Underlying code works the same as public source versions. According to Fortran rules, overlap isn't permitted, as if the prototypes were defined with *restrict. In practice, it will likely work OK with overlap, if copying toward smaller addresses with positive increment parameters, or toward larger addresses with negative increments.
memmove() is fairly efficient in most current implementations (maybe better than ?copy for the case of both increments negative), so it may not make sense to use ?copy in such cases.
