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The result might be compiler/platform specific. I think that there's no any warranty that the compiler will put local variables on 2 sequential adjacent addresses.The difference would be 1 if a and b were elements of the same array, for local two local variables the result might be surprising.

In this case this will come as 1 or -1 depending on compiler.
but if tweak the code a bit to
d1 = &a;
d2 = &b;
and diff = d1-d2, then result will =/-4 for int.
This gets lil confusing.

I think output for all four cases will be 1.
In C difference between two addresses of same data type is usually 1, because addresses for memory is allocated to variable from heap memory.
And thus the difference between two consecutive addresses is 1

Memory allocation is done on stack. So, the difference will be 1 in all the cases.

The answer I get from my compiler is 3, 3, 12, and 2. The C Standard makes no guarantees about packing of local variables. Moreover, as for subtracting two pointers the Standard states "Unless both pointers point to elements of the same array object, or one past the last element of the array object, the behavior is undefined." So, we have undefined behavior. And don't get me started on "void main".

The reason for 1 is , in C the pointer will be incremented always by the size of pointer type, for example if the pointer type is int and size of int is 4 bytes, and you increment the pointer it always increment the pointer by 4 bytes, and similarly if you do addition or subtraction it is always %sizeofpointer, in this case it's %4.
This below example gave me bit clarity.

#include<stdio.h>

main()
{
	int arr[2]={10,20};
	
   	printf("%0x\n",&arr[0]);
   	printf("%0x\n",&arr[1]);
   	printf("%d\n",(char*)&arr[1]- (char*)&arr[0]);
}

declare diff variable as a void pointer like.....void *diff...it won,t throw error..

It seems to be size of int, float, char etc.

I think error because how can u decler variable again and again in c