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Extended file attributes (xattrs) are named metadata values attached to files and other filesystem objects. They are separate from a file’s ordinary contents and can store information used by the operating system, security systems, or applications. Their names, permissions, capacity, and portability depend on the operating system and filesystem.
What an extended file attribute is
An extended attribute is a name:value pair associated with a filesystem object. The name identifies the attribute, and its value may be text or binary data. Linux documentation describes xattrs as metadata associated with files and directories; its APIs also cover inodes and symbolic links. Linux man-pages: xattr(7) and setxattr(2)
Think of a file as having its ordinary filesystem metadata and contents, with optional, separately named metadata values attached. An xattr is not another stream of ordinary file contents.
What extended attributes are used for
Xattrs can support operating-system and filesystem functions as well as application-specific metadata. On Linux, documented examples include access-control information, security labels, file capabilities, and user-defined details such as a MIME type or character encoding. Linux man-pages: xattr(7)
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Linux namespaces
Linux qualifies attribute names with a namespace, written as namespace.attribute. The documented namespaces have different purposes and access rules:
| Namespace | Documented role | Example |
|---|---|---|
user |
Arbitrary additional information; access is governed by file permissions. | user.mime_type |
system |
System objects managed by the kernel or filesystem. | system.posix_acl_access |
security |
Security-module information and file capabilities. | security.selinux |
trusted |
Information intended for user-space mechanisms that ordinary processes should not access; visibility requires CAP_SYS_ADMIN. |
trusted.md5sum is a naming example in the manual. |
These names and access descriptions are Linux-specific; they should not be assumed to apply on other operating systems. Linux man-pages: xattr(7)
How xattrs are listed, read, and changed on Linux
Linux provides the getfattr and setfattr utilities, as well as system calls to list, retrieve, set, and remove extended attributes. Consult the manuals installed for your system for command syntax and permission requirements. xattr(7), getxattr(2), and setxattr(2)
listxattrlists attribute names.getxattrretrieves the value for a named attribute.setxattrcreates or updates an attribute; by default it creates the attribute if absent and replaces it if present.removexattrremoves an attribute.
The XATTR_CREATE and XATTR_REPLACE flags let a caller require creation of a new attribute or replacement of an existing one, respectively. The non-l calls follow a symbolic link and operate on its target; Linux also provides lgetxattr and lsetxattr variants that operate on the link itself. setxattr(2) and getxattr(2)
Limits and filesystem-specific behavior
The Linux man-pages project’s xattr(7), version 6.19 dated February 8, 2026, 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 not promises that every filesystem accepts values up to those limits: filesystem-specific restrictions can be tighter. 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 also require the user_xattr mount option. Check the documentation for the filesystem and mount configuration you actually use before relying on a particular capacity or availability. xattr(7)
On ext4, the kernel documentation describes attributes as typically stored in a separate data block referenced from an inode, with some available space also possible within the inode entry. That describes ext4’s implementation, not a universal storage rule for xattrs. Linux kernel documentation: ext4 extended attributes
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Why an operation can fail
An xattr operation may fail when the filesystem does not support the feature or it is disabled, the namespace is invalid, the value exceeds a filesystem’s limit, storage or quota is exhausted, or the caller lacks permission. The applicable system-call documentation describes errors and behavior. getxattr(2) and setxattr(2)
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How extended attributes differ across platforms
Linux and Apple use the broad idea of named metadata attached to filesystem objects, but their APIs, naming conventions, permissions, synchronization behavior, and limits differ. Linux xattrs are not specified by POSIX.1, so software should not assume that every system supports them or implements them identically. Linux man-pages: xattr(7)
Apple’s File Provider protocol exposes an extendedAttributes dictionary as metadata. The system decides which attributes sync, and Apple says that syncable extended attributes are capped at about 32 KiB total per item; attributes may become nonsyncable when that total is exceeded. This is a File Provider behavior, not a general limit for all Apple xattrs. The dictionary also excludes the resource fork and does not replace metadata represented in other protocol fields. Apple Developer: NSFileProviderItemProtocol.extendedAttributes
Apple’s archived Mac OS X setxattr(2) manual documents an older API model and flags; because it dates to October 19, 2004, it should not be treated as evidence of current platform-wide limits. Apple archived setxattr(2) manual
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What to check before relying on xattrs
If an application, backup, archive, or file-transfer workflow depends on extended attributes, verify the details at both ends rather than assuming the metadata will travel with the file.
Quick Recap
- Support: Confirm that the target filesystem and its mount configuration support the attributes you need.
- Names and scope: Check which names or namespaces are recognized and whether attributes can be attached to the relevant object, including a symbolic link if applicable.
- Permissions: Determine which users or capabilities can list, read, and write the attributes.
- Capacity: Check per-name, per-value, aggregate, and filesystem-specific limits.
- Transfer and sync: Verify that the particular copy, backup, archive, or cloud/file-provider process preserves the attributes you rely on.
- API behavior: Confirm how the API handles symbolic links, missing attributes, and create-versus-replace operations.
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