The Unix tradition encourages developers to anticipate this problem by designing for transparency — in particular, designing programs in such a way that their internal data flows are readily monitored with the naked eye and simple tools, and readily mentally modeled. This is a topic we covered in detail in Chapter6. Design for transparency is valuable both for preventing bugs and for easing the runtime-debugging task.

Design for transparency is not, however, sufficient in itself. When you are debugging a program at runtime, it's extremely useful to be able to examine the state of your program at runtime, set breakpoints, and execute pieces of it down to the single-statement level in a controlled way. Unix has a long tradition of hosting programs to help you with this. Open-source Unixes feature a powerful one called gdb (yet another FSF project) that supports C and C++ debugging.

Perl, Python, Java, and Emacs Lisp all support standard packages or programs (included with their base distributions) that allow you to set breakpoints, control execution, and do general runtime-debugger things. Tcl, designed as a small language for small projects, has no such facility (though it does have a trace facility that can be used to watch variables at runtime).