NULL pointer gets randomly modified to 0x62 or similar - c

Question

I'm making a regex parser. The instruction "t->left = tmp;" the instruction "t->left = tmp;" creates a segmentation fault, but not always ! Try executing the code several times, you'll see that it doesn't always happen. The segmentation fault occurs when printing the tree, because one of the nodes has a child with address "0x62" or "0x54" or something similar. It's really weird because when "tmp" is created I check that both children are NULL but somehow one of them gets modified to "0x.." while executing.
Thanks to anyone that could help me solve this !
I spent quite a while trying to figure out why just adding a "left children" while parsing the tree will create a segmentation fault. I really don't understand because it's a simple pointer creation and assignment ! Comment the instruction "t->left = tmp;" and the segmentation fault is gone ! The weirdest issue so far..
Best regards !
/*
* Includes
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
/*
* Typedefs
*/
struct node {
char val;
struct node* left;
struct node* right;
};
typedef struct node NODE;
typedef struct node * TREE;
/*
* Tree handling
* - create
* - free
* - print
*/
TREE createNode(char val) {
TREE t = (TREE) malloc(sizeof(TREE));
t->val = val;
t->left = NULL;
t->right = NULL;
return t;
}
void freeTree(TREE t) {
if (t != NULL) {
if (t->left != NULL)
freeTree(t->left);
if (t->right != NULL)
freeTree(t->right);
free(t);
}
}
void printNode(TREE t) {
printf("------ NODE ------\n");
printf("t == %p\n", t);
if (t != NULL) {
printf("val = %c\n", t->val);
printf("left : %p\n", t->left);
printf("right : %p\n", t->right);
}
}
void printTree(TREE t) {
if (t != NULL) {
printf("%c\t[%p]\n", t->val, t);
printf("%p\n", t->left);
if (t->left != NULL) {
printf("----------------begin left----------------\n");
printTree(t->left);
printf("----------------end left----------------\n");
}
printf("%p\n", t->right);
if (t->right != NULL) {
printTree(t->right);
}
}
}
/*
* Misc functions
*/
char* concat(char *s1, char *s2)
{
char *result = malloc(strlen(s1)+strlen(s2)+1);//+1 for the zero-terminator
//in real code you would check for errors in malloc here
strcpy(result, s1);
strcat(result, s2);
return result;
}
char* substr(char* s, int start) {
int i = 0;
char* sub = malloc(4*(strlen(s) - start) + 1); // +1 for the zero-terminator
while (s[i+start] != '\0') {
sub[i] = s[i+start];
i++;
}
return sub;
}
char* substr_(char* s, int start, int end) {
int i = 0;
char* sub = malloc(4*(strlen(s) - start) + 1); // +1 for the zero-terminator
while (s[i+start] != '\0' && i < (end-start+1)) {
sub[i] = s[i+start];
i++;
}
return sub;
}
/*
* Regex handling
*/
TREE parseParenthesis(char* regex) {
if (regex[0] == '\0') return NULL;
printf("%s\n",regex);
TREE tree = createNode(regex[0]);
int i = 0;
int start = 1;
int lastParenthesisPos = 0;;
switch (regex[0]) {
case '(' :
while (regex[i] != '\0') {
if (regex[i] == ')')
lastParenthesisPos = i;
i++;
}
tree->left = parseParenthesis(substr_(regex, 1, lastParenthesisPos-1));
start = lastParenthesisPos + 1;
break;
case '|' :
case ')' : // Handled by case ')'
case '*' :
case '+' :
case '?' :
default : break;
}
tree->right = parseParenthesis(substr(regex, start));
return tree;
}
void parseExtras(TREE t, TREE parent) {
if (t == NULL) return;
TREE tmp = NULL;
switch (t->val) {
case '*' :
case '+' :
case '?' :
parseExtras(t->right, t);
tmp = createNode(parent->val);
t->left = tmp;
break;
case '(' :
case ')' :
case '|' :
default :
parseExtras(t->left, t);
parseExtras(t->right, t);
break;
}
}
/*
* Main
*/
int main() {
//char* regex = "a|(b|c*)?d|ef";
char* regex = "ab*dd*";
// Parse ()
TREE t = parseParenthesis(regex);
printf("************************************ OK - Parse parenthesis\n");
// Parse * + ?
parseExtras(t, NULL);
printf("************************************ OK - Parse extras\n");
printTree(t);
printf("************************************ OK - Print tree\n");
freeTree(t);
printf("************************************ OK - Free tree\n");
return 0;
}

Answer 1

You are allocating space only for a pointer, not the actual struct:
TREE t = (TREE) malloc(sizeof(TREE));
After this, you are modifying memory that is beyond the size of TREE (which is a pointer).
As a comment above said, don't use typedef unnecessarily.