为何__android_log_print未定义？Android CMake编译报错排查

问题现象 #

编译时出现如下链接错误：

ld: error: undefined symbol: __android_log_print
>>> referenced by android_native_app_glue.c:51 (C:/dev/android/sdk/ndk/22.1.7171670/sources/android/native_app_glue\android_native_app_glue.c:51)
>>>               CMakeFiles/app-glue.dir/C_/dev/android/sdk/ndk/22.1.7171670/sources/android/native_app_glue/android_native_app_glue.c.o:(android_app_read_cmd)

当前使用的CMakeLists.txt内容：

cmake_minimum_required(VERSION 3.22.1)
project(MyGame)

set (APP_GLUE_DIR ${ANDROID_NDK}/sources/android/native_app_glue)

add_library(app-glue SHARED ${APP_GLUE_DIR}/android_native_app_glue.c)
target_include_directories(app-glue PUBLIC ${APP_GLUE_DIR})

add_library(native-lib SHARED ./native-lib.cpp)
target_include_directories(native-lib PUBLIC ${APP_GLUE_DIR})

add_library(libMyGame SHARED IMPORTED )

find_library(log-lib log)

set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -u ANativeActivity_onCreate")
target_link_libraries(native-lib app-glue android ${log-lib} libMyGame)

注：native-lib中同样使用__android_log_print但未出现类似错误。

错误原因 #

• android_native_app_glue.c中调用了__android_log_print，该符号属于Android NDK的log库
• 你将app-glue编译为共享库（SHARED），编译这个共享库时没有链接log库和android库，导致链接器无法找到该符号
• native-lib没有报错是因为它本身链接了log库，自身调用的__android_log_print符号能被正常解析，但app-glue作为独立共享库，编译阶段就需要解决自身的符号依赖

修复方法 #

方法一：为共享库app-glue添加依赖链接 #

修改CMakeLists.txt，为app-glue库添加log和android库的链接依赖：

cmake_minimum_required(VERSION 3.22.1)
project(MyGame)

set (APP_GLUE_DIR ${ANDROID_NDK}/sources/android/native_app_glue)

find_library(log-lib log)

add_library(app-glue SHARED ${APP_GLUE_DIR}/android_native_app_glue.c)
target_include_directories(app-glue PUBLIC ${APP_GLUE_DIR})
# 为app-glue添加依赖库链接
target_link_libraries(app-glue android ${log-lib})

add_library(native-lib SHARED ./native-lib.cpp)
target_include_directories(native-lib PUBLIC ${APP_GLUE_DIR})

add_library(libMyGame SHARED IMPORTED )

set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -u ANativeActivity_onCreate")
target_link_libraries(native-lib app-glue libMyGame)

方法二：将app-glue编译为静态库 #

如果不需要将app-glue作为独立共享库，可将其改为静态库（STATIC），这样在链接native-lib时会把app-glue的代码合并进去，只需保证native-lib已链接log和android库即可：

# 将app-glue改为静态库
add_library(app-glue STATIC ${APP_GLUE_DIR}/android_native_app_glue.c)
target_include_directories(app-glue PUBLIC ${APP_GLUE_DIR})

这种方式下，原有的target_link_libraries(native-lib app-glue android ${log-lib} libMyGame)无需修改，即可解决符号未定义问题。

内容的提问来源于stack exchange，提问作者Bungles