CMake库搜索函数居然不搜索LD_LIBRARY_PATH
摘要: 本文通过编译后运行找不到库文件的问题引入,首先分析了find_package(JNI)的工作流程,而后针对cmake不搜索LD_LIBRARY_PATH的问题,提出了一种通用的解决办法。
本文分享自华为云社区《CMake库搜索函数居然不搜索LD_LIBRARY_PATH? 由编译工具使用体验而引发的思考》,作者: 蜉蝣与海 。
最近产品要使用JNI技术,CMake编译C++代码时需要对外链接libjvm.so库。代码编译倒是正常,系统中也有libjvm.so, 然而使用时却报了如下异常:
error while loading shared libraries: libjvm.so: cannot open shared object file: No such file or directory
这个报错表示,操作系统并没有找到libjvm.so, 我们的操作系统是从LD_LIBRARY_PATH中搜索这些动态链接库,很显然目前libjvm.so并不在这个目录下。
问题的解决倒是简单,直接在LD_LIBRARY_PATH里加入libjvm.so的库即可。但是这却引发了我的思考:
为什么构建时可以找到libjvm.so, 运行时却找不到呢?
这个问题的回答,既可以有简明扼要版解释,又可以刨根问底深挖。
先来看简明扼要版解释:
代码的CMakeList中使用了下列语句,在编译过程中寻找并链接libjvm.so,这个搜索方式和操作系统的搜索方式不同:
find_package(JNI) get_filename_component(JVM_LIB_PATH ${JAVA_JVM_LIBRARY} DIRECTORY) get_filename_component(JAVA_LIB_PATH ${JVM_LIB_PATH} DIRECTORY) link_directories(${JVM_LIB_PATH} ${JAVA_LIB_PATH}) set_target_properties(${NAME} PROPERTIES LINK_FLAGS "-ljvm")
其中find_package(JNI)会搜索libjvm.so可能存在的路径,通过get_filename_component来获得libjvm.so的文件夹,并把这个文件夹设为默认搜索库路径。而后set_target_properties会进行链接工作。
这个答案只能告诉我们“是什么”,但是作为一只程序猿,还要了解“为什么”,这里引申几个问题讨论:
- find_package(JNI)的工作过程是怎样的?为什么LD_LIBRARY_PATH里没找到的依赖库,cmake可以找到
- cmake的库搜索函数find_library会搜索LD_LIBRARY_PATH吗,如果不会,可以通过设置来搜索LD_LIBRARY_PATH吗?
问题一:find_package(JNI)的工作过程是怎样的
为了方便开发者引用外部包,cmake官方预定义了许多寻找依赖包的Module, 他们存储在cmake的/share/-cmake-<version>/Modules目录下。每个以Find<LibraryName>.cmake命名的文件都可以帮我们找到一个包[1]。在本地计算机执行以下指令,即可找到find_package(JNI)使用的脚本文件。
find / -name FindJNI.cmake
打开自己的cmake对应的FindJNI文件,可以看到密密麻麻的注释和脚本,通过阅读这些脚本,我们得以得知FindJNI是如何工作的。
分析问题前,先看问题带来的结果,文件最上方注释有如下说明:
This module sets the following result variables: ``JNI_INCLUDE_DIRS`` the include dirs to use ``JNI_LIBRARIES`` the libraries to use (JAWT and JVM) ``JNI_FOUND`` TRUE if JNI headers and libraries were found. Cache Variables ^^^^^^^^^^^^^^^ The following cache variables are also available to set or use: ``JAVA_AWT_LIBRARY`` the path to the Java AWT Native Interface (JAWT) library ``JAVA_JVM_LIBRARY`` the path to the Java Virtual Machine (JVM) library ``JAVA_INCLUDE_PATH`` the include path to jni.h ``JAVA_INCLUDE_PATH2`` the include path to jni_md.h and jniport.h ``JAVA_AWT_INCLUDE_PATH`` the include path to jawt.h
