YACC

Introduction What is YACC ? a tool for automatically generating a parser given a grammar written in a yacc specification (.y file) YACC (Yet Another Compiler Compiler) is a program designed to compile a LALR(1) grammar and to produce the source code of the syntactic analyzer of a language produced by this grammar. A grammar specifies a set of production rules, which define a language. A production rule specifies a sequence of symbols, sentences, which are legal in the language.

History Yacc original written by Stephen C. Johnson, Variants: – lex, yacc (AT&T) – bison: a yacc replacement (GNU) – flex: fast lexical analyzer (GNU) – BSD yacc – PCLEX, PCYACC (Abraxas Software)

How YACC Works YACC source (translate.y) yacc y.tab.c (1) Parse cc/gcc y.tab.c a.out (2) Compile a.out Token stream0utput (3) Run

Skeleton of a yacc specification (.y file) translate.y %{ %} [DEFINITION SECTION] % [PRODUCTION RULES SECTION] % y.tab.c is generated after running This part will be embedded into y.yab.c contains token declarations. Tokens are recognized in lexer. define how to “understand” the input language, and what actions to take for each “sentence”. Any user code. For example, a main function to call the parser function yyparse()

YACC File Format Definition section – declarations of tokens – type of values used on parser stack Rules section – list of grammar rules with semantic routines User code Comments in /*... */ may appear in any of the sections

Declaration Section Two optional sections – Ordinary C declarations delimited by %{ and %} – Declarations of grammar tokens % token DIGIT Declares DIGIT to be a token Tokens specified in this section can be used as terminal in the second and third sections.

Translation Rules Section Each rule consists of a grammar production and associated semantic action. → | | …. | would be written in YACC as : {semantic action1} | {semantic action2} |… | {semantic action n} ;

Translation Rules Section Rule section is a grammar Example expr : expr '+' term | term; term : term '*' factor | factor; factor : '(' expr ')' | ID | NUM; Semantic action is a sequence of C statements. Semantic action is performed when we reduce by the associated production Normally the semantic action computes a value for $$ in terms of $i s.

The Position of Rules expr : expr '+' term { $$ = $1 + $3; } | term{ $$ = $1; } ; term : term '*' factor { $$ = $1 * $3; } | factor { $$ = $1; } ; factor : '(' expr ')‘ { $$ = $2; } | ID | NUM ;

Supporting C Routines A lexical analyzer by the name yylex() should be provided. – yylex() produces a pair consisting of a token and its attribute value. – If a token such as DIGIT is returened it must be declared in the first section. – The attribute value associate with a token is communicated to the parser through a Yacc defined variable yylval. Error recovery routines may be added as necessary.

Sample yacc program %{ #include %} %token DIGIT line:expr ‘\n’{printf(“%D\n”,$1);} ; Expr: expr ‘+’ term{$$=$1+$3} | term ; Term: term ‘*’ factor {$$=$1*$3} | factor ; Factor: ‘(‘ expr{$$=$2} | DIGIT ; %

Sample yacc program %{ #include %} %token DIGIT % line:expr ‘\n’ {printf(“%d\n”,$1);} ; expr: expr ‘+’ term{$$=$1+$3} | term ; term: term ‘*’ factor {$$=$1*$3} | factor ; factor: ‘(‘ expr{$$=$2} | DIGIT ; % yylex() { int c; c=getchar(); if isdigit(c ) { yylval=c-’0’; return DIGIT; } return c; }

Yacc with ambiguous grammar Precedence / Association %token NUMBER %left '+' '-' %left '*' '/' %right UMINUS % Lines : expr ‘\n’ {printf(“%d\n”, $1);} expr : expr ‘+’ expr { $$ = $1 + $3; } | expr ‘-’ expr { $$ = $1 - $3; } | expr ‘*’ expr { $$ = $1 * $3; } | expr ‘/’ expr{ if($3==0) yyerror(“divide 0”); else | ‘-’ expr %prec UMINUS {$$ = -$2; } |NUMBER ; %

Conflicts shift/reduce conflict – occurs when a grammar is written in such a way that a decision between shifting and reducing can not be made. ex: IF-ELSE ambigious. To resolve this conflict, yacc will choose to shift reduce/reduce Conflicts: start : expr | stmt ; expr : CONSTANT ; stmt : CONSTANT ; Yacc resolves the conflict by reducing using the rule that occurs earlier in the grammar.