当所有需要的库和引用都被考虑在内时，为什么我会收到 `undefined reference to xxx` 错误？

Question

本质上，我正在尝试使用 redland rdf 库，但我无法对它们进行 link。当我尝试使用 redland 库的简单基本程序时，出现以下错误：

/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，您可能只是认为我丢失了一个 link 库，在我进一步检查之前我是这么想的（见下文），但是，这些库都已计算在内。

在这种情况下，一个适当的最小工作示例很困难，因为它需要获取和构建我正在尝试使用的库。但是，我创建了一个 GitHub repository

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

其中包含构建 Redland 库所需的源文件以及我正在使用的示例代码（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

请注意，我正在为 Linux 开发 windows 子系统上的 Ubuntu-18.04。

要获取、构建和安装我正在使用这些终端命令的库：

#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

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

还有我的最小 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}

")

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

为了构建，我使用的是 CLion，它只是在后台执行此操作：

mkdir build && cd build
CMake ..
make

因此给我 linker 错误。通过使用 nm 检查 Redland 库的内容，我进行了更深入的挖掘。

$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里面，是librdf.a里面定义的引用

# 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，我们发现它在librdf.a..

中确实是未定义的

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

但这无论如何都应该在 libraptor2.a 中，如果我们看一下，我们会发现它确实存在并且应该被定义为：

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

我的理解是 linking 的过程本质上应该用 libraptor.a 中的定义填充 librdf.a 中的未定义引用，但这显然没有发生。

为什么会这样？

Answer 1

当您的静态库相互依赖时，link 命令很重要（参见this 回复）。

如果 librdf 依赖于 libraptor 库（如 link 错误所示），libraptor 库应列在 [之后] librdf 当指定给 linker 时。尝试根据您的库依赖项重新排列 target_link_libraries() 命令中的库列表以遵守此顺序。

当所有需要的库和引用都被考虑在内时，为什么我会收到 `undefined reference to xxx` 错误？

Why am I getting `undefined reference to xxx` errors when all libraries and references needed are accounted for?

gcc

cmake

linker

g++

raptor2