Question

I have a Visual C++ solution, which consists out of 3 projects.

One of these projects, project “A” is used by both other projects and it has some global data which should always be the same.

However when I link project A into both other projects it seems that two instances of project A are working on different data.

Can this be the case and how can I set up the linking process to prevent this from happending?

— Update to make things more clear

- Project 1 -

main () {
  init();
  test();
}

- Project 2 -

test () {
  cout << get_data();
}

- Project A -

int data;

init() {
  data = 123;
}

get_data() {
  return data;
}

---

As you can see in this exaple I am initializeing the data of project A in the first project and I am accessing it from the second project. My observation is that the data is not initialized when the acces from the second project takes place.

Both projects A and 2 are linked statically into project 1 so the output is a single executable.

Answer 1

The symbols from project A in the static library are linked into both project 1 and project 2, separately. Getting them merged involves compiler-specific mechanisms.

Basically, you must make project 2 re-export project A’s symbols, and have project 1 import those instead of importing project A directly.

If you can’t do that (e.g. because you don’t have control over either project 1 or 2), you must write workarounds inside project A. One option (the easiest usually) is to convert project A to a dynamic library. Then both project 1 and 2 load the same instance of project A and the data is shared.

Another option is to change project A so that it doesn’t have a global variable, but instead registers a process-global data item that contains the data you want; for example, you could abuse the local atom table[1] to store a pointer to dynamic memory.

[1] http://msdn.microsoft.com/en-us/library/windows/desktop/ms649053%28v=vs.85%29.aspx#_win32_Integer_Atoms