How to add "-l" (ell) compiler flag in CMake

Work on Ubuntu 16

I used g++ main.cpp -lpq command for compiler my small project. Now I use Clion and wanna do same what I do with g++. But I can't add compiler flags in cmake file and get compile error.

cmake_minimum_required(VERSION 3.5.1)
project(day_g)

set(CMAKE_CXX_FLAGS "-lpq")

add_definitions(-lpq)

message("CMAKE_CXX_FLAGS is ${CMAKE_CXX_FLAGS}")

set(CMAKE_CXX_STANDARD 11)

set(SOURCE_FILES main.cpp)
add_executable(day_g ${SOURCE_FILES})

Also I run only cmake file and get CMAKE_CXX_FLAGS with -lpq flag.

CMAKE_CXX_FLAGS is -lpq
-- Configuring done
-- Generating done

How properly add compiler flags to cmake file?

Flag -l is for linker, not for compiler. This flag is used for link with libraries. CMake has special command target_link_libraries for that purpose:

target_link_libraries(day_g pq)

-lq is not a compiler flag (CFLAGS) but a linker flag.

To pass a library in a CMake project you should use:

target_link_libraries(target_name libraries...)

Note that if you specify 'q' as library the project will link with libq.a or, if you are on windows q.dll.

... in your CMakeLists.txt the correct line to add is:

target_link_libraries(day_g pq)

Note also that when you add a CFLAG you should also "remember" the previous ones that may be added by libraries or by your platform, ie:

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O3")

To check the exact flags cmake is passing to compiler or linker you can always run, from the build directory, the following command:

make VERBOSE=1