while(1) {
  nanosleep(&ts,NULL);
  MyTimerHandler();
  while( XPending(dpy) ) {
    XNextEvent(m_dpy, &xev);
    MyEventHandler(&xev);
  }
}

while(1) {
  nanosleep(&ts,NULL);
  MyTimerHandler();
  while( XCheckWindowEvent(dpy, win, ExposureMask|KeyMask..., &xev) ) {
    MyEventHandler(&xev);
  }
}

void* XInputThread(void*) {
  XEvent &xev;
  MyEvent &ev;

  while( someCondition ) {
    XNextEvent(dpy, &xev);
    pthread_mutex_lock(&queueMutex);
    MyEventFromXEvent(&ev, &xev)
    queue.push_back(ev);
    pthread_mutex_unlock(&queueMutex);
    sem_post(&queueSem);
  }
}

void TimerSigHandler(int signum) {
  MyEvent &ev;

  pthread_mutex_lock(&queueMutex);
  MyEventFromTime(&ev, timeval);
  queue.push_back(ev);
  pthread_mutex_unlock(&queueMutex);  
  sem_post(&queueSem);
}

int main() {
  ...

  pthread_create( &xinputThread, NULL, XInputThread, NULL );

  memset (&sa, 0, sizeof (sa));
  sa.sa_handler = &TimerSigHandler;
  sigaction (SIGALRM, &sa, NULL);

  timer.it_value.tv_sec = 0;
  timer.it_value.tv_usec = 10000;
  timer.it_interval.tv_sec = 0;
  timer.it_interval.tv_usec = 10000;
  setitimer (ITIMER_REAL, &timer, NULL);

  ...

  while(1) {
    sem_wait(&queueSem);
    pthread_mutex_lock(&queueMutex);
    ev = queue.top();
    queue.pop();
    pthread_mutex_unlock(&queueMutex);

    ProcessTheEvent(&ev);
  }
  ...

}

#include <stdio.h>
#include <stdlib.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>

Display *dis;
Window win;
int x11_fd;
fd_set in_fds;

struct timeval tv;
XEvent ev;

int main() {
    dis = XOpenDisplay(NULL);
    win = XCreateSimpleWindow(dis, RootWindow(dis, 0), 1, 1, 256, 256, \
        0, BlackPixel (dis, 0), BlackPixel(dis, 0));
    
    // You don't need all of these. Make the mask as you normally would.
    XSelectInput(dis, win, 
        ExposureMask | KeyPressMask | KeyReleaseMask | PointerMotionMask |
        ButtonPressMask | ButtonReleaseMask  | StructureNotifyMask 
        );

    XMapWindow(dis, win);
    XFlush(dis);
    
    // This returns the FD of the X11 display (or something like that)
    x11_fd = ConnectionNumber(dis);

    // Main loop
    while(1) {
        // Create a File Description Set containing x11_fd
        FD_ZERO(&in_fds);
        FD_SET(x11_fd, &in_fds);

        // Set our timer.  One second sounds good.
        tv.tv_usec = 0;
        tv.tv_sec = 1;

        // Wait for X Event or a Timer
        if (select(x11_fd+1, &in_fds, 0, 0, &tv))
            printf("Event Received!\n");
        else
            // Handle timer here
            printf("Timer Fired!\n");

        // Handle XEvents and flush the input 
        while(XPending(dis))
            XNextEvent(dis, &ev);
    }
    return(0);
}

#include <stdio.h>
#include <stdlib.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <sys/time.h>

#ifndef timeradd
# define timeradd(a, b, result)                           \
   do {                                        \
     (result)->tv_sec = (a)->tv_sec + (b)->tv_sec;                 \
     (result)->tv_usec = (a)->tv_usec + (b)->tv_usec;                  \
     if ((result)->tv_usec >= 1000000)                         \
       {                                       \
     ++(result)->tv_sec;                           \
     (result)->tv_usec -= 1000000;                         \
       }                                       \
   } while (0)
#endif
#ifndef timersub
 # define timersub(a, b, result)                           \
   do {                                        \
     (result)->tv_sec = (a)->tv_sec - (b)->tv_sec;                 \
     (result)->tv_usec = (a)->tv_usec - (b)->tv_usec;                  \
     if ((result)->tv_usec < 0) {                          \
       --(result)->tv_sec;                             \
       (result)->tv_usec += 1000000;                       \
     }                                         \
   } while (0)
#endif


Display *dis;
Window win;
int x11_fd;
fd_set in_fds;

int main() {
    struct timeval tv;
    struct timeval stv;
    struct timeval tv_period;

    XEvent ev;

    dis = XOpenDisplay(NULL);
    win = XCreateSimpleWindow(dis, RootWindow(dis, 0), 1, 1, 256, 256, \
        0, BlackPixel (dis, 0), BlackPixel(dis, 0));
    
    // You don't need all of these. Make the mask as you normally would.
    XSelectInput(dis, win, 
        ExposureMask | KeyPressMask | KeyReleaseMask | PointerMotionMask |
        ButtonPressMask | ButtonReleaseMask  | StructureNotifyMask 
        );

    XMapWindow(dis, win);
    XFlush(dis);
    
    // This returns the FD of the X11 connection (or something like that)
    x11_fd = ConnectionNumber(dis);

    // Set Timer Period (this is how you configure the frequency):
    tv_period.tv_sec = 0;
    tv_period.tv_usec = 500000; // 500000us = 500ms = 0.5s 


    tv.tv_sec = tv_period.tv_sec;       // Set tv=1 sec so select() will timeout.
    tv.tv_usec = tv_period.tv_usec;
    gettimeofday(&stv, 0);              // Get the time of day and
    timeradd(&stv, &tv_period, &stv);   // Trust my math for now.. :)

    // Main loop
    while(1) {
        // Create a File Description Set containing x11_fd
        FD_ZERO(&in_fds);
        FD_SET(x11_fd, &in_fds);


        // Wait for X Event or a Timer
        if (select(x11_fd+1, &in_fds, 0, 0, &tv)) {
            printf("Event Received!\n");

            gettimeofday(&tv, 0);
            timersub(&stv, &tv, &tv);  // set tv = remaining time.
        }
        else {
            printf("Timer Fired!\n");
            // Initialize timer variables again.
            tv.tv_sec = tv_period.tv_sec;       // Set tv=1 sec so select() will timeout.
            tv.tv_usec = tv_period.tv_usec;
            gettimeofday(&stv, 0);
            timeradd(&stv, &tv_period, &stv);  // Trust my math for now.. :)
        }

        // Handle XEvents and flush the input 
        while(XPending(dis))
            XNextEvent(dis, &ev);
    }
    return(0);
}