Hello Everybody!
I am writing a networking application in python for a small piece of
hardware, in which there could sometimes be timeouts. I am using
sockets to communicate to this device. Data that is sent to the device
is instructions for that piece of hardware, and data recieved is just
responses from that hardware.
When a timeout happens, for some reason extra data is stored inside
the buffer, so when the timeout is over, that extra data (remember this
data is instructions) is executed by the hardware, which I don't want.
For timeout purposes, I want the socket to be nonblocking, but at the
same time this means that I can't flush the buffer, which I think would
solve the problem.
I realize this is probably quite unclear, so below is a tcpdump which
will explain what I mean.
15:10:37.667963 aurl06f2.39979 > 192.168.128.161.46667: P 481:493(12)
ack 481 win 5840 <nop,nop,timestamp 157458905 528016> (DF)
4500 0040 090c 4000 4006 599f 8fd1 06f2
c0a8 80a1 9c2b b64b 30c7 9289 a552 55df
8018 16d0 c3ec 0000 0101 080a 0962 a1d9
0008 0e90 0000 0001 f800 0110 0000 0000
15:10:51.107769 aurl06f2.39979 > 192.168.128.161.46667: P 481:493(12)
ack 481 win 5840 <nop,nop,timestamp 157460249 528016> (DF)
4500 0040 090d 4000 4006 599e 8fd1 06f2
c0a8 80a1 9c2b b64b 30c7 9289 a552 55df
8018 16d0 beac 0000 0101 080a 0962 a719
0008 0e90 0000 0001 f800 0110 0000 0000
15:10:51.108259 192.168.128.161.46667 > aurl06f2.39979: P 481:493(12)
ack 493 win 5792 <nop,nop,timestamp 533605 157460249> (DF)
4500 0040 5580 4000 3f06 0e2b c0a8 80a1
8fd1 06f2 b64b 9c2b a552 55df 30c7 9295
8018 16a0 dbf5 0000 0101 080a 0008 2465
0962 a719 8000 0001 f800 0110 0000 4d05
15:10:51.108283 aurl06f2.39979 > 192.168.128.161.46667: P 493:565(72)
ack 493 win 5840 <nop,nop,timestamp 157460249 533605> (DF)
4500 007c 090e 4000 4006 5961 8fd1 06f2
c0a8 80a1 9c2b b64b 30c7 9295 a552 55eb
8018 16d0 cb29 0000 0101 080a 0962 a719
0008 2465 0000 0001 f800 0110 0000 0000
0000 0001 f800 0110 0000 0000 0000 0001
f800
************* NOTICE THE EXTRA DATA *****************
15:10:51.108731 192.168.128.161.46667 > aurl06f2.39979: P 493:505(12)
ack 565 win 5792 <nop,nop,timestamp 533605 157460249> (DF)
4500 0040 5581 4000 3f06 0e2a c0a8 80a1
8fd1 06f2 b64b 9c2b a552 55eb 30c7 92dd
8018 16a0 db99 0000 0101 080a 0008 2465
0962 a719 8000 0001 f800 0110 0000 4d0d
15:10:51.111760 aurl06f2.39979 > 192.168.128.161.46667: P 565:577(12)
ack 505 win 5840 <nop,nop,timestamp 157460249 533605> (DF)
4500 0040 090f 4000 4006 599c 8fd1 06f2
c0a8 80a1 9c2b b64b 30c7 92dd a552 55f7
8018 16d0 a867 0000 0101 080a 0962 a719
0008 2465 0000 0001 f800 0114 0000 0000
The last 3 words (32 bits/word) of the hex printout shows the
condition. The first of the last 3 words is the instruction, in this
case a read instruction: 0000_0001. The second is the address:
f800_0110 or f800_0114. The third is the data recieved.
As you can see, the first 2 printouts are a timeout (it is much longer
than this, but i didnt want to post all of that) because one machine is
communicating to the other without a response. The third printout is
when the machine recovers from the timeout, and it works fine.
However, the printout after the machine recovers has some extra data at
the end. This is the data I want to flush out of the buffer, but I
cant really figure out how to do so.
Sorry for the long post and thanks a lot for your help
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