Re: Problem with intertask comm

[Ted Dennison<dennison@telepath.com>]

In article <3ad5ecf3$0$191$456d72a3@news.skynet.be>, Tom De Muer says...
>I'm doing the same assignment as Pieter Thysebaert who posted a message some
>days ago but I came across another problem:
>
>"The prefix to 'ACCESS shall not be statically deeper than that of the
>expected type, Continuing"
..
>** ** patient_data_a : aliased Patient_Data := (id => 1, my_monitor =>
>monitor_machine_a'access); ** **
> patient_a   : PatientTask(info_for_patient => patient_data_a'access);

The problem is more or less what the error message says it is. However, the
message is written in LRM-eese. I'll try to translate.

Your pointed-to object (the type of monitor_machine_a) is declared inside a
subprogram somewhere. That means that the object will cease to exist when that
subprogram exits. If that happens, any pointers to that object will suddenly be
pointing to unallocated (or worse, allocated for other purposes) memory. That
would clearly be a Bad Thing. The language would like to prevent that from
happening, by creating rules that make that impossible. Restricting what objects
you can copy it into wouldn't work, as that would force the compiler to track
the flow of that value through your code. So instead it restricts the *type* of
objects you can copy that value into. Specificly, you can't copy that value into
an object whose type was declared at a higher scope than the object being
pointed to (and thus might exist when the object itself no longer does).

To get around this, you can do one of the following:

o  Move the declaration of Patient_Data_A into a higher scope.
o  Move the declaration of the type that the Info_For_Patient field uses into a
lower scope.
o  Use 'Unchecked_Access instead of 'Access. Note that if you use this option
you must be able to certify that the object can't possibly ever be used after it
has gone out of scope. Your instructor might still not be happy about it.

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