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 lists. The security namespace is used by SELinux, for example. Support for the extended attribute concept from a POSIX.1e draft that had been withdrawn in 1997 was added to Linux around 2002. As of 2016, they are not yet in widespread use by user-space Linux programs, but are used by Beagle, OpenStack Swift, Dropbox, KDE's semantic metadata framework (Baloo), Chromium, Wget and cURL. The Linux kernel allows extended attribute to have names of up to 255 bytes and values of up to 64 KiB, as do XFS and ReiserFS, but ext2/3/4 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, 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. Extended attributes can be accessed and modified using the getfattr and setfattr commands from the attr package on most distributions. The APIs are called and . macOS Mac OS X 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 macOS imposes any limit on the number of extended attributes. Each attribute consists of a name and the associated data. The name is a null-terminated Unicode string. No namespace restrictions are present (making this an open xattr system) and the convention is to use a reverse DNS string (similar to Uniform Type Identifiers) as the attribute name. macOS supports listing, getting, setting, and removing extended attributes from files or directories using a Linux-like API. From the command line, these abilities are exposed through the xattr utility. Since macOS 10.5, files originating from the