# 搭建你的第一个应用（Hello Shy）

本文主要说明如何创建并运行你的第一个板载应用程序。
它涵盖了 PX4 应用程序开发所需的所有基本概念和 API。

:::info
For simplicity, more advanced features like start/stop functionality and command-line arguments are omitted.
These are covered in [Application/Module Template](../modules/module_template.md).
:::

## 系统必备组件

以下内容是您需要提前准备的：

- [PX4 SITL Simulator](../simulation/index.md) _or_ a [PX4-compatible flight controller](../flight_controller/index.md).
- [PX4 Development Toolchain](../dev_setup/dev_env.md) for the desired target.
- [Download the PX4 Source Code](../dev_setup/building_px4.md#download-the-px4-source-code) from Github

The source code [PX4-Autopilot/src/examples/px4_simple_app](https://github.com/PX4/PX4-Autopilot/tree/main/src/examples/px4_simple_app) directory contains a completed version of this tutorial that you can review if you get stuck.

- Rename (or delete) the **px4_simple_app** directory.

## 最小的应用程序

In this section we create a _minimal application_ that just prints out `Hello Sky!`.
This consists of a single _C_ file and a _cmake_ definition (which tells the toolchain how to build the application).

1. Create a new directory **PX4-Autopilot/src/examples/px4_simple_app**.

2. Create a new C file in that directory named **px4_simple_app.c**:
  - Copy in the default header to the top of the page.
    该注释应出现在所有贡献的文件中！

    ```c
    /****************************************************************************
     *
     *   Copyright (c) 2012-2022 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.
     *
     ****************************************************************************/
    ```

  - 将下面的代码复制到头部注释的下方，
    该注释应出现在所有贡献的文件中！

    ```c
    /**
     * @file px4_simple_app.c
     * Minimal application example for PX4 autopilot
     *
     * @author Example User <mail@example.com>
     */

    #include <px4_platform_common/log.h>

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

    int px4_simple_app_main(int argc, char *argv[])
    {
    	PX4_INFO("Hello Sky!");
    	return OK;
    }
    ```

    :::tip
    The main function must be named `<module_name>_main` and exported from the module as shown.

:::

    :::tip
    `PX4_INFO` is the equivalent of `printf` for the PX4 shell (included from **px4_platform_common/log.h**).
    There are different log levels: `PX4_INFO`, `PX4_WARN`, `PX4_ERR`, `PX4_DEBUG`.
    Warnings and errors are additionally added to the [ULog](../dev_log/ulog_file_format.md) and shown on [Flight Review](https://logs.px4.io/).

:::

3. Create and open a new _cmake_ definition file named **CMakeLists.txt**.
  复制下面的文本：

  ```cmake
  ############################################################################
  #
  #   Copyright (c) 2015 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.
  #
  ############################################################################
  px4_add_module(
  	MODULE examples__px4_simple_app
  	MAIN px4_simple_app
  	STACK_MAIN 2000
  	SRCS
  		px4_simple_app.c
  	DEPENDS
  	)
  ```

  The `px4_add_module()` method builds a static library from a module description.

  - The `MODULE` block is the Firmware-unique name of the module (by convention the module name is prefixed by parent directories back to `src`).
  - The `MAIN` block lists the entry point of the module, which registers the command with NuttX so that it can be called from the PX4 shell or SITL console.

