDLE,FUNCTION,SCALAR |
| 1941 | |
| 1942 | Implements the ioctl(2) function. You'll probably have to say |
| 1943 | |
| 1944 | require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph |
| 1945 | |
| 1946 | first to get the correct function definitions. If F<ioctl.ph> doesn't |
| 1947 | exist or doesn't have the correct definitions you'll have to roll your |
| 1948 | own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>. |
| 1949 | (There is a Perl script called B<h2ph> that comes with the Perl kit that |
| 1950 | may help you in this, but it's nontrivial.) SCALAR will be read and/or |
| 1951 | written depending on the FUNCTION--a pointer to the string value of SCALAR |
| 1952 | will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR |
| 1953 | has no string value but does have a numeric value, that value will be |
| 1954 | passed rather than a pointer to the string value. To guarantee this to be |
| 1955 | TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()> |
| 1956 | functions are useful for manipulating the values of structures used by |
| 1957 | C<ioctl()>. The following example sets the erase character to DEL. |
| 1958 | |
| 1959 | require 'ioctl.ph'; |
| 1960 | $getp = &TIOCGETP; |
| 1961 | die "NO TIOCGETP" if $@ || !$getp; |
| 1962 | $sgttyb_t = "ccccs"; # 4 chars and a short |
| 1963 | if (ioctl(STDIN,$getp,$sgttyb)) { |
| 1964 | @ary = unpack($sgttyb_t,$sgttyb); |
| 1965 | $ary[2] = 127; |
| 1966 | $sgttyb = pack($sgttyb_t,@ary); |
| 1967 | ioctl(STDIN,&TIOCSETP,$sgttyb) |
| 1968 | || die "Can't ioctl: $!"; |
| 1969 | } |
| 1970 | |
| 1971 | The return value of C<ioctl()> (and C<fcntl()>) is as follows: |
| 1972 | |
| 1973 | if OS returns: then Perl returns: |
| 1974 | -1 undefined value |
| 1975 | 0 string "0 but true" |
| 1976 | anything else that number |
| 1977 | |
| 1978 | Thus Perl returns TRUE on success and FALSE on failure, yet you can |
| 1979 | still easily determine the actual value returned by the operating |
| 1980 | system: |
| 1981 | |
| 1982 | ($retval = ioctl(...)) || ($retval = -1); |
| 1983 | printf "System returned %d\n", $retval; |
| 1984 | |
| 1985 | The special string "C<0> but true" is excempt from B<-w> complaints |
| 1986 | about improper numeric conversions. |
| 1987 | |
| 1988 | =item join EXPR,LIST |
| 1989 | |
| 1990 | Joins the separate strings of LIST into a single string with |
| 1991 | fields separated by the value of EXPR, and returns the string. |
| 1992 | Example: |
| 1993 | |
| 1994 | $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); |
| 1995 | |
| 1996 | See L</split>. |
| 1997 | |
| 1998 | =item keys HASH |
| 1999 | |
| 2000 | Returns a list consisting of all the keys of the named hash. (In a |
| 2001 | scalar context, returns the number of keys.) The keys are returned in |
| 2002 | an apparently random order. The actual random order is subject to |
| 2003 | change in future versions of perl, but it is guaranteed to be the same |
| 2004 | order as either the C<values()> or C<each()> function produces (given |
| 2005 | that the hash has not been modified). As a side effect, it resets |
| 2006 | HASH's iterator. |
| 2007 | |
| 2008 | Here is yet another way to print your environment: |
| 2009 | |
| 2010 | @keys = keys %ENV; |
| 2011 | @values = values %ENV; |
| 2012 | while ($#keys >= 0) { |
| 2013 | print pop(@keys), '=', pop(@values), "\n"; |
| 2014 | } |
| 2015 | |
| 2016 | or how about sorted by key: |
| 2017 | |
| 2018 | foreach $key (sort(keys %ENV)) { |
| 2019 | print $key, '=', $ENV{$key}, "\n"; |
| 2020 | } |
| 2021 | |
| 2022 | To sort a hash by value, you'll need to use a C<sort()> function. |
| 2023 | Here's a descending numeric sort of a hash by its values: |
| 2024 | |
| 2025 | foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { |
| 2026 | printf "%4d %s\n", $hash{$key}, $key; |
| 2027 | } |
| 2028 | |
| 2029 | As an lvalue C<keys()> allows you to increase the number of hash buckets |
| 2030 | allocated for the given hash. This can gain you a measure of efficiency if |
| 2031 | you know the hash is going to get big. (This is similar to pre-extending |
| 2032 | an array by assigning a larger number to $#array.) If you say |
| 2033 | |
| 2034 | keys %hash = 200; |
| 2035 | |
| 2036 | then C<%hash> will have at least 200 buckets allocated for it--256 of them, |
| 2037 | in fact, since it rounds up to the next power of two. These |
| 2038 | buckets will be retained even if you do C<%hash = ()>, use C<undef |
| 2039 | %hash> if you want to free the storage while C<%hash> is still in scope. |
| 2040 | You can't shrink the number of buckets allocated for the hash using |
| 2041 | C<keys()> in this way (but you needn't worry about doing this by accident, |
| 2042 | as trying has no effect). |
| 2043 | |
| 2044 | See also C<each()>, C<values()> and C<sort()>. |
| 2045 | |
| 2046 | =item kill LIST |
| 2047 | |
| 2048 | Sends a signal to a list of processes. The first element of |
| 2049 | the list must be the signal to send. Returns the number of |
| 2050 | processes successfully signaled. |
| 2051 | |
| 2052 | $cnt = kill 1, $child1, $child2; |
| 2053 | kill 9, @goners; |
| 2054 | |
| 2055 | Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills |
| 2056 | process groups instead of processes. (On System V, a negative I<PROCESS> |
| 2057 | number will also kill process groups, but that's not portable.) That |
| 2058 | means you usually want to use positive not negative signals. You may also |
| 2059 | use a signal name in quotes. See L<perlipc/"Signals"> for details. |
| 2060 | |
| 2061 | =item last LABEL |
| 2062 | |
| 2063 | =item last |
| 2064 | |
| 2065 | The C<last> command is like the C<break> statement in C (as used in |
| 2066 | loops); it immediately exits the loop in question. If the LABEL is |
| 2067 | omitted, the command refers to the innermost enclosing loop. The |
| 2068 | C<continue> block, if any, is not executed: |
| 2069 | |
| 2070 | LINE: while (<STDIN>) { |
| 2071 | last LINE if /^$/; # exit when done with header |
| 2072 | #... |
| 2073 | } |
| 2074 | |
| 2075 | C<last> cannot be used to exit a block which returns a value such as |
| 2076 | C<eval {}>, C<sub {}> or C<do {}>. |
| 2077 | |
| 2078 | See also L</continue> for an illustration of how C<last>, L</next>, and |
| 2079 | L</redo> work. |
| 2080 | |
| 2081 | See also L<perlsyn>. |
| 2082 | |
| 2083 | =item lc EXPR |
| 2084 | |
| 2085 | =item lc |
| 2086 | |
| 2087 | Returns an lowercased version of EXPR. This is the internal function |
| 2088 | implementing the C<\L> escape in double-quoted strings. |
| 2089 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. |
| 2090 | |
| 2091 | If EXPR is omitted, uses C<$_>. |
| 2092 | |
| 2093 | =item lcfirst EXPR |
| 2094 | |
| 2095 | =item lcfirst |
| 2096 | |
| 2097 | Returns the value of EXPR with the first character lowercased. This is |
| 2098 | the internal function implementing the C<\l> escape in double-quoted strings. |
| 2099 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. |
| 2100 | |
| 2101 | If EXPR is omitted, uses C<$_>. |
| 2102 | |
| 2103 | =item length EXPR |
| 2104 | |
| 2105 | =item length |
| 2106 | |
| 2107 | Returns the length in characters of the value of EXPR. If EXPR is |
| 2108 | omitted, returns length of C<$_>. |
| 2109 | |
| 2110 | =item link OLDFILE,NEWFILE |
| 2111 | |
| 2112 | Creates a new filename linked to the old filename. Returns TRUE for |
| 2113 | success, FALSE otherwise. |
| 2114 | |
| 2115 | =item listen SOCKET,QUEUESIZE |
| 2116 | |
| 2117 | Does the same thing that the listen system call does. Returns TRUE if |
| 2118 | it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">. |
| 2119 | |
| 2120 | =item local EXPR |
| 2121 | |
| 2122 | A local modifies the listed variables to be local to the enclosing |
| 2123 | block, file, or eval. If more than one value is listed, the list must |
| 2124 | be placed in parentheses. See L<perlsub/"Temporary Values via local()"> |
| 2125 | for details, including issues with tied arrays and hashes. |
| 2126 | |
| 2127 | You really probably want to be using C<my()> instead, because C<local()> isn't |
| 2128 | what most people think of as "local". See L<perlsub/"Private Variables |
| 2129 | via my()"> for details. |
| 2130 | |
| 2131 | =item localtime EXPR |
| 2132 | |
| 2133 | Converts a time as returned by the time function to a 9-element array |
| 2134 | with the time analyzed for the local time zone. Typically used as |
| 2135 | follows: |
| 2136 | |
| 2137 | # 0 1 2 3 4 5 6 7 8 |
| 2138 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = |
| 2139 | localtime(time); |
| 2140 | |
| 2141 | All array elements are numeric, and come straight out of a struct tm. |
| 2142 | In particular this means that C<$mon> has the range C<0..11> and C<$wday> has |
| 2143 | the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of |
| 2144 | years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year. |
| 2145 | |
| 2146 | If EXPR is omitted, uses the current time (C<localtime(time)>). |
| 2147 | |
| 2148 | In scalar context, returns the ctime(3) value: |
| 2149 | |
| 2150 | $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" |
| 2151 | |
| 2152 | This scalar value is B<not> locale dependent, see L<perllocale>, but |
| 2153 | instead a Perl builtin. Also see the C<Time::Local> module, and the |
| 2154 | strftime(3) and mktime(3) function available via the POSIX module. To |
| 2155 | get somewhat similar but locale dependent date strings, set up your |
| 2156 | locale environment variables appropriately (please see L<perllocale>) |
| 2157 | and try for example: |
| 2158 | |
| 2159 | use POSIX qw(strftime); |
| 2160 | $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; |
| 2161 | |
| 2162 | Note that the C<%a> and C<%b>, the short forms of the day of the week |
| 2163 | and the month of the year, may not necessarily be three characters wide. |
| 2164 | |
| 2165 | =item log EXPR |
| 2166 | |
| 2167 | =item log |
| 2168 | |
| 2169 | Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log |
| 2170 | of C<$_>. |
| 2171 | |
| 2172 | =item lstat FILEHANDLE |
| 2173 | |
| 2174 | =item lstat EXPR |
| 2175 | |
| 2176 | =item lstat |
| 2177 | |
| 2178 | Does the same thing as the C<stat()> function (including setting the |
| 2179 | special C<_> filehandle) but stats a symbolic link instead of the file |
| 2180 | the symbolic link points to. If symbolic links are unimplemented on |
| 2181 | your system, a normal C<stat()> is done. |
| 2182 | |
| 2183 | If EXPR is omitted, stats C<$_>. |
| 2184 | |
| 2185 | =item m// |
| 2186 | |
| 2187 | The match operator. See L<perlop>. |
| 2188 | |
| 2189 | =item map BLOCK LIST |
| 2190 | |
| 2191 | =item map EXPR,LIST |
| 2192 | |
| 2193 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each |
| 2194 | element) and returns the list value composed of the results of each such |
| 2195 | evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST |
| 2196 | may produce zero, one, or more elements in the returned value. |
| 2197 | |
| 2198 | @chars = map(chr, @nums); |
| 2199 | |
| 2200 | translates a list of numbers to the corresponding characters. And |
| 2201 | |
| 2202 | %hash = map { getkey($_) => $_ } @array; |
| 2203 | |
| 2204 | is just a funny way to write |
| 2205 | |
| 2206 | %hash = (); |
| 2207 | foreach $_ (@array) { |
| 2208 | $hash{getkey($_)} = $_; |
| 2209 | } |
| 2210 | |
| 2211 | Note that, because C<$_> is a reference into the list value, it can be used |
| 2212 | to modify the elements of the array. While this is useful and |
| 2213 | supported, it can cause bizarre results if the LIST is not a named |
| 2214 | array. See also L</grep> for an array composed of those items of the |
| 2215 | original list for which the BLOCK or EXPR evaluates to true. |
| 2216 | |
| 2217 | =item mkdir FILENAME,MODE |
| 2218 | |
| 2219 | Creates the directory specified by FILENAME, with permissions |
| 2220 | specified by MODE (as modified by C<umask>). If it succeeds it |
| 2221 | returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). |
| 2222 | |
| 2223 | In general, it is better to create directories with permissive MODEs, |
| 2224 | and let the user modify that with their C<umask>, than it is to supply |
| 2225 | a restrictive MODE and give the user no way to be more permissive. |
| 2226 | The exceptions to this rule are when the file or directory should be |
| 2227 | kept private (mail files, for instance). The perlfunc(1) entry on |
| 2228 | C<umask> discusses the choice of MODE in more detail. |
| 2229 | |
| 2230 | =item msgctl ID,CMD,ARG |
| 2231 | |
| 2232 | Calls the System V IPC function msgctl(2). You'll probably have to say |
| 2233 | |
| 2234 | use IPC::SysV; |
| 2235 | |
| 2236 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, |
| 2237 | then ARG must be a variable which will hold the returned C<msqid_ds> |
| 2238 | structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but |
| 2239 | true" for zero, or the actual return value otherwise. See also |
| 2240 | C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation. |
| 2241 | |
| 2242 | =item msgget KEY,FLAGS |
| 2243 | |
| 2244 | Calls the System V IPC function msgget(2). Returns the message queue |
| 2245 | id, or the undefined value if there is an error. See also C<IPC::SysV> |
| 2246 | and C<IPC::SysV::Msg> documentation. |
| 2247 | |
| 2248 | =item msgsnd ID,MSG,FLAGS |
| 2249 | |
| 2250 | Calls the System V IPC function msgsnd to send the message MSG to the |
| 2251 | message queue ID. MSG must begin with the long integer message type, |
| 2252 | which may be created with C<pack("l", $type)>. Returns TRUE if |
| 2253 | successful, or FALSE if there is an error. See also C<IPC::SysV> |
| 2254 | and C<IPC::SysV::Msg> documentation. |
| 2255 | |
| 2256 | =item msgrcv ID,VAR,SIZE,TYPE,FLAGS |
| 2257 | |
| 2258 | Calls the System V IPC function msgrcv to receive a message from |
| 2259 | message queue ID into variable VAR with a maximum message size of |
| 2260 | SIZE. Note that if a message is received, the message type will be |
| 2261 | the first thing in VAR, and the maximum length of VAR is SIZE plus the |
| 2262 | size of the message type. Returns TRUE if successful, or FALSE if |
| 2263 | there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation. |
| 2264 | |
| 2265 | =item my EXPR |
| 2266 | |
| 2267 | A C<my()> declares the listed variables to be local (lexically) to the |
| 2268 | enclosing block, file, or C<eval()>. If |
| 2269 | more than one value is listed, the list must be placed in parentheses. See |
| 2270 | L<perlsub/"Private Variables via my()"> for details. |
| 2271 | |
| 2272 | =item next LABEL |
| 2273 | |
| 2274 | =item next |
| 2275 | |
| 2276 | The C<next> command is like the C<continue> statement in C; it starts |
| 2277 | the next iteration of the loop: |
| 2278 | |
| 2279 | LINE: while (<STDIN>) { |
| 2280 | next LINE if /^#/; # discard comments |
| 2281 | #... |
| 2282 | } |
| 2283 | |
| 2284 | Note that if there were a C<continue> block on the above, it would get |
| 2285 | executed even on discarded lines. If the LABEL is omitted, the command |
| 2286 | refers to the innermost enclosing loop. |
| 2287 | |
| 2288 | C<next> cannot be used to exit a block which returns a value such as |
| 2289 | C<eval {}>, C<sub {}> or C<do {}>. |
| 2290 | |
| 2291 | See also L</continue> for an illustration of how L</last>, C<next>, and |
| 2292 | L</redo> work. |
| 2293 | |
| 2294 | See also L<perlsyn>. |
| 2295 | |
| 2296 | =item no Module LIST |
| 2297 | |
| 2298 | See the L</use> function, which C<no> is the opposite of. |
| 2299 | |
| 2300 | =item oct EXPR |
| 2301 | |
| 2302 | =item oct |
| 2303 | |
| 2304 | Interprets EXPR as an octal string and returns the corresponding |
| 2305 | value. (If EXPR happens to start off with C<0x>, interprets it as |
| 2306 | a hex string instead.) The following will handle decimal, octal, and |
| 2307 | hex in the standard Perl or C notation: |
| 2308 | |
