---
name: "acl"
section: "9"
platform: "linux"
package: "freebsd-manpages"
version: "6.0-5"
release: "Debian 4.0 (Etch)"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/acl/9"
---

# acl(9)

virtual file system access control lists

## NAME

**acl** – virtual file system access control lists

## SYNOPSIS

**#include \<sys/param.h\>**

**#include \<sys/vnode.h\>**

**#include \<sys/acl.h\>**

In the kernel configuration file: **options UFS\_ACL**

## DESCRIPTION

Access control lists, or ACLs, allow fine-grained specification of rights for vnodes representing files and directories. However, as there are a plethora of file systems with differing ACL semantics, the vnode interface is aware only of the syntax of ACLs, relying on the underlying file system to implement the details. Depending on the underlying file system, each file or directory may have zero or more ACLs associated with it, named using the *type* field of the appropriate vnode ACL calls: [VOP\_ACLCHECK(9)](/VOP_ACLCHECK/9.md), [VOP\_GETACL(9)](/VOP_GETACL/9.md), and [VOP\_SETACL(9)](/VOP_SETACL/9.md).

Currently, each ACL is represented in-kernel by a fixed-size *acl* structure, defined as follows:

```
struct acl {
        int                     acl_cnt;
        struct acl_entry        acl_entry[ACL_MAX_ENTRIES];
};
```

An ACL is constructed from a fixed size array of ACL entries, each of which consists of a set of permissions, principal namespace, and principal identifier.

Each individual ACL entry is of the type *acl\_entry\_t*, which is a structure with the following members:

- *acl\_tag\_t* *ae\_tag*

  The following is a list of definitions of ACL types to be set in *ae\_tag*:

  - ACL\_UNDEFINED\_FIELD

    Undefined ACL type.

  - ACL\_USER\_OBJ

    Discretionary access rights for processes whose effective user ID matches the user ID of the file's owner.

  - ACL\_USER

    Discretionary access rights for processes whose effective user ID matches the ACL entry qualifier.

  - ACL\_GROUP\_OBJ

    Discretionary access rights for processes whose effective group ID or any supplemental groups match the group ID of the file's owner.

  - ACL\_GROUP

    Discretionary access rights for processes whose effective group ID or any supplemental groups match the ACL entry qualifier.

  - ACL\_MASK

    The maximum discretionary access rights that can be granted to a process in the file group class.

  - ACL\_OTHER

    Discretionary access rights for processes not covered by any other ACL entry.

  - ACL\_OTHER\_OBJ

    Same as ACL\_OTHER. Each ACL entry must contain exactly one ACL\_USER\_OBJ, one ACL\_GROUP\_OBJ, and one ACL\_OTHER. If any of ACL\_USER, ACL\_GROUP, or ACL\_OTHER are present, then exactly one ACL\_MASK entry should be present.

- *uid\_t* *ae\_id*

  The ID of user for whom this ACL describes access permissions.

- *acl\_perm\_t* *ae\_perm*

  This field defines what kind of access the process matching this ACL has for accessing the associated file.

  - ACL\_EXECUTE

    The process may execute the associated file.

  - ACL\_WRITE

    The process may write to the associated file.

  - ACL\_READ

    The process may read from the associated file.

  - ACL\_PERM\_NONE

    The process has no read, write or execute permissions to the associated file.

## IMPLEMENTATION NOTES

```
typedef mode_t  *acl_permset_t;
/* internal ACL structure */
struct acl {
        int                     acl_cnt;
        struct acl_entry        acl_entry[ACL_MAX_ENTRIES];
};
/* external ACL structure */
struct acl_t_struct {
        struct acl              ats_acl;
        int                     ats_cur_entry;
};
typedef struct acl_t_struct *acl_t;
/*
 * Possible valid values for ae_tag field.
 */
#define ACL_UNDEFINED_TAG       0x00000000
#define ACL_USER_OBJ            0x00000001
#define ACL_USER                0x00000002
#define ACL_GROUP_OBJ           0x00000004
#define ACL_GROUP               0x00000008
#define ACL_MASK                0x00000010
#define ACL_OTHER               0x00000020
#define ACL_OTHER_OBJ           ACL_OTHER
/*
 * Possible valid values for acl_type_t arguments.
 */
#define ACL_TYPE_ACCESS         0x00000000
#define ACL_TYPE_DEFAULT        0x00000001
#define ACL_TYPE_AFS            0x00000002
#define ACL_TYPE_CODA           0x00000003
#define ACL_TYPE_NTFS           0x00000004
#define ACL_TYPE_NWFS           0x00000005
/*
 * Possible flags in ae_perm field.
 */
#define ACL_EXECUTE             0x0001
#define ACL_WRITE               0x0002
#define ACL_READ                0x0004
#define ACL_PERM_NONE           0x0000
#define ACL_PERM_BITS           (ACL_EXECUTE | ACL_WRITE | ACL_READ)
#define ACL_POSIX1E_BITS        (ACL_EXECUTE | ACL_WRITE | ACL_READ)
/*
 * Possible entry_id values for acl_get_entry()
 */
#define ACL_FIRST_ENTRY         0
#define ACL_NEXT_ENTRY          1
/*
 * Undefined value in ae_id field
 */
#define ACL_UNDEFINED_ID        ((uid_t)-1)
```

## SEE ALSO

[acl(3)](/acl/3.md), [vaccess\_acl\_posix1e(9)](/vaccess_acl_posix1e/9.md), [VFS(9)](/VFS/9.md), vnaccess(9), [VOP\_ACLCHECK(9)](/VOP_ACLCHECK/9.md), [VOP\_GETACL(9)](/VOP_GETACL/9.md), [VOP\_SETACL(9)](/VOP_SETACL/9.md)

## AUTHORS

This manual page was written by Robert Watson.