mm.c

/*
 * mm-naive.c - The fastest, least memory-efficient malloc package.
 * 
 * In this naive approach, a block is allocated by simply incrementing
 * the brk pointer.  A block is pure payload. There are no headers or
 * footers.  Blocks are never coalesced or reused. Realloc is
 * implemented directly using mm_malloc and mm_free.
 *
 * NOTE TO STUDENTS: Replace this header comment with your own header
 * comment that gives a high level description of your solution.
 */
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <unistd.h>
#include <string.h>

#include "mm.h"
#include "memlib.h"

/*********************************************************
 * NOTE TO STUDENTS: Before you do anything else, please
 * provide your team information in the following struct.
 ********************************************************/

/* single word (4) or double word (8) alignment */
#define ALIGNMENT 8

/* rounds up to the nearest multiple of ALIGNMENT */
#define ALIGN(size) (((size) + (ALIGNMENT - 1)) & ~0x7)

#define SIZE_T_SIZE (ALIGN(sizeof(size_t)))

/* Basic constants and macros */
#define WSIZE 4
#define DSIZE 8
#define CHUNKSIZE (1 << 12)

#define MAX(x, y) ((x) > (y) ? (x) : (y))

/* Pack a size and allocated bit into a word */
#define PACK(size, prev_alloc, alloc) ((size) | (alloc) | (prev_alloc << 1))

/* Read and write a word at address p */
#define GET(p) (*(unsigned int *)(p))
#define PUT(p, val) (*(unsigned int *)(p) = (val))

/* Read the size and allocated fileds from address p */
#define GET_SIZE(p) (GET(p) & ~0x7)
#define GET_ALLOC(p) (GET(p) & 0x1)
#define GET_PREV_ALLOC(p) ((GET(p) & 0x2) >> 1)

/* Given block ptr bp, compute address of its header and footer */
#define HDRP(bp) ((char *)(bp)-WSIZE)
#define FTRP(bp) ((char *)(bp) + GET_SIZE(HDRP(bp)) - DSIZE)

/* Given block ptr bp, compute address of next and previous blocks */
#define NEXT_BLKP(bp) ((char *)(bp) + GET_SIZE((char *)(bp)-WSIZE) - WSIZE)
#define PREV_BLKP(bp) ((char *)(bp) - (GET_SIZE(HDRP((char *)bp) - WSIZE) + WSIZE))

static char *heap_listp;
static char *free_chunk_head;

void ins(void *bp)
{
    PUT(bp, NULL);
    PUT(bp + WSIZE, free_chunk_head);
    if (free_chunk_head != NULL)
        PUT(free_chunk_head + WSIZE, HDRP(bp));
    free_chunk_head = HDRP(bp);
}

void del(void *bp)
{
    if (GET(bp) != NULL)
        PUT(GET(bp) + (2 * WSIZE), GET(bp + WSIZE));
    else
        free_chunk_head = GET(bp + WSIZE);

    if (GET(bp + WSIZE) != NULL)
        PUT(GET(bp + WSIZE) + WSIZE, GET(bp));
}

static void *coalesce(void *bp)
{
    char *p;
    size_t size = GET_SIZE(HDRP(bp));
    if (GET_ALLOC(NEXT_BLKP(bp)) == 0)
    {
        p = NEXT_BLKP(bp);
        size += GET_SIZE(p);
        PUT(HDRP(bp), PACK(size, GET_PREV_ALLOC(HDRP(bp)), 0));
        PUT(FTRP(bp), PACK(size, GET_PREV_ALLOC(HDRP(bp)), 0));
        del(p + WSIZE);
    }
    if (GET_PREV_ALLOC(HDRP(bp)) == 0)
    {
        p = PREV_BLKP(bp);
        size += GET_SIZE(p);
        PUT(p, PACK(size, GET_PREV_ALLOC(p), 0));
        PUT(FTRP(p + WSIZE), PACK(size, GET_PREV_ALLOC(p), 0));
        del(bp);
        bp = p + WSIZE;
    }
    return bp;
}

