Hello Mandeep,
On Mon, 31 Jan 2005 16:29:52 +0530, Mandeep Sandhu
<Mandeep_Sandhu@xxxxxxxxxxx> wrote:
> On Mon, 2005-01-31 at 16:20 +0530, cranium 2003 wrote:
> > Hello,
> > In linux kernel source etheernet header is defined as structure as
> > struct ethhdr
> > {
> > unsigned char h_dest[ETH_ALEN]; /* destination eth addr */
> > unsigned char h_source[ETH_ALEN]; /* source ether addr */
> > unsigned short h_proto; /* packet type ID field */
> > } __attribute__((packed));
> > I want to know what __attribute__((packed)) meance?
> > It is not looking as a structure object then what is thati?
>
> Your ethernet header is 14 bytes long so the "packed" attribute
> tells the compiler to to pack the structure to 14 bytes and not
> try to byte align it on a word boundary (as it would do without
> specifying this keyword).
Why its necessary to tell compiler to consider it as 14 bytes?
becasue sizeof(struct ethhdr) is 14 bytes so while putting ethhdr in
packet 14 bytes are always put so why question arises to have word
boundry alignment?
regards,
cranium
> from "info gcc"
>
> `packed'
> The `packed' attribute specifies that a variable or structure field
> should have the smallest possible alignment--one byte for a
> variable, and one bit for a field, unless you specify a larger
> value with the `aligned' attribute.
>
> Here is a structure in which the field `x' is packed, so that it
> immediately follows `a':
>
> struct foo
> {
> char a;
> int x[2] __attribute__ ((packed));
> };
>
> HTH,
> -mandeep
>
> > regards,
> > cranium
> >
> > --
> > Kernelnewbies: Help each other learn about the Linux kernel.
> > Archive: http://mail.nl.linux.org/kernelnewbies/
> > FAQ: http://kernelnewbies.org/faq/
> >
>
--
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