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Discussion on: I'm an Expert in Memory Management & Segfaults, Ask Me Anything!

mortoray profile image
edA‑qa mort‑ora‑y

Given that you're writing a fairly low-level, perhaps wait-free concurrent algorithm, it's possible that you have a bug that trips up about every millionth execution of the code. It's a race condition that corrupts a vital structure. Any attempt to trace upsets the condition leading to the error, thus making it go away. Your only choice is a tedious hand execution and logical reasoning.

My question is, how do you avoid throwing the computer out the window?

codemouse92 profile image
Jason C. McDonald Ask Me Anything

Avoid? I love tedious hand execution and logical reasoning! (No, seriously.) One of my absolute favorite things to do in programming is to print off the source, sit down with a pen, a hot beverage, a blank notebook, and a jazz soundtrack...and then spend the next hour or three just desk-checking the entire thing.

Mmmmmmmmmmmmmmmmmmmmmm, bliss. ,^

Why do you think I specialized in memory management and undefined behavior? I ADORE it!

Now, if you don't have my particular mental condition, and actually don't enjoy desk-checking for Heisenbugs, my advice is this: get off the computer. Print off the source, cozy up in your favorite chair in a relaxing environment, and desk-check it.

liulk profile image
Likai Liu

I also recommend writing unit tests that makes the race condition more likely to happen. For example, if the code normally runs with < 10 threads, test it with 1000 threads. Sometimes code is well-behaved when the data entered are far apart, so try testing with consecutive values. If it's the opposite, test with random values.

What I learned over the years is that race-freedom is not composeable: code using several mutexes incorrectly could still suffer race condition, even though a single mutex is race-free on its own. When testing wait-free algorithms, start with very small primitives and gradually add onto it. And write plenty of assert() on the non-volatile local variables of the shared volatile variables the code might be using. When assert triggers under the debugger, you'll be able to see which invariants are violated in that snapshot.