Checking type safety of foreign function calls

Foreign function interfaces (FFIs) allow components in different languages to communicate directly with each other. While FFIs are useful, they often require writing tricky low-level code and include little or no static safety checking, thus providing a rich source of hard-to-find programming errors. In this article, we study the problem of enforcing type safety across the OCaml-to-C FFI and the Java Native Interface (JNI). We present O-Saffire and J-Saffire, a pair of multilingual type inference systems that ensure C code that uses these FFIs accesses high-level data safely. Our inference systems use representational types to model C's low-level view of OCaml and Java values, and singleton types to track integers, strings, memory offsets, and type tags through C. J-Saffire, our Java system, uses a polymorphic flow-insensitive, unification-based analysis. Polymorphism is important because it allows us to precisely model user-defined wrapper functions and the more than 200 JNI functions. O-Saffire, our OCaml system, uses a monomorphic flow-sensitive analysis because, while polymorphism is much less important for the OCaml FFI flow-sensitivity is critical to track conditional branches, which are used when pattern matching OCaml data in C. O-Saffire also tracks garbage collection information to ensure that local C pointers to the OCaml heap are registered properly, which is not necessary for the JNI. We have applied O-Saffire and J-Saffire to a set of benchmarks and found many bugs and questionable coding practices. These results suggest that static checking of FFIs can be a valuable tool in writing correct multilingual software.