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384 :: pointer casting question

• stefandd (orginal) [2014-02-26T20:45:00+01:00] view original

  I am completely new to Nimrod, and unfortunately had to start playing with the unsafe stuff first. Could somebody kindly point out why the following does not print "hello, world!"? .. code-block:: nimrod

  var
    tststr: string = "hello, world!"
    tstptr: ptr string = addr(tststr)
    tstptr2: pointer
    memad: int
  
  echo tstptr[] # prints hello, world!
  memad = cast[int](tstptr[]) # address, same as the one listed in repr(tstptr)!
  tstptr2 = cast[pointer](memad) # generic pointer to address?
  tstptr = cast[ptr string](tstptr2) # converting back to pointer to string?
  echo tstptr[] # SIGSEV :-(
  

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• micklat (orginal) [2014-02-26T21:40:54+01:00] view original

  Are you sure that castint is supposed to give you a pointer to a string? remember that [] is the dereference operator. It seem like you're trying to reinterpret the string itself as an int, so i don't see why that should give you a pointer.

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• stefandd (orginal) [2014-02-26T21:47:50+01:00] view original

  castint works fine. When I display it, it is a 32-bit integer with the same value as the memory address of the string.

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• stefandd (orginal) [2014-02-26T21:51:27+01:00] view original

  @micklat: oops - my bad. I think you are right that this is my problem. Thanks for clarifying.

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• filwit (orginal) [2014-02-26T21:55:45+01:00] view original

  the [] operator gets the value of the pointed memory. So you're line with memad isn't getting the address of tstptr, it's casting the first 32/64bit memory block tstptr references to an int. Since tstptr is a pointer type you can directly cast it to an int. Change the following line and your code works.

  
  memad = cast[int](tstptr) # cast pointer to int
  Run

  [EDIT]

  ps. int's in Nimrod match the size of pointers by default. so no need to worry about casting to int32/int64 on different platforms.

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• stefandd (orginal) [2014-02-27T03:30:19+01:00] view original

  Yes, the code works now. Thanks for the explanations.

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