使用fopen用c语言写入txt

I am studying computer science and I am supposed to create a c shell for a project.In this part of the project I have to write the code for these types of commands : "sort < in.txt > out.txt". My problem is that after using execlp so as to sort my "in.txt", i cannot write the result in the "out.txt" that I create. My code is like:I am studying computer science and I am suppose