HOME
        TheInfoList






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 File formats may be either proprietary or free and may be either unpublished or open.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.

Specifications

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.

Patents

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.

Identifying file type

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.

Filename extension

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 .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.

Hiding the extension, however, can create the appearance of two or more identical filenames in the same folder. For example, a company logo may be needed both in .eps format (for publishing) and .png format (for web sites). With the extensions visible, these would appear as the unique filenames: "CompanyLogo.eps" and "CompanyLogo.png". On the other hand, hiding the extensions would make both appear as "CompanyLogo", which can lead to confusion.

Hiding extensions can also pose a securi

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.

Patents

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.

Identifying file type

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.

Filename extension

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 .html (or .htm), and GIF images by .gif. In the original 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.

Identifying file type

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 .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 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.

Hiding the extension, however, can create the appearance of two or more identical filenames in the same folder. For example, a company logo may be needed both in .eps format (for publishing) and .png format (for web sites). With the extensions visible, these would appear as the unique filenames: "CompanyLogo.eps" and "CompanyLogo.png". On the other hand, hiding the extensions would make both appear as "CompanyLogo", which can lead to confusion.

Hiding extensions can also pose a security risk.[1] For example, a malicious user could create an executable program with an innocent name such as "Holiday photo.jpg.exe". The ".exe" would be hidden and an unsuspecting user would see "Holiday photo.jpg", which would appear to be a JPEG image, usually unable to harm the machine. However, the operating system would still see the ".exe" extension and run the program, which would then be able to cause harm to the computer. The same is true with files with only one extension: as it is not shown to the user, no information about the file can be deduced without explicitly investigating the file. To further trick users, it is possible to store an icon inside the program, in which case some operating systems' icon assignment for the executable file (.exe) would be overridden with an icon commonly used to represent JPEG images, making the program look like an image. Extensions can also be spoofed: some Microsoft Word macro viruses create a Word file in template format and save it with a .doc extension. Since Word generally ignores extensions and looks at the format of the file, these would open as templates, execute, and spread the virus.[citation needed] These issues require users with extensions hidden to be vigilant and never let the operating system choose with what program to open a file not known to be trustworthy (which contradicts the idea of making things easier for the user). This represents a practical problem for Windows systems where extension-hiding is turned on by default.

A second way to identify a file format is to use information regarding the format stored inside the file itself, either information meant for this purpose or binary strings that happen to always be in specific locations in files of some formats. Since the easiest place to locate them is at the beginning, such area is usually called a file header when it is greater than a few bytes, or a magic number if it is just a few bytes long.

File header

The metadata co

The metadata contained in a file header are usually stored at the start of the file, but might be present in other areas too, often including the end, depending on the file format or the type of data contained. Character-based (text) files usually have character-based headers, whereas binary formats usually have binary headers, although this is not a rule. Text-based file headers usually take up more space, but being human-readable, they can easily be examined by using simple software such as a text editor or a hexadecimal editor.

As well as identifying the file format, file headers may contain metadata about the file and its contents. For example, most image files store information about image format, size, resolution and image files store information about image format, size, resolution and color space, and optionally authoring information such as who made the image, when and where it was made, what camera model and photographic settings were used (Exif), and so on. Such metadata may be used by software reading or interpreting the file during the loading process and afterwards.

File headers may be used by an operating system to quickly gather information about a file without loading it all into memory, but doing so uses more of a computer's resources than reading directly from the directory information. For instance, when a graphic file manager has to display the contents of a folder, it must read the headers of many files before it can display the appropriate icons, but these will be located in different places on the storage medium thus taking longer to access. A folder containing many files with complex metadata such as thumbnail information may require considerable time before it can be displayed.

If a header is binary hard-coded such that the header itself needs complex interpretation in order to be recognized, especially for metadata content protection's sake, there is a risk that the file format can be misinterpreted. It may even have been badly written at the source. This can result in corrupt metadata which, in extremely bad cases, might even render the file unreadable.[clarification needed]

A more complex example of file headers are those used for wrapper (or container) file formats.

One way to incorporate file type metadata, often associated with Unix and its derivatives, is just to store a "magic number" inside the file itself. Originally, this term was used for a specific set of 2-byte identifiers at the beginnings of files, but since any 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, 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 HTML>, or, for XHTML, the XML identifier, which begins with <?xml. The files can also begin with HTML comments, random text, or several empty lines, but still be usable HTML.

