As the title indicates, I'm trying to solve a fairly basic problem. I'm dealing with a function that prints to STDOUT at a 'C' level, which means that stuff like perlio, in memory filehandles, localized globs and such don't work. So I decided on tempfiles. I would create the tempfile, redirect STDOUT, call my C function, it writes to the tempfile, I close STDOUT, open it again appropiately and read from the tempfile. Everything will be peachy keen! Unfortunately I'm getting a fairly strange error. When I close the redirected STDOUT, it empties the tempfile! Here's the code:

my $old_stdout;
my( $fh, $fn );
sub redir_stdout
{
  open $old_stdout,">&STDOUT" or die "Couldn't open old_out to &STDOUT
+ $!";
  ( $fh, $fn ) = tempfile();
  print "tempfile: $fn\n";
  open STDOUT, '>&=', $fh or die "Couldn't open STDOUT to tmpfile $fn:
+ $!";
}

sub undir_stdout
{
  close STDOUT; 
  close $fh;
  open STDOUT, ">&=", $old_stdout or die "Couldn't reopen STDOUT to ol
+d_out: $!";

  open $fh, "<$fn" or die "Couldn't open temp file, $fn : $!";
  if(wantarray)
  {
    return <$fh>;
  }

  else
  {
    local $/;
    return <$fh>;
  }
}
[download]

I use it like so:

redir_stdout();
stupid_c_function();
die undir_stdout();
[download]

Which of course returns nothing. When I print the filename of the tempfile and comment out undir_stdout, I can examine the tempfile after the script exits and find that it contains all of the expected data. I played with comments in the undir_stdout function and it seems to be the 'close STDOUT' line specifically that clears the file. (The reason these things are actual functions is a tad complicated, mostly having to do with perl code that generates perlcode).

Update:

I drastically simplified my code, so now it looks like:

  open my $old_out, ">&STDOUT" or die $!; 
  open STDOUT, "> sc_out" or die $!; 
  send_command('i', "@_", 0 );
  #close STDOUT;
[download]

That works, in that ./sc_out contains the expected output when the script terminates. If I uncomment "close STDOUT", sc_out is empty.

Update the second:

Solved the problem. It turns out the C code in send_command was buffering the output, so when I closed STDOUT, the buffer couldn't flush to the file when the script terminated. Adding a fflush makes it all work nicely.