# include "stdio.h"
 void sub1(char a,char b)
 { char c;   c=a; a=b; b=c; }
 void sub2(char *a,char b)
{ char c;   c=*a; *a=b; b=c; }
 void sub3(char *a,char *b)
{ char c;   c=*a; *a=*b; *b=c; }
void main()
 { char a,b;   a='A'; b='B';  
 sub3(&a,&b); putchar(a); putchar(b);
  sub2(&a,b);  putchar(a); putchar(b);
  sub1(a,b);   putchar(a); putchar(b);
}  答案为什么会是BAAAAA

sub3(&a,&b);a,b交换值，得a = 'B',b = 'A';
sub2(&a,b);b的值再赋给a,得a = 'A',b作为形参，值不变，b = 'A'
sub1(a,b);a,b都为形参，a,b的值都不变，a= 'A',b = 'A'.