3 perlfaq5 - Files and Formats ($Revision: 1.9 $, $Date: 2002/02/11 19:30:21 $)
7 This section deals with I/O and the "f" issues: filehandles, flushing,
10 =head2 How do I flush/unbuffer an output filehandle? Why must I do this?
12 The C standard I/O library (stdio) normally buffers characters sent to
13 devices. This is done for efficiency reasons so that there isn't a
14 system call for each byte. Any time you use print() or write() in
15 Perl, you go though this buffering. syswrite() circumvents stdio and
18 In most stdio implementations, the type of output buffering and the size of
19 the buffer varies according to the type of device. Disk files are block
20 buffered, often with a buffer size of more than 2k. Pipes and sockets
21 are often buffered with a buffer size between 1/2 and 2k. Serial devices
22 (e.g. modems, terminals) are normally line-buffered, and stdio sends
23 the entire line when it gets the newline.
25 Perl does not support truly unbuffered output (except insofar as you can
26 C<syswrite(OUT, $char, 1)>). What it does instead support is "command
27 buffering", in which a physical write is performed after every output
28 command. This isn't as hard on your system as unbuffering, but does
29 get the output where you want it when you want it.
31 If you expect characters to get to your device when you print them there,
32 you'll want to autoflush its handle.
33 Use select() and the C<$|> variable to control autoflushing
34 (see L<perlvar/$|> and L<perlfunc/select>):
36 $old_fh = select(OUTPUT_HANDLE);
40 Or using the traditional idiom:
42 select((select(OUTPUT_HANDLE), $| = 1)[0]);
44 Or if don't mind slowly loading several thousand lines of module code
45 just because you're afraid of the C<$|> variable:
48 open(DEV, "+</dev/tty"); # ceci n'est pas une pipe
51 or the newer IO::* modules:
54 open(DEV, ">/dev/printer"); # but is this?
59 use IO::Socket; # this one is kinda a pipe?
60 $sock = IO::Socket::INET->new(PeerAddr => 'www.perl.com',
61 PeerPort => 'http(80)',
63 die "$!" unless $sock;
66 print $sock "GET / HTTP/1.0" . "\015\012" x 2;
67 $document = join('', <$sock>);
68 print "DOC IS: $document\n";
70 Note the bizarrely hard coded carriage return and newline in their octal
71 equivalents. This is the ONLY way (currently) to assure a proper flush
72 on all platforms, including Macintosh. That's the way things work in
73 network programming: you really should specify the exact bit pattern
74 on the network line terminator. In practice, C<"\n\n"> often works,
75 but this is not portable.
77 See L<perlfaq9> for other examples of fetching URLs over the web.
79 =head2 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?
81 The short answer is to use the Tie::File module, which is included
82 in the standard distribution since Perl 5.8.0.
84 The long answer is that those are operations of a text editor. Perl
85 is not a text editor. Perl is a programming language. You have to
86 decompose the problem into low-level calls to read, write, open,
89 Although humans have an easy time thinking of a text file as being a
90 sequence of lines that operates much like a stack of playing cards--or
91 punch cards--computers usually see the text file as a sequence of bytes.
92 In general, there's no direct way for Perl to seek to a particular line
93 of a file, insert text into a file, or remove text from a file.
95 (There are exceptions in special circumstances. You can add or remove
96 data at the very end of the file. A sequence of bytes can be replaced
97 with another sequence of the same length. The C<$DB_RECNO> array
98 bindings as documented in L<DB_File> also provide a direct way of
99 modifying a file. Files where all lines are the same length are also
102 The general solution is to create a temporary copy of the text file with
103 the changes you want, then copy that over the original. This assumes
107 $new = "$file.tmp.$$";
110 open(OLD, "< $old") or die "can't open $old: $!";
111 open(NEW, "> $new") or die "can't open $new: $!";
113 # Correct typos, preserving case
115 s/\b(p)earl\b/${1}erl/i;
116 (print NEW $_) or die "can't write to $new: $!";
119 close(OLD) or die "can't close $old: $!";
120 close(NEW) or die "can't close $new: $!";
122 rename($old, $bak) or die "can't rename $old to $bak: $!";
123 rename($new, $old) or die "can't rename $new to $old: $!";
125 Perl can do this sort of thing for you automatically with the C<-i>
126 command-line switch or the closely-related C<$^I> variable (see
127 L<perlrun> for more details). Note that
128 C<-i> may require a suffix on some non-Unix systems; see the
129 platform-specific documentation that came with your port.
