Segmentation fault (Core Dump) while creating stack using pointer

Could you please help me to find out why I'm getting the segmentation fault (Core Dumped) error?

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

struct stack {
    int size;
    int top;
    int *arr;
};

int main(){
struct stack *ptr;
ptr->size = 10; //Code is not going ahead after this line(Segmentation fault)
ptr->top = -1;
ptr->arr = (int *)malloc(ptr->size * sizeof(int));

return 0;
}

Trying to create a stack using pointers but while I run this code I m getting this error and I m not able to go ahead without solving this error. Wants to know what is the reason behind it. I read all the reasons for the segmentation error but was not able to solve the error.

Solution

You are getting segmentation fault because you have no ptr allocated.

ptr is struct stack*, a pointer.

And it was not even initialized.

from your code

    struct stack* ptr;
    ptr->size = 10;

But ptr points to what?

A safe way:

#include<stdlib.h>

typedef struct st_stack
{
    int  size;
    int  top;
    int* arr;
} Stack;

int main(void)
{
    Stack* ptr = (Stack*)malloc(sizeof(Stack));
    ptr->size = 10;
    ptr->top  = -1;
    ptr->arr  = (int*)malloc(ptr->size * sizeof(int));

    free(ptr->arr); // free array
    free(ptr); // free stack

    return 0;
}

Note to code police: I always cast the return of malloc(). Yes, I know about the C-FAQ from the 90's (never updated). I do this because, as many others, I do no like implicit conversions. For the compiler void* is ok. The cast is a reminder for the programmer of what is being allocated at each call, and helps to avoid many bugs. malloc() accepts any expression that evaluates to a size_t and in many cases it is hard to know, in a series of allocations, which is which.

C-FAQ list a big advantage of not using a cast: the compiler will alert you if you forget to #include stdlib.h. Well, if someone needs this kind of reminder there are bigger problems to solve. ;)