[R] help debugging segfaults

Liaw, Andy andy_liaw@merck.com
Thu, 13 Jun 2002 12:48:09 -0400

OK, so I linked my code against libefence and then run my test code under
gdb.  It still finishes without reporting any problem.  (This is with
Calloc() and Free(), instead of S_alloc().)  Can anyone tell me what to do
or look for next?  I'll be eternally grateful!


> -----Original Message-----
> From: Thomas Lumley [mailto:tlumley@u.washington.edu]
> Sent: Thursday, June 13, 2002 12:05 PM
> To: Liaw, Andy
> Cc: 'r-devel@stat.math.ethz.ch'
> Subject: RE: [R] help debugging segfaults
> On Thu, 13 Jun 2002, Liaw, Andy wrote:
> >
> > Everyone suggested using some malloc debugger (such as 
> Electric Fence).  All
> > I did was following half of what BDR suggested below, i.e., 
> changing all the
> > S_alloc() calls to Calloc() and Free().  I didn't get to 
> try efence, and the
> > problem seems to have disappeared!
> No, the *symptoms* have disappeared. This is not necessarily an
> improvement.
> > As I read it from the R-exts manual, the difference between 
> S_alloc and
> > Calloc is that S_alloc takes memory from the heap, whereas 
> Calloc takes
> > memory addtional to the heap.  So can anyone explain what's 
> going on?  It's
> > still kind of mysterious to me...
> It looked as if your code was writing to/reading from memory 
> that didn't
> belong to it. What happens then depends on who the memory 
> does belong to,
> and what they are doing with it. There's no guarantee that 
> just because it
> doesn't crash today that it won't in the future, or worse, 
> start giving
> subtly wrong answers.
> A segmentation fault is a good thing in this context -- it's 
> a sign that
> the operating system has protected you from the consequences of your
> actions by killing the program.
> 	-thomas

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