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Functions break large computing tasks into smaller ones, and enable people to build on what others have done instead of starting over from scratch. Appropriate functions hide details of operation from parts of the program that don't need to know about them, thus clarifying the whole, and easing the pain of making changes.
C has been designed to make functions efficient and easy to use; C programs generally consist of many small functions rather than a few big ones. A program may reside in one or more source files. Source files may be compiled separately and loaded together, along with previously compiled functions from libraries. We will not go into that process here, however, since the details vary from system to system.
Function declaration and definition is the area where the ANSI standard has made the most changes to C. It is now possible to declare the type of arguments when a function is declared. The syntax of function declaration also changes, so that declarations and definitions match. This makes it possible for a compiler to detect many more errors than it could before. Furthermore, when arguments are properly declared, appropriate type coercions are performed automatically.
Every function is defined exactly once. A program can declare and call a function as many times as necessary.
The definition of a function consists of a function head (or the declarator) and a function block . The function head specifies the name of the function, the type of its return value, and the types and names of its parameters, if any. The statements in the function block specify what the function does. The general form of a function definition is as follows:
//function head
type function-name(parameter declarations)//function block
{declarations and statements
}
In the function head, name is the function's name, while type (return-type) consists of at least one type specifier, which defines the type of the function's return value. The return type may be void or any object type, except array types. Furthermore, type may include the function specifier inline, and/or one of the storage class specifiers extern and static.
A function cannot return a function or an array. However, you can define a function that returns a pointer to a function or a pointer to an array.
The parameter declarations are contained in a comma-separated list of declarations of the function's parameters. If the function has no parameters, this list is either empty or contains merely the word void.
The type of a function specifies not only its return type, but also the types of all its parameters. The following listing is a simple function to calculate the volume of a cylinder.
// The cylinderVolume( ) function calculates the volume of a cylinder.
// Arguments: Radius of the base circle; height of the cylinder.// Return value: Volume of the cylinder.
extern double cylinderVolume( double r, double h ){
const double pi = 3.1415926536; // Pi is constantreturn pi * r * r * h;
}
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