In general, threads are identified to the run-time system by thread handles. Thread handles are provided to:
The thread handle uniquely identifies the thread to the various multi-threading COBOL verbs or to CBL_THREAD_n routines. With reasonable care, thread handles can be used by any of the active threads in the run-unit, not just the creator of a thread. It is important to remember that the lifetime of a thread and that of its thread handle might not be the same. Depending on how a thread was created and what operations have been done on it, a thread handle could still be valid even if the thread has terminated. In order to enable the system to recover the resources associated with the handle, it is up to the user to explicitly detach the handle for a thread.
A handle can become detached from its thread in one of several ways. When a thread is created with the START verb or CBL_THREAD_CREATE routine, its handle is, by default, created as detached. When another-language thread becomes known to the run-time system, its handle is automatically and always created as detached. A non-detached handle can be detached with a call to CBL_THREAD_DETACH. In any case, a detached handle must always be used with care, as when the thread terminates the handle immediately becomes invalid. If a thread with a non-detached handle has already terminated, then a call to CBL_THREAD_DETACH with that thread handle will immediately cause the handle to become invalid.
A non-detached handle is useful for retrieval of return values (via the WAIT verb or CBL_THREAD_WAIT routine), and for inspection of thread identification data (via the CBL_THREAD_IDDATA_GET routine) after a thread has potentially terminated. The START verb provides a way of identifying a newly created thread by returning a non-detached thread handle. The CBL_THREAD_CREATE routine provides a flag to indicate that the thread should be created and a non-detached handle returned.