The magic number approach offers better guarantees that the format will be identified correctly, and can often determine more precise information about the file. Since reasonably reliable "magic number" tests can be fairly complex, and each file must effectively be tested against every possibility in the magic database, this approach is relatively inefficient, especially for displaying large lists of files (in contrast, file name and metadata-based methods need to check only one piece of da

The magic number approach offers better guarantees that the format will be identified correctly, and can often determine more precise information about the file. Since reasonably reliable "magic number" tests can be fairly complex, and each file must effectively be tested against every possibility in the magic database, this approach is relatively inefficient, especially for displaying large lists of files (in contrast, file name and metadata-based methods need to check only one piece of data, and match it against a sorted index). Also, data must be read from the file itself, increasing latency as opposed to metadata stored in the directory. Where file types don't lend themselves to recognition in this way, the system must fall back to metadata. It is, however, the best way for a program to check if the file it has been told to process is of the correct format: while the file's name or metadata may be altered independently 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. On the other hand, a valid magic number does not guarantee that the file is not corrupt or is of a correct 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.

Another operating system using magic numbers is AmigaOS, where magic numbers were called "Magic Cookies" and were adopted as a standard system to recognize executables in Hunk executable file format and also to let single programs, tools and utilities deal automatically with their saved data files, or any other kind of file types when saving and loading data. This system was then enhanced with the Amiga standard Datatype recognition system. Another method was the FourCC method, originating in OSType on Macintosh, later adapted by Interchange File Format (IFF) and derivatives.

A final way of storing the format of a file is to explicitly store information about the format in the file system, rather than within the file itself.

This approach keeps the metadata separate from both the main data and the name, but is also less portable than either filename 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 exten

This approach keeps the metadata separate from both the main data and the name, but is also less portable than either filename 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 utility 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 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.

The Mac OS' 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. These codes could be any 4-byte sequence, but were often selected so that the ASCII representation formed a sequence of meaningful characters, such as an abbreviation of the application's name or the developer's initials. For instance a HyperCard "stack" file has a creator of WILD (from Hypercard's previous name, "WildCard") and a type of STAK. The BBEdit text editor has a creator code of R*ch referring to its original programmer, Rich Siegel. The type code specifies the format of the file, while the creator code specifies the default program to open it with when double-clicked by the user. For example, the user could have several text files all with the type code of TEXT, but which each open in a different program, due to having differing creator codes. This feature was intended so that, for example, human-readable plain-text files could be opened in a general purpose text editor, while programming or HTML code files would open in a specialized editor or IDE. However, this feature was often the source of user confusion, as which program would launch when the files were double-clicked was often unpredictable. 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.

Mac OS X uniform type identifiers (UTIs)

MIME types are widely used in many Interne

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 (managed by IANA) 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 identify files on BeOS, AmigaOS 4.0 and MorphOS, as well as store unique application signatures for application launching. In AmigaOS and MorphOS the Mime type system works in parallel with Amiga specific Datatype system.

There are problems with the MIME types though; several organisations and people have created their own MIME types without registering them properly with IANA, which makes the use of this standard awkward in some cases.

NNNNNNNNN-XX-YYYYYYY. The first part indicates the organisation origin/maintainer (this number represents a value in a company/standards organisation database), the 2 following digits categorize the type of file in hexadecimal. The final part is composed of the usual filename extension of the file or the international standard number of the file, padded left with zeros. For example, the PNG file specification has the FFID of 000000001-31-0015948 where 31 indicates an image file, 0015948 is the standard number and 000000001 indicates the International Organization for Standardization (ISO).

File content based format identification

Another but less popular

Another but less popular way to identify the file format is to examine the file contents for distinguishable patterns among file types. The contents of a file are a sequence of bytes and a byte has 256 unique permutations (0–255). Thus, counting the occurrence of byte patterns that is often referred as byte frequency distribution gives distinguishable patterns to identify file types. There are many content-based file type identification schemes that use byte frequency distribution to build the representative models for file type and use any statistical and data mining techniques to identify file types [2]

File structure

There are several types of ways to structure data in a file. The most usual

There are several types of ways to structure data in a file. The most usual ones are described below.

Unstructured formats (raw memory dumps)

Navigation menu