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I found little to no examples online about sending absolute events (EV_ABS) using libevdev.

I was able to create a device and send some keystrokes, but no luck with absolute events.

The device is shown by sudo libinput list-devices but when I try to record events with sudo evemu-record nothing is shown.

here is my testing code:

#include <iostream>
#include <assert.h>
#include <errno.h>
#include <fcntl.h>
#include <linux/input.h>
#include <linux/uinput.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <libevdev-1.0/libevdev/libevdev-uinput.h>
#include <unistd.h>

int main() {


int err;
struct libevdev dev;
struct libevdev_uinput uidev;
struct input_absinfo absinfo{
    .value = 0,
    .minimum = 0,
    .maximum = 12372,
    .fuzz = 0,
    .flat = 0,
    .resolution = 40
};


dev = libevdev_new();
libevdev_set_name(dev, "eye tracker pointer");
libevdev_enable_event_type(dev, EV_ABS);
libevdev_enable_event_code(dev, EV_ABS, ABS_X, &absinfo);
libevdev_enable_event_code(dev, EV_ABS, ABS_Y, &absinfo);


err = libevdev_uinput_create_from_device(dev, LIBEVDEV_UINPUT_OPEN_MANAGED, &uidev);
if (err != 0) return err;


usleep(10000000UL);//s
libevdev_uinput_write_event(uidev, EV_ABS, ABS_X, 100);
libevdev_uinput_write_event(uidev, EV_ABS, ABS_Y, 100);
libevdev_uinput_write_event(uidev, EV_ABS, SYN_REPORT, 0);
libevdev_uinput_write_event(uidev, EV_ABS, ABS_X, 200);
libevdev_uinput_write_event(uidev, EV_ABS, ABS_Y, 200);
libevdev_uinput_write_event(uidev, EV_ABS, SYN_REPORT, 0);


usleep(2000000UL);


libevdev_uinput_destroy(uidev);


return 0;

Maybe I am doing something wrong with the input_absinfo initialization?

I'm on fedora 36

Marcus Müller
  • 28,836
olbotta
  • 1

1 Answers1

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The error was simply that i wasn't calling sync the right way (EV_SYN). It took me hours to realize.

Code should have been:

libevdev_uinput_write_event(uidev, EV_ABS, ABS_X, 100);
libevdev_uinput_write_event(uidev, EV_ABS, ABS_Y, 100);
libevdev_uinput_write_event(uidev, EV_SYN, SYN_REPORT, 0);
libevdev_uinput_write_event(uidev, EV_ABS, ABS_X, 200);
libevdev_uinput_write_event(uidev, EV_ABS, ABS_Y, 200);
libevdev_uinput_write_event(uidev, EV_SYN, SYN_REPORT, 0);
olbotta
  • 1