Header files contain information needed by the compiler telling it about functions and macros not present in a .c file. By convention, they are named with a .h extention.
The contain only prototype statments and macro definitions.
We include the information in a header file in a .c file using the #include compiler directive. This directive can include system header files or user header files. We have already seen one instance of this directive with system header files:
#include <stdio.h>The < > brackets say that the header file is a system file, located in "the usual place" in the file system where header files are stored (usually /usr/include on unix systems).
We can include our own header files as well:
#include "tfdef.h"This tells the compiler to find the header file in the current directory.
The include directive tells the C preprocessor to get that header and put it in place of the directive as if it were typed there before beginning to translate the code.