3 perlopentut - tutorial on opening things in Perl
7 Perl has two simple, built-in ways to open files: the shell way for
8 convenience, and the C way for precision. The choice is yours.
10 =head1 Open E<agrave> la shell
12 Perl's C<open> function was designed to mimic the way command-line
13 redirection in the shell works. Here are some basic examples
16 $ myprogram file1 file2 file3
17 $ myprogram < inputfile
18 $ myprogram > outputfile
19 $ myprogram >> outputfile
20 $ myprogram | otherprogram
21 $ otherprogram | myprogram
23 And here are some more advanced examples:
25 $ otherprogram | myprogram f1 - f2
26 $ otherprogram 2>&1 | myprogram -
30 Programmers accustomed to constructs like those above can take comfort
31 in learning that Perl directly supports these familiar constructs using
32 virtually the same syntax as the shell.
36 The C<open> function takes two arguments: the first is a filehandle,
37 and the second is a single string comprising both what to open and how
38 to open it. C<open> returns true when it works, and when it fails,
39 returns a false value and sets the special variable $! to reflect
40 the system error. If the filehandle was previously opened, it will
41 be implicitly closed first.
45 open(INFO, "datafile") || die("can't open datafile: $!");
46 open(INFO, "< datafile") || die("can't open datafile: $!");
47 open(RESULTS,"> runstats") || die("can't open runstats: $!");
48 open(LOG, ">> logfile ") || die("can't open logfile: $!");
50 If you prefer the low-punctuation version, you could write that this way:
52 open INFO, "< datafile" or die "can't open datafile: $!";
53 open RESULTS,"> runstats" or die "can't open runstats: $!";
54 open LOG, ">> logfile " or die "can't open logfile: $!";
56 A few things to notice. First, the leading less-than is optional.
57 If omitted, Perl assumes that you want to open the file for reading.
59 The other important thing to notice is that, just as in the shell,
60 any white space before or after the filename is ignored. This is good,
61 because you wouldn't want these to do different things:
63 open INFO, "<datafile"
64 open INFO, "< datafile"
65 open INFO, "< datafile"
67 Ignoring surround whitespace also helps for when you read a filename in
68 from a different file, and forget to trim it before opening:
70 $filename = <INFO>; # oops, \n still there
71 open(EXTRA, "< $filename") || die "can't open $filename: $!";
73 This is not a bug, but a feature. Because C<open> mimics the shell in
74 its style of using redirection arrows to specify how to open the file, it
75 also does so with respect to extra white space around the filename itself
76 as well. For accessing files with naughty names, see
77 L<"Dispelling the Dweomer">.
81 In C, when you want to open a file using the standard I/O library,
82 you use the C<fopen> function, but when opening a pipe, you use the
83 C<popen> function. But in the shell, you just use a different redirection
84 character. That's also the case for Perl. The C<open> call
85 remains the same--just its argument differs.
87 If the leading character is a pipe symbol, C<open> starts up a new
88 command and open a write-only filehandle leading into that command.
89 This lets you write into that handle and have what you write show up on
90 that command's standard input. For example:
92 open(PRINTER, "| lpr -Plp1") || die "can't run lpr: $!";
93 print PRINTER "stuff\n";
94 close(PRINTER) || die "can't close lpr: $!";
96 If the trailing character is a pipe, you start up a new command and open a
97 read-only filehandle leading out of that command. This lets whatever that
98 command writes to its standard output show up on your handle for reading.
101 open(NET, "netstat -i -n |") || die "can't fun netstat: $!";
102 while (<NET>) { } # do something with input
103 close(NET) || die "can't close netstat: $!";
105 What happens if you try to open a pipe to or from a non-existent
106 command? If possible, Perl will detect the failure and set C<$!> as
107 usual. But if the command contains special shell characters, such as
108 C<E<gt>> or C<*>, called 'metacharacters', Perl does not execute the
109 command directly. Instead, Perl runs the shell, which then tries to
110 run the command. This means that it's the shell that gets the error
111 indication. In such a case, the C<open> call will only indicate
112 failure if Perl can't even run the shell. See L<perlfaq8/"How can I
113 capture STDERR from an external command?"> to see how to cope with
114 this. There's also an explanation in L<perlipc>.
