C is an unsafe language. Researchers have been developing tools to port C to safer languages such as Rust, Checked C, or Go. Existing tools, however, resort to preprocessing the source file first, then porting the resulting code, leaving barely recognizable code that loses macro abstractions. To preserve macro usage, porting tools need analyses that understand macro behavior to port to equivalent constructs. But macro semantics differ from typical functions, precluding simple syntactic transformations to port them. We introduce the first comprehensive framework for analyzing the portability of macro usage. We decompose macro behavior into 26 fine-grained properties and implement a program analysis tool, called Maki, that identifies them in real-world code with 94% accuracy. We apply Maki to 21 programs containing a total of 86,199 macro definitions. We found that real-world macros are much more portable than previously known. More than a third (37%) are easy-to-port, and Maki provides hints for porting more complicated macros. We find, on average, 2x more easy-to-port macros and up to 7x more in the best case compared to prior work. Guided by Maki's output, we found and hand-ported macros in three real-world programs. We submitted patches to Linux maintainers that transform eleven macros, nine of which have been accepted.

Variability-aware analysis is critical for ensuring the quality of configurable C software. An important step toward the development of variability-aware analysis at scale is to transform real-world C software that uses both C and preprocessor into pure C code, by replacing the preprocessor's compile-time variability with C's runtime-variability. In this work, we design and implement a desugaring tool, SugarC, that transforms away real-world preprocessor usage. SugarC augments C's formal grammar specification with translation rules, performs simultaneous type checking during desugaring, and introduces numerous optimizations to address challenges that appear in real-world preprocessor usage. The experiments on DesugarBench, a benchmark consisting of 108 manually-created programs, show that SugarC supports many more language features than two existing desugaring tools. When applied on three real-world configurable C software, SugarC desugared 774 out of 813 files in the three programs, taking at most ten minutes in the worst case and less than two minutes for 95% of the C files.