Implementing Ada Exceptions

Abstract

Can exceptionls be x n the words of the 1983 Ada stand-fore important that exception handling not r^* l ard,Iexceptions are "errors or other impose significant overhead on execution implemented for Ada exceptional situations that arise dur-time or program size. This principle guided without imposing ing program execution. " The standard our consideration of implementation techoverhead on normal goes on: "To raise an exception is to aban-niques. don normal program execution so as to e.wcutiom? Yes, draw attention to the fact that the coras long as certain responding situation has arisen. Executing Exceptions in Ada rules are followed, some actions, in response to the arising of Ada permits a programmer to declare ' an exception, is called handling the exceptions local to any package, procedure, exception." or other declarative unit. It also provides This article discusses the implementa-some predefined exceptions. Any exception tion of exceptions and exception handling. can be raised explicitly by execution of the Provision for handling exceptions is not a raise statement. The predefined exceptions common feature in programming lan-can also be raised implicitly through runguages. In Ada, exception handling has time checks. peculiar interactions with other language For example, CONSTRAINT-ERROR features. We will explain enough of the Ada is a predefined exception. It can be raised semantics of exceptions to expose the explicitly by the statement "raise implementation problems. We will then CONSTRAINT-ERROR;". It can also be describe several approaches to implemen-raised implicitly, as when attempting to tation and compare their merits. Algo-evaluate an indexed component of an array rithms for the various strategies are given for an index outside the index range of the in an Ada-like pseudocode.array. This check, to determine whether an This discussion is based on our experi-index is within bounds, is called ence in designing and implementing excep-INDEX&CHECK. tion handling for an Ada compiler atThe basic approach to exception hanFlorida State University. The compiler is a dling followed in Ada is described by its cross-compiler, written in Pascal, hosted designers as "local termination. '2 They on a CDC Cyber 170/760 system, and tar-cite Bron, Fokkinga, and De Haas3 as geted to a bare Zilog Z8002 microcomputer inspiration for their design and note its system with 64K bytes of memory. The similarity to the Bliss signal enable concompiler, which supports the full 1983 struct.4 Additional studies of exception Ada standard , successfully completed vali-handling have been made by Gooddation testing on ACVC Version 1.6 in Sep-enough5 and Levin.6 tember 1985. It was developed under When an exception is raised in a task or contract to the USAF Armament Labora-main program, "normal program executory at Eglin AFB for use in development tion is abandoned and control is transof embedded real-time systems, such as ferred to an exception handler."1 Each those used in aircraft guidance.handler is associated with the sequence of ...

Mixing coroutines and processes in an Ada tasking implementation

“…It is possible to propagate exceptions during rendezvous and to abort virtual task s without any significant changes [Bake85,Bake86,Jha87] . It is not difficult to extend the implementatio n to support nested rendezvous [Habe8O, Vest87a] or selective wait statements with terminate alternative s [Hilf82,Bake85,Vest87a] .…”

Edo

“…Language-supported exception handling mechanisms, such as in Ada [Baker and Riccardi 1986], Modula-3 [Cardelli et al 1989], and C++ [Koenig and Stroustrup 1990], allow a programmer to write exception handlers for a region of the program. Java ] is one of the most popular languages today that support such a mechanism.…”