Python ctypes argument errors

Question

I wrote a test dll in C++ to make sure things work before I start using a more important dll that I need. Basically it takes two doubles and adds them, then returns the result. I've been playing around and with other test functions I've gotten returns to work, I just can't pass an argument due to errors. My code is:

import ctypes
import string

nDLL = ctypes.WinDLL('test.dll')
func = nDLL['haloshg_add']
func.restype = ctypes.c_double
func.argtypes = (ctypes.c_double,ctypes.c_double)
print(func(5.0,5.0))

It returns the error for the line that called "func":

ValueError: Procedure probably called with too many arguments (8 bytes in excess)

What am I doing wrong? Thanks.

Answer 1

You probably got the calling conventions mixed up. I'm guessing you have a C function declared something like this:

double haloshg_add(double d1, double s2)
{
    return d1+d2;
}

This will use the C calling convention by default. The simplest approach would be to change the calling convention in your ctypes code:

nDLL = ctypes.CDLL('test.dll')

If you wanted to change the calling convention in the C code to stdcall (to match ctypes.WinDLL) then you would do this:

double __stdcall haloshg_add(double d1, double s2)

Whatever you do, only do one of these changes. If you do both you'll have the reverse failure!

If it were me, I'd just change the Python code to use C calling convention (use CDLL). That change has the least impact.