Assertive debugging: correcting software as if we meant it -

Assertive debugging: correcting software as if we meant it


Assertive debugging is a new way to make embedded systems ensure their own health by having your code monitor itself.

Debugging is an art that needs much further study …. The most effective debugging techniques seem to be those which are designed and built into the program itself—many of today's best programmers will devote nearly half of their programs to facilitating the debugging process on the other half; the first half… will eventually be thrown away, but the net result is a surprising gain in productivity.

—Donald Knuth, The Art of Computer Programming 1

As Don Knuth implies, debugging is a much-neglected subject, and we're paying a terrible price for that neglect. We've made little progress in debugging methods in half a century, with the result that projects everywhere are bogged down because of buggy software. The price in lost time and wasted resources, when the projects are commercial, must run into the billions; the price when the projects are military is paid not only in dollars but in lives. This situation is intolerable; new ideas and approaches must be found. This articles offers one such new approach.

I propose that a new system for debugging software called the Assertive Debugging System (ADS) can transform debugging from a minor art form to a modern industrial process. ADS exploits an old idea—the assertions were first suggested by John von Neumann in 1947.2 ADS, however, does something with assertions that neither he nor anyone else, to my knowledge, has proposed, much less done: it uses them systematically and exhaustively rather than as ad hoc tools that are employed only when the programmer remembers them and feels like using them. In doing so, ADS transforms assertions from an idea that's been floating around for half a century without achieving much, into a technology that could effect a revolution in program development. And unlike the methods Knuth had in mind, it doesn't throw away that part of the program devoted to debugging, but preserves it as valuable documentation of the state of the subject program and for later reuse when that program is modified.

Bugs: the major bottleneck
It's nearly impossible to find a scientific or engineering project these days that doesn't depend on computing, and almost as hard to find one that's not slipping its schedule because of buggy software. The debugging problem is a critical one for nearly all our projects. The penalties we pay for buggy software are already high: lost business when our customers are dissatisfied and lost sales when our products are tardy coming to market; these penalties will get much higher as we increasingly use computers for critical applications—mission-critical and even life-critical.

In such critical applications, being able to take and prove you've taken serious debugging measures will become much more important, even legally required. For applications on which so much depends, today's half-hearted gestures at debugging will no longer be acceptable. ADS represents an approach to program debugging that directly addresses these issues: it enables developers to shorten the debugging process, and it supports the systematic and documentable debugging of software objects that I contend will soon be required just to stay in business—perhaps required just to stay out of jail.

Debugging today
The most remarkable thing about debugging today is how little it differs from debugging at the dawn of the modern computing age, half a century ago. We still do it by letting a faulty program run up to what we conjecture is a critical point, then stop execution and look at the state of what we think are the key variables. If one of these variables differs in value from what we expected, we try to understand how it could have assumed that value. If we can't understand where it went wrong, we repeat the process, stopping at some earlier point. After an unpredictable number of iterations of this process, we stop the program close enough to the location of the bug, and the standard revelation occurs: we find that we have forgotten to reset some counter, flush some buffer, allow for the overflow of some data structure, or have committed one of the other half-dozen classic programming errors.

This is how software was debugged in the mid '50s, and how it's debugged today. It's a process that will always, if time and customer patience permit, eventually find the bug that's troubling you—but, usually, only that particular occurrence of it, and only after a debugging effort of unpredictable length, and without leaving anyone the wiser about the program being debugged, or about how to find other such bugs.

What is a bug?
To make clear which bugs are the really troublesome bugs—the ones that ADS is meant to deal with—I offer here a rough taxonomy of software problems in general, with estimates of their relative gravity. You'll find nothing original in this taxonomy; all it does is gather and organize some common truths and put them in a form convenient for understanding ADS. Only programmer errors are considered here—problems caused by hardware failure, operator error, or other conditions not under the programmer's control are not nearly so difficult to deal with, nor so serious a problem. Programmer errors are:

  1. Algorithm design errors. The programmer (or his client) has misunderstood the problem, and hence the way to solve it; consequently, his algorithm, even if implemented perfectly, will not work. For example, he may be trying to compute the orbit of an artificial Earth satellite on the assumptions that the planet is perfectly spherical. His error has nothing to do with computing, but rather with his or his client's understanding of the problem he is dealing with.
  2. Program design errors. Although the programmer's understanding of his problem and his approach to solving it are correct, he has blundered in designing a program to implement that solution. For example, he has failed to realize that the program he is expecting the computer to execute would take longer to run than the expected lifetime of the universe. This error is computer-related; it reflects a defect in the programmer's understanding, not of any specific computer or programming language, but of computing in general.
  3. Program implementation errors. The programmer has erred in generating the instructions to be executed by his computer. Of this type, there are two varieties.

