Structure of a YACC File Has the same three-part structure as Lex Each part is separated by a symbol The three parts are even identical: definition section rules section code section (copied directly into the generated program) Definiton
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Presentation Transcript
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Structure of a YACC File Has the same three-part structure as Lex
Each part is separated by a %% symbol
The three parts are even identical:
definition section
rules section
code section (copied directly into the generated program)<br>
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Definiton Section Declare tokens used in the grammar and types of values used on the stack here
Tokens that are single quoted characters like “=“ or “+” need not be declared.
Literal C code can be included in a block in this section using %{…%}<br>
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Declaring Tokens The tokens that are used in the grammar must be declared
Include lines like the one below in the definition section: %token CHARSTRING INT IDENTIFIER
%token LPAREN RPAREN<br>
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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<br>
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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.<br>
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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.<br>
Where do Tokens and Their Values Come From? Typically from the lexer. yyparse yylex YACC LEX<br>
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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.<br>
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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;}
[1-9][0-9]* {yylval = atoi(yytext); return INTEGER;}<br>
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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.<br>
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The %union Declaration %union {
int intValue;
float floatValue;
}
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.html#toc6<br>
