How do I compile and link C++ code with compiled C code?

Question

I want to be able to use Cmockery to mock C functions called from C++ code I'm testing. As a step towards that, I've renamed the Cmockery example run_tests.c to run_tests.cpp, and am attempting to compile and link it with cmockery.c:

g++ -m32 -DHAVE_CONFIG_H -DPIC -I ../cmockery-0.1.2 -I /usr/include/malloc -c run_tests.cpp -o obj/run_tests.o
gcc -m32 -DHAVE_CONFIG_H -DPIC -Wno-format -I ../cmockery-0.1.2 -I /usr/include/malloc -c ../cmockery-0.1.2/cmockery.c -o obj/cmockery.o
g++  -m32 -o run_tests obj/run_tests.o obj/cmockery.o

The first two command lines (to compile) are successful, but after the last I get:

Undefined symbols:
  "_run_tests(UnitTest const*, unsigned long)", referenced from:
      _main in run_tests.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

That undefined symbol is from line 29 of run_tests.cpp:

return run_tests(tests);

The run_tests() function is defined in cmockery.c.

After reading "Linking C++ code with 'gcc' (without g++)", I tried:

gcc -lstdc++ -m32 -o run_tests obj/run_tests.o obj/cmockery.o

But got the same result:

Undefined symbols:
  "_run_tests(UnitTest const*, unsigned long)", referenced from:
      _main in run_tests.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

How do I compile and link C++ code so it finds the symbols in C code?

Answer 1

I think that you can get thinkgs to link from C++ by adding the following around the contents of the cmockery.h file:

At or near the beginning:

#if defined(__cplusplus)
extern "C" {
#endif

At or near the end:

#if defined(__cplusplus)
}
#endif

That way, use of the header in C sources will ignore the extern "C" part of the declaration, but when the header is include in C++ builds, the compiler will be properly told that the linkage for the declarations in that header use C semantics.

For a quick-n-dirty test or if you'd rather not modify the header, you can try:

extern "C" {
#include "cmockery.h"
}

but my preference would be to put the extern "C" block in the header (and only around the stuff that's required - that might need a bit of analysis).

Answer 2

In your header files included by your C++ code, you need extern "C" declarations for all the functions that are compiled in C.