FALLTHROUGH —— ArduPilot —— switch_case代码理解

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switch (effective_mode) 
    {
    case AUTO:
        handle_auto_mode();
        break;

    case AVOID_ADSB:
    case GUIDED:
        if (auto_state.vtol_loiter && quadplane.available()) {
            quadplane.guided_update();
            break;
        }
        FALLTHROUGH;  //中文翻译：落到

    case RTL:
    case LOITER:
        calc_nav_roll();
        calc_nav_pitch();
        calc_throttle();
        break;

 switch(control_mode) {
    case AUTO:
        if (ahrs.home_is_set()) {
            mission.update();
        }
        break;
            
    case RTL:
        if (quadplane.available() && quadplane.rtl_mode == 1 &&
            (nav_controller->reached_loiter_target() ||
             location_passed_point(current_loc, prev_WP_loc, next_WP_loc) ||
             auto_state.wp_distance < MAX(qrtl_radius, quadplane.stopping_distance())) &&
            AP_HAL::millis() - last_mode_change_ms > 1000) {
            /*
              for a quadplane in RTL mode we switch to QRTL when we
              are within the maximum of the stopping distance and the
              RTL_RADIUS
             */
            set_mode(QRTL, MODE_REASON_UNKNOWN);
            break;
        } else if (g.rtl_autoland == 1 &&
            !auto_state.checked_for_autoland &&
            reached_loiter_target() && 
            labs(altitude_error_cm) < 1000) {
            // we've reached the RTL point, see if we have a landing sequence
            if (mission.jump_to_landing_sequence()) {
                // switch from RTL -> AUTO
                set_mode(AUTO, MODE_REASON_UNKNOWN);
            }

            // prevent running the expensive jump_to_landing_sequence
            // on every loop
            auto_state.checked_for_autoland = true;
        }
        else if (g.rtl_autoland == 2 &&
            !auto_state.checked_for_autoland) {
            // Go directly to the landing sequence
            if (mission.jump_to_landing_sequence()) {
                // switch from RTL -> AUTO
                set_mode(AUTO, MODE_REASON_UNKNOWN);
            }

            // prevent running the expensive jump_to_landing_sequence
            // on every loop
            auto_state.checked_for_autoland = true;
        }
        radius = abs(g.rtl_radius);
        if (radius > 0) {
            loiter.direction = (g.rtl_radius < 0) ? -1 : 1;
        }
        // fall through to LOITER  落到LOITER
        FALLTHROUGH;

    case LOITER:
    case AVOID_ADSB:
    case GUIDED:
        update_loiter(radius);
        break;

    case CRUISE:
        update_cruise();
        break;

    case MANUAL:
    case STABILIZE:
    case TRAINING:
    case INITIALISING:
    case ACRO:
    case FLY_BY_WIRE_A:
    case AUTOTUNE:
    case FLY_BY_WIRE_B:
    case CIRCLE:
    case QSTABILIZE:
    case QHOVER:
    case QLOITER:
    case QLAND:
    case QRTL:
        // nothing to do
        break;
    }

针对上述两段代码中的FALLTHROUGH以及注释分析，可以大致推测出，switch case语句中如果出现FALLTHROUGH语句，就相当于直接继续执行下一个case，直到遇见break，退出swich语句。

继续追踪FALLTHROUGH，可以追踪到AP_common.h文件中的如下代码段

#ifdef __has_cpp_attribute
#  if __has_cpp_attribute(fallthrough)
#    define FALLTHROUGH [[fallthrough]]
#  elif __has_cpp_attribute(gnu::fallthrough)
#    define FALLTHROUGH [[gnu::fallthrough]]
#  endif
#endif
#ifndef FALLTHROUGH
#  define FALLTHROUGH
#endif

可以看到如果定义__has_cpp_attribute 且如果 __has_cpp_attribute(fallthrough)为真，那么就定义FALLTHROUGH 为 [[fallthrough]]。 如果没有定义FALLTHROUGH，那么就定义FALLTHROUGH。

仅仅从字面理解，我们可以知道这是一种c++的属性。

继续对上述代码查询搜索，我们可以查到如下，可以证实我们上述的分析。

网址：http://gcc.gnu.org/projects/cxx-status.html