How to get an X11 Window from a Process ID?

Question

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 ?

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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 <[email protected]>  #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; } 

Answer 1

The only way I know to do this is to traverse the tree of windows until you find what you're looking for. Traversing isn't hard (just see what xwininfo -root -tree does by looking at xwininfo.c if you need an example).

But how do you identify the window you are looking for? Some applications set a window property called _NET_WM_PID.

I believe that OpenOffice is one of the applications that sets that property (as do most Gnome apps), so you're in luck.

Answer 2

Check if /proc/PID/environ contains a variable called WINDOWID