116 If you would like to open a bidirectional pipe, the IPC::Open2
117 library will handle this for you. Check out
118 L<perlipc/"Bidirectional Communication with Another Process">
120 =head2 The Minus File
122 Again following the lead of the standard shell utilities, Perl's
123 C<open> function treats a file whose name is a single minus, "-", in a
124 special way. If you open minus for reading, it really means to access
125 the standard input. If you open minus for writing, it really means to
126 access the standard output.
128 If minus can be used as the default input or default output, what happens
129 if you open a pipe into or out of minus? What's the default command it
130 would run? The same script as you're currently running! This is actually
131 a stealth C<fork> hidden inside an C<open> call. See
132 L<perlipc/"Safe Pipe Opens"> for details.
134 =head2 Mixing Reads and Writes
136 It is possible to specify both read and write access. All you do is
137 add a "+" symbol in front of the redirection. But as in the shell,
138 using a less-than on a file never creates a new file; it only opens an
139 existing one. On the other hand, using a greater-than always clobbers
140 (truncates to zero length) an existing file, or creates a brand-new one
141 if there isn't an old one. Adding a "+" for read-write doesn't affect
142 whether it only works on existing files or always clobbers existing ones.
144 open(WTMP, "+< /usr/adm/wtmp")
145 || die "can't open /usr/adm/wtmp: $!";
147 open(SCREEN, "+> /tmp/lkscreen")
148 || die "can't open /tmp/lkscreen: $!";
150 open(LOGFILE, "+>> /tmp/applog"
151 || die "can't open /tmp/applog: $!";
153 The first one won't create a new file, and the second one will always
154 clobber an old one. The third one will create a new file if necessary
155 and not clobber an old one, and it will allow you to read at any point
156 in the file, but all writes will always go to the end. In short,
157 the first case is substantially more common than the second and third
158 cases, which are almost always wrong. (If you know C, the plus in
159 Perl's C<open> is historically derived from the one in C's fopen(3S),
160 which it ultimately calls.)
162 In fact, when it comes to updating a file, unless you're working on
163 a binary file as in the WTMP case above, you probably don't want to
164 use this approach for updating. Instead, Perl's B<-i> flag comes to
165 the rescue. The following command takes all the C, C++, or yacc source
166 or header files and changes all their foo's to bar's, leaving
167 the old version in the original file name with a ".orig" tacked
170 $ perl -i.orig -pe 's/\bfoo\b/bar/g' *.[Cchy]
172 This is a short cut for some renaming games that are really
173 the best way to update textfiles. See the second question in
174 L<perlfaq5> for more details.
178 One of the most common uses for C<open> is one you never
179 even notice. When you process the ARGV filehandle using
180 C<< <ARGV> >>, Perl actually does an implicit open
181 on each file in @ARGV. Thus a program called like this:
183 $ myprogram file1 file2 file3
185 Can have all its files opened and processed one at a time
186 using a construct no more complex than:
189 # do something with $_
192 If @ARGV is empty when the loop first begins, Perl pretends you've opened
193 up minus, that is, the standard input. In fact, $ARGV, the currently
194 open file during C<< <ARGV> >> processing, is even set to "-"
195 in these circumstances.
197 You are welcome to pre-process your @ARGV before starting the loop to
198 make sure it's to your liking. One reason to do this might be to remove
199 command options beginning with a minus. While you can always roll the
200 simple ones by hand, the Getopts modules are good for this.
