PRB: #pragma pack Can Generate Compiler Warnings (98314)


The information in this article applies to:

  • Microsoft Visual C++ for Windows, 16-bit edition 1.0
  • Microsoft Visual C++ for Windows, 16-bit edition 1.5
  • Microsoft Visual C++ for Windows, 16-bit edition 1.51
  • Microsoft Visual C++, 32-bit Editions 1.0
  • Microsoft Visual C++, 32-bit Editions 2.0
  • Microsoft Visual C++, 32-bit Editions 2.1
  • Microsoft Visual C++, 32-bit Editions 4.0
  • Microsoft Visual C++, 32-bit Editions 4.1
  • Microsoft Visual C++, 32-bit Editions 4.2
  • Microsoft Visual C++, 32-bit Editions 5.0
  • Microsoft Visual C++, 32-bit Editions 6.0
This article was previously published under Q98314

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)
{}
Modification Type:MinorLast Reviewed:7/5/2005
Keywords:kbCompiler kbprb KB98314
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