The most important macro in ansi.h is _PROTOTYPE. This macro allows us to write function prototypes in the form

   _PROTOTYPE (return-type function-name, (argument-type argument, ... ) )



and have this transformed by the C preprocessor into

   return-type function-name(argument-type, argument, ...)



if the compiler is an ANSI Standard C compiler, or

   return-type function-name()



if the compiler is an old-fashioned (i.e., Kernighan & Ritchie) compiler.