What Is .xar

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Last updated: April 11, 2026

Quick Answer: XAR (eXtensible ARchive) is an open-source archive format developed by Apple in 2004 and introduced in macOS 10.5 Leopard in 2007. It is primarily used for macOS software installation packages, browser extensions, and other compressed files, allowing each contained file to be independently compressed using different methods. Unlike traditional formats such as tar or ZIP, XAR uses a more flexible structure with a table of contents stored at the beginning of the file.

Key Facts

XAR was developed by Apple in 2004 and open-sourced in 2007 as a modern replacement for gzipped pax files
The format was introduced in macOS 10.5 Leopard in 2007 and became the basis for flat installer packages (.pkg files)
XAR files have a three-part structure: a binary header, a zlib-compressed XML table of contents, and a heap containing actual file data
Each file within a XAR archive can be independently compressed using different methods such as gzip or bzip2, providing superior flexibility
XAR is used for macOS software installations and Safari browser extensions (Safari 5.0+), though the archiver is no longer under active development

Overview

XAR, short for eXtensible ARchive format, is an open-source file archiver and compression format developed by Apple in 2004. It was officially introduced with macOS 10.5 Leopard in 2007 and has since become the default archive format for macOS systems. The format was designed as a modern replacement for traditional archive methods like gzipped pax files, offering greater flexibility and better support for complex file structures.

XAR was created within Apple's OpenDarwin project and was subsequently open-sourced in 2007, allowing developers outside the Apple ecosystem to utilize and modify the format. While the xar archiver tool is no longer under active development by Apple, the format remains widely used today, particularly for macOS software installation packages (.pkg files) and browser extensions such as those used in Safari 5.0 and later versions.

How It Works

XAR files are structured with three distinct sections that work together to create an efficient and flexible archive:

Header Structure: Every XAR file begins with a binary header containing the magic sequence "xar!" (0x78617221 in hexadecimal). This identifier allows systems to recognize and properly process the file as a valid XAR archive.
Table of Contents (TOC): Unlike tar or cpio archives, XAR stores its table of contents at the beginning of the file as a zlib-compressed, UTF-8 encoded XML document. This design eliminates the need to scan through the entire archive to locate and extract individual files, making random file access significantly faster.
Independent Compression: Each file contained within a XAR archive is independently compressed or encoded. This allows different files in the same archive to use different compression methods, such as gzip for one file and bzip2 for another, optimizing space and performance.
Heap Section: The heap is the final section of the XAR file and contains the actual compressed file data. The flexible structure allows for efficient storage and retrieval of various file types and formats.

Key Comparisons

Feature	XAR	ZIP	TAR
Table of Contents Location	Beginning (compressed XML)	End of file	No built-in TOC
Independent Compression	Yes, per-file compression	Per-file compression	Whole archive compression
Metadata Support	Extensive (XML-based)	Limited	Limited
Open Source Status	Open source (2007)	Open format	Open source
Primary Use Cases	macOS packages, Safari extensions	General cross-platform files	Unix/Linux system backups
File Access Speed	Fast (TOC at start)	Slow (TOC at end)	Sequential (must scan)

Why It Matters

macOS Ecosystem Integration: XAR is fundamental to how Apple distributes software updates and system components, making it essential for anyone working with macOS installation packages or managing software distribution on Apple systems.
Efficient Resource Management: The ability to use different compression algorithms for different files within a single archive provides optimal balance between file size and access speed, crucial for large software installations.
Metadata Preservation: XAR's XML-based table of contents preserves detailed file metadata and ownership information, which is critical for proper installation and system integration on macOS, including permission preservation and extended attributes.
Security and Integrity: The format supports cryptographic signatures and checksums, allowing software distributors to verify the authenticity and integrity of archived files during installation.

Although active development has ceased, XAR remains relevant and widely used within the Apple ecosystem. The format proved so effective for macOS package management that Apple chose to maintain the flat installer package format (.pkg) as a core part of macOS, even as the underlying archiver tool was sunset. Understanding XAR is valuable for macOS developers, system administrators, and anyone working with Apple software distribution and deployment.