FAQ

FAQ

Frequently asked questions and troubleshooting.

General

What is ZARSharp?

A pure-C# port of the ZArchive 0.1.2 library: directory-tree archives with per-block zstd compression. It also bundles a complete dependency-free RFC 8878 zstd codec, a zeekstd-compatible seekable zstd implementation, and a batch pipeline engine.

Why "byte-identical" output? Why does it matter?

Archives created by ZARSharp are bit-for-bit the same as those created by the original C++ zarchive.exe for the same API call sequence. This means:

• Files pack to the exact same size, no regression risk when switching implementations
• Existing tooling, hash checks and distribution workflows keep working
• Any .zar from the ecosystem extracts here, and vice versa

The test suite enforces this with thousands of parity vectors and committed golden files.

What are the runtime requirements?

.NET 8.0, 9.0, or 10.0. No native dependencies, no unsafe code on the zstd path, BCL only. The library is trimmable and AOT-compatible.

Is it production ready?

The port is complete (Steps 1–6 of the port plan), with 3800+ green tests including native-tool parity matrices and committed goldens. Performance is the main known gap (see Benchmarks).

Compatibility

Can zarchive.exe read my archives? Can I read zarchive.exe archives?

Yes, both directions. The suite tests interop both ways, and a native-packed .zar is committed as a golden artifact.

Can standard zstd tools read the standalone frames?

Yes — frames follow RFC 8878 and decode with the official zstd CLI. Conversely, ZARSharp decodes anything standard tools produce at levels 1–22 (no dictionaries, no legacy frames).

Why does my output differ from zarchive.exe in rare cases?

The shipped zarchive.exe bundles libzstd 1.5.2; ZARSharp targets the frozen 1.5.7 reference. On some multi-transition heterogeneous 64 KiB blocks, level 6 differs between those libzstd versions (a 1.5.2-vs-1.5.7 upstream change, not a bug). Homogeneous blocks are identical; extract interops both ways regardless.

Can I read seekable files made by zeekstd / the C reference?

Yes. The two write different flavors (zeekstd sets the frame content-checksum flag, the C library writes plain frames) — both are valid and both decode here. Our writer emits the zeekstd flavor by default.

Usage

Which compression level should I pick?

Goal	Level
Match zarchive.exe default	6 (the default)
Maximum speed	1–3
Balanced	6–9
Small archives, time budget exists	12–15
Archival, size over everything	19–22

See Zstd-Compression#level-selection-guidelines.

How do I make archives reproducible?

ZArchiveTool.Pack sorts entries ordinally by default (deterministicOrder: true) — same input tree yields byte-identical output. Do not combine with Checksum/custom compressors if you need stable bytes across machines.

How do I store files without compression?

ZArchiveTool.Pack(dir, out, compressor: new ZarRawCompressor());
// or CLI: zar --no-compress <dir>

How do I get native-libzstd-class speed?

Plug a block compressor into IZarBlockCompressor (e.g. a native interop adapter). Note byte parity with zarchive.exe is only guaranteed with the built-in ZstdCompressor — custom compressors are opt-in for a reason.

Does it support multi-threaded compression?

Inside one frame, no (like upstream). Across items, yes: the pipeline packs batches in parallel (MaxDegreeOfParallelism). Each 64 KiB block is an independent frame, so batch parallelism scales cleanly.

Are dictionaries supported?

No — zstd dictionaries are out of scope. Negative levels are also rejected (locked by tests).

Errors

ZArchiveReader.TryOpen returned null — what do I do?

TryOpen never throws; null means the file failed validation (bad magic/version, hash mismatch, truncated, malformed tables). Common causes:

• The file is not a .zar at all (check for a download wrapper, e.g. HTML)
• Truncated transfer — re-download; truncations always fail the open
• Newer format revision (0.1.2 only is supported)

Flipping bytes in an archive doesn't always throw — why?

Data blocks carry no per-block checksums (same as native). The footer SHA-256 catches most corruption, but a flip inside a data block may simply decode to different bytes. If integrity matters, hash your files independently.

My pack fails with exit code -16 / an IOException

-16 (PackOutputFailed) is an output I/O error — disk full, permission denied, or the destination vanished mid-run. ZARSharp fails the pack here where native zarchive.exe would silently pack a truncated file (a documented intentional deviation).

Extraction fails with -12 (ExtractionFailed)

The archive is corrupt or the I/O failed mid-extract. Extraction lines printed before the failure show how far it got (preorder, including directories).

Project

What is the license?

MIT. The frozen reference sources it ports from (ZArchive, libzstd, zeekstd, ZarManager) are used as specification/oracle only; see LICENSE.

How do I contribute?

Open an issue or PR on the issue tracker. If you touch compression logic, keep byte parity: the parity tests and goldens in ZARSharp.Tests/Goldens/ must stay green, and CI holds the line with no native toolchain installed.

How are versions managed?

Versions derive from git tags via MinVer (v2.7.1-style tags). Every push builds and tests on Ubuntu/Windows/macOS; tag pushes publish to NuGet.