m.dem.extract stuff...

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m.dem.extract stuff...

John E. Parks-3

Regarding the m.dem.extract talk... I believe that a tape device
  is bassically regarded as one file under that particular device
  name (directory/file)  In short there funtionally is no difference
  between specifying a tape device or a file.

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Re: m.dem.extract stuff...

Michael Shapiro-3
In response to the following:
|
|
|Regarding the m.dem.extract talk... I believe that a tape device
|  is bassically regarded as one file under that particular device
|  name (directory/file)  In short there funtionally is no difference
|  between specifying a tape device or a file.
|

(1) m.dem.extract does assume a tape drive and probably won't work with
files. A lot of the GRASS tape programs make assumptions about tapes
(mutiple files with EOFs, read asking for x bytes will succeed if the
record lenght is less than x and will return the correct count of the
number of bytes in a physical tape record, etc.)

(2) The 4.0 version of m.dem.extract is defective. It has been fixed
but hasn't been re-released yet. We are going to start putting fixes
on the OGI ftp site (moon.cecer.army.mil) shortly and we will announce
these fix upgrades on this list and the grassp-list.


Michael Shapiro (CERL)

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Re: m.dem.extract stuff...

Michael Shapiro-3
In reply to this post by John E. Parks-3
In response to this comment:

|Regarding the m.dem.extract talk... I believe that a tape device
|  is bassically regarded as one file under that particular device
|  name (directory/file)  In short there funtionally is no difference
|  between specifying a tape device or a file.

There is a major difference between tapes and  files.  Tapes  can
have  multiple  files  on  them (embedded EOFs between files) and
have true physical record lengths. This means that a program  can
keep  reading  after one EOF and get more data (if there are more
files) and that read() requests for more data than  is  contained
in  a  physical  record only read what is in that physical record
(and return the number of bytes read).  It  will  not  read  data
from two physical records in one read() request.  This is clearly
not the case for  files.  The  implication  is  that  GRASS  tape
reading  commands  which  make theses assumptions about the input
device won't work on files (ie data copied from  the  tape  to  a
file to get it onto a system which doesn't have a tape drive).