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<H1> C.7.2 The Package Task_Attributes</H1>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> The following language-defined
generic library package exists: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="Examples"><TT><B>with</B> Ada.Task_Identification; <B>use</B> Ada.Task_Identification;<BR>
<B>generic</B><BR>
<B>type</B> Attribute <B>is</B> <B>private</B>;<BR>
Initial_Value : <B>in</B> Attribute;<BR>
<B>package</B> Ada.Task_Attributes <B>is</B><A NAME="I6465"></A><A NAME="I6466"></A><A NAME="I6467"></A></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Examples"><TT> <B>type</B> <A NAME="I6468"></A><A NAME="I6469"></A>Attribute_Handle <B>is</B> <B>access</B> <B>all</B> Attribute;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Examples"><TT> <B>function</B> <A NAME="I6470"></A><A NAME="I6471"></A>Value(T : Task_ID := Current_Task)<BR>
<B>return</B> Attribute;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Examples"><TT> <B>function</B> <A NAME="I6472"></A><A NAME="I6473"></A>Reference(T : Task_ID := Current_Task)<BR>
<B>return</B> Attribute_Handle;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Examples"><TT> <B>procedure</B> <A NAME="I6474"></A><A NAME="I6475"></A>Set_Value(Val : <B>in</B> Attribute;<BR>
T : <B>in</B> Task_ID := Current_Task);<BR>
<B>procedure</B> <A NAME="I6476"></A><A NAME="I6477"></A>Reinitialize(T : <B>in</B> Task_ID := Current_Task);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Examples"><TT><B>end</B> Ada.Task_Attributes;</TT></DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal"> When an instance of Task_Attributes is elaborated
in a given active partition, an object of the actual type corresponding
to the formal type Attribute is implicitly created for each task (of
that partition) that exists and is not yet terminated. This object acts
as a user-defined attribute of the task. A task created previously in
the partition and not yet terminated has this attribute from that point
on. Each task subsequently created in the partition will have this attribute
when created. In all these cases, the initial value of the given attribute
is Initial_Value.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Normal"> The Value operation returns the value of the corresponding
attribute of T.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Normal"> The Reference operation returns an access value
that designates the corresponding attribute of T.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Normal"> The Set_Value operation performs any finalization
on the old value of the attribute of T and assigns Val to that attribute
(see <A HREF="RM-5-2.html">5.2</A> and <A HREF="RM-7-6.html">7.6</A>).</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Normal"> The effect of the Reinitialize operation is the
same as Set_Value where the Val parameter is replaced with Initial_Value.
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6478"></A>For all the operations declared
in this package, Tasking_Error is raised if the task identified by T
is terminated. <A NAME="I6479"></A>Program_Error is raised if the value
of T is Null_Task_ID. </DIV>
<H4 ALIGN=CENTER>Bounded (Run-Time) Errors</H4>
<DIV Class="Paranum"><FONT SIZE=-2>13.1/1</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6480"></A>If the package Ada.Task_Attributes
is instantiated with a controlled type and the controlled type has user-defined
Adjust or Finalize operations that in turn access task attributes by
any of the above operations, then a call of Set_Value of the instantiated
package constitutes a bounded error. The call may perform as expected
or may result in forever blocking the calling task and subsequently some
or all tasks of the partition. </DIV>
<H4 ALIGN=CENTER>Erroneous Execution</H4>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6481"></A>It is erroneous to dereference
the access value returned by a given call on Reference after a subsequent
call on Reinitialize for the same task attribute, or after the associated
task terminates. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6482"></A>If a value of Task_ID is
passed as a parameter to any of the operations declared in this package
and the corresponding task object no longer exists, the execution of
the program is erroneous.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15.1/1</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6483"></A>Accesses to task attributes
via a value of type Attribute_Handle are erroneous if executed concurrently
with each other or with calls of any of the operations declared in package
Task_Attributes. </DIV>
<H4 ALIGN=CENTER>Implementation Requirements</H4>
<DIV Class="Paranum"><FONT SIZE=-2>16/1</FONT></DIV>
<DIV Class="Normal"> For a given attribute of a given task, the
implementation shall perform the operations declared in this package
atomically with respect to any of these operations of the same attribute
of the same task. The granularity of any locking mechanism necessary
to achieve such atomicity is implementation defined. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>17</FONT></DIV>
<DIV Class="Normal"> When a task terminates, the implementation shall
finalize all attributes of the task, and reclaim any other storage associated
with the attributes. </DIV>
<H4 ALIGN=CENTER>Documentation Requirements</H4>
<DIV Class="Paranum"><FONT SIZE=-2>18</FONT></DIV>
<DIV Class="Normal"> The implementation shall document the limit on
