Compiler Design Yacc Example "Yet Another Compiler Compiler" Kanat Bolazar

Lex and Yacc Two classical tools for compilers: Lex: A Lexical Analyzer Generator Yacc: “Yet Another Compiler Compiler” (Parser Generator) Lex creates programs that scan your tokens one by one. Yacc takes a grammar (sentence structure) and generates a parser. Lexical Rules Grammar Rules Lex Yacc Input yylex() yyparse() Parsed Input

Lex and Yacc Lex and Yacc generate C code for your analyzer & parser. Lexical Rules Grammar Rules C code Lex C code Yacc Parsed Input Input yylex() yyparse() token stream char stream C code C code Lexical Analyzer (Tokenizer) Parser

Flex, Yacc, Bison, Byacc Often, instead of the standard Lex and Yacc, Flex and Bison are used: Flex: A fast lexical analyzer (GNU) Bison: A drop-in replacement for (backwards compatible with) Yacc Byacc is Berkeley implementation of Yacc (so it is Yacc). Resources: http://en.wikipedia.org/wiki/Flex_lexical_analyser http://en.wikipedia.org/wiki/GNU_Bison The Lex & Yacc Page (manuals, links): http://dinosaur.compilertools.net/

Yacc: A Standard Parser Generator Yacc is not a new tool, and yet, it is still used in many projects. Yacc syntax is similar to Lex/Flex at the top level. Lex/Flex rules were regular expression – action pairs. Yacc rules are grammar rule – action pairs. declarations %% rules programs

Yacc Examples: Calculator A standard Yacc example is the int-valued calculator. Appendix A of Yacc manual at Lex and Yacc Page shows such a calculator. We'll examine this example in parts. Let's start with four operations: E -> E + E | E – E | E * E | E / E Note that this grammar is ambiguous because 2 + 5 * 7 could be parsed 2 + 5 first or 5 * 7 first.

Yacc Calculator Example: Declarations %{ # include <stdio.h> # include <ctype.h> int regs[26]; int base; %} %start list %token DIGIT LETTER %left '+' '-' %left '*' '/' '%' %left UMINUS /* precedence for unary minus */ Directly included C code list is our start symbol; a list of one-line statements / expressions. DIGIT & LETTER are tokens; (other tokens use ASCII codes, as in '+', '=', etc) Precedence and associativity (left) of operators: +, - have lowest precedence *, / have higher precedence Unary minus has highest precedence

Yacc Calculator Example: Rules %% /* begin rules section */ list : /* empty */ | list stat '\n' | list error '\n' { yyerrok; } ; stat : expr { printf( "%d\n", $1 ); } | LETTER '=' expr { regs[$1] = $3; } number: DIGIT { $$ = $1; base = ($1==0) ? 8 : 10; } | number DIGIT { $$ = base * $1 + $2; } list: a list of one-line statements / expressions. Error handling allows a statement to be corrupt, but list continues with next statement. statement: expression to calculate, or assignment number: made up of digits (tokenizer should handle this, but this is a simple example).

Yacc Calculator Example: Rules, cont'd expr : '(' expr ')' { $$ = $2; } | expr '+' expr { $$ = $1 + $3; } | expr '-' expr { $$ = $1 - $3; } | expr '*' expr { $$ = $1 * $3; } | expr '/' expr { $$ = $1 / $3; } | '-' expr %prec UMINUS { $$ = - $2; } | LETTER { $$ = regs[$1]; } | number ; Unary minus Letter: Register/var

Yacc Calculator Example: Programs (C Code) %% /* start of programs */ yylex() { /* lexical analysis routine */ /* returns LETTER for a lower case letter, yylval = 0 through 25 */ /* return DIGIT for a digit, yylval = 0 through 9 */ /* all other characters are returned immediately */ int c; while( (c=getchar()) == ' ' ) {/* skip blanks */ } /* c is now nonblank */ if( islower( c ) ) { yylval = c - 'a'; return ( LETTER ); } if( isdigit( c ) ) { yylval = c - '0'; return( DIGIT ); return( c );