  :::tip
  The `px4_add_module()` format is documented in [PX4-Autopilot/cmake/px4_add_module.cmake](https://github.com/PX4/PX4-Autopilot/blob/main/cmake/px4_add_module.cmake). <!-- NEED px4_version -->

:::

  ::: info
  If you specify `DYNAMIC` as an option to `px4_add_module`, a _shared library_ is created instead of a static library on POSIX platforms (these can be loaded without having to recompile PX4, and shared to others as binaries rather than source code).
  Your app will not become a builtin command, but ends up in a separate file called `examples__px4_simple_app.px4mod`.
  You can then run your command by loading the file at runtime using the `dyn` command: `dyn ./examples__px4_simple_app.px4mod`

:::

4. Create and open a new _Kconfig_ definition file named **Kconfig** and define your symbol for naming (see [Kconfig naming convention](../hardware/porting_guide_config.md#px4-kconfig-symbol-naming-convention)).
  复制下面的文本：

  ```
  menuconfig EXAMPLES_PX4_SIMPLE_APP
  	bool "px4_simple_app"
  	default n
  	---help---
  		Enable support for px4_simple_app
  ```

## 编译应用程序/固件

应用程序的编写至此完成。
为了运行它，您首先需要确保它是作为 PX4 的一部分构建的。
Applications are added to the build/firmware in the appropriate board-level _px4board_ file for your target:

- PX4 SITL (Simulator): [PX4-Autopilot/boards/px4/sitl/default.px4board](https://github.com/PX4/PX4-Autopilot/blob/main/boards/px4/sitl/default.px4board)
- Pixhawk v1/2: [PX4-Autopilot/boards/px4/fmu-v2/default.px4board](https://github.com/PX4/PX4-Autopilot/blob/main/boards/px4/fmu-v2/default.px4board)
- Pixracer (px4/fmu-v4): [PX4-Autopilot/boards/px4/fmu-v4/default.px4board](https://github.com/PX4/PX4-Autopilot/blob/main/boards/px4/fmu-v4/default.px4board)
- _px4board_ files for other boards can be found in [PX4-Autopilot/boards/](https://github.com/PX4/PX4-Autopilot/tree/main/boards)

To enable the compilation of the application into the firmware add the corresponding Kconfig key `CONFIG_EXAMPLES_PX4_SIMPLE_APP=y` in the _px4board_ file or run [boardconfig](../hardware/porting_guide_config.md#px4-menuconfig-setup) `make px4_fmu-v4_default boardconfig`:

```
examples  --->
    [x] PX4 Simple app  ----
```

:::info
The line will already be present for most files, because the examples are included in firmware by default.
:::

使用特定板的命令构建示例：

- jMAVSim Simulator: `make px4_sitl_default jmavsim`
- Pixhawk v1/2: `make px4_fmu-v2_default` (or just `make px4_fmu-v2`)
- Pixhawk v3: `make px4_fmu-v4_default`
- Other boards: [Building the Code](../dev_setup/building_px4.md#building-for-nuttx)

## 测试应用（硬件）

### 将固件上传至飞控板

启用上传器，然后重启飞控板：

- Pixhawk v1/2: `make px4_fmu-v2_default upload`
- Pixhawk v3: `make px4_fmu-v4_default upload`

在您重启飞控板之前，它应该打印一些编译消息，并在最后打印：

```sh
Loaded firmware for X,X, waiting for the bootloader...
```

一旦飞控板被重启并完成了固件的上传，命令行界面将输出：

```sh
Erase  : [====================] 100.0%
Program: [====================] 100.0%
Verify : [====================] 100.0%
Rebooting.

[100%] Built target upload
```

### 连接至控制台

Now connect to the [system console](../debug/system_console.md) either via serial or USB.
Hitting **ENTER** will bring up the shell prompt:

```sh
nsh>
```

输入“help”并按回车键

```sh
nsh> help
  help usage:  help [-v] [<cmd>]

  [           df          kill        mkfifo      ps          sleep
  ?           echo        losetup     mkrd        pwd         test
  cat         exec        ls          mh          rm          umount
  cd          exit        mb          mount       rmdir       unset
  cp          free        mkdir       mv          set         usleep
  dd          help        mkfatfs     mw          sh          xd

Builtin Apps:
  reboot
  perf
  top
  ..
  px4_simple_app
  ..
  sercon
  serdis
```

Note that `px4_simple_app` is now part of the available commands.
Start it by typing `px4_simple_app` and ENTER:

```sh
nsh> px4_simple_app
Hello Sky!
```

该应用程序现在已正确注册到系统中，并且可以扩展以实际执行实用的任务。

## 测试应用（SITL）

If you're using SITL the _PX4 console_ is automatically started (see [Building the Code > First Build (Using a Simulator)](../dev_setup/building_px4.md#first-build-using-a-simulator)).
As with the _nsh console_ (see previous section) you can type `help` to see the list of built-in apps.

