Yes and no. No, because ISO never said it must behave this way. Yes, because every libc I've personally encountered acts this way. At a glance, glibc's x86 im

Yes and no. No, because ISO never said it must behave this way. Yes, because every libc I've personally encountered acts this way. At a glance, glibc's x86 implementation[1, 2], musl, and picolibc all handle 0-length memcpy as you'd expect. I'm sure other folks could dig up the code for Newlib, uclibc, and others, and they'd see the same thing. On a related note, ISO C has THREE different things that most people tend to lump together as "undefined behavior." They are: Implementation-defined behavior: ISO doesn't require any particular behavior, but they do require implementations to consistently apply a particular behavior, and document that behavior. Unspecified behavior: ISO doesn't require any particular behavior, but they do require implementations to consistently use a particular behavior, but they don't require that behavior to be documented. Undefined behavior: ISO doesn't require any particular behavior, and they don't require implementations to define any particular behavior either. [1]: https://github.com/lattera/glibc/blob/master/string/memcpy.c https://github.com/lattera/glibc/blob/master/string/memcpy.c [2]: https://github.com/lattera/glibc/blob/895ef79e04a953cac1493863bcae29ad85657ee1/sysdeps/i386/memcopy.h#L26 https://github.com/lattera/glibc/blob/895ef79e04a953cac14938...