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Setting Contexts

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After creating a new context, a minifilter driver can attach it to an object by calling one of the following set routines:

The set routine does the following, depending on the value of the Operation parameter:

  • If Operation == FLT_SET_CONTEXT_KEEP_IF_EXISTS:

    • If the minifilter has not already set a context of the same type for the object, the set routine:
      • Attaches the newly allocated context to the object.
      • Increments the reference count.
    • Else if the minifilter has already set a context, the set routine:
      • Returns STATUS_FLT_CONTEXT_ALREADY_DEFINED (an NTSTATUS error code).
      • Does not replace the existing context.
      • Does not increment the reference count.
      • Stores a pointer to the existing context in the OldContext parameter if it is non-NULL. When this pointer is no longer needed, the minifilter driver must release it by calling FltReleaseContext.

  • If Operation == FLT_SET_CONTEXT_REPLACE_IF_EXISTS:

    • The set routine always attaches the new context to the object.
    • If the minifilter driver has already set a context, the set routine:
      • Deletes the existing context, sets the new context, and increments the reference count on the new context.
      • If the OldContext parameter is non-NULL, it receives a pointer to the deleted context. When this pointer is no longer needed, the minifilter driver must release it by calling FltReleaseContext.

Once a context type has been set, a minifilter can get a context during subsequent I/O operation(s) to determine whether it needs to take any action.

Every successful context set must eventually be deleted.

In the following code example, taken from the CTX sample minifilter, the CtxInstanceSetup routine creates and sets an instance context:

status = FltAllocateContext(
           FltObjects->Filter,              //in: Filter
           FLT_INSTANCE_CONTEXT,            //in: ContextType
           CTX_INSTANCE_CONTEXT_SIZE,       //in: ContextSize
           NonPagedPool,                    //in: PoolType
           &instanceContext);               //out: ReturnedContext
...
status = FltSetInstanceContext(
           FltObjects->Instance,            //in: Instance
           FLT_SET_CONTEXT_KEEP_IF_EXISTS,  //in: Operation
           instanceContext,                 //in: NewContext
           NULL);                           //out: OldContext

if (instanceContext != NULL) {
  FltReleaseContext(instanceContext);
}
return status;

Note that after the call to FltSetInstanceContext, there is a call to FltReleaseContext to release the reference count that was set by FltAllocateContext (not FltSetInstanceContext). This is explained in Releasing Contexts.