=head1 NAME
-perlfaq5 - Files and Formats ($Revision: 8075 $)
+perlfaq5 - Files and Formats ($Revision: 10126 $)
=head1 DESCRIPTION
=head2 How do I flush/unbuffer an output filehandle? Why must I do this?
X<flush> X<buffer> X<unbuffer> X<autoflush>
-Perl does not support truly unbuffered output (except
-insofar as you can C<syswrite(OUT, $char, 1)>), although it
-does support is "command buffering", in which a physical
-write is performed after every output command.
-
-The C standard I/O library (stdio) normally buffers
-characters sent to devices so that there isn't a system call
-for each byte. In most stdio implementations, the type of
-output buffering and the size of the buffer varies according
-to the type of device. Perl's print() and write() functions
-normally buffer output, while syswrite() bypasses buffering
-all together.
-
-If you want your output to be sent immediately when you
-execute print() or write() (for instance, for some network
-protocols), you must set the handle's autoflush flag. This
-flag is the Perl variable $| and when it is set to a true
-value, Perl will flush the handle's buffer after each
-print() or write(). Setting $| affects buffering only for
-the currently selected default file handle. You choose this
-handle with the one argument select() call (see
+Perl does not support truly unbuffered output (except insofar as you
+can C<syswrite(OUT, $char, 1)>), although it does support is "command
+buffering", in which a physical write is performed after every output
+command.
+
+The C standard I/O library (stdio) normally buffers characters sent to
+devices so that there isn't a system call for each byte. In most stdio
+implementations, the type of output buffering and the size of the
+buffer varies according to the type of device. Perl's C<print()> and
+C<write()> functions normally buffer output, while C<syswrite()>
+bypasses buffering all together.
+
+If you want your output to be sent immediately when you execute
+C<print()> or C<write()> (for instance, for some network protocols),
+you must set the handle's autoflush flag. This flag is the Perl
+variable C<$|> and when it is set to a true value, Perl will flush the
+handle's buffer after each C<print()> or C<write()>. Setting C<$|>
+affects buffering only for the currently selected default filehandle.
+You choose this handle with the one argument C<select()> call (see
L<perlvar/$E<verbar>> and L<perlfunc/select>).
-Use select() to choose the desired handle, then set its
+Use C<select()> to choose the desired handle, then set its
per-filehandle variables.
$old_fh = select(OUTPUT_HANDLE);
select($old_fh);
Some modules offer object-oriented access to handles and their
-variables, although they may be overkill if this is the only
-thing you do with them. You can use IO::Handle:
+variables, although they may be overkill if this is the only thing you
+do with them. You can use C<IO::Handle>:
use IO::Handle;
- open(DEV, ">/dev/printer"); # but is this?
- DEV->autoflush(1);
+ open my( $printer ), ">", "/dev/printer"); # but is this?
+ $printer->autoflush(1);
-or IO::Socket:
+or C<IO::Socket> (which inherits from C<IO::Handle>):
use IO::Socket; # this one is kinda a pipe?
my $sock = IO::Socket::INET->new( 'www.example.com:80' );
$sock->autoflush();
+You can also flush an C<IO::Handle> object without setting
+C<autoflush>. Call the C<flush> method to flush the buffer yourself:
+
+ use IO::Handle;
+ open my( $printer ), ">", "/dev/printer");
+ $printer->flush; # one time flush
+
+
=head2 How do I change, delete, or insert a line in a file, or append to the beginning of a file?
X<file, editing>
close $out;
To change only a particular line, the input line number, C<$.>, is
-useful. Use C<next> to skip all lines up to line 5, make a change and
-print the result, then stop further processing with C<last>.
+useful. First read and print the lines up to the one you want to
+change. Next, read the single line you want to change, change it, and
+print it. After that, read the rest of the lines and print those:
- while( <$in> )
+ while( <$in> ) # print the lines before the change
{
- next unless $. == 5;
- s/\b(perl)\b/Perl/g;
print $out $_;
- last;
+ last if $. == 4; # line number before change
}
+ my $line = <$in>;
+ $line =~ s/\b(perl)\b/Perl/g;
+ print $out $line;
+
+ while( <$in> ) # print the rest of the lines
+ {
+ print $out $_;
+ }
+
To skip lines, use the looping controls. The C<next> in this example
skips comment lines, and the C<last> stops all processing once it
encounters either C<__END__> or C<__DATA__>.
