AW: clumsy cast in dlopen.3

[Walter Harms <wharms@bfs.de>]

Hello,
I agree the cast is not nice, (someone for a extension of C standard ?)
but i have to admit that i have never seen the trick with the union.
But it needs some explaination. The comment in the example is already huge,
i would ask for a comment subsektion for this behavier here.

btw: the original code in the example looks like this ...
cosine = (typeof(double (double)) *) dlsym(handle, "cos");

my2c
 wh

________________________________________
Von: Alejandro Colomar <alx@kernel.org>
Gesendet: Donnerstag, 14. Mai 2026 13:29:20
An: Bruno Haible
Cc: linux-man@vger.kernel.org; Martin Uecker
Betreff: Re: clumsy cast in dlopen.3

Hi Bruno,

On 2026-05-14T12:56:55+0200, Bruno Haible wrote:
> The dlopen.3 man page contains this text:
>
>                   *(void **) &cosine = dlsym(handle, "cos");
>
>               This (clumsy) cast conforms with the ISO C standard and will
>               avoid any compiler warnings.
>
> However, such a cast violates the strict aliasing rules of ISO C, no?

I think I agree.  Dereferencing the pointer &cosine with a type
different than the type of the object is not allowed.  I've CCed Martin,
who might be able to confirm.

>
> The proper workaround is to use a union:
>
>   union { double (*cosine) (double); void *pointer; } u;
>
>   u.pointer = dlsym(handle, "cos");
>   ...
>   printf("%f\n", u.cosine(2.0));

This is seems much better, indeed.


Have a lovely day!
Alex

>
> Bruno
>
>
>
>

--
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