Linking - Initiates after successful compilation
    A linker starts on the first line of main() and resolves all references, 
    i.e. it hooks calls of functions to their actual declaration (and code).

A linker's operation is based on locating functions by their signature.
- A function's signature is its name and parameters, in order, by type.

The prevalent linker error is 'undefined reference'. 
  It means the linker can't hook up a function call to the actual function's code
 - Most common cause is a mismatch between the prototype and the actual declaration.