libc_group.rs

// gr_mem: Vec<String>
let mut char_vec: Vec<*mut c_char> = Vec::new();

for mem in self.gr_mem.into_iter() {
    let c_str = CString::new(mem).unwrap();
    let cstr_len = c_str.to_bytes_with_nul().len();
    let dst = buf.as_mut_ptr().offset(offset as isize) as *mut i8;
    std::ptr::copy_nonoverlapping(c_str.as_ptr(), dst , cstr_len);
    char_vec.push(dst);
    offset += cstr_len;
}
char_vec.push(std::ptr::null_mut());
let dst = buf.as_mut_ptr().offset(offset as isize) as *mut i8;
std::ptr::copy(char_vec.as_ptr(), dst as *mut *mut i8, char_vec.len());

g.gr_mem = dst as *mut *mut i8;