> Functions like memcpy or malloc cannot legally be written in C without breaking this behavior

> Also not true.

You cannot implement malloc in C because malloc always returns a pointer that's not a part of an existing allocation. The malloc in libc has special dispensation from the compiler to do this (GCC "malloc" attribute).

Similarly you can't implement pthread mutexes in C because they imply optimization barriers (all global memory might change) that a C function with a visible implementation wouldn't have.