Re: Questions about reading binary files - Managed C++/CLI Discussion Boards

Re: Questions about reading binary files

GentooGuy29-Sep-07 9:20

GentooGuy

29-Sep-07 9:20

okay that's strange.

thanks you for the help, I really appreciated it Smile | :)

Re: Questions about reading binary files

GentooGuy29-Sep-07 10:27

GentooGuy

29-Sep-07 10:27

it works fine now Smile | :)

thanks a lot!

Re: Questions about reading binary files

GentooGuy29-Sep-07 10:52

GentooGuy

29-Sep-07 10:52

I'm sorry, but I still think they're swapped.
My reason for this is the fact that I'm looking up 0x9003, which is a tag in a JPEG file indicating the date of the picture.
When using 'od', I see this:

<br />
$ od -x file.jpg  | grep 9003<br />
0000520      0004    0000    3230    3030    9003    0002    0014    0000<br />

This is the only occurence of '9003' in a file which does contain the information (so this must be the instance I'm looking for).
But, when running my program, and printing some lines, I get this output:

<br />
4<br />
0<br />
0<br />
0<br />
30<br />
32<br />
30<br />
30<br />
3<br />
90<br />
2<br />
0<br />
14<br />
0<br />

This is produced by the loop you've proposed. When comparing both outputs, I see this one has the bytes swapped when compared to od and (!) the exif standard. So I guess, od isn't wrong. Or am I indeed wrong?

Re: Questions about reading binary files [modified*2]

Mark Salsbery29-Sep-07 11:08

Mark Salsbery

29-Sep-07 11:08

I think we're confusing two different issues here...

First, your "od" is lumping 2 byte pairs and is assuming little-endian
byte order so, as you can see from the sample listings you've posted,
each pair of bytes appears swapped in the od-generated listing.

Second, you have to parse your file properly, depending on the byte order.
exif has some kind of tag to indicate whether multi-byte integers are stored
in "motorola" or "intel" order. This doesn't mean you can just go through the
entire file and swap every pair of bytes. This means when you encounter
multibyte-integer data in the file, you may need to swap bytes to work with
the data on your platform.

You need to parse the file bytes following its type and format. I don't have the jpeg/jfif
format memorized but it's well documented all over the internet Smile | :)

Again, the only swapping going on here is by your "od" utility. In your code you simply
have the bytes in the same order they occur in the file.

Mark

*edit* LOL I really meant "sample", not "ample"....ample sounded snotty LMAO

Last modified: 17mins after originally posted --

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Re: Questions about reading binary files [modified*2]

GentooGuy30-Sep-07 2:40

GentooGuy

30-Sep-07 2:40

okay that sounds possible.

But when having a look at http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/EXIF.html
I can see the tags about the creation date are 0x9003 and 0x9004 (still have to decide which one to take).

when a having a look at the jpg itself (which has a date and time of 01-01-1998) i see the strings about the date in a proper sequence.... but there's no 9003. Just a 0390 before it.

I've looked it up in the exif documentation, and I haven't found anything about inverting the bytes on such markers. What am I missing?

Re: Questions about reading binary files

Mark Salsbery30-Sep-07 6:31

Mark Salsbery

30-Sep-07 6:31

GentooGuy wrote:
but there's no 9003. Just a 0390 before it.

Where are you seeing that? In your od results? If so, then that IS 9003
because od is swapping avery pair of bytes.

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Re: Questions about reading binary files

GentooGuy30-Sep-07 6:37

GentooGuy

30-Sep-07 6:37

Nop, in the VS binary editor

Re: Questions about reading binary files

Mark Salsbery30-Sep-07 6:47

Mark Salsbery

30-Sep-07 6:47

I don't know what to tell you....

There's only three possibilities here:

1) The file was written incorrectly (not following specs)
2) There's a tag somewhere that indicates the byte order and you need to use it
3) You're interpreting the binary bytes incorrectly.

Which is it? Smile | :)

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Re: Questions about reading binary files

GentooGuy30-Sep-07 6:55

GentooGuy

30-Sep-07 6:55

option 2.

I've read something about it yesterday, currently trying to find the document which described it.

Re: Questions about reading binary files

Mark Salsbery30-Sep-07 7:10

Mark Salsbery

30-Sep-07 7:10

JPEG is big-endian.

EXIF follows TIFF specs which can be big or little endian.
This is usually determined by the first two bytes of the file:
"II" (0x49 0x49) for little endian, "MM" (0x4D 0x4D) for big endian.

For a file with "MM" byte order, tags (and all other multi-byte fields
of tags) will need to be swapped on Intel machines.

For the tag 0x9003, I would expect the following storage in the file:

Big endian: 0x90 0x03
Little endian: 0x03 0x90

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Re: Questions about reading binary files

GentooGuy30-Sep-07 14:51

GentooGuy

30-Sep-07 14:51

thanks, I've got it working Smile | :)

Re: Questions about reading binary files

Mark Salsbery29-Sep-07 11:11

Mark Salsbery

29-Sep-07 11:11

BTW are you using Visual Studio? If so, open the jpeg file in the binary editor window.
It won't swap any bytes in the display.