    • Formal or syntactic errors.
    • His program has violated a rule imposed by his program-development tools—but the violation is of the type caught by those tools.

    • Substantive or logic errors.
    • The program compiles but doesn't run to completion or runs but yields bad output. The programmer has either made some mechanical error (such as a typo), a formal error of a type not caught by the development system, or—and this is the critical type—an error in detailed program logic, such as neglecting to flush a buffer or writing beyond the end of a data area. (Conceivably, she may have encountered a bug in her program-development tools. This, of course, is not her fault but the fault of a programmer—another programmer.)

Type 1 errors have nothing to do with computing; they're just plain old ignorance, carelessness, or stupidity, for which no general remedy is known. Type 2 errors are computer-related, but aren't particularly troublesome; they're so gross that they're usually found early in the program's design stage and are relatively uncommon. Type 3a are already reasonably well handled—most modern program-development systems detect all the common syntactic errors and closely pinpoint them. Sometimes they can even fix them, as the program used to compose this article silently changes hte to the.

Really dangerous bugs
Type 3b errors are the real villains: easy to introduce, hard to notice, and patient in waiting for the worst possible moment to manifest themselves. The reason they're so great a problem is that they're so trivial, so inconspicuous, so hard to focus on. Type 3b bugs (henceforth just “bugs”) are dangerous precisely because they're seldom immediately troublesome. A program infected with them is often asymptomatic until it crashes disastrously or yields obviously faulty output. Generally these bugs let programs run with no sign of trouble long after they have in fact corrupted the results. By the time it's evident that something is wrong, much has happened to delete or corrupt the evidence needed to determine just where the problem originated; hence the long and painful period of backtracking that the debugging process almost always begins with.

What is needed, then, to deal with the debugging problem is some way to make bugs manifest themselves quickly, so as to give us the earliest possible warning of their existence, and let us take action before continued program execution can obliterate their traces. Ideally, we would like bugs to become so blatant that their presence can be detected even before they have acted; we want to catch them when they are just about to do their dirty work. That is what ADS is designed to do.

How ADS works
The way to catch bugs while they're fresh and out in the open is by monitoring the behavior of a great variety of variables at run time, looking for violations of assertions made by the programmer when he defined them. “Variable” means here not just those quantities a mathematician would think of and label as such, but any program construct any of whose properties change in a predictable way, either absolutely or relative to some other program construct. Among these would be the numeric variables that specify how often a loop is to be traversed, how many characters a buffer can hold before it's to be written out, how many states a switch can assume, and so on; they define, collectively, the route the program is meant to follow. It's the major premise of ADS that no bug can take effect without soon causing some variable to violate a constraint, and that if such violations are systematically detected, virtually every bug will cause an alarm to sound while it's still “fresh,” easily found, and understood.

The rigorous and systematic testing of such assertions throughout execution amounts to erecting walls on both sides of the narrow path that a program must take if its results are to be correct, so that the slightest deviation from that path causes an almost immediate collision between the running program and some assertion. Consequently, something valuable is learned from every execution-time failure: a bug is found (or at least its hiding area is narrowed down significantly) or a programmer's misconception is uncovered.

Using ADS
For each of his program constructs, the programmer asserts at definition time all the constraints on its behavior he can think of. The possible constraints include the following; others will doubtless suggest themselves as experience in the use of ADS grows:

  • its maximum and minimum size
  • the step size by which it will vary
  • whether it's cyclic, monotonic, or “random” in the sequence of values it can assume
  • the relationships it bears to one or more other variables
  • an explicit list of the values it may take on or those it may not
  • for a pointer or link, the type of construct it can point to

These assertions are expressed in a notation that's a natural extension of the source language the programmer uses, and they may be grouped in various ways, so that the programmer can activate or deactivate sets of related assertions with one command.

At each compilation of a subject program, the activated assertions generate into the object program code that can be used to check the variables to which they apply, at every change of value, for violations of any of the constraints so imposed (“can be used” because not every test needs to be executed every time). When the monitoring code detects that any variable has violated (or in some cases, is about to violate) an assertion, it halts execution of the program, and takes the exception action specified by the programmer.

