We describe twinning and its applications to adapting programs to alternative APIs. Twinning is a simple technique that allows programmers to specify a class of program changes, in the form of a mapping, without modifying the target program directly. Using twinning, programmers can specify changes that transition a program from using one API to using an alternative API.

The C language definition leaves the sizes and layouts of types partially unspecified. When a C program makes assumptions about type layout, its semantics is defined only on platforms (C compilers and the underlying hardware) on which those assumptions hold. Previous work on formalizing C-like languages has ignored this issue, either by assuming that programs do not make such assumptions or by assuming that all valid programs target only one platform. In the latter case, the platform's choices are hard-wired in the language semantics.