On some operating systems, each file can be associated with arbitrary extended file attributes. At present, Emacs supports querying and setting two specific sets of extended file attributes: Access Control Lists (ACLs) and SELinux contexts. These extended file attributes are used, on some systems, to impose more sophisticated file access controls than the basic Unix-style permissions discussed in the previous sections.
A detailed explanation of ACLs and SELinux is beyond the scope of this manual. For our purposes, each file can be associated with an ACL, which specifies its properties under an ACL-based file control system, and/or an SELinux context, which specifies its properties under the SELinux system.
This function returns the ACL for the file filename. The exact
Lisp representation of the ACL is unspecified (and may change in
future Emacs versions), but it is the same as what set-file-acl
takes for its acl argument (see Changing File Names and Attributes).
The underlying ACL implementation is platform-specific; on GNU/Linux and BSD, Emacs uses the POSIX ACL interface, while on MS-Windows Emacs emulates the POSIX ACL interface with native file security APIs.
If ACLs are not supported or the file does not exist,
then the return value is nil.
This function returns the SELinux context of the file filename,
as a list of the form (user role type
range). The list elements are the context’s user, role, type,
and range respectively, as Lisp strings; see the SELinux documentation
for details about what these actually mean. The return value has the
same form as what set-file-selinux-context takes for its
context argument (see Changing File Names and Attributes).
If SELinux is not supported or the file does not exist,
then the return value is (nil nil nil nil).
This function returns an alist of the Emacs-recognized extended
attributes of file filename. Currently, it serves as a
convenient way to retrieve both the ACL and SELinux context; you can
then call the function set-file-extended-attributes, with the
returned alist as its second argument, to apply the same file access
attributes to another file (see Changing File Names and Attributes).
One of the elements is (acl . acl), where acl has
the same form returned by file-acl.
Another element is (selinux-context . context), where
context is the SELinux context, in the same form returned by
file-selinux-context.