C Preprocessor & Macro

Introduction

The C preprocessor is a tool which filters your source code before it is compiled.

The preprocessor allows constants to be named using the # notation.

The preprocessor provides several other facilities which will be described here.

It is particularly useful for selecting machine dependent pieces of code for different computer types, allowing a single program to be compiled and run on several different computers.

The C preprocessor isn't restricted to use with C programs only. Programmers who use other languages may also find it useful, however it is tuned to recognize features of the C language like comments and strings, so its use may be restricted in other circumstances.

Set of commonly used Preprocessor Directives and their Functions:

#define defines a macro substitution. #undef undefined a macro. #include specifies the files to be include. #ifdef tests for a macro definition #ifndef tests whether a macro is not defined. #if tests a compile-time condition. #endif specifies the end of #if. #else specifies alternatives when #if test fails.

These Preprocessor or directives can be divided into three categories:

1. Macro Substitution Directives

2. File Inclusion Directives

3. Compiler Control Directives