Follow a process across .Fn fork calls. This flag is returned if the system was unable to attach an event to the child process, usually due to resource limitations. On return, .Va fflags contains the events which triggered the filter. Takes the signal number to monitor as the identifier and returns when the given signal is delivered to the process. This coexists with the .Fn signal and .Fn sigaction facilities, and has a lower precedence.
Event notification happens after normal signal delivery processing.
Otherwise, .Dv -1 will be returned, and .Dv errno will be set to indicate the error condition. If the time limit expires, then .Fn kevent returns 0. There was an error reading or writing the .Va kevent structure. The specified time limit is invalid. The process does not have permission to register a filter.