tasking: GNAT semaphores

Author: leogermond

courses/fundamentals_of_ada/240_tasking/21_gnat_semaphores.rst

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+===============
+GNAT Semaphores
+===============
+
+----------
+Semaphores
+----------
+
+* Shared counters
+* Multitask-safe
+
+    - Support priorities from "Real-time Systems" LRM Annex D
+
+* :ada:`Counting_Semaphore` and :ada:`Binary_Semaphore`
+
+    - :ada:`protected` types
+    - Counting holds an Integer
+    - Binary holds a Boolean
+
+* Priority ceiling (LRM D.3)
+
+    - For :ada:`pragma Locking_Policy (Ceiling_Locking)`
+    - Protects against priority inversions
+
+
+---------
+Interface
+---------
+
+.. code:: Ada
+
+   protected type Counting_Semaphore
+      (Initial_Value : Natural;
+      [...]
+
+      entry Seize;
+      --  Blocks caller until/unless the semaphore's internal counter is
+      --  greater than zero. Decrements the semaphore's internal counter when
+      --  executed.
+
+      procedure Release;
+      --  Increments the semaphore's internal counter
+
+.. code:: Ada
+
+   protected type Binary_Semaphore
+     (Initially_Available : Boolean;
+
+.. code:: Ada
+
+   subtype Mutual_Exclusion is Binary_Semaphore
+     (Initially_Available => True,
+      Ceiling             => Default_Ceiling);
+
+------------------
+Idiom: Scope Locks
+------------------
+
+* Automatic release
+
+.. code:: Ada
+
+    type Scope_Lock (Lock : access Mutual_Exclusion) is
+       new Ada.Finalization.Limited_Controlled with null record;
+
+    procedure Initialize (This : in out Scope_Lock) is
+    begin
+      This.Lock.Seize;
+    end Initialize;
+
+    procedure Finalize (This : in out Scope_Lock) is
+    begin
+      This.Lock.Release;
+    end Finalize;
+    
+    Mutex : aliased Mutual_Exclusion;
+
+    State : Integer := 0;
+
+    procedure Operation_1 is
+       S : Scope_Lock (Mutex’Access);
+    begin
+       State := State + 1;  -- for example...
+       Put_Line ("State is now" & State'Img);
+    end Operation_1;