Q60252: INFO: Initializing Bitfields as Integers in C

Article: Q60252
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,2.0,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbcode kbLangC kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600
Last Modified: 29-NOV-2001

-------------------------------------------------------------------------------
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, 2.0, 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 
-------------------------------------------------------------------------------

SUMMARY
=======

In Microsoft C, you can initialize the values of an entire bitfield structure as
an integer in several ways:

- Declare the bitfield structure as part of a union with an integer. (This is
  the preferred method).

- Use an integer pointer by setting the pointer to the address of the structure
  and then changing what the pointer points to.

- Enforce the bitfield type constraints to get a copy of the bitfield into an
  integer variable.

For examples of these three methods, see below.

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

In Microsoft C, bitfields are stored in word-sized blocks with the least
significant bit representing the first bit of the bitfield. For example, the
bitfields in bitstruct, defined below in the example, are stored as follows:

          <  p4   > <  p3   > < p2> <p1>
       |?|?|?|?|?|?|?|?|?|?|?|?|?|?|?|?|

Assigning the integer 0x4c to this structure results in the following bit
pattern:

       |0|0|0|0|0|0|0|0|0|1|0|0|1|1|0|0|

The bitfields are given the following respective values:

  p1=0 p2=3 p3=2 p4=0

If the number of bits needed for a bitfield structure exceeds 16 in code compiled
for MS-DOS or Windows, words will be added as needed to provide room for the
structure with no single bitfield crossing a word boundary. Microsoft C/C++,
32-bit Edition, stores bitfield structures in double word values. If the field
requires more than 32 bits, the compiler uses additional double words as
needed.

NOTE: There is no "standard" for storing bitfields in memory; therefore, any
program that depends on a particular storage method is not portable to systems
that use a different method.

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

  struct strtype
     {
     unsigned p1:2;
     unsigned p2:3;
     unsigned p3:5;
     unsigned p4:5;
     } bitstruct;

  union untype
     {
     struct strtype un_bitstruct;
     unsigned bit_integer;
     } bitunion;

  unsigned *intptr;
  unsigned intgr;

  void main(void)
  {
     /*  Using the bitfield structure only */ 

        /* Set the pointer to address of bitfield */ 
     intptr = (unsigned *)&bitstruct;

        /* Change the bitfield */ 
     *intptr = 0x4c;

        /* Get the new value */ 
     intgr = *(unsigned *)&bitstruct;

     /* Using an union makes this much easier (syntactically) */ 

        /* Set the pointer */ 
     intptr = &bitunion.bit_integer;

        /* Change the bitfield */ 
     bitunion.bit_integer = 0x4c;

        /* Get the new value */ 
     intgr = bitunion.bit_integer;
  }


Additional query words: 8.00 8.00c 9.00

======================================================================
Keywords          : kbcode kbLangC kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbZNotKeyword8 kbvc150 kbvc100 kbCCompSearch kbZNotKeyword3 kbVC500 kbVC600 kbVC200 kbVC32bitSearch kbVC500Search
Version           : :1.0,1.5,2.0,4.0,5.0,6.0
Issue type        : kbinfo

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

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.