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Q98314: PRB: #pragma pack Can Generate Compiler Warnings

Article: Q98314
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,1.51,2.0,2.1,4.0,4.1,4.2,5.0,6.0
Operating System(s): 
Keyword(s): kbCompiler kbVC100 kbVC150 kbVC151 kbVC200 kbVC210 kbVC400 kbVC410 kbVC420 kbVC500 kbVC
Last Modified: 29-NOV-2001

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The information in this article applies to:

- Microsoft C/C++ for MS-DOS 
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51 
- Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 4.1, 4.2, 5.0, 6.0 
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SYMPTOMS
========

When an application uses the pack pragma, C/C++ the compiler generates the
following message:

  warning C4103 used #pragma pack to change alignment

Microsoft Visual C++ 5.0 generates a different warning:

  warning C4653: compiler option 'structure packing (/Zp)' inconsistent with
  precompiled header; current command-line option ignored

CAUSE
=====

The pragma pack argument differs from the structure packing option specified on
the compiler command line. This warning was added to the compiler because
third-party tools can change the performance of user code by including a pragma
pack message in library include files. The warning message is designed to
indicate potential bugs in code under development.

RESOLUTION
==========

If the code restores the pack argument to the structure packing option specified
on the compiler command line, the warning does not occur. The sample code below
demonstrates restoring the compiler default.

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

The pack pragma and the /Zp compiler option each pack data structures to a
specified byte boundary. For example, if a structure requires 9 bytes of storage
and the compiler command line includes the /Zp1 option or the code includes the
#pragma pack(1) statement, the compiler reads and writes 9 bytes of information
when it accesses the structure. If the compiler command line includes the /Zp2
option or the code includes the #pragma pack(2) statement, the compiler reads
and writes 10 bytes when it accesses the structure.

Adding the /Zp option to the command line may generate a different warning if the
PCH file is not updated after the change:

  warning C4653: compiler option 'structure packing (/Zp)' inconsistent with
  precompiled header; current command-line option ignored

If the code includes the pack pragma, the compiler determines whether the
/Zp<x> option is specified, where <x> is 1 (the default when no
number is specified), 2, 4, 8, or 16. If the compiler command line does not
specify the /Zp option, the compiler packs structures on 2-, 4-, or 8-byte
boundaries depending on the compiler version.

The compiler generates a warning if a header file changes the structure packing
boundary and does not restore it to the value it before the end of the file.

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

  /*
  * Compile options needed: /Zp2 or /Zp4
  */ 

  FILE.H
  ------

   #pragma pack(1)

   // Remove the comment from the following line to eliminate this
   // warning:
   // #pragma pack()

  FILE.C
  ------

  #include "file.h"

  void main(void)
  {}

Additional query words: 8.00 8.00c 9.00 9.10

======================================================================
Keywords          : kbCompiler kbVC100 kbVC150 kbVC151 kbVC200 kbVC210 kbVC400 kbVC410 kbVC420 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbZNotKeyword8 kbvc150 kbvc100 kbZNotKeyword3 kbVC410 kbVC420 kbVC500 kbVC600 kbVC151 kbVC200 kbVC210 kbVC32bitSearch kbVC16bitSearch kbVC500Search
Version           : :1.0,1.5,1.51,2.0,2.1,4.0,4.1,4.2,5.0,6.0
Issue type        : kbprb

=============================================================================

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