linux uinput: simple example?

I’m having some problems getting both sides of code using uinput working.

Based on Getting started with uinput: the user level input subsystem^{[dead link; archived]} I put together the following writer (minus error handling):

int main(int ac, char **av)
{
    int fd = open("/dev/uinput", O_WRONLY | O_NONBLOCK);
    int ret = ioctl(fd, UI_SET_EVBIT, EV_ABS);
    ret = ioctl(fd, UI_SET_ABSBIT, ABS_X);

    struct uinput_user_dev uidev = {0};
    snprintf(uidev.name, UINPUT_MAX_NAME_SIZE, "uinput-rotary");
    uidev.absmin[ABS_X] = 0;
    uidev.absmax[ABS_X] = 255;
    ret = write(fd, &uidev, sizeof(uidev));
    ret = ioctl(fd, UI_DEV_CREATE);

    struct input_event ev = {0};
    ev.type = EV_ABS;
    ev.code = ABS_X;
    ev.value = 42;

    ret = write(fd, &ev, sizeof(ev));

    getchar();

    ret = ioctl(fd, UI_DEV_DESTROY);
    return EXIT_SUCCESS;
}

That seems to work, at least the full input_event structure seems to be written.

I then wrote the most naive reader of events I could come up with:

int main(int ac, char **av)
{
    int fd = open(av[1], O_RDONLY);

    char name[256] = "unknown";
    ioctl(fd, EVIOCGNAME(sizeof(name)), name);
    printf("reading from %sn", name);

    struct input_event ev = {0};
    int ret = read(fd, &ev, sizeof(ev));
    printf("Read an event! %in", ret);
    printf("ev.time.tv_sec: %lin", ev.time.tv_sec);
    printf("ev.time.tv_usec: %lin", ev.time.tv_usec);
    printf("ev.type: %hin", ev.type);
    printf("ev.code: %hin", ev.code);
    printf("ev.value: %lin", ev.value);

    return EXIT_SUCCESS;
}

Unfortunately the reader side doesn’t work at all; only manages to read 8 bytes each time, which isn’t nearly a full input_event structure.

What silly mistake am I making?

Answer

You should also be writing a sync event after the actual event. In your writer side code:

struct input_event ev = {0};
ev.type = EV_ABS;
ev.code = ABS_X;
ev.value = 42;

usleep(1500);

memset(&ev, 0, sizeof(ev));
ev.type = EV_SYN;
ev.code = 0;
ev.value = 0;

ret = write(fd, &ev, sizeof(ev));

getchar();