Structure integrity example code

structure.c

#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <stdio.h>

u_int32_t wordhash(u_int32_t a)
{
    a = (a+0x7ed55d16) + (a<<12);
    a = (a^0xc761c23c) ^ (a>>19);
    a = (a+0x165667b1) + (a<<5);
    a = (a+0xd3a2646c) ^ (a<<9);
    a = (a+0xfd7046c5) + (a<<3);
    a = (a^0xb55a4f09) ^ (a>>16);
    return a;
}

struct _heap {
	void *a;
	void *b;
	int c;
	char d;
	void *e;
};

union heap {
	struct _heap heap;
	u_int32_t ints[(sizeof(struct _heap) / sizeof(u_int32_t)) + 1];
};

u_int32_t hash(union heap *heap)
{
	u_int32_t ret;
	int i;

	ret = (u_int32_t)(heap); // initialize with mem addr
	for(i = 0; i < sizeof(struct _heap) / sizeof(u_int32_t); i++) {
		ret ^= wordhash(ret - heap->ints[i]);
	}
	return ret;
}

void sethash(union heap *heap)
{
	heap->ints[sizeof(struct _heap) / sizeof(u_int32_t)] = hash(heap);
}

void checkhash(union heap *heap)
{
	u_int32_t expected;
	u_int32_t got;

	expected = hash(heap);
	got = heap->ints[sizeof(struct _heap) / sizeof(u_int32_t)];

	if(got != expected) {
		printf("heap corruption present :( got %08x, expected %08x\n", got, expected);
		exit(EXIT_FAILURE);
	}
}

int main(int argc, char **argv)
{
	union heap *h;

	printf("Allocating heap structure\n");
	h = calloc(sizeof(union heap), 1);

	printf("Setting hash\n");
	sethash(h);
	printf("  %p %p %d %c %p (should be all 0's)\n", h->heap.a, h->heap.b, h->heap.c, h->heap.d, h->heap.e);
	printf("  checksum: %08x\n", h->ints[sizeof(struct _heap) / sizeof(u_int32_t)]);
	printf("Checking hash\n");
	checkhash(h);

	printf("  Check passed. Corrupting h->heap.a\n");
	h->heap.a += 1;

	printf("Checking hash (should error)\n");
	checkhash(h);

	printf("Corruption detection failed\n");

}