Reading binary using a struct - behavour not as expected?

[richardd]

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