Function Calling By value or By reference

Calling a Function:

While creating a C function, you give a definition of what the function has to do. To use a function, you will have to call that function to perform the defined task.

When a program calls a function, program control is transferred to the called function. A called function performs defined task and when its return statement is executed or when its function-ending closing brace is reached, it returns program control back to the main program.

To call a function, you simply need to pass the required parameters along with function name, and if function returns a value, then you can store returned value. For example:

#include <stdio.h>
 
/* function declaration */
int max(int num1, int num2);
 
int main ()
{
   /* local variable definition */
   int a = 100;
   int b = 200;
   int ret;
 
   /* calling a function to get max value */
   ret = max(a, b);
 
   printf( "Max value is : %d\n", ret );
 
   return 0;
}
 
/* function returning the max between two numbers */
int max(int num1, int num2) 
{
   /* local variable declaration */
   int result;
 
   if (num1 > num2)
      result = num1;
   else
      result = num2;
 
   return result; 
}

I kept max() function along with main() function and compiled the source code. While running final executable, it would produce the following result:

Max value is : 200

Function Arguments:

If a function is to use arguments, it must declare variables that accept the values of the arguments. These variables are called the formal parameters of the function.

The formal parameters behave like other local variables inside the function and are created upon entry into the function and destroyed upon exit.

While calling a function, there are two ways that arguments can be passed to a function:

Call Type	Description
Call by value	This method copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument.
Call by reference	This method copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument.

By default, C uses call by value to pass arguments. In general, this means that code within a function cannot alter the arguments used to call the function and above mentioned example while calling max() function used the same method.

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