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 is a process that reduces the size of data files for efficient storage or transmission. It uses various algorithms to condense data by identifying and eliminating redundancy, which can often substantially decrease the size of the data without losing the original information.
There are two main types of file compression: lossless and lossy. Lossless compression allows the original data to be perfectly reconstructed from the compressed data, which is ideal for files where every bit of data is important, like text or database files. Common examples include ZIP and RAR file formats. On the other hand, lossy compression eliminates less important data to reduce file size more significantly, often used in audio, video, and image files. JPEGs and MP3s are examples where some data loss does not substantially degrade the perceptual quality of the content.
File compression is beneficial in a multitude of ways. It conserves storage space on devices and servers, lowering costs and improving efficiency. It also speeds up file transfer times over networks, including the internet, which is especially valuable for large files. Moreover, compressed files can be grouped together into one archive file, assisting in organization and easier transportation of multiple files.
However, file compression does have some drawbacks. The compression and decompression process requires computational resources, which could slow down system performance, particularly for larger files. Also, in the case of lossy compression, some original data is lost during compression, and the resultant quality may not be acceptable for all uses, especially professional applications that demand high quality.
File compression is a critical tool in today's digital world. It enhances efficiency, saves storage space and decreases download and upload times. Nonetheless, it comes with its own set of drawbacks in terms of system performance and risk of quality degradation. Therefore, it is essential to be mindful of these factors to choose the right compression technique for specific data needs.
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.