Can't push after using pop - c

Question

I've been at this for about 7 hours trying to get this to work on my own, but I can't figure it out. With my testing earlier I was able to successfully fully push all the values of my char[] into the stack and I could properly pop them off and return the value.
However, if I try to push something back onto the stack then the whole program crashes.
Notes:
The code is supposed to be used to create nodes for an Expression Tree later on
The commented out code in the switch statement is code that I had when the error occurred, was just saving it for when I could have fixed it.
-When pushing after using a pop, the function is properly called, however it does not enter either the if or the else statement within push.
Here is the code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct Stack;
typedef struct StackNode;
typedef struct StackPtr;
struct StackPtr
{
struct StackPtr *prev;
struct StackNode *ptr;
};
struct StackNode
{
char c;
struct StackNode *lchild;
struct StackNode *rchild;
};
struct Stack
{
struct StackPtr *top;
};
// Prototypes
void initStack(struct Stack*);
struct StackNode* createNode(char);
struct StackNode* pop(struct Stack*);
void push(struct Stack*, struct StackNode*);
int isEmpty(struct Stack*);
int main()
{
struct Stack *s = malloc(sizeof(s));
initStack(s);
char exp[7] = "45+67+*";
int length = 0;
for(length; length<3; length++) // push the expression onto the stack
{
struct StackNode *exp1 = malloc(sizeof(exp1));
struct StackNode *exp2 = malloc(sizeof(exp2));
struct StackNode *c = malloc(sizeof(c));
if(exp[length] > 47 && exp[length] < 58) // is a number
{
c->c = exp[length];
push(s,c);
}
else
{
switch(exp[length])
{
default:
printf("exp[length] error\n");
break;
case '+':
exp2 = pop(s);
exp1 = pop(s);
c->c = '7';
push(s, c);
//push(s, exp1->ptr);
//push(s, exp2->ptr);
//c->c = exp[length];
//c->rchild = exp2;
//c->lchild = exp1;
//printf("test ");
//push(s, c);
//printf("test ");
break;
case '-':
break;
case '*':
break;
case '/':
break;
}
}
}
return 0;
}
void initStack(struct Stack *s)
{
struct StackNode *n = malloc(sizeof(n));
struct StackPtr *p = malloc(sizeof(p));
n->c = NULL;
n->lchild = NULL;
n->rchild = NULL;
p->prev = NULL;
p->ptr = n;
s->top = p;
}
struct StackNode* createNode(char c)
{
struct StackNode *n = malloc(sizeof(n));
n->c = c;
n->lchild = NULL;
n->rchild = NULL;
return n;
}
void push(struct Stack *s, struct StackNode *n)
{
if(s->top->ptr->c == NULL) // First item being pushed
{
s->top->ptr = n;
printf("1Added: %c to the stack\n", n->c);
}
else
{
struct StackPtr *o = malloc(sizeof(o));
o->prev = s->top;
s->top = o;
s->top->ptr = n;
printf("2Added: %c to the stack\n", n->c);
}
}
struct StackNode* pop(struct Stack *s)
{
if(isEmpty(s) == 1)
{
printf("Stack is empty!\n");
return;
}
else
{
struct StackPtr *tmp = malloc(sizeof(tmp));
tmp = s->top;
s->top = s->top->prev;
printf("Popped: %c from the stack.\n",tmp->ptr->c);
return tmp->ptr;
}
}
int isEmpty(struct Stack *s)
{
if(s->top == NULL)
return 1;
else
return 0; // false

Answer 1

your POP function is setting
s->top = s->top->prev;
after two nodes are popped out the Prev value is NULL so top is pointing to NULL, which is causing the segmentation fault during the next PUSH
Few more things to consider, change value to 8 instead of 7 as \0 needs to be fit. or use it as
char exp[]
char exp[8] = "45+67+*";