Clone this repo:
  1. 872df74 Use -Werror in system/chre by Chih-Hung Hsieh · 3 weeks ago master
  2. 1355b80 [NanoSensorCal] Improved Tuning (GyroCal/OTC-Gyro) am: 56697ebadb by David Jacobs · 3 months ago
  3. 56697eb [NanoSensorCal] Improved Tuning (GyroCal/OTC-Gyro) by David Jacobs · 3 months ago
  4. 15e4632 [IMU_CAL] Don't dynamically change gyro latency am: 8294943c6e by Brian Duddie · 3 months ago
  5. 8294943 [IMU_CAL] Don't dynamically change gyro latency by Brian Duddie · 3 months ago

Context Hub Runtime Environment (CHRE)

Build Instructions

Build targets are arranged in the form of a variant triple consisting of:


The vendor is the provider of the CHRE implementation (ex: google, qcom). The arch is the CPU architecture (ie: hexagonv60, x86, cm4). The variant is the target platform (ie: slpi, nanohub, linux, googletest).

A debug build can be obtained by appending _debug to the variant triple. As an example:

make google_hexagonv62_slpi make google_hexagonv62_slpi_debug


CHRE is compatible with Linux as a simulator.

Linux Build/Run

The build target for x86 linux is google_x86_linux. You can build/run the simulator with the following command:


Linux Unit Tests

You can run all unit tests with the following command. Pass arguments to this script and they are passed to the gtest framework. (example: --gtest_filter=DynamicVector.*)


SLPI Hexagon

First, setup paths to the Hexagon Tools (v8.x.x), SDK (v3.0), and SLPI source tree, for example:

export HEXAGON_TOOLS_PREFIX=~/Qualcomm/HEXAGON_Tools/8.0
export HEXAGON_SDK_PREFIX=~/Qualcomm/Hexagon_SDK/3.0
export SLPI_PREFIX=~/Qualcomm/msm8998/slpi_proc

Then use the provided Makefiles to build:

make google_hexagonv62_slpi -j

Directory Structure

The CHRE project is organized as follows:

  • chre_api
    • The stable API exposed to nanoapps
  • core
    • Common code that applies to all CHRE platforms, most notably event management.
  • pal
    • An abstraction layer that implementers must supply to access device-specific functionality (such as GPS and Wi-Fi). The PAL is a C API which allows it to be implemented using a vendor-supplied library.
  • platform
    • Contains the system interface that all plaforms must implement, along with implementations for individual platforms. This includes the implementation of the CHRE API.
    • platform/shared
      • Contains code that will apply to multiple platforms, but not necessarily all.
    • platform/linux
      • This directory contains the canonical example for running CHRE on desktop machines, primarily for simulation and testing.
  • apps
    • A small number of sample applications are provided. These are intended to guide developers of new applications and help implementers test basic functionality quickly.
    • This is reference code and is not required for the CHRE to function.
  • util
    • Contains data structures used throughout CHRE and common utility code.
  • variant/simulator
    • Contains the CHRE variant for the simulator. This is a good example to start from when porting to new devices. Variants are explained in more detail below.

Within each of these directories, you may find a tests subdirectory containing tests written against the googletest framework.

Platform Directory Structure

The platform directory contains an interface that common code under core leverages to implement the runtime. All platforms are required to implement the interface provided in platform/include.

The following gives a more detailed explanation of the directory structure.

  • platform - The top-level directory for platform-specific code.
    • include - The interface that platforms are required to implement.
    • shared - Code that may be shared by more than one platform but not necessarily required for all.
    • slpi - The implementation of the common interface for the SLPI and any SLPI-specific code.
    • linux - The implementation of the common interface for the simulator running on Linux and any simulator-specific code.

Common CHRE code that is expected to run across all platforms is located in core. This code must have a stable way to access the platform-specific implementation of the common platform API. This is handled by providing a stable include path and changing the search path for the platform implementation. Here is an example directory layout:

  • platform
    • <platform_name>
      • include
        • chre
          • target_platform

The build system will add platform/<platform_name>/include to the include search path allowing common code to find the implementation of the platform interface. Here is an example of core code including a platform-specific header in this way:

#include "chre/target_platform/log.h"

When building for the linux platform, the file is included from:


Supplied Nanoapps

This project includes a number of nanoapps that serve as both examples of how to use CHRE, debugging tools and can perform some useful function.

All nanoapps in the apps directory are placed in a namespace when built statically with this CHRE implementation. When compiled as standalone nanoapps, there is no outer namespace on their entry points. This allows testing various CHRE subsystems without requiring dynamic loading and allows these nanoapps to coexist within a CHRE binary. Refer to apps/hello_world/ for a minimal example.


Any of the nanoapps that end with the term World are intended to test some feature of the system. The HelloWorld nanoapp simply exercises logging functionality, TimerWorld exercises timers and WifiWorld uses wifi, for example. These nanoapps log all results via chreLog which makes them effective tools when bringing up a new CHRE implementation.


This nanoapp implements IMU calibration.

Porting CHRE

This codebase is intended to be ported to a variety of operating systems. If you wish to port CHRE to a new OS, refer to the platform directory. An example of the Linux port is provided under platform/linux.

There are notes regarding initialization under platform/include/chre/platform/init.h that will also be helpful.

Important Considerations

Platforms are required to implement support for invoking the constructors and destructors of global, non-POD types at load and unload time, respectively. This is required for both the runtime and nanoapps.

Coding conventions

There are many well-established coding standards within Google. The official C++ style guide is used with the exception of Android naming conventions for methods and variables. This means 2 space indents, camelCase method names, an mPrefix on class members and so on. Style rules that are not specified in the Android style guide are inherited from Google.

CHRE Variants

A CHRE variant allows injecting additional source files into the build on a per-device basis. This can be used to inject:

  • A version string
    • Set to undefined if not specified
  • A static nanoapp list
    • Empty if left undefined
  • Additional static nanoapp includes
    • Vendor-specific nanoapps could be specified in the variant

Export the CHRE_VARIANT_MK_INCLUDES containing the mk files that you wish to be included the CHRE variant build. Refer to and the variant/simulator subdirectory for an example as used by the simulator.

Use of C++

This project uses C++11, but with two main caveats:

  1. General considerations for using C++ in an embedded environment apply. This means avoiding language features that can impose runtime overhead should be avoided, due to the relative scarcity of memory and CPU resources, and power considerations. Examples include RTTI, exceptions, overuse of dynamic memory allocation, etc. Refer to existing literature on this topic including this Technical Report on C++ Performance and so on.
  2. Support of C++ standard libraries are not generally expected to be extensive or widespread in the embedded environments where this code will run. That means that things like and should not be used, in favor of simple platform abstractions that can be implemented directly with less effort (potentially using those libraries if they are known to be available).