Calling C code from C++ in a CMake project. Undefined symbol. Have extern C

Question

I\'m trying to build a CMake project that calls C code from C++, and I\'m getting undefined symbols, even though I\'m (AFAIK) properly using \"extern C\".

CMakeLists.txt

Answer 1

extern "C" is C++ syntax. Your header lib.h therefore cannot be used from C. If you change it as follows it can be used from C++ and C as well.

#ifndef LIB_H_HEADER
#define LIB_H_HEADER

#ifdef __cplusplus
extern "C" 
{
#endif

int printit();

#ifdef __cplusplus
}
#endif

#endif /* LIB_H_HEADER */

As you have both C and CXX sources your project call should enable C as well project(CTest LANGUAGES C CXX) in your CMakeLists.txt.