diff --git a/drivers/pipes/pipe_common.c b/drivers/pipes/pipe_common.c
index 3bb5fc405e0..2bc153efc7f 100644
--- a/drivers/pipes/pipe_common.c
+++ b/drivers/pipes/pipe_common.c
@@ -785,11 +785,18 @@ int pipecommon_ioctl(FAR struct file *filep, int cmd, unsigned long arg)
         }
         break;
 
-      case FIONREAD:
+      case FIONWRITE:  /* Number of bytes waiting in send queue */
+      case FIONREAD:   /* Number of bytes available for reading */
         {
           int count;
 
-          /* Determine the number of bytes available in the buffer */
+          /* Determine the number of bytes written to the buffer.  This is,
+           * of course, also the number of bytes that may be read from the
+           * buffer.
+           *
+           *   d_rdndx - index to remove next byte from the buffer
+           *   d_wrndx - Index to next location to add a byte to the buffer.
+           */
 
           if (dev->d_wrndx < dev->d_rdndx)
             {
@@ -805,11 +812,17 @@ int pipecommon_ioctl(FAR struct file *filep, int cmd, unsigned long arg)
         }
         break;
 
-      case FIONWRITE:
+      /* Free space in buffer */
+
+      case FIONSPACE:
         {
           int count;
 
-          /* Determine the number of bytes free in the buffer */
+          /* Determine the number of bytes free in the buffer.
+           *
+           *   d_rdndx - index to remove next byte from the buffer
+           *   d_wrndx - Index to next location to add a byte to the buffer.
+           */
 
           if (dev->d_wrndx < dev->d_rdndx)
             {
diff --git a/drivers/serial/pty.c b/drivers/serial/pty.c
index 29b752fac90..ae5ac1619be 100644
--- a/drivers/serial/pty.c
+++ b/drivers/serial/pty.c
@@ -774,17 +774,48 @@ static int pty_ioctl(FAR struct file *filep, int cmd, unsigned long arg)
         }
         break;
 #endif
+
+      /* Get the number of bytes that are immediately available for reading
+       * from the source pipe.
+       */
+
+      case FIONREAD:
+        {
+          ret = file_ioctl(&dev->pd_src, cmd, arg);
+        }
+        break;
+
+      /* Get the number of bytes waiting in the sink pipe (FIONWRITE) or the
+       * number of unused bytes in the sink pipe (FIONSPACE).
+       */
+
+      case FIONWRITE:
+      case FIONSPACE:
+        {
+          ret = file_ioctl(&dev->pd_sink, cmd, arg);
+        }
+        break;
+
       /* Any unrecognized IOCTL commands will be passed to the contained
        * pipe driver.
+       *
+       * REVISIT:  We know for a fact that the pipe driver only supports
+       * FIONREAD, FIONWRITE, FIONSPACE and PIPEIOC_POLICY.  The first two
+       * are handled above and PIPEIOC_POLICY should not be managed by
+       * applications -- it can break the PTY!
        */
 
       default:
         {
+#if 0
           ret = file_ioctl(&dev->pd_src, cmd, arg);
           if (ret >= 0 || ret == -ENOTTY)
             {
               ret = file_ioctl(&dev->pd_sink, cmd, arg);
             }
+#else
+          ret = ENOTTY;
+#endif
         }
         break;
     }
diff --git a/drivers/serial/serial.c b/drivers/serial/serial.c
index 566cf834e6f..49d6a970b7d 100644
--- a/drivers/serial/serial.c
+++ b/drivers/serial/serial.c
@@ -910,12 +910,16 @@ static int uart_ioctl(FAR struct file *filep, int cmd, unsigned long arg)
     {
       switch (cmd)
         {
+          /* Get the number of bytes that may be read from the RX buffer
+           * (without waiting)
+           */
+
           case FIONREAD:
             {
               int count;
               irqstate_t flags = enter_critical_section();
 
