when do global variables allocate their memory?

Question

It's been bothering me for a while but I didn't find any good resource about this matter. I have some global variables in my code. It's obvious that they are initialized in some order but is the memmory needed for all those objects reserved before any initialization take place?

here is the simple example of what might go wrong in my code and how I can use the answer:

I had a map<RTTI, object*> objectPool which holds samples of every class in my code, which i used to load objects from a file. To create those samples I use some global variables just to introduce class instance to objectPool. But sometimes those sample instances were initialized before ObjectPool itself. And that generated runtime error.

To fix that error I used some delayed initializer map<RTTI,object*>* lateInitializedObjectPool;. Now every instance first check if the objectPool is initialized and initilize it if not and then intoduce itself to the object pool. It seems to work fine but I'm worried if even the memmory needed for object pool pointer is not reserved before other classes begin to introduce themselves and that may cause access violation.

Answer 1

Variables declared at namespace scope (as opposed to in classes or functions) have the space for the objects themselves (sizeof(ObjectType)) allocated by the executable (or DLL) loader. If the object is a POD that uses aggregate initialization, then it typically gets its values set by having the linker write those values directly into the executable and the exe's loader simply blasting all of those into memory. Objects that don't use aggregate initialization get their values zeroed out initially.

After all of that, if any of these objects have constructors, then those constructors are executed before main is run. Thus, if any of those constructors dynamically allocate memory, that is when they do it. After the executable is loaded, but before main is run.

Answer 2

There's usually separate memory areas for variables that the compiler:

• worked out initially contain all 0s - perhaps with a pre-main() constructor running to change their content
• predetermined have a specific non-0 value, such that they can be written in a pre-constructed form into the executable image and page faulted in ready for use.

When I say a "separate memory area", I mean some memory the OS executable loader arranges for the process, just as per the stack or heap, but different in that these areas are of fixed pre-determined size. In UNIX, the all-0 memory region mentioned above is commonly known as the "BSS", the non-0 initialised area as "data" - see http://en.wikipedia.org/wiki/Data_segment for details.