Re: memory.profile() messes up the vector heap on Alpha/Linux?

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Subject: Re: memory.profile() messes up the vector heap on Alpha/Linux?
From: Peter Dalgaard BSA (p.dalgaard@biostat.ku.dk)
Date: Sat 27 Nov 1999 - 07:27:54 EST


Message-ID: <x2vh6pndt1.fsf@blueberry.kubism.ku.dk>

Naoki Takebayashi <ntakebay@bio.indiana.edu> writes:

> Hello,
> I have been trying to debug a problem with R-0.90.0 (this bug was in
> 0.65.1, too). The following code results in seg faults. It doesn't
> seg-fault on Linux/intel.
>
> > memory.profile()
> > gc()
>
> As long as I don't execute memory.profile(), there is no problem with
> garbage collection. So I think that memory.profile() screws up the
> heap in some way.
>
> When it seg-faults, it dies in the function compactPhase() in memory.c,
>
> while (vfrom < R_VTop) {
> s = BACKPOINTER(*vfrom);
> switch (TYPEOF(s)) { /* get size in bytes */ /* <- dies here */
>
> it segfaults becuase s becomes a pointer to 0x0, when the while loop is
> almost finishing, i.e., vfrom is close to R_VTop (vfrom = 0x20001840ef8
> and R_VTop = 0x20001841198).
>
> I tried to see what's going on in do_memoryprofile() in memory.c by
> stepping with gdb, but I couldn't see what was going wrong (well, to be
> honest, I didn't fully understand what was going on).
>
> I thought someone may have some insights about the problem or strategy to
> debug this problem. I wanted to set some kind of watch point, so the
> gdb breaks when the address 0x20001840ef8 is changed. But I'm not
> really good at gdb yet, so I'm wondering if someone can help.

The gdb command would be 'watch *0x20001840ef8' (it's important to use
the address as a number so that one gets a hardware watchpoint), but
are you sure that's the right address to watch? I'd try the address of
s itself ('p &s' to print it). Also, what happens if you turn
gctorture() on just before the call to memory.profile? Usually slows
things down horribly, but causes a crash much closer to the actual
damage.

Happy hunting...

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)             FAX: (+45) 35327907
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