VeriFast is a prototype verification tool for single-threaded and multithreaded C and Java programs. In this paper, we first describe the basic symbolic execution approach in some formal detail. Then we zoom in on two technical aspects: the approach to permission accounting, including fractional permissions, precise predicates, and counting permissions; and the approach to lemma function termination in the presence of dynamically-bound lemma function calls. Finally, we describe three ongoing efforts: application to JavaCard programs, integration of shape analysis, and application to Linux device drivers.

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We present a sound verification approach for verifying input/output properties of programs. Our approach supports defining highlevel I/O actions on top of low-level ones (compositionality), defining input/output actions without taking into account which other actions exist (modularity), and other features. As the key ingredient, we developed a separation logic over Petri nets. We also show how with the same specification style we can elegantly modularly verify "I/O-like" code that uses the Template Pattern. We have implemented our approach in the VeriFast verifier and applied it to a number of challenging examples.