无法在 CMake 中创建动态库并将其链接到另一个动态库？

Question

I'm currently struggling in developing a small application under Windows 10 which uses different libraries. First of all I'd like to say that I've been a professional developer for quite a few years but I've always developed on different environments (mostly GNU/Linux and MacOS) and when it comes to Windows for me it has always been more of a gaming OS than a working one; now that Windows 10 seems to be more open-source friendly I'd like to give it a shot and see if by combining WSL-2 and vcpkg I can have something working. So I thought that developing a simple cross-platform 2D application using the allegro library could have been a good idea.

I've created a new CMake project using Visual Studio 2019 Community Edition, and set-up a vcpkg clone inside the project itself (I don't feel like this is a good idea but I kind of liked the idea of having everything in a single place). Using vcpkg I installed the 2 libraries I'd wish to use: allegro and sqlite3. Both installations went fine and I'm actually quite surprised by vcpkg itself.

I setup my project structure: since this will be a modular project I started creating a bunch of shared libraries in CMake via the add_library(lib_name SHARED sources) function and I ended up having a structure similar to the following one:

project_root/CMakeLists.txt
project_root/src/main.cpp

project_root/core
project_root/core/CMakeLists.txt
project_root/core/include/core/bunch_of_headers.hpp
project_root/core/src/bunch_of_sources.cpp

project_root/model
project_root/model/CMakeLists.txt
project_root/model/include/model/bunch_of_headers.hpp
project_root/model/src/bunch_of_sources.cpp

and so on..

Every folder within the project_root contains a module, and all the CMakeLists file look like the following one:

add_library(model SHARED all_the_source_files.cpp)

include(DeclareNewSharedLibrary)
declare_new_shared_library(model)

find_package(sqlite3 CONFIG REQUIRED)
target_link_libraries(model sqlite3)

where declare_new_shared_library() is defined as the following function in a separate cmake module

function(declare_new_shared_library lib_name)
    message(STATUS "Declaring new library ${lib_name}")
    include(GenerateExportHeader)
    generate_export_header(${lib_name})

    target_include_directories(${lib_name}
        PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}/include"
        PUBLIC "${CMAKE_CURRENT_BINARY_DIR}"
    )

    set_target_properties(${lib_name} PROPERTIES
        IMPORTED_IMPLIB "${CMAKE_CURRENT_BINARY_DIR}/${lib_name}.lib"
        IMPORTED_LOCATION "${CMAKE_CURRENT_BINARY_DIR}/${lib_name}.dll"
    )
endfunction(declare_new_shared_library)

Nothing fancy so far.

Now, every module compiles fine, even when I link the shared library against a library which has been downloaded through vcpkg (as it happens in the example above). The ld on Windows correctly outputs the needed files ( .lib and .dll ) and copies them in the ${CMAKE_RUNTIME_OUTPUT_DIRECTORY} as expected.

The issue I'm having here, is that there is one library which doesn't compile. The CMake file is identical to the other ones, except that I'm linking against the allegro library (which has been installed via vcpkg like all the others). Here's the CMake file for comparison:

add_library(core SHARED all_the_sources.cpp)

include(DeclareNewSharedLibrary)
declare_new_shared_library(core)

find_package(allegro CONFIG REQUIRED)
target_link_libraries(core allegro)

When I try to compile this target (either directly or via the dependencies system) I get the following error

TL;DR:

C:\Dev\ConsultantLife\msvcrtd.lib(exe_main.obj) : error LNK2019: unresolved external symbol main referenced in function "int __cdecl invoke_main(void)" (?invoke_main@@YAHXZ)
C:\Dev\ConsultantLife\bin\core.dll : fatal error LNK1120: 1 unresolved externals

Here is the long version for the braves:

