I discovered a problem in the file drivers/serial/serial.c concerning the function uart_close(…). In the case that a serial device is opened with the flag O_NONBLOCK the function uart_close(…) blocks until all data in the buffer is transmitted. The function close(…) called on an handle opened with O_NONBLOCK should not block. The problem occurred with a CDC/ACM device.

Documentation/README.html

@@ -8,7 +8,7 @@
   <tr align="center" bgcolor="#e4e4e4">
     <td>
       <h1><big><font color="#3c34ec"><i>NuttX README Files</i></font></big></h1>
-      <p>Last Updated: April 18, 2017</p>
+      <p>Last Updated: May 9, 2017</p>
     </td>
   </tr>
 </table>

drivers/serial/serial.c

@@ -1,7 +1,7 @@
 /************************************************************************************
  * drivers/serial/serial.c
  *
- *   Copyright (C) 2007-2009, 2011-2013, 2016 Gregory Nutt. All rights reserved.
+ *   Copyright (C) 2007-2009, 2011-2013, 2016-2017 Gregory Nutt. All rights reserved.
  *   Author: Gregory Nutt <gnutt@nuttx.org>
  *
  * Redistribution and use in source and binary forms, with or without
@@ -1207,6 +1207,10 @@ static int uart_close(FAR struct file *filep)
 
   uart_disablerxint(dev);
 
+  /* Prevent blocking if the device is opened with O_NONBLOCK */
+
+  if ((filep->f_oflags & O_NONBLOCK) == 0)
+    {
       /* Now we wait for the transmit buffer to clear */
 
       while (dev->xmit.head != dev->xmit.tail)
@@ -1228,6 +1232,7 @@ static int uart_close(FAR struct file *filep)
           up_mdelay(HALF_SECOND_MSEC);
 #endif
         }
+    }
 
   /* Free the IRQ and disable the UART */