如何将输出数据写入文本文件？

Question

我是verilog的新用户。我想在verilog中读取和写入文本文件。成功地，我使用$fscan从文本文件中读取输入。现在的问题是我无法将数据放入或写入输出文本文件。当我运行程序输出文件为空时 我在这里附上程序的源代码：

    integer              data_file    ; // file handler
    integer              scan_file    ; // file handler
    reg                         [21:0] captured_data;
    integer                     f; 
    reg                  [21:0] Add_num;
    `define NULL 0  

    initial begin     
        data_file = $fopen("r.txt","r");
        f = $fopen("output.txt","w"); 

    if (data_file == `NULL) 
          begin
             $display("data_file handle was NULL");  
            // $finish ;
            end
    end      

   always @(posedge clk) begin
    scan_file = $fscanf(data_file, "%d\n", captured_data); 
    if (!$feof(data_file)) 
        begin
            //use captured_data as you would any other wire or reg value;
                Add_num = captured_data + captured_data;
                //out<=Add_num; 
                $display (" Data is = %d", Add_num );        
             $fwrite(f,"%d\n",   Add_num);
        end
    end