Signal delivery also depends on the signal delivery key. Each process has one signal delivery key. The signal delivery key is set to the PSW key of the caller of the first z/OS UNIX call that created the process. A process created by the fork or exec service has key 8. The attach_exec service works differently from the exec and fork service; it creates a process with a signal delivery key equal to the PSW key of the Attach_exec caller. Key zero is not a valid signal delivery key. Therefore, if the caller's PSW key is zero when mvssigsetup created the process, the mvssigsetup call fails and signal catchers cannot be invoked in this process.