What is the type of Yylval?

By default yylval is of type int, but you can override that from the YACC file by re#defining YYSTYPE. The Lexer needs to be able to access yylval.

What is Yylval in bison?

yylval is a union which you can either declare or the bison will declare automatically.

Where is Yylval defined?

The data type for yylval , YYSTYPE , is typically defined in a header file, included in section 1 of the flex input file. For a list of functions and macros available, See bison-functions. The declaration of yylex becomes, int yylex ( YYSTYPE * lvalp, yyscan_t scanner );

What is Yywrap function in Lex?

Function yywrap is called by lex when input is exhausted. Return 1 if you are done or 0 if more processing is required. Every C program requires a main function. In this case we simply call yylex that is the main entry-point for lex . The input file for lex is yyin and defaults to stdin .

What is Yytext?

yytext holds the text matched by the current token. So yytext[0] holds the first character of the text matched by the current token. Sometimes you have a rule which can match different texts so you need to get the real text matched like for variable names or you have a rule to match all arithmetic operations.

What is echo in lex?

You can print or manipulate the contents of this array as you like. In fact, lex provides a macro called ECHO that is equivalent to printf (“\%s”, yytext). When your action consists of a long C statement, or two or more C statements, you might write it on several lines.

What is the use of Yywrap in lex?

A lex library routine that you can redefine is yywrap() , which is called whenever the scanner reaches the end of file. If yywrap() returns 1, the scanner continues with normal wrapup on the end of input.

What is Yylex and Yytext?

Variable yytext is a pointer to the matched string (NULL-terminated) and yyleng is the length of the matched string. Variable yyout is the output file and defaults to stdout . Function yywrap is called by lex when input is exhausted. In this case we simply call yylex that is the main entry-point for lex .

What is the default type of yylval?

The type of yylval is by default int, but this may be changed by the use of the yacc \%union statement. lex assumes that the programmer defines yylval correctly; yacc writes a definition for yylval to the file y.tab.h if the \%union statement is used.

How to add yylval to lexer?

The Lexer needs to be able to access yylval. In order to do so, it must be declared in the scope of the lexer as an extern variable. The original YACC neglects to do this for you, so you should add the following to your lexter, just beneath #include : Bison, which most people are using these days, does this for you automatically.

What does the extern yylval mean?

5 extern yylval;means that yylvalis defined somewhere else. So you have to do that. Usually it is defined in the yacc/bison generated parser, so the name can be resolved when you link the scanner and the parser. If you aren’t using bison/yacc, you will have to define yylvalyourself.

What is the return type of INT in yylex?

Function yylex has a return type of int that returns a token. Values associated with the token are returned by lex in variable yylval. For example, would store the value of the integer in yylval, and return token INTEGER to yacc. The type of yylval is determined by YYSTYPE. Since the default type is integer this works well in this case.