Structs unpack and pack changes data

I am currently trying to pack some data in a script "p1.py", and then unpack it in another script "p2.py" via a pipe. The pipe communication works perfectly, and I'm able to send data between the two scripts. However, upon unpacking the data gets changed. In this case, I do this in script "p2.py" in order to pack the data and send it over to the "p1.py" script:

COMMAND_STRUCT = struct.Struct(">fB3s")    packed_data = struct.pack(">fB3s",29.1,1,b'3s')  print(packed_data)  #lateral_airspeed_input, drop_package_commanded_byte, _ = COMMAND_STRUCT.unpack(packed_data)  #print(lateral_airspeed_input)  

As we can see, I print "packed_data" which is packed to be a float, a byte and then 3 bytes. Now if I simply print the packed data to my terminal, I get this output:

b'A\xe8\xcc\xcd\x013s\x00'

Which is simply the packed form of the 29.1,1 and 3 bytes. If I uncomment the last two lines, lateral_airspeed_input will be 29.1 -indicating that the unpacking and packing works, as I can pack the data and unpack and get the same data after unpacking.

Now, here's where it get interesting. When I bring in the other script, I do the same thing as seen here:

        cmd = pilot.stdout.read(COMMAND_STRUCT.size)          #print(cmd)          if len(cmd) != COMMAND_STRUCT.size:              result = CRASHED  # The pilot process must have exited              break          lateral_airspeed_input, drop_package_commanded_byte, _ = COMMAND_STRUCT.unpack(cmd)          print(lateral_airspeed_input)          lateral_airspeed = max(-30.0, min(30.0, lateral_airspeed_input))          drop_package_commanded = bool(drop_package_commanded_byte)  

Here, the packed data is read onto the variable cmd, and then cmd is unpacked to the lateral_airspeed_input, drop_package_commanded_byte and the _ variables. Ideally, if we print lateral_airspeed_input we should be getting 29.1. However, I get 7.713289749049545e+20 upon printing it. However, if I just print cmd straight away, I get this:

b"b'A\xe8\" b'xcc\xcd\' b'x013s\x0' b"0'\r\n"

which looks similar to the packed data from before - when I was just using one py script. For some reason, the packed data gets all these extra , b" and 0\r\n things on it. I think this is messing up the communication and I was wondering if there's a way to avoid this. I'd rather avoid this than stripping the codes off the lines.

Thanks for the help. I am using subprocess pipes to manage the communication between the two scripts, and simply running the subprocess through my windows cmd.

https://stackoverflow.com/questions/66898122/structs-unpack-and-pack-changes-data April 01, 2021 at 11:40AM