BPF question

Robert Watson rwatson at FreeBSD.org
Thu Nov 6 09:06:38 PST 2008

On Thu, 6 Nov 2008, Ivo Vachkov wrote:

> I am using simple write() calls to send packets over BPF file descriptor. 
> The BPF file descriptor is in buffered read mode (I assume this is the 
> default and I do not set it explicitly). From what I see my write() calls 
> are somewhat buffered. Since timing is relatively important for my project 
> I'd like to ask if there is a way "flush" the write buffer. Setting O_DIRECT 
> flag on the file descriptor doesn't seem to have any effect.

The write(2) system call does no buffering in userspace (unlike, say, 
fwrite(3)), and when you write to a BPF device it essentially goes straight 
into the network interface output queue, so there should be no need for a 
flush mechanism.  Could you describe the buffering effect you're seeing a bit 

Robert N M Watson
Computer Laboratory
University of Cambridge

More information about the freebsd-net mailing list