Compiler auto correct uninitialized variable? (gcc does and VC++ doesn't)

[acppdummy]

I had a flawed function like this:

fn(){
char c;
if (runFirstTime){
#ifdef VC
c='\\';
#else
c='/';
#endif
}
... // c is used in the rest of the function to construct some pathnames
}

The problem is that the value of c is not defined the 2nd time the function is called (and subsequently). It somehow worked fine under CygWin compiled with gcc. So I didn't spot the flaw until it ran incorrectly under Windows complied with VC++ 2010. Then I found the error and changed the code to something like

fn(){
#ifdef VC
const char c='\\';
#else
const char c='/';
#endif
...
}

So now it works correctly under Windows. Then I re-compiled the new code with gcc and to my surprise gcc produced exactly the same binary! How can this be? Does the gcc compiler see my flaw and fix it for me somehow? If so I am truly amazed.