Source code contains preprocessor directives. There are numerous instructions to choose from. #include is one of them, and it's used to include a header file. Header files contain declarations, most commonly for functions and types (and occasionally variables) present in a library.
A preprocessor (or precompiler) in computer science is a programme that processes its input data to produce output that can be used as input by another programme. The output is a preprocessed version of the input data that is frequently used by other programmes, such as compilers.
For the computer programming languages C, Objective-C, and C++, the C preprocessor is the macro preprocessor. Header files, macro expansions, conditional compilation, and line control are all available through the preprocessor.
Header files have two functions. The interfaces to portions of the operating system are declared in system header files. They are included in your programme to provide the definitions and declarations required to call system calls and libraries.
The Standard C Library has 19 header files. The. h file extension is used for all files.