int modbus_send_raw_request(modbus_t *ctx, uint8_t *raw_req, int raw_req_length)
{
    sft_t sft;
    uint8_t req[MAX_MESSAGE_LENGTH];
    int req_length;

    if (ctx == NULL) {
        errno = EINVAL;
        return -1;
    }

    if (raw_req_length < 2 || raw_req_length > (MODBUS_MAX_PDU_LENGTH + 1)) {
        /* The raw request must contain function and slave at least and
           must not be longer than the maximum pdu length plus the slave
           address. */
        errno = EINVAL;
        return -1;
    }

    sft.slave = raw_req[0];
    sft.function = raw_req[1];
    /* The t_id is left to zero */
    sft.t_id = 0;
    /* This response function only set the header so it's convenient here */
    req_length = ctx->backend->build_response_basis(&sft, req);

    if (raw_req_length > 2) {
        /* Copy data after function code */
        memcpy(req + req_length, raw_req + 2, raw_req_length - 2);
        req_length += raw_req_length - 2;
    }

    return send_msg(ctx, req, req_length);
}
int modbus_send_raw_request(modbus_t *ctx, uint8_t *raw_req, int raw_req_length)
{
    sft_t sft;
    uint8_t req[MAX_MESSAGE_LENGTH];
    int req_length;

    if (ctx == NULL) {
        errno = EINVAL;
        return -1;
    }

    if (raw_req_length < 2 || raw_req_length > (MODBUS_MAX_PDU_LENGTH + 1)) {
         
        errno = EINVAL;
        return -1;
    }

    sft.slave = raw_req[0];
    sft.function = raw_req[1];
     
    sft.t_id = 0;
     
    req_length = ctx->backend->build_response_basis(&sft, req);

    if (raw_req_length > 2) {
         
        memcpy(req + req_length, raw_req + 2, raw_req_length - 2);
        req_length += raw_req_length - 2;
    }

    return send_msg(ctx, req, req_length);
}