131 # Renumber a series of tests from the command line
132 perl -pi -e 's/(^\s+test\s+)\d+/ $1 . ++$count /e' t/op/taint.t
135 local($^I, @ARGV) = ('.orig', glob("*.c"));
138 print "This line should appear at the top of each file\n";
140 s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case
142 close ARGV if eof; # Reset $.
145 If you need to seek to an arbitrary line of a file that changes
146 infrequently, you could build up an index of byte positions of where
147 the line ends are in the file. If the file is large, an index of
148 every tenth or hundredth line end would allow you to seek and read
149 fairly efficiently. If the file is sorted, try the look.pl library
150 (part of the standard perl distribution).
152 In the unique case of deleting lines at the end of a file, you
153 can use tell() and truncate(). The following code snippet deletes
154 the last line of a file without making a copy or reading the
155 whole file into memory:
157 open (FH, "+< $file");
158 while ( <FH> ) { $addr = tell(FH) unless eof(FH) }
161 Error checking is left as an exercise for the reader.
163 =head2 How do I count the number of lines in a file?
165 One fairly efficient way is to count newlines in the file. The
166 following program uses a feature of tr///, as documented in L<perlop>.
167 If your text file doesn't end with a newline, then it's not really a
168 proper text file, so this may report one fewer line than you expect.
171 open(FILE, $filename) or die "Can't open `$filename': $!";
172 while (sysread FILE, $buffer, 4096) {
173 $lines += ($buffer =~ tr/\n//);
177 This assumes no funny games with newline translations.
179 =head2 How do I make a temporary file name?
181 Use the File::Temp module, see L<File::Temp> for more information.
183 use File::Temp qw/ tempfile tempdir /;
185 $dir = tempdir( CLEANUP => 1 );
186 ($fh, $filename) = tempfile( DIR => $dir );
188 # or if you don't need to know the filename
190 $fh = tempfile( DIR => $dir );
192 The File::Temp has been a standard module since Perl 5.6.1. If you
193 don't have a modern enough Perl installed, use the C<new_tmpfile>
194 class method from the IO::File module to get a filehandle opened for
195 reading and writing. Use it if you don't need to know the file's name:
198 $fh = IO::File->new_tmpfile()
199 or die "Unable to make new temporary file: $!";
201 If you're committed to creating a temporary file by hand, use the
202 process ID and/or the current time-value. If you need to have many
203 temporary files in one process, use a counter:
207 my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
208 my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
212 until (defined(fileno(FH)) || $count++ > 100) {
213 $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
214 sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
216 if (defined(fileno(FH))
217 return (*FH, $base_name);
224 =head2 How can I manipulate fixed-record-length files?
226 The most efficient way is using pack() and unpack(). This is faster than
227 using substr() when taking many, many strings. It is slower for just a few.
229 Here is a sample chunk of code to break up and put back together again
230 some fixed-format input lines, in this case from the output of a normal,
234 # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
235 $PS_T = 'A6 A4 A7 A5 A*';
239 ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
240 for $var (qw!pid tt stat time command!) {
241 print "$var: <$$var>\n";
243 print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),
247 We've used C<$$var> in a way that forbidden by C<use strict 'refs'>.
248 That is, we've promoted a string to a scalar variable reference using
249 symbolic references. This is okay in small programs, but doesn't scale
250 well. It also only works on global variables, not lexicals.
252 =head2 How can I make a filehandle local to a subroutine? How do I pass filehandles between subroutines? How do I make an array of filehandles?
254 The fastest, simplest, and most direct way is to localize the typeglob
255 of the filehandle in question:
259 Typeglobs are fast (especially compared with the alternatives) and
260 reasonably easy to use, but they also have one subtle drawback. If you
261 had, for example, a function named TmpHandle(), or a variable named
262 %TmpHandle, you just hid it from yourself.
266 open(HostFile, "</etc/hosts") or die "no /etc/hosts: $!";
267 local $_; # <- VERY IMPORTANT
269 print if /\b127\.(0\.0\.)?1\b/;
271 # *HostFile automatically closes/disappears here
274 Here's how to use typeglobs in a loop to open and store a bunch of
275 filehandles. We'll use as values of the hash an ordered
276 pair to make it easy to sort the hash in insertion order.
278 @names = qw(motd termcap passwd hosts);
280 foreach $filename (@names) {
282 open(FH, "/etc/$filename") || die "$filename: $!";
283 $file{$filename} = [ $i++, *FH ];
286 # Using the filehandles in the array
287 foreach $name (sort { $file{$a}[0] <=> $file{$b}[0] } keys %file) {
288 my $fh = $file{$name}[1];
290 print "$name $. $line";
293 For passing filehandles to functions, the easiest way is to
294 preface them with a star, as in func(*STDIN).
