malloc lab

Malloc Lab是一个实验项目，要求我们实现一个动态内存分配器，使其具有与标准C库中的malloc、free、realloc函数相同的功能。我们需要自行定义块的空间结构，并实现find_fit函数和place函数，这两个函数都涉及到指针的操作，需要特别注意，以避免出现BUG。

在实现动态内存分配器时，我们需要考虑以下几个方面：
1. 内存块的分配和释放：我们需要实现malloc和free函数，前者用于分配内存块，后者用于释放内存块。
2. 内存块的合并：当一个内存块被释放时，我们需要将其与相邻的空闲块合并，以便后续的内存分配。
3. 内存块的查找：在分配内存块时，我们需要查找一个合适的空闲块，以满足用户的需求。

以下是一个简单的动态内存分配器的实现，仅供参考：

/* Basic constants and macros */
#define WSIZE       4       /* Word and header/footer size (bytes) */
#define DSIZE       8       /* Double word size (bytes) */
#define CHUNKSIZE  (1<<12)  /* Extend heap by this amount (bytes) */

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

/* 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 fields from address p */
#define GET_SIZE(p)  (GET(p) & ~0x7)
#define GET_ALLOC(p) (GET(p) & 0x1)

/* 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)))
#define PREV_BLKP(bp)  ((char *)(bp) - GET_SIZE(((char *)(bp) - DSIZE)))

/* Global variables */
static char *heap_listp = 0;

/* Function prototypes for internal helper routines */
static void *extend_heap(size_t words);
static void *coalesce(void *bp);
static void *find_fit(size_t asize);
static void place(void *bp, size_t asize);

/* 
 * mm_init - Initialize the memory manager 
 */
int mm_init(void) 
{
    /* Create the initial empty heap */
    if ((heap_listp = mem_sbrk(4*WSIZE)) == (void *)-1)
        return -1;
    PUT(heap_listp, 0);                            /* Alignment padding */
    PUT(heap_listp + (1*WSIZE), PACK(DSIZE, 1));   /* Prologue header */ 
    PUT(heap_listp + (2*WSIZE), PACK(DSIZE, 1));   /* Prologue footer */ 
    PUT(heap_listp + (3*WSIZE), PACK(0, 1));       /* Epilogue header */
    heap_listp += (2*WSIZE);

    /* Extend the empty heap with a free block of CHUNKSIZE bytes */
    if (extend_heap(CHUNKSIZE/WSIZE) == NULL)
        return -1;
    return 0;
}

/* 
 * mm_malloc - Allocate a block with at least size bytes of payload 
 */
void *mm_malloc(size_t size) 
{
    size_t asize;      /* Adjusted block size */
    size_t extendsize; /* Amount to extend heap if no fit */
    char *bp;

    /* Ignore spurious requests */
    if (size == 0)
        return NULL;

    /* Adjust block size to include overhead and alignment reqs. */
    if (size <= DSIZE)
        asize = 2*DSIZE;
    else
        asize = DSIZE * ((size + (DSIZE) + (DSIZE-1)) / DSIZE);

    /* Search the free list for a fit */
    if ((bp = find_fit(asize)) != NULL) {
        place(bp, asize);
        return bp;
    }

    /* No fit found. Get more memory and place the block */
    extendsize = MAX(asize, CHUNKSIZE);
    if ((bp = extend_heap(extendsize/WSIZE)) == NULL)
        return NULL;
    place(bp, asize);
    return bp;
}

/*
 * mm_free - Free a block 
 */
void mm_free(void *bp)
{
    size_t size = GET_SIZE(HDRP(bp));

    PUT(HDRP(bp), PACK(size, 0));
    PUT(FTRP(bp), PACK(size, 0));
    coalesce(bp);
}

/*
 * mm_realloc - naive implementation of mm_realloc
 */
void *mm_realloc(void *ptr, size_t size)
{
    void *newptr;
    size_t copySize;

    /* If size == 0 then this is just free, and we return NULL. */
    if (size == 0) {
        mm_free(ptr);
        return 0;
    }

