When can a memory leak occur?

Question

I don't know what to think here...

We have a component that runs as a service. It runs perfectly well on my local machine, but on some other machine (on both machine RAM's are equal to 2GB) it starts to generate bad_alloc exceptions on the second and consecutive days. The thing is that the memory usage of the process stays the same at aproximately 50Mb level. The other weird thing is that by means of tracing messages we have localized the exception to be thrown from a stringstream object which does but insert no more than 1-2 Kb data into the stream. We're using STL-Port if that matters.

Now, when you get a bad_alloc exception, you think it's a memory leak. But all our manual allocations are wrapped into a smart pointer. Also, I can't understand how a stringstream object lacks memory when the whole process uses only ~50Mb (the memory usage stays approximtely constant(and sure doesn't rise) from day to day).

I can't provide you with code, because the project is really big, and the part which throws the exception really does nothing else but create a stringstream and << some data and then log it.

So, my question is... How can a memory leak/bad_alloc occur when the process uses only 50Mb memory out of 2GB ? What other wild guesses do you have as to what could possibly be wrong?

Thanks in advance, I know the question is vague etc., I'm just sort of desperate and I tried my best to explain the problem.

Answer 1

One likely reason within your description is that you try to allocate a block of some unreasonably big size because of an error in your code. Something like this;

 size_t numberOfElements;//uninitialized
 if( .... ) {
    numberOfElements = obtain();
 }
 elements = new Element[numberOfElements];

now if numberOfElements is left uninitialized it can contain some unreasonably big number and so you effectively try to allocate a block of say 3GB which the memory manager refuses to do.

So it can be not that your program is short on memory, but that it tries to allocate more memory than it could possibly be allowed to under even the best condition.

Answer 2

bad_alloc doesn't necessarily mean there is not enough memory. The allocation functions might also fail because the heap is corrupted. You might have some buffer overrun or code writing into deleted memory, etc.

You could also use Valgrind or one of its Windows replacements to find the leak/overrun.