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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Subject: Re: How do package body initializers get run in GNU Ada?
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Date: 2002-11-02T16:00:47+00:00
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Van Snyder <vsnyder@math.jpl.nasa.gov> writes:

> In GNU Ada, what mechanism is used to cause execution of package
> body initializers, i.e., the code between begin and end in a
> package body?
> 
> Is it done using some kind of back-patching/overlaying done
> by an otherwise garden-variety ld, by special features added
> to ld to support Ada, by first-time flags at the entries
> to procedures, or by something else?

I believe most Ada compilers do not use such features of the linker.
Instead, they provide a program called the "builder" or "prelinker".
The compiler generates a procedure for each compilation unit,
containing the elaboration code.  The builder constructs a "main" that
calls all those elaboration procedures in a correct order, and then
calls the user-defined main procedure.  The builder normally then 
invokes the system linker (e.g. 'ld'), with all that code, telling it
the automatically constructed "main" is the program's entry point (*not*
the user-defined main procedure).

Note that the elaboration code for a package body is not just the code
between "begin" and "end".  It also includes initialization code for all
kinds of declarations.  E.g., if you say "X: Integer := Func(...);", the
elaboration code must call Func and initialize X to the result.
Similarly, package specs also have elaboration code.

- Bob