swig python interfacing to function using void **

Question

BACKGROUND. I have an API (third party provided) consisting of C header files and a shared library. I have managed to create a shell script for the build environment, along with a simple interface file for swig. I am trying to make this API accessible to an IPython environment such that I don't have to compile C code all the time to communicate with the associated hardware that leverages this API for I/O.

PROBLEM. The first function call I need to do creates a board handle (some arbitrary "object" that is used for all other function calls in the C-side. The function accepts a void **, assuming the underlying function is probably malloc-ing memory, has some sort of internal structure, and allows accessing this memory by some of the other functions. Anyhow, I can't seem to properly interface to this from Python due to the lack of support for void * and receive a typeError.

The offending C code snippet, with typedef's/defines extracted from the underlying header files is:

    #define WD_PVOID  void*
    typedef WD_PVOID   WD_BOARD;
    typedef WD_UINT32  WD_RetCode;
    #define WD_EXPORT  extern
    #define WD_CHAR8  char
    #define WD_UINT32  unsigned int
    #---------------------------------------
    //prototype
    WD_EXPORT WD_RetCode wd_CreateBoardHandle( WD_BOARD *pBoardHandle, const WD_CHAR8  *pUrl );

    //interpreted prototype
    //extern unsigned int wd_CreateBoardHandle( void* *pBoardHandle, const char  *pUrl );

A third party provided provided example (written in C) uses the function as so (I removed superfluous stuff) :

  int main(int argc, char *argv [])
  {
     WD_RetCode rc;
     Vhdl_Example_Opts  cmdOpts = VHDL_EXAMPLE_DEFAULTS;
     char urlBoard[VHDL_SHORT_STRING_LENGTH];
     WD_BOARD           BoardHandle;
     sprintf(urlBoard, "/%s/%s/wildstar7/board%d", cmdOpts.hostVal, cmdOpts.boardDomain, cmdOpts.boardSlot);

     rc = wd_CreateBoardHandle(&BoardHandle,urlBoard);
  }

and lastly, my watered down swig interface file (I have been trying swig typedef's and *OUTPUT with no success):

    %module wdapi
    %{
    #include "wd_linux_pci.h"
    #include "wd_types.h"
    #include "wd_errors.h"
    %}
    %import "wd_linux_pci.h"
    %import "wd_types.h"
    %import "wd_errors.h"
    %include <typemaps.i>

    WD_EXPORT WD_RetCode wd_CreateBoardHandle( WD_BOARD *pBoardHandle, const WD_CHAR8 *pUrl );

    WD_EXPORT WD_RetCode wd_OpenBoard( WD_BOARD  BoardHandle );

What I would like to be able to do is to call that function in python as so:

    rslt,boardHandle = wdapi.wd_CreateBoardHandle("/foo/bar/etc")

Please let me know if I can provide any other information and I greatly appreciate your help/guidance towards a solution! I have spent days trying to review other similar issues posted.

EDIT. I manipulated some typedefs from other posts with similar issues. I am able to now call the functions and receive both a value in rslt and boardHandle as an object; however, it appears the rslt value is gibberish. Here is the new swig interface file (any thoughts as to the problem?):

      %module wdapi 
      %{
      #include "wd_linux_pci.h"
      #include "wd_types.h"
      #include "wd_errors.h"
      %}

      %import "wd_linux_pci.h"
      %import "wd_types.h"
      %import "wd_errors.h"
      %include <python/typemaps.i>


      %typemap(argout) WD_BOARD *pBoardHandle 
      {
        PyObject *obj = PyCObject_FromVoidPtr( *$1, NULL );
        $result = PyTuple_Pack(2, $result, obj);
      }

      %typemap(in,numinputs=0) WD_BOARD *pBoardHandle (WD_BOARD temp) 
      {
        $1 = &temp;
      }

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


      WD_EXPORT WD_RetCode wd_CreateBoardHandle( WD_BOARD *pBoardHandle, const WD_CHAR8 *pUrl );

      WD_EXPORT WD_RetCode wd_OpenBoard( WD_BOARD  BoardHandle );

      WD_EXPORT WD_RetCode wd_DeleteBoardHandle( WD_BOARD  BoardHandle );

      WD_EXPORT WD_RetCode wd_IsBoardPresent( const WD_CHAR8 *pUrl, WD_BOOL *OUTPUT );

Christopher Zanchi · Accepted Answer · 2015-09-08 13:31:41Z

I resolved my own question. The edited swig interface file, listed above in my original post, turned out to correct my issue. Turns out that somewhere along the way, I mangled the input to my function call in python and the error code returned was "undefined" from the API.

On another note, while investigating other options, I also found "ctypes" which brought me to a solution first. Rather than dealing with wrapper code and building a 2nd shared library (that calls another), ctypes allowed me to access it directly and was much easier. I will still evaluate which I will move forward with. ctypes python code is listed below for comparison (look at the c-code example I listed in the original post) :

    from ctypes import cdll
    from ctypes import CDLL
    from ctypes import c_void_p
    from ctypes import addressof
    from ctypes import byref

    import sys

    #Update Library Path for shared API library
    sys.path.append('/usr/local/lib');

    #Load the API and make accessible to Python
    cdll.LoadLibrary("libwdapi.so")
    wdapi = CDLL("libwdapi.so")

    #Create the url for the board
    urlBoard='/<server>/<boardType>/<FGPAType>/<processingElement>'

    #Lets create a void pointer for boardHandle object
    pBoardHandle=c_void_p()


    #now create & open the board
    rtn = wdapi.wd_CreateBoardHandle(byref(pBoardHandle),urlBoard)
    if (rtn) :
      print "Error"
    else :
      print "Success"

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