204 # -v, -D, -o ARG, sets $opt_v, $opt_D, $opt_o
207 # -v, -D, -o ARG, sets $args{v}, $args{D}, $args{o}
208 getopts("vDo:", \%args);
210 Or the standard Getopt::Long module to permit named arguments:
213 GetOptions( "verbose" => \$verbose, # --verbose
214 "Debug" => \$debug, # --Debug
215 "output=s" => \$output );
216 # --output=somestring or --output somestring
218 Another reason for preprocessing arguments is to make an empty
219 argument list default to all files:
221 @ARGV = glob("*") unless @ARGV;
223 You could even filter out all but plain, text files. This is a bit
224 silent, of course, and you might prefer to mention them on the way.
226 @ARGV = grep { -f && -T } @ARGV;
228 If you're using the B<-n> or B<-p> command-line options, you
229 should put changes to @ARGV in a C<BEGIN{}> block.
231 Remember that a normal C<open> has special properties, in that it might
232 call fopen(3S) or it might called popen(3S), depending on what its
233 argument looks like; that's why it's sometimes called "magic open".
236 $pwdinfo = `domainname` =~ /^(\(none\))?$/
241 or die "can't open $pwdinfo: $!";
243 This sort of thing also comes into play in filter processing. Because
244 C<< <ARGV> >> processing employs the normal, shell-style Perl C<open>,
245 it respects all the special things we've already seen:
247 $ myprogram f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
249 That program will read from the file F<f1>, the process F<cmd1>, standard
250 input (F<tmpfile> in this case), the F<f2> file, the F<cmd2> command,
251 and finally the F<f3> file.
253 Yes, this also means that if you have a file named "-" (and so on) in
254 your directory, that they won't be processed as literal files by C<open>.
255 You'll need to pass them as "./-" much as you would for the I<rm> program.
256 Or you could use C<sysopen> as described below.
258 One of the more interesting applications is to change files of a certain
259 name into pipes. For example, to autoprocess gzipped or compressed
260 files by decompressing them with I<gzip>:
262 @ARGV = map { /^\.(gz|Z)$/ ? "gzip -dc $_ |" : $_ } @ARGV;
264 Or, if you have the I<GET> program installed from LWP,
265 you can fetch URLs before processing them:
267 @ARGV = map { m#^\w+://# ? "GET $_ |" : $_ } @ARGV;
269 It's not for nothing that this is called magic C<< <ARGV> >>.
272 =head1 Open E<agrave> la C
274 If you want the convenience of the shell, then Perl's C<open> is
275 definitely the way to go. On the other hand, if you want finer precision
276 than C's simplistic fopen(3S) provides, then you should look to Perl's
277 C<sysopen>, which is a direct hook into the open(2) system call.
278 That does mean it's a bit more involved, but that's the price of
281 C<sysopen> takes 3 (or 4) arguments.
283 sysopen HANDLE, PATH, FLAGS, [MASK]
285 The HANDLE argument is a filehandle just as with C<open>. The PATH is
286 a literal path, one that doesn't pay attention to any greater-thans or
287 less-thans or pipes or minuses, nor ignore white space. If it's there,
288 it's part of the path. The FLAGS argument contains one or more values
289 derived from the Fcntl module that have been or'd together using the
290 bitwise "|" operator. The final argument, the MASK, is optional; if
291 present, it is combined with the user's current umask for the creation
292 mode of the file. You should usually omit this.
294 Although the traditional values of read-only, write-only, and read-write
295 are 0, 1, and 2 respectively, this is known not to hold true on some
296 systems. Instead, it's best to load in the appropriate constants first
297 from the Fcntl module, which supplies the following standard flags:
301 O_RDWR Read and write
302 O_CREAT Create the file if it doesn't exist
303 O_EXCL Fail if the file already exists
304 O_APPEND Append to the file
305 O_TRUNC Truncate the file
306 O_NONBLOCK Non-blocking access
308 Less common flags that are sometimes available on some operating
309 systems include C<O_BINARY>, C<O_TEXT>, C<O_SHLOCK>, C<O_EXLOCK>,
310 C<O_DEFER>, C<O_SYNC>, C<O_ASYNC>, C<O_DSYNC>, C<O_RSYNC>,
311 C<O_NOCTTY>, C<O_NDELAY> and C<O_LARGEFILE>. Consult your open(2)
312 manpage or its local equivalent for details. (Note: starting from
313 Perl release 5.6 the O_LARGEFILE flag, if available, is automatically
314 added to the sysopen() flags because large files are the default.)
