How do you use extern "C" to call C++ functions in C

Question

So I've read about a dozen pages explaining how to do this, and for the life of me I can't get it to work.

I have a library written in C++, and a set of code written in C that is compiled with the gnu compiler, and I just can't seem to be able to call the C++ code from C.

I have a library written in C++:

printHeadOO.hpp

#ifdef __cplusplus
extern "C" 
#endif

void printThis(void);

printSrcOO.cpp

#include "printHeadOO.hpp"
#include <iostream>

void printThis(void){
    std::cout<<"This is Printed Using C++ \n";
}

okay cool, if I call this from a C++ program , it compile, runs and everything is happy. But suppose instead I call it from a C program:

printMain.c

#include <stdio.h>
#include "printHeadOO.hpp" 

int main(void){
    printf("This is printed from C Main \n");
    printThis();

    return 0;   
}

so everything compiles just fine with:

#g++ -c printSrcOO.cpp
#gcc -c printMain.c

but then at link-time:

#gcc printSrcOO.o printMain.o -o myProgram

gcc yaks up the following:

printSrcOO.o: In function `__static_initialization_and_destruction_0(int, int)':
printSrcOO.cpp:(.text+0x23): undefined reference to `std::ios_base::Init::Init()'
printSrcOO.o: In function `__tcf_0':
printSrcOO.cpp:(.text+0x6c): undefined reference to `std::ios_base::Init::~Init()'
printSrcOO.o: In function `printThis':
printSrcOO.cpp:(.text+0x83): undefined reference to `std::cout'
printSrcOO.cpp:(.text+0x88): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
printSrcOO.o:(.eh_frame+0x11): undefined reference to `__gxx_personality_v0'
collect2: ld returned 1 exit status

How do I get g++ to create hooks that gcc will use? Am I just doing something stupid/missing something?

TIA Dan

Answer 1

You can compile your C code with gcc and make use of those C++ functions, but you have to link the final executable with g++, because the C++ functions need the C++ standard library:

g++ printSrcOO.o printMain.o -o myProgram

Alternatively, you can try to manually link against the C++ library with gcc:

gcc printSrcOO.o printMain.o -o myProgram -lstdc++

But I'd stick to linking with g++ since I'm not sure whether it sets other important link flags.