FFI wrapper

C's strtol converts text to a number in a given base and reports where it stopped reading through an output pointer. The raw interface is very error-prone; the task is to wrap it in a safe Rust function:

unsafe extern "C" {
    fn strtol(s: *const c_char, son: *mut *mut c_char, taban: c_int) -> c_long;
}

fn sayiya(s: &str, taban: i32) -> Result<i64, String> { .. }

The safe interface's responsibilities: converting the text into a CString (inner null byte error), finding the invalid character by looking at how far the pointer advanced, and returning the result in Rust types.

offset_from gives the element difference between two pointers within the same allocation:

fn sayiya(s: &str, taban: i32) -> Result<i64, String> {
    let c = CString::new(s).map_err(|_| String::from("metinde boş bayt var"))?;
    let mut son: *mut c_char = std::ptr::null_mut();
    // SAFETY: c is a valid C string; son is a writable pointer.
    let n = unsafe { strtol(c.as_ptr(), &mut son, taban) };
    // SAFETY: strtol sets son to somewhere inside c.
    let okunan = unsafe { son.cast_const().offset_from(c.as_ptr()) } as usize;
    match s[okunan..].chars().next() {
        None if okunan > 0 => Ok(n as i64),
        None => Err(String::from("boş girdi")),
        Some(k) => Err(format!("geçersiz karakter: {k}")),
    }
}