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. See section 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.
See section Parser C-Language Interface.
Aside from the token type names and the symbols in the actions you
write, all variable and function names used in the Bison parser file
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.
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