使用C++写一个异步的TCP客户端

由于异步编程需要使用操作系统提供的异步IO接口，因此需要使用操作系统相关的库函数。下面给出一个使用Windows Socket编写的异步TCP客户端的示例代码：

#include <stdio.h>
#include <winsock2.h>
#include <windows.h>

#define BUF_SIZE 1024

void error_handling(const char* msg);

int main(int argc, char* argv[])
{
    WSADATA wsaData;
    SOCKET sock;
    SOCKADDR_IN serv_addr;
    HANDLE hIocp;
    OVERLAPPED overlapped;
    DWORD bytesTransferred;
    DWORD flags = 0;
    char buf[BUF_SIZE];
    int str_len, addr_size;

    if (argc != 3) {
        printf("Usage: %s <IP> <port>\n", argv[0]);
        return 1;
    }

    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
        error_handling("WSAStartup() error");
    }

    sock = WSASocket(AF_INET, SOCK_STREAM, 0, NULL, 0, WSA_FLAG_OVERLAPPED);
    if (sock == INVALID_SOCKET) {
        error_handling("WSASocket() error");
    }

    memset(&serv_addr, 0, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = inet_addr(argv[1]);
    serv_addr.sin_port = htons(atoi(argv[2]));

    if (connect(sock, (SOCKADDR*)&serv_addr, sizeof(serv_addr)) == SOCKET_ERROR) {
        if (WSAGetLastError() != WSAEWOULDBLOCK) {
            error_handling("connect() error");
        }
    }

    hIocp = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0);
    if (hIocp == NULL) {
        error_handling("CreateIoCompletionPort() error");
    }

    if (CreateIoCompletionPort((HANDLE)sock, hIocp, (DWORD)sock, 0) == NULL) {
        error_handling("CreateIoCompletionPort() error");
    }

    memset(&overlapped, 0, sizeof(overlapped));
    overlapped.hEvent = WSACreateEvent();
    if (overlapped.hEvent == NULL) {
        error_handling("WSACreateEvent() error");
    }

    while (1) {
        str_len = scanf("%s", buf);
        if (str_len == EOF) {
            break;
        }

        WSASend(sock, buf, str_len, &bytesTransferred, 0, &overlapped, NULL);
        memset(&overlapped, 0, sizeof(overlapped));
        overlapped.hEvent = WSACreateEvent();
        if (overlapped.hEvent == NULL) {
            error_handling("WSACreateEvent() error");
        }

        if (WSARecv(sock, buf, BUF_SIZE, &bytesTransferred, &flags, &overlapped, NULL) == SOCKET_ERROR) {
            if (WSAGetLastError() != WSA_IO_PENDING) {
                error_handling("WSARecv() error");
            }
        }

        if (WaitForSingleObject(overlapped.hEvent, INFINITE) == WAIT_FAILED) {
            error_handling("WaitForSingleObject() error");
        }

        if (WSAGetOverlappedResult(sock, &overlapped, &bytesTransferred, FALSE, &flags) == FALSE) {
            error_handling("WSAGetOverlappedResult() error");
        }

        buf[bytesTransferred] = '\0';
        printf("Received message: %s\n", buf);
    }

    closesocket(sock);
    WSACleanup();
    return 0;
}

void error_handling(const char* msg)
{
    fprintf(stderr, "%s\n", msg);
    exit(1);
}

该程序使用了Windows Socket的异步IO接口，通过CreateIoCompletionPort函数创建了一个IOCP对象，然后将客户端套接字关联到该IOCP对象上。在发送消息时，使用WSASend函数发送数据，然后在接收消息时，使用WSARecv函数进行异步接收，然后等待接收完成的事件被触发，再使用WSAGetOverlappedResult函数获取接收到的数据。由于使用了异步IO接口，因此程序能够在数据发送和接收时继续执行其他操作，从而实现了异步功能。