Yalantis/uCrop · error · CImgDisplayException

CImgDisplay::screen_width(): Failed to open X11 display.

Error message

CImgDisplay::screen_width(): Failed to open X11 display.

What it means

CImgDisplay::screen_width() queries the X server for the screen width; if no display is currently open it tries XOpenDisplay(NULL) itself, and throws CImgDisplayException when that connection fails. Same root causes as [26] but triggered by the static screen-size query.

Source

Thrown at ucrop/src/main/jni/CImg.h:10544

              colormap[index].green = (unsigned short)(g<<8);
              colormap[index].blue = (unsigned short)(b<<8);
              colormap[index++].flags = DoRed | DoGreen | DoBlue;
            }
      }
      }
      XStoreColors(X11_attr.display,cmap,colormap,256);
      delete[] colormap;
    }

    // Public functions.
    static int screen_width() {
      cimg::X11_attr &X11_attr = cimg::X11_attr::ref();
      Display *const dpy = X11_attr.display;
      int res = 0;
      if (!dpy) {
        Display *const _dpy = XOpenDisplay(0);
        if (!_dpy)
          throw CImgDisplayException("CImgDisplay::screen_width(): Failed to open X11 display.");
        res = DisplayWidth(_dpy,DefaultScreen(_dpy));
        XCloseDisplay(_dpy);
      } else {

#ifdef cimg_use_xrandr
        if (X11_attr.resolutions && X11_attr.curr_resolution)
          res = X11_attr.resolutions[X11_attr.curr_resolution].width;
        else res = DisplayWidth(dpy,DefaultScreen(dpy));
#else
        res = DisplayWidth(dpy,DefaultScreen(dpy));
#endif
      }
      return res;
    }

    static int screen_height() {
      cimg::X11_attr &X11_attr = cimg::X11_attr::ref();
      Display *const dpy = X11_attr.display;

View on GitHub (pinned to f788b534b4)

Solutions

1. Ensure DISPLAY is set and an X server is reachable before calling (xset -q as a probe)
2. Use Xvfb in headless/CI environments: Xvfb :99 & export DISPLAY=:99
3. Avoid the query in headless mode — hardcode/default the window size or read resolution from your own config

Example fix

// before
int w = cimg::CImgDisplay::screen_width(); // throws headless
// after
const char *dpy = getenv("DISPLAY");
int w = (dpy && *dpy) ? cimg::CImgDisplay::screen_width() : 1280; // fallback size

const char *d = getenv("DISPLAY");
bool xReady = d && *d && (system("xset -q > /dev/null 2>&1") == 0);

bool canQueryScreen() { const char *d = getenv("DISPLAY"); return d && *d; }

try { w = cimg::CImgDisplay::screen_width(); } catch (const cimg_library::CImgDisplayException &e) { w = 1280; // default fallback size
}

When it happens

Trigger: Calling cimg::CImgDisplay::screen_width() (or screen_height()) in an environment with no reachable X server: DISPLAY unset/invalid, headless CI/container, SSH without forwarding, or X server down.

Common situations: Code that queries screen size at startup to size windows, running on headless servers or in Docker; cron/systemd jobs without DISPLAY; Wayland sessions without XWayland.

Understand the failure class

Background: "environment variable is not set" and "Missing keys in environment" errors: what missing required env var messages mean and how to fix them — this error's family across 28 libraries.

Related errors

• CImgDisplay::assign(): Failed to open X11 display.
• CImgDisplay(): No display available.
• CImgDisplay::assign(): Invalid %u bits screen mode detected
• CImgDisplay::assign(): Max number of displays (512) already
• init_fullscreen(): Xrandr extension not supported by the X s

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/fa141536395ef3a4. Report an issue: GitHub.