[lwip-users] Checksum problem with lwip_chksum()

[Chris WIlliams]

I have had problems with checksums, and traced a problem in
lwip_chksum().

The problem is to do with alignment:

I am using Hitachi H8 which is 2 byte aligned and I have set

#define MEM_ALIGNMENT           2

The problem is that a definition of the kind:

char *text = "This is text";

is BYTE aligned.

If you try to send this with the following call:

tcp_write(port,text,strlen(text),0);

that is without copy, you can fail to get the correct checksum from
lwip_chksum(). This assumes that ALL the data it receives will be 2 byte
aligned. 

This caused one of my applications to always fail. If you change the
call to:

tcp_write(port,terminal,length,1);

then it works OK because it copies the data into a pbuf first which IS
correctly aligned.

I have reworked lwip_chksum() to work in bytes. This is slower, but it
does now work correctly.

Has anybody else seen this problem? Is it a particular problem of the H8
(Paul C, did you have any problem here?). Would anybody like me to post
my new code?

Chris.
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