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883 :: writeBuffer noob question

• kuba (orginal) [2015-02-17T02:49:04+01:00] view original

  Hi there, How do I properly use writeBuffer procedure? I can't find how to pass a pointer to the array which I guess is the problem... Thanks.

  var f = open("mypath", fmWrite)
  let aaa = [1,2,3]
  writeBuffer(f, aaa ,cast[int](3))
  Run

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• bozaloshtsh (orginal) [2015-02-17T03:01:09+01:00] view original

  Does addr aaa work?

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• def (orginal) [2015-02-17T03:02:55+01:00] view original

  var f = open("mypath", fmWrite)
  var aaa = [1,2,3]
  let written = f.writeBuffer(addr aaa, sizeof(aaa))
  Run

  You need to use var instead of let so that aaa can even have an address. The address of a let would not always make sense, so it doesn't work. The size of the buffer is in bytes, so it has to be 3 * sizeof(int) or simply sizeof(aaa).

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• kuba (orginal) [2015-02-17T03:19:19+01:00] view original

  Thanks, that works. One thing which still confuses me is the following:

  f.writeBuffer (addr aaa ,sizeof(aaa))    # Error: value of type 'int' has to be discarded
  let written = f.writeBuffer (addr aaa ,sizeof(aaa)) # compiles fine
  Run

  I don't get why I'm expected to store the result in the variable?

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• def (orginal) [2015-02-17T03:21:57+01:00] view original

  Because writeBuffer is not guaranteed to actually write all you want it to write. You're supposed to check the result, which is the number of bytes written. If you really don't want to check whether written == sizeof(aaa) you can just use discard f.writeBuffer(addr aaa, sizeof aaa).

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• kuba (orginal) [2015-02-17T03:24:07+01:00] view original

  Got it. Thanks!

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• fadg44a3w4fe (orginal) [2015-02-17T03:37:10+01:00] view original

  discard f.writeBuffer(addr aaa, sizeof aaa) is a terrible idea and can lead to silent data loss. doAssert f.writeBuffer(addr aaa, sizeof aaa) == sizeof(aaa) is a better choice, it'll fail loudly.

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