Index: lib/libc/sys/send.2
===================================================================
--- lib/libc/sys/send.2
+++ lib/libc/sys/send.2
@@ -122,6 +122,7 @@
 #define	MSG_OOB		0x00001 /* process out-of-band data */
 #define	MSG_DONTROUTE	0x00004 /* bypass routing, use direct interface */
 #define MSG_EOR		0x00008 /* data completes record */
+#define	MSG_DONTWAIT	0x00080	/* this message should be nonblocking */
 #define	MSG_EOF		0x00100 /* data completes transaction */
 #define	MSG_NOSIGNAL	0x20000 /* do not generate SIGPIPE on EOF */
 .Ed
Index: sys/kern/uipc_socket.c
===================================================================
--- sys/kern/uipc_socket.c
+++ sys/kern/uipc_socket.c
@@ -1522,7 +1522,7 @@
 		}
 		if (space < resid + clen &&
 		    (atomic || space < so->so_snd.sb_lowat || space < clen)) {
-			if ((so->so_state & SS_NBIO) || (flags & MSG_NBIO)) {
+			if ((so->so_state & SS_NBIO) || (flags & MSG_NBIO) || (flags & MSG_DONTWAIT)) {
 				SOCKBUF_UNLOCK(&so->so_snd);
 				error = EWOULDBLOCK;
 				goto release;