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

This fixes bug #1510.

diff --git a/configure.ac b/configure.ac
index 4d643e2ba2ae0b89fe92f99f52a1c354d7733b12..b1af16f08727e9bea3e339a083c224f051f741c7 100644 (file)
--- a/configure.ac
+++ b/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.])], [])

diff --git a/libserialport_internal.h b/libserialport_internal.h
index 5b811cc3bef665c02764fb4bbba0f0105cfe182b..ecf8fe95fa51ce4b7993df81d39f873538a5718e 100644 (file)
--- a/libserialport_internal.h
+++ b/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>

diff --git a/serialport.c b/serialport.c
index c22e94fa4bbbf52f988855378c0297585a5cd8dd..51fe0788fc4abd015adc9768d63c9288ac9e0593 100644 (file)
--- a/serialport.c
+++ b/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);