Find memory leaks caused by smart pointers
Does anybody know a "technique" to discover memory leaks caused by smart pointers? I am currently working on a large project written in C++ that heavily uses smart pointers with reference counting. Obviously we have some memory leaks caused by smart pointers, that are still referenced somewhere in the code, so that their memory does not get free'd. It's very hard to find the line of code with the "needless" reference, that causes the corresponding object not to be free'd (although it's not of use any longer).
I found some advice in the web, that proposed to collect call stacks of the increment/decrement operations of the reference counter. This gives me a good hint, which piece of code has caused the reference counter to get increased or decreased.
But what I need is some kind of algorithm that groups the corresponding "increase/decrease call stacks" together. After removing these pairs of call stacks, I hopefully have (at least) one "increase call stack" left over, that shows me the piece of code with the "needless" reference, that caused the corresponding object not to be freed. Now it will be no big deal to fix the leak!
But has anybody an idea for an "algorithm" that does the grouping?
Development takes place under Windows XP.
(I hope someone understood, what I tried to explain ...)
EDIt: I am talking about leaks caused by circular references.
Note that one source of leaks with reference-counting smart pointers are pointers with circular dependancies. For example, A have a smart pointer to B, and B have a smart pointer to A. Neither A nor B will be destroyed. You will have to find, and then break the dependancies.
If possible, use boost smart pointers, and use shared_ptr for pointers which are supposed to be owners of the data, and weak_ptr for pointers not supposed to call delete.
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The way I do it is simply: - on every AddRef() record call-stack, - matching Release() removes it. This way at the end of the program I'm left with AddRefs() without maching Releases. No need to match pairs,
If you can reproduce the leak in a deterministic way, a simple technique I often used is to number all your smart pointers in their order of construction (use a static counter in the constructor), and report this ID together with the leak. Then run the program again, and trigger a DebugBreak() when the smart pointer with the same ID gets constructed.
You should also consider this great tool : http://www.codeproject.com/KB/applications/visualleakdetector.aspx
What I do is wrap the smart pointer with a class that takes FUNCTION and LINE parameters. Increment a count for that function and line every time the constructor is called, and decrement the count every time the destructor is called. then, write a function that dumps the function/line/count information. That tells you where all of your references were created
To detect reference cycles you need to have a graph of all reference-counted objects. Such a graph is not easy to construct, but it can be done.
Create a global
set<CRefCounted*> to register living reference-counted objects. This is easier if you have common AddRef() implementation - just add
this pointer to the set when object's reference count goes from 0 to 1. Similarly, in Release() remove object from the set when it's reference count goes from 1 to 0.
Next, provide some way to get the set of referenced objects from each
CRefCounted*. It could be a
virtual set<CRefCounted*> CRefCounted::get_children() or whatever suits you. Now you have a way to walk the graph.
Finally, implement your favorite algorithm for cycle detection in a directed graph. Start the program, create some cycles and run cycle detector. Enjoy! :)
What I have done to solve this is to override the malloc/new & free/delete operators such that they keep track in a data structure as much as possible about the operation you are performing.
For example, when overriding malloc/new, You can create a record of the caller's address, the amount of bytes requested, the assigned pointer value returned and a sequence ID so all your records can be sequenced (I do not know if you deal with threads but you need to take that into account, too).
When writing the free/delete routines, I also keep track of the caller's address and the pointer info. Then I look backwards into the list and try to match the malloc/new counterpart using the pointer as my key. If I don't find it, raise a red flag.
If you can afford it, you can embed in your data the sequence ID to be absolutely sure who and when allocation call was made. The key here is to uniquely identify each transaction pair as much as we can.
Then you will have a third routine displaying your memory allocations/deallocation history, along with the functions invoking each transaction. (this can be accomplished by parsing the symbolic map out of your linker). You will know how much memory you will have allocated at any time and who did it.
If you don't have enough resources to perform these transactions (my typical case for 8-bit microcontrollers), you can output the same information via a serial or TCP link to another machine with enough resources.