I extracted the X selection making code from xclip and it resulted in this gist.

scrot-select.c

/*
   Code is taken from scrot, and is currently having some
   redundant code in it.

   Permission is hereby granted, free of charge, to any person obtaining a copy
   of this software and associated documentation files (the "Software"), to
   deal in the Software without restriction, including without limitation the
   rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
   sell copies of the Software, and to permit persons to whom the Software is
   furnished to do so, subject to the following conditions:

   The above copyright notice and this permission notice shall be included in
   all copies of the Software and its documentation and acknowledgment shall be
   given in the documentation and software packages that this Software was
   used.

   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
   IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
   THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
   IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
   CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

 * compile with -O2 -L/usr/X11R6/lib -lX11 -L/usr/lib -lgiblib -L/usr/lib -lImlib2 -lfreetype -lz -L/usr/lib -lX11 -lXext -ldl -lm -lm
 */
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <X11/Xatom.h>
#include <X11/Xos.h>
#include <X11/keysym.h>
#include <X11/Xresource.h>
#include <X11/cursorfont.h>

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <ctype.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/time.h>
#include <errno.h>
#include <dirent.h>
#include <stdarg.h>
#include <time.h>
#include <signal.h>
#include <sys/wait.h>

/* Imlib stuff */
Display *disp;
Visual *vis;
Colormap cm;
int depth;

/* Thumbnail sizes */
Window root;
Screen *scr;

Window
scrot_get_window(Display * display,
                 Window window,
                 int x,
                 int y)
{
  Window source, target;

  int status, x_offset, y_offset;

  source = root;
  target = window;
  if (window == None)
    window = root;
  while (1) {
    status =
      XTranslateCoordinates(display, source, window, x, y, &x_offset,
                            &y_offset, &target);
    if (status != True)
      break;
    if (target == None)
      break;
    source = window;
    window = target;
    x = x_offset;
    y = y_offset;
  }
  if (target == None)
    target = window;
  return (target);
}

/* Taken from scrot's main.c, formely caleld scrot_sel_and_grab_image */
int
main(void)
{
   disp = XOpenDisplay(NULL);
   if (!disp)
      printf("Can't open X display. It *is* running, yeah?");
    scr = ScreenOfDisplay(disp, DefaultScreen(disp));

   vis = DefaultVisual(disp, XScreenNumberOfScreen(scr));
   depth = DefaultDepth(disp, XScreenNumberOfScreen(scr));
   cm = DefaultColormap(disp, XScreenNumberOfScreen(scr));
   root = RootWindow(disp, XScreenNumberOfScreen(scr));

   imlib_context_set_display(disp);

  static int xfd = 0;
  static int fdsize = 0;
  XEvent ev;
  fd_set fdset;
  int count = 0, done = 0;
  int rx = 0, ry = 0, rw = 0, rh = 0, btn_pressed = 0;
  int rect_x = 0, rect_y = 0, rect_w = 0, rect_h = 0;
  Cursor cursor, cursor2;
  Window target = None;
  GC gc;
  XGCValues gcval;

  xfd = ConnectionNumber(disp);
  fdsize = xfd + 1;

  cursor = XCreateFontCursor(disp, XC_left_ptr);
  cursor2 = XCreateFontCursor(disp, XC_lr_angle);

  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 =
    XCreateGC(disp, root,
              GCFunction | GCForeground | GCBackground | GCSubwindowMode,
              &gcval);

  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:");


  while (1) {
    /* handle events here */
    while (!done && XPending(disp)) {
      XNextEvent(disp, &ev);
      switch (ev.type) {
        case MotionNotify:
          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;
          target =
            scrot_get_window(disp, ev.xbutton.subwindow, ev.xbutton.x,
                             ev.xbutton.y);
          if (target == None)
            target = root;
          break;
        case ButtonRelease:
          done = 1;
          break;
        case KeyPress:
          fprintf(stderr, "Key was pressed, aborting shot\n");
          done = 2;
          break;
        case KeyRelease:
          /* ignore */
          break;
        default:
          break;
      }
    }
    if (done)
      break;

    /* now block some */
    FD_ZERO(&fdset);
    FD_SET(xfd, &fdset);
    errno = 0;
    count = select(fdsize, &fdset, NULL, NULL, NULL);
    if ((count < 0)
        && ((errno == ENOMEM) || (errno == EINVAL) || (errno == EBADF)))
      printf("Connection to X display lost");
  }
  if (rect_w) {
    XDrawRectangle(disp, root, gc, rect_x, rect_y, rect_w, rect_h);
    XFlush(disp);
  }
  XUngrabPointer(disp, CurrentTime);
  XUngrabKeyboard(disp, CurrentTime);
  XFreeCursor(disp, cursor);
  XFreeGC(disp, gc);
  XSync(disp, True);


  if (done < 2) {
    if (rect_w > 5) {
      /* if a rect has been drawn, it's an area selection */
      rw = ev.xbutton.x - rx;
      rh = ev.xbutton.y - ry;

      if (rw < 0) {
        rx += rw;
        rw = 0 - rw;
      }
      if (rh < 0) {
        ry += rh;
        rh = 0 - rh;
      }
    } else {
      Window child;
      XWindowAttributes attr;
      int stat;

      /* else it's a window click */
      /* get geometry of window and use that */
      /* get windowmanager frame of window */
      if (target != root) {
        unsigned int d, x;
        int status;

        status = XGetGeometry(disp, target, &root, &x, &x, &d, &d, &d, &d);
        if (status != 0) {
          Window rt, *children, parent;

          for (;;) {
            /* Find window manager frame. */
            status = XQueryTree(disp, target, &rt, &parent, &children, &d);
            if (status && (children != None))
              XFree((char *) children);
            if (!status || (parent == None) || (parent == rt))
              break;
            target = parent;
          }
          /* Get client window. */
          if (/* !opt.border */ 0)
            target = scrot_get_client_window(disp, target);
          XRaiseWindow(disp, target);
        }
      }
      stat = XGetWindowAttributes(disp, target, &attr);
      if ((stat == False) || (attr.map_state != IsViewable))
        return -1;
      rw = attr.width;
      rh = attr.height;
      XTranslateCoordinates(disp, target, root, 0, 0, &rx, &ry, &child);
    }

    /* clip rectangle nicely */
    if (rx < 0) {
      rw += rx;
      rx = 0;
    }
    if (ry < 0) {
      rh += ry;
      ry = 0;
    }
    if ((rx + rw) > scr->width)
      rw = scr->width - rx;
    if ((ry + rh) > scr->height)
      rh = scr->height - ry;

    printf("x:%i\ny:%i\nw:%i\nh:%i\n", rx, ry, rw, rh);
  }

  return 0;
}