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From: Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de>
Newsgroups: comp.arch.embedded,comp.lang.ada
Subject: Re: Certified C compilers for safety-critical embedded systems
Date: Tue, 6 Jan 2004 13:33:36 +0000 (UTC)
Organization: GMUGHDU
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In comp.arch.embedded Dave Hansen <iddw@hotmail.com> wrote:
: On Wed, 31 Dec 2003 14:27:08 +0000 (UTC), Georg Bauhaus
: <sb463ba@l1-hrz.uni-duisburg.de> wrote:
: 
:>In comp.lang.ada Dave Hansen <iddw@hotmail.com> wrote:
: 
: Note: I'm reading from comp.arch.embedded.  Just so you know where I'm
: coming from...

Sorry. I need to adjust my reader software.

: Is the "'access" tag (or whatever you call it)

It's an attribute, like 'address, 'size, 'range, etc.

: Is the "'access" tag (or whatever you call it) _required_ on the call
: if a function is going to modify the parameter?  I like it even better
: if that's true.

Yes, as Stephen has explained. Let me add that if you want a variable
to be modifiably through some pointer you will have to say so.
You declare the variable aliased (using the "aliased" keyword
in the declaration), and point to it using 'access values.

There is also an 'unchecked_access attribute which allows pointers to
aliased local objects to be passed around. The intention is to remind
the programmer/maintainer of the pointer validity check that cannot
in general be done by the compiler as in the following example:

   type Reg16_Ptr is access all Interfaces.Unsigned_16;
   --  pointer to a 16 bit register


   function trig_reg return Reg16_Ptr is
      
      v: aliased Interfaces.Unsigned_16;
      --  16 bits from the toaster's eject trigger, v is local to function
      
      pragma Import(Ada, v);
      --  do not initialise v
      
      for v'address use System.Storage_Elements.To_Address(16#A022#);
      --  hardware address
   begin
      return v'unchecked_access;
      --  points to local v which is really a register, not a value on the
      -- stack
   end trig_reg;


-- Georg