起因是Csapp第三版，5.17，自己手動實現高階memset函式

程式碼如下：

#include<stdlib.h>
#include<cstdio>
#include<iostream>
#include<stack>
#include<ctime>
#include<cmath>
#include<windows.h>
#include<vector>
#include<map>
#include <limits.h>
using namespace std;
void
 
 *my_memset(void *s, int c, size_t n)
{
    size_t K=sizeof(unsigned long);
    if(n<K)
    {
          size_t cnt = 0;
          unsigned char *schar = (unsigned char *)s;
          while (cnt < n)
          {
              *schar++ = (unsigned char)c;
              cnt++;
          }
    }
    
 
else
    {
          unsigned long comb=0;
        for (int i=1;i<=K;i++)
        {
              comb<<=CHAR_BIT;
              comb+=(unsigned char)c;
        }
        size_t cnt = 0;
        unsigned long *slong = (unsigned long *)s;
        while (cnt+K<=n)
        {
              *slong=comb;
            slong
 
+=1;
              cnt+=K;
        }
        unsigned char *schar =(unsigned char *)slong;
        while(cnt<n)
        {
              *schar++ = (unsigned char)c;
              cnt++;
        }
  }
  return s;
}
int main()
{
    char a[15];
    my_memset(a,65,sizeof(a));
    cout<<a<<endl;
    memset(a,65,sizeof(a));
    cout<<a<<endl;
    system("pause");
}

　　關注

　　這段程式碼，slong是unsigned long *型別的指標。

　　我們想要指標指向範圍跨越sizeof(unsigned long)，直接++，就能實現。不需要slong+=sizeof(unsigned long)