Unix – Wait for a Unix Domain socket to be bound

I know I am very late to the party and that this was answered a long time ago but I just encountered this searching for something else and I have an alternate proposal.

When you encounter the EADDRINUSE return from bind() you can enter an error checking routine that connects to the socket. If the connection succeeds, there is a running process that is at least alive enough to have done the accept(). This strikes me as being the simplest and most portable way of achieving what you want to achieve. It has drawbacks in that the server that created the UDS in the first place may actually still be running but "stuck" somehow and unable to do an accept(), so this solution certainly isn't fool-proof, but it is a step in the right direction I think.

If the connect() fails then go ahead and unlink() the endpoint and try the bind() again.

Yes, this is possible on Linux, but it won't be very portable. It's achieved using what is called "ancillary data" with sendmsg / recvmsg.

• Use SO_PASSCRED with setsockopt
• Use SCM_CREDENTIALS and the struct ucred structure

This structure is defined in Linux:

struct ucred {
    pid_t pid;    /* process ID of the sending process */
    uid_t uid;    /* user ID of the sending process */
    gid_t gid;    /* group ID of the sending process */
};

Note you have to fill these in your msghdr.control, and the kernel will check if they're correct.

The main portability hindrance is that this structure differs on other Unixes - for example on FreeBSD it's:

struct cmsgcred {
    pid_t   cmcred_pid;          /* PID of sending process */
    uid_t   cmcred_uid;          /* real UID of sending process */
    uid_t   cmcred_euid;         /* effective UID of sending process */
    gid_t   cmcred_gid;          /* real GID of sending process */
    short   cmcred_ngroups;      /* number or groups */
    gid_t   cmcred_groups[CMGROUP_MAX];     /* groups */
};