Hi,

0 means both STDERR and STDOUT were buffered

No, that's not correct, it simply reflects the state of the "autoflush" setting. From $|:

Default is 0 (regardless of whether the channel is really buffered by the system or not; $| tells you only whether you've asked Perl explicitly to flush after each write).

ie the default is they are buffered

The issue here is not whether they are buffered, as they always are in one form or another, but the issue is when that buffer is flushed. According to Suffering from Buffering:

When a filehandle is attached to the terminal, ... it is in line buffered mode by default. A filehandle in line buffered mode has two special properties: It's flushed automatically whenever you print a newline character to it, and it's flushed automatically whenever you read from the terminal. ... But now let's try it with STDOUT attached to a file instead of to the terminal... it isn't line-buffered; only filehandles attached to the terminal are line-buffered by default. ... The filehandle STDERR, which is normally used for error logging, is always in line buffered mode by default.

And the autoflush setting

forces a flush right away and after every write or print on the currently selected output channel.

So STDERR is always in line-buffered mode by default, and Perl decides whether STDOUT should be line-buffered or not depending on whether it's attached to a terminal or not.

Regards,
-- Hauke D

Consider Program 1:

#!/usr/bin/perl
use warnings;

for my $i (1..5)
{
   $x+1; # here just to throw an error to STDERR
   print "$i\n";
}

__END__
Useless use of addition (+) in void context at C:\Projects_Perl\testin
+g\demoStdErrorTiming1.pl line 6.
Name "main::x" used only once: possible typo at C:\Projects_Perl\testi
+ng\demoStdErrorTiming1.pl line 6.
Use of uninitialized value $x in addition (+) at C:\Projects_Perl\test
+ing\demoStdErrorTiming1.pl line 6.
Use of uninitialized value $x in addition (+) at C:\Projects_Perl\test
+ing\demoStdErrorTiming1.pl line 6.
Use of uninitialized value $x in addition (+) at C:\Projects_Perl\test
+ing\demoStdErrorTiming1.pl line 6.
Use of uninitialized value $x in addition (+) at C:\Projects_Perl\test
+ing\demoStdErrorTiming1.pl line 6.
Use of uninitialized value $x in addition (+) at C:\Projects_Perl\test
+ing\demoStdErrorTiming1.pl line 6.
1
2
3
4
5
[download]

#!/usr/bin/perl
use warnings;
$|=1;               ### BIG DEAL HERE ### Unbuffers STDOUT

for my $i (1..5)
{
   $x+1; #here just to throw an error to STDERR
   print "$i\n";
}

__END__
Useless use of addition (+) in void context at C:\Projects_Perl\testin
+g\demoStdErrorTiming2.pl line 7.
Name "main::x" used only once: possible typo at C:\Projects_Perl\testi
+ng\demoStdErrorTiming2.pl line 7.
Use of uninitialized value $x in addition (+) at C:\Projects_Perl\test
+ing\demoStdErrorTiming2.pl line 7.
1
Use of uninitialized value $x in addition (+) at C:\Projects_Perl\test
+ing\demoStdErrorTiming2.pl line 7.
2
Use of uninitialized value $x in addition (+) at C:\Projects_Perl\test
+ing\demoStdErrorTiming2.pl line 7.
3
Use of uninitialized value $x in addition (+) at C:\Projects_Perl\test
+ing\demoStdErrorTiming2.pl line 7.
4
Use of uninitialized value $x in addition (+) at C:\Projects_Perl\test
+ing\demoStdErrorTiming2.pl line 7.
5
[download]

Program 2's error messages are interleaved with "normal" prints because the normal prints are being prematurely flushed out. There is a big,(can be HUGE) performance penalty for this but, this makes debugging easier.

Hi Marshall,

Program 1's error messages come out right way while stdout messages come later because the stdout's filehandle's output buffer is not ready to be "flushed", "printed".

Sorry, but that's not quite correct either. If run from the terminal, your Program 1's STDOUT will be line-buffered and will flush its STDOUT on every newline, but the order in which STDOUT and STDERR output is intermixed on the terminal is AFAIK not well-defined. On my machine, the output of both programs looks the same when run from the terminal.

See also my post here.

Regards,
-- Hauke D

Ok, fair enough. There are some fine details here, one of which is whether the std output is going to a terminal or not? I did a copy and paste from my dev environment which is not exactly the same as running from the command line directly.

My demo is not "bullet proof", but I think it gives the idea of what can happen.

Thanks for your good points.