Re: AW: clumsy cast in dlopen.3

[Alejandro Colomar <alx@kernel.org>]

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Hi Walter,

On 2026-05-15T08:35:49+0000, Walter Harms wrote:
> 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.

The thing about unions is that the only two ways for type punning that
are blessed by ISO C are unions and memcpy(3).  Everything else isn't
allowed.

Perfectly valid:

	static_assert(sizeof(int) == sizeof(float));

	union u {int i; float f;};

	float    f;
	union u  u;

	u.i = 42;
	f = u.f;

Perfectly valid:

	static_assert(sizeof(int) == sizeof(float));

	int    i;
	float  f;

	i = 42;
	memcpy(&f, &i, sizeof(float));

UB:

	static_assert(sizeof(int) == sizeof(float));

	int    i;
	float  f;

	i = 42;
	f = *(float *) &i;


Have a lovely day!
Alex

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

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

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