On Tue, Jan 30, 2024 at 10:45:11PM +0100, Alejandro Colomar wrote:
> Hi,
>
> I'm trying to do something like the following:
>
> $ cat const.c
> union u {
> int a;
> const int b;
> };
>
> int
> main(void)
> {
> union u u, v;
>
> u.a = 42;
> v = u;
> }
> $ cc -Wall -Wextra const.c
> const.c: In function ‘main’:
> const.c:12:11: error: assignment of read-only variable ‘v’
> 12 | v = u;
> | ^
> const.c:9:21: warning: variable ‘v’ set but not used [-Wunused-but-set-variable]
> 9 | union u u, v;
> | ^
>
> The actual data I'm using is not just an int, but that serves to
> reproduce the problem easily. In reality, the union is more like this:
>
> struct rstr {
> const size_t length;
> const char *const start;
> };
>
> union str {
> struct {
> size_t length;
> char *start;
> } w;
> struct rstr r;
> };
>
> I don't see anywhere in C11 that makes this a constraint violation, and
Ahh, I didn't find it. It's specified undex 6.3 (Conversions),
6.3.2.1 (Lvalues, arrays, and function designators)
<http://port70.net/~nsz/c/c11/n1570.html#6.3.2.1>:
< A modifiable lvalue is an lvalue that does not have array type, does
< not have an incomplete type, does not have a const- qualified type,
< and if it is a structure or union, does not have any member
< (including, recursively, any member or element of all contained
< aggregates or unions) with a const- qualified type.
I'm wondering if this is a bit conservative, and a union like this could
be a modifiable lvalue. I find it useful, since it allows inheriting a
non-modifiable version of the string which cannot be made modifiable
again, but would let you edit the string as long as you keep the union.
I added the named member 'w' to allow copying v.w = u.w, but when the
union is part of a larger structure and I need to copy the entire
structure, that doesn't help. memcpy(3) does help, but it looses all
type safety.
Maybe this could be allowed as an extension. Any thoughts?
Cheers,
Alex
--
<https://www.alejandro-colomar.es/>
Looking for a remote C programming job at the moment.
Attachment:
signature.asc
Description: PGP signature
