Test the GLIBC MALLOC_PERTURB_ feature

malloc-perturb.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <malloc.h>

struct Rect {
	char a, b;
};
typedef struct Rect Rect;

enum {
	N = 11,
};

void do_test(void)
{
	int i;
	Rect *p;
	unsigned char *cp;
	size_t k;

	k = sizeof(typeof(*p)) * N;
	cp = malloc(k);
	p = (Rect*)cp;

	printf("p: {%lld, %lld}\n", (long long)p->a, (long long)p->b);
	for (i=0; i<k; i++)
		printf("%02x", cp[i]);
	putchar('\n');

	free(p);
	printf("p: {%lld, %lld}\n", (long long)p->a, (long long)p->b);
	for (i=0; i<k; i++)
		printf("%02x", cp[i]);
	putchar('\n');
}

int main()
{
	/* same as "export MALLOC_PERTURB_=0" */
	 mallopt(M_PERTURB, 0);
	do_test();

	/* same as "export MALLOC_PERTURB_=$((0x7f))" */
	mallopt(M_PERTURB, 0x7f);
	do_test();
	return 0;
}