C言語の関数に関するスコープルールを説明します。

#include<stdio.h>
main ( ){
   int a=10, b = 20;
   printf ("before swapping a=%d, b=%d", a,b);
   swap (a,b);
   printf ("after swapping a=%d, b=%d", a,b);
}
swap (int a, int b){
   int c;
   c=a;
   a=b;
   b=c;
}

Before swapping a=10, b=20
After swapping a = 10, b=20

include<stdio.h>
int a=10, b = 20;
main(){
   printf ("before swapping a=%d, b=%d", a,b);
   swap ( );
   printf ("after swapping a=%d, b=%d", a,b);
}
swap ( ){
   int c;
   c=a;
   a=b;
   b=c;
}

Before swapping a = 10, b =20
After swapping a = 20, b = 10