[lwip-users] Re: [lwip] uIP: writev() and tap

[Adam Dunkels]

Hi Felipe!

Yes, the Linux code for uIP 0.6 was taken from lwIP without even looking at 
it :-)

On Tuesday 19 February 2002 01.06, you wrote:
> it seems that the tap device under Linux gets a packet per write. So the
> writev in tapdev_send breaks one packet in 2. Well, I replaced the writev
> with
>
>   ret = write(fd,uip_buf,uip_len);
>
> Even thinking "well, there must be a reason to use writev instead of
> write"... I suppose it's because sometimes there's a "hole" between the
> headers and the data.

The writev will gather the two pieces of data in a single write() since the 
headers and data can be from different places in memory. There is a bug in 
Linux < 2.4.18 that makes writev() on a tapdev misbehave. Eddie C. Dost 
reported this uIP bug a few days ago and also sent in a bug report to Linus 
so that upcoming Linux versions will work better with writev() and the tap 
device.

The simplest workaround is just to copy the data into a new buffer and do a 
write() on that buffer:

#ifdef linux
  {
    char tmpbuf[UIP_BUFSIZE];
    int i;

    for(i = 0; i < 40 + UIP_LLH_LEN; i++) {
      tmpbuf[i] = uip_buf[i];
    }
    
    for(; i < uip_len; i++) {
      tmpbuf[i] = uip_appdata[i - 40 - UIP_LLH_LEN];
    }
    
    ret = write(fd, tmpbuf, uip_len);
  }  
#else 

/adam
-- 
Adam Dunkels <address@hidden>
http://www.sics.se/~adam

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