INFO: Initializing Bitfields as Integers in C

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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.


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:
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;


Article ID: 60252 - Last Review: February 27, 2014 - Revision: 2.1
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