295 See L<perlfaq7/"Passing Filehandles"> for details.
297 If you want to create many anonymous handles, you should check out the
298 Symbol, FileHandle, or IO::Handle (etc.) modules. Here's the equivalent
299 code with Symbol::gensym, which is reasonably light-weight:
301 foreach $filename (@names) {
304 open($fh, "/etc/$filename") || die "open /etc/$filename: $!";
305 $file{$filename} = [ $i++, $fh ];
308 Here's using the semi-object-oriented FileHandle module, which certainly
313 foreach $filename (@names) {
314 my $fh = FileHandle->new("/etc/$filename") or die "$filename: $!";
315 $file{$filename} = [ $i++, $fh ];
318 Please understand that whether the filehandle happens to be a (probably
319 localized) typeglob or an anonymous handle from one of the modules
320 in no way affects the bizarre rules for managing indirect handles.
321 See the next question.
323 =head2 How can I use a filehandle indirectly?
325 An indirect filehandle is using something other than a symbol
326 in a place that a filehandle is expected. Here are ways
327 to get indirect filehandles:
329 $fh = SOME_FH; # bareword is strict-subs hostile
330 $fh = "SOME_FH"; # strict-refs hostile; same package only
331 $fh = *SOME_FH; # typeglob
332 $fh = \*SOME_FH; # ref to typeglob (bless-able)
333 $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
335 Or, you can use the C<new> method from the FileHandle or IO modules to
336 create an anonymous filehandle, store that in a scalar variable,
337 and use it as though it were a normal filehandle.
340 $fh = FileHandle->new();
342 use IO::Handle; # 5.004 or higher
343 $fh = IO::Handle->new();
345 Then use any of those as you would a normal filehandle. Anywhere that
346 Perl is expecting a filehandle, an indirect filehandle may be used
347 instead. An indirect filehandle is just a scalar variable that contains
348 a filehandle. Functions like C<print>, C<open>, C<seek>, or
349 the C<< <FH> >> diamond operator will accept either a read filehandle
350 or a scalar variable containing one:
352 ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
353 print $ofh "Type it: ";
355 print $efh "What was that: $got";
357 If you're passing a filehandle to a function, you can write
358 the function in two ways:
362 print $fh "Sending to indirect filehandle\n";
365 Or it can localize a typeglob and use the filehandle directly:
369 print FH "Sending to localized filehandle\n";
372 Both styles work with either objects or typeglobs of real filehandles.
373 (They might also work with strings under some circumstances, but this
379 In the examples above, we assigned the filehandle to a scalar variable
380 before using it. That is because only simple scalar variables, not
381 expressions or subscripts of hashes or arrays, can be used with
382 built-ins like C<print>, C<printf>, or the diamond operator. Using
383 something other than a simple scalar variable as a filehandle is
384 illegal and won't even compile:
386 @fd = (*STDIN, *STDOUT, *STDERR);
387 print $fd[1] "Type it: "; # WRONG
388 $got = <$fd[0]> # WRONG
389 print $fd[2] "What was that: $got"; # WRONG
391 With C<print> and C<printf>, you get around this by using a block and
392 an expression where you would place the filehandle:
394 print { $fd[1] } "funny stuff\n";
395 printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
396 # Pity the poor deadbeef.
398 That block is a proper block like any other, so you can put more
399 complicated code there. This sends the message out to one of two places:
402 print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
403 print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n";
405 This approach of treating C<print> and C<printf> like object methods
406 calls doesn't work for the diamond operator. That's because it's a
407 real operator, not just a function with a comma-less argument. Assuming
408 you've been storing typeglobs in your structure as we did above, you
409 can use the built-in function named C<readline> to reads a record just
410 as C<< <> >> does. Given the initialization shown above for @fd, this
411 would work, but only because readline() require a typeglob. It doesn't
412 work with objects or strings, which might be a bug we haven't fixed yet.
414 $got = readline($fd[0]);
416 Let it be noted that the flakiness of indirect filehandles is not
417 related to whether they're strings, typeglobs, objects, or anything else.
418 It's the syntax of the fundamental operators. Playing the object
419 game doesn't help you at all here.
421 =head2 How can I set up a footer format to be used with write()?
423 There's no builtin way to do this, but L<perlform> has a couple of
424 techniques to make it possible for the intrepid hacker.
426 =head2 How can I write() into a string?
