Re: ocaml sefault in bytecode: unanswered questions

[Sylvain Le Gall <sylvain@le-gall.net>]

On 07-08-2009, ivan chollet <ivan.chollet@free.fr> wrote:
>
> This GDB was configured as "i386-marcel-freebsd"...(no debugging symbols
> found)...
>
> Not very informative. So here are my questions:

I suppose you are running freebsd ? Which version of freebsd, of ocaml ? 

>
>  
>
> -          What is the best way to produce and analyze core dumps in ocaml?
> Should I compile in bytecode or native? Is there any special gdb "trick"
> that gives you more information? Is there any special "trick" while
> compiling the ocaml runtime to make it throw more information?
>

gdb is not the perfect tool to debug ocaml program. You should give a
try to ocamldebug which is a better option for bytecode (see below for
options). Bytecode is more informative when coming to reporting
backtrace (at least with old version of ocaml). 

Compile every program with "-g" option (just like gcc). 

If you have compiled everything with "-g" option, you can also use the
environment variable OCAMLRUNPARAM="b" to get a backtrace for your
exception, at runtime.

> -          Then, my main question is actually: in bytecode, what can produce
> segfaults? My ocaml code is completely standard, excepted that I use the
> Marshal module. So my question is rather: outside the Marshal module, what
> can cause segfault?

Some part of the bytecode are just standard C, everything can cause a
segfault just as C. These errors are not very common but it is possible
that some case are not well handled on freebsd. Most probably a porting
issue.

Marshal module can easily trigger a segfault when you map the loaded data
to a type which doesn't match the dumped data.

Example: 
List.length (Marshal.from_string (Marshal.to_string 1234 []) 0);;

Here the integer value is marshalled and then unmarshalled as a list ->
segfault.

>
> -          Slightly unrelated question: I have been able to generate
> segfaults by running ocaml code that: 1) iterates recursively through a list
> reference 2) changes the reference while still iterating on it. For example,
> you just do a "List.iter myfun !myref", and within the function myfun, you
> do stuff like "myref := List.filter somefilterfunction !myref". It is not
> good to program like this, but for some reason I thought ocaml would not
> segfault on that. Is this expected behavior? If it's not, I'll be happy to
> provide some simple source code that illustrates it. (nevermind I have
> actually cleaned all my code base from these dirty uses of references)
>

Could you provide a minimal example code for this error ? I don't think
this should generate a segfault.


Regards
Sylvain Le Gall