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- /**
- QUESTION1: bp < p->s.ptr 什么时候出现,插入到哪里?
- ANSWER1:
- QUESTION2: join to lower, freep好像指向的位置比其他情况不一样,这会导致什么问题,会导致morecore多一次调用吗.
- ANSWER2: 会导致循环开始位置不一样,不会导致morecore多一次调用,假如只能使用这次释放的内存块的话,
- 会循环过去,指到freep位置时,大小满足了,就拿到数据返回.
-
- QUESTION3: &base地址在ubuntu64bit上测试确实比sbrk获得的地址低,这依靠了系统相关特性,假如在相反的平台上程序是否可以正常运行.
- ANSWER3:
- */
- #include <stdio.h>
- typedef long Align; /* for alignment to long boundary */
- union header { /* block header */
- struct {
- union header *ptr; /* next block if on free list */
- unsigned size; /* size of this block */
- } s;
- Align x; /* force alignment of blocks */
- };
- typedef union header Header;
- static Header base; /* empty list to get started */
- static Header *freep = NULL; /* start of free list */
- /* free: put block ap(AccessPoint) in free list */
- void free(void *ap)
- {
- Header *bp, *p;
- bp = (Header *)ap - 1; /* point to block header */
- for (p = freep; !(bp > p && bp < p->s.ptr); p = p->s.ptr)
- {
- if (p >= p->s.ptr && (bp > p || bp < p->s.ptr)) /* QUESTION1 */
- {
- break; /* freed block at start or end of arena */
- }
- }
-
- if (bp + bp->s.size == p->s.ptr) { /* join to upper(next) nbr */
- bp->s.size += p->s.ptr->s.size;
- bp->s.ptr = p->s.ptr->s.ptr;
- }
- else
- bp->s.ptr = p->s.ptr;
-
- if (p + p->s.size == bp) { /* join to lower(prev) nbr */
- p->s.size += bp->s.size;/* QUESTION2 */
- p->s.ptr = bp->s.ptr;
- }
- else
- p->s.ptr = bp;
-
- freep = p;
- }
- #define NALLOC 1024 /* minimum #units to request */
- /* morecore: ask system for more memory */
- static Header *morecore(unsigned nu)
- {
- char *cp, *sbrk(int);
- Header *up;
- if (nu < NALLOC)
- nu = NALLOC;
- cp = sbrk(nu * sizeof(Header));
- if (cp == (char *)-1) /* no space at all */
- return NULL;
- up = (Header *)cp;
- up->s.size = nu;
- free((void *)(up + 1));
- return freep;
- }
- /* malloc: general-purpose storage allocator */
- void *malloc(unsigned nbytes)
- {
- Header *p, *prevp;
- Header *moreroce(unsigned);
- unsigned nunits;
- nunits = (nbytes + sizeof(Header) - 1) / sizeof(Header) + 1;
- if ((prevp = freep) == NULL) { /* no free list yet */
- base.s.ptr = freep = prevp = &base; /* QUESTION3 */
- base.s.size = 0;
- }
- for (p = prevp->s.ptr;; prevp = p, p = p->s.ptr) {
- if (p->s.size >= nunits) { /* big enough */
- if (p->s.size == nunits) /* exactly */
- {
- prevp->s.ptr = p->s.ptr;
- }
- else { /* allocate tail end */
- p->s.size -= nunits;
- p += p->s.size;
- p->s.size = nunits;
- }
- freep = prevp;
-
- return (void *)(p + 1);
- }
- if (p == freep) /* wrapped around free list */
- if ((p = morecore(nunits)) == NULL)
- return NULL; /* none left */
- }
- }
- int main(int argc, char* argv[])
- {
- char* p0=NULL;
- char* p1=NULL;
- char* p2=NULL;
- char* p3=NULL;
- char* p4=NULL;
-
- p0=malloc(4096);
- if(NULL!=p0)
- {
- free(p0);
- }
-
- p1=malloc(1024);
- p2=malloc(1024);
- p3=malloc(1024);
- p4=malloc(1024);
-
- if(NULL!=p1)
- {
- free(p1);
- }
-
- if(NULL!=p2)
- {
- free(p2);
- }
- if(NULL!=p3)
- {
- free(p3);
- }
- if(NULL!=p4)
- {
- free(p4);
- }
- return 0;
- }
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