[pixhawk笔记]6-uORB流程及关键函数解析

本文中将结合代码、文档及注释，给出uORB执行流程及关键函数的解析，由于uORB的机制实现较为复杂，所以本文主要学习如何使用uORB的接口来实现通信。回到上一篇笔记中的代码：

#include <px4_config.h>
#include <px4_tasks.h>
#include <px4_posix.h>
#include <unistd.h>
#include <stdio.h>
#include <poll.h>
#include <string.h>

#include <uORB/uORB.h>
#include <uORB/topics/sensor_combined.h>
#include <uORB/topics/vehicle_attitude.h>

__EXPORT int px4_simple_app_main(int argc, char *argv[]);

int px4_simple_app_main(int argc, char *argv[])
{
    PX4_INFO("Hello Sky!");
    int sensor_sub_fd = orb_subscribe(ORB_ID(sensor_combined));
        orb_set_interval(sensor_sub_fd,200);/* limit the update rate to 5 Hz */

    struct vehicle_attitude_s att;
    memset(&att,0,sizeof(att));
    orb_advert_t att_pub = orb_advertise(ORB_ID(vehicle_attitude),&att);

    px4_pollfd_struct_t fds[] = {
        {.fd = sensor_sub_fd, .events = POLLIN},
    };        

    int error_counter = 0;

    for(int i=0;i<5;i++){
        int poll_ret = px4_poll(fds,1,1000);

        if(poll_ret == 0){
            PX4_ERR("Got no data within a second");
        }else if(poll_ret<0){
            if(error_counter<10 || error_counter % 50 == 0){
                PX4_ERR("ERROR return value from poll():%d",poll_ret);
            }

            error_counter++;
        }else{
            if(fds[0].revents & POLLIN){
                struct sensor_combined_s raw;
                orb_copy(ORB_ID(sensor_combined),sensor_sub_fd,&raw);
                PX4_INFO("Accelerometer:\t%8.4f\t%8.4f\t%8.4f",
                    (double)raw.accelerometer_m_s2[0],
                    (double)raw.accelerometer_m_s2[1],
                    (double)raw.accelerometer_m_s2[2]);

                att.rollspeed = raw.accelerometer_m_s2[0];
                att.pitchspeed = raw.accelerometer_m_s2[1];
                att.yawspeed = raw.accelerometer_m_s2[2];

                orb_publish(ORB_ID(vehicle_attitude),att_pub,&att);
            }
        }
    }
    PX4_INFO("exiting");
    return OK;
}

• 订阅
  可以看出，订阅一个并获取一个主题的信息主要流程及其中关键函数如下：

• #include <uORB/topics/sensor_combined.h>
  ..
  int sensor_sub_fd = orb_subscribe(ORB_ID(sensor_combined));

该语句先包含一个主题头文件，该文件内容如下：

/****************************************************************************
 *
 *   Copyright (C) 2013-2016 PX4 Development Team. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name PX4 nor the names of its contributors may be
 *    used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
 * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 *
 ****************************************************************************/

/* Auto-generated by genmsg_cpp from file /home/spy/src/Firmware/msg/sensor_combined.msg */

#pragma once

#include <stdint.h>
#ifdef __cplusplus
#include <cstring>
#else
#include <string.h>
#endif

#include <uORB/uORB.h>

#ifndef __cplusplus
#define RELATIVE_TIMESTAMP_INVALID 2147483647

#endif

#ifdef __cplusplus
struct __EXPORT sensor_combined_s {
#else
struct sensor_combined_s {
#endif
    uint64_t timestamp; // required for logger
    float gyro_rad[3];
    uint32_t gyro_integral_dt;
    int32_t accelerometer_timestamp_relative;
    float accelerometer_m_s2[3];
    uint32_t accelerometer_integral_dt;
    int32_t magnetometer_timestamp_relative;
    float magnetometer_ga[3];
    int32_t baro_timestamp_relative;
    float baro_alt_meter;
    float baro_temp_celcius;

#ifdef __cplusplus
    static constexpr int32_t RELATIVE_TIMESTAMP_INVALID = 2147483647;

#endif
};

/* register this as object request broker structure */
ORB_DECLARE(sensor_combined);