| 2309 | $val = oct($val) if $val =~ /^0/; |
| 2310 | |
| 2311 | If EXPR is omitted, uses C<$_>. This function is commonly used when |
| 2312 | a string such as C<644> needs to be converted into a file mode, for |
| 2313 | example. (Although perl will automatically convert strings into |
| 2314 | numbers as needed, this automatic conversion assumes base 10.) |
| 2315 | |
| 2316 | =item open FILEHANDLE,EXPR |
| 2317 | |
| 2318 | =item open FILEHANDLE |
| 2319 | |
| 2320 | Opens the file whose filename is given by EXPR, and associates it with |
| 2321 | FILEHANDLE. If FILEHANDLE is an expression, its value is used as the |
| 2322 | name of the real filehandle wanted. If EXPR is omitted, the scalar |
| 2323 | variable of the same name as the FILEHANDLE contains the filename. |
| 2324 | (Note that lexical variables--those declared with C<my()>--will not work |
| 2325 | for this purpose; so if you're using C<my()>, specify EXPR in your call |
| 2326 | to open.) |
| 2327 | |
| 2328 | If the filename begins with C<'E<lt>'> or nothing, the file is opened for input. |
| 2329 | If the filename begins with C<'E<gt>'>, the file is truncated and opened for |
| 2330 | output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>, |
| 2331 | the file is opened for appending, again being created if necessary. |
| 2332 | You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that |
| 2333 | you want both read and write access to the file; thus C<'+E<lt>'> is almost |
| 2334 | always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the |
| 2335 | file first. You can't usually use either read-write mode for updating |
| 2336 | textfiles, since they have variable length records. See the B<-i> |
| 2337 | switch in L<perlrun> for a better approach. The file is created with |
| 2338 | permissions of C<0666> modified by the process' C<umask> value. |
| 2339 | |
| 2340 | The prefix and the filename may be separated with spaces. |
| 2341 | These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>, |
| 2342 | C<'w+'>, C<'a'>, and C<'a+'>. |
| 2343 | |
| 2344 | If the filename begins with C<'|'>, the filename is interpreted as a |
| 2345 | command to which output is to be piped, and if the filename ends with a |
| 2346 | C<'|'>, the filename is interpreted See L<perlipc/"Using open() for IPC"> |
| 2347 | for more examples of this. (You are not allowed to C<open()> to a command |
| 2348 | that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, |
| 2349 | and L<perlipc/"Bidirectional Communication"> for alternatives.) |
| 2350 | |
| 2351 | Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns |
| 2352 | nonzero upon success, the undefined value otherwise. If the C<open()> |
| 2353 | involved a pipe, the return value happens to be the pid of the |
| 2354 | subprocess. |
| 2355 | |
| 2356 | If you're unfortunate enough to be running Perl on a system that |
| 2357 | distinguishes between text files and binary files (modern operating |
| 2358 | systems don't care), then you should check out L</binmode> for tips for |
| 2359 | dealing with this. The key distinction between systems that need C<binmode()> |
| 2360 | and those that don't is their text file formats. Systems like Unix, MacOS, and |
| 2361 | Plan9, which delimit lines with a single character, and which encode that |
| 2362 | character in C as C<"\n">, do not need C<binmode()>. The rest need it. |
| 2363 | |
| 2364 | When opening a file, it's usually a bad idea to continue normal execution |
| 2365 | if the request failed, so C<open()> is frequently used in connection with |
| 2366 | C<die()>. Even if C<die()> won't do what you want (say, in a CGI script, |
| 2367 | where you want to make a nicely formatted error message (but there are |
| 2368 | modules that can help with that problem)) you should always check |
| 2369 | the return value from opening a file. The infrequent exception is when |
| 2370 | working with an unopened filehandle is actually what you want to do. |
| 2371 | |
| 2372 | Examples: |
| 2373 | |
| 2374 | $ARTICLE = 100; |
| 2375 | open ARTICLE or die "Can't find article $ARTICLE: $!\n"; |
| 2376 | while (<ARTICLE>) {... |
| 2377 | |
| 2378 | open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) |
| 2379 | # if the open fails, output is discarded |
| 2380 | |
| 2381 | open(DBASE, '+<dbase.mine') # open for update |
| 2382 | or die "Can't open 'dbase.mine' for update: $!"; |
| 2383 | |
| 2384 | open(ARTICLE, "caesar <$article |") # decrypt article |
| 2385 | or die "Can't start caesar: $!"; |
| 2386 | |
| 2387 | open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id |
| 2388 | or die "Can't start sort: $!"; |
| 2389 | |
| 2390 | # process argument list of files along with any includes |
| 2391 | |
| 2392 | foreach $file (@ARGV) { |
| 2393 | process($file, 'fh00'); |
| 2394 | } |
| 2395 | |
| 2396 | sub process { |
| 2397 | my($filename, $input) = @_; |
| 2398 | $input++; # this is a string increment |
| 2399 | unless (open($input, $filename)) { |
| 2400 | print STDERR "Can't open $filename: $!\n"; |
| 2401 | return; |
| 2402 | } |
| 2403 | |
| 2404 | local $_; |
| 2405 | while (<$input>) { # note use of indirection |
| 2406 | if (/^#include "(.*)"/) { |
| 2407 | process($1, $input); |
| 2408 | next; |
| 2409 | } |
| 2410 | #... # whatever |
| 2411 | } |
| 2412 | } |
| 2413 | |
| 2414 | You may also, in the Bourne shell tradition, specify an EXPR beginning |
| 2415 | with C<'E<gt>&'>, in which case the rest of the string is interpreted as the |
| 2416 | name of a filehandle (or file descriptor, if numeric) to be |
| 2417 | duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>, |
| 2418 | C<+E<gt>E<gt>>, and C<+E<lt>>. The |
| 2419 | mode you specify should match the mode of the original filehandle. |
| 2420 | (Duping a filehandle does not take into account any existing contents of |
| 2421 | stdio buffers.) |
| 2422 | Here is a script that saves, redirects, and restores STDOUT and |
| 2423 | STDERR: |
| 2424 | |
| 2425 | #!/usr/bin/perl |
| 2426 | open(OLDOUT, ">&STDOUT"); |
| 2427 | open(OLDERR, ">&STDERR"); |
| 2428 | |
| 2429 | open(STDOUT, ">foo.out") || die "Can't redirect stdout"; |
| 2430 | open(STDERR, ">&STDOUT") || die "Can't dup stdout"; |
| 2431 | |
| 2432 | select(STDERR); $| = 1; # make unbuffered |
| 2433 | select(STDOUT); $| = 1; # make unbuffered |
| 2434 | |
| 2435 | print STDOUT "stdout 1\n"; # this works for |
| 2436 | print STDERR "stderr 1\n"; # subprocesses too |
| 2437 | |
| 2438 | close(STDOUT); |
| 2439 | close(STDERR); |
| 2440 | |
| 2441 | open(STDOUT, ">&OLDOUT"); |
| 2442 | open(STDERR, ">&OLDERR"); |
| 2443 | |
| 2444 | print STDOUT "stdout 2\n"; |
| 2445 | print STDERR "stderr 2\n"; |
| 2446 | |
| 2447 | |
| 2448 | If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an |
| 2449 | equivalent of C's C<fdopen()> of that file descriptor; this is more |
| 2450 | parsimonious of file descriptors. For example: |
| 2451 | |
| 2452 | open(FILEHANDLE, "<&=$fd") |
| 2453 | |
| 2454 | If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then |
| 2455 | there is an implicit fork done, and the return value of open is the pid |
| 2456 | of the child within the parent process, and C<0> within the child |
| 2457 | process. (Use C<defined($pid)> to determine whether the open was successful.) |
| 2458 | The filehandle behaves normally for the parent, but i/o to that |
| 2459 | filehandle is piped from/to the STDOUT/STDIN of the child process. |
| 2460 | In the child process the filehandle isn't opened--i/o happens from/to |
| 2461 | the new STDOUT or STDIN. Typically this is used like the normal |
| 2462 | piped open when you want to exercise more control over just how the |
| 2463 | pipe command gets executed, such as when you are running setuid, and |
| 2464 | don't want to have to scan shell commands for metacharacters. |
| 2465 | The following pairs are more or less equivalent: |
| 2466 | |
| 2467 | open(FOO, "|tr '[a-z]' '[A-Z]'"); |
| 2468 | open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]'; |
| 2469 | |
| 2470 | open(FOO, "cat -n '$file'|"); |
| 2471 | open(FOO, "-|") || exec 'cat', '-n', $file; |
| 2472 | |
| 2473 | See L<perlipc/"Safe Pipe Opens"> for more examples of this. |
| 2474 | |
| 2475 | NOTE: On any operation that may do a fork, any unflushed buffers remain |
| 2476 | unflushed in both processes, which means you may need to set C<$|> to |
| 2477 | avoid duplicate output. On systems that support a close-on-exec flag on |
| 2478 | files, the flag will be set for the newly opened file descriptor as |
| 2479 | determined by the value of $^F. See L<perlvar/$^F>. |
| 2480 | |
| 2481 | Closing any piped filehandle causes the parent process to wait for the |
| 2482 | child to finish, and returns the status value in C<$?>. |
| 2483 | |
| 2484 | The filename passed to open will have leading and trailing |
| 2485 | whitespace deleted, and the normal redirection characters |
| 2486 | honored. This property, known as "magic open", |
| 2487 | can often be used to good effect. A user could specify a filename of |
| 2488 | F<"rsh cat file |">, or you could change certain filenames as needed: |
| 2489 | |
| 2490 | $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; |
| 2491 | open(FH, $filename) or die "Can't open $filename: $!"; |
| 2492 | |
| 2493 | However, to open a file with arbitrary weird characters in it, it's |
| 2494 | necessary to protect any leading and trailing whitespace: |
| 2495 | |
| 2496 | $file =~ s#^(\s)#./$1#; |
| 2497 | open(FOO, "< $file\0"); |
| 2498 | |
| 2499 | If you want a "real" C C<open()> (see L<open(2)> on your system), then you |
| 2500 | should use the C<sysopen()> function, which involves no such magic. This is |
| 2501 | another way to protect your filenames from interpretation. For example: |
| 2502 | |
| 2503 | use IO::Handle; |
| 2504 | sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) |
| 2505 | or die "sysopen $path: $!"; |
| 2506 | $oldfh = select(HANDLE); $| = 1; select($oldfh); |
| 2507 | print HANDLE "stuff $$\n"); |
| 2508 | seek(HANDLE, 0, 0); |
| 2509 | print "File contains: ", <HANDLE>; |
| 2510 | |
| 2511 | Using the constructor from the C<IO::Handle> package (or one of its |
| 2512 | subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous |
| 2513 | filehandles that have the scope of whatever variables hold references to |
| 2514 | them, and automatically close whenever and however you leave that scope: |
| 2515 | |
| 2516 | use IO::File; |
| 2517 | #... |
| 2518 | sub read_myfile_munged { |
| 2519 | my $ALL = shift; |
| 2520 | my $handle = new IO::File; |
| 2521 | open($handle, "myfile") or die "myfile: $!"; |
| 2522 | $first = <$handle> |
| 2523 | or return (); # Automatically closed here. |
| 2524 | mung $first or die "mung failed"; # Or here. |
| 2525 | return $first, <$handle> if $ALL; # Or here. |
| 2526 | $first; # Or here. |
| 2527 | } |
| 2528 | |
| 2529 | See L</seek> for some details about mixing reading and writing. |
| 2530 | |
| 2531 | =item opendir DIRHANDLE,EXPR |
| 2532 | |
| 2533 | Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>, |
| 2534 | C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful. |
| 2535 | DIRHANDLEs have their own namespace separate from FILEHANDLEs. |
| 2536 | |
| 2537 | =item ord EXPR |
| 2538 | |
| 2539 | =item ord |
| 2540 | |
| 2541 | Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If |
| 2542 | EXPR is omitted, uses C<$_>. For the reverse, see L</chr>. |
| 2543 | |
| 2544 | =item pack TEMPLATE,LIST |
| 2545 | |
| 2546 | Takes an array or list of values and packs it into a binary structure, |
| 2547 | returning the string containing the structure. The TEMPLATE is a |
| 2548 | sequence of characters that give the order and type of values, as |
| 2549 | follows: |
| 2550 | |
| 2551 | A An ascii string, will be space padded. |
| 2552 | a An ascii string, will be null padded. |
| 2553 | b A bit string (ascending bit order, like vec()). |
| 2554 | B A bit string (descending bit order). |
| 2555 | h A hex string (low nybble first). |
| 2556 | H A hex string (high nybble first). |
| 2557 | |
| 2558 | c A signed char value. |
| 2559 | C An unsigned char value. Only does bytes. See U for Unicode. |
| 2560 | |
| 2561 | s A signed short value. |
| 2562 | S An unsigned short value. |
| 2563 | (This 'short' is _exactly_ 16 bits, which may differ from |
| 2564 | what a local C compiler calls 'short'.) |
| 2565 | |
| 2566 | i A signed integer value. |
| 2567 | I An unsigned integer value. |
| 2568 | (This 'integer' is _at_least_ 32 bits wide. Its exact |
| 2569 | size depends on what a local C compiler calls 'int', |
| 2570 | and may even be larger than the 'long' described in |
| 2571 | the next item.) |
| 2572 | |
| 2573 | l A signed long value. |
| 2574 | L An unsigned long value. |
| 2575 | (This 'long' is _exactly_ 32 bits, which may differ from |
| 2576 | what a local C compiler calls 'long'.) |
| 2577 | |
| 2578 | n A short in "network" (big-endian) order. |
| 2579 | N A long in "network" (big-endian) order. |
| 2580 | v A short in "VAX" (little-endian) order. |
| 2581 | V A long in "VAX" (little-endian) order. |
| 2582 | (These 'shorts' and 'longs' are _exactly_ 16 bits and |
| 2583 | _exactly_ 32 bits, respectively.) |
| 2584 | |
| 2585 | q A signed quad (64-bit) value. |
| 2586 | Q An unsigned quad value. |
| 2587 | (Available only if your system supports 64-bit integer values |
| 2588 | _and_ if Perl has been compiled to support those. |
| 2589 | Causes a fatal error otherwise.) |
| 2590 | |
| 2591 | f A single-precision float in the native format. |
| 2592 | d A double-precision float in the native format. |
| 2593 | |
| 2594 | p A pointer to a null-terminated string. |
| 2595 | P A pointer to a structure (fixed-length string). |
| 2596 | |
| 2597 | u A uuencoded string. |
| 2598 | U A Unicode character number. Encodes to UTF-8 internally. |
| 2599 | Works even if C<use utf8> is not in effect. |
| 2600 | |
| 2601 | w A BER compressed integer. Its bytes represent an unsigned |
| 2602 | integer in base 128, most significant digit first, with as |
| 2603 | few digits as possible. Bit eight (the high bit) is set |
| 2604 | on each byte except the last. |
| 2605 | |
| 2606 | x A null byte. |
| 2607 | X Back up a byte. |
| 2608 | @ Null fill to absolute position. |
| 2609 | |
| 2610 | Each letter may optionally be followed by a number giving a repeat |
| 2611 | count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the |
| 2612 | pack function will gobble up that many values from the LIST. A C<*> for the |
| 2613 | repeat count means to use however many items are left. The C<"a"> and C<"A"> |
| 2614 | types gobble just one value, but pack it as a string of length count, |
| 2615 | padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips |
| 2616 | trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B"> |
| 2617 | fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a |
| 2618 | string that many nybbles long. The C<"p"> type packs a pointer to a null- |
| 2619 | terminated string. You are responsible for ensuring the string is not a |
| 2620 | temporary value (which can potentially get deallocated before you get |
| 2621 | around to using the packed result). The C<"P"> packs a pointer to a structure |
| 2622 | of the size indicated by the length. A NULL pointer is created if the |
| 2623 | corresponding value for C<"p"> or C<"P"> is C<undef>. |
| 2624 | Real numbers (floats and doubles) are |
| 2625 | in the native machine format only; due to the multiplicity of floating |
| 2626 | formats around, and the lack of a standard "network" representation, no |
| 2627 | facility for interchange has been made. This means that packed floating |
| 2628 | point data written on one machine may not be readable on another - even if |
| 2629 | both use IEEE floating point arithmetic (as the endian-ness of the memory |
| 2630 | representation is not part of the IEEE spec). Note that Perl uses doubles |
