为什么在考虑到所有需要的库和引用时，我得到了对am `错误的“未定义的引用”？

Question

本质上，我试图使用雷德兰rdf库，但我无法链接到它们。当我尝试使用redland库的一个简单的basic程序时，我会得到以下错误：

/usr/local/lib/librdf.a(rdf_init.o): In function `librdf_free_memory':
/home/ciaran/Software/redland/redland-1.0.17/src/rdf_init.c:671: undefined reference to `raptor_free_memory'
/usr/local/lib/librdf.a(rdf_init.o): In function `librdf_alloc_memory':
/home/ciaran/Software/redland/redland-1.0.17/src/rdf_init.c:689: undefined reference to `raptor_alloc_memory'
/usr/local/lib/librdf.a(rdf_init.o): In function `librdf_calloc_memory':
/home/ciaran/Software/redland/redland-1.0.17/src/rdf_init.c:707: undefined reference to `raptor_calloc_memory'

乍一看，您可能只是认为我缺少了一个链接库，这就是我在进一步检查之前所想的(请参阅下面)，但是，所有的库都已经说明了。

在这种情况下，一个适当的最小工作示例是很困难的，因为它需要获取和构建我试图使用的库。但是，我已经创建了一个GitHub 存储库

$ git clone git@github.com:CiaranWelsh/RedlandBuildTest.git

它包含构建Redland库所需的源文件以及我使用的示例代码(这会中断)。

为了构建库，您还需要

    $ sudo apt install automake autoconf libtool gtk-doc-tools
    $ sudo apt install libxml2 libxml2-dev libxslt libxslt-dev libcurl4-openssl-dev libltdl-dev

注意，我正在开发用于Linux的windows子系统的Ubuntu-18.04。

为了获取、构建和安装库，我使用以下终端命令：

#raptor2
wget "http://download.librdf.org/source/raptor2-2.0.15.tar.gz" 
tar -xvf raptor2-2.0.15.tar.gz
cd raptor2-2.0.15
./autogen.sh 
make
sudo make install

# redland (librdf)
wget "http://download.librdf.org/source/redland-1.0.17.tar.gz" 
tar -xvf redland-1.0.17.tar.gz
cd redland-1.0.17 
./autogen.sh
make
sudo make install

# rasqal
wget "http://download.librdf.org/source/rasqal-0.9.33.tar.gz" 
tar -xvf rasqal-0.9.33.tar.gz
cd rasqal-0.9.33
./autogen.sh 
make 
sudo make install

它们作为shell脚本(get-raptor.sh、get-rasqal.sh和get-librdf.sh)在github存储库中可用。

以及我的最小CMake脚本(也在存储库中)：

cmake_minimum_required(VERSION 3.15)
project(RedlandBuildTest)

set(CMAKE_CXX_STANDARD 14)

find_library(RAPTOR2_STATIC_LIBRARY
        NAMES libraptor2.a
        PATHS /usr/local/lib
        )

find_path(RAPTOR2_INCLUDE_DIR
        NAMES raptor2.h
        PATHS /usr/local/include/raptor2
        )
find_library(RASQAL_STATIC_LIBRARY
        NAMES librasqal.a
        PATHS /usr/local/lib
        )

find_path(RASQAL_INCLUDE_DIR
        NAMES rasqal.h
        PATHS /usr/local/include/rasqal
        )

find_library(LIBRDF_STATIC_LIBRARY
        NAMES librdf.a
        PATHS /usr/local/lib
        )

find_path(LIBRDF_INCLUDE_DIR
        NAMES librdf.h
        PATHS /usr/local/include
        )

add_executable(RedlandBuildTest main.c)

target_include_directories(RedlandBuildTest PRIVATE
        ${RAPTOR2_INCLUDE_DIR}
        ${RASQAL_INCLUDE_DIR}
        ${LIBRDF_INCLUDE_DIR}
        )

target_link_libraries(RedlandBuildTest PRIVATE
        ${RAPTOR2_STATIC_LIBRARY}
        ${RASQAL_STATIC_LIBRARY}
        ${LIBRDF_STATIC_LIBRARY}
        curl
        xml2
        xslt
        ltdl
        )

get_target_property(LINK_LIBRARIES RedlandBuildTest LINK_LIBRARIES)
get_target_property(INCLUDE_DIRECTORIES RedlandBuildTest INCLUDE_DIRECTORIES)

message(STATUS "

