C type naming conventions, _t or ALLCAPS

Question

I've always wondered if there are any naming convetions as when to use ALLCAPS for a type and when to append _t (and when to not use anything?). I know back in the days K&R published all kinds of documents about how to use C, but I couldn't find anything about this.

Among the C standard library types, _t seem prettys dominant

time_t
clock_t
uint32_t
size_t
sig_atomic_t
...

, as opposed to FILE, va_list or struct tm. Are there actually rules to this or is it completely arbitrary? Microsoft always uses typenames in ALLCAPS in their Windows API, which at least seems more consistent than the C library, frankly...

Answer 1

Actually according to the POSIX standard, all type names ending in _t are reserved, as defined here:

Names that end with ‘_t’ are reserved for additional type names.

Answer 2

I would strive to minimize your use of typedef. Use the struct keyword for structures and only use typedef when you're either (a) defining an opaque type that might be implemented as anything, or (b) defining an alias for an arithmetic type you might want to change in the future or in different configurations (for example if you want to be able to choose float or double).

In any case, don't use ALL CAPS because it's hideously ugly, and don't use _t because it's reserved by POSIX. That probably doesn't matter if you prefix your type names appropriately, but then the _t on the end is just unnecessary ugliness and gratuitous nonportability. Just prefixing them like this should be fine: foo_scalar (where foo is the name of your library/module).