Netmap: head vs cur vs tail?

[Matthew P. Grosvenor]

Thanks! This has tons more info. I'll have a read. 

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Sent from my phone, sorry about the typos. 

> On 17 Oct 2014, at 18:27, Luigi Rizzo <rizzo at iet.unipi.it> wrote:
> 
> 
> 
>> On Fri, Oct 17, 2014 at 9:55 AM, Matthew P. Grosvenor <matthew.grosvenor at cl.cam.ac.uk> wrote:
>> Hi all,
>> I’m trying to understand how to use the netmap framework, specifically how the head, tail and current “pointers” interact with each other.
>> 
>> Looking in man NETMAP(4) (http://www.freebsd.org/cgi/man.cgi?query=netmap&sektion=4) under data structures, struct netmap_ring   it says: "  contains the index of he current read or write slot (cur), “. In the example code, the following pattern is used:
> 
> ​the default netmap manpage at the above URL is the old one,
> please use the one for 10-stable or 11-current 
> 
> http://www.freebsd.org/cgi/man.cgi​?​query=netmap&manpath=FreeBSD+10.0-stable
> 
> ​which​ explains in more detail the role
> of the three pointers (with some ascii graphics too).
> 
> Feel free to ask for more details if the page is not clear
> 
> cheers
> luigi
> 
>> 
>>         i = ring->cur;
>>         ...
>>         ring->cur = NETMAP_RING_NEXT(ring, i);
>> 
>> However, in the example that ships with the netmap source (https://code.google.com/p/netmap/source/browse/examples/bridge.c#72 & https://code.google.com/p/netmap/source/browse/examples/pkt-gen.c#660) the following pattern is used:
>> 
>>         j = rxring->cur;
>>         while(…){
>>                 j = nm_ring_next(rxring, j);
>>         …
>>         }
>>         rxring->head = rxring->cur = j;
>> 
>> So the obvious question is, what is the relationship between head and current? Do I believe the man page (and man page example) that head is not necessary, or do I believe the example code that head is necessary and should be set to the same value as current? And if so, what is the point of head? And why is it updated outside of the loop in both of the examples?
>> 
>> At a high level, I’m looking for a better understanding of what head, tail and current mean and how they affect the processing of rings.
> 
>  
>> Cheers,
>> Matt
>> 
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> 
> 
> 
> -- 
> -----------------------------------------+-------------------------------
>  Prof. Luigi RIZZO, rizzo at iet.unipi.it  . Dip. di Ing. dell'Informazione
>  http://www.iet.unipi.it/~luigi/        . Universita` di Pisa
>  TEL      +39-050-2211611               . via Diotisalvi 2
>  Mobile   +39-338-6809875               . 56122 PISA (Italy)
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