Some signal handlers cannot be changed; that is, a user cannot catch these signals: E.g, SIGKILL. The default signal handler for SIGKILL terminates the process and cannot be replaced.

The default signal handler for most signals terminates the receiving process.

The SIGCHLD signal is sent to the parent when a child terminates, is stopped, or is continued. The default signal handler for SIGCHLD does nothing - so the default action is to ignore this signal.