CMake undefined symbol: pthread_create compiling dynamic library - c++

Question

My project was working fine before but I changed it to be compiled as a dynamic library in order to have it perform self-update, like so:
Launcher -> Executable
Server -> Library (Core project, multi-threaded with std::thread)
Updater -> Library
Now, on Linux (Debian Bullseye), I get a runtime error undefined symbol: pthread_create when the launcher tries to dlopen the Server library. Windows equivalent works fine.
I've tried splitting my big CMakeLists in several subprojects, thinking it could be a CMake bug, but that was of course not the problem.
I've included below a simplified version of the parent CMakeLists and the subproject CMakeLists for the Server and the launcher.
Parent:
################################
# Project settings
################################
cmake_minimum_required (VERSION 3.8)
set(TARGET_NAME "Server")
set(SERVER_VERSION "1.0.0")
set(CMAKE_TRY_COMPILE_TARGET_TYPE "STATIC_LIBRARY")
project(${TARGET_NAME} VERSION ${SERVER_VERSION} DESCRIPTION "My Server")
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
################################
# Sources
################################
configure_file(
${CMAKE_SOURCE_DIR}/src/Resource.h
${CMAKE_CURRENT_BINARY_DIR}/src/Resource.h
COPYONLY)
configure_file(
${CMAKE_SOURCE_DIR}/src/Resource.rc.in
${CMAKE_CURRENT_BINARY_DIR}/src/Resource.rc
#ONLY)
set_source_files_properties("../Server.ico" PROPERTIES LANGUAGE RC)
include_directories("includes")
################################
# Subprojects
################################
add_subdirectory("src/Server")
add_subdirectory("src/Launcher")
add_subdirectory("src/Updater")
Server lib Subproject:
################################
# Project settings
################################
cmake_minimum_required (VERSION 3.8)
set(TARGET_NAME "Server-Core")
if (LIB)
set(EXE_NAME "ServerCore")
else ()
set(EXE_NAME "Server")
endif ()
project(${TARGET_NAME} VERSION ${SERVER_VERSION} DESCRIPTION "My Server")
################################
# Sources
################################
configure_file(
Server.hh.in
Server.hh
#ONLY)
include_directories(. "${CMAKE_BINARY_DIR}/src/Server/")
file(GLOB_RECURSE SRC "*.hh" "*.hpp" "*.cpp" "${CMAKE_BINARY_DIR}/src/*.hh")
if (LIB)
add_library(${TARGET_NAME} SHARED ${SRC})
elseif (WIN32)
add_executable(${TARGET_NAME} ${SRC} ${CMAKE_BINARY_DIR}/src/Resource.rc)
else ()
add_executable(${TARGET_NAME} ${SRC})
endif ()
################################
# Libs
################################
set(THREADS_PREFER_PTHREAD_FLAG ON)
find_package(Threads REQUIRED)
target_link_libraries(${TARGET_NAME} Threads::Threads)
if(WIN32)
target_link_libraries(${TARGET_NAME} wsock32 ws2_32)
target_link_libraries(${TARGET_NAME} Crypt32)
target_link_libraries(${TARGET_NAME} ${CMAKE_SOURCE_DIR}/libs/libssl.lib)
target_link_libraries(${TARGET_NAME} ${CMAKE_SOURCE_DIR}/libs/libcrypto.lib)
elseif(UNIX)
target_link_libraries(${TARGET_NAME} -static-libgcc -static-libstdc++)
target_link_libraries(${TARGET_NAME} ${CMAKE_SOURCE_DIR}/libs/libssl.a)
target_link_libraries(${TARGET_NAME} ${CMAKE_SOURCE_DIR}/libs/libcrypto.a)
endif()
################################
# Compiler settings
################################
set_target_properties(${TARGET_NAME} PROPERTIES VERSION ${SERVER_VERSION})
set_target_properties(${TARGET_NAME} PROPERTIES PREFIX "")
set_property(TARGET ${TARGET_NAME} PROPERTY CXX_STANDARD 20)
set_property(TARGET ${TARGET_NAME} PROPERTY CXX_STANDARD_REQUIRED ON)
set_property(TARGET ${TARGET_NAME} PROPERTY OUTPUT_NAME ${EXE_NAME})
if(MSVC)
target_compile_options(${TARGET_NAME} PUBLIC /std:c++latest)
target_compile_options(${TARGET_NAME} PUBLIC /Zc:__cplusplus)
set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} /MANIFEST:NO")
endif()
Launcher subproject:
################################
# Project settings
################################
cmake_minimum_required (VERSION 3.8)
set(LAUNCHER_TARGET "Launcher")
set(LAUNCHER_EXE_NAME "Server")
set(LAUNCHER_VERSION "1.0.0")
project(${LAUNCHER_TARGET} VERSION ${LAUNCHER_VERSION} DESCRIPTION "My Server")
################################
# Sources
################################
if (LIB)
file(GLOB_RECURSE SRC "*.hh" "*.hpp" "*.cpp"
"../Server/Utils/DynamicLibrary/DynamicLibraryWindows.cpp" "../Server/Utils/DynamicLibrary/DynamicLibraryLinux.cpp"
"../Server/Utils/DynamicLibrary/IDynamicLibrary.cpp")
if (WIN32)
add_executable(${LAUNCHER_TARGET} ${SRC} ${CMAKE_BINARY_DIR}/src/Resource.rc)
else ()
add_executable(${LAUNCHER_TARGET} ${SRC})
endif ()
endif ()
################################
# Libs
################################
if (UNIX)
if (LIB)
target_link_libraries(${LAUNCHER_TARGET} ${CMAKE_DL_LIBS})
endif ()
endif ()
################################
# Compiler settings
################################
set(THREADS_PREFER_PTHREAD_FLAG ON)
find_package(Threads REQUIRED)
target_link_libraries(${LAUNCHER_TARGET} Threads::Threads)
if (LIB)
set_target_properties(${LAUNCHER_TARGET} PROPERTIES VERSION ${LAUNCHER_VERSION})
set_property(TARGET ${LAUNCHER_TARGET} PROPERTY CXX_STANDARD 20)
set_property(TARGET ${LAUNCHER_TARGET} PROPERTY CXX_STANDARD_REQUIRED ON)
set_property(TARGET ${LAUNCHER_TARGET} PROPERTY OUTPUT_NAME ${LAUNCHER_EXE_NAME})
endif ()
I'm using CMake 3.16.3 and gcc 9.3.0
What did I do wrong? I'm still not extremely comfortable with CMake

Answer 1

I have been able to fix the problem by adding
target_link_libraries(${TARGET} pthread)
after
target_link_libraries(${TARGET} Threads::Threads)
for the Launcher, when compiling for Linux.
And I need to include <thread> in the launcher and have a piece of code such as
std::thread t([](){});
t.join();
This is extremely weird