Cannot Call C++ Code from C without Error [duplicate] - c++

Question

This question already has answers here:
How to mix C++ and C correctly
(2 answers)
Closed 4 years ago.
I'm trying to write a C++ library that can be called from C. However, whenever I try to even write a bare minimum example, it crashes with undefined references. Here is my code:
mylibrary.h
#ifndef __MY_CPP_THING_H
#define __MY_CPP_THING_H
#ifdef __cplusplus
extern "C" {
#endif
void printSomething();
#ifdef __cplusplus
}
#endif
#endif
mylibrary.cpp
#include <iostream>
#include "mylibrary.h"
extern "C" {
void printSomething() {
std::cout << "PLEASE PRINT\n";
}
}
main.c
#include "mylibrary.h"
int main() {
printSomething();
return 0;
}
The compiling process goes something like this:
g++ -c mylibrary.cpp -o mylibrary.o (create "mylibrary.o")
ar rcs libmylibrary.a mylibrary.o (create static library "libmylibrary.a")
gcc main.c -L. -lmylibrary (link static library and compile C source file)
However, I receive this error dump:
mylibrary.o:mylibrary.cpp:(.text+0x17): 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*)'
mylibrary.o:mylibrary.cpp:(.text+0x32): undefined reference to `std::ios_base::Init::~Init()'
mylibrary.o:mylibrary.cpp:(.text+0x62): undefined reference to `std::ios_base::Init::Init()'
mylibrary.o:mylibrary.cpp:(.rdata$.refptr._ZSt4cout[.refptr._ZSt4cout]+0x0): undefined reference to `std::cout'
collect2.exe: error: ld returned 1 exit status
Any suggestions on how to resolve the error?

Answer 1

mylibrary.o still depends on C++ standard library and gcc doesn't know about it. Call gcc with -lstdc++ in the last step.

Answer 2

Creating a dynamic library instead of a static library should do the trick :
$ gcc -c main.c
$ g++ -fPIC -shared -o mylibrary.so mylibrary.cpp
$ gcc -o main main.o mylibrary.so
and then :
$ LD_LIBRARY_PATH=".:${LD_LIBRARY_PATH}" ./main
PLEASE PRINT
with :
$ objdump -p main | grep NEEDED
NEEDED mylibrary.so
NEEDED libc.so.6
$ objdump -p mylibrary.so | grep NEEDED
NEEDED libstdc++.so.6
NEEDED libc.so.6