This program shows you how your process heap can become fragmented under some memory allocators!

heap-fragmentor.cpp

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
 
 
// Define constants to make sure strings are not allocated at the top of the heap
#define HIT_ENTER  "Press Enter to continue...\n"
#define ALLOCATED  "500k 5KB chunks were just provisioned\n"
#define FREED      "The first allocations were just free()'d.\n"
#define FREED_NEXT "The 2nd allocations were just free()'d.\n"
 
 
void press_enter_to_continue(void) {
   printf(HIT_ENTER);
   getchar();
   return;
}
 
int main() {
    printf("Hello!  This program will fragment its process heap.  Run top -p %d to follow along!\n", getpid());
    press_enter_to_continue();
 
    int i;
    // Arbitrary value
    int ARRAY_SIZE = 5*1024*102;
 
    // Because we're asking for a large size at the get-go, the backing memory for these arrays will be mmap'd.  But the pointers stored here will be for
    // small allocations that we expect to be brk()'d.  We keep the mapping so we can free them later.
    char *p1[ARRAY_SIZE];
    char *p2[ARRAY_SIZE];
 
    int mallocSize = 5 * 1024;
 
    for(i=0; i < ARRAY_SIZE; i++){
        // malloc in small chunks such that we are always below the mmap threshold for these allocations.
        p1[i] = malloc(mallocSize);
        // Write something to make sure the page is backed by physical RAM
        *p1[i] = 'a';
    }
 
    printf(ALLOCATED);
    press_enter_to_continue();
 
    for(i=0; i < ARRAY_SIZE; i++){
        // Again, malloc in small chunks such that we are always below the mmap threshold for these allocations
        p2[i] = malloc(mallocSize);
       *p2[i] = 'a'; 
    }
 
    printf(ALLOCATED);
    press_enter_to_continue();
 
    // Free the allocations
    for(i=0; i < ARRAY_SIZE; i++){
        free(p1[i]);
    }
 
    printf(FREED);
    press_enter_to_continue();
 
    // Free the allocations
    for(i=0; i < ARRAY_SIZE; i++){
        free(p2[i]);
    }
 
    printf(FREED_NEXT);
    press_enter_to_continue();
 
    return 0;
}