Where do I put all these function-like #defines, in C?

Question

I'm working with an embedded system, and I'm ending up with a ton of HW-interfacing #define macros. I want to put all of these into a separate file (for OOP-ness), but I don't know the best way to #include that. Do I just put them all into a .c file, then include that? Seems silly to put these in a .h file.

Answer 1

I don't see anything wrong with the .h file.

Answer 2

These should go in the .h files. The other option is a .c file, and that would require using #include to include a .c file, which will very definitely confuse people -- as well as confusing your makefile, if it uses the standard assumption that every .c file will correspond directly to a compiled .o file.

The normal pattern is that .h files are for things that are included in other places (and, particularly, in multiple other places), and that .c files are for things that are compiled once into object files.

Thus, the following things normally go into .h files:

• Function prototypes
• Constant declarations
• Global variable extern declarations
• Inline function definitions
• Type definitions
• and macro definitions, such as what you're asking about.

Conversely, the following things normally go into .c files:

• Global variable definitions
• Function definitions that will be compiled into object code and linked

The case of "function definitions only go into .c files" is simply the degenerate case when you don't have any inline functions.

In C++, where lots of functions are defined in templated form and thus the definitions need to be included whenever they're used, those definitions very often go in the .h (or .hpp, or whatever) file. So this sort of thing definitely has precedent.

Answer 3

I'm not necessarily recommending this but have seen it in quite a few embedded projects over the last 10+ years: include inline functions as .inl.

Brooks breaks down the responsibilities nicely. You might consider separating inline and macro definitions from ordinary function prototypes and such:

#include "prototypes.h"
#include "macros.inl"

int foo(void);
int bar(char);

Your end goal is consistency: any layout decisions should assist those who succeed you.