On Fri, 6 Feb 2026 15:36:37 +0100 Morten Brørup <[email protected]> wrote:
> > From: Stephen Hemminger [mailto:[email protected]] > > Sent: Friday, 6 February 2026 02.49 > > > > Why are the header structures marked packed, that is bogus, BSD and > > Linux don't do it. > > They have been packed since the first public release in 2013 [1]. > > I guess it's because the IP and TCP headers contain 4-byte fields, which make > those structures 4-byte aligned; but since the IP header follows a 14 byte > Ethernet header (without the magic 2-byte pre-padding done by the kernel), > the instances of the IP header are not 4-byte aligned, but 2-byte aligned. > Marking them packed is a way of stripping the alignment. > > BTW, the IPv4 header was bumped (from no alignment) to 2-byte alignment with > patch [2]. > > [1]: > https://github.com/DPDK/dpdk/commit/af75078fece3615088e561357c1e97603e43a5fe#diff-620c2b2031359304a7f26328a52035c9f8ddf722b9280f957047dcb81467777f > [2]: > https://github.com/DPDK/dpdk/commit/c14fba68edfa4aeba7c0dfb5dbc3b4f23affbb81 > > > > Windows probably does > > Yes, probably. > The Microsoft compiler is more pedantic (leading to fewer bugs), and many of > those structures should formally be packed (or more correctly: unaligned). > > > but Windows code seems to love packed even when > > it is not necessary. > > I guess packing (without thinking about the need for it) has become a bad > habit for some Windows programmers. > Making structure packed (in the past) made code slower on some architectures because it required generating multiple load/store operations.
