ACL

ACL

The ACL structure is the header of an access-control list (ACL). A complete ACL consists of an ACL structure followed by an ordered list of zero or more access-control entries (ACEs).

typedef struct _ACL { // acl

BYTE AclRevision;

BYTE Sbz1;

WORD AclSize;

WORD AceCount;

WORD Sbz2;

} ACL;

Members

AclRevision

Specifies the ACL's revision level. This value should be ACL_REVISION. All ACEs in an ACL must be at the same revision level.

Sbz1

Specifies a zero byte of padding that aligns the AclRevision member on a 16-bit boundary.

AclSize

Specifies the size, in bytes, of the ACL. This value includes both the ACL structure and all the ACEs.

AceCount

Specifies the number of ACEs stored in the ACL.

Sbz2

Specifies two zero bytes of padding that align the ACL structure on a 32-bit boundary.

Remarks

An ACL includes a sequential list of zero or more ACEs. The individual ACEs in an ACL are numbered from 0 to n, where n+1 is the number of ACEs in the ACL. When editing an ACL, an application refers to an ACE within the ACL by its index.

There are two types of ACL: discretionary and system.

A discretionary ACL is controlled by the owner of an object or anyone granted WRITE_DAC access to the object. It specifies the access particular users and groups can have to an object. For example, the owner of a file can use a discretionary ACL to control which users and groups can and cannot have access to the file.

An object may also have system-level security information associated with it, in the form of a system ACL controlled by a system administrator. A system ACL can allow the system administrator to audit any attempts to gain access to an object.

Three ACE structures are currently defined:

ACE structure	Description
ACCESS_ALLOWED_ACE	Grants specified rights to a user or group. This ACE is stored in a discretionary ACL.
ACCESS_DENIED_ACE	Denies specified rights to a user or group. This ACE is stored in a discretionary ACL.
SYSTEM_AUDIT_ACE	Specifies what types of access will cause system-level audits. This ACE is stored in a system ACL.

A fourth ACE structure, SYSTEM_ALARM_ACE, is not currently supported by Windows NT.

The ACL structure is to be treated as though it were opaque and applications are not to attempt to work with its members directly. To ensure that ACLs are semantically correct, applications can use the functions listed in the SeeAlso section to create and manipulate ACLs.

Each ACL and ACE structure begins on a doubleword boundary.