| 2631 | internally for all numeric calculation, and converting from double into |
| 2632 | float and thence back to double again will lose precision (i.e., |
| 2633 | C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>). |
| 2634 | |
| 2635 | Examples: |
| 2636 | |
| 2637 | $foo = pack("CCCC",65,66,67,68); |
| 2638 | # foo eq "ABCD" |
| 2639 | $foo = pack("C4",65,66,67,68); |
| 2640 | # same thing |
| 2641 | $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); |
| 2642 | # same thing with Unicode circled letters |
| 2643 | |
| 2644 | $foo = pack("ccxxcc",65,66,67,68); |
| 2645 | # foo eq "AB\0\0CD" |
| 2646 | |
| 2647 | $foo = pack("s2",1,2); |
| 2648 | # "\1\0\2\0" on little-endian |
| 2649 | # "\0\1\0\2" on big-endian |
| 2650 | |
| 2651 | $foo = pack("a4","abcd","x","y","z"); |
| 2652 | # "abcd" |
| 2653 | |
| 2654 | $foo = pack("aaaa","abcd","x","y","z"); |
| 2655 | # "axyz" |
| 2656 | |
| 2657 | $foo = pack("a14","abcdefg"); |
| 2658 | # "abcdefg\0\0\0\0\0\0\0" |
| 2659 | |
| 2660 | $foo = pack("i9pl", gmtime); |
| 2661 | # a real struct tm (on my system anyway) |
| 2662 | |
| 2663 | sub bintodec { |
| 2664 | unpack("N", pack("B32", substr("0" x 32 . shift, -32))); |
| 2665 | } |
| 2666 | |
| 2667 | The same template may generally also be used in the unpack function. |
| 2668 | |
| 2669 | =item package |
| 2670 | |
| 2671 | =item package NAMESPACE |
| 2672 | |
| 2673 | Declares the compilation unit as being in the given namespace. The scope |
| 2674 | of the package declaration is from the declaration itself through the end of |
| 2675 | the enclosing block (the same scope as the C<local()> operator). All further |
| 2676 | unqualified dynamic identifiers will be in this namespace. A package |
| 2677 | statement affects only dynamic variables--including those you've used |
| 2678 | C<local()> on--but I<not> lexical variables created with C<my()>. Typically it |
| 2679 | would be the first declaration in a file to be included by the C<require> |
| 2680 | or C<use> operator. You can switch into a package in more than one place; |
| 2681 | it merely influences which symbol table is used by the compiler for the |
| 2682 | rest of that block. You can refer to variables and filehandles in other |
| 2683 | packages by prefixing the identifier with the package name and a double |
| 2684 | colon: C<$Package::Variable>. If the package name is null, the C<main> |
| 2685 | package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>. |
| 2686 | |
| 2687 | If NAMESPACE is omitted, then there is no current package, and all |
| 2688 | identifiers must be fully qualified or lexicals. This is stricter |
| 2689 | than C<use strict>, since it also extends to function names. |
| 2690 | |
| 2691 | See L<perlmod/"Packages"> for more information about packages, modules, |
| 2692 | and classes. See L<perlsub> for other scoping issues. |
| 2693 | |
| 2694 | =item pipe READHANDLE,WRITEHANDLE |
| 2695 | |
| 2696 | Opens a pair of connected pipes like the corresponding system call. |
| 2697 | Note that if you set up a loop of piped processes, deadlock can occur |
| 2698 | unless you are very careful. In addition, note that Perl's pipes use |
| 2699 | stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE |
| 2700 | after each command, depending on the application. |
| 2701 | |
| 2702 | See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication"> |
| 2703 | for examples of such things. |
| 2704 | |
| 2705 | On systems that support a close-on-exec flag on files, the flag will be set |
| 2706 | for the newly opened file descriptors as determined by the value of $^F. |
| 2707 | See L<perlvar/$^F>. |
| 2708 | |
| 2709 | =item pop ARRAY |
| 2710 | |
| 2711 | =item pop |
| 2712 | |
| 2713 | Pops and returns the last value of the array, shortening the array by |
| 2714 | 1. Has a similar effect to |
| 2715 | |
| 2716 | $tmp = $ARRAY[$#ARRAY--]; |
| 2717 | |
| 2718 | If there are no elements in the array, returns the undefined value. |
| 2719 | If ARRAY is omitted, pops the |
| 2720 | C<@ARGV> array in the main program, and the C<@_> array in subroutines, just |
| 2721 | like C<shift()>. |
| 2722 | |
| 2723 | =item pos SCALAR |
| 2724 | |
| 2725 | =item pos |
| 2726 | |
| 2727 | Returns the offset of where the last C<m//g> search left off for the variable |
| 2728 | is in question (C<$_> is used when the variable is not specified). May be |
| 2729 | modified to change that offset. Such modification will also influence |
| 2730 | the C<\G> zero-width assertion in regular expressions. See L<perlre> and |
| 2731 | L<perlop>. |
| 2732 | |
| 2733 | =item print FILEHANDLE LIST |
| 2734 | |
| 2735 | =item print LIST |
| 2736 | |
| 2737 | =item print |
| 2738 | |
| 2739 | Prints a string or a comma-separated list of strings. Returns TRUE |
| 2740 | if successful. FILEHANDLE may be a scalar variable name, in which case |
| 2741 | the variable contains the name of or a reference to the filehandle, thus introducing one |
| 2742 | level of indirection. (NOTE: If FILEHANDLE is a variable and the next |
| 2743 | token is a term, it may be misinterpreted as an operator unless you |
| 2744 | interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is |
| 2745 | omitted, prints by default to standard output (or to the last selected |
| 2746 | output channel--see L</select>). If LIST is also omitted, prints C<$_> to |
| 2747 | the currently selected output channel. To set the default output channel to something other than |
| 2748 | STDOUT use the select operation. Note that, because print takes a |
| 2749 | LIST, anything in the LIST is evaluated in list context, and any |
| 2750 | subroutine that you call will have one or more of its expressions |
| 2751 | evaluated in list context. Also be careful not to follow the print |
| 2752 | keyword with a left parenthesis unless you want the corresponding right |
| 2753 | parenthesis to terminate the arguments to the print--interpose a C<+> or |
| 2754 | put parentheses around all the arguments. |
| 2755 | |
| 2756 | Note that if you're storing FILEHANDLES in an array or other expression, |
| 2757 | you will have to use a block returning its value instead: |
| 2758 | |
| 2759 | print { $files[$i] } "stuff\n"; |
| 2760 | print { $OK ? STDOUT : STDERR } "stuff\n"; |
| 2761 | |
| 2762 | =item printf FILEHANDLE FORMAT, LIST |
| 2763 | |
| 2764 | =item printf FORMAT, LIST |
| 2765 | |
| 2766 | Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\> |
| 2767 | (the output record separator) is not appended. The first argument |
| 2768 | of the list will be interpreted as the C<printf()> format. If C<use locale> is |
| 2769 | in effect, the character used for the decimal point in formatted real numbers |
| 2770 | is affected by the LC_NUMERIC locale. See L<perllocale>. |
| 2771 | |
| 2772 | Don't fall into the trap of using a C<printf()> when a simple |
| 2773 | C<print()> would do. The C<print()> is more efficient and less |
| 2774 | error prone. |
| 2775 | |
| 2776 | =item prototype FUNCTION |
| 2777 | |
| 2778 | Returns the prototype of a function as a string (or C<undef> if the |
| 2779 | function has no prototype). FUNCTION is a reference to, or the name of, |
| 2780 | the function whose prototype you want to retrieve. |
| 2781 | |
| 2782 | If FUNCTION is a string starting with C<CORE::>, the rest is taken as |
| 2783 | a name for Perl builtin. If builtin is not I<overridable> (such as |
| 2784 | C<qw//>) or its arguments cannot be expressed by a prototype (such as |
| 2785 | C<system()>) - in other words, the builtin does not behave like a Perl |
| 2786 | function - returns C<undef>. Otherwise, the string describing the |
| 2787 | equivalent prototype is returned. |
| 2788 | |
| 2789 | =item push ARRAY,LIST |
| 2790 | |
| 2791 | Treats ARRAY as a stack, and pushes the values of LIST |
| 2792 | onto the end of ARRAY. The length of ARRAY increases by the length of |
| 2793 | LIST. Has the same effect as |
| 2794 | |
| 2795 | for $value (LIST) { |
| 2796 | $ARRAY[++$#ARRAY] = $value; |
| 2797 | } |
| 2798 | |
| 2799 | but is more efficient. Returns the new number of elements in the array. |
| 2800 | |
| 2801 | =item q/STRING/ |
| 2802 | |
| 2803 | =item qq/STRING/ |
| 2804 | |
| 2805 | =item qr/STRING/ |
| 2806 | |
| 2807 | =item qx/STRING/ |
| 2808 | |
| 2809 | =item qw/STRING/ |
| 2810 | |
| 2811 | Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">. |
| 2812 | |
| 2813 | =item quotemeta EXPR |
| 2814 | |
| 2815 | =item quotemeta |
| 2816 | |
| 2817 | Returns the value of EXPR with all non-alphanumeric |
| 2818 | characters backslashed. (That is, all characters not matching |
| 2819 | C</[A-Za-z_0-9]/> will be preceded by a backslash in the |
| 2820 | returned string, regardless of any locale settings.) |
| 2821 | This is the internal function implementing |
| 2822 | the C<\Q> escape in double-quoted strings. |
| 2823 | |
| 2824 | If EXPR is omitted, uses C<$_>. |
| 2825 | |
| 2826 | =item rand EXPR |
| 2827 | |
| 2828 | =item rand |
| 2829 | |
| 2830 | Returns a random fractional number greater than or equal to C<0> and less |
| 2831 | than the value of EXPR. (EXPR should be positive.) If EXPR is |
| 2832 | omitted, the value C<1> is used. Automatically calls C<srand()> unless |
| 2833 | C<srand()> has already been called. See also C<srand()>. |
| 2834 | |
| 2835 | (Note: If your rand function consistently returns numbers that are too |
| 2836 | large or too small, then your version of Perl was probably compiled |
| 2837 | with the wrong number of RANDBITS.) |
| 2838 | |
| 2839 | =item read FILEHANDLE,SCALAR,LENGTH,OFFSET |
| 2840 | |
| 2841 | =item read FILEHANDLE,SCALAR,LENGTH |
| 2842 | |
| 2843 | Attempts to read LENGTH bytes of data into variable SCALAR from the |
| 2844 | specified FILEHANDLE. Returns the number of bytes actually read, |
| 2845 | C<0> at end of file, or undef if there was an error. SCALAR will be grown |
| 2846 | or shrunk to the length actually read. An OFFSET may be specified to |
| 2847 | place the read data at some other place than the beginning of the |
| 2848 | string. This call is actually implemented in terms of stdio's fread(3) |
| 2849 | call. To get a true read(2) system call, see C<sysread()>. |
| 2850 | |
| 2851 | =item readdir DIRHANDLE |
| 2852 | |
| 2853 | Returns the next directory entry for a directory opened by C<opendir()>. |
| 2854 | If used in list context, returns all the rest of the entries in the |
| 2855 | directory. If there are no more entries, returns an undefined value in |
| 2856 | scalar context or a null list in list context. |
| 2857 | |
| 2858 | If you're planning to filetest the return values out of a C<readdir()>, you'd |
| 2859 | better prepend the directory in question. Otherwise, because we didn't |
| 2860 | C<chdir()> there, it would have been testing the wrong file. |
| 2861 | |
| 2862 | opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!"; |
| 2863 | @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR); |
| 2864 | closedir DIR; |
| 2865 | |
| 2866 | =item readline EXPR |
| 2867 | |
| 2868 | Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line |
| 2869 | is read and returned. In list context, reads until end-of-file is |
| 2870 | reached and returns a list of lines (however you've defined lines |
| 2871 | with C<$/> or C<$INPUT_RECORD_SEPARATOR>). |
| 2872 | This is the internal function implementing the C<E<lt>EXPRE<gt>> |
| 2873 | operator, but you can use it directly. The C<E<lt>EXPRE<gt>> |
| 2874 | operator is discussed in more detail in L<perlop/"I/O Operators">. |
| 2875 | |
| 2876 | $line = <STDIN>; |
| 2877 | $line = readline(*STDIN); # same thing |
| 2878 | |
| 2879 | =item readlink EXPR |
| 2880 | |
| 2881 | =item readlink |
| 2882 | |
| 2883 | Returns the value of a symbolic link, if symbolic links are |
| 2884 | implemented. If not, gives a fatal error. If there is some system |
| 2885 | error, returns the undefined value and sets C<$!> (errno). If EXPR is |
| 2886 | omitted, uses C<$_>. |
| 2887 | |
| 2888 | =item readpipe EXPR |
| 2889 | |
| 2890 | EXPR is executed as a system command. |
| 2891 | The collected standard output of the command is returned. |
| 2892 | In scalar context, it comes back as a single (potentially |
| 2893 | multi-line) string. In list context, returns a list of lines |
| 2894 | (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>). |
| 2895 | This is the internal function implementing the C<qx/EXPR/> |
| 2896 | operator, but you can use it directly. The C<qx/EXPR/> |
| 2897 | operator is discussed in more detail in L<perlop/"I/O Operators">. |
| 2898 | |
| 2899 | =item recv SOCKET,SCALAR,LEN,FLAGS |
| 2900 | |
| 2901 | Receives a message on a socket. Attempts to receive LENGTH bytes of |
| 2902 | data into variable SCALAR from the specified SOCKET filehandle. |
| 2903 | Actually does a C C<recvfrom()>, so that it can return the address of the |
| 2904 | sender. Returns the undefined value if there's an error. SCALAR will |
| 2905 | be grown or shrunk to the length actually read. Takes the same flags |
| 2906 | as the system call of the same name. |
| 2907 | See L<perlipc/"UDP: Message Passing"> for examples. |
| 2908 | |
| 2909 | =item redo LABEL |
| 2910 | |
| 2911 | =item redo |
| 2912 | |
| 2913 | The C<redo> command restarts the loop block without evaluating the |
| 2914 | conditional again. The L</continue> block, if any, is not executed. If |
| 2915 | the LABEL is omitted, the command refers to the innermost enclosing |
| 2916 | loop. This command is normally used by programs that want to lie to |
| 2917 | themselves about what was just input: |
| 2918 | |
| 2919 | # a simpleminded Pascal comment stripper |
| 2920 | # (warning: assumes no { or } in strings) |
| 2921 | LINE: while (<STDIN>) { |
| 2922 | while (s|({.*}.*){.*}|$1 |) {} |
| 2923 | s|{.*}| |; |
| 2924 | if (s|{.*| |) { |
| 2925 | $front = $_; |
| 2926 | while (<STDIN>) { |
| 2927 | if (/}/) { # end of comment? |
| 2928 | s|^|$front\{|; |
| 2929 | redo LINE; |
| 2930 | } |
| 2931 | } |
| 2932 | } |
| 2933 | print; |
| 2934 | } |
| 2935 | |
| 2936 | C<redo> cannot be used to retry a block which returns a value such as |
| 2937 | C<eval {}>, C<sub {}> or C<do {}>. |
| 2938 | |
| 2939 | See also L</continue> for an illustration of how L</last>, L</next>, and |
| 2940 | C<redo> work. |
| 2941 | |
| 2942 | See also L<perlsyn>. |
| 2943 | |
| 2944 | =item ref EXPR |
| 2945 | |
| 2946 | =item ref |
| 2947 | |
| 2948 | Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR |
| 2949 | is not specified, C<$_> will be used. The value returned depends on the |
| 2950 | type of thing the reference is a reference to. |
| 2951 | Builtin types include: |
| 2952 | |
| 2953 | REF |
| 2954 | SCALAR |
| 2955 | ARRAY |
| 2956 | HASH |
| 2957 | CODE |
| 2958 | GLOB |
| 2959 | |
| 2960 | If the referenced object has been blessed into a package, then that package |
| 2961 | name is returned instead. You can think of C<ref()> as a C<typeof()> operator. |
| 2962 | |
| 2963 | if (ref($r) eq "HASH") { |
| 2964 | print "r is a reference to a hash.\n"; |
| 2965 | } |
| 2966 | if (!ref($r)) { |
| 2967 | print "r is not a reference at all.\n"; |
| 2968 | } |
| 2969 | |
| 2970 | See also L<perlref>. |
| 2971 | |
| 2972 | =item rename OLDNAME,NEWNAME |
| 2973 | |
| 2974 | Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will |
| 2975 | not work across file system boundaries. |
| 2976 | |
| 2977 | =item require EXPR |
| 2978 | |
| 2979 | =item require |
| 2980 | |
| 2981 | Demands some semantics specified by EXPR, or by C<$_> if EXPR is not |
| 2982 | supplied. If EXPR is numeric, demands that the current version of Perl |
| 2983 | (C<$]> or $PERL_VERSION) be equal or greater than EXPR. |
| 2984 | |
| 2985 | Otherwise, demands that a library file be included if it hasn't already |
| 2986 | been included. The file is included via the do-FILE mechanism, which is |
| 2987 | essentially just a variety of C<eval()>. Has semantics similar to the following |
| 2988 | subroutine: |
| 2989 | |
