On 22/7/23 12:22 pm, Thorsten Glaser via austin-group-l at The Open
Group wrote:
https://www.austingroupbugs.net/view.php?id=561#c6085 (the accepted
text) suggests that…
versions, the size is typically in the range 92 to 108. An
application can
deduce the size by using <tt>sizeof(((struct sockaddr_un
*)0)->sun_path)</tt>.
… but I was recently told that attempting that is UB because it
dereferences a nil pointer, even though it’s only within a sizeof,
(Apologies, I meant to reply list; doing so now.)
The text of the standard doesn't support this (6.5.3.4):
"The sizeof operator yields the size (in bytes) of its operand, which
may be an
expression or the parenthesized name of a type. The size is determined
from the type of
the operand. The result is an integer. If the type of the operand is a
variable length array
type, the operand is evaluated; otherwise, the operand is not evaluated
and the result is an
integer constant."
Since the operand isn't evaluated, there is no null pointer dereference.
It is the act of dereference (via evaluation) which causes the UB, not
its potential presence in the code. Consider that when a potential null
pointer exists on a conditional branch that isn't taken, it is
universally agreed that there is no UB due to the "potential"
dereference. Otherwise null pointer checks in code would be useless.
Regards.
