bison's reduction didn't work as expected

Question

I'm trying to write a EPL parser,so I'm learning flex and bison.I try using it with following rules(SQL):

SELECT { cout<<"SELECT detected"<<endl;return SELECT; }
FROM { cout<<"FROM detected"<<endl;return FROM;}
[a-zA-Z][0-9a-zA-Z]* { cout<<"IDENTIFIER detected"<<endl;yylval.c=yytext;
                         return IDENTIFIER; }
'$' { return DOL;}
[ \t] { cout<<"space founded:"<<int(yytext[0])<<endl; }
\n { return EOL;}
. {}

and bison rules are:

sel_stmt : {cout<<"VOID"<<endl;}
         | SELECT identifier_expr FROM identifier_expr {    cout<<"select statement founded"<<endl; }
         ;

identifier_expr : DOL IDENTIFIER {
$$=$2;
cout<<"ident_expr:"<<$$<<endl;
}
;

all tokens and non-terminals have type "char*"

as the input from stdin is "select $abc from $ddd" reudction happend when lexer returned token FROM,in "identifier_expr" action ,output is "ident_expr:abc from" why this happened?

rici rici · Accepted Answer · 2013-05-21T05:12:47

You must create a copy of the token string (yytext) if you want to use it outside of the flex action. The string pointed to by yytext is a temporary value, and will be modified as soon as the lexer is re-entered.

See the bison FAQ, the flex manual, or any number of SO questions (which are harder to search for because many of the questioners misdiagnose the problem).

bison's reduction didn't work as expected

1 Answers