| 2990 | sub require { |
| 2991 | my($filename) = @_; |
| 2992 | return 1 if $INC{$filename}; |
| 2993 | my($realfilename,$result); |
| 2994 | ITER: { |
| 2995 | foreach $prefix (@INC) { |
| 2996 | $realfilename = "$prefix/$filename"; |
| 2997 | if (-f $realfilename) { |
| 2998 | $result = do $realfilename; |
| 2999 | last ITER; |
| 3000 | } |
| 3001 | } |
| 3002 | die "Can't find $filename in \@INC"; |
| 3003 | } |
| 3004 | die $@ if $@; |
| 3005 | die "$filename did not return true value" unless $result; |
| 3006 | $INC{$filename} = $realfilename; |
| 3007 | return $result; |
| 3008 | } |
| 3009 | |
| 3010 | Note that the file will not be included twice under the same specified |
| 3011 | name. The file must return TRUE as the last statement to indicate |
| 3012 | successful execution of any initialization code, so it's customary to |
| 3013 | end such a file with "C<1;>" unless you're sure it'll return TRUE |
| 3014 | otherwise. But it's better just to put the "C<1;>", in case you add more |
| 3015 | statements. |
| 3016 | |
| 3017 | If EXPR is a bareword, the require assumes a "F<.pm>" extension and |
| 3018 | replaces "F<::>" with "F</>" in the filename for you, |
| 3019 | to make it easy to load standard modules. This form of loading of |
| 3020 | modules does not risk altering your namespace. |
| 3021 | |
| 3022 | In other words, if you try this: |
| 3023 | |
| 3024 | require Foo::Bar; # a splendid bareword |
| 3025 | |
| 3026 | The require function will actually look for the "F<Foo/Bar.pm>" file in the |
| 3027 | directories specified in the C<@INC> array. |
| 3028 | |
| 3029 | But if you try this: |
| 3030 | |
| 3031 | $class = 'Foo::Bar'; |
| 3032 | require $class; # $class is not a bareword |
| 3033 | #or |
| 3034 | require "Foo::Bar"; # not a bareword because of the "" |
| 3035 | |
| 3036 | The require function will look for the "F<Foo::Bar>" file in the @INC array and |
| 3037 | will complain about not finding "F<Foo::Bar>" there. In this case you can do: |
| 3038 | |
| 3039 | eval "require $class"; |
| 3040 | |
| 3041 | For a yet-more-powerful import facility, see L</use> and L<perlmod>. |
| 3042 | |
| 3043 | =item reset EXPR |
| 3044 | |
| 3045 | =item reset |
| 3046 | |
| 3047 | Generally used in a C<continue> block at the end of a loop to clear |
| 3048 | variables and reset C<??> searches so that they work again. The |
| 3049 | expression is interpreted as a list of single characters (hyphens |
| 3050 | allowed for ranges). All variables and arrays beginning with one of |
| 3051 | those letters are reset to their pristine state. If the expression is |
| 3052 | omitted, one-match searches (C<?pattern?>) are reset to match again. Resets |
| 3053 | only variables or searches in the current package. Always returns |
| 3054 | 1. Examples: |
| 3055 | |
| 3056 | reset 'X'; # reset all X variables |
| 3057 | reset 'a-z'; # reset lower case variables |
| 3058 | reset; # just reset ?? searches |
| 3059 | |
| 3060 | Resetting C<"A-Z"> is not recommended because you'll wipe out your |
| 3061 | C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables |
| 3062 | are unaffected, but they clean themselves up on scope exit anyway, |
| 3063 | so you'll probably want to use them instead. See L</my>. |
| 3064 | |
| 3065 | =item return EXPR |
| 3066 | |
| 3067 | =item return |
| 3068 | |
| 3069 | Returns from a subroutine, C<eval()>, or C<do FILE> with the value |
| 3070 | given in EXPR. Evaluation of EXPR may be in list, scalar, or void |
| 3071 | context, depending on how the return value will be used, and the context |
| 3072 | may vary from one execution to the next (see C<wantarray()>). If no EXPR |
| 3073 | is given, returns an empty list in list context, an undefined value in |
| 3074 | scalar context, or nothing in a void context. |
| 3075 | |
| 3076 | (Note that in the absence of a return, a subroutine, eval, or do FILE |
| 3077 | will automatically return the value of the last expression evaluated.) |
| 3078 | |
| 3079 | =item reverse LIST |
| 3080 | |
| 3081 | In list context, returns a list value consisting of the elements |
| 3082 | of LIST in the opposite order. In scalar context, concatenates the |
| 3083 | elements of LIST, and returns a string value with all the characters |
| 3084 | in the opposite order. |
| 3085 | |
| 3086 | print reverse <>; # line tac, last line first |
| 3087 | |
| 3088 | undef $/; # for efficiency of <> |
| 3089 | print scalar reverse <>; # character tac, last line tsrif |
| 3090 | |
| 3091 | This operator is also handy for inverting a hash, although there are some |
| 3092 | caveats. If a value is duplicated in the original hash, only one of those |
| 3093 | can be represented as a key in the inverted hash. Also, this has to |
| 3094 | unwind one hash and build a whole new one, which may take some time |
| 3095 | on a large hash. |
| 3096 | |
| 3097 | %by_name = reverse %by_address; # Invert the hash |
| 3098 | |
| 3099 | =item rewinddir DIRHANDLE |
| 3100 | |
| 3101 | Sets the current position to the beginning of the directory for the |
| 3102 | C<readdir()> routine on DIRHANDLE. |
| 3103 | |
| 3104 | =item rindex STR,SUBSTR,POSITION |
| 3105 | |
| 3106 | =item rindex STR,SUBSTR |
| 3107 | |
| 3108 | Works just like index except that it returns the position of the LAST |
| 3109 | occurrence of SUBSTR in STR. If POSITION is specified, returns the |
| 3110 | last occurrence at or before that position. |
| 3111 | |
| 3112 | =item rmdir FILENAME |
| 3113 | |
| 3114 | =item rmdir |
| 3115 | |
| 3116 | Deletes the directory specified by FILENAME if that directory is empty. If it |
| 3117 | succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If |
| 3118 | FILENAME is omitted, uses C<$_>. |
| 3119 | |
| 3120 | =item s/// |
| 3121 | |
| 3122 | The substitution operator. See L<perlop>. |
| 3123 | |
| 3124 | =item scalar EXPR |
| 3125 | |
| 3126 | Forces EXPR to be interpreted in scalar context and returns the value |
| 3127 | of EXPR. |
| 3128 | |
| 3129 | @counts = ( scalar @a, scalar @b, scalar @c ); |
| 3130 | |
| 3131 | There is no equivalent operator to force an expression to |
| 3132 | be interpolated in list context because it's in practice never |
| 3133 | needed. If you really wanted to do so, however, you could use |
| 3134 | the construction C<@{[ (some expression) ]}>, but usually a simple |
| 3135 | C<(some expression)> suffices. |
| 3136 | |
| 3137 | =item seek FILEHANDLE,POSITION,WHENCE |
| 3138 | |
| 3139 | Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>. |
| 3140 | FILEHANDLE may be an expression whose value gives the name of the |
| 3141 | filehandle. The values for WHENCE are C<0> to set the new position to |
| 3142 | POSITION, C<1> to set it to the current position plus POSITION, and C<2> to |
| 3143 | set it to EOF plus POSITION (typically negative). For WHENCE you may |
| 3144 | use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the |
| 3145 | C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise. |
| 3146 | |
| 3147 | If you want to position file for C<sysread()> or C<syswrite()>, don't use |
| 3148 | C<seek()> -- buffering makes its effect on the file's system position |
| 3149 | unpredictable and non-portable. Use C<sysseek()> instead. |
| 3150 | |
| 3151 | On some systems you have to do a seek whenever you switch between reading |
| 3152 | and writing. Amongst other things, this may have the effect of calling |
| 3153 | stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving |
| 3154 | the file position: |
| 3155 | |
| 3156 | seek(TEST,0,1); |
| 3157 | |
| 3158 | This is also useful for applications emulating C<tail -f>. Once you hit |
| 3159 | EOF on your read, and then sleep for a while, you might have to stick in a |
| 3160 | seek() to reset things. The C<seek()> doesn't change the current position, |
| 3161 | but it I<does> clear the end-of-file condition on the handle, so that the |
| 3162 | next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope. |
| 3163 | |
| 3164 | If that doesn't work (some stdios are particularly cantankerous), then |
| 3165 | you may need something more like this: |
| 3166 | |
| 3167 | for (;;) { |
| 3168 | for ($curpos = tell(FILE); $_ = <FILE>; |
| 3169 | $curpos = tell(FILE)) { |
| 3170 | # search for some stuff and put it into files |
| 3171 | } |
| 3172 | sleep($for_a_while); |
| 3173 | seek(FILE, $curpos, 0); |
| 3174 | } |
| 3175 | |
| 3176 | =item seekdir DIRHANDLE,POS |
| 3177 | |
| 3178 | Sets the current position for the C<readdir()> routine on DIRHANDLE. POS |
| 3179 | must be a value returned by C<telldir()>. Has the same caveats about |
| 3180 | possible directory compaction as the corresponding system library |
| 3181 | routine. |
| 3182 | |
| 3183 | =item select FILEHANDLE |
| 3184 | |
| 3185 | =item select |
| 3186 | |
| 3187 | Returns the currently selected filehandle. Sets the current default |
| 3188 | filehandle for output, if FILEHANDLE is supplied. This has two |
| 3189 | effects: first, a C<write()> or a C<print()> without a filehandle will |
| 3190 | default to this FILEHANDLE. Second, references to variables related to |
| 3191 | output will refer to this output channel. For example, if you have to |
| 3192 | set the top of form format for more than one output channel, you might |
| 3193 | do the following: |
| 3194 | |
| 3195 | select(REPORT1); |
| 3196 | $^ = 'report1_top'; |
| 3197 | select(REPORT2); |
| 3198 | $^ = 'report2_top'; |
| 3199 | |
| 3200 | FILEHANDLE may be an expression whose value gives the name of the |
| 3201 | actual filehandle. Thus: |
| 3202 | |
| 3203 | $oldfh = select(STDERR); $| = 1; select($oldfh); |
| 3204 | |
| 3205 | Some programmers may prefer to think of filehandles as objects with |
| 3206 | methods, preferring to write the last example as: |
| 3207 | |
| 3208 | use IO::Handle; |
| 3209 | STDERR->autoflush(1); |
| 3210 | |
| 3211 | =item select RBITS,WBITS,EBITS,TIMEOUT |
| 3212 | |
| 3213 | This calls the select(2) system call with the bit masks specified, which |
| 3214 | can be constructed using C<fileno()> and C<vec()>, along these lines: |
| 3215 | |
| 3216 | $rin = $win = $ein = ''; |
| 3217 | vec($rin,fileno(STDIN),1) = 1; |
| 3218 | vec($win,fileno(STDOUT),1) = 1; |
| 3219 | $ein = $rin | $win; |
| 3220 | |
| 3221 | If you want to select on many filehandles you might wish to write a |
| 3222 | subroutine: |
| 3223 | |
| 3224 | sub fhbits { |
| 3225 | my(@fhlist) = split(' ',$_[0]); |
| 3226 | my($bits); |
| 3227 | for (@fhlist) { |
| 3228 | vec($bits,fileno($_),1) = 1; |
| 3229 | } |
| 3230 | $bits; |
| 3231 | } |
| 3232 | $rin = fhbits('STDIN TTY SOCK'); |
| 3233 | |
| 3234 | The usual idiom is: |
| 3235 | |
| 3236 | ($nfound,$timeleft) = |
| 3237 | select($rout=$rin, $wout=$win, $eout=$ein, $timeout); |
| 3238 | |
| 3239 | or to block until something becomes ready just do this |
| 3240 | |
| 3241 | $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); |
| 3242 | |
| 3243 | Most systems do not bother to return anything useful in C<$timeleft>, so |
| 3244 | calling select() in scalar context just returns C<$nfound>. |
| 3245 | |
| 3246 | Any of the bit masks can also be undef. The timeout, if specified, is |
| 3247 | in seconds, which may be fractional. Note: not all implementations are |
| 3248 | capable of returning theC<$timeleft>. If not, they always return |
| 3249 | C<$timeleft> equal to the supplied C<$timeout>. |
| 3250 | |
| 3251 | You can effect a sleep of 250 milliseconds this way: |
| 3252 | |
| 3253 | select(undef, undef, undef, 0.25); |
| 3254 | |
| 3255 | B<WARNING>: One should not attempt to mix buffered I/O (like C<read()> |
| 3256 | or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even |
| 3257 | then only on POSIX systems. You have to use C<sysread()> instead. |
| 3258 | |
| 3259 | =item semctl ID,SEMNUM,CMD,ARG |
| 3260 | |
| 3261 | Calls the System V IPC function C<semctl()>. You'll probably have to say |
| 3262 | |
| 3263 | use IPC::SysV; |
| 3264 | |
| 3265 | first to get the correct constant definitions. If CMD is IPC_STAT or |
| 3266 | GETALL, then ARG must be a variable which will hold the returned |
| 3267 | semid_ds structure or semaphore value array. Returns like C<ioctl()>: the |
| 3268 | undefined value for error, "C<0> but true" for zero, or the actual return |
| 3269 | value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation. |
| 3270 | |
| 3271 | =item semget KEY,NSEMS,FLAGS |
| 3272 | |
| 3273 | Calls the System V IPC function semget. Returns the semaphore id, or |
| 3274 | the undefined value if there is an error. See also C<IPC::SysV> and |
| 3275 | C<IPC::SysV::Semaphore> documentation. |
| 3276 | |
| 3277 | =item semop KEY,OPSTRING |
| 3278 | |
| 3279 | Calls the System V IPC function semop to perform semaphore operations |
| 3280 | such as signaling and waiting. OPSTRING must be a packed array of |
| 3281 | semop structures. Each semop structure can be generated with |
| 3282 | C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore |
| 3283 | operations is implied by the length of OPSTRING. Returns TRUE if |
| 3284 | successful, or FALSE if there is an error. As an example, the |
| 3285 | following code waits on semaphore C<$semnum> of semaphore id C<$semid>: |
| 3286 | |
| 3287 | $semop = pack("sss", $semnum, -1, 0); |
| 3288 | die "Semaphore trouble: $!\n" unless semop($semid, $semop); |
| 3289 | |
| 3290 | To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV> |
| 3291 | and C<IPC::SysV::Semaphore> documentation. |
| 3292 | |
| 3293 | =item send SOCKET,MSG,FLAGS,TO |
| 3294 | |
| 3295 | =item send SOCKET,MSG,FLAGS |
| 3296 | |
| 3297 | Sends a message on a socket. Takes the same flags as the system call |
| 3298 | of the same name. On unconnected sockets you must specify a |
| 3299 | destination to send TO, in which case it does a C C<sendto()>. Returns |
| 3300 | the number of characters sent, or the undefined value if there is an |
| 3301 | error. |
| 3302 | See L<perlipc/"UDP: Message Passing"> for examples. |
| 3303 | |
| 3304 | =item setpgrp PID,PGRP |
| 3305 | |
| 3306 | Sets the current process group for the specified PID, C<0> for the current |
| 3307 | process. Will produce a fatal error if used on a machine that doesn't |
| 3308 | implement setpgrp(2). If the arguments are omitted, it defaults to |
| 3309 | C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any |
| 3310 | arguments, so only setpgrp C<0,0> is portable. |
| 3311 | |
| 3312 | =item setpriority WHICH,WHO,PRIORITY |
| 3313 | |
| 3314 | Sets the current priority for a process, a process group, or a user. |
| 3315 | (See setpriority(2).) Will produce a fatal error if used on a machine |
| 3316 | that doesn't implement setpriority(2). |
| 3317 | |
| 3318 | =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL |
| 3319 | |
| 3320 | Sets the socket option requested. Returns undefined if there is an |
| 3321 | error. OPTVAL may be specified as C<undef> if you don't want to pass an |
| 3322 | argument. |
| 3323 | |
| 3324 | =item shift ARRAY |
| 3325 | |
| 3326 | =item shift |
| 3327 | |
| 3328 | Shifts the first value of the array off and returns it, shortening the |
| 3329 | array by 1 and moving everything down. If there are no elements in the |
| 3330 | array, returns the undefined value. If ARRAY is omitted, shifts the |
| 3331 | C<@_> array within the lexical scope of subroutines and formats, and the |
| 3332 | C<@ARGV> array at file scopes or within the lexical scopes established by |
| 3333 | the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs. |
| 3334 | See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the |
| 3335 | same thing to the left end of an array that C<pop()> and C<push()> do to the |
| 3336 | right end. |
| 3337 | |
| 3338 | =item shmctl ID,CMD,ARG |
| 3339 | |
| 3340 | Calls the System V IPC function shmctl. You'll probably have to say |
| 3341 | |
| 3342 | use IPC::SysV; |
| 3343 | |
| 3344 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, |
