3.1.1 The prologue

The Prologue section contains macro definitions and declarations of functions and variables that are used in the actions in the grammar rules. These are copied to the beginning of the parser file so that they precede the definition of yyparse. You can use ‘#include’ to get the declarations from a header file. If you don't need any C declarations, you may omit the ‘%{’ and ‘%}’ delimiters that bracket this section.

The Prologue section is terminated by the first occurrence of ‘%}’ that is outside a comment, a string literal, or a character constant.

You may have more than one Prologue section, intermixed with the Bison declarations. This allows you to have C and Bison declarations that refer to each other. For example, the %union declaration may use types defined in a header file, and you may wish to prototype functions that take arguments of type YYSTYPE. This can be done with two Prologue blocks, one before and one after the %union declaration.

%{
  #define _GNU_SOURCE
  #include <stdio.h>
  #include "ptypes.h"
%}

%union {
  long int n;
  tree t;  /* tree is defined in ‘ptypes.h’. */
}

%{
  static void print_token_value (FILE *, int, YYSTYPE);
  #define YYPRINT(F, N, L) print_token_value (F, N, L)
%}

…

When in doubt, it is usually safer to put prologue code before all Bison declarations, rather than after. For example, any definitions of feature test macros like _GNU_SOURCE or _POSIX_C_SOURCE should appear before all Bison declarations, as feature test macros can affect the behavior of Bison-generated #include directives.