没有发送（或接收）队列的Unix套接字

Question

I have a sender/receiver pair of codes written in C and Python. The client is producing data and send it over a unix socket to receiver. Receiver is not always listening to the socket, because it should do something with received data.

In my current code all messages sent by sender are waiting in a queue to be received by receiver, but I don't want this behavior. I want the sender know that the receiver is busy now (maybe even by raising an error). So how can I set the queue length of receiver to zero?

Please consider that I don't want the send or receive procedure be blocking.

My receiver and sender parts of codes are these:

Receiver (in Python):

self.app_socket = socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM)
self.app_socket.settimeout(0.01)
self.app_socket.bind(APP_SOCKET)

def poll(self, *args):
    try:
        message = self.app_socket.recv(1024)
    except socket.timeout:
        return

Sender (in C):

int openSocket(char *path)
{
    int sock;
    struct sockaddr_un addr;
    int size;

    sock = socket(AF_UNIX, SOCK_DGRAM, 0);
    if (sock < 0)
        exit (EXIT_FAILURE);

    addr.sun_family = AF_UNIX;
    strcpy(addr.sun_path, path);
    size = strlen(addr.sun_path) + sizeof(addr.sun_family);

    if (connect(sock, (struct sockaddr *) &addr, size) < 0)
        return -1;

    return sock;
}

int sendMessage(int sock, unsigned char *message, int length)
{
    int count;

    count = send(sock, message, length, 0);
    if (count < 0)
        return -1;

    return count;
}

Answer 1

You can't tell on sender side what is going on on the receiver. May be receiver process is not even scheduled now. The only reasonable way is to answer from receiver back to sender, where you can wait some timeout for this answer and decide whether receiver busy or not.

Answer 2

You need some way for the receiver to signal its state (read or not) to the sender. There are several IPC primitives you might use for this purpose.

Or you could make up your own. For example the receiver could so something along these line:

    mkdir("/path/to/my/flag/dir", 0555);  /* ready */
    read();
    rmdir("/path/to/my/flag/dir");       /* busy */

and the sender could periodically check for existence of this with something along these lines:

    oktogo = (access("/path/to/my/flag/dir", F_OK) == 0);