Re: AW: clumsy cast in dlopen.3

[Martin Uecker <uecker@tugraz.at>]

There is no such thing needed here, you just do

double (*cosine)(double);

cosine = dlsym(handle, "cos");


Best,
Martin

Am Freitag, dem 15.05.2026 um 11:56 +0200 schrieb Alejandro Colomar:
> 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>