Add iob_tailroom

I plan to use this for tcp.

include/nuttx/mm/iob.h

@@ -478,6 +478,17 @@ int iob_trycopyin(FAR struct iob_s *iob, FAR const uint8_t *src,
 int iob_copyout(FAR uint8_t *dest, FAR const struct iob_s *iob,
                 unsigned int len, unsigned int offset);
 
+/****************************************************************************
+ * Name: iob_tailroom
+ *
+ * Description:
+ *  Return the number of bytes at the tail of the I/O buffer chain which
+ *  can be used to append data without additional allocations.
+ *
+ ****************************************************************************/
+
+unsigned int iob_tailroom(FAR struct iob_s *iob);
+
 /****************************************************************************
  * Name: iob_clone
  *

mm/iob/Make.defs

@@ -27,7 +27,7 @@ CSRCS += iob_concat.c iob_copyin.c iob_copyout.c iob_contig.c iob_free.c
 CSRCS += iob_free_chain.c iob_free_qentry.c iob_free_queue.c
 CSRCS += iob_initialize.c iob_pack.c iob_peek_queue.c iob_remove_queue.c
 CSRCS += iob_statistics.c iob_trimhead.c iob_trimhead_queue.c iob_trimtail.c
-CSRCS += iob_navail.c iob_free_queue_qentry.c
+CSRCS += iob_navail.c iob_free_queue_qentry.c iob_tailroom.c
 
 ifeq ($(CONFIG_IOB_NOTIFIER),y)
   CSRCS += iob_notifier.c

mm/iob/iob_tailroom.c

@@ -0,0 +1,52 @@
+/****************************************************************************
+ * mm/iob/iob_tailroom.c
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.  The
+ * ASF licenses this file to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the
+ * License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ ****************************************************************************/
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <nuttx/config.h>
+
+#include <nuttx/mm/iob.h>
+
+#include "iob.h"
+
+/****************************************************************************
+ * Public Functions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Name: iob_tailroom
+ *
+ * Description:
+ *  Return the number of bytes at the tail of the I/O buffer chain which
+ *  can be used to append data without additional allocations.
+ *
+ ****************************************************************************/
+
+unsigned int iob_tailroom(FAR struct iob_s *iob)
+{
+  while (iob->io_flink != NULL)
+    {
+      iob = iob->io_flink;
+    }
+
+  return CONFIG_IOB_BUFSIZE - (iob->io_offset + iob->io_len);
+}