Question

I am trying to access elements of a structure from ctypes. The structure is created in an init function in the C code and pointer to it is returned to Python. The problem I am having is that I get a segfault when trying to access elements of the returned structure. Here is the code:

The C code (which I’ve called ctypes_struct_test.c):

#include <stdio.h>
#include <stdbool.h>

typedef struct {
    bool flag;
} simple_structure;

simple_structure * init()
{
    static simple_structure test_struct = {.flag = true};
    if (test_struct.flag) {
        printf("flag is set in C\n");
    }
    return &test_struct;
}

The Python code (which I’ve called ctypes_struct_test.py):

#!/usr/bin/env python

import ctypes
import os

class SimpleStructure(ctypes.Structure):
    _fields_ = [('flag', ctypes.c_bool)]

class CtypesWrapperClass(object):

    def __init__(self):

        cwd = os.path.dirname(os.path.abspath(__file__))
        library_file = os.path.join(cwd,'libctypes_struct_test.so')

        self._c_ctypes_test = ctypes.CDLL(library_file)

        self._c_ctypes_test.init.restypes = ctypes.POINTER(SimpleStructure)
        self._c_ctypes_test.init.argtypes = []

        self.simple_structure = ctypes.cast(\
                self._c_ctypes_test.init(),\
                ctypes.POINTER(SimpleStructure))


a = CtypesWrapperClass()
print 'Python initialised fine'
print a.simple_structure.contents
print a.simple_structure.contents.flag

The C is compiled under Linux as follows:

gcc -o ctypes_struct_test.os -c --std=c99 -fPIC ctypes_struct_test.c
gcc -o libctypes_struct_test.so -shared ctypes_struct_test.os

On running python ctypes_struct_test.py, I get the following output:

flag is set in C
Python initialised fine
<__main__.SimpleStructure object at 0x166c680>
Segmentation fault

Is there some problem with what I am trying to do, or the way I am trying to do it?

Answer 1

To make this work, replace the wrong line

self._c_ctypes_test.init.restypes = ctypes.POINTER(SimpleStructure)

by the correct line

self._c_ctypes_test.init.restype = ctypes.POINTER(SimpleStructure)

Also consider to remove the pointless cast

self.simple_structure = ctypes.cast(
    self._c_ctypes_test.init(), ctypes.POINTER(SimpleStructure))

It’s casting from ctypes.POINTER(SimpleStructure) to ctypes.POINTER(SimpleStructure)!