Can't link a C shared object to a C++ program - c++

Question

I am trying to write a makefile for a project. The project involves a test program that defines a main function written in C++11 that is supposed to call a shared object library written in c99 and run some tests.
My makefile successfully compiles the c99 library and produces "libhanoi.so".
When I try to link the C99 library to the C++11 part, I get the following error:
g++ -std=gnu++11 -L. -lhanoi -o main tests/testing.cpp tests/main.cpp
/tmp/cctHQTcW.o: In function `binarion_constructor(unsigned long*)':
main.cpp:(.text+0x28): undefined reference to `binarion64_t'
collect2: error: ld returned 1 exit status
Makefile:29: recipe for target 'tests' failed
make: *** [tests] Error 1
However, the output of "nm -C libhanoi.so" shows that the binarion64_t function is being exported by libhanoi.so:
0000000000000610 T binarion64_t(long long, long long)
When I introduce a typo into the name of libhanoi.so, it introduces an error saying it can't find libhanoi.so.
So it must be able to find libhanoi.so and libhanoi.so is exporting the unimplemented function in main.cpp, yet it still is giving an undefined reference. What's going on?
Minimal example:
hanoi.h:
#ifndef HANOI_H
#define HANOI_H
#include <inttypes.h>
// binarion (base -1+i) constructor
uint64_t binarion64_t(long long A, long long B);
#endif // HANOI_H
binarion.c:
#include "hanoi.h"
uint64_t binarion64_t(long long A,long long B){
return 0;
}
main.cpp:
#include <stdio.h>
extern "C" {
#include "hanoi.h"
};
uint64_t binarion_constructor(uint64_t * args){
return binarion64_t(args[0], args[1]);
}
int main(void){
return 0;
}
Compile:
g++ -std=c99 -c binarion.c
g++ -std=c99 -shared -o libhanoi.so binarion.o -lm
g++ -std=gnu++11 -L. -lhanoi -o main main.cpp
output:
/tmp/ccjoRmCg.o: In function `binarion_constructor(unsigned long*)':
main.cpp:(.text+0x28): undefined reference to `binarion64_t'
collect2: error: ld returned 1 exit status
EDIT:
The commands I'm running are:
gcc -std=c99 -c binarion.c
gcc -std=c99 -shared -o libhanoi.so binarion.o -lm
g++ -std=gnu++11 -L. -lhanoi -o main main.cpp
The files are exactly the ones in the question. The output of "readelf -s libhanoi.so | grep binarion" is:
12: 0000000000000660 19 FUNC GLOBAL DEFAULT 11 binarion64_t
33: 0000000000000000 0 FILE LOCAL DEFAULT ABS binarion.c
46: 0000000000000660 19 FUNC GLOBAL DEFAULT 11 binarion64_t
and the output of "g++ -std=gnu++11 -L. -lhanoi -o main main.cpp" is:
/tmp/cczfgY8M.o: In function `binarion_constructor(unsigned long*)':
main.cpp:(.text+0x28): undefined reference to `binarion64_t'
collect2: error: ld returned 1 exit status

Answer 1

TL; DR:
Use:
gcc -std=c99 -c binarion.c
gcc -std=c99 -shared -o libhanoi.so binarion.o -lm
g++ -std=gnu++11 -L. -lhanoi -o main main.cpp
Explanation:
You should be using gcc to compile a C file, g++ is for C++. When you do g++ -std=c99 -c binarion.c the compiler gives you a hint with:
cc1plus: warning: command line option ‘-std=c99’ is valid for C/ObjC but not for C++
This means that you end up compiling your library as a C++ library. You can verify that by calling readelf -s libhanoi.so | grep binarion:
9: 00000000000005da 19 FUNC GLOBAL DEFAULT 9 _Z12binarion64_txx
29: 0000000000000000 0 FILE LOCAL DEFAULT ABS binarion.c
44: 00000000000005da 19 FUNC GLOBAL DEFAULT 9 _Z12binarion64_txx
As you can see, the function has been name-mangled, which is something C++ does, and C doesn't.
However, when compiling main.cpp you tell the compiler that binarion_t has C linkage:
extern "C" {
#include "hanoi.h"
};
So it is searching for unmangled binarion_t (instead of _Z12binarion64_txx).