Linking C and CXX files in CMake

I'm building C++ app with CMake. But it uses some source files in C. Here is simplified structure: trunk/CMakeLists.txt: ``` project(myapp) set(CMAKE_CXX_FLAGS "${CMAKE_C_FLAGS} -g -Wall") add_subdirectory (src myapp) ``` trunk/src/main.cpp: ``` #include "smth/f.h" int main() { f(); } ``` trunk/src/CMakeLists.txt: ``` add_subdirectory (smth) link_directories (smth) set(APP_SRC main) add_executable (myapp ${APP_SRC}) target_link_libraries (myapp smth) ``` trunk/src/smth/f.h: ``` #ifndef F_H #define F_H void f(); #endif ``` trunk/src/smth/f.c: ``` #include "f.h" void f() {} ``` trunk/src/smth/CMakeLists.txt ``` set (SMTH_SRC some_cpp_file1 some_cpp_file2 f) add_library (smth STATIC ${SMTH_SRC}) ``` The problem is: i run gmake, it compiles all the files and when it links all libs together, i get: ``` undefined reference to `f()` in main.cpp ``` if i rename f.c into f.cpp everything goes just fine. What's the difference and how to handle it? Thanks