Re: clumsy cast in dlopen.3

[Alejandro Colomar <alx@kernel.org>]

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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
> 
> 
> 
> 

-- 
<https://www.alejandro-colomar.es>

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