LINK_LIBRARIES      ${LINK_LIBRARIES}
INCLUDE_DIRECTORIES ${INCLUDE_DIRECTORIES}

")

message(STATUS "

    RAPTOR2_STATIC_LIBRARY   ${RAPTOR2_STATIC_LIBRARY}
    RAPTOR2_INCLUDE_DIR      ${RAPTOR2_INCLUDE_DIR}

    RASQAL_STATIC_LIBRARY    ${RASQAL_STATIC_LIBRARY}
    RASQAL_INCLUDE_DIR       ${RASQAL_INCLUDE_DIR}

    LIBRDF_STATIC_LIBRARY    ${LIBRDF_STATIC_LIBRARY}
    LIBRDF_INCLUDE_DIR       ${LIBRDF_INCLUDE_DIR}

")

以及CMake命令的输出：

#(looks good)
LINK_LIBRARIES      /usr/local/lib/libraptor2.a;/usr/local/lib/librasqal.a;/usr/local/lib/librdf.a;curl;xml2;xslt;ltdl 
INCLUDE_DIRECTORIES /usr/local/include/raptor2;/usr/local/include/rasqal;/usr/local/include


    RAPTOR2_STATIC_LIBRARY   /usr/local/lib/libraptor2.a
    RAPTOR2_INCLUDE_DIR      /usr/local/include/raptor2

    RASQAL_STATIC_LIBRARY    /usr/local/lib/librasqal.a
    RASQAL_INCLUDE_DIR       /usr/local/include/rasqal

    LIBRDF_STATIC_LIBRARY    /usr/local/lib/librdf.a
    LIBRDF_INCLUDE_DIR       /usr/local/include

为了进行构建，我使用了CLion，它只在后台执行以下操作：

mkdir build && cd build
CMake ..
make

这就给了我链接错误。通过使用nm检查Redland库的内容，我已经挖掘得更深了一点。

$nm -A librdf.a > librdf.a.nmoutput.txt
$nm -A libraptor2.a > libraptor2.a.nmoutput.txt
$nm -A librasqal.a > librasqal.a.nmoutput.txt 

第一次冒犯undfined reference错误

/usr/local/lib/librdf.a(rdf_init.o): In function `librdf_free_memory':
/home/ciaran/Software/redland/redland-1.0.17/src/rdf_init.c:671: undefined reference to `raptor_free_memory'

在函数librdf_free_memory中，它是在librdf.a中定义的引用。

# librdf.a.nmoutput.txt
...
librdf.a:rdf_init.o:0000000000000740 T librdf_free_memory
...

当我们查找对raptor_free_memory的未定义引用时，我们认为它在librdf.a.中确实是未定义的。

#librdf.a.nmoutput.txt
...
librdf.a:rdf_init.o:                 U raptor_free_memory
...

但是无论如何，这应该在libraptor2.a中，如果我们看一看，我们就会发现它确实存在，并且定义为它应该是的：

# libraptor2.a.nmoutput.txt
...
libraptor2.a:raptor_general.o:0000000000000863 T raptor_free_memory
...

我的理解是，链接的过程本质上应该用librdf.a内部的定义填充libraptor.a中未定义的引用，但这显然是没有发生的。

为什么会发生这种情况？

Answer 1

当您的静态库相互依赖时，链接顺序关系到(参见这响应)。

如果librdf依赖于libraptor库(如链接错误所指示的那样)，那么当指定给链接器时，libraptor库应该在librdf之后列出。尝试根据库依赖关系重新排列target_link_libraries()命令中的库列表，以遵循此顺序。