Makefile

xrrtest: xrrtest.cpp
	${CXX} -lX11 -lXinerama -lXrandr -o xrrtest xrrtest.cpp

xrrtest.cpp

#include <cstdlib>
#include <cstdio>

#include <X11/Xlib.h>
#include <X11/extensions/Xrandr.h>

bool get_width_height(XRRScreenResources *res, RRMode id,
                      unsigned int *width, unsigned int *height) {
  for (int i = 0; i < res->nmode; i++) {
    if (res->modes[i].id == id) {
      *width = res->modes[i].width;
      *height = res->modes[i].height;
      return true;
    }
  }
  return false;
}

int main(void) {
  Display *dpy = XOpenDisplay(NULL);
  if (!dpy) abort();

  XRRScreenResources *res = XRRGetScreenResources(dpy, RootWindow(dpy, 0));
  if (!res) abort();

  for (int output_number = 0; output_number < res->noutput; output_number++) {
    XRROutputInfo *output_info = XRRGetOutputInfo(dpy, res, res->outputs[output_number]);
    if (!output_info) abort();

    if (output_info->connection == RR_Connected) {
      printf("%s\n", output_info->name);
      for (int mode_number = 0; mode_number < output_info->nmode; mode_number++) {
        unsigned int width, height;
        if (get_width_height(res, output_info->modes[mode_number],
                             &width, &height))
        {
          printf("\t%dx%d\n", width, height);
        }
      }
    }

    XRRFreeOutputInfo(output_info);
  }
  XRRFreeScreenResources(res);

  return 0;
}