What is the use of header files in C language?

Header files are used in the C programming language to provide information to the compiler about various functions, data types, and other entities used in a C programme. They are used to declare and define the interfaces to libraries or modules in a programme.

The following are the primary functions of header files in C:

1. Function Prototypes: Function prototypes are declarations that provide the names, return types, and parameter types of functions in header files. When functions are utilised wrongly, the compiler can do type checking and detect mistakes.

2. Constants and Macros: Using the '#define' directive, header files can declare constants or offer preprocessor macros. These constants and macros can be used throughout the programme to help with code reuse.

3. Data Types: In a programme, header files can define new data types or structures. The compiler learns about these data types and may handle them effectively by incorporating the appropriate header file.

4. External Libraries and Modules: Declarations and definitions from external libraries or modules are typically included in header files. The compiler knows how to interact with these external entities since the proper header files are included.

5. Conditional Compilation: Conditional compilation directives such as '#ifdef' and '#ifndef' are frequently used in header files to include or omit specified pieces of code based on certain criteria. Different code paths are possible based on the target platform, configuration, or feature set.

Overall, header files in C enable for the separation of code interface and implementation, allowing for modular and reusable programming. They enable communication between distinct portions of a programme or external libraries, facilitate appropriate type verification, and specify shared constants.