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Re: Separating stdout and stderr

From: Maxim Kuvyrkov
Subject: Re: Separating stdout and stderr
Date: Wed, 30 Mar 2016 16:44:29 +0300

Hi Ben,
Hi Christophe,

[TL;DR] Will it be a problem for dejagnu to merge test's stderr into stderr 
with an equivalent of bash's 2>&1 ?

From my investigations of the sanitizer testsuite, it seems to be hitting 
several unrelated issues in DejaGnu.  Ben's patch is addressing one of those, 
but there is at least one other issue that also related to handling of stdout 
and stderr.

The sanitizer testsuite is maintained as part of LLVM project, and it is using 
a different testing framework.  In the LLVM copy of the sanitizer testsuite 
every testcase has an annotation to call itself with stderr merged into stdout 
via 2>&1 shell syntax.  This is done [at least in part] to avoid races between 
output to stdout and stderr streams.  [Note that tests tend to print using 
printf, while sanitizer libraries print to stderr with fprintf.]

Dejagnu, on the other hand, seems to be giving the test process separate stdout 
and stderr streams, and then merges the output from the two internally.  This 
causes non-determinism in output due to races in how kernel flushes the two 

I understand that this is, primarily, an issue with the test itself, and, 
indeed, it can be avoided by the test using exclusively stderr or exclusively 
stdout, but I don't think any dejagnu testsuite is relying on separate stdout 
and stderr.  So maybe we should just merge the two?

Maxim Kuvyrkov

> On Mar 30, 2016, at 11:47 AM, Ben Elliston <address@hidden> wrote:
> On Wed, Mar 30, 2016 at 10:15:46AM +0200, Christophe Lyon wrote:
>> Well, I re-ran it, and I still have no evidence of using standard_wait.
>> Are you running native validations?
> Yes.
>> I remind you that we are using a cross-testing configuration, so we might be
>> using a different code path than the one you are looking at.
> Oh, I see. Sorry.
>> As I said earlier, I'm looking at local_exec in remote.exp and it
>> does contain a code fragment similar to the one you've just
>> patched. Does it need the same change?
> Yes, probably. Can you give it a try?
> Cheers, Ben
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