Select on a file

select_example.c

#include <stdio.h>
#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/select.h>


int main(void) {
  int read_fd = open("example.txt", O_NONBLOCK | O_RDONLY);
  int read_fd_nfds = read_fd + 1;
  
  fd_set set1;
  struct timeval timeout;

  // initialize fd set1
  FD_ZERO(&set1);
  FD_SET(read_fd, &set1);

  int select_val = 0;
  select_val = select(read_fd_nfds, &set1, NULL, NULL, NULL);
  printf("Got %d\n", select_val);
  printf("Our fd is set? %d\n", FD_ISSET(read_fd, &set1));
  // Read nothing from the FD, to satisfy things (?)
  read(read_fd, NULL, 0);
  // Hoping this select will block forever; unfortunately, it
  // immediately succeeds.
  FD_ZERO(&set1);
  FD_SET(read_fd, &set1);
  select_val = select(read_fd_nfds, &set1, NULL, NULL, NULL);
  printf("Second select: got %d\n", select_val);
  printf("Our fd is set? %d\n", FD_ISSET(read_fd, &set1));
  return 0;
}

Actual output:

Got 1
Our fd is set? 8
Second select: got 1
Our fd is set? 8

Expected output:

./a.out  
Got 1
Our fd is set? 8
(block forever)