On 02/20/15 04:43, Jonathan Wakely wrote:
On 20 February 2015 at 11:06, Florian Weimer wrote:
On 02/19/2015 09:56 PM, Sandra Loosemore wrote:
Hmmmm, Passing the additional option in user code would be one thing,
but what about library code? E.g., using memcpy (either explicitly or
implicitly for a structure copy)?
The memcpy problem isn't restricted to embedded architectures.
size_t size;
const unsigned char *source;
std::vector<char> vec;
…
vec.resize(size);
memcpy(vec.data(), source, size);
std::vector<T>::data() can return a null pointer if the vector is empty,
which means that this code is invalid for empty inputs.
I think the C standard is wrong here. We should extend it, as a QoI
matter, and support null pointers for variable-length inputs and outputs
if the size is 0. But I suspect this is still a minority view.
I'm inclined to agree.
Most developers aren't aware of the preconditions on memcpy, but GCC
optimizes aggressively based on those preconditions, so we have a
large and potentially dangerous gap between what developers expect and
what actually happens.
But that's always true -- this isn't any different than aliasing,
arithmetic overflow, etc. The standards define the contract between
the compiler/library implementors and the developers. Once the contract
is broken, all bets are off.
jeff
