Question

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 memcpy(0,0,0) is safe.

The only restriction I could find is that if the memory regions overlap, then the behavior is undefined…

But can we consider that the memory regions overlap here ?

Answer 1

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 memcpy that

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.

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.