connect函数在阻塞和非阻塞模式下的行为

`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socketint 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connectstd::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socketint 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connectstd::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;elsestd::cout << "connect to server error." << std::endl;close(clientfd);return 0;
}