Pipe autoflush

Zielony has asked for the wisdom of the Perl Monks concerning the following question:

Hello, Monks,

I'm trying to autoflush pipe, but it doesn't work.

#!/usr/bin/perl -w

use strict;
use IO::Handle;
use POSIX qw /mkfifo/;

mkfifo "fifo", 0777 unless (-p "fifo");

pipe (PARENTR, SAPW);
pipe (SAPR, PARENTW);
SAPW->autoflush(1);
PARENTW->autoflush(1);

my $pid = fork;
if ($pid) {
    close PARENTR; close PARENTW;

    while (1) {
        open FIFOR, "fifo";
        my $line = <FIFOR>;
        close FIFOR;
        print SAPW $line;
        close SAPW; # don't want this
        print <SAPR>;
    }
} else {
    close SAPR; close SAPW;

    open STDIN , '<&PARENTR';
    open STDOUT, '>&PARENTW';

    exec "cat";
    die $!;
}
[download]

As you see, I have to close pipe SAPW, but I need it for next whiles.

Thank for your help.

Comment on Pipe autoflush Download Code

Replies are listed 'Best First'.
Re: Pipe autoflush by almut (Canon) on Aug 23, 2007 at 12:33 UTC
I think your problem is that you're reading from `SAPR` in list context (because of the `print`), so it's trying to read too much, and thus blocks... Try `print scalar <SAPR>;` [download] or `my $sap_line = <SAPR>; # scalar context print $sap_line;` [download] In that case you don't have to close `SAPW`.	[reply] [d/l] [select]
Re^2: Pipe autoflush by Zielony (Acolyte) on Aug 23, 2007 at 12:37 UTC
Works. Thank you very much! UPDATE ...but only for cat. For other programs, for example md5sum, not.	[reply]
Re^3: Pipe autoflush by almut (Canon) on Aug 23, 2007 at 15:57 UTC
Smooth communication depends on both sides agreeing on a common protocol. Not all programs operate in a line based fashion like `cat`. For example, if you're having `md5sum` read its data from stdin (instead of supplying a list of filenames), it is reading the entire input (which may include newlines), then prints the digest of that data to stdout, and quits. IOW, simply replacing `cat` with `md5sum` in the code above can't work. If you let us in on what you're ultimately trying to achieve, we might be able to provide better assistance... :)	[reply] [d/l] [select]
Re^4: Pipe autoflush by Zielony (Acolyte) on Aug 23, 2007 at 16:33 UTC
Re^5: Pipe autoflush by almut (Canon) on Aug 23, 2007 at 20:04 UTC
Some notes below your chosen depth have not been shown here
Re: Pipe autoflush by technojosh (Priest) on Aug 23, 2007 at 14:18 UTC
I try to make a habit, when I need autoflush on, of setting: `$\| = 1;` [download] near the start of my script	[reply] [d/l]
Re^2: Pipe autoflush by ikegami (Patriarch) on Aug 23, 2007 at 14:54 UTC
That would autoflush the currently `select`ed file handle, usually `STDOUT`. That would be totally useless here.	[reply] [d/l] [select]