可以看出，该文件定义了一个sensor_combined的结构体，该结构体中包含了陀螺仪、加速度计、磁罗盘和气压计的数据。并使用ORB_DECLARE()宏声明一个sensor_combined的主题。该文件由genmsg.cpp由/msg/sensor_combined.msg中的内容生成，该msg文件内容如下：

#
# Sensor readings in SI-unit form.
#
# These fields are scaled and offset-compensated where possible and do not
# change with board revisions and sensor updates.
#

int32 RELATIVE_TIMESTAMP_INVALID = 2147483647 # (0x7fffffff) If one of the relative timestamps is set to this value, it means the associated sensor values are invalid

# gyro timstamp is equal to the timestamp of the message
float32[3] gyro_rad			# average angular rate measured in the XYZ body frame in rad/s over the last gyro sampling period
uint32 gyro_integral_dt		# gyro measurement sampling period in us

int32 accelerometer_timestamp_relative	# timestamp + accelerometer_timestamp_relative = Accelerometer timestamp
float32[3] accelerometer_m_s2		# average value acceleration measured in the XYZ body frame in m/s/s over the last accelerometer sampling period
uint32 accelerometer_integral_dt	# accelerometer measurement sampling period in us

int32 magnetometer_timestamp_relative	# timestamp + magnetometer_timestamp_relative = Magnetometer timestamp
float32[3] magnetometer_ga		# Magnetic field in NED body frame, in Gauss

int32 baro_timestamp_relative		# timestamp + baro_timestamp_relative = Barometer timestamp
float32 baro_alt_meter			# Altitude, already temp. comp.
float32 baro_temp_celcius		# Temperature in degrees celsius

可以看出，在.msg文件中主要定义了一些成员变量项，自动生成的.h文件使用这些成员变量生成了结构体，　并使用ORB_DECLARE()宏声明了这些主题，ORB_DECLARE()代码如下：读者也可以按该方法加入自定义的.msg文件，并编写对应的CMakeLists.txt，来生成消息，通过uORB来交换数据。

/**
 * Declare (prototype) the uORB metadata for a topic (used by code generators).
 *
 * @param _name		The name of the topic.
 */
#if defined(__cplusplus)
# define ORB_DECLARE(_name)		extern "C" const struct orb_metadata __orb_##_name __EXPORT
#else
# define ORB_DECLARE(_name)		extern const struct orb_metadata __orb_##_name __EXPORT
#endif

可以看出，ORB_DECLARE()宏实际上定义了一些orb_metadata 的结构体变量，变量名为__orb_prototype_name。
订阅时指定了ORB_ID，ORB_ID宏如下：

/**
 * Generates a pointer to the uORB metadata structure for
 * a given topic.
 *
 * The topic must have been declared previously in scope
 * with ORB_DECLARE().
 *
 * @param _name		The name of the topic.
 */
#define ORB_ID(_name)		&__orb_##_name

其实是取了ORB_DECLARE()宏生成__orb_prototype_name结构体的地址。orb_subscribe的原型如下：

	/**
	 * Subscribe to a topic.
	 *
	 * The returned value is a file descriptor that can be passed to poll()
	 * in order to wait for updates to a topic, as well as topic_read,
	 * orb_check and orb_stat.
	 *
	 * Subscription will succeed even if the topic has not been advertised;
	 * in this case the topic will have a timestamp of zero, it will never
	 * signal a poll() event, checking will always return false and it cannot
	 * be copied. When the topic is subsequently advertised, poll, check,
	 * stat and copy calls will react to the initial publication that is
	 * performed as part of the advertisement.
	 *
	 * Subscription will fail if the topic is not known to the system, i.e.
	 * there is nothing in the system that has declared the topic and thus it
	 * can never be published.
	 *
	 * Internally this will call orb_subscribe_multi with instance 0.
	 *
	 * @param meta    The uORB metadata (usually from the ORB_ID() macro)
	 *      for the topic.
	 * @return    ERROR on error, otherwise returns a handle
	 *      that can be used to read and update the topic.
	 */
	int  orb_subscribe(const struct orb_metadata *meta) ;

