dd seek problem Post: 302230820

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Top Forums UNIX for Advanced & Expert Users dd seek problem Post 302230820 by silvermoon on Sunday 31st of August 2008 03:35:36 PM

08-31-2008

Registered User

Quote:

Originally Posted by era

Can you examine the resulting file? I assume you get the result from the first experiment in the first 281,881 bytes after the second operation, and its contents are 2 x 281,881 bytes (but of course, in the absence of a file system, it's hard to tell). What about when you copy it back, do you get padding on either end of the file, or the wrong number of bytes, or the wrong contents?

When it's copied back, I get the same size file. When I view the contents of the file, it has got the last few hundred bytes of the jpg. The rest of the file is zero's that were placed on the memory stick before the copy operation occurred.

I finally realized that default 550 obs was the issue when combined with skip, whereas seek used 281881.

The program I'm writing is in Python. My goal is to write a file at any location I want on any drive. It's part of an encryption idea, where the encryption decides where to store the file rather than the file management system. That's when I need to seek a number of bytes that isn't the ibs. I'm not sure the dd is going to help me with this. But I can't say that with full confidence, because I don't fully understand how it works yet.

silvermoon

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seek

seek(n) 						       Tcl Built-In Commands							   seek(n)

__________________________________________________________________________________________________________________________________________________

NAME

       seek - Change the access position for an open channel

SYNOPSIS

       seek channelId offset ?origin?
_________________________________________________________________

DESCRIPTION

       Changes	the  current access position for channelId.  ChannelId must be a channel identifier such as returned from a previous invocation of
       open or socket.	The offset and origin arguments specify the position at which the next read or write will occur for channelId. Offset must
       be an integer (which may be negative) and origin must be one of the following:

       start	 The new access position will be offset bytes from the start of the underlying file or device.

       current	 The  new access position will be offset bytes from the current access position; a negative offset moves the access position back-
		 wards in the underlying file or device.

       end	 The new access position will be offset bytes from the end of the file or device.  A negative offset places  the  access  position
		 before the end of file, and a positive offset places the access position after the end of file.

       The origin argument defaults to start.

       The  command  flushes  all buffered output for the channel before the command returns, even if the channel is in nonblocking mode.  It also
       discards any buffered and unread input.	This command returns an empty string.  An error occurs if this	command  is  applied  to  channels
       whose underlying file or device does not support seeking.

       Note  that  offset  values  are	byte offsets, not character offsets.  Both seek and tell operate in terms of bytes, not characters, unlike |
       read.

SEE ALSO

       file(n), open(n), close(n), gets(n), tell(n)

KEYWORDS

       access position, file, seek

Tcl									8.1								   seek(n)