Question

I added the line extern "C" void perlinTest(void); to a C++ header along with the include of the c header file hoping that was all I needed but the compiler complains:

Undefined symbols for architecture i386:
  "perlinTest()", referenced from:
      CreateRenderer3(IResourceManager*) in Renderer.o

Answer 1

Your C++ code needs to be aware that the function is a C function. To do so, you need to declare it this way:

extern "C" [prototype];

A realistic example for your situation would be:

extern "C" void perlinTest();

The reason for this is that C++ function names are mangled to something that tells about the types of the parameters. At the lowest level, this is what allows overloading: it never really is legal to have two visible symbols that share the same name, so C++ allows them by embedding markers that indicate the types of the parameters in the function names. For instance, void perlinTest() gets mangled as _Z10perlinTestv on my Lion box with g++ (and probably clang++), though this is ABI-specific and will not necessarily be the same on other platforms.

However, C doesn’t support overloading, and functions aren’t subject to name mangling, so when your C++ code tries to call one, it needs to know that it must not use a mangled name. This is what extern "C" tells the compiler.

If your header files need to be readable from both C and C++, the common practice is to wrap them in an extern "C" block (extern "C" { /* declarations */ }) itself wrapped in an #ifdef __cplusplus preprocessor directive (so the C code doesn’t see the extern "C" code).

#ifdef __cplusplus
extern "C" {
#endif

/* header body */

#ifdef __cplusplus
}
#endif