Re: Ada "library only" compiler ?

["Peter C. Chapin" <PChapin@vtc.vsc.edu>]

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

Of course the system interface isn't standardized whereas the C 
interface is (POSIX). If the raw interface to the underlying system 
changed the Ada interfacing code would have to change. Of course the 
same could be said for clib (the clients of clib wouldn't have to change 
but clib itself would).

I believe there is, in fact, an Ada standard interface to POSIX-like 
systems. My understanding is that there is no requirement that says the 
implementation of that interface must be done on top of C.

Peter