監聽套接字是非阻塞的，accept()返回的連接套接字是否也是阻塞的？

以muduo爲例，在類Acceptor中，成員變量監聽套接字acceptSocket_在Acceptor構造時調用socket::createNonblockingOrDie()， 它的實現如下：

int sockets::createNonblockingOrDie(sa_family_t family)
{
#if VALGRIND
	int sockfd = ::socket(family, SOCK_STREAM, IPPROTO_TCP);
	if (sockfd < 0)
	{
		LOG_SYSFATAL << "sockets::createNonblockingOrDie";
	}

	setNonBlockAndCloseOnExec(sockfd);
#else
	int sockfd = ::socket(family, SOCK_STREAM | SOCK_NONBLOCK | SOCK_CLOEXEC, IPPROTO_TCP);
	if (sockfd < 0)
	{
		LOG_SYSFATAL << "sockets::createNonblockingOrDie";
	}
#endif
	return sockfd;
}

其中又調用了setNonBlockAndCloseOnExec()，注意其中設置套接字的方法，先通過fcntl()獲取原屬性，然後原屬性 | 上O_NONBLOCK，再通過fcntl()設置修改後的新屬性。

#if VALGRIND || defined (NO_ACCEPT4)
void setNonBlockAndCloseOnExec(int sockfd)
{
	// non-block
	int flags = ::fcntl(sockfd, F_GETFL, 0);
	flags |= O_NONBLOCK;
	int ret = ::fcntl(sockfd, F_SETFL, flags);
	// FIXME check

	// close-on-exec
	flags = ::fcntl(sockfd, F_GETFD, 0);
	flags |= FD_CLOEXEC;
	ret = ::fcntl(sockfd, F_SETFD, flags);
	// FIXME check

	(void)ret;
}
#endif

}  // namespace

監聽套接字就被設置成了非阻塞。

當有新的連接請求到來時，套接字可讀，觸發調用Acceptor::handleRead()，其實現如下：

void Acceptor::handleRead()
{
	loop_->assertInLoopThread();
	InetAddress peerAddr;
	//FIXME loop until no more
	int connfd = acceptSocket_.accept(&peerAddr);
	if (connfd >= 0)
	{
		// string hostport = peerAddr.toIpPort();
		// LOG_TRACE << "Accepts of " << hostport;
		if (newConnectionCallback_)
		{
			newConnectionCallback_(connfd, peerAddr);
		}
		else
		{
			sockets::close(connfd);
		}
	}
	else
	{
		LOG_SYSERR << "in Acceptor::handleRead";
		// Read the section named "The special problem of
		// accept()ing when you can't" in libev's doc.
		// By Marc Lehmann, author of libev.
		if (errno == EMFILE)
		{
			::close(idleFd_);
			idleFd_ = ::accept(acceptSocket_.fd(), NULL, NULL);
			::close(idleFd_);
			idleFd_ = ::open("/dev/null", O_RDONLY | O_CLOEXEC);
		}
	}
}

其中又調用了sockets::accept(int sockfd, struct sockaddr_in6* addr)，其實現如下：

int sockets::accept(int sockfd, struct sockaddr_in6* addr)
{
	socklen_t addrlen = static_cast<socklen_t>(sizeof *addr);
#if VALGRIND || defined (NO_ACCEPT4)
	int connfd = ::accept(sockfd, sockaddr_cast(addr), &addrlen);
	setNonBlockAndCloseOnExec(connfd);
#else
	int connfd = ::accept4(sockfd, sockaddr_cast(addr),
		&addrlen, SOCK_NONBLOCK | SOCK_CLOEXEC);
#endif
	if (connfd < 0)
	{
		int savedErrno = errno;
		LOG_SYSERR << "Socket::accept";
		switch (savedErrno)
		{
		case EAGAIN:
		case ECONNABORTED:
		case EINTR:
		case EPROTO: // ???
		case EPERM:
		case EMFILE: // per-process lmit of open file desctiptor ???
					 // expected errors
			errno = savedErrno;
			break;
		case EBADF:
		case EFAULT:
		case EINVAL:
		case ENFILE:
		case ENOBUFS:
		case ENOMEM:
		case ENOTSOCK:
		case EOPNOTSUPP:
			// unexpected errors
			LOG_FATAL << "unexpected error of ::accept " << savedErrno;
			break;
		default:
			LOG_FATAL << "unknown error of ::accept " << savedErrno;
			break;
		}
	}
	return connfd;
}

從其實現來看，監聽套接字爲非阻塞時，對其調用accept()返回的套接字應該還是爲阻塞的，不然也不會接着調用setNonBlockAndCloseOnExec()。如果要使accept()返回的套接字直接就是非阻塞的，可以調用accept4()[如果支持的話]，它比accept()多了一個參數[SOCK_NONBLOCK | SOCK_CLOEXEC]，可以通過設置該參數使返回的連接套接字直接就是非阻塞的。