c++ - 如何在超时内从另一个线程唤醒 select()

Question

根据“man select”信息:

     "On  success,  select() and pselect() return the number of file descrip‐
   tors contained in the three returned  descriptor  sets which may be zero 
   if the timeout expires  before  anything  interesting happens.  On error, 
   -1 is returned, and errno is set appropriately; the sets and timeout become
   undefined, so do not  rely  on  their  contents after an error."

选择将唤醒因为:

       1)read/write availability       
       2)select error                 
       3)descriptoris closed.  

但是，如果没有可用数据并且select仍在超时，我们如何从另一个线程唤醒select()？

[更新]
伪代码

          // Thread blocks on Select
          void *SocketReadThread(void *param){
               ...
               while(!(ReadThread*)param->ExitThread()) {
                   struct timeval timeout;
                   timeout.tv_sec = 60; //one minute
                   timeout.tv_usec = 0;

                   fd_set rds;
                   FD_ZERO(&rds);
                   FD_SET(sockfd, &rds)'

                   //actually, the first parameter of select() is 
                    //ignored on windows, though on linux this parameter
                   //should be (maximum socket value + 1)
                   int ret = select(sockfd + 1, &rds, NULL, NULL, &timeout );
                   //handle the result
                   //might break from here

               }
               return NULL;
          }

          //main Thread
          int main(){
                //create the SocketReadThread
                ReaderThread* rthread = new ReaderThread;
                pthread_create(&pthreadid, NULL, SocketReaderThread, 
                          NULL, (void*)rthread);

                 // do lots of things here
                 ............................

                //now main thread wants to exit SocketReaderThread
                //it sets the internal state of ReadThread as true
                rthread->SetExitFlag(true);
                //but how to wake up select ??????????????????
                //if SocketReaderThread currently blocks on select
          }

[更新]
1)@trojanfoe提供了一种方法来实现，他的方法将socket数据(可能是脏数据或退出消息数据)写入wakeup select。我将进行测试并在那里更新结果。
2) 另外要提的是，关闭套接字并不能保证唤醒选择函数调用，请参阅 this post .

[更新2]
经过多次测试，这里有一些关于唤醒选择的事实:
1) 如果 select 监视的套接字被另一个应用程序关闭，则 select() 调用会立即醒来。此后，读取或写入套接字将获得返回值 0，errno = 0
2) 如果 select 监视的套接字被同一应用程序的另一个线程关闭，如果没有数据可读或可写，则 select() 直到超时才会唤醒。选择超时后，进行读/写操作会导致错误 errno = EBADF(因为socket在超时期间已经被另一个线程关闭了)

Answer 1

我使用基于 pipe() 的事件对象:

IoEvent.h:

#pragma once

class IoEvent {
protected:
    int m_pipe[2];
    bool m_ownsFDs;

public:
    IoEvent();              // Creates a user event
    IoEvent(int fd);        // Create a file event

    IoEvent(const IoEvent &other);

    virtual ~IoEvent();

    /**
     * Set the event to signalled state.
     */
    void set();

    /**
     * Reset the event from signalled state.
     */
    void reset();

    inline int fd() const {
        return m_pipe[0];
    }
};

IoEvent.cpp:

#include "IoEvent.h"
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <poll.h>

using namespace std;

IoEvent::IoEvent() : 
    m_ownsFDs(true) {
    if (pipe(m_pipe) < 0)
        throw MyException("Failed to create pipe: %s (%d)", strerror(errno), errno);

    if (fcntl(m_pipe[0], F_SETFL, O_NONBLOCK) < 0)
        throw MyException("Failed to set pipe non-blocking mode: %s (%d)", strerror(errno), errno);
}

IoEvent::IoEvent(int fd) : 
    m_ownsFDs(false) {
    m_pipe[0] = fd;
    m_pipe[1] = -1;
}

IoEvent::IoEvent(const IoEvent &other) {
    m_pipe[0] = other.m_pipe[0];
    m_pipe[1] = other.m_pipe[1];
    m_ownsFDs = false;
}

IoEvent::~IoEvent() {
    if (m_pipe[0] >= 0) {
        if (m_ownsFDs)
            close(m_pipe[0]);

        m_pipe[0] = -1;
    }

    if (m_pipe[1] >= 0) {
        if (m_ownsFDs)
            close(m_pipe[1]);

        m_pipe[1] = -1;
    }
}

void IoEvent::set() {
    if (m_ownsFDs)
        write(m_pipe[1], "x", 1);
}

void IoEvent::reset() {
    if (m_ownsFDs) {
        uint8_t buf;

        while (read(m_pipe[0], &buf, 1) == 1)
            ;
    }
}

您可以放弃 m_ownsFDs 成员；我什至不确定我是否会再使用它。