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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: Address to function pointer conversion
Date: 1997/06/14
Message-ID: <dewar.866305487@merv>#1/1
X-Deja-AN: 248381501
References: <5nrq5h$13cm@info4.rus.uni-stuttgart.de>
Organization: New York University
Newsgroups: comp.lang.ada
Date: 1997-06-14T00:00:00+00:00
List-Id: <comp.lang.ada>


Michael says

<<I just stumbled over a little problem. I have a system address and I would 
like to
convert it to an access to function type. How can this be done correctly? 
There is
a generic package System.Address_To_Access_Conversions which takes an
object type and then provides an access to object type and the appropriate
conversion routines from address to access type and  vice versa. But, what is
the object of an access to function type here which I need to instantiate 
this
package? Any suggestions are welcome.
 
Michael
 
PS: I currently just use an unchecked conversion which just works fine, but
that's not the answer I am looking for.
 >>



Well, it is the only possible answer! It is clear that the language provides
no such capability, because the very *idea* of converting an address to
an access-to-function type is highly implementation dependent. What makes
you think that an access-to-function value is anything like an address?

It often is not. For example, in gcc it is often the address of a trampoline
on a local stack. In the DEC ABI, an access to function value is a pointer
to a procedure descriptor.

Unchecked conversion may work on one particular implementation, and you feel
nervous using it, because you are afraid that it might be implementation
dependent. GOOD! You should feel nervous, whatever you feel you need this
for should be reconsidered carefully, since you are venturing into the
areas of highly non-portable code.