- From the documentation:
*GNU MP is a library for arbitrary precision arithmetic, operating on signed integers, rational numbers,
and floating point numbers. It has a rich set of functions, and the functions have a regular interface.*

So, anyway, it does bignums.
I felt like learning the API to this library, so I wrote this fun little program for factoring arbitrary integers today:

#include <stdio.h>
#include <gmp.h>
void factors(mpz_t);
void firstfactor(mpz_t, mpz_t, mpz_t);
int main(int argc, char **argv)
{
mpz_t x;
mpz_init(x);
while(!feof(stdin)) {
mpz_inp_str(x, stdin, 10);
factors(x);
}
return 0;
}
void firstfactor(mpz_t x, mpz_t left, mpz_t fact)
{
mpz_t n, quot, rem;
mpz_init(quot);
mpz_init(rem);
mpz_init_set_si(n, 2);
while(mpz_cmp(n, x) < 0) {
mpz_tdiv_qr(quot, rem, x, n);
if(mpz_cmp_si(rem, 0)==0) {
mpz_set(left, quot);
mpz_set(fact, n);
return;
}
mpz_add_ui(n, n, 1);
}
mpz_set_si(left, -1);
mpz_set(fact, x);
return;
}
void factors(mpz_t x)
{
mpz_t fact, left;
mpz_init(fact);
mpz_init(left);
while(1){
firstfactor(x, left, fact);
mpz_out_str(stdout, 10, fact);
printf(" ");
if(mpz_cmp_si(left, -1)==0)
break;
mpz_set(x, left);
}
printf("\n");
return;
}