| Issue |
74205
|
| Summary |
`clang` feature request: warn on incorrect tagged union value access
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
matheusmoreira
|
Currently `clang` does not produce a warning if the value of a tagged union that doesn't correspond to its type tag is accessed.
For example, this code:
```C
// warn.c
#include <stdio.h>
enum T { I, F };
union U { int i; float f; };
struct S { enum T t; union U u; };
int main(void) {
struct S s = { .t = F, .u.f = 12345.67890f };
switch (s.t) {
case I:
printf("%d\n", s.u.i);
break;
case F:
// copied the above case
// but neglected to update the code
printf("%d\n", s.u.i);
break;
}
}
```
Does not produce any warning when compiled, leading to incorrect results:
```shell
$ clang -Weverything -o warn warn.c && ./warn
1178658487
```
I understand that `union`s are typically used for type punning and that such accesses are often intended by the programmer but compiler checks would still be beneficial when that's not the case. [People have created C preprocessor solutions][1] to use tagged unions safely in C. I've also seen support for safe tagged unions in newer languages like Zig.
A compiler mechanism to establish a relationship between the `union` values and their corresponding `enum` tags would be extremely useful. Something like this, perhaps:
```C
struct S {
enum T t;
union U {
int i __attribute__((tag(t, I)));
float f __attribute__((tag(t, F)));
} u;
};
```
Then `clang` would be able to warn when `union` values are accessed in a context where their specified tags are not known to be the correct value:
```C
switch (s.t) {
case I:
// i is accessed
// the tag of i is t
// t is supposed to equal I
// compiler knows t equals I because of switch case
// correct, no warning is emitted
printf("%d\n", s.u.i);
break;
case F:
// i is accessed
// the tag of i is t
// t is supposed to equal I
// compiler knows t equals F because of switch case
// incorrect, a warning is emitted
printf("%d\n", s.u.i);
break;
}
```
Relevant links:
- lone-lang/lone#1
[1]: https://github.com/Hirrolot/datatype99
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