m.dem.extract stuff...

Michael Shapiro shapiro at zorro.cecer.army.mil
Fri Jan 10 10:32:36 EST 1992

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).

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