Re^2: 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.)

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

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Re^3: how to unpack a C struct with length array preceding data array by AnomalousMonk (Archbishop) 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...	[reply] [d/l] [select]
Re^4: how to unpack a C struct with length array preceding data array by BrowserUk (Patriarch) on May 23, 2013 at 22:18 UTC
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.	[reply]
Re^4: how to unpack a C struct with length array preceding data array by Anonymous Monk on May 23, 2013 at 15:54 UTC
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.	[reply] [d/l] [select]


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