INFO: Initializing Bitfields as Integers in C

• 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, versions 1.0, 1.5
• Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 4.0, 5.0

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.

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>
        |?|?|?|?|?|?|?|?|?|?|?|?|?|?|?|?|

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

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

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;
   }