C and C++ linkage with extern "C" - c++

Question

I have a C++ function defined in a .h file as follows and implemented in a .cpp file:
extern "C" void func(bool first, float min, float* state[6], float* err[6][6])
{
//uses vectors and classes and other C++ constructs
}
How can I call func in a C file? How do I set up my file architecture / makefile to compile this?
Thanks!

Answer 1

You call the function from C in the normal way. However, you need to wrap the extern "C" in an preprocessor macro to prevent the C compiler from seeing it:
#ifndef __cplusplus
extern "C"
#endif
void func(bool first, float min, float* state[6], float* err[6][6]);
Assuming you're working with GCC, then compile the C code with gcc, compile the C++ code with g++, and then link with g++.

Answer 2

To call it in C, all you need to do is call it normally. Because you told the compiler to use the C calling conventions and ABI with extern "C", you can call it normally:
func(args);
To compiler, use this for the C++:
g++ -c -o myfunc.o myfunc.cpp
Then this for the C:
gcc -c -o main.o somec.c
Than link:
g++ -o main main.o myfunc.o
Make sure that the C++ header for the function uses ONLY C CONSTRUCTS. So include things like <vector> in the .cpp file instead.

Answer 3

call it in C using
func(/* put arguments here */);
By saying extern "C" you are asking the compiler not to mangle your names. Otherwise, C++ compiler would tend to mangle them (i.e. add additional symbols to make them unique) before the linker.
You'll also want to make sure you have setup to use C calling convention.

Answer 4

//header file included from both C and C++ files
#ifndef __cplusplus
#include <stdbool.h> // for C99 type bool
#endif
#ifdef __cplusplus
extern "C" {
#endif
void func(bool first, float min, float* state[6], float* err[6][6]);
#ifdef __cplusplus
} // extern "C"
#endif
// cpp file
#include "the_above_header.h"
#include <vector>
extern "C" void func(bool first, float min, float* state[6], float* err[6][6]);
{
//uses vectors and classes and other C++ constructs
}
// c file
#include "the_above_header.h"
int main() {
bool b;
float f;
float *s[6];
float *err[6][6];
func(b,f,s,err);
}