What are pinned objects?
I am trying to find a memory leak using ants memory profiler, and I've encountered in a new term:
Pinned objects.
Can some one give me a good & simple explanation about what this objects are, How can I pinn/Unpinn objects, and detect who pinned objects?
Thanks
A pinned object is one that is not allowed to move. The garbage collector is normally compacting the memory in that it moves all objects to "one or more clusters". This is to create large chunks of free space.
This basically means if someone else (outside) has a pointer to the memory address of an object, this may point to random content - as the object has moved.
Pinning an object tells the GC to NOT MOVE IT. This is normally useless and ONLY makes sense when working with pointers - like when using PInvoke. Sometimes you need to turn in an address to a structure (in the memory layout term), and if that is implemented in a class, you have to pin that.
To answer concrete:
- You can not find out who pinned an obiect.
- Pinning is done with the FIXED statement. This is only allowed in unsafe code.
Check:
http://msdn.microsoft.com/en-us/library/f58wzh21%28VS.80%29.aspx
A pinned object is one that cannot be moved around by the garbage collector, meaning its address has to be kept the same because someone else, usually some piece of non managed code, depends upon the object being at a definite memory address.
Usually the garbage collector has freedom to relocate objects in memory. In managed code, as the garbage collector has the ability to access all references, it can freely remap an object to a different location and then update all references to that object so that the process is transparent to the running code. That way, the GC has the ability to better organize the program's memory and compact it if needed.
When an non-managed object is interacting with your code (in unsafe sections,) a situation may arise where there's a pointer somewhere to a piece of your code — for example, to a piece of memory addressed in your code that is being handled by an external COM call. This memory can't be remapped because the COM call is expecting the object to be in a given address and thus, if it was moved, the GC wouldn't have any way to notify the COM object of that change, resulting in an access violation or worse.
The reason you might pin an object is if you are making calls to unmanaged code.
When the garbage collector runs it may remove an object that's no longer required. This leaves a "hole" of free space in the heap. The GC then compacts the heap by moving the remaining objects together to make sure the free space is in one continous block (a bit like defragmenting your hard disk).
It also updates all the references (in the managed code) to any objects that have been moved as part of the compaction.
If you are working with unmanaged code (e.g. some external C++) and give it a pointer to an object, there is no way for the GC to tell the unmanaged code that the object has moved after it has run. Therefore you might mark the object your sharing with the external code as pinned so that you don't have the problem of the pointer becoming invalid.
Pinned objects are used when communicating with non-managed code. In managed code the garbage collector is free to move memory blocks around, as it knows about all references to the memory block and can update those accordingly.
When communicating with non-managed code (e.g. Win-API) pointers to data or buffers are often passed as argument. If the garbage collector was free to move that data, the pointers would suddenly become invalid. As the pointer is transferred to unmanaged code, it is not possible for the GC to update the pointer - or even know of where it is used. To prevent memory moving and make sure that the data stays in the place known by the unmanaged code's pointer the object can be pinned.