Lab6 Malloc Lab
CSAPPMalloc Lab的目的是设计一个通用分配器。它基于隐式空闲链表，使用立即边界标记合并方式。
分配器包含在一个源文件mm.c中，这次Lab需要完成四个函数:
int mm_init(void); /* 初始化分配器 */
void *mm_malloc(size_t size); /* malloc */
void mm_free(void *ptr); /* free */
void *mm_realloc(void *ptr, size_t size); /* realloc */
堆的示意图如下所示。
最后发现这个lab的答案书上竟然已经写出来了，写的特别好，借鉴一下：
/*
* Simple, 32-bit and 64-bit clean allocator based on implicit free
* lists, first-fit placement, and boundary tag coalescing, as described
* in the CS:APP3e text. Blocks must be aligned to doubleword (8 byte)
* boundaries. Minimum block size is 16 bytes.
*/
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "mm.h"
#include "memlib.h"
/*
* If NEXT_FIT defined use next fit search, else use first-fit search
*/
#define NEXT_FITx
/* $begin mallocmacros */
/* Basic constants and macros */
#define WSIZE 4 /* Word and header/footer size (bytes) */ //line:vm:mm:beginconst
#define DSIZE 8 /* Double word size (bytes) */
#define CHUNKSIZE (1<<12) /* Extend heap by this amount (bytes) */ //line:vm:mm:endconst
#define MAX(x, y) ((x) > (y)? (x) : (y))
/* Pack a size and allocated bit into a word */
#define PACK(size, alloc) ((size) | (alloc)) //line:vm:mm:pack
/* Read and write a word at address p */
#define GET(p) (*(unsigned int *)(p)) //line:vm:mm:get
#define PUT(p, val) (*(unsigned int *)(p) = (val)) //line:vm:mm:put
/* Read the size and allocated fields from address p */
#define GET_SIZE(p) (GET(p) & ~0x7) //line:vm:mm:getsize
#define GET_ALLOC(p) (GET(p) & 0x1) //line:vm:mm:getalloc
/* Given block ptr bp, compute address of its header and footer */
#define HDRP(bp) ((char *)(bp) - WSIZE) //line:vm:mm:hdrp
#define FTRP(bp) ((char *)(bp) + GET_SIZE(HDRP(bp)) - DSIZE) //line:vm:mm:ftrp
/* Given block ptr bp, compute address of next and previous blocks */
#define NEXT_BLKP(bp) ((char *)(bp) + GET_SIZE(((char *)(bp) - WSIZE))) //line:vm:mm:nextblkp
#define PREV_BLKP(bp) ((char *)(bp) - GET_SIZE(((char *)(bp) - DSIZE))) //line:vm:mm:prevblkp
/* $end mallocmacros */
/* Global variables */
static char *heap_listp = 0; /* Pointer to first block */
#ifdef NEXT_FIT
static char *rover; /* Next fit rover */
#endif
/* Function prototypes for internal helper routines */
static void *extend_heap(size_t words);
static void place(void *bp, size_t asize);
static void *find_fit(size_t asize);
static void *coalesce(void *bp);
static void printblock(void *bp);
static void checkheap(int verbose);
static void checkblock(void *bp);
/*
* mm_init - Initialize the memory manager
*/
/* $begin mminit */
int mm_init(void)
{
/* Create the initial empty heap */
if ((heap_listp = mem_sbrk(4*WSIZE)) == (void *)-1) //line:vm:mm:begininit
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); //line:vm:mm:endinit
/* $end mminit */
#ifdef NEXT_FIT
rover = heap_listp;
#endif
/* $begin mminit */
/* Extend the empty heap with a free block of CHUNKSIZE bytes */
if (extend_heap(CHUNKSIZE/WSIZE) == NULL)
return -1;
return 0;
}
/* $end mminit */
/*
* mm_malloc - Allocate a block with at least size bytes of payload
*/
/* $begin mmmalloc */
void *mm_malloc(size_t size)
{
size_t asize; /* Adjusted block size */
size_t extendsize; /* Amount to extend heap if no fit */
char *bp;
/* $end mmmalloc */
if (heap_listp == 0){
mm_init();
}
/* $begin mmmalloc */
/* Ignore spurious requests */
if (size == 0)
return NULL;
/* Adjust block size to include overhead and alignment reqs. */
if (size <= DSIZE) //line:vm:mm:sizeadjust1
asize = 2*DSIZE; //line:vm:mm:sizeadjust2
else
asize = DSIZE * ((size + (DSIZE) + (DSIZE-1)) / DSIZE); //line:vm:mm:sizeadjust3
/* Search the free list for a fit */
if ((bp = find_fit(asize)) != NULL) { //line:vm:mm:findfitcall
place(bp, asize); //line:vm:mm:findfitplace
return bp;
}
/* No fit found. Get more memory and place the block */
extendsize = MAX(asize,CHUNKSIZE); //line:vm:mm:growheap1
if ((bp = extend_heap(extendsize/WSIZE)) == NULL)
return NULL; //line:vm:mm:growheap2
place(bp, asize); //line:vm:mm:growheap3
return bp;
}
/* $end mmmalloc */
/*
* mm_free - Free a block
*/
/* $begin mmfree */
void mm_free(void *bp)
{
/* $end mmfree */
