On Mon, 2005-01-31 at 16:44 +0530, cranium 2003 wrote:
> 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?
for faster access. on most cpu archs, accessing byte aligned (word/
double word) mem is much faster. hence the compiler by default does
this optimization, unless and until explicitly told NOT to...so your
14 byte structure would otherwise be aligned on a 16 byte boundary.
It pads two extra bytes to your structure....since you don't want these
extra bytes when you are transmitting your packet...you tell the
compiler not to pad it ( via the "packed" attribute).
-mandeep
> 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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Archive: http://mail.nl.linux.org/kernelnewbies/
FAQ: http://kernelnewbies.org/faq/
