Operation Contexts

Contexts is a placeholder for application-defined data. The data, placed to Context, is not used by CBFS Filter in any way.

Why Contexts are useful

Contexts can be used to store information about the file or directory, and this information can be re-used later to speed-up subsequent operations.
For example, if you open some file on the file system, you can place file handle to Context. Next, when the application handles OnReadFileC callback, it can use this handle instead of the file name. This speeds-up the operation, because there is no need to search some hash tables for a handle of the file.

How long Contexts live

During file operations, contexts live from file open to file close calls. Contexts are stored with other information, related to opened file. During directory enumeration, contexts live from the call to OnEnumerateDirectory to the call to OnCloseEnumeration.
Contexts are local to the process in which they were created. They can't be passed to other processes.

There exist two different contexts for file operations - first (so-called file context) is allocated on the first file open operation and is deallocated on the last file close operation. Another one (so-called handle context) is allocated for each file open operation and is deallocated during corresponding file close operation. For example, consider the following sequence of operations:

  1. File X is opened by process A. File context FX is allocated. Handle context HXA is allocated.
  2. File X is opened by process B. Handle context HXB is allocated.
  3. File X is opened by process C. Handle context HXC is allocated. At this point, the file is opened three times, and we have three handle contexts and just one file context allocated.
  4. File X is closed by process C. HXC is deallocated, FX is alive.
  5. File X is closed by process A. HXA is deallocated, FX is alive.
  6. File X is closed by process B. HXB is deallocated, FX is deallocated as well.
File Context is available in all callbacks where file operations are performed on opened files. Handle Context, however, is not available when file read or write operations are performed. This is because memory mapping manager and cache manager act as proxies, and they can either issue one write request with combined data or postpone the writing operation to later moment (it's even possible that the application that involved the operation has been closed, and the data is still not written).

Use of contexts

Use of Contexts in VCL and C++/Lib API is quite straightforward - you just put pointers to Context.

With .NET API the situation is more complicated, as .NET uses garbage collection and improper typecasting can cause problems with object lifetime. To avoid this problem, we have declared Contexts to be IntPtr. If you want to store the reference to the object in Context, you use GCHandle as follows (C# syntax):

Stream stream; // the object we put to Context
GCHandle ctx = GCHandle.Alloc(stream);
Context = (IntPtr) ctx;
To access the stream from Context, use the following code:
GCHandle ctx = (GCHandle) Context;
Stream stream = (Stream ) ctx.Target; // the object we get from Context

To store numbers or other types, that are not objects, use "boxing" (.NET concept, when a value type is "packed" into an object).