| 3345 | then ARG must be a variable which will hold the returned C<shmid_ds> |
| 3346 | structure. Returns like ioctl: the undefined value for error, "C<0> but |
| 3347 | true" for zero, or the actual return value otherwise. |
| 3348 | See also C<IPC::SysV> documentation. |
| 3349 | |
| 3350 | =item shmget KEY,SIZE,FLAGS |
| 3351 | |
| 3352 | Calls the System V IPC function shmget. Returns the shared memory |
| 3353 | segment id, or the undefined value if there is an error. |
| 3354 | See also C<IPC::SysV> documentation. |
| 3355 | |
| 3356 | =item shmread ID,VAR,POS,SIZE |
| 3357 | |
| 3358 | =item shmwrite ID,STRING,POS,SIZE |
| 3359 | |
| 3360 | Reads or writes the System V shared memory segment ID starting at |
| 3361 | position POS for size SIZE by attaching to it, copying in/out, and |
| 3362 | detaching from it. When reading, VAR must be a variable that will |
| 3363 | hold the data read. When writing, if STRING is too long, only SIZE |
| 3364 | bytes are used; if STRING is too short, nulls are written to fill out |
| 3365 | SIZE bytes. Return TRUE if successful, or FALSE if there is an error. |
| 3366 | See also C<IPC::SysV> documentation. |
| 3367 | |
| 3368 | =item shutdown SOCKET,HOW |
| 3369 | |
| 3370 | Shuts down a socket connection in the manner indicated by HOW, which |
| 3371 | has the same interpretation as in the system call of the same name. |
| 3372 | |
| 3373 | shutdown(SOCKET, 0); # I/we have stopped reading data |
| 3374 | shutdown(SOCKET, 1); # I/we have stopped writing data |
| 3375 | shutdown(SOCKET, 2); # I/we have stopped using this socket |
| 3376 | |
| 3377 | This is useful with sockets when you want to tell the other |
| 3378 | side you're done writing but not done reading, or vice versa. |
| 3379 | It's also a more insistent form of close because it also |
| 3380 | disables the filedescriptor in any forked copies in other |
| 3381 | processes. |
| 3382 | |
| 3383 | =item sin EXPR |
| 3384 | |
| 3385 | =item sin |
| 3386 | |
| 3387 | Returns the sine of EXPR (expressed in radians). If EXPR is omitted, |
| 3388 | returns sine of C<$_>. |
| 3389 | |
| 3390 | For the inverse sine operation, you may use the C<POSIX::asin()> |
| 3391 | function, or use this relation: |
| 3392 | |
| 3393 | sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } |
| 3394 | |
| 3395 | =item sleep EXPR |
| 3396 | |
| 3397 | =item sleep |
| 3398 | |
| 3399 | Causes the script to sleep for EXPR seconds, or forever if no EXPR. |
| 3400 | May be interrupted if the process receives a signal such as C<SIGALRM>. |
| 3401 | Returns the number of seconds actually slept. You probably cannot |
| 3402 | mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented |
| 3403 | using C<alarm()>. |
| 3404 | |
| 3405 | On some older systems, it may sleep up to a full second less than what |
| 3406 | you requested, depending on how it counts seconds. Most modern systems |
| 3407 | always sleep the full amount. They may appear to sleep longer than that, |
| 3408 | however, because your process might not be scheduled right away in a |
| 3409 | busy multitasking system. |
| 3410 | |
| 3411 | For delays of finer granularity than one second, you may use Perl's |
| 3412 | C<syscall()> interface to access setitimer(2) if your system supports it, |
| 3413 | or else see L</select> above. |
| 3414 | |
| 3415 | See also the POSIX module's C<sigpause()> function. |
| 3416 | |
| 3417 | =item socket SOCKET,DOMAIN,TYPE,PROTOCOL |
| 3418 | |
| 3419 | Opens a socket of the specified kind and attaches it to filehandle |
| 3420 | SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the |
| 3421 | system call of the same name. You should "C<use Socket;>" first to get |
| 3422 | the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">. |
| 3423 | |
| 3424 | =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL |
| 3425 | |
| 3426 | Creates an unnamed pair of sockets in the specified domain, of the |
| 3427 | specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as |
| 3428 | for the system call of the same name. If unimplemented, yields a fatal |
| 3429 | error. Returns TRUE if successful. |
| 3430 | |
| 3431 | Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call |
| 3432 | to C<pipe(Rdr, Wtr)> is essentially: |
| 3433 | |
| 3434 | use Socket; |
| 3435 | socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); |
| 3436 | shutdown(Rdr, 1); # no more writing for reader |
| 3437 | shutdown(Wtr, 0); # no more reading for writer |
| 3438 | |
| 3439 | See L<perlipc> for an example of socketpair use. |
| 3440 | |
| 3441 | =item sort SUBNAME LIST |
| 3442 | |
| 3443 | =item sort BLOCK LIST |
| 3444 | |
| 3445 | =item sort LIST |
| 3446 | |
| 3447 | Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK |
| 3448 | is omitted, C<sort()>s in standard string comparison order. If SUBNAME is |
| 3449 | specified, it gives the name of a subroutine that returns an integer |
| 3450 | less than, equal to, or greater than C<0>, depending on how the elements |
| 3451 | of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp> |
| 3452 | operators are extremely useful in such routines.) SUBNAME may be a |
| 3453 | scalar variable name (unsubscripted), in which case the value provides |
| 3454 | the name of (or a reference to) the actual subroutine to use. In place |
| 3455 | of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort |
| 3456 | subroutine. |
| 3457 | |
| 3458 | In the interests of efficiency the normal calling code for subroutines is |
| 3459 | bypassed, with the following effects: the subroutine may not be a |
| 3460 | recursive subroutine, and the two elements to be compared are passed into |
| 3461 | the subroutine not via C<@_> but as the package global variables C<$a> and |
| 3462 | C<$b> (see example below). They are passed by reference, so don't |
| 3463 | modify C<$a> and C<$b>. And don't try to declare them as lexicals either. |
| 3464 | |
| 3465 | You also cannot exit out of the sort block or subroutine using any of the |
| 3466 | loop control operators described in L<perlsyn> or with C<goto()>. |
| 3467 | |
| 3468 | When C<use locale> is in effect, C<sort LIST> sorts LIST according to the |
| 3469 | current collation locale. See L<perllocale>. |
| 3470 | |
| 3471 | Examples: |
| 3472 | |
| 3473 | # sort lexically |
| 3474 | @articles = sort @files; |
| 3475 | |
| 3476 | # same thing, but with explicit sort routine |
| 3477 | @articles = sort {$a cmp $b} @files; |
| 3478 | |
| 3479 | # now case-insensitively |
| 3480 | @articles = sort {uc($a) cmp uc($b)} @files; |
| 3481 | |
| 3482 | # same thing in reversed order |
| 3483 | @articles = sort {$b cmp $a} @files; |
| 3484 | |
| 3485 | # sort numerically ascending |
| 3486 | @articles = sort {$a <=> $b} @files; |
| 3487 | |
| 3488 | # sort numerically descending |
| 3489 | @articles = sort {$b <=> $a} @files; |
| 3490 | |
| 3491 | # sort using explicit subroutine name |
| 3492 | sub byage { |
| 3493 | $age{$a} <=> $age{$b}; # presuming numeric |
| 3494 | } |
| 3495 | @sortedclass = sort byage @class; |
| 3496 | |
| 3497 | # this sorts the %age hash by value instead of key |
| 3498 | # using an in-line function |
| 3499 | @eldest = sort { $age{$b} <=> $age{$a} } keys %age; |
| 3500 | |
| 3501 | sub backwards { $b cmp $a; } |
| 3502 | @harry = ('dog','cat','x','Cain','Abel'); |
| 3503 | @george = ('gone','chased','yz','Punished','Axed'); |
| 3504 | print sort @harry; |
| 3505 | # prints AbelCaincatdogx |
| 3506 | print sort backwards @harry; |
| 3507 | # prints xdogcatCainAbel |
| 3508 | print sort @george, 'to', @harry; |
| 3509 | # prints AbelAxedCainPunishedcatchaseddoggonetoxyz |
| 3510 | |
| 3511 | # inefficiently sort by descending numeric compare using |
| 3512 | # the first integer after the first = sign, or the |
| 3513 | # whole record case-insensitively otherwise |
| 3514 | |
| 3515 | @new = sort { |
| 3516 | ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] |
| 3517 | || |
| 3518 | uc($a) cmp uc($b) |
| 3519 | } @old; |
| 3520 | |
| 3521 | # same thing, but much more efficiently; |
| 3522 | # we'll build auxiliary indices instead |
| 3523 | # for speed |
| 3524 | @nums = @caps = (); |
| 3525 | for (@old) { |
| 3526 | push @nums, /=(\d+)/; |
| 3527 | push @caps, uc($_); |
| 3528 | } |
| 3529 | |
| 3530 | @new = @old[ sort { |
| 3531 | $nums[$b] <=> $nums[$a] |
| 3532 | || |
| 3533 | $caps[$a] cmp $caps[$b] |
| 3534 | } 0..$#old |
| 3535 | ]; |
| 3536 | |
| 3537 | # same thing using a Schwartzian Transform (no temps) |
| 3538 | @new = map { $_->[0] } |
| 3539 | sort { $b->[1] <=> $a->[1] |
| 3540 | || |
| 3541 | $a->[2] cmp $b->[2] |
| 3542 | } map { [$_, /=(\d+)/, uc($_)] } @old; |
| 3543 | |
| 3544 | If you're using strict, you I<MUST NOT> declare C<$a> |
| 3545 | and C<$b> as lexicals. They are package globals. That means |
| 3546 | if you're in the C<main> package, it's |
| 3547 | |
| 3548 | @articles = sort {$main::b <=> $main::a} @files; |
| 3549 | |
| 3550 | or just |
| 3551 | |
| 3552 | @articles = sort {$::b <=> $::a} @files; |
| 3553 | |
| 3554 | but if you're in the C<FooPack> package, it's |
| 3555 | |
| 3556 | @articles = sort {$FooPack::b <=> $FooPack::a} @files; |
| 3557 | |
| 3558 | The comparison function is required to behave. If it returns |
| 3559 | inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and |
| 3560 | sometimes saying the opposite, for example) the results are not |
| 3561 | well-defined. |
| 3562 | |
| 3563 | =item splice ARRAY,OFFSET,LENGTH,LIST |
| 3564 | |
| 3565 | =item splice ARRAY,OFFSET,LENGTH |
| 3566 | |
| 3567 | =item splice ARRAY,OFFSET |
| 3568 | |
| 3569 | Removes the elements designated by OFFSET and LENGTH from an array, and |
| 3570 | replaces them with the elements of LIST, if any. In list context, |
| 3571 | returns the elements removed from the array. In scalar context, |
| 3572 | returns the last element removed, or C<undef> if no elements are |
| 3573 | removed. The array grows or shrinks as necessary. |
| 3574 | If OFFSET is negative then it start that far from the end of the array. |
| 3575 | If LENGTH is omitted, removes everything from OFFSET onward. |
| 3576 | If LENGTH is negative, leave that many elements off the end of the array. |
| 3577 | The following equivalences hold (assuming C<$[ == 0>): |
| 3578 | |
| 3579 | push(@a,$x,$y) splice(@a,@a,0,$x,$y) |
| 3580 | pop(@a) splice(@a,-1) |
| 3581 | shift(@a) splice(@a,0,1) |
| 3582 | unshift(@a,$x,$y) splice(@a,0,0,$x,$y) |
| 3583 | $a[$x] = $y splice(@a,$x,1,$y) |
| 3584 | |
| 3585 | Example, assuming array lengths are passed before arrays: |
| 3586 | |
| 3587 | sub aeq { # compare two list values |
| 3588 | my(@a) = splice(@_,0,shift); |
| 3589 | my(@b) = splice(@_,0,shift); |
| 3590 | return 0 unless @a == @b; # same len? |
| 3591 | while (@a) { |
| 3592 | return 0 if pop(@a) ne pop(@b); |
| 3593 | } |
| 3594 | return 1; |
| 3595 | } |
| 3596 | if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } |
| 3597 | |
| 3598 | =item split /PATTERN/,EXPR,LIMIT |
| 3599 | |
| 3600 | =item split /PATTERN/,EXPR |
| 3601 | |
| 3602 | =item split /PATTERN/ |
| 3603 | |
| 3604 | =item split |
| 3605 | |
| 3606 | Splits a string into an array of strings, and returns it. By default, |
| 3607 | empty leading fields are preserved, and empty trailing ones are deleted. |
| 3608 | |
| 3609 | If not in list context, returns the number of fields found and splits into |
| 3610 | the C<@_> array. (In list context, you can force the split into C<@_> by |
| 3611 | using C<??> as the pattern delimiters, but it still returns the list |
| 3612 | value.) The use of implicit split to C<@_> is deprecated, however, because |
| 3613 | it clobbers your subroutine arguments. |
| 3614 | |
| 3615 | If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted, |
| 3616 | splits on whitespace (after skipping any leading whitespace). Anything |
| 3617 | matching PATTERN is taken to be a delimiter separating the fields. (Note |
| 3618 | that the delimiter may be longer than one character.) |
| 3619 | |
| 3620 | If LIMIT is specified and positive, splits into no more than that |
| 3621 | many fields (though it may split into fewer). If LIMIT is unspecified |
| 3622 | or zero, trailing null fields are stripped (which potential users |
| 3623 | of C<pop()> would do well to remember). If LIMIT is negative, it is |
| 3624 | treated as if an arbitrarily large LIMIT had been specified. |
| 3625 | |
| 3626 | A pattern matching the null string (not to be confused with |
| 3627 | a null pattern C<//>, which is just one member of the set of patterns |
| 3628 | matching a null string) will split the value of EXPR into separate |
| 3629 | characters at each point it matches that way. For example: |
| 3630 | |
| 3631 | print join(':', split(/ */, 'hi there')); |
| 3632 | |
| 3633 | produces the output 'h:i:t:h:e:r:e'. |
| 3634 | |
| 3635 | The LIMIT parameter can be used to split a line partially |
| 3636 | |
| 3637 | ($login, $passwd, $remainder) = split(/:/, $_, 3); |
| 3638 | |
| 3639 | When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT |
| 3640 | one larger than the number of variables in the list, to avoid |
| 3641 | unnecessary work. For the list above LIMIT would have been 4 by |
| 3642 | default. In time critical applications it behooves you not to split |
| 3643 | into more fields than you really need. |
| 3644 | |
| 3645 | If the PATTERN contains parentheses, additional array elements are |
| 3646 | created from each matching substring in the delimiter. |
| 3647 | |
| 3648 | split(/([,-])/, "1-10,20", 3); |
| 3649 | |
| 3650 | produces the list value |
| 3651 | |
| 3652 | (1, '-', 10, ',', 20) |
| 3653 | |
| 3654 | If you had the entire header of a normal Unix email message in C<$header>, |
| 3655 | you could split it up into fields and their values this way: |
| 3656 | |
| 3657 | $header =~ s/\n\s+/ /g; # fix continuation lines |
| 3658 | %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header); |
| 3659 | |
| 3660 | The pattern C</PATTERN/> may be replaced with an expression to specify |
| 3661 | patterns that vary at runtime. (To do runtime compilation only once, |
| 3662 | use C</$variable/o>.) |
| 3663 | |
| 3664 | As a special case, specifying a PATTERN of space (C<' '>) will split on |
| 3665 | white space just as C<split()> with no arguments does. Thus, C<split(' ')> can |
| 3666 | be used to emulate B<awk>'s default behavior, whereas C<split(/ /)> |
| 3667 | will give you as many null initial fields as there are leading spaces. |
| 3668 | A C<split()> on C</\s+/> is like a C<split(' ')> except that any leading |
| 3669 | whitespace produces a null first field. A C<split()> with no arguments |
| 3670 | really does a C<split(' ', $_)> internally. |
| 3671 | |
| 3672 | Example: |
| 3673 | |
| 3674 | open(PASSWD, '/etc/passwd'); |
| 3675 | while (<PASSWD>) { |
| 3676 | ($login, $passwd, $uid, $gid, |
| 3677 | $gcos, $home, $shell) = split(/:/); |
| 3678 | #... |
| 3679 | } |
| 3680 | |
| 3681 | (Note that C<$shell> above will still have a newline on it. See L</chop>, |
| 3682 | L</chomp>, and L</join>.) |
| 3683 | |
| 3684 | =item sprintf FORMAT, LIST |
| 3685 | |
| 3686 | Returns a string formatted by the usual C<printf()> conventions of the |
| 3687 | C library function C<sprintf()>. See L<sprintf(3)> or L<printf(3)> |
| 3688 | on your system for an explanation of the general principles. |
| 3689 | |
| 3690 | Perl does its own C<sprintf()> formatting -- it emulates the C |
| 3691 | function C<sprintf()>, but it doesn't use it (except for floating-point |
| 3692 | numbers, and even then only the standard modifiers are allowed). As a |
| 3693 | result, any non-standard extensions in your local C<sprintf()> are not |
| 3694 | available from Perl. |
| 3695 | |
| 3696 | Perl's C<sprintf()> permits the following universally-known conversions: |
| 3697 | |
| 3698 | %% a percent sign |
| 3699 | %c a character with the given number |
| 3700 | %s a string |
| 3701 | %d a signed integer, in decimal |
| 3702 | %u an unsigned integer, in decimal |
