非阻塞Connect详解-优快云博客

本文详细介绍了如何使用非阻塞方式实现socket连接，并通过代码示例展示了如何处理EINPROGRESS错误，监听连接状态，以及如何确认连接是否成功。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

非阻塞connect

connect系统调用的man手册中一段话描述了connect出错时的一种errno值：EINPROGRESS。这种错误发生在对非阻塞的socket调用connect，而连接又没有立即建立时。根据man文档的解释，在这种情况下，我们可以调用select，poll等函数来监听这个连接失败的socket上的可写事件。当select，poll等函数返回后，在利用getsockopt来读取错误码并清除该socket上的错误。如果错误码为0，表示连接成功建立，否则连接失败。

代码为：

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <assert.h>
#include <stdio.h>
#include <time.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <string.h>

#define BUFFER_SIZE 1023

int setnonblocking( int fd )
{
    int old_option = fcntl( fd, F_GETFL );
    int new_option = old_option | O_NONBLOCK;
    fcntl( fd, F_SETFL, new_option );
    return old_option;
}

int unblock_connect( const char* ip, int port, int time )
{
    int ret = 0;
    struct sockaddr_in address;
    bzero( &address, sizeof( address ) );
    address.sin_family = AF_INET;
    inet_pton( AF_INET, ip, &address.sin_addr );
    address.sin_port = htons( port );

    int sockfd = socket( PF_INET, SOCK_STREAM, 0 );
    int fdopt = setnonblocking( sockfd );
    ret = connect( sockfd, ( struct sockaddr* )&address, sizeof( address ) );
    if ( ret == 0 )
    {
        printf( "connect with server immediately\n" );
        fcntl( sockfd, F_SETFL, fdopt );
        return sockfd;
    }
    else if ( errno != EINPROGRESS )
    {
        printf( "unblock connect not support\n" );
        return -1;
    }

    fd_set readfds;
    fd_set writefds;
    struct timeval timeout;

    FD_ZERO( &readfds );
    FD_SET( sockfd, &writefds );

    timeout.tv_sec = time;
    timeout.tv_usec = 0;

    ret = select( sockfd + 1, NULL, &writefds, NULL, &timeout );
    if ( ret <= 0 )
    {
        printf( "connection time out\n" );
        close( sockfd );
        return -1;
    }

    if ( ! FD_ISSET( sockfd, &writefds  ) )
    {
        printf( "no events on sockfd found\n" );
        close( sockfd );
        return -1;
    }

    int error = 0;
    socklen_t length = sizeof( error );
    if( getsockopt( sockfd, SOL_SOCKET, SO_ERROR, &error, &length ) < 0 )
    {
        printf( "get socket option failed\n" );
        close( sockfd );
        return -1;
    }

    if( error != 0 )
    {
        printf( "connection failed after select with the error: %d \n", error );
        close( sockfd );
        return -1;
    }
    
    printf( "connection ready after select with the socket: %d \n", sockfd );
    fcntl( sockfd, F_SETFL, fdopt );
    return sockfd;
}

int main( int argc, char* argv[] )
{
    if( argc <= 2 )
    {
        printf( "usage: %s ip_address port_number\n", basename( argv[0] ) );
        return 1;
    }
    const char* ip = argv[1];
    int port = atoi( argv[2] );

    int sockfd = unblock_connect( ip, port, 10 );
    if ( sockfd < 0 )
    {
        return 1;
    }
    shutdown( sockfd, SHUT_WR );
    sleep( 200 );
    printf( "send data out\n" );
    send( sockfd, "abc", 3, 0 );
    //sleep( 600 );
    return 0;
}