Listen to changes in brightness using epoll.

epolltest.c

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/epoll.h>
#include <sys/types.h>

int main(int argc, char** argv) {
  int n;
  int epfd = epoll_create(1);
  int fd = open("/sys/class/backlight/intel_backlight/actual_brightness", O_RDONLY | O_NONBLOCK);
  printf("open returned %d: %s\n", fd, strerror(errno));
  if(fd > 0) {
    char buf = 0;

    struct epoll_event ev;
    struct epoll_event events;
    ev.events = EPOLLPRI;
    ev.data.fd = fd;

    n = epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev);
    printf("epoll_ctl returned %d: %s\n", n, strerror(errno));

    while(1) {
      n = epoll_wait(epfd, &events, 1, -1);
      printf("epoll returned %d: %s\n", n, strerror(errno));

      if(n > 0) {
        n = lseek(fd, 0, SEEK_SET);
        printf("seek %d bytes: %s\n", n, strerror(errno));
        n = read(fd, &buf, 1);
        printf("read %d bytes: %s\n", n, strerror(errno));
        printf("buf = 0x%x\n", buf);
      }
    }
  }

  return(0);
}