[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:
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- px4_poll
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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)。
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- 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。
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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
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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:
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- 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
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- 发布
发布主题主要使用两个函数:
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- 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。
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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);
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