fibg.c

#include <gmpxx.h>
#include <charconv>
#include <string>
#include <iomanip>
#include <iostream>

using namespace std;

int main(int argc, char **argv) {
    for(int i=1; i<argc; i++) {
        size_t ai;
        if (std::from_chars(argv[i], argv[i] + strlen(argv[i]), ai).ec == std::errc{}) {
            const mpz_class v = mpz_class::fibonacci(ai);
            cout << setw(15) << ai << ": "
                 << mpz_sizeinbase(v.get_mpz_t(), 10) << " digits,"
                 << mpz_sizeinbase(v.get_mpz_t(), 2) << " bits\n"
            ;
        }
    }
}