428 See L<perlform/"Accessing Formatting Internals"> for an swrite() function.
430 =head2 How can I output my numbers with commas added?
432 This one will do it for you:
436 1 while ($number =~ s/^([-+]?\d+)(\d{3})/$1,$2/);
440 $n = 23659019423.2331;
441 print "GOT: ", commify($n), "\n";
443 GOT: 23,659,019,423.2331
447 s/^([-+]?\d+)(\d{3})/$1,$2/g;
449 because you have to put the comma in and then recalculate your
452 Alternatively, this code commifies all numbers in a line regardless of
453 whether they have decimal portions, are preceded by + or -, or
456 # from Andrew Johnson <ajohnson@gpu.srv.ualberta.ca>
459 $input = reverse $input;
460 $input =~ s<(\d\d\d)(?=\d)(?!\d*\.)><$1,>g;
461 return scalar reverse $input;
464 =head2 How can I translate tildes (~) in a filename?
466 Use the <> (glob()) operator, documented in L<perlfunc>. Older
467 versions of Perl require that you have a shell installed that groks
468 tildes. Recent perl versions have this feature built in. The
469 File::KGlob module (available from CPAN) gives more portable glob
472 Within Perl, you may use this directly:
475 ^ ~ # find a leading tilde
477 [^/] # a non-slash character
478 * # repeated 0 or more times (0 means me)
483 : ( $ENV{HOME} || $ENV{LOGDIR} )
486 =head2 How come when I open a file read-write it wipes it out?
488 Because you're using something like this, which truncates the file and
489 I<then> gives you read-write access:
491 open(FH, "+> /path/name"); # WRONG (almost always)
493 Whoops. You should instead use this, which will fail if the file
496 open(FH, "+< /path/name"); # open for update
498 Using ">" always clobbers or creates. Using "<" never does
499 either. The "+" doesn't change this.
501 Here are examples of many kinds of file opens. Those using sysopen()
506 To open file for reading:
508 open(FH, "< $path") || die $!;
509 sysopen(FH, $path, O_RDONLY) || die $!;
511 To open file for writing, create new file if needed or else truncate old file:
513 open(FH, "> $path") || die $!;
514 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!;
515 sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!;
517 To open file for writing, create new file, file must not exist:
519 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
520 sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
522 To open file for appending, create if necessary:
524 open(FH, ">> $path") || die $!;
525 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!;
526 sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;
528 To open file for appending, file must exist:
530 sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
532 To open file for update, file must exist:
534 open(FH, "+< $path") || die $!;
535 sysopen(FH, $path, O_RDWR) || die $!;
537 To open file for update, create file if necessary:
539 sysopen(FH, $path, O_RDWR|O_CREAT) || die $!;
540 sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!;
542 To open file for update, file must not exist:
544 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!;
545 sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!;
547 To open a file without blocking, creating if necessary:
549 sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT)
550 or die "can't open /tmp/somefile: $!":
552 Be warned that neither creation nor deletion of files is guaranteed to
553 be an atomic operation over NFS. That is, two processes might both
554 successfully create or unlink the same file! Therefore O_EXCL
555 isn't as exclusive as you might wish.
557 See also the new L<perlopentut> if you have it (new for 5.6).
559 =head2 Why do I sometimes get an "Argument list too long" when I use <*>?
561 The C<< <> >> operator performs a globbing operation (see above).
562 In Perl versions earlier than v5.6.0, the internal glob() operator forks
563 csh(1) to do the actual glob expansion, but
564 csh can't handle more than 127 items and so gives the error message
565 C<Argument list too long>. People who installed tcsh as csh won't
566 have this problem, but their users may be surprised by it.
568 To get around this, either upgrade to Perl v5.6.0 or later, do the glob
569 yourself with readdir() and patterns, or use a module like File::KGlob,
570 one that doesn't use the shell to do globbing.
572 =head2 Is there a leak/bug in glob()?
574 Due to the current implementation on some operating systems, when you
575 use the glob() function or its angle-bracket alias in a scalar
576 context, you may cause a memory leak and/or unpredictable behavior. It's
577 best therefore to use glob() only in list context.
579 =head2 How can I open a file with a leading ">" or trailing blanks?
581 Normally perl ignores trailing blanks in filenames, and interprets
582 certain leading characters (or a trailing "|") to mean something
583 special. To avoid this, you might want to use a routine like the one below.
584 It turns incomplete pathnames into explicit relative ones, and tacks a
585 trailing null byte on the name to make perl leave it alone:
594 $badpath = "<<<something really wicked ";
595 $fn = safe_filename($badpath");
596 open(FH, "> $fn") or "couldn't open $badpath: $!";
598 This assumes that you are using POSIX (portable operating systems
599 interface) paths. If you are on a closed, non-portable, proprietary
600 system, you may have to adjust the C<"./"> above.
