在Windows下使用fstream :: seekg在Unix下创建的文件 [英] Using fstream::seekg under windows on a file created under Unix

问题描述

我有C ++一个跨平台程序（在Linux下使用g ++编译，在PC下使用Visual Studio编译）。此程序将行写入文本文件（使用< 运算符和 std :: endl ），从生成的文本文件中读取数据（使用 std :: getline ）。

I have C++ a cross-platform program (compiled with g++ under Linux and with Visual Studio under PC). This program writes lines to a text file (using << operator and std::endl) but can also read data back from the generated text file (using std::getline).

内存，当读取数据文件，我第一次读它，并保存在我的程序中的数据位置。当需要数据时，我稍后使用 seekg 移动到特定位置并读取数据。

To optimize data access and save memory, when reading the data file, I read it a first time and save data position in my program. When data is needed, I later use seekg to move to a specific position and read the data.

• 在PC上创建和阅读文件很好。


• 在Linux上创建和阅读文件工作正常。

在PC下，seekg有时无法移动光标。我可以隔离在下面的例子中的问题。它读取文件一次，保存第二个分区和值，然后返回到保存的位置并再次读取该行。

Under PC, seekg sometime fails to move the cursor accordingly. I could isolate the problem in the example below. It reads the file once, saves second lineposition and value, then moves back to the saved position and reads the line again.

#include <fstream>
#include <iostream>
#include <string>
#include <assert.h>
int main()
{
    std::fstream file;
    file.open( "buglines.txt", std::ios_base::in );
    if ( file.is_open() )
    {
        std::streampos posLine2;
        std::string lineStr;
        std::string line2Str;
        int line = 1;
        while ( std::getline( file, lineStr ) )
        {
            if ( line == 1 )
                posLine2 = file.tellg(); // save line 2 position
            if ( line == 2 )
                line2Str = lineStr; // save line 2 content

            ++line;
            std::cout << lineStr <<std::endl;
        }
        std::cout << "Reached EOF, trying to read line 2 a second time" << std::endl;
        file.clear(); // clear EOF flag
        file.seekg(posLine2); // move to line 2
        std::getline( file, lineStr ); // read the line
        assert( lineStr == line2Str ); // compare

    }
    return 0;
}

我从Windows运行。

I'm running this from Windows.

• 如果在Windows下创建了 buglines.txt （十六进制编辑器显示行分隔符为2个字符 0x0D 0x0A ），它工作（ lineStr == line2Str ）。


• 如果 buglines.txt 是在Linux下创建的（十六进制编辑器将行分隔符显示为1个字符 0x0A ），它不工作（lineStr是空字符串） 。

• If buglines.txt was created under Windows (hexadecimal editor shows line separators as 2 characters 0x0D 0x0A), it works (lineStr == line2Str).
• If buglines.txt was created under Linux (hexadecimal editor shows line separators as 1 character 0x0A), it does not works (lineStr is empty string). Even if the getline loop worked perfectly.

我知道两个系统的处理方式与EOL不同，但是我只是使用 getline 阅读功能，我希望它会巧妙地工作...我错过了什么？

I know both system deals differently with EOL, but as I'm just using getline function for reading, I was hoping that it would smartly work...am I missing something?

推荐答案

我无法轻易升级我的项目的运行时库，显然没有其他的解决方案。

I can't easily upgrade the runtime library for my project and as, apparently, there is no other "solution".

std :: ios_base :: binary 属性。它修复了报告的问题，但引入了一个新的：当使用 getline 读取文件时，我们获得额外的 \r 。

I tried to set std::ios_base::binary attribute upon file open. It fixes the reported problem but introduces a new one: We get extra \r chacters when reading the file with getline.

所以如果任何人有相同的问题，需要一个修复，这里是一个解决方法：只需关闭文件，重新打开它，然后吃前n个字符将读指针移动到正确的位置：

So if anyone has the same problem and needs a fix, here is a workaround: simply close the file, re-open it, and then eat the first n characters to move the read pointer to the good location:

#include <fstream>
#include <iostream>
#include <string>
#include <assert.h>

int main()
{
    std::fstream file;
    const std::string fileName = "buglines.txt";
    file.open( fileName.c_str(), std::ios_base::in );
    if ( file.is_open() )
    {
        std::streampos posLine2;
        std::string lineStr;
        std::string line2Str;
        int line = 1;
        while ( std::getline( file, lineStr ) )
        {
            if ( line == 1 )
                posLine2 = file.tellg(); // save line 2 position
            if ( line == 2 )
                line2Str = lineStr; // save line 2 content

            ++line;
            std::cout << lineStr << std::endl;
        }
        std::cout << "Reached EOF, trying to read line 2 a second time" << std::endl;
        //file.clear(); // clear EOF flag
        //file.seekg(posLine2); // move to line 2
        file.close();
        file.open( fileName.c_str(), std::ios_base::in );
        assert( file.is_open() );
        char* temp = new char[static_cast<int>(posLine2)+1];
        file.read( temp, static_cast<int>(posLine2)+1 ); // if posLine2 is too big, consider splitting with in a loop
        delete [] temp;
        assert( file.tellg() == posLine2 );

        std::getline( file, lineStr ); // read the line
        assert( lineStr == line2Str ); // compare
    }
    return 0;
}

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