Convert erlang binary pattern match to Elixir

Questions / Help
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1

I’m currently working on writing an SFTP server in Elixir, leveraging the OTP ssh daemon and sftp server functionality.

I need to improve my logging, and part of that is deciphering the messages sent via SSH.

Looking at the erlang code, I can see where it is happening in ssh_sftp.erl in handle_data/3 … but I’m sort of stumped when trying to pattern match out the op code/data from the raw binary in Elixir (mainly because I’m a little unfamiliar on where the Len is getting declared in these matches, I know that the ?UINT32 is unsigned-big-integer-32)

2

In elixir it is msg::binary-size(123) and rest::binary

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3

I always struggle with the conversion from erlang to elixir pattern matching and I never remember where to find the documentation.

I think I’ve used this page in the past:
https://hexdocs.pm/elixir/Kernel.SpecialForms.html

and more specifically this section:

https://hexdocs.pm/elixir/Kernel.SpecialForms.html#<<>>/1

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4

Awesome! Thank you both so much, this definitely helped unlock it for me.

Just to close the loop, this code is working for me. Looks like (based on your link @cmkarlsson), that Len is just a regular variable assigned, specifying the binary length of the message:

        <<message_length::unsigned-integer-32, message::binary-size(message_length),
            _rest::binary>> = raw
    <<op_code, _req_id::unsigned-big-integer-32, data::binary>> = message

Thanks again!

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