Ftp的断点下载实现

思路：首先获取本地临时文件的大小，在通过FTp的REST命令获取远程文件的偏移，然后再RETR命令在偏移处下载。while循环对比本地文件和远程文件的字节大小，如此

不断的反复以上过程，直到远程文件字节和本地文件字节一样，退出循环，下载完成。

    long remote_file_size = m_ftp->getFileLength(remoteFileName.toStdString());
     long recvSize = ;

     if (remote_file_size == - )
     {
         return false;
     }

     int conn_count = ;

     while (conn_count !=  && local_file_size != remote_file_size)
     {
         recvSize = m_ftp->Get2(remoteFileName.toStdString(), tmp_save_file_name.toStdString(), local_file_size);
         if (recvSize <= )
         {
             msleep();
             if (!reConnect())
             {
                 break;
             }
             conn_count--;
         }

         local_file_size = getLocalFileSize(tmp_save_file_name);
     }

 FTP_API CFTP::Get2(const std::string &strRemoteFile, const std::string &strLocalFile, const int pos)
 {
     return downLoad2(strRemoteFile, strLocalFile, pos);
 }

 FTP_API CFTP::downLoad2(const std::string &strRemoteFile, const std::string& strLocalFile, const int pos /*= 0*/, const unsigned int length /*= 0*/)
 {
     assert(length >= );

     FILE *file = NULL;
     unsigned long nDataLen = FTP_DEFAULT_BUFFER;
     char strPos[MAX_PATH] = {  };
     int data_fd = socket(AF_INET, SOCK_STREAM, );

     assert(data_fd != -);

     if ((length != ) && (length < nDataLen))
     {
         nDataLen = length;
     }
     char *dataBuf = new char[nDataLen];
     if (dataBuf == NULL)
     {
         return -;
     }
     memset(dataBuf, , sizeof(dataBuf));

     //assert(dataBuf != NULL);

     sprintf(strPos, "%d", pos);

     if (createDataLink(data_fd) < )
     {
         trace("@@@@ Create Data Link error!!!\n");
         return -;
     }

     std::string strCmdLine = parseCommand(FTP_COMMAND_DOWNLOAD_POS, strPos);//param 1 为REST命令
     if (Send(m_cmdSocket, strCmdLine) < )
     {
         return -;
     }
     trace("@@@@Response: %s\n", serverResponse(m_cmdSocket).c_str());

     strCmdLine = parseCommand(FTP_COMMAND_DOWNLOAD_FILE, strRemoteFile);//param 1为RETR命令

     if (Send(m_cmdSocket, strCmdLine) < )
     {
         return -;
     }

     std::string strResponse = serverResponse(m_cmdSocket);
     trace("@@@@Response: %s\n", strResponse.c_str());

     file = createLocalFile2(std::string(strLocalFile));
     assert(file != NULL);

     int len = ;
     int nReceiveLen = ;
     while ((len = getData(data_fd, dataBuf, nDataLen)) > )
     {
         nReceiveLen += len;

         int num = fwrite(dataBuf, , len, file);
         memset(dataBuf, , sizeof(dataBuf));

         trace("Num:%d, nReceiveLen:%d\n", num, nReceiveLen);

     }

     Close(data_fd);
     fclose(file);
     delete[]dataBuf;

     return nReceiveLen;

 }

references:

http://blog.chinaunix.net/uid-7377299-id-112977.html