可看出，其接受参数就是一个orb_metadata的常值指针，从注释看，其返回一个int类型的fd，表示文件描述符，该描述符作为一个句柄，可以用来读取并更新数据，如果订阅失败，则返回ERROR。
还有个orb_subscribe_multi函数可以用于订阅主题有多个实例的情况：

	/**
	 * Subscribe to a multi-instance of a topic.
	 *
	 * The returned value is a file descriptor that can be passed to poll()
	 * in order to wait for updates to a topic, as well as topic_read,
	 * orb_check and orb_stat.
	 *
	 * Subscription will succeed even if the topic has not been advertised;
	 * in this case the topic will have a timestamp of zero, it will never
	 * signal a poll() event, checking will always return false and it cannot
	 * be copied. When the topic is subsequently advertised, poll, check,
	 * stat and copy calls will react to the initial publication that is
	 * performed as part of the advertisement.
	 *
	 * Subscription will fail if the topic is not known to the system, i.e.
	 * there is nothing in the system that has declared the topic and thus it
	 * can never be published.
	 *
	 * If a publisher publishes multiple instances the subscriber should
	 * subscribe to each instance with orb_subscribe_multi
	 * (@see orb_advertise_multi()).
	 *
	 * @param meta    The uORB metadata (usually from the ORB_ID() macro)
	 *      for the topic.
	 * @param instance  The instance of the topic. Instance 0 matches the
	 *      topic of the orb_subscribe() call, higher indices
	 *      are for topics created with orb_advertise_multi().
	 * @return    ERROR on error, otherwise returns a handle
	 *      that can be used to read and update the topic.
	 *      If the topic in question is not known (due to an
	 *      ORB_DEFINE_OPTIONAL with no corresponding ORB_DECLARE)
	 *      this function will return -1 and set errno to ENOENT.
	 */
	int  orb_subscribe_multi(const struct orb_metadata *meta, unsigned instance) ;

　　　　该函数可以通过instance参数来指定实例索引。

订阅了之后可以获取该主题的数据，获取主题数据有如下几个函数可用：

• • • px4_poll

本文开头的代码中即用的该函数，该函数其实是调用的nuttx系统的poll函数，该函数原型如下：

/****************************************************************************
 * Name: poll
 *
 * Description:
 *   poll() waits for one of a set of file descriptors to become ready to
 *   perform I/O.  If none of the events requested (and no error) has
 *   occurred for any of  the  file  descriptors,  then  poll() blocks until
 *   one of the events occurs.
 *
 * Inputs:
 *   fds  - List of structures describing file descriptors to be monitored
 *   nfds - The number of entries in the list
 *   timeout - Specifies an upper limit on the time for which poll() will
 *     block in milliseconds.  A negative value of timeout means an infinite
 *     timeout.
 *
 * Return:
 *   On success, the number of structures that have non-zero revents fields.
 *   A value of 0 indicates that the call timed out and no file descriptors
 *   were ready.  On error, -1 is returned, and errno is set appropriately:
 *
 *   EBADF  - An invalid file descriptor was given in one of the sets.
 *   EFAULT - The fds address is invalid
 *   EINTR  - A signal occurred before any requested event.
 *   EINVAL - The nfds value exceeds a system limit.
 *   ENOMEM - There was no space to allocate internal data structures.
 *   ENOSYS - One or more of the drivers supporting the file descriptor
 *     does not support the poll method.
 *
 ****************************************************************************/

int poll(FAR struct pollfd *fds, nfds_t nfds, int timeout)

可以看出，该函数用于阻塞等待文件描述符更新，等待时间为timeout毫秒。该函数返回值大于0表示等到数据更新，0表示没有等到数据更新，小于0则表示发生错误。（返回值的详细情况参见注释）。

• • • orb_check
      该函数原型及注释如下：

      	/**
      	 * Check whether a topic has been published to since the last orb_copy.
      	 *
      	 * This check can be used to determine whether to copy the topic when
      	 * not using poll(), or to avoid the overhead of calling poll() when the
      	 * topic is likely to have updated.
      	 *
      	 * Updates are tracked on a per-handle basis; this call will continue to
      	 * return true until orb_copy is called using the same handle. This interface
      	 * should be preferred over calling orb_stat due to the race window between
      	 * stat and copy that can lead to missed updates.
      	 *
      	 * @param handle  A handle returned from orb_subscribe.
      	 * @param updated Set to true if the topic has been updated since the
      	 *      last time it was copied using this handle.
      	 * @return    OK if the check was successful, ERROR otherwise with
      	 *      errno set accordingly.
      	 */
      	int  orb_check(int handle, bool *updated) ;
      

