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<H1> 9.7.3 Conditional Entry Calls</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> [A <FONT FACE="Arial, Helvetica">conditional_entry_call</FONT>
issues an entry call that is then cancelled if it is not selected immediately
(or if a requeue-with-abort of the call is not selected immediately).]
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>1.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>To be honest: </B>In the case
of an entry call on a protected object, it is OK if the entry is closed
at the start of the corresponding protected action, so long as it opens
and the call is selected before the end of that protected action (due
to changes in the Count attribute). </FONT></DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">conditional_entry_call<A NAME="I3778"></A>
::= </FONT><BR>
<B>select</B><BR>
<A NAME="I3779"></A><FONT FACE="Arial, Helvetica">entry_call_alternative</FONT><BR>
<B>else</B><BR>
<A NAME="I3780"></A><FONT FACE="Arial, Helvetica">sequence_of_statements</FONT><BR>
<B>end select</B>;</DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3781"></A>The execution of a <FONT FACE="Arial, Helvetica">conditional_entry_call</FONT>
is defined to be equivalent to the execution of a <FONT FACE="Arial, Helvetica">timed_entry_call</FONT>
with a <FONT FACE="Arial, Helvetica">delay_alternative</FONT> specifying
an immediate expiration time and the same <FONT FACE="Arial, Helvetica">sequence_of_statements</FONT>
as given after the reserved word <B>else</B>. </DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>37 A <FONT FACE="Arial, Helvetica">conditional_entry_call</FONT>
may briefly increase the Count attribute of the entry, even if the conditional
call is not selected.</FONT></DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <I>Example of a
conditional entry call:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Examples"><TT><B>procedure</B> Spin(R : <B>in</B> Resource) <B>is</B><BR>
<B>begin</B><BR>
<B>loop</B><BR>
<B>select</B><BR>
R.Seize;<BR>
<B>return</B>;<BR>
<B>else</B><BR>
<B>null</B>; <I>-- busy waiting</I><BR>
<B>end</B> <B>select</B>;<BR>
<B>end</B> <B>loop</B>;<BR>
<B>end</B>;</TT></DIV>
<H4 ALIGN=CENTER>Wording Changes from Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>6.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>This clause comes after the one
for Timed Entry Calls, so we can define conditional entry calls in terms
of timed entry calls. We do that so that an "expiration time"
is defined for both, thereby simplifying the definition of what happens
on a requeue-with-abort. </FONT></DIV>
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