LWIP之lwip_select函数

最新推荐文章于 2025-05-26 14:29:43 发布

glassine

最新推荐文章于 2025-05-26 14:29:43 发布

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本文解析了 lwip_select 函数的实现原理，该函数用于检查一组文件描述符的活动状态，并在没有活动时等待指定的时间间隔。文章详细介绍了如何通过扫描套接字列表来确定当前就绪状态，并在必要时等待事件的发生。

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代码如下：

/**
 * Processing exceptset is not yet implemented.
 */
int
lwip_select(int maxfdp1, fd_set *readset, fd_set *writeset, fd_set *exceptset,
            struct timeval *timeout)
{
  u32_t waitres = 0;
  int nready;
  fd_set lreadset, lwriteset, lexceptset;
  u32_t msectimeout;
  struct lwip_select_cb select_cb;
  err_t err;
  int i;
  SYS_ARCH_DECL_PROTECT(lev);

  LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_select(%d, %p, %p, %p, tvsec=%"S32_F" tvusec=%"S32_F")\n",
                  maxfdp1, (void *)readset, (void *) writeset, (void *) exceptset,
                  timeout ? (s32_t)timeout->tv_sec : (s32_t)-1,
                  timeout ? (s32_t)timeout->tv_usec : (s32_t)-1));

  /* Go through each socket in each list to count number of sockets which
     currently match */
  nready = lwip_selscan(maxfdp1, readset, writeset, exceptset, &lreadset, &lwriteset, &lexceptset);

  /* If we don't have any current events, then suspend if we are supposed to */
  if (!nready) {
    if (timeout && timeout->tv_sec == 0 && timeout->tv_usec == 0) {
      LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_select: no timeout, returning 0\n"));
      /* This is OK as the local fdsets are empty and nready is zero,
         or we would have returned earlier. */
      goto return_copy_fdsets;
    }

    /* None ready: add our semaphore to list:
       We don't actually need any dynamic memory. Our entry on the
       list is only valid while we are in this function, so it's ok
       to use local variables. */

    select_cb.next = NULL;
    select_cb.prev = NULL;
    select_cb.readset = readset;
    select_cb.writeset = writeset;
    select_cb.exceptset = exceptset;
    select_cb.sem_signalled = 0;
    err = sys_sem_new(&select_cb.sem, 0);
    if (err != ERR_OK) {
      /* failed to create semaphore */
      set_errno(ENOMEM);
      return -1;
    }

    /* Protect the select_cb_list */
    SYS_ARCH_PROTECT(lev);

    /* Put this select_cb on top of list */
    select_cb.next = select_cb_list;
    if (select_cb_list != NULL) {
      select_cb_list->prev = &select_cb;
    }
    select_cb_list = &select_cb;
    /* Increasing this counter tells even_callback that the list has changed. */
    select_cb_ctr++;

    /* Now we can safely unprotect */
    SYS_ARCH_UNPROTECT(lev);

    /* Increase select_waiting for each socket we are interested in */
    for(i = 0; i < maxfdp1; i++) {
      if ((readset && FD_ISSET(i, readset)) ||
          (writeset && FD_ISSET(i, writeset)) ||
          (exceptset && FD_ISSET(i, exceptset))) {
        struct lwip_sock *sock = tryget_socket(i);
        LWIP_ASSERT("sock != NULL", sock != NULL);
        SYS_ARCH_PROTECT(lev);
        sock->select_waiting++;
        LWIP_ASSERT("sock->select_waiting > 0", sock->select_waiting > 0);
        SYS_ARCH_UNPROTECT(lev);
      }
    }

    /* Call lwip_selscan again: there could have been events between
       the last scan (whithout us on the list) and putting us on the list! */
    nready = lwip_selscan(maxfdp1, readset, writeset, exceptset, &lreadset, &lwriteset, &lexceptset);
    if (!nready) {
      /* Still none ready, just wait to be woken */
      if (timeout == 0) {
        /* Wait forever */
        msectimeout = 0;
      } else {
        msectimeout =  ((timeout->tv_sec * 1000) + ((timeout->tv_usec + 500)/1000));
        if (msectimeout == 0) {
          /* Wait 1ms at least (0 means wait forever) */
          msectimeout = 1;
        }
      }

      waitres = sys_arch_sem_wait(&select_cb.sem, msectimeout);
    }
    /* Increase select_waiting for each socket we are interested in */
    for(i = 0; i < maxfdp1; i++) {
      if ((readset && FD_ISSET(i, readset)) ||
          (writeset && FD_ISSET(i, writeset)) ||
          (exceptset && FD_ISSET(i, exceptset))) {
        struct lwip_sock *sock = tryget_socket(i);
        LWIP_ASSERT("sock != NULL", sock != NULL);
        SYS_ARCH_PROTECT(lev);
        sock->select_waiting--;
        LWIP_ASSERT("sock->select_waiting >= 0", sock->select_waiting >= 0);
        SYS_ARCH_UNPROTECT(lev);
      }
    }
    /* Take us off the list */
    SYS_ARCH_PROTECT(lev);
    if (select_cb.next != NULL) {
      select_cb.next->prev = select_cb.prev;
    }
    if (select_cb_list == &select_cb) {
      LWIP_ASSERT("select_cb.prev == NULL", select_cb.prev == NULL);
      select_cb_list = select_cb.next;
    } else {
      LWIP_ASSERT("select_cb.prev != NULL", select_cb.prev != NULL);
      select_cb.prev->next = select_cb.next;
    }
    /* Increasing this counter tells even_callback that the list has changed. */
    select_cb_ctr++;
    SYS_ARCH_UNPROTECT(lev);

    sys_sem_free(&select_cb.sem);
    if (waitres == SYS_ARCH_TIMEOUT)  {
      /* Timeout */
      LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_select: timeout expired\n"));
      /* This is OK as the local fdsets are empty and nready is zero,
         or we would have returned earlier. */
      goto return_copy_fdsets;
    }

    /* See what's set */
    nready = lwip_selscan(maxfdp1, readset, writeset, exceptset, &lreadset, &lwriteset, &lexceptset);
  }

  LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_select: nready=%d\n", nready));
return_copy_fdsets:
  set_errno(0);
  if (readset) {
    *readset = lreadset;
  }
  if (writeset) {
    *writeset = lwriteset;
  }
  if (exceptset) {
    *exceptset = lexceptset;
  }


  return nready;
}