316 Here's how to use C<sysopen> to emulate the simple C<open> calls we had
317 before. We'll omit the C<|| die $!> checks for clarity, but make sure
318 you always check the return values in real code. These aren't quite
319 the same, since C<open> will trim leading and trailing white space,
320 but you'll get the idea:
322 To open a file for reading:
325 sysopen(FH, $path, O_RDONLY);
327 To open a file for writing, creating a new file if needed or else truncating
331 sysopen(FH, $path, O_WRONLY | O_TRUNC | O_CREAT);
333 To open a file for appending, creating one if necessary:
335 open(FH, ">> $path");
336 sysopen(FH, $path, O_WRONLY | O_APPEND | O_CREAT);
338 To open a file for update, where the file must already exist:
340 open(FH, "+< $path");
341 sysopen(FH, $path, O_RDWR);
343 And here are things you can do with C<sysopen> that you cannot do with
344 a regular C<open>. As you see, it's just a matter of controlling the
345 flags in the third argument.
347 To open a file for writing, creating a new file which must not previously
350 sysopen(FH, $path, O_WRONLY | O_EXCL | O_CREAT);
352 To open a file for appending, where that file must already exist:
354 sysopen(FH, $path, O_WRONLY | O_APPEND);
356 To open a file for update, creating a new file if necessary:
358 sysopen(FH, $path, O_RDWR | O_CREAT);
360 To open a file for update, where that file must not already exist:
362 sysopen(FH, $path, O_RDWR | O_EXCL | O_CREAT);
364 To open a file without blocking, creating one if necessary:
366 sysopen(FH, $path, O_WRONLY | O_NONBLOCK | O_CREAT);
368 =head2 Permissions E<agrave> la mode
370 If you omit the MASK argument to C<sysopen>, Perl uses the octal value
371 0666. The normal MASK to use for executables and directories should
372 be 0777, and for anything else, 0666.
374 Why so permissive? Well, it isn't really. The MASK will be modified
375 by your process's current C<umask>. A umask is a number representing
376 I<disabled> permissions bits; that is, bits that will not be turned on
377 in the created files' permissions field.
379 For example, if your C<umask> were 027, then the 020 part would
380 disable the group from writing, and the 007 part would disable others
381 from reading, writing, or executing. Under these conditions, passing
382 C<sysopen> 0666 would create a file with mode 0640, since C<0666 &~ 027>
385 You should seldom use the MASK argument to C<sysopen()>. That takes
386 away the user's freedom to choose what permission new files will have.
387 Denying choice is almost always a bad thing. One exception would be for
388 cases where sensitive or private data is being stored, such as with mail
389 folders, cookie files, and internal temporary files.
391 =head1 Obscure Open Tricks
393 =head2 Re-Opening Files (dups)
395 Sometimes you already have a filehandle open, and want to make another
396 handle that's a duplicate of the first one. In the shell, we place an
397 ampersand in front of a file descriptor number when doing redirections.
398 For example, C<< 2>&1 >> makes descriptor 2 (that's STDERR in Perl)
399 be redirected into descriptor 1 (which is usually Perl's STDOUT).
400 The same is essentially true in Perl: a filename that begins with an
401 ampersand is treated instead as a file descriptor if a number, or as a
402 filehandle if a string.
404 open(SAVEOUT, ">&SAVEERR") || die "couldn't dup SAVEERR: $!";
405 open(MHCONTEXT, "<&4") || die "couldn't dup fd4: $!";
407 That means that if a function is expecting a filename, but you don't
408 want to give it a filename because you already have the file open, you
409 can just pass the filehandle with a leading ampersand. It's best to
410 use a fully qualified handle though, just in case the function happens
411 to be in a different package:
413 somefunction("&main::LOGFILE");
415 This way if somefunction() is planning on opening its argument, it can
416 just use the already opened handle. This differs from passing a handle,
417 because with a handle, you don't open the file. Here you have something
418 you can pass to open.
