Re: Support for memcpy with equal source and destination

[Paul Eggert <eggert@cs.ucla.edu>]

I see several areas of possible confusion, so if we make this change to 
the glibc documentation, the new documentation should make the following 
clear:

* This is a GNU extension, and other C libraries might not guarantee 
this (not surprising). Also, other C compilers might not guarantee this 
even when used with glibc (somewhat more surprising).

* GCC and other compilers might warn about memcpy (X, X, SIZE) even if 
it is supported.

* This an exception to the usual rule about "restrict", since the 
prototype says "restrict" but it's OK if the two pointers are the same 
(so "restrict" now means that they cannot overlap other than being 
equal, just for this particular function).

* The pointers must point to SIZE bytes even when SOURCE == DESTINATION. 
For example, memcpy (&errno, &errno, SIZE_MAX) is not valid.

* If SIZE is nonzero, the destination must be writable even when SOURCE 
== DESTINATION.

* mempcpy, strcpy, and stpcpy are like memcpy in the above respects. 
(Are there other functions for which we want to make similar guarantees?)

The issue with null-or-invalid pointers and size zero is independent of 
this, as far as I can see, so it can be treated separately.