I was trying unsuccessfully half the day to find out how to make Perl flush and commit write of a logfile, for showing the progress on a web page.

This is my code:

sub printtolog
{
    my $txt = shift;
    my $logf;

    $txt = getdirdatetag() . ': ' . $txt . "\n";
    # see https://perldoc.perl.org/functions/open
    open($logf, ">>:utf8", $logging_logfile)
        || die "$0: can't open $logging_logfile for appending: $!";

    # see https://perldoc.perl.org/functions/select
    # this does not work
#     my $oldfh = select $logf;
#     $| = 1;
#     select($oldfh);
    # this also does not work
#     $logf->autoflush(1);
    print $logf $txt;
    # this does not work
#     $logf->flush;
#     $logf->sync;
    # this also does not work
#     $select()->flush();
#     $select()->sync();
    close $logf;
}

[download]

Finally, I found a comment on StackOverflow, saying that the UTF-8 encoding layer adds another buffering layer.
(See the discussion between Ikegami and Ωmega on https://stackoverflow.com/questions/33812618/can-you-force-flush-output-in-perl)

Is this the reason why the logfile only gets created and written after either writing some kilobytes of many messages, or when the program much later terminates?

Does there exist a way to circumvent this buffering and to force creation and flushing of the file at all?

In reply to Is it possible at all to flush buffer and commit file write, if UTF-8 encoding is being used?? by dissident

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