RE: Static, Automatic and Heap Storage

[watern@xxxxxxxxxxxxxx]

Alan,

Thanks for your explanation.

I think what you are calling static and automatic storage I have come
across in the past, in other languages, as global and local storage. If I
remember rightly, in Quickbasic you could declare variables as being either
global or local. I think the only reason that variables in a subprocedure
would be global would be if other parts of the program needed to know about
them.

>The term static storage means that the storage is allocated at the start
of
>a program or module.
.....
>Automatic storage is different. Automatic storage only exists inside of a
>sub-procedure and only for the life of the sub-procedure. When you call
that
>function, any storage, i.e. variables, required are allocated from the
>system heap, .....

Trying to make sense of my original thinking, I can see that if
sflsiz=sflpag, since the compiler would know exactly how much storage was
required for subfile fields it would be possible for the subfile data to be
accomodated in static storage. If sflsiz>sflpag, then automatic storage
would have to be used as the amount of space is not known until run-time.

I thought automatic storage was used where the amount of space required was
unknown at compilation time, but you seem to be saying it is also used
where the program/procedure uses the same amount of space, but does not
need to retain values between calls.

I had the idea that when a program is called in a job, it uses space in the
PAG for holding file i/o handles etc and variables contained in static
storage.  Since automatic storage would always be outside the PAG, there
would be an overhead associated to handling data there.  I may have this
completely wrong. I am sure that compilation listing used to give
information about static storage and automatic storage areas used by the
program object. (PSSA and PASA).

>This reason this all important is that languages like ILE RPG (C type
>languages) are very efficient at managing automatic storage and very
>inefficient at managing static storage.

I am very surpised at this. I would have expected that static storage would
always be more efficient to manage as the amount of space required never
changes for different invocations of the same program.

As a footnote, the dsppgm command will show you space size for both static
and automatic storage for OPM programs, but will only show static storage
size for ILE programs.

I'm not quite sure where I am going with this one, other than confusing
myself more. Sorry !

I didnt previously understand the similarity between static/automatic and
global/local variables, so thanks for explaining that.

Rgds,
Nigel


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