call by reference

1. call by reference：函式呼叫時傳入變數的位址，因此呼叫函式後，函式中若變更變數，結束函式後變數也跟著變動。
2. 範例：輸入兩個數字，將其對換，呼叫時傳入變數的儲存位址 &a, &b (變數前加 & 符號)，函式內取該位址上的值 *x, *y (變數前加 * 符號)。

   [dywang@dyw219 zzz]$ vim function3.c
   [dywang@dyw219 zzz]$ cat function3.c
   #include <stdio.h>
   
   void swap(int *x, int *y) {
   	int temp;
   
   	temp = *x;
   	*x = *y;
   	*y = temp;
   	return;
   }
    
   main() {
   	int a, b;
   
   	printf("Enter TWO integers: ");
   	scanf("%d %d", &a, &b);
   
   	printf("Before swap, a = %d, b = %d\n", a, b);
   	swap(&a, &b);
   	printf("After swap, a = %d, b = %d\n", a, b);
   }
   

3. 編譯成功

   [dywang@dyw219 zzz]$ gcc -o function3 function3.c
   

4. 執行結果：呼叫對換函式後，原變數 a b 的數值被調換了。

   [dywang@dyw219 zzz]$ ./function3
   Enter TWO integers: 10 20
   Before swap, a = 10, b = 20
   After swap, a = 20, b = 10
   [dywang@dyw219 zzz]$ ./function3
   Enter TWO integers: 21 11
   Before swap, a = 21, b = 11
   After swap, a = 11, b = 21