mbuf: document how to set length when attaching ext buffer

Enhance API documentation of rte_pktmbuf_attach_extbuf() to
explain that the attached mbuf is initialized with length = 0.

Link: https://bugs.dpdk.org/show_bug.cgi?id=362
Signed-off-by: Jörg Thalheim <joerg@thalheim.io>
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>

diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index 219b110..2d4bda2 100644 (file)
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -924,10 +924,14 @@ rte_pktmbuf_ext_shinfo_init_helper(void *buf_addr, uint16_t *buf_len,
  * provided via shinfo. This callback function will be called once all the
  * mbufs are detached from the buffer (refcnt becomes zero).
  *
- * The headroom for the attaching mbuf will be set to zero and this can be
- * properly adjusted after attachment. For example, ``rte_pktmbuf_adj()``
+ * The headroom length of the attaching mbuf will be set to zero and this
+ * can be properly adjusted after attachment. For example, ``rte_pktmbuf_adj()``
  * or ``rte_pktmbuf_reset_headroom()`` might be used.
  *
+ * Similarly, the packet length is initialized to 0. If the buffer contains
+ * data, the user has to adjust ``data_len`` and the ``pkt_len`` field of
+ * the mbuf accordingly.
+ *
  * More mbufs can be attached to the same external buffer by
  * ``rte_pktmbuf_attach()`` once the external buffer has been attached by
  * this API.