[Rd] debugging strange segfault

[Prof Brian Ripley]

The symptom is that of the compiled code overrunning the storage it was 
allocated.  I tracked down an instance (in SuppDists) yesterday and have 
seen quite a few in my time.

I would expect you to find a write to memory off one or other end of an
array, and compiling with bounds checking in place *may* help.  (g77 is
less useful than some commercial Fortran compilers at this, and some
people write code that cannot be checked.)

Increasing some storage areas for output arrays will often make the 
problem go away, but it is not actually a solution since it may just 
relocate an illegal write to a non-fatal place.

Hope that helps enough,

Brian

On Thu, 8 Jan 2004, Liaw, Andy wrote:

> Dear R-devel,
> 
> Can anyone give me some hints on how to go about debugging a strange
> segfault in my randomForest package?  Here's the scoop:
> 
> A user reported segfault when running predict() in the randomForest package.
> I asked for the data and code.  The combination runs fine under WinXPPro,
> but does give segfault on one of our Linux boxes running R (1.7.0 through
> R-devel_2004-01-08) on Mandrake 9.0. 
> 
> The predict.randomForest() function calls a C function "runforest" via
> .C(..., DUP=FALSE, ...), which in turns calls a Fortran subroutine
> "testreebag" within a for loop.  The segfault seems to occur right after
> finishing the runforest() function in C and returning to R.  I inserted the
> line:
> 
> Rprintf("Done!\n");
> 
> as the last line of the runforest() function and got the following output:
> 
> > library(randomForest, lib.loc="~/rlibs")
> > arabid <- read.table('arabidopsis.out', sep=' ', header=T)
> > arabid <- arabid[,-which(names(arabid) == "X0")]
> > set.seed(1)
> > fit <- randomForest(arabid[,-1], arabid[,1], ntree=100)
> > predict(fit, arabid[,-1])
> Done!
> 
> Program received signal SIGSEGV, Segmentation fault.
> 0x40152a48 in malloc () from /lib/libc.so.6
> 
> [If I change the DUP=FALSE in the .C() call to TRUE, I get the following:
> Program received signal SIGSEGV, Segmentation fault.
> 0x080b412b in Rf_duplicate (s=0x1) at duplicate.c:75
> 75          switch (TYPEOF(s)) {
> ]
> 
> At this point I'm clueless as to what to do next, and would very much
> appreciate any help!
> 
> Best,
> Andy
> 
> Andy Liaw, PhD
> Biometrics Research      PO Box 2000, RY33-300     
> Merck Research Labs           Rahway, NJ 07065
> mailto:andy_liaw at merck.com        732-594-0820
> 
> 
> 
> 
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595