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Extended file attributes is a file system feature that enables users to associate computer files with metadata not interpreted by the filesystem, whereas regular attributes have a purpose strictly defined by the filesystem (such as permissions or records of creation and modification times). Unlike forks, which can usually be as large as the maximum file size, extended attributes are usually limited in size to a value significantly smaller than the maximum file size. Typical uses can be storing the author of a document, the character encoding of a plain-text document, a checksum, cryptographic hash or digital signature.
In FreeBSD 5.0 and later, the UFS1 and UFS2 filesystems support extended attributes. Any regular file may have a list of extended attributes. Each attribute consists of a name and the associated data. The name must be a null-terminated string, and exists in a namespace identified by a small-integer namespace identifier. Currently, two namespaces exist: user and system. The user namespace has no restrictions with regard to naming or contents. The system namespace is primarily used by the kernel for access control lists and mandatory access control. Since FreeBSD 8.0, extended attributes are also supported on ZFS filesystem.
In Linux, the ext2, ext3, ext4, JFS, ReiserFS, XFS, Btrfs and OCFS2 1.6 filesystems support extended attributes (abbreviated xattr) when enabled in the kernel configuration. Any regular file or directory may have extended attributes consisting of a name and associated data. The name must be a null-terminated string prefixed by a namespace identifier and a dot character. Currently, four namespaces exist: user, trusted, security and system. The user namespace has no restrictions with regard to naming or contents. The system namespace is primarily used by the kernel for access control lists. The security namespace is used by SELinux, for example.
Extended attributes are not widely used in user-space programs in Linux, although they are supported in the 2.6 and later versions of the kernel. Beagle and Dropbox do use extended attributes, and freedesktop.org published recommendations for their use.
For ext2/3/4 and btrfs, each extended attribute is limited to a filesystem block (e.g. 4 KiB), and in practice in ext2/3/4 all of them must fit together on a single block (including names and values). ReiserFS allow attributes of arbitrary size. In XFS the names can be up to 256 bytes in length, terminated by the first 0 byte, and the values can be up to 64KB of arbitrary binary data.
Extended attributes can be accessed and modified using the 'attr' command on most distributions.
Mac OS X 10.4 and later support extended attributes by making use of the HFS+ filesystem Attributes file B*-tree feature which allows for named forks. Although the named forks in HFS+ support arbitrarily large amounts of data through extents, the OS support for extended attributes only supports inline attributes, limiting their size to that which can fit within a single B*-tree node. Any regular file may have a list of extended attributes. HFS+ supports an arbitrary number of named forks, and it is unknown if OS X imposes any limit on the number of extended attributes. Each attribute consists of a name and the associated data. The name is a null-terminated Unicode string. The Mac OS X APIs support listing, getting, setting, and removing extended attributes from files or directories. The xattr utility may be used from the Terminal as well. Since MacOS 10.6, user space extended attributes are not preserved on save in common Cocoa applications (TextEdit, Preview etc.).
In OS/2 version 1.2 and later, the High Performance File System was designed with extended attributes in mind, but support for them was also retro-fitted on the FAT filesystem of DOS. For compatibility with other operating systems using a FAT partition, OS/2 attributes are stored inside a single file "EA DATA. SF" located in the root directory. This file is normally inaccessible when an operating system supporting extended attributes manages the disk, but can be freely manipulated under, for example, DOS. Files and directories having extended attributes use one or more clusters inside this file. The logical cluster number of the first used cluster is stored inside the owning file's or directory's directory entry, in two previously unused bytes. These two bytes are used for other purposes on the FAT32 filesystem, and hence OS/2 extended attributes cannot be stored on this filesystem.
Parts of OS/2 version 2.0 and later such as the Workplace Shell uses several standardized extended attributes (also called EAs) for purposes like identifying the filetype, comments, computer icons and keywords about the file. Programs written in the interpreted language Rexx store an already parsed version of the code as an extended attribute, to allow faster execution.
The Solaris operating system version 9 and later allows files to have "extended attributes", which are actually forks; the maximum size of an "extended attribute" is the same as the maximum size of a file, and they are read and written in the same fashion as files. Internally, they are actually stored and accessed like normal files, so their names cannot contain "/" characters and their ownership and permissions can differ from those of the parent file.
Version 4 of the Network File System supports extended attributes in much the same way as Solaris.
On Windows NT, limited-length extended attributes are supported by FAT, HPFS, and NTFS. This was implemented as part of the OS/2 subsystem. They are notably used by the NFS server of the Interix POSIX subsystem in order to implement Unix-like permissions.