Manipulate numeric data in binary files

pkipe · March 3rd, 2008, 06:37 PM

I have a binary file (written by a C++ app) that contains a numeric data field. The decimal value of the field is 6187 as displayed in an app GUI, however when I open the binary file in a hex editor, the field is shown as 2b 18 00 00.

When I convert 6187 to hex in a calculator, the result is 182b, so it is clear that the number is being stored as four bytes in reverse order (instead of 00 00 18 2b).

I'm working with VB6. I need to get the value out of the record, increment its value, and update the record with the new value. Is there a simple (straight-forward) way to do this?

Any help would be greatly appreciated.

gbianchi · March 4th, 2008, 07:07 AM

Hi there... why don't you just read the first byte, add 1 to it (as a byte number) and then store it again where it should (that can be a pain in the ass, since you have to write all the file to do that)..

HTH

Gonzalo

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BrianWren · March 4th, 2008, 05:52 PM

Gonzalo, that would give trouble when there is a carry...

I think you will need to convert this to decimal (in Integer), increment it, then convert back to Least-Significant-Byte/Most-Significant-Byte, and write it back to the file.

So get the first value, then add the second value times 256 to it.
Then increment.
Then write the value Mod 255
Subtract the value Mod 255, then divide the results of that by 255, and write it.

gbianchi · March 4th, 2008, 09:24 PM

You won't believe me Brian, but I was thinking in the carry a second before I wrote the post.. then I forget about it... damm...

Thanks Brian...

HTH

Gonzalo

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marcostraf · March 11th, 2008, 08:06 PM

declare a variabile of type Long, and read that from the file:

Code:

    Dim f As Integer
    f = FreeFile
    Open "c:\foo" For Binary Access Read As #f
    Dim k As Long
    Get f, ,k
    debug.print k, "0x" & hex(k)
    close f

and you'll see that you get 6187 0x182B as results, that is what you want
Increment k by 1, open the file as readwrite, and put back k

Code:

    Open "c:\foo" For Binary Access Read Write As #f
    k = k + 1
    Put f, , k
    Close f

"There are two ways to write error-free programs. Only the third one works."
Unknown

BrianWren · March 12th, 2008, 03:28 PM

Quote:

quote:Originally posted by gbianchi
You wonât believe me Brian, but...

But, I do believe you. (Would it be too over the top to say, âGlad I could help carry the solution to the poster?â)

BrianWren · March 12th, 2008, 03:30 PM

I didn't realize that reading out of a file into a long handled the LSB-MSB order of things properly. That's a good thing to know. Kudos.

gbianchi · March 12th, 2008, 03:35 PM

yeah Marco great solution..

HTH

Gonzalo

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