The C Preprocessor is a translation phase that processes the source code before the compiler. It is a text substitution tool separate from the compiler and provides its own powerful language for programmers. Preprocessor directives start with the symbol #. The preprocessor simplifies program development, readability, and modification, and makes C code portable across different architectures.

The preprocessor performs three main functions:

1. File inclusion: Inserts contents of one file into another.
2. Macro substitution: Replaces text with another.
3. Conditional compilation: Compiles certain parts of the code based on conditions.

The syntax of the preprocessor differs from the rest of the C program, and while it is primarily designed for C, it can be useful in other programming languages as well.

Objective:

After going through this unit, you will be able to:

1. define, declare preprocessor directives;
2. discuss various preprocessing directives, for example file inclusion, macro substitution, and conditional compilation; and
3. discuss various syntaxes of preprocessor directives and their applications.