Concept explainers
Explanation of Solution
Modify the allocator to perform constant-time coalescing requires both a header and a footer for each block:
In the “Section 9.9.12 (mm.c)”, remove the red color text and add the highlighted lines which is represented in the below code. The modified “mm.c” file is as follows:
#define MAX(x, y) ((x) > (y)? (x) : (y))
/* Pack a size and allocated bit into a word */
#define PACK(size, alloc) ((size) | (alloc))
// Define a pack with size and allocated bit
#define PACK(size, alloc, prev_alloc) ((size) | (alloc) | (prev_alloc << 1))
/* Read and write a word at address p */
#define GET(p) (*(unsigned int *)(p))
/* Read the size and allocated fields from address p */
#define GET_SIZE(p) (GET(p) & ~0x7)
#define GET_ALLOC(p) (GET(p) & 0x1)
// Define allocated fields
#define GET_PREV_ALLOC(p) ((GET(p) >> 1) & 0x1)
/* Given block ptr bp, compute address of its header and footer */
#define HDRP(bp) ((char *)(bp) - WSIZE)
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 */
// Call PUT() function for Prologue header
PUT(heap_listp + (1 * WSIZE), PACK(DSIZE, 1, 1)); /* Prologue header */
// Call PUT() function for Prologue footer
PUT(heap_listp + (2 * WSIZE), PACK(DSIZE, 1, 1));
// Call PUT() function for Epilogue header
PUT(heap_listp + (3 * WSIZE), PACK(0, 1, 1));
heap_listp += (2 * WSIZE);
/* $end mminit */
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);
// Check size to adjust block
if (size <= WSIZE)
// Assign size value
asize = DSIZE;
// Otherwise
else
// Compute size to add overhead and alignment requirements
asize = DSIZE * ((size + (WSIZE)+(DSIZE - 1)) / DSIZE);
/* Search the free list for a fit */
if ((bp = find_fit(asize)) != NULL) {
}
/* $begin mmfree */
PUT(HDRP(bp), PACK(size, 0));
PUT(FTRP(bp), PACK(size, 0));
// Call PUT() function with size and allocated bit
PUT(HDRP(bp), PACK(size, 0, GET_PREV_ALLOC(HDRP(bp))));
PUT(FTRP(bp), PACK(size, 0, GET_PREV_ALLOC(HDRP(bp))));
// Check allocated bit
if (GET_ALLOC(HDRP(NEXT_BLKP(bp))))
// Call PUT() function
PUT(HDRP(NEXT_BLKP(bp)), PACK(GET_SIZE(HDRP(NEXT_BLKP(bp))), 1, 0));
// Otherwise
else {
// Call PUT() function for HDRP
PUT(HDRP(NEXT_BLKP(bp)), PACK(GET_SIZE(HDRP(NEXT_BLKP(bp))), 0, 0));
// Call PUT() function for FTRP
PUT(FTRP(NEXT_BLKP(bp)), PACK(GET_SIZE(HDRP(NEXT_BLKP(bp))), 0, 0));
}
// Call coalesce() function to fill values
coalesce(bp);
}
/* $begin mmfree */
static void *coalesce(void *bp)
{
size_t prev_alloc = GET_ALLOC(FTRP(PREV_BLKP(bp)));
// Call GET_PREV_ALLOC() function and the return value is assign to prev_alloc
size_t prev_alloc = GET_PREV_ALLOC(HDRP(bp));
size_t next_alloc = GET_ALLOC(HDRP(NEXT_BLKP(bp)));
size_t size = GET_SIZE(HDRP(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));
// Call PUT() function for HDRP with PACK size
PUT(HDRP(bp), PACK(size, 0, 1));
// Call PUT() function for FTRP with PACK size
PUT(FTRP(bp), PACK(size, 0, 1));
}
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));
// Call PUT() function for HDRP with PACK size
PUT(FTRP(bp), PACK(size, 0, 1));
// Call PUT() function for FTRP with PACK size
PUT(HDRP(PREV_BLKP(bp)), PACK(size, 0, 1));
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));
// Call PUT() function for HDRP with PACK size
PUT(HDRP(PREV_BLKP(bp)), PACK(size, 0, 1));
// Call PUT() function for FTRP with PACK size
PUT(FTRP(NEXT_BLKP(bp)), PACK(size, 0...
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