求大神解析一下这串代码
#include<stdio.h>
#include<string.h>
void fun(char *w,int m)
{char s,*p1,*p2;
p1=w;p2=w+m-1;
while(p1<p2){
s=*p1++;*p1=*p2--;*p2=s;}
}
main()
{char a[]="AAAAGGG";
fun(a,strlen(a));
puts(a);
}