query:definition of func in C

Hi all

 include<stdio.h>
 #define ....

 void xyz();        <---- i am referring to this defn of xyz()

 void abc()
  {
   -:statements:-
  }

 void main()
  {
     abc();
     xyz();
  }

 void xyz()
  {
   -:statements:-
  }
      

Is it compulsory to define all the Routines,which are called from main ?(except extern)
ie xyz() was defined at the begginng of the program,but abc() was not defined.
is it so because during compilation,compiler have already met with abc()
before being called from main().

Thanks
Arjun