Input and Output concurrently

This is a discussion on Input and Output concurrently within the C++ Programming forums, part of the General Programming Boards category; Hi, I would like to know if it is possible to read a file and write to the same file ...

  1. #1
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    Input and Output concurrently

    Hi,

    I would like to know if it is possible to read a file and write to the same file at the same time.
    For example, if I have

    Code:
    #include <fstream>
    // and other headers
    
    
    using namespace std;
    
    string temp_string;
    char result;
    
    
    fstream studfile;
    studfile.open("student.dat", ios::in | ios::out); // Is this correct?
    
    while (!studfile.eof() ) {
         getline ( studfile, temp_string);
         studfile << "Results: " << result << endl;
    }
    Is there any way to manipulate a .dat file while reading it?
    I've checked quite a couple of sites, but there are only solutions for binary files..

    Thanks! =)

  2. #2
    and the hat of sweating
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    It should be possible. Are you having problems?
    Do you mean doing both reading & writing in a single thread, or trying to read and write at exactly the same time? If it's the latter, then you should be using some kind of mutex to serialize either reading or writing but not both.

  3. #3
    and the hat of wrongness Salem's Avatar
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    If you're reading and writing fixed length records, yes.
    If you're reading a text file and trying to make each line longer, NO!

    Oh, and read the FAQ on why your use of eof() is bad.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  4. #4
    Big & Little Wong Tin-Bar Jackie Chan's Avatar
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    here it should be enough testing as:
    Code:
    while (studfile)
    {
        //...
    }
    but why studfile.eof() is bad ?
    and is my suggestion better ?

  5. #5
    and the hat of wrongness Salem's Avatar
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    > but why studfile.eof() is bad ?
    Hello! - Read the FAQ!
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  6. #6
    Big & Little Wong Tin-Bar Jackie Chan's Avatar
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    kindly can you point me to the FAQ? because the FAQ of this board only mentions that C File I/O feof() is bad. it says nothing about C++ eof().

  7. #7
    and the hat of wrongness Salem's Avatar
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    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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