During the execution of an event handler, CMC runs the implementation code directly, automatically catching errors such as pointer access violations and program assertion failures present in the code. In addition, CMC detects use-after-free bugs by overwriting any freed memory with a random value.
Once a state is generated, CMC checks for violations of user-provided system invariants (such as the absence of global routing loops). Also, CMC detects memory leaks in each generated state. While this can be achieved by a standard mark-and-sweep algorithm to find all reachable memory, such an algorithm is not yet implemented in CMC. Instead, in our case study, CMC detects memory leaks as follows: starting from a copy of the current state, CMC calls various cleanup functions present in the implementation itself. Any heap memory that is left allocated is reported as leaked. This approach, while requiring additional manual effort, can also potentially find bugs in the cleanup code.
In the future, the CMC approach could easily be coupled with other dynamic debugging tools such as Purify or StackGuard. These tools can catch run-time errors such as uses of uninitialized memory, stack overflows, etc. Such tools would be more effective when used with CMC than with ordinary testing, because CMC would achieve greater effective test coverage for a given level of user effort than conventional software testing methods.