.LZMA

LZMA compressed file

Introduction

LZMA is the compression method Igor Pavlov created for 7-Zip’s 7z archives. An .lzma file is a single file compressed with it, in the simple container from his LZMA SDK, known as LZMA_Alone. LZMA Utils made it popular on Linux for a few years, but the container has no magic number and no integrity check, so it was replaced by xz from 2009. Old .tar.lzma files and some firmware images still use it.

Structure

Numbers are little-endian.

  1. 0Properties lc, lp and pb packed into one byte as (pb × 5 + lp) × 9 + lc1 byte
  2. 1Dictionary size4 bytes
  3. 5Uncompressed size or all ones if unknown8 bytes
  4. 13LZMA datathe rest
An .lzma file: 13-byte header, then the compressed data.

With the usual settings (lc 3, lp 0, pb 2) the first byte is 5D, which is the nearest thing the format has to a signature. When the size is unknown, the data ends with an end marker instead.

Links

Software

  • XZ Utils, whose unlzma and xz --format=lzma read and write it (macOS, Linux, Windows)
  • 7-Zip (Windows)

Developers

  • LZMA SDK, Igor Pavlov’s code in C, C++, C# and Java
  • lzma, Python standard library module, with FORMAT_ALONE

Reference

  • The LZMA specification, lzma-specification.txt, in the LZMA SDK