How does the linker know where is the definition of an extern function?

Question

I read a few posts and concluded that extern tells compiler that \"This function exists, but the code for it is somewhere else. Don\'t panic.\" But how does the linker know

Answer 1

Linking usually happens this way: The command line is iterated over and every argument given is

1. used directly if it is an object file,
2. used in the extent needed (=to fulfill all references which are unresolved till now).

At the end, every reference has to be fulfilled in order to successfully link. The order of lines given at the linker command line is important.