x                  The address of the function previously
                       established as the action for the signal.
                       The address may be the value SIG_DFL (0)  or
                       SIG_IGN (1).
    0                  Indicates errors. For this reason, there is
                       no way to know whether a return status of 0
                       indicates failure, or whether it indicates
                       that a previous action was SIG_DFL (0).