Thread: Header file include guards

Hi all, odd question time again.

I see include guards (I believe that is what they are called) used all the time. But I am not 100% sure of their use and syntax. There is nothing in the books that I own, and I can't find a great deal on the net, (but I may be searching for the wrong terms). I am pretty sure they are used to prevent a header file being included multiple times.

a: Why the preceding double underscore?
b: I read somewhere that the double underscore shouldn't be
used for user defined header files, as these are reserved for
internal use, and users should use a single underscore.
Is this true?

Code:

#ifndef __GAMEVERSION_H
#define __GAMEVERSION_H

#define VERSION_NUMBER 1

#endif

Inclusion guards prevent a given header file from being included in a given .c file more than once. This can happen when include files become nested (foo.c includes bar.h and baz.h where baz.h *also* includes bar.h).

What you read regarding the leading double underscore (or even a single underscore) is also correct. Preceding identifiers with an underscore invades the implementation's namespace and should therefore only be done by implementors.

For my inclusion guards, I take the name of the header file in upper case and replace the dot by an underscore. So the inclusion guard for foo.h would look like this:

#ifndef FOO_H
#define FOO_H

/* ... */

#endif

Thanks for that - that was exactly what I was looking for.

Just another question though, why the underscore and not a ".", What is the actual reason from the preprocessors point of view, that the underscore is used in lieu of a full stop?

I guess I didn't make the connection, that is is a variable (or at least requiring that it followed the conventions of variable declarations).

Duh!

Thanks Quzah