Question

I need to construct the following data type in Python for passing to a C function:

struct {
  unsigned a,b,c;
  char data[8];
};

However, I need to actually pass a pointer to the data field to the function, not a pointer to a struct, and I can’t figure out how to do this.

Here is what I have so far:

from ctypes import *                                                           
class MyStruct(Structure):                                              
  _fields_ = [("a",c_uint), ("b",c_uint), ("c",c_uint), ("data",c_char*8)]
mystruct = MyStruct(0,1,8,"ABCDEFGH")
external_c_function(mystruct.data)

Now in C I have this function:

int external_c_function(char *data) {
  int a = ((unsigned *)data)[-1];
  int b = ((unsigned *)data)[-2];
  int c = ((unsigned *)data)[-3];
  ...
}

The problem is, when the function gets called, “data” correctly points to “ABCDEFGH”, but when I try to get the rest of the struct data preceding it, it is garbage. What am I doing wrong? Isn’t mystruct held sequentially in memory like a real C struct? I suspect something funny is going on with the array: am I actually doing something silly like this?

struct {
  unsigned a,b,c;
  char *data; // -> char[8]
};

and if so, how do I do it correctly?

Answer 1

You pass a pointer to an element of a structure by reference, using the offset of the element:

external_c_function(byref(mystruct,MyStruct.data.offset))