这段代码表明,执行find_package(JNI)之后,会有一系列变量被设置,其中包括表示JNI是否被找到的变量JNI_FOUND,以及表示libjvm.so的变量JAVA_JVM_LIBRARY。这些变量在设定之后,通过FindPackageHandleStandardArgs导出,返回调用处,FindPackageHandleStandardArgs是cmake专门用来导出变量的宏[2]:
include(${CMAKE_CURRENT_LIST_DIR}/FindPackageHandleStandardArgs.cmake)
FIND_PACKAGE_HANDLE_STANDARD_ARGS(JNI DEFAULT_MSG JAVA_AWT_LIBRARY
JAVA_JVM_LIBRARY
JAVA_INCLUDE_PATH
JAVA_INCLUDE_PATH2
JAVA_AWT_INCLUDE_PATH)
在文件中定位JAVA_JVM_LIBRARY, 可以追踪到下述代码片段:
foreach(search ${_JNI_SEARCHES}) find_library(JAVA_JVM_LIBRARY ${_JNI_${search}_JVM}) find_library(JAVA_AWT_LIBRARY ${_JNI_${search}_JAWT}) if(JAVA_JVM_LIBRARY) break() endif() endforeach()
由此可知,JAVA_JVM_LIBRARY这个变量,是通过逐个搜索${_JNI_${search}_JVM}里的文件夹进而确定JAVA_JVM_LIBRARY的。而${_JNI_${search}_JVM}相关的定义语句如图:
set(_JNI_FRAMEWORK_JVM NAMES JavaVM) set(_JNI_NORMAL_JVM NAMES jvm PATHS ${JAVA_JVM_LIBRARY_DIRECTORIES} )
其中JAVA_JVM_LIBRARY_DIRECTORIES中涉及了大量可能的libjvm.so存在的路径。
set(JAVA_JVM_LIBRARY_DIRECTORIES) foreach(dir ${JAVA_AWT_LIBRARY_DIRECTORIES}) list(APPEND JAVA_JVM_LIBRARY_DIRECTORIES "${dir}" "${dir}/client" "${dir}/server" # IBM SDK, Java Technology Edition, specific paths "${dir}/j9vm" "${dir}/default" ) endforeach() set(JAVA_AWT_LIBRARY_DIRECTORIES) if(_JAVA_HOME) JAVA_APPEND_LIBRARY_DIRECTORIES(JAVA_AWT_LIBRARY_DIRECTORIES ${_JAVA_HOME}/jre/lib/{libarch} ${_JAVA_HOME}/jre/lib ${_JAVA_HOME}/lib/{libarch} ${_JAVA_HOME}/lib ${_JAVA_HOME} ) endif() JAVA_APPEND_LIBRARY_DIRECTORIES(JAVA_AWT_LIBRARY_DIRECTORIES ${_JNI_JAVA_AWT_LIBRARY_TRIES} ) foreach(_java_dir IN LISTS _JNI_JAVA_DIRECTORIES_BASE) list(APPEND _JNI_JAVA_AWT_LIBRARY_TRIES ${_java_dir}/jre/lib/{libarch} ${_java_dir}/jre/lib ${_java_dir}/lib/{libarch} ${_java_dir}/lib ${_java_dir} ) list(APPEND _JNI_JAVA_INCLUDE_TRIES ${_java_dir}/include ) endforeach()
如上图所示,变量依赖顺序如下:
JAVA_JVM_LIBRARY_DIRECTORIES => JAVA_AWT_LIBRARY_DIRECTORIES => _JNI_JAVA_AWT_LIBRARY_TRIES & _JAVA_HOME => _JNI_JAVA_DIRECTORIES_BASE
最终发现JAVA_JVM_LIBRARY_DIRECTORIES变量的值,是由JAVA_HOME变量的值和_JNI_JAVA_DIRECTORIES_BASE变量的值共同决定的。而JNI_JAVA_DIRECTORY_BASE预置了大量预定义路径:
