问题
This question follows this previous question about the definedness of memcpy(0, 0, 0), which has been conclusively determined to be undefined behavior.
As the linked question shows, the answer hinges on the contents of C11's clause 7.1.4:1
Each of the following statements applies unless explicitly stated otherwise in the detailed descriptions that follow: 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, […]) […] the behavior is undefined. […]
The standard function memcpy() expects pointers to void and const void, as so:
void *memcpy(void * restrict s1, const void * restrict s2, size_t n);
The question is worth asking at at all only because there are two notions of “valid” pointers in the standard: there are the pointers that can validly be obtained through pointer arithmetics and can validly be compared with <, > to other pointers inside the same object. And there are pointers that are valid for dereferencing. The former class includes “one-past” pointers such as &a + 1 and &b + 1 in the following snippet, whereas the latter class does not include these as valid.
char a;
const char b = '7';
memcpy(&a + 1, &b + 1, 0);
Should the above snippet be considered defined behavior, in light of the fact that the arguments of memcpy() are typed as pointers to void anyway, so the question of their respective validities cannot be about dereferencing them. Or should &a + 1 and &b + 1 be considered “outside the address space of the program”?
This matters to me because I am in the process of formalizing the effects of standard C functions. I had written one pre-condition of memcpy() as requires \valid(s1+(0 .. n-1));,until it was pointed to my attention that GCC 4.9 had started to aggressively optimize such library function calls beyond what is expressed in the formula above (indeed). The formula \valid(s1+(0 .. n-1)) in this particular specification language is equivalent to true when n is 0, and does not capture the undefined behavior that GCC 4.9 relies on to optimize.
回答1:
C11 says:
(C11, 7.24.2.1p2) "The memcpy function copies n characters from the object pointed to by s2 into the object pointed to by s1."
&a + 1 itself is a valid pointer to integer addition but &a + 1 is not a pointer to an object, so the call invokes undefined behavior.
回答2:
While the "correct" answer according to the standard appears to disagree, I can find it only disingenuous that after int a[6]; int b[6]; all of
memcpy(a+0, b+0, 6);
memcpy(a+1, b+1, 5);
memcpy(a+2, b+2, 4);
memcpy(a+3, b+3, 3);
memcpy(a+4, b+4, 2);
memcpy(a+5, b+5, 1);
should be valid (and copy an area ending at the end of the arrays) while
memcpy(a+6, b+6, 0);
is valid in light of the count but not of the addresses. It's the same end of the copied area!
Personally, I'd lean towards defining memcpy(0,0,0) being valid as well (with the rationale of just demanding valid pointers but no objects) but at least it's a singular case while the "end of array" case is an actual exception to an otherwise regular pattern for copying an area at the end of an array.
来源：https://stackoverflow.com/questions/25390577/is-memcpya-1-b-1-0-defined-in-c11