Re: Problems with nested structures

Stefan Schukat <SSchukat@xxxxxxxxx> writes:

> Hello, 
>
> I have a problem getting nested structures from a C-API interface.
> The call succeeds but all members in the nested elements are empty.
>
> Could my definition of the call be wrong or is there any restriction in
> C-Types that 
> structures with structures with union are not transported ? 
>
> I'm working with ctypes 0.6.3 and Python 2.2.1.
>
>       Thanks 
>
>               Stefan
>
>
>
> Background:
>
> The call I want to wrap is defined as
>
> int GetData( unsigned char** pRecvBuffer, unsigned short* pObjNum); 
>
> where the first element is a Structure with nested structures in C, e.g.
> sample C-Code
>
>   unsigned char*     RecvBuffer;
>   WORD               nObj;
>   MyStruct*              Data;
>   iRet = GetData(&RecvBuffer, &nObj);
>
>   if (iRet &&  nObj>0)
>   {
>       Data = (MyStruct*)(RecvBuffer); 
>
> where the structure is defined as:
>
> typedef struct {
>   unsigned char        Length;
>   LogStruct            Log;
> } MyStruct;
>
> with:
>
> typedef struct {
>   unsigned char     Length;
>   unsigned char     Data[8];
>   IDUnion               ID;
> } LogStruct;
>
> with:
>
> typedef union
> {
>   unsigned long Dword;
>   unsigned short Word[2];
>   unsigned char Byte[4];
> } IDUnion;
>
>
> and I wrapped it with:
>
> class MyStruct(ctypes.Structure):
>     _fields_ = [
>                 ("Length", ctypes.c_ubyte),
>                 ("Log", LogStruct),
>                 ]
>
>
> class LogStruct(ctypes.Structure):
>     _fields_ = [("Length", ctypes.c_ubyte),
>                 ("Data", ctypes.c_ubyte * 8),
>                 ("ID",   IDUnion ),
>                 ] 
>
> class IDUnion(ctypes.Union):
>     _fields_ = [("Dword", ctypes.c_ulong),
>                 ("Word", ctypes.c_ushort * 2),
>                 ("Byte", ctypes.c_ubyte * 4)
>                 ]
>
> def GetData(self):
>     LocalpRecvBuffer = ctypes.pointer(MyStruct())
>     LocalpObjNum = ctypes.c_ushort()
>     self._Handle.GetData(ctypes.byref(LocalpRecvBuffer), 
> ctypes.byref(LocalpObjNum))
>     if LocalpObjNum.value > 0:
>        return LocalpRecvBuffer.contents, LocalpObjNum.value
>     else:
>        return None, 0 
>
> All elements of Logstruct and IDUnion aree still empty after the call.

I think the problem is that you pass a pointer to MyStruct with byref.
This should work:

def GetData(self):
    LocalpRecvBuffer = MyStruct()
    LocalpObjNum = ctypes.c_ushort()
    self._Handle.GetData(ctypes.byref(LocalpRecvBuffer), 
ctypes.byref(LocalpObjNum))
    if LocalpObjNum.value > 0:
       return LocalpRecvBuffer.contents, LocalpObjNum.value
    else:
       return None, 0 

Thomas



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