usr/klibc/malloc.c

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

#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>
#include "malloc.h"

/* Both the arena list and the free memory list are double linked
   list with head node.  This the head node. Note that the arena list
   is sorted in order of address. */
struct free_arena_header __malloc_head = {
	{
		ARENA_TYPE_HEAD,
		0,
		&__malloc_head,
		&__malloc_head,
	},
	&__malloc_head,
	&__malloc_head
};

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);
}

static struct free_arena_header *__free_block(struct free_arena_header *ah)
{
	struct free_arena_header *pah, *nah;

	pah = ah->a.prev;
	nah = ah->a.next;
	if (pah->a.type == ARENA_TYPE_FREE &&
	    (char *)pah + pah->a.size == (char *)ah) {
		/* Coalesce into the previous block */
		pah->a.size += ah->a.size;
		pah->a.next = nah;
		nah->a.prev = pah;

#ifdef DEBUG_MALLOC
		ah->a.type = ARENA_TYPE_DEAD;
#endif

		ah = pah;
		pah = ah->a.prev;
	} else {
		/* Need to add this block to the free chain */
		ah->a.type = ARENA_TYPE_FREE;

		ah->next_free = __malloc_head.next_free;
		ah->prev_free = &__malloc_head;
		__malloc_head.next_free = ah;
		ah->next_free->prev_free = ah;
	}

	/* In either of the previous cases, we might be able to merge
	   with the subsequent block... */
	if (nah->a.type == ARENA_TYPE_FREE &&
	    (char *)ah + ah->a.size == (char *)nah) {
		ah->a.size += nah->a.size;

		/* Remove the old block from the chains */
		nah->next_free->prev_free = nah->prev_free;
		nah->prev_free->next_free = nah->next_free;
		ah->a.next = nah->a.next;
		nah->a.next->a.prev = ah;

#ifdef DEBUG_MALLOC
		nah->a.type = ARENA_TYPE_DEAD;
#endif
	}

	/* Return the block that contains the called block */
	return ah;
}

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

	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 */
	fsize = (size + MALLOC_CHUNK_MASK) & ~MALLOC_CHUNK_MASK;

#if _KLIBC_MALLOC_USES_SBRK
	fp = (struct free_arena_header *)sbrk(fsize);
#else
	fp = (struct free_arena_header *)
	    mmap(NULL, fsize, PROT_READ | PROT_WRITE,
		 MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
#endif

	if (fp == (struct free_arena_header *)MAP_FAILED) {
		return NULL;	/* Failed to get a block */
	}

	/* Insert the block into the management chains.  We need to set
	   up the size and the main block list pointer, the rest of
	   the work is logically identical to free(). */
	fp->a.type = ARENA_TYPE_FREE;
	fp->a.size = fsize;

	/* We need to insert this into the main block list in the proper
	   place -- this list is required to be sorted.  Since we most likely
	   get memory assignments in ascending order, search backwards for
	   the proper place. */
	for (pah = __malloc_head.a.prev; pah->a.type != ARENA_TYPE_HEAD;
	     pah = pah->a.prev) {
		if (pah < fp)
			break;
	}

	/* Now pah points to the node that should be the predecessor of
	   the new node */
	fp->a.next = pah->a.next;
	fp->a.prev = pah;
	pah->a.next = fp;
	fp->a.next->a.prev = fp;

	/* Insert into the free chain and coalesce with adjacent blocks */
	fp = __free_block(fp);

	/* Now we can allocate from this block */
	return __malloc_from_block(fp, size);
}

void free(void *ptr)
{
	struct free_arena_header *ah;

	if (!ptr)
		return;

	ah = (struct free_arena_header *)
	    ((struct arena_header *)ptr - 1);

#ifdef DEBUG_MALLOC
	assert(ah->a.type == ARENA_TYPE_USED);
#endif

	__free_block(ah);

	/* Here we could insert code to return memory to the system. */
}