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CMake library search function does not search LD_LIBRARY_PATH

2022-07-30 12:33:00 【51CTO】

摘要： This article is introduced by the problem that the library file cannot be found after compiling and running,首先分析了find_package(JNI)的工作流程,and then forcmake不搜索LD_LIBRARY_PATH的问题,A general solution is proposed.

本文分享自华为云社区《 CMakeThe library search function doesn't actually searchLD_LIBRARY_PATH? Thinking triggered by the experience of using the compilation tool》,作者：蜉蝣与海 .

Product to be used recentlyJNI技术,CMake编译C++The code requires external linkslibjvm.so库.The code compiles fine,系统中也有libjvm.so, However, the following exception was reported when using it：

       
       error while loading shared libraries: libjvm.so: cannot open shared object file: No such file or directory
      
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This error message indicates,The operating system was not foundlibjvm.so, Our operating system is from LD_LIBRARY_PATHSearch for these dynamic link libraries,Obviously currentlylibjvm.sonot in this directory.

The solution to the problem is simple,直接在LD_LIBRARY_PATH里加入libjvm.so的库即可.But this got me thinking：

Why can be found when buildinglibjvm.so, It can't be found at runtime？

这个问题的回答,Both can have a concise and concise version of the explanation,You can also dig deeper.

Let’s look at the brief explanation first：

代码的CMakeListThe following statements are used in ,Look for and link during compilationlibjvm.so,This search method is different from the search method of the operating system:

       
       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")
      
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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,And set this folder as the default search library path.而后set_target_propertiesLinking will work.

The answer can only tell us“是什么”,But as a programmer,还要了解“为什么”,Several issues are raised here：

find_package(JNI)的工作过程是怎样的？为什么LD_LIBRARY_PATHDependency library not found,cmake可以找到
cmakelibrary search functionfind_library会搜索LD_LIBRARY_PATH吗,如果不会,You can search by settingLD_LIBRARY_PATH吗？

问题一：find_package(JNI)的工作过程是怎样的

In order to facilitate developers to reference external packages,cmakeThere are many official pre-defined packages to look for dependenciesModule, 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].Execute the following instructions on your local computer,即可找到find_package(JNI)使用的脚本文件.

       
       find / -name FindJNI.cmake
      
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打开自己的cmake对应的FindJNI文件,You can see dense comments and scripts,by reading these scripts,We got to knowFindJNI是如何工作的.

分析问题前,Look at the results of the problem first,The comments at the top of the file have the following description：

       
       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
      
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这段代码表明,执行find_package(JNI)之后,A series of variables will be set,This includes representationJNIWhether the variable was foundJNI_FOUND,以及表示libjvm.so的变量JAVA_JVM_LIBRARY.After these variables are set,通过FindPackageHandleStandardArgs导出,返回调用处,FindPackageHandleStandardArgs是cmakeA macro dedicated to exporting variables[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)
      
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在文件中定位JAVA_JVM_LIBRARY, The following code snippet can be traced：

       
       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()
      
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由此可知,JAVA_JVM_LIBRARY这个变量,by searching one by one${_JNI_${search}_JVM}Folder in and then confirmJAVA_JVM_LIBRARY的.而${_JNI_${search}_JVM}The relevant definition sentences are shown in the figure：

       
       set(_JNI_FRAMEWORK_JVM NAMES JavaVM)
       
set(_JNI_NORMAL_JVM
       
  NAMES jvm
       
  PATHS ${JAVA_JVM_LIBRARY_DIRECTORIES}
       
  )
      
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其中JAVA_JVM_LIBRARY_DIRECTORIESThere are a lot of possibilities involvedlibjvm.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()
      
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如上图所示,The variable dependency order is as follows：

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_BASEThe values of the variables are jointly determined.而JNI_JAVA_DIRECTORY_BASEA large number of predefined paths are preset：

       
       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
       
  )
      
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通过以上分析可以看出,JAVA_JVM_LIBRARY的搜索,依赖JAVA_HOMEand lots of predefined paths.

问题二：cmakeLibrary search functionfind_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_libraryThe default is not to searchLD_LIBRARY_PATH, And can't find it online eithercmake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,Then injectfind_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()
      
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Compilation fails if the desired library cannot be found,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()
      
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小结

This article is introduced by the problem that the library file cannot be found after compiling and running,首先分析了find_package(JNI)的工作流程,and then forcmake不搜索LD_LIBRARY_PATH的问题,A general solution is proposed.