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6065 :: Casting basics (a few questions)

• mantielero (orginal) [2020-03-11T23:37:03+01:00] view original

  I am trying to read c-array into a sequence. If somebody could confirm if my understanding is correct.

  In one hand, I have in C world: an address, the array size and the type. Something like: bufferptr, size and uint8 (it contains an image).

  In order to load the buffer into the seq, is the following right?

  var data = newSeq[uint8](size)
  for i in 0..<size:
    data[i] = cast[seq[uint8]](bufferptr)[i]
  Run

  Another thing that I would like to understand is pointer airithmetic. Is the following right?

  1. In order to operate with pointer, first cast the pointer to an int in order of being able to operate with it. This is done with cast[int](bufferptr).
  2. Once we have done pointer arithmetic, we cast it back to a pointer. That would be address.
  3. Finally, we get the content from that address casting it into a uint8. That would be value.

  let address = cast[pointer]( cast[int](bufferptr) + offset )
  let value = cast[uint8](address)
  Run

  Is my understanding correct?

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• yglukhov (orginal) [2020-03-12T10:27:41+01:00] view original

  Alternative:

  let mybuf: ptr uint8 = ...
  let mybufLenInBytes: int = ...
  var s = newSeq[uint8](mybufLenInBytes)
  if mybufLenInBytes != 0:
    copyMem(addr s[0], mybuf, mybufLenInBytes)
  Run

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• mantielero (orginal) [2020-03-12T17:12:16+01:00] view original

  Thank you both. I got it working.

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