XRandR Verify Active Rate

[re.mcclue]

A XRRScreenResources contains multiple XRRModeInfo and RROutput (which with XRRGetOutputInfo() effectively means XRROutputInfo). Therefore, a screen can have many outputs which in turn, can have many modes. The rate is a property of the mode. Because of this many-to-many relationship, how can I determine what the active rate is?
In other words:

XRRScreenResources *res = XRRGetScreenResources(display, default_root_window);
  for (int i = 0; i < res->nmode; ++i) {
    XRRModeInfo *mode_info = &res->modes[i];
    // How do you verify this is the active rate?
    double rate = (double)mode_info->dotClock / ((double)mode_info->hTotal * (double)mode_info->vTotal));
  }

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