How to avoid cpp adding spaces to macro parameters

Question

My company has 1000s of source files that use a macro to specify the include path. I cannot change that and it works with the current compilers. Now, I try to compile that with GCC (4.8.2 or 3.4.6) and it fails.

Basically there is a macro like that:

#define COMP_INC(comp, file) <comp/include/file>

That is used like that:

#include COMP_INC(posix, inttypes.h)

My problem is that GCC is for some unknown reason translating my macro call into that:

#include <posix/include/ inttypes.h>

Notice the space before "inttypes.h"? Because of that, the header file is not found.

Why does the preprocessor insert a space? Can I avoid it? I can only change the macro, I cannot change the source files that uses it.

Answer 1

Remove the space between posix and inttypes.h (after the comma). The problem goes away.

This answer tells us why this occurs:

From the C99 standard:

A preprocessing directive of the form

# define identifier replacement-list new-line

defines an object-like macro that causes each subsequent instance of the macro name to be replaced by the replacement list of preprocessing tokens that constitute the remainder of the directive.

---

So macros work on tokens and the whitepace is to be expected.

EDIT:

According to this answer, you can use the traditional-cpp flag to mimic the old behavior of not adding the space. I am not sure how that will affect your build, but you can try it:

Only much older preprocessors didn't insert that extra space -- note that original entry was submitted over 20 years ago in 1988, before the 1989 version of the C standard was standardized. You can pass the -traditional-cpp flag to the GCC preprocessor, which causes it to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors.

Answer 2

If you can accept to predefine all possible 'comp'-names and 'file'-names:

#define MAKE_INCLUDE(comp, file) <comp/include/file>

#define NAME_posix posix
#define NAME_inttypes inttypes

#define MAKE_NAME(x) NAME_##x

#define COMP_INC(comp, file) MAKE_INCLUDE(MAKE_NAME(comp), MAKE_NAME(file))

#include COMP_INC(posix, inttypes.h)

Note: A suffix like '.h' can not be part of a macro. Hence, directories like lib-1.0-A and lib-1.0-B will be indistinguishable (but it might be no issue, here).