[gambit-list] My New Segfault (Re: Tracking Down a Segfault)

Joel J. Adamson <adamsonj@email.unc.edu> adamsonj at email.unc.edu
Fri Sep 5 12:35:49 EDT 2008

>>>>> "Marc" == Marc Feeley <feeley at iro.umontreal.ca> writes:

    Marc> The source of the problem is that you are storing references
    Marc> to Scheme objects (i.e. the ___SCMOBJ type) in places that the
    Marc> garbage collector has no knowledge of (i.e. C variables), so
    Marc> when there is a GC these references are not updated when the
    Marc> object referenced is moved by the GC.

I had a feeling this might be the problem, but not the vocabulary to
express it ;)    

    Marc>   [Side note: amusingly the code actually works for f64vectors
    Marc> of length greater or equal to 128 because those vectors are
    Marc> allocated as "still" objects that aren't moved by the GC!]

Yeah, I tried testing it with larger and larger vectors and couldn't get
the segfault ;)

    Marc> 1) call the macro ___F64VECTORREF (from gambit.h) which does
    Marc> not allocate a flonum, and hence does not invoke the GC

Okay, duh: I guess I'll check gambit.h next time I write a C function.    

    Marc> 2) use an inline code c-lambda where the vector is passed in
    Marc> one of the ___argxxx variables.  These variables ***are know
    Marc> to the GC and will be updated properly***.  But don't store
    Marc> their content in another C variable!

Good to know the difference.

Now I'm getting a segfault at mark_continuation() (mem.c:2430).  

   if (ra1 == ___GSTATE->internal_return)
            ___WORD actual_ra = ___FP_STK(fp,___RETI_RA);
here===>    ___RETN_GET_FS_LINK_GCMAP(ra1,fs,link,gcmap,nextgcmap)

This happened before when I was accumulating a list instead of
outputting and then throwing away each run of the simulation --- am I
just using up memory and the GC can't handle it? (is that even
possible?)  Or does it mean that I have other unreachable objects
unknown to the garbage collector?


Joel J. Adamson
University of North Carolina at Chapel Hill
CB #3280, Coker Hall
Chapel Hill, NC 27599-3280

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