#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>
#include <gnu/lib-names.h>  /* Defines LIBM_SO (which will be a
                              string such as "libm.so.6") */
int
main(void)
{
   void *handle;
   double (*cosine)(double);
   char *error;

   handle = dlopen(LIBM_SO, RTLD_LAZY);
   if (!handle) {
       fprintf(stderr, "%s\n", dlerror());
       exit(EXIT_FAILURE);
   }

   dlerror();    /* Clear any existing error */

   cosine = (double (*)(double)) dlsym(handle, "cos");

   /* According to the ISO C standard, casting between function
      pointers and 'void *', as done above, produces undefined results.
      POSIX.1-2001 and POSIX.1-2008 accepted this state of affairs and
      proposed the following workaround:

          *(void **) (&cosine) = dlsym(handle, "cos");

      This (clumsy) cast conforms with the ISO C standard and will
      avoid any compiler warnings.

      The 2013 Technical Corrigendum 1 to POSIX.1-2008 improved matters
      by requiring that conforming implementations support casting
      'void *' to a function pointer.  Nevertheless, some compilers
      (e.g., gcc with the '-pedantic' option) may complain about the
      cast used in this program. */

   error = dlerror();
   if (error != NULL) {
       fprintf(stderr, "%s\n", error);
       exit(EXIT_FAILURE);
   }

   printf("%f\n", (*cosine)(2.0));
   dlclose(handle);
   exit(EXIT_SUCCESS);
}