Re: [lwip-users] alignment error of 4 bytes alignment

[ÕÅÔö²¨£¬Nodinfo Technology]

Çdzç:

        I have encountered this problem and I walked around it.my solution is 
as below:

1.In the ethernet driver,after get the actual length of input packtet in 
argument len,allocate 2 more bytes of pbuf's length:
    p = pbuf_alloc(PBUF_RAW, len+2, PBUF_POOL);
    pbuf_header(p, -2);
2.After 1,in most cases fields in incoming ip packets are 4 bytes alinged.but 
the field sipaddr in struct etharp_hdr is not aligned.(this is the only case 
that not aligned)so I change etharp.c a little to avoid this problem.btw: 
before the definition of struct etharp_hdr,I use #pragma pack(1).

3.In opt.h,#define PBUF_LINK_HLEN 16
4.In function arp_query,resolve the output packet's alignemnt problem in the 
same way as 1.

I hope this may help you.

======= 2003-11-12 13:21:00 you write£º=======

>Dear Sir:
>       lwip's pbuf address's alignment can be set 4 aligned.But after
>    case ETHTYPE_IP:
>    q=etharp_ip_input(netif, p);
>    pbuf_header(p, -14);
>               ~~~~~~~~~~~~~
>    netif->input(p, netif);
>p->payload is not 4 byte aligned now,which generates ip.c,tcp_in.c many 
>alignment error.
>So I wish lwip suport a 14 bytes pbuf chain to left pbufs.every pbuf is 4 
>bytes aligned.
>After pbuf_header(p, -14);,the p->payload change to p->next->payload,which is 
>aligned.
>Now lwip does not support this.
>
>
>                               qiaochong
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                           Zhangzb,Nodinfo Technology
address@hidden
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡2003-11-12