I have a C interface that looks like this (simplified):
extern bool Operation(void ** ppData);
extern float GetFieldValue(void* pData);
extern void Cleanup(p);
which is used as follows:
void * p = NULL;
float theAnswer = 0.0f;
if (Operation(&p))
{
theAnswer = GetFieldValue(p);
Cleanup(p);
}
You'll note that Operation() allocates the buffer p, that GetFieldValue queries p, and that Cleanup frees p. I don't have any control over the C interface -- that code is widely used elsewhere.
I'd like to call this code from Python via SWIG, but I was unable to find any good examples of how to pass a pointer to a pointer -- and retrieve its value.
I think the correct way to do this is by use of typemaps, so I defined an interface that would automatically dereference p for me on the C side:
%typemap(in) void** {
$1 = (void**)&($input);
}
However, I was unable to get the following python code to work:
import test
p = None
theAnswer = 0.0f
if test.Operation(p):
theAnswer = test.GetFieldValue(p)
test.Cleanup(p)
After calling test.Operation(), p always kept its initial value of None.
Any help with figuring out the correct way to do this in SWIG would be much appreciated. Otherwise, I'm likely to just write a C++ wrapper around the C code that stops Python from having to deal with the pointer. And then wrap that wrapper with SWIG. Somebody stop me!
Edit:
Thanks to Jorenko, I now have the following SWIG interface:
% module Test
%typemap (in,numinputs=0) void** (void *temp)
{
$1 = &temp;
}
%typemap (argout) void**
{
PyObject *obj = PyCObject_FromVoidPtr(*$1, Cleanup);
$result = PyTuple_Pack(2, $result, obj);
}
%{
extern bool Operation(void ** ppData);
extern float GetFieldValue(void *p);
extern void Cleanup(void *p);
%}
%inline
%{
float gfv(void *p){ return GetFieldValue(p);}
%}
%typemap (in) void*
{
if (PyCObject_Check($input))
{
$1 = PyCObject_AsVoidPtr($input);
}
}
The python code that uses this SWIG interface is as follows:
import test
success, p = test.Operation()
if success:
f = test.GetFieldValue(p) # This doesn't work
f = test.gvp(p) # This works!
test.Cleanup(p)
Oddly, in the python code, test.GetFieldValue(p) returns gibberish, but test.gfv(p) returns the correct value. I've inserting debugging code into the typemap for void*, and both have the same value of p! The call Any ideas about that?