Inclusion guards keep the same file from accidently including itself, which would be bad.

They don't protect you from having multiple copies of a variable. If you declare a variable in a header, every file that includes it is going to get a copy of it. When link time comes around it's going to go 'Oh my god! There's 30 dozen variables all named foo! Which one do I use? What do I do? Ahhh!', promptly followed by it running screaming off the edge of a cliff.

And that's where 'extern' comes in, which you've already discovered.

Ok, in your example you said you use #ifndef #define etc, but you didn't put any names after them. Just incase thats how your useing them read on: if you have the qualifier bit then ignore me [grin]

You need to actually define something for this to work. #define BLAH defines whatever BLAH is to the precompiler, and later you can check if it is defined. Thats how the header guard thing works. First time the file is included it says "if BLAH is not defined then continue", and it goes on. Then the first thing you do is define BLAH. Next time you try to include the file it will say "if blah is not defined ... oh wait it is don't bother" and skip down to the #endif tag. So you need to put something after the #define call - the thing you'll be defining as shown below:



EDIT: too slow