I am not so well-versed in the C standard, so please bear with me.
I would like to know if it is guaranteed, by the standard, that
I have a draft version of the C standard (ISO/IEC 9899:1999), and it has some fun things to say about that call. For starters, it mentions (§7.21.1/2) in regards to
Where an argument declared as
size_tn 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.
The reference indicated here points to this:
If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer outside the address space of the program, or a null pointer, or a pointer to non-modifiable storage when the corresponding parameter is not const-qualified) or a type (after promotion) not expected by a function with variable number of arguments, the behavior is undefined.
So it looks like according to the C spec, calling
memcpy(0, 0, 0)
results in undefined behavior, because null pointers are considered "invalid values."
That said, I would be utterly astonished if any actual implementation of
memcpy broke if you did this, since most of the intuitive implementations I can think of would do nothing at all if you said to copy zero bytes.