Extended file attributes
Encyclopedia
Extended file attributes is a file system
feature that enables users to associate computer file
s 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
.
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 list
s and mandatory access control
. Since FreeBSD
8.0, extended attributes are also supported on ZFS
filesystem.
, the ext2
, ext3
, ext4
, JFS, ReiserFS
, XFS
, Btrfs
and OCFS2 1.6 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 list
s. 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
publishes recommendations for their use.
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 the Mac OS imposes any limit on the number of extended attributes. Each attribute is denoted by 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 10.6, user space extended attributes are not preserved on save in common Cocoa
applications (TextEdit, Preview etc.).
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 icon
s 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.
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.
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.
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
feature that enables users to associate computer file
Computer file
A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...
s with metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
not interpreted by the filesystem, whereas regular attributes have a purpose strictly defined by the filesystem (such as permissions
File system permissions
Most current file systems have methods of administering permissions or access rights to specific users and groups of users. These systems control the ability of the users to view or make changes to the contents of the filesystem....
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
Character encoding
A character encoding system consists of a code that pairs each character from a given repertoire with something else, such as a sequence of natural numbers, octets or electrical pulses, in order to facilitate the transmission of data through telecommunication networks or storage of text in...
of a plain-text document, or a checksum
Cyclic redundancy check
A cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data...
.
FreeBSD
In FreeBSDFreeBSD
FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...
5.0 and later, the UFS1
Unix File System
The Unix file system is a file system used by many Unix and Unix-like operating systems. It is also called the Berkeley Fast File System, the BSD Fast File System or FFS...
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
Null-terminated string
In computer programming, a null-terminated string is a character string stored as an array containing the characters and terminated with a null character...
, 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 list
Access control list
An access control list , with respect to a computer file system, is a list of permissions attached to an object. An ACL specifies which users or system processes are granted access to objects, as well as what operations are allowed on given objects. Each entry in a typical ACL specifies a subject...
s and mandatory access control
Mandatory access control
In computer security, mandatory access control refers to a type of access control by which the operating system constrains the ability of a subject or initiator to access or generally perform some sort of operation on an object or target...
. Since FreeBSD
FreeBSD
FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...
8.0, extended attributes are also supported on ZFS
ZFS
In computing, ZFS is a combined file system and logical volume manager designed by Sun Microsystems. The features of ZFS include data integrity verification against data corruption modes , support for high storage capacities, integration of the concepts of filesystem and volume management,...
filesystem.
Linux
In LinuxLinux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
, the ext2
Ext2
The ext2 or second extended filesystem is a file system for the Linux kernel. It was initially designed by Rémy Card as a replacement for the extended file system ....
, ext3
Ext3
The ext3 or third extended filesystem is a journaled file system that is commonly used by the Linux kernel. It is the default file system for many popular Linux distributions, including Debian...
, ext4
Ext4
The ext4 or fourth extended filesystem is a journaling file system for Linux, developed as the successor to ext3.It was born as a series of backward compatible extensions to ext3, many of them originally developed by Cluster File Systems for the Lustre file system between 2003 and 2006, meant to...
, JFS, ReiserFS
ReiserFS
ReiserFS is a general-purpose, journaled computer file system designed and implemented by a team at Namesys led by Hans Reiser. ReiserFS is currently supported on Linux . Introduced in version 2.4.1 of the Linux kernel, it was the first journaling file system to be included in the standard kernel...
, XFS
XFS
XFS is a high-performance journaling file system created by Silicon Graphics, Inc. It is the default file system in IRIX releases 5.3 and onwards and later ported to the Linux kernel. XFS is particularly proficient at parallel IO due to its allocation group based design...
, Btrfs
Btrfs
Btrfs is a GPL-licensed copy-on-write file system for Linux.Development began at Oracle Corporation in 2007....
and OCFS2 1.6 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
Null-terminated string
In computer programming, a null-terminated string is a character string stored as an array containing the characters and terminated with a null character...
, and must be prefixed by a namespace
Namespace (computer science)
A namespace is an abstract container or environment created to hold a logical grouping of unique identifiers or symbols . An identifier defined in a namespace is associated only with that namespace. The same identifier can be independently defined in multiple namespaces...
