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cmsg.3: ensure buf is suitably aligned in sending example 

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.

Signed-off-by: David Wragg <david@wragg.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
David Wragg 2014-11-02 23:08:35 +00:00 committed by Michael Kerrisk
parent e0f1f1765b
commit a7bd3dab97
1 changed files with 8 additions and 3 deletions
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11
man3/cmsg.3
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@ -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;