420 If you have one of those tricky, newfangled I/O objects that the C++
421 folks are raving about, then this doesn't work because those aren't a
422 proper filehandle in the native Perl sense. You'll have to use fileno()
423 to pull out the proper descriptor number, assuming you can:
426 $handle = IO::Socket::INET->new("www.perl.com:80");
427 $fd = $handle->fileno;
428 somefunction("&$fd"); # not an indirect function call
430 It can be easier (and certainly will be faster) just to use real
434 local *REMOTE = IO::Socket::INET->new("www.perl.com:80");
435 die "can't connect" unless defined(fileno(REMOTE));
436 somefunction("&main::REMOTE");
438 If the filehandle or descriptor number is preceded not just with a simple
439 "&" but rather with a "&=" combination, then Perl will not create a
440 completely new descriptor opened to the same place using the dup(2)
441 system call. Instead, it will just make something of an alias to the
442 existing one using the fdopen(3S) library call This is slightly more
443 parsimonious of systems resources, although this is less a concern
444 these days. Here's an example of that:
446 $fd = $ENV{"MHCONTEXTFD"};
447 open(MHCONTEXT, "<&=$fd") or die "couldn't fdopen $fd: $!";
449 If you're using magic C<< <ARGV> >>, you could even pass in as a
450 command line argument in @ARGV something like C<"<&=$MHCONTEXTFD">,
451 but we've never seen anyone actually do this.
453 =head2 Dispelling the Dweomer
455 Perl is more of a DWIMmer language than something like Java--where DWIM
456 is an acronym for "do what I mean". But this principle sometimes leads
457 to more hidden magic than one knows what to do with. In this way, Perl
458 is also filled with I<dweomer>, an obscure word meaning an enchantment.
459 Sometimes, Perl's DWIMmer is just too much like dweomer for comfort.
461 If magic C<open> is a bit too magical for you, you don't have to turn
462 to C<sysopen>. To open a file with arbitrary weird characters in
463 it, it's necessary to protect any leading and trailing whitespace.
464 Leading whitespace is protected by inserting a C<"./"> in front of a
465 filename that starts with whitespace. Trailing whitespace is protected
466 by appending an ASCII NUL byte (C<"\0">) at the end off the string.
468 $file =~ s#^(\s)#./$1#;
469 open(FH, "< $file\0") || die "can't open $file: $!";
471 This assumes, of course, that your system considers dot the current
472 working directory, slash the directory separator, and disallows ASCII
473 NULs within a valid filename. Most systems follow these conventions,
474 including all POSIX systems as well as proprietary Microsoft systems.
475 The only vaguely popular system that doesn't work this way is the
476 proprietary Macintosh system, which uses a colon where the rest of us
477 use a slash. Maybe C<sysopen> isn't such a bad idea after all.
479 If you want to use C<< <ARGV> >> processing in a totally boring
480 and non-magical way, you could do this first:
482 # "Sam sat on the ground and put his head in his hands.
483 # 'I wish I had never come here, and I don't want to see
484 # no more magic,' he said, and fell silent."
493 But be warned that users will not appreciate being unable to use "-"
494 to mean standard input, per the standard convention.
496 =head2 Paths as Opens
498 You've probably noticed how Perl's C<warn> and C<die> functions can
499 produce messages like:
501 Some warning at scriptname line 29, <FH> line 7.
503 That's because you opened a filehandle FH, and had read in seven records
504 from it. But what was the name of the file, not the handle?
