使用select处理外带数据

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外带数据使用recv接受，只是最后一个参数要是MSG_OOB，但是怎么判断这次的数据是MSG_OOB，那就需要使用select来处理.

#include <iostream>
#include <string>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <cassert>
#include <unistd.h>
#include <fcntl.h>
#include <cstdlib>
#include <cstring>
#include <cstdio>

using namespace std;

int main(int argc, char *argv[])
{
    if (argc <= 2)
        return -1;
    const char* ip = argv[1];
    int port = atoi(argv[2]);
    int ret = 0;
    struct sockaddr_in address;
    bzero(&address, sizeof(address));
    address.sin_family = AF_INET;
    address.sin_port = htons(port);
    if (1 != ::inet_pton(AF_INET, ip, &address.sin_addr))
        return -1;
    int listen_fd = ::socket(AF_INET, SOCK_STREAM, 0);
    if (listen_fd == -1)
        return -1;
    ret = ::bind(listen_fd, (struct sockaddr*)&address, sizeof(address));
    if (ret == -1)
        return -1;
    ret = ::listen(listen_fd, 5);
    if (ret == -1)
        return -1;
    struct sockaddr_in client_address;
    socklen_t client_address_len = sizeof(client_address);
    int conn_fd = ::accept(listen_fd, (struct sockaddr*)&client_address, &client_address_len);
    if (conn_fd==-1)
    {
        ::close(listen_fd);
        return -1;
    }
    char buf[1024] = {0};
    fd_set read_fds, exception_fds;
    FD_ZERO(&read_fds);
    FD_ZERO(&exception_fds);

    while (true)
    {
        bzero(buf, sizeof(buf));
        FD_SET(conn_fd, &read_fds);
        FD_SET(conn_fd, &exception_fds);
        //最后一个参数传NULL，表示一直等待
        ret = ::select(conn_fd+1, &read_fds, NULL, &exception_fds, NULL);
        if (ret <= 0)
            break;
        if (FD_ISSET(conn_fd, &read_fds))
        {
            ret = ::recv(conn_fd, buf, 1024-1, 0);
            if (ret <= 0)
                break;
            else
                cout << buf << endl;
        }
        else if (FD_ISSET(conn_fd, &exception_fds))
        {
            ret = ::recv(conn_fd, buf, 1024-1, MSG_OOB);
            if (ret <= 0)
                break;
            else
                cout << buf << endl;
        }
        //68~75处理外带数据
    }
    ::close(conn_fd);
    ::close(listen_fd);
    return 0;
}