file id

This is a discussion on file id within the C Programming forums, part of the General Programming Boards category; i want my program to be able to figure out if an input file is one that was generated from ...

  1. #1
    Registered User trekker's Avatar
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    file id

    i want my program to be able to figure out if an input file
    is one that was generated from it.
    How can i use the magic number technique ?
    Are there any other portable ways ?

    TIA
    to boldy code where...

  2. #2
    Skunkmeister Stoned_Coder's Avatar
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    c++ imposes no structure on files it is entirely up to the programmer to structure his files exactly as he sees fit. So invent your own file format that will uniquely identify data files as belonging to your app.
    Free the weed!! Class B to class C is not good enough!!
    And the FAQ is here :- http://faq.cprogramming.com/cgi-bin/smartfaq.cgi

  3. #3
    Registered User trekker's Avatar
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    that's exactly what i want to do.
    i know the structure should be of my choice,
    but i'm asking for any common or popular ways to do so.
    I just want the doc for them
    to boldy code where...

  4. #4
    Obsessed with C chrismiceli's Avatar
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    One of my books told me ways to make magic numbers for files, I will look at it and post it when I get home this afternoon.
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  5. #5
    ATH0 quzah's Avatar
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    Much like graphics files or other common file formats, write a file header first. The first N bytes are this, the next N bytes are that... etc. Then add your data after it. You may want to jump over to Wotsit and look at a few file formats to get an idea of how things are commonly done.

    Quzah.
    Hope is the first step on the road to disappointment.

  6. #6
    Super Moderator VirtualAce's Avatar
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    Well there is a simple way with structures:

    Code:
    struct MyFileHeader
    {
      char  ID[10];       
      ...
      ..
    };

    ID could be any 10 characters. If the ID does not match your program's ID that it places in the string, it was not created by your program.

  7. #7
    Obsessed with C chrismiceli's Avatar
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    One method for choosing a magic number is choosing the first four letters of a program name(i.e, list) and then convert them to hexadecimal: 0x6C607374 then add 0x808080 to the number, producing the magic number of 0xECE0F3F4

    this algorythm generates a magic number that is probably unique. The high bit is set on each byte to make the byte non-ANCII and avoid confusion between caps and ascii files
    from Practicle C by, Steve Oualline

    That is one way to go
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