National Instruments 320685D-01 ユーザーズマニュアル

Chapter 3

Windows 95/NT Compiler/Linker Issues

LabWindows/CVI Programmer Reference Manual

3-6

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 National Instruments Corporation

The compilers differ in their default maximum alignment of elements within structures.

If your DLL API uses structures, you can guarantee compatibility among the different 
compilers by using the 

pack

 pragma to specify a specific maximum alignment factor. Place 

this pragma in the DLL include file, before the definitions of the structures. You can choose 
any alignment factor. After the structure definitions, reset the maximum alignment factor back 
to the default, as in the following example:

#pragma pack (4)  /* set maximum alignment to 4 */

typedef struct {

      char a;

      int b;

   } MyStruct1;

typdef struct {

      char a;

      double b;

   } MyStruct2;

#pragma pack ()  /* reset max alignment to default */

LabWindows/CVI predefines the 

__DEFALIGN

 macro to the default structure alignment of 

the current compatible compiler.

Borland C/C++ uses the smallest number of bytes necessary to hold the bit fields you specify 
in a structure. The other compilers always use 4-byte elements. You can force compatibility 
by adding a dummy bit field of the correct size to pad the set of contiguous bit fields so that 
they fit exactly into a 4-byte element. Example:

typedef struct {

   int  a:1;

   int  b:1;

   int  c:1;

   int  dummy:29;      /* pad to 32 bits */

} MyStruct;

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