Calling C from C++

extern_C.cpp

#include <iostream>

extern "C" int sum(int, int);

int main()
{
    std::cout << sum(1, 2) << "\n";
}

Makefile

CC = clang
CFLAGS = -Wall -Wextra

CXX = clang++
CXXFLAGS = $(CFLAGS) -std=c++11

extern_C: extern_C.o sum.o
	$(CXX) $(CXXFLAGS) $^ -o $@

clean:
	rm -f extern_C extern_C.o sum.o

.PHONY: clean

sum.c

int sum(int a, int b)
{
    return a + b;
}