In the C and C++ programming languages, an #include guard, sometimes called a macro guard, header guard or file guard, is a way to avoid the problem of double inclusion when dealing with the include directive.
The C preprocessor processes inclusion directives like #include "foo.h" to include "foo.h" and transcludes the code of that file into a copy of the main file often called the translation unit.
However, if an #include directive for a given file appears multiple times during compilation, often times constructs are then defined twice, and the resulting translation unit is invalid. #include guards prevent this by defining a preprocessor macro when a header is first included, and detecting its presence to ignore the file's contents next time it is included.
The addition of #include guards to a header file is one way to make that file idempotent and the program safer. Another construct to combat double inclusion is
, which is non-standard but nearly universally supported directive among C and C++ compilers.
The following C code demonstrates a real problem that can arise if #include guards are missing:
Here, the file "child.c" has indirectly included two copies of the text in the header file "grandparent.h". This causes a compilation error, since the structure type foo will thus be defined twice. In C++, this would be called a violation of the one definition rule.
The same code as the previous section is used with the addition of #include guards. The C preprocessor preprocesses the header files, including and further preprocessing them recursively. This will result in a working source file.
struct foo ;
struct foo ;
Here, the first inclusion of "grandparent.h" has the macro GRANDPARENT_H defined. When "child.c" includes "grandparent.h" at the second time (while including "parent.h"), as the #ifndef test returns false, the preprocessor skips down to the #endif, thus avoiding the second definition of struct foo. The program compiles correctly.
Different naming conventions for the guard macro may be used by different programmers. Other common forms of the above example include GRANDPARENT_INCLUDED, CREATORSNAME_YYYYMMDD_HHMMSS (with the appropriate time information substituted), and names generated from a UUID. (However, names starting with one underscore and a capital letter (C and C++) or any name containing double underscore (C++ only), such as _GRANDPARENT_H and GRANDPARENT__H, are reserved to the language implementation and should not be used by the user.[1] [2])
Of course, it is important to avoid duplicating the same include-guard macro name in different header files, as including the 1st will prevent the 2nd from being included, leading to the loss of any declarations, inline definitions, or other #includes in the 2nd header.
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, most C and C++ implementations provide a non-standard [[pragma once|#pragma once]] directive. This directive, inserted at the top of a header file, will ensure that the file is included only once. The Objective-C language (which is a superset of C) has an [[Objective-C##import|#import]] directive, which works exactly like #include, except that it includes each file only once, thus obviating the need for #include guards.[3]
Some languages support specifying that the code should be included only once, in the including file, rather than in the included one (as with C/C++ include guards and #pragma once):
%INCLUDE statement as the equivalent to C's #include directive. IBM Enterprise PL/I also supports the %XINCLUDE statement which will "incorporate external text into the source program if it has not previously been included." (It also offers an XPROCEDURE statement, similar to a PROCEDURE statement, which will ignore the second and subsequent occurrences of an XPROCEDURE with the same name.) [4]#import directive (see above)include_once[5][[Pragma once | #pragma once]]