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 list
Access control list
An access control list , with respect to a computer file system, is a list of permissions attached to an object. An ACL specifies which users or system processes are granted access to objects, as well as what operations are allowed on given objects. Each entry in a typical ACL specifies a subject...
s. 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
Linux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
. Beagle
Beagle (software)
Beagle is a search system for Linux and other such modern Unix-like systems, enabling the user to search documents, chat logs, email and contact lists in a similar way to Spotlight in Mac OS X, and Windows Search or Google Desktop under Linux or Microsoft Windows...
and Dropbox do use extended attributes, and freedesktop.org
Freedesktop.org
freedesktop.org is a project to work on interoperability and shared base technology for free software desktop environments for the X Window System on Linux and other Unix-like operating systems. It was founded by Havoc Pennington from Red Hat in March 2000.The organisation focuses on the user....
publishes recommendations for their use.
Mac OS X
Mac OS XMac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
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 the Mac OS imposes any limit on the number of extended attributes. Each attribute is denoted by a name and the associated data. The name is a null-terminated
Null-terminated string
In computer programming, a null-terminated string is a character string stored as an array containing the characters and terminated with a null character...
Unicode
Unicode
Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...
string.
The Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
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 10.6, user space extended attributes are not preserved on save in common Cocoa
Cocoa (API)
Cocoa is Apple's native object-oriented application programming interface for the Mac OS X operating system and—along with the Cocoa Touch extension for gesture recognition and animation—for applications for the iOS operating system, used on Apple devices such as the iPhone, the iPod Touch, and...
applications (TextEdit, Preview etc.).
OpenBSD
OpenBSD's native ffs filesystem supports extended attributes; they are referred to as flags, and are controlled by the chflags command.OS/2
In OS/2OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...
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
File Allocation Table
File Allocation Table is a computer file system architecture now widely used on many computer systems and most memory cards, such as those used with digital cameras. FAT file systems are commonly found on floppy disks, flash memory cards, digital cameras, and many other portable devices because of...
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
Cluster (file system)
In computer file systems, a cluster or allocation unit is the unit of disk space allocation for files and directories. To reduce the overhead of managing on-disk data structures, the filesystem does not allocate individual disk sectors, but contiguous groups of sectors, called clusters.On a disk...
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
Workplace Shell
The Workplace Shell is a object-oriented desktop shell produced by IBM's Boca Raton development lab for OS/2 2.0. It is based on Common User Access and made a radical shift away from the Program Manager type interface that earlier versions of OS/2 shared with Windows 3.x or the...
uses several standardized extended attributes (also called EAs) for purposes like identifying the filetype, comments, computer icon
Computer icon
A computer icon is a pictogram displayed on a computer screen and used to navigate a computer system or mobile device. The icon itself is a small picture or symbol serving as a quick, intuitive representation of a software tool, function or a data file accessible on the system. It functions as an...
s and keywords about the file.
Programs written in the interpreted language Rexx
REXX
REXX is an interpreted programming language that was developed at IBM. It is a structured high-level programming language that was designed to be both easy to learn and easy to read...
store an already parsed version of the code as an extended attribute, to allow faster execution.
Solaris
The Solaris operating systemSolaris Operating System
Solaris is a Unix operating system originally developed by Sun Microsystems. It superseded their earlier SunOS in 1993. Oracle Solaris, as it is now known, has been owned by Oracle Corporation since Oracle's acquisition of Sun in January 2010....
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.
Windows NT
Windows NTWindows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
supports extended attributes on FAT
File Allocation Table
File Allocation Table is a computer file system architecture now widely used on many computer systems and most memory cards, such as those used with digital cameras. FAT file systems are commonly found on floppy disks, flash memory cards, digital cameras, and many other portable devices because of...
and HPFS filesystems in the same way as OS/2
OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...
does. The NTFS
NTFS
NTFS is the standard file system of Windows NT, including its later versions Windows 2000, Windows XP, Windows Server 2003, Windows Server 2008, Windows Vista, and Windows 7....
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
Interix
Interix is the name of an optional, full-featured POSIX and Unix environment subsystem for Microsoft's Windows NT-based operating systems.Interix is a component of the Services for Unix release 3.0 and 3.5...
POSIX subsystem in order to implement Unix-like permissions.
External links
- The extattr(2) man pageManual page (Unix)Man pages are the extensive documentation that comes preinstalled with almost all substantial Unix and Unix-like operating systems. The Unix command used to display them is man. Each page is a self-contained document.- Usage :...
for FreeBSD. - 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 pageManual page (Unix)Man pages are the extensive documentation that comes preinstalled with almost all substantial Unix and Unix-like operating systems. The Unix command used to display them is man. Each page is a self-contained document.- Usage :...
for Solaris 10