source: bootcd/isolinux/syslinux-6.03/dos/malloc.c

/*
 * malloc.c
 *
 * Very simple linked-list based malloc()/free().
 */

#include <stdlib.h>
#include <string.h>
#include "malloc.h"

struct free_arena_header __malloc_head = {
    {
     ARENA_TYPE_HEAD,
     0,
     &__malloc_head,
     &__malloc_head,
     },
    &__malloc_head,
    &__malloc_head
};

extern void *__mem_end;         /* In argv.c */

void __init_memory_arena(void)
{
    extern char __heap_end[];
    struct free_arena_header *fp;

    fp = (struct free_arena_header *)__mem_end;
    fp->a.type = ARENA_TYPE_FREE;
    fp->a.size = __heap_end - (char *)__mem_end;

    /* Insert into chains */
    fp->a.next = fp->a.prev = &__malloc_head;
    fp->next_free = fp->prev_free = &__malloc_head;
    __malloc_head.a.next = __malloc_head.a.prev = fp;
    __malloc_head.next_free = __malloc_head.prev_free = fp;
}

static void *__malloc_from_block(struct free_arena_header *fp, size_t size)
{
    size_t fsize;
    struct free_arena_header *nfp, *na;

    fsize = fp->a.size;

    /* We need the 2* to account for the larger requirements of a free block */
    if (fsize >= size + 2 * sizeof(struct arena_header)) {
        /* Bigger block than required -- split block */
        nfp = (struct free_arena_header *)((char *)fp + size);
        na = fp->a.next;

        nfp->a.type = ARENA_TYPE_FREE;
        nfp->a.size = fsize - size;
        fp->a.type = ARENA_TYPE_USED;
        fp->a.size = size;

        /* Insert into all-block chain */
        nfp->a.prev = fp;
        nfp->a.next = na;
        na->a.prev = nfp;
        fp->a.next = nfp;

        /* Replace current block on free chain */
        nfp->next_free = fp->next_free;
        nfp->prev_free = fp->prev_free;
        fp->next_free->prev_free = nfp;
        fp->prev_free->next_free = nfp;
    } else {
        /* Allocate the whole block */
        fp->a.type = ARENA_TYPE_USED;

        /* Remove from free chain */
        fp->next_free->prev_free = fp->prev_free;
        fp->prev_free->next_free = fp->next_free;
    }

    return (void *)(&fp->a + 1);
}

void *malloc(size_t size)
{
    struct free_arena_header *fp;

    if (size == 0)
        return NULL;

    /* Add the obligatory arena header, and round up */
    size = (size + 2 * sizeof(struct arena_header) - 1) & ~ARENA_SIZE_MASK;

    for (fp = __malloc_head.next_free; fp->a.type != ARENA_TYPE_HEAD;
         fp = fp->next_free) {
        if (fp->a.size >= size) {
            /* Found fit -- allocate out of this block */
            return __malloc_from_block(fp, size);
        }
    }

    /* Nothing found... need to request a block from the kernel */
    return NULL;                /* No kernel to get stuff from */
}

void *calloc(size_t nmemb, size_t size)
{
    void *p;
    size *= nmemb;
    p = malloc(size);
    if (p)
        memset(p, 0, size);
    return p;
}