When a new thread is created, no ON-units are assumed to be established. The ON-units which are in effect at the time a thread is created are not inherited by the new attached thread. Conditions that occur within a thread are handled within the thread and are not handled across thread boundaries.
For example, assume that thread A opens file F; then, A creates thread T. T then causes the ENDFILE condition to be raised. If an ON ENDFILE condition, is not established in thread T itself, the ERROR condition is raised in T and the usual condition handling takes place all within thread T. Whether or not A has established ON-units for ENDFILE or ERROR does not affect the execution of thread T.
A thread must establish ON-units for appropriate conditions if it wishes to handle them. There is no mechanism to signal conditions across threads.
If CTRL-BREAK is used to raise the ATTENTION condition, the ATTENTION condition is raised only in the main thread, not in any threads created by ATTACH statements.