alloc.c

/*
 *
 * alloc.c
 *
 * Created at:  Thu 17 Oct 2013 17:23:59 CEST 17:23:59
 *
 * Author:  Szymon Urbaś <[email protected]>
 *
 * License: the MIT license
 *
 * Description: In this program I'm trying to implement the 'one big malloc' (as
 *              opposed to 'many small mallocs').
 *
 *              One malloc/free instead of POOLSZ many, now that's what's called
 *              optimization.
 *
 *              Nemo will use that knowledge aqquired here.
 *              Because, suprisingly, this is actually working.
 *
 * Update: Now there are two different 'object' types (of different sizes) that
 *         are supported. Aaand it still works, which is nice.
 *
 */

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

#define POOL_SIZE 20

/* the base of the two different 'objects' */
struct base {
  char id;          /* 1 byte  */
};

struct integer {
  struct base base; /* 1 byte  */
  short i;          /* 2 bytes */
};           /* total: 3 bytes */

struct floating {
  struct base base; /* 1 byte  */
  double f;         /* 8 bytes */
};           /* total: 9 bytes */

/* pointer to the malloced block of memory */
static void *pool;
/* pointer to the current element in the pool */
static void *curr;
/* the current 'id', pretty much not essential to the program */
static int id;

/*
 * Short and simple. This two functions pretend to be the new 'malloc'.
 * There's two separate functions for each 'object' kind.
 *
 * Some optimizations may follow, but for now I'm KISSing it.
 */
struct integer *new_integer(short i)
{
  void *save = curr;

  ((struct integer *)curr)->base.id = id + 97;
  ((struct integer *)curr)->i = i;

  curr += sizeof(struct integer);
  id++;

  return (struct integer *)save;
}

struct floating *new_floating(double f)
{
  void *save = curr;

  ((struct floating *)curr)->base.id = id + 97;
  ((struct floating *)curr)->f = f;

  curr += sizeof(struct floating);
  id++;

  return (struct floating *)save;
}

int main(void)
{
  /* a few dummy 'objects' */
  struct integer  *five, *seven;
  struct floating *pi, *e;

  /* initialize the pool */
  pool = malloc(sizeof(void *) * POOL_SIZE);

  if (!pool){
    fprintf(stderr, "malloc failed\n");
    return 1;
  }

  /* set 'curr' to the first 'object' in the pool */
  curr = pool;

  /* 'malloc' the first 'object' */
  five = new_integer(5);
  /* same for the second */
  pi = new_floating(3.14);
  /* and for the third one */
  e = new_floating(2.71);
  /* aaaaand the last one */
  seven = new_integer(7);

  /* make sure it actually works, and that the struct members are accessible */
  assert(five->i == 5);
  assert(pi->f == 3.14);
  assert(e->f == 2.71);
  assert(seven->i == 7);

  /* some debugging info */
  printf("pool's place in the memory: %p\n", (void *)pool);
  printf("sizeof(struct integer): %lu, sizeof(struct floating): %lu\n\n", sizeof(struct integer), sizeof(struct floating));

  printf("%p id %c:  five: %d\n", (void *)five, five->base.id, five->i);
  printf("%p id %c:    pi: %g\n", (void *)pi, pi->base.id, pi->f);
  printf("%p id %c:     e: %g\n", (void *)e, e->base.id, e->f);
  printf("%p id %c: seven: %d\n", (void *)seven, seven->base.id, seven->i);

  printf("\ncurr ended up pointing to %p\n", (void *)curr);

  /* release the <s>kraken</s> memory */
  free(pool);

  return 0;
}