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<H1> D.11 Asynchronous Task Control</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> [This clause introduces a language-defined package
to do asynchronous suspend/resume on tasks. It uses a conceptual <I>held
priority</I> value to represent the task's <I>held</I> state.] </DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> The following language-defined
library package exists: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Examples"><TT><B>with</B> Ada.Task_Identification;<BR>
<B>package</B> Ada.Asynchronous_Task_Control <B>is</B><A NAME="I6927"></A><A NAME="I6928"></A><A NAME="I6929"></A><BR>
<B>procedure</B> <A NAME="I6930"></A><A NAME="I6931"></A>Hold(T : <B>in</B> Ada.Task_Identification.Task_ID);<BR>
<B>procedure</B> <A NAME="I6932"></A><A NAME="I6933"></A>Continue(T : <B>in</B> Ada.Task_Identification.Task_ID);<BR>
<B>function</B> <A NAME="I6934"></A><A NAME="I6935"></A>Is_Held(T : Ada.Task_Identification.Task_ID)<BR>
<B>return</B> Boolean;<BR>
<B>end</B> Ada.Asynchronous_Task_Control;</TT></DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6936"></A><A NAME="I6937"></A><A NAME="I6938"></A>After
the Hold operation has been applied to a task, the task becomes <I>held</I>.
For each processor there is a conceptual <I>idle task</I>, which is always
ready. The base priority of the idle task is below System.Any_Priority'First.
The <I>held priority</I> is a constant of the type integer whose value
is below the base priority of the idle task. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>The held state
should not be confused with the blocked state as defined in <A HREF="AA-9-2.html">9.2</A>;
the task is still ready. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal"> The Hold operation sets the state of T to held.
For a held task: the task's own base priority does not constitute an
inheritance source (see <A HREF="AA-D-1.html">D.1</A>), and the value
of the held priority is defined to be such a source instead. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>For example,
if T is currently inheriting priorities from other sources (e.g. it is
executing in a protected action), its active priority does not change,
and it continues to execute until it leaves the protected action. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> The Continue operation resets the state of T to
not-held; T's active priority is then reevaluated as described in <A HREF="AA-D-1.html">D.1</A>.
[This time, T's base priority is taken into account.]</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal"> The Is_Held function returns True if and only
if T is in the held state. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>Note that the
state of T can be changed immediately after Is_Held returns. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal"> As part of these operations, a check is made that
the task identified by T is not terminated. <A NAME="I6939"></A>Tasking_Error
is raised if the check fails. <A NAME="I6940"></A>Program_Error is raised
if the value of T is Null_Task_ID.</DIV>
<H4 ALIGN=CENTER>Erroneous Execution</H4>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6941"></A>If any operation in this package
is called with a parameter T that specifies a task object that no longer
exists, the execution of the program is erroneous. </DIV>
<H4 ALIGN=CENTER>Implementation Permissions</H4>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Normal"> An implementation need not support Asynchronous_Task_Control
if it is infeasible to support it in the target environment. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>A direct implementation
of the Asynchronous_Task_Control semantics using priorities is not necessarily
efficient enough. Thus, we envision implementations that use some other
mechanism to set the ``held'' state. If there is no other such mechanism,
support for Asynchronous_Task_Control might be infeasible, because an
implementation in terms of priority would require one idle task per processor.
On some systems, programs are not supposed to know how many processors
are available, so creating enough idle tasks would be problematic. </FONT></DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>33 It is a consequence of
the priority rules that held tasks cannot be dispatched on any processor
in a partition (unless they are inheriting priorities) since their priorities
are defined to be below the priority of any idle task.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>34 The effect of calling
Get_Priority and Set_Priority on a Held task is the same as on any other
task.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>35 Calling Hold on a held
task or Continue on a non-held task has no effect.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Notes" Style="margin-bottom: 0.4em"><FONT SIZE=-1>36 The
rules affecting queuing are derived from the above rules, in addition
to the normal priority rules: </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<UL Class="NotesBulleted"><FONT SIZE=-1><LI TYPE=DISC>When a held task is on the ready queue, its priority is
so low as to never reach the top of the queue as long as there are other
tasks on that queue.</LI></FONT></UL>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<UL Class="NotesBulleted"><FONT SIZE=-1><LI TYPE=DISC>If a task is executing in a protected action, inside a
rendezvous, or is inheriting priorities from other sources (e.g. when
activated), it continues to execute until it is no longer executing the
corresponding construct.</LI></FONT></UL>
<DIV Class="Paranum"><FONT SIZE=-2>17</FONT></DIV>
<UL Class="NotesBulleted"><FONT SIZE=-1><LI TYPE=DISC>If a task becomes held while waiting (as a caller) for
a rendezvous to complete, the active priority of the accepting task is
not affected.</LI></FONT></UL>
<DIV Class="Paranum"><FONT SIZE=-2>18/1</FONT></DIV>
<UL Class="NotesBulleted"><FONT SIZE=-1><LI TYPE=DISC>{<I><A HREF="defect1.html#8652/0077">8652/0077</A></I>}
If a task becomes held while waiting in a <FONT FACE="Arial, Helvetica">selective_accept</FONT>,
and a<U>n</U> entry call is issued to one of the open entries, the corresponding
<U><FONT FACE="Arial, Helvetica">accept_alternative</FONT></U><S>accept
body</S> executes. When the rendezvous completes, the active priority
of the accepting task is lowered to the held priority (unless it is still
inheriting from other sources), and the task does not execute until another
Continue.</LI></FONT></UL>
<DIV Class="Paranum"><FONT SIZE=-2>19</FONT></DIV>
<UL Class="NotesBulleted"><FONT SIZE=-1><LI TYPE=DISC>The same holds if the held task is the only task on a protected
entry queue whose barrier becomes open. The corresponding entry body
executes.</LI></FONT></UL>
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