Header files contain definitions of functions and variables which can be incorporated into any C program by using the pre-processor #include statement. Standard header files are provided with each compiler, and cover a range of areas, string handling, mathematical, data conversion, printing and reading of variables.
To use any of the standard functions, the appropriate header file should be included. This is done at the beginning of the C source file. For example, to use the function printf() in a program, the line
should be at the beginning of the source file, because the definition for printf() is found in the file stdio.h All header files have the extension .h and generally reside in the /include subdirectory.
#include <stdio.h> #include "mydecls.h"
The use of angle brackets <> informs the compiler to search the compilers include directory for the specified file. The use of the double quotes "" around the filename inform the compiler to search in the current directory for the specified file.