At this point the programmer using ADS is in a very different position from that of the programmer of today whose program has stopped at a breakpoint. When ADS stops an execution, it's not because the program has just come to some point where the programmer hoped that an examination of some of his variables will reveal something; it's at a point—which may be far earlier or far later than the point at which that programmer would have inserted a breakpoint—where an anomaly has definitely been detected and almost certainly detected very close to its origin. Nor is the programmer limited, when ADS reports such an event, to the kind of hit-or-miss search that his counterpart today typically performs when at a conventional breakpoint; if the bug is not immediately apparent, the ADS user's next step is to rerun his program with a greater degree of monitoring enabled for all code dynamically preceding the point at which the anomaly was detected, so as to catch it at an even earlier moment.

There is unfortunately no practical way to demonstrate the validity of these claims for ADS short of building and using that system, but a thought-experiment may be helpful, if not conclusive. Draw on your experience for a real bug that you've recently been involved with or create an imaginary bug based on experience. Recreate on paper the state of the program variables just before the faulty instruction caused the first deviation from correct behavior, but with assertion checking, as just described, enabled. Consider, that is, that every variable—every predictably varying construct—in the program was being monitored at every change of value for violation of any the conditions you would have specified if you'd been using ADS. See how close to the point where the bug first manifests itself ADS would have stopped and raised a red flag—and imagine how much easier it would be to pinpoint that bug with such help than without it. In almost all cases, I think you will find that the difference between conventional debugging and ADS debugging is so great as to amount a difference in kind.

Cost of assertive debugging
Most programmers exposed to the idea seem to agree that ADS would enable them to find bugs much more quickly, but many protest that the cost would be prohibitive; program execution, burdened with all that run-time checking, could cost hundreds of times more cycles than ordinary execution. Many also quail at the thought of supplying all the assertions that would enable ADS to rigorously monitor the execution of the program. These are not unreasonable concerns, but they're more imposing at first glance than after a hard look.

To arrive at a true estimate of the cost of the ADS approach, the main requirement is to make sure that it's being compared against a thoroughly realistic estimate of the cost of the alternative, the present method of debugging. Present-day debugging runs often yield little or no knowledge about the bug being sought, and the cost of those runs, in the full sense of cost, must be counted in. With ADS, on the other hand, every execution results in useful, documented knowledge; either it finds a violation of an assertion or it runs to completion, reporting that with respect at least to the assertions activated, the program is bug free. Even if ADS reports a violation, but it turns out that the program code is correct, and it's the programmer's assertion that was mistaken, something of value is learned. In fact, it may be that the information gained in that case is the most valuable of all; it's not an individual bug that's been found, but a misconception in the programmer's mind about her program—something even more important to correct. Note, too, that the cost of ADS is almost entirely in machine cycles; what it saves is project schedule slippage, software-engineer time, and time-to-market. In short, it preserves assets that are growing ever more expensive, and it does so by using assets that are growing ever cheaper, and are already so cheap as to be in many cases not even worth metering.

The formulation of assertions is done when the programmer declares the variables and data structures; that is, at the time when his intentions for them are clearer in his mind than they will ever be again. Present-day implementation systems already require him to give full static definitions of his variables and structures; ADS requires him to add explicit statements about how they're allowed or forbidden to change at run-time. In effect, the user does a lot of his debugging at the ideal time: when he's not under pressure to get a specific bug fixed to make a specific delivery date, and when his mind is clear and his knowledge fresh.

Philosophies compared
The great difference between debugging with ADS and with conventional tools is that ADS, once primed, takes the initiative and, within the limits set by its user, does a completely systematic job. In conventional debugging, the system in effect says to the user, “I know of no reason to stop execution at this point, but you have ordered a halt here by setting a breakpoint, so here's a window through which you can look at whatever variables you, in your present state of knowledge, think might be relevant to your bug hunt. If an anomaly does exist in the current state of your program, you're responsible for recognizing it; I wouldn't know an anomaly if I tripped over one.”

ADS, by contrast, says, “Way back in program design and development days, you told me what you meant by an anomaly for each of many of the variables and other constructs in this program; more recently, you told me which of these anomalies I was to keep looking for, and what I was to do when I found one. I have now found one and am reporting as instructed. The details are as follows ….” With ADS, the software engineer does the planning, the debugging system does the heavy lifting.

