QUESTION 03 · Functions

What is a function prototype?

2 marks

Answer

A function prototype declares a function and specifies the types of its parameters.

Example: int add(int a, int b);
The compiler uses it to check calls and apply the required argument conversions.

Distinguish a function declaration, definition and call with an example.

5 marks

Answer in brief

A declaration describes a function, a definition supplies its body, and a call executes it.

Declaration or prototype

int square(int number);

Meaning of the prototype

int before the name is the return type. square is the function name. int number specifies one integer parameter; the parameter name is optional in a prototype. The semicolon ends the declaration. A prototype does not contain the function body.

Definition

int square(int number)
{
    return number * number;
}

Function call

In square(5), the value 5 is supplied as an argument. Control enters the definition, number receives 5 and the expression number * number produces 25. return sends the value back to the caller. The example declares the function before main and defines it afterwards.

Important rules

Declare the function before its first call. Modern C does not provide an implicit function declaration.
Keep declarations and the definition compatible, including the return type and parameter types.
A prototype alone is not a definition; a used non-library function still needs a definition.
In C11, write f(void) to declare no parameters. A declaration f() does not specify the parameter types.

Why prototypes are useful

The compiler can detect many wrong argument counts and incompatible types when a prototype is visible. A prototype does not prove the function computes the right answer; the body must still be tested. The sample square value is deliberately small enough for int arithmetic.

C program and output

Download .c Open compiler

/* CodeBhavya | PPS AR25 | Learning example */
#include <stdio.h>
int square(int number);
int main(void)
{
    printf("Square = %d\n", square(5));
    return 0;
}
int square(int number)
{
    return number * number;
}

Sample input and output

This example uses values already written in the program.

Output

Square = 25