#include<stdio.h>void main(){ int sum(); int diff(); printf("%d+%d=%d\n",5,3,sum(5,3)); printf("%d-%d=%d\n",6,4,diff(6,4));}int sum(int x,int y){ int sum; sum=x+y; }int diff(int x,int y){ return(x-y);}C语言环境下 说缺少返回值 但是运算结果没错 什么是返回值啊