the number of attributes per task, if any, and the limit on the total
storage for attribute values per task, if such a limit exists.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>19</FONT></DIV>
<DIV Class="Normal"> In addition, if these limits can be configured,
the implementation shall document how to configure them. </DIV>
<H4 ALIGN=CENTER>Metrics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>20</FONT></DIV>
<DIV Class="Normal"> The implementation shall document the following
metrics: A task calling the following subprograms shall execute in a
sufficiently high priority as to not be preempted during the measurement
period. This period shall start just before issuing the call and end
just after the call completes. If the attributes of task T are accessed
by the measurement tests, no other task shall access attributes of that
task during the measurement period. For all measurements described here,
the Attribute type shall be a scalar whose size is equal to the size
of the predefined integer size. For each measurement, two cases shall
be documented: one where the accessed attributes are of the calling task
(that is, the default value for the T parameter is used), and the other,
where T identifies another, non-terminated, task.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>21</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> The following calls
(to subprograms in the Task_Attributes package) shall be measured: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>22</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a call to Value, where the return value is Initial_Value;</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>23</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a call to Value, where the return value is not equal to
Initial_Value;</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>24</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a call to Reference, where the return value designates
a value equal to Initial_Value;</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>25</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a call to Reference, where the return value designates
a value not equal to Initial_Value;</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>26</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a call to Set_Value where the Val parameter is not equal
to Initial_Value and the old attribute value is equal to Initial_Value.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>27</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a call to Set_Value where the Val parameter is not equal
to Initial_Value and the old attribute value is not equal to Initial_Value.</LI></UL>
<H4 ALIGN=CENTER>Implementation Permissions</H4>
<DIV Class="Paranum"><FONT SIZE=-2>28</FONT></DIV>
<DIV Class="Normal"> An implementation need not actually create the
object corresponding to a task attribute until its value is set to something
other than that of Initial_Value, or until Reference is called for the
task attribute. Similarly, when the value of the attribute is to be reinitialized
to that of Initial_Value, the object may instead be finalized and its
storage reclaimed, to be recreated when needed later. While the object
does not exist, the function Value may simply return Initial_Value, rather
than implicitly creating the object. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>29</FONT></DIV>
<DIV Class="Normal"> An implementation is allowed to place restrictions
on the maximum number of attributes a task may have, the maximum size
of each attribute, and the total storage size allocated for all the attributes
of a task.</DIV>
<H4 ALIGN=CENTER>Implementation Advice</H4>
<DIV Class="Paranum"><FONT SIZE=-2>30</FONT></DIV>
<DIV Class="Normal"> Some implementations are targeted to domains
in which memory use at run time must be completely deterministic. For
such implementations, it is recommended that the storage for task attributes
will be pre-allocated statically and not from the heap. This can be accomplished
by either placing restrictions on the number and the size of the task's
attributes, or by using the pre-allocated storage for the first N attribute
objects, and the heap for the others. In the latter case, N should be
documented.</DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>31</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>12 An attribute always exists
(after instantiation), and has the initial value. It need not occupy
memory until the first operation that potentially changes the attribute
value. The same holds true after Reinitialize.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>32</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>13 The result of the Reference
function should be used with care; it is always safe to use that result
in the task body whose attribute is being accessed. However, when the
result is being used by another task, the programmer must make sure that
the task whose attribute is being accessed is not yet terminated. Failing
to do so could make the program execution erroneous.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>33</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>14 As specified in <A HREF="RM-C-7-1.html">C.7.1</A>,
if the parameter T (in a call on a subprogram of an instance of this
package) identifies a nonexistent task, the execution of the program
is erroneous. </FONT></DIV>
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