Mouse pointer centering for X11

Makefile

PREFIX = /usr/local
bindir = /bin

LDLIBS := $(shell pkg-config --libs-only-l x11 xi)

all: mouse-spring

install:
    install -Dm0755 mouse-spring $(DESTDIR)$(PREFIX)$(bindir)/mouse-spring

mouse-spring.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <err.h>

#include <X11/Xlib.h>
#include <X11/extensions/XInput2.h>

int
main(void)
{
    Window window;
    Display *display;
    int screen;

    int xi_opcode;
    int error;
    int event;

    unsigned int width;
    unsigned int height;

    int fd;
    fd_set fdset;
    struct timeval tv;

    XIEventMask mask[2];
    XIEventMask *m;

    display = XOpenDisplay(NULL);
    window = DefaultRootWindow(display);
    screen = DefaultScreen(display);

    if(!XQueryExtension(display, "XInputExtension", &xi_opcode, &event, &error))
        err(1, "XINPUT not available\n");

    m = &mask[0];
    m->deviceid = XIAllDevices;
    m->mask_len = XIMaskLen(XI_LASTEVENT);
    m->mask = calloc(m->mask_len, sizeof(unsigned char));

    XISetMask(m->mask, XI_Motion);

    XISelectEvents(display, window, &mask[0], 1);
    XSync(display, False);
    free(mask[0].mask);

    fd = ConnectionNumber(display);

    width = DisplayWidth(display, screen);
    height = DisplayHeight(display, screen);

    for(;;){
        FD_ZERO(&fdset);
        FD_SET(fd, &fdset);

        /* The goal here is to allow for some slop in the movement rather than
         * pinning the pointer.  By using a timeout for the event window we can
         * allow the pointer to move before forcing it back to the centre.
         */
        tv.tv_usec = 20000;

        if(select(fd + 1, &fdset, NULL, NULL, &tv) == 0){
            XWarpPointer(display, None, window, 0, 0, 0, 0, width/2, height/2);
        }

        XEvent event;
        XGenericEventCookie *cookie = (XGenericEventCookie*)&event.xcookie;

        XNextEvent(display, (XEvent*)&event);
#if 0
        if(XGetEventData(display, cookie) && cookie->type == GenericEvent && cookie->extension == xi_opcode){
            XIDeviceEvent *device = cookie->data;
            printf("%.0f %.0f\n", device->root_x, device->root_y);
        }
#endif
        XFreeEventData(display, cookie);
    }

    XDestroyWindow(display, window);
    return 0;
}