LDAPv3 operations can be extended through the use of controls.
    Controls may be sent to a server or returned to the client
    with any LDAP message. These controls are referred to as server
    controls.

    The LDAP API also supports a client-side extension mechanism
    through the use of client controls. These controls affect the
    behavior of the LDAP API only and are never sent to a server.
    A common data structure is used to represent both types of
    controls:

            typedef struct ldapcontrol {
                    char                            *ldctl_oid;
                    struct berval                   ldctl_value;
                    char                            ldctl_iscritical;
            } LDAPControl, *PLDAPControl;

    The fields in the ldapcontrol structure have the following
    meanings:

    ldctl_oid      The control type, represented as a string.
    ldctl_value    The data associated with the control (if any).
                   To specify a zero-length value, set ldctl_
                   value.bv_len to zero and ldctl_value.bv_val to
                   a zero-length string. To indicate that no data is
                   associated with the control, set ldctl_value.bv_
                   val to NULL.
    ldctl_         Indicates whether the control is critical or not.
    iscritical     If this field is non-zero, the operation will only
                   be carried out if the control is recognized by the
                   server and/or client.

    Some LDAP API calls allocate an ldapcontrol structure or a
    NULL-terminated array of ldapcontrol structures. The following
    functions can be used to dispose of a single control or an array
    of controls:

         void ldap_control_free( LDAPControl *ctrl );

         void ldap_controls_free( LDAPControl **ctrls );

    A set of controls that affect the entire session can be set using
    the ldap_set_option() function. A list of controls can also be
    passed directly to some LDAP API calls, such as ldap_search_
    ext(), in which case any controls set for the session through
    the use of ldap_set_option() are ignored. Control lists are
    represented as a NULL-terminated array of pointers to ldapcontrol
    structures.

    Server controls are defined by LDAPv3 protocol extension
    documents; for example, a control has been proposed to support
    paging of search results. No client controls are currently
    implemented in this version of the API.