XINU -需要帮助理解系统调用- getstk.c

Question

我在概念上无法理解这个系统调用接近尾声时发生了什么，以及为什么。我知道getstk.c方法返回可用空间的最高内存地址，但不理解一些代码在做什么。对此进行一些澄清将是非常好的。我不完全理解的代码区域用星号强调。

/* getstk.c - getstk */

#include <xinu.h>

/*------------------------------------------------------------------------
 *  getstk  -  Allocate stack memory, returning highest word address
 *------------------------------------------------------------------------
 */
char    *getstk(
          uint32        nbytes          /* size of memory requested     */
        )
{
        intmask mask;                   /* saved interrupt mask         */
        struct  memblk  *prev, *curr;   /* walk through memory list     */
        struct  memblk  *fits, *fitsprev; /* record block that fits     */

        mask = disable();
        if (nbytes == 0) {
                restore(mask);
                return (char *)SYSERR;
        }

        nbytes = (uint32) roundmb(nbytes);      /* use mblock multiples */

        prev = &memlist;
        curr = memlist.mnext;
        fits = NULL;
        fitsprev = NULL;  /* to avoid a compiler warning */

        while (curr != NULL) {                  /* scan entire list     */
                if (curr->mlength >= nbytes) {  /* record block address */
                        fits = curr;            /*   when request fits  */
                        fitsprev = prev;
                }
                prev = curr;
                curr = curr->mnext;
        }

        if (fits == NULL) {                     /* no block was found   */
                restore(mask);
                return (char *)SYSERR;
        }
        if (nbytes == fits->mlength) {          /* block is exact match */
                fitsprev->mnext = fits->mnext;
        **} else {                                /* remove top section   */
                fits->mlength -= nbytes;
                fits = (struct memblk *)((uint32)fits + fits->mlength);
        }**
        memlist.mlength -= nbytes;
        restore(mask);
        **return (char *)((uint32) fits + nbytes - sizeof(uint32));**
}

struct memblk可以在这里找到：

struct  memblk  {           /* see roundmb & truncmb    */
    struct  memblk  *mnext;     /* ptr to next free memory blk  */
    uint32  mlength;        /* size of blk (includes memblk)*/
    };
extern  struct  memblk  memlist;    /* head of free memory list */

为什么他们返回fits + nbytes - sizeof(uint32)？为什么他们要将fits(结构)转换为uint32类型？

Answer 1

    if (nbytes == fits->mlength) {          /* block is exact match */
            fitsprev->mnext = fits->mnext;
    **} else {                                /* remove top section   */
            fits->mlength -= nbytes;
            fits = (struct memblk *)((uint32)fits + fits->mlength);
    }**
    memlist.mlength -= nbytes;
    restore(mask);
    **return (char *)((uint32) fits + nbytes - sizeof(uint32));**

如果找到了一个完美的匹配，该块就会从空闲列表中删除。如果找到更大的块，则将该块拆分为两个块:比原始块小的空闲块nbytes (fits->mlength -= nbytes)；以及从新的空闲块(fits = (struct memblk *)((uint32)fits + fits->mlength))开始的已分配的nbytes块，由函数返回。

为什么他们要返回

+ nbytes - sizeof(uint32)？为什么他们要将fits(结构)转换为uint32类型？

由于堆栈在这种情况下向下增长，因此该函数返回指向堆栈顶部的指针，该指针是位于已分配块末尾的字，即：

(uint32)fits /* start of allocated block */ + nbytes /* size of allocated block */ - sizeof(uint32) /* size of a word */

对(uint32)的强制转换是使用整数算法而不是指针算法，否则fits+1将生成一个指向sizeof(struct memblk)超出fits的指针。转换为(char *)可能更惯用。