#endif /* TCP_LISTEN_BACKLOG */
  }

  return newconn;
}

/**
 * Receive data (in form of a netbuf containing a packet buffer) from a netconn
 *
 * @param conn the netconn from which to receive data
 * @return a new netbuf containing received data or NULL on memory error or timeout
 */
struct netbuf *
netconn_recv(struct netconn *conn)
{
  struct api_msg msg;
  struct netbuf *buf = NULL;
  struct pbuf *p;
  u16_t len;

  LWIP_ERROR("netconn_recv: invalid conn",  (conn != NULL), return NULL;);

  if (conn->recvmbox == SYS_MBOX_NULL) {
    /* @todo: should calling netconn_recv on a TCP listen conn be fatal (ERR_CONN)?? */
    /* TCP listen conns don't have a recvmbox! */
    conn->err = ERR_CONN;
    return NULL;
  }

  if (ERR_IS_FATAL(conn->err)) {
    return NULL;
  }

  if (conn->type == NETCONN_TCP) {
#if LWIP_TCP
    if (conn->state == NETCONN_LISTEN) {
      /* @todo: should calling netconn_recv on a TCP listen conn be fatal?? */
      conn->err = ERR_CONN;
      return NULL;
    }

    buf = memp_malloc(MEMP_NETBUF);

    if (buf == NULL) {
      conn->err = ERR_MEM;
      return NULL;
    }

#if LWIP_SO_RCVTIMEO
    if (sys_arch_mbox_fetch(conn->recvmbox, (void *)&p, conn->recv_timeout)==SYS_ARCH_TIMEOUT) {
      conn->err = ERR_TIMEOUT;
      p = NULL;
    }
#else
    sys_arch_mbox_fetch(conn->recvmbox, (void *)&p, 0);
#endif /* LWIP_SO_RCVTIMEO*/

    if (p != NULL) {
      len = p->tot_len;
      SYS_ARCH_DEC(conn->recv_avail, len);
    } else {
      len = 0;
    }

    /* Register event with callback */
    API_EVENT(conn, NETCONN_EVT_RCVMINUS, len);

    /* If we are closed, we indicate that we no longer wish to use the socket */
    if (p == NULL) {
      memp_free(MEMP_NETBUF, buf);
      /* Avoid to lose any previous error code */
      if (conn->err == ERR_OK) {
        conn->err = ERR_CLSD;
      }
      return NULL;
    }

    buf->p = p;
    buf->ptr = p;
    buf->port = 0;
    buf->addr = NULL;

    /* Let the stack know that we have taken the data. */
    msg.function = do_recv;
    msg.msg.conn = conn;
    if (buf != NULL) {
      msg.msg.msg.r.len = buf->p->tot_len;
    } else {
      msg.msg.msg.r.len = 1;
    }
    TCPIP_APIMSG(&msg);
#endif /* LWIP_TCP */
  } else {
#if (LWIP_UDP || LWIP_RAW)
#if LWIP_SO_RCVTIMEO
    if (sys_arch_mbox_fetch(conn->recvmbox, (void *)&buf, conn->recv_timeout)==SYS_ARCH_TIMEOUT) {
      buf = NULL;
    }
#else
    sys_arch_mbox_fetch(conn->recvmbox, (void *)&buf, 0);
#endif /* LWIP_SO_RCVTIMEO*/
    if (buf!=NULL) {
      SYS_ARCH_DEC(conn->recv_avail, buf->p->tot_len);
      /* Register event with callback */
      API_EVENT(conn, NETCONN_EVT_RCVMINUS, buf->p->tot_len);
    }
#endif /* (LWIP_UDP || LWIP_RAW) */
  }

  LWIP_DEBUGF(API_LIB_DEBUG, ("netconn_recv: received %p (err %d)\n", (void *)buf, conn->err));

  return buf;
}

/**
 * Send data (in form of a netbuf) to a specific remote IP address and port.
 * Only to be used for UDP and RAW netconns (not TCP).
 *
 * @param conn the netconn over which to send data
 * @param buf a netbuf containing the data to send
 * @param addr the remote IP address to which to send the data
 * @param port the remote port to which to send the data
 * @return ERR_OK if data was sent, any other err_t on error
 */
err_t
netconn_sendto(struct netconn *conn, struct netbuf *buf, struct ip_addr *addr, u16_t port)
{
  if (buf != NULL) {
    buf->addr = addr;
    buf->port = port;
    return netconn_send(conn, buf);
  }
  return ERR_VAL;
}

/**
 * Send data over a UDP or RAW netconn (that is already connected).
 *
 * @param conn the UDP or RAW netconn over which to send data
 * @param buf a netbuf containing the data to send
 * @return ERR_OK if data was sent, any other err_t on error
 */
err_t
netconn_send(struct netconn *conn, struct netbuf *buf)
{
  struct api_msg msg;

