Re: please set both MALLOC_PERTURB_ and MALLOC_CHECK_ envvars

[Jim Meyering <jim@meyering.net>]

Chong Yidong wrote:

> Jim Meyering <jim@meyering.net> writes:
>
>>> If you develop emacs, or even if you just build from sources regularly,
>>> on a glibc-based system, do us all a favor and build with these envvar
>>> settings:
>>>
>>>     export MALLOC_PERTURB_=$((RANDOM % 255 + 1))
>>>     export MALLOC_CHECK_=3
>>>
>>> Why?  Because that helps you expose malloc-related problems far earlier.
>>> I've found numerous bugs that way.
>>
>> Most of that argument is valid, but I have to confess this time
>> a compiler problem appears to be at fault.
>> [...]
>>     gcc version 4.7.0 20110528 (experimental) (GCC)
>>
>> everything works fine, and yes!, even with these envvar settings:
>>
>>     MALLOC_PERTURB_=117
>>     MALLOC_CHECK_=3
>>
>> So at least for a little while, I'll be using that.
>
> Maybe we should add these environment variables to the Hydra build.

Good idea.

> But
> what to use for MALLOC_PERTURB---117, or the value in your previous
> message?

You can use any value between 1..255 for MALLOC_PERTURB_
That chooses the byte that glibc will use to memset all freed buffers.
I set it like this in a shell start-up file:

    export MALLOC_PERTURB_=$(($RANDOM % 255 + 1))

Using a random value is slightly better than using a fixed one
in case your fixed value is someday just the right/wrong value
to mask a problem.  At least with a random value, if you rerun
the test in a different shell, the odds are good you won't use
the unfortunate setting again.