OS support for ATC's

OS support for ATC's

[sbelmont700]

The LRM 9.7.4 defines an "Asynchronous Transfer of Control" (otherwise known as an abortable select statment), where one part can be preempted by the acceptance of an entry in the other.  GNAT, however, defines a compiler-specific "polling" pragma that is ostensibly required to support ATC's on "targets that cannot otherwise support it", and Windows is given as the example.

I suppose it's more a GNAT-specific question, but how does one know if ATC's are supported on some arbitray target (DOS, Vxworks, brand-X linux, OS/2, etc)?  The LRM makes no mention that they might not be supported, so it seems odd to me that GNAT requires specific action to make it so, and moreso that they provide no particular guidance on when it's required and presumably would compile a program that would not work (i.e. if the polling pragma was not turned on for windows, I assume it would simply not abort when required).

-sb