Thread safety of alloca

[Denis Koroskin]

On Sun, 25 Oct 2009 03:34:24 +0300, dsimcha <dsimcha at yahoo.com> wrote:

> Does anyone know if alloca() is by some chance not thread safe?  I'm  
> working
> on improving my parallelization library and I keep running into all  
> kinds of
> erratic behavior (but not stack overflows) when I use alloca() instead  
> of heap
> as an optimization, but when I disable this optimization, everything  
> seems to
> work.  I do know that several threads are likely calling alloca() at the  
> same
> time and with requests large enough that their stacks might need to be  
> expanded.
>
> At first, I thought this was just a symptom of a subtle race condition,  
> but I
> tried inserting sleep statements and synchronized blocks in various  
> places
> around the alloca() calls to change the timing of things and the  
> conclusion is
> the same:  malloc() works, alloca() doesn't.

You are probably misusing alloca (e.g. returning it from a function) or  
overwriting the stack space in some other way. This is strange otherwise.  
I don't think it is thread-unsafe since it doesn't access any global state.

Is the problematic source code available for inspection online?

btw, alloca is implemented in druntime\src\compiler\dmd\alloca.d