Example RNG api

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

//Give the function pointer a pretty name
//Defines the API that users must fit
typedef void(*RNGFunc)(char*,int);

void gen_rsa_key(RNGFunc rng) {
    //I want 10 random bytes!
    char buff[10];
    rng(buff, 10);
    //use those random bytes...
    //etc
}

//Fill arr with len random bytes
void my_bad_rng(char *arr, int len) {
    srand(time(NULL));
    for(int i=0; i < len; i++) {
        arr[i] = (char)(rand() % 255);
    }
}

void better_unix_rng(char *arr, int len) {
    FILE* urandom = fopen("/dev/urandom", "r");
    if (urandom != NULL) {
        fread(arr, 1, 10, urandom);
    } else {
        //couldn't open /dev/urandom
        //PANIC!!
        abort();
    }
}

int main() {
    gen_rsa_key(my_bad_rng);
    gen_rsa_key(better_unix_rng);
}