static const to an object in a shared library in C++. Is it share between processes?

Question

I've been writing a shared library in C++, but I want to share some instance of a class through users of the library. I mean, a read-only object loaded just one time from the library and used by every process linked to the library.

As far as I know this can be made using const or static const, but it doesn't work as expected.

For example:

#include <iostream>
static const int x = 1;

int main()
{
    std::cout << x << std:endl;

    *(const_cast<int *>(&x)) = 2;
    std::cout << x << std:endl;

    return 0;
}

Using GCC 4.8.1 the code compiles well but, obviously, it fails at runtime because the x variable is read-only (it produces a segmentation fault on my Linux).

However, lets see this code:

#include <iostream>
struct A
{
    A() : x(1) {}
    int x;
}

static const A a;

int main()
{
    std::cout << a.x << std:endl;

    const_cast<A *>(&a)->x = 2;
    std::cout << x << std:endl;

    return 0;
}

The last code compiles and run well. The executable prints

1
2

I was able to modify the const data! So I guess the const modifier does NOT work properly with classes.

Then my questions are:

What do the const static modifiers do to an instance of a class?
How could I put an instance in a shared library to share it through different processes, loading once and sharing the same RAM memory?

The class I want to instantiate inherits from an abstract one. I am using C++11 but codes shown before were tested without the C++11 support.

sorry if I made any english mistake

`const_cast(&a)->x = 2;` is undefined behavior. Anything might happen including setting the value to 2. — Asha, Jun 14 '13 at 05:20
"*I was able to modify the const data!"*. Pesky compiler tricked you! This is UnBelievable! — Mark Garcia, Jun 14 '13 at 05:22
@Asha I read about it, but how can I know if the object is or isn't in a read-only memory and is shared between procesess? — Daniel, Jun 14 '13 at 05:41

score 3 · Accepted Answer · answered Jun 14 '13 at 05:47

Static variables are not shared between processes. Each process gets its own data segment, which is where variables are placed. It doesn't matter whether they're const or not.

About the const_cast, you seem to be confused about what it's actually there for. It is able to remove the const attribute from any variable. The whole point of it is to get the compiler to allow writing to a const variable. You can use it on anything, and you do so at your own risk. If you strip the const from something that really isn't writable, you're into undefined behavior.

A const static variable has the usual meaning. It's static, and it's constant. The type doesn't matter; const static int or const static A. Means the same thing for both.

If you want to share the object between processes, then you will need to put it into POSIX shared memory and synchronize access to it. You use shm_open() for this. There are some tutorials on shared memory online, like this one for example.

OK Thank you! I know that const_cast remove the const attribute at compiler's sight, but I thought (apparently wrong) the OS stops me if I try to modify the data (just like in first example). So, if I put a very big binary array in a const section of the library it will be copied to any process? — Daniel, Jun 14 '13 at 06:07
@Daniel Yes, each process always gets its own copy of all static variables. — Nikos C., Jun 14 '13 at 06:16

static const to an object in a shared library in C++. Is it share between processes?

1 Answers1