This topic explains how to create and run your first onboard application. It covers all the basic concepts and APIs required for app development on PX4.
:::note For simplicity, more advanced features like start/stop functionality and command-line arguments are omitted. These are covered in Application/Module Template. :::
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).
Create a new directory PX4-Autopilot/src/examples/px4_simple_app.
Create a new C file in that directory named px4_simple_app.c:
Copy in the default header to the top of the page. This should be present in all contributed files!
/**************************************************************************** * * 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. * ****************************************************************************/
Copy the following code below the default header. This should be present in all contributed files!
/** * @file px4_simple_app.c * Minimal application example for PX4 autopilot * * @author Example User <mail@example.com> */#include<px4_platform_common/log.h>__EXPORT intpx4_simple_app_main(int argc,char*argv[]);intpx4_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 and shown on Flight Review. :::
Create and open a new cmake definition file named CMakeLists.txt. Copy in the text below:
############################################################################## 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.
:::note 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 :::
Create and open a new Kconfig definition file named Kconfig and define your symbol for naming (see Kconfig naming convention). Copy in the text below:
The application is now complete. In order to run it you first need to make sure that it is built as part of PX4. Applications are added to the build/firmware in the appropriate board-level px4board file for your target:
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 boardconfigmake px4_fmu-v4_default boardconfig:
examples --->
[x] PX4 Simple app ----
:::note The line will already be present for most files, because the examples are included in firmware by default. :::
Build the example using the board-specific command:
jMAVSim Simulator: make px4_sitl_default jmavsim
Pixhawk v1/2: make px4_fmu-v2_default (or just make px4_fmu-v2)
The application can now be extended to actually perform useful tasks.
Subscribing to Sensor Data
To do something useful, the application needs to subscribe inputs and publish outputs (e.g. motor or servo commands).
:::tip The benefits of the PX4 hardware abstraction comes into play here! There is no need to interact in any way with sensor drivers and no need to update your app if the board or sensors are updated. :::
Individual message channels between applications are called topics. For this tutorial, we are interested in the SensorCombined topic, which holds the synchronized sensor data of the complete system.
The sensor_sub_fd is a topic handle and can be used to very efficiently perform a blocking wait for new data. The current thread goes to sleep and is woken up automatically by the scheduler once new data is available, not consuming any CPU cycles while waiting. To do this, we use the poll() POSIX system call.
Adding poll() to the subscription looks like (pseudocode, look for the full implementation below):
#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 */structsensor_combined_sraw; /* 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]); }}
Compile the app again by entering:
make
Testing the uORB Subscription
The final step is to start your application as a background process/task by typing the following in the nsh shell:
px4_simple_app&
Your app will display 5 sensor values in the console and then exit:
To use the calculated outputs, the next step is to publish the results. Below we show how to publish the attitude topic.
:::note 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:
/**************************************************************************** * * 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 intpx4_simple_app_main(int argc,char*argv[]);intpx4_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"); } elseif (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");return0;}
Running the Complete Example
And finally run your app:
px4_simple_app
If you start QGroundControl, you can check the sensor values in the real time plot (Analyze > MAVLink Inspector).
Wrap-Up
This tutorial covered everything needed to develop a basic PX4 autopilot application. Keep in mind that the full list of uORB messages/topics is available here and that the headers are well documented and serve as reference.
Further information and troubleshooting/common pitfalls can be found here: uORB.
The next page presents a template for writing a full application with start and stop functionality.
