用c++写个连接ftp服务器获取文件的代码

以下是使用C语言连接FTP服务器并获取文件的示例代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <winsock2.h>

#pragma comment (lib, "ws2_32.lib")

int main(int argc, char *argv[]) {
    if (argc < 5) {
        printf("Usage: %s <ftp_server> <ftp_username> <ftp_password> <remote_file_path>\n", argv[0]);
        return 1;
    }

    WSADATA wsaData;
    if (WSAStartup(MAKEWORD(2,2), &wsaData) != 0) {
        printf("WSAStartup failed with error: %d\n", WSAGetLastError());
        return 1;
    }

    SOCKET controlSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (controlSocket == INVALID_SOCKET) {
        printf("socket failed with error: %d\n", WSAGetLastError());
        WSACleanup();
        return 1;
    }

    struct sockaddr_in serverAddress;
    serverAddress.sin_family = AF_INET;
    serverAddress.sin_port = htons(21);
    serverAddress.sin_addr.s_addr = inet_addr(argv[1]);

    if (connect(controlSocket, (SOCKADDR*)&serverAddress, sizeof(serverAddress)) == SOCKET_ERROR) {
        printf("connect failed with error: %d\n", WSAGetLastError());
        closesocket(controlSocket);
        WSACleanup();
        return 1;
    }

    char buffer[2048];
    int bytesReceived = recv(controlSocket, buffer, sizeof(buffer), 0);
    buffer[bytesReceived] = '\0';
    printf("%s", buffer);

    char username[100], password[100];
    sprintf(username, "USER %s\r\n", argv[2]);
    sprintf(password, "PASS %s\r\n", argv[3]);

    if (send(controlSocket, username, strlen(username), 0) == SOCKET_ERROR) {
        printf("send failed with error: %d\n", WSAGetLastError());
        closesocket(controlSocket);
        WSACleanup();
        return 1;
    }

    bytesReceived = recv(controlSocket, buffer, sizeof(buffer), 0);
    buffer[bytesReceived] = '\0';
    printf("%s", buffer);

    if (send(controlSocket, password, strlen(password), 0) == SOCKET_ERROR) {
        printf("send failed with error: %d\n", WSAGetLastError());
        closesocket(controlSocket);
        WSACleanup();
        return 1;
    }

    bytesReceived = recv(controlSocket, buffer, sizeof(buffer), 0);
    buffer[bytesReceived] = '\0';
    printf("%s", buffer);

    char pasvCommand[] = "PASV\r\n";

    if (send(controlSocket, pasvCommand, strlen(pasvCommand), 0) == SOCKET_ERROR) {
        printf("send failed with error: %d\n", WSAGetLastError());
        closesocket(controlSocket);
        WSACleanup();
        return 1;
    }

    bytesReceived = recv(controlSocket, buffer, sizeof(buffer), 0);
    buffer[bytesReceived] = '\0';
    printf("%s", buffer);

    int ip1, ip2, ip3, ip4, port1, port2;
    sscanf(buffer, "227 Entering Passive Mode (%d,%d,%d,%d,%d,%d)\r\n", &ip1, &ip2, &ip3, &ip4, &port1, &port2);

    struct sockaddr_in dataAddress;
    dataAddress.sin_family = AF_INET;
    char dataIp[20];
    sprintf(dataIp, "%d.%d.%d.%d", ip1, ip2, ip3, ip4);
    dataAddress.sin_addr.s_addr = inet_addr(dataIp);
    dataAddress.sin_port = htons((port1 << 8) | port2);

    SOCKET dataSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (dataSocket == INVALID_SOCKET) {
        printf("socket failed with error: %d\n", WSAGetLastError());
        closesocket(controlSocket);
        WSACleanup();
        return 1;
    }

    if (connect(dataSocket, (SOCKADDR*)&dataAddress, sizeof(dataAddress)) == SOCKET_ERROR) {
        printf("connect failed with error: %d\n", WSAGetLastError());
        closesocket(dataSocket);
        closesocket(controlSocket);
        WSACleanup();
        return 1;
    }

    char retrCommand[100];
    sprintf(retrCommand, "RETR %s\r\n", argv[4]);

    if (send(controlSocket, retrCommand, strlen(retrCommand), 0) == SOCKET_ERROR) {
        printf("send failed with error: %d\n", WSAGetLastError());
        closesocket(dataSocket);
        closesocket(controlSocket);
        WSACleanup();
        return 1;
    }

    bytesReceived = recv(controlSocket, buffer, sizeof(buffer), 0);
    buffer[bytesReceived] = '\0';
    printf("%s", buffer);

    FILE *fp = fopen(argv[4], "wb");
    if (fp == NULL) {
        printf("Unable to open file '%s'\n", argv[4]);
        closesocket(dataSocket);
        closesocket(controlSocket);
        WSACleanup();
        return 1;
    }

    while (1) {
        bytesReceived = recv(dataSocket, buffer, sizeof(buffer), 0);
        if (bytesReceived == 0) {
            break;
        }
        fwrite(buffer, sizeof(char), bytesReceived, fp);
    }

    fclose(fp);

    closesocket(dataSocket);
    closesocket(controlSocket);
    WSACleanup();

    printf("File '%s' downloaded successfully\n", argv[4]);

    return 0;
}

请注意，此代码仅适用于Windows操作系统，因为它使用了Windows特定的API。如果您想在其他操作系统上运行此代码，您需要进行相应的修改。