CMake: Make a static library works but dynamic doesn't - c++

Question

I am trying to make a shared library using CMake, but its wired that if I make it statically, it works well, but dynamic doesn't:
CMAKE_MINIMUM_REQUIRED(VERSION 3.6)
PROJECT(TestDemo)
SET(CMAKE_CXX_STANDARD 11)
SET(CMAKE_CXX_COMPILER clang++)
# Headers
INCLUDE_DIRECTORIES(src)
INCLUDE_DIRECTORIES(src/xxx)
INCLUDE_DIRECTORIES(3rdparty/zstd)
INCLUDE_DIRECTORIES(/usr/local/include)
# CORE LIB
FILE(GLOB CORE_SRC src/xxx/*.cpp
src/xxx/io/*.cpp
src/xxx/util/*.cpp
src/xxx/thread/*.cpp)
LIST(REMOVE_ITEM CORE_SRC ${CMAKE_CURRENT_SOURCE_DIR}/src/xxx/io/BzipStream.cpp
${CMAKE_CURRENT_SOURCE_DIR}/src/xxx/io/GzipStream.cpp)
# This works
ADD_LIBRARY(xxx-core STATIC ${CORE_SRC})
# This doesn't
#ADD_LIBRARY(xxx-core SHARED ${CORE_SRC})
The error message is:
[100%] Linking CXX shared library libxxxdb-core.dylib
Undefined symbols for architecture x86_64:
"_mpool_get_global", referenced from:
xxx::mpoolxx<xxx::alloc_to_mpool_bridge<xxx::mpoolxx<long>, 1>::MemBlock>::get_vtab() in trb_cxx.cpp.o
"_sfixed_mpool_destroy", referenced from:
xxx::fixed_mpool_wrapper<24>::~fixed_mpool_wrapper() in trb_cxx.cpp.o
"_sfixed_mpool_init", referenced from:
xxx::fixed_mpool_wrapper<24>::fixed_mpool_wrapper() in trb_cxx.cpp.o
"_trb_destroy", referenced from:
xxx::trbstrmap_imp<int, unsigned char, &(xxx_trb_compare_less_tag), xxx::mpoolxx<long>, 0, 16>::~trbstrmap_imp() in trb_cxx.cpp.o
xxx::trbtab<int const, std::__1::pair<int const, int>, 0, &(xxx_trb_compare_less_tag), xxx::fixed_mpoolxx<long>, 0, 16>::~trbtab() in trb_cxx.cpp.o
"_trb_erase", referenced from:
xxx::trbtab<int const, std::__1::pair<int const, int>, 0, &(xxx_trb_compare_less_tag), xxx::fixed_mpoolxx<long>, 0, 16>::erase(int const&) in trb_cxx.cpp.o
xxx::trbtab<int const, std::__1::pair<int const, int>, 0, &(xxx_trb_compare_less_tag), xxx::fixed_mpoolxx<long>, 0, 16>::erase(xxx::trb_iterator<std::__1::pair<int const, int>, 16, 0>) in trb_cxx.cpp.o
"_trb_iter_first", referenced from:
xxx::trbtab<int const, std::__1::pair<int const, int>, 0, &(xxx_trb_compare_less_tag), xxx::fixed_mpoolxx<long>, 0, 16>::begin() const in trb_cxx.cpp.o
"_trb_iter_next", referenced from:
xxx::trb_iterator<std::__1::pair<int const, int>, 16, 0>::operator++() in trb_cxx.cpp.o
"_trb_probe", referenced from:
xxx::trbtab<int const, std::__1::pair<int const, int>, 0, &(xxx_trb_compare_less_tag), xxx::fixed_mpoolxx<long>, 0, 16>::insert(std::__1::pair<int const, int> const&) in trb_cxx.cpp.o
xxx::trbmap<int, int, &(xxx_trb_compare_less_tag), xxx::fixed_mpoolxx<long>, 0, 16>::operator[](int const&) in trb_cxx.cpp.o
"_trb_probe_node", referenced from:
xxx::trbstrmap_imp<int, unsigned char, &(xxx_trb_compare_less_tag), xxx::mpoolxx<long>, 0, 16>::probe_raw(char const*, unsigned long, char const*) in trb_cxx.cpp.o
"_trb_vtab_init", referenced from:
xxx::trbxx_vtab_init_by_cxx_type(trb_vtab*, field_type_t, int (*)(trb_vtab const*, trb_tree const*, void const*, void const*), int (*)(trb_vtab const*, trb_tree const*, void const*, void const*)) in trb_cxx.cpp.o
xxx::trbxx_vtab_init(trb_vtab*, field_type_t, int (*)(trb_vtab const*, trb_tree const*, void const*, void const*)) in trb_cxx.cpp.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Answer 1

No linkage happens in the creation of a static library because a static library
is not produced by the linker: it is simply an archive of object files created
with the archiver ar. So there can't
be any undefined references or other linkage errors when you create a static library,
just like there can't be if you create a .tar or .zip archive of object files.
See static-libraries,
A shared library, like a program, is produced by the linker and the Mac OS/Darwin
linker (unlike the GNU/Linux linker) by default does not permit undefined symbol
references in a shared library.
You have two options:
You can specify all the libraries that xxx-core depends upon to its linkage,
with target_link_libraries
Or, you can override the linker's default behaviour with the linkage option
-undefined=dynamic_lookup, indicating
that undefined references in the shared library are to be resolved by the loader at runtime.
In your CMakeLists.txt, use set_target_properties
after you have defined the xxx-core target:
ADD_LIBRARY(xxx-core SHARED ${CORE_SRC})
SET_TARGET_PROPERTIES(xxx-core LINK_FLAGS Wl,-undefined=dynamic_lookup)