We study in this paper the problem of analyzing implementations of open systems --- systems in which only some of the components are present. We present an algorithm for automatically closing an open concurrent reactive system with its most general environment, i.e., the environment that can provide any input at any time to the system. The result is a nondeterministic closed (i.e., self-executable) system which can exhibit all the possible reactive behaviors of the original open system. These behaviors can then be analyzed using VeriSoft, an existing tool for systematically exploring the state spaces of closed systems composed of multiple (possibly nondeterministic) processes executing arbitrary code. We have implemented the techniques introduced in this paper in a prototype tool for automatically closing open programs written in the C programming language. We discuss preliminary experimental results obtained with a large telephone-switching software application developed at Lucent Technologies.

VeriSoft is a tool for systematically exploring the state spaces of systems composed of several concurrent processes executing arbitrary code written in full-fledged programming languages such as C or C++. The state space of a concurrent system is a directed graph that represents the combined behavior of all concurrent components in the system. By exploring its state space, VeriSoft can automatically detect coordination problems between the processes of a concurrent system.We report in this paper our analysis with VeriSoft of the "Heart-Beat Monitor" (HBM), a telephone switching application developed at Lucent Technologies. The HBM of a telephone switch determines the status of different elements connected to the switch by measuring propagation delays of messages transmitted via these elements. This information plays an important role in the routing of data in the switch, and can significantly impact switch performance.We discuss the steps of our analysis of the HBM using VeriSoft. Because no modeling of the HBM code is necessary with this tool, the total elapsed time before being able to run the first tests was on the order of a few hours, instead of several days or weeks that would have been needed for the (error-prone) modeling phase required with traditional model checkers or theorem provers.We then present the results of our analysis. Since VeriSoft automatically generates, executes and evaluates thousands of tests per minute and has complete control over nondeterminism, our analysis revealed HBM behavior that is virtually impossible to detect or test in a traditional lab-testing environment. Specifically, we discovered flaws in the existing documentation on this application and unexpected behaviors in the software itself. These results are being used as the basis for the redesign of the HBM software in the next commercial release of the switching software.