gistfile1.h

#include <stdio.h>
#include <sys/select.h>
#include <fcntl.h>
#include <stdlib.h>

int main(int argc, char ** argv) {
  char * dev_short = (argc > 1) ? argv[1] : "sda1";
  char dev[512];
  snprintf(dev, sizeof(dev), "/sys/block/%s/size", dev_short);

  int fd = open(dev, O_RDONLY);
  if (fd == -1) {
    char msg[512];
    snprintf(msg, sizeof(msg), "open %s", dev);

    perror(msg);
    exit(1);
  }
  printf("fd = %d\n", fd);

  fd_set fdr;
  FD_ZERO(&fdr);
  FD_SET(fd, &fdr);

  struct timeval timestart, timeend;
  struct timeval timeout;
  timeout.tv_sec = 5L;
  timeout.tv_usec = 0L;

  gettimeofday(&timestart, NULL);

  int selret = select(fd + 1, &fdr, NULL, NULL, &timeout);
  printf("selret = %d\n", selret);
  printf("FD_ISSET(%d) is %d\n", fd, FD_ISSET(fd, &fdr));

  gettimeofday(&timeend, NULL);

  struct timeval timediff;
  timediff.tv_sec = timeend.tv_sec - timestart.tv_sec;
  timediff.tv_usec = timeend.tv_usec - timestart.tv_usec;
  if (timediff.tv_usec < 0L) {
    timediff.tv_usec += 1000000L;
    timediff.tv_sec -= 1;
  }
  printf("time elapsed = %ld.%06ld seconds\n", timediff.tv_sec, timediff.tv_usec);
}
/*
good: 
fd = 3
selret = 1
FD_ISSET(3) is 1
time elapsed = 0.000016 seconds

bad: 
fd = 3
selret = 0
FD_ISSET(3) is 0
time elapsed = 5.000043 seconds
*/

gistfile2.builder

f = File.open("/sys/block/sda/sda2/size")
# => #<File:/sys/block/sda/sda2/size>
IO.select([f],nil,nil,nil)

gistfile3.h

#include <stdio.h>
#include <sys/select.h>
#include <fcntl.h>
#include <stdlib.h>

int main(int argc, char ** argv) {
  char * dev_short = (argc > 1) ? argv[1] : "sda1";
  char dev[512];
  snprintf(dev, sizeof(dev), "/sys/block/%s/size", dev_short);

  int fd = open(dev, O_RDONLY);
  if (fd == -1) {
    char msg[512];
    snprintf(msg, sizeof(msg), "open %s", dev);

    perror(msg);
    exit(1);
  }
  printf("fd = %d\n", fd);

  fd_set fdr;
  FD_ZERO(&fdr);
  FD_SET(fd, &fdr);

  struct timeval timestart, timeend;
  struct timeval timeout;
  timeout.tv_sec = 5L;
  timeout.tv_usec = 0L;

  gettimeofday(&timestart, NULL);

  int selret = select(fd + 1, &fdr, NULL, NULL, &timeout);
  printf("selret = %d\n", selret);
  printf("FD_ISSET(%d) is %d\n", fd, FD_ISSET(fd, &fdr));

  gettimeofday(&timeend, NULL);

  struct timeval timediff;
  timediff.tv_sec = timeend.tv_sec - timestart.tv_sec;
  timediff.tv_usec = timeend.tv_usec - timestart.tv_usec;
  if (timediff.tv_usec < 0L) {
    timediff.tv_usec += 1000000L;
    timediff.tv_sec -= 1;
  }
  printf("time elapsed = %ld.%06ld seconds\n", timediff.tv_sec, timediff.tv_usec);

  if (selret == 0 || !FD_ISSET(fd, &fdr)) {
    fprintf(stderr, "Your kernel /sys select is borked. selret should == 1 and FD_ISSET(%d, &fdr) should be 1!\n", fd);
  }
}
/*
BAD:

[root@localhost tmp]# ./fuuuuuuuu  sda/sda2
fd = 3
selret = 0
FD_ISSET(3) is 0
time elapsed = 5.001028 seconds
Your kernel /sys select is borked. selret should == 1 and FD_ISSET(3, &fdr) should be 1!
*/