Find string length using pointers
type of difference between pointers

pointer_one - pointer_two


The result of this expression is a ptrdiff_t (§6.5.6/9).  Assigning it back to a pointer converts an integer type to a pointer, which IIRC should give you a compiler warning as it can lead to all kind of trouble. 

relational operators with pointers

Comparing pointers with relative operators is undefined behavior if the pointers don't point into the same array (§6.5.8/5).

else if(s1&gt;s2) // undefined behavior
{
  return 1;
}
else if(s1&lt;s2) // undefined behavior
{
  return -1;
} 


naming

cmpstr isn't really a suitable name for a function that compares the lengths of two strings.

comments


  // if s1 string is equal to s2 string


You don't compare strings for equality.  You (try to) compare their lengths.

Also, your comments should explain why you wrote the code the way you did. Don't tell what the code is doing. With well written code that should be self apparent.

do one thing per function

cmpstr calculates the length of two strings (code duplication!) and then compares these.  That's too much.  Delegate that length calculation to another function: strlen.  Or - for learning purpose only - write your own replacement.

ptrdiff_t vs size_t

Wondered why strlen returns a size_t instead of a ptrdiff_t?  That's because a string might be longer than the maximum difference that can be represented by ptrdiff_t.  Thus end - start may actually be undefined behavior when done with really long strings.  Better keep a size_t counter that you increase while searching for the '\0'.

program output should end with a '\n'

This is only minor, but otherwise it may corrupt the controlling terminal a bit.  Or like in ideone drop the output completely.

printf("%d\n", cmpstr(hello, world));
//        ^^


test your code

You really should!  Every (in an ideal world) piece of code that you write should be tested. You might want to look into TDD (test driven development) and build up good habits from the start.

making it faster

Do you really need to obtain the length of both strings?  Take these two strings as example:

""
"the quick brown fox did something but I don't remember what"


When you start at the first char of each of those, at how many chars do you need to look to determine which string is longer?