Re: fixed point types over an interface

[Laurent Pautet <pautet@inf.enst.fr>]

    >> Another possibility is to use a target independent
    >> implementation of Stream_IO, such as is provided with GLADE
    >> (the GNAT implementation of the distributed annex PCS). GLADE
    >> uses XDR for stream representation, which avoids any endian (or
    >> other target dependent) problems.

    nassera> good choice. I just wanted to also add that Sun NFS and
    nassera> Sun RPC also uses XDR.

    nassera> man xdr: (on solaris 2.5)

    nassera> DESCRIPTION XDR routines allow C programmers to describe
    nassera> arbitrary data structures in a machine-independent
    nassera> fashion.  Data for remote procedure calls (RPC) are
    nassera> transmitted using these routines.

    nassera> by the way, I wonder why the man output above mentions
    nassera> only "C programmers".

Because it doesn't apply to Ada :-) For instance:

XDR says :

The representation of all items requires a multiple of four bytes (or
32 bits) of data.

LRM says :

For composite types, the Write or Read attribute for each component is
called in a canonical order ... Read and Write should use the smallest
number of stream elements needed to represent all values in the base
range of the scalar type.

So a string of 7 characters is stored on 8 bytes with XDR and 7 with Ada.

GLADE is only 95% XDR in order to follow LRM requirements. The 5%
comes from the XDR 4 bytes rule (basically strings).