Signal handling is distinct from a different thread because it blocks the execution of the “preempted thread” until the signal handler completes. As a example,

Signal handling is distinct from a different thread because it blocks the execution of the “preempted thread” until the signal handler completes. As a example, if the preempted code grabs a lock for a resource, then signal handler completion can not depend on grabbing that lock because that lock will never be released until the preempted code runs again and the preempted code can never run again until the signal handler completes. A correct signal handler can never wait for a resource held by regular code. This precludes coordination or sharing via normal locks or critical sections.