Reading binary using a struct - behavour not as expected?

[Larry Bates]

Rich,

Ok, your problem is this:

When you define attributes immediately after
the class definition they are SHARED among
all instances of the class. Basically they are
global (actually this can come in handy if you
want to keep counters across class instances).
You were copying into lastcopy, but everytime
you unpacked, you overwrote your shared attributes.

Try this instead:

class PCAPPacketHdr:
    #
    # Attributes defined here are global across
    # instances of this class.
    #
    FormatString = "LLLL"

    def __init__(self):
        #
        # Attributes defined here are local to
        # the class instance.
        #
        self.TSSec = None
        self.TSUSec = None
        self.InclclLen = None
        self.OrigLen = None
        return

This will share FormatString (it doesn't change),
but have individual attributes for the other
values you wish to be unique between two different
instances of the class.

Regards,
Larry Bates
Syscon, Inc.

"richardd" <richardd at hmgcc.gov.uk> wrote in message
news:40712544$1 at mail.hmgcc.gov.uk...
> Hi,
> I am writing code to deal with PCAP files. I have a PCAP dump and I am
> looking at the timestamps in the PCAP packet headers to see if they are in
> the correct order in the file. To do this I have a class called
> PCAPPacketHdr as follows
>
> import struct
>
> class PCAPPacketHdr:
>     FormatString = "LLLL"
>     TSSec = None
>     TSUSec = None
>     InclLen = None
>     OrigLen = None
>
>     def Pack(self):
>         return struct.pack( self.FormatString, self.TSSec, self.TSUSec,
> self.InclLen, self.OrigLen )
>
>     def Unpack(self, buffer):
>         self.TSSec, self.TSUSec, self.InclLen, self.OrigLen =
> struct.unpack( self.FormatString, buffer )
>
>     def Size(self):
>         return struct.calcsize (self.FormatString)
>
>
> I then have code which opens up the file (skipping the PCAP file magic
> number and PCAP file header), and reads in each packet header as follows:
>
> while not eof:
>      #read in PCAPPacketHdr
>      buf = curFile.read(packetHdr.Size())
>      if len(buf) == packetHdr.Size():
>          packetHdr.Unpack(buf)
>          if lastPacketHdr != None:
>              if lastPacketHdr.TSSec > packetHdr.TSSec:
>                  outputFile.write("ERROR: Packet TSSec earlier than last
> one: \n")
>                  outputFile.write("       Last Packet
> "+repr(lastPacketHdr.TSSec)+"."+repr(lastPacketHdr.TSUSec)+"\n")
>              elif lastPacketHdr.TSSec == packetHdr.TSSec:
>                  if lastPacketHdr.TSUSec > packetHdr.TSUSec:
>                      outputFile.write("ERROR: Packet TSUSec earlier than
> last one\n")
>                      outputFile.write("       Last Packet
> "+repr(lastPacketHdr.TSSec)+"."+repr(lastPacketHdr.TSUSec)+"\n")
>          outputFile.write("       Packet
> "+repr(packetHdr.TSSec)+"."+repr(packetHdr.TSUSec)+"\n")
>          lastPacketHdr = copy.deepcopy(packetHdr)
>          #skip packet payload
>          packetPayload = curFile.read(packetHdr.InclLen)
>      else:
>          eof = True
>
>
> This code appears to work fine for extracting the timestamps from the
file,
> the repr( ) calls on the timestamps allow me to write them to the output
> file correctly, it's just the comparison operators don't appear to be
> working as I would expect. It appears than when the TSUSec timestamp is
the
> same as the previous one in the data I input, it reports "ERROR: Packet
> TSUSec earlier than last one".
>
> This makes me think that the comparison operators aren't acting on the
data
> as longs as I expected.
>
> Can anyone shed some light on what I'm doing wrong, I'm still very new to
> Python.
>
> Thanks in advance,
>
> Rich
>
>