muduo网络库学习笔记(四) 通过eventfd实现的事件通知机制
上篇文章为EventLoop添加了一个定时器Fd,为EventLoop增加了3个接口：runAfter()、runAt()、runEvery()、这三个接口用于处理定时任务和周期任务. 底层通过封装TimerFd实现。
TimerId runAt(const TimeStamp& time, const NetCallBacks::TimerCallBack& cb);
TimerId runAfter(double delay, const NetCallBacks::TimerCallBack& cb);
TimerId runEvery(double interval, const NetCallBacks::TimerCallBack& cb);
今天为EventLoop添加另一个Fd：EventFd, 用于实现线程间的事件通知机制.本文会先介绍eventfd的使用，然后给出muduo中EventLoop对eventfd的封装.
eventfd的使用
eventfd系统函数
eventfd - 事件通知文件描述符
#include <sys/eventfd.h>
int eventfd（unsigned int initval ，int flags ）;
创建一个能被用户应用程序用于时间等待唤醒机制的eventfd对象.
initval :
eventfd（）创建一个可用作事件的“eventfd对象”用户空间应用程序和内核等待/通知机制通知用户空间应用程序的事件。该对象包含一个由内核维护的无符号64位整型（uint64_t）计数器。此计数器的初始值通过initval指定。一般设0.
flags ：
以下标志中按位OR运算以更改eventfd()的行为，(文件中常用的这两个flags肯定都懂意思吧，就不翻译了,第三个信号量的不管它.)：
EFD_CLOEXEC (since Linux 2.6.27)
Set the close-on-exec (FD_CLOEXEC) flag on the new file
descriptor. See the description of the O_CLOEXEC flag in
open(2) for reasons why this may be useful.
EFD_NONBLOCK (since Linux 2.6.27)
Set the O_NONBLOCK file status flag on the new open file
description. Using this flag saves extra calls to fcntl(2) to
achieve the same result.
EFD_SEMAPHORE (since Linux 2.6.30)
Provide semaphore-like semantics for reads from the new file
descriptor. See below.
read（2）
成功读取返回一个8byte的整数。read（2）如果提供的缓冲区的大小小于8个字节返回错误EINVAL
write (2）
将缓冲区写入的8字节整形值加到内核计数器上。可以写入的最大值
是计数器中是最大的无符号64位值减1（即0xfffffffffffffffe）。
返回值：
On success, eventfd() returns a new eventfd file descriptor. On error, -1 is returned and errno is set to indicate the error.
使用示例
#include <iostream>
#include <assert.h>
#include <poll.h>
#include <signal.h>
#include <sys/eventfd.h>
#include <unistd.h>
#include <string.h>
#include <thread>
static int s_efd = 0;
int createEventfd()
{
int evtfd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
std::cout << "createEventfd() fd : " << evtfd << std::endl;
if (evtfd < 0)
{
std::cout << "Failed in eventfd
";
abort();
}
return evtfd;
}
void testThread()
{
int timeout = 0;
while(timeout < 3) {
sleep(1);
timeout++;
}
uint64_t one = 1;
ssize_t n = write(s_efd, &one, sizeof one);
if(n != sizeof one)
{
std::cout << " writes " << n << " bytes instead of 8
";
}
}
int main()
{
s_efd = createEventfd();
fd_set rdset;
FD_ZERO(&rdset);
FD_SET(s_efd, &rdset);
struct timeval timeout;
timeout.tv_sec = 1;
timeout.tv_usec = 0;
std::thread t(testThread);
while(1)
{
if(select(s_efd + 1, &rdset, NULL, NULL, &timeout) == 0)
{
std::cout << "timeout
";
timeout.tv_sec = 1;
timeout.tv_usec = 0;
FD_SET(s_efd, &rdset);
continue;
}
uint64_t one = 0;
ssize_t n = read(s_efd, &one, sizeof one);
if(n != sizeof one)
{
std::cout << " read " << n << " bytes instead of 8
";
}
std::cout << " wakeup ！
";
break;
}
t.join();
close(s_efd);
return 0;
}
./test.out
createEventfd() fd : 3
timeout
timeout
timeout
wakeup ！
eventfd 单纯的使用文件描述符实现的线程间的通知机制，可以很好的融入select、poll、epoll的I/O复用机制中.
EventLoop对eventfd的封装
所增加的接口及成员:
typedef std::function<void()> Functor;
void runInLoop(const Functor& cb);
void wakeup(); //是写m_wakeupFd 通知poll 处理读事件.
void queueInLoop(const Functor& cb);
private:
//used to waked up
void handleRead();
void doPendingFunctors();
int m_wakeupFd;
std::unique_ptr<Channel> p_wakeupChannel;
mutable MutexLock m_mutex;
bool m_callingPendingFunctors; /* atomic */
std::vector<Functor> m_pendingFunctors; // @GuardedBy mutex_
工作时序
(runInLoop() -> quueInLoop())/queueInLoop() -> wakeup() -> poll() -> handleRead() -> doPendingFunctors()
runInLoop()
如果用户在当前IO线程调用这个函数， 回调会同步进行； 如果用户在其他线程调用runInLoop()，cb会被加入队列， IO线程会被唤醒来调用这个Functor.
void EventLoop::runInLoop(const Functor& cb)
{
if(isInloopThread())
cb();
else
queueInLoop(cb);
}
queueInLoop()
会将回调添加到容器，同时通过wakeup()唤醒poll()调用容器内的回调.
void EventLoop::queueInLoop(const Functor& cb)
{
LOG_TRACE << "EventLoop::queueInLoop()";
{
MutexLockGuard lock(m_mutex);
m_pendingFunctors.push_back(std::move(cb));
}
if(!isInloopThread())
{
wakeup();
}
}
内部实现，
wakeup()
写已注册到poll的eventfd 通知poll 处理读事件.
// m_wakeupFd(createEventfd()),
// p_wakeupChannel(new Channel(this, m_wakeupFd)),
void EventLoop::wakeup()
{
uint64_t one = 1;
ssize_t n = sockets::write(m_wakeupFd, &one, sizeof one);
if(n != sizeof one)
{
LOG_ERROR << "EventLoop::wakeup() writes " << n << " bytes instead of 8";
}
}
handleRead()
poll回调读事件,处理eventfd.
void EventLoop::handleRead() //handle wakeup Fd
{
LOG_TRACE << "EventLoop::handleRead() handle wakeup Fd";
uint64_t one = 1;
ssize_t n = sockets::read(m_wakeupFd, &one, sizeof one);
if(n != sizeof one)
{
LOG_ERROR << "EventLoop::handleRead() reads " << n << "bytes instead of 8";
}
doPendingFunctors();
}
doPendingFunctors()
处理挂起的事件.
void EventLoop::doPendingFunctors()
{
LOG_TRACE << "EventLoop::doPendingFunctors()";
std::vector<Functor> functors;
m_callingPendingFunctors = true;
{
MutexLockGuard lock(m_mutex);
functors.swap(m_pendingFunctors);
}
for(size_t i = 0; i < functors.size(); ++i)
{
functors[i]();
}
m_callingPendingFunctors = false;
}
总结
本文主要介绍了muduo中EventLoop通过 通过封装一层eventfd实现的runInLoop()函数，使得其他线程想往EventLoop所在的I/O线程注册任务成为可能.
下篇文章会写Connector和Acceptor，链接器和监听器 实现第一条链接。