Clearing out stdin in C when it may or may not be empty

Question

I am a programming student looking for a way to get rid of characters that may be hanging around in stdin. I have tried a technique that has been given here in various forms, wh

Answer 1

Flushing an input stream (in a portable way) without blocking could be done like this:

#include 
#include 
#include 
#include 
#include 

int flush_inputstream(int fd)
{
  int result = 0;

  int flags = fcntl(fd, F_GETFL);
  if (-1 == flags)
  {
    perror("fcntl() failed getting flags");

    result = -1;
    goto lblExit;
  }

  if (!(flags & O_NONBLOCK)) /* If stream isn't non-blocking */
  {                          /* set it to be non-blocking. */
    result = fcntl(fd, F_SETFL, O_NONBLOCK);
    if (-1 == result)
    {
      perror("fcntl() failed setting O_NONBLOCK");

      goto lblExit;
    }
  }

  /* Loop reading from the stream until it is emtpy: */
  do
  {
    char c = 0;
    ssize_t bytesRead = read(fd, &c, 1);
    if (-1 == bytesRead)
    {
      if ((EAGAIN != errno) && (EWOULDBLOCK != errno))
      {
        perror("read() failed");

        result = -1;
      }

      break;
    }
  } while (1);

  if (!(flags & O_NONBLOCK)) /* If stream had not be non-blocking */
  {                          /* re-set it to not be non-blocking. */
    int result_fcntl = fcntl(fd, F_SETFL, flags & ~O_NONBLOCK);
    if (-1 == result_fcntl)
    {
      perror("fcntl() failed setting flags");

      if (0 == result) /* Do not overwrite prvious error! */
      {
        result = result_fcntl;
      }

      goto lblExit;
    }
  }

lblExit:

  return result;
}

/* To test this: */
int main(void)
{
  int fd = fileno(stdin);

  printf("Feed some chars via the keyboard now!\n");

  sleep(3);

  printf("Game Over! Press enter to see stdin is empty\n");

  if (-1 == flush_inputstream(fd))
  {
    fprintf(stderr, "flush_inputstream() failed");
    return EXIT_FAILURE;
  }

  char s[16] = "";
  if (NULL == fgets(s, sizeof(s), stdin))
  {
    perror("fgets() failed");
  }

  printf("%s\n", s);

  return EXIT_SUCCESS;
}