pack and unpack problems

[Steven Taschuk]

Quoth Justin Johnson:
> Would you mind explaining this for me?  What do you mean by check
> alignment?  Sorry, this is kinda new for me.  In the meantime I'll go
> read my orielly tcp book.  :-)

Some architectures constrain (some) values to occur "aligned" at
certain boundaries in memory.  Structs on such platforms might
need padding between their members to ensure proper alignment;
e.g., instances of
    struct foo {
        char a;
        int b;
    }
might be organized in memory by the C compiler as
    0     1     2     3     4     5     6     7     8
    |  a  |     padding     |           b           |
where the padding makes sure b is aligned properly, but is not
otherwise used.  (Obviously here the whole struct must also be
aligned.)

But if you're parsing UDP packets, you know the exact layout and
don't care what the platform's C compiler would do with a struct
declaration.  Thus you want '!' for "standard alignment", which is
to say, no alignment required at all.  ('!' also implies network
byte order; see the docs.)

(Header layouts are, I suspect, designed with this kind of thing
in mind, so values are aligned in sensible ways anyway.  Thus
"parsing" a UDP packet in C could be as simple as casting a void *
to a struct udpheader *.)

-- 
Steven Taschuk                                     staschuk at telusplanet.net
Receive them ignorant; dispatch them confused.  (Weschler's Teaching Motto)