read() and write() are what everyone expects. 

They are what programmers are used to at a low level.

while (len > 0) {
     ssize_t ret;
     ret = write(fd, buf, len);
     if (ret < 0) {
  if (errno == EAGAIN || errno == EINTR)
              continue;
          err(1, "write failed: ");
     }
     buf += ret;
     len -= ret;
}