範圍原則

1. Local 本地變數：變數宣告在函式或區塊中，程式離開函式或區塊後就失效。
   

   #include <stdio.h>
    
   int main ()
   {
     /* local variable declaration */
     int a, b;
     int c;
    
     /* actual initialization */
     a = 10;
     b = 20;
     c = a + b;
    
     printf ("value of a = %d, b = %d and c = %d\n", a, b, c);
    
     return 0;
   }
   

2. Global 全域變數：宣告在函式外，通常在程式的最上方，變數保持其值直到程式結束，即使程式進入函式執行，依然可以存取這個變數。
   

   #include <stdio.h>
    
   /* global variable declaration */
   int g;
    
   int main ()
   {
     /* local variable declaration */
     int a, b;
    
     /* actual initialization */
     a = 10;
     b = 20;
     g = a + b;
    
     printf ("value of a = %d, b = %d and g = %d\n", a, b, g);
    
     return 0;
   }
   

3. 程式中相同名子的本地變數及全域變數可以同時存在。
   

   #include <stdio.h>
    
   /* global variable declaration */
   int g = 20;
    
   int main ()
   {
     /* local variable declaration */
     int g = 10;
    
     printf ("value of g = %d\n",  g);
    
     return 0;
   }
   

4. 範例
   

   #include <stdio.h>
    
   /* global variable declaration */
   int a = 20;
    
   int main ()
   {
     /* local variable declaration in main function */
     int a = 10;
     int b = 20;
     int c = 0;
   
     printf ("value of a in main() = %d\n",  a);
     c = sum( a, b);
     printf ("value of c in main() = %d\n",  c);
   
     return 0;
   }
   
   /* function to add two integers */
   int sum(int a, int b)
   {
       printf ("value of a in sum() = %d\n",  a);
       printf ("value of b in sum() = %d\n",  b);
   
       return a + b;
   }