A type system for format strings

Most programming languages support format strings, but their use is error-prone. Using the wrong format string syntax, or passing the wrong number or type of arguments, leads to unintelligible text output, program crashes, or security vulnerabilities. This paper presents a type system that guarantees that calls to format string APIs will never fail. In Java, this means that the API will not throw exceptions. In C, this means that the API will not return negative values, corrupt memory, etc. We instantiated this type system for Java’s Formatter API, and evaluated it on 6 large and well-maintained open-source projects. Format string bugs are common in practice (our type system found 104 bugs), and the annotation burden on the user of our type system is low (on average, for every bug found, only 1.0 annotations need to be written).