XINU - 需要帮助了解系统调用 - getstk.c

Question

我在概念上理解本系统调用结束时会发生什么问题，以及为什么。我明白getstk.c方法返回可用空间的最高内存地址，但不明白一些代码在做什么。这方面的一些清晰度会很好。星号强调我不完全理解的代码区域。

/* 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));** 
} 

的结构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 */ 

为什么他们返回拟合+的nbytes - 的sizeof（UINT32）？为什么他们适合（一个结构）来键入uint32？

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匹配，该块就从空闲列表中删除。如果找到更大的块，则该块被分成两个块：比原始块（fits->mlength -= nbytes）小的空闲块nbytes;并且在新的空闲块（fits = (struct memblk *)((uint32)fits + fits->mlength)）之后开始分配的块nbytes，由该功能返回。

他们为什么要返回fits + nbytes - sizeof（uint32）？为什么他们适合（一个结构）来键入uint32？

由于堆栈在此情况下向下增长，该函数返回一个指针到堆栈的顶部，这是在所分配的块的末端的话，即：

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

铸到(uint32)是使用整数算术代替指针算术，否则fits+1会产生指向sizeof(struct memblk)超出fits的指针。铸造到(char *)可能会更习惯。