Integrating another program into existing program compiled with CMake - c

Question

I'm trying to integrate a side program (Program B) into an existing program (Program A) compiled/built with CMake. Currently CMake handles finding all the resources for and compiling Program A. I would like to include a couple .h files that Program B points to, so I can embed some of Program B's .c code in Program A.
I've tried playing around with one of the CMakeLists.txt files and even added some .c files to the add_library() block, but I still receive an "undefined reference to THIS_FUNCTION" error on compile.
Thank you for your time!
EDIT: Here's part of the CMakeList.txt file I updated on the RPi
add_library(rtlsdr_shared SHARED
librtlsdr.c
tuner_e4k.c
tuner_fc0012.c
tuner_fc0013.c
tuner_fc2580.c
tuner_r82xx.c
gpu_fft.c #Added this and a couple other .c files
)
target_link_libraries(rtlsdr_shared
${LIBUSB_LIBRARIES}
)
EDIT 2:
Proj A => rtl-sdr
Proj B => gpu_fft
rtl-sdr/
CMakeLists.txt
build/
cmake/
include/
m4/
src/
CMakeLists.txt
gpu_fft/
makefile

Answer 1

I think you better should build program B as a shared library, and add to the include_directories of program A the includes of program B. Then target_link_libraries to your program A.
Edit:
What I have in mind is a project with this folder structure:
projA/
CMakeLists.txt
include/
src/
B/
CMakeLists.txt
include/
src/
The CMakeLists.txt in B is very classical and build the shared library libB.so (e.g. on linux, .dll on WIN32).
In the CMakeLists.txt of projA folder put:
# first build project B
add_subdirectory(B)
# add include directory of project B
include_directories("${CMAKE_CURRENT_SOURCE_DIR}/B/include")
# [...] build here your project
# and finally:
target_link_libraries(A B)
I hope it helps
EDIT2:
To build shared libraries you can set:
OPTION(BUILD_SHARED_LIBS TRUE)
or simply
SET(BUILD_SHARED_LIBS TRUE)
EDIT 3:
You can also use it simply as an external library. Set libraries search path first:
LINK_DIRECTORIES(${yourPathToLibB})
Don't forget to
include_directories("${yourPathToLibB}/include")
too. And then just do
TARGET_LINK_LIBRARIES(A B)