Of Memory Management, Heap Corruption, and C++

Question

So, I need some help. I am working on a project in C++. However, I think I have somehow managed to corrupt my heap. This is based on the fact that I added an std::string to a class and assigning it a value from another std::string:

std::string hello = "Hello, world.\n";
/* exampleString = "Hello, world.\n" would work fine. */
exampleString = hello;

crashes on my system with a stack dump. So basically I need to stop and go through all my code and memory management stuff and find out where I've screwed up. The codebase is still small (about 1000 lines), so this is easily do-able.

Still, I'm over my head with this kind of stuff, so I thought I'd throw it out there. I'm on a Linux system and have poked around with valgrind, and while not knowing completely what I'm doing, it did report that the std::string's destructor was an invalid free. I have to admit to getting the term 'Heap Corruption' from a Google search; any general purpose articles on this sort of stuff would be appreciated as well.

(In before rm -rf ProjectDir, do again in C# :D)

EDIT: I haven't made it clear, but what I'm asking for are ways an advice of diagnosing these sort of memory problems. I know the std::string stuff is right, so it's something I've done (or a bug, but there's Not A Problem With Select). I'm sure I could check the code I've written up and you very smart folks would see the problem in no time, but I want to add this kind of code analysis to my 'toolbox', as it were.

Answer 1

Other than tools like Boundschecker or Purify, your best bet at solving problems like this is to just get really good at reading code and become familiar with the code that you're working on.

Memory corruption is one of the most difficult things to troubleshoot and usually these types of problems are solved by spending hours/days in a debugger and noticing something like "hey, pointer X is being used after it was deleted!".

If it helps any, it's something you get better at as you gain experience.

Your memory allocation for the array looks correct, but make sure you check all the places where you access the array too.

Answer 2

Oh, if you want to know how to debug the problem, that's simple. First, get a dead chicken. Then, start shaking it.

Seriously, I haven't found a consistent way to track these kinds of bugs down. Because there's so many potential problems, there's not a simple checklist to go through. However, I would recommend the following:

1. Get comfortable in a debugger.
2. Start tromping around in the debugger to see if you can find anything that looks fishy. Check especially to see what's happening during the exampleString = hello; line.
3. Check to make sure it's actually crashing on the exampleString = hello; line, and not when exiting some enclosing block (which could cause destructors to fire).
4. Check any pointer magic you might be doing. Pointer arithmetic, casting, etc.
5. Check all of your allocations and deallocations to make sure they are matched (no double-deallocations).
6. Make sure you aren't returning any references or pointers to objects on the stack.

There are lots of other things to try, too. I'm sure some other people will chime in with ideas as well.

Answer 3

Also, I actually fixed the std::string problem. How? By replacing it with a vector, compiling, then replacing the string again. It was consistently crashing there, and that fixed even though it...couldn't. There's something nasty there, and I'm not sure what.

That sounds like you really did shake a chicken at it. If you don't know why it's working now, then it's still broken, and pretty much guaranteed to bite you again later (after you've added even more complexity).

Answer 4

Run Purify.

It is a near-magical tool that will report when you are clobbering memory you shouldn't be touching, leaking memory by not freeing things, double-freeing, etc.

It works at the machine code level, so you don't even have to have the source code.

One of the most enjoyable vendor conference calls I was ever on was when Purify found a memory leak in their code, and we were able to ask, "is it possible you're not freeing memory in your function foo()" and hear the astonishment in their voices.

They thought we were debugging gods but then we let them in on the secret so they could run Purify before we had to use their code. :-)

http://www-306.ibm.com/software/awdtools/purify/unix/

(It's pretty pricey but they have a free eval download)

Answer 5

Your code as I can see has no errors. As has been said more context is needed.

If you haven't already tried, install gdb (the gcc debugger) and compile the program with -g. This will compile in debugging symbols which gdb can use. Once you have gdb installed run it with the program (gdb ). This is a useful cheatsheat for using gdb.

Set a breakpoint for the function that is producing the bug, and see what the value of exampleString is. Also do the same for whatever parameter you are passing to exampleString. This should at least tell you if the std::strings are valid.

I found the answer from this article to be a good guide about pointers.

Answer 6

It could be heap corruption, but it's just as likely to be stack corruption. Jim's right. We really need a bit more context. Those two lines of source don't tell us much in isolation. There could be any number of things causing this (which is the real joy of C/C++).

If you're comfortable posting your code, you could even throw all of it up on a server and post a link. I'm sure you'd gets lots more advice that way (some of it undoubtedly unrelated to your question).