I have this simplified file structure:

Component
|----hdr/
      |-------sut.h
|----src/
      |-------sut.c
|---test/
      |-------TestMain.cpp
      |-------CMakeLists.txt
      |-------build/

Now my TestMain.cpp includes sut.h (subject under test) and performs function calls to functions declared in sut.h.
This is my CMakeLists.txt file:

cmake_minimum_required(VERSION 3.24)
project(CppTest)

set(PROJECT_SOURCES
  ${CMAKE_CURRENT_SOURCE_DIR}/../../../Common/common/src/file.c
  ${CMAKE_CURRENT_SOURCE_DIR}/../src/sut.c
)

# GoogleTest requires at least C++14
set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

include(FetchContent)
FetchContent_Declare(
  googletest
  URL https://github.com/google/googletest/archive/03597a01ee50ed33e9dfd640b249b4be3799d395.zip
)
# For Windows: Prevent overriding the parent project's compiler/linker settings
#set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
FetchContent_MakeAvailable(googletest)

add_executable(CppTest ${CMAKE_CURRENT_SOURCE_DIR}/TestMain.cpp)
target_sources(CppTest PUBLIC ${PROJECT_SOURCES})

target_include_directories(CppTest
  PUBLIC
  ${CMAKE_CURRENT_SOURCE_DIR}/../hdr                        # include sut.h
  ${CMAKE_CURRENT_SOURCE_DIR}/../../../Database/hdr/        # needed for sut.c
  ${CMAKE_CURRENT_SOURCE_DIR}/../../../Common/common/hdr/   # needed for sut.c
)

target_link_libraries(CppTest GTest::gtest_main)

Now everything compiles fine, however I get a linker error that a function defined in sut.h and used in TestMain.cpp has an undefined reference. But I added its source sut.c in PROJECT_SOURCES so sut.c should be linked to TestMain.cpp – right?

What am I doing wrong here?