in reply to Re: Need help with (un)pack templates
in thread Need help with (un)pack templates

Hey BrowserUk!

I've read the docs on (un)pack, so I understand what the characters in the template represent. I just don't understand when I'm supposed to use those characters. For example, in the above code for decoding an IP packet, which is network data-- How am I supposed to know I shouldn't be using "n" or "N" to decode the network data?

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Re^3: Need help with (un)pack templates
by BrowserUk (Patriarch) on Dec 20, 2009 at 14:32 UTC
    How am I supposed to know I shouldn't be using "n" or "N" to decode the network data?

    That's a tough question to answer in full. Essentially, you have to know what format the data you are trying to decode is in.

    For example,

    • Fields 1 & 2 are both 4-bits, hence the template you have extracts them both as a single 8.bit number, which is presumably broken down further later in the code.
    • Field 3 is is an 8-bit number, and extracted as such.
    • Field 4 is a 16-bit number, hence they use 'n' to extract it.
    • Field 5 Ditto.
    • Fields 6 & 7 are 3 & 13 bits respectively. But as #7 crosses a byte boundary, they extract the two as a single 16-bit number (the third 'n'), and then (presumably) break that down furtehr later in the code.
    • etc. ...

    Without knowing what the format of the data you have is, there is no way to know what templates are applicable.

    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
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