What's the difference between memmove and bcopy?

bcopy and memmove do exactly the same thing. However, they take arguments in a different order: bcopy(src, dest, n) vs memmove(dest, src, n). So they can't be two names for the same function.

Historically, bcopy is the older of the two; it was present in 3BSD if memory serves. memmove was invented by the C committee in 1988 or so.

New code should use memmove, as it is required by the C standard and therefore more portable than bcopy.

The main difference normally comes from their origin. bcopy comes from Berkeley Source Distribution (BSD) systems, while memcpy and their family come mainly from AT&T's System V. And finally, by complying with legacy code, all those functions were included in the standards.

There's a good explanation about the use of memset vs bzero in Richard Steven's book "UNIX network programming" that somewhat explains about the use of either one.

Today implementations just resolve to calling the same internal implementation but providing both interfaces.

Functionally they are equivalent. The difference is in portability. memmove is part of the C standard while bcopy is part of POSIX, and is therefore less portable.

bcopy is deprecated. The man page says:

This function is deprecated (marked as LEGACY in POSIX.1-2001): use memcpy(3) or memmove(3) in new programs. Note that the first two arguments are interchanged for memcpy(3) and memmove(3). POSIX.1-2008 removes the specification of bcopy().