pthread_kill(3C) is the thread analog of kill(2). A pthread_kill() call sends a signal to a specific thread. A signal that is sent to a specified thread is different from a signal that is sent to a process. When a signal is sent to a process, the signal can be handled by any thread in the process. A signal sent by pthread_kill() can be handled only by the specified thread.
You can use pthread_kill() to send signals only to threads in the current process. Because the thread identifier, type thread_t, is local in scope, you cannot name a thread outside the scope of the current process.
On receipt of a signal by the target thread, the action invoked (handler, SIG_DFL, or SIG_IGN) is global, as usual. If you send SIGXXX to a thread, and SIGXXX to kill a process, the whole process is killed when the target thread receives the signal.