How to detect unconnected AF_UNIX sockets
jb.1234abcd at gmail.com
Mon Sep 10 12:41:37 UTC 2012
Sam Varshavchik <mrsam <at> courier-mta.com> writes:
> I'm porting existing code from Linux where a connect() to an AF_UNIX socket
> that exists, but does not have a listener, fails with ECONNREFUSED. This is
> quite agreeable with the comparable scenario in AF_INET, with a connection
> attempt to a port without a listener on it. So the same code handles both
> situations, immediately reporting an error, and without caring much about
> the type of the socket.
> But on FreeBSD, according to truss, it seems that a connect() to an AF_UNIX
> socket without a listener still succeeds. A subsequent write() also
> succeeds, and read() blocks.
> The fall-out is quite unfortunate, and I was hoping for a way to detect that
> my allegedly connected socket is lying to me, and it's not really connected
> to anything. How would I go about doing that? I tried using the
> LOCAL_CONNWAIT option as documented in unix(4), but that does not appear to
> make any difference, in this situation.
It seems to work here:
$ uname -a
... FreeBSD 9.1-RC1 #0 ...
$ ls -al /tmp/server
srwxr-xr-x 1 jb wheel 0 Sep 10 14:22 /tmp/server
$ ps auxww |grep -i server-af_unix
client af_unix: connect() errno = 61
connect() failed: No such file or directory
$ grep 61 /usr/include/errno.h
#define ECONNREFUSED 61 /* Connection refused */
rc = connect(sd, (struct sockaddr *)&serveraddr, SUN_LEN(&serveraddr));
if (rc < 0)
printf("client af_unix: connect() errno = %d\n", errno);
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