A foundation for refactoring C with macros

Abstract

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.

Un-preprocessing

“…Most approaches [10,12,39,41] try to find a suitable model that represent both the C program and the preprocessor directives. A recent approach [33] suggests an alternative: perform refactoring on one variant and prevent the refactoring if problems may be caused in other variants. This is based on the observation that changes on one variant seldom causes problems in other variant.…”

“…In the future we may combine our approach with these approaches to deal with multiple variants. However, handling only one variant is already useful in many cases: (1) many programs, though with conditional compilation, do not have many variants; (2) as revealed by Overbey et al [33], changes in one variant often do not cause problems in other variants.…”