setacl, fsetacl - set access control list (ACL) information
const char *path,
const struct acl_entry *acl
const struct acl_entry *acl
setacl() sets an existing file's access control list (ACL) or deletes
optional entries from it. path points to a path name of a file.
Similarly, fsetacl() sets an existing file's access control list for
an open file known by the file descriptor fildes.
The effective user ID of the process must match the owner of the file
or be the super-user to set a file's ACL.
A successful call to setacl() deletes all of a file's previous
optional ACL entries (see explanation below), if any. nentries
indicates how many valid entries are defined in the acl parameter. If
nentries is zero or greater, the new ACL is applied to the file. If
any of the file's base entries (see below) is not mentioned in the new
ACL, it is retained but its access mode is set to zero (no access).
Hence, routine calls of setacl() completely define the file's ACL.
As a special case, if nentries is negative (that is, a value of
ACL_DELOPT (defined in <<<<sys/acl.h>>>>), the acl parameter is ignored, all
of the file's optional entries, if any, are deleted, and its base
entries are left unaltered.
Some of the miscellaneous mode bits in the file's mode might be turned
off as a consequence of calling setacl(). See chmod(2).
Access Control Lists
An ACL consists of a series of entries. Entries can be categorized in
four levels of specificity:
(u.g, mode) applies to user u in group g
(u.%, mode) applies to user u in any group
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(%.g, mode) applies to any user in group g
(%.%, mode) applies to any user in any group
Entries in the ACL must be unique; no two entries can have the same
user ID (uid) and group ID (gid) (see below). Entries can appear in
any order. The system orders them as needed for access checking.
The <<<<sys/acl.h>>>> header file defines ACL_NSUSER as the non-specific uid
value and ACL_NSGROUP as the non-specific gid value represented by %
above. If uid in an entry is ACL_NSUSER, it is a %.g entry. If gid
in an entry is ACL_NSGROUP, it is a u.% entry. If both uid and gid
are non-specific, the file's entry is %.%.
The <<<<unistd.h>>>> header file defines meanings of mode bits in ACL
entries (R_OK, W_OK, and X_OK). Irrelevant bits in mode values must
Every file's ACL has three base entries which cannot be added or
deleted, but only modified. The base ACL entries are mapped directly
from the file's permission bits.
(<file's owner> . ACL_NSGROUP, <file's owner mode bits>)
(ACL_NSUSER . <file's group>, <file's group mode bits>)
(ACL_NSUSER . ACL_NSGROUP, <file's other mode bits>)
In addition, up to 13 optional ACL entries can be set to restrict or
grant access to a file.
Altering a base ACL entry's modes with setacl() changes the file's
corresponding permission bits. The permission bits can be altered
also by using chmod() (see chmod(2)) and read using stat() (see
The number of entries allowed per file (see NACLENTRIES in
<<<<sys/acl.h>>>>) is small for space and performance reasons. User groups
should be created as needed for access control purposes. Since
ordinary users cannot create groups, their ability to control file
access with ACLs might be somewhat limited.
Upon successful completion, setacl() and fsetacl() return a value of
zero. If an error occurs, they return -1, the file's ACL is not
modified, and errno is set to indicate the error.
setacl() and fsetacl() fail if any of the following conditions are
[ENOTDIR] A component of the path prefix is not a
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[ENOENT] The named file does not exist (for example,
path is null or a component of path does not
[EBADF] fildes is not a valid file descriptor.
[EACCES] A component of the path prefix denies search
[EPERM] The effective user ID does not match the
owner of the file and the effective user ID
is not super-user.
[EROFS] The named file resides on a read-only file
[EFAULT] path or acl points outside the allocated
address space of the process, or acl is not
as large as indicated by nentries.
[EINVAL] There is a redundant entry in the ACL, or acl
contains an invalid uid, gid, or mode value.
[E2BIG] An attempt was made to set an ACL with more
than NACLENTRIES entries.
[EOPNOTSUPP] The function is not supported on remote files
by some networking services.
[ENOSYS] The function is not supported by this file
[ENOSPC] Not enough space on the file system.
[ENFILE] System file table is full.
[ENAMETOOLONG] The length of path exceeds PATH_MAX bytes, or
the length of a component of path exceeds
NAME_MAX bytes while _POSIX_NO_TRUNC is in
[ELOOP] Too many symbolic links were encountered in
translating the path name.
[EDQUOT] User's disk quota block or inode limit has
been reached for this file system.
The following code fragment defines and sets an ACL on file ../shared
which allows the file's owner to read, write, and execute or search
the file, and allows user 103, group 204 to read the file.
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char *filename = "../shared";
struct acl_entry acl ;
struct stat statbuf;
if (stat (filename, &&&& statbuf) <<<< 0)
acl  . uid = statbuf . st_uid; /* file owner */
acl  . gid = ACL_NSGROUP;
acl  . mode = R_OK | W_OK | X_OK;
acl  . uid = 103;
acl  . gid = 204;
acl  . mode = R_OK;
if (setacl (filename, 2, acl))
The following call deletes all optional ACL entries from file1:
setacl ("file1", ACL_DELOPT, (struct acl_entry *) 0);
setacl() and fsetacl() are not supported on remote files.
ACLs are only supported on HFS file systems.
setacl() and fsetacl() were developed by HP.
access(2), chmod(2), getaccess(2), getacl(2), stat(2), acl(5),
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