Error in the semantic values returned by bison

Question

A part of my bison grammar is as shown

head: OPEN statement CLOSE 
    {
       $$=$2;
    }
     ;
statement: word
    {
         $$=$1;
     }
    | statement word
     {
         $$=$1;
          printf("%s",$$);
     }
   ;

Now if my input is [hai hello] where [ is the OPEN & ] is the CLOSE respectively,then in the printf statement I get the output as "hai hello" itself..but in the $$ of head I get "hai hello]". Same happens with other grammars too.i.e., if i try to print valye of $1,the values of $2,$3,... are also printed.. why is it so.

Answer 1

The problem is probably in your lexer -- you probably have lexer actions that do something like yylval.str = yytext; to return a semantic value. The problem is that yytext is a pointer into the scanner's read buffer and is only valid until the next call to yylex. So all your semantic values in the parser quickly become dangling pointers and what they point at is no longer valid.

You need to make a copy of the token string in the lexer. Use an action something like
yylval.str = strdup(yytext);. Of course, then you have potential memory leak issues in your parser -- you need to free the $n values you don't need anymore.