lz4_flex 0.11.1

Fastest LZ4 implementation in Rust, no unsafe by default.

Rust Docs Crates.io



Fastest LZ4 implementation in Rust. Originally based on redox-os' lz4 compression, but now a complete rewrite. The results in the table are from a benchmark in this project (66Kb JSON, 10MB dickens) with the block format.

AMD Ryzen 7 5900HX, rustc 1.69.0 (84c898d65 2023-04-16), Manjaro, CPU Boost Disabled, CPU Governor: Performance


Compressor Compression Decompression Ratio
lz4_flex unsafe w. unchecked_decode 1615 MiB/s 5973 MiB/s 0.2284
lz4_flex unsafe 1615 MiB/s 5512 MiB/s 0.2284
lz4_flex safe 1272 MiB/s 4540 MiB/s 0.2284
lzzz (lz4 1.9.3) 1469 MiB/s 5313 MiB/s 0.2283
lz4_fear 662 MiB/s 939 MiB/s 0.2283
snap 1452 MiB/s 1649 MiB/s 0.2242

10 Mb dickens

Compressor Compression Decompression Ratio
lz4_flex unsafe w. unchecked_decode 347 MiB/s 3168 MiB/s 0.6372
lz4_flex unsafe 347 MiB/s 2734 MiB/s 0.6372
lz4_flex safe 259 MiB/s 2338 MiB/s 0.6372
lzzz (lz4 1.9.3) 324 MiB/s 2759 MiB/s 0.6372
lz4_fear 201 MiB/s 370 MiB/s 0.6372
snap 286 MiB/s 679 MiB/s 0.6276


  • Very good logo
  • LZ4 Block format
  • LZ4 Frame format (thanks @arthurprs)
  • High performance
  • 1,5s clean release build time
  • Feature flags to configure safe/unsafe code usage
  • no-std support with block format (thanks @coolreader18)
  • 32-bit support


Compression and decompression uses no usafe via the default feature flags "safe-encode" and "safe-decode". If you need more performance you can disable them (e.g. with no-default-features).


lz4_flex = { version = "0.11" }


lz4_flex = { version = "0.11", default-features = false }

Block Format

The block format is only valid for smaller data chunks as as block is de/compressed in memory. For larger data use the frame format, which consists of multiple blocks.

use lz4_flex::block::{compress_prepend_size, decompress_size_prepended};

fn main(){
    let input: &[u8] = b"Hello people, what's up?";
    let compressed = compress_prepend_size(input);
    let uncompressed = decompress_size_prepended(&compressed).unwrap();
    assert_eq!(input, uncompressed);

no_std support

no_std support is currently only for the block format, since the frame format uses std::io::Write, which is not available in core.


The benchmark is run with criterion, the test files are in the benches folder.

Currently 4 implementations are compared, this one, lz-fear, the c version via rust bindings and snappy. The lz4-flex version is tested with the feature flags safe-decode and safe-encode switched on and off.

Tested on AMD Ryzen 7 5900HX, rustc 1.69.0 (84c898d65 2023-04-16), Manjaro, CPU Boost Disabled, CPU 3GHZ

Results v0.11.0 02-06-2023 (safe-decode and safe-encode off)

cargo bench --no-default-features



Results v0.11.0 02-06-2023 (safe-decode and safe-encode on)

cargo bench




Miri can be used to find issues related to incorrect unsafe usage:

MIRIFLAGS="-Zmiri-disable-isolation -Zmiri-disable-stacked-borrows" cargo +nightly miri test --no-default-features --features frame


This fuzz target generates corrupted data for the decompressor. cargo +nightly fuzz run fuzz_decomp_corrupt_block and cargo +nightly fuzz run fuzz_decomp_corrupt_frame

This fuzz target asserts that a compression and decompression rountrip returns the original input. cargo +nightly fuzz run fuzz_roundtrip and cargo +nightly fuzz run fuzz_roundtrip_frame

This fuzz target asserts compression with cpp and decompression with lz4_flex returns the original input. cargo +nightly fuzz run fuzz_roundtrip_cpp_compress

Bindings in other languages


  • High compression

Migrate from v0.10 to v0.11.1

To migrate, just remove the checked-decode feature flag if you used it.