完整示例代码，研究连接失败时poll()的返回值。

poll_connection_refused.cpp

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <errno.h>
#include <unistd.h>
#include <poll.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define ASYNC_CONTINUE() ( errno == EAGAIN || errno == EINTR || errno == EINPROGRESS)

int fcntl_enable(int sockfd, int feature)
{
    int flags = fcntl(sockfd, F_GETFL, 0);
    if ( flags < 0 )
    {
        return flags;
    }

    int ret = fcntl(sockfd, F_SETFL, flags | feature );
    if ( ret < 0 )
    {
        return ret;
    }
    return 0;
}

int async_socket()
{
    int sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if ( sockfd < 0 )
    {
        fprintf(stderr, "socket fail: %m");
        return sockfd;
    }

    int ret = fcntl_enable(sockfd, O_NONBLOCK);
    if ( ret < 0 )
    {
        fprintf(stderr, "fcntl enable nonblock fail: %m");
        close(sockfd);
        return ret;
    }
    return sockfd;
}

int async_connect(int sockfd, const char* host, short port)
{
    struct sockaddr_in servaddr;
    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons(port);
    inet_pton(AF_INET, host, &servaddr.sin_addr);

    int ret = connect(sockfd, (const struct sockaddr *)&servaddr, sizeof(servaddr));
    if ( ret < 0 and not ASYNC_CONTINUE() )
    {
        fprintf(stderr, "connect to [%s:%d] fail: %m", host, port);
        return ret;
    }
    return 0;
}

int async_net(const char* host, short port)
{
    int sockfd = async_socket();
    if ( sockfd < 0 )
    {
        return sockfd;
    }

    int ret =async_connect(sockfd, host, port);
    if ( ret < 0 )
    {
        close(sockfd);
        return ret;
    }
    return sockfd;
}

int connection_refused()
{
    int sockfd = async_net("127.0.0.1", 11111);
    struct pollfd pfd;
    pfd.fd = sockfd;
    pfd.events = POLLIN|POLLOUT;
    int ready = poll(&pfd, 1, -1);
    printf("ready=%d, POLLIN=%d, POLLOUT=%d, POLLERR=%d\n",
            ready,
            ( pfd.revents&POLLIN ) != 0,
            ( pfd.revents&POLLOUT ) != 0,
            ( pfd.revents&POLLERR ) != 0);

    int error;
    socklen_t errlen = sizeof(error);
    getsockopt(sockfd, SOL_SOCKET, SO_ERROR, &error, &errlen);
    printf("error=%d, msg=%s\n",
            error, strerror(error) );
    return 0;
}

int main(int argc, const char *argv[])
{
    connection_refused();
    return 0;
}