602 It would be a lot clearer to use sysopen(), though:
605 $badpath = "<<<something really wicked ";
606 sysopen (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
607 or die "can't open $badpath: $!";
609 For more information, see also the new L<perlopentut> if you have it
612 =head2 How can I reliably rename a file?
614 If your operating system supports a proper mv(1) utility or its functional
615 equivalent, this works:
617 rename($old, $new) or system("mv", $old, $new);
619 It may be more portable to use the File::Copy module instead.
620 You just copy to the new file to the new name (checking return
621 values), then delete the old one. This isn't really the same
622 semantically as a rename(), which preserves meta-information like
623 permissions, timestamps, inode info, etc.
625 Newer versions of File::Copy export a move() function.
627 =head2 How can I lock a file?
629 Perl's builtin flock() function (see L<perlfunc> for details) will call
630 flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and
631 later), and lockf(3) if neither of the two previous system calls exists.
632 On some systems, it may even use a different form of native locking.
633 Here are some gotchas with Perl's flock():
639 Produces a fatal error if none of the three system calls (or their
640 close equivalent) exists.
644 lockf(3) does not provide shared locking, and requires that the
645 filehandle be open for writing (or appending, or read/writing).
649 Some versions of flock() can't lock files over a network (e.g. on NFS file
650 systems), so you'd need to force the use of fcntl(2) when you build Perl.
651 But even this is dubious at best. See the flock entry of L<perlfunc>
652 and the F<INSTALL> file in the source distribution for information on
653 building Perl to do this.
655 Two potentially non-obvious but traditional flock semantics are that
656 it waits indefinitely until the lock is granted, and that its locks are
657 I<merely advisory>. Such discretionary locks are more flexible, but
658 offer fewer guarantees. This means that files locked with flock() may
659 be modified by programs that do not also use flock(). Cars that stop
660 for red lights get on well with each other, but not with cars that don't
661 stop for red lights. See the perlport manpage, your port's specific
662 documentation, or your system-specific local manpages for details. It's
663 best to assume traditional behavior if you're writing portable programs.
664 (If you're not, you should as always feel perfectly free to write
665 for your own system's idiosyncrasies (sometimes called "features").
666 Slavish adherence to portability concerns shouldn't get in the way of
667 your getting your job done.)
669 For more information on file locking, see also
670 L<perlopentut/"File Locking"> if you have it (new for 5.6).
674 =head2 Why can't I just open(FH, ">file.lock")?
676 A common bit of code B<NOT TO USE> is this:
678 sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
679 open(LCK, "> file.lock"); # THIS BROKEN CODE
681 This is a classic race condition: you take two steps to do something
682 which must be done in one. That's why computer hardware provides an
683 atomic test-and-set instruction. In theory, this "ought" to work:
685 sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
686 or die "can't open file.lock: $!":
688 except that lamentably, file creation (and deletion) is not atomic
689 over NFS, so this won't work (at least, not every time) over the net.
690 Various schemes involving link() have been suggested, but
691 these tend to involve busy-wait, which is also subdesirable.
693 =head2 I still don't get locking. I just want to increment the number in the file. How can I do this?
695 Didn't anyone ever tell you web-page hit counters were useless?
696 They don't count number of hits, they're a waste of time, and they serve
697 only to stroke the writer's vanity. It's better to pick a random number;
698 they're more realistic.
700 Anyway, this is what you can do if you can't help yourself.
702 use Fcntl qw(:DEFAULT :flock);
703 sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
704 flock(FH, LOCK_EX) or die "can't flock numfile: $!";
706 seek(FH, 0, 0) or die "can't rewind numfile: $!";
707 truncate(FH, 0) or die "can't truncate numfile: $!";
708 (print FH $num+1, "\n") or die "can't write numfile: $!";
709 close FH or die "can't close numfile: $!";
711 Here's a much better web-page hit counter:
713 $hits = int( (time() - 850_000_000) / rand(1_000) );
715 If the count doesn't impress your friends, then the code might. :-)
717 =head2 All I want to do is append a small amount of text to the end of a file. Do I still have to use locking?
719 If you are on a system that correctly implements flock() and you use the
720 example appending code from "perldoc -f flock" everything will be OK
721 even if the OS you are on doesn't implement append mode correctly (if
722 such a system exists.) So if you are happy to restrict yourself to OSs
723 that implement flock() (and that's not really much of a restriction)
724 then that is what you should do.
