Two dll function calls only work with random print() in between

Question

I use a camera SDK with a DLL (ctypes.WinDLL).

camera_path = 'cam://0'.encode('utf-8')
handle = xdll.XDLL.open_camera(camera_path, 0, 0)
# (The handle returned is 1)
xdll.XDLL.set_property_value_f(handle, b'IntegrationTime', c_double(2500))

This gives an the following error:

OSError: exception: access violation reading 0x0000000000000001

The weird thing is, that the code works as expected with a random print in between:

camera_path = 'cam://0'.encode('utf-8')
handle = xdll.XDLL.open_camera(camera_path, 0, 0)
# (The handle returned is 1)
print('random print')
xdll.XDLL.set_property_value_f(handle, b'IntegrationTime', c_double(2500))

Any idea what print() does to prevent such an error?

If time.sleep(1) is used instead of print() it shows the error, so the time spent on printing should not make a difference.

EDIT: The interesting lines from the header file:

typedef int            XCHANDLE;    ///< Handle type used to identify an initialised session to a camera.

XCHANDLE    IMPEXPC XC_OpenCamera                   (const char * pCameraName = "cam://default", XStatus pCallBack = 0, void * pUser = 0);  ///< @sa XCamera::Create
ErrCode     IMPEXPC XC_SetPropertyValueF            (XCHANDLE h, const char * pPrp, double dValue, const char * pUnit);

methods in xdll.XDLL:

    open_camera = _xenethDLL.XC_OpenCamera
    open_camera.restype = c_int32  # XCHANDLE


    set_property_value_f = _xenethDLL.XC_SetPropertyValueF
    set_property_value_f.restype = c_ulong  # ErrCode
    set_property_value_f.argtypes = (c_int32, c_char_p, c_double)

How about `sys.stdout.flush()`? I _think_ that's what print calls. — Peter, Nov 21 '22 at 15:02
Most likely, a duplicate: [\[SO\]: C function called from Python via ctypes returns incorrect value (@CristiFati's answer)](https://stackoverflow.com/a/58611011/4788546). — CristiFati, Nov 21 '22 at 15:09
You are not supplying the required `pUnit` parameter to `XC_SetPropertyValueF`. — jasonharper, Nov 21 '22 at 15:58

riffraff · Answer 1 · 2022-11-21T16:16:43.580

0

I forgot to add a required parameter (char * pUnit) to the argtypes. Therefore, i got some weird/undefined behaviour.

Now, after i added the parameter the code executes as expected.

edited Nov 21 '22 at 16:16

answered Nov 21 '22 at 16:14

riffraff

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Two dll function calls only work with random print() in between

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