[lwip-devel] [bug #36283] PPP struct used on header size computation and not packed

[Sylvain Rochet]

Follow-up Comment #6, bug #36283 (project lwip):

I finally troubleshooted that and found why pppInputHeader must be packed.

If a structure is not packed, I guess the compiler optimize any use of this
structure in a way that the structure must be aligned everywhere it is used.

With PPPoE, if not packed:

Ethernet => +14
PPPoE header => +6
sizeof(*pih) - sizeof(inProtocol) => -6

So, pppInputHeader replaces the PPPoE header, which is good (except that we
double the inProtocol field, which is what -sizeof(inProtocol) tried to
avoid), but the *pih ptr is not aligned and some uC die because of that.

Of course, if the structure pppInputHeader is not packed, it may work I guess
if ETH_PAD_SIZE is set to 2 (if the driver support it, which mine not at the
moment), but this is just because the PPPoE header is 6 bytes long and
pppInputHeader too.

I am not a compiler expert, I guess if the structure is packed, that the
compiler does not add alignment optimisation then the structure can be used on
unaligned address.

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