The difference between the two could spell the difference between watching every project slip because of buggy software, and turning programming into a respectable and reliable industrial discipline.

Mark Halpern is a programmer and software designer whose experience goes back to the days when Fortran was the latest thing in computing systems. You can find an account of his career and links to other articles he's written at He welcomes comments; send them to .

End notes

  1. The epigraph is from Knuth's The Art of Computer Programming (ed. 1), Vol 1, Addison-Wesley, 1998, p.189.
  2. Goldstine, Herman H. The Computer from Pascal to von Neumann . Princeton University Press, 1972, p. 268.

Reader Response

ok…A couple of comments here.

1. Diagnostics, at least 'good' diagnostics need to follow the philosophy of being deeply embedded within the project. The most successful systems usually employ the notion of diagnostics/debugging as an integral part of the architecture.

2. Diagnostics/debugging can follow a couple of forms. The most common being reactive and proactive. Of course proactive will usually consume code resources, and reactive will in general get activated after a problem is detected.

3. It is generally better to try and implement and architect the system so that bugs are found, as much as possible, at compile/build time. This in general will save a lot of grief and headache later.

4. Wherever possible, always try to implement unhandled exception error and post-mortem diagnostics. Doing so allows developers to find that nasty pointer dereferencing/overrun errors after the fact. There are many ways to do this with modern processors and compilers today.

Ken Wada
Sr Embedded Systems Consultant
Aurium Technologies Inc.

I have been using ADS in my embedded applications for a number of years, and it has paid off significantly.

In order to address the performance concerns (CPU cycles) on critical applications, I produce two builds of the final object code. The first build has all the debugging assertions included and is used during development and in-house product and stress testing. The second build is used for the actual final product, which has the assertions effectively “compiled out.” This allows the final product to have optimum performance, but still allows the source code to contain assertions that can be used for future changes and debugging.

Even if the assertions are not completely removed from the final product, usually the action taken when an assertion is found is more tame than the action taken in the development build.

Bob Weinberger
Emerson Process Management
Sr Embedded Software Engineer

Your article in Embedded Systems Programming was interesting to me, however it left me hanging a bit. In the Using ADS section, you list types of constraints that assertions can be defined for and in the next paragraph you state, “they may be grouped in various ways, so that the programmer can activate or deactivate sets of related assertions with one command.”

I was hoping to see some examples of this later in the article, such as #define macros, but there were none. Do you have any references to articles or header files that illustrate the types of assertions you covered and categories for making them conditional?

One feature I've never seen in assert() libraries, even for embedded systems, is code that, instead of quitting the program, logs the occurrence in a buffer or sends it out over a comm. link, then continues. Or tells users where to set a software breakpoint in the assertion code and an explanation of how to use an SLD (source-level debugger) to back track to find the cause of the fired-off assertion.

I am in the business of HW-based debuggers (e.g. JTAG controlled) that includes processor trace. Another technique if the system is real-time is to cause the trace to trigger without halting the CPU (like a logic analyzer) and look back in the trace for the program error.

– Bruce Ableidinger

Author response:
Thanks for your note on my recent ADS article.

First, I have not implemented an ADS-enabled compiler, nor do I know of any implemented by anyone else, so I cannot give actual examples of the usage of assertions in the way I propose. That's why I had to describe a thought-experiment in my article.

Second, I don't regard the technical details of such a compiler as being of any great interest or difficulty; for the most part, they're just extensions of features that are currently available in several modern programming systems. Practically all systems today enable the user to give a name to a set of commands and to include or exclude any named set in a given compilation. The kinds of assertions that can be made, beyond those I listed, will be learned by experience.

Third, what I see as the great difficulty is changing the programming culture. I wrote my article as a step toward making programmers understand that: (1) traditional debugging methods are becoming, or already are, completely unacceptable; (2) that the thorough, systematic use of assertions goes a long way to solving the debugging problem; and (3) that the cost of using ADS is small compared with the cost of sending out buggy software. If these lessons can be driven home, the rest is easy; the creation of the necessary software could be done within six months by a team of two or three good system programmers.

I'm puzzled by your reference to a feature that would tell programmers where to set a breakpoint in the assertion code–as I envision ADS usage, the programmer does not set breakpoints; the assertions themselves implicitly create potential breakpoints.

