The use of extern

[Vaderman]

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

extern HANDLE hMutex;

dwWaitResult = WaitForSingleObject(hMutex, 20);

now, this is where the confusion lies...

in Module2.C

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

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

Regards

[NMTop40]

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

[miteshpandey]

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

Handle hMutex;

[RoboTact]

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.

[Vaderman]

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.