On Sun, Apr 19, 2026 at 7:53 AM Mohammad-Reza Nabipoor
<[email protected]> wrote:
>
> The following program produces wrong result on CentOS 7.9 (cfarm112),
> and on Alpine Linux (cfarm27 and cfarm437):
>
> ```c
> #include "c-strtod.h"
>
> #include <stdio.h>
> #include <stdint.h>
>
> int
> main()
> {
> union
> {
> float f;
> uint32_t i;
> } u;
>
> u.f = c_strtof ("-nan", NULL);
> printf ("f:%f x:0x%08x\n", u.f, u.i);
> return 0;
> }
> ```
>
> Output:
>
> f:nan x:0x7fc00000
>
> Expected output:
>
> f:-nan x:0xffc00000
>
>
> You can find a sample package on the mentioned machines on Compile Farm:
> ~mnabipoor/x-0.1.tar.gz
The sample program smells of undefined behavior. I don't think you
can access both u.f and u.i members of the union. You have to pick
one or the other, and the one selected is the one that was written to,
which is u.f due to `u.f = c_strtof ("-nan", NULL);`.
The finding may be valid, but the sample program looks off.
Jeff