– Mark Halpern

I just read your article in Embedded Systems Programming about assertive debugging. I thought it was an interesting idea but I am not sure I understand enough about it to evaluate it. Specifically, I understand that you are suggesting a systematic approach to building a set of invariants for the variables within a program. What I don't understand is:

a) when constraint checking is done against invariants,
b) what constraint checking is done when invoked, and
c) who controls when and how much constraint checking is done, and by what mechanism.

If you happen to have a ready example of the use of your method, I would probably be able to find all my answers there.

Thanks for the article and I look forward to understanding your idea better!

– Jody Glider
STSM, Storage Systems and Servers Research

Author response:
(a) constraint-checking, or assertion testing, is done at execution time

(b) in the default case, all assertions that have been compiled into the program are executed; if the programmer has exercised his power to disable any particular set(s) of such assertions for this particular execution, those assertions are not tested during this execution of the program

(c) the programmer who causes the execution of the program controls just how much assertion-checking is done, and does so by means of compiler directives such as “Skip assertion-sets B, D-G, and M.”

Since no ADS-capable programming system yet exists, I cannot give you a realistic example of its use, but my proposal does not include anything highly novel in technology; the gist of my proposal is that assertions, a feature that has been known since the days of von Neumann, should be used thoroughly and systematically. The features that have to be added to programming systems to make them ADS-capable are simple, and mostly extensions of well-known existing features.

– Mark Halpern

Mark, you appear scarred by trying to debug with a less than capable debugger such as the GNU debugger, gdb.

You should try Borland's or Watcom's debuggers. They provide immensely greater debugging productivity. I won't use gdb, its not as good as adding printf() statements. While Borland and Watcom show ALL the local variables at the same time, and allow the user to ask for global variables and to single step with a single key stroke, going to functions or not going to functions in detail depending on which key is truck. Watcom's allows setting break points AND logical tests for those break points so one can delay the break until a numeric value is in or out of some range or whatever else one can set in that logical selection.

My programs tend to have many nested loops and these capabilities of not hitting a break point until some value is in range sure saves me millions of key strokes and frustration. Such sophistication is not in gdb, probably won't ever be. I don't know if Watcom's logical will compare strings, when I need that I put in source code and set my break inside that source code string compare. I do much with weather and sometimes its a 100,000 lines of input data before trash affects my programs. With gdb I could not debug. With Watcom I do it regularly.

Open Watcom is coming to Linux also as well as supporting OS/2, DOS, and windoze.

– Gerald N. Johnson

Author response:
I'm glad to learn that there are some better debuggers out there than there were back when Fortran was the hottest programming language available, but the facilities you describe don't achieve anything like what I tried to describe in my article.

What I want, and think we can achieve with advanced assertions, is a state in which we can say to our programming system “I've told you how I expect my program to work; now I want you to monitor it, and notify me immediately if it tries to do anything else. And I want you to document everything–what assertions were in effect during this execution, what exceptions you detected, what the state of all key variables was when the exceptions were detected, and so on.”

The programmer using such a system would not set breakpoints, not try to guess where the program might have bugs, not try to “save time” (his own or the computer's) by cutting corners in making assertions. He would assume that his program will contain bugs as originally written, and by means of systematic and thorough use of assertions, transfer almost all the burden of finding those bugs to the programming system.

I think we can get to that position anytime we're prepared to make the necessary effort, and that it's a damned shame it wasn't done years and years ago. I hope it won't take some disasters caused by buggy software to make us take the necessary steps.

– Mark Halpern

I enjoyed very much reading your article on ADS at

As a former lead programmer, high-speed hardware debugging tools designer, CPU architect, and someone else that has been around the block several times since Fortran was hot, your perspective is welcome–it is based on reality instead of hype.

Allow me to share a couple alternatives/complements that might make ADS less objectionable to the average programmer and in turn, more valuable to all of us.

First, I am a believer in full trace, as deep as you can afford. Not the assembly-language reconstruction type, but full reconstruction up to the source language level. Combine that ADS and you would unravel the type 3b problems much more quickly than the iterative approach that will inevitably occur even with ADS's power.

Second, I have at times, constructed “observers” that run in parallel with the mainline program. In the observer, one codifies the information that you propose be included directly in the mainline program via ADS. By moving the overhead of assuring the expected behavior of the program onto a separate processor (“hyper-threaded CPUs” make this almost free at run-time) we remove the objections about CPU overhead (mostly). We get the benefits of ADS without the impact on run-time performance

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.