inserting string in stack using linked list - c

Question

i am confused with the code below. why does the input overwrite the elements in the stack.
for example i enter "abc" first
then the stack will be "123"
but when i enter another string, let's say "234"
the stack will be "234" "234"
but when i used int data type instead of char for my input, there's no error.
thanks!
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct node{
char* info;
struct node *ptr;
}*top,*top1,*temp;
int count = 0;
/* Push data into stack */
void pushy(char* data){
if (top == NULL)
{
top =(struct node *)malloc(1*sizeof(struct node));
top->ptr = NULL;
strcpy(top->info, data);
}
else
{
temp =(struct node *)malloc(1*sizeof(struct node));
temp->ptr = top;
strcpy(temp->info, data);
top = temp;
}
count++;
}
/* Display stack elements */
void display(){
top1 = top;
if (top1 == NULL)
{
printf("Stack is empty");
return;
}
while (top1 != NULL)
{
printf("%s ", top1->info);
top1 = top1->ptr;
}
}
/* Pop Operation on stack */
void pop(){
top1 = top;
if (top1 == NULL)
{
printf("\n Error : Trying to pop from empty stack");
return;
}
else
top1 = top1->ptr;
printf("\n Popped value : %s", top->info);
free(top);
top = top1;
count--;
}
/* Return top element */
char* topelement(){
return(top->info);
}
int main()
{
int ch=0;
printf("\n 1 - Push");
printf("\n 2 - Pop");
printf("\n 3 - Top");
printf("\n 4 - Display");
printf("\n 5 - Exit");
top = NULL;
while (1)
{
char no[10]={NULL};
char* e;
printf("\n Enter choice : ");
scanf("%d", &ch);
if(ch==1){
printf("Enter data : ");
scanf("\n\n%s", &no);
pushy(no);
}
else if(ch==2){
pop();
}
else if(ch==3){
if (top == NULL)
printf("No elements in stack");
else
{
e = topelement();
printf("\n Top element : %d", e);
}
}
else if(ch==4)
display();
else if(ch==5)
exit(0);
else
printf("Invalid");
}
}

Answer 1

At least function pushy has undefined behaviour beacuse it tries to overwrite memory that was not allocated by the function
void pushy(char* data){
if (top == NULL)
{
top =(struct node *)malloc(1*sizeof(struct node));
top->ptr = NULL;
strcpy(top->info, data);
^^^^^^^^^^^^^^^^^^^^^^^
}
//...
You should at first allocate memory that will be pointed to by data member top->info where you are going to copy the string pointed to by parameter data
For example
void pushy(char* data){
if (top == NULL)
{
top =(struct node *)malloc(1*sizeof(struct node));
top->ptr = NULL;
top->info = malloc( ( strlen( data ) + 1 ) * sizeof( char ) );
strcpy(top->info, data);
}
//...
The function could be written the following way
void pushy( const char* data )
{
temp = ( struct node * )malloc( sizeof( struct node ) );
temp->ptr = top;
temp->info = ( char * )malloc( ( strlen( data ) + 1 ) * sizeof( char ) );
strcpy( temp->info, data );
top = temp;
++count;
}
Of cpurse you have to free also the memory allocated for top->info when you will free the node itself in function pop().
Take into account that there is no need to declare variables top1 and temp like global.
Also in this call
scanf("\n\n%s", &no);
the second argument shall be specified simply like no instead of &no
And in this call
printf("\n Top element : %d", e);
you have to use format specifier %s instead of %d

Answer 2

scanf("\n\n%s", &no);
Remove '&' from the above statement.
Also, as per you code better to take info as char array instead of char *
struct node{
char info[20];
struct node *ptr;
}*top,*top1,*temp;

Answer 3

No memory is being allocated for the string in the node:
top =(struct node *)malloc(1*sizeof(struct node));
top->ptr = NULL;
strcpy(top->info, data);
When strcpy is called top->info pointer is invalid. You will have to allocate the memory, assing it to the pointer top->info and then copy the string into that memory.
Don't forget to free the memory held my info pointer before you free the entire node.