Tom Lane wrote:
> Stephan Szabo <[EMAIL PROTECTED]> writes:
> > On Mon, 2 Feb 2004, Tom Lane wrote:
> >> This isn't a bug, and I see no reason to clutter the code just to shut
> >> up valgrind.
> > Isn't memcpy on overlapping (even entirely overlapping) buffers undefined
> > behavior unless the count is 0?
> The reason that the spec describes overlapped memcpy as undefined is
> that it does not want to restrict which direction the copy occurs in
> (proceeding from lower to higher memory addresses or vice versa).
> memmove is constrained to do the copy in the direction that will avoid
> failure when the source and destination partially overlap.  But memcpy
> is expected to do whichever is fastest on the particular architecture,
> without concern for possible overlap.  (Offhand I've never heard of a
> machine where memcpy doesn't work lower-to-higher, but maybe there is
> one.)
> However, when the source and destination buffers are the same, it does
> not matter which direction you copy in; you are picking up and putting
> down the same bytes at the same addresses no matter what order you
> process the bytes in.  There is no implementation in existence that will
> produce an unwanted result.
> If you want to argue about dependencies on implementation details that
> are theoretically undefined according to the ANSI C spec, we have tons
> more beyond this one.  F'r instance, depending on twos-complement
> arithmetic is illegal per spec also ...

Isn't memmove() for overlaping regions?  That's what my BSD manual page

  Bruce Momjian                        |
  [EMAIL PROTECTED]               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

---------------------------(end of broadcast)---------------------------
TIP 3: if posting/reading through Usenet, please send an appropriate
      subscribe-nomail command to [EMAIL PROTECTED] so that your
      message can get through to the mailing list cleanly

Reply via email to