When to use function-like macros in C

Question

I was reading some code written in C this evening, and at the top of the file was the function-like macro HASH:

#define HASH(fp) (((unsigned long)fp)%NHASH)
         


        

Answer 1

The most common (and most often wrong) reason people give for using macros (in "plain old C") is the efficiency argument. Using them for efficiency is fine if you have actually profiled your code and are optimizing a true bottleneck (or are writing a library function that might be a bottleneck for somebody someday). But most people who insist on using them have Not actually analyzed anything and are just creating confusion where it adds no benefit.

Macros can also be used for some handy search-and-replace type substitutions which the regular C language is not capable of.

Some problems I have had in maintaining code written by macro abusers is that the macros can look quite like functions but do not show up in the symbol table, so it can be very annoying trying to trace them back to their origins in sprawling codesets (where is this thing defined?!). Writing macros in ALL CAPS is obviously helpful to future readers.

If they are more than fairly simple substitutions, they can also create some confusion if you have to step-trace through them with a debugger.