Boost.Log with CMake causing undefined reference error

For CMake 3.15 (and some earlier versions) the following appears to be sufficient to build Boost.Log with CMake without the original linker error:

cmake_minimum_required(VERSION 3.15)
project(boost_log_tutorial)

SET(Boost_USE_STATIC_LIBS ON)           # link statically
#ADD_DEFINITIONS(-DBOOST_LOG_DYN_LINK)  # or, link dynamically

find_package(Boost 1.69.0 COMPONENTS log REQUIRED)

add_executable(boost_log_tutorial main.cpp)
target_link_libraries(boost_log_tutorial Boost::log_setup Boost::log)

The key things seem to be linking against Boost:log_setup and Boost::log, and specifying either static or dynamic linking with SET(Boost_USE_STATIC_LIBS ON) or ADD_DEFINITIONS(-DBOOST_LOG_DYN_LINK).

It looks like it boils down to linking to the shared version of Boost.Log.

There is a bit of detail on the issue in the docs for Boost.Log Your error message mentions the namespace boost::log::v2s_mt_posix and from the docs, this implies the linker is expecting to link to a static version of Boost.Log.

If you want to link to the shared version, it seems you need to define BOOST_LOG_DYN_LINK or BOOST_ALL_DYN_LINK, i.e. in your CMakeLists.txt add:

ADD_DEFINITIONS(-DBOOST_LOG_DYN_LINK)

If you want to link to the static version of Boost.Log, instead you need to add a CMake variable before calling FIND_PACKAGE(Boost ...):

SET(Boost_USE_STATIC_LIBS ON)
FIND_PACKAGE(Boost 1.54 COMPONENTS log REQUIRED)

For further variables which affect how CMake finds Boost, see the docs for FindBoost.