How do you define the definition section of a c program?

Answer 1

C programs don't have a definition section as such. C source files are composed from declarations and definitions, however the only requirement is that all user-defined identifiers be declared before they are used; the definition itself can literally be placed anywhere. However, a definition is itself a declaration so we really only need declarations when a definition is used by two or more source files or where two or more definitions refer to each other (cyclic dependants).
Typically, we place declarations in header files which can then be included (via the #include compiler directive) in any source files that require them, thus ensuring all declarations are consistent. By organising declarations by their function we can include all related identifiers with a single #include directive. Declarations that are not actually used don't cost anything because declarations do not generate code. Definitions do generate code, but if the code is not used the only cost is in generating the code because unused code has neither internal or external linkages.