https://stackoverflow.com/questions/55191553/what-actually-happens-when-two-shared-libraries-define-the-same-symbol

_same_symbol_in_so.cpp

void PrintA(void);
void PrintB(void);

int main(int argc, char *argv[])
{
  PrintA();
  PrintB();
  return 0;
}

command.sh

g++ libA.cpp -fPIC -shared -o libA.so
g++ libB.cpp -fPIC -shared -o libB.so
# https://www.cprogramming.com/tutorial/shared-libraries-linux-gcc.html
g++ -L/home/xq -Wl,-rpath=/home/xq -Wall -o a.out tt.cpp -lA -lB

# ./a.out
Constructor
Constructor
LibA:0x7f21c8087049 0x7f21c808417a 1
LibB:0x7f21c8087049 0x7f21c808417a 1
Destructor
Destructor

The results above means only only Data instance, but constructor and destructor has been run twice.

data_.h

#ifndef SHARED_HEADER_H_
#define SHARED_HEADER_H_

#include <iostream>

struct Data {
  Data(void) {std::cout << "Constructor" << std::endl;}
  ~Data(void) {std::cout << "Destructor" << std::endl;}
  int FuncDefinedByLib(void) const;
};

extern const Data data;

#endif

libA.cpp

#include "data_.h"

const Data data;

int Data::FuncDefinedByLib(void) const {return 2;}

void PrintA(void) {
  std::cout << "LibA:" << &data << " "
    << (void*)&Data::FuncDefinedByLib <<  " "
    << data.FuncDefinedByLib() << std::endl;
}

libB.cpp

#include "data_.h"

const Data data;

int Data::FuncDefinedByLib(void) const {return 2;}

void PrintB(void) {
  std::cout << "LibB:" << &data << " "
    << (void*)&Data::FuncDefinedByLib <<  " "
    << data.FuncDefinedByLib() << std::endl;
}

• Each process has its own address space, meaning that there is never any memory being shared between processes (unless you use some inter-process communication library or extensions).
• The One Definition Rule (ODR) still applies, meaning that you can only have one definition of the global variable visible at link-time (static or dynamic linking).

https://stackoverflow.com/a/19374253/6074780