On 11/03/2014 12:08 AM, David Wragg wrote:
> Inspection of the definition of CMSG_FIRSTHDR (both in glibc and the
> suggested definition in RFC3542) shows that it yields the msg_control
> field. So when sending, the pointer placed in msg_control should be
> suitably aligned as a struct cmsghdr. In the sending example, buf was
> declared as a bare char array, and so is not necessarily suitably
> aligned.
>
> The solution here involves placing buf inside a union, and is based on
> the sockets/scm_rights_send.c sample from The Linux Programming
> Interface "dist" source code collection.
Thanks, David. Applied.
Cheers,
Michael
> Signed-off-by: David Wragg <david@xxxxxxxxx>
> ---
> man3/cmsg.3 | 11 ++++++++---
> 1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/man3/cmsg.3 b/man3/cmsg.3
> index f355e6b..03f542e 100644
> --- a/man3/cmsg.3
> +++ b/man3/cmsg.3
> @@ -198,11 +198,16 @@ UNIX domain socket using
> struct msghdr msg = {0};
> struct cmsghdr *cmsg;
> int myfds[NUM_FD]; /* Contains the file descriptors to pass. */
> -char buf[CMSG_SPACE(sizeof myfds)]; /* ancillary data buffer */
> +union {
> + /* ancillary data buffer, wrapped in a union in order to ensure it is
> + suitably aligned */
> + char buf[CMSG_SPACE(sizeof myfds)];
> + struct cmsghdr align;
> +} u;
> int *fdptr;
>
> -msg.msg_control = buf;
> -msg.msg_controllen = sizeof buf;
> +msg.msg_control = u.buf;
> +msg.msg_controllen = sizeof u.buf;
> cmsg = CMSG_FIRSTHDR(&msg);
> cmsg\->cmsg_level = SOL_SOCKET;
> cmsg\->cmsg_type = SCM_RIGHTS;
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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