There are two ways to pass the data into the function in C language:

•  method :call by value 
• method :call by reference.

Call by value in C

• In this method, we pass the values of variables (the value of the actual parameters) into the formal parameters.
• Here, the formal parameter cannot modify the value of the actual parameter.
• The formal parameter gets a copy of the value of the actual parameter, therefore the memory allocation is different for actual and formal parameters.
• The actual parameter is the argument that is used in the function call, whereas the formal parameter is the argument that is used in the function definition.

Example: Swapping values on integers

#include <stdio.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); // printing the value of a and b in main  
    swap(a,b);  
    printf("After swapping values in main a = %d, b = %d\n",a,b); // The value of actual parameters do not change by changing the formal parameters in call by value, a = 10, b = 20  
}  
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, a = 20, b = 10   
}  

Output:

Before swapping the values in main a = 10, b = 20
After swapping values in function a = 20, b = 10
After swapping values in main a = 10, b = 20

Here, the value of a and b remain unchanged in the main function, but that of function swap() is getting exchanged.

Call by reference in C

• In call by reference, the addresses of actual arguments in the calling function are copied into the formal arguments of the called function
• Using this method, we have access to the actual arguments and hence we can be able to manipulate them.
• The memory allocation is the same for both formal parameters and actual parameters.

Example: Swapping integer values

#include <stdio.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); // printing the value of a and b in main  
    swap(&a,&b);  
    printf("After swapping values in main a = %d, b = %d\n",a,b); // The value of actual parameters do not change by changing the formal parameters in call by value, a = 10, b = 20  
}  
void swap (int *a, int *b)  
{  
    int temp;   
    temp = *a;  
    *a= *b;  
    *b=temp;  
   
}  

Output

Before swapping the values in main a = 10, b = 20
After swapping values in main a = 20, b = 10

The above code manages to exchange the values of actual parameters.

Note: also read about the Passing arguments between functions in C & Functions in C

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