In computing, an archive file is a computer file that is composed of one or more files along with metadata. Archive files are used to collect multiple data files together into a single file for easier portability and storage, or simply to compress files to use less storage space. Archive files often store directory structures, error detection and correction information, arbitrary comments, and sometimes use built-in encryption.[1][2][3] Archive files are particularly useful in that they store file system data and metadata within the contents of a particular file, and thus can be stored on systems or sent over channels that do not support the file system in question, only file contents – examples include sending a directory structure over email, files with names unsupported on the target file system due to length or characters, and retaining files' date and time information.[4] Additionally, it facilitates transferring high numbers of small files such as resources of saved web pages, since a container file is transferred using a single file operation, whereas transferring many small files requires the computer to modify the file system structure for each file individually, making it considerably slower.[5][6] Beyond archival purposes, archive files are frequently used for packaging software for distribution, as software contents are often naturally spread across several files; the archive is then known as a package. While the archival file format is the same, there are additional conventions about contents, such as requiring a manifest file, and the resulting format is known as a package format.[7] Examples include deb for Debian, JAR for Java, APK for Android, and self-extracting Windows Installer executables.
Features supported by various kinds of archives include:
Some archive programs have self-extraction, self-installation, source volume and medium information, and package notes/description.
The file extension or file header of the archive file are indicators of the file format used. Computer archive files are created by file archiver software, optical disc authoring software, and disk image software.[8] An archive format is the file format of an archive file. Some formats are well-defined by their authors and have become conventions supported by multiple vendors and communities.[9] Types
ExamplesFilename extensions used to distinguish different types of archives include zip, rar, 7z, and tar, the first of which is the most widely implemented.[10] Java also introduced a whole family of archive extensions such as jar and war (j is for Java and w is for web). They are used to exchange entire byte-code deployment. Sometimes they are also used to exchange source code and other text, HTML and XML files. By default they are all compressed.[11] Archive files often include parity checks and other checksums for error detection, for instance zip files use a cyclic redundancy check (CRC). RAR archives may include additional error correction data (called recovery records).[12] Archive files that do not natively support recovery records can use separate parchive (PAR) files that allows for additional error correction and recovery of missing files in a multi-file archive.[13]
Page 27z is a compressed archive file format that supports several different data compression, encryption and pre-processing algorithms. The 7z format initially appeared as implemented by the 7-Zip archiver. The 7-Zip program is publicly available under the terms of the GNU Lesser General Public License. The LZMA SDK 4.62 was placed in the public domain in December 2008. The latest stable version of 7-Zip and LZMA SDK is version 22.01.[2]
.7z
application/x-7z-compressed The official, informal 7z file format specification is distributed with 7-Zip's source code since 2015. The specification can be found in plain text format in the 'doc' sub-directory of the source code distribution.[3] There have been additional third-party attempts at writing more concrete documentation based on the released code.[4] The 7z format provides the following main features:
The format's open architecture allows additional future compression methods to be added to the standard. Compression methodsThe following compression methods are currently defined:
A suite of recompression tools called AdvanceCOMP contains a copy of the DEFLATE encoder from the 7-Zip implementation; these utilities can often be used to further compress the size of existing gzip, ZIP, PNG, or MNG files. Pre-processing filtersThe LZMA SDK comes with the BCJ and BCJ2 preprocessors included, so that later stages are able to achieve greater compression: For x86, ARM, PowerPC (PPC), IA-64 Itanium, and ARM Thumb processors, jump targets are 'normalized' [5] before compression by changing relative position into absolute values. For x86, this means that near jumps, calls and conditional jumps (but not short jumps and conditional jumps) are converted from the machine language "jump 1655 bytes backwards" style notation to normalized "jump to address 5554" style notation; all jumps to 5554, perhaps a common subroutine, are thus encoded identically, making them more compressible.
Similar executable pre-processing technology is included in other software; the RAR compressor features displacement compression for 32-bit x86 executables and IA-64 executables, and the UPX runtime executable file compressor includes support for working with 16-bit values within DOS binary files. EncryptionThe 7z format supports encryption with the AES algorithm with a 256-bit key. The key is generated from a user-supplied passphrase using an algorithm based on the SHA-256 hash function. The SHA-256 is executed 218 (262144) times,[6] which causes a significant delay on slow PCs before compression or extraction starts. This technique is called key stretching and is used to make a brute-force search for the passphrase more difficult. Current GPU-based, and custom hardware attacks limit the effectiveness of this particular method of key stretching,[7] so it is still important to choose a strong password. The 7z format provides the option to encrypt the filenames of a 7z archive. LimitationsThe 7z format does not store filesystem permissions (such as UNIX owner/group permissions or NTFS ACLs), and hence can be inappropriate for backup/archival purposes. A workaround on UNIX-like systems for this is to convert data to a tar bitstream before compressing with 7z. But it is worth noting that GNU tar (common in many UNIX environments) can also compress with the LZMA2 algorithm ("xz") natively, without the use of 7z, using the "-J" switch. The resulting file extension is ".tar.xz" or ".txz" and not ".tar.7z". This method of compression has been adopted with many distributions for packaging, such as Arch, Debian (deb), Fedora (rpm) and Slackware. (The older "lzma" format is less efficient.)[8] On the other hand, it is important to note, that tar does not save the filesystem encoding, which means that tar compressed filenames can become unreadable if decompressed on a different computer. The 7z format does not allow extraction of some "broken files"—that is (for example) if one has the first segment of a series of 7z files, 7z cannot give the start of the files within the archive—it must wait until all segments are downloaded. The 7z format also lacks recovery records, making it vulnerable to data degradation unless used in conjunction with external solutions, like parchives, or within filesystems with robust error-correction. By way of comparison, zip files also lack a recovery feature while the rar format has one.
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