(bison.info)Bison Parser

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Bison Output: the Parser File

   When you run Bison, you give it a Bison grammar file as input.  The
output is a C source file that parses the language described by the
grammar.  This file is called a "Bison parser".  Keep in mind that the
Bison utility and the Bison parser are two distinct programs: the Bison
utility is a program whose output is the Bison parser that becomes part
of your program.

   The job of the Bison parser is to group tokens into groupings
according to the grammar rules--for example, to build identifiers and
operators into expressions.  As it does this, it runs the actions for
the grammar rules it uses.

   The tokens come from a function called the "lexical analyzer" that
you must supply in some fashion (such as by writing it in C).  The
Bison parser calls the lexical analyzer each time it wants a new token.
It doesn't know what is "inside" the tokens (though their semantic
values may reflect this).  Typically the lexical analyzer makes the
tokens by parsing characters of text, but Bison does not depend on
this.  Note: The Lexical Analyzer Function `yylex'.

   The Bison parser file is C code which defines a function named
`yyparse' which implements that grammar.  This function does not make a
complete C program: you must supply some additional functions.  One is
the lexical analyzer.  Another is an error-reporting function which the
parser calls to report an error.  In addition, a complete C program must
start with a function called `main'; you have to provide this, and
arrange for it to call `yyparse' or the parser will never run.  Note:
Parser C-Language Interface.

   Aside from the token type names and the symbols in the actions you
write, all symbols defined in the Bison parser file itself begin with
`yy' or `YY'.  This includes interface functions such as the lexical
analyzer function `yylex', the error reporting function `yyerror' and
the parser function `yyparse' itself.  This also includes numerous
identifiers used for internal purposes.  Therefore, you should avoid
using C identifiers starting with `yy' or `YY' in the Bison grammar
file except for the ones defined in this manual.

   In some cases the Bison parser file includes system headers, and in
those cases your code should respect the identifiers reserved by those
headers.  On some non-GNU hosts, `<alloca.h>', `<stddef.h>', and
`<stdlib.h>' are included as needed to declare memory allocators and
related types.  Other system headers may be included if you define
`YYDEBUG' to a nonzero value (Note: Debugging Your Parser.).

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