为什么使用 CMake 时我的共享库无法正确链接？

Question

这是一个非常微不足道的问题，可能是由于我对 CMake 缺乏经验。我已按照http://fabzter.com/blog/cmake-tutorial-1上的教程进行操作，但遇到链接问题。

基本上我有一个 MathFuncs 库和一个使用 MathFuncs 的可执行文件。MathFuncs 的 CMakeList 是：

cmake_minimum_required(VERSION 2.8)
project(MathFuncs)

include_directories(${PROJECT_SOURCE_DIR})

SET (HEADER_FILES mysqrt.hpp)
add_library(MathFuncs SHARED mysqrt.cpp ${HEADER_FILES})

可执行的 CMakeLists 是：

cmake_minimum_required (VERSION 2.6)
project (Tutorial)

set (Tutorial_VERSION_MAJOR 1)
set (Tutorial_VERSION_MINOR 0)
set (Tutorial_VERSION_BUGFIX 0)

#configure header file to pass some of the CMake settings 
#to the source code
configure_file(
    "${PROJECT_SOURCE_DIR}/TutorialConfig.h.in"
    "${PROJECT_BINARY_DIR}/TutorialConfig.h"
) 

# add the binary tree to the search path for include files
# so that we will find TutorialConfig.h
include_directories("${PROJECT_BINARY_DIR}")

# add the include directories necessary to build library
include_directories("${PROJECT_SOURCE_DIR}/MathFuncs}")
add_subdirectory(MathFuncs)

SET (MATHFUNCTIONS_DIR ${PROJECT_SOURCE_DIR}/MathFuncs)
add_executable(tutorial tutorial.cpp ${MATHFUNCTIONS_DIR}/mysqrt.hpp)

target_link_libraries(tutorial MathFuncs)

CMake 运行良好。但是，当我尝试使用 Visual Studio 进行编译时，出现链接器错误，指出它无法打开 MathFuncs.lib。当我从 MathFuncs CMakeList 中删除“共享”选项时，它会运行，因为它正在构建静态库，但是对于我的应用程序，我需要一个共享库 DLL。

如何让 CMake 将库引用设置为共享？

谢谢你，

Answer 1

看来您没有正确从 DLL 导出类/函数。请参阅这篇文章以获取详细说明。

这并不完全简单，但幸运的是 CMake 可以在这里提供一些帮助。您可以使用该GenerateExportHeader模块自动生成标头，其中包含正确导出库的公共函数所需的所有预处理器定义。

这是我如何更改您的文件的示例。希望每个内容都有足够的评论，让您了解正在发生的事情。如果没有，请添加评论，我将进行扩展。

顶级 CMakeLists.txt：

cmake_minimum_required (VERSION 3.0)
project(Tutorial)

add_subdirectory(MathFuncs)

set(Tutorial_VERSION_MAJOR 1)
set(Tutorial_VERSION_MINOR 0)
set(Tutorial_VERSION_BUGFIX 0)

# Configure header file to pass some of the CMake settings to the source code.
set(TutorialConfigHeader "${PROJECT_BINARY_DIR}/TutorialConfig.h")
configure_file(TutorialConfig.h.in "${TutorialConfigHeader}")

# Include the output file of 'configure_file' to ensure it gets configured.
add_executable(tutorial tutorial.cpp "${TutorialConfigHeader}")

# Add TutorialConfig.h's path to include dirs for 'tutorial'.
target_include_directories(tutorial PRIVATE "${PROJECT_BINARY_DIR}")

target_link_libraries(tutorial MathFuncs)

# If 'MathFuncs' is a shared lib, copy it to 'tutorial's bin dir so
# it can be found at runtime.
get_target_property(MathFuncsType MathFuncs TYPE)
if(MathFuncsType STREQUAL "SHARED_LIBRARY")
  add_custom_command(TARGET tutorial POST_BUILD
                     COMMAND ${CMAKE_COMMAND} -E copy_if_different
                         $<TARGET_FILE:MathFuncs> $<TARGET_FILE_DIR:tutorial>
                     COMMENT "Copying MathFuncs shared lib alongside tutorial."
                     VERBATIM)
endif()

MathFuncs/CMakeLists.txt：

cmake_minimum_required(VERSION 3.0)
project(MathFuncs)

add_library(MathFuncs SHARED
            mysqrt.cpp
            mysqrt.hpp
            "${PROJECT_BINARY_DIR}/mathfuncs_export.h")

# Write appropriate export PP definitions in a file called
# 'mathfuncs_export.h' in the current binary dir
include(GenerateExportHeader)
generate_export_header(MathFuncs)

# Add the current source dir as a PUBLIC include dir
# (to allow mysqrt.hpp to be found by dependent targets)
# Add the current binary dir as a PUBLIC include dir
# (to allow mathfuncs_export.h to be found by dependent targets)
target_include_directories(MathFuncs PUBLIC
                           ${PROJECT_SOURCE_DIR} ${PROJECT_BINARY_DIR})

然后要使用生成的导出标头，您只需让 mysqrt.hpp 包含类似以下内容：

#ifndef MYSQRT_HPP_
#define MYSQRT_HPP_

#include "mathfuncs_export.h"

double MATHFUNCS_EXPORT MySqrt(double input);

#endif  // MYSQRT_HPP_

现在这应该会导致 VS 创建一个导出库${CMAKE_BINARY_DIR}/MathFuncs/Debug/MathFuncs.lib（修复链接器问题）和实际的 DLL ${CMAKE_BINARY_DIR}/MathFuncs/Debug/MathFuncs.dll。