Why does Platform SDK define union LARGE_INTEGER in such a way?

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I found these links on MSDN(Implementation of LARGE_INTEGER & ULARGE_INTEGER)
http://msdn.microsoft.com/en-us/library/windows/desktop/aa383713(v=vs.85).aspx[^]
http://msdn.microsoft.com/en-us/library/windows/desktop/aa383742(v=vs.85).aspx[^]

typedef union _LARGE_INTEGER {
  struct {
    DWORD LowPart;
    LONG  HighPart;
  };
  struct {
    DWORD LowPart;
    LONG  HighPart;
  } u;
  LONGLONG QuadPart;
} LARGE_INTEGER, *PLARGE_INTEGER;

Who can tell me what the consideration for the former definition is,
and what does it mean by the struct name "u" (means "unsigned" ?, I'm
not sure.)? and Why/Where should we use LARGE_INTEGER instead of _int64
Thanks in advance

Posted 20-Nov-14 15:40pm

Buddhi Chaturanga

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User 59241 · Answer 1 · 2014-11-20T17:55:00

Firstly the structure u seems redundant. Microsoft supports anonymous structures: http://msdn.microsoft.com/en-us/library/z2cx9y4f.aspx[^]

Thus in a Microsoft environment you could write:

C#

LARGE_INTEGER li;
li.LowPart = 1;

This is non-portable so to make it portable the named structure u is added. Now you can write:

C++

li.u.LowPart = 1;

Use it where _int64 is not supported.