DOCUMENT:Q60252  29-NOV-2001  [visualc]
TITLE   :INFO: Initializing Bitfields as Integers in C
PRODUCT :Microsoft C Compiler
PROD/VER::1.0,1.5,2.0,4.0,5.0,6.0
OPER/SYS:
KEYWORDS:kbcode kbLangC kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600

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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 
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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

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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

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