Re: Naive question about system dependencies

[stt@spock.camb.inmet.com (Tucker Taft)]

In article <hholm.110.2E77EA6F@clark.net>, Howard Holm <hholm@clark.net> wrote:

>I have some experience using Ada in a classroom setting, but unfortunately 
>little experience using it in any real-world settings.  I was considering how 
>I would go about writing a program that was intended to be used in several 
>different operating system environments (e.g. OS/2 and UNIX).  If I were using 
>C, I would either use compiler generated macros, if they existed, or create 
>some macros in the makefile that could be used to determine which system was 
>the target (i.e. #ifdef OS2).  The most likely equivalent seemed to me to be 
>the System_Name constant in the System package.  However, when I checked, the 
>Gnat compiler simply uses "GNAT" for that definition.  So, my question is how 
>do you go about accounting for differences in GUI's and the like?  Do you have 
>a build directory that you copy implementation specific low-level packages 
>into and out of for each build, or do you define a search path and have a 
>collection of implementation specific directories and an independent 
>directory? Or, did I overlook some other language featue to address this 
>question?

Some people use the C preprocessor (or equivalent) with Ada source
code.  However, in our environment we have found it is more
maintainable to avoid using the preprocessor approach, and define
interfaces to isolate target/host independent code from target/host 
dependent code, and then have separate bodies to implement
the interface for each target/host.

Although this might seem like more work, in the long run
it vastly simplifies the process of moving to a new target
or host, and keeps the code much more readable than when
ifdef's and associated target dependences are littered around the code.

>Howard Holm
>hholm@clark.net

-Tucker Taft  stt@inmet.com