Re: String concatenation?

[Hans Aberg]

At 17:03 -0500 2003/06/02, Kevin Milner wrote:
>Hello, I'm using Bison and Flex to make a specialized XML parser to handle a
>SOAP response. The problem I am having is that when I try to assign a value
>to a nonterminal, as in the following:
>
>xmlopen: OPENTAG tagname paramlist CLOSETAG {     $$.value = $2.value;
>                        sprintf(buff, "xmlopen(%s)-> < tagname paramlist >",
>$$.value);
>                        PrintRule(buff);
>       }
>
>I get $2.value = "whatevertagname whateverparams >", as if xmlopen is
>assigned the value of everything from the point I am interested in to the
>end of the statement.

This is one of the most common questions in this list: Probably you use
Flex to generate a lexer. This lexer then merely supplies a pointer in a
buffer to the identified text and nullterminates it. At the next call, it
restores the buffer for the next identification.

Thus, if you want to use the strings the lexer identifies at a later point
in time, you must make a clone of it.

>I'm using a struct that simply contains a char * called value. I tried this
>thinking that it would solve the problem, but the same thing happens if I
>simply use %type char* on everything.
>Can anyone help me understand the way that strings are assigned to
>nonterminals in Bison?

The Bison generated parser does absolutely nothing, except what tell it to
do in the actions. The parser just reads the numbers it gets from the lexer
and lets you to execute actions at the appropriate times.

  Hans Aberg