C Programming

FUNCTIONS
A function in C can perform a particular task, and supports the concept of modular programming design techniques.

We have already been exposed to functions. The main body of a C program, identified by the keyword main, and enclosed by the left and right braces is a function. It is called by the operating system when the program is loaded, and when terminated, returns to the operating system.

Functions have a basic structure. Their format is


	return_data_type  function_name  ( arguments, arguments )
	data_type_declarations_of_arguments;
	{
		function_body
	}

It is worth noting that a return_data_type is assumed to be type int unless otherwise specified, thus the programs we have seen so far imply that main() returns an integer to the operating system.

ANSI C varies slightly in the way that functions are declared. Its format is


	return_data_type function_name (data_type variable_name, data_type variable_name, .. )
	{
		function_body
	}

This permits type checking by utilizing function prototypes to inform the compiler of the type and number of parameters a function accepts. When calling a function, this information is used to perform type and parameter checking.

ANSI C also requires that the return_data_type for a function which does not return data must be type void. The default return_data_type is assumed to be integer unless otherwise specified, but must match that which the function declaration specifies.

A simple function is,


	void print_message( void )
	{
		printf("This is a module called print_message.\n");
	}

Note the function name is print_message. No arguments are accepted by the function, this is indicated by the keyword void in the accepted parameter section of the function declaration. The return_data_type is void, thus data is not returned by the function.

An ANSI C function prototype for print_message() is,


	void print_message( void );

Function prototypes are listed at the beginning of the source file. Often, they might be placed in a users .h (header) file.