On 04/06/2017 08:12 PM, Bernd Edlinger wrote:
On 04/06/17 19:47, Florian Weimer wrote:
On 04/06/2017 07:39 PM, Bernd Edlinger wrote:
On 04/06/17 16:17, Florian Weimer wrote:
Here is what I want to write in the doc:
@item typeless_storage
@cindex @code{typeless_storage} type attribute
A type declared with this attribute behaves like a character type
with respect to aliasing semantics.
This is attribute is similar to the @code{may_alias} attribute,
except that it is not restricted to pointers.
As Jakub pointed out, this is not what we need here. An object of type
char does *not* have untyped storage. Accessing it as a different type
is still undefined.
but, do you agree that this is valid in C11?
typedef char char_a[4];
int
main (void)
{
char_a a = {1,2,3,4};
short *b = (short *) &a;
b[1] = 0;
if (a[0] == 1 && a[1] == 2 && a[2] == 3 && a[3] == 4)
abort();
exit(0);
}
all I want to do is replace "char" with a different type.
Thanks a lot for posting a concrete example.
The effective type of a[2] and [3] is char. The character type wildcard
in 6.5(7) only applies to the type of the lvalue expression ysed for the
access, not the effective type of the object being accessed. The type
of the LHS of the assignment expression is short. So the access is
undefined.
exactly *that* is what I want to make valid with that attribute, which
would be also useful in C and kernel code, IMHO.
And I think we all agree that this is a laudable goal.
But isn't the effective type changed by the assignment b[1] = 0;
as described in 6.5(6):
"If a value is stored into an object having no declared type through an
lvalue having a type that is not a character type, then the type of the
lvalue becomes the effective type of the object for that access and for
subsequent accesses that do not modify the stored value."
I don't know what your patch does, but your proposed documentation does
not make this valid because “declared as char” is still not “having no
declared type”. Or put differently, “behaves like a character type” is
not what we actually want here.
Let me repeat that I don't know if this is merely a documentation issue.
Thanks,
Florian
