BISON AST production prints scrambled values - c

Question

I'm trying to make a simple parser. It's for a homework assignment but also for own experimentation. I have completed the lexer and the parser and I'm trying now to output an AST. The problem is that when I'm adding, for example, two integers, the result tree is printed with unrecognizable symbols. A valid input should be +(1,1) and a valid output should be (+ 1 1). Instead of this, I'm getting ( + �|k �|k ). I've tried many things, without actually any significant result. The sprintf function returns a null terminator, so probably this is not the problem. Below is the parser code (.y file):
%{
#define YYDEBUG 1
%}
%start program
%token NUMBER
%token ID
%token PLUS MINUS TIMES
%token LP RP EQUALS COMMA
%token END
%token LET IN AND
%left PLUS MINUS
%left TIMES
%left LET IN AND
%left EQUALS
%%
program:{printf("Empty Input\n");} /* empty */
| program line /* do nothing */
line: expr END { printtree($1); printf("\n");}
;
expr : /*Empty*/
| LET deflist IN expr {}
| ID { printf("Found ID\n"); $$ = make_id_leaf($1);}
| NUMBER { printf("Found NUMBER\n"); $$ = make_number_leaf($1);}
| PLUS LP expr COMMA expr RP {$$ = make_plus_tree($3,$5); printf("Found expr PLUS expr.\n"); }
| TIMES LP expr COMMA expr RP {$$ = make_times_tree($3,$5); printf("Found expr TIMES expr. Result:%d\n", $$);}
| MINUS ID
| MINUS NUMBER { printf("found MINUS NUMBER\n"); }
;
deflist : definition
| definition AND deflist
;
definition : /*Empty*/
| ID EQUALS expr {printf("Found EQ\n");}
;
%%
/*int main (void) {return yyparse ( );}*/
int yyerror (char *s) {fprintf (stderr, "%s\n", s);}
The lexer file:
%{
#include "parser.h"
%}
DIGIT [0-9]
LETTER [a-zA-Z]
%%
LET {printf("Encountered LET\n"); return(LET);}
IN {printf("Encountered IN\n"); return(IN);}
AND {printf("Encountered AND\n"); return(AND);}
{DIGIT}+ {yylval = atoi(yytext); return NUMBER;}
{LETTER}* { if (strlen(yytext) <= 8){
yylval = strlen(yytext);
printf( "<ID, %s> ", yytext );
return(ID);
} else {
yytext[8] = '\0';
printf("WARNING! Long identifier. Truncating to 8 chars\n");
printf( "<ID, %s> ", yytext );
return(ID);
}
}
[ \t] ;
[\n] return(END);
"+" return(PLUS);
"-" return(MINUS);
"*" return(TIMES);
"=" return(EQUALS);
"(" return(LP);
")" return(RP);
"," return(COMMA);
<<EOF>> return(0);
%%
int yywrap (void) {return 1;}
The main.c which includes the yyparse() function:
#include <stdio.h>
#include <stdlib.h>
#include "tree.h"
#include "treedefs.h"
int main(int argc, char **argv){
yyparse();
return 0;
}
And the treedefs.h file which includes the function definitions (I've included only the struct definition, the number leaf and the plus tree):
typedef struct tree{
char *token;
TREE *l;
TREE *r;
TREE *child;
}TREE;
/* Make number leaves */
TREE *make_number_leaf(int n){
TREE *leafNum = malloc(sizeof(TREE));
char *c, ch[8];
sprintf(ch, "%d", n); /* Effective way to convert int to string */
c = ch;
leafNum->token = c;
leafNum->l = NULL;
leafNum->r = NULL;
leafNum->child = NULL;
printf("NUM Leaf is: %s\n", leafNum->token);
return (leafNum);
}
/* Addition tree */
TREE *make_plus_tree(TREE *l, TREE *r){
TREE *plusTree = malloc(sizeof(TREE));
plusTree->token = "+";
plusTree->l = l;
plusTree->r = r;
plusTree->child = NULL;
return (plusTree);
}
void printtree(TREE *tree)
{
if (tree->l || tree->r){
printf("(");
}
printf(" %s ", tree->token);
if (tree->l){
printtree(tree->l);
}
if (tree->r){
printtree(tree->r);
}
if (tree->l || tree->r){
printf(")");
}
}
The file tree.h includes only some declarations, no big deal, and definitely not related to the issue.
Why the numbers look like this? And how can I fix it? Any help will be greatly appreciated.

Answer 1

This problem actually has nothing to do with bison or flex. It's in your make_number_leaf implementation:
TREE *make_number_leaf(int n){
TREE *leafNum = malloc(sizeof(TREE));
char *c, ch[8];
// ^ local variable
sprintf(ch, "%d", n); /* Effective way to convert int to string */
c = ch;
leafNum->token = c;
// ^ dangling pointer
// Remainder omitted
}
As indicated in the comments above, ch is a local (stack-allocated) variable, whose lifetime ends when the function returns. Assigning its address to the variable c does nothing to change that. So the value of c which is stored into leafNum->token will become a dangling pointer as soon as the function returns.
So when you later attempt to print out the token, you are printing out the contents of random memory.
You need to malloc a character buffer, and remember to free it when you are freeing the TREE. (However, in the case where leafNum->token is a string literal, you cannot call free, so you need to be a bit cleverer.)