[erlang-questions] How to track down intermittent segfaults in a threaded NIF

[Dominic Morneau]

Can you give it a try with "+Mea min" in erl options? This should make
Erlang fall back to malloc for all allocators, hopefully making guard
malloc more effective.

Dominic

2018年5月30日(水) 5:15 Igor Clark <igor.clark@REDACTED>:

> OK. Thanks very much Scott. I've got all this working using both those
> extra options, and it does seem to make the NIF crash a lot sooner than
> previously, which is great. But I'm still only seeing "process_main" in
> the crashed thread, so I'm not much closer to knowing where the illegal
> access is. I wonder if it's in lots of places because of what I'm doing
> with the callback and the thread. I hope not.
>
> I'll do some more digging, and tomorrow I'll try out a debug emulator
> build as well.
>
> Thanks very much for helping me get this far!
>
> On 29/05/2018 16:31, Scott Ribe wrote:
> >> On May 29, 2018, at 9:16 AM, Igor Clark <igor.clark@REDACTED> wrote:
> >>
> >> So, do I have this right: the point of the Guard Malloc is to make the
> crash happen at the time of allocation, rather than delayed until something
> trying to access it triggers the segfault; so if I get a crash while
> running like this, I should be able to just check in the Console debug log,
> and the stack trace should show where the bug actually is?
> > At the time of the illegal access, not the allocation. Yes, that's the
> point, you get a stack trace showing you illegal access.
> >
> > However, the BEAM allocator will reduce its effectiveness. When you
> malloc in your C code, you get a block set up such that accessing just past
> it (or potentially before it) will cause an immediate crash. When you free
> it, it's then set up such that accessing will cause an immediate crash. But
> if you use Erlang's allocation routines, Erlang may malloc a bigger block
> with those protections, then hand out multiple suballocations, and access
> beyond the end of one of those can simply corrupt the next one without
> crashing at that point.
> >
> > You should also be using MallocScribble & MallocPreScribble.
> >
> >
> >
>
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