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. Many formats still use three-character extensions even though modern operating systems and application programs no longer have this limitation. There are a limited number of three-letter extensions, which can cause a given extension to be used by more than one program. 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. For example, HTML documents are identified by names that end with. This portion of the filename is known as the 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. 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.
However, the GIF patent expired in the US in mid-2003, and worldwide in mid-2004.ĭifferent operating systems have traditionally taken different approaches to determining a particular file's format, with each approach having its own advantages and disadvantages. This has resulted in a significant decrease in the use of GIFs, and is partly responsible for the development of the alternative PNG format. 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.
Although patents for file formats are not directly permitted under US law, some formats encode data using patented algorithms. Patent law, rather than copyright, is more often used to protect a file format. Both strategies require significant time, money, or both therefore, file formats with publicly available specifications tend to be supported by more programs. The latter approach is possible only when a formal specification document exists.
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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. 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. 4.1 Unstructured formats (raw memory dumps)įile formats often have a published specification describing the encoding method and enabling testing of program intended functionality.3.3.8 File content based format identification.
3.3.5 PRONOM unique identifiers (PUIDs).
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