C语言 按引用函数调用

int sum(int a, int b);

int s = sum(10, 20); //Here 10 and 20 are actual parameters
or 
int s = sum(n1, n2); //Here n1 and n2 are actual parameters

#include <stdio.h>
void increment(int  *var)
{
    /* Although we are performing the increment on variable
     * var, however the var is a pointer that holds the address
     * of variable num, which means the increment is actually done
     * on the address where value of num is stored.
     */
    *var = *var+1;
}
int main()
{
     int num=20;
     /* This way of calling the function is known as call by
      * reference. Instead of passing the variable num, we are
      * passing the address of variable num
      */
     increment(&num);
     printf("Value of num is: %d", num);
   return 0;
}

Value of num is: 21

#include 
void swapnum ( int *var1, int *var2 )
{
   int tempnum ;
   tempnum = *var1 ;
   *var1 = *var2 ;
   *var2 = tempnum ;
}
int main( )
{
   int num1 = 35, num2 = 45 ;
   printf("Before swapping:");
   printf("\nnum1 value is %d", num1);
   printf("\nnum2 value is %d", num2);

   /*calling swap function*/
   swapnum( &num1, &num2 );

   printf("\nAfter swapping:");
   printf("\nnum1 value is %d", num1);
   printf("\nnum2 value is %d", num2);
   return 0;
}

Before swapping:
num1 value is 35
num2 value is 45
After swapping:
num1 value is 45
num2 value is 35