Program Verification in the Presence of I/O

Software veri?cation tools that build machine-checked proofs of functional correctness usually focus on the algorithmic content of the code. Their proofs are not grounded in a formal semantic model of the environment that the program runs in, or the program’s interaction with that environment. As a result, several layers of translation and wrapper code must be trusted. In contrast, the CakeML project focuses on endto-end veri?cation to replace this trusted code with veri?ed code in a cost-e?ective manner. In this paper, we present infrastructure for developing and verifying impure functional programs with I/O and imperative ?le handling. Specifically, we extend CakeML with a low-level model of ?le I/O, and verify a high-level ?le I/O library in terms of the model. We use this library to develop and verify several Unix-style command-line utilities: cat, sort, grep, di? and patch. The work?ow we present is built around the HOL4 theorem prover, and therefore all our results have machine-checked proofs.