for（;x!=‘@’;)括号里是啥意思啊？scanf可以写在for循环外面嘛？

#include<stdio.h>

int main() {
char x=0;
int a=0, b=0, c=0, d=0, e=0, f=0;
for (;x!='@';) {
scanf_s("%c", &x);
switch (x){
case'(':a++; break;
case')':b++; break;
case'[':c++; break;
case']':d++; break;
case'{':e++; break;
case'}':f++; break;
default:break;
}
}

if (e != f) printf("{}"); 

if (c != d) printf("[]");

if (a != b) printf("()");

if (a==b&&c==d&&e==f){
    printf("NULL");
}
return 0;

}