      该函数也是用于检查主题是否有更新，与px4_poll不同的是没有阻塞等待，如果从上次orb_copy函数执行之后，主题发生了更新，则将第二个参数设置为true，否则为false。若函数正确执行，返回OK，否则返回ERROR。相比于px4_poll，该函数不会阻塞等待，所以不会造成系统过载。

    • orb_stat

      	/**
      	 * Return the last time that the topic was updated. If a queue is used, it returns
      	 * the timestamp of the latest element in the queue.
      	 *
      	 * @param handle  A handle returned from orb_subscribe.
      	 * @param time    Returns the absolute time that the topic was updated, or zero if it has
      	 *      never been updated. Time is measured in microseconds.
      	 * @return    OK on success, ERROR otherwise with errno set accordingly.
      	 */
      	int  orb_stat(int handle, uint64_t *time) ;
      

      该函数用于获得指定的主题上一次更新的绝对时间，为毫秒数。返回值为OK表示成功执行，ERROR为有错误发生，具体错误可以查看errno。从orb_check的注释中可以看出，该函数有缺陷，因为在orb_stat和orb_copy之间的时间窗口有可能发生更新，而根据返回的毫秒数来确定可能会错过该更新。

　　　　　　　　　使用以上三个函数可以确定主题是否发生更新，如有更新发生，则可以使用orb_copy来从主题中获得更新数据：

• • • orb_copy
      该函数原型如下：

      	/**
      	 * Fetch data from a topic.
      	 *
      	 * This is the only operation that will reset the internal marker that
      	 * indicates that a topic has been updated for a subscriber. Once poll
      	 * or check return indicating that an updaet is available, this call
      	 * must be used to update the subscription.
      	 *
      	 * @param meta    The uORB metadata (usually from the ORB_ID() macro)
      	 *      for the topic.
      	 * @param handle  A handle returned from orb_subscribe.
      	 * @param buffer  Pointer to the buffer receiving the data, or NULL
      	 *      if the caller wants to clear the updated flag without
      	 *      using the data.
      	 * @return    OK on success, ERROR otherwise with errno set accordingly.
      	 */
      	int  orb_copy(const struct orb_metadata *meta, int handle, void *buffer) ;
      

      代码中调用时形式如下：

       struct sensor_combined_s raw;
       orb_copy(ORB_ID(sensor_combined),sensor_sub_fd,&raw);
      

      第一个参数为ORB_ID获取的主题ID（orb_metadata的指针），第二个参数为文件描述符，可用订阅时返回值，第三个参数为更新数据的缓存地址，可以为一个主题对应结构体类型的地址。
      此外，在订阅时还有其他函数：

    • orb_set_interval(sensor_sub_fd,200);该函数用于设置主题更新速率。

• 发布
  发布主题主要使用两个函数：
  • • orb_advertise(ORB_ID(vehicle_attitude),&att);
      该函数原型及注释如下：

      	// ==== uORB interface methods ====
      	/**
      	 * Advertise as the publisher of a topic.
      	 *
      	 * This performs the initial advertisement of a topic; it creates the topic
      	 * node in /obj if required and publishes the initial data.
      	 *
      	 * Any number of advertisers may publish to a topic; publications are atomic
      	 * but co-ordination between publishers is not provided by the ORB.
      	 *
      	 * Internally this will call orb_advertise_multi with an instance of 0 and
      	 * default priority.
      	 *
      	 * @param meta    The uORB metadata (usually from the ORB_ID() macro)
      	 *      for the topic.
      	 * @param data    A pointer to the initial data to be published.
      	 *      For topics updated by interrupt handlers, the advertisement
      	 *      must be performed from non-interrupt context.
      	 * @param queue_size  Maximum number of buffered elements. If this is 1, no queuing is
      	 *      used.
      	 * @return    nullptr on error, otherwise returns an object pointer
      	 *      that can be used to publish to the topic.
      	 *      If the topic in question is not known (due to an
      	 *      ORB_DEFINE with no corresponding ORB_DECLARE)
      	 *      this function will return nullptr and set errno to ENOENT.
      	 */
      	orb_advert_t orb_advertise(const struct orb_metadata *meta, const void *data, unsigned int queue_size = 1)
      	{
      		return orb_advertise_multi(meta, data, nullptr, ORB_PRIO_DEFAULT, queue_size);
      	}
      