    /* If oldptr is NULL, then this is just malloc. */
    if (ptr == NULL) {
        return mm_malloc(size);
    }

    newptr = mm_malloc(size);

    /* If realloc() fails the original block is left untouched  */
    if (!newptr) {
      return 0;
    }

    /* Copy the old data. */
    copySize = GET_SIZE(HDRP(ptr));
    if (size < copySize) {
      copySize = size;
    }
    memcpy(newptr, ptr, copySize);

    /* Free the old block. */
    mm_free(ptr);

    return newptr;
}

/* 
 * extend_heap - Extend heap with free block and return its block pointer
 */
static void *extend_heap(size_t words) 
{
    char *bp;
    size_t size;

    /* Allocate an even number of words to maintain alignment */
    size = (words % 2) ? (words+1) * WSIZE : words * WSIZE;
    if ((long)(bp = mem_sbrk(size)) == -1)
        return NULL;

    /* Initialize free block header/footer and the epilogue header */
    PUT(HDRP(bp), PACK(size, 0));         /* Free block header */
    PUT(FTRP(bp), PACK(size, 0));         /* Free block footer */
    PUT(HDRP(NEXT_BLKP(bp)), PACK(0, 1)); /* New epilogue header */

    /* Coalesce if the previous block was free */
    return coalesce(bp);
}

/* 
 * coalesce - Boundary tag coalescing. Return ptr to coalesced block
 */
static void *coalesce(void *bp) 
{
    size_t prev_alloc = GET_ALLOC(FTRP(PREV_BLKP(bp)));
    size_t next_alloc = GET_ALLOC(HDRP(NEXT_BLKP(bp)));
    size_t size = GET_SIZE(HDRP(bp));

    if (prev_alloc && next_alloc) {            /* Case 1 */
        return bp;
    }

    else if (prev_alloc && !next_alloc) {      /* Case 2 */
        size += GET_SIZE(HDRP(NEXT_BLKP(bp)));
        PUT(HDRP(bp), PACK(size, 0));
        PUT(FTRP(bp), PACK(size,0));
    }

    else if (!prev_alloc && next_alloc) {      /* Case 3 */
        size += GET_SIZE(HDRP(PREV_BLKP(bp)));
        PUT(FTRP(bp), PACK(size, 0));
        PUT(HDRP(PREV_BLKP(bp)), PACK(size, 0));
        bp = PREV_BLKP(bp);
    }

    else {                                     /* Case 4 */
        size += GET_SIZE(HDRP(PREV_BLKP(bp))) + 
            GET_SIZE(FTRP(NEXT_BLKP(bp)));
        PUT(HDRP(PREV_BLKP(bp)), PACK(size, 0));
        PUT(FTRP(NEXT_BLKP(bp)), PACK(size, 0));
        bp = PREV_BLKP(bp);
    }

    return bp;
}

/* 
 * find_fit - Find a fit for a block with asize bytes 
 */
static void *find_fit(size_t asize)
{
    void *bp;

    for (bp = heap_listp; GET_SIZE(HDRP(bp)) > 0; bp = NEXT_BLKP(bp)) {
        if (!GET_ALLOC(HDRP(bp)) && (asize <= GET_SIZE(HDRP(bp)))) {
            return bp;
        }
    }

    return NULL; /* No fit */
}

/* 
 * place - Place block of asize bytes at start of free block bp 
 *         and split if remainder would be at least minimum block size
 */
static void place(void *bp, size_t asize)
{
    size_t csize = GET_SIZE(HDRP(bp));

    if ((csize - asize) >= (2*DSIZE)) {
        PUT(HDRP(bp), PACK(asize, 1));
        PUT(FTRP(bp), PACK(asize, 1));
        bp = NEXT_BLKP(bp);
        PUT(HDRP(bp), PACK(csize-asize, 0));
        PUT(FTRP(bp), PACK(csize-asize, 0));
    }
    else {
        PUT(HDRP(bp), PACK(csize, 1));
        PUT(FTRP(bp), PACK(csize, 1));
    }
}