Version: 2022.2
Language : English
Thread safe types
Copying NativeContainer structures

Implement a custom native container

To implement a custom native container, you must annotate your type with the the NativeContainer attribute. You should also understand how native containers are integrated with the safety system.

There are two major elements to implement:

  • Usage tracking: Allows Unity to keep track of scheduled jobs that use a NativeContainer instance, so that it can detect and prevent potential conflicts, such as two jobs writing to the same native container at the same time.
  • Leak tracking: Detects when a NativeContainer isn’t disposed of properly. In this situation, a memory leak happens, where the memory allocated to the NativeContainer becomes unavailable for the entire remaining lifetime of the program.

Implement usage tracking

To access usage tracking in your code, use the AtomicSafetyHandle class. AtomicSafetyHandle holds a reference to the central information that the safety system stores for a given native container, and is the main way that the methods of a NativeContainer interact with the safety system. Because of this, every NativeContainer instance must contain an AtomicSafetyHandle field named m_Safety.

Each AtomicSafetyHandle stores a set of flags that indicate what types of operation can be performed on the native container in the current context. When a job contains a NativeContainer instance, the job system automatically configures the flags in the AtomicSafetyHandle to reflect the way that the native container can be used in that job.

When a job tries to read from a NativeContainer instance, the job system calls the CheckReadAndThrow method before reading, to confirm that the job has read access to the native container. Similarly, when a job tries to write to a native container, the job system calls CheckWriteAndThrow before writing, to check that the job has write access to the native container. Two jobs that have been assigned the same NativeContainer instance have separate AtomicSafetyHandle objects for that native container, so although they both reference the same set of central information, they can each hold separate flags that indicate what read and write access each job has to the native container.

Implement leak tracking

Unity’s native code primarily implements leak tracking. It uses the UnsafeUtility.MallocTracked method to allocate the memory needed to store NativeContainer data, and then uses UnsafeUtility.FreeTracked to dispose of it.

In earlier versions of Unity the DisposeSentinel class provides leak tracking. Unity reports a memory leak when the garbage collector collects the DisposeSentinel object. To create a DisposeSentinel, use the Create method, which also initializes the AtomicSafetyHandle at the same time. When you use this method, you don’t need to initialize the AtomicSafetyHandle. When the NativeContainer is disposed of, the Dispose method disposes of both the DisposeSentinel and the AtomicSafetyHandle in a single call.

To identify where the leaked NativeContainer was created, you can capture the stack trace of where the memory was originally allocated. To do this, use the NativeLeakDetection.Mode property. You can also access this property in the Editor. To do this, go to Preferences > Jobs > Leak Detection Level and choose the leak detection level you need.

Nested native containers

The safety system doesn’t support nested native containers in jobs, because the job system can’t correctly configure the AtomicSafetyHandle for each individual NativeContainer inside the larger NativeContainer instance.

To prevent scheduling jobs that use nested native containers, use SetNestedContainer, which flags a NativeContainer as nested when they contain other NativeContainer instances.

Safety IDs and error messages

The safety system provides error messages that indicate when your code doesn’t adhere to safety constraints. To help make the error messages clearer, you can register a NativeContainer object’s name with the safety system.

To register a name, use NewStaticSafetyId, which returns a safety ID that you can pass to SetStaticSafetyId. Once you create a safety ID, you can reuse it for all instances of the NativeContainer, so a common pattern is to store it in a static member of the container class.

You can also override the error messages for specific safety constraint violations with SetCustomErrorMessage.

Additional resources

Thread safe types
Copying NativeContainer structures