> For me, the most surprising part is Handlers[signum].tripped which is declared as volatile *and* an atomic variable.

I think it's just following this part of the C spec (Some background in bpo-12060):

> or refers to any object with static storage duration other than by assigning a value to a static storage duration variable of type volatile sig_atomic_t. Furthermore, if such a call fails, the value of errno is unspecified.

https://pubs.opengroup.org/onlinepubs/007904875/functions/sigaction.html / https://wiki.sei.cmu.edu/confluence/display/c/SIG31-C.+Do+not+access+shared+objects+in+signal+handlers

> I'm not sure if Handlers[signum].func must be volatile neither.

I think your assessment here is right considering it's never used in the signal handler itself.

> Also, wakeup.fd is declared as sig_atomic_t on Unix. Could we use an atomic variable instead, or is it important to use "sig_atomic_t"?

Again looking at the https://wiki.sei.cmu.edu/confluence/display/c/SIG31-C.+Do+not+access+shared+objects+in+signal+handlers recommendation it seems like using an atomic variable is fine but only if it's lock-free.