Question

I am porting visual c++ 6 application to python using ctypes.
It uses same windows library dll. and I am only porting windows forms to python application.

and I am stucked when I am sending cpp structure to dll in python.

I have tried to find solution on the struct differeces. from cpp code, there is a struct tagging(tagTest) but there is no equivalent in python code. is there any solution for that?

this is cpp that I want to port from.

========================cpp part================================

BOOL CSMessage(HWND hWnd, const char* string1, const char* string2, const char* ltagTest, int structlen);

typedef struct tagTest
{
    char item1                     [  8];   char _item1;
    char item2                     [  1];   char _item2;
} TcTest, *LPTest;  

void Initiator()
{
    TcTest  ctest   ={0};
    memset(&ctest,0x20,sizeof TcTest);
    move(ctest.item1,"TESTSTRI");
    move(ctest.item2,"1");

    mhandler.CSMessage(GetSafeHwnd(),"String","Call",(char*)&ctest,sizeof TcTest);
}

=========================pythonpart============================

csMessageProto=ctypes.WINFUNCTYPE(BOOL, HWND, c_char_p, c_char_p, c_char_p, c_int)
csMessageParams=((1,"hWnd",0),(1,"string1",0),(1,"string2",0),(1,"ltagTest",0),(1,"structlen",0))
CSMessage=csMessageProto(("CSMessage",testDll), csMessageParams)

class tagTest(ctypes.Structure):
    _fields_ = [
        ('item1', c_char*8),('_item1', c_char),
        ('item2', c_char*1),('_item2', c_char)
    ]
PtagTest=ctypes.POINTER(tagTest)

utestblock=tagTest()
utestblock.item1=str("TESTSTRI")
utestblock.item2=str(1)

CSMessage(hwnd, "String", "Call", ctypes.byref(utestblock), ctypes.sizeof(utestblock));

---

revision history

1. added declaration of dll function both cpp and python
2. revised with @avl_sweden suggestions

=====================result==============================

after applying @avl_sweden’s suggestion,
After test, the error msg is this.

ctypes.ArgumentError: argument 4: <type 'exceptions.TypeError'>: wrong type

and for the type

print "Is Type : "+str(ctypes.byref(utestblock))

Is Type : <cparam 'P' (023e5968)>

=========================================================

so it’s looks like types are different in def and instance.

in first line in python code

1: csMessageProto=ctypes.WINFUNCTYPE(BOOL, HWND, c_char_p, c_char_p, c_char_p, c_int)

the fourth c_char_p is not matching

16: CSMessage(hwnd, "String", "Call", ctypes.byref(utestblock), ctypes.sizeof(utestblock));

to match the differences.

I changed code to

1: csMessageProto=ctypes.WINFUNCTYPE(BOOL, HWND, c_char_p, PtagTest, c_char_p, c_int)

and code returns no error. but I cannot find if it is sent through. becuase there was no return from dll.

do you think this is the right solution?

=====================================================================================

i have been tested and it seems it is right solution.

Thank you again.

Answer 1

I see four things wrong here:

First, your python Structure definition doesn’t match the one in the C-code snippet. The python one should be:

class tagTest(ctypes.Structure):
    _fields_ = [
        ('item1', c_char*8), ('_item1',c_char),
        ('item2', c_char*1), ('_item2',c_char)
    ]

The second thing I see, is that you’re sending a pointer to a pointer to the CSMessage function.

Could you try:

utestblock=tagTest()
utestblock.item1=str("TESTSTRI")
utestblock.item2=str(1)

loadeddll.CSMessage(hwnd, "String", "Call", ctypes.byref(utestblock),
                    ctypes.sizeof(utestblock))

The third issue is less severe, and this is that the string "TESTSTRING" does not fit into 8 bytes.

The fourth issue I see, is that the types of the default parameters is wrong.

Try this instead:

csMessageParams=((1, "hWnd", 0), (1, "string1", ""), (1, "string2", ""),
                 (1, "ltagTest", ""), (1, "structlen", 0))