Magic numbers '&' operator problem

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# 1  
Old 07-04-2011
Magic numbers '&' operator problem

Hello everyone,

on the man page of "magic(5)"
There is explanation
"&, to specify that the value from the file must have set all of the bits that are set in the specified value" .

My question is that what is the difference between '&' and equal operator '=' ? I tested it with file command but I do not understand the work way of it. For example, I prepared a magic.mime named 'mymagic' including only one line that is " 0 ubyte &0x00 filetypededected" and I create a file named 'myfile' setting its first byte to \x4e . when i call 'file -m mymagic myfile' it prints filetypededected so how can it be?
# 2  
Old 07-05-2011
Originally Posted by segmentation
"&, to specify that the value from the file must have set all of the bits that are set in the specified value" .

" 0 ubyte &0x00 filetypededected"
I don't have ubyte on my man page, what system are you using? Ignoring that, &0x00 says to scan 0x00 looking only at the 1 bits. Any 1 bits appearing in 0x00 must be matched by the corresponding bit in the byte under test. Since &0x00 has no 1 bits, all bytes will match it. In contrast, =0x00 means that only a byte of zero matches.
# 3  
Old 07-05-2011
Thank you very much friend it works now. "ubyte" is not writing on man pages but it is mentioned in somewhere in magic.mime database files and ulong, ubelong ... , as well. So "^" operator is vice versa of "&" we care about 0 bits, isn't it?

Aaa, I tested "^" operator for 0 bits this time but I have not idea about it.

In magic file, I inserted 0xd3 which is 11010011 in binary form
and in my file which I will test has 0xb1 on his first byte which is 10110001.
I operate ^ operator as "Or" and "exOr" but i could not get any idea.Could you help for this operator?

Last edited by segmentation; 07-05-2011 at 04:50 PM.. Reason: wrong guess
# 4  
Old 07-05-2011
"&0x1" means low order bit must be 1
"^0x1" means low order bit must be 0

And you're right, I have ubyte in my files but not on my man page.
# 5  
Old 07-05-2011
Sorry, I do not understand functionality of ^ operator by this sentences
" "^0x1" means low order bit must be 0 "

I posted above and will write again clearly.

In my magic.mime there is only one line
"0 byte ^0xd3 file\ detected"

and in my test file named myfile, the first byte is 0xb1.

0xd3 = 11010011
0xb1 = 10110001

when I write 'file -m magic.mime myfile'
it prints "file detected".

According to your explanation there should be 0 bits on testing byte corresponding 1 bits in magic.mime but in example above there is nothing like that.
# 6  
Old 07-05-2011
Xor is a bitwise operator:

If you have multiple 1 bits it is no longer a simple test.
0xd3 = 11010011
0xb1 = 10110001
0x62 = 01100010

It's not real obvious why you would do that. But the result is a match. Do you know C language? If so it is like:
if (a ^ b)   printf("match\n");

Make them both 0xb1 and the test will fail.
$ cat /tmp/mymagic
0 byte ^0xb1 filedetected
$ od -tx1 /tmp/x
0000000 b1 0a
$ file -m /tmp/mymagic /tmp/x
/tmp/x: ISO-8859 text

# 7  
Old 07-06-2011
I choose a=0x22 and b=0x32.
In C they match but in magic test not matched. When we think like C language it is similar to not equal but it does not work every time. I am implementing filetype detection in erlang so I need functionalities of magic operators.I still have problem with ^ operator. Smilie

The problem is solved.
At least one 0 bit should be placed in the test file instead of corresponding 1 bits in magic file.

Thank you very much for your helps.

Last edited by segmentation; 07-06-2011 at 08:41 AM.. Reason: Problem Solved
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