How to write custom events to /dev/uinput in Linux?

Question

linux

I'm trying to emulate a gamepad by sending my own events to uinput. The keyboard is emulated perfectly, but for some reason the gamepad is in trouble. Its events are not registered. What's wrong? Here is the code:

Spoiler: C code

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <linux/input.h>
#include <linux/uinput.h>

#define die(str, args...) do { \
        perror(str); \
        exit(EXIT_FAILURE); \
    } while(0)

int
main(void)
{
    int                    fd;
    struct uinput_user_dev uidev;
    struct input_event     ev;
    int                    dx, dy;
    int                    i;

    fd = open("/dev/uinput", O_WRONLY | O_NONBLOCK);
    if(fd < 0)
        die("error: open");

    if(ioctl(fd, UI_SET_EVBIT, EV_KEY) < 0)
        die("error: ioctl");
    if(ioctl(fd, UI_SET_KEYBIT, KEY_0) < 0)
        die("error: ioctl");
    if(ioctl(fd, UI_SET_KEYBIT, BTN_A) < 0)
        die("error: ioctl");

    if(ioctl(fd, UI_SET_EVBIT, EV_SYN) < 0)
        die("error: ioctl");

    if(ioctl(fd, UI_SET_EVBIT, EV_REL) < 0)
        die("error: ioctl");
    if(ioctl(fd, UI_SET_RELBIT, REL_X) < 0)
        die("error: ioctl");
    if(ioctl(fd, UI_SET_RELBIT, REL_Y) < 0)
        die("error: ioctl");

  if(ioctl(fd, UI_SET_EVBIT, EV_ABS) < 0)
        die("error: ioctl");
    if(ioctl(fd, UI_SET_RELBIT, ABS_X) < 0)
        die("error: ioctl");
    if(ioctl(fd, UI_SET_RELBIT, ABS_Y) < 0)
        die("error: ioctl");

    memset(&uidev, 0, sizeof(uidev));
    snprintf(uidev.name, UINPUT_MAX_NAME_SIZE, "uinput-sample");
    uidev.id.bustype = BUS_USB;
    uidev.id.vendor  = 0x1;
    uidev.id.product = 0x1;
    uidev.id.version = 1;

    if(write(fd, &uidev, sizeof(uidev)) < 0)
        die("error: write");

    if(ioctl(fd, UI_DEV_CREATE) < 0)
        die("error: ioctl");

    sleep(2);

    srand(time(NULL));

    while(1) {
            memset(&ev, 0, sizeof(struct input_event));
            ev.type = EV_KEY;
            ev.code = KEY_0;
            ev.value = 1;
            if(write(fd, &ev, sizeof(struct input_event)) < 0)
                die("error: write");

            memset(&ev, 0, sizeof(struct input_event));
            ev.type = EV_KEY;
            ev.code = KEY_0;
            ev.value = 0;
            if(write(fd, &ev, sizeof(struct input_event)) < 0)
                die("error: write");

      memset(&ev, 0, sizeof(struct input_event));
            ev.type = EV_KEY;
            ev.code = BTN_A;
            ev.value = 1;
            if(write(fd, &ev, sizeof(struct input_event)) < 0)
                die("error: write");

            memset(&ev, 0, sizeof(struct input_event));
            ev.type = EV_SYN;
            ev.code = 0;
            ev.value = 0;
            if(write(fd, &ev, sizeof(struct input_event)) < 0)
                die("error: write");


        sleep(1);
    }

    sleep(2);

    if(ioctl(fd, UI_DEV_DESTROY) < 0)
        die("error: ioctl");

    close(fd);

    return 0;
}

Actually, I open the file:

fd = open("/dev/uinput", O_WRONLY | O_NONBLOCK);
    if(fd < 0)
        die("error: open");

I allow specific events:

if(ioctl(fd, UI_SET_KEYBIT, BTN_A) < 0)
        die("error: ioctl");
     
    if(ioctl(fd, UI_SET_EVBIT, EV_SYN) < 0)
        die("error: ioctl");

Registering the device:

memset(&uidev, 0, sizeof(uidev));
    snprintf(uidev.name, UINPUT_MAX_NAME_SIZE, "uinput-sample");
    uidev.id.bustype = BUS_USB;
    uidev.id.vendor  = 0x1;
    uidev.id.product = 0x1;
    uidev.id.version = 1;

    if(write(fd, &uidev, sizeof(uidev)) < 0)
        die("error: write");

And send events:

memset(&ev, 0, sizeof(struct input_event));
            ev.type = EV_KEY;
            ev.code = BTN_A;
            ev.value = 1;
            if(write(fd, &ev, sizeof(struct input_event)) < 0)
                die("error: write");

            memset(&ev, 0, sizeof(struct input_event));
            ev.type = EV_SYN;
            ev.code = 0;
            ev.value = 0;
            if(write(fd, &ev, sizeof(struct input_event)) < 0)
                die("error: write");

What's wrong?