Information about Extended File Attributes

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, or a checksum.

OS/2

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.

Windows NT

Windows NT supports extended attributes on FAT and HPFS filesystems in the same way as OS/2 does. The NTFS file system was also designed to store them, as one of the many possible file forks, to accommodate the OS/2 subsystem. OS/2 extended attributes are accessible to any OS/2 programs the same way as in native OS/2 and to any Windows program through the BackupRead and BackupWrite system calls. They are notably used by the NFS server of the Interix POSIX subsystem in order to implement Unix-like permissions.

Linux

In Linux, the ext2, ext3, JFS, ReiserFS and XFS filesystems support extended attributes (abbreviated xattr) if the libattr feature is enabled in the kernel configuration. Any regular file may have a list of extended attributes. Each attribute is denoted by a name and the associated data. The name must be a null-terminated string, and must be 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. The Beagle search tool, however, does use extended attributes, and freedesktop.org publishes for their use.

FreeBSD

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 is denoted by 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.

Extended attributes are not widely used in user-space programs in FreeBSD.

Solaris

The Solaris operating system version 9 and later allows files to have "extended attributes", which are effectively forks. Internally, they are actually stored and accessed like normal files, so their names cannot contain "/" characters, their size is practically unlimited 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.

Mac OS X

In Mac OS X 10.4 and later, the HFS+ filesystem supports extended attributes. Any regular file may have a list of extended attributes. Each attribute is denoted by a name and the associated data. The name must be a null-terminated string.

See also

• fork (filesystem)
• attribute-value pair
• File attribute

External links

• http://acl.bestbits.at/ - Linux Extended Attributes and ACLs or libattr
• Implementation of OS/2 extended attributes on the FAT file system
• The fsattr(5) man page for Solaris 10
• The SetFile(1) man page for OSX