Hi,
> Having written that, I did test that 'sizeof(((struct
> sockaddr_un*)0)->sun_path)' compiles with gcc, although I'm less
> certain of whether the C standard permits that (or even if that
> permission has changed over time) - the expression argument to sizeof
> is unevaluated, which counters the argument that you can't normally
> evaluate a dereference of a NULL pointer.
According to ISO/IEC 9899, versions 1999 to 2017 at least, the answer seems
clear from section 6.5.3.4 ("The sizeof operator" or "The sizeof and _Alignof
operators"), paragraph 2:
"""
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.
"""
Other relevant sections, especially 6.5.2.3 and in particular its paragraphs 2
and 4, are not in contradiction with the above text.
So in short, yes, IMHO, the standard allows something like:
'sizeof(((struct sockaddr_un*)0)->sun_path)'
where in fact 0 could be replaced by any other invalid pointer.
Regards.
--
Olivier Certner
