Exception Usage (EXU)












Have a plan for managing the use of Ada exceptions at the application level.


EXU01, EXU02, EXU03, EXU04

Exceptions in modern languages present the software architect with a dilemma. On one hand, exceptions can increase integrity by allowing components to signal specific errors in a manner that cannot be ignored, and, in general, allow residual errors to be caught. (Although there should be no unexpected errors in high integrity code, there may be some such errors due, for example, to unforeseeable events such as radiation-induced single-event upsets.) On the other hand, unmanaged use of exceptions increases verification expense and difficulty, especially flow analysis, perhaps to an untenable degree. In that case overall integrity is reduced or unwarranted.

In addition, programming languages may define some system-level errors in terms of language-defined exceptions. Such exceptions may be unavoidable, at least at the system level. For example, in Ada, stack overflow is signalled with the language-defined Storage_Error exception. Other system events, such as bus error, may also be mapped to language-defined or vendor-defined exceptions.

Complicating the issue further is the fact that, if exceptions are completely disallowed, there will be no exception handling code in the underlying run-time library. The effects are unpredictable if any exception actually does occur.

Therefore, for the application software the system software architect must decide whether to allow exceptions at all, and if they are to be used, decide the degree and manner of their usage. At the system level, the architect must identify the exceptions that are possible and how they will be addressed.