求大神解说一下，我刚开始学c不太理解这个调用过程#include <stdio.h>int x1=30,x2=40;void swap(int x,int y);void main(){ int x3=10,x4=20; swap(x3,x4); swap(x2,x1); printf("%d,%d,%d,%d\n",x3,x4,x1,x2);}void swap(int x,int y){ x1=x; x=y; y=x1;}