[hunson]

Aha! That's how you use this function. I have read in the manual but did not understand it right away how and for what to use it. Thank you.
I did some testing, that's exactly what I needed :)

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <gmp.h>

int main()
{
  mpz_t n;
  mpz_t a;
  mpz_t b;
  mpz_t c;
  mpz_t base;
  int x;
  int i;
  mpz_init(n);
  mpz_init(a);
  mpz_init(b);
  mpz_init(c);
  mpz_init_set_ui(base,2);
  time_t sec;

  sec=time(NULL);
  for (x = 1;x <1000000;x++)
  {
    //2^a+2^b-1
    //2^5000+2^1000-1
    mpz_setbit(n,5000);
    for(i=0;i<=999;i++)
   {
     mpz_setbit(n,i);
   }
  }
  printf("time=%ld sec.\n",time(NULL)-sec);

  sec=time(NULL);
  for (x = 0;x <1000000;x++)
  {
    //2^a+2^b-1
    //2^5000+2^1000-1
    mpz_pow_ui(a,base,5000);
    mpz_pow_ui(b,base,1000);
    mpz_add(c,a,b);
    mpz_sub_ui(c,c,1);
  }
  printf("time=%ld sec.\n",time(NULL)-sec);

  mpz_clear(n);
  mpz_clear(a);
  mpz_clear(b);
  mpz_clear(c);
  return 0;
}

I guess the idea was not half bad, but its the slower solution. The loop method takes 3 sec. on my machine and the GMP implementation 0 sec.
At least my idea works


hunson