How to get an X11 Window from a Process ID?

http://stackoverflow.com/questions/151407/how-to-get-an-x11-window-from-a-process-id

Under Linux, my C++ application is using fork() and execv() to launch multiple instances of OpenOffice so as to view some powerpoint slide shows. This part works.

Next I want to be able to move the OpenOffice windows to specific locations on the display. I can do that with the XMoveResizeWindow() function but I need to find the Window for each instance.

I have the process ID of each instance, how can I find the X11 Window from that ?

UPDATE - Thanks to Andy's suggestion, I have pulled this off. I'm posting the code here to share it with the Stack Overflow community.

Unfortunately Open Office does not seem to set the _NET_WM_PID property so this doesn't ultimately solve my problem but it does answer the question.

// Attempt to identify a window by name or attribute.
// by Adam Pierce <adam@doctort.org>

#include <X11/Xlib.h>
#include <X11/Xatom.h>
#include <iostream>
#include <list>

using namespace std;

class WindowsMatchingPid
{
public:
WindowsMatchingPid(Display *display, Window wRoot, unsigned long pid)
: _display(display)
, _pid(pid)
{
// Get the PID property atom.
_atomPID = XInternAtom(display, "_NET_WM_PID", True);
if(_atomPID == None)
{
cout << "No such atom" << endl;
return;
}

search(wRoot);
}

const list<Window> &result() const { return _result; }

private:
unsigned long  _pid;
Atom           _atomPID;
Display       *_display;
list<Window>   _result;

void search(Window w)
{
// Get the PID for the current Window.
Atom           type;
int            format;
unsigned long  nItems;
unsigned long  bytesAfter;
unsigned char *propPID = 0;
if(Success == XGetWindowProperty(_display, w, _atomPID, 0, 1, False, XA_CARDINAL,
&type, &format, &nItems, &bytesAfter, &propPID))
{
if(propPID != 0)
{
// If the PID matches, add this window to the result set.
if(_pid == *((unsigned long *)propPID))
_result.push_back(w);

XFree(propPID);
}
}

// Recurse into child windows.
Window    wRoot;
Window    wParent;
Window   *wChild;
unsigned  nChildren;
if(0 != XQueryTree(_display, w, &wRoot, &wParent, &wChild, &nChildren))
{
for(unsigned i = 0; i < nChildren; i++)
search(wChild[i]);
}
}
};

int main(int argc, char **argv)
{
if(argc < 2)
return 1;

int pid = atoi(argv[1]);
cout << "Searching for windows associated with PID " << pid << endl;

// Start with the root window.
Display *display = XOpenDisplay(0);

WindowsMatchingPid match(display, XDefaultRootWindow(display), pid);

// Print the result.
const list<Window> &result = match.result();
for(list<Window>::const_iterator it = result.begin(); it != result.end(); it++)
cout << "Window #" << (unsigned long)(*it) << endl;

return 0;
}

[/code]
chingliu-desktop tmp # g++ -o getwindow getwindow.cpp -I/usr/X11R6/include -L/usr/X11R6/lib -lX11