Is it guaranteed to be safe to perform memcpy(0,0,0)?

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.

Just for fun, the release-notes for gcc-4.9 indicate that its optimizer makes use of these rules, and for example can remove the conditional in

int copy (int* dest, int* src, size_t nbytes) {
    memmove (dest, src, nbytes);
    if (src != NULL)
        return *src;
    return 0;
}

which then gives unexpected results when copy(0,0,0) is called (see https://gcc.gnu.org/gcc-4.9/porting_to.html).

I am somewhat ambivalent about the gcc-4.9 behaviour; the behaviour might be standards compliant, but being able to call memmove(0,0,0) is sometimes a useful extension to those standards.