C #define Directive: Purpose & Usage

In the C language, #define is a preprocessor directive used to define a constant or a macro. The defined constant or macro can be used anywhere in the program and will be replaced with its corresponding value at compile time.

The functions of #define include:

1. Define
2. Define
3. Declare or define.
4. Conditional compilation allows for defining or undefining certain constants or macros based on conditional compilation directives, allowing for choosing different code paths based on circumstances during compilation.

Generally speaking, the purpose of #define is to define constants or macros in the code to improve its readability, maintainability, and flexibility.