[Info-vax] VMS internals design, was: Re: BASIC and AST routines

[Stephen Hoffman]

On 2021-12-01 19:41:00 +0000, Dave Froble said:

> On 12/1/2021 10:52 AM, Stephen Hoffman wrote:
>> On 2021-12-01 02:11:19 +0000, Phil Howell said:
>> 
>>> You are not  alone in your confusion
>>> See this post from a long time ago
>>> https://community.hpe.com/t5/Operating-System-OpenVMS/AST-routine-and-C-language-va-count-va-start-va-end-etc/td-p/4878940#.YabUqew8arU 
>>> 
>>> 
>> 
>> I'd forgotten about that thread.
>> 
>> What a wonderfully inconsistent trashfire ASTs are.
>> 
>> Somebody at VSI probably now has some (more) writing to do, and some 
>> (more) of the existing documentation to review.
>> 
>> And it seems some BASIC declaration somewhere for the AST API is 
>> arguably busted.
>> 
>> Ah, well.
>> 
>> 
> 
> I'm a bit afraid to ask another question.  The last question I asked 
> seemed to start weeks of, not sure what to call it, but some of it was 
> rather nasty.
> 
> Oh, well, another question.
> 
> I haven't done any research, so the question might have a simple answer.
> 
> When as AST is specified while calling a system service, and an AST 
> parameter can be specified, other than following the docs, what causes 
> me to need to specify 5 parameters in the AST subroutine?  Unless I 
> declare the subroutine with 5 parameters, I don't know what might 
> enforce such a requirement.
> 
> Ok, I really should just go and try it myself, but, I'm lazy.  Anyone 
> have a simple answer?

I usually declare the subroutine with one argument for an AST routine, 
and that's the context pointer. That's worked in C and C++ for an aeon 
or three.

Though whether it breaks with x86-64 port?

And I usually use a pointer to some app-local data structure, as that's 
where I stash the IOSB or whatever other connection-specific details 
are required for the AST.

It's also where I stash the "unwind in progress" flag, if I'm 
cancelling some operation and it's unclear whether the cancel or the 
AST will arrive first.

If that one-argument declaration is tolerated by BASIC, use it.

The Linker isn't particularly sensitive to API declarations, and will 
probably not notice any API differences. API contract "enforcement" 
here is usually by app failure.

Otherwise—if BASIC won't play nice with a one-argument AST 
declaration—specify the context pointer and whatever other four values 
will be tolerated by BASIC and the Linker.



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