Add FIONWRITE to allow applications to sniff the amount of writable space on a descriptor. Implement this for serial devices only.

nuttx/drivers/serial/serial.c

@@ -688,6 +688,27 @@ static int uart_ioctl(FAR struct file *filep, int cmd, unsigned long arg)
             *(int *)arg = count;
           }
 
+          case FIONWRITE:
+          {
+            int count;
+            irqstate_t state = irqsave();
+
+            /* determine the number of bytes free in the buffer */
+
+            if (dev->xmit.head <= dev->xmit.tail)
+              { 
+                count = dev->xmit.tail - dev->xmit.head - 1;
+              }
+            else
+              {
+                count = dev->xmit.size - (dev->xmit.head - dev->xmit.tail) - 1;
+              }
+
+            irqrestore(state);
+
+            *(int *)arg = count;
+          }
+
 #ifdef CONFIG_SERIAL_TERMIOS
           case TCGETS:
           {

nuttx/include/nuttx/fs/ioctl.h

@@ -110,6 +110,10 @@
                                            * OUT: Bytes readable from this fd
                                            */
 
+#define FIONWRITE       _FIOC(0x0005)     /* IN:  Location to return value (int *)
+                                           * OUT: Bytes writable to this fd
+                                           */
+
 /* NuttX file system ioctl definitions **************************************/
 
 #define _DIOCVALID(c)   (_IOC_TYPE(c)==_DIOCBASE)