Question

I have a structure which contains character array on C side

stuct s
{
    int x;
    char buffer[100];
}

and on my python side I define

class myS(ctypes.Structure):
    _fields_ = [("x", c_int),
         ("buffer",type(create_string_buffer(100)))]

Now, when I do

buf = create_string_buffer(64)
s1 = myS(10,buf)

It gives me error

TypeError: expected string or Unicode object, c_char_Array_100 found

I want a string which will be changed by my C function. how to do it?

Answer 1

You don’t have to create a buffer. The buffer is in the structure when you instantiate it.

Here’s a quick DLL:

#include <string.h>

struct s
{
    int x;
    char buffer[100];
};

__declspec(dllexport) void func(struct s* a)
{
    a->x = 5;
    strcpy(a->buffer,"here is the contents of the string.");
}

And here’s Python code to call it:

import ctypes

class myS(ctypes.Structure):
    _fields_ = [
        ("x", ctypes.c_int),
        ("buffer",ctypes.c_char * 100)]

s1 = myS()
dll = ctypes.CDLL('test')
dll.func(ctypes.byref(s1))
print s1.buffer
print s1.x

Output:

here is the contents of the string.
5