1# Building an NDK Project with CMake 2 3 4In many complex application projects, C++ projects are compiled and built in command line mode through build systems such as CMake. The following describes how to switch an existing CMake project to the OpenHarmony toolchain so that CMake can be used to build the project. 5 6 7## Downloading the NDK 8 9You can download the NDK in either of the following modes: 10 111. (Recommended) Acquire source code from mirrors for an officially released version. For details, see [release notes](../../release-notes/OpenHarmony-v4.0-release.md). 122. Download the NDK from the SDK Manager in DevEco Studio. 13 14 15## Decompressing the NDK 16 17Place the downloaded NDK in a folder you prefer and decompress it. Below shows the directory structure after decompression. 18 19 20 21Configure the Linux environment as follows: (Skip them if the NDK is downloaded from DevEco Studio.) 22 231. Add the CMake tool that comes with the NDK to the environment variables. 24 ``` 25 #Open the .bashrc file. 26 vim ~/.bashrc 27 #Append the custom CMake path to the file. 28 export PATH=~/ohos-sdk/ohos-sdk/linux/native/build-tools/cmake/bin:$PATH 29 #Run the source ~/.bashrc command to make the environment variables take effect. 30 source ~/.bashrc 31 ``` 32 332. Check the default CMake path. 34 ``` 35 #Run the which cmake command. 36 which cmake 37 #The result should be the same as the custom path previously appended to the .bashrc file. 38 ~/ohos-sdk/ohos-sdk/linux/native/build-tools/cmake/bin/cmake 39 ``` 40 41 42## Using the NDK to Compile a Native Program 43 44You can use the NDK to quickly develop a native program, including native dynamic libraries, static libraries, and executable files. The following exemplifies how to use the NDK to compile an executable program and a dynamic library in a C/C++ demo project. 45 46 47### Demo Project 48 49The following is a CMake demo project. This project contains two directories. The **include** directory contains the header files of the library, and the **src** directory contains all source code. Specifically, the **src** directory contains two files: **sum.cpp** (algorithm file) and **main.cpp** (main entry file for invoking algorithms). The two files are compiled into an executable program and an algorithm dynamic library. 50 51**Demo Project Directory** 52 53``` 54demo 55 ├── CMakeLists.txt 56 ├── include 57 └── sum.h 58 └── src 59 ├── CMakeLists.txt 60 ├── sum.cpp 61 └── main.cpp 62``` 63 64**CMakeLists.txt in the demo Directory** 65 66``` 67# Specify the minimum CMake version. 68CMAKE_MINIMUM_REQUIRED(VERSION 3.16) 69 70# Set the project name, which is HELLO in this example. 71PROJECT(HELLO) 72 73#Add a subdirectory and build the subdirectory. 74ADD_SUBDIRECTORY(src) 75``` 76 77**CMakeLists.txt in the src Directory** 78 79``` 80SET(LIBHELLO_SRC hello.cpp) 81 82# Set compilation flags. 83SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O0") 84 85# Set the link parameter. The value below is only for exemplary purposes. 86SET(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -Wl,--emit-relocs --verbose") 87 88# Add a libsum dynamic library target. If the compilation is successful, a libsum.so file is generated. 89ADD_LIBRARY(sum SHARED sum.cpp) 90 91# Add the executable target called Hello. If the compilation is successful, a Hello executable is generated. 92ADD_EXECUTABLE(Hello ${LIBHELLO_SRC}) 93 94# Specify the path to the include directory of the Hello target. 95TARGET_INCLUDE_DIRECTORIES(Hello PUBLIC ../include) 96 97# Specify the name of the library to be linked to the Hello target. 98TARGET_LINK_LIBRARIES(Hello PUBLIC sum) 99``` 100 101**Source Code** 102 103**hello.cpp** source code: 104 105``` 106#include <iostream> 107#include "sum.h" 108 109int main(int argc,const char **argv) 110{ 111 std::cout<< "hello world!" <<std::endl; 112 int total = sum(1, 100); 113 std::cout<< "Sum 1 + 100=" << total << std::endl; 114 return 0; 115} 116``` 117 118**sum.h** source code: 119 120``` 121int sum(int a, int b); 122``` 123 124**sum.cpp** source code: 125 126``` 127#include <iostream> 128 129int sum(int a, int b) 130{ 131 return a + b; 132} 133``` 134 135 136### Compiling and Building the Demo Project 137 138In the project directory, create the **build** directory to store the intermediate files generated during CMake building. 139 140> **NOTE** 141> 142> In the following commands, **ohos-sdk** is the root directory of the downloaded SDK. Replace it with the actual directory. 143 1441. Use **OHOS_STL=c++_shared** to dynamically link the C++ library to build a project. If **OHOS_STL** is not specified, **c++_shared** is used by default. 145 146 ``` 147 >mkdir build && cd build 148 >cmake -DOHOS_STL=c++_shared -DOHOS_ARCH=armeabi-v7a -DOHOS_PLATFORM=OHOS -DCMAKE_TOOLCHAIN_FILE={ohos-sdk}/linux/native/build/cmake/ohos.toolchain.cmake .. 149 >cmake --build . 150 ``` 151 1522. Use **OHOS_STL=c++_static** to link a static C++ library to build the project. 153 154 ``` 155 >mkdir build && cd build 156 >cmake -DOHOS_STL=c++_static -DOHOS_ARCH=armeabi-v7a -DOHOS_PLATFORM=OHOS -DCMAKE_TOOLCHAIN_FILE={ohos-sdk}/linux/native/build/cmake/ohos.toolchain.cmake .. 157 >cmake --build . 158 ``` 159 160 In the command, the **OHOS_ARCH** and **OHOS_PLATFORM** variables generate the **--target** command parameters of Clang++. In this example, the two parameters are **--target=arm-linux-ohos** and **--march=armv7a**. **CMAKE_TOOLCHAIN_FILE** specifies the toolchain file. In this file, **--sysroot={ndk_sysroot directory}** is set for Clang++ by default, instructing the compiler to search for the root directory of system header files. 161
