FAQ 5.1 How do I flush/unbuffer an output filehandle? Why must I do this?


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5.1: How do I flush/unbuffer an output filehandle? Why must I do this?

(contributed by brian d foy)

You might like to read Mark Jason Dominus's "Suffering From Buffering"
at http://perl.plover.com/FAQs/Buffering.html .

Perl normally buffers output so it doesn't make a system call for every
bit of output. By saving up output, it makes fewer expensive system
calls. For instance, in this little bit of code, you want to print a dot
to the screen for every line you process to watch the progress of your
program. Instead of seeing a dot for every line, Perl buffers the output
and you have a long wait before you see a row of 50 dots all at once:

# long wait, then row of dots all at once
while( <> ) {
print ".";
print "\n" unless ++$count % 50;

#... expensive line processing operations

To get around this, you have to unbuffer the output filehandle, in this
case, "STDOUT". You can set the special variable $| to a true value
(mnemonic: making your filehandles "piping hot"):


# dot shown immediately
while( <> ) {
print ".";
print "\n" unless ++$count % 50;

#... expensive line processing operations

The $| is one of the per-filehandle special variables, so each
filehandle has its own copy of its value. If you want to merge standard
output and standard error for instance, you have to unbuffer each
(although STDERR might be unbuffered by default):

my $previous_default = select(STDOUT); # save previous default
$|++; # autoflush STDOUT
$|++; # autoflush STDERR, to be sure
select($previous_default); # restore previous default

# now should alternate . and +
while( 1 )
sleep 1;
print STDOUT ".";
print STDERR "+";
print STDOUT "\n" unless ++$count % 25;

Besides the $| special variable, you can use "binmode" to give your
filehandle a ":unix" layer, which is unbuffered:

binmode( STDOUT, ":unix" );

while( 1 ) {
sleep 1;
print ".";
print "\n" unless ++$count % 50;

For more information on output layers, see the entries for "binmode" and
"open" in perlfunc, and the "PerlIO" module documentation.

If you are using "IO::Handle" or one of its subclasses, you can call the
"autoflush" method to change the settings of the filehandle:

use IO::Handle;
open my( $io_fh ), ">", "output.txt";

The "IO::Handle" objects also have a "flush" method. You can flush the
buffer any time you want without auto-buffering



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