File format

A file format is a standard way that information is encoded for storage in a computer file. It specifies how bits are used to encode information in a digital storage medium. File formats may be either proprietary or free and may be either unpublished or open. A file format is a standard way that information is encoded for storage in a computer file. It specifies how bits are used to encode information in a digital storage medium. File formats may be either proprietary or free and may be either unpublished or open. Some file formats are designed for very particular types of data: PNG files, for example, store bitmapped images using lossless data compression. Other file formats, however, are designed for storage of several different types of data: the Ogg format can act as a container for different types of multimedia including any combination of audio and video, with or without text (such as subtitles), and metadata. A text file can contain any stream of characters, including possible control characters, and is encoded in one of various character encoding schemes. Some file formats, such as HTML, scalable vector graphics, and the source code of computer software are text files with defined syntaxes that allow them to be used for specific purposes. File formats often have a published specification describing the encoding method and enabling testing of program intended functionality. Not all formats have freely available specification documents, partly because some developers view their specification documents as trade secrets, and partly because other developers never author a formal specification document, letting precedent set by other already existing programs that use the format define the format via how these existing programs use it. If the developer of a format doesn't publish free specifications, another developer looking to utilize that kind of file must either reverse engineer the file to find out how to read it or acquire the specification document from the format's developers for a fee and by signing a non-disclosure agreement. The latter approach is possible only when a formal specification document exists. Both strategies require significant time, money, or both; therefore, file formats with publicly available specifications tend to be supported by more programs. Patent law, rather than copyright, is more often used to protect a file format. Although patents for file formats are not directly permitted under US law, some formats encode data using patented algorithms. For example, using compression with the GIF file format requires the use of a patented algorithm, and though the patent owner did not initially enforce their patent, they later began collecting royalty fees. This has resulted in a significant decrease in the use of GIFs, and is partly responsible for the development of the alternative PNG format. However, the GIF patent expired in the US in mid-2003, and worldwide in mid-2004. Different operating systems have traditionally taken different approaches to determining a particular file's format, with each approach having its own advantages and disadvantages. Most modern operating systems and individual applications need to use all of the following approaches to read 'foreign' file formats, if not work with them completely. One popular method used by many operating systems, including Windows, macOS, CP/M, DOS, VMS and VM/CMS is to determine the format of a file based on the end of its name, more specifically the letters following the final period. This portion of the filename is known as the filename extension. For example, HTML documents are identified by names that end with .mw-parser-output .monospaced{font-family:monospace,monospace}.html (or .htm), and GIF images by .gif. In the original FAT file system, file names were limited to an eight-character identifier and a three-character extension, known as an 8.3 filename. There are only so many three-letter extensions, so, often any given extension might be linked to more than one program. Many formats still use three-character extensions even though modern operating systems and application programs no longer have this limitation. Since there is no standard list of extensions, more than one format can use the same extension, which can confuse both the operating system and users. One artifact of this approach is that the system can easily be tricked into treating a file as a different format simply by renaming it—an HTML file can, for instance, be easily treated as plain text by renaming it from filename.html to filename.txt. Although this strategy was useful to expert users who could easily understand and manipulate this information, it was often confusing to less technical users, who could accidentally make a file unusable (or 'lose' it) by renaming it incorrectly. This led more recent operating system shells, such as Windows 95 and Mac OS X, to hide the extension when listing files. This prevents the user from accidentally changing the file type, and allows expert users to turn this feature off and display the extensions.