[ctypes-users] Using python ctypes to call a c function in a dll that returns a pointer to a structure

[John M. <te...@bl...>]

Hello,

I am new to ctypes and I am very excited about its capabilities.
Unfortunately, I have not found any examples 
that have helped me solve my issue so I thought I would post it here.

I have a C-DLL 'userlib_win64.dll' as a test 
function that multiplies 2 numbers and returns a pointer to a structure
which as elements has the inputs and their 
product as follows,

typedef struct {
double  a;
double  b;          
double  c;
} MULT;

_declspec(dllexport) MULT 
*multNumbers(_int32,_int32);

MULT *multNumbers(_int32 a,_int32 b) {
MULT *m = malloc(sizeof(MULT));
m->a = (double) 
a;          
m->b = (double) b;          
m->c = m->a * m->b;
return(m);
}

I am using a python script called 
'mult_test.py'to call this function as follows,

from ctypes import *
ulib = cdll.userlib_win64
mult = ulib.multNumbers

class MULT(Structure): pass
MULT._fields_ = [
              ("a", c_double), 
              ("b", c_double), 
              ("c", c_double)]
mult.argtypes = [c_int,c_int]      
mult.restype = pointer(MULT)
#mult(1,2)
p = MULT()

p.a = 3.
p.b = 2.
p.c = p.a*p.b
print p.a, p.b, p.c
q = mult(4,5)

I receive an error message as follows,

D:\codes\Simlib-1.0\bin\lib>mult_test
Traceback (most recent call last):
  File "D:\codes\Simlib-1.0\bin\lib\mult_test.py", line 10, in <module>
    mult.restype = pointer(MULT)
TypeError: _type_ must have storage info

If I change line 
10 to,

mult.restype = POINTER(MULT),

I receive a different error,

D:\codes\Simlib-1.0\bin\lib>mult_test
3.0 2.0 6.0
Traceback (most recent call last):
  File "D:\codes\Simlib-1.0\bin\lib\mult_test.py", line 18, in <module>
    q = mult(4,5)
WindowsError: exception: access violation reading 0x0000000000000004

I would be very thankful for any help 
someone out there may be able to offer ,

Cheers, John McElhaney