How to access global variable from .so in executable - c++

Question

Please help me, How to access run time varibale's data(att) in stub.exe of att variable that is defined in proc.so,
I have created proc.so and linked with attol.exe and stub.exe and
attol.exe updates 'att' variable and stub.exe is accessing 'att' variable and prints att's value.
I have used below commands to compile the code :
g++ -Wall -c attol.cc proc.cc stub.cc
g++ -shared -dynamiclib -fPIC -o libproc.so proc.o -ldl
g++ -rdynamic -o attol.exe attol.o /users/hbharti/DLOPEN/proc/libproc.so -ldl
g++ -rdynamic -o stub.exe stub.o /users/hbharti/DLOPEN/proc/libproc.so -ldl
When i am running both .exe at different terminal then attol.exe showing 'att:4' value but stub.exe is showing incorrect value,
But stub.exe should display '4' value or updated value.
out put attol.exe:
./attol.exe
Value of att is : 4
Enter the value of att :
out put stub.exe:
./stub.exe
Att : 0
----Complete Code Details----
proc.h:
#ifndef __X_H_INCLUDED__
#define __X_H_INCLUDED__
extern int att;
int fun();
#endif
proc.cc:
#include<iostream.h>
#include "proc.h"
int att;
int fun ()
{
att=4;
return 0;
}
Above code is generating proc.o and then this proc.o will converted into proc.so with below commands:
g++ -Wall -c attol.cc proc.cc stub.cc
g++ -shared -dynamiclib -fPIC -o libproc.so proc.o -ldl
attol.cc:
#include <iostream.h>
#include "proc.h"
using namespace std;
int main ()
{
int ch=1;
fun();
cout<<"\n Value of att is : "<<att;
do{
cout<<"\n Enter the value of att : ";
cin>>att;
cout<<"\n Do you want to continue the : ";
cin>>ch;
}while(ch!=0);
return 0;
}
attol.cc file creates attol.exe by using below command
g++ -rdynamic -o attol.exe attol.o /users/hbharti/DLOPEN/proc/libproc.so -ldl
out put:
Value of att is : 4
Enter the value of att :
stub.cc:
#include <iostream.h>
#include <dlfcn.h>
int main ()
{
void *handle;
char *error;
handle = dlopen ("/users/hbharti/DLOPEN/proc/libproc.so", RTLD_LAZY);
if (!handle) {
fputs (dlerror(), stderr);
exit(1);
}
int *att =(int*) dlsym(handle, "att");
if ((error = dlerror()) != NULL) {
fputs(error, stderr);
exit(1);
}
cout<<"\n Att : " <<*att;
cout<<"\n " ;
dlclose(handle);
}
stub.cc file creates stub.exe by using below command
g++ -rdynamic -o stub.exe stub.o /users/hbharti/DLOPEN/proc/libproc.so -ldl

Answer 1

Judging by the code, there seems to be a fundamental issue with the core-logic.
A shared-object (*.so) is loaded into the executing process's memory address space.
However it is NOT shared across multiple processes. When 2 or more executables attempt to access the same shared-object (*.so), they both get independent copies of it mapped into their respective memory address spaces.
Data (even globals) within a shared-object (*so) are NOT shared across 2 or more executables.