Question

I’ve always seen the C include extension as .h.

I am working on a project with include extensions of .r. I believe the extension .r refers to Mac resource forks.

I’m wondering if C allows the extension on an include file to be anything?

The code compiles without any errors.

examples:

#include "HostTypes.h" 
#include "IdentifySpec.r"

Answer 1

There is no restriction on the name (including extension) of an include file.
All of the following are valid

#include "foo.bar"
#include "look_ma_no_extension"
#include "1.2.3.4.5.6.7.8.9"

However, using .h is preferred because it is the usual extension 🙂