[Swig-user] typemap for C error return pattern

[Joel B. M. <jm...@ga...>]

Hi,

I have a C(++) library which propagates errors by returning error codes 
in an integer return from every function.  Thus the typical functions 
look like:

long DoSomething(...);  // do an operation with no result other than 
exceptional error
long GetValue(..., RESULT *lpResult); // get the RESULT structure with 
possible exceptional error

I'm evaluating wrapping this library with swig for access from python. 
I want to map the non-zero exceptional error returns to python 
exceptions.  To this end, I've made an out typemap for long:

%typemap(out) long {
     if( ERROR_SUCCESS != $1 )
         SWIG_exception_fail(SWIG_RuntimeError, ExceptionToString($1));
}

This appears promising, but the DoSomething (as above) should return 
PyNone.  Instead I get a python exception "SystemError: error return 
without exception set" because the resultobj gets initialized to 0 and 
is never touched.  I could probably set resultobj to Py_RETURN_NONE in 
my typemap(out), but that's not correct for functions like GetValue (as 
above) should return the lpResult value through a typemap(argout).

I would assume this is fairly common to have integer error returns in a 
C library, but the closest example I could find is 
http://stackoverflow.com/questions/9456723/typemap-and-exception-for-error-codes-from-c-functions-for-swig-python 
where the exceptional error code is a parameter which a 
typemap(in/argout) can suppress the error parameter nicely from the 
higher level API.

How can I have my generated resultobj be PyNone or the appropriate 
result in general?

Thanks,
Joel