  Cleaning... 3 files.
  [1/2] C:\PROGRA~2\MICROS~1\2019\COMMUN~1\VC\Tools\Llvm\80D306~1.0\bin\clang-cl.exe  /nologo -TP -Dcore_EXPORTS -I..\..\..\core\include -Icore -I..\..\..\vcpkg\installed\x64-windows\include -m64 -fdiagnostics-absolute-paths  /DWIN32 /D_WINDOWS /W3 /GR /EHsc /MDd /Zi /Ob0 /Od /RTC1 /showIncludes /Focore\CMakeFiles\core.dir\Game.cpp.obj /Fdcore\CMakeFiles\core.dir\ -c ..\..\..\core\Game.cpp
  [2/2] cmd.exe /C "cd . && "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\CommonExtensions\Microsoft\CMake\CMake\bin\cmake.exe" -E vs_link_dll --intdir=core\CMakeFiles\core.dir --rc=C:\PROGRA~2\WI3CF2~1\10\bin\100183~1.0\x64\rc.exe --mt=C:\PROGRA~2\WI3CF2~1\10\bin\100183~1.0\x64\mt.exe --manifests  -- C:\PROGRA~2\MICROS~1\2019\COMMUN~1\VC\Tools\MSVC\1422~1.279\bin\Hostx64\x64\link.exe /nologo core\CMakeFiles\core.dir\Game.cpp.obj  /out:bin\core.dll /implib:core\core.lib /pdb:bin\core.pdb /dll /version:0.0 /machine:x64 /debug /INCREMENTAL  ..\..\..\vcpkg\installed\x64-windows\debug\lib\allegro-debug.lib kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib  && cmd.exe /C "cd /D C:\Dev\ConsultantLife\out\build\x64-Debug\core && powershell -noprofile -executionpolicy Bypass -file C:/Dev/ConsultantLife/vcpkg/scripts/buildsystems/msbuild/applocal.ps1 -targetBinary C:/Dev/ConsultantLife/out/build/x64-Debug/bin/core.dll -installedDir C:/Dev/ConsultantLife/vcpkg/installed/x64-windows/debug/bin -OutVariable out""
  FAILED: bin/core.dll core/core.lib 
  cmd.exe /C "cd . && "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\CommonExtensions\Microsoft\CMake\CMake\bin\cmake.exe" -E vs_link_dll --intdir=core\CMakeFiles\core.dir --rc=C:\PROGRA~2\WI3CF2~1\10\bin\100183~1.0\x64\rc.exe --mt=C:\PROGRA~2\WI3CF2~1\10\bin\100183~1.0\x64\mt.exe --manifests  -- C:\PROGRA~2\MICROS~1\2019\COMMUN~1\VC\Tools\MSVC\1422~1.279\bin\Hostx64\x64\link.exe /nologo core\CMakeFiles\core.dir\Game.cpp.obj  /out:bin\core.dll /implib:core\core.lib /pdb:bin\core.pdb /dll /version:0.0 /machine:x64 /debug /INCREMENTAL  ..\..\..\vcpkg\installed\x64-windows\debug\lib\allegro-debug.lib kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib  && cmd.exe /C "cd /D C:\Dev\ConsultantLife\out\build\x64-Debug\core && powershell -noprofile -executionpolicy Bypass -file C:/Dev/ConsultantLife/vcpkg/scripts/buildsystems/msbuild/applocal.ps1 -targetBinary C:/Dev/ConsultantLife/out/build/x64-Debug/bin/core.dll -installedDir C:/Dev/ConsultantLife/vcpkg/installed/x64-windows/debug/bin -OutVariable out""
  LINK Pass 1: command "C:\PROGRA~2\MICROS~1\2019\COMMUN~1\VC\Tools\MSVC\1422~1.279\bin\Hostx64\x64\link.exe /nologo core\CMakeFiles\core.dir\Game.cpp.obj /out:bin\core.dll /implib:core\core.lib /pdb:bin\core.pdb /dll /version:0.0 /machine:x64 /debug /INCREMENTAL ..\..\..\vcpkg\installed\x64-windows\debug\lib\allegro-debug.lib kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib /MANIFEST /MANIFESTFILE:core\CMakeFiles\core.dir/intermediate.manifest core\CMakeFiles\core.dir/manifest.res" failed (exit code 1120) with the following output:
     Creating library core\core.lib and object core\core.exp
C:\Dev\ConsultantLife\msvcrtd.lib(exe_main.obj) : error LNK2019: unresolved external symbol main referenced in function "int __cdecl invoke_main(void)" (?invoke_main@@YAHXZ)
C:\Dev\ConsultantLife\bin\core.dll : fatal error LNK1120: 1 unresolved externals
  ninja: build stopped: subcommand failed.

Now, from what I understand, the linker is trying to link my library as an executable and since it's unable to find a main function (yep, that's a library, darling..) it complains and gives an error.

The compilation of the other libraries in the project has exactly the same link.exe command (except, different libraries other than the system ones) and it doesn't fail..

Now I would like to ask: is there something I'm doing terribly wrong, is there a bug in allegro-5.2 (even though I can't see how a library could look for a main method and how the linker could.. well, you see my point) or should I give up everything?

Could it be an issue on the Allegro.cmake files? I have to admit I've never used CMake for professional projects (we're a bunch of oldies still using GNU Autotools&Co.) so I might have missed something. Here is the files I've created (copied/pasted from the other cmake files I found in the vcpkg folders)

### Allegro-config.cmake
include(${CMAKE_CURRENT_LIST_DIR}/Allegro-targets.cmake)


### Allegro-targets.cmake
# Generated by CMake

if("${CMAKE_MAJOR_VERSION}.${CMAKE_MINOR_VERSION}" LESS 2.5)
   message(FATAL_ERROR "CMake >= 2.6.0 required")
endif()
cmake_policy(PUSH)
cmake_policy(VERSION 2.6)
#----------------------------------------------------------------
# Generated CMake target import file.
#----------------------------------------------------------------

