/**
 * Phase 01 - Get a Window that works and can be closed.
 *
 * This code won't be structured very well, just trying to get stuff working.
 */
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>

// ~16.6 ms between frames is ~60 fps.
#define RATE_LIMIT 16.6

// Forward declaration of this function so we can use it in main().
double timespec_diff(struct timespec *a, struct timespec *b);

int main(int argc, char *argv[])
{
  // Create application display.
  Display *dpy = XOpenDisplay(NULL);

  if (dpy == NULL) {
    return EXIT_FAILURE;
  }

  // Create the application Window.
  unsigned long black = WhitePixel(dpy, DefaultScreen(dpy));
  Window win = XCreateSimpleWindow(dpy, DefaultRootWindow(dpy), 0, 0, 350, 200, 0, black, black);

  // Setup the Window Manager hints.
  XWMHints *wmhints = XAllocWMHints();
  // This basically tells other functions that this contains a value for input and initial state.
  wmhints->flags = InputHint | StateHint;
  // And these are the values for input and initial state.
  wmhints->input = True;
  wmhints->initial_state = NormalState;

  // Setup the Size Hints (also for the Window Manager).
  XSizeHints *sizehints = XAllocSizeHints();
  // This tells other functions that the value for min width and height.
  sizehints->flags = PMinSize;
  // And these are the values for min width and height.
  sizehints->min_width = 100;
  sizehints->min_height = 60;

  /*
   * This particular function does some allocating that doesn't ever get freed.
   * Valgrind reports 27,262 bytes in 384 blocks as still reachable because of this.
   * It's possible that this "leak" could be avoided by using XSetWMProperties()
   * and creating our own XTextProperty's.
   */
  // Sets Window properties that are used by the Window Manager.
  Xutf8SetWMProperties(dpy, win, "test keyboard", "", NULL, 0, sizehints, wmhints, NULL);

  // Tell X that we want to be notified of the Exposure event, so we can know when our window is initially visible.
  XSelectInput(dpy, win, ExposureMask);

  // Grab a copy of X's representation of WM_PROTOCOLS, used in checking for window closed events.
  Atom wm_protocol = XInternAtom(dpy, "WM_PROTOCOLS", True);
  // Let the Window Manager know that we want the event when a user closes the window.
  Atom wm_delete = XInternAtom(dpy, "WM_DELETE_WINDOW", True);
  XSetWMProtocols(dpy, win, &wm_delete, 1);

  // Map the window to the display.
  XMapWindow(dpy, win);

  // This variable will be used to examine events thrown to our application window.
  XEvent e;

  // Block execution until the window is exposed.
  XWindowEvent(dpy, win, ExposureMask, &e);

  // After being exposed, we'll tell X what input events we want to know about here.
  XSelectInput(dpy, win, KeyPressMask);

  // The loop
  // @TODO: Use sleeping to avoid taking up all CPU cycles.
  Bool done = False;

  // We need to track very small periods of time (nanoseconds), so we use the struct timespec.
  struct timespec prev, curr;

  /*
   * Get the current time with CLOCK_MONOTONIC_RAW, which gets the time past since a certain time.
   * CLOCK_MONOTONIC_RAW is not subject to adjustments to the system clock.
   */
  clock_gettime(CLOCK_MONOTONIC_RAW, &curr);
  // Initialize the previous time with the current time, that way our current vs. previous comparison is valid.
  prev.tv_sec = curr.tv_sec;
  prev.tv_nsec = curr.tv_nsec;

  // This variable will be used to normalize our loop to a specific rate.
  double mill_store = 0;

  // A couple of variables used to deal with KeyPress and KeyRelease events.
  KeySym event_key;
  char *key_string = NULL;

  while(!done) {
    // Get the current time.
    clock_gettime(CLOCK_MONOTONIC_RAW, &curr);
    // Store the difference in ms between curr and prev, store it in mill_store for use later.
    mill_store += timespec_diff(&curr, &prev);

    // @TODO: Determine if this should happen before updating curr.
    // Handle events in the event queue.
    while(XPending(dpy) > 0) {
      XNextEvent(dpy, &e);
      switch(e.type) {
        case ClientMessage:
          // This client message is a window manager protocol.
          if (e.xclient.message_type == wm_protocol) {
            // Somehow this checks if the protocol was a WM_DELETE protocol, so we can exit the loop and be done.
            if (e.xclient.data.l[0] == wm_delete) {
              done = True;
            }
          }
          break;
        case KeyPress:
          // Handle KeyPress events.
          // @TODO: set the second value (index) properly
          event_key = XLookupKeysym(&(e.xkey), 0);
          key_string = XKeysymToString(event_key);
          printf("Key pressed: %s\n", key_string);
          break;
      }
    }

    // Only do stuff if the ms passed is greater than our rate limit.
    if (mill_store > RATE_LIMIT && !done) {
      /*
       * This loop counts down the mill_store, so if we have more ms stored than the Rate limit,
       * we run all the processes once. If we have three times the rate limit, we run all the
       * processes thrice. If the mill_store is less than the rate limit, then we pass on
       * processing for this time around the loop (which shouldn't really happen).
       *
       * This helps us have predictable numbers when we do things dependent on numbers, like physics.
       */
      for (; mill_store > RATE_LIMIT && ! done; mill_store -= RATE_LIMIT) {
        // Things that should run once per tick/frame will go here.
      }
    }

    // Update the previous timespec with the most recent timespec so we can calculate the diff next time around.
    prev.tv_sec = curr.tv_sec;
    prev.tv_nsec = curr.tv_nsec;

    /**
     * Make our process sleep to avoid locking up the CPU.
     *
     * From what I understand, the following sleep code will not work on Windows.
     * It works on Linux, it probably works on OSX, but a different approach is needed
     * for Windows.
     */
    if (mill_store < RATE_LIMIT && !done) {
      // We'll need a couple of timespecs, and an int to check for errors.
      struct timespec sleep_required, sleep_remaining;
      int was_error = 0;

      // initialize the remaining sleep time with the value in mill_store.
      sleep_remaining.tv_sec = mill_store / 1000.0;
      sleep_remaining.tv_nsec = ((int)mill_store % 1000) * 1000000;

      do {
        // Set the required sleep time using the remaining time, so we can continue sleeping if nanosleep is interrupted.
        sleep_required.tv_sec = sleep_remaining.tv_sec;
        sleep_required.tv_nsec = sleep_remaining.tv_nsec;

        // Clear out the errno variable before calling nanosleep so we can catch errors.
        errno = 0;
        // Try sleeping for the required time, if nanosleep is interrupted, sleep_remaining will have the time left to sleep.
        was_error = nanosleep(&sleep_required, &sleep_remaining);
        // Keep looping if nanosleep was interrupted and there is some sleep time remaining.
      } while (was_error == -1 && errno == EINTR);
    }
  }

  // Free all the things.
  XFree(sizehints);
  sizehints = NULL;
  XFree(wmhints);
  wmhints = NULL;

  XDestroyWindow(dpy, win);
  XCloseDisplay(dpy);
  dpy = NULL;

  return EXIT_SUCCESS;
}

/**
 * This returns the difference between the values of two timespecs.
 */
double timespec_diff(struct timespec *a, struct timespec *b)
{
	return (((a->tv_sec * 1000000000) + a->tv_nsec) - ((b->tv_sec * 1000000000) + b->tv_nsec)) / 1000000.0;
}
 

編譯

$ gcc main.c -std=gnu99 -g -Wall `pkg-config x11 --cflags --libs`