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.