connect函数在阻塞和非阻塞模式下的行为
当socket使用阻塞模式时,connect函数会阻塞到有明确结果才会返回,如果网络环境较差,可能要等一会,影响体验,
为了解决这个问题,我们使用异步connect技术
- 创建
socket,将socket设置为非阻塞模式 - 调用
connect函数,此时无论connect函数是否连接成功,都会立即返回,如果返回-1,不一定表示连接出错,如果此时错误码为EINPROGRESS表示正在尝试连接 - 调用
select函数,在指定时间内判断该socket是否可写,可写说明连接成功,反之,连接失败
上述流程代码
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <iostream>
#include <string.h>
#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#define SERVER_ADDRESS "127.0.0.1"
#define SERVER_PORT 3000
#define SEND_DATA "helloworld"
int main(int argc, char* argv[])
{
//1.创建一个socket
int clientfd = socket(AF_INET, SOCK_STREAM, 0);
if (clientfd == -1)
{
std::cout << "create client socket error." << std::endl;
return -1;
}
//将clientfd设置成非阻塞模式
int oldSocketFlag = fcntl(clientfd, F_GETFL, 0);
int newSocketFlag = oldSocketFlag | O_NONBLOCK;
if (fcntl(clientfd, F_SETFL, newSocketFlag) == -1)
{
close(clientfd);
std::cout << "set socket to nonblock error." << std::endl;
return -1;
}
//2.连接服务器
struct sockaddr_in serveraddr;
serveraddr.sin_family = AF_INET;
serveraddr.sin_addr.s_addr = inet_addr(SERVER_ADDRESS);
serveraddr.sin_port = htons(SERVER_PORT);
for (;;)
{
int ret = connect(clientfd, (struct sockaddr *)&serveraddr, sizeof(serveraddr));
if (ret == 0)
{
std::cout << "connect to server successfully." << std::endl;
close(clientfd);
return 0;
}
else if (ret == -1)
{
if (errno == EINTR)
{
//connect 动作被信号中断，重试connect
std::cout << "connecting interruptted by signal, try again." << std::endl;
continue;
}
else if (errno == EINPROGRESS)
{
//连接正在尝试中
break;
}
else
{
//真的出错了，
close(clientfd);
return -1;
}
}
}
fd_set writeset;
FD_ZERO(&writeset);
FD_SET(clientfd, &writeset);
struct timeval tv;
tv.tv_sec = 3;
tv.tv_usec = 0;
//3.调用select函数判断socket是否可写
if (select(clientfd + 1, NULL, &writeset, NULL, &tv) == 1)
{
std::cout << "[select] connect to server successfully." << std::endl;
}
else
{
std::cout << "[select] connect to server error." << std::endl;
}
close(clientfd);
return 0;
}首先先用nc命令启动一个服务端程序并执行
nc -v -l -n 0.0.0.0 3000然后运行程序,我用的clion
把服务端关掉,在重新启动客户端,一看结果,还是
为什么连接不上也会输出同样的结果?原因如下:
- 在Windows上,一个
socket没有建立连接之前,我们用select检测是否可写,是可以得到正确结果的,即不可写;连接成功后在检测,就会变为可写 - 在Linux上一个
socket没有建立连接之前,用select函数检测是否可写,我们也会得到可写的结果,**所以,在Linux上,我们不仅要用select检测socket是否可写还要用getsocketopt检测socket此时是否出错
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <iostream>
#include <string.h>
#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#define SERVER_ADDRESS "127.0.0.1"
#define SERVER_PORT 3000
#define SEND_DATA "helloworld"
int main(int argc, char* argv[])
{
//1.创建一个socket
int clientfd = socket(AF_INET, SOCK_STREAM, 0);
if (clientfd == -1)
{
std::cout << "create client socket error." << std::endl;
return -1;
}
//将clientfd设置成非阻塞模式
int oldSocketFlag = fcntl(clientfd, F_GETFL, 0);
int newSocketFlag = oldSocketFlag | O_NONBLOCK;
if (fcntl(clientfd, F_SETFL, newSocketFlag) == -1)
{
close(clientfd);
std::cout << "set socket to nonblock error." << std::endl;
return -1;
}
//2.连接服务器
struct sockaddr_in serveraddr;
serveraddr.sin_family = AF_INET;
serveraddr.sin_addr.s_addr = inet_addr(SERVER_ADDRESS);
serveraddr.sin_port = htons(SERVER_PORT);
for (;;)
{
int ret = connect(clientfd, (struct sockaddr *)&serveraddr, sizeof(serveraddr));
if (ret == 0)
{
std::cout << "connect to server successfully." << std::endl;
close(clientfd);
return 0;
}
else if (ret == -1)
{
if (errno == EINTR)
{
//connect 动作被信号中断，重试connect
std::cout << "connecting interruptted by signal, try again." << std::endl;
continue;
}
else if (errno == EINPROGRESS)
{
//连接正在尝试中
break;
}
else
{
//真的出错了，
close(clientfd);
return -1;
}
}
}
fd_set writeset;
FD_ZERO(&writeset);
FD_SET(clientfd, &writeset);
struct timeval tv;
tv.tv_sec = 3;
tv.tv_usec = 0;
//3.调用select函数判断socket是否可写
if (select(clientfd + 1, NULL, &writeset, NULL, &tv) != 1)
{
std::cout << "[select] connect to server error." << std::endl;
close(clientfd);
return -1;
}
int err;
socklen_t len = static_cast<socklen_t>(sizeof err);
//4.调用getsockopt检测此时socket是否出错
if (::getsockopt(clientfd, SOL_SOCKET, SO_ERROR, &err, &len) < 0)
{
close(clientfd);
return -1;
}
if (err == 0)
std::cout << "connect to server successfully." << std::endl;
else
std::cout << "connect to server error." << std::endl;
close(clientfd);
return 0;
}TCP网络编程的基本流程
Linux与C++11多线程编程(学习笔记)
Linux select函数用法和原理
socket的阻塞模式和非阻塞模式(send和recv函数在阻塞和非阻塞模式下的表现)
connect函数在阻塞和非阻塞模式下的行为
获取socket对应的接收缓冲区中的可读数据量
