send, sendto, sendmsg, recv, recvfrom and recvmsg failed with EOPNOTSUPP
for MSG_DONTWAIT (0x80), so code that polls a blocking socket with it
got an error on every call instead of its data or EWOULDBLOCK.
FreeBSD 11.4 defines the flag in sys/sys/socket.h:
#define MSG_DONTWAIT 0x80 /* this message should be nonblocking */
and soreceive_generic() (sys/kern/uipc_socket.c) treats it as a
non-blocking socket for that one call:
if ((so->so_state & SS_NBIO) ||
(flags & (MSG_DONTWAIT|MSG_NBIO))) {
SOCKBUF_UNLOCK(&so->so_rcv);
error = EWOULDBLOCK;
goto release;
On Linux and macOS the flag now maps to the host's MSG_DONTWAIT, next to
MSG_PEEK on a receive and MSG_NOSIGNAL on a send. Winsock has no
per-call non-blocking flag, so Windows keeps failing with EOPNOTSUPP
for it; TechnicalDebt records that. Other flags still fail with
EOPNOTSUPP.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
