Getting Started with YACC

1
Getting Started with YACC
2
Structure of a YACC File

• Has the same three-part structure that Lex uses.
• Each part is seperated by a symbol.
• The three parts are even identical
• Definition section
• Rules section
• Code section (copied directly into the generated
  program)

3
The Definiton Section

• Declare the tokens used in the grammar and the
  types of values used on the stack here.
• Tokens that are single quoted characters like
  or do not need to be declared.
• Literal C code can be included in a block in this
  section using

4
Declaring Tokens

• The tokens that are used in the grammar must be
  declared.
• This is done by including lines like the one
  below in the definition section

token CHARACTERSTRING INTEGER IDENTIFIER
5
The Rules Section

• The rules of the grammar are placed here.
• Here is an example of the basic syntax

Expr ? INTEGER INTEGER INTEGER - INTEGER
expr INTEGER INTEGER action INTEGER
INTEGER action
YACC grammar definition
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YACC Actions

• Simiar to Lex, actions can be defined that will
  be performed whenever a production is applied in
  the stream of tokens.
• These are usually included after the production
  whose action is to be defined.
• Since every symbol in the grammar has a
  corresponding value, it will be necessary to
  access those values.
• Accessing the YACC stack will be the way to do
  this.

7
Accessing the Stack

• Since YACC generates an LR parser, it will push
  the symbols that it reads along with their values
  on a stack until it is ready to reduce.
• To access these values, include a dollar sign
  with a number to get at each value in the
  production in the action definition.

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Accessing the Stack
expr INTEGER INTEGER 1 3
INTEGER INTEGER 1 - 3
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Where do Tokens and Their Values Come From?

• Typically from the lexer.

LEX
YACC
yyparse
yylex
10
Revisiting Lex

• The Lex file will have to be modified to work
  with the YACC parser in two main places.
• In the definition section, include this
  statement include y.tab.h
• That is a header file automatically created by
  YACC when the parser is generated.
• The actions for the rules need to be changed too.

11
Revisiting Lex Actions

• For tokens with a value, assign that value to
  yylval. YACC can read the value from that
  variable.
• Include a return statement for the token name
  (this is the same name that is defined at the top
  of the YACC file).

if return IF 1-90-9
yylval atoi(yytext) return INTEGER
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The union Declaration

• Different tokens have different data types.
• INTEGER are integers, FLOAT are floats,
  CHARACTERSTRING are char , IDENTIFIER are
  pointers to the entry in the symbol table for
  that identifier.
• The union will allow the parser to apply the
  right data type to the right token.

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The union Declaration
YACC Definition Section
union int intValue float
floatValue token ltintValuegt INTEGER token
ltfloatValuegt FLOAT
Lex Rules Section
yylval.intValue atoi(yytext) return
INTEGER yylval.floatValue atof(yytext)
return FLOAT
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References That Might Be Useful

• Levine J R , Manson T , Brown D, Lex Yacc,
  (2Ed , O'Reilly, 1992)
• Stephen C. Johnson, Yacc Yet Another
  Compiler-Compiler, http//www.cs.utexas.edu/users
  /novak/yaccpaper.htm
• Bert Hubert, Lex and YACC primer/HOWTO,
  http//www.tldp.org/HOWTO/Lex-YACC-HOWTO.htmltoc6