memcpy(), memmove(), and memset() with zero length

If I copy or set zero bytes with memcpy(), memmove(), or memset(), can the <dest> and <src> arguments be null? Can they be invalid pointers?

Regards,
Jon

This is covered in 7.21.1.p2 of C99 (C++ just points to the C standard):

Where an argument declared as size_t n specifies the length of the array for a
function, n can have the value zero on a call to that function. Unless explicitly stated
otherwise in the description of a particular function in this subclause, pointer arguments
on such a call shall still have valid values, as described in 7.1.4. On such a call, a
function that locates a character finds no occurrence, a function that compares two
character sequences returns zero, and a function that copies characters copies zero
characters.