# Commands may need to know the format version.
set(CMAKE_IMPORT_FILE_VERSION 1)

# Protect against multiple inclusion, which would fail when already imported targets are added once more.
set(_targetsDefined)
set(_targetsNotDefined)
set(_expectedTargets)
foreach(_expectedTarget sqlite3)
  list(APPEND _expectedTargets ${_expectedTarget})
  if(NOT TARGET ${_expectedTarget})
    list(APPEND _targetsNotDefined ${_expectedTarget})
  endif()
  if(TARGET ${_expectedTarget})
    list(APPEND _targetsDefined ${_expectedTarget})
  endif()
endforeach()
if("${_targetsDefined}" STREQUAL "${_expectedTargets}")
  unset(_targetsDefined)
  unset(_targetsNotDefined)
  unset(_expectedTargets)
  set(CMAKE_IMPORT_FILE_VERSION)
  cmake_policy(POP)
  return()
endif()
if(NOT "${_targetsDefined}" STREQUAL "")
  message(FATAL_ERROR "Some (but not all) targets in this export set were already defined.\nTargets Defined: ${_targetsDefined}\nTargets not yet defined: ${_targetsNotDefined}\n")
endif()
unset(_targetsDefined)
unset(_targetsNotDefined)
unset(_expectedTargets)


# Compute the installation prefix relative to this file.
set(_IMPORT_PREFIX "${PROJECT_SOURCE_DIR}/vcpkg/installed/${VCPKG_TARGET_TRIPLET}")

# Create imported target allegro
add_library(allegro SHARED IMPORTED)

set_target_properties(allegro PROPERTIES
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"
)

# Load information for each installed configuration.
file(GLOB CONFIG_FILES "${CMAKE_CURRENT_LIST_DIR}/Allegro-targets-*.cmake")
foreach(f ${CONFIG_FILES})
  include(${f})
endforeach()

# Cleanup temporary variables.
set(_IMPORT_PREFIX)

# Loop over all imported files and verify that they actually exist
foreach(target ${_IMPORT_CHECK_TARGETS} )
  foreach(file ${_IMPORT_CHECK_FILES_FOR_${target}} )
    if(NOT EXISTS "${file}" )
      message(FATAL_ERROR "The imported target \"${target}\" references the file
   \"${file}\"
but this file does not exist.  Possible reasons include:
* The file was deleted, renamed, or moved to another location.
* An install or uninstall procedure did not complete successfully.
* The installation package was faulty and contained
   \"${CMAKE_CURRENT_LIST_FILE}\"
but not all the files it references.
")
    endif()
  endforeach()
  unset(_IMPORT_CHECK_FILES_FOR_${target})
endforeach()
unset(_IMPORT_CHECK_TARGETS)

# This file does not depend on other imported targets which have
# been exported from the same project but in a separate export set.

# Commands beyond this point should not need to know the version.
set(CMAKE_IMPORT_FILE_VERSION)
cmake_policy(POP)



### Allegro-targets-debug.cmake
#----------------------------------------------------------------
# Generated CMake target import file for configuration "Debug".
#----------------------------------------------------------------

# Commands may need to know the format version.
set(CMAKE_IMPORT_FILE_VERSION 1)

# Import target "allegro" for configuration "Debug"
set_property(TARGET allegro APPEND PROPERTY IMPORTED_CONFIGURATIONS DEBUG)
set_target_properties(allegro PROPERTIES
    IMPORTED_IMPLIB_DEBUG "${_IMPORT_PREFIX}/debug/lib/allegro-debug.lib"
    IMPORTED_LOCATION_DEBUG "${_IMPORT_PREFIX}/debug/bin/allegro-debug-5.2.dll"
  )

list(APPEND _IMPORT_CHECK_TARGETS allegro )
list(APPEND _IMPORT_CHECK_FILES_FOR_allegro 
    "${_IMPORT_PREFIX}/debug/lib/allegro-debug.lib" 
    "${_IMPORT_PREFIX}/debug/bin/allegro-debug-5.2.dll"
)

# Commands beyond this point should not need to know the version.
set(CMAKE_IMPORT_FILE_VERSION)

Since this behaviour has been driving me crazy for the last few days, I tested the same configuration on a Linux environment (vcpkg, cmake, ...) and it worked like a charm, I compiled everything and it worked. The only difference I see is that vcpkg on Linux uses only static libraries, while it uses dll on Windows.

Sorry for the wall of text!

Thank you for your help!

Answer 1

And as it usually happens, I've been able to find the answer by myself just a couple of minutes after asking for help.

It seems like the liballegro tries to find a main method, which is the reason why the linking was failing.

The solution was to add the following line to the Allegro-config.cmake file

target_compile_definitions(allegro INTERFACE ALLEGRO_NO_MAGIC_MAIN)