I just found https://www.opengl.org/discussion_boards/showthread.php/157014-Compiling-with-gcc, and I modified the code to get rid of the 100% CPU hog; it seems to work fine on Ubuntu 14.04 - simply run it from a terminal, draw your rectangle area, and the program will exit, printing the coordinates in the terminal:

xrectsel.c:

#include<stdio.h>

#include<stdlib.h>

#include<X11/Xlib.h>

#include<X11/cursorfont.h>

#include<unistd.h> // added for sleep/usleep



// original from [https://bbs.archlinux.org/viewtopic.php?id=85378 Select a screen area with mouse and return the geometry of this area? / Programming & Scripting / Arch Linux Forums]

// build with (Ubuntu 14.04):

// gcc -Wall xrectsel.c -o xrectsel -lX11



int main(void)

{

  int rx = 0, ry = 0, rw = 0, rh = 0;

  int rect_x = 0, rect_y = 0, rect_w = 0, rect_h = 0;

  int btn_pressed = 0, done = 0;



  XEvent ev;

  Display *disp = XOpenDisplay(NULL);



  if(!disp)

    return EXIT_FAILURE;



  Screen *scr = NULL;

  scr = ScreenOfDisplay(disp, DefaultScreen(disp));



  Window root = 0;

  root = RootWindow(disp, XScreenNumberOfScreen(scr));



  Cursor cursor, cursor2;

  cursor = XCreateFontCursor(disp, XC_left_ptr);

  cursor2 = XCreateFontCursor(disp, XC_lr_angle);



  XGCValues gcval;

  gcval.foreground = XWhitePixel(disp, 0);

  gcval.function = GXxor;

  gcval.background = XBlackPixel(disp, 0);

  gcval.plane_mask = gcval.background ^ gcval.foreground;

  gcval.subwindow_mode = IncludeInferiors;



  GC gc;

  gc = XCreateGC(disp, root,

                 GCFunction | GCForeground | GCBackground | GCSubwindowMode,

                 &gcval);



  /* this XGrab* stuff makes XPending true ? */

  if ((XGrabPointer

       (disp, root, False,

        ButtonMotionMask | ButtonPressMask | ButtonReleaseMask, GrabModeAsync,

        GrabModeAsync, root, cursor, CurrentTime) != GrabSuccess))

    printf("couldn't grab pointer:");



  if ((XGrabKeyboard

       (disp, root, False, GrabModeAsync, GrabModeAsync,

        CurrentTime) != GrabSuccess))

    printf("couldn't grab keyboard:");



  // see also: http://stackoverflow.com/questions/19659486/xpending-cycle-is-making-cpu-100

  while (!done) {

    //~ while (!done && XPending(disp)) {

      //~ XNextEvent(disp, &ev);

    if (!XPending(disp)) { usleep(1000); continue; } // fixes the 100% CPU hog issue in original code

    if ( (XNextEvent(disp, &ev) >= 0) ) {

      switch (ev.type) {

        case MotionNotify:

        /* this case is purely for drawing rect on screen */

          if (btn_pressed) {

            if (rect_w) {

              /* re-draw the last rect to clear it */

              XDrawRectangle(disp, root, gc, rect_x, rect_y, rect_w, rect_h);

            } else {

              /* Change the cursor to show we're selecting a region */

              XChangeActivePointerGrab(disp,

                                       ButtonMotionMask | ButtonReleaseMask,

                                       cursor2, CurrentTime);

            }

            rect_x = rx;

            rect_y = ry;

            rect_w = ev.xmotion.x - rect_x;

            rect_h = ev.xmotion.y - rect_y;



            if (rect_w < 0) {

              rect_x += rect_w;

              rect_w = 0 - rect_w;

            }

            if (rect_h < 0) {

              rect_y += rect_h;

              rect_h = 0 - rect_h;

            }

            /* draw rectangle */

            XDrawRectangle(disp, root, gc, rect_x, rect_y, rect_w, rect_h);

            XFlush(disp);

          }

          break;

        case ButtonPress:

          btn_pressed = 1;

          rx = ev.xbutton.x;

          ry = ev.xbutton.y;

          break;

        case ButtonRelease:

          done = 1;

          break;

      }

    }

  }

  /* clear the drawn rectangle */

  if (rect_w) {

    XDrawRectangle(disp, root, gc, rect_x, rect_y, rect_w, rect_h);

    XFlush(disp);

  }

  rw = ev.xbutton.x - rx;

  rh = ev.xbutton.y - ry;

  /* cursor moves backwards */

  if (rw < 0) {

    rx += rw;

    rw = 0 - rw;

  }

  if (rh < 0) {

    ry += rh;

    rh = 0 - rh;

  }



  XCloseDisplay(disp);



  printf("%dx%d+%d+%d

",rw,rh,rx,ry);



  return EXIT_SUCCESS;

}