math - C++ - fmod return the wrong answer -

first off, i'm doing project school , not allowed use external libraries , hence cannot use gmp. problem is, have function requires "tough" calculations. ie,

m^e mod n 

here's code

#include <iostream> #include <math.h>  using namespace std;  int main() {     int e = 17, n = 3233, m = 65;     long double p, mod;      p = pow(m, e); // gives 6.59974e+30 correct      mod = fmodl(p, n);      cout<<mod; // gives 887, when correct answer 2790      return 0; } 

as can see fmod (fmodl) function not return correct value there workaround ? again, without using external libraries.

you can write own modulo power function.

int modpow(int a,int b,int mod) {     int product,pseq;     product=1;     pseq=a%mod;     while(b>0)     {         if(b&1)             product=(product*pseq)%mod;         pseq=(pseq*pseq)%mod;         b>>=1     }     return product; } 

refer http://en.wikipedia.org/wiki/modular_exponentiation explanation