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Gzip compression |
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Gzip is a software application used for file compression. Gzip is short for GNU zip; the program is a free software replacement for the compress program used in early Unix systems, intended for use by the GNU Project. Gzip was created by Jean-Loup Gailly and Mark Adler. Version 0.1 was first publicly released on October 31, 1992. Version 1.0 followed in February 1993. OpenBSD's version of gzip is actually the compress program, to which support for the gzip format was added in OpenBSD 3.4 - the 'g' in this specific version stands for gratis. |
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Gzip is based on the DEFLATE algorithm, which is a combination of LZ77 and Huffman coding. DEFLATE was intended as a replacement for LZW and other patent-encumbered data compression algorithms which, at the time, limited the usability of compress and other popular archivers. Gzip is often also used to refer to the gzip file format, which is:
Although its file format also allows for multiple such streams to be concatenated (zipped files are simply decompressed concatenated as if they were originally one file), gzip is normally used to compress just single files. Compressed archives are typically created by assembling collections of files into a single tar archive, and then compressing that archive with gzip. The final .tar.gz or .tgz file is usually called a tarball. Whenever possible, each file is replaced by one with the extension `.gz', while keeping the same ownership modes, access and modification times. gzip will only attempt to compress regular files. In particular, it will ignore symbolic links. By default, gzip keeps the original file name and timestamp in the compressed file. Gzip is not to be confused with the Zip archive format, which also uses DEFLATE. The Zip format can hold collections of files without an external archiver, but is less compact than compressed tarballs holding the same data, because it compresses files individually and cannot take advantage of redundancy between files (solid compression). |
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zlib is an abstraction of the DEFLATE algorithm in library form which includes support both for the gzip file format and a lightweight stream format in its API. The zlib stream format, DEFLATE, and the gzip file format were standardized respectively as RFC 1950, RFC 1951, and RFC 1952. The Content-Encoding header in HTTP/1.1 allows clients to optionally receive compressed HTTP responses and (less commonly) to send compressed requests. The standard itself specifies two compression methods: gzip (RFC 1952; the content wrapped in a gzip stream) and deflate (RFC 1950; the content wrapped in a zlib-formatted stream). Compressed responses are supported by many HTTP client libraries, almost all modern browsers and both of the major HTTP server platforms, Apache and Microsoft IIS. Many server implementations, however, incorrectly implement the protocol by using the raw DEFLATE stream format (RFC 1951) instead. The bug is sufficiently pervasive that most modern browsers will accept both RFC 1951 and RFC 1950-formatted data for the deflate compressed method. |
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When reading archives, WinTar can automatically detect archive formats and its compression method, including gzip method. When writing archive files, WinTar requires a compression method before archiving starts. It also can be specified a None compression method which means the files will be written into arhive, but no compression apply to it. See Using WinTar... |
Related compression methods
See also:gzip home |
a 10-byte header, containing a magic number, a version number and a timestamp
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