How to prevent stdin stream from reading data from associated file descriptor on program start?

Question

I'm using select() call to detect input presence in the main cycle of my program. This makes me use raw file descriptor (0) instead of stdin.

While working in this mode I've noticed that my software occasionally loses a chunk of input at the beginning. I suspect that stdin consumes some of it on the program start. Is there a way to prevent this behavior of stdin or otherwise get the whole input data?

The effect described can be reproduced only with some data on standard input at the very moment of program start. My executable should be used as xinetd service in a way that it always has some input on the start.

Standard input is read in the following way:

Error processInput() {
  struct timeval ktimeout;
  int fd=fileno(stdin);
  int maxFd=fd+1;
  FD_ZERO(&fdset);
  FD_SET(fd, &fdset);
  ktimeout.tv_sec = 0;
  ktimeout.tv_usec = 1;
  int selectRv=-1;
  while ((selectRv=select(maxFd, &fdset, NULL, NULL, &ktimeout)) > 0) {
    int left=MAX_BUFFER_SIZE-position-1;
    assert(left>0);
    int bytesCount=read(fd, buffer+position, left);
    //Input processing goes here
  }
}

Answer 1

Don't mix cooked and raw meat together. Try replacing the read() call with the equivalent fread() call.

It is very likely that fileno(stdin) is initializing the stdin object, causing it to read and buffer some input. Or perhaps you are already calling something that causes it to initialize (scanf(), getchar(), etc...).