File format

A file format is a particular way to encode information for storage in a computer file.

Since a disk drive, or indeed any computer storage, can store only bits, the computer must have some way of converting information to 0s and 1s and vice-versa. There are different kinds of formats for different kinds of information. Within any format type, e.g., word processor documents, there will typically be several different formats. Sometimes these formats compete with each other.

Generality

Some file formats are designed to store very particular sorts of data: the JPEG format, for example, is designed only to store static images. Other file formats, however, are designed for storage of several different types of data: the GIF format supports storage of both still images and simple animations, and the QuickTime format can act as a container for many different types of multimedia. A text file is simply one that stores any text, in a format such as ASCII or Unicode, with few if any control characters. Some file formats, such as HTML, or the source code of some particular programming language, are in fact also text files, but adhere to more specific rules which allow them to be used for specific purposes.

It is sometimes possible to cause a program to read a file encoded in one format as if it were encoded in another format. For example, one can play a Microsoft Word document as if it were a song by using a music-playing program that deals in "headerless" audio files. The result does not sound very musical, however. This is so because a sensible arrangement of bits in one format is almost always nonsensical in another.

Specifications

Many file formats, including some of the most well-known file formats, have a published specification document (often with a reference implementation) 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.

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.

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 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.

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.

Filename extension

One popular method in use by several operating systems, including Mac OS X CP/M, DOS, and Windows, is to determine the format of a file based on the section of its name 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 .html (or .htm on older systems), and GIF images by .gif. In the original FAT filesystem, filenames were limited to an eight-character identifier and a three-character extension, which is known as 8-dot-3. Many formats thus 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 the operating system and consequently 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 more recent operating system shells, such as Windows 95 and Mac OS X, to hide the extension when displaying lists of recognized files. This separates the user from the complete filename, preventing the accidental changing of a file type, while allowing expert users to still retain the original functionality through enabling the displaying of file extensions.

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 upon the standard to which they adhere. Many file types, most especially plain-text files, are harder to spot by this method. HTML files, for example, might begin with the string <html> (which is not case sensitive), or an appropriate document type definition that starts with <!DOCTYPE, or, for XHTML, the XML identifier which begins with <?xml. The files could also begin with any random text or several empty lines, but still be usable 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. Since reliable "magic number" tests can be fairly complex, and each file must be tested against every possibility in the "magic file", this approach is also relatively inefficient, especially for displaying large lists of files (in contrast, filename and metadata based methods need check only one piece of data, and match it against a sorted index). 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.

So-called shebang lines in script files are a special case of magic numbers. Here, the magic number is human-readable text that identifies a specific command interpreter and options to be passed to the command interpreter.

Explicit metadata

A final way of storing the format of a file is to explicitly store information about the format in the file system.

This approach keeps the metadata separate from both the main data and the name, but is also less portable than either file extensions or "magic numbers", since the format has to be converted from filesystem to filesystem. 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.

Note that zip files or archive files solve the problem of handling metadata. A utiliy program collects multiple files together along with metadata about each file and the folders/directories they came from all within one new file (e.g. a zip file with extension .zip). The new file is also compressed and possibly encrypted, but now is transmissible as a single ascii/text file across operating systems by ftp systems or attached to email. At the destination, it must be unzipped by a compatible utility to be useful, but the problems of transmission are solved this way.

Apple Macintosh type-codes

The Macintosh's Hierarchical File System stores codes for creator and type as part of the directory entry 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.

OS/2 and Windows NT extended attributes

The HPFS, NTFS, FAT12 and FAT16 (but not FAT32) filesystems allow the storage of extended attributes with files. These comprise an arbitrary set of triplets with a name, a coded type for the value and a value, where the names are unique and values can be up to 64 KB long. There are standardized meanings for certain types and names. One such is that the ".TYPE" extended attribute is used to determine the file type. Its value comprises a list of one or more file types associated with the file, each of which is a string, such as "Plain Text" or "HTML document". Thus a file may have several types.

POSIX extended attributes

On Unix and Unix-like systems, the ext2, ext3, ReiserFS version 3, XFS, JFS, FFS, and HFS+ filesystems allow the storage of extended attributes with files. These include an arbitrary list of "name=value" strings, where the names are unique, which can be accessed by their "name" parts.

MIME types

MIME types are widely used in many Internet-related applications, and increasingly elsewhere, although their usage for on-disc type information is rare. These consist of a standardised system of identifiers consisting of a type and a sub-type, separated by a slash — for instance, text/html or image/gif. These 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. MIME types are used to identify files on BeOS, as well as store unique application signatures for application launching.

References

External links

DllDrive Free Downloadable Dll File Archives
The File Extension Archives - Large archive of file extensions
File extensions database - Large database of file extensions and file types
FileInfo.net - The File Extensions Resource
File Extensions and File Formats Listed Alphabetically
The Programmer's File Format Collection
Format wars File formats for websites and print explained
File signatures (aka magic numbers) found in files to indicate their file type

Computer file formats

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