What is the difference between actual and formal arguments in C?

Before going to the difference between actual and formal arguments in C, first understand the two terms - argument and parameter. Dennis M Ritchie in his classic book on C language mentioned, "We will generally use parameter for a variable named in the parenthesized list in a function definition, and argument for the value used in a call of the function. The terms formal argument and actual argument are sometimes used for the same distinction." So don't get confused if you find the question like difference between actual and formal arguments or difference between actual and formal parameters. They both are the same.

The major difference between actual and formal arguments is that actual arguments are the source of information; calling programs pass actual arguments to called functions. The called functions access the information using corresponding formal arguments.

The following piece of code demonstrates actual and formal arguments.

/* Demonstrating difference between actual and formal arguments */
#include <stdio.h>
 
int addTwoInts(int, int); /* Prototype */
 
int main() /* Main function */
{
  int n1 = 10, n2 = 20, sum;
 
  /* n1 and n2 are actual arguments. They are the source
     of data. Caller program supplies the data to called 
     function in form of actual arguments. */
  sum = addTwoInts(n1, n2); /* function call */
 
  printf("Sum of %d and %d is: %d \n", n1, n2, sum);
}
 
/* a and b are formal parameters. They receive the values from
   actual arguments when this function is called. */
int addTwoInts(int a, int b)
{
  return (a + b);
}
 
OUTPUT
======
Sum of 10 and 20 is: 30

In above piece of code, the variables n1 and n2 are called actual arguments, whereas the variables a and b are called formal arguments. When values are passed to a called function the values present in actual arguments are copied to the formal arguments. In case of above program the values of n1 and n2 are 10 and 20 respectively. The main() would call addTwoInts with n1 and n2 as its actual arguments, and addTwoInts will send back the computed sum to the main().

Conclusively, when a function is called all actual arguments (those supplied by the caller) are evaluated and each formal argument is initialized with its corresponding actual argument.

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