CMake无法找到OpenSSL：Android NDK工具链引发的问题排查

核心原因 #

Android NDK的android.toolchain.cmake会配置交叉编译环境，修改CMake的查找根路径规则，导致默认的FindOpenSSL.cmake优先在NDK的sysroot中查找，忽略了你指定的OPENSSL_ROOT_DIR；同时BoringSSL与官方OpenSSL的目录结构存在差异，进一步干扰了自动查找逻辑。

具体解决思路 #

1. 确保BoringSSL是针对目标Android架构编译的 #

交叉编译环境下，BoringSSL的编译产物必须和你要构建的Android ABI（如arm64-v8a、armeabi-v7a）完全匹配，不能用x86/x86_64平台的编译结果。编译BoringSSL时需指定NDK工具链，示例命令（以arm64-v8a为例）：

export ANDROID_NDK=/path/to/ndk/25.1.8937393
export TOOLCHAIN=$ANDROID_NDK/toolchains/llvm/prebuilt/darwin-x86_64
export TARGET=aarch64-linux-android
export API=24 # 目标Android API版本

export AR=$TOOLCHAIN/bin/llvm-ar
export CC=$TOOLCHAIN/bin/$TARGET$API-clang
export CXX=$TOOLCHAIN/bin/$TARGET$API-clang++

mkdir build-arm64 && cd build-arm64
cmake ../boringssl-source/ \
  -DCMAKE_SYSTEM_NAME=Android \
  -DCMAKE_SYSTEM_VERSION=$API \
  -DCMAKE_ANDROID_ARCH_ABI=arm64-v8a \
  -DCMAKE_ANDROID_NDK=$ANDROID_NDK \
  -DCMAKE_C_COMPILER=$CC \
  -DCMAKE_CXX_COMPILER=$CXX \
  -DCMAKE_INSTALL_PREFIX=/path/to/boringssl-install/arm64-v8a

make install

2. 手动指定OpenSSL变量，绕过自动查找 #

既然find_package(OpenSSL)自动查找失效，直接在CMakeLists.txt中手动设置所需变量，放在find_package(OpenSSL REQUIRED)之前：

# 根据你的BoringSSL安装路径和目标ABI调整
set(OPENSSL_ROOT_DIR "/path/to/boringssl-install/${CMAKE_ANDROID_ABI}")
set(OPENSSL_INCLUDE_DIR "${OPENSSL_ROOT_DIR}/include")
set(OPENSSL_CRYPTO_LIBRARY "${OPENSSL_ROOT_DIR}/lib/libcrypto.a")
set(OPENSSL_SSL_LIBRARY "${OPENSSL_ROOT_DIR}/lib/libssl.a")
set(OPENSSL_FOUND TRUE)

find_package(OpenSSL REQUIRED)

如果BoringSSL库按ABI分目录存放，${CMAKE_ANDROID_ABI}会自动匹配当前编译目标。

3. 自定义FindBoringSSL模块 #

由于BoringSSL与官方OpenSSL结构有差异，可编写自定义FindBoringSSL.cmake模块，放在项目的cmake/Modules目录下，让CMake优先使用这个模块：

# cmake/Modules/FindBoringSSL.cmake
find_path(BORINGSSL_INCLUDE_DIR
  NAMES openssl/ssl.h openssl/crypto.h
  PATHS "${OPENSSL_ROOT_DIR}/include"
  NO_CMAKE_FIND_ROOT_PATH # 跳过sysroot查找，直接用指定路径
)

find_library(BORINGSSL_CRYPTO_LIBRARY
  NAMES crypto
  PATHS "${OPENSSL_ROOT_DIR}/lib/${CMAKE_ANDROID_ABI}"
  NO_CMAKE_FIND_ROOT_PATH
)

find_library(BORINGSSL_SSL_LIBRARY
  NAMES ssl
  PATHS "${OPENSSL_ROOT_DIR}/lib/${CMAKE_ANDROID_ABI}"
  NO_CMAKE_FIND_ROOT_PATH
)

set(OPENSSL_INCLUDE_DIR ${BORINGSSL_INCLUDE_DIR})
set(OPENSSL_CRYPTO_LIBRARY ${BORINGSSL_CRYPTO_LIBRARY})
set(OPENSSL_SSL_LIBRARY ${BORINGSSL_SSL_LIBRARY})
set(OPENSSL_FOUND TRUE)

include(FindPackageHandleStandardArgs)
find_package_handle_standard_args(BoringSSL DEFAULT_MSG
  BORINGSSL_INCLUDE_DIR
  BORINGSSL_CRYPTO_LIBRARY
  BORINGSSL_SSL_LIBRARY
)

然后在主CMakeLists.txt中添加模块路径并查找：

set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/Modules" ${CMAKE_MODULE_PATH})
find_package(BoringSSL REQUIRED)

4. 调整CMake查找根路径规则 #

NDK工具链默认设置CMAKE_FIND_ROOT_PATH_MODE_INCLUDE和CMAKE_FIND_ROOT_PATH_MODE_LIBRARY为ONLY，即只在sysroot中查找。可修改这两个变量，让CMake同时在指定路径查找：

# 在find_package之前设置
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE BOTH)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY BOTH)

set(OPENSSL_ROOT_DIR "/path/to/boringssl-install")
find_package(OpenSSL REQUIRED)

注意：该设置会影响所有依赖的查找逻辑，可能导致其他系统库查找出现问题，需测试验证。

内容的提问来源于stack exchange，提问作者Kostya Vasilyev