File menu -> Open/File...
Select the file
Click the little drop arrow on the "Open" button and choose "Open with..."
Choose binary editor

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Re: Questions about reading binary files

Mark Salsbery29-Sep-07 11:22

Mark Salsbery

29-Sep-07 11:22

I looked up od

You should be using something like "-t x1" (or is it "-t xC") instead of "-x" on your command line since
you want single-byte hex integers, not double-byte integers.

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Re: Questions about reading binary files

Luc Pattyn29-Sep-07 11:49

Luc Pattyn

29-Sep-07 11:49

right! forget od in 16-bit mode.

Luc Pattyn [Forum Guidelines] [My Articles]

this weeks tips:
- make Visual display line numbers: Tools/Options/TextEditor/...
- show exceptions with ToString() to see all information
- before you ask a question here, search CodeProject, then Google

Re: Questions about reading binary files

Luc Pattyn30-Sep-07 4:31

Luc Pattyn

30-Sep-07 4:31

FYI:

I checked some JPEG documents and can confirm JPEG is "big-endian" which means multi-byte
quantities need their byte order reversed on a "little-endian" machine such as Intel's x86.

From the doc: "For parameters which are 2 bytes (16 bits) in length, the most significant
byte shall come first in the compressed data’s ordered sequence of bytes."

As already stated by Mark, this does NOT mean you should swap every pair of bytes; it does
mean if two bytes are to represent a 16-bit integer, then you should swap both bytes; if 4 bytes
represent a 32-bit integer, then you should swap all four bytes.

Since 2B and 4B integers will not always be properly aligned in memory, you will need
a method that interprets 2 bytes in the right order, whereever they are, as in the following
C# code snippet:

// unsigned !
private byte getByte() {
    return bytes[p++];
}

// unsigned and big-endian !
private ushort getShort() {
    ushort bHi=getByte();
    ushort bLo=getByte();
    return (ushort)((bHi<<8)+bLo);
}

where bytes[] holds the entire file content, and p is the "current position".

Smile | :)

Luc Pattyn [Forum Guidelines] [My Articles]

C++ Byte Level Code.. [modified]

spalanivel27-Sep-07 21:26

spalanivel

27-Sep-07 21:26

Hi,
What is "Byte Level Code" in C++?? When and How we are using?? Any tutorial regarding this topic...Thanks

-- modified at 9:25 Friday 28th September, 2007

Re: C++ Byte Level Code..

led mike28-Sep-07 5:19

led mike

28-Sep-07 5:19

spalanivel wrote:
What is "Byte Level Code" in C++??

Byte level code is a term mostly associated with Java. Java compilers do not produce "machine code", they produce a machine independent code that was often called "byte code". To execute the byte code you need an interpreter specific to the machine (processor / architecture) because the processor can only execute machine code.

The original intent of C/C++ is to produce machine code. If there are non machine code C++ compilers then they would produce something that might be called byte level code meaning that it needs an interpreter because the processor can't execute it.

Re: C++ Byte Level Code..

spalanivel2-Oct-07 5:24

spalanivel

2-Oct-07 5:24

Thanks Mike..

Simple ESC Key

Michael10127-Sep-07 18:18

Michael101

27-Sep-07 18:18

Hey everyone,

I've got a C++ console application and it goes into a loop while that never ends except when the user presses the ESC key! Right now my loop is coded like this:

while (!kbhit())
{ code }

I have to change the kbhit bit to make it so it only ends the loop when I press the ESC key!

Thanks for your help in advance, I appreciate it!

Michael Wink | ;)

Re: Simple ESC Key

Luc Pattyn28-Sep-07 7:44

Luc Pattyn

28-Sep-07 7:44

This seems to be unmanaged C++, if so you posted in the wrong forum.
Maybe this will do it: while(!kbhit()||getch()!=27) {...}

Smile | :)

Luc Pattyn [Forum Guidelines] [My Articles]

Another DataGridView Question

BuckBrown27-Sep-07 11:37

BuckBrown

27-Sep-07 11:37

Damn it. I just discussed this last week and I can't find the answer in my notes. I want to set the current cell. The documentation has an example in C# is...

myDataGridView->CurrentCell = myDataGridView(1, 1);

I can't do this in C++/CLI. What is the correct syntax to set the current cell?

Buck

Re: Another DataGridView Question

BuckBrown27-Sep-07 11:49

BuckBrown

27-Sep-07 11:49

Don't bother, I've got it. Amazing how I can spend an hour trying to do something and then figure it out 5 minutes after posting. It's...

myDataGridView->CurrentCell = myDataGridView->Rows[row]->Cells[col];

Re: Another DataGridView Question

Luc Pattyn27-Sep-07 13:19

Luc Pattyn

27-Sep-07 13:19