eventfd实现线程事件通知机制

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eventfd的使用

eventfd系统函数

eventfd  - 事件通知文件描述符

#include
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 
#include  #include  #include  #include  #include  #include  #include  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\n"; 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\n"; } } 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\n"; 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\n"; } std::cout << " wakeup ！\n"; 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 p_wakeupChannel; mutable MutexLock m_mutex; bool m_callingPendingFunctors; /* atomic */ std::vector m_pendingFunctors; // @GuardedBy mutex_

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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);
}

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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 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; }

 

 

转载于:https://www.cnblogs.com/developing/p/10887779.html