A Foundation for Refactoring C with Macros

This paper establishes the concept of preprocessor dependences as a foundation for building automated refactoring tools that transform source code containing lexical macros and conditional compilation directives, such as those provided by the C preprocessor (CPP). We define a preprocessor dependence graph (PPDG) that models the relationships among macro definitions, macro invocations, and conditional compilation directives in a file-the relationships that must be maintained for the semantics of the C preprocessor to be preserved. For many refactorings, a tool can construct a PPDG from the code before and after it is transformed, then perform a linear-time comparison of the two graphs to determine whether the refactoring will operate correctly in the presence of macros and conditional compilation directives. The proposed technique was implemented in OpenRefactory/C and tested by applying refactorings to GNU Coreutils version 8.21. Empirical results indicate that the technique is effective; it successfully handled refactoring scenarios in which Eclipse CDT, Visual Assist X, and XRefactory all refactored code incorrectly.