This paper shows how one can easily transform K definitions of programming languages into runtime verification tools. To increase the confidence that these runtime verification tools can be used for testing real-world programs, the paper uses KERNELC, a subset of the C programming language containing functions, memory allocation, pointer arithmetic, and input/output, which can be used to execute and test real C programs. KERNELC is extended with threads and synchronization constructs, and two concurrent semantics are derived from its sequential semantics. The first semantics, defining a sequentially consistent memory model, can be easily transformed into a runtime verification tool for checking datarace and deadlock freeness. The second semantics defines in a relatively minimal fashion a relaxed memory model based on the x86-TSO memory model. By exploring the executions of an implementation of Peterson’s mutual exclusion algorithm for both definitions, it is shown that the algorithm guarantees mutual exclusion for the sequentially consistent model, but cannot guarantee it for the relaxed model, but also that by allowing fence operations in the language, the algorithm can be fixed and proven correct for the TSO model, too.