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From: Stephen Leake <Stephen.Leake@gsfc.nasa.gov>
Subject: Re: Big-endian vs little-endian
Date: 1999/02/09
Message-ID: <ulni7e04i.fsf@gsfc.nasa.gov>#1/1
X-Deja-AN: 442549404
References: <36B155D2.2E8573BB@wvu.edu> <7982p9$nll$3@plug.news.pipex.net>
 <36BB9A6E.D472A709@wvu.edu>
Organization: NASA Goddard Space Flight Center -- Greenbelt, Maryland USA
Newsgroups: comp.lang.ada
Date: 1999-02-09T00:00:00+00:00
List-Id: <comp.lang.ada>

Mike Werner <mwerner@wvu.edu> writes:

> Here's the relevant data structure:
> 
>    type Sys_type is (Zarya, Unity, PMA1, PMA2);
>    type Subsys_type is (CDH, CT, ECLSS, EPS, GNC, SM);
>    subtype Desc_type is String(1..256);
>    subtype Dur_Min_Type is Integer;
>    subtype Dur_Sec_type is Integer;
>    type Apm_Rec is 
>       record
> 	 Description : Desc_Type;      
> 	 System : Sys_Type;
> 	 Subsystem : Subsys_Type;
> 	 Dur_Min : Dur_Min_Type;
> 	 Dur_Sec : Dur_Sec_Type;  
>       end record;
> 
> The problematic parts were the Apm_Rec.Dur_Min and the Apm_Rec.Dur_Sec -
> all the others read in just fine.

You have a byte-endianness problem. System.Bit_Order address a
bit-endiannes problem. They are similar, but different.

The best Ada solution is to use streams to read the binary file. You
have to define your own Integer type (you should do this anyway, to
make sure it is the same size as the school's server Integer type!).
Then you can define the stream read and write functions to do byte
swapping.

If you're not up to streams (quite understandable :), you can just use
Unchecked_Conversion. Assuming 32 bit integers, do something like:

type Network_4_Bytes is record
    Hi_Byte : Interfaces.Unsigned_8;
    Byte_3 : Interfaces.Unsigned_8;
    Byte_2 : Interfaces.Unsigned_8;
    Low_Byte : Interfaces.Unsigned_8;
end record;
pragma Pack (Network_4_Bytes);
for Network_4_Bytes'size use 32; -- confirm size

function To_Network is new Unchecked_conversion
    (Source => Interfaces.Integer_32,
     Target => Network_4_Bytes);

Of course, to make your code portable to your school computer, you'll
have to hide this in a body, and set a compile-time flag to decide
whether to swap bytes or not. I define a package Endianness to handle
the compile-time flag.

Good luck!

-- Stephe