Question

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?

Update: I’ve decided to use ctypes. MUCH easier.

Answer 1

I agree with theller, you should use ctypes instead. It’s always easier than thinking about typemaps.

But, if you’re dead set on using swig, what you need to do is make a typemap for void** that RETURNS the newly allocated void*:

%typemap (in,numinputs=0) void** (void *temp) {     $1 = &temp; }  %typemap (argout) void** {     PyObject *obj = PyCObject_FromVoidPtr(*$1);     $result = PyTuple_Pack(2, $result, obj); } 

Then your python looks like:

import test success, p = test.Operation() theAnswer = 0.0f if success:    theAnswer = test.GetFieldValue(p)    test.Cleanup(p) 

Edit:

I’d expect swig to handle a simple by-value void* arg gracefully on its own, but just in case, here’s swig code to wrap the void* for GetFieldValue() and Cleanup():

%typemap (in) void* {     $1 = PyCObject_AsVoidPtr($input); }