Realloc on NULL-valued (or undefined) pointer

Question

I was reading about realloc and got confused about a point mentioned there. Consider the code below:

#include 
#include 

int          


        

Answer 1

Is there any danger in allocating memory using realloc using the initially NULL-valued ptr?

No, that would exactly be like a malloc.

If instead of:

int* ptr = NULL;

I had this:

int* ptr; // no value given to ptr

would it be a problem to call realloc using ptr?

Yes, there would be a problem. If realloc doesn't get a NULL, it will try to expand memory starting from that location, or may try to free and malloc another part of memory. Since uninitialized variables can have any value, chances are very high, they are not a value realloc likes. If you are lucky, your program would immediately crash.