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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Where to get Zero Footprint Profile?
Date: Sun, 25 May 2014 20:39:16 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2014-05-25T20:39:16-04:00
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droiddermo@gmail.com writes:

> ...For example I don't quite understand what is secondary stack
> and what it is used for.

The secondary stack is used for caller-unknown-sized function results.
That is, function results that are of indefinite subtype.
For example, if you have:

    function F(...) return String is
        Length: Natural := ...; -- who knows what?
        Result: String(1..Length) := (others => 'A');
    begin
        return Result;
    end F;

Then somewhere else:

    X: constant String := F(...);

then F will return its result on the secondary stack.
The caller (declaration of X) doesn't know the size of the
result of F.  So F allocates its result on the secondary stack,
and then returns the address of that result to the caller.

You can look at s-secsta.ad[sb] to see how it works by default.

If the result subtype of F were "subtype S is String(1..80);",
then it would not use the secondary stack.

- Bob