I am trying to implement stack in C using pointer , but when I create a pointer and try and assign s->top = -1, it shows segmentation fault

Question

I declared a structure like this

#define max_size 100

struct lifo
{
    int top;
    int st[max_size];
};

I made a function like so:

void create(stack *s)
{
    s->top=-1;
}

I declared a pointer in the main() function:

struct lifo *s;

I called this function like so:

create(s);

but it shows "segmentation fault" at

s->top=-1

I am not sure what is the error