The use of extern

My understanding of using extern is;
1) It used as a declaration without being defined;
2) Any new declarion with definition of the same name will be treated as being the same as the extern declaration. For example.

In Module1.c

Code:

---

extern HANDLE hMutex;

dwWaitResult = WaitForSingleObject(hMutex, 20);

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now, this is where the confusion lies...

in Module2.C

Code:

---

extern HANDLE hMutex;  // does this take the value from within Module1.c?

// so that I can then do the following...
ReleaseMutex(hMutex);

---

Regards

There isn't an hMutex in Module1.c

The linker will try to find an instance of hMutex and will use it for both Module1.c and Module2.C

Both of your declarations tells that the hMutex variable is declared but defined somewhere else. If the linker doesn't find the definition of hMutex somewhere else then u will have a link error.

So somewhere u should have:

Moudule3.cpp

Code:

---

Handle hMutex;

---

You'll have 'unresolved external symbol' linker error. When you declare variable with specifier extern, you effectively say "there is a variable there somewhere, I want to access it using this symbol". If linker can't find that variable, it can't link usage of symbol with actual variable, and so program code is incomplete.

Sorry folks, the hMutex is indeed defined and should've been defined in Module1, but I guess I must've omitted that out by accident. But the explanition given by NMTop40 has sufficed.

Thanx guys.