The ACCEPT BEGIN Procedure

Selective waits and simple

accept

statements are compiled into either one or two procedure calls. The rst procedure is ACCEPT BEGIN, whose declaration is shown in Figure 37. The second procedure, which is called if an

accept

alternative is selected, is ACCEPT END,

procedure ENTRY CALL(MY NAME : in TASK NAME CALLEE T NAME : in TASK NAME CALLEE E NAME : in ENTRY NAME IN PARAMS : in PARAM LIST

TIME OUT : in DURATION

OUT PARAMS : out PARAM LISTREF

ACCEPTED : out BOOLEAN

ABORTED : out BOOLEAN

MY NODE : in out NODE IDTYPE) ;;jwhere

;; MY NAME's scope is unchanged:

;;j CURRENT SCOPE (MY NAME)= inCURRENT SCOPE (MYNAME), ;; A task cannot rendezvous with itself:

;;j out(ACCEPTED;

>

MY NAME

=

=CALLEE T NAME),

;; For a simple entry call by a caller that is not abnormal, ENTRY CALL ;; will not terminate until the call is accepted:

;;j out(TIME OUT =DURATION'LAST and notABORTED ;

>

;;j ACCEPTED),

;; The called entry must have been declared by CALLEE T NAME: ;;j 1

<

=CALLEE ENAME

<

= ENTRYCOUNT (CALLEE T NAME), ;; An unaccepted timed entry call by a caller that is not abnormal ;; lasts at least as long as TIME OUT:

;;j out(notABORTED and notACCEPTED ;

>

;;j CALENDAR.CLOCK; inCALENDAR.CLOCK

>

=TIME OUT),

;; An aborted task is no longer CALLABLE i.e., any evaluation of the CALLABLE ;; attribute on MY NAME should return FALSE if ABORTED is TRUE:

;;j out(ABORTED ;

>

;;j (for all T1 : TASK NAME N1: NODE IDTYPE =

>

;;j notCALLABLEATTR(T1,MY NAME, N1))), ;; Execution of an entry call can cause TASKING ERROR to be ;; propagated to the caller:

;;j raiseTASKING ERROR

4.6. THE SUPERVISOR INTERFACE 87

procedure ACCEPT BEGIN (MYNAME : in TASK NAME GUARDS : in GUARD ARRAY INDICES : in INDEXARRAY TIMEOUT : in DURATION TERMALT : in BOOLEAN

IN PARAMS: out PARAM LIST REF INDEX : out NATURAL

ABORTED : out BOOLEAN MY NODE : in out NODE IDTYPE) ;;jwhere

;; MY NAME's scope is unchanged:

;;j CURRENT SCOPE (MY NAME)= inCURRENT SCOPE (MYNAME), ;; GUARDS and INDICES have one component per entry of MY NAME ;; plus an extra one:

;;j GUARDS'LAST= INDICES'LAST= ENTRYCOUNT (MY NAME)+1, ;; Execution of the delay alternative of a selective wait must occur

;; after TIME OUT has elapsed:

;;j out(notABORTED and notTERM ALT and ;;j inTIME OUT

<

DURATION'LAST and ;;j GUARDS(GUARDS'LAST) and

;;j INDEX=INDICES(INDICES'LAST);

>

;;j CALENDAR.CLOCK; inCALENDAR.CLOCK

>

=TIME OUT), ;; Execution of a selective wait with all alternatives closed results in the ;; propagation of PROGRAM ERROR to the accepting task:

;;j (for all I1: GUARDS'RANGE =

>

not GUARDS(I1)) =

>

;;j raise PROGRAM ERROR,

;; The value returned in INDEX must be a component of INDICES: ;;j out(notABORTED ;

>

;;j exist I1: INDICES'RANGE=

>

INDEX= INDICES(I1)), ;; An aborted task is no longer CALLABLE:

;;j out(ABORTED ;

>

;;j (for all T1 : TASK NAME N1: NODE IDTYPE =

>

;;j notCALLABLEATTR(T1,MY NAME, N1)))

described in the next section.

MY NAME is set to the name of the task executing the simple

accept

or selective wait. The possible forms of

accept

statement or selective wait are dierentiated by the values of the parameters TERM ALT, TIME OUT and GUARDS. GUARDS and INDICES are arrays of

N

+ 1 components|the rst

N

components for the

N

entries of MY NAME, and the

N

+ 1

th

component for an

else

part,

delay

alternative or

terminate

alterna- tive. TERM ALT is set to TRUE only when executing a selective wait with a

terminate

alternative.

A simple

accept

statement for entry number

K

will have GUARDS (

K

) set to TRUE and all other GUARDS set to FALSE. If an entry is named as an

accept

alternative in a selective wait, the GUARDS component for the entry is set to the value of the guard on the alternative, or to TRUE if the alternative is unguarded. If an entry is not named in a selective wait, its GUARDS component is set to FALSE. If an entry is named in multiple

accept

alternatives of a selective wait, a single open alternative for the entry (if one exists) is chosen at runtime in an unspecied manner.

For a selective wait with an

else

part, GUARDS (

N

+1) is set to TRUE and TIME OUT is set to zero. For a selective wait with a

terminate

alternative, TERM ALT is set to TRUE, TIME OUT is set to DURATION'LAST (\innity") and GUARDS (

N

+ 1) is set to the value of the guard on the

terminate

alternative (or TRUE if there is no guard). For a selective wait with one or more

delay

alternatives, a single open alternative with the smallest delay value (if one exists) is chosen at runtime in an unspecied manner TIME OUT is set to the chosen delay value. For a selective wait that contains only

accept

alternatives, GUARDS (

N

+ 1) is set to FALSE.

The function of INDICES was explained in Section 4.6.1 the component of INDICES corresponding to the chosen alternative is returned in INDEX. The values of

in

-mode parameters to a selected

accept

are available in IN PARAMS upon termination of the procedure. Since an

accept

statement and selective wait are both synchronization points for completion of an abnormal task, an ABNORMAL parameter is provided.

ACCEPT BEGIN performs the beginning of the rendezvous message sequence of the Weatherly protocol. Characteristics of ACCEPT BEGIN which are a consequence of the semantics of Ada are formally specied in Anna in Figure 37.

4.6. THE SUPERVISOR INTERFACE 89

procedure ACCEPT END (MY NAME : in TASK NAME OUT PARAMS : in PARAM LIST LIST

ABORTED : out BOOLEAN

MY NODE : in out NODE IDTYPE) ;;jwhere

;; MY NAME's scope is unchanged:

;;j CURRENT SCOPE (MY NAME)= inCURRENT SCOPE (MYNAME), ;; An aborted task is no longer CALLABLE:

;;j out(ABORTED ;

>

;;j (for all T1 : TASK NAME N1: NODE IDTYPE =

>

;;j notCALLABLEATTR(T1,MY NAME, N1)))

Figure 38: The Supervisor ACCEPT END Procedure.

In document Design and verification of distributed tasking supervisors for concurrent programming languages (Page 103-107)