506 If you aren't running with C<strict refs>, or if you've turn them off
507 temporarily, then all you have to do is this:
509 open($path, "< $path") || die "can't open $path: $!";
514 Since you're using the pathname of the file as its handle,
515 you'll get warnings more like
517 Some warning at scriptname line 29, </etc/motd> line 7.
519 =head2 Single Argument Open
521 Remember how we said that Perl's open took two arguments? That was a
522 passive prevarication. You see, it can also take just one argument.
523 If and only if the variable is a global variable, not a lexical, you
524 can pass C<open> just one argument, the filehandle, and it will
525 get the path from the global scalar variable of the same name.
528 open FILE or die "can't open $FILE: $!";
533 Why is this here? Someone has to cater to the hysterical porpoises.
534 It's something that's been in Perl since the very beginning, if not
537 =head2 Playing with STDIN and STDOUT
539 One clever move with STDOUT is to explicitly close it when you're done
542 END { close(STDOUT) || die "can't close stdout: $!" }
544 If you don't do this, and your program fills up the disk partition due
545 to a command line redirection, it won't report the error exit with a
548 You don't have to accept the STDIN and STDOUT you were given. You are
549 welcome to reopen them if you'd like.
551 open(STDIN, "< datafile")
552 || die "can't open datafile: $!";
554 open(STDOUT, "> output")
555 || die "can't open output: $!";
557 And then these can be read directly or passed on to subprocesses.
558 This makes it look as though the program were initially invoked
559 with those redirections from the command line.
561 It's probably more interesting to connect these to pipes. For example:
563 $pager = $ENV{PAGER} || "(less || more)";
564 open(STDOUT, "| $pager")
565 || die "can't fork a pager: $!";
567 This makes it appear as though your program were called with its stdout
568 already piped into your pager. You can also use this kind of thing
569 in conjunction with an implicit fork to yourself. You might do this
570 if you would rather handle the post processing in your own program,
571 just in a different process:
579 my $lines = shift || 20;
580 return unless $pid = open(STDOUT, "|-");
581 die "cannot fork: $!" unless defined $pid;
584 last if --$lines < 0;
589 This technique can be applied to repeatedly push as many filters on your
590 output stream as you wish.
592 =head1 Other I/O Issues
594 These topics aren't really arguments related to C<open> or C<sysopen>,
595 but they do affect what you do with your open files.
597 =head2 Opening Non-File Files
599 When is a file not a file? Well, you could say when it exists but
600 isn't a plain file. We'll check whether it's a symbolic link first,
603 if (-l $file || ! -f _) {
604 print "$file is not a plain file\n";
607 What other kinds of files are there than, well, files? Directories,
608 symbolic links, named pipes, Unix-domain sockets, and block and character
609 devices. Those are all files, too--just not I<plain> files. This isn't
610 the same issue as being a text file. Not all text files are plain files.
611 Not all plain files are textfiles. That's why there are separate C<-f>
612 and C<-T> file tests.
614 To open a directory, you should use the C<opendir> function, then
615 process it with C<readdir>, carefully restoring the directory
618 opendir(DIR, $dirname) or die "can't opendir $dirname: $!";
619 while (defined($file = readdir(DIR))) {
620 # do something with "$dirname/$file"
624 If you want to process directories recursively, it's better to use the
625 File::Find module. For example, this prints out all files recursively,
626 add adds a slash to their names if the file is a directory.
628 @ARGV = qw(.) unless @ARGV;
630 find sub { print $File::Find::name, -d && '/', "\n" }, @ARGV;
632 This finds all bogus symbolic links beneath a particular directory:
634 find sub { print "$File::Find::name\n" if -l && !-e }, $dir;
636 As you see, with symbolic links, you can just pretend that it is
637 what it points to. Or, if you want to know I<what> it points to, then
638 C<readlink> is called for:
641 if (defined($whither = readlink($file))) {
642 print "$file points to $whither\n";
644 print "$file points nowhere: $!\n";
648 Named pipes are a different matter. You pretend they're regular files,
649 but their opens will normally block until there is both a reader and
650 a writer. You can read more about them in L<perlipc/"Named Pipes">.
