Learning Xlib: Get next Button Press on the display

hydraMax · 01-04-2012, 06:20 PM

For learning purposes, I was hoping to write a small C program that would wait for the next Button Press anywhere on the display. After reading through a bunch of the Xlib documentation, I thought this would work:

Code:

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

int main() {
  Display * dpy = XOpenDisplay(NULL);
  if(dpy == NULL) {
    fprintf(stderr, "error: could not open display.");
    exit(EXIT_FAILURE);
  }
  Window rootw = XDefaultRootWindow(dpy);
  XSelectInput(dpy, rootw, ButtonPressMask);
  XEvent * e;
  XNextEvent(dpy, e);
  /*
    Display some information about the event probably...
   */
}

But that always dies with:

Code:

X Error of failed request:  BadAccess (attempt to access private resource denied)
  Major opcode of failed request:  2 (X_ChangeWindowAttributes)
  Serial number of failed request:  7
  Current serial number in output stream:  7

Edit: I think the problem above is that I am not grabbing the pointer first. This seems to work:

Code:

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

int main() {
  Display * dpy = XOpenDisplay(NULL);
  if(dpy == NULL) {
    fprintf(stderr, "error: could not open display.");
    exit(EXIT_FAILURE);
  }
  Cursor csr = XCreateFontCursor(dpy, XC_crosshair);
  switch(csr) {
  case BadAlloc:
    fprintf(stderr, "error: BadAlloc during XCreateFontCursor");
    exit(EXIT_FAILURE);
  case BadValue:
    fprintf(stderr, "error: BadValue during XCreateFontCursor");
    exit(EXIT_FAILURE);
  }
  Window rootw = XDefaultRootWindow(dpy);
  XEvent evt;
  int err = XGrabPointer(dpy,
  			 rootw,
  			 False,
  			 ButtonPressMask,
  			 GrabModeAsync,
  			 GrabModeAsync,
  			 None,
  			 csr,
  			 CurrentTime);
  switch(err) {
  case BadCursor:
    fprintf(stderr, "error: BadCursor during XGrabPointer");
    exit(EXIT_FAILURE);
  case BadValue:
    fprintf(stderr, "error: BadCursor during XGrabPointer");
    exit(EXIT_FAILURE);
  case BadWindow:
    fprintf(stderr, "error: BadCursor during XGrabPointer");
    exit(EXIT_FAILURE);
  }
  XNextEvent(dpy, &evt);
  printf("got it!\n");
  /*
    Display some information about the event probably...
   */
}

Is there anything technically wrong with this approach?

kalleanka · 01-09-2012, 05:14 PM

I think your window is not done and some event happens. Use XSync() before event loop. X is really messy.

Try something like...

....

XSync(dpy, 0); // to avoid chase

while(XPending(dpy))
{
XNextEvent(dpy, &evt);
printf("got it!\n");
/*
Display some information about the event probably...
*/
}

....

hydraMax · 01-10-2012, 03:19 AM

@kalleanka: Thank you for the reply, but I am slightly confused: why do I need an event loop? (I literally only need the first mouse click that occurs after starting the program.)

kalleanka · 01-10-2012, 10:14 AM

well because normally when you set an eventmask like this XSelectInput(dpy, rootw, ButtonPressMask); you get a flow of events and you only catch the first or the one in the buffert. You nead a switch statement to and sometimes some if statements to select you event you are looking for. Your program just runs to the end only catching the first event of a flow. The XPending is not needed but makes the program more efficient as i understand it.

I see now that i missed the event loop. It should be like this.

....

XSync(dpy, 0); // to avoid chase
while(1)
{
while(XPending(dpy))
{
XNextEvent(dpy, &evt);
printf("got it!\n");

switch(evt.type)
{
case ....

case .....

}

/*
Display some information about the event probably...
*/
}
}

If you need timeouts you insert a select in your loop. If you need critical timeout checks or tasks you make an new thread before the main event loop to handle the critical tasks or checks. I do not think X is threadsafe so be aware.