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Up-to-date trunk clang has introduced a built-in operator called _Countof, which is like the 'lenof' macro in this code (returns the number of elements in a statically-declared array object) but with the safety advantage that it provokes a compile error if you accidentally use it on a pointer. In this commit I add a cmake-time check for it, and conditional on that, switch over the definition of lenof. This should add a safety check for accidental uses of lenof(pointer). When I tested it with new clang, this whole code base compiled cleanly with the new setting, so there aren't currently any such accidents. clang cites C2y as the source for _Countof: WG14 document N3369 initially proposed it under a different name, and then there was a big internet survey about naming (in which of course I voted for lenof!), and document N3469 summarises the results, which show that the name _Countof and/or countof won. Links: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3369.pdf https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3469.htm My reading of N3469 seems to say that there will _either_ be _Countof by itself, _or_ lowercase 'countof' as a new keyword, but they don't say which. They say they _don't_ intend to do the same equivocation we had with _Complex and _Bool, where you have a _Countof keyword and an optional header file defining a lowercase non-underscore macro wrapping it. But there hasn't been a new whole draft published since N3469 yet, so I don't know what will end up in it when there is. However, as of now, _Countof exists in at least one compiler, and that seems like enough reason to implement it here. If it becomes 'countof' in the real standard, then we can always change over later. (And in that case it would probably make sense to rename the macro throughout the code base to align with what will become the new standard usage.)