Extended file attributes: Difference between revisions

Revision as of 23:35, 10 December 2020

Revision as of 15:05, 12 January 2021

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| work=IBM [[AIX]] V7.2 documentation

| publisher=[[IBM]]

| accessdateaccess-date=2017-07-11}}</ref> The {{mono|getea}},<ref>{{cite web

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| work=IBM AIX V7.2 documentation: Base Operating System and Extensions, Volume 1

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| work=IBM AIX V7.2 documentation: Base Operating System and Extensions, Volume 2

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| work=IBM AIX V7.2 documentation: Base Operating System and Extensions, Volume 2

| publisher=IBM

| accessdateaccess-date=2017-07-11}}</ref> and {{mono|removeea}}<ref>{{cite web

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| work=IBM AIX V7.2 documentation: Base Operating System and Extensions, Volume 2

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| accessdateaccess-date=2017-07-11}}</ref> APIs support fetching, setting, listing, getting information about, and removing extended attributes.

=== OpenBSD ===

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| last=Kim

| date=2012-10-05

| accessdateaccess-date=2017-07-11}}</ref> 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 list]]s. The security namespace is used by [[SELinux]], for example.

Support for the extended attribute concept from a POSIX.1e draft{{cn|date=July 2020}} that had been withdrawn in 1997 was added to Linux around 2002.<ref>{{cite web

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| url=https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/include/?id=1ea864f1c53bc771294e61cf9be43b1d22e78f4c

| date=2002-02-05

| accessdateaccess-date=2017-07-11}}</ref><ref>{{cite web

| url=https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/3/html/Release_Notes/as-x86/index.html

| title=Red Hat Enterprise Linux AS 3 Release Notes (x86 Edition)

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| archive-date=2015-12-21

| accessdateaccess-date=2017-07-11}}</ref>

As of 2016, they are not yet in widespread use by user-space Linux programs, but are used by [[Beagle (software)|Beagle]], [[OpenStack#Object Storage .28Swift.29|OpenStack Swift]], [[Dropbox (service)|Dropbox]], [[KDE]]'s semantic metadata framework (Baloo), [[Chromium (web browser)|Chromium]], [[Wget]] and [[cURL]]. A set of recommendations for using them is available at [[freedesktop.org]].<ref>{{cite web

| url=https://www.freedesktop.org/wiki/CommonExtendedAttributes/

| title=Guidelines for extended attributes

| date=2013-05-18

| accessdateaccess-date=2017-07-11}}</ref>

The Linux kernel allows extended attribute to have names of up to 255 bytes and values of up to 64 KiB,<ref>{{cite web

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| title=linux/include/uapi/linux/limits.h

| date=2012-10-13

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as do [[XFS#Extended attributes|XFS]] and [[ReiserFS]], but [[ext2]]/[[ext3|3]]/[[ext4|4]] and [[btrfs]] impose much smaller limits, requiring all the attributes (names and values) of one file to fit in one "filesystem block" (usually 4 KiB). Per POSIX.1e,{{cn|date=July 2020}} the names are required to start with one of ''security'', ''system'', ''trusted'', and ''user'' plus a period. This defines the four namespaces of extended attributes.<ref>{{man|7|xattr|Linux}}</ref>

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| url=https://savannah.nongnu.org/projects/attr/

| website=[[GNU Savannah]]

| accessdateaccess-date=2017-07-11}}</ref> The APIs are called {{code|getxattr}} and {{code|setxattr}}.

===macOS===

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| date=2000-10-28

| accessdateaccess-date=2017-07-11}}</ref> 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.

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| date=2016-06-15

| publisher=[[MSDN]]

| accessdateaccess-date=2017-07-11}}</ref>

Additionally, [[NTFS]] can store arbitrary-length extended attributes in the form of [[NTFS#Alternate data streams .28ADS.29|alternate data streams]] (ADS), a type of [[Fork (file system)|resource fork]]. Plugins for the file manager [[Total Commander]], like ''NTFS Descriptions'' and ''QuickSearch eXtended'' support filtering the file list by or searching for metadata contained in ADS.<ref>{{cite web|url=https://totalcmd.net/plugring/NTFS_diz.html|title=NTFS Descriptions 1.2.1|website=TotalCmd.net|accessdateaccess-date=16 April 2018}}</ref><ref>{{cite web|url=http://totalcmd.net/plugring/google_like_qs.html|title=QuickSearch eXtended|website=TotalCmd.net|accessdateaccess-date=16 April 2018}}</ref> [[NTFS-3G]] supports mapping ADS to extended attributes in [[Filesystem in Userspace|FUSE]]; it also maps file attributes that way.<ref>{{cite web |title=Extended Attributes |url=https://www.tuxera.com/community/ntfs-3g-advanced/extended-attributes/ |website=Tuxera}}</ref>

==See also==