Hi Ze,
I haven't heard of anyone porting Cap'n Proto to QNX, but it should be possible.
`sigaltstack()` is only used as part of `printStackTraceOnCrash()`, which is a totally optional helper that is very OS-specific. You could start by adding some #if/#else such that this function is empty on QNX. There's probably a number of other places around the library where you'll need to do similar things, but not too many, as long as you're sticking to just libkj and libcapnp. libkj-async will probably be a lot more complicated to support, but it's only needed for RPC -- if you only care about serialization then you can ignore that part.
-Kenton