Question

This question is very similar (or almost identical) to In a non blocking socket connect, select() always returns 1; however, I can’t seem to find where my code is faltering.

I am using non-blocking sockets and want to use select() when connecting a client to a server to check for timeout/success. The problem is select() is always returning 1 almost immediately, even when I don’t even have the server running and there is nothing to connect to. Thanks in advance for the help, code snippet is as follows:

//Loop through the addrinfo structs and try to connect to the first one we can
for(p = serverinfo; p != NULL; p = p->ai_next) {
    if ((sockfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol)) == -1) 
    {
        //We couldn't create the socket, try again
        perror("client: socket");
        continue;
    }

    //Set the socket to non-blocking
    int flags = fcntl(sockfd, F_GETFL, 0);
    fcntl(sockfd, F_SETFL, flags | O_NONBLOCK);

    if (connect(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
        //The error was something other than non-block/in progress, try next addrinfo
        if(errno != EINPROGRESS) 
        {
            close(sockfd);
            perror("client: connect");
            continue;
        }

        fd_set write_fds;
        FD_ZERO(&write_fds);            //Zero out the file descriptor set
        FD_SET(sockfd, &write_fds);     //Set the current socket file descriptor into the set

        //We are going to use select to wait for the socket to connect
        struct timeval tv;              //Time value struct declaration
        tv.tv_sec = 5;                  //The second portion of the struct
        tv.tv_usec = 0;                 //The microsecond portion of the struct

        //DEBUG: This is ALWAYS 1
        int select_ret = select(sockfd + 1, NULL, &write_fds, NULL, &tv);
        cout << select_ret << endl;

        //Check return, -1 is error, 0 is timeout
        if(select_ret == -1 || select_ret == 0)
        {
            //We had an error connecting
            cout << "Error Connecting\n";
            close(sockfd);
            continue;
        }
    }

    //We successfully connected, break out of loop
    break;
}

Answer 1

What do you expect select() to return? Consider that select() is normally used to wait for multiple file descriptors – if you were connecting two, how would you know which one succeeded/failed based purely on the return value of select? You wouldn’t, obviously.

Which is why select() just tells you which file descriptors have changed in some way, and you’re supposed to determine independently what that was. In the case of connect(), you should call getsockopt() to retrieve the result of the connection attempt. See this answer where I explain how to do a non-blocking connect().