Q40027: INFO: #pragma pack() Affects Declarations, Not Definitions

Article: Q40027
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,1.51,1.52,2.0,2.1,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbLangC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Last Modified: 29-NOV-2001

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The information in this article applies to:

- Microsoft C for MS-DOS 
- Microsoft C/C++ for MS-DOS 
- Microsoft Visual C++, versions 1.0, 1.5, 1.51, 1.52, 2.0, 2.1, 4.0 
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SUMMARY
=======

The pack pragma affects declarations, not definitions. Place #pragma pack()
prior to data declarations.

MORE INFORMATION
================

Sample Code
-----------

The following program demonstrates usage of #pragma pack():

  /* compile options needed */ 
  /* Elements of variables of type struct x will be byte-aligned. */ 
  #pragma pack(1)
  struct x { int a; char b; int c; };

  /* Elements of variables of type struct y will be word-aligned. */ 
  #pragma pack(2)
  struct y { int a; char b; int d; };

  /* The pragma below does NOT affect the definitions that follow. */ 
  #pragma pack(4)

  struct x X;
  struct y Y;

  void main (void)
  { /* dummy main */ }

NOTE: The default packing value for the 16-bit products mentioned above is 2
bytes. The default packing value for Visual C++ 32-bit, versions 1.0 and 2.xx,
is 4 bytes. The default packing value for Visual C++ 32-bit, version 4.0, is 4
bytes for bit fields and 8 bytes otherwise.

Additional query words:

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Keywords          : kbLangC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbZNotKeyword8 kbvc150 kbvc100 kbCCompSearch kbZNotKeyword3 kbVC500 kbVC600 kbVC151 kbVC200 kbVC210 kbVC32bitSearch kbVC152 kbVC500Search
Version           : :1.0,1.5,1.51,1.52,2.0,2.1,4.0,5.0,6.0
Issue type        : kbinfo

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