      该函数用来广播一个主题发布者，返回一个对象指针，该对象可用来发布主题，若出现错误，则返回空指针。
      该函数接收参数为有三个：第一个ORB_ID，表示要发布的主题ID；第二个为结构体指针，指向要发布的数据结构体；第三个为队列大小，默认为1，表示没有消息队列。
      通过查看代码，发现其实该函数调用了orb_advertise_multi函数，使用空指针0作为主题的实例索引，表示只广播主题的第一个实例，并使用默认优先级来广播。
      也可以使用orb_advertise_multi函数。其原型和注释如下：

      /**
      	 * Advertise as the publisher of a topic.
      	 *
      	 * This performs the initial advertisement of a topic; it creates the topic
      	 * node in /obj if required and publishes the initial data.
      	 *
      	 * Any number of advertisers may publish to a topic; publications are atomic
      	 * but co-ordination between publishers is not provided by the ORB.
      	 *
      	 * The multi can be used to create multiple independent instances of the same topic
      	 * (each instance has its own buffer).
      	 * This is useful for multiple publishers who publish the same topic. The subscriber
      	 * then subscribes to all instances and chooses which source he wants to use.
      	 *
      	 * @param meta    The uORB metadata (usually from the ORB_ID() macro)
      	 *      for the topic.
      	 * @param data    A pointer to the initial data to be published.
      	 *      For topics updated by interrupt handlers, the advertisement
      	 *      must be performed from non-interrupt context.
      	 * @param instance  Pointer to an integer which will yield the instance ID (0-based)
      	 *      of the publication. This is an output parameter and will be set to the newly
      	 *      created instance, ie. 0 for the first advertiser, 1 for the next and so on.
      	 * @param priority  The priority of the instance. If a subscriber subscribes multiple
      	 *      instances, the priority allows the subscriber to prioritize the best
      	 *      data source as long as its available. The subscriber is responsible to check
      	 *      and handle different priorities (@see orb_priority()).
      	 * @param queue_size  Maximum number of buffered elements. If this is 1, no queuing is
      	 *      used.
      	 * @return    ERROR on error, otherwise returns a handle
      	 *      that can be used to publish to the topic.
      	 *      If the topic in question is not known (due to an
      	 *      ORB_DEFINE with no corresponding ORB_DECLARE)
      	 *      this function will return -1 and set errno to ENOENT.
      	 */
      	orb_advert_t orb_advertise_multi(const struct orb_metadata *meta, const void *data, int *instance,
      					 int priority, unsigned int queue_size = 1) ;
      

      该函数可以指定int*类型的instance参数，可以在一个主题存在多个实例时使用。

    • orb_publish(ORB_ID(vehicle_attitude),att_pub,&att);
      该函数用于在广播主题之后发布主题，使用广播时返回的主题句柄。函数原型及注释如下：

      	/**
      	 * Publish new data to a topic.
      	 *
      	 * The data is atomically published to the topic and any waiting subscribers
      	 * will be notified.  Subscribers that are not waiting can check the topic
      	 * for updates using orb_check and/or orb_stat.
      	 *
      	 * @param meta    The uORB metadata (usually from the ORB_ID() macro)
      	 *      for the topic.
      	 * @handle    The handle returned from orb_advertise.
      	 * @param data    A pointer to the data to be published.
      	 * @return    OK on success, ERROR otherwise with errno set accordingly.
      	 */
      	int  orb_publish(const struct orb_metadata *meta, orb_advert_t handle, const void *data) ;
      

      该函数用于发布主题，发布之后会通知所有订阅者主题已更新（例如使用px4_poll进行阻塞等待的订阅者），而orb_check需要手动检查是否已更新。