if (bp == 0)
return;
/* $begin mmfree */
size_t size = GET_SIZE(HDRP(bp));
/* $end mmfree */
if (heap_listp == 0){
mm_init();
}
/* $begin mmfree */
PUT(HDRP(bp), PACK(size, 0));
PUT(FTRP(bp), PACK(size, 0));
coalesce(bp);
}
/* $end mmfree */
/*
* coalesce - Boundary tag coalescing. Return ptr to coalesced block
*/
/* $begin mmfree */
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);
}
/* $end mmfree */
#ifdef NEXT_FIT
/* Make sure the rover isn't pointing into the free block */
/* that we just coalesced */
if ((rover > (char *)bp) && (rover < NEXT_BLKP(bp)))
rover = bp;
#endif
/* $begin mmfree */
return bp;
}
/* $end mmfree */
/*
* mm_realloc - Naive implementation of realloc
*/
void *mm_realloc(void *ptr, size_t size)
{
size_t oldsize;
void *newptr;
/* 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. */
oldsize = GET_SIZE(HDRP(ptr));
if(size < oldsize) oldsize = size;
memcpy(newptr, ptr, oldsize);
/* Free the old block. */
mm_free(ptr);
return newptr;
}
/*
* mm_checkheap - Check the heap for correctness
*/
void mm_checkheap(int verbose)
{
checkheap(verbose);
}
/*
* The remaining routines are internal helper routines
*/
/*
* extend_heap - Extend heap with free block and return its block pointer
*/
/* $begin mmextendheap */
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; //line:vm:mm:beginextend
if ((long)(bp = mem_sbrk(size)) == -1)
return NULL; //line:vm:mm:endextend
/* Initialize free block header/footer and the epilogue header */
PUT(HDRP(bp), PACK(size, 0)); /* Free block header */ //line:vm:mm:freeblockhdr
PUT(FTRP(bp), PACK(size, 0)); /* Free block footer */ //line:vm:mm:freeblockftr
PUT(HDRP(NEXT_BLKP(bp)), PACK(0, 1)); /* New epilogue header */ //line:vm:mm:newepihdr
/* Coalesce if the previous block was free */
return coalesce(bp); //line:vm:mm:returnblock
}
/* $end mmextendheap */
/*
* place - Place block of asize bytes at start of free block bp
* and split if remainder would be at least minimum block size
*/
/* $begin mmplace */
/* $begin mmplace-proto */
static void place(void *bp, size_t asize)
/* $end mmplace-proto */
{
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));
}
}
/* $end mmplace */
/*
* find_fit - Find a fit for a block with asize bytes
*/
/* $begin mmfirstfit */
/* $begin mmfirstfit-proto */
static void *find_fit(size_t asize)
/* $end mmfirstfit-proto */
{
/* $end mmfirstfit */
#ifdef NEXT_FIT
/* Next fit search */
char *oldrover = rover;
/* Search from the rover to the end of list */
for ( ; GET_SIZE(HDRP(rover)) > 0; rover = NEXT_BLKP(rover))
if (!GET_ALLOC(HDRP(rover)) && (asize <= GET_SIZE(HDRP(rover))))
return rover;
/* search from start of list to old rover */
for (rover = heap_listp; rover < oldrover; rover = NEXT_BLKP(rover))
if (!GET_ALLOC(HDRP(rover)) && (asize <= GET_SIZE(HDRP(rover))))
return rover;
return NULL; /* no fit found */
#else
/* $begin mmfirstfit */
/* First-fit search */
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 */
#endif
}
/* $end mmfirstfit */
static void printblock(void *bp)
{
size_t hsize, halloc, fsize, falloc;
checkheap(0);
hsize = GET_SIZE(HDRP(bp));
halloc = GET_ALLOC(HDRP(bp));
fsize = GET_SIZE(FTRP(bp));
falloc = GET_ALLOC(FTRP(bp));
if (hsize == 0) {
printf("%p: EOL\n", bp);
return;
}
printf("%p: header: [%ld:%c] footer: [%ld:%c]\n", bp,
hsize, (halloc ? 'a' : 'f'),
fsize, (falloc ? 'a' : 'f'));
}
static void checkblock(void *bp)
{
if ((size_t)bp % 8)
printf("Error: %p is not doubleword aligned\n", bp);
if (GET(HDRP(bp)) != GET(FTRP(bp)))
printf("Error: header does not match footer\n");
}
/*
* checkheap - Minimal check of the heap for consistency
*/
void checkheap(int verbose)
{
char *bp = heap_listp;
if (verbose)
printf("Heap (%p):\n", heap_listp);
if ((GET_SIZE(HDRP(heap_listp)) != DSIZE) || !GET_ALLOC(HDRP(heap_listp)))
printf("Bad prologue header\n");
checkblock(heap_listp);
for (bp = heap_listp; GET_SIZE(HDRP(bp)) > 0; bp = NEXT_BLKP(bp)) {
if (verbose)
printblock(bp);
checkblock(bp);
}
if (verbose)
printblock(bp);
if ((GET_SIZE(HDRP(bp)) != 0) || !(GET_ALLOC(HDRP(bp))))
printf("Bad epilogue header\n");
}