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From Marcel Offermans <marcel.offerm...@luminis.nl>
Subject Re: Exception Handling in C
Date Sun, 13 Feb 2011 23:03:30 GMT
Hello Bob,

First of all, welcome to the list!

On Feb 13, 2011, at 19:01 , J.A. Fels wrote:

> This is my first commit on this mailing list, so Ill briefly introduce
> myself: I'm Bob Fels, software engineer in embedded real time systems.
> For that I mainly use C, but I am also experienced in C++, Java, Python,
> Perl, various assembly flavors etc.

That's definitely one of the areas we're targetting with Celix. Just out of interest, are
you familiar with OSGi?

> I am every interested in using a service approach in embedded software,
> so Celix drew my interest.
> * Using returns
> For exception handling the most easy way is using states as return
> values of functions. This will keep your code really simple and you can
> still have predictable returns (and can still comply with Misra rule
> about one return per function).
> The try-catch macros look nice indeed and looks like a good way to let
> it like like the Java manner of exception handling, but I think
> something like the following is very readable:
> switch (my_function())
> {
> 		break;
> 		/* Do someting */
> 		break;
> 		/* Do someting */
> 		break;
> 	default:
> 		/* This is bad, return value not expected */
> 		break;
> }
> Don't forget to keep Celix simple however ;).

I agree that it's quite common in C to use return values to signal success or failure of a
function. Maybe by adopting exceptions, we can keep some of the service API's a bit closer
to their Java/OSGi equivalents, but the question is if that's worth the extra hassle of using
those macros and adopting something that is less common to C programmers. I have a slight
personal preference to sticking with using return values.

> * setjmp/longjmp
> Using setjmp and longjmp is possible even for real time systems as
> exceptions should not be part of the normal flow of the application and
> thus in exceptional situations the main thing that matters is getting
> back to a stable state.
> With setjmp and longjmp will make more return points in functions, so
> code flow will be harder to determine, as you can jump up multiple
> functions. Also you need to drag along that structure which has saved
> your stack, so it will be in every function call.
> Its no problem at all for threads in principle as you just save the
> stack and instruction pointer. Don't use it between threads however, but
> that will not happen with try-catch either. The other danger is that
> because of the sudden "return" its very easy for example to forget to
> release locks.
> Advantage is that it indeed does exactly the same as a throw in Java.

I have little experience with setjmp/longjmp (it's been a while since I've used C).

> * Conclusion
> I agree with your idea to go for the easy solution. It will keep your
> code simple. As I showed, even without libex you can still make the code
> look very recognizable and if you really want to, with some defines you
> can make it look more TRY-CATCHY.

+1 :)

> I hope this information/thoughts helps,
> best regards,
> Bob Fels
> On Sun, 2011-02-13 at 13:36 +0100, Alexander Broekhuis wrote:
>> Hello everyone,
>> Now that everything is in SVN, I would like to start working on the code again.
>> There is one important aspect which still needs a solution: Exception handling
>> Celix follows the OSGi spec (Java), but since C doesn't have
>> exceptions like Java, a solution is needed to be able to report
>> errors/exceptions.
>> I've been looking on the web, and found 2 possible solutions.
>> 1) Using a library that uses setjmp/longjmp:
>> Examples are:
>> - http://libexcept.sourceforge.net/
>> - http://www.nicemice.net/cexcept/
>> - http://code.google.com/p/exceptions4c/
>> 2) Use the return value of functions:
>> - For example by simply returning a int value indicating the error
>> - Use a library which supplies try/catch macros: http://code.google.com/p/libex/
>> The first solution makes it possible to have an API very similar to
>> the OSGi spec, but I am not sure how setjmp and longjmp interact with
>> threads and real time behavior.
>> The second solution is the most simple, and with the help of a library
>> it is still possible to have clear code with try/catch constructs.
>> Does anyone have any experience with these kind of libraries?
>> For now I tend to lean towards using a simple solution, either simply
>> returning the error code, or using something like libex (which is
>> lgpl, which might be a problem..).

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