POSIX

RT-Thread POSIX支持

发布于 2016-05-08 12:19:34 浏览：4599 订阅该版

4 个回答

bernard 2016-05-10

这家伙很懒，什么也没写！

bernard 2016-05-22

这家伙很懒，什么也没写！

如果需要在lwip之外支持select接口（例如能够让fd_set支持到文件描述符，以及后续扩展的UNIX套接字），需要放弃lwip目前已有的select接口

lwIP的select实现类似于：

在创建一个socket时就默认设置一个事件callback函数，在一些事件触发时回调这个函数：

```

/**
 * Callback registered in the netconn layer for each socket-netconn.
 * Processes recvevent (data available) and wakes up tasks waiting for select.
 */
static void
event_callback(struct netconn *conn, enum netconn_evt evt, u16_t len)
{
  int s;
  struct lwip_sock *sock;
  struct lwip_select_cb *scb;
  int last_select_cb_ctr;
  SYS_ARCH_DECL_PROTECT(lev);

LWIP_UNUSED_ARG(len);

/* Get socket */
  if (conn) {
    s = conn->socket;
    if (s < 0) {
      /* Data comes in right away after an accept, even though
       * the server task might not have created a new socket yet.
       * Just count down (or up) if that's the case and we
       * will use the data later. Note that only receive events
       * can happen before the new socket is set up. */
      SYS_ARCH_PROTECT(lev);
      if (conn->socket < 0) {
        if (evt == NETCONN_EVT_RCVPLUS) {
          conn->socket--;
        }
        SYS_ARCH_UNPROTECT(lev);
        return;
      }
      s = conn->socket;
      SYS_ARCH_UNPROTECT(lev);
    }

sock = get_socket(s);
    if (!sock) {
      return;
    }
  } else {
    return;
  }

SYS_ARCH_PROTECT(lev);
  /* Set event as required */
  switch (evt) {
    case NETCONN_EVT_RCVPLUS:
      sock->rcvevent++;
      break;
    case NETCONN_EVT_RCVMINUS:
      sock->rcvevent--;
      break;
    case NETCONN_EVT_SENDPLUS:
      sock->sendevent = 1;
      break;
    case NETCONN_EVT_SENDMINUS:
      sock->sendevent = 0;
      break;
    case NETCONN_EVT_ERROR:
      sock->errevent = 1;
      break;
    default:
      LWIP_ASSERT("unknown event", 0);
      break;
  }

if (sock->select_waiting == 0) {
    /* noone is waiting for this socket, no need to check select_cb_list */
    SYS_ARCH_UNPROTECT(lev);
    return;
  }

/* Now decide if anyone is waiting for this socket */
  /* NOTE: This code goes through the select_cb_list list multiple times
     ONLY IF a select was actually waiting. We go through the list the number
     of waiting select calls + 1. This list is expected to be small. */

/* At this point, SYS_ARCH is still protected! */
again:
  for (scb = select_cb_list; scb != NULL; scb = scb->next) {
    if (scb->sem_signalled == 0) {
      /* semaphore not signalled yet */
      int do_signal = 0;
      /* Test this select call for our socket */
      if (sock->rcvevent > 0) {
        if (scb->readset && FD_ISSET(s, scb->readset)) {
          do_signal = 1;
        }
      }
      if (sock->sendevent != 0) {
        if (!do_signal && scb->writeset && FD_ISSET(s, scb->writeset)) {
          do_signal = 1;
        }
      }
      if (sock->errevent != 0) {
        if (!do_signal && scb->exceptset && FD_ISSET(s, scb->exceptset)) {
          do_signal = 1;
        }
      }
      if (do_signal) {
        scb->sem_signalled = 1;
        /* Don't call SYS_ARCH_UNPROTECT() before signaling the semaphore, as this might
           lead to the select thread taking itself off the list, invalidagin the semaphore. */
        sys_sem_signal(&scb->sem);
      }
    }
    /* unlock interrupts with each step */
    last_select_cb_ctr = select_cb_ctr;
    SYS_ARCH_UNPROTECT(lev);
    /* this makes sure interrupt protection time is short */
    SYS_ARCH_PROTECT(lev);
    if (last_select_cb_ctr != select_cb_ctr) {
      /* someone has changed select_cb_list, restart at the beginning */
      goto again;
    }
  }
  SYS_ARCH_UNPROTECT(lev);
}

```

在这个其中，依据事件的类型来设置一些参数：

```

/* Set event as required */
  switch (evt) {
    case NETCONN_EVT_RCVPLUS:
      sock->rcvevent++;
      break;
    case NETCONN_EVT_RCVMINUS:
      sock->rcvevent--;
      break;
    case NETCONN_EVT_SENDPLUS:
      sock->sendevent = 1;
      break;
    case NETCONN_EVT_SENDMINUS:
      sock->sendevent = 0;
      break;
    case NETCONN_EVT_ERROR:
      sock->errevent = 1;
      break;
    default:
      LWIP_ASSERT("unknown event", 0);
      break;
  }

```

以及确实触发了事件，则发送一个信号量，这个在lwip中是以sys_sem_signal的方式进行：

```

if (do_signal) {
        scb->sem_signalled = 1;
        /* Don't call SYS_ARCH_UNPROTECT() before signaling the semaphore, as this might
           lead to the select thread taking itself off the list, invalidagin the semaphore. */
        sys_sem_signal(&scb->sem);
      }

```

因为lwIP为了减少移植接口，所以基本上只采取一些最基本的任务间同步的机制，这里如果使用event flag的方式会更好。

即，如果要在外部实现对lwIP的select操作，需要对socket设置不同的event callback回调函数，并根据类似的情况来设置socket中的rcvevent/sendevent值。