6.5.1 Pragma No_Return
1/2
{
AI95-00329-01}
{
AI95-00414-01}
A pragma No_Return indicates
that a procedure cannot return normally[; it may propagate an exception
or loop forever]. 1.a/2
Discussion: Pragma
No_Deposit will have to wait for Ada 2017. :-)
Syntax
2/2
3/2
pragma
No_Return(procedure_local_name{,
procedure_local_name});
Legality Rules
4/2
{
AI95-00329-01}
{
AI95-00414-01}
{non-returning}
Each procedure_local_name
shall denote one or more procedures or generic procedures; the denoted
entities are non-returning. The procedure_local_name
shall not denote a null procedure nor an instance of a generic unit.4.a/2
Reason: A null
procedure cannot have the appropriate non-returning semantics, as it
does not raise an exception or loop forever.
4.b/2
Ramification: The
procedure can be abstract. The denoted declaration can be a renaming_declaration
if it obeys the usual rules for representation pragmas: the renaming
has to occur immediately within the same declarative_region
as the renamed subprogram. If a non-returning procedure is renamed (anywhere)
calls through the new name still have the non-returning semantics.
5/2
6/2
{
AI95-00414-01}
A procedure shall be non-returning if it overrides
a dispatching non-returning procedure. {generic
contract issue [partial]} In addition
to the places where Legality Rules normally apply (see 12.3),
this rule applies also in the private part of an instance of a generic
unit.6.a/2
Reason: This ensures
that dispatching calls to non-returning procedures will, in fact, not
return.
7/2
{
AI95-00414-01}
If a renaming-as-body completes a non-returning
procedure declaration, then the renamed procedure shall be non-returning.7.a/2
Reason: This ensures
that no extra code is needed to implement the renames (that is, no wrapper
is needed) as the body has the same property.
Static Semantics
8/2
{
AI95-00329-01}
{
AI95-00414-01}
If a generic procedure is non-returning, then so
are its instances. If a procedure declared within a generic unit is non-returning,
then so are the corresponding copies of that procedure in instances.
Dynamic Semantics
9/2
{
AI95-00329-01}
{
AI95-00414-01}
If the body of a non-returning procedure completes
normally, Program_Error is raised at the point of the call. {Program_Error
(raised by failure of run-time check)} 9.a/2
Discussion: Note
that there is no name for suppressing this check, since the check represents
a bug, imposes no time overhead, and minimal space overhead (since it
can usually be statically eliminated as dead code).
9.b/2
Implementation Note:
If a non-returning procedure tries to return, we raise Program_Error.
This is stated as happening at the call site, because we do not wish
to allow the procedure to handle the exception (and then, perhaps, try
to return again!). However, the expected run-time model is that the compiler
will generate raise Program_Error at the end of the procedure
body (but not handleable by the procedure itself), as opposed to doing
it at the call site. (This is just like the typical run-time model for
functions that fall off the end without returning a value). The reason
is indirect calls: in P.all(...);, the compiler cannot know whether
P designates a non-returning procedure or a normal one. Putting the raise
Program_Error in the procedure's generated code solves this problem neatly.
9.c/2
Similarly, if one passes
a non-returning procedure to a generic formal parameter, the compiler
cannot know this at call sites (in shared code implementations); the
raise-in-body solution deals with this neatly.
Examples
10/2
{
AI95-00433-01}
procedure Fail(Msg : String); -- raises Fatal_Error exception
pragma No_Return(Fail);
-- Inform compiler and reader that procedure never returns normallyExtensions to Ada 95
10.a/2
Sponsored by Ada-Europe