Enter `px4_simple_app` to run the minimal app.

```sh
pxh> px4_simple_app
INFO  [px4_simple_app] Hello Sky!
```

现在可以扩展该应用程序以实际执行实用的任务

## 订阅传感器数据

为了做一些实用的事情，应用程序需要订阅输入和发布输出（例如电机或伺服命令）。

:::tip
The benefits of the PX4 hardware abstraction comes into play here!
无需以任何方式与传感器驱动程序交互，如果板或传感器更新，也无需更新您的应用程序。
:::

Individual message channels between applications are called [topics](../middleware/uorb.md). For this tutorial, we are interested in the [SensorCombined](https://github.com/PX4/PX4-Autopilot/blob/main/msg/SensorCombined.msg) topic, which holds the synchronized sensor data of the complete system.

订阅主题很简单：

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

The `sensor_sub_fd` is a topic handle and can be used to very efficiently perform a blocking wait for new data.
当前线程进入休眠状态，一旦有新数据可用就会被调度器自动唤醒，等待时不消耗任何 CPU 周期。
To do this, we use the [poll()](https://pubs.opengroup.org/onlinepubs/007908799/xsh/poll.html) POSIX system call.

Adding `poll()` to the subscription looks like (_pseudocode, look for the full implementation below_):

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

/* one could wait for multiple topics with this technique, just using one here */
px4_pollfd_struct_t fds[] = {
    { .fd = sensor_sub_fd,   .events = POLLIN },
};

while (true) {
	/* wait for sensor update of 1 file descriptor for 1000 ms (1 second) */
	int poll_ret = px4_poll(fds, 1, 1000);
	..
	if (fds[0].revents & POLLIN) {
		/* obtained data for the first file descriptor */
		struct sensor_combined_s raw;
		/* copy sensors raw data into local buffer */
		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]);
	}
}
```

再次编译应用程序可以输入：

```sh
make
```

### 测试 uORB 消息订阅

最后一步是通过在 nsh shell 中键入以下内容来启动您的应用程序作为后台进程/任务：

```sh
px4_simple_app &
```

您的应用程序将在控制台中显示 5 个传感器值（译者注：需要使用后面的完整示例中的代码，如果使用上面的伪代码会连续输出并无法退出），然后退出：

```sh
[px4_simple_app] Accelerometer:   0.0483          0.0821          0.0332
[px4_simple_app] Accelerometer:   0.0486          0.0820          0.0336
[px4_simple_app] Accelerometer:   0.0487          0.0819          0.0327
[px4_simple_app] Accelerometer:   0.0482          0.0818          0.0323
[px4_simple_app] Accelerometer:   0.0482          0.0827          0.0331
[px4_simple_app] Accelerometer:   0.0489          0.0804          0.0328
```

:::tip
The [Module Template for Full Applications](../modules/module_template.md) can be used to write background process that can be controlled from the command line.
:::

## 发布数据

To use the calculated outputs, the next step is to _publish_ the results.
下面我们将展示如何发布姿态主题。

:::info
We've chosen `attitude` because we know that the _mavlink_ app forwards it to the ground control station - providing an easy way to look at the results.
:::

The interface is pretty simple: initialize the `struct` of the topic to be published and advertise the topic:

```c
#include <uORB/topics/vehicle_attitude.h>
..
/* advertise attitude topic */
struct vehicle_attitude_s att;
memset(&att, 0, sizeof(att));
orb_advert_t att_pub_fd = orb_advertise(ORB_ID(vehicle_attitude), &att);
```

在主循环中，随时发布信息：

