In C, I did not notice any effect of the extern keyword used before function declaration.
At first, I thought that when defining extern int f(); in a single file forces you to implement it outside of the file's scope. …
For many questions the answer seems to be found in "the standard". However, where do we find that? Preferably online.
Googling can sometimes feel futile, again especially for the C standards, since they are drowned in the flood of discussions …