About Relative Debugging

Traditional debuggers allow a user to control a program and examine its state at any point of the execution. The user sets breakpoints in the code, interactively examines program variables, and verifies that these variables have expected values. Erroneous values can be traced to erroneous code by using information about program data flow. Relative debugging differs from traditional debugging in two important respects. First, program variables are compared not with user expectations, but with variables in another reference program that is known to be correct. Second, because the reference program is available to compute correct values, the comparison process can be automated. Hence, the relative debugging process proceeds as follows. The user first formulates a set of assertions about key data structures in the reference and the development versions. These assertions specify locations at which data structures should be identical: violations of the assertions indicate errors. The relative debugger is then responsible for managing the execution of the two program versions, for validating the supplied assertions by comparing the data structures, and for reporting any differences. If differences are reported, the user proceeds to isolate erroneous code by repeatedly refining the assertions and reinvoking the relative debugger. Once the erroneous region is small enough, traditional debugging techniques can be used to correct the development version. Thus, the relative debugger provides a quick and effective way of locating problems in the development version.

A relative debugger provides all the functionality of a traditional debugger, including commands for program control, state access and breakpoints. However, the heart of the relative debugger is a set of new commands, not available in conventional debuggers. These commands support the relative debugging methodology.

Because the reference and development versions of the program are executed concurrently, a relative debugger must be capable of handling two programs at the same time. It is useful for the relative debugger to support the debugging of programs written in different programming languages and executing on different computers in a heterogeneous network. This makes it possible to use the relative debugger when porting programs from one language or computer to another.

A relative debugger checks user-supplied assertions by comparing data structures in the reference and development versions. It performs necessary transformations of different internal data representations on different computers or in different languages. When performing comparisons, the debugger must take into account different data types, allowing for such issues as inexact equality in floating point numbers, and differences in dynamic pointer values. This aspect of the debugger will be illustrated in the next section. Violations of assertions are reported to the user. A number of approaches are possible for reporting differences in data structures, ranging from text to advanced data visualization techniques. If there are only a few differences, then the numeric values of differences are printed out. If differences are numerous, then visualization techniques are required to present them in a meaningful way.