  LWIP_ERROR("netconn_send: invalid conn",  (conn != NULL), return ERR_ARG;);

  LWIP_DEBUGF(API_LIB_DEBUG, ("netconn_send: sending %"U16_F" bytes\n", buf->p->tot_len));
  msg.function = do_send;
  msg.msg.conn = conn;
  msg.msg.msg.b = buf;
  TCPIP_APIMSG(&msg);
  return conn->err;
}

/**
 * Send data over a TCP netconn.
 *
 * @param conn the TCP netconn over which to send data
 * @param dataptr pointer to the application buffer that contains the data to send
 * @param size size of the application data to send
 * @param apiflags combination of following flags :
 * - NETCONN_COPY (0x01) data will be copied into memory belonging to the stack
 * - NETCONN_MORE (0x02) for TCP connection, PSH flag will be set on last segment sent
 * @return ERR_OK if data was sent, any other err_t on error
 */
err_t
netconn_write(struct netconn *conn, const void *dataptr, size_t size, u8_t apiflags)
{
  struct api_msg msg;

  LWIP_ERROR("netconn_write: invalid conn",  (conn != NULL), return ERR_ARG;);
  LWIP_ERROR("netconn_write: invalid conn->type",  (conn->type == NETCONN_TCP), return ERR_VAL;);

  msg.function = do_write;
  msg.msg.conn = conn;
  msg.msg.msg.w.dataptr = dataptr;
  msg.msg.msg.w.apiflags = apiflags;
  msg.msg.msg.w.len = size;
  /* For locking the core: this _can_ be delayed on low memory/low send buffer,
     but if it is, this is done inside api_msg.c:do_write(), so we can use the
     non-blocking version here. */
  TCPIP_APIMSG(&msg);
  return conn->err;
}

/**
 * Close a TCP netconn (doesn't delete it).
 *
 * @param conn the TCP netconn to close
 * @return ERR_OK if the netconn was closed, any other err_t on error
 */
err_t
netconn_close(struct netconn *conn)
{
  struct api_msg msg;

  LWIP_ERROR("netconn_close: invalid conn",  (conn != NULL), return ERR_ARG;);

  msg.function = do_close;
  msg.msg.conn = conn;
  tcpip_apimsg(&msg);
  return conn->err;
}

#if LWIP_IGMP
/**
 * Join multicast groups for UDP netconns.
 *
 * @param conn the UDP netconn for which to change multicast addresses
 * @param multiaddr IP address of the multicast group to join or leave
 * @param interface the IP address of the network interface on which to send
 *                  the igmp message
 * @param join_or_leave flag whether to send a join- or leave-message
 * @return ERR_OK if the action was taken, any err_t on error
 */
err_t
netconn_join_leave_group(struct netconn *conn,
                         struct ip_addr *multiaddr,
                         struct ip_addr *interface,
                         enum netconn_igmp join_or_leave)
{
  struct api_msg msg;

  LWIP_ERROR("netconn_join_leave_group: invalid conn",  (conn != NULL), return ERR_ARG;);

  msg.function = do_join_leave_group;
  msg.msg.conn = conn;
  msg.msg.msg.jl.multiaddr = multiaddr;
  msg.msg.msg.jl.interface = interface;
  msg.msg.msg.jl.join_or_leave = join_or_leave;
  TCPIP_APIMSG(&msg);
  return conn->err;
}
#endif /* LWIP_IGMP */

#if LWIP_DNS
/**
 * Execute a DNS query, only one IP address is returned
 *
 * @param name a string representation of the DNS host name to query
 * @param addr a preallocated struct ip_addr where to store the resolved IP address
 * @return ERR_OK: resolving succeeded
 *         ERR_MEM: memory error, try again later
 *         ERR_ARG: dns client not initialized or invalid hostname
 *         ERR_VAL: dns server response was invalid
 */
err_t
netconn_gethostbyname(const char *name, struct ip_addr *addr)
{
  struct dns_api_msg msg;
  err_t err;
  sys_sem_t sem;

  LWIP_ERROR("netconn_gethostbyname: invalid name", (name != NULL), return ERR_ARG;);
  LWIP_ERROR("netconn_gethostbyname: invalid addr", (addr != NULL), return ERR_ARG;);

  sem = sys_sem_new(0);
  if (sem == SYS_SEM_NULL) {
    return ERR_MEM;
  }

  msg.name = name;
  msg.addr = addr;
  msg.err = &err;
  msg.sem = sem;

  tcpip_callback(do_gethostbyname, &msg);
  sys_sem_wait(sem);
  sys_sem_free(sem);

  return err;
}
#endif /* LWIP_DNS*/

#endif /* LWIP_NETCONN */