# Writing packet payloads to file **URL:** \ **Category:** GNU Radio\ **Created:** [November 17, 2012, 12:29am UTC](https://www.ruby-forum.com/t/writing-packet-payloads-to-file/224565 "2012-11-17T00:29:01Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![aris](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/258eb7/32.png) [@aris](https://www.ruby-forum.com/u/aris)\ **Post date:** [November 17, 2012, 12:29am UTC](https://www.ruby-forum.com/t/writing-packet-payloads-to-file/224565/1 "2012-11-17T00:29:01Z")
Hello all, I am researching a new coding algorithm. I am transmitting and coding a text file using payload\_tx.py and then write the binary of the packet payload to a file to do offline analysis. I am having trouble writing packet payloads to file, however, because the I can’t seem to find a way to get data out of a gr\_message except for the to\_string() method. If I try and convert the string back to binary (my attempt involved python’s binascii module), I get an “Error: illegal char” message. This makes sense, since the data is still coded, and should not be a valid ascii string. But how can I get around this problem and write binary to file? ---
**Author:** ![Colleen\_J](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/5e9695/32.png) [@Colleen\_J](https://www.ruby-forum.com/u/Colleen_J)\ **Post date:** [November 20, 2012, 7:09pm UTC](https://www.ruby-forum.com/t/writing-packet-payloads-to-file/224565/2 "2012-11-20T19:09:12Z")
On Fri, Nov 16, 2012 at 6:28 PM, Colleen J. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > message. > Class of 2013 > Colleen, Have you tried to use struck.unpack? Tom ---
**Author:** ![Colleen\_J](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/5e9695/32.png) [@Colleen\_J](https://www.ruby-forum.com/u/Colleen_J)\ **Post date:** [November 21, 2012, 8:22pm UTC](https://www.ruby-forum.com/t/writing-packet-payloads-to-file/224565/3 "2012-11-21T20:22:46Z")
Thanks for the response. I’m not sure I understand how struct.unpack will help, since the only data I can get seems to already be a python string. If I transmit a packet with “hello world” as my payload, the only thing I ever see in the receive script is the python string “hello world”. I don’t know how to access the intermediate binary representation. ---
**Author:** ![Colleen\_J](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/5e9695/32.png) [@Colleen\_J](https://www.ruby-forum.com/u/Colleen_J)\ **Post date:** [November 24, 2012, 4:31pm UTC](https://www.ruby-forum.com/t/writing-packet-payloads-to-file/224565/4 "2012-11-24T16:31:54Z")
On Wed, Nov 21, 2012 at 2:21 PM, Colleen J. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Thanks for the response. I’m not sure I understand how struct.unpack will > help, since the only data I can get seems to already be a python string. If > I transmit a packet with “hello world” as my payload, the only thing I ever > see in the receive script is the python string “hello world”. I don’t know > how to access the intermediate binary representation. Colleen, Take a look at how we handle this stuff in gr-digital. Specifically, in gr-digital/python/pkt.py, the method run() of \_queue\_watcher\_thread extracts the message contents as a string and passes it to packet\_utils.unmake\_packet (in packet\_utils.py). We have a bunch of helper functions in there to handle the string data (such as string\_to\_hex\_list and the like). Finally, the paylad (As a string) is passed up to the example scripts like gr-digital/examples/narrowband/benchmark\_rx.py where the rx\_callback metho uses struct.unpack to extract the packet number, which is non-ASCII. Hopefully, following the way we handle the messages as strings and the various methods we use to manipulate them here will help you with your tasks. Tom