prob with struct and byte order

[John Machin]

Marc 'BlackJack' Rintsch wrote:
> In <1153785622.870353.85960 at b28g2000cwb.googlegroups.com>, nephish wrote:
>
> > tohex gave me
> > '53 54 58
>
>    S  T  X
>
> > 00 00 00 34
>
> Length!?  Decimal 57.

3 * 16 + 4 -> 52 where I come from -- assuming hex means hexadecimal
and not witchcraft :-)

>
> > 00 00 00 c8
>
> Type!?  Decimal 200.
>
> > 70 69 76 6f 74 72 61 63 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > 74 72 61 63 31 70 69 76 6f 74 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>
> Payload!?
>
> > 45 4e 58'
>
>   E  N  X
>
> > this is the login message (message type 200)
>
>
> The problem I see is the length.  The payload without STX, ENX and the two
> numbers in front is 47 bytes so there's a 5 byte difference.

I don't think so.

>  You have to
>  look at some more messages to get an idea how the length corresponds to
>  the actual payloads length.

Yes, but not because of the 5-difference problem. The OP has favoured
us with 3 messages (in 3 different formats), 2 x login and 1 of some
sort of data. See below.

8<--- script start
import struct

def unhex(s, spaced):
    return ''.join([chr(int(s[x:x+2], 16)) for x in xrange(0, len(s), 2
+ spaced)]) # gasp

hex1 =
"5354580000002c000000ea3137353834363638353500000000000000000000000000000000000000000000d6090d5400000000454e58"
txt1 = unhex(hex1, 0)

txt2 =
'STX\x00\x00\x004\x00\x00\x00\xc8stateman\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00state1man\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00ENX'


hex3 = '53 54 58 00 00 00 34 00 00 00 c8 70 69 76 6f 74 72 61 63 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 74 72 61 63 31 70 69 76 6f 74
00 00 00 00 00 00 00 00 00 00 00 00 00 00 45 4e 58'
txt3 = unhex(hex3, 1)

for msgno, msg in enumerate((txt1, txt2, txt3)):
    print "\nMessage %d: length of actual string is %d" % (msgno + 1,
len(msg))
    print "Has STX/ENX:", msg.startswith("STX") and msg.endswith("ENX")
    print "repr:", repr(msg)
    print "hex :", ' '.join(["%02x" % ord(x) for x in msg])
    msg_len, msg_type = struct.unpack('>II', msg[3:11])
    print "Internal len: %d; type: %d" % (msg_len, msg_type)
8<--- end script, start output

Message 1: length of actual string is 54
Has STX/ENX: True
repr:
'STX\x00\x00\x00,\x00\x00\x00\xea1758466855\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xd6\t\rT\x00\x00\x00\x00ENX'
hex : 53 54 58 00 00 00 2c 00 00 00 ea 31 37 35 38 34 36 36 38 35 35 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 d6 09 0d
54 00 00 00 00 45 4e 58
Internal len: 44; type: 234

Message 2: length of actual string is 61
Has STX/ENX: True
repr:
'STX\x00\x00\x004\x00\x00\x00\xc8stateman\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00state1man\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00ENX'
hex : 53 54 58 00 00 00 34 00 00 00 c8 73 74 61 74 65 6d 61 6e 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 73 74 61 74 65 31 6d 61 6e 00 00
00 00 00 00 00 00 00 00 00 00 00 00 45 4e 58
Internal len: 52; type: 200

Message 3: length of actual string is 62
Has STX/ENX: True
repr:
'STX\x00\x00\x004\x00\x00\x00\xc8pivotrac\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00trac1pivot\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00ENX'
hex : 53 54 58 00 00 00 34 00 00 00 c8 70 69 76 6f 74 72 61 63 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 74 72 61 63 31 70 69 76 6f 74 00
00 00 00 00 00 00 00 00 00 00 00 00 00 45 4e 58
Internal len: 52; type: 200
8<---
Messages 1 and 3 tend to indicate that external_len == internal_len +
10 ... maybe there's been a copy/paste problem somewhere along the line
with message 2; perhaps the OP could check this.

Cheers,
John