Checking if a string is a palindrome using a stack

Question

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>

typedef struct node
{
    char data;
    struct node *link;
}StackNode;

void insertData(StackNode **);
void push(StackNode **, char);
void checkData(StackNode **);
bool pop(StackNode **,char *);

char sent[20] = "";

void main()
{
   StackNode *stackTop;
   insertData(&stackTop);
   checkData(&stackTop);
   printf("\n");
   return;
}

void insertData(StackNode **stackTop)
{
    char c;
    int len;

    printf("Enter the Sentence\n");
    while( ( ( c = getchar() ) != '\n'))
    {   
        if( ( ( c>='a' &&c<='z') || (c>='A' && c<='Z')))
        {
            if((c>='A' && c<='Z'))
            {
                int rem;
                rem = c-'A';
                c='a' + rem;
            }
            push(stackTop,c);
            len = strlen(sent);
            sent[len++]=c;
            sent[len]='\0';
        }
    }
    printf("Letters are %s\n\n",sent);
}

void push(StackNode **stackTop,char c)
{
    StackNode *pNew;
    pNew = (StackNode*) malloc(sizeof(StackNode));
    if(!pNew)
    {
        printf("Error 100:Out of memory\n");
        exit(100);
    }
    pNew->data = c;
    pNew->link = *stackTop;
    *stackTop = pNew;
}

void checkData(StackNode **stackTop)
{
    char c;
    int i=0;
    while(pop(stackTop,&c))
    {
        if( c !=sent[i++])
        {
            printf("Not palindrome");
            return;
        }
    }
    printf("Palindrome");
}

bool pop(StackNode **stackTop,char *c)
{
    StackNode *pNew;
    pNew = *stackTop;
    if(pNew == NULL)
        return false;
    *c = pNew->data;
    *stackTop = pNew->link;
    printf("char poped %c\n",*c);
    free(pNew);
    return true;
}

I am able to pop each letter, after that the exe file is not working, I think I'm unable to check if the letters are same in reverse order. please help

I've been working all day to write this program,but I'm stuck at this point.

Answer 1

When programming in C, you should always initialize pointers to NULL. Because you didn't initialize the StackNode *stackTop (in main) it is pointing to garbage and is not null. So when you are checking for a palindrome, your pop method just keeps going off the end of the stack and never returns false because it never hits a null. StackNode *stackTop = NULL; should fix your problem.