4.6.2 Semantic Values of Tokens

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, yylval has that type. Thus, if the type is int (the default), you might write this in yylex:

  …
  yylval = value;  /* Put value onto Bison stack.  */
  return INT;      /* Return the type of the token.  */
  …

When you are using multiple data types, yylval's type is a union made from the %union declaration (see section The Collection of Value Types). So when you store a token's value, you must use the proper member of the union. If the %union declaration looks like this:

%union {
  int intval;
  double val;
  symrec *tptr;
}

then the code in yylex might look like this:

  …
  yylval.intval = value; /* Put value onto Bison stack.  */
  return INT;            /* Return the type of the token.  */
  …