uORB消息机制

    uORB是一种用于进程间进行异步发布和订阅的消息机制API。

    在中可以学习通过C++如何使用uORB。

    由于很多应用都是基于uORB的，因此在系统刚启动时uORB就自动运行了。uORB通过启动。可以使用uorb test进行单位测试。

    添加新的主题(topic)

    要想增加新的topic，你需要在msg/目录下创建一个新的.msg 文件并在msg/CMakeLists.txt下添加该文件。这样C/C++编译器自动在程序中添相应的代码。

    可以先看看现有的msg文件了解下都支持那些类型。一个消息也可以嵌套在其他消息当中。

    每一个生成的C/C++结构体中，会多出一个uint64_t timestamp 字段。这个变量用于将消息记录到日志当中。

    首先需要在文件.msg中，通过添加类似如下的一行代码,一个消息定义就可以用于多个独立的主题.

    1. # TOPICS mission offboard_mission onboard_mission

    然后在代码中, 把它们作为主题ID用:ORB_ID(offboard_mission).

    在系统的任何地方都可以发布一个主题, 包括在中断上下文中(被hrt_call接口调用的函数). 但是, 公告(advertise)一个主题仅限于在中断上下文之外.

    一个主题只能由同一个进程进行公告, 并作为其之后的发布(publish).

    列出所有主题并进行监听

    接收者(listener)命令仅在Pixracer（FMUv4）以及Linux/OS X上可用。

    要列出一个主题中的5个消息, 执行以下监听命令:

    1. listener sensor_accel 5

    得到的输出就是关于该主题的n次内容:

    The command uorb top shows the publishing frequency of each topic in
    real-time:

    1. TOPIC NAME                        INST #SUB #MSG #LOST #QSIZE
    2. actuator_armed                       0    6    4     0 1
    4. battery_status                       0    6  500  2694 1
    5. commander_state                      0    1   98    89 1
    6. control_state                        0    4  242   433 1
    7. ekf2_innovations                     0    1  242   223 1
    8. ekf2_timestamps                      0    1  242    23 1
    9. estimator_status                     0    3  242   488 1
    10. sensor_accel                         0    1  242     0 1
    11. sensor_accel                         1    1  249    43 1
    12. sensor_baro                          0    1   42     0 1
    13. sensor_combined                      0    6  242   636 1

    The columns are: topic name, multi-instance index, number of subscribers,
    publishing frequency in Hz, number of lost messages (all subscribers combined), and
    queue size.

    uORB provides a mechanism to publish multiple independent instances of the same
    topic through orb_advertise_multi. It will return an instance index to the
    publisher. A subscriber will then have to choose to which instance to subscribe
    to using orb_subscribe_multi (orb_subscribe subscribes to the first
    instance).
    Having multiple instances is useful for example if the system has several
    sensors of the same type.

    Make sure not to mix orb_advertise_multi and orb_advertise for the same
    topic.

    Troubleshooting and common Pitfalls

    The following explains some common pitfalls and corner cases:

    • The topic is not published: make sure the ‘s of each call match. It
      is also important that orb_subscribe and orb_unsubscribe are called from
      the same task       as orb_publish. This applies to px4_task_spawn_cmd(), but
      also when using work queues (work_queue()).
    • Make sure to clean up: use orb_unsubscribe and orb_unadvertise.
    • A successful orb_check() or px4_poll() call requires an orb_copy(),
      otherwise the next poll will return immediately.
    • It is perfectly ok to call orb_subscribe before anyone advertised the topic.
    • orb_check() and px4_poll() will only return true for publications that are
      done after orb_subscribe(). This is important for topics that are not
      published regularly. If a subscriber needs the previous data, it should just
      do an unconditional orb_copy() right after orb_subscribe() (Note that
      will fail if there is no advertiser yet).