Abstract Fast Breakpoints: Design and Implementation

We have designed and implemented a fast breakpoint facility. Breakpoints are usually thought of as a feature of an interactive debugger, in which case the break-points need not be particularly fast. In our environment breakpoints are often used for non-interactive informa-tion gathering: for example, procedure call count and statement execution count profiling [Swinehart, et al.]. When used non-interactively, breakpoints should be as fast as possible, so as to perturb the execution of the program as little as possible. Even in interactive debuggers, a conditional breakpoint facility would benefit from breakpoints that could transfer to the evaluation of the condition rapidly, and continue expeditiously if the condition were not satisfied. Such conditional breakpoints could be used to check assertions, etc. Program advising could also make use of fast breakpoints [Teitelman]. Examples of advising include tracing, timing, and even animation, all of which should be part of an advanced programming environment. We have ported the Cedar environment from a machine with microcode support for breakpoints [Lampson and Pier] to commercial platforms running C code [Atkinson, et al.]. Most of our ports run under the Unix * operating system, so one choice for implementing breakpoints for Cedar was to use the breakpoint facility provided by that system. The breakpoints provided by the Unix operating system are several orders of magnitude too slow (and also several process switches too complicated) for the applications we have in mind. * Unix is a trademark of AT&T Bell Laboratories. Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct corn-mercial advantage, the ACM copyright notice and the titie of the publication and its date appear, and notice is given that copying is by permission of the Association for Computing Machinery. To copy otherwise, or to republish, requires a fee and/or specific permission