accept() in message queues IPC UNIX

Question

for (;;)
{
    if (msgrcv(msqid, &flag, sizeof(struct flags) - sizeof(long), 1, IPC_NOWAIT)>0)
           break;
}
    msgsnd(msqid, &message , sizeof(struct messages) - sizeof(long), 0);

Is there any accept() function like in sockets IPC, but for message queues IPC? My server should wait client connection and only when client connected send data back to it. I special send from client some useless data and check data in infinity loop(that means client is connected, I know that is very stupid algorithm).

Answer 1

No, there is nothing directly analogous. Message queues are more like connectionless datagram sockets than they are like connection-oriented stream sockets (which support accept() and so forth).

Some implementations (z/OS?) expose the number of processes blocked on a msgrcv , which is similar to what you are looking for, but this is not portable.

As I see it, you have two easy options.

First, do what you are doing but don't IPC_NOWAIT in the server's msgrcv . No point in spinning in a loop if you're not doing anything. Just block until a client announces itself. (And use different message types for communication from client-to-server and server-to-client — you don't want the clients consuming their messages to the server, nor vice-versa.)

Second, switch to AF_UNIX stream sockets, which will give you accept() able semantics.