XAR (eXtensible ARchive) is a file format developed by Apple Inc. for bundling and distributing software on macOS. It serves as a replacement for older formats like .pkg and .dmg, offering several advantages such as improved security, smaller file sizes, and better performance. XAR files use the .xar file extension and can be created and extracted using the xar command-line utility included with macOS.
The XAR format is based on the XML (eXtensible Markup Language) standard. An XAR archive consists of three main components: a table of contents (TOC) in XML format describing the archive's contents, the actual files and directories stored in the archive, and digital signatures for security. The TOC acts as an index, specifying the path, size, and other metadata for each file in the archive. This XML-based structure allows for extensibility, as Apple or third parties can add custom tags to support new features.
One key aspect of the XAR format is its use of compression. By default, XAR uses zlib compression to reduce the size of the archived files. The TOC itself is also compressed. This results in smaller archive sizes compared to older formats like .pkg, which store files uncompressed. However, XAR also supports storing files uncompressed if desired. The compression applied to each file can be specified individually in the TOC.
To ensure the integrity and authenticity of XAR archives, the format incorporates digital signatures. Each XAR file includes one or more signatures that cover the entire TOC. These signatures are created using public-key cryptography, typically with RSA or DSA algorithms. The signatures allow recipients to verify that the archive hasn't been tampered with and that it originates from a trusted source. Apple uses XAR signatures for distributing software updates and applications on the Mac App Store.
When an XAR archive is opened, the TOC is first decompressed and parsed. The TOC provides a directory structure and file metadata, similar to the 'tar' format used on Unix systems. The actual file data is stored after the TOC in the archive. Each file's data can be compressed or uncompressed, as indicated by the corresponding entry in the TOC. To extract a file, its data is located using the offset and size information from the TOC.
The XAR format supports several advanced features beyond basic archiving. One such feature is the ability to include multiple TOCs in a single archive. This allows for creating incremental updates where only the changed files need to be included in the update archive. The multiple TOCs can describe the archive's state across different versions of the software. Smart updating mechanisms can use this information to apply incremental patches efficiently.
Additionally, XAR archives can store extended attributes and access control lists (ACLs) associated with the archived files. Extended attributes are key-value pairs that can store app-specific metadata. ACLs define granular permissions for accessing files. By preserving this information in the archive, XAR ensures that the original file attributes are restored when extracted on the target system.
The XAR format also includes provisions for code signing. In addition to the archive-level signatures covering the TOC, individual files within the archive can have their own signatures. This is useful for distributing software components that need to be independently verified. For example, a plugin architecture can use code signing to ensure that only trusted plugins are loaded by an application.
Another feature of XAR is its ability to store hard links. Hard links allow multiple directory entries to reference the same file data on disk. In the XAR TOC, hard links are represented using special XML elements that point to the original file entry. When the archive is extracted, the hard links are recreated, preserving disk space and maintaining the original directory structure.
To work with XAR archives programmatically, developers can use the xar command-line tool or libraries like libxar. The xar tool provides commands for creating, extracting, and manipulating XAR archives. It supports various options for compression, signing, and verification. Libxar is a C library that implements the XAR format and provides an API for reading and writing XAR archives. It allows developers to integrate XAR support into their own applications.
In summary, the XAR format offers a modern and extensible approach to software packaging and distribution on macOS. Its use of XML for the table of contents, compression for smaller file sizes, digital signatures for security, and support for advanced features like incremental updates and code signing make it a powerful tool for developers and system administrators. As Apple continues to improve and promote the format, XAR is likely to become the standard for software distribution on macOS.
File compression reduces redundancy so the same information takes fewer bits. The upper bound on how far you can go is governed by information theory: for lossless compression, the limit is the entropy of the source (see Shannon’s source coding theorem and his original 1948 paper “A Mathematical Theory of Communication”). For lossy compression, the trade-off between rate and quality is captured by rate–distortion theory.
Most compressors have two stages. First, a model predicts or exposes structure in the data. Second, a coder turns those predictions into near-optimal bit patterns. A classic modeling family is Lempel–Ziv: LZ77 (1977) and LZ78 (1978) detect repeated substrings and emit references instead of raw bytes. On the coding side, Huffman coding (see the original paper 1952) assigns shorter codes to more likely symbols. Arithmetic coding and range coding are finer-grained alternatives that squeeze closer to the entropy limit, while modern Asymmetric Numeral Systems (ANS) achieves similar compression with fast table-driven implementations.
DEFLATE (used by gzip, zlib, and ZIP) combines LZ77 with Huffman coding. Its specs are public: DEFLATE RFC 1951, zlib wrapper RFC 1950, and gzip file format RFC 1952. Gzip is framed for streaming and explicitly does not attempt to provide random access. PNG images standardize DEFLATE as their only compression method (with a max 32 KiB window), per the PNG spec “Compression method 0… deflate/inflate… at most 32768 bytes” and W3C/ISO PNG 2nd Edition.
