0                  Indicates success.
    -1                 Indicates an error; A new signal handler is
                       not installed. errno is set to one of the
                       following values:

                       o  EFAULT - The action or o_action argument
                          points to a location outside of the
                          allocated address space of the process.

                       o  EINVAL - The sig argument is not a valid
                          signal number. Or an attempt was made to
                          ignore or supply a handler for the SIGKILL,
                          SIGSTOP, and SIGCONT signals.