Relative Debugging Using Multiple Program Versions

Incremental software development is the process of building new programs by modifying old ones. This method of programming is extremely common, and takes advantage of existing pieces of software which are known to be correct. The result is that are many software versions in existance at the same time. To date, most software development tools have addressed only one aspect of incremental software development, namely, source code control. An equally important issue is in verifying that future versions of a program have the same dynamic behaviour as the original. Currently, there are no tools which address this issue. In this paper we introduce a new concept called relative debugging, which supports the incremental development of computer programs. Relative debugging makes it possible to compare the execution of two programs with the same functionality. The programs can run on the same computer, or on di erent computers connected by a network. They can di er in their source programming language or in implementation details. One of the programs usually serves as a reference version which produces the desired results. The execution of the other process is compared to the reference version in order to nd discrepancies. We describe basic primitives for relative debugging and their implementation in a debugger called Guard. Guard is built on top of our portable debugging interface which provides a debugging platform for heterogeneous environments. The utility of Guard is illustrated by a practical example, in which a program written in Fortran is used to nd errors in a version of the same code written in the C programming language.