Q:

How to use FFTW library in cmake?

I have C++ code which uses FFTW 3.3.4. Ubuntu 16.04, cmake version 3.7.2
$ locate *fftw*.so

/usr/lib/libsfftw.so
/usr/lib/libsfftw_mpi.so
/usr/lib/libsfftw_threads.so
/usr/lib/libsrfftw.so
/usr/lib/libsrfftw_mpi.so
/usr/lib/libsrfftw_threads.so
/usr/lib/x86_64-linux-gnu/libfftw3.so
/usr/lib/x86_64-linux-gnu/libfftw3_mpi.so
/usr/lib/x86_64-linux-gnu/libfftw3_omp.so
/usr/lib/x86_64-linux-gnu/libfftw3_threads.so
/usr/lib/x86_64-linux-gnu/libfftw3f.so
/usr/lib/x86_64-linux-gnu/libfftw3f_mpi.so
/usr/lib/x86_64-linux-gnu/libfftw3f_omp.so
/usr/lib/x86_64-linux-gnu/libfftw3f_threads.so
/usr/lib/x86_64-linux-gnu/libfftw3l.so
/usr/lib/x86_64-linux-gnu/libfftw3l_mpi.so
/usr/lib/x86_64-linux-gnu/libfftw3l_omp.so
/usr/lib/x86_64-linux-gnu/libfftw3l_threads.so
/usr/lib/x86_64-linux-gnu/libfftw3q.so
/usr/lib/x86_64-linux-gnu/libfftw3q_omp.so
/usr/lib/x86_64-linux-gnu/libfftw3q_threads.so

$ locate fftw3.h
/usr/include/fftw3.h

I can compile it in this way:
g++ main.cpp -o main -lfftw3

but I have a problem with cmake.
This is my CMakeLists.txt:
cmake_minimum_required(VERSION 3.5.1)
project (main)
SET(CMAKE_C_COMPILER gcc)
SET(CMAKE_CXX_COMPILER g++)
file(GLOB SOURCES "*.cpp") 
SET(CMAKE_CXX_FLAGS "-lm -lfftw3")
SET(CMAKE_C_FLAGS   "-lm -lfftw3")
INCLUDE_DIRECTORIES(/usr/include)
LINK_DIRECTORIES(/usr/lib/x86_64-linux-gnu)
add_library(fftw3 STATIC IMPORTED)
set(CMAKE_C_OUTPUT_EXTENSION_REPLACE 1) 
set(CMAKE_CXX_OUTPUT_EXTENSION_REPLACE 1)
add_executable(main ${SOURCES})

cmake . && make

gives 
undefined reference to `fftw_malloc'

and the same for the other fftw functions.

A:

The command add_library will create a library in your project (CMake -
 add_library). I assume that is not what you want. 
The command: g++ main.cpp -o main -lfftw3 will link the executable to the fftw library. In CMake you can reproduce the linking with:
add_executable(main ${SOURCES})
target_link_libraries(main fftw3)

Docu: CMake - target_link_libraries
Notice: It is important that the add_executable command comes before the linking.
Have fun with FFTW :)

