Querypoint Debugging

To fix a bug, software developers have to examine the bug-gy execution to locate defects. They employ different ap-proaches (e.g., setting breakpoints, inserting printing statements in the code) to navigate over buggy execution and inspect program state at suspicious points. In this pa-per we describe Querypoints, a new kind of compound conditional breakpoint relating two execution points. De-velopers specify Querypoints relative to a successfully paused conventional breakpoint or conventional watchpoint or to another Querypoint. The relative conditions contain runtime data values, like the last time a value was changed, or program statements, like the last conditional branch. The Querypoint combines the program state information from two execution points in the same execution; Querypoints can be chained to work backwards from effects to causes in a program. We present basic Querypoint concepts, two sample Que-rypoints, lastChange and lastCondition, and a description of our implementation of these Querypoints. To demon-strate that Querypoints are feasible we describe a prototype that implements an alternative approach to finding a bug in a graphical program analyzed previously with Whyline, one of the new logging-based debuggers.