NuttX follows OpenGroup.org: https://pubs.opengroup.org/onlinepubs/009695399/functions/recv.html : [EAGAIN] or [EWOULDBLOCK] The socket's file descriptor is marked O_NONBLOCK and no data is waiting to be received; or MSG_OOB is set and no out-of-band data is available and either the socket's file descriptor is marked O_NONBLOCK or the socket does not support blocking to await out-of-band data. ETIMEDOUT is not a valid error to be returned from recv() or recvfrom().