Maybe it will run on SunOS 5.8 now.

bin/9l

@@ -205,6 +205,19 @@ case "$tag" in
 			;;
 		esac
 	done
+	case "${SYSVERSION:-`uname -r`}" in
+	5.[67])
+		echo do not know how to link right thread library on "$tag" 1>&2
+		;;
+	5.8)
+		# Some trickery is needed to force use of
+		# alternate thread lib from /usr/lib/lwp
+		# Likely, this only works with sun cc,
+		# for other compiler/loader we would need other flags.
+		ld="$ld -i"
+		extralibs="$extralibs /usr/lib/lwp/libthread.so -R/usr/lib/lwp:/usr/lib"
+		;;
+	esac
 	;;
 *)
 	echo do not know how to link on "$tag" 1>&2

src/cmd/9term/SunOS.c

@@ -9,8 +9,14 @@
 int
 getpts(int fd[], char *slave)
 {
+	void (*f)(int);
+	int r;
+
 	fd[1] = open("/dev/ptmx", ORDWR);
-	if ((grantpt(fd[1]) < 0) || (unlockpt(fd[1]) < 0))
+	f = signal(SIGCLD, SIG_DFL);
+	r = grantpt(fd[1]);
+	signal(SIGCLD, f);
+	if(r < 0 || unlockpt(fd[1]) < 0)
 		return -1;
 	fchmod(fd[1], 0622);