Lex & Yacc AST homework - abstract-syntax-tree

Question

I need to write this function in AST, preorder, but when I run my yacc file, it prints "Segmentatio fault(core dumped)". If you can please help me resolve my problem, because it as been a few days and I still do not understand what to do. I checked my syntax and it is working, but for some reason when I add mknode and printtree to it, it prints this message. Please help me.
void foo(int x, y, z; real f){
if (x>y) {
x = x + f;
}
else {
y = x + y + z;
x = f*2;
z = f;
}
This is my yacc file, including my function printtree and mknode.
%{
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct node{
char *token;
struct node *left;
struct node *right;
}node;
node *mknode(char *token, node *left, node *right);
void printtree(node *tree);
%}
%union
{
char *s;
struct node *node;
}
%token IF ELSE INT CHAR VOID REAL RETURN GUI
%left '*'
%left '+'
%token <s> NUM ID FUNC
%type <node> S start function func args args1 body if_st ret_st expr block ass calc
%type <s> type
%%
S: start {printtree($1);};
start: function {$$ = mknode("CODE",$1,NULL);};
function: func { $$ = mknode("FUNC",$1, NULL); };
func: type ID '(' args ')' '{' body '}' {$$ = mknode($2,NULL, mknode("ARGS", $4,mknode($1, NULL,$7)));};
type: INT {$$ = "INT";}
| CHAR {$$ = "CHAR";}
| VOID {$$ = "VOID";}
| REAL {$$ = "REAL";};
args: type args1 args {$$ = mknode($1,$2,$3);} | type args1 {$$ = mknode($1,$2,NULL);} ;
args1: ID {$$ = mknode($1,NULL,NULL);}
| ID ';' {$$ = mknode($1,NULL,NULL);}
| ID ',' args1 {$$ = mknode($1,NULL,$3);}
| { $$ = NULL; };
body: if_st {$$ = mknode("BODY", $1, NULL);}
| ret_st {$$ = mknode("BODY", $1, NULL);};
if_st: IF'(' expr ')' '{'block'}' ELSE '{'block'}' {$$ = mknode("IF-ELSE",mknode(NULL,$3,mknode(NULL,$6,$10)),NULL);}
| IF '(' expr ')' '{'block'}'{$$ = mknode("IF",$3,$6);} ;
expr: ID '<' ID {$$ = mknode("<",mknode($1,NULL,NULL),mknode($3,NULL,NULL));}
| ID '>' ID {$$ = mknode(">",mknode($1,NULL,NULL),mknode($3,NULL,NULL));}
| ID '=' ID {$$ = mknode("==",mknode($1,NULL,NULL),mknode($3,NULL,NULL));}
| ID '<' NUM {$$ = mknode("<",mknode($1,NULL,NULL),mknode($3,NULL,NULL));}
| ID '>' NUM {$$ = mknode(">",mknode($1,NULL,NULL),mknode($3,NULL,NULL));}
| ID '=' NUM {$$ = mknode("==",mknode($1,NULL,NULL),mknode($3,NULL,NULL));};
block: block ass {$$ = mknode(NULL,$1,$2);}
| ass {$$ = mknode(NULL,$1,NULL);};
ass: ID '=' calc ';'{$$ = mknode("=",mknode($1,NULL,NULL),mknode(NULL,$3,NULL));};
calc: ID '+' calc {$$ = mknode("+",mknode($1,NULL,NULL),mknode(NULL,$3,NULL));}
| ID '*' calc {$$ = mknode("*",mknode($1,NULL,NULL),mknode(NULL,$3,NULL));}
| NUM '+' calc {$$ = mknode("+",mknode($1,NULL,NULL),mknode(NULL,$3,NULL));}
| NUM '*' calc {$$ = mknode("*",mknode($1,NULL,NULL),mknode(NULL,$3,NULL));}
| NUM {$$ = mknode($1,NULL,NULL);}
| ID {$$ = mknode($1,NULL,NULL);};
ret_st: RETURN GUI calc GUI ';' { $$ = mknode("RET", $3, NULL); };
%%
#include "lex.yy.c"
int main()
{
return yyparse();
}
node *mknode(char *token,node *left,node *right)
{
node *newnode = (node*)malloc(sizeof(node));
char *newstr = (char*)malloc(sizeof*(token)+1);
strcpy(newstr,token);
newnode->left = left;
newnode->right = right;
newnode->token = newstr;
return newnode;
}
void printtree(node *tree)
{
printf("%s\n",tree->token);
if(tree->left)
printtree(tree->left);
if(tree->right)
printtree(tree->right);
}
int yyerror()
{
printf("ERROR\n");
return 0;
}

Answer 1

Most likely cause of the crash:
you call mknode in a couple of places (eg, the block rule) with NULL as the first argument, but mknode calls strcpy with this argument as the source string, so it will crash
Other problems:
you use sizeof(token) where token is a char * (getting the size of a pointer, not the length of the string. You need strlen(token). Better yet, use strdup(token) to do the malloc+strcpy all in one.
your grammar is inflexible, with almost-dupliacted rules and limited nesting. You're better off using fewer rules -- get rid of all the calc/expr stuff and just have
expr: expr '+' expr
| expr '*' expr
| expr '<' expr
| expr '>' expr
| expr '=' expr
| ID
| NUM
| '(' expr ')'
and set precedence of your operators appropriately. Similarly block and body should be combined into one non-terminal and a couple of rules.