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
considering that the const member is fully overlapped by a non-const
member, I don't see why this assignment would be bad. I couldn't find
what's the exact semantics of assignment to a union, but I guess it's
similar to initialization, and since the first member is non-const, this
should be doable. Maybe it's a bit of unspecified behavior, precisely
because nothing seems to specify it.
Is that really a constraint violation, or should the compiler accept the
code?
Have a lovely day,
Alex
--
<https://www.alejandro-colomar.es/>
Looking for a remote C programming job at the moment.
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