How do I bring a processes window to the foreground on X Windows? (C++)

Question

I have the PID for the process (and the name), I want to bring it to the front on linux (ubuntu). On mac I would simply do SetFrontProcess(pid), on windows I'd enumerate the windows, pick out the one I wanted, and call SetWindowPos(hwnd, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE); but I'm at a loss of what to do on linux. I've looked at X Lib a bit, but most/all of those functions seem to operate on windows inside your process.

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Edit: Using bdk's answer I added these helpers to my code to get the Window

bool searchHelper(Display* display, Window w, Atom& atomPID, unsigned long pid, Window& result)
{
    bool ret = false;

    Atom atomType;
    int format;
    unsigned long nItems;
    unsigned long bytesAfter;
    unsigned char* propPID = 0;
    if (Success == XGetWindowProperty(display,w,atomPID,0,1,False,XA_CARDINAL,&atomType,&format,&nItems,&bytesAfter,&propPID))
    {
        if (propPID != 0)
        {
            if (pid == *((unsigned long *)propPID))
            {
                result = w;
                ret = true;
            }
            XFree(propPID);
        }
    }

    if (ret)
        return ret; //we found we can stop

    //check the children of the window
    Window wRoot;
    Window wParent;
    Window *wChild=NULL;
    unsigned nChildren=0;
    if (XQueryTree(display, w, &wRoot, &wParent, &wChild, &nChildren) != 0 )
    {
        for (unsigned i=0; i<nChildren; ++i)
        {
            ret = searchHelper(display, wChild[i], atomPID, pid, result);
            if (ret)
                break;
        }
    }
    return ret;
}

bool getWindowFromPid(unsigned long pid, Display* display, Window& result)
{
    Window window = XDefaultRootWindow(display);
    Atom atomPID = XInternAtom(display, "_NET_WM_PID", true);
    if (atomPID == None)
    {
        qDebug("XInternAtom failure");
        return false;
    }
    return searchHelper(display, window, atomPID, pid, result);
}

Now I get the window successfully, but when I do the following

if (getWindowFromPid(pid,display,window))
{
    qDebug("Found window ID:%d", window);
    int result = XRaiseWindow(display,window);
    qDebug("XRaiseWindow returned:%d", result);
}

XRaiseWindow returns 1 (BadRequest). The documentation for XRaiseWindow does not mention the return code of BadRequest being a possible result. I'm not sure what is wrong. Am I not allowed to call it for windows in a different process? Is this focus steeling prevention hampering me? Any thoughts?

Edit edit:

So looking at what xwininfo.c does when you call it with -frame I changed my code as follows based on bdk's suggestion.

if (getWindowFromPid(pid,display,window))
    {
        qDebug("Found window ID:%d", window);

        //Need the windowmanger frame (or parent) id not window id
        Window root, parent;
        Window *childlist;
        unsigned int ujunk;
        int status = XQueryTree(display, window, &root, &parent, &childlist, &ujunk);
        if (status && parent && parent != root)
        {
            qDebug("Found frame window ID:%d",parent);
            window = parent;
        }

        XSetWindowAttributes xswa;
        xswa.override_redirect=True;
        int result = XChangeWindowAttributes (display,window,CWOverrideRedirect,&xswa);
        qDebug("XChangeWindowAttributes returned:%d", result);
        result = XRaiseWindow(display,window);
        qDebug("XRaiseWindow returned:%d", result);
    }
    else
        qDebug("unable to find the window for the pid");

At this point I do find the window frame ID, but I get a return code of "1" from both XChangeWindowAttributes and XRaiseWindow. Am I just not allowed to modify another process' window?

Answer 1

I haven't tried this myself, but putting these two methods together may work:

The XRaiseWindow API Call in xlib lets you raise a Window to the front if you know the Window ID.

http://www.unix.com/man-page/Linux/3/XRaiseWindow/

This stackoverflow answer explains how to get a Window ID from a PID:

How to get an X11 Window from a Process ID?

EDIT:

I've had limited success with XRaiseWindow. The Following program does work under twm window manager, but not ion which I usually use. The Window Manager must have ways of preventing applications from 'popping up'. To make this work, i also had to pass it the Window ID of the Window Manager's frame for the window, not the window itself. run xwininfo -frame and click on the window and you get the frame ID instead, compile this program with gcc test.c -lX and pass it that hexid on the command line and it will raise the window.

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

 int main(int argc, char **argv)
 {
   Display *dsp = XOpenDisplay(NULL);
   long id = strtol(argv[1], NULL, 16);
   XSetWindowAttributes xswa;
   xswa.override_redirect=True;
   XChangeWindowAttributes (dsp,id,CWOverrideRedirect, &xswa);
   XRaiseWindow ( dsp, id );
   XCloseDisplay ( dsp );
 }