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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Ada  "library only" compiler ?
Date: Sat, 21 Jul 2012 14:05:26 +0300
Organization: Tidorum Ltd
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Date: 2012-07-21T14:05:26+03:00
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On 12-07-21 01:25 , Peter C. Chapin wrote:
> On 2012-07-20 17:11, Niklas Holsti wrote:
>
>> GNAT runs on operating systems (Windows, Linux, etc.) in which the
>> Application Programming Interface is defined in C. In order to open a
>> file under Linux, the application (here GNAT) *must* use the open() or
>> fopen() functions, which are defined in the API as C functions. Trying
>> to open a file using "low-level Ada" would be breaking the OS interface
>> which is obviously a bad idea (and probably impossible because an
>> application does not have the necessary low-level access rights for
>> that).
>
> I don't think this is quite right. Entrance into the operating system is
> ultimately done in machine language. The C API functions you're talking
> about are thin wrappers that present a C callable interface and then do
> what must be done to actually invoke the system.

You are right, in principle, and I was simplifiying. With "low-level 
Ada" I meant accessing machine-level stuff like I/O ports.

> For some systems "doing what must be done" involves executing exotic
> instructions that (standard) C compilers can't generate.

I would guess that they mostly use assembly-code insertions in C source.

> Thus one supposes that functions  like open(), etc, are written in
> assembly language or its moral equivalent.
>
> This could be done for Ada. An Ada procedure could be written that wraps
> the same assembly language magic but that provides an Ada interface to
> the system... no need to use clib in that case.

True, and Randy Brukardt says in a later post they did it in that way 
for the Janus/Ada compiler on SCO Unix.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .