What is the type of Yylval?
By default, the yylval variable has the int type. Up until now, this has been satisfactory; however, yylval should be able to represent the value of any token you find, which means that in some programs it should be able to represent more than just the int type.
What is Yystype?
The YYSTYPE is a union with a member for each type of token defined within the yacc source file.
What is %union in bison?
The %union declaration specifies the entire collection of possible data types for semantic values. They are given names val and tptr ; these names are used in the %token , %nterm and %type declarations to pick one of the types for a terminal or nonterminal symbol (see Nonterminal Symbols).
What is Atoi Yytext?
yylval = atoi(yytext); uses the C atoi() library function to convert this text into an integer value and assigns that integer to yylval. After this conversion has taken place, the action returns the defined value INTEGER to indicate that an integer has been obtained. Defining tokens talks about this kind of definition.
What is Yylval in bison?
yylval is a union which you can either declare or the bison will declare automatically.
What is Yyin and Yyout?
yyin. A variable that determines the input stream for the yylex() and input functions. yyout. A variable that determines the output stream for the output macro, which processes input that does not match any rules.
What is Yyout?
Variable yyout is the output file and defaults to stdout . 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 .
What is Yyparse?
4.1 The Parser Function yyparse You call the function yyparse to cause parsing to occur. This function reads tokens, executes actions, and ultimately returns when it encounters end-of-input or an unrecoverable syntax error.
How do you run a Bison?
10.3. Running Bison
- File y.tab.c contains the parsing tables and a definition of function yyparse, with heading void yyparse(void) Calling yyparse will run the parser.
- Option -d asks Bison to write a file y. tab.
- Option -v asks Bison to write file y. output.
What is Yylex in lex?
4.3 The Lexical Analyzer Function yylex The lexical analyzer function, yylex , recognizes tokens from the input stream and returns them to the parser. Bison does not create this function automatically; you must write it so that yyparse can call it. The function is sometimes referred to as a lexical scanner.
What is Yyout in lex?
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 . Some implementations of lex include copies of main and yywrap in a library thus eliminating the need to code them explicitly.
What is the role of Yylval?
The yylval global variable is used to pass the semantic value associated with a token from the lexer to the parser. The semantic values of symbols are accessed in yacc actions as $1 , $2 , etc and are set for non-terminals by assigning to $$ .
How is the yylval variable declared in Yacc?
This union defines all the possible values tokens may have. Yacc creates a typedef of YYSTYPE for this union. All token types (see %type declarations below) are taken from the field names of this union. The global variable yylval which lex uses to return token values is declared as a YYSTYPE union.
How is the type of yylval determined by YYSTYPE?
The type of yylval is determined by YYSTYPE. Since the default type is integer this works well in this case. Token values 0-255 are reserved for character values. For example, if you had a rule such as
How does the% uniondeclaration modifies the type of yylval?
The %uniondeclaration modifies the type of yylval. The bisonmanual explains: In an ordinary (nonreentrant) parser, the semantic value of the token must be stored into the global variable yylval. When you are using just one data type for semantic values, yylvalhas that type.
How is Flex used in Yacc parser generator?
One of the main uses of flex is as a companion to the yacc parser-generator. yacc parsers expect to call a routine named yylex() to find the next input token. The routine is supposed to return the type of the next token as well as putting any associated value in the global yylval.