[ANNOUNCE] libevdev - a library to wrap the evdev kernel interface

[Peter Hutterer]

For the last month or so I've been spending some time on a helper library
for evdev devices. The motivation is two-fold:

* users of evdev currently issue ioctls directly, checking bit masks
  manually. all this is a source for error, especially as there are some
  inconsistencies in the ioctl APIs. For example, EV_REP does not behave the
  same way as EV_KEY. this can be abstracted through a library.

* SYN_DROPPED signals that the process is not reading events fast enough.
  The process must re-sync the device, calculating the delta and processing
  it accordingly. This is similar across all drivers, but needlessly
  complicated.

libevdev provides API calls for the various bits, so a client can work this
way:

  struct libevdev *dev;

  fd = open("/dev/input/event0", O_RDONLY);
  rc = libevdev_new_from_fd(fd, &dev);

  if (libevdev_has_event_code(dev, EV_REL, REL_X))
        /* do something */

including the various checks that prevent OOM access for invalid types/codes.

For event handling, libevdev provides one call that returns the next event
in the queue:

  rc = libevdev_next_event(dev, flags, &ev);

if a SYN_DROPPED is the next event in the queue, libevdev syncs the device
state and returns 1. The process can then handle the delta by simply parsing
events until the device is fully synced again:

  while ((rc = libevdev_next_event(dev, LIBEVEV_READ_SYNC, &ev)) == 1)
     /* this is a synced event */

  if (rc == -EAGAIN)
     /* we are now fully synced */
  else
     /* error */

For a more complete example: evtest written with libevdev would look like
this:
https://github.com/whot/libevdev/blob/master/tools/libevdev-events.c

The code is still in its early stages and some parts are incomplete, but I'm
at the point where I'd like some comments and eyeballs to help find any
issues with the lot. Ideally, I'd like all of the X drivers and wayland to
eventually use libevdev. Plans to switch evemu, mtdev, etc. are on the todo
list somewhere too.

Repository sits here:
    https://github.com/whot/libevdev
The API documentation is published here:
    http://whot.github.io/libevdev/doc/html/modules.html

Suggestions, comments, criticism, etc. welcome.

Cheers,
  Peter