pointers.c

#include <stdio.h>

int
main(int argc, char **argv) {
    int x = 1;
    int y = 2;
    int *xp = &x;
    int *yp = &y;
    printf("1: xp = %p; *xp = %d\n", xp, *xp);
    printf("2: yp = %p; *xp = %d\n", yp, *yp);

    int *zp = NULL;
    int **pp = &zp;
    printf("3: pp = %p; *pp = %p; zp = %p\n", pp, *pp, zp);
    *pp = xp;
    printf("4: pp = %p; *pp = %p; **pp = %d; zp = %p\n", pp, *pp, **pp, zp);
    *pp = yp;
    printf("5: pp = %p; *pp = %p; **pp = %d; zp = %p\n", pp, *pp, **pp, zp);
}

pointers.md

Expectations

1. 3:pp is 4:pp is 5:pp.
2. 3:*pp is 3:zp is null.
3. 4:*pp is 1:xp is 4:zp.
4. 5:*pp is 2:yp is 5:zp.
5. 4:**pp is xp is 1.
6. 5:**pp is yp is 2.

pointers.py

import ctypes

def main():
    x = ctypes.c_int(1)
    y = ctypes.c_int(2)
    xp = ctypes.pointer(x)
    yp = ctypes.pointer(y)
    print('1: xp = {}; *xp = {}'.format(ptr(xp), xp.contents))
    print('2: yp = {}; *yp = {}'.format(ptr(yp), yp.contents))

    zp = ctypes.POINTER(ctypes.c_int)()
    pp = ctypes.pointer(zp)
    print('3: pp = {}; *pp = {}; zp = {}'.format(ptr(pp), ptr(pp.contents), ptr(zp)))
    pp[0] = xp
    print('4: pp = {}; *pp = {}; **pp = {}; zp = {}'.format(ptr(pp), ptr(pp.contents), pp.contents.contents, ptr(zp)))
    pp[0] = yp
    print('5: pp = {}; *pp = {}; **pp = {}; zp = {}'.format(ptr(pp), ptr(pp.contents), pp.contents.contents, ptr(zp)))

def ptr(pointer):
    '''Format a pointer into a string.'''
    return ptr1(pointer)
    #return ptr2(pointer)

def ptr1(pointer):
    address = ctypes.cast(pointer, ctypes.c_void_p).value
    if address is None:
        return '(null)'
    else:
        return '{:#018x}'.format(address)

def ptr2(pointer):
    address = ctypes.addressof(pointer)
    if address is None:
        return '(null)'
    else:
        return '{:#018x}'.format(address)

main()