651 Unix-domain sockets are rather different beasts as well; they're
652 described in L<perlipc/"Unix-Domain TCP Clients and Servers">.
654 When it comes to opening devices, it can be easy and it can tricky.
655 We'll assume that if you're opening up a block device, you know what
656 you're doing. The character devices are more interesting. These are
657 typically used for modems, mice, and some kinds of printers. This is
658 described in L<perlfaq8/"How do I read and write the serial port?">
659 It's often enough to open them carefully:
661 sysopen(TTYIN, "/dev/ttyS1", O_RDWR | O_NDELAY | O_NOCTTY)
662 # (O_NOCTTY no longer needed on POSIX systems)
663 or die "can't open /dev/ttyS1: $!";
664 open(TTYOUT, "+>&TTYIN")
665 or die "can't dup TTYIN: $!";
667 $ofh = select(TTYOUT); $| = 1; select($ofh);
669 print TTYOUT "+++at\015";
672 With descriptors that you haven't opened using C<sysopen>, such as a
673 socket, you can set them to be non-blocking using C<fcntl>:
676 fcntl(Connection, F_SETFL, O_NONBLOCK)
677 or die "can't set non blocking: $!";
679 Rather than losing yourself in a morass of twisting, turning C<ioctl>s,
680 all dissimilar, if you're going to manipulate ttys, it's best to
681 make calls out to the stty(1) program if you have it, or else use the
682 portable POSIX interface. To figure this all out, you'll need to read the
683 termios(3) manpage, which describes the POSIX interface to tty devices,
684 and then L<POSIX>, which describes Perl's interface to POSIX. There are
685 also some high-level modules on CPAN that can help you with these games.
686 Check out Term::ReadKey and Term::ReadLine.
688 What else can you open? To open a connection using sockets, you won't use
689 one of Perl's two open functions. See
690 L<perlipc/"Sockets: Client/Server Communication"> for that. Here's an
691 example. Once you have it, you can use FH as a bidirectional filehandle.
694 local *FH = IO::Socket::INET->new("www.perl.com:80");
696 For opening up a URL, the LWP modules from CPAN are just what
697 the doctor ordered. There's no filehandle interface, but
698 it's still easy to get the contents of a document:
701 $doc = get('http://www.linpro.no/lwp/');
705 On certain legacy systems with what could charitably be called terminally
706 convoluted (some would say broken) I/O models, a file isn't a file--at
707 least, not with respect to the C standard I/O library. On these old
708 systems whose libraries (but not kernels) distinguish between text and
709 binary streams, to get files to behave properly you'll have to bend over
710 backwards to avoid nasty problems. On such infelicitous systems, sockets
711 and pipes are already opened in binary mode, and there is currently no
712 way to turn that off. With files, you have more options.
714 Another option is to use the C<binmode> function on the appropriate
715 handles before doing regular I/O on them:
719 while (<STDIN>) { print }
721 Passing C<sysopen> a non-standard flag option will also open the file in
722 binary mode on those systems that support it. This is the equivalent of
723 opening the file normally, then calling C<binmode>ing on the handle.
725 sysopen(BINDAT, "records.data", O_RDWR | O_BINARY)
726 || die "can't open records.data: $!";
728 Now you can use C<read> and C<print> on that handle without worrying
729 about the system non-standard I/O library breaking your data. It's not
730 a pretty picture, but then, legacy systems seldom are. CP/M will be
731 with us until the end of days, and after.
733 On systems with exotic I/O systems, it turns out that, astonishingly
734 enough, even unbuffered I/O using C<sysread> and C<syswrite> might do
735 sneaky data mutilation behind your back.
737 while (sysread(WHENCE, $buf, 1024)) {
738 syswrite(WHITHER, $buf, length($buf));
741 Depending on the vicissitudes of your runtime system, even these calls
742 may need C<binmode> or C<O_BINARY> first. Systems known to be free of
743 such difficulties include Unix, the Mac OS, Plan9, and Inferno.
