Re: Dynamic allocation in the predefined language environment

[Matthias-Christian Ott <ott@mirix.org>]

On 06/07/15 14:56, Bob Duff wrote:
> Matthias-Christian Ott <ott@mirix.org> writes:
> 
>> Then to finish this discussion: If it is required to raise
>> Storage_Error, then it should be in the standard.
> 
> It is in the standard.  See 11.1(6):
> 
> 6   The execution of any construct raises Storage_Error if there is
> insufficient storage for that execution. The amount of storage needed for the
> execution of constructs is unspecified.

I think that answers the first part of my question adequately, although
its in a surprising part of the specification and the "any construct" is
a bit ambiguous because it's in the dynamic semantics section of
exception declarations but seems to refer to any construct in the language.

>> ... There are only a
>> handful of Ada compilers which most likely all raise Storage_Error but I
>> think the history of C shows that if there is an ambiguity in the
>> standard, both compiler writers and software developers will interpret
>> it differently, so that both different compilers and the assumptions of
>> software developers about these compilers will differ.
> 
> Yes, all true, but 11.1(6) seems pretty unambiguous to me.
> 
> Actually handling Storage_Error can be tricky, though.  For one thing,
> in theory, the handler itself could raise Storage_Error, defeating the
> purpose.  More importantly, since Storage_Error can be raised by
> anything, it is essentially asynchronous with the rest of the program,
> so can leave data structures in a corrupted state.

If you follow the strong exception safety principle from C++, corrupted
data structures should not be a problem. However, you can only do this
in code that is under your control.

You can somewhat avoid Storage_Error exception while handling
Storage_Error exceptions by having "spare" memory that you free directly
after the exception handler is invoked. Otherwise there are many
application-specific possibilities on how to deal with memory allocation
errors in a spectrum ranging from simply aborting to degradation of
service/user experience.

- Matthias-Christian