Now that I know I can read a single group, the next step is to iterate.

Iteration of this C API requires the ability to test for the end of iteration. For this, I use the std::ptr to test for a null pointer. To include this in the rust file:

use std::ptr;

To test for the a null pointer in a while loop:

This style of while loop requires calling the getgrpent function twice. I don’t love that, but it seems to be the clearest code. here is the whole loop:

fn enumerate_groups(){
    let mut groupent: * const GroupEnt;
 
    unsafe{
        setgrent();
        groupent = getgrent();
    }
    while groupent != ptr::null(){
        let c_str: &CStr = unsafe { CStr::from_ptr((*groupent).gr_name) };
        println!("{}", c_str.to_str().unwrap());
        unsafe{
            groupent = getgrent();
        }        
    }       
    unsafe{
        endgrent();
    }
}

The multiple unsafe blocks are to try an isolate the unsafe portions, but also to enable refactoring as a follow on step.