Re: question about mbuf intialization

[Beverly Schwartz <bschwart%bbn.com@localhost>]

On Sep 18, 2013, at 12:16 PM, Matt Thomas <matt%3am-software.com@localhost> 
wrote:

> 
> On Sep 18, 2013, at 8:33 AM, Beverly Schwartz <bschwart%bbn.com@localhost> 
> wrote:
> 
>> Any reason why we can't add
>>      m->m_len = 0;
>> to m_get, and
>>      m->m_pkthdr.len = 0;
>> to m_gethdr?
> 
> Why not m->m_len = MLEN or MHLEN and m->m_pkthdr.len = MHLEN?
> 
> That way you know how much space is left.

m_len and m_pkthdr.len report on the data that is there, not on how much space 
exists.  At initialiation, there is no data, so should be initialized to 0.



> But in reality, it's expected the user knows that the length
> fields and/or m_data need to be adjusted.

As long as functions like m_copyback exist which provide a different way to 
load up data (and these function *do* update m_len and pkthdr.len), it seems 
that we just add a burden to the user that doesn't have to be there.  At the 
very least, the mbuf man page should indicate that these fields need to be 
initialized by the user.

Is there a downside to providing the initialization?  If not, why not do it?  
It reduces the chances of error because some user doesn't realize these fields 
are not initialized.

-Bev