If no signal in set is pending at the time of the call, the thread shall be suspended until one or more becomes pending. The signals defined by set shall have been blocked at the time of the call to sigwait(); otherwise, the behavior is undefined. The effect of sigwait() on the signal actions for the signals in set is unspecified.
This is really ambiguous ,what's the difference between pending and block here?
And its conclusion on how to choose between sigwait and sigaction is not clear at all:
In summary, when it is necessary for code run in response to an asynchronous signal to notify a thread, sigwait() should be used to handle the signal. Alterna- tively, if the implementation provides semaphores, they also can be used, either following sigwait() or from within a signal handling routine previously registered with sigaction().
Can someone make the reason of sigwait more rational?