[Voted into WP at October, 2009 meeting.]

Proposed resolution (September, 2009):

1. Change 6.9.1 [intro.execution] paragraph 3 as follows:

...An instance of the abstract machine can thus have more than one possible execution sequence for a given program and a given input.

2. Change 6.9.1 [intro.execution] paragraph 5 as follows:

A conforming implementation executing a well-formed program shall produce the same observable behavior as one of the possible execution sequences executions of the corresponding instance of the abstract machine with the same program and the same input. However, if any such execution sequence contains an undefined operation, this International Standard places no requirement on the implementation executing that program with that input (not even with regard to operations preceding the first undefined operation).

3. Delete 6.9.1 [intro.execution] paragraph 6, including the footnote:

The observable behavior of the abstract machine is its sequence of reads and writes to volatile data and calls to library I/O functions. [Footnote: An implementation can offer additional library I/O functions as an extension. Implementations that do so should treat calls to those functions as “observable behavior” as well. —end footnote]

4. Change 6.9.1 [intro.execution] paragraph 9 as follows:

The least requirements on a conforming implementation are:

• Access to volatile objects are evaluated strictly according to the rules of the abstract machine.

• At program termination, all data written into files shall be identical to one of the possible results that execution of the program according to the abstract semantics would have produced.

• The input and output dynamics of interactive devices shall take place in such a fashion that prompting messages actually appear prior to a program waiting prompting output is actually delivered before a program waits for input. What constitutes an interactive device is implementation-defined.

These collectively are referred to as the observable behavior of the program. [Note: more stringent correspondences between abstract and actual semantics may be defined by each implementation. —end note]

(Note; this resolution also resolves issue 612.)

In the presence of threads, it is no longer appropriate to characterize the abstract machine as having an “execution sequence.”