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10845 :: #pragma pack(push, n) equivalent for Nim Objects

• emg (orginal) [2024-01-04T01:23:41+01:00] view original

  Hi,

  I'm trying to create Nim bindings for an arcane C API that uses struct packing specified using #pragma pack(push, n) all over the headers. What is the most effective way to achieve such struct packing in Nim Objects. Here's an example in C

  
  #pragma pack(push, 8)
  struct A {
  int m_0;
  double m_1;
  void* m_2;
  /* so on */
  ...
  char m_28
  };
  #pragma pack(pop)
  Run

  Ideally it would be great if they can be set at a scope or object level.

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• demotomohiro (orginal) [2024-01-04T06:45:15+01:00] view original

  I think packed pragma can be used for your case: https://nim-lang.org/docs/manual.html#foreign-function-interface-packed-pragma

  There is push and pop pragmas: https://nim-lang.org/docs/manual.html#pragmas-push-and-pop-pragmas

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• emg (orginal) [2024-01-04T17:21:47+01:00] view original

  Thanks for your reply. I don't necessarily need them always tightly packed, but with a byte boundary. The 'n' byte part in the C pragma compiler directive.

  Is it possible to do like this? I tried but it doesn't seem to compile.

  
  type
    TestObj {.packed(8).} = object
        m_1 : int64
        m2  : float64
        m3  : uint8
  Run

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• emg (orginal) [2024-01-04T22:22:20+01:00] view original

  Ok I think the following compiles and works as expected.

  
  type
    TestObj {.packed.} = object
        m_1 {.align(8).} : int64
        m2  {.align(8).}: float64
        m3  {.align(8).}: uint8
  Run

  It would be great if we could set the alignment for all object fields with one pragma, but I guess one can write macros. If there is a better way to do this please let me know.

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