Zstandard (zstd): a newer general-purpose compressor designed for high ratios with very fast decompression. The format is documented in RFC 8878 (also HTML mirror) and the reference spec on GitHub. Like gzip, the basic frame doesn’t aim for random access. One of zstd’s superpowers is dictionaries: small samples from your corpus that dramatically improve compression on many tiny or similar files (see python-zstandard dictionary docs and Nigel Tao’s worked example). Implementations accept both “unstructured” and “structured” dictionaries (discussion).
Brotli: optimized for web content (e.g., WOFF2 fonts, HTTP). It mixes a static dictionary with a DEFLATE-like LZ+entropy core. The spec is RFC 7932, which also notes a sliding window of 2WBITS−16 with WBITS in [10, 24] (1 KiB−16 B up to 16 MiB−16 B) and that it does not attempt random access. Brotli often beats gzip on web text while decoding quickly.
ZIP container: ZIP is a file archive that can store entries with various compression methods (deflate, store, zstd, etc.). The de facto standard is PKWARE’s APPNOTE (see APPNOTE portal, a hosted copy, and LC overviews ZIP File Format (PKWARE) / ZIP 6.3.3).
LZ4 targets raw speed with modest ratios. See its project page (“extremely fast compression”) and frame format. It’s ideal for in-memory caches, telemetry, or hot paths where decompression must be near RAM speed.
XZ / LZMA push for density (great ratios) with relatively slow compression. XZ is a container; the heavy lifting is typically LZMA/LZMA2 (LZ77-like modeling + range coding). See .xz file format, the LZMA spec (Pavlov), and Linux kernel notes on XZ Embedded. XZ usually out-compresses gzip and often competes with high-ratio modern codecs, but with slower encode times.
bzip2 applies the Burrows–Wheeler Transform (BWT), move-to-front, RLE, and Huffman coding. It’s typically smaller than gzip but slower; see the official manual and man pages (Linux).
“Window size” matters. DEFLATE references can only look back 32 KiB (RFC 1951 and PNG’s 32 KiB cap noted here). Brotli’s window ranges from about 1 KiB to 16 MiB (RFC 7932). Zstd tunes window and search depth by level (RFC 8878). Basic gzip/zstd/brotli streams are designed for sequential decoding; the base formats don’t promise random access, though containers (e.g., tar indexes, chunked framing, or format-specific indexes) can layer it on.
The formats above are lossless: you can reconstruct exact bytes. Media codecs are often lossy: they discard imperceptible detail to hit lower bitrates. In images, classic JPEG (DCT, quantization, entropy coding) is standardized in ITU-T T.81 / ISO/IEC 10918-1. In audio, MP3 (MPEG-1 Layer III) and AAC (MPEG-2/4) rely on perceptual models and MDCT transforms (see ISO/IEC 11172-3, ISO/IEC 13818-7, and an MDCT overview here). Lossy and lossless can coexist (e.g., PNG for UI assets; Web codecs for images/video/audio).
Theory: Shannon 1948 · Rate–distortion · Coding: Huffman 1952 · Arithmetic coding · Range coding · ANS. Formats: DEFLATE · zlib · gzip · Zstandard · Brotli · LZ4 frame · XZ format. BWT stack: Burrows–Wheeler (1994) · bzip2 manual. Media: JPEG T.81 · MP3 ISO/IEC 11172-3 · AAC ISO/IEC 13818-7 · MDCT.
Bottom line: choose a compressor that matches your data and constraints, measure on real inputs, and don’t forget the gains from dictionaries and smart framing. With the right pairing, you can get smaller files, faster transfers, and snappier apps — without sacrificing correctness or portability.
File compression is a process that reduces the size of a file or files, typically to save storage space or speed up transmission over a network.
File compression works by identifying and removing redundancy in the data. It uses algorithms to encode the original data in a smaller space.
The two primary types of file compression are lossless and lossy compression. Lossless compression allows the original file to be perfectly restored, while lossy compression enables more significant size reduction at the cost of some loss in data quality.
A popular example of a file compression tool is WinZip, which supports multiple compression formats including ZIP and RAR.
With lossless compression, the quality remains unchanged. However, with lossy compression, there can be a noticeable decrease in quality since it eliminates less-important data to reduce file size more significantly.
Yes, file compression is safe in terms of data integrity, especially with lossless compression. However, like any files, compressed files can be targeted by malware or viruses, so it's always important to have reputable security software in place.
Almost all types of files can be compressed, including text files, images, audio, video, and software files. However, the level of compression achievable can significantly vary between file types.
A ZIP file is a type of file format that uses lossless compression to reduce the size of one or more files. Multiple files in a ZIP file are effectively bundled together into a single file, which also makes sharing easier.
Technically, yes, although the additional size reduction might be minimal or even counterproductive. Compressing an already compressed file might sometimes increase its size due to metadata added by the compression algorithm.
To decompress a file, you typically need a decompression or unzipping tool, like WinZip or 7-Zip. These tools can extract the original files from the compressed format.