What causes "save_jxl(): Specified filename is (null)." in Yalantis/uCrop?

Pass a concrete path with a .jxl extension: img.save_jxl("out.jxl").

Yalantis/uCrop · error · CImgArgumentException

save_jxl(): Specified filename is (null).

Error message

save_jxl(): Specified filename is (null).

What it means

CImg's save_jxl() requires a destination filename; _save_jxl throws CImgArgumentException immediately when the filename pointer is null. JPEG XL saving in this code path is filename-based only — there is no FILE* overload to fall back on.

Solutions

Pass a concrete path with a .jxl extension: img.save_jxl("out.jxl").
Validate the filename (non-null, non-empty) before calling save_jxl and fail early.
Ensure the variable holding the path is actually initialized before the save call.

Example fix

// before
const char *out = config.outputPath; // may be null
img.save_jxl(out);
// after
if (!config.outputPath || !*config.outputPath) throw std::invalid_argument("outputPath required");
img.save_jxl(config.outputPath);

Defensive patterns

Strategy: validation

Validate before calling

if (!outPath || !*outPath) {
  throw std::invalid_argument("jxl output path is required");
}
img.save_jxl(outPath);

Prevention

Validate output paths at configuration load time.
Never store filenames as raw pointers that can be null.
Check strlen/emptiness before format-specific save calls.

When it happens

Trigger: Calling img.save_jxl(nullptr) or passing a null/empty-derived filename variable to _save_jxl.

Common situations: Filenames computed at runtime (from user input, config, or string operations) that end up null; also calling internal overloads directly with an uninitialized char*.

Related errors

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/84664b6c997ef54b. Report an issue: GitHub.

Appendix: source

Thrown at ucrop/src/main/jni/CImg.h:61766

#endif
    }

    //! Save image as a JPEG XL file.
    /**
      \param filename Filename, as a C-string.
      \param distance Sets the level for lossy compression: lower = higher quality.
        Range: 0 .. 25. 0.0 = mathematically lossless
      \param bytes_per_pixel Force the number of bytes per pixels for the saving, when possible.
    **/
    const CImg<T>& save_jxl(const char *const filename, const float distance=1.0f,
                            const unsigned int bytes_per_pixel=0) const {
      return _save_jxl(filename,distance,bytes_per_pixel);
    }

    const CImg<T>& _save_jxl(const char *const filename, const float distance=1.0f,
                             const unsigned int bytes_per_pixel=0) const {
      if (!filename)
        throw CImgArgumentException(_cimg_instance
                                    "save_jxl(): Specified filename is (null).",
                                    cimg_instance);
      if (_spectrum > 4)
        throw CImgArgumentException(_cimg_instance
                                    "save_jxl(): JPEG XL only supports at most 4 channels.",
                                    cimg_instance);
      double stmin, stmax = (double)max_min(stmin);
      if (stmin<0 || (bytes_per_pixel==1 && stmax>=256) || stmax>=65536)
        cimg::warn(_cimg_instance
                   "save_jxl(): Instance has pixel values in [%g,%g], probable type overflow in file '%s'.",
                   cimg_instance,
                   stmin,stmax,filename);
      if (bytes_per_pixel>2 || sizeof(T)<bytes_per_pixel)
        throw CImgArgumentException(_cimg_instance
                                    "save_jxl(): bytes_per_pixel must be in [0, 2] and less than or equal to sizeof(T)",
                                    cimg_instance);
      if (_depth>1)
        cimg::warn(_cimg_instance

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