On multithread operating systems, drivers must be thread safe. That is, it must be possible for applications to use the same handle on more than one thread. How this is achieved is driver-specific, and it is likely that drivers will serialize any attempts to concurrently use the same handle on two different threads.
Applications commonly use multiple threads instead of asynchronous processing. The application creates a separate thread and calls an ODBC function on it, and then continues processing on the main thread. Rather than having to continually poll the asynchronous function, as is the case when the SQL_ATTR_ASYNC_ENABLE statement attribute is used, the application can simply let the newly created thread finish.
Functions that accept a statement handle and are running on one thread can be canceled by calling SQLCancel with the same statement handle from another thread. Although drivers should not serialize the use of SQLCancel in this manner, there is no guarantee that calling SQLCancel will actually cancel the function running on the other thread.