uORB消息机制

简介

uORB是一种用于进程间进行异步发布和订阅的消息机制API。

在教程中可以学习通过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uorb start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。

为了在代码中使用"topic"需要添加头文件:

#include <uORB/topics/topic_name.h>

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.

# TOPICS mission offboard_mission onboard_mission

【按】这里这一步将产生三个主题ID- mission、 offboard_mission 、以及 onboard_mission (第一个ID务必与.msg文件名相同)

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

发布主题

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。

要列出所有主题, 先列出文件句柄:

ls /obj

要列出一个主题中的5个消息, 执行以下监听命令:

listener sensor_accel 5

得到的输出就是关于该主题的n次内容:

TOPIC: sensor_accel #3
timestamp: 84978861
integral_dt: 4044
error_count: 0
x: -1
y: 2
z: 100
x_integral: -0
y_integral: 0
z_integral: 0
temperature: 46
range_m_s2: 78
scaling: 0

TOPIC: sensor_accel #4
timestamp: 85010833
integral_dt: 3980
error_count: 0
x: -1
y: 2
z: 100
x_integral: -0
y_integral: 0
z_integral: 0
temperature: 46
range_m_s2: 78
scaling: 0

uorb top Command

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

update: 1s, num topics: 77
TOPIC NAME                        INST #SUB #MSG #LOST #QSIZE
actuator_armed                       0    6    4     0 1
actuator_controls_0                  0    7  242  1044 1
battery_status                       0    6  500  2694 1
commander_state                      0    1   98    89 1
control_state                        0    4  242   433 1
ekf2_innovations                     0    1  242   223 1
ekf2_timestamps                      0    1  242    23 1
estimator_status                     0    3  242   488 1
mc_att_ctrl_status                   0    0  242     0 1
sensor_accel                         0    1  242     0 1
sensor_accel                         1    1  249    43 1
sensor_baro                          0    1   42     0 1
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.

Multi-instance

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.

The full API is documented in src/modules/uORB/uORBManager.hpp.

Troubleshooting and common Pitfalls

The following explains some common pitfalls and corner cases:

• The topic is not published: make sure the ORB_ID()'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 orb_copy() will fail if there is no advertiser yet).