Manual

Chapter 3: Bison Grammar Files 41
3 Bison Grammar Files
Bison takes as input a context-free grammar specification and produces a C-language function
that recognizes correct instances of the grammar.
The Bison grammar input file conventionally has a name ending in ‘.y’. See Chapter 9
[Invoking Bison], page 97.
3.1 Outline of a Bison Grammar
A Bison grammar file has four main sections, shown here with the appropriate delimiters:
%{
Prologue
%}
Bison declarations
%%
Grammar rules
%%
Epilogue
Comments enclosed in ‘/* ... */’ may appear in any of the sections. As a GNU extension,
‘//’ introduces a comment that continues until end of line.
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 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.
%{
#include
#include "ptypes.h"
%}
%union {
long int n;
tree t; /* tree is defined in ‘ptypes.h’. */
}
%{