getrandom.c

   1 /*
   2  * Copyright (C) 2016 Etienne Champetier <champetier.etienne@gmail.com>
   3  *
   4  * This program is free software; you can redistribute it and/or modify
   5  * it under the terms of the GNU General Public License as published by
   6  * the Free Software Foundation; either version 2 of the License, or
   7  * (at your option) any later version.
   8  *
   9  * This program is distributed in the hope that it will be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12  * GNU General Public License for more details.
  13  */
  14 #define _GNU_SOURCE
  15 #include <errno.h>
  16 #include <linux/random.h>
  17 #include <stdio.h>
  18 #include <stdlib.h>
  19 #include <string.h>
  20 #include <sys/syscall.h>
  21 #include <unistd.h>
  22
  23 #define ERROR_EXIT(fmt, ...) do { \
  24                 fprintf(stderr, fmt, ## __VA_ARGS__); \
  25                 return EXIT_FAILURE; \
  26         } while (0)
  27
  28 static int usage(char *name)
  29 {
  30         fprintf(stderr, "Usage: %s <nb>\n", name);
  31         fprintf(stderr, " => return <nb> bytes from getrandom()\n");
  32         return EXIT_FAILURE;
  33 }
  34
  35 int main(int argc, char *argv[])
  36 {
  37         if (argc != 2)
  38                 return usage(argv[0]);
  39
  40         if (isatty(STDOUT_FILENO))
  41                 ERROR_EXIT("Not outputting random to a tty\n");
  42
  43         int nbtot = atoi(argv[1]);
  44         if (nbtot < 1)
  45                 ERROR_EXIT("Invalid <nb> param (must be > 0)\n");
  46
  47         char buf[256];
  48         int len = sizeof(buf);
  49         while (nbtot > 0) {
  50                 if (nbtot <= sizeof(buf))
  51                         len = nbtot;
  52                 if (syscall(SYS_getrandom, buf, len, 0) == -1)
  53                         ERROR_EXIT("getrandom() failed: %m\n");
  54                 if (write(STDOUT_FILENO, buf, len) != len)
  55                         ERROR_EXIT("write() failed: %m\n");
  56                 nbtot -= sizeof(buf);
  57         }
  58
  59         return 0;
  60 }