Error in printing the stack - c

Question

I am trying to implement a program using stacks.But the stack is not getting displayed
#include<stdio.h>
int size=0,count=1,test=0;
struct Car
{
int registrationNo;
char *name;
};
struct ParkingLot
{
struct Car C[10];
};
struct stack
{
struct ParkingLot P;
int top;
} st;
int stfull()
{
if(st.top >= size-1)
return 1;
else
return 0;
}
void push(struct Car item) {
st.top++;
st.P.C[st.top] = item;
}
int stempty() {
if (st.top == -1)
return 1;
else
return 0;
}
void display() {
int i;
if (stempty())
printf("\nStack Is Empty!");
else {
//printf("%d\n",st.top);
for (i = 0; i<=st.top; i++)
printf("\n%s", st.P.C[i].name);
}
}
void Enter_ParkingLot()
{
struct Car CC;
int checkFull=stfull();
if(checkFull==1)
printf("Parking Lot is FUll\n");
else
{
CC.registrationNo=count;count++;
char ch[100];
printf("Enter name of owner\n");
scanf("%s",ch);
CC.name=ch;
push(CC);
}
}
int main()
{
printf("Enter size of Parking Lot\n");
st.top=-1;
scanf("%d",&size);
Enter_ParkingLot();
Enter_ParkingLot();
display();
return 0;
}
This was my input on terminal-
Enter size of Parking Lot
2
Enter name of owner
ABCD
Enter name of owner
EFGH
This was my output-
`#
`#
There was a blank line before first # in the output.

Answer 1

If you assign the pointer field in struct Car to a local variable, it will not work, you need to redeclare your struct Car like this
struct Car
{
int registrationNo;
char name[100];
};
and then instead of
CC.name=ch;
do it this way
strcpy(CC.name, ch);
also, it's better to write
scanf("%99s",ch);
to prevent overflowing ch, and in your case it would be even better to do
scanf("%99s", CC.name);
I fixed your code
#include <stdio.h>
#include <string.h>
struct Car
{
int registrationNo;
char name[100];
};
struct ParkingLot
{
struct Car C[10];
};
struct stack
{
struct ParkingLot P;
int top;
} st;
int stfull(int size)
{
if(st.top >= size - 1)
return 1;
return 0;
}
void push(struct Car item)
{
st.P.C[++(st.top)] = item;
}
int stempty()
{
if (st.top == -1)
return 1;
return 0;
}
void display()
{
int i;
if (stempty() != 0)
printf("\nStack Is Empty!");
else {
for (i = 0 ; i <= st.top ; i++)
printf("\n%s", st.P.C[i].name);
}
}
int Enter_ParkingLot(int count, int size)
{
struct Car CC;
if (stfull(size) == 1)
printf("Parking Lot is FUll\n");
else
{
CC.registrationNo = count;
printf("Enter name of owner\n");
scanf("%99s", CC.name);
push(CC);
}
return count + 1;
}
int main()
{
int size = 0, count = 1;
printf("Enter size of Parking Lot\n");
st.top = -1;
scanf("%d", &size);
count = Enter_ParkingLot(count, size);
count = Enter_ParkingLot(count, size);
display();
return 0;
}
I removed global variables, they where not needed.
I fixed some if/else's which made no sense.
I also applied my previously suggested fixes related to your original problem.

Answer 2

The Problem:
You store the name of the car into an array that is allocated on the stack.
You then copy the pointer to that array to your car object.
The original array then goes out of scope.
You then try to print that array, which no longer exists, via the pointer.
Undefined behavior occurs.
The Solution:
You need to allocate memory for the name of the car with malloc() and use strcpy() to copy it to the car object.