Re: AW: clumsy cast in dlopen.3

[Alejandro Colomar <alx@kernel.org>]

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Hi Martin,

On 2026-05-15T16:05:24+0200, Martin Uecker wrote:
> 
> There is no such thing needed here, you just do
> 
> double (*cosine)(double);
> 
> cosine = dlsym(handle, "cos");

Hmmmm, yes.  I don't know why the manual page uses a cast in the POSIX
version.

The above wouldn't be enough in ISO C, since void* isn't guaranteed to
work with function pointers.  However, any systems where dlsym(3) works
must support holding function pointers in void* (because otherwise,
dlsym(3) itself wouldn't work), so ISO C shouldn't be a concern at all.

Hmmm; I'll simplify the page.

Thanks!


Have a lovely day!
Alex

> 
> 
> 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>

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

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