int (* 函数指针名) (参数类型 参数名1，参数类型 参数名2，...);
 int (* p) (int x, int y);

int 函数名(参数类型 参数名1，参数类型 参数名2，...){
    /*do something*/ 
}
int add(int num1 ,int num2){
    return (nm1 + num2);
}

p = add;

int num = p (1 , 1);

#include <stdio.h>

int fun1 (int num1, int num2){
    return (num1+num2);
}

int fun2 (int num1, int num2){
    return (num1-num2);
}

int main (void ){
    int (*p) (int number1, int number2 );
    p = fun1;
    printf("The fun1 answer is %d.\n",p(200, 100));

    p = fun2;
    printf("The fun2 answer is %d.\n",p(200, 100));
    return 0;
}