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Strange MD5 behaviour

 


perl4me2
New User

Jan 19, 2009, 1:40 PM

Post #1 of 2 (551 views)
Strange MD5 behaviour Can't Post
Hello,

I have a strange problem I need help with.

Scenario:

Program1:
opens a binary file,
:loop
reads 1024 bytes with OFFSET,
runs a MD5,
save MD5 to a textfile,
increment OFFSET by bytes read,
loop until end of file.

Program2:
:loop
opens a binary file,
reads 1024 bytes with OFFSET,
runs a MD5,
save MD5 to a textfile,
close the file,
increment OFFSET by bytes read,
loop until end of file.

Only the first MD5's are equal between textfiles, any idea why?
Note the difference between program 1 and 2, number 1 reads while the file is open, 2 re-opens and positions with OFFSET.
If in program2 I read the exact previous block and then read the following block the MD5 equals.

So it sounds like OFFSET is not working (neither is seek) as it should.


perl4me2
New User

Jan 20, 2009, 2:28 AM

Post #2 of 2 (544 views)
Re: [perl4me2] Strange MD5 behaviour [In reply to] Can't Post
Solved... a very stupid mistake to make...

$buffer = "";
seek (INF,$position,0);
$fsize = read (INF, $buffer, $size, $position);


When reading a binary block from a file, seek is not broken, but you can't use a position parameter at both functions, if you do you're buffer will get pre-pended with zero bytes until the start of the data read.

Ie. read with position 100:

0,0,0,0,0,0,0,0(up to 99) then the data read. (this way a md5 always fails and the buffer gets larger and larger)

$buffer = "";
seek (INF,$position,0);
$fsize = read (INF, $buffer, $size);


This works SlyLaugh me happy camper.

 
 


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