Linking a C++ exe to a C lib in cmake - cmake

Question

I thought I understood cmake pretty well until I came accross this problem that I just can't figure out. I've built a static library in C, and I'm trying to run a unit test on it in C++, but I can't seem to link to any of the static functions in that library and I can't figure out why. I've reproduced the problem in the skeleton project below:
CMakeLists.txt:
cmake_minimum_required(VERSION 3.6)
project(myproj)
add_library(mylib STATIC mylib.c)
find_package(Boost 1.64.0 COMPONENTS unit_test_framework)
add_executable(mytestapp mytest.cpp)
target_include_directories(mytestapp PRIVATE .)
target_link_libraries(mytestapp mylib)
enable_testing()
add_test( mytest mytestapp)
mylib.c:
int add(int a, int b)
{
return a + b;
}
mylib.h
int add(int a, int b);
mytest.cpp:
#define BOOST_TEST_MODULE mylib_test
#include <boost/test/included/unit_test.hpp>
#include "mylib.h"
BOOST_AUTO_TEST_CASE(mylib_test)
{
BOOST_TEST( add(2,2) == 4);
}
Then my output is:
$ make
Scanning dependencies of target mylib
[ 25%] Building C object CMakeFiles/mylib.dir/mylib.c.o
[ 50%] Linking C static library libmylib.a
[ 50%] Built target mylib
Scanning dependencies of target mytestapp
[ 75%] Building CXX object CMakeFiles/mytestapp.dir/mytest.cpp.o
[100%] Linking CXX executable mytestapp
CMakeFiles/mytestapp.dir/mytest.cpp.o: In function `mylib_test::test_method()':
mytest.cpp:(.text+0x1e411): undefined reference to `add(int, int)'
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/mytestapp.dir/build.make:96: mytestapp] Error 1
make[1]: *** [CMakeFiles/Makefile2:105: CMakeFiles/mytestapp.dir/all] Error 2
make: *** [Makefile:95: all] Error 2
If I compile mylib in C++, then it links fine, but not in C. That's a problem for me because I have a huge library in C and I'm trying to use the boost_test_framework (in C++) to load this lib and test it.

Answer 1

Solution:
I needed to import the library with extern "C" like so:
#define BOOST_TEST_MODULE mylib_test
#include <boost/test/included/unit_test.hpp>
extern "C" {
#include "mylib.h"
}
BOOST_AUTO_TEST_CASE(mylib_test)
{
BOOST_TEST( add(2,2) == 4);
}