Most common reasons for unstable bugs in C++?

Question

I am currently working on a large project, and I spend most of the time debugging. While debugging is a normal process, there are bugs, that are unstable, and these bugs are the greatest pain for the developer. The program does not work, well, sometimes... Sometimes it does, and there is nothing you can do about it.

What can be done about these bugs? Most common debugging tools (interactive debuggers, watches, log messages) may lead you nowhere, because the bug will disappear ... just to appear once again, later. That is why I am asking for some heuristics: what are the most common reasons for such bugs? What suspicious code should we investigate to locate such a bugs?

Let me start the list:

1. using uninitialized variables. Common misprints like mMember = mMember;
2. thread synchronization. Sometimes it can be a matter of luck;
3. working with non-smart pointers, dereferencing invalid ones;

what else?

Answer 1

• Reading from uncached memory while a cache line is being written back over the memory (This is a right bastard to find).
• Buffer overwrites
• Stack overflows!

The only 3 i can think of at the mo ... may edit later :)

Answer 2

• buffer overflows
• using pointers to deleted objects
• returning invalid references or references to out of scope objects
• unhandled exceptions
• resource leaks (not only memory)

• infinite recursion

• dynamic libraries version mismatch

Answer 3

IME the underlying problem in many projects is that developers use low-level features of C++ like manual memory management, C-style string handling, etc. even though they are very rarely ever necessary (and then only well encapsulated in classes). This leads to memory corruption, invalid pointers, buffer overflows, resource leaks and whatnot. All the while nice and clean high-level constructs are available.

I was part of the team for a large (several MLoC) application for several years and the number of crashing bugs for different parts of the application nicely correlated to the programming style used within these parts. When asked why they wouldn't change their programming style some of the culprits answered that their style in general yields more performance. (Not only is this wrong, it's also a fact that customers rather have a more stable but slower program than a fast one that keeps crashing on them. Also, most of their code wasn't even required to be fast...)

As for multi-threading: I don't feel expert enough to offer solutions here, but I think Herb Sutter's Effective Concurrency columns are a very worthwhile read on the subject.

Edit to address the discussions in the comments:

I did not write that "C-style string handling is not more performant". (Certainly a lot of negation in this sentence, but since I feel misread, I try to be precise.) What I said is that high level constructs are not in general less performant: std::vector isn't in general slower than manually doing dynamically allocated C arrays, since it is a dynamically allocated C array. Of course, there are cases where something coded according to special requirements will perform better than any general solution -- but that doesn't necessarily mean you'll have to resort to manual memory management. This is why I wrote that, if such things are necessary, then only well-encapsulated in classes.

But what's even more important: in most code the difference doesn't matter. Whether a button depresses 0.01secs after someone clicked it or 0.05secs simply doesn't matter, so even a factor 5 speed gain is irrelevant in the button's code. Whether the code crashes, however, always matters.

To sum up my argument: First make it work correctly. This is best done using well-proven off-the-shelf building blocks. Then measure. Then improve performance where it matters, using well-proven off-the-shelf idioms.

Answer 4

I was actually going to post a question that asked exactly the opposite - do others find, as I do, that you spend almost no time using the debugger when working with C++? I honestly cannot remember the last time I used one - it must have been about six months ago.

Frankly, if you spend most of the time in the debugger, I think there is something very wrong with your basic coding practices.

Answer 5

If you are really in a position where you already have bad code that breaks, the best plan is probably to throw as many tools at it as you can (OS/lib-level memory checking, automated testing, logging, core dumps, etc) to find the problem areas. Then rewrite the code to do something more deterministic. Most of the bugs come from people doing things that mostly work most of the time, but C++ offers stronger guarantees if you use the right tools and approaches.

Answer 6

Not really a C++ issue but seen in a C/C++ project.

The trickiest issue I had to deal with was an initialization issue when starting up the OS on our platform that lead to unusual crashes. It took years before we found out what happened. Before that we ran the system overnight and if it didn't crash, then it was normally okay. Luckily, the OS isn't sold anymore.

Answer 7

Race conditions.

These are one of the few things that still sends a shiver down my spine when it comes up in debugging (or in the issue tracker). Inherently horrible to debug, and extremely easy to create. The three most common causes of bugs in my C++ software have been race conditions, reliance on uninitialised memory, and reliance on static constructor order.

And if you don't know what race conditions are, chances are they're the cause of your instability ;)

Answer 8

Haven't seen this one mentioned yet:

• Inheriting from a class that does not have a virtual destructor.

Answer 9

addresses and memory used before allocation or after deallocation, segmentation faults, arrayoutofbounds, offset, threadlocks, unintelligible operator overloading, inline assembly, void exit and void in general where return values are desired complicates where math.h functions are worth a look since all math.h functions both have working arguments and return values compared to other library overly void, emptiness tests, nils, nulls and voids. 4 general conventions I recommend are return values, arguments, ternary choices and invertible changes. Faultprone to avoid are vectors (use arrays instead) void with empty arguments and in my subjective opinion I avoid the switch statement in favor of more intelligible or readable if...elseif or more abstract "is".

C++ also has rather lousy forward compatibility compared to scripts and interpreted, to try a decade old Java it still runs unchanged and safe in later vm.