What causes "Error writing package dat file. " in nodejs/node?

Ensure the output directory exists and is writable by the build user.

Error writing package dat file.

Error message

Error writing package dat file.

What it means

writePackageDatFile() returned non-zero while emitting the assembled .dat. pkgdata prints the generic failure and propagates that result code. The error originates in the file write (I/O), not in option parsing — by this point all arguments were accepted.

Source

Thrown at deps/icu-small/source/tools/pkgdata/pkgdata.cpp:622

        uprv_strcpy(targetDir, o->targetDir);
        uprv_strcat(targetDir, PKGDATA_FILE_SEP_STRING);

        uprv_strcpy(tmpDir, o->tmpDir);
        uprv_strcat(tmpDir, PKGDATA_FILE_SEP_STRING);

        uprv_strcpy(datFileNamePath, tmpDir);

        uprv_strcpy(datFileName, o->shortName);
        uprv_strcat(datFileName, UDATA_CMN_SUFFIX);

        uprv_strcat(datFileNamePath, datFileName);

        if(o->verbose) {
          fprintf(stdout, "# Writing package file %s ..\n", datFileNamePath);
        }
        result = writePackageDatFile(datFileNamePath, o->comment, o->srcDir, o->fileListFiles->str, nullptr, U_CHARSET_FAMILY ? 'e' :  U_IS_BIG_ENDIAN ? 'b' : 'l');
        if (result != 0) {
            fprintf(stderr,"Error writing package dat file.\n");
            return result;
        }

        if (IN_COMMON_MODE(mode)) {
            char targetFileNamePath[LARGE_BUFFER_MAX_SIZE] = "";

            uprv_strcpy(targetFileNamePath, targetDir);
            uprv_strcat(targetFileNamePath, datFileName);

            /* Move the dat file created to the target directory. */
            if (uprv_strcmp(datFileNamePath, targetFileNamePath) != 0) {
                if (T_FileStream_file_exists(targetFileNamePath)) {
                    if ((result = remove(targetFileNamePath)) != 0) {
                        fprintf(stderr, "Unable to remove old dat file: %s\n",
                                targetFileNamePath);
                        return result;
                    }
                }

View on GitHub (pinned to 1b2de5e052)

Solutions

Ensure the output directory exists and is writable by the build user.
Free disk space on the output volume.
Run pkgdata with --verbose to see the exact datFileNamePath being written, then verify that location.

Defensive patterns

Strategy: validation

Validate before calling

# bash: writable output dir + free disk before writing the .dat
OUT_DIR="$(dirname "$OUT_DAT")"
[ -w "$OUT_DIR" ] || { echo "output dir not writable: $OUT_DIR" >&2; exit 2; }
dfree=$(df -k --output=avail "$OUT_DIR" | tail -1)
[ "$dfree" -gt 10240 ] || { echo "low disk on $OUT_DIR" >&2; exit 2; }

Try / catch

# treat a write failure as retryable after cleanup
pkgdata "$@" || { rc=$?; rm -f "$OUT_DAT".tmp; pkgdata "$@"; exit $?; }

Prevention

Create and chmod the output directory in the build setup step.
Monitor free disk on the output volume in CI.

When it happens

Trigger: Output directory missing or not writable, disk full, path too long, or permission denied during the write.

Common situations: Target directory not created yet; read-only build output; out of disk space; antivirus locking the file on Windows.

Related errors

AI-assisted analysis of nodejs/node@1b2de5e052 (2026-08-13). Data as JSON: /api/errors/f559d57b2d83f399. Report an issue: GitHub.