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Re: Stumped with select->can_read (buffered)

by tye (Sage)
on Aug 03, 2012 at 19:17 UTC ( #985326=note: print w/replies, xml ) Need Help??

in reply to Stumped with select->can_read

while(@ready = $sel->can_read) #... $line = <$fh>;

Don't mix buffered I/O with select. can_read() doesn't return a handle because there is nothing currently left to read from the file handle. But <$fh> left plenty of stuff to be read in the buffer.

- tye        

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Re^2: Stumped with select->can_read (buffered)
by w1r3d (Initiate) on Aug 03, 2012 at 20:18 UTC

    That makes sense. Would I have to use read() in this case?

    I tried doing:

    while(@ready = $sel->can_read) { foreach my $fh(@ready) { my $line = ""; my $buf = ""; while(read($fh, $buf, 1024)) { $line .= $buf; } } }

    Didn't work for me, maybe because LENGTH (=1024) is too big? Sorry, I'm not familiar with read(), as you can see :/

    EDIT: nvm. I'm reading the link you posted now. Will report back if I still can't figure it out. Thanks!

      It's best to use sysread, but you will have to figure out a way to detect line endings. Maybe after concating the sysread data into a temp buffer then pull off lines with a regex or split. Read perldoc -f sysread. It will read as much as it can in a non-blocking manner. This will try to read in 1024 byte chunks, but will read less if that is all that is there.
      while(@ready = $sel->can_read) { foreach my $fh(@ready) { my $line = ""; my $buf = ""; # while(read($fh, $buf, 1024)) while( my $bytes_read = sysread( $fh, $buf, 1024 ) > 0 ) ) { print "$bytes_read\n"; { $line .= $buf; } } }

      I'm not really a human, but I play one on earth.
      Old Perl Programmer Haiku ................... flash japh

        I tried doing that, but it didn't seem to work. My guess is that because one particular $fh is very busy, the while loop never ends?

        This *seems* to work at the moment:

        while(@ready = $sel->can_read) { foreach my $fh(@ready) { my $line = ""; sysread( $fh, $line, 1024 ); } }

        My concern now is whether 1024 is too small of a buffer that I could miss something (if that's possible), or missing a newline character because of the buffer size. Would it be horrible (performance wise) to read one byte at a time until I reach a newline character, thus forming a real $line?

        Thanks again!


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