BEGIN {
use Fcntl;
my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
- my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
+ my $base_name = sprintf "%s/%d-%d-0000", $temp_dir, $$, time;
sub temp_file {
local *FH;
my $count = 0;
- until (defined(fileno(FH)) || $count++ > 100) {
- $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
- # O_EXCL is required for security reasons.
- sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
+ until( defined(fileno(FH)) || $count++ > 100 ) {
+ $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
+ # O_EXCL is required for security reasons.
+ sysopen FH, $base_name, O_WRONLY|O_EXCL|O_CREAT;
+ }
+
+ if( defined fileno(FH) ) {
+ return (*FH, $base_name);
+ }
+ else {
+ return ();
+ }
}
-
- if (defined(fileno(FH))
- return (*FH, $base_name);
- }
- else {
- return ();
- }
- }
+
}
=head2 How can I manipulate fixed-record-length files?
=head2 How can I write() into a string?
X<write, into a string>
-See L<perlform/"Accessing Formatting Internals"> for an swrite() function.
+See L<perlform/"Accessing Formatting Internals"> for an C<swrite()> function.
+
+=head2 How can I open a filehandle to a string?
+X<string>, X<open>, X<IO::Scalar>, X<filehandle>
+
+(contributed by Peter J. Holzer, hjp-usenet2@hjp.at)
+
+Since Perl 5.8.0, you can pass a reference to a scalar instead of the
+filename to create a file handle which you can used to read from or write to
+a string:
+
+ open(my $fh, '>', \$string) or die "Could not open string for writing";
+ print $fh "foo\n";
+ print $fh "bar\n"; # $string now contains "foo\nbar\n"
+
+ open(my $fh, '<', \$string) or die "Could not open string for reading";
+ my $x = <$fh>; # $x now contains "foo\n"
+
+With older versions of Perl, the C<IO::String> module provides similar
+functionality.
=head2 How can I output my numbers with commas added?
X<number, commify>
Unless you have a particular reason to use the two argument form you
should use the three argument form of open() which does not treat any
-charcters in the filename as special.
+characters in the filename as special.
open FILE, "<", " file "; # filename is " file "
open FILE, ">", ">file"; # filename is ">file"
except that lamentably, file creation (and deletion) is not atomic
over NFS, so this won't work (at least, not every time) over the net.
Various schemes involving link() have been suggested, but
-these tend to involve busy-wait, which is also subdesirable.
+these tend to involve busy-wait, which is also less than desirable.
=head2 I still don't get locking. I just want to increment the number in the file. How can I do this?
X<counter> X<file, counter>
Error checking, as always, has been left as an exercise for the reader.
=head2 How do I close a file descriptor by number?
-X<file, closing file descriptors>
+X<file, closing file descriptors> X<POSIX> X<close>
+
+If, for some reason, you have a file descriptor instead of a
+filehandle (perhaps you used C<POSIX::open>), you can use the
+C<close()> function from the C<POSIX> module:
-This should rarely be necessary, as the Perl close() function is to be
+ use POSIX ();
+
+ POSIX::close( $fd );
+
+This should rarely be necessary, as the Perl C<close()> function is to be
used for things that Perl opened itself, even if it was a dup of a
-numeric descriptor as with MHCONTEXT above. But if you really have
+numeric descriptor as with C<MHCONTEXT> above. But if you really have
to, you may be able to do this:
require 'sys/syscall.ph';
$rc = syscall(&SYS_close, $fd + 0); # must force numeric
die "can't sysclose $fd: $!" unless $rc == -1;
-Or, just use the fdopen(3S) feature of open():
+Or, just use the fdopen(3S) feature of C<open()>:
{
- local *F;
- open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
- close F;
+ open my( $fh ), "<&=$fd" or die "Cannot reopen fd=$fd: $!";
+ close $fh;
}
=head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
=head1 REVISION
-Revision: $Revision: 8075 $
+Revision: $Revision: 10126 $
-Date: $Date: 2006-11-15 02:26:49 +0100 (mer, 15 nov 2006) $
+Date: $Date: 2007-10-27 21:29:20 +0200 (Sat, 27 Oct 2007) $
See L<perlfaq> for source control details and availability.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
+Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
projects / p5sagit/p5-mst-13.2.git / blobdiff