2008/9/4 Joel J. Adamson <adamsonj@email.unc.edu> <adamsonj@email.unc.edu>

Dear Gambit-List,

I've been accumulating a library of functions for simulating population
processes (called "genxic"), and I'm running into some pesky errors.
This code includes quite a lot of foreign functions, interfacing with
the GNU Scientific Library.

1) When running numerically intense simulations, I'll get a segfault:

,----
| Process scheme segmentation fault
`----

This might be after 50 iterations of a simulation when I'd planned to do
1152.

Where do I go from there? I've tried running gsc and gsi in GDB, but
pretty often I can't get a backtrace after the segfault. The conditions
under which the segfault occurs varies. Sometimes I get it after
loading the library.

Recompile Gambit with debug options flipped on:

Unpack the Gambit sources.

Run ./configure --enable-single-host

(Should we flip on --enable-debug here also ?? )

Replace the two occurrences -O1 in Makefile with -O0 -g (more important than -O0 is -g, I heard a rumor that debugging info may be better if you do -O1 -g but have no idea).

make; make install

Ensure your sourcecode is compiled with (declare (safe)) (that's the default setting). I don't remember if the -O0 -g will be reflected to gcc/g++ when you do compile-file from within your app, so consider adding this to cc-opts of compile-file.

2) I've gotten this bizarre error from one of my procedures, and would
like to know where to go to debug it:

,----
| *** ERROR IN | genxic.o1#31| -- (Argument 1) F64VECTOR expected
| (f64vector-ref
| '#u16(47806 51966 47806 51966 47806 51966 47806 51966 47806 51966 47806 519...
| 1)
`----

I am not initializing any u16 vectors (that I know of), so how can I
tell where this comes from, and how do I investigate, or find what
procedure genxic.o1#31 is? I tried repeating the compilation, and this
is what I got

Dumping:
#<primitive | genxic.o2|>
#<primitive | genxic.o2#0|>
#<primitive | genxic.o2#1|>
#<primitive | genxic.o2#2|>
#<primitive | genxic.o2#3|>
...
#<primitive | genxic.o2#30|>
#<primitive | genxic.o2#31|>
#<procedure 1+>
#<procedure 1->
#<procedure atom?>
#<procedure xcons>
#<procedure make-list>...

How are these procedures labeled, and how do I figure out which one it
is?

Using the instructions above, this should be fixed.

In order to track down the source of your bugs, it may be important for you to note that

- Throwing exceptions with no handler/catcher may sigsegv your app.

- You should presuppose that ___argXX variables inside Scheme-to-C calls are trashed after C-to-Scheme calls. I.e.: If you have declared a c-define or c-lambda that takes parameters, these will appear as ___arg0, ___arg1 etc. in the C code of the c-define / c-lambda. These will have correct contents all until you invoke Scheme functions from the C code. not afterwards. So back them up to the stack if you do.

- Ensure you didn't call the version of any Gambit-internal function with broken parameters. The names of them start with ##. Very simple example:

>(cadr 4)
*** ERROR IN (console)@3.1 -- (Argument 1) PAIR expected
(cadr 4)
1>
>(##cadr 4)
/usr/local/bin/gsc: line 5: 17999 Segmentation fault

If your problems persist, ask the list further.