726 If you know you are only going to use a system that does correctly
727 implement appending (i.e. not Win32) then you can omit the seek() from
730 If you know you are only writing code to run on an OS and filesystem that
731 does implement append mode correctly (a local filesystem on a modern
732 Unix for example), and you keep the file in block-buffered mode and you
733 write less than one buffer-full of output between each manual flushing
734 of the buffer then each bufferload is almost guaranteed to be written to
735 the end of the file in one chunk without getting intermingled with
736 anyone else's output. You can also use the syswrite() function which is
737 simply a wrapper around your systems write(2) system call.
739 There is still a small theoretical chance that a signal will interrupt
740 the system level write() operation before completion. There is also a
741 possibility that some STDIO implementations may call multiple system
742 level write()s even if the buffer was empty to start. There may be some
743 systems where this probability is reduced to zero.
745 =head2 How do I randomly update a binary file?
747 If you're just trying to patch a binary, in many cases something as
748 simple as this works:
750 perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
752 However, if you have fixed sized records, then you might do something more
755 $RECSIZE = 220; # size of record, in bytes
756 $recno = 37; # which record to update
757 open(FH, "+<somewhere") || die "can't update somewhere: $!";
758 seek(FH, $recno * $RECSIZE, 0);
759 read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
761 seek(FH, -$RECSIZE, 1);
765 Locking and error checking are left as an exercise for the reader.
766 Don't forget them or you'll be quite sorry.
768 =head2 How do I get a file's timestamp in perl?
770 If you want to retrieve the time at which the file was last read,
771 written, or had its meta-data (owner, etc) changed, you use the B<-M>,
772 B<-A>, or B<-C> file test operations as documented in L<perlfunc>. These
773 retrieve the age of the file (measured against the start-time of your
774 program) in days as a floating point number. To retrieve the "raw"
775 time in seconds since the epoch, you would call the stat function,
776 then use localtime(), gmtime(), or POSIX::strftime() to convert this
777 into human-readable form.
781 $write_secs = (stat($file))[9];
782 printf "file %s updated at %s\n", $file,
783 scalar localtime($write_secs);
785 If you prefer something more legible, use the File::stat module
786 (part of the standard distribution in version 5.004 and later):
788 # error checking left as an exercise for reader.
791 $date_string = ctime(stat($file)->mtime);
792 print "file $file updated at $date_string\n";
794 The POSIX::strftime() approach has the benefit of being,
795 in theory, independent of the current locale. See L<perllocale>
798 =head2 How do I set a file's timestamp in perl?
800 You use the utime() function documented in L<perlfunc/utime>.
801 By way of example, here's a little program that copies the
802 read and write times from its first argument to all the rest
806 die "usage: cptimes timestamp_file other_files ...\n";
809 ($atime, $mtime) = (stat($timestamp))[8,9];
810 utime $atime, $mtime, @ARGV;
812 Error checking is, as usual, left as an exercise for the reader.
814 Note that utime() currently doesn't work correctly with Win95/NT
815 ports. A bug has been reported. Check it carefully before using
816 utime() on those platforms.
818 =head2 How do I print to more than one file at once?
820 If you only have to do this once, you can do this:
822 for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
824 To connect up to one filehandle to several output filehandles, it's
825 easiest to use the tee(1) program if you have it, and let it take care
828 open (FH, "| tee file1 file2 file3");
832 # make STDOUT go to three files, plus original STDOUT
833 open (STDOUT, "| tee file1 file2 file3") or die "Teeing off: $!\n";
834 print "whatever\n" or die "Writing: $!\n";
835 close(STDOUT) or die "Closing: $!\n";
837 Otherwise you'll have to write your own multiplexing print
838 function--or your own tee program--or use Tom Christiansen's,
839 at http://www.cpan.org/authors/id/TOMC/scripts/tct.gz , which is
840 written in Perl and offers much greater functionality
841 than the stock version.
843 =head2 How can I read in an entire file all at once?
845 The customary Perl approach for processing all the lines in a file is to
846 do so one line at a time:
848 open (INPUT, $file) || die "can't open $file: $!";
851 # do something with $_
853 close(INPUT) || die "can't close $file: $!";
855 This is tremendously more efficient than reading the entire file into
856 memory as an array of lines and then processing it one element at a time,
857 which is often--if not almost always--the wrong approach. Whenever
858 you see someone do this:
862 you should think long and hard about why you need everything loaded
863 at once. It's just not a scalable solution. You might also find it
864 more fun to use the standard DB_File module's $DB_RECNO bindings,
865 which allow you to tie an array to a file so that accessing an element
866 the array actually accesses the corresponding line in the file.
