correct_my_mem_allocator.c

#include <stdio.h>
#include <unistd.h>

#define END_OF_LIST (void *)-1


typedef union FreeListCell {
  union FreeListCell *next;
  char str[CELL_SIZE_IN_BYTES];
} cell;

// global pointer, for free list management
cell *next_free;

void init_mem_pool() {
  int i;
  cell *curr_ptr;
  
  next_free = (cell *)sbrk(sizeof(cell)*NUM_CELLS);
  if (next_free == END_OF_LIST) {
    // unable to allocate memory
    fprintf(stderr, "sbrk failed");
    exit(-1);
  }
  curr_ptr = next_free;
  for (i=0; i<NUM_CELLS-1; i++) {
    // set up the initial free-list links
    curr_ptr->next = (curr_ptr+1); // point to next cell in list
    curr_ptr++;
  }
  // last one - set END_OF_LIST marker
  (next_free[NUM_CELLS-1]).next = END_OF_LIST;
  
}

void *my_malloc(int num_bytes) {
  cell *ptr;

  // out of mem
  if (next_free == END_OF_LIST)
    return NULL;

  // if there is a free cell, then advance in linked list
  ptr = next_free;
  next_free = next_free->next;
  return (void *)ptr;
}

void my_free(void *ptr) {
  // prepend just free'd cell onto linked list...
  ((cell *)ptr)->next = next_free;
  next_free = ptr;
  return;
}

void out_of_mem() {
    fprintf(stderr, "out of memory!\n");
    exit(-1);
}