为什么这个数据流在字节26结束？ [英] Why does this Stream of data end at byte 26?

问题描述

我仍然在处理一个星期前的位图I / O问题。我再次陷入困境，所以我决定开始一种类型的I / OI熟悉，并使它更像我需要的稳定（它是一次检查每个字节（像素），并输出到一个文件基于字节的值）。

我开始使用一个程序来读取和检查文本文件的每个字符，如果它高于某个阈值则输出一个'Z'，并输出'A'如果它低于它。

该程序工作得很好，所以我决定将其从字符更改为文件中的字节。

现在，我遇到了问题。文件的第一个26（字节0-25）字节是正确的，但其余的是0或-1，这取决于我使用 ifstream.get（）还是 ifstream.read 。

输入文件 Input.FILE 是在十六进制编辑器中创建的，只包含 0x00-0xFF 。长度为256个字节。

代码：

  #include< iostream> 
 #include< fstream> 
 #include< vector> 
 
 using namespace std; 
 
 int main（）
 {
 ifstream sourcefile; 
 ofstream output; 
 ofstream output2; 
 
 output.open（output.txt）; 
 output2.open（messages.txt）; 
 sourcefile.open（Input.FILE）; 
 
矢量< char>数据（256）; 
 sourcefile.read（& data [0]，256）; 
 sourcefile.seekg（0，ios :: beg）; 
 
 for（int i =（int）0x00; i  {
 output2< Value data [<< i<< ] =< （int）data [i]< endl; 
 
 if（（int）data [i] <0）
 {
 //因为我不能创建一个unsigned char向量，我用它来转换
 //它到正确的数字。这可能是问题吗？ 
 data [i] = 256 + data [i]; 
 output2<< 小于零。 << endl; 
} 
 
 if（data [i]> 64）
 {
 data [i] = 0xFF; 
 output2<< 大于64，设置为0xFF。 << endl; 
} 
 else if（data [i] <64）
 {
 data [i] = 0x00; 
 output2<< 小于64，设置为0x00。 << endl; 
} 
 else 
 {
 //这很可能毫无意义，但我不想冒险
 data [i] = 0x75; 
 output2<< 既不大于也不小于64？设置为0x75。 << endl; 
} 
 
 output2<< endl; 
} 
 
 output.write（& data [0]，256）; 
} 
  

输出（来自 message.txt ）：

注意： data [0-25] 正确的值

...

值数据[19] = 19小于64，设置为0x00。 br>
值数据[20] = 20小于64，设置为0x00。

值数据[21] = 21小于64，设置为0x00。

值数据[22] = 22小于64，设置为0x00。

值数据[23] = 23小于64，设置为0x00。

值数据[24] = 24小于64，设置为0x00。

值数据[25] = 25小于64，设置为0x00。

值数据[26] = 0小于64，如果您查找

www.asciitable.com/\"> ascii code 25是你会看到它表示中等结束，所以很有可能，如果你正在阅读ascii模式，任何后续的读取都无效。

尝试指定您使用的是二进制文件：

  sourcefile.open（Input.FILE，ios :: binary）; 
  

I'm still working on that bitmap I/O problem from a week ago. I got stuck again, so I decided to start with a type of I/O I was familiar with, and make it more like what I needed steadily (which is checking each byte (pixel) at a time and outputting to a file based on that byte's value).

I started out with a program that read and checked each character of a text file, and output a 'Z' if it was above some threshold and output an 'A' if it was below it.

That program worked great, so I decided to change it from characters to bytes in a file.

Now, I've been having problems with it. The first 26 (bytes 0-25) bytes of the file are correct, but the rest are 0 or -1, depending on whether I use ifstream.get() or ifstream.read.

The input file Input.FILE was made in a hex editor, and just contains 0x00-0xFF. It's 256 bytes in length.

Code:

#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

int main()
{
   ifstream sourcefile;
   ofstream output;
   ofstream output2;

   output.open("output.txt");
   output2.open("messages.txt");
   sourcefile.open("Input.FILE");

   vector<char> data(256);
   sourcefile.read(&data[0],256);
   sourcefile.seekg(0,ios::beg);

   for(int i = (int) 0x00 ; i < data.size() ; i++)
   {
      output2 << "Value data[" << i << "] = " << (int) data[i] << endl;

      if((int)data[i] < 0)
      {
         // since I can't make an unsigned char vector, I use this to convert
         // it to the correct number. Could this be the problem?
         data[i] = 256 + data[i];
         output2 << "less than zero." << endl;
      }

      if(data[i] > 64)
      {
         data[i] = 0xFF;
         output2 << "greater than 64, set to 0xFF." << endl;
      }
      else if(data[i] < 64)
      {
         data[i] = 0x00;
         output2 << "less than 64, set to 0x00." << endl;
      }
      else
      {
         // This is most likely pointless, but I didn't want to take a chance
         data[i] = 0x75;
         output2 << "neither greater nor less than 64? Set to 0x75." << endl;
      }

      output2 << endl;
   }

   output.write(&data[0],256);
}

Output (from message.txt):

Note: data[0-25] contain the correct values

...
Value data[19] = 19 less than 64, set to 0x00.
Value data[20] = 20 less than 64, set to 0x00.
Value data[21] = 21 less than 64, set to 0x00.
Value data[22] = 22 less than 64, set to 0x00.
Value data[23] = 23 less than 64, set to 0x00.
Value data[24] = 24 less than 64, set to 0x00.
Value data[25] = 25 less than 64, set to 0x00.
Value data[26] = 0 less than 64, set to 0x00.

解决方案

If you look up what ascii code 25 is you'll see it means end of medium so there's a good chance that if you're reading in ascii mode, any subsequent reads aren't going to work.

Try specifying that you're using a binary file:

sourcefile.open("Input.FILE", ios::binary);

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