Re: [libpcap][patch] appending to a capture

[Darren Reed <darren.reed () oracle com>]

On  1/06/11 08:10 AM, Mark Johnston wrote:
> Hi Darren,
>
> On Tue, May 31, 2011 at 03:53:22PM -0700, Darren Reed wrote:
>    
> > Hi Mark,
> >
> > I must admit that I don't see the point of this patch.
> >
> > A pcap data file, with packets in it, is something that
> > I would create using tcpdump over a specific period
> > of time. The data file is thus associated with a very
> > specific set of actions. To then append data to that
> > file without that data being associated with the
> > original action seems wrong.
> >      
> In my tree at work, the function in used in a program quite similar
> to tcpflow. I agree that it doesn't make sense to cat the output of
> multiple tcpdump sessions into a single capture file, but it depends on
> what I want to do with the recorded packets... in my case they're used
> by another program to replay captured flows, so the context in which
> they were originally captured doesn't really matter.
>
>    
> > That said, I can also imagine people using this function
> > and running into huge performance problems.
> >      
> I don't see how that is. The function essentially does the following:
>
> - Check if we're writing to stdout. If so, write a header and return.
> - Open the file, read the header and make sure it matches the one passed
>    into the function.
> - If it does, lseek to EOF, and if the file was empty, write a header.
> - Return the descriptor.
>
> How could this cause a performance problem?
>    

Because for every packet that is appended you need to do:
1. open(2)
2. read(2)
3. seek(2)
4. write(2)
5. close(2)

That's an awful lot of overhead to write 100 bytes out to a file.

My fear is that various people will see this function and end up using 
it in place of the other functions in libpcap because they like the name 
of this one.

Michael's idea of pcap_reopen() is much better. It supports everything 
from doing a single packet write to multiple packets with only a single 
open/read/seek.

Darren

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