}
}
pollfd->revents = mask;
return mask;
}
这个函数很简单，先根据socket描述符或者是文件句柄找到进程对应的struct file *file结构，然后调用file->f_op->poll(file,pwait)，这是这个函数的核心调用，这其实也是linux的VFS的一部分，这会根据当前的文件是什么类型的文件来选择调用的入口，如file是socket网络文件，此时调用的就是由网络驱动设备来实现的poll，如果file是ext3等文件系统上打开的一个文件，那就会调用由该文件系统来实现的poll函数，我们以tcp_poll为例来了解一般poll完成什么工作；
注意下面的参数，file和wait是由file->f_op->poll调用传入的参数，而struct socket为socket连接的进程方面表示。
[cpp]
unsigned int tcp_poll(struct file *file, struct socket *sock, poll_table *wait)
{
unsigned int mask;
struct sock *sk = sock->sk;
struct tcp_sock *tp = tcp_sk(sk);
poll_wait(file, sk->sk_sleep, wait);
if (sk->sk_state == TCP_LISTEN)
return inet_csk_listen_poll(sk);
/* Socket is not locked. We are protected from async events
by poll logic and correct handling of state changes
made by another threads is impossible in any case.
*/
mask = 0;
if (sk->sk_err)
mask = POLLERR;
/*
* POLLHUP is certainly not done right. But poll() doesn't
* have a notion of HUP in just one direction, and for a
* socket the read side is more interesting.
*
* Some poll() documentation says that POLLHUP is incompatible
* with the POLLOUT/POLLWR flags, so somebody should check this
* all. But careful, it tends to be safer to return too many
* bits than too few, and you can easily break real applications
* if you don't tell them that something has hung up!
*
* Check-me.
* Check number 1. POLLHUP is _UNMASKABLE_ event (see UNIX98 and
* our fs/select.c). It means that after we received EOF,
* poll always returns immediately, making impossible poll() on write()
* in state CLOSE_WAIT. One solution is evident --- to set POLLHUP
* if and only if shutdown has been made in both directions.
* Actually, it is interesting to look how Solaris and DUX
* solve this dilemma. I would prefer, if PULLHUP were maskable,
* then we could set it on SND_SHUTDOWN. BTW examples given
* in Stevens' books assume exactly this behaviour, it explains
* why PULLHUP is incompatible with POLLOUT. --ANK
*
* NOTE. Check for TCP_CLOSE is added. The goal is to prevent
* blocking on fresh not-connected or disconnected socket. --ANK
*/
if (sk->sk_shutdown == SHUTDOWN_MASK || sk->sk_state == TCP_CLOSE)
mask |= POLLHUP;
if (sk->sk_shutdown & RCV_SHUTDOWN)
mask |= POLLIN | POLLRDNORM | POLLRDHUP;
/* Connected */
if ((1 << sk->sk_state) & ~(TCPF_SYN_SENT | TCPF_SYN_RECV)) {
/* Potential race condition. If read of tp below will
* escape above sk->sk_state, we can be illegally awaken
* in SYN_* states. */
if ((tp->rcv_nxt != tp->copied_seq) &&
(tp->urg_seq != tp->copied_seq ||
tp->rcv_nxt != tp->copied_seq + 1 ||
sock_flag(sk, SOCK_URGINLINE) || !tp->urg_data))
mask |= POLLIN | POLLRDNORM;
if (!(sk->sk_shutdown & SEND_SHUTDOWN)) {
if (sk_stream_wspace(sk) >= sk_stream_min_wspace(sk)) {
mask |= POLLOUT | POLLWRNORM;
} else { /* send SIGIO later */
set_bit(SOCK_ASYNC_NOSPACE,
&sk->sk_socket->flags);
set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
/* Race breaker. If space is freed after
* wspace test but before t