Function sbuf_drain

freebsd/kern/subr_sbuf.c:386–428

* Call the drain and process the return. */

 * Call the drain and process the return.
 */
static int
sbuf_drain(struct sbuf *s)
{
	int len;

	KASSERT(s->s_len > 0, ("Shouldn't drain empty sbuf %p", s));
	KASSERT(s->s_error == 0, ("Called %s with error on %p", __func__, s));

	if (SBUF_DODRAINTOEOR(s) && s->s_rec_off == 0)
		return (s->s_error = EDEADLK);
	len = s->s_drain_func(s->s_drain_arg, s->s_buf,
	    SBUF_DODRAINTOEOR(s) ? s->s_rec_off : s->s_len);
	if (len <= 0) {
		s->s_error = len ? -len : EDEADLK;
		return (s->s_error);
	}
	KASSERT(len > 0 && len <= s->s_len,
	    ("Bad drain amount %d for sbuf %p", len, s));
	s->s_len -= len;
	s->s_rec_off -= len;
	/*
	 * Fast path for the expected case where all the data was
	 * drained.
	 */
	if (s->s_len == 0) {
		/*
		 * When the s_buf is entirely drained, we need to remember if
		 * the last character was a '\n' or not for
		 * sbuf_nl_terminate().
		 */
		if (s->s_buf[len - 1] == '\n')
			SBUF_SETFLAG(s, SBUF_DRAINATEOL);
		else
			SBUF_CLEARFLAG(s, SBUF_DRAINATEOL);
		return (0);
	}
	/*
	 * Move the remaining characters to the beginning of the
	 * string.
	 */
	memmove(s->s_buf, s->s_buf + len, s->s_len);
	return (0);
}

/*
 * Append bytes to an sbuf.  This is the core function for appending