| 3703 | %o an unsigned integer, in octal |
| 3704 | %x an unsigned integer, in hexadecimal |
| 3705 | %e a floating-point number, in scientific notation |
| 3706 | %f a floating-point number, in fixed decimal notation |
| 3707 | %g a floating-point number, in %e or %f notation |
| 3708 | |
| 3709 | In addition, Perl permits the following widely-supported conversions: |
| 3710 | |
| 3711 | %X like %x, but using upper-case letters |
| 3712 | %E like %e, but using an upper-case "E" |
| 3713 | %G like %g, but with an upper-case "E" (if applicable) |
| 3714 | %p a pointer (outputs the Perl value's address in hexadecimal) |
| 3715 | %n special: *stores* the number of characters output so far |
| 3716 | into the next variable in the parameter list |
| 3717 | |
| 3718 | Finally, for backward (and we do mean "backward") compatibility, Perl |
| 3719 | permits these unnecessary but widely-supported conversions: |
| 3720 | |
| 3721 | %i a synonym for %d |
| 3722 | %D a synonym for %ld |
| 3723 | %U a synonym for %lu |
| 3724 | %O a synonym for %lo |
| 3725 | %F a synonym for %f |
| 3726 | |
| 3727 | Perl permits the following universally-known flags between the C<%> |
| 3728 | and the conversion letter: |
| 3729 | |
| 3730 | space prefix positive number with a space |
| 3731 | + prefix positive number with a plus sign |
| 3732 | - left-justify within the field |
| 3733 | 0 use zeros, not spaces, to right-justify |
| 3734 | # prefix non-zero octal with "0", non-zero hex with "0x" |
| 3735 | number minimum field width |
| 3736 | .number "precision": digits after decimal point for |
| 3737 | floating-point, max length for string, minimum length |
| 3738 | for integer |
| 3739 | l interpret integer as C type "long" or "unsigned long" |
| 3740 | h interpret integer as C type "short" or "unsigned short" |
| 3741 | |
| 3742 | There is also one Perl-specific flag: |
| 3743 | |
| 3744 | V interpret integer as Perl's standard integer type |
| 3745 | |
| 3746 | Where a number would appear in the flags, an asterisk ("C<*>") may be |
| 3747 | used instead, in which case Perl uses the next item in the parameter |
| 3748 | list as the given number (that is, as the field width or precision). |
| 3749 | If a field width obtained through "C<*>" is negative, it has the same |
| 3750 | effect as the "C<->" flag: left-justification. |
| 3751 | |
| 3752 | If C<use locale> is in effect, the character used for the decimal |
| 3753 | point in formatted real numbers is affected by the LC_NUMERIC locale. |
| 3754 | See L<perllocale>. |
| 3755 | |
| 3756 | =item sqrt EXPR |
| 3757 | |
| 3758 | =item sqrt |
| 3759 | |
| 3760 | Return the square root of EXPR. If EXPR is omitted, returns square |
| 3761 | root of C<$_>. |
| 3762 | |
| 3763 | =item srand EXPR |
| 3764 | |
| 3765 | =item srand |
| 3766 | |
| 3767 | Sets the random number seed for the C<rand()> operator. If EXPR is |
| 3768 | omitted, uses a semi-random value supplied by the kernel (if it supports |
| 3769 | the F</dev/urandom> device) or based on the current time and process |
| 3770 | ID, among other things. In versions of Perl prior to 5.004 the default |
| 3771 | seed was just the current C<time()>. This isn't a particularly good seed, |
| 3772 | so many old programs supply their own seed value (often C<time ^ $$> or |
| 3773 | C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more. |
| 3774 | |
| 3775 | In fact, it's usually not necessary to call C<srand()> at all, because if |
| 3776 | it is not called explicitly, it is called implicitly at the first use of |
| 3777 | the C<rand()> operator. However, this was not the case in version of Perl |
| 3778 | before 5.004, so if your script will run under older Perl versions, it |
| 3779 | should call C<srand()>. |
| 3780 | |
| 3781 | Note that you need something much more random than the default seed for |
| 3782 | cryptographic purposes. Checksumming the compressed output of one or more |
| 3783 | rapidly changing operating system status programs is the usual method. For |
| 3784 | example: |
| 3785 | |
| 3786 | srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`); |
| 3787 | |
| 3788 | If you're particularly concerned with this, see the C<Math::TrulyRandom> |
| 3789 | module in CPAN. |
| 3790 | |
| 3791 | Do I<not> call C<srand()> multiple times in your program unless you know |
| 3792 | exactly what you're doing and why you're doing it. The point of the |
| 3793 | function is to "seed" the C<rand()> function so that C<rand()> can produce |
| 3794 | a different sequence each time you run your program. Just do it once at the |
| 3795 | top of your program, or you I<won't> get random numbers out of C<rand()>! |
| 3796 | |
| 3797 | Frequently called programs (like CGI scripts) that simply use |
| 3798 | |
| 3799 | time ^ $$ |
| 3800 | |
| 3801 | for a seed can fall prey to the mathematical property that |
| 3802 | |
| 3803 | a^b == (a+1)^(b+1) |
| 3804 | |
| 3805 | one-third of the time. So don't do that. |
| 3806 | |
| 3807 | =item stat FILEHANDLE |
| 3808 | |
| 3809 | =item stat EXPR |
| 3810 | |
| 3811 | =item stat |
| 3812 | |
| 3813 | Returns a 13-element list giving the status info for a file, either |
| 3814 | the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, |
| 3815 | it stats C<$_>. Returns a null list if the stat fails. Typically used |
| 3816 | as follows: |
| 3817 | |
| 3818 | ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, |
| 3819 | $atime,$mtime,$ctime,$blksize,$blocks) |
| 3820 | = stat($filename); |
| 3821 | |
| 3822 | Not all fields are supported on all filesystem types. Here are the |
| 3823 | meaning of the fields: |
| 3824 | |
| 3825 | 0 dev device number of filesystem |
| 3826 | 1 ino inode number |
| 3827 | 2 mode file mode (type and permissions) |
| 3828 | 3 nlink number of (hard) links to the file |
| 3829 | 4 uid numeric user ID of file's owner |
| 3830 | 5 gid numeric group ID of file's owner |
| 3831 | 6 rdev the device identifier (special files only) |
| 3832 | 7 size total size of file, in bytes |
| 3833 | 8 atime last access time since the epoch |
| 3834 | 9 mtime last modify time since the epoch |
| 3835 | 10 ctime inode change time (NOT creation time!) since the epoch |
| 3836 | 11 blksize preferred block size for file system I/O |
| 3837 | 12 blocks actual number of blocks allocated |
| 3838 | |
| 3839 | (The epoch was at 00:00 January 1, 1970 GMT.) |
| 3840 | |
| 3841 | If stat is passed the special filehandle consisting of an underline, no |
| 3842 | stat is done, but the current contents of the stat structure from the |
| 3843 | last stat or filetest are returned. Example: |
| 3844 | |
| 3845 | if (-x $file && (($d) = stat(_)) && $d < 0) { |
| 3846 | print "$file is executable NFS file\n"; |
| 3847 | } |
| 3848 | |
| 3849 | (This works on machines only for which the device number is negative under NFS.) |
| 3850 | |
| 3851 | In scalar context, C<stat()> returns a boolean value indicating success |
| 3852 | or failure, and, if successful, sets the information associated with |
| 3853 | the special filehandle C<_>. |
| 3854 | |
| 3855 | =item study SCALAR |
| 3856 | |
| 3857 | =item study |
| 3858 | |
| 3859 | Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of |
| 3860 | doing many pattern matches on the string before it is next modified. |
| 3861 | This may or may not save time, depending on the nature and number of |
| 3862 | patterns you are searching on, and on the distribution of character |
| 3863 | frequencies in the string to be searched -- you probably want to compare |
| 3864 | run times with and without it to see which runs faster. Those loops |
| 3865 | which scan for many short constant strings (including the constant |
| 3866 | parts of more complex patterns) will benefit most. You may have only |
| 3867 | one C<study()> active at a time -- if you study a different scalar the first |
| 3868 | is "unstudied". (The way C<study()> works is this: a linked list of every |
| 3869 | character in the string to be searched is made, so we know, for |
| 3870 | example, where all the C<'k'> characters are. From each search string, |
| 3871 | the rarest character is selected, based on some static frequency tables |
| 3872 | constructed from some C programs and English text. Only those places |
| 3873 | that contain this "rarest" character are examined.) |
| 3874 | |
| 3875 | For example, here is a loop that inserts index producing entries |
| 3876 | before any line containing a certain pattern: |
| 3877 | |
| 3878 | while (<>) { |
| 3879 | study; |
| 3880 | print ".IX foo\n" if /\bfoo\b/; |
| 3881 | print ".IX bar\n" if /\bbar\b/; |
| 3882 | print ".IX blurfl\n" if /\bblurfl\b/; |
| 3883 | # ... |
| 3884 | print; |
| 3885 | } |
| 3886 | |
| 3887 | In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f"> |
| 3888 | will be looked at, because C<"f"> is rarer than C<"o">. In general, this is |
| 3889 | a big win except in pathological cases. The only question is whether |
| 3890 | it saves you more time than it took to build the linked list in the |
| 3891 | first place. |
| 3892 | |
| 3893 | Note that if you have to look for strings that you don't know till |
| 3894 | runtime, you can build an entire loop as a string and C<eval()> that to |
| 3895 | avoid recompiling all your patterns all the time. Together with |
| 3896 | undefining C<$/> to input entire files as one record, this can be very |
| 3897 | fast, often faster than specialized programs like fgrep(1). The following |
| 3898 | scans a list of files (C<@files>) for a list of words (C<@words>), and prints |
| 3899 | out the names of those files that contain a match: |
| 3900 | |
| 3901 | $search = 'while (<>) { study;'; |
| 3902 | foreach $word (@words) { |
| 3903 | $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; |
| 3904 | } |
| 3905 | $search .= "}"; |
| 3906 | @ARGV = @files; |
| 3907 | undef $/; |
| 3908 | eval $search; # this screams |
| 3909 | $/ = "\n"; # put back to normal input delimiter |
| 3910 | foreach $file (sort keys(%seen)) { |
| 3911 | print $file, "\n"; |
| 3912 | } |
| 3913 | |
| 3914 | =item sub BLOCK |
| 3915 | |
| 3916 | =item sub NAME |
| 3917 | |
| 3918 | =item sub NAME BLOCK |
| 3919 | |
| 3920 | This is subroutine definition, not a real function I<per se>. With just a |
| 3921 | NAME (and possibly prototypes), it's just a forward declaration. Without |
| 3922 | a NAME, it's an anonymous function declaration, and does actually return a |
| 3923 | value: the CODE ref of the closure you just created. See L<perlsub> and |
| 3924 | L<perlref> for details. |
| 3925 | |
| 3926 | =item substr EXPR,OFFSET,LEN,REPLACEMENT |
| 3927 | |
| 3928 | =item substr EXPR,OFFSET,LEN |
| 3929 | |
| 3930 | =item substr EXPR,OFFSET |
| 3931 | |
| 3932 | Extracts a substring out of EXPR and returns it. First character is at |
| 3933 | offset C<0>, or whatever you've set C<$[> to (but don't do that). |
| 3934 | If OFFSET is negative (or more precisely, less than C<$[>), starts |
| 3935 | that far from the end of the string. If LEN is omitted, returns |
| 3936 | everything to the end of the string. If LEN is negative, leaves that |
| 3937 | many characters off the end of the string. |
| 3938 | |
| 3939 | If you specify a substring that is partly outside the string, the part |
| 3940 | within the string is returned. If the substring is totally outside |
| 3941 | the string a warning is produced. |
| 3942 | |
| 3943 | You can use the C<substr()> function |
| 3944 | as an lvalue, in which case EXPR must be an lvalue. If you assign |
| 3945 | something shorter than LEN, the string will shrink, and if you assign |
| 3946 | something longer than LEN, the string will grow to accommodate it. To |
| 3947 | keep the string the same length you may need to pad or chop your value |
| 3948 | using C<sprintf()>. |
| 3949 | |
| 3950 | An alternative to using C<substr()> as an lvalue is to specify the |
| 3951 | replacement string as the 4th argument. This allows you to replace |
| 3952 | parts of the EXPR and return what was there before in one operation. |
| 3953 | |
| 3954 | =item symlink OLDFILE,NEWFILE |
| 3955 | |
| 3956 | Creates a new filename symbolically linked to the old filename. |
| 3957 | Returns C<1> for success, C<0> otherwise. On systems that don't support |
| 3958 | symbolic links, produces a fatal error at run time. To check for that, |
| 3959 | use eval: |
| 3960 | |
| 3961 | $symlink_exists = eval { symlink("",""); 1 }; |
| 3962 | |
| 3963 | =item syscall LIST |
| 3964 | |
| 3965 | Calls the system call specified as the first element of the list, |
| 3966 | passing the remaining elements as arguments to the system call. If |
| 3967 | unimplemented, produces a fatal error. The arguments are interpreted |
| 3968 | as follows: if a given argument is numeric, the argument is passed as |
| 3969 | an int. If not, the pointer to the string value is passed. You are |
| 3970 | responsible to make sure a string is pre-extended long enough to |
| 3971 | receive any result that might be written into a string. You can't use a |
| 3972 | string literal (or other read-only string) as an argument to C<syscall()> |
| 3973 | because Perl has to assume that any string pointer might be written |
| 3974 | through. If your |
| 3975 | integer arguments are not literals and have never been interpreted in a |
| 3976 | numeric context, you may need to add C<0> to them to force them to look |
| 3977 | like numbers. This emulates the C<syswrite()> function (or vice versa): |
| 3978 | |
| 3979 | require 'syscall.ph'; # may need to run h2ph |
| 3980 | $s = "hi there\n"; |
| 3981 | syscall(&SYS_write, fileno(STDOUT), $s, length $s); |
| 3982 | |
| 3983 | Note that Perl supports passing of up to only 14 arguments to your system call, |
| 3984 | which in practice should usually suffice. |
| 3985 | |
| 3986 | Syscall returns whatever value returned by the system call it calls. |
| 3987 | If the system call fails, C<syscall()> returns C<-1> and sets C<$!> (errno). |
| 3988 | Note that some system calls can legitimately return C<-1>. The proper |
| 3989 | way to handle such calls is to assign C<$!=0;> before the call and |
| 3990 | check the value of C<$!> if syscall returns C<-1>. |
| 3991 | |
| 3992 | There's a problem with C<syscall(&SYS_pipe)>: it returns the file |
| 3993 | number of the read end of the pipe it creates. There is no way |
| 3994 | to retrieve the file number of the other end. You can avoid this |
| 3995 | problem by using C<pipe()> instead. |
| 3996 | |
| 3997 | =item sysopen FILEHANDLE,FILENAME,MODE |
| 3998 | |
| 3999 | =item sysopen FILEHANDLE,FILENAME,MODE,PERMS |
| 4000 | |
| 4001 | Opens the file whose filename is given by FILENAME, and associates it |
| 4002 | with FILEHANDLE. If FILEHANDLE is an expression, its value is used as |
| 4003 | the name of the real filehandle wanted. This function calls the |
| 4004 | underlying operating system's C<open()> function with the parameters |
| 4005 | FILENAME, MODE, PERMS. |
| 4006 | |
| 4007 | The possible values and flag bits of the MODE parameter are |
| 4008 | system-dependent; they are available via the standard module C<Fcntl>. |
| 4009 | For historical reasons, some values work on almost every system |
| 4010 | supported by perl: zero means read-only, one means write-only, and two |
| 4011 | means read/write. We know that these values do I<not> work under |
| 4012 | OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to |
| 4013 | use them in new code. |
| 4014 | |
| 4015 | If the file named by FILENAME does not exist and the C<open()> call creates |
| 4016 | it (typically because MODE includes the C<O_CREAT> flag), then the value of |
| 4017 | PERMS specifies the permissions of the newly created file. If you omit |
| 4018 | the PERMS argument to C<sysopen()>, Perl uses the octal value C<0666>. |
| 4019 | These permission values need to be in octal, and are modified by your |
| 4020 | process's current C<umask>. |
| 4021 | |
