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

ID: Q40027

The information in this article applies to:

Microsoft C for MS-DOS, versions 6.0, 6.0a, 6.0ax
Microsoft C/C++ for MS-DOS, version 7.0
Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52
Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0, 6.0

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:

Keywords : kbLangC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Version : MS-DOS:6.0,6.00a,6.00ax,7.0; WINDOWS:1.0,1.5,1.51,1.52; WINDOWS NT:1.0,2.0,2.1,4.0,5.0
Platform : MS-DOS NT WINDOWS
Issue type :