Make mouse and trackball connected to Pi400 work for laptop over netcat session

mstr.c

/*  gcc -Wall -pedantic -Wextra mstr.c -o mstr

    Works together with "Raspberry Pi400 as a USB HID Keyboard":
    https://gist.github.com/Hermann-SW/19c7522220a4ec3c8d6ecc89de2b0a60

    Raspberry mouse and trackball, both connected to Pi400, work on laptop!

    On laptop:
    $ nc -l 8888 | sudo ./mstr -nodelay

    On Pi400:
    ( cat /dev/input/event0 & cat /dev/input/event3 & wait ) | nc laptopip 8888
*/
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <linux/uinput.h>
#include <assert.h>

int main(int argc, char *argv[])
{
   struct uinput_setup usetup;

   int nodelay = (argc==2) && !strcmp(argv[1], "-nodelay");

   int fd = open("/dev/uinput", O_WRONLY | O_NONBLOCK);

   ioctl(fd, UI_SET_EVBIT, EV_KEY);
   ioctl(fd, UI_SET_KEYBIT, BTN_LEFT);
   ioctl(fd, UI_SET_KEYBIT, BTN_MIDDLE);
   ioctl(fd, UI_SET_KEYBIT, BTN_RIGHT);

   ioctl(fd, UI_SET_EVBIT, EV_REL);
   ioctl(fd, UI_SET_RELBIT, REL_X);
   ioctl(fd, UI_SET_RELBIT, REL_Y);
   ioctl(fd, UI_SET_RELBIT, REL_WHEEL);
   ioctl(fd, UI_SET_RELBIT, REL_HWHEEL);

   memset(&usetup, 0, sizeof(usetup));
   usetup.id.bustype = BUS_USB;
   usetup.id.vendor = 0x1234; /* sample vendor */
   usetup.id.product = 0x5678; /* sample product */
   strcpy(usetup.name, "mouse replay");

   ioctl(fd, UI_DEV_SETUP, &usetup);
   ioctl(fd, UI_DEV_CREATE);

   sleep(1);

   const int pi_ie_sz = 16;
   const int ie_sz = sizeof(struct input_event);
   char buf[ie_sz];
   void *q = buf + ie_sz - pi_ie_sz;
   while (!feof(stdin))
   {
     if (fread(q, pi_ie_sz, 1, stdin) == 0)  break;
     bzero(buf, ie_sz - 8);
     assert(write(fd, buf, ie_sz) == ie_sz);
     if (!nodelay)  usleep(15000);
   }

   sleep(1);

   ioctl(fd, UI_DEV_DESTROY);
   close(fd);

   return 0;
}

https://www.raspberrypi.org/forums/viewtopic.php?f=140&t=295074&p=1884806#p1884806