868 On very rare occasion, you may have an algorithm that demands that
869 the entire file be in memory at once as one scalar. The simplest solution
874 Being in scalar context, you get the whole thing. In list context,
875 you'd get a list of all the lines:
877 @lines = `cat $file`;
879 This tiny but expedient solution is neat, clean, and portable to
880 all systems on which decent tools have been installed. For those
881 who prefer not to use the toolbox, you can of course read the file
882 manually, although this makes for more complicated code.
886 open (INPUT, $file) || die "can't open $file: $!";
890 That temporarily undefs your record separator, and will automatically
891 close the file at block exit. If the file is already open, just use this:
893 $var = do { local $/; <INPUT> };
895 =head2 How can I read in a file by paragraphs?
897 Use the C<$/> variable (see L<perlvar> for details). You can either
898 set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">,
899 for instance, gets treated as two paragraphs and not three), or
900 C<"\n\n"> to accept empty paragraphs.
902 Note that a blank line must have no blanks in it. Thus
903 S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.
905 =head2 How can I read a single character from a file? From the keyboard?
907 You can use the builtin C<getc()> function for most filehandles, but
908 it won't (easily) work on a terminal device. For STDIN, either use
909 the Term::ReadKey module from CPAN or use the sample code in
912 If your system supports the portable operating system programming
913 interface (POSIX), you can use the following code, which you'll note
914 turns off echo processing as well.
928 use POSIX qw(:termios_h);
930 my ($term, $oterm, $echo, $noecho, $fd_stdin);
932 $fd_stdin = fileno(STDIN);
934 $term = POSIX::Termios->new();
935 $term->getattr($fd_stdin);
936 $oterm = $term->getlflag();
938 $echo = ECHO | ECHOK | ICANON;
939 $noecho = $oterm & ~$echo;
942 $term->setlflag($noecho);
943 $term->setcc(VTIME, 1);
944 $term->setattr($fd_stdin, TCSANOW);
948 $term->setlflag($oterm);
949 $term->setcc(VTIME, 0);
950 $term->setattr($fd_stdin, TCSANOW);
956 sysread(STDIN, $key, 1);
965 The Term::ReadKey module from CPAN may be easier to use. Recent versions
966 include also support for non-portable systems as well.
969 open(TTY, "</dev/tty");
970 print "Gimme a char: ";
972 $key = ReadKey 0, *TTY;
974 printf "\nYou said %s, char number %03d\n",
977 =head2 How can I tell whether there's a character waiting on a filehandle?
979 The very first thing you should do is look into getting the Term::ReadKey
980 extension from CPAN. As we mentioned earlier, it now even has limited
981 support for non-portable (read: not open systems, closed, proprietary,
982 not POSIX, not Unix, etc) systems.
984 You should also check out the Frequently Asked Questions list in
985 comp.unix.* for things like this: the answer is essentially the same.
986 It's very system dependent. Here's one solution that works on BSD
991 vec($rin, fileno(STDIN), 1) = 1;
992 return $nfd = select($rin,undef,undef,0);
995 If you want to find out how many characters are waiting, there's
996 also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that
997 comes with Perl tries to convert C include files to Perl code, which
998 can be C<require>d. FIONREAD ends up defined as a function in the
999 I<sys/ioctl.ph> file:
1001 require 'sys/ioctl.ph';
1003 $size = pack("L", 0);
1004 ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
1005 $size = unpack("L", $size);
1007 If I<h2ph> wasn't installed or doesn't work for you, you can
1008 I<grep> the include files by hand:
1010 % grep FIONREAD /usr/include/*/*
1011 /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
1013 Or write a small C program using the editor of champions:
1016 #include <sys/ioctl.h>
1018 printf("%#08x\n", FIONREAD);
1021 % cc -o fionread fionread.c
1025 And then hard code it, leaving porting as an exercise to your successor.
1027 $FIONREAD = 0x4004667f; # XXX: opsys dependent
1029 $size = pack("L", 0);
1030 ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
1031 $size = unpack("L", $size);
1033 FIONREAD requires a filehandle connected to a stream, meaning that sockets,
1034 pipes, and tty devices work, but I<not> files.
1036 =head2 How do I do a C<tail -f> in perl?
1042 The statement C<seek(GWFILE, 0, 1)> doesn't change the current position,
1043 but it does clear the end-of-file condition on the handle, so that the
1044 next <GWFILE> makes Perl try again to read something.
