Gossip about redis source code 121

rio Of connwrite and connread How to write ？ Toss me for a long time , After referring to the open source code , Get it done ：

static size_t rioConnWrite(rio *r, const void *buf, size_t len) {
    UNUSED(r);
    UNUSED(buf);
    UNUSED(len);
    return 0; /* Error, this target does not yet support writing. */
}
 
/* Returns 1 or 0 for success/failure. */
static size_t rioConnRead(rio *r, void *buf, size_t len) {
    size_t avail = sdslen(r->io.conn.buf)-r->io.conn.pos;
 
    /* If the buffer is too small for the entire request: realloc. */
    if (sdslen(r->io.conn.buf) + sdsavail(r->io.conn.buf) < len)
        r->io.conn.buf = sdsMakeRoomFor(r->io.conn.buf, len - sdslen(r->io.conn.buf));
 
    /* If the remaining unused buffer is not large enough: memmove so that we
     * can read the rest. */
    if (len > avail && sdsavail(r->io.conn.buf) < len - avail) {
        sdsrange(r->io.conn.buf, r->io.conn.pos, -1);
        r->io.conn.pos = 0;
    }
 
    /* If we don't already have all the data in the sds, read more */
    while (len > sdslen(r->io.conn.buf) - r->io.conn.pos) {
        size_t buffered = sdslen(r->io.conn.buf) - r->io.conn.pos;
        size_t needs = len - buffered;
        /* Read either what's missing, or PROTO_IOBUF_LEN, the bigger of
         * the two. */
        size_t toread = needs < PROTO_IOBUF_LEN ? PROTO_IOBUF_LEN: needs;
        if (toread > sdsavail(r->io.conn.buf)) toread = sdsavail(r->io.conn.buf);
        if (r->io.conn.read_limit != 0 &&
            r->io.conn.read_so_far + buffered + toread > r->io.conn.read_limit)
        {
            /* Make sure the caller didn't request to read past the limit.
             * If they didn't we'll buffer till the limit, if they did, we'll
             * return an error. */
            if (r->io.conn.read_limit >= r->io.conn.read_so_far + len)
                toread = r->io.conn.read_limit - r->io.conn.read_so_far - buffered;
            else {
                errno = EOVERFLOW;
                return 0;
            }
        }
        int retval = connRead(r->io.conn.conn,
                          (char*)r->io.conn.buf + sdslen(r->io.conn.buf),
                          toread);
        if (retval <= 0) {
            if (errno == EWOULDBLOCK) errno = ETIMEDOUT;
            return 0;
        }
        sdsIncrLen(r->io.conn.buf, retval);
    }
 
    memcpy(buf, (char*)r->io.conn.buf + r->io.conn.pos, len);
    r->io.conn.read_so_far += len;
    r->io.conn.pos += len;
    return len;
}