set(_JNI_JAVA_DIRECTORIES_BASE /usr/lib/jvm/java /usr/lib/java /usr/lib/jvm /usr/local/lib/java /usr/local/share/java /usr/lib/j2sdk1.4-sun /usr/lib/j2sdk1.5-sun /opt/sun-jdk-1.5.0.04 /usr/lib/jvm/java-6-sun /usr/lib/jvm/java-1.5.0-sun /usr/lib/jvm/java-6-sun-1.6.0.00 # can this one be removed according to #8821 ? Alex /usr/lib/jvm/java-6-openjdk /usr/lib/jvm/java-1.6.0-openjdk-1.6.0.0 # fedora # Debian specific paths for default JVM /usr/lib/jvm/default-java # Arch Linux specific paths for default JVM /usr/lib/jvm/default # Ubuntu specific paths for default JVM /usr/lib/jvm/java-11-openjdk-{libarch} # Ubuntu 18.04 LTS /usr/lib/jvm/java-8-openjdk-{libarch} # Ubuntu 15.10 /usr/lib/jvm/java-7-openjdk-{libarch} # Ubuntu 15.10 /usr/lib/jvm/java-6-openjdk-{libarch} # Ubuntu 15.10 # OpenBSD specific paths for default JVM /usr/local/jdk-1.7.0 /usr/local/jre-1.7.0 /usr/local/jdk-1.6.0 /usr/local/jre-1.6.0 # SuSE specific paths for default JVM /usr/lib64/jvm/java /usr/lib64/jvm/jre )
通过以上分析可以看出,JAVA_JVM_LIBRARY的搜索,依赖JAVA_HOME和大量预定义路径。
问题二:cmake库搜索函数find_library会搜索LD_LIBRARY_PATH吗
通过阅读Does CMake’s find_library search LD_LIBRARY_PATH可以知道,find_library默认不搜索LD_LIBRARY_PATH, 并且网上也找不到让cmake搜索LD_LIBRARY_PATH的文章。
那cmake能搜索LD_LIBRARY_PATH吗?
答案是可以的,通过cmake获取LD_LIBRARY_PATH环境变量,并转为cmake可理解的list格式,而后注入find_library即可,代码如下:
string(REPLACE ":" ";" RUNTIME_PATH "$ENV{LD_LIBRARY_PATH}") find_library(JVM_API NAMES jvm HINTS ${RUNTIME_PATH}) if (JVM_API STREQUAL "JVM_API-NOTFOUND") message(WARNING "found libjvm.so only in ${JAVA_JVM_LIBRARY} but not in LD_LIBRARY_PATH. environment variable LD_LIBRARY_PATH must include its' directory.") endif()
如果希望找不到这个库时编译失败,可以将WARNING改为fatal_error, 代码如下:
string(REPLACE ":" ";" RUNTIME_PATH "$ENV{LD_LIBRARY_PATH}") find_library(JVM_API NAMES jvm HINTS ${RUNTIME_PATH}) if (JVM_API STREQUAL "JVM_API-NOTFOUND") message(FATAL_ERROR "found libjvm.so only in ${JAVA_JVM_LIBRARY} but not in LD_LIBRARY_PATH. environment variable LD_LIBRARY_PATH must include its' directory.") endif()
小结
本文通过编译后运行找不到库文件的问题引入,首先分析了find_package(JNI)的工作流程,而后针对cmake不搜索LD_LIBRARY_PATH的问题,提出了一种通用的解决办法。
参考文献:
[1] Cmake之深入理解find_package()的用法://zhuanlan.zhihu.com/p/97369704?utm_source=wechat_session
[2] Cmake中find_package命令的搜索模式之模块模式(Module mode)://www.jianshu.com/p/f983a90bcf91
[3]Does CMake’s find_library search LD_LIBRARY_PATH?://stackoverflow.com/questions/41566316/does-cmakes-find-library-search-ld-library-path