Compile-Time Debugging of C Programs Working on Trees

We exhibit a technique for automatically verifying the safety of simple C programs working on tree-shaped data structures. We do not consider the complete behavior of programs, but only attempt to verify that they respect the shape and integrity of the store. A verified program is guaranteed to preserve the tree-shapes of data structures, to avoid pointer errors such as NULL dereferences, leaking memory, and dangling references, and furthermore to satisfy assertions specified in a specialized store logic. A program is transformed into a single formula in WSRT, an extension of WS2S that is decided by the MONA tool. This technique is complete for loop-free code, but for loops and recursive functions we rely on Hoarestyle invariants. A default well-formedness invariant is supplied and can be strengthened as needed by programmer annotations. If a program fails to verify, a counterexample in the form of an initial store that leads to an error is automatically generated. This extends previous work that uses a similar technique to verify a simpler syntax manipulating only list structures. In that case, programs are translated into WS1S formulas. A naive generalization to recursive datatypes determines an encoding in WS2S that leads to infeasible computations. To obtain a working tool, we have extended MONA to directly support recursive structures using an encoding that provides a necessary state-space factorization. This extension of MONA defines the new WSRT logic together with its decision procedure.