Creating an object in shared memory inside a Shared Lib (so) in C++

Question

Is it possible to share a single 'god' instance among everyone that links to this code, to be placed in a shared object?

god* _god = NULL;
extern "C" 
{

int set_log_level(int level)
{
    if(_god == NULL) return -1;
    _stb->log_level(level);
    return 0;
}

int god_init(){
    if(_god == NULL){
        _god = new god(); //Magic happens here
    }
}

}

Provided that I perform a lock synchronization at the beginning of every function, and considering that God itself can new/malloc other things, but those things will never be returned themselves to the caller (God mallocs only for internal use), what is the simplest way of doing this, if possible.

How can that be extended to an arbitrary number of programs linked to this shared library?

Answer 1

This isn't the correct approach at all. By doing what you suggest, the variable, yes, is global to the library, and thus the program, but the data is private to the actual running process. You won't be able to share the values across running programs. @grieve is referring to a global accessed by multiple threads, but threads share the same parent process instance.

Across actual processes, you need to break out to an OS specific shared memory facility. Take a look at Shared Memory for details. It's a doable issue, but it's not particularly trivial to pull off. You'll also need a interprocess synchronization system like Semaphores as well to coordinate usage.

Answer 2

I have feeling that god will be a server of some kind. Consider using a proper client/server architecture, so as to keep god away from the masses.

Answer 3

Boost Interprocess library has high(er) level, portable shared memory objects.