在数据报Unix套接字上ECONNREFUSED

Question

What are the possible reasons for ECONNREFUSED when sending over a connectionless datagram Unix socket?

Also any advice on how to debug this is welcomed as this problem is reproducible.

I get the error regardless if sendto() or sendmsg() is used.

if ((sock = socket(PF_UNIX, SOCK_DGRAM, 0)) < 0)
{
    return 0;
}
unlink("/tmp/serv");
addr.sun_family = AF_UNIX;    

strncpy(&addr.sun_path[0], "/tmp/serv", sizeof(addr.sun_path));

if (bind(sock, (struct sockaddr *)&addr, sizeof(addr)) < 0)
{
    return 0;
}

sockaddr_un from;
int fromlen = sizeof(from);
if (recvfrom(sock, &i, sizeof(i),0,(sockaddr*)&from,(socklen_t*)&fromlen) < 0 )
{
    //some error handling code
}

printf("from.sun_family=%d, from.sun_path=%s",from.sun_family,from.sun_path); // this prints, as expected "from.sun_family=1, from.sun_path=/tmp/client"
strncpy(&addr.sun_path[0], "/tmp/client", sizeof(addr.sun_path));
sendto(sock,"abc",3,0,(sockaddr*)&addr, sizeof(addr)); //this fails with ECONNREFUSED

Answer 1

From man 7 unix :

ECONNREFUSED The remote address specified by connect(2) was not a listening socket. This error can also occur if the target filename is not a socket.

In Linux, sendto on Unix socket does the following :

1548         if (sock_flag(other, SOCK_DEAD)) {
1549                 /*
1550                  *      Check with 1003.1g - what should
1551                  *      datagram error
1552                  */
1553                 unix_state_unlock(other);
1554                 sock_put(other);
1555 
1556                 err = 0;
1557                 unix_state_lock(sk);
1558                 if (unix_peer(sk) == other) {
1559                         unix_peer(sk) = NULL;
1560                         unix_state_unlock(sk);
1561 
1562                         unix_dgram_disconnected(sk, other);
1563                         sock_put(other);
1564                         err = -ECONNREFUSED;
1565                 } else {
1566                         unix_state_unlock(sk);
1567                 }
1568 
1569                 other = NULL;
1570                 if (err)
1571                         goto out_free;
1572                 goto restart;
1573         }

In other words, there is no reader on the other end of the socket you send to, or the socket does not longer exist in the filesystem.