-              /* Determine the number of bytes available in the buffer */
+              /* Determine the number of bytes available in the RX buffer */
 
               if (dev->recv.tail <= dev->recv.head)
                 {
@@ -933,12 +937,39 @@ static int uart_ioctl(FAR struct file *filep, int cmd, unsigned long arg)
             }
             break;
 
+          /* Get the number of bytes that have been written to the TX buffer. */
+
           case FIONWRITE:
             {
               int count;
               irqstate_t flags = enter_critical_section();
 
-              /* Determine the number of bytes free in the buffer */
+              /* Determine the number of bytes waiting in the TX buffer */
+
+              if (dev->xmit.head < dev->xmit.tail)
+                {
+                  count = dev->xmit.head - dev->xmit.tail;
+                }
+              else
+                {
+                  count = dev->xmit.size - (dev->xmit.tail - dev->xmit.head);
+                }
+
+              leave_critical_section(flags);
+
+              *(FAR int *)((uintptr_t)arg) = count;
+              ret = 0;
+            }
+            break;
+
+          /* Get the number of free bytes in the TX buffer */
+
+          case FIONSPACE:
+            {
+              int count;
+              irqstate_t flags = enter_critical_section();
+
+              /* Determine the number of bytes free in the TX buffer */
 
               if (dev->xmit.head < dev->xmit.tail)
                 {
diff --git a/drivers/usbdev/cdcacm.c b/drivers/usbdev/cdcacm.c
index 28d498363b0..64e2e68075d 100644
--- a/drivers/usbdev/cdcacm.c
+++ b/drivers/usbdev/cdcacm.c
@@ -1982,12 +1982,16 @@ static int cdcuart_ioctl(FAR struct file *filep, int cmd, unsigned long arg)
       break;
 #endif
 
+    /* Get the number of bytes that may be read from the RX buffer (without
+     * waiting)
+     */
+
     case FIONREAD:
       {
         int count;
         irqstate_t flags = enter_critical_section();
 
-        /* Determine the number of bytes available in the buffer. */
+        /* Determine the number of bytes available in the RX buffer. */
 
         if (serdev->recv.tail <= serdev->recv.head)
           {
@@ -2005,12 +2009,39 @@ static int cdcuart_ioctl(FAR struct file *filep, int cmd, unsigned long arg)
       }
       break;
 
+    /* Get the number of bytes that have been written to the TX buffer. */
+
     case FIONWRITE:
       {
         int count;
         irqstate_t flags = enter_critical_section();
 
-        /* Determine the number of bytes free in the buffer. */
+        /* Determine the number of bytes waiting in the TX buffer. */
+
+        if (serdev->xmit.tail <= serdev->xmit.head)
+          {
+            count = serdev->xmit.head - serdev->xmit.tail;
+          }
+        else
+          {
+            count = serdev->xmit.size - (serdev->xmit.tail - serdev->xmit.head);
+          }
+
+        leave_critical_section(flags);
+
+        *(int *)arg = count;
+        ret = 0;
+      }
+      break;
+
+    /* Get the number of free bytes in the TX buffer */
+
+    case FIONSPACE:
+      {
+        int count;
+        irqstate_t flags = enter_critical_section();
+
+        /* Determine the number of bytes free in the TX buffer */
 
         if (serdev->xmit.head < serdev->xmit.tail)
           {
diff --git a/include/nuttx/fs/ioctl.h b/include/nuttx/fs/ioctl.h
index ba7586e3da6..63074a3a9bc 100644
--- a/include/nuttx/fs/ioctl.h
+++ b/include/nuttx/fs/ioctl.h
@@ -137,7 +137,10 @@
                                            * OUT: Bytes readable from this fd
                                            */
 #define FIONWRITE       _FIOC(0x0006)     /* IN:  Location to return value (int *)
-                                           * OUT: Bytes writable to this fd
+                                           * OUT: Number bytes in send queue
+                                           */
+#define FIONSPACE       _FIOC(0x0007)     /* IN:  Location to return value (int *)
+                                           * OUT: Free space in send queue.
                                            */
 
 /* NuttX file system ioctl definitions **************************************/