Yalantis/uCrop · error · CImgIOException

cimg::stderr(): Reference to 'stderr' stream not allowed in…

Error message

cimg::stderr(): Reference to 'stderr' stream not allowed in R mode ('cimg_use_r' is defined).

What it means

cimg::stderr() in an R-mode build (cimg_use_r defined) throws CImgIOException when asked for the raw stderr FILE*, since R must own error/diagnostic output; writing to stderr directly bypasses R's error handling. It sets exception_mode(0) before throwing so CImg does not double-report.

Solutions

  1. Route diagnostics through R's REprintf/error() API instead of cimg::stderr().
  2. Adjust cimg::exception_mode() / output settings so code does not request stderr in R mode.
  3. Build without cimg_use_r if native stdio diagnostics are required.
  4. Catch CImgIOException around diagnostics-triggering calls.

Example fix

// before
std::FILE* err = cimg::stderr(); // throws in R mode
// after
#ifdef cimg_use_r
REprintf("%s", diag.c_str());
#else
std::fprintf(cimg::stderr(), "%s", diag.c_str());
#endif
Defensive patterns

Strategy: try-catch

Validate before calling

bool stderrAllowed() {
#ifndef cimg_use_r
  return true;
#else
  return false;
#endif
}

Type guard

bool stdioAvailable() { return
#ifndef cimg_use_r
  true;
#else
  false;
#endif
}

Try / catch

try {
  std::FILE* err = cimg::stderr();
} catch (CImgIOException& e) {
  // R mode: use REprintf / error() instead
}

Prevention

When it happens

Trigger: Calling cimg::stderr() directly, or triggering CImg code paths that report warnings/errors to stderr, inside a cimg_use_r build with throw_exception enabled.

Common situations: CImg exception/warning handlers in R-mode builds; porting command-line CImg tools into R; debug builds that log diagnostics to cimg::stderr().

Understand the failure class

Background: "unsupported platform" / "not supported on this platform" errors: what they mean and how to fix them — this error's family across 47 libraries.

Related errors

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/c4ff850f4fb6210c. Report an issue: GitHub.

Appendix: source

Thrown at ucrop/src/main/jni/CImg.h:69133

      return stdout;
#else
      if (throw_exception) {
        cimg::exception_mode(0);
        throw CImgIOException("cimg::stdout(): Reference to 'stdout' stream not allowed in R mode "
                              "('cimg_use_r' is defined).");
      }
      return 0;
#endif
    }

    inline FILE* _stderr(const bool throw_exception) {
#ifndef cimg_use_r
      cimg::unused(throw_exception);
      return stderr;
#else
      if (throw_exception) {
        cimg::exception_mode(0);
        throw CImgIOException("cimg::stderr(): Reference to 'stderr' stream not allowed in R mode "
                              "('cimg_use_r' is defined).");
      }
      return 0;
#endif
    }

    // Open a file (similar to std:: fopen(), but with wide character support on Windows).
    inline std::FILE *std_fopen(const char *const path, const char *const mode) {
      std::FILE *const res = std::fopen(path,mode);
      if (res) return res;
#if cimg_OS==2
      // Try alternative method, with wide-character string.
      int err = MultiByteToWideChar(CP_UTF8,0,path,-1,0,0);
      if (err) {
        CImg<wchar_t> wpath((unsigned int)err);
        err = MultiByteToWideChar(CP_UTF8,0,path,-1,wpath,err);
        if (err) { // Convert 'mode' to a wide-character string
          err = MultiByteToWideChar(CP_UTF8,0,mode,-1,0,0);

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