C programs are divided into modules or functions. Some functions are written by users and some are stored in C library. Library functions are grouped category wise and stored in different files known as header files. To access the file stored in the library, it is necessary to tell the compiler about the files to be accessed. This is achieved by the pre-processor directive #include as follows,
Filename is the name of the library file that contains the required function definition. Pre-processor directives are placed at the beginning of the program.