lz4_flex 0.9.3

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) with the block format.

AMD Ryzen 7 5800H, rustc 1.57.0-nightly (5ecc8ad84 2021-09-19), Linux Mint.

Compressor Compression Decompression Ratio
lz4_flex unsafe 1897 MiB/s 7123 MiB/s 0.2289
lz4_flex unsafe w. checked_decode 1897 MiB/s 6637 MiB/s 0.2289
lz4_flex safe 1591 MiB/s 5163 MiB/s 0.2289
lzzz (lz4 1.9.3) 2235 MiB/s 7001 MiB/s 0.2283
lz4_fear 886 MiB/s 1359 MiB/s 0.2283
snap 1886 MiB/s 1649 MiB/s 0.2242


  • 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 = "0.9.3" }


lz4_flex = { version = "0.9.3", default-features = false }

Warning: If you don't trust your input and your are using the Block format, use checked-decode in order to avoid out of bounds access. When using the Frame format make sure to enable checksums.

lz4_flex = { version = "0.9.3", default-features = false, features = ["checked-decode"] }
use lz4_flex::{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 and 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.

Results v0.8.0 17-05-2021 (safe-decode and safe-encode off)

cargo bench --no-default-features

Executed on Core i7-6700 Linux Mint.



Results v0.8.0 17-05-2021 (safe-decode and safe-encode on)

cargo bench

Executed on Core i7-6700 Linux Mint.




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. Make sure to switch to the checked_decode version in fuzz/Cargo.toml before testing this. 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