What is the function of the `#include` directive in C?

The #include directive in C serves the critical function of including the contents of a header file in a C source file. By using this directive, programmers can leverage predefined data types, constants, and function declarations that are contained within these header files, which are essential for using various libraries and APIs efficiently.

For instance, including the standard input-output header file <stdio.h> allows the programmer to write functions like printf() and scanf(), which handle input and output operations. The directive essentially directs the preprocessor to take the specified file and prepend its contents to the current source file before the actual compilation process begins. This promotes code reuse and modular design, making it easier to manage large codebases by separating logically related declarations and definitions into separate files.

Using the #include directive effectively enables a standardized way to access and incorporate reusable components, making it a fundamental feature in C programming.