根据“男人选择”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wakup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仍在超时范围内,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

}

[UPDATE]

1)@trojanfoe提供了一种实现此目的的方法,他的方法将套接字数据(可能是脏数据或退出消息数据)写入唤醒选择.我将进行测试并在那里更新结果.

2)另外要提一下,关闭套接字并不能保证唤醒select函数调用,请参见this post.

[UPDATE2]

做了很多测试后,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

会立刻醒来.此后,读取或写入套接字将使用errno = 0获得返回值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,

如果没有要读取或写入的数据,则select()将在超时之前不会唤醒.选择超时后,使用errno = EBADF进行读/写操作会导致错误

(因为套接字在超时期间已被另一个线程关闭)

解决方法:

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

#include

#include

#include

#include

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)

;

}

}

你可以抛弃m_ownsFDs成员;我甚至不确定我是否会再使用它了.

标签：c-2,linux,tcp,multithreading,sockets

来源： https://codeday.me/bug/20190529/1177712.html