display-available.c

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////////////////////////////////////////////////////////////////////////
//
// Copyright (C) 2012-2025 The Octave Project Developers
//
// See the file COPYRIGHT.md in the top-level directory of this
// distribution or <https://octave.org/copyright/>.
//
// This file is part of Octave.
//
// Octave is free software: you can redistribute it and/or modify it
// under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// Octave is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with Octave; see the file COPYING.  If not, see
// <https://www.gnu.org/licenses/>.
//
////////////////////////////////////////////////////////////////////////
 
#if defined (HAVE_CONFIG_H)
#  include "config.h"
#endif
 
#include <stdlib.h>
 
#if defined (OCTAVE_USE_WINDOWS_API)
#  include <windows.h>
#elif defined (HAVE_FRAMEWORK_CARBON)
#  include <Carbon/Carbon.h>
#elif defined (HAVE_X_WINDOWS)
#  include <X11/Xlib.h>
#endif
 
#include "display-available.h"
 
const char *
display_available (int *dpy_avail)
{
  *dpy_avail = 0;
 
  const char *err_msg = "";
 
#if defined (OCTAVE_USE_WINDOWS_API)
 
  HDC hdc = GetDC (0);
 
  if (hdc)
    *dpy_avail = 1;
  else
    err_msg = "no graphical display found";
 
#elif defined (HAVE_FRAMEWORK_CARBON)
 
  CGDirectDisplayID display = CGMainDisplayID ();
 
  if (display)
    *dpy_avail = 1;
  else
    err_msg = "no graphical display found";
 
#elif defined (HAVE_X_WINDOWS)
 
  const char *display_name = getenv ("DISPLAY");
 
  if (display_name && *display_name)
    {
      Display *display = XOpenDisplay (display_name);
 
      if (display)
        {
          Screen *screen = DefaultScreenOfDisplay (display);
 
          if (! screen)
            err_msg = "X11 display has no default screen";
 
          XCloseDisplay (display);
 
          *dpy_avail = 1;
        }
      else
        err_msg = "unable to open X11 DISPLAY";
    }
  else
    err_msg = "X11 DISPLAY environment variable not set";
 
#else
 
  err_msg = "no graphical display found";
 
#endif
 
  return err_msg;
}