Synchronous Barriers
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

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# D.10.1 Synchronous Barriers

1/3
This subclause introduces a language-defined package to synchronously release a group of tasks after the number of blocked tasks reaches a specified count value.

#### Static Semantics

2/3
The following language-defined library package exists:
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package Ada.Synchronous_Barriers is
pragma Preelaborate(Synchronous_Barriers);
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subtype Barrier_Limit is Positive range 1 .. implementation-defined;
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type Synchronous_Barrier (Release_Threshold : Barrier_Limit) is limited private;
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procedure Wait_For_Release (The_Barrier : in out Synchronous_Barrier;
Notified : out Boolean);
7/3
private
-- not specified by the language
end Ada.Synchronous_Barriers;
8/3
Type Synchronous_Barrier needs finalization (see 7.6).

#### Dynamic Semantics

9/3
Each call to Wait_For_Release blocks the calling task until the number of blocked tasks associated with the Synchronous_Barrier object is equal to Release_Threshold, at which time all blocked tasks are released. Notified is set to True for one of the released tasks, and set to False for all other released tasks.
10/3
The mechanism for determining which task sets Notified to True is implementation defined.
11/3
Once all tasks have been released, a Synchronous_Barrier object may be reused to block another Release_Threshold number of tasks.
12/3
As the first step of the finalization of a Synchronous_Barrier, each blocked task is unblocked and Program_Error is raised at the place of the call to Wait_For_Release.
13/3
It is implementation defined whether an abnormal task which is waiting on a Synchronous_Barrier object is aborted immediately or aborted when the tasks waiting on the object are released.
14/3
Wait_For_Release is a potentially blocking operation (see 9.5.1).

#### Bounded (Run-Time) Errors

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It is a bounded error to call Wait_For_Release on a Synchronous_Barrier object after that object is finalized. If the error is detected, Program_Error is raised. Otherwise, the call proceeds normally, which may leave a task blocked forever.

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