Slightly rewritten test from http://bugs.python.org/issue19246 that demonstrates memory fragmentation issue.

stress_heap.c

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


int create_huge_linked_list(void **last_item) {
    int i;
    void *prev_item = NULL;

    for(i = sizeof(void *); i < 1000000; i++) {
        void *new_item = malloc(i);
        if(new_item == NULL) {
            break;
        }
        *(void **)new_item = prev_item;
        prev_item = new_item;
    }
    *last_item = prev_item;
    return i;
}

void free_linked_list(void *last_item) {
    while(last_item != NULL) {
        void *prev_item = *(void **)last_item;
        free(last_item);
        last_item = prev_item;
    }
}

int stress_heap() {
    void *last_item;
    int amount = create_huge_linked_list(&last_item);
    free_linked_list(last_item);
    return amount;
}

void stress_twice(void) {
    int first = stress_heap();
    int second = stress_heap();
    printf("%i %i %f%%\n", first, second, 100.0 * second / first);
}

void stress_and_alloc_1_mb() {
    void *ptr;

    ptr = malloc(1000000);
    if(ptr != NULL) {
        printf("Successfully allocated 1 MB before stress\n");
        free(ptr);

        stress_heap();

        ptr = malloc(1000000);
        if(ptr != NULL) {
            printf("Successfully allocated 1 MB after stress\n");
            free(ptr);
        } else {
            printf("Failed to allocate 1 MB after stress\n");
        }
    } else {
        printf("Failed to allocate 1 MB before stress\n");
    }
}

int main() {
    stress_and_alloc_1_mb();
    stress_twice();
    return 0;
}