Re: best practice: error handling

["Vinzent Hoefler" <0439279208b62c95f1880bf0f8776eeb@t-domaingrabbing.de>]

milouz wrote:

> I'm wondering about the best way to handle errors in Ada when calling
> a function/procedure that may fails.

Well, I'd say, there is no "best way". It depends on the application and
the precise context.

> For example, when programming on Unix systems in C, the called
> functions usually return a zero or positive value on success and -1 on
> failure, while setting a global variable (errno) to give some
> informations about the failure.

As rule of thumb I'd suggest the following:

If the error should not occur under (almost) all sane circumstances (for
instance, waiting on a semaphore should always succeed, unless the handle
is invalid), then raise an exception.

If you kind of expect the call to fail under normal conditions, a proper
out-Parameter (like "Success" ;) can be added.

Keep in mind, that returning some error requires a function, and until now
functions can't have "out"-parameters, so returning an "error code" is only
possible for those subroutines which do not need to have "out"-parameters.
So for the sake of having a consistent interface, you might want to to use an
"out"-parameter, regardless if the subroutine in question could otherwise be
expressed as function or not.

On the other hand, if you want a function to return its result, because it
looks more natural, you cannot use an "Success : out Boolean", so it may be
more convenient to raise an exception for errors instead of using an "out"-
parameter for the result.


Vinzent.

-- 
f u cn rd ths, u cn gt a gd jb n cmptr prgrmmng.