<div dir="ltr">Darn it - typo: the pointer to the start of the data room in a mbuf was incorrectly described earlier.<br>I should have written: rte_pktmbuf_mtod(mbuf, ....); // provide a pointer to the start of the data room.<br><br><br><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">From: <strong class="gmail_sendername" dir="auto">fwefew 4t4tg</strong> <span dir="auto"><<a href="mailto:7532yahoo@gmail.com">7532yahoo@gmail.com</a>></span><br>Date: Tue, Feb 1, 2022 at 1:16 PM<br>Subject: mbuf memory layout<br>To:  <<a href="mailto:users@dpdk.org">users@dpdk.org</a>><br></div><br><br><div dir="ltr">I create a mempool thusly with 0 bytes private size and 1024 bytes data room size:<br><br>rte_mempool *pool = rte_pktmbuf_pool_create("test", 1024, 0, 0, 1024, 0);<br><br>from which I allocate 1 mbuf.<div><br>which of the following three memory layouts best describes mbufs I allocate from this <br>pool? I can't tell if it's the 2nd or 3rd one. I believe the first one is wrong:<br><br><pre style="margin-top:0px;margin-bottom:0px;padding:0px;overflow:hidden;line-height:1.2em;color:rgb(0,0,0)">                                                                             
 mbuf - RTE_PKTMBUF_HEADROOM not in data room                                
 +--------------------------------+                                          
 |+------------------------------+|                                          
 ||struct rte_mbuf               ||                                          
 |+------------------------------+|                                          
 ||RTE_PKTMBUF_HEADROOM bytes    ||                                          
 |+------------------------------+| Data room starts here which will hold    
 ||1024 bytes of data room size  || L2/3/4 headers and actual payload. A     
 |+------------------------------+| pointer to the start of this field is    
 |               .                | given by rte_pktmbuf(mbuf, ...)          
 |               .                |                                          
 |               .                |                                          
 +--------------------------------+                                          
                                                                             
                                                                             
 mbuf - RTE_PKTMBUF_HEADROOM adds to data room size:                         
 +--------------------------------+                                          
 |+------------------------------+|                                          
 ||struct rte_mbuf               ||                                          
 |+------------------------------+|Data room starts here which will hold     
 ||1024 bytes + RTE_PKTMBUF_HEAD\||L2/3/4 headers and actual payload. A      
 ||ROOM bytes                    ||pointer to the start of this field is     
 ||                              ||given by rte_pktmbuf(mbuf, ...)           
 ||                              ||                                          
 |+------------------------------+|                                          
 |               .                |                                          
 |               .                |                                          
 |               .                |                                          
 +--------------------------------+                                          
                                                                             
 mbuf - RTE_PKTMBUF_HEADROOM included in data room size                      
 +--------------------------------+                                          
 |+------------------------------+|                                          
 ||struct rte_mbuf               ||                                          
 |+------------------------------+|Data room starts here which will hold     
 ||1024 bytes of data room size  ||L2/3/4 headers and actual payload. Since  
 ||                              ||data room size was >= RTE_PKTMBUF_HEADROOM
 ||                              ||size the data room size of 1024 was valid 
 |+------------------------------+|and that's all that is reserved for the   
 |                                |data room                                 
 |                                |                                          
 |                                |                                          
 |                                |                                          
 +--------------------------------+                                          </pre><br><br></div></div>
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