Re: [Caml-list] Matching exceptions in C code

[John Whitington <john@coherentgraphics.co.uk>]

Hi.

> On 21 Jun 2017, at 16:39, Richard W.M. Jones <rich@annexia.org> wrote:
> 
> On Sat, Jun 03, 2017 at 07:44:31AM +0100, Richard W.M. Jones wrote:
>> I'm trying to catch a Unix_error exception in C code, and extract the
>> errno from it.  Getting the errno is fine, but the problem is matching
>> on the Unix.Unix_error exception.
>> 
>> My original code used the documented method:
>> 
>>  retv = caml_callbackN_exn (*cb, 4, args);
>> 
>>  if (Is_exception_result (retv)) {
>>    retv = Extract_exception (retv);
>>    if (Field (retv, 0) == *caml_named_value ("Unix.Unix_error"))
>>      ...
>> 
>> However the if statement never matched the exception.  (I verified on
>> the OCaml side that the correct exception is being thrown).
>> 
>> After reading:
>> 
>>  http://caml.inria.fr/pub/ml-archives/caml-list/2006/05/097f63cfb39a80418f95c70c3c520aa8.en.html
>>  http://caml.inria.fr/pub/ml-archives/caml-list/2009/06/797e2f797f57b8ea2a2c0e431a2df312.en.html
>> 
>> it seems I'm not the first person to have this problem.  It may be
>> that Unix is linked twice, but I'm not really sure about that.  I'm
>> using -output-obj with unix.cmxa linked into the object file, and also
>> -lunix in the link step, which is the documented way to do things, but
>> I don't know if that means the Unix module initializes itself twice.
>> 
>> Anyway, I modified the code to this, which *works* and is actually
>> more convenient than the documented method (the real code matches on
>> several other system exceptions as well):
>> 
>>    ...
>>    retv = Extract_exception (retv);
>>    exn_name = String_val (Field (Field (retv, 0), 0));
>>    if (strcmp (exn_name, "Unix.Unix_error") == 0) {
>>      int errcode = code_of_unix_error (Field (retv, 1));
> 
> So I found out today that this doesn't work for every exception.
> 
> For 'End_of_file' in particular, 'exn_name' ends up pointing to a bit
> of random memory (unfortunately it's not NULL or otherwise easily
> detectable), and so the program crashes in the strcmp.

End_of_file, Invalid_argument, and Failure are not defined in pervasives.ml, they are “internal” in some sense. Perhaps this is the cause?

John

-- 
John Whitington
Director, Coherent Graphics Ltd