static void *extend_heap(size_t words)
{
    char *bp;
    size_t size;
    size = (words % 2) ? (words + 1) * WSIZE : words * WSIZE;
    if ((long)(bp = mem_sbrk(size)) == -1)
        return NULL;
    PUT(HDRP(bp), PACK(size, GET_PREV_ALLOC(HDRP(bp)), 0));
    PUT(FTRP(bp), PACK(size, GET_PREV_ALLOC(HDRP(bp)), 0));
    ins(bp);
    PUT(NEXT_BLKP(bp), PACK(0, 0, 1));
    return coalesce(bp);
}

static void *find_fit(size_t size)
{
    size_t f = 0;
    char *p = free_chunk_head;
    char *addr;
    while (p != NULL)
    {
        if (GET_SIZE(p) >= size + 4 * WSIZE)
        {
            addr = p + WSIZE;
            f = 1;
            break;
        }
        p = GET(p + 2 * WSIZE);
    }
    if (f)
        return addr;
    else
        return NULL;
}

static void place(void *bp, size_t size)
{
    size_t lsize;
    char *p;
    lsize = GET_SIZE(HDRP(bp)) - size;
    PUT(HDRP(bp), PACK(size, GET_PREV_ALLOC(HDRP(bp)), 1));
    p = NEXT_BLKP(bp);
    PUT(p, PACK(lsize, 1, 0));
    PUT(FTRP(p + WSIZE), PACK(lsize, 1, 0));
    ins(p + WSIZE);
    del(bp);
}

/* 
 * mm_init - initialize the malloc package.
 */
int mm_init(void)
{
    free_chunk_head = NULL;
    if ((heap_listp = mem_sbrk(4 * WSIZE)) == (void *)(-1))
        return -1;
    PUT(heap_listp, 0);
    PUT(heap_listp + (1 * WSIZE), PACK(DSIZE, 1, 1));
    PUT(heap_listp + (2 * WSIZE), PACK(DSIZE, 1, 1));
    PUT(heap_listp + (3 * WSIZE), PACK(0, 1, 1));
    heap_listp += (2 * WSIZE);
    if (extend_heap(CHUNKSIZE / WSIZE) == NULL)
        return -1;
    return 0;
}

/* 
 * mm_malloc - Allocate a block by incrementing the brk pointer.
 *     Always allocate a block whose size is a multiple of the alignment.
 */
void *mm_malloc(size_t size)
{
    size_t asize;
    size_t extendsize;
    char *bp;

    if (size == 0)
        return NULL;

    if (size <= DSIZE)
        asize = 2 * DSIZE;
    else
        asize = DSIZE * ((size + (DSIZE) + (DSIZE - 1)) / DSIZE);

    if ((bp = find_fit(asize)) != NULL)
    {
        place(bp, asize);
        return bp;
    }

    extendsize = MAX(asize, CHUNKSIZE);
    if ((bp = extend_heap(extendsize / WSIZE)) == NULL)
        return NULL;
    place(bp, asize);
    return bp;
}

/*
 * mm_free - Freeing a block does nothing.
 */
void mm_free(void *bp)
{
    size_t size = GET_SIZE(HDRP(bp));
    char *np;
    PUT(HDRP(bp), PACK(size, GET_PREV_ALLOC(HDRP(bp)), 0));
    PUT(FTRP(bp), PACK(size, GET_PREV_ALLOC(HDRP(bp)), 0));
    np = NEXT_BLKP(bp);
    PUT(np, PACK(GET_SIZE(np), 0, GET_ALLOC(np)));
    ins(bp);
    coalesce(bp);
}

/*
 * mm_realloc - Implemented simply in terms of mm_malloc and mm_free
 */
void *mm_realloc(void *ptr, size_t size)
{
    void *oldptr = ptr;
    void *newptr;
    size_t copySize;

    newptr = mm_malloc(size);
    if (newptr == NULL)
        return NULL;
    copySize = *(size_t *)((char *)oldptr - SIZE_T_SIZE);
    if (size < copySize)
        copySize = size;
    memcpy(newptr, oldptr, copySize);
    mm_free(oldptr);
    return newptr;
}