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Extended file attributes (xattrs) are named metadata values attached to a file or other filesystem object. They supplement ordinary file metadata without becoming a second stream of file contents. Depending on the operating system and filesystem, they can store application details or support system functions such as access control and security.
How extended file attributes work
An xattr is a name:value pair associated with a filesystem object. The name identifies the kind of metadata; its value may be text or binary data. Linux documents xattrs for files, directories, inodes, and symbolic links. They are separate from the file’s ordinary contents and are not a universal format that every filesystem or operating system must support. Linux xattr(7)
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On Linux, attribute names are organized into namespaces and written in the form namespace.attribute. The namespace identifies the broad purpose and, in some cases, which processes may access the attribute.
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|---|---|---|
user |
Arbitrary additional information, with access governed by file permissions. | user.mime_type, or metadata such as character set or encoding. |
system |
System-managed information, including ACL-related metadata. | system.posix_acl_access |
security |
Security information used by kernel security modules and file capabilities. | security.selinux |
trusted |
Information for user-space mechanisms that ordinary processes should not access; Linux documents visibility for processes with CAP_SYS_ADMIN. |
trusted.md5sum is a naming example in the manual. |
These namespace names and access rules describe Linux, not a universal convention for other platforms. Linux xattr(7)
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What xattrs are used for
Xattrs can hold application metadata, such as a MIME type or character encoding, or system metadata used for access control and security. Their purpose depends on the attribute name, namespace, platform, and software that reads or writes them. An attribute is useful only when the relevant software and filesystem recognize and preserve it.
How to view, set, and remove xattrs on Linux
Linux provides the getfattr and setfattr utilities, as well as system calls to list, retrieve, create or replace, and remove attributes. Consult the documentation installed for your distribution for command syntax and required permissions. The system-call interfaces are documented in getxattr(2) and setxattr(2).
At the API level, getxattr retrieves the value for a named attribute, while setxattr writes a text or binary value. By default, setting an attribute creates it if it does not exist and replaces it if it does; the XATTR_CREATE and XATTR_REPLACE flags can require one behavior or the other. Listing and removal are handled by listxattr and removexattr. setxattr(2) getxattr(2)
Symbolic links need particular care: Linux’s lgetxattr and lsetxattr variants operate on the link itself, whereas the corresponding functions without the initial l follow the link and operate on its target. getxattr(2) setxattr(2)
Limits and common reasons an operation fails
The Linux man-pages project’s xattr(7), version 6.19 dated 2026-02-08, documents VFS limits of 255 bytes per attribute name, 64 kB per attribute value, and 64 kB for the returned list of attribute names. These are Linux VFS limits, not guarantees that a particular filesystem can store values up to those maxima. Filesystem-specific constraints can be tighter. Linux xattr(7)
For example, the manual says current ext2, ext3, and ext4 implementations require the total bytes used by a file’s attribute names and values to fit in one filesystem block; Btrfs has a different nodesize-related limit. Some older or other filesystem configurations may require a user_xattr mount option. Check the documentation for the filesystem and mount configuration you actually use before relying on a particular capacity or availability. Linux xattr(7)
An xattr operation can fail because the filesystem does not support the feature or it is disabled, the namespace or name is invalid, the value exceeds a filesystem-specific limit, storage or quota is exhausted, or the caller lacks permission. The system-call documentation describes these interfaces and their errors. getxattr(2) setxattr(2)
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How xattrs differ across platforms and filesystems
Extended attributes are not specified by POSIX.1, so software should not assume that every platform supports them or uses identical names, permissions, APIs, or limits. Linux and Apple documentation share the broad idea of named metadata attached to filesystem items, but that does not make their implementations interchangeable. The cited Apple API manual is an archived Mac OS X page dated 2004-10-19, so it documents an API model and flags rather than establishing current limits for every Apple system. Linux xattr(7) Apple archived setxattr(2)
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Apple’s File Provider protocol exposes an extendedAttributes dictionary as metadata. The system determines which attributes sync; Apple says syncable attributes are capped at about 32 KiB total per item, and some attributes may become nonsyncable when the total exceeds that amount. This is a File Provider syncing behavior, not a general xattr size limit. The protocol also represents some metadata through other fields and excludes the resource fork from this dictionary. Apple File Provider extendedAttributes
On ext4 specifically, the kernel documentation says xattrs are typically stored in a separate data block referenced from an inode, with some available space also possible within the inode entry. That is an implementation detail of ext4, not part of the general definition of an xattr. Linux kernel ext4 extended attributes
The cited documentation does not establish current Windows-specific equivalents or limits. Do not infer that Windows attributes, Linux xattrs, and Apple extended attributes share names, behavior, or portability. When moving files between systems, verify that the particular copy, backup, archive, or sync workflow preserves the specific attributes you rely on.
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