1046 If that doesn't work (it relies on features of your stdio implementation),
1047 then you need something more like this:
1050 for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
1051 # search for some stuff and put it into files
1054 seek(GWFILE, $curpos, 0); # seek to where we had been
1057 If this still doesn't work, look into the POSIX module. POSIX defines
1058 the clearerr() method, which can remove the end of file condition on a
1059 filehandle. The method: read until end of file, clearerr(), read some
1060 more. Lather, rinse, repeat.
1062 There's also a File::Tail module from CPAN.
1064 =head2 How do I dup() a filehandle in Perl?
1066 If you check L<perlfunc/open>, you'll see that several of the ways
1067 to call open() should do the trick. For example:
1069 open(LOG, ">>/tmp/logfile");
1070 open(STDERR, ">&LOG");
1072 Or even with a literal numeric descriptor:
1074 $fd = $ENV{MHCONTEXTFD};
1075 open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
1077 Note that "<&STDIN" makes a copy, but "<&=STDIN" make
1078 an alias. That means if you close an aliased handle, all
1079 aliases become inaccessible. This is not true with
1082 Error checking, as always, has been left as an exercise for the reader.
1084 =head2 How do I close a file descriptor by number?
1086 This should rarely be necessary, as the Perl close() function is to be
1087 used for things that Perl opened itself, even if it was a dup of a
1088 numeric descriptor as with MHCONTEXT above. But if you really have
1089 to, you may be able to do this:
1091 require 'sys/syscall.ph';
1092 $rc = syscall(&SYS_close, $fd + 0); # must force numeric
1093 die "can't sysclose $fd: $!" unless $rc == -1;
1095 Or, just use the fdopen(3S) feature of open():
1099 open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
1103 =head2 Why can't I use "C:\temp\foo" in DOS paths? What doesn't `C:\temp\foo.exe` work?
1105 Whoops! You just put a tab and a formfeed into that filename!
1106 Remember that within double quoted strings ("like\this"), the
1107 backslash is an escape character. The full list of these is in
1108 L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't
1109 have a file called "c:(tab)emp(formfeed)oo" or
1110 "c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.
1112 Either single-quote your strings, or (preferably) use forward slashes.
1113 Since all DOS and Windows versions since something like MS-DOS 2.0 or so
1114 have treated C</> and C<\> the same in a path, you might as well use the
1115 one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++,
1116 awk, Tcl, Java, or Python, just to mention a few. POSIX paths
1117 are more portable, too.
1119 =head2 Why doesn't glob("*.*") get all the files?
1121 Because even on non-Unix ports, Perl's glob function follows standard
1122 Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden)
1123 files. This makes glob() portable even to legacy systems. Your
1124 port may include proprietary globbing functions as well. Check its
1125 documentation for details.
1127 =head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
1129 This is elaborately and painstakingly described in the
1130 F<file-dir-perms> article in the "Far More Than You Ever Wanted To
1131 Know" collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz .
1133 The executive summary: learn how your filesystem works. The
1134 permissions on a file say what can happen to the data in that file.
1135 The permissions on a directory say what can happen to the list of
1136 files in that directory. If you delete a file, you're removing its
1137 name from the directory (so the operation depends on the permissions
1138 of the directory, not of the file). If you try to write to the file,
1139 the permissions of the file govern whether you're allowed to.
1141 =head2 How do I select a random line from a file?
1143 Here's an algorithm from the Camel Book:
1146 rand($.) < 1 && ($line = $_) while <>;
1148 This has a significant advantage in space over reading the whole
1149 file in. A simple proof by induction is available upon
1150 request if you doubt the algorithm's correctness.
1152 =head2 Why do I get weird spaces when I print an array of lines?
1158 joins together the elements of C<@lines> with a space between them.
1159 If C<@lines> were C<("little", "fluffy", "clouds")> then the above
1160 statement would print
1162 little fluffy clouds
1164 but if each element of C<@lines> was a line of text, ending a newline
1165 character C<("little\n", "fluffy\n", "clouds\n")> then it would print:
1171 If your array contains lines, just print them:
1175 =head1 AUTHOR AND COPYRIGHT
1177 Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
1178 All rights reserved.
1180 This documentation is free; you can redistribute it and/or modify it
1181 under the same terms as Perl itself.
1183 Irrespective of its distribution, all code examples here are in the public
1184 domain. You are permitted and encouraged to use this code and any
1185 derivatives thereof in your own programs for fun or for profit as you
1186 see fit. A simple comment in the code giving credit to the FAQ would
1187 be courteous but is not required.