在Unix下创建的文件的Windows下使用fstream :: seekg

Question

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 ).

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.

• Creating and reading the file on PC works fine.
• Creating and reading the file on Linux works fine.
• But creating the file on Linux and reading on PC fails.

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;
}

I'm running this from Windows.

• 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.

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?

Answer 1

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

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 .

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;
}