Many existing systems are written in C and are not re-entrant or thread safe. Sometimes these systems are required in a context for which they were not first designed, possibly meaning they now need to be re-entrant. This article introduces a program that filters C source code, modifying shared resources (the global variables) to make the code re-entrant: virtualizing the code. The code is then compiled as normal. This approach allows programmatic virtualization with little cost at runtime. Copyright © 2007 John Wiley & Sons, Ltd.