B.6 Using DECthreads with Asynchronous System Trap (AST) Routines

An AST is an OpenVMS mechanism for reporting an asynchronous event to a process. The following are restrictions concerning the use of ASTs with DECthreads:

• Avoid blocking ASTs using any mechanism other than $SETAST.

• Be aware that blocking ASTs in one thread may prevent delivery of ASTs which are actually intended for other threads. Therefore, it is best to avoid blocking ASTs for an extended period of time. Also, it is best to avoid calling DECthreads functions which may block the thread while it has disabled ASTs.

• Do not call DECthreads routines, except those that have the _int (interrupt) suffix in their names, from within an AST routine. Calling any other DECthreads routines from code running in an AST can be unreliable or cause unexpected behavior.

• For OpenVMS Alpha, ASTs are handed off to DECthreads by the operating system. This allows ASTs to be delivered in the context of the appropriate thread. On a multiprocessor machine it may be possible to have a thread executing an AST routine in parallel with another thread's execution. When a thread disables ASTs, not only does it block out its own ASTs, but it prevents delivery of any ASTs that do not specifically belong to a particular thread as well.