Can't understand why its not working when using two variables, the same works if i use just a single variable

#include <stdio.h>

int main()
{
char *a,*b;
scanf("%s %s",a,b);
printf("%s %s",a,b);
return 0;
} 

It works like this,

#include <stdio.h> 
int main(){
    char *a;
    scanf("%s",a);
    printf("%s",a);
    return 0;
}

I think its something with memory becoz when i use malloc and assign some memory it works.

>Solution :

It is undefined behaviour (UB) in both cases as you pass the pointer which was not assigned with reference to the allocated valid memory, large enough to accommodate the scanned strings. As it is a UB it does not have to express itself in any particular way. So single variable version does not work as well.

int main(void)
{
   char x[10], y[10];
   char *a = x,*b = y;
   /* .... */
}

int main(void)
{
   
   char *a = malloc(10),*b = malloc(10);
   /* .... */
   free(a);
   free(b);
}

int main(void)
{
   char a[10], b[10;
   /* .... */
}