如何在Android项目CMAKE_TOOLCHAIN_FILE中正确设置CMAKE_SYSROOT？

问题 #

为Android项目编写CMAKE_TOOLCHAIN_FILE时遇到路径错误：

设置的CMAKE_SYSROOT：

set(CMAKE_SYSROOT "${ANDROID_TOOLCHAIN_ROOT}/sysroot")

该路径展开为${CMAKE_SOURCE_DIR}/external/cmdline-tools/latest/../.sdk/ndk/25.2.9519653/toolchains/llvm/prebuilt/linux-x86_64/sysroot，是实际存在的目录。

但执行CMake配置时出现错误：

-- CMAKE_SYSROOT /home/user/demo_project/external/cmdline-tools/latest/../.sdk/ndk/25.2.9519653/toolchains/llvm/prebuilt/linux-x86_64/sysroot
-- Android: Selected Clang toolchain 'aarch64-linux-android-clang' with GCC toolchain ''
-- The C compiler identification is Clang 14.0.7
-- The CXX compiler identification is Clang 14.0.7
-- Check for working C compiler: /home/user/demo_project/external/cmdline-tools/latest/../.sdk/ndk/25.2.9519653/toolchains/llvm/prebuilt/linux-x86_64/bin/clang
CMake Error at /usr/share/cmake-3.10/Modules/Platform/Android-Initialize.cmake:31 (message):
  Android: The system root directory needed for the selected Android version
  and architecture does not exist:

    /home/user/demo_project/CMakeFiles/CMakeTmp/external/cmdline-tools/latest/../.sdk/ndk/25.2.9519653/toolchains/llvm/prebuilt/linux-x86_64/sysroot

操作步骤：

~/demo_project$ mkdir build
~/demo_project$ cd build
~/demo_project$ cmake ../ -DCMAKE_TOOLCHAIN_FILE=../tool/buildtools/cmake_toolchain/android.cmake

注意到错误中的路径比STATUS输出多了CMakeFiles/CMakeTmp/前缀，导致路径无效。完整的CMAKE_TOOLCHAIN_FILE内容如下：

set(HOST_TAG "linux-x86_64")
set(ANDROID_API 31)
set(SDK "../.sdk")
set(ANDROID_CMD ${CMAKE_SOURCE_DIR}/external/cmdline-tools/latest) #for android_sdkmanager_install macro to work, set this
set(ANDROID_PACKAGES "")
set(NDK_PATH ${ANDROID_CMD}/${SDK}/ndk/25.2.9519653)
set(CMAKE_ANDROID_NDK ${NDK_PATH})
set(CMAKE_ANDROID_API ${ANDROID_API})
set(ANDROID_TARGET aarch64-linux-android${ANDROID_API})
set(CMAKE_ANDROID_NDK_TOOLCHAIN_VERSION clang)

set(ANDROID_TARGET_TRIPLE "aarch64-linux-android")
set(ANDROID_TARGET_ARCH "armv8")
set(CMAKE_ANDROID_ARCH_ABI "arm64-v8a")

set(CMAKE_SYSTEM_NAME Android)

SET(ANDROID_TOOLCHAIN_ROOT ${NDK_PATH}/toolchains/llvm/prebuilt/linux-x86_64)
set(CMAKE_SYSROOT "${ANDROID_TOOLCHAIN_ROOT}/sysroot")
# Disable some features that are not available on Android
SET(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
SET(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
SET(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)

用于配置NDK的脚本：

SCRIPT_DIR="$( dirname -- "${BASH_SOURCE[0]}"; )"
SCRIPT_DIR=$(realpath $SCRIPT_DIR)

ANDROID_API=31

cd $SCRIPT_DIR/../external/cmdline-tools/latest/

echo Handle android licenses
yes | ./bin/sdkmanager --sdk_root=../.sdk --licenses

sdk_install() {
    PACKAGE_NAME=$1
    echo Handle package "$PACKAGE_NAME"
    ./bin/sdkmanager --sdk_root=../.sdk --install "$PACKAGE_NAME"

}

sdk_install "build-tools;34.0.0-rc4"
sdk_install "ndk;25.2.9519653"
sdk_install "platforms;android-$ANDROID_API"

PLATFORMS_DIR=$(realpath $SCRIPT_DIR/../external/cmdline-tools/.sdk/platforms)
ln -sf $PLATFORMS_DIR ../.sdk/ndk/25.2.9519653/platforms

