Parameter Passing in C

 When a function gets executed in the program, the execution control is transferred from calling-function to called function and executes function definition, and finally comes back to the calling function. When the execution control is transferred from calling-function to called-function it may carry one or number of data values. These data values are called as parameters.

In C, there are two types of parameters and they are as follows...

• Actual Parameters
• Formal Parameters

The actual parameters are the parameters that are speficified in calling function. The formal parameters are the parameters that are declared at called function. When a function gets executed, the copy of actual parameter values are copied into formal parameters.

There are two methods to pass the data into the function in C language, i.e., call by value and call by reference.

Call by Value

 

• In call by value method, the value of the actual parameters is copied into the formal parameters. In other words, we can say that the value of the variable is used in the function call in the call by value method.
• In call by value method, we can not modify the value of the actual parameter by the formal parameter.
• In call by value, different memory is allocated for actual and formal parameters since the value of the actual parameter is copied into the formal parameter.
• The actual parameter is the argument which is used in the function call whereas formal parameter is the argument which is used in the function definition.

Example

 #include<stdio.h>  
 #include<conio.h>
 void swap(int , int); //prototype of the function   
    int main()  
    {  
        int a = 10;  
        int b = 20;   
        printf("Before swapping the values in main a = %d, b = %d\n",a,b);
        swap(a,b);  // calling function
        printf("After swapping values in main a = %d, b = %d\n",a,b); // Actual parameters                    getch();  
    }  
    void swap (int a, int b)  // called function
    {  
        int temp;   
        temp = a;  
        a=b;  
        b=temp;  
        printf("After swapping values in function a = %d, b = %d\n",a,b); // Formal parameters
    }  

 Output

 

 

Call by Reference

 

• In call by reference, the address of the variable is passed into the function call as the actual parameter.
• The value of the actual parameters can be modified by changing the formal parameters since the address of the actual parameters is passed.
• In call by reference, the memory allocation is similar for both formal parameters and actual parameters. All the operations in the function are performed on the value stored at the address of the actual parameters, and the modified value gets stored at the same address.

Example

 #include<stdio.h>  
 #include<conio.h>
 void swap(int *, int *); //prototype of the function   
int main()  
{  
    int a = 10;  
    int b = 20;   
    printf("Before swapping the values in main a = %d, b = %d\n",a,b);  
    swap(&a,&b);  
    printf("After swapping values in main a = %d, b = %d\n",a,b); // Actual parameters
    getch();
}  
void swap (int *a, int *b)  
{  
    int temp;   
    temp = *a;  
    *a=*b;  
    *b=temp;  
    printf("After swapping values in function a = %d, b = %d\n",*a,*b); // Formal parameters  
}  

Output