The core issue here is the weak linking of the symbol, and in certain environments--for instance, using the latest Xcode (8.1) with the latest SDK (10.12)--os_unfair_lock may resolve even though you're compiling on a host that doesn't support it (10.11). We can use the availability macros to circumvent this problem, and detect that we're not compiling for a target that is going to support them and error out at compile time. The other alternative is to do a runtime check, but that presents issues for cross-compiling.