Include guard

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In the C programming language, an #include guard is a particular construct used to avoid the problem of double inclusion when dealing with the #include directive, as demonstrated in the following sample C code:

Double inclusion

File \"grandfather.h\"

struct foo ;

File \"father.h\"

#include "grandfather.h"

File \"child.c\"

#include "grandfather.h"
#include "father.h"

Here, the file "child.c" has indirectly included two copies of the text in the header file "grandfather.h". This causes a compilation error, since the structure foo is apparently defined twice!

Use of #include guards

File \"grandfather.h\"

#ifndef H_GRANDFATHER
#define H_GRANDFATHER


struct foo ;


#endif

File \"father.h\"

#include "grandfather.h"

File \"child.c\"

#include "grandfather.h"
#include "father.h"

Here, the first inclusion of "grandfather.h" causes the macro H_GRANDFATHER to be defined. Then, when "father.h" includes "grandfather.h" the second time, the #ifndef test fails, and the compiler skips down to the #endif, thus avoiding the second definition of struct foo. The program compiles correctly.

Difficulties

In order for #include guards to work properly, each guard must test and conditionally set a different preprocessor macro. Therefore, a project using #include guards must work out a coherent naming scheme for its include guards, and make sure its scheme doesn't conflict with that of any third-party headers it uses, or with the names of any globally visible macros.

For this reason, many C and C++ implementations provide the non-standard directive #pragma once. This directive, inserted at the top of a header file, will ensure that the file is only included once.

See also

• C preprocessor

 

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