error: extern declaration of 'i' follows declaration with no linkage

Question

In the following program, I thought that extern int i; will change the following i to refer to the i defined outside main

Answer 1

Once you define a variable named i inside your main function, the i at file scope is masked and cannot be accessed (unless you have its address).

When you later add the declaration extern int i, this conflicts with the local variable named i at the same scope since locals can't have external linkage. It does not give you access to the global i.

When you remove the local i, the extern int i declaration matches up with the definition at file scope, so there is no error. As for the warning on extern int i=1;, that did not go away for me on gcc 4.1.2, so that depends on the compiler.