Python Ctypes passing a structure pointer defined in .h file.

Question

I'm new to both ctypes and C and having trouble passing a struct pointer variable into a c-function called in python via ctypes. Please bear with me if it is too basic and obvious. Below is how my c-code looks like.

#include "mylib.h"   (inside this mylib.h file MYSTRUCT is defined)

struct MYSTRUCT* modifystruct(a,b,c,d,e)
{
MYSTRUCT *mystpointer;
.....
.....
return mystpointer;
} 


int mycfunction(mystpointer)
MYSTRUCT *mystpointer;
{
.........
.........
.........
}

Like above, modifystruct function update *mystpointer which is a pointer to MYSTRUCT and returns it. And mycfunction is to pass the returned mystpointer. In C, this works fine inside main function. But when I try to load the ".so" file using ctypes into python, it fails and I think I'm not correctly defining the argtype for mystpointer. Below is the brief python code I wrote. Let's say the above c-code is compiled to "mycmodule.so".

mylib=cdll.LoadLibrary("mycmodule.so")
mycfunction=mylib.mycfunction
mycfunction.restype=c_int
mycfunction.argtypes=[c_void_p]
mystpointer=c_void_p()

In the C code, I have to define mystpointer type as "MYSTRUCT *mystpointer;" However, I do not know how to do so in ctypes... instead, I defined the types as c_void_p but this triggers the failure. Thanks in advance!

Answer 1

I think what you may be missing is knowing exactly where you want the struct memory to be allocated. The c code below provides a function that allocates memory for the struct and returns a pointer to it (new_struct()).

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct {
    int a;
    int b;
} my_struct;

my_struct *new_struct()
{
    my_struct *struct_instance = (my_struct *)malloc(sizeof(my_struct));
    memset(struct_instance, 0, sizeof(my_struct));
    return struct_instance;
}

int modify_struct(my_struct *ms) {
    ms->a = 1;
    ms->b = 2;
    return 0;
}

void print_struct_c(my_struct *ms) {
    printf("my_struct {\n"
           "    a = %d\n"
           "    b = %d\n"
           "}\n", ms->a, ms->b);
}

From Python, to get the pointer, call the C function that does the allocation, then you can pass that to the other C functions that take it as a parameter.

import ctypes

lib_file_path = <<< path to lib file >>>

# Very simple example of how to declare a ctypes structure to twin the
# C library's declaration. This doesn't need to be declared if the Python
# code isn't going to need access to the struct's data members.
class MyStruct(ctypes.Structure):
    _fields_ = [('a', ctypes.c_int),
                ('b', ctypes.c_int)]

def print_struct(s):
    # Print struct that was allocated via Python ctypes.
    print("my_struct.a = %d, my_struct.b = %d" % (s.a, s.b))

def print_struct_ptr(sptr):
    # Print pointer to struct. Note the data members of the pointer are 
    # accessed via 'contents'.
    print("my_struct_ptr.contents.a = %d, my_struct_ptr.contents.b = %d" 
          % (sptr.contents.a, sptr.contents.b))


my_c_lib = ctypes.cdll.LoadLibrary(lib_file_path)

# If you don't need to access the struct's data members from Python, then 
# it's not necessary to declare MyStruct above. Also, in that case,
# 'restype' and 'argtypes' (below) can be set to ctypes.c_void_p instead.
my_c_lib.new_struct.restype     =  ctypes.POINTER(MyStruct)
my_c_lib.modify_struct.argtypes = [ctypes.POINTER(MyStruct)]

# Call C function to create struct instance.
my_struct_c_ptr = my_c_lib.new_struct()
print_struct_ptr(my_struct_c_ptr)

my_c_lib.modify_struct(my_struct_c_ptr)
print_struct_ptr(my_struct_c_ptr)

# Allocating struct instance from Python, then passing to C function.
my_struct_py = MyStruct(0, 0)
print_struct(my_struct_py)

my_c_lib.modify_struct(ctypes.byref(my_struct_py))
print_struct(my_struct_py)

# Data members of Python allocated struct can be acessed directly.
my_struct_py.a = 555

my_c_lib.print_struct_c(ctypes.byref(my_struct_py)) # Note use of 'byref()'
                                                    # to invoke c function.

The code above has been updated to include an example of how to allocate a struct instance via Python, and how to access the data members of either the C allocated or Python allocated struct (note differences in the print functions).