Fork (filesystem)

In a computer file system, a fork is a set of data associated with a file system object. File systems without forks only allow a single such set of data for the contents, while file systems with forks allow multiple such contents. Every non-empty file must have at least one fork, often of default type, and depending on the file system, a file may have one or more other associated forks, which in turn may contain primary data integral to the file, or just metadata. Unlike extended attributes, a similar file system feature which is typically of fixed size, forks can be of variable size, possibly even larger than the file's primary data fork. The size of a file is the sum of the sizes of each fork. Forks are also known as streams, since they are of variable size (unlike fixed size metadata), but this differs from other uses of "stream" to mean "data available over time, potentially infinite".

On file systems without forks, one may instead use multiple separate files that are associated with each other, particularly sidecar files for metadata. However, the connection between these files is not automatically preserved by the file system, and must instead be handled by each program that works on files. Another alternative is a container file, which stores additional data within a given file format, or an archive file, which allows storing several files and metadata within a file (within a single fork). This requires that programs process the container file or archive file, rather than the file system handling forks. These alternatives require additional work by programs using the data, but benefit from portability to file systems that do not support forks.

File system forks are associated with Apple's Hierarchical File System (HFS). Apple's HFS, and the original Apple Macintosh file system MFS, allowed a file system object to have several kinds of forks: a data fork, a resource fork, and multiple named forks.

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