Error in ctypes with a function and a structure pointer

Question

I am trying to use C++ function (in a dll) with python. To do this, i use ctypes library.

My C++ code is library to use a webcam which exports a set of C functions.

This the function that I want use:

/*! Release the grabber object. Must be called, if the calling application
    does no longer need the grabber.
    @param hGrabber The handle to grabber to be released.
    @sa IC_CreateGrabber
*/
void AC IC_ReleaseGrabber( HGRABBER *hGrabber ); ///< Releas an HGRABBER object.

It is the release memory function

this the HGRABBER structure:

//////////////////////////////////////////////////////////////////////////
/*! This is the handle of an grabber object. Please use the HGRABBER type to access
    this object.
*/
typedef struct HGRABBER_t__ { int unused; } HGRABBER_t; ///<Internal structure of the grabber object handle.
#define HGRABBER HGRABBER_t* ///< Type of grabber object handle. Used for all functions. 

My code is:

Necessary Structure HGRABBER (it is named HGRABBER_TYPE in my case)

class HGRABBER_T(ctypes.Structure):
    _fields_ = [("unused", ctypes.c_int)] 

HGRABBER_TYPE = ctypes.POINTER(HGRABBER_T)

The call function:

self._dllref =  ctypes.windll.LoadLibrary(DLL_PATH)
self._grabber_handle = self._dllref.IC_CreateGrabber() 
 ....
 ...
 ....
self._dllref.IC_ReleaseGrabber(ctypes.pointer(HGRABBER_TYPE(self._grabber_handle)))

and finally, the error that i receive:

self._dllref.IC_ReleaseGrabber(ctypes.byref(HGRABBER_TYPE(self._grabber_handle)))
TypeError: expected HGRABBER_T instead of int

I checked other related posts, for example this but it didn't help me..

I appreciate any help!

UPDATE:

I applied restype and argtypes in order to specify argument and return values (thanks!).

With the modifications, the code is:

self._dllref =  ctypes.windll.LoadLibrary(DLL_PATH)
self._dllref.IC_CreateGrabber.restype = HGRABBER_TYPE        
self._grabber_handle = self._dllref.IC_CreateGrabber() 
...
..
self._dllref.IC_ReleaseGrabber.argtypes = [HGRABBER_TYPE]
self._dllref.IC_ReleaseGrabber(self._grabber_handle)

I should have multiple mistakes, now my error is:

self._dllref.IC_ReleaseGrabber(self._grabber_handle)
WindowsError: exception: access violation writing 0x6E657137

I checked the argument for the function (HGRABBER *hGrabber), argtypes for the release function should be:

self._dllref.IC_ReleaseGrabber.argtypes = [ctypes.POINTER(HGRABBER_TYPE)]

With this modification, i get another different error:

self._dllref.IC_ReleaseGrabber(self._grabber_handle)
WindowsError: exception: access violation reading 0x6B0F1FE0

I am searching these errors, it seems a bad conversion of the pointer which i don't understand, the structure seems very simple to cast and i don't see what i miss..

UPDATE 2

I missed to add ctypes.byref when i call the function, it has to be:

 self._dllref.IC_ReleaseGrabber(ctypes.byref(self._grabber_handle))

UPDATE 3

Unfortunately, i am getting a random error related with pointer argument ( (ctypes.byref(self._grabber_handle)) ), sometimes the release function accepts the object but sometimes give this error:

    _dllref.IC_ReleaseGrabber(ctypes.byref(_grabber_handle))
WindowsError: exception: access violation reading 0x5A694F44

Answer 1

You can set the return type of IC_CreateGrabber such that you don't need the recast when you call IC_ReleaseGrabber .

For example:

self._dllref =  ctypes.windll.LoadLibrary(DLL_PATH)

# here set the return type
self._dllref.IC_CreateGrabber.restype = HGRABBER_TYPE

# here set the argtypes
self._dllref.IC_ReleaseGrabber.argtypes = [ctypes.POINTER(HGRABBER_TYPE)]

self._grabber_handle = self._dllref.IC_CreateGrabber() 

self._dllref.IC_ReleaseGrabber(ctypes.byref(self._grabber_handle))

By setting the restype and argtypes of the library functions, ctypes knows how to handle the values from the C side of things.