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@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;
--
1.9.3
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