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::stri

Answer 1

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 2

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 3

As far as I can tell your code is correct. Assuming exampleString is an std::string that has class scope like you describe, you ought to be able to initialize/assign it that way. Perhaps there is some other issue? Maybe a snippet of actual code would help put it in context.

Question: Is exampleString a pointer to a string object created with new?

Answer 4

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).

Answer 5

The code was simply an example of where my program was failing (it was allocated on the stack, Jim). I'm not actually looking for 'what have I done wrong', but rather 'how do I diagnose what I've done wrong'. Teach a man to fish and all that. Though looking at the question, I haven't made that clear enough. Thank goodness for the edit function. :')

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. I did want to check the one time I manually allocate memory on the heap, though:

 this->map = new Area*[largestY + 1];
 for (int i = 0; i < largestY + 1; i++) {
     this->map[i] = new Area[largestX + 1];
 }

and deleting it:

for (int i = 0; i < largestY + 1; i++) {
    delete [] this->map[i];
}
delete [] this->map;

I haven't allocated a 2d array with C++ before. It seems to work.

Answer 6

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)