原因分析 #

错误核心是相对路径在CMake测试编译阶段被错误解析：

• 你设置的CMAKE_SYSROOT包含../相对路径，在项目根目录下解析正确。
• 但CMake检测编译器可用性时，会切换到临时目录build/CMakeFiles/CMakeTmp执行测试编译，此时../会指向demo_project/CMakeFiles而非项目根目录，导致拼接后的路径多出CMakeFiles/CMakeTmp前缀，最终路径不存在。

解决方法 #

将所有路径转换为绝对路径，避免相对路径在不同工作目录下的解析错误，有两种实现方式：

方式1：基于toolchain文件位置构建绝对路径 #

修改CMAKE_TOOLCHAIN_FILE中路径相关配置，用CMAKE_CURRENT_LIST_DIR（当前toolchain文件的绝对路径）定位项目根目录，再构建其他绝对路径：

set(HOST_TAG "linux-x86_64")
set(ANDROID_API 31)

# 获取当前toolchain文件的绝对路径，假设该文件在 tool/buildtools/cmake_toolchain/ 下
set(TOOLCHAIN_ABS_DIR ${CMAKE_CURRENT_LIST_DIR})
# 向上三级目录到达项目根目录（根据实际目录结构调整层级）
set(PROJECT_ROOT_ABS ${TOOLCHAIN_ABS_DIR}/../../..)

# 用绝对路径定义所有路径变量
set(ANDROID_CMD ${PROJECT_ROOT_ABS}/external/cmdline-tools/latest)
set(SDK ${PROJECT_ROOT_ABS}/.sdk)
set(NDK_PATH ${SDK}/ndk/25.2.9519653)

set(CMAKE_ANDROID_NDK ${NDK_PATH})
set(CMAKE_ANDROID_API ${ANDROID_API})
set(ANDROID_TARGET aarch64-linux-android${ANDROID_API})
set(CMAKE_ANDROID_NDK_TOOLCHAIN_VERSION clang)

set(ANDROID_TARGET_TRIPLE "aarch64-linux-android")
set(ANDROID_TARGET_ARCH "armv8")
set(CMAKE_ANDROID_ARCH_ABI "arm64-v8a")

set(CMAKE_SYSTEM_NAME Android)

SET(ANDROID_TOOLCHAIN_ROOT ${NDK_PATH}/toolchains/llvm/prebuilt/linux-x86_64)
set(CMAKE_SYSROOT "${ANDROID_TOOLCHAIN_ROOT}/sysroot")

# Disable some features that are not available on Android
SET(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
SET(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
SET(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)

方式2：用file(REAL_PATH)转换相对路径为绝对路径 #

如果不想调整目录层级，可以用CMake的file(REAL_PATH)命令将相对路径解析为绝对路径：

set(HOST_TAG "linux-x86_64")
set(ANDROID_API 31)
set(SDK "../.sdk")
set(ANDROID_CMD ${CMAKE_SOURCE_DIR}/external/cmdline-tools/latest)
set(ANDROID_PACKAGES "")

# 将相对路径转换为绝对路径
file(REAL_PATH "${ANDROID_CMD}/${SDK}/ndk/25.2.9519653" NDK_PATH)

set(CMAKE_ANDROID_NDK ${NDK_PATH})
set(CMAKE_ANDROID_API ${ANDROID_API})
set(ANDROID_TARGET aarch64-linux-android${ANDROID_API})
set(CMAKE_ANDROID_NDK_TOOLCHAIN_VERSION clang)

set(ANDROID_TARGET_TRIPLE "aarch64-linux-android")
set(ANDROID_TARGET_ARCH "armv8")
set(CMAKE_ANDROID_ARCH_ABI "arm64-v8a")

set(CMAKE_SYSTEM_NAME Android)

SET(ANDROID_TOOLCHAIN_ROOT ${NDK_PATH}/toolchains/llvm/prebuilt/linux-x86_64)
set(CMAKE_SYSROOT "${ANDROID_TOOLCHAIN_ROOT}/sysroot")

# Disable some features that are not available on Android
SET(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
SET(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
SET(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)

两种方式都能确保CMAKE_SYSROOT是绝对路径，无论CMake在哪个目录执行测试编译，都能正确定位到NDK的sysroot目录。

内容的提问来源于stack exchange，提问作者Silver