```c
orb_publish(ORB_ID(vehicle_attitude), att_pub_fd, &att);
```

## 完整的示例代码

The [complete example code](https://github.com/PX4/PX4-Autopilot/blob/main/src/examples/px4_simple_app/px4_simple_app.c) is now:

```c
/****************************************************************************
 *
 *   Copyright (c) 2012-2019 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.
 *
 ****************************************************************************/

/**
 * @file px4_simple_app.c
 * Minimal application example for PX4 autopilot
 *
 * @author Example User <mail@example.com>
 */

#include <px4_platform_common/px4_config.h>
#include <px4_platform_common/tasks.h>
#include <px4_platform_common/posix.h>
#include <unistd.h>
#include <stdio.h>
#include <poll.h>
#include <string.h>
#include <math.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!");

	/* subscribe to sensor_combined topic */
	int sensor_sub_fd = orb_subscribe(ORB_ID(sensor_combined));
	/* limit the update rate to 5 Hz */
	orb_set_interval(sensor_sub_fd, 200);

	/* advertise attitude topic */
	struct vehicle_attitude_s att;
	memset(&att, 0, sizeof(att));
	orb_advert_t att_pub = orb_advertise(ORB_ID(vehicle_attitude), &att);

	/* one could wait for multiple topics with this technique, just using one here */
	px4_pollfd_struct_t fds[] = {
		{ .fd = sensor_sub_fd,   .events = POLLIN },
		/* there could be more file descriptors here, in the form like:
		 * { .fd = other_sub_fd,   .events = POLLIN },
		 */
	};

	int error_counter = 0;

	for (int i = 0; i < 5; i++) {
		/* wait for sensor update of 1 file descriptor for 1000 ms (1 second) */
		int poll_ret = px4_poll(fds, 1, 1000);

		/* handle the poll result */
		if (poll_ret == 0) {
			/* this means none of our providers is giving us data */
			PX4_ERR("Got no data within a second");

		} else if (poll_ret < 0) {
			/* this is seriously bad - should be an emergency */
			if (error_counter < 10 || error_counter % 50 == 0) {
				/* use a counter to prevent flooding (and slowing us down) */
				PX4_ERR("ERROR return value from poll(): %d", poll_ret);
			}

			error_counter++;

		} else {

			if (fds[0].revents & POLLIN) {
				/* obtained data for the first file descriptor */
				struct sensor_combined_s raw;
				/* copy sensors raw data into local buffer */
				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]);

				/* set att and publish this information for other apps
				 the following does not have any meaning, it's just an example
				*/
				att.q[0] = raw.accelerometer_m_s2[0];
				att.q[1] = raw.accelerometer_m_s2[1];
				att.q[2] = raw.accelerometer_m_s2[2];

				orb_publish(ORB_ID(vehicle_attitude), att_pub, &att);
			}

			/* there could be more file descriptors here, in the form like:
			 * if (fds[1..n].revents & POLLIN) {}
			 */
		}
	}

	PX4_INFO("exiting");

	return 0;
}
```

## 运行完整的示例

最后运行你的应用程序：

```sh
px4_simple_app
```

If you start _QGroundControl_, you can check the sensor values in the real time plot ([Analyze > MAVLink Inspector](https://docs.qgroundcontrol.com/master/en/qgc-user-guide/analyze_view/mavlink_inspector.html)).

## 总结

本教程涵盖了所有开发基本 PX4 自动驾驶仪应用程序的内容。
Keep in mind that the full list of uORB messages/topics is [available here](https://github.com/PX4/PX4-Autopilot/tree/main/msg/) and that the headers are well documented and serve as reference.

Further information and troubleshooting/common pitfalls can be found here: [uORB](../middleware/uorb.md).

下一页提供了一个模板，用于编写具有启动和停止功能的完整应用程序。
