diff --git a/sys/kern/sys_socket.c b/sys/kern/sys_socket.c
index ecfb4adca6d3..c29ecc6445f6 100644
--- a/sys/kern/sys_socket.c
+++ b/sys/kern/sys_socket.c
@@ -174,6 +174,14 @@ soo_ioctl(struct file *fp, u_long cmd, void *data, struct ucred *active_cred,
 		*(int *)data = so->so_snd.sb_cc;
 		break;
 
+	case FIONSPACE:
+		if ((so->so_snd.sb_hiwat < so->so_snd.sb_cc) ||
+		    (so->so_snd.sb_mbmax < so->so_snd.sb_mbcnt))
+			*(int *)data = 0;
+		else
+			*(int *)data = sbspace(&so->so_snd);
+		break;
+
 	case FIOSETOWN:
 		error = fsetown(*(int *)data, &so->so_sigio);
 		break;
diff --git a/sys/sys/filio.h b/sys/sys/filio.h
index 68e7852a4ceb..1710ba45e6c5 100644
--- a/sys/sys/filio.h
+++ b/sys/sys/filio.h
@@ -56,6 +56,7 @@ struct fiodgname_arg {
 };
 #define	FIODGNAME	_IOW('f', 120, struct fiodgname_arg) /* get dev. name */
 #define	FIONWRITE	_IOR('f', 119, int)	/* get # bytes (yet) to write */
+#define	FIONSPACE	_IOR('f', 118, int)	/* get space in send queue */
 /* Handle lseek SEEK_DATA and SEEK_HOLE for holey file knowledge. */
 #define	FIOSEEKDATA	_IOWR('f', 97, off_t)	/* SEEK_DATA */
 #define	FIOSEEKHOLE	_IOWR('f', 98, off_t)	/* SEEK_HOLE */