Quick test using epoll to wait on GPIO events

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 epoll_create(int size)
  // number of file descriptors a process wants to monitor, 
  // which helps the kernel to decide the size of the epoll instance
  int epfd = epoll_create(1); 
  int fd = open("/sys/class/gpio/gpio34/value", O_RDWR | 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; // used to receive urgent data. 
    ev.data.fd = fd;

    // int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);
    // epfd: is the file descriptor returned by epoll_create
    // op: refers to the operation to be performed on the file descriptor fd. 
    // In general, three operations are supported:
    // - Register fd with the epoll instance (EPOLL_CTL_ADD) and get notified about events that occur on fd
    // - Delete/deregister fd from the epoll instance. 
    //   This would mean that the process would no longer get any notifications about events 
    //   on that file descriptor (EPOLL_CTL_DEL). 
    //   If a file descriptor has been added to multiple epoll instances, 
    //   then closing it will remove it from all of the epoll interest lists to which it was added.
    // - Modify the events fd is monitoring (EPOLL_CTL_MOD)
    // fd: is the file descriptor we want to add to the epoll list/interest list.
    // event: is a pointer to a structure called epoll_event which stores the event we actually want to monitor fd for.
    n = epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev);
    printf("epoll_ctl returned %d: %s\n", n, strerror(errno));

    while(1) {
      // int epoll_wait(int epfd, struct epoll_event *evlist, int maxevents, int timeout);
      // evlist: is an array of epoll_event structures. 
      // evlist is allocated by the calling process and when epoll_wait returns, 
      // this array is modified to indicate information about the subset of file descriptors 
      // in the interest list that are in the ready state (this is called the ready list)
      // * when timeout is set to -1, epoll_wait will block “forever”
      // * return: -1 if error, 0 if timeout, 
      //       a positive integer which indicates the total number of file descriptors in the evlist array
      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);
}

example-output.txt

root@beaglebone:/var/lib/cloud9# gcc -o epolltest epolltest.c && ./epolltest 
open returned 4: Success
epoll_ctl returned 0: Success
epoll returned 1: Success
seek 0 bytes: Success
read 1 bytes: Success
buf = 0x31
epoll returned 1: Success
seek 0 bytes: Success
read 1 bytes: Success
buf = 0x30
epoll returned 1: Success
seek 0 bytes: Success
read 1 bytes: Success
buf = 0x31
epoll returned 1: Success
seek 0 bytes: Success
read 1 bytes: Success
buf = 0x30
epoll returned 1: Success
seek 0 bytes: Success
read 1 bytes: Success
buf = 0x31
^C
root@beaglebone:/var/lib/cloud9#