从主机名和端口创建套接字，但 ：：select（） 表示没有数据 [关闭]

问：

我正在尝试使用主机名和端口创建套接字。但是，这意味着套接字没有数据。::select()

有人可以帮忙吗？

下面的独立示例（和 Godbolt 链接）。

https://godbolt.org/z/r7G3aczdK

#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/ioctl.h>
#include <net/if.h>
#include <netinet/tcp.h>

#include <string>
#include <iostream>

bool isSocketReady(int sockfd)
{
    fd_set rfds;
    struct timeval tv;

    FD_ZERO(&rfds);
    FD_SET(0, &rfds);

    tv.tv_sec = 2;
    tv.tv_usec = 0;

    const int retval = ::select(sockfd, &rfds, NULL, NULL, &tv);

    if (retval == -1)
    {
        printf("select() error");
        return false;
    }
    else if (retval)
    {
        printf("Data is available now.\n");
        return true;
    }
    else
    {
        printf("No data within timeout.\n");
        return false;
    }
}

int connectByHostName(const std::string& host, int port)
{
    int sockfd = -1;

    struct addrinfo hints;
    memset(&hints, 0, sizeof(hints));
    hints.ai_flags = AI_ADDRCONFIG | AI_NUMERICSERV;
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;

    const std::string port_str = std::to_string(port);

    struct addrinfo* res{nullptr};
    const int r =  getaddrinfo(host.c_str(), port_str.c_str(), &hints, &res);
    
    if(r < 0)
    {
        return -1;
    }

    for (struct addrinfo* address = res; address != nullptr; address = address->ai_next)
    {
        sockfd = socket(address->ai_family, address->ai_socktype, address->ai_protocol);
        
        if (sockfd < 0)
        {
            continue;
        }

        int flag = 1;
        setsockopt(sockfd, IPPROTO_TCP, TCP_NODELAY, (char*) &flag, sizeof(flag));

        if(::connect(sockfd, address->ai_addr, address->ai_addrlen) < 0)
        {
            continue;
        }
        else if(isSocketReady(sockfd))
        {
            break;
        }            
    }

    freeaddrinfo(res);
    return sockfd;
}

int main()
{
    int sock = connectByHostName("example.com", 80);
    std::cout << sock << std::endl;
}