What are data breakpoints?

Question

I just came to know that there are data breakpoints. I have worked for the last 5 years in C++ using Visual Studio, and I have never used data breakpoints.

Answer 1

So far we've got a great definition and a bunch of great theoretical explanations.

Let's have a concrete example!

I'm currently working on a rather large and convoluted codebase. I made a small safe change to one bit of code and started getting - in a completely unrelated chunk of the codebase - crashes in the memory allocator. This is generally a sign that you're doing something Very Wrong with memory management - either double-deletion or writing out-of-bounds.

Thankfully, we have an option to turn on a debug memory manager that checks for things like this. I turned it on and it immediately started reporting a memory block guard violation, which means that something wrote out of bounds. The problem is that this report shows up only once the memory is deallocated - essentially saying "hey, something was broken. Hope you can figure out what!"

Unfortunately this particular chunk of memory, at the point of deallocation, is completely indistinguishable from literally thousands of other chunks of memory. Fortunately, our debug framework tags each allocation with a consecutive ID, and the memory that got corrupted had a consistent ID (#9667, if you're curious.) One quick breakpoint in the memory manager later and I was able to find where that memory was allocated. Which, as it turned out, wasn't immediately helpful either.

But at that point, I had several important components:

• I knew the address of a block of memory
• I knew the intended length of that memory
• I knew that, at some point in the future, a specific byte past the intended length of that memory would be overwritten

Given this, I could set up a data breakpoint on that specific byte, then hit "go" and find out where the corruption occured.

Which I did - it led to an off-by-one error which I am now in the process of fixing.

And that's a concrete example of how data breakpoints can be useful. :)