Pointer [HELP]

Question

#include <stdio.h>

int main()
{
int num = 10;
int * ptr = NULL;
int *pt = &num;

ptr = &num;

printf("Address of num = %x\n", &num);
printf("Value of num = %d\n", num);

printf("\n");

printf("Address of ptr = %x\n", &ptr);
printf("Value of ptr = %x\n", ptr);
printf("Value pointed by ptr = %d\n", *ptr);

printf("\n");

printf("Address of pt = %x\n", &pt);
printf("Value of pt = %x\n", pt);
printf("Value pointed by pt = %d\n", *pt);

return 0;
}

what's the difference between storing address of a value to a pointer while declaring pointer and after declaring pointer?

After declaring pointer why can't i assign ,like:
int *ptr = NULL;
*ptr = &num ?

what's the difference between say, &ptr, and ptr?

Does ptr store an address and value temporary and it's real address &ptr(?) doesn't change?

Answer 1

Tincture haven't gone through your entire code, but quick check gives me some hint to post...

*ptr = &num is illegal ... why ? because &num is address of num variable which is of type pointer. so, ptr = & num will work due to ptr is of type pointer... now coming to your point of *ptr = &num... *ptr is no more pointer... it represents value pointed by pointer variable... so, there you can assign value like *ptr = 10 but ptr can't have assigned value of type int...

regarding int* ptr = &num ; is declaration and definition in a single line...

int* ptr; // declaration
ptr = &num; // definition

both are same... just a different way...