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: %8.4f %8.4f %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。
- orb_check
-
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。
- orb_copy
-
- 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。
- orb_advertise(ORB_ID(vehicle_attitude),&att);
-