Preprocessor "macro function" vs. function pointer - best practice?

Question

I recently started a small personal project (RGB value to BGR value conversion program) in C, and I realised that a function that converts from RGB to BGR can not only perform the conversion but also the inversion. Obviously that means I don't really need two functions rgb2bgr and bgr2rgb. However, does it matter whether I use a function pointer instead of a macro? For example:

int rgb2bgr (const int rgb);

/*
 * Should I do this because it allows the compiler to issue
 * appropriate error messages using the proper function name,
 * not to mention possible debugging benefits?
 */
int (*bgr2rgb) (const int bgr) = rgb2bgr;

/*
 * Or should I do this since it is merely a convenience
 * and they're really the same function anyway?
 */
#define bgr2rgb(bgr) (rgb2bgr (bgr))

I'm not necessarily looking for a change in execution efficiency as it's more of a subjective question out of curiosity. I am well aware of the fact that type safety is neither lost nor gained using either method. Would the function pointer merely be a convenience or are there more practical benefits to be gained of which I am unaware?

Answer 1

Another possibility would be to just have the second function call the first and let the compiler worry about optimizing it (by inlining or generating a tail call).

Answer 2

I would use the macro. It's more common and more idiomatic, and runs into fewer problems crossing translation units (i.e. you don't have to worry about declaring the macro static).

Additionally, by using a function pointer, you prevent inlining on most compilers.

Finally, with the function pointer, it's possible for clients to do this:

int evil(const int bgr) { /* Do something evil */ }

bgr2rgb = evil

Sure, they probably don't want this, but it's possible there could be a variable named similar to bgr2rgb down the line where it only takes one typo....

The macro is safer, though I would define it this way -- there's no need for a function-like macro here:

#define bgr2rgb rgb2bgr