#include<stdio.h>
void sub(double x, double *y, double *z)
{
    *y = *y - 1.0; *z = *z + x;
}
main()
{
    double a = 2.5, b = 9.0, *pa, *pb;
    pa = &a; pb = &b;
    sub(b - a, pa, pa);
    printf("%f\n", a);
}

相当于：
double x;
x = b - a;
a = a - 1.0;
a = a + x;
printf("%f\n", a);

嗯~~
*y其实就是*z,二者都是a
所以a=a-1.0;a=a+x;我知道了  谢谢！