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Concept
Extended file attributes
Summary
Extended file attributes are features that enable users to associate s with metadata not interpreted by the filesystem, whereas regular attributes have a purpose strictly defined by the filesystem (such as or records of creation and modification times). Unlike , 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 include storing the author of a document, the character encoding of a plain-text document, or a checksum, cryptographic hash or digital certificate, and discretionary access control information.
In Unix-like systems, extended attributes are usually abbreviated as xattr.
In AIX, the JFS2 v2 filesystem supports extended attributes, which are accessible using the command. The , , , , and APIs support fetching, setting, listing, getting information about, and removing extended attributes.
In the now-defunct BeOS and successors like Haiku, extended file attributes are widely used in base and third-party programs.
The allows the indexing and querying of attributes, essentially giving the filesystem database-like characteristics. The uses of extended attributes in Be-like systems are varied: For example, and OpenTracker, the file-managers of BeOS and Haiku respectively, both store the locations of file icons in attributes, Haiku's "Mail" service stores all message content and metadata in extended file attributes, and the MIME types of files are stored in their attributes. Extended file attributes can be viewed and edited in Be-like systems' GUI through the file-manager, often Tracker or derivatives thereof.
In FreeBSD 5.0 and later, the , UFS2, and ZFS filesystems support extended attributes, using the family of system calls. 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.
Official source
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