747 In a multitasking environment, you may need to be careful not to collide
748 with other processes who want to do I/O on the same files as others
749 are working on. You'll often need shared or exclusive locks
750 on files for reading and writing respectively. You might just
751 pretend that only exclusive locks exist.
753 Never use the existence of a file C<-e $file> as a locking indication,
754 because there is a race condition between the test for the existence of
755 the file and its creation. Atomicity is critical.
757 Perl's most portable locking interface is via the C<flock> function,
758 whose simplicity is emulated on systems that don't directly support it,
759 such as SysV or WindowsNT. The underlying semantics may affect how
760 it all works, so you should learn how C<flock> is implemented on your
761 system's port of Perl.
763 File locking I<does not> lock out another process that would like to
764 do I/O. A file lock only locks out others trying to get a lock, not
765 processes trying to do I/O. Because locks are advisory, if one process
766 uses locking and another doesn't, all bets are off.
768 By default, the C<flock> call will block until a lock is granted.
769 A request for a shared lock will be granted as soon as there is no
770 exclusive locker. A request for an exclusive lock will be granted as
771 soon as there is no locker of any kind. Locks are on file descriptors,
772 not file names. You can't lock a file until you open it, and you can't
773 hold on to a lock once the file has been closed.
775 Here's how to get a blocking shared lock on a file, typically used
779 use Fcntl qw(:DEFAULT :flock);
780 open(FH, "< filename") or die "can't open filename: $!";
781 flock(FH, LOCK_SH) or die "can't lock filename: $!";
784 You can get a non-blocking lock by using C<LOCK_NB>.
786 flock(FH, LOCK_SH | LOCK_NB)
787 or die "can't lock filename: $!";
789 This can be useful for producing more user-friendly behaviour by warning
790 if you're going to be blocking:
793 use Fcntl qw(:DEFAULT :flock);
794 open(FH, "< filename") or die "can't open filename: $!";
795 unless (flock(FH, LOCK_SH | LOCK_NB)) {
797 print "Waiting for lock...";
798 flock(FH, LOCK_SH) or die "can't lock filename: $!";
803 To get an exclusive lock, typically used for writing, you have to be
804 careful. We C<sysopen> the file so it can be locked before it gets
805 emptied. You can get a nonblocking version using C<LOCK_EX | LOCK_NB>.
808 use Fcntl qw(:DEFAULT :flock);
809 sysopen(FH, "filename", O_WRONLY | O_CREAT)
810 or die "can't open filename: $!";
812 or die "can't lock filename: $!";
814 or die "can't truncate filename: $!";
817 Finally, due to the uncounted millions who cannot be dissuaded from
818 wasting cycles on useless vanity devices called hit counters, here's
819 how to increment a number in a file safely:
821 use Fcntl qw(:DEFAULT :flock);
823 sysopen(FH, "numfile", O_RDWR | O_CREAT)
824 or die "can't open numfile: $!";
826 $ofh = select(FH); $| = 1; select ($ofh);
828 or die "can't write-lock numfile: $!";
832 or die "can't rewind numfile : $!";
833 print FH $num+1, "\n"
834 or die "can't write numfile: $!";
836 truncate(FH, tell(FH))
837 or die "can't truncate numfile: $!";
839 or die "can't close numfile: $!";
843 The C<open> and C<sysopen> function in perlfunc(1);
844 the standard open(2), dup(2), fopen(3), and fdopen(3) manpages;
845 the POSIX documentation.
847 =head1 AUTHOR and COPYRIGHT
849 Copyright 1998 Tom Christiansen.
851 This documentation is free; you can redistribute it and/or modify it
852 under the same terms as Perl itself.
854 Irrespective of its distribution, all code examples in these files are
855 hereby placed into the public domain. You are permitted and
856 encouraged to use this code in your own programs for fun or for profit
857 as you see fit. A simple comment in the code giving credit would be
858 courteous but is not required.
862 First release: Sat Jan 9 08:09:11 MST 1999