Mailinglist Archive: opensuse-packaging (80 mails)

[Cristian Morales Vega <cmorve69@xxxxxxxx>]

2010/8/14 Philipp Thomas <Philipp.Thomas2@xxxxxxx>:
> On Fri, 13 Aug 2010 19:44:38 +0200, Cristian Morales Vega
> <cmorve69@xxxxxxxx> wrote:
>
> > The thing is the offending code is
> >
> > "*(void **)(&comp_compress) = dlsym(handle, "comp_compress");"
>
> > So, the question: could gcc -O2 really break that code? Or that's a
> > false warning and I can be 100% safe ignoring it?
>
> Yes, it can. The C compiler assumes the normal aliasing rules, i.e.
> that objects of one type can only be accessed through pointers of the
> same type or char/void pointers. Therefor the compiler will organize
> the code depending on whether it finds other accesses or not.
>
> And before you remark it, there is a difference between 'void **' and
> 'void *' and the standard does only cover 'void *'. The only way I
> know of to correct the code would be to use a union, i.e. for the
> csine example
>
>
>          {
>           void *handle;
>           double (*cosine)(double);
>           char *error;
>           union{ void *vp; double (*cos)(double); }pun.
>
>           handle = dlopen("libm.so", RTLD_LAZY);
>
>           if (!handle) {
>               fprintf(stderr, "%s\n", dlerror());
>               exit(EXIT_FAILURE);
>           }
>
>           dlerror();    /* Clear any existing error */
>
>           pun.vp = dlsym(handle, "cos);
>           cosine = pun.cos.
>
>           if ((error = dlerror()) != NULL)  {
>               fprintf(stderr, "%s\n", error);
>               exit(EXIT_FAILURE);
>           }
>
> GCC guarantees that the union trick works.

a) I hate the world
b) It worked, thanks.

Since I'm going to bookmark this for future reference, for
completeness: any idea about if this is different in C++? Is there
"void *" to a function pointer defined?
Also, I'm not sure about how the Single UNIX Specification, IEEE and
The Open Group are mixed. But at
http://www.opengroup.org/onlinepubs/009695399/functions/dlsym.html it
says

"Due to the problem noted here, a future version may either add a new
function to return function pointers, or the current interface may be
deprecated in favor of two new functions: one that returns data
pointers and the other that returns function pointers."

And at the header it says "IEEE Std 1003.1, 2004 Edition". Wikipedia
says there exists POSIX:2008... so, there is any new function that
returns function pointers?
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