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Re^2: how to unpack a C struct with length array preceding data array

by BrowserUk (Pope)
on May 23, 2013 at 12:11 UTC ( #1034934=note: print w/ replies, xml ) Need Help??


in reply to Re: how to unpack a C struct with length array preceding data array
in thread how to unpack a C struct with length array preceding data array

Problem with that is it only works(*) where the structure has len/data len/data; but the OP has len1/len2 data1/data2.

(*Barring playing games with relative or absolute positioning per AnomalousMonk ig's reply.)


With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
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".
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Comment on Re^2: how to unpack a C struct with length array preceding data array
Re^3: how to unpack a C struct with length array preceding data array
by AnomalousMonk (Monsignor) on May 23, 2013 at 14:03 UTC

    I'm playing games with relative or absolute positioning?!? Have you seen ig's reply? I haven't even begun to understand what's going on there, although to be fair, I haven't really had time yet to study it: I don't know the significance of the  'W' specifier, and the  '@1W@0Wx/x/a' bit looks interesting, and the whole thing is enclosed in a  '(...)' group to which no quantification seems to be applied... Hmmm, interesting...

      ig's reply was quite instructive, I had some trouble understanding it, but it was worth the trouble.

      The (..) could be omitted, but since @ is a position "counted from the start of the innermost ()-group", by putting them there you can reuse this template as is in a bigger structure. (see pack)

      W is "An unsigned char value " so 'W W' takes the two first chars and adds them to the output list : (5,6). '@0Wx' goes back to the beginning of the structure, reads one byte as a number (5), and skips one. So we're now at the Beginning of "Hello" and the output list should be (5,6,5). '/a' takes the last element in the output list as a length (5), and reads that many characters: we get (5,6,"Hello").

      Now for the best part. With '@1W@0Wx' we go back to pos 1, read "\006", go back to 0, read "\005" and skip one. We are at the beginning of "Hello" again, and the ouput list is (5,6,"Hello",6,5). '/x' takes the last value in the output list and skips as many bytes, so we end up just before " World" with ouput list: (5,6,"Hello",6). Another '/a' takes the last value in the output list, and reads a 6 bytes long string. We end up with (5,6,"Hello", " World").

      What I learned with this exemple is that the '/' template is not necessarily lenth/type, but that /type pops the last value read, so that you may stack more than one, and then make as many /type readings. And well, that's not too much of a surprise, it was in pack's documentation all along :).

      I do agree with ig though, you should avoid that solution if possible, it's hard to read, and it would be difficult to expand it for a string of three parts or more.

      I'm playing games ...

      Sorry to both you & ig for the misattribution.


      With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
      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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