> very exotic requirements
I'd be interested to know what you are thinking.
The primary exotic thing I can imagine is an architecture lacking the ability to do atomic operations. But even in that case, C11 has atomic operations [1] built in. So worst case, the C library for the target architecture would likely boil down to mutex operations.
Which platforms might that be? Even MIPS has atomics (at least pointer sized last i checked).
Well, basically, yeah, if your platform lacks support for atomics, or if you'd need some extra functionality around the shared pointer like e.g. logging the shared pointer refcounts while enforcing consistent ordering of logs (which can be useful if you're unfortunate enough to have to debug a race condition where you need to pay attention to refcounts, assuming the extra mutex won't make your heisenbug disappear), or synchronizing something else along with the refcount (basically a "fat", custom shared pointer that does more than just shared-pointering).