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scarypajamas

help with python ctypes

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I'm trying to call a C function using python ctypes. I want to pass and receive a struct from the function but am having a lot of trouble with it. I have a C dll with the following code:
-- test.h --
struct vec3f
{
	float x;
	float y;
	float z;
};

__declspec(dllexport) struct vec3f addvec(struct vec3f a, struct vec3f b);




-- test.c --
struct vec3f addvec(struct vec3f a, struct vec3f b)
{
	a.x += b.x;
	a.y += b.y;
	a.z += b.z;
	return a;
}


My python code looks like this:
-- tester.py --
from ctypes import *
import os
libtest = cdll.LoadLibrary(os.getcwd() + '/libpydlltest.dll')


class vec3f(Structure):
	_fields_ =  [("x", c_float),
				("y", c_float),
				("z", c_float)]
libtest.addvec.argtypes = [vec3f, vec3f]
libtest.addvec.restype = vec3f
				
a = vec3f(10, 15, 20)
b = vec3f(4, 5, 6)

print a.x, a.y, a.z
print b.x, b.y, b.z

a = libtest.addvec(a,b)

print a.x, a.y, a.z


The problem is, when I run my python script, I get the following error:
Quote:
Traceback (most recent call last): File "tester.py", line 20, in <module> a = libtest.addvec(a,b) ValueError: Procedure called with not enough arguments (4 bytes missing) or wrong calling convention

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The latter part of the error message may be your issue (wrong calling convention). Perhaps give windll.LoadLibrary a try. Let me know if that works.

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Odd...

I replaced cdll.LoadLibrary with windll.LoadLibrary and got the following message:

Quote:

Traceback (most recent call last):
File "tester.py", line 9, in <module>
print libtest.add(a, b)
ValueError: Procedure probably called with too many arguments (8 bytes in excess)

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Hmm, I'm not particularly familiar with ctypes, but I'll see what I can do to help you out some. I was reading through the documentation and when I came across the section on structure alignment, I was thinking that this may be your issue. If your DLL is packing things together tightly, and ctypes is packing things together on 8-byte boundaries, that would explain the "8 bytes in excess" (4 for each argument). The best advice I can give at the moment is to play around with the structure packing.

P.S. I quickly copied and pasted your code and it works fine on OS X, so it's just a simple little calling convention/packing issues I would expect.

[Edited by - aryx on March 1, 2010 5:03:55 AM]

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