Equating with NULL in C

Question

I was trying simple stack implementation in C. Here is my simple code:

1    #include "stdio.h"
2    #include "limits.h"
3    #include "malloc.h"
4    
5    typedef struct StackEntry StackEntry;
6    
7    struct StackEntry
8    {
9        int data;
10        StackEntry *next;
11    };
12    
13    StackEntry* createStack(){
14        return NULL;
15    }
16    
17    int isEmptyStack(StackEntry **pStackTop)
18    {
19        return *pStackTop == NULL;
20    }
21    
22    void push(StackEntry **pStackTop, int pData)
23    {
24        StackEntry *lTempStackEntry;
25        lTempStackEntry = (StackEntry*)malloc(sizeof(StackEntry));
26    
27        if(!lTempStackEntry)
28            return;
29    
30        lTempStackEntry->data = pData;
31        lTempStackEntry->next = *pStackTop;
32    
33        *pStackTop = lTempStackEntry;
34    }
35    
36    int pop(StackEntry **pStackTop)
37    {
38        int lTempData;
39        StackEntry *lTempStackEntry;
40    
41        if(isEmptyStack(pStackTop))
42           return INT_MIN;
43    
44        lTempStackEntry = *pStackTop;
45        *pStackTop = (*pStackTop)->next;
46        lTempData = lTempStackEntry->data;
47        free(lTempStackEntry);
48    
49        return lTempData;
50    }
51    
52    int stackTop(StackEntry **pStackTop)
53    {
54        if(isEmptyStack(pStackTop))
55            return INT_MIN;
56    
57        return (*pStackTop)->data;
58    }
59    
60    void deleteStack(StackEntry ** pStackTop)
61    {
62        StackEntry *secondTopNode, *topNode;
63        topNode = *pStackTop;
64        /*
65         * In this we free all the nodes from second top to bottom,
66         * by one by one attaching them to top->next. At the end
67         * we free the top node.
68         */
69        while(topNode->next)
70        {
71            secondTopNode = topNode->next;
72            topNode->next = secondTopNode->next;  //make third top node
73                                                  //second top node
74            free(secondTopNode);
75            secondTopNode = NULL;
76        }
77        free(topNode);
78        topNode = NULL;
79    }
80    
81    int main(void) {
82      // your code goes here
83        StackEntry *stack = createStack();  //stack: 0x0
84        printf("\n push 1");
85        push(&stack,1);
86        printf("\n push 2");
87        push(&stack,2);
88        printf("\n push 3");
89        push(&stack,3);
90        printf("\n stack top: %d", stackTop(&stack));
91        printf("\n pop: %d  ",pop(&stack));
92        printf("\n stack top: %d", stackTop(&stack));
93        printf("\n pop: %d  ",pop(&stack));
94        printf("\n stack top: %d", stackTop(&stack));
95    
96        deleteStack(&stack);
97        printf("\n stack deleted.");
98        printf("\n is stack empty: %d", isEmptyStack(&stack));   //here it should print 1, but its printing 0
99    
100        printf("\n push 1");
101        push(&stack,1);
102        printf("\n push 2");
103        push(&stack,2);
104    
105        printf("\n pop: %d  ",pop(&stack));
106        printf("\n pop: %d  ",pop(&stack));
107     return 0;
108    }

Output

push 1
push 2
push 3
stack top: 3
pop: 3  
stack top: 2
pop: 2  
stack top: 1
stack deleted.
is stack empty: 0
push 1
push 2
pop: 2  
pop: 1 

In deleteStack(), I free up all of the stack entry and set stack top to NULL. In isEmptyStack() check if the stack top is empty. So it should evaluate to 1 and line 98 should print 1 (i.e. true), but it prints 0. Whats wrong here? Here is the code on ideone.

Debugging efforts I took:

Answer 1

In function deleteStack, you don't set stack to null instead a copy of stack pointer to null.

What you do is:

topNode = *pStackTop;
topNode = NULL;

Which is incorrect.

What you should instead do is:

*pStackTop = NULL;