How to spot undefined behavior

Question

Is there any way to know if you program has undefined behavior in C++ (or even C), short of memorizing the entire spec?

The reason I ask is that I\'ve noticed a lot

Answer 1

Unfortunately, there is no way way to detect all UB. You'd have to solve the Halting Problem to do that.

The best you can do is to know as many of the rules as possible, look it up when you're in doubt, and check with other programmers (through pair programming, code reviews or just SO questions)

Compiling with as many warnings as possible, and under multiple compilers can help. And running the code through static analysis tools such as Valgrind can detect many issues.

But ultimately, no tool can detect it all.

An additional problem is that many programs actually have to rely on UB. Some API's require it, and just assume that "it works on all sane compilers". OpenGL does that in one or two cases. The Win32 API won't even compile under a standards compliant compiler.

So even if you had a magic UB-detecting tool, it would still be tripped up by the cases that aren't under your control.

Answer 2

A good compiler, such as the Intel C++ compiler, should be able to spot 99% of cases of undefined behaviour. You'll need to investigate the flags and switches to use. As ever, read the manual.

Answer 3

Simple: Don't do things that you don't know that you can do.

• When you are unsure or have a fishy feeling, check the reference

Answer 4

I'm not aware of any software tool to detect all forms of UB. Obviously using your compiler's warnings and possibly lint or another static code checker can help a lot.

The other thing that helps a lot is simply experience: The more you program the language, the more you'll see constructs that appear suspect and be able to catch them earlier in the process.