Segmentation fault on simple Bison script - c++

Question

OK, I'm doing a few experiments with Lex/Bison(Yacc), and given that my C skills are rather rusty (I've once created compilers and stuff with all these tools and now I'm lost in the first few lines... :-S), I need your help.
This is what my Parser looks like :
%{
#include <stdio.h>
#include <string.h>
void yyerror(const char *str)
{
fprintf(stderr,"error: %s\n",str);
}
int yywrap()
{
return 1;
}
main()
{
yyparse();
}
%}
%union
{
char* str;
}
%token <str> WHAT IS FIND GET SHOW WITH POSS OF NUMBER WORD
%type <str> statement
%start statements
%%
statement
: GET { printf("get\n"); }
| SHOW { printf("%s\n",$1); }
| OF { printf("of\n"); }
;
statements
: statement
| statements statement
;
The Issue :
So, basically, whenever the parser comes across a "get", it prints "get". And so on.
However, when trying to print "show" (using the $1 specifier) it gives out a segmentation fault error.
What am I doing wrong?

Answer 1

Lex returns a number representing the token, you need to access yytext to get the text of what is parsed.
something like
statement : GET { printf("get\n"); }
| SHOW { printf("%s\n",yytext); }
| OF { printf("of\n"); }
;
to propogate the text of terminals, I go ahead associate a nonterminal with a terminal and pass back the char* and start building the parse tree for example. Note I've left out the type decl and the implementation of create_sww_ASTNode(char*,char*,char*); However, importantly not all nonterminals will return the same type, for number is an integer, word return char* sww return astNode (or whatever generic abstract syntax tree structure you come up with). Usually beyond the nonterminal representing terminals, it's all AST stuff.
sww : show word word
{
$$ = create_sww_ASTNode($1,$2,$3);
}
;
word : WORD
{
$$ = malloc(strlen(yytext) + 1);
strcpy($$,yytext);
}
;
show : SHOW
{
$$ = malloc(strlen(yytext) + 1);
strcpy($$,yytext);
}
;
number : NUMBER
{
$$ = atoi(yytext);
}
;

Answer 2

You don't show your lexer code, but the problem is probably that you never set yylval to anything, so when you access $1 in the parser, it contains garbage and you get a crash. Your lexer actions need to set yylval.str to something so it will be valid:
"show" { yylval.str = "SHOW"; return SHOW }
[a-z]+ { yylval.str = strdup(yytext); return WORD; }

Answer 3

OK, so here's the answer (Can somebody tell me what it is that I always come up with the solution once I've already published a question here in SO? lol!)
The problem was not with the parser itself, but actually with the Lexer.
The thing is : when you tell it to { printf("%s\n",$1); }, we actually tell it to print yylval (which is by default an int, not a string).
So, the trick is to convert the appropriate tokens into strings.
Here's my (updated) Lexer file :
%{
#include <stdio.h>
#include "parser.tab.h"
void toStr();
%}
DIGIT [0-9]
LETTER [a-zA-Z]
LETTER_OR_SPACE [a-zA-Z ]
%%
find { toStr(); return FIND; }
get { toStr(); return GET; }
show { toStr(); return SHOW; }
{DIGIT}+(\.{DIGIT}+)? { toStr(); return NUMBER; }
{LETTER}+ { toStr(); return WORD; }
\n /* ignore end of line */;
[ \t]+ /* ignore whitespace */;
%%
void toStr()
{
yylval.str=strdup(yytext);
}