Python ctypes arguments with DLL - pointer to array of doubles

Question

I am a newbie coder working with ctypes in Python, and attempting to use functions from a DLL written in C. I have found a lot of similar questions to mine on SO but nothing that answers this type of conundrum.

I have loaded the DLL just fine, but one of the functions I need to call asks for a pointer to an array of 6 doubles to dump values into, and I cannot figure out how to give the function what it needs via Python. (My failed attempts to just do this in C are another story.)

I've tried various permutations of ctypes.POINTER(c_double), using byref(), POINTER(c_double * 6), etc and I get type errors at worst, or access violations at best, for all of them.

From the DLL documentation:

int swe_calc_ut (double tjd_ut, int ipl, int iflag, double* xx, char* serr)

The function does calculations using the time in Julian Days to return the longitude, latitude, etc, of a planetary body as doubles.

The closest I've gotten to passing in data types that the DLL will accept is using this code, just trying to get any of the 6 doubles out of swe_calc_ut:

dll = windll.LoadLibrary(# file path)

# retype the args for swe_calc_ut
py_swe_calc_ut = dll.swe_calc_ut
py_swe_calc_ut.argtypes = [c_double, c_int, c_int, c_double, c_char_p]
py_swe_calc_ut.restype = None

tjd = c_double(# some value from elsewhere)

returnarray = c_double()
errorstring = create_string_buffer(126)

py_swe_calc_ut(tjd, c_int(0), c_int(64*1024), returnarray, errorstring)

When I try to run this as-is, I get the error:

OSError: exception: access violation writing 0x0000000000000000

Using byref() gives me a type error, etc.

If anyone could point me in the right direction for getting the desired doubles out of the original DLL function, I would be eternally grateful; I'm stumped and can't get this off the ground.

Answer 1

This (untested) should work. The 4th parameter is type POINTER(c_double). The equivalent of C's double returnarray[6] type is c_double * 6 and an instance of that type is returnarray= (c_double * 6)(). Also, if you've declared the argument types you don't need to wrap the input parameters such as int(0); passing 0 is fine:

dll = windll.LoadLibrary(# file path)

# retype the args for swe_calc_ut
py_swe_calc_ut = dll.swe_calc_ut
py_swe_calc_ut.argtypes = [c_double, c_int, c_int, POINTER(c_double), c_char_p]
py_swe_calc_ut.restype = None

tdj = 1.5 # some value
returnarray = (c_double * 6)()
errorstring = create_string_buffer(126)

py_swe_calc_ut(tjd, 0, 64*1024, returnarray, errorstring)