qsort() practice

The practice is actually a question from my OOP344 TEST1. Requirement is as follows:
$ Q2 hello i am writing a text
a
am
hello
i
text
writing
$_
Write a program to produce the output above, using qsort() fucntion.

The solution is here. It's really a simple function, however I didn't do it completely correct during the exam. I think there are two reasons: first, I am not familiar with the qsort() function, though the spec is given; second, I didn't practice how to pass a function pointer as a parameter to a function.