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

I came across a strange problem earlier today: when switching to a new filehandle with select(), sometimes output will go to the new filehandle before it goes to the old one. Here is some relevant code to help explain: 
# Create the TEE filehandle, which will print to both STDOUT and to a 
+file
open (TEE, "| tee $name") or die "Teeing off: $!\n";
select(TEE); # Now print goes to the TEE handle by default
$| = 1; # unbuffer TEE

my $stmt = $dbh->prepare("$sql") or die $DBI::errstr; # $sql takes 30-
+40 secs. to run
$stmt->execute() or die $DBI::errstr;

while (my @array = $stmt->fetchrow()) {
    print;
}
$stmt->finish();

select(STDOUT);
&printfooter(); # prints a here-doc, about 10 lines
close(STDOUT) or die "Error closing: $!\n";

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The SQL query that gets printed to the TEE filehandle is pretty huge - it generally takes 35-40 seconds for that block to execute, according to a benchmark. And, it doesn't seem to matter whether the filehandle is buffered. So, can anyone explain why this is happening?

Replies are listed 'Best First'.
Re: Order of output when switching filehandles
by Shendal (Hermit) on Aug 14, 2000 at 22:44 UTC
    Instead of using tee, have you considered just printing directly to the file? I assume you are wanting the results to print to screen as well as to a log file. Why not do something like this:
    open(FILE, ">$name") or die "Failed file open: $!\n";

my $stmt = $dbh->prepare("$sql") or die $DBI::errstr; # $sql takes 30-
+40 secs. to run
$stmt->execute() or die $DBI::errstr;

while (my @array = $stmt->fetchrow()) {
    print;
    print FILE "$_";
}
$stmt->finish();

close(FILE);

    [download]
    I didn't test this, but I'd bet that it's faster than running tee -- not to mention quite a bit easer to customize.
    Hope that helps,
    Shendal
[reply]
[d/l]
      I ended up using tee because I use more print's than are in this example, and it becomes pretty unwieldy when you have to duplicate them all.
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(crazyinsomniac) Re: Order of output when switching filehandles
by crazyinsomniac (Prior) on Aug 14, 2000 at 22:45 UTC
    I'm not sure, but you should close ( TEE ) when you're done with it.

    "cRaZy is co01, but sometimes cRaZy is cRaZy".
                                                          - crazyinsomniac

[reply]
      Erk. That seems to have fixed it. Thanks.

      Just out of curiosity, can any of you gurus explain why that happens?

[reply]
        I may be smoking something, but usually $| = 1; doesn't completely disable buffering, just most of Perl's buffering. You still need to close it to force the OS buffer to be flushed.
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Re: Order of output when switching filehandles
by lhoward (Vicar) on Aug 14, 2000 at 23:27 UTC
    Have you considered using IO::Tee instead of launching tee as separate process? 
    #!/usr/bin/perl -w

use strict;

use IO::Tee;

open FH,">somefile" or die "error opening $!";
my $tee=IO::Tee->new(\*STDOUT,\*FH);
select $tee;
print "foo\n";
print "bar\n";

close FH;

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[reply]
[d/l]
      I considered that, but the machine I'm using does not have IO::Tee installed on it, so launching it as a seperate process is easier for all parties involved - it might not be the "right way" to do it, but theres no reason for it to not work and security is not a concern.
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