恢复从某个位置下载

Question

暂停后我需要下载该文件。但是我不知道如何使用 https://github.com/yhirose/yhirose/cpp-httplib 。但是问题是，当下载恢复时，服务器开始首先向我发送文件。问题：如何告诉服务器我已经下载的件的大小，以便服务器仅发送必要的部分？ 我的代码：

   std::string body;
   httplib::Client cli( url, port );
   cli.set_follow_location( true );
   
   int file_size = is_part_of_file ? GetFileSize( result_file_name.c_str() ) : 0; // it is downloaded part of the file
   int last_percent;
   auto res = cli.Get(
      file_path.c_str(), httplib::Headers(),
      [ & ]( const httplib::Response& response )
      {
         ( void )response;
         return *is_download;
      },
      [ & ]( const char* data, size_t data_length )
      {
         body.append( data, data_length );
         return *is_download;
      },
      [ & ]( uint64_t len, uint64_t total )
      {
         int percent = ( int )( len * 100 / total );
         if( last_percent != percent )
         {
            *p_percent = ( ( float )percent / 100 );
         }
         last_percent = percent;
         return *is_download;
      } );

   if( res )
   {
      std::ofstream out( result_file_name, std::ios::binary | std::ios::app );
      out << body;
      out.close();
   }
   else
   {
      if( is_part_of_file )
      {
         std::ofstream out( result_file_name, std::ios::binary | std::ios::app );
         out << body;
         out.close();
      }
      return false;
   }