C_Header-Files

Software-Engineering

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# What are Header files?

Header files contain C declarations, C_Macros definitions & functions. File extension: .h.

Are shared between several source files - allow you to declare functions, C_Macros, … in one place & you can use them across different parts of your programm.

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# Why use header files

• reusability
• seperate declaration from implementation
• encapsulation
• conditional choices

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# Include header files

using the include directive

#include <unistd.h>          // unistd.h - used for system calls
#include "mylibs/mylib.h"    // own header files

The #include directive copies the content of header file into source file - during preprocessing stage. Shows compiler, that declarations are available when processing each source file.

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# Search path

" " - compiler first looks for header file in same directory as source file not found - checks list of standard system directories.

< > - compiler only looks in standard system directories.

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# Conditinal compilation directives

To avoid errors/conflicts by multiple inclusions of same header file

• use conditional compilation directives

#ifndef FILE_FOO_SEEN 
#define FILE_FOO_SEEN 
 
/* the entire header file */ 
 
#endif /* !FILE_FOO_SEEN */

How this works:

• If FILE_FOO_SEEN has not been defined before (first time header is included) - gets defined & header is included
• If FILE_FOO_SEEN has been defined (header has already been included) - content in #ifndef to #endif is skipped

# sample keywords

#ifndef (if not defined) - checks, whether a macro has been defined.

• is not - code between #ifndef to #endif is included
• is - code is skipped

#define - defines macro.

#endif - marks end of conditional block.

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