A header file is a file with extension .h which contains C function declarations and macro definitions to be shared between several source files.
A simple practice in C programs is that we keep all the constants, macros, system wide global variables, and function prototypes in the header files and include that header file wherever it is required.
There are two types of header files: the files that the programmer writes and the files that comes with your compiler.
Include Syntax:
Header files are included in source files using the preprocessing directive
This form is used for system header files. It searches for a file named 'file' in a standard list of system directories.
This form is used for header files of your own program. It searches for a file named 'file' in the directory containing the current source file.
You can prepend directories to this list with the -I option while compiling your source code.
Include Operation:
The
Only Once Include:
If a header file happens to be included twice, the compiler will process its contents twice and it will result in an error. The standard way to prevent this is using conditional directive in header file.
Computed Include:
It may be required to select one of the several different header files to be included into your program.
The preprocessor offers the ability to use a macro for the header name. This is called a computed include.
A simple practice in C programs is that we keep all the constants, macros, system wide global variables, and function prototypes in the header files and include that header file wherever it is required.
There are two types of header files: the files that the programmer writes and the files that comes with your compiler.
Include Syntax:
Header files are included in source files using the preprocessing directive
#include. It has the following two forms.
#include <file>
This form is used for system header files. It searches for a file named 'file' in a standard list of system directories.
#include "file" This form is used for header files of your own program. It searches for a file named 'file' in the directory containing the current source file.
You can prepend directories to this list with the -I option while compiling your source code.
Include Operation:
The
#include directive directs the preprocessor to scan specified file and copy the content of the header file in to source file.Only Once Include:
If a header file happens to be included twice, the compiler will process its contents twice and it will result in an error. The standard way to prevent this is using conditional directive in header file.
#ifndef HEADER_FILE
#define HEADER_FILE
the entire header file file
#endif
Computed Include:
It may be required to select one of the several different header files to be included into your program.
#if SYSTEM_1
# include "system_1.h"
#elif SYSTEM_2
# include "system_2.h"
#elif SYSTEM_3
...
#endif
The preprocessor offers the ability to use a macro for the header name. This is called a computed include.
#define SYSTEM_H "system_1.h"
...
#include SYSTEM_H
| ANSI C Standard Library Header Files | |
| assert.h | Diagnostics |
| ctype.h | Character Class Tests |
| errno.h | Error Number |
| float.h | Implementation-defined Floating-Point Limits |
| limits.h | Implementation-defined Limits |
| locale.h | location specific information |
| math.h | Mathematical Functions |
| setjmp.h | Non-local Jumps |
| signal.h | Signals |
| stdarg.h | Variable Argument Lists |
| stddef.h | Standard definitions |
| stdio.h | Input and Output |
| stdlib.h | Utility functions |
| string.h | String functions |
| time.h | Time and Date functions |
Related topics:
Standard Input-Output in C | File Handling in C | Preprocessors in C | Type Casting in C | Error Handling in C
List of topics: C Programming
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