/*
 * Copyright (C) 2016 Etienne Champetier <champetier.etienne@gmail.com>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 */
#define _GNU_SOURCE
#include <errno.h>
#include <linux/random.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/syscall.h>
#include <unistd.h>

#define ERROR_EXIT(fmt, ...) do { \
        fprintf(stderr, fmt, ## __VA_ARGS__); \
        return EXIT_FAILURE; \
        } while (0)

static int usage(char *name)
{
    fprintf(stderr, "Usage: %s <nb>\n", name);
    fprintf(stderr, " => return <nb> bytes from getrandom()\n");
    return EXIT_FAILURE;
}

int main(int argc, char *argv[])
{
    if (argc != 2)
        return usage(argv[0]);

    if (isatty(STDOUT_FILENO))
        ERROR_EXIT("Not outputting random to a tty\n");

    int nbtot = atoi(argv[1]);
    if (nbtot < 1)
        ERROR_EXIT("Invalid <nb> param (must be > 0)\n");

    char buf[256];
    int len = sizeof(buf);
    while (nbtot > 0) {
        if (nbtot <= sizeof(buf))
            len = nbtot;
        if (syscall(SYS_getrandom, buf, len, 0) == -1)
            ERROR_EXIT("getrandom() failed: %s\n", strerror(errno));
        if (write(STDOUT_FILENO, buf, len) != len)
            ERROR_EXIT("write() failed: %s\n", strerror(errno));
        nbtot -= sizeof(buf);
    }
}