| 4022 | Seldom if ever use C<0644> as argument to C<sysopen()> because that |
| 4023 | takes away the user's option to have a more permissive umask. Better |
| 4024 | to omit it. See the perlfunc(1) entry on C<umask> for more on this. |
| 4025 | |
| 4026 | The C<IO::File> module provides a more object-oriented approach, if you're |
| 4027 | into that kind of thing. |
| 4028 | |
| 4029 | =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET |
| 4030 | |
| 4031 | =item sysread FILEHANDLE,SCALAR,LENGTH |
| 4032 | |
| 4033 | Attempts to read LENGTH bytes of data into variable SCALAR from the |
| 4034 | specified FILEHANDLE, using the system call read(2). It bypasses stdio, |
| 4035 | so mixing this with other kinds of reads, C<print()>, C<write()>, |
| 4036 | C<seek()>, C<tell()>, or C<eof()> can cause confusion because stdio |
| 4037 | usually buffers data. Returns the number of bytes actually read, C<0> |
| 4038 | at end of file, or undef if there was an error. SCALAR will be grown or |
| 4039 | shrunk so that the last byte actually read is the last byte of the |
| 4040 | scalar after the read. |
| 4041 | |
| 4042 | An OFFSET may be specified to place the read data at some place in the |
| 4043 | string other than the beginning. A negative OFFSET specifies |
| 4044 | placement at that many bytes counting backwards from the end of the |
| 4045 | string. A positive OFFSET greater than the length of SCALAR results |
| 4046 | in the string being padded to the required size with C<"\0"> bytes before |
| 4047 | the result of the read is appended. |
| 4048 | |
| 4049 | =item sysseek FILEHANDLE,POSITION,WHENCE |
| 4050 | |
| 4051 | Sets FILEHANDLE's system position using the system call lseek(2). It |
| 4052 | bypasses stdio, so mixing this with reads (other than C<sysread()>), |
| 4053 | C<print()>, C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause |
| 4054 | confusion. FILEHANDLE may be an expression whose value gives the name |
| 4055 | of the filehandle. The values for WHENCE are C<0> to set the new |
| 4056 | position to POSITION, C<1> to set the it to the current position plus |
| 4057 | POSITION, and C<2> to set it to EOF plus POSITION (typically negative). |
| 4058 | For WHENCE, you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and |
| 4059 | C<SEEK_END> from either the C<IO::Seekable> or the POSIX module. |
| 4060 | |
| 4061 | Returns the new position, or the undefined value on failure. A position |
| 4062 | of zero is returned as the string "C<0> but true"; thus C<sysseek()> returns |
| 4063 | TRUE on success and FALSE on failure, yet you can still easily determine |
| 4064 | the new position. |
| 4065 | |
| 4066 | =item system LIST |
| 4067 | |
| 4068 | =item system PROGRAM LIST |
| 4069 | |
| 4070 | Does exactly the same thing as "C<exec LIST>" except that a fork is done |
| 4071 | first, and the parent process waits for the child process to complete. |
| 4072 | Note that argument processing varies depending on the number of |
| 4073 | arguments. If there is more than one argument in LIST, or if LIST is |
| 4074 | an array with more than one value, starts the program given by the |
| 4075 | first element of the list with arguments given by the rest of the list. |
| 4076 | If there is only one scalar argument, the argument is |
| 4077 | checked for shell metacharacters, and if there are any, the entire |
| 4078 | argument is passed to the system's command shell for parsing (this is |
| 4079 | C</bin/sh -c> on Unix platforms, but varies on other platforms). If |
| 4080 | there are no shell metacharacters in the argument, it is split into |
| 4081 | words and passed directly to C<execvp()>, which is more efficient. |
| 4082 | |
| 4083 | The return value is the exit status of the program as |
| 4084 | returned by the C<wait()> call. To get the actual exit value divide by |
| 4085 | 256. See also L</exec>. This is I<NOT> what you want to use to capture |
| 4086 | the output from a command, for that you should use merely backticks or |
| 4087 | C<qx//>, as described in L<perlop/"`STRING`">. |
| 4088 | |
| 4089 | Like C<exec()>, C<system()> allows you to lie to a program about its name if |
| 4090 | you use the "C<system PROGRAM LIST>" syntax. Again, see L</exec>. |
| 4091 | |
| 4092 | Because C<system()> and backticks block C<SIGINT> and C<SIGQUIT>, killing the |
| 4093 | program they're running doesn't actually interrupt your program. |
| 4094 | |
| 4095 | @args = ("command", "arg1", "arg2"); |
| 4096 | system(@args) == 0 |
| 4097 | or die "system @args failed: $?" |
| 4098 | |
| 4099 | You can check all the failure possibilities by inspecting |
| 4100 | C<$?> like this: |
| 4101 | |
| 4102 | $exit_value = $? >> 8; |
| 4103 | $signal_num = $? & 127; |
| 4104 | $dumped_core = $? & 128; |
| 4105 | |
| 4106 | When the arguments get executed via the system shell, results |
| 4107 | and return codes will be subject to its quirks and capabilities. |
| 4108 | See L<perlop/"`STRING`"> and L</exec> for details. |
| 4109 | |
| 4110 | =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET |
| 4111 | |
| 4112 | =item syswrite FILEHANDLE,SCALAR,LENGTH |
| 4113 | |
| 4114 | =item syswrite FILEHANDLE,SCALAR |
| 4115 | |
| 4116 | Attempts to write LENGTH bytes of data from variable SCALAR to the |
| 4117 | specified FILEHANDLE, using the system call write(2). If LENGTH is |
| 4118 | not specified, writes whole SCALAR. It bypasses |
| 4119 | stdio, so mixing this with reads (other than C<sysread())>, C<print()>, |
| 4120 | C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause confusion |
| 4121 | because stdio usually buffers data. Returns the number of bytes |
| 4122 | actually written, or C<undef> if there was an error. If the LENGTH is |
| 4123 | greater than the available data in the SCALAR after the OFFSET, only as |
| 4124 | much data as is available will be written. |
| 4125 | |
| 4126 | An OFFSET may be specified to write the data from some part of the |
| 4127 | string other than the beginning. A negative OFFSET specifies writing |
| 4128 | that many bytes counting backwards from the end of the string. In the |
| 4129 | case the SCALAR is empty you can use OFFSET but only zero offset. |
| 4130 | |
| 4131 | =item tell FILEHANDLE |
| 4132 | |
| 4133 | =item tell |
| 4134 | |
| 4135 | Returns the current position for FILEHANDLE. FILEHANDLE may be an |
| 4136 | expression whose value gives the name of the actual filehandle. If |
| 4137 | FILEHANDLE is omitted, assumes the file last read. |
| 4138 | |
| 4139 | =item telldir DIRHANDLE |
| 4140 | |
| 4141 | Returns the current position of the C<readdir()> routines on DIRHANDLE. |
| 4142 | Value may be given to C<seekdir()> to access a particular location in a |
| 4143 | directory. Has the same caveats about possible directory compaction as |
| 4144 | the corresponding system library routine. |
| 4145 | |
| 4146 | =item tie VARIABLE,CLASSNAME,LIST |
| 4147 | |
| 4148 | This function binds a variable to a package class that will provide the |
| 4149 | implementation for the variable. VARIABLE is the name of the variable |
| 4150 | to be enchanted. CLASSNAME is the name of a class implementing objects |
| 4151 | of correct type. Any additional arguments are passed to the "C<new()>" |
| 4152 | method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>, |
| 4153 | or C<TIEHASH>). Typically these are arguments such as might be passed |
| 4154 | to the C<dbm_open()> function of C. The object returned by the "C<new()>" |
| 4155 | method is also returned by the C<tie()> function, which would be useful |
| 4156 | if you want to access other methods in CLASSNAME. |
| 4157 | |
| 4158 | Note that functions such as C<keys()> and C<values()> may return huge lists |
| 4159 | when used on large objects, like DBM files. You may prefer to use the |
| 4160 | C<each()> function to iterate over such. Example: |
| 4161 | |
| 4162 | # print out history file offsets |
| 4163 | use NDBM_File; |
| 4164 | tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); |
| 4165 | while (($key,$val) = each %HIST) { |
| 4166 | print $key, ' = ', unpack('L',$val), "\n"; |
| 4167 | } |
| 4168 | untie(%HIST); |
| 4169 | |
| 4170 | A class implementing a hash should have the following methods: |
| 4171 | |
| 4172 | TIEHASH classname, LIST |
| 4173 | FETCH this, key |
| 4174 | STORE this, key, value |
| 4175 | DELETE this, key |
| 4176 | CLEAR this |
| 4177 | EXISTS this, key |
| 4178 | FIRSTKEY this |
| 4179 | NEXTKEY this, lastkey |
| 4180 | DESTROY this |
| 4181 | |
| 4182 | A class implementing an ordinary array should have the following methods: |
| 4183 | |
| 4184 | TIEARRAY classname, LIST |
| 4185 | FETCH this, key |
| 4186 | STORE this, key, value |
| 4187 | FETCHSIZE this |
| 4188 | STORESIZE this, count |
| 4189 | CLEAR this |
| 4190 | PUSH this, LIST |
| 4191 | POP this |
| 4192 | SHIFT this |
| 4193 | UNSHIFT this, LIST |
| 4194 | SPLICE this, offset, length, LIST |
| 4195 | EXTEND this, count |
| 4196 | DESTROY this |
| 4197 | |
| 4198 | A class implementing a file handle should have the following methods: |
| 4199 | |
| 4200 | TIEHANDLE classname, LIST |
| 4201 | READ this, scalar, length, offset |
| 4202 | READLINE this |
| 4203 | GETC this |
| 4204 | WRITE this, scalar, length, offset |
| 4205 | PRINT this, LIST |
| 4206 | PRINTF this, format, LIST |
| 4207 | CLOSE this |
| 4208 | DESTROY this |
| 4209 | |
| 4210 | A class implementing a scalar should have the following methods: |
| 4211 | |
| 4212 | TIESCALAR classname, LIST |
| 4213 | FETCH this, |
| 4214 | STORE this, value |
| 4215 | DESTROY this |
| 4216 | |
| 4217 | Not all methods indicated above need be implemented. See L<perltie>, |
| 4218 | L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar> and L<Tie::Handle>. |
| 4219 | |
| 4220 | Unlike C<dbmopen()>, the C<tie()> function will not use or require a module |
| 4221 | for you--you need to do that explicitly yourself. See L<DB_File> |
| 4222 | or the F<Config> module for interesting C<tie()> implementations. |
| 4223 | |
| 4224 | For further details see L<perltie>, L<"tied VARIABLE">. |
| 4225 | |
| 4226 | =item tied VARIABLE |
| 4227 | |
| 4228 | Returns a reference to the object underlying VARIABLE (the same value |
| 4229 | that was originally returned by the C<tie()> call that bound the variable |
| 4230 | to a package.) Returns the undefined value if VARIABLE isn't tied to a |
| 4231 | package. |
| 4232 | |
| 4233 | =item time |
| 4234 | |
| 4235 | Returns the number of non-leap seconds since whatever time the system |
| 4236 | considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS, |
| 4237 | and 00:00:00 UTC, January 1, 1970 for most other systems). |
| 4238 | Suitable for feeding to C<gmtime()> and C<localtime()>. |
| 4239 | |
| 4240 | =item times |
| 4241 | |
| 4242 | Returns a four-element list giving the user and system times, in |
| 4243 | seconds, for this process and the children of this process. |
| 4244 | |
| 4245 | ($user,$system,$cuser,$csystem) = times; |
| 4246 | |
| 4247 | =item tr/// |
| 4248 | |
| 4249 | The transliteration operator. Same as C<y///>. See L<perlop>. |
| 4250 | |
| 4251 | =item truncate FILEHANDLE,LENGTH |
| 4252 | |
| 4253 | =item truncate EXPR,LENGTH |
| 4254 | |
| 4255 | Truncates the file opened on FILEHANDLE, or named by EXPR, to the |
| 4256 | specified length. Produces a fatal error if truncate isn't implemented |
| 4257 | on your system. Returns TRUE if successful, the undefined value |
| 4258 | otherwise. |
| 4259 | |
| 4260 | =item uc EXPR |
| 4261 | |
| 4262 | =item uc |
| 4263 | |
| 4264 | Returns an uppercased version of EXPR. This is the internal function |
| 4265 | implementing the C<\U> escape in double-quoted strings. |
| 4266 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. |
| 4267 | Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It |
| 4268 | does not attempt to do titlecase mapping on initial letters. See C<ucfirst()> for that.) |
| 4269 | |
| 4270 | If EXPR is omitted, uses C<$_>. |
| 4271 | |
| 4272 | =item ucfirst EXPR |
| 4273 | |
| 4274 | =item ucfirst |
| 4275 | |
| 4276 | Returns the value of EXPR with the first character |
| 4277 | in uppercase (titlecase in Unicode). This is |
| 4278 | the internal function implementing the C<\u> escape in double-quoted strings. |
| 4279 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. |
| 4280 | |
| 4281 | If EXPR is omitted, uses C<$_>. |
| 4282 | |
| 4283 | =item umask EXPR |
| 4284 | |
| 4285 | =item umask |
| 4286 | |
| 4287 | Sets the umask for the process to EXPR and returns the previous value. |
| 4288 | If EXPR is omitted, merely returns the current umask. |
| 4289 | |
| 4290 | The Unix permission C<rwxr-x---> is represented as three sets of three |
| 4291 | bits, or three octal digits: C<0750> (the leading 0 indicates octal |
| 4292 | and isn't one of the the digits). The C<umask> value is such a number |
| 4293 | representing disabled permissions bits. The permission (or "mode") |
| 4294 | values you pass C<mkdir> or C<sysopen> are modified by your umask, so |
| 4295 | even if you tell C<sysopen> to create a file with permissions C<0777>, |
| 4296 | if your umask is C<0022> then the file will actually be created with |
| 4297 | permissions C<0755>. If your C<umask> were C<0027> (group can't |
| 4298 | write; others can't read, write, or execute), then passing |
| 4299 | C<sysopen()> C<0666> would create a file with mode C<0640> (C<0666 &~ |
| 4300 | 027> is C<0640>). |
| 4301 | |
| 4302 | Here's some advice: supply a creation mode of C<0666> for regular |
| 4303 | files (in C<sysopen()>) and one of C<0777> for directories (in |
| 4304 | C<mkdir()>) and executable files. This gives users the freedom of |
| 4305 | choice: if they want protected files, they might choose process umasks |
| 4306 | of C<022>, C<027>, or even the particularly antisocial mask of C<077>. |
| 4307 | Programs should rarely if ever make policy decisions better left to |
| 4308 | the user. The exception to this is when writing files that should be |
| 4309 | kept private: mail files, web browser cookies, I<.rhosts> files, and |
| 4310 | so on. |
| 4311 | |
| 4312 | If umask(2) is not implemented on your system and you are trying to |
| 4313 | restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a |
| 4314 | fatal error at run time. If umask(2) is not implemented and you are |
| 4315 | not trying to restrict access for yourself, returns C<undef>. |
| 4316 | |
| 4317 | Remember that a umask is a number, usually given in octal; it is I<not> a |
| 4318 | string of octal digits. See also L</oct>, if all you have is a string. |
| 4319 | |
| 4320 | =item undef EXPR |
| 4321 | |
| 4322 | =item undef |
| 4323 | |
| 4324 | Undefines the value of EXPR, which must be an lvalue. Use only on a |
| 4325 | scalar value, an array (using "C<@>"), a hash (using "C<%>"), a subroutine |
| 4326 | (using "C<&>"), or a typeglob (using "<*>"). (Saying C<undef $hash{$key}> |
| 4327 | will probably not do what you expect on most predefined variables or |
| 4328 | DBM list values, so don't do that; see L<delete>.) Always returns the |
| 4329 | undefined value. You can omit the EXPR, in which case nothing is |
| 4330 | undefined, but you still get an undefined value that you could, for |
| 4331 | instance, return from a subroutine, assign to a variable or pass as a |
| 4332 | parameter. Examples: |
| 4333 | |
| 4334 | undef $foo; |
| 4335 | undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; |
| 4336 | undef @ary; |
| 4337 | undef %hash; |
| 4338 | undef &mysub; |
| 4339 | undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. |
| 4340 | return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; |
| 4341 | select undef, undef, undef, 0.25; |
| 4342 | ($a, $b, undef, $c) = &foo; # Ignore third value returned |
| 4343 | |
| 4344 | Note that this is a unary operator, not a list operator. |
| 4345 | |
| 4346 | =item unless (EXPR) BLOCK |
| 4347 | |
| 4348 | The negative counterpart of L</if>. If the EXPR returns false the |
