Mmm, I remember doing this, with select(3) for maximum portability and all... then learning that neither Linux nor Windows actually follow what POSIX says selec

Mmm, I remember doing this, with select(3) for maximum portability and all... then learning that neither Linux nor Windows actually follow what POSIX says select(3) should do, e.g. how to report a socket with non-blocking connect(3) operation when it finally fails to connect: it should be reported as readable (reading from it would fail immediately and would not block), writeable (same reason), having exceptional condition pending (it has a pending error). Nope, Linux reports it as readable/writeable but not having exceptional condition, while Windows reports it as only having exceptional condition pending but not as readable nor writeable. Beautiful, just beautiful. In the end you have to put sockets in such scenarios into all three sets and check what getsockopt(s, SOL_SOCKET, SO_ERROR) returns for them, that's the portable way.