Listening for sysfs battery events - c

Question

I'm writing a daemon which checks the battery capacity. This is for a solar powered embedded device running Linux. I've read that it's a bad idea to use sleep() in daemons, thus I'm trying to use events. So I wrote some PoCs, but I'm not getting any events! My first implementation, as they recommended to me, uses libudev and poll():
#include <fcntl.h>
#include <libudev.h>
#include <poll.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
int main(void)
{
struct udev *udev;
struct udev_monitor *mon;
struct pollfd fds[1];
int fd;
udev = udev_new();
if (udev == NULL)
return 1;
mon = udev_monitor_new_from_netlink(udev, "udev");
udev_monitor_filter_add_match_subsystem_devtype(mon, "power_supply", NULL);
udev_monitor_enable_receiving(mon);
fd = udev_monitor_get_fd(mon);
fds[0].fd = fd;
fds[0].events = POLLIN;
fds[0].revents = 0;
if (poll(fds, 1, -1) > 0) {
/* Never gets here! */
struct udev_device *const dev = udev_monitor_receive_device(mon);
if (dev != NULL) {
puts(udev_device_get_sysname(dev));
udev_device_unref(dev);
}
else
fputs("udev_monitor_receive_device() failed\n", stderr);
}
udev_unref(udev);
return 0;
}
They only event I get, is when I plug/unplug the charger! Then I thought that the status bar I use in my laptop's installation does show the battery capacity. I looked at the source and they're using inotify to monitor the battery's uevent. But I've read everywhere that I shouldn't use inotify for sysfs! I tried nonetheless:
#include <stdio.h>
#include <sys/inotify.h>
#include <unistd.h>
#define BAT_PATH "/sys/class/power_supply/BAT0"
int main(void)
{
struct inotify_event ev = {0};
int wd, ret = 1;
ssize_t len;
const int fd = inotify_init1(IN_CLOEXEC);
if (fd < 0) {
perror("inotify_init() failed");
return ret;
}
/* else */
wd = inotify_add_watch(fd, BAT_PATH "/uevent", IN_ACCESS);
if (wd < 0)
goto end;
/* else */
len = read(fd, &ev, sizeof(ev));
/* Again... never gets here. */
if (len > 0 && (ev.mask & IN_ACCESS))
puts("It worked!");
inotify_rm_watch(fd, wd);
ret = 0;
end:
close(fd);
return ret;
}
Turns out that doesn't work either! How can it work for my status bar but not work when I try it? Am I doing something horribly wrong? Thank you.

Answer 1

Regarding your first implementation (would comment but not enough rep. as i know nothing about libudev): the guide i followed to successfully use sysfs to poll() a GPIO for interrupt suggests to look for a POLLPRI event, instead of POLLIN as you show in the first implementation (see man poll for event types).
More importantly, you say you get a single event when you connect/disconnect charger, do you mean a single event per software execution? If this is the case, it might be due to the fact that you don't clear the interrupt flag: after poll() hits, in sysfs one needs to int len = read(fds[0].fd, *buf, SIZE); to mark the interrupt as served, and also lseek(fds[0].fd, 0, 0); in order for the next read() to be succesful (see my other answer here for a code example).

Answer 2

i dont know whether this answer will help you or not, but i am writing answer because if any other users face same issue they can get it resolved.
the solution is:
you need to monitor kernel Events, so you need to change line
form
mon = udev_monitor_new_from_netlink(udev, "udev");
to
mon = udev_monitor_new_from_netlink(udev, "kernel");
then you will get events.