Problems using struct pack/unpack in files, and reading them.

[kent nyberg]

Hi there,
Im deeply sorry for yet another question to this list.  I have come across a problem to which google seems not 
to eager to supply the anwser.

The problem is the following.
First I do this:

def setup_drive():
    test = pack('>HH',  0b1000000000000000, 0b1000000100000001)
    file = open('drive.bin', 'wb')
    for x in range(640000):
        file.write(test)
    file.close()


I have a pack, which I fill the file with. The data is not of interrest right now.

This all works great.
Then I do this:


def LoadCommandAndReact(place_to_read):
    global RegisterAX
    
    tmp = place_to_read.read()[RegisterAX:calcsize('HH')]
    klar = unpack('>HH', tmp)

    if place_to_read.closed:
       print("Drive error. Drive closed.")
    else:
       pass

    if checkfirstbit(klar[RegisterAX]):
       print("First bit is set. Move Cmd.")
       if (klar[0] & 0b0111111111111111):
          print("Cmd ERROR: Multiple commands is set.")
          pass
       else:
          #Here something to do with mv command.
          print("Adress to read to, from the offset of the command")
          print(klar[RegisterAX+1])  #Variable is always Offset+1          
    else:
       print("First bit is not set.")
    #Change RegisterAX offset+2bytes, 
    RegisterAX=+2   #We read two bytes per cycle. Becaus command is first, and variables are second.    




This all works If I run the LoadCommand..  only once.
But when I  run it for the second time,  it complains:

Traceback (most recent call last):
  File "core.py", line 98, in <module>
    LoadCommandAndReact(readfile)
  File "core.py", line 30, in LoadCommandAndReact
    klar = unpack('>HH', tmp)
struct.error: unpack requires a string argument of length 4



Im having trouble understanding the error.   unpack requires a string argument of length 4.
I have filled the file with data that should be readable, and is readable by the function 
when I run it for the first time. And since the file is filled with duplicates; should it not be readable the second
time aswell? I change the RegisterAX =+2. The amount I add should not matter, Its just my "offset" of where to read in the file.
The problem seems that the second time, unpack doesnt get the size of '>HH' to read. But surely 
.read()[RegisterAX:calcsize('HH')]  should assure that its fed with correct data?

I have filled the file with data of size '>HH' and first command reads that.  So next read should read the next
duplicate of that data that has been written to the file?


I understand its hard to read, and a long email. So well, its just a cry out in the dark evening.  If some one can
share some light.. I would be much happy about it.

/Kent.