Declaring & Defining Functions In C

Function Declaration in C

• A function declaration, also known as a function prototype, is a statement that specifies the return type, name, and parameters of a function, but does not provide the implementation or working or actual body of the function.

• Function declarations are used to inform the compiler about the signature of the function, so that it can check the correctness of function calls.

• The syntax of a function declaration in C is:

return_type function_name(argument_list);

For example:

int sum(int a, int b);

Explanation

This function declaration states that the function sum returns an integer value and takes two integer arguments.

built-in functions

Function declarations of built-in functions are usually placed in header files, so that they can be easily shared and reused in multiple source files. The actual implementation of these function is provided in a separate source file.

Function Definition in C

A function definition provides the implementation,working, actual body of the function of a function. In simple words function definition specifies the tasks it performs.

return_type function_name(argument_list) 
{
   // function body
}

For example:

int sum(int a, int b)
{
   int result;
   result = a + b;
   return result;
}

Explanation

• This function definition specifies the implementation of the function sum, which takes two integer arguments and returns their sum as an integer value.

• The function definition must match the function declaration in terms of the return type, name, and parameters.

• The function body contains one or more statements that specify the tasks performed by the function.

• The return statement is used to return the result of the function to the caller.

Differences between function declaration and definition

Function Declaration	Function Definition
Provides the compiler with the name, return type, and parameter list of a function.	Provides the complete implementation of a function, including its code.
Also known as a function prototype.	Also known as an implementation or definition of a function.
Uses the syntax return_type function_name(parameter list);.	Uses the syntax return_type function_name(parameter list) { function body }.
Can appear in multiple places throughout the code, allowing for early binding of functions.	Must appear exactly once in the code, usually in a separate file or at the bottom of the main file.
The function body is not included in the declaration.	The function body is included in the definition.

tip

• In C, it is generally a good practice to declare functions before they are called in the program, so that the compiler knows their signature and can check for any errors.

• The definition of a function should be placed in a separate file or at the bottom of the main file, after all the functions that call it have been declared. This helps to make the code more readable and maintainable.