9.5.1 Protected Subprograms and Protected Actions
A
protected subprogram is a subprogram declared immediately within
a
protected_definition.
Protected procedures provide exclusive read-write access to the data
of a protected object; protected functions provide concurrent read-only
access to the data.
Static Semantics
Within the body of a protected function (or a function
declared immediately within a
protected_body),
the current instance of the enclosing protected unit is defined to be
a constant (that is, its subcomponents may be read but not updated).
Within the body of a protected procedure (or a procedure declared immediately
within a
protected_body),
and within an
entry_body,
the current instance is defined to be a variable (updating is permitted).
Dynamic Semantics
For the execution of a call on
a protected subprogram, the evaluation of the
name
or
prefix
and of the parameter associations, and any assigning back of
in out
or
out parameters, proceeds as for a normal subprogram call (see
6.4). If the call is an internal call (see
9.5), the body of the subprogram is executed
as for a normal subprogram call. If the call is an external call, then
the body of the subprogram is executed as part of a new
protected
action on the target protected object; the protected action completes
after the body of the subprogram is executed. A protected action can
also be started by an entry call (see
9.5.3).
A
new protected action is not started on a protected object while another
protected action on the same protected object is underway, unless both
actions are the result of a call on a protected function. This rule is
expressible in terms of the execution resource associated with the protected
object:
Starting
a protected action on a protected object corresponds to
acquiring
the execution resource associated with the protected object, either for
concurrent read-only access if the protected action is for a call on
a protected function, or for exclusive read-write access otherwise;
Completing
the protected action corresponds to
releasing the associated execution
resource.
After performing an operation on a protected object
other than a call on a protected function, but prior to completing the
associated protected action, the entry queues (if any) of the protected
object are serviced (see
9.5.3).
Bounded (Run-Time) Errors
During
a protected action, it is a bounded error to invoke an operation that
is
potentially blocking.
The
following are defined to be potentially blocking operations:
task creation or activation;
an external call on a protected subprogram (or
an external requeue) with the same target object as that of the protected
action;
a call on a subprogram whose body contains a potentially
blocking operation.
If the bounded error is detected,
Program_Error is raised. If not detected, the bounded error might result
in deadlock or a (nested) protected action on the same target object.
Certain language-defined subprograms are potentially
blocking. In particular, the subprograms of the language-defined input-output
packages that manipulate files (implicitly or explicitly) are potentially
blocking. Other potentially blocking subprograms are identified where
they are defined. When not specified as potentially blocking, a language-defined
subprogram is nonblocking.
19 If two tasks both try to start a protected
action on a protected object, and at most one is calling a protected
function, then only one of the tasks can proceed. Although the other
task cannot proceed, it is not considered blocked, and it might be consuming
processing resources while it awaits its turn. There is no language-defined
ordering or queuing presumed for tasks competing to start a protected
action — on a multiprocessor such tasks might use busy-waiting;
for monoprocessor considerations, see
D.3,
“
Priority Ceiling Locking”.
20 The body of a protected unit may contain
declarations and bodies for local subprograms. These are not visible
outside the protected unit.
21 The body of a protected function can
contain internal calls on other protected functions, but not protected
procedures, because the current instance is a constant. On the other
hand, the body of a protected procedure can contain internal calls on
both protected functions and procedures.
22 From within a protected action, an internal
call on a protected subprogram, or an external call on a protected subprogram
with a different target object is not considered a potentially blocking
operation.
23 The
pragma
Detect_Blocking may be used to ensure that all executions of potentially
blocking operations during a protected action raise Program_Error. See
H.5.
Examples
Examples of protected
subprogram calls (see 9.4):
Shared_Array.Set_Component(N, E);
E := Shared_Array.Component(M);
Control.Release;
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