[Perl.sig] Perl tip: Buffering and IO::Handle

Litss Coordinator litss.coord at anu.edu.au
Tue Sep 13 15:19:29 EST 2005

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           What:  Melbourne Perl Mongers
           When:  6:30pm, Wednesday 14th September, 2005
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==== Buffering and IO::Handle ====

    Sooner or later most Perl programmers encounter the concept of buffering
    on filehandles, and more often than not it's by accident. Perl's rule
    for buffering output is very simple: if output is sent to anything
    that's not a terminal, then it will be buffered by default.

    Buffering is generally a good thing, by sending output to the operating
    system in managable chunks we reduce the amount of context switching and
    overheads associated with output. That makes our system more efficient
    and responsible, and all without any effort on behalf of the programmer.

    Our problems arise when we want output to happen now, rather than when
    it becomes efficient to do so. Common situations include when we're
    writing to something that in turn is outputting to a terminal, or to a
    logfile where it's critical we know exactly what stage our program has
    reached. In these situations, we'd like to turn buffering off.

    The traditional way to disable buffering on a filehandle is to use a
    ``select'' statement, and Perl's special $| variable:

                my $old_fh = select($fh_to_unbuf);  # Select FH to unbuffer.
                $| = 1;                             # Unbuffer that FH.
                select($old_fh);                    # Re-select the old FH.

    Unfortunately the code above isn't particularly clear, even with
    comments explaining each step. If we leave out the last step, we can
    accidentally change the default output for ``print'' statements, which
    can be a maintenance nightmare.

    Luckily, there's a better way, without requiring changes to your
    existing code. All of Perl's filehandles act like members of the
    ``IO::Handle'', whether they're contained in a scalar, or an old
    bareword handle like ``STDOUT'' or ``LOGFILE''. That's right, anything
    you can do using ``IO::Handle'' can be done on a regular filehandle.

    Now unbuffering a filehandle can be done easily and cleanly:

            use IO::Handle;

            # Unbuffer a scalar filehandle.
            $fh_to_unbuf->autoflush(1);     # Make unbuffered.

            # Unbuffer an old-style filehandle.

    ``IO::Handle'' also provides a number of methods for common problems,
    such as explicitly flushing an output handle:


    or printing a single record in an unbuffered fashion:

            $fh->printflush("Checkpoint: Photon torpedos armed");

    ``IO::Handle'' is a standard Perl module, and provides many methods that
    make working with filehandles simpler and more maintainable. To learn
    more, simply use ``perldoc IO::Handle''.

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