| 4349 | BLOCK is entered. |
| 4350 | |
| 4351 | See also L<perlsyn>. |
| 4352 | |
| 4353 | =item unlink LIST |
| 4354 | |
| 4355 | =item unlink |
| 4356 | |
| 4357 | Deletes a list of files. Returns the number of files successfully |
| 4358 | deleted. |
| 4359 | |
| 4360 | $cnt = unlink 'a', 'b', 'c'; |
| 4361 | unlink @goners; |
| 4362 | unlink <*.bak>; |
| 4363 | |
| 4364 | Note: C<unlink()> will not delete directories unless you are superuser and |
| 4365 | the B<-U> flag is supplied to Perl. Even if these conditions are |
| 4366 | met, be warned that unlinking a directory can inflict damage on your |
| 4367 | filesystem. Use C<rmdir()> instead. |
| 4368 | |
| 4369 | If LIST is omitted, uses C<$_>. |
| 4370 | |
| 4371 | =item unpack TEMPLATE,EXPR |
| 4372 | |
| 4373 | C<Unpack()> does the reverse of C<pack()>: it takes a string representing a |
| 4374 | structure and expands it out into a list value, returning the array |
| 4375 | value. (In scalar context, it returns merely the first value |
| 4376 | produced.) The TEMPLATE has the same format as in the C<pack()> function. |
| 4377 | Here's a subroutine that does substring: |
| 4378 | |
| 4379 | sub substr { |
| 4380 | my($what,$where,$howmuch) = @_; |
| 4381 | unpack("x$where a$howmuch", $what); |
| 4382 | } |
| 4383 | |
| 4384 | and then there's |
| 4385 | |
| 4386 | sub ordinal { unpack("c",$_[0]); } # same as ord() |
| 4387 | |
| 4388 | In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that |
| 4389 | you want a E<lt>numberE<gt>-bit checksum of the items instead of the items |
| 4390 | themselves. Default is a 16-bit checksum. For example, the following |
| 4391 | computes the same number as the System V sum program: |
| 4392 | |
| 4393 | while (<>) { |
| 4394 | $checksum += unpack("%16C*", $_); |
| 4395 | } |
| 4396 | $checksum %= 65536; |
| 4397 | |
| 4398 | The following efficiently counts the number of set bits in a bit vector: |
| 4399 | |
| 4400 | $setbits = unpack("%32b*", $selectmask); |
| 4401 | |
| 4402 | =item unshift ARRAY,LIST |
| 4403 | |
| 4404 | Does the opposite of a C<shift()>. Or the opposite of a C<push()>, |
| 4405 | depending on how you look at it. Prepends list to the front of the |
| 4406 | array, and returns the new number of elements in the array. |
| 4407 | |
| 4408 | unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/; |
| 4409 | |
| 4410 | Note the LIST is prepended whole, not one element at a time, so the |
| 4411 | prepended elements stay in the same order. Use C<reverse()> to do the |
| 4412 | reverse. |
| 4413 | |
| 4414 | =item until (EXPR) BLOCK |
| 4415 | |
| 4416 | =item do BLOCK until (EXPR) |
| 4417 | |
| 4418 | Enter BLOCK until EXPR returns false. The first form may avoid entering |
| 4419 | the BLOCK, the second form enters the BLOCK at least once. |
| 4420 | |
| 4421 | See L</do>, L</while>, and L</for>. |
| 4422 | |
| 4423 | See also L<perlsyn>. |
| 4424 | |
| 4425 | =item untie VARIABLE |
| 4426 | |
| 4427 | Breaks the binding between a variable and a package. (See C<tie()>.) |
| 4428 | |
| 4429 | =item use Module LIST |
| 4430 | |
| 4431 | =item use Module |
| 4432 | |
| 4433 | =item use Module VERSION LIST |
| 4434 | |
| 4435 | =item use VERSION |
| 4436 | |
| 4437 | Imports some semantics into the current package from the named module, |
| 4438 | generally by aliasing certain subroutine or variable names into your |
| 4439 | package. It is exactly equivalent to |
| 4440 | |
| 4441 | BEGIN { require Module; import Module LIST; } |
| 4442 | |
| 4443 | except that Module I<must> be a bareword. |
| 4444 | |
| 4445 | If the first argument to C<use> is a number, it is treated as a version |
| 4446 | number instead of a module name. If the version of the Perl interpreter |
| 4447 | is less than VERSION, then an error message is printed and Perl exits |
| 4448 | immediately. This is often useful if you need to check the current |
| 4449 | Perl version before C<use>ing library modules that have changed in |
| 4450 | incompatible ways from older versions of Perl. (We try not to do |
| 4451 | this more than we have to.) |
| 4452 | |
| 4453 | The C<BEGIN> forces the C<require> and C<import()> to happen at compile time. The |
| 4454 | C<require> makes sure the module is loaded into memory if it hasn't been |
| 4455 | yet. The C<import()> is not a builtin--it's just an ordinary static method |
| 4456 | call into the "C<Module>" package to tell the module to import the list of |
| 4457 | features back into the current package. The module can implement its |
| 4458 | C<import()> method any way it likes, though most modules just choose to |
| 4459 | derive their C<import()> method via inheritance from the C<Exporter> class that |
| 4460 | is defined in the C<Exporter> module. See L<Exporter>. If no C<import()> |
| 4461 | method can be found then the error is currently silently ignored. This |
| 4462 | may change to a fatal error in a future version. |
| 4463 | |
| 4464 | If you don't want your namespace altered, explicitly supply an empty list: |
| 4465 | |
| 4466 | use Module (); |
| 4467 | |
| 4468 | That is exactly equivalent to |
| 4469 | |
| 4470 | BEGIN { require Module } |
| 4471 | |
| 4472 | If the VERSION argument is present between Module and LIST, then the |
| 4473 | C<use> will call the VERSION method in class Module with the given |
| 4474 | version as an argument. The default VERSION method, inherited from |
| 4475 | the Universal class, croaks if the given version is larger than the |
| 4476 | value of the variable C<$Module::VERSION>. (Note that there is not a |
| 4477 | comma after VERSION!) |
| 4478 | |
| 4479 | Because this is a wide-open interface, pragmas (compiler directives) |
| 4480 | are also implemented this way. Currently implemented pragmas are: |
| 4481 | |
| 4482 | use integer; |
| 4483 | use diagnostics; |
| 4484 | use sigtrap qw(SEGV BUS); |
| 4485 | use strict qw(subs vars refs); |
| 4486 | use subs qw(afunc blurfl); |
| 4487 | |
| 4488 | Some of these these pseudo-modules import semantics into the current |
| 4489 | block scope (like C<strict> or C<integer>, unlike ordinary modules, |
| 4490 | which import symbols into the current package (which are effective |
| 4491 | through the end of the file). |
| 4492 | |
| 4493 | There's a corresponding "C<no>" command that unimports meanings imported |
| 4494 | by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import()>. |
| 4495 | |
| 4496 | no integer; |
| 4497 | no strict 'refs'; |
| 4498 | |
| 4499 | If no C<unimport()> method can be found the call fails with a fatal error. |
| 4500 | |
| 4501 | See L<perlmod> for a list of standard modules and pragmas. |
| 4502 | |
| 4503 | =item utime LIST |
| 4504 | |
| 4505 | Changes the access and modification times on each file of a list of |
| 4506 | files. The first two elements of the list must be the NUMERICAL access |
| 4507 | and modification times, in that order. Returns the number of files |
| 4508 | successfully changed. The inode modification time of each file is set |
| 4509 | to the current time. This code has the same effect as the "C<touch>" |
| 4510 | command if the files already exist: |
| 4511 | |
| 4512 | #!/usr/bin/perl |
| 4513 | $now = time; |
| 4514 | utime $now, $now, @ARGV; |
| 4515 | |
| 4516 | =item values HASH |
| 4517 | |
| 4518 | Returns a list consisting of all the values of the named hash. (In a |
| 4519 | scalar context, returns the number of values.) The values are |
| 4520 | returned in an apparently random order. The actual random order is |
| 4521 | subject to change in future versions of perl, but it is guaranteed to |
| 4522 | be the same order as either the C<keys()> or C<each()> function would |
| 4523 | produce on the same (unmodified) hash. |
| 4524 | |
| 4525 | As a side effect, it resets HASH's iterator. See also C<keys()>, C<each()>, |
| 4526 | and C<sort()>. |
| 4527 | |
| 4528 | =item vec EXPR,OFFSET,BITS |
| 4529 | |
| 4530 | Treats the string in EXPR as a vector of unsigned integers, and |
| 4531 | returns the value of the bit field specified by OFFSET. BITS specifies |
| 4532 | the number of bits that are reserved for each entry in the bit |
| 4533 | vector. This must be a power of two from 1 to 32. C<vec()> may also be |
| 4534 | assigned to, in which case parentheses are needed to give the expression |
| 4535 | the correct precedence as in |
| 4536 | |
| 4537 | vec($image, $max_x * $x + $y, 8) = 3; |
| 4538 | |
| 4539 | Vectors created with C<vec()> can also be manipulated with the logical |
| 4540 | operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is |
| 4541 | desired when both operands are strings. |
| 4542 | |
| 4543 | The following code will build up an ASCII string saying C<'PerlPerlPerl'>. |
| 4544 | The comments show the string after each step. Note that this code works |
| 4545 | in the same way on big-endian or little-endian machines. |
| 4546 | |
| 4547 | my $foo = ''; |
| 4548 | vec($foo, 0, 32) = 0x5065726C; # 'Perl' |
| 4549 | vec($foo, 2, 16) = 0x5065; # 'PerlPe' |
| 4550 | vec($foo, 3, 16) = 0x726C; # 'PerlPerl' |
| 4551 | vec($foo, 8, 8) = 0x50; # 'PerlPerlP' |
| 4552 | vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' |
| 4553 | vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" |
| 4554 | vec($foo, 21, 4) = 7; # 'PerlPerlPer' |
| 4555 | # 'r' is "\x72" |
| 4556 | vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" |
| 4557 | vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" |
| 4558 | vec($foo, 94, 1) = 1; # 'PerlPerlPerl' |
| 4559 | # 'l' is "\x6c" |
| 4560 | |
| 4561 | To transform a bit vector into a string or array of 0's and 1's, use these: |
| 4562 | |
| 4563 | $bits = unpack("b*", $vector); |
| 4564 | @bits = split(//, unpack("b*", $vector)); |
| 4565 | |
| 4566 | If you know the exact length in bits, it can be used in place of the C<*>. |
| 4567 | |
| 4568 | =item wait |
| 4569 | |
| 4570 | Waits for a child process to terminate and returns the pid of the |
| 4571 | deceased process, or C<-1> if there are no child processes. The status is |
| 4572 | returned in C<$?>. Note that a return value of C<-1> could mean that |
| 4573 | child processes are being automatically reaped, as described in L<perlipc>. |
| 4574 | |
| 4575 | =item waitpid PID,FLAGS |
| 4576 | |
| 4577 | Waits for a particular child process to terminate and returns the pid |
| 4578 | of the deceased process, or C<-1> if there is no such child process. The |
| 4579 | status is returned in C<$?>. If you say |
| 4580 | |
| 4581 | use POSIX ":sys_wait_h"; |
| 4582 | #... |
| 4583 | waitpid(-1,&WNOHANG); |
| 4584 | |
| 4585 | then you can do a non-blocking wait for any process. Non-blocking wait |
| 4586 | is available on machines supporting either the waitpid(2) or |
| 4587 | wait4(2) system calls. However, waiting for a particular pid with |
| 4588 | FLAGS of C<0> is implemented everywhere. (Perl emulates the system call |
| 4589 | by remembering the status values of processes that have exited but have |
| 4590 | not been harvested by the Perl script yet.) |
| 4591 | |
| 4592 | Note that a return value of C<-1> could mean that child processes are being |
| 4593 | automatically reaped. See L<perlipc> for details, and for other examples. |
| 4594 | |
| 4595 | =item wantarray |
| 4596 | |
| 4597 | Returns TRUE if the context of the currently executing subroutine is |
| 4598 | looking for a list value. Returns FALSE if the context is looking |
| 4599 | for a scalar. Returns the undefined value if the context is looking |
| 4600 | for no value (void context). |
| 4601 | |
| 4602 | return unless defined wantarray; # don't bother doing more |
| 4603 | my @a = complex_calculation(); |
| 4604 | return wantarray ? @a : "@a"; |
| 4605 | |
| 4606 | =item warn LIST |
| 4607 | |
| 4608 | Produces a message on STDERR just like C<die()>, but doesn't exit or throw |
| 4609 | an exception. |
| 4610 | |
| 4611 | If LIST is empty and C<$@> already contains a value (typically from a |
| 4612 | previous eval) that value is used after appending C<"\t...caught"> |
| 4613 | to C<$@>. This is useful for staying almost, but not entirely similar to |
| 4614 | C<die()>. |
| 4615 | |
| 4616 | If C<$@> is empty then the string C<"Warning: Something's wrong"> is used. |
| 4617 | |
| 4618 | No message is printed if there is a C<$SIG{__WARN__}> handler |
| 4619 | installed. It is the handler's responsibility to deal with the message |
| 4620 | as it sees fit (like, for instance, converting it into a C<die()>). Most |
| 4621 | handlers must therefore make arrangements to actually display the |
| 4622 | warnings that they are not prepared to deal with, by calling C<warn()> |
| 4623 | again in the handler. Note that this is quite safe and will not |
| 4624 | produce an endless loop, since C<__WARN__> hooks are not called from |
| 4625 | inside one. |
| 4626 | |
| 4627 | You will find this behavior is slightly different from that of |
| 4628 | C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can |
| 4629 | instead call C<die()> again to change it). |
| 4630 | |
| 4631 | Using a C<__WARN__> handler provides a powerful way to silence all |
| 4632 | warnings (even the so-called mandatory ones). An example: |
| 4633 | |
| 4634 | # wipe out *all* compile-time warnings |
| 4635 | BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } |
| 4636 | my $foo = 10; |
| 4637 | my $foo = 20; # no warning about duplicate my $foo, |
| 4638 | # but hey, you asked for it! |
| 4639 | # no compile-time or run-time warnings before here |
| 4640 | $DOWARN = 1; |
| 4641 | |
| 4642 | # run-time warnings enabled after here |
| 4643 | warn "\$foo is alive and $foo!"; # does show up |
| 4644 | |
| 4645 | See L<perlvar> for details on setting C<%SIG> entries, and for more |
| 4646 | examples. |
| 4647 | |
| 4648 | =item while (EXPR) BLOCK |
| 4649 | |
| 4650 | =item do BLOCK while (EXPR) |
| 4651 | |
| 4652 | Enter BLOCK while EXPR is true. The first form may avoid entering the |
| 4653 | BLOCK, the second form enters the BLOCK at least once. |
| 4654 | |
| 4655 | See also L<perlsyn>, L</for>, L</until>, and L</continue>. |
| 4656 | |
| 4657 | =item write FILEHANDLE |
| 4658 | |
| 4659 | =item write EXPR |
| 4660 | |
| 4661 | =item write |
| 4662 | |
| 4663 | Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, |
| 4664 | using the format associated with that file. By default the format for |
| 4665 | a file is the one having the same name as the filehandle, but the |
| 4666 | format for the current output channel (see the C<select()> function) may be set |
| 4667 | explicitly by assigning the name of the format to the C<$~> variable. |
| 4668 | |
| 4669 | Top of form processing is handled automatically: if there is |
| 4670 | insufficient room on the current page for the formatted record, the |
| 4671 | page is advanced by writing a form feed, a special top-of-page format |
| 4672 | is used to format the new page header, and then the record is written. |
| 4673 | By default the top-of-page format is the name of the filehandle with |
| 4674 | "_TOP" appended, but it may be dynamically set to the format of your |
| 4675 | choice by assigning the name to the C<$^> variable while the filehandle is |
| 4676 | selected. The number of lines remaining on the current page is in |
| 4677 | variable C<$->, which can be set to C<0> to force a new page. |
| 4678 | |
| 4679 | If FILEHANDLE is unspecified, output goes to the current default output |
| 4680 | channel, which starts out as STDOUT but may be changed by the |
| 4681 | C<select()> operator. If the FILEHANDLE is an EXPR, then the expression |
| 4682 | is evaluated and the resulting string is used to look up the name of |
| 4683 | the FILEHANDLE at run time. For more on formats, see L<perlform>. |
| 4684 | |
| 4685 | Note that write is I<NOT> the opposite of C<read()>. Unfortunately. |
| 4686 | |
| 4687 | =item y/// |
| 4688 | |
| 4689 | The transliteration operator. Same as C<tr///>. See L<perlop>. |
| 4690 | |
| 4691 | =back |