Converting a pointer into an integer

I am trying to adapt an existing code to a 64 bit machine. The main problem is that in one function, the previous coder uses a void* argument that is converted into suitable type in the function itself. A short example:

void function(MESSAGE_ID id, void* param)
{
    if(id == FOO) {
        int real_param = (int)param;
        // ...
    }
}

Of course, on a 64 bit machine, I get the error:

error: cast from 'void*' to 'int' loses precision

I would like to correct this so that it still works on a 32 bit machine and as cleanly as possible. Any idea ?