---
 unix/serial.c |   38 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 36 insertions(+), 2 deletions(-)

--- unix/serial.c
+++ unix/serial.c	2023-07-12 09:28:06.155507529 +0000
@@ -57,11 +57,10 @@ const char serial_rcsid[] = "$Id: serial
 
 #if HAVE_FCNTL_H
 #include <fcntl.h>
-#else
+#endif
 #if HAVE_SYS_FILE_H
 #include <sys/file.h>
 #endif
-#endif
 
 #ifndef O_RDONLY
 #define O_RDONLY 0
@@ -817,6 +816,34 @@ fsserial_lock (qconn, fin, fuser)
 	(void) fsserial_lockfile (FALSE, qconn);
 	return FALSE;
       }
+#else
+#if HAVE_TIOCEXCL
+    /* If we can't mark it in use, return FALSE to indicate that the
+       lock failed.  */
+    if (ioctl (qsysdep->o, TIOCEXCL) < 0)
+      {
+	if (errno != EALREADY)
+	  ulog (LOG_ERROR, "ioctl (TIOCEXCL): %s", strerror (errno));
+#ifdef TIOCNOTTY
+	(void) ioctl (qsysdep->o, TIOCNOTTY, (char *) NULL);
+#endif
+	(void) close (qsysdep->o);
+	qsysdep->o = -1;
+	(void) fsserial_lockfile (FALSE, qconn);
+	return FALSE;
+      }
+#if defined(LOCK_EX) && defined(LOCK_NB)
+    if (flock(qsysdep->o, LOCK_EX|LOCK_NB) < 0)
+      {
+	if (errno != EWOULDBLOCK)
+	  ulog (LOG_ERROR, "flock (LOCK_EX|LOCK_NB): %s", strerror (errno));
+	(void) close (qsysdep->o);
+	qsysdep->o = -1;
+	(void) fsserial_lockfile (FALSE, qconn);
+	return FALSE;
+      }
+#endif
+#endif
 #endif
 
 #if HAVE_DEV_INFO
@@ -2220,6 +2247,13 @@ fsysdep_modem_end_dial (qconn, qdial)
 		q->iflags = iflags;
 #if HAVE_TIOCSINUSE
 		(void) ioctl (onew, TIOCSINUSE, 0);
+#else
+#if HAVE_TIOCEXCL
+		(void) ioctl (onew, TIOCEXCL);
+#if defined(LOCK_EX) && defined(LOCK_NB)
+		(void) flock(onew, LOCK_EX|LOCK_NB);
+#endif
+#endif
 #endif
 	      }
 	  }