Thread: Including source file twice, but with include guard

Originally Posted by thetinman

Will the code from the fourth source file exist in two locations in the compiled code? Is this something that is compiler dependent?

Short answers: Yes. No.

Longer answer:

In your example, the definition of myFunction() will be in both translation units. That breaches the one-definition rule. Practically, when linking, the linker will usually complain about that function being defined multiple times (the fact the definitions will be equivalent doesn't change that - basic linker technology is simply not that smart).

Generally speaking, #including source files (.c) is usually not done, precisely because it causes breaches of the one-definition rule. Include guards do not change that.

There is an exception for inline functions, but inline functions are conventionally defined in header files, not in source files.

IIRC, the double underline prefix is NOT allowed to be used by normal C programmers; not sure, But, I think only Compiler Writers that is allowed to use it.

Code:

#ifndef __SOURCE3__

Tim S.

Originally Posted by stahta01

IIRC, the double underline prefix is NOT allowed to be used by normal C programmers; not sure, But, I think only Compiler Writers that is allowed to use it.

Yes, indeed. The C standard reserves such identifiers (and others with leading underscores) for use by the implementation.

Do not #include source files. The conventional way to do what you are trying to do is to make all source files independent (.c) and then compile them all together. Only the include files typically have include guards for convenience.

Code:

gcc -o myprogram main.c source1.c source2.c source3.c

main.c:

Code:

#include "source1.h"
#include "source2.h"
#include "source3.h"

int main()
{
    myFunction();
}