Header files contain definitions of functions and variables, which is imported or used into any C program by using the pre-processor #include statement. Header file have an extension ".h" which contains C function declaration and macro definition.
Each header file contains information (or declarations) for a particular group of functions. Like stdio.h header file contains declarations of standard input and output functions available in C which is used for get the input and print the output. Similarly, the header file math.h contains declarations of mathematical functions available in C.
Types of Header files
- System header files: It is comes with compiler.
- User header files: It is written by programmer.
Why need of header files
When we want to use any function in our c program then first we need to import their definition from c library, for importing their declaration and definition we need to include header file in program by using #include. Header file include at the top of any C program.
For example if we use printf() in C program, then we need to include, stdio.h header file, because in stdio.h header file definition of printf() (for print message on screen) is written in stdio.h header file.
How to use header file in Program
Both user and system header files are include using the pre-processing directive #include. It has following two forms:
This form is used for system header files. It searches for a file named file in a standard list of system directives.
This form used for header files of our own program. It searches for a file named file in the directive containing the current file.
Note: 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.