How to generate random seed for random numbers

Question

I'm trying to generate big random numbers for the public key and private key. I have problem with the initial seed to generate a random 256-bit private key on client-side. as you may know, we shouldn't use rand or srand function in C because it's easy to break.
how can I generate a random seed to generate a random 256-bit private key?
I use GMP's Linear congruential Algorithm to generate random number in C.

Answer 1

On unix systems, you can read from /dev/random and /dev/urandom files to get some "randomness" byte sequences. Those sequences are based on your system entropy. See this post for more details about their differences.

#include <unistd.h> // read
#include <fcntl.h>  // open
#include <stdio.h>  // printf

int main(void)
{
    int             fd;
    unsigned int    seed;

    fd = open("/dev/urandom", O_RDONLY);

    read(fd, &seed, sizeof seed);

    printf("%u\n", seed);

    //  Then you can use srand with your new random seed

    return (0);
}

Note: Don't forget to check for errors after open and read, and to close fd after use.