丹尼斯.里奇的c语言教程malloc例子两个疑问
/**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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