Lex and Yacc

 This is a simple sample how Lex and Yacc cowork.

• 1) The agreement is Yacc gets tokens by calling a function called yylex(). We define Yacc's yylex() to be static who just calls Lex' getToken(). Lex will generate "#define yylex cminuslex" while cminuslex() is the real working function. So in Lex' getToken(), it really goes into cminuslex().
• 2) YYSTYPE is the type for $1(the first item right after :), $2.... We define it to be (TreeNode *). So all $1, $2 have the type (TreeNode *).

 

Lex file – cminus.l

Yacc file – cminus.y

%{  #include "cminus.tab.h"  #include "globals.h"  #include "scan.h"  // lexeme of identifier or reserved word char tokenString[MAXTOKENLEN+1];  %}    digit       [0-9]  number      {digit}+  letter      [a-zA-Z]  identifier {letter}+  newline     \n|\r\n  whitespace [ \t]+    %option nounistd    %%    "if"            {return IF_YY;}  ......  {number}        {return NUM_YY;}  {identifier}    {return ID_YY;}  {newline}       {g_lineno++;}  {whitespace}    {/* skip whitespace */}  "//".*{newline} {g_lineno++;}  .               {return ERROR_YY;}    %%  // Token values IF_YY... are exported from  // Yacc defined as enum or macro. We can  // only assume it is an int.  TokenType_YY getToken(void)  {   TokenType_YY currentToken;     // Calls into Lex' yylex, not Yacc's.   currentToken = yylex();   strncpy(tokenString,yytext,MAXTOKENLEN);      return currentToken;  }  int yywrap()  {     return 1;  }

%{  #include "globals.h"  #include "util.h"  #include "scan.h"  #include "parse.h"    #define YYSTYPE TreeNode *  static char * savedString; static int savedLineNo;  static int savedNum;  // stores syntax tree for main program  static TreeNode *savedTree;     static int yylex(void);    %}    %token IF_YY ELSE_YY RETURN_YY ... ......  %token ERROR_YY    %% // Grammar for Cminus    program : declaration_list           { savedTree = $1; }          ;  ......    %%    int yyerror(char * message)  {     fprintf(g_listing,          "Syntax error at line %d: %s\n",           g_lineno,message);     fprintf(g_listing,"Current token: ");     printToken(yychar,tokenString);     g_error = TRUE;     return 0;  }    // Calls Lex's getToken().  static int yylex(void)  {     return getToken();  }    TreeNode * parse(void)  {     yyparse();     return savedTree;  }