Article ID: 60252
Article Last Modified on 7/5/2005
APPLIES TO
- Microsoft C Professional Development System 6.0a
- Microsoft Visual C++ 1.0 Professional Edition
- Microsoft Visual C++ 1.5 Professional Edition
- Microsoft Visual C++ 2.0 Professional Edition
- Microsoft Visual C++ 4.0 Standard Edition
- Microsoft Visual C++ 5.0 Enterprise Edition
- Microsoft Visual C++ 6.0 Enterprise Edition
- Microsoft Visual C++ 5.0 Professional Edition
- Microsoft Visual C++ 6.0 Professional Edition
- Microsoft Visual C++ 6.0 Standard Edition
This article was previously published under Q60252
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: kbinfo kblangc kbcode KB60252
