套接字绑带设备

这里，我只需要ifr_name这个成员域就够了。代码修改成了下面这样：
    struct ifreq if_ppp0;
    struct ifreq if_ppp1;
    strncpy(if_ppp0.ifr_name, "ppp0", IFNAMSIZ);
    strncpy(if_ppp1.ifr_name, "ppp1", IFNAMSIZ);

    sock1 = socket(AF_INET, SOCK_DGRAM, 0);
    sock2 = socket(AF_INET, SOCK_DGRAM, 0);
    
if (setsockopt(sock1, SOL_SOCKET, SO_BINDTODEVICE,
            (char *)&if_ppp0, sizeof(if_ppp0)) 
        /*error handling*/
    }

    if (setsockopt(sock2, SOL_SOCKET, SO_BINDTODEVICE,
            (char *)&if_ppp1, sizeof(if_ppp1)) 
        /*error handling*/
    }