如何在yacc中使用yylval和字符串

%union {
    int int_val;
    char *str_val;
}

digit   [0-9]
number  {digit}+
string  \"(\\.|[^\\"])*\"

"number"  {
    yylval.int_val = atoi(yytext);
    return NUMBER;
}

"string"  {
    yylval.str_val = strdup(yytext);
    return STRING;
}

expression:
    expression "+" expression  { $$ = $1 + $3; }
    | expression "-" expression  { $$ = $1 - $3; }
    | expression "*" expression  { $$ = $1 * $3; }
    | expression "/" expression  { $$ = $1 / $3; }
    | "number"  { $$ = $1; }
    | "string"  { printf("String: %s\n", $1); }
    ;