FILE I/O. data written at wrong location.

This is a discussion on FILE I/O. data written at wrong location. within the C++ Programming forums, part of the General Programming Boards category; I got a series of functions that are connected too eachother, and that write in a certain data file, but ...

  1. #1
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    FILE I/O. data written at wrong location.

    I got a series of functions that are connected too eachother, and that write in a certain data file, but when I look into the data file, the data entered isnt where it is supposed too be! it always writes itself in the back of the file

    this is the code:

    Code:
      std::ofstream WriteIt ("C:\Datacab.dat", ios::app | ios::binary );
      WriteIt.seekp ( ( (Location)* sizeof (member) ), ios::beg );
      std::cout <<"location in bytes: "<<  (Location)* sizeof (member) << std::endl;
      WriteIt.write ( ( reinterpret_cast < char * > (&Wthis) ),sizeof (member) );
    I am 100&#37; sure that somthin is wrong RIGHT THERE since this is what i got as output checking:

    Code:
    Location: LOOP -> 1
    Registered!!
    location in bytes: 0
    wheras Location represents the times looped untill the place is found where the data must written, we do minus 1 (-1) to get the exact location in the file that needs too be overwritten. this is shown by : location in bytes -> 0

    but it just doesnt write there! help?!
    instead it just allways APPENDS on the back of the file, increasing it size too!! which isnt what i want

  2. #2
    and the hat of int overfl Salem's Avatar
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    > std::ofstream WriteIt ("C:\Datacab.dat"
    Erm, two backslashes perhaps?

    > ios::app
    AFAIK, if you open a file in append mode, then all writes go to the end, no matter where you've previously seeked to.
    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.
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    how am i supposed to write to certain locations then? ... all i know is ios::app
    *edit* i have changed my program and now it writes at the right location... but i still dont know why
    std::fstream WriteIt ("C:\Datacab.dat", ios::in | ios:ut | ios::binary );
    Last edited by epidemic; 04-05-2007 at 05:28 PM.

  4. #4
    and the hat of int overfl Salem's Avatar
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    Code:
    #include <iostream>
    #include <fstream>
    using namespace std;
    
    int main ( ) {
        ofstream f("foo.bin");
        f.write("hello",5);  // 5 chars at the start 
        f.seekp(5,ios::cur);  // a gap of 5 (will be zero filled)
        f.write("world",5);  // 5 more chars
        return 0;
    }
    
    Result
    $ od -Ax -t x1z foo.bin
    000000 68 65 6c 6c 6f 00 00 00 00 00 77 6f 72 6c 64     >hello.....world<
    00000f
    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.

  5. #5
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    can i ask, what the diffrence between .bin files and .dat files are?
    and i also solved my problem this way:

    Code:
    std::fstream WriteIt ("C:\Datacab.dat", ios::in | ios:ut | ios::binary );
    but i can explain why that does work.

  6. #6
    and the hat of int overfl Salem's Avatar
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    .bin, .dat, .foo, .bar, .xxx - doesn't make a bean of difference to the code.

    The only thing which counts is ios::binary

    Why are you still using a single \ ?

    Just because \D isn't defined just makes you lucky if you're seeing "Datacab.dat" in the file system. Just try "C:\newfile.dat" and enter the land of fun.
    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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