non blocking and without roundtrip to userspace

copy-stdin2stdout

#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <limits.h>
int
main(int argc, char *argv[])
{
  int len;
  int devnull = open("/dev/null", O_WRONLY);
  fd_set rfds;
  do {
    FD_ZERO(&rfds);
    FD_SET(STDIN_FILENO, &rfds);
    select(STDIN_FILENO+1, &rfds, NULL, NULL, 0);
    if (FD_ISSET(STDIN_FILENO, &rfds)) {
      len = splice(STDIN_FILENO, NULL, STDOUT_FILENO, NULL, INT_MAX, SPLICE_F_NONBLOCK);
      if (len < 0) {
        if (errno == EAGAIN) {
          splice(STDIN_FILENO, NULL, devnull, NULL, INT_MAX, SPLICE_F_MOVE);
          continue;
        }
        break;
      }
    }
  } while (1);
  close(devnull);
  exit(EXIT_SUCCESS);
}