ctypes passing a pointer to a field in a struct

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?

Mark Tolonen · Accepted Answer · 2012-11-15 04:36:28Z

5

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))

answered Nov 15, 2012 at 4:36

Mark Tolonen

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Comments

DaveP · Accepted Answer · 2012-11-15 06:13:00Z

It seems that when you reference mystruct.data, a copy of the data is made. I say this because the python command type(mystruct.data), returns str, rather than a C type.

I presume that you are not able to modify the external_c_function to accept the pointer at the start of the structure, as this would be the most obvious solution. Therefore you need to somehow do C style pointer arithmetic in python - i.e. get the address of mystruct (possibly using ctypes.pointer), then figure out a way to increment this pointer by the appropriate number of bytes.

I don't know how you can do such pointer arithmetic in python, or if it's even possible to do in any robust manner. However, you could always wrap external_c_function in another C function which does the necessary pointer arithmetic.

edit

Mark's answer solves the problem neatly. My comment about why the error occurs is still correct.

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