save_cimg(): Failed to save compressed data for file '%s', s

Error message

save_cimg(): Failed to save compressed data for file '%s', saving them uncompressed.

What it means

During save_cimg() of compressed data (zlib enabled), the deflate compression of a tile/strip failed (failed_to_compress set). The library warns and writes the data uncompressed for that item instead, so the save still completes.

Source

Thrown at ucrop/src/main/jni/CImg.h:68199

              cbuf = new Bytef[csiz];
              failed_to_compress = (bool)compress(cbuf,&csiz,(Bytef*)buf,siz);
              if (!failed_to_compress) {
                std::fprintf(nfile," #%lu\n",csiz);
                cimg::fwrite(cbuf,csiz,nfile);
              }
              delete[] buf;
            } else { // Non-boolean data
              const ulongT siz = sizeof(T)*ref.size();
              csiz = siz + siz/100 + 16;
              cbuf = new Bytef[csiz];
              failed_to_compress = (bool)compress(cbuf,&csiz,(Bytef*)ref._data,siz);
              if (!failed_to_compress) {
                std::fprintf(nfile," #%lu\n",csiz);
                cimg::fwrite(cbuf,csiz,nfile);
              }
            }
            if (failed_to_compress)
              cimg::warn(_cimglist_instance
                         "save_cimg(): Failed to save compressed data for file '%s', saving them uncompressed.",
                         cimglist_instance,
                         filename?filename:"(FILE*)");
            delete[] cbuf;
#endif
          }
          if (failed_to_compress) { // Write non-compressed
            std::fputc('\n',nfile);
            if (is_bool) { // Boolean data (bitwise)
              ulongT siz;
              const unsigned char *const buf = ref._bool2uchar(siz,false);
              cimg::fwrite(buf,siz,nfile);
              delete[] buf;
            } else cimg::fwrite(ref._data,ref.size(),nfile); // Non-boolean data
          }
        } else std::fputc('\n',nfile);
      }
      if (!file) cimg::fclose(nfile);

View on GitHub (pinned to f788b534b4)

Solutions

Check available memory; compress smaller images or reduce resolution before saving
Verify the zlib library is correctly built/linked (zlibVersion() sanity check)
Save uncompressed (is_compressed=false) if compression keeps failing
Free other allocations before saving to avoid compress() failing on allocation

Example fix

// before
images.save_cimg("out.cimgz", true);
// after
try {
  images.save_cimg("out.cimgz", true);
} catch (...) { /* may have fallen back to uncompressed */ }
// or avoid compression entirely:
images.save_cimg("out.cimg", false);

Defensive patterns

Strategy: fallback

Validate before calling

// preflight zlib
if (zlibVersion() == nullptr) { /* zlib broken: save uncompressed */ images.save_cimg("out.cimg", false); }

Prevention

Ensure adequate memory for compression buffers
Sanity-check zlib linkage (zlibVersion) at startup
Fall back to uncompressed saving when compression is unreliable

When it happens

Trigger: cimg_use_zlob... i.e. zlib-enabled build where compress()/deflate returns Z_* error — typically out-of-memory for the compression buffer, or zlib returning an error on very large buffers; internal buffer allocation failure for cbuf.

Common situations: Very large images exhausting memory during compression; corrupted/limited zlib installation; resource-constrained embedded environments (Android NDK builds of uCrop).

Understand the failure class

Background: "failed to write file", "Could not save figure", "Error saving remote file" — file write failed: causes and fixes across languages and libraries — this error's family across 38 libraries.

Related errors

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/0761cc3b52a5d026. Report an issue: GitHub.