What is the significance of forward declaration in C programming?

Question

I am now learning C programming through Learn C the Hard Way by Zed A. Shaw. There is this code (taking from his website):

#include 
#include          


        

Answer 1

Forward declarations of functions are unavoidable whenever your call graph is cyclic; that is, whenever you have (direct or indirect) recursion between functions.

They are useful if you want to separate your program into more than one translation unit, as they allow separation of declaration and definition of functions (placing the declaration in a .h header and the definition in a .c file).