2 Specifications

Many file formats, including some of the most well-known file formats, have a published specificationIn engineering and manufacturing, the term specification has the following meanings: Technical requirement An essential technical requirement for items, materials, or services, including the procedures to be used to determine whether the requirement has b document (often with a reference implementationIn computing, a reference implementation (or, infrequently, sample implementation is a software example of a standard for use in helping others implement their own versions of the standard. A standard is much easier to understand with a working example to) that describes exactly how the data is to be encoded, and which can be used to determine whether or not a particular program treats a particular file format correctly. There are, however, two reasons why this is not always the case. First, some file format developers view their specification documents as trade secrets, and therefore do not release them to the public. A prominent example of this exists in several formats used by the Microsoft Office suite of applications. Second, some file format developers never spend time writing a separate specification document; rather, the format is defined only implicitly, through the program(s) that manipulate data in the format.

Note that using file formats without a publicly available specification can be costly. Learning how the format works will require either reverse-engineering it from a reference implementation or acquiring the specification document for a fee from the format developers. This second approach is possible only when there is a specification document, and typically requires the signing of a non-disclosure agreement. Both strategies require significant time, money, or both. Therefore, as a general rule, file formats with publicly available specifications are supported by a large number of programs, while non-public formats are supported by only a few programs.

The most useful part of intellectual property law for protecting ownership of a file format appears to be patent law. Although patents for file formats are not directly permitted under US law, some formats require the encoding of data with patented algorithms. For example, the GIF file format requires the use of a patented algorithm, and although initially the patent owner did not enforce it, they later began collecting fees for use of the algorithm. 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 patent expired in the US in mid-2003, worldwide in mid-2004; algorithms are themselves not currently patentable under European law.

3 Identifying the type of a file

Since files are seen by programs as streams of data, a method is required to determine the format of a particular file within the filesystem — an example of metadata. Different operating systems have traditionally taken different approaches to this problem, with each approach having its own advantages and disadvantages.

Of course, most modern operating systems, and individual applications, need to use all of these approaches to process various files, at least to be able to read 'foreign' file formats, if not work with them completely.

3.1 By file extension

One popular method — used by several operating systems including some of those produced by DEC, CP/M, and consequently DOS and Windows — is to determine the format of a file based on the section of its name following the final appearance of "." - referred to as the filename extension. For example, HTML documents are identified by names ending .htm or .html, and GIF images by .gif. In the original FAT filesystem, this extension was limited to three characters, and thus many formats still use three-character identifiers even though most operating systems and application programs no longer have this limitation. Since there is no standard list of extensions, this can cause confusion due to more than one relatively rare format using the same extension, causing the operating system to misidentify files, confusing users.

One advantage 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 frequently confusing to less technical users, who might accidentally make a file unusable (or 'lose' it) by renaming it incorrectly. This led to more recent operating system shells, such as Windows 95 and MacOS X, to hide the extension when displaying lists of files - thus negating the advantages of having the format accessible in the name.

3.2 By "magic number"

An alternative method, often associated with Unix and its derivatives, is to store a magic number inside the file itself. Originally, this term was used for a specific set of 2- byte identifiers at the beginning of a file, but since any undecoded binary sequence can be regarded as a number, any feature of a file format which uniquely distinguishes it can be used for identification. GIF images, for instance, always begin with the ASCII representation of either GIF87a or GIF89a, depending which standard they adhere to. HTML files are harder to spot by this method: they might begin with the ASCII characters , or an appropriate document type definition beginning , or, for XHTML, the XML identifier which begins - or they might just launch straight in with some text, but still be valid HTML.

This approach offers better guarantees that the format will be identified correctly, and can often determine more precise information about the file. This is only useful, however, if the interface used to access the files allows the user to easily manipulate any file in a variety of ways - as opposed to double clicking automatically doing the "right" thing; it is therefore more often associated with command line interfaces than graphical ones. A further disadvantage is that it requires scanning of both the file in question and a "magic file" listing known identifiers, making it less efficient, especially for displaying large lists of files. And, as with the example of HTML, some filetypes just don't lend themselves to recognition in this way. It is, however, the best way for a program to check if a file it has been told to process is of the correct format: while the file's name or metadata may be altered indepently of its content, failing a well-designed magic number test is a pretty sure sign that the file is either corrupt or of the wrong type.

3.3 By explicit metadata

A final way of storing the format of a file is to store an explicit representation alongside the filename. A well-known example of this is the Macintosh's Hierarchical File System, which stores codes for creator and type within the listing for each file. These codes are referred to as OSTypes, and for instance an application written by Apple would have a creator of AAPL and a type of APPL. RISC OS uses a similar system, consisting of a 12- bit number which can be looked up in a table of descriptions - e.g. the hexadecimal number FF5 is "aliased" to PoScript, representing a PostScript file.

This approach keeps the metadata separate from both the main data and the name, but is also less portable than either of the above, since the format has to be converted from system to system. While this is also true to an extent with filename extensions - for instance, for compatibility with MS-DOS's three character limit - most forms of storage have a roughly equivalent definition of a file's data and name, but may have varying or no representation of further metadata.

Related to this approach is that of MIME types, which are widely used in many Internet-related applications, and increasingly elsewhere. These consist of a standardised system of identifiers consisting of a type and a sub-type, sperated by a slash - for instance, text/html or image/gif. These avoid the standardisation problems inherent in the other approaches, and were originally intended as a way of identifying what type of file was attached to an e-mail, independent of the source and target operating systems. In Macintosh terms, the MIME type encodes the file type information but not the file creator.

4 See also

• List of file formats
• Graphics file format
• Audio file format
• Video file format
• Object file format
• Document file format

5 External link

• The Programmer's File Format Collection

Computer file formats

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