x Upon successful completion, the signal number
selected is returned.
-1 Indicates that an error occurred; errno is set
to one of the following values:
o EINVAL - The timeout argument specified a
tv_nsec value less than 0 or greater than
or equal to 1 billion.
o EINTR - The wait was interrupted by an
unblocked, intercepted signal.
o EAGAIN - No signal specified by set was
generated within the specified timeout
period.