Open the file descriptor of a serial port on POSIX systems exclusively

This fixes bug #1510.

configure.ac

@@ -129,6 +129,10 @@ AC_CHECK_TYPES([struct serial_struct],,, [[#include <linux/serial.h>]])
 # Check for realpath().
 AC_CHECK_FUNC([realpath], [AC_DEFINE(HAVE_REALPATH, 1, [realpath is available.])], [])
 
+# Check for flock().
+AC_CHECK_HEADER([sys/file.h], [AC_DEFINE(HAVE_SYS_FILE_H, 1, [sys/file.h is available.])], [])
+AC_CHECK_FUNC([flock], [AC_DEFINE(HAVE_FLOCK, 1, [flock is available.])], [])
+
 # Check for clock_gettime().
 AC_CHECK_FUNC([clock_gettime],
 	[AC_DEFINE(HAVE_CLOCK_GETTIME, 1, [clock_gettime is available.])], [])

libserialport_internal.h

@@ -82,6 +82,9 @@
 #include <time.h>
 #include <poll.h>
 #include <unistd.h>
+#ifdef HAVE_SYS_FILE_H
+#include <sys/file.h>
+#endif
 #endif
 #ifdef __APPLE__
 #include <CoreFoundation/CoreFoundation.h>

serialport.c

@@ -551,6 +551,39 @@ SP_API enum sp_return sp_open(struct sp_port *port, enum sp_mode flags)
 
 	if ((port->fd = open(port->name, flags_local)) < 0)
 		RETURN_FAIL("open() failed");
+
+	/*
+	 * On POSIX in the default case the file descriptor of a serial port
+	 * is not opened exclusively. Therefore the settings of a port are
+	 * overwritten if the serial port is opened a second time. Windows
+	 * opens all serial ports exclusively.
+	 * So the idea is to open the serial ports alike in the exclusive mode.
+	 *
+	 * ioctl(*, TIOCEXCL) defines the file descriptor as exclusive. So all
+	 * further open calls on the serial port will fail.
+	 *
+	 * There is a race condition if two processes open the same serial
+	 * port. None of the processes will notice the exclusive ownership of
+	 * the other process because ioctl() doesn't return an error code if
+	 * the file descriptor is already marked as exclusive.
+	 * This can be solved with flock(). It returns an error if the file
+	 * descriptor is already locked by another process.
+	 */
+#ifdef HAVE_FLOCK
+	if (flock(port->fd, LOCK_EX | LOCK_NB) < 0)
+		RETURN_FAIL("flock() failed");
+#endif
+
+#ifdef TIOCEXCL
+	/*
+	 * Before Linux 3.8 ioctl(*, TIOCEXCL) was not implemented and could
+	 * lead to EINVAL or ENOTTY.
+	 * These errors aren't fatal and can be ignored.
+	 */
+	if (ioctl(port->fd, TIOCEXCL) < 0 && errno != EINVAL && errno != ENOTTY)
+		RETURN_FAIL("ioctl() failed");
+#endif
+
 #endif
 
 	ret = get_config(port, &data, &config);