multiple protocol

        struct header1 {
            uint16_t a1;
            int16_t a2;
            int16_t a3;

            int16_t a4;
            int16_t a5;
            int16_t a6;

            int16_t a7;
            int16_t a8;
            int16_t a9;
        };
        struct header2 {
            uint32_t length : 24; 
            int16_t a2;
            int16_t a3;

            int16_t a4;
            int16_t a5;
            int16_t a6;

            int16_t a7;
            int16_t a8;
            int16_t a9;
        };

        union protocol {
            header1 protocol1;
            header2 protocol2; 
        } p;

        
        if(해더에 따라...) {
            p.protocol1 = (temp*)&read_msg_[0];